Hydrodeoxygenation of guaiacol into cyclohexane over mesoporous silica supported Ni-ZrO2 catalyst

Ordered mesoporous silica SBA-15 and KIT-6 supported Ni/ZrO2 catalysts are evaluated for the hydrodeoxygenation of guaiacol (stirred batch reactor, 300 degrees C, 50 bar of H-2), guaiacol being a model compound of bio-oil issued from the lignin fraction of biomass liquefaction. ZrO2 and Ni are dispersed using incipient-wetness impregnation (IWI) or two-solvent (TS) method over mesoporous silica. Mesoporous zirconia (nanocasting method) and conventional zirconia were also used as supports. Cyclohexane, cyclohexanol and methoxycyclohexanol are the main products of reaction. Significant differences in cyclohexane yield are measured (after 8 h reaction, with conversion in most cases above 85%): NiZr-KIT-TS, 71.6% > NiZr-SBA-TS, 63.3% >> Ni/meso-Zr, 7.8% > NiZr-SBA-IWI, 5.2% > Ni/ZrO2, 1.6%. Tetragonal ZrO2 phase, acting as oxophilic phase for guaiacol/oxygenated intermediates adsorption and activation, is observed whatever the approach selected for the preparation. However, when nickel is supported over pure zirconia, low selectivity to cyclohexane is obtained, while the main product becomes cyclohexanol with yields reaching similar to 40%. Better activities and selectivities toward total deoxygenation (cyclohexane production) are obtained for the silica supported Ni-ZrO2 catalysts, especially when the catalyst is prepared by the two-solvent approach. Improvement is associated to the proximity between Ni and ZrO2 particles when confined in the silica mesopores. Finally, conversion and yield to cyclohexane obtained with NiZr-KIT-TS are stable during 5 successive catalytic cycles demonstrating the absence of significant modification of the catalyst during reaction. These results are evidencing that the confining strategy is efficient to improve stability of active elements during HDO reaction.