Sustainable Valorization of CO₂ through Nuclear Power-to-X Pathways

Abstract

Some of the issues concerning energy security and climate change can be addressed by employing nuclear power (NP) to supply the energy required for the conversion of carbon dioxide (CO₂) into chemicals, products, and materials. Nuclear energy represents a neutral carbon source that can be generated sustainably, reliably, and consistently. Nuclear power plants (NPPs) could supply energy in the form of heat, electricity, and ionizing radiation to drive CO₂ chemical reactions underpinning NP-to-X type of pathways. CO₂ conversion processes are either commercially available or emerging technologies at different developmental maturity stages. This work reviews the published literature (articles and patents) that reports R&D results and the understanding and development of chemical reactions and processes, as well as the efforts in integrating NPPs and chemical processes (CPs). As will be made evident, a new industrial era for the manufacturing of decarbonized chemicals, products, and materials will be possible by developing and implementing new (more energy- and carbon-efficient) processes responding to the NP-to-X pathways. This new decarbonizing platform not only contributes to achieving net zero goals but also broadens the NPP product beyond electricity.

1. Introduction

Some of the most significant concerns in the 21st century have been energy security and climate change. Ensuring energy availability to underpin economic growth has been based on an exacerbated reliance on fossil resources, which are readily available and globally accessible. However, the detrimental impacts on the climate of the consequential CO₂ emissions have driven R&D activities on the development of mitigation and removal technologies. Therefore, the mitigation of carbon emissions and the use of renewable or carbon (C) neutral energy systems are intrinsically connected to attaining energy security under stable future climate systems [1]. However, renewable energy generation inherently depends on weather conditions causing power fluctuations, intermittency, and/or interruptions, which in turn have brought about the development of energy storage technologies. In this regard, chemical compounds could be employed as energy carriers to facilitate storage and transportation (energy transmission). The identification of compounds exhibiting high energy density and with low production costs started over a decade ago when methane was identified. However, the development of production processes with low energy and carbon intensity is still needed. A new R&D area emerged and was coined “Power-to-Gas” (PtG), with the electrochemical methanation reaction as the process of choice. Thus, the production of methane using renewable energy and a renewable resource, such as CO₂, gave rise to a new renewable fuel, i.e., renewable natural gas (RNG). Alternatively, and in terms of their energy density and transportability, other energy carriers became of interest, such as dimethyl ether (DME) and, more particularly, liquids, e.g., methanol and Fischer–Tropsch products. The economic benefits and competitive value-added of chemicals production broadened the range of energy carriers and expanded the topic beyond methane-to-“X” products, i.e., PtX pathways. In some instances, other acronyms include “V” for value in PtV and “L” for liquids in PtL, in which case, attention was focused on fuels and, more specifically, methanol. In summary, the requirements for the selected energy carrier include high energy density, low energy intensity, low carbon intensity, low production and storage costs, and easy transportation. Since power is supplied in the form of electricity, the type of chemical process (CP) receiving the most attention is the electrochemical process.

Opposite to renewable energy, nuclear power (NP) appeals include consistency, reliability, and security, and NP is an energy resource considered C-neutral. These characteristics make nuclear energy an excellent solution to replace part of the energy demand currently being met by fossil sources (e.g., coal and natural gas). Additionally, nuclear power plants (NPP) provide a more versatile energy mix in the form of heat (thermal load), electricity (off-peak current), and radiation (neutrons, fast electrons, and gamma/X-rays). The progress and deployment potentialities of micro- and small modular nuclear reactors enable the application of multiple approaches for the implementation of decarbonization strategies. The various nuclear reactor technologies (e.g., pebble bed reactors, accelerator-driven systems, nuclear battery reactors, advanced nuclear fuel designs, super-critical CO₂ reactors, and others operated at higher temperatures, such as fusion reactors, molten salt reactors, liquid metal-cooled fast reactors, fluoride salt-cooled high-temperature reactors, and high-temperature gas reactors) [2] offer a variety of options to harness and adapt to energy integration into CPs. Larger-scale nuclear reactors of Generation IV [3] are specifically interested in providing heat within a temperature range of 550 °C–850 °C [4]. More demanding chemical reactions may find grounds for integration into Very-High-Temperature Reactors (VHTR) for temperatures in the range of 1000 °C or the Ultra-High-Temperature Reactor (UHTR) for temperatures of 1300 °C [5]. The heat provided at these temperature ranges might represent a cost-effective solution and a more competitive and environmentally amicable manner for industrial heat than fossil fuel combustion. The safety features of many of the Generation IV reactors are thought to be the worthiest characteristics to consider for integration into CPs. Regardless of the great R&D advances reached by these attractive reactors, their commercial implementation is further into the future, and the testing of built prototypes has brought up both developmental insights but also challenges to be addressed. Nevertheless, all these advances and potentialities justify R&D efforts in the development of NP-to-X pathways, which will require the development of integrated energy systems (IES) coupling NP to chemical processes. Currently, these efforts are minimal and nonexistent within the CO₂ valorization area.

Japan [6] and, more recently, Europe [7] initiated efforts on PtG pathways to develop long-term storage solutions for renewable electricity. The goal was the production of high energy-containing gases, such as CH₄ and H₂, from carbon sources and renewable electricity. In the Japanese conception, CO₂ is recycled [8] via the methanation reaction ((R1), the Sabatier reaction) [9,10] using H₂ produced by the electrolysis of seawater [11] and powered by renewable energy [12]. Process development and pilot plant testing took place during the 1980s–1990s [12].

CO₂ + 4H₂ → CH₄ + 2H₂O

(R1)

The European process concept follows the Japanese one, though their R&D efforts broaden the water electrolysis window to a wider range of electrolyzers. In this case, rather than considering a recycling of CO₂, they also generalize it to emission valorization. Thus, the PtG concept developed in Japan, as well as in Germany, Switzerland, Denmark, and France, can be represented by the scheme shown in Figure 1. The main drawbacks of PtG are relatively low efficiency and high costs. The process economy was largely influenced by RNG production costs, which were strongly dependent on the annual operational time and the electricity price [13].

As mentioned above, this work directly concerns NP–chemical-processing IESs, which integrate the energy offered by an NPP with that demanded by a chemical plant (CP), which forms the basis for NP-to-X pathways. This type of IES has a first consequence in the diversification of the NPP product slate beyond electricity into chemicals, which can reach more distant markets than those accessible by transmission lines. NPP integration into CPs is not new, and a long history of efforts dates back to the 1970s, with two significant projects, namely Nuclear Hydrogen (NH) [14] and ADAM and EVA (A&E) [15]. These two programs are the most relevant examples of devoted efforts on the technology development of NP-CP IES that underpinned the potentialities for the development of NP-to-X pathways. The early studies on NH were high-temperature applications and were based on the thermochemical production of bulk hydrogen. Hydrogen can be produced from the splitting of the water molecule, which is an energy-intense reaction. In fact, at standard conditions, DG° = 237.2 kJ/mol and DH° = 285.8 kJ/mol, and only at temperatures above 4400 °C does DG become negative. This thermodynamic limitation has been overcome by designing multi-reaction cyclic pathways, which enable the process chemistry to drive at temperatures above 550 °C [16]. Examples of the investigated thermochemical water splitting cycles include (i) iodine–sulfur, (ii) iron chlorides, (iii) hybrid sulfur, (iv) Ca–Br, and (v) Cu–Cl. To lower the process temperature, a cycle is created, coupling a high-temperature endothermic reaction with a low-temperature exothermic one [16]. These production pathways were considered by the Nuclear Hydrogen Program and are characterized as being stoichiometric reactions, and the required energy was supplied by an NPP [17,18,19,20,21,22,23,24,25,26]. However, the energy intensity of these hydrogen production technologies is not better than that of the existing commercially operated technologies. Importantly, then, it is important for intensification/integration efforts to avoid leveraging inefficiencies of either the candidate CP or nuclear resources. Therefore, the development of CPs with better energy efficiency is needed.

Another early study was the A&E process based on the chemical heat pipe concept (see Figure 2), which involves producing a chemical energy carrier through an endothermic reaction and transporting it to a remote location where it could be converted back to energy or to other chemical products. It was first tested in Germany using heat from a high-temperature gas-cooled nuclear reactor for the first half cycle (EVA, from the German term “Einzelrohr-Versuchs-Anlage”, meaning single-tube experimental facility) to produce synthesis gas (syngas, a mixture of carbon monoxide and hydrogen), by methane steam reforming (SMR, the endothermic reaction (R2)). The Adam half-cycle of the process was run in a distant location and consisted of the methanation reaction of syngas (an exothermic reaction, (R3)). The methanation heat of reaction could be used for electricity generation or any other process heat [27].

CH₄ + H₂O → CO + 3H₂     ΔH_298K = 206 kJ/mol

(R2)

CO + 3H₂ → CH₄ + H₂O     ΔH_298K = −206 kJ/mol

(R3)

As can be derived from the above-referred description of the NH and A&E projects, great advances have been achieved in NP-CP IES, which can be used to undertake further development of NP-to-X pathways. In fact, current efforts are devoted to green hydrogen production, but unfortunately, nothing has been reported in regard to CO₂ valorization. In this regard and for the purpose of this work, CO₂ valorization will be seen as its chemical transformation into value-added products. In connection with NE, the reported CO₂ valorization work using available nuclear energy is related to driving or inducing reactions by means of ionizing radiation, for which chemistry and chemical reactions were revised recently [28]. In particular, dry reforming, the reaction of methane with CO₂ to produce syngas, has been proven to be radiation-induced [29]. Additionally, the opportunities for employing the energy mix (heat, electricity, and radiation) available from NPPs have been identified [30], which further emphasizes the existing potential of implementing NP-to-X pathways in the immediate future.

It is important to point out that CO₂ exhibits high stability and/or low reactivity, and its chemical transformation involves significant energy consumption, ergo energy-intense processes. Accordingly, to accomplish its conversion typically requires severe conditions, such as high temperatures and pressures. Therefore, any new process cannot rely on fossil energy resources (e.g., coal and natural gas), in which case a significant quantity of greenhouse gases (GHGs) would be released. Consequently, new processes need to be developed not only to take advantage of neutral-C energy generation but also to attain cost-effectiveness through more energy-efficient processes. In fact, efficiency improvement is one of the recommended approaches for emission reduction purposes. However, emissions from industrial processes (e.g., cement, steel, refining, etc.) and from high-temperature heat (e.g., >1000 °C) have been recognized as being difficult to cost-effectively avoid [31]. Technological solutions have been developed, but these are unsustainable, absurdly expensive, or have not reached commercial scale [32].

Thus, CO₂ keeps accumulating in the atmosphere, reaching a concentration of 417 parts per million (ppm), with global emissions of over 37 Gton_CO2/y in 2022 [33]. The industrial and energy sectors are responsible for nearly 70% of these emissions [34]. The magnitude of CO₂ emissions and the difficulties in their reduction support their categorization as renewable feedstock [35,36,37,38,39]. However, prior to feeding it to any chemical reactor, emissions need to be captured. Although many capture technologies have been commercially available for over 50 years, the installed capacity was about 40–50 Mton/y in 2019. Since then, this capacity has undergone a steady growth of over 57% (57–68%) interannually to reach a level of 361 Mton/y by 31 July 2023 [40]. The CCS projects tracking that the International Energy Agency (IEA) carries out indicated that, most likely, the total amount of CO₂ potentially captured by 2030 would be around 435 Mton/y, and the storage capacity would increase to around 615 Mton/y [41]. Therefore, in the best-case scenario and considering that during the last five years, emissions have remained very close to the 37 Gton/y range, CCS alone will contribute to mitigating less than 2% of the global emissions. This number increases favorably (up to 60%) when the 2050 net-zero goal is considered since this goal assigned only 1 Gton/y to be captured and stored by 2050 [41]. The significant role that carbon capture (CC) plays in decarbonization is clear, but it is not enough, as has already been established by the Intergovernmental Panel on Climate Change (IPCC) [42] and the IEA [43]. Consequently, they have recommended the incorporation of additional approaches to the decarbonization strategy, including energy demand reduction, energy efficiency improvements, and commercial implementation of conversion processes [42,43,44]. In fact, using emitted CO₂ to produce fuels, chemicals, and materials could complement CCS role while creating revenues, which will incentivize industry by providing economic sustainability [45], particularly those recognized as difficult to decarbonize (e.g., cement, steel, refining, and high-temperature heat) [31].

The integration of chemical CO₂-utilization processes to CC technologies might serve as a vehicle to close the C-cycle and underpin a C-based circular economy (CCE). Although closing the C-cycle represents a big challenge on its own, doing it sustainably requires the use of particular methods and tools to implement the circularization strategy [46]. A goal to establish a sustainable CCE (SCCE) needs to keep the circular economy (CE) definition in our mind all the way through: “a CE is one where nothing is wasted and where natural resources are managed sustainably, and biodiversity is protected, valued and restored in ways that enhance our society’s resilience; and our low-carbon growth has long been decoupled from resource use, setting the pace for a safe and sustainable global society” [47].

This work will review the published literature concerning R&D and technology development to valorize CO₂ using energy (heat, electricity, or radiation) that could be provided by NPPs. Special attention will be given to compounds whose production might attain sustainability. The factors considered in the search for sustainability are (i) the use or consumption of massive amounts of CO₂, (ii) a long lifecycle of the product, (iii) the economy of scale, and/or (iv) a reduction in energy intensity from the currently employed production process technology. The chemical conversion of CO₂ into intermediates or building blocks and final products will be reviewed in the following sections.

2. Chemical Pathways to Valorize CO₂

Abundant chemical reactions of CO₂ have been the subject of research and the core of the development of process technologies. Although CO₂ utilization includes rendering a service and/or an application, the present work will only consider its chemical conversion into value-added products, such as fuels, bulk or commodity chemicals, materials, and specialty products. Vast R&D efforts have been devoted to the study and technological development of these reactions [48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95], though the level of progress and maturity differ among them, with only a few becoming commercially available [96,97]. Currently, some of the proposed technological solutions are unsustainable, extremely expensive, or have not reached commercial scale [32].

2.1. Synthesis Gas (Syngas)

Synthesis gas or syngas is a mixture of carbon monoxide (CO) and hydrogen (H₂), which, under certain conditions, reacts to produce a variety of products, both oxygenates and hydrocarbons (Figure 3). Some of these compounds are raw materials in the synthesis of other chemicals and materials, giving rise to multiple chemical pathways and a broad product slate.

Evidently, syngas is a very versatile and valuable intermediate as well as the preferred source of H₂. Commercially available technologies are based on the reforming of hydrocarbons, partial oxidation, or gasification of residues or coal. The selection of technology is based on the final product desired, which depends on the H₂/CO ratio (syngas ratio, see Figure 4) or the application given to the syngas. Production is mostly carried out either by SMR (R2) or coal gasification. SMR is the preferred technology when H₂ is the desired product, while coal gasification is selected for power generation applications.

By 2019, global production of syngas was 261.02 million normal cubic meters per hour (Mnm³/h), with a value of $43.6 bln [98]. Global syngas production consumes about 500 exaJ, constituting 29% from natural gas, 39% from crude oil, and 32% from coal [99].

The development of cost-effective technology for its production from CO₂ would open a path for a vast consumption of this compound while satisfying the market of a great variety of products. Several pathways involving different forms of energy have been proposed for the production of syngas from CO₂, such as thermocatalytic pathways and chemical loops (driven by heat), electrochemical (driven by electricity) pathways, and radiation-induced (driven by ionizing radiation) pathways, which are discussed in the following sections.

2.1.1. Thermocatalytic Pathway

The thermocatalytic pathway for producing syngas from CO₂ is based on the methane dry reforming (MDR) reaction (R4). The favorable environmental impact of this reaction is obvious since it consumes the two top carbon-containing GHGs (CO₂ and methane, CH₄). However, MDR has severe thermodynamic limitations, starting with a high thermodynamic potential (ΔG°₂₉₈ = 174.6 kJmol⁻¹) that becomes negative only at very high temperatures, e.g., ΔG°₁₀₇₃ = −44.76 kJ mol⁻¹ [100,101]. Additionally, the endothermicity of the reaction (ΔH° = 247 kJ/mol) also sets a requirement for high temperatures (800–1000 °C) [102]. Under these conditions, not only does the employed catalyst deactivate rapidly, but selectivity is also very low, leading to undesirable products and reducing economic competitiveness.

CO₂ + CH₄ → 2CO + 2H₂     ΔH_298K = 247 kJ/mol

(R4)

The MDR intensification with the partial oxidation reaction, adding small amounts of oxygen or water, contributes to overcoming thermodynamic limitations through the exothermicity of the oxidation reactions (R5). The O₂ partial pressure needs to be carefully controlled to avoid total oxidation (R6). A net reduction in energy requirements results in a more energy-efficient MDR process [103] that also contributes to the reduction of carbon formation [104,105,106].

CH₄ + ½O₂ → CO + 2H₂     ΔH_298K = −36 kJ/mol

(R5)

CH₄ + 2O₂ → CO₂ + 2H₂O     ΔH_298K = −802 kJ/mol

(R6)

Carbon formation is one of the reasons behind catalyst deactivation. The reacting mixture containing CO₂, CO, and CH₄ is prompted to form carbon through the methane decomposition reaction (R7) and the Boudouard reaction (R8) [107]. While deactivation caused by carbon deposition could be remediated with catalyst regeneration, the high operating temperatures also trigger irreversible deactivation, shortening the life cycle of the catalyst. Irreversible structural transformations, such as sintering and recombination of the active components, take place at high temperatures and accelerate deactivation. More stable catalysts [108,109] have been studied, although the problem still remains.

CH₄ → C + 2H₂     ΔH_298K = 74.85 kJ/mol

(R7)

2CO → C + CO₂     ΔH_298K = −172 kJ/mol

(R8)

Among the undesirable products, water is formed through a side reaction, the reverse water gas shift (RWGS, (R9)), consuming part of the produced H₂ and forming additional CO that results in H₂/CO ratios < 1 [110,111,112]. However, water is known to suppress coke formation probably through the steam carbon gasification reaction (R10), and from this viewpoint, RWGS could be beneficial. In fact, a really “dry” MDR will be difficult to realize at a large scale with realistic feedstocks and at the high severity the MDR reaction requires [113].

CO₂ + H₂ → CO + H₂O     ΔH_298K = 41.15 kJ/mol

(R9)

C + H₂O → CO + H₂     ΔH_298K= 131 kJ/mol

(R10)

The lack of selectivity and the fast deactivation are the main drivers for the vast efforts devoted to catalysis R&D. New catalyst designs and formulations [104,114,115,116,117,118,119,120,121,122,123,124], new synthetic methods [108,109,125,126,127,128,129], and the study of deactivation [100,114,117,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143] have been the main R&D subjects. Noble metal catalysts are more stable since, by exhibiting higher activity, they can perform at lower temperatures [144]. However, regardless of their instability, the low cost of base metals places Co and Ni at the top of the preferences list for industrial catalyst development [145], particularly as good prospects for H₂ production [146].

These catalyst issues, together with the energy demands of the MDR reaction, contribute to an unfavorable economy. While technological improvements are being addressed but not yet achieved, alternative approaches have considered integration and/or intensification of other processes. These intensified, combined, and/or integrated technologies have been techno-economically assessed, and some examples are cited here. As can be deduced from the (R4) stoichiometry, the H₂/CO ratio of MDR syngas is lower than that produced from SMR (~3) and could be increased by the intensification with POX. A lower ratio favors alkenes, oxygenates, hydroformylation and acetic acid [147], and methanol synthesis, including so its derivative products (e.g., gasoline and paraffins, olefins, and aromatics) [148] (see Figure 3). Experimental data of a combined MDR–oxidative coupling of methane (OCM) process from a mini-scale unit showed good potentialities by lowering energy consumption through a 40% autothermal behavior [149]. Lower OPEX and CAPEX were assessed for methanol production when MDR instead of SMR was the source of syngas to the methanol synthesis [134]. Although MDR is preferably considered for syngas production, there might be favorable situations in which to incentivize H₂ production [146]; for instance, a membrane reactor can be used to produce ultra-pure H₂ [150].

Regarding technology development, two processes were developed in the 1990s: the Sulfur PAssivated ReforminG (SPARG™) process [113,151,152] and the CALCOR™ process [153,154,155]. And more recently, two others are under development: the R&D activities from the Gas Technology Institute (GTI) [156] and the DRYREF™ process [157,158]. In the SPARG™ process (originally designed by Haldor-Topsoe), sulfur is used to selectively poison active sites, leading to disassembling metal clusters, favoring reforming and inhibiting coke formation [152]. The poisoning effect of sulfur on Ni SMR catalysts has been reported [159,160], as well as in MDR [132]. Although the retarding effect on CH₄ decomposition was shown [132], absolute proof of a dual effect on MDR and coke formation has not been published. The inhibiting carbon formation feature was demonstrated in an SMR-type pilot unit [161]. This process is operated within the temperature range of 875–945 °C to produce syngas at a ratio between 0.55 and 3.2 (preferably 1.8–3) by using a H₂O/CH₄ ratio of 0.1–0.26 in the pre-reformer unit and a CO₂/CH₄ ratio of 0.07–2.5 in the MDR unit. In this process, the severity of the operation determines the carbon-formation tendency. Since carbon deposition increases with decreasing CO₂/CH₄ and H₂O/CH₄ ratios [162], high severity is defined by an operation far beyond the carbon limit curve (the carbon limit curves are plots indicative of the deposition of carbon normalized to the amount of converted carbon sources at the thermodynamic equilibrium, as a function of process conditions) [163]. Sterling Chemical Inc. built, started (1987), and continuously operated a commercial demonstration unit for four years in Texas, USA [152]. The coke suppression of the SPARG™ process was better than the use of water for the same purposes. It was estimated that 133,000 Nm³/h of water would have been required to attain a similar effect [113]. However, as mentioned above, the total absence of water was not recommended, and most SPARG plants were designed to feed significant amounts of water to decrease severity and mitigate carbon formation as dictated by the limit for carbon limit curve. More details on the SPARG process and the industrial experience on the different sites where it was tested and demonstrated can be found in [113].

Caloric GmbH developed the multistage CALCOR™ process based on a Ni catalyst to produce CO-enriched syngas (H₂/CO < 1, ~0.42) [154,155]. Coke deposition is minimized by varying feedstock compositions and catalysts (reactivity and geometry) through the stages of the process. Since CO₂ in the feed exceeds the stoichiometric requirements, the gas product is mainly CO, with a minor proportion of CO₂, H₂, water, and traces of CH₄. This unreacted CO₂ is then separated and recycled back into the reformer. Hydrogen is also recovered separately to finally collect a stream that might contain up to 99.95% CO [164], depending on the downstream application (e.g., acetic acid and phosgene production [165]).

The GTI’s R&D activities have been focused on the study, design, and development of a nanocatalyst that would enable smooth and stable operations in the absence of steam [156]. The process would convert captured CO₂ in a compact reactor, utilizing a nano-engineered catalyst on hollow fiber tubes [114,115,116,117,125]. A long-run (400-h) laboratory-scale continuous test showed stable catalyst performance [118]. The process is thought to be integrated into a dimethyl ether (DME) synthesis process. Based on energy density, pipeline transportability, and use as a drop-in fuel, DME was considered. However, further research was announced to be needed to prove the integrated concept at the same scale [156].

The development of the DRYREF™ process has been carried out by a large industrial academia alliance that includes Lynde, BASF, hte, Karlsruhe Institute of Technology, the Technical University of Munich, the University of Leipzig, and DECHEMA Forschungsinstitut [158] through a €1 billion project funded by the German Ministry for Economic Affairs and Energy [166]. This technology can be seen as an attempt to improve the energy efficiency of the SMR process by using an innovative Ni-based oxide-supported catalyst (SYNSPIRE™ G1-110) that can operate at higher pressures (20–40 bar) and by including steam (at a steam-to-carbon (S/C) ratio of 0.9–1.8) to minimize coke formation. The presence of steam, besides promoting the RWGS reaction (R9), also brings the possibility of the occurrence of the SMR (R2), resulting in a more variable syngas H₂/CO ratio depending on the S/C ratio [167].

A simulation of a relevant MDR process (DRYREF type) integrated into the Fischer–Tropsch synthesis indicated that the production of 93 ton/h of a synthetic fuel composed of 72% diesel, 26% gasoline, and 2% LPG looks environmentally attractive by consuming 330 ton/h of CO₂ even though the economy did not show great promises [168]. Linde advanced the development to the pilot stage at Pullach, Germany, in 2015–2017 and demonstrated its suitability for integration into downstream processes such as aldehydes, methanol, acetic, and formic acid synthesis [169]. The pilot test at an S/C of 0.9 not only demonstrated energy savings but also an extended catalyst lifecycle due to a performance exempted from carbon deposition. With these results, Linde Engineering estimated savings of $12 M in its first five years of operation in the costs for building a new plant with a capacity of 50,000 scf/h. Compared to SMR, Linde’s DRYREF represented overall reductions in OPEX and CAPEX of, respectively, 5% and 3% [157]. Although Linde announced the technology would be commercially ready by 2018 after the Linde Pilot Reformer commissioning [169], no further information has been provided.

Although processes have reached a commercial scale, no investment has been made in their implementation. One of the reasons is the materials needed to handle the required high temperatures. Therefore, technological improvements in both catalysts and processes that either can withstand reaction temperatures or perform at less severe conditions without detrimental effects on competitiveness would be essential for a commercial application. Nuclear power might have an opportunity, particularly to lower reaction temperatures, as it has been recently demonstrated that radiation could induce the MDR reaction at conditions close to ambient (room temperature and absolute pressures of 100 psi) (more details are in Section 2.1.4) [29].

2.1.2. Chemical Looping Pathway

Chemical looping (CL) is a type of thermochemical cycle to transport oxygen within a reacting system. The original conception of the thermochemical cycles was to combine heat sources with chemical reactions that disaggregate molecules. Chemical compounds are continuously recycled in these processes, creating closed loops by integrating two hemi-cycles [170]. Buelens et al. [171] have described the particular case involving CO₂ in CL through three types of processes, namely, thermal redox, chemical redox, and carbonation. As illustrated in Figure 5, in the first half cycle (hemicycle 1, (R11)), an oxidant (redox CLs) or CO₂ (carbonation CL) reacts with a metal (Me) or a metal oxide (MeO), respectively, in the so-called “Air Reactor”. Meanwhile, in hemicycle 2 (R12), a reductant (chemical) or heat (thermal and carbonation) is used to restore the initial compounds in the Fuel Reactor.

	Hemicycle 1 (Reduction step)	Hemicycle 2 (Oxidation step)
	(R11)	(R12)
Thermal Redox	Me + Oxidant → MeO + Reduced-oxidant	MeO + Heat → Me + ½O2
Chemical Redox	Me + Oxidant → MeO + Reduced-oxidant	MeO + Reductant → Me + Oxidant
Carbonation	MeO + (CO2)sol → MeCO3	MeCO3 + Heat → MeO + Pure CO2

The carbonation CL is applied as CC means, with a direct recovery of concentrated CO₂. Meanwhile, the redox-based CLs can be applied to CO, H₂, or syngas production. Carbonation CL reduces the energy and costs of a CC application, and CO production has been mentioned as being economically sustainable [171]. The high efficiency and reduction in costs and energy demands for the power and industrial sectors, as well as the lack of commercial experience, have created barriers to CL growth [172].

Syngas production through CL may require the combination of multiple reactions, including methane oxidation (R13), CO₂ reduction (R14), and steam/water reduction (R15)—reactions that are described below and seen in the net reaction (R16). Hemicycle 2 is the reoxidation of the metal (R17) to restore the starting oxide. A CL-MDR process requires oxygen carriers with high oxygen storage capacity (e.g., perovskite-type materials and oxides and mixed oxides of Fe, Co, Ni, Cu, Ce, etc.) [173,174]. It has also been postulated that methane POX (R18) can occur rather than methane total oxidation (R13), in which case a H₂-rich syngas could be recovered (H₂/CO ~ 2) from hemicycle 1. In this case, the CO₂ reduction reaction (R14) can be used as hemicycle 2 [175], from which pure CO can be separated, and a high syngas ratio of hemicycle 1 is desired.

4MeO + CH₄ → 4Me + CO₂ + 2H₂O     Methane oxidation

(R13)

2Me + 2CO₂ → 2MeO + 2CO     CO₂ reduction

(R14)

2Me + 2H₂O → 2MeO + 2H₂     Steam reduction

(R15)

CH₄ + CO₂ → 2CO + 2H₂     Net reaction (R13 + R14 + R15)

(R16)

2Me + O₂ → 4MeO     Metal oxidation with air

(R17)

MeO + CH₄ → Me + CO + 2H₂     Methane POX

(R18)

An investigation of mixed oxides of Fe, Co, or Ni with ceria or zirconia showed the superiority of the Ni-CeO₂ system in terms of both activity and stability (over 60 cycles). Co ranked second with slightly lower activity and stability (over 12 cycles), but Fe also showed poor performance and stability (over 12 cycles). The analysis of the spent materials indicated that in the case of Ni and Co, these metals act as catalysts and ceria as the oxygen-transporting material since the former metals, once reduced during hemicycle 1, do not re-oxidize again when going through hemicycle 2 [176].

One of the benefits of CL MDR derives from taking advantage of the redox properties of a Me/MeO pair rather than limiting MDR to thermodynamics and CO₂ reactivity. A thermodynamic assessment indicated that for a MeO/CH₄ ratio < 1 and Fe₂O₃ as MeO, the POX reaction dominates (R18). However, MeO/CH₄ ratio > 1 would be undesirable since methane combustion would overcome CO and H₂ production, and CO₂ and H₂O would become the major products [177].

Another advantage of CL MDR comes from the stepwise operation. During hemicycle 1 ((R16) or (R18)), some decomposition might take place, and coke will deposit on the MeO/Me, but it will be burnt off during hemicycle 2 by the presence of the oxidant. Since H₂ is produced in hemicycle 1, it will never be in contact with CO₂, and the RWHS reaction (R9) could be avoided, with concomitant H₂ savings and increased syngas ratio [178,179]. Trimetallic mixed oxides (Ni-Fe-Al) exhibited better stability than binary systems, as proven over 20 CL MDR cycles and in conventional MDR [180]. A perovskite-type system (La_0.9Sr_0.1FeO₃ supported on yttria stabilized-zirconia) also showed stable performance over 7 h operation of 20 min CL MDR cycles [181].

No report on the environmental impact of standalone CL units could be found. Most of the reported assessments regard applications such as a combustion system (chemical looping combustion, CLC) [182,183,184,185], IESs [186,187,188], decarbonization of power plants (coal or natural gas) [182,183,184,185,189,190], and on reforming [191,192]. All these assessments report environmental benefits ranging from very small (e.g., thermal natural gas power plants) to becoming negative carbon-emitting facilities (e.g., coal power plant–CL CC–Sequestration). Additionally, economic and environmental sustainability was evaluated through a simulation of an IES consisting of biomass gasification and syngas CL to poly-generate power, DME, and ammonia. The results showed that the system thermal and exergy efficiencies were 60.15% and 46.70%, respectively, with a payback time of 8 years and net present value of nearly $1.6 M for a 20-year life cycle [188]. Regarding CL reforming, both hydrogen [191] and syngas [192] production have been evaluated. Compared to conventional reforming technologies (SMR, POX or autothermal, ATR), CL reforming offers higher energy efficiencies, lower carbon emissions, and lower operating costs [192]. The carbon footprint, acidification, freshwater eutrophication, ozone depletion, photochemical ozone formation, and depletion of minerals and metals have been evaluated for a CL MDR process as a measure of its environmental performance. The leakage of biogas from the anaerobic digestion plants was the most determinant factor for the environmental viability of green hydrogen production by CL MDR. The perovskite-based material employed in this system environmentally outperformed when benchmarked against a conventional DRM [191].

Regardless of the progress reached by CL in general and CL MDR specifically, further R&D is needed in heat integration, new reactor designs, process integration, oxygen carriers with increased capacity by improved formulations and/or structures, further testing for longer runs at larger scales (pilot and demonstration), and comprehensive TEA and LCA. Regarding heat integration, a couple of works on radiation-induced reductive decomposition of carbonates and oxides could be considered to couple NP to thermochemical cycles as a means to improve energy efficiency [193,194].

2.1.3. Electrochemical Pathway

The electrification strategy for decarbonization has been pushing high pressure into the development of electrochemical processes for CO₂ conversion. CO₂ electrolysis is based on its reductive decomposition to produce CO and O₂, taking advantage of catalysts and electric power to overcome the energy barrier for CO₂ activation and reduce the energy required for its reduction reaction [195]. CO₂ electrochemical reduction has been an R&D subject for decades, and the thousands of published articles have been the basis for the developed electrolyzers. Commercial electrolyzers are available for bench-top scale for both liquid- and gas-phase CO₂ reduction R&D, e.g., [196]. On the industrial scale, some companies have announced commercial availability for electrolyzers producing higher reduction products, e.g., [197,198]. Regardless of these announcements, technology development is still in progress and trying to overcome identified challenges such as high selectivity electrocatalysts at high current density (high conversion), reliability, durability, productivity, and energy efficiency [199]. Although the combination of a water electrolyzer with another electrolyzer for CO₂ can be used for syngas production and/or green hydrogen production by electrolysis is receiving great attention, the revision of this work will fall away from the scope of the present work (various colors are used to categorize the magnitude of the net carbon emissions during a production process: a brown process technology uses heat and energy generated from the combustion of fossil fuel; in a blue process technology, CC, CCS, and/or CCU technologies are incorporated to mitigate emissions; in a green process technology, the heat and energy are generated from renewable, low-C or neutral-C sources; and in some instances, gray is used as an intermediate emitting scale between brown and blue—other, less-used colors include pink, turquoise, white, and yellow);. It is the simultaneous or co-electrolysis of CO₂ and water that is relevant here, and currently, within the net zero proposed strategies, co-electrolysis represents the most studied electrochemical alternative.

The electrochemical CO₂ reduction processes, in general, can be separated into two ranges of operating temperatures: low-temperature electrolysis (LTE) (T < 100 °C) and high-temperature electrolysis (HTE) (T ≤ 600 °C) processes [200]. The development of suitable ion-conductive materials is defined by the selected temperature range.

In HTE, the co-electrolysis of steam and CO₂ produces syngas [201]. Initially, many studies were focused on this operation mode [201,202,203,204,205,206,207,208,209,210,211], introducing improvements by considering co-production (e.g., chlorine [212]) or integration with other plants (e.g., biomass gasification [205]). In order to decrease the syngas ratio, part of the electrochemically produced H₂ can be used in RWGS reaction to simultaneously increase C-utilization, for instance [213]. CO₂ HTE is the option of choice when a combination with thermocatalytic processes is desired, e.g., CO₂ total reduction [214]. The reduction of the equilibrium potential and the improved reaction kinetics are significant advantages of HTE operation compared with LTE. However, in cases of unavailability of external heat or when fossil-sourced heat is undesired, LTE might become the only option.

LTE follows two approaches based on the CO₂ phase employed as feedstock. The first concerns the gas phase, and the other concerns the liquid phase. The gas-phase operation requires high CO₂ concentrations to avoid electrode flooding (deactivation) and ensure good conversion [215,216]. Reactor architectures to deliver gas-phase CO₂ to the cathode rather than dissolved in a liquid electrolyte are recommended to improve current density [217]. Similarly, electrodes that facilitate the mass transport of CO₂ gas to catalytic sites pushed the development of gas diffusion electrodes (GDEs), which also control water flux and inhibit side reactions [218]. GDEs enable operationsat current densities over 200 mA/cm² to produce CO and formate [218].

The high over-potential of CO₂ LTE probably represents the greatest challenge faced in advancing it toward scaling. The high activation energy (slow kinetics) and other sources of energy losses and inefficiencies are responsible for that overpotential. The design and development of new electrocatalysts represents only one of the approaches that address the issues and contribute to overcoming the challenge. Extensive catalysis R&D has focused on understanding the catalytic phenomenon and the development of new catalysts [219,220,221]. Catalysts based on metals, metal alloys, metal oxides/chalcogenides, carbon nanotubes, and enzymes have been proposed [222].

Both HTE and gas-phase LTE will require CO₂ to be captured, recovered, and purified from emitting sources prior to feeding to the electrochemical cell. Under the current state of technology, the low conversion and poor selectivity leave unconverted reactants mixed into the products, creating needs for separation, recovery, purification, and/or recycling, which affect the economy of the technology [200,223,224]. Process intensification under the LTE mode of operation may be the improvement approach to choose. The reported studies include the electrolysis of bicarbonate (mostly KHCO₃) solutions [225,226] and the direct co-electrolysis of captured CO₂ solutions. Bicarbonate solutions were tested to evaluate the change of the anodic reaction in decreasing the overpotential. An operating voltage of 2.3 V was achieved by running the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER) [227]. The electrocatalytic selectivity of Zn-Al layered double hydroxides for bicarbonate reduction at a current density of 15 mA/cm² and −1.4 V vs. RHE was 77% for CO and 94% for syngas [228]. The lifecycle and economic assessment of the electrochemical conversion of bicarbonate solutions for formate and CO production was reported, and in both cases, this capture–conversion integration rendered economic and environmental benefits, leading to an eco-efficient categorization of this C-utilization approach [229]. In general, electrochemical testing using bicarbonate solutions has gained acceptance as a rapid method for assessing the impact of different variables on the performance during the direct conversion of captured CO₂, facilitating the evaluation and understanding of catalytic parameters [230,231,232,233] of feedstock composition [226,234,235,236,237], and of operating conditions [225,238]. In a demonstration of direct co-electrolysis of captured CO₂, the intensification of solvent recovery and conversion within the electrochemical unit was considered [239], achieving conversions better than 70% under a current density of 200 mA/cm² [240]. In this case, a switch polarity solvent (1-cyclohexylpiperidine) was used as capture media. The performance for the co-electrolysis of monoethanolamine-captured CO₂ was not as good as the previously discussed, with a lower current density of 50 mA/cm² and faradaic efficiency (FE) of 72% [241]. (The faradaic efficiency (FE) of the CO₂ electrochemical reduction is the ratio of the measured amount of produced main reduction product (e.g., CO) and its theoretical amount evaluated using Faraday’s Law. Thus, it can be calculated as FE(%) = Fn_iχ_iṅ/I, where n_i is the number of the electrons involved in the considered CO₂ reduction, F is the Faraday constant, χ_i is the volume fraction of the considered product, I is the electrical current, and ṅ is the molar CO₂ gas flow rate. The FE was lower (45%) when 2-amino-2-methyl-1-propanol (AMP) was used as a capture solvent and electrolyte for CO₂ reduction to CO [242]. Syngas was selectively produced from CO₂ captured in 1.0 M alcohol amine solutions, using electrocatalysts of Au or Cu supported on MgAl-layered double hydroxides. The hydrogen evolution reaction (HER) was enhanced toward negative potential under employed conditions [226]. The effect of different amine-based deep eutectic solvents used as electrolytes for CO₂ reduction on the cell performance was evaluated. A synergistic effect among nano-size agglomerate dispersion on Ag-surface, bicarbonate formation, exchange current density, and Cl⁻ ions present in the deep eutectic solvent facilitated CO₂ reduction to CO [243]. Recovery of the capture solvent was the focus of the study as a way to leverage intensification [244].

The electrochemical pathways and electrocatalytic reduction of CO₂ to produce syngas have been the subject of patents and patent applications. Reported concepts and schemes include multistep, integrated, and intensified processes [245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268].

The lifecycle of a power-to-syngas integrated system (renewable energy-based) was assessed for an HTE operation mode. This study considered CO₂ from direct air capture (DAC) as feedstock. In comparison with an SMR-based system, the HTE showed lower global warming potential (GWP) and fossil fuel depletion, as well as negative net GWP in cradle-to-gate LCA when HTE was exclusively operated on renewable power. However, continuous HTE operation would require additional fossil electricity, under which circumstances achievement of negative net GWP is limited, and other environmental issues (human toxicity, acidification, particulate matter, or metal depletion) became worse than for SMR [269].

2.1.4. Radiation-Induced Pathways

The effects of ionizing radiation on molecules include excitation, ionization, neutralization, activation, and conversion via chemical reactions. At this point, it is worth defining some radiation-related parameters. The absorbed dose is the amount of radiation absorbed by an atom or molecule, and it is measured either using the gray (Gy, SI unit, J/kg) or the rad (CGS unit, 100 erg/g, 0.01 Gy). The magnitude of the radiation effect on a chemical transformation is parametrically defined by the G-value. The G-value is the number of molecules, atoms, or free radicals formed or converted per 100 eV of energy absorbed.

The radiolysis of CO₂ has been studied for over a century, and the main reaction that occurs is reductive decomposition (R19), with CO as the main product [270]. The maximum decomposition observed was 0.1% on solid CO₂, with a G value of 9 to 10. This low yield (G value) has been explained as due to a greater reverse reaction, known as backreaction, i.e., recombination of products. Nonetheless, a reaction network has been proposed to explain the low yields, as well [271,272].

CO₂ → CO + O

(R19)

Since ionizing radiation has additional effects than just chemical conversion, the presence of other molecules can result in the production of other species besides CO. Some of the various chemistries that ionizing radiation can induce on CO₂ have been described and discussed previously [28]. Regarding syngas production and based on the (R4)–(R6) and (R9) reactions described above, it seems possible to combine the CO₂ radiolytic reactions with those of CH₄, O₂, and/or water to induce the formation of the desired product. The radiation-induced reactivity and reactions of this reacting system have not been studied systematically. Instead, the reaction with water and with methane have been studied in more detail.

The individual radiolysis of CO₂ and H₂O was found to be affected by the phase state of the reactant, the dose rate, and the ionizing radiation type. The effect of irradiation conditions was stronger for CO₂ than for water [273]. Both atomic hydrogen [274,275] and H₂ [276,277,278,279,280] have been observed to be formed from water radiolysis. It can be expected that the ion radicals or even the nascent hydrogen formed under irradiation will be highly reactive and react with other molecules present. Conflicting G(CO) results have been reported for the reaction with water; in one instance, water increased G(CO) [281], while in another, a decrease was observed [282]. These studies did not report H₂ formation. High reported CO yields were explained as due to the formation of weakly bound H₂O···CO₂ complexes that decomposed into CO. This study also postulated the presence of hydrogen atoms [283]. The reported radiolysis studies of CO₂ and H₂O mixtures indicated the formation of CO and H₂, though these products were supposed to be formed from the radiolysis of the individual molecules [284,285,286]. These radiation-induced reactions (probably due to the potentiality of the formed products) have given rise to patents [287,288,289].

In the reaction with methane (or with heavier hydrocarbons [290,291]), yields and product distribution varied. Reported results include the deposition of reactive carbon [292], the formation of a formaldehyde polymer wax [293], and a mixture of hydrogen, ethane, higher alkanes, ethene, acetylene, and polymeric (C_nH_2n) liquid [293,294]. More recently, the radiation-induced MDR reaction (R4) was proven at temperatures close to ambient, demonstrating further that a small amount of methane can stop the backreactions from occurring, with a consequent CO yield increase. Furthermore, radiation-induced catalytic promotion was also proven to take place at the same operating conditions [29].

2.2. Methanol

Conventionally, methanol synthesis is carried out from syngas as a ratio of 2 (R20) [295], using commercially available and currently operative, mature catalytic (e.g., [296,297,298,299,300]) process (e.g., [71,301]) technologies. However, the fact that methanol synthesis is an energy- and carbon-intense process has prompted the search for a sustainable alternative [302].

CO + 2H₂ ⇌ CH₃OH     ΔH_298K = −91 kJ/mol

(R20)

The uses of methanol (e.g., fuel, chemical, solvent, etc.) [303] as well as its applications (reactant, building block, fuel cells, etc.) [304] are increasing and pressing its industrial demand into a continuous growth cycle. An important methanol attribute is to be a liquid at ambient conditions that makes it pipeline transportable, handled, and distributed using existing infrastructure. It can be used as a fuel or blendstock in the gasoline range. Otherwise, it can be converted into dimethyl ether (DME) that can be used as a fuel or blendstock in the diesel range or replace liquefied natural gas (LNG) and liquified petroleum gas (LPG) [154]. For instance, methanol is the main component in feedstock for the synthesis of other intermediates and building blocks (e.g., olefins, ethanol, acetic acid, etc., see Figure 3). Technologies for olefin production from methanol (MTO process) are already mature technology [305,306,307]. In summary, methanol can be used as vehicular fuel, it can generate electricity in fuel cells, and it is an important raw material in the chemical and petrochemical industry [153]. Hence, methanol is an important energy carrier for a clean and sustainable energy future, which, within the NPtX scenario, appears as a convenient solution for efficient energy storage on a large scale and has an emission reduction potential if produced from CO₂. Methanol and these derivative products already have a well-established (large) market, and if produced from CO₂, both the intermediate product and its derivatives will represent a vast consumption of this GHG compound. Commercially operating technologies are currently used for the production of methanol derivatives; hence, no further discussion is provided here on these mature technologies. Instead, the work regarding methanol synthesis from CO₂ follows.

2.2.1. Thermocatalytic Pathways

The thermo-catalytic CO₂ hydrogenation can produce methanol in a single step characterized by reaction (R21) or in two steps involving the production of CO by RWGS (R9), as the first step, and feeding additional H₂, in a second step, after intermediate H₂O removal. This second step proceeds in a conventional methanol synthesis reactor.

CO₂ + 3H₂ → CH₃OH + H₂O     ΔH_298K = −49.4 kJ/mol

(R21)

At this point, it is worth noting that CO₂ hydrogenation gives rise to the so-called “hydrogen conundrum” (the term conundrum is used here to refer to a problem that is difficult to solve because its solution might take it back to the causes of the problem) since, traditionally, H₂ is produced by SMR, which, as mentioned above, is a CO₂-emitting process. Hydrogenation reactions are generally exothermic, and in the case of the methanol-producing (R21) reaction, the methanation reaction (shown as reaction (R1)) can also take place under similar conditions. Therefore, promoting either reaction requires the development of highly selective catalysts, which in the case of CO₂ hydrogenation has not received enough effort, and most of the published results concern the use of conventional methanol synthesis catalysts.

Carbon Recycling International (CRI) licenses technology based on the Olah single-step process (R21), the Emissions-to-Liquids (EtL) technology [308]. An EtL demonstration plant was commissioned in 2005 and has been operative since then in Svartsengi, Iceland [309]. Renewable (geothermal) power and green hydrogen produced from water electrolysis are employed to produce methanol using CO₂ captured from the geo-power plant. In February 2024, CRI started up a larger EtL plant in Anyang City, Henan Province, China, to produce 110 kton of methanol to be used as a renewable fuel in the plant fleet [310]. The plant is expected to consume 160 kton_CO2/y, captured from an adjacent coal power plant.

Lurgi also offers another single-step option [311]. This process has undergone subsequent improvements, including the addition of stages, reactors, heat exchangers, separators, etc. [312,313,314,315,316], after its initial development.

Regarding two-step processes, a technology option is commercially available: the CAMERE technology [317,318]. In the first step, the RWGS reactor is coated with a water perm-selective membrane to improve water removal. In this way, CO₂ reduction increases, improving CO yield [319,320]. As mentioned above, the second step considers a hydrogen addition to proceed according to typical methanol synthesis conditions (R20) [321].

The use of conventional methanol synthesis catalysts (CO hydrogenation catalysts) in all these processes creates drawbacks derived from poor selectivity and low conversion that jeopardize the process economy. Some of the side reactions involved from the lack of selectivity include the Boudouard (R8), the methanation (R1), and WGS/RWGS (R9) reactions, which have been mentioned already. Worth emphasizing is the Sabatier methanation reaction (R1) [322], with ∆H = −165 kJ/mol, which, although being an energy-intense reaction, has received great attention due to its role in reviving the PtG program [323]. Other side reactions lead to the formation of alcohols, ketones, esters, and oligomers, such as the following:

CO₂ + 2H₂ → CH₄ + O₂     3CO₂ + 3H₂O → C₃H₆O + 4O₂

nCO₂ + 3nH₂ → (CH₂)_n + 2nH₂O     2CO₂ + 2H₂O → 2CH₃OH + 3O₂

CO₂ + 2H₂O → CH₄ + 2O₂     2CO₂ + 2H₂O → 2HCOOH + O₂

Additionally, water formation not only limits the CO yield but also represents hydrogen and exergy losses [324]. Therefore, more active and selective catalysts (not for CO but rather for CO₂ hydrogenation) are needed together with more energy-efficient processes. Some of the efforts devoted to heterogeneous catalysts can be found in [325,326,327,328,329,330,331,332], photocatalysts [333], biocatalysts [334,335], and processes [58,222,336,337,338,339,340,341,342,343,344,345,346,347]. Additional improvements have been reached by integration to make power plants more energy-efficient [348] and by CO₂ recycling [349].

2.2.2. Electrochemical Pathways

The hydrogen conundrum has prompted the incorporation of green hydrogen in CO₂ hydrogenation applications [350]. The production of green hydrogen falls outside the scope of the present work, and although these pathways are sometimes regarded as electrochemical pathways, in this work, we will consider only those in which CO₂ is electrochemically converted.

The electrocatalytic processes have been gaining track and include CO₂ reduction (e.g., [351]), co-electrolysis (to syngas followed by methanol synthesis), and hydrogenation processes [352,353,354,355,356]. The electrochemical CO₂ reduction to form methanol has been mainly studied for gas-phase systems [357,358,359], with substantial efforts devoted to electrocatalytic studies. Improved gas dispersion was attained when using electrocatalyst support conjugated microporous polymers (e.g., TPE-CMP) mixed with carbon nanotubes (CNT) to ensure a high electronic conductivity. Pt nanoparticles were doped onto the support material as an active phase [360]. CO₂ adsorption may be strongly enhanced due to the pore structure of the p-conjugated polymer. Good performance measured as liquid product yield was thought to be due to the high local concentration of CO₂ on the polymer surface, enabling access to the active metal nanoparticles [360].

Improvements have been attempted by the incorporation of biocatalysts [334,335] and the development of bio-electrochemical processes [222]. One of the former examples of bio-electrocatalyst consisted of immobilized enzymes for the reduction of CO₂ and reported a very low FE (<10%) for methanol formation [334,361]. The low solubility of CO₂ in aqueous media is a barrier to high throughput operation of bio-electrocatalytic technologies.

Electrocatalytic hydrogenation to higher-reduced products has been claimed in the patent literature [250,251,253,254,256,260,352,353,354,355,356,362] based on studies carried out at bench-top scale. Results can be characterized by low yields, poor selectivity, high over-potential, low FE, and current densities. Based on the revised literature (articles and patents), the electrochemical route needs improvements on electrocatalysts [360,363,364] and on cell potential, current density, and durability of materials and components [359].

2.2.3. Economic and Environmental Assessments

The methanol economy [71,365] centers this molecule as the core to drive the production of a variety of products (fuels, chemicals, and materials). It has been around for more than 15 years, and although all products are commercially produced, such an economy has not emerged. Implementing this economy based on the existing technologies will bear drastic environmental consequences due to their energy intensity and carbon footprint. Instead, using CO₂ for its production represents a compelling alternative to massively utilize the carbon emissions [71]. Nevertheless, when considering CO₂ as a feedstock, only a few of the needed technologies are commercially available; some need improvements [366], and many are under development [357,367]; however, many for derivative products and materials are mature [71]. The technological maturity of both thermocatalytic [357,368,369,370,371,372,373,374,375] and electrocatalytic pathways [247,248,357,376,377,378] vary but still offer a myriad of opportunities for a decarbonized and decarbonizing technology platform to start implementing the methanol economy.

The economic and environmental assessment of the CO₂-based processes are rarely reported. Improved energetic efficiency was assessed for methanol as an energy carrier considering the overall energy conversion-storage system, including methanol as the storage medium. The study, based on thermodynamic data for methanol synthesis, showed the exergy of the heat released in the synthesis reactions of 16.2–20.0%, depending on the applied conversion technology [324].

The capture costs and the energy (electricity) costs contributed together to make the electrochemical production of methanol from CO₂ extremely expensive in Germany [379]. This study demonstrated how capture costs (and thus feedstock), energy source, and location affect the methanol production economy. Nevertheless, a production cost comparison among different technologies and feedstocks was reported without including capture cost, feedstock production costs, and geographical location impacts [380].

Exothermic reactions like methanol synthesis offer an opportunity for energy integration between the capture and the synthesis units. A supply chain optimization in Germany assessed economic profits of 999.62–1568.17 €/ton_CO2 captured and utilized. Utilization was exemplified with the production of methanol, DME, formic acid, acetic acid, urea, and polypropylene carbonate [381]. However, in some instances, these energy savings are not enough to compensate the economy, and it was shown that a 33% energy saving in solvent regeneration still resulted economically unfeasible when the energy cost was 70 €/kWh since the production cost was 1.5 times the evaluated revenues [344]. Energy savings were also mentioned for the integration of a biomass-fired oxy-combustor and a solid oxide electrochemical electrolyzer, but economic and/or environmental impacts were not reported [382].

Seven different integrating configurations/scenarios were considered in the environmental performance of methanol (and urea) production using CO₂ emitted from a blast furnace of a steel mill. The global warming impact (GWI) reductions evaluated across the whole functional unit (FU) varied from 0.70 to 4.62 MtCO_2-eq/FU within the seven scenarios [383].

2.3. Urea

In nature, mammals synthesize urea (CO(NH₂)₂) when metabolizing proteins. Industrially, it is produced using the Bosch–Meiser process [384,385], which was developed in 1922. This technology consists of a two-step process characterized by reactions (R22) and (R23). The first step is the reaction between liquid ammonia (NH₃) and gaseous CO₂ (R22) to form carbamate that decomposes through reaction (R23) in the second step, yielding urea and water. While the carbamate formation reaction (R22) is an exothermic reaction, its decomposition (R23) to yield urea is endothermic. Currently, the industrial production of urea consumes the largest CO₂ volume by any CO₂ conversion process (about 130 Mton_CO2/y, globally) [386].

2NH₃ + CO₂ ⇌ H₂N-COONH₄     ΔH_298K = −117 kJ/mol

(R22)

H₂N-COONH₄ ⇌ (NH₂)₂CO + H₂O     ΔH_298K = 15.5 kJ/mol

(R23)

The urea market is determined mainly by its use as a source of nitrogen in fertilizers and, in a minor proportion, by its use as a co-reactant or intermediate in the chemical industry. In fact, around 90% of the global urea production (~184 ton/y in 2022) is consumed by the fertilizer sector [385]. Excessive consumption of urea for fertilizer applications would have an environmental impact directly in the agricultural sector and indirectly through its production in the industrial sector. In the chemical industry, urea is applied as a building block for producing two materials: urea–formaldehyde and urea–melamine–formaldehyde resins [52,387,388,389,390,391].

The large CO₂ consumption during urea synthesis cannot compensate for the volume emitted from the ammonia production processes. The energy and carbon intensity of the ammonia-producing technology is partially due to the high severity employed and the large hydrogen consumption. Hydrogen production is also an energy- and carbon-intense process. The ammonia synthesis process is one of the top energy-intense technologies. Ammonia synthesis, together with benzene, methanol, and steam cracking, is one of the four top processes responsible for the high CO₂ emissions of the chemical industry [392]. Not all urea plants include an ammonia plant, and not all ammonia plants include a hydrogen plant; therefore, only a few cases can recycle part of the ammonia plant-emitted CO₂ to feed the urea synthesis process. Green ammonia production will represent a big step toward the sustainability of the urea synthesis technology. The search for more energy-efficient and less severe processes for ammonia synthesis has received significant R&D efforts for many decades. More recently, the electrochemical synthesis of ammonia appeared as one of the green alternatives that could emerge in the near future (e.g., [393,394,395,396,397,398]). Cleaner alternatives have been announced by the major technology licensor for converting some ammonia plants from brown to blue and building new commercial-scale plants of green ammonia (e.g., [399,400]). Then, it seems that the efforts on green ammonia and hydrogen production will shorten the wait for sustainable urea production.

These opportunities toward large-scale sustainable production of urea (and so of its derivatives) appear with barriers and limitations, some of which have been identified in sustainability assessments and studies. For instance, the unfeasibility of completing the decarbonization of the ammonia industry by 2050 was revealed in a prospective LCA study [401]. Regardless of the potentialities of electrolysis-based green ammonia, replacing fossil-based ammonia will require expansion of renewable energy, reduction in the urea demand, infrastructure for CO₂ transport and storage, and securing supply of raw materials. Additionally, considering the longevity of existing plants, the growth of ammonia as a fuel will favor the economy [401]. Other points of attention for the urea plant that require improvements include the following: the material preparation stage, synthesis stage, and waste treatment stage, while for the hydrogen production unit, steam generation, replacement of fossil fuels burning, and energy efficiency measures [402]. Optimization to maximize efficiency and minimize costs is needed for the amine CC unit to avoid exergy destruction since an exergoeconomic analysis showed this unit as responsible for the environmental impact of the plant [403]. The environmental impact of urea production has been assessed as a way to reflect its potential in GHG emission mitigation. As an example, steel mills might reduce more than 65% of their emissions by integrating them into a urea plant run on wind energy [383].

The environmental impact of urea agriculture application could be the most relevant. The environmental impact of urea production and use collectively includes not only GHG emissions but also eutrophication and acidification [404]. Synthetic fertilizers (including urea) were found to be environmentally better than organic-derived ones when 18 environmental indicators were evaluated in terms of kg of nutrients rather than per kg of product [405]. The incorporation of renewable resources and sustainable management are needed to attain sustainable agriculture, as revealed by an integrated LCA and Emergy. (Emergy is a thermodynamics-based concept that provides a holistic environmental accounting system. An emergy assessment identifies and measures all energy and matter inputs into a given system to evaluate the environmental cost of a given resource, expressed in a common unit, the solar emergy joule (sej). The unit emergy value, UEV, is the emergy per unit product.) The authors of [406] evaluated the environmental costs and impacts of resources (including urea) in agricultural production [407]. The environmental impact of urea production has also been evaluated for other fertilizer/agricultural applications. One of these was the production of a bio-based binder derived from hemp, cultivated using urea-based fertilizer. In this case, urea production topped the contributions to carbon emissions (21%) due to the energy consumption of the urea plant (30.1 GJ/ton_urea, [402]) [408]. Nonetheless, some benefits have also been reported, this time for the blending of controlled-release urea that increased agronomic product yield and decreased associated emissions [409]. At a global scale, sustainability improvement measures might include the use of renewable feedstocks and raw materials, eco-efficient upstream processes, and renewable energy sources [410]; here, one may add C-neutral sources and a note of attention for NP.

Versatile resins can be synthesized by the co-polymerization of urea with formaldehyde. The complex chemistry involved in the polymerization reaction includes addition and condensation reactions. The urea molecule undergoes the addition of hydroxymethyl oligomers formed by condensation and cross-linking reactions of the hydroxymethyl moiety of formaldehyde [411,412]. Both the kinetics and the properties of the produced resin are affected by temperature, pH, and the formaldehyde-to-urea (F/U) molar ratio [413], which determine the extent of branching and type and proportion of chemical bonds [414]. Urea-formaldehyde (UF) resins are mostly used as wood adhesives (85% of the ~11 Mton/y global adhesive market [415]) in the manufacture of plywood, particleboard, fiberboard, and oriented strand board [416,417]. The post-war housing efforts created a market opportunity for plywood and particleboard [418]. However, soon after this popular application escalated, the worst environmental impact caused by formaldehyde releases over time was discovered, leading to indoor pollution that threatened the health of exposed humans [419,420,421,422,423,424,425]. Mitigation of formaldehyde emissions and improvements to product properties were achieved through changes in the polymerization conditions and in the manufacturing process [415,419,426,427,428,429].

The possibilities for green/renewable urea production leave the challenge to sustainable formaldehyde production before any attempt for sustainability can be made for UF resin production. Pathways from CO₂ [222,352,430,431] and from biomass [432,433,434] have been investigated for UF resin production, but development is in its infancy. The comparison of the environmental impact of UF resins with bio-based adhesives is controversial. LCA studies showed lower environmental impact and functional advantages on composite adhesives [435,436], but another study concerned with bio-cementitious materials found a greater environmental (eutrophication) impact [437].

Abating formaldehyde emissions was also attempted by reformulating the resin to improve stability through the incorporation of melamine, a cyanamide trimer (C₃H₆N₆) [438,439,440,441,442]. Due to its toxicity, melamine use and commercialization are highly regulated [443]. Though a wide range of applications is found in tableware and building materials [444], additional applications are given to the products manufactured with melamine copolymerized UF resins [445,446,447,448,449,450] in reinforced wood panels, construction, shopfitting industry furniture, interior decoration, etc. These low-cost melamine-UF resins advantageously offer high strength, high dimensional stability, isotropicity, and enhanced processability. Compared to UF resins, melamine-UF resins exhibit lower photochemical oxidation, ecotoxicity, and human toxicity [451], and the environmental impact of its production is mainly due to energy consumption and the factor derived from the UF resin production [452,453]. Therefore, sustainable melamine-UF resins will pass first through the sustainable production of UF resins and then sustainable melamine production, involving (green) urea (e.g., [388,391,454]) and (green) NH₃ (e.g., [387,389,390]), will be pursued.

2.4. Polymers

The synthesis of oxygenated building blocks can be achieved by carboxylation reactions, which add a carboxylic group (-COO formed, for instance, from CO₂) in other organic molecules. Various organic compounds can be obtained, e.g., acids [455], esters, lactones, carbamates [456,457,458], and carbonates [459]. Among the chemical reactions of CO₂, carboxylation is the least energy-intense, and, in return, compounds with high energy value, e.g., strained cyclic molecules, or unsaturated compounds can be produced. Similarly, in hydrocarboxylation, CO₂ reacts with olefins and/or alcohols in the presence of H₂ to yield carboxylic acids or esters [460].

Polymeric materials such as polyacids, polyurethanes, and polycarbonates can be derived, respectively, from carboxylic acids, esters, and organic carbonates produced by carboxylation [461,462]. These high-molecular-weight materials fixate CO₂ in their matrix as long as their lifecycle or service cycle lasts. The equivalent materials, currently produced from fossil feedstocks, could be replaced by these decarbonized options that are additionally biodegradable, renewable materials with low toxicity. In conclusion, carboxylation reactions can use massive volumes of CO₂ to produce commodity and special materials.

2.4.1. Thermocatalytic Pathways

Carboxylation of alcohols produces dialkyl carbonates, according to reaction (R24), which occurs under base-catalyzed conditions at high pressures. Tin oxide, zirconia [463], and alkoxides of Sn (IV) and Ti (IV) [464] are examples of studied catalysts.

CO₂ + 2ROH → RO-COO-R + H₂O

(R24)

Dimethyl carbonate (DMC) constitutes the most relevant example of an organic carbonate monomer based on its possibilities for sustainable manufacturing via the carboxylation of methanol (R25). Since green methanol can be synthesized sustainably, as discussed above, and CO₂ is considered a renewable feedstock then, DMC can become sustainable in the short term.

CO₂ + 2CH₃OH → CH₃OCO₂CH₃ + H₂O     DH°_298K = −27.90 kJ/mol | DG°_298K = 26.21 kJ/mol

(R25)

The exothermicity of the (R25) reaction could advantageously be used for energy integration and to develop a more energy-efficient process. However, thermodynamic limitations (DG°_298K) preclude the reaction from occurring spontaneously at ambient temperatures and pressures [465], calling for process intensification. Reactive distillation increased conversion from 10% to 99.5% and also reduced energy consumption [466] (reactive distillation is an intensified process that combines distillation with chemical conversion in a single unit, the distillation tower; a catalytic system is placed conveniently within the tower to separate one of the products from the reacting mixture and favor conversion or shift the equilibrium to the right). Product recovery is another process difficulty since DMC and methanol form an azeotrope. Extractive distillation processes, commonly used for azeotropic separations, consume large volumes of solvents and energy. Economic benefits (70% savings in total annual costs) and positive environmental impact were found when product recovery was carried out with ionic liquids, though some evaluated solvents thermally decomposed under employed conditions [467]. DMC electrochemical synthesis, in the presence of 1-butyl-3-methylimidazolium bromide and potassium methoxide, showed better performance (within the limitations of product recovery) and environmental impact than the currently commercially used technology (ENI process) [468]. Commercial DMC production is based on the carbonylation of methanol reaction (R26), which has been replacing the original technology based on the reaction between phosgene and methanol, forming methyl chloroformate as intermediate (R27) that then decomposes into DMC and hydrochloric acid (R28).

CO + ½O₂ + 2CH₃OH → CH₃OCO₂CH₃ + H₂O

(R26)

COCl₂ + CH₃OH → CH₃OCOCl + HCl

(R27)

CH₃OCOCl + CH₃OH → CH₃OCO₂CH₃ + HCl

(R28)

The catalytic active phase for the DMC synthesis is an amphoteric pair of sites, on which the acid sites promote methylation from methanol while the basic sites activate CO₂. Examples of this type of catalysts include CeO₂ [469], ZrO₂ [470], hydrotalcite [471], Y₂O₃ [465], and [Nb(OMe)₅]₂ [472].

The DMC market is largely determined by its use as a methylating agent [473,474]. However, this use is threatened by its consequent CO₂ formation, which increases atmospheric releases, though new developments in methylating processes to mitigate emissions have started [475]. Regardless of these threats, DMC demand has been continuously growing, underpinned by its advantageous properties and attributes that include non-toxicity, reactivity, and derivatives (e.g., aromatic polycarbonates, solvents, and fuels) [476]. The search for a sustainable production pathway is incentivized by this growing market and by the interest in DMC’s advantageous properties. One example is urea methanolysis [477], which produces methyl carbamate, from which DMC results through a reaction with methanol [478]. This route showed slight environmental advantages but more technical limitations. The sustainability of studied/developed pathways (six different pathways, Path 1 through 6, described below) has been compared by assessing the environmental impact based on the stoichiometry of involved reactions [479]. As can be noticed, CO₂ can be consumed directly in Paths 4 (for urea synthesis), 5, and 6.

Path 1		(R27) + (R28)	Phosgene, coproducing HCl
Path 2	(R29)	2NO + 2CH3OH → 2CH3ONO + H2	via methyl nitrite, coproducing CO and NO
	(R30)	2CH3ONO + CO → CH3OCO2CH3 + 2NO
Path 3		(R26)	Methanol carbonylation
Path 4	(R31)	CO(NH2)2 + CH3OH → CH3OCONH2 + NH3	Urea pathway, coproducing NH3
	(R32)	CH3OCONH2 + CH3OH → CH3OCO2CH3 + NH3
Path 5	(R33)	(CH2)2O + CO2 → (CH2)2OCO2	Ethylene oxide (EO) pathway, coproducing ethylene glycol (EG)
	(R34)	(CH2)2OCO2 + 2CH3OH → CH3OCO2CH3 + (CH2)2(OH)2
Path 6		(R24)	Methanol carboxylation

The indicators used in the assessment of the environmental impact were the potential for ozone depletion; for photochemical oxidation; and for acidification; GWP; and human, terrestrial, and aquatic toxicity. A total score was evaluated for each path, considering the environmental indicators and the economic and toxicity analyses. Based on these total scores, the paths were ranked 3 → 6 → 2 → 5 → 4 → 1. The economics of Path 5 was found to be the best, while the environmental assessment favored Paths 3 and 6. However, Paths 1, 2, and 3 do not represent decarbonizing options and should not be considered for further development if industrial decarbonization is the purpose. In fact, Path 1 is already being phased out. Therefore, methanol carboxylation (Path 6) and the EO Path 5 pathways should be kept on the developmental plate, although the authors ruled out Path 6 on unexplained industrial unfeasibility grounds [479]. Optimized data were obtained from Path 4 and Path 5 simulations and used for evaluating sustainability indexes such as material index, energy index, and ecoefficiency. This new evaluation confirmed the superiority of Path 5 over Path 4 [480]. By-product recycling was considered for improving Path 5 and resulted in environmental (10% impact reduction) and economic benefits of a 20% increase in DMC yield, a five-fold increase in the net present value, and a reduction of the payback period from 8.3 years to 3.6 years [481].

DMC can be one of the starting reactants in the production of polyurethanes (PUs) if a sustainable pathway is desired. Its reaction with amines produces carbamates [482], which can be converted into isocyanates [483]. The isocyanate NCO groups react with polyol OH groups to form PUs. Alternatively, DMC could react with aliphatic diols to form oligo-carbonate diols and also polycarbonates [484,485]. The nearly 26 Mton/y PUs’ global demand is defined by their uses in manufacture (e.g., footwear, sponges, pillows, mattresses, seats, cast objects, bezels, structural parts, etc.) and in construction (e.g., thermal insulation boards, structure and sandwich panels, foams, etc.). Nevertheless, construction applications are limited by their flammability. New applications are also emerging, such as the chemical uses of oxadiazinetriones [486]. The positive environmental impact of PUs lies inarises from their chemical inertia and non-carcinogenicity. However, their decomposition products are hazardous and toxic, and some can even be lethal. Therefore, the assessment of the Pus’ environmental impact needs to involve both production methods and applications and to consider human exposure to toxic and hazardous raw materials and/or compounds [487]. The evaluation of these health impacts should be as comprehensive as possible, including all materials, products, and compounds and their life cycle stages (e.g., production stage, storage stage, service life, disposal stage, etc.). Comprehensive evaluations have not been reported; instead, specific stages for particular applications can be found, and some examples are as follows: PU-containing tiles [488] (e.g., gypsum ceiling tiles [489]), PUs’ production stage (e.g., as building material [490], insulating panels [491], waterborne PU dispersions [492]), service life stage (e.g., insulation applications) [493]. All these analyses concerned petrochemically produced PUs, and the takeaway message is that environmental benefits can be obtained from PU applications, but a need for a sustainable production pathway exists. One approach is forming urethane groups from the reaction of cyclic carbonates with bifunctional primary or secondary amines. This approach has been considered for the synthesis of thermosetting plastic networks [494], which can be used as binder to replace Portland cement in concrete for building applications [495]. Another approach that considers biomass and CO₂ as starting materials has been investigated to formulate new composite materials. Some of these investigated pathways have been economically and environmentally assessed [496,497,498,499,500,501]. Glycerol carbonate emerges as a potential candidate for renewable starting material, particularly considering the increasing glycerol surplus arising from biodiesel production [502]. In fact, glycerol appears economically attractive to underpin a production platform for renewable chemicals and products [503,504]. Glycerol carbonate can be obtained from the reaction of glycerol with CO₂ [505] using tin complexes as homogenous catalysts [506,507] or with urea in the presence of metal-impregnated zeolites [508], zinc salts [509], and lanthanum oxide [510]. Glycerol carbonate can be used in the manufacture of PUs, polyesters, and polyamides as well as solvent, emulsifier, and chemical intermediate [511]. The economic and environmental impacts of biomass-involving pathways and polymer products have been reported [496,497,512,513,514,515,516,517,518,519].

Specialty chemicals and polycarbonates can be produced from cyclic carbonates, economically and environmentally. The thermocatalytic production of polycarbonates from the reaction of CO₂ with epoxides has been studied using organic bases, alkalis, metal halides, metal complexes, ionic liquids, zeolites, and metal oxides [481]. The industrial scalability of the potential processes was challenged by catalyst stability and/or recovery [520]. Reported studies on the catalysts did not address these scalability-associated issues; instead, kinetics and selectivity were focused on [458,521]. R&D activities leading to the development of catalysts and processes for CO₂ chemical utilization started at Bayer during the first decade of the 2000s [522]. An overview of the considered chemical technologies was reported in [459]. A variety of polymers could be produced from the developed and scaled-up processes, e.g., polycarbonates, polyethercarbonates, polyacrylates, and PUs [82]. The reaction of cyclic acid anhydrides and epoxides with CO₂ produced terpolymers and polyalkylenecarbonates, respectively [462].

2.4.2. Radiation-Induced Pathways

Additionally, to thermocatalytic pathways and methods described above for the production of polymers, ionizing radiation has been considered in the search for more energy-efficient processes. The radiation-induced carboxylation reactions have been employed to produce salicylic acid from phenol [523,524], malonic acid from acetic acid [524,525,526,527], glycolic acid [523], and amino acids from amines [528]. The radiation-induced carboxylation of methanol produces formic, glycolic, and oxalic acids, as well as ethylene glycol and formaldehyde [529]. Meanwhile, radiation-induced carboxylation of formic acid produced preferentially oxalic acid [530]. The conversion of CO₂ has been found to produce transient species such as COO⁻/COOH and organic (hydrocarbonaceous) radicals, leading to the production of carboxylic acids, esters, or organic carbonates [528].

Although polymerization reactions are known to be initiated by free radicals and these key reactive species can be created by γ-radiation, only a few works have been reported for radiation-induced polymerization, e.g., vinyl monomers [531], styrene and methyl styrene [532], succinic acid [533], methylmethacrylate [531], and carboxylic acids [534]. On the other hand, CO₂ can be grafted as carboxylated functional groups on linear low-density polyethylene by γ-irradiation under supercritical conditions. Thus, radiation-induced carboxylation can be used to functionalize polymers [535]. Other organic compounds have been grafted on polymers, e.g., maleic anhydride [536], that can be synthesized using CO₂ as a (co-)reactant.

Assuming that (i) carboxylation reactions can be induced by ionizing radiation, (ii) the products of the carboxylation reactions can polymerize, and (iii) polymerization reactions can be induced by radiation, it could be expected that there might be reaction conditions under which carboxylation and polymerization can be carried out in a single chemical process step under the effects of ionizing radiation. The monomer production and its consumption through polymerization in one pot will overcome the low carboxylation yield that is thermodynamically limited by shifting the equilibrium to the right. Even though the production of value-added polymers is one of the economically rewarding massive chemical utilization of CO₂ routes and all the involved reactions have been proven to occur, the development of CPs to carry these reactions out is still in its infancy. Moreover, the recent discovery that polymers containing CO₂ moiety are responsive to a variety of stimuli/effects has highly increased the value of these polymers.

2.5. Carbonates and Bicarbonates

Carbonation is a naturally occurring reaction between CO₂ and alkaline or alkaline earth elements or their oxides, leading to the formation of mineral materials, which is also known as mineralization. Carbonation moves carbon from atmospheric gases to Earth’s solid constituents as part of the C-cycle. This reaction on mineral deposits in geological formations has been used industrially with sequestration purposes to store CO₂ [56,59,87,537,538,539,540,541,542,543]. The experience gained from sequestration identified kinetic limitations and high energy consumption as the top challenges to address for a wider range of applications to be developed.

Inexpensive silicate minerals (e.g., serpentine, olivine, and wollastonite) are considered raw materials for the production of carbonates and bicarbonates using CO₂ in an acid solution. Metal cations (Me) lixiviate from the mineral matrix and react with the dissolved CO₂, resulting in the consumption of protons (R35) and solid precipitation (R36).

2H⁺ + H₂O + MeSiO₃ → Me²⁺ + H₄SiO₄

(R35)

H₂CO₃ + Me²⁺ → MeCO₃ + 2H⁺

(R36)

The carbonation heat of reaction (ΔH_298K of (R36)) ranges from −60 to −100 kJ/mol. This exothermicity does not compensate the kinetic limitations [544]. Cations leaching is the reason for the slow kinetics, moving R&D to investigate the effect of mineral type and composition, metal element, pH, and temperature on considered minerals and approaches: reacting phases (gas–solid vs. aqueous), resource (raw material and CO₂) selection, and raw material pre-treatment and/or activation [540]. For instance, ammonium salts can be used to pretreat the raw material to facilitate Mg extraction [545], and serpentinite and metaperidotite minerals are selected based on their high Mg content [546]. Increasing the temperature to improve the kinetics is not a solution for the aqueous-phase option since dissolved CO₂ will decrease. Thus, increasing temperature has to be accompanied by an increase in pressure as well to avoid the CO₂ move from the liquid to the gas phase, e.g., pressurized fluidized bed suggested by Abo Akademi [547]. However, solving the kinetic problems (by increasing severity) seems to aggravate the energy intensity problem.

The energy intensity problem was originally due to waste and mineral treatments (mechanical separation, crushing, milling, and grinding), material flow management, CO₂ compression, and/or process conditions [548]. Energy intensity has been addressed through efficiency improvements, process integration, and intensification. The integration of the heat released by (R36) could be used to balance the energy demanded by metal extraction/digestion, i.e., lixiviation (R35), in a sort of autothermal operation [549]. The economic benefits of cheap mineral resources bring other inefficiencies to the process. The metal content of these minerals and the carbonation stoichiometry imply that a large amount of mineral is required to remove or bind CO₂ (2–3 ton_rock/ton_CO2).

The development of the direct aqueous mineralization method started in the 1990s at Los Alamos National Laboratory [550] and continued at the National Energy Technology Laboratory (NETL), leading to [551,552,553] to produce Ca and/or Mg carbonates. This process follows the two steps described by (R35) and (R36) reactions [554]. The high energy consumed by the process was supplied by a coal-fired power plant, which made the process impracticable due to the large CO₂ emissions [553]. The economic drawback was identified as associated with the product’s low price and elevated production costs [555].

This perception of economic disadvantages was later confirmed by sequestration experiences and reported analyses/assessments [556]. For instance, carbonation costs using olivine ranged from $50 to $100/ton_CO2, which increased power generation by 30–50% when applied as a CCS method for power plants [78]. The process economy could be improved by enhancing energy efficiency [557], improving materials management [558], and by intensification [559]. Cambridge Carbon Capture claims to have improved both economic [560] and environmental [561] impacts by incorporating an alkaline pretreatment step [562,563], using a cheaper mineral [564] and CO₂ source [565], co-producing electricity [566], and valorizing all products and by-products [567]. This company emphasized the most value-adding aspects: the optimization of production costs and the valorization of all products [560]. The former aspect considers the use of cheaper materials [564], mostly industrial wastes (e.g., cement bypass and kiln dust, waste cement, construction and demolition mineral waste, red mud, fly ash [568], steel/blast furnace/electric arc furnace slags [569], mine tailings, municipal waste incinerator ash, and water treatment sludge [570]). Meanwhile, for the latter, the relevant products include calcium and magnesium oxides, carbonate mineral powders, silica, and high-purity metals [567]. Products’ value and valorization have been considered in the development of methods to produce specialty nanoparticle materials, e.g., calcium, vanadium, silver, copper, nickel, etc., by integration into different industrial processes and carbonating industrial wastes [571,572,573], e.g., steel mills [574], combination with RWGS [561], cement [575], and incineration plants [576]. Besides the economic benefits of nanomaterials production, the environmental assessment also had a positive result.

Electrochemical carbonation pathways have been employed to produce bicarbonates (e.g., ammonium [577]) and co-produce electricity [578] or hydrogen [579]. This type of pathway has been considered for recycling scrap metal sources, such as wasted aluminum foil [578,579].

Carbonation of industrial wastes might bear logistic limitations since, in some instances, wastes might be generated in a different location from where CO₂ is emitted. However, the metal (steel) and cement industries, two of the largest CO₂ emitters, are excellent candidates for implementing carbonation within their decarbonization strategies. In these cases, the direct use of flue gases [545,580] constitutes an intensification option by eliminating the capture step [560] and will contribute to economic improvements by reducing the required Capex.

Carbonation technological developments have been covered in patents, e.g., [563,581,582,583,584]. Assuming that economic and technical hurdles are overcome and carbonation processes are implemented, the market of the products is limited by their current uses and applications. Among the multiple uses and applications of carbonates and bicarbonates (health, cosmetics, pulp and paper, chemicals, hygiene, glass, photography, textiles, construction, etc. [45,585,586]), most relevant to this work is the nuclear sector, for which molten salts are used as cooling fluid in certain reactors [587,588,589,590,591,592]. Regardless of these many uses and applications, implementation of a potentially sustainable and decarbonizing production option will change market dynamics and will affect product prices and, in consequence, process economy. Therefore, new applications and uses need to be developed to contribute to demand growth and to keep the price from at least not dropping.

3. Integration to Nuclear Power

Although the idea of integrating an NPP into a CP is not new, it has not been accomplished, and the remaining challenges need to be addressed to further develop and realize NP-CP IES that could underpin the implementation of NP-to-X pathways. These challenges, for the purpose of this work, concern energy integration, which, in the case of heat and electricity integration, can be considered a mature technical area. However, the particularities of the heat and electrical current (intensity and voltage, for instance) that the NPP might offer and the CP might demand will be the subject of analysis, design, and implementation for the corresponding integration project.

Nevertheless, relevant experience was gained in Germany, Japan, Russia, and the USA on the heat integration of chemical heat pipes (Figure 2) during the development and demonstration of A&E and NH projects [593]. In German EVA-I tests, the primary loop of the high-temperature gas (helium) reactor (HTGR) was directly integrated into an SMR reactor. Although these tests provided valuable data on heat transfer, mechanical design, and behavior of reformer materials in helium with regard to H₂ and tritium permeation and corrosion [594], they also raised questions on the validity of a single-tube reactor. Thus, a multi-tubular (18 and 30 tubes) EVA-II reactor was then built and tested under normal (nuclear) operation conditions to assess thermal, process, and mechanical design; material properties and selection; as well as transient behavior under fluctuating conditions (load variations, start-up, and shut down) [595], over 5200 h [596]. Heat transfer was studied for two different heating arrangements: a baffle structure (30 tubes), an annular structure (18 tubes), and an external electric heater. In the case of He, which was used as heating medium, it entered the reformer tubes at 950 °C and exited the reactor at 650 °C [597]. It was found that heat transfer occurred at >90% by convection and less than 10% by radiation and that it could be improved by increasing He temperature or pressure, increasing pressure drop, or incorporating fins on the tubes [598]. The selection of materials for building the different process components was based on tests run for over 10,000 h [599,600,601]. Similarly, the ADAM I and II units were built and tested at 650 °C for 2344 h and 6000 h, respectively [602]. The integrated A&E plant transported energy of 5.72 MW(th) through the pipeline, receiving an input of 10.78 MW(e) total power, mostly provided by the helium flow (heat). The ADAM plant was operated with an energy efficiency of 72% (5.42 MW of heat released and 2.34 MW used heat). This experience demonstrated the possibilities of transporting 300 kW of nuclear power [603,604,605] and nuclear heat over long distances [15,593,597,606,607] for 10,150 h. The drawbacks of the technology included contamination by solid fission products and increased corrosion of the core and the fuel elements [608].

In the A&E German development, an intermediate heat exchanger (IHX) was deemed unnecessary within the initial assumptions [595]. After thorough analyses of the found drawbacks, this initial assumption was ruled as wrong. Instead, the Japanese decided to include an IHX in their HTTR project, which also involved commercially available SMR. The use of the IHX reduced hydrogen and tritium permeation rates to negligible values, avoiding product contamination. In this case, the Japanese could concentrate the R&D efforts on hydrogen (and tritium) permeation [609,610] as well as acquiring heat transfer data while still addressing materials design, selection, and testing [611,612]. The observation of He leaks [613,614] and the possibilities of explosion and fires [615,616] elevated safety concerns [617] and were favorably used to develop guidelines to design safer and more reliable reformers [618,619,620,621]. Scaling (1/30 of industrial scale) and integration to demonstrate performance were carried out, using electrical heating to model nuclear heat and to produce 110 m³/h of H₂ [622]. Testing of this unit was used to develop start-up and shut-down protocols and to collect data on materials’ behavior, performance (under transient and steady state), and operating conditions and to define SMR design premises [622], including control instrumentation [623]. An SMR to produce 4200 Nm³/h of hydrogen, using a conventional Ni-catalyst and consuming about 10 MW of thermal energy was designed [619], and a dynamic simulation code to predict heat and temperature fluctuations of the system was developed [623]. Although drastic drawbacks were never reported, the shifting of hydrogen production R&D to the study of the sulfur–iodine thermochemical cycle indicates that some would have been found [624,625].

Heat integration has been proposed to be carried out either using spent nuclear fuel (SNF) [626] or employing open cycles [627] rather than the closed-cycle type used by A&E or any other heating pipe. In summary, heat management within energy integration between NP and CPs might not be as simple as expected from the maturity of the technology but instead would create new challenges to be addressed in the immediate future.

Energy integration of electrochemical reactions to NPPs would bring benefits of a combined option of heat and electrical power to the electrochemical plant. The scale-up attempts of emerging CO₂ electrolysis technologies were recently compared [361,628], pointing out some of the scalability issues of concepts that have been demonstrated at a laboratory scale. High over-potential, low FE, and low current densities are the main challenges to overcome to avoid additional costs prior to any attempt to scale up [629]. Current density can be improved in HTE CO₂ reduction, leading to performance improvements as well in solid oxide electrochemical cells [200]. In HTE operations, part of the energy requirements is provided by thermal energy, and lower electric power is consumed by the electrolysis process. In fact, HTE energy consumption, in CO₂ co-electrolysis, can be 30–35% lower than that of LTE, and the estimated H₂ production cost was as low as $0.660/gge (gallon of gasoline equivalent) [630]. The high temperature employed creates opportunities to manufacture other products beyond CO as well as to process intensification by introducing a combination of electro- and heterogeneous catalysts and/or a combination of physical and chemical processes. The experience gained through the integrating efforts of LTE or HTE for H₂ production to NPPs [23,631,632,633,634] should serve as a knowledge platform when other electrochemical units need to be integrated, and the barriers of economic feasibility have to be overcome. In the H₂ case, cogeneration of electricity by reversible operation of an LTE or HTE as fuel cell (FC) has been suggested as an economic improvement of the integrated system [635]. HTE operations are most benefitted from integration to NPPs that would provide both heat and electricity. However, for HTE, material stability represents higher risks as operating temperature increases [636,637,638,639]. NPP’s integration will require endurance of long-term stability of the electrolysis system under industrial operation conditions [200]. Finally, combination of electrochemical process with radiation-induced chemistry could further contribute to decreasing the activation potential for electrochemical reduction.

Meanwhile, the use of radiation as energy source through integration of the chemical process to an NPP is new and not a straightforward step. The use of ionizing radiation to drive CPs gives rise to new and complex challenges. The first question to ask is whether such integration is worth an investment. Schmeda-Lopez et al. tried to answer this question through an economic evaluation of a process concept that combines a propane radiolysis process (to produce propylene) with a molten salt nuclear power reactor. The ionizing radiation employed was γ-ray from the radioactive gaseous fission products that is either dissipated or wasted within the NPP. The economy of the NP-integrated process resulted in unfavorable returns on investment for small NPPs, but it was more than acceptable for larger NPPs [640].

Livingstone proposed to supply the γ-radiation from rod assemblies of SNFs to reduce CO₂ [641]. The theoretical assumptions for designing the chemical reactor were (i) direct contact between the spent fuel and the reactor walls and (ii) the critical role of the fast Compton electrons in inducing the reaction [641]. This proposal takes advantage of employing the radiation emitted by SNF during its cooling process in the storage pond. The activity of a conventional ⁶⁰Co source (~10¹⁴ Bq), typically used in commercial applications, could be four orders of magnitude lower than that of SNFs (~10¹⁹ Bq). Additional advantageous characteristics of the SNF γ-radiation are the almost neutron-free nature of the radiation, its mean energy of 578.4 keV, and an energy spectrum extending from a few hundred keV to over 2 MeV. Comparatively, the energy (γ-radiation) from SNFs will induceto completion chemical reactions in seconds or minutes that might take hours to complete by exposure to conventional radiation sources.

In terms of availability, the NE sector follows and takes strict measures for the safe management, storage, and disposal of SNFs. As can be deduced from the previous paragraph, once the energy generation lifecycle is complete (15–18 months), the SNF rod assemblies are removed from the reactor core with a significant remaining residual radioactivity since only about 2% of the fissionable fuel is consumed. The residual radioactivity (~98%) would take decades to be released in the form of heat from the SNF rods, which are regarded as “hot”. These hot SNF assemblies need to be “cool” enough to safely be handled and transported. During this cooling process, substantial amounts of energy are released from the SNF rods, and this energy is safely dissipated and unused. Thus, this cooling is carried out in water pools that every NPP provides to initially store the SNF assemblies. While the pool temperature increases, its water content decreases by evaporation losses, so energy (to cool down the pool) and water are consumed. Water needs to be replenished since, by regulation, storage has to be at least under20 feet of water [642]. Since the SNF storage in the cooling pools lasts for 10–15 years, the storage capacity of existing cooling pools has begun to be exhausted [643]. In the United States (US), for instance, 60,000 metric tons of heavy metal (MTHM) of SNF were stored at nationwide sites by 2008 [643], for a yearly accumulation of 2000 MTHM [644]. Thus, if the 104 nuclear reactors in the US remain operative by 2050, then the inventory would increase to about 130,000 MTHM [645]. From the initial storage at the NPP location, the SNF should be moved to a permanent storage facility, which commonly is a geological formation where it should remain for at least 10,000 years. However, these permanent geological repositories are difficult to license and implement, leaving the SNF at their generating NPP for longer times than the cooling requires. Nonetheless, at this point, it is worth mentioning the NE sector’s efforts of SNF reprocessing for recycling purposes [645,646]. Reprocessing facilities are operative in France, the United Kingdom, and Russia. France reprocesses SNFs domestically and has reprocessed SNFs from Japan, Germany, Switzerland, Belgium, Italy, Spain, the Netherlands, and Italy. SNFs from Japan, Germany, Switzerland, Belgium, Italy, the Netherlands, Sweden, and Canada have been reprocessed in the United Kingdom, while Russia has reprocessed SNFs from Ukraine. The recycling in the NE sector is currently limited by economics, making storage the disposal alternative [647].

It is clear that the ionizing radiation emitted by SNF in cooling ponds, otherwise wasted, could offer a reliable energy source to drive CPs. Additionally, if the considered chemical reaction is endothermic, the heat of reaction could contribute to cooling the pond and will not only save on energy consumption but also prevent water losses, an important move toward eco-efficiency and sustainability.

Radiation-induced conversion pathways for CO₂ could be more energy-efficient processes, benefiting from the added synergy, which lowers the overall energy requirements for CO₂ conversion by utilizing the limited value of SNF ionizing radiation at NPP facilities. However, this integration concept brings about new challenges, such as the need to increase the radiolytic conversion efficiency of CO₂ and to find means for controlling the propagation over the termination reactions. The radiolysis of CO₂ has found very little to no application due to the lack of fundamental understanding of these two main scientific challenges. It is known that the low radical yield is mainly due to recombination reactions (backreactions but also to the inherent low reactivity of CO₂. Backreactions are termination reactions that compete with the propagation reactions that cascade into more radical formation. Knowledge on the mechanisms to improve radical yields, either by the use of additives or identification of favorable conditions for radical formation, is still needed. Radicals are highly reactive but rather than recombining reactions (backreactions or termination reactions), what is needed are conditions to favor the propagation reaction to increase radical yield.

Additionally, integration also demands attention regarding certain aspects of the radiation-induced CP. On the one hand, the rational design of chemical pathways and on the other, radiation-tolerant materials and components are technical areas calling for R&D. Controlling reaction pathways, besides the issues pointed out above, also includes taking apart, tuning, or stopping undesired pathways while accelerating specific ones. This might require (radiation-derived) heat management and/or equipment and reactor design [648,649]. The effects of radiation-induced reacting mixture and reaction products on the corrosion of reactor materials [650] also require further investigation.

The CO₂ valorization through NP-to-X pathways represents decarbonizing approaches that connect the NE industry to a decarbonized chemical sector and enable a smooth, consistent energy transition of a hard-to-decarbonize industry. This work has examined a variety of possible conversion pathways for CO₂, as well as the importance of framing conversion within the carbon cycle and using it as the cycle-closing mechanism to enable a circular economy, which has been reported as a way to procure sustainability [96,651]. Along the text, scientific questions, issues, gaps, and challenges that need to be addressed through R&D are mentioned. The challenges to be addressed, as well as the issues that need to be solved, have also been discussed. The unique energy mix provided by NPPs creates a platform of opportunities for the development of processes and methodologies that support the NP-to-X pathways. CO₂ valorization provides the economic incentives for decarbonization.

The enormous gap between current emissions and utilization calls for a suite of chemical process technologies since there is not a single answer or a single solution. Then, there cannot be a single processing solution for the 37 Gton of emitted CO₂, and instead, a suite of process technologies meeting certain requirements is needed. Thus, these processes are required to minimize hydrogen consumption and produce value-added products using low-carbon-emitting energy sources, ultimately resulting in cost-effective technologies. In order to avoid leveraging inefficiencies, the new processes being developed should be more energy-efficient than those currently operative. Process intensification needs to be present in these NP-to-X pathways through the combination, coupling, and/or integration of processes that take advantage of potential synergies among the available forms of energy (heat, electrons, and radiation).

Since energy is the key integrating factor in the NP-CP IES to enable NP-to-X pathways, no risk should be taken to jeopardize energy security, affordability, and accessibility, which are the basis for human development. Finally, in the valorization of CO₂ through NP-to-X pathways, sustainability has to be the core driver since the lack of sustainable pathways in the past is the reason for having to face the current situation.

4. Future Potentialities

CO₂ valorization, as part of a decarbonizing strategy, relies on the development of process technologies with the highest energy and carbon efficiency. In terms of energy, and due to the intrinsic energy intensity involved in CO₂ chemical reactions, not only is high efficiency required, but also the energy has to be provided by a C-neutral source. Then, regarding C-efficiency, it can be satisfied partially by the use of C-neutral sources and additionally by reactions consuming large CO₂ amounts and rendering products that retain carbon for long periods of time. Furthermore, the development of these CP targets a sustainable future on the horizon by addressing the identified economic and environmental challenges. For instance, hydrogenation reactions should employ green H₂ and additionally should either minimize its consumption or develop an intensified process that generates hydrogen in the reacting medium.

Concerning CO₂ as feedstock, a first consideration is the holistic examination of the magnitude of the emissions (Gton/y scale) and of the (high) emitters (global 10⁴ scale), the CO₂ concentration in the different effluents, emitters’ throughput, contaminants present, and geographical location. It becomes evident that a universal technology of the one-size-fits-all solution type does not exist and probably cannot be developed. Moreover, those feedstock attributes give rise to composition, concentration, and logistic mismatches between the emitter and the potential conversion plant. The mismatches in composition and concentration are defined by a potential discrepancy between the feedstock specification to be utilized and the concentration of undesired compounds and CO₂ in the emitted stream. A logistic mismatch is found when the emitter is located at an uneconomical distance from the using location. Finally, the dynamics and scale of the emissions throughput at a given emitter location may differ from that required to satisfy the market size of the proposed product(s).

This work has examined some chemical pathways that fall within the above-mentioned preferred attributes, though improvements in energy and C-efficiency are required to further develop into sustainable process technologies. Examples of awarded patents and published patent applications have been included in the discussion. The chemical reactions discussed were categorized according to the type of energy employed to satisfy the high energy requirements of CO₂ conversion reactions, i.e., thermal/heat, electricity, and ionizing radiation. These three types of energy can become available from NPPs and create a new (decarbonizing) platform for the production of (decarbonized) chemicals, products, and materials derived from CO₂. However, the integration of CPs to NPPs will bring new challenges. Although heat or electricity integration is a mature technical area, the particularities of each considered plant, as has been pointed out above, need to be addressed and sorted out. The former involves further development and implementation of chemical heat pipes, and the application of gained experiences and knowledge on new processes. Similarly, electrochemical process integration is not just a plug-and-play step, and some of the particularities of each process have already become an R&D subject. The low deployment of electrochemical processes is a result of their lack of economic sustainability, mainly due to a continuously increasing electricity cost. Thus, the challenges found within the electrochemical pathways concern those aspects associated with improving energy efficiency, such as overcoming reactions overpotential by developing more active electrocatalysts, improving faradaic efficiency by developing more selective electrocatalysts and intensifying processes by reducing number of process steps, for instance.

Radiation-induced chemistries go beyond the study of radiolytic reactions into tailor-designed pathways for the production of value-added products. New concepts for either new synthesis methods, new deconstruction methods, and/or new ways of approaching the development of radiation-induced CPs can be created by addressing identified scientific challenges. The use of catalysts and/or electrocatalysts calls for dynamic experimentation under continuous flow at high dose rates. Still, the definition of the fundamental principles behind the competition between propagation and termination reaction needs to be investigated, for which new approaches will need to be defined. Therefore, the broadest challenges probably come from the development and integration of radiation-induced CPs. Regarding the chemical process itself, there are aspects in which fundamental knowledge needs to be created. A comprehensive understanding of the interactions between ionizing radiation with the reacting molecules, with the surface of (radiation-induced-activated?) catalysts or electrocatalysts, and/or with adsorbed species on catalysts/electrocatalysts is key for controlling the propagation reactions (formation of ion radicals) and the termination reactions (backreactions or disappearance of ion radicals). The application of the knowledge derived from the understanding of the chemical transformations of ion radicals/free radicals could be directed to design reaction pathways for the production of specific compounds. The catalysis knowledge is not only restricted on the activating (increase in catalytic activity) effect of ionizing radiation but also catalyst development needs to address effects on stability and on integrity, as well as the safe use and management of catalysts that have been exposed to ionizing radiation. The identification of knowledge gaps and the approaches and attempts to fill these have been reviewed not long ago [28]. The development of protocols for the safe use of ionizing radiation expands into the full development of radiation-induced CPs. The application of ionizing radiation sourced from SNFs during the cooling process has been proposed but needs further efforts and attention before considering scaling to a commercial application. Similar to the suggested R&D work with catalysts, products and materials that have been exposed to radiation will require handling under strict safety protocols. Radiation-induced processes might open new opportunities for other wastes besides CO₂ since the combination of feedstocks is also possible, as it would be the use of CO₂ for radiolytically functionalizing plastic wastes [28] as an upcycling mechanism.

Overall, the NP-to-X pathways are of particular interest in the development of new CPs for dealing with emitted CO₂ and also for handling renewable, non-recyclable wastes, for which radiation, heat, and/or electricity can drive such reactions. NPPs’ energy mix would also give opportunities for finding synergies between the energy forms by combining heat and electricity (e.g., high-temperature electrolysis), radiation and heat (e.g., radiation-induced thermocatalytic reactions), radiation and electricity (e.g., radiation-induced electrocatalytic reactions), etc.

Finally, the synergistic use of the three different types of energy (e.g., radiation and (electro)catalysis, radiation and thermal-catalysis, or radiation and high-temperature electrocatalysis) collectively gives rise to new and overwhelming R&D challenges. Process design and development will lead to the definition of whether simultaneous or consecutive application of the radiation type is applied. In some instances, simultaneity might result obvious when lifetime or reactivity of ion radicals implies so. However, in cases where the stability and integrity of components, catalysts/electrocatalysts, products, and materials are suspected or jeopardized, then overcoming the existing challenges in the development of consecutive processes becomes the option.

5. Concluding Remarks

Nuclear power represents one of the means to reliably provide three different forms of C-free energy, namely, heat, electricity, and ionizing radiation, at a large scale from a single point. The utilization of part of this energy to drive chemical reactions to produce a variety of CO₂-derived products not only broadens the market of NPPs beyond electricity generation but also significantly benefits the environment, giving use to noxious emissions while creating value from it. The new markets will include industrial and commercial sectors such as manufacturing, chemical, construction, and buildings. An additional benefit of the diversification of nuclear energy interests is its revitalization, supported by new partners, markets, customers, and end-users.

However, the integration of NP with CP is not a straightforward step, and scientific and technical problems are found along the way. Greater benefits would be obtained from energy-intense chemical transformations. Currently, high temperatures and heat are provided by the combustion of fossil fuels, leading to significant CO₂ emissions. Therefore, heat integration into an NPP will bring economic and environmental benefits. These benefits could be better if, in addition to heat, the desired chemical transformation can be induced by ionizing radiation and/or carried out electrochemically.

Although low yields are observed from radiation-induced CO₂ reactions, the presence of other compounds (e.g., methane or water) and catalysts in the reacting medium improve yields. Catalyst and electrocatalyst research can be designed to guide radical reactions, scale the potential of electrochemical processes to final conversion into desirable products, and control the propagation and termination reactions to attain the desired product distribution. Ideally, full synergy among heat, electricity, and ionizing radiation would make the best out of the NP-to-X pathways.

The development of decarbonizing processes responding to the NP-to-X pathways gives rise to a new industrial era for the manufacture of decarbonized chemicals, products, and materials with higher efficiency that will lower the contribution of industry to the emission of polluting compounds into the atmosphere. The progress made in the development of nuclear reactors dealing with higher-temperature heat or delivering smaller and/or modular amounts of energy can be advantageously used to implement distributed platforms for these new manufacturing schemes while contributing to the achievement of net zero goals.