Tailoring the acid strength of microporous silicoaluminophosphates through the use of mixtures of templates: Control of the silicon incorporation mechanism

Abstract

SAPO-5 samples have been synthesized with triethylamine (TEA), benzylpyrrolidine (BP) and mixtures of them as structure directing agents (SDAs). It has been observed that in the as-synthesised materials the concentration of SDAs (molecules per unit cell) and the Si content are similar. According to the different molecular size of both SDAs, the samples exhibit higher organic weight and lower water content as the molar fraction of BP in the synthesis gel increases. These differences in selectivity for organic/water incorporation influences the mechanism of Si substitution in the AlPO lattice: the higher organic content obtained at higher BP/TEA ratio leads to the formation of large Si islands, while the higher water content of the sample obtained with TEA enhances the formation of isolated Si(OAl) 4 environments. Interestingly, it has been found that these two opposite trends can be tailored to a certain point by using mixtures of both SDAs, TEA and BP, in the required ratio. The catalytic activity of the samples has been tested in the isomerisation of m-xylene. It has been observed a higher activity of samples obtained with BP as the main SDA, thus evidencing the better performance of large Si islands in this reaction.