Ind. Eng. Chem. Res. 1992, 31,1167-1170
1167
Dialkyl and Diaryl Carbonates by Carbonate Interchange Reaction with Dimethyl Carbonate? Abbas-Alli G. Shaikh and Swaminathan Sivaram* Division of Polymer Chemistry, National Chemical Laboratory, Pune 411 008, India
Phenols, substituted phenols, and alcohols undergo carbonate interchange reaction with dimethyl carbonate in the presence of tin and titanium catalysts. Tin catalysts were found t o be superior for both phenols and alcohols. In the case of substituted phenols, conversion and selectivity to diaryl carbonate was higher when the substitutent was located para to the hydroxy group. The results have been explained on the basis of the known chemistry of reaction of organotin compounds with alcohols, phenols, and dialkyl carbonates. Diaryl and dialkyl carbonates have come to occupy an important position as useful organic chemicals for a variety of industrial and synthetic applications. Diaryl carbonates can be used for the manufacture of polycarbonate (Clagett and Shafer, 1989). Dialkyl carbonates are useful alkylating agents and safe substitutes for dimethyl sulfate (Iori and Romano, 1980;Merger et al., 1979;Lissel et al., 1989). They can also be used to introduce a carbonate linkage by suitable carbonate interchange reactions and hence are a safe replacement for phosgene (Illuminati et al., 1976; Bolon and Hallgren, 1984). Although dialkyl carbonate can be conveniently produced by carbonylation of the corresponding alcohol (Romano et al., 1980),carbonylation of phenols does not yield diphenyl carbonates in high yield and selectivities (Hallgren and Mathews, 1979). A variety of other methods are also described in the literature for the synthesis of dialkyl carbonates. They include reaction of alcohol with ethylene carbonate (Frevel and Gilpin, 1972;Tobita and Niizeki, 1980;Green, 1986;Knifton, 1987),with dimethyl carbonate (Fischer et al., 1979;Proux and Pellergrina, 1989), and with urea (Ball et al., 1980, 1984); phasetransfer-catalyzed reaction of alkyl halides with potassium hydrogen carbonate and potassium carbonate (Lissel and Dehmlow, 1981;Fujinami et al., 1981;Cella and Bacon, 1984);reaction of alcohol with diethyl azodicarboxylate in the presence of carbon dioxide and triphenylphosphine (Hoffman, 1982);reaction of oxirane with carbon dioxide (Baba et al., 1987);reaction of alcohols with carbon monoxide in the presence of elemental sulfur (Mizuno et al., 1989);and carbonate interchange reaction of alcohols with dimethyl carbonate in the presence of potassium carbonate coated with 5 w t % PEG 6000 (Tundo et al., 1988). However, none of these methods can be applied to the synthesis of diaryl carbonates. The most promising approach to diaryl carbonate appears to be through carbonate interchange reaction of aromatic hydroxy compounds with dimethyl carbonate (DMC). Thermodynamically, the carbonate interchange reaction between phenol and DMC is not favored (Kep= 3X a t 453 K) (Tundo et al., 1988). Diphenyl carbonate was reported to be produced in about 14-46% yield from the reaction of phenol and di-n-butyl carbonate at 200 "C (550mmHg/8 h) using an alkyltin aryl oxide catalyst (Yamazaki et al., 1979). However, the selectivity was low and substantial amounts of phenyl butyl carbonate was also produced. A number of patents are extant on the catalytic carbonate interchange reaction of phenol with DMC. They reveal the use of titanium and organotin
* To whom correspondence may be addressed. NCL Communication No. 5222.
catalysts at high temperature and pressure. Using phenol, the best selectivity that has been reported is about 78% at a total conversion of 42% and a reaction time of 24 h (Victor, 1985). Use of phenyl acetate and a tin catalyst in the carbonate interchange reaction leads to better selectivities. Diphenyl carbonate was produced in 95% selectivity with a total conversion of 100% at 220 "C/200 psig (Bolon et al., 1985). We therefore undertook a systematic study of the carbonate interchange reaction between dimethyl carbonate and alcohols and phenols. The objective was to comparatively evaluate various catalysts and study ita influence on both yield and selectivity. The generality of this reaction to various substituted phenols was also examined. There is no prior report on the synthesis of substituted aryl carbonate by carbonate interchange reaction. This paper reports the results of this study.
Experimental Section Materials. Dimethyl carbonate (E. Merck) was fractionally distilled and stored over molecular sieves (4A). Various alcohols and phenols were of laboratory reagent grade and subjected to drying and purification by standard procedures (Perrin and Armarego, 1988). Di-n-butyltin oxide (FASCAT M&T Chemicals, Rahway, NJ), di-n-butyltin dilaurate (Wilson Laboratory, Bombay, India), aluminum isopropoxide (Aldrich, Milwaukee, WI), and magnesium methoxide (Huls Troidorf A.G. Werk, Rheinfelden, Germany) were used as received. Titanium phenoxide was synthesized using a literature procedure (Yoshino et al., 1957). Tetrabutyl-l,3-diphenoxydistannoxane was synthesized using the reported procedure (Considine et al., 1963). Analysis. Products were characterized by gas chromatography (Hewlett Packard Series S-710A/30A), NMR spectroscopy (Varian T60 and FT 80A), IR spectroscopy (?erkin Elmer 5999 B), and mass spectroscopy (Finnigan Mat 1020 B). Where possible, products were isolated by either crystallization or distillation, and melting points/ boiling points were determined and compared with literature. General Procedure: Reaction of Phenol with DMC. The reaction was carried out in a 100-mL three-neck round-bottomed flask, equipped with a nitrogen inlet, dropping funnel, and a fractionating column connected to a liquid dividing head. Phenol (18.8g, 0.2 mol) was added followed by di-n-butyltin oxide (0.5g, 0.002mol) under a stream of nitrogen. The mixture was heated to 95-100 "C when the catalyst completely dissolved in phenol. DMC (21.3g, 0.23 mol) was added dropwise at 100 OC over a period of 30 min to the reaction mixture. The pot temperature was between 110 and 120 "C after complete ad-
0888-5885/92/2631-ll67$03.00/00 1992 American Chemical Society
1168 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 Table I. Effect of Catalvsts on the Carbonate Interchange Reaction of Dimethyl Carbonate with Phenol diphenyl phenyl methyl sel to diphenyl entry catalysta conv, % carbonate, % carbonate, % carbonate, % 50 44 6 88 1 n-Bu2Sn0 2 ~-BU~S~(OC(O)C~~H 44~ ~ ) ~ ~ 41 3 93 53 41 12 77 3 n-[Bu2Sn(OPh)12' 33 27 6 82 4 Ti(OPhI4 5 Ti(OBu), 39 33 6 85 2 2 0 100 6 Al(i-OPrI3
mol of product/ mol of catal 28 23 58 19 23 1
1 mol % based on phenol, reaction time 24 h. bReaction time 48 h. '0.5 mol % baaed on phenol.
Table XI. Reaction of Phenol and Substituted Phenols with Dimethyl Carbonate in the Presence of II -BusSnO Catalysta conv, diaryl aryl methyl sel to diaryl mol of product/ entrv subst % carbonate. (%) carbonate, % carbonate, % mol of catal 4 1 H 37 33 90 20 9 77 24 2 P-CH3 39 30 11 82 36 3 p-OCH3 63 52 6 88 28 4 p-c1 51 45 17 38 23 5 p-Br 28 11 25 0 25 6 m-CH3 25 0 25 25 7 m-OCH3 25 11 0 11 8 o-CH, 11 9 0 9 9 0-Clb 9 a
1 mol % catalyst baaed on phenol and substituted phenols; reaction time 12 h.
Reaction time 24 h.
Table 111. Reaction of Alcohols (R-OH)with Dimethyl Carbonate in the Presence of n-Bu2Sn0Catalysta reaction conv, dialkyl alkyl methyl sel to dialkyl mol of product/ entrv Rtime. h % carbonate, % carbonate, % carbonate, % mol of catal 1 ArCH28 93 64 29 69 61 12 86 2 n-C4Hs12 84 72 48 3 sec-C,H918 84 55 29 65 56 4 tert-C4Hs24 5 CH24HCH212 70 41 29 58.5 50 a 1 mol
% catalyst baaed on alcohol.
dition of DMC. The occurrence of reaction was indicated by attainment of a temperature of 62-63 "C at the top of the column, which corresponds to the 70:30 constant boiling azeotrope of methanol and DMC. The azeotrope was collected slowly in a receiver flask attached to a liquid dividing head. The pot temperature was gradually increased from 120 to 180 "C. When the azeotrope had completely distilled out, the overhead temperature of the column attained a value of 82-84 "C. The pot temperature was raised to 220 "C to enable removal of excess DMC. The composition of methanol in the azeotrope was determined by gas chromatography [5% SE-30 on Chromosorb AW (80-100 mesh) stainless steel column 6 f t X 1/8 in.; column temperature 70 "C; injector temperature 150 "C; detector temperature 200 "C, flame ionization detector; carrier gas N2,15 mL/min]. On the basis of this the total material balance could be established and was always >96%. The product diphenyl carbonate was isolated by extraction into an organic solvent. The organic layer was washed free of phenol using dilute alkali, dried, and solvent evaporated to yield 11.23 g (50% yield) of a mixture of diphenyl carbonate and phenyl methyl carbonate. The relative proportion of these two products was estimated by NMR. Pure diphenyl carbonate could be obtained by recrystallization from absolute ethanol: yield 9.41 g (88% selectivity); mp 78 "C. Reactions of other phenols and alcohols were conducted in the same manner.
Results and Discussion Effect of Catalysts. A series of tin, titanium, aluminum, and magnesium compounds were screened as catalysts for the carbonate interchange reaction of phenol with DMC. The results are shown in Table I. Tin and tita-
nium were both found to be effective. Selectivities to diphenyl carbonate were in the range of 80-909'0 depending on catalyst and condition. No reaction occurred in the presence of magnesium methoxide and n-butylmagnesium chloride as catalysts. Of the tin catalysts, tetrabutyl-1,3-diphenoxydistannoxane (entry 3) and di-n-butyltin oxide (entry 1) were found to be more active than di-n-butyltin dilaurate (entry 2). Titanium catalyst was hydrolyzed to TiOp during aqueous workup after reaction and was separated by filtration. Tin catalyst was recovered without change after the reaction during workup. This was confirmed by comparing the IR spectra of recovered catalyst with that of the fresh catalyst. Effect of Substituent on Phenol. A series of substituted phenols were examined. The results are shown in Table 11. The substituent exerted a significant effect on both conversion and selectivity. Para substituents were more reactive and gave predominantly diaryl carbonates. Ortho and meta substituents gave only aryl methyl carbonates in low conversions. Electronic effects of the substituent appeared less important as both p-methoxy and p-chloro gave good conversion and selectivity. Under these conditions, 0- and p-nitrophenol failed to undergo carbonate interchange reaction. Similarly o-hydroxyphenol was unreactive. Carbonate Interchange Reaction with Alcohol. Carbonate interchange reaction with alcohols does not suffer from the severe thermodynamic constraint experienced with phenols (K, = 2.45 at 453 K) (Tundo et al., 1988). These reactions are therefore more facile and yield a mixture of carbonates in good conversion (Table 111). However, the selectivities are moderate. The influences of steric effects are evident by the increased selectivities
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1169
@$.;""
Scheme I. Mechanism of Carbonate Interchange Reaction in the Presence of Tin Catalyst
Bu
8
phenols as well as branched alkanols can be attributed to steric hindrance to the approach of the DMC to the reaction field in the presence of the sterically crowded aryloxy/alkoxy group on stannoxane (la). The complete absence of diaryl carbonate in the case of ortho- and meta-substituted phenols indicates that steric factor also plays an important role in the second nucleophilic attack involving intermediate 5.
I
r
[
Bu1)n
1
'
ocH3
1
OR ;0/c-0cH3
-
0
\
Sn-OCH3
/
Bu
-4
BU
1
II
RO-C-OCH3
'>-OR
i n - 0 eu
-c02 Bu
+
+
H20
OR
OR
I
-0-;-OCH3
i
0 II RO-C-OR
B +
s+
BuaSnO
BuZSnO
+ROH+CH30H
a -co2 OR Bu
\
Sn-OR
/
BU
3.
to dialkyl carbonate in the case of n-butyl alcohol relative to sec-butyl alcohol and the total absence of reactivity of tert-butyl alcohol. Similar results have been previously reported using gas-liquid phase-transfer catalysis (PTC) catalysts (Tundo et al., 1988). However, the levels of conversion obtained using gas-liquid PTC (180"C/lO h, conversion of n-butyl alcohol = 27.5%) are inferior to those observed in our present work. Steric considerations have also been reported to be important in the reaction of bis(trialky1tin) oxides with alcohols (Davies et al., 1971).
Reaction Mechanism (Scheme I) The observed reactivities of phenols and alcohols toward carbonate interchange reaction with dimethyl carbonate in the presence of tin catalyst can be rationalized on the basis of the known chemistry of reaction of organotin compounds with alcohols, phenols, and dialkyl carbonate (Davies et al., 1971). Dialkyltin oxides react with phenols and alcohols in solution to give (aryloxy)/(alkoxy)distannoxane (1). Coordination of the carbonyl group of DMC with (aryloxy)/(alkoxy)distannoxane followed by a nucleophilic attack of the aryloxy or alkoxy group on the carbonyl group leads to (aryloxy)/(alkoxy)tin carbonates 2 and 5 (Davies et al., 1971). Such carbonyl activation by tin coordination has been recently reported to play an important role in tin-catalyzed transesterification of esters (Otera et al., 1991). Intermediates 2 and 5, by a second nucleophilic attack of the aryloxy/alkoxy group on the carbonyl group, lead to the final products 3 and 6. Partial decarboxylation of intermediates 2 and 5 under the reaction conditions employed cannot be ruled out (Davies et al., 1971). This would result in the loss of catalyst from the reaction. The complete lack of reactivity of nitro-substituted phenols can be attributed to the poor nucleophilicity of the phenolic group. The lack of reactivity of o-hydroxyphenol is presumably due to the formation of a stable cyclic tin ester (8) which is inert to further reaction (Zuckermann, 1963). The lower reactivity of ortho- and meta-substituted
Conclusion Diphenyl carbonate can be obtained by carbonate interchange reaction of phenol with dimethyl carbonate using a variety of organotin catalysts. Para-substituted phenols give diaryl carbonates in good selectivity. However, orthoand meta-substituted phenols give only aryl methyl carbonate in poor conversions. Tin catalysts are also active for the conversion of alcohol to dialkyl carbonates. However, selectivitiesare somewhat lower. The results are best explained in terms of a mechanism involving the formation of a (aryloxy)/ (alkoxy)tin carbonate intermediate. Registry No. Dimethyl carbonate, 616-386; phenol, 108-95-2; diphenyl carbonate, 102-09-0; methyl phenyl carbonate, 1350927-8; p-cresol, 106-44-5; di-p-tolyl carbonate, 621-02-3; methyl p-tolyl carbonate, 1848-01-7; p-methoxyphenol, 150-76-5; di-panisyl carbonate, 5676-71-1; p-anisyl methyl carbonate, 22159-41-7; p-chlorophenol, 106-48-9; bis@-chlorophenyl) carbonate, 216753-5; p-chlorophenyl methyl carbonate, 24260-28-4; p-bromophenol, 106-41-2; bis(p-bromophenyl) carbonate, 5676-69-7; p bromophenyl methyl carbonate, 1847-93-4; rn-cresol, 108-39-4; methyl m-tolyl carbonate, 1848-02-8; m-methoxyphenol, 150-19-6; m-ankyl methyl carbonate, 54644-49-4; o-cresol, 95-48-7; methyl o-tolyl carbonate, 1847-92-3; o-chlorophenol, 95-57-8; o-chlorophenyl methyl carbonate, 1847-97-8; benzyl alcohol, 100-51-6; dibenzyl carbonate, 3459-92-5; benzyl methyl carbonate, 1332610-8;1-butanol, 71-36-3; dibutyl carbonate, 542-52-9;butyl methyl carbonate, 4824-75-3; 2-butanol, 78-92-2; di-sec-butyl carbonate, 623-63-2; sec-butyl methyl carbonate, 35363-41-8; allyl alcohol, 107-18-6; diallyl carbonate, 15022-08-9; allyl methyl carbonate, 35466-83-2; dibutyltin oxide, 818-08-6.
Literature Cited Baba, A.; Nazaki, T.; Matsuda, H. Carbonate Formation from Oxiranes and C02 Catalyzed by Organotinhalide-tetralkylPhosphonium Halide Complexes. Bull. Chem. SOC.Jpn. 1987, 60, 1552-1554. Ball, P.; Fullmann, H.; Heitz, W. Carbonates and Polycarbonates from Urea and Alcohol. Angew. Chem. 1980, 19 (9), 718-720. Ball, P.; Fullmann, H.; Schwalm, R.; Heitz, W. Synthesis of Carbonates and Polycarbonates by Reaction of Urea with Hydroxy Compounds. C1 Mol. Chem. 1984,1,95-108. Bolon, D. A.; Hallgren, J. E. Synthesia of Polycarbonate from Dialkyl Carbonate and Bisphenol Diester. US Patent 4,452,968, 1984; Chem. Abstr. 1984, 101, 55679n. Bolon, D. A.; Gorczyca, T. B.; Hallgren, J. E. Diary1 Carbonates. US Patent 4,553,504, 1985; Chem. Abstr. 1985, 104, 50677n. Cella, J. A.; Bacon, S. W. Preparation of Dialkyl Carbonates via Phase Transfer Catalyzed Alkylation of Alkali-Metal Carbonate and Bicarbonate Salts. J. Org. Chem. 1984,49 (6), 1122-1125. Clagett, D. C.; Shafer, S. J. Polycarbonates. In Comprehensive Polymer Science; Allen, G., Bevington, J. C., Eds.;Pergamon: Oxford, England, 1989; Vol. 5, Chapter 20, pp 345-356. Considine, Wm. J.; Ventura, J. J.; Gibbons, A. J.; Ross, A. Organotin Chemistry 11. The Reaction of Diorganotin Oxides with Phenols. Can. J. Chem. 1963,41,1239-1243. Davies, A. G.; Kleinschmidt, D. C.; Palan, P. R.; Vasishtha, S. C. Organotin Chemistry. Part XI.'**The Preparation of Organotin Alkoxides. J. Chem. SOC.(C) 1971, 3972-3976. Fischer, K.; Himmele, W.; Kaibe, G.; Barl, M.; Schneider, K. Carbonic Acid Esters. Ger. Offen. 2,749,754, 1979; Chem. Abstr. 1979, 91, 38943b.
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Frevel, L. K.; Gilpin, J. 0. A. Carbonate Synthesis from Alkylene Carbonates. US Patent 3,642,858, 1972; Chem. Abstr. 1972, 76, 991402. Fujinami, T.; Sato, S.; Sakai, S. A Facile Preparation of Dialkyl Carbonates from Potassium Carbonate and Alkyl Bromides by using Organostannyl Compound as a Catalyst. Chem. Lett. 1981, 749-752. Green, M. J. Transesterification Process. Eur. Pat. Appl. EP 150,962, 1986; Chem. Abstr. 1986,104,129505~. Hallgren, J. E.; Mathews, D. R. 0. The Reactions of Carbon Monoxide and Phenols Promoted by Palladium Complexes. J. Organomet. Chem. 1979, 175 (l), 135-142. Hoffman, W. A,, 111. Convenient Preparation of Carbonates from Alcohols and Carbon Dioxide. J. Org. Chem. 1982,47,5209-5210. Illuminati, G.; Romano, U.; Tesei, R. Aromatic Carbonates. Ger. Offen. 2,528,412, 1975; Chem. Abstr. 1976, 84, 121495h. Iori, G.; Romano, U. Preparation of Phenolic Ethers. UK Patent Appl. 2026484, 1980; Chem. Abstr. 1980, 93,167894b. Knifton, J. F. Process for Cosynthesis of Ethylene Glycol and Dimethyl Carbonate. US Patent 4,661,609,1987; Chem. Abstr. 1987, 107,39225e. Lissel, M.; Dehmlow, E. V. Phasentransfer-Katalytische Herstellung von Kohlensaureestern Ohne Verwendung von Phosgen. Chem. Ber. 1981,114, 1210-1215. Lissel, M.; Rohani-Derfuli, A.; Vogt, G. Reactions with Dimethyl Carbonate. Part 3. 1Applications and Mechanisms of Mono or Bis Methylation of Aromatic Amines with Dimethyl Carbonate. J. Chem. Res. (S) 1989, 312. Merger, F.; Towae, F.; Schroff, L. Aralkyl and Alkyl Phenol Ethers. Ger. Offen. 2,729,031, 1979; Chem. Abstr. 1979, 90,137482m. Mizuno, T.; Nakamura, F.; Egashira, Y.; Nishiguchi, I.; Hirashima, T.; Ogawa, A.; Kambe, N.; Sonoda, N. Facile Synthesis of Carbonates from Alcohols and Carbon Monoxide Promoted by Elemental Sulfur. Synthesis 1989,636-638.
Otera, J.; Dan-oh, N.; Nozaki, H. Novel Template Effects of Distannoxane Catalysts in Highly Efficient Transesterification and Esterification. J. Org. Chem. 1991, 56, 5307-5311. Perrin, D. D.; Armarego, W. L. F. Purification of Organic Chemicals. In Purification of Laboratory Chemicals, 3rd ed.; Pergamon: Oxford, England, 1988; Chapter 3, pp 65-309. Proux, Y.; Pellergrina, M. Preparation of Organic Carbonates by Transesterification of Carbonic Acid Diesters in the Presence of Crown Ethers or Cryptand Phase Transfer Catalysts. Fr. Pat. 2,2608,812, 1989; Chem. Abstr. 1989, 110, 215192q. Romano, U.; Tesei, R.; Mauri, M. M.; and Rebora, P. Synthesis of Dimethyl Carbonate from Methanol, Carbon Monoxide and Oxygen Catalysed by Copper Compounds. Ind. Eng. Chem. Prod. Res. Deu. 1980, 19, 396-403. Tobita, M.; Niizeki, J. Dialkyl Carbonic Acid Esters. Jpn. Kokai Tokkyo Koho J P 79148,726,1980; Chem. Abstr. l980,92,214901t. Tundo, P.; Trotta, F.; Morgalio, G.; Ligorati, F. Continuous-Flow Processes under Gas-Liquid Phase-Transfer Catalysis (GL-PTC) Conditions: The Reaction of Dialkyl Carbonates with Phenols, Alcohols, and Mercaptans. Ind. Eng. Chem. Res. 1988, 27, 1565-1571. Victor, M. Aromatic Carbonates. Ger. Offen. DE 3,445,552 1985; Chem. Abstr. 1985, 104, 50647~. Yamazaki, N.; Nakahama, S.; Higashi, F. Polymers Derived From Carbon Dioxide and Carbonates. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18 (4), 249-252. Yoshino, T.; Kijima, I.;Ochi, M.; Sampei, A.; Sai, S. Synthesis of Aryl Titanate. Kogyo Kagaku Zasshi 1957,60, 1124-1125. Zuckermann, J. J. The Reaction Tin with Dihydric Phenols. The Direct Synthesis of Tin (11) Heterocycles. J. Chem. SOC.1963, 1322-1324.
Receiued for reuiew July 9, 1991 Accepted January 6, 1992
Development of a Pulse-Injection Short Contact Time Coal Liquefaction Flow Reactor William D. Provine, Nicholas D. Porro, Concetta LaMarca, Michael T. Klein,* Henry C. Foley, and Kenneth B. Bischoff Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Charles G. Scouten, Donald C. Cronauer, and A. Martin Tait Research and Development Department, Amoco Oil Company, Naperville, Illinois 60566
A well-mixed reactor was developed to study the initial reaction pathways and mechanisms of direct coal liquefaction. This pulse-injection flow vessel exploits the reactor residence time distribution to obtain an isothermal series of samples of well-defined holding times from 10 to 900 s. The reactor design also allowed for quick and efficient procurement of optimally spaced and sized samples. The operation of this reador is illustrated using both model compound and coal liquefaction experiments.
Direct coal liquefaction typically involves mixing the coal with a “donor” solvent to form a slurry that is then heated for 1800-5400 s (30-90 min) at 400-450 “C under 500-2000 psig of hydrogen pressure. The hydrogen-deficient coal abstracts hydrogen from the donor solvent or hydrogenrich structures in the coal to form liquids that are subsequently upgraded to more desirable light distillates. Throughout, retrograde reactions unfavorably compete by producing heavy products. Retrograde reactions convert desirable small product molecules into less-desirable larger macromolecules. This can occur either by recombination of two small product molecules or by addition of these product molecules back onto the unconverted coal macrostructure. Work by Shim (1984) and others shows that retrograde reactions are important even at short reaction times. Indeed, retrograde reactions can have the net effect of converting relatively weak bonds that are potentially cleavable into strong bonds 0888-588519212631-1170$03.00/0
that are more difficult to cleave. As a result, retrograde reactions can influence the liquefaction behavior observed at longer reaction times. This motivated keen interest in the chemistry of both the coal and its liquid products at short reaction times, where it would be most fruitful to attempt to unravel the competing contributions of forward and retrograde reactions. Such previous studies have been hampered by equipment limitations. Batch liquefaction studies often use tubing bomb or autoclave reactors (Cassidy et al., 1989; Maa et al., 1984). Results from these reactors can be confounded by large temperature fluctuations during the initial stages of liquefaction. Flow liquefaction has been studied in tubular or autoclave reactors (Brunson, 1979, Laine et al., 1985; Gibbins et al., 1990). Generally, these provide a single global residence time for a given system flow rate. Also, donor solvent vaporization through tubular flow reactors can obscure the actual residence time within 1992 American Chemical Society
