Fast pyrolysis converts biomass into a liquid fuel intermediate known as bio-oil. Due to oxygen containing functional groups, bio-oil species are reactively unstable, tending to polymerize during long term storage and upgrading. The high temperatures and pressures used for conventional stabilizing and upgrading of bio-oil lead to coke production and catalyst deactivation. Milder treatments are needed to reduce coke formation and increase catalyst lifetime while lowering costs.
We have recently developed a mild strategy for bio-oil upgrading using electrocatalytic hydrogenation (ECH). During ECH, atomic hydrogen adsorbed on the catalyst surface reacts with adsorbed organic compounds. The availability of such surface hydrogen is controlled by the applied current and potential at ambient pressure, rather than by external hydrogen pressure, as in the classical hydrogenation. Useful temperature lowering is known as well since most ECH reactions proceed below the boiling point of water. We have now demonstrated the ECH of furfural and guaiacol, two model compounds derived respectively from pyrolysis of cellulose and lignin.
Cost is a key issue for any intended biomass upgrading process. Screening of catalysts for ECH of furfural therefore focused on inexpensive metals: nickel, iron, aluminum, copper and stainless steel. With nickel catalyst at room temperature and ambient pressure, 78% of furfural underwent ECH to furfuryl alcohol along with small amounts of hydrogenolysis product 2-methylfuran (selectivities 95:5). Further trials revealed that acid favored formation of 2-methylfuran, raising its selectivity from 5% to 14% as pH was decreased from 5.0 to 1.0. Additional factors affecting the product selectivity, such as temperature and initial furfural concentration, were also investigated.
Unlike the reactive aldehyde furfural, the electron rich aromatic guaiacol (2-methoxyphenol) resisted ECH with simple nickel catalysts. However, development of a carbon-supported ruthenium electrocatalyst enabled complete conversion of guaiacol to cyclohexanol and 2-methoxycyclohexanol at 80°C and ambient pressure. Effects of metal loading, electrolyte, temperature and substrate concentrations were explored. These model compounds studies highlight the promise of ECH as a strategy for stabilizing and upgrading bio-oils from biomass fast pyrolysis.