Behavior and Local Structure of Surface Sites in Microporous Silicoaluminophosphates

Chemical Behavior and Local Structure of Surface Sites in Microporous SiLICOALUMINOPHOSPHATES 

In the preceding decade, microporous silicoaluminophosphates have drawn increasing interest as solid catalysts in chemical technology, because of their acidic and shape-selective properties. H-SAPO-34 with the chabasite structure, for example, is a suitable catalyst for the conversion of MTO (210). H-SAPO-37 with the faujasite structure was applied for the isomerization of -decane (211) and the isobutylene/2-butene alkylation (212). 

Investigations performed by Minchev et al (215) indicated that the framework of crystalline silicoaluminophosphates can be damaged upon the rehydration of the template-free material. In the case of rehydrated template-free H-SAPO-5 and H-SAPO-34, for example, a strong loss of the crystallinity occurs in the presence of water. However, the crystallinity can be completely restored after an additional dehydration at 823 K. Hydration of H-SAPO-37 at room temperature causes irreversible structural changes and leads to a material that is totally amorphous to X-ray diffraction (216). At temperatures of more than 345 K, template-free H-SAPO-37 exhibits a high stability toward hydration (216). 

To investigate the hydration and dehydration processes of H-SAPO-34 and H-SAPO-37, H and Al MAS NMR spectroscopy was applied under CF conditions with the equipment shown in Fig. 12 (217). The chemical behavior and the change of the silicoaluminophosphate framework were monitored as nitrogen loaded with water or dry nitrogen was injected into the MAS NMR rotor filled with the silicoaluminophosphates. By this approach, the primary adsorption sites of water in silicoaluminophosphates and the variation of the aluminum coordination were observed. Furthermore, the formation of framework defects and the conditions of water desorption were characterized. 

At the beginning of the hydration of calcined H-SAPO-34 (i.e., after the start of the injection of nitrogen loaded with water vapor into the spinning MAS NMR rotor), only a change in the H MAS NMR spectra was observed (Figs 23b and c, left). After a water adsorption of 0.8 mmol/g, a significant decrease of the signal of 

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