Structural charge transfer in the aluminophosphate molecular sieves by diffuse reflectance spectroscopy

Influence of water adsorption in AlPO-5, SAPO-5, AlPO-11 and SAPO-11 has been studied with UV diffuse reflectance spectroscopy. The observed UV absorption spectra in the as-synthesized, template free and hydrated materials are related to the charge transfer processes between aluminum and oxygen atoms of the aluminophosphate and water molecules. As-synthesised materials show two distinct and well-defined bands at about 220 and 260-280 nm correlated to framework aluminum and organic templates, respectively. Upon calcination, the band of occluded template disappears and the band assigned to the framework aluminum shifts at about 240 nm. When the calcined samples are completely hydrated, broadening of the aluminum charge transfer band is observed. This is due to coordination of water molecules to the part of the framework aluminum. Broadening occurs more in AlPO-5 possibly because of higher water capacity and homogenity with respect to SAPO-5. (C) 1999 Elsevier Science B.V. All rights reserved.