A Review on Green Hydrogen Valorization by Heterogeneous Catalytic Hydrogenation of Captured CO₂ into Value-Added Products

Abstract

The catalytic hydrogenation of captured CO₂ by different industrial processes allows obtaining liquid biofuels and some chemical products that not only present the interest of being obtained from a very low-cost raw material (CO₂) that indeed constitutes an environmental pollution problem but also constitute an energy vector, which can facilitate the storage and transport of very diverse renewable energies. Thus, the combined use of green H₂ and captured CO₂ to obtain chemical products and biofuels has become attractive for different processes such as power-to-liquids (P2L) and power-to-gas (P2G), which use any renewable power to convert carbon dioxide and water into value-added, synthetic renewable E-fuels and renewable platform molecules, also contributing in an important way to CO₂ mitigation. In this regard, there has been an extraordinary increase in the study of supported metal catalysts capable of converting CO₂ into synthetic natural gas, according to the Sabatier reaction, or in dimethyl ether, as in power-to-gas processes, as well as in liquid hydrocarbons by the Fischer-Tropsch process, and especially in producing methanol by P2L processes. As a result, the current review aims to provide an overall picture of the most recent research, focusing on the last five years, when research in this field has increased dramatically.

1. Introduction

Nowadays, there is a serious concern about the danger caused by the high GHG (greenhouse gas emissions) produced by our way of life, and in particular by the anthropogenic emissions of carbon dioxide (CO₂). Thus, if current trends continue, the planets temperature could rise dangerously, accelerating the climate change and resulting in an increase in the level of the oceans and their acidification. This scenario is already having a very negative impact on the ecosystems of the planet, as well as having a deep negative influence on the economic and social development of many countries all over the world [1]. To reduce the impact of this atmospheric pollutant, an important effort is being made, embodied in different international treaties, to carry out the substitution of fossil fuels with different renewable energy sources [2]. Considering that CO₂ alone accounts for around 77% of total greenhouse gases and that natural removal of CO₂ through forests and oceans is not enough to remove the excessive amount of CO₂ present in the atmosphere, other CO₂ mitigation strategies are required. Thus, in addition to renewable energies such as hydropower, wind, and solar energy, which are being considered as alternatives for fossil fuel mitigation, the use of various technologies for the Carbon Capture and Storage (CCS) of CO₂ [3], as well as its subsequent transformation into useful chemicals [4], is being considered, because CO₂ is a nontoxic chemical that is widely used as a C1 building block in the synthesis of highly important chemicals (Figure 1) [5,6].

In addition, CO₂ is being used not only in chemical transformation but also in mineralization and biological processes, such as those of an autotrophic biota and some microorganisms such as algae, cyanobacteria, and chemoautotrophic bacteria that have CO₂ fixing mechanisms [7], following routes such as thermochemical, electrochemical, and photocatalytic conversion, with different levels of maturity and performance [8,9].

Hence, catalytic conversion of CO₂ into chemicals and fuels is a “two birds, one stone” approach to fighting climate change, contributing also to solving the energy and green supply deficits in the modern world, as shown in Figure 2 [10].

In this way, as a complementary strategy to the capture and storage of CO₂, it is also necessary to consider the capture, storage, and utilization (CCS/U) of CO₂ as a feedstock for the synthesis of different fine chemical products, such as urea, methanol, formic acid, dimethyl ether, dimethyl or diethyl carbonates, and many others [5,11,12,13].

Therefore, it is completely pertinent to evaluate the technical possibilities to hydrogenate the CO₂ after its capture as one of the most promising ways to transform it into fine chemicals or biofuels. According to data in

Table 1. A comparative summary of hydrogen production processes and hydrogen color codes.

Hydrogen	Brown	Grey	Blue	Green
Feedstock	Coal	Natural Gas	Natural Gas	Renewable electricity
Carbon Capture	Gasification No CCS	Steam methane reforming No CCS	Advanced gas reforming CCS	Electrolysis
GHG: Emissions (tonCO2/tonH2)	Highest 19	High emissions 11	Low emissions 0.2	Potential for zero GHG emissions
Estimated Cost (per kg H2)	$1.2–$2.1	$1–$2.1	$1.5–$2.9	$3–$7.5

CCS: carbon capture and storage; GHG: greenhouse gas; tCO₂/tH₂—ton of carbon dioxide per ton of hydrogen.

, considering blue hydrogen’s low emissions level, blue and green hydrogen could be used together to begin the transition to net zero emissions, which is planned for 2050. As can be seen, the blue hydrogen could constitute an intermediate for use in the CO₂ hydrogenation processes, also considering that, at this time, the European Union Commission has labeled as sustainable some energy raw materials, such as nuclear and natural gas [14].

In addition, it must be taken into account that the production costs of both types of hydrogen will continue to decline over the next few decades, favoring the CCS/U process, as can be seen in Figure 3 [15].

Furthermore, it is necessary to consider the necessary complementarity between both types of hydrogen over a long period of time to guarantee a stable supply of this raw material over time for any CO₂ hydrogenation process carried out on an industrial scale. Besides, green hydrogen is greatly influenced by climatic factors, mainly wind intensity and solar radiation, so a supply of hydrogen obtained by a technology controlled by a safe raw material, such as natural gas, that generates very low CO₂ emissions is foreseeable [16].

On the other hand, the massive production of electricity, obtained by renewable technologies, can be used as another strategy that could complement the decarbonization process. However, this requires an effective and economically viable method that allows its storage, given the extreme dependence of these technologies on the climate. In this sense, the transformation of CO₂ and water by either power-to-liquids (P2L) and/or power-to-gas (P2G) processes, employing this Renew Energy, has recently gained much attention as an efficient way for CO₂ mitigation and obtaining value-added synthetic crude and/or synthetic natural gas [17]. Hence, combining the use of CO₂ and renewable H₂, obtained by water electrolysis, to produce chemicals and biofuels seems to be the most promising way for a larger H₂ utilization as an energy vector, providing, for instance, methane or methanol, as well as other light oxygenated hydrocarbons for fuel cell (FC) applications in electric engines [18]. In this way, a closed loop between Renew Energy sources and CO₂ reuse can be obtained, coupled with the benefits of clean energy sources and fossil fuels. The main drawback of these processes is that they are not economically competitive yet in comparison with processes carried out with conventional hydrogen (blue or gray), which is obtained from fossil fuels, with the consequent CO₂ emissions [19,20].

Up to date, there are still several applications more economically described for CO₂ capture and utilization than catalytic hydrogenation processes, such as methane recovery from hydrates [21], the production of biofuel and biomaterials by bacteria for the production of value-added products such as biodiesel, bioplastics, extracellular polymeric substances, biosurfactants, and other related biomaterials [22], or CO₂ utilization in agricultural greenhouses. However, the CCS methods are recognized as the most useful procedures to reduce CO₂ emissions while using fossil fuels in power generation [23]. Furthermore, the economic cost of producing green hydrogen is expected to fall rapidly, allowing the activation of various catalytic hydrogenation procedures on an industrial scale capable of significantly reducing GHG emissions. Thus, several studies are being conducted to develop innovative hydrogen generation systems by using low-carbon energy like wind and solar, which could enable the wide use, effective storage, and full market penetration of green hydrogen [24].

In this regard, either natural gas steam-methane reforming (SMR) or blue hydrogen could be considered a viable partner for accelerating hydrogen penetration in CO₂ capture, storage, and utilization (CCSU) [25].

Thus, catalytic hydrogenation processes close the cycle that allows the recovery of green hydrogen, obtained in very different amounts depending on seasonal fluctuations in renewable energy production. In addition, through these catalytic hydrogenation processes, a portfolio of useful chemicals and renewable fuels can be obtained, such as methane, methanol, ethanol [26], cyclic carbonates [27,28], or higher hydrocarbons such as aromatics [29,30], etc. Figure 4 [31].

Among these processes, methanol obtained by hydrogenation of CO₂ using electro-catalytically generated green hydrogen presents a great interest for the development of a complete strategy for the application of renewable energies since it can be used to convert and store the excess of electrical energy into chemical energy, contributing to smooth the natural fluctuation in the Renew Energy supply [32].

In fact, as can be seen in Figure 5, about 48% of the total methanol demand is for chemical intermediate uses, whereas the remaining 52% is for energy uses. The predominant use of methanol, at 29%, is in the production of formaldehyde, followed by its use in alternative fuels, such as gasoline blending, DME, and biodiesel, which make up 21% of the total demand.

Therefore, methanol could become a central compound in the worldwide energy landscape.

Summarizing, in the near future, the utilization of CO₂ and H₂ obtained by water electrolysis to produce P2L and P2G is one of the most promising strategies for CO₂ mitigation. However, due to economic limitations, CCS methods are recognized as the most useful procedures to reduce CO₂ emissions. In this sense, catalytic hydrogenation of CO₂ to obtain different products with added-value presents great interest. This review provides an overview of the various CO₂ heterogeneous catalytic hydrogenation reactions that can be used for the storage and transport, with a focus on the so-called liquid and gaseous organic hydrogen carriers, that is, the processes known as power-to-gas (P2G) and power-to-liquids (P2L).

2. Catalytic Hydrogenation of CO₂ to Renewable Methane

The catalytic hydrogenation of CO₂ to methane and water is a thermochemical process described over a hundred years ago and is known as the Sabatier reaction, or CO₂ methanation (Equation (1)) [33]. This is carried out catalytically at high temperatures (300–400 °C) and pressures (30 bar) in the presence of a suitable catalyst [34], although low-catalytic low temperature methanation has been studied in recent years [35]. By using green hydrogen, synthetic natural gas (SNG) is obtained, which can be used directly or stored for later use, allowing the transfer of electrical energy to a useful renewable fuel.

CO₂ + 4H₂ → CH₄ + 2H₂O, ΔH⁰_298K = −165 kJ/mol

(1)

The reaction is thermodynamically favored (ΔG_298K = −113.2 kJ/mol), but it involves an eight-electron process to reduce the fully oxidized carbon to methane, with important kinetic limitations, so that a catalyst is required in order to achieve high selectivity and conversion. Thus, catalysts based on noble- and transition-metal materials (Ru, Rh, Pd, and Ni) supported on metal oxides (Al₂O₃, CeO₂, ZrO₂, TiO₂, SiO₂, La₂O₃) are usually applied for the CO₂ methanation process [36,37,38,39]. Therefore, even though Ru is the most active metal, its high cost limits the large-scale application of Ru-based catalysts. In contrast, Ni-based systems are the most widely investigated for industrial purposes, as they combine a reasonable high selectivity for methane with lower costs [40,41].

Nevertheless, sintering of Ni nanoparticles and carbon deposition on the support surfaces of catalysts usually lead to their deactivation [42]. The carbon deposition phenomenon occurs either through CO disproportionation reactions (Equation (2)), or via CH₄ decomposition (Equation (3)), ref. [43]:

2CO → C + CO₂, ΔH⁰_298K = −173 kJ mol⁻¹

(2)

CH₄ → C + 2 H₂, ΔH⁰_298K = 75 kJ mol⁻¹

(3)

Thus, an incessant number of investigations currently aim at achieving the CO₂ conversion to methane via hydrogenation, using deposited Ni on very different supports and applying it with very different methodologies, since the tandem Ni/support exhibit a higher performance/cost ratio. To optimize the efficiency of these supported Ni catalysts, various inorganic materials, including Al₂O₃, SiO₂, TiO₂, CeO₂, and ZrO₂, that favor the dispersion of Ni particles and enhance their activity and stability, have been studied. In this respect, the sintering of Ni particles supported on Al₂O₃ can be inhibited to some extent, so that Al₂O₃ seems to be superior to the other supports [42,44,45,46,47,48].

On the other hand, in order to optimize these Ni/Al₂O₃ systems, extensive studies about the influence of operating conditions on carbon deposition, with special emphasis on the effects of the operating temperature, reaction time, and H₂/CO ratio, are being carried out, given that all are significant factors in the morphology and amount of carbon deposits, because until now it has not been possible to satisfactorily eliminate this carbon deposition in the Ni/Al₂O₃ catalysts [44,45,49,50,51,52]. Furthermore, there is a substantial amount of research being conducted on other factors that may influence both the catalytic performance and the intensity of carbon deposition during successive reactions with reused, supported Ni catalysts. Figure 6 collects a number of parameters influencing the catalyst design to improve the low-temperature catalytic performance of supported Ni catalysts toward the CO₂ methanation process [53].

On the other hand, mixtures of inorganic solids as supports, in many cases together with Al₂O₃, as well as other metals as co-catalysts with Ni, such as Co, La, Ru, and many others, have also been studied [46,55,56,57,58,59]. In this regard, the bimetallic systems Ni and Ru have shown promising results [60,61], outperforming the reaction over monometallic Ru or Ni catalysts. Thus, the formation of Ni-Ru alloys or the synergy between two adjacent metallic phases open the door to new high-performance and low-cost methanation catalysts [62,63], as illustrated in Figure 7.

The determining steps in the reaction, using Ni and Ru catalysts, are the HCO* dissociation to CH* and O* and the CH₃* hydrogenation to CH₄, respectively. Hence, as the selectivity of the reaction on Ni is higher than that on Ru, the activity attained on Ru catalysts is higher. Therefore, it can be concluded that the combination of both metals allows for a characteristic synergistic activity, in which Ru drastically improves the reducibility of Ni catalysts, also improving the Ni metallic dispersion and providing additional methanation sites [62]. In addition, it is an important finding that the addition of certain promoters, such as CeO₂, La₂O₃, Sm₂O₃, Y₂O₃, and ZrO₂ is clearly beneficial, not only because the corresponding metal-oxide promoted catalysts exhibited higher catalytic performance than Ni/Al₂O₃, but also because the stability along the successive uses is clearly increased [64]. CO₂ methanation has recently been achieved on Mg-promoted Fe catalysts, where Mg/Fe₂O₃ catalysts exhibiting the highest yield of 32% (400 °C) in CH₄ production, under practical operation conditions (8 bar, 10,000 h⁻¹) [65]. Thus, the competitive advantage presented by the low price of these materials in comparison with those usually used, Ni and Ru, should be highlighted.

Considering the huge amount of work regarding CO₂ methanation, a review of different catalysts usually investigated in carbon dioxide methanation by using different noble and non-noble metals supported on different materials is collected in

Table 2. A comparative summary of different metal-supported catalysts studied in the carbon dioxide methanation process.

Metal	Promoter	Catalyst Support	Conversion (%)	T (°C)	P (atm) *	Ref.
Ni	CeO2	γ-Al2O3	>60	300	0–10	[34]
Ru	---	TiO2	>60	210–300	1.0	[66]
Ru	---	Al2O3, CeO2, MnOx, ZnO	25–80	400	1.5	[67]
Ni	---	CeO2, Al2O3, Y2O3	60–80	250−500	1.0	[42]
Ni	TiO2	Al2O3	40	550	3.0	[43]
Ni	Mn	TiO2	95	350	---	[68]
Ni, Ni-Co	---	Al2O3	90	350–400	1.0	[44]
Ni	---	CeO2-ZrO2	55–99.8	200–350	---	[49]
Ni	---	ZrO2	72	300	---	[69]
Ni	---	Al2O3	5–75	350–450	1.0	[45]
Ni	Ce	SiC	80–95	600	10–15	[47]
Ni	La	Mg-Al	61	250	1.0	[50]
Ni	La	γ-Al2O3	4–14	377–400	1.0	[46]
Ni-Ru	---	CaO-Al2O3	84	380–550	1.0	[70]
Ni	CeO2	Al2O3-ZrO2-TiO2	83	300–400	5.0	[51]
Ni-ZrO2	---	Carb. Nanotubes	10–50	200–500	1.0	[52]
Ni	---	γ-Al2O3	66–92	250–350	----	[71]
Ni/Al2O3	---	3D-copper	3–45	300–500	1.0	[72]
Ni	---	Al2O3, CeO2, CeO2-ZrO2	60–80	200–450	1.0	[73]
Ni/GDC	---	Ceramic monolith, Open-cell foam	70–52	300–600	1.0	[74]
Ni	---	Attapulgite	60–80	200–600	1.0	[75]
Ni	---	Al2O3, CeO2, ZrO2	70–90	200–600	1.9	[76]
Ni	Sm2O3, Pr2O3, MgO	CeO2	55	200−500	1.0	[77]
Ni	---	CeO2 nanocatalyst	84	220	1.0	[78]
Ni	----	CeO2	81	250	1.0	[79]
Ni	La	Hydrotalcite-Al2O3	60–80	225–425	1.0	[55]
Ni	---	Sepiolite, Todorokite	70–100	250–450	1.0	[80]
Ni	---	ZSM-5@MCM-41	80	400	1.0	[81]
Ni	La	MgO, Al2O3	60–80	200–400	1.0	[82]
Ni-Ru	---	Al2O3	70–85	200–500	1.0	[60]
Ni	---	Al2O3	30	400	15.8	[48]
Ni	Mg, Ca, Sr, Ba	Al2O3	40–60	200–600	1.0	[56]
Ni	---	Al2O3, CeO2	60–80	200–500	1.0	[83]
Ni	---	Al2O3-ZrO2	60–77	160–460	1.0	[57]
Ni-Ru	---	Al2O3	40–85	250–550	1.0	[61]
Ni-Ru	---	MgO-Al2O3	40–65	650–550	1.0	[84]
Ni	Pt, Ru, Rh	CeO2, CeZrO4, CeO2/SiO2	65–70	200–400	1.0	[85]
Ni-Ru		CeO2-ZrO2	50–80	200–450	1.0	[86]
Ni-Ru		MgAl2O4	55–70	200–400	1.0	[87]
Ni	CeO2, La2O3, Sm2O3, Y2O3, ZrO2	Al2O3	75–95	200–300	5.0	[64]

* Atm = 1, indicates atmospheric pressure, in flow process.

.

As can be seen, very good CO₂ conversion values have been obtained, as the 99.8% value reported by Ashok et al. over a Ni supported on CeO₂-ZrO₂ [49]. In addition to the importance of CO₂ methanation by itself, the possibility of integrating the water electrolysis and CO₂ methanation is a highly effective way to store the excess of renewable electricity produced by whichever renewable sources, such as wind and photovoltaic power generation, are intermittent due to weather conditions [88]. Therefore, storage of the electric excess is closely related to the power-to-gas (P2G) systems, so they are promising technologies to achieve this purpose. Thus, the transformation of green energy into synthetic natural gas (SNG) is carried out, which, as it comes from CO₂ obtained by capture and storage (CCS), exhibits a renewable character. Besides, the generated SNG can be stored or directly injected into the existing natural gas network.

Figure 8 shows the integrated co-electrolysis and syngas methanation for the direct production of synthetic natural gas from CO₂ and H₂O using a hydrotalcite-derived 20%Ni-2%Fe/(Mg, Al)O_x catalyst and a commercial methanation catalyst (Ni/Al₂O₃) [89].

Likewise, a general plant scheme to transform CO₂ and water into synthetic natural gas is shown in Figure 9 [90]. Thus, in a circular power to gas process, renewable hydrogen, or green hydrogen, produced by water electrolysis powered by renewable electricity, such as solar or wind, will be critical to achieving net-zero emissions, and major advances in electrolyzer technologies are being developed in this regard [91,92,93,94,95,96,97,98,99]. However, the efficiency of the plant processes, regardless of the technological methodology or the experimental conditions used, strongly depends on the efficiency of metal-supported catalysts, Table 2.

3. Catalytic Hydrogenation of CO₂ in Power-to-Liquid (P2L) Processes

Various liquid organic hydrogen carriers (LOHCs) compounds, such as hydrocarbons or high molecular weight alcohols, can be obtained through catalytic hydrogenation of CO_2, but the most prominent power-to-liquid (P2L) processes at this time are methanol synthesis, DME production [100], and Fischer-Tropsch fuels [101].

3.1. Catalytic Hydrogenation of CO₂ to Renewable Methanol

Methanol is supposed to have potential not only as a hydrogen-alternative energy vector (i.e., direct use as a fuel), but also as a hydrogen storage material. In addition, given that methanol is already synthesized on a large scale, there is the possibility of using existing infrastructure and production plants [102]. Accordingly, methanol is the simplest C1 liquid product that can be obtained from CO₂ (Figure 10). Therefore, methanol can be considered a key component of the anthropogenic carbon cycle in the framework of a “Methanol Economy” [103,104,105]. Indeed, the versatility of methanol, currently used to obtain multiple chemical products such as formaldehyde, acetic acid, methyl tertiary-butyl ether (MTBE), dimethyl ether (DME), or even olefins, as well as the possibility of its use as renewable fuel, while also taking advantage of the existing infrastructures for the transport and distribution of fuels, is what justifies the so-called “Methanol Economy” [106]. Thus, even though most of the methanol is currently produced from natural-gas-derived syngas, its alternative production using CO₂, water, and renewable electricity could present an opportunity to advance toward carbon neutrality [107].

Regarding the different possibilities to obtain methanol from captured CO₂, such as electrochemical, photochemical, photoelectrochemical, and catalytic conversion, it is the heterogeneous catalysis that attracts the most attention.

The heterogeneously catalyzed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to obtain methanol was described nearly 100 years ago, and the standard catalyst Cu/ZnO/Al₂O₃, currently applied in the methanol industrial synthesis reaction, has been used for the last 50 years [108,109]. This industrial reaction is currently taking place over Cu-ZnO/Al₂O₃ catalysts at pressures of 5 and 100 atm and temperatures in the 220–300 °C interval. Despite the fact that the reaction is exothermic, the conversion of CO₂ to methanol is kinetically limited, only obtaining a methanol conversion of around 15–25%. Thus, methanol is produced from synthesis gas (syngas) on an industrial scale, which is obtained from the steam reforming of fossil methane with a certain CO/H₂ ratio called metgas, which also contains about 3% by volume of CO₂. When this metgas is treated with H₂ at high pressures and moderate temperatures in the presence of conventional catalysts, Cu/ZnO/Al₂O₃, methanol is obtained (Equation (4)) [108]:

CO₂ + 3H₂ ⇋ CH₃OH + H₂O ΔH = −49.16 kJmol⁻¹

(4)

However, by starting with pure CO₂ and H₂, rather than a mixture of CO, CO₂, and H₂ as in the syngas procedure, the chemical process is simplified, so the reaction and purification processes in conventional methanol-producing industrial plants could also be simplified. That is because, despite the direct synthesis of methanol from CO₂ is less exothermic (Equation (5)) and it is also accompanied by the reverse water–gas-shift (RWGS) as a secondary reaction (Equation (6)), the high exothermic character of the methanol formation from syngas (Equation (4)) necessitates the use of a very complex reactor capable of providing efficient cooling for the heat generated. Conversely, the thermal control inside the reactor during the methanol synthesis from CO₂ is easier due to the lower heat profile of this process.

CO₂ + 3H₂ ⇋ CH₃OH + H₂O ΔH = −49.16 kJmol⁻¹

(5)

CO₂ +H₂ ⇋ CO + H₂O ΔH = 41.22 kJmol⁻¹

(6)

Another advantage of this process is that the only reaction impurities are essentially limited to water and dissolved CO₂ in the crude methanol. In this way, it is possible to diminish the cost and improve the efficiency of the process in comparison with the process of methanol production from syngas. Another important issue is the overall cost of the two processes, given that today, syngas is cheaper than green hydrogen and captured CO₂. For this reason, great efforts are being made to obtain green hydrogen on an industrial scale or other low-carbon hydrogen production methods, such as aqua hydrogen or blue hydrogen (obtained via new technologies from fossil fuels but with a lower carbon footprint).

Given that there is a general motivation regarding the use of captured CO₂ for the methanol synthesis as a liquid-to-power process, a great effort is being devoted to improving the current Cu-based catalysts employed to get more active, selective, and stable heterogeneous catalysts [110]. On the other hand, new noble metal-supported catalysts, able to increase the efficiency of this process for direct CO₂ hydrogenation to obtain bio methanol are being researched [111,112].

3.1.1. Hydrogenation CO₂ to Methanol using Cu-Based Catalysts

Once the convenience of advancing the catalytic processes of direct hydrogenation of CO₂ to obtain methanol was accepted, the first candidates were copper-based catalysts due to both their low cost and their good efficiency for methanol synthesis from synthesis gas, or Syngas [113,114]. However, some disadvantages, such as the formation of CO as a byproduct of the reverse water−gas shift (RWGS) reaction (Equation (6)), and the sintering of copper particles, which are responsible for catalyst deactivation after several reuses, determine the need to get better catalytic systems [115]. That is why numerous studies are currently being carried out in an attempt to improve the catalytic behavior of Cu, for which the role of various supports and/or additives that work as promoters is being investigated. The supports mainly consist of several metal oxides, such as Al₂O₃, SiO₂, ZnO, ZrO₂, CeO₂, TiO₂, or In₂O₃. The main role of these supports and promoters is to alter the electronic and geometric properties of active centers, thereby altering the metal-support interactions. Thus, various authors demonstrate that the yield to methanol is determined by active sites modulated by metal-support interaction as well as the influence of promoters [116]. This electronic interaction between supports and catalytically active metals is manifested through its influence on the energy levels of the frontier orbitals of the corresponding metal. The closer they are to the corresponding HOMO-LUMO of the CO₂ and H₂ molecules, the greater the catalytic efficiency of the reaction.

Thus, in a relatively short period of time, numerous studies have appeared on the performance of Cu catalysts by examining the effects of the composition of the support and the influence of the method of catalyst synthesis (

Table 3. Comparative performance of different Cu supported catalysts in the carbon dioxide hydrogenation to methanol reaction obtained by using different solid supports and/or different synthesis.

Catalyst Support	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm)	Ref.
ZnO/Al2O3	60–90	50–80	210–250	60	[109]
ZnO/ZrO2	50–80	65–85	220	30	[117] a
ZnO/ZrO2	20–60	30–90	200–300	1–40	[113] b
Al2O3	50–70	50–65	180−300	1–360	[114] a
ZnO	2–9	45–95	220–300	30	[118]
Al2O3/MgO	0–25	20–30	150–250	10	[119]
ZrO2/CeO2	3–10	40–82	220−260	30	[120] c
SiO2	5	79	190−250	30	[121]
ZrO2/ZnO	30–70	30–70	190–250	10–30	[122]
Na-ZSM-5/ZnOx	2–12	25–100	200–300	30	[123]
CuO/ZrO2	5–15	15–70	230	10	[124]
Al2O3/MgO	20–35	5–35	200–400	20	[125]
Al2O3/ZnO	5–35	5–70	200–400	20	[126]
SiO2/TiIV Surf.	4–18	49–85	230	25.0	[127]
SiO2/ZnII Surf	1–5	48–86	230	50.0	[128]
ZnO/ZrO2/Mg-Al (LDH)	1–7	50–100	200–300	30.0	[129]
ZnO/MnO/SBA-15 silica	4–8	100	180	40.0	[130]
ZnO	9–13	65–80	240	30.0	[131]
ZnO/SiO2	90–100	65–100	250	40.0	[132]
ZrO2	2–7	30–70	350	10.0	[133]
ZnO/Attapulgite	12–18	7–25	320	6.0	[134]
ZnGa/LDH nanosheet	17–20	30–50	270	50.0	[135]
Sr-Perovskite	1–16	36–63	200–280	20–50	[136]
CexZryOz	5–16	45–95	200–300	30	[137]
ZrOx	13.1	78.8	260	45	[138] c
ZrO2	1.0–5.0	68–75	220	30	[139]
ZnO	1.0–25.0	10–90	200–300	20	[140]
ZnO/Faujasite	2.0	27–35	240−260	15	[141] a,c
ZnO/CeO2	1.0–3.5	20–70	250	30	[142] c
ZnO/ZrO2/C-nanofibers	8–14	78–92	180	30	[143]
ZnO/Al2O3	10–28	33–85	240	40	[144]
ZnO/SiO2	8–14	50–59	220	30	[145]
AlCeO	2–24	12–95	200–280	30	[146]
AlCeO	6–22	25–97	200–280	30	[147]
ZnO	1–14	1–60	150–300	1.0	[148]
CeO2	1–7	20–90	240–300	20	[149]
Al2O3/ZrO2/ZnO	<7.0	43–59	230	30	[150]
ZnO/ZrO2	19.6	50	280	50	[151]
ZnO/ZrO2	9–15	87–98	250	50	[152]
Al2O3	50	-	325	1.25	[153]

^a This study mainly deals with the effects of the CO₂ hydrogenation reaction mechanism. ^b The catalysts’ ability to be reused is determined. ^c Special attention is paid to the existence of strong metal-support interaction effects (SMSI).

), the influence of several additives used as promotors (

Table 4. Influence of the use of different additives on the performance of the carbon dioxide hydrogenation to methanol reaction using different Cu-supported catalysts.

Promoter	Catalyst Support	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm)	Ref.
Pr2O3	ZnO	80–100	75–100	200–260	30	[115]
In2O3	ZrO2	1–6	60–90	210–290	10–250	[153]
CeO2	Al2O3	16–23	59–94	220–280	40	[147]
LaOx	Silica SBA-15	45–85	45–81	220–280	30	[154]
W	CeO2	13	87	250	35	[155]
Pd	Ce0.3Zr0.7O2	15–25	90–95	250	50	[156] a
Sm2O3	ZrO2	8–14	50–80	230	10	[157]
Al + Ga	ZnO	16–18	99	250	30	[158]
Al	ZnO	1–17	99	250	30	[159] b
Zn	Graphene	18–20	50–80	250	15.0	[160]
Hydrotalcite	ZnO-Al2O3	6	64–73	250	15–30	[161]
ZnO-ZrO2	Hydrotalcite	3–6	35–65	250	25.0	[162]
Zn, Ga	SiO2	0.5–5.0	10–80	220–280	8.0	[163]
Pd	SiO2	6.6–3.7	12–30	300	41	[164]
Pd	CexZr1-xO2	13–20	10–25	250–300	30–60	[156]
Ni	CeO2-nanotube	2–18	75–86	220–300	20–40	[165]
ZnO	Al2O3	10–20	50–100	160–250	10–25	[137] a
MgO	ZnO	4–16	25–100	200–300	30	[166]
MgO, CaO, SrO, BaO, ZnO	Al2O3	2–9	10–100	200–400	20	[167]

^a This study focuses on the effects of the CO₂ hydrogenation reaction mechanism. ^b The catalysts’ ability to be reused is determined.

), as well as other factors of interest in the final behavior of the catalyst, including the use of CuO instead of Cu metal as a catalytically active species (

Table 5. Comparative performance in the carbon dioxide hydrogenation to methanol reaction, of different CuO supported catalysts, using different solid as supports, different synthesis methods or different additives.

Promoter	Catalyst Support	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm)	Ref.
	ZrO2	5–15	15–70	230	10	[124]
ZnO/ZrO2	SBA-15 silica	10–25	20–35	250	30	[168]
ZnO/ZrO2	Mg-Al (LDH)	1–7	50–100	200–300	30.0	[129]
In2O3	CuO	5–12	50–90	220–280	5–30	[169]
	CuO/ZrO2	2–12	20–70	230	10.0	[170]
	CuO/CeO2/TiO2	1.5–6.5	28–52	190–235	30	[171]
MoO3/WO3/Cr2O3	CuO/ZnO/ZrO2	18–20	40–48	240	40	[172]
	CuO/ZnO/Al2O3	10–16	>99	250	50	[173]
Ag	CuO/ZrO2	4–8	25–45	270	10	[174]
	CuO/Ce0.4Zr0.6O2	7–13	72–96	220–280	30	[175]
	CuO/ZnO	38	70	270	50	[176]
	CuO/ZnO/CeO2	14–20	95–98	240	1.0	[177] a
	Cu/Zn/Ce/TiOx	4–7	25–45	275	30	[178]
	CuO/ZnO/TiO2/Zr	3–25	15–85	200–280	30	[179]
CuO/ZnO/CeO2	TiO2 nanotubes	10–20	25–80	220–300	30	[180]
	CuO/ZnO/ZrO2	9–17	40–54	300–600	30	[181]
Graphene oxide	CuO/ZnO/ZrO2	2–25	10–76	200–280	20	[182]
Carbon	CuO/ZnO	8–24	18–60	230–290	30	[183]
WO3	CuO-ZnO-ZrO2	5–20	42–64	240	30	[184]
ZrO2/Al2O3	CuO/ZnO	20–25	40–95	200−260	27.6	[185]
In2O3, Pd	CuO/ZnO/Al2O3	7–16	99–100	250	50	[186]
La2O3	CuO/ZnO/Al2O3	1–18	5–100	160–260	1.0	[187]
SiO2	CuO/ZnO/ZrO2	2–5	10–70	200–280	20	[188]
	CuO/ZnO/ZrO2	4–15	45–85	240	30	[189]
	CuO/CeO2/ZrO2	5–20	2–8	200–260	30	[190] b
Ag	CuO/ZrO2	1–7	30–70	230	10	[191]
Zeolite	CuO/ZnO/ZrO2	5–20	5–12	260	30	[192]
CuO-ZnO	Al2O3, SiO2	2–14	46–59	250, 270	30, 50	[193]
	Ce1-xZrxO2	2–15	10–95	200–300	30	[194]
LaxSr1-xCuO	Perovskite	1–16	4–55	250–300	30	[195]
ZrO2, MnO2	CuO-ZnO/SBA-15	8–9	10–25	250	30	[196]
CuO/ZnO	Oyster Shells	1–2	50–70	250	30	[197]
Pd	CuO/ZnO/Al2O3	1–10	10–90	180–240	50	[198]
La, Ti or Y	CuZnIn/MZrOx	2–6	40–80	225	20	[199]
La, Ce, or Sm	CuZnO/Zn-AlOx	25	54	250	40	[200]

^a The reusability of the catalysts is determined. ^b This study mainly deals with the effects of the CO₂ hydrogenation reaction mechanism.

). In Table 3, Table 4 and Table 5, the comparative performance of different Cu-supported catalyst systems in the carbon dioxide hydrogenation to methanol reaction is collected. In general, the catalytic performance of the catalysts was tested in a fixed-bed stainless-steel tubular reactor at different pressures and temperatures. Furthermore, the conversion and selectivity to methanol limits values obtained are collected. In this regard, an extensive review of supported Cu catalysts studied is presented, focusing on the last five years and highlighting the special importance that the CO₂ hydrogenation catalytic process has gained.

The influence of copper form, according to its size, i.e., bulk, nanoparticle, and cluster, has been deeply studied. In general, copper nanoparticles would enable higher activity due to having an overall higher Cu surface exposure, although more energy is generally required to increase the reaction kinetics. Regarding copper clusters, small clusters of small metallic particles tend to perform better at higher dispersion. For instance, the Cu₄ clusters exhibited a lower activation barrier to CO₂ hydrogenation than bulk Cu(111) [201].

Promoters are added to the catalyst to achieve three possible outcomes: increasing the number of available active sites, maintaining the Cu surface stability, particularly by increasing the Cu dispersion, and increasing electron transfer to the active sites, all of which can improve the catalyst activity. For their part, support materials are used to immobilize Cu particles in order to increase active site dispersion and maintain high thermal stability. Active metal–support interaction can promote a high synergy, increasing the reaction activity, particularly if the support can adsorb and transfer the reactants to the active sites without taking part in the reaction itself [116]. As can be seen in Table 4 and Table 5, a great number of additives, supports, and promoters have been tested. The results obtained with this huge number of Cu catalysts studied usually fall within those usually described for the Cu-ZnO/Al₂O₃ catalysts used in the catalyzed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to obtain methanol in the industrial synthesis reaction. This special reactivity of this industrial catalyst for methanol synthesis is attributed to the effects of strong metal-support interactions (SMSI) that allow a favorable synergy between Cu and the Zn atoms. This means that it is feasible to optimize the choice of catalysts, considering other parameters of technical and economic importance, such as the cost of the catalyst and the possibility of its reuse in successive reactions.

However, despite the fact that using waste CO₂ should decrease the methanol production cost, the significantly low price of this CO₂ gas means that this process remains a challenge, mainly caused by the lack of an efficient catalyst that can perform this reaction in a successful way in terms of kinetics and methanol selectivity. For this reason, it is an objective of extreme priority to advance in the development of new catalysts with good activity and high selectivity for methanol synthesis through CO₂ hydrogenation.

Despite the fact that to date, Cu-based catalysts are the most important catalysts for the conversion of syngas to methanol due to their excellent reactivity and low cost, they exhibit serious shortcomings when CO₂ replaces CO because CO₂ is more inert than CO, leading to lower CO₂ conversion. Besides, the water produced during this reaction results in the sintering of catalytically active copper sites. For this reason, numerous investigations are being directed to the search for efficient catalysts in this process using non-Cu-based heterogeneous catalysts such as noble or rare metals or mixed oxide catalysts represented by M-ZrOx (M = Zn, Ga, and Cd) solid solution catalysts, which present high methanol selectivity and catalytic activity as well as excellent stability to improve the catalytic activity, selectivity, and durability of catalysts in CO₂ hydrogenation [202].

3.1.2. CO₂ Hydrogenation to Methanol by Noble or Rare Metal-Based Catalysts

Despite their limited availability and high cost, many supported noble metal-based catalysts (Pd, Pt, Au, and Ag) can achieve high methanol selectivity and catalytic activity even at low temperatures with excellent stability. Hence, a large amount of research has been conducted in recent years to optimize the catalytic behavior of systems based on noble metals for the CO₂ hydrogenation to methanol. In this sense, various factors can influence the final catalytic behavior, such as the composition of the support, the synthesis method, or the influence of various additives used as promoters (

Table 6. A comparative summary of different noble metals and rare earth supported catalysts studied in the carbon dioxide hydrogenation process to produce methanol.

Noble Metal	Promoter	Catalyst Support	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm)	Ref.
Au		In2O3	5–13	60–100	250–300	50	[203] a
Ir		In2O3	18	70	300	50	[204]
Pd	Ga	SiO2	1–5	81	230	25	[205]
Pd		CeO2	2–18	4–100	200–280	10–250	[206]
Pd	Al	ZnO	2–14	15–70	250	30	[207]
Pd		In2O3	3	100	280	50	[208]
Pt		In2O3	37	63	30.0	1.0	[209]
Pd		In2O3/SBA-15	13	83	260	50	[210]
Ni5Ga3		SiO2	3–35	11–16	200–300	1.0	[211]
Ni		Ga2O3	0.5–1	10–100	160–300	5.0	[212]
Re		TiO2	1–2	82	150	50	[213]
Ti		MoOx/TiO2	80	70	150	50	[214]
ReOx		TiO2	18	98	200		[215]
Co		SiO2	2–14	10–80	260–320	20	[216]
Ag		In2O3	5–30	75–100	200–275	50	[217]
Au		ZrO2	5–9	40–70	140–220	30	[218]
Au		MxOy b	5–45	10–95	200–350	1.0	[219]
Au		In2O3-ZrO2	2–15	65–100	200–300	50	[220]
Au		CuO/CeO2	4–10	30	200–300	30	[221]
Au		CeO2	1∓2	5–45	240	5∓50	[222]
Ru		In2O3	1–30	70–97	200–300	50	[223]
Au		ZrO2	4–6	48–75	240	40	[224]
Au		ZnO-ZrO2	4.5–6	82–95	320	55	[225]
Rh		In2O3	1–17	56–100	250–300	50	[226]
Rh		In2O3–ZrO2	1–18	65–100	250–300	50	[227]
Pt		In2O3	1–15	54–100	225–300	50	[228] a
Pd		In2O3	1–20	70–100	200–300	50	[229]
Ni		In2O3	1–18	60–100	200–300	50	[230]
Au		In2O3	2–14	70–100	225–300	50	[203] a
Rh		In2O3	4–10	60–80	270–320	50	[231] a
Ni	ZrO2	In2O3	1–18	43–100	200–300	50	[232]
Ni		In2O3	6–15	30–80	280	50	[233]
Pd		In2O3	10	72	295	30	[234] a
Pd		SiO2	---	64–71	200	30	[235]
Pd	Ga2O3	SiO2	10	17–65	220–250	30	[236]
Pd		ZnZrOx	4–35	5–90	200–400	50	[237]
Pd		CeO2	2–10	40–78	200–260	50	[238]
Pd		SiO2	1–20	1–28	220–280	8.0	[239]

^a Special attention is paid to the existence of strong metal-support interaction effects (SMSI). ^b MxOy: Al₂O₃, TiO₂, Fe₂O₃, CeO₂, and ZnO.

). In this respect, the catalytic behaviour of bimetalic catalysts has also piqued the interest of researchers. In this respect, the most interesting results are collected in Table 6 and

Table 7. A comparative summary of different bimetallic-supported catalysts studied in the carbon dioxide hydrogenation process to obtain methanol.

Metal	Catalyst Support	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm)	Ref.
Ni/In/Al	SiO2	1.6–3.8	1–12	210–290	1.0	[240] a
Ni/In	SiO2-SBA-15	1–17	1–90	300	5−50	[241]
Co/In	In2O3	19	69	300	50	[242]
In/Pd	SiO2	2–5	61	300	40	[243]
Rh/In	Al2O3	1–10	5–90	270	45	[244] a
Pd/Zn	CeO2	8–17	65–98	220–270	20	[245]
Ca/Pd/Zn	ZrO2	2–10	97–100	220–270	20–30	[246] a
In/Ru	SiO2	1–5	20–85	200−240	34	[247]
Ni/Ga	SiO2, CeO2, ZrO2	1–6	5–30	180–270	1–30	[248] a
Pd/Cu	SiO2	1.6–2.8	18–27	300	30–50	[249]
Pd/Cu	MxOy b	7–16	28–34	300	40	[250] a
Pd/Cu	SiO2	3–7	12–40	300	40	[251]
Pd/Cu	SiO2	3–6	12–40	300	40	[252]
Cu/Ni	Graphene	7.87	98.7	225	40	[253]
Pd/Cu/Zn	SiC	1–11	10–100	150–300	1.0	[254]
Cu/Ni	Mordenite	100	30–60	220	30	[255]
Ru/Mo	Ru−Mo Phosphide	0.5–4.5	5–75	180–220	65–72	[256]
Rh/Co	nanospheres	100	96	150	24	[257]
Pd/Zn/Al	ZnO, Al2O3	0.5–4.0	15–70	250	30	[207]
Ni/Sn	InZrO2	1–5	55–100	225–275	25	[258]
Pd/Cu	TiO2-MO2 c	7–16	25–40	250	40	[259]
Ni/Ga	Hydrotalcite	2–3.5	60–100	200–300	30	[72]
Pd/Cu	CeO2	2–17	24–84	190–270	30	[260] a
Cu/Zn	Coord. polymer	13–20	25–59	220–260	40	[261]
Cu/Zn	UiO-66 (Zr) MOF	12–22	28–54	220–300	30	[262]
Cu/ZnO	Al2O3	5–11	64–87	220–260	30	[263] a
Ag/Cu	Mordenite	----	48–61	230	30	[264]
Cu/Zn	UiO-66 (Zr) MOF	25–30	15–24	230	50	[265]
Cu/Pd	SiO2	2–32	1–4	220–360	40	[266] a
Pd/In	Unsupported nanoparticles	<3.0	25–90	190–270	50	[267]

^a Special attention is paid to the existence of strong metal support interaction (SMSI), and/or geometric and/or electronic effects. ^b MxOy: TiO₂, ZrO₂, CeO₂, Al₂O₃, SiO₂. ^c TiO₂-MO₂: TiO₂-CeO₂ and TiO₂-ZrO₂.

.

As can be seen, a wide screening of heterogeneous catalysts containing noble or rare metals (e.g., Pd, Pt, Au, Rh, Ru, Ir, and Re based catalysts), as well as bimetallic systems have been studied. In general, the results obtained have revealed their excellent catalytic activity, stability, and resistance compared with Cu-based catalysts. In this regard, special attention has been paid to the effects of metal content exhibit on the activation degree for the hydrogen adsorption in the active centers, as well as the effect of support in metal dispersion and the resistance of the catalyst deactivation along the susceptive uses.

From the results depicted in Table 6, the In₂O₃ seems to be one of the best options as a support for noble metals, achieving CO₂ conversion between 10–20% and selectivity values up to 100% with Pt, Pd, and Ni as metals.

On the other hand, metal sintering, as well as the effect of different synthesis methods in the development of different morphological and/or metal-support interaction effects have also been considered, as well as the Single-Atom Catalysts (SACs) technique, which promotes atomically distributed active metal sites on the support surfaces. In this way, SACs provide great advantages in minimizing the usage of precious metals with a 100% atom-utilization efficiency, which thus results in improved catalytic reactivity [268].

It is noteworthy that bimetallic catalysts are best suited for methanol hydrogenation in comparison to their monometallic counterparts. Herein, a summary regarding the advances of the bimetallic catalysts (Ni, Cu, Pd, Rh, Ru, Zn, and In-based bimetallic systems) for methanol production in recent years, as well as the different strategies to enhance the catalytic activity, including regulating the active species, nanoparticle size, and catalyst support, have been included, Table 7.

3.1.3. CO₂ Hydrogenation to Methanol over Mixed Oxide-Based Catalysts

Despite the fact that catalysts based on the use of supported Cu [269] or noble metals [270] have demonstrated their ability to produce methanol as a product of CO₂ hydrogenation, the investigation of simple metal oxides or diverse metal oxides mixtures (MOs) to obtain heterogeneous systems with enough catalytic activity, selectivity, and durability for the synthesis of methanol from CO₂ hydrogenation is currently an important line of research, collecting a large number of publications in recent years. The most interesting results are summarized in Table 8. In this regard, a rational design of Mos has been proposed, i.e., the general optimization framework followed to fine-tune non-precious metal oxide sites and their surrounding environment through appropriate synthetic and promotional or modification routes, as shown in Figure 11 [271].

The multiple studies collected in Table 8 show that there are fundamentally two options that meet expectations. On the one hand, results obtained using binary metal oxides ZnO-ZrO₂ and, and on the other hand, heterogeneous catalysts including In₂O₃, either alone or in various hybrid mixtures. In this sense, some research is currently directed to the optimization of the ZnO-ZrO₂ mixtures, evaluating parameters that implement the highly selective production of CH₃OH [272,273], whereas the other line intends the evaluation of the promoting effect of incorporating Ga or small amounts of Cu into ZnZrOx solid solutions [274,275].

Table 8. A comparative summary of different active metal oxides catalysts studied in the carbon dioxide hydrogenation process to obtain methanol.

Metal Oxides	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm.)	Ref.
NiO/In2O3	1–3	50–60	250	30	[276] a
In2O3	1–10	45–95	240–330	30	[277] b
InOx/ZrO2	0.5–2.5	70−80	250–300	50	[278] a
ZnO/ZrO2	10	86–91	315–320	50	[279] c
ZrO2/In2O3	0.4–5.0	85	220–300	50	[280] a
GaxIn2−xO3	7–35	0.5–35	320–400	30	[281] a
In2O3-ZrO2	3–11	53–91	255–300	40	[282]
MaZrOx d	4.3–12.4	80	250–300	50	[283]
GaZnZrOx	7.7–8.8	86–88	320	50	[274]
In2O3/ZrO2	e	e	270–310	30–55	[284]
In2O3	17	92.4	300	50	[285]
In2O3/Support f	0.1–6.0	5–40	220–300	1.0	[286] a
In2O3/Support g	1–20	5–51	260–360	30	[287]
ZnO/ZrO2	9.2	50–95	320	30	[288]
MnOx/Co3O4	3–57	2–22	250	10	[289]
GaxIn2−xO3	7–38	---	320–400	30	[281]
ZnO/ZrO2	10	10–85	320	50	[272]
Co3O4/In2O3	10	30–70	300	40	[290]
ZnZrOx h	1–18	30–90	200–360	45	[291]
In2O3/ZrO2	3–8	65–90	300	50	[292]
CoxOy/MgO	7–35	8–30	i	1.0	[293]
InNi3C0.5/ZrO2	25.7	90.2	325	60	[294] a
In2O3/ZrO2	5–30	---	320–400	20	[295]
ZrZnOx/zeolite	1–8	5–30	400	30	[273]
In2O3/GO j	1–14	5–100	200–450	30	[296]
In2O3	4–18	20–85	260–360	40	[297]

^a Special attention is paid to the existence of strong metal support interaction (SMSI) and/or geometric and/or electronic effects. ^b A phase-mixing strategy is used in the synthesis of catalysts. ^c The ability of the catalysts for their reuse is determined. ^d (Ma = Cd, Ga). ^e Results are expressed in terms of methanol space-time yield. ^f Supports ZrO₂ and CeO₂. ^g Supports MnO and MgO. ^h Promoters, small amounts (<2%) of Cu, Pd, or Pt. ⁱ Non thermal plasma-catalysis DBD reactor. ^j Graphene oxide, GO.

Table 8. A comparative summary of different active metal oxides catalysts studied in the carbon dioxide hydrogenation process to obtain methanol.

Metal Oxides	Conversion (%)	SCH3OH (%)	T (°C)	P (Atm.)	Ref.
NiO/In2O3	1–3	50–60	250	30	[276] a
In2O3	1–10	45–95	240–330	30	[277] b
InOx/ZrO2	0.5–2.5	70−80	250–300	50	[278] a
ZnO/ZrO2	10	86–91	315–320	50	[279] c
ZrO2/In2O3	0.4–5.0	85	220–300	50	[280] a
GaxIn2−xO3	7–35	0.5–35	320–400	30	[281] a
In2O3-ZrO2	3–11	53–91	255–300	40	[282]
MaZrOx d	4.3–12.4	80	250–300	50	[283]
GaZnZrOx	7.7–8.8	86–88	320	50	[274]
In2O3/ZrO2	e	e	270–310	30–55	[284]
In2O3	17	92.4	300	50	[285]
In2O3/Support f	0.1–6.0	5–40	220–300	1.0	[286] a
In2O3/Support g	1–20	5–51	260–360	30	[287]
ZnO/ZrO2	9.2	50–95	320	30	[288]
MnOx/Co3O4	3–57	2–22	250	10	[289]
GaxIn2−xO3	7–38	---	320–400	30	[281]
ZnO/ZrO2	10	10–85	320	50	[272]
Co3O4/In2O3	10	30–70	300	40	[290]
ZnZrOx h	1–18	30–90	200–360	45	[291]
In2O3/ZrO2	3–8	65–90	300	50	[292]
CoxOy/MgO	7–35	8–30	i	1.0	[293]
InNi3C0.5/ZrO2	25.7	90.2	325	60	[294] a
In2O3/ZrO2	5–30	---	320–400	20	[295]
ZrZnOx/zeolite	1–8	5–30	400	30	[273]
In2O3/GO j	1–14	5–100	200–450	30	[296]
In2O3	4–18	20–85	260–360	40	[297]

^a Special attention is paid to the existence of strong metal support interaction (SMSI) and/or geometric and/or electronic effects. ^b A phase-mixing strategy is used in the synthesis of catalysts. ^c The ability of the catalysts for their reuse is determined. ^d (Ma = Cd, Ga). ^e Results are expressed in terms of methanol space-time yield. ^f Supports ZrO₂ and CeO₂. ^g Supports MnO and MgO. ^h Promoters, small amounts (<2%) of Cu, Pd, or Pt. ⁱ Non thermal plasma-catalysis DBD reactor. ^j Graphene oxide, GO.

Besides, it has also been verified that several mixed systems with oxides [287,296], including ZrO₂, or transition metals, such as Co, Ni, Sn, Pd [298,299], or CuO [300]. Moreover, PdZn alloy catalysts supported on ZnFe composite oxides [301] or molybdenum phosphide catalysts have also been studied, attaining very promising results [302].

Nevertheless, from the different options covered to date, indium oxide-based catalysts are attracting the highest interest due to their excellent selectivity to methanol and high activity for CO₂ conversion. Therefore, most of the new high-performance catalysts are described over ternary Cu-based catalysts with several promotor compounds, including In, Ce, Zn, or Zr [303,304].

3.1.4. Methanol Reaction Process for CO₂ Hydrogenation to Fuels and Chemicals

By coupling two successive reactions using a bifunctional catalyst, the hydrogenation of CO₂ to methanol can be applied to obtain C2+ compounds, including dimethyl ether (DME), light olefins, and gasoline-type hydrocarbons [59]. Thus, after the conversion of CO₂ and H₂ to CH₃OH on the surface of a suitable catalyst, the methanol is dehydrated or coupled on zeolites, alumina, or some other suitable acid-base catalyst, according to the scheme shown in Figure 12. Consequently, the synthesis of products with two or more carbons (C2+) from CO₂ hydrogenation can be achieved by first converting of CO₂ to carbon monoxide or methanol and then conducting a C–C or C–O coupling reaction with a bifunctional or hybrid catalyst [305].

In this respect, dimethyl ether (DME) is a versatile raw material and an interesting alternative fuel that can be produced directly by catalytic hydrogenation of CO₂ [105,306]. Therefore, this process is considered a potential vector to contribute to the CO₂ reduction because of its lower operating costs compared to the classic two-step synthesis of DME, CO, and hydrogen. Figure 13 shows a general scheme of the DME formation. In recent years, a great number of studies have been carried out with the aim of finding a good catalyst for the production of DME from syngas. However, multiple investigations are currently comparing direct CO₂-to-DME to bifunctional/hybrid catalytic systems.

Table 9. Comparative summary of different bifunctional/hybrid catalytic systems for improving the direct conversion of CO₂ to DME.

Metal Catalyst	Acid Catalyst	Conversion (%)	SCO (%)	SCH3OH (%)	SDME (%)	T (°C)	P (Atm.)	Ref.
CZZA a	HZSM-5	25–28	20–80	5–7	10–70	220–280	27.6	[185]
CuO/ZnO/ZrO2	Zeolite	2–20	15–20	5–12	1–30	260	30	[192]
CuO/ZnO/Al2O3	SiO2-Al2O3	5–9	49–65	11–17	22–35	260	30	[307]
Cu-BTC MOF b	Al2O3	2–26	15–25	5–50	14–90	260	30	[308]
CuO/ZnO/ZrO2	Zr(SO4)2	14–17	60–80	7–20	14–28	260	20	[309]
CuZnAlZrCe	ZSM-5	13–19	59–63	9–11	26–33	250	30	[310]
In2O3	HNT c	1–5	0.0	20–80	10–70	200–300	10–40	[311]
CZA/HPW d	TiO2	5–22	14–17	5–99	7–59	250	30	[306]
CuZnAlSi/Sn	------	10	50–60	5–9	80–85	280	40	[312]
CuO/ZnO/ZrO2	SAPO-11	40–50	0.0	10–50	50–90	250–325	10–50	[313]
CZA e	HZSM-5	25	25–28	7.0	65–70	220–280	21–42	[314]
CuO/ZnO	HZSM-5	15–35	10–30	5–88	10–80	200–260	15–20	[315]
CuZnOZrO2	WOx/Al2O3	10–20	64–69	8–17	15–28	300	20	[316]
Cu/ZnO/MOx f	SAPO-34	5–20	50–90	19–24	25–31	200–260	10	[317]
Cu/ZnO/ZrO2	HZSM-5	1–11	9–90	6–18	5–75	200–330	30	[318]
GaZrOx	------	1–9	10–88	10–100	10–25	240–380	30	[319]
CuO/ZnO/Al2O3	SAPO-18	1–8	5–8	3–15	85–90	250–350	20–40	[320]
CuO/ZnO/Al2O3	MCM-41-TPA g	2–7	23–65	18–50	18–25	220–250	45	[321]
Cu/ZnO/ZrO2	ZSM5	8–11	15–49	24–26	38–58	240	30	[322]
nano-Pd/In2O3	H-ZSM-5	6–11	40–45	17–19	36–42	280–300	30	[323]
Gallium nitride		1–25	40–82	18–42	0–80	300–450	20	[324]

^a CZZA: CuO/ZnO/ZrO₂/Al₂O₃. ^b Cu-BTC MOF: Cu-1,3,5-benzenetricarboxylate metal–organic framework (Cu-BTC MOF). ^c HNT: natural clay halloysite nanotubes, and HNT modified with Al-MCM-41 silica arrays. ^d CZA-HPW: Cu/ZnO/Al₂O₃-H₃PW₁₂O₄₀. ^e CZA: CuO/ZnO/Al₂O₃. ^f MO_x: Al₂O₃, CeO₂, or ZrO₂. ^g TPA: tungstophosphoric acid.

collects a comparative summary of the different bifunctional/hybrid catalytic systems recently studied in the carbon dioxide hydrogenation process to obtain DME.

In summary, it can be said that DME is currently considered a firm candidate for its application in the circular process of capturing and using CO₂, not only to carry out an effective mitigation of environmental problems [325], but also to contribute to obtaining chemical products of interest to society [326].

3.2. One-Step Process for the Conversion of CO₂ to Light Olefins

Light olefins such as ethylene, propylene, and butylene are currently among the top petrochemicals and fuels produced. These olefins are used to produce a wide variety of polymers, plastics, solvents, and cosmetics. Moreover, light olefins can be oligomerized into long-chain hydrocarbons that can be used as fuels, making them a desirable product with high potential. Thus, their production from CO₂ hydrogenation can contribute to a great extent to the elimination of CO₂ emissions. Nowadays, there are mainly two methods for the synthesis of light olefins from captured CO₂. The first one is the modified Fischer-Tropsch synthesis (FTS), where carbon monoxide is obtained by the reverse water gas shift (RWGS) reaction in a first step and, in a second step; CO is hydrogenated to lower hydrocarbons (HCs) [327,328]. On the other hand, the production of light olefins can be obtained by a different two-step process, usually called the methanol to olefins process, consisting of the hydrogenation of CO₂ into methanol and subsequently a dehydration-condensation process, as shown in Figure 14. These pathways will be discussed in more detail in the following subsections.

3.2.1. CO₂ Hydrogenation in a One-Step Process over Bifunctional or Hybrid Catalysts

At present, the production and marketing of low molecular weight olefins is already being carried out through the MTO process, which has high selectivity values for C2–C4 olefins [329], since this process, along with the methanol-to-gasoline (MTG) process, are technological discoveries in the synfuels arena, first introduced by Mobil Oil Corporation [330]. However, although considerable progress has been made in the hydrogenation of carbon dioxide to various C1 chemicals, it is still a great challenge to synthesize value-added products with two or more carbons directly from CO₂, given the technical and economic interest of the process. In this regard, a great number of investigations have been carried out. Most of these studies aimed at evaluating the experimental conditions and/or the bifunctional catalysts able to couple two successive reactions, the hydrogenation of CO₂ to methanol, followed by its dehydration or coupling on zeolites, alumina, or some other suitable acid-base catalyst. A comparative summary of the different bifunctional/hybrid catalysts recently studied in the MTO process with high selectivity for light olefins is collected in

Table 10. Comparative summary of different bifunctional/hybrid catalytic systems for the MTO process for improving the direct conversion of CO₂ to light olefins.

Metal Catalyst	Acid Catalyst	Conversion (%)	SCO (%)	SC2-C4 (%)	SC5 (%)	T (°C)	P (Atm)	Ref.
In2O3	HZSM-5	12–15	45–50	20–25	79	340	30	[331]
Cu/CeO2	SAPO-34	4–20	30–75	30–65	4–9	300–500	20	[332]
ZnZrOx	Zeolites a	18–24	-----	1.5–2.8	2–9	325–400	10	[333]
InCo	Zn-zeolite beta	8.0	6	8	85	300	50	[334]
In2O3/ZrO2	SAPO-34	17–26	64–70	65–82	2–5	380	30	[335]
ZnO/ZrO2	SAPO-34	42–45 b	4–22	76–85	2–3	375	15	[336]
ZnGaOx spinel	SAPO-34	7–50	-----	15–76	5–7	400	40	[337]
In2O3/ZrO2	SAPO-34	15–21 c	-----	3–6	----	400	30	[338]
In2O3/ZrO2	SAPO-34	23–25	2–6	2–7	----	400	30	[339]
In-Zr	SAPO-34	35	-----	93	----	400	30	[340]
Mn2O3-ZnO	SAPO-34	9–30	50–91	86–92	3–13	380	30	[341]
Fe/Co	K-Al2O3	37–42	12–16	67	17–21	320	20	[342]
Fe5C2	Zeolite d	8–56	1–49	12–93 e	1–56	300	10	[343]
In2O3	SAPO-34	18–35	18–37	16–34	----	340–400	10–25	[344]
ZnZrOx	SAPO-34	9–14	40–43	82–83	----	380	30	[345]
In2O3/ZrO2	SAPO–34	29–38	45–90	68–85	3–5	400	10–30	[331]
ZnZrO	SAPO-34	10–15	-----	80.0	1–3	330–380	20	[346]
InCeOx/InCrOx	SAPO-34	5–20	15–60	70–90	3–7	300–350	10–35	[347]
CuZnZr(CZZ)	SAPO-34	10–20	57–86	70–88	0.5–5	400	20	[348]
NiCu/CeO2	SAPO-34	12–20	55–85	62–79	2–4	350–450	20	[349]
ZnO/Y2O3	SAPO-34	6–28	75–97	90–94	1–5	390	40	[350]
ZrS/Fe2O3@KO2	SAPO-34	46–48	24–27	42–55	25–38	375	30	[351]
10K13Fe2Co100Zr	Polymetallic fibers	10–48	-----	70–80	----	400	30	[352]
ZnO/ZrO2	MnSAPO-34 f	15–21	----- g	90–99	0.4–7.6	380	20	[353]
GaZrOx	SAPO-34	5–12	50–60	92–95	1–3	370–410	30	[354]
CuO/ZnO/Al2O3	SAPO-34	50–56	4–10	50–56	---- h	250–450	30	[355]
In2O3	SAPO-34 i	27–51	3–75	50–92	5–20	360	25	[356]
CuO/ZnO	kaolin/SAPO-34	33–58	7–10	78–81	---- j	400	30	[357]
Y2O3/Fe/Co	SAPO-34	7–18	31–35	75–85	1–3	300–400	10–25	[358]
FeZnK	SAPO-34	42–50	14–20	54–61	8–25	280–360	15	[359]
FeNa	Supports k	19–33	10–60	17–73	----	320	20	[360]

^a Zeolites and silicoaluminophosphates with different topologies, MOR, FER, MFI, BEA, CHA, and ERI. ^b CH₄ selectivity, 2–9%. ^c oxygenates (MeOH and DME): 0.0–0.5%. ^d Containing K, Ce or La. ^e CH₄ Conversion 7–86%. ^f Polymetallic fibers. ^g CH₄: 2.4–8.6. ^h CH₄: 15–18. ⁱ With Fe-Co/K-Al₂O₃ as composite. ^j CH₄: 11–112. ^k SiO₂, Al₂O₃, ZrO₂ and CNT (multi-walled carbon nanotube).

.

According to recent research developed, in this tandem catalytic process, methanol is obtained as the product of CO₂ hydrogenation in this tandem catalytic process by using various metal oxides; however, in the second acid-catalyzed C–C coupling reactions, zeolites SAPO-34 are the main catalysts used. In this respect, the acidity and pore structure of the zeolites seem to be decisive factors in obtaining this coupling process among silicoaluminophosphate (SAPO) zeotype materials. Current research seems to show that SAPO-34 is the best acidic catalyst for obtaining C2–C4 olefins and is superior to other catalysts such as ZSM-5 or SSZ-13 [361,362,363].

On the other hand, they are also being evaluated with promising results, including the use of Fe-based catalysts promoted with K, Na, Mn, Zn, and Ce to increase lower olefin selectivity, owing to their enhanced CO₂ adsorption ability and facilitation of the formation and stability of active species Fe₅C₂. Furthermore, the favorable effect of Fe-Co bimetallic systems on the formation of C2+ hydrocarbons in these supported catalysts has been demonstrated [364]. Thus, the combination of these various factors—the application of Fe catalysts supported on different solids, activated by metals such as Co and alkali metals—constitute promising lines of research for obtaining light olefins from the hydrogenation of CO₂ [365]. Finally, using this tandem technique of hydrogenation of CO₂ in the presence of supports of an acid nature, attempts are also being made to obtain various aromatic compounds. Thus, the use of a series of metal oxides (In₂O₃, Cu-Zn-Al, and ZnZr_xO) with different spherical HZSM-5 zeolites has been investigated to obtain the direct conversion of CO₂ to aromatics [366].

3.2.2. Modified Fischer-Tropsch Synthesis Route

The modified Fisher-Tropsch synthesis route has a clear controlling factor, determined by the RWGS reaction, since it exhibits an endothermic and reversible character, which limits the CO₂ conversion to CO at values around 20% [367]. However, the FTS process, on the other hand, is a well known route for the transformation of syngas (CO + H₂) into C2+ hydrocarbons, that proceeds on catalyst surfaces through the following steps: (1) adsorption and dissociation of CO and H₂; (2) formation of CH_x (x = 0–3) species on catalyst surface; (3) C–C bond formation through coupling of CHx species, that leading to chain growth and surface CnHm intermediates or CH₄ by the hydrogenation of CH_x species; (4) dehydrogenation or hydrogenation of C_nH_m into olefins or paraffins compounds [368].

Fe, Co, and Ru metals are conventionally employed as active catalyst components owing to their capabilities in both CO dissociation and C–C coupling or chain growth [369]. However, the C–C coupling is uncontrollable on these metal surfaces, leading to a statistical distribution of products, i.e., the Anderson-Schulz-Flory (ASF) distribution. Consequently, one of the main objectives of the research on these catalytic systems is finding supports, either metal or bimetallic. In this sense, it has been shown that Ni-Fe catalysts improved selectivity towards CO without significantly compromising FTS process activity, coupling the high activity of Ni catalysts with the high CO selectivity of Fe [370]. Similarly, a large number of studies have recently evaluated the behavior of different supports, different metals, and different operating conditions in the CO₂-FT process, as collected in several reviews [328,371,372]. Besides, a comparative summary of the different FTS catalysts recently studied are collected in

Table 11. Comparative summary of different catalytic systems for FTS process activity improving the direct conversion of CO₂ to light olefins.

Metal Catalysts	Alkali Metal	Support	Conversion (%)	SCO (%)	SCH4 (%)	SC2-C5 (%)	T (°C)	P (Atm)	Ref.
Fe	---	Carbon	14–52	5–49	3–8	5–38	300	25	[373]
Co	---	SAPO-34	----	64–74	24–28	65–70	220	20	[374]
Fe3O4/Mn	Na	-----	22–30	14–32	12–36	64–88	320	5.0	[375]
Co	K	Al2O3	15–97	1–34	2–33	2–57	200–350	1–50	[376]
Fe5C2	---	-----	41–50	3–10	20–46	51–70	320	30	[377]
Fe/Co	K	-----	32–58	2–10	8–36	62–82	300	25	[378]
Fe/Mn	K	-----	38.2	5.6	10.4	22.3	300	10	[379]
Co/Mn	Na	SiO2	45–47	18–20	2.0	52–54	260–270	50	[380]
Fe/Co (Ru)	K	-----	30–57	2–16	7–30	54–84	450	2.0	[381]
Co3O4/MnO2	---	-----	42–48	2–39	4–23	90–96	270	1.0	[382]
Co/Pt	---	ZSM-5	10–28	----	52–100	10–48	200–500	1–30	[383]
Fe	Na	ZSM-5	18–22	28–32	22–41	30–54	450	20	[384]
Fe/C	K	X-ZSM-5 a	34–36	18–20	10–15	85–89	320	20	[385]
CuFeO2	---	----	13–18	28–32	1–60	40–95	300	10	[386]
Cu/Fe	---	Al2O3	35–42	23–42	28–38	51–91	300–400	30	[387]
Fe		SMC b	8–45	16–86	5–11	70–89	260	10	[388]
Ru/Ni(NPs) c	----		2–30	0–47	1–100	7–76	150	2–8.5	[389]
Raney-Fe, Fe	----	SiO2	4–12	14–27	5–22	22–78	220–265	20	[390]
Fe/Ti d	K	----	30–35	38–85	23–25	72–75	320	20	[391]
FeMn	---	HZSM-5	28–40	64–68	-----	58–69	280	10	[392]
RuCl3/Ru	---	----	----	0–85	14–100	17.5	180	50	[393]
Co, CoO, Co3O4	----	SixAlyOz	3–35	0–12	6–31	15–93	220–260	20	[394]
Fe3O4	----	SiO2	5–7	2–5	57–93	7–43	220	1.0	[395]
Fe3O4/FexCy	Na		36–46	8–11	36–60	16–52	320	20	[396]
Fe3O4/FeCx		Mesop. C	15–54	5–31	13–75	25–87	320	30	[397]
Co/Ce/La	----	Al2O3	13	49–52	15	99–100	230	20	[398]
Co@CoOx/Co2C-Mn	Na		1–62	1–92	45–70	29–54	230–310	40	[399]
Fe-/Co	----	SiO2	4–20	11–13	26–45	43–70	260–280	20	[400]
FeCo X,(X: La, Mn, Zn)	K	Al2O3	65–100	30–38	0–58	44–100	300	10	[401]
Co	----	C/SiO2 e	2–8	6–14	21–32	62–70	250	5	[402]
Co6/MnOx	----	-------	15	0–0.7	-----	0–99	200	8	[403]
Fe2O3	K	Al2O3	40–47	19–33	20–31	40–50	400	30	[404]
Fe-Co	K	Al2O3	37–49	9–29	14–23	58–68	320–360	20–30	[405]
Fe-Cu	K	-----	24–41	6–16	5–10	79–88	250–340	20	[406]
X-Fe5C2/ZnO	Na	-----	2–28	15–36	10–16	68–89	280–370	25	[407]
Ni		MgAl2O4	6–70	2–96	3–98	----	330–400	1.0	[408]
FeAlOx	Na	HZSM-5/SiO2	29–48	8–18	10–35	47–88 f	335–400	35	[409]
Fe-Zn	K	SAPO	43–48	14–18	15–40	36–57	320	15	[359]
Fe-Zn	Na	----	15–39	14–30	12–48	52–88	340	25	[410]
ZnCoxFe2-xO4		SiO2	24–52	6–16	16–21	36.1	260–340	25	[275]
Fe (Cu, Mn, V, Zn, Co)	K	Al2O3	29–40	10–20	15–22	65–74	340	20	[411]

^a X: K⁺, Na⁺, Cu²⁺, Mn²⁺, Mg²⁺, Ce²⁺, La³⁺, or Cs²⁺. ^b spherical mesoporous carbon: (SMC). ^c nanoparticles (2–3 nm), in a hydrophobic ionic liquid (IL). ^d K–Fe–Ti layered metal oxides (LMO). ^e oxygenates, including alcohols and aldehydes Sel.(%): 2–8. ^f Selectivity to aromatics: 7–30.

. Obviously, this table expresses very summarized values of several selected parameters, obtained from very extensive studies addressing different goals but aiming to carry out an approximate comparison between the different catalysts currently evaluated in FTS reactions.

As can be seen, the majority of catalysts investigated in the two consecutive processes for CO₂ Fischer-Tropsch synthesis (CO₂–FTS) contain metallic Fe as the active species, enhanced with different inorganic supports, other transition metals, and/or alkali metals. Therefore, these are similar catalysts to those employed in the last few years to obtain olefins from syngas. The main handicap is that these catalysts also work for the water-gas-shift reaction (WGSR), producing large amounts of CO₂ as areaction product [412,413,414]. Some recent research using different catalysts as well as different kinds of feedstocks (coal, biomass, methane via reforming, and nonconventional energy sources) to obtain the syn-gas (CO and H₂) is shown in

Table 12. Comparative summary of different catalytic systems for Fischer-Tropsch (FTS) improving the direct conversion of the water-gas-shift reaction (WGSR) to light olefins.

Metal Catalysts	Alkali Metal	Support	Conversion (%)	SCO2 (%)	SCH4 (%)	SC2-C5 (%)	T (°C)	P (Atm)	Ref.
α-Fe2O3		SiO2, Al2O3	18–65	25–36	16–19	81–94	280	10	[415]
Fe-Mn, Cu		SiO2	75–96	23–45	15–19	80–85	200	20	[416]
CoMnAlOx a	----	SiO2	5–14	9–48	2–24	45–85 b	260	10	[417]
Co-Re, Pt-ZSM-5	----	Al2O3	5–75	----	-----	24–43 c	225–255	20–30	[418]
Fe	Na	ZSM-5	24–87	26–42	16–41	57–85	300	10	[419]
Co, Re	----	Al2O3, CNT d	2–4	----	42–56	44–58	210	1.9	[420]
Fe-Zn	Na	Zeolites e	47–44	88–95	11–16	75–80	360	1.0	[421]
CoO-Co		SiO2, TiO2, Al2O3	24–75	----	----	45–83	210	20	[422]
Fe-Cu	K		65–90	16	19–37	63–81	340	15	[423]
Fe1Zn1.2Ox	Na		38–95	31–37	15–19	85–87	340	20	[424]
Fe, Fe3C		Carbon	80–90	10–14	7–9	88–90	250–350	34–85	[425]
Fe	K	Al2O3	7–90	18–70	7–8	12–74	300–420	20	[426]

^a Composite oxides. ^b Oxygenates, including alcohols and aldehydes Sel.(%): 6–14. ^c Selectivity C₁₀-C₂₀ (%). ^d γ-alumina, α-alumina and carbon nanotube (CNT). ^e Zeolites: HY, NaY, ZSM-5, SAPO-34, Hβ, Liβ, Naβ, Kβ, and Rbβ.

.

Therefore, from the conventional Fischer-Tropsch reaction, it is possible to access catalytic systems that could be tested in modified Fischer-Tropsch processes capable of using CO₂ as a raw material to access light olefins (C2-C4) widely used in different fields such as the synthesis of polymers and pharmaceutical intermediates. However, the chain length distributions are given by the Anderson-Flory (ASF) distribution, which limit the C2-C4 range to less than 58% [427]. Finally, it is possible to integrate waste CO₂ in synthesis using Fe-based Fischer-Tropsch with green H₂ as well as olefin oligomerization, thereby increasing the production of value-added liquid hydrocarbons [428].

4. Concluding Remarks, Challenges, and Research Outlook

The use of CO₂ as a raw material for the production of various chemicals via catalytic hydrogenation is currently a necessary eventuality, not only because global warming is a risk, but also, and more importantly, because there is a real possibility of technologically accessing sufficient amounts of green hydrogen at an affordable, economical cost. In this respect, the use of hydrogen as an energy vector, not only in a significant number of heavy industries but also in transportation fuels, is expected to decisively contribute to meeting decarbonization goals to achieve net zero emissions in the next two decades. However, these objectives do not only mean to address efficient hydrogen production but also its trustworthy transportation and storage. For this purpose, it is currently considered that the use of different liquid organic hydrogen carriers (LOHCs) is a valuable solution to making available a reliable and on-demand hydrogen supply. However, green hydrogen can also be stored and transported as a ‘green’ feedstock for the synthesis of biofuels and several fine chemicals.

Therefore, the production of green hydrogen via electrolysis and its storage and transportation using some hydrogen carriers such as ammonia or methanol must be considered as part of sustainable chemical and biofuel manufacturing. Thus, the power-to-ammonia concept allows producing ammonia by the Haber–Bosch process, the currently second most produced industrial chemical, from air, water, and (renewable) electricity. Besides, methanol synthesis, with a global production capacity of around 85 million metric tons per year, which is expected to rise in the coming years, can be obtained by catalytic CO₂ hydrogenation. In this regard, methanol is one of the most important industrial chemicals, serving as a feedstock for a wide range of chemical products. Besides, it is also being used increasingly as a fuel additive and as a transportation fuel alternative. This assumption is confirmed by the high number of investigations carried out in recent decades. As can be seen in Figure 15, the production of methanol from CO₂ hydrogenation is the option most investigated, followed by CO₂ methanation.

The primary industrial process relevant to the current scenario, developed to reduce global warming, is CO₂ to methanol conversion. However, extensive commercialization of green methanol from CO₂ hydrogenation is still seriously limited by its economic viability due to various factors. These include the difficulty in accessing renewable H₂ and sources of CO₂ recovered from industrial processes, in enough quantity and purity. In addition, it must be added to these factors that the current low price of methanol, due to the low price of natural gas, has been used until now for its industrial production. Despite this, in the last decade there has been widespread industrial interest in the development of technologies in this field, probably encouraged by the increasing implementation of legal regulations on fossil fuels to mitigate climate change and the general introduction of a strict carbon tax.

Furthermore, the actual introduction of Renew Energy technologies in many countries have made solar and wind the cheapest sources of energy in many parts of the world. This has not only caused the rapid decarbonization of the electricity sector but also opened the possibility of obtaining several chemicals by CO₂ hydrogenation via electrolysis.

This new scenario allows us to consider that an exemplary carbon capture and utilization cycle based on mature technologies can meet the energy requirements of the “industrial carbon cycle”, an emerging paradigm in which industrial CO₂ emissions are captured and reprocessed into chemicals and E-fuels. In this context, methanol would come to occupy a central role as a platform molecule from which most chemical commodities could be obtained (Figure 16), partially replacing the ethanol role granted by the paradigm associated with so-called green chemistry, which is primarily based on biomass feedstocks.

At the very least, the massive use of anthropogenic CO₂ would free up huge amounts of agricultural land that, in the paradigm of green chemistry, should be used for crops destined for industrial uses. For this reason, the application of CO₂ as a raw material for obtaining methanol and other chemicals could constitute the main chemical reaction to be developed in the 21st century, similar to what happened in the past 20th century with the catalytic hydrogenation reaction of nitrogen gas for the production of ammonia by the Haber–Bosch process.

Therefore, based on the existing investigations, it can be concluded that there are two priority directions that should be followed in the immediate future. On the one hand, the performance of the electrolysis processes to obtain green hydrogen must be increased as much as possible. On the other hand, implement the processes for catalytic hydrogenation of CO₂ to obtain green methanol. In this sense, it would be necessary to consider the development of more efficient heterogeneous catalysts, both in the yield obtained and in their behavior over successive uses. Likewise, it is a priority to try to obtain catalytic systems that are as economical as possible, through the use of non-noble metals, in order to obtain, in a viable technical and economic way, green methanol, which would be the platform molecule on which, in the present century, the fine chemistry will foreseeably rest.