Pyrolysis of Aldehydes and Ketones

C H A P T E R

10 Pyrolysis of Aldehydes and Ketones

S U B C H A P T E R

10.1

Aldehydes

GENERAL ASPECTS Aldehydes are carbonyl compounds with the general formula RdCH]O, where R is a hydrocarbon radical. The carbonyl functional group >C]O is bonded in aldehydes to a hydrogen atom and a carbon atom. Several naming procedures are used for aldehydes. A standard procedure starts with the name of the longest carbon chain containing the aldehyde group and replacing the suffix -e with -al (e.g., methanal, ethanal, etc.). It is also common to name the aldehyde starting with the name of the acid from which the aldehyde derives (by the replacement of the COOH group with the CHO group). This is done by changing the suffix -ic acid or -oic acid with -aldehyde (e.g., formaldehyde from formic acid, acetaldehyde from acetic acid, etc.) In other procedures, the CHO group is indicated by the suffix carboxaldehyde or by the prefix oxo-. The aldehydes are very common compounds that are found in nature and synthesized for numerous applications. The aldehyde group can be present in more complex molecules,

Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00010-X

391

# 2019 Elsevier B.V. All rights reserved.

392

10. PYROLYSIS OF ALDEHYDES AND KETONES

including carbohydrates (where the CHO group is free or present in acetal form). Due to the importance and complexity of carbohydrates, their pyrolysis is discussed separately in Chapter 11. The aldehydes are the result of many oxidation processes, including the oxidation of carburants that takes place in flames. Pyrolysis of aldehydes has been the subject of numerous studies.

FORMALDEHYDE Formaldehyde (HCHO or CH2O) starts to decompose at temperatures above 450°C. Pyrolysis performed in the temperature range 466–516°C showed that the decomposition reaction has approximately a second-order rate [1]. The main pyrolysis products are CO and H2, and the reaction can be written as follows: HCHO ! CO + H2 ΔG ¼ 0:47  0:026 T kcal=mol

(10.1.1)

In addition to CO and H2, lower levels of methanol also were detected in the pyrolysate. The level of methanol is higher at lower temperatures of pyrolysis. Among the compounds that were not formed by pyrolysis at lower temperatures were CO2, formic acid, glyoxal, dimethyl ether, and methane. At lower temperatures, the initiation reaction appears to be the following: 2 HCHO ! H2 C  OH + CHO ΔG ¼ + 58:0  0:007 T kcal=mol

(10.1.2)

The rate constants for the formation of CO, H2, and CH3OH in the temperature range 466– 516°C were found to be described by the following Arrhenius equations (cc is cubic centimeter):
kCO ¼ 10+11:80:4 exp ½ð34; 400  1300Þ=RT  cc0:8 = mol0:8 s
kH2 ¼ 10+8:71:0 exp ½ð34; 000  3600Þ=RT  cc0:6 = mol0:6 s

(10.1.4)

kCH3 OH ¼ 10+12:60:9 exp ½ð36;800  3600Þ=RT cc=ðmol sÞ

(10.1.5)

(10.1.3)

The variation of CO yield at 516.6°C and different CH2O pressures as a function of decomposition time are shown in Fig. 10.1.1. As temperature increases, methanol is formed only in traces, and in the range between 930°C and 1730°C, the initiation takes place by the following reaction (M is an inert molecule): HCHOð+MÞ ! H + CHO ð+MÞ

(10.1.6)

This reaction is followed by propagation reactions as follows: HCHO + H ! CHO + H2

(10.1.7)

CHO ð+MÞ ! CO + H ð+MÞ

(10.1.8)

393

FORMALDEHYDE

FIG. 10.1.1 The variation of CO yield at 516.6°C and different CH2O pressures as a function of decomposition time [1]. The pressure is expressed in Torr.

30

CO (mol 10–6)

25 50 Torr

20

71 Torr 115 Torr

15

143.5 Torr

10

162 Torr

5 0 0

1

2 3 Time (s)

4

5

and by the termination reactions: CHO + H ! CO + H2

(10.1.9)

CHO + CHO ! CH2 O + CO

(10.1.10)

The parameters for the Arrhenius equation in the form k ¼ ATn exp( E#/RT) (units: cm, mol, cal, K) for the reactions (10.1.6)–(10.1.10) are given in Table 10.1.1 [2,3]. Some other studies were performed regarding the pyrolysis of formaldehyde. These include shockwave experiments [4,5], modeling studies [6], laser heating pyrolysis [7], etc. Oxidation of formaldehyde has been studied in various conditions [8,9]. The oxidation starts with the initiation reaction of the type: CH2 ¼ O + O2 ! CHO + HOO

(10.1.11)

The free radicals formed in these reactions or directly from oxygen that can generate under the influence of heat-free O: atoms continue with propagation reactions that generate CO, CO2, water, and other free radicals, as shown in the following examples: CH2 ¼ O + O : ! CHO + OH 

CH2 ¼ O + HOO ! CO + H2 O + OH

(10.1.12) 

(10.1.13)

CO + HOO ! CO2 + OH

(10.1.14)

CH2 ¼ O + OH ! CHO + H2 O

(10.1.15)

Specific values for the kinetic parameters for these reactions are reported in the literature [2]. When sufficient oxygen is present, the overall result of the process can be written as follows:

394

10. PYROLYSIS OF ALDEHYDES AND KETONES

TABLE 10.1.1 The Parameters for the Arrhenius Equation in the Form k ¼ ATn exp(E#/RT) (units: cm, mol, cal, K) for the reactions (10.1.6)–(10.1.10) [2,3] n

E#

5.54  10+15

0

75,000

(10.1.7)

2.18  10

1.77

3000

(10.1.8)

6.95  10

1

17,000

(10.1.9)

2.16  10

0

0

(10.1.10)

4.50  10

0

0

Reaction

A

(10.1.6)

+8 +17 +14 +13

CH2 ¼ O + O2 ! CO2 + H2 O

(10.1.16)

At lower temperatures, reaction (10.1.16) can take place without the formation of CO as an intermediary species [9].

ACETALDEHYDE Pyrolysis of acetaldehyde at temperatures higher than 725°C generates mainly CO and CH4 with lower levels of H2, C2H6, and C2H4. The results obtained by performing pyrolysis in a turbulent flow reactor at 778°C using CH3CHO at a level of about 1% in nitrogen at different exposure times are shown in Fig. 10.1.2 [10]. Traces of acetone, propane, propene, and acetylene also were detected in the pyrolysate above 725°C. The pyrolysis process starts with the following initiation reaction: H3 C  CH ¼ O ! CH3  + CHO ΔG ¼ + 84:8  0:037 T kcal=mol

(10.1.17)

–7

Log conc (mol/cc)

–7.5 C2H4

–8

C2H6 H2

–8.5

CH4

–9

CO

–9.5 –10 0.1

0.12

0.14

0.16

0.18

0.2

Time (s)

FIG. 10.1.2 Results for pyrolysis of acetaldehyde in a turbulent flow reactor at 778°C using CH3CHO at a level of about 1% in nitrogen at different exposure times [10].

395

ACETALDEHYDE

At temperatures in the range of 525–725°C, the Arrhenius equation for reaction (10.1.17) was found to be [10]: k ¼ 10+15:850:2 exp ½ð81; 775  1000Þ=RT

(10.1.18)

The process continues with various propagation reactions, as shown below: CH ¼ O ð +MÞ ! CO + H ð +MÞ

(10.1.19)

H + H3 C  CH ¼ O ! H2 + CH3  C ¼ O

(10.1.20)

CH3  + H3 C  CH ¼ O ! CH4 + CH3  C ¼ O

(10.1.21)

CH3  C ¼ O ð +MÞ ! CO + CH3  ð +MÞ

(10.1.22)

Also, termination reactions lead to the formation of CO, H2, CH4, and C2H6, as shown below: CH ¼ O + H ! CO + H2

(10.1.23)

2CH3  ! C2 H6

(10.1.24)





CH3 + CH ¼ O ! CH4 + CO

(10.1.25)

At higher temperatures, acetaldehyde pyrolysis has been evaluated using shock-tube experiments with partial pressures in the range of 40–500 Torr and temperatures in the range of 1275–2125°C [11–13]. The diluting gas used in the experiment was krypton. With reaction (10.1.17) being unimolecular, a limiting high pressure rate constant k∞ as well as a low pressure rate constant k0 were determined. The values for these constants are given in Table 10.1.2. The parameters for the Arrhenius equation in the form k ¼ ATn exp( E#/RT) for the reactions (10.1.19)–(10.1.25) also are given in Table 10.1.2 [11]. Other studies also were performed on acetaldehyde pyrolysis and are reported in the literature [14–16]. TABLE 10.1.2 The Parameters for the Arrhenius Equation k ¼ ATn exp(E#/RT) for the Reactions (10.1.17)–(10.1.25) [11] Log A

n

E# (kcal/mol)

22.632

1.88

85.48

76.346

11.81

95.04

(10.1.19)

13.602

0

15.535

(10.1.20)

13.375

0

3.642

(10.1.21)

3.150

4.58

1.966

(10.1.22)

15.780

0

14.070

(10.1.23)

?

0

0

(10.1.24)

13.301

0

0

(10.1.25)

13.023

0

0

Reaction (10.1.17) k∞ (10.1.17) k

0

Note: “?” indicates unknown value.

396

10. PYROLYSIS OF ALDEHYDES AND KETONES

BENZALDEHYDE Benzaldehyde decomposes at temperatures above 500°C with the formation of CO and benzene in a reaction, as shown below: H5 C6  CH ¼ O ! C6 H6 + CO ΔG ¼ +1:99  0:003 T kcal=mol

(10.1.26)

At higher temperatures, besides CO and benzene, small amounts of H2, biphenyl, and char are generated. At lower temperatures, in the range of 350–370°C, pyrolysis in a glass-sealed tube of benzaldehyde leads to the formation of benzyl benzoate in a reaction similar to a Cannizzaro disproportionation:

ð10:1:27Þ

A Cannizzaro reaction typically takes place in a basic medium with the formation of an anion, as shown below:

ð10:1:28Þ

In the pyrolysis process, with no OH added (by purpose), it is possible that either impurities or the glass walls of the pyrolysis vessel serve as a source of OH anions. Also, because an autocatalytic effect on this reaction was proven for benzyl alcohol, the formation of small amounts of this compound may be the reason for the formation of benzyl benzoate. In this case, the reaction takes place as shown below:

ð10:1:29Þ

where the benzylate ion is formed from the dissociation of benzyl alcohol.

OTHER SIMPLE ALDEHYDES

397

OTHER SIMPLE ALDEHYDES Propionaldehyde decomposes upon heating above 500°C, mainly with the formation of CO and ethane (in a similar reaction with that of acetaldehyde). Relative to the composition of acetaldehyde pyrolysate, the levels of hydrogen and methane are higher because of a higher extent of side reactions. At temperatures between 450 and 600°C, the reaction has a unimolecular mechanism, and at higher temperatures the pyrolysis process becomes more complex with an increase in the methane yield. Pyrolysis of some other aldehydes that generate stable hydrocarbons decompose similarly to acetaldehyde with a few side reactions [17]. This is the case, for example, for 2,2,2triphenylethanal, which generates CO and triphenylmethane, and that of cinnamic aldehyde, which generates mainly CO and styrene (with lower levels of benzene, acetylene, ethylene, and distyrene [18]). On the other hand, pyrolysis of 2-methylpropanal (isobutyraldehyde) around 600°C, besides producing CO, does not primarily form propane, as expected. A mixture of gases is generated, including H2, CH4, C2H6 as the main components, and also some propene and ethylene [17]. Similarly, 3-methylbutanal (isovaleraldehyde) forms by pyrolysis CO, CH4, H2, C2H6, and a mixture of other saturated and unsaturated hydrocarbons. The formation of free radicals by reactions similar to reaction (10.1.17) and further interactions of these free radicals explain the result of the process. Acrolein (2-propenal) also generates by pyrolysis at 530°C in a static system a mixture of products including CO, H2, CH4, C2H6, C2H4, C4H8, and other hydrocarbons. For example, pyrolysis of acrolein diluted in the mole ratio 2:1 with toluene or with benzene generated by pyrolysis the results shown in Table 10.1.3. These results are expressed in % mole product/mole acrolein reacted [19]. In addition to the compounds listed in Table 10.1.3, acetylene, propyne, propane, 2butenes, isoprene, cyclohexane, and dihydropyran were detected in the pyrolysate. TABLE 10.1.3 Composition of Acrolein Pyrolysate Expressed in % Mole Product/Mole Acrolein Reacted [19]

Compound

Acrolein in Toluene Heating Time 4 s, Temp. 600°C

Acrolein in Benzene Heating Time 9.8 s, Temp. 600°C

CO

86–97

100

C2H4

22–24

20

C3H6

8.7–9.2

12

H2

7.2–8.3

2

CH4

5.4–6.2

2

C2H6

3.3–4.4

3

1,3-Butadiene

3.4

2

1-Butene

2.3–3.0

3

C3H8

–

2

398

10. PYROLYSIS OF ALDEHYDES AND KETONES

The initiation of acrolein decomposition is similar to that for acetaldehyde and is described by the following reaction: ð10:1:30Þ The reaction is followed by propagation reactions of the type: ð10:1:31Þ ð10:1:32Þ ð10:1:33Þ The free radicals formed in these reactions are the source of the compounds listed in Table 10.1.3 and of other trace compounds found in acrolein pyrolysate. In the presence of oxygen, the pyrolysis of acrolein generates similar compounds as pyrolysis in a nonoxidizing atmosphere, plus some acetaldehyde and water. The formation of C2 H3  free radicals during acrolein pyrolysis and their reaction with O2 may be the source of acetaldehyde. Other free radicals such as OH• and OOH• may also play a role in this process.

HYDROXYALDEHYDES Hydroxyaldehydes are common compounds, with the class of carbohydrates being included in this type of compound. Pyrolysis of carbohydrates is discussed separately in Chapter 11. Even excluding carbohydrates, numerous other compounds have both the OH and the CHO functionalities. These functionalities can be positioned on the same carbon (e.g., glycolaldehyde), or they can be in β, γ, etc., position. Glycolaldehyde (or hydroxyacetaldehyde) is not stable to heating and, when kept around 100°C for several hours, undergoes a condensation of the type:

n

ð10:1:34Þ

At higher temperatures, glycolaldehyde decomposes with the formation of various small molecules, including CO and CH3OH. Lactaldehyde by low temperature heating forms some acetol and at higher temperatures generates various decomposition products, including CO, ethanol, crotonaldehyde, etc. Pyrolysis of glycolaldehyde in the presence of tetramethylammonium hydroxide (TMAH) shows that the substance undergoes an aldol condensation in a strong basic medium provided by TMAH. The resulting condensation products include C4, C5, and C6 carbohydrates. Further reactions during thermally assisted hydrolysis and methylation generate compounds

399

ALDEHYDES CONTAINING HYDROXY AND ETHER GROUPS

similar to those obtained from glucose, which include several methylated metasaccharinic (deoxyaldonic) acids [20] (see Chapter 11). β-Hydroxyaldehydes easily eliminate water by mild heating (around 165°C), with a typical reaction being the transformation of aldols in homologs of crotonaldehyde, as shown below for aldol (3-hydroxybutyraldehyde): ð10:1:35Þ The reaction is also associated with the decomposition and formation of acetaldehyde by a retro-aldol condensation (see Subchapter 1.2). Heating at higher temperatures leads to decomposition with the formation of small molecules, including CO, CH4, small chain alkanes, alkenes, etc. Other aldols undergo reactions similar to reaction (10.1.35) upon mild heating, and at higher temperatures suffer decompositions with the formation of H2O, CO, and small chain hydrocarbons [21].

ALDEHYDES CONTAINING HYDROXY AND ETHER GROUPS Compounds with various functional groups such as alcohol, ether, and aldehyde are common in nature. For example, 4-hydroxy-3-methoxybenzaldehyde (vanillin) contains aldehyde, alcohol, and ether groups. This compound is present in nature and also synthesized as a flavorant. Pyrolysis of a sample of vanillin was performed on a 0.50 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 pyrolysate was done by GC/MS under the conditions given in Table 1.4.1. The window between 36.0 and 60.0 min from the pyrogram is given in Fig. 10.1.3, and the compound Abundance

52.94

O 41.10

3e+07 2.5e+07 2e+07

O OH

41.55

1.5e+07

CH3 55.66 50.72

1e+07

54.78

39.95 39.53

50,00,000

57.25

49.16

43.84

36.45 0 Time-->

36.0

38.0

40.0

42.0

44.0

46.0

48.0

50.0

52.0

54.0

56.0

58.0

FIG. 10.1.3 Window 36–60 min of the pyrogram for 4-hydroxy-3-methoxy benzaldehyde (vanillin).

60.0

400

10. PYROLYSIS OF ALDEHYDES AND KETONES

TABLE 10.1.4 Identification of the Main Peaks in the Chromatogram Shown in Fig. 10.1.3 for the Pyrolysis of 4-Hydroxy-3-Methoxybenzaldehyde (Vanillin) Retention Time (Min)

CAS#

Moles % Pyrolysate

44

124-38-9

2.73

5.86

54

106-99-0

0.50

9.55

66

542-92-7

0.23

Benzene

17.88

78

71-43-2

0.20

5

Toluene

24.41

92

108-88-3

0.31

6

Styrene

31.26

104

100-42-5

0.20

7

2-Cyclopenten-1-one

31.75

82

930-30-3

0.22

8

Benzofuran

36.45

118

271-89-6

0.88

9

2-Hydroxybenzaldehyde

39.53

122

90-02-8

3.96

10

Phenol

39.95

94

108-95-2

7.49

11

2-Methoxyphenol

41.10

124

90-05-1

24.70

12

2-Methylphenol

41.55

108

95-48-7

12.92

13

2,3-Dimethylphenol

42.34

122

526-75-0

0.23

14

4-Methylphenol

42.64

108

106-44-5

0.42

15

2-Ethylphenol

43.84

122

90-00-6

3.22

16

3,4-Dimethylphenol

44.11

122

95-65-8

0.42

17

2-Methoxy-5-methylphenol

44.22

138

1195-09-1

0.20

18

2,3-Dihydrobenzofuran

44.88

120

496-16-2

0.45

19

3-Methoxybenzaldehyde

45.08

136

591-31-1

0.33

20

3-Methyl-para-anisaldehyde

47.57

150

32723-67-4

0.20

21

2,3-Dihydro-1H-inden-1-one

48.09

132

83-33-0

0.27

22

1,2-Benzenediol (catechol)

49.16

110

120-80-9

6.84

23

3,4-(Methylenedioxy)benzaldehyde (piperonal)

50.72

150

120-57-0

9.66

24

2-Methylbenzofuran

50.98

132

4265-25-2

0.22

25

4-Hydroxy-3-methoxybenzaldehyde (vanillin)

52.94

152

121-33-5

Not included

26

3,4-Dimethoxybenzaldehyde

53.28

166

120-14-9

1.10

27

4-Hydroxybenzaldehyde

54.78

122

123-08-0

6.93

28

4-Hydroxy-3-methylbenzaldehyde

55.66

136

15174-69-3

10.94

29

9H-xanthene

56.63

182

92-83-1

0.25

30

4-Methyl-para-anisaldehyde

57.25

150

32723-67-4

3.95

No.

Compound

1

Carbon dioxide

4.71

2

1,3-Butadiene

3

1,3-Cyclopentadiene

4

Hydrogen, CO, methane, ethane, and water not included.

MW

ALDEHYDES CONTAINING HYDROXY AND ETHER GROUPS

401

identifications and their relative molar content in 100 moles of pyrolysate are given in Table 10.1.4. The calculation of the mole % was obtained based solely on peak areas, and because differences in the MS response factors can be quite large for different compounds, the estimations may have large errors. The peak of vanillin dominates the pyrogram because a large amount of compound vaporizes before reaching the decomposition temperature. The vanillin peak area was not included in the calculation of molar contribution of pyrolysate. From Table 10.1.4, it can be seen that the elimination of CO from the aldehyde group in vanillin, similar to reaction (10.1.26), is an important component of the pyrolysis process. This reaction is shown below:

ð10:1:36Þ

The level of 2-methoxyphenol that can be found in the pyrolysate is 27.4%, and it is possible that more 2-methoxyphenol was initially generated but that it was further decomposed in the pyrolytic process. The ether group does not decompose before the aldehyde to generate a phenol and a hydrocarbon (see reaction 10.4.2). The corresponding compound that would be generated for vanillin is 3,4-dihydroxy-benzaldehyde, which was not detected in the pyrolysate. On the other hand, 1,2-benzenediol (catechol) was detected in the pyrolysate, with the likely reactions that generate this compound being the decomposition of 2-methoxyphenol in a similar decomposition to that of methoxybenzene (see reaction 10.4.1). Another reaction taking place during vanillin pyrolysis is that leading to the formation of piperonal, shown as follows:

ð10:1:37Þ

The elimination of hydrogen and the formation of 9.66% piperonal in the pyrolysate indicate that the phenol group also decomposes in a reaction similar to that for phenol (see reaction 9.3.1) and the phenoxyl radical further generates piperonal. The formation of 4-hydroxy-3-methyl-benzaldehyde and of other compounds with a methyl group attached to the benzene ring is additional proof that complex radicalic reactions take place during vanillin pyrolysis. Pyrolysis of vanillin deposited on Al2O3 has the effect of decomposing a much larger proportion of the parent molecule, with the formation of H2O, CO2, CH3dOdCH3, CH3-OH, and 3,4-dimethoxy-benzaldehyde as the main decomposition products. Another aldehyde containing alcohol and ether groups, which is similar to vanillin, is syringaldehyde or 4-hydroxy-3,5-dimethoxy-benzaldehyde. This compound is found in some types of wood (spruce, maple, etc.). Pyrolysis of a sample of syringaldehyde was

402

10. PYROLYSIS OF ALDEHYDES AND KETONES

Abundance

49.54

OH OCH3

H3CO

2e+07

59.33

1.8e+07 1.6e+07 54.48

1.4e+07

CH

1.2e+07

O

1e+07

56.22

80,00,000

40,00,000 20,00,000 0

55.64

52.62 41.55 42.33 43.28 41.08 39.76

60,00,000

38.0

40.0

42.0

44.0

45.51

46.0

48.81

48.0

58.04 50.19 52.73 53.81 50.0

52.0

54.0

56.0

58.0

60.20

60.0

62.0

Time-->

FIG. 10.1.4 Window 38–62 min of the pyrogram for 4-hydroxy-3,5-dimethoxy benzaldehyde (syringaldehyde).

performed on a 0.75 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 pyrolysate was done by GC/MS under the conditions given in Table 1.4.1. The window between 38.0 and 62.0 min from the pyrogram is given in Fig. 10.1.4, and the compound identifications and their relative molar content in 100 moles of pyrolysate are given in Table 10.1.5. The calculation of the mole % was obtained based solely on peak areas.

TABLE 10.1.5 Identification of the Main Peaks in the Chromatogram Shown in Fig. 10.1.4 for the Pyrolysis of 4-Hydroxy-3,5-Dimethoxybenzaldehyde (Syringaldehyde) No.

Compound

Retention Time (Min)

MW

CAS#

Moles % Pyrolysate

1

Carbon dioxide

4.70

44

124-38-9

7.00

2

Propene

4.97

42

115-07-1

0.37

3

Formaldehyde

5.20

30

50-00-0

0.77

4

Propyne

5.31

40

74-99-7

0.39

5

2-Butene

5.61

56

107-01-7

0.12

6

1,3-Butadiene

5.85

54

106-99-0

0.18

7

1,2-Butadiene

6.26

54

590-19-2

0.18

8

Methanol

7.05

32

67-56-1

2.59

9

Cyclopentene

8.12

68

142-29-0

0.06 Continued

403

ALDEHYDES CONTAINING HYDROXY AND ETHER GROUPS

TABLE 10.1.5 Identification of the Main Peaks in the Chromatogram Shown in for the Pyrolysis of 4-Hydroxy-3,5-Dimethoxybenzaldehyde(Syringaldehyde)—Cont’d No.

Compound

Retention Time (Min)

MW

CAS#

Moles % Pyrolysate

10

1,4-Pentadiene

8.83

68

591-93-5

0.09

11

1,3-Cyclopentadiene

9.53

66

542-92-7

0.28

12

1,4-Cyclohexadiene

14.90

80

628-41-1

0.13

13

1-Methyl-1,3-cyclopentadiene

15.47

80

96-39-9

0.10

14

3-Methylenecyclopentene

17.20

80

930-26-7

0.07

15

Benzene

17.88

78

71-43-2

0.23

16

Toluene

24.41

92

108-88-3

0.39

17

Ethylbenzene

29.15

106

100-41-4

0.02

18

1,3-Dimethylbenzene (m-xylene)

29.55

106

108-38-3

0.10

19

1,4-Dimethylbenzene (p-xylene)

30.86

106

106-42-3

0.02

20

Styrene

31.25

104

100-42-5

0.03

21

2-Methyl-2-cyclopenten-1-one

33.98

96

1120-73-6

0.27

22

Benzofuran

36.45

118

271-89-6

0.27

23

3-Hydroxybenzaldehyde

39.52

122

100-83-4

0.23

24

2-Methylbenzofuran

39.76

132

4265-25-2

0.80

25

Phenol

39.95

94

108-95-2

0.93

26

7-Methylbenzofuran

40.21

132

17057-52-8

0.03

27

2-Methoxyphenol

41.08

124

90-05-1

1.09

28

2-Methylphenol

41.55

108

95-48-7

4.12

29

2-Hydroxy-4-methylbenzaldehyde

42.05

136

698-27-1

0.78

30

2,6-Dimethylphenol

42.33

122

576-26-1

3.26

31

3-Hydroxybenzeneethanol

42.66

138

13398-94-2

0.51

32

2,7-Dimethylbenzofuran

43.15

146

28715-26-6

0.07

33

3-Methyl-4-methoxyphenol

43.28

138

14786-82-4

1.90

34

2-Ethylphenol

43.83

122

90-00-6

1.35

35

2,4-Dimethylphenol

44.10

122

105-67-9

0.41

36

2-Ethyl-5-methylphenol

44.39

136

1687-61-2

1.67

37

1-(2-Hydroxy-5-methylphenyl)ethanone

44.52

150

1450-72-2

0.51

38

1,4-Dimethoxy-2-methylbenxene

44.62

152

24599-58-4

0.18

39

2,3-Dihydrobenzofuran

44.88

120

496-16-2

0.07 Continued

404

10. PYROLYSIS OF ALDEHYDES AND KETONES

TABLE 10.1.5 Identification of the Main Peaks in the Chromatogram Shown in for the Pyrolysis of 4-Hydroxy-3,5-Dimethoxybenzaldehyde(Syringaldehyde)—Cont’d No.

Compound

Retention Time (Min)

MW

CAS#

Moles % Pyrolysate

40

2,3,6-Trimethylphenol

44.96

136

2416-94-6

0.30

41

3-Hydroxy-4-methylbenzaldehyde

45.08

136

57295-30-4

0.03

42

4-Ethyl-2-methoxyphenol

45.51

152

2785-89-9

1.18

43

7-Methoxybenzofuran

46.07

148

7168-85-6

0.25

44

2-Ethyl-4,5-dimethylphenol

46.18

150

2219-78-5

0.27

45

1,2,3-Trimethoxybenzene

46.94

168

634-36-6

0.09

46

2,2,-Bifuran

48.08

134

5905-00-0

0.51

47

3-Methoxy-1,2-benzenediol

48.40

140

934-00-9

0.89

48

2-Hydroxy-3-methoxybenzaldehyde

48.78

152

148-53-8

0.82

49

2,6-Dimethoxyphenol

49.54

154

91-10-1

25.24

50

2-Methyl-benzofuran-3-carboxaldehyde

51.81

160

55581-61-8

0.87

51

4-Hydroxy-3-methoxybenzaldehyde (vanillin)

52.62

152

121-33-5

4.74

52

6-Hydroxy-4-methoxy-2,3Dimethylbenzaldehyde

53.80

180

34883-12-0

0.76

53

4,6-Dihydroxy-2,3-dimethylbenzaldehyde

54.48

166

2990-31-0

9.87

54

1-(p-Methoxyphenyl)-2-methoxyprop-1ene

54.56

178

64304-85-4

0.92

55

4-Hydroxy-3-methylbenzaldehyde

55.64

136

15174-69-3

4.50

56

3-Methyl-methoxybenzaldehyde

55.68

150

32723-67-4

2.21

57

3,4,5-Trimethoxybenzaldehyde

56.11

196

86-81-7

1.74

58

3-Methoxy-4,5methylenedioxybenzaldehyde

56.22

180

5780-07-4

6.46

59

2,3-Dimethyl-para-anisaldehyde

57.02

164

38998-17-3

1.51

60

4-Vinyl-2-methoxyphenol

57.25

150

7786-61-0

1.55

61

3-Methoxycinnamic acid

57.82

178

6099-4-3

0.29

62

4-Methyl-2,5-dimethoxybenzaldehyde

58.04

180

N/A

2.70

63

4-Hydroxy-3,5-dimethoxybenzaldehyde (syringaldehyde)

59.33

182

134-96-3

Not included

64

1-(4-Hydroxyphenyl)-3-phenyl-2-propen1-one ?

59.83

224

2657-25-2

0.48

65

4-Hydroxy-3,5-dimethoxybenzoic acid

60.20

212

884-35-5

0.24

Hydrogen, CO, methane, ethane, and water not included. Note: “?” indicates uncertain compound assignment.

REFERENCES 10.1

405

Similar to the case of vanillin, the peak of syringaldehyde dominates the pyrogram because a large amount of compound vaporizes before reaching the decomposition temperature. The syringaldehyde peak area was not included in the calculation of the molar contribution of pyrolysate. A variety of decomposition products are seen in Table 10.1.5, very similar to those found in vanillin pyrolysate. These include compounds resulting from the elimination of CO from the aldehyde group (2,6-dimethoxyphenol) and compounds resulting from the cleavage of the ether group as well as numerous compounds resulting from various rearrangement reactions. The phenolic OH group is more resistant to heating, as seen from the large number of phenolic compounds listed in Table 10.1.5. However, heating can eliminate even these groups, and simple molecules such as benzene and toluene are detected in the pyrolysate. It was not possible to determine if, in these molecules, the initial benzene ring was preexistent (to pyrolysis) or was synthesized from other smaller fragments.

References 10.1 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

C.J. Chen, D.J. McKenney, Can. J. Chem. 50 (1972) 992. Y. Hidaka, T. Taniguchi, T. Kamesawa, H. Masaoka, K. Inami, H. Kawano, Int. J. Chem. Kinet. 25 (1993) 305. Y. Hidaka, T. Taniguchi, H. Tanaka, T. Kamesawa, K. Inami, H. Kawano, Combust. Flame 92 (1993) 365. D. Gay, G.P. Glass, G.B. Kistiakowsky, H. Niki, J. Chem. Phys. 43 (1965) 4017. A.M. Dean, R.L. Johnson, D.C. Steiner, Combust. Flame 37 (1980) 41. J. Troe, J. Phys. Chem. A 111 (2007) 3862. Y.N. Samsonov, A.K. Petrov, A.V. Baklanov, React. Kinet. Catal. Lett. 5 (1976) 435. Y. Hidaka, K. Sato, Y. Hemi, H. Tanaka, K. Inami, Combust. Flame 118 (1999) 340. A.M. Markevich, R.I. Mishkina, L.F. Filipova, Russ. Chem. Bull. 7 (1958) 480. M.B. Colket III, D.W. Naegeli, I. Glassman, Int. J. Chem. Kinet. 7 (1975) 223. K.S. Gupte, J.H. Kiefer, R.S. Tranter, S.J. Klippenstrin, L.B. Harding, Proc. Sci. Inst. 31 (2007) 167. J. Ernst, K. Spindler, Ber. Bunsenges 79 (1975) 1163. J. Ernst, K. Spindler, H.G. Wagner, Ber. Bunsenges 80 (1976) 645. L. Batt, J. Chem. Phys. 47 (1967) 3674. K.J. Laidler, M.T.H. Liu, Proc. Roy. Soc. London, Ser. A 297 (1967) 365. C.A. McDowell, J.H. Thomas, J. Chem. Phys. 17 (1949) 588. E.R. Sias, S.L. Cole, B.J. Warner, E.M. Wright, L.R. McCunn, J. Anal. Appl. Pyrolysis 123 (2017) 340. C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [19] C.E. Castro, F.F. Rust, J. Am. Chem. Soc. 93 (1961) 4928. [20] C. Schwarzinger, J. Anal. Appl. Pyrolysis 68–69 (2003) 137. [21] D. Shen, S. Yao, S. Wu, Q. Liu, R. Xiao, J. Anal. Appl. Pyrolysis 113 (2015) 665.

406

10. PYROLYSIS OF ALDEHYDES AND KETONES

S U B C H A P T E R

10.2

Ketones

GENERAL ASPECTS Ketones are carbonyl compounds with the general formula Rd(CO)dR0 , where R and R0 are hydrocarbon radicals. The carbonyl functional group >C]O is bonded in ketones to two carbon atoms. Several naming procedures are used for ketones. A common name is derived from the name of the two radical groups R and R0 and the word ketone. The IUPAC name recognizes the ]O group (not C]O) and changes the suffix “e” of the corresponding hydrocarbon (longest carbon chain) with the suffix “one,” possibly indicating the position of the ] O group with the lowest number (e.g., methyl ethyl ketone is butan-2-one). In some compounds with more important structural descriptors than ketone, the group ]O can be indicated as “oxo” or “keto.” Some traditional names also are used (e.g., acetone). Ketones are important chemical compounds. They are present in nature and are synthesized for many applications. Ketones are used mainly as solvents and intermediates in the chemical industry. Pyrolysis of ketones may occur in incinerators, in unintentional fires, and in some synthetic procedures such as the preparation of ketene [1], etc. (The name ketene is used for CH2]CO, which is the simplest member of the class of compounds named ketenes.)

ACETONE Pyrolysis of acetone, CH3dCOdCH3, has been performed in various conditions, which are reported in the literature [2–8]. The pyrolysis process generates CO, H2, CH4, C2H4, C2H6, C2H2, and various other short-chain hydrocarbons as well as ketene. In the interval from 1100 to 1570 K, besides CH4, C2H4, C2H6, C2H2 and ketene, the short-chain hydrocarbons included propyne, allene, propene, and 1,3-butadiene. The main reactions in acetone decomposition can be written as follows: H3 C  CO  CH3 ! CO + H2 + C2 H4

(10.2.1)

H3 C  CO  CH3 ! CH4 + H2 C ¼ CO

(10.2.2)

In a shock-tube experiment performed on 1% acetone vapors in argon, the variation in the ratios of concentration C to the initial acetone concentration Co for several pyrolysis

407

ACETONE

(CH3)2CO

1

CO

0.9

CH4

C/C o

0.8

C2H2

0.7

C2H4

0.6

C2H6

0.5

2300 Heating time (ms)

0.4 0.3 0.2 0.1 0 1000

1100

1200 1300 1400 Temperature (K)

1500

1600

2100

1900

1700 1000

1200

1400

1600

Temperature (K)

FIG. 10.2.1 Variation with temperature in the ratios of concentration C to the initial acetone concentration Co of several components of acetone pyrolysate. The linear dependence of temperatures for the effective heating time also is shown.

components is given in Fig. 10.2.1 (the effective heating time was not the same for different temperatures, varying linearly between 2287 μs at 1090 K and 1724 μs at 1566 K) [9]. The decomposition of acetone occurs starting with the radical initiation reactions, as shown below: ðCH3 Þ2 C ¼ O ! CH3 CO + CH3  ΔH ¼ + 79:9kcal=mol

(10.2.3)

The kinetic of this reaction is described by the Arrhenius equation with parameters given in Table 10.2.1. The initiation is followed by several propagation steps: CH3 CO ! CO + CH3 

(10.2.4)

ðCH3 Þ2 C ¼ O + CH3  ! CH4 + H3 C  CO  CH2 

(10.2.5)

H3 C  CO  CH2  ! H2 C ¼ CO + CH3 

(10.2.6)

The corresponding A, n, and E parameters in the Arrhenius equation k ¼ AT exp( E#/ RT) for reactions (10.2.3)–(10.2.6) and for several other reactions important in acetone pyrolysis are given in Table 10.2.1. A number of termination reactions, also shown in Table 10.2.1, lead to the formation of C2H4, C2H6, and other short-chain hydrocarbons. In addition to the main initiation reaction (15.1.3), the decomposition of acetone also may start with the following initiation: #

ðCH3 Þ2 CO ! CH3  CO  CH2  + H

n

(10.2.7)

However, this reaction has a much lower role in acetone pyrolysis, with the dissociation enthalpy for the CdH bond being considerably higher than that in reaction (10.2.3).

408

10. PYROLYSIS OF ALDEHYDES AND KETONES

TABLE 10.2.1 A, n, and E# Parameters in the Arrhenius Equation k ¼ ATn exp( E#/RT) for Reactions (10.2.3)–(10.2.6) and for Several Other Reactions Important in Acetone Pyrolysis n

E# (kcal/mol)

1.13  10+16

0

81.7

2.3  10

Reaction

A

CH3 COCH3 ! CH3 CO + CH3  



CH3 COCH3 + H ! CH3 COCH2 + H2 

+7



CH3 CO + ðMÞ ! CH3 + CO + ðMÞ 



CH3 COCH3 + CH3 ! CH3 COCH2 + CH4 

CH3 COCH2 ! CH2 CO + CH3 



0





∞

C2 H6 + H ! C2 H5 + H2

CH3 + CH3 ! C2 H6 k CH3 + CH3 ! C2 H6 k

5.0

5.36  10

3.4

18.9

9.50  10

+3

2.5

8.4

1.0  10

+13









2

+27



C2 H5 + H ! C2 H4 + H2

0

28.0

2.12  10

+50

9.7

5.2

2.12  10

+16

1.0

0.62

1.15  10

+8

1.9

7.53

4.50  10

+13

0

0.0

The formation of ketene and CH4 in acetone pyrolysis is similar to the formation of CO and CH4 in the pyrolysis of acetaldehyde. Ketene can further decompose under the influence of heat, and it is difficult to determine the true extent of reaction (10.2.6) (ΔH ¼ + 31.6 kcal/mol) that leads to the amount of ketene detected in the pyrolysate. The formation of ketene during acetone pyrolysis can be used for the preparation of this compound [10]. The study of acetone pyrolysis at very high temperature (4000 K) using shockwave experiments shows that the compound is completely decomposed within 3 μs with the formation of C2 [6]. Acetone oxidation below 800°C also was reported in the literature [9,11–13]. Table 10.2.2 shows three main reactions taking place during acetone oxidation and the corresponding A, n, and E# parameters in the Arrhenius equation k ¼ ATn exp( E#/RT). Once the oxidation process starts and a large number of free radicals are generated, numerous other propagation and termination reactions will take place. Acetone oxidation was simulated using computer modeling that involved 164 elementary reactions, including oxidation for formaldehyde, methane, ethane, ethylene, acetylene, and ketene [9]. Acetone has an enthalpy of combustion of 402 kcal/mol (at 25°C) when the reaction generates CO2 and H2O.

TABLE 10.2.2 Several Main Reactions Taking Place During Acetone Oxidation and the Corresponding A, n, and E# Parameters in the Arrhenius Equation k ¼ ATn exp( E#/RT) n

E# (kcal/mol)

6.0  10+13

0

51.9

1.0  10

0

5.96

2.0  10

0

3.0

Reaction

A

CH3 COCH3 + O2 ! CH3 COCH2  + HOO 



CH3 COCH3 + O : ! CH3 COCH2 + OH 



CH3 COCH3 + OH ! CH3 COCH2 + H2 O

+13 +13

OTHER KETONES

409

OTHER KETONES Pyrolysis of methyl ethyl ketone and of diethyl ketone takes place similarly to that of acetone. The compounds generated include CO, H2, CH4, C2H4, C2H6, C2H2, and other shortchain unsaturated hydrocarbons. The formation of ketenes also is noticed during pyrolysis of these ketones around 700°C, but the yield of methyl ketene (CH3dCH]C]O) is low, and ketene is formed by a reaction as shown below: ð10:2:8Þ Decomposition of tert-butyl methyl ketone occurs only at temperatures above 700°C. The reaction products are CO, H2, and some other small hydrocarbons. The formation of ketene also was noticed but at a very low level. Methane is not a main reaction product in the pyrolysis of tert-butyl methyl ketone. Acetophenone (C6H5-CO-CH3) decomposes with the formation of CO (70%–75%), CO2, benzene, and lower levels of toluene, diphenyl, 1,4-diphenylbenzene, H2, CH4, and C2H4. The absence of a hydrogen atom on the aromatic carbon bound to the carbonyl group leads to the low levels of CH4 that are formed from more than one reaction involving free radicals. Triphenylbenzene, 1,3-diphenyl-2-buten-1-one, and traces of 2,4-diphenylfuran were generated when acetophenone was heated for a long time at a low temperature (200°C) [2]. Thermal decomposition of dibenzyl ketone, or 1,3-diphenylacetone (C5H5dCH2)2CO, at temperatures above 400°C leads to the formation of CO, toluene, and char. Small levels of short chain aliphatic hydrocarbons also are generated [2]. Cyclohexanone at temperatures above 700°C decomposes with the formation of CO and various alkenes including C2H4, C3H6, etc. Smaller levels of other hydrocarbons such as CH4, H2, C2H6, etc., also are formed. Depending on the heating temperature, other reactions also may take place during cyclohexanone thermal decomposition. At lower temperatures (around 200°C and a long heating time), the formation of 2-cyclohexylidenecyclohexan-1one was noticed [2]:

ð10:2:9Þ

At higher temperatures, the elimination of water takes place from a single cyclohexanone molecule with the formation of cyclohexadiene as shown below:

ð10:2:10Þ

410

10. PYROLYSIS OF ALDEHYDES AND KETONES

Benzophenone (C6H5)2CO is the simplest aromatic ketone. This compound, at temperatures above 500°C, generates mainly CO, benzene, diphenylmethane, and char. Dibenzosuberone generates by pyrolysis anthracene, CO, and H2 as shown in reaction (2.3.7) [14]. When the molecule of the ketone becomes more complicated, the overall structure starts to play a more important role in the production of various components in the pyrolysate. Two examples of ketones that are found in rose oil are β-damascone and β-damascenone. βDamascone is (E)-1-(2,6,6-trimethyl-1-cyclohexenyl)but-2-en-1-one, and β-damascenone is (E)-1-(2,6,6-trimethyl-1-cyclohexa-1,3-dienyl)but-2-en-1-one.

b-Damascone

b-Damascenone

In the pyrolysis products of the two compounds at 900°C using conditions described in Subchapter 1.4, the parent compound is present at more than 75% from the initial material. This is caused, in part, by the relatively good thermal stability of both β-damascone and βdamascenone, but mainly by their volatility such that during pyrolysis, a large amount of the parent compound volatizes before reaching the equilibrium temperature Teq. Among the pyrolysis products of both these ketones are present low levels of simple aromatic hydrocarbons such as toluene, 1,3-dimethylbenzene, 1,1,6-trimethyl-1,2,3,4-tetrahydronaphthalene, and some isomers of the parent molecule. Although the two ketones differ by only one additional double bond in the six-carbon cycle, the pyrolysis products show some qualitative and quantitative differences.

DIKETONES AND QUINONES Two (or more) ketone groups can be present in the same molecule, and when present on an aliphatic chain, the ketone groups can be in positions 1,2(α), 1,3(β), or separated by more carbon atoms. The simplest α-diketone is diacetyl or butandione, CH3(CO)(CO)CH3. The decomposition of this compound at temperatures between 600°C and 675°C leads to the formation of CO, CH4, and ketene. The maximum yield of ketene is obtained around 615°C [15]. The simplest β-diketone is acetylacetone or pentane-2,4-dione, CH3(CO)CH2(CO)CH3. The decomposition of this compound also leads to the formation of ketene and CH4. The optimum temperature for the generation of ketene is around 635°C, when only part of the diketone is decomposed. At higher temperatures, the ketene itself is decomposed (see Subchapter 10.3). Diketones with the two CO groups separated by more carbon atoms typically show less interaction between these groups, and the thermal decomposition is influenced independently by

411

DIKETONES AND QUINONES

each group. However, a ketone such as 1,4-diphenylbutane-1,4-dione heated for 18 h at 310°C generates 2,5-diphenylfuran [16]. A special group of diketones is the quinones. p-Benzoquinone (C6H4O2) or cyclohexa-2,5diene-1,4-dione and its homologs do not have an aromatic character. These compounds are very stable to heating. The decomposition begins to be noticed above 750°C, but stronger decomposition can be achieved only at higher temperatures. p-Benzoquinone is also volatile, and pyrolysis in a pyrolyzer used for solid samples leads to the volatilization of a large amount of compound before reaching the decomposition temperature. The main pyrolysis products of benzoquinone above 900°C are CO2, hydroquinone, and char, with low levels of phenol and traces of benzene, toluene, and naphthalene. The formation of hydroquinone in the thermal decomposition is explained by the high thermal stability of the aromatic ring and the oxidative character of benzoquinone. 2,6-Dimethyl-1,4-benzoquinone decomposes similarly to benzoquinone. The main decomposition product in this case is 2,6-dimethylhydroquinone (2,6-dimethyl-1,4-benzenediol). The pyrolysis of a 0.6 mg sample of 2,6-dimethylbenzoquinone leads to the pyrogram shown in Fig. 10.2.2. The pyrolysis was performed using Teq ¼ 1100°C, β ¼ 20°C/ms, THt ¼ 10 s, and a housing temperature of Thou ¼ 280°C. The analysis of pyrolysate was done by GC/MS under conditions given in Table 1.4.1. The compound identifications in the pyrogram and their relative molar content in 100 moles of pyrolysate are given in Table 10.2.3. The calculation of the mole % was obtained based solely on peak areas. In addition to the compounds listed in Table 10.2.3, the pyrolysis generates a certain amount of char. The presence of naphthalene and acenaphthylene in detectable amounts by direct GC/MS analysis indicates that other PAHs are likely to be in the pyrolysate, but at levels below the detection capability of the technique used for the analysis.

Abundance

42.15

O H3C

35,00,000

53.87

CH3

30,00,000 25,00,000

O

20,00,000 17.92 15,00,000

52.75 24.45

10,00,000 4.74 5,00,000 0 Time-->

10.0

15.0

38.01

29.60

14.96 5.0

45.20

9.58

20.0

25.0

30.0

FIG. 10.2.2 Pyrogram of 2,6-dimethyl-1,4-benzoquinone at 1100°C.

35.0

40.0

46.87 45.0

50.0

55.0

412

10. PYROLYSIS OF ALDEHYDES AND KETONES

TABLE 10.2.3 Identification of the Main Peaks in the Chromatogram Shown in Fig. 10.2.2 for the Pyrolysis of 2,6-Dimethylbenzoquinone at 1100°C Retention Time (Min)

CAS#

Moles % Pyrolysate

44

124-38-9

4.97

9.59

66

542-92-7

2.23

1-Methyl-cyclopentadiene

14.96

80

96-39-9

0.88

4

1,3-Cyclohexadiene

15.52

80

592-57-4

0.66

5

Benzene

17.92

78

71-43-2

6.70

6

Toluene

24.45

92

108-88-3

2.62

7

p-Xylene

29.60

106

95-47-6

0.69

8

3,4-Bis(methylene)cyclopentanone

36.99

108

27646-73-7

0.44

9

Indene

38.01

116

95-13-6

0.56

10

Phenol

40.07

94

108-95-2

0.52

11

2,5-Dimethyl-2,5-cyclohexadiene-1,4dione

41.60

136

137-18-8

0.98

12

2-Methylphenol

41.71

108

95-48-7

0.88

13

2,6-Dimethyl-1,4-benzoquinone

42.22

108

527-61-7

Not included

14

3,5-Dimethylphenol

42.37

122

108-68-9

3.75

15

3-Methylphenol

42.73

108

108-39-4

2.37

16

4-Hydroxy-2,4,5-trimethyl-2,5cyclohexadien-1-one

43.55

152

14353-72-1

0.26

17

Naphthalene

43.65

128

91-20-3

0.50

18

2,4-Dimethylphenol

44.19

122

105-67-9

0.34

19

3,4-Dimethylphenol

45.20

122

95-65-8

1.58

20

1-Methylnaphthalene

46.87

142

90-12-0

0.44

21

2-Methylnaphthalene

47.42

142

91-57-6

0.27

22

Resorcinol

51.88

110

108-46-3

0.64

23

Acenaphthylene

52.14

152

208-96-8

0.25

24

2-Methyl-5-hydroxybenzofuran

52.75

148

6769-56-8

1.95

25

2-Methyl-1,4-benzenediol

53.02

124

95-71-6

2.61

26

2,6-Dimethyl-1,4-benzenediol

53.87

138

654-42-2

59.82

27

2,5-Dimethyl-1,4-benzenediol

54.13

138

1321-28-4

2.65

28

5-Hydroxy-4-methylindan-1-one

54.87

162

N/A

0.43

No.

Compound

1

Carbon dioxide

4.74

2

1,3-Cyclopentadiene

3

Hydrogen, CO, methane, ethane, and water not included. Note: Bold number indicates major constituent in the pyrolyzate.

MW

413

KETONES CONTAINING OTHER FUNCTIONAL GROUPS

KETONES CONTAINING OTHER FUNCTIONAL GROUPS Numerous compounds contain ketone groups in addition to other functional groups. These may include halogen, alcohol, ether, and other groups. In some reactions, the other functional group may be eliminated before the carbonyl group is affected. For example, pyrolysis of phenyl 2,4,6-tribromophenyl ketone (tribromobenzophenone) pyrolyzed around 500°C generates 1,3-dibromofluoren-9-one and eliminates HBr. The same elimination of the halogen is seen, for example, during the pyrolysis of 4-chlorobutanone, which takes place by the following reaction: ð10:2:11Þ The kinetic parameters for reaction (10.2.11) were determined in the temperature range of 402–424.4°C and found to follow the Arrhenius equation [17]:
log k s1 ¼ ð13:67  0:69Þ  ð225:2  8:6Þ kJ=mol=2:303RT (10.2.12) Another common additional group to carbonyl is hydroxyl. The OH group relative to the C]O group can be in vicinal positions (α-hydroxyketones) or separated by one (β-hydroxyketones) or more carbon atoms. Acetol (3-hydroxy-2-butanone) is the simplest hydroxyketone, and the OH group is in α-position to the carbonyl. Pyrolysis of this compound around 450°C generates acetaldehyde and formaldehyde as well as low levels of CO, H2, CH4, and crotonaldehyde. Another common α-hydroxyketone is benzoin (2-hydroxy1,2-diphenylethan-1-one). This compound decomposes above 300°C with the formation of benzyl phenyl ketone (deoxybenzoin), 1,2-diphenylethane-1,2-dione (dibenzoyl), and 1,2diphenylethane-1-ol [18]. β-Hydroxyketones are somewhat more stable than β-hydroxyaldehydes (aldols), but they decompose in the same manner with elimination of water upon heating at temperatures above 250–300°C. For example, 4-hydroxypentan-2-one changes at temperatures higher than 250°C into 3-penten-2-one.

ð10:2:13Þ

Decomposition of 3-hydroxy-3-methyl-2-butanone at temperatures around 600 K leads to the formation of acetaldehyde and acetone, as shown below:

ð10:2:14Þ

414

10. PYROLYSIS OF ALDEHYDES AND KETONES

The Arrhenius equation describing reaction (10.2.14) is given by the formula [19]:
log 10 k s1 ¼ 12:4  53, 060=ð4:574T Þ (10.2.15) Decomposition of hydroxyketones with the OH group at carbon atoms situated farther from the carbonyl group starts to show similarities with the decomposition of carbohydrates, with the water elimination remaining a major reaction under the influence of heat. The formation of stable heterocycles is sometimes the reason why the pyrolysis of some compounds takes a specific path. For example, 6-hydroxy-6-methylheptan-2-one decomposes upon distillation in vacuum around 125°C to form 2,2,6-trimethyl-2H-3,4-dihydropyran, as shown in the following reaction [2]: ð10:2:16Þ

References 10.2 [1] L. A. Nicolai, E. W. S. Nicholson, J. O. Smith, U.S. Patent 2537079 (1951). [2] C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [3] J.R. McNesby, T.W. Davis, A.S. Gordon, J. Chem. Phys. 21 (1953) 965. [4] M. Szwarc, J.W. Taylor, J. Phys. Chem. 23 (1955) 2310. [5] V.I. Bodrov, Y.L. Muromtsev, V.N. Shamkin, O.Y. Zhukhovitskii, Teor. Osn. Khim. Technol. 19 (1885) 336. [6] B.P. Levitt, N. Wright, Trans. Faraday Soc. 63 (1967) 282. [7] B.C. Capelin, G. Ingram, J. Kokolis, Microchem. J. 19 (1974) 229. [8] J. Ernst, K. Spindler, H.G. Wagner, Ber. Bunsenges Phys. Chem. 80 (1976) 654. [9] K. Sato, Y. Hidaka, Combust. Flame 122 (2000) 291. [10] C.D. Hurd, W.H. Tallyn, J. Am. Chem. Soc. 47 (1925) 1427. [11] D.E. Hoare, T.M. Li, Combust. Flame 12 (1968) 136. [12] D.E. Hoare, T.M. Li, Combust. Flame 12 (1968) 145. [13] D.E. Hoare, E.E. Lill, J. Chem. Soc. Faraday Trans. I 69 (1973) 603. [14] W.S. Trahanovsky, J.L. Tunkel, J.C. Thoen, Y. Wang, J. Org. Chem. 60 (1995) 8407. [15] C.D. Hurd, W.H. Tallyn, J. Am. Chem. Soc. 47 (1925) 1779. [16] S. Skraup, S. Guggenheimer, Ber. Deutch. Chem. Gesell. 58 (1925) 2488. [17] R.M. Dominguez, G. Chuchani, Int. J. Chem. Kinet. 13 (1981) 403. [18] A. Lachman, J. Am. Chem. Soc. 46 (1924) 708. [19] N.A. Al-Awadi, O.M.E. El-Dusouqui, Int. J. Chem. Kinet. 29 (1999) 295.

KETENE (ETHENONE)

415

S U B C H A P T E R

10.3

Ketenes

GENERAL ASPECTS Ketenes are carbonyl compounds containing the CO group connected by a double bond to a carbon atom. Ketenes have the general formula RR0 C]CO, where R and R0 are hydrocarbon radicals. The simplest ketene (CH2]C]O, ethenone) also is named just ketene. Ketenes are reactive compounds that dimerize to diketenes. Some ketenes such as ethenone dimerize, forming a lactone (oxetan-2-one). Methyl ketene generates two dimers, one with a lactone structure and the other having a cyclopbutandione structure. The formation of the two possible dimers is shown in the following reaction: ð10:3:1Þ

Ketene (ethenone) is present mainly as a lactone. Ketenes and ketene dimers have various practical applications such as paper sizing.

KETENE (ETHENONE) The study of ketene pyrolysis is related particularly with the pyrolysis of acetone [1] and with the combustion of different fuels [2,3], particularly of acetylene [4]. Pyrolysis results generated using shock-tube experiments were reported for ketene pyrolysis [5] as well as oxidation [6]. The study for pyrolysis was performed on mixtures containing 0.26% and 2.2% ketene diluted with argon in the temperature range 1102–1921 K. The main pyrolysis products were CO, H2, CH4, C2H2, and C2H4. The reaction times varied between 1.7 and 2.1 ms. The variation of the ratio C/Co of the concentration of the reaction product (C) vs initial ketene concentration (Co) as a function of temperature is shown in Fig. 10.3.1 [5]. The whole process was modeled with a computer simulation using 38 elementary reactions [5]. The ketene consumption took place following seven reactions.

416

10. PYROLYSIS OF ALDEHYDES AND KETONES

1

temperature for ketene (initial conc. Co) and some ketene pyrolysis products [5].

0.8

CH2CO

0.6

CO

C/Co

FIG. 10.3.1 Variation of C/Co as a function of

C2H4 0.4

CH4

0.2

H2

0 1000

1200 1400 Temperature (K)

1600

The experimental and calculated values for A, n, and E# in the Arrhenius equation k ¼ ATn exp( E#/RT) as well as the heats of reaction ΔH (kcal/mol) for these reactions are given in Table 10.3.1. The oxidation of ketene was evaluated in several gas mixtures containing between 0.26% and 2.26% acetone, oxygen, or N2O as oxidant from 0.14% to 2%, all diluted to 100% with argon [6]. The temperature range for the study was between 1050 and 2050 K. The main reaction product during oxidation was CO. Lower levels of CO2. C2H4, and CH4 also were generated. The initiation is probably controlled by the formation of free O: atoms and of OH• free radicals that further react with ketene by reactions, as shown below: H2 C ¼ CO + O : ! CO + HCHO ΔH ¼ 102kcal=mol

(10.3.2)

H2 C ¼ CO + O : ! CO2 + CH2 : ΔH ¼ 50kcal=mol

(10.3.3)





H2 C ¼ CO + OH ! HCHO + CHO

ΔH ¼ 15kcal=mol

(10.3.4)

A computer simulation using 85 reactions with rate constants available in the literature or generated during the study was performed to explain the experimental data obtained from ketene oxidation [6]. TABLE 10.3.1 The Values for the Frequency Factor A (s1), Temperature Exponent n, and Activation Energy E# (cal/mol) in the Arrhenius Equation and for the Heats of Formation (kcal/mol) for the Main Reactions Responsible for Ketene Consumption [5] No.

Reaction

A (s21)

1

CH2CO + M ! CH2 : + CO + M 

n

E# (cal/mol)

ΔH (kcal/mol)

3.96  10

0

59,300

+75.8

+15



2

CH2 CO + H ! CH3 + CO

1.11  10

2

2000

32.5

3

CH2CO + H ! CHCO + H2

1.80  10

0

8600

0.7

+12

•

+7

•

+14



4

CH2 CO + CH2 : ! C2 H4 + CH3

1.00  10

0

0

94.2

5

CH2 CO + CH2 : ! CHCO + CH3 

3.60  10+13

0

11,000

4.3

6

CH2 CO + CH3  ! C2 H5  + CO

9.00  10+10

0

0

23.0

7

CH2 CO + CH3  ! CHCO + CH4

7.50  10+12

0

13,000

+0.7

GENERAL ASPECTS

417

OTHER KETENES Besides pyrolysis of ethenone (ketene), several studies on other ketenes and on ketene dimers were performed [7,8]. One of these studies evaluated the thermal decomposition of dichloroketene [8]. This compound decomposes similarly to ketene, with the main initiation reaction being the formation of CCl2: and CO. Another study was directed toward pyrolysis of the dimers of ketenes with tetradecyl, hexadecyl, and octadecyl substituents [7].

References 10.3 [1] [2] [3] [4] [5] [6] [7] [8]

K. Sato, Y. Hidaka, Combust. Flame 122 (2000) 291. P. Dagaut, M. Cathonnet, J. Boettner, Combust. Sci. Technol. 83 (1992) 167. J.A. Miller, C.F. Melius, Combust. Flame 91 (1992) 21. Y. Hidaka, W.C. Gardiner Jr., C.S. Eubank, J. Mol. Sci. 2 (1982) 141. Y. Hidaka, K. Kimura, H. Kawano, Combust. Flame 99 (1994) 18. Y. Hidaka, K. Kimura, K. Hattori, T. Okuno, Combust. Flame 106 (1996) 155. S. Moldoveanu, Q. Zha, N. Kulshreshtha, K. Agyei-Aye, http://tobaccodocuments.org/bw/948940.html. A.S. Shestov, S.A. Kostina, V.D. Knyazev, Proc. Combust. Inst. 30 (2005) 975.

S U B C H A P T E R

10.4

Thioaldehydes and Thioketones

GENERAL ASPECTS The replacement of the oxygen atom in the formula of an aldehyde or ketone with a sulfur atom leads to the formation of thioaldehydes and thioketones, respectively (Experimentally, these compounds can be obtained from the aldehydes and ketones by saturating their alcoholic solutions with H2S in the presence of a hydracid such as HCl). Aliphatic thioaldehydes and thioketones have a strong tendency to polymerize, and their isolation is problematic. Aromatic thioaldehydes and ketones with at least one aromatic substituent are more stable. However, these compounds still have the tendency to polymerize, for example, with the formation of trimers (in a reaction similar to the formation of paraacetaldehyde from acetaldehyde).

418

10. PYROLYSIS OF ALDEHYDES AND KETONES

Pyrolysis of thioaldehydes and thioketones does not show much similarity with that of typical aldehydes and ketones because they have the tendency to generate sulfur upon pyrolysis. For example, tri-thiobenzaldehyde (which does not contain any more C]S groups) around 190°C decomposes, as shown in the following reaction:

ð10:4:1Þ

The tendency to produce stable heterocycles is seen at higher temperatures, when trithiobenzaldehyde generates tetraphenylthiophene:

ð10:4:2Þ

Aromatic or partly aromatic thioketone trimers such as tri-thioacetophenone generate the monomers by distillation. Upon heating at the boiling point for longer periods of time, the decomposition takes place with the formation of styrene, ethylbenzene, diphenylthiophene, and sulfur. Thiobenzophenone decomposes at the distillation temperature, generating tetraphenylethylene, sulfur, H2S, and char [1].

Reference 10.4 [1] C.D. Nenitzescu, Chimie Organica, vol. 1, Tehnica, Bucuresti, 1960.