Small-pore molecular sieves SAPO-57 and SAPO-59: synthesis, characterization, and catalytic properties in methanol-to-olefins conversion

Abstract

The synthesis and characterization of SAPO-57 (AFV) and SAPO-59 (AVL), two new members of the ABC-6 family of zeolite structures, are presented. Both SAPO-57 and SAPO-59 can crystallize over a relatively wide range of Si/(Si + Al + P) ratios in the presence of diethyldimethylammonium and ethyltrimethylammonium ions as an organic structure-directing agent (OSDA), respectively, when the SAPO gels are homogeneously mixed during synthesis. The substitution of Si atoms on the crystallographically distinct tetrahedral sites in the framework of these two materials during the crystallization process, as well as their partial extraction caused by calcination for OSDA removal, was found to be non-random in nature. The catalytic properties of H-SAPO-57 and H-SAPO-59 with similar Si contents, crystallite sizes, and acidic properties are investigated in the methanol-to-olefins (MTO) conversion and compared with those observed for H-SAPO-34 and H-SAPO-35. The overall results of our study demonstrate that the pore dimensionality of cage-based small-pore molecular sieves is one of the most crucial factors influencing their MTO stability.