Wim Souverijns

Molecular shape selectivity of EUO zeolites

Abstract The micropores of the EUO zeolite consist of 10-membered ring (10-MR) channels with deep side-pockets circumscribed by 12-MRs. In decane isomerization and hydrocracking EUO zeolites crystallized in the presence of hexamethonium (HM) and dibenzyldimethylammonium (DBDMA) exhibit different molecular shape selective properties. The catalytic sites of EUO specimens crystallized with DBDMA are located in more spacious cavities than those of EUO crystals synthesized with HM. This disparity in molecular shape selectivity is explained by a different location of the framework aluminium atoms in the two materials, viz. by the absence of aluminium in the 10-MR channels in EUO-DBDMA. Such aluminium siting is in agreement with the location of the organic template cations in the pores of the as-synthesized zeolites suggested in literature and revealed by a molecular graphics modeling.

Selective key-lock catalysis in dimethylbranching of alkanes on ton type zeolites

The Pt/H-ZSM-22 catalyst exhibits peculiar dimethylbranching selectivities in the skeletal isomerization of C 8 -C 11 n-alkanes. Out of the large number of theoretically possible dibranched skeletal isomerization products, a few dimethylbranched isomers having methylbranchings at specific carbon atom positions are formed selectively. The observed selectivity is rationalized with stereospecific catalysis at the [001] crystallographic planes at the external crystal surface. Computer generated molecular models show that only the preferred dimethylbranched isomers can maximize their van der Waals interaction with specific sites on the zeolite external surface. Since these adsorption sites bridge two (or more) adjacent pore mouths, the ensemble allows the adsorption of only preferred isomers in a key-lock type of interaction.

Mechanism of the paring reaction of naphtenes

Abstract1-cyclohexyloctane is isomerized on the large pore, bifunctional zeolite catalyst Pt/H-Y. The detailed isomerization mechanism of this naphtene molecule is revealed through a thorough analysis of the numerous isomerization products. It is found that methyl side-chains are generated on the ring of the molecule via a ring contraction-expansion mechanism, which are subsequently transferred to then-alkyl substituent via methyl shifts. Direct branching of then-alkyl chain is a much slower process compared to the generation of methyl substituents on the ring.

Micropore structure of zeolite MCM-22 as determined by the decane catalytic test reaction

According to the decane catalytic test reaction, the pore architecture of zeolite MCM-22, with unknown topology, contains two micropore systems circumscribed by 10- and 12-rings; the 12-ring channel system is monodimensional and has no connections with the 10-ring pores, where the strongest acid sites are located.