Nanomaterials and hybrid nanocomposites for CO₂ capture and utilization: environmental and energy sustainability

Abstract

Anthropogenic carbon dioxide (CO₂) emissions have dramatically increased since the industrial revolution, building up in the atmosphere and causing global warming. Sustainable CO₂ capture, utilization, and storage (CCUS) techniques are required, and materials and technologies for CO₂ capture, conversion, and utilization are of interest. Different CCUS methods such as adsorption, absorption, biochemical, and membrane methods are being developed. Besides, there has been a good advancement in CO₂ conversion into viable products, such as photoreduction of CO₂ using sunlight into hydrocarbon fuels, including methane and methanol, which is a promising method to use CO₂ as fuel feedstock using the advantages of solar energy. There are several methods and various materials used for CO₂ conversion. Also, efficient nanostructured catalysts are used for CO₂ photoreduction. This review discusses the sources of CO₂ emission, the strategies for minimizing CO₂ emissions, and CO₂ sequestration. In addition, the review highlights the technologies for CO₂ capture, separation, and storage. Two categories, non-conversion utilization (direct use) of CO₂ and conversion of CO₂ to chemicals and energy products, are used to classify different forms of CO₂ utilization. Direct utilization of CO₂ includes enhanced oil and gas recovery, welding, foaming, and propellants, and the use of supercritical CO₂ as a solvent. The conversion of CO₂ into chemicals and energy products via chemical processes and photosynthesis is a promising way to reduce CO₂ emissions and generate more economically valuable chemicals. Different catalytic systems, such as inorganics, organics, biological, and hybrid systems, are provided. Lastly, a summary and perspectives on this emerging research field are presented.

---

1. Introduction

The research related to organic–inorganic hybrid nanocomposites is gaining momentum, and large amounts of data are being generated from different research groups and real fields to tailor the properties of nanomaterials towards their optimal performance. Energy production via fossil fuel combustion plays a critical role in carbon dioxide (CO₂) emissions. In an unprecedented way, there is a fast growth of CO₂ levels that disturb the earth's climatic conditions.¹ Therefore, controlling and balancing CO₂ emissions while meeting global energy demand is critical. CO₂ capture, utilization, and storage (CCUS) technologies are the best options for reducing global warming while addressing the energy crisis.² Therefore, it is essential to create renewable energy sources to reduce the effects of global warming and satisfy the rising demand for energy.^3–5 The CO₂ released by industries and the burning of fossil fuels is collected and stored using carbon management technologies such as CCUS. The captured CO₂ can also be transformed into various high-value chemicals and fuels, addressing pollution control and energy supply issues. This review introduces the CO₂ cycle with emission sources, storage, and utilization. Different forms of CO₂ utilization are divided into two categories: non-conversion utilization (direct use) and conversion of CO₂ to chemicals and energy products.

2. CO₂ sources and emission

Several sources of CO₂ emission cause an increase in its concentrations in the atmosphere. The CO₂ sources can be classified into natural and human sources. Natural sources include decomposition, plant and animal respiration, ocean release and respiration, and volcanic eruptions. Human sources, which are sources of anthropogenic emissions, include activities such as cement production, deforestation, and the burning of fossil fuels like coal, oil, and natural gas. Some other sources include electricity, heat sector, transportation, and industries. Human activities such as burning oil, coal, and gas, as well as deforestation, are the primary sources of CO₂ emission. The atmospheric concentration of CO₂ has been rising extensively due to human activities, including the industrial revolution. Human sources of emissions have upset the natural balance by adding extra carbon dioxide to the atmosphere without removing any.

The increase in the global atmospheric concentration of anthropogenic greenhouse gases (GHG), including CO₂, leads to global warming and climate change. The increase in such gases causes ozone layer depletion, which in turn causes the greenhouse effect and global warming, and ultimately climate change. Climate change decreases worldwide agricultural productivity because of low rainfall, changing seasons, and rising temperatures. Global warming, ocean acidification, desertification, and changing weather conditions could all be made worse by the high level of industrial pollution and the indiscriminate production of greenhouse gases into the atmosphere. But among the immediate effects of climate change are issues with food security, migration, health issues, rising sea levels, and severe storms that affect coastal regions.⁴ The increase in CO₂ may also indicate a negative impact on the environment, which indicate a decrease in trees and an increase in air pollution, causing several consequences. Environmental degradation is defined as the depletion of environmental resources such as air, water, and soil; the elimination of ecosystems; extinction of wildlife; pollution perceived to be harmful to the ecosystem. Another factor for CO₂ is deforestation, which arises due to the excessive consumption and waste of paper products that come from the trees.

There are two ways to minimize the emission of CO₂. First, fossil fuels can continue to be used with the recommendation to use different CO₂ capture, storage, and utilization methods. Second, people can switch to using alternative clean energies and biomaterials.⁶ However, this way seems economically unviable as people cannot bear the high cost of such technologies. So, a combination of both can be a good option until new technologies are developed, Fig. 1.

Fig. 1 Possible mitigation of CO₂ emissions.

3. Strategies for minimizing CO₂ emissions

The strategies that can be used to minimize anthropogenic CO₂ emissions include:⁷

(i) Decreasing the production of CO₂ with the use of the sources of renewable energy, such as solar, hydro, and wind, and by increasing energy efficiency.

(ii) Extracting produced CO₂ from the climate by reforestation and geological storage in deep-sea sediments and underground storage.

(iii) Converting and utilizing CO₂.

In Fig. 2, several of the crucial procedures are displayed. The chemical sector, for example, has a growing demand for carbon, as does the desire to reduce its reliance on fossil fuels. This, combined with the desire to reach net-zero emissions in the future, has sped up feedstock development. As a result, the industry is aggressively developing carbon capture and utilization (CCU) technology to utilize CO₂ as a feedstock alongside new generations of bio-based feedstocks and improved recycled raw materials. In recent years, several encouraging advancements have been made, and commercial items such as cosmetic Plastic Bottles manufactured from waste CO₂ are now accessible.

Fig. 2 Illustration of CO₂ steps of emission, capture, storage, and utilization.

4. CO₂ sequestration

Carbon sequestration is capturing, securing, and storing CO₂ from the atmosphere. The goal is to prevent CO₂ from warming the atmosphere by stabilizing it in both solid and dissolved forms. The method has great potential for cutting CO₂. There are two basic forms of carbon sequestration; geological and biological.

Generally, sequestration can be done in three different ways: post-combustion capture, pre-combustion capture, and oxy-combustion. In addition, many other separation methods, such as gas phase separation, absorption into a liquid, and adsorption on a solid, as well as hybrid procedures like adsorption/membrane systems, are being researched. As companies move to restorative farming practices, they use the aforementioned procedures to collect the carbon emissions from power plants, factories, fuel-burning enterprises, and new generation animal production facilities.

Storage of CO₂ in vegetation, such as grasslands or forests, as well as in soils and oceans, is known as biological carbon sequestration. While the process of storing CO₂ in underground geologic formations, such as rocks, is known as geological carbon sequestration. Classically, CO₂ is extracted from an energy-related source, such as a power plant or a natural gas processing facility, or an industrial source, such as steel or cement production, and then injected into porous rocks for long-term storage. Carbon capture and storage may allow utilizing fossil fuels until another energy source is introduced on a large scale.

New ways of technological carbon sequestration are explored to remove and store CO₂ from the atmosphere using innovative technologies. Direct air capture of CO₂ can be done directly from the air using advanced technology plants. By developing novel types of chemicals that are capable of identifying and trapping carbon dioxide from the air, scientists are building molecules that can alter the shape. The engineered molecules act as a filter, only attracting CO₂. It is highly of interest to have efficient methods for the CO₂ removal and utilization of a resource for several applications. This includes the use of CO₂ as a raw material to produce graphene.

5. CO₂ capture and separation technologies

Fig. 3 depicts the procedures and materials utilized for CO₂ capture and separation. Briefly, various methods were applied, including absorption, adsorption, separation, membrane, cryogenic distillation, chemical looping, and geothermal and biochemical technologies. CO₂ separation can be achieved by ionic liquid membranes, polymeric membranes, and MOF mixed matrix membranes. CO₂ capture can be achieved using biological separation/fixation techniques such as microalgae, bacteria, and their enzymes.

Fig. 3 Classification of CO₂ capture technologies.

One of the main CO₂ separation technologies that can be applied to isolate the CO₂ from the fuel gas stream prior to transportation is absorption which can be done using liquid or solid sorbents. Liquor sorbents are absorbed to separate the CO₂ from the flue gas. The sorbent can be regenerated through a stripping or regenerative process by heating or depressurization. CO₂ absorption can be performed by common solvents, ionic liquids, deep eutectic solvents, phase-change ionic liquids, solvents modified with nanoparticles, encapsulated liquid sorbents, novel gas–liquid contactors, and mini- and micro-channels, and rotating packed beds.

Adsorption using a solid sorbent binds the CO₂ on its surfaces. CO₂ adsorption can be performed by carbonaceous materials and nanomaterials, conventional and nanosized zeolites, functionalized sorbents, and swing technologies. Large specific surface area, high selectivity, and higher generation ability are the main criteria for sorbent selection. Another separation method is chemical looping combustion, in which a metal oxide is used as an oxygen carrier instead of using pure oxygen directly for the combustion, as in the case of oxyfuel combustion. During the process, the metal oxide is reduced to metal while the fuel is oxidized to CO₂ and water. In membrane separation, various membranes can allow only CO₂ to pass through, while excluding other flue gas components. Hydrate-based CO₂ separation is a new technology by which the exhaust gas containing CO₂ is exposed to water under high pressure forming hydrates. The CO₂ in the exhaust gas is selectively engaged in the hydrate cages and is separated from other gases. Cryogenic distillation is a gas separation process using distillation at very low temperatures and high pressure. It is similar to other conventional distillation processes except that it is used to separate components of gaseous instead of liquid. Many adsorption procedures are used to achieve CO₂ separation based on regeneration methods. This includes (1) vacuum as well as pressure swing adsorption (VSA and PSA), (2) temperature swing adsorption (TSA), (3) electric swing adsorption (ESA), (4) simulated moving beds (SMB), and (5) purge displacement.^8,9

Organic cage frameworks (OCFs), pCage-1, and pCage-2 with controlled porosity synthesized from bicyclooxacalixarene cages were reported to have high CO₂ adsorption capacities and adsorption selectivity of CO₂/N₂, that can be the reason for the introduction of N-doped triazine unit.¹⁰ A cross-linked organo-magnesium complex (MTF-Mg) has been reported for selective CO₂ adsorption over N₂.¹¹ The CO₂ preferential adsorption arises from its strong interaction with the exposed magnesium atoms.

In addition, porous organic polymer (HAT-TP) synthesized through a condensation reaction to introduce hexaazatriphenylene (HAT) units to triptycene (TP)-based microporous polymer was reported as an effective recyclable catalyst for chemical conversions of CO₂ to cyclic carbonates with epoxide.¹²

In brief, CO₂ capture technologies include:

➢ Pre-combustion: in this process, the fuel is pretreated before combustion.

➢ Post-combustion: this process removes CO₂ from the flue gas after combustion.

➢ Oxyfuel combustion: in which oxygen, instead of air, is used for combustion.

Most promising capture technologies include capture at large point sources, which is the extraction of carbon dioxide from the flue gases of industrial sources (e.g., power plants, cement, or steel factories). Direct air capture requires more energy than direct capturing CO₂ in flue gases. In addition, capturing the CO₂ emitted by bioenergy production can reduce the atmospheric concentration of CO₂. The general process is called bioenergy with carbon capture and sequestration.

6. CO₂ storage

The CO₂ capture and separation are followed by CO₂ transportation into the storage location. CO₂ storage is the last step in the CCS chain. CO₂ storage can be implemented in different modes classified into natural and man-made modes of storage. Natural modes include terrestrial sequestration, while man-made storage includes storage in geologic formations. The storage strategies include:

Oceanic underground geological storage is considered the most viable sequestration strategy. In comparison to carbonation and oceanic storage, geological storage is preferable due to several factors, including economic considerations, site accessibility (in the case of ocean and mineral sequestration), and associated worries about the security of the stored CO₂ and detrimental environmental effects of mineralization and ocean storage. There are several potential geological storage options, including saline aquifers, depleted oil and gas reservoirs, unmineable coal seams, basalt formations, organic-rich shales, hydrate storage of CO₂ within the subsurface environment, and CO₂-based enhanced geothermal systems.

Deep ocean storage includes the CO₂ injection into deep ocean water. The main proposed approaches for ocean storage are based on the direct dissolution of CO₂ into the seawater.

Mineralization or mineral carbonation: in this strategy, the captured CO₂ is sequestered through the process of mineralization, where CO₂ is reacted with alkaline earth metal oxides or hydroxides, such as calcium-and magnesium-rich minerals, to produce stable carbonates.¹³

Terrestrial sequestration is the capture of CO₂ from the atmosphere and storing of it in soils and vegetation. CO₂ removal from the atmosphere through photosynthesis and prevention of the emission of CO₂ from terrestrial sources are the mechanisms for terrestrial storage.¹⁴

7. CO₂ utilization

The scientific community opposes CO₂ capture and storage (CCS) because of long-term liability matters, limited cost-effective storage capacity, the probability of leakage, and public acceptance of onshore storage locations.¹⁵ At the same time, deep ocean sequestration would instantly influence lowering pH, enhancing water acidity, and potentially causing ecosystem imbalance.¹⁶

Carbon capture and utilization (CCU) is a viable alternative to CCS that is gaining traction owing to sustainable progress and is thought to be a long-term solution to the CO₂ problems. CCU explores the use of CO₂ in applications other than storage. CO₂ is a vital raw material for products that continue to require carbon sources. This is because it provides their structure and properties (carbon-containing chemicals). Another reason is using carbon-free energy carriers, such as electricity or hydrogen. CO₂ is one of the few carbon alternative sources to fossil fuels. The chemical feedstocks,¹⁷ refrigerants,¹⁸ cleaning liquids,¹⁹ solvents media,^20,21 inserting agents, and packing gas are examples of CO₂ use attempts that have been demonstrated in recent years.

CO₂ utilization can be classified into non-conversion utilization (direct use) and conversion utilization, Fig. 4. The entire CCU process can be broken down into two steps:

Fig. 4 Classification of CO₂ utilization in various fields and industries.

• The initial step in CCU is the capture of CO₂ and separation at the production source.

• The next step is CO₂ utilization performed by (i) non-conversion process to use CO₂ as a solvent, a working fluid, and a heat transfer agent or by (ii) conversion process to use it as a feedstock for the production of fuels, chemicals, and polymers, Fig. 3. The advantages and disadvantages of some of the potential technological ways of CO₂ use are listed in Table 1.

Table 1 Potential technological ways of CO₂ uses

Technology	Description	Advantages	Disadvantages
(i) Non-conversion
Desalination	CO2 is combined with brine at high temperature and pressure forming hydrates, which may then be extracted to reveal clean water	➢ Produces potable water or treats water that has been contaminated by a procedure	➢ Costs of power and equipment, however, similar to existing water treatment substitutes
		➢ Enables a revenue stream or cost offset in a system that has already committed to CCS
Enhanced oil recovery	CO2 is pumped into existing oil wells to boost pressure and reduce oil viscosity, allowing for more oil to be retrieved	➢ It is a mature technology with permanent storage and a great potential for CO2 utilization, as well as an income source to pay the costs of carbon capture	➢ Facilitates further fossil fuel use, producing more CO2
Enhanced geothermal systems	Supercritical CO2 is used to transmit geothermal heat or to create electricity directly using a supercritical CO2 turbine	➢ Improves the efficiency of a renewable energy source and	➢ It takes a long time for a product to be commercialized
		➢ Provides long-term storage	➢ Transporting supercritical CO2 is expensive
			➢ A geothermal site requires a grid connection
Enhanced coal bed methane	CO2 is pumped into partially depleted coal seams, where it is absorbed by coal, causing methane to be displaced to the surface, where it can be recovered and used as a fuel	➢ Methane may replace more carbon-intensive fuel sources	➢ CO2 adsorbed in coal might cause it to swell and impede routes, causing methane recovery to be hampered
		➢ It is with permanent storage	➢ A low cost of methane
			➢ CO2 transportation costs
(ii) Conversion
Mineralization
CO2 mineralization	CO2 reacts with minerals or industrial waste products. This results in new compounds utilized in constructions, as consumer products, or as a substitute to CCS	➢ Abundant materials (minerals or industrial wastes)	➢ To speed up the process, high energy is used
		➢ Substitute to CCS	➢ High material requirements
			➢ Minerals and processing costs
Concrete curing	Precast concrete is cured using waste CO2 flue gas. CO2 is kept in the concrete as a non-reactive limestone	➢ Low cost versus traditional curing	➢ The product must meet quality requirements
		➢ Flue gases can be used directly in the cement industry	➢ The expense of modifying the curing process
		➢ Carbon offset opportunity for the cement industry, which produces a lot of pollution
		➢ A low-carbon consumer item that has the potential to grow beyond its current market
Bauxite residue carbonation	The alkalinity of aluminum mining slurry is reduced by CO2	➢ Aluminum mine closure and reclamation expenses can be reduced	➢ Cost of concentrating CO2
			➢ Need access to CO2
Biological
Algae cultivation	CO2 is absorbed by microalgae, which can then be transformed into proteins, fertilizers, and biomass for biofuels	➢ Competitive source of biofuel	➢ Algae are sensitive to impurities, pH
		➢ Can use flue gas directly	➢ Cost of controlling growth and drying conditions
		➢ Can result in permanent storage	➢ Large area and sunny climate needed for ponds
		➢ A tonne of microalgae can fix about two tonnes of CO2	➢ High energy needs for photobioreactors
Chemicals
Liquid fuels – methanol	Methanol is produced by catalytically converting CO2 and hydrogen into methanol, which can be combined with gasoline	➢ The energy carrier eventually replaces fossil fuels, lessening our reliance on them for transportation and other purposes	➢ Inefficient process; requires renewable or low emissions energy to have net CO2 abatement benefit
			➢ Needs low-cost renewable hydrogen
			➢ Cost of purifying CO2
Liquid fuels – formic acid	Formic acid is made by electro-reducing CO2 in water	➢ Formic acid is a preservative and antibacterial agent that can be utilized as an energy carrier (with hydrogen as the major fuel)	➢ Inefficient process; requires renewable or low emissions energy to have net CO2 abatement benefit
			➢ Chemistry needs to be perfected
			➢ Cost of purifying CO2
Polymers/chemical feedstock	CO2 is converted into polycarbonates with the use of a zinc-based catalyst	➢ Flue gas can be used directly; CO2 has a significant potential usage; a wide range of products (plastic bags, laminates, automobiles, medical components, and so on) are possible	➢ Non-permanent storage; some CO2 can be re-emitted as soon as six months
		➢ Existing infrastructure can be employed
Urea yield boosting	Urea fertilizer is made from ammonia and CO2	➢ Process emissions intensity is reduced	➢ CO2 is re-emitted when urea is broken down as fertilizer
			➢ Non-permanent storage

---

7.1. CO₂ non-conversion utilization (direct use)

CO₂ can be used directly in several applications such as dry ice, carbonated drinks, fire extinguisher, solvent, refrigerants, process fluids, welding mediums, or in algae farms for photosynthesis. It can also be used in large-scale industries to enhance oil recovery, gas recovery, and geothermal systems indirectly. Also, the direct use of CO₂ includes its use as heat transfer fluid in refrigeration, power systems, and other industries. Another example of CO₂ non-conversion utilization is in desalination, where CO₂ is combined with brine at high temperature and pressure, forming hydrates, which may then be extracted to reveal clean water.

In such uses, CO₂ molecules remain pure or dissolved in a mixture and do not react or crack further. However, such direct applications for CO₂ are limited in scale and have a small effect on the overall CO₂ abatement.²²

7.2. CO₂ conversion utilization

A potential solution to environmental issues like global warming and the energy crisis is the conversion of CO₂ to sustainable and green solar fuel. Technologies like CO₂ capture, storage, and usage provide appealing methods to lower CO₂ emissions. Although CO₂ can be effectively separated via CO₂ collection and storage, converting CO₂ into chemicals and fuels is more practical. It converts CO₂ into crucial molecules like carbonate, methanol, salicylic acid, and other substances.

In the conversion process, a carbon–carbon bond or a carbon–hydrogen bond has to be established. Such bond formation requires energy input to activate CO₂ or the other substrates such as reductive coupling. Inappropriately, the carbon–oxygen double bond is very stable (energy of CO₂ 750 kJ mol⁻¹) compared with C–H and C–C chemical bonds (430 and 336 kJ mol⁻¹). Commonly, chemical fixation of CO₂ requires: (i) high input of energy to perform reaction activity of substrate (unsaturated compounds, small-ring compounds, and organometallic compounds) and (ii) high reaction temperature and pressure. There are several pathways for CO₂ conversion. These can be classified as shown in Fig. 5.

Fig. 5 Conversion strategies of CO₂.

In brief, the CO₂ conversion reactions can be categorized into two classes:

(i) The first class includes the reactions that do not need a substantial amount of external energy. Hence, the reaction occurs by attaching the CO₂ molecule to the other reactant. These reactions are usually called carboxylation reactions. Consequently, this class involves the production of carboxylates and lactones (RCOOR), carbamates (R₁R₂NCOOR₃), ureas (RRNCONRR), isocyanates (RNCO), and carbonates (ROC(O)OR).

(ii) The second class includes the reactions that produce reduced forms of CO₂. Such reduction reactions require a significant amount of external energy. They involve products including; HCOO⁻ (formates), [C(O)O]₂²⁻ (oxalates), H₂CO (formaldehyde), CO, CH₃OH, CH₄, and C₂H₄. The external energy needed in these reactions is supplied as heat, electrons, or irradiation/photons to break the bonds in CO₂. Such processes are named thermal, electrochemical, and photochemical, respectively. Consequently, catalysis, and high temperature and pressure, are required for lowering the energy barrier toward the formation of C1-building block chemicals, for instance.^23,24

Such catalytic process of CO₂ includes the formation of chemicals such as urea, methane, methanol, formic acid, formaldehyde, ethylene carbonate, dimethyl carbonate, cyclic carbonates, salicylic acid, cyclic carbonates, polymer building blocks, and polycarbonates.

7.2.1. CO₂ thermal conversion. Thermochemical conversion of CO₂ utilizes catalysts and a combination of heat and pressure to convert CO₂ into valuable products. However, the reversibility of reactions and thermodynamic restrictions on reactions are obstacles to this method. In contrast to procedures like catalytic hydrogenation for the synthesis of methanol, which is created to be decentralized to take advantage of point sources of CO₂, methods like Fischer–Tropsch are developed in large-scale factories to maximize economies of scale. An example is the reverse water gas shift to convert CO₂ and H₂ to CO and H₂O. It may be followed by other processes such as Fischer–Tropsch to produce liquid synthetic fuels such as e-diesel, e-gasoline, and e-kerosene.

7.2.2. CO₂ electrochemical conversion. In electrochemical CO₂, electricity is applied to induce a nonspontaneous reaction of CO₂ reduction. This is similar to water electrolysis for green hydrogen production when the electricity is from a renewable source. Electrolysis is performed at low or high temperatures. The low temperature is more flexible but less energy efficient. High-temperature electrolysis is performed in solid oxide electrolyzer cells and is used to reduce CO₂ into C1 products such as CO or its mixture with hydrogen (syngas). CO₂ is split into CO and O₂, and the process is performed in the gaseous phase at 600–800 °C. With a low-temperature reduction of CO₂, C2+ products such as ethanol, ethylene, and propylene can be produced using an aqueous electrolyte design or membrane electrode assembly.

In bioelectrochemical processes, CO₂ is converted with the support of microorganisms directly in one step. However, it can also be converted with separate production of H₂ in a two-step process. In the first step, hydrogen is produced, then in the second step, hydrogen and CO₂ are fed to a bioreactor containing anaerobic methanogenic species for methane production.

7.2.3. CO₂ bio-chemical/enzymatic conversion. CO₂ contains carbon, which is the major component of biofuels. Therefore, CO₂ can be turned into biofuels and bioenergy, making it a potential candidate for renewable energy production. In bioconversion to energy, CO₂ is first captured into a biological system, and then converted into biofuels or bioenergy. Engineered microbes convert CO₂ into value-added bio-based chemicals.²⁵

Due to the limitations of natural CO₂ fixing pathways, synthetic CO₂ fixation routes have been developed at levels of in silico prediction, in vitro testing, and in vivo implantation of a synthetic CO₂ fixation pathway. Multiple criteria are considered while designing a synthetic CO₂ fixing pathway. Oxygen-tolerant and kinetically fast carboxylases are considered to incorporate into the pathways. The undesired side reactions are kept at a minimum. After passing the topological, energetic, and kinetic assessment, the resulting designed pathway is anticipated to have quick overall pathway kinetics, high flux, and improved performance compared to natural CO₂ fixing pathways.

The photosynthetic recycling of CO₂ can be used to produce products such as 2-methyl-1-butanol, isobutyraldehyde, and isobutanol, ethanol, butanol, malate, formate, lipids, sugars (hexose, pentose, and triose), biomass, succinate, limonene, isopropanol, acetic acid, isobutanol, and 3-methyl1-butanol, succinic acid, isobutyraldehyde, 1,3-propanediol.²⁶

7.2.4. CO₂ catalytic conversion. The catalytic conversion of CO₂ into chemicals is of high interest. Both homogeneous and heterogeneous catalysis processes can be used. Valuable chemicals such as carbonates, carbamates, urethanes, lactones, pyrones, and formic acid and their derivatives can be synthesized by homogeneous catalysts. Heterogeneous catalysis can offer several technical advantages such as stability, separation, handling, and reusing of the catalyst and reactor design. Although CO₂ activation by heterogeneous catalytic routes is still limited, efforts have been made toward synthesizing dimethyl carbonate, cyclic carbonates, and synthetic gas (CO, H₂) and methanol synthesis from CO₂ hydrogenation.²⁷

Examples of CO₂ catalytic conversion include CO₂ reforming of CH₄, CO₂ hydrogenation to methanol, formation of dimethyl carbonate from CO₂ and methanol, formation of cyclic carbonate from CO₂ and epoxide, and formation of cyclic carbonate from CO₂ in the presence of ammonium salt, and the reaction of CO₂ and propylene glycol.²⁸

Catalytic conversion of CO₂ can be used to form hydrocarbon and non-hydrocarbon fuels. Because of the high stability of CO₂ molecule , CO₂ conversion into valuable products requires high energy depending on the downward steps for 4+ oxidation state of carbon. CO₂ conversion to HC fuels involves a high energy process, where the C-oxidation state is +2 or lower for possible production of compounds such as HCOOH, H₂CO, CH₃OH, CH₄, and other HCs.²⁹

There are two ways to convert CO₂ into hydrocarbon-based fuels; (1) transformation using classical Fischer–Tropsch synthesis [Ramirez] through reverse water-gas shift (RWGS) reaction followed by HC cracking, isomerization and aromatization, (2) CO₂ transformation into methanol (MeOH) followed by the MeOH conversion to HC. The CO₂ is first converted via the Fischer–Tropsch synthesis and then followed by coupling with the Fischer–Tropsch catalyst system containing zeolite. This is to allow production distribution and selectivity to olefins and or aromatic. Similarly, combining the CO₂ with MeOH catalyst system, i.e., zeolite, can also allow the generation of useful chemicals via the CO hydrogenation.³⁰

Catalysts such as metals and their oxides, carbon nanostructures, metal–organic frameworks (MOFs), and polymer functionalized materials have been reported for CO₂ conversions.³¹ For CO₂ reduction, the one-electron reduction is endothermic and produced (CO₂˙) is highly reactive. Electrochemical potentials of possible CO₂ reduction reactions in aqueous solutions for the production of different chemicals and hydrocarbon fuels are in equations:

CO₂ + 2H⁺ + 2e⁻ → HCOOH, E° = −0.61 V (2)

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O, E° = −0.53 V (3)

CO₂ + 4H⁺ + 4e⁻ → HCHO + H₂O, E° = −0.48 V (4)

CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O, E° = −0.38 V (5)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O, E° = −0.24 V (6)

2H⁺ + 2e⁻ → H₂, E° = −0.41 V (7)

2H₂O → O₂ + 4H⁺ + 4e⁻, E° = +0.81 V (8)

Fig. 6 lists several reactions for the transformation of CO₂ into products that are commercially valuable.³²

Fig. 6 Directions of the CO₂ chemical transformation to various chemicals.³

7.2.4.1. Nanomaterials used for CO₂ catalytic conversion. The general classification of the materials and nanomaterials used for CO₂ conversion can be listed in Fig. 7. These materials can be prepared, modified, and used individually or prepared in a hybrid or combination of two or more chemicals. Hybridization of two or more types of these materials may further improve efficiency. For example, carbon materials like graphene, graphene oxide (GO), reduced GO, carbon nanotubes, and graphitic carbon nitride (g-C₃N₄) are commonly used for CO₂ applications. By varying the precursor, performing post-synthetic functionalization, and modification with metal nanoparticles or polymeric branches, the electronic band structures of the carbon nanomaterials and their light absorption features can be tuned, therefore, enhancing the CO₂ conversion and making them attractive for CO₂ applications.^33–35

Fig. 7 Examples of building blocks that form hybrid materials for CO₂ application.

Solid substrates are functionalized or modified with amines and ionic liquids^36,37 to have more affinity toward CO₂. Nevertheless, polymers such as amidine and guanidine-based polymers, poly(ethylene-imine), poly(ether imide), and poly(ionic liquid)s (PILs) interact more with CO₂.³⁸ Other polymers, including polystyrene, polysulfone, or poly(ether-b-amide-6), can be utilized due to their controllable porosity, good CO₂ permeability, and selectivity.

Porous organic polymers prepared from specific multidentate organic building blocks are good candidates for CO₂ applications.^39,40 However, polymerization conditions play a key role in their porosity. In addition, cross-linking of polymers is optimized and controlled to get macroporous (>50 nm), mesoporous (2–50 nm), and microporous (<2 nm) materials. This can also impact the pore volume and surface area, which are important parameters affecting the affinity toward CO₂.⁴¹

Another class of materials is the inorganic materials which can be synthesized with good morphology and high surface area. Examples include spherical nanoparticles, nanowires, nanocubes, and nanosheets. These materials can be easily prepared with controlled properties. For instance, nanoparticles of numerous shapes display various efficiencies for identical catalytic processes owing to the exposure of several crystal facets.^42–44 Metal such as palladium, gold, silver, and copper are the most efficient. With the ability to stabilize the reaction intermediates, metals such as copper are outstandingly active for CO₂ reduction to hydrocarbons. However, in some cases, palladium, gold, and silver form CO.^45,46 MOFs are another class of materials that showed excellent performance in CO₂ applications owing to their crystallinity and high surface area.^47,48 Such materials with crystals show high internal cavities interconnected mainly by small channels to play a key role in several CO₂ applications.^49,50 Heterogeneous catalytic conversion of CO₂ to chemicals and fuels is gaining particular interest because of its better stability, durability, simplicity in separation, handling, and simpler reactor design.

7.2.4.2. Homogeneous and heterogeneous metal complex-based catalysts. Homogeneous and heterogeneous metal complex-based catalysts are used for the electroreduction of CO₂, Fig. 8. In homogeneous catalysis, the metal complex acts as a redox shuttle between the electrode and CO₂. Mostly, the catalyst is in a more highly reduced state due to its acceptance of electrons from the electrode. As a result of the reduction, the catalyst transfers electrons to CO₂ in the solution and returns to its initial state, resulting in an indirect electrolysis reaction.³¹ Catalysts are often single atom transition-metal complexes stabilized by ligands in environments that tune the redox potentials of the catalyst. In the stable coordination environments found for typical Ir and Ru metal complexes, there is often a high catalytic selectivity after optimization in the design of the catalyst. However, the difficulties of catalyst recovery after catalytic cycles with considerable energy demands from distillation, filtration, or crystallization to recover the catalyst is a significant impediment in large-scale synthesis.

Fig. 8 Reduction of CO₂ by various catalytic mechanisms: (a) homogeneous, (b) immobilized, and (c) heterogeneous way in an electrocatalytic film.⁵²

Catalyst immobilization in surface or electrochemical reactions combines the benefits of homogenous catalysis and selectivity with the capacity to reuse the catalysts. With the reduction of CO₂ as our primary goal, we devised many techniques, including polymerization, noncovalent surface binding, and surface-chemical binding, to make chemically modified electrodes.⁵¹ Surface binding facilitates electron transfer from the electrode to metal complex catalysts on the surface. The electrochemical reduction of CO₂ using heterogeneous catalysts with high surface area is an appealing method. Nanostructured catalysts can produce much improved reactive surfaces with a significant portion of the reactive sites available for catalysis compared to traditional electrodes.

7.2.5. CO₂ photocatalytic conversion. Photocatalytic systems can potentially conduct difficult transformations, which thermal reactions cannot realize. Several materials and nanomaterials used as photocatalysts have been tested to convert CO₂ into valuable products. These can be classified into different classes, including nanoparticles, nanocomposites, and hybrid materials.^2,23 Understanding the mechanics of CO₂ photoreduction reduces reaction barriers and increases selectivity toward the desired products, resulting in increased CO₂ lowering efficiency. The CO₂ photocatalytic conversion using semiconductor-based catalysts involves, Fig. 9: (1) CO₂ adsorption on the surface of the catalyst, (2) generation of electron and hole pairs after the absorption of light, (3) separation and transfer of charges to the surface active sites, and the generated charges may recombine back in the (4) bulk or (5) on the surface, (6) reduction of the adsorbed CO₂ by accepting an electron, and (7) oxidation of water to give oxygen.

Fig. 9 Steps of photocatalytic CO₂ conversion. The absorption of light energy equal to or greater than the bandgap (E_g) results in the excitation of the electrons from the valence band (VB) to the conduction band (CB), leaving behind holes in the VB. The electrons and holes promote the reduction and oxidation of the reactant molecules.⁵³

The influencing factors for CO₂ photocatalytic reactions,^54,55 rate-limiting steps,⁵⁶ thermodynamic and kinetic requirements,⁵⁷ and reaction processes^58,59 have all been reviewed in several reports. For example, the adsorption of reactants on the surface of the catalysts is critical in heterogeneous catalytic reactions, and it is often regarded as the first step in initiating the reaction. CO₂ adsorption on photocatalysts is the first stage in CO₂ photoreduction, and the amounts and states of CO₂ adsorption on photocatalysts can have a big impact on CO₂ reduction processes, efficiency, and product selectivity.^60,61 This component has previously been disregarded. The number of papers looking at the impact of CO₂ adsorption on CO₂ photoreduction has increased in recent years. There are, however, few assessments that summarize these attempts.

Generally, the mechanisms for CO₂ photoreduction can be explained in different ways.^62–65 These mechanisms can be summaries in the steps including:

Step 1: CO₂ and H₂O diffuse into the photocatalyst surface

Step 2: CO₂ and H₂O do move along the photocatalyst surface by surface diffusion to occupy the active sites

Step 3: Light absorption and formation of electrons holes on the surface of the photocatalyst

Step 4: Interaction between adsorbed reactants and charged particles and then recombination of charged particles

Step 5: Desorption of the formed products

7.2.5.1. Photocatalysts requirements. Catalysts should be with: (i) photosensitizer component (to initiate photochemical one-electron transfer), such as organic compounds semiconductors and metal complexes, and (ii) catalyst (to convert one-electron transfer into a multi-electron reduction of CO₂), such as complexes, metal particles, and enzymes. Excellently designed photocatalysts (rhenium(I) complexes) can fill both roles. The photocatalytic efficiency can be evaluated by: (i) product selectivity, (ii) quantum yield = (product/mol)/(absorbed photons/einstein), (iii) the stability of the photocatalyst given by turnover number (TON) = (product/mol)/(photocatalyst/mol or unit mass), (iv) the photocatalytic cycle speed given by turnover frequencies (TOF) = TON/(reaction time/min).
7.2.5.2. Homogeneous catalysts. Homogenous redox photocatalysts consist of a light-harvesting unit (photosensitizer) and two catalytic sites. One site is for the oxidation process, where a donor provides the electrons, and the other is the reduction site, where the electrons are transferred to an acceptor. Sometimes, the photosensitizer works as photosensitizer and reduction site.⁶⁶

Visible light absorption by the photosensitizer unit leads to the excited state. Commonly used photosensitizers for CO₂ reduction are ruthenium (polypyridyl)-and (bipyridyl)-rhenium(I) complexes (2.1 Chromophores). So, the light absorption gives rise to a single metal to ligand charge transfer (¹MLCT), which produces a triplet excited state (³MLCT) by inter-system crossing. This ³MLCT state has different redox properties than the ground state. Owing to this state's reducing properties, direct interaction with an acceptor can lead to the one-electron oxidized ground state of the photosensitizer via oxidative quenching. In terms of CO₂ reduction, reductive quenching is the more important process. With an electron from the oxidation site, a ground state of electron reduced species is formed, capable of transferring electron to the reduction site to recover the d⁶ neutral ground state of the photosensitizer.⁶⁷

When exposed to light radiations of suitable frequencies, the ruthenium-or iridium-based complexes convert CO₂ into CO or formate.⁴ A series of anthracene-substituted mononuclear and dinuclear rhenium complexes have been reported for photocatalytic CO₂ reduction showing a good TON value and long life.⁵ The anthracene moiety also functions as a sterically bulky group that may inhibit deleterious intermolecular catalyst deactivation pathways.

Examples of the multi-component system include ruthenium(I) diimine photosensitizers, rhenium(II) diimine, photosensitizer, and organic photosensitizers.⁴¹ Moreover, some supramolecular photocatalysts are also used, such as Ru(II)–Ni(I), Ru(II)–Co(III), and Ru(II)–Re(I). An example of the selective catalytic reduction site for the photoreduction of CO₂ over rhenium(I) systems is given in Fig. 10.²⁸

Fig. 10 Catalytic cycle for the photoreduction of CO₂ with (bipyridyl)Re(CO)₃X complexes by Hawecker et al.⁶⁸

7.2.5.3. Heterogeneous catalysts. Several heterogeneous catalysts have been developed, including metal-supported catalysts, perovskites, and solid solution catalysts.⁶⁹ The heterogeneous catalyst consists of metal, supports, and promotors. Precious metals (Pt, Rh, or Ru) are known to have high activity and durability and are resistant to coke deposition, although at a high price. The precious metal-based catalysts show high activity in spite of the very small amount of metal catalyst used.

Non-precious metals (Ni or Co) have been widely investigated as well. Non-precious metals such as Ni or Co have been used more for DRM due to their low price and abundance.⁷⁰ Ni catalysts have shown a level of activity comparable to precious metals. Alloyed metal catalysts are widely used because they have a different electronic structure than monometallic materials. Monometallic Ni or Fe catalysts have shown poor durability because the monometallic Ni catalyst is easily deactivated by coke deposition, and Fe is not much active for CO₂ conversion.

TiO₂, BaLa₄TiO₁₅, SrTiO₃, WO₃, NaNbO₄, and Zn₂GeO₄ are a few of the materials that show promise for use in these applications. Most of these inorganic semiconductors have good structural controllability and stability (the morphology, size, and surface). Titania-based materials, including 1-D (TiO₂ nanowires, nanorods, nanobelts, and nanotubes), 2-D (TiO₂ nanolayers and nanosheets), 3-D, and hierarchical nanostructures, are the most commonly employed catalysts.¹⁶ Enhanced efficiency is attributed to factors such as high surface area, good photogenerated-charges separation, directional charge transport, enhanced light-harvesting owing to light trapping/scattering, and low photon influence, considering the configuration and geometry of the photocatalysts. Semiconductors such as ZnO, CdS, GaP, SiC, WO₃, Ga₂O₃, GaP, InTaO₄, MgO, ZrO₂, BiVO₄, and ATaO₃ are also used. Semiconductors with more negative CB potentials convert CO₂ to CH₃OH. Water worked as a reductant since no reducing reagents were introduced into the reaction solutions.^71,72

For example, it was reported that AgI/GCN nanocomposite's charge separation process follows the double-transfer mechanism (conventional charge separation mechanism) and Z-scheme process.⁷³ Upon irradiation, the AgI and GCN material's VB electrons are excited to CB. Therefore, the CO₂ photoconversion occurs at the surface of AgI if the double charge transfer mechanism is employed (Fig. 11a). However, when the Z-scheme mechanism is adopted, the photoconversion of CO₂ occurs on the surface of GCN because the CB value of GCN (−1.12 eV vs. NHE) is more negative than the standard reduction potential for the formation of O₂⁻, methane, ethanol and acetone, Fig. 11b.

Fig. 11 Charge separation mechanism for photocatalytic conversion of CO₂ over AgI/GCN composite. (a) Double charge separation mechanism; (b) Z-scheme mechanism.⁷³

7.2.5.4. Hybrid nanocatalysts. Hybrid materials are composites made by a synergistic combination of organic and inorganic components at the nanometer or molecular level. New properties are created by new electron orbitals formed between them. Hybrid materials such as a photocatalyst consisting of photosensitizer (CdSe quantum dots) and catalyst (Pt/TiO₂) were reported.⁷⁴ CdSe quantum dots show a negative shift of CB energy where electron injection into titania can proceed under visible light in an aqueous media forming CH₄ and CH₃OH. Nanoparticles supported on carbon nanotubes, g-C₃N₄, graphene nanosheets, and carbon dots are used for CO₂ photoreduction under vis-light into CO.^75–77 The influence of dye-based organic linkers on CO₂ photoreduction efficiency has been reported using UiO-67. Re₃-MOFs displayed high photocatalytic activity. When Ren-MOFs and Re3-MOFs were coated with silver nanoparticles, the efficiency was seven times enhanced, Fig. 12.⁷⁸

Fig. 12 Structures of Re_n-MOF and Ag-Re_n-MOF for plasmon-enhanced photocatalytic CO₂ conversion (a) and mechanism of conversion (b).⁷⁸

Covalently bonded microporous organic polymers (MOPs) showed high physical and chemical stability. Cooper's group reported on several monomers synthesizing pyrene-based conjugated microporous polymers with 1.94–2.95 eV band gaps, exhibiting high photocatalytic activity.⁷⁹ Covalent triazine frameworks (CTFs) are a distinguished subclass of contemporary high-performance nanoporous materials. CTFs with aromatic triazine linkages are constructed by covalently linked light elements (C, N, and H) and boast high porosity, high nitrogen content, and good thermal/chemical stability. Furthermore, a triazine ring connected with electron-rich units can easily form p–n heterojunction via the conjugated structure, imparting superior nature to conjugated CTFs, particularly in terms of bandgap tunability and rapid electron separation rates.^80,81

8. Pilot-scale projects and commercialization

Several well-known industries have expanded their funding for CCUS work. Also, various demonstration initiatives at the pilot scale are underway.⁸² Examples are:

• CarbFix in Iceland for CO₂ capture from geothermal fluid and air.

• Drax in the UK for CO₂ capture from biomass power generation.

• STEPWISE in Sweden for sorption-enhanced water gas shift separation testing in the iron as well as the steel industry.

• CIUDEN in Spain for CO₂ storage.

• Geothermal plants for CCUS in Croatia for electricity generation from geothermal hot brine.

CCS is projected to play a key role in achieving global warming goals and turning CO₂ into useful goods.^83,84 As a result, several technologies are being developed to capture, transport, store, and utilize CO₂. Typically, technological development proceeds in stages: (i) bench or laboratory size, (ii) pilot scale, (iii) demonstration scale, and (iv) commercial scale.

These steps can be easily classified into numerous technology readiness levels (TRL), Fig. 13. At the TRL 3, TRL 6, and TRL 7 development stages, there is a glut of technology. Beyond TRL 3, improving a technology necessitates further research money, but methods beyond TRL 5 and TRL 7 necessitate major financial commitment and commercial interests, for example, in the case of some polymeric membranes. The following sections of this article go into greater detail about the technical development of the various CCS technologies.

Fig. 13 Carbon collection, storage, and utilization technologies progressing in terms of technology readiness (TRL).⁸⁵

9. CO₂ use market

Although it is difficult to assess the future market of CO₂ uses, there are some areas and applications of CO₂. These can be listed as:

9.1. CO₂-derived fuels

The carbon in CO₂ can be used to produce fuels that are in use today. The process involves using CO₂ in combination with hydrogen, which is highly energy-intensive to produce carbon-containing fuels that are easier to handle and use than pure hydrogen. Examples are methane, methanol, gasoline, and aviation fuels.

9.2. CO₂-derived chemicals

The carbon and oxygen in CO₂ are promising to be used as an alternative to fossil fuels in producing chemicals, and materials including plastics, fibers, and synthetic rubber. Converting CO₂ to methanol and methane is the most technologically mature pathway.

9.3. CO₂ to building materials

CO₂ can be used to produce building materials to replace the water in concrete. It is called CO₂ curing or as a raw material in its constituents, cement, and construction aggregates. This process involves the reaction of CO₂ with calcium or magnesium to form low-energy carbonate molecules, the form of carbon that makes up concrete. While incorporating CO₂ in the manufacture of cement is still in its early stages of development, CO₂-cured concrete is one of the most established and promising applications of CO₂ utilization.

Construction aggregates, tiny particles used in building materials, can be made by combining CO₂ with industrial or waste products from power plant activities. These include coal fly ash and iron slag, which would otherwise be heaped or kept in landfills, Fig. 14.

Fig. 14 CO₂ conversion pathways into CO₂-derived building materials (building aggregates or CO₂-cured concrete).

9.4. Crop yield boosting

CO₂ can be utilized to increase the yields of biological processes, such as algae growth and greenhouse crop cultivation. The most developed use for increasing yields is using CO₂ with low-temperature heat in industrial greenhouses.

10. Conclusion and future perspective

In summary, one of the key issues facing our world is the rapid increase in atmospheric carbon dioxide (CO₂) concentrations. As a result, considerable interest has been in regulating CO₂ emissions and lowering atmospheric CO₂ concentrations to avoid a worldwide disaster. To reduce CO₂ emissions, various approaches have emerged including:

➢ Developing and converting to environmentally friendly energy sources;

➢ Improving energy efficiency and utilizing current processes;

➢ CO₂ collection and sequestration;

➢ Converting CO₂ to valuable products.

There has recently been a surge in interest in capturing CO₂ emissions and either permanently immobilizing them or chemically converting them into useful goods. This has prompted the creation of many strategies, approaches, and techniques aimed at reducing CO₂ levels through green energy alternatives as well as capturing and converting CO₂ into value-added products.

The use of CO₂ in commercial technology is based mostly on a trade-off between performance and environmental benefits.

Overall, this review is based on containing and utilizing CO₂ using organic–inorganic hybrid materials. The range of potential CO₂ uses involves direct use, by which CO₂ is not chemically altered (non-conversion), and the use of CO₂ by transformation, via multiple chemical and biological conversion processes to fuels, chemicals, and building materials. The strategy/technologies come under CO₂-Capture Utilization and Storage (CCUS). CCUS is far better than CCS because the former utilizes/transforms CO₂ into valuable products like ethylene, ethanol, methanol, formic acid, and formaldehyde. The transformations can be achieved using various materials, such as organic–inorganic hybrid-nanocomposites. Contemporary research should primarily focus on sustainable science to immediately address global warming.

The market for CO₂ use is expected to increase, with opportunities related to building materials, and as a carbon source for fuels and chemicals. However, some key factors affect the future market for CO₂-derived products, including scalability, competitiveness and climate benefits, and the visibility of the technologies.

Conflicts of interest

There are no conflicts to declare.

References

1. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzerland, 2014.
2. A. J. Nathanael, K. Kannaiyan, A. K. Kunhiraman, S. Ramakrishna and V. Kumaravel, Global opportunities and challenges on net-zero CO₂ emissions towards a sustainable future, React. Chem. Eng., 2021, 6, 2226–2247.
3. N. MacDowell, et al., An overview of CO₂ capture technologies, Energy Environ. Sci., 2010, 3, 1645–1669.
4. S. VijayaVenkataRaman, S. Iniyan and R. Goic, A review of climate change, mitigation and adaptation, Renewable Sustainable Energy Rev., 2012, 16(1), 878–897.
5. N. P. Liyanage, W. Yang, S. Guertin, S. Sinha Roy, C. A. Carpenter, R. E. Adams, R. H. Schmehl, J. H. Delcamp and J. W. Jurss, Photochemical CO₂ Reduction with Mononuclear and Dinuclear Rhenium Catalysts Bearing a Pendant Anthracene Chromophore, Chem. Commun., 2019, 55(7), 993–996.
6. K. O. Yoro, Numerical Simulation of CO₂ Adsorption Behaviour of Polyaspartamide Adsorbent for PostCombustion CO₂ Capture, MSc thesis, University of the Witwatersrand, Johannesburg, South Africa, 2017.
7. A. Gulzar, et al., Carbon dioxide utilization: a paradigm shift with CO₂ economy, Chem. Eng. J. Adv., 2020, 3, 100013.
8. E. J. Granite and T. O'Brien, Review of novel methods for carbon dioxide separation from flue and fuel gases, Fuel Process. Technol., 2005, 86, 1423–1434.
9. H. Yang, Z. Xu, M. Fan, R. Gupta, R. B. Slimane, A. E. Bland and I. Wright, Progress in carbon dioxide separation and capture, J. Environ. Sci., 2008, 20, 14–27.
10. Z. Wang, Q. Ou, H. Ma, G. Cheng, Q.-P. Zhang, B. Tan and C. Zhang, Molecular Engineering for Organic Cage Frameworks with Fixed Pore Size to Tune Their Porous Properties and Improve CO₂ Capture, ACS Appl. Polym. Mater., 2021, 3(1), 171–177.
11. K. Pareek, Q. Zhang, R. Rohan and H. Cheng, Highly selective carbon dioxide adsorption on exposed magnesium metals in a cross-linked organo-magnesium complex, J. Mater. Chem. A, 2014, 2, 13534–13540.
12. H. Ma, Z. Wang, Y.-H. Zhao, Q. Ou, B. Tan and C. Zhang, Microporous polymer based on hexaazatriphenylene-fused triptycene for CO₂ capture and conversion, Sci. China Mater., 2020, 63, 429–436.
13. J. T. Litynski, S. M. Klara, H. G. McIlvried and R. D. Srivastava, An overview of terrestrial sequestration of carbon dioxide: The United States Department of Energy's fossil energy R&D program, Clim. Change, 2006, 74(1–3), 81–95.
14. M. Bonchio, M. Carraro, M. Gardan, G. Scorrano, E. Drioli and E. Fontananova, Hybrid photocatalytic membranes embedding decatungstate for heterogeneous photooxygenation, Top. Catal., 2006, 40, 133.
15. C. Oltra, P. Upham, H. Riesch, À. Boso, S. Brunsting, E. Dütschke and A. Lis, Public responses to CO₂ storage sites: lessons from five European cases, Energy Environ., 2012, 23, 227–248.
16. R. L. Newmark, S. J. Friedmann and S. A. Carroll, Water challenges for geologic carbon capture and sequestration, Environ. Manage., 2010, 45, 651–661.
17. T. Sakakura, J.-C. Choi and H. Yasuda, Transformation of carbon dioxide, Chem. Rev., 2007, 107, 2365–2387.
18. A. Pearson, Carbon dioxide—new uses for an old refrigerant, Int. J. Refrig., 2005, 28, 1140–1148.
19. W. Dale Spall and K. E. Laintz, A Survey on the use of supercritical carbon dioxide as a cleaning solvent, ed. M. John and P. S. Samuel, William Andrew Publishing, Westwood, NJ, 1998, pp. 162–194.
20. A. Baiker, Supercritical fluids in heterogeneous catalysis, Chem. Rev., 1998, 99, 453–474.
21. A. Laury and J. G. Sebranek, Use of carbon monoxide combined with carbon dioxide for modified atmosphere packaging of pre- and postrigor fresh pork sausage to improve shelf life, J. Food Prot., 2007, 70, 937–942.
22. N. Muradov, Industrial Utilization of CO₂: a Win–Win Solution, in Liberating Energy From Carbon: Introduction to Decarbonization, Springer New York, New York, NY, 2014, pp. 325–383.
23. T. Inoue, A. Fujishima, S. Konishi and K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature, 1979, 277, 637.
24. E. Alper and O. Orhan, CO₂ utilization: developments in conversion processes, Petroleum, 2017, 3(1), 109–126.
25. A. N. Mistry, U. Ganta, J. Chakrabarty and S. Dutta, A review on biological systems for CO₂ sequestration: organisms and their pathways, Environ. Prog. Sustainable Energy, 2019, 38, 127–136.
26. A. Nisar, S. Khan, M. Hameed, A. Nisar, H. Ahmad and S. Azhar Mehmood, Bio-conversion of CO₂ into biofuels and other value-added chemicals via metabolic engineering, Microbiol. Res., 2021, 251, 126813.
27. B. Cornils and W. A. Hermann, Applied Homogeneous Catalysis with Organometallic Compounds, VCH, Weinheim, 1996, vol. 2, p. 1048.
28. J. Ma, N. Sun, X. Zhang, N. Zhao, F. Xiao, W. Wei and Y. Sun, A short review of catalysis for CO₂ conversion, Catal. Today, 2009, 148(3–4), 221–231.
29. M. D. Porosoff, B. Yan and J. G. Chen, Catalytic reduction of CO₂ by H₂ for synthesis of CO, methanol and hydrocarbons: challenges and opportunities, Energy Environ. Sci., 2016, 9, 62–73.
30. A. Ramirez, L. Gevers, A. Bavykina, S. Ould-Chikh and J. Gascon, Metal organic framework-derived Iron catalysts for the direct hydrogenation of CO₂ to short chain olefins, ACS Catal., 2018, 8, 9174–9182.
31. R. Francke, B. Schille and M. Roemelt, Homogeneously Catalyzed Electroreduction of Carbon Dioxide—Methods, Mechanisms, and Catalysts, Chem. Rev., 2018, 118, 4631–4701.
32. G. K. Ramesha, J. F. Brennecke and P. V. Kamat, Origin of Catalytic Effect in the Reduction of CO₂ at Nanostructured TiO₂ Films, ACS Catal., 2014, 4(9), 3249–3254.
33. G. Liu, W. Jin and N. Xu, Graphene-based membranes, Chem. Soc. Rev., 2015, 44, 5016–5030.
34. J. H. Lee, H. J. Lee and J. W. Choi, Unveiling anomalous CO₂-to-N₂ selectivity of graphene oxide, Phys. Chem. Chem. Phys., 2017, 19, 22743–22748.
35. J. Y. Tang, X. Y. Kong and B. J. Ng, et al) Midgap-state-mediated two-step photoexcitation in nitrogen defect-modified g-C3N4 atomic layers for superior photocatalytic CO₂ reduction, Catal. Sci. Technol., 2019, 9, 2335–2343.
36. G. Qi, L. Fu and E. P. Giannelis, Sponges with covalently tethered amines for high-efficiency carbon capture, Nat. Commun., 2014, 5, 5796.
37. T. Endo, D. Nagai, T. Monma, H. Yamaguchi and B. Ochiai, A Novel Construction of a Reversible Fixation–Release System of Carbon Dioxide by Amidines and Their Polymers, Macromolecules, 2004, 37, 2007–2009.
38. S. Supasitmongkol and P. Styring, High CO₂ solubility in ionic liquids and a tetraalkylammonium-based poly(ionic liquid), Energy Environ. Sci., 2010, 3, 1961–1972.
39. D. Wu, F. Xu, B. Sun, R. Fu, H. He and K. Matyjaszewski, Design and Preparation of Porous Polymers, Chem. Rev., 2012, 112, 3959–4015.
40. P. Bhanja, A. Modak and A. Bhaumik, Porous Organic Polymers for CO₂ Storage and Conversion Reactions, ChemCatChem, 2019, 11, 244–257.
41. P. Kaur, J. T. Hupp and S. T. Nguyen, Porous Organic Polymers in Catalysis: Opportunities and Challenges, ACS Catal., 2011, 1, 819–835.
42. S. Cao, F. Tao, Y. Tang, Y. Li and J. Yu, Size- and shape-dependent catalytic performances of oxidation and reduction reactions on nanocatalysts, Chem. Soc. Rev., 2016, 45, 4747–4765.
43. Y. Tamaki and O. Ishitani, Supramolecular Photocatalysts for the Reduction of CO₂, ACS Catal., 2017, 7(5), 3394–3409.
44. D. Raciti and C. Wang, Recent Advances in CO₂ Reduction Electrocatalysis on Copper, ACS Energy Lett., 2018, 3, 1545–1556.
45. R. Shi, G. I. N. Waterhouse and T. Zhang, Recent Progress in Photocatalytic CO₂ Reduction Over Perovskite Oxides, Sol. RRL, 2017, 1, 1700126.
46. J. Hou, S. Cao, Y. Wu, Z. Gao, F. Liang, Y. Sun, Z. Lin and L. Sun, Inorganic Colloidal Perovskite Quantum Dots for Robust Solar CO₂ Reduction, Chem.–Eur. J., 2017, 23, 9481–9485.
47. M. S. Denny Jr, J. C. Moreton, L. Benz and S. M. Cohen, Nat. Rev. Mater., 2016, 1, 16078 , Metal–organic frameworks for membrane-based separations..
48. X. Y. Chen, V.-T. Hoang, D. Rodrigue and S. Kaliaguine, Postsynthetic Methods for the Functionalization of Metal–Organic Frameworks, RSC Adv., 2013, 3, 24266–24279.
49. A. Razzaq and S. Il In, TiO₂ Based Nanostructures for Photocatalytic CO₂ Conversion to Valuable Chemicals, Micromachines, 2019, 10, 326.
50. M. Rebber, C. Willa and D. Koziej, Organic–inorganic hybrids for CO₂ sensing, separation and conversion, Nanoscale Horiz., 2020, 5, 431–453.
51. A. S. Varela, W. Ju and P. Strasser, Molecular Nitrogen-Carbon Catalysts, Solid Metal Organic Framework Catalysts, and Solid Metal/Nitrogen-Doped Carbon (MNC) Catalysts for the Electrochemical CO₂ Reduction, Adv. Energy Mater., 2018, 8, 1703614.
52. S. Zhang, Q. Fan, R. Xia and T. J. Meyer, CO₂ Reduction: From Homogeneous to Heterogeneous Electrocatalysis, Acc. Chem. Res., 2020, 53(1), 255–264.
53. S. A. Rawool, K. K. Yadav and V. Polshettiwar, Defective TiO₂ for photocatalytic CO₂ conversion to fuels and chemicals, Chem. Sci., 2021, 12, 4267.
54. X. Chang, T. Wang and J. Gong, CO₂ photo-reduction: insights into CO₂ activation and reaction on surfaces of photocatalysts, Energy Environ. Sci., 2016, 9, 2177–2196.
55. C.-C. Wang, Y.-Q. Zhang, J. Li and P. Wang, Photocatalytic CO₂ reduction in metal–organic frameworks: a mini review, J. Mol. Struct., 2015, 1083, 127–136.
56. W.-N. Wang, J. Soulis, Y. J. Yang and P. Biswas, Comparison of CO₂ photoreduction systems: a review, Aerosol Air Qual. Res., 2014, 14, 533–549.
57. X. Li, J. Yu, S. Wageh, A. A. Al-Ghamdi and J. Xie, Graphene in photocatalysis: a review, Small, 2016, 48, 6640–6696.
58. L. Liu and Y. Li, Understanding the reaction mechanism of photocatalytic reduction of CO₂ with H₂O on TiO₂-based photocatalysts: a review, Aerosol Air Qual. Res., 2014, 14, 453–469.
59. S. N. Habisreutinger, L. Schmidt-Mende and J. K. Stolarczyk, Photocatalytic reduction of CO₂ on TiO₂ and other semiconductors, Angew. Chem., Int. Ed., 2013, 52, 7372–7408.
60. M. Marszewski, S. Cao, J. Yu and M. Jaroniec, Semiconductor-based photocatalytic CO₂ conversion, Mater. Horiz., 2015, 2, 261–278.
61. X. Xiang, F. Pan and Y. Li, A review on adsorption-enhanced photoreduction of carbon dioxide by nanocomposite materials, Adv. Compos. Hybrid Mater., 2018, 1, 6–31.
62. L.-L. Tan, W.-J. Ong, S.-P. Chai and A. R. Mohamed, Photocatalytic reduction of CO₂ with H₂O over graphene oxide-supported oxygen-rich TiO₂ hybrid photocatalyst under visible light irradiation: Process and kinetic studies, Chem. Eng. J., 2017, 308, 248–255.
63. J. C. Wu, H.-M. Lin and C.-L. Lai, Photo reduction of CO₂ to methanol using optical-fiber photoreactor, Appl. Catal., A, 2005, 296, 194–200.
64. S. Delavari and N. A. S. Amin, Photocatalytic conversion of CO₂ and CH4 over immobilized titania nanoparticles coated on mesh: Optimization and kinetic study, Appl. Energy, 2016, 162, 1171–1185.
65. A. Khalilzadeh and A. Shariati, Photoreduction of CO₂ over heterogeneous modified TiO₂ nanoparticles under visible light irradiation: Synthesis, process and kinetic study, Sol. Energy, 2018, 164, 251–261.
66. R. Reithmeier, C. Bruckmeier and B. Rieger, Conversion of CO₂ via Visible Light Promoted Homogeneous Redox Catalysis, Catalysts, 2012, 2(4), 544–571.
67. H. Takeda and O. Ishitani, Development of efficient photocatalytic systems for CO₂ reduction using mononuclear and multinuclear metal complexes based on mechanistic studies, Coord. Chem. Rev., 2010, 254, 346–354.
68. J. Hawecker, J.-M. Lehn and R. Ziessel, Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′-Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts, Helv. Chim. Acta, 1986, 69, 1990–2012.
69. H. S. Whang, J. Lim and M. S. Choi, et al., Heterogeneous catalysts for catalytic CO₂ conversion into value-added chemicals, BMC Chem., 2019, 1, 9.
70. H. Miura, K. Endo, R. Ogawa and T. Shishido, Supported palladium-gold alloy catalysts for efficient and selective hydrosilylation under mild conditions with isolated single palladium atoms in alloy nanoparticles as the main active site, ACS Catal., 2017, 7(3), 1543–1553.
71. H. Pan and D. Michael, Heagy, Photons to Formate: A Review on Photocatalytic Reduction of CO₂ to Formic Acid, Nanomaterials, 2020, 10, 2422.
72. S. R. Lingampalli, M. M. Ayyub and C. N. R. Rao, Recent Progress in the Photocatalytic Reduction of Carbon Dioxide, ACS Omega, 2017, 2(6), 2740–2748.
73. P. Murugesan, S. Narayanan and M. Matheswaran, Visible light photocatalytic conversion of CO₂ in aqueous solution using 2D-structured carbon-based catalyst-coated β,γ-AgI nanocomposite, J. Mater. Sci., 2019, 54, 7798–7810.
74. C. Wang, R. L. Thompson, J. Baltrus and C. Matranga, Visible Light Photoreduction of CO₂ Using CdSe/Pt/TiO₂ Heterostructured Catalysts, J. Phys. Chem. Lett., 2010, 1, 48–53.
75. M. Li, L. Zhang, X. Fan, Y. Zhou, M. Wu and J. Shi, Highly selective CO₂ photoreduction to CO over g-C3N4/Bi2WO6 composites under visible light, J. Mater. Chem. A, 2015, 3, 5189–5196.
76. J. Yu, J. Jin, B. Cheng and M. Jaroniec, A noble metal-free reduced graphene oxide–CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO₂ to solar fuel, J. Mater. Chem. A, 2014, 2, 3407–3416.
77. Y.-F. Xu, M.-Z. Yang, B.-X. Chen, X.-D. Wang, H.-Y. Chen, D.-B. Kuang and C.-Y. Su, A CsPbBr3 Perovskite Quantum Dot/Graphene Oxide Composite for Photocatalytic CO₂ Reduction, J. Am. Chem. Soc., 2017, 139(16), 5660–5663.
78. K. M. Choi, D. Kim, B. Rungtaweevoranit, C. A. Trickett, J. T. D. Barmanbek, A. S. Alshammari, P. Yang and O. M. Yaghi, Plasmon-enhanced photocatalytic CO₂ conversion within metal–organic frameworks under visible light, J. Am. Chem. Soc., 2017, 139(1), 356–362.
79. R. S. Sprick, J.-X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M. A. Zwijnenburg, D. J. Adams and A. I. Cooper, Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution, J. Am. Chem. Soc., 2015, 137(9), 3265–3270.
80. T. Zhao, Q. Niu, G. Huang, Q. Chen, Y. Gao, J. Bi and L. Wu, Rational construction of Ni(OH)2 nanoparticles on covalent triazine-based framework for artificial CO₂ reduction, J. Colloid Interface Sci., 2021, 602, 23–31.
81. C. Zhu, Q. Fang, R. Liu, D. Wen, S. Song and Y. Shen, Insights into the Crucial Role of Electron and Spin Structures in Heteroatom-Doped Covalent Triazine Frameworks for Removing Organic Micropollutants, Environ. Sci. Technol., 2022, 56(10), 6699–6709.
82. A. J. Nathanael, K. Kannaiyan, A. K. Kunhiraman, S. Ramakrishna and V. Kumaravel, Global opportunities and challenges on net-zero CO₂ emissions towards a sustainable future, React. Chem. Eng., 2021, 6, 2226–2247.
83. IPCC Climate Change 2014: Mitigation of Climate Change. Working Group III Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2014.
84. A. Cousins, L. Wardhaugh and A. Cottrell, Pilot plant operation for liquid absorption-based post-combustion CO₂ capture, Absorption-based Post-combustion Capture of Carbon Dioxide, Woodhead Publishing, Cambridge, UK, 2016, pp. 649–684.
85. M. Bui, et al., Carbon capture and storage (CCS): the way forward, Energy Environ. Sci., 2018, 11, 1062–1176.

---