SAPO-11, SAPO-31, and SAPO-41 molecular sieves: Synthesis, characterization, and catalytic properties in n-octane hydroisomerization

Large-pore SAPO-5 and medium-pore SAPO-11, SAPO-31, and SAPO-41 have been synthesized using a hydrothermal method. These catalysts were characterized by chemical analysis, XRD, SEM, IR, TPD of NH3, and MAS NMR; Pt dispersion was measured by Hz adsorption. n-Octane hydroconversion over the Pt-SAPO catalysts has been tested. High selectivity for n-octane isomerization has been observed on medium-pore PtSAPO-11, -31, and -41, while preferential hydrocracking has been found for large-pore Pt-SAPO-5. Isomerization products consisted of monobranched isomers with a negligible amount of dibranched isomers. Interestingly, among the medium-pore SAPOs, SAPO-41 exhibits the highest isomerization selectivity. The selectivity decreases in the order SAPO-41 > SAPO-31 > SAPO-11. The differences in isomerization selectivities of the SAPOs are explained by considering diffusional restriction and steric constraints, SAPO-41 channel dimensions adequately fitting the dimensions of the monobranched isomers. (C) 1997 Academic Press.