Molecular shape selectivity of ZSM-5, modified ZSM-5 and ZSM-11 type zeolites

Abstract

In order to ascertain the relative importance of different shape-selective effects in the conversion of methanol to higher hydrocarbons on Pentasil-family zeolites, several catalysts were used in the present investigation of the methanol conversion and alkylation of p-xylene by methanol. These catalysts are ZSM-5 zeolites, differing in their acidification pretreatments, modified by the incorporation of phosphorus, or embedded in a silica filler. H-ZSM-5 and H-ZSM-11 zeolites with very similar Al contents and particle sizes were also compared in order to assess the effect of channel tortuosity. It appears from the selectivities of aromatic hydrocarbons (methanol conversion at 370 °C) that H-ZSM-5 yields preferentially C₈-aromatics while H-ZSM-11 produces larger amounts of C₉-aromatics (and C₆₊-aliphatics). Both zeolites are stable catalysts as a function of time on stream. The production of aromatic para-isomers increases when additional constraints are imposed by the channel structure or when the outer surface of the crystallites is deactivated. A comparative study of H-ZSM-5 and H-ZSM-11 in the alkylation of p-xylene with methanol shows activities and selectivities in agreement with these observations. ZSM-11 is also found to have more alkylation activity in the p-xylene-methanol reaction than a corresponding ZSM-5 catalyst.

These observations are discussed in terms of molecular shape-selective properties involving the nature and geometry of the channel network and the dimensions of the channel interconnections.