Synthesis and characterization of the large-pore molecular sieve SAPO-46

Abstract

Synthesis of a novel large-pore SAPO-46 molecular sieve has been described. The effect of the organic template (di-n-propylamine) and silica concentration in the synthesis gel on the crystallinity and phase purity of SAPO-46 was studied. Samples were characterized by X-ray diffraction (XRD), magic-angle spinning (MAS) NMR spectroscopy, FTIR spectroscopy, thermal analysis (TG and DTA), chemical analysis, scanning electron microscopy and EDAX microprobe analysis. SAPO-46 had a lower thermal stability than SAPO-5, SAPO-37 and MgAPO-36. Silicon substitution for phosphorus at crystallographically distinct sites has been suggested to cause the creation of two different silicon environments in SAPO-46. The extent of silicon substitution was large and temperature-programmed desorption (TPD) of ammonia has shown high concentration of acid sites. SAPO-46 showed high activity for alkylation of toluene with methanol that was comparable with that of ZSM-5, but showed no para selectivity. However, as the reaction proceeded, para selectivity developed due coking at the pore mouth.