Diffusion of cumene in H‐ZSM‐8 and modified H‐ZSM‐8 Zeolites

Abstract

ZSM-8 zeolite is a medium-pore (pore size = 0.6 nm) high-silica zeolite, belonging to a pentad family of zeolites, introduced by Mobil Oil Corporation in 1971. The X-ray diffraction spectrum of ZSM-8 is somewhat similar to that of ZSM-5, which is a very widely investigated zeolite, except for a splitting in the highest peak in the region 28 = 22-25O observed for ZSM-8 (Lechert, 1984). Earlier studies (Akolekar and Choudhary, 1987) have indicated that the influence of degree of Hi -exchange and calcination temperature on strong acid sites and catalytic activity and paraselectivity (in hydrocarbon conversion and alcohol-to-aromatics conversion reactions) of ZSM-8 are quite similar to those observed for ZSM-5. For ZSM-8, however, the number of strong sites per framework A1 (Choudhary and Akolekar, 1989) and the rate of catalyst deactivation due to deposition of large hydrocarbon molecules in zeolite channels (Akolekar and Choudhary, 1987;Akolekar, 1987) are found to be higher. Also the diffusion of cyclohexane (Chon et al., 1985) in H-ZSM-8 is found to be faster.Extensive studies have been reported on ZSM-5-type zeolites for the sorption/diffusion properties (Ruthven, 1984; Choudhary and Srinivasan, 1986a, b) and also on the zeolite modified with MgO, P,O,, and B,O, (Kaeding and Butter, 1980;Kaeding et al., 1984;Mamman, 1988). For ZSM-5, intracrystalline species like MgO can produce a two order of magnitude change in diffusivity and also significant changes in catalytic properties (Olson and Haag, 1984).No detailed investigation on the diffusion in ZSM-8 and modified ZSM-8 zeolites has been reported so far. The present investigation was undertaken with the objective of studying the influence of cations ( H + or N a + ) and modification by MgO, P,O,, and B,O, on the diffusion in ZSM-8-type zeolite, using a model sorbate (viz., Cumene).Correapndcncc concerning this paper should be addrcsd to V. R. Choudhary. Experimental StudiesThe preparation and characterization of H-ZSM-8 and H . N a -Z S M -8 zeolites (Si/AI = 29.6 and crystal size = 3.5pm) have been described earlier (Akolekar and Choudhary, 1987). The degrees of H+-exchange of the H-ZSM-8 and H . Na-ZSM-8 zeolites were 0.96 and 0.08, respectively. The magnesium, phosphorous and boron modified H-ZSM-8 zeolites have been prepared by contacting the H-ZSM-8 zeolite with aqueous solutions of magnesium acetate, ortho-phosphoric acid, and boric acid (all BDH AnalaR), respectively, at room temperature for two weeks while shaking the flask containing the zeolite and the solution occasionally. The zeolite was then filtered using a Gooch crucible and washed quickly to remove the material impregnated on the external surface of the zeolite crystals. The zeolite was then dried in a vacuum oven at 373 K for 12 h, calcined in air at 773 K for 12 h and analyzed for its magnesium, phosphorous or boron content by atomic absorption spectroscopy or spectrophotometry. All the zeolites were pressed (without any binder) and crushed into particles of about 0.2 mm size. This has not cau...

Single-Component Sorption/Diffusion of Cyclic Compounds from Their Bulk Liquid Phase in H-ZSM-5 Zeolite

“…However, sorption/diffusion in the zeolite from the liquid phase is relatively less investigated. Liquid-phase singlecomponent sorption/diffusion of o-and m-xylenes and mesitylene (Choudhary and Singh, 1986), C 1 -C 4 alcohols (Lin and Ma, 1988), cumene Choudhary and Mamman, 1990), and C 2 -C 8 alcohols, n-hexane, and n-butylamine (Choudhary et al, 1992) in ZSM-5 zeolite has been reported earlier. The diffusion in ZSM-5 from the liquid phase is expected to differ from the gas-phase diffusion because of the lower degree of freedom for the orientation of liquid sorbate molecules and also due to the requirement of additional energy (equivalent to enthalpy of vaporization) for pulling out the sorbate molecules from their bulk liquid during their entrance in the zeolite channels.…”

“…Apart from the sorbate factors, the diffusion in ZSM-5 zeolite has been found to be strongly influenced by a number of zeolite factors, such as Si/Al ratio, degree of cation exchange, thermal/hydrothermal treatments, presence of strongly sorbed species or foreign materials, etc. (Post et al, 1984;Wu and Ma, 1984;Choudhary and Srinivasan, 1986b;Choudhary and Mamman, 1990). Hence, the diffusivi- ties in different ZSM-5 type zeolites can seldom be compared.…”

Synthesis and Characterization of Zeolitic Materials Using Phosphorous Organic Structure Directing Agents

“…[143] The incorporation of phosphorus has been carried out by several methods like wet impregnation with H3PO4 [144] and NH4H2PO4, [138] or by gas deposition of PCl3 [145] P(OCH3)3 [146] or P(CH3)3, [136] among others. The most studied materials include zeolites ZSM-5, [136,147,148] USY, [149,150] Beta, [151] ZSM-8, [152] MCM-22, [153] and mesoporous MCM-41 zeolite. [154] Despite the large number of studies on phosphorus incorporation, the phosphorus-zeolite interaction is still unclear, and several interaction theories have been proposed to explain the observed properties of the final catalysts.…”