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Retrogressive Pyrolysis Pathways for Surface-Constrained Diarylmethanes A. C. Buchanan, III,* Phillip F. Britt, and Larry J. Koran Oak Ridge National Laboratory, Chemical & Analytical Sciences Division, P.O. Box 2008, MS-6130, Oak Ridge, Tennessee 37831-6130 Received October 22, 2001. Revised Manuscript Received December 10, 2001
Pyrolysis studies of silica-immobilized diarylmethanes, which are models for related structures in fuel resources, have been conducted at 425 °C to explore the impact of restricted mass transport on reaction pathways with particular attention to the formation of PAHs. Two retrogressive free radical pathways are observed in the pyrolysis of silica-immobilized diphenylmethane and 2-naphthylphenylmethane that involve the diarylmethyl radical as a key intermediate. The major pathway is cyclization-dehydrogenation that forms fluorene and benzofluorene (two isomers), respectively. This pathway can be attenuated, but not eliminated, by the presence of a neighboring hydrogen donor (tetralin) on the surface in contrast to fluid-phase studies. The second pathway, which has not been reported in fluid phases, is a radical displacement path that forms crosslinked triarylmethane products. The selectivity for these two pathways is found to be a function of the orientation of the diarylmethane on the surface, as well as the presence of neighboring spacer molecules.
Introduction Diarylmethanes serve as models for analogous thermally robust linkages present in fuel resources such as coal,1-12 and diphenylmethane units are also thought to be important components in acidic or alkaline treated lignins.13 As a consequence of the high C-C and C-H bond strengths (ca. 89 and 82 kcal mol-1, respectively, for diphenylmethane1), most studies have focused on the hydrogenolysis chemistry for diarylmethanes at 400450 °C in the presence of hydrogen donor solvents and/ or hydrogen atmosphere. In the absence of added hydrogen sources, pyrolytically induced cyclizationdehydrogenation reactions have the potential for generating polycyclic aromatic hydrocarbons (PAHs) through establishment of the fluorene (and its analogues) skel* Author to whom correspondence should be addressed. (1) Poutsma, M. L. Energy Fuels 1990, 4, 113, and references therein. (b) Poutsma, M. L. J. Anal. Appl. Pyrolysis 2000, 54, 5. (2) Murata, S.; Nakamura, M.; Miura, M.; Nomura, M. Energy Fuels 1995, 9, 849. (3) Futamura, S.; Koyanagi, S.; Kamiya, Y. Fuel 1988, 67, 1436. (4) McMillen, D. F.; Malhotra, R.; Chang, S.-J. Fuel 1987, 66, 1611. (b) Malhotra, R.; McMillen, D. F. Energy Fuels 1990, 4, 184. (5) Autrey, T.; Alborn, E. A.; Franz, J. A.; Camaioni, D. M. Energy Fuels 1995, 9, 420. (6) Mitchell, S. C.; Lafferty, C. J.; Garcia, R.; Snape, C. E.; Buchanan, III, A. C.; Britt, P. F.; Klavetter, E. Energy Fuels 1993, 7, 331. (7) Shi, B.; Ji, Y.; Guthrie, R. D.; Davis, B. H. Energy Fuels 1994, 8, 1268. (8) McMillen, D. F.; Ogier, W. C.; Ross, D. S. J. Org. Chem. 1981, 46, 3322. (9) Benjamin, B. M.; Raaen V. F.; Maupin, P. H.; Brown, L. L.; Collins, C. J. Fuel 1978, 57, 269. (10) Petrocelli, F. P.; Klein, M. T. Macromolecules 1984, 17, 161. (11) Sweeting, J. W.; Wilshire, J. F. K. Aust. J. Chem. 1962, 15, 89. (12) Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707. (13) Lai, Y.-Z.; Xu, H.; Yang, R. In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W. G., Northey, R. A., Schultz, T. P., Eds.; American Chemical Society Symposium Series 742: Washington, DC, 2000; pp 239-249.
eton. There continues to be considerable environmental interest in the formation of PAHs during the pyrolysis and combustion of coal and other fuel resources.14,15 Fluorene, an EPA priority pollutant,15 and benzo[a]fluorene have been recently identified as products from pyrolysis of Yallourn Brown coal,14a as well as from the fuel-rich combustion of coal primary tar.14b Direct observation of fluorene formation from pyrolysis of diphenylmethane itself has been reported, but only at high temperatures (ca. 550-700 °C).10,11 In our research, we have been exploring the effects of restricted mass transport on pyrolysis reactions of model compounds for related structural units in fossil and renewable energy resources.16 Restricted mass transport, which can be important in the thermochemical processing of macromolecular energy resources,16a has been simulated through the use of model compounds that are covalently linked to the surface of silica nanoparticles through a thermally robust Si-O-Caryl linkage. This research has uncovered examples where product selectivities and reaction rates are significantly altered compared with corresponding fluid phase models. In particular, retrogressive rearrangement and (14) Wornat, M. J.; Mikolajczak, C. J.; Vernaglia, B. A.; Kalish, M. A. Energy Fuels 1999, 13, 1092. (b) Ledesma, E. B.; Kalish, M. A.; Nelson, P. F.; Wornat, M. J.; Mackie, J. C. Fuel 2000, 79, 1801. (15) Mastral, A. M.; Callen, M. S. Environ. Sci. Technol. 2000, 34, 3051. (16) Buchanan, III, A. C.; Britt, P. F. J. Anal. Appl. Pyrolysis 2000, 54, 127. (b) Buchanan, III, A. C.; Britt, P. F.; Thomas, K. B.; Biggs, C. A. J. Am. Chem. Soc. 1996, 118, 2182. (c) Buchanan, III, A. C.; Britt, P. F.; Thomas, K. B. Energy Fuels 1998, 12, 649. (d) Buchanan, III, A. C.; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895. (e) Buchanan, III, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. Soc. 1986, 108, 7703. (f) Ismail, K.; Mitchell, S. C.; Brown, S. D.; Snape, C. E.; Buchanan, III, A. C.; Britt, P. F.; Franco, D. V.; Maes, I. I.; Yperman, J. Energy Fuels 1995, 9, 707.
10.1021/ef0102521 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/22/2002
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Experimental Section
Figure 1. Silica-immobilized diarylmethane model compounds investigated.
cyclization pathways can be promoted under restricted mass transport conditions.16 For example, pyrolysis of silica-immobilized bibenzyl at 400 °C resulted in a significantly enhanced rate for a cyclization-dehydrogenation pathway that forms the PAH, phenanthrene, compared with fluid phases.16e Furthermore, in studies of the pyrolysis of the silica-immobilized (denoted by the symbol “≈”) model compounds, ≈PhCH2XPh [X ) CH2,16e O,16d S16f], a radical rearrangement path via a 1,2-aryl shift (eq 1) was promoted that generates products containing the diphenylmethane skeleton. For the ether and
≈PhCH‚XPh f ≈PhCH(Ph)X‚
(1)
sulfide models, this chemistry occurs at temperatures of only 275-300 °C. Hence, during the thermal processing of coal, there are low-temperature pathways to generate additional diarylmethane linkages beyond those in the native material. We have now examined the pyrolysis chemistry for the silica-immobilized diarylmethanes, diphenylmethane (≈SiOC6H4CH2C6H5; ≈DPM) and 2-naphthylphenylmethane (≈SiOC6H4CH2C10H7; ≈NPM), to explore the potential for PAH formation at modest reaction temperatures (400-425 °C).17 The influence of surface orientation of the DPM molecules on the reaction pathways has been probed, as well as the influence of small amounts of co-attached radical initiator (bibenzyl) and large amounts of co-attached hydrogen donor (tetralin), whose structures are shown in Figure 1. These studies reveal the occurrence of two retrogressive reaction pathways, cyclization-dehydrogenation and aromatic substitution, that involve diarylmethyl radicals as key intermediates. (17) Aspects of this work have been previously communicated. (a) Buchanan, III, A. C.; Britt, P. F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43 (3), 630. (b) Buchanan, III, A. C.; Britt, P. F.; Koran, L. J. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2001, 46 (1), 225.
General. GC analysis was performed on a Hewlett-Packard 5890 Series II gas chromatograph employing a J & W Scientific 30 m × 0.25 mm DB-5, 5% diphenyl 95% dimethyl polysiloxane column (0.25 µm film thickness), and flame-ionization detection. Product detector response factors were determined relative to cumene (hydrocarbon products) or 2,5-dimethylphenol and 4-phenylphenol (phenolic products) as internal standards. Mass spectra were obtained at 70 eV with a Hewlett-Packard 5972A/5890 Series II GC-MS equipped with a capillary column matched to that used for GC analyses. High purity acetone, methylene chloride, and water were commercially available and used as received. Benzene was distilled from sodium under argon before use. Cumene was fractionally distilled (2×), and 2,5-dimethylphenol and 4-phenylphenol were recrystallized (3×) from hexanes and benzene/hexanes, respectively, prior to use. Preparation of Surface-Attached Materials. The DPM model compound was attached to the silica surface through both a para- and meta- linkage (Figure 1). The precursor phenol, p-HOC6H4CH2C6H5, was commercially available. Purification involved elution from a silica gel column with benzene, followed by multiple recrystallizations from hot benzene/hexane (1:4) to give the desired phenol in 99.9% purity by GC. The isomeric phenol, m-HOC6H4CH2C6H5, was synthesized by the reaction of benzene with m-HOC6H4CH2OH in the presence of AlCl3. Following addition of water and then additional benzene, the benzene layer was separated, washed with saturated NaCl solution, dried over Na2SO4, filtered, and the solvent removed under reduced pressure. Vacuum distillation (135-140 °C at 0.25 Torr) gave the desired phenol with a purity of 99.6% by GC. The precursor phenol for ≈NPM (pHOC6H4CH2-2-C10H7) was synthesized by reaction of p-bromoanisole with sec-butyllithium in THF at -78 °C, coupling with 2-naphthaldehyde, reduction of the resulting alcohol with H2 over 10% Pd/C in acetic acid, and demethylation of the methyl ether with Me3SiCl/NaI in CH3CN. Purification involved elution from a silica gel column (toluene) followed by multiple recrystallizations from hot toluene/hexane (1:2) giving the desired phenol with a GC purity of 99.8%. The precursor phenol for ≈TET (5,6,7,8-tetrahydro-2-naphthol) was commercially available and purified as previously described.16c Synthesis of the precursor phenol for ≈BB (p-HOC6H4CH2CH2C6H5) has also been reported.16e Chemical attachment of the phenols to the surface of a nonporous silica (Cabosil M-5; 12 nm particle size; 200 m2 g-1; ca. 1.5 mmol SiOH g-1) was accomplished as described below for the p-DPM isomer. For the two-component surfaces, the phenols were attached in a single step as described previously.16b,c p-HOC6H4CH2C6H5 (6.085 g; 33.0 mmol) was dissolved in dry benzene (50-100 mL) and added to a benzene slurry (300 mL) of silica (9.57 g; 14.4 mmol SiOH) that had been dried at 200 °C for 4 h in an oven. Following stirring and benzene removal on a rotovap, the solid was sealed in a Pyrex tube that was evacuated to 4 × 10-6 Torr. The attachment reaction was conducted in a fluidized sand bath at 200 °C for 1 h. Unattached phenol was removed by temperatureramped sublimation under dynamic vacuum (225-275 °C; 1 h; 0.02 Torr). For ≈NPM synthesis, an additional Soxhlet extraction (benzene, 4 h) was required to remove unattached precursor phenol. Surface coverage analysis was accomplished by dissolution of the solid (ca. 200 mg) in 30 mL of 1 N NaOH overnight. 4-Phenylphenol was added as an internal standard. The solution was neutralized by the addition of HCl, and extracted thoroughly with CH2Cl2 (3 × 10 mL). The solution was dried over MgSO4, filtered, the solvent removed under reduced pressure, and the resulting material silylated with N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) in pyridine (2.5 M). GC analysis gave the following surface coverages: ≈DPM (0.43 mmol g-1), ≈m-DPM (0.29 mmol g-1), ≈NPM (two
Pyrolysis of Surface-Constrained Diarylmethanes
Energy & Fuels, Vol. 16, No. 2, 2002 519
Figure 2. Major products from pyrolysis of ≈DPM (0.43 mmol g-1) at 425 °C. The yields represent averages of 6 runs. Table 1. Composition and Pyrolysis of Surface-Attached Diarylmethanes surface composition
coveragea (mmol g-1)
≈DPM
0.43
≈m-DPM ≈NPM
0.29 0.50 0.49 0.12/0.38 0.38/0.089
≈DPM/≈TET ≈DPM/≈BB
no. of pyrolyses
temp (°C)
conversion range (%)
rateb (% h-1)
corr. coeff.c (r)
selectivityd (cycl.:displ.)
6 1 10 6 4 5 4
425 400 425 425 425 425 400
0.76-18.0 1.8 2.3-6.6 1.3-6.0 1.4-3.6 1.5-3.8g 0.33-2.0h
1.1 0.45 (0.21)e 0.51 0.31 0.43 0.41 2.3h
0.986 f 0.920 0.989 0.981 0.991 0.981
4.6 ( 0.4 3.5 3.3 ( 0.4 3.0 ( 0.3 2.9 ( 0.3 cyclization only 16.4 ( 1.7
a Component surface coverage on a per gram of derivatized silica basis. b Initial rates for total decomposition of ≈DPM from the slopes of the linear regressions of conversion versus reaction time. c Correlation coefficient from linear regression analysis of initial rates. d Selectivity for cyclization path (fluorene-based products) relative to the displacement path (triarylmethane products). e Rate in parentheses is for conversion solely to cyclization and displacement products. f Does not apply. g The ≈TET conversion over this range was 17-32%. h Conversion and rate are based solely on the cyclization and displacement products, because hydrocracked products are obscured by the same products formed from ≈BB pyrolysis. ≈BB conversion range is 1.8-14.0%.
batches at 0.50 and 0.49 mmol g-1), ≈DPM/≈TET (0.12/0.38 mmol g-1), ≈DPM/≈BB (0.38/0.089 mmol g-1). Chemical purities were typically > 99%. Pyrolysis Procedure. The pyrolysis apparatus and procedure have been described previously.16 In brief, a weighed amount of sample (0.5-0.7 g) was placed in one end of a T-shaped Pyrex tube, evacuated, and sealed at ca. 2 × 10-6 Torr. The sample was inserted into a preheated temperaturecontrolled ((1.0 °C) tube furnace, and the other end placed in a liquid nitrogen bath. The volatile products collected in the trap were dissolved in acetone (0.1-0.3 mL) containing internal standards (vide supra) and analyzed by GC and GCMS. Surface-attached pyrolysis products were similarly analyzed after separation by digestion of the silica in aqueous base and silylation of the resulting phenols to the corresponding trimethylsilyl ethers as described above for the surface coverage analysis. Product Assignments. Product assignments were confirmed with authentic samples except as noted. For ≈DPM, triarylmethane product 2 (see Figure 2) was identified from the mass spectrum of the recovered phenol, Mr ) 260, which added one trimethylsilyl group upon silylation with BSTFA to give a derivative with Mr ) 332. The mass spectrum of product 3 gave Mr ) 276 for the recovered di-phenol, which added two trimethylsilyl groups upon silylation to give a derivative with Mr ) 420. The mass spectra for 2 and 3 were characterized by their parent ions (the base peaks), and major fragment ions corresponding to loss of a phenyl group (m/z ) 77) consistent with the proposed structures. For ≈m-DPM, 4-hydroxyfluorene (4) was identified from its mass spectra (Mr ) 182 that silylates once to a derivative with Mr ) 254), which were similar to those for the 2-hydroxyfluorene isomer (2) that was previously synthesized.16b Triarylmethane products 6 (Mr ) 260; silylated, Mr ) 332), and 7
(Mr ) 276; silylated, Mr ) 420) gave mass spectra similar to the isomeric products 2 and 3, respectively, as expected. For ≈NPM, the benzofluorene isomers 8 and 9 gave similar mass spectra with the expected parent ions corresponding to Mr ) 232 for the phenol and Mr ) 304 for the silylated derivative. The mass spectrum for triarylmethane product 10 gave Mr ) 360 which added one trimethylsilyl group to give a derivative with Mr ) 432, while the mass spectrum of 11 gave Mr ) 326 which added two trimethylsilyl groups to give a derivative with Mr ) 470. For both 10 and 11, the mass spectra were characterized by intense parent ions and major fragment ions for loss of a naphthyl group (m/z ) 127) consistent with the proposed structures.
Results The silica-attached diarylmethane models prepared are shown in Figure 1, and their surface coverages are reported in Table 1. The symbol “≈” will be used to denote a silica-attached molecule. For the ≈NPM model, two different batches were prepared that gave similar pyrolysis rates and product distributions. Typically 4-6 pyrolyses were conducted on each sample in sealed, evacuated tubes. Gas-phase and surface-attached products were separately analyzed as described in the Experimental Section. Pyrolysis studies focused on low diarylmethane conversions (based on recovered products) to identify primary products and probe inherent reaction path selectivities. The initial rates of pyrolysis were determined from the slopes of linear regressions of diarylmethane conversion versus reaction time, and these data are collected in Table 1.
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Figure 3. Major products from pyrolysis of ≈m-DPM (0.29 mmol g-1) at 425 °C. The yields represent averages of 10 runs.
Figure 4. Major products from pyrolysis of ≈NPM (two batches at 0.50 and 0.49 mmol g-1) at 425 °C. The yields represent averages of 10 runs.
The principal pyrolysis products determined by GC and GC-MS for ≈DPM, ≈m-DPM, and ≈NPM are shown in Figures 2, 3, and 4, respectively. Product identities were confirmed with authentic samples except for 2-4 and 6-11 which were identified on the basis of their mass spectra (see Experimental Section). For each substrate, several additional products were detected in small amounts that could not be clearly identified. As expected, mass balances are typically good at low conversions (ca. 97% at < 4% conversion), but become progressively worse at higher conversions (e.g., ca. 87% for ≈DPM at 18% conversion). In Figures 2-4, the yields of the products (in mol %) are presented, and represent the averages of at least 4-6 pyrolysis runs. Standard deviations in the product yields ranged from 4 to 15% of the average value. For each substrate, reaction path selectivity (vide infra) was calculated on the basis of the yields of the fluorene-based cyclization products relative to the triarylmethane-based displacement products, and the values with their associated errors are reported in Table 1. In the case of the two component surface, ≈DPM/ ≈BB, additional products were observed arising from the pyrolysis of the bibenzyl moiety. These were consistent with ≈BB pyrolysis products identified pre-
viously.16e However, because of the significant ≈BB conversion (2-14%) that occurred, and the fact that this reaction also produces the hydrocracked pyrolysis products (gas-phase and surface-attached benzene and toluene in Figure 2), an accurate total conversion of ≈DPM could not be obtained. The conversion and rate reported in Table 1 for this sample are based on the yields of the cyclization and displacement products 1-3. For the twocomponent surface, ≈DPM/≈TET, the tetralin moiety underwent significant conversion (17-32%) and generated a complex array of products that are consistent with those previously detected in tetralin pyrolysis.1,16c The major products arose from ring contraction (methylindan) and dehydrogenation (naphthalene) as expected. ≈DPM mass balances were ca. 94%. The impacts of the ≈BB and ≈TET molecules on ≈DPM pyrolysis rate and product selectivity will be discussed in more detail below. Discussion Pyrolysis Rates. Pyrolysis of the diarylmethanes occurred slowly at 425 °C with rates in the range of 0.31.1% h-1. The slow rates are expected on the basis of the strong C-C and C-H bonds present in the mol-
Pyrolysis of Surface-Constrained Diarylmethanes Scheme 1
ecule.1 Petrocelli and Klein10 have reported that the pyrolysis of DPM occurs readily in biphenyl (0.1 M) at 550 °C. However, surprisingly, Suzuki et al. have reported DPM to pyrolyze at a rate of ca. 1% h-1 in an argon atmosphere at 425 °C, and Murata et al.,2 report modest conversions for dibenzylnaphthalenes at 430 °C. In our earlier studies of the temperature-programmed reduction (under nitrogen flow) of ≈DPM (0.45 mmol g-1),18 benzene and toluene evolution was detected at 425 °C with peak evolution occurring at ca. 520 °C. This peak maximum shifted to lower temperatures in the presence of hydrogen gas and a dispersed MoS2 catalyst. Hence, it appears that diarylmethanes can undergo very slow pyrolysis at 425 °C. However, the pyrolysis rate can be accelerated in the presence of free-radical initiators as discussed below. ≈DPM. Pyrolysis of ≈DPM leads to the principal products shown in Figure 2. The major product observed is surface-attached fluorene (1) formed via a cyclization-dehydrogenation path. The fluorene yield is slightly larger than reported in the fluid phase.10 This reaction liberates hydrogen that results in hydrogenolysis of ≈DPM to form benzene and toluene (both gas-phase and surface-attached), and hydrogenolysis of the SiOPhCH2Ph surface linkage to produce the diphenylmethane. Interestingly, two triphenylmethane products (2 and 3) were detected that have not been previously reported in DPM pyrolysis, and appear to arise from mass transfer restrictions on the surface. The ≈DPM pyrolysis products can be explained by a free radical mechanism in which the surface-attached diphenylmethyl radical, ≈PhCH‚Ph, serves as the key intermediate. The initiation step for this chemistry remains uncertain,1 but homolytic scission of the C-C bond (log k/s-1 ) 15.3 - 82.3/θ)19 or benzylic C-H bond (log k/s-1 ) 15.3 - 81.4/θ)19 appears unlikely in this temperature regime. The possible involvement of a lower energy reverse radical disproportionation (RRD) step (Scheme 1) has been suggested,2,20a and can be important in low-temperature pyrolysis regimes where there are no weak bonds for homolysis.1b,21 This reaction would form diphenylmethyl radicals directly, which have been detected by ESR spectroscopy during heating DPM at 430 °C.22 Furthermore, the concentration of (18) Mitchell, S. C.; Lafferty, C. J.; Garcia, R.; Snape, C. E.; Buchanan, III, A. C.; Britt, P. F.; Klavetter, E. Energy Fuels 1993, 7, 331. (19) Rossi, M. J.; McMillen, D. F.; Golden, D. M. J. Phys. Chem. 1984, 88, 5031. (20) King, H. H.; Stock, L. M. Fuel 1982, 61, 257. (b) Ceylan, K.; Stock, L. M. Fuel 1990, 69, 1386. (21) The role of RRD, or retrodisproportionation as preferred by the authors, in solution-phase transfer hydrogenation and hydrogenolysis reactions has been recently reviewed [Ruchardt, C.; Gerst, M.; Ebenhoch, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 1406]. Thermochemical kinetic estimates using the bond additivity approach16c,29 give ∆H ) 61.0 kcal mol-1 for the RRD process for two vapor-phase DPM molecules (similar to that shown in Scheme 1), which can be used as an estimate of the activation energy for the process.
Energy & Fuels, Vol. 16, No. 2, 2002 521 Scheme 2
Scheme 3
diphenylmethyl radicals increased substantially upon the addition of benzophenone, which can participate as a hydrogen acceptor in RRD.21,22 The RRD step may be facilitated on the surface as a consequence of the preorganization of the reactants on the surface. Rapid bimolecular hydrogen transfer steps (hydrogen transfer, radical relay process) have been indicated in the pyrolysis of surface-attached 1,3-diphenylpropane in the presence of DPM spacer molecules.16b Hydrogen abstraction by gas-phase and surfaceattached benzyl radicals (denoted by “∼” in Scheme 2, step 2), or hydrogen atoms (step 6) also form surfaceattached diphenylmethyl radicals. Cyclization of these radicals to produce fluorene (step 3) appears to proceed through radical 12. In support of this premise, Siskos et al.23 recently conducted a laser flash photolysis study of N-(triphenylmethyl)anilines that probed the mechanism of cyclization of the related triphenylmethyl radical to form 9-phenylfluorene (Scheme 3). This study provided spectroscopic evidence for the formation of intermediate radical 13, analogous to 12 in Scheme 2, as well as the nonaromatic isomer intermediate 14. The cyclization process liberates hydrogen atoms that induce hydrogenolysis of ≈DPM via standard ipsosubstitution to produce the hydrocracked products (Scheme 2, steps 4-5).24 On the basis of the yields of surface-attached benzene relative to benzene (Figure 2), (22) Livingston, R.; Zeldes, H.; Conradi, M. S. J. Am. Chem. Soc. 1979, 101, 4312. (23) Siskos, M. G.; Zarkadis, A. K.; Steenken, S.; Karakostas, N.; Garas, S. K. J. Org. Chem. 1998, 63, 3251. (24) Kinetic modeling studies have also examined the various possible mechanisms of hydrogenolysis for diarylmethanes in hydrogen donor solvents.5 For benzylarenes such as diphenylmethane, the model predicts that the free hydrogen atom pathway dominates the hydrogenolysis activity.
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Energy & Fuels, Vol. 16, No. 2, 2002 Scheme 4
Scheme 5
there is a slight selectivity of 1.2 ( 0.1 in favor of step 4a relative to 4b that is not observed in the ≈m-DPM case described below. This selectivity is consistent with a slight stabilization of the intermediate ipso-substituted radical 15 by the p-silyloxy linking group to the surface (Scheme 4) compared with radical 16. Recent theoretical calculations on hydroxy-substituted cyclohexadienyl radicals support this conclusion.25 Hydrocracking of the surface linkage to produce gas-phase DPM (step 5) presumably occurs through a similar ipsosubstitution process. The amount of cleavage of the surface linkage for the three model compounds investigated (Figures 2-4) is proportional to the amount of cyclized product formed, but further analysis is limited by the lack of understanding of the potential role of surface packing and steric effects that may impact the accessibility to this attachment point. The formation of the new triphenylmethane products, 2 and 3, is also consistent with a radical reaction involving diphenylmethyl radical intermediates, which undergo an ipso-aromatic substitution reaction on a neighboring molecule of ≈DPM in competition with cyclization (Scheme 5). No products were detected that could arise from substitution at non-ipso-positions. There is little, if any, selectivity apparent in the substitution on the A or B rings based on the 2:3 product ratio of 1.1 ( 0.1. This radical displacement pathway is observed on the surface, and not in fluid phases, as a consequence of the surface confinement for the diphenylmethyl radicals and their close proximity to other ≈DPM molecules on the surface. The selectivity for cyclization relative to displacement (Table 1) was found to be 4.6 ( 0.4. This path selectivity should be sensitive to the distance between ≈DPM molecules on the surface, which will be discussed below in the studies of the twocomponent surfaces. The radical displacement pathway can also be considered a retrogressive reaction pathway, since products such as 3 are formally cross-linked on the surface. ≈DPM/≈BB and ≈DPM/≈TET. Pyrolysis of ≈DPM/ ≈BB (0.38/0.089 mmol g-1) was investigated at 400 °C
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to examine the impact of a surface-dispersed free radical initiator on the pyrolysis rate, as well as the presence of a small amount of a surface diluent on the cyclization: displacement path selectivity. The lower temperature was chosen to limit the conversion of the reactive bibenzyl moiety, which generates both surface-attached and gas-phase benzyl radicals with a rate constant of 8 × 10-6 s-1.16c,e Pyrolysis of ≈BB also generates the surface-attached and gas-phase benzene and toluene products observed for ≈DPM.16c,e For the purpose of the rate comparison, the ≈DPM pyrolysis rate in Table 1 was based solely on the unique cyclization and rearrangement products 1-3. For comparison, a ≈DPM pyrolysis was also conducted at 400 °C and the rate calculated on the basis of the same products. The rate enhancement (2.3/0.21% h-1 ) 11) is clearly consistent with the free-radical mechanism presented above. Hence, the rates of the retrogressive reactions for diarylmethanes can be significantly accelerated in the presence of free-radical sources. Interestingly, the path selectivity increased to 16.4 in the presence of the ≈BB. We attribute this change in selectivity to the fact that ≈BB contributes ca. 20% of the surface-bound molecules. Hence, ≈DPM molecules are diluted on the surface, which retards the rate of the displacement reaction. This path can be completely eliminated by the presence of excess spacer molecules as described below.26 One might expect that, if a pure gas-phase radical initiator were employed, the ≈DPM path selectivity would be unaltered. These experiments were conducted on ≈NPM (vide infra) with 2,3-dimethyl-2,3-diphenylbutane as the initiator, and showed the expected rate acceleration without altering the ≈NPM product selectivity. Pyrolysis of ≈DPM/≈TET (0.12/0.38 mmol g-1) was investigated to examine the effect of a neighboring hydrogen donor on the surface on the ≈DPM pyrolysis. We find the ≈DPM pyrolysis rate at 425 °C is reduced ca. 2.7-fold compared with surfaces containing ≈DPM alone. This rate reduction is a consequence of dilution of ≈DPM molecules on the surface, and conversion of some fraction of diphenylmethyl radicals into tetralyl radicals by hydrogen transfer. The presence of the tetralin spacer molecules reduces the cyclization reaction 2.3-fold (fluorene yield of 14 ( 1%), but does not eliminate it. However, the 3-fold excess of ≈TET leads to complete elimination of the displacement pathway for ≈DPM that forms triphenylmethane products 2 and 3. Instead, the reaction for ≈DPM is dominated by the hydrocracked products shown in Figure 2, which account for the remainder of the observed ≈DPM products. In solution-phase pyrolysis studies of diarylmethanes in the presence of hydroaromatic solvents, typically only products from hydrocracking are reported with no evidence for the cyclization path indicated.1-5 ≈m-DPM. Pyrolysis of ≈m-DPM was examined to probe the impact of surface molecular orientation on the product selectivity. The product mixture obtained (Fig(25) Autrey, T,; Linehan, J. C.; Kaune, L.; Powers, T. R.; McMillan, E. F.; Stearns, C.; Franz, J. A. Energy Fuels 1999, 13, 927. (26) The cyclization:displacement path selectivity would also be expected to increase, albeit not as dramatically, even in the absence of spacer molecules as the initial surface coverage of ≈DPM decreases. We have not investigated this surface coverage dependence for single component surfaces of ≈DPM.
Pyrolysis of Surface-Constrained Diarylmethanes
ure 3) was very similar to that obtained for the paraisomer (Figure 2). However, cyclization of the intermediate radical, m-PhCH‚Ph, gives rise to two possible surface-attached fluorene isomers 4 and 5. The identity of 5 has been confirmed with an authentic sample of 2-hydroxyfluorene16b (which is formed following detachment of 5 from the surface). The slight preference observed for formation of isomer 4 (4:5 selectivity of 1.13 ( 0.06) is consistent with predictions from HF/6-31G*// AM1 and DFT/pBP/DN*//AM1 calculations on the corresponding phenols.27 Little selectivity is expected in the formation of the hydrogenolysis products, since the meta-substitution pattern removes the silyloxy-substituent from direct interaction with the radical center in the cyclohexadienyl radical intermediates (vide supra). The yields of ≈PhH relative to PhH, which were 1.2 for the para-case above, are found to be 0.97 ( 0.10 for the meta-isomer as expected. The radical displacement pathway is again detected, and produces triphenylmethanes 6 and 7 in comparable yields. However, the different orientation of these molecules on the surface results in a slightly lower selectivity for the cyclization versus displacement path (3.3 ( 0.4) compared with the para-isomer (Table 1). This suggests that the meta-orientation may provide a better geometry for the radical displacement reaction to occur, and/or the cyclization reaction is slower due to lack of conjugation for the radical intermediate with the m-silyloxy surface linkage. ≈NPM. Pyrolysis of ≈NPM was explored to examine the generality of the radical cyclization and displacement chemistry for a larger ring system diarylmethane. As seen from Figure 4, pyrolysis of ≈NPM gives an analogous set of products to those obtained from the ≈DPM and ≈m-DPM systems, whose yields were similar (maximum deviations were (10%) for the two different batches. Furthermore, addition of a gas-phase radical initiator, Ph(CH3)2CC(CH3)2Ph, accelerated the pyrolysis rate without altering the product distribution. The retrogressive cyclization path is again prominent producing the benzo[b]fluorene and benzo[c]fluorene isomers, 8 and 9, respectively. DFT calculations (pBP/ DN*//AM1) on the corresponding phenols suggest that 8 would be more the more stable isomer.27 With the presence of the more reactive naphthalene ring, we expected to see more selectivity in the hydrocracking reaction for ≈NPM that is the analogue to step 4 in Scheme 2 for ≈DPM.28 The enthalpy for hydrogen atom addition to naphthalene at the 2-position (-28 kcal mol-1) is more exothermic than addition to benzene (-22 kcal mol-1).4a For highly exothermic hydrogen atom additions such as these, an Evans-Polanyi factor of 0.1-0.2 (early transition state) is recommended.4a This predicts a naphthalene: PhH selectivity ratio of 1.5-2.4 at 425 °C, which would be slightly reduced by the presence of the p-silyloxy substituent as described above for ≈DPM. The observed naphthalene: surface(27) Calculations were performed using PC Spartan Pro software from Wavefunction, Inc. Hartree-Fock and density functional calculations were single-point energy calculations on AM1 optimized geometries. (28) One might also expect that the presence of the naphthalene ring would accelerate the initial RRD step. However, we observe the overall pyrolysis rate for ≈NPM to actually be slower than for ≈DPM as shown in Table 1. The origin of this effect is not currently understood.
Energy & Fuels, Vol. 16, No. 2, 2002 523 Scheme 6
attached benzene ratio was measured to be 2.1 ( 0.22 which is consistent with this analysis. The radical displacement path is also observed for ≈NPM and produces the triarylmethanes 10 and 11. The path selectivity for cyclization relative to displacement was found to be 3.0 ( 0.3, which is lower than that measured for the structurally related para-substituted isomer of DPM (Table 1). This modest difference apparently reflects altered reactivity for the intermediate diarylmethyl radical, which in the case of ≈NPM is both more delocalized and sterically congested. A more detailed investigation of the influence of diarylmethyl radical structure is required to gain a more complete understanding of the selectivity for partitioning between these two reaction pathways. We note that, in contrast to the behavior of ≈DPM and ≈m-DPM where no selectivity was observed, the radical displacement by the intermediate diarylmethyl radical favors the more reactive naphthalene ring of ≈NPM resulting in a 10: 11 selectivity of 6.6 ( 0.62. This displacement pathway, proceeding through radicals 17 and 18 in Scheme 6, will be considerably less exothermic than the corresponding hydrogenolysis reaction, since a C-C bond rather than a C-H bond is formed in the cyclohexadienyl-type radical intermediates. In fact, using the bond additivity approach, we estimate that the addition of benzyl radical to benzene (analogue of 18) is actually slightly endothermic (2.6 kcal mol-1).29 Hence, the displacement reaction will involve a much later transition state described by an Evans-Polanyi factor of 0.4-0.5 for a near thermoneutral reaction.1b,4a Assuming 17 is 6.0 kcal mol-1 more stable than 18 as in the case of the hydrogen adducts (vide supra), an Evans-Polanyi factor of 0.4-0.5 gives a predicted 10:11 selectivity of 5.68.6 consistent with the measured value.30 Conclusions This research has shown that diarylmethane structures in coal and other fuel resources are possible sources of PAHs during pyrolysis and combustion. Through the use of silica-immobilized diarylmethanes, we have found that PAH formation can occur, albeit slowly, at temperatures as low as 400-425 °C. PAH formation occurs by a retrogressive cyclization path that establishes the fluorene skeleton and involves diarylmethyl radicals as the key intermediates. This reaction can be significantly accelerated by the presence of free radical initiators, both surface-confined and in the gas phase, which are also present during pyrolysis and (29) NIST Structures and Properties, NIST Database 25, Version 2.02. (30) The kinetics and theory of the reactions of carbon-centered radicals with alkenes in solution has been recently reviewed. For many of these reactions, an Evans-Polanyi factor of ca. 0.25 is recommended. Fischer, H.; Radom, L. Angew. Chem., Int. Ed. 2001, 40, 1340.
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combustion of fuels. Furthermore, the presence of neighboring hydrogen donors on the surface, such as tetralin, attenuates but does not eliminate the cyclization path in contrast to fluid-phase models. The potential for PAH formation is only accentuated under restricted mass transport conditions, where unimolecular radical reactions such as cyclization are promoted as a consequence of reduced rates of radical termination. Moreover, the precursor diarylmethane structures are also readily formed at temperatures as low as 275 °C from, for example, benzyl phenyl ethers16d and sulfides16f by free-radical rearrangement pathways that are promoted by restricted mass transport conditions. Interestingly, the formation of fluorene and benzo[a]fluorene have been reported in the pyrolysis and fuel-rich combustion of coal and coal primary tar.14 The benzo[a]fluorene could arise from an indigenous structure in the coal, or from a naphthyl phenyl methane substituted at the 1-position on the naphthalene. Our current work suggests that other benzofluorene isomers may be present in coal pyrolysis products potentially arising from naphthyl phenyl methanes substituted at the 2-position of the naphthalene. This research has also uncovered the presence of a second retrogressive pathway for surface-confined di-
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arylmethanes, which leads to cross-linked triarylmethanes on the silica surface. This radical displacement path involves ipso-aromatic substitution of diarylmethylradicalsonneighboringdiarylmethanemolecules on the surface, a pyrolysis path that has not been reported for diarylmethanes in fluid phases. The cyclization:displacement path selectivity is found to depend on the structure and orientation of the diarylmethane molecule on the surface, as well as the proximity of neighboring molecules. For example, dilution of ≈DPM with ca. 20 mol % of an attached spacer molecule (≈BB) hindered the displacement reaction and increased the path selectivity 3.5-fold. This research has provided new fundamental insights into the pyrolysis behavior of diarylmethanes, particularly under conditions of restricted mass transport, and the free-radical pathways that lead to PAH formation and cross-linking. Acknowledgment. This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. EF0102521
