A THEORETICAL-STUDY OF ALUMINUM SUBSTITUTION IN MFI ZEOLITES

The substitution of aluminium for silicon in sodium and proton forms o f MFI zeolites has been studied by theoretical force field calculation s. The parameters of the cff91_czeo force field developed by Hill and Sauer for zeolites were adopted in this calculation. The molecular str ucture was optimized according to an energy minimization algorithm wit h the Newton-Raphson method. The results indicate that there seems lit tle energetic preference for aluminium location in Na-MFI, and that th e smaller is the Si-O-Al angle, the smaller is the proton-aluminium su bstitution energy in H-MFI. The acidity is related to the framework st ructure and the strong acid sites are correlated with large Si-O-Al an gles. These conclude that the electrostatic interaction plays a minor role in aluminium substitution and the locations of aluminium atoms an d of Bronsted acid sites depend mainly on the framework geometry.