Polymerization during low-temperature electrochemical upgrading of bio-oil: Multi-technique characterization of bio-oil evolution

The electrochemical method provides a green route to upgrade bio-oil, but it is still challenged by the issue of bio-oil polymerization which restrains its application. This study investigates the polymerization behaviors of the bio-oil during its electrochemical upgrading under various current densities and different reaction time using macrophotography and multiple characterizations. The results show that the polymerization is not only electroinitiated on the cathode surface, but also electro-propagated in the bulk solution. The large molecules (MW > 800 Da) originally existed in the bio-oil are consumed in the polymerization. The small molecules (MW < 800 Da) can be produced from the polymerization of aromatics with 1-3 rings, furans and levoglucosan through cycloaddition, dehydration and demethoxylation. These small molecules can directly form solid products (coke) rather than leading to the increase of the large molecules. The polymerization can be enhanced by the increasing current density and reaction time. The rate of bio-oil polymerization can surpass the rate of coke formation as the reaction proceeds. The polymer layer also forms on the anode surface by polymerization during the electrochemical upgrading of bio-oil.