(592d) Deoxygenation of Bio-Oil Intermediates: Examination of Methane As a Direct Reducing Agent

Thermochemical conversion of biomass into bio-oil is a promising way to create renewable liquid transportation fuels.  However, untreated bio-oil is not suitable for use a fuel, because it is thermally unstable, corrosive, has low heating value, and is often immiscible with hydrocarbon fuels.  Upgrading of bio-oil to remove oxygen, saturate double bonds, and increase energy density is therefore required; such upgrading currently is done via catalytic hydrodeoxygenation with molecular hydrogen.  Another possible source of reducing equivalents for upgrading is methane, available either from anaerobic digestion of biomass or from abundant fossil reserves.  Each mole of methane is capable of providing up to four reducing equivalents; however, methane is the most stable hydrocarbon having strong C-H bonds (104 kcal/mol), stable tetrahedral geometry, and no reactive functional groups to facilitate reaction.  

The present work examines methane activation over supported metal catalysts as a first step in using methane directly for bio-oil upgrading.  In addition to methane as a source of reducing equivalents, the possibility of methane incorporation into bio-oil species, which would further increase fuel value because of methane’s high energy content (~50 MJ/kg), is explored.  Initial studies of methane activation over Pt and Ru catalysts at 150°-250°C showed that methane readily undergoes H-D exchange between CH₄ and D₂, and between CH₄ and CD₄ to give mixed CH_xD_4-x species. The CH₄ – CD₄ exchange takes place without H₂ or D₂ present.  

Direct incorporation of methane into benzaldehyde and other species as model bio-oil compounds over Pt and Pt/Cu catalysts was not observed. However, it was shown that over Pt/Cu/SiO₂ benzaldehdye was quantitatively converted to toluene in the presence of hydrogen, and into benzene in the presence of methane or nitrogen.  Copper was confirmed as the active catalyst species for deoxygenation.