Renewable feedstocks and bio-refinery concepts are the key to a successful transition to a sustainable chemical industry. One conceivable refinery concept is based on pyrolysis oils from biomass, though these oils are quite difficult to handle. A well investigated approach to upgrade pyrolysis oil and turn it into a valuable products is catalytic hydrodeoxygenation (HDO). However, this process has to be optimized and new ideas are needed to make the hydrodeoxygenation process attractive sustainable and economically competitive. With regard to the many successful applications of gas-expanded liquids in heterogeneous catalysis, the expansion of pyrolysis oil with carbon dioxide was applied in the context of a hydrodeoxygenation reaction. The catalyst used for HDO was Ni/Al2O3 (nickel loading 20 %wt).
The influence of CO2 on the viscosity was found to be quite strong at low temperature. At 52 °C and a CO2 pressure of 0.5 MPa the viscosity is reduced by 30 %. With 4.0 MPa of CO2 the viscosity decreases by 60 %. With supercritical CO2 a volume expansion of 5 % was observed. The hydrodeoxygenation showed best ... mehr results at 340 °C and autogenous pressure. The experiments were started at a total pressure of 8.0 MPa at room temperature (H2 + CO2), with a respective partial pressure of CO2 of 0 MPa, 2.0 MPa or 4.0 MPa. A deoxygenation degree of around 70 % could be reached (dry basis) under each atmosphere. The analysis of the upgraded products by different techniques indicated a slight decrease of hydrogenation with increasing the pressure of CO2.
Despite we observed a change in the physical properties when expanding the pyrolysis oil with CO2, no real improvement of the catalytic hydrodeoxygenation reaction (e.g. deoxygenation degree) could be found yet. Possible reasons for the absence of gas-expanded liquid effects could be the polar nature of the used pyrolysis oil and the high temperature. We assume that a viscous and more tar-like, but less polar pyrolysis oil will be more influenced. Gas-expansion with CO2 tends to be less effective with polar liquids due to the unpolar nature of CO2. The only observed effect in our actual system was a decrease of the hydrogenation with decreasing partial pressure of hydrogen