AB-INITIO PERIODIC HARTREE-FOCK INVESTIGATION OF A ZEOLITE ACID SITE

We have studied theoretical models of a zeolite Bronsted acid site using ab initio periodic Hartree-Fock (PHF) theory. We present results for both one and two acid sites inside a sodalite structure (beta-cage). We optimized the geometry of the bridging hydroxyl groups using the STO-3G basis set. All other reported crystal properties were evaluated using the more complete 6-21G* (in which the outermost orbital exponents were reoptimized for the solid) level of theory. The results of the periodic calculations are compared to available experimental data, similar periodic calculations of silica sodalite in which there are no acid sites, and calculations of clusters that mimic parts of the periodic structure. The optimized geometry of the single bridging hydroxyl group has internal coordinates of Si-O = 1.64 Angstrom, Al-0 = 1.72 Angstrom, O-H = 0.97 Angstrom, Si-O(H)-Al = 136.3 degrees, and Si-O-H = 110.7 degrees, and the hydrogen is bent 25.2 degrees out of the Si-O-Al plane. The optimized geometry of the bridging hydroxyl group in the two-acid site model is very similar, with Si-O = 1.65 Angstrom, Al-0 = 1.72 Angstrom, O-H = 0.97 Angstrom, Si-O(H)-Al = 135.5 degrees, Si-O-H = 112.7 degrees, and an out-of-plane angle of 21.5 degrees. We find that similar geometries can be obtained using isolated clusters that mimic the periodic lattice; however, there are significant differences. In particular, the optimized values of the internal coordinates oscillate with increasing cluster size, giving little confidence in our ability to determine the size of the cluster needed to obtain convergence. We also compare our results to a PHF calculation of the pure SiO2 lattice (devoid of the acid site) to illustrate the changes in electronic properties that occur due to the introduction of the acid site.