Pyrolysis-gas chromatography-mass spectrometry of two trimeric lignin model compounds with alkyl-aryl ether structure

135

Journal of Analytical and Applied Pyrolysis, 14 (1988) 135-148 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

PYROLYSIS-GAS CHROMATOGRAPHY-MASS SPECTROMETRY OF TWO TRIMERIC LIGNIN MODEL COMPOUNDS WITH ALKYL-ARYL ETHER STRUCTURE

OSKAR

FAIX

*. DIETRICH

MEIER

and INGRID

FORTMANN

Federal Research Centre for Forestry and Forest Products, Institute for Wood Chemistry and Chemical Technology of Wood, Leuschnerstr. 91, D-2050 Hamburg 80 (F.R.G.) (Received

February

9th, 1988; accepted

June 20th, 1988)

ABSTRACT Thermal degradation of w-guaiacoxy-acetoguaiacone-benzylether (A) and w-syringoxyacetoguaiacone-benzylether (B) has been investigated for a better understanding of the thermolytic behaviour of lignins. Earlier differential scanning calorimetric studies revealed that degradation starts below 200 o C. Direct insertion probe experiments also demonstrated that splitting of linkages between aroxy and acetoguaiacone-benzylether (C) moieties commences at 175 o C. As a consequence the presence of the following compounds was observed: guaiacol and C (from A) and syringol and C (from B). Simultaneously to these splitting mechanisms a rearrangement occurs between the w-carbon of compound C and the phenolic oxygen of the aroxy residues, resulting in the formation of 2-methoxybenzaldehyde and acetovanillone-benzylether (from A) and 2,6_dimethoxybenzaldehyde and acetovanillone-benzylether (from B). Pyrolysis-gas chromatography-mass spectrometry experiments between 240 to 600” C corroborated these results. The higher the pyrolysis temperature the more peaks are displayed in the pyrograms. The pyrograms obtained at 600 0 C show 40 to 70 major peaks which were evaluated. The relative retention times, M+ values and in most cases the assignments to defined compounds are given. Many ‘unknown’ substances could be identified as condensation products derived from benzyl radicals. Flow diagrams illustrate the main splitting mechanisms of compounds A and B. Aryl ethers; gas chromatography; thermal degradation mechanisms.

lignin model

compounds;

mass spectrometry;

pyrolysis;

INTRODUCTION

Lignins accompany the polysaccharides and are considered to be the second most abundant natural polymers produced by terraneous plants. Lignins have complicated structures and their analysis is important both from a scientific and a technical point of view [l]. There are many tech0165-2370/88/$03.50

0 1988 Elsevier

Science Publishers

B.V.

136

niques for lignin characterization by means of chemical and thermal degradation as well as degradation-free spectroscopical methods [1,2]. Pyrolytic (Py) methods have been also applied for lignin characterization [3-121 in combination with gas chromatography (GC) and/or mass spectrometry (MS). It would be desirable to develop these rapid microanalytical tools to standard analytical techniques for lignin analysis. In the direct Py-MS approach the sample is pyrolyzed in the ion source and mass fragments evolved are detected preferably by soft ionization techniques - such as field ionization, field desorption, chemical ionization and fast atom bombardment - to get high molecular fragments without further degradation. These techniques have not been applied frequently for lignin analysis [ll], although its analytical power has been demonstrated successfully on humic substances and coal [12]. Reproducible ‘fingerprints’ of the samples can be obtained but it is difficult to identify the structure of the fragments. Py-thermogravimetry-MS was described for the detection of dynamic processes in the low mass range (m/z < 54) of thermal degradation products [12]. However, the direct (‘on-line’) combination of Py with capillary GC-MS is the most common analytical device for lignin pyrolysis. In contrast to the Py-MS approach, Py-GC-MS supplies additional information about nature and thermal fragments of pyrolyzed substances. Furthermore, GC retention times are valuable parameters to identify unknown compounds. Even quantification of single lignin degradation products is possible using the ‘off-line’ approach [6], so that semi-quantitative classification of lignins can be made. Usually, electron impact energy at 70 eV is used for ionization but soft photoionization at about 11 eV energy has been applied as well [8]. The merit and limitations of analytical pyrolysis of natural polymers, including lignins, are described in the literature [15,16]. The advantages are: simplicity of sample preparation and rapidity of performance. On the other hand results from pyrolysis studies are influenced by the Py condition (kind of heating, geometry of the pyrolysis chamber, heat capacity of purging gas, temperature, heating rate). Even small details in the physical state of the samples (powder, film, thermal history, amount and thickness etc.) are important. The great number of degradation products is a disadvantage in routine applications. In lignin pyrolysis the complexity of thermal degradation processes requires a step by step approach starting the studies with small molecules and advancing to larger ones. Therefore, in this paper the Py-GC-MS at different Py temperatures of two trimeric lignin model compounds is described, which represent the frequent alkyl-aryl ether linkages between monomeric units in softwood lignins (model A) and hardwood lignins (model B). The guaiacyl nuclei (G units) in both models possess ethyl side-chains for the sake of simplicity (rather than propyl side-chains in lignins) with a carbonyl group adjacent to the aromatic ring. In both cases

137

the phenolic OH groups are etherified with a benzyl group. Moreover, in model A the G unit is etherified with another G and in model B with a syringyl (S) nucleus. Benzyl groups are usually not present in lignins. Benzylation is frequently used, however, to label or protect free phenolic hydroxyl groups. The model compounds A and B represent typical lignin structures, in which one aromatic nucleus is etherified with two other units. Ether linkages with benzyl groups increase the transparency of the fragmentation mechanisms because they can easily be differentiated from G and S units in the pyrogram.

EXPERIMENTAL

Materials Compounds A and B were obtained by reaction of w-bromoacetoguaiacone-benzylether with guaiacol or syringol, respectively [17]. Recrystallization from ethyl acetate-ethanol. M.p. A: 104.8” C (lit. 103-104” C); m.p. B: 85.9’C. The elemental analytical data correspond well to the theoretical values. Py-GC-MS

experiments

A platinum coil probe was used for pyrolysis together with a quartz tube containing ca. 0.25 mg substance. To control pyrolysis a Chemical Data System Pyroprobe 100 was used. With thermocouple temperature measurements and repeated calibrations the relation between nominal and real temperature in the quartz tube was established. Experiments were carried out at different temperatures but the ‘ramp off’ position of the instruments was always maintained, corresponding to approx. 600 ms heat-up time of the coil. The heating rate within the tube was not determined. In experiments at temperatures below 280’ C the interface and injector temperature was also lowered to the corresponding pyrolysis temperature. However, in experiments over 280” C the interface and injector block temperature is maintained at 280” C. A disadvantage of our equipment is that samples are exposed for 5 s to the interface temperature before pyrolysis started. Helium was used as carrier gas to sweep volatile products from the interface onto the fused silica column. The pyrolysis time was set to 10 s and the purging time of the interface to 30 s (including pyrolysis time). Gas chromatograph: Carlo Erba Fractovap 4160 with a split injector and a split ratio of 1 : 20. Column: 30 m fused-silica, DB 5 (0.25 pm film thickness; J & W Scientific). Temperature programme: 45 o C isothermal for 4 min; 4O C/min up to 280 o C. Carrier: helium with linear velocity of 32 cm/s. Electron impact mass spectra were obtained on a Kratos MS 25 at 70

m

6 a

9

5’

139

Fig. 3. Mass chromatograms showing the low temperature degradation of compound B observed in the ion source. The m/z values presented are typical for the degradation products Nos. 28, 65, 36, and 62 (for substance names see Table 2).

Mettler TA 3000 with a DSC 20 standard cell (purged with nitrogen before an experiment) controlled by a TC 10A processor was used. Heating rate: 1 o C/ min. The DSC curves are depicted in Fig. 2.

Pyroly& in the ion source (Py-MS) Pyrolysis experiments were also carried out in the ion source using a direct insertion probe to investigate the beginning of thermal degradation at low temperatures. The temperature of the ion source was set to 50°C and compound B was heated from 100 to 37O’C at about 40”C/min heating rate. The m/z values, whose intensity profiles are presented in Fig. 3, were selected according to the results of Py-GC-MS experiments.

RESULTS

AND DISCUSSION

The mass spectra of compounds A and B and the origin of the fragments with positive charge are depicted in Fig. 1. To get first orientation about the beginning and the course of thermal degradation DSC curves were recorded. However, one should be aware of the fact that DSC with low and Py with extremely high heating rates are obeying different laws. The same is true for vacuum pyrolysis in the ion source. The results of these experiments can not be compared directly. According to the DSC curves in Fig. 2 thermal changes occur even below 200” C. The intense exotherm peaks around 450°C indicate the most important thermal changes leading to the degradation of the molecules. Py-GC-MS experiments were carried out a different temperature levels and the results are presented in Table 1. At 200” C no degradation products could be detected. At 240” C, however, in both cases four degradation fragments appeared in the pyrogramme: guaiacol, 2-methoxybenzaldehyde, vanillin-benzylether, and acetoguaiaconebenzylether from compound A; syringol, 2,6-dimethoxybenzaldehyde, vanil-

140

141

lin-benzylether and acetoguaiacone-benzylether from compound B. As is shown in Table 1 the intensity of the first mentioned two compounds is essentially higher than those of the last ones. At 28OOC the same compounds are visible but toluene also appears in traces as a result of the splitting of benzyl ethers. At 380” C the yields of toluene, vanillin-benzylether and guaiacol-benzylether increase for both compounds. There are only gradual differences between the pyrograms of substances A and B up to this the peaks of vanillin-benzylether and temperature level. For example acetoguaiacone-benzylether are more pronounced in the case of compound B. Furthermore, the comparison of the intensities in Table 1 demonstrates the reduction in yields of the major degradation products combined with the occurrence of low weight, less specific degradation products. Above 280 OC the recombination of benzyl radicals begins giving rise to 1,2-diphenylethane (dibenzyl). At temperatures around 380” C considerable amounts of this compound can be observed. The first four degradation products arising between 240 and 280” C indicate unequivocally the weakest bonds of these molecules. First, the bond between the aromatic oxygen of nucleus 2 and carbon p of the side chain of unit 1, and second, the linkage between the carbon atoms cr and /? of the side chain of nucleus 1, are equally prone to thermal rupture. In the latter case a rearrangement is proposed to explain the formation of the compounds 2-methoxybenzaldehyde (no. 23 in Table 1) and 2,6_dimethoxybenzaldehyde (no. 36 in Table 1) even at very low temperatures. The structure of compound no. 23 is proved by CC-MS of an authentic compound. In the case of compound 36 the proof is indirect: 3,5- and 2,3_dimethoxybenzaldehydes as authentic reference substances have different retention times on CC but very similar mass spectra to that of compound 36. The mechanism of this rearrangement is not yet known. The presence of these aldehydes in the degradation mixture is unexpected. They are not detectable in lignin pyrograms, indicating that structures without side-chain, as the nuclei 2 of compounds A and B, are not present in lignins. The thermal fragmentation of compound B in vacuum investigated by Py-MS is depicted in Fig. 3. The mass chromatograms display the typical molecular ions and fragments of 2,6_dimethoxybenzaldehyde + vanillinbenzylether and syringol + acetoguaiacone-benzylether which appear simultaneously at about 170°C. Although the thermal behaviour of substances in vacuum Py and Py under normal pressure is not exactly the same, this finding is a corroboration for the low temperature Py-GC-MS experiments: The two splitting mechanisms between nucleus 1 and 2, as described above, are equally probable and occur in vacuum pyrolysis below 200 o C. As a consequence of the tendencies described in Table 1, Py-GC-MS studies at 600°C display a further increase of unspecific degradation products. The yields and number of recombination products increase as well. The pyrograms are depicted in Fig. 4 and the major products are listed

142

R.T.

, lrY3-l’

lmm

secl

zz:58

-

a3

FACTOR

.6

’ FACTOR

B

RT

[mm

“

Pl8

’ FACTOR

’ FACTOR

s la

L9

secl-

Fig. 4. Total ion pyrograms from compounds Factor = gain for better peak presentation.

!,: 57

A and B. Pyrolysis

I5’ 26 temperature:

600 “C.

in Table 2. According to Fig. 4 and Table 2 up to seventy peaks (69 from B and 42 from A) of the pyrograms were analyzed. Relative retention times of the total ion current peaks are presented with guaiacol as reference peak (relative retention time = 1). These data can be helpful to identify compounds without mass spectrometer. Additionally, the retention times of n-alkanes are presented as time markers to simplify orientation. in similar pyrograms (if repeated elsewhere). These alkanes would be appear at the given retention times in the chromatogram, if the separation would be realized under the same chromatographic conditions as described in this paper, and thus, can support the orientation. The identification of substances marked with * * is assured by authentic compounds. The structure of compounds marked with * is plausible based

143

TABLE 2 Data from the pyrograms Peak No.

Relative retention time (guaiacol = 1)

0.22 0.39 0.46 0.50 0.53 0.64 0.73 0.79 0.87 0.92 1.00 1.03 1.04 1.10 1.13 1.16 1.20 1.23 1.24 1.24 1.29 1.29 1.31 1.33 1.37 1.40 1.44 1.45 1.54 1.56 1.57 1.67 1.75 1.78 1.83 1.84

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1.88 1.94 1.96 1.97 2.02 2.07 2.08

35 36 37

-

Alkanes

in Fig. 4 Total ion current int. (W) A

Symbol

M+’

Compound

**

B 73 28 23

100 19 15

** ** **

78 92 106 104

Benzene Benzene, methylBenzene, methylBenzene, vinyl-

<5 17 42

14 7 26

** ** t*

108 106 94

Benzene, methoxyBenzaldehyde Phenol

72 30 59

9 23 21

** ** **

122 108 124

Benzaldehyde, 2-hydroxyPhenol, 2-methylGuaiacol

7

** * ** ** ** ** * ?

122 136 122 122 138 128 136 218

Phenol, 2,6-dimethylBenzaldehyde, hydroxymethylPhenol, 2-ethylPhenol, 2,4-/2,5-dimethylBenzene, 1,3-dimethoxyNaphthalene Phenol, methylethylImpurity from a synthesis step

** *t * ** ** ** t

136 110 150 124 136 140 124

Benzaldehyde, 3-methoxyCatechol (Benzene, 1,2-dihydro-) Phenol, C4Benzene, I-methoxy-3-hydroxyBenzaldehyde, 2-methoxyGuaiacol. 4-hydroxyCatechol, methyl-

9 6 29

* *t

152 152 154

Benzaldehyde, Unknown Syringol

9 7 11

** ** **

152 168 164

**

Vanillin Diphenylmethane Acetophenone, 3,4_methylenedioxyAcetophenone, 4-hydroxyUnknown Acetoguaiacone

c9

Cl0

Cl1 15 6

9 <5 14 8 15

13 Cl2 13

22 12 10 <5

65 <5 (5 Cl3

hydroxymethoxy-

Cl4 <5 23 15 18

<5 20

t*

136 164 166

42 25 28

** * **

182 166 150

Cl5 100

Dibenzyl (Ethane, 1,2-diphenyl-) Benzaldehyde, 2,6-dimethoxyAcetophenone. 4-hydroxy3-methyl(Continued

on p. 144)

144 TABLE 2 (continued) Peak No.

38

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Relative retention time (guaiacol = 1)

Alkanes

A

2.52 2.53 2.55 2.57 2.60 2.63 2.65 2.67 2.72 2.73 2.74 2.77 2.80 2.87 2.92 2.93 3.08 3.16 3.21 3.22 3.23 3.26 3.30 3.37 3.45 3.46 3.52 3.59 3.63

M+’

Compound

168/ 180

Overlapping

164 164 198 184

Spectrum similar to peak Nos. 31,40 Spectrum similar to peak Nos. 31, 39 Unknown Diphenylmethane, 2-hydroxy-

180 194 198 184 214 208/ 210 198 194

Phenanthrene, 9,10-dihydroRecombination product Unknown Diphenylmethane, 4-hydroxyGuaiacol-benzylether Overlapping spectrum

212 198 212 214 214 200/ 212 226

Recombination product Recombination product Recombination product Recombination product Recombination product Overlapping spectrum

226 228 196

Unknown Guaiacol, 4-methyl-benzyletherCH,OH-CO-veratrol

242 242 272 254

Benzophenone, 2,6-dimethoxyVanillin-benzylether Recombination product Unknown

256 254 240/ 256 256

Acetoguaiacone-benzylether Unknown Overlapping spectrum

286

Unknown

288 288

Acetoguiaiacone, Unknown

B <5

2.16 2.17 2.18 2.20 2.25 2.34 2.37 2.39 2.42 2.44 2.48 2.49 2.50

Symbol

Total ion current int. (W)

spectrum

Cl6 <5 15 14

<5 <5 8

*lt

14

t*

7 <5 ~5 <5

+* t

Cl7 19 <5

25

21 5

11 <5

**

Phenol, 4-methyl-benzyletherRecombination product

Cl8 <5 <5 5 6

<5 5 <5 5 10 (5 7

Unknown

Cl9 <5 <5

11 (5

* *

(5

* *

c20 19 <5 5 c22 27 5 <5

6* <5 8

<5

5

Recombination

product

C23 (5 C24 14

* <5

w-guaiacoxy-

145 TABLE 2 (continued) Peak No.

Relative retention time (guaiacol = 1)

Alkanes

Total ion current int. (W) A

Symbol

M+’

Compound

288/ 304 9

Overlapping

B

12

3.66

15

73 74 75 76

3.68 3.68 3.15 3.78

<5 <5 (5 (5

? *

77 78 79 80 81

3.80 3.84 4.06 4.13 4.50

4 <5

? ? 3

<5

? **

(5

* Structure evident according to mass spectra; uct of the benzyl radical. See text. l * Assured by authentic compounds.

270 288 302,’ 304 7 ? 1

? 378

“recombination

spectrum

Incomplete spectrum CHO-CO-G-O-benzene Unknown Overlapping spectrum Incomplete spectrum Incomplete spectrum Plasticizer Incomplete spectrum Model compound A product”

= reaction

prod-

on mass spectra. The mass spectra of compounds designated as “recombinarefer to unknown structures. According to comparative tion products” studies with many model compounds it is obvious that these peaks appear only in the pyrograms when benzyl ether linkages are present in the compounds. Hence, they are recombination product of benzyl radicals with other splitting moieties of the molecule. Of course, for analytical purposes the low temperature pyrolysis studies are more valuable than degradations at higher temperatures. Nevertheless, listings, as presented in Table 2, can be justified by the following: Lignin macromolecules are frequently pyrolyzed up to temperatures around 600 o C in order to obtain high yields of valuable pyrolysis products. Ideal conditions, which avoid the formation of unspecific low weight products, tar, charcoal and secondary recombination products (condensation products), can only be approximated but never realized. Therefore, it is important to study pyrolysis also under realistic (technically feasible) conditions and identify products formed at higher pyrolysis temperatures. Thus, pyrolysis of well defined low molecular compounds makes assignments to degradation products much easier. The peak intensities in pyrograms of compounds A and B at 620 o C differ considerably from each other. The intensities presented in Table 2 are normalized to the highest peak ( = 100) in the pyrogram (1,2-diphenylethane for compound A and toluene for compound B). These values are presented to give a rough estimation of the quantitative distribution of degradation products. So, the course of pyrolytic degradation can be studied by comparison of the relative peak intensities in Tables 1 and 2.

146

REARRANG

al

7 OH

Fig. 5. Flow diagram about origin and yield of 21 to 25 thermal degradation compounds A and B. The first (upper) numerals refer to substance numbers (lower) ones to peak intensities from Table 2.

products from and the second

Table 2 and the pyrograms from Fig. 4 demonstrate the fingerprinting ability of Py-GC-MS: The two compounds differ only by one methoxyl group and this difference has a profound effect on the type and amount of degradation products. Repetitive experiments under the same conditions proved the good reproducibility of this kind of fingerprinting. The peak heights oscillate within 5% relative intensities. The flow diagrams in Fig. 5 display possible splitting routes. It is not possible to construct a complete degradation scheme of compounds A and B at a pyrolysis temperature of 600 o C because there are too many possibilites and intermediates inclusive ramifications and alternatives of the degradation route. In Fig. 5 only 20 to 25 of the most important cleavage products are presented in a form, that allows the assignment of a degradation product to a structural detail in the mother compound. Relevant sub-structures of compounds A and B for thermal cleavage are framed. The most probable thermal cleavages - observed by thermolysis below 380” C, as discussed

147

above - are indicated in the diagram with the thickest frames, and the less frequent ones with dashed lines. The formulas of the degradation products are marked with two numbers which refer to the substance numbers (upper or first numeral) and the intensities (lower or second numeral) listed in the Table 2. Arrows lead from framed sub-structural details to the most important degradation products. Hence, the differences and similarities between the degradation patterns of compounds A and B are visible at a glance. Less specific products, such as phenol, catechol, anisole, dihydrophenanthrene, and naphthalene, are located on the right side of the diagram behind the big parenthesis, indicating that these products could have arised from nearly all sub-units and degradation intermediates.

CONCLUSIONS

Py-GC-MS of low molecular weight lignin model compounds should be done for structural studies preferably at the lowest possible temperatures. At higher temperatures many unspecific degradation products and secondary condensation products are formed. Nevertheless, the high-temperature pyrograms can be used for fingerprinting. Lignins with benzylated phenolic OH groups would probably also give rise to toluene and recombination products of them. Therefore, for the distinction of phenolic OH groups other kind of labelling is recommended (e.g. methylation with diazomethane). According to the results in this paper Py-GC-MS could contribute important data for structural elucidation of oligomeric lignin fragments. Our knowledge of the relation between pyrograms and structural details in lignins will become probably deeper by further studies and with model compounds whose similarity with lignins is higher. Direct comparison of pyrolytic data from model compounds A and B and lignins is not possible. The formation of 2-methoxyand 2,6-dimethoxybenzaldehyde from these compounds is an evidence for differences to pyrolytic degradation of lignins.

REFERENCES 1 K.V. Sarkanen and C.H. Ludwig (Editors), Lignins - Occurrence, Formation, Structure and Reactions, Wiley-Interscience, New York, London, Sydney, Toronto, 1971. 2 B.L. Browning, Methods of Wood Chemistry, Vol. II, Interscience, New York, London, Sydney, 1967. 3 J.R. Obst, J. Wood Chem. Technol., 3 (1983) 377. 4 J.R. Obst and L.L. Landucci, J. Wood Chem. Technol., 6 (1986) 311. 5 C. Saiz-Jimenez and J.W. de Leeuw, Org. Geochem., 6 (1984) 417. 6 0. Faix, D. Meier and I. Grobe, J. Anal. Appl. Pyrolysis, 11 (1987) 403. 7 0. Faix and D. Meier, Proceedings of the Fourth International Symposium on Wood and Pulping Chemistry, April 27-30, 1987, Paris, France.

148 8 9 10 11 12 13 14

W. Genuit, J.J. Boon and 0. Faix, Anal. Chem., 59 (1987) 508. J.J. Boon, A.D. Pouwels and G.B. Eijkel, Trans. Biochem. Sot., 15 (1987) 170. A.D. Pouwels and J.J. Boon, J. Wood Chem. Technol., 7 (1987) 197. J. Metzger, Fresenius Z. Anal. Chem., 295 (1979) 45. 0. Faix, E. Jakab, F. Till and T. SzCkely, Wood Sci. Technol., 22, No. 4 (1988) in press. H.-R. Schulten, J. Anal. Appl. Pyrolysis, 12 (1987) 149. W.J. Irwin, Analytical Pyrolysis, A Comprehensive Guide, Marcel Dekker, New York, Basel, 1982. 15 K.J. Voorhees (Editor), Analytical Pyrolysis, Techniques and Applications, Butterworth, London, Boston, 1984. 16 K. Kratzl, W. Kisser, J. Gratzl and H. Silbemagel, Monatsh. Chem., 90 (1959) 771.