| MCM-22 possesses an unique crystal structure containing two independent pore systems. The Mo loaded MCM-22 catalyst has been shown to be an excellent catalyst for methane dehydroaromatization with higher benzene selectivity. The acidic strength of Bronsted acid site of zeolites is strongly dependent on the distribution of framework Al atoms. It has been proposed that the acid sites in supercage of MCM-22 play an important role for its shape-selective catalysis.In the present work, DFT calculations were performed on the acidity of MCM-22 zeolite with double Si/Al substitutions at NNN site in the 12 member-ring. The properties of Si/Al substitution energy, proton affinity, bridging hydroxyl stretching frequency and atomic charge on proton, and interaction energy of probe molecules adsorbed on the acidic centers were calculated and used to measure the BrQnsted acid strength. Through ONIOM2 method, the influence of framework of zeolite was investigated. The adsorption of ethene molecule on the protonic acid center of Tl site was also studies by ONIOM2 method. The main results in this dissertation are summarized as follows:1. Through our calculations it was confirmed that the acidity in NNNN-substitution is stronger than NNN-substitution. For NNN-structure, HAW-HAW model shows a slightly higher acidity than HA11-HA11 model. For NNNN-structure, if two protons reside outside of two Al atoms, the acidity is the strongest; in other cases, if the second hydrogen is positioned between two Al atoms, the acidity decreases.2. Three nitrogenous probe molecules (N2, NH3, C5H5N) were selected to study the adsorption on the Bronsted acid sites in NNN- and NNNN-positions located at the 12 member-ring in MCM-22 zeolite. The calculated results of zeolites acidity in NNN-position strongly depend on the basicity and the size of selected probe molecules. The adsorption of N2 molecules on the neighbor acid sites shows no difference with the single acid site, and the adsorption type is weak... |
