The Effect Of Crystal Size And Morphology Of SAPO-34on Its Catalytic Performance Of Methanol-to-Olefin Reactions

In this work, we systematically investigated the size and morphology effect ofsilicoaluminophosphate molecular sieve—SAPO-34on the catalytic performance ofMethanol-to-Olefin (MTO) reactions. Catalytic test of MTO reactions over theseSAPO-34s demonstrates that the catalyst deactivation and consequently the catalystlifetime are strongly dependent on the crystallite size and morphology. The nanosizedSAPO-34catalysts exhibit longer lifetime in the methanol conversion reactionscompared with those with ordinary crystallite size. Particularly, the nano sheet-likeSAPO-34with thickness of20nm gives rise to remarkably enhanced lifetime. Theevaluation of coking rate and the fraction of methanol consumption on coke indicatevery slight coke deposition over nanosized SAPO-34s during methanol conversion,which ensures the operation of long-term methanol conversion.The SAPO-34catalysts with different particle size distributions were obtainedmainly through fractionation or synthesized with different structure-directing agents(SDAs) in the previous reports. In the present study, four highly crystalline SAPO-34catalysts with different crystallite sizes and morphologies were synthesized from thestarting gel with the same composition under conventional hydrothermal andmicrowave-assisted heating conditions, using TEAOH as the sole SDA. Textualproperties and chemical environments of framework atoms as well as acidconcentration were characterized by N2adsorption and NMR measurements. Thedifferences in the textural properties, chemical environments of framework atoms andacid site distributions of the four catalysts were correlated to the catalytic lifetime andproduct selectivity. Based on the measurement of coke formation and thedetermination of retained coke species, a scheme is proposed to elucidate thereduction in coke deposition, consequently the remarkably enhanced lifetime on thenanosized SAPO-34catalysts in methanol conversion. This work will provide usefulguidance for the improvement of methanol conversion process by utilizing nanosizedSAPO-34catalysts.