THEORETICAL DETERMINATION OF RELATIVE ACIDITY IN ZEOLITE (FAUJASITE)

A series of ab initio calculations on clusters H3Si-OH-AlH3 and (OH)3Si-OH-Al(OH)3 modeling zeolite systems is performed in order to investigate their acidic properties. Comparison of interaction energies of these clusters with those of various base compounds allows one to establish a scale of acidity. Proton transfer between the clusters and base species OH2, SH2, NH3, and PH3 are also examined. At our level of accuracy, the calculations fail to reproduce the formation of ionic species. In the case of NH3 the complex H3Si-OH-AlH3..NH3 is embedded into a discrete distribution of electric charges. Calculation at the SCF level taking account of this electrostatic perturbation favors the formation of NH4+.