1278
Energy & Fuels 1997, 11, 1278-1287
Pyrolysis of Aromatic Carboxylic Acids: Potential Involvement of Anhydrides in Retrograde Reactions in Low-Rank Coal Thomas P. Eskay, Phillip F. Britt,* and A. C. Buchanan III Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS-6197, Oak Ridge, Tennessee 37831-6197 Received May 27, 1997. Revised Manuscript Received August 26, 1997X
The pyrolysis of 1-(3-carboxyphenyl)-2-(4-biphenyl)ethane (3) has been studied in the liquid phase at 400 °C neat and diluted in hydrogen-donor and nondonor solvents to determine the role of decarboxylation of aromatic carboxylic acids in the cross-linking processes in low-rank coal. Decarboxylation was the dominant reaction pathway in the pyrolysis of this model compound, and decarboxylation occurred primarily by an acid-promoted ionic mechanism that does not lead to cross-linking. However, pyrolysis in a nondonor solvent produced a small amount of products containing a new aryl-aryl bond between 3 and the solvent that represents the formation of a cross-link associated with the decarboxylation process. These cross-linked products were found to be formed by a free-radical pathway and could be decreased by the addition of H2O or tetralin to the pyrolysis medium. It is proposed that the cross-linked products arise from the formation and subsequent decomposition of anhydrides during the pyrolysis of the acid. Pyrolysis of di3,3′-(2-(4-biphenyl)ethyl)benzoic anhydride and 1-(3-carboxaldehydephenyl)-2-(4-biphenyl)ethane was investigated to support the proposed free-radical formation of the cross-linked products. These results suggest that cross-linking processes in low-rank coals may not be directly related to the decarboxylation process but may indirectly result from intermediates formed from reactions of the aromatic carboxylic acids.
Introduction In recent years, oxygen functional groups in low-rank coals have clearly been shown to be the major actors in retrograde reactions that inhibit efficient thermochemical processing of low-rank coals. In the pyrolysis and liquefaction of low-rank coals, low-temperature crosslinking reactions have been correlated with the loss of carboxyl groups and the evolution of CO2 and H2O.1,2 Pretreatments such as methylation, demineralization, or ion exchange of the inorganic cations reduce crosslinking and CO2 evolution in pyrolysis,2a,3a while the exchange of Na+, K+, Ca2+, or Ba2+ into demineralized coal increases cross-linking and CO2 evolution in pyrolysis and liquefaction.3,4 Solomon et al. have modeled cross-linking in coals by including one cross-link for every CO2 evolved,2a while Niksa has modeled the evolution rates and yields of oxygen-containing species (CO2, CO, and H2O) by including char links when noncondensable gases are expelled.5 These results suggest that decarboxylation may occur by a pathway that initiates retrograde (cross-linking) reactions in the coal polymer. However, the reaction mechanisms by which decarboxylation occur in low-rank coals are not known. It is not clearly understood how decarboxylation X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (2) (a) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42. (b) Ibarra, J. V.; Moliner, R.; Gavilan, M. P. Fuel 1991, 70, 408. (3) (a) Serio, M. A.; Kroo, E.; Chapernay, S.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc. Div., Fuel Chem. 1993, 38 (3), 1021. (b) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 577. (4) Joseph, J. T.; Forria, T. R. Fuel 1992, 71, 75. (5) Niksa, S. Energy Fuels 1996, 10, 173.
S0887-0624(97)00074-1 CCC: $14.00
leads to cross-linking beyond the suggestion that decarboxylation could be a radical process that involves radical recombination or radical addition reactions. The role of decarboxylation in cross-linking processes has recently been brought into question by Manion et al. by their observation that the decarboxylation of benzoic acid derivatives in naphthalene or tetralin yielded only small amounts of aryl-aryl coupling products, which would be stable at t < 400 °C and would constitute a low-temperature cross-link.6 In this study, it was proposed that unactivated carboxylic acids (such as benzoic acid) primarily decarboxylate via the carboxylate anion, while activated carboxylic acids (such as 4-hydroxybenzoic acid) primarily decarboxylate by an acid-catalyzed mechanism. The small portion of decarboxylation that results in coupling appears to proceed by the addition of an aryl radical to the solvent, since the yields of the cross-linked products were reduced by tetralin. Manion et al. proposed that the phenyl radicals were produced by the oxidation of the anion of benzoic acid by an unknown electron-acceptor to form C6H5CO2•, which would rapidly decarboxylate to form the phenyl radical and CO2. We have recently conducted a study of the pyrolysis of neat 1,2-di(3,3′-dicarboxyphenyl)ethane (1) and 1,2di(4,4′-dicarboxyphenyl)ethane (2) under conditions where free radicals were present and found that decarboxylation occurs readily between 350 and 425 °C with no evidence of coupling products or products representative of cross-links.7 We proposed that decarboxylation oc(6) (a) Manion, J. A.; McMillen, D. F.; Malhotra, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37 (4), 1720. (b) Manion, J. A.; McMillen, D. F.; Malhotra, R. Energy Fuels 1996, 10, 776.
© 1997 American Chemical Society
Pyrolysis of Aromatic Carboxylic Acids
curred primarily by an acid-promoted cationic pathway, and protonation by a second carboxylic acid (eq 1) was the rate-determining step.
The possible involvement of a free-radical decarboxylation pathway was ruled out with our finding that no deuterium (99% pure by GC), mp 61-62.5 °C. MS m/z (relative intensity): 286 (M+, 18), 167 (100), 119 (7). 1H NMR (acetoned6): δ 10.00 (s, 1H), 7.83 (s, 1H), 7.77 (d, 1H, J ) 7.8 Hz), 7.65 (d, 2H, J ) 7.7 Hz), 7.61-7.57 (m, 3H), 7.52 (t, 1H, J ) 7.6 Hz), 7.45 (t, 2 H, J ) 7.5 Hz), 7.36-7.32 (m, 3H), 3.00 (m, 4H). 1-Naphthylboronic Acid. 1-Bromonaphthalene (5.00 g, 24.1 mmol) and Et2O (20 mL) were placed in an oven-dried flask under argon. The solution was cooled to 0 °C and secbutyllithium (18.5 mL, 1.3 M in cyclohexane, 24.1 mmol) was added with stirring. After 10 min, the solution was transferred by cannula to a second oven-dried flask containing triisopropyl borate (15 mL, 48 mmol) and Et2O (10 mL) at -78 °C. The resulting solution was stirred for 10 min, warmed to room temperature, and quenched with the careful addition of 10% aqueous HCl. The ethereal extract was washed with H2O (25 mL), and the solvent was removed under reduced pressure until the product precipitated. The resulting slurry was filtered, and a white powder was collected (2.52 g, 40%), mp 202 °C (dec). 1H NMR (acetone-d6, with D2O): δ 8.59 (d, 1H, J ) 8.0 Hz), 7.89-7.83 (m, 3H), 7.46-7.44 (m, 3H). 1-(3-(1-Naphthyl)phenyl)-2-(4-biphenyl)ethene. 1-Naphthylboronic acid (0.50 g, 2.9 mmol), 1-(3-bromophenyl)-2-(4biphenyl)ethene (0.635 g, 1.9 mmol), triethylamine (0.8 mL, 7.85 mmol), Pd(Ph3P)4 (0.066 g, 0.06 mmol), and DMF (7 mL) were placed in an oven-dried reaction flask under argon, equipped with a reflux condenser. The solution was stirred and heated to 100 °C. After 3 h, the solution was cooled, and the solvent was removed under reduced pressure. The residue was partitioned between CH2Cl2 (15 mL) and 10% aqueous NH3 (15 mL), and the organic layer was collected and dried over Na2SO4. The solvent was removed under reduced pressure to produce an oily solid (0.630 g, 87%). The compound was purified by recrystallization from hexane:toluene (98:2) to give a white powder, mp 155-156 °C. MS, m/z (relative intensity): 382 (M+, 100), 305 (3), 255 (4). 1H NMR (CDCl3): δ 7.94-7.87 (m, 3H), 7.67-7.17 (m, 19H). 1-(3-(1-Naphthyl)phenyl)-2-(4-biphenyl)ethane. Crude 1-(3-(1-naphthyl)phenyl)-2-(4-biphenyl)ethene (0.530 g, 1.3 mmol), 10% Pd/C (0.070 g), and EtOH/isopropyl alcohol (1:1, 30 mL) were placed in a Parr hydrogenation bottle and shaken under 50 psi of H2 until 1-(3-(1-naphthyl)phenyl)-2-(4-biphenyl)ethene could no longer be detected by GC analysis (96 h). The solution was vacuum filtered, and the Pd/C was washed with CH2Cl2. The solution was evaporated to dryness to produce an oil. A portion of the product was further purified by flash chromatography using hexane as the eluent to produce a clear oil. MS, m/z (relative intensity): 384 (M+, 17), 217 (5), 167 (100). 1H NMR (CDCl3): δ 7.88 (d, 1H, J ) 8.1 Hz), 7.85 (d, 1H, J ) 8.2 Hz), 7.84 (d, 1H, J ) 8.0 Hz), 7.57 (d, 2H, J ) 7.3 Hz), 7.52-7.23 (m, 15H), 3.00 (s, 4H). 1-(4-Biphenyl)-2-(phenyl)ethene. 1-(3-Bromophenyl)-2(4-biphenyl)ethene (9.50 g, 28.3 mmol) and THF (300 mL) were placed in an oven-dried flask under argon. The resulting solution was stirred and cooled to -78 °C, and n-butyllithium (11 mL, 2.5 M in hexanes, 28 mmol) was added dropwise over a period of 30 min. The solution was stirred for 30 min, and 2-propanol (40 mL, 522 mmol) was slowly added over a period of 10 min followed by the addition of H2O (200 mL). The solution was extracted with CH2Cl2 (500 mL), and the solvent was removed under reduced pressure to produce a white solid (7.2 g, 100%). The product was further purified by recrystallization from toluene, mp 215-218 °C. MS, m/z (relative intensity): 256 (M+, 100), 255 (24), 178 (23). 1H NMR (THFd8) δ 7.66-7.63 (m, 6H), 7.58 (d, 2H, J ) 7.4 Hz), 7.45-7.20 (m, 8H).
Pyrolysis of Aromatic Carboxylic Acids
Energy & Fuels, Vol. 11, No. 6, 1997 1281
Table 1. Product Distributions from the Pyrolysis of 1-(3-Carboxyphenyl)-2-(4-biphenyl)ethane (3) at 400 °C for 1 h with Various Diluentsd entry
1
producta (% yield)
neat
PhCO2H 3-CH3PhCO2H 3-CH3CH2PhCO2H 4-Ph-PhCH3 4-Ph-PhCH2CH3 4-Ph-PhCH(CH3)Ph 4-Ph-PhCH2CH2Ph (4) 4-PhsPhCHdCHPh 4-Ph-PhCH2PhCO2H 4-Ph-PhCH(CH3)PhCO2H 4-PhsPhCHdCHPhCO2H 4-Ph-PhCH2CH2Ph-3-(1-naphthyl) (5) 4-Ph-PhCH2CH2Ph-3-(2-naphthyl) (6) ratio 4:(5 + 6) conversionb mass balance
0.03 1.70 0.08 1.90 0.06 0.06 5.65 0.13 0.10 0.90 1.90
12.6 98.1
2 10:1 N:3
3 10:1 N:3
4 5:5:1 N:Bp:3
5 5:5:1 N:T:3
6 10:1:1.6 N:3:H2O
7 10:1:3.5 N:3:H2O
0.02 2.66 0.09 2.78 0.08 0.02 2.91 0.06 0.04 0.50 1.33 0.14 0.16 9.7 10.8 100.1
0.027 2.56 0.094 2.64 0.082 nd 1.91 0.043 0.039 0.47 1.33 0.13 0.14 7.1 9.5 99.7
0.04 3.00 nd 3.40 0.03 0.02 4.80 0.10 0.05 0.28 1.30 0.08 0.10 18.0c 13.6 102.1
nd 2.50 nd 2.80 nd nd 4.40 nd nd nd 0.14 0.06 0.06 41.0 9.0 99.0
0.02 2.73 0.09 2.83 0.08 0.01 2.26 0.05 0.05 0.51 1.54 0.07 0.08 16.0 10.35 99.8
nd 2.60 0.06 2.71 0.04 nd 2.52 0.07 nd 0.36 1.51 0.05 0.04 26.8 10.0 100.0
a (mol product formed/mol starting material) × 100. b Conversion based on products recovered. c Three isomers of biphenyl coupled to 4 were also detected (0.08% yield) and are included in the ratio. d N ) naphthalene. Bp ) biphenyl. T ) tetralin. nd ) not detected.
1-(4-Biphenyl)-2-(phenyl)ethane. 1-(4-Biphenyl)-2-(phenyl)ethene (4.0 g, 15.6 mmol), ethanol (60 mL), and 5% Pd/C (0.40 g) were placed in a Parr hydrogenation vessel. The vessel was pressurized with 50 psi of H2 and shaken for 24 h. The solution was filtered and the solvent was removed under reduced pressure, producing a white solid (4.0 g, 99%). The solid was further purified by recrystallization from 2-propanol, mp 109-110 °C. MS, m/z (relative intensity): 258 (M+, 22), 167 (100), 152 (12), 91 (14). 1H NMR (CDCl3): δ 7.57 (d, 2H, J ) 7.6 Hz), 7.51 (d, 2H, J ) 7.9 Hz), 7.41 (t, 2H, J ) 7.6 Hz), 7.33-7.19 (m, 8H), 2.95 (m, 4H). Pyrolyses. Pyrolysis of the compounds was performed by loading Pyrex tubes with the appropriate amounts of substrate and diluent and conducting three freeze-pump-thaw cycles prior to sealing the tube at ca. 10-5 Torr. Tube volumes were kept to a minimum with the solid filling roughly one-half of the sealed pyrolysis tube. The pyrolyses were performed in a Carbolite tube furnace, which maintained a temperature of 400 ( 1 °C. After the pyrolysis, the samples were quickly removed from the furnace and cooled in liquid N2. The tubes were cracked open, and the solid products were removed with a 2:1 mixture of pyridine:N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA, Pierce). Internal standards (2-phenylbenzoic acid and 3,5-dimethylbenzoic acid) were added and the reaction mixtures analyzed by GC and GC-MS. For toluene analysis, the solids from the pyrolysis were extracted with CH2Cl2, internal standards (cumene and those mentioned above) were added, and the sample was analyzed by GC. The remaining products were analyzed as the trimethylsilyl esters by removal of the CH2Cl2 under reduced pressure and dissolving the residue in BSTFA:pyridine and reanalyzing. The identities of products from the thermolysis of 3 were determined by GCMS analysis and were further confirmed by comparison with commercially available or synthesized authentic materials. Mass balances were calculated by comparing the ratios (×100) of recovered starting material and product equivalents to the initial charge of starting material. For CO/CO2 analysis, the cooled pyrolysis tubes were placed into a tightly fitted Tygon tube containing a septum on one end. The tubes were cracked open within the Tygon tubing, and the gas was drawn out via syringe through the septum. One sample was analyzed immediately, and the remainder of the gas in the pyrolysis tube was quickly drawn out and stored in a crimped capped autosampler vial with a Teflon-lined septum. The gas was stored in this manner because it was found that CO2 slowly leaked through the Tygon tubing. Additional GC analyses of the gas were performed using the gas stored in the crimp top vial. For HPLC analysis, the reaction products were dissolved
in CH2Cl2/THF (1:1) and were analyzed without derivatization of the acids.
Results and Discussion Thermolysis of Monoacid 3. The major products from the thermolysis of neat 3 at 400 °C are shown in eq 2, and a typical product distribution for a 1 h pyrolysis is given in Table 1, entry 1.
In addition to the major products, we have also identified minor amounts of 3-ethylbenzoic acid, benzoic acid, 4-ethylbiphenyl, 1-(4-biphenyl)-1-phenylethane, 1-(3carboxyphenyl)-1-(4-biphenyl)ethane, and 3-carboxyphenyl-4-biphenylmethane. A series of liquid-phase thermolyses were run at 400 °C, and the normalized product yields of the major products are plotted as a function of conversion in Figure 1. Throughout the large conversion range studied, decarboxylation to 1-(4-biphenyl)2-phenylethane (4) was the dominant product, and the mass balances were good even at high conversion (97% at 40% conversion). The 4-phenyltoluene, m-toluic acid, substituted stilbenes, and rearranged products were produced from the C-C homolysis of the ethylene linkage in 3 and subsequent reactions of the benzyl radicals. Full details of the mechanism of formation of these products have been described in previous publications.7a,8 Several other products were also formed in the pyrolysis that have not been identified, but based upon the GC peak area and the good mass balance, the amounts of these products are very small (80%) in the thermolysis of 3 cannot take place by a radical pathway and must occur by another mechanism, such as the acidpromoted cationic mechanism previously described.7a (13) (a) Fahr, A.; Stein, S. E. J. Phys. Chem. 1988, 92, 495. (b) Chen, R. H.; Kafafi, S. A.; Stein, S. E. J. Am. Chem. Soc. 1989, 111, 1418. (14) The bond dissociation energy of the aldehydic C-H bond is similar to that of the benzylic C-H bond (see ref 20) so that hydrogen abstraction of the aldehydic and benzylic C-H will be competatitive. (15) Rate constant calculated from log k (s-1) ) 14.6 - 29.4/ (2.303RT) in the following. Solly, R. K.; Benson, S. W. J. Am. Chem. Soc. 1971, 93, 2127. (16) The alcohol is proposed to be formed as a byproduct of the arylation reaction, which initially forms a cyclohexadienyl radical intermediate (see eq 3). This intermediate can transfer a hydrogen to the aldehyde to form the hydroxybenzyl radical or lose a hydrogen atom (by β-scission), which can also reduce the aldehyde. Hydrogen abstraction of the hydroxybenzyl radical produces the alcohol. The activation barrier for the addition of a hydrogen atom to formaldehyde, to form the hydroxymethyl radical, has been estimated as 1.2 kcal mol-1,17 which is similar to the intrinsic activation barrier for the addition of a hydrogen atom to an alkene (1-2 kcal mol-1).18 Therefore, reduction should have a lower activation barrier than hydrogen abstraction. In the thermolysis of 7 in naphthalene-d8, GC-MS analysis of the silylated alcohol found only 15% d1, indicating that the initial hydrogen addition occurs primarily at the oxygen of the aldehyde to make the resonance-stabilized hydroxybenzyl radical rather than at the benzylic carbon, which produces an unstabilized alkoxy radical.
Energy & Fuels, Vol. 11, No. 6, 1997 1283
In the thermolysis of aldehyde 7, the yields of arylated products were found to be dependent on the concentration and structure of the aromatic acceptor molecule, since arylation must compete with hydrogen abstraction. For example, the ratio of arylation to hydrogen abstraction ((5 + 6):4) in the thermolysis of 7 diluted in a 10-fold excess of naphthalene was 2.1:1, while in a 3.5-fold excess of naphthalene or in a 10-fold excess of biphenyl, the ratio was 1:1 and 1:1.25, respectively. The proportion of 4:5:6 was not affected by the addition of H2O (1.5 equiv:7) to the pyrolysis mixture. When thermolysis of 7 was run in naphthalene-d8, the rate of arylation dropped by a factor of 2, which is consistent with the isotope effect reported by Chen et al. for the phenylation of naphthalene-d8 in the gas phase.13b This confirms that hydrogen loss is the rate-determining step in the arylation reaction and that aryl radical addition is reversible. The yields of arylated products also depend on the concentration of hydrogen donors. For example, the ratio of arylation to hydrogen abstraction in the thermolysis of 7 diluted 10-fold in a 1:1 mixture of naphthalene:biphenyl is 1.4:1.0 while the ratio for a mixture of naphthalene:tetralin is 1.0:4.0. Therefore, the yields of cross-linked products will be highly dependent on the environment in which the aryl radical is generated. Formation of Anhydrides in the Thermolysis of 3. Although the results discussed above show that the bulk of decarboxylation in the thermolysis of 3 is ionic and does not lead to retrograde reactions, under certain reaction conditions, a small amount of cross-linking chemistry can occur in association with decarboxylation. These results suggest that in the thermolysis of 3, the 3-(2-(4-biphenyl)ethyl)phenyl radical, is produced and reacts with naphthalene to form arylated products 5 and 6. The most straightforward route to produce this aryl radical is by hydrogen abstraction from the carboxy group to produce a benzoyloxy radical followed by rapid decarboxylation to produce the aryl radical (k ≈ 6.4 × 109 s-1 for decarboxylation of the benzoyloxyl radical at 400 °C).19 However, on the basis of bond strengths, the benzylic radicals (from C-C homolysis of 3) would favor hydrogen abstraction from the four benzylic hydrogens of 3 (86 kcal/mol)20 as opposed to the one carboxylic acid hydrogen (O-H, 101 kcal/mol).21 The electron-transfer pathway proposed by Manion et al.6 for the formation of cross-linked products did not seem like a viable pathway either, in light of our recent results on the thermolysis of benzoic acid9 and our previous results on the pyrolysis of the diacids,7 which indicated that decarboxylation occurs by a cationic mechanism rather than an anionic mechanism. However, on the basis of the reduction of cross-linked products by the addition of water, water must inhibit (17) Tsuboi, T.; Katoh, M.; Kikuchi, S.; Hashimoto, K. Jpn. J. Appl. Phys. 1981, 20, 985. (18) Poutsma M. L. A Review of Thermolysis Studies of Model Comounds Relevant to Processing of Coal. Report ORNL/TM-10637; ORNL: Oak Ridge, TN, 1987; p 28; available through NTIS. (19) Chateauneif, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1988, 110, 2886. (20) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493. (21) The bond strength for the O-H bond was calculated using additivity tables in the following. Benson, S. Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976. The heat of formation of PhCO2• (-21 ( 2 kcal/mol) was obtained from the following. Mortimer, C. T. Reaction Heats and Bond Strengths; Pergamon Press: New York, 1962; p 142.
1284 Energy & Fuels, Vol. 11, No. 6, 1997
the formation of an intermediate in the thermolysis of monoacid 3 that leads to the production of aryl radicals, since water has no affect on the arylation reaction. Considering the known thermal chemistry of carboxylic acids, the only water sensitive intermediate that could be formed in the thermolysis of 3 is an anhydride, 8, formed by the condensation of two equivalents of acid (eq 5).
Eskay et al. Table 2. Major Products from the Pyrolysis of Anhydride 8 in Naphthalene (1:10) at 400 °C for 15 min producta (mol %)
run 1 (%)
run 2 (%)
4-Ph-PhCH3 4-Ph-PhCH2CH2Ph (4) 4-PhsPhCHdCHPh 4-Ph-PhCH2CH2Ph-3-(1-naphthyl) (5) 4-Ph-PhCH2CH2Ph-3-(2-naphthyl) (6) conversionb
2.3 35.9 5.2 40.3 16.4 22.3
4.2 25.5 2.8 49.3 17.9 21.0
a A small amount of aldehyde 7 was also detected. b Conversion based on number of moles of 8 needed to decompose to form products detected.
Anhydrides are known to be formed if carboxylic acids (such as benzoic acid) are heated under conditions in which H2O is removed.22 Therefore, we initially tried to determined if anhydride 8 could be formed and identified under our reaction conditions. Since it is unlikely that a large molecule such as 8 would elute through the GC, the reaction mixtures from the pyrolysis of 3 in naphthalene were analyzed by reverse-phase HPLC. From the sealed tube pyrolysis of 3:naphthalene (1:3.5 or 1:10 equiv) at 400 °C for 1.5 h, a small amount of 8 (ca. 1.5%, or 0.4% of the starting carboxylic acid) was detected in addition to the previously described products. The identity of the anhydride was confirmed by comparing the HPLC retention time of the product formed in the pyrolysis of 3 with an authentic sample of 8 prepared by independent synthesis (see Experimental Section) and by its mass spectrum obtained by LC-MS. The detection of only a small amount of anhydride is also consistent with the excellent mass balances obtained in these pyrolyses (Table 1), which do not include the anhydride. On the other hand, in the thermolysis of 3 in naphthalene with H2O (1:3.5:1.5 equiv), anhydride 8 was not observed and the cross-linked products 5 and 6 were reduced 3-fold when compared to the run without H2O. To determine if the formation of anhydride was unique to the thermolysis of 3, the thermolysis of benzoic acid in naphthalene (1: 2.5 equiv) was also investigated. It was discovered that a small amount of benzoic anhydride was formed (∼2% of total starting acid) in the thermolysis of benzoic acid. However, as with 3, when H2O was added to the thermolysis of benzoic acid in naphthalene, the anhydride was not observed. These results show that anhydride formation can occur in the pyrolysis of carboxylic acids under the sealed tube thermolysis conditions used in this study and that the anhydride formation was inhibited by the presence of H2O. Thermolysis of Anhydride 8. Now that the formation of the anhydride of 3 has been established, the thermal decomposition pathways of anhydride 8 under our reaction conditions were investigated. Unfortunately, there have been very few studies on the thermolysis of aromatic anhydrides. The high-temperature (ca. 700 °C) pyrolysis of phthalic anhydride has been investigated as a method to produce benzyne.23 The vapor-phase pyrolysis of benzoic anhydride has been investigated in a flow system at lower temperatures (22) Davison, D.; Newman, P. J. Am. Chem. Soc. 1952, 74, 1515. (23) (a) Fields, E. K.; Meyer, S. J. Chem. Soc., Chem. Commun. 1965, 474. (b) Fields, E. K.; Meyer, S. J. Org. Chem. 1966, 31, 3307. (c) Patterson, J. M.; Shiue, C.-Y.; Smith, W. T., Jr. J. Org. Chem. 1973, 38, 387.
Table 3. Products from the Pyrolysis of Benzoic Anhydride in Naphthalene (1:10) at 400 °C for 1 h producta (mol %)
run 1 (%)
run 2 (%)
benzene 1-phenylnaphthalene 2-phenylnaphthalene phenyl benzoate conversionb
29.5 40.4 26.4 6.2 7.5
27.9 43.1 25.2 6.1 11.0
a Small amounts of 1- and 2-naphthylphenyl ketone and 1,1′-, 2,2′-, and 1,2′-binaphthyl were also detected. b Conversion based on number of moles of benzoic anhydride needed to decompose to form products detected.
(525-565 °C with a contact time of ca. 1-2 min).24 In this study, the anhydride was found to be remarkably thermostable, but at 550 °C, a complex mixture of products were formed, containing benzene and benzoic acid as the dominant products and smaller amounts of biphenyl, benzaldehyde, and benzophenone. The low reactivity of the anhydride is consistent with our thermochemical estimates of the C(dO)sO bond strength of 84 kcal mol-1 for benzoic anhydride.25 To determine if the thermolysis of an anhydride could lead to the observed cross-linked products, the thermolysis of 8 and benzoic anhydride diluted in a 10-fold excess of naphthalene was investigated at 400 °C and the data are presented in Tables 2 and 3, respectively. Surprisingly, each anhydride produced significant amounts of arylated naphthalenes, although our thermochemical estimates predict that benzoic anhydride would be relatively stable at 400 °C. The ratio of arylation to abstraction for the anhydrides, ca. 2:1, was similar to that found in the pyrolysis of aldehyde 7, indicating that aryl radicals are probably formed in the decomposition of the anhydrides. Small amounts of binaphthyls were also produced in the pyrolyses, which is also consistent with the formation of aryl radicals.26 The rate of decomposition of benzoic anhydride and 8 was surprisingly large (9% h-1 and 88% h-1, respectively) in light of our estimate that conversion would be ,1% h-1 based on the C(dO)sO bond strength.27 (24) Allan, R. J. P.; Jones, E.; Ritchie, P. D. J. Chem. Soc. 1957, 524. (25) The (Od)CsO bond strength was calculated using the thermochemical data avalable in ref 21. (26) Binaphthyls are formed by abstraction of hydrogen by an aryl radical or hydrogen atom from naphthalene to form the naphthyl radical followed by coupling with a second naphthalene. Pyrolysis of naphthalene alone under these reaction conditions failed to produce any binaphthyls, establishing that the formation of the binaphthyls is a result of the chemistry of the decomposition of the anhydride. (27) Anhydride 8 was synthesiszed by two independent pathways (see Experimental Section), and pyrolysis of 8 from each preparation produced identical results. This result suggests that the high conversions observed are not due to an impurity from any reagents used in the synthesis.
Pyrolysis of Aromatic Carboxylic Acids Scheme 1
Thus, it seems unlikely that the anhydrides are decomposing exclusively by C(dO)sO homolysis to produce the benzoyl and benzoyloxyl radicals that would rapidly decarbonylate and decarboxylate to produce aryl radicals (Scheme 1, path a). However, decomposition could occur by induced homolysis, i.e., by a chain reaction as observed in the decomposition of benzoyl peroxide.28 The decomposition of anhydride 8 could be enhanced by the addition of benzylic radicals (formed by C-C homolysis of 8), hydrogen atoms (from the arylation reaction), or aryl radicals to the (Od)CsOsC(dO) linkage. In support of the induced decomposition pathway (Scheme 1, path b), a small amount of phenyl benzoate (ca. 6.0 mol %) was found in the pyrolysis of benzoic anhydride in naphthalene. This product could arise from phenyl radical addition to the carbonyl oxygen followed by β-scission. We also observe a small amount of benzoic acid in the pyrolysis of benzoic anhydride. This could arise from a small amount of hydrolysis of the anhydride or from addition of a hydrogen atom to the anhydride.32 Support for the formation of benzoic acid by the addition of hydrogen atoms to the anhydride is provided by the pyrolysis of aldehyde 7 in which it was proposed that the alcohol was formed by the addition of a hydrogen atom to the aldehyde.16 One consequence of the involvement of the induced decomposition pathway is that more CO should be produced than CO2, while for the pathway involving homolysis of the C(dO)sO bond, a CO:CO2 ratio of unity is expected (Scheme 1). In the pyrolysis of benzoic anhydride and 8, the ratio of CO:CO2 was measured as (28) Studies on the thermal decomposition of benzoyl peroxide ((PhCO2-)2) found that the rate of decomposition varied considerably from solvent to solvent while radical scavengers reduced the rate.29 It was determined that a chain reaction occurred in certain solvents in which the phenyl radical, formed from decarboxylation of the benzoyloxyl radical formed from the homolysis of the O-O bond, could abstract hydrogen from the solvent. The solvent radical would add to the benzoyl peroxide to form the benzoyloxyl radical and an ester (in diethyl ether, 1-ethoxyethyl benzoate was isolated in 95% yield).30 Thermolysis of 18O-labeled benzoyl peroxide, in which the carbonyl oxygen was labeled, and triphenylmethane in benzene produced triphenylmethyl benzoate as the major product in which most of the 18O remained in the carbonyl group, indicating that the triphenylmethyl radical attacked the peroxidic oxygen atom.31 In the thermolysis of 8, thermochemical estimates predict20,21 the addition of the phenyl radical to the carbonyl oxygen is favored over radical addition to the ether oxygen as observed for benzoyl peroxide. (29) Koenig, T. In Free Radicals; Kochi, J. K., Ed.; John Wiley and Sons: New York, 1973; Vol. 1, Chapter 3, p 116. (30) Bartlett, P. D.; Nozaki, K. J. Am. Chem. Soc. 1947, 69, 2299. (31) von Doering, W. E.; Okamoto, K.; Krauch, H. J. Am. Chem. Soc. 1960, 82, 3579. (32) We have quantitated the amount of benzoic acid produced in the pyrolysis of benzoic anhydride in order to determine if the quantities produced could be sucessfully fit to the mechanism depicted in path b of Scheme 1. However, at this point, the interpretation has been complicated by our finding that some benzoic acid can be produced by hydrolysis of benzoic anhydride by trace amounts water present in the pyrolysis tubes or in the materials themselves. In addition, in the presence of hydrogen atoms and phenyl radicals, benzoic acid can decarboxylate by a free-radical pathway. Therefore, it is difficult to determine how much benzoic acid was produced by each pathway in the pyrolysis.
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3:1 and 25:1, respectively. The excess CO detected in these pyrolyses is consistent with the induced pyrolysis mechanism and inconsistent with a pure homolysis mechanism. The small amount of CO2 most likely arises from decarboxylation of the acid formed by hydrolysis of the anhydride or hydrogen addition to the carbon atom of the anhydride followed by β-scission to form the aldehyde and the benzoyloxyl radical. However, it is currently unclear why the induced decomposition reaction is more favorable for 8 than for benzoic anhydride.33 Regardless of the exact pathways involved in the decomposition of the anhydrides, these studies show that the rate of decomposition of the aromatic anhydrides (up to 88% h-1) is faster than that of decarboxylation (∼4% h-1) by the ionic pathway. Formation and decomposition of
