A COMPARATIVE-STUDY OF ACIDIC PROPERTIES OF SAPO-5, SAPO-11, SAPO-34 AND SAPO-37 MOLECULAR-SIEVES

Silicon substituted AlPO molecular sieves (SAPO) of the structure types 5, 11, 34 and 37 have been synthesized and characterized for crystallinity and phase purity using X-ray diffraction, scanning electron microscopy, thermal analysis, BET surface area measurements and chemical analysis. The incorporation of silicon into the framework of SAPO materials was demonstrated by Al-27, P-31 and Si-29 magic angle spinning nuclear magnetic resonance. The acidic properties of these materials have been studied by N1s X-ray photoelectron, Fourier transform infrared and temperature programmed desorption spectra of chemisorbed pyridine. Two types of N1s XPS peaks with binding energy values (401.5-402.7 eV) and (399.6-400.5 eV) corresponding to pyridine chemisorbed on Bronsted and Lewis acid sites, respectively were detected. The IR spectra in the pyridine region of AlPO4-5 showed an absence of a Bronsted band and in the case of SAPO-34 molecular sieve absence of both, Bronsted and Lewis bands. However, N1s XP spectra clearly showed the presence of Bronsted N1s component peaks in AlPO4-5 and the presence of Bronsted and Lewis bound pyridine on the surface of SAPO-34 molecular sieve. The N/Si ratio of close to one observed for SAPO-37 was interpreted to mean that in this case all silicon atoms were involved in generating Bronsted acid sites. The lower N/Si ratios observed for SAPO-5, SAPO-11 and SAPO-34 were explained on the basis of either formation of silicon rich domains in the structure or pore size considerations. The presence of silicon rich domains in some of these structures was confirmed by Si-29 NMR study. Comparison of the XPS data with FTIR and Si-29 NMR results, allowed us to conclude that not all silicon atoms present in the framework were responsible for generating the Bronsted acid sites in SAPO-n molecular sieves.