Key parameters in hydrothermal synthesis and characterization of low silicon content SAPO-34 molecular sieve

Low silicon content SAPO-34 was successfully synthesized using the conventional hydrothermal crystallization in the static condition. Effects of different synthesis conditions including crystallization temperature and the silicon source were investigated through X-ray diffraction patterns. It was concluded that the silicon source had a significant effect on silicon incorporation into the alumino-phosphate building blocks in the course of crystallization. Using precipitated silica instead of silica sol resulted in the formation of impurities of alumino-phosphates phases whose crystalline structures collapse at the high temperature of calcination. For the low silicon SAPO-34 synthesis, the optimum crystallization temperature, as a less critical parameter than silicon source, was about 190 °C. Acidity of successfully synthesized low silicon content SAPO-34 has been compared with those of medium and high silicon content SAPO-34s through NH3–TPD, TG and in-situ FTIR. The measured amounts of adsorbed ammonia on the samples, using a thermogravimetric (TG) instrument, indicated that the total acidity of the low silicon content sample was only slightly lower than those of higher silicon content SAPO-34s, and this was in agreement with in-situ FTIR data. The lack of acidity increase by increasing the silicon content of the synthesis gels was explained by the formation of silicon islands as revealed by the 29Si NMR data.