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 degrees 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 Si-29 NMR data. (C) 2008 Elsevier Inc. All rights reserved.