Towards Sustainable Oxalic Acid From CO 2 And Biomass - PubMed

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Abstract

To quickly and drastically reduce CO2 emissions and meet our ambitions of a circular future, we need to develop carbon capture and storage (CCS) and carbon capture and utilization (CCU) to deal with the CO2 that we produce. While we have many alternatives to replace fossil feedstocks for energy generation, for materials such as plastics we need carbon. The ultimate circular carbon feedstock would be CO2 . A promising route is the electrochemical reduction of CO2 to formic acid derivatives that can subsequently be converted into oxalic acid. Oxalic acid is a potential new platform chemical for material production as useful monomers such as glycolic acid can be derived from it. This work is part of the European Horizon 2020 project "Ocean" in which all these steps are developed. This Review aims to highlight new developments in oxalic acid production processes with a focus on CO2 -based routes. All available processes are critically assessed and compared on criteria including overall process efficiency and triple bottom line sustainability.

Keywords: CO2 conversion; carbon capture and utilization; catalysis; formate coupling; oxalic acid.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

**Figure 1** Annual CO2 emissions caused by the release of fossil carbon and land‐use change and development of global mean atmospheric CO2 level.[ 6 , 7 ]

**Figure 2** Value tree: Oxalic acid as starting compound for a variety of high‐value products.

**Figure 3** Overall there are six feedstocks directly used for oxalic acid‐producing processes. These feedstocks include (1) CO2, (2) CO, (3) alkali formate, (4) ethylene glycol, (5) propylene, and (6) carbohydrates. Except for CO2, a commercially used route exists for all of those feedstocks.

**Scheme 1** Biomass can be (a) directly converted into oxalic acid by oxidation or used as a feedstock for oxalic acid precursors including (b) propylene, (c) ethanol and CO2, (d) CO, and (e) glucose. CO can be converted into oxalic acid (f) directly or (g) via the formation of formate. Glucose can be (h) oxidized directly or (i) first converted to ethylene glycol, which is subsequently oxidized.

**Scheme 2** Fossil carbon can be converted to oxalic acid via four pathways: (a) naphtha can be converted to ethylene glycol (via ethylene), which can be oxidized to oxalic acid; (b) propylene is obtained from naphtha cracking and can be converted to oxalic acid; (c) fossil carbon is converted to CO in a gasification process, which can be converted to oxalic acid via the dialkyl oxalate process; and alternatively (d) CO can be converted to formate, which is turned into oxalic acid using formate coupling followed by an acidification step.

**Scheme 3** CO2 can be converted to oxalic acid via four main pathways: (a) through direct conversion of CO2 to alkali oxalate; (b) through a metal formate intermediate, which can be obtained from the electrocatalytic or photocatalytic reduction of CO2; (c) via CO and the dialkyl oxalate process; and (d) via ethylene glycol and subsequent oxidation (in practice not done because ethylene glycol would be obtained from oxalic acid, not vice versa).

**Scheme 4** Direct conversion of CO2 to oxalate followed by acidification to oxalic acid: (a) direct electrochemistry; (b) CO2 reduction catalyzed homogeneously by metal complexes; (c) reduction of CO2 with Ca‐ascorbate and electrochemical regeneration.

**Figure 4** Flow sheet of zinc oxalate process as developed by Fischer et al. In the bottom left in the first cycle, CO2 is converted to zinc oxalate and removed by filtration. The oxalate is dissolved in sulfuric acid in the second cycle, and oxalic acid is extracted from the zinc sulfate solution, which is recycled to sulfuric acid and zinc in the zinc electrolysis cell shown at the top. Pure oxalic acid is obtained by evaporation of the extractant in the last step in the bottom right. Reproduced with permission from Ref. [147]. Copyright 1981, Springer.

**Figure 5** Structure of the triangular rhodium complex [(RhCp*)3(μ3‐S)2]2. Reproduced with permission from Ref. [152]. Copyright 1994, The Chemical Society of Japan.

**Figure 6** Catalytic cycle of copper complex for CO2 activation as proposed by Bouwman and co‐workers. The initial copper(II) complex [4]4+ is first reduced at −0.03 V vs. NHE to the copper(I) complex [1]2+. This is subsequently reduced by two CO2 to copper (ii) again. Two of the complexes merge and the bound CO2 .− radical anions couple to form bridging oxalate molecules [2]4+. Lithium ions and acetonitrile liberate the oxalate as lithium oxalate and the initial complex is formed again. Reproduced with permission from Ref. [153]. Copyright 2010, American Association for the Advancement of Science.

**Figure 7** Revised version of the original three‐step reaction cycle for reduction of CO2 to oxalic acid. The starting CuII complex (1 or 2) is reduced to a CuI complex by sodium ascorbate (3). In the presence of oxygen, the ascorbate is reduced to oxalate to give oxalate‐bridged complex (4). In the presence of CO2 and absence of ascorbate, however, a stable three‐valent carbonate complex is formed. Reproduced with permission form Ref. [155]. Copyright 2021, Nature Publishing Group.

**Scheme 5** CO can be converted to oxalic acid via the dialkyl oxalate process, where first the dialkyl ester of oxalic acid is formed, which is then hydrolyzed to oxalic acid.

**Scheme 6** CO can be obtained from fossil carbon, biomass, and CO2. Fossil routes include (a) Boudouard reaction, (b) steam reforming of gas, and (c) partial oxidation of hydrocarbons. Biomass can be (d) thermochemically converted and CO2 can be reduced to CO via (e) reverse water‐gas‐shift reactions, (f) direct electrochemical reduction, or (g) electrolysis.

**Figure 8** Flow diagram of UBE liquid phase process for oxalic acid production in (CO2Bu)2. Reproduced with permission from Ref. [195]. Copyright 1999, Elsevier.

**Scheme 7** Alkali formates can be converted to oxalic acid via formate coupling to oxalate and subsequent acidification to oxalic acid.

**Scheme 8** Alkali formates can be obtained directly or indirectly from CO2. The two indirect ways include (a) CO reduction with caustics and (b) hydrogenation of carbonates. The direct conversion of CO2 can be either (c) electrochemical, (d) photochemical reduction, or (e) enzymatic conversion.

**Figure 9** Trassati's volcano plot shows the relationship between metal‐hydrogen bonding energies (on the x‐axis) and the exchange current for hydrogen evolution (on the y‐axis) for several metals. Reproduced with permission from Ref. [219]. Copyright 2014, Beilstein Institut.

**Figure 10** Proposed mechanism for formate production under the influence of Ru catalyst and hydrous conditions. The cycle begins at the top left with the pre‐catalyst Y, which is activated by the electron transfer from the sacrificial electron donor (SED) to the photosensitizer (PS) to yield Z. With the dissociation of Cl−, the active catalyst A becomes available. A chemically transforms by complexation with a proton to form B. The dicationic complex B subsequently is reduced again by the PS to form C, which can react with CO2 to form formate and the dicationic complex D. Reduction of the dicationic complex D by the PS regenerates the initial complex A. Reproduced with permission from Ref. [224]. Copyright 2019, American Chemical Society.

**Figure 11** Yields of potassium oxalate from KOH‐catalyzed reactions as reported in literature and patents.[ 34 , 256 , 281 , 286 , 288 ]

**Scheme 9** Mechanism for hydride‐catalyzed formate coupling reaction as postulated by Lakkaraju et al.

**Figure 12** DFT free‐energy calculations of the catalytic conversion of formate into oxalate as salts of sodium (red 663 K, orange 298 K) and potassium (navy 713 K, blue 298 K). Figure adapted with permission from Ref. [34]. Copyright 2016, Wiley‐VCH.

**Figure 13** Simple electrolysis cell‐design (A) uses two cation‐exchange membranes (blue) to create three compartments. In the anodic compartment, the oxygen evolution reaction on the anode produces protons and oxygen. The protons migrate to the middle compartment, where they exchange potassium for a proton to form oxalic acid. The potassium migrates through the cation‐exchange membrane to the cathodic compartment, where it forms potassium hydroxide with the hydroxide ions produced on the cathode during the hydrogen evolution reaction. In the advanced multifunctional cell (B), which has the fourth compartment by adding a bi‐polar membrane, the salt splitting can be coupled with the production of high‐value chemicals. A reductant is reduced in the cathodic compartment and an oxidant is oxidized in the anodic compartment. The proton for the reduction is drawn from the bipolar membrane in which water splitting is taking place.

**Scheme 10** (a) Ethylene glycol can be oxidized to oxalic acid by catalytic oxidation with oxygen or nitric acid. (b) A newer alternative route uses electrochemical oxidation.

**Scheme 11** Oxidation of ethylene glycol using mixed acids in water.

**Scheme 12** Oxidation of ethylene glycol by nitric acid and oxygen.

**Scheme 13** Oxalic acid production from propylene or ethylene via catalytic oxidation.

**Scheme 14** Propylene production from glucose via fermentation, dehydration, and metathesis. Glucose is converted to alcohols including isopropanol, 1‐butanol, and ethanol. The alcohols are co‐dehydrated to the corresponding olefins, and 1‐butene and ethylene are sent to a metathesis system to form more propylene.

**Scheme 15** Oxalic acid production from propylene starts with (a) propylene conversion to α‐nitratolactic acid with nitric acid, followed by (b) oxidation by oxygen to oxalic acid.

**Scheme 16** Oxalic acid can be obtained from biomass via (a) oxidation, (b) alkali heating, and (c) fermentation.

**Scheme 17** Oxidation of glucose to oxalic acid with nitric acid, catalyzed by vanadium pentoxide.

**Scheme 18** Alkali fusion process to obtain oxalic acid from biomass. Steps include the heating of biomass with alkali solution, magnesium oxalate precipitation, and finally acidification.

**Scheme 19** Fermentation of sugars to oxalic acid.

**Figure 14** Overview of all potential routes that connect the three feedstocks (slate: fossil; green: biomass; yellow: CO2) with the product side. All processes are graded on their sustainability in a three‐color scheme. The colors are based on the grading as in Table 4 (red:<2; yellow:<4, green:>4) The arrow style indicates the maturity of the process. Fossil pathways were kept separate from biomass and CO2 pathways.

All figures (33) See this image and copyright information in PMC

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Towards Sustainable Oxalic Acid From CO 2 And Biomass - PubMed

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