A DFT study of the structural units in SBA-15 mesoporous molecular sieve

Geometries of silica rings in SBA-15 mesoporous molecular sieve were investigated by the density functional theory (DFT). The stable conformations of model clusters with monocyclic and bicyclic silica rings were obtained. By comparing the calculated values of Si-O bond length and O-Si-O, Si-O-Si bond angles with the experimental data of amorphous silica, we found that all monocyclic rings are not the reasonable model to represent the structure of SBA-15 mesoporous molecular sieve. After having investigated the bicyclic model clusters theoretically, we inferred that 5-, 6- and 7-membered rings should be the main structural elements in the SBA-15 framework. Among the model clusters employed in this study the model 5-6-s was proved to be the best one for the SBA-15 molecular sieve. A comparison between the calculated IR frequencies of the model clusters and the measured values of SBA-15 further verified that the bicyclic model clusters employed here are reasonable and the model 5-6-s is the most suitable model cluster for the SBA-15 mesoporous molecular sieve.