Heterogeneous Hydrocarbon Oxidation - American Chemical Society

Chapter 31

Selective Photooxidation of Small Hydrocarbons by O with Visible Light in Zeolites Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch031

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Hai Sun, Fritz Blatter, and Heinz Frei

Structural Biology Division, Calvin Laboratory, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 Small alkenes, alkanes, or alkyl substituted benzenes loaded with O gas into cation-exchanged zeolite Y react upon irradiation with visible light to yield corresponding carbonyl products at very high selectivity. Alkyl (alkenyl) hydroperoxides are formed as intermediates, in the case of isobutane as the final product. This was observed when monitoring the reactions in situ by Fourier-transform infrared spectroscopy. Experiments were typically run at room temperature, in some cases at zeolite temperatures as low as -100°C to elucidate mechanisms. Chemistry was induced by light from a tungsten lamp. Frequently, studies were also conducted with the emission of an Ar ion or cw dye laser in order to determine the visible wavelengths responsible for the reaction. Diffuse reflectance spectra revealed a visible absorption tail which originates from a hydrocarbon•O collision complex. It is attributed to the hydrocarbon•O charge-transfer absorption whose onset is shifted from the UV into the visible region by the very high electrostatic field of the zeolite (shifts of the order of 1.5 to 3 eV). Quantum efficiencies are in the region 10-30%, and selectivities remain high even upon conversion of more than 50% of the hydrocarbon loaded into the zeolite matrix. Many of the reactions studied are of commercial importance: toluene to benzaldehyde, propylene to acrolein or propylene oxide, isobutane to t-butyl hydroperoxide, cyclohexane to cyclohexanone, ethane to acetaldehyde. 2

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Oxidation by 0 is the single most important process for the conversion of abundant hydrocarbons to oxygenated derivatives such as organic building blocks for die manufacture of plastics and synthetic fibers, and industrial intermediates for the synthesis of fine chemicals (2-6). In large-scale synthesis, the use of molecular oxygen as oxidant is dictated primarily by economic factors. Yet, autoxidation of small hydrocarbons is inherently unselective, whether conducted in the gas or liquid phase, or whether catalyzed 2

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Corresponding author 0097-6156/96/0638-0409$15.00/0 © 1996 American Chemical Society In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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HETEROGENEOUS HYDROCARBON OXIDATION

by transition metals or not (1-4). One reason is that the desired products such as alcohols or carbonyls are more easily oxidized by 0 than the parent hydrocarbon. Overoxidation can only be minimized by keeping conversions low (at a few percent). Another factor is diversion of the radical chain reaction leading to the primary product (alkyl or alkenyl hydroperoxide) by termination steps which result in the formation of oxy radicals. This is especially a problem in the case of olefin oxidations. The highly reactive oxy radicals can undergo several competing reactions that lead to a multitude of products. Hence, oxidations by O2 exhibit most often little chemo- or regioselectivity. A major challenge in the field of hydrocarbon + O2 chemistry is, therefore, to find reaction paths that afford the primary product with high selectivity at high conversion. We have developed a method that affords partial oxidation of small alkenes, alkanes, and substituted aromatics by O2 at very high selectivity. The approach is based on photoexcitation of hydrocarbon«02 pairs in a large-pore zeolite (faujasite). The 3-dimensional network of molecular-size cages of the latter offer a natural environment for the formation of hydrocarbon»02 collisional pairs at high concentration (Fig. 1). The key to selectivity is a low-energy reaction path that is opened up by a very strong stabilization of the hydrocarbon«02 excited charge-transfer state by the high electrostatic field of the zeolite cage. The stabilization causes a red-shift of the charge-transfer absorption from the UV into the visible region. Access to this low-energy excited state, coupled with the positional constraint imposed by the zeolite nanocage furnishes a new,tightlycontrolled reaction path for small hydrocarbon + O2 systems.

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch031

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Experimental Self-supporting zeolite wafers of 5-10 mg (1.2 cm diameter) were placed in a miniature infrared or UV-Vis vacuum cell (7-14). For infrared measurements, the cell was mounted inside a variable temperature vacuum system (Oxford Model DN1714 or DN1724). The zeolite was dehydrated by heating the cell to 200°C for 12-15 h while evacuating with a turbomolecular pump. The reactants were subsequently loaded from the gas phase into the zeolite. The loading level was adjusted by the gas pressure and the zeolite temperature. Both laboratory-synthesized and commercial zeolite NaY (Aldrich) were used. Alkaline-earth exchanged zeolite Y (BaY, CaY) was prepared by repeated ionexchange of NaY at 90°C in 0.5 M solution of the corresponding chloride salt (15). The degree of exchange (ICP) was typically 95% or better. Photochemistry was monitored in situ by Fourier-transform infrared spectroscopy using a Bruker Model IFS 113 or IR 44 instrument. Zeolite Y is transparent in the infrared except for the region 1200-920 cm* and below 800 cm . For photolysis, a prism-tuned cw Ar ion laser (Coherent Model Innova 90) or the emission of a tungsten lamp was used (equipped with a UV cut-off filter). The light beam was expanded to cover the entire zeolite pellet. Experiments were conducted at temperatures between 100 K and room temperature. UV-Vis spectra were recorded by the diffuse reflectance method using an integrating sphere (Shimadzu Model 2100 equipped with a Model ISR260 sphere). 1

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In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

31. SUN ET AL.

Selective Photooxidation of Small Hydrocarbons by 0

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Results Our work has focused on the partial oxidation of small olefins, aromatics, and alkanes. A partial overview of the reactions studied thus far is given in Scheme 1. We will present, in turn, one or two examples for each class of hydrocarbons. Toluene to Benzaldehyde Conversion. Upon loading of toluene (5 Torr) and 0 (760 Torr) into a BaY or CaY matrix, a continuous absorption with a tail extending into the visible region is observed (Fig. 2) (9). Alkali or alkaline earth exchanged zeolite Y has no optical absorption in this spectral range (16). The band appears only when the hydrocarbon and O2 are simultaneously present in the zeolite. The absorption can be reversibly removed by pumping off the oxygen gas, which constitutes direct evidence that it originates from the hydrocarbon«02 complex. Use of the diffuse reflectance method is required because of the strong scattering behavior of the zeolite pellet (type Y crystallites have a size of about one micron). The onset of the lowest energy absorption of toluene»02 contact complexes in the oxygen-saturated liquid lies in the UV region around 370 nm (17). It originates from excitation of the hydrocarbon«02 charge-transfer state. By contrast, the diffuse reflectance spectra indicate onset of the toluene»C>2 absorption in BaY and CaY at 500 and 600 nm, respectively. Speculating that the very large red shift of the hydrocarbon»02 absorption is caused by high electrostatic fields inside the cage (18), we have determined experimentally the magnitude of the field in the exchanged zeolite Y samples used in our work (19). The method consists of measuring the induced infrared fundamental absorption of O2 or N2, or the symmetric stretch vibration of C H loaded into the matrix. These infrared-forbidden transitions become active in the presence of an electrostatic field, and the band intensity is proportional to the square of the field (20). The method has previously been used by Cohen de Lara in the study of zeolite A (21). Fig. 3 shows the induced infrared absorption of O2 at 1550 cm" upon loading of the gas into a NaY or BaY pellet. Pressures were adjusted so as to obtain 1.5 molecules O2 per supercage on average. These intensity measurements revealed very high electrostatic fields. For example, in NaY at -50°C the molecules experience a field of 0.3 V A - , or 0.9 V A in BaY (19). When shining green or blue light on zeolite BaY loaded with toluene and 02, benzaldehyde and H2O grew in under concurrent depletion of toluene. Fig. 4 shows an infrared difference spectrum after photolysis with 488 nm light from an Ar ion laser at room temperature (400 mW cm , 3 h). The same result was obtained when using the visible emission of a tungsten lamp. Yields were independent of the light source and simply reflect the number of light quanta absorbed by the reactants. The use of monochromatic laser light had the advantage of furnishing the wavelength-dependence of product yields. The positive bands agree completely with the spectrum of an authentic sample of benzaldehyde in BaY. Aside from a shoulder at 1640 cm" which is due to H2O coproduct, no other infrared absorption grew in even upon prolonged photolysis. This signals completely selective oxidation of toluene to benzaldehyde by 62. Observation of some thermal growth of benzaldehyde after toluene + O2


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Figure 1. Photo-induced reaction of hydrocarbon«02 collisional pairs inside a zeolite Y supercage.

)

300

1

400

1

500

1

600

f

700

W a v e l e n g t h (nm) Figure 2. Toluene«C>2 diffuse reflectance spectra in zeolite BaY and CaY.


31. SUN ET AL.


CH / H

C

CH

H

3

C

"

+

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Selective Photooxidation of Small Hydrocarbons by CL

0

CH

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NaY, BaY*

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2

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2

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X98% at ambient 2


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1.6

C H - C H =CH /0 /BaY 3

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-100°C 1.2

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1

0.8

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temperature, >99.8% at -100°C) (12). The selectivity is undiminished even upon consumption of 20% of the propylene loaded into the zeolite. Note that since the strong scattering of the visible photolysis light restricts penetration to the front section of the zeolite, the conversion of propylene in the irradiated section of the pellet is substantially larger. On the basis of diffuse reflectance spectroscopy of the visible propylene«0 charge-transfer absorption and the measured infrared product growth, a rather high reaction quantum yield of 20% was estimated (12).


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Selective Oxidation of Isobutane and Ethane. Particularly interesting is the finding that even small alkanes can be partially oxidized by oxygen in cationexchanged zeolite Y under visible light. The corresponding alkyl hydroperoxides and carbonyl compounds are produced with very high selectivity at high conversion of the hydrocarbon. When loading isobutane (1.6 Torr) and 0 (900 Torr) into zeolite BaY and irradiating with visible light, f-butyl hydroperoxide grew in at 98% selectivity (13). As in all hydrocarbon oxidations studied thus far, the linear dependence of the yield with photolysis light intensity indicated a single photon process. Fig. 6 shows the infrared difference spectrum upon photochemical conversion of more than half (57%) of the isobutane loaded into the matrix. All positive bands originate from f-butyl hydroperoxide growth except for the small absorption at 1686 cm" , which indicates the formation of 2% acetone. (CH ) C=0 and CH OH (which was observed when accelerating the f-butyl hydroperoxide rearrangement at 50°C (13)) are established thermal products of (CH ) COOH (23). Hence, the trace amount of acetone and methanol in the zeolite is secondary thermal products of the hydroperoxide. The absorption of the isobutane»0 complex in BaY was observed by diffuse reflectance spectroscopy (13). While the band was weak because only a fraction of the reactant pairs are probed by visible light in the highly scattering pellet, it was sufficiently strong to allow a quantum efficiency estimate of 15% (blue and green photons). An interesting aspect of the synthesis of f-butyl hydroperoxide in the zeolite is the opportunity of in situ use for olefin epoxidation. This allows us to avoid accumulation of the important but hazardous oxidizer in bulk quantities. After photochemical synthesis of the hydroperoxide in BaY, the remaining oxygen and isobutane were pumped off andte/w-2-butenewas loaded into the zeolite. Growth of £rans-2,3-epoxybutane and f-butanol was observed in the dark at room temperature only a few minutes after adsorption of the olefin. Infrared spectroscopic analysis showed that no cis-epoxide was formed upon conversion of as much as one third of the butene. Similarly, complete stereospecificity was obtained when conducting the epoxidation reaction with cis-2-butene (Scheme 3) (13). Ethane was found to react with 0 in zeolite CaY under irradiation with blue light when several hundred Torr C H£ and one atm of 0 were loaded into the dehydrated pellet (24). Fig. 7 shows the infrared product growth after 3 hours of photolysis at room temperature with the 488 nm line of an Ar ion laser (500 mW cm" ). Acetaldehyde (1707, 1419, 1357 cm" ) and H 0 (3400, 1650 cm ) are the only observed products. Not even a trace of carbon dioxide was produced. The latter could have easily been detected by its very strong infrared 2

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3

2

3

3

2

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419


H

3

N

c=c

> =
2 charge-transfer state by the electrostatic field of the zeolite cage. The charge-transfer state can be accessed by low-energy visible instead of the more energetic UV photons, with the result that primary products emerge with minimal excess energy. This, coupled with the confinement imposed by the zeolite cage, prevents random radical coupling reactions and homolytic fragmentation of the primary products. Specifically, no homolytic OO bond rupture of the hydroperoxide intermediates occurs which would lead to the formation of OH and alkoxy radicals. These are the two species that destroy the selectivity upon thermal autoxidation of small hydrocarbons (1). Moreover, the use of visible light insures that no photodissociation or further oxidation of hydroperoxide or carbonyl products by 0 takes place. The main reason in the case of alkene and toluene systems is that the ionization potential (IP) increases upon partial oxidation of the hydrocarbon. For example, IP of propylene is 9.73 eV, compared to 10.10 eV for acrolein or 10.22 eV for propylene oxide; IP of toluene is 8.82 eV, compared to 9.52 eV for benzaldehyde (37). The result is that the onset of the chargetransfer absorption of the product«02 complexes lies at shorter wavelengths than for the alkene»02 or toluene»02 systems, which renders secondary photooxidation of the primary products unlikely. Hence, the method of hydrocarbon oxidation by O2 with visible light is inherently stable against further reaction with oxygen. Overoxidation is a major obstacle to selectivity in the case of thermal catalytic autoxidation (1-6). Only for light alkanes like propane and ethane are die ionization potentials of the corresponding carbonyl products (acetone, acetaldehyde) lower than that of the parent hydrocarbon (37). However, we have not observed overoxidation under visible light irradiation in these cases thus far (24). 2

Conclusions Very high selectivities have been achieved upon oxidation of olefins, alkanes, and alkyl substituted benzenes by O2 in zeolites under visible light. For example, toluene to benzaldehyde oxidation by O2 without overoxidation or side reactions is unprecedented. Cobalt(III) catalyzed autoxidation of toluene in solution used currently in an industrial process for benzaldehyde synthesis lacks this selectivity, mainly because of continued oxidation of the aldehyde to benzoic acid (1,2). Yet, toluene to benzaldehyde conversion by oxygen is an important process. The aldehyde serves as an intermediate in the manufacture of agrochemicals, fragrances, and other specialty chemicals (38). Among the olefin photooxidations demonstrated thus far, the propylene to acrolein and propylene oxide conversion is perhaps the most interesting one from an applications standpoint. While there is a commercially important method for


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Selective Photooxidation of Small Hydrocarbons by 0>

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propylene to acrolein oxidation by 0 using a Bi molybdate catalyst (39), no selective transformation of propylene to propylene oxide by oxygen through thermal catalysis has been reported thus far. Selective propylene epoxidation schemes still use H 0 2 or organic hydroperoxides as O donors (1). A recent breakthrough in the area of epoxidation with aqueous H2O2 are Ti silicalite catalysts (40). The products of all alkane photooxidations studied thus far are of commercial importance. For example, f-butyl hydroperoxide is a widely used oxidizing agent, even in large-scale processes like propylene epoxidation (1,22,41). Cyclohexanone is an intermediate in the manufacture of nylons (2) as well as a variety of fine chemicals (38). Acetaldehyde and acetone are both bulk chemicals. Yet, existing thermal autoxidation processes are very unselective. Practical selectivities (« 70%) of desired hydroperoxide (t-butyl hydroperoxide) or carbonyl compounds (cyclohexanone, acetone, acetaldehyde) can only be obtained at low conversion (below 10%) (1,2). Even then, carbonyls are often co-produced along with the corresponding alcohols. By contrast, the new method presented here demonstrates that these alkane oxidations to the corresponding hydroperoxides or carbonyls can be accomplished at very high selectivity even at high conversion of the reactants. In the case of the isobutane oxidation an additional attractive feature is that the hydroperoxide product can be used in situ for subsequent epoxidation reactions. Especially intriguing is the completely selective ethane to acetaldehyde conversion. Thermal catalytic methods produce substantial amounts of C 0 without exception. By contrast, none was observed in our experiments. It may open up ethane, a constituent of natural gas, as a new feedstock for the aldehyde (instead of the currently used ethylene). More generally, the photochemical oxidations by O2 in zeolites constitute a new and highly selective method for activation of alkane C-H bonds (3). Successful upscaling of our experiments with micromolar quantities reported here requires a reduction of die scattering of photolysis light, and choice of operating conditions that allow continuous desorption of the products from the zeolite host. The preferred solution for the light scattering problem would be translucent zeolite membranes. Such membranes have been reported very recently for pentasil-type zeolites (ZSM-5) (42). Although release of small oxygenated hydrocarbons from zeolite Y by polar organic solvents is routine (43), a solvent-free method would be preferable. Use of a carrier gas and modestly elevated temperatures may be sufficient to effect desorption of the polar products at acceptable rates. 2


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Acknowledgment This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U. S. Department of Energy under Contract No. DE-AC03-76SF00098. Literature Cited [1]

Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981.


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Parshall, G. W.; Ittel, S. Homogeneous Catalysis; 2nd ed.; Wiley: New York, 1992. Activation and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley: New York, 1989. Sheldon, R. Α.; Dakka, J. Catal. Today 1994, 19, 215. Lyons, J. E.; Parshall, G. W. Catal. Today 1994, 22, 313. Dartt, C. B.; Davis, M. E. Ind. Eng. Chem. Res. 1994, 33, 2887. Blatter, F.; Frei, H. J. Am. Chem. Soc. 1993, 115, 7501. Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 1812. Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 7951. Blatter, F.; Moreau, F.; Frei, H. J. Phys. Chem. 1994, 98, 13403. Blatter, F.; Sun, H.; Frei, H. U. S. Patent filed, 1995. Blatter, F., Sun, H., and Frei, H. Catal. Lett. 1995, 35, 1. Blatter, F.; Sun, H.; Frei, H. Angew. Chem. Int. Ed. Engl. (Chemistry), in press. Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc., submitted. Bellat, J. P.; Simonot-Grange, M. H.; Jullian, S. Zeolites 1995, 15, 124. Engel, S.; Kynast, U.; Unger, K. K.; Schüth, F. In Zeolites and Related Microporous Materials, Studies in Surface Science and Catalysis; Weitkamp, J.; Karge, H. G.; Pfeifer, H.; Hölderich, W., Eds.; Elsevier: Amsterdam, 1994, Vol. 84; p. 477. Chien, J. C. W. J. Phys. Chem. 1965, 69, 4317. Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; Wiley: New York, 1974. Blatter, F.; Frei, H. J. Phys. Chem., to be submitted. Condon, E. U. Phys. Rev. 1932, 41, 759. Barrachin, B.; Cohen de Lara, E. J. Chem. Soc., Farad. Trans. 2 1986, 82, 1953. Sheldon, R. A. In The Chemistry of Functional Groups - Peroxides; Patai, S., Ed.; Wiley: New York, 1983; Ch. 6. Hiatt, R. In: Organic Peroxides; Swern, D., Ed.; Wiley: New York, 1971, Vol. 2; p. 81. Sun, H.; Blatter, F.; Frei, H., to be submitted. Murov, S.L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993. Ramamurthy, V. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; Ch. 10. Mulliken, R. S.; Pearson, W. B. Molecular Complexes; Wiley: New York, 1969. Davis, K. M . C. In Molecular Association; Foster, R., Ed.; Academic Press: New York, 1975, Vol. 1; p. 151. Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis; Van Bekkum, H.; Flanigen, E. M.; Jansen, J. C.; Eds.; Elsevier: Amsterdam ,1991, Vol. 58. Spackman, Μ. Α.; Weber, H. P. J. Phys. Chem. 1988, 92, 794. Liu, S. B.; Fung, Β. M.; Yang, T. C.; Hong, E. C.; Chang, C. T.; Shih, P. C.; Tong, F. H.; Chen, T. L. J. Phys. Chem. 1994, 98, 4393. Coope, J. A. R.; Gardner, C. L.; McDowell, C. Α.; Pelman, A. I. Mol. Phys. 1971, 21, 1043. Sugihara, Α.; Shimokoshi, K.; Yasimori, I. J. Phys. Chem. 1977, 81, 669.


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Iwasaki, M.; Toriyama, K.; Nunome, K. J. Am. Chem. Soc. 1981, 103, 3591. Sauer, M. C.; Werst, D. W.; Jonah, C. D.; Trifunac, A. D. Radiat. Phys. Chem. 1991, 37, 461. Hammerich, O.; Parker, V. D. Adv. Phys. Org. Chem. 1984, 20, 55. CRC Handbook of Chemistry and Physics; 53rd ed.; Weast, R. C., Ed.; The Chemical Rubber Co.: Cleveland, 1972; p. E-62. Szmant, H. H. Organic Building Blocks of the Chemical Industry; Wiley: New York, 1989. Grasselli, R. K.; Centi, G.; Trifiro, F. Appl. Cat. 1990, 57, 149. Notari, B. Stud. Surf. Sci. Cat. 1987, 37, 413. Indictor, N.; Brill, W. F. J. Org. Chem. 1965, 30, 2074. Kiyozumi, Y.; Maeda, K.; Mizukami, F. Workshop on Cluster Science; National Institute for Advanced Interdisciplinary Research: Tsukuba, Japan, 1995. Thibault-Starzyk, F.; Parton, R. F.; Jacobs, P. A. In Zeolites and Related Microporous Materials, Studies in Surface Science and Catalysis; Weitkamp, J.; Karge, H. G.; Pfeifer, H.; Hölderich, W., Eds.; Elsevier: Amsterdam, 1994, Vol. 84; p. 1419.


Heterogeneous Hydrocarbon Oxidation - American Chemical Society

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