S.A. Zygmunt

An assessment of density functional methods for studying molecular adsorption in cluster models of zeolites

Abstract The performance of six density functional theory (DFT) methods has been tested for a zeolite cluster containing three tetrahedral atoms (3T) and the complexes it forms with water and methanol molecules. The DFT methods (BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91) give results in good agreement with second-order perturbation theory (MP2). The results in this paper provide evidence of the suitability of DFT methods for studying hydrogen-bonded adsorption complexes in zeolites. Generally, the hybrid DFT methods are in closer agreement with experiment and MP2 than the pure DFT methods for geometrical parameters. The only exception is the Z − geometry, perhaps due to its anionic character. All DFT methods give results in good overall agreement with MP2 for intramolecular geometrical parameters of the adsorption complexes, intramolecular vibrational frequencies, and adsorption energies. The B3LYP method gives intermolecular geometries and intermolecular vibrational frequencies which are closest to those obtained from the MP2 method. Thus, the B3LYP method seems to be the best choice for a density functional treatment of molecular adsorption in zeolite systems.

Evidence for dimeric and tetrameric water clusters in HZSM-5.

Temperature-programmed desorption shows that water desorbs from HZSM-5 (Si/Al = 13) {center_dot} nH2O in a two-step process. The amount of water evolved at each step corresponds to about two H2O per acidic proton, indicating that the tetrameric cluster formed at full loading consists of a strongly bound pair and a weakly bound pair of water molecules. Quantum mechanical studies using cluster models of aluminosilicate systems indicate that the desorption energies for the first and second water molecules of a dimer adsorbed at the acid site are 9.7 and 12.6 kcal/mol, respectively. These values are within the energy resolution of the TPD experiment and are consistent with both water molecules from the dimer seeming to desorb simultaneously. Evidence for a tetramer is consistent with earlier findings on dealuminated mordenites.

A theoretical study of the oxygen K absorption edge in cluster models of sodalite

Abstract The X-ray absorption near-edge spectrum (XANES) has been calculated for the oxygen K-edge in cluster models of pure silica sodalite and a related sodalite containing Al. Results are reported which describe the dependence of the calculated near-edge spectrum on cluster size as well as the presence of Al and the charge-balancing Na cations. Prominent features of the calculated spectra appear to be correlated with the underlying structure of the cluster models. In agreement with other calculations and measurements of near-edge structure in oxides, backscattering of the outgoing photoelectron from 0 atom neighbors is the dominant source of XANES structure, whereas backscattering from Si atoms has a relatively minor effect. Also calculated was the spectrum of a small cluster model of α-SiO 2 , which compared favorably with a previously reported experimental near-edge spectrum.

Computational studies of water adsorption in zeolites

We have performed high-level ab initio calculations using Hartree-Fock (HF) theory, Moller-Plesset perturbation theory (MP2), and density-functional theory (DFT) to study the geometry and energetics of the adsorption complex involving H{sub 2}O and the Bronsted acid site in the zeolite H-ZSM-5. These calculations use aluminosilicate cluster models for the zeolite framework with as many as 28 T atoms (T=Si, Al). We included geometry optimization in the local vicinity of the acid site at the MP2 and DFT levels of theory for the smallest cluster, while in the larger clusters this was done at the HF/6-31G(d) level of theory. We have also calculated corrections for zero-point energies, extensions to higher basis sets, and higher levels of electron correlation. Results for the adsorption energy and geometry of this complex are reported and compared with previous theoretical and experimental values.