Copper- and silver-containing monolithic silica-supported preparations for selective propene-propane adsorption from the gas phase

Cylindrically shaped macroscopic-in-size bodies of aluminum-doped silica possessing uniformly sized mesopores have been used as supports for copper- and silver-based formulations in the selective adsorption of propene against propane in the gas phase. The metal addition was achieved using AgNO3 or Cu(NO3)(2) in aqueous solution by either postsynthesis incipient wetness impregnation or direct incorporation in the synthesis stage, the overall metal content in samples being 2, 5, 10, and 20 wt %. The effect of metal addition on the degree of Al insertion into the silica matrix was evaluated by Al-27 magic-angle spinning NMR. The nature of surface metal species was studied by X-ray photoelectron spectroscopy and temperature-programmed reduction by H-2. Individual adsorption of propene and propane was measured at room temperature, and the selectivity coefficient was calculated as a function of the pressure and molar fraction of propene in the binary gas mixture. Within the series of Cu-derivatized adsorbents, the equilibrium selectivity is a complex function of the nature, amount, and dispersion degree of the copper species available on the surface. Contrary to expectations, both Cu(I) and Cu(II) states may give good propene-propane selectivity performance. For Ag-containing adsorbents the selectivity is mainly related to the nature and distribution of the silver species exposed on the surface, showing little dependence upon the overall Ag(I) content. The shaped sample functionalized with only 5 wt % of silver in the synthesis stage (Ag5SiAl20) appears to be the best monolithic adsorbent since it combines good selectivity performance (selectivity ratio of 37) and high specific surface area (ca. m(2) g(-1) g-1). Compared to fine-particle porous materials previously reported in the literature, the monolithic adsorbent form is more useful for industrial uses.