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 acidity of a series of molecules that model a catalytic site within zeolite 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 theoretical predictions of the siliceous models agree well with the crystal structure of silicalite, the siliceous ZSM-5 analog. We show how increases in the size of the zeolite model affect the calculated results. In particular, internal coordinates converge quickly with. increases in model size, while deprotonation energies do not converge within the range of models tested. We also show the effects on structure and acidity caused by replacing Si with trivalent metals. The most accurate calculations predict acidity increases in the order [B]-ZSM-5 much less than [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 calculations 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 functional theory for the study zeolite structure, reactivity, and the substitution of transition metals within the zeolite framework.