Transport, sorption, and catalytic conversion of aromatic molecules in zeolites

Understanding the elementary chemistry of catalytic transformations in zeolites is the key requirement to developing better catalysts and processes for the shape selective conversion of aromatic molecules in medium pore zeolites, e.g., H-ZSM5. The sorption of these rigid molecules into zeolite pores occurs via a precursor step at the outer surface. Building a microporous few nanometer thick silica overlayer improves adsorption rate for molecules with a small radius of gyration, e.g., benzene, while it decreases the rate for larger ones, e.g., p-xylene. All acid sites have the same catalytic activity for reactions, e.g., xylene isomerization and toluene alkylation. Kinetic measurements show that in unmodified materials reactions rapidly attain the thermodynamic equilibrium minimizing potential effects of shape selective properties of the zeolite. Decreasing the effective particle size by combined leaching with acids and bases creates a zeolite material with three-dimensional mesopores. The reduction of the particle size results in significantly higher rates. Surface modification reduces the catalytic activity, but increases selectivity. Acid sites at the pore entrances are blocked by this treatment, thus, reducing so the potential isomerization of products. The potential and limitations of this approach to generate the new class of shape selective catalysts are elucidated. This is an abstract of a paper submitted at the 21st Annual Saudi-Japan Symposium on Catalysts in Petroleum Refining and Petrochemicals (Dhahran, Saudi Arabia 11/27-28/2011).