Recent Developments on CO₂ Hydrogenation Performance over Structured Zeolites: A Review on Properties, Synthesis, and Characterization

Abstract

This review focuses on an extensive synopsis of the recent improvements in CO₂ hydrogenation over structured zeolites, including their properties, synthesis methods, and characterization. Key features such as bimodal mesoporous structures, surface oxygen vacancies, and the Si/Al ratio are explored for their roles in enhancing catalytic activity. Additionally, the impact of porosity, thermal stability, and structural integrity on the performance of zeolites, as well as their interactions with electrical and plasma environments, are discussed in detail. The synthesis of structured zeolites is analyzed by comparing the advantages and limitations of bottom-up methods, including hard templating, soft templating, and non-templating approaches, to top-down methods, such as dealumination, desilication, and recrystallization. The review addresses the challenges associated with these synthesis techniques, such as pore-induced diffusion limitations, morphological constraints, and maintaining crystal integrity, highlighting the need for innovative solutions and optimization strategies. Advanced characterization techniques are emphasized as essential for understanding the catalytic mechanisms and dynamic behaviors of zeolites, thereby facilitating further research into their efficient and effective use. The study concludes by underscoring the importance of continued research to refine synthesis and characterization methods, which is crucial for optimizing catalytic activity in CO₂ hydrogenation. This effort is important for achieving selective catalysis and is paramount to the global initiative to reduce carbon emissions and address climate change.

1. Introduction

Global warming is a significant concern as it is exacerbated by the excessive emissions of greenhouse gases, specifically carbon dioxide (CO₂), which contribute to mounting climate-related issues [1]. CO₂ hydrogenation is gradually emerging as a critical technology for carbon neutrality, which provides a route for CO₂ valorization into valuable chemicals and fuels [2,3]. The process offers a two-fold solution; besides addressing the pivotal issue of CO₂ emissions, it also assists in the sustainable production of heavy hydrocarbons [4]. The selective and optimized transformation of CO₂ represents a current challenge considering its thermodynamic stability and inert properties [5].

There are two primary methods for converting CO₂ into hydrocarbons, both of which have also been industrially applied for decades, reflecting their proven scalability and commercial viability. The first method, modified Fischer–Tropsch synthesis (modified-FTS), combines the reverse water-gas shift (RWGS) reaction and the Fischer–Tropsch reaction [6]. This process not only synergistically utilizes CO₂ but also transforms it into various hydrocarbons, as evidenced by the long-standing operation of SASOL plants in South Africa and the ORYX GTL plant in Qatar, which all utilize FTS technology [7,8]. The second method is the methanol-mediated route, where CO₂ is first converted into methanol and then transformed into hydrocarbons through the methanol-to-hydrocarbon (MTH) process. This pathway is exemplified by the operation of the world’s largest methanol plant in Iran, which uses CO₂ as a feedstock [9]. Given the thermal hydrogenation foundation of both technologies, unless otherwise stated, this review will therefore primarily focus on the thermal hydrogenation of CO₂. This approach has been comprehensively explored over structured zeolites in terms of mechanisms, kinetics, catalyst development, and industrial improvements and applications, as reviewed extensively by Ye et al. [10], Ojelade and Zaman [11], and Wang et al. [12], who have detailed the underlying processes and implementations of both the modified-FTS and MTH routes. Structured zeolites, with their unequaled porous structures and acid-base properties, are used as catalysts in a whole range of chemical processes, among them CO₂ hydrogenation. Figure 1 shows the classes of well-known zeolites (e.g., natural [13], and synthetic [14,15,16]). Both zeolite types have been widely applied not just in catalysis [17], but even in other industries such as water and wastewater treatment, agriculture, biomedicine, laundry detergents, and construction [18,19,20,21,22].

For CO₂ hydrogenation, zeolite application has received considerable attention over the last decade. Figure 2 shows the distribution of several articles displayed on Google Scholar when the phrase “CO₂ hydrogenation over zeolites” is used to search articles. The trend indicates unprecedented growth in the application of zeolites for CO₂ conversion reactions, particularly over the last decade, where an exponential trend has been seen. The recent signs of progress have mainly concentrated on the improvement of the zeolites through careful control of their compositions, porous characteristics, and coupling of thermal and mechanical properties that render promotion of reaction selectivity and enhance the production of longer-chained hydrocarbons such as aromatics, liquid fuels, alcohols, and heavy olefins [5,23]. It should be noted that zeolites by themselves have been widely studied for various applications as aforementioned; nonetheless, the recent developments in the field of CO₂ catalysis and the growing interest of researchers in the application of zeolites for CO₂ conversion create a demand for a review covering the growth in the field, primarily over the last decade, which is the motivation behind this article. This study intends to summarize the recent advances in the area of CO₂ hydrogenation over structured zeolites by offering a synoptic view of the different influential parameters, synthesis techniques, characterization requirements, and catalytic functions. The review critically evaluates the current literature that should bring out new ideas, findings of key innovations, and significant breakthroughs. It will embrace the rudiments of the process, the specific traits of structured zeolites, and their role in augmenting catalytic activity. In addition, there will be reflections on the challenges and prospects in this field; hence, one can get ideas on structured zeolites and the way they may be used to reduce the global CO₂ problem.

2. Thermal CO₂ Hydrogenation over Other Methods: A Brief Overview

In addition to the well-established thermal routes for CO₂ hydrogenation, such as FTS and MTH mentioned in the introduction section, various alternative methods have been explored to expand the range of technologies for CO₂ reduction.

Table 1. State-of-the-art overview of CO₂ hydrogenation technologies: assessing advantages and limitations.

Method	Advantages	Disadvantages
Thermal [10,11,12]	Proven scalability, high product yields, extensively used in industrial settings	Can be energy-intensive; less environmentally friendly without renewable energy integration; catalyst stability issues
Electrochemical [24,25]	Facilitates CO2 reduction using electricity, can be integrated with renewable electricity sources; scalable; stable long-term	Often a broad product distribution at high conversions; high energy demand; economically challenging
Photocatalytic [26,27]	Utilizes sunlight, environmentally friendly; no need for external energy inputs; economically feasible	Lower efficiency and stability, which hinders scalability; lower yields
Biochemical and Chemo-enzymatic [28,29,30]	High selectivity; operates under mild conditions; efficient for producing bulk chemicals	Slow reaction rates; complex maintenance of biocatalytic activity; high temperatures can inhibit enzyme activity

shows the alternatives, which include electrochemical, photocatalytic, biochemical, and chemo-enzymatic methods, each characterized by distinct mechanisms and specific applications. While each technology has its own merits and limitations, its implementation in industries largely depends on the type of industry, the purity level of the product, and the scale of processing.

However, despite the diversity of available technologies, the foundation of hydrocarbon processing industrial plants still predominantly relies on thermal conversion methods. This is not only evident in traditional sectors like chemical synthesis via the FTS and MTH routes but is also a staple in the oil and gas industries. Processes such as Gas to Liquid (GTL) and Natural Gas Reforming (NGR) for hydrogen production extensively employ thermal methods due to their robustness and scalability [31,32]. These processes are integral to the production workflows in these industries, underlining the predominance of thermal techniques.

The thermal CO₂ hydrogenation process appears more energy-efficient than the solar route. For example, the thermal method benefits from a novel membrane reactor that enhances methanol yield and requires a smaller reactor, which exhibits superior energy efficiency at 70.3% [33]. In contrast, a solar CO₂ hydrogenation process achieves an energy efficiency of 15.5%, with a 26% improvement over such previous methods possible under optimal conditions [34]. Despite the advancements in solar hydrogenation, the thermal approach is suggested to be superior in terms of energy efficiency. This sharp contrast highlights the larger energy savings characteristic of thermal CO₂ hydrogenation. In addition, Zeng et al. [35] highlighted that plasma power (a form of thermal hydrogenation) is more efficient than heating power (a form of solar hydrogenation), even at lower temperatures, showing the highest conversions in the presence of plasma.

Unlike thermal processes, electrochemical processes are energy-intensive, and the lifespan of catalytic materials is low for such processes [36]. Hence, the application of electrochemical conversion in industries remains limited, even today, due to practical challenges in optimizing key parameters such as Faradaic efficiency (FE), Energetic Efficiency (EE), and Current Density (CD), which are crucial for the success of this approach [37,38]. To overcome these obstacles, catalysts must possess rapid electron transmission capabilities, superior electron transport ability, and robust electron-transporting features [36,39].

The primary distinction between photocatalytic CO₂ reduction and electrochemical CO₂ reduction is in their electron sources. In photocatalytic reduction, electrons are generated by exposing semiconductors to light, whereas in electrochemical reduction, electrons are sourced through the application of an electrical current. However, since photocatalytic methods depend on light absorption, they often suffer from limited efficiency, poor selectivity, and insufficient product variety [40]. In addition, such processes are intricate, encompassing various mechanisms like electron and proton transfer as well as the formation and breaking of chemical bonds, which remain poorly comprehended [37]. Furthermore, the process of photocatalytic conversion necessitates substantial capital investment and energy, especially when using semiconductors with wide band gaps [41]. Substantial challenges in substrate-based photocatalysts include the degradation of polymers, the creation of volatile by-products, and the complexities in separating the catalyst from the final product [41]. Therefore, the catalysts used in such a conversion process need to be optimized and treated, which can additionally increase the overall expenses [42].

Biochemical and chemo-enzymatic methods, though innovative and sustainable, face challenges due to the high cost of cofactors, and complex biological systems and demand a need for efficient cofactor regeneration for large-scale applications [43,44]. One of the challenges that the biochemical conversion process faces is the maintenance of an adequate nutrient supply, the collection of end products, and the sustainability of the microbial cultures. It has been suggested that genetic modification of these organisms could overcome such problems [43]. However, this approach can fail in some cases, as the insertion of many gene copies can accidentally inactivate some genes—a situation called genetic knockout [41]. Hence, pretreatment is required, and productivity is lower when compared to that of the thermo-chemical process. Generally, enzymes, often costly and characterized by limited stability, activity, and reusability, face constraints in their industrial usage. Technologies like enzyme modification and immobilization require further advancement to decrease costs and boost enzyme performance and recyclability, thereby enhancing economic viability [45]. Additionally, the slow reaction rate significantly hinders the industrial application of enzymatic CO₂ conversion, necessitating the deployment of biocatalysts to address kinetic issues and achieve a faster, more effective conversion process.

Therefore, considering the extensive application, proven scalability, and integration within the hydrocarbon industrial processes, the thermal route remains the preferred method for CO₂ hydrogenation due to its robustness and higher reaction rates, especially when efficiency and selectivity are paramount. This assertion is corroborated by the above case studies, which clearly illustrated the limitations inherent in alternative methods and the comparative strengths of the thermal approach. Drawing upon these insights,

Table 2. Efficacy of thermal conversion against electrochemical, photocatalytic, biochemical, and chemo-enzymatic methods in CO₂ hydrogenation.

Hydrogenation Feature	Thermal Conversion [33,34,35]	Electrochemical Conversion [36,37,38,39]	Photocatalytic Conversion [37,40,41,42]	Biochemical and Chemo-Enzymatic Conversion [41,43,44,45]
Energy Efficiency	High, but varies based on the used catalytic materials	Energy-intensive with usually low catalytic material lifespan	Limited by light absorption efficiency	High cofactor costs impact overall efficiency
CO2 Conversion Rate	High at moderate to high temperatures	Can be high based on applied potential and choice of catalyst	Hampered by poor selectivity and product variety	Less productive, slow reaction rates
Catalyst Durability	Robust; choice of reducible supports enhances durability	Often short lifespan due to rigorous operational conditions	Challenges with catalyst degradation and by-product formation	Enzyme stability and recyclability need enhancement
Capital and Operational Costs	Relatively cost-effective due to higher efficiencies, and easy integration with reforming industries	Relatively high due to the need for optimizing efficiency and current density. Still requires technological maturity for CO2 conv.	Significant capital investment and energy required, especially for wide bandgap semiconductors	Costly due to the high price of enzymes and cofactors
Technological Maturity	Advanced	Remains limited in industry due to practical challenges	Processes are complex and mechanisms are not fully understood	Requires advancements in enzyme modification and immobilization
Reaction Mechanisms	Complex but widely studied on a variety of catalysts	Complex with limited understanding. Requires intricate tools to monitor reaction intermediates near electrodes in liquid medium	Involves intricate mechanisms that are poorly understood	Constrained by the complexity of biological systems
Selectivity	High and can be tailored for desired outcomes	Low; produces a mix of products. High conversion is often reported with broad product distribution	Often poor due to multiple possible reactions	Can be very selective. Dependent on the specificity of biological pathways
Scalability	Highly scalable and suitable for industrial scale-up	Faces practical challenges in scaling. Requires further technological development.	Limited scalability due to technical and efficiency constraints	Challenging due to the need to maintain microbial cultures and nutrient delivery
Downstream Processing	Simplified due to few by-products	Could require separation of multiple by-products	Complexities in separating the catalyst from the product increase expenses	Demands efficient cofactor regeneration and faces genetic knockout issues
Potential for improvement	Relatively well-established with room for incremental improvements based on catalyst and reactor design	Requires catalysts with fast electron transfer and robust transport features	Needs optimization of photocatalysts and treatment processes	Genetic engineering is suggested but leads to challenges like genetic knockout

encapsulates the array of advantages that thermal methods hold over their electrochemical, photocatalytic, biochemical, and chemo-enzymatic counterparts.

Recently, the literature documents the generation of liquid fuels, aromatics, and olefin compounds via the thermal catalytic hydrogenation process applied to CO₂ [12,46,47]. In addition, studies indicate that the inclusion of structured zeolites in thermo-catalytic hydrogenation processes has the potential to enhance the selectivity of hydrocarbon products [48,49]. For example, by integrating a Fe-Zn-Zr catalyst with H-ZSM-5, a notable selectivity for isoalkane products, reaching up to 91.9%, can be achieved [49]. Hence, utilizing the thermal CO₂ hydrogenation approach for zeolites as a catalytic medium brings forth a spectrum of advantages, underscored by their structural features that dictate the pathways of the hydrogenation reaction (more in Section 3.1). In addition, zeolites are highly effective in thermal CO₂ hydrogenation due to their extensive surface area and porosity, which provide numerous active sites, enhancing reaction rates and energy efficiency (more in Section 3.2). Their selective pore structure also plays a crucial role by ensuring molecular sieving, allowing only specific hydrocarbon production, thus increasing selectivity and reducing the need for further separation processes. This makes zeolites exceptionally suitable for controlled and efficient chemical processes. The acidity of zeolites is adjustable, which can be tailored to enhance catalytic activity for thermal CO₂ hydrogenation (more in Section 3.3).

Furthermore, zeolites’ thermal stability is also pivotal, as they withstand the necessary high temperatures for thermal hydrogenation, preserving catalytic activity (more in Section 3.4). Additionally, zeolites’ ability to be regenerated through thermal treatments allows for their reuse, ensuring economic viability. The ability to regenerate zeolites thermally without significant loss of structural integrity or catalytic activity is a crucial advantage over other catalysts that might degrade or require complex regeneration processes. This is particularly aligned with the characteristics emphasized by the synthesis methods for zeolites; bottom-up and top-down approaches (more in Section 4). This renderability, along potential cost savings and a reduced environmental impact due to less frequent catalyst replacement and decreased waste, positions thermal CO₂ hydrogenation with zeolites as a method that is both efficient and sustainable compared to the aforementioned hydrogenation approaches. Furthermore, zeolites are already widely used in various industrial applications, including oil refining and petrochemical processes [50]. Thus, their use in CO₂ hydrogenation does not require radically new technologies or setups. This compatibility with existing industrial infrastructure and processes reduces the barriers to implementation and capital investment, making it a more feasible option compared to other methods that might require specialized equipment or conditions.

3. Influential Parameters

Zeolites, with their tunable structural and functional properties, are key catalysts in CO₂ hydrogenation processes [51,52]. These structured zeolites offer specific pore structures, a balanced density of acid/basic sites, and the capacity to support and stabilize active metal sites, making them essential for controlled catalytic activity and improved product selectivity. This section explores complex parameters influencing zeolite performance in CO₂ hydrogenation, such as their composition, bimodal mesoporosity, oxygen vacancies, Si/Al ratio, thermal stability, structural integrity, and electrical and plasma interactions. Understanding these factors is crucial not only for defining their role in developing effective and sustainable processes for converting CO₂ into valuable chemicals but also for optimizing zeolites for their potential in industrial applications. Selected examples are presented further to highlight this key aspect of structured zeolites in CO₂ conversion.

3.1. Interplay of Composition and Preparation

Structured zeolites, typically modified with metal-based catalysts, present a multifaceted platform where the interplay between the zeolite topology, metal characteristics, and preparation methods determines the efficiency and outcome of the hydrogenation process. Metal-based catalysts are instrumental in defining product selectivity. These catalysts, when integrated with zeolites, leverage the unique hybrid nature of active sites within the zeolite framework. This configuration supports specific reaction intermediates that dictate the pathways of the hydrogenation reaction, thereby influencing the selectivity toward different hydrocarbons [5]. Wei et al.’s [53] investigation found that the topology of various zeolites, including H-Y and H-BEA, influenced the distribution of hydrocarbons during fuel production, although CO₂ conversion rates remained consistent. Zeolites with ten-membered rings, such as HZSM-5, were notably efficient in enhancing the selectivity towards larger hydrocarbon molecules in the gasoline range (C₅–C₁₁), with HZSM-5 demonstrating a preference for heavier fractions. Acidity levels were critical, with medium acidity in HZSM-5 (160) favoring the desired reaction, whereas stronger acidity prompted excessive cracking of larger hydrocarbons. Popova et al. [54] developed both mono- and bimetallic catalysts with Ni and Ru on ZSM-5 zeolites, noting the impact of nickel (Ni) content and the introduction of ruthenium (Ru) on their properties. Characterization techniques (XRD, TPR-TGA, TEM, XPS) revealed a fine dispersion of Ni and ruthenium oxide (e.g., RuO₂, RuO₄) across the zeolite’s surface and pores (more in Section 5). Tests in CO₂ hydrogenation showed that the 10Ni5RuZSM-5 variant exhibited excellent methane production efficiency, achieving 100% conversion with complete selectivity at 400 °C, and displayed promising stability for practical use. Furthermore, the composition of zeolites, particularly when doped with metals such as lithium (Li), can significantly alter catalyst reactivity. For instance, Kitamura et al. [55] investigated CO₂ hydrogenation using Rh zeolites enhanced with Li (see Figure 3a,b). They found that increasing Li levels shifted the primary product from methane to CO and ethanol, with a notable increase in ethanol production when CO was present. FTIR and TPR analyses suggested that Li additives fostered new reaction pathways on the catalyst surface, leading to improved CO₂ adsorption and stabilization of adsorbed CO species. Similarly, incorporating molecules like Na⁺, N, and Ge into the ZSM-5 zeolite framework was demonstrated to lower the energy barriers for CO₂ hydrogenation, thereby enhancing the overall catalytic efficacy [56].

Preparative methods also significantly contribute to tuning the characteristics of zeolites for the best performance in CO₂ hydrogenation. The synthesis method of zeolite, such as the choice of template, calcination conditions, and metal loading approaches, can also significantly affect the structural and chemical properties of the catalyst. Liu et al. [57] investigated how alkali metal promotion in Co-zeolite catalysts affects product selectivity, comparing IWI with the IE approach (see Figure 3c,d). Their findings revealed distinct differences: IWI led to a higher yield of olefins and C₅₊ hydrocarbons, attributed to an elevated Si/Al ratio (see Figure 3e). In contrast, the IE method’s selectivity remained largely unaffected by changes in the Si/Al ratio, likely due to alkaline poisoning (see Figure 3f). Hence, the zeolite framework topology determines the behavior of CO₂ and hydrogen (H₂) in the active sites. Specific zeolite structures with specific synthesis procedures can selectively adsorb these reactants and promote their transformation into the desired products under the appropriate conditions. For instance, Ramirez et al. [58] explored the significant role of zeolite topology in the direct conversion of CO₂ into valuable hydrocarbons. They reported that catalysts using different zeolite frameworks, specifically MOR and ZSM-5, exhibit markedly different behaviors in their ability to convert CO₂ to olefins and aromatics, respectively. The study also highlighted that the unique structural characteristics of MOR favor olefin production, while the properties of ZSM-5 enhance aromatic synthesis under the same reaction conditions. This variation is attributed to how these zeolite structures interact with potassium superoxide-doped iron oxide catalysts, influencing the formation of surface formate species and the activity of carbocation intermediates (see Figure 4).

3.2. Bimodal Mesoporous Structure and Surface Oxygen Vacancies

Bimodal mesoporous zeolites, with their dual-sized pores, optimize CO₂ hydrogenation by boosting mass transfer and catalytic site availability. The adjustment of the zeolite’s pore architecture can be accomplished by developing a hierarchical pore system. This system ensures that the zeolite not only retains its intrinsic micropores but also incorporates supplementary meso- or macropores, which may be located either within or between the crystals [59,60]. In this way, the mesopores enhance the ingress of reactants like CO₂ and H₂, while surface oxygen vacancies act as active sites or alter adjacent metal sites to better activate CO₂ and promote essential reduction steps in the hydrogenation process. Song et al. [61] explored the influence of zeolite morphology and pore structure, focusing on aromatics production using a Cu-Fe₂O₃-HZSM-5 catalyst. The study involved commercial zeolites and those synthesized via various methods such as dry-gel, hydrothermal, and phase-transfer techniques. Their research found a preference for mesoporous structures in enhancing aromatic production, particularly noting that the HZSM-5 produced by the phase-transfer method showed an improved selectivity for aromatics from 56.61% to 61.94%, attributed to its rich 5–12 nm pores, which facilitate the formation of aromatics like BTX. In the same investigation, they also proposed an “H recycling mechanism” for CO₂ hydrogenation to aromatics, suggesting that hydrogen species from the zeolite can move to the oxide interface and interact with adsorbed CO₂ at oxygen vacancies. Similarly, Tian et al. [62] developed M(Z, Ca, In)-UIO-66/palygorskite-HZSM-5 zeolite tandem catalysts for converting CO₂ directly into aromatics, employing Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and Density Functional Theory (DFT) to investigate the reaction mechanisms. The standout, 6%Zn-UIO-662/Z5 at 550 °C, achieved the highest specific surface area of 437.50 m²/g, optimal acidity, and numerous oxygen vacancies, leading to an impressive CO₂ conversion rate and aromatics selectivity of 88.8%.

The adsorption properties and pore size distributions of Zn-UIO-66/Z5 tandem catalysts across varying calcination temperatures and Zn contents are depicted in Figure 5. The catalysts exhibited type IV isotherms with H4-type hysteresis at higher pressures (P/P0 = 0.7–1.0), indicating the presence of both micropores and mesopores, the latter attributed to the agglomeration state of acidified palygorskite nanofibers. Adjusting crystallization times during Z5 zeolite preparation allowed for controlled interstitial pore formation between nanofiber crystals, thus improving reactant and product diffusion. Notably, the addition of Zn enhanced the adsorption capacities, with optimal results at 6% Zn inclusion. However, excessive calcination temperatures can compromise structural integrity and reduce adsorption capacities. The integration of Zn and UIO-66 into Z5 zeolite and the presence of acidified palygorskite were key to enhancing pore structure and surface area, which in turn improved the catalyst’s performance in CO₂ hydrogenation to aromatics. The pore size distribution is predominantly centered around 10–20 nm, contributing to efficient catalysis. Therefore, the mesoporous structure within zeolites enhances the movement of olefin intermediates towards acidic sites. This improved diffusion pathway leads to increased production yields of aromatic compounds. Furthermore, in the work by Yan et al. [63], they discovered that Ni catalysts supported on zeolites with lower Si/Al ratios (such as 4A, Ni-5A, and Ni-13X) possess large Ni nanoparticles that engage weakly with the support and exhibit lower activation temperatures, leading to notable CO₂ methanation activity, with conversions between 70.4–70.9% and a high methane selectivity of 92.4–96.4%. In contrast, zeolites with higher Si/Al ratios and mesoporous structures (such as Ni-ZSM-5 and Ni-BEA), while demonstrating smaller Ni particles and stronger metal-support interaction, resulted in lower CO₂ conversion rates (71.2–73.4%) and methane selectivity, indicating another crucial role of mesopores in catalytic performance. When selectivity decreases in conjunction with the mesoporous structure of zeolites, it suggests that while the mesopores may enhance the accessibility of reactants to the catalytic sites by improving diffusion, they may also allow for a wider range of reactions to occur, which can lead to a broader array of products. In other words, the larger mesopores are not as selective in facilitating only the desired reaction pathways; instead, they might enable side reactions, leading to an assortment of products rather than selectively yielding the target molecule. For CO₂ hydrogenation specifically, if a mesoporous zeolite results in decreased selectivity, it means that while more CO₂ might be converted (enhanced activity), the proportion of the desired product (such as methane in the case of methanation in Yan et al.’s study [63]) is lower because other products are also formed. This could be because mesopores allow not only the desired reactants to access the active sites more easily but also potentially allow unwanted reactants to participate in the reactions, or they might not effectively stabilize the transition states leading to the desired products. Overall, while bimodal mesoporosity and surface oxygen vacancies can improve catalytic activity, their influence on selectivity is a delicate balance that requires careful optimization to favor the desired CO₂ hydrogenation pathways.

3.3. Si/Al Ratio

Zeolites with minimal acidity tend to promote hydrogen transfer reactions, often leading to the generation of longer-chain hydrocarbons. In contrast, an excessive number of active sites may be conducive to the formation of saturated hydrocarbons, eventually leading to coking [11]. Thus, the Si/Al ratio must be adjusted with high precision, because this ratio is a direct measure of acidity, to maximize the number of Brønsted acid sites for the best performance. Da Costa-Serra et al. [64] found that Ni-based catalysts mounted on delaminated ITQ-2 zeolite, when presenting higher Si/Al ratios, demonstrated enhanced catalytic activity attributed to increased hydrophobicity. The catalytic outcomes for the ITO-2 and ZSM-5 frameworks varied based on their aluminum content, with high aluminum leading to consistent catalytic behaviors across different zeolite types. Notably, a reduction in aluminum content in the support contributes significantly to catalytic efficiency, as seen in delaminated ITO-2, identified by XRD (see Figure 6a) to have smaller NiO crystallite sizes, resulting in higher CO₂ conversion (50% improvement) and CH₄ selectivity (98%). However, the size distribution of Ni⁰ particles in the 5Ni/ITQ-2(∞) sample remains tightly controlled and closely matches the pre-reaction size, measuring 2.8 nm after the reaction compared to 2.4 nm beforehand (see Figure 6b).

The study by the same authors revealed that higher aluminum content supports (lower Si/Al ratio) yielded similar performances in catalysts due to enhanced Brønsted acidity and hydrophobicity, which are beneficial for reaction progress. Additionally, a high Si/Al ratio improves the dispersion and stability of Ni nanoparticles, influencing their initial dispersion and subsequent thermal redistribution. This relationship highlights the crucial role of aluminum content and Si/Al ratios in determining the properties and effectiveness of these catalysts, which were also previously reported to be significant for methanation processes [65]. Delving deeper, Tada et al. [66] developed multifunctional catalysts, ZnO/ZrO₂, by blending mordenite (MOR) zeolites with varying Si/Al ratios (9, 14, and 104) with ZnO/ZrO₂ and assessed their impact on CO₂ hydrogenation efficiency using a high-pressure flow reactor. Figure 7 illustrates the space-time yield of various hydrocarbons for three different catalysts, namely MOR009, MOR014, and MOR104, over a span of time.

For MOR009 (see Figure 7a), the production levels of methane and C₂–C₄ olefins are fairly consistent, though the yield of methanol shows slight fluctuations. Notably, the production of C₅ olefins and paraffins is almost non-existent, which indicates a limited ability to generate higher carbon number hydrocarbons. In contrast, MOR014 in Figure 7b demonstrates a growing yield of C₅ olefins that peaks midway through the assessment period and then modestly declines, while the output of C₅ paraffins remains stable. This behavior suggests a heightened capacity for creating heavier olefins compared to MOR009. Lastly, the MOR104 catalyst in Figure 7c maintains a stable output of C₅ olefins throughout the period, with only minor fluctuations observed. The production rates for C₂–C₄ paraffins are consistent, although C₅ paraffins exhibit some variability. Overall, MOR104 displays a robust capability for producing C₅ olefins, highlighting its effectiveness for sustained light olefin production. For CO₂ aromatization, Cui et al. [67] assessed the impact of zeolite acidity by varying the Si/Al ratio using a Na-modified spinel oxide ZnFeOx coupled with H-ZSM-5 zeolites. They observed that an increased Si/Al ratio led to a decrease in aromatic selectivity due to the reduced density of Brønsted acid sites, which are pivotal for aromatization. Consequently, a higher density of Brønsted acid sites results in greater selectivity for aromatics [68].

However, greater CO₂ adsorption does not necessarily imply greater hydrogenation activity, which is the main problem with typical zeolite catalysts. The hydrogenation process is not solely dependent upon the amount of CO₂ adsorbed. Instead, it is also dependent upon the availability and reactivity of H₂ within the catalyst system and the catalyst’s ability to help the reaction between CO₂ and H₂ to form hydrocarbons. A zeolite with a higher CO₂ adsorption capacity, in consequence of its higher basicity (linked to a lower Si/Al ratio), may be able to retain CO₂ more tightly, thus hindering the reaction with H₂, if the CO₂ is not easily released to react with H₂. The equilibrium between the strength of the adsorption and the reactivity is fragile, whereby strong adsorption could interfere with the catalytic process, while weak adsorption might not capture CO₂ efficiently enough for the reaction. Therefore, the ideal scenario for enhanced hydrogenation activity is not just high CO₂ adsorption but also an optimal level of adsorption that allows for effective interaction with H₂, leading to the formation of the desired hydrocarbon products.

3.4. Porosity, Thermal Stability, and Structural Integrity

The porosity, thermal stability, and structural integrity of structured zeolites are also pivotal for optimizing CO₂ hydrogenation processes. These factors are integral to the zeolite’s ability to adsorb reactants, withstand high temperatures, and maintain its structure under reaction conditions, which, in turn, affects catalytic performance, product selectivity, and potentially its commercial applicability. In a pioneering study by Xiang et al. [69], an AFX-type SAPO-56 zeolite was innovatively augmented with copper (Cu) nanoparticles, crafting a multifaceted catalyst aimed at converting CO₂ to methanol. This novel catalyst achieved a CO₂ to methanol conversion rate of 16.4% and showcased an impressive 80.9% selectivity for methanol production. In this study, the strategic dispersion of Cu nanoparticles across the extensive pore network of the SAPO-56 zeolite markedly boosted H₂ adsorption and its subsequent dissociation, facilitating widespread H₂ atom availability. These atoms interacted with the zeolite’s intrinsic Lewis acid sites, effectively catalyzing methanol synthesis. The catalyst’s hierarchically structured porosity was pivotal in steering product selectivity while ensuring exceptional stability, highlighting its potential for industrial deployment in CO₂ reduction initiatives. Similarly, Cui et al. [70] made significant strides in the catalytic conversion of CO₂ to methanol by creating a zeolite-encapsulated Cu/ZnOx@Na-ZSM-5 catalyst with a core–shell structure derived from bimetallic CuZn-HKUST-1 nanoparticles via hydrothermal synthesis. This design also effectively prevented Cu sintering and enhanced the catalyst’s thermal stability. The zeolite framework not only contributes to a higher methanol yield by ensuring a synergistic effect and intimate interaction of the encapsulated nanoparticles but also upholds the structural integrity of the catalyst, promoting sustained activity over prolonged durations. These innovations demonstrate the critical role of porosity and the architectural solidity of zeolites in fostering stable and efficient CO₂ hydrogenation processes.

3.5. Electrical and Plasma Interactions

The control of electrical and plasma properties in catalyst design is also essential for achieving high CO₂ conversion rates and selectivity to the desired products operating under various operational conditions. Figure 8 illustrates a setup for plasma-assisted CO₂ hydrogenation using a dielectric barrier discharge (DBD) reactor [71]. Feliz et. al. [72] observed that zeolite structures affect the ionic conductivity and dielectric constant of the catalyst in plasma-assisted reactions. Lower ionic conductivity facilitates a better CO₂ conversion (34%) because of decreased plasma density and CO₂ dissociation at ambient pressure and temperatures under 100 °C. Owing to zeolites’ unique structures, they have been recognized as prospective catalyst supports for DBD plasma systems intended for CO₂-to-methane conversion. Attention has been especially given to Ni-based catalysts within these plasma systems [73,74]. For example, the utilization of Ni/beta zeolite catalysts has been associated with a greater selectivity for methane production, with up to 95% conversion for either CO or CO₂ [75]. It was noted that Ni particles could be redistributed under DBD plasma exposure, which might improve the catalytic process. In addition, this treatment was recommended to promote the desorption of the adsorbed molecules from the catalyst, whereby these adsorbed molecules have much weaker bonds compared to their gaseous counterparts. Reactive species that were produced by plasma were believed to play a role in removing the adsorbed CO molecules, which is a key step for achieving higher methane yield, while the accumulation of water on the catalyst’s active sites, emerging as a reaction byproduct, poses a significant limitation for thermal CO₂ methanation by obstructing access to these sites, in line with the insights provided by Sabatier’s principle [76,77].

When discussing electrical and plasma interactions in such catalytic systems, it is useful to look at the wider meaning of ionic conductivity and its impact on catalytic processes. The work of Henckel et al. [78] is a relevant example of this, as they used Electrochemical Impedance Spectroscopy (EIS) to investigate the interaction between the catalyst and ionomer, with a particular interest in the transport of hydroxide ions in the electrochemical CO to ethylene conversion. The results confirmed that fine-tuning ionic conductivity can substantially promote the Faradaic efficiency of ethylene and the cell voltage. Although the study did not directly concern zeolite materials and hydrogenation of CO₂, the principles that can be derived from their work are of great interest to CO₂ hydrogenation over structured zeolites. The principle of modulating ionic behavior to maximize catalyst performance is a general concept that is observed in diverse catalyst systems, including zeolites for CO₂ hydrogenation. As it happens in electrochemical systems where both ionic transport and electrical interactions are crucial for the reaction outcomes, in zeolite catalysts, these characteristics can affect the diffusion of reactants and the distribution of active sites and subsequently determine the efficiency and selectivity of the hydrogenation reactions.

4. Synthesis Methods

Structured zeolites can be synthesized in many ways by utilizing precursors in soluble form and reducing them, or by using existing solid compounds and modifying their structures. These methods can be broadly grouped into two categories: bottom-up and top-down approaches [79], as shown in Figure 9a.

4.1. Bottom-Up Approach

The bottom-up approach focuses on creating mesopores within the zeolite structure using a variety of templating methods. Hard-templating involves using rigid carbonaceous materials like carbon black, carbon nanotubes/nanofibers, or 3DOM carbon templates to shape the forming zeolite structure [80,81,82]. In a study by Wu et al. [83], Ni nanoparticles were encapsulated within microporous graphene-like carbon derived from a NaY zeolite template. The resulting Ni-loaded catalyst demonstrated efficient CO₂ hydrogenation to methane, with 50% conversion and 96% methane selectivity at high space-time yields, showcasing remarkable long-term stability. Subsequently, soft-templating is less rigid and facilitates the crystallization of zeolites by forming interactions with zeolite precursors or frameworks through a range of covalent and electrostatic forces, including van der Waals interactions, hydrogen bonds, and ionic attractions [84]. It utilizes quaternary ammonium surfactants such as cetyltrimethylammonium bromide (CTAB) or cationic polyelectrolytes such as polydiallyldimethylammonium chloride (PDDA-Cl) and polydiallyldimethylammonium chloride (PDADMAC) [85,86,87]. These create microporous zeolite crystallization, while the organosilane’s hydrophobic alkyl chains direct mesopore formation [88]. However, the conventional hydrothermal synthesis method (see Figure 9b), is notable for its substantial byproduct generation, including waste materials and gases such as CO₂ and N₂, and its prolonged crystallization process typically leads to reduced production efficiency [89]. Therefore, alternative ‘green’ synthesis routes have been sustainably employed to overcome such concerns. For example, Tian et al. [90] described an innovative approach for crafting SAPO-34 zeolites with layered porosity using only rice husk as the silicon source. The resulting bio-SAPO-34 exhibited high efficiency in converting CO₂ to olefins, with an impressive C₂–C₄ olefin selectivity of 94.5%. The study found that CH₃O* species were pivotal intermediates, with DRIFTS analysis showing their formation on the ZnZrOx catalyst surface and subsequent transfer to the acid sites of bio-SAPO-34 for effective carbon–carbon coupling. In addition, and interestingly, Din et al. [91] performed the green synthesis of bimetallic catalysts supported on zeolites, whereby it involved the deposition of Cu and Co onto the zeolite rather than the synthesis of the zeolite framework itself, which did not require extensive washing of precipitates. The process included dissolving the metal acetates in water, adding zeolite to the solution, adjusting the pH with sodium hydroxide, and maintaining the mixture at a specified temperature while stirring. The final product was then dried, resulting in the formation of bimetallic Cu-Co/zeolite catalysts. The ideal balance of Cu:Co at a 1:1 ratio was determined to be the most effective among the various metal combinations tested, leading to the highest observed rate of methanol synthesis.

Furthermore, non-templating methods rely on alternative strategies like one-pot synthesis, dry-gel conversion, or seed-induced crystallization without using traditional templates [54,89]. For instance, Wei et al. [53] developed a Na–Fe₃O₄/HZSM-5 catalyst using a one-pot synthesis technique. This catalyst demonstrated the ability to transform CO₂ into hydrocarbons within the gasoline range (C₅–C₁₁), achieving a selectivity of 78% among hydrocarbons and limiting methane to just 4%, with a CO₂ conversion rate of 22% under conditions pertinent to industry. They built the catalyst at the molecular level and relied on chemical reactions between iron salts and NaOH in solution to spontaneously form the catalyst without a structural template.

The bottom-up approach to structured zeolite synthesis is not without difficulties. A principal issue is the intrinsic fragility of hierarchical zeolite structures, which could result in loose intermolecular interactions, thereby affecting mechanical stability and charge transport [92]. This can be an especially serious issue when zeolites are used under the high-temperature and high-pressure conditions characteristic of CO₂ hydrogenation processes. One possible approach is to optimize the strength and performance of the materials by linking the molecules through strong covalent bonding that involves the reaction directly on the surfaces [92]. Furthermore, the bottom-up approach sometimes struggles with the controlled physical and chemical transformations within liquid droplets, a process that is integral to the synthesis of zeolites [93]. An alternative method, encompassing sol-gel and aerosol-assisted approaches, is proposed due to its enhanced focus on controlling the physical and chemical properties during the synthesis of nanomaterials, particularly resultant zeolites [94,95]. Another challenge in the bottom-up synthesis of zeolites is managing intercrystallite diffusion, which can impede reactant and product movement within the crystalline lattice [96]. To address this, an optimal strategy involves enhancing porosity through methods like dealumination, desilication, and recrystallization. These techniques, which now fall under the top-down approach, modify existing zeolite structures to improve molecular accessibility and diffusion pathways.

4.2. Top-Down Approach

The top-down approach involves modifying pre-existing zeolite structures through post-synthesis treatments to enhance their properties, rather than constructing new materials from the ground up. This method focuses on altering existing materials to develop structures with novel characteristics. Top-down approaches play a vital role in the development of hierarchical zeolite structures, which boast improved catalytic properties due to increased surface area and pore volume, as well as the surface properties of the zeolites. Dealumination is a process where aluminum atoms are removed from the zeolite framework through methods such as acid leaching or steam treatment, impacting the Si/Al ratio [97,98,99]. Its counterpart, desilication, involves the removal of silica atoms, typically by treating them with a base solution [100]. This procedure enhances mesoporosity within the zeolite structure [101]. Recrystallization typically involves dissolving the zeolite in a suitable solvent and then causing the material to precipitate out of the solution, forming a more ordered crystalline structure [102,103]. The parameters of the process, such as temperature, concentration, and choice of solvent, can significantly influence the outcome [89]. For example, Oishi et al. [48] employed a multi-top-down approach to modify MOR zeolite, initially transforming it into a slurry via milling and subsequently recrystallizing it under heat in an autoclave, resulting in nanosized MOR particles. The processed MOR underwent ion exchange with NH₄Cl, fine-tuning its ionic makeup. They then performed dealumination, using different sulfuric acid concentrations to adjust the Si/Al ratio, thus tailoring the zeolite’s acidity and enhancing its catalytic abilities. Lastly, defect healing with a fluoride solution was conducted on the dealuminated MOR to mend structural imperfections, optimizing the material. Following the modifications, the enhanced MOR sample exhibited improved catalysis in CO₂ hydrogenation to lower olefins, notably doubling the olefin-to-paraffin ratio from 0.69 to 1.4, thus signifying a more selective and efficient reaction. Furthermore, Liu et al. [57] demonstrated alternative top-down approaches such as incipient wetness impregnation (IWI) and ion exchange (IE) for a series of Co/ZSM-5 catalysts to facilitate CO₂ hydrogenation to value-added chemicals via the modified-FTS pathway. They found that the selectivity for CO₂ hydrogenation on catalysts prepared by IWI strongly depends on the Si/Al ratio. In contrast, such a ratio has little impact on catalysts produced via IE. Enhancing the Si/Al ratio in impregnated catalysts usually improves olefin production, which is associated with acidic features and cobalt reducibility. However, in the same study, altering the Si/Al ratio in ion-exchanged catalysts did not influence the acid properties of K-ZSM-5 for catalytic performance. This lack of effect can be attributed to the introduction of alkali metals, which increased the number of basic sites and reduced the crystallinity. Therefore, it is essential to avoid excessive modification of atoms, as this can lead to the destruction of the crystal structure, thus underscoring the necessity for controlled and balanced treatment processes [50].

The top-down methodology for synthesizing zeolites also encapsulates a set of distinctive challenges that command innovative solutions to further enhance their catalytic performance. Firstly, inadequate characterization methods used for zeolites synthesized by the top-down approach prove to be one of the serious limitations whereby it is crucial to disclose the specific T-sites where the metal centers are situated, as these locations dictate the geometry of the active centers within the zeolites and, consequently, define their reactivity [104,105]. To address this challenge, Dapsens et al. [106] emphasized the utilization of advanced X-ray Absorption Spectroscopy methods, such as the integration of Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES), in overcoming analytical limitations. These techniques were pivotal in the studies conducted by Liu et al. [57] and Ding et al. [107], which focused on the characterization of Co/ZSM-5 and Cu@NaBeta catalysts, respectively. Their investigations demonstrated that the synergistic interaction between metal nanoparticles and the zeolitic framework plays a critical role in forming high-performance catalysts for CO₂ hydrogenation to olefins and ethanol accordingly. Next, the structural integrity of catalysts can be compromised by water generated during reactions. Specifically, during a CO₂ methanation reaction, zeolites are vulnerable to dealumination, a process in which aluminum atoms are stripped from the framework. This occurs under high operating temperatures and pressures in the equipment, particularly in the presence of water molecules, leading to partial degradation or impairment of the zeolite structure [108]. Such conditions can also result in the clogging of the zeolite pores as the released aluminum accumulates within the structure, further compromising its effectiveness. To mitigate these concerns, Bacariza et al. [109] suggested that commercial USY zeolites, typically used in fluid catalytic cracking (FCC) industrial processes and subjected to ultra-stabilization treatments including controlled steaming for dealumination, could serve as effective supports for CO₂ methanation catalysts. The same authors also pointed out that employing USY zeolite as a support demonstrated promising results, with no structural damage observed in the samples after both conventional and deactivation tests, even when exposed to water during the reaction. In addition, Verboekend et al. noted desilication issues like limited Si/Al ratio control, over-extraction of silicon, structural damage, ineffective pore agents, and uneven mesopores affecting zeolite performance [110]. Optimizing zeolite desilication involves careful control of alkali treatment conditions and the use of pore-directing agents to ensure consistent pore size while maintaining the zeolite’s structure and acid sites [111]. Selective treatment is key to achieving ideal mesoporosity without compromising catalytic efficiency. Using tetrapropylammonium hydroxide (TPAOH) for selective desilication, for example, Dai et al. [112] engineered silicalite-1 with precise mesoporosity, achieving nanocube sizes up to 600 nm. This approach also improved CO₂–hydrocarbon conversion, with a marked increase in C₅₊ hydrocarbon selectivity when iron-based catalysts were supported on these tailored zeolites. Similarly, Sharma et al. [113] designed a novel two-bed catalytic system for CO₂ hydrogenation via the MTH pathway, leveraging a desilicated HZSM-5 zeolite to boost hydrocarbon yields, with a notable hydrocarbon selectivity of up to 71.2%. This system, through selective desilication, enhanced the production of longer-chain liquid hydrocarbons, impressively improving C₈–C₁₂ selectivity from 29.2% to 42.4%. Finally, recrystallization concerns are not singled out because they can be part of a broader set of synthesis conditions that also include dealumination and desilication. Overall, these conditions collectively impact the structural and compositional modifications of zeolites during the top-down approach.

5. Characterization Techniques

From the preceding sections, it is clear that catalyst characterization techniques are indispensable tools for gaining a comprehensive understanding of the catalytic mechanisms, performance, and intricate behavior of zeolites in catalysis, such as CO₂ hydrogenation processes. These advanced techniques in

Table 3. Advanced characterization techniques for structured zeolites in catalytic reactions like CO₂ hydrogenation.

Category	Method	Application
Molecular and Chemical Structure Analysis [5,114,115]	Fourier-transform infrared (FTIR) spectroscopy	Quantify absorption spectra in chemical bonds and functional groups in molecules
	Raman spectroscopy	Postulate information about molecular vibrations, crystal structures, and phase transitions
	Nuclear Magnetic Resonance (NMR) spectroscopy	Provide detailed information on the framework aluminum distribution and the nature of acid sites
	Small-angle X-ray Scattering (SAXS)	Analyze the structural details of materials at the nanoscale level by detecting inhomogeneities and phase separation within
Crystallographic and Phase Analysis [54,115,116]	X-ray diffraction (XRD)	Assess crystalline structures, crystal phases, and crystal defects
	Powder X-ray Diffraction (PXRD)	Analyze powdered crystalline materials for crystal structure identification
	Selected Area Electron Diffraction (SAED)	Obtain crystallographic information from a sample area
Surface and Elemental Analysis [91,117,118]	X-ray photoelectron spectroscopy (XPS)	Examine the chemistry of the surface, including aspects such as elemental composition, chemical and empirical states, and the electronic state of elements
	X-ray Absorption Spectroscopy (XAS)	Determine local geometric/electronic structural order
	Auger Electron Spectroscopy (AES)	Detect emitted energy of electrons from the catalyst surface
Microscopy and Imaging [115,119]	Scanning Electron Microscopy (SEM)	Generate high-resolution images of the surface, internal structure, morphology, and crystallography of nanomaterials
	Transmission Electron Microscopy (TEM)
Thermal Analysis [4,5,54,115,120]	Temperature-Programmed Reduction-Thermogravimetric Analysis (TPR-TGA)	TPR: Measure the change in chemical state upon heating TGA: Measure changes in physical and chemical states upon heating
	Temperature-Programmed Desorption (TPD)	Investigate adsorption and desorption behaviors on surface interactions and binding energies
	Temperature-Programmed Oxidation (TPO)	Evaluate oxidation behaviors, particularly in carbonaceous materials, catalyst deactivation investigations
	Temperature-Programmed Reaction (TPRe)	Study reaction kinetics, and catalytic stability under different thermal environments
	Temperature-Programmed Surface Reaction (TPSR)	Focus on surface reactions; mechanisms of surface-mediated reactions
	Temperature-Programmed Reduction/Oxidation (TPR-O)	Explore redox properties for redox reactions
	Temperature Programmed Reduction-Differential Thermogravimetry (TPR-DTG)	Determine the temperatures at which reduction events occur and the quantitative aspects amount of oxygen removed from an oxide
	Temperature-Programmed Ammonia Desorption (TPAD)	Observe ammonia-desorption for acid catalysis

help elucidate the structural and chemical properties of catalysts, which are critical for optimizing their activity and stability under tailored reaction conditions. In this section, we will delve deeper into the specific methods and their applications in detailing the structural adaptations and chemical dynamics of zeolites, which facilitate their efficacy in CO₂ hydrogenation. We will explore how these characterization methods contribute to a better understanding of the interaction between catalysts and reactants and how this knowledge can lead to the development of more efficient catalytic systems. For the subsequent discussions, we will only highlight specific characterization methods from each category, providing detailed illustrations of how these techniques are applied to enhance our understanding of zeolite catalysis in CO₂ hydrogenation.

Molecular and chemical structure analysis techniques, such as FTIR and Raman spectroscopy, measure molecular vibrations and identify chemical bonds to reveal the composition of substances. NMR spectroscopy offers a detailed view of atomic-level structure, while SAXS enables the investigation of nanoscale inhomogeneities and phase distribution within materials. In a study by Ramirez et al. [5], NMR spectroscopy provided detailed insights into the selective catalytic roles of various zeolites (MOR, ZSM-22, FER, and ZSM-5) to explore the role of CO at the RWGS reaction phase of the modified-FTS route for CO₂ hydrogenation to alkenes, aromatic compounds, and alkanes. Figure 10a highlights how MOR zeolites are inclined towards producing lighter olefins, whereas ZSM-22 zeolites preferentially lead to heavier olefin production. Additionally, ZSM-5 achieves complete ethylene conversion, while MOR and ZSM-22 exhibit the lowest, with MOR specifically favoring paraffin production when ¹³CO is absent. However, the introduction of ¹³CO sees MOR attain the highest conversion rate at around 4%, with ZSM-5 and ZSM-22 at roughly 1% (Figure 10b). Solid-state NMR confirms COs direct incorporation into the zeolite structure (Figure 10c), and a notable carbonyl peak at 177.5 ppm associates with methyl acetate suggests a ketene intermediate in the reaction (Figure 10d). This set of NMR figures paints a comprehensive picture of how each zeolite type uniquely influences the reaction pathway and product distribution.

Crystallographic and phase analysis methods encompass techniques like XRD, which examines the arrangement of atoms within crystals and identifies phases and defects. PXRD is tailored to analyze powdered samples for structural identification, while SAED provides crystallographic information from targeted areas within a specimen. On the other hand, microscopy and imaging techniques like SEM and TEM are pivotal for capturing high-resolution images that reveal surface features, internal structures, morphologies, and the crystallography of zeolites. Krachuamram et al. [115] conducted a study to fine-tune the synthesis of NaA/NaX zeolite to maximize surface area and pore volume, finding the best conditions to be a 5 day aging and a 9 h crystallization process in the presence of CTAB and heptane. When used as a support for Ni impregnation at various loadings, the zeolite synthesized under these conditions showed the highest catalytic activity for CO₂ methanation at a 15% Ni level, attributed to the optimal dispersion of Ni on the zeolite surface. Their XRD analyses indicated a notable reduction in the crystallinity of the zeolite, with peak intensities for NaX and NaA decreasing upon Ni impregnation (Figure 11a), and NiO reflections becoming more pronounced. The specific surface area of the zeolite increased significantly from 166.5 m²/g to 1411 m²/g with Ni addition (Figure 12b), suggesting a dispersion of Ni that did not disrupt the zeolite’s structure, as confirmed by SEM images (Figure 12a). Despite these structural changes, the catalytic efficiency remained primarily reliant on gas adsorption capacities rather than on the surface area, with the presence of Ni-enhancing adsorption sites for H₂ and CO₂ (Figure 11b).

Surface and elemental analysis for zeolites typically employ techniques such as XPS, which analyzes surface chemistry, including elemental composition and states; XAS, for assessing the local structure and electronic state order; and AES, which detects the energies of electrons emitted from surfaces, providing details on the surface’s elemental composition. Conversely, under varying thermal conditions, TPR-TGA allows for the study of reduction processes and zeolite stability. TPD can help determine the strength and nature of adsorption sites by measuring the gases desorbed as the temperature increases. Similarly, TPR-O and TPR-DTG provide insight into the oxidative and reductive characteristics of zeolites, while TPSR and TPAD are employed to examine surface interactions and acidity, respectively.

Popova et al. [54] developed micro-mesoporous ZSM-5 catalysts modified with Ni and Ru through wet impregnation, examining how Ni content and the addition of Ru influenced the catalysts’ properties by utilizing TPR-DTG to analyze, and they observed distinct reduction peaks. For monometallic samples with 10%Ru, a significant reduction peak occurred at 190 °C, while samples with 5%Ni and 10%Ni exhibited two peaks spanning temperatures of 195–205 °C and 325–360 °C, respectively, suggesting Ni’s influence on the reduction process and dispersion on the zeolite (Figure 13a). The XPS analysis post-reduction offered insights into the surface chemistry, indicating a prevalence of RuO₂ for the monometallic samples (e.g., 5RuZSM-5) and a more balanced presence of reduced and oxidized Ru states in the bimetallic samples, such as 10Ni5RuZSM-5 (Figure 13c). These findings aligned with their TPR-DTG results, revealing that bimetallic samples with higher Ni content have more Ni accessible on the surface, which enhanced catalytic activity due to a greater number of active sites, whereby 10Ni5RuZSM-5 exhibits a single, wider peak reaching its apex at 268 °C (Figure 13b). As a result, the 10Ni5RuZSM-5 catalyst showcased high efficiency, achieving 100% selectivity for methane production from CO₂ at 400 °C. This performance, coupled with its stability and the potential for reuse, underscores the catalyst’s applicability in practical settings.

In common practice, a combination of these characterization techniques in Table 3 often provides a more complete picture. For instance, SEM might be paired with XRD to correlate surface morphology with crystalline structure, or TPR-TGA might be used alongside XPS to evaluate how thermal treatments affect surface chemistry. The choice depends on whether the study aims to optimize the catalyst’s structural properties, understand the chemical processes occurring during the reaction, or improve the catalyst’s stability and reusability. A multi-technique approach often yields the most comprehensive understanding of the catalytic system.

6. Conclusions

CO₂ hydrogenation over structured zeolites has emerged as a significant technological approach for carbon neutrality, leveraging the conversion of CO₂ into valuable hydrocarbons. Two main methods, the modified-FTS and the MTH route, effectively channel CO₂ into a variety of hydrocarbons, with zeolites playing a pivotal role due to their unique catalytic properties. The thermal hydrogenation method stands out over electrochemical, photocatalytic, biochemical, and chemo-enzymatic approaches for its high efficiency, selectivity, and proven scalability. This positions zeolites as an optimal catalyst for CO₂ hydrogenation, capable of undergoing regeneration and withstanding the high temperatures required for these processes.

Subsequently, the efficacy of structured zeolites in CO₂ hydrogenation depends on the synergy between their composition, preparation, and unique structural properties. Careful calibration of the Si/Al ratio is critical, as it influences acidity and CO₂ adsorption, which are key factors for selectivity and conversion efficiency. Bimodal mesoporous zeolites improve catalytic activity due to enhanced mass transfer and accessible sites but require careful optimization to maintain product specificity. The integration of metals such as Cu and Ni within zeolite frameworks, coupled with plasma treatments, has been shown to boost performance, balancing strong adsorption with necessary reactivity. Attention to electrical and plasma interactions within the catalyst design is therefore paramount, ensuring high conversion rates and desired selectivity in CO₂ hydrogenation processes.

In addition, optimizing zeolite synthesis through bottom-up and top-down methods is critical for enhancing CO₂ hydrogenation. Bottom-up strategies, such as various templating techniques, improve mass transfer by overcoming the diffusion limitations inherent in microporous zeolites and by embedding metals like Cu and Co to augment hydrocarbon production. To combat issues like stability under extreme conditions, these methods are evolving to reinforce molecular bonds and manage intracrystalline diffusion. Top-down methods, including dealumination and recrystallization, further refine zeolite structures, bolstering mesoporosity and catalytic selectivity without altering the crystal integrity. Advanced spectroscopy (e.g., EXAFS and XANES) plays a key role in this fine-tuning, guiding the precise distribution of metal nanoparticles and ensuring optimal catalytic functionality. Furthermore, CO₂ hydrogenation over zeolites benefits from a suite of characterization techniques. FTIR and NMR reveal molecular intricacies and active sites critical for the catalytic process, while phase analyses like XRD detail crystalline structures that influence functionality. Elemental analyses like XPS provide surface analysis crucial for CO₂ interaction, and microscopic imaging like SEM gives insight into morphology and metal distribution, affecting activity and selectivity. Thermal methods, like TPR-TGA gauge stability and reducibility, are essential for catalyst durability.

Looking forward, the trajectory of CO₂ hydrogenation using structured zeolites is set towards transformative progress. Further development of synthesis approaches (bottom-up and top-down), together with sophisticated characterization methods as listed in Table 3, is fundamental for realizing the whole potential of zeolite as an efficient catalyst. With the world moving towards greener and more sustainable options, structured zeolites play a more and more significant role in holistically converting CO₂ into high-value-added chemical products. Achieving optimal selectivity is a delicate balance that is centered on numerous factors, including the accessibility of active sites, metal particle dynamics, support structure interactions, and overall porosity, each shaped by synthesis and characterization methods. Future research will focus on overcoming current limitations, as remarked in this review, and innovating to enhance catalyst efficiency and selectivity, thereby contributing to the wider goal of reducing the carbon footprint and addressing climate change.