Microporous heteropoly compounds and their shape selective catalysis

Abstract

Recent development for shape selective catalysis by microporous heteropoly compounds has been reviewed. Synthesis and characterization of microporous heteropoly compounds with different cations and compositions have been described first. Pore-size distributions of the heteropoly compounds and their Pt-promoted bifunctional catalysts have been determined by means of adsorption of Ar and N2. It was revealed that M2.1H0.9PW12O40 and 0.5wt.% Pt-M2.1H0.9PW12O40 (M=Cs, Rb, and K) possessed only ultramicropores, while the pores of Pt-CsxH3−xPW12O40 (x=2.3, 2.5, 2.8 and 3.0) showed bimodal distributions in the range from micropore to mesopore. From the adsorption of various molecules including n-butane and isobutane, the pore width of Pt-Cs2.1H0.9PW12O40 was determined to be close to the molecular size of n-butane, that is, 0.43nm. The fraction of external surface area in the total surface area of Pt-Cs2.1H0.9PW12O40 was estimated to only 0.06. The micropore-sizes of Rb2.1H0.9PW12O40 and Pt-Rb2.1H0.9PW12O40 were estimated to be about 0.6nm. Cs2.1H0.9PW12O40 and Pt-Cs2.1H0.9PW12O40 exhibited reactant shape selectivity for acid-catalyzed reaction, and hydrogenation and oxidation of smaller molecules, respectively, due to the uniform ultramicropores and small external surface areas. Furthermore, Pt-Rb2.1H0.9PW12O40 exhibited shape selectivity for hydrogenation of aromatic compounds.