Insights on the seed selection criteria of SAPO-34 synthesis: structural units and their chemical microenvironment

Seed-assisted synthesis shows great potential to modify the properties of inorganic microporous materials and enhance synthetic efficiency. However, the understanding of the related mechanism is insufficient, especially in silicoaluminophosphate (SAPO) molecular sieve systems. In this work, the seed-assisted synthesis of SAPO-34, an important industrial methanol-to-olefins (MTO) catalyst with CHA topology, was investigated by using various seeds with different structures, compositions and morphologies. It reveals that both double 6-ring (d6r) units and complex Si environments are key features for a highly effective SAPO seed, which may provide small fragments containing complete or broken d6r units with Si-O-Al domains after the dissolution of seeds and thus derive the nucleation and crystallization of SAPO-34. Moreover, small crystal size and high Si content are positive factors for a qualified SAPO seed. Heteronuclear seeds, such as SAPO-18, SAPO-35 and SAPO-56, have been found to be effective for the synthesis of SAPO-34 with improved MTO catalytic performance. For SAPO materials without d6r units, their effect strongly depends on the growth solution and there is no preferential phase selectivity for SAPO-34. These findings could prompt the efficient synthesis and property control of SAPO molecular sieves.