The interactions of sorbates with gallosilicates and alkali-metal exchanged gallosilicates |
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Authors: | J Limtrakul M Kuno P Treesukol |
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Institution: | Laboratory for Computational and Applied Chemistry, Chemistry Department, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand |
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Abstract: | Structures, energetics and vibrational frequencies of the interaction of adsorbates with H-aluminosilicates (H-AlZ), H-gallosilicates (H-GaZ), alkali-metal exchanged aluminosilicates (X-AlZ) and alkali-metal exchanged gallosilicates (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 compensating alkali-metal 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, ΔENSE, of the naked alkali-metal/H2O adducts with those of the alkali-metal 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 zeolitic oxygen frameworks surrounding the cations. The interaction of sorbates with the alkali-metal exchanged gallosilicates can be employed to probe the field strength inside the catalytic frameworks as indicated by the plot of the binding energy, ΔE, versus 1/RX–Ow2, with R(X–Ow) being the distance between the cationic nucleus and the oxygen atom of the adsorbate. 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 IR spectra of water adsorbed on Na-ZSM-5 of Zecchina et al. (J. Phys. Chem., 100 (1996) 16 484). Other important features, i.e. the correlation between ΔνOH and, ΔE, R(X–Ow), and 1/RX–Ow2, cationic size, demonstrate that the interactions of sorbates with alkali-metal exchanged gallosilicates are well approximated by electrostatic contribution. |
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Keywords: | Gallosilicates Alkali-metal exchanged gallosilicates B3LYP and HF calculations Zeolites Aluminosilicates |
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