An alternative to petrochemicals: biomass electrovalorization

Abstract

Replacing petrochemicals with refined waste biomass as a sustainable chemical source has become an attractive option to lower global carbon emissions. Popular methods of refining lignocellulosic waste biomass use thermochemical processes, which have significant environmental downsides. Using electrochemistry instead would overcome many of these downsides, directly driving chemical reactions with renewable electricity and revolutionizing the way many chemicals are produced today. This review mainly focuses on two furanic platform chemicals that are produced from the dehydration of cellulose, 5-hydroxymethylfurfural and furfural, which can be electrochemically reduced or oxidized to replace fuels and monomers that today are obtained from petrochemicals. Critical parameters such as electrode materials and electrolyte pH are discussed in relation to their influence on conversion efficiency and product distribution.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

Keywords: biomass; electrochemistry; electrovalorization; platform chemicals.

Figure 1.

Annual production of biomass and crude oil feedstock, compared with annual production of the platform chemicals EG, furfural and LA. Data points were extracted from references [–,–16].

Figure 2.

Possible oxidation and reduction products available from (a) furfural and (b) HMF. Red labels depict electrochemical oxidation products, and blue labels depict electrochemical hydrogenation products. BHMF (2,5-bishydroxymethylfuran), 5-MF (5-methylfurfural), DFF (2,5-diformylfuran), HMFCA (5-hydroxymethyl−2-furancarboxylic acid), FFCA (5-formyl−2-furoic acid). Δ_fG⁰ values for calculating electrode potentials (U⁰) obtained from references [29,30].

Figure 3.

Summary of major reported furfural electrochemical reduction products on monometallic electrodes at moderate cathodic potentials. The ratio of the shaded squares depicts the major and minor products. The pinacol product represents hydrofuroin. Cu electrodes are the only monometallic species that produce 2-MF with high Faradaic efficiencies [–45].

Figure 4.

Electroreduction products of furanic compounds on different electrodes. (a) Co-plotted linear sweep voltammograms of different metal cathodes for the electrochemical reduction of furfural or HMF in acidic electrolytes. Shaded areas depict a major reduction in productions reported [–51]. (b) Adsorption energies of furfural and hydrogen obtained using density functional theory (DFT) and reported products. Co/CuPc depict metal phthalocyanines [30].

Figure 5.

Comparison of the onset potentials of (a) transition metals and (b) post-transition metals with regard to the HER and HMF reduction, in acidic (0.5 M H₂SO₄) and neutral (pH 7 phosphate buffer) media. The ‘product’ on the legend describes HMF reduction products. Onset potentials of the HER and HMF reduction were reported by recording the potential at which a current density of −0.5 mA cm⁻² was achieved [55].

Figure 6.

Products from electrochemical furfural reduction with changing initial furfural concentration. Reaction conditions: 0.5 M H₂SO₄ electrolyte with 4:1 ratio of acid to acetonitrile co-solvent, Cu foil electrode, 1.5 h reaction time at −0.80 V_RHE. Figure adapted from reference [57].

Figure 7.

Qualitative selectivity predictions of furfural reduction products on Cu electrodes. The size of the molecules represents the relative abundance expected for each parameter [19].

Figure 8.

Proposed electrochemical reduction pathways of carbonyl compounds. The red pathway depicts the ECH pathway using adsorbed hydrogens, whereas the blue pathway depicts the PCET pathway [51].

Figure 9.

Electrochemical reduction of furfural and benzaldehyde on Pb foil. Faradaic efficiencies of (a) coupled products and (b) reduced alcohol products after electrolysis. Reaction conditions: 0.25 M sodium phosphate buffer (pH 6.7), 20 mM of hydrofuroin and benzaldehyde [66].

Scheme 1.

Competing pathways for HMF oxidation to FDCA.

Figure 10.

LSVs of NiOOH, CoOOH and FeOOH before (dashed line) and after (solid line) the addition of 5 mM HMF. Reaction condition: 0.1 M KOH (pH 13), at room temperature, scan rate was 5 mV s⁻¹. Data points were taken from reference [76].

Figure 11.

The electrochemical oxidation of HMF in acidic electrolyte at 1.60 V_RHE using MnO_x electrodes. (a) Products obtained from HMF oxidation at different amounts of charge passed. (b) Precipitation of FDCA in solution after cooling from 60°C. Figure reprinted with permission from the publisher [79].

Figure 12.

Electrochemical oxidation of HMF on Au_xPd_y electrodes. (a) Reported favoured reaction routes of HMF oxidation on Au/C and Pd/C anodes. (b) LSVs of various Pd and Au anodes using different starting substrates HMF, HMFCA and FFCA [82]. Reaction conditions: 0.1 M KOH (pH 13), room temperature, scan rate 50 mV s⁻¹.

Scheme 2.

Electrochemical catalytic cycle of TEMPO demonstrating alcohol oxidation into aldehydes.

Scheme 3.

Mechanism for the base-catalysed Cannizzaro reaction starting from HMF.