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

Silicoaluminophosphate SAPO-41 was synthesized either in the presence of su rfactant or without added surfactant. Under the synthesis conditions with a dded surfactant small-size molecular sieve crystals were formed, while larg er crystals were obtained in the absence of surfactant. Magic-angle spinnin g nuclear magnetic resonance spectroscopic studies showed that silica-rich domains existed for silicon-rich samples. Infrared spectra in the OH stretc hing vibrations and temperature-programmed desorption of ammonia revealed t he presence of Bronsted acid sites. Samples prepared in the presence of sur factant exhibited higher acid strength. The hydroconversion of n-octane was performed over Pt-Pd-containing SAPO-41 samples. These catalysts showed hi gh selectivity for the hydroisomerization reaction. Among the Cs isomers, m onomethylheptanes were preferentially formed. The selectivity towards hydro isomerization 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 me dium-pore SAPO-41 played the major role. Pore mouth catalysis at the interi or of the pore is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.