Silicon-substituted aluminophosphate molecular sieves: Studies on SAPO-5, -11, -34 and -44 (original) (raw)

Silicon-substituted aluminophosphates with different framework structures SAPO-5,-11,-34 and-44 have been synthesized by the hydrothermal method using organic templates. The extent of silicon substitution in these materials has been investigafed by chemical analysis, measurement of acidity through temperature-programmed desorption of ammonia and silicon MAS-NMR spectroscopy. It has been found that for SAPO-5, and SAPO-11 a small amount of silicon substitution takes place at the phosphorus sites and most of the silicon atoms are present either as silica-rich regions in the structure or as amorphous silica. The amount of silicon in them is quite low. For SAPO-34 and-44, on the other hand, the major part of the silicon randomly substitutes the phosphorus atoms and the rest of it forms small silica patches in the structure. The last two structures cannot be prepared in pure form unless the concentration of the silica source in the synthesis gel is quite high. Keywords. Silicon-substituted aluminophosphates; MAS-NMR; temperature-programmed desorption of ammonia.