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R e x i u e d for review October 23, 1985 Accepted July 15, 1986
Role of Nickel Crystallite in Nickel Zeolite Catalysts: n-Hexane Isomerization Laxml Naraln Shll and Subhash Bhatla' Department of Chemical Engineering, Indian Institute of Technology, Kanpur 2080 16, India
The isomerization of n-hexane has been studied in the presence of excess hydrogen at 643 K in a flow reactor over NiCa-Y (nickel calcium Y type faujasite) and NiH-M (nickel hydrogen mordenite) catalysts. Catalysts were prepared by ion exchange, and nickel concentration was varied from 1 to 5 wt % Nickel crystallite size distribution was affected with the degree of nickel exchange and the type of parent cation present in the zeolite. NiCa-Y containing 1.86 wt % nickel exhibited maximum isomerization activity and selectivity. NiH-M catalysts showed lower conversion and activity compared to NiCa-Y catalysts because of higher acidity and the presence of largssized nickel crystallites. The average nickel crystallite size was calculated by using electron microscopy and X-ray line-broadening techniques. The isomerization activity of nickel over the zeolites was probably due to the interaction of small nickel crystallites and the polarizing entities of the intrazeolitic matrix.
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Introduction Isomerization of n-paraffins is an industrially important octane-upgrading process. Various bifunctional catalysts containing noble metals like Pt and Pd over various supports such as silica-alumina, chlorinated alumina, and crystalline aluminosilicates (zeolites) have been widely studied and proven to possess high isomerization selectivity (Weisz, 1963). Thermodynamic consideration of isomerization reactions reveals that the lower the reaction temperature the greater the increase in octane number (Kouwenhoven, 1973; Bolton, 1976),and thus chlorinated alumina base catalysts, which can be operated at lower temperatures (383-453 K), are found to be the best bifunctional catalysts. However, these catalysts represent potential corrosion problems and are susceptible to
* Author to whom correspondence should be addressed. 0196-4321/86/1225-0530$01.50/0
deactivation, and their usage requires severe pretreatment of feedstocks. The noble-metal-loaded acid zeolites are free from this drawback and can be operated at relatively low temperatures (Kouwenhoven, 1971). Pt and Pd supported over faujasite type Y and mordenite in decationized alkaline earth and rare earth exchanged forms exhibited excellent isomerization activity and selectivity in a number of cases (Ftabo, 1961,1965;Lanewalla, 1967;Voorhies, 1968 Beecher, 1969; Minachev, 1969; Bolton, 1970; Hopper, 1972; Ribeiro, 1980). A number of extensive reviews on the isomerization of C5 or C6 paraffins using bifunctional catalysts are reported (Minachev, 1976; Chevalier, 1976; Chick, 1977). Most of these studies are centered on the modification and improvement of isomerization and the re-forming of catalysts containing noble metals over zeolites. A few studies using nickel zeolites have been reported in the literature (Koradia, 1980; Law, 1980; Rubenstein, 1986 American Chemical Society
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1968; Richardson, 1971; Braun, 1977). Minachev et al. (1966,1972)reported n-pentane isomerization activity and selectivity of nickel over hydrogen mordenite (NiH-M) containing 5 w t % nickel at a temperature of 550 K and a total preesure of 30 atm. Koradia and co-workers (1980) reported the relative activity for pentane isomerization, which declined in the order Ni > Pd >> Rh. Pentane isomerization kinetics over nickel-loaded faujasite is reported more recently by Law et al. (1980). Minachev et al. (1968)studied the isomerization activity of NiNa-Y and NiCa-Y catalysts and observed NiCa-Y containing 7.2 w t % nickel and having a Si02/A1203ratio of 4.2 had maximum n-hexane isomerization activity under the reaction conditions of 30 atm of pressure and a hydrogen/hexane ratio of 3:2. Richardson (1971) studied the bifunctional activity of NiCa-Y zeolites for hexane conversion and concluded that the overall activity of catalyst is due to the activity of the zeolite matrix and the activity of dispersed nickel crystallites rather than to a metal-support interaction. Braun et al. (1977) studied n-hexane isomerization over Ni, Pd, and Pt containing hydrogen mordenite and a calcium-exchanged form of faujasite Y type (Ca-Y) zeolitesand reported the effect of temperature on n-hexane conversion and isomer yield. The nickel catalysts in the high-dispersion state were prepared by ion-exchange technique (Anderson, 1975). The reduction of nickel ion to nickel metal and the characterization of nickel metal by X-ray line broadening, electron microscopy, magnetic measurements, and chemisorption have been the subject of many investigations. Rabo et al. (1966) and Herd et al. (1973) both reported reducibility of nickel ions in zeolites to nickel metal in separate studies. Bager (1977) observed the effect of other cations (e.g., Ca2+)on the reduction of nickel ions in zeolite. Dispersion of nickel metal can also be influenced by the choice of cation present. Briend-Faure et al. (1978a,b) observed that nickel ions located at hexagonal prisms (SI sites) were difficult to reduce as compared with those in the sodalite cages ( S i ) and in the supercages (SII,SI
