Review of carbon dioxide utilization technologies and their potential for industrial application

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Journal of CO2 Utilization 65 (2022) 102239

Available online 30 September 2022

Review article

Review of carbon dioxide utilization technologies and their potential for industrial application

Changsoo Kim

^a

, Chun-Jae Yoo

^a

, Hyung-Suk Oh

^a^,^b

, Byoung Koun Min

^a^,^c

, Ung Lee

^a^,^b^,^c^,^*

aClean Energy Research Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seoul 02792, Republic of Korea

bDivision of Energy and Environmental Technology, KIST School, Korea University of Science and Technology (UST), Hwarang-ro 14-gil 5, Seoul 02792, Republic of Korea

cGreen School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

A R T I C L E I N F O Keywords:

Carbon dioxide utilization Level of development Industrial application

A B S T R A C T

With the evermore increasing interest in climate change world-wide, various studies are being conducted with the aim of reducing CO2 emissions. The Paris Agreement vows a decrease of carbon emissions by 32 % until 2050, and thus strategic pathways have to be set to achieve this immense goal. Carbon dioxide utilization (CDU) technologies are deemed as one of the most practical methods to achieve large amounts of CO2 reduction, but careful analysis and application plans are required to obtain the maximum efficiency regarding CO2 emissions reduction. In this paper, various CDU technologies are reviewed in terms of efficiency, maturity, and cost. First the various CDU technologies are sorted according to their application status and the CO2 utilization capacity of the technology. Then each of the technologies are reviewed in detail, then their current maturity and potential for long-term usage are reviewed regarding the various studies conducted world-wide. Finally, insight is provided on how the various methods can be integrated to practically achieve the CO2 emission reduction goal. The purpose of this review is to provide an overview of the current CDU status, and to possibly give insight into the practical applications and future strategies for achieving a net-zero environment.

1. Introduction

Controlling the amount of greenhouse gas emissions and consequently mitigating global warming and related climate problems is deemed as a global task. CO₂is the main contributor to global warming and climate change, of which the global emissions from energy combustion and industrial processes added up to 36.3 Gt in 2021 [81]. The United Nations Framework Convention on Climate Change (UNFCCC) proposed a treaty to limit global warming to below 2 ^∘C, preferably to 1.5 ^∘C, namely the Paris Agreement in 2015 [170]. The consequences of global temperature increase larger than 1.5^∘C are serious and extensive, as provided in the recent report from the Intergovernmental Panel on Climate Change (IPCC) [83]. While immediate and intense actions are required to achieve the goal set by the Paris Agreement, practical applications which are capable of reducing CO₂on a large scale are far from realizable, and many countries are still conducting research and searching for the right option to delve into as the main strategy for tackling the climate crisis.

Among the various options for CO2 mitigation, carbon dioxide utilization (CDU) is considered as a readily-applicable option in the short to mid-term. The idea of CDU is to convert the captured CO2 into valuable products such as fuels, chemicals, and minerals, or utilize them for business applications such as enhanced oil recovery (EOR). While various studies suggest a bright future for the different CDU applications, not many of the options qualify for the three important aspects of CDU: long-term carbon fixation, economic feasibility, and large-scale application. Often is the case where seemingly sustainable CDU applications fail to achieve sufficient profitability, or is not suitable for large- scale CO2 reductions in the scale of millions of tonnes per year. To effectively choose mitigation options and commercialize processes in association with the industrial sector, it is important to acknowledge the development stages of the different CDU options, and their potential for industrial application.

Along with the technological aspects, it is essential to discuss CDU applications in accordance with the related market and policy trends.

Since CDU applications tend to be operated at a national scale, the

* Corresponding author at: Clean Energy Research Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seoul 02792, Republic of Korea.

E-mail addresses: [email protected] (C. Kim), [email protected] (C.-J. Yoo), [email protected] (H.-S. Oh), [email protected] (B.K. Min), [email protected].

kr (U. Lee).

Contents lists available at ScienceDirect

Journal of CO2 Utilization

journal homepage: www.elsevier.com/locate/jcou

https://doi.org/10.1016/j.jcou.2022.102239

Received 29 July 2022; Received in revised form 9 September 2022; Accepted 19 September 2022

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policies regarding the organization of a greener economy, such as carbon credits, carbon tax, and government subsidies to the industries, are essential for motivating the projects. Formation of CDU product markets are also greatly affected by these policies, since additional taxation of non-CDU products can invertly promote the profitability of CDU products. Overall, to determine specific CDU projects of interest and to focus on selected technologies for development, it is essential to understand the market and policy trends related to CDU, along with the technological development of the separate technologies.

In this review, CDU technologies regarding the production of chemicals and fuels, are reviewed based on their stage of development and industrial application potential. Also, the market and policy trends of leading countries in the development of CDU technologies are presented, to provide insight into the global association of CDU development. Details of each technology are provided in comparison with the conventional methods, usually incorporating petrochemical products, along with their carbon mitigation capacity and trends within the academia and the industrial sector. The various CDU technologies are divided into three categories: technologies in the demonstration phase, technologies in the development stage having potential for large-scale applications, and technologies in the development stage having potential for small-scale applications. In Section 2, CDU options in the demonstration phase are introduced, including specific projects regarding the actual pilot plant under operation. In Section 3, CDU options in the development phase, such as carbon monoxide (CO) or fuels, having the potential of large scale application, are introduced. In Section 4, CDU technologies in the development phase with small markets are introduced. These options are introduced because although the current market size is comparably small, it has the potential for exponential growth in the future, due to development of related technologies and changes in government policies. In Section 5, the market and policy trend is considered for the CDU technologies, and related projects are reviewed. In Section 6 the future perspectives of CDU technologies are provided, and the paper is finalized with conclusions in section 7.

2. CDU technologies in the demonstration phase 2.1. Methanol synthesis

2.1.1. Overview of technology

Methanol is one of the most widely used chemicals throughout the world, with annual demand of 98 Mt in 2019 and global production capacity of 150 Mt [84]. Methanol is mainly used as a raw material for synthesizing hydrocarbon compounds such as formaldehyde, ethylene, and propylene, via the methanol-to-olefins (MTO) reaction. Also, it is incorporated as fuel by itself, or as a blend with gasoline or diesel components. As mentioned in the work by Olah [132], methanol has the potential to become a base chemical for the economy, serving as raw material for chemical compounds and as a fuel. There is also a growing market for direct methanol fuel cells (DMFCs), or the use of methanol as a hydrogen carrier, in preparation for the hydrogen economy. Accord- ingly, the methanol market has large potential for growth and, despite the technical barriers that need to be overcome, such as low conversion and selectivity during synthesis, methanol is currently one of the most probable candidates for large-scale CDU application.

Mainly two strategies exist for CDU-based methanol production: (1) direct hydrogenation of captured CO2 to produce methanol, and (2) producing CO by utilizing CO2, via either electrochemical reduction or the reverse water-gas shift reaction, and reacting it with H₂obtained via an environmental pathway, such as water electrolysis.

2.1.2. Current stage of development and research trends

Currently most of the conventional methanol production rely on coal gasification and natural gas reforming processes. The related equations for methanol production are presented in Eqs. 1-4. Eq. 1 shows the steam

methane reforming (SMR) reaction, where methane is reacted with water to generate a mixture of CO and H₂, or syngas. Eqs. 2 and 3 shows the methanol production via hydrogenation of CO and CO2, respectively. Eq. 4 shows the reverse water-gas shift (RWGS) reaction, which is a competing reaction of Eq. 3. Most of the conventional methanol is produced by sequentially reacting Eq. 1 and then Eqs. 2 and 3, where a small amount of CO2 is added to the syngas produced from Eq. 1, to increase the reaction rate.

For CDU methanol, the main reaction is Eq. 3, where CO2 is directly reacted with 3 moles of H2 to produce methanol and water. However, since the enthalpy change is larger negatively compared to the CDU pathway, the conventional pathway is preferred from a thermodynamic point of view. Thus the equilibrium yield of methanol production via Eq.

2 is over 80 % at 200 ^∘C, while equilibrium yield of the Eq. 3 only shows 40 %. Additionally, at higher temperatures, the reverse water-gas shift reaction (Eq. 4) is promoted, resulting in lower methanol selectivity.

CH4+H2O→CO+3H2, ΔH298K= − 206.2kJ∕mol (1) CO+2H2→CH3OH, ΔH298K= − 90.5kJ∕mol (2) CO2+3H2→CH3OH+H2O, ΔH298K= − 49.3kJ∕mol (3) CO2+H2→CO+H2O, ΔH298K=41.2kJ∕mol (4)

Due to these limitations, the main focus of development for increasing the yield of direct CO2 hydrogenation are the development of novel catalysts and design of new process configurations for efficient energy usage.

CDU methanol is produced by incorporating the commercialized Cu/

ZnO/Al2O3 catalyst developed by the Imperial Chemical Industries (ICI) [36,176]. The operating conditions of the reaction are in the temperature range of 230–300^∘C, and a pressure range of 50–100 bar. However, the Cu/ZnO/Al2O3 catalyst has numerous limitations, mainly where water produced as a byproduct significantly deactivates the catalytic performance [112]. Thus various studies have conducted research on improving the Cu/ZnO/Al2O3 catalyst, by identifying the active sites of the catalyst and providing variations to enhance performance. Behrens et al. [39] identified the catalyst active sites using a combination of experiments, imaging methods, and density functional theory (DFT) calculations. They showed that the active sites consist of Cu steps with Zn atoms, and that they are stabilized by bulk defects and surface spe- cies. Kuld et al. [105] have presented results on quantifying the performance enhancement effect provided by ZnO during catalyst reactions. By combining experiments with DFT calculations, they showed that the Zn coverage on the Cu surface is essential for methanol productivity, showing a monotonic increase up to the level of 0.5. Wu et al. [183] proposed the use of inverse ZrO2/Cu catalysts with tunable Zr/Cu ratios for CO2 hydrogenation to methanol. The prepared catalyst consisting of 10 % ZrO2 supported over 90 % of Cu showed a methanol formation rate 3.3 times higher than a conventional Cu/ZrO2 catalyst.

Schematic description and the performance of the inverse catalyst is shown in Fig. 1 (a) and (b), respectively.

Additional studies have been conducted to revise process configurations capable of maximizing energy efficiency. Such studies include efficient removal of heat emitted during methanol production [201], and increasing the equilibrium yield by selectively removing water from the product via membrane reactors [54].

The second strategy for CDU methanol production, to first produce syngas using CO2, and then to convert the produced syngas to methanol, is being studied from various viewpoints as well. Joo et al.[90] developed the Carbon Dioxide Hydrogenation to Methanol via RWSG (CAMERE) process, which sequentially produces CO via Eq. 4, and then produces methanol by adding H2. The developed process was built as a pilot plant with 66 % methanol yield, producing 75 kg/d of methanol [137]. Another example is the complex reforming of methane-H2O-CO2. Since the methane-H2O reforming and methane-CO2 reforming

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processes both produce syngas with inappropriate ratios of CO and H2, the two reactions are incorporated simultaneously to produce syngas products with CO:H2 ratio of 1:2.

2.1.3. Industrial applications

Industrially, research has been focused on using renewable energy and CO2 to produce methanol, since it has the effect of mitigating carbon emissions. An industrial scale version of such a process has been built by the Carbon Recycling International (CRI) Inc. in 2012, incorporating the fluegas obtained from a nearby geothermal power plant, and the H2

obtained by electrolysis of water to produce 4 Gt/y of methanol [6]. The methanol plant uses the Cu/ZnO/Al2O3 catalyst [73], and the produced methanol is mainly utilized for gasoline blending or for biodiesel production [59]. A commercial-scale facility of CRI is currently under construction in Anyang, China, with an expected capacity of 110, 000 t/y of methanol production.

Progress has been made in terms of catalysts as well. In 1996, the Research Institute of Innovative Technology for the Earth (RITE) and the National Agriculture and Food Research Organization (NIRE) of Japan co-created a commercial catalyst by combining ZrO2 and SiO2 to the widely used Cu/ZnO/Al2O3 catalyst [149]. By incorporating the developed catalyst and recycling the unreacted syngas, they were able to produce methanol at 250^∘C and 50 bar, with a purity of 99.9 wt%. Based on this accomplishment, the Mitsui Chemicals Inc. constructed a pilot plant producing 100 Mt/y of methanol in 2008. This plant consists of CO2 from fluegas and H2 acquired from solar energy driven electrolysis, making it a environment-friendly process [18].

In 2011, Air Liquide Inc. published a study regarding the production of methanol from pure CO2 [139]. The commercial Cu/ZnO/Al2O3

catalyst was used, and the constructed pilot plant included a loop reactor with a methanol liquefier, showing a carbon conversion of 94–96.5%.

While the productivity of the proposed process was lower than conventional methanol production processes using syngas, compared to the conventional process it has the advantage of producing high purity methanol in similiar conditions of gas hourly space velocity, temperature, and pressure, where approximately 65% of the side products such as esters, hydrocarbons, ketones, and C2+alcohols, were reduced.

2.2. Polymer synthesis 2.2.1. Overview of technology

The advantages of polymer synthesis with regards to carbon emission mitigation is that it can fix carbon as a product in longer durations than other CDU-based products, and that the potential market size is large

due to variety of products. Unlike CDU-based fuels or chemicals which are combusted during usage, polymers are used as their end-product forms. Also, after the end of product life polymers may be recycled to reproduce polymers, which prevents re-emission of CO2 back into the atmosphere. Regarding the related markets, CDU polymers are capable of displacing the petrochemical-based polymer production market, and also have the potential to create new markets with development of novel materials. The overall polymer market was valued at 533.6 billion USD in 2019, and is expected to expand with an annual growth rate of 5.1%

from 2020 to 2030 [25], having potential as a suitable market for CDU products.

Various materials can be incorporated for polymer production via CO₂, including the already commercialized polymer monomer precursors with carboxylate, ester, lactone, carbonate, and carbonyl functional groups. These are all in the form of containing the molecular structure of carbon dioxide as it is or maintaining at least one C––O bond, so it is possible to reduce the complex process of converting to an active state such as excessive hydrogenation of carbon dioxide when producing a carbon dioxide polymer, so it is possible to secure a rela- tively economical process. A representative example of a technology for converting CO2 to a monomer is the ethylene carbonate (EC) manufacturing technology, which can be a raw material for polycarbonate, and a representative example of a technology for direct conversion to a polymer compound is the production of polyether carbonate polyol, which is the main component of polyurethane.

Since many CDU-based polymer products have been commercialized, recent studies are focused on developing new pathways for polymer precursors, to reduce energy usage and to enhance profitability and carbon footprint of the products. Various studies have focused on copolymerization of epoxides and CO2 which leads to the production of polyalkylene carbonates or cyclic organic carbonates, which can be further processed into polyether(-polycarbonate)s, or polyhydroxyur- ethanes [188]. The various routes of epoxide and CO2 copolymerization, are provided in Fig. 2. The reaction of epoxides and CO2 may follow the reaction route of (a) polyalkylene carbonates production, or (b) cyclic organic carbonates production, depending on the catalyst type and reaction conditions [120,126]. While polymers resulting from route a) have found applications in biomedicine and polyurethane foams, new applications are being investigated for the reaction route b), where various forms of polymers can be synthesized depending on the type of epoxides. Representative applications of route b) are the preparation of polyhydroxurethanes to produce non-isocyante polyurethanes (NIPUs), Fig. 1.(a) Schematic of the reaction behavior of the inverse ZrO2/Cu catalyst proposed by Wu et al. [183], compared to the conventional Cu/ZrO2 catalyst. (b) Catalytic performance of the inverse catalyst with respect to the change in ZrO2 composition, at reaction conditions of 220^∘C, 30 bar, and CO2 to H2 ratio of 1:3.

Reprinted from Wu et al. [183].

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which allows mitigating the use of isocyanates, which are toxic material used for conventional production of polyurethane [40,95,156].

Various economically viable CDU-based polymers have been commercialized, such as polycarbonate (PC), polyol and polyurethane (PU) production. Covestro Inc. is one of the companies which have commercialized carbon-based polymer production. Their polyether- polycarbonate polyol product, cardyon™, is known to reduce CO2

emissions by 20 % compared to the conventional polyol production process [140], and is used to create flexible polyurethane foams. Their core work was to develop a process to displace conventional polyol with a carbon-based polyol, which was based on the development of a double metal cyanide catalyst. Covestro AG built a demo sized plant in 2016, producing 5,000 ton/y of polyol using CO2 obtained from a neighboring chemical facility [10].

Novomer Inc. is also one of the leading companies in the production of polyol. Their research is focused on the catalytic conversion of CO2 to produce polycarbonate polyol, polypropylene carbonate, and poly- ethylene carbonate. Development of the Salen-Co based homogeneous catalyst sparked the research on CO2 conversion, and their recent output of polypropylene carbonate for polyurethane production (Converge®) can displace the conventional petrochemical-based polyol, including up to 50 % of CO2 within their structure. Also, LCA of their polyol process shows carbon emission mitigation of 67% compared to conventional polyol production processes [53]. The technology and production rights of Converge®have been taken over by Saudi Aramco on 2016, and global market production is currently under development [29].

The Mg-based catalyst for efficient production of polyols, developed by Econic Technologies, is currently in its pre-commercialization stage [12]. The Mg-based catalyst promotes epoxy copolymerization of polyolefins and cyclohexene oxides, which results in the production of polyols and polyurethane. A key point of their technology is that the CO2

composition within polyol is increased, due to the use of a homogeneous polyol catalyst.

Another application of CDU-based polymer is the process developed

by Asahi Kasei, where CO₂is utilized to produce polycarbonates (PC) and high purity ethylene carbonates (EC). Their process producing CDU PC was commercialized with a plant capacity of 150,000 t/y, and is capable of reducing CO₂emissions (0.173 t/tPC) as well as substituting the use of highly toxic phosgenes used in the conventional process [65].

A more recently developed process for producing high purity EC was constructed with a capacity of 38,000 t/y EC, produced for developing the solvent for lithium-ion batteries [1].

Newlight Technologies, Inc. developed a CDU-based high performance thermoplastic, named AirCarbon®. AirCarbon®is produced by re-assembling carbon, oxygen, and hydrogen, where carbon is obtained by extraction from CH4 or CO2, incorporating a biocatalyst developed by Newlight Technologies [19]. The process for producing AirCarbon®was commercialized in 2013, and the products are being provided to customer companies for use as packaging material.

2.3. Urea synthesis

Urea was first obtained by Friedrich Wohler in 1828 by reacting silver cyanate with ammonium chloride. Mass production became available later on when Basarov started producing urea using ammonia and CO2, and process development enabled economically efficient production of urea. It is mainly used as agricultural fertilizer, and is also utilized as a NOx reduction agent in diesel vehicles, a monomer for polyurethane and an additive for cosmetics. The worldwide production of urea was 233 Mt in 2019, and is estimated to increase to 300 Mt by 2030 [30].

Recently, studies are being conducted to increase the yield rate of urea and also to produce it while reducing carbon emissions. Urea production via ammonium carbamate (AC) decomposition is one of the representative methods. The related reactions are shown in Eqs. 5 and 6.

2NH3+CO2→NH2COONH4 (5) Fig. 2. Various routes for CO2 polymerization to epoxides. Reprinted with permission from Yadav et al. [188] Copyright (2019) John Wiley and Sons.

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NH2COONH4⇌H2N(C=O)NH2+H2O (6) As shown in the reaction equations the overall process is a 2-step reaction. In the first step, AC is produced in an exothermic reaction, then in the second step, the AC is dehydrated to form urea in an endo- thermic reaction. While Eq. 5 reacts spontaneously with an ammonia to CO2 ratio of 2–3 vol% in a temperature region of 180–210^∘C and a pressure of 150 bar without a catalyst, Eq. 6 is a slow equilibrium reaction, and a commercial catalyst for intensifying the yield rate is yet to be developed. The created urea is retrieved in low pressure, then the unreacted AC is recycled back into Eq. 5. Thus it is essential to enhance the process performance of the recycled NH₃and CO₂, to maximize the yield rate. Accordingly, recent studies are focused on enhancing Eq. 6, by means of inorganic catalysts such as Cu(II) and Zn(II), or organic catalyst such as DBU. One example is the study by Barzagli et al. [37], where the production of urea at a mild condition of 100^∘C and 3.7 bar was reported. Results of the study show that, using an organic catalyst DBU with the DMSO solute, an yield rate of 35% was achieved.

Other research efforts are being made from a more eco-friendly perspective, such as urea synthesis via an electrochemical reaction of N2 and CO2 in aqueous conditions. Chen et al. [45] published a study to electrochemically generate urea under ambient conditions, by simultaneously fixing N2 and CO2. A flow reactor cell was fabricated with a working electrode with Pd-Cu nanoparticles supported on TiO2 nanosheets as catalysts, a Ni foam anode, and an electrolyte based on an aqueous KHCO3 solution. The proposed reaction mechanism is illus- trated in Fig. 3(a). First, the N2 molecules are chemisorbed onto the catalyst surface, forming activated N₂with reduced N–N bond order.

The existence of activated N2 promotes CO2 reduction to occur on adjacent metal sites, forming CO. Then the intermediate *NCON* is formed, which is subsequently hydrogenated to produce urea, at a Faradaic efficiency of 8.92 % at − 0.4 V vs. reversible hydrogen electrode (RHE). This research shows the potential for converting the conventional process, which requires high initial investment costs and energy input, into a low-energy, low-cost small scale process, which allows localized production of urea in different parts of the world.

Various forms of research are being conducted to commercialize an alternative route to produce urea. One of the representative research results is to produce disubstituted urea, such as 1,3-dialkyl urea or 1,3- diaryl urea, via carbonylation of CO or CO2 [75,99–101,154]. These reactions are shown in the equations below:

2RNH2+CO+1

2O2→RNH(C=O)NHR+H2O (7) 2RNH2+CO2→RNH(C=O)NHR+H2O (8)

These types of substitution reactions utilize Cs-based or K-based catalysts. When an amine is deprotonated the basic condition of the catalyst is relevant, and thus CO2 utilization in urea production is ad- vantageous when alkylamine is the substrate. However, when the substrate is an aromatic amine, only benzylamine is available. CO utilization shows higher reactivity compared to CO2 utilization, in the presence of a catalyst and an operating condition of 100–120^∘C. How- ever CO is more expensive compared to CO2 and also toxic, raising safety issues during process operation. A CO2 utilization process has to be operated at a temperature of 170–180^∘C, to activate the thermody- namically stable CO2. Also, H2O is produced as a byproduct, meaning that the catalyst has to be stable enough not to undergo hydration in high temperatures. By optimizing the process, eliminating the water byproduct can increase the yield rate by promoting the forward reaction. Recent studies show that with the appropriate selection of anions, even Cs-based catalysts are stable in hydration conditions, providing high yield of over 95% and a turnover frequency (TOF) of 858h⁻¹. Various efforts have been made on the production of disubstituted urea via the production of a novel catalyst. Choi et al. [47] proposed the use of K3PO4 to catalyze the amine-CO2 carboxylation for producing 1, 3-disubstituted ureas with high yields. The schematic of the reaction and the catalyst configuration is shown in Fig. 3(b) Truong et al. [167, 168] presented the use of alkali metal azolides as catalysts for amine-CO2 carboxylation to disubstituted ureas. Nguyen et al. [129] proposed the use of polystyrene-functionalized basic ionic liquids (PS-BILs) as catalysts for producing disubstituted ureas. Among the various candidates the [bis-imidazolium]/[bis-bicarbonate] showed the highest activity and good recyclability of up to seven consecutive runs.

CDU-based urea production is a mature technology, where all of the urea production incorporates CO2 as raw material. The expected demand of urea is estimated to expand at a growth rate of 2.1% between 2021 and 2026, forming a market size of 212 million USD [32]. Thus urea production is expected to play a significant role as a main CDU application. The role of industries would be to develop optimized processes for cost efficient production of urea, such as integrating the energy loss from the ammonia production process.

2.4. Mineral carbonation 2.4.1. Overview of technology

Incorporating CO2 to mineral as a CDU application was first proposed by Seifritz [153]. Mineral carbonation technologies produce carbon-based minerals by reacting CO2 with industrially developed

Fig. 3. Two examples of recent approaches for CDU-based urea production. (a) The use of Pd-Cu alloy nanoparticles on TiO2 nanosheets as catalyst in electrochemical conversion of CO2 to urea under ambient conditions. Reprinted from Chen et al. [45] Copyright (2020) Springer Nature. (b) Using K3PO4 as a catalyst for CO2 carboxylation of amines to urea. Reprinted from Choi et al. [47] Copyright (2014) Elsevier.

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inorganic byproducts. Main products of mineral carbonation include calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). Min- eral carbonation processes can be categorized into two pathways: direct and indirect carbonation. Direct carbonation is the process of mimicking the natural process of mineral carbonation, where CO₂is directly injected to minerals within the geological formations, and heat and pressure is applied to accelerate the weathering process. The economic feasibility of the direct carbonation process depends greatly on the actual amount of heat and pressure required, and thus it is important to design processes so that waste heat can be utilized.

Indirect mineral carbonation includes an extraction process, where calcium or magnesium is extracted from minerals in acidic conditions.

Although an additional preprocess is required, indirect carbonation requires milder carbonation conditions and shows higher carbonation efficiency, compared to the direct carbonation process [116].

Mineral carbonation is considered the most effective option for long- term carbon storage, since it has less effect on the environment compared to geological or marine storage, has low possibility of re- emission into the atmosphere, and the derived products are applicable as a variety of industrial materials. To improve the economic performance of the carbonation process, research efforts have been focused on recovering valuable byproducts from the process, such as zeolites or precipitated calcium carbonates (PCC). A preliminary economic analysis provided in the studies by Chu et al. [48] and Liu et al. [115] have shown that by selling byproducts such as NH4Al(SO4)2, economic profitability can be achieved.

MgCO3 is mainly used as a desiccant, and also used as raw material in various industries, such as pharmaceutical, cosmetics, and the ink industry. CaCO3 is widely used in the paper-making, paint, and concrete industry. Also, it has low reactiveness and high absorption rate within the body, making it a viable option as an inert filler and as a coating agent in capsules used in the pharmaceutical industry.

Since the reaction of mineral carbonation does not require the use of catalysts or complex reaction paths, studies focus on developing pathways for incorporating various types of waste materials as sources of Ca/

Mg, or integrating the obtained CaCO3 or MgCO3 with processes for producing value-added products. Different types of wastes include blast furnace slag, steel slag, waste gypsum, industrial caustics, coal fly ash, etc., and potential final product applications include cement ingredient, paper filler, fertilizer, etc. Typical process flow diagram of CO2 mineralization, and various options for converting the produced CaCO3 and MgCO₃into other chemicals, are presented in Fig. 4(a) and (b), respectively.

Ren et al. [142] employed different salt solutions to enhance the direct carbonation process of blast furnace slag. Under reaction conditions of 150^∘C, CO2 pressure of 30 bar and using 1 mol/L NaCl solution as the reagent, the CO2 mineralization capacity of 280 kgCO2/tBFS was obtained. It was shown that the saline solution promotes the dissolution of calcium and enhances the CO2 mineralization process, by increasing the effective acidity of the solution. Ghacham et al. [71] studied the efficiency of steel slag carbonation, considering the effect of various parameters such as gas pressure, liquid/solid ratio (L/S), gas/liquid (G/L) ratio, and reaction time on the CO2 carbonation capacity.

Parameter analysis showed that the G/L and CO2 pressure have a posi- tive effect on the conversion of CO₂, and by optimizing the parameters the system achieved a carbonation capacity of 52 kg per tonne of slag.

Yu et al. [194] proposed to employ the coal fly ash carbonation process as an amine regeneration process, integrating it with a sterically hin- dered amine-based CO2 absorption to maximize the energy efficiency.

The integrated CO2 absorption-mineralization (IAM) process was able to eliminate the need for an amine regeneration process, greatly decreasing the overall energy usage. Park et al. [136] reported the use of waste concrete as the source of calcium, and the operation results of the pilot-scale plant with a capacity of producing 20 kg/d of CaCO3. The

developed process was able to produce CaCO3 with a purity of 99.0 %, mitigating CO2 by 0.465 kg per process cycle, taking into account the energy usage.

Studies focused on enhancing the economic feasibility include the approaches for recovering silica and aluminum resources from industrial solid waste, while using CaO and MgO for CO2 fixation. Tong et al. [166]

developed a new process for simultaneously obtaining CaCO3, Fe, and glass ceramics without wastewater, via reaction of steel slag and CO2. NH4Cl was used for leaching residue of the steel slag. Chu et al. [48] used NH₄HSO₄to leach blast furnace slag, producing valuable resources such as ammonium alum and silica, and achieving CO2 mitigation of 361 kgCO2/tBFS. Other studies focus on processes for producing precipitated calcium carbonate (PCC), which is basic raw material for plastics, coatings, and pesticides. Song et al. [158] studied the production of high-purity PCC via mineral carbonation of flue gas desulfur- ization gypsum. Analysis of operating conditions revealed that the maximum formation efficiency for PCC was affected by the ammonia content, and the S/L ratio. He et al. [78] proposed a novel process for utilizing coal fly ash to produce high-purity PCC, where recyclable ammonium salt, such as NH4Cl, NH4NO3, and CH3COONH4, were used the extract calcium from coal fly ash prior to the carbonation process.

Although in its early stage of development, the use of ultrasound is gaining attention as a process intensification method for mineral carbonation. Ultrasound can promote the mineral carbonation process by applying sound waves in the range of 16–100 kHz to enhance the mixing process, and accelerate the particle breakage and carbonate passivation layer removal [150,152]. Several studies have provided Fig. 4.(a) Process flow diagram for producing NaHCO3, MgCO3, or CaCO3

from industrial caustics. The HE block represents the heat exchanger. Reprinted from Kelly et al. [96] Copyright (2011) Elsevier. (b) Possible usages of CaCO3

and MgCO3 via integration with other processes. Reprinted from Woodall et al. [182].

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promising results regarding mineral carbonation using ultrasound. Said et al. [148] showed the enhanced performance of calcium extraction from steel slag using ultrasound. Their results showed that ultrasound can remove the porous layer which forms after calcium is liberated from the slag, extracting 96 % of the Ca in the smallest particles of the slag.

Santos et al. [151] presented results on the improved performance of mineral carbonation to produce aragonite, with the use of ultrasound.

The use of ultrasound reduced the required concentration of Mg, as well as decreasing the reaction temperature to 24–70^∘C. A recent study by Wang et al. [173] presented the conversion of gypsum to CaCO3 via ultrasonic carbonation, where pure vaterite was obtained by applying 50 % ultrasonic amplitude for 30 min. While scale-up of the ultrasound-integrated process is yet to be realized, and current limitations such as large amounts of electricity usage [133] must be dealt with, ultrasound may prove to be an effective mineral carbonation route in the near future.

The carbonated mineral market consisting of products produced from conventional processes is estimated to be 10–60 billion USD/y, and is expected to increase to 550 billion USD/y in 2030. Due to this large market potential, various construction companies are investing on the CDU mineral carbonation process, such as Blue Planet, Calera, Skyonic, Carbicrete, Carbon Capture Machine, and Carbon8 Systems. The Calera Corp has published over 250 patents on the mineral carbonation process, in which electrolysis is used to separate Ca and Mg used for carbonation.

The Sleipner project injects CO2 into the Utsira saline formation of the North Sea [38]. It is estimated that 30 million tonnes of CO2 will be stored in the Utsira saline formation over the project life time of 30 years.

CarbonCure Technologies developed a CO2 curing process to produce concrete, where liquid CO2 is injected into a pressurized tank where wet concrete is being mixed. The efficiency of CO₂absorption into the concrete is known to be approximately 50–80 % [124]. The process can be retrofitted into existing concrete plants, and is currently being widely applied in ready-mix concrete plants in North America [8].

Carbon8 Systems developed the Accelerated Carbonization Tech- nology (ACT), which utilizes CO2 to mineralize various thermal wastes, including steel slags, incinerator ash, etc. [7] The first ACT plant was built in 2012, which produces more than 65,000 t/y of carbonated products, namely CircaBuild®, and CircaGrow® [203].

Carbonfree Chemicals developed a CO2-mineralization process for manufacturing inorganic products, such as NaHCO₃, baking soda, caustic soda, etc. The process, namely the SkyMine®process, incorporates an electrochemical method to produce low-concentration NaOH from salt and water [172]. This solution is used to capture CO2

to produce high-purity NaHCO3. The first SkyMine®plant was constructed in 2015, utilizing CO2 at a capacity of 83,000 t/y [9].

Carbfix, which is a carbon fixation frim in Iceland, injects CO2 into basaltic rock, after mixing CO2 with water. Carbfix recently partnered up with Climeworks, which is a direct air capture (DAC) firm, to capture 4000 tonnes of CO2 per year out of air, and to inject it into basaltic formations in Iceland [3]. Currently an accumulated amount of 81,165 tonnes of CO2 have been injected by this process [4].

3. CDU in the development phase – Technologies with potential for large scale reduction

In this section, various technologies that retain potential for large scale CO2 reduction, but with less technological maturity compared to technologies in the demonstration phase, are introduced. While these technologies are still in the development phase and various hurdles need to be overcome for commericialization, the large market available for these technologies are attracting many corporations and research groups.

3.1. Carbon monoxide 3.1.1. Overview of technology

Carbon monoxide (CO) is obtainable from CO2 reduction, and has high industrial value while retaining large CO₂mitigation potential.

Stoichiometrically, 1 mol of converted CO2 produces 1 mol of CO, which means that 1.57 tCO2 can be consumed by producing 1 tonne of CO.

Upon mixing with H2 to obtain syngas, the widely utilized Fisher- Tropsch process allows the conversion of syngas to a variety of hydrocarbons and other value-added chemicals, making CO an attractive option for commercial CDU application. Also, syngas can be processed to produce methanol, which is a valuable chemical on its own, and can also be further processed to produce olefins via the MTO reaction, as introduced in Section 2.1. Furthermore, 99 % of the CO produced within the steel industry is collected and reused within the facility or used to create electricity in power plants, providing diverse applications for large scale CO2 mitigation.

CO can be obtained from CO₂through various pathways, such as electrochemical, catalytic, photochemical, and plasma conversion.

Electrochemical conversion of CO2 can be considered a green process if the electricity produced from renewable energy sources, such as solar or wind power, can be utilized. Photochemical conversion processes directly incorporate solar energy, and allows production of chemicals with minimal CO2 emissions given the appropriate light source and catalyst. Plasma conversion allows rapid heating required for the methane reforming process, by adjusting the frequency of plasma generation. While the carbon abatement potential of these technologies are clear, additional research is required for large scale demonstration of these technologies to take part in the CO2 mitigation objectives.

Electrochemical CO2 reduction technologies producing CO can be categorized according to the operating temperature. Low temperature electrochemical conversion is processed at temperatures below 100^∘C, and make use of polymer membranes and acidic or basic aqueous electrolytes. Water is first decomposed into O2 and hydrogen ions at the anode, then the produced hydrogen ion is transferred through the membrane to participate in reduction reactions with CO2, producing valuable chemicals such as CO, formic acid, methane, and alcohols [64].

The specific products obtained from electrochemical systems differ according to the catalyst and membrane type. Among the different catalysts, Au, Ag, and Zn catalysts are known to provide the highest selectivity for CO production [79]. High temperature electrochemical conversion systems operate at approximately 800^∘C, and incorporate solid oxides as electrodes and the electrolyte. Upon operation, the electrical energy provided decomposes the CO2 provided to the cathode into CO and oxygen ion. Then the oxygen ion is transferred to the anode through the electrolyte to produce oxygen gas by reaction with electrons. High temperature electrochemical conversion incorporate the cermet family of cathodes such as Ni and Yttria Stabilized Zirconia (YSZ), and tend to have a higher conversion rate of CO2 compared to the low temperature electrolyzers [200], regardless of the type of electrode used. Recent research regarding electrochemical CO production focus on the development and tuning of catalysts to enhance conversion and selectivity, the design of electrolyzers [109], and the design of processes integrated with the capture process to maximize energy efficiency and improve profitability. Chung et al. [49] reported the development of a catalyst for decomposing CO2 at low temperatures, which is produced by sputtering of a liquid polymer and shows high stability and produc- ibility, and also high conversion rate from CO₂to CO. Nguyen et al.

[128] proposed a method for increasing the Faradaic efficiency of CO production by adjusting the coverage of reduced graphene oxide layers on porous Zn electrocatalysts. Park et al. [138] recently developed a metal-ceramic complex cathodic catalyst which operates at high temperatures and shows stable performance even with certain amounts of impurities included. Characterization images of the developed catalyst

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are shown in Fig. 5. Rosen et al. [146] reported that the CO conversion rate is extremely enhanced when ionic liquid is applied to the low temperature electrolysis system.

Catalytic conversion technologies include a wide range of technologies that utilize catalysts and a reducing agent for converting CO₂to CO. Dry methane reforming and CO2 hydrogenation reactions are representative techniques. Dry methane reforming makes use of methane as the reducing agent to convert CO2 to syngas, which has the effect of mitigating two different types of greenhouse gases [85], methane and CO2. Typical catalyst selections include transition metals such as Ni, Ru, Rh, Pd, Ir, and Pt, which all show high activity when used for methane reforming reactions. Among these candidates, Ni shows the highest cost efficiency and is widely studied for commercialization of the process. CO2 hydrogenation is the process of employing H2 as the reducing agent to reduce CO₂to CO, and different chemicals may be obtained according to the type of catalyst used. During CO2 hydrogenation two reactions occur competitively, the reverse water-gas shift reaction (Eq. 4) and the methanol synthesis reaction (Eq. 3). The selectivity of the reaction products are determined by the type of catalyst used. CuO-ZnO/TiO2, Cu/ZnO/Cr2O3, and Cu/ZnO/Al2O3 all show high selectivity of over 70% towards the production of CO [162]. Song et al. [159] recently published a study on the development of the NiMoCat catalyst, which is prepared by placing Ni-Mo alloy nanoparticles on the magnesium oxide support. The NiMoCat catalyst is reported to have a long life time when used for dry methane reforming, and shows no sign of coking or sintering. The synthesizing process and images related to catalyst production is shown in Fig. 6. Joo et al. [91]

implemented the atomic layer deposition (ALD) method to produce a catalyst for dry methane reforming, showing high conversion rate of methane and improved life-time.

Photochemical conversion is the process of converting CO₂using the excited state electron obtained by solar energy, and various products can be obtained according to the number of electrons and photons incorporated in the reactions. A schematic of the photocatalytic CO₂reduction process is shown in Fig. 7 Typically for CO production, a homogeneous reaction system based on a transition metal complex is used. Various options of sensitizers, dopants, radiation sources are considered for new catalyst development. An early example of such a

system is the Re-carbonyl system, where it was reported to selectively produce CO when used in the form of a single component molecular catalyst, acting both as a photosensitizer for absorbing light and as a photocatalyst for accelerating the reaction [77]. Recently, Liu et al.

[117] proposed the use of a Cu^IIcomplex molecular catalyst for reduction of CO2 to CO. Using [Ru(phen)3](PF6)2) as the photosensitizer, the reaction was carried out with visible light, and the developed catalyst showed a high TON of 9900 and a CO selectivity of 98 %. Zhang et al.

[197] developed a method for deriving single-atom catalyst from the metal-organic framework. Using Co as the base metal, pyrolysis of the ultrathin Co metal-organic layers during g-C3N4 formation resulted in the anchoring of single Co(II) sites. The produced photocatalyst showed a CO evolution rate of 464.1μmol/gh, which was three times faster than the bulky Co-MOF catalyst. Nguyen et al. [127] developed a novel catalyst, Ag-decorated reduced TiO₂/WO₃hybrid nanoparticles, for converting CO2 to oxygen and CO when exposed to visual rays. The catalyst performance was shown to have an apparent quantum yield of 34.8 %, with maximum selectivity of 100 % CO. Won et al. [181] greatly enhanced the efficiency and life time of a photocatalytic system by applying porphyrin dye.

Plasma conversion of CO₂is the process of using non-thermal plasma to split CO2 to CO and O2, or via the dry reforming of methane (DRM) process, where CO2 and methane are converted to CO and H2. Plasma conversion has the advantage over catalytic conversions that carbon adsorption on catalysts are prevented [157], which is a critical factor of catalyst deactivation. Aerts et al. [34] presented the experiments and computational analysis results of CO2 splitting via dielectric barrier discharge (DBD) plasma. Investigation of the operating conditions showed that the discharge gap and the specific energy input (SEI) has a significant effect on the conversion and energy efficiency. For DRM, recent studies focus on the hybrid application of plasma and catalysts to maximize the process efficiency. Chu et al. [48] presented the use of microwave plasma coupled with a catalytic reactor to convert CO2 and CH₄to syngas under atmospheric pressure. The hybrid system showed approximately twice as high conversion levels for both CO2 and CH4, compared to the system using only the microwave plasma.

Martin-del-Campo et al. [42] proposed the integrated use of the rotating gliding arc plasma and Ni-supported catalysts in a spouted bed reactor

Fig. 5. SEM image of the catalyst developed by Park et al. [138]. Reprinted from Choi et al. [47] Copyright (2014) Elsevier. (a) Image of the La0.6Sr0.4Co0.5-

Ni0.2Mn0.3O3(LSCNM) and (b) the CoNi-R.P.LSCM catalyst obtained by reducing LSCNM. (c) The TEM and EDS mappings of each of the elements consisting the CoNi-R.P.LSCM powder.

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for DRM. The reactor image and performance of the proposed system is given in Fig. 8. Upon experimenting with two different catalysts, Ni supported on Al2O3 or SiO2, maximum selectivity of 96.7 % was obtained for CO using a combination of plasma and passivated 30 wt%

Ni/SiO2, and maximum selectivity of 84.9% for H2 was obtained using plasma and fresh 15 wt% NiO/SiO2.

Progress in CO2 conversion to CO in the industry has been intense over the last few years. Considering electrocatalytic conversion, the Sunfire GmbH integrated the CO conversion process to a fuel production process, where syngas is produced by co-electrolysis of CO2 and water in a high-temperature electrochemical system, then the produced syngas is converted to a synthetic fuel named “the blue crude.” Efforts are being made to upscale the process for industrial commercialization [31].

Studies on catalytic conversion of CO2 are being conducted mainly by corporations in Germany and Norway. Linde developed a green process for dry methane reforming, and received funding from the German government for the construction and operation of a pilot-scale

plant [17]. The Nordic Blue Crude is currently constructing a pilot plant for producing blue crude at a capacity of 10 million liters by 2025.

The syngas which is used as material for blue crude production is obtained by reacting CO2 with H2 in a RWGS reaction, where the H2 is produced by alkaline electrolysis [20].

Regarding photochemical CO2 conversion, the Sandia National Laboratory (SNL) of the U.S. Department of Energy (DOE) have demonstrated a Sunshine to Petro (S2P) reactor, which incorporates solar energy to produce syngas from CO2 and water [121]. The thermochemical heat engine, i.e., the Counter-Rotating Ring Receiver Reactor Recuperator (CR5), produces CO with input of CO2, and H2 with input of H2. Thus syngas can be produced with adjusting the ratio of the operations. However, the current system is incapable of achieving Fig. 6. Preparation of the NiMoCat proposed in Song et al. [159]. (a) Synthesizing sequence of NiMoCat. MgO is obtained from combustion synthesis of Mg chips and CO2, then the MgO nanopowder is dispersed into a Ni and Mo containing salt solution, which is then reduced with hydrazine. Images on the right show TEM and SEM images of (b) MgO cubes, (c) freshly created NiMoCat, and (d) NiMoCat after reaction. Reprinted from Song et al. [159] Copyright (2020) The American Association for the Advancement of Science.

Fig. 7. Schematic figure showing the photocatalytic CO2 reduction process.

Reprinted from Wu et al. [184].

Fig. 8.Reactor image and performance of the system proposed by Martin-del- Campo et al. [42]. By integrating the rotating gliding arc plasma with Ni-supported catalysts CO is produced with high selectivity. Reprinted from Martin-del-Campo et al. [42] Copyright (2021) Elsevier.

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efficiencies high enough to compete with other CO producing technologies, and further improvements are required for commercialization of the process [122].

ReCarbon Inc. made progress on converting methane-CO2 to syngas via plasma conversion at a foodwaste site, and is recently working on the construction of a commercialized power plant based on these results.

The Emission Blade, a microwave plasma generation device, conducts DRM to produce syngas in a Plasma Carbon Conversion Unit (PCCU) [26].

3.2. Light olefins

Light olefins are carbon-based chemicals that compose the building blocks for various polymers. Light olefins include ethylene, propylene, and butadiene, and are conventionally obtained by the naphtha cracking process, ethane cracking process, and coal-to-olefin process from natural gas, coal, and crude oil. The basic conversion equations of this process are shown in Eqs. 9 and 10.

CO2+H2⇄CO+H2O (9)

CO2+H2⇄(−CH2− )_n+H2O (10)

Olefins can be obtained from CO2 via thermochemical and electrochemical pathways. For the thermochemical pathway, direct and indirect hydrogenation are available options. For direct hydrogenation, multi-functional catalysts are used to produce C2-C3 olefins. As for indirect hydrogenation, CO2 is first converted to CO, then proceeds with the FT reaction, or is further converted to methanol to undergo the MTO reaction. A summary of the thermochemical reaction routes from CO₂to olefins is provided in Fig. 9. For the electrochemical pathway, Cu-based catalysts are incorporated to directly produce light olefins by CO2

reduction.

Since the reactions related to thermochemical conversion of CO2 to olefins is mature, most of the recently published studies focus on the development of catalysts to improve the overall performance.

Dorner et al. [56,57] developed catalysts based on Fe, Mn, and K for CO2 hydrogenation to C2-C5+olefins. The developed catalyst combines the RWGS and FT chain growth reactions, and the Fe/Mn/K catalyst acts as a bifunctional catalyst. Sai Prasad et al. [147] studied the FT reaction of CO/CO₂/H₂bio-syngas with Fe-based catalysts. At reaction conditions of 300^∘C and 10 bar, they obtained 39.19 % distribution of C2-C4 products, with an olefin selectivity of 77.23 % within the C2-C4 products, using a Fe/Cu/Al/K catalyst. Zhang et al. [199] developed the Na and Zn promoted Fe catalyst to achieve C2-C12 olefin selectivity of 80 % at 39 % CO2 conversion. Analysis showed that using Zn enhances the Fe

catalyst performance by enhancing H2 adsorption, and that the Na promoter increases the CO₂adsorption and inhibits hydrogenation of olefin, increasing selectivity. Witoon et al. [180] implemented the Fe/Co/K/Al oxides catalyst for CO2 hydrogenation to olefins, with alternating preparation methods to alter CO₂and H₂adsorption. The results showed that by preparing catalysts by precipitation with NH4OH as the reducing agent, followed by reduction using NaBH4 as the reducing agent, showed the highest olefin yield of 16.58 % at 350 ^∘C and 20 bar.

Kolesnichenko et al. [104] studied the conversion of methanol and DME to olefins using zeolite catalysts modified with Rh compounds.

Their results show that the implementation of Rh centers increases the selectivity of lower olefins synthesis. Hanaoka et al. [76] studied the performance of zeolite catalysts for the DEM-to-olefins reaction. The results showed that ferrierite zeolite with NH₄⁺ showed the highest n-butene yield.

Another significant pathway for converting CO2 to olefins is the use of electrochemical reactions, where CO₂is reduced to produce olefins of various carbon numbers, depending on the catalyst used.

Tomboc et al. [165] proposed the electrochemical conversion mechanism of CO₂to ethylene, using Cu-based electrocatalysts. They categorized the Cu-based electrocatalysts into four groups, namely metallic Cu, Cu alloys, Cu compounds, and supported Cu-based catalysts, and identified the detailed mechanisms for selective CO2 reduction reaction for each of the different types of catalysts. Kim et al. [102]

synthesized a monodisperse nanoparticle of CoFe2O4 to produce Na-promoted CoFe2O4 catalysts supported on carbon nanotubes, which showed high CO2 conversion of 34% and light olefin selectivity of 39 %.

Jeong et al. [86] proposed the method of controlling the spacing between Cu facets within the atomic-scale to maximize binding energies of CO₂reduction intermediates, enhancing the selectivity of C2+products during CO2 electrochemical reduction. Yang et al. [191] showed that olefin production from CO and CO2 is possible using Na- and K-promoted Fe-Zn catalyst. They found that the use of Na- and K-promoters alters the balance between iron oxides and iron carbides in the catalyst, affecting the CO and CO2 conversion. Gao et al. [67] proposed the use of bifunctional catalyst composed of indium-zirconium composite oxide and SAPO-34 zeolite, with the former composite responsible for CO2

activation and the latter composite responsible for selective C-C coupling. The results showed high olefin selectivity as high as 80% at CO2 conversions higher than 30 %. Yuan et al. [195] made use of CoFe2O4 as a catalyst precursor for CO2 precursor, which showed high selectivity for C2+hydrocarbons. Analysis of the results showed that high dispersion of Fe and Co helped inhibit the formation of methane, and consequently promoted the formation of longer chain products.

While various corporations produce olefins via the MTO reaction, currently none of the processes are integrated with CDU. This is due to the fact that overall processes need to be retrofitted and revised to implement CDU-based applications of olefin production, and that currently it is not economical to produce olefins as the end-product of an entire plant. Future developments should be focused on integrating the existing MTO processes with the CDU-based methanol processes.

3.3. Aromatic compounds 3.3.1. Overview of technology

While the current level of research compared to previously reviewed technologies is at a novice stage, the conversion of CO₂to aromatic compounds, such as benzene, toluene and xylene (BTX), is gaining considerable attention. Two main pathways exist for producing BTXs from CO₂: the Fischer-Tropsch (FT) synthesis pathway, where CO₂is first converted to C2+hydrocarbons, then transformed into aromatic compounds on zeolites, and the methanol pathway, where methanol is first synthesized from CO2, converted to olefins on zeolites, then finally Fig. 9. Various routes for producing light olefins from CO2 and H2 obtained

with renewable energy. Reprinted with permission from Centi et al. [43]

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converted to aromatic compounds.

The FT pathway first produces CO from RWGS reactions, then the CO is converted to C2+compounds. The most widely used catalysts are Fe and Co-based, but has the limitation in terms of selectivity, where the selectivity towards C2+compounds are limited by the Anderson- Schulz-Flory (ASF) distribution limitation[62,63]. The typical selectivity value of the FT pathway to C2+hydrocarbons is below 60%.

The methanol pathway is free from the ASF distribution limitation, and typically shows higher selectivity compared to the FT pathway. The C2+compounds produced via both the FT and methanol pathway undergo C-C coupling reactions, ring formation and dehydration reactions on the surface of the zeolite to produce aromatic compounds.

Catalytic conversion of CO₂directly to aromatic compounds is at its very early stage of development. A majority of the studies conducted on the subject focus on the development of effective catalysts. Conventional catalysts of CO₂-to-methanol, such as ZnCrOx/HZSM-5, ZnZrO₂/HZSM-

5, CR2O3/HZSM-5, ZnAlOx/HZSM-5, and ZnCrOx/HZSM-5, typically show low conversion of 10–20 %. Also, since conventional catalysts show low selectivity towards aromatic compounds, various studies are focused on compositing HZSM-5 to increase the selectivity of BTX. For the methanol pathway, the conversion tends to be low (9–22 %) and selectivity high (56–74 %), whereas for the FT pathway, CO2 conversion is higher (27–34 %) compared to the methanol pathway but the selectivity is lower (20–53 %).

Regarding the methanol pathway, the Liu group of Dalian Institute of Chemical Physics (DICP) conducted various studies on incorporating the spinel type ZnAlOx&HZSM-5 catalyst. When the catalyst was used to react H2:CO2:Ar with a ratio of 3:1:0.2 at 30 bar and 320^∘C, the conversion of the reaction was 9 %, and showed an aromatic compound selectivity of 76% [130]. The Tsubaki group conducted studies on the subject as well, where reaction of H₂:CO₂ratio of 3:1 at 30 bar and 350^∘C, using a Cr2O3/HZSM-5 catalyst showed 34 % of CO2 conversion and 76% aromatic compound selectivity [35]. In another study by the Tsubaki group using Cr₂O₃/Zn-doped HZSM-5 @SiO₂catalyst, 22 %

Fig. 10. Schematic illustrations and representative performances of the direct CO2 hydrogenation and CO-mediated pathways for BTX production. (a) Reaction mechanism of CO2 hydrogenation on the ZnZrO/ZSM-5 tandem catalyst proposed by Li et al. [111], and (b) the performance of the variations of the catalyst, in terms of product hydrocarbon distribution. (c) Schematic flow diagram of the CO-mediated production of BTX. (d) Performance of the Na/Fe and HZSM-5 catalyst proposed by Xu et al. [187] in terms of the aromatic distributions according to temperature change, and (e) time-dependent stability test of the developed catalyst. (a) and (b) reprinted from Li et al. [111] Copyright (2019) Elsevier. (c) and (d) reprinted from Xu et al. [187] Copyright (2019) Royal Society of Chemistry.