A Comparison of Different Approaches to the Conversion of Carbon Dioxide into Useful Products: Part II
A Comparison of Different Approaches to the Conversion of Carbon Dioxide into Useful Products: Part II
More routes to CO2 reduction
In this review, we consider a range of different technological approaches to carbon dioxide conversion, their current status and the molecules which each approach is best suited to making. Part II presents the photochemical, photoelectrochemical, plasma and microbial electrosynthetic routes to CO2 reduction and discusses the technological options together with proposals for future research needs from an industry perspective.
1. Other Approaches to Carbon Dioxide Conversion
1.1 Photochemistry and Photoelectrochemistry
In addition to the CO2 conversion technologies described in Part I (1), there are several other approaches that are being investigated, although in general these are currently at a lower technology readiness level (TRL). With the sun being a desirable source of renewable power, the direct use of solar energy to produce chemicals from CO2 akin to artificial photosynthesis is instinctively appealing. This is exploited in photochemistry using a photocatalytic (PC) reactor, which uses photons from the visible and ultraviolet spectrum to generate electrons from irradiating photosensitive semiconductors and transfers the electrons from a valence band to a conducting band in connection with a photocatalyst to promote the reduction of CO2. Thus, an immediate advantage stems from not converting the solar energy to electricity first to supply the electrons, which brings in the associated upstream inefficiencies, as in the power-to-X type of conversion routes or electrochemical reduction. Commonly observed products from the photochemical reduction of CO2 include CO, methanol, formic acid and methane (Figure 1). However, the process is complex to understand involving many mechanisms, especially for multiple proton and electron transfers for products such as methanol and methane with ‘CO2 activation’ being a rate-determining step for many products. Much research effort is being concentrated on developing photocatalysts from earth-abundant, low-cost and less-toxic catalysts such as those based on iron, nickel, manganese and copper that are more scalable than those metals on the second and third row of the Periodic Table, such as rhenium, ruthenium and iridium (3). There has also been recent work looking at using metal-free photocatalysts such as graphene, nitrides and carbides (4). Using solar energy for CO2 conversion does impart a degree of inflexibility regarding the energy source compared with for example an electrochemical route, with the energy conversion efficiency being key for cost and scalability. Although much promising research is being conducted in this area, the CO2 conversion rates often observed in such systems under development appear impractically low for industrial implementation with some of the technical research challenges resting with developing photocatalysts that promote a high CO2 reduction efficiency and product selectivity (5, 6).
The attributes of photochemistry and electrochemistry can be combined in the approach known as photoelectrochemistry, conducted in a photoelectrochemical (PEC) cell, and has been investigated for CO2 reduction to fuels and chemicals. A major benefit over a pure electrochemical approach is that a significant part of the energy necessary for the conversion is supplied by light, omitting or at least lowering the transmission and rectification losses associated with using solely an electrical supply (7). Fundamentally, PEC cells comprise an anode and cathode immersed in an electrolyte, with at least one of the electrodes being a photosensitive semiconductor that absorbs light and through photoexcitation provides some of the required electron flow (the current). The technology still faces challenges from complex reaction pathways, mass transfer conditions and large photovoltage requirements to generate sufficient product selectivities and solar-to-chemical efficiencies (8).
1.2 Plasma Technologies
A process that would utilise renewable electricity for CO2 conversion is non-thermal plasma (NTP) technology using a configured plasma reactor. Similar to a low-temperature electrochemical reactor, a plasma reactor has the ability to be switched on or off in response to the intermittency characteristic of the energy supply and requirements for grid-balancing. It also exhibits high reaction rates, attains steady-state quickly and can produce a range of hydrocarbon and oxygenated species, operating at near atmospheric temperature and pressure. The ability to enable thermodynamically uphill reactions to occur at ambient conditions arises from the generation of a low temperature ionised gas with electrons energised by applying an electric field to typically between 1 eV and 10 eV. This range is ideal for exciting molecular and atomic species and breaking chemical bonds, with the OC=O in CO2 requiring 5.5 eV (9). Different types of plasma are being investigated for CO2 conversion, involving various reactor configurations and how the electricity is supplied and its power rating, with the main three being dielectric barrier discharge (DBD), microwave (MW) plasmas and gliding arc (GA) plasmas, generating different performance for CO2 conversion (10). The DBD configuration is probably the most widely investigated in this area. Some advantages and disadvantages of the main plasma technologies for CO2 reduction are presented in Figure 2.
One of the main CO2 conversion reactions studied is CO2 splitting and dissociation to CO and molecular oxygen, with CO being potentially used as a product itself or as a feedstock for fuels and chemicals. Other reactions involve a hydrogen source co-reactant such as methane (dry reforming of methane mainly to syngas but also forming hydrocarbons and oxygenates), hydrogen or water (hydrogenation to added-value fuels and chemicals). Akin to many processes the energy efficiency, reactant conversion and product selectivity are important parameters to evaluate from experiment and target achieving sufficient values (for example energy efficiency of >60% (10)) to merit further scale-up towards industrial commercialisation, with the specific energy input (SEI) being considered an important factor for influencing such performance for plasma technologies. NTP naturally involves radical-based chemistries, so improving selectivity of the desired product and lessening the burden on downstream separation costs is undoubtedly a challenge. The reported product yields and energy efficiencies for producing added-value hydrocarbons and oxygenates are generally lower than for CO and syngas type products, with the presence of molecular oxygen (from CO2 dissociation) leading to the undesired oxidation of formed hydrocarbons (12). The use of heterogeneous catalysts such as metal oxides or zeolites to improve performance including selectivity is one such option, notably used in a packed-bed DBD configuration (13, 14). Any observation of high energy efficiencies appears to come at the expense of low conversions with augmenting both representing a long-term challenge involving further understanding of the complex plasma chemistry (11, 15). As with many novel CO2 conversion reactors, the scale-up of plasma technology would lend itself to being modular, addressing the needs of small-scale, distributed production. When considering scale-up there will however be the need for hazard management regarding the in situ generation of molecular oxygen from the CO2 together with flammable products from the CO2 conversion, with potential ignition sources from the plasma generation equipment requiring the necessary safeguarding.
In general, the technologies described in this section are at lower TRLs than those mentioned in Part I of this review (1), especially when compared with the thermochemical routes. They each share some of the overall performance challenges of improving the energy efficiency and product selectivity to make them viable as contenders for future industrial scale-up. The use of realistic CO2 feeds should be evaluated at the laboratory scale to ascertain the performance impact of inert or reactive gaseous components and seek to define tolerable catalyst poison levels, depending on the CO2 source. Supporting studies such as technoeconomic assessment (TEA) and life cycle assessment (LCA) at the early stage are also recommended for these technologies to assist in understanding their viability for further development in the CO2 utilisation space.
1.3 Microbial Electrosynthesis
Microbial electrosynthesis (MES) is a combined biological and electrocatalytic process, where reducing energy cells in the form of electricity is provided to microbial for the synthesis of organic materials from CO2. In MES, bioreactors fitted with electrodes transfer electrons to host organisms via hydrogen, a reduced mediator, or by direct transfer (Figure 3). This is thought to be an efficient means of providing reducing energy compared to the classic fermentative routes (16).
MES was first demonstrated in 2010 with the production of acetate from CO2 by a biofilm of the acetogenic Sporomusa ovata on a graphite cathode, which generated hydrogen (17). This was at a rate of 1.6 g acetate m–2 day–1 (relative to the cathode surface area). Since then, several examples using different wild type or engineered microbes converting CO2 to chemical products have been published. With improvement in electrode design, microbial engineering and strain selection, improved yields of acetate from CO2 have been achieved. Using a mixed culture biofilm and three-dimensional macroporous cathodes, acetate production at 685 g m–2 day–1 was demonstrated (18).
Apart from acetogens, the Knallgas bacterium Cupriavidus necator, which can use hydrogen or formate from the cathode as a source of reducing energy, has also been used in MES. Through genetic engineering and MES, C. necator has been used for the production of branched chain alcohols, alkanes and terpenes.
The use of MES for CO2 conversion is still far from industrial use due to limitations in productivities and expense. Electron uptake by microbes is low, needing a wider cathode surface area and to be made with more biocompatible and cheaper materials. Limitations of the biological pathways themselves (slow carboxylation kinetics, high acetate products when using acetogens) also lead to low titres. How successfully these bottlenecks are addressed will determine whether MES will be more widely adopted as a means of CO2 fixation in the future (16).
In this section, a comparison is made between the various technology options available for producing different molecules, which is summarised in Figure 4.
2.1 Technology Options for C1 Molecules
For the majority of C1 molecules, well developed routes exist for their manufacture from syngas which can be adapted to use CO2 and renewable hydrogen to give a sustainable process. It is therefore hard for new processes to displace these existing technologies. Even in the case of a product such as formaldehyde, which is made commercially through highly efficient methanol oxidation processes (19), it is difficult to imagine a new process displacing the two well-established methods.
There are a couple of exceptions to this thinking. The first is the production of CO or syngas, which are very important intermediates in the production of a range of fuels and chemicals. The hydrogenation of CO2 to CO is not a well-established technology: it runs at high temperatures and suffers from equilibrium limitations, problems with carbon laydown and formation of unwanted hydrocarbons. Hence there is an opportunity to use new processes such as electrochemical CO2 reduction to drive these reactions.
Another molecule which could be of interest to develop new routes to is formic acid. Currently, formic acid is largely produced by hydrolysis of methyl formate, which is in turn made by carbonylation of methanol (20). Direct routes from CO2 have been developed based on ruthenium homogeneous catalysts and on a range of heterogeneous catalysts (21) as well as electrochemical methods (22, 23). One challenge here is in the low current demand for formic acid. Its current uses are mainly as a preservative for animal feeds and silage, as well as in the leather and textile industries (20). Proposed future uses include as a fuel, a hydrogen storage and transport medium and also as a feedstock for biological processes to make higher carbon chain length products. Given the toxic and corrosive nature of formic acid (20), it remains to be seen to what extent these applications are realised.
2.2 Technology Options for C2–C4 Molecules
The next classification of molecules to consider is those with a few carbon atoms. The key challenge here is typically selectivity to the chosen product. Unlike for C1 atoms, there are very few well-established catalytic processes for selectively converting C1 feeds such as syngas into C2–C4 products. The exception to this is dimethylether, which is produced in two steps from CO2 by dehydration of methanol using acidic catalysts such as zeolites or alumina (24). Dimethylether is unusual as a C2 molecule which does not contain a C–C bond. It is proposed as a fuel of the future due to having similar combustion properties to diesel, although as a gas at room temperature it would require handling in a different way to current fuels.
Ethylene is an important intermediate in the production of chemicals, notably polyethylene but also a wide range of other molecules, and so a route from CO2 to ethylene would be very useful to decarbonise ethylene production. There is no established direct heterogeneous or homogeneous catalytic conversion route from CO2 to ethylene. By far the most progress has been made with electrochemical conversion, where in contrast to other molecules such as methanol, ethylene has been made with 70% Faradaic efficiency (FE) over copper catalysts (25, 26) at practical current densities of 500–1100 mA cm–2. With further commercialisation, this could provide a viable pathway to a range of fuels and chemicals using ethylene as an intermediate. The main competing technology is a two-step method using the methanol-to-olefins process.
A range of other C2–C4 molecules have been prepared using electrochemistry, but only at laboratory scale and generally as part of a mixture of products. For example, Opus 12 reports having made eleven different compounds (27) in this range. Of other molecules formed electrochemically, ethanol is the best studied but is still only reported in modest selectivity. Ethanol is also reported as being produced by heterogeneous catalysis and biological approaches (see box).
Beyond C2, propanol has been reported as being made electrochemically from CO2 on a molybdenum sulfide electrode (28) but only at <5% FE. At C4, biological systems can be very efficient in producing a number of molecules, particularly oxygenates such as butanediols or butyrates. Some systems are able to produce C3 molecules such as propionates, but generally even number chains are preferred due to the C2 coupling mechanism employed.
2.3 Technology Options for C5+ Molecules
For larger molecules, direct synthesis from CO2 becomes increasingly challenging. Biological systems offer perhaps the best route to making C6 molecules, but only a small number of examples have been published. However, the biological approach seems to offer a more selective direct conversion than is possible by other routes such as catalysis or electrochemistry. Interestingly, a number of the examples of C6 and even C8 production have been given through MES (29, 30).
As the product’s carbon chain becomes longer, using coupled processes becomes more relevant (Figure 5). It is important to note that not all molecules need to be accessed in a single step from CO2, and in many cases it will be preferable to use two efficient steps rather than one less efficient one. Some examples of this are already well-known, for example reduction of CO2 to CO which can then be converted to fuel range hydrocarbons by the Fischer-Tropsch (FT) process.
Efficient processes to make larger molecules tend to be based on a biological coupling of C1 or C2 molecules which can be prepared in different ways. Catalytic coupling processes such as Guerbet coupling of ethanol (31) or FT coupling of syngas tend to give a range of products, which is acceptable for end uses in fuels and in some cases solvents, but not for the majority of applications. Two-step processes with a second biological step can be based on a wide range of feedstocks, including formic acid (32), methanol (33) or syngas (34). An advantage of this approach is that all these molecules can be made readily by catalytic or electrochemical reduction of CO2, enabling efficient processes. One possible concern is that the feedstock can be consumed by the microorganism and be converted back into CO2. Approaches which have the potential to overcome this include MES, where feeding (renewable) electrons into the reaction supplies the microbes with energy without feedstock being consumed.
There are many viable routes to convert CO2 using sustainable technologies, and many products are accessible, from C1 through to C10 and beyond. Different technological approaches suit different molecules (Figure 4) and have their own advantages and drawbacks. The use of two-step processes widens the range of molecules even further, and in some cases allows more energy-efficient syntheses to be developed. Key to making sustainable processes based on CO2 is the supply of renewable energy as light, electricity or another source. In the case of electricity, intermittency is an important issue when power from solar panels or wind turbines is used. Chemical processes in general do not like to be switched on and off or turned up and down, although low temperature electrochemical and biological processes are more tolerant to this than most. However, low utilisation rates will stop processes being viable from a capital cost perspective.
Comparing Methods for Conversion of Carbon Dioxide into Ethanol
Ethanol is one of the few molecules where viable catalytic, electrochemical and biological routes exist, and so it is possible to compare processes for their manufacture (Table I). The range of coproducts made varies significantly for the different approaches, due to the different mechanisms which are in operation. This is significant because it impacts the separations required. Removal of water will be a challenge in all cases, although the biological system is often especially dilute. Despite the differences in mechanism, CO plays an important role in three of the four processes. Ruthenium- or iron-based reverse water gas shift catalysts are deliberately added to the methanol coupling and electrochemical reactions, while the co-feeding of a modest amount of CO increases the rate of the biological process significantly. The direct catalytic conversion route was shown by Fourier transform infrared to proceed via formate-like intermediates, but CO was produced in small amounts by less optimised catalysts. The catalytic and biological reactions were reliant on dissolved CO2 (and hydrogen), which was increased in the catalytic systems by the use of pressure. The electrochemical flow reactor meanwhile made use of a humidified CO2 stream to increase the partial pressure at the cathode. Each approach has its merits in terms of performance and scalability, and there are features such as working with gas flows and easier separations which could be transferred from one approach to another.
|Catalytic (35)||Catalytic two-step (36)||Biological (37)||Electrochemical (38)|
|Reagents||CO2:H2 = 1:3||CO2:H2 = 1:3||CO2:H2 = 1:2||CO2 and water|
|Catalyst||CoAlOx, 10 g l–1||[Ru(CO)3Cl2]2 with Co4(CO)12, 0.5 gRu l–1||Clostridium autoethanogenum, 0.2 g l–1||Copper metal with iron porphyrin|
|Reactor type||Batch reactor||Batch reactor||Flow reactor||Membrane electrode assembly (MEA)|
|Other reagents||Water as solvent||Methanol, lithium iodide promoter, 1-ethyl-2-pyrrolidone solvent||A very dilute aqueous medium containing a range of nutrients (39)||0.1 M KHCO3 electrolyte|
|Ethanol selectivity, C-mol%||92||65||55||32|
|Other products made||Methanol, propanol, small amounts of CO||Methane, CO||Acetate, biomass||Ethylene, CO, hydrogen, propanol, formic acid and acetic acid|
3. Conclusions and Research Needs
A wide range of molecules have been reported to be prepared from CO2, ranging from those derived from existing large scale processes such as methanol or methane production, through to more embryonic yet promising methods such as MES or coupled catalytic-biological or electrochemical-biological processes for the production of longer carbon chain molecules. While research is certainly needed in this area, we propose that it should be focussed in areas which will make the greatest impact, both in terms of economics and scalability and of climate change mitigation.
Research should be focussed on developing routes to molecules where routes do not currently exist, rather than competing with existing high TRL processes. This is especially true in the C1 space which is dominated by traditional syngas-based processes, although these often need to be optimised for CO2-based feedstocks, different contaminants and scales. Consideration should be given to the whole process, from CO2 purification to the downstream separations needed to give the desired products in good purity. It is worth being mindful of the specifications required for different products. Which impurities can be tolerated and which are not acceptable? Can reaction mixtures be simplified? LCA and TEA should also be used at an early stage to identify the most promising routes.
We also suggest that most of the C1 molecules can be made efficiently by adaptation of existing catalytic processes, with the exception of CO and formic acid where electrochemical routes also offer promise. Electrochemistry appears well suited to making molecules in the C2–C4 range, especially C2 at present. Biological processes offer direct routes to molecules in the C2–C8 range, and compete with two step electrochemical-biological and catalytic-biological processes for efficiency. For fuel synthesis, coupling processes such as FT or methanol to gasoline are well-established, although biological routes are interesting too.
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Annette Alcasabas is a Lead Scientist at the Biotechnology division of Johnson Matthey. She has a PhD in Biochemistry from the Baylor College of Medicine, USA and a background in microbial genetics. Annette worked in industrial synthetic biology prior to joining Johnson Matthey in 2016. At Johnson Matthey, she is responsible for the molecular biology workflow used in commercial protein production and protein engineering. Annette also leads a number of external collaborative projects with the goal of advancing skills, opportunities and applications for Johnson Matthey in biotechnology.
Peter Ellis was awarded his BSc and PhD in Chemistry from Durham University, UK. Following postdoctoral research with Professor Robbie Burch into the direct synthesis of hydrogen peroxide, he joined Johnson Matthey in 2001. With Johnson Matthey, he has worked extensively on the synthesis of heterogeneous catalysts, notably for the Fischer-Tropsch process, and also in related materials characterisation methods. In 2017 he moved to lead a new project identifying opportunities for Johnson Matthey in electrochemistry. He is currently the Technology Director of Johnson Matthey’s Green Hydrogen business.
Iain Malone is a Research Scientist in the Electrochemistry and Materials group at Johnson Matthey. He joined Johnson Matthey in 2019 as a placement student for his final year Masters project with the Department of Chemistry, University of York, UK working on synthesis and testing of catalyst materials for electrochemical CO2 reduction. After graduating with MChem from the University of York in 2020 Iain has continued research on CO2 reduction with Johnson Matthey.
Gareth Williams is a Senior Process Engineer in the Electrochemistry and Materials Group at Johnson Matthey. He obtained his BEng and PhD in chemical engineering from the University of Bath, UK, and joined Johnson Matthey in 2003, gaining experience in syngas catalysts and processes. Within Johnson Matthey he has been involved in research projects for low carbon technologies including chemical looping. He is a current member of the Electrochemical Transformations research team who are evaluating new opportunities in electrochemistry including bulk and fine chemicals production.
Chris Zalitis is a Research Scientist in the Electrochemical Transformations team designed to introduce electrochemical techniques into new and existing markets of Johnson Matthey. He earned his Master of Chemistry degree at Southampton University, UK, in 2008 and his PhD from Imperial College London, UK, in 2012 in the area of fuel cells looking at the electrocatalytic performance of anode and cathode catalysts under idealised high current density operation. He joined Johnson Matthey in 2013 where he has worked in areas of electrochlorination, battery materials, fuel cells and electrosynthesis, with recent focus on water and CO2 electrolysers.