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Pyrolysis of Carbohydrates
C H A P T E R
11 Pyrolysis of Carbohydrates
S U B C H A P T E R
11.1
Monosaccharides
GENERAL ASPECTS In nature, the most abundant class of biomolecules is that of carbohydrates, including compounds from plants (glucose, fructose, cellulose, starch, pectin, etc.), animals (glucose, glycogen, etc.), bacteria, fungi, archaea, and protists (microbial and fungal polysaccharides, etc.). Carbohydrates (saccharides or sugars) are organic compounds containing in their molecule one or more carbonyl groups and two or more alcohol groups on an aliphatic hydrocarbon chain. These compounds can be classified as monosaccharides, oligosaccharides, and polysaccharides. The general formula for monosaccharides is CnH2nOn (n ¼ 3 for trioses, n ¼ 4 for tetroses, n ¼ 5 for pentoses, etc.). The oligosaccharides and polysaccharides are generated by the elimination of water from two or more monosaccharide molecules, with the sugar units being connected by an ether group. Besides simple carbohydrates that contain only carbonyl and alcohol functional groups (as well as ether for oligosaccharides and polysaccharides), more complex carbohydrates also are known. These may include other functional groups such as halogen, carboxyl (dCOOH), amino (dNH2), amido (dNHCO-R), etc. Several deoxy sugars (with the hydroxyl group replaced by hydrogen) also are known (deoxyribose, rhamnose, etc.). Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00011-1
419
# 2019 Elsevier B.V. All rights reserved.
420
11. PYROLYSIS OF CARBOHYDRATES
Besides natural carbohydrates, many synthetic carbohydrates were produced. Some are related to chemically modified polysaccharides (e.g., ethyl cellulose, carboxymethyl cellulose). Others are synthetic small organic molecules. Glyceraldehyde and dihydroxyacetone (n ¼ 3) can be considered the simplest monosaccharides. (Glycolaldehyde or hydroxyacetaldehyde HOCH2dCHO also is considered by some a two-carbon monosaccharide.) Glyceraldehyde has one asymmetric carbon. Consequently, for this compound there are two possible enantiomers indicated as D and L. The D form is shown with the OH to the right of the carbon chain when the CH]O group is at the top, and the L form with the OH to the left, in a so-called Fisher formula:
For glyceraldehyde, the D-form is an (R)-enantiomer while the L form is an (S)-enantiomer. Monosaccharides with a larger number of carbons may have more asymmetric carbons but they can be conventionally derived from glyceraldehyde (or dihydroxyacetone) by including more H C OH units in the carbon chain between the CH]O group and the lowest H C OH unit. Glyceraldehyde leads to the series of aldoses, and dihydroxyacetone to the series of ketoses. The aldoses with the same stereochemistry as D-glyceraldehyde at the asymmetric carbon that is the most distant from the carbonyl group will form the D-series of aldoses. The monosaccharides with the same stereochemistry as L-glyceraldehyde at the most distant asymmetric carbon will form the L-series. The D-series of aldoses is shown below:
GENERAL ASPECTS
421
In monosaccharides with n > 4, it is common that the carbonyl group forms with one of the dOH groups an internal hemiacetal, such that many monosaccharides have a cyclic structure. The open chain (linear) form of monosaccharides with n > 4 can be found only in water solutions as a small proportion of the dissolved compound. The cyclic hemiacetal can have the form of a five-atom ring (furanose) or a six-atom ring (pyranose).
The proportion of the furanose form versus the pyranose form at equilibrium depends on the structure of the sugar. Glucose, for example, is present almost completely in pyranose form while talose is about 69% pyranose and 31% furanose. The formation of the cyclic hemiacetals of monosaccharides adds one more asymmetric carbon to their structure compared to the linear form. For example, linear glucose has four asymmetric carbons while cyclic glucose has five asymmetric carbons. The new asymmetric C-1 has two possible steric forms (two anomers), indicated as α and β. The configurations of cyclic monosaccharides are described by several types of formulas. As an example, the following formulas are shown for α-D-glucopyranose: Fischer, modified Fischer, Haworth, cyclic, and structural (shorter bonds in the Haworth and structural formulas indicate dH).
Fischer formulas have the same orientation of OH groups as the linear ones. In the Haworth formulas (of aldoses), the side chain (e.g., CH2OH group) is up when it is attached to an R carbon (the original OH to this carbon was to the right in the linear formula), and the side chain is down when the carbon to which it is attached in the ring is S. If the side chain is up, the position of the other substituents (e.g., OH) is reversed to what it was in the Fischer formula (see Fischer and modified Fischer formulas for glucose). In this way, the OH groups on the right (R) from the Fischer formula (and from the linear form) are down in the Haworth formula. For the side chain down, the OH (and other) groups on the right in the Fischer formula are up in the Haworth formula, and those to the left are down. For D-aldoses the steric assignment for the C-1 carbon is α when this is an S carbon (and it is shown down) and is β when this is an R carbon.
422
11. PYROLYSIS OF CARBOHYDRATES
The nomenclature of carbohydrates includes common names as well as systematic names. The common names are sometimes indicated by a three-letter convention: arabinose (Ara), glucose (Glc), galactose (Gal), etc. The IUPAC name of carbohydrates can be found in the literature [1]. In IUPAC nomenclature the numbering of atoms includes not only the carbons but also the oxygen atom (which is numbered as (1). For this reason, the IUPAC name of α-Dglucopyranose is (2S,3R,4S,5R,6R)-6-(hydroxymethyl)tetrahydro-2H-pyran-2,3,4,5-tetraol. Monosaccharides and the lower oligosaccharides (e.g., di-, trisaccharides), being small organic molecules, are included among the molecules further discussed in this chapter regarding their thermal decomposition. The presentation of pyrolysis of polysaccharides (such as cellulose, starch, and chitin) is beyond the purpose of this book and can be found in various other publications (see, e.g., Ref. [2]). The importance of studying the pyrolysis of carbohydrates is related to the wide distribution in nature of this class of compounds. Pyrolysis can occur in a variety of intentional and unintentional processes such as pyrolysis in incinerators, forest fires, cigarette smoking, and food cooking [3].
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS There is very little direct information regarding the pyrolysis of trioses and tetroses. Under initial mild heating, condensation products of these molecules takes place, and further pyrolysis generates compounds similar to those of higher sugars. In basic conditions and heating (e.g., in the presence of NaOH or by treatment with tetramethylammonium hydroxide (TMAH)), these compounds generate carboxylic acids (formic, acetic), hydroxycarboxylic acids (glycolic, lactic), and various lactones [4,5]. Pentoses pyrolysis has been more frequently reported in the literature [5–8]. The pyrolysis of these compounds at temperatures higher than 550°C typically generates as the main products H2O, CO2, aldehydes (hydroxyacetaldehyde, formaldehyde, acetaldehyde, etc.), ketones (butanone, 1-hydroxypropanone, etc.), furan and pyran derivatives (high levels of furfural), and several deoxy-sugars. Smaller molecules that do not have chiral centers and are generated during pyrolysis are very similar for different pentoses. As an example, the time window between start and 41 min of the pyrograms for D-()-arabinose, D-()-ribose, and D-(+)-xylose is shown in Fig. 11.1.1. Arabinose and ribose are present mainly in furanose form while xylose is mainly present in pyranose form. Pyrolysis of all samples shown in Fig. 11.1.1 was performed on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of mixture that includes only the compounds eluting in the first 41 min in the pyrogram are shown in Table 11.1.1. In addition, the calculation of the mole % was obtained solely based on peak areas, and because differences in the MS response factors can be quite large for different compounds, the estimations may have large errors. As shown in Table 11.1.1, for the compounds generated by pyrolysis and eluting from the chromatographic column in the first 41 min, both the nature and the relative concentration of
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
423
(A)
(B)
(C) FIG. 11.1.1 First 41 min from the pyrograms obtained at 900°C from 1.0 mg D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C).
the compounds generated from the three analyzed pentoses is very similar, in spite of the difference in the cycle type (furanose or pyranose) assumed by the sugar. At longer retention times in the pyrograms, several heavier and more polar compounds start eluting. Fig. 11.1.2 displays the time window from 42 to 58 min for the pyrograms of 1.0 mg D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C) (the first part of these chromatograms is shown in Fig. 11.1.1).
424
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.1 Identification of the Main Peaks in the Chromatograms Shown in Figs. 11.1.1A–C for Pyrolysis at 900°C of D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C) Moles % (for Compounds Eluting in the First 42 min)
No. Compound
Retention Time (Min)
MW CAS#
Arabinose Ribose Xylose
1
Carbon dioxide
4.24
44
124-38-9
14.25
11.36
11.14
2
Propene
4.47
42
115-07-1
0.83
0.67
0.86
3
Formaldehyde
4.65
30
50-00-0
6.20
6.50
6.53
4
Acetaldehyde
5.81
44
75-07-0
1.61
1.17
1.38
5
Furan
7.63
68
110-00-9
1.32
1.42
1.31
6
2-Propenal
8.69
56
107-02-8
0.76
0.45
0.85
7
Acetone
9.10
58
67-64-1
1.14
0.50
0.63
8
Pyruvaldehyde
9.63
72
78-98-8
4.29
5.17
5.43
9
2-Methylfuran
12.06
82
534-22-5
0.73
0.59
0.82
10
Butenal
13.85
72
123-72-8
0.20
0.21
0.28
11
2,3-Butanedione
14.48
86
431-03-8
0.31
0.34
0.33
12
Hydroxyacetaldehyde
17.77
60
141-46-8
5.74
8.52
6.72
13
Formic acid
18.50
46
64-18-6
0.23
0.23
0.13
14
Acetic acid
20.07
60
64-19-7
0.84
0.88
0.67
15
Ethyl-1-propenyl ether
22.33
86
928-55-2
0.15
0.15
0.18
16
1-Hydroxy-2-propanone
22.48
74
116-09-6
1.39
1.60
1.48
17
Toluene
23.07
92
108-88-3
0.09
0.09
0.06
18
Methyl formate ?
23.80
60
107-31-3
0.43
0.44
0.25
19
Hydroxyacetic acid (glycolic acid)
24.63
76
79-14-1
0.08
0.08
Trace
20
2,2’-Bioxirane ?
26.71
86
1464-53-5
0.04
0.07
0.02
21
1,4-Dioxadiene
28.92
84
N/A
4.44
4.27
3.49
22
3-Furaldehyde
29.13
96
498-60-2
0.27
0.04
0.32
23
2-Oxopropionic acid methyl ester
29.36
102
600-22-6
0.12
0.34
0.12
24
2,5-Furandione
29.65
98
108-31-6
0.51
0.63
0.45
25
2-Cyclopentene-1,4-dione
30.30
96
930-60-9
3.43
4.60
1.37
26
Furancarboxaldehyde (furfural)
30.44
96
98-01-1
26.22
30.62
32.24
27
2-Propylfuran
30.83
110
4229-91-8
trace
0.08
0.31
28
2-Furanmethanol
31.70
98
98-00-0
0.76
1.03
0.85 Continued
425
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
TABLE 11.1.1 Identification of the Main Peaks in the Chromatograms Shown in Figs. 11.1.1A–C for Pyrolysis at 900°C of D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C)—Cont’d Moles % (for Compounds Eluting in the First 42 min)
No. Compound
Retention Time (Min)
MW CAS#
Arabinose Ribose Xylose
29
1-(2-Furanyl)ethanone
33.16
110
1192-62-7
0.22
0.24
0.23
30
5,6-Dihydro-2H-pyran-2-one
33.65
98
3393-45-1
0.38
Trace
0.39
31
2-Methylcyclopenete-1-one
33.93
96
1120-73-6
0.02
0.06
0.02
32
Dihydro-4-hydroxy-2(3H)-furanone
34.12
102
5469-16-9
0.74
0.74
0.60
33
Dihydro-3-methylene-2(3H)-furanone
34.40
98
547-65-9
0.64
0.24
0.21
34
2-Hydroxy-2-cyclopenten-1-one
34.53
98
10493-98-8
0.41
0.33
0.46
35
2-Methylenecyclopentanol
35.41
98
20461-31-8
1.12
0.54
1.28
36
2,3-Dihydroxypropanal
35.67
90
367-47-5
5.28
2.41
6.45
37
Butyrolactone
36.54
86
96-48-0
0.42
0.17
0.53
38
Tetrahydro-4H-pyran-4-one
36.77
100
29943-42-8
0.34
0.12
0.33
39
5-Methyl-2,3-dihydrofuran-2,4-dione
36.89
114
N/A
0.50
0.44
0.39
40
Ethyl-1-propenyl ether ?
37.20
86
928-55-2
0.47
0.23
0.21
41
6,8-Dioxabicyclo[3.2.1]octane
37.44
114
280-16-0
2.45
1.38
2.58
42
1,3-Dihydroxy-2-propanone
37.77
90
96-26-4
4.59
5.25
3.91
43
2-Hydroxy-3-methyl-2-cyclopenten-1-one
37.99
112
80-71-7
0.81
0.79
0.81
44
2H-pyran-2,6(3H)-dione
38.13
112
5926-95-4
0.62
1.43
0.61
45
4,5-Dimethyl-1,3-dioxol-2-one ?
39.38
114
37830-90-3
0.78
0.72
0.44
46
Unknown
40.16
114
N/A
1.94
0.66
0.58
47
3-Furancarboxylic acid methyl ester
40.88
126
13129-23-2
0.20
0.20
0.19
48
Unknown
41.03
116
N/A
0.32
0.33
0.14
49
Unknown
41.29
130
N/A
0.61
0.62
0.65
50
Unknown
40.55
98
N/A
0.06
0.24
0.02
51
3-Furancarboxylic acid methyl ester
40.88
126
13129-23-2
0.10
0.19
0.18
52
Unknown
41.29
130
N/A
0.61
0.62
0.60
Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification.
The correct identity of the peaks from these time windows in the pyrograms was more difficult to obtain through searches in the available mass spectral libraries. The peak eluting 43.15 min in the three pyrograms was tentatively identified as resulting from the molecules of pentoses by the elimination of two water molecules to generate a 5,7-dioxabicyclo[2.2.1]
426
11. PYROLYSIS OF CARBOHYDRATES
Abundance 8,000,000
45.56
7,000,000
HO
46.52
O OH
6,000,000 5,000,000 4,000,000 3,000,000 2,000,000
OH
43.15 46.20 46.94
1,000,000 0 42.0 (A)
OH
44.0
46.0
53.45
49.58 48.0
50.0
52.0
54.0
55.92 56.0
58.0
8,000,000
HO
7,000,000
O
6,000,000 5,000,000
45.51
4,000,000
OH
47.80
OH
OH
3,000,000
53.69 54.45 54.66
2,000,000 1,000,000
(B)
42.0
46.18 46.93
43.15 44.0
46.0
49.58 48.0
51.20
50.0
52.0
54.0
56.0
58.0
56.0
58.0
8,000,000 7,000,000
50.11
6,000,000
OH
5,000,000 4,000,000 3,000,000
OH 43.15
45.51 46.93 46.17 47.65
2,000,000 1,000,000 0 Time--> 42.0
44.0
46.0
48.0
51.21 50.0
O OH OH
52.42 52.0
54.0
(C) FIG. 11.1.2 Time window between 42 and 58 min from the pyrograms obtained at 900°C from 1.0 mg D-()arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C).
heptenol. Several isomers and diasteroisomers of this compound are possible. The peak at 45.51 min for xylose was tentatively identified as tetrahydro-3,4-furandiol (available in the mass spectral libraries). The peak at 45.56 min for arabinose and at 45.51 min for ribose is not pure, being formed from tetrahydro-3,4-furandiol mixed with 2,5-anhydroarabinofuranose (3,6-dioxabicyclo[2.2.1]heptane-2.7-diol) and 2,5-ribofuranose, respectively. The reaction of the formation of 2,5-anhydro-arabinofuranose is shown below:
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
Abundance
FIG. 11.1.3 Mass spectrum of 3,6-dioxa-bicyclo[2.2.1]
74
100
427
OH
heptane-2.7-diol obtained from arabinose.
56
O
80
O 43
60
OH
61
29 40 20 84 0 m/z-->
20
40
60
80
101 114 100
120
ð11:1:1Þ
The mass spectrum of 3,6-dioxabicyclo[2.2.1]heptane-2.7-diol is shown in Fig. 11.1.3. This compound is not formed from xylose. By the loss of one water molecule, all three sugars form 5,7-dioxabicyclo[2.2.1]heptane-2,3diols in reactions shown below for ribose and xylose. ð11:1:2Þ
ð11:1:3Þ
5,7-Dioxabicyclo[2.2.1]heptane-2,3-diol has four chiral centers (and therefore potentially 16 stereoisomers). The stereoisomers of this compound contain part of the chiral centers of the initial molecule and can be indicated as 1,5-anhydro-arabinose when generated from arabinose, 1,5-anhydro-ribose when generated from ribose, etc. The diastereoisomers can be separated on nonchiral chromatographic columns, and their mass spectra are identical (or very similar). The diastereoisomer from arabinose elutes at 46.52 min, the one from ribose at 47.8 min, and the one from xylose at 50.11 min. The spectrum of 5,7-dioxabicyclo[2.2.1] heptane-2,3-diol (1,4-anhydro-xylopyranose) obtained from xylose is shown in Fig. 11.1.4. The spectra of 1,5-anhydro-arabinofuranose and 1,5-anhydro-ribofuranose are virtually identical with that of 1,4-anhydro-xylopyranose, except for the fragment m/z ¼ 110, which is absent in these spectra.
428 FIG. 11.1.4
11. PYROLYSIS OF CARBOHYDRATES
Mass spectrum of 5,7-dioxa-bicyclo-[2.2.1] heptane-2,3-diol obtained from xylose.
Abundance
57
OH
100
O O
80
OH
73
60
29 43
40 39
20 m/z-->
0
20
40
86
68 47
103 60
80
110
100
Pentoses pyrolyzate contains a number of compounds with more than two OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg of the sugars at 900°C followed by collection of the pyrolyzate and derivatization with bis(trimethylsilyl)trifluoroacetamide (BSTFA) in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done with a GC/MS technique in conditions described in Table 1.4.2. Besides the compounds already identified in Table 11.1.1, the analysis of the silylated pyrolyzate showed the presence of some undecomposed initial sugar. Also, the formation of dimers generated by the elimination of water from two molecules of the initial sugars was detected.
GLUCOSE D-(+)-Glucose is probably the most common monosaccharide. The compound is a hexose present mainly in pyranose form, and the ratio of the anomers in solution is 36/64 α/β. Glucose decomposition starts at melting temperature at 153–156°C with the formation of 1,6-anhydroglucose and of several di- and oligosaccharides [9]. Among the disaccharides (see Subchapter 11.2) detected in glucose heated around the melting temperatures for 2.5 h were 2-O-α-D-glucopyranosyl-D-glucose (kojibiose Glc α(1 !2) Glc), 2-O-β-glucopyranosylD-glucose (sophorose Glc β(1 !2) Glc), 3-O-α-D-glucopyranosyl-D-glucose (nigerose Glc α(1! 3) Glc), 3-β-D-glucosyl-D-glucose (laminaribiose Glc β(1 !3) Glc), 4-O-α-Dglucopyranosyl-D-glucose (maltose Glc α(1 !4) Glc), 4-O-β-D-glucopyranosyl-D-glucose (cellobiose Glc β(1 ! 4) Glc), 6-O-α-D-glucopyranosyl-D-glucose (isomaltose Glc α(1 ! 6) Glc), 6-O-β-D-glucopyranosyl-D-glucose (gentiobiose Glc β(1 !6) Glc), and α-D-glucopyranosylα-D-glucopyranoside (trehalose Glc α(1 ! 1)α Glc) [10]. Among the trisaccharides, maltotriose (Glc α(1! 4) Glc α(1 !4) Glc) and panose (Glc α(1 ! 6) Glc α(1 ! 4) Glc) were identified (tentatively).
429
GLUCOSE
OH Abundance
CH 2
4.23
HO
OH
6,000,000
HO
28.91
2,000,000 1,000,000 0 Time-->
42.99
34.13
4.65 7.63 9.64 5.80 12.08 5.0
10.0
49.91
17.76
15.0
35.60 22.48 18.58 20.0
59.90
46.37
OH
4,000,000 3,000,000
56.71
O
7,000,000
5,000,000
47.68
30.39
25.0
30.83 30.0
40.87 45.68 37.73 42.66
35.0
40.0
45.0
49.38 48.26 50.0
55.0
60.0
FIG. 11.1.5 Pyrogram of 1.0 mg D-(+)-glucose at 900°C.
At higher temperatures, glucose decomposition generates a large number of molecular fragments [6,11,12,14,15]. Pyrolysis of a sample of D-(+)-glucose performed on a 1.0 mg compound using Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.1.5. Pyrolysis of glucose at temperatures as low as 550°C generated a pyrogram very similar to that at 900°C, including the nature of the pyrolytic compounds and the relative peak intensities in the pyrogram. This indicates that for glucose (and other similar carbohydrates) the heating at temperatures beyond the decomposition point brings minor changes in the pyrolyzate composition. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.1.2. Because the main pyrolysis products of glucose and fructose are very similar, Table 11.1.2 also contains the identification of the main compounds from the pyrolysis of fructose with the pyrogram shown later in Fig. 11.1.13. The calculation of the mole % was obtained based solely on peak areas. The correct identification of various compounds in glucose pyrolysis is sometimes a problem. The complexity of the pyrolyzate mixture as well as variations in the pyrolysis conditions may lead to some variations in the results reported in the literature. For example, besides the compounds listed in Table 11.1.2, a few compounds such as 2-oxopropanal (MW ¼ 72) or 1-hydroxybutane-2,3-dione (MW ¼ 102) were not identified in the pyrogram, although reported in the literature [16]. Also the correct identification of anhydrosugars is sometimes a problem because the corresponding mass spectra are not available in common mass spectral libraries and different anhydrosugars have similar spectra. For comparison, the spectra of 1,6anhydro-β-D-glucopyranose (levoglucosan) and of 1,6-anhydro-β-D-glucofuranose are shown in Figs. 11.1.6 and 11.1.7, respectively.
430
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
1
Carbon dioxide
4.23
44
124-38-9
10.37
10.12
2
Formaldehyde
4.65
30
50-00-0
6.73
9.00
3
1-Butene
5.02
56
106-98-9
0.04
Trace
4
1-Butyne
5.21
54
107-00-6
0.04
Trace
5
Acetaldehyde
5.80
44
75-07-0
1.41
0.85
6
Furan
7.63
68
110-00-9
1.69
0.45
7
1,3-Cyclopentadiene
8.53
66
542-92-7
0.20
Trace
8
2-Propenal (acrolein)
8.68
56
107-02-8
0.82
0.44
9
Propanal
8.70
58
123-38-6
0.21
0.11
10
Acetone
9.11
58
67-64-1
0.50
0.70
11
Pyruvaldehyde
9.64
72
78-98-8
2.52
1.86
12
2-Methylfuran
12.08
82
534-22-5
0.57
0.60
13
2-Propen-1-ol
13.55
58
107-18-6
0.04
Trace
14
Butenal
13.88
72
123-72-8
0.03
Trace
15
Methyl vinyl ketone
14.24
70
78-94-4
0.16
Trace
16
2,3-Butanedione (diacetyl)
14.50
86
431-03-8
0.21
Trace
17
Benzene
16.44
78
71-43-2
0.02
Trace
18
Hydroxyacetaldehyde (glycol aldehyde)
17.76
60
141-46-8
4.89
0.99
19
2,5-Dimethylfuran
18.58
96
625-86-5
0.23
0.35
20
Formic acid
18.64
46
64-18-6
0.02
Trace
21
2-Butenal (Z)
19.43
70
15798-64-8
0.15
Trace
22
2-Methyl-2-propenal
19.52
70
78-85-3
0.21
Trace
23
Ethanol
19.86
46
64-17-5
0.01
Trace
24
Acetic acid
20.11
60
64-19-7
0.37
1.08
25
Vinylfuran
20.87
94
1487-18-9
0.16
Trace
26
2,3-Pentandione
21.20
100
600-14-6
0.01
Trace
27
Ethyl-1-propenyl ether
22.33
86
928-55-2
0.21
Trace
28
1-Hydroxy-2-propanone (acetol)
22.48
74
116-09-6
0.71
0.52
29
Toluene
23.07
92
108-88-3
0.12
Trace
30
2-Ethyl-5-methylfuran + methyl formate ?
23.80
110
1703-52-2
0.14
Trace Continued
431
GLUCOSE
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
31
Hydroxyacetic acid (glycolic acid)
24.63
76
79-14-1
0.01
Trace
32
2-Hydroxypropanoic acid (lactic acid)
24.96
90
50-21-5
Trace
Trace
33
2,3-Dihydro-1,4-dioxin
25.11
86
543-75-9
0.03
Trace
34
3-Methylfuran
25.70
82
930-27-8
0.22
Trace
35
2-Propenoic acid methyl ester ?
26.69
86
96-33-3
0.44
0.41
36
1-Hydroxy-2-butanone
27.61
88
5077-67-8
0.08
Trace
37
1,4-Dioxadiene
28.91
84
N/A
5.79
0.78
38
3-Furaldehyde
29.12
96
498-60-2
0.24
Trace
39
2-Oxopropionic acid methyl ester ?
29.33
102
600-22-6
0.13
0.12
40
Butandial
29.65
86
638-37-9
0.25
0.14
41
2-Cyclopentene-1,4-dione
30.27
96
930-60-9
0.23
Trace
42
Furancarboxaldehyde (furfural)
30.39
96
98-01-1
11.49
26.74
43
2-Propylfuran
30.83
110
4229-91-8
0.85
Trace
44
2-Furanmethanol
31.70
98
98-00-0
0.12
0.10
45
5-Methyl-2(3H)-furanone
32.05
98
591-12-8
0.18
Trace
46
Methylglyoxal + unknown
32.65
72
78-98-8
0.13
Trace
47
1-(2-Furanyl)ethanone
33.15
110
1192-62-7
0.31
0.31
48
5-Methylidenffuran-2-one (protoanemonin)
33.93
96
108-28-1
0.11
Trace
49
2,4-Dihydroxy-2,5-dimethyl-2(2H)-furan-3-one
33.95
144
10230-62-3
Trace
Trace
50
Dihydro-4-hydroxy-2(3H)-furanone
34.13
102
5469-16-9
3.07
0.22
51
Dihydro-3-methylene-2(3H)-furanone
34.40
98
547-65-9
0.12
0.13
52
2-Hydroxy-2-cyclopenten-1-one
34.53
98
10493-98-8
0.48
0.39
53
Unknown
34.65
112
N/A
0.12
0.14
54
Isomaltol
34.92
126
3420-59-5
0.12
Trace
55
Benzaldehyde + unknown
34.99
106
100-52-7
0.14
0.17
56
2-Methylenecyclopentanol
35.39
98
20461-31-8
0.25
Trace
57
5-Methyl-2-furancarboxaldehyde
35.60
110
620-02-0
1.04
1.38
58
3-Methyl-2-cyclopenten-1-one
36.35
96
2758-18-1
0.13
Trace
59
7,8-Dioxabicyclo[3.2.1]oct-2-ene (mix)
36.53
112
N/A
0.16
Trace
60
Tetrahydro-4H-pyran-4-one
36.75
100
29943-42-8
0.46
Trace Continued
432
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
61
Dihydro-5-propyl-2(3H)furanone ?
36.96
128
105-21-5
0.14
0.16
62
6,8-Dioxabicyclo[3.2.1]octane
37.43
114
280-16-0
0.51
0.32
63
1,3-Dihydroxy-2-propanone
37.74
90
96-26-4
1.41
2.66
64
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.97
112
80-71-7
0.53
0.24
65
Isomer of 2-hydroxy-3-methyl-2-cyclopenten-1- 38.21 one
112
N/A
0.32
Trace
66
Phenol
38.67
94
108-95-2
0.19
0.11
67
1,5-Hexadien-3-ol
38.85
98
924-41-4
0.29
Trace
68
4,5-Dimethyl-1,3-dioxol-2-one ?
39.36
114
37830-90-3
0.25
0.39
69
2,5-Dimethyl-4-hydroxy-2-hydrofuran-3-one ?
39.72
128
3658-77-3
0.09
Trace
70
Unknown
39.89
144
N/A
0.29
0.60
71
2-Methylphenol (o-cresol)
40.25
108
95-48-7
0.09
Trace
72
Unknown
40.41
82
N/A
0.08
Trace
73
3-Furancarboxylic acid methyl ester
40.87
126
13129-23-2
0.87
0.84
74
3-Hydroxy-2-methyl-4H-pyran-4-one (maltol)
41.18
126
118-71-8
0.41
Trace
75
2,5-Furandicarboxaldehyde
41.42
124
823-82-5
0.49
0.61
76
Unknown
41.61
144
N/A
0.24
0.31
77
4-Methylene-1,2-dioxolan-3-one ?
41.94
114
N/A
0.41
Trace
78
2,3-Dihydro-3,5-dihydroxy-6-methyl-4Hpyran-4-one
42.66
144
28564-83-2
0.46
1.34
79
Levoglucosenone
42.99
126
37112-31-5
2.19
0.10
80
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
43.17
142
1073-96-7
0.23
0.15
81
2-Methyl-2-pentenoic acid
43.75
114
16957-70-3
0.16
Trace
82
Dianhydrosugar
44.89
144
N/A
0.06
Trace
83
Unknown
45.47
98
N/A
0.39
0.98
84
5-Acetoxymethyl-2-furaldehyde
45.68
168
10551-58-3
0.74
0.46
85
1,4:3,6-Dianhydro-α-D-glucopyranose
46.37
144
N/A
2.67
Trace
86
Unknown
46.96
116
N/A
Trace
0.21
87
5-Acetoxymethyl-2-furancarboxaldehyde
47.35
126
10551-58-3
Trace
0.26
88
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.68
126
67-47-0
10.33
30.13 Continued
433
GLUCOSE
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
89
3-Methyl-1,2-cyclopentanediol
48.26
116
27583-37-5
0.39
Trace
90
2’,3’-Dideoxyribonolactone ?
48.43
116
32780-06-6
Trace
0.25
91
1,2-Cyclohexanediol ?
49.38
116
1792-81-0
1.07
0.44
92
Dianhydrosugar ?
49.91
144
N/A
1.30
Trace
93
Unknown
56.04
198
N/A
Trace
0.45
94
Unknown
56.30
198
N/A
Trace
0.42
95
1,6-Anhydro-β-D-glucopyranose (levoglucosan) 56.72
162
498-07-7
11.00
0.32
96
1,6-Anhydro-β-D-glucofuranose
162
7425-74-3
3.03
0.12
59.90
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
FIG. 11.1.6 Mass spectrum of 1,6-anhydro-β-D-
Abundance
glucopyranose (levoglucosan).
60
100 90
O
HO HO
80
O
70
OH
60
57
50
73
40 30
43
69
20
98
10 0 m/z--> 40
115
89
60
80
100
127
120
144
140
162
160
As shown in Table 11.1.2, glucose pyrolysis generates small molecules such as formaldehyde, hydroxy-acetaldehyde, acetaldehyde, furan derivatives (five-ring cycle containing one oxygen), pyran derivatives (six-ring cycle containing one oxygen), and dioxane derivatives (six-ring cycle containing two oxygen atoms). The formation of small molecules such as H2O, CO, and CO2 and organic compounds with C1–C4 atoms such as HCHO, CH3CHO, and CH3COCH3 takes place from a variety of reactions, including eliminations, fragmentation, extrusions, rearrangements, etc. For example, hydroxyacetaldehyde may be formed by a retro-aldolization reaction, as shown below:
434
11. PYROLYSIS OF CARBOHYDRATES
FIG. 11.1.7 Mass spectrum of 1,6-anhydro-β-Dglucofuranose.
Abundance 100
73
90
OH O
80 70
O OH
60 50
OH
40
20
69
44
30 19
61
29
85
10
91
0 m/z--> 20
40
60
80
98
100
115 127
120
ð11:1:4Þ
A common reaction during carbohydrate pyrolysis is water elimination. When the elimination takes place between OH groups at vicinal carbons, the result is the formation of an aldol type structure. The elimination of water from the diol group is favored if the OH groups are in syn positions, but this is not a strict requirement. The aldol can further undergo a retroaldol fragmentation, as shown below:
ð11:1:5Þ
Because in a glucose molecule there are several pairs of vicinal carbons with OH substituents, the reaction may take place for any of these pairs with a multitude of reaction products. The reaction is shown below for the pair C3–C4 and the elimination of the OH from C3 of the glucose molecule:
ð11:1:6Þ
GLUCOSE
435
The resulting compounds continue the decomposition process, as shown below for one of the resulting aldehydes:
ð11:1:7Þ
The elimination of water can occur also following a Grob fragmentation that takes place for a 1,3-alcohol [17]. Because glucose has several pairs of 1,3-carbons with OH substituents, the reaction may generate a multitude of reaction products (as was mentioned for the water elimination between OH groups at vicinal atoms). For the structural formula of glucose, the reaction is exemplified below between the OH groups at the C2 and C4 atoms and the cleavage of C3–C4 bond in the molecule:
ð11:1:8Þ
The initially formed fragments can further pyrolyze to generate CO2 and small aldehydes, ketones, hydroxyaldehydes, etc. The water elimination from OH groups bound to 1,3-carbons is influenced by their position as syn or anti (syn favored), but water can be eliminated from both types. The formation of formaldehyde by reaction (11.1.8) does not account for the whole amount of this compound formed from glucose. As shown in Table 11.1.2, formaldehyde represents about 9% (mole) of pyrolyzate, and its generation is likely to occur from other reactions. Addition of diammonium phosphate during glucose pyrolysis was shown to diminish the formation of formaldehyde [18]. It is likely that diammonium phosphate (NH4)2HPO4 has a catalytic effect on the opening of the internal hemiacetal structure [19], which leads to somewhat different reactions during glucose pyrolysis while also generating ammonia that reacts with HCHO, diminishing its apparent level. Water elimination also can have as a result the formation of various cycles. The formation of a furan derivative, 5-(hydroxymethyl)-2-furancarboxaldehyde (hydroxymethylfurfural), is shown as an example in the following sequence of reactions:
ð11:1:9Þ
Hydroxymethylfurfural may further decompose to generate furancarboxaldehyde (furfural) and formaldehyde, as shown in reaction (11.1.10):
436
11. PYROLYSIS OF CARBOHYDRATES
ð11:1:10Þ Other reactions may also be responsible for the formation of furfural, with one example shown below:
ð11:1:11Þ
Another path also starts with formaldehyde elimination, but the cyclization process is assumed to occur in a later stage, as shown in the following sequence of reactions [20]:
ð11:1:12Þ
The formation of furfural also can be explained starting with the linear form of glucose [20], although the low level of this form in solid glucose makes this mechanism unlikely. The formation of pyran derivatives can result from various reactions affecting the OH and CH2OH groups without affecting the initial pyran cycle of glucose. However, the formation of new six-atom rings containing one oxygen is also possible. An example of a reaction with the preservation of the pyran cycle from glucose (and with the formation of a new 1,3-dioxolane cycle) is the formation of levoglucosan by the elimination of one water molecule between the OH in the 1-position of glucose and the H from the OH group connected to the 6-position:
ð11:1:13Þ
Further decomposition of levoglucosan generates pyrolysis products similar to those from glucose [21,22]. Fig. 11.1.8 shows the comparison between the pyrogram of 1 mg glucose (the same shown in Fig. 11.1.5) and the pyrogram of 1 mg levoglucosan (1,6-anhydro glucose) with the pyrolysis performed in identical conditions with that of glucose pyrolyzed at 900°C. Glucose pyrolyzate is shown in Trace A and levoglucosan pyrolyzate in Trace B. Detailed inspection of the two pyrograms from Fig. 11.1.8 (using the data analysis capability of a GC/MS system) shows the qualitative identity of most compounds, although some
437
GLUCOSE
Abundance 1.4e+07 1.2e+07 1e+07 8,000,000 6,000,000 4,000,000 2,000,000 0
(A)
30.39
47.68
4.23 28.91 4.65 5.0
7.63 10.0
17.76 15.0
42.99 46.37
34.13 35.60
20.0
25.0
30.0
35.0
56.71
49.91
40.87 40.0
59.90
45.0
50.0
55.0
60.0
65.0
56.71 1.4e+07 1.2e+07 1e+07 8,000,000 6,000,000 4,000,000 2,000,000 0
(B)
Time-->
46.37
17.76 4.32 10.47 6.04 5.0
10.0
22.34 15.0
20.0
25.0
30.0
49.91
42.99
30.39 28.91
47.68
34.76
37.79 40.87
35.0
40.0
59.91
46.73 45.0
50.0
55.0
60.0
65.0
FIG. 11.1.8 Pyrogram of 1 mg glucose (Trace A) and of 1.0 mg levoglucosan (Trace B) at 900°C.
differences are seen in peak intensities. This indicates that pyrolysis of levoglucosan, initially formed during glucose pyrolysis, is a likely contributor to the formation of other molecules in glucose pyrolyzate. However, this does not indicate that other reactions do not occur independently during pyrolysis of glucose, starting directly from the parent molecule. The lack of levoglucosan formation in fructose pyrolysis (see further), and the similarity of the nature of compounds generated from glucose and fructose are additional proof that levoglucosan formation is not the unique path of glucose pyrolysis. Besides levoglucosan, several other anhydrosugars are generated during glucose pyrolysis. The formulas for several of these compounds (including levoglucosan) detected in glucose pyrolyzate are shown below:
438
11. PYROLYSIS OF CARBOHYDRATES
Besides the furan and pyran cycles, the anhydrosugars contain other oxygenated cycles. A reaction leading to the formation of a dioxane derivative (1,4-dioxadiene) is shown below:
ð11:1:14Þ
With the formation of a double bond at the bridgehead not being allowed (Bredt’s rule), the elimination of the second water molecule leads to the fragmentation that gives dioxandiene and hydroxyacetaldehyde. Dioxane derivatives also can be formed from other reactions during glucose pyrolysis. Glucose pyrolyzate contains a number of compounds with multiple OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg glucose at 900°C followed by the collection of the pyrolyzate and derivatization with BSTFA in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done with a GC/MS technique in conditions described in Table 1.4.2. The chromatogram of the derivatized pyrolyzate is shown in Fig. 11.1.9. The peak identification in the chromatogram from Fig. 11.1.9 was done using mass spectra library searches. Only some of the peaks were identified and are listed in Table 11.1.3 as a function of their retention time. The results shown in Table 11.1.3 indicate that besides the compounds already identified in Table 11.1.1, three new groups of compounds are present in the pyrolyzate. The first group
Abundance 39.43 40.69
1.3e+07 1.2e+07 1e+07 8,000,000
22.52 17.46
6,000,000 4,000,000
18.35 14.05
35.58 19.96
2,000,000 12.42
23.76 24.93
33.08 30.11
39.73
25.0
30.0
40.0
19.21 0 Time-->
15.0
20.0
35.0
44.10 44.90 46.32 46.89
45.0
50.0
64.6165.92 55.0
60.0
65.0
70.0
FIG. 11.1.9 Chromatogram of the silylated pyrolyzate of 1 mg D-(+)-glucose. Pyrolysis performed at 900°C and derivatization done with BSTFA.
439
GLUCOSE
TABLE 11.1.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.9 for the Pyrolyzate of Glucose Derivatized With Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and Their Molar Levels Relative to Levoglucosan No. Compound
Ret. Time
MW Fragment Ions (m/z)
%
1
Lactic acid 2 TMS
12.65
262
73(100), 147(50), 131(38), 205(11), 247(7)
3.09
2
Ethylene glycol 2 TMS
13.09
206
73(100), 147(85), 191(78)
2.49
3
Glycolic acid 2 TMS
13.53
220
73(100), 147(78), 66 (20), 205(17), 177(11)
3.80
4
2-Hydroxy-cyclopenten-1-one TMS
16.07
170
155(100), 81(25), 75(17), 73(17)
1.89
a
99(100), 44(30), 69(25)
—
5
Hydrolyzed reagent
18.35
?
6
Dihydroxyacetone 2 TMS
19.97
234
73(100), 103(44), 189(10), 219(4)
6.56
7
1,4-Dioxan-2,5-diol 2TMS
22.52
264
117(100), 161(79), 73(76), 191(8), 263(1)
18.58
8
2-(2-Hydroxyethoxy)ethan-1-ol 2 TMS
22.79
250
117(100), 161(85), 73(85), 191(12), 245(3)
5.77
9
5-(Hydroxymethyl)furan-carboxaldehyde 23.76 TMS
198
183(100), 109(34), 111(28), 169(14), 198(4)
9.76
10
Glyceric acid 3TMS
24.63
322
73(100), 147(72), 189(50), 292(38)
4.21
11
2-Furanhydroxyacetic acid 2TMS
26.56
286
169(100), 73(58), 147(30), 243(13), 286(3)
1.83
12
1,4:3,6-Anhydro-a-D-glucopyranose TMS
27.57
216
73(100), 75(50), 201(45), 191(15)
1.54
13
2-[2-(2-Hydroxyethoxy)ethoxy]ethan-1-ol 30.12 2 TMS
308
117(100), 73(55), 103 (20) 161(20)
3.96
14
1,2,3-Trihydroxybenzene 3 TMS
32.96
342
239(100), 73(100), 342(54), 327(10)
0.05
a
73(100), 258(55), 103(35), 185(30)
—
239(100), 73(100), 342(54), 327(10)
0.05
15
Unknown
33.08
?
16
Trihydroxybenzene 3 TMS
35.52
17
I.S.tert-butylhydroquinone 2 TMS
35.58
310
310(100), 295(99), 73(53)
8.32
18
Ribonic acid 5 TMS ?b
37.11
526
73(100), 292(30), 147(25), 103(24), 421(1)
0.91
19
1,6-Anhydroglucofuranose 3 TMS
37.97
378
73(100), 147(55), 217(43), 205(30)
1.89
20
Anhydrosugar 3 TMS
38.55
378
73(100), 217(70), 204(66), 333(20)
1.16
21
Levoglucosan 3 TMS
39.43
378
73(100), 204(48), 217(42), 147(18), 333(19)
100.00
22
Anhydrosugar 3 TMS
39.73
378
73(100), 217(88), 191(64), 204(21), 430(10)
2.71
23
1,4-Anhydroglucopyranose 3TMS
40.70
378
217(100), 73(20), 319(10)
43.56
24
Hexose 5 TMS
44.10
540
217(100), 73 (62), 204(56), 103(37), 363(1)
4.20
25
Hexose 5 TMS
44.90
540
73(100), 204(76), 103(52), 217(40), 259(13), 363(1)
6.17
26
Glucose 5 TMS
45.93
540
204(100), 191(40), 73(37), 217(19), 147(20), 435(2)
0.75
Continued
440
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.9 for the Pyrolyzate of Glucose Derivatized With Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and Their Molar Levels Relative to Levoglucosan—Cont’d No. Compound
Ret. Time
MW Fragment Ions (m/z)
%
46.09
466
73(100), 217(46), 147(38), 333(12), 466(10)
0.05
46.33
540
217(100), 73(80), 103(33), 247(14), 319(2)
3.03
27
Gluconic acid lactone 4 TMS
28
β-D-Galactofuranose 5 TMS ?
29
Hexose 5 TMS
46.89
540
217(100), 73(49), 103(18), 243(11)
1.69
30
Gluconic acid 6TMS
48.60
628
73(100), 333(25), 292(23), 147(20), 423(4)
0.15
31
Glucose 5 TMS
48.96
540
204(100), 191(47), 73(49), 217(19), 147(19), 435(2)
0.81
32
Hexose 5 TMS
50.30
540
73(100), 217( 70), 204(53), 103(34), 243(32), 0.82 407(8)
33
Hexose 5 TMS
50.82
540
73(100), 217(78), 243(43), 205(40), 204(37), 407(14)
0.50
34
Hexose 5 TMS
52.55
540
73(100), 217(63), 205(53), 204(49), 147(35), 243(16)
0.70
35
Disaccharide 8 TMS
64.61
918
204(100), 217(57), 73(55), 361(29), 147(18), 451(1)
0.51
36
Disaccharide 8 TMS
64.95
918
204(100), 73(93), 217(80), 361(43), 271(31), 407(7)
0.38
37
Disaccharide 8 TMS
65.09
918
217(100), 73(98) 271(44), 204(31), 205(24), 489(2)
0.23
38
Disaccharide 8 TMS
65.77
918
73(100), 361 (84), 217(80), 308(20), 319(10), 0.38 407(4)
39
Disaccharide 8 TMS
65.92
918
73(100), 217(60), 205(52), 246(42), 233(32), 473(2)
0.42
40
Disaccharide 8 TMS
66.02
918
204(100), 73(93), 361(74), 217(69), 147(24), 451(1)
0.30
41
Disaccharide 8 TMS
66.25
918
73(100), 217(74), 205(55), 233(44), 189(23), 473(3)
0.15
a b
b
Note: Unknown MW. Note: “?” indicates uncertain compound identification.
consists of several sugar fragment molecules, including traces of ethylene glycol and ethylene glycol ethers; traces of a few sugar-related acids, which include glyceric acid and ribonic acid; a gluconic acid lactone; and two isomers of trihydroxybenzene. No gluconic acid or deoxygluconic acid were detected in the pyrolyzate. The presence of trihydroxybenzenes indicates the possibility of the formation of aromatic compounds in sugar pyrolysis. The second group consists of several monosaccharides. The compounds from this group were present at levels between 0.5% and 6% (molar) from that of levoglucosan (taken as 100%). Glucose is
GLUCOSE
PAH m g/g glucose
10
441
FIG. 11.1.10 The level of PAHs in glucose pyrolyzate first heated at 300°C and then at 600°C [25].
8 6 4 2
Ac
en ap ht hy l Fl ene Ph uo r en e an ne An thr th en Fl rac e uo e ra ne nt he Py ne Be re nz Ch ne ry [a se ]a Be nth ne nz rac e o[ a] ne py re ne
0
present in this group at about 0.8% compared to levoglucosan. The third group consists of several disaccharides (including maltose). The presence of disaccharides shows that under the influence of heat, two molecules of glucose can eliminate water to form disaccharides. The compounds from this group were present at levels around 0.3% (molar) from that of levoglucosan and represented about 2% from the total area in the chromatogram. Similar results regarding silylated pyrolyzate using hexamethyldisilazane [23] and online silylation [24] were reported in the literature. Another group of compounds that was not listed in Table 11.1.2 and also not identified by the extension of the analyzed products by silylation and GC/MS analysis (Table 11.1.3) is the condensation products that have a high molecular weight. A considerable amount of char is generated by glucose pyrolysis, and besides carbon, the char contains a tar-type material. This tar is made from molecules resulting from the condensation of glucose molecules by the elimination of water or by reactions between various fragments formed in the initial stages of pyrolysis. Also, this tar contains traces of other nonvolatile compounds such as PAHs. In an experiment designed to determine the levels of PAHs in glucose pyrolysis, a glucose sample was charred at 300°C for 60 min and then heated at 600°C for 10 min [25]. The analysis of the resulting levels of PAHs is shown in Fig. 11.1.10. The formation of PAHs in glucose pyrolyzates is similar to the formation of PAHs in cellulose pyrolyzate [26]. A considerable effort has been made in the study of thermochemolysis of carbohydrates in the presence of TMAH [5]. In strong basic conditions and elevated temperature, carbohydrates have the tendency to form deoxyaldonic acids (saccharinic acids) in a reaction as shown below for a hexose (see reaction 2.5.2): ð11:1:15Þ
442
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.4 Main Compounds Identified in Glucose Pyrolyzate at 700°C in the Presence of TMAH [5] No. Compound
MW
Fragment Ions (m/z)
Rel. Peak Height
1
2-Methoxypropanoic acid methyl ester
118
59, 60, 75, 103, 118
65
2
2,4-Dimethoxy butanoic acid methyl ester
162
45, 72, 73, 75, 132
14
3
2,4,5-Trimethyoxy-3-deoxy-D-threo-pentonic acid methyl ester
206
75, 101, 115, 129, 206
5
4
2,4,5-Trimethyoxy-3-deoxy-D-erythro-pentonic acid methyl ester
206
75, 101, 115, 129, 206
9
5
Trimethoxybenzene
168
93, 110, 125, 153, 168
15
6
2,4,5,6-Tetramethoxy-3-deoxy-D-arabino-hexonic acid methyl ester
234
75, 101, 129, 159, 191
70
7
2,4,5,6-Tetramethoxy-3-deoxy-D-ribo-hexonic acid methyl ester
234
75, 101, 129, 159, 191
100
The resulting acid is further methylated by TMAH. Molecular fragmentation also is produced by the elevated temperatures and the result is the formation of numerous methylated compounds, with the main ones formed from the thermochemolysis/methylation of glucose being given in Table 11.1.4 [5]. Thermochemolysis/methylation in the presence of TMAH has been applied on various carbohydrates with the goal of the characterization of saccharides in biomaterials [5,27].
MANNOSE AND GALACTOSE The pyrograms of two other hexoses, mannose and galactose, which are typically found mainly in pyranose form (mannose 100% and galactose 93%) are given in Fig. 11.1.11 Trace A and B, respectively. The pyrolysis and the pyrograms were obtained in identical conditions as for glucose at 900°C. The peaks eluting up to 50 min in the pyrograms shown in Fig. 11.1.11 Trace A and B are identical in nature with those generated from glucose and can be identified by their retention times in Table 11.1.2. The major peaks belong to CO2. Similar to the case of pentoses, toward the end of the pyrogram are eluting anhydrosugars that retain some of the chiral centers of the parent molecule. Anhydrosugar diastereoisomers were generated from glucose, mannose, and galactose, respectively. These compounds elute at different retention times, as seen by comparing the pyrograms shown in Fig. 11.1.5 for glucose and Fig. 11.1.11A and B for mannose and galactose, respectively. The identification of the main peaks eluting at higher retention times than 50 min in the pyrograms of mannose and galactose is given in Table 11.1.5. The GC/MS analysis of anhydrosugars formed by pyrolysis of monosaccharides and retaining OH groups bound to chiral carbons is better performed using offline derivatization of these compounds after pyrolysis. A time window (37.0–46.0 min) from the chromatograms
443
MANNOSE AND GALACTOSE
Abundance
30.39
55.13
47.68
1e+07
OH CH 2
9,000,000 8,000,000 7,000,000
28.91
O
4.23
HO
OH
6,000,000
HO
5,000,000
OH
4,000,000
34.13
3,000,000 2,000,000 1,000,000
(A)
0
17.76 49.91
7.63 9.64 12.08 5.80
4.65
5.0
10.0
30.83 35.60
40.87
1e+07
45.68
56.64 59.30
50.80
22.48
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
47.68
30.39
9,000,000
55.65
42.99 44.89
55.0
60.0
53.77
OH CH 2
4.23
O
8,000,000
HO
7,000,000 6,000,000
OH HO
60.08
28.91
OH
5,000,000 56.13
4,000,000 3,000,000 2,000,000 1,000,000
34.13 4.65 9.64 7.63 5.80 12.08
49.91
17.76 22.48
42.67 37.73 40.87 35.60 45.68 30.83
51.32
0
(B)
Time-->
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
FIG. 11.1.11 Pyrogram of 1 mg mannose (Trace A) and of 1.0 mg galactose (Trace B) at 900°C.
of trimethylsilyl (TMS) derivatives of the pyrolyzates of (a) mannose, (b) allose, (c) galactose, and (d) glucose is shown in Fig. 11.1.12, with the peak identification in Table 11.1.6. Pyrolysis of these hexoses was done at 590°C [2]. As shown in Fig. 11.1.12, the differences in the retention times for 1,6-anhydrosugars or 1,4anhydrosugars from different parent molecules are noticeable. This is proof that the anhydrosugars maintain part of the initial steric structure of the parent molecule.
444
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.5 Identification of the Peaks Eluting at Retention Times Higher Than 50 min in the Pyrograms of Mannose and Galactose Shown in Fig. 11.1.11 Trace A and B Retention Time (Min)
No. Compound
Mannose MW Area (106)
Galactose Area (106)
1
1,450.80 Anhydromannopyranose
162
34.2
–
2
1,6-Anhydro-βgalactopyranose
53.78
162
–
1156.5
3
1,6-Anhydro-βmannopyranose
55.14
162
744.5
–
4
1,6-Anhydro-αgalactofuranose
56.13
162
–
210.8
5
1,6-Anhydro-αmannofuranose
55.63
162
200.4
–
6
Anhydropyranose
56.64
162
27.0
–
7
1,6-Anhydro-β-Dglucofuranose ?
59.85
162
19.9
–
8
Anhydrofuranose ?
60.08
162
8.8
–
9
1,6-Anhydro-βgalactopyranose
60.08
162
–
402.2
Note: “?” indicates uncertain compound identification.
Abundance
b1 d3
4e+07 c1 c1
3.5e+07
a1
3e+07
b5 b5
d4
c2
2.5e+07
d1
2e+07
c3
d2
b2
1.5e+07
b3
1e+07
a2
a3
b4
5,000,000 Time -->
FIG. 11.1.12
37.0
38.0
39.0
40.0
41.0
42.0
43.0
44.0
45.0
46.0
d c b a
Time window between 40 and 50 min from the chromatogram of TMS derivatives of pyrolyzates of (a) mannose, (b) allose, (c) galactose, and (d) glucose showing anhydrosugars. Peak identification is given in Table 11.1.6.
FRUCTOSE
445
TABLE 11.1.6 Identification of the Peaks in the Chromatogram Shown in Fig. 11.1.12 for the Anhydrosugars Formed During Pyrolysis of Four Monosaccharides Saccharide
Peak
Assignment
Mannose
a1
1,6-β-Anhydromannopyranose
a2
1,4-Anhydromannopyranose
a3
1,6-α-Anhydromannofuranose
b1
1,6-Anhydroallopyranose
b2
1,6-Anhydroglucopyranose (impurity in allose)
b3
An anhydrofuranose ?
b4
1,6-Anhydroallofuranose
b5
1,4-Aanhydroallopyranose
c1
1,6-Anhydrogalactopyranose
c2
1,6-Anhydrogalactofuranose
c3
1,4-Anhydrogalactopyranose
d1
1,6-Anhydroglucofuranose
d2
1,4-Anhydroglucopyranose type ?
d3
1,6-Anhydroglucopyranose (levoglucosan)
d4
1,4-Anhydroglucopyranose
Allose
Galactose
Glucose
Note: “?” indicates uncertain compound identification.
FRUCTOSE Fructose is a hexose that is found mainly in furanose form. The pyrolysis products of fructose are similar in nature with those of other monosaccharides. An example of a pyrogram generated online at 900°C from D-()-fructose is shown in Fig. 11.1.13. The pyrolysis was done in conditions identical to those for glucose. The peak identification and relative molar content in 100 mol of pyrolyzate are given in Table 11.1.2, together with those of glucose. The calculation of the mole % was obtained based solely on peak areas. Qualitatively, the composition of fructose pyrolyzate is very similar to that of glucose. However, the proportion of various compounds in the two pyrograms is very different. Particularly, the dianhydrosugars seen in glucose pyrolyzate are absent in that of fructose, and levoglucosan and 1,6-anhydro-β-D-glucofuranose are present at significantly lower levels for fructose. The experiment performed on fructose using pyrolysis at 900°C followed by offline derivatization with BSTFA and GC/MS analysis showed, except for the anhydrosugars, similar compounds as in the case of glucose, but with the levels very different. Another significant difference was noticed in the level of disaccharide generated from fructose in the specific conditions used for this experiment. The level of disaccharides was for fructose about 10 times
446
11. PYROLYSIS OF CARBOHYDRATES
Abundance
47.68
30.39
4.23
HO
7,000,000
CH2
6,000,000 5,000,000
O HO
4.65
OH
CH2 OH
4,000,000
OH 35.60 37.75
3,000,000
40.87
2,000,000
9.64 28.91
1,000,000
42.66
5.80
12.08
17.76
22.48
39.90
56.30
45.47
26.69
59.90
0 Time-->
FIG. 11.1.13
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
Pyrogram of 1.0 mg D-()-fructose at 900°C.
higher than in the case of glucose (and represented about 16% from the total peak areas in the chromatogram). It was not possible to identify individual disaccharides in the mixture using mass spectral library searches. The mechanisms involved in fructose pyrolysis are similar to those proposed for explaining the composition of glucose pyrolyzate. It is likely that other hexoses have similar pyrolysis products with the compounds discussed in this section, although the percentage of various components in the pyrolyzate is likely to be different from compound to compound. Differentiation between different monosaccharides for analytical purposes has been achieved successfully using the differences in the anhydrosugars generated by pyrolysis [7,12] or using thermally assisted hydrolysis and methylation (e.g., with TMAH as a reagent) followed by GC/MS analysis [5]. Formation of different anhydrosugars from various monosaccharides has been booth experimentally and theoretically investigated [28–34].
References 11.1 [1] http://old.iupac.org/reports/provisional/abstract04/BB-prs310305/chapter10.pdf. [2] S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998. [3] M.J. Kleeman, M.A. Robert, S.G. Riddle, P.M. Fine, M.D. Hays, J.J. Schauer, M.P. Hannigan, Atm. Environ. 42 (2008) 3059. [4] O. Novotn y, K. Cejpek, J. Velisˇek, Czech J. Food Sci. 26 (2008) 117. [5] D. Fabbri, R.J. Helleur, J. Anal. Appl. Pyrolysis 49 (1999) 277. [6] R.J. Helleur, J. Anal. Appl. Pyrolysis 11 (1987) 297. [7] D.R. Budgell, E.R. Heyes, R.J. Helleur, Anal. Chim. Acta 192 (1987) 243. [8] N. Mitsuo, N. Nakayama, H. Matsumoto, T. Satoh, Chem. Pharm. Bull. 37 (1989) 1624. ˜ rsi, J. Therm. Anal. 5 (1973) 329. [9] F. O [10] H. Sugisawa, H. Edo, J. Food Sci. 31 (1966) 561. [11] H. Sugisawa, J. Food Sci. 31 (1966) 381.
GENERAL ASPECTS
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
447
E.R.J. Evans, D. Wang, F.A. Agblevor, H.L. Chum, S.D. Baldwin, Carbohydr. Res. 23 (1996) 219. E.B. Sanders, A.I. Goldsmith, J.I. Seeman, J. Anal. Appl. Pyrolysis 66 (2003) 29. S.L. Morgan, C.A. Jacques, Anal. Chem. 54 (1982) 741. J.W. Laskowitz, B. Carroll, Carbohydr. Res. 5 (1967) 245. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 82 (2008) 42. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 82 (2008) 10. R.R. Baker, S. Coburn, C. Liu, J. Anal. Appl. Pyrolysis 77 (2006) 12. K. Agyei-Aye, M.X. Chian, J.H. Lauterbach, S.C. Moldoveanu, Carbohydr. Res. 337 (2002) 2273. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 83 (2008) 37. A. Lorene, S. Nanbu, K. Fukuda, Mem. Fac. Eng. Kyushu Univ. 67 (2007) 67. T. Hosoya, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrolysis 83 (2008) 64. D. Fabbri, G. Chiavari, Anal. Chim. Acta 449 (2001) 271. D. Scalarone, O. Chiantore, C. Riedo, J. Anal. Appl. Pyrolysis 83 (2008) 157. T.E. McGrath, W.G. Chan, M.J. Hajaligol, J. Anal. Appl. Pyrolysis 66 (2003) 51. O.T. Chrotyk, W.S. Schlotzhauer, Betr. Tabak. 7 (1973) 165. C. Schwarzinger, J. Anal. Appl. Pyrolysis 71 (2004) 501. A. Fukutome, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrolysis 124 (2017) 666. M. Wang, C. Liu, X. Xu, Q. Li, J. Anal. Appl. Pyrolysis 120 (2016) 464. Q. Lu, Y. Zhang, C.-q. Dong, Y.-P. Yang, H.-Z. Yu, J. Anal. Appl. Pyrolysis 110 (2014) 34. H. Kawamoto, T. Hosoya, Y. Ueno, T. Shoji, S. Saka, J. Anal. Appl. Pyrolysis 109 (2014) 41. Y. Zhang, C. Liu, H. Xie, J. Anal. Appl. Pyrolysis 105 (2014) 23. S. Matsuoka, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrol. 103 (2013) 300. S.-S. Choi, M.-C. Kim, Y.-K. Kim, J. Anal. Appl. Pyrolysis 90 (2011) 56.
S U B C H A P T E R
11.2
Disaccharides
GENERAL ASPECTS Disaccharides are formed by the elimination of a water molecule between two monosaccharides (different or identical) with the formation of an ether bond. Disaccharides have common names and systematic names and can be indicated by the names of the participating monosaccharides and the bond type. For example, maltose or 4-O-α-D-glucopyranosylD-glucose is indicated as Glc α(1 ! 4) Glc.
448
11. PYROLYSIS OF CARBOHYDRATES
The elimination of water can take place between any of the OH groups of the participating monosaccharides, and more than one disaccharide can be formed, even starting with identical monosaccharide molecules. For example, two glucose molecules connected α(1 ! 4) form maltose and connected α(1! 1)α form trehalose. When a disaccharide is formed involving one OH group from the anomeric carbon, the same monosaccharide even connected to the OH at the same carbon on the other molecule can generate two different disaccharides. For example, two molecules of glucose connected α(1 !4) form maltose, and two molecules of glucose connected β(1 !4) form cellobiose. When the water molecule is eliminated between anomeric carbons in both monosaccharides, the resulting disaccharide does not have reducing properties (monosaccharides and disaccharides with a free acetal OH group have reducing properties). The name of the nonreducing disaccharides replaces the end “ose” with “oside,” suggesting the formation of a glycoside (see Subchapter 11.3). The structure of disaccharides is amply described in the literature (see e.g., [1]).
MALTOSE, LACTOSE, AND SUCROSE Pyrolysis of disaccharides takes place following reactions very similar to those of monosaccharides. The nature of pyrolysis products from a disaccharide depends on the nature of the component monosaccharides and on the type of connection between the two monosaccharide molecules. For several 1 !2 or 1 !4 connected monosaccharides, the nature of the pyrolysis products is not very different from that of the pyrolysis of individual monosaccharides, but the proportion of various compounds in the pyrolyzate may show differences. The decomposition starts around 190°C with losing H2O and it is continued with further decompositions as the temperature increases. Flash pyrolysis at temperatures above 650–700°C up to about 1000°C generates various fragments of the molecules, similar to the case of monosaccharides. Pyrolysis of three disaccharides—maltose, lactose, and sucrose—was performed in identical conditions as for glucose, and the pyrograms are shown in Fig. 11.2.1 for maltose (Trace A), lactose (Trace B), and sucrose (Trace C). Maltose is 4-O-α-D-glucopyranosyl-D-glucose (Glc α(1! 4) Glc), lactose is β-D-galactopyranosyl-(1$ 4)-β-D-glucopyranose (Gal β(1! 4) Glc), and sucrose is α-D-glucopyranosyl-(1$2)-β-D-fructofuranoside (Glc α(1! 2) Fru). Pyrolysis conditions were Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. Because the nature of the compounds in the three pyrolyzates is the same as for glucose for all the compounds eluting before 50 min in the pyrogram, the compound identifications based on their retention time can be obtained from Table 11.1.2 (list for glucose). This window of the pyrogram is dominated by the peaks of CO2 (2.23 min),
449
MALTOSE, LACTOSE, AND SUCROSE A bundance
OH
9,000,000 8,000,000 7,000,000
30.39
OH
O
1e+07
OH
OH
O
OH
OH
4.23 49.91
6,000,000
42.9946.36
5,000,000 28.91
4,000,000 3,000,000 2,000,000 1,000,000 0
(A)
56.71
O
OH HO
47.68
34.14 4.65
7.63 9.64 22.48 5.80 12.07 14.50 18.58 5.0
10.0
15.0
20.0
30.83
25.0
30.0
35.60 40.88 37.44 35.0
40.0
60.08 45.0
50.0
OH
9,000,000 4.23 OH HO
7,000,000
O O
OH
OH
O
OH
OH
6,000,000
49.91
28.91
OH
5,000,000
45.71 46.36
4,000,000 34.14
3,000,000
1,000,000
4.65 5.80
7.63 9.64 12.07
17.76 22.48
30.83 35.60
59.90 60.08
42.99 43.25 40.88
51.33
0
(B)
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
30.43 1e+07 9,000,000 8,000,000 7,000,000
5,000,000
4.23 HO
O
HO
OH
O
0 Time-->
60.0
56.72
OH OH
4.65 28.91
3,000,000
1,000,000
47.68
55.0
O OH
OH
4,000,000
2,000,000
50.0
OH
6,000,000
(C)
60.0
30.39
1e+07
2,000,000
55.0
53.77 56.71
47.68
8,000,000
59.90
45.67 49.38
17.76
9.63
17.76 7.63 22.48 12.07 5.80 20.04 26.69 14.49 5.0
10.0
15.0
20.0
25.0
30.0
59.90 35.60 42.67 34.13 37.76 46.37 40.87
35.0
40.0
45.0
49.91
50.0
55.0
60.0
FIG. 11.2.1 Pyrogram of 1.0 mg maltose Glc α(1! 4) Glc (Trace A), 1.0 mg lactose Gal β(1! 4) Glc, (Trace B), and 1.0 mg sucrose Glc α(1!2) Fru (Trace C) at 900°C.
450
11. PYROLYSIS OF CARBOHYDRATES
1,4-dioxadiene (28.91 min), furancarboxaldehyde (30.39 min), and 5-(hydroxymethyl)-2furancarboxaldehyde (47.68 min). For the anhydrosugars that are formed from glucose, their identification is also given in Table 11.1.2, and for those generated from the galactose moiety, the identification can be done based on Table 11.1.5. Pyrolysis of disaccharides typically starts with the cleavage of the ether bond between the two monosaccharide units [2]. This leads to the formation of a monosaccharide and a dehydrated derivative of the other monosaccharide. In the case of lactose, for example, the two molecules formed are 1,6-anhydro-β-D-galactopyranose (levoglucosan equivalent for galactose) and glucose. The resulting molecules continue to decompose under the influence of heat. Levoglucosan was shown to decompose very similarly to glucose (see Subchapter 11.1), and the same properties can be assumed valid for other 1,6-anhydrosugars; therefore it is not possible to distinguish the origin of the resulting fragments as generated from a monosaccharide or from its 1,6-anhydro derivative. The formation of fragments by the cleavage of the ether bond between the monosaccharide units allows the formation of new di- or trisaccharides, typically identified at low levels in the pyrolyzates of disaccharides. This formation of new condensation products between the disaccharide fragments was also verified in an experiment with sucrose thermal degradation, where the addition of erythritol ((2R,3S)-butane-1,2,3,4-tetraol) led to the formation of an erythritol fructoside [3]. Disaccharide pyrolysis also leads to the formation of a considerable amount of char, with most of this material consisting of carbon but also containing various oligosaccharides. The formation of PAHs from disaccharide pyrolysis has been found to be very similar to the formation of these compounds from monosaccharides or from cellulose [3]. The levels of several PAHs generated in sucrose charred at 300°C for 60 min and then heated at 600°C for 10 min are shown in Fig. 11.2.2 [3].
PAH mg/g sucrose
8 6 4 2
Ac en a
ph t
hy
le ne F Ph luo en ren an e An thre th n e Fl rac uo e ra ne nt he n Py e re Be nz Ch ne r [a ]a yse B e nth n e nz rac e o[ a] ne py re ne
0
FIG. 11.2.2 The level of PAHs in sucrose pyrolyzate first at 300°C and then at 600°C [3].
451
RUTINOSE
35.35 38.39
Abundance 1.6e+07
HO
1.4e+07
H3C
1.2e+07 1e+07
OH 4.07
O O
CH2 O
OH HO HO
36.06
OH
8,000,000 6,000,000
47.68
OH
5.57
22.18
4,000,000 2,000,000 0 Time--> 5.0
30.08
39.95 42.72
40.54 44.76
28.61
17.36
50.15
11.70 10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
56.70 55.0
60.0
FIG. 11.2.3 Pyrogram of 1.0 mg of rutinose Ram α(1!6) Glc at 900°C. Note: The retention times of the early peaks in the pyrogram from Fig. 11.2.3 are shifted with about 0.3 min earlier compared to those from Fig. 11.1.5 of glucose.
RUTINOSE Rutinose, or 6-O-α-L-rhamnosyl-D-glucose or 6-O-(6-deoxy-α-L-mannopyranosyl)-D-glucose, is a disaccharide consisting of a rhamnose and a glucose molecule indicated as (Ram α(1 !6) Glc). This compound is present in some glycosides such as rutin. Rutinose starts decomposing similarly to other disaccharides around 190°C. Pyrolysis of a sample of rutinose performed on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.2.3. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.2.1. The calculation of the mole % was obtained based solely on peak areas.
TABLE 11.2.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.2.3 for the Pyrolysis of Rutinose No. Compound
Retention Time (Min) MW CAS#
Moles %
1
Carbon dioxide
4.07
44
124-38-9
10.86
2
Formaldehyde
4.49
30
50-00-0
1.96
3
Acetaldehyde
5.57
44
75-07-0
5.90
4
Ethanol
7.85
46
64-17-5
1.30
5
Pyruvaldehyde
9.32
72
78-98-8
3.70 Continued
452
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.2.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.2.3 for the Pyrolysis of Rutinose—Cont’d No. Compound
Retention Time (Min) MW CAS#
Moles %
6
Hydroxyacetaldehyde (glycol aldehyde)
17.36
60
141-46-8
3.81
7
Acetic acid
19.77
60
64-19-7
2.45
8
Ethyl-1-propenyl ether
22.03
86
928-55-2
0.71
9
1-Hydroxy-2-propanone (acetol)
22.18
74
116-09-6
5.07
10
1,4-Dioxadiene
28.61
84
N/A
1.90
11
2-Oxopropionic acid methyl ester ?
29.04
102
600-22-6
0.95
12
Furancarboxaldehyde (furfural)
30.08
96
98-01-1
2.59
13
Furanyl ethanone
32.87
110
1192-62-7
0.10
14
Dihydro-4-hydroxy-2(3H)-furanone
33.86
102
5469-16-9
0.65
15
3-Hepten-2-one ?
34.65
112
1119-44-4
0.59
16
5-Methyl-2-furancarboxaldehyde
35.34
110
620-02-0
10.60
17
3-Acetyldihydro-2(3H)-furanone
36.06
128
517-23-7
4.48
18
Tetrahydro-3,6-dimethyl-2H-pyran-2-one
36.25
128
3720-22-7
1.43
19
4-Penten-2-ol
37.35
86
625-31-0
2.03
20
1,3-Dihydroxy-2-propanone
37.52
90
96-26-4
3.60
21
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.74
112
80-71-7
3.02
22
3-Hydroxy-5-methyl-5,6-dihydroxypyran-4-one
38.39
128
N/A
10.38
23
2,5-Dimethyl-4-hydroxy-3(2H)-furanone
39.45
128
3658-77-3
0.84
24
Unknown
39.95
128
N/A
3.67
25
Anhydrosugar from rhamnose ?
40.54
144
N/A
1.42
26
Anhydrosugar from rhamnose ?
42.36
144
N/A
1.37
27
Anhydrosugar from rhamnose ?
42.72
144
35810-56-1
4.45
28
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
42.95
142
1073-96-7
0.85
29
Dianhydromannitol
44.02
146
N/A
0.20
30
Unknown
44.77
128
N/A
1.78
31
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.68
126
67-47-0
6.05
32
Unknown
50.15
146
N/A
0.81
33
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.70
162
498-07-7
0.47
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
453
REFERENCES 11.2
Abundance 128
100 O
H 3C
OH 113
80 58
40 29
87
O
O
CH 3 Si CH 3 CH 3
O
40
69
20
H3C
60
O
41
185 73
80
60
0 m/z--> 20
Abundance 100
20
157 59
101 115 129 143
169
200
0 40
60
(A)
80
100
120
140
m/z-->
60
80
100
120
140
160
180
200
(B)
FIG. 11.2.4 Mass spectrum of 3-hydroxy-5-methyl-5,6-dihydroxypyran-4-one (spectrum A) and that of its TMS derivative (spectrum B).
The pyrogram of rutinose is rather different from the one of the other disaccharides. The main peaks in the pyrogram are CO2, 5-methyl-2-furancarboxaldehyde, and 3-hydroxy-5methyl-5,6-dihydroxypyran-4-one. Because there are compounds that retain several OH groups from the parent compound among the pyrolysis products of rutinose, it can be expected that some pyrolysis products may not be seen in the pyrogram generated under conditions given in Table 1.4.1 because they do not elute from the chromatographic column. For the analysis of these compounds, the silylation of the pyrolyzate followed by GC/MS analysis under conditions described in Table 1.4.2 is necessary. However, except for some undecomposed rutinose, no additional compounds were detected in the silylated pyrolyzate. A confirmation of the formation of 3-hydroxy-5-methyl-5,6-dihydroxypyran-4-one was obtained following the analysis of the silylated pyrolyzate because this compound also generates a sizable peak in the chromatogram of the silylated pyrolyzate. The mass spectra of 3hydroxy-5-methyl-5,6-dihydroxypyran-4-one and of its TMS derivative are shown in Fig. 11.2.4A and B, respectively. Important differences can be seen between the pyrolysis of other disaccharides and that of rutinose in the formation of anhydrosugars. The level of levoglucosan is very low in rutinose pyrolyzate because the OH group from C6 of glucose is involved in the ether link with the rhamnose unit, and it is not available to eliminate a water molecule between the OH groups at atoms 1 and 6. Also, instead of furfural formation, 5-methylfurfural is among the major pyrolysis products of rutinose. The pyrolysis of this compound shows that besides the component monosaccharides in a disaccharide, the type of bond between the monosaccharide units is also important for the pyrolyzate composition.
References 11.2 [1] C.A. Stortz, A.D. French, Mol. Simul. 34 (2008) 373. [2] G.N. Richards, F. Shafizadeh, Aust. J. Chem. 31 (1978) 1825. [3] T.E. McGrath, W.G. Chan, M.J. Hajaligol, J. Anal. Appl. Pyrolysis 66 (2003) 51.
454
11. PYROLYSIS OF CARBOHYDRATES
S U B C H A P T E R
11.3
Carbohydrates With Additional Functional Groups
GENERAL CONCEPTS A large variety of compounds are derived from carbohydrates. These compounds are generated by the modification of the initial carbohydrate molecule, either adding or subtracting functionalities. Several more common carbohydrate derivatives are indicated below. – The replacement of an OH group with hydrogen generates a special type of compound known as doxy sugars. Examples of these molecules are 2-deoxy-D-ribose((2R,4S,5R)-5hydroxymethyltetrahydrofuran-2,4-diol), 6-deoxy-L-galactose (fucose), and 6-deoxy-Lmannose (rhamnose). – Reduction (hydrogenation) of the aldehyde group(s) of a carbohydrate into alcohol generates a sugar alcohol. These compounds can be classified as polyols, and a discussion on their pyrolysis can be found in Subchapter 4.1 (sorbitol or glucitol is discussed with some details). In the case of disaccharides (or trisaccharides, etc.) that are formed, leaving the OH free at only one anomeric carbon, it is possible to change one sugar unit into a polyol while the other(s) remains unaffected. For example, partial hydrogenation of maltose leads to 4-O-α-D-glucopyranosyl-D-glucitol (maltitol). These compounds are part carbohydrate and part polyol. – Oxidation of a monosaccharide leads to the formation of sugar acids. There are numerous sugar acids that may have one or more carboxyl groups, and some may have additional modifications of the carbohydrate structure. Examples of sugars acids are gluconic acid, glucuronic acid, and ascorbic acid. – Replacement of an OH group with chlorine (or other halogen) leads to halogenated sugars. Sucralose or 4,10 ,60 -trichloro-4,10 ,60 -trideoxygalactosucrose is a well-known halogenated sugar used as an artificial sweetener (sold under the trade name Splenda). – Ethers and silyl ethers are sugar derivatives obtained by the replacement of the active hydrogens from the OH groups with R or Si(R)3 where R is an alkyl or aryl radical. These derivatives of sugars are frequently used in various syntheses and for analytical purposes. Making of trimethylsilyl ethers for analytical purposes after pyrolysis of sugars was previously described (see Subchapter 11.1). – Glycosides are a special class of ethers formed between the OH group from the anomeric carbon of a saccharide and a nonsugar molecule (this class does not include nonreducing disaccharides that involve the anomeric carbons in their formation). The nonsugar group of
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL
455
a glycoside is indicated as an aglycone. When the sugar moiety is glucose, the compounds are indicated as glucosides. Numerous glycosides are natural compounds. – Sugar esters are compounds where one or more OH groups are esterified. The esterification can be done with a variety of acids, with the most common esters being acetylated sugars. – Simple amino sugars are compounds with an OH group replaced by an NH2 group. The most common amino sugar is 2-amino-2-deoxy-D-glucose or (3R,4R,5S,6R)-3-amino(hydroxymethyl)oxane-2,4,5-triol. The amino group can be acetylated to form N-acetyl glucosamine, another common amino sugar. These molecules play a major role in biological systems, being involved in various glycosylation mechanisms. More complicated molecules are also known in this group of sugar derivatives. Among these are the derivatives that contain both amino and carboxyl groups (e.g., neuraminic acid).
DEOXYSUGARS There is very little information in the literature regarding the pyrolysis of deoxysugars. Pyrolysis of a rhamnose (6-deoxy-L-mannose) sample performed at 900°C in conditions similar to that for glucose showed that the compound behaves very similarly to other sugars during pyrolysis. Most small fragments generated from rhamnose are similar to those generated, for example, from glucose.
Among the small fragments that were different in rhamnose pyrolyzate compared to glucose were 1,2- and 1,3-propanediol and 2,5-dimethyl-4-hydroxy-3(2H)-furanone. The peak intensities were also relatively different. A number of compounds tentatively identified as anhydro-deoxysugars resulting from the elimination of one or two water molecules from rhamnose were also present in the pyrolyzate. However, only tentative assignments were possible because the mass spectra of these anhydrosugars were not available in common mass spectra libraries. Silylation of the rhamnose pyrolyzate showed that some rhamnose is still present in the pyrolyzate as well as several disaccharides resulting from the elimination of water between two rhamnose molecules. These disaccharides were present at levels around 0%–1% in the pyrolyzate.
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL A typical example of a partially hydrogenated sugar is maltitol. This compound is formed from a glucose unit connected through the glucosidic OH with a glucitol (sorbitol) unit. The compound could also be classified as a glucoside. Pyrolysis of a sample of maltitol performed
456
11. PYROLYSIS OF CARBOHYDRATES
A bundance
OH
56.95 OH
8,000,000 CH 2OH
7,000,000
OH
6,000,000
O
OH
O
OH
30.54
OH
4.30 5,000,000
OH
50.06 29.08
OH
60.08
4,000,000 3,000,000 2,000,000
4.73
1,000,000 0 Time-->
17.97
9.78 7.74 5.90 8.81
10.0
45.82 47.77
31.00 37.58 34.28
15.0
41.01 43.12
25.86 20.0
25.0
30.0
35.0
59.36 60.53
49.23
33.31
12.21 5.0
22.67
40.0
45.0
53.84 50.0
55.0
60.0
FIG. 11.3.1 Pyrogram of 1 mg maltitol at 900°C.
on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.1. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.1. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxide
4.30
44
124-38-9
8.31
2
Propene
4.55
42
115-07-1
1.37
3
Formaldehyde
4.73
30
50-00-0
10.06
4
1,3-Butadiene
5.33
54
106-99-0
0.44
5
Acetaldehyde
5.90
44
75-07-0
4.49
6
Furan
7.74
68
110-00-9
2.77
7
1,3-Cyclopentadiene
8.66
66
542-92-7
0.71
8
2-Propenal (acrolein)
8.81
56
107-02-8
2.68
9
Acetone
9.23
58
67-64-1
0.33
10
Butanal
9.78
72
123-72-8
3.17
11
2,3-Dihydrofuran
11.12
70
1191-99-7
0.15 Continued
457
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL
TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
12
2-Methylfuran
12.21
82
534-22-5
0.57
13
Methyl vinyl ketone
14.39
70
78-94-4
0.44
14
2,3-Butanedione (diacetyl)
14.66
86
431-03-8
0.53
15
Hydroxyacetaldehyde (glycol aldehyde)
17.97
60
141-46-8
7.36
16
2-Butenal
19.61
70
15798-64-8
0.20
17
2-Methyl-2-propenal
19.69
70
78-85-3
0.36
18
Acetic acid
20.24
60
64-19-7
0.14
19
Vinylfuran
21.05
94
1487-18-9
0.17
20
Ethyl-1-propenyl ether
22.52
86
928-55-2
0.49
21
1-Hydroxy-2-propanone (acetol)
22.67
74
116-09-6
2.76
22
3-Methylfuran
25.86
82
930-27-8
0.40
23
2-Propenoic acid methyl ester ?
26.86
86
96-33-3
0.29
24
1,4-Dioxadiene
29.08
84
N/A
3.81
25
2-Oxopropionic acid methyl ester ?
29.49
102
600-22-6
0.15
26
Butandial
29.84
86
638-37-9
0.93
27
Furancarboxaldehyde (furfural)
30.54
96
98-01-1
3.73
28
2-Propylfuran
31.00
110
4229-91-8
1.66
29
2-Furanmethanol
31.84
98
98-00-0
0.18
30
5-Methyl-2(3H)-furanone
32.21
98
591-12-8
0.15
31
Vinyl-2(3H)-furanone
32.65
110
N/A
0.10
32
Methylglyoxal + unknown
32.81
72
78-98-8
0.19
33
1-(2-Furanyl)ethanone
33.31
110
1192-62-7
0.66
34
Dihydro-4-hydroxy-2(3H)-furanone
34.28
102
5469-16-9
1.33
35
2-Hydroxy-2-cyclopenten-1-one
34.68
98
10493-98-8
0.80
36
2-Cyclohexen-1-ol
35.54
98
822-67-3
0.19
37
5-Methyl-2-furancarboxaldehyde
35.75
110
620-02-0
0.56
38
1,3-Benzenediol
35.07
110
108-46-3
0.13
39
5-Acetyldihydro-2(3H)-furanone
37.11
128
29393-32-6
0.15
40
6,8-Dioxabicyclo[3.2.1]octane
37.58
114
280-16-0
1.03
41
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
38.11
112
80-71-7
0.40 Continued
458
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
42
Phenol
38.80
94
108-95-2
0.24
43
1,5-Hexadien-3-ol
38.96
98
924-41-4
0.30
44
2,5-Dimethyl-4-hydroxy-3(2H)-furanone ?
39.85
128
3658-77-3
0.14
45
3-Furancarboxylic acid methyl ester
41.01
126
13129-23-2
0.39
46
Unknown
41.15
116
N/A
0.16
47
2,3-Dihydro-3,5-dihydroxy-6-methyl-4H-pyran- 42.79 4-one
144
28564-83-2
0.16
48
(Z)-penta-2,4-dienoic acid ?
43.12
98
N/A
0.40
49
1-Hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one
45.82
144
113781-138
1.12
50
1,4:3,6-Dianhydro-α-D-glucopyranose
46.49
144
N/A
0.91
51
Unknown
47.20
144
N/A
0.13
52
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.77
126
67-47-0
1.20
53
1,4:3,6-Dianhydro-D-glucitol (isosorbide)
49.23
146
652-67-5
0.65
54
1,2-Cyclohexanediol ?
49.50
116
1792-81-0
0.34
55
3-Hydroxy-6-(hydroxymethyl)-5H-6hydropyran-4-one
50.06
144
N/A
2.50
56
Unknown
50.27
144
N/A
0.16
57
Unknown
53.84
128
N/A
0.16
58
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.95
162
498-07-7
17.84
59
Unknown
58.15
162
N/A
0.22
60
1,4-Anhydro-D-glucitol
59.36
164
N/A
3.00
61
Methyl-D-glucofuranoside ?
59.65
164
N/A
1.57
62
1,6-Anhydro-β-D-glucofuranose
60.08
162
7425-74-3
2.68
63
Anhydrosugar
60.53
162
N/A
1.40
Hydrogen, CO, methane, ethylene, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
The results of the analysis of maltitol pyrolyzate from Table 11.3.1 show a number of compounds that were generated in both glucose (see Subchapter 11.1) and sorbitol (glucitol) (see Subchapter 4.1) as well as compounds typical for glucose pyrolysis (e.g., a larger level of levoglucosan) and compounds typical for sorbitol (e.g., isosorbide). Maltitol pyrolyzate
SUGAR ACIDS
459
composition is basically an average of the composition of the pyrolyzates of the two molecular moieties (glucose and glucitol), such that little interaction appears to occur between these two parts during pyrolysis. This effect is very likely caused by the relatively weak ether bond between the sugar and the alcohol.
SUGAR ACIDS Sugar acids can be obtained either by the direct oxidation of a carbohydrate or by indirect methods such as the oxidation of a monosaccharide with specifically protected groups. The main groups of acids generated from carbohydrates are the following: – Aldonic acids that can be generated by the direct oxidation of the aldehyde group into the COOH group. The general formula of these compounds is HOCH2-(HC(OH))n-COOH. A typical example of aldonic acid is gluconic acid (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanoic acid. The formation of cyclic acetals is not possible for these compounds because no carbonyl group is available. Aldonic acids are typical hydroxy acids, and their pyrolysis is presented in Subchapter 12.3 together with other hydroxy acids. – Aldaric acids generated by the transformation of both the aldehyde group and the terminal CH2OH group in COOH. The general formula for these compounds is HOOC-(HC(OH))nCOOH. A typical example in this group is the saccharic acid (glucaric acid) or (2S,3S,4S,5R)-2,3,4,5-tetrahydroxyhexandioic acid. The formation of cyclic acetals is not possible for these compounds because no carbonyl group is available. Aldaric acids also are typical hydroxy acids, and their pyrolysis is presented in Subchapter 12.3. – Uronic acids generated by the transformation of the terminal CH2OH group in COOH. – The general formula is HOOC-(HC(OH))n-CHO for these compounds. A typical example in this group is glucuronic acid or 3,4,5,6-tetrahydroxytetrahydropyran-2carboxylic acid. Because the carbonyl group remains unaffected in uronic acids, they form cyclic acetals similar to the saccharide to which they are related. This is possible by modifying the carbonyl group into an internal acetal, and uronic acids are presented in this section. – Saccharinic acids or 3-deoxyaldonic acids that are formed in strong basic media from carbohydrates. The reaction of formation of saccharinic acids plays a particularly important role in the pyrolysis of carbohydrates in the presence of TMAH when thermochemolysis and methylation processes takes place (see reaction 11.1.12). – Ketoaldonic acids with the general formula HOCH2-(HC(OH))n-C(O)-COOH (also known as uronic acids), which are also derivatives of certain sugars and can be generated by their oxidation with strong oxidants (e.g., chromic acid) in the presence of catalysts. – Ascorbic acid, which is in fact a lactone that can ionize and has acidic properties. Pyrolysis of ascorbic acid is discussed in Subchapter 14.3. Only uronic acids have a chemical structure closely related to that of carbohydrates. Pyrolysis of a sample of D-glucuronic acid performed on a 1.0 mg compound at Teq ¼ 700°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.2. Pyrolysis in identical conditions except for Teq ¼ 900°C generates almost
460
11. PYROLYSIS OF CARBOHYDRATES
Abundance
56.24
10,000,000 4.23
9,000,000
CO O H
8,000,000
OH
7,000,000
30.39
O OH OH
OH
6,000,000 5,000,000 4,000,000
28.91
3,000,000
7.63
2,000,000 1,000,000
5.80
0
5.0
Time-->
9.0411.98 14.50 10.0
15.0
34.40 20.11 20.0
51.10
34.53 25.0
30.0
35.0
40.0
45.0
50.0
54.15
56.85 60.0
55.0
FIG. 11.3.2 Pyrogram of D-glucuronic acid at 700°C.
identical results. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.2. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.2 for the Pyrolysis of D-glucuronic Acid at 700°C No. Compound 1 2 3 4 5 6 7
Carbon dioxidea Acetaldehyde
a
a
Furan
a
2-Propenal (acrolein) a
Acetone
a
2-Methylfuran
a
2,3-Butanedione (diacetyl) a
8
Acetic acid
9
2(5H)-furanone
10 11
a
2-Cyclopentandione a
1,4-Dioxadiene
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
4.23
44
124-38-9
29.53
5.80
44
75-07-0
0.71
7.63
68
110-00-9
2.14
8.68
56
107-02-8
0.15
9.04
58
67-64-1
1.16
11.98
82
534-22-5
0.93
14.5
86
431-03-8
0.13
20.11
60
64-19-7
0.42
28.27
84
497-23-4
0.19
28.60
98
3008-40-0
0.15
28.91
84
N/A
2.58 Continued
461
SUGAR ACIDS
TABLE 11.3.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.2 for the Pyrolysis of D-glucuronic Acid at 700°C—Cont’d No. Compound 12 13 14 15 16 17 18 19 20
Butanedial Furancarboxaldehyde (furfural)
a
a
5-Methyl-2(3H)-furanone
a
Methylglyoxal + unknown
a
2-Methylcyclopentene-1-one
a
Dihydro-3-methylene-2(3H)-furanone a
2-Hydroxy-2-cyclopenten-1-one a
Benzaldehyde + unknown
a
2-Methylenecyclopentanol
a
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
29.60
86
638-37-9
0.14
30.39
96
98-01-1
4.90
31.98
98
591-12-8
0.17
32.58
72
78-98-8
0.10
33.85
96
1120-73-6
0.19
34.40
98
547-65-9
0.67
34.53
98
10493-98-8
0.32
35.02
106
100-52-7
0.07
35.33
98
20461-31-8
0.11
21
2-Hydroxy-3-methyl-2-cyclopenten-1-one
37.94
112
80-71-7
0.17
22
2H-pyran-2,6(3H)-dione
38.05
112
5926-95-4
0.18
a
23
Phenol
38.60
94
108-95-2
0.09
24
3-Methylphenol (m-cresol)
41.31
108
108-39-4
0.12
25
1,3-Cyclopentandione
41.70
98
3859-41-4
0.11
26
Furylformate ?
41.88
126
13493-97-5
0.09
27
5-Oxotetrahydrofuran-2-carboxylic acid methyl ester
42.20
144
21461-85-8
0.09
28
(3E)-4-(2-furyl)-3-buten-2-one
43.24
136
41438-24-8
0.06
29
3-Hydroxybenzaldehyde
51.10
122
100-83-4
0.41
30
Unknown
52.42
162
N/A
0.07
31
1-(2-Hydroxyphenyl)ethanone
53.10
136
118-93-4
0.06
32
Deoxy-glucuronic acid
54.15
176
N/A
0.38
33
Glucuronic acid ?-lactone
55.44
176
N/A
0.12
34
Deoxy-glucuronic acid
55.65
176
N/A
0.31
35
Glucuronic acid 1,6-lactone
56.24
176
N/A
52.54
36
Unknown
56.85
192
N/A
0.19
37
Trihydroxyperhydropyran-2-carboxylic acid
58.38
178
N/A
0.06
38
3,5,7-Trihydroxy-2H-1-benzopyran-2-one
60.73
194
22065-07-2
0.07
39
Dihydroxydihydropyran-2-carboxylic acid
60.91
160
N/A
0.12
a
Compounds identical with those generated from the pyrolysis of glucose. Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
462
11. PYROLYSIS OF CARBOHYDRATES
FIG. 11.3.3 Mass spectrum of D-glucuronic acid-1,6lactone (MW ¼ 176).
Abundance 100
57 O
90
71
80
O
86
70 60
OH
29
50
OH
40 30
O
OH
43
20 10 0 m/z--> 20
101114 40
60
80
129 141 159
100 120 140 160
As shown in Table 11.3.2, some of the compounds generated in the pyrolysis of glucuronic acid are identical to those generated from glucose pyrolysis. These are small fragment molecules such as acetaldehyde, furan, and furfural. A few minor pyrolyzate constituents are different, but they also consist of furan or pyran derivatives. The main reaction taking place during D-glucuronic acid pyrolysis is the formation of a 1,6-lactone (2,3,4-trihydroxy-7,8dioxabicyclo[3,2,1] octan-6-one), as shown in the reaction below:
ð11:3:1Þ
The reaction is similar to the formation of levoglucosan from glucose (see Subchapter 11.1). The mass spectrum of D-glucuronic acid-1,6-lactone is given in Fig. 11.3.3. The GC/MS analysis of silylated pyrolyzate shows, in addition to the compounds listed in Table 11.3.2, low levels of undecomposed glucuronic acid and traces of a few disaccharide type compounds.
HALOGENATED SUGARS Among halogenated sugars, sucralose or 1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4chloro-4-deoxy-α-D-galactopyranoside is of more interest, with it being a common sweetener. Pyrolysis of a sample of sucralose performed on a 1.0 mg compound at Teq ¼ 700°C, β ¼ 10°C/ ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.4. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.3. The calculation of the mole % was obtained based on peak areas.
463
HALOGENATED SUGARS
Abundance 9,000,000
4.20
Cl OH
8,000,000
HO
O
7,000,000
O OH
6,000,000
53.64
OH
CH2OH
O
Cl
Cl
5,000,000 9.02
4,000,000
42.96
3,000,000
55.11
2,000,000
20.04 17.45 11.99 14.41
1,000,000 0
5.0
Time-->
10.0
15.0
20.0
30.34 25.0
30.0
34.36
35.0
40.84 43.85 49.91 48.22 45.65
55.50
40.0
55.0
45.0
50.0
60.0
FIG. 11.3.4 Pyrogram of 1 mg sucralose at 700°C.
TABLE 11.3.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.4 for the Pyrolysis of Sucralose at 700°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxidea
4.20
44
124-38-9
20.78
2
Propene
4.45
42
115-07-1
0.85
3
2-Methylpropene
4.96
56
115-11-7
0.33
4
Acetaldehydea
5.8
44
75-07-0
0.27
5
Furana
7.58
68
110-00-9
0.41
6
Hydrochloric acid
8.13 peak extended
36
7647-01-0
8.68
7
Acetonea
9.02
58
67-64-1
6.51
11.99
82
534-22-5
0.60
8
2-Methylfuran
a a
9
2,3-Butanedione (diacetyl)
14.5
86
431-03-8
0.24
10
Chloroacetaldehyde
14.62
78
107-20-0
0.53
16.44
78
71-43-2
0.16
17.45
102
N/A
0.63
18.52
96
625-86-5
0.21
a
11
Benzene
12
2-Chlorofuran
13
a
2,5-Dimethylfuran
Continued
464
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.4 for the Pyrolysis of Sucralose at 700°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
14
Acetic acida
20.04
60
64-19-7
5.49
15
1-Chloro-2-propanone
22.70
92
78-95-5
0.28
23.02
92
108-88-3
0.21
30.34
96
98-01-1
0.66
16 17
a
Toluene
a
Furancarboxaldehyde (furfural) a
18
1-(2-Furanyl)ethanone
33.12
110
1192-62-7
0.58
19
2,4-Pentanedione ?
33.32
100
123-54-6
0.46
20
Dihydro-3-methylene-2(3H)-furanone
34.36
98
547-65-9
0.72
34.62
112
N/A
0.32
35.57
110
620-02-0
0.32
37.11
110
N/A
0.27
37.95
112
80-71-7
0.16
38.66
94
108-95-2
0.20
21 22 23 24 25
a
Unknown
a
5-Methyl-2-furancarboxaldehyde a
Unknown
a
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene) a
Phenol
a
26
3-Furancarboxylic acid methyl ester
40.84
126
13129-23-2
1.36
27
3-Methylphenol
41.37
108
108-39-4
0.31
a
28
Levoglucosenone
42.96
126
37112-31-5
2.06
29
Unknown
43.85
126
N/A
0.92
30
Unknown
48.22
144
N/A
0.48
a
31
Dianhydrosugar ?
49.91
144
N/A
1.07
32
4-Chloro-7,8-dioxabicyclo[3.2.1]octane-2,3-diol (1,6anhydro-β-D-4-chloro-4-deoxygalactopyranose)
53.65
180
N/A
41.94
33
2,5-Bis(chloromethyl)-4-hydroxy-2,4,5-trihydrofuran3-one ?
55.11
198
N/A
0.82
34
Unknown chlorinated sugar fragment
55.51
198
N/A
0.64
35
Unknown sugar related
55.74
?
N/A
0.21
36
Unknown sugar related
55.97
?
N/A
0.32
a
Compounds identical in nature with those generated from the pyrolysis of glucose. Hydrogen, CO, methane, ethylene, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
As shown in Table 11.3.3, a number of compounds from sucralose pyrolysis are similar to those from other (not chlorinated) carbohydrates. Several chlorinated compounds are also generated in the pyrolyzate, including HCl, chloroacetaldehyde, chloropropanone, etc. The main pyrolysis product (in the conditions of the experiment) is the formation of a
HALOGENATED SUGARS
Abundance 100
FIG. 11.3.5 Mass spectrum of 1,6-anhydroβ-D-4-chloro-4-deoxygalactopyranose (MW ¼ 180).
60 89
90 80
O Cl
70
OH
60 50
OH 29 41
99
20
105 116
10 0 m/z--> 20
O
75
40 30
465
60
100
127 145 163 181 140
180
chlorinated equivalent to levoglucosan from the pyrolysis of other sugars. The reaction is shown below:
ð11:3:2Þ
The identification of 1,6-anhydro-β-D-4-chloro-4-deoxygalactopyranose is more certain while the structure of the other fragment is only tentative. The mass spectrum of 1,6anhydro-β-D-4-chloro-4-deoxygalactopyranose is shown in Fig. 11.3.5. The mass spectrum of the trimethylsilylated (TMS) derivative of this compound and that of the TMS derivative of 2,5-bis-(chloromethyl)-4-hydroxy-2,4,5-trihydrofuran-3-one are shown in Figs. 11.3.6 and 11.3.7, respectively. The carbonyl group in the ketone 2,5-bis-(chloromethyl)-4-hydroxy-2,4,5-tri-hydrofuran-3-one is assumed involved in a keto-enol equilibrium, and the ketone becomes 2,5-bis-(chloromethyl)-4,5-dihydrofuran-3,4-diol, which generated a TMS derivative [1] by derivatization. The analysis of sucralose pyrolyzate indicates that compounds similar to those generated from other sugars also are produced from the pyrolysis of this compound. The elimination of HCl from the molecule is, in many respects, similar to the elimination of H2O from simple sugar molecules.
466
11. PYROLYSIS OF CARBOHYDRATES
Abundance
161
100 90
O
O
73
CH3
80
Cl
70 60
CH3
O
40
CH3
CH3
CH3
204
30
Si
CH3
Si
50
O
117 243
20
103
45
10
147
217 278
0 40
m/z-->
60
80
100
120
140
160
180
200
220
240
260
324
280
300
320
FIG. 11.3.6 Mass spectrum of the TMS derivative of 1,6-anhydro-β-D-4-chloro-4-deoxygalactopyranose
(MW ¼ 324).
Abundance
CH3
73
100
CH3
90
H 3C
80
O O
Si
CH3
CH3
70
O
60 50
Cl
258
Cl
103
40
185
30 20
CH3
Si
147
45
10
243 169
59
271
215
288
0 m/z-->
40
60
80
100
120
140
160
180
200
220
240
260
280
342
306 300
320
340
FIG. 11.3.7 Mass spectrum of the TMS derivative of 2,5-bis(chloromethyl)-4,5-dihydrofuran-3,4-diol (MW ¼ 342).
ETHERS AND SILYL ETHERS OF SACCHARIDES Ethers and silyl ethers of carbohydrates are somewhat more stable to higher temperatures compared to the unmodified compounds. Pyrolysis of these compounds is reported in the literature only in relation with their formation by pyrolysis in the presence of a derivatization reagent such as TMAH [2–5] or hexamethyldisilazane [5]. The resulting compounds (methyl ethers of trimethylsilyl ethers) are stable in the conditions in which they are generated (700°C for the TMAH thermo-chemolysis/methylation).
467
GLYCOSIDES
GLYCOSIDES Glycosides are compounds frequently found in nature (e.g., [6–8]) (glycosides formed with glucose are named glucosides). Examples of glucosides are salicin or 2-(hydroxy-methyl)phenyl-β-D-glucopyranoside (related to aspirin) found in willow bark; naringin, a flavonoid glycoside that gives the bitter taste to grapefruit; neohesperidin dihydrochalcone, which is an artificial sweetener derived from bitter neohesperidin found in citrus juices; and rutin, a flavonoid glycoside also known as vitamin P1. Among other natural glycosides are those formed with 27-carbon steroids or with 30-carbon triterpenes that form the group of saponins. This group includes the cardiac steroid glycyrrhizin, which is a glycoside (with a sugar acid) of glycyrrhetinic acid and is found in licorice root, and many other plant glycosides. Pyrolysis of glycosides typically gives a mixture of compounds, some resulting from the saccharide and some from the aglycon moieties with little interaction of the two parts of the molecule. This effect, also seen for example in disaccharides or in compounds that are part carbohydrate and part polyol (maltitol), is caused by the relatively weak ether bond between the sugar and the aglycon. The glycosidic bond is cleaved faster than other parts of the molecule and generates fragments that undergo further pyrolytic decompositions. An example of a glycoside pyrolysis is given in Fig. 11.3.8 for neohesperidin dihydrochalcone. From the name of this compound, 2-O-(6-deoxy-2-O-{3,5-dihydroxy-4-[3(3-hydroxy-4-methoxyphenyl)propanoyl]phenyl}-α-L-mannopyranosyl-β-D-glucopyranose, the aglycon part (chalcone derivative) as well as the sugar moiety (rhamnoglucose) can be seen. The compound is closely related to naringin (40 ,5,7-trihydroxyflavanone-7-rhamnoglucoside). Pyrolysis of a sample of 1.0 mg neohesperidin dihydrochalcone at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.8. Abundance
2e+07 1.8e+07
57.59
OH O HO HO
1.6e+07 1.4e+07 1.2e+07 1e+07
OH
O
OCH3
42.59
O
56.74 OH
O
H3C HO
OH HO
O
45.24
38.61
OH
47.38
4.20
8,000,000 42.94
6,000,000
9.03
4,000,000 2,000,000 0 Time-->
34.37 36.29
12.01 5.76 7.59 5.0
10.0
14.56 17.68 15.0
22.45
20.0
25.0
40.18
65.38 49.88
28.88 30.0
35.0
40.0
FIG. 11.3.8 Pyrogram of neohesperidin dihydrochalcone at 900°C.
45.0
50.0
55.48 59.52
55.0
60.0
65.0
468
11. PYROLYSIS OF CARBOHYDRATES
The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.4. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
1
Carbon dioxide
4.21
44
124-38-9
15.68
2
Propene
4.45
42
115-07-1
1.08
3
Formaldehyde
4.63
30
50-00-0
0.53
4
Propynal
5.29
54
624-67-9
Trace
5
Acetaldehyde
5.76
44
75-07-0
2.02
6
Methanol
6.34
32
67-56-1
0.20
7
Furan
7.59
68
110-00-9
0.23
8
Propanal
8.65
58
123-38-6
0.20
9
Acetone
9.03
58
67-64-1
5.19
10
2-Methylfuran
12.01
82
534-22-5
2.15
11
2,3-Butanedione (diacetyl)
14.56
86
431-03-8
0.27
12
Hydroxyacetaldehyde (glycol aldehyde)
17.68
60
141-46-8
0.37
13
2,5-Dimethylfuran
18.54
96
625-86-5
0.19
14
Acetic acid
20.03
60
64-19-7
0.82
15
1-Hydroxy-2-propanone (acetol)
22.45
74
116-09-6
0.65
16
1,4-Dioxadiene
28.88
84
N/A
0.62
17
2,5-Furandione
29.62
98
N/A
0.11
18
Furancarboxaldehyde (furfural)
30.38
96
98-01-1
0.21
19
Dihydro-3-methylene-2(3H)-furanone
34.37
98
547-65-9
1.65
20
5-Methyl-2-furancarboxaldehyde
35.58
110
620-02-0
0.66
21
Unknown
36.29
128
N/A
1.44
22
3,5-Dimethyl-2,4(3H,5H)-furandione
36.49
128
5460-81-1
0.17
23
Resorcinol
37.12
110
108-46-3
0.10
24
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene) 37.96
112
80-71-7
0.16
25
5-Methyl-6,7-dioxabicyclo[2.2.1]heptan-2-one
38.61
128
N/A
4.31
26
Isomaltol
39.41
126
3420-59-5
0.32 Continued
469
GLYCOSIDES
TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
27
2-Methoxyphenol + unknown
39.70
124
90-05-1
0.62
28
Unknown from rhamnose
40.18
102
N/A
2.41
29
Unknown
40.76
144
N/A
0.33
30
Maltol
41.18
126
118-71-8
0.05
31
4-Methylphenol
41.31
108
106-44-5
0.26
32
3-Methylphenol
41.36
108
108-39-4
0.34
33
Unknown from sugar
42.17
128
N/A
0.63
34
2-Methoxy-5-methylphenol
42.59
138
1195-09-1
6.36
35
2,4-Dimethylphenol
42.81
122
105-67-9
0.60
36
Levoglucosenone
42.94
126
37112-31-5
3.48
37
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
43.15
142
1073-96-7
0.33
38
Unknown
44.99
128
N/A
0.50
39
4-Ethyl-2-methoxyphenol
45.24
152
2785-89-9
3.81
40
1,4:3,6-Dianhydro-α-D-glucopyranose
46.33
144
N/A
0.15
41
Unknown
46.97
180
N/A
0.20
42
2-Methoxy-4-vinylphenol
47.38
150
7786-61-0
2.87
43
2’,4’-Dihydroxy-3’-methylacetophenone + unknown
47.62
166
10139-84-1
0.04
44
Resorcinol monoacetate
48.73
152
102-29-4
0.12
45
Mixture
49.34
164
N/A
0.18
46
6-(Hydroxymethyl)-2-methyl-2H-3,5,6trihydropyran-4-one
49.88
144
N/A
0.70
47
4-(1,2-Propandienyl)-guaiacol
50.79
162
N/A
0.67
48
1-(2,6-Dihydroxyphenyl)butan-1-one
51.20
180
N/A
0.06
49
Unknown aromatic compound
51.36
180
N/A
0.09
50
Unknown
51.61
180
N/A
0.13
51
2-Ethyl-1,4-benzodioxin ?
51.87
162
27549-01-5
0.17
52
4-(4-Methoxyphenyl)-2-butanone
52.09
178
104-20-1
0.15
53
Anhydrosugar (from rhamnose ?)
53.45
146
N/A
0.23
54
1-(4-Hydroxy-3-methoxyphenyl) ethanone
55.28
166
498-02-2
0.13
55
4-Hydroxy-2-methyloxycinnamaldehyde
55.48
178
127321-19-1
0.63 Continued
470
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
56
4-(4-Hydroxyphenyl)-2-butanone
56.26
164
5471-51-2
0.30
57
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.75
162
498-07-7
11.04
58
Ethylhomovanillate ?
57.13
210
60563-13-5
0.21
59
3,4-Methylenedioxyphenyl acetone
57.27
178
4676-39-5
0.21
60
4-(4-Hydroxy-3-methoxyphenyl)butan-2-one
57.59
194
122-48-5
19.44
61
Unknown
58.66
192
N/A
0.06
62
5-Hydroxy-6-methoxy-1-indanone
59.52
178
127933-78-4
0.69
63
1,6-Anhydro-β-D-glucofuranose
59.86
162
7425-74-3
0.45
64
4,5-Dimethoxy-2-(2-propenyl)phenol ?
60.09
194
N/A
0.07
65
1-(4-Methoxymethyl-2,6-dimethylphenyl)ethanone
62.18
192
1000202-02-2
0.11
66
6-(3-Hydroxy-4-methoxyphenyl)hexane-2,4-dione ?
65.38
236
N/A
1.84
Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
Pyrolysis products listed in Table 11.3.4 can be grouped easily into fragments resulting from the sugar (furan derivatives, 6-methyl-5,7-dioxabicyclo[2.2.1]heptan-2-one, levoglucosenone, levoglucosan, etc.) and those resulting from the aglycon (most phenol derivatives) including 4-(4-hydroxy-3-methoxyphenyl)butan-2-one. Even some of the compounds resulting from the rhamnose fragment (5-methyl-6,7-dioxabicyclo[2.2.1]-heptan-2one) and those from the glucose fragment (levoglucosenone, levoglucosan) can be identified. A “model” reaction for the formation of some of the main pyrolysis products from neohesperidin dihydrochalcone is shown below:
ð11:3:3Þ
471
GLYCOSIDES
Abundance
OH
38.38 HO
1.4e+07 1.2e+07 1e+07
OH HO O
4.09
8,000,000
H3C OH
OH
O
OH
O
O
O HO HO
35.32
6,000,000
49.64
OH
4,000,000 5.58 2,000,000
56.48
O
36.04 39.93 42.69
8.77 28.60
11.68 17.33
47.55
34.11
22.13
53.82
59.61
0 Time-->
10.0
20.0
30.0
40.0
50.0
60.0
FIG. 11.3.9 Pyrogram of rutin obtained at 900°C.
Further pyrolysis of the intermediate compounds generated in reaction (11.3.3) as well as other pyrolysis products generated directly from the parent molecule leads to a complex pyrolyzate composition. Char is also formed during pyrolysis, as is a considerable amount of CO2, which shows that the reaction model (11.3.3) is far from complete. Another example of the pyrolysis of a glycoside is that of rutin. The compound is quercetin3-O-β-D-glucose-α-L-rhamnose or 3-{[6-O-(6-deoxy-α-L-mannopyranosyl)-β-D-glucopyranosyl] oxy}-2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-1-benzopyran-3-one. Pyrolysis of a sample of 1.0 mg rutin at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.9. The analysis of pyrolyzate was done by GC/ MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.5. The calculation of the mole % was obtained based solely on peak areas. Typical for a glycoside, the peaks in the pyrogram of rutin can be related to the peaks resulting from the pyrolysis of rutinose (see Table 11.2.1) or even glucose (see Table 11.1.2) and of quercetin (see Subchapter 16.1). Some free quercetin is also present in the pyrolyzate of rutin, but it can be detected only in the silylated pyrolyzate. Pyrolysis of rutin also produces a considerable amount of char. The pyrolyzates of glycosides are frequently similar in composition with that of a mixture of the pyrolysis products of the sugar and the aglycon. This was shown in the previous two examples for neohesperidin dihydrochalcone and also for rutin. The same pattern is seen during the pyrolysis of anthocyanins, where the pyrolyzate is made from a mixture of compounds resulting from the sugar pyrolysis and from the anthocyanidin pyrolysis. For the glycosides with a thermally stable aglycon, the capability of releasing the aglycon upon heating has been investigated with the purpose of developing nonvolatile flavor precursors. These compounds are intended to release the flavor only when heated, for example during cigarette smoking. A typical example of this type of glycoside is menthyl glucoside
472
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.5 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.9 for the Pyrolysis of Rutin at 900°C No. Compound
Retention Time (Min) MW CAS#
Moles %
1
Carbon dioxide
4.09
44
124-38-9
24.74
2
Propene
4.30
42
115-07-1
0.86
3
Formaldehyde
4.50
30
50-00-0
0.75
4
Acetaldehyde
5.58
44
75-07-0
2.55
5
Furan
7.36
68
110-00-9
0.30
6
Propanal
8.38
58
123-38-6
0.32
7
Acetone
8.77
58
67-64-1
3.23
8
2-Methylfuran
11.68
82
534-22-5
1.35
9
Methyl vinyl ketone
13.81
70
78-94-4
0.12
10
2,3-Butanedione (diacetyl)
14.07
86
431-03-8
0.09
11
2-Butenone
14.19
72
78-93-3
0.36
12
Hydroxyacetaldehyde (glycol aldehyde)
17.33
60
141-46-8
0.96
13
2,5-Dimethylfuran
18.20
96
625-86-5
0.41
14
2-Methyl-3-buten-2-one
18.85
84
814-78-8
0.47
15
Acetic acid
19.87
60
64-19-7
1.11
16
Ethyl-1-propenyl ether
22.00
86
928-55-2
0.44
17
1-Hydroxy-2-propanone (acetol)
22.13
74
116-09-6
0.75
18
2,4-Pentandione
26.12
100
123-54-6
0.48
19
1,4-Dioxadiene
28.60
84
N/A
1.69
20
Furancarboxaldehyde (furfural)
30.08
96
98-01-1
0.69
21
Dihydro-4-hydroxy-2(3H)-furanone
33.84
102
5469-16-9
0.36
22
Dihydro-3-methylene-2(3H)-furanone
34.11
98
547-65-9
2.10
23
5,6-Dihydro-4-methyl-2H-pyran-2-one
34.19
112
2381-87-5
0.41
24
1-Methyl-7-oxabicyclo[4.1.0]heptane
34.64
112
1713-33-3
0.41
25
6-Methylcyclohex-2-en-1-ol ?
35.11
112
N/A
0.28
26
5-Methyl-2-furancarboxaldehyde
35.32
110
620-02-0
3.74
27
3-Methyl-1,2-cyclopentanedione
35.43
112
765-70-8
0.18
28
Unknown
35.91
112
N/A
0.53
29
3-Acetyldihydro-2(3H)-furanone
36.04
128
517-23-7
3.75
30
Tetrahydro-3,6-dimethyl-2H-pyran-2-one
36.23
128
3720-22-7
0.35 Continued
473
GLYCOSIDES
TABLE 11.3.5 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.9 for the Pyrolysis of Rutin at 900°C—Cont’d No. Compound
Retention Time (Min) MW CAS#
Moles %
31
Unknown
36.86
110
N/A
0.51
32
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.72
112
80-71-7
1.03
33
3-Hydroxy-5-methyl-5,6-dihydroxypyran-4-one
38.37
128
N/A
9.47
34
2,5-Dimethyl-4-hydroxy-3(2H)-furanone
39.44
128
3658-77-3
0.68
35
Unknown
39.93
128
N/A
2.21
36
Anhydrosugar from rhamnose
40.52
144
N/A
1.10
37
4-Methylphenol (p-cresol)
41.08
108
106-44-5
0.88
38
1,3-Benzodioxol-2-one
41.40
136
2171-74-6
0.16
39
Anhydrosugar from rhamnose
42.39
144
N/A
0.68
40
Anhydrosugar from rhamnose
42.69
144
N/A
2.47
41
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
42.94
142
1073-96-7
1.31
42
Dianhydromannitol
44.02
146
N/A
0.34
43
Unknown
44.75
128
N/A
0.53
44
4,7-Dihydro-1,3-isobenzofurandione
44.88
150
4773-89-1
0.41
45
2-Propoxyphenol
46.79
152
6280-96-2
0.75
46
1-(2-Hydroxy-6-methoxyphenyl)ethanone ?
47.39
166
703-23-1
1.11
47
1,2-Benzenediol (catechol)
47.55
110
120-80-9
2.25
48
6-Hydroxy-4-methoxy-2,3-dimethylbenzaldehyde
49.09
180
34883-12-0
1.71
49
4-Methyl-1,2-benzenediol in mix
49.64
124
452-86-8
5.44
50
1,3-Benzenediol (resorcinol)
50.99
110
108-46-3
0.27
51
2,4-Dihydroxybenzeneacetic acid ?
53.82
168
N/A
0.30
52
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.47
162
498-07-7
12.11
53
2,5-Dihydroxybenzeneacetic acid ?
58.65
168
451-13-8
0.14
54
1,6-Anhydro-α-D-glacatofuranose
59.61
162
N/A
0.34
Hydrogen, CO, methane, ethane, and water not included. Note: "?" indicates uncertain compound identification.
(menthyl-β-D-glucopyranoside) [9]. The heating of this compound between ambient to a temperature of 500°C at a heating rate of 10°C/min showed that, at 200°C, the compound does not decompose, and between 250°C and 300°C the decomposition takes place mainly with the formation of menthol. At 400°C, besides menthol, various byproducts generated mainly from the glucose moiety are produced. The formation of the unmodified aglycon from the pyrolysis of a glucoside is still dependent on the stability of the resulting compound. For some
474
11. PYROLYSIS OF CARBOHYDRATES
compounds, the first stage of the decomposition reaction affects the aglycon molecule and not the sugar. A comparison of the main pyrolysis reactions for menthyl glucoside and phenylethanol glucoside is shown in reactions 11.3.4 and 11.3.5, respectively:
ð11:3:4Þ
ð11:3:5Þ
Further decomposition of the sugar and the aglycon is not uncommon and depends on the pyrolysis conditions (Teq in particular).
SUGAR ESTERS Sugar esters are compounds resulting from the esterification reaction between a carbohydrate (acting as an alcohol) and an organic or inorganic acid. A wide range of compounds can be obtained as esters, even considering one type of acyl group. The acyl can be attached to a different number of OH groups and at different positions in the carbohydrate. Pyrolysis of glucose pentaacetate at Teq ¼ 700°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C leads to only about 15% decomposition of the parent molecule, with the rest of it transferring intact to the GC/MS system. Some of the sample may have been transferred before reaching the nominal Teq value, being vaporized without decomposition and transferred out of the heating zone. The decomposition products of glucose pentaacetate include acetic acid, acetic anhydride, and compounds such as 1,5-anhydro-D-arabino-hex-1-enitol tetraacetate:
ð11:3:6Þ
The elimination of the acetic acid molecule can take place from positions other than C1, leading to analogous compounds.
AMINO SUGARS
475
The carbohydrates only partially esterified are less stable to heating. The pyrolysis products generated from these esters include the free acids and sugar decomposition products. The amount of acid released from sugar esters depends on the number of esterified OH groups, the position of esterification, and the heating temperature. The compounds with a high degree of esterification have the tendency to generate molecules that retain the acyl group, such that they do not yield the highest level of free acid [10]. Among the inorganic esters of carbohydrates, glucose-6-phosphate is probably the most important compound because it is involved in cell metabolism. Besides the formation of H3PO4, the pyrolysis of glucose-6-phosphate is not expected to generate fragments different from those generated from glucose.
AMINO SUGARS The replacement of the OH group from the C2 of glucose with an NH2 group leads to the formation of 2-amino-2-deoxy-D-glucose (glucosamine). Glucosamine is a precursor in the synthesis of glycosylated proteins and is the building block for chitosan and (in acetylated form) for chitin. The amino group, having a basic character, can form salts with various acids. Pyrolysis at 900°C of glucosamine in its salt form with HCl generates three types of compounds. The first type includes small molecules that are similar to those generated in glucose pyrolysis. These molecules include CO2, aldehydes, furans, and other small molecules. Levoglucosenone is also present in the pyrolyzate (at about 0.2%), but levoglucosan is absent. No aminated equivalent of levoglucosan was detected in the pyrolyzate. The second type of compound is nitrogen containing. This includes acetonitrile, propanenitrile, and low levels of alkyl pyridines and pyrazines. The third group of compounds includes a few chlorinated compounds. Besides HCl that was expected in the pyrolyzate, compounds such as methylene chloride and 3-chloro-2-cyclopenten-1-one also were detected. This indicates that the formation of an excess of HCl during pyrolysis leads to the interactions of this compound with organic fragments and the formation of organochlorine compounds. N-Acetyl-D-glucosamine (GlcNAc) is an important amino sugar in the bacterial cell wall as a component of peptidoglycans. In these polymers, N-acetyl-D-glucosamine is typically connected on one side with other sugars and on the other side to 2-acetamido-2deoxymuramic acid and further to the amino acids in a protein chain (see, e.g., [11]). From a chemical point of view, N-acetyl-D-glucosamine is an amide of acetic acid with 2-amino2-deoxy-D-glucose. Having a large sugar moiety, it is interesting to compare its pyrolysis with other sugars. For this purpose, a sample of 1.0 mg N-acetyl-D-glucosamine was pyrolyzed at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The pyrogram is shown in Fig. 11.3.10, and the compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.6. The calculation of the mole % was obtained based solely on peak areas. The compounds from Table 11.3.6 indicate that the pyrolysis of N-acetylglucosamine is in many respects similar to that of other sugars. The formation of small molecules, including CO2, (H2O not shown in the table), small aldehydes and ketones, and furan derivatives, is
476
11. PYROLYSIS OF CARBOHYDRATES
A bundance
19.92
OH
48.42
8,000,000 7,000,000
O
31.77
4.20
HO
NH
6,000,000
O
5,000,000
CH3
4,000,000
60.71
3,000,000 2,000,000
17.67
1,000,000 0 Time-->
OH
HO
5.76
10.83 9.04
5.0
10.0
14.39 15.0
59.36 60.84 56.25 54.57 51.04 63.91
36.11 21.67
20.0
41.61 42.88 38.08
27.36
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
FIG. 11.3.10
Pyrogram of 1 mg N-acetyl-D-glucosamine at 900°C. Peak identifications by their retention times given in Table 11.3.6.
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxide
4.20
44
124-38-9
10.16
2
Cyclopropane
4.47
42
75-19-4
0.75
3
Formaldehyde
4.63
30
50-00-0
1.37
4
Acetaldehyde
5.76
44
75-07-0
0.35
5
Furan
7.58
68
110-00-9
0.07
6
2-Propenal (acrolein)
8.64
56
107-02-8
0.20
7
Acetone
9.04
58
67-64-1
0.66
8
Acetonitrile
10.83
41
75-05-8
2.33
9
2-Methylfuran
11.97
82
534-22-5
0.05
10
2,3-Butanedione (diacetyl)
14.39
86
431-03-8
0.13
11
2-Butanone
14.53
72
78-93-3
0.17
12
Propanenitrile
16.20
55
107-12-0
0.08
13
Benzene
16.35
78
71-43-2
0.06
14
2-Methyloxazole
16.61
83
N/A
0.05
15
Hydroxyacetaldehyde (glycol aldehyde)
17.68
60
141-46-8
3.26 Continued
477
AMINO SUGARS
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
16
2,5-Dimethylfuran
18.50
96
625-86-5
0.06
17
Acetic acid
19.92
46
64-17-5
34.07
18
2,4-Dimethyloxazole
21.68
97
7208-05-1
0.09
19
1-Hydroxy-2-propanone (acetol)
22.43
74
116-09-6
0.11
20
Toluene
22.99
92
108-88-3
0.07
21
Pyridine
23.94
79
110-86-1
0.12
22
Acetic acid anhydride
24.87
102
108-24-7
0.07
23
2-Methylpyridine
26.99
93
109-06-8
0.14
24
Acetic acid methyl ester
27.36
74
79-20-9
0.29
25
Unknown
27.55
111
N/A
0.11
26
1H-pyrrole
27.60
67
109-97-7
0.21
27
Ethylbenzene
28.16
106
100-41-4
0.04
0
28
3,3 -oxybispropanenitrile
28.84
124
1656-48-0
0.29
29
Pyruvic acid methyl ester
29.26
102
600-22-6
0.14
30
Furancarboxaldehyde (furfural)
30.31
96
98-01-1
0.12
31
Acetamide
31.77
59
60-35-5
26.76
32
1-Acetyloxy-2-propanone
31.90
116
592-20-1
0.22
33
Dihydro-4-hydroxy-2(3H)-furanone
34.08
102
5469-16-9
0.31
34
2-Hydroxy-2-cyclopenten-1-one
34.50
98
10493-98-8
0.10
35
N-acetylpyrrole
34.85
109
N/A
0.13
36
5-Ethyl-2,4-dimethyl-1,3-oxazole
36.11
125
N/A
0.92
37
2,4,5-Trimethyl-1,3-oxazoline
36.29
113
N/A
0.41
38
N-acetylacetamide
38.08
101
625-77-4
0.20
39
Phenol
38.59
94
108-95-2
0.07
40
2-Methyl-4-butyloxazole
39.17
139
N/A
0.13
41
1H-pyrrole-2-carboxaldehyde
39.25
95
1003-29-8
0.10
42
2-Methylbenzoxazole
41.02
133
95-21-6
0.01
43
1-(1H-pyrrol-2-yl)ethanone
40.00
109
1072-83-9
0.04
44
3,4-Dimethyl-2-propyloxazole
41.01
139
53833-32-2
0.03 Continued
478
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
45
Acetamidoacetaldehyde
41.61
101
64790-08-5
1.07
46
Unknown
41.83
133
N/A
0.16
47
Oxazole derivative ?
41.95
153
N/A
0.10
48
Unknown
42.15
133
N/A
0.15
49
Unknown
42.60
141
N/A
0.06
50
5-Methyl-2(1H)-pyridinone
42.75
109
1003-68-5
0.12
51
2-Furanmethanol + unknown
42.88
98
98-00-0
0.38
52
2-Pyridinemethanol acetate
43.32
151
1007-49-4
0.07
53
6-Methyl-2(1H)-pyridinone
44.15
109
3279-76-3
0.06
54
1,2-Dihydro-6-methyl-2-oxo-3-pyridinecarboxaldehyde
44.90
137
784440-89-8
0.09
55
Unknown
46.19
151
N/A
0.08
56
2,4,4-Trimethyl-2-enolide
46.43
126
4182-41-6
0.07
57
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.55
126
67-47-0
0.24
58
Unknown
47.84
139
N/A
0.09
59
3-Acetamidofuran
48.42
125
59445-85-1
6.32
60
1-Acetamidobicyclo[3.2.0]hetan-2-one
48.54
167
N/A
0.23
61
Pyridin-2,6-diol, diacetate ?
50.83
195
N/A
0.16
62
2-Acetamido-2-deoxyglucono-1,4-lactone ?
51.04
219
N/A
0.11
63
Unknown
52.83
149
N/A
0.07
64
Unknown
53.10
169
N/A
0.11
65
N-(2,4-dihydroxyphenyl)acetamide
54.57
167
71516-07-9
0.36
66
Unknown
54.72
128
N/A
0.20
67
Unknown
54.93
152
N/A
0.11
68
6-(Hydroxymethyl)-2-methyl-4,5,6,7,3a,7a-hexahydro7-oxabenzoxazole-4,5-diol
56.25
203
N/A
0.51
69
Anhydro-N-acetyl-glucosamine ?
59.36
203
N/A
0.72
70
3-Acetamido-5-acetylfuran
60.71
167
95598-28-0
2.05
71
3-Acetamido-5-acetylfuran isomer
60.84
167
N/A
1.17
72
Unknown
61.42
181
N/A
0.10
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: "?" indicates uncertain compound identification.
AMINO SUGARS
479
expected. However, the levels of furfural and 5-hydroxymethylfurfural are much lower than those for typical carbohydrates. Small molecules containing nitrogen also are generated, such as several nitriles. Acetic acid and acetamide are found in large proportions in the pyrolyzate. Also, 3-acetamidofuran is present in the pyrolyzate. One interesting reaction generating oxazoles derivatives also occur during pyrolysis. The first step of this reaction can be written as follows:
ð11:3:7Þ
Similar to the case of glucose, it is expected that N-acetylglucosamine pyrolyzate contains a number of compounds with multiple OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg N-acetyl-Dglucosamine at 900°C followed by the collection of the pyrolyzate and derivatization with bis(trimethylsilyl)trifluoroacetamide (BSTFA) in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done using a GC/MS technique in conditions described in Table 1.4.2. Only the window between 42 and 52 min from the chromatogram of the derivatized pyrolyzate is shown in Fig. 11.3.11 because this portion contains compounds with OH silylated groups. The identification of the peaks is given in Table 11.3.7, based on their retention times.
Abundance 52.38
1.2e+07 1e+07 8,000,000 43.13 6,000,000 4,000,000
52.46
48.80
43.39 44.52
2,000,000 46.30 0 42.0 Time-->
44.0
46.0
50.94 48.0
50.0
52.0
FIG. 11.3.11 Time window 42–52 min in the pyrogram of N-acetylglucosamine.
480
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.7 Identification of the Main Peaks in the Time Window 42–52 min in the Chromatogram Shown in Fig. 11.3.11 for the Pyrolyzate of N-acetylglucosamine Derivatized to TMS Derivative With Bis(trimethylsilyl) trifluoroacetamide (BSTFA) No. Compound
Ret. Time
No. of TMS
MW
1
2-(Acetylamino)-3,6-anhydro-2-deoxy-D-glucopyranose
43.13
2
347
2
6-(Hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta[2,1-d]1,3-oxazole-4,5-diol
43.40
3
401
3
5-(Hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol 44.52
3
419
4
Unknown
46.30
3
?
5
N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1]octan-4-yl)acetamide
48.80
2
347
6
N-acetyl-2-deoxy-2-aminoglucose
52.38
4
509
7
N-acetyl-2-deoxy-2-aminohexose
52.46
4
509
The identification of silylated pyrolysis products of N-acetylglucosamine encountered problems due to the absence of their spectra in common mass spectral libraries. The mass spectra of three TMS 6-(hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta[2,1-d]1,3oxazole-4,5-diol is shown in Fig. 11.3.12, that of three TMS derivative of 5(hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol is shown in Fig. 11.3.13, and that of two TMS derivative of of N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1] octan-4-yl)acetamide, which is the equivalent of two TMS levoglucosan generated from glucose is shown in Fig. 11.3.14. Abundance
TM S 298
O 100
TM S
N
O
CH 3
80
O 131
73
60
O
TM S
40
20 147 45 0 m/z-->
40
103 80
311 210
120
160
200
386 401
269 240
280
320
360
400
FIG. 11.3.12 Mass spectrum of three TMS derivative of 6-(hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta [2,1-d]1,3-oxazole-4,5-diol (MW ¼ 401).
481
AMINO SUGARS
Abundance
73
100
TMS O
80
TMS 60
N
O
CH3 O
O
217
189 40
O
TMS
147
316
20 117
300
45 0 m/z-->
329
272
40
80
120
160
200
240
280
404
358
320
360
400
FIG. 11.3.13 Mass spectrum of three TMS derivative of 5-(hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol (MW ¼ 419). Abundance
186
100
TMS 80
O
TMS
O
NH
CH3
144
73 60
O
O O
40 212 20
116 101
43
129
59 0 m/z-->
40
80
120
332
169
160
242 200
240
288
280
320
FIG. 11.3.14 Mass spectrum of two TMS derivative of N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1]octan-4-yl) acetamide (MW ¼ 347).
As shown in Table 11.3.7, some of the initial compound is transferred unmodified by pyrolysis, and also some isomerization process takes place. The other compounds are generated from water elimination of one or more OH groups of the parent compound in reactions either similar to that seen for simple sugars or as shown in reaction (11.3.7).
482
11. PYROLYSIS OF CARBOHYDRATES
References 11.3 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.C. Moldoveanu, V. David, Sample Preparation in Chromatography, Elsevier, Amsterdam, 2002. D. Fabbri, R.J. Helleur, J. Anal. Appl. Pyrolysis 49 (1999) 277. M. Blazsό, S. Janitsek, A. Gelencser, P. Artaxo, B. Graham, M.O. Andreae, J. Anal. Appl. Pyrolysis 68-69 (2003) 351. I. Tanczos, C. Schwarzinger, H. Schmidt, J. Balla, J. Anal. Appl. Pyrolysis 68-69 (2003) 151. D. Fabbri, G. Chiavari, Anal. Chim. Acta 449 (2001) 271. M. Stengele, E. Stahl-Biskup, J. Essent. Oil Res. 5 (1993) 13. B. Bennini, A.J. Chulia, M. Kaouadji, F. Thomasson, Pytochemistry 31 (1992) 2483. E. Stahl-Biskup, Flav. Fragr. J. 2 (1987) 75. W.-c. Xie, J. Tang, X.-h. Gu, C.-r. Luo, G.-y. Wang, J. Anal. Appl. Pyrolysis 78 (2007) 180. W. W. Weeks, S. C. Moldoveanu, 50th Tobacco Res. Conf, (1996) paper 55. S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998, 300.
11 Pyrolysis of Carbohydrates
S U B C H A P T E R
11.1
Monosaccharides
GENERAL ASPECTS In nature, the most abundant class of biomolecules is that of carbohydrates, including compounds from plants (glucose, fructose, cellulose, starch, pectin, etc.), animals (glucose, glycogen, etc.), bacteria, fungi, archaea, and protists (microbial and fungal polysaccharides, etc.). Carbohydrates (saccharides or sugars) are organic compounds containing in their molecule one or more carbonyl groups and two or more alcohol groups on an aliphatic hydrocarbon chain. These compounds can be classified as monosaccharides, oligosaccharides, and polysaccharides. The general formula for monosaccharides is CnH2nOn (n ¼ 3 for trioses, n ¼ 4 for tetroses, n ¼ 5 for pentoses, etc.). The oligosaccharides and polysaccharides are generated by the elimination of water from two or more monosaccharide molecules, with the sugar units being connected by an ether group. Besides simple carbohydrates that contain only carbonyl and alcohol functional groups (as well as ether for oligosaccharides and polysaccharides), more complex carbohydrates also are known. These may include other functional groups such as halogen, carboxyl (dCOOH), amino (dNH2), amido (dNHCO-R), etc. Several deoxy sugars (with the hydroxyl group replaced by hydrogen) also are known (deoxyribose, rhamnose, etc.). Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00011-1
419
# 2019 Elsevier B.V. All rights reserved.
420
11. PYROLYSIS OF CARBOHYDRATES
Besides natural carbohydrates, many synthetic carbohydrates were produced. Some are related to chemically modified polysaccharides (e.g., ethyl cellulose, carboxymethyl cellulose). Others are synthetic small organic molecules. Glyceraldehyde and dihydroxyacetone (n ¼ 3) can be considered the simplest monosaccharides. (Glycolaldehyde or hydroxyacetaldehyde HOCH2dCHO also is considered by some a two-carbon monosaccharide.) Glyceraldehyde has one asymmetric carbon. Consequently, for this compound there are two possible enantiomers indicated as D and L. The D form is shown with the OH to the right of the carbon chain when the CH]O group is at the top, and the L form with the OH to the left, in a so-called Fisher formula:
For glyceraldehyde, the D-form is an (R)-enantiomer while the L form is an (S)-enantiomer. Monosaccharides with a larger number of carbons may have more asymmetric carbons but they can be conventionally derived from glyceraldehyde (or dihydroxyacetone) by including more H C OH units in the carbon chain between the CH]O group and the lowest H C OH unit. Glyceraldehyde leads to the series of aldoses, and dihydroxyacetone to the series of ketoses. The aldoses with the same stereochemistry as D-glyceraldehyde at the asymmetric carbon that is the most distant from the carbonyl group will form the D-series of aldoses. The monosaccharides with the same stereochemistry as L-glyceraldehyde at the most distant asymmetric carbon will form the L-series. The D-series of aldoses is shown below:
GENERAL ASPECTS
421
In monosaccharides with n > 4, it is common that the carbonyl group forms with one of the dOH groups an internal hemiacetal, such that many monosaccharides have a cyclic structure. The open chain (linear) form of monosaccharides with n > 4 can be found only in water solutions as a small proportion of the dissolved compound. The cyclic hemiacetal can have the form of a five-atom ring (furanose) or a six-atom ring (pyranose).
The proportion of the furanose form versus the pyranose form at equilibrium depends on the structure of the sugar. Glucose, for example, is present almost completely in pyranose form while talose is about 69% pyranose and 31% furanose. The formation of the cyclic hemiacetals of monosaccharides adds one more asymmetric carbon to their structure compared to the linear form. For example, linear glucose has four asymmetric carbons while cyclic glucose has five asymmetric carbons. The new asymmetric C-1 has two possible steric forms (two anomers), indicated as α and β. The configurations of cyclic monosaccharides are described by several types of formulas. As an example, the following formulas are shown for α-D-glucopyranose: Fischer, modified Fischer, Haworth, cyclic, and structural (shorter bonds in the Haworth and structural formulas indicate dH).
Fischer formulas have the same orientation of OH groups as the linear ones. In the Haworth formulas (of aldoses), the side chain (e.g., CH2OH group) is up when it is attached to an R carbon (the original OH to this carbon was to the right in the linear formula), and the side chain is down when the carbon to which it is attached in the ring is S. If the side chain is up, the position of the other substituents (e.g., OH) is reversed to what it was in the Fischer formula (see Fischer and modified Fischer formulas for glucose). In this way, the OH groups on the right (R) from the Fischer formula (and from the linear form) are down in the Haworth formula. For the side chain down, the OH (and other) groups on the right in the Fischer formula are up in the Haworth formula, and those to the left are down. For D-aldoses the steric assignment for the C-1 carbon is α when this is an S carbon (and it is shown down) and is β when this is an R carbon.
422
11. PYROLYSIS OF CARBOHYDRATES
The nomenclature of carbohydrates includes common names as well as systematic names. The common names are sometimes indicated by a three-letter convention: arabinose (Ara), glucose (Glc), galactose (Gal), etc. The IUPAC name of carbohydrates can be found in the literature [1]. In IUPAC nomenclature the numbering of atoms includes not only the carbons but also the oxygen atom (which is numbered as (1). For this reason, the IUPAC name of α-Dglucopyranose is (2S,3R,4S,5R,6R)-6-(hydroxymethyl)tetrahydro-2H-pyran-2,3,4,5-tetraol. Monosaccharides and the lower oligosaccharides (e.g., di-, trisaccharides), being small organic molecules, are included among the molecules further discussed in this chapter regarding their thermal decomposition. The presentation of pyrolysis of polysaccharides (such as cellulose, starch, and chitin) is beyond the purpose of this book and can be found in various other publications (see, e.g., Ref. [2]). The importance of studying the pyrolysis of carbohydrates is related to the wide distribution in nature of this class of compounds. Pyrolysis can occur in a variety of intentional and unintentional processes such as pyrolysis in incinerators, forest fires, cigarette smoking, and food cooking [3].
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS There is very little direct information regarding the pyrolysis of trioses and tetroses. Under initial mild heating, condensation products of these molecules takes place, and further pyrolysis generates compounds similar to those of higher sugars. In basic conditions and heating (e.g., in the presence of NaOH or by treatment with tetramethylammonium hydroxide (TMAH)), these compounds generate carboxylic acids (formic, acetic), hydroxycarboxylic acids (glycolic, lactic), and various lactones [4,5]. Pentoses pyrolysis has been more frequently reported in the literature [5–8]. The pyrolysis of these compounds at temperatures higher than 550°C typically generates as the main products H2O, CO2, aldehydes (hydroxyacetaldehyde, formaldehyde, acetaldehyde, etc.), ketones (butanone, 1-hydroxypropanone, etc.), furan and pyran derivatives (high levels of furfural), and several deoxy-sugars. Smaller molecules that do not have chiral centers and are generated during pyrolysis are very similar for different pentoses. As an example, the time window between start and 41 min of the pyrograms for D-()-arabinose, D-()-ribose, and D-(+)-xylose is shown in Fig. 11.1.1. Arabinose and ribose are present mainly in furanose form while xylose is mainly present in pyranose form. Pyrolysis of all samples shown in Fig. 11.1.1 was performed on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of mixture that includes only the compounds eluting in the first 41 min in the pyrogram are shown in Table 11.1.1. In addition, the calculation of the mole % was obtained solely based on peak areas, and because differences in the MS response factors can be quite large for different compounds, the estimations may have large errors. As shown in Table 11.1.1, for the compounds generated by pyrolysis and eluting from the chromatographic column in the first 41 min, both the nature and the relative concentration of
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
423
(A)
(B)
(C) FIG. 11.1.1 First 41 min from the pyrograms obtained at 900°C from 1.0 mg D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C).
the compounds generated from the three analyzed pentoses is very similar, in spite of the difference in the cycle type (furanose or pyranose) assumed by the sugar. At longer retention times in the pyrograms, several heavier and more polar compounds start eluting. Fig. 11.1.2 displays the time window from 42 to 58 min for the pyrograms of 1.0 mg D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C) (the first part of these chromatograms is shown in Fig. 11.1.1).
424
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.1 Identification of the Main Peaks in the Chromatograms Shown in Figs. 11.1.1A–C for Pyrolysis at 900°C of D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C) Moles % (for Compounds Eluting in the First 42 min)
No. Compound
Retention Time (Min)
MW CAS#
Arabinose Ribose Xylose
1
Carbon dioxide
4.24
44
124-38-9
14.25
11.36
11.14
2
Propene
4.47
42
115-07-1
0.83
0.67
0.86
3
Formaldehyde
4.65
30
50-00-0
6.20
6.50
6.53
4
Acetaldehyde
5.81
44
75-07-0
1.61
1.17
1.38
5
Furan
7.63
68
110-00-9
1.32
1.42
1.31
6
2-Propenal
8.69
56
107-02-8
0.76
0.45
0.85
7
Acetone
9.10
58
67-64-1
1.14
0.50
0.63
8
Pyruvaldehyde
9.63
72
78-98-8
4.29
5.17
5.43
9
2-Methylfuran
12.06
82
534-22-5
0.73
0.59
0.82
10
Butenal
13.85
72
123-72-8
0.20
0.21
0.28
11
2,3-Butanedione
14.48
86
431-03-8
0.31
0.34
0.33
12
Hydroxyacetaldehyde
17.77
60
141-46-8
5.74
8.52
6.72
13
Formic acid
18.50
46
64-18-6
0.23
0.23
0.13
14
Acetic acid
20.07
60
64-19-7
0.84
0.88
0.67
15
Ethyl-1-propenyl ether
22.33
86
928-55-2
0.15
0.15
0.18
16
1-Hydroxy-2-propanone
22.48
74
116-09-6
1.39
1.60
1.48
17
Toluene
23.07
92
108-88-3
0.09
0.09
0.06
18
Methyl formate ?
23.80
60
107-31-3
0.43
0.44
0.25
19
Hydroxyacetic acid (glycolic acid)
24.63
76
79-14-1
0.08
0.08
Trace
20
2,2’-Bioxirane ?
26.71
86
1464-53-5
0.04
0.07
0.02
21
1,4-Dioxadiene
28.92
84
N/A
4.44
4.27
3.49
22
3-Furaldehyde
29.13
96
498-60-2
0.27
0.04
0.32
23
2-Oxopropionic acid methyl ester
29.36
102
600-22-6
0.12
0.34
0.12
24
2,5-Furandione
29.65
98
108-31-6
0.51
0.63
0.45
25
2-Cyclopentene-1,4-dione
30.30
96
930-60-9
3.43
4.60
1.37
26
Furancarboxaldehyde (furfural)
30.44
96
98-01-1
26.22
30.62
32.24
27
2-Propylfuran
30.83
110
4229-91-8
trace
0.08
0.31
28
2-Furanmethanol
31.70
98
98-00-0
0.76
1.03
0.85 Continued
425
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
TABLE 11.1.1 Identification of the Main Peaks in the Chromatograms Shown in Figs. 11.1.1A–C for Pyrolysis at 900°C of D-()-arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C)—Cont’d Moles % (for Compounds Eluting in the First 42 min)
No. Compound
Retention Time (Min)
MW CAS#
Arabinose Ribose Xylose
29
1-(2-Furanyl)ethanone
33.16
110
1192-62-7
0.22
0.24
0.23
30
5,6-Dihydro-2H-pyran-2-one
33.65
98
3393-45-1
0.38
Trace
0.39
31
2-Methylcyclopenete-1-one
33.93
96
1120-73-6
0.02
0.06
0.02
32
Dihydro-4-hydroxy-2(3H)-furanone
34.12
102
5469-16-9
0.74
0.74
0.60
33
Dihydro-3-methylene-2(3H)-furanone
34.40
98
547-65-9
0.64
0.24
0.21
34
2-Hydroxy-2-cyclopenten-1-one
34.53
98
10493-98-8
0.41
0.33
0.46
35
2-Methylenecyclopentanol
35.41
98
20461-31-8
1.12
0.54
1.28
36
2,3-Dihydroxypropanal
35.67
90
367-47-5
5.28
2.41
6.45
37
Butyrolactone
36.54
86
96-48-0
0.42
0.17
0.53
38
Tetrahydro-4H-pyran-4-one
36.77
100
29943-42-8
0.34
0.12
0.33
39
5-Methyl-2,3-dihydrofuran-2,4-dione
36.89
114
N/A
0.50
0.44
0.39
40
Ethyl-1-propenyl ether ?
37.20
86
928-55-2
0.47
0.23
0.21
41
6,8-Dioxabicyclo[3.2.1]octane
37.44
114
280-16-0
2.45
1.38
2.58
42
1,3-Dihydroxy-2-propanone
37.77
90
96-26-4
4.59
5.25
3.91
43
2-Hydroxy-3-methyl-2-cyclopenten-1-one
37.99
112
80-71-7
0.81
0.79
0.81
44
2H-pyran-2,6(3H)-dione
38.13
112
5926-95-4
0.62
1.43
0.61
45
4,5-Dimethyl-1,3-dioxol-2-one ?
39.38
114
37830-90-3
0.78
0.72
0.44
46
Unknown
40.16
114
N/A
1.94
0.66
0.58
47
3-Furancarboxylic acid methyl ester
40.88
126
13129-23-2
0.20
0.20
0.19
48
Unknown
41.03
116
N/A
0.32
0.33
0.14
49
Unknown
41.29
130
N/A
0.61
0.62
0.65
50
Unknown
40.55
98
N/A
0.06
0.24
0.02
51
3-Furancarboxylic acid methyl ester
40.88
126
13129-23-2
0.10
0.19
0.18
52
Unknown
41.29
130
N/A
0.61
0.62
0.60
Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification.
The correct identity of the peaks from these time windows in the pyrograms was more difficult to obtain through searches in the available mass spectral libraries. The peak eluting 43.15 min in the three pyrograms was tentatively identified as resulting from the molecules of pentoses by the elimination of two water molecules to generate a 5,7-dioxabicyclo[2.2.1]
426
11. PYROLYSIS OF CARBOHYDRATES
Abundance 8,000,000
45.56
7,000,000
HO
46.52
O OH
6,000,000 5,000,000 4,000,000 3,000,000 2,000,000
OH
43.15 46.20 46.94
1,000,000 0 42.0 (A)
OH
44.0
46.0
53.45
49.58 48.0
50.0
52.0
54.0
55.92 56.0
58.0
8,000,000
HO
7,000,000
O
6,000,000 5,000,000
45.51
4,000,000
OH
47.80
OH
OH
3,000,000
53.69 54.45 54.66
2,000,000 1,000,000
(B)
42.0
46.18 46.93
43.15 44.0
46.0
49.58 48.0
51.20
50.0
52.0
54.0
56.0
58.0
56.0
58.0
8,000,000 7,000,000
50.11
6,000,000
OH
5,000,000 4,000,000 3,000,000
OH 43.15
45.51 46.93 46.17 47.65
2,000,000 1,000,000 0 Time--> 42.0
44.0
46.0
48.0
51.21 50.0
O OH OH
52.42 52.0
54.0
(C) FIG. 11.1.2 Time window between 42 and 58 min from the pyrograms obtained at 900°C from 1.0 mg D-()arabinose (Trace A), D-()-ribose (Trace B), and D-(+)-xylose (Trace C).
heptenol. Several isomers and diasteroisomers of this compound are possible. The peak at 45.51 min for xylose was tentatively identified as tetrahydro-3,4-furandiol (available in the mass spectral libraries). The peak at 45.56 min for arabinose and at 45.51 min for ribose is not pure, being formed from tetrahydro-3,4-furandiol mixed with 2,5-anhydroarabinofuranose (3,6-dioxabicyclo[2.2.1]heptane-2.7-diol) and 2,5-ribofuranose, respectively. The reaction of the formation of 2,5-anhydro-arabinofuranose is shown below:
MONOSACCHARIDES WITH FEWER THAN SIX CARBON ATOMS
Abundance
FIG. 11.1.3 Mass spectrum of 3,6-dioxa-bicyclo[2.2.1]
74
100
427
OH
heptane-2.7-diol obtained from arabinose.
56
O
80
O 43
60
OH
61
29 40 20 84 0 m/z-->
20
40
60
80
101 114 100
120
ð11:1:1Þ
The mass spectrum of 3,6-dioxabicyclo[2.2.1]heptane-2.7-diol is shown in Fig. 11.1.3. This compound is not formed from xylose. By the loss of one water molecule, all three sugars form 5,7-dioxabicyclo[2.2.1]heptane-2,3diols in reactions shown below for ribose and xylose. ð11:1:2Þ
ð11:1:3Þ
5,7-Dioxabicyclo[2.2.1]heptane-2,3-diol has four chiral centers (and therefore potentially 16 stereoisomers). The stereoisomers of this compound contain part of the chiral centers of the initial molecule and can be indicated as 1,5-anhydro-arabinose when generated from arabinose, 1,5-anhydro-ribose when generated from ribose, etc. The diastereoisomers can be separated on nonchiral chromatographic columns, and their mass spectra are identical (or very similar). The diastereoisomer from arabinose elutes at 46.52 min, the one from ribose at 47.8 min, and the one from xylose at 50.11 min. The spectrum of 5,7-dioxabicyclo[2.2.1] heptane-2,3-diol (1,4-anhydro-xylopyranose) obtained from xylose is shown in Fig. 11.1.4. The spectra of 1,5-anhydro-arabinofuranose and 1,5-anhydro-ribofuranose are virtually identical with that of 1,4-anhydro-xylopyranose, except for the fragment m/z ¼ 110, which is absent in these spectra.
428 FIG. 11.1.4
11. PYROLYSIS OF CARBOHYDRATES
Mass spectrum of 5,7-dioxa-bicyclo-[2.2.1] heptane-2,3-diol obtained from xylose.
Abundance
57
OH
100
O O
80
OH
73
60
29 43
40 39
20 m/z-->
0
20
40
86
68 47
103 60
80
110
100
Pentoses pyrolyzate contains a number of compounds with more than two OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg of the sugars at 900°C followed by collection of the pyrolyzate and derivatization with bis(trimethylsilyl)trifluoroacetamide (BSTFA) in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done with a GC/MS technique in conditions described in Table 1.4.2. Besides the compounds already identified in Table 11.1.1, the analysis of the silylated pyrolyzate showed the presence of some undecomposed initial sugar. Also, the formation of dimers generated by the elimination of water from two molecules of the initial sugars was detected.
GLUCOSE D-(+)-Glucose is probably the most common monosaccharide. The compound is a hexose present mainly in pyranose form, and the ratio of the anomers in solution is 36/64 α/β. Glucose decomposition starts at melting temperature at 153–156°C with the formation of 1,6-anhydroglucose and of several di- and oligosaccharides [9]. Among the disaccharides (see Subchapter 11.2) detected in glucose heated around the melting temperatures for 2.5 h were 2-O-α-D-glucopyranosyl-D-glucose (kojibiose Glc α(1 !2) Glc), 2-O-β-glucopyranosylD-glucose (sophorose Glc β(1 !2) Glc), 3-O-α-D-glucopyranosyl-D-glucose (nigerose Glc α(1! 3) Glc), 3-β-D-glucosyl-D-glucose (laminaribiose Glc β(1 !3) Glc), 4-O-α-Dglucopyranosyl-D-glucose (maltose Glc α(1 !4) Glc), 4-O-β-D-glucopyranosyl-D-glucose (cellobiose Glc β(1 ! 4) Glc), 6-O-α-D-glucopyranosyl-D-glucose (isomaltose Glc α(1 ! 6) Glc), 6-O-β-D-glucopyranosyl-D-glucose (gentiobiose Glc β(1 !6) Glc), and α-D-glucopyranosylα-D-glucopyranoside (trehalose Glc α(1 ! 1)α Glc) [10]. Among the trisaccharides, maltotriose (Glc α(1! 4) Glc α(1 !4) Glc) and panose (Glc α(1 ! 6) Glc α(1 ! 4) Glc) were identified (tentatively).
429
GLUCOSE
OH Abundance
CH 2
4.23
HO
OH
6,000,000
HO
28.91
2,000,000 1,000,000 0 Time-->
42.99
34.13
4.65 7.63 9.64 5.80 12.08 5.0
10.0
49.91
17.76
15.0
35.60 22.48 18.58 20.0
59.90
46.37
OH
4,000,000 3,000,000
56.71
O
7,000,000
5,000,000
47.68
30.39
25.0
30.83 30.0
40.87 45.68 37.73 42.66
35.0
40.0
45.0
49.38 48.26 50.0
55.0
60.0
FIG. 11.1.5 Pyrogram of 1.0 mg D-(+)-glucose at 900°C.
At higher temperatures, glucose decomposition generates a large number of molecular fragments [6,11,12,14,15]. Pyrolysis of a sample of D-(+)-glucose performed on a 1.0 mg compound using Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.1.5. Pyrolysis of glucose at temperatures as low as 550°C generated a pyrogram very similar to that at 900°C, including the nature of the pyrolytic compounds and the relative peak intensities in the pyrogram. This indicates that for glucose (and other similar carbohydrates) the heating at temperatures beyond the decomposition point brings minor changes in the pyrolyzate composition. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.1.2. Because the main pyrolysis products of glucose and fructose are very similar, Table 11.1.2 also contains the identification of the main compounds from the pyrolysis of fructose with the pyrogram shown later in Fig. 11.1.13. The calculation of the mole % was obtained based solely on peak areas. The correct identification of various compounds in glucose pyrolysis is sometimes a problem. The complexity of the pyrolyzate mixture as well as variations in the pyrolysis conditions may lead to some variations in the results reported in the literature. For example, besides the compounds listed in Table 11.1.2, a few compounds such as 2-oxopropanal (MW ¼ 72) or 1-hydroxybutane-2,3-dione (MW ¼ 102) were not identified in the pyrogram, although reported in the literature [16]. Also the correct identification of anhydrosugars is sometimes a problem because the corresponding mass spectra are not available in common mass spectral libraries and different anhydrosugars have similar spectra. For comparison, the spectra of 1,6anhydro-β-D-glucopyranose (levoglucosan) and of 1,6-anhydro-β-D-glucofuranose are shown in Figs. 11.1.6 and 11.1.7, respectively.
430
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
1
Carbon dioxide
4.23
44
124-38-9
10.37
10.12
2
Formaldehyde
4.65
30
50-00-0
6.73
9.00
3
1-Butene
5.02
56
106-98-9
0.04
Trace
4
1-Butyne
5.21
54
107-00-6
0.04
Trace
5
Acetaldehyde
5.80
44
75-07-0
1.41
0.85
6
Furan
7.63
68
110-00-9
1.69
0.45
7
1,3-Cyclopentadiene
8.53
66
542-92-7
0.20
Trace
8
2-Propenal (acrolein)
8.68
56
107-02-8
0.82
0.44
9
Propanal
8.70
58
123-38-6
0.21
0.11
10
Acetone
9.11
58
67-64-1
0.50
0.70
11
Pyruvaldehyde
9.64
72
78-98-8
2.52
1.86
12
2-Methylfuran
12.08
82
534-22-5
0.57
0.60
13
2-Propen-1-ol
13.55
58
107-18-6
0.04
Trace
14
Butenal
13.88
72
123-72-8
0.03
Trace
15
Methyl vinyl ketone
14.24
70
78-94-4
0.16
Trace
16
2,3-Butanedione (diacetyl)
14.50
86
431-03-8
0.21
Trace
17
Benzene
16.44
78
71-43-2
0.02
Trace
18
Hydroxyacetaldehyde (glycol aldehyde)
17.76
60
141-46-8
4.89
0.99
19
2,5-Dimethylfuran
18.58
96
625-86-5
0.23
0.35
20
Formic acid
18.64
46
64-18-6
0.02
Trace
21
2-Butenal (Z)
19.43
70
15798-64-8
0.15
Trace
22
2-Methyl-2-propenal
19.52
70
78-85-3
0.21
Trace
23
Ethanol
19.86
46
64-17-5
0.01
Trace
24
Acetic acid
20.11
60
64-19-7
0.37
1.08
25
Vinylfuran
20.87
94
1487-18-9
0.16
Trace
26
2,3-Pentandione
21.20
100
600-14-6
0.01
Trace
27
Ethyl-1-propenyl ether
22.33
86
928-55-2
0.21
Trace
28
1-Hydroxy-2-propanone (acetol)
22.48
74
116-09-6
0.71
0.52
29
Toluene
23.07
92
108-88-3
0.12
Trace
30
2-Ethyl-5-methylfuran + methyl formate ?
23.80
110
1703-52-2
0.14
Trace Continued
431
GLUCOSE
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
31
Hydroxyacetic acid (glycolic acid)
24.63
76
79-14-1
0.01
Trace
32
2-Hydroxypropanoic acid (lactic acid)
24.96
90
50-21-5
Trace
Trace
33
2,3-Dihydro-1,4-dioxin
25.11
86
543-75-9
0.03
Trace
34
3-Methylfuran
25.70
82
930-27-8
0.22
Trace
35
2-Propenoic acid methyl ester ?
26.69
86
96-33-3
0.44
0.41
36
1-Hydroxy-2-butanone
27.61
88
5077-67-8
0.08
Trace
37
1,4-Dioxadiene
28.91
84
N/A
5.79
0.78
38
3-Furaldehyde
29.12
96
498-60-2
0.24
Trace
39
2-Oxopropionic acid methyl ester ?
29.33
102
600-22-6
0.13
0.12
40
Butandial
29.65
86
638-37-9
0.25
0.14
41
2-Cyclopentene-1,4-dione
30.27
96
930-60-9
0.23
Trace
42
Furancarboxaldehyde (furfural)
30.39
96
98-01-1
11.49
26.74
43
2-Propylfuran
30.83
110
4229-91-8
0.85
Trace
44
2-Furanmethanol
31.70
98
98-00-0
0.12
0.10
45
5-Methyl-2(3H)-furanone
32.05
98
591-12-8
0.18
Trace
46
Methylglyoxal + unknown
32.65
72
78-98-8
0.13
Trace
47
1-(2-Furanyl)ethanone
33.15
110
1192-62-7
0.31
0.31
48
5-Methylidenffuran-2-one (protoanemonin)
33.93
96
108-28-1
0.11
Trace
49
2,4-Dihydroxy-2,5-dimethyl-2(2H)-furan-3-one
33.95
144
10230-62-3
Trace
Trace
50
Dihydro-4-hydroxy-2(3H)-furanone
34.13
102
5469-16-9
3.07
0.22
51
Dihydro-3-methylene-2(3H)-furanone
34.40
98
547-65-9
0.12
0.13
52
2-Hydroxy-2-cyclopenten-1-one
34.53
98
10493-98-8
0.48
0.39
53
Unknown
34.65
112
N/A
0.12
0.14
54
Isomaltol
34.92
126
3420-59-5
0.12
Trace
55
Benzaldehyde + unknown
34.99
106
100-52-7
0.14
0.17
56
2-Methylenecyclopentanol
35.39
98
20461-31-8
0.25
Trace
57
5-Methyl-2-furancarboxaldehyde
35.60
110
620-02-0
1.04
1.38
58
3-Methyl-2-cyclopenten-1-one
36.35
96
2758-18-1
0.13
Trace
59
7,8-Dioxabicyclo[3.2.1]oct-2-ene (mix)
36.53
112
N/A
0.16
Trace
60
Tetrahydro-4H-pyran-4-one
36.75
100
29943-42-8
0.46
Trace Continued
432
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
61
Dihydro-5-propyl-2(3H)furanone ?
36.96
128
105-21-5
0.14
0.16
62
6,8-Dioxabicyclo[3.2.1]octane
37.43
114
280-16-0
0.51
0.32
63
1,3-Dihydroxy-2-propanone
37.74
90
96-26-4
1.41
2.66
64
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.97
112
80-71-7
0.53
0.24
65
Isomer of 2-hydroxy-3-methyl-2-cyclopenten-1- 38.21 one
112
N/A
0.32
Trace
66
Phenol
38.67
94
108-95-2
0.19
0.11
67
1,5-Hexadien-3-ol
38.85
98
924-41-4
0.29
Trace
68
4,5-Dimethyl-1,3-dioxol-2-one ?
39.36
114
37830-90-3
0.25
0.39
69
2,5-Dimethyl-4-hydroxy-2-hydrofuran-3-one ?
39.72
128
3658-77-3
0.09
Trace
70
Unknown
39.89
144
N/A
0.29
0.60
71
2-Methylphenol (o-cresol)
40.25
108
95-48-7
0.09
Trace
72
Unknown
40.41
82
N/A
0.08
Trace
73
3-Furancarboxylic acid methyl ester
40.87
126
13129-23-2
0.87
0.84
74
3-Hydroxy-2-methyl-4H-pyran-4-one (maltol)
41.18
126
118-71-8
0.41
Trace
75
2,5-Furandicarboxaldehyde
41.42
124
823-82-5
0.49
0.61
76
Unknown
41.61
144
N/A
0.24
0.31
77
4-Methylene-1,2-dioxolan-3-one ?
41.94
114
N/A
0.41
Trace
78
2,3-Dihydro-3,5-dihydroxy-6-methyl-4Hpyran-4-one
42.66
144
28564-83-2
0.46
1.34
79
Levoglucosenone
42.99
126
37112-31-5
2.19
0.10
80
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
43.17
142
1073-96-7
0.23
0.15
81
2-Methyl-2-pentenoic acid
43.75
114
16957-70-3
0.16
Trace
82
Dianhydrosugar
44.89
144
N/A
0.06
Trace
83
Unknown
45.47
98
N/A
0.39
0.98
84
5-Acetoxymethyl-2-furaldehyde
45.68
168
10551-58-3
0.74
0.46
85
1,4:3,6-Dianhydro-α-D-glucopyranose
46.37
144
N/A
2.67
Trace
86
Unknown
46.96
116
N/A
Trace
0.21
87
5-Acetoxymethyl-2-furancarboxaldehyde
47.35
126
10551-58-3
Trace
0.26
88
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.68
126
67-47-0
10.33
30.13 Continued
433
GLUCOSE
TABLE 11.1.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.5 for the Pyrolysis of Glucose and in Fig. 11.1.13 for the Pyrolysis of Fructose at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Glucose Moles %
Fructose Moles %
89
3-Methyl-1,2-cyclopentanediol
48.26
116
27583-37-5
0.39
Trace
90
2’,3’-Dideoxyribonolactone ?
48.43
116
32780-06-6
Trace
0.25
91
1,2-Cyclohexanediol ?
49.38
116
1792-81-0
1.07
0.44
92
Dianhydrosugar ?
49.91
144
N/A
1.30
Trace
93
Unknown
56.04
198
N/A
Trace
0.45
94
Unknown
56.30
198
N/A
Trace
0.42
95
1,6-Anhydro-β-D-glucopyranose (levoglucosan) 56.72
162
498-07-7
11.00
0.32
96
1,6-Anhydro-β-D-glucofuranose
162
7425-74-3
3.03
0.12
59.90
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
FIG. 11.1.6 Mass spectrum of 1,6-anhydro-β-D-
Abundance
glucopyranose (levoglucosan).
60
100 90
O
HO HO
80
O
70
OH
60
57
50
73
40 30
43
69
20
98
10 0 m/z--> 40
115
89
60
80
100
127
120
144
140
162
160
As shown in Table 11.1.2, glucose pyrolysis generates small molecules such as formaldehyde, hydroxy-acetaldehyde, acetaldehyde, furan derivatives (five-ring cycle containing one oxygen), pyran derivatives (six-ring cycle containing one oxygen), and dioxane derivatives (six-ring cycle containing two oxygen atoms). The formation of small molecules such as H2O, CO, and CO2 and organic compounds with C1–C4 atoms such as HCHO, CH3CHO, and CH3COCH3 takes place from a variety of reactions, including eliminations, fragmentation, extrusions, rearrangements, etc. For example, hydroxyacetaldehyde may be formed by a retro-aldolization reaction, as shown below:
434
11. PYROLYSIS OF CARBOHYDRATES
FIG. 11.1.7 Mass spectrum of 1,6-anhydro-β-Dglucofuranose.
Abundance 100
73
90
OH O
80 70
O OH
60 50
OH
40
20
69
44
30 19
61
29
85
10
91
0 m/z--> 20
40
60
80
98
100
115 127
120
ð11:1:4Þ
A common reaction during carbohydrate pyrolysis is water elimination. When the elimination takes place between OH groups at vicinal carbons, the result is the formation of an aldol type structure. The elimination of water from the diol group is favored if the OH groups are in syn positions, but this is not a strict requirement. The aldol can further undergo a retroaldol fragmentation, as shown below:
ð11:1:5Þ
Because in a glucose molecule there are several pairs of vicinal carbons with OH substituents, the reaction may take place for any of these pairs with a multitude of reaction products. The reaction is shown below for the pair C3–C4 and the elimination of the OH from C3 of the glucose molecule:
ð11:1:6Þ
GLUCOSE
435
The resulting compounds continue the decomposition process, as shown below for one of the resulting aldehydes:
ð11:1:7Þ
The elimination of water can occur also following a Grob fragmentation that takes place for a 1,3-alcohol [17]. Because glucose has several pairs of 1,3-carbons with OH substituents, the reaction may generate a multitude of reaction products (as was mentioned for the water elimination between OH groups at vicinal atoms). For the structural formula of glucose, the reaction is exemplified below between the OH groups at the C2 and C4 atoms and the cleavage of C3–C4 bond in the molecule:
ð11:1:8Þ
The initially formed fragments can further pyrolyze to generate CO2 and small aldehydes, ketones, hydroxyaldehydes, etc. The water elimination from OH groups bound to 1,3-carbons is influenced by their position as syn or anti (syn favored), but water can be eliminated from both types. The formation of formaldehyde by reaction (11.1.8) does not account for the whole amount of this compound formed from glucose. As shown in Table 11.1.2, formaldehyde represents about 9% (mole) of pyrolyzate, and its generation is likely to occur from other reactions. Addition of diammonium phosphate during glucose pyrolysis was shown to diminish the formation of formaldehyde [18]. It is likely that diammonium phosphate (NH4)2HPO4 has a catalytic effect on the opening of the internal hemiacetal structure [19], which leads to somewhat different reactions during glucose pyrolysis while also generating ammonia that reacts with HCHO, diminishing its apparent level. Water elimination also can have as a result the formation of various cycles. The formation of a furan derivative, 5-(hydroxymethyl)-2-furancarboxaldehyde (hydroxymethylfurfural), is shown as an example in the following sequence of reactions:
ð11:1:9Þ
Hydroxymethylfurfural may further decompose to generate furancarboxaldehyde (furfural) and formaldehyde, as shown in reaction (11.1.10):
436
11. PYROLYSIS OF CARBOHYDRATES
ð11:1:10Þ Other reactions may also be responsible for the formation of furfural, with one example shown below:
ð11:1:11Þ
Another path also starts with formaldehyde elimination, but the cyclization process is assumed to occur in a later stage, as shown in the following sequence of reactions [20]:
ð11:1:12Þ
The formation of furfural also can be explained starting with the linear form of glucose [20], although the low level of this form in solid glucose makes this mechanism unlikely. The formation of pyran derivatives can result from various reactions affecting the OH and CH2OH groups without affecting the initial pyran cycle of glucose. However, the formation of new six-atom rings containing one oxygen is also possible. An example of a reaction with the preservation of the pyran cycle from glucose (and with the formation of a new 1,3-dioxolane cycle) is the formation of levoglucosan by the elimination of one water molecule between the OH in the 1-position of glucose and the H from the OH group connected to the 6-position:
ð11:1:13Þ
Further decomposition of levoglucosan generates pyrolysis products similar to those from glucose [21,22]. Fig. 11.1.8 shows the comparison between the pyrogram of 1 mg glucose (the same shown in Fig. 11.1.5) and the pyrogram of 1 mg levoglucosan (1,6-anhydro glucose) with the pyrolysis performed in identical conditions with that of glucose pyrolyzed at 900°C. Glucose pyrolyzate is shown in Trace A and levoglucosan pyrolyzate in Trace B. Detailed inspection of the two pyrograms from Fig. 11.1.8 (using the data analysis capability of a GC/MS system) shows the qualitative identity of most compounds, although some
437
GLUCOSE
Abundance 1.4e+07 1.2e+07 1e+07 8,000,000 6,000,000 4,000,000 2,000,000 0
(A)
30.39
47.68
4.23 28.91 4.65 5.0
7.63 10.0
17.76 15.0
42.99 46.37
34.13 35.60
20.0
25.0
30.0
35.0
56.71
49.91
40.87 40.0
59.90
45.0
50.0
55.0
60.0
65.0
56.71 1.4e+07 1.2e+07 1e+07 8,000,000 6,000,000 4,000,000 2,000,000 0
(B)
Time-->
46.37
17.76 4.32 10.47 6.04 5.0
10.0
22.34 15.0
20.0
25.0
30.0
49.91
42.99
30.39 28.91
47.68
34.76
37.79 40.87
35.0
40.0
59.91
46.73 45.0
50.0
55.0
60.0
65.0
FIG. 11.1.8 Pyrogram of 1 mg glucose (Trace A) and of 1.0 mg levoglucosan (Trace B) at 900°C.
differences are seen in peak intensities. This indicates that pyrolysis of levoglucosan, initially formed during glucose pyrolysis, is a likely contributor to the formation of other molecules in glucose pyrolyzate. However, this does not indicate that other reactions do not occur independently during pyrolysis of glucose, starting directly from the parent molecule. The lack of levoglucosan formation in fructose pyrolysis (see further), and the similarity of the nature of compounds generated from glucose and fructose are additional proof that levoglucosan formation is not the unique path of glucose pyrolysis. Besides levoglucosan, several other anhydrosugars are generated during glucose pyrolysis. The formulas for several of these compounds (including levoglucosan) detected in glucose pyrolyzate are shown below:
438
11. PYROLYSIS OF CARBOHYDRATES
Besides the furan and pyran cycles, the anhydrosugars contain other oxygenated cycles. A reaction leading to the formation of a dioxane derivative (1,4-dioxadiene) is shown below:
ð11:1:14Þ
With the formation of a double bond at the bridgehead not being allowed (Bredt’s rule), the elimination of the second water molecule leads to the fragmentation that gives dioxandiene and hydroxyacetaldehyde. Dioxane derivatives also can be formed from other reactions during glucose pyrolysis. Glucose pyrolyzate contains a number of compounds with multiple OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg glucose at 900°C followed by the collection of the pyrolyzate and derivatization with BSTFA in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done with a GC/MS technique in conditions described in Table 1.4.2. The chromatogram of the derivatized pyrolyzate is shown in Fig. 11.1.9. The peak identification in the chromatogram from Fig. 11.1.9 was done using mass spectra library searches. Only some of the peaks were identified and are listed in Table 11.1.3 as a function of their retention time. The results shown in Table 11.1.3 indicate that besides the compounds already identified in Table 11.1.1, three new groups of compounds are present in the pyrolyzate. The first group
Abundance 39.43 40.69
1.3e+07 1.2e+07 1e+07 8,000,000
22.52 17.46
6,000,000 4,000,000
18.35 14.05
35.58 19.96
2,000,000 12.42
23.76 24.93
33.08 30.11
39.73
25.0
30.0
40.0
19.21 0 Time-->
15.0
20.0
35.0
44.10 44.90 46.32 46.89
45.0
50.0
64.6165.92 55.0
60.0
65.0
70.0
FIG. 11.1.9 Chromatogram of the silylated pyrolyzate of 1 mg D-(+)-glucose. Pyrolysis performed at 900°C and derivatization done with BSTFA.
439
GLUCOSE
TABLE 11.1.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.9 for the Pyrolyzate of Glucose Derivatized With Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and Their Molar Levels Relative to Levoglucosan No. Compound
Ret. Time
MW Fragment Ions (m/z)
%
1
Lactic acid 2 TMS
12.65
262
73(100), 147(50), 131(38), 205(11), 247(7)
3.09
2
Ethylene glycol 2 TMS
13.09
206
73(100), 147(85), 191(78)
2.49
3
Glycolic acid 2 TMS
13.53
220
73(100), 147(78), 66 (20), 205(17), 177(11)
3.80
4
2-Hydroxy-cyclopenten-1-one TMS
16.07
170
155(100), 81(25), 75(17), 73(17)
1.89
a
99(100), 44(30), 69(25)
—
5
Hydrolyzed reagent
18.35
?
6
Dihydroxyacetone 2 TMS
19.97
234
73(100), 103(44), 189(10), 219(4)
6.56
7
1,4-Dioxan-2,5-diol 2TMS
22.52
264
117(100), 161(79), 73(76), 191(8), 263(1)
18.58
8
2-(2-Hydroxyethoxy)ethan-1-ol 2 TMS
22.79
250
117(100), 161(85), 73(85), 191(12), 245(3)
5.77
9
5-(Hydroxymethyl)furan-carboxaldehyde 23.76 TMS
198
183(100), 109(34), 111(28), 169(14), 198(4)
9.76
10
Glyceric acid 3TMS
24.63
322
73(100), 147(72), 189(50), 292(38)
4.21
11
2-Furanhydroxyacetic acid 2TMS
26.56
286
169(100), 73(58), 147(30), 243(13), 286(3)
1.83
12
1,4:3,6-Anhydro-a-D-glucopyranose TMS
27.57
216
73(100), 75(50), 201(45), 191(15)
1.54
13
2-[2-(2-Hydroxyethoxy)ethoxy]ethan-1-ol 30.12 2 TMS
308
117(100), 73(55), 103 (20) 161(20)
3.96
14
1,2,3-Trihydroxybenzene 3 TMS
32.96
342
239(100), 73(100), 342(54), 327(10)
0.05
a
73(100), 258(55), 103(35), 185(30)
—
239(100), 73(100), 342(54), 327(10)
0.05
15
Unknown
33.08
?
16
Trihydroxybenzene 3 TMS
35.52
17
I.S.tert-butylhydroquinone 2 TMS
35.58
310
310(100), 295(99), 73(53)
8.32
18
Ribonic acid 5 TMS ?b
37.11
526
73(100), 292(30), 147(25), 103(24), 421(1)
0.91
19
1,6-Anhydroglucofuranose 3 TMS
37.97
378
73(100), 147(55), 217(43), 205(30)
1.89
20
Anhydrosugar 3 TMS
38.55
378
73(100), 217(70), 204(66), 333(20)
1.16
21
Levoglucosan 3 TMS
39.43
378
73(100), 204(48), 217(42), 147(18), 333(19)
100.00
22
Anhydrosugar 3 TMS
39.73
378
73(100), 217(88), 191(64), 204(21), 430(10)
2.71
23
1,4-Anhydroglucopyranose 3TMS
40.70
378
217(100), 73(20), 319(10)
43.56
24
Hexose 5 TMS
44.10
540
217(100), 73 (62), 204(56), 103(37), 363(1)
4.20
25
Hexose 5 TMS
44.90
540
73(100), 204(76), 103(52), 217(40), 259(13), 363(1)
6.17
26
Glucose 5 TMS
45.93
540
204(100), 191(40), 73(37), 217(19), 147(20), 435(2)
0.75
Continued
440
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.1.9 for the Pyrolyzate of Glucose Derivatized With Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and Their Molar Levels Relative to Levoglucosan—Cont’d No. Compound
Ret. Time
MW Fragment Ions (m/z)
%
46.09
466
73(100), 217(46), 147(38), 333(12), 466(10)
0.05
46.33
540
217(100), 73(80), 103(33), 247(14), 319(2)
3.03
27
Gluconic acid lactone 4 TMS
28
β-D-Galactofuranose 5 TMS ?
29
Hexose 5 TMS
46.89
540
217(100), 73(49), 103(18), 243(11)
1.69
30
Gluconic acid 6TMS
48.60
628
73(100), 333(25), 292(23), 147(20), 423(4)
0.15
31
Glucose 5 TMS
48.96
540
204(100), 191(47), 73(49), 217(19), 147(19), 435(2)
0.81
32
Hexose 5 TMS
50.30
540
73(100), 217( 70), 204(53), 103(34), 243(32), 0.82 407(8)
33
Hexose 5 TMS
50.82
540
73(100), 217(78), 243(43), 205(40), 204(37), 407(14)
0.50
34
Hexose 5 TMS
52.55
540
73(100), 217(63), 205(53), 204(49), 147(35), 243(16)
0.70
35
Disaccharide 8 TMS
64.61
918
204(100), 217(57), 73(55), 361(29), 147(18), 451(1)
0.51
36
Disaccharide 8 TMS
64.95
918
204(100), 73(93), 217(80), 361(43), 271(31), 407(7)
0.38
37
Disaccharide 8 TMS
65.09
918
217(100), 73(98) 271(44), 204(31), 205(24), 489(2)
0.23
38
Disaccharide 8 TMS
65.77
918
73(100), 361 (84), 217(80), 308(20), 319(10), 0.38 407(4)
39
Disaccharide 8 TMS
65.92
918
73(100), 217(60), 205(52), 246(42), 233(32), 473(2)
0.42
40
Disaccharide 8 TMS
66.02
918
204(100), 73(93), 361(74), 217(69), 147(24), 451(1)
0.30
41
Disaccharide 8 TMS
66.25
918
73(100), 217(74), 205(55), 233(44), 189(23), 473(3)
0.15
a b
b
Note: Unknown MW. Note: “?” indicates uncertain compound identification.
consists of several sugar fragment molecules, including traces of ethylene glycol and ethylene glycol ethers; traces of a few sugar-related acids, which include glyceric acid and ribonic acid; a gluconic acid lactone; and two isomers of trihydroxybenzene. No gluconic acid or deoxygluconic acid were detected in the pyrolyzate. The presence of trihydroxybenzenes indicates the possibility of the formation of aromatic compounds in sugar pyrolysis. The second group consists of several monosaccharides. The compounds from this group were present at levels between 0.5% and 6% (molar) from that of levoglucosan (taken as 100%). Glucose is
GLUCOSE
PAH m g/g glucose
10
441
FIG. 11.1.10 The level of PAHs in glucose pyrolyzate first heated at 300°C and then at 600°C [25].
8 6 4 2
Ac
en ap ht hy l Fl ene Ph uo r en e an ne An thr th en Fl rac e uo e ra ne nt he Py ne Be re nz Ch ne ry [a se ]a Be nth ne nz rac e o[ a] ne py re ne
0
present in this group at about 0.8% compared to levoglucosan. The third group consists of several disaccharides (including maltose). The presence of disaccharides shows that under the influence of heat, two molecules of glucose can eliminate water to form disaccharides. The compounds from this group were present at levels around 0.3% (molar) from that of levoglucosan and represented about 2% from the total area in the chromatogram. Similar results regarding silylated pyrolyzate using hexamethyldisilazane [23] and online silylation [24] were reported in the literature. Another group of compounds that was not listed in Table 11.1.2 and also not identified by the extension of the analyzed products by silylation and GC/MS analysis (Table 11.1.3) is the condensation products that have a high molecular weight. A considerable amount of char is generated by glucose pyrolysis, and besides carbon, the char contains a tar-type material. This tar is made from molecules resulting from the condensation of glucose molecules by the elimination of water or by reactions between various fragments formed in the initial stages of pyrolysis. Also, this tar contains traces of other nonvolatile compounds such as PAHs. In an experiment designed to determine the levels of PAHs in glucose pyrolysis, a glucose sample was charred at 300°C for 60 min and then heated at 600°C for 10 min [25]. The analysis of the resulting levels of PAHs is shown in Fig. 11.1.10. The formation of PAHs in glucose pyrolyzates is similar to the formation of PAHs in cellulose pyrolyzate [26]. A considerable effort has been made in the study of thermochemolysis of carbohydrates in the presence of TMAH [5]. In strong basic conditions and elevated temperature, carbohydrates have the tendency to form deoxyaldonic acids (saccharinic acids) in a reaction as shown below for a hexose (see reaction 2.5.2): ð11:1:15Þ
442
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.4 Main Compounds Identified in Glucose Pyrolyzate at 700°C in the Presence of TMAH [5] No. Compound
MW
Fragment Ions (m/z)
Rel. Peak Height
1
2-Methoxypropanoic acid methyl ester
118
59, 60, 75, 103, 118
65
2
2,4-Dimethoxy butanoic acid methyl ester
162
45, 72, 73, 75, 132
14
3
2,4,5-Trimethyoxy-3-deoxy-D-threo-pentonic acid methyl ester
206
75, 101, 115, 129, 206
5
4
2,4,5-Trimethyoxy-3-deoxy-D-erythro-pentonic acid methyl ester
206
75, 101, 115, 129, 206
9
5
Trimethoxybenzene
168
93, 110, 125, 153, 168
15
6
2,4,5,6-Tetramethoxy-3-deoxy-D-arabino-hexonic acid methyl ester
234
75, 101, 129, 159, 191
70
7
2,4,5,6-Tetramethoxy-3-deoxy-D-ribo-hexonic acid methyl ester
234
75, 101, 129, 159, 191
100
The resulting acid is further methylated by TMAH. Molecular fragmentation also is produced by the elevated temperatures and the result is the formation of numerous methylated compounds, with the main ones formed from the thermochemolysis/methylation of glucose being given in Table 11.1.4 [5]. Thermochemolysis/methylation in the presence of TMAH has been applied on various carbohydrates with the goal of the characterization of saccharides in biomaterials [5,27].
MANNOSE AND GALACTOSE The pyrograms of two other hexoses, mannose and galactose, which are typically found mainly in pyranose form (mannose 100% and galactose 93%) are given in Fig. 11.1.11 Trace A and B, respectively. The pyrolysis and the pyrograms were obtained in identical conditions as for glucose at 900°C. The peaks eluting up to 50 min in the pyrograms shown in Fig. 11.1.11 Trace A and B are identical in nature with those generated from glucose and can be identified by their retention times in Table 11.1.2. The major peaks belong to CO2. Similar to the case of pentoses, toward the end of the pyrogram are eluting anhydrosugars that retain some of the chiral centers of the parent molecule. Anhydrosugar diastereoisomers were generated from glucose, mannose, and galactose, respectively. These compounds elute at different retention times, as seen by comparing the pyrograms shown in Fig. 11.1.5 for glucose and Fig. 11.1.11A and B for mannose and galactose, respectively. The identification of the main peaks eluting at higher retention times than 50 min in the pyrograms of mannose and galactose is given in Table 11.1.5. The GC/MS analysis of anhydrosugars formed by pyrolysis of monosaccharides and retaining OH groups bound to chiral carbons is better performed using offline derivatization of these compounds after pyrolysis. A time window (37.0–46.0 min) from the chromatograms
443
MANNOSE AND GALACTOSE
Abundance
30.39
55.13
47.68
1e+07
OH CH 2
9,000,000 8,000,000 7,000,000
28.91
O
4.23
HO
OH
6,000,000
HO
5,000,000
OH
4,000,000
34.13
3,000,000 2,000,000 1,000,000
(A)
0
17.76 49.91
7.63 9.64 12.08 5.80
4.65
5.0
10.0
30.83 35.60
40.87
1e+07
45.68
56.64 59.30
50.80
22.48
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
47.68
30.39
9,000,000
55.65
42.99 44.89
55.0
60.0
53.77
OH CH 2
4.23
O
8,000,000
HO
7,000,000 6,000,000
OH HO
60.08
28.91
OH
5,000,000 56.13
4,000,000 3,000,000 2,000,000 1,000,000
34.13 4.65 9.64 7.63 5.80 12.08
49.91
17.76 22.48
42.67 37.73 40.87 35.60 45.68 30.83
51.32
0
(B)
Time-->
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
FIG. 11.1.11 Pyrogram of 1 mg mannose (Trace A) and of 1.0 mg galactose (Trace B) at 900°C.
of trimethylsilyl (TMS) derivatives of the pyrolyzates of (a) mannose, (b) allose, (c) galactose, and (d) glucose is shown in Fig. 11.1.12, with the peak identification in Table 11.1.6. Pyrolysis of these hexoses was done at 590°C [2]. As shown in Fig. 11.1.12, the differences in the retention times for 1,6-anhydrosugars or 1,4anhydrosugars from different parent molecules are noticeable. This is proof that the anhydrosugars maintain part of the initial steric structure of the parent molecule.
444
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.1.5 Identification of the Peaks Eluting at Retention Times Higher Than 50 min in the Pyrograms of Mannose and Galactose Shown in Fig. 11.1.11 Trace A and B Retention Time (Min)
No. Compound
Mannose MW Area (106)
Galactose Area (106)
1
1,450.80 Anhydromannopyranose
162
34.2
–
2
1,6-Anhydro-βgalactopyranose
53.78
162
–
1156.5
3
1,6-Anhydro-βmannopyranose
55.14
162
744.5
–
4
1,6-Anhydro-αgalactofuranose
56.13
162
–
210.8
5
1,6-Anhydro-αmannofuranose
55.63
162
200.4
–
6
Anhydropyranose
56.64
162
27.0
–
7
1,6-Anhydro-β-Dglucofuranose ?
59.85
162
19.9
–
8
Anhydrofuranose ?
60.08
162
8.8
–
9
1,6-Anhydro-βgalactopyranose
60.08
162
–
402.2
Note: “?” indicates uncertain compound identification.
Abundance
b1 d3
4e+07 c1 c1
3.5e+07
a1
3e+07
b5 b5
d4
c2
2.5e+07
d1
2e+07
c3
d2
b2
1.5e+07
b3
1e+07
a2
a3
b4
5,000,000 Time -->
FIG. 11.1.12
37.0
38.0
39.0
40.0
41.0
42.0
43.0
44.0
45.0
46.0
d c b a
Time window between 40 and 50 min from the chromatogram of TMS derivatives of pyrolyzates of (a) mannose, (b) allose, (c) galactose, and (d) glucose showing anhydrosugars. Peak identification is given in Table 11.1.6.
FRUCTOSE
445
TABLE 11.1.6 Identification of the Peaks in the Chromatogram Shown in Fig. 11.1.12 for the Anhydrosugars Formed During Pyrolysis of Four Monosaccharides Saccharide
Peak
Assignment
Mannose
a1
1,6-β-Anhydromannopyranose
a2
1,4-Anhydromannopyranose
a3
1,6-α-Anhydromannofuranose
b1
1,6-Anhydroallopyranose
b2
1,6-Anhydroglucopyranose (impurity in allose)
b3
An anhydrofuranose ?
b4
1,6-Anhydroallofuranose
b5
1,4-Aanhydroallopyranose
c1
1,6-Anhydrogalactopyranose
c2
1,6-Anhydrogalactofuranose
c3
1,4-Anhydrogalactopyranose
d1
1,6-Anhydroglucofuranose
d2
1,4-Anhydroglucopyranose type ?
d3
1,6-Anhydroglucopyranose (levoglucosan)
d4
1,4-Anhydroglucopyranose
Allose
Galactose
Glucose
Note: “?” indicates uncertain compound identification.
FRUCTOSE Fructose is a hexose that is found mainly in furanose form. The pyrolysis products of fructose are similar in nature with those of other monosaccharides. An example of a pyrogram generated online at 900°C from D-()-fructose is shown in Fig. 11.1.13. The pyrolysis was done in conditions identical to those for glucose. The peak identification and relative molar content in 100 mol of pyrolyzate are given in Table 11.1.2, together with those of glucose. The calculation of the mole % was obtained based solely on peak areas. Qualitatively, the composition of fructose pyrolyzate is very similar to that of glucose. However, the proportion of various compounds in the two pyrograms is very different. Particularly, the dianhydrosugars seen in glucose pyrolyzate are absent in that of fructose, and levoglucosan and 1,6-anhydro-β-D-glucofuranose are present at significantly lower levels for fructose. The experiment performed on fructose using pyrolysis at 900°C followed by offline derivatization with BSTFA and GC/MS analysis showed, except for the anhydrosugars, similar compounds as in the case of glucose, but with the levels very different. Another significant difference was noticed in the level of disaccharide generated from fructose in the specific conditions used for this experiment. The level of disaccharides was for fructose about 10 times
446
11. PYROLYSIS OF CARBOHYDRATES
Abundance
47.68
30.39
4.23
HO
7,000,000
CH2
6,000,000 5,000,000
O HO
4.65
OH
CH2 OH
4,000,000
OH 35.60 37.75
3,000,000
40.87
2,000,000
9.64 28.91
1,000,000
42.66
5.80
12.08
17.76
22.48
39.90
56.30
45.47
26.69
59.90
0 Time-->
FIG. 11.1.13
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
Pyrogram of 1.0 mg D-()-fructose at 900°C.
higher than in the case of glucose (and represented about 16% from the total peak areas in the chromatogram). It was not possible to identify individual disaccharides in the mixture using mass spectral library searches. The mechanisms involved in fructose pyrolysis are similar to those proposed for explaining the composition of glucose pyrolyzate. It is likely that other hexoses have similar pyrolysis products with the compounds discussed in this section, although the percentage of various components in the pyrolyzate is likely to be different from compound to compound. Differentiation between different monosaccharides for analytical purposes has been achieved successfully using the differences in the anhydrosugars generated by pyrolysis [7,12] or using thermally assisted hydrolysis and methylation (e.g., with TMAH as a reagent) followed by GC/MS analysis [5]. Formation of different anhydrosugars from various monosaccharides has been booth experimentally and theoretically investigated [28–34].
References 11.1 [1] http://old.iupac.org/reports/provisional/abstract04/BB-prs310305/chapter10.pdf. [2] S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998. [3] M.J. Kleeman, M.A. Robert, S.G. Riddle, P.M. Fine, M.D. Hays, J.J. Schauer, M.P. Hannigan, Atm. Environ. 42 (2008) 3059. [4] O. Novotn y, K. Cejpek, J. Velisˇek, Czech J. Food Sci. 26 (2008) 117. [5] D. Fabbri, R.J. Helleur, J. Anal. Appl. Pyrolysis 49 (1999) 277. [6] R.J. Helleur, J. Anal. Appl. Pyrolysis 11 (1987) 297. [7] D.R. Budgell, E.R. Heyes, R.J. Helleur, Anal. Chim. Acta 192 (1987) 243. [8] N. Mitsuo, N. Nakayama, H. Matsumoto, T. Satoh, Chem. Pharm. Bull. 37 (1989) 1624. ˜ rsi, J. Therm. Anal. 5 (1973) 329. [9] F. O [10] H. Sugisawa, H. Edo, J. Food Sci. 31 (1966) 561. [11] H. Sugisawa, J. Food Sci. 31 (1966) 381.
GENERAL ASPECTS
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
447
E.R.J. Evans, D. Wang, F.A. Agblevor, H.L. Chum, S.D. Baldwin, Carbohydr. Res. 23 (1996) 219. E.B. Sanders, A.I. Goldsmith, J.I. Seeman, J. Anal. Appl. Pyrolysis 66 (2003) 29. S.L. Morgan, C.A. Jacques, Anal. Chem. 54 (1982) 741. J.W. Laskowitz, B. Carroll, Carbohydr. Res. 5 (1967) 245. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 82 (2008) 42. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 82 (2008) 10. R.R. Baker, S. Coburn, C. Liu, J. Anal. Appl. Pyrolysis 77 (2006) 12. K. Agyei-Aye, M.X. Chian, J.H. Lauterbach, S.C. Moldoveanu, Carbohydr. Res. 337 (2002) 2273. J.B. Paine III, Y.B. Pithawalla, J.D. Naworal, J. Anal. Appl. Pyrolysis 83 (2008) 37. A. Lorene, S. Nanbu, K. Fukuda, Mem. Fac. Eng. Kyushu Univ. 67 (2007) 67. T. Hosoya, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrolysis 83 (2008) 64. D. Fabbri, G. Chiavari, Anal. Chim. Acta 449 (2001) 271. D. Scalarone, O. Chiantore, C. Riedo, J. Anal. Appl. Pyrolysis 83 (2008) 157. T.E. McGrath, W.G. Chan, M.J. Hajaligol, J. Anal. Appl. Pyrolysis 66 (2003) 51. O.T. Chrotyk, W.S. Schlotzhauer, Betr. Tabak. 7 (1973) 165. C. Schwarzinger, J. Anal. Appl. Pyrolysis 71 (2004) 501. A. Fukutome, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrolysis 124 (2017) 666. M. Wang, C. Liu, X. Xu, Q. Li, J. Anal. Appl. Pyrolysis 120 (2016) 464. Q. Lu, Y. Zhang, C.-q. Dong, Y.-P. Yang, H.-Z. Yu, J. Anal. Appl. Pyrolysis 110 (2014) 34. H. Kawamoto, T. Hosoya, Y. Ueno, T. Shoji, S. Saka, J. Anal. Appl. Pyrolysis 109 (2014) 41. Y. Zhang, C. Liu, H. Xie, J. Anal. Appl. Pyrolysis 105 (2014) 23. S. Matsuoka, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrol. 103 (2013) 300. S.-S. Choi, M.-C. Kim, Y.-K. Kim, J. Anal. Appl. Pyrolysis 90 (2011) 56.
S U B C H A P T E R
11.2
Disaccharides
GENERAL ASPECTS Disaccharides are formed by the elimination of a water molecule between two monosaccharides (different or identical) with the formation of an ether bond. Disaccharides have common names and systematic names and can be indicated by the names of the participating monosaccharides and the bond type. For example, maltose or 4-O-α-D-glucopyranosylD-glucose is indicated as Glc α(1 ! 4) Glc.
448
11. PYROLYSIS OF CARBOHYDRATES
The elimination of water can take place between any of the OH groups of the participating monosaccharides, and more than one disaccharide can be formed, even starting with identical monosaccharide molecules. For example, two glucose molecules connected α(1 ! 4) form maltose and connected α(1! 1)α form trehalose. When a disaccharide is formed involving one OH group from the anomeric carbon, the same monosaccharide even connected to the OH at the same carbon on the other molecule can generate two different disaccharides. For example, two molecules of glucose connected α(1 !4) form maltose, and two molecules of glucose connected β(1 !4) form cellobiose. When the water molecule is eliminated between anomeric carbons in both monosaccharides, the resulting disaccharide does not have reducing properties (monosaccharides and disaccharides with a free acetal OH group have reducing properties). The name of the nonreducing disaccharides replaces the end “ose” with “oside,” suggesting the formation of a glycoside (see Subchapter 11.3). The structure of disaccharides is amply described in the literature (see e.g., [1]).
MALTOSE, LACTOSE, AND SUCROSE Pyrolysis of disaccharides takes place following reactions very similar to those of monosaccharides. The nature of pyrolysis products from a disaccharide depends on the nature of the component monosaccharides and on the type of connection between the two monosaccharide molecules. For several 1 !2 or 1 !4 connected monosaccharides, the nature of the pyrolysis products is not very different from that of the pyrolysis of individual monosaccharides, but the proportion of various compounds in the pyrolyzate may show differences. The decomposition starts around 190°C with losing H2O and it is continued with further decompositions as the temperature increases. Flash pyrolysis at temperatures above 650–700°C up to about 1000°C generates various fragments of the molecules, similar to the case of monosaccharides. Pyrolysis of three disaccharides—maltose, lactose, and sucrose—was performed in identical conditions as for glucose, and the pyrograms are shown in Fig. 11.2.1 for maltose (Trace A), lactose (Trace B), and sucrose (Trace C). Maltose is 4-O-α-D-glucopyranosyl-D-glucose (Glc α(1! 4) Glc), lactose is β-D-galactopyranosyl-(1$ 4)-β-D-glucopyranose (Gal β(1! 4) Glc), and sucrose is α-D-glucopyranosyl-(1$2)-β-D-fructofuranoside (Glc α(1! 2) Fru). Pyrolysis conditions were Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. Because the nature of the compounds in the three pyrolyzates is the same as for glucose for all the compounds eluting before 50 min in the pyrogram, the compound identifications based on their retention time can be obtained from Table 11.1.2 (list for glucose). This window of the pyrogram is dominated by the peaks of CO2 (2.23 min),
449
MALTOSE, LACTOSE, AND SUCROSE A bundance
OH
9,000,000 8,000,000 7,000,000
30.39
OH
O
1e+07
OH
OH
O
OH
OH
4.23 49.91
6,000,000
42.9946.36
5,000,000 28.91
4,000,000 3,000,000 2,000,000 1,000,000 0
(A)
56.71
O
OH HO
47.68
34.14 4.65
7.63 9.64 22.48 5.80 12.07 14.50 18.58 5.0
10.0
15.0
20.0
30.83
25.0
30.0
35.60 40.88 37.44 35.0
40.0
60.08 45.0
50.0
OH
9,000,000 4.23 OH HO
7,000,000
O O
OH
OH
O
OH
OH
6,000,000
49.91
28.91
OH
5,000,000
45.71 46.36
4,000,000 34.14
3,000,000
1,000,000
4.65 5.80
7.63 9.64 12.07
17.76 22.48
30.83 35.60
59.90 60.08
42.99 43.25 40.88
51.33
0
(B)
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
30.43 1e+07 9,000,000 8,000,000 7,000,000
5,000,000
4.23 HO
O
HO
OH
O
0 Time-->
60.0
56.72
OH OH
4.65 28.91
3,000,000
1,000,000
47.68
55.0
O OH
OH
4,000,000
2,000,000
50.0
OH
6,000,000
(C)
60.0
30.39
1e+07
2,000,000
55.0
53.77 56.71
47.68
8,000,000
59.90
45.67 49.38
17.76
9.63
17.76 7.63 22.48 12.07 5.80 20.04 26.69 14.49 5.0
10.0
15.0
20.0
25.0
30.0
59.90 35.60 42.67 34.13 37.76 46.37 40.87
35.0
40.0
45.0
49.91
50.0
55.0
60.0
FIG. 11.2.1 Pyrogram of 1.0 mg maltose Glc α(1! 4) Glc (Trace A), 1.0 mg lactose Gal β(1! 4) Glc, (Trace B), and 1.0 mg sucrose Glc α(1!2) Fru (Trace C) at 900°C.
450
11. PYROLYSIS OF CARBOHYDRATES
1,4-dioxadiene (28.91 min), furancarboxaldehyde (30.39 min), and 5-(hydroxymethyl)-2furancarboxaldehyde (47.68 min). For the anhydrosugars that are formed from glucose, their identification is also given in Table 11.1.2, and for those generated from the galactose moiety, the identification can be done based on Table 11.1.5. Pyrolysis of disaccharides typically starts with the cleavage of the ether bond between the two monosaccharide units [2]. This leads to the formation of a monosaccharide and a dehydrated derivative of the other monosaccharide. In the case of lactose, for example, the two molecules formed are 1,6-anhydro-β-D-galactopyranose (levoglucosan equivalent for galactose) and glucose. The resulting molecules continue to decompose under the influence of heat. Levoglucosan was shown to decompose very similarly to glucose (see Subchapter 11.1), and the same properties can be assumed valid for other 1,6-anhydrosugars; therefore it is not possible to distinguish the origin of the resulting fragments as generated from a monosaccharide or from its 1,6-anhydro derivative. The formation of fragments by the cleavage of the ether bond between the monosaccharide units allows the formation of new di- or trisaccharides, typically identified at low levels in the pyrolyzates of disaccharides. This formation of new condensation products between the disaccharide fragments was also verified in an experiment with sucrose thermal degradation, where the addition of erythritol ((2R,3S)-butane-1,2,3,4-tetraol) led to the formation of an erythritol fructoside [3]. Disaccharide pyrolysis also leads to the formation of a considerable amount of char, with most of this material consisting of carbon but also containing various oligosaccharides. The formation of PAHs from disaccharide pyrolysis has been found to be very similar to the formation of these compounds from monosaccharides or from cellulose [3]. The levels of several PAHs generated in sucrose charred at 300°C for 60 min and then heated at 600°C for 10 min are shown in Fig. 11.2.2 [3].
PAH mg/g sucrose
8 6 4 2
Ac en a
ph t
hy
le ne F Ph luo en ren an e An thre th n e Fl rac uo e ra ne nt he n Py e re Be nz Ch ne r [a ]a yse B e nth n e nz rac e o[ a] ne py re ne
0
FIG. 11.2.2 The level of PAHs in sucrose pyrolyzate first at 300°C and then at 600°C [3].
451
RUTINOSE
35.35 38.39
Abundance 1.6e+07
HO
1.4e+07
H3C
1.2e+07 1e+07
OH 4.07
O O
CH2 O
OH HO HO
36.06
OH
8,000,000 6,000,000
47.68
OH
5.57
22.18
4,000,000 2,000,000 0 Time--> 5.0
30.08
39.95 42.72
40.54 44.76
28.61
17.36
50.15
11.70 10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
56.70 55.0
60.0
FIG. 11.2.3 Pyrogram of 1.0 mg of rutinose Ram α(1!6) Glc at 900°C. Note: The retention times of the early peaks in the pyrogram from Fig. 11.2.3 are shifted with about 0.3 min earlier compared to those from Fig. 11.1.5 of glucose.
RUTINOSE Rutinose, or 6-O-α-L-rhamnosyl-D-glucose or 6-O-(6-deoxy-α-L-mannopyranosyl)-D-glucose, is a disaccharide consisting of a rhamnose and a glucose molecule indicated as (Ram α(1 !6) Glc). This compound is present in some glycosides such as rutin. Rutinose starts decomposing similarly to other disaccharides around 190°C. Pyrolysis of a sample of rutinose performed on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.2.3. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.2.1. The calculation of the mole % was obtained based solely on peak areas.
TABLE 11.2.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.2.3 for the Pyrolysis of Rutinose No. Compound
Retention Time (Min) MW CAS#
Moles %
1
Carbon dioxide
4.07
44
124-38-9
10.86
2
Formaldehyde
4.49
30
50-00-0
1.96
3
Acetaldehyde
5.57
44
75-07-0
5.90
4
Ethanol
7.85
46
64-17-5
1.30
5
Pyruvaldehyde
9.32
72
78-98-8
3.70 Continued
452
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.2.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.2.3 for the Pyrolysis of Rutinose—Cont’d No. Compound
Retention Time (Min) MW CAS#
Moles %
6
Hydroxyacetaldehyde (glycol aldehyde)
17.36
60
141-46-8
3.81
7
Acetic acid
19.77
60
64-19-7
2.45
8
Ethyl-1-propenyl ether
22.03
86
928-55-2
0.71
9
1-Hydroxy-2-propanone (acetol)
22.18
74
116-09-6
5.07
10
1,4-Dioxadiene
28.61
84
N/A
1.90
11
2-Oxopropionic acid methyl ester ?
29.04
102
600-22-6
0.95
12
Furancarboxaldehyde (furfural)
30.08
96
98-01-1
2.59
13
Furanyl ethanone
32.87
110
1192-62-7
0.10
14
Dihydro-4-hydroxy-2(3H)-furanone
33.86
102
5469-16-9
0.65
15
3-Hepten-2-one ?
34.65
112
1119-44-4
0.59
16
5-Methyl-2-furancarboxaldehyde
35.34
110
620-02-0
10.60
17
3-Acetyldihydro-2(3H)-furanone
36.06
128
517-23-7
4.48
18
Tetrahydro-3,6-dimethyl-2H-pyran-2-one
36.25
128
3720-22-7
1.43
19
4-Penten-2-ol
37.35
86
625-31-0
2.03
20
1,3-Dihydroxy-2-propanone
37.52
90
96-26-4
3.60
21
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.74
112
80-71-7
3.02
22
3-Hydroxy-5-methyl-5,6-dihydroxypyran-4-one
38.39
128
N/A
10.38
23
2,5-Dimethyl-4-hydroxy-3(2H)-furanone
39.45
128
3658-77-3
0.84
24
Unknown
39.95
128
N/A
3.67
25
Anhydrosugar from rhamnose ?
40.54
144
N/A
1.42
26
Anhydrosugar from rhamnose ?
42.36
144
N/A
1.37
27
Anhydrosugar from rhamnose ?
42.72
144
35810-56-1
4.45
28
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
42.95
142
1073-96-7
0.85
29
Dianhydromannitol
44.02
146
N/A
0.20
30
Unknown
44.77
128
N/A
1.78
31
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.68
126
67-47-0
6.05
32
Unknown
50.15
146
N/A
0.81
33
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.70
162
498-07-7
0.47
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
453
REFERENCES 11.2
Abundance 128
100 O
H 3C
OH 113
80 58
40 29
87
O
O
CH 3 Si CH 3 CH 3
O
40
69
20
H3C
60
O
41
185 73
80
60
0 m/z--> 20
Abundance 100
20
157 59
101 115 129 143
169
200
0 40
60
(A)
80
100
120
140
m/z-->
60
80
100
120
140
160
180
200
(B)
FIG. 11.2.4 Mass spectrum of 3-hydroxy-5-methyl-5,6-dihydroxypyran-4-one (spectrum A) and that of its TMS derivative (spectrum B).
The pyrogram of rutinose is rather different from the one of the other disaccharides. The main peaks in the pyrogram are CO2, 5-methyl-2-furancarboxaldehyde, and 3-hydroxy-5methyl-5,6-dihydroxypyran-4-one. Because there are compounds that retain several OH groups from the parent compound among the pyrolysis products of rutinose, it can be expected that some pyrolysis products may not be seen in the pyrogram generated under conditions given in Table 1.4.1 because they do not elute from the chromatographic column. For the analysis of these compounds, the silylation of the pyrolyzate followed by GC/MS analysis under conditions described in Table 1.4.2 is necessary. However, except for some undecomposed rutinose, no additional compounds were detected in the silylated pyrolyzate. A confirmation of the formation of 3-hydroxy-5-methyl-5,6-dihydroxypyran-4-one was obtained following the analysis of the silylated pyrolyzate because this compound also generates a sizable peak in the chromatogram of the silylated pyrolyzate. The mass spectra of 3hydroxy-5-methyl-5,6-dihydroxypyran-4-one and of its TMS derivative are shown in Fig. 11.2.4A and B, respectively. Important differences can be seen between the pyrolysis of other disaccharides and that of rutinose in the formation of anhydrosugars. The level of levoglucosan is very low in rutinose pyrolyzate because the OH group from C6 of glucose is involved in the ether link with the rhamnose unit, and it is not available to eliminate a water molecule between the OH groups at atoms 1 and 6. Also, instead of furfural formation, 5-methylfurfural is among the major pyrolysis products of rutinose. The pyrolysis of this compound shows that besides the component monosaccharides in a disaccharide, the type of bond between the monosaccharide units is also important for the pyrolyzate composition.
References 11.2 [1] C.A. Stortz, A.D. French, Mol. Simul. 34 (2008) 373. [2] G.N. Richards, F. Shafizadeh, Aust. J. Chem. 31 (1978) 1825. [3] T.E. McGrath, W.G. Chan, M.J. Hajaligol, J. Anal. Appl. Pyrolysis 66 (2003) 51.
454
11. PYROLYSIS OF CARBOHYDRATES
S U B C H A P T E R
11.3
Carbohydrates With Additional Functional Groups
GENERAL CONCEPTS A large variety of compounds are derived from carbohydrates. These compounds are generated by the modification of the initial carbohydrate molecule, either adding or subtracting functionalities. Several more common carbohydrate derivatives are indicated below. – The replacement of an OH group with hydrogen generates a special type of compound known as doxy sugars. Examples of these molecules are 2-deoxy-D-ribose((2R,4S,5R)-5hydroxymethyltetrahydrofuran-2,4-diol), 6-deoxy-L-galactose (fucose), and 6-deoxy-Lmannose (rhamnose). – Reduction (hydrogenation) of the aldehyde group(s) of a carbohydrate into alcohol generates a sugar alcohol. These compounds can be classified as polyols, and a discussion on their pyrolysis can be found in Subchapter 4.1 (sorbitol or glucitol is discussed with some details). In the case of disaccharides (or trisaccharides, etc.) that are formed, leaving the OH free at only one anomeric carbon, it is possible to change one sugar unit into a polyol while the other(s) remains unaffected. For example, partial hydrogenation of maltose leads to 4-O-α-D-glucopyranosyl-D-glucitol (maltitol). These compounds are part carbohydrate and part polyol. – Oxidation of a monosaccharide leads to the formation of sugar acids. There are numerous sugar acids that may have one or more carboxyl groups, and some may have additional modifications of the carbohydrate structure. Examples of sugars acids are gluconic acid, glucuronic acid, and ascorbic acid. – Replacement of an OH group with chlorine (or other halogen) leads to halogenated sugars. Sucralose or 4,10 ,60 -trichloro-4,10 ,60 -trideoxygalactosucrose is a well-known halogenated sugar used as an artificial sweetener (sold under the trade name Splenda). – Ethers and silyl ethers are sugar derivatives obtained by the replacement of the active hydrogens from the OH groups with R or Si(R)3 where R is an alkyl or aryl radical. These derivatives of sugars are frequently used in various syntheses and for analytical purposes. Making of trimethylsilyl ethers for analytical purposes after pyrolysis of sugars was previously described (see Subchapter 11.1). – Glycosides are a special class of ethers formed between the OH group from the anomeric carbon of a saccharide and a nonsugar molecule (this class does not include nonreducing disaccharides that involve the anomeric carbons in their formation). The nonsugar group of
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL
455
a glycoside is indicated as an aglycone. When the sugar moiety is glucose, the compounds are indicated as glucosides. Numerous glycosides are natural compounds. – Sugar esters are compounds where one or more OH groups are esterified. The esterification can be done with a variety of acids, with the most common esters being acetylated sugars. – Simple amino sugars are compounds with an OH group replaced by an NH2 group. The most common amino sugar is 2-amino-2-deoxy-D-glucose or (3R,4R,5S,6R)-3-amino(hydroxymethyl)oxane-2,4,5-triol. The amino group can be acetylated to form N-acetyl glucosamine, another common amino sugar. These molecules play a major role in biological systems, being involved in various glycosylation mechanisms. More complicated molecules are also known in this group of sugar derivatives. Among these are the derivatives that contain both amino and carboxyl groups (e.g., neuraminic acid).
DEOXYSUGARS There is very little information in the literature regarding the pyrolysis of deoxysugars. Pyrolysis of a rhamnose (6-deoxy-L-mannose) sample performed at 900°C in conditions similar to that for glucose showed that the compound behaves very similarly to other sugars during pyrolysis. Most small fragments generated from rhamnose are similar to those generated, for example, from glucose.
Among the small fragments that were different in rhamnose pyrolyzate compared to glucose were 1,2- and 1,3-propanediol and 2,5-dimethyl-4-hydroxy-3(2H)-furanone. The peak intensities were also relatively different. A number of compounds tentatively identified as anhydro-deoxysugars resulting from the elimination of one or two water molecules from rhamnose were also present in the pyrolyzate. However, only tentative assignments were possible because the mass spectra of these anhydrosugars were not available in common mass spectra libraries. Silylation of the rhamnose pyrolyzate showed that some rhamnose is still present in the pyrolyzate as well as several disaccharides resulting from the elimination of water between two rhamnose molecules. These disaccharides were present at levels around 0%–1% in the pyrolyzate.
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL A typical example of a partially hydrogenated sugar is maltitol. This compound is formed from a glucose unit connected through the glucosidic OH with a glucitol (sorbitol) unit. The compound could also be classified as a glucoside. Pyrolysis of a sample of maltitol performed
456
11. PYROLYSIS OF CARBOHYDRATES
A bundance
OH
56.95 OH
8,000,000 CH 2OH
7,000,000
OH
6,000,000
O
OH
O
OH
30.54
OH
4.30 5,000,000
OH
50.06 29.08
OH
60.08
4,000,000 3,000,000 2,000,000
4.73
1,000,000 0 Time-->
17.97
9.78 7.74 5.90 8.81
10.0
45.82 47.77
31.00 37.58 34.28
15.0
41.01 43.12
25.86 20.0
25.0
30.0
35.0
59.36 60.53
49.23
33.31
12.21 5.0
22.67
40.0
45.0
53.84 50.0
55.0
60.0
FIG. 11.3.1 Pyrogram of 1 mg maltitol at 900°C.
on a 1.0 mg compound at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.1. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.1. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxide
4.30
44
124-38-9
8.31
2
Propene
4.55
42
115-07-1
1.37
3
Formaldehyde
4.73
30
50-00-0
10.06
4
1,3-Butadiene
5.33
54
106-99-0
0.44
5
Acetaldehyde
5.90
44
75-07-0
4.49
6
Furan
7.74
68
110-00-9
2.77
7
1,3-Cyclopentadiene
8.66
66
542-92-7
0.71
8
2-Propenal (acrolein)
8.81
56
107-02-8
2.68
9
Acetone
9.23
58
67-64-1
0.33
10
Butanal
9.78
72
123-72-8
3.17
11
2,3-Dihydrofuran
11.12
70
1191-99-7
0.15 Continued
457
COMPOUNDS THAT ARE PART CARBOHYDRATE AND PART POLYOL
TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
12
2-Methylfuran
12.21
82
534-22-5
0.57
13
Methyl vinyl ketone
14.39
70
78-94-4
0.44
14
2,3-Butanedione (diacetyl)
14.66
86
431-03-8
0.53
15
Hydroxyacetaldehyde (glycol aldehyde)
17.97
60
141-46-8
7.36
16
2-Butenal
19.61
70
15798-64-8
0.20
17
2-Methyl-2-propenal
19.69
70
78-85-3
0.36
18
Acetic acid
20.24
60
64-19-7
0.14
19
Vinylfuran
21.05
94
1487-18-9
0.17
20
Ethyl-1-propenyl ether
22.52
86
928-55-2
0.49
21
1-Hydroxy-2-propanone (acetol)
22.67
74
116-09-6
2.76
22
3-Methylfuran
25.86
82
930-27-8
0.40
23
2-Propenoic acid methyl ester ?
26.86
86
96-33-3
0.29
24
1,4-Dioxadiene
29.08
84
N/A
3.81
25
2-Oxopropionic acid methyl ester ?
29.49
102
600-22-6
0.15
26
Butandial
29.84
86
638-37-9
0.93
27
Furancarboxaldehyde (furfural)
30.54
96
98-01-1
3.73
28
2-Propylfuran
31.00
110
4229-91-8
1.66
29
2-Furanmethanol
31.84
98
98-00-0
0.18
30
5-Methyl-2(3H)-furanone
32.21
98
591-12-8
0.15
31
Vinyl-2(3H)-furanone
32.65
110
N/A
0.10
32
Methylglyoxal + unknown
32.81
72
78-98-8
0.19
33
1-(2-Furanyl)ethanone
33.31
110
1192-62-7
0.66
34
Dihydro-4-hydroxy-2(3H)-furanone
34.28
102
5469-16-9
1.33
35
2-Hydroxy-2-cyclopenten-1-one
34.68
98
10493-98-8
0.80
36
2-Cyclohexen-1-ol
35.54
98
822-67-3
0.19
37
5-Methyl-2-furancarboxaldehyde
35.75
110
620-02-0
0.56
38
1,3-Benzenediol
35.07
110
108-46-3
0.13
39
5-Acetyldihydro-2(3H)-furanone
37.11
128
29393-32-6
0.15
40
6,8-Dioxabicyclo[3.2.1]octane
37.58
114
280-16-0
1.03
41
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
38.11
112
80-71-7
0.40 Continued
458
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.1 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.1 for Pyrolysis of Maltitol at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
42
Phenol
38.80
94
108-95-2
0.24
43
1,5-Hexadien-3-ol
38.96
98
924-41-4
0.30
44
2,5-Dimethyl-4-hydroxy-3(2H)-furanone ?
39.85
128
3658-77-3
0.14
45
3-Furancarboxylic acid methyl ester
41.01
126
13129-23-2
0.39
46
Unknown
41.15
116
N/A
0.16
47
2,3-Dihydro-3,5-dihydroxy-6-methyl-4H-pyran- 42.79 4-one
144
28564-83-2
0.16
48
(Z)-penta-2,4-dienoic acid ?
43.12
98
N/A
0.40
49
1-Hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one
45.82
144
113781-138
1.12
50
1,4:3,6-Dianhydro-α-D-glucopyranose
46.49
144
N/A
0.91
51
Unknown
47.20
144
N/A
0.13
52
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.77
126
67-47-0
1.20
53
1,4:3,6-Dianhydro-D-glucitol (isosorbide)
49.23
146
652-67-5
0.65
54
1,2-Cyclohexanediol ?
49.50
116
1792-81-0
0.34
55
3-Hydroxy-6-(hydroxymethyl)-5H-6hydropyran-4-one
50.06
144
N/A
2.50
56
Unknown
50.27
144
N/A
0.16
57
Unknown
53.84
128
N/A
0.16
58
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.95
162
498-07-7
17.84
59
Unknown
58.15
162
N/A
0.22
60
1,4-Anhydro-D-glucitol
59.36
164
N/A
3.00
61
Methyl-D-glucofuranoside ?
59.65
164
N/A
1.57
62
1,6-Anhydro-β-D-glucofuranose
60.08
162
7425-74-3
2.68
63
Anhydrosugar
60.53
162
N/A
1.40
Hydrogen, CO, methane, ethylene, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: “?” indicates uncertain compound identification.
The results of the analysis of maltitol pyrolyzate from Table 11.3.1 show a number of compounds that were generated in both glucose (see Subchapter 11.1) and sorbitol (glucitol) (see Subchapter 4.1) as well as compounds typical for glucose pyrolysis (e.g., a larger level of levoglucosan) and compounds typical for sorbitol (e.g., isosorbide). Maltitol pyrolyzate
SUGAR ACIDS
459
composition is basically an average of the composition of the pyrolyzates of the two molecular moieties (glucose and glucitol), such that little interaction appears to occur between these two parts during pyrolysis. This effect is very likely caused by the relatively weak ether bond between the sugar and the alcohol.
SUGAR ACIDS Sugar acids can be obtained either by the direct oxidation of a carbohydrate or by indirect methods such as the oxidation of a monosaccharide with specifically protected groups. The main groups of acids generated from carbohydrates are the following: – Aldonic acids that can be generated by the direct oxidation of the aldehyde group into the COOH group. The general formula of these compounds is HOCH2-(HC(OH))n-COOH. A typical example of aldonic acid is gluconic acid (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanoic acid. The formation of cyclic acetals is not possible for these compounds because no carbonyl group is available. Aldonic acids are typical hydroxy acids, and their pyrolysis is presented in Subchapter 12.3 together with other hydroxy acids. – Aldaric acids generated by the transformation of both the aldehyde group and the terminal CH2OH group in COOH. The general formula for these compounds is HOOC-(HC(OH))nCOOH. A typical example in this group is the saccharic acid (glucaric acid) or (2S,3S,4S,5R)-2,3,4,5-tetrahydroxyhexandioic acid. The formation of cyclic acetals is not possible for these compounds because no carbonyl group is available. Aldaric acids also are typical hydroxy acids, and their pyrolysis is presented in Subchapter 12.3. – Uronic acids generated by the transformation of the terminal CH2OH group in COOH. – The general formula is HOOC-(HC(OH))n-CHO for these compounds. A typical example in this group is glucuronic acid or 3,4,5,6-tetrahydroxytetrahydropyran-2carboxylic acid. Because the carbonyl group remains unaffected in uronic acids, they form cyclic acetals similar to the saccharide to which they are related. This is possible by modifying the carbonyl group into an internal acetal, and uronic acids are presented in this section. – Saccharinic acids or 3-deoxyaldonic acids that are formed in strong basic media from carbohydrates. The reaction of formation of saccharinic acids plays a particularly important role in the pyrolysis of carbohydrates in the presence of TMAH when thermochemolysis and methylation processes takes place (see reaction 11.1.12). – Ketoaldonic acids with the general formula HOCH2-(HC(OH))n-C(O)-COOH (also known as uronic acids), which are also derivatives of certain sugars and can be generated by their oxidation with strong oxidants (e.g., chromic acid) in the presence of catalysts. – Ascorbic acid, which is in fact a lactone that can ionize and has acidic properties. Pyrolysis of ascorbic acid is discussed in Subchapter 14.3. Only uronic acids have a chemical structure closely related to that of carbohydrates. Pyrolysis of a sample of D-glucuronic acid performed on a 1.0 mg compound at Teq ¼ 700°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.2. Pyrolysis in identical conditions except for Teq ¼ 900°C generates almost
460
11. PYROLYSIS OF CARBOHYDRATES
Abundance
56.24
10,000,000 4.23
9,000,000
CO O H
8,000,000
OH
7,000,000
30.39
O OH OH
OH
6,000,000 5,000,000 4,000,000
28.91
3,000,000
7.63
2,000,000 1,000,000
5.80
0
5.0
Time-->
9.0411.98 14.50 10.0
15.0
34.40 20.11 20.0
51.10
34.53 25.0
30.0
35.0
40.0
45.0
50.0
54.15
56.85 60.0
55.0
FIG. 11.3.2 Pyrogram of D-glucuronic acid at 700°C.
identical results. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.2. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.2 for the Pyrolysis of D-glucuronic Acid at 700°C No. Compound 1 2 3 4 5 6 7
Carbon dioxidea Acetaldehyde
a
a
Furan
a
2-Propenal (acrolein) a
Acetone
a
2-Methylfuran
a
2,3-Butanedione (diacetyl) a
8
Acetic acid
9
2(5H)-furanone
10 11
a
2-Cyclopentandione a
1,4-Dioxadiene
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
4.23
44
124-38-9
29.53
5.80
44
75-07-0
0.71
7.63
68
110-00-9
2.14
8.68
56
107-02-8
0.15
9.04
58
67-64-1
1.16
11.98
82
534-22-5
0.93
14.5
86
431-03-8
0.13
20.11
60
64-19-7
0.42
28.27
84
497-23-4
0.19
28.60
98
3008-40-0
0.15
28.91
84
N/A
2.58 Continued
461
SUGAR ACIDS
TABLE 11.3.2 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.2 for the Pyrolysis of D-glucuronic Acid at 700°C—Cont’d No. Compound 12 13 14 15 16 17 18 19 20
Butanedial Furancarboxaldehyde (furfural)
a
a
5-Methyl-2(3H)-furanone
a
Methylglyoxal + unknown
a
2-Methylcyclopentene-1-one
a
Dihydro-3-methylene-2(3H)-furanone a
2-Hydroxy-2-cyclopenten-1-one a
Benzaldehyde + unknown
a
2-Methylenecyclopentanol
a
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
29.60
86
638-37-9
0.14
30.39
96
98-01-1
4.90
31.98
98
591-12-8
0.17
32.58
72
78-98-8
0.10
33.85
96
1120-73-6
0.19
34.40
98
547-65-9
0.67
34.53
98
10493-98-8
0.32
35.02
106
100-52-7
0.07
35.33
98
20461-31-8
0.11
21
2-Hydroxy-3-methyl-2-cyclopenten-1-one
37.94
112
80-71-7
0.17
22
2H-pyran-2,6(3H)-dione
38.05
112
5926-95-4
0.18
a
23
Phenol
38.60
94
108-95-2
0.09
24
3-Methylphenol (m-cresol)
41.31
108
108-39-4
0.12
25
1,3-Cyclopentandione
41.70
98
3859-41-4
0.11
26
Furylformate ?
41.88
126
13493-97-5
0.09
27
5-Oxotetrahydrofuran-2-carboxylic acid methyl ester
42.20
144
21461-85-8
0.09
28
(3E)-4-(2-furyl)-3-buten-2-one
43.24
136
41438-24-8
0.06
29
3-Hydroxybenzaldehyde
51.10
122
100-83-4
0.41
30
Unknown
52.42
162
N/A
0.07
31
1-(2-Hydroxyphenyl)ethanone
53.10
136
118-93-4
0.06
32
Deoxy-glucuronic acid
54.15
176
N/A
0.38
33
Glucuronic acid ?-lactone
55.44
176
N/A
0.12
34
Deoxy-glucuronic acid
55.65
176
N/A
0.31
35
Glucuronic acid 1,6-lactone
56.24
176
N/A
52.54
36
Unknown
56.85
192
N/A
0.19
37
Trihydroxyperhydropyran-2-carboxylic acid
58.38
178
N/A
0.06
38
3,5,7-Trihydroxy-2H-1-benzopyran-2-one
60.73
194
22065-07-2
0.07
39
Dihydroxydihydropyran-2-carboxylic acid
60.91
160
N/A
0.12
a
Compounds identical with those generated from the pyrolysis of glucose. Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
462
11. PYROLYSIS OF CARBOHYDRATES
FIG. 11.3.3 Mass spectrum of D-glucuronic acid-1,6lactone (MW ¼ 176).
Abundance 100
57 O
90
71
80
O
86
70 60
OH
29
50
OH
40 30
O
OH
43
20 10 0 m/z--> 20
101114 40
60
80
129 141 159
100 120 140 160
As shown in Table 11.3.2, some of the compounds generated in the pyrolysis of glucuronic acid are identical to those generated from glucose pyrolysis. These are small fragment molecules such as acetaldehyde, furan, and furfural. A few minor pyrolyzate constituents are different, but they also consist of furan or pyran derivatives. The main reaction taking place during D-glucuronic acid pyrolysis is the formation of a 1,6-lactone (2,3,4-trihydroxy-7,8dioxabicyclo[3,2,1] octan-6-one), as shown in the reaction below:
ð11:3:1Þ
The reaction is similar to the formation of levoglucosan from glucose (see Subchapter 11.1). The mass spectrum of D-glucuronic acid-1,6-lactone is given in Fig. 11.3.3. The GC/MS analysis of silylated pyrolyzate shows, in addition to the compounds listed in Table 11.3.2, low levels of undecomposed glucuronic acid and traces of a few disaccharide type compounds.
HALOGENATED SUGARS Among halogenated sugars, sucralose or 1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4chloro-4-deoxy-α-D-galactopyranoside is of more interest, with it being a common sweetener. Pyrolysis of a sample of sucralose performed on a 1.0 mg compound at Teq ¼ 700°C, β ¼ 10°C/ ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.4. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.3. The calculation of the mole % was obtained based on peak areas.
463
HALOGENATED SUGARS
Abundance 9,000,000
4.20
Cl OH
8,000,000
HO
O
7,000,000
O OH
6,000,000
53.64
OH
CH2OH
O
Cl
Cl
5,000,000 9.02
4,000,000
42.96
3,000,000
55.11
2,000,000
20.04 17.45 11.99 14.41
1,000,000 0
5.0
Time-->
10.0
15.0
20.0
30.34 25.0
30.0
34.36
35.0
40.84 43.85 49.91 48.22 45.65
55.50
40.0
55.0
45.0
50.0
60.0
FIG. 11.3.4 Pyrogram of 1 mg sucralose at 700°C.
TABLE 11.3.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.4 for the Pyrolysis of Sucralose at 700°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxidea
4.20
44
124-38-9
20.78
2
Propene
4.45
42
115-07-1
0.85
3
2-Methylpropene
4.96
56
115-11-7
0.33
4
Acetaldehydea
5.8
44
75-07-0
0.27
5
Furana
7.58
68
110-00-9
0.41
6
Hydrochloric acid
8.13 peak extended
36
7647-01-0
8.68
7
Acetonea
9.02
58
67-64-1
6.51
11.99
82
534-22-5
0.60
8
2-Methylfuran
a a
9
2,3-Butanedione (diacetyl)
14.5
86
431-03-8
0.24
10
Chloroacetaldehyde
14.62
78
107-20-0
0.53
16.44
78
71-43-2
0.16
17.45
102
N/A
0.63
18.52
96
625-86-5
0.21
a
11
Benzene
12
2-Chlorofuran
13
a
2,5-Dimethylfuran
Continued
464
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.4 for the Pyrolysis of Sucralose at 700°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
14
Acetic acida
20.04
60
64-19-7
5.49
15
1-Chloro-2-propanone
22.70
92
78-95-5
0.28
23.02
92
108-88-3
0.21
30.34
96
98-01-1
0.66
16 17
a
Toluene
a
Furancarboxaldehyde (furfural) a
18
1-(2-Furanyl)ethanone
33.12
110
1192-62-7
0.58
19
2,4-Pentanedione ?
33.32
100
123-54-6
0.46
20
Dihydro-3-methylene-2(3H)-furanone
34.36
98
547-65-9
0.72
34.62
112
N/A
0.32
35.57
110
620-02-0
0.32
37.11
110
N/A
0.27
37.95
112
80-71-7
0.16
38.66
94
108-95-2
0.20
21 22 23 24 25
a
Unknown
a
5-Methyl-2-furancarboxaldehyde a
Unknown
a
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene) a
Phenol
a
26
3-Furancarboxylic acid methyl ester
40.84
126
13129-23-2
1.36
27
3-Methylphenol
41.37
108
108-39-4
0.31
a
28
Levoglucosenone
42.96
126
37112-31-5
2.06
29
Unknown
43.85
126
N/A
0.92
30
Unknown
48.22
144
N/A
0.48
a
31
Dianhydrosugar ?
49.91
144
N/A
1.07
32
4-Chloro-7,8-dioxabicyclo[3.2.1]octane-2,3-diol (1,6anhydro-β-D-4-chloro-4-deoxygalactopyranose)
53.65
180
N/A
41.94
33
2,5-Bis(chloromethyl)-4-hydroxy-2,4,5-trihydrofuran3-one ?
55.11
198
N/A
0.82
34
Unknown chlorinated sugar fragment
55.51
198
N/A
0.64
35
Unknown sugar related
55.74
?
N/A
0.21
36
Unknown sugar related
55.97
?
N/A
0.32
a
Compounds identical in nature with those generated from the pyrolysis of glucose. Hydrogen, CO, methane, ethylene, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
As shown in Table 11.3.3, a number of compounds from sucralose pyrolysis are similar to those from other (not chlorinated) carbohydrates. Several chlorinated compounds are also generated in the pyrolyzate, including HCl, chloroacetaldehyde, chloropropanone, etc. The main pyrolysis product (in the conditions of the experiment) is the formation of a
HALOGENATED SUGARS
Abundance 100
FIG. 11.3.5 Mass spectrum of 1,6-anhydroβ-D-4-chloro-4-deoxygalactopyranose (MW ¼ 180).
60 89
90 80
O Cl
70
OH
60 50
OH 29 41
99
20
105 116
10 0 m/z--> 20
O
75
40 30
465
60
100
127 145 163 181 140
180
chlorinated equivalent to levoglucosan from the pyrolysis of other sugars. The reaction is shown below:
ð11:3:2Þ
The identification of 1,6-anhydro-β-D-4-chloro-4-deoxygalactopyranose is more certain while the structure of the other fragment is only tentative. The mass spectrum of 1,6anhydro-β-D-4-chloro-4-deoxygalactopyranose is shown in Fig. 11.3.5. The mass spectrum of the trimethylsilylated (TMS) derivative of this compound and that of the TMS derivative of 2,5-bis-(chloromethyl)-4-hydroxy-2,4,5-trihydrofuran-3-one are shown in Figs. 11.3.6 and 11.3.7, respectively. The carbonyl group in the ketone 2,5-bis-(chloromethyl)-4-hydroxy-2,4,5-tri-hydrofuran-3-one is assumed involved in a keto-enol equilibrium, and the ketone becomes 2,5-bis-(chloromethyl)-4,5-dihydrofuran-3,4-diol, which generated a TMS derivative [1] by derivatization. The analysis of sucralose pyrolyzate indicates that compounds similar to those generated from other sugars also are produced from the pyrolysis of this compound. The elimination of HCl from the molecule is, in many respects, similar to the elimination of H2O from simple sugar molecules.
466
11. PYROLYSIS OF CARBOHYDRATES
Abundance
161
100 90
O
O
73
CH3
80
Cl
70 60
CH3
O
40
CH3
CH3
CH3
204
30
Si
CH3
Si
50
O
117 243
20
103
45
10
147
217 278
0 40
m/z-->
60
80
100
120
140
160
180
200
220
240
260
324
280
300
320
FIG. 11.3.6 Mass spectrum of the TMS derivative of 1,6-anhydro-β-D-4-chloro-4-deoxygalactopyranose
(MW ¼ 324).
Abundance
CH3
73
100
CH3
90
H 3C
80
O O
Si
CH3
CH3
70
O
60 50
Cl
258
Cl
103
40
185
30 20
CH3
Si
147
45
10
243 169
59
271
215
288
0 m/z-->
40
60
80
100
120
140
160
180
200
220
240
260
280
342
306 300
320
340
FIG. 11.3.7 Mass spectrum of the TMS derivative of 2,5-bis(chloromethyl)-4,5-dihydrofuran-3,4-diol (MW ¼ 342).
ETHERS AND SILYL ETHERS OF SACCHARIDES Ethers and silyl ethers of carbohydrates are somewhat more stable to higher temperatures compared to the unmodified compounds. Pyrolysis of these compounds is reported in the literature only in relation with their formation by pyrolysis in the presence of a derivatization reagent such as TMAH [2–5] or hexamethyldisilazane [5]. The resulting compounds (methyl ethers of trimethylsilyl ethers) are stable in the conditions in which they are generated (700°C for the TMAH thermo-chemolysis/methylation).
467
GLYCOSIDES
GLYCOSIDES Glycosides are compounds frequently found in nature (e.g., [6–8]) (glycosides formed with glucose are named glucosides). Examples of glucosides are salicin or 2-(hydroxy-methyl)phenyl-β-D-glucopyranoside (related to aspirin) found in willow bark; naringin, a flavonoid glycoside that gives the bitter taste to grapefruit; neohesperidin dihydrochalcone, which is an artificial sweetener derived from bitter neohesperidin found in citrus juices; and rutin, a flavonoid glycoside also known as vitamin P1. Among other natural glycosides are those formed with 27-carbon steroids or with 30-carbon triterpenes that form the group of saponins. This group includes the cardiac steroid glycyrrhizin, which is a glycoside (with a sugar acid) of glycyrrhetinic acid and is found in licorice root, and many other plant glycosides. Pyrolysis of glycosides typically gives a mixture of compounds, some resulting from the saccharide and some from the aglycon moieties with little interaction of the two parts of the molecule. This effect, also seen for example in disaccharides or in compounds that are part carbohydrate and part polyol (maltitol), is caused by the relatively weak ether bond between the sugar and the aglycon. The glycosidic bond is cleaved faster than other parts of the molecule and generates fragments that undergo further pyrolytic decompositions. An example of a glycoside pyrolysis is given in Fig. 11.3.8 for neohesperidin dihydrochalcone. From the name of this compound, 2-O-(6-deoxy-2-O-{3,5-dihydroxy-4-[3(3-hydroxy-4-methoxyphenyl)propanoyl]phenyl}-α-L-mannopyranosyl-β-D-glucopyranose, the aglycon part (chalcone derivative) as well as the sugar moiety (rhamnoglucose) can be seen. The compound is closely related to naringin (40 ,5,7-trihydroxyflavanone-7-rhamnoglucoside). Pyrolysis of a sample of 1.0 mg neohesperidin dihydrochalcone at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.8. Abundance
2e+07 1.8e+07
57.59
OH O HO HO
1.6e+07 1.4e+07 1.2e+07 1e+07
OH
O
OCH3
42.59
O
56.74 OH
O
H3C HO
OH HO
O
45.24
38.61
OH
47.38
4.20
8,000,000 42.94
6,000,000
9.03
4,000,000 2,000,000 0 Time-->
34.37 36.29
12.01 5.76 7.59 5.0
10.0
14.56 17.68 15.0
22.45
20.0
25.0
40.18
65.38 49.88
28.88 30.0
35.0
40.0
FIG. 11.3.8 Pyrogram of neohesperidin dihydrochalcone at 900°C.
45.0
50.0
55.48 59.52
55.0
60.0
65.0
468
11. PYROLYSIS OF CARBOHYDRATES
The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.4. The calculation of the mole % was obtained based solely on peak areas. TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
1
Carbon dioxide
4.21
44
124-38-9
15.68
2
Propene
4.45
42
115-07-1
1.08
3
Formaldehyde
4.63
30
50-00-0
0.53
4
Propynal
5.29
54
624-67-9
Trace
5
Acetaldehyde
5.76
44
75-07-0
2.02
6
Methanol
6.34
32
67-56-1
0.20
7
Furan
7.59
68
110-00-9
0.23
8
Propanal
8.65
58
123-38-6
0.20
9
Acetone
9.03
58
67-64-1
5.19
10
2-Methylfuran
12.01
82
534-22-5
2.15
11
2,3-Butanedione (diacetyl)
14.56
86
431-03-8
0.27
12
Hydroxyacetaldehyde (glycol aldehyde)
17.68
60
141-46-8
0.37
13
2,5-Dimethylfuran
18.54
96
625-86-5
0.19
14
Acetic acid
20.03
60
64-19-7
0.82
15
1-Hydroxy-2-propanone (acetol)
22.45
74
116-09-6
0.65
16
1,4-Dioxadiene
28.88
84
N/A
0.62
17
2,5-Furandione
29.62
98
N/A
0.11
18
Furancarboxaldehyde (furfural)
30.38
96
98-01-1
0.21
19
Dihydro-3-methylene-2(3H)-furanone
34.37
98
547-65-9
1.65
20
5-Methyl-2-furancarboxaldehyde
35.58
110
620-02-0
0.66
21
Unknown
36.29
128
N/A
1.44
22
3,5-Dimethyl-2,4(3H,5H)-furandione
36.49
128
5460-81-1
0.17
23
Resorcinol
37.12
110
108-46-3
0.10
24
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene) 37.96
112
80-71-7
0.16
25
5-Methyl-6,7-dioxabicyclo[2.2.1]heptan-2-one
38.61
128
N/A
4.31
26
Isomaltol
39.41
126
3420-59-5
0.32 Continued
469
GLYCOSIDES
TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
27
2-Methoxyphenol + unknown
39.70
124
90-05-1
0.62
28
Unknown from rhamnose
40.18
102
N/A
2.41
29
Unknown
40.76
144
N/A
0.33
30
Maltol
41.18
126
118-71-8
0.05
31
4-Methylphenol
41.31
108
106-44-5
0.26
32
3-Methylphenol
41.36
108
108-39-4
0.34
33
Unknown from sugar
42.17
128
N/A
0.63
34
2-Methoxy-5-methylphenol
42.59
138
1195-09-1
6.36
35
2,4-Dimethylphenol
42.81
122
105-67-9
0.60
36
Levoglucosenone
42.94
126
37112-31-5
3.48
37
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
43.15
142
1073-96-7
0.33
38
Unknown
44.99
128
N/A
0.50
39
4-Ethyl-2-methoxyphenol
45.24
152
2785-89-9
3.81
40
1,4:3,6-Dianhydro-α-D-glucopyranose
46.33
144
N/A
0.15
41
Unknown
46.97
180
N/A
0.20
42
2-Methoxy-4-vinylphenol
47.38
150
7786-61-0
2.87
43
2’,4’-Dihydroxy-3’-methylacetophenone + unknown
47.62
166
10139-84-1
0.04
44
Resorcinol monoacetate
48.73
152
102-29-4
0.12
45
Mixture
49.34
164
N/A
0.18
46
6-(Hydroxymethyl)-2-methyl-2H-3,5,6trihydropyran-4-one
49.88
144
N/A
0.70
47
4-(1,2-Propandienyl)-guaiacol
50.79
162
N/A
0.67
48
1-(2,6-Dihydroxyphenyl)butan-1-one
51.20
180
N/A
0.06
49
Unknown aromatic compound
51.36
180
N/A
0.09
50
Unknown
51.61
180
N/A
0.13
51
2-Ethyl-1,4-benzodioxin ?
51.87
162
27549-01-5
0.17
52
4-(4-Methoxyphenyl)-2-butanone
52.09
178
104-20-1
0.15
53
Anhydrosugar (from rhamnose ?)
53.45
146
N/A
0.23
54
1-(4-Hydroxy-3-methoxyphenyl) ethanone
55.28
166
498-02-2
0.13
55
4-Hydroxy-2-methyloxycinnamaldehyde
55.48
178
127321-19-1
0.63 Continued
470
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.8 for the Pyrolysis of Neohesperidin Dihydrochalcone at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % (pyrolyzate)
56
4-(4-Hydroxyphenyl)-2-butanone
56.26
164
5471-51-2
0.30
57
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.75
162
498-07-7
11.04
58
Ethylhomovanillate ?
57.13
210
60563-13-5
0.21
59
3,4-Methylenedioxyphenyl acetone
57.27
178
4676-39-5
0.21
60
4-(4-Hydroxy-3-methoxyphenyl)butan-2-one
57.59
194
122-48-5
19.44
61
Unknown
58.66
192
N/A
0.06
62
5-Hydroxy-6-methoxy-1-indanone
59.52
178
127933-78-4
0.69
63
1,6-Anhydro-β-D-glucofuranose
59.86
162
7425-74-3
0.45
64
4,5-Dimethoxy-2-(2-propenyl)phenol ?
60.09
194
N/A
0.07
65
1-(4-Methoxymethyl-2,6-dimethylphenyl)ethanone
62.18
192
1000202-02-2
0.11
66
6-(3-Hydroxy-4-methoxyphenyl)hexane-2,4-dione ?
65.38
236
N/A
1.84
Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound identification. Note: Bold numbers indicate compound with high content in the pyrolyzate.
Pyrolysis products listed in Table 11.3.4 can be grouped easily into fragments resulting from the sugar (furan derivatives, 6-methyl-5,7-dioxabicyclo[2.2.1]heptan-2-one, levoglucosenone, levoglucosan, etc.) and those resulting from the aglycon (most phenol derivatives) including 4-(4-hydroxy-3-methoxyphenyl)butan-2-one. Even some of the compounds resulting from the rhamnose fragment (5-methyl-6,7-dioxabicyclo[2.2.1]-heptan-2one) and those from the glucose fragment (levoglucosenone, levoglucosan) can be identified. A “model” reaction for the formation of some of the main pyrolysis products from neohesperidin dihydrochalcone is shown below:
ð11:3:3Þ
471
GLYCOSIDES
Abundance
OH
38.38 HO
1.4e+07 1.2e+07 1e+07
OH HO O
4.09
8,000,000
H3C OH
OH
O
OH
O
O
O HO HO
35.32
6,000,000
49.64
OH
4,000,000 5.58 2,000,000
56.48
O
36.04 39.93 42.69
8.77 28.60
11.68 17.33
47.55
34.11
22.13
53.82
59.61
0 Time-->
10.0
20.0
30.0
40.0
50.0
60.0
FIG. 11.3.9 Pyrogram of rutin obtained at 900°C.
Further pyrolysis of the intermediate compounds generated in reaction (11.3.3) as well as other pyrolysis products generated directly from the parent molecule leads to a complex pyrolyzate composition. Char is also formed during pyrolysis, as is a considerable amount of CO2, which shows that the reaction model (11.3.3) is far from complete. Another example of the pyrolysis of a glycoside is that of rutin. The compound is quercetin3-O-β-D-glucose-α-L-rhamnose or 3-{[6-O-(6-deoxy-α-L-mannopyranosyl)-β-D-glucopyranosyl] oxy}-2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-1-benzopyran-3-one. Pyrolysis of a sample of 1.0 mg rutin at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C generates the pyrogram shown in Fig. 11.3.9. The analysis of pyrolyzate was done by GC/ MS under conditions given in Table 1.4.1. The compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.5. The calculation of the mole % was obtained based solely on peak areas. Typical for a glycoside, the peaks in the pyrogram of rutin can be related to the peaks resulting from the pyrolysis of rutinose (see Table 11.2.1) or even glucose (see Table 11.1.2) and of quercetin (see Subchapter 16.1). Some free quercetin is also present in the pyrolyzate of rutin, but it can be detected only in the silylated pyrolyzate. Pyrolysis of rutin also produces a considerable amount of char. The pyrolyzates of glycosides are frequently similar in composition with that of a mixture of the pyrolysis products of the sugar and the aglycon. This was shown in the previous two examples for neohesperidin dihydrochalcone and also for rutin. The same pattern is seen during the pyrolysis of anthocyanins, where the pyrolyzate is made from a mixture of compounds resulting from the sugar pyrolysis and from the anthocyanidin pyrolysis. For the glycosides with a thermally stable aglycon, the capability of releasing the aglycon upon heating has been investigated with the purpose of developing nonvolatile flavor precursors. These compounds are intended to release the flavor only when heated, for example during cigarette smoking. A typical example of this type of glycoside is menthyl glucoside
472
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.5 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.9 for the Pyrolysis of Rutin at 900°C No. Compound
Retention Time (Min) MW CAS#
Moles %
1
Carbon dioxide
4.09
44
124-38-9
24.74
2
Propene
4.30
42
115-07-1
0.86
3
Formaldehyde
4.50
30
50-00-0
0.75
4
Acetaldehyde
5.58
44
75-07-0
2.55
5
Furan
7.36
68
110-00-9
0.30
6
Propanal
8.38
58
123-38-6
0.32
7
Acetone
8.77
58
67-64-1
3.23
8
2-Methylfuran
11.68
82
534-22-5
1.35
9
Methyl vinyl ketone
13.81
70
78-94-4
0.12
10
2,3-Butanedione (diacetyl)
14.07
86
431-03-8
0.09
11
2-Butenone
14.19
72
78-93-3
0.36
12
Hydroxyacetaldehyde (glycol aldehyde)
17.33
60
141-46-8
0.96
13
2,5-Dimethylfuran
18.20
96
625-86-5
0.41
14
2-Methyl-3-buten-2-one
18.85
84
814-78-8
0.47
15
Acetic acid
19.87
60
64-19-7
1.11
16
Ethyl-1-propenyl ether
22.00
86
928-55-2
0.44
17
1-Hydroxy-2-propanone (acetol)
22.13
74
116-09-6
0.75
18
2,4-Pentandione
26.12
100
123-54-6
0.48
19
1,4-Dioxadiene
28.60
84
N/A
1.69
20
Furancarboxaldehyde (furfural)
30.08
96
98-01-1
0.69
21
Dihydro-4-hydroxy-2(3H)-furanone
33.84
102
5469-16-9
0.36
22
Dihydro-3-methylene-2(3H)-furanone
34.11
98
547-65-9
2.10
23
5,6-Dihydro-4-methyl-2H-pyran-2-one
34.19
112
2381-87-5
0.41
24
1-Methyl-7-oxabicyclo[4.1.0]heptane
34.64
112
1713-33-3
0.41
25
6-Methylcyclohex-2-en-1-ol ?
35.11
112
N/A
0.28
26
5-Methyl-2-furancarboxaldehyde
35.32
110
620-02-0
3.74
27
3-Methyl-1,2-cyclopentanedione
35.43
112
765-70-8
0.18
28
Unknown
35.91
112
N/A
0.53
29
3-Acetyldihydro-2(3H)-furanone
36.04
128
517-23-7
3.75
30
Tetrahydro-3,6-dimethyl-2H-pyran-2-one
36.23
128
3720-22-7
0.35 Continued
473
GLYCOSIDES
TABLE 11.3.5 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.9 for the Pyrolysis of Rutin at 900°C—Cont’d No. Compound
Retention Time (Min) MW CAS#
Moles %
31
Unknown
36.86
110
N/A
0.51
32
2-Hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene)
37.72
112
80-71-7
1.03
33
3-Hydroxy-5-methyl-5,6-dihydroxypyran-4-one
38.37
128
N/A
9.47
34
2,5-Dimethyl-4-hydroxy-3(2H)-furanone
39.44
128
3658-77-3
0.68
35
Unknown
39.93
128
N/A
2.21
36
Anhydrosugar from rhamnose
40.52
144
N/A
1.10
37
4-Methylphenol (p-cresol)
41.08
108
106-44-5
0.88
38
1,3-Benzodioxol-2-one
41.40
136
2171-74-6
0.16
39
Anhydrosugar from rhamnose
42.39
144
N/A
0.68
40
Anhydrosugar from rhamnose
42.69
144
N/A
2.47
41
3,5-Dihydroxy-2-methyl-4H-pyran-4-one (hydroxymaltol)
42.94
142
1073-96-7
1.31
42
Dianhydromannitol
44.02
146
N/A
0.34
43
Unknown
44.75
128
N/A
0.53
44
4,7-Dihydro-1,3-isobenzofurandione
44.88
150
4773-89-1
0.41
45
2-Propoxyphenol
46.79
152
6280-96-2
0.75
46
1-(2-Hydroxy-6-methoxyphenyl)ethanone ?
47.39
166
703-23-1
1.11
47
1,2-Benzenediol (catechol)
47.55
110
120-80-9
2.25
48
6-Hydroxy-4-methoxy-2,3-dimethylbenzaldehyde
49.09
180
34883-12-0
1.71
49
4-Methyl-1,2-benzenediol in mix
49.64
124
452-86-8
5.44
50
1,3-Benzenediol (resorcinol)
50.99
110
108-46-3
0.27
51
2,4-Dihydroxybenzeneacetic acid ?
53.82
168
N/A
0.30
52
1,6-Anhydro-β-D-glucopyranose (levoglucosan)
56.47
162
498-07-7
12.11
53
2,5-Dihydroxybenzeneacetic acid ?
58.65
168
451-13-8
0.14
54
1,6-Anhydro-α-D-glacatofuranose
59.61
162
N/A
0.34
Hydrogen, CO, methane, ethane, and water not included. Note: "?" indicates uncertain compound identification.
(menthyl-β-D-glucopyranoside) [9]. The heating of this compound between ambient to a temperature of 500°C at a heating rate of 10°C/min showed that, at 200°C, the compound does not decompose, and between 250°C and 300°C the decomposition takes place mainly with the formation of menthol. At 400°C, besides menthol, various byproducts generated mainly from the glucose moiety are produced. The formation of the unmodified aglycon from the pyrolysis of a glucoside is still dependent on the stability of the resulting compound. For some
474
11. PYROLYSIS OF CARBOHYDRATES
compounds, the first stage of the decomposition reaction affects the aglycon molecule and not the sugar. A comparison of the main pyrolysis reactions for menthyl glucoside and phenylethanol glucoside is shown in reactions 11.3.4 and 11.3.5, respectively:
ð11:3:4Þ
ð11:3:5Þ
Further decomposition of the sugar and the aglycon is not uncommon and depends on the pyrolysis conditions (Teq in particular).
SUGAR ESTERS Sugar esters are compounds resulting from the esterification reaction between a carbohydrate (acting as an alcohol) and an organic or inorganic acid. A wide range of compounds can be obtained as esters, even considering one type of acyl group. The acyl can be attached to a different number of OH groups and at different positions in the carbohydrate. Pyrolysis of glucose pentaacetate at Teq ¼ 700°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C leads to only about 15% decomposition of the parent molecule, with the rest of it transferring intact to the GC/MS system. Some of the sample may have been transferred before reaching the nominal Teq value, being vaporized without decomposition and transferred out of the heating zone. The decomposition products of glucose pentaacetate include acetic acid, acetic anhydride, and compounds such as 1,5-anhydro-D-arabino-hex-1-enitol tetraacetate:
ð11:3:6Þ
The elimination of the acetic acid molecule can take place from positions other than C1, leading to analogous compounds.
AMINO SUGARS
475
The carbohydrates only partially esterified are less stable to heating. The pyrolysis products generated from these esters include the free acids and sugar decomposition products. The amount of acid released from sugar esters depends on the number of esterified OH groups, the position of esterification, and the heating temperature. The compounds with a high degree of esterification have the tendency to generate molecules that retain the acyl group, such that they do not yield the highest level of free acid [10]. Among the inorganic esters of carbohydrates, glucose-6-phosphate is probably the most important compound because it is involved in cell metabolism. Besides the formation of H3PO4, the pyrolysis of glucose-6-phosphate is not expected to generate fragments different from those generated from glucose.
AMINO SUGARS The replacement of the OH group from the C2 of glucose with an NH2 group leads to the formation of 2-amino-2-deoxy-D-glucose (glucosamine). Glucosamine is a precursor in the synthesis of glycosylated proteins and is the building block for chitosan and (in acetylated form) for chitin. The amino group, having a basic character, can form salts with various acids. Pyrolysis at 900°C of glucosamine in its salt form with HCl generates three types of compounds. The first type includes small molecules that are similar to those generated in glucose pyrolysis. These molecules include CO2, aldehydes, furans, and other small molecules. Levoglucosenone is also present in the pyrolyzate (at about 0.2%), but levoglucosan is absent. No aminated equivalent of levoglucosan was detected in the pyrolyzate. The second type of compound is nitrogen containing. This includes acetonitrile, propanenitrile, and low levels of alkyl pyridines and pyrazines. The third group of compounds includes a few chlorinated compounds. Besides HCl that was expected in the pyrolyzate, compounds such as methylene chloride and 3-chloro-2-cyclopenten-1-one also were detected. This indicates that the formation of an excess of HCl during pyrolysis leads to the interactions of this compound with organic fragments and the formation of organochlorine compounds. N-Acetyl-D-glucosamine (GlcNAc) is an important amino sugar in the bacterial cell wall as a component of peptidoglycans. In these polymers, N-acetyl-D-glucosamine is typically connected on one side with other sugars and on the other side to 2-acetamido-2deoxymuramic acid and further to the amino acids in a protein chain (see, e.g., [11]). From a chemical point of view, N-acetyl-D-glucosamine is an amide of acetic acid with 2-amino2-deoxy-D-glucose. Having a large sugar moiety, it is interesting to compare its pyrolysis with other sugars. For this purpose, a sample of 1.0 mg N-acetyl-D-glucosamine was pyrolyzed at Teq ¼ 900°C, β ¼ 10°C/ms, THt ¼ 10 s, and housing temperature Thou ¼ 280°C. The analysis of pyrolyzate was done by GC/MS under conditions given in Table 1.4.1. The pyrogram is shown in Fig. 11.3.10, and the compound identifications and their relative molar content in 100 mole of pyrolyzate are given in Table 11.3.6. The calculation of the mole % was obtained based solely on peak areas. The compounds from Table 11.3.6 indicate that the pyrolysis of N-acetylglucosamine is in many respects similar to that of other sugars. The formation of small molecules, including CO2, (H2O not shown in the table), small aldehydes and ketones, and furan derivatives, is
476
11. PYROLYSIS OF CARBOHYDRATES
A bundance
19.92
OH
48.42
8,000,000 7,000,000
O
31.77
4.20
HO
NH
6,000,000
O
5,000,000
CH3
4,000,000
60.71
3,000,000 2,000,000
17.67
1,000,000 0 Time-->
OH
HO
5.76
10.83 9.04
5.0
10.0
14.39 15.0
59.36 60.84 56.25 54.57 51.04 63.91
36.11 21.67
20.0
41.61 42.88 38.08
27.36
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
FIG. 11.3.10
Pyrogram of 1 mg N-acetyl-D-glucosamine at 900°C. Peak identifications by their retention times given in Table 11.3.6.
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
1
Carbon dioxide
4.20
44
124-38-9
10.16
2
Cyclopropane
4.47
42
75-19-4
0.75
3
Formaldehyde
4.63
30
50-00-0
1.37
4
Acetaldehyde
5.76
44
75-07-0
0.35
5
Furan
7.58
68
110-00-9
0.07
6
2-Propenal (acrolein)
8.64
56
107-02-8
0.20
7
Acetone
9.04
58
67-64-1
0.66
8
Acetonitrile
10.83
41
75-05-8
2.33
9
2-Methylfuran
11.97
82
534-22-5
0.05
10
2,3-Butanedione (diacetyl)
14.39
86
431-03-8
0.13
11
2-Butanone
14.53
72
78-93-3
0.17
12
Propanenitrile
16.20
55
107-12-0
0.08
13
Benzene
16.35
78
71-43-2
0.06
14
2-Methyloxazole
16.61
83
N/A
0.05
15
Hydroxyacetaldehyde (glycol aldehyde)
17.68
60
141-46-8
3.26 Continued
477
AMINO SUGARS
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
16
2,5-Dimethylfuran
18.50
96
625-86-5
0.06
17
Acetic acid
19.92
46
64-17-5
34.07
18
2,4-Dimethyloxazole
21.68
97
7208-05-1
0.09
19
1-Hydroxy-2-propanone (acetol)
22.43
74
116-09-6
0.11
20
Toluene
22.99
92
108-88-3
0.07
21
Pyridine
23.94
79
110-86-1
0.12
22
Acetic acid anhydride
24.87
102
108-24-7
0.07
23
2-Methylpyridine
26.99
93
109-06-8
0.14
24
Acetic acid methyl ester
27.36
74
79-20-9
0.29
25
Unknown
27.55
111
N/A
0.11
26
1H-pyrrole
27.60
67
109-97-7
0.21
27
Ethylbenzene
28.16
106
100-41-4
0.04
0
28
3,3 -oxybispropanenitrile
28.84
124
1656-48-0
0.29
29
Pyruvic acid methyl ester
29.26
102
600-22-6
0.14
30
Furancarboxaldehyde (furfural)
30.31
96
98-01-1
0.12
31
Acetamide
31.77
59
60-35-5
26.76
32
1-Acetyloxy-2-propanone
31.90
116
592-20-1
0.22
33
Dihydro-4-hydroxy-2(3H)-furanone
34.08
102
5469-16-9
0.31
34
2-Hydroxy-2-cyclopenten-1-one
34.50
98
10493-98-8
0.10
35
N-acetylpyrrole
34.85
109
N/A
0.13
36
5-Ethyl-2,4-dimethyl-1,3-oxazole
36.11
125
N/A
0.92
37
2,4,5-Trimethyl-1,3-oxazoline
36.29
113
N/A
0.41
38
N-acetylacetamide
38.08
101
625-77-4
0.20
39
Phenol
38.59
94
108-95-2
0.07
40
2-Methyl-4-butyloxazole
39.17
139
N/A
0.13
41
1H-pyrrole-2-carboxaldehyde
39.25
95
1003-29-8
0.10
42
2-Methylbenzoxazole
41.02
133
95-21-6
0.01
43
1-(1H-pyrrol-2-yl)ethanone
40.00
109
1072-83-9
0.04
44
3,4-Dimethyl-2-propyloxazole
41.01
139
53833-32-2
0.03 Continued
478
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.6 Identification of the Main Peaks in the Chromatogram Shown in Fig. 11.3.10 for the Pyrolysis of N-acetyl-D-glucosamine at 900°C—Cont’d No. Compound
Retention Time (Min)
MW CAS#
Moles % Pyrolyzate
45
Acetamidoacetaldehyde
41.61
101
64790-08-5
1.07
46
Unknown
41.83
133
N/A
0.16
47
Oxazole derivative ?
41.95
153
N/A
0.10
48
Unknown
42.15
133
N/A
0.15
49
Unknown
42.60
141
N/A
0.06
50
5-Methyl-2(1H)-pyridinone
42.75
109
1003-68-5
0.12
51
2-Furanmethanol + unknown
42.88
98
98-00-0
0.38
52
2-Pyridinemethanol acetate
43.32
151
1007-49-4
0.07
53
6-Methyl-2(1H)-pyridinone
44.15
109
3279-76-3
0.06
54
1,2-Dihydro-6-methyl-2-oxo-3-pyridinecarboxaldehyde
44.90
137
784440-89-8
0.09
55
Unknown
46.19
151
N/A
0.08
56
2,4,4-Trimethyl-2-enolide
46.43
126
4182-41-6
0.07
57
5-(Hydroxymethyl)-2-furancarboxaldehyde
47.55
126
67-47-0
0.24
58
Unknown
47.84
139
N/A
0.09
59
3-Acetamidofuran
48.42
125
59445-85-1
6.32
60
1-Acetamidobicyclo[3.2.0]hetan-2-one
48.54
167
N/A
0.23
61
Pyridin-2,6-diol, diacetate ?
50.83
195
N/A
0.16
62
2-Acetamido-2-deoxyglucono-1,4-lactone ?
51.04
219
N/A
0.11
63
Unknown
52.83
149
N/A
0.07
64
Unknown
53.10
169
N/A
0.11
65
N-(2,4-dihydroxyphenyl)acetamide
54.57
167
71516-07-9
0.36
66
Unknown
54.72
128
N/A
0.20
67
Unknown
54.93
152
N/A
0.11
68
6-(Hydroxymethyl)-2-methyl-4,5,6,7,3a,7a-hexahydro7-oxabenzoxazole-4,5-diol
56.25
203
N/A
0.51
69
Anhydro-N-acetyl-glucosamine ?
59.36
203
N/A
0.72
70
3-Acetamido-5-acetylfuran
60.71
167
95598-28-0
2.05
71
3-Acetamido-5-acetylfuran isomer
60.84
167
N/A
1.17
72
Unknown
61.42
181
N/A
0.10
Hydrogen, CO, methane, ethane, and water not included. Note: Bold numbers indicate compound with high content in the pyrolyzate. Note: "?" indicates uncertain compound identification.
AMINO SUGARS
479
expected. However, the levels of furfural and 5-hydroxymethylfurfural are much lower than those for typical carbohydrates. Small molecules containing nitrogen also are generated, such as several nitriles. Acetic acid and acetamide are found in large proportions in the pyrolyzate. Also, 3-acetamidofuran is present in the pyrolyzate. One interesting reaction generating oxazoles derivatives also occur during pyrolysis. The first step of this reaction can be written as follows:
ð11:3:7Þ
Similar to the case of glucose, it is expected that N-acetylglucosamine pyrolyzate contains a number of compounds with multiple OH groups, and some of these compounds may not elute from the chromatographic column when conditions given in Table 1.4.1 are used. For this reason, a second experiment was performed by pyrolyzing 1.0 mg N-acetyl-Dglucosamine at 900°C followed by the collection of the pyrolyzate and derivatization with bis(trimethylsilyl)trifluoroacetamide (BSTFA) in conditions described in Subchapter 1.4. The analysis of the derivatized pyrolyzate was done using a GC/MS technique in conditions described in Table 1.4.2. Only the window between 42 and 52 min from the chromatogram of the derivatized pyrolyzate is shown in Fig. 11.3.11 because this portion contains compounds with OH silylated groups. The identification of the peaks is given in Table 11.3.7, based on their retention times.
Abundance 52.38
1.2e+07 1e+07 8,000,000 43.13 6,000,000 4,000,000
52.46
48.80
43.39 44.52
2,000,000 46.30 0 42.0 Time-->
44.0
46.0
50.94 48.0
50.0
52.0
FIG. 11.3.11 Time window 42–52 min in the pyrogram of N-acetylglucosamine.
480
11. PYROLYSIS OF CARBOHYDRATES
TABLE 11.3.7 Identification of the Main Peaks in the Time Window 42–52 min in the Chromatogram Shown in Fig. 11.3.11 for the Pyrolyzate of N-acetylglucosamine Derivatized to TMS Derivative With Bis(trimethylsilyl) trifluoroacetamide (BSTFA) No. Compound
Ret. Time
No. of TMS
MW
1
2-(Acetylamino)-3,6-anhydro-2-deoxy-D-glucopyranose
43.13
2
347
2
6-(Hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta[2,1-d]1,3-oxazole-4,5-diol
43.40
3
401
3
5-(Hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol 44.52
3
419
4
Unknown
46.30
3
?
5
N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1]octan-4-yl)acetamide
48.80
2
347
6
N-acetyl-2-deoxy-2-aminoglucose
52.38
4
509
7
N-acetyl-2-deoxy-2-aminohexose
52.46
4
509
The identification of silylated pyrolysis products of N-acetylglucosamine encountered problems due to the absence of their spectra in common mass spectral libraries. The mass spectra of three TMS 6-(hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta[2,1-d]1,3oxazole-4,5-diol is shown in Fig. 11.3.12, that of three TMS derivative of 5(hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol is shown in Fig. 11.3.13, and that of two TMS derivative of of N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1] octan-4-yl)acetamide, which is the equivalent of two TMS levoglucosan generated from glucose is shown in Fig. 11.3.14. Abundance
TM S 298
O 100
TM S
N
O
CH 3
80
O 131
73
60
O
TM S
40
20 147 45 0 m/z-->
40
103 80
311 210
120
160
200
386 401
269 240
280
320
360
400
FIG. 11.3.12 Mass spectrum of three TMS derivative of 6-(hydroxymethyl)-2-methyl-4,5,6-trihydrocyclopenta [2,1-d]1,3-oxazole-4,5-diol (MW ¼ 401).
481
AMINO SUGARS
Abundance
73
100
TMS O
80
TMS 60
N
O
CH3 O
O
217
189 40
O
TMS
147
316
20 117
300
45 0 m/z-->
329
272
40
80
120
160
200
240
280
404
358
320
360
400
FIG. 11.3.13 Mass spectrum of three TMS derivative of 5-(hydroxymethyl)-2-methyl-3a,5,6,7,7a-pentahydropyrano[3,2-d]oxazole-6,7-diol (MW ¼ 419). Abundance
186
100
TMS 80
O
TMS
O
NH
CH3
144
73 60
O
O O
40 212 20
116 101
43
129
59 0 m/z-->
40
80
120
332
169
160
242 200
240
288
280
320
FIG. 11.3.14 Mass spectrum of two TMS derivative of N-(2,3-dihydroxy-6,8-dioxabicyclo[3.2.1]octan-4-yl) acetamide (MW ¼ 347).
As shown in Table 11.3.7, some of the initial compound is transferred unmodified by pyrolysis, and also some isomerization process takes place. The other compounds are generated from water elimination of one or more OH groups of the parent compound in reactions either similar to that seen for simple sugars or as shown in reaction (11.3.7).
482
11. PYROLYSIS OF CARBOHYDRATES
References 11.3 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.C. Moldoveanu, V. David, Sample Preparation in Chromatography, Elsevier, Amsterdam, 2002. D. Fabbri, R.J. Helleur, J. Anal. Appl. Pyrolysis 49 (1999) 277. M. Blazsό, S. Janitsek, A. Gelencser, P. Artaxo, B. Graham, M.O. Andreae, J. Anal. Appl. Pyrolysis 68-69 (2003) 351. I. Tanczos, C. Schwarzinger, H. Schmidt, J. Balla, J. Anal. Appl. Pyrolysis 68-69 (2003) 151. D. Fabbri, G. Chiavari, Anal. Chim. Acta 449 (2001) 271. M. Stengele, E. Stahl-Biskup, J. Essent. Oil Res. 5 (1993) 13. B. Bennini, A.J. Chulia, M. Kaouadji, F. Thomasson, Pytochemistry 31 (1992) 2483. E. Stahl-Biskup, Flav. Fragr. J. 2 (1987) 75. W.-c. Xie, J. Tang, X.-h. Gu, C.-r. Luo, G.-y. Wang, J. Anal. Appl. Pyrolysis 78 (2007) 180. W. W. Weeks, S. C. Moldoveanu, 50th Tobacco Res. Conf, (1996) paper 55. S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998, 300.