DENSITY-FUNCTIONAL STUDIES OF ZEOLITES .2. STRUCTURE AND ACIDITY OF [T]-ZSM-5 MODELS (T=B, AL, GA, AND FE)

We use local density functional theory to study the structure and acid ity of a series of molecules that model a catalytic site within zeolit e ZSM-5. We consider models of the form (X)(3)Si-O(H)-T(X)(3) where X = OH, OSiH3, and OSi(OH)(3) and T = Si, Al, B, Ga, or Fe. The theoreti cal predictions of the siliceous models agree well with the crystal st ructure of silicalite, the siliceous ZSM-5 analog. We show how increas es in the size of the zeolite model affect the calculated results. In particular, internal coordinates converge quickly with. increases in m odel size, while deprotonation energies do not converge within the ran ge of models tested. We also show the effects on structure and acidity caused by replacing Si with trivalent metals. The most accurate calcu lations predict acidity increases in the order [B]-ZSM-5 much less tha n [Fe]-ZSM-5 < [Ga]-ZSM-5 < [A]-ZSM-5, in agreement with experimental results. However, the correct prediction of the acidity trend requires both the largest models and extensive geometry relaxation. The calcul ations also elucidate the experimentally observed change from trigonal to tetrahedral coordination between the neutral and anionic forms of [B]-ZSM-5. This work demonstrates the great potential of density funct ional theory for the study zeolite structure, reactivity, and the subs titution of transition metals within the zeolite framework.