Interaction of Water with Brønsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes

Abstract

The adsorption of one and two water molecules on cluster models of Brønsted 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 Møller-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 1 H NMR chemical shifts change.

The heterogeneity of the hydroxyl groups in chabazite

“…For the open 7T clusters the difference almost disappears. All values for adsorption energies listed in Table II are within the range of previous theoretical studies, 8,10,14,17,19 from Ϫ45.6 to Ϫ92.5 kJ/mol. Geometries.…”

“…Cluster calculations as described above have been shown to give good results for the interaction of molecules with zeolites. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In the calculations of the shifts of the frequency of the zeolite OH stretch vibration the anharmonicity has been included. This has been done by representing the potential energy in the 1-dimensional vibrational Hamiltonian by a polynomial:…”

“…The adsorption of water and methanol has been studied both on clusters [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] representing a small part of the zeolite and in periodic calculations [19][20][21][22][23][24][25][26] including the zeolite crystal structure. The role of the zeolite structure on the activation of methanol has been studied in periodic approach but the conclusions are somewhat contradictory.…”

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…[50][51][52][53][54][55][56][57][58][59][60][61] Water and methanol are two crucial molecules during the MTO process. In some cases water is added to the methanol feed in the form of process condensate or steam to tune the product selectivity.…”

In situ solid state NMR observation of the methanol-to-hydrocarbons (MTH) process over nanosized and microsized HZSM-5 zeolites

“…14 ppm is due to the hydroxy groups of the methanol remaining on the catalysts and/or water adsorbed on Brønsted acid sites: water produced during the methanol transformation will be adsorbed on the Brønsted acid sites as ion pairs, which will result in a 14 ppm peak. 47 The signal at 6-10 ppm in Fig. 3b and g is characteristic of adsorption of water on the acid bridging OH groups of zeolites.…”