Shape-selective catalysis of zeolites

Abstract

Cation exchange will modify the diffusional characteristics of zeolite catalysts. It will also alter the acidic properties of the catalysts. In addition exchanged cation may serve as a major component. Interaction of hydrocarbons with Cu(II) exchanged zeolite was investigated using EPR and ESCA techniques. Acidic properties of zeolite catalysts were studied by i.r. and temperature programmed desorption techniques. It was shown that the acid site strength in ZSM-5 zeolite decreased with increasing radius of exchanged alkalimetal cations. Adsorption and diffusion of cyclohexane, xylene isomers and $C_5-C_9$ paraffins were studied using HZSM-5,-8 and -11 zeolite samples of uniform size. Uptake of p-xylene was rapid in HZSM zeolites but diffusion of m-and o-xylene was substantially slow. The amount taken up indicated that most of the internal surface was not accessible by m-xylene. Reactions of toluene were studied over HZSM-5, -8, -11 and cation-exchanged as well as Mg-modified pentasil zeotites. o-Xylene isomerization was one order of magnitude faster than toluene disproportionation over HZSM-5 at 400℃. It seems that the bimolecular transition state required for transalkylation are not readily formed within the ZSM-5 structure. Only Cs-ZSM-5 showed increased p-xylene selectivity. The para-selectivity increased with increasing degree of cation exchange and with decreasing reaction temperature. It seems that the internal pore structure of Cs-ZSM-5 cannot provide an enough room to form the restricted transition state that leads to m-and o-xylene. The overall conversion of toluene, however, decreased with increasing degree of cation exchange and this was attributed to the reduced acidity. High p-ethyltoluene selectivity was observed over Cs-exchanged and Mg-modified HZSM zeolites. The increased para-selectivity was interpreted in terms of selective diffusion of the smaller para-isomer combined possibly with the sterically favored formation of para-isomer...