QUANTUM CHEMICAL STUDY ON STRUCTURAL ACIDITY OF HZSM-5 ZEOLITE

The basic units of the framework structure of HZSM-5 zeolite are the five-membered rings which are linked by bridging oxygen atoms and as a result form units of four-membered rings or six membered rings. Semiempirical CNDO/2 quantum chemical method is used to calculate the charge density distribution for all these basic units, i.e., 4-, 5- and 6-membered rings of HZSM-5 zeolite. Quantitative relations between the structural parameter α_0 of the acidity of HZSM-5 zeolite and the number of aluminum atoms N_(Al) per unit cell are obtained. Zeolite may be considered as solids acting more or less as protonic polyacids in solution. This protonic acidity is expressed as activity, a_(H~+)= [H~+]f_(H~+) ,were =n_(H~+)aN_(Al)/Z_(cat) and the activity coefficient f_(H~+) is equal to a constant multiplied by the structural parameter, f_(H~+)= Kα_0, K is taken to be 32. Upder different (SiO_2/Al_2O_3) ratios, therefore, the protonic activities could be obtained theoretically, and the calculation results are essent