1. Introduction
Global warming is a significant concern as it is exacerbated by the excessive emissions of greenhouse gases, specifically carbon dioxide (CO
2), which contributes to mounting climate-related issues [
1]. CO
2 hydrogenation is gradually emerging as a critical technology for carbon neutrality, which provides a route for CO
2 valorization into valuable chemicals and fuels [
2,
3]. The process offers a two-fold solution; besides addressing the pivotal issue of CO
2 emissions, it also assists in the sustainable production of heavy hydrocarbons [
4]. The selective and optimized transformation of CO
2 represents a current challenge considering its thermodynamic stability and inert properties [
5].
Structured zeolites, with their unequaled porous structures and acid-base properties, are used as a catalyst in a whole range of chemical processes, among them is CO
2 hydrogenation. Their ability to catalyze a breadth of reactions—from the Fischer-Tropsch Synthesis (FTS) process to the direct synthesis of aromatics—highlights their generality in catalysis [
6].
Figure 1 shows the classes of well-known zeolites (e.g., natural [
7], and synthetic [
8,
9,
10]). Both zeolite types have been widely applied not just in catalysis [
11], but even in other industries such as water and wastewater treatment, agriculture, biomedical, laundry detergents, and construction [
12,
13,
14,
15,
16]. For CO
2 hydrogenation, zeolites application has received considerable attention over the last decade.
Figure 2 shows the distribution of number of articles displayed on google scholar when the phrase “CO
2 hydrogenation over zeolites” is used to search articles. The trend clearly indicates an unprecedented growth in the application of zeolites for CO
2 conversion reactions, particularly over the last decade where an exponential trend is 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, and alcohols, and heavy olefins [
5,
17]. It should be noted that zeolites by themselves have been widely studied for various applications, nonetheless, the recent developments in the field of CO
2 catalysis and growing interest of researchers in the application of zeolites for CO
2 conversion creates 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 CO
2 hydrogenation over structured zeolites by offering a synoptic view of the different 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 upon the challenges and prospects in this field whence one can get ideas on structured zeolites and the way they may be used to reduce the global CO
2 problem.
4. Characterization Techniques
Catalyst characterization techniques have now become essential instruments in acquiring a high level of understanding regarding the catalytic mechanism, performance, and the complex behavior of catalysts in operation. The methods illustrated in
Table 1 have been used to identify the structural and electronic properties of catalysts, the nature of active sites, and the interaction of CO
2 and other reaction species on the surface of the catalyst, and understand the adsorption and redox properties of the catalytic system. These techniques have enabled researchers to observe real-time changes in catalyst composition and structure during the hydrogenation process, providing valuable information that guides the design of more efficient and selective catalysts.
Building upon the fundamental knowledge acquired from spectroscopic techniques, recent studies have achieved significant progress in the process of converting CO
2 into high-value-added products using structured zeolites. Song and co-workers [
81] designed zeolites with nanocrystals of 60-150 nm via the soft-template approach. Analyzed using advanced techniques like XRD and FTIR in
Figure 4a,b, the zeolites exhibit superior catalytic performance in various chemical reactions, including benzene alkylation and polymer cracking, due to their extensive surface area and acid sites. A study by Ticali et. al. [
73] revealed the suitability of the combination coating of Zn-doped ZrO
2 with H-ZSM-5 and H-SAPO-34 zeolites to methanol-to-hydrocarbon reaction. The test results disclosed that the 30% ZnZrZnOX amended together with the zeolite ZSM-5 provides the perfect performance of the system, which allowed for the C
3 production to be 1.5 mol kg⁻¹ h⁻¹. This study, presumably employed NMR spectroscopy in addition to FTIR spectroscopy, PXRD, and XAS (see
Figure 4c,d) to conduct a thorough structural examination, serves as evidence for how Zn-doping can give the solution a boost in terms of the quality of the catalyst performed with no zinc leaching observed.
As Popova et. al. [
20] analyzed in
Figure 5, the effect of the reaction conditions on CO
2 hydrogenation was investigated using Ni- and Ru-based ZSM-5 modified catalysts. It appeared that 10Ni5Ru pure zeolite type ZSM-5 was among the most active and highly selective materials in the process of methane production and achieved equilibrium conversion, and 100% selectivity at an operating temperature of 400°C. Among other methods used were Raman spectroscopy, XRD, SAXS, and SAED. Xiang et. al. [
74], while primarily using XRD, SEM, and XPS. also included NMR spectroscopy in its comprehensive analysis of creating a Ni-doped ETS-10 zeolite catalyst for CO
2 methanation with a CO
2 conversion rate of 39.7% and CH
4 selectivity at 280°C. The catalyst of the hierarchical structure and its CO
2 adsorption-activation capability was realized to be of great importance through the study and a new way for the fabrication of highly efficient catalysts. By utilizing FTIR spectroscopy, Azzolina-Jury and Thibault-Starzyk [
82] presented a new catalyst where Ni-doped Zeolite H-USY was used for plasma-assisted CO
2 hydrogenation. Mainly, the study identified CO, which became the product, with methane and other linear hydrocarbons being the result of the hydrogenation of linear carbonyls on nickel surfaces. These findings therefore give us a novel insight into what role the plasma plays in CO
2 hydrogenation. García-Hurtado et. al. [
83] addressed zeolite structure and crystal size impact via XRD and NMR spectroscopy on CO
2 hydrogenation with K-promoted iron oxide and zeolite-based bifunctional catalyst. The study established that the sample with K/Fe
3O
4-MFI gave the highest hydrocarbon yield while CHA and BEA favored the production of C
1-C
4 fractions with high olefin selectivity. The outcome of this research where zeolite structure affected product selectivity provides evidence for the crucial role in this process. Lin et al. [
84] manufactured an advanced catalyst characterized by XRD and NMR spectroscopy, which contains Cr
2O
3 and twin-structured ZSM-5 zeolite that was used for the direct conversion of CO
2 and H
2 into valuable aromatics such as para-xylene.
Zhang et. al. [
85] reported an in-situ spectroscopic study, utilizing Raman spectroscopy and XRD, on the chemical reaction structure of iron active phases regarding CO
2 hydrogenation. This study provided a panoramic view of the transformation of iron oxides (α-Fe
2O
3 and γ- Fe
2O
3) to iron carbides, with χ-Fe
5C
2 showing higher selectivity to lower olefins. Wang and co-workers [
28] accelerated methanol-mediated CO
2 hydrogenation into aromatics by increasing the oxygen vacancy density in chromium oxide (Cr
2O
3). The study proved that the combination of Cr
2O
3 with the acidic zeolite H-ZSM-5 yields high levels of CO
2 conversion as well as selectivity to aromatics. Therefore, this study presented a breakthrough method of combinatorial co-tuning of the catalyst parts for better performance as illustrated in
Figure 6. Finally, the characterization of structured zeolites via temperature-programmed methods has provided substantial insights into their adsorptive and redox capabilities, alongside their catalytic efficiency. Cimino et. al. [
86] have highlighted a paradigm on how temperature-programmed catalytic approaches can be employed in the pursuit of better catalytic performance. The study on Ru/Ce/Ni foams for CO
2 methane fixation highlighted that the porous CeO
2 construction and the dispersion of the Ru particles, in the Ni foam was the cause for much performance improvement, and showed the efficiency of this kind of optimization methodology.
The in-depth characterization of structured zeolites, leveraging advanced techniques such as spectroscopy, microscopy, and thermal analysis, has been instrumental in decoding their intricate structural and chemical nuances. Pioneering case studies have shown the extremely crucial role of metal-metal oxide interaction and transition metal doping in making the catalyst more stable and efficient [
87,
88]. These insights are highly important, as they make known the molecular behavior of zeolites and provide understanding for the development of catalysts of much higher activity and selectivity. On the performance aspect, the optimization knowledge of reaction parameters such as temperature, pressure, CO
2/H
2 feed ratio, etc., as well as the construction of modified kinetic models have provided a more detailed method for the optimizing production of desired hydrogenation products [
83,
89,
90,
91]. Overall, these techniques not only lead to a deeper understanding of catalytic materials and processes but also define the basis for the development of more productive and environmentally friendly ways to convert CO
2 to valuable substances.
5. Conclusion
This review has discussed the recent achievements of CO2 hydrogenation performances of structured zeolites, emphasizing influencing properties, synthesis pathways, and characterization methodologies. The interaction between composition and preparation methods is a significant consideration in the activity of structured zeolites in CO2 hydrogenation. The development of a bimodal mesoporous structure combined with the existence of surface oxygen vacancies results in a substantial improvement of the catalytic activity. Further, the Si/Al ratio, porosity, thermal stability, and structural integrity of zeolites are also crucial parameters, which determine performance and durability in catalysis. Moreover, electrical and plasma interactions add novel ways to enhance the reactivity, selectivity, and efficiency of these catalysts. In terms of catalyst syntheses for structured zeolites, we have explored both bottom-up and top-down approaches. The bottom-up techniques like hard templating, soft templating and non-templating are crucial in dealing with the diffusion limitations that are commonly associated with the microporous zeolites. The formation of the hierarchical structures and mesopore integration technology are the prospective routes for improving catalytic activity. However, dealumination, desilication, and recrystallization, among other top-down approaches, though bound by thermodynamic constraints as well as morphological restrictions, represent opportunities to boost the catalytic performance of structured zeolites for CO2 hydrogenation. The introduction of advanced characterization tools, morphological optimization, and modern thermal reduction technologies can overcome these shortcomings making the catalysts more efficient and commercially viable. The role of catalyst characterization using spectroscopic techniques allows continuous evaluation of the real-time features that the catalyst composition and structure possess, which is imperative in designing more selective and efficient catalysts.
Looking forward, the field of CO2 hydrogenation over structured zeolites is poised for significant advancements. Further development of synthesis approaches, together with sophisticated characterization methods, 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 part in holistically converting CO2 to high-value-added chemical products. Future investigations should be aimed at the elimination of the current drawbacks and the development of novel innovations for increasing the efficiency and selectivity of these catalysts, thus promoting the global strategy of carbon footprint reduction and climate change mitigation.