Investigations of Formation of Molecular Sieve SAPO-34

Crystallization of a catalytically important molecular sieve SAPO-34 was examined under dry gel conversion conditions including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The progressive changes in the structure of solid gel phases were monitored by powder XRD, SEM, and several solid-state NMR techniques involving 31P, 27Al, 29Si, and 13C. The results suggest that the formation pathways under VPT and SAC appear to be similar: SAPO-34 crystallizes from a semicrystalline precursor with a layered structure held mainly by weak nonbonding interactions. AFM data indicate that the crystal growth mechanism is "birth and spread" and that the fast nucleation on the (001) crystal face likely results from simultaneous transformation of many small domains of precursor to SAPO-34 via cooperative bond breaking and reforming. 29Si NMR results suggest that Si incorporation occurs slowly via amorphous aluminosilicate species. These findings provide new physical insights into the formation of SAPO-34.