Molecular modelling study of solid acidity in molybdenum oxide supported on silica-alumina

Factors influencing the solid acidity in molybdenum oxides supported on sil ica-alumina were studied theoretically by means of self-consistent Hartree- Fock (HF), electron-correlated Moller-Plesset (MP2) and local density funct ional (LDF) quantum mechanical calculations of acid site models. This work was aimed at elucidating relationships between the chemical composition, su rface geometry and the electronic properties of the solid acid catalysts. Local structures of unsupported and supported molybdenum oxide inferred fro m earlier experimental results were studied using cluster models. Relations hips between the structural transformations in the molybdenum oxide tetrahe dra and the changes in Bronsted-Lewis acidity were identified. The experime ntally observed effect of the catalyst support composition on the acid stre ngth of the supported molybdenum oxide catalyst was interpreted in terms of the calculated charge redistribution and molecular orbital energies. An ac id site structure is proposed to explain the effect of Mo lending on the Br onsted acidity. The calculated changes in infrared vibration frequencies ag ree with the measured ones and support the mechanism proposed for the acidi ty changes on Mo loading.