Influence of cation exchange on M-Pt-ETS-10 molecular sieve: correlation between ab initio results, catalytic activity, and physicochemical investigations

Abstract

The electronic property of Pt supported on cation-exchanged Pt-M-ETS-10 [where M = Li, Na, K, Rb, Cs, Mg(OH), Ca(OH), Sr(OH), and Ba(OH) ions] depends on the location of Pt and the nature of the exchanged metal ion. Electronic changes on the Pt cluster are highly influenced when it is near TiO₆ rather than SiO₄ units. The benzene selectivity in the transformation of n-hexane over Pt-M-ETS-10 molecular sieves is found to correlate with the basicity of the exchanged cations and the average electron density of Pt. The relationship between electron density on Pt and the amount of CO₂ adsorbed (from TPD) and the frequency of the ν₃ band of CO₂ adsorbed in FTIR is established. We report here the results of ab initio Hartree-Fock calculations on model clusters representing the Pt (active) site, molecular sieve with cations, Pt-M-molecular sieve, benzene, and H₂S adsorbed over Pt-molecular sieve. Generally, the charge density on Pt decreases in the order Cs > Rb > K > Ba(OH) > Na > Sr(OH) > Ca(OH) > Li > Mg(OH) and indicates that the electron transfer is from the support to the Pt. Catalytic activity of Pt-M-ETS-10 corroborates well and explains the experimentally observed higher activity as compared to commercial Pt-Al₂O₃ catalyst. The larger benzene yield in the case of Pt supported over basic zeolites could be attributed to the ease of desorption of benzene, and it is supported by a decreasing binding energy of benzene from Li to Cs and Mg(OH) to Ba(OH) in Pt-M-ETS-10. The electron density on Pt decreases drastically in the presence of sulfur, in the order Cs < Rb < K < Na < Li and Ba(OH) < Sr(OH) < Ca(OH) < Mg(OH).