A DFT Exploration of the Adsorption of Methanol on the Brönsted Acid Site of Nanostructured H-ZSM-5 Catalyst: Probing the Effect of Cluster Size on the O–H···O Hydrogen Bond Based on NMR and NQR Parameters

DFT calculations have been performed at the B3LYP/6-31++g** level to investigate the effect of the cluster size of ZSM-5 on the hydrogen atom of the Brönsted acid site via NQR and NMR parameters in methanol adsorption. ZSM-5 clusters have been modeled as 1T, 2T, 5T and 8T. T represents a general tetrahedral unit. Chemical shifts, δiso, of 1H atom in the Brönsted acid site as well as quadrupole coupling constant, CQ, and asymmetry parameter, ηQ, of 2H atom in the Brönsted acid site have been calculated. Spectrometric parameters showed that these parameters are influenced by the size and the shell of surrounding central T. Results indicate that inclusion of the next shell of T atoms (silicon atoms) around bridged hydroxyl group in the bare cluster models raises CQ and 1H δiso from 1T to 5T, 2T to 8T, while no significant changes are observed in the same electronic environment. The calculated the natural charge distribution of hydrogen atom in the Brönsted acid site analyzed by the NBO method for comparison of the cluster models.