Introduction

The atmospheric concentration of poisonous gases, due to human activities, has been rising extensively since the Industrial Revolution. It has reached dangerous levels not seen in the last 3 million years and has continually increased. Without significant control and conversion to valuable products, it will become a major disaster for the ecosystem. CO2, which accounts for over 70% of all greenhouse gases, is prominent among these poisonous gases. According to the IEA 2021,1 global CO2 emissions declined by 5.8% in 2020. CO2 emissions were lower than energy demand in 2020 due to the pandemic hitting interest for other fossil-derived fuels, especially oil and coal, while renewables increased. Despite the decrease in 2020, global energy-related CO2 emissions remained at 31.5 Gt, which added to CO2 reaching its highest-ever average annual concentration in the atmosphere of 412.5 parts per million in 2020, around half higher than when the industrial revolution began. In 2021 worldwide, energy-related CO2 emissions were projected to bounce back and increase by 4.8% as coal, oil, and gas demand returned with the economy. The increment of more than 1500 Mt CO2 would be the most significant increase since the carbon-intensive economic recovery from the worldwide financial crisis over ten years. It leaves worldwide emissions in 2021 of around 1.2% (about 400 Mt), below the 2019 peak. However, carbon dioxide is now a recognized carbon feedstock for the chemical supply chain, becoming an alternative to oil and gas.

Key industries required for industrialization, like cement, petrochemicals, oil and gas, power, steel, chemicals, and other heavy-emitting industries, are responsible for a large proportion of CO2 emissions globally. As the world strives to meet the net-zero emission target in the coming decades, balancing sustainability becomes highly imperative for societies to thrive. Sustainability is most likely to be achieved by capturing and utilizing the stored CO2 as fuel, electricity, chemical feedstocks, and other value-added products. One proven and established technology by which CO2 is captured is carbon capture utilization and storage (CCUS). CCUS has the potential to sequester about 90% of harmful CO2 emissions from heavy-emitting-related industries2 and help create a sustainable environment. It will, therefore, be crucial and is expected to play a critical role in the clean energy transition agenda. The main types of CCUS are pre-combustion, post-combustion, and oxy-fuel technologies. Detailed descriptions and processes of CCUS technologies can be found elsewhere.3, 4

The utilization of the captured CO2 provides an economic justification for the carbon capture and storage (CCS) process. The utilization of CO2 is basically through direct (non-conversion) and indirect (conversion) methods. Various review publications on CO2 utilization have analyzed the multiple techniques of CO2 utilization (simulations, biochemical, catalytic, and electrochemical) to obtain numerous value-added products. While some studies centered on a few products,5,6,7 others analyzed only a single product.8,9,10,11,12,13,14,15 Published studies on a wide variety of CO2-derived products where up to six (6) and more products are analyzed and compared simultaneously are scarce in the open literature. This broad coverage is significant for selecting the best possible technique(s), process conditions, and products at a glance.

In general, there are three methods by which CO2 can be converted into chemical products. They include.

  • The application of physical energy such as electricity.

  • Choosing suitable oxidized low-energy synthetic targets such as organic carbonates.

  • Using high-energy materials such as hydrogen.

Forecasting and identifying market opportunities in CO2 utilization technology in terms of viability, readiness, markets, and momentum is highly imperative. According to the CO2U16 research report, there have been tremendous advancements in CO2 utilization recently, and many methods have proven to be scalable. Accordingly, four critical markets have been identified as viable priorities for CO2 utilization. They are building materials, chemical intermediates, fuels, and polymers. The CO2U16 report also identifies eight product categories within the four critical markets that should be pursued based on their technological maturity, commercial potential, and potential impact on carbon emission reduction. These product categories are:

  1. (a)

    Building materials

    • Concrete

    • Carbonate aggregates

  2. (b)

    Chemical intermediates

    • Methanol

    • Formic acid

    • Syngas

  3. (c)

    Fuels

    • Liquid fuels and

    • Methane

  4. (d)

    Polymers

    • Polyols

    • Polycarbonates

The majority of the conversion process in the open literature is by hydrogenation, chemical, electrochemical, photocatalytic, and thermal conversion. Consequently, this study will focus on CO2 conversion literature via hydrogenation, chemical, electrochemical, photocatalytic, and thermal conversion. The primary reason for this study is the massive amount of literature being churned out on the subject without proper synthesis of the available knowledge. For instance, a good way of measuring CO2 emission mitigation compliance globally can be in terms of existing CCUS facilities. Also, the percentage share of the total number of research publications on the value creation of CO2 conversion will provide valuable insight into the various products derivable from CO2 conversion. Hence, there is a need to summarize findings so researchers, operators, and other stakeholders can understand the best prospects. Understanding the best means of converting CO2 to value-added products relative to other sources of producing the same products will help shape the direction of the CO2 utilization research area. Also, the knowledge of the global distribution of CO2 utilization facilities will help identify the regions/countries at the forefront of CO2 capture and utilization and those lagging behind. This approach helps measure regions' and countries' commitments and compliance in mitigating CO2 emissions. These are essential gaps in knowledge that need to be explored. The basis of the research publication literature analysis will be articles published on CO2 utilization between 2016 and 2022.

Research methodology

This study is a systematic review of published articles or data on existing CO2 utilization facilities and published research articles on CO2 utilization (from 2016 to 2022). The research papers, including peer-reviewed journal publications, books, study reports, newsletters, and conference proceedings, were sourced from the Google search engine. After removing duplicates and screening, 69 CO2 conversion facilities and 425 research publications on CO2 utilization, obtained from over 500 published articles, were used for the study. The Cochran model17 was adopted to determine the representative sample sizes for the study. Table 1 shows the values of the statistical parameters used in determining the representative sample size by Cochran’s model.

Table 1 Statistical parameters for sample size determination.
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From the sample size analysis, it can be seen that a minimum sample size of 69 is needed to analyze the data on CO2 utilization facilities, with a confidence limit of 90%, a population proportion of 0.5, and a margin of error of 9.93%. At the same time, a minimum number of 385 research articles is required to investigate the data on published research on CO2 utilization, with a confidence limit of 95%, a standard deviation of 0.5, and a margin of error of 5%. The margin of error for the CO2 conversion facilities was raised to 9.93% while reducing the confidence limit to 90% due to the scarcity of the required data. A confidence limit of 90% and above and a margin of error of less than 10% is considered adequate in obtaining representative samples for data analysis. For the published articles on CO2 conversion to value-added products, this study will focus on the products identified in the CO2U16 report as viable materials. In addition, research publications on the conversion of CO2 to DME and urea will be included in this study to broaden the scope of products that are derivable from CO2.

Furthermore, these products were selected because of their premium significance in sustainable industrialization, economic development, and energy security of any society. The outputs of each product of CO2 utilization are presented in two sections. The first section analyses the various existing facilities for CO2 conversion, while the second section analyses the various research publications on CO2 utilization. The method of production and output generated is also stated. The key process parameters at which the optimum values are achieved are also reported for better specificity of the results.

Analysis of CO2 conversion facilities and published articles

This section highlights and synthesizes published data on existing CO2 conversion facilities as well as published articles on CO2 conversion to value-added products selected in line with the objectives of this study between 2016 and 2022. The analyses are presented as follows:

CO2 conversion facilities

Different facilities are used to obtain various value-added products using CO2 as feed. Some facilities convert CO2 to single products like methanol, DME, Urea, polymer, formic acid (FA), syngas, fuels, and building materials; others convert CO2 to more than one product. The main aim of this section is to present and analyze data on the existing conversion facilities to determine their location, quantity, and the specific product they are designed to produce. The conversion facilities are presented as follows.

CO2–methanol facilities

Methanol is a common feedstock for several synthetic chemicals. The main value-added products obtained from methanol include formaldehyde, acetic acid, methyl tertiary-butyl ether, and dimethyl ether.18 Its transformation into olefins is an emerging sector. Figure 1 shows a block diagram of methanol production from CO2 hydrogenation.

Figure 1
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Methanol production process via CO2 hydrogenation.

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Tables 2 and 3 show the existing and planned methanol industrial and commercial facilities worldwide. It is worthy of note that the planned methanol facilities in Table 3 were not used in the data analysis but rather to highlight the anticipated growth of CO2 to methanol conversion facilities in the near future.

Table 2 Existing facilities for CO2 to methanol production.
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Table 3 Planned facilities for CO2 to methanol production.
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CO2–DME facilities

Dimethyl ether (DME) is a natural compound primarily utilized as airborne fuel and as a reagent for creating broadly applied mixtures such as dimethyl sulfate and acetic acid.33 Dimethyl ether is a new and reasonable synthetic fuel that can substitute liquefied petroleum gas (LPG) or mixed in fuel mixture to its excellent combustion properties (cetane number = 55–60). DME has the potential to be fed into diesel engines, which would be only slightly modified, and its combustion prevents soot formation.34 The one-step and two-step Dimethyl ether production processes are relatively well established. Figure 2 shows the schematic of the production process of DME from CO2 hydrogenation, while Table 4 shows the existing DME industrial and commercial facilities worldwide.

Figure 2
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DME production process via CO2 hydrogenation.

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Table 4 Existing facilities for DME production.
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CO2–polymer facilities

About 400 million tonnes of polymers are created worldwide annually. The amount has been developing by 3–4% each year for quite a long time, and they have become a vital and significant piece of the advanced world. Regardless of their valuable and adaptable material properties, polymers face a significant issue: 90% of them are produced from fossil carbon and end up as CO2 emissions.39

Generally, the plastics industry cannot be decarbonized because carbon is the primary atom in their material structures. In this context, renewable alternatives to fossil feedstocks are required. Chemicals and polymers produced today now utilize renewable carbon, mostly from biomass. However, as biomass and the recycling of plastics alone will not suffice in bridging the gap in demand for polymers and plastics, carbon-dioxide utilization could be the solution to polymer production in the future.40 One such way of producing polymers from CO2 is hydrogenation. Figure 3 shows the schematic of the polymer production process from CO2 hydrogenation, while Table 5 shows the existing industrial and commercial facilities worldwide.

Figure 3
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Polymer production process via CO2 hydrogenation.

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Table 5 Existing facilities for CO2 to polymer production.
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CO2–urea facilities

Due to its significance in fertilizer production, urea is industrially produced on an immense scale. In 2019, for instance, annual world production was around 218 million tonnes. Urea is constantly synthesized from ammonia and carbon dioxide. Large quantities of CO2 are often generated during ammonia production, beginning from nitrogen available in the air and hydrogen produced from natural gas. As a result, urea production facilities are almost always located adjacent to the site where ammonia is produced.47 Aside from the CO2 generated during ammonia production, captured CO2 from other carbon-intensive processes can also be used with hydrogen to make urea. Figure 4 shows the schematic of the urea production process from CO2 hydrogenation, while Table 6 shows the existing CO2 to urea industrial and commercial facilities worldwide.

Figure 4
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Urea production process via CO2 hydrogenation.

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Table 6 Existing facilities for CO2 to Urea production.
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CO2–FA facilities

Within the limits of our literature search, there is no carbon dioxide conversion facility to FA, probably because methanol can serve as a feedstock for producing FA. FA is used as a chemical intermediate in adhesives, preservatives, dimethylformamide (DMF), and other products.16 Figure 5 shows the schematic of the process of producing FA from carbon-dioxide hydrogenation.

Figure 5
figure 5

Formic-acid production process (electrochemical reduction) via CO2 hydrogenation.

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CO2–building materials facilities

One proven way to lower emissions from the building and construction industry is the conversion of CO2 into concrete and other building material aggregates. It is one of the most mature and efficient CCU technology. Businesses are effectively transforming CO2 into building materials thanks to research and innovation. Project activities that capture waste CO2 that would have otherwise been released into the atmosphere can utilize the gas as a feedstock in concrete construction. By creating a type of concrete that sequesters CO2 into the material itself, which has an added benefit, and by developing concrete that uses less Portland cement, these project activities aim to minimize greenhouse gas emissions. The sequestering of CO2 lowers emissions by taking gas that would have otherwise been emitted and capturing, compressing, and transporting it to a site where it can be embedded into the concrete. Lowering the cement required in the concrete further reduces emissions because cement production is a high-energy and carbon-intensive process.58 Figure 6 shows the schematic of converting CO2 to calcium carbonate for cement and concrete production. Table 7 shows the existing facilities for concrete production.

Figure 6
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Typical schematic of a calcium carbonate facility via CO2 and industrial waste.

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Table 7 Existing facilities for building materials production.
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CO2–syngas and fuels

The conversion of CO2 to syngas and fuels offers an alternative pathway to waste utilization and decarbonization. Conventional syngas are produced from fossil-based sources like natural gas and coal via various chemical processes, including the electrochemical reduction of the produced CO2. However, sustainable and clean syngas can also be made from CO2 gasification of multiple materials, using specific catalysts to obtain the desired products. Carbon–neutral fuels like jet fuels, more efficient gasoline, and liquid hydrocarbons can be produced from CO2 conversion via several chemical processes. The conversion of CO2 to these fuels, including syngas, can enhance energy security while helping to achieve the net-zero emission targets. Despite the numerous benefits, only a few commercial conversion facilities are available globally (see Tables 8 and 9).

Table 8 Existing facilities for syngas production.
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Table 9 Existing facilities for fuel production.
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Published articles on CO2 conversion

This section contains the data on the published literature on the conversion of carbon dioxide to different value-added products, including methanol, urea, DME, FA, polymer, building materials, syngas, and fuels. There are various methods by which CO2 can be converted into other products. These methods are photochemical, biochemical, electrochemical, thermal, and chemical. The pros and cons of the different techniques of CO2 conversion into other products can be found elsewhere.9 Published literature from 2016 to 2022 on the conversion of CO2 to the selected products chosen for this study is analyzed and presented in the following sections. This literature analysis was done to synthesize the available knowledge on CO2 conversion studies to clearly determine the significant areas of focus for researchers on the subject matter in recent times.

CO2–methanol

Catalytic hydrogenation of a mixture of CO2 is the basis of syngas processes. These syngas processes make it possible to produce a variety of chemical products, including methanol. A careful survey of published works revealed two main research approaches for CO2 to methanol conversion. They are theoretical (simulation) and experimentation. Table 10 shows the carbon dioxide conversion to methanol analysis via different methods.

Table 10 Studies on CO2 conversion to methanol.
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CO2–dimethyl ether

Despite numerous studies on DME production, there is still a lack of process simulations for directly converting carbon dioxide to DME. Its distinguishing feature at the most conceptual level is that DME is synthesized directly from synthesis gas and is called a “direct” or “one-step” process. By contrast, the conventional process is called the “indirect” or “two-step” process because DME is produced from an intermediate product, methanol. Table 11 shows the results of the literature analysis on CO2 to DME for the period under investigation.

Table 11 Studies on CO2 to DME.
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Compared to the indirect method, the direct method of converting CO2 to DME is more cost-effective, with a higher return on investment and CO2 utilization impact.145, 146, 152

CO2–polymer

Different CO2-based monomers and polymers like polyester, polyureas, polyurethanes, polyols, and polycarbonates are produced from CO2 and have been commercialized. Still, it remains to be seen if the technology can compete. Reduction in the cost of CO2 and/or greater incentives to reduce carbon emissions must be implemented for CO2 utilization to polymers to compete with conventional feedstock.169 The data on the published work on CO2 to polymer is presented in Table 12.

Table 12 Studies of CO2 to polymer.
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CO2–FA

Currently, FA is used as a chemical intermediate in adhesives, preservatives, DMF, and other products. Because it’s more reactive than methanol, FA is more suitable as a chemical intermediate. Research in the reduction of carbon dioxide to FA (CH2OH) is still early-stage. FA also has been proposed as a fuel source for fuel cells. This application is still in the proof-of-concept phase.273 Converting harmful CO2 to beneficial FA is a step in the right direction. Consequently, several researchers have been working on CO2 conversion to FA. Some of the results are presented in Table 13.

Table 13 Studies of CO2 to FA.
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CO2–urea

The production of urea uses large amounts of CO2, which can be obtained from any CO2-generating process, including carbon sequestration. A few researchers have published works on CO2 to urea conversion. Table 14 shows the literature analysis of the published works within the period under investigation.

Table 14 Studies on CO2 to urea.
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CO2-building materials

The conversion of CO2 to concrete and other building aggregates is one of the primary sectors identified by the CO2 utilization roadmap's concentration of active developers.328 Solid carbonates are created from converting gaseous CO2 from industrial point sources or ambient air, and they can either supplement or replace concrete components. The three leading CO2 utilization technologies used in the building materials industry for the production of concrete via CO2 mineralization include58;

  • CO2 curing: CO2 replaces energy-intensive steam curing to decrease cement content and boost strength while mineralizing and storing carbon in precast and ready-mix concrete.

  • Carbonation: A solid carbonate is created when CO2 interacts with Ca or MgO. This substance supplements or replaces traditional concrete components (e.g., aggregates, SCMs).

  • CO2-based-cements: In making cement, CO2 is used as a raw material, replacing traditional Portland cement.

Since the nineteenth century, Portland cement has been the industry standard. Limestone, a primary raw material for Portland cement, emits carbon dioxide when burned in a cement kiln. More than 55% of emissions associated with cement production are attributable to this process. Portland cement can be replaced with other cements that emit significantly fewer greenhouse gases for any application.328 Table 15 shows the published works on CO2 to building materials within the period under investigation.

Table 15 Studies of CO2 to building materials.
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CO2–syngas and fuel

Syngas, a vital fuel, is critical for synthesizing many chemicals, including hydrogen, and the various chemicals and fuels considered in this study. It consists primarily of H2 and CO. Syngas differs chemically from other gases produced by gasification processes typically carried out at low temperatures because it uses higher pressures to synthesize chemicals and fuels.272 Global syngas production accounts for about 2% of primary energy use. However, the syngas market, primarily from fossil fuels, is dominated by the ammonia industry globally.402, 403 Jet fuels, methane, gasoline, and liquid hydrocarbon fuels are other fuels that can be produced from CO2 conversion via several chemical processes. Tables 16 and 17 show published literature on syngas and fuel synthesis.

Table 16 Studies on CO2 to syngas.
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Table 17 Studies on CO2 to fuels.
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Results and discussion

This section provides an analysis of the results. The data on CO2 utilization were compared in two ways. The first was to obtain vital information on the various existing conversion facilities, and the second was to provide information on the products of core interests to researchers through published articles.

Analysis of the existing carbon dioxide utilization facilities

Based on the sampled data, the distribution of the CO2 conversion facilities globally showed that while some continents are leading in the CO2 conversion processes to mitigate greenhouse gas emissions, others are still lagging. From the results of the analysis in Fig. 7, most conversion facilities are located in Asia (mainly in China), followed by Europe (Western Europe), and then North America (Mainly in the US). The high number of CO2 conversion facilities in Asia, driven mainly by China and India, can be attributed to the commitment of both countries (and other countries in the region) to minimize emissions from their rapid industrialization drive in recent times. China, the second-largest economy globally, has been on a steady path of rapid industrialization. This rapid industrialization growth rate comes with massive emissions of GHGs, especially CO2. Hence, China has become one of the industrialized hubs with the highest emission index. Likewise, the stringent net-zero emissions targets and environmental sustainability regulations and policies may be responsible for the relatively significant share of carbon capture facilities in Europe and North America. Most of the facilities in Asia convert CO2 to polymer, urea, and chemical intermediates. In Europe, the facilities mainly produce chemical intermediates from CO2. Building materials feedstocks are the predominant products obtained from CO2 conversion in North America. The facility distribution and CO2 conversion options in the regions are mainly driven by stringent regulatory policies occasioned by the need to decarbonize and mitigate greenhouse gas emissions for environmental sustainability, meeting net-zero emission targets, providing an economic justification for CCS projects, and the increasing demand for raw materials, chemicals, and fuels, which are in high demand in any industrialized societies.

Figure 7
figure 7

Global distribution of CO2 conversion facilities.

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The relatively insignificant or absence of CO2 conversion facilities in many developing countries in Africa, South America, Eastern Europe, other developing Asian countries, and the Middle East (excluding Saudi Arabia) show the weakness or lack of clear policy and stringent greenhouse gas emission mitigation regulations. It can also be attributed to the reluctance of many countries in these regions to meet net-zero emission targets since they contribute the least emissions globally. Most of these developing countries depend primarily on fossil fuels for energy, and their lack of decarbonization strategies could undermine global efforts to combat climate change due to greenhouse emissions.

Figure 8 shows the percentage distribution of the global carbon dioxide utilization facilities based on value-added products. The results show that there are more facilities converting CO2 to chemical intermediate products like methanol, DME, and syngas (29%), urea (25%), and polymers (23%) than there are for building materials (16%) and fuels (7%). The data suggests that over 90% of the global CO2 conversion facilities produce chemical intermediates, urea, polymers, and building materials. The development of CCUS facilities is driven primarily by the need to decarbonize and create a sustainable environment. Minimizing the high carbon footprints associated with the conventional processes from which these materials are produced could be responsible for the relatively high amount of CO2 conversion facilities. As the global population rises, more people will require more energy for sustainability. More energy will most likely result in industrialization, which requires certain feedstocks as building blocks. Hence, the demand for these feedstocks is expected to rise. The rising demand for these feedstocks and the need for environmental sustainability (in this case, carbon neutrality) explains why the majority of CO2 conversion facilities produce chemical intermediates (methanol, DME, Syngas, FA) as well as urea, polymers, and building materials.

Figure 8
figure 8

Global CO2 conversion facilities based on products.

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The inadequate or lack of facilities for FA (an important chemical intermediate) production from CO2 could be due to several technical and economic reasons. Its demand remains low, and unless more specific beneficial applications are developed, its utilization might remain challenging.169 Overall, CO2 conversion to raw materials, chemicals, and fuels is mainly governed by economics, especially when there are alternative feedstocks for producing the same chemicals and fuels. The economics and availability of alternative materials may be responsible for the relatively low number of CO2 conversion facilities to fuels globally. Therefore, the number of CO2 conversion facilities to raw materials, chemicals, and fuels will only increase when it becomes cost-effective and sustainable to use CO2 as feedstock for the conversion processes.

Analysis of published articles on CO2 conversion

Figure 9 compares the volume of published research on the conversion of CO2 to various products under investigation. Based on the sampled data, the results showed that the published articles in the last 6 years were more on chemical intermediates (45%), polymers (24%), and building materials (17%) than there were for fuels (11%) and urea (3%). The relatively high number of published articles on chemical intermediates (methanol, DME, FA, and Syngas) and polymers could result from their high demand and usefulness as a feedstock in many chemical and industrial processes. The statistical results from the analysis of published articles in this study align with that of existing conversion facilities except for urea. The high number of published articles on chemical intermediates, followed by polymers and building materials, can be attributed to the fact that since most of the existing conversion facilities produce these materials, researchers focus more on improving and optimizing their production processes from the existing facilities. In addition, the high volume of published articles could be due to the increased market demand for these products and the urgent need to decarbonize the industrial sector. Furthermore, the special attention on chemical intermediates, polymers, and building materials may be that they provide the outlets for making CCUS economically viable.16 Although there are also many other efforts to convert other greenhouse gases, like methane CH4, to some chemical intermediates like methanol,485, 486 chemicals,487,488,489 and solar fuels,490 most research interests tilt towards CO2 conversion rather than CH4, most likely because CO2 accounts for a more significant percentage of greenhouse gasses (76%) than methane (16%).491 Hence, the general focus, like this study, is more on CO2 conversion. The attention of researchers on the transformation of CO2 to building materials (17%), especially concrete in the building materials industry, stems from the need to decarbonize the concrete (a critical raw material required for industrialization) production industry, which is energy-intensive with a high carbon footprint.

Figure 9
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Percentage share of published articles.

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The absence or inadequacy of direct or indirect CO2 to FA facilities could be responsible for the relatively higher volume of research on the area. The relatively low volumes of research on urea (3%) could be because the technologies for converting CO2 to these products are pretty well established, and there are other efficient and cost-effective feedstocks for their production. As a result, there is little novelty in using CO2 as a feedstock, resulting in little work to optimize the grey areas. In addition, the conventional urea production process is an energy- and carbon-intensive technology that contradicts the idea of carbon neutrality. Furthermore, since urea was not listed as one of the priorities for CO2 utilization16 and hence might not be a viable economic route for the CCUS process, researchers may be unwilling to conduct studies on CO2 conversion to urea, explaining why there is a low volume of publications in this area in recent times. Nonetheless, continuous efforts should be made to develop novel, cost-effective, and sustainable technologies for converting CO2 to these and many other products. This is a sure way of making the decarbonization process attractive, cost-effective, and competitive.

Analysis of processes and catalysts used for CO2 conversion

CO2 transformation into various value-added products can be classified under the following major processes: Photochemical, Electrochemical, Thermal, and Chemical.9, 492,493,494,495 The characteristics of these processes are shown in Table 18. The catalysts used in these various processes to obtain a desired product are further highlighted.

Table 18 Characteristics of the various methods of CO2 conversion.
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Catalyst for CO2 conversion to methanol

The methods of converting carbon dioxide into methanol are affected by two significant factors. They include reaction conditions and catalyst properties.496 The various categories of catalysts used for the production of methanol include496;

  • Cu-based catalyst

  • Precious metal-base catalyst

  • Other catalysts

Cu-based catalyst

Cu-based catalyst is the most used catalyst for methanol production via Carbon dioxide496; however, in order to achieve total activity, they require pre-treatment in hydrogen, which is inconvenient for small-scale onboard systems. This method relies on heterogeneous Cu–ZnO catalysts, where Cu is the active phase, and ZnO is a crucial promoter to increase the system's activity. Commercial Cu/ZnO/Al2O3 catalyst development has advanced noticeably in recent years despite numerous disputes on the reaction mechanism.497,498,499,500 With the application of Cu–ZnO catalysts, high deactivation and low activities are observed. This has led to significant efforts in developing catalysts to improve the high deactivation of Cu–ZnO catalysts and the low activity.501 Another disadvantage is that Cu/ZnO catalysts have a heightened sensitivity to a few parts per million of sulfur and have the potential to exhibit pyrophoric activity when exposed to air.502 This type of catalyst has good activity, high methanol selectivity, low operating pressure, and temperature, so it is an excellent choice for hydrogenation catalysts in carbon dioxide conversion to methanol.503

Precious metal-based catalyst

Precious metal-based catalysts (Au, Pd, Pt, Ga, Rh) have drawn a lot of interest in converting carbon dioxide to methanol due to their high activity in adsorption and dissociation of hydrogen. Some metal-based catalysts, such as Palladium (Pd), as the active metal, exhibit high results for methanol production. However, to produce methanol using pure Pd, they are usually combined with other metals as promoters. In this case, the methanol selectivity is affected by the type of promoters used.496 This type of catalyst will generate a high methanol selectivity at a very high temperature.496

Other catalyst

High temperatures act as an equilibrium restriction on the hydrogenation of carbon dioxide to methanol. A catalyst often functions most effectively above room temperature, necessitating extraordinary thermal stability. Therefore, the thermal stability criterion must be closely followed if catalysts other than Cu and precious metal-based catalysts are to be utilized.496

Catalyst for CO2 conversion to DME

Compared to photocatalytic or electrocatalytic methods, carbon dioxide's one-step heterogeneous catalytic conversion to value-added compounds shows better efficiency. Nevertheless, in practical demonstrations and applications, conventional catalysts for the one-step carbon dioxide hydrogenation to DME still have inadequate space–time yield and stability. From this angle, the recent progress in the one-step carbon dioxide hydrogenation to DME is concentrated on various catalytic systems through an analysis of published experimental findings and the reaction mechanism, which includes the catalytic properties of carbon dioxide molecules, activation modes and active sites under specific conditions.504 The various categories of catalysts used for the production of DME include496;

  • Cu-based catalyst

  • Precious metal-base catalyst

  • Oxide-derived bifunctional catalyst

Cu-based catalyst

Cu-based catalysts are the most thoroughly researched catalysts in early studies. The well-known bifunctional catalyst (CZA) coupling with acidic sites is first used in carbon dioxide hydrogenation to DME.504 Cu-based catalysts have not yet found practical usage in the carbon dioxide hydrogenation of DME. Nevertheless, there is still room for improvement in the catalytic performance, including DME selectivity, catalytic activity, and stability. To achieve this, investigating the nature of active sites and the reaction mechanism becomes necessary.504

Precious metal-based catalyst

Most relevant publications500 agree that the decreased Pd species serve as active sites for the hydrogenation of carbon dioxide. But when it comes to the reaction mechanism, no one can agree. Based on the existing results, we may hypothesize that the nature of active sites causes this difference. The activated carbon dioxide can then be moved to the Pd surface and undergo hydrogenation to produce DME.

Oxide-derived bifunctional catalyst

Despite increased stability, oxide-based catalysts still require improvements in DME selectivity and catalytic activity. Prior research has consistently demonstrated the critical role that oxygen vacancies play. Nevertheless, the quantity of oxygen vacancies and the catalytic activity/DME selectivity cannot be correlated. To rationally design and advance oxide-based catalysts for carbon dioxide hydrogenation to DME with improved catalytic performance, mechanistic studies on the formation/consumption of oxygen vacancies and the kinetics of the oxygen vacancies participating in elementary steps during carbon dioxide hydrogenation are therefore necessary.505

Catalyst for CO2 conversion to polymer

The well-known catalysts used for polymer production are heterogeneous and homogeneous. For the copolymerization of carbon dioxide and propylene oxide, the catalysts for synthesizing carbon dioxide-based polymer polyol are typically chosen from double-metal cyanide and salen systems with high activity and immortal polymerization character. Depending on the catalyst, the resulting polyols exhibit various structures and features. For the alternating copolymerization of carbon dioxide and propylene oxide to yield polymers with a carbonate linkage concentration greater than 99%, SalenCo is the perfect catalyst.506 While heterogeneous catalysts for the carbon dioxide reaction with epoxides have received less attention than homogeneous catalysts up to this point, research on the latter is expanding. In particular, heterogeneous catalysts are more desirable than homogeneous catalysts due to their ease of separation from products and reutilization, especially with the growing interest in the widespread industrialization of this carbon dioxide conversion method.507 At the same time, the disadvantages are often limited activity and selectivity.508

Catalyst for CO2 conversion to FA

Solid catalysts are thought to be the most effective for this purpose due to their efficiency and possibility for recycling. Scientists have investigated the catalytic potential of numerous metal–organic frameworks (MOFs) based on Co, Sn, Mn, Ni, Bi, Hg, Cd, Pb, and Fe over time. In the production of FA, most of the iron-based catalysts reported thus far exclusively produce carbon monoxide as the primary product.509

Various studies have shown the production of formate through nano-structures of these metals, such as nanostructured Sn/SnOx thin film,510 Sn or Sn oxide nanoparticles with < 5 nm,511 hierarchical mesoporous SnO2 nanosheets,512 atomic layer deposited Sn or Sn sulfide on nanoneedle templates,513 ultrathin Bi nanosheets,514 and oxide-derived Pb.515 Due to Hg, Cd, and Pb toxicity, most studies have mainly used Sn or Bi. The CO2 reduction reaction's selectivity on different metals can be regulated by adjusting the reaction conditions to yield FA. For instance, high-pressure studies on CO2 reduction reactions on W, Fe, Co, Ni, Zn, Pt, Rh, and Ir have been conducted.516 Despite having lower selectivity than Sn or Bi, they nevertheless demonstrated considerable activity toward formate formation. Pd produced a substantial amount of CO, but it also performed well in the synthesis of FA.517, 518

Various methods for producing FA include electrochemical, photochemical, and hydrogenation. In contrast to the other methods, the electrochemically facilitated carbon dioxide conversion to FA requires lower temperatures and pressures, lowering operating and production costs. This explains why most of the CO2 conversion to FA studies are via the electrochemical method. Nevertheless, there is still a limit to the metal catalysts' selectivity when converting carbon dioxide into certain compounds.519 The current photocatalytic system has several drawbacks that prevent it from being used in industrial settings. These drawbacks include low visible light consumption, rapid charge recombination, and poor photogenerated electron and hole migration capabilities.520

Catalyst for CO2 conversion to urea

The conventional urea production process is an energy- and carbon-intensive technology that contradicts the idea of carbon neutrality. Electrocatalytic urea manufacturing is a promising and sustainable method since, fortunately, using renewable energy in electrochemical synthesis has demonstrated considerable potential for producing high-value nitrogen compounds. However, its large-scale industrial growth is limited by its poor yield and Faraday efficiency, as well as the uncertain process of C-N bond formation. Researchers are looking for electrocatalysts with improved performance.521 The various categories of catalysts used to produce urea are521 Metal and metal alloys, Metal compounds, and Metal–organic compounds. Electrocatalytic synthesis is a promising technique with the potential to produce urea more efficiently and sustainably. Using an electrocatalyst to speed up chemical reactions can save energy by removing the high pressure and temperature requirement during electrocatalytic synthesis.521 However, there are still several difficulties with electrocatalytic urea production. First, urea is the final target product. Secondly, the creation of novel electrocatalysts is the most significant obstacle. Thirdly, a thorough and in-depth investigation of the synthesis process is the component of mutual reinforcement.521 According to one study, electrocatalytic urea synthesis requires a Faraday efficiency of 56.2% to compete with the conventional urea synthesis industry at an average battery voltage of 2.7 V.522 PdCu/CBC has achieved the highest Faraday efficiency record of 69.1%.523

Catalyst for CO2 conversion to syngas

The various categories of catalysts used to produce Syngas are primarily homogeneous and heterogeneous. Because of their accessibility and low cost, non-noble metal-based catalysts like nickel (Ni) and cobalt (Co) were frequently considered for industrial-scale application.524 The addition of promoters was analyzed to support the catalyst's catalytic activity and prevent catalyst deactivation due to carbon deposition. Nonetheless, the prior research was primarily concerned with characterizing the catalysts' surfaces to synthesize syngas. The techno-economic sustainability of homogeneous catalysts for industrial production will require additional evaluations.525

Catalyst for CO2 conversion to methane

CO2 methanation over a variety of metal catalysts, Ru,526 Ni,527 Co,528 and Fe,529 has been studied for effective and economical production. There are two types of catalytic systems: homogeneous and heterogeneous.525 Controlling single-site catalyst performance during design and synthesis has proven to be a difficult part of developing homogeneous catalysts.525 The assessments reveal that electrocatalytic homogenous conversion of carbon dioxide to fuel is still under development for optimizing catalyst characteristics and operating conditions for larger applications. Furthermore, recent studies mainly evaluated heterogeneous catalysts for direct carbon dioxide conversion to syngas.525 Heterogeneous catalysis powered by sunlight has proven to be a viable and economical approach for converting CO2 to methane.525

Several new metal catalysts and production techniques known as the Sabetier reaction have been discovered. Studies revealed that Ru was the most active catalyst at the time.526 Thermodynamics dictates that moderate temperatures (T ≤ 300 °C, P ≤ 10 bar) are required to obtain the desired quality without needlessly increasing pressure. In such cases, the design of the catalyst is crucial for overcoming kinetic constraints, achieving adequate reaction rates, and minimizing reactor volumes. Nickel is an affordable and readily available active material that can accomplish these reaction rates. On nickel catalysts, high values (> 99%) of selectivity are readily attained.527 However, enhancing low-temperature activity is challenging when utilizing Ni-based catalysts for carbon dioxide methanation since high-temperature reactions are constrained by chemical equilibrium and energy conservation. Due to their excellent thermal stability and low cost, traditional metal oxides continue to be the most promising support for optimizing Ni-based catalysts for carbon dioxide methanation.466 Ni exhibits strong action toward methane and excellent selectivity. However, as heat evolves from the extremely exothermic methanation reaction, conventional Ni-catalysts experience deactivation due to the sintering of the Ni particles. Further catalyst degradation is caused by coke deposition and the production of volatile nickel carbonyls. Additionally, Ni raises toxicological concerns. On the other hand, Fe has poor selectivity but a very high activity for CO2 activation. Fe is 180 times less expensive than nickel since it is far more abundant and nontoxic than nickel.529 Besides Ni, there are other metals that are active in methane production via carbon dioxide. These metals for methane catalysts are based on their activity (Ru > Fe > Ni > Co) and selectivity to methane (Ni > Co > Fe > Ru).529

Future perspective

Carbon dioxide utilization is a possible way of reducing the cost of the CCS value chain. Converting the captured and stored carbon dioxide into value-added products could reduce the costs of building new CCS facilities or retrofitting existing ones. This process is known as integrated carbon capture storage and utilization (CCSU). The utilization pathway of carbon dioxide could be economically sound, and its usage is increasing drastically worldwide in various sectors: chemical, fuel, agriculture, and mineralization through direct or indirect processes. However, the research analysis showed that the successful conversion of CO2 to value-added products depends on the type of catalyst, hydrogen availability, energy requirements, and economics, among other factors. This implies that the energy requirements, the type of catalyst used, and the sources of hydrogen production could go a long way in determining the process's cost-effectiveness, yield, efficiency, and sustainability.

The literature analysis showed that synthesizing chemical intermediate products like methanol DME and polymers often occurs at high temperatures and pressures, except in the study of Gonzalez-Garay et al.,79 which produced methanol at low temperatures (25 °C) and pressures (1 bar). The increased cost is one significant implication of producing these materials from CO2 at high temperatures and pressures. Reducing the energy requirements will help drive down the associated costs of the process, thereby making a good economic case for the CCSU process. Consequently, more research on reducing the energy requirements while improving the yields for chemical intermediates and polymer synthesis from CO2 should be investigated. Furthermore, more emphasis should be placed on Gonzalez-Garay et al.79's work on methanol synthesis, where they simulated methanol production at near atmospheric conditions of 25 °C and 1 bar. Their work should be further investigated to see how the energy requirements of methanol and, by extension, DME and polymers could be reduced.

The development of competitive catalytic technologies for the selective catalytic hydrogenation and other carbon dioxide conversion processes to products offers a path forward to reducing the enormous carbon dioxide emissions from fossil fuels by converting them to valuable materials and products. Catalysis and catalyst development is a growing research interest in converting CO2 to fuels and chemical products. However, there is still no preferable approach and a better class of efficient catalysts for converting CO2 to fuels and chemicals in terms of cost-efficiency and optimum production.530 From the analysis of the published literature, most of the catalyst used in the conversions are inefficient and requires some reducing agents/metal-based materials. It is anticipated that cost-effective, efficient, and sustainable catalyst development represents a challenging problem in the different CO2 to chemical/fuel synthesis. Therefore, researchers should focus on developing efficient and cost-effective catalysts for CO2 conversion processes.

Hydrogen is vital to the CO2 hydrogenation process, syngas synthesis, and urea production. A cleaner, cost-effective, and efficient technology for hydrogen production will go a long way in enhancing a sustainable CO2 conversion process. Various categories of hydrogen production techniques, including the conventional hydrogen production process from fossil fuels, thermolysis, solar energy processes, water electrolysis, and other novel methods, can be further researched to find a reliable, suitable, sustainable, and cost-effective way of hydrogen production required for an efficient CO2 conversion process. Researchers should optimize proper hydrogen production techniques to make the CCS process economically viable.

Calcium carbonate (CaCO3) can be used in many applications, including building materials. The conversion of CO2 to CaCO3 can help protect the depleting limestone (natural) resources. Published works on this subject have shown the possibility of converting CO2 to calcium carbonate (CaCO3). In one such study, CO2 was converted to CaCO3 when passed through a Nickel catalyst.531 In another study, CO2 was converted to CaCO3 using genetically altered yeast.532 Other studies on CO2 conversion to CaCO3 used the amine looping strategy,533 membrane gas absorption,530 and ultrasonic carbonization.534 The application of CO2 in construction materials is gaining traction. Even if the industrial trials mentioned in the above research publications demonstrate that CO2 may be employed in construction components, scalability and market viability are still impacted by a variety of factors. Some challenges in the production of building materials via CO2 usually include the unavailability of raw materials, the cost of carbonated products, and the distance of CO2 source and suitability. Others are carbonation reaction and processing and the construction codes and standards. It is challenging to commercialize these items on a broad scale due to the lack of strict adherence to construction standards and rules. Another issue is that construction codes frequently describe the materials' composition rather than the performance criteria, making them more prescriptive than performance-based. In the short to medium term, there is a tremendous opportunity for CO2 usage in the market for construction materials. Although several chemical and biological use approaches are more advanced in process technology than carbonation-based concrete curing, additional study is needed to address significant issues with carbonation technology, processing, and market adoption. Breakthrough studies on the transformation of CO2 to CaCO3 can proffer a profitable pathway for the carbon capture process and should, therefore, be investigated.

The study showed that most of the existing conversion facilities and published research articles in recent times focused more on CO2 conversion to chemical intermediates, polymers, and building materials. This suggests that converting CO2 to these products could be critical to CCUS’s economic viability. It also indicates the viability of the chemical intermediates, polymers, and building materials markets. Consequently, more studies should be conducted to create energy-efficient processes and technologies for improving and optimizing CO2 conversion to chemical intermediates, polymers, and building materials. Furthermore, facilities for converting CO2 to fuels are the least globally. Also, the research publications on fuel production from CO2 are relatively low. Having been identified as one of the potentially viable and promising materials derivable from CO2,16 fuel synthesis from CO2 can become a reliable decarbonization strategy for the transport sector, especially the aviation and maritime industries. These transport industry sectors will continue to play a vital role in globalization and trade and are expected to grow significantly as the global population increases. The aviation and maritime industries currently contribute significant GHG emissions, resulting in global warming and severe environmental consequences. Hence, utilizing gaseous or liquid fuels in both sectors can help achieve net-zero targets and carbon neutrality in critical transport industry sectors.

Consequently, more studies are required in CO2 conversion to aviation and maritime fuels to decarbonize the sectors and gain more insights into CO2 conversion to fuels. Furthermore, the importance of urea fertilizer in crop production and food and energy security cannot be over-emphasized. Hence, further investigations are needed to expand the frontiers of CO2 conversion to these products.

Conclusion

Carbon dioxide utilization offers a pathway for reducing the costs of the CCS process, producing carbon–neutral products, and endearing a sustainable environment. This review has analyzed the existing global CO2 utilization facilities based on location and product type. Furthermore, published articles on CO2 conversion to materials identified in the CO2U16 report as viable priorities for CO2 utilization were also analyzed. The analysis of the conversion facilities was done based on a 90% confidence limit within a 9.93% margin of error. Likewise, the study on the published articles was done based on a 95% confidence limit within a 5% margin of error. The following conclusions can be drawn from the study.

  1. 1.

    Over 90% of global CO2 conversion facilities produce chemical intermediates, urea, polymers, and building materials, and less than 10% produce fuels.

  2. 2.

    More than half of the global CO2 conversion facilities are in South-East Asia (mainly China), with the remaining in Western Europe (23%), North America (20%), and Oceania (3%). Developing countries in Africa, Central and South America, and others are far behind in the CCSU process as there are currently too few or no CO2 conversion facilities in those regions. This is a call for concern with respect to the global clean energy initiative.

  3. 3.

    The analysis of the research publications from 2016 to 2022 shows that the research focus is currently on CO2 conversion to chemical intermediates, polymers, building materials, and fuels (over 95%) and less on urea.

  4. 4.

    A future perspective on CO2 conversion includes increasing research output and conversion facilities on fuels to specifically help decarbonize the aviation and maritime sectors while adopting energy-efficient processes and technologies for cost-effective and efficient CO2 conversion.