Nanosize-enhanced lifetime of SAPO-34 catalysts in methanol-to-olefin reactions

In this work, we systematically investigated the size effect of silicoaluminophosphate molecular sieve catalysts SAPO-34 on the catalytic performance of methanol-to-olefin (MTO) reactions. Four highly crystalline SAPO-34 molecular sieves with different crystallite sizes were synthesized under hydrothermal conditions carried out in conventional or microwave ovens with the same starting gel composition using TEAOH as the structure-directing agent. The as-prepared SAPO-34s have similar composition, and their average crystal size can be controlled between 20 nm and 8 μm. Textural properties and chemical environments of framework atoms as well as acid concentration were characterized by N₂ adsorption and NMR measurements. The MTO reactions were carried out over these four SAPO-34 catalysts to study their catalytic performances dependent on the crystal size. The occurrence of catalyst deactivation varied considerably with the crystal size of SAPO-34s. Significantly, the nanosized catalysts, especially the sheetlike SAPO-34 catalyst with 20 nm thickness, exhibited the longest catalyst lifetime and lowest coking rate in MTO reactions. On the basis of the measurement of coke formation and the determination of retained coke species, a scheme is proposed to elucidate the reduction in coke deposition and consequently the remarkably enhanced lifetime on the nanosized SAPO-34 catalysts in methanol conversion.