Characterization and catalytic function of Re 0 and Re 4+ in [formula omitted] and [formula omitted] catalysts

Abstract

Electron spin resonance (ESR) spectroscopy and CO adsorption measurements have shown that Re 0 and Re 4+ species coexist on the surface of reduced 0.2 wt% Re Al 2 O 3 and 0.3−0.2 wt% Pt Re Al 2 O 3 catalysts. The catalysts were prepared by incipient wetness impregnation with metallic salt precursors, calcined in air at 500 °C, and dried in He at 500 °C prior to reduction. The Re 4+ species, which is stabilized by the Al 2O 3 and resists reduction up to 500 °C, yields an ESR signal which exhibits hyperfine splitting and accounts for less than 20% of the total Re in Re Al 2 O 3 and less than 10% of the Re in PtRe Al 2 O 3 . This species does not chemisorb CO but interacts weakly with O 2. The Re 0 does not give rise to a resonance signal, but adsorbs CO strongly. The double bond shift (DBS) of 1-pentene, hydrogenation of 1-pentene, and the skeletal isomerization (SI) of 3,3-dimethyl-l-butene were tested using mild reaction conditions. The results indicate that Re 4+ slightly enhances the Brønsted acidity in Re Al 2 O 3 , although this effect is negligible in comparison with the acidity induced by Cl − ions. The Re 0 sites are very active for olefin hydrogenation; however, after presulfiding the Re Al 2 O 3 , this hydrogenation activity is drastically lowered, but the DBS activity is still high which indicates that surface ReS species are active for the DBS at the Conditions employed.