Thermal stability and dehydroxylation of Bronsted acid sites in silicoaluminophosphates H-SAPO-11, H-SAPO-81 H-SAPO-31, and H-SAPO-34 investigated by multi-nuclear solid-state NMR spectroscopy

The thermal stability and dehydroxylation of the microporous silicoalummophosphates H-SAPO-11, H-SAPO-18, H-SAPO-31, and H-SAPO-34 were investigated after thermal treatment at temperatures of 773-1173 K. While no change of the crystallinity upon thermal treatment at 1173 K was found by X-ray diffraction, H-1 MAS NMR spectroscopy indicated a dehydroxylation of 40-50% of the initially existing bridging OH groups. As shown by Al-27 and Si-29 MAS NMR spectroscopy, this dehydroxylation is not accompanied by a dealumination but rather by a removal of silicon (desilication) in the local structures of the former bridging OH groups. The dehydroxylation and desilication of the silicoaluminophosphate framework do not lead to the formation of defect OH groups as evidenced by H-1 MAS NMR spectroscopy. To explain the high stability of the silicoaluminophosphates after dehydroxylation and desilication, a healing process is proposed which is based on a migration of phosphorus atoms to framework vacancies and their transformation to P(OAl)(4) species. (C) 2002 Elsevier Science Inc. All rights reserved.