Electrochemical Studies of Bio-Oils: Conversion and Upgrading
Authors: Silva Tiago , Santos Diogo M.F. , Condeco José 
Published: February 2020

Biomass is an abundant resource and is mostly used as a source of energy by direct combustion. However, there are other alternatives for the efficient use of this renewable resource: sustainable production of liquid and solid fuels, hydrogen, synthetic gases and valuable chemicals (1). Liquid fuels are advantageous because of their elevated high heating value (HHV) (2), being obtained by a solvolysis process at moderate temperatures and atmospheric pressure (3), and referred to as liquefaction. Recently, the innovative generation of syngas uses the water electrolysis process with liquefied biomass as a carbon source (necessary to obtaining carbon monoxide and carbon dioxide) (4). Syngas has many applications like the intermediate production of transport fuels, gas fuels and various chemicals.
Biomass can also be a source of valuable chemicals, for example levulinic acid, which is predominant in liquefied cork and of extreme importance as an intermediate in the synthesis of many other compounds. Nilges et al. (5) made use of electrochemistry for the production of renewable chemicals and biofuels, specifically a two-step electrochemical conversion of levulinic acid to octane via valeric acid. The conversion of levulinic acid into hydrocarbons is usually achieved via multi-step processes (under harsh conditions of temperature and pressure, 250 – 400 ºC and 10 – 35 bar of H2), while the electrochemical reaction is performed at room temperature and in aqueous solutions, with a natural phase separation allowing a simple separation of the hydrocarbon. In the electrochemical processes, the selectivity for the reaction products are dependent on the electrolyte composition, electrode material and current density (6).
This work identifies the potential of the different biomass under study, namely pinewood, olive stones, cork and almond shells, to produce bio-oil with the highest prospective to continue further studies regarding two aspects: the electrochemical conversion to industrial relevant compounds and the electrocatalytic upgrading of biomass-derived intermediates.
Herein, the liquefied products of the mentioned biomass samples were characterized by several physicochemical methods (e.g., density, viscosity and conductivity). Then, the electrochemical behavior of the bio-oils were analyzed using platinum (Pt) electrodes both as anode and as cathode. Voltammetric methods were used to assess the liquefied biomass samples anodic oxidation at the Pt electrode using different potential scan rates. The hydrogen evolution reaction in the samples was also evaluated at the same electrode. Chronoamperometry measurements were used to check the effect of different applied potentials in the anodic and cathodic currents. To increase the samples’ conductivity, the effect of adding different concentrations of H2SO4 was also assessed using the same methods. A small-scale laboratory electrolyzer using Ni plates for both cathode and anode was assembled and the operation parameters were evaluated, specifically the cell voltage applied and the electrolyzer operation time.
The bio-oil, before and after electrolysis, and any compounds deposited on the electrodes surface were characterized. The as deposited compounds were observed using optical microscope (Figure 1). The bio-oils and the deposits were chemically analyzed using FTIR spectroscopy and mass spectrometry.
Figure 1. Optical microscope image of the solid compound deposited in the Ni anode during the electrolysis of liquefied cork (cell voltage = 2.5 V; time = 1 h).

References

W. Yang, T. Shimanouchi, M. Iwamura, Y. Takahashi, R. Mano, K. Takashima, et al. Elevating the fuel properties of Humulus lupulus, Plumeria alba and Calophyllum inophyllum L. through wet torrefaction. Fuel. 2015, Vol. 146, pp. 88-94.

M. M. Mateus, J. C. Bordado, R. G. dos Santos. Potential biofuel from liquefied cork – Higher heating value comparison. Fuel. 2016, Vol. 174, pp. 114-117.

H. Pan. Synthesis of polymers from organic solvent liquefied biomass: A review. Renew. Sustain. Energy Reviews. 2011, Vol. 15, pp. 3454-3463.

L. Guerra, K. Moura, J. Rodrigues, J. Gomes, J. Puna, J. Bordado, T. Santos. Synthesis gas production from water electrolysis, using the Electrocracking concept. J. Environ. Chem. Eng. 2018, Vol. 6, pp. 604-609.

P. Nilges, T. R. dos Santos, F. Harnisch, U. Schröder. Electrochemistry for biofuel generation: Electrochemical conversion of levulinic acid to octane. Energy Environ. Sci. 2012, Vol. 5, pp. 5231–5235.

T. R. dos Santos, P. Nilges, W. Sauter, F. Harnisch, U. Schröder Electrochemistry for the generation of renewable chemicals: electrochemical conversion of levulinic acid.
RSC Advances. 2015, Vol. 5, pp. 26634-26643.

Figure 1