Polymer-Induced Selectivity Enhancement in the Hydrogenation of 2-Hexyne Catalyzed by Poly(vinylpyrrolidone)-Stabilized Platinum Colloids in an Amorphous Mixed Metal Oxide Support

Poly(vinylpyrrolidone) (PVP)-stabilized Pt clusters (1.4 nm average particle size) were embedded in an amorphous microporous titania-silica mixed oxide (AMM-(Si30Ti)-Si-Me) by means of a modified sol-gel procedure. The Pt particles were shown by transmission electron microscopy to retain their approximate original average diameter. The catalytic activity of the Pt-colloid-containing titania-silica mixed oxide catalysts was tested with the hydrogenation of 2-hexyne at 1 bar in decane or 1-propanol in a batch reactor. Selectivities to cis/trans-2-hexene were higher than those for PVP-stabilized colloids dispersed in the liquid phase. In colloidal dispersions of the PVP-stabilized catalyst in propanol, an initial a-hexene selectivity of 80% at a conversion of < 10% fell to ca. 45% at 100% conversion. With the AMM-supported catalyst, a 2-hexene selectivity of > 94% was observed at hexyne conversions up to 50%, and at complete conversion a selectivity to 2-hexenes of 88% was achieved (cis-2-hexene selectivity of 91%), Thermal treatment of the platinum-containing AMM-(Si30Ti)-Si-Me in an oxygen atmosphere leads to the removal of the protective PVP shell surrounding the colloids inside the sol-gel material, as detected by C-13-MAS NMR spectroscopy, resulting in an increase in both surface area and Pt particle size. The resulting catalysts showed lower selectivity for 2-hexene. A control experiment showed that the presence of PVP was responsible for the high selectivities to cis/trans-2-hexene exhibited by the AMM-supported colloidal Pt catalysts.