Shape-selective oxidation catalysed by a Pt-promoted ultramicroporous heteropoly compound

Abstract

Pt-promoted porous heteropoly compounds, Pt-Cs_xH_3-xPW₁₂O₄₀, have been synthesized and their micro- and meso-pore size distributions determined by adsorption of variously sized molecules and the Dollimore–Heal N₂ isotherm method, respectively. 0.5 wt.% Pt-Cs_2.1H_0.9PW₁₂O₄₀ showed a Type I N₂ adsorption isotherm with a plateau, indicating the presence of nearly uniform micropores; 1.0 wt.% Pt-Cs_2.1H_0.9PW₁₂O₄₀, 1.5 wt.% Pt-Cs_2.1H_0.9PW₁₂O₄₀ and 0.5 wt.% Pt-Cs_2.5H_0.5PW₁₂O₄₀ gave Type IV isotherms, as usually observed for mesoporous materials. Adsorption of molecules such as N₂ [molecularsize (MS) = 0.36 nm], n-butane (MS = 0.43 nm), isobutane (MS = 0.50 nm), benzene (MS = 0.59 nm), 2,2-dimethylpro-pane (MS = 0.62 nm), and 1,3,5-trimethylbenzene (MS = 0.75 nm) demonstrated that the pore width of 0.5 wt.%Pt-Cs_2.1H_0.9PW₁₂O₄₀ was in the range 0.43–0.50 nm and that the external surface area was very small. Owing to the restricted pore size, 0.5 wt.% Pt-Cs_2.1H_0.9PW₁₂O₄₀ exhibited efficient shape selectivity for catalytic oxidation; smaller molecules such as methane (MS = 0.38 nm) and CO (MS = 0.38 nm) were readily oxidized, whereas benzene (MS = 0.59 nm) was not oxidized. This efficient shape selectivity indicates that Pt is present exclusively in the pores and not on the external surface of 0.5 wt.%Pt-Cs_2.1H_0.9PW₁₂O₄₀.