杂原子MFI分子筛羟基结构与Brφnsted酸强度的研究

Theoretical studies of the relationship between the hydroxyl group structure and Brφnsted acid strength in the isomorphously substituted MFI zeolite

应用从头算方法和密度泛函理论计算杂原子MFI分子筛(Al,Ga,B)的Brφnsted(Br)酸结构与酸强度的关系.建立了2T桥羟基簇(T12-O24 H-Si(T12=Al,Ga,B))和末端硅羟基簇结构模型,应用质子亲合势,羟基的键长,羟基中氢的电荷数,羟基的离子性,相对电负性,氨的吸附热作为酸强度参数.计算结果表明:应用RBLYP方法和DNP基组对杂原子H-MFI分子筛(Al,Ga,B)的Br酸强度可以进行合理的预测,其强度顺序为:Si(OH)<B(OH)Si<Ga(OH)Si<Al(OH)Si.

The Bronsted（Br） acid sites have important influence on the activity of the MFI catalyst. The ah initio and density functional theory simulations were carried out to investigate the Br acid strength of a series of clusters that approximates a catalytic acid site within isomorphously substituted MFI. The calculations were performed with the 2T bridged hydroxyl cluster models （T12-O24H-Si where T12=A1,Ga,B） and terminal silanol groups. The proton affinity,the distance of OH bond,the charge on the proton,the ionicity of the OH bond, the relative electrophilicity, as well as NH3 adsorption energy are good indicator of acid strength. It is observed that with the RBLYP method and DNP basis set,the simulation results predict that acid strength increases in the order Si（OH）〈B（OH）Si〈Ga（OH）Si〈AI（OH）Si.