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

2002; Elsevier BV; Volume: 57; Issue: 2 Linguagem: Inglês

10.1016/s1387-1811(02)00562-0

1873-3093

A. Buchholz, Wei Wang, A. Arnold, Mingyu Xu, Michael Hunger,

Crystal Structures and Properties

Applying 1H and 27Al 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 Brønsted 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⩾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 P–O–Al bonds in the aluminophosphate regions of the framework. Upon hydration of H-SAPO-37 at T⩾353 K, only a hydration of bridging OH groups occurs, and no hydrolysis of P–O–Al bonds responsible for a damage of the framework was found.