Structures and potential energy surface of faujasitic zeolite/water

Abstract

The structures and the potential energy surface of the system faujasitic zeolite/water have been investigated by Hartree-Fock, second-order Moller-Plesset (MP2) and by the density functional theory (DFT) calculations, using five basis sets 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p) and 6-311 + G(d,p). The DFT calculations employ the Becke-3-Lee-Yang-Parr (B3LYP) and Becke-Lee-Yang-Parr (BLYP) density functional, and, for comparisons, the local density approximation with the Vosko-Wilk-Nusair (VWN) functional. The B3LYP approach is found to yield better agreement with the corresponding experimental results than the VWM and BLYP functionals. The B3LYP amd MP2 levels of theory yield basically the same results. Results of B3LYP with a 6-311 + G(3df,2p) basis set are also very close to those of the very accurate coupled pair functional (CPF) method. Also proton affinities (PA) computed by B3LYP reproduce the corresponding CPF and G1 results very well. The predicted PA of faujasitic catalyst is estimated to be 294 ± 3 kcal/mol, which is in the range of the experimentally determined value of 291–300 kcal/mol. The interaction of faujasite catalyst with water has revealed that the structures can be stabilized by the formation of two hydrogen bonds with water molecules adsorbed at the bridging hydroxyl groups which can act either as a proton acceptor or as a proton donor. Comparison of the faujasite complexes with silanol and hydrogen halides has demonstrated that the faujasitic zeolite is a strong acid. The potential energy surfaces of faujasite zeolite/water has been investigated and analytical interaction potentials have been derived.