The atomic positions and interactions between adsorbed guest molecules, such as ammonia in H-ZSM-5 microporous solids, are for the first time revealed by using in situ synchrotron powder X-ray diffraction combined with refinement within experimental errors.
Two series of SAPO-5 molecular sieves were synthesized: (i) Using tripropylamine as a template and varying the Si content in the gel, and (ii) varying the template molecule at constant silicon content in the gel. The products were characterized physico-chemically and tested in toluene disproportionation. It was found that the rising silica content in the synthesis gel has only little influence on the SiO2 incorporation into the SAPO. Investigation of the acid character revealed that the concentrations of the adsorbed ammonia molecules are nearly constant and independent of the total silicon content of the crystals. The catalytic activity showed no correlation either with the silicon content of the catalysts or with the acidity measured by temperature-programmed desorption of ammonia. It is concluded that the incorporated silicon exists not only in a monomeric form and that the TPD of ammonia reflects not only the strong Bronsted acidity. The SAPO-5 sample synthesized with triethylamine possessed the strongest acidity corresponding to the highest catalytic activity. It is supposed that in the presence of TEA molecules silicon replaces mainly P atoms in the AlPO4-5 framework.