Have you heard of sustainable chemistry, which is able to reduce CO2?

Technology and science is advancing every day and can benefit us in tackling climate change with new terms that have been trending namely, climate technology or climate science. A term that is even interesting is sustainable chemistry. Just imagine, fossil feedstock being replaced by renewable feedstocks by just converting carbon dioxide (CO2). The activation of CO2 is however, energy-intensive and challenging as one oxalate molecule requires two CO2 molecules to be reduced. There are currently inventions in using chemistry sustainably for climate actions.

An insight into sustainable chemistry for climate action

An article by Science Direct written by Schuler, Morana and other researchers in March 2022 mentions that there is a possibility of using sustainable chemistry to take on the battle against climate change. An organic acid that is taking on this battle and has become of interest recently is oxalic acid. Oxalic acid is generally used in health care products and pharmaceuticals. This organic acid can also be produced by CO2, formate, sugars, ethylene, glycol and CO. Around the world many environmental non-governmental organisations are trying their best to reach out to the public and governments to tackle climate change in a sustainable manner. Now though, scientists have taken action on climate change by reducing CO2 and having sustainable alternatives to fossil feedback, which is renewables. This is achieved through reducing CO2 electrochemically to subsequent carbon-carbon coupling and format in direction to oxalate to produce an oxalate coupling reaction (FOCR). The formation of FOCR is an interesting carbon, capture and utilisation (CCU) option. By converting oxalate to oxalic acid is simple, as a strong acid is needed through a process of acidification or electrochemical acidification. The electrochemical CO2 reduction is quite promising but requires an investment of two electrons per CO2 molecule and is an expensive process. 

The alternatives in reaching sustainable chemistry 

Alternatives have been experimented, such as exploring potential coupling partners based on their reactivity, attractiveness and side reactions of the prospective product. The result is that CO2 was known to be the most desirable coupling partner as it matched with characteristics of having low potential for side reactions and broad availability. The CO2 reduction has been supported by many studies in previous years, hence, a desirable match for the prospective product. Through a trial, CO2 acted as a poison to FOCR, where there has been a reaction to hydride and carbonite to form oxalate and formate. This has been the motivation to do further experiments based on the positive reaction. The aim of this research taken on by Schuler, Morana and other researchers is to discover the potential outcomes of coupling format with transforming CO2 to oxalate that is supported by investigations, proof, experiment, which can prove the mechanisms and influence various process parameters by using isotope labelling studies. Isotope labelling according to another article written by Science Direct mentions that this method occurs through chemical modification or metabolic labelling that forms peptides by incorporating heavy isotope-molecules. Further information on the research is explained in the results and discussion section.



An overview of the results and discussion

The five schemes of the experiment

Five schemes were used to experiment with CO2. Scheme 1) “Coupling of two formate molecules to oxalate and hydrogen via the carbonite dianion (CO2 2−) intermediate with the regeneration of the hydride catalyst in the FOCR. M+ indicates the presence of a metal counter-ion for the salts and was either potassium- or sodium cation in this work”. Scheme 2: “alternative coupling of formate and CO2 to oxalate via carbonite dianion (CO2 2−) intermediate. M+ indicates the presence of a metal counter-ion for the salts and was either potassium- or sodium cation in this work”. Thirdly, scheme 3, “decomposition of oxalate to carbonate, CO2, CO, and elemental carbon in the presence of hydride. M+ indicates the presence of a metal counter-ion for the salts and was either potassium- or sodium cation in this work.” Fourthly, Scheme 4, “decompositions of formate and oxalate to carbonate as proposed by Gorski et al. [40] In both decomposition reactions carbonate is formed from metal oxide and CO2 as in step I. The decomposition of formate starts with step II where formate decomposes to metal hydride and CO2” Lastly, Scheme 5, “potential reaction pathways for oxalate formation in a system with 13C-formate and 12CO2. Overall, four steps I-IV can be combined to lead to three different outcomes.”

Alternatively, the experiments show that there is a possibility of providing CO2 in a gaseous form. It is however proven that there are challenges to have abundant CO2 ready in a short space of reaction time between carbonite and formate in the general formate to oxalate formation. To achieve a fast reaction time to supply abundant CO2, CO2 is supplied in a supercritical form or in a liquid. To achieve this - potassium formate or sodium is used to get the desired result. Through the Proof-of-principle in equimolar formate and hydride mixtures four reactions were established: “1) The intended reaction of formate, hydride, and CO2 (FHC),  2) The positive control reaction of formate coupling with hydride in nitrogen atmosphere (FOCR), 3) Control reactions between CO2 + hydride (COH) and 4) Control reactions between CO2 + formate (COF).” The result of the experiment that had most of the carbons was as follows: “format (81%), oxalate (46 %) and carbonate (19 %)”.

The next phase in the experiment involved the influence of temperature, hydride loading, CO2 pressure and reaction time. This is where the temperature, hydride loading, CO2 pressure and reaction time was altered to investigate the reaction towards oxalate formation. The third phase discovered mechanistic proof via isotope labelling. In this phase, oxalate can only be formed from captured CO2 and formate and splitted into three pathways as diagramised in figure 1 below. 

Figure 1. Three fundamental pathways for oxalate production are consisting of either A) without or B), C) and D) with the involvement of CO2.

The conclusion of the study

In conclusion, supercritical CO2  in oxalate has been incorporated by reaction at low reaction temperatures with formate (200 degrees celsius) with a catalyst and coreactant, hydride superbase. This process achieved 52%, which is a maximum theoretical incorporation efficiency. The isotopic labelling studies produced CO2 in the form of  formate and oxalate. Further, for CO2 to be reduced, higher reaction temperatures and long reaction times were imperative. This resulted in an increase and incorporation in gaseous or non-soluble carbon compounds and the decomposition of oxalate to carbonates. Further, concluded experiments show, “the production of carbonate as a side product contributes to 15% of the overall carbon balance and could not be avoided in our system”. Addittionally, “overall, our results open the door for a new CCU pathway from CO2 to oxalic acid. It allows to increase the obtainable oxalate or oxalic acid amount per electrochemically reduced CO2 significantly. However, the reaction towards oxalate also requires the use of stoichiometric amounts of alkali hydrides. Alkali hydrides are currently produced from the reaction of their alkali metals with hydrogen and are therefore energy intensive to produce.” The above processes were just experiments and further understanding is needed for the reaction between hydride, CO2 and formate. To allow for sustainable processes the crucial reaction parameters are crucial. The researchers state that, “our work will help to accelerate the development of new processes starting from CO2-derived formates as carbon sources”.

A personal viewpoint on sustainable chemistry



In today’s time, science and technology has dominated traditional methods of taking actions against global issues. For instance, the research above shows and experiments how sustainable chemistry can be used for climate action. It is a new method that still needs a lot of clarification and understanding, however we are using science in tackling CO2 which is a natural chemical compound produced in the atmosphere. Plants for instance in-take CO2 from the air during photosynthesis. Simultaneously, CO2 is also produced through human-induced actions with industrial pollution, smoking, deforestation, agriculture and other land-use alterations. This means that humans have added more CO2 into our atmosphere that the earth or even plants can not even handle. Now science comes into the picture with sustainable chemistry to reduce CO2 as an action against climate change. I personally, would not like to alter nature through science, as nature would take its stance on humans. We should not alter the environment in any fixed way to hide our damaging actions on the environment.