Generation of Mesoporosity in LTA Zeolites by Organosilane Surfactant for Rapid Molecular Transport in Catalytic Application
Résumé
Hydrothermal crystallization of LTA zeolite was performed at a gel composition containing organosilane surfactant as a mesopore-generating agent. The zeolite was constructed of microporous crystalline zeolite frameworks that were penetrated by a three-dimensional disordered network of mesopores. Pore size analysis showed a gradual shift from 6 to 10 nm as the amount of the surfactant was increased. The detailed study indicated that the mesopores were templated by the organosilane surfactant, where the micelles were expanded by the excessive organosilane. Pore diameters could be further expanded to 24 nm by the addition of EO20PO70EO20 triblock copolymers as pore-expanding agents. Xenon uptake and 129Xe NMR measurements at 297 K revealed that the xenon diffusion into the highly mesoporous LTA zeolite could occur 200 times more rapidly compared with that of a solely microporous zeolite. The two zeolites showed a dramatic difference in product selectivity, catalytic activity, and lifetime, when they were compared after Ca2+ ion exchange as a catalyst for the conversion of methanol to dimethyl ether and hydrocarbons. These results are attributed to rapid transport into and out of the zeolitic micropores via mesopores. Fully open micropore−mesopore connectivity would make such hierarchically porous zeolites very attractive for applications in adsorption and catalysis.