Energy & Fuels 1992,6,109-110
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Communxattons Reduction of Coal Reactivity by Conformational Rearrangements
Sir: Conformational rearrangements of the organic portion of Illinois No. 6 coal caused by mild solvent treatment sharply reduce its reactivity with tetralin at 350 "C. Illinois No. 6 coal was heated in tetralin (N2atmosphere) at 350 "C for periods of up to 1 h and its conversion to pyridine-extractable material was measured. The conversion data were compared with the results of analogous reactions of the pyridine extract and of the insoluble extraction 30 40 0 20 40 60 80 residue. Pyridine extraction is known to cause conformational rearrangements.' The conversion of the whole TIME [min] coal was significantly (between 10 and 20%)greater than Figure 1. Conversion to pyridine extractables in tetralin at 350 the s u m of conversions of the extract and the residue. O C in a N2 atmosphere for Illinois No. 6 coal and products: (0) Another sample of Illinois No. 6 was heated in chloropyridine extract; ( 0 ) residue from pyridine extraction; (A) benzene for 300 h at 115 "C, conditions known to induce weighted s u m of extract and residue; (A)Illinois No. 6 coal; ( 0 ) conformationalrearrangements within coals.2 Subsequent Illinois No. 6 coal heated in chlorobenzene at 115 "C for 300 h. reactions of this rearranged coal with tetralin at 350 "C gave significantly lower conversions to pyridine solubles the coal was clear and water white. The extractability of than did the whole coal. For shorter reaction times, the the coal was 31 wt % measured as the weight loss of the conversions were the lowest of the three systems and were solid. It is well known that small and variable amounts as much as 25% less than those of the unaltered coal. of pyridine are tenaciously retained by Small A number of different experimental approaches have amounts of pyridine added to the reactions of the whole shown that coals undergo conformational rearrangements coal with tetralin had no effect, showing that the retained when exposed to solvents. Brenner demonstrated that pyridine does not influence the measured reactivity. Rerapid rearrangements occur in coal thin sections swollen actions were carried out in tubing bombs using 2 g of coal with THF.3 Hsieh and Duda showed that diffusion rates and 4 g of tetralin in dry N2atmospheres. The bombs were were sensitive to the previous exposure of coals to solvents.4 heated at 350 "C in a Techne sand bath and reached 75% Cody and co-workers observed that the first swelling of six of their reaction temperature in 40 s. Vertical agitation coals was anisotropic and that the coals rearranged to a at 220 cycles/min was used. Reaction times were measured new shape when the solvent was r e m ~ v e d . Two ~ papers from the moment of immersion in the hot bath. After the are directly relevant to the present work. It has been reaction, the bombs were rapidly cooled in a blast of cold argued that extraction with pyridine causes conformational air. Coal samples were warmed in chlorobenzene at 115 rearrangements in coals, rearrangements which result in "C for 300 h. The chlorobenzene was removed under enhanced association through increased noncovalent invacuum and analysis for chlorine showed that less than teractions between ar0matics.l Evidence has also been 0.1 w t % of the rearranged coal was retained chloropresented that warming a coal in chlorobenzene at 115 "C benzene. All handling of substrates and reaction products results in similar conformational rearrangements leading was performed in dry N2 atmospheres. to a more highly associated, lower free energy ~ o a l . ~ ~ ~ Figure 1 shows the conversion of Illinois No. 6 coal to The effect of these conformational rearrangements on pyridine extractables by heating in tetralin at 350 "C for coal reactivity is the subject of this paper. Two recent the indicated times. Conversion reaches a maximum of papers report the enhancement of coal liquefaction by 90% after 40 min and then decreases to 82% after 1 h, preswelling with organic solvents or tetrabutylammonium probably due to retrograde reactions occurring. IndeBoth studies used hydrogen donor solvents, pendently, the pyridine extract and the insoluble extracH2atmospheres, and removal of the swelling solvents betion residue were reacted under the same conditions and fore liquefacti~n.~.~ their independent conversions are shown. It is noteworthy Illinois No. 6 coal from the Argonne Premium Coal that, for longer reaction times, the extract undergoes reSample Bank was used. Dry pyridine Soxhlet extractions trograde reactions which decrease its pyridine extractaof previously dried coal were carried out under a slight bility. The sum of conversions of these two materials, positive pressure of dry N2until the solvent draining from weighted by their relative amounts in the coal, is also plotted in the figure. The pyridine-treated material is significantly less reactive than the unextracted coal. Also (1) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107-110. shown in the figure are conversion data for the reactions (2) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. which used chlorobenzene-rearranged coal as a substrate. (3) Brenner, D. Fuel 1985, 64, 167-173. For shorter reaction times this is the least reactive material (4) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 17b-178. (5) Cody, G. D. Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1990,2, studied. However, its conversion after 1h approaches that 340-344. of untreated coal. (6) Larsen, J. W. Proc. ICCS Int. Conf. Coal Sci., Tokyo, Jpn. 1989,
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1 , 9-12. (7) Joseph, J. T. Fuel 1991, 70, 139-144.
(8)Baldwin, R. M.; Kennar, D. R.; Nguanprasert, 0.; Miller, R. L. Fuel
(9) Collins, C. J.; Hagaman, E. W.; Jones, R. M.; Raaen, V. F. Fuel
1991, 70, 429-433.
1981, 60, 359-360.
0887-0624/92/2506-OlO9$03.00/00 - 1992 American Chemical Societv ~
Energy &Fuels 1992,6, 110-112
110
Both pyridine extraction and chlorobenzene treatment have been reported to induce the conformational rearrangement of coals to more highly associated, lower free energy states. The data presented here demonstrate that, under the mild reaction conditions used in this work, the rearranged coal is much less reactive than the parent coal. Whether this is due to alterations in mass transfer or chemical effects is not known at present and is under active investigation. All who cany out chemical reactions on coals should be aware that exposure to solvents can have significant effects on reactivity. In a forthcoming series of papers we will be discussing the nature and thermodynamics of the conformational rearrangements of coals. Acknowledgment. This material was prepared with the support of the US.Department of Energy, Grant No. DE-FG22-87PC79926. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE. We are grateful to Drs. Peter Hall and Robert Flowers for many stimulating discussions and wise suggestions. Registry No. ClPh, 108-90-7; pyridine, 110-86-1; tetralin, 119-64-2.
John W. Larsen,* Murat Azik, Anna Korda Department of Chemistry Lehigh University Bethlehem, Pennsylvania 18015 Received June 28,1991 Revised Manuscript Received September 30, 1991
Dispersed Catalysts: Evidence for a Role for Solid-state Interactions
Sir: Recent research on the development of novel catalysts for coal liquefaction and hydropyrolysis has focused heavily on small particle size, dispersed catalysts that, in some instances, possess an acidic fun~tionality.'-~The advantage of such catalysts is improved contact between the reagents and catalyst surface, particularly in the critical early stages of coal dissolution. The role of solid-state interactions between coal and catalyst under such conditions is not easily addressed. We recently developed a method for immobilizing model compounds on an inert silica surface that has allowed the study of reactions at ca. 400 "C under conditions of restricted mass We now report the first use of these immobilized model
(1) Derbyshire, F. J. Energy Fuels 1989, 3, 273. (2) Utz, B. R.; Cugini, A. V.; Frommell, E. A. ACS Symp. Ser. 1990,
Figure 1. (a)Primary products from free-radical decomposition of =Ph(CH2)3Ph.(b)Products from acid-catalyzed cracking of ;.Ph(CH2)3Phin dispersed solids.
compounds as solid-state molecular probes in heterogeneous catalytic reactions relevant to coal processing. We find that solid mixtures of surface-immobilized 1,3-diphenylpropane (=DPP) and a 15nm particle size Si02-l% A1203 undergo facile acid-catalyzed reaction chemistry at 310-375 "C under vacuum. The results provide evidence that solid-state interactions between substrate and catalyst can occur in well-dispersed solids. Two batches of =DPP (surface coverages of 0.53 and 0.32 mmol/g) were prepared on a silica support as described previously.6 Solid mixtures of =DPP and catalyst8pg were prepared from dilute benzene slurries by solvent removal at 70 "C on a rotovap. SEM-EDX analysis showed the aluminum to be uniformly distributed in the dispersed mixture. Reactions were performed in sealed tubes under vacuum (ca. 5 X lo4 T ~ r r ) . ~ , ~ Thermolysis of =DPP in the absence of catalyst occurs by a free-radical chain mechanism to afford the simple product mixture shown in Figure la? When =DPP (0.53 mmol/g) is dispersed with a fumed silica: the thermolysis products are unchanged, and the rate of conversion of =DPP is reduced (ca. 4-fold) as a result of dilution (Table I). On the other hand, thermolysis of =DPP in the presence of dispersed, colloidal Si02-l% A1203leads to a rate acceleration (factors of ca. 2 and 8 at 375 "C compared with undiluted and Si02-diluted =DPP) and a remarkably different set of reaction products (Figure lb).'O Representative data from reactions conducted at different temperatures and catalyst loadings are given in Table I. Significantly, the reaction is also facile at 310 "C, where =DPP is thermally stable. The products result primarily from aromatic dealkylation reactions and are characteristic of acid-catalyzed reactions over aluminosilicates involving carbocation intermediates-'l Another indication of acidcatalyzed chemistry is that the surface-attached products are extensively isomerized. Although products such as
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(3) (a) Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman, G. P.; Huggins, F. E. Energy Fuels 1990,4, 231. (b) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffman, G. P. Ibid. 1991,5, 497. (4) (a) Snape, C. E.; Lafferty, C. J.; Stephens, H. P.; Dosch, R. G.; Klavetter, E. Fuel 1991, 70,393. (b) Snape, C. E.; Bolton, C.; Dosch, R. G.; Stephens, H. P. Energy Fuels 1989, 3, 421. (5) Buchanan, 111, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. SOC.1986, 108,7703. (6) The Si-(Tc I covalent linkage is formed by the condensation of ~ - H O C G H , ( C H ~ ) ~ with C & , the surface hydroxyls of a fumed silica (surface area of 200 mZ/g). Buchanan, 111, A. C.; Biggs, C. A. J. Org. Chem. 1989,54, 517. (7) Buchanan, 111, A. C.; Britt, P. F.; Biggs, C. A. Energy Fuels 1990, 4 , 415.
(8) The Si02-l%Al2O3employed (Aerosil MOX-170; Degussa Coip.) is an amorphous co-fumed oxide with a surface area of 170 m2/g and average primary particle size of 15 nm. Aerosil 200 fumed silica was employed for control experiments, which has a surface area of 200 m2/g and average primary particle size of 12 nm. These solids were dried at 200 OC for 4 h prior to use. (9) Buchanan, 111, A. C.; Britt, P. F.; Biggs, C. A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991, 36 (2), 536. (10) Vapor-phase products were collected m a cold trap (77 K) as they formed and were analyzed by GC and GC-MS with the aid of internal standards. Products on the surface were detached by base hydrolysis, and the resulting phenols were analyzed in a similar fashion. (11)Pines, H. The Chemistry of Catalytic Hydrocarbon Conuersions; Academic Press: New York, 1981; Chapter 1.
0887-0624/92/2506-Ol10$03.00/00 1992 American Chemical Society
