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Energy & Fuels 2006, 20, 54-60
Pyrolysis of Benzyl Phenyl Ether Confined in Mesoporous Silica Michelle K. Kidder, Phillip F. Britt, and A. C. Buchanan, III* Chemical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, MS-6197, Oak Ridge, Tennessee 37831-6197 ReceiVed August 15, 2005. ReVised Manuscript ReceiVed September 21, 2005
Benzyl phenyl ether (PhOCH2Ph, BPE), a model for related structural features in lignin and low-rank coal, has been immobilized in MCM-41 hexagonal mesoporous silicas having pore diameters of 2.7, 2.2, and 1.7 nm. Pyrolysis studies conducted at 275 °C have revealed that the free-radical reaction pathways previously observed on nonporous Cabosil silica remain operative in the MCM-41 silicas. The BPE pyrolysis rate is slightly slower (ca. 2- fold) in these nanoporous solids than on Cabosil, which is correlated with a corresponding decrease in the rate of radical formation by homolysis of the weak O-C bond. Two rearrangement pathways dominate the chemistry and account for ca. 67 mol % of the products at all pore sizes studied, which is markedly increased compared with the corresponding Cabosil case (ca. 50 mol %). The first pathway is recombination of the incipient benzyl and phenoxyl radicals to form surface-immobilized benzylphenol isomers, 1. The second pathway involves an O-C phenyl shift in surface-attached PhOCH‚Ph radicals leading to formation of surface-attached benzhydrol (4) and benzophenone (5) products. The selectivity between these two rearrangement pathways shows only a small dependence on pore size, and comparisons with new data obtained for BPE on Cabosil indicate that the density of grafted BPE molecules on the surface is the most important factor in determining this product selectivity.
Introduction Our prior research probing the pyrolysis kinetics and mechanisms of fuel model compounds showed that restrictions on mass transport can play an important role in altering the freeradical pathways involved.1 This work primarily involved studies of molecules covalently immobilized on the surface of nonporous silica nanoparticles (Cabosil). Ordered mesoporous silicas such as SBA-15 and MCM-41, as well as their organic derivatives, are being widely investigated for many potential applications in catalysis,2 separations,3 and synthesis of nanostructured materials.4 Recently we showed that these mesoporous silicas could be derivatized with organic molecules using a * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Buchanan, A. C., III; Britt, P. F.; Thomas, K. B.; Biggs, C. A. J. Am. Chem. Soc. 1996, 118, 2182. (b) Buchanan, A. C., III; Britt, P. F. J. Anal. Appl. Pyrol. 2000, 54, 127. (c) Britt, P. F.; Buchanan, A. C., III; Malcolm, E. A. Energy Fuels 2000, 14, 1314. (d) Buchanan, A. C., III; Britt, P. F.; Koran, L. J. Energy Fuels 2002, 16, 517. (g) Buchanan, A. C., III; Britt, P. F.; Thomas, K. B. Energy Fuels 1998, 12, 649. (2) (a) Haller, G. L. J. Catal. 2003, 216, 12. (b) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgely, P. A. Acc. Chem. Res. 2003, 36, 20. (c) Brunel, D.; Blanc, A. C.; Galarneau, A.; Fajula, F. Catal. Today 2002, 73, 139. (d) Dufaud, V.; Davis, M. E. J. Am. Chem. Soc. 2003, 125, 9403. (e) Nozaki, C.; Lugmair, C. G.; Bell, A. T.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 13194. (f) McKittrick, M. W.; Jones, C. W. J. Am. Chem. Soc. 2004, 126, 3052. (3) (a) Ding, J. D.; Hudalla, C. J.; Cook, J. T.; Walsh, D. P.; Boissel, C. E.; Iraneta, P. C.; O’Gara, J. E. Chem. Mater. 2004, 16, 670. (b) Ueno, Y.; Tate, A.; Niwa, O.; Zhou, H.-S.; Yamada, T.; Honma, I. Chem. Commun. 2004, 746. (c) Lin, Y.; Fryxell, G. E.; Wu, H.; Engelhard, M. EnViron. Sci. Technol. 2001, 35, 3962. (d) Newalkar, B. L.; Choudary, N. V.; Kumar, P.; Kormarneni, S.; Bhat, T. S. G. Chem. Mater. 2002, 14, 304. (e) Lee, B.; Bao, L.-L.; Im, H.-J.; Dai, S.; Hagaman, E. W.; Lin, J. S. Langmuir 2003, 19, 4246. (4) (a) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 19, 1403. (b) Inagaki, S.; Guan, S.; Ohsuno, T.; Terasaki, O. Nature 2002, 416, 304. (c) Li, Z.; Jaroniec, M. J. Am. Chem. Soc. 2001, 123, 9208. (d) Asefa, T.; Kruk, M.; Coombs, N.; Grondey, H.; MacLachlan, M. J.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 11662.
thermally robust ≡Si-O-Caryl linkage, which permits investigations of the effects of pore confinement and pore size on pyrolysis reactions.5 We reported that the pyrolysis of 1,3diphenylpropane (DPP) confined in mesoporous silicas resulted in altered reaction rates and product selectivities compared with confinement on the nonporous silica, Cabosil. In particular, the pyrolysis rate was enhanced in the mesoporous silicas and increased slightly as the pore size was decreased from 5.6 to 1.7 nm. Further analysis at corresponding DPP surface densities on MCM-41 and Cabosil indicated a rate enhancement of 4-5 at 375 °C.5b The DPP reaction is a radical chain process whose rate is controlled by bimolecular hydrogen-transfer steps. It was proposed that pore confinement increased the rate of these steps through enhanced encounter frequencies and, perhaps, improved geometries for hydrogen transfer on the surface. Utilizing isomeric hydrogen-donor molecules on a silica surface we recently demonstrated that improved orientations for hydrogen transfers on the surface results in faster rates for pyrolysis reactions involving free-radical intermediates.6 In the current work we extend our investigations on pore confinement to benzyl phenyl ether (PhOCH2Ph; BPE), which serves as a model for related R-aryl ether linkages in lignin7 and low-rank coals.8 We immobilized BPE on the surface of three hexagonal mesoporous MCM-41 silicas having mean pore diameters of 2.7, 2.2, and 1.7 nm. The attachment reaction involves condensation of a phenolic derivative of BPE (m(5) (a) Kidder, M. K.; Britt, P. F.; Zhang, Z.; Dai, S.; Buchanan, A. C., III Chem. Commun. 2003, 2804. (b) Kidder, M. K.; Britt, P. F.; Zhang, Z.; Dai, S.; Hagaman, E. W.; Chaffee, A. L.; Buchanan, A. C., III J. Am. Chem. Soc. 2005, 127, 6353. (6) (a) Buchanan, A. C., III; Kidder, M. K.; Britt, P. F. J. Am. Chem. Soc. 2003, 125, 11806. (b) Buchanan, A. C., III; Kidder, M. K.; Britt, P. F. J. Phys. Chem. B 2004, 108, 16772. (7) Lignin: Historical, Biological, and Materials PerspectiVes; Glasser, W. G., Northey, R. A., Schultz, T. P., Eds.; ACS Symposium Series 742; American Chemical Society: Washington, DC, 2000.
10.1021/ef0502608 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/21/2005
Pyrolysis of Benzyl Phenyl Ether
Energy & Fuels, Vol. 20, No. 1, 2006 55 Table 1. Maximum BPE Surface Coverages Obtained for Modified Silicas silica
surface areaa (m2 g-1)
pore diametera (nm)
BPE coverageb (mmol g-1)
BPE densityc (nm-2)
Cabosil MCM-41 MCM-41 MCM-41
200 1150 1194 1285
n.a. 2.7 2.2 1.7
0.52 1.55 1.56 1.53
1.71 1.14 1.10 1.00
a For native silicas. b Average of results from chemical analysis (see text) and carbon elemental analysis on a per gram of derivatized silica basis (error (3%). c Molecular density of BPE groups (molecules/nm2 surface area) corrected for the weight of attached BPE.
Experimental Section
Figure 1. Representation of benzyl phenyl ether (BPE) immobilized in MCM-41 mesoporous silica by a Si-O-Caryl linkage.
HOC6H4OCH2C6H5) with the surface silanols of the mesoporous silicas, which establishes a thermally robust Si-O-Caryl linkage to the surface (Figure 1). In contrast to DPP, the rate of BPE pyrolysis is controlled by the initial unimolecular homolysis of the weak (ca. 52 kcal mol-1) O-C bond.8,9 Hence, BPE reacts rapidly at 275-300 °C by a different free-radical mechanism, and pore confinement may have a different influence on the pyrolysis rate. In prior studies of pyrolysis of BPE immobilized on Cabosil9 we discovered a new rearrangement pathway not previously reported in solution phases that involved an O-C phenyl shift in surface-attached PhOCH‚Ph radical. This pathway produced surface-attached benzhydrol (PhCH(OH)Ph) and benzophenone (PhCOPh) as major products. A second rearrangement pathway, radical recombination of benzyl and phenoxyl radicals at the phenoxyl ring carbons to form benzylphenol (PhCH2PhOH) isomers, was also promoted by the restricted mass transport conditions. These rearrangement pathways, which accounted for ca. 50% of the pyrolysis products, are retrogressive in nature since the products contain more refractory diphenylmethanetype structures compared with the labile ether bridge in BPE. In the current study we seek to determine if these rearrangement pathways are still significant when confined in nanoporous solids and whether the selectivity between the pathways is impacted by pore size. (8) The pyrolysis of BPE in fluid phases has been extensively investigated. (a) Meyer, D.; Nicole, D.; Delpuech, J. J. Fuel Process. Technol. 1986, 12, 255. (b) Meyer, D.; Oviawe, P.; Nicole, D.; Lauer, J. C.; Clement, J. Fuel 1990, 69, 1309. (c) King, H.-H.; Stock, L. M. Fuel 1984, 63, 810. (d) King, H.-H.; Stock, L. M. Fuel 1982, 61, 1172. (e) Korobkov, V. Y.; Grigorieva, E. N.; Bykov, V. I.; Senko, O. V.; Kalechitz, I. V. Fuel 1988, 67, 657. (f) Korobkov, V. Y.; Grigorieva, E. N.; Bykov, V. I.; Kalechitz, I. V. Fuel 1988, 67, 663. (g) Schlosberg, R. H.; Davis, W. H., Jr.; Ashe, T. R. Fuel 1981, 60, 201. (h) Ozawa, S.; Sasaki, K.; Ogina, Y. Fuel 1986, 65, 707. (i) Wu, B. C.; Klein, M. T.; Sandler, S. I. Energy Fuels 1991, 5, 453. (j) Siskin, M.; Brons, G.; Vaughn, S. N.; Katrizky, A. R.; Balasubramamian, M. Energy Fuels 1990, 4, 488. (k) Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707. (l) Makabe, M.; Itoh, H.; Ouchi, K. Fuel 1990, 69, 575. (m) Sato, Y.; Yamakawa, T. Ind. Eng. Chem. Fundam. 1985, 24, 12. (n) Wu, B. C.; Klein, M. T.; Sandler, S. I. AIChE J. 1990, 36, 1129. (9) Buchanan, A. C., III; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895.
Materials. The MCM-41 silicas were synthesized as previously described.5 Controlled variation in the size of the structure directing template, CnH2n+1N(CH3)3Br (n ) 16, 14, 12), produced three structurally related hexagonal mesoporous silicas with mean pore diameters of 2.7, 2.2, and 1.7 nm, respectively. The nitrogen BET surface areas and BJH pore diameters for these materials are given in Table 1. The phenol m-HOC6H4OCH2C6H5 (HOBPE) was synthesized and purified as previously described.9 Benzene was distilled from sodium immediately prior to use. High-purity acetone and diethyl ether were commercially available and used as received. Cumene was fractionally distilled (2×), 3,4-dimethylphenol was recrystallized from ethanol, and 4-phenylphenol was recrystallized from benzene/hexanes. Surface-Attachment Reaction. A sample of MCM-41 was dried in an oven at 200 °C for 4 h and then cooled to room temperature in a desiccator prior to use. The silica (1 g, ca. 4.5 mmol of SiOH assuming a maximum of ca. 2.2 SiOH nm-2 derivatizable by small molecules5,10) was slurried with dry benzene (15 mL), excess HOBPE (2.6 g, 13.0 mmol) was added, the mixture was stirred for 10 min, and the solvent was removed on a rotary evaporator. The solid was transferred to a Pyrex glass reaction tube, evacuated overnight, and sealed at
