Synthesis and characterization of SAPO-41: effect of the silicon content and the crystal size on the hydroisomerization of n-octane over Pt–Pd/SAPO-41

Abstract

Silicoaluminophosphate SAPO-41 was synthesized either in the presence of surfactant or without added surfactant. Under the synthesis conditions with added surfactant small-size molecular sieve crystals were formed, while larger crystals were obtained in the absence of surfactant. Magic-angle spinning nuclear magnetic resonance spectroscopic studies showed that silica-rich domains existed for silicon-rich samples. Infrared spectra in the OH stretching vibrations and temperature-programmed desorption of ammonia revealed the presence of Brønsted acid sites. Samples prepared in the presence of surfactant exhibited higher acid strength. The hydroconversion of n-octane was perfomed over Pt–Pd-containing SAPO-41 samples. These catalysts showed high selectivity for the hydroisomerization reaction. Among the C8 isomers, monomethylheptanes were preferentially formed. The selectivity towards hydroisomerization at constant conversion level did not change with the SAPO-41 crystal size. However, a small-crystal SAPO-41 sample was considerably more active than large-crystal SAPO-41. The shape selectivity of the tubular medium-pore SAPO-41 played the major role. Pore mouth catalysis at the interior of the pore is also discussed.