Study Of The Adsorption On External Br(?)nsted Acid Sites In H-MCM-22 Zeolites By Theorical Calculation

Density functional theory calculation was performed for study on the acid strength of two acid sites in the 12 member-ring of MCM-22 zeolite. The dependency of the acid strength on the spacial distances of the framework Al atoms and the adsorption of ethylene and benzene on the adjacent acid sites were investigated. All calculations were mainly based on 52T cluster model including the 12 member-ring. The geometry optimizations were performed by using ONIOM2 method. The adsorptions of different probe molecules on T1 sites of the 12 member-ring were studied. The influences of different intercepting strategies on the calculation results were discussed. In addition, the natural bond orbital analysis was used to explore the electronic structures and molecular orbital of the adsorption complexes. The calculation results indicated that:1 . For the case of two Br?nsted-acid sites existed in the 12-member-ring, if one Si atom exists between two framework Al atoms, the acidity descends obviously. If the Si atom number between the framework Al atoms increases, the acidity rises up. When the Si atom number is more than three, the acid strength reaches a steady value, being equivalent to the isolated system.2.For ethylene adsorption, its adsorption energy of all cases are 8 kcal/mol, independent on the spacial distance between the framework Al atoms. For benzene adsorption, its adsorption energy on next nearest neighbour acid sites is higher than that on single acid site, and if the Si atom number is more than three, its adsorption energy is between 5~7 kcal/mol. The adsorption energy of either two ethylene molecules or two benzene molecules on the adjacent acid sites is equivalent to that on the single acid site.3.By using ONIOM2 method to investigate the adsorption complex on T1 site, different intercepting strategies lead to variable calculation results. With the active framework oxygen atom as the central atom, the high layer has to contain at least three shells of framework atoms for a reasonable result. If the high layer is too small, which do not include the important active atoms, it will lead to lower results. The atoms in low layer are responsible for the non-bond interaction, the number of low layer atoms show minor influence on the calculation results. In addition, the intercepting strategy can influence the calculation results. For high layer, the dangling atom of silicon leads to smaller potential energy4 . Density functional theory calculation was performed for investigation 8T class model of the adsorption of basic probe molecules on the Br?nsted acid sites of H-MCM-22 zeolite. The hydrogen bonding interaction fin the adsorption complexes was confirmed by using natural bond orbital analysis. It was found that for the weak basic probe molecules such as nitrogen, carbon monoxide, ethylene, and benzene, the adsorbed molecules donate long pair electrons orπorbital electrons into the unoccupiedσ* orbital of the bridging O-H group. For the stronger basic molecules such as ammonia and pyridine, the acidic proton is transferred to the probe, forming a conjugated acidic cation and the zeolite anion. In this case the strong hydrogen bonding is contributed by the electron density donation from the long pair orbital of framework basic oxygen atoms to the unoccupied N-Hσ* orbital. In addition, there exists obvious electron donation from the long pair orbital of framework basic oxygen to the unoccupied molecular orbital of adsorbed molecule, causing the probe molecule deflecting toward the neighboring basic oxygen.