Successive steps of hydration and dehydration of silicoaluminophosphates H-SAPO-34 and H-SAPO-37 investigated by in situ CF MAS NMR spectroscopy

Applying H-1 and Al-27 CF MAS NMR spectroscopy (CF: continuous flow), the hydration and dehydration of silicoaluminophosphates were studied under in situ conditions in a flow of nitrogen loaded with water vapor or in a flow of dry nitrogen. The hydration of H-SAPO-34 and H-SAPO-37 is composed of two successive steps, viz. (i) hydration of Bronsted acidic bridging OH groups (SiOHAl) followed by (ii) coordination of water to aluminum atoms. In hydrated H-SAPO-34, the hydration-induced transformation of tetrahedrally coordinated framework aluminum atoms into octahedrally coordinated aluminum atoms is a reversible process at T = 298 K and on the time scale of the experiments. In the case of hydrated H-SAPO-37, the dehydration of octahedrally coordinated aluminum atoms requires temperatures of T greater than or equal to 373 K corresponding to a stronger bonding of the water molecules to the aluminum atoms than in H-SAPO-34. This indicates an immediate hydrolysis of the H-SAPO-37 framework. Since no defect SiOH groups were observed upon hydration and dehydration of H-SAPO-37 at T = 298 K, the hydrolysis of this materials starts at dropP-O-Aldrop bonds in the aluminophosphate regions of the framework. Upon hydration of H-SAPO-37 at T greater than or equal to, 353 K, only a hydration of bridging OH groups occurs, and no hydrolysis of dropP-O-Aldrop bonds responsible for a damage of the framework was found. (C) 2002 Elsevier Science Inc. All rights reserved.