Interaction of water with Bronsted acidic sites of zeolite catalysts. Ab initio study of 1:1 and 2:1 surface complexes

The adsorption of one and two water molecules on cluster models of Bronsted acid sites of zeolite catalysts has been investigated by ab initio quantum chemical methods at the Hartree-Fock self-consistent field (HF), at the second-order Moller-Plesset perturbation theory (MP2), and at the density functional theory (DFT) levels. Among the two possible structures of the 1:1 adsorption complex, the water H-bonded to the zeolitic OH group (neutral complex) and the hydroxonium ion attached to the negatively charged zeolite surface (ion pair), only the former is a minimum. The ion pair complex is a transition structure for the proton transfer from one lattice oxygen to a neighboring one via the adsorbed water. However, the energy difference between both structures is only a few kJ/mol. For the neutral 1:1 adsorption complex we predict an average shift of the three protons involved of 7-8 ppm; the observed shifts are 6-7 ppm for one water molecule per site. The vibrational frequencies calculated for the ion pair structure do not permit an interpretation of the observed infrared spectrum. For the neutral structure (MP2) we predict frequencies of 1317 and 1022 cm(-1) for the zeolitic in-plane and out-of-plane modes, respectively, while the zeolitic OH stretching mode is strongly red-shifted down to 2740-2850 cm(-1). These data support a recent interpretation of the IR spectrum which explains the observed triplet of bands as a result of Fermi resonance between the strongly perturbed zeolitic OH stretch and the OH bending overtones. The MP2 calculations for the neutral complex also provide a complete assignment of the peaks observed by inelastic neutron scattering for water on H-mordenite. Inclusion of electron correlation proves crucial, and comparison of MP2 and DFT (gradient corrected functionals) methods is made. While energy differences are very similar, the DFT approach yields by far too large frequency shifts for OH donor groups in H bonds, When a second water molecule is added (2:1 complex), both the neutral and the ion pair structure prove to be local minima on the potential energy surface. The adsorption energy is found to drop by 25%, and the ion pair structure becomes the more stable one. Predictions are made on how the vibrational spectra and the H-1 NMR chemical shifts change.