Pore structure and shape selectivity of platinum-promoted cesium salts of 12-tungstophosphoric acid

摘要

Pore-widths and pore-size distributions of 0.5 wt% Pt-Cs_xH_(3-x)PW_12O_40 have been studied by means of adsorption of various molecules. For the distributions of micropore and mesopore, isotherms of Ar and N_22 adsorption were analyzed, respectively. Pt-Cs_2.1H_0.9PW_12O_40 possessed only ultramicropores. On the other hand, the pores of Pt-Cs_xPW_12O_40 (x = 2.3, 2.5, 2.8 and 3.0) showed bimodal distributions in the range from micropore to mesopore, and the widths of both pores tendd to increase as the Cs content increased. From the amounts and rates of adsorption for n-butane and isobutane, the pore width of Pt-Cs_2.1H_0.9PW_12O_40 was determined to be close to the molecular size of n-butane, that is, 0.43 nm. The fraction of external surface area in the total surface area of Pt-Cs_2.1H_0.9PW_12O_40 was estimated to be only 0.06 from the adsorption of 1,3,5-trimethylbenzene and t-plot of N_2 adsorption. Pt-Cs_2.1H_0.9PW_12O_40 exhibited a shape selectivity due to the uniform ultramicropores and small external surface area; it catalyzed the oxidation of n-butane but not that of isovbutane. SEM and TEM measurements revealed the primary crystallites and their aggregated states.