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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 27-32
Registry No. 02,7782-44-7;methacrolein, 78-85-3;methyl ethyl ketone, 78-93-3. Literature Cited -6
1,2
1.4 1.6 103.~-lK - ~
1,8
Figure 22. Arrhenius plots of the rate of methane oxidation over
L~.*Sro.zFeyCol-y03-6. Fe is less active. Accordingly, the results obtained here suggest a possibility of finding more excellent catalysts by selecting a proper combination of B site cations. Finally, we would briefly refer to the surface structure of perovskite-type oxides. According to our XPS measurements, the surface of Sr-substituted LaCoO3 is subject to compositional changes during high-temperature calcination. These changes can lead to a deactivation of catalytic properties of the compounds.
Eguchi, K.; Aso, 1.; Yamazoe, N.; Seiyama, T. Chem. Lett. 1878, 1345. Eguchi, K.; Toyozawa, Y.; Yamazoe, N.; Seiyama, T. J . Catal. 1883, 83, 32. Hall, W. K.; Lalacono, M. "Proceedings, 6th International Congress on Catalysis", London, 1977; p 246. Iwamoto, M.; Yoda, Y.; Egashira, M.; Seiyama, T. J . phys. Chem. 1876, 80, 1989. Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. Bull. Chem. SOC.Jpn. 1878, 5 1 , 2765. Katsuki, S.;Inokuchi, M. J . Phys. SOC.Jpn. 1882, 5 1 , 3652. Lunsford, J. H. Catal. Rev. 1873. 8 , 135. Lyhamn, L.; Cyvin, S. J.; Cyvin, B. N.; Brunvoil, J. Z . Naturforsch. 1876, A31, 1589. Meadowcroft, D. B. Nature (London) 1870, 226, 847. Ohara, T. Shokubai(Catalyst) 1877, 19, 157. Seiyama, T.; Nita, K.; Maehara, T.; Yamazoe, N.; Takita, Y. J . Catal. 1877, 49, 164. Takita, Y.; Nita, K.; Maehara, T.; Yamazoe, N.; Seiyama, T. J . Catal. 1877, 50, 364. Takita, Y.; Nita, K.; Yamazoe, N.; Seiyama, T. Engineering Sciences Report, Kyushu University 1979; Vol. I , p 1. Tsigdinos, 0. A. Top. Curr. Chem. 1878, 76, 1. Voorhoeve, R. J. H.; Remeika, J. P.; Freeland, P. E.; Matthias, B. T. Science 1872, 177, 353. Voorhoeve, R. J. H. "Advanced Materials in Catalysis", Academic Press: New York, 1977; p 129. Yamazoe, N.; Hidaka, S.;Arai, H.; Seiyama, T. Oxld. Commun. 1883, 4 , 287. Yamazoe, N.; Noguchl, M.; Seiyama, T. Nippon Kagaku Kalshi 1883, 1983, 470.
Receioed for review April 26, 1984 Accepted September 4, 1984
Nature of Active Sites and Coking Reactions in a Pillared Clay Mineral Marlo L. Occelll' and Joseph E. Lester Gulf Research d Development Company, Pittsburgh, Pennsylvania 15230
FTIR studies of chemisorbed pyridine confirm that (Na or Ca) bentonites pillared with alumina clusters contain both Lewis and Brernsted acid sites. At 400 OC, acidity is mainly of the Lewis type and acid site strength is comparable to that observed in HY. Formation of catalytic coke from 1-hexene and toluene on these materials is significantly slower than that measured for zeolites. However, coke make from gas oil conversion at microactivity test (MAT) conditions is at least a factor of 2 greater than on HY zeolite. The pillared clay high coke make during gas oil cracking is attributed to its open, two-dimensional microporous structure and high Lewis acidity, which promotes polycondensation reactions and coke formation.
Introduction
Montmorillonitespillared with N+(CH3)*and N+(CzH6)4 cations were first reported by Barrer and MacLeod (1955). While the molecular sieving properties of these materials were known (Barrer, 1978), their catalytic potential was ignored, probably because of their limited thermal stability. In the late 1970's, synthesis of heat-stable (500 to 600 "C), high-surface-area bentonites pillared with large inorganic proppants was reported by several workers (Brindley and Sempels, 1977; Lahav et al., 1978; Yamanaka and Brindley, 1979; Vaughan et al., 1979; Shabtai et al., 1980). Literature recognition of the catalytic properties of clays pillared kith alumina began appearing in 1979-1980 (Vaughan et al., 1979; Vaughan and Lussier, 1980). Their high cracking activity is believed to be associated with the'nature of the 0196-432118511224-0027$01.50/0
acidity introduced by the aluminum oligomers used to expand the clay structure. Similar catalysts were synthesized by Shabtai (1980) and used in studying the rate of cumene and isopropyl naphthalene dealkylation. Of particular interest to the petroleum industry is their selectivity to light cycle gas oil (LCGO) production when cracking gas oil fractions (Lussier et al., 1980; Occelli, 1983). In addition to cracking, pillared clays are active catalysts for methanol conversion, for alkylating toluene with ethylene (Occelli et al., 1984), and may represent a new class of versatile heterogeneous catalysts. Thermal stability of pillared clays in 95% steam-5% nitrogen mixtures is limited to temperatures below 600 "C. They also deactivate readily because of coke formation in most hydrocarbon conversion reactions. The high coke 0 1985 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
make has been attributed to the presence of iron (3-570 Fe203)impurities (Lussier et al., 1980). At cracking temperatures (under vacuum), Na-bentonite pillared with hydroxyaluminum oligomers has acidity of the Lewis type (Occelli and Tindwa, 1983). These acid sites catalyze coke formation (Hall et al., 1964). Carbonaceous deposits on fluid cracking catalysts (FCC) have been attributed to the metals-catalyzed cracking of reaction products as well as to cracking reactions at the zeolite acid centers. In addition to this catalytic coke formation, carbon deposits result from the pyrolytic distillation of hydrocarbons in the cracking zone (Conradson Carbon) and from hydrocarbons' sorption and retention in the catalyst pores (Cimbalo et al., 1972). This paper reports the results of our investigation of the role the acid sites have on the pillared Ca-bentonite coke make while cracking gas oil fractions. Experimental Section A calcium-rich bentonite (containing 95% montmorillonite and 4-570 Fe203impurities) was ion-exchanged with an aluminum chlorohydroxide (ACH) solution (Reheis Chemical Co.'s Chlorhydrol) as described elsewhere (Vaughan et al., 1979) to form ACH-bentonite. Clay properties have been discussed by Occelli (1983). Catalyst surface area reduction was accomplished by passing a mixture of 95% steam and 5% nitrogen for 10 h at 565-730 "C. The Barrett-Joyner-Halenda (1950) method was used to calculate the pillared-clay pore size distribution from nitrogen adsorption isotherms. Infrared spectra of sorbed pyridine were obtained with a Nicolet 7199 FTIR spectrometer a t 1 cm-l resolution. Samples were pressed at 350 atm into self-suportingwafers -0.25 cm in diameter with density of -10 mg/cm2. The wafers were then mounted in an optical cell, evacuated at 5 0.01 torr, and degassed by heating for 3 h at 400 "C; after cooling to room temperature, pyridine was sorbed. The sample was then evacuated at progressively higher temperatures (100 to 400 O C ) and the spectra were recorded. Infrared studies of coke formation were performed according to the procedure described by Blackmond et al. (1982a). Pillared clays samples were mounted in a heatable, flow-through cell in the spectrometer, permitting the surface species being formed to be followed in situ at temperature. (One advantage of FTIR is that the sample emission is unmodulated and therefore should have minimal effect on the absorbance peaks.) The samples were calcined in air at 350 "C (the maximum temperature for which the cell was designed), purged with helium at 60 cm3/min for 30 min (-30 cell volumes) at 350 "C, and cooled to the initial reaction temperature. Hydrocarbonsaturated gas (either H2 or He) flow was started and spectra were obtained at a sequence of increasing temperatures. Typically the sample was maintained at a given temperature for 30 min or until no change was apparent in sequential IR spectra. A t the end of the run, the hydrocarbon flow was interrupted and a spectrum was obtained at temperature with an He or H2 purge. Although this procedure utilizes a relatively low calcination temperature, it provided a consistent comparison among zeolites and presumably should do likewise for clays. Because of the large background absorbance of the clay itself, it was necessary to subtract the spectrum of the calcined wafer from those of the wafer exposed to the reactant gas in order to obtain spectra where differences in the additional peaks could be seen. In the 1300 to 2000-cm-l region, background absorbance of the clay samples ranged from 1 to 1.7. Since the wafers were not of equal optical density over their surface, small changes in
Table I. Surface Properties of Bentonite and HY (Linde LZY-82) after Drying at 100 "C CaACHbentonite bentonite HY BET area, m2/g 46.6 270 508 average pore radius, 35.7 13.4 12.7 0.18 0.32 pore volume, cm3/g 0.08 pore volume distribution, area 70 R
