CO₂ to Methanol Conversion: A Bibliometric Analysis with Insights into Reaction Mechanisms, and Recent Advances in Catalytic Conversion

Abstract

The rising levels of atmospheric carbon dioxide (CO₂) necessitate urgent and effective strategies for its capture and utilization. Among the various CO₂ valorization pathways, the conversion of CO₂ into methanol has gained considerable attention due to its dual role in reducing greenhouse gas emissions and serving as a renewable fuel and chemical feedstock. This review uniquely combines bibliometric analysis of 13,289 peer-reviewed publications (2012–2023) with an evaluation of Cu-based catalyst advancements, addressing critical gaps in the literature. A bibliometric analysis highlights the key trends, collaborations, and research gaps in the field. Among the catalytic systems, noble metals, though highly active, are uneconomical for large-scale applications, while non-noble metals, such as nickel, exhibit limited activity due to undesired reaction pathways. In comparison, Cu-based catalysts overcome these challenges by offering a balance of activity, selectivity, and cost-effectiveness. Special emphasis is placed on the CO₂ to methanol conversion pathways, with insights into thermodynamic constraints, emerging solutions, and potential directions for future research. By consolidating the current state of knowledge, this review identifies significant opportunities for advancing CO₂ conversion technologies, particularly in methanol synthesis, positioning it as a promising strategy for sustainable carbon management and energy production.

1. Introduction

Climate change poses a formidable environmental challenge, principally driven by increased atmospheric concentrations of carbon dioxide (CO₂). This elevation in CO₂, primarily resulting from the combustion of fossil fuels, has seen levels rise from 381 ppm in 2006 to 423 ppm by 2024, with annual increases ranging from 1.5 to 3 ppm [1]. Addressing this issue demands sustainable approaches to mitigating CO₂ emissions while also advancing renewable fuel production. Among the strategies employed, Carbon Capture and Storage (CCS) offers a pathway by collecting and sequestering CO₂ through methods like direct air capture, saline aquifers, and depleted oil reservoirs. However, the scalability of CCS is limited by substantial spatial and financial demands. Alternatively, Carbon Capture and Utilization (CCU) proposes a solution by converting captured CO₂ into less harmful, value-added products [2,3].

In the quest for sustainable industrial solutions, the thermo-catalytic conversion of CO₂ has shown considerable promise. Specifically, CO₂ hydrogenation is a critical process where hydrogen (H₂), produced sustainably via the electrolysis of water using renewable energy sources such as wind and solar power, serves as a vital reagent [4,5]. This process can yield several crucial chemical products, including methane [2,6], alkanes [7], carbon monoxide [8], olefins [9], and alcohols [10]. Among these, methanol is particularly significant due to its extensive industrial demand exceeding 110 million tons annually. Methanol is utilized as a precursor in the production of formaldehyde, acetic acid, methyl tert-butyl ether (MTBE), and gasoline [11,12]. It also offers potential as a fuel alternative in modified diesel engines due to its high-octane number and is utilized in direct methanol fuel cells (DMFC) to generate electricity [13]. The conversion of CO₂ to methanol not only has environmental benefits but also presents economic advantages. This process is highly selective, generating fewer byproducts and operating under milder reaction conditions compared to traditional methods. It offers a sustainable solution for CO₂ recycling, supporting global efforts to reduce greenhouse gas emissions and address climate change. Methanol synthesized from CO₂ is a clean fuel and a versatile raw material for producing various chemicals, benefiting from an ideal stoichiometric ratio that enhances conversion efficiency. Furthermore, it requires less energy, avoids the need for high-pressure storage like hydrogen, and can be safely utilized as a fuel without requiring modifications to engines.

The transformation of CO₂ into methanol necessitates catalysts due to the molecule’s stability. Advances in catalyst development have significantly enhanced the efficiency of this conversion process. Various metal-based catalytic systems have been explored, including noble metals (e.g., Pd, Pt) and transition metals (e.g., Fe, Ni, Co). While noble metals demonstrate excellent catalytic activity and selectivity, their high cost and limited availability hinder widespread application [14]. Transition metal catalysts, on the other hand, are more abundant and cost-effective but often favor competing reactions such as methane or carbon monoxide formation, reducing methanol selectivity [1,15]. Additionally, these systems tend to lack the thermal stability and tunable active sites necessary for efficient CO₂ hydrogenation under industrially relevant conditions [16]. In contrast, Cu-based catalysts provide an optimal balance of cost, availability, and catalytic efficiency [17]. Their ability to generate and stabilize active Cu⁰/Cu⁺ sites facilitates the activation of CO₂ and selective methanol synthesis under milder conditions [18,19].

The scholarly work surrounding CO₂ hydrogenation to methanol has been growing annually, indicating an active field of research with substantial progress [17,19,20]. Although several reviews have explored the catalytic conversion of CO₂ to methanol, most focus on general overviews of catalytic systems, reaction mechanisms, and thermodynamic aspects [21,22,23,24]. However, they often lack detailed bibliometric analyses along with highlighting the emerging role of Cu-based catalysts in this field. Additionally, many existing studies do not comprehensively examine the recent technological advancements and trends driving progress in CO₂ to-methanol conversion. This review addresses these gaps by combining a bibliometric analysis of publication trends with an in-depth evaluation of thermodynamic and reaction pathways, with a particular focus on the recent developments in Cu-based catalysts. By doing so, this manuscript provides insights into both academic and industrial advancements in CO₂ conversion technologies. This assessment will critically explore these advancements and propose directions for future research. By consolidating existing knowledge, this review aims to illuminate pathways for future enhancements in CO₂ conversion technologies, positioning methanol synthesis as an effective strategy for sustainable carbon management and energy production.

2. Search Strategy and Bibliometric Evaluation Technique

The bibliometric analysis of CO₂ conversion to methanol was conducted using a robust search strategy to capture the breadth of relevant research publications. Data for this analysis was extracted from the Scopus database, a widely recognized platform for high-quality, peer-reviewed academic publications (See Figure 1). The initial data set comprised 14,517 documents, spanning multiple disciplines and types of publications. To ensure a focused and high-quality analysis, a systematic filtering process was employed, narrowing the data set to 13,289 relevant publications. This filtering excluded non-peer-reviewed content and other less relevant document types, ensuring the data set consisted of high-impact research outputs.

The search string used for data retrieval included combinations of key terms relevant to CO₂ conversion to methanol, such as “CO₂ methanol”, “carbon dioxide to methanol”, “CO₂ hydrogenation to methanol”, “CO₂ conversion to methanol”, and “carbon dioxide hydrogenation to methanol”. The search was limited to documents published between 2012 and 2023, ensuring that only the most recent research on CO₂ methanol conversion was included. Furthermore, the search was restricted to English language publications, although a few significant contributions in Chinese, Polish, Portuguese, and Russian were also considered to reflect the global nature of research in this field.

To further understand the thematic focus within the CO₂ to-methanol research community, a keyword analysis was conducted. A total of 9363 distinct keywords were identified across the selected articles, providing insights into the prevalent themes and areas of focus within the literature. To process and visualize the collected data, VOSviewer (version 1.6.20) was employed to construct co-authorship networks, co-citation analyses, and keyword co-occurrence maps. This software allowed for the identification of collaborative networks among researchers, institutions, and countries, while also illuminating the key research topics that have emerged over the past decade. Microsoft Excel was used for supplementary data management and statistical analysis, particularly in generating publication trends and other performance indicators.

This comprehensive bibliometric approach provides a detailed view of the global research landscape surrounding CO₂ conversion to methanol. The findings not only highlight the rapid growth of research activity in this field but also reveal key collaborations, influential contributors, and emerging research themes. These insights are invaluable for identifying research gaps, fostering collaboration, and guiding future research efforts in the pursuit of efficient and scalable CO₂ methanol conversion technologies.

2.1. Publication Trends and Top Research Fields

A comprehensive analysis of the research landscape on CO₂ conversion to methanol revealed a total of 14,517 documents indexed in the Scopus database. After applying filters to ensure data quality, a final selection of 13,289 documents was made for further analysis. These filtered results comprised 11,058 original research articles, 2584 review articles, 200 conference proceedings, and 803 books, indicating a robust and diverse body of work addressing various aspects of CO₂ to methanol conversion.

Most of the publications were in English, accounting for 14,135 documents, underscoring the dominance of English as the primary language for academic discourse in this field. There was also a notable, albeit smaller, contribution from non-English languages, with 59 publications in Chinese, seven each in Polish and Portuguese, and five in Russian. This diversity in language reflects the global interest in research on CO₂ conversion, though English language publications dominate the field.

Additionally, the publication was analyzed with the inclusion of the query term “Cu-Based Catalyst”. A total of 9361 papers were identified that featured Cu-based catalysts, representing a subset of the initial 13,283 publications. These included 2063 review articles and 7298 research articles.

2.1.1. Publication Trend Analysis

The trend of publications over time indicates a consistent and significant rise in research output, particularly in recent years. From 2013 to 2023, the number of published works has increased dramatically (Figure 2a). In 2013, the field saw only 219 publications, while in 2023, this number surged to 2833, a more than tenfold increase over a decade. This upward trend underscores the growing importance of CO₂ conversion to methanol in the context of global efforts to mitigate climate change and develop sustainable energy solutions.

There was a marked acceleration in research output after 2016, with the number of publications rising from 567 in 2016 to 2462 in 2022. Several factors likely contributed to this surge, including heightened global awareness of the need for carbon capture technologies and sustainable fuel-production methods. Moreover, the year 2020 saw 1623 publications, reflecting a continued growth in research activity despite global disruptions caused by the COVID-19 pandemic. The largest yearly increase in research output occurred between 2019 and 2020, where the number of publications rose from 1129 to 1623, an increase of approximately 44%. This spike is possibly attributable to increased investments in sustainable energy research and the ongoing development of catalytic processes and new technologies for CO₂ to methanol conversion. The upward trend continued post-pandemic, with 2078 articles published in 2021 and 2462 in 2022, reflecting a sustained interest in this research area.

Interestingly, a similar publication trend is observed specifically for Cu-based catalysts, as shown in Figure 2b. From 2013 to 2023, research activity in this area increased substantially, rising from 113 publications in 2013 to 2182 in 2023. This exponential growth demonstrates the intensifying global focus on Cu-based catalysts, particularly after 2020, likely due to their crucial role in advancing catalytic efficiency.

2.1.2. Research Areas

The field of CO₂ conversion to methanol spans a broad range of scientific disciplines, reflecting its inherently interdisciplinary nature. The distribution of research areas demonstrates the multifaceted approach required to address the technical and scientific challenges associated with converting CO₂ into value-added products like methanol.

Figure 3 exhibits that the most prominent research domain is Chemistry, with 7722 publications, representing the central role that chemical processes, catalysis, and reaction mechanisms play in CO₂ methanol conversion. Chemistry is essential for developing and optimizing catalytic systems, understanding reaction pathways, and improving efficiency in the conversion process. Closely following is Chemical Engineering, with 7073 publications. Chemical engineering research contributes significantly to the practical implementation and scaling of CO₂ to methanol conversion technologies. This includes the design of reactors, process intensification, and the integration of CO₂ conversion systems into existing industrial frameworks. The near parity in the number of publications between chemistry and chemical engineering highlights the close collaboration between these disciplines in tackling CO₂ conversion challenges.

The Energy sector, with 3258 publications, underscores the importance of this research in the broader context of renewable energy and sustainable fuel production. Research in this area focuses on developing energy-efficient processes, integrating renewable H₂ sources, and scaling up methanol production for use as an alternative fuel. Materials Science contributes 3102 publications, reflecting its critical role in the development of advanced catalysts and novel materials that enhance CO₂ conversion efficiency. The design and synthesis of materials with high surface area, stability, and catalytic activity are key to improving the conversion rates of CO₂ to methanol. Environmental Science, with 3013 publications, represents the growing recognition of CO₂ conversion as a crucial strategy for reducing greenhouse gas emissions and mitigating climate change. Research in this area focuses on the environmental impact of CO₂ utilization technologies and their potential role in circular carbon economies. Engineering more broadly, excluding chemical engineering, also contributes significantly, with 1913 publications. This research area includes mechanical and industrial engineering approaches to process optimization, system integration, and the practical deployment of CO₂ conversion technologies on a commercial scale. Physics and Astronomy contribute 1565 publications, primarily in theoretical studies, modeling, and simulations of reaction mechanisms at the atomic and molecular levels. Understanding the fundamental physical principles of CO₂ conversion is essential for improving reaction efficiency and developing new catalytic processes. The field of Biochemistry, Genetics, and Molecular Biology has 876 publications, highlighting the potential for biological approaches to CO₂ conversion, such as using enzymes or microorganisms for methanol production. These studies often explore the intersection of biocatalysis and chemical processes for CO₂ utilization. Other contributing fields include Multidisciplinary research (317 publications), which brings together insights from various scientific and engineering domains to address the complex challenges of CO₂ methanol conversion.

This diverse distribution of research areas demonstrates that the successful conversion of CO₂ to methanol requires collaboration across multiple scientific disciplines. Chemistry, chemical engineering, and energy research form the core of this effort, while contributions from materials science, environmental science, and engineering ensure that these technologies can be effectively scaled and integrated into broader energy systems.

2.2. Top Countries

A country-wise analysis of the research output in the field of CO₂ conversion to methanol provides insights into the global distribution of expertise and contributions (Figure 4a). The analysis reveals that research in this domain is concentrated in a few key regions, with several countries emerging as global leaders in the field.

China leads the world by a significant margin, with 6247 publications, making it the dominant force in CO₂ conversion to methanol research. This prominence reflects China’s significant investment in sustainable energy technologies and its emphasis on reducing greenhouse gas emissions through innovative chemical conversion processes. China’s research output in this area accounts for a substantial portion of the global literature, underscoring its pivotal role in advancing CO₂ utilization technologies.

The United States follows with 1657 publications, maintaining its position as a key player in this domain. The U.S. has a well-established research infrastructure and a history of innovation in chemical engineering and catalysis, which are critical to the development of CO₂ to methanol technologies. While the number of publications is considerably lower than China’s, the U.S. remains a major contributor to both fundamental and applied research in this field. India ranks third with 878 publications, highlighting the country’s growing focus on sustainable energy and CO₂-reduction technologies. India’s increasing research output is indicative of its rising prominence in global efforts to address climate change and develop alternative energy sources. Germany and Japan are also significant contributors, with 797 and 645 publications, respectively. Both countries are known for their advancements in chemical engineering, catalysis, and sustainable technologies, making them key players in the CO₂ conversion research landscape. Germany’s strong research focus on catalysis and renewable energy aligns well with the goals of CO₂ to methanol conversion, while Japan’s contributions reflect its innovation in material sciences and energy conversion processes. The United Kingdom follows closely with 624 publications, showcasing its contributions to both fundamental research and industrial applications of CO₂ conversion. South Korea (611 publications) and Spain (549 publications) have also established themselves as important contributors to this field, with research focusing on catalysis, renewable energy integration, and innovative conversion technologies. Rounding out the top 10 are Australia, with 426 publications, and Italy, with 423 publications. Both countries have increasingly invested in renewable energy research, with a particular focus on CO₂ utilization and the development of sustainable chemical processes.

Upon comparing the country-wise research outputs in CO₂ conversion to methanol and CO₂ conversion to methanol specified with Cu-based catalysts, a notable correlation emerges (see Figure 4b). China leads both, with 6247 publications in CO₂ to methanol research and 4774 in Cu-based catalyst studies, underscoring its significant advancements in CO₂ conversion as sustainable energy technologies. Similarly, the United States and India maintain prominent positions in both fields, reflecting their commitment to advancing catalytic processes for CO₂ utilization. This parallel trend suggests that nations prioritizing CO₂ conversion research also focus on developing Cu-based catalysts, highlighting the integral role of such catalysts in efficient CO₂-to-methanol conversion processes.

In total, these top 10 countries account for a significant majority of the global research output in CO₂ conversion to methanol, reflecting the strategic importance of this field in the context of global efforts to mitigate climate change. This analysis underscores the collaborative nature of this research, with multiple countries contributing to the development of scalable and efficient CO₂ utilization technologies.

2.3. Top Journals

To assess the influence and contribution of various journals to the field of CO₂ conversion to methanol, an in-depth journal analysis was conducted. A total of 10 prominent journals emerged as key platforms for publishing research in this area, as shown in Figure 5. The data reflects a broad distribution across multidisciplinary and specialized journals, highlighting the interdisciplinary nature of this research, which spans catalysis, energy technologies, and material sciences. The most prolific journals, ranked by the number of articles published, are presented below.

ACS Catalysis leads with 509 publications, positioning itself as the most significant contributor to research on CO₂-to-methanol conversion. As one of the premier journals in the field of catalysis, its focus on both theoretical and applied research makes it a preferred outlet for cutting-edge studies in this domain. Following closely is the Journal of CO₂ Utilization, with 469 publications, underscoring its pivotal role in disseminating research specifically targeting CO₂ conversion and valorization technologies. These journals collectively represent the forefront of innovation in catalysis and CO₂ utilization. Applied Catalysis B: Environmental ranks third, with 420 publications, highlighting its contribution to sustainable catalysis and environmental applications of CO₂ conversion. Chemical Engineering Journal and Catalysts follow, with 321 and 290 articles, respectively, reflecting their importance in both theoretical advancements and practical implementations of catalytic processes. Other notable contributors include Catalysis Science and Technology and the International Journal of Hydrogen Energy, each publishing 284 articles. Their focus spans across catalysis, energy conversion, and H₂ production, reflecting the growing interconnection between CO₂-reduction technologies and the H₂ economy. Fuel contributed 253 articles, reinforcing its relevance in the study of alternative fuels and energy systems. Meanwhile, Angewandte Chemie International Edition, a prestigious journal known for publishing pioneering chemical research, produced 235 publications on CO₂-to-methanol conversion, reflecting its influence in high-impact, foundational research. Finally, Journal of Materials Chemistry A contributed 225 articles, indicating the significant role of advanced materials in catalysis and CO₂ utilization.

This analysis demonstrates that CO₂ conversion to methanol research is disseminated through a variety of high-impact journals, spanning catalysis, chemical engineering, energy systems, and materials science. These journals not only shape the current research landscape but also drive the future directions of CO₂ conversion technologies.

2.4. Top Keywords

The analysis of top keywords in the field of CO₂ conversion to methanol highlights the prominent research areas and emerging trends in this domain. From a total of 9363 keywords identified across the selected publications, 102 keywords were chosen for detailed analysis based on their frequency of occurrence (Figure 6). These keywords were further divided into five distinct clusters, each representing key themes within the research landscape.

The most frequently used keyword, CO₂ hydrogenation, appeared 460 times and falls under Cluster 1, indicating its central role in research focusing on the catalytic conversion processes necessary for producing methanol from CO₂. This keyword also reflects the high level of interest in using hydrogenation as a method to convert CO₂ into valuable chemicals. The total link strength of this term, 481, highlights its significance in the ongoing discussions surrounding CO₂-reduction technologies and efficient methanol synthesis. Another critical term, CO₂ reduction, appeared 339 times and is part of Cluster 3, emphasizing the growing attention toward reducing CO₂ emissions through chemical and catalytic processes. This keyword is closely linked with the study of advanced catalysts and reaction mechanisms, particularly in the context of sustainable and energy-efficient solutions. As CO₂ reduction plays a pivotal role in mitigating climate change, its frequent occurrence underscores its relevance to carbon capture and utilization strategies.

The keyword methanol was cited 290 times, positioning it within Cluster 5. Its high frequency reflects the emphasis on methanol as the primary product of interest in CO₂ conversion research. Methanol synthesis, a key component of carbon utilization efforts, has been the focus of extensive research aiming to optimize production processes and improve catalyst efficiency. Similarly, carbon dioxide, mentioned 244 times in the same cluster, illustrates the ongoing research efforts to utilize CO₂ as a feedstock, highlighting the environmental focus of this field. In addition to these dominant keywords, terms such as photocatalysis (226 occurrences in Cluster 3) and electrocatalysis (146 occurrences in Cluster 2) demonstrate the importance of exploring alternative, renewable energy-driven methods for CO₂ conversion. Photocatalysis represents a promising approach using light to drive CO₂-reduction reactions, while electrocatalysis focuses on electrochemical methods powered by sustainable energy sources. These areas represent innovative pathways for improving the sustainability of methanol production.

Other noteworthy keywords include methanol synthesis (135 occurrences in Cluster 1), reflecting the focus on improving the efficiency of converting CO₂ into methanol, and CO₂ conversion (154 occurrences in Cluster 1), which encompasses a broad spectrum of research dedicated to the transformation of CO₂ into value-added products. Together, these terms illustrate a diverse and multifaceted research landscape, with efforts ranging from fundamental catalytic studies to applied engineering solutions. The keyword clusters reveal the diversity of research approaches: Cluster 1 emphasizes hydrogenation and catalytic conversion, Cluster 2 focuses on electrocatalysis and sustainable energy, Cluster 3 delves into photocatalysis, Cluster 4 explores chemical engineering aspects, and Cluster 5 involves materials science and catalytic system advancements. This comprehensive keyword analysis offers a clear understanding of the primary research directions, showcasing the field’s focus on catalysis, sustainable processes, and innovative chemical engineering solutions in CO₂-to-methanol conversion.

3. CO₂ Conversion to Methanol

The transformation of CO₂ into alcohols, particularly methanol, is a topic of significant interest both in the chemical industry and the scientific community. As a vital C1 alcohol, methanol serves as a precursor to a diverse array of chemical compounds. The concept of a “methanol economy”, introduced by Nobel Laureate George Olah, centers around the hydrogenation of CO₂ into methanol and its derivatives. This approach encompasses the capture of CO₂ from various sources, including natural, human-made, or industrial, followed by its effective catalytic conversion into methanol, illustrated in Figure 7.

In recent developments, research and development (R&D) in methanol synthesis have been increasingly geared towards more sustainable methodologies. This includes the reduction of CO₂ using H₂ derived from technologies that utilize natural and renewable energy sources. Prominent companies in this field, such as Mitsui Chemicals and Carbon Recycling International (CRI) Inc., have pioneered such methods for methanol production. Specifically, CRI’s facility in Iceland boasts a production capacity of approximately 5 million liters of methanol annually (4 kta). Remarkably, the H₂ required for this process is generated through water electrolysis, utilizing energy sourced primarily from natural elements like geothermal, hydroelectric, and wind power [26].

3.1. Thermodynamics and Reaction Mechanism of CO₂ Conversion to Methanol

The commercial production of methanol predominantly relies on syngas (a mixture of CO and H₂), which is derived from fossil resources such as coal and natural gas through processes like coal gasification and natural gas steam reforming [27]. Typically, small amounts of CO₂ (2–8%) are introduced to the CO/H₂ stream to adjust the H/C ratio to achieve the required stoichiometry and enhance the reaction rate.

$${\mathrm{CO}+2\mathrm{H}}_2{=\mathrm{CH}}_3\mathrm{OH}\Delta H_{298K}=-90{.6\mathrm{kJ}\mathrm{mol}}^{-1}$$

(1)

$${\mathrm{CO}}_2+{3\mathrm{H}}_2{=\mathrm{CH}}_3{\mathrm{OH}+\mathrm{H}}_2\mathrm{O}\Delta H_{298K}=-49{.5\mathrm{kJ}\mathrm{mol}}^{-1}$$

(2)

Reverse water-gas-shift reaction (RWGS)

$${\mathrm{CO}}_2+{\mathrm{H}}_2{=\mathrm{COH}+\mathrm{H}}_2\mathrm{O}\Delta H_{298K}=-41{.2\mathrm{kJ}\mathrm{mol}}^{-1}$$

(3)

Compared to syngas-based methanol production (Equation (1)), the hydrogenation of CO₂ to methanol demands an additional amount of H₂ due to the need to remove one oxygen atom from CO₂ via water formation as a byproduct (Equation (2)). This results in thermodynamically less favorable conditions for methanol production from CO₂ compared to CO, resulting in lower one-pass methanol yields (Figure 8). For instance, at 200 °C, the equilibrium methanol yield from CO exceeds 80%, while from CO₂, it is less than 40% [28]. Additionally, the formation of side products like ethers, ketones, higher alcohols, or hydrocarbons is thermodynamically more favorable under typical synthesis conditions [5,29].

The synthesis of methanol from CO₂ and H₂ is an exothermic reaction that reduces the number of gas-phase molecules. According to Le Chatelier’s principle, high pressure and low temperature should thermodynamically favor methanol production. However, due to the inert nature of CO₂, a higher reaction temperature (typically above 240 °C) is needed to increase the rate of CO₂ activation and methanol formation [19,30,31]. At these temperatures, the reverse water-gas shift (RWGS) reaction (Equation (3)) becomes thermodynamically favorable, reducing methanol yields and wasting H₂ [31,32].

The RWGS reaction will not only wasteH₂ supplies but also reduce the yield of methanol.

Although the thermodynamic equilibrium limits the maximum methanol yield, this limitation can be mitigated through optimization of reaction conditions, reactor design, and innovations such as recycling unconverted feed gas after product separation or in situ product removal (e.g., water removal via distillation or membranes) [28].

Methanol production from CO₂ is highly exothermic and limited by kinetics, resulting in only 15–25% conversion under typical conditions [33]. The endothermic RWGS reaction competes with methanol synthesis, further complicating the process. Efficient reactor design is essential to remove the heat generated during methanol synthesis and maintain isothermal conditions [34]. Water, a byproduct of this reaction, can deactivate conventional copper-based catalysts, highlighting the importance of catalyst design and kinetic studies [35,36].

Methanol synthesis from CO₂ can proceed through various reaction pathways, with the formate, RWGS, and trans-COOH* mechanisms being the most prominent (Figure 9a). In the classical formate pathway, CO₂ reacts with adsorbed atomic H, forming a formate intermediate (HCOO*), which subsequently transforms into intermediates such as dioxomethylene (H₂COO*), formaldehyde (H₂CO*), methoxy (CH₃O*), and finally, methanol (CH₃OH) [5,36]. Alternatively, formate can hydrogenate into formic acid (HCOOH*) and follow a similar sequence leading to methanol through the methoxy intermediate.

The RWGS pathway involves the initial transformation of CO₂ into CO*, which is then hydrogenated to form methanol via intermediates such as formyl (HCO*), formaldehyde (H₂CO*), and methoxy (CH₃O*). A variation of this mechanism involves the formation of hydrocarboxyl (COOH*) intermediates, which can convert into CO* and proceed through a similar route to methanol. In the trans-COOH mechanism, COOH* can also produce dihydroxycarbene (COHOH*), which dissociates into hydroxymethylidyne (COH*) and subsequently into hydroxymethylene (HCOH*), leading to methanol formation [5,36].

Methanol is an attractive product due to its versatility as a clean, biodegradable fuel that can be converted into other valuable chemicals such as dimethyl ether, olefins, and longer-chain hydrocarbons [38,39,40]. However, two major thermodynamic challenges hinder methanol generation: high-pressure requirements (up to 200 bar) and low reaction kinetics at lower temperatures [41], which reduce methanol yield. Moreover, to maintain high methanol selectivity and minimize CO production, the RWGS reaction must be suppressed [10,41,42,43,44].

The methanol formation mechanism via CO₂ hydrogenation is complex and not yet fully understood [31]. Effective catalysts are required to reduce operating pressure while ensuring high methanol yields. Copper (Cu) is the most widely used catalyst due to its high activity at low temperatures, affordability, diverse oxidation states, and stable interactions with oxygen, which prevent surface poisoning and unstable intermediates [45,46,47]. Cu also exhibits strong interactions with other materials (e.g., Zn), improving catalyst stability and selectivity while lowering operational pressures [28,48]. However, Cu-based catalysts face challenges such as deactivation from impurities, a high activation barrier for H₂, and limited CO₂ activation [49,50]. To address these issues, supports, promoters, and bimetallic catalyst are often added to modulate the active sites and surface properties of Cu catalysts, improving their performance. For example, adding these materials enhances Cu’s ability to dissociate H₂ and provides smaller particle sizes that increase the surface area available for reactions [51,52].

Kinetic models, such as the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms, have been proposed to describe the methanol synthesis process. The formate mechanism is widely believed to dominate on Cu-based catalysts [53]. For instance, DFT calculations on Cu/ZnO/Al₂O₃ catalysts, particularly on the Cu (111) surface, showed that CO₂ hydrogenation occurs via the formate pathway (CO₂* → HCOO* → HCOOH* → CH₃O₂* → CH₂O* → CH₃O* → CH₃OH*). Despite the thermodynamic preference for methanol production from CO, studies indicate that CO₂ hydrogenation contributes to approximately two-thirds of the methanol yield, contrary to the predictions of potential energy surface (PES) analysis, which suggest CO hydrogenation is more favorable. The promotion of CO₂ hydrogenation by CO* may involve its role as an H-donor, facilitating the formation of HCO* and enhancing CO₂ conversion [53].

Most recently, Cao et al. [54] investigated CO₂ hydrogenation to methanol on Cu-based catalysts using DFT calculations and kinetic modeling. Their work revealed that the formate (HCOO*) pathway is the dominant reaction mechanism, with a reaction rate four to six orders of magnitude higher than the carboxyl (COOH*) pathway (see Figure 9b). The rate-determining step involves the hydrogenation of HCOOH* to H₂COOH*, leading to methanol via CH₃O* intermediates. CO* acts primarily as a byproduct due to its high hydrogenation barrier.

3.2. Recent Advances in Cu-Based Catalyst for CO₂ Hydrogenation to Methanol

The conversion of CO₂ to methanol is a highly promising route, offering dual benefits of mitigating greenhouse gas emissions and reducing dependence on fossil fuels. The hydrogenation of CO₂ to methanol (CO₂ + 3H₂ ⇌ CH₃OH + H₂O) presents several challenges, including the formation of water vapor as a byproduct, which can negatively impact catalyst performance [44,55]. This is a significant difference compared to methanol synthesis from CO hydrogenation, where no water is produced. Therefore, developing catalysts that maintain stability under high-water partial pressure and resist the reverse water–gas shift (RWGS) reaction remains a primary research focus [44]. Among the transition metal, Noble metals are limited and unfavorable for industrial applications, and non-noble metals like Ni or Fe are not effective due to reaction limitations. Based on our bibliometric analysis and effective catalytic activity of Cu-based catalyst, this review further explores the recent advancement in the Cu-based catalyst. The Cu-based catalysts for CO₂ to methanol, along with the recently reported reaction promoters, are summarized in Table 1.

Historically, the first industrial process for methanol synthesis was developed in the 1920s by Badische Anilin Soda and Fabrik (BASF), using ZnO–Cr₂O₃ catalysts under harsh reaction conditions (300–400 °C, 25–30 MPa). In the 1960s, Imperial Chemical Industries (ICI) introduced the Cu/ZnO/Al₂O₃ catalyst, which enabled methanol synthesis under milder conditions (220–300 °C, 5–10 MPa). This catalyst quickly became the industry standard due to its higher activity and stability [44,56,57]. However, despite its success, Cu/ZnO/Al₂O₃ catalysts are not without limitations, as the water produced during the reaction can facilitate the agglomeration of ZnO and the oxidation of Cu⁰ to Cu²⁺, leading to catalyst deactivation [58].

Recent research has focused on developing highly effective Cu-based catalysts that demonstrate better activity, selectivity, water tolerance, and stability. Cu and ZnO remain the primary active components in these catalysts, often supported by various modifiers and metal oxides such as Al₂O₃, ZrO₂, and CeO₂ [1,3,59,60,61]. These catalysts typically operate at 220–300 °C and 5–10 MPa, with varying degrees of conversion, selectivity, and space–time yield (STY) depending on the composition and preparation method [59,62]. Cu-based catalysts are the most extensively studied systems for CO₂ hydrogenation to methanol. They are widely recognized for their high activity, operational longevity, and resistance to poisoning under industrial conditions. The primary mechanism for methanol synthesis involves the direct hydrogenation of CO₂, although the RWGS reaction (CO₂ + H₂ ⇌ CO + H₂O) often competes with methanol formation, which can lower methanol yield [63,64]. Over the years, scientists have developed a wide range of catalysts to mitigate this issue, focusing primarily on Cu-based systems supported by metal oxides [65].

The primary strategy for improving Cu-based catalysts has been the incorporation of metal oxides such as ZnO, Al₂O₃, and ZrO₂, which act as carriers and promoters, optimizing the interaction between Cu, the support and bimetallic/alloy [66]. The addition of ZrO₂, for instance, has been shown to enhance methanol selectivity by stabilizing reaction intermediates like formate and methoxide [63,67]. Additionally, CeO₂, with its unique redox properties, has proven to be an excellent promoter, increasing the number of oxygen vacancies and improving CO₂ activation [68]. Research trends over the past decade show that Cu-based catalysts continue to dominate, with approximately 79% of publications on CO₂ hydrogenation to methanol focusing on Cu–ZnO systems [69,70]. Among these, the addition of Al₂O₃ and ZrO₂ remains the most popular approach for enhancing catalyst performance.

3.2.1. Role of Supports or Support Effect in Cu-Based Catalysts

Selecting an appropriate support is essential for optimizing catalytic activity, stability, and selectivity for CO₂ hydrogenation to methanol. Originally, supports were used primarily to ensure well-dispersed metal particles and reduce sintering [71,72]. However, it has become clear that the SMSI effect can significantly influence catalyst morphology, chemical states, and activity levels [73,74].

ZnO is one of the most studied supports due to its dual function as a structural and electronic promoter in Cu-based catalysts. ZnO acts as a geometric spacer between Cu NPs, enhancing Cu dispersion and increasing the exposed Cu surface area [75,76]. Additionally, ZnO modulates electronic properties through SMSI with Cu, impacting the electron density at the Cu–ZnO interface. Tsang et al. investigated these morphology-dependent electronic interactions, finding that the polar (002) facet in plate-like ZnO, with higher oxygen defect density, increases electron transfer from ZnO to Cu, resulting in methanol selectivity above 70% in Cu/plate–ZnO/Al₂O₃ catalysts [48]. Further enhancements in electron density were demonstrated with the encapsulation of CdSe quantum dots in ZnO rods, leading to an impressive 75% methanol selectivity [77].

Most recently, Xuan et al. [78] explored the impact of solvent choice on the preparation and catalytic performance of CuO–ZnO–ZrO₂–Al₂O₃ (CZZA) catalysts using the citrate complexing method for CO₂ hydrogenation to methanol. The improved results are attributed to the better dispersion of active Cu species and the increased oxygen vacancy content, which are critical for CO₂ activation and methanol synthesis. The CZZA catalyst achieved a CO₂ conversion rate of 21.8% and a methanol selectivity of 51.0% under optimal reaction conditions (240 °C, 3 MPa). The study also underscores the formate pathway as the reaction mechanism and highlights hydrogen spillover as a pivotal factor in the reaction.

ZrO₂ is recognized for its high thermal stability, mechanical strength, and specific surface area, making it a preferred support for Cu-based catalysts in CO₂ hydrogenation. Comparative studies have shown that Cu/ZrO₂-based catalysts outperform Cu/Al₂O₃ counterparts due to ZrO₂’s lower hydrophilicity and its ability to facilitate a dual-site reaction mechanism for CO₂ adsorption and hydrogenation. Grabowski et al. observed that catalysts with a higher proportion of tetragonal ZrO₂ (t-ZrO₂) phase, stabilized by oxygen vacancies, demonstrated increased methanol production rates by enhancing the availability of Cu⁺ ions at the surface [79]. Similarly, nanocrystalline ZrO₂, with more surface edges and defects, was shown to strengthen Cu–ZrO₂ interactions, enhancing CO₂ adsorption and promoting catalyst reduction [70].

Spinel-structured supports, such as ZnFe₂O₄, enable fine control over Cu nanoparticle size and distribution, influencing CO adsorption and selectivity. Recent studies have demonstrated that Cu/ZnFe₂O₄ spinel catalysts with a 33Cu/ZnFe-0.5 configuration achieved a methanol selectivity of 71.6% and CO₂ conversion of 9.4% at 260 °C and 4.5 MPa, attributed to the SMSI effect and the stabilization of ZnO adjacent to surface Cu nanoparticles [80,81,82]. Despite their effectiveness, spinel catalysts typically have lower surface areas due to high calcination temperatures. However, microwave-hydrothermal synthesis methods have improved their surface area and thermal stability, allowing the preparation of Cu-based catalysts with stable microstructures and smaller particle sizes [80,83].

Metal-organic frameworks (MOFs) offer a novel approach by stabilizing metal NPs within their porous structures, providing unique catalytic properties. For instance, Cu-based catalysts encapsulated within Zr-based MOF UiO-66 achieve 100% methanol selectivity and an eightfold increase in catalytic activity compared to traditional Cu/ZnO/Al₂O₃ catalysts with the help of SMSI effects between Zr oxide secondary building units and Cu NPs [84]. This structure enables efficient H₂ activation by metallic Cu and stabilizes intermediates, enhancing both reaction rates and stability [85,86].

Commonly used supports like SiO₂ provide large surface areas but suffer from thermal instability at elevated temperatures, limiting their long-term applicability in methanol synthesis due to their transformation into less stable forms in the presence of water vapor [54]. Despite these limitations, advanced support materials such as carbon-based carriers, MOFs, and structured metal oxides continue to expand the potential for durable and efficient methanol synthesis from CO₂.

The role of supports in Cu-based catalysts for CO₂ hydrogenation to methanol is multifaceted, encompassing structural, electronic, and chemical effects. By optimizing the choice of support material, researchers can significantly enhance the catalytic performance, selectivity, and stability of Cu-based systems. The use of advanced supports, such as mixed oxides, spinel structures, and rare earth oxides, has opened new avenues for improving methanol synthesis, offering better resistance to deactivation and higher methanol yields. Moving forward, continued research into the design of tailored support materials will be key to further advancements in this field.

3.2.2. Role of Promoters in Cu-Based Catalysts

Promoters in Cu-based catalysts for CO₂ hydrogenation enhance both structural and electronic properties, optimizing catalytic activity, stability, and methanol selectivity. By influencing factors such as metal dispersion, redox properties, and acid-base interactions, promoters enable more efficient pathways for CO₂ reduction to methanol [87].

Alkali metals, particularly potassium (K), act as promoters in Cu–Zn–Al catalysts by modifying the Cu⁺/u⁰ ratio, which is critical for enhanced CO₂ hydrogenation activity. For instance, potassium-promoted Cu–Zn–Al (CZA-K) catalysts show a higher Cu⁺/u⁰ ratio than sodium-promoted systems, which correlates with improved methanol selectivity. The increased Cu⁺ concentration is associated with the formation of K-O-(CO)-O surface species, which limits the reverse water-gas shift (RWGS) reaction and increases methanol production [88]. This effect arises because the K-O species inhibit CO₂ dissociation, thereby promoting methanol over CO formation.

Ga₂O₃ is extensively studied as a promoter in Cu–ZnO-based catalysts due to its ability to increase specific surface area, reduce particle size, and stabilize Cu⁺ species. These structural and electronic modifications contribute to a higher methanol yield. Studies have shown that Ga₂O₃ in Cu–ZnO/HZSM-5 catalysts enhance the dispersion of Cu and stabilizes intermediate Cu⁺ states, which are essential for CO₂ hydrogenation [32,55,89,90]. For instance, Schumann et al. observed that Ga³⁺ incorporation increases ZnO conductivity and generates redox-active defect sites, resulting in improved methanol synthesis rates [91]. Furthermore, Ga₂O₃ facilitates the formation of a ZnGa₂O₄ spinel phase, enabling type II heterojunctions in ZnO–Ga₂O₃ mixtures, which drive the reduction of ZnO and the formation of CuZn alloys, both of which are highly active for methanol synthesis [48].

Promotion of Indium (In) has demonstrated significant effects in Cu/CeO₂ catalysts, enhancing methanol yields by promoting smaller Cu particle sizes and improving dispersion and stability. Loading only 1 wt% indium in Cu/CeO₂ resulted in marked improvements in the methanol yield rate due to the enhanced stability and distribution of Cu on the catalyst surface [92]. The reduction in particle size and improved distribution likely results from In’s role in preventing Cu agglomeration, which stabilizes the catalyst’s active sites during CO₂ hydrogenation.

Rare earth metals, such as lanthanum (La), calcium (Ca), and cerium (Ce), have shown substantial promotional effects on Cu/ZnO/ZrO₂ catalysts. La₂O₃, for example, not only increases Cu dispersion but also introduces oxygen vacancies by partially substituting Zr⁴⁺ with La⁴⁺, thereby enhancing CO₂ adsorption and methanol selectivity up to 72% [93]. Additionally, the enhanced basicity from La₂O₃ increases the number of basic sites, favoring formate adsorption and hydrogenation, which are crucial for methanol formation. Recent studies using La-modified CZA catalysts demonstrated up to a 30% increase in CH₃OH production, attributed to the improved porosity and surface area, which facilitate intermediate adsorption and CO₂ hydrogenation steps [94,95].

Recent literature [14,18,19] has explored the significant influence of catalyst design strategies and support interactions, as evidenced by the varied results across systems such as Cu/TiO₂ and Cu-Ca_0.8La_0.2TiO₃. Cu/TiO₂-600, prepared using a lattice confinement strategy, exhibited superior performance with a methanol selectivity of 55.5% and a CO₂ conversion of 45.2% under reaction conditions of 240 °C, 3 MPa, and a feed ratio of CO₂/H₂/N₂ = 1/3/1. This high efficiency is attributed to the enhanced dispersion of Cu species within the TiO₂ lattice, which not only improves sintering resistance but also maintains a high density of active sites for CO₂ activation and methanol synthesis. In comparison, Cu/TiO₂-500, operated under harsher conditions of 300 °C and 4 MPa, showed lower methanol selectivity (43.3%) and CO₂ conversion (12.5%), suggesting that the catalytic efficiency is sensitive to both the reaction environment and the interplay between Cu and its support. In this system, the role of oxygen vacancies in TiO₂ was found to be crucial for activating CO₂, but excessive metal-support interaction (SMSI) likely hindered catalytic activity by partially covering Cu active sites, underscoring the importance of balancing metal-support interactions for optimal performance. On the other hand, Cu-Ca_0.8La_0.2TiO₃, a perovskite-structured catalyst, displayed a well-balanced performance with a methanol selectivity of 58.5% and a CO₂ conversion of 22.5% at 300 °C and 3 MPa, using a higher hydrogen-rich feed ratio of CO₂/H₂/N₂ = 6/18/1. The incorporation of La into the perovskite lattice enhanced the dispersion of Cu species and optimized the ratio of Cu⁰/Cu⁺, with Cu⁰ playing a pivotal role in H₂ activation and Cu⁺ facilitating methanol synthesis. This dual functionality, combined with the stability of the perovskite structure, enabled efficient CO₂ activation and selectivity toward methanol. Comparatively, the results indicate that while lattice confinement strategies in Cu/TiO₂-600 maximize methanol selectivity and thermal stability, the Cu–Ca_0.8La_0.2TiO₃ system strikes a balance between selectivity and conversion by leveraging perovskite properties and active site tuning. These findings collectively highlight the need for precise control over catalyst composition and support interactions to achieve high-efficiency methanol production in CO₂ hydrogenation processes.

Noble metals, including palladium (Pd) and gold (Au), have been used to boost hydrogenation rates in Cu-based catalysts by facilitating H₂ spillover. This mechanism enhances reducibility, improves surface hydrogenation reactions, and stabilizes Cu sites, resulting in increased methanol production [20,96,97,98]. For instance, Martin et al. reported that Au acts as an electron-withdrawing agent, promoting electronic transfer from ZnO to Cu and stabilizing Cu⁰ species against reoxidation, thus enhancing methanol yield by increasing resistance to CO₂ or H₂O-induced oxidation [99].

Interestingly, methanol itself may act as a promoter in Cu/ZnO/MgO catalysts. The presence of a small amount of methanol in the reaction feed has been shown to reduce the apparent activation energy from 117.9 kJ/mol to 67.9 kJ/mol, suggesting that methanol interacts with the catalyst in a way that lowers the energy barrier for CO₂ hydrogenation. Although the precise mechanism of this effect remains unclear, it may relate to methanol’s interaction with active sites, enhancing reaction kinetics under certain conditions.

The study by He et al. [100] investigates the catalytic performance of Mg-modified Cu–ZnO–ZrO₂ catalysts with a co-precipitation method for CO₂ hydrogenation to methanol, the incorporation of MgO into the Cu–ZnO–ZrO₂ system optimized the catalyst’s microstructure, enhanced the dispersion of active species, and introduced appropriate basic sites, which significantly improved the adsorption and activation of CO₂. Among the catalysts tested, the optimized Cu–ZnO–ZrO₂/MgO (CZZ_0.8M_0.2) achieved a CO₂ conversion of 7.3% and methanol selectivity of 71.8% under reaction conditions of 220 °C, 3 MPa. This performance represents nearly double the conversion rate compared to unmodified Cu–ZnO–ZrO₂ catalysts. The MgO as a promoter has showed significant potential for improving CO₂ hydrogenation catalysts, making CZZ_0.8M_0.2 a highly promising candidate for efficient methanol production.

In summary, the incorporation of promoters, whether alkali metals, metal oxides, rare earth elements, or noble metals, significantly impacts the performance of Cu-based catalysts in CO₂ hydrogenation to methanol. Each promoter provides unique benefits, such as improved Cu dispersion, stabilization of reactive intermediates, and enhanced electronic properties, all of which contribute to greater methanol yields and selectivity. The strategic use of these promoters thus forms a critical approach in designing more effective and sustainable catalysts for methanol synthesis from CO₂.

Table 1. Cu-based catalysts for CO₂ to methanol.

Catalysts	T (°C)	P (MPa)	Gas Composition	Space Velocity (mL·g−1 h−1)	CO2 Conversion (%)	CH3OH Selectivity (%)	Ref.
Cu-ZrO2	230	1	CO2/H2/N2 = 1/3/1	50,000	1.6	72.2	[101]
Cu/ZrO2	260	3	CO2/H2/N2 = 23/69/8	6000	17.1	58.5	[102]
Cu-Zn-Zr/CuBr2	250	5	CO2/H2/N2 = 23/69/8	3000	10.7	97.1	[103]
Cu/TiO2-600	240	3	CO2/H2/N2 = 1/3/1	3600	45.2	55.5	[18]
Cu/TiO2-500	300	4	CO2/H2/N2 = 1/3/1	10,000	12.5	43.3	[14]
Cu-Ca0.8La0.2TiO3	300	3	CO2/H2/N2 = 6/18/1	3000	22.5	58.5	[19]
0.5% Cu-Zn-Zr	290	4.5	CO2/H2 = 1/3	10,800	9.5	76	[104]
Cu-Zn-Al-Ce	250	3	CO2/H2 = 1/3	12,000	14.2	37.8	[105]
20% ZnO-ZrO2	320	2	CO2/H2 = 24/72	24,000	6.4	78.5	[63]
inverse-ZrO2/Cu	220	3	CO2/H2 = 1/3	48,000	<5	~70	[65]
Cu@ZnO-1.0	240	3	CO2/H2 = 1/3	12,000	19.6	76.9	[61]
CuO/Ce0.4 Zr0.6 O2	220	3	CO2/H2 = 1/3	10,000	7	96.4	[66]
Cu-Zn-Al-K	240	3	CO2/H2 = 1/4	2400	14	96	[106]
Cu-ZnO-A12O3	260	3.6	CO2/H2 = 1/10	18,000	65.8	77.3	[107]
Cu-ZnO-A12O3	280	4	CO2/H2 = 1/3	10,000	65.3	91.9	[108]
Cu-ZnO	250	3	CO2/H2 = 1/3	18,000	2.3	100	[109]
Cu-ZnO-ZrO2	200	1	CO2/H2/N2 = 3/9/1	4000	5.8	55.2	[110]
Cu-ZnO-ZrO2	230	3	-	3000	15.2	35.1	[111]
Cu-ZnO-ZrO2/MgO	220	3	CO2/H2/N2 = 23/69/8	18,000	7.3	71.8	[100]
Cu-ZnO-ZrO2	240	3	CO2/H2 = 1/3	3600	12.1	54.1	[112]
Cu/ZrO2	280	3	CO2/H2 = 1/3	7200	12	32	[113]
30Cu-ZnO-ZrO2	280	5	-	10,000	23	33	[114]
Fe-Cu-ZnO-ZrO2	250	3	CO2/H2 = 1/4	60,000	18.7	53.8	[115]
Cu-Ga2O3-ZrO2	250	2	CO2/H2 = 1/3	2500	13.7	75.6	[116]
7.7 Ga203/IE/Cu/ZrO2	250	3	CO2/H2 = 22/75	20,000	1.3	74	[117]
Cu/Al2O3	280	9.5	CO2/H2 = 1/4	12,000	30	80	[118]
LaO0.8Zr0.2Cu0.7Zn0.3Ox	250	5	CO2/H2 = 1/3	3600	12.6	52.5	[119]
30Cu-ZnO-ZrO2	280	5	-	10,000	21	34	[120]
Cu-ZnO-ZrO20-Al2O3	240	3	CO2/H2/N2 = 23.5/64.5/12	2400	21.8	51	[78]

Table 1. Cu-based catalysts for CO₂ to methanol.

Catalysts	T (°C)	P (MPa)	Gas Composition	Space Velocity (mL·g−1 h−1)	CO2 Conversion (%)	CH3OH Selectivity (%)	Ref.
Cu-ZrO2	230	1	CO2/H2/N2 = 1/3/1	50,000	1.6	72.2	[101]
Cu/ZrO2	260	3	CO2/H2/N2 = 23/69/8	6000	17.1	58.5	[102]
Cu-Zn-Zr/CuBr2	250	5	CO2/H2/N2 = 23/69/8	3000	10.7	97.1	[103]
Cu/TiO2-600	240	3	CO2/H2/N2 = 1/3/1	3600	45.2	55.5	[18]
Cu/TiO2-500	300	4	CO2/H2/N2 = 1/3/1	10,000	12.5	43.3	[14]
Cu-Ca0.8La0.2TiO3	300	3	CO2/H2/N2 = 6/18/1	3000	22.5	58.5	[19]
0.5% Cu-Zn-Zr	290	4.5	CO2/H2 = 1/3	10,800	9.5	76	[104]
Cu-Zn-Al-Ce	250	3	CO2/H2 = 1/3	12,000	14.2	37.8	[105]
20% ZnO-ZrO2	320	2	CO2/H2 = 24/72	24,000	6.4	78.5	[63]
inverse-ZrO2/Cu	220	3	CO2/H2 = 1/3	48,000	<5	~70	[65]
Cu@ZnO-1.0	240	3	CO2/H2 = 1/3	12,000	19.6	76.9	[61]
CuO/Ce0.4 Zr0.6 O2	220	3	CO2/H2 = 1/3	10,000	7	96.4	[66]
Cu-Zn-Al-K	240	3	CO2/H2 = 1/4	2400	14	96	[106]
Cu-ZnO-A12O3	260	3.6	CO2/H2 = 1/10	18,000	65.8	77.3	[107]
Cu-ZnO-A12O3	280	4	CO2/H2 = 1/3	10,000	65.3	91.9	[108]
Cu-ZnO	250	3	CO2/H2 = 1/3	18,000	2.3	100	[109]
Cu-ZnO-ZrO2	200	1	CO2/H2/N2 = 3/9/1	4000	5.8	55.2	[110]
Cu-ZnO-ZrO2	230	3	-	3000	15.2	35.1	[111]
Cu-ZnO-ZrO2/MgO	220	3	CO2/H2/N2 = 23/69/8	18,000	7.3	71.8	[100]
Cu-ZnO-ZrO2	240	3	CO2/H2 = 1/3	3600	12.1	54.1	[112]
Cu/ZrO2	280	3	CO2/H2 = 1/3	7200	12	32	[113]
30Cu-ZnO-ZrO2	280	5	-	10,000	23	33	[114]
Fe-Cu-ZnO-ZrO2	250	3	CO2/H2 = 1/4	60,000	18.7	53.8	[115]
Cu-Ga2O3-ZrO2	250	2	CO2/H2 = 1/3	2500	13.7	75.6	[116]
7.7 Ga203/IE/Cu/ZrO2	250	3	CO2/H2 = 22/75	20,000	1.3	74	[117]
Cu/Al2O3	280	9.5	CO2/H2 = 1/4	12,000	30	80	[118]
LaO0.8Zr0.2Cu0.7Zn0.3Ox	250	5	CO2/H2 = 1/3	3600	12.6	52.5	[119]
30Cu-ZnO-ZrO2	280	5	-	10,000	21	34	[120]
Cu-ZnO-ZrO20-Al2O3	240	3	CO2/H2/N2 = 23.5/64.5/12	2400	21.8	51	[78]

3.2.3. Role of Alloy in Cu-Based Catalysts

The Cu/ZnO/Al₂O₃ (CZA) system has been commercially employed for over 60 years, but the exact nature of its active sites remains a topic of debate. The active sites for CO₂ to methanol conversion in these catalysts are believed to be located at the metal-oxide interfaces (Cu–ZnO) and surface Cu–Zn alloys (Alloys). Nakamura and Fujitani first provided evidence using XRD that Cu–Zn alloys form in Cu/ZnO catalysts, which significantly improve the specific activity and indicate that these alloys are the active sites for methanol synthesis [121]. Other researchers, such as Malte Behrens, observed through HRTEM that Cu nanoparticles in these systems are covered by disordered ZnOx layers, forming a surface Cu–Zn alloy. DFT calculations further confirmed that these surface alloying sites can enhance methanol synthesis [122].

Further support comes from Topsøe researchers, who used H₂-TPD and XPS to provide definitive evidence of surface Cu–Zn alloy formation [123]. Their findings suggest that increased H₂ partial pressure leads to more Zn in the metallic state, while DFT calculations indicated that Cu–Zn alloying weakens H₂ adsorption, promoting methanol production [124]. The group also proposed that ZnO particle size plays a key role in controlling Zn coverage on Cu, affecting the overall catalytic activity [59].

In contrast, a team at Brookhaven National Laboratory provided evidence that the metal-oxide interface (ZnO–Cu) is the primary active site. They found that Zn/Cu catalysts initially exhibited low activity but were gradually oxidized to form ZnO/Cu interfaces during the reaction, achieving high activity levels comparable to directly prepared ZnO/Cu systems [125].

Palladium (Pd) has been introduced into Cu–ZnO catalysts to increase methanol yield, not by creating new active sites, but by promoting H₂ spillover and stabilizing active Cu sites, preventing their oxidation by CO₂ [50]. Pd and Cu can form stable alloys after reduction, which has been demonstrated to provide significant synergistic effects for methanol synthesis [126]. For instance, Chunshan Song’s team developed a series of SiO₂ supported Cu–Pd bimetallic catalysts, showing that an optimized atomic ratio of Pd/(Pd + Cu) between 0.25 and 0.34 doubled the methanol yield compared to monometallic Cu and Pd catalysts [126,127]. Their results indicated that Cu–Pd alloy nanoparticles increase the surface coverage of formate species, a key intermediate in methanol synthesis. Furthermore, PdCu-enriched catalysts outperformed PdCu3-enriched catalysts in methanol production [128].

The effect of support materials on the performance of Cu–Pd catalysts has also been investigated. Supports such as TiO₂, ZrO₂, and Al₂O₃ were found to enhance methanol production by creating moderate MSI. In contrast, CeO₂, which created stronger MSI, hindered the formation of Cu–Pd alloys and led to competitive adsorption of H₂ and CO₂, resulting in poorer performance [129].

Although CZA catalysts perform well, their efficiency in CO₂ hydrogenation can be negatively affected by water formation and the hydrophilic nature of alumina [110,126]. As a result, ZrO₂ has been introduced as a less hydrophilic promoter to improve catalytic performance [130]. Studies have shown that adding ZrO₂ increases Cu dispersion, enhancing methanol selectivity [111]. Various synthesis methods have been developed to create Cu–ZnO–ZrO₂ catalysts, including surfactant-assisted coprecipitation, which enhances the interaction between Cu and Zr, resulting in higher methanol selectivity [131]. Other studies have focused on optimizing the crystallographic structure of ZrO₂ to increase catalytic activity [79]. For example, tetrahedral ZrO₂ has been found to be more beneficial than amorphous forms for methanol synthesis [113].

In addition to ZrO₂, Ga₂O₃ and CeO₂ have also been used as bimetallic promoters. Ga₂O₃ has been shown to improve catalytic performance by increasing SACu and enhancing the Cu–ZnO interaction [114], while CeO₂ provides new sites for CO₂ activation, significantly increasing methanol production [132]. Furthermore, ternary Cu/Ga₂O₃/ZrO₂ systems have outperformed binary Cu–ZrO₂ and Cu–Ga₂O₃ catalysts, particularly when specific preparation conditions are followed [117].

4. Conclusions

The conversion of CO₂ to methanol is a pivotal strategy for addressing global carbon emissions while enabling the production of a versatile chemical and energy carrier. This review integrates a comprehensive bibliometric analysis with a detailed examination of thermodynamic constraints, reaction mechanisms, and advances in catalyst development. The bibliometric analysis, encompassing over 13,000 publications from 2012 to 2023, highlights the significant growth in research on CO₂-to-methanol technologies, particularly the central role of Cu-based catalysts. Notably, 9361 of these publications focus specifically on Cu-based catalysts, underscoring the growing interest in their potential for CO₂ conversion. The analysis underscores China’s leadership in the field, along with substantial contributions from the United States, Germany, and other nations, reflecting the global urgency to address climate change.

Cu-based catalysts, enhanced through doping with elements such as ZrO₂, CeO₂, and Ga₂O₃, and innovations in composite supports like Cu–ZnO–ZrO₂–Al₂O₃, have emerged as the cornerstone for industrial-scale CO₂ hydrogenation. These systems balance activity, stability, and cost-effectiveness, addressing the challenges posed by the reverse water-gas shift reaction, water-induced deactivation, and the thermodynamic stability of CO₂. The review also explores advanced catalytic strategies like perovskite-type oxides, highlighting their potential to overcome existing limitations and unlock new avenues for catalyst design. Emerging materials, including metal-organic frameworks and bimetallic alloys, offer further opportunities for optimizing reaction pathways and improving selectivity under industrial conditions.

Further challenges and outlook for industrial applications are discussed in the next section. Addressing these challenges requires an interdisciplinary approach that integrates renewable energy sources, innovative reactor designs, and policy-driven incentives to enable sustainable industrial adoption. By consolidating current knowledge and identifying critical gaps, this review not only provides a roadmap for future research but also emphasizes the transformative potential of CO₂-to-methanol conversion in realizing a circular carbon economy. Continued innovation and collaboration across disciplines will be essential in transitioning this technology from the laboratory to large-scale implementation, making it a cornerstone of sustainable energy and carbon management.

5. Challenges and Outlook

While significant strides have been made in advancing CO₂ hydrogenation to methanol, several technical and economic challenges remain unresolved, limiting the scalability and industrial adoption of this promising technology. The activation of CO₂ remains a primary obstacle, driven by the molecule’s inherent thermodynamic stability, which necessitates substantial energy input. In Cu-based catalytic systems, this challenge is compounded by the weak interaction between CO₂ and the catalyst surface, leading to a reliance on high operational pressures—up to 200 bar—to drive reactions forward. This reliance significantly increases energy consumption and operational costs, forming a barrier to large-scale commercialization. Addressing this requires innovative approaches to catalyst design. Emerging strategies, such as the integration of machine learning and quantum simulations, offer the potential to predict and engineer advanced catalytic materials with enhanced CO₂ affinity, enabling activation at lower pressures and temperatures. By optimizing the electronic and structural properties of catalysts, such as doping Cu-based systems with promoters that create active surface sites for CO₂ adsorption, future research can directly target these bottlenecks.

In addition to activation challenges, the reverse water-gas shift (RWGS) reaction poses a persistent issue, particularly at elevated temperatures where it competes with methanol production. The RWGS reaction not only reduces methanol selectivity but also wastes valuable hydrogen, undermining process efficiency. To overcome this, catalyst systems must be designed to suppress RWGS while favoring methanol formation under milder conditions. Recent advances in nanostructured and multifunctional catalysts have shown promise in achieving this balance, particularly through the stabilization of intermediates that favor methanol synthesis pathways. Further research should explore the synergy between catalyst structure and reaction engineering, such as optimizing reactor configurations to enable selective intermediate stabilization. For example, reactors that facilitate in situ water removal could minimize the impact of water as a byproduct, which otherwise deactivates Cu-based catalysts by promoting agglomeration and oxidation of active sites. Enhancing the resistance of catalysts to water-induced deactivation through surface modifications or the incorporation of hydrophobic supports is another critical avenue for improving process longevity and efficiency.

Scaling up CO₂ to methanol conversion remains a key challenge due to the economic and logistical barriers associated with CO₂ capture and hydrogen production. While small-scale demonstration plants, such as Carbon Recycling International’s facility in Iceland, have validated the feasibility of this technology, achieving economic viability at an industrial scale demands substantial advancements. Current CO₂ capture methods, whether from industrial emissions or direct air capture (DAC) systems, are energy-intensive and costly, with DAC technologies exhibiting low technological readiness levels. Innovations in capture materials and processes, such as adsorption-based systems with higher efficiency or bioengineered solutions leveraging microorganisms for CO₂ fixation, could significantly lower costs. Coupling these advancements with decentralized production hubs that integrate CO₂ capture and methanol synthesis at emission sources offers a practical pathway to scalability.

Hydrogen production remains another critical bottleneck. Conventional methods like steam methane reforming and biomass gasification generate carbon byproducts, undercutting the environmental benefits of methanol synthesis. Electrolysis, powered by renewable energy sources such as wind or geothermal energy, provides a more sustainable alternative, as demonstrated by the geothermal-powered production model in Iceland. However, the high costs and relatively low efficiencies of current electrolysis systems, typically ranging between 40–60%, limit their widespread adoption. Increasing the efficiency of electrolysis through advances in membrane technologies, such as solid oxide electrolysis cells, and scaling renewable energy sources to supply hydrogen at competitive costs, are essential for the long-term viability of CO₂-to-methanol conversion. Policy incentives and investment in renewable energy infrastructure will also play a pivotal role in reducing the economic barriers to green hydrogen production.

Catalyst development continues to be a central focus in advancing CO₂ hydrogenation to methanol, addressing the limitations of Cu-based systems that, while effective and affordable, are prone to deactivation under industrial conditions, particularly in the presence of water. Recent progress has demonstrated the potential of composite supports, bimetallic systems, and structural modifications to overcome these challenges and enhance catalyst performance. Composite supports such as Cu–ZnO–ZrO₂–Al₂O₃ have shown improved stability by optimizing the dispersion of active Cu species and resisting sintering and deactivation, critical for sustained industrial applications. Bimetallic catalysts, such as Pd–Cu or Ga–Cu systems, exploit synergistic interactions between metals to stabilize intermediates, improve reaction kinetics, and suppress competing reactions like the reverse water-gas shift (RWGS). Structural innovations, including perovskite-type oxides like Ca_0.8La_0.8TiO₃, introduce dynamic lattice oxygen and defect sites that facilitate CO₂ activation and enhance hydrogenation efficiency, while layered double hydroxides (LDHs) offer a high density of tunable active sites and robust thermal stability, creating a supportive environment for Cu nanoparticles. These advances collectively aim to enhance catalyst durability, activity, and selectivity under practical operating conditions, addressing key industrial requirements. Moving forward, hybrid systems that integrate the complementary benefits of these approaches hold significant promise, and their validation through pilot-scale studies will be essential to establish their feasibility and performance under real-world conditions, paving the way for resilient and efficient catalytic technologies for CO₂ hydrogenation.

Finally, the economic viability of methanol production from CO₂ remains a critical hurdle. Although market demand for methanol is growing, the cost of production through CO₂ hydrogenation currently exceeds that of conventional methods due to the expenses associated with CO₂ capture and hydrogen generation. Integrating methanol synthesis plants with renewable energy sources and industries emitting high CO₂ volumes could reduce costs and improve economic feasibility. Additionally, exploring the valorization of methanol into higher-value products, such as olefins or specialty chemicals, could provide supplementary revenue streams. Participation in carbon credit markets through Certified Emissions Reduction (CER) programs may also offset production costs, further incentivizing industrial adoption.

The successful commercialization of CO₂-to-methanol technologies will ultimately depend on a confluence of scientific innovation, strategic integration of renewable energy, and supportive policy frameworks. By setting measurable performance targets—such as achieving methanol selectivity above 90% at pressures below 50 bar—and fostering interdisciplinary collaborations, the field can overcome its current limitations. As catalyst systems evolve and production processes are optimized, CO₂ hydrogenation to methanol has the potential to transition from a laboratory-scale innovation to a cornerstone of sustainable carbon management and green energy production.