Quantum Chemistry Investigation Of Adsorption In Type-A Zeolite

Type-A zeolites are widely utilized in catalysis, adsorption and the separation of gases because of their regular micro-pore strcture and the different interaction with the sorbates of different size and configurations. The interaction effect of zeolites and the sorbate, pore structure of zeolites and orientation of the sorbate in the zeolites are very important to their adsorption characters and potential catalytic activity. But up to date, the information of the sorbate in the zeolites has not been obtained precisely by experiments. So a mass of the theoretical investigations, including quantum chemistry computation, have been brought into existence. It is known from present literatures that the cluster consisted of tetrahedron of silicon and aluminum which is near the adsorption position has been used as the computation model. The enlarging zeolite configuration and its basic structure cell -βcage were not considered. In this paper, by using Gaussian software, the eight-membered ring cluster andβcage cluster of type-A zeolites were constructed and calculated. The structure, stability, charges and frequencies of the clusters of Si₂₄O₃₆; Si₁₂Al₁₂O₃₆ and Si₁₂Al-(12)O₃₆^12- were analysed in order to investigate the interaction effect of zeolites and the sorbate. The distribution characters of the sorbate in zeolites were provided in theory which contributes to guiding the modification, characterization, development and design of molecular sieves.Gas chromatography is utilized to modify and characterize the thermodynamic parameters of the sorbate in the zeolites for its simplicity and acceptable accuracy. In this paper the adsorption energy, entropy and Gibbs free energy of type-A zeolite were obtained by using gas chromatography. The computational results were attested by the gas chromatography experiments. The concrete results are as follows:1. By means of quantum chemistry calculation, the three kinds of clusters ofβcage were analyzed with their structure and energy. From the optimized configuration, geometry parameter, distributing of Mulliken charge, frontier molecular orbital energy and total energy, the conclusion were made out that in theβcage when silicon was replaced by aluminium at the ratio of 1to1 and the charge was increased, the bond length of silicon and oxygen and the distance of atom aluminum were increased. It was showed that the structure cell energy of type-A zeolite was increased and the stability was decreased. The Si-O binary system was the steadiest by analyzing the results.2. The models of eight-membered ring of the type-A zeolite with probe molecules - nitrogen, carbon monoxide and water - were simulated and calculated by quantum chemistry method. The concrete results are followings:(1) The bond length of nitrogen on the type-A zeolite was not changed. But the bond length of carbon monoxide on the type-A zeolite was changed a little. The framework of the model of the type-A zeolite and carbon monoxide was also changed a little. There were strong interactions between water and the model of the type-A zeolite. There was a tendency of forming bonds between the hydrogen atom of water molecule and the oxygen atom in the model. So the framework of the type-A zeolite was changed visibly.(2) The Mulliken charges of silicon and aluminium were not changed when nitrogen and carbon monoxide were put on the model of the type-A zeolite. But the Mulliken charges of silicon and aluminium near water were declined visibly.(3) The adsorption heat values of nitrogen, carbon monoxide and water on the eight member ring of the type-A zeolite were -26.8kJ/mol, -28.7kJ/mol and -81.8kJ/mol respectively. The data showed that the adsorption of nitrogen, carbon monoxide and water on type-A molecular sieve belonged to physical reaction.3. The clusters ofβcage of the type-A zeolite with probe molecules -nitrogen, carbon monoxide and water - were simulated and calculated by quantum chemistry method. The concrete results are as follows:(1) The distance between atmoes of nitrogen, carbon monoxide and water and silicon was nearer in the type-A zeolite than aluminium was. The distance is longer than the bond length of the normal covalent bond. It showed that the forms of the complex formation were the result of effect of Van de Waals force. So the adsorption belonged to the physical reaction.(2) The Mulliken charge ofβcages showed that the distributing of the charge was more uniform when silicon was replaced by aluminium at the ratio of 1to1 and the charge was increased. The stability of the systems was stronger.(3) The sequence of the LUMO energy value of the frontier molecular orbital energy of three kinds ofβcages was Si₁₂Al₁₂O₃₆ < Si₂₄O₃₆ < Si₁₂Al₁₂O₃₆^12-. The LUMO energy value ofβcages of Si₁₂Al₁₂O₃₆ and Si₂₄O₃₆ showed that they could accept charges.(4) The data of entropy value of nitrogen, carbon monoxide and water were decreased when they were capsuled inβcages. It indicated that the type-A zeolite could adsorb small molecules availably.4. The conclusions of the investigation of quantum chemistry were validated by the parameters of the gas chromatography exprement. The quantum chemistry method and group of eight-membered ring were accurate. But that of theβcage will be ameliorated in the future.