Crystal Growth Kinetics as a Tool for Controlling the Catalytic Performance of a FAU-Type Basic Catalyst

This study reports on the catalytic performance of nanosized zeolite X crystals and their precursors in the reaction of benzaldehyde with ethyl cyanoacetate. Crystal growth kinetics of FAU-type zeolite is studied at low temperature (35 degrees C) in order to discriminate different crystallization stages. First X-ray crystalline material is detected after 6 days of hydrothermal treatment. The formation of the crystalline phase is preceded by changes in the ring structure of an aluminosilicate precursor as revealed by the combined Raman-HEXRD-solid-state NMR analyses. The set of experimental data shows that these changes are related to the reorganization of the gel structure and the formation of zeolite units. Prior to the appearance of crystalline material, the apparently amorphous solid exhibits chemical composition and short-range order organization similar to that of a crystalline FAU-type zeolite. Knoevenagel condensation was used to test the catalytic activity of a series of zeolite intermediates and nanosized zeolite crystals. The amorphous precursor obtained after 5 days of hydrothermal treatment showed the highest yield of ethyl alpha-cyanocinnamate. Superior catalytic performance of this material was attributed to the combination of strong basic sites and less restricted and more accessible structure of the semicrystalline zeolite units. Thus, the crystal growth kinetics of FAU-type zeolite can be used as a tool to tune the properties of a catalyst used in Knoevenagel condensation.