Influence of crystallinity on the physico-chemical properties of SAPO-31 and hydroconversion of n-octane over Pt loaded catalysts

Abstract

SAPO-31 samples of different crystallinity were prepared at various stages of hydrothermal synthesis and characterized by XRD, SEM, chemical analysis, TGA/DTA, N 2 adsorption, IR spectra of adsorbed CO, and solid state NMR. The obtained data showed that the adsorptive and acidic properties of SAPO-31 are related to the crystallinity of the material. The concentration of strong acid sites increased simultaneously with the growth of the content of the microporous phase and attained a maximum value at a crystallinity of 80% with no further change. During earlier stages of crystallization, a microporous material deficient in silica is formed, and Si atoms are predominantly introduced into the framework by the substitution of P by Si according to a SM2 mechanism. At later stages, silica domains are formed in SAPO-31 due to the combined SM2 + SM3 substitution mechanisms. NMR data showed that the environment of the Si atoms changes after the maximum crystallinity is attained, which points to the continuous modification of the acidic properties of SAPO-31. The hydroconversion of n-octane was performed over Pt-containing silicoaluminophosphates with different crystallinity. Catalytic experiments showed that an increased time of hydrothermal synthesis of the silicoaluminophosphate gives rise to a further increase of both the catalytic activity and isomerization selectivity of Pt loaded catalysts. It was supposed that the acidic sites located in the near-surface layer of the SAPO-31 crystals are responsible for the improved catalytic properties of Pt/SAPO-31.