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
4.77 9AQ
. 1 . , --I.
(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
Energy & Fuels, Vol. 6, No. 1, 1992 111
Communications
Table I. Effect of Dispersed Acid Catalyst on Thermolysis of =Ph(CH,),Ph z D P P coverage, mmol/g catalvst" catalist: =DPP (by wt) temp, OC time," min conversion: % products, mol % PhH PhCH3 PhCH2CH3 PhCHXCH2 PhCHZCHzCH3 PhCHdHCH3 indan indene =Ph =PhCH3 ( 0 - ) =PhCH3 (m-) zPhCH3 (p-) =PhCH&H3 ( 0 - ) =PhCHZCH3 (m-) =PhCHzCH3 (p-) z P h C H d H 2 @-) zindan (5-) Zindan (4-)
0.532 none 0 375 60 10.3 0.0 21.5 0.0 28.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 28.4 0.0 0.0 0.2 21.3 0.0 0.0
0.532
0.532
0.532
0.532
0.532
A
B
B
B
B
2.80 375 60 2.7
2.80 375 60 23.4
2.80 375 10 17.3
2.80 345 10 11.8
2.80 310 10 4.8
0.0 23.8 0.0 24.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.3 0.0 0.0 1.2 25.6 0.0 0.0
11.1 0.6 3.8 0.4 1.6 0.6 34.7 5.5 32.4 1.6 2.5 1.4 0.3 0.9 0.7 0.0 0.8 1.1
10.1 0.4 3.6 0.3 1.4 0.4 37.2 4.1 32.3 1.6 1.7 1.3 0.3 0.9 0.8 0.0 1.4 2.0
8.4 0.3 2.8 0.2 0.9 0.2 39.3 3.7 34.6 1.6 1.2 1.5 0.4 0.9 0.6 0.0 1.5 2.1
7.7 0.2 2.1 0.2 0.6 0.2 39.1 5.7 34.0 1.2 0.6 2.0 0.5 0.8 1.1 0.0 1.7 2.1
0.320 none
0.320
B
0.320
B
0 375 60 5.5
0.20 375 60 11.5
0.61 375 60 17.3
0.0 22.3 0.0 25.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 27.9 0.0 0.0 0.6 24.2 0.0 0.0
8.3 0.8 3.4 0.9 0.8 0.9 29.9 9.2 35.8 1.5 1.5 1.7 0.4 1.0 1.0 0.0 1.2 1.7
9.5 0.7 3.6 0.5 0.9 0.5 34.2 5.9 35.4 1.5 1.9 1.4 0.3 0.9 0.7 0.0 1.0 1.1
" A = SiOz; B = SiOz-l% A1203(see ref 8). *Heat-up time is 1 min. cSum of products on a CI5 equivalent basis; mass balances 195%.
=PhCH3 and PhCH3 are formed in the free-radical decomposition of =DPP, the results at 310 OC demonstrate that they can also be formed by an acid-catalyzed path. This is consistent with recent results reported for the hydrocracking of fluid-phase 1,3-diphenylpropane over a supported ZnC12catalyst.12 The acid-catalyzed cracking of =DPP is fairly selective with PhH, =PhH, indan, = indan, and indene accounting for 85-90% of the products independent of reaction temperature or catalyst:=DPP ratio. The rate of =DPP conversion (0.53 mmol/g) at 310-375 "C (including data not shown in Table I) shows the expected increase with increasing temperature. At all temperatures, a fast initial rate is observed that quickly levels off with increasing =DPP conversion. For example, at 345 "C, reactions run for 5,10,30, and 60 min gave conversions of 8.0, 11.9, 13.7, and 16.2%, respectively. This behavior may be indicative of mass transport limitations as a result of surface immobilization and a lack of mixing of the solids during the reaction. In addition, some sites on the catalyst may be becoming deactivated as a consequence of coke formation, as suggested by blackening of the solids at higher =DPP conversions. The effect of catalyst loading levels on the rate of cracking of =DPP was examined with the 0.32 mmol/g batch. Even in the presence of small quantities of catalyst, the reaction is accelerated, and the product mixture is dominated by the acid-derived products. As shown in Table I, the =DPP conversion increases with increased catalyst loading. However, at higher catalyst:=DPP ratios of 1.29,2.77, and 4.79, increasing =DPP conversions of only 20.6, 21.0, and 22.8%, respectively, were observed indicating that the =DPP cracking rate is reaching a plateau. Cracking of hydrocarbons over aluminosilicate catalysts occurs through the generation of carbocation intermediates at Bronsted and/or Lewis acid ~ites."J~-'~ Acid-catalyzed cracking of alkanes proceeds through a chain reaction involving hydride transfer from reactant molecules to car(12) Ohon, E. s.;Diehl, J. W.; Sharma, R. K. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36 (2), 578. (13) Corma, A.; Wojciechowski, B. W. Catal. Reu. 1985,27, 29. (14) Guisnet, M. R.-Acc. Chem. Res. 1990, 23, 392. (15) Rabo, J. A.; Gajda, G. J. Catal. Rev. 1990, 31, 385.
I
bocation intermediate~."J~-'~ In the case of alkylbenzenes, a pathway involving ipso protonation on the benzene ring to give an arenium ion intermediate followed by cleavage of the alkyl substituent is generally inv0ked.l' In the present study, although the ipso protonation pathway appears likely, formation of major products such as =Ph and indan can be accounted for by both reaction pathways.16 Since hydrogen gas is not present during the reaction, the formation of products such as PhC3H7, PhCH3, and =PhCH3 is indicative of the occurrence of hydride-transfer steps. Hence, both the ipso protonation and benzylic cation/hydride transfer paths appear to contribute to the cracking of =DPP. The key result of the current study is the fact that the catalyzed cracking of =DPP occurs readily in dispersed solids. The reaction occurs at low catalyst loadings (e.g., 17 wt %) with an aluminosilicate having a much lower density of acidic sites @/A1 ratio of 84) than in typical amorphous aluminosilicate cracking catalysts (Si/Al ratios of 3-81," Moreover, the reaction relies on solid-state contacts (no solvent or cover gas), the solids are not mixed during the reaction, and the DPP substrate and s o m of the cationic intermediates are subject to diffusional restraints. Experiments in which =DPP was separated from the aluminosilicate by a plug of fumed silica show no indication of macroscopic transport of protons across the silica support. Hence, it appears that the small particle size of the catalyst (15 nm) is a key factor contributing to the facile acid-catalyzed reaction of =DPP in the solid state. The degree to which organic chain reactions contribute to the efficiency of this reaction will be probed in future studies with other substrates. This study provides new evidence that activation of a substrate by a catalyst can occur in the solid state. This suggests that, in the early stages of coal liquefaction/hydropyrolysis, solid-state interactions between highly dispersed, small particle size catalysts and coal particles may make important contributions to changes in conversion (16) A benzylic cation chain mechanism has been demonstrated in the acid-catalyzedcracking of 1,3-diphenylpropaneto benzene and indan in SbCla and SbCl3-A1Cl3 melts, albeit at the considerably lower temperature range of 1OC-130 "C. See: Buchanan, 111, A. C.; Dworkin, A. s.; Smith, G. P. J. Am. Chem. SOC.1983, 105, 2843.
112 Energy & Fuels, Vol. 6, No. 1, 1992
efficiency and product composition. The results also suggest a broader role for the use of surface-immobilized model compounds as solid-state molecular probes in studies of heterogeneous reactions related to coal conversion.
Acknowledgment. Research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-MOR21400 with Martin Marietta Energy Systems, Inc. Registry No. P h H , 71-43-2; PhCH3, 108-88-3; PhCH2CH3, 100-41-4;PhCH=CH2,100-42-5; Ph(CH2)2CH3,103-65-1;PHC-
Communications H = C H C H 3 , 637-50-3; indan, 496-11-7; indene, 95-13-6; 1,3-diphenylpropene, 1081-75-0; silica, 7631-86-9.
A. C. Buchanan, III,* P. F. Britt, C.A. Biggs
Chemistry Division Oak Ridge National Laboratory P.O. Box 2008 Oak Ridge, Tennessee 37831-6197 Received August 5,1991 Revised Manuscript Received September 12, 1991
