应用理论计算研究MCM-22分子筛相邻酸性位上乙烯和苯的吸附

应用ONIOM计算方法研究了MCM-22分子筛超笼12元环上存在两个酸性位时的酸强度及其与骨架铝之间距离的关系,并研究了乙烯和苯分子吸附的规律.计算采用52T簇模型和B3LYP/6-31G**/MNDO方法.结果表明,存在两个酸性位且两个骨架铝之间间隔1个骨架硅时,酸强度比孤立的酸性位明显降低;当间隔的硅原子数增加时,酸强度呈上升趋势,间隔3个以上骨架硅时,其酸强度与孤立的酸性位几乎没有差别.对于乙烯的吸附,当两个骨架铝之间间隔1~4个骨架硅时,其吸附能几乎没有差别(31~35 kJ/mol);对于苯的吸附,当两个骨架铝之间间隔1个骨架硅时,其吸附能有所提高,因为两个桥羟基同时对苯分子产生氢键吸附作用.当两个骨架铝之间的距离增大时,苯的吸附能几乎相同(21~29 kJ/mol).若两个乙烯分子或苯分子同时吸附在双酸性位上,其吸附能与单个分子在孤立酸性位吸附时几乎没有差别.应用自然键轨道计算分析了吸附配合物的电子结构,进一步探明了乙烯和苯在分子筛酸性位上吸附的本质.

Study of Ethylene and Benzene Adsorption on Br(o)nsted Acid Sites in MCM-22 Zeolites by Theoretical Calculation

The ONIOM method was used to perform the computational study on the acid strength of double acid sites in the 12 member-ring of MCM-22 zeolite.The calculation was carried out by the B3LYP/6-31G**/MNDO method based on the 52T cluster model.The dependency of the acid strength on the interval distance of the framework Al atoms was examined,and the adsorption of ethylene and benzene on the adjacent acid sites was investigated.The calculation results indicated that when two acid sites existed near each other and spaced out by one Si atom, the acidity descends obviously with respect to the isolated acid site.If the Si atom number between two 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.For C2H4 adsorption,its adsorption energy in all cases is 31-35 kJ/mol,independent on the interval distance between the framework Al atoms.For C6H6 benzene adsorption,its adsorption energy on next nearest neighbour acid sites is higher than that on single acid site,and when the Si atom number is more than three,its adsorption energy is 2129 kJ/mol.The adsorption energy of either two C2H4 molecules or two C6H6 molecules at the adjacent acid sites is equivalent to that of single molecule on the isolated acid site.The natural bond orbital analysis for the adsorption complexes ascertained their electronic structure,and the nature of C2H4 and C6H6 adsorption on the acid sites was explored.