Microporous cesium salts of tetravalent Keggin-type polyoxotungstates Cs4[SiW12O40], Cs4[PW11O39(Sn-n-C4H9)], and Cs4[PW11O39(Sn-OH)] and their adsorption properties

Microporous cesium salts of modified and unmodified tetravalent Keggin-type polyoxometalates, including Cs4[SiW12O40], Cs4[PW11O39(Sn-n-C4H9)], and Cs4[PW11O39(Sn-OH)], were synthesized. The crystalline structures, which had body-centered cubic (bcc) arrangements, the lattice constants, and the pore-size distributions of the three Cs salts were similar, regardless of the presence or absence and types of functional groups introduced. The Cs salts had only micropores and no mesopores. The micropore size distributions were determined from adsorption isotherms of Ar, which showed a sharp peak at 0.59 nm and a shoulder at 0.62 nm. The fractions of the external surface areas to the total surface areas of the Cs salts were less than 6%. It is plausible that the micropores originate from the heteropoly anion defects in the crystallite, which form to avoid mismatches in the Cs+/(heteropoly anion) ratio required for charge balance (=4) and for a bcc structure (=3). The surface of the Cs salt introduced with n-butyl groups was hydrophobic, although the surface density of the n-butyl groups was low. On the other hand, the hydroxyl groups present on the surface of Cs4[PW11O39(Sn-OH)] had little effect on the adsorption of water, methanol, ethanol, and hexane but a great impact on that for benzene due to the interactions between the -OH groups and the aromatic rings (-OH···π).

Tetravalent Keggin-type polyoxotungstates.Figure optionsDownload as PowerPoint slideHighlights
► Cesium salts of tetravalent Keggin-type polyoxotungstates had only micropores.
► The micropores originated from the polyoxotungstate anion defects in the crystallite.
► The surface of the cesium salt introduced with n-butyl groups was hydrophobic.