The interactions of sorbates with gallosilicates and alkali-metal exchanged gallosilicates

Structures, energetics and vibrational frequencies of the interaction of ad sorbates with H-aluminosilicates (H-AIZ), H-gallosilicates (H-GaZ), alkali- metal exchanged aluminosilicates (X-AlZ) and alkali-metal exchanged gallosi licates (X-GaZ), where X being Li, Na, or K, have been carried out at B3LYP and HF levels of theory with 6-31G(d) as the basis set. The charge compens ating alkali-melal ions can affect the catalytically active site (=Si-O-T= where T = Al or Ga) by weakening the Si-O, Al-O, and Ga-O bonds as compared to their anionic frameworks. Comparing the net stabilization energies, Del ta E-NSE, of the naked alkali-metal/H2O adducts with those of the alkali-me tal exchanged zeolite/H2O systems, the latter amounts only to about 50% of the former, which is partly due to the destabilizing role of the negative z eolitic oxygen frameworks surrounding the cations. The interaction of sorba tes with the alkali-metal exchanged gallosilicates can be employed to probe the field strength inside the catalytic frameworks as indicated by the plo t of the binding energy, Delta E; versus 1/R-X-Ow(2), with R(X-Ow) being th e distance between the cationic nucleus and the oxygen atom of the adsorbat e. The IR spectra of H2O adsorbed on Na-AlZ are calculated to be 3584, 3651 , and 1686 cm(-1). The obtained results are in excellent agreement with the very recent experimental LR spectra of water adsorbed on Na-ZSM-5 of Zecch ina et al. (J. Phys, Chem., 100 (1996) 16 484). Other important features, i .e. the correlation between Delta nu(OH) and, Delta E, R(X-Ow), and 1/R-X-O w(2), cationic sizer demonstrate that the interactions of sorbates with alk ali-metal exchanged gallosilicates are well approximated by electrostatic c ontribution. (C) 1999 Elsevier Science B.V. All rights reserved.