HYDROGENATION CATALYSIS BY THERMALLY ACTIVATED SILICA

Conversion of alkenes to alkanes under D-2 or H-2 (5.5 MPa cold pressu re) at temperatures as low as 200-350 degrees C is promoted by thermal ly-activated fumed silica. There is no reaction in the absence of sili ca and its activity is increased by the thermal pretreatment (330 to 4 30 degrees C for 16 h under Ar). Stilbene (1,2-diphenylethene) treated as described with D-2 gives 1,2-diphenylethane consisting mainly of m olecules containing two atoms of deuterium at aliphatic sites (H-2 NMR ). At 250 degrees C diphenylacetylene is converted to stilbene-d(2) wh ich is predominantly the cis-isomer (cis/trans > 3) showing that the h ydro enation is stereoselective. Other compounds for which hydrogenati on is promoted include 1-nonene and anthracene. In some cases, hydroge nation of isolated aromatic rings is observed. For example, 1,2-diphen ylethane in the absence of stilbene gives 1-cyclohexyl-2-phenylethane along with some 1,2-dicyclohexylethane. It is shown that surface OH gr oups on the silica are converted to OD by D-2 under the reaction condi tions.