Properties of T1-OH-T2 (T1 = Al, B and T2 = P, Si) Bridges on Metal-Substituted AlPO4-5 Molecular Sieves : MNDO Calculations

摘要

Semiempirical MNDO calculations are employed to study ionicity of OH groups and stability in T1-OH-T2 bridges(T1 = Al, B and T2 = P, Si) such as found in aluminophosphate family(AlPO4-5, BAPO-5, and SAPO-5) molecular sieves. Dimeric model clusters of Al-OH-P, B-OH-P and Al-OH-Si bridges were considered. It is shown that the elongation of the T-O bond, upon replacement of Al by B, occurs preferentially by a local deformation of the Al-O-P bridge. But the elongation of the T-O bond occurs preferentially by a rotation of Al-O-Si bridge upon substitution P for Si. Also, the ionicity of OH groups and stability increase in order to B-OH-P < Al-OH-P < Al-OH-Si bridge.