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Radical Chemistry under Diffusional Constraints: Impact of Spacer Molecules on the Thermolysis of Surface-Immobilized Bibenzyl A. C. Buchanan, III,* Phillip F. Britt, and Kimberly B. Thomas Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6197 Received January 12, 1998. Revised Manuscript Received January 15, 1998
Silica nanoparticle surfaces have been chemically modified to contain a molecular probe, 1,2diphenylethane (bibenzyl), as well as a second spacer molecule of variable structure. Thermolysis kinetics and products at 400 °C have been determined for bibenzyl, and the impact of diffusional constraints and spacer molecular structure on the multipathway radical chemistry has been analyzed relative to fluid phases. Unimolecular homolysis rate constants, k ) (7-9) × 10-6 s-1, are found to be independent of spacer molecular structure (naphthalene, diphenylmethane, tetralin) and similar to values measured in fluid phases, indicating the lack of a cage effect on the silica surface. However, the total rate of bibenzyl thermolysis and the resulting product selectivities are profoundly affected by both surface immobilization and the structure of the spacer molecule. Naphthalene spacers behave as “molecular walls” serving as physical barriers to bimolecular hydrogen transfer steps on the surface and augmenting the effects of diffusional constraints. In contrast, diphenylmethane spacers are found to serve as hydrogen transfer, radical relay catalysts that translocate radical sites across the surface and diminish the impact of diffusional constraints. Tetralin spacers undergo significant reaction with free-radical intermediates, selectively producing the isomeric methylindan via a chain pathway that is promoted by the restrictions on diffusion.
Introduction The chemistry and spectroscopy of molecules in constrained media is an important area of current chemical research. The constraining media investigated have been quite varied and include, for example, micelles, liquid crystals, organic crystals and polymers, layered clays, silica surfaces, and microporous zeolites.1 In our research, we have been interested in exploring organic free-radical reactions at high temperatures on chemically modified silica surfaces.2-4 Radical processes are important in many areas of chemistry and biochemistry, and the kinetics and mechanisms of these processes and their synthetic utilization continue to be widely investigated.5 Our use of covalent attachment (1) Leading references: (a) Klafter, J.; Drake, J. M., Eds. Molecular Dynamics in Restricted Geometries; Wiley: New York, 1989. (b) Ramamurthy, V., Ed. Photochemistry in Organized and Constrained Media; VCH: New York, 1991. (c) Endo, K.; Koike, T.; Sawaki, T.; Hayashida, O.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119, 4117. (d) Pitchumani, K.; Warrier, M.; Ramamurthy, V. J. Am. Chem. Soc. 1996, 118, 9428. (e) Turro, N. J.; Buchachenko, A. L.; Tarasov, V. F. Acc. Chem. Res. 1995, 28, 69. (f) Toda, F. Acc. Chem. Res. 1995, 28, 480. (g) Lem, G.; Kaprinidis, N. A.; Schuster, D. I.; Ghatlia, N. D.; Turro, N. J. J. Am. Chem. Soc. 1993, 115, 7009. (h) Laszlo, P. Acc. Chem. Res. 1986, 19, 121. (i) Leon, J. W.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 2226. (2) Buchanan, III, A. C.; Britt, P. F.; Thomas, K. B.; Biggs, C. A. J. Am. Chem. Soc. 1996, 118, 2182. (3) Buchanan, III, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. Soc. 1986, 108, 7703. (4) (a) Buchanan, III, A. C.; Biggs, C. A. J. Org. Chem. 1989, 54, 517. (b) Britt, P. F.; Buchanan, III, A. C. J. Org. Chem. 1991, 56, 6132. (c) Britt, P. F.; Buchanan, III, A. C.; Malcolm, E. A.; Biggs, C. A. J. Anal. Appl. Pyrol. 1993, 25, 407. (d) Buchanan, III, A. C.; Britt, P. F. Biggs, C. A. Energy Fuels 1990, 4, 415.
of organic molecules to the surface of silica nanoparticles (12 nm), through a Si-O-Caryl linkage, has provided a unique opportunity to study radical reactions under diffusional constraints over a wide temperature range up to ca. 450 °C. This permits a fundamental examination of high-temperature radical reactions under conditions of restricted diffusion that are also significant in the thermochemical processing of organic energy resources (fossil and renewable)6 that possess complex, cross-linked macromolecular structures.7 Studies of the thermolysis of silica-immobilized diarylalkanes at 350-400 °C have shown that reaction rates and product selectivities can be altered by restricted diffusion compared with liquid- or vapor-phase (5) Leading references: (a) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; Wiley: New York, 1995. (b) Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical Reactions: Concepts, Guidelines, and Synthetic Applications; VCH: New York, 1995. (c) Chatgilialoglu, C. In Chemical Synthesis; Chatgilialoglu, C., Snieckus, V., Eds.; Kluwer Academic: Netherlands, 1996; pp 263276. (d) Newcomb, M. Tetrahedron 1993, 49, 1151, (e) Beckwith, A. J.; Bowry, V. W. J. Am. Chem. Soc. 1994, 116, 2710. (f) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91, 1237. (g) Curran, D. P.; Shen, W. J. Am. Chem. Soc. 1993, 115, 6051. (h) Tanner, D. D., Ed. Advances in Free Radical Chemistry; JAI: London, 1990. (i) Fischer, H. In Free Radicals in Biology and the Environment; Minisci, F., Ed.; Kluwer Academic: Netherlands, 1997; 63-78. (6) (a) Gavalas, G. R. Coal Pyrolysis; Elsevier: Amsterdam, 1982. (b) Schlosberg, R. H. Chemistry of Coal Conversion; Plenum: New York, 1985. (c) Bridgwater, A. V., Ed. Thermochemical Processing of Biomass; Butterworth: London, 1984. (7) (a) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: NewYork, 1982; Chapter 6. (b) Berkowitz, N. The Chemistry of Coal; Elsevier: New York, 1985; Chapter 14. (c) Faulon, J.-L.; Hatcher, P. G. Energy Fuels, 1994, 8, 402.
S0887-0624(98)00005-X CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
650 Energy & Fuels, Vol. 12, No. 3, 1998
Buchanan et al.
Scheme 1
behavior.2-4 Recently, we began designing two-component surfaces that were tailored to include a probe reporting molecule (1,3-diphenylpropane or DPP) and a spacer molecule of variable structure.2 The rate of the free-radical chain decomposition of DPP at 375 °C, which generates toluene and styrene as essentially the only products, was found to be very sensitive (factor of 40 variation) to the structure and orientation of the neighboring spacer molecules on the surface. Enhanced thermolysis rates occurred for surfaces with spacer molecules containing benzylic C-H bonds that could become involved in a novel hydrogen transfer, radical relay mechanism as illustrated in Scheme 1. This process, which was characterized with the use of isotopic labels and spacer molecules of varying benzylic C-H bond strengths, is facilitated by preorganization of the reactants on the surface. The radical relay process provides an alternative mechanism for radical centers to rapidly relocate in a diffusionally constrained environment without the need for physical diffusion. In this paper, we report the second study of thermolysis of two-component, chemically modified surfaces, which employs a different probe molecule, 1,2-diphenylethane (bibenzyl). In contrast to the single path, long kinetic chain, radical mechanism observed for thermolysis of DPP, bibenzyl undergoes a more complex, multipath free-radical decomposition mechanism.3,8 Thermolysis of bibenzyl in gas and liquid phases has been extensively investigated, and the results have been recently reviewed.8 The rate of decomposition is controlled by homolysis of the weak bibenzylic bond (ca. 62 kcal mol-1) that produces benzyl radicals (eq 1), with log k/s-1 ) 15.3-62.3/2.303RT measured at 375-475 °C in the gas phase.9 In the absence of an added hydrogen donor, hydrogen abstraction from bibenzyl by the benzyl radicals (eq 2) leads to induced decomposition through formation of 1,2-diphenyl-1-ethyl radicals, which terminate by typical radical-radical coupling and disproportionation steps.
PhCH2CH2Ph f PhCH2• + PhCH2•
(1)
PhCH2• + PhCH2CH2Ph f PhCH3 + PhCH‚CH2Ph (2) However, when bibenzyl was immobilized on a silica surface, denoted ≈BB where “≈” represents the Si-OCaryl linkage to the surface, additional reaction channels (8) Poutsma, M. L. Energy Fuels 1990, 4, 113. (9) Stein, S. E.; Robaugh, D. A.; Alfieri, A. D.; Miller, R. E. J. Am. Chem. Soc. 1982, 104, 6567.
Figure 1. Silica-immobilized substrates that were synthesized and subjected to thermolysis at 400 °C.
became significant.3 These included rearrangement of ≈BB to form surface-attached 1,1-diphenylethane, cyclization-dehydrogenation to form surface-attached 9,10-dihydrophenanthrene and phenanthrene, and hydrodealkylation to form surface-attached benzene and gas-phase ethylbenzene as well as the complementary product pair. The onset of these new reaction channels could be attributed to a decreased termination rate for the surface-attached analogue of PhCH‚CH2Ph as a consequence of restrictions on diffusion for these radicals on the surface. This conclusion was supported by the virtual absence of the tetraphenylbutane radical termination product formed from coupling of the surfacebound radicals. As a consequence of this multipath radical decay mechanism governing the thermal decomposition of ≈BB, we anticipated that both the thermolysis rate and product distribution could show a marked dependence on the structure of neighboring molecules on the surface. Hence, we have prepared the two-component surfaces shown in Figure 1 containing an aromatic spacer (naphthalene), a spacer containing benzylic C-H bonds capable of participating in a radical relay mechanism analogous to that shown in Scheme 1 (diphenylmethane), and a hydroaromatic spacer widely investigated as a hydrogen donor in fluid phases to inhibit radical-based induced decomposition steps (tetralin). The results reported herein demonstrate a strong sensitivity of ≈BB reaction rate and product distribution to the spacer molecular structure, which would not be predicted from previous studies of corresponding fluidphase analogues without the diffusional constraints. Results Preparation of Chemically Modified Surfaces. The two-component surfaces shown in Figure 1 were
Radical Chemistry under Diffusional Constraints
Energy & Fuels, Vol. 12, No. 3, 1998 651 Scheme 2
Table 1. Composition of Silica-Immobilized Substrates surface composition ≈BB ≈BB/≈NAP ≈BB/≈DPM ≈BB/≈TET
surface coveragea (mmol g-1)
total coverageb (molecules nm-2)
purity (%)
0.51 0.083 0.084 0.073/0.33 0.089/0.38 0.099/0.34
1.7 0.25 0.26 1.3 1.5 1.4
99.7 99.8 99.3 99.9 99.9 99.3
a Component surface coverage on a per gram of derivatized silica basis. b Based on a silica surface area of 200 ( 25 m2 g-1.
prepared by the cocondensation of the precursor phenols with the surface silanol groups of a nonporous, fumed silica (200 m2 g-1; ca. 4.5 SiOH nm-2) as illustrated in Scheme 2 for the ≈BB/≈TET surface.10 Reagent stoichiometries were adjusted to obtain comparable ≈BB surface coverages in the range of 0.07-0.10 mmol g-1 with the balance of the available surface area covered by spacer molecules. Surface coverages and chemical purities for these two-component chemically modified silica surfaces are given in Table 1. In addition, singlecomponent surfaces of ≈BB were prepared at saturated surface coverage (0.51 mmol g-1), and at a low surface coverage (ca. 0.08 mmol g-1) that permits direct comparison with ≈BB behavior in the presence of the spacer molecules. For the two-component surfaces, total surface coverages obtained were 1.3-1.5 molecules nm-2, which are comparable to but slightly smaller than those obtained previously for 1,3-diphenylpropane surfaces with related spacers (1.5-1.8 molecules nm-2), indicating slightly less efficient packing on the surface.2 Thermolysis Products. Typically, 4-6 thermolyses were performed for each surface composition at 400 °C in sealed, evacuated tubes. Volatile products (Table 2, 1-5) were collected in a cold trap for analysis, while silica-attached products (Table 2, 6-15) were analyzed as the corresponding phenols following liberation from the surface by base hydrolysis of the silica.11 Thermolysis of ≈BB alone produced a complex product mixture with key products shown in Scheme 3, which are consistent with previously identified products from thermolysis of ≈BB.3 In the presence of spacer molecules, no new products from thermolysis of ≈BB were observed. However, the distribution of products was (10) Analogously, fumed silicas have been derivatized with aliphatic alcohols. See: Zaborski, M.; Vidal, A.; Ligner, G.; Balard, H.; Papirer, E.; Burneau, A. Langmuir 1989, 5, 447. (11) The volatile products collected in the trap also contain small quantities (4-8%) of unreacted parent phenols that are produced by a condensation reaction with residual adjacent, underivatized surface silanol groups to form the phenols and a siloxane linkage. This process occurs upon heat up to reaction temperature and is analogous to the dehydroxylation of silica itself to form water and a siloxane linkage at these elevated temperatures. See: Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
Figure 2. Conversion of tetralin spacer to principal products as a function of bibenzyl conversion.
found to depend on the presence and structure of spacer molecules. This is illustrated in Table 2 where product distributions for the 15 products, accounting for ca. 99% of reacted ≈BB, are compared at 400 °C for 30 min reaction times.12 The principal reaction pathways for product formation will be discussed below. From Table 2, ≈BB conversion based on consumed starting material corresponds to ≈BB conversion based on total product yield calculated on a C14-equivalent basis, indicating that material balances are excellent. In the thermolysis of ≈BB/≈NAP, no products were detected involving reaction of the ≈NAP spacer. In the case of the ≈DPM spacer, a small amount of ≈DPM reacted by cyclization-dehydrogenation to form surfaceattached fluorene. The formation of the surface-attached fluorene increased with increasing ≈BB conversion, accounting for 0.1-1.8% of the ≈DPM over a ≈BB conversion range of 2-14%. The ≈TET spacer exhibited considerably higher reactivity, reaching a conversion of ca. 34% at a ≈BB conversion of 23%. The dominant product formed from ≈TET was the ring contracted, surface-attached methylindan, consisting of two isomers formed in comparable amounts. Dehydrogenated, surface-attached naphthalene was produced as the minor product, and trace amounts (
