Crystallization and catalytic properties of molecular sieve SAPO-34 by a vapor-phase transport method†
Formation of molecular sieve SAPO-34 has been examined under vapor-phase transport (VPT) conditions. Powder XRD, SEM/EDX, and solid-state NMR were used to follow the evolution of gel samples as a function of crystallization time. The results indicate that the initial VPT dry gel is a mixture of an amorphous material (major component) and a dense phase (minor component). The amorphous material is aluminophosphate in nature, but its structure is not suitable for transformation into SAPO-34 since it only has octahedral Al. Introducing morpholine (the structure directing agent) onto the surface of the dry gel via a vapor phase leads to changes in the structure of the amorphous phase as is evidenced by the conversion of octahedral Al into the tetrahedral Al sites. The dense phase is transformed into the tridymite phase. Upon further heating under VPT conditions, the equilibrium between tridymite and amorphous phases shifts to the amorphous material which is subsequently reorganized to form SAPO-34. The 29Si NMR results show that Si was incorporated into the framework at a later stage of crystallization via the formation of aluminosilicates. Catalytic tests indicate that the SAPO-34 product obtained by the VPT method had a longer lifetime for 100% methanol conversion and similar product selectivity to the SAPO-34 prepared by the normal hydrothermal synthesis.