J. Phys. Chem. 1993, 97, 5897-5900
5897
Ab Initio Study of CO Adsorption on Zeolites Simon Bates and John h e r ’ Department of Chemistry, University of Manchester Institute of Science & Technology, P.O. Box 88, Sackville Street, Manchester M60 I QD. U.K. Received: January 20, 1993; In Final Form: March 17, I993
Ab initio self-consistent field Hartree-Fock (SCF-HF) calculations have been performed, using the 6-3 l G * basis set, to investigate the nature of the complexes formed between carbon monoxide and three cluster models, chosen to represent the acid sites in zeolites: Al(0H)j for a Lewis site, H$i(OH)A1H3 for a Brornsted site, and H3SiOH for a terminal hydroxyl. Three types of interaction with CO are considered for each model: through the carbon lone pair, through the oxygen lone pair, and through the *-electron cloud. The geometries of the complexes are fully described. HF interaction energies are calculated, and corrections are made for BSSE and electron correlation (MP2 method). Stretching frequencies for C O and the model hydroxyls are determined. The theoretical data are compared to the experimental data available in the literature.
Introduction There is a great deal of effort, both experimentall-ll and theoretical,l~5J1-15 invested in trying to understand the nature and propertiesof acid sites in zeolites, by using “probe”adsorbate molecules, since these sites are responsible for the catalytic properties of zeolites. Experimental techniques, such as Fourier transform infrared spectroscopy (FTIR), have been used with a range of probe molecules, including hydrogen,l-3sll nitrogen,4 and carbon monoxide.5-10 Theoretical methods, both ab initiol~5~11-15 and semiempirical? have been used to study the interactions of probe molecules such as hydrogenl3l19l4J5 and carbon monoxides with various cluster models. In this paper, we present ab initio self-consistent field HartreeFock (SCF-HF) results for the interaction of carbon monoxide with cluster models chosen to represent the acid sites present in zeolites: Lewis sites, Brornsted sites, and terminal hydroxyls. We also discuss these results in light of the published experimental data, to assess the effectiveness of the theoretical methods used.
Calculation Details and Models All calculations were performed on the Amdahl Vector Processor VP1100 at the Manchester ComputingCentre, running the GAUSSIAN9Ol6suite of programs. The standard 6-31G* basis set was used for the calculations. Supplementary calculations using the 6-3 1G** basis set, which includes polarization functionson the hydrogen atoms, produced SCF binding energies that differed little from those obtained using 6-3 1G*. This result has also recently been observed in a study of Lewis acid-base complexes of AlCl3.17 Geometries were optimized using the Bernyl8 gradient optimization method, which tests convergence on maximum/RMS internal coordinate forces (
