Quantum Chemistry Study On Catalytic Performance Of Ion-exchanged ZSM-5 Zeolite

ZSM-5 zeolite is a very important shape selective catalyst in petrochemical industry, such as:olefin cracking and isomerization. And it is very important to increase the profits of petrochemical industry. In this thesis, the structure, adsorption and catalytic performance of Ag-ZSM-5, La-ZSM-5 zeolites were investigated by using density functional theory (DFT) with cluster model. The main conclusions are summarized as follows:1,The Infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite were investigated by using density function theory (DFT). The changes of NBO charge show that the electron transfers from CO molecule to the Ag+ cation to form an a-bond, and accompanies with the back donation of d-electrons from Ag+ cation to the CO (π*) orbital as one and two CO molecules are adsorbed on Ag-ZSM-5. The free energy changes△G,-5.55kcal/mol and 6.52kcal/mol for one and two CO molecules, illustrated that the Ag+ (CO)2 complex is unstable at the room temperature. The vibration frequency of C-O stretching of one CO molecule bonded to Ag+ ion at 2211cm-1 is in good agreement with the experimental results. The calculated C-O symmetric and antisymmetric stretching frequencies in the Ag+(CO)2 complex are shifted to 2231 and 2205cm-1 when the second CO molecule is adsorbed. The calculated C-O stretch frequency in CO-Ag-ZSM-5-H2O complex shifts to 2199cm-1, the symmetric and antisymmetric O-H stretching frequencies are 3390cm-1 and 3869cm-1 respectively. The Gibbs free energy change (△GH2O) is-6.58kcal/mol as the H2O molecule adsorbed on CO-Ag-ZSM-5 complex at 298K. The results show that CO-Ag-ZSM-5-H2O complex is more stable at room temperature.2,The structure, acidity and stability of lanthanum species of modified zeolites have been investigated by using the density functional theory. The mechanism of 1-butene activation has also been studied with the effect of the channel wall in the zeolites. The acidity of acidic site slightly decreases when the lanthanum ions are introduced. The lanthanum species are very stable in modified zeolites even at the temperature of 650K. The average frequency of the O-H stretching in lanthanum species is 3758 cm-1, and the frequency of the O-H stretching in H-ZSM-5 is 3646 cm-1. The reaction path of more stable carbenium formation is considered. The activation barrier is 42.93 kcal/mol. There is noπ-complex presented in the adsorption states and the reaction mechanism is different from the catalytic cracking reactions on H-ZSM-5 zeolites.