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

Abstract

The activity of Pt-M–ETS-10 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 calculations on cluster models representing Pt-M-ETS-10 corresponding to the presence of Pt nearer to [TiO₆]²⁻ and [SiO₄] groups reveal that the presence of Pt nearer to [TiO₆]²⁻ is energetically more favored and results in a gain of electron charge by Pt. The studies also suggest an increasing amount of electron transfer from the model clusters of ETS-10 to the Pt with increase in the electropositive character of the exchanged metal. It appears therefore, that small Pt clusters residing nearer to [TiO₆]²⁻ accessible from the 12-MR channels are the likely active sites in n-hexane aromatization over Pt-M-ETS-10.