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  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.
PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS, PTS A-C