Investigation of hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 by diffuse reflectance infrared spectroscopy

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Abstract
Hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 have been investigated by diffuse reflectance IR spectroscopy. By carefully comparing the intensities of stretching vibrations with the appropriate combination and overtone bands the two types of bridged Si—OH—Al group were assigned to the fundamental stretching vibrations at 3625 and 3600 cm–1. Both types of group exhibited approximately the same acidic properties but differed in their localization. The bridged groups with ν= 3600 cm–1 were assumed to be localized in the hexagonal prisms, forming an H-bond with adjacent oxygen anions of the framework. In-plane and out-of-plane bending vibrations were observed for the first time in SAPOs.