Influence of the nature of the exchanged ion on n-hexane aromatization activity of Pt-M-ETS-10: Ab initio calculations on the location of Pt

The activity of Pt-M-ETS-IO samples exchanged with different alkali metals (M = Li, Na, K, Rb, or Cs) in the aromatization of n-hexane depends on the nature of the exchanged metal ion (M). A distinct relationship between the intermediate electronegativity (S-int) of the different metal-exchanged ETS -10 samples and benzene yield is reported, suggesting the activation of Pt by the basicity of the exchanged metal. Results of ab initio Hartree-Fock c alculations an cluster models representing Pt-M-ETS-10 corresponding to the presence of Pt nearer to [TiO6](2-) and [SiO4] groups reveal that the pres ence of Pt nearer to [TiO6](2-) is energetically more favored and results i n a gain of electron charge by Pt. The studies also suggest an increasing a mount of electron transfer from the model clusters of ETS-10 to the Pt with increase in the electropositive character of the exchanged metal. It appea rs therefore, that small Pt clusters residing nearer to [TiO6](2-) accessib le from the 12-MR channels are the likely active sites in n-hexane aromatiz ation over Pt-M-ETS-10. (C) 1999 Academic Press.