Energy & Fuels 2008, 22, 1121–1125
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Thermolysis of the Benzophenone Ketyl Cheryl D. Stevenson* and Yong Seol Kim Department of Chemistry, Illinois State UniVersity, Normal, Illinois 61790-4160 ReceiVed October 5, 2007. ReVised Manuscript ReceiVed December 4, 2007
Benzophenone, a material that is common in biomass and coal, resists pyrolysis up to 400 °C. However, when the compound is reduced with potassium metal in tetrahydrofuran (THF) followed by solvent removal, the C-H bonds, as well as the CO bond, are activated, and the solid K+C12H10CO•– · THF salt begins to gasify at less than 250 °C. The noncondensable (at liquid nitrogen temperature) gases consist of hydrogen and methane. Condensable pyrolysis products include the following: benzene, toluene, biphenyl, xylenes, diphenylmethane, and ethylbenzene. When the temperature is raised to 380 °C and held there for several hours, CO is also liberated. Isotopic labeling studies suggest that the CH4 comes from a carbene intermediate, and the CO does not originate from the carbonyl moiety of the ketyl.
Introduction Benzophenone is one of the most commonly used reagents in the organic chemistry laboratory, and it is routinely employed in its reduced form (the ketyl) to dry organic solvents.1 In fact, essentially every organic chemistry laboratory, where dry solvents are used, is equipped with solvent stills containing the benzophenone ketyl. The reactivity of the deep blue benzophenone ketyl (C12H10CO•–) toward protic material, coupled with its apparent thermal stability, accounts for the fact that refluxing organic solvents (ethers, benzene, etc.) in the presence of C12H10CO•– is a standard means of producing laboratory quantities of dry solvent. The importance of the thermal integrity of this ketyl constituted part of our motivation for studying its pyrolysis chemistry. Alkali metal reduction of benzophenone in tetrahydrofuran (THF) leads to the ketyl. Removal of the solvent under vacuum leaves the solid anion radical salt, which is coordinated to a molecule of THF (K+C12H10CO•– · THF).2 It was anticipated that heating this material, under high vacuum conditions, would allow observation of the decomposition products without the overpressurization problems encountered upon heating the fluid THF solutions. The thermal stability of neutral benzophenone is well-documented, and heating neutral 2-methylbenzophenone under flash vacuum conditions in the gas phase to temperatures as high as 800 °C allows complete recovery of the starting material,3a and no evidence of the anticipated decarbonylation is observed. As pointed out by a reviewer, condensed phase pyrolysis can take place at lower temperatures, and, in fact, such pyrolysis of benzophenone has yielded benzaldehyde, methylphenylketone, and phenylphenol.3b These are, however, not products that we observed during the pyrolysis of the corresponding anion radical. * Corresponding author. E-mail:
[email protected]. (1) Robertson, G. M. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, L. I., Eds.; Pergamon: New York, 1991; Chapter 2.6, Vol. 3. (2) Stevenson, C. D.; Hashim, R. T. J. Am. Chem. Soc. 1985, 107, 5794– 5795. (3) (a) Gu, T. Y.; Weber, P. W. J. Org. Chem. 1980, 45, 2541–2544. (b) Stanislav, S.; Jana, M.; Jana, V.; Jiri, M. Sbornik Vysoke Skoly ChemickoTechnologicke V Praze, D: Technologie PaliV 1984, D49, 335–52. (4) (a) Poutsma, M. L.; Dyer, C. W. J. Org. Chem. 1982, 47, 3367– 3377. (b) Mann, B. E.; Turner, M. L.; Quyoum, R.; Marsih, N.; Maitlis, P. M J. Am. Chem. Soc. 1999, 121, 6497–6498.
Due to its thermal integrity, molten benzophenone has even been used as a solvent for the pyrolysis of model compounds for coal at temperatures up to 400 °C.4a These pyrolytic studies in molten benzophenone (C12H10CO) normally lead to the production of Fischer–Tropsch gases (which include CO and H2),4b but the Fischer–Tropsch gases do not originate from the C12H10CO. Attempts to decarbonylate benzophenone have been unsuccessful, but decarbonylations (to yield free CO) of aldehydes and acid chlorides are routinely accomplished with the use of organometallic catalysts such as rhodium complexes.5 An interesting discovery by Ridge and Pan6 shows that the gas phase oxidative addition of benzophenone to the nickel tricarbonyl anion yields a complex, in which the benzophenone ostensibly undergoes decarbonylation to yield CO. The liberated CO, however, proved not to come from the benzophenone. Here, we report that the thermolysis of, arguably, the most important anion radical in organic chemistry (C12H10CO•–) also yields a Fischer–Tropsch gas mixture (methane, hydrogen, carbon monoxide) along with a of host of interesting organic products including benzene and biphenyl. Other than three reports from our laboratory,7 concerning the pyrolysis (gasification) of the naphthalene anion radical, benzoquinone anion radical, and anion radical-ammonia complexes, there have been no reports of anion radical pyrolysis. However, these previous reports indicate that combustible gases are much easier to liberate from a molecular system after the addition of an extra electron. The current interest in gasification processes to yield usable fuels motivated us in this study.8 In fact, commercial research and development into hydrogen production technologies including reforming, CO conversion, and gasification methods are under considerable study.8 Experimental Section Reaction bulbs, at ca. 10-4 torr, containing about 1 mmol of solid potassium benzophenone anion radical salt (K+C12H10(5) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; pp 768–769.. (6) Pan, Y. H.; Ridge, D. P. J. Am. Chem. Soc. 1992, 114, 2773–2780.
10.1021/ef700591d CCC: $40.75 2008 American Chemical Society Published on Web 01/16/2008
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Figure 1. Vacuum apparatus used for the analysis of the gases produced from the thermolysis of the benzophenone anion radical. The system was pumped down to 10-4 Torr prior to opening the Teflon stopcock to allow the gases into the vacuum line.
Figure 2. Plot of the number of moles of noncondensable gases evolved per mole of solid benzophenone anion radical salt vs time (hours: minutes). The ketyl was generated in THF.
CO•– · THF)9 were heated to up to 380 °C, and on occasion a little higher, with monitoring of the evolving gas pressure for 2–3 h; see Figure 1. After the pressure readings indicated that no more gases were being evolved, the Teflon stopcock was closed, and the reaction apparatus was disconnected from the vacuum system. However, in some cases, the reaction bulb was left at 380 °C for periods up to 1 week.
Results and Discussion As the heating jacket warmed up, noncondensable gases appeared in the manifold prior to the temperature reaching 250 °C, Figure 2. This was somewhat surprising given that neutral benzophenone can be heated to 800 °C without decomposition.3 (7) (a) Stevenson, C. D.; Espe, M. P.; Emanuelson, T. J. Org. Chem. 1985, 50, 4289–4291. (b) Stevenson, C. D.; Rice, C. V.; Garland, P. M.; Clark, B. K. J. Org. Chem. 1997, 62, 2193–2197. (c) Stevenson, C. D.; Heinle, L. J.; Reiter, R. C. J. Org. Chem. 2002, 67, 119. (8) (a) Kordesch, K. V.; Simader, G. R. Chem. ReV. 1995, 95, 191– 207. (b) For a recent example see: Ohman, M.; Pommer, L.; Nordin, A. Energy Fuels 2005, 19, 1742–1748.
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Figure 3. Matrix assisted laser desorption mass spectrum (MALDMS) confirming the formation of polymeric product found in the reaction bulb after thermolysis of the benzophenone anion radical salt. The parent peak at 410 m/e corresponds to five phenyl moieties and a carbonyl. Also, regular peaks appearing at 167 m/e intervals refer to the addition of phenyltropelium.
By the time the temperature reached 380 °C, 1 mol of volatile gas per mole of anion radical had evolved yielding a total pressure of 20-40 Torr. Further heating did not produce significantly more gas. Heating of the Cu-CuO furnace resulted in an immediate drop in the total pressure (CuO + H2 f Cu + H2O) to about 35% of its original value, indicating that (65 ( 3)% of the gas consisted of H2. It seems reasonable that the hydrogen originates from the formation of new C-C bonds from previous C-H bonds in the ketyl with concomitant polymerization. Consistent with this idea is the polymeric material found in the reaction bulb. Indeed, toluene extracts of the black sooty material left in the reaction bulb exhibit a laser desorption mass spectroscopy base peak at 410 charge/mass. This corresponds to five phenyl moieties with one carbonyl. After the 410 m/e peak, a prominent peak appears at intervals of 167 charge/mass units. These peaks represent successive losses of phenyltropylium (Figure 3). Lighter organic materials, including benzene, toluene, xylenes, biphenyl, diphenylmethane, and unreacted benzophenone, were found in the frozen U-tube connected to the reaction vessel (Utube no. 1). Benzene is, by far, the most abundant condensable fragmentation product. The water produced in the CuO furnace was frozen in the adjacent U-tube (Figure 1) as it was produced. IR spectra of the gases remaining after removal of the H2, via the Cu-CuO furnace, produced the IR spectrum shown in Figure 4. The IR spectral analysis of the noncondensable gases obtained during the first 2–3 h of the thermolysis reveals only methane (the CO2 level is not above ambient), Figure 4 upper. Given that only one or no hydrogens are associated with each carbon in the ketyl, the appearance of carbons with four hydrogens may seem surprising. It is not difficult to imagine reaction pathways where two hydrogens gather on a single carbon, but four hydrogens ending up on a single carbon most likely requires the involvement of carbenes. Scott and co-
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Figure 5. IR spectra, in the carbonyl range, of the evolved noncondensable gases from the pyrolysis of K+C12H1013CO•– · THF. The absorption of the mixture of 13CO and 12CO appears at 2000–2200 cm-1. (top) Two close vibration frequencies scrambled together. (middle) Authentic spectrum for 12CO. (bottom) Middle spectrum subtracted from the top spectrum. This procedure revealed the IR spectrum of pure 13CO and showed that the ratio of 13CO/12CO in the top spectrum is 1.6.
Figure 4. IR spectra of the noncondensable gases generated during the thermolysis of benzophenone ketyl. (upper) Spectrum obtained after 75 min of heating (see the gas evolution profile in Figure 2). (lower) Spectrum obtained after 1 week at 380 °C. Note the intense signal for CO. Scheme 1
workers considered,10a but subsequently dismissed,10b a mechanism for the high temperature automerization of 1,2-di-13Cbenzene that involves a carbene intermediate, Structure 1. On the other hand, carbenes were definitely formed during the pyrolysis of the naphthalene anion radical. This was evidenced by the formation of cylopropane when the pyrolysis was carried out in the presence of ethylene.7 The analogous intermediate for generating carbene in the benzophenone anion radical thermolysis is thus easily predicted, Structure 2 in Scheme 1.
The above argument, concerning methane production, suggests that the static pyrolysis of a mixture of K+C12H10CO•– · THF and perdeuteriated ketyl (K+C12D10CO•– · THF) should lead to isotopically scrambled methanes (CH4 + CH2D2 + CD4). This proved to be the case, but interestingly, CH3D and CHD3
were also generated. The latter two species probably originated from an exchange process. Indeed, deuterium scrambling under heterolytic hydrogenation conditions is common. When pure perdeuteriated benzophenone ketyl was thermolyzed as described and the evolved noncondensable gases exposed to the cyclopentene-Pt, perprotiated, monodeuteriated, and dideuteriated cyclopentane were formed. This means that the THF, as well as the benzophenone, served as a source of hydrogen. Involvement of the coordinating solvent in the H2 formation is not surprising. In fact, attempts to pyrolyize the benzene anion radical yielded gas phase products, all of which originated from the coordinating solvent (18-crown-6).11 The composition of the condensable and noncondensable materials does not change up to 1 h of heating. After that period, the evolution of the gas is very slow. Even so, when the temperature was maintained at 380 °C for several days, IR analysis revealed the surprising evolution of carbon monoxide. It seemed remotely possible that the CO came from the decarbonylation of benzophenone. This would be consistent with the formation of the biphenyl, but the biphenyl appeared during the first few minutes, while the CO did not appear until days later. The observation of CO initiated our curiosity concerning the decarbonylation problem. The pyrolysis procedure was repeated with K+C12H1013CO•– · THF, and the anion radical salt was thermolyzed at 380 °C for 3 days. Surprisingly, the IR revealed a mixture of 12CO and 13CO, Figure 5. If the carbon monoxide was generated via the decarbonylation of the ketyl, there should have been no 12CO produced. Further, when K+C12H1013CO•– · NH3 (produced via the reduction of labeled benzophenone with potassium metal in liquid ammonia),7c 12CO and 13CO was, again, produced from the subsequent pyrolysis. Carbon monoxide is slowly formed from the polymeric materials after days of heating, and it originates from chance meetings of heat activated oxygen and carbon sites. Mass spectral data of the polymeric products from the K+C12H1013CO•– · THF pyrolysis is nearly identical to that from (9) Prior to solvent removal, the THF solutions yielded the appropriate EPR spectra. See: Ayscough, P. B.; Wilson, R. J. Chem. Soc. 1961, 83, 1330–1333. (10) (a) Scott, L. T.; Roelofs, N. H.; Tsang, T. H. J. Am. Chem. Soc. 1987, 109, 5456–5461. (b) Merz, K. M., Jr.; Scott, L. T J. Chem. Soc. Chem. Commun. 1993, 412–414. (11) Stevenson, C. D.; Morgan, G. J. Org. Chem. 1998, 63, 7694.
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peaks. However, every parent peak from 154 (for C12H10) to 164 m/e (for C12D10) is present. This shows that the C-H bonds in the aromatic moieties are activated in the anion radicals and are broken during the heating. In contrast to the thermal stability of neutral benzophenone, the static heating of the benzophenone anion radical gives rise to noncondensable gases such as Fischer–Tropsch gases (H2/ CO) and CH4. The methane appears to originate from a carbene intermediate, which reacts with the hydrogen. The C-C and C-H bonds are strongly activated in the anion radical, as both are disrupted upon heating to just 250 °C. The hydrogen gas, which makes up 65–70% of the noncondensable gases, originates from polymerization processes. It is believed that the carbene intermediate generated from the thermolysis reacts with this H2 to produce the methane. Although the static heating of this ketyl produces a little over 1 mol of syngas per mole of ketyl, most of the carbon and hydrogen is “lost” to polymeric materials. Hence, this is not an efficient process for the production of syngas, but knowledge concerning syngas production from pure materials is important. There is considerable hope that, in the not so distant future, syngas (Fischer–Tropsch gases) will represent a precursor to a significant portion of our usable fuel and that much of the syngas will originate from the conversion of substrates over long time periods (as opposed to just initial products).
Figure 6. GC output obtained from the trapped materials in the U-tube after K+C12H1013CO•– pyrolysis.
Figure 7. Mass spectrum (y axis is m/z) of benzene fragmentation species generated via the pyrolysis of a 1:1 mixture of K+C12H10CO•- · THF and K+C12D10CO•- · THF. Note that the spectrum shows the parent peaks of all possible isotopic isomers of benzene.
the K+C12H1012CO•– · THF except that the major peaks are shifted to slightly higher mass/charge units. C6H513CH3, C6H513CO-C H , and meta- and para-CH -C H -13CH were trapped 6 5 3 6 4 3 in the U-tube during the pyrolysis along with unisotopically substituted benzene, biphenyl, toluene, and ethylbenzene, Figure 6. It is clear that many of the 13C carbonyl carbon atoms end up in the polymeric materials, and the remainder of the 13Cs ends up as methyl or methylene units connected to aromatic moieties. Just as the methane molecules, produced via the static pyrolysis of the C12H1012CO + C12D1012CO anion radical mixture, are isotopically scrambled, so are the benzenes (Figure 7) and other small organic compounds found in U-tube no. 1. In the case of biphenyl, the mass spectral peaks at 158 (from C12H6D4) and 159 m/e (from C12H5D5) are the most abundant
Conclusions The heating the solid potassium anion radical salt of benzophenone to just 250 °C results in the liberation of two noncondensable (liquid nitrogen) gases: methane and hydrogen. Carbon monoxide also results from the pyrolysis but only after extended heating at 380 °C, and it does not come from the decarbonylation of the ketyl but is emitted, much later in the pyrolysis, from the previously formed polymers. In fact, some of the carbon in the CO originates from the tetrahydrofuran that is coordinated in the ketyl salt. Fast and slow pyrolysis of biomass also produces very different products.12 Along with syngas and polymer, several small organic systems are formed including (with relative amounts indicated) the following: benzene (200), toluene (100), ethyl benzene (10), xylenes (12), biphenyl (8), and diphenylmethane (3). The carbon framework of the aromatic moieties in these systems originate intact from the phenyl groups in the ketyl, but the methyl and methylene groups are derived from isolated fragments from electron activated carbon-carbon and carbon-hydrogen bonds. The organic pyrolysis products do not form prior to the first sign of hydrogen and methane evolution (see Figure 2). After that, their nature does not change except that CO eventually evolves. The benzophenone pyrolysis described here is simply the beginning of a plethora of possible ketyl pyrolysis systems (including the substituted benzosemiquinones) which could yield millions of interesting products and possibly fuel stocks. Keep in mind that a variety of benzophenones are found in plant extracts,13 and the majority (12) Mohan, D.; Pittman, C. U., Jr; Steele, P. H. Energy Fuels 2006, 20, 848–889.
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of the processes for the conversion of biomass to fuels begin with pyrolysis.12,14
Acknowledgment. We thank the National Science Foundation for support of this work.
(13) For example see: (a) Tanaka, N.; Takaishi, Y.; Shikishima, Y.; Nakanishi, Y.; Bastow, K.; Lee, K.-H.; Honda, G.; Ito, M.; Takeda, Y.; Kodzhimatov, O. K.; Ashurmetov, O. J. Nat. Prod. 2004, 67, 1870–1875. (b) Bernardi, A. P. M.; Ferraz, A. B. F.; Albring, D. V.; Bordignon, S. A. L.; Schripsema, J.; Bridi, R.; Dutra-Filho, C. S.; Henriques, A. T.; von Poser, G. L. J. Nat. Prod. 2005, 68, 784–786.
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(14) Buchanan, A. C., III; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895–9903.
