Incorporation of Silicon into AlPO-5 Framework Sites: Higher Thermal Stability and Lower Extra-Framework Aluminum Concentration

Abstract

Diffuse reflectance spectroscopy was used to investigate the structural modification of AlPO-5 microporous aluminophosphate synthesized in the presence of a silicon source and subjected to calcination in air. Extra-framework aluminium species, formed upon calcination and expulsion of aluminium from the framework, are detected by DRS because they are involved in aluminium-oxygen charge transfer transitions. Impregnation of the calcined AlPO-5 and SAPO-5 samples with ethanolic acetylacetone converted all the extra-framework aluminium atoms with distorted symmetry to a highly ordered symmetry, as evidenced by a distinct and well-defined 280 nm band in the DR spectra. The intensity of the 280 nm absorption band in AlPO-5 is much higher than those of all SAPO-5 samples with different silicon content. This is an indication of the presence ofa considerably higher amount of extra-framework aluminium in AlPO-5. Washing the acetylacetone treated samples with hot ethanol leads to extraction of some of the complexed aluminium. The progress of extraction could be monitored by both DR and UV-vis spectrophotometry.