J . Org. Chem. 1989, 5 4 , 517-525
517
Articles Thermolysis of Surface-Attached 1,3-Diphenylpropane: Radical Chain Induced Decomposition under Conditions of Restricted Diffusion A. C. Buchanan, III,* and Cheryl A. Biggs C h e m i s t r y Diuision, Oak Ridge N a t i o n a l Laboratory, P.O. Box 2008, O a k Ridge, T e n n e s s e e 37831-6197
Received August 16, 1988
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Surface-immobilized1,3-diphenylpropane(- -Ph(CHJ3Ph, or -DPP) has been prepared by the condensation of p(3-phenylpropy1)phenol with the surface hydroxyl groups of an amorphous, fumed silica. Thermolysis studies of the covalently attached -DPP have been conducted to explore the effects of restricted radical and substrate diffusion on a free-radical chain induced decomposition reaction, which has been reported to underlie the thermal reactivity of fluid-phase DPP a t 9400 "C. Thermolysis of -DPP has been studied at 345-400 "C as a function of -DPP conversion (0.3-23%) and initial surface coverage (0.132-0.586 mmol g-l; 0.43-1.97 DPP molecules nm-2). Decomposition of surface-immobilized -DPP occurs readily in this temperature range by parallel free-radical chain pathways, whose rates are quite sensitive to surface coverage, producing PhCH3 + -PhCH=CH, and -PhCH3 + PhCH=CH2 as the major product pairs. An unusual regiospecific hydrogen transfer process is observed that favors formation of -PhCH2CHzCHPh over -PhCHCH2CH2Ph as the proximity of -DPP molecules and hydrogen abstracting radicals on the surface decreases. This phenomenon results in regioselective cracking of -DPP favoring the -PhCH:, + PhCH=CH2 product pair. Implications are discussed for the thermal degradation of related structural features in a diffusionally restricted, macromolecular network such as occurs in coal.
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Introduction is also not perturbed by the mode of surface attachment. For example, at 375 "C, the first-order rate constants for Thermolysis studies of organic compounds that serve as the initial homolysis of gaseous, liquid (in tetralin), and models for structural features present in coal have provided surface-immobilized bibenzyl are 2.2, 1.1, and 1.2 x IO4 valuable insights into the kinetic and mechanistic behavior s-l, respectively.16a However, we found that restricted of many key structural element~.l-~ The resulting radical and substrate mobility can cause dramatic changes knowledge has enhanced current understanding of the in free-radical reaction pathways compared with those in thermal reactivity of coal, which underpins many techfluid phases. In particular, complex free-radical chain nologically important processes such as coal p'yolysis and hydropyrolysis2bJ0J1b,cJ2 and liquefa~tion.~~Jl~~~J~J~ Our research interest has turned toward modeling the con(1) Stein, S.E. ACS S y m p . Ser. 1981, No. 169, 97. straints on free-radical reactions that might be imposed (2) (a) Stein, S. E. Chemistry of Coal Conversion; Schlosberg, R. H., in coal as a consequence of its cross-linked macromokecular Ed.; Plenum Press: New York, 1985; Chapter 2. (b) Solomon, P. R.; Hamblen, D. G. Ibid. Chapter 5. (c) Stock, L. M. Ibid. Chapter 6. structure.llaJ4J5 This complicating feature could signif(3) (a) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Ogier, W. C.; Niicantly perturb the extrapolation of simple model comgenda, S.E.; Fleming, R. H. Fuel 1987, 66, 1611. (b) McMillen, D. F.; pound behavior in fluid phases to that of similar features Ogier, W. C.; Ross, D. S.J . Org. Chem. 1981, 46, 3322. (4) Franz, J. A.; Camaioni, D. M.; Beishline, R. R.; Dalling, D. K. J . in coal itself. In order to study this restricted-mobility Org. Chem. 1984, 49, 3563. phenomenon at temperatures relevant to coal thermolysis, (5)Bockrath, B.; Bittner, E.; McGrew, J. J. Am. Chem. SOC.1984,106, which begins to occur at 350-400 "C, we have developed 135. (6) Hung, M.-M.; Stock, L. M. Fuel 1982, 61, 1161. procedures that allow the investigation of the thermal (7) (a) Benjamin, B. M.; Raaen, V. F.; Maupin, P. H.; Brown, L. L.; reactivity of substituted aromatic model compounds that Collins, C. J. Fuel 1978,57,269. (b) Collins, C. J.; h e n , V. F.; Benjamin, are covalently anchored to an inert silica surface.16 The B. M.; Maupin, P. H.; Roark, W. H. J . Am. Chem. SOC.1979,101,5009. (8) (a) Poutsma, M. L. Fuel 1980,59,1960. (b) Poutsma, M. L.; Dyer, = S i O C , linkage employed is thermally robust in the C. W. J. Org. Chem. 1982,47,3367. temperature range of interest. (9) Squire, K.R.; Solomon, P. R.; Carangelo, R. M.; DiTaranto, M. B. We recently showed that, as in the liquid and vapor Fuel 1986, 65, 833. (10) Gavalas, G. R. Coal Pyrolysis; Elsevier: Amsterdam, 1982. phases, surface-immobilized biphenyl (denoted WPhPh) (11) (a) Wender, 1.; Heredy, L. A.; Neuworth, M. B.; Dryden, I. G. C. and diphenylmethane (- -PhCH2Ph) do not react after Chemistry o f Coal Utilization; Elliot, M. A., Ed.; Wiley-Interscience: several hours of heating in vacuum at 400 "C.16 This New York, 1981; Suppl. Vol. 2, Chapter 8. (b) Howard, J. B. Ibid. Chapter 12. (c) Seglin, L.; Bresler, S.A. Ibid. Chapter 13. (d) Gorin, E. indicated that the remaining underivatized surface hyIbid. Chapter 27. (e) Alpert, S. B.; Wolk, R. H. Ibid. Chapter 28. droxyl groups on the high purity, fumed silica do not (12) Berkowitz, N. T h e Chemistry o f Coal, Coal Science and Techcatalyze any new reaction pathways. Moreover, a detailed nology; Elsevier: Amsterdam, 1985; Vol. 11. study of the thermolysis of surface-immobilized bibenzyl (13) Whitehurst, D. D.; Mitchell, T. 0.;Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (- -PhCH2CH2Ph) showed that the rate of unimolecular (14) Davidson, R. M. In Coal Science; Gorbaty, M. L., Larsen, J. W., C-C homolysis is similar to that in fluid phases and, hence, Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1.
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* Research sponsored by the Division of Chemical Sciences/Office of Basic Energy Sciences, US.Department of Energy under contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc.
(15) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; Chapter 6. (16) (a) Buchanan, A. C., 111; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. SOC.1986,108,7703. (b) Poutsma, M.L.; Douglas, E. C.; Leach, J. E. J. Am. Chem. SOC.1984, 106, 1136.
0022-3263/89/1954-0517$01.50/00 1989 American Chemical Society
Buchanan and Biggs
518 J. Org. Chem., Vol. 54, No. 3, 1989
pathways became major thermal decay routes on the surface leading to an actual acceleration in the total rate of decomposition for the immobilized bibenzyl as well as a vastly different product mixture. These enhanced radical chain pathways on the surface resulted in substantial isomerization [to form -PhCH(Ph)CH3], cyclizationdehydrogenation (to form -dihydrophenanthrene and -phenanthrene), and hydrogenolysis (to form -PhH PhCzH5and -PhCzH5 + PhH) of bibenzyl groups. In this paper we examine the influence of surface immobilization on the thermolysis of 1,3-diphenylpropane (DPP)," whose fluid-phase behavior has been extensively studied.'JgZ1 Thermolysis of liquid DPP occurs at a faster rate than that of bibenzyl despite the fact that DPP contains no bond that is as thermally labile as the central C-C bond in bibenzyl.22 It is now well documented through mechanistic studies that the facile thermal cracking of DPP a t 350-400 "C (eq 1) occurs via an induced decomPh(CHz)3Ph PhCH3 + PhCH=CHZ (1) position reaction in a long chain free-radical process rather than by rate-controlling C-C homolysis.1B,1g8Kinetic orders of approximately 3/2 (1.59,181.551g8),activation energies of ca. 52 kcal/mol (52.3,1B51.41ga),rate acceleration by addition of free-radical initiator^,'^^^^ results of deuterium labeling experiments,lBand thermochemical analysislBwere all consistent with the mechanistic scheme shown in eq 2-6, where eq 4 and 5 are the chain propagation steps and the simplified steady-state rate expression, rate = ( k 2 / k6)1/zk4[1]3'2,results from the fact that k , >> K4[1]. Ph(CHJ3Ph --* PhCH2' + PhCHzCHz' (2)
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condensation of excess p-HOC6H4(CHz)3C,jH5with the surface hydroxyls of a high-purity fumed silica.
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HOPh(CHZ),Ph
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PhCHz'
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PhCH,
PhCH=CH2
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2PhCH2' PhCHzCHzPh (6) These fluid-phase studies suggest that three-carbon bridges (and probably longer6JB)between aromatic moieties in coal are likely to cleave under mild thermal conditions as a result of similar induced decomposition pathways that involve 0-scission of radicals such as 2. However, it is not clear to what extent these processes may be perturbed in a macromolecular framework such as occurs in coal. Hence, we have prepared surface-immobilized 1,3-diphenylpropane (- -DPP) as a model and studied its reactivity a t 345-400 "C as a function of conversion and surface coverage to examine the influence of restricted diffusion on the radical chain induced decomposition process. +
Results Preparation of Surface-Attached 1,3-Diphenylpropane (- -DPP, or 3). Two batches of -DPP were prepared at saturation coverage as shown in eq 7 by the
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(1'7) For a preliminary communication of the present work, see: Buchanan, A. C., 111; Biggs, C. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32(3), 175. (18) Poutsma, M.L.;Dyer, C. W. J. Org. Chem. 1982, 47, 4903. (19) (a) Gilbert, K. E.; Gajewski, J. J. J. Org. Chem. 1982, 47, 4899. (b) Gilbert, K. E. J. Org. Chem. 1984, 49,6. (20) Sweeting, J. W.;Wilshire, J. F. K. Aust. J. Chem. 1962, 15, 89. (21) (a) King, H.-H.; Stock, L. M. Fuel 1982,61, 1172; (b) Ibid. 1984, 63, 810. (22) The bond dissociation energies, D o , for PhCHz-CHzPh and PhCHz-CHZCH2Ph are about 62 and 72 kcal mol-', respectively, predicting a rate depression of >lo3 for bond homoloysis at 400 "C in going from the dimethylene to the trimethylene bridge.18
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Analysis of the surface coverage, by GC of parent phenol liberated from a base hydrolysis procedure, gave consistent coverages of 0.586 (batch A) and 0.566 (batch B) mmol of -DPP/g of derivatized silica with purities of 99.7 and 99.9%, respectively. The average coverage of 0.576 mmol/g corresponds to ca. 1.97 DPP molecules/nm2 of surface area, indicating that approximately 40-50% of the surface hydroxyls have been derivatized. The efficiency of packing of substituted benzenes attached to the amorphous silica surface was considered previously by using crystalline silica surfaces as models.16 The present level of "saturation" surface coverage for -DPP, which is slightly larger than that for surface-attached bibenzyl, is consistent with a "liquid-like" monolayer with steric constraints prohibiting substantially higher coverages. Two batches with approximately one-fourth the saturation surface coverage were prepared in a similar fashion, by limiting the initial phenol to surface hydroxyl ratio. These two batches were analyzed to have surface coverages of 0.142 and 0.132 mmol g-' (ca. 0.43 DPP molecule nm-? with purities of 99.8%. Thermolysis of -DPP. Thermolyses were performed in sealed evacuated Pyrex tubes with a T-shaped configuration such that volatile products could be collected in a cold trap as they formed. The volatile products in the trap were analyzed by GC and GC-MS. In a separate procedure, surface-attached products were liberated as phenols following digestion of the silica in base, silylated to the corresponding trimethylsilyl ethers, and analyzed as above. Thermolysis of -DPP at saturation (or high) coverages was studied at 345-400 "C from 1.5 to 23% conversion, and quantitative data for the products described below are presented in Table I. Phenolic products described below were obtained after the digestion procedure and correspond to the surface-attached products indicated in parentheses and also shown in Table I. A t low conversions (
