Energy & Fuels 2005, 19, 365-373
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Pyrolysis of Aromatic Carboxylic Acid Salts: Does Decarboxylation Play a Role in Cross-Linking Reactions? Reza Dabestani,* Phillip F. Britt, and A. C. Buchanan III Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS-6197, Oak Ridge, Tennessee 37831-06197 Received August 18, 2004. Revised Manuscript Received November 9, 2004
The pyrolysis of sodium benzoate, potassium benzoate, and calcium benzoate has been investigated between 435 and 500 °C as models for carboxylic acid salts in low-rank coals to determine if the decarboxylation of benzoate salts can contribute to cross-linking reactions during the thermal processing of low-rank coals. Decarboxylation was the dominant reaction pathway for all three salts. However, char formation (leading to poor mass balance) was also observed in all cases. The reaction proceeds via an anionic pathway, where the initial decarboxylation of benzoate salt leads to the formation of a phenyl anion as the key intermediate. Proton abstraction by the phenyl anion from another molecule of benzoate leads to the formation of benzene and benzoate dianion, which can react with CO2 to form a di-salt of phthalic acid (i.e., the Henkel reaction). Both benzene and phthalate account for >95% of the total products in the pyrolysis of sodium and potassium benzoates. The minor products diphenylmethane, benzophenone, triphenylmethane, and 9-phenylfluorene can form by a mechanism involving the reaction of phenyl anion with alkali benzoate to generate benzophenone, which reacts further to form the other products. Contrary to previous reports, calcium benzoate pyrolysis proceeds via an anionic mechanism to form benzene and benzophenone as the major products. No evidence of any calcium phthalate formation is observed in the pyrolysis of calcium benzoate. Furthermore, the reaction of benzophenone with the phenyl anion to form diphenylmethane, triphenylmethane, and 9-phenylfluorene seems to be slowed considerably (compared to sodium and potassium benzoates), based on the observed yield of these minor products. The rate constant for the pyrolysis of these benzoate salts, which decarboxylate slower than corresponding acids, decreases in the following order: potassium benzoate > sodium benzoate . calcium benzoate. The pyrolysis rate is increased by at least a factor of 3 in the presence of water. Water traps the intermediate phenyl anion very efficiently to form benzene almost exclusively. Based on these findings, it can be concluded that the presence of water can drastically reduce cross-linking reactions and significantly improve liquefaction.
Introduction Oxygen functional groups (i.e., carboxylic acids and their alkali and alkaline-earth metal salts, phenols, and aryl esters) present in low-rank coals are believed to be major contributors to retrograde reactions that inhibit efficient thermochemical conversion of low-rank coals to liquid fuels. Although low-temperature (T e 400 °C) cross-linking reactions have been correlated with the loss of carboxyl groups and the evolution of carbon dioxide (CO2) and water,1,2a-c,3 the role of decarboxylation, leading to cross-linked products, has been questioned by Manion et al.,4 who observed that the decarboxylation of benzoic acid derivatives in naphthalene or tetralin yielded only small amounts of aryl-aryl coupling products (a potential crosslink). It was proposed (1) Derbyshire, F.; Davis, A.; Lin, R. Energy Fuels 1989, 3, 431. (2) (a) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42 and references therein. (b) Serio, M. A.; Kroo, E.; Chapernay, S.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 1021. (c) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (d) Joseph, T. J.; Forrai, T. R. Fuel 1992, 71, 75. (3) Ibarra, J. V.; Moliner, R.; Gavilan, M. P. Fuel 1991, 70, 408. (4) (a) Manion, J. A.; McMillen, D. F.; Malhotra, R. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1992, 37 (4), 1720. (b) Manion, J. A.; McMillen, D. F.; Malhorta, R. Energy Fuels 1996, 10, 776.
that benzoic acid decarboxylates via the carboxylate anion intermediate, whereas activated carboxylic acids (e.g., 4-hydroxybenzoic acid) decarboxylate via an acidcatalyzed mechanism.4 It was further proposed that aryl-aryl coupling products are formed by the addition of an aryl radical, produced by the oxidation of benzoate anion by an unknown electron acceptor, to the solvent (because cross-linked product yields were reduced in tetralin).4 Cross-linking reactions (determined by a loss of solvent swelling) and the evolution of CO2 are reduced in coal pyrolysis by methylation, demineralization, or ion exchange of the inorganic cations.2a,b The oxidation or ion exchange of Na+, K+, Ca2+, or Ba2+ cations into demineralized coal, on the other hand, was observed to increase cross-linking and CO2 evolution in pyrolysis and liquefaction.2b,2d,5 The observed increase in liquefaction yields and the decrease in tar has been attributed to the electrostatic and/or covalent interactions of divalent cations, which act as initial crosslinks in the coal network.2b The aforementioned notion has been further (5) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 577.
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supported by the work of Wornat and Sakurovs,6 who reported that the proton magnetic resonance thermal analysis of cation-exchanged coals exhibit a lower extent of fusion during heating, compared to the protonexchanged form. Furthermore, the divalent cations show a more pronounced effect than the monovalent cations.6 Although the role of carboxylic acids and their corresponding salts on the pyrolysis, combustion, and liquefaction of low-rank coals has been examined, the formation of primary products, reaction pathways, and kinetics was not well-understood. Thus, we initiated a systematic study of the pyrolysis of simple and polymeric model compounds containing aromatic carboxylic acids to provide a fundamental understanding of the decomposition pathways and reaction kinetics for aromatic carboxylic acids at temperatures relevant to coal processing.7 The results of our studies on the pyrolysis of carboxylic acids have revealed that, at 325-425 °C, decarboxylation proceeds primarily via an acid-promoted cationic pathway that does not lead to a significant amount of cross-linked products.7a However, under certain reaction conditions, anhydrides, which are lowtemperature crosslinks, can be formed by the condensation of aromatic carboxylic acids.7b,c Anhydrides can decompose via a radical-induced decomposition pathway to produce aryl radicals, which can lead to arylated (cross-linked) products. Because many of the carboxylic acids in coal are ion-exchanged as alkali and alkalineearth metal salts,5,8,9 we recently investigated the pyrolysis of the salts of aromatic carboxylic acids via thermogravimetry, coupled with mass spectrometry (TG-MS) analysis of the evolved gases. These studies have shown that the salts decompose at higher temperatures than the corresponding acids and different products are formed.8 Furthermore, our initial results indicated that divalent salts (e.g., calcium salts) may decompose via a different pathway than the monovalent salts, such as sodium and potassium. Hites and Biemann10 have reported that the pyrolysis of calcium benzoate at 500 °C proceeds via a free-radical mechanism, to produce mainly benzene and benzophenone (in equal amounts) and small amounts of biphenyl and 9-fluorenone. Artok and Schobert11 have reported that sodium benzoate undergoes an inefficient decomposition reaction at 450 °C (3.5% conversion) to produce predominately benzaldehyde (2.1%) and biphenyl (0.9%), with small amounts of benzene (0.2%) and toluene (0.3%), in addition to carbon monoxide (CO) (0.6%) and CO2 (1.2%); however, no mechanistic details were provided for product formation. We have undertaken a new systematic study on the pyrolysis of alkali and alkaline-earth metal salts of benzoic acid in an attempt to provide mechanistic insights into the pathways of product formation to determine if decarboxylation leads to crosslinking. (6) Wornat, M. J.; Sakurovs, R. Fuel 1996, 75, 867. (7) (a) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1996, 10, 1257. (b) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1997, 11, 1278. (c) Britt, P. F.; Mungall, W. S.; Buchanan, A. C., III. Energy Fuels 1998, 12, 660. (d) Britt, P. F.; Buchanan, A. C., III; Eskay, T. P.; Mungall, W. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44 (3), 533. (8) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75. (9) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427. (10) Hites, R. A.; Biemann, K. J. Am. Chem. Soc. 1972, 94, 5772. (11) Artok, L.; Schobert, H. H. J. Anal. Appl. Pyrolysis 2000, 54, 215.
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Experimental Section Sodium benzoate (Aldrich, 99%) and potassium benzoate (Aldrich, 99%) were used as received, and calcium benzoate was synthesized according to a previously reported procedure.12 For comparison purposes, another source of calcium benzoate (City Chemicals, >99% purity) was also used without further purification. Benzene (EM Science), acetone (EMD, spectral grade), ethyl acetate (Fisher), methylene chloride (EM Science), and D2O (99.8%, MSD Isotopes) were all used as received. Biphenyl (Aldrich, 99%), diphenylmethane (Aldrich, 99%), benzophenone (Aldrich, 99%), triphenylmethane (Aldrich, 99%), diphenylmethanol (Aldrich, 99%), triphenylmethanol (Aldrich, 98%), fluorenone (Aldrich, 99%), benzoic acid (Mallinckrodt), phthalic acid (Aldrich, 99%), 4-phenylbenzoic acid (Aldrich, 95%), 2-methylnaphthalene (Aldrich, 99%), and p-toluic acid (Aldrich, 98%) were all recrystallized at least once before use. Gas chromatography (GC) analysis was performed using a Hewlett-Packard 5890 Series II gas chromatograph that was equipped with a J&W Scientific 30 m × 0.25 mm inner diameter (i.d.), 0.25-µm film thickness DB-5 column and a flame ionization detector. Mass spectra were obtained at 70 eV on a Hewlett-Packard 5972 gas chromatograph/mass spectrometer that was equipped with a capillary column identical to that used for GC analysis. Pyrolysis of the compounds was conducted by loading small Pyrex tubes with 20-50 mg of substrate and performing five freeze-pump-thaw cycles prior to flame sealing the tube at ca. 10-5 Torr. Tube volumes were kept to a minimum (total volume of 95%) have been obtained, based on the weight of the acetone-insoluble solids and the GC yields (see column 11 in Table 1) of the acetonesoluble products (i.e., hydrocarbons), as well as based on the GC yields of both acetone-soluble products and acidified solid residue (see column 10 in Table 1), indicating good recovery of all of the products from the reaction mixture (except at the highest conversion). Thus, based on the observed mass balances, no major products are unaccounted for. Figure 1 shows the conversion as a function of time for the pyrolysis of sodium benzoate at 450 °C. From the slope of the line, a rate constant of k ) 1.7 × 10-5 s-1 is obtained for the pyrolysis of sodium benzoate (note that additional data points that are not shown in Table 1 were used in Figure 1). The fact that we observe benzene and sodium phthalate as the dominant reaction products suggests that they are most likely formed by an uncatalyzed Henkel(13) Dabestani, R.; Britt, P. F.; Buchanan, A. C., III. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2003, 48 (2), 565.
Figure 1. Plot of normalized total products yield (conversion), as a function of time, for the pyrolysis of degassed sodium benzoate at 450 °C.
type reaction.14,15 The Henkel reaction is an industrialscale thermal rearrangement or disproportionation (350-500 °C) of alkaline salts of aromatic carboxylic acids to symmetrical diacids in the presence of cadmium or other metallics and carbon dioxide. For example, potassium benzoate reacts under these conditions to form terephthalic acid and benzene, whereas phthalic acid forms terephthalic acid. The role of cadmium is to facilitate the decarboxylation.15 In the Henkel reaction, decarboxylation of sodium benzoate (1) leads to the formation of the phenyl anion, which can abstract a proton from another molecule of sodium benzoate to form benzene and a phenyl anion, which can react with the CO2 to form PHNa (Scheme 2). The fact that we observe PHNa instead of terephthalate in the pyrolysis of sodium benzoate may suggest that, in the absence of a catalyst, phthalate formation is favored. The remaining products could also be formed by an anionic reaction involving the phenyl anion. Because the reaction tubes are sealed under vacuum, the reaction of the benzene anion with CO2 will be slow. Thus, the anion could react with a sodium benzoate to form BP and sodium oxide, as shown in Scheme 3. BP could undergo additional reaction with a benzene anion to produce the sodium salt of triphenylmethanol. However, triphenylmethanol was not observed as a product. Nevertheless, an independent pyrolysis of triphenylmethanol, which proceeds much more rapidly than that of sodium benzoate at 450 °C, produced TPM and 9PF, (14) Ogeta, Y.; Hojo, M.; Morikawa, M. J. Org. Chem. 1960, 25, 2082. (15) McNelis, E. J. Org. Chem. 1965, 30, 1209 and references therein.
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Dabestani et al. Scheme 2
Scheme 3
in addition to carbonaceous solids. To test the intermediacy of BP in the formation of these products, a sample of sodium benzoate containing 5 mol % BP (co-mixed) was pyrolyzed side-by-side with a sample of sodium benzoate under identical conditions. Comparison of the pyrolysis products revealed that, in the sample containing BP, the yield of TPM and 9PF was 2-3 times higher and the yield of benzene was ∼30% lower, compared to that for sodium benzoate alone. These results suggest that BP is an intermediate to the formation of TPM and 9PF via the triphenylmethanol route. It was also shown that BP was stable at 450 °C, under the pyrolysis
conditions. Although the exact mechanism for the formation of BIP and DPM is not known, a possible pathway involves nucleophilic aromatic substitution of phenylsodium on benzene to produce BIP and a hydride, which is captured by BP to produce the sodium salt of diphenyl methanol. Figure 2 shows the change in the yield of products as a function of pyrolysis time. It is clear from the plot that the PHNa yield increases, at the expense of B, as the pyrolysis time is increased. Such behavior suggests that the reaction of the benzene anion with another molecule of sodium benzoate to form a sodium benzoate dianion (intermediate 3 in Scheme 2) becomes more competitive with protonation of the benzene anion as the pyrolysis time is increased. This is not too surprising, in view of the fact that the CO2 concentration, which is required for the formation of PHNa, increases as the pyrolysis time is increased. However, experimental difficulties associated with introduction of CO2 into the Pyrex pyrolysis tubes have hampered our attempt to test this hypothesis. B can also form via reaction of the phenyl anion with trace amounts of water present in the sample and/or in the Pyrex reaction vessel. As the water is consumed, the PHNa yield increases. The significant run-to-run variation (∼10%) in the conversion and product distribution was probably a consequence of residual water presence (vide infra). The fact that no significant change in the yield of other products is observed at higher pyrolysis times indicates that the mechanism of their formation is unchanged.
Figure 2. Changes in the product yields, as a function of time, for the pyrolysis of degassed sodium benzoate at 450 °C.
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Table 2. Product Yields as a Function of Time for the Pyrolysis of Potassium Benzoate at 450 °C Normalized Product Yields (mol %)a
Mass Balance (%)
pyrolysis time (s)
lossb (%)
B
BIP
DPM
BP
TPM
9PF
PHNa
Ac
A + Wd
300 600 900 1800 3600 5400
7.2 12.5 15.8 24.8 31.7 33.9
82.3 83.9 79.2 81.3 83.5 84.1
0.4 0.7 1.0 1.3 2.2 3.9
0.0 0.0 0.0 0.1 0.2 0.2
0.0 0.0 0.0 0.1 0.3 0.4
0.0 0.0 0.0 0.0 0.03 0.04
0.0 0.0 0.0 0.05 0.3 0.6
17.3 15.4 19.8 17.2 13.5 10.9
104 76.0 77.0 64.3 52.6 48.5
119 94.0 94.3 93.4 95.6 94.6
a Abbreviations are as follows: B, benzene; BIP, biphenyl; DPM, diphenylmethane; BP, benzophenone; TPM, triphenylmethane; 9PF, 9-phenylfluorene; and PHNa, sodium phthalate. b Loss (percent conversion) values are based on the total moles of products, relative to starting material. c Mass balance obtained from GC quantitation of the products. d Mass balance obtained from GC quantitation of the acetone-soluble products and the weight of solid residue from initial acetone wash after the pyrolysis.
Figure 3. Changes in the product yields, as a function of time, for the pyrolysis of degassed potassium benzoate at 450 °C.
When the pyrolysis of sodium benzoate was performed in the presence of water or D2O (30 mol %), benzene or deuterated benzene, respectively, accounted for g99% of the products, with BIP, DPM, and BP accounting for the remaining 1%. There was no evidence of any TPM and/or 9PF formation. The rate of benzene/deuterated benzene formation was ∼3 times faster than that observed in the absence of water or D2O. Thus, water could react with the salt to form benzoic acid, which can decompose via an electrophilic pathway (as evidenced by multiple deuteriums in the benzene) and/or the water is protonating the anion before other reactions can occur (i.e., rate-limiting protonation). Based on these findings, it seems that the pyrolysis of sodium benzoate proceeds via an anionic mechanism involving both a Henkel-type reaction that leads to the formation of B and PHNa (Scheme 2) and a second pathway (Scheme 3) leading to BP, BIP, DPM, phenylbenzoate, TPM, and 9PF. 2. Pyrolysis of Potassium Benzoate. The pyrolysis of deaerated potassium benzoate at 450 °C proceeds much more readily than that of sodium benzoate to produce B and potassium phthalate (PHK) as the major products. Only BIP is observed as the minor product at
the early stages of the pyrolysis (up to 15 min). As the pyrolysis time is increased (to >900 s), the other minor products (DPM, BP, TPM, and 9PF) are also detected. At reaction times of >300 s, a black material was formed in the reaction tube. As opposed to sodium benzoate, the mass balances are poor, indicating that cross-linked (or non-acetone-soluble) products are formed. TGA data shows that potassium benzoate decomposes at a lower temperature than sodium benzoate.12 Table 2 shows the change in yield of products, as a function of pyrolysis time. The fact that identical reaction products are observed for the sodium and potassium benzoates suggest that the mechanisms of pyrolysis are most likely the same. Note that the product distribution for the major products at the lowest conversion (7.2%) is similar to that for sodium benzoate at low conversion (8.5% in Table 1). However, no significant change in the yield of B is observed as the pyrolysis time is increased. Similarly, the yield of minor products, which account for e5% of the total product yield, is essentially unaffected with increases in the pyrolysis time. The PHK yield, on the other hand, seems to decrease as the pyrolysis time is
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Figure 4. Plot of normalized total products yield (conversion), as a function of time, for the pyrolysis of degassed potassium benzoate at 450 °C.
Figure 5. Plot of normalized total product yields (conversion), as a function of time, for the pyrolysis of degassed potassium benzoate at 435 °C.
increased, contrary to the PHNa case. Figure 3 shows the change in the yield of products, as a function of the pyrolysis time, for potassium benzoate at 450 °C. At higher pyrolysis times (>300 s), a black solid mass was formed in the pyrolysis tube. After extraction of the acetone-soluble products, the black solid residue did not completely dissolve in hot hydrochloric acid (HCl) during the acidification process. Thus, either PHK is reacting to form insoluble products or the anion is reacting with another hydrogen source to form B. The decrease in PHK yield may suggest that the availability of CO2 to the potassium benzoate anion is hampered by trapping of this intermediate in a cage of black solid residue formed at pyrolysis times of >300 s. If some polymeric materials are formed as the pyrolysis proceeds, they can mask the intermediate from accessing CO2 to form PHK. The poor mass balances observed by analysis (see column 10 in Table 2) at pyrolysis times of >300 s further support the aforementioned notion. Figure 4 shows a plot of the normalized total product yields, as a function of time for the pyrolysis of potassium benzoate at 450 °C. The data shows linear behavior at short pyrolysis times (
