Synthesis and Applications of Zeolite-Encapsulated Metal Catalysts

Abstract

Supported metal catalysts are extensively applied in the heterogeneous catalysis field. However, metal species are prone to migration and aggregation during catalytic reactions due to their high surface energy, which leads to deactivation. In recent years, the use of porous materials, particularly zeolites, to anchor metal species has gained significant attention. By confining metal single atoms, subnanometer metal clusters, and nanoparticles within the pores or nanocavities of these materials, the dispersion and stability of the metal species can be greatly enhanced, thereby improving the catalytic performance. This review systematically discussed the synthesis principles and diverse methodologies to fabricate zeolite-encapsulated metal catalysts. It further outlined their catalytic applications across various catalysis fields, emphasizing enhanced stability and selectivity enabled by confinement effects. Finally, the review provided critical perspectives on future developments, addressing challenges in precise structural control and scalability for industrial implementation.

1. Introduction

Heterogeneous catalysis is indispensable in various fields, such as energy conversion, petrochemical processing, and environmental protection. It is estimated that approximately 90% of chemical product manufacturing processes involve catalytic reactions. Among these, metal catalysts, with their unique electronic structures and excellent catalytic activity, play a crucial role in numerous catalytic processes. However, metal catalysts face numerous challenges in practical applications. Particularly under harsh reaction conditions such as high temperatures and pressures, metal species often exhibit high surface energy, which makes them prone to migration and aggregation. This leads to the loss of active sites on the catalyst, ultimately resulting in catalyst deactivation [1]. Additionally, metal nanoparticles may leach or become poisoned due to strong interactions with reactants or products, further compromising the stability of the catalyst. For example, in certain reforming reactions, metal catalysts are prone to coke deposition, which covers the active sites and leads to deactivation. These issues significantly limit the industrial application and long-term stability of metal catalysts. Therefore, the development of a new structure of metal catalyst to improve their dispersion, stability, and resistance to sintering has become a key challenge in the field of catalytic research [2].

To address the issues of sintering and instability of metal catalysts at high temperatures, the strategy of encapsulating active metals and anchoring them within porous materials has gained significant attention in recent years [3]. Zeolites, as a class of porous materials with regular channel structures, high surface areas, tunable pore sizes, and abundant active sites, provide an ideal support for encapsulating metal catalysts. By confining metal single atoms; subnanometer metal clusters; and nanoparticles within the framework, nanochannels, and/or nanoshells of zeolites, the sintering and migration of metal species can be effectively suppressed [4,5]. Moreover, the confinement effect of zeolites can alter the electronic structure and surface properties of metal nanoparticles, thereby tuning the selectivity and activity of catalytic reactions. Therefore, the study of zeolite-encapsulated metal catalysts holds not only significant scientific value but also broad application prospects. Scheme 1 presents several typical structures of zeolite-encapsulated metal-based catalysts.

When metal species are loaded with zeolite, their structure and performance will change significantly. First, the pore structure of the zeolite can limit the growth and agglomeration of metal species to maintain small size and uniform dispersion. Secondly, the confinement effect of the zeolite can change the electronic environment of the metal species and enhance the interaction between the metal and the support zeolite [6,7]. This interaction can improve the stability and anti-sintering performance of the metal species, and can also regulate their catalytic activity and selectivity. In addition, the pores of the zeolite can also serve as a transmission channel for reactants and products, affecting the adsorption, diffusion, and reaction process of the reactants, thereby achieving the regulation of the catalytic reaction.

A variety of preparation methods have been developed, including impregnation, ion exchange, ship in a bottle, solvent-free crystallization, hydrothermal, dissolution–recrystallization, etc. [6,7]. Each method has its own unique principles, operating steps, and scope of application, and can prepare zeolite-loaded metal catalysts with different structures and properties. These catalysts show excellent activity, selectivity, and stability in a variety of catalytic reactions, such as hydrogenation, oxidation, reforming, and dehydrogenation. Therefore, in-depth research on the preparation technology, structure–performance relationship, and application fields of zeolite-loaded metal catalysts is of great significance for promoting the development of catalytic science and the innovation of industrial catalysts.

Moreover, to address the challenges of metal aggregation and enhance their dispersion within zeolites, various zeolite modification strategies have been explored. These modifications are crucial for improving the performance of zeolite-encapsulated metal catalysts [8,9,10]. One common approach is the generation of mesopores in zeolites through post-synthesis methods such as dealumination and desilication. These mesopores not only facilitate the diffusion of reactants and products but also provide additional space for metal particle dispersion. Another approach for mesopores’ introduction is the use of templating methods during the synthesis of zeolites, including hard templates and soft templates. Other approaches include solvent-free crystallization, hydrothermal method, etc. These modification strategies not only increase the dispersion of metal nanoparticles but also strengthen the metal–support interaction, which is crucial for improving the stability and catalytic performance of zeolite-encapsulated metal catalysts. By carefully selecting and optimizing these methods, it is possible to tailor the properties of zeolite-encapsulated metal catalysts.

Thus, zeolite-loaded metal catalysts, as a new type of heterogeneous catalyst, have attracted widespread attention due to their unique structural advantages and excellent catalytic performance. This paper systematically reviews the preparation methods of zeolite-loaded metal catalysts and their applications in the field of catalysis; analyzes the principles and characteristics of different preparation methods; explores the mechanism of the influence of zeolite coating structure on the performance of metal catalysts; looks forward to the future development trend of this field; and offers valuable guidance and decision-making insights for the rational design and scalable application of next-generation heterogeneous catalytic materials.

2. Preparation Method of Zeolite-Encapsulated Metal Catalyst

The synthesis of zeolite-encapsulated metal catalysts is a critical step in tailoring their catalytic performance, as the preparation method directly influences the size, dispersion, and stability and metal–support interaction of metal species within the zeolite. Especially, the strength of the metal–support interaction is significantly influenced by the preparation method. This interaction plays a crucial role in determining the stability and catalytic performance of the metal species within the zeolite. Different synthesis strategies can lead to varying degrees of metal encapsulation, dispersion, and electronic coupling with the zeolite support, which in turn affect the strength of the metal–support interaction. These interactions can enhance the stability of the metal nanoparticles, prevent their aggregation or leaching, and modulate their electronic properties to improve catalytic activity and selectivity. Various strategies have been developed to achieve precise control over metal encapsulation, ranging from conventional techniques, such as impregnation and ion exchange, to advanced approaches, like “ship-in-a-bottle” and solvent-free crystallization [11]. These methods differ in their mechanisms, metal-loading efficiency, and ability to prevent sintering or leaching under reaction conditions. Selecting an appropriate synthetic route is essential to optimize the interaction between metal nanoparticles and the zeolite matrix, thereby enhancing catalytic activity, selectivity, and long-term stability. This section systematically reviews the principles, advantages, and limitations of key preparation techniques, providing insights into their suitability for different catalytic applications.

2.1. Impregnation

The impregnation method is the simplest and most widely used method to introduce metal components into the zeolite structure, and it can be applied to almost all zeolite materials. The impregnation method mainly involves fully mixing the prepared zeolite support with a solution containing the metal precursor and then drying the mixture by heating, and a catalyst constructed with metal species and zeolite can be obtained by calcination and/or reduction.

Li et al. [12] selected hierarchical ZSM-5 as a support, achieved Ni metal coating via a simple impregnation method, and used the prepared hierarchical Ni/HZSM-5 catalyst for the catalytic hydrodeoxygenation (HDO) reaction of biomass oil (Figure 1). From the TEM image, it can be seen that for the Ni/HZSM-5 catalyst, the Ni particles are relatively uniformly distributed, with an average size of about 6 nm. The Ni particles on the traditional HZSM-5 zeolite show a larger particle size, with a diameter range of 8–13 nm. With the combination of electron microscopy characterization, it further demonstrated that the Ni species was successfully encapsulated in the mesopores of the zeolite. The study found that the introduction of hierarchical pores and strong acid centers can greatly improve the hydrogenation activity of the catalyst, and its stable initial selectivity can be maintained after seven cycles of regeneration.

With the impregnation method and ligand protection strategy, Miah et al. [13] used the mesoporous structure of SBA-15 to control the size and dispersibility of Au nanoparticles, preventing them from agglomerating and leaching during the preparation and reaction process. In addition, the use of ammonia ligands to replace chloride ligands of the Au precursor can further inhibit the generation of Au(OH)₃ and the precipitation of Au, successfully confining monodisperse Au (~2.3 nm) particles in the SBA-15 pores. The results of the 4-nitrophenol (4-NP) reduction reaction showed that the as-prepared encapsulated Au/SBA-15 catalyst had excellent catalytic reduction activity and stability, and it exhibited excellent performance in five catalytic cycle reactions.

Kockrick et al. [14] also used the impregnation method to successfully encapsulate FePt nanoparticles in SBA-15. The growth of particles was controlled by the pore size and shape of the mesopores, and the particles were isolated from each other in the pores, so that the FePt particles were transformed from the cubic phase to the tetragonal phase. The particle diameter was well controlled, and the sintering and aggregation of metal species during the thermally induced phase transition were effectively inhibited. The authors deduced that after high-temperature treatment (550–1050 °C), the FePt particles were still stably encapsulated in the SBA-15 pores.

The advantages of the impregnation method are that the metal content is controllable and the preparation process is simple, and the zeolite pores can be completely filled with metal precursors [15]. This method has been successfully used to prepare zeolite-loaded single-atom noble metal catalysts such as Pt, Au, Pd, etc. [16,17]. The strength of the metal–support interaction in impregnation-derived, zeolite-encapsulated catalysts depends on factors like metal precursor penetration depth and adhesion to the zeolite. In impregnation, metal precursors are introduced into the zeolite pores via diffusion. Weak chemical bonds would result in weaker interactions, making metal species prone to migration or leaching. However, by optimizing parameters such as precursor concentration, impregnation time, and calcination conditions, the metal–support interaction can be strengthened. However, this method has some limitations. When the metal loading is high, it is easy to be unevenly distributed inside and outside the pores, or even block the pores of the zeolite, which lowers dispersion and accelerates sintering. The metal component gradually sinters in the reaction environment, and surface area loss and/or metal aggregation occur, resulting in a shortened catalyst life.

2.2. Ion Exchange Method

When using the impregnation method, the metal precursor is usually not easy to be completely encapsulated inside the pores of the zeolite, resulting in some species being unable to form an encapsulated structure. The ion exchange method can solve the above problem. The ion exchange method is also carried out in a solution, using a metal precursor as an ion exchanger to exchange ions of the zeolite with the active metal sites to promote the entry of metal species into the pores.

Pd²⁺ was introduced into the micropores of ZSM-5 by ion exchange by Huang et al. [18], forming monodispersed Pd₁O₄ sites with the Pd loading of 0.01 wt%. TEM, XPS, and XRD characterizations showed that only single atomic species existed. Cai et al. [19] used the interaction between hydroxyl groups and Au nanoclusters in HY zeolite supercages to achieve the encapsulation of Au nanoclusters in the size of 1 nm in HY zeolite via the ion exchange method (Figure 2). Since it has a large number of ion exchange sites (i.e., proton sites) in HY, metal species with relatively large atomic mass (such as Au and Ag) can obtain a higher metal loading. XRD, TEM, and other analyses showed that the Au nanoclusters are well encapsulated in the HY zeolite supercage and can limit and avoid further aggregation of the Au nanoclusters. This catalyst exhibits excellent catalytic performance in the selective oxidation of 5-hydroxymethyl-2-furfural (HMF) to 2,5-furandicarboxylic acid (FDCA).

Kim et al. [20] exchanged the Pd(NH₃)₄Cl₂ with H⁺ or NH₄⁺ in SSZ-13 zeolite through an ion exchange process, achieving complete dispersion of isolated Pd²⁺ ions in SSZ-13 zeolite and successfully generating uniformly dispersed nanoscale PdO clusters in SSZ-13. The formation process of PdO clusters was analyzed by cryo-electron microscopy and ultramicrotomy techniques, revealing that the spatial confinement effect of SSZ-13 zeolite can effectively inhibit the growth of PdO clusters during heat treatment.

Ion exchange stands out for its ability to drive metal precursors deep into the micropores, yielding highly dispersed, often atomically distributed metal sites with minimal external deposition. This method is particularly effective for low Si/Al ratio zeolites that possess abundant exchangeable protons, and it allows for the accurate tuning of ultra-low metal loadings. In this method, the strength of the metal–support interaction is often enhanced due to the intimate contact between metal ions and the zeolite framework during the exchange process. As metal ions replace protons or other cations within the zeolite, they might form strong ionic bonds with the oxygen atoms of the zeolite lattice. This results in stronger metal–support interaction compared to impregnation method, where metal precursors may primarily reside on the external surfaces or shallow pores. The strong electrostatic attraction between the metal ions and the zeolite facilitates deeper penetration and uniform dispersion of metal species within the pores. Consequently, the anchored metal ions are less prone to migration or aggregation during subsequent reduction and calcination steps. The stability of the metal nanoparticles is further improved, as the zeolite framework acts as a protective matrix, shielding the metal from harsh reaction conditions. However, the above-mentioned ion exchange method for preparing encapsulated zeolite catalysts has certain limitations: The SiO₂/Al₂O₃ ratio of the as-used zeolite needs to be relatively low with more ion exchange sites. In addition, the zeolite-encapsulated metal species obtained by the ion exchange method may have the problem of poor thermal stability.

2.3. Ship-in-a-Bottle Approach

The ship-in-a-bottle approach, first reported by Ichikawa et al. [21], is a conceptually elegant strategy for constructing zeolite-encapsulated catalysts in which the active species, typically metal complexes, clusters, or even larger polyoxometalates, are assembled from molecular precursors that can freely diffuse through the zeolite pores, yet the final entity is too bulky to escape once formed. Mimicking the nautical analogy, the “ship” is built piece by piece inside the “bottle” (zeolite cavity or supercage), relying on sequential coordination, ligand exchange, or condensation reactions that are spatially restricted by the surrounding crystalline framework [22].

In recent years, the ship-in-a-bottle technology has been further developed. Dai et al. successfully encapsulated phosphotungstic acid (HPW) into a hollow Silicalite-1 (S-1) zeolite via the ship-in-a-bottle method [23]. By introducing Na₂WO₄ and Na₂HPO₄ as templates into the interior of the zeolite, and then using ion exchange to introduce metals into the hollow S-1, HPW nanoparticles with a pore size larger than that of S-1 were formed, thus limiting the leakage of active species and thereby successfully encapsulating HPW inside the zeolite. The catalyst showed high catalytic activity and stability for the synthesis reaction of ethyl acetate.

Okemoto et al. [24] successfully prepared a Cu-based Y zeolite catalyst by using the ship-in-a-bottle method, encapsulating the Cu complex in the supercage of Y zeolite. In the catalytic oxidation of benzene to phenol, the catalytic activity and reproducibility of the prepared encapsulated Cu catalyst were better than those of the corresponding supported Cu catalyst. Thermogravimetric analysis (DTA-TG) of proton-type Y-type zeolite showed that all Cu complex-loaded catalysts showed the decomposition stage and weight loss of the corresponding complex, indicating that the complex was stably formed in the supercage of Y zeolite.

Besides conventional SiO₂-based zeolite, zeolite-like MOFs have also been reported to encapsulate metals using the ship-in-a-bottle strategy. Yu et al. [25] confined a Pd–phosphine complex inside the pores of MOF-808 through a surfactant-free ship-in-a-bottle route (Figure 3). The pre-formed precursor complex was entrapped during MOF crystallization and then briefly heated to detach the ligands and anchor atomically dispersed Pd without harming the framework. This spatial separation of Pd sites from the MOF’s acidic nodes allows two electronically mismatched steps, namely nitro reduction and β-ketoenamine condensation, to proceed in one pot with near-quantitative yield and selectivity for a broad set of substrates, while the catalyst remains intact over repeated runs.

The ship-in-a-bottle strategy excels at producing truly encapsulated, leach-resistant catalytic species that remain strictly confined within the zeolite cages, thereby maximizing stability and recyclability. Its mild, solution-based conditions minimize framework damage and allow for precise control over the nuclearity and ligand environment of the entrapped complex. For the catalysts prepared via this method, the strength of the metal–support interaction is typically strong due to the unique encapsulation process. As metal complexes or clusters form within the confined space of the zeolite pores or supercages, they develop strong chemical bonds with the surrounding zeolite framework. This encapsulation leads to a tight spatial confinement that restricts the movement of metal species, thereby enhancing the metal–support interaction. The interaction strength can be further increased by the precise coordination of metal precursors with the oxygen atoms in the zeolite lattice during the assembly process. This strong interaction not only prevents the agglomeration and migration of metal species but also alters their electronic structure, improving catalytic performance. However, the method is intrinsically limited to zeolites possessing sufficiently large cavities (primarily Y-type and, to a lesser extent, β or EMT), suffers from low metal loadings dictated by the number of available supercages, and involves multistep syntheses that can be time-consuming and difficult to scale [22,26,27,28,29].

2.4. Solvent-Free Crystallization

Solvent-free crystallization is an emerging green route to prepare zeolite-encapsulated metal catalysts without using any solvent. In this protocol, a physical mixture of a dry, amorphous silicate or aluminosilicate precursor; a structure-directing agent (SDA); and a pre-formed metal source (metal salt, oxide, or pre-reduced nanoparticles) is simply ground or pressed into a solid compact. Upon mild heating under autogenous pressure or in a sealed autoclave, the amorphous matrix dissolves locally and recrystallizes around the metal particles, yielding a zeolite shell (typically MFI, BEA, or MEL) that physically entraps the active phase. Because no aqueous or organic solvent is present, diffusion distances are short, supersaturation is high, and crystallization is exceptionally fast, while the absence of solvent minimizes waste, simplifies product recovery, and allows for synthesis temperatures well above the decomposition limits of conventional hydrothermal routes [30,31,32].

Wang et al. [33] adopted a solvent-free crystallization strategy using solid mixtures (Pd nanoparticles, amorphous silica, and an organic structure-directing agent) as the precursors to form a Silicalite-1 (S-1)-encapsulated Pd metal nanoparticle catalyst with a core–shell structure (Pd@S-1). The as-prepared encapsulated Pd@S-1 and the Pd/S-1 prepared by traditional impregnation were analyzed by TEM, and the Pd nanoparticles encapsulated in the Pd@S-1 could be directly observed. In addition, selective catalytic reactions were carried out using molecules with different molecular diameters, such as large molecular benzaldehyde (BA) and 3,5-isopropylbenzaldehyde (DPBA), which are unable to enter the pores of the S-1. The results showed that the activity of Pd/S-1 was higher than that of Pd@S-1, further confirming that the Pd nanoparticles were completely encapsulated inside the S-1. This catalyst not only fully exerted the high activity of Pd nanoparticles but also used the zeolite microporous structure to change the diffusion properties of substrate molecules and product selectivity, which was of potential significance for the design and development of efficient heterogeneous catalysts.

Zhang et al. [34] successfully encapsulated bimetallic AuPd nanoparticles in S-1 through a solvent-free strategy (Figure 4). Compared with the traditional hydrothermal method, the precipitation of AuPd species caused by the dissolution of amorphous SiO₂ was greatly reduced under solvent-free conditions. The as-prepared Au_0.4Pd_0.6@S-1 catalyst was characterized by TEM, which confirmed that the AuPd nanoparticles were encapsulated in the S-1. The metal-encapsulated structure of the catalyst Au_0.4Pd_0.6@S-1 was further confirmed by the shape-selective reaction using 3,5-dimethylbenzyl alcohol (DMBA). Since the molecular size of DMBA is larger than that of the micropore size of S-1, Au_0.4Pd_0.6@S-1 showed a very low conversion (about 10%), while the Au_0.4Pd_0.6/S-1 prepared by the impregnation method still had a high catalytic performance, about nine times that of the encapsulation method. This also confirmed the successful preparation of encapsulation AuPd-based catalyst.

Tang et al. prepared a series of SBA-15-encapsulated metal oxide (Co₃O₄, NiO, and CeO₂) catalysts with high loading (≥20 wt%) by a solvent-free method [35]. Since there is no solvent in the preparation process, the redispersion of the metal precursor is suppressed, thus effectively enhancing the interaction between the precursor and the support. XRD and TEM characterizations show that this method can obtain well-dispersed, non-aggregated metal nanoparticles, which can be uniformly dispersed in the mesoporous channels of SBA-15.

Since the solvent-free crystallization method does not require the introduction of solvents during the synthesis process, the synthesis temperature can be effectively increased, breaking through the limitation of conventional hydrothermal synthesis that the organic template decomposes at high temperatures. In addition, a large number of experiments have shown that the solvent-free environment makes the reaction more efficient.

In solvent-free crystallization, the strength of the metal–support interaction arises from the intimate mixing of metal precursors with the amorphous precursors of the zeolite. In this method, metal precursors are physically mixed with amorphous silicate or aluminosilicate precursors and structure-directing agents. Upon heating, the amorphous zeolite precursors dissolves and recrystallizes around the metal species, creating a zeolite shell that tightly encapsulates the metal. This process ensures that the metal species are uniformly dispersed and firmly anchored within the zeolite framework. The absence of solvents minimizes the diffusion distance and maximizes the contact area between the metal and the zeolite, leading to a strong metal–support interaction. Compared to impregnation and ion exchange methods, solvent-free crystallization typically results in a stronger metal–support interaction due to the in situ formation of the zeolite matrix around the metal particles. However, whether the reaction can occur, how the heat diffuses, whether the physical properties can be controlled, and how the products can be separated need further exploration [36,37,38,39].

2.5. Hydrothermal Crystallization

The hydrothermal crystallization method uses a metal coordination complex as the precursor and performs hydrothermal treatment in a closed container to form a dispersed nanocrystalline core and encapsulate the metal component, ultimately forming a zeolite-encapsulated metal catalyst. This is the most common method for zeolite-encapsulated metal catalysts’ preparation.

Otto et al. [40] used a protective ligand to stabilize the metal cation precursor, improving the pre-precipitation of the zeolite under hydrothermal conditions. They successfully achieved the formation of almost-monodisperse bimetallic clusters with a particle size of 1-2 nm in LTA, and these bimetallic clusters did not affect the crystallinity of LTA during the continuous heat treatment and alloying process in O₂ and H₂. Metal encapsulation and alloying were achieved by introducing mercaptosilane-stabilized metal cation precursors into the zeolite synthesis gel, and the metal cations were then encapsulated in the pores of LTA. Wang et al. [41] used [Pd(NH₂CH₂CH₂CH₂NH₂)₂]Cl₂ as the precursor and interacted with the zeolite precursor gel to achieve the uniform dispersion of ultrasmall palladium clusters in the cross-channels of the MFI zeolite by hydrothermal crystallization. HRTEM images showed that the Pd clusters encapsulated in the zeolite were well dispersed and evenly distributed throughout the zeolite crystals. The average particle size was in the range of 1.5–1.8 nm, which was much smaller than the Pd/S-1-im catalyst prepared by impregnation (2.7 nm) and the commercial Pd/C catalyst (3.8 nm). In the reaction of methane decomposition to H₂, the in situ confinement effect of the metal clusters enabled the catalyst to show excellent catalytic activity, thermal stability, and reproducibility.

Cu nanoclusters inside an Al-rich ZSM-5 were synthesized by a one-pot hydrothermal crystallization strategy reported by Dai et al. [42]. By co-assembling a Cu-amine precursor with zeolite nutrients under alkaline hydrothermal conditions, the metal was entrapped during crystal growth, yielding 2.3 nm clusters that resisted sintering up to 600 °C (Figure 5). The resulting Cu@HZ40 integrates Brønsted acid sites with ultrasmall metallic Cu, enabling furfuryl alcohol ring-opening to pentanediols at 100% conversion and 92% selectivity, and it has a far superior performance compared to conventionally impregnated Cu/HZ40.

The group of Li [43] also used the hydrothermal crystallization method to in situ confine the metal–ethylenediamine (M-EDA) complex in the β-cage of the Y-type zeolite during the crystallization process, and then they thermally reduced the metal atoms to fix them in the zeolite, thus anchoring isolated metal atoms (M-ISAS, M = Pt, Pd, Ru, Rh, Co, Ni, and Cu) in the Y-type zeolite. The as-synthesized catalyst has the maximum metal atom utilization and the minimum volume occupancy. By combining density functional theory (DFT) calculations with EXAFS, it was found that the stable position of the metal atom on the six-membered ring plane of the zeolite framework was determined to be the most stable configuration, which was conducive to the catalytic interaction between the metal–ISAS and the reaction substrate. The low-contrast zeolite support was observed from the HAADF-STEM image, confirming the existence of atomically dispersed metal sites in M-ISAS@Y. Taking Pt-ISAS@Y as an example, in the n-hexane isomerization reaction, continuous catalytic dehydrogenation/hydrogenation and isomerization at the Brønsted acid site were achieved, with ultra-high activity and selectivity, and high development potential in the field of industrial applications.

The advantages of the hydrothermal crystallization method are that it is a simple operation; it is wide applicability; it has a relatively low energy consumption, without high-temperature treatment; and the as-prepared metal-based catalyst possesses uniform particle size distribution and high metal dispersion, and it can effectively avoid the precipitation of metal hydroxides in the synthesis process. Hydrothermal crystallization also facilitates a strong metal–support interaction by forming zeolite crystals around the metal precursors in solution, similar to that of solvent-free crystallization. The mild hydrothermal conditions allow for uniform dispersion and encapsulation of metal species within the zeolite, creating a stable interaction. However, the hydrothermal crystallization method has disadvantages, such as a long reaction time and species leaching, and currently only the preparation of oxide powders can be achieved. For example, many species can be leached into the supernatant during the process, leading to material waste and increased cost. To mitigate this, optimizing synthesis conditions such as pH, temperature, and time can enhance metal incorporation and reduce leaching. In addition, the method requires high temperature and high pressure, making it highly dependent on production equipment, which also hinders the development of the hydrothermal crystallization method for industrial application [42,44,45].

2.6. Dissolution–Recrystallization Method

Dissolution–recrystallization is a two-stage, self-templating route to zeolite-encapsulated metal catalysts that exploits the reversible solubility of the zeolite framework itself. In the first stage, a parent zeolite (or pre-impregnated metal/zeolite composite) is partially dissolved in a mild alkaline solution containing structure-directing ions (e.g., TPA⁺ and TBA⁺) and a metal precursor. During the second stage, controlled re-crystallization is induced, either hydrothermally or by simply aging, whereby silicate or aluminosilicate species leached from the zeolite interior reassemble on the outer surface, forming a new microporous or mesoporous shell that physically entraps the metal nanoparticles within hollow or macroporous cavities. Because the dissolution zone propagates inward and re-precipitation occurs outward, the method naturally produces core–void–shell architectures with tunable shell thickness and cavity size, while the original zeolite crystal serves simultaneously as silicon/aluminum source, shape-directing scaffold, and protective barrier against metal sintering.

Dai et al. [46] first prepare the FeK/S-1 catalyst by an impregnation method. Then, tetrapropylammonium hydroxide (TPAOH) was added to preferentially attach to the zeolite surface through electrostatic interaction, reducing the dissolution of the outer surface of the zeolite. Meanwhile, silicate oligomers leached from the interior of the zeolite, then interacted with TPA⁺ on the zeolite surface, and recrystallized to form a Fe-based catalyst with hollow structure (or macroporous structure) encapsulated Fe species. The use of hollow Silicalite-1 (or macroporous Silicalite-1) as the support can significantly increase the dispersion of iron oxide which further enhanced the conversion of CO₂ and the selectivity to C5 higher hydrocarbons in CO₂ hydrogenation. This strategy can effectively prevent the migration and aggregation of active metals at high temperatures and improve the dispersion of metals. In addition, the author also used the dissolution–recrystallization strategy to encapsulate a series of Cu-Pd [47], Ni-Pt [48], and Fe-Cu [49] bimetallic nanoparticles in hollow Silicalite-1 and further explored their catalytic performance.

Employing a dissolution–recrystallization strategy, the Beta zeolite framework was first mildly etched in alkaline media to generate soluble silicate/aluminate oligomers by Zhan et al. [50]. These materials then reassembled around pre-impregnated Ni precursors during synchronized hydrothermal recrystallization, yielding a hierarchical meso-microporous structure with narrowly dispersed 5.4 nm Ni clusters (Figure 6). The mesopores acted as highways for bulky low-density polyethylene (LDPE) formed on external Brønsted acid sites to reach internal Ni sites for hydrogenation, while the intact micropores imposed a second layer of shape selectivity that sieved out C5–C9 naphtha molecules. This 10Ni@Beta catalyst prepared by single-step confinement route delivered 88% liquid yield and 99% naphtha selectivity at 250 °C, outperforming noble-metal benchmarks by tightly coupling metal and acid functions and suppressing both coking and Ni sintering.

The dissolution–recrystallization method leverages the reversible dissolution–reprecipitation of the zeolite to create tunable “core–shell” and “yolk–shell” structure in a single step, markedly improving metal dispersion, sintering resistance, and macromolecular diffusion, all without external Si/Al sources and under relatively mild conditions. This method would strengthen the metal–support interaction by reassembling the zeolite framework around pre-impregnated metal species. This process creates a new zeolite shell that encapsulates the metal, similar to the encapsulation achieved in solvent-free and hydrothermal crystallization. The interaction strength is enhanced by the intimate contact between the metal and the newly formed zeolite crystals during recrystallization. Nevertheless, parent-zeolite loss and alkaline effluent generation hinder green scale-up, while shell thickness and cavity size are highly sensitive to base concentration, temperature, and time, narrowing the process window. In addition, high metal loadings readily compromise shell integrity, and solid–liquid separation coupled with mother-liquor recycling in batch mode still requires further optimization.

From above, it can be concluded that, among these preparation strategies, impregnation remains unrivaled for its operational simplicity and tunable high metal loadings yet suffers from facile sintering and pore blockage, whereas ion exchange excels in furnishing atomically dispersed sites at ultra-low loadings but is hampered by the requirement of low-silica zeolites and limited uptake. The ship-in-a-bottle technique provides genuine cage-locked species with exceptional leaching resistance, although it is confined to large-cavity zeolites and low capacities; by contrast, solvent-free crystallization offers a green, solventless route that embeds metals uniformly within freshly grown zeolite crystals, yet demands tight control over solid-state heat and mass transfer. Hydrothermal crystallization delivers narrow particle-size distributions under mild one-pot conditions, but it is penalized by long batch times and high pressures, while dissolution–recrystallization uniquely generates hollow core–shell architectures that protect metals from sintering, albeit at the cost of partial zeolite loss and alkaline effluents. This information is summarized in

Table 1. Comparison of preparation methods for zeolite-encapsulated metal catalysts.

Preparation Method	Principle	Advantages	Disadvantages
Impregnation	Metal precursors are mixed with zeolite, followed by drying and calcination.	Simple operation, controllable metal content, and complete pore filling.	Non-uniform metal distribution, pore blockage, and easy sintering.
Ion Exchange	Metal precursors replace exchangeable ions in zeolites, introducing metals into pores.	Intimate contact between metal ions and zeolite framework, achieving uniform dispersion and even atomic distribution.	Requires low-silica zeolites, limited metal loading, and potential poor thermal stability.
Ship-in-a-bottle approach	Metal complexes or clusters form within zeolite pores or supercages, too large to escape once formed.	Creates truly encapsulated, leak-resistant species with strong metal–support interaction and high stability.	Confined to large-cavity zeolites (mainly Y-type), low metal loading, and multi-step synthesis.
Solvent-free Crystallization	Amorphous silicate or aluminosilicate precursors and metal sources are mixed, with zeolite crystallizing around metal particles under heating without solvents.	Green process, uniform metal encapsulation within freshly grown zeolite crystals.	Requires strict control of solid-state heat and mass transfer; synthesis temperatures must exceed conventional hydrothermal limits.
Hydrothermal crystallization	Metal coordination complexes are used as precursors, and hydrothermal treatment forms dispersed nanocrystalline cores encapsulating metal components.	Simple operation, wide applicability, uniform particle size distribution, and high metal dispersion.	Long reaction times, high temperature and pressure requirements, and equipment dependency.
Dissolution–recrystallization	Parent zeolite or pre-impregnated metal/zeolite composite is partially dissolved in mild alkaline solution, followed by controlled re-crystallization to form a new zeolite shell encapsulating metal species.	Creates tunable core–shell and yolk–shell structures in one step, enhancing metal dispersion and sintering resistance.	Partial zeolite loss, alkaline effluent generation, and sensitivity of shell thickness and cavity size to base concentration, temperature, and time.

. Future efforts will likely pivot toward next-generation, hybrid synthetic paradigms that transcend the classical boundaries, ultimately enabling atom-precise, kilogram-scale fabrication of zeolite-encapsulated metals with unprecedented catalytic performance [2,3,4,5].

3. Relationship Between the Structure and Performance of Zeolite-Encapsulated Metal Catalysts

Understanding how the structure of zeolite-encapsulated metal catalysts governs their catalytic performance is the cornerstone for rational design. For metal species, its size, shape, and alloy composition would modulate the density of active sites and the electronic structure. Beyond merely hosting active metals, the zeolite framework acts as a multipurpose regulator. Its pore system imposes geometric constraints that dictate particle size and shape, and its acid sites create electronic and chemical synergies with the metal [3]. In addition, its crystalline lattice can channel reactants, intermediates, and products in ways that dramatically influence activity, selectivity, and stability [51,52]. This section dissects these structure–performance relationships to reveal how these parameters can be tuned, individually or in concert, to craft catalysts that not only resist sintering and poisoning but also unlock new reaction pathways unattainable with conventional supported catalysts.

3.1. Effects of Metal Nanoparticle Size, Shape, and Alloy Composition on Catalytic Performance

The size, shape, and alloy composition of metal nanoparticles directly govern their intrinsic catalytic performance. Within the zeolites, these structural parameters are co-modulated by channel geometry and framework micro-environments, amplifying their influence on activity, selectivity, and stability. This section sequentially addresses the size effect, shape effect, and alloy synergy, and clarifies their inter-relationships in zeolite-encapsulated catalysts.

3.1.1. Size Effect of Metal Nanoparticles

The size of metal nanoparticles has a significant effect on their catalytic performance. Smaller metal nanoparticles have higher dispersion and surface energy, which can provide more active sites and thus improve catalytic activity. In addition, small-sized metal nanoparticles also have quantum effects, and their electronic structure and energy band structure will change, which will also affect their interaction with the reactants, thereby changing the selectivity and activity of the catalytic reaction.

The research group of Lu [53] studied the size effect of metal Pd in the selective oxidation reaction of benzyl alcohol and found that the catalytic activity and selectivity showed a “volcano-type” trend with the particle size. When the catalyst size was greater than 4 nm, the larger the size, the lower the proportion of low-coordinated atoms and the better the selectivity. When the Pd particle size was lower than 4 nm, although the proportion of low-coordinated atoms increased, the selectivity became better. The characterizations results showed that the electronic structure of Pd for different particle sizes had changed significantly, indicating that the electronic effect may have reversed the trend of selectivity changes.

Subnanometer PtSn clusters were entrapped within the 10-membered-ring channels of MFI zeolite and revealed a pronounced size–composition interplay that dictates both intrinsic activity and longevity in propane dehydrogenation, as illustrated in Figure 7a,b [54]. By systematically modulating the Pt loading from 0.006 to 0.44 wt%, the authors map a continuous transition from isolated Pt atoms to Pt3Sn1 and Pt6Sn1 clusters. The smallest bimetallic ensembles (0.3–0.5 nm) exhibit the highest turnover frequency because the under-coordinated Pt centers effectively weaken the C-H bond, whereas larger clusters (>0.6 nm) suffer from a lower density of active sites and accelerated coke formation. This size-governed reactivity–stability trade-off underscores the necessity of subnanometer precision when engineering zeolite-encapsulated bimetallic catalysts for alkane dehydrogenation.

3.1.2. Shape Effects of Metal Nanoparticles

The shape of metal nanoparticles determines the exposed crystal planes and edge sites, which have different abilities to adsorb and activate reactants, thereby affecting the catalytic performance.

For metal-encapsulated catalysts, the shape effect of metal nanoparticles is also crucial. As shown in Figure 7c,d, Zhang’s group encapsulated Pt nanoparticles within pure silica zeolite with MFI topology to construct Pt@S-1-encapsulated catalysts [55]. The research found that the geometric confinement of the zeolite channels maintained the Pt nanoparticles in a specific shape, increasing the proportion of low-coordination atoms with high activity and significantly enhancing the catalytic performance. The conversion of cinnamaldehyde and the selectivity to cinnamyl alcohol reached as high as 99.8% and 98.7%, respectively.

Furthermore, Yu’s group [57] utilized the confinement effect of zeolite to successfully in situ prepare Silicalite-1-encapsulated subnanometer bimetallic Pd-M(OH)₂ (M = Ni, Co) catalysts under hydrothermal conditions. Characterizations revealed that the bimetallic Pd-Ni(OH)₂ had subnanometer dimensions and were confined at the intersection of 10-ring channels in Silicalite-1. This precise control over shape and position enabled the catalyst to exhibit ultra-high performance in the formic acid decomposition hydrogen evolution reaction, with initial and complete decomposition TOFs reaching as high as 5803 h⁻¹ and 1879 h⁻¹, respectively, without any CO by-products, and demonstrated excellent catalytic cycle stability.

3.1.3. Synergistic Effect of Alloy Composition

Alloy composition can induce a strong synergistic effect between metal components, significantly changing the catalytic activity and selectivity of metal nanoparticles. Bimetallic or multimetallic alloy catalysts can combine the advantages of different metals to achieve better catalytic performance. Pt-Zn bimetallic nanoclusters were encapsulated in SAPO-11 zeolites by Wang et al. [53]. The catalyst showed high stability and excellent catalytic performance in the hydrogenation reaction of levulinic acid, with a yield of more than 80%, which was attributed to the synergistic effect between Pt and Zn.

In a ligand-assisted hydrothermal synthesis, Pt-Mn alloy clusters were immobilized inside ZSM-5, creating a confined bimetallic structure that outperforms single-metal or post-impregnated counterparts (Figure 7e,f) [56]. The intimate alloy contact, driven by simultaneous nucleation of Pt and Mn within the growing zeolite framework, triggers electron flow from Mn to Pt, which stabilized Pt⁰ and simultaneously enriches surface-adsorbed oxygen. Because the alloy was spatially locked by the rigid zeolite lattice, particle growth and leaching can be suppressed, and after ten redox cycles at 60000 mL g⁻¹ h⁻¹, activity remains unchanged.

In constructing high-performance VOC oxidation systems, the synergistic effect between alloying and zeolite encapsulation is gaining increasing recognition [58]. The incorporation of a PdAg alloy inside the Silicalite-1 exemplifies this trend. The electron transfer from Ag to Pd generated electron-rich Pd sites that markedly weakened the C-H bond of adsorbed toluene, while Ag simultaneously activated gas-phase O₂ to reactive oxygen species. This bifunctional behavior, reinforced by the confinement effect of the zeolite pores, yielded a catalyst that not only attained complete toluene removal at 179 °C, but also retained its activity after 100 h on stream and under 5% H₂O. Beyond simple dispersion, the alloy phase stabilized ultrasmall nanoparticles against sintering and carbon deposition, demonstrating that alloying within an encapsulated architecture can be a decisive lever for both intrinsic activity and long-term robustness in VOC abatement.

In summary, metal size, shape tailoring, and alloy synergy within zeolite confinement allow for concurrent optimization of active-site density, electronic structure, and stability. This multidimensional structural control not only offers a model platform for elucidating metal–support interaction but also lays the theoretical and experimental groundwork for developing highly efficient, robust, and scalable heterogeneous catalysts.

3.2. Effects of Zeolite Structure and Properties on Catalysts

Having elucidated the influence of metal-side structural parameters, this section turns to the intrinsic pore structure and acidity of zeolites. Topology, hierarchical porosity and acid-site distribution jointly govern reactant diffusion, metal positioning, and product selectivity. Hence, their interaction with catalytic performance is systematically discussed.

3.2.1. The Pore Structure of Zeolites

In encapsulated metal–zeolite catalysts, introducing hierarchical pore structures (including mesopores and/or macropores) and varying the topology of zeolite can significantly enhance catalytic activity by regulating reactant diffusion and the localization of metal active species (Figure 8A) [59,60,61,62,63]. Different types of zeolites, including three-dimensional, mesoporous (such as SBA-15 and MCM-41), and lamellar/exfoliated structures, have distinct impacts on metal incorporation, affecting metal dispersion, performance, and loading amounts [8,9,10]. Traditional microporous zeolites, with their highly ordered channel systems, are adept at confining metal species within their frameworks, enhancing stability and selectivity. However, their small pore sizes can restrict metal loading and mass transfer. In contrast, mesoporous zeolites like SBA-15, MCM-41, etc., with pore sizes ranging from 2 to 50 nm, offer larger pores and higher surface areas, facilitating the dispersion of metal nanoparticles and enabling higher metal loadings. This is particularly beneficial for reactions involving large molecules. Lamellar or exfoliated zeolites, with their expanded interlayer spaces, provide unique advantages for metal incorporation. These structures allow for the intercalation of metal species between layers, increasing interaction and potentially altering the electronic properties of the metal. This can lead to enhanced catalytic activity and selectivity. The exfoliated structure also offers shorter diffusion paths for reactants and products, improving reaction efficiency. Overall, the choice of zeolite type should be guided by the specific requirements of the target reaction.

Tian et al. [64] successfully prepared hollow Pt@ZSM-5 catalysts with large cavities and mesoporous shells through alkali post-treatment and recrystallization of traditional Pt/ZSM-5. Their study found that this unique zeolite cage structure effectively enriches benzene through its porous shell while leveraging the internal acid sites of the zeolite to enhance benzene adsorption and storage, creating a high-concentration benzene environment that significantly boosts catalytic activity. The as-prepared catalyst achieved a T₉₀ value (temperature required for 90% conversion) as low as 178 °C, comparable to that of conventional high-Pt-content Pt/ZSM-5 catalysts. Additionally, the catalyst exhibited excellent cycling stability and resistance to steam interference. Zhu et al. [65] achieved spatial configurational construction of Pt-based catalysts by coating HZSM-5 with resorcinol-formaldehyde resin. By controlling the localization of Pt species, they achieved atomic- and nanoscale proximity between metal and acid sites. The results showed that the ZSM-5/Pt@mSiO₂ catalyst, with metal sites located on the ZSM-5 core, exhibited superior activity and isomerization selectivity. This indicates that atomically proximate active sites are more conducive to synergistic effects when zeolite channel diffusion limitations are mitigated, providing a new direction for designing efficient bifunctional catalysts.

The topology of zeolite could influence the catalytic performance during the reaction. Confining Rh clusters inside different zeolite topologies reveals that framework architecture, not merely pore size, dictates both activity and regioselectivity in long-chain olefin hydroformylation [66]. Within the sinusoidal 10-membered-ring channels of MFI, the rigid lattice enforces a steric template that orients the Rh center and incoming olefin, suppressing branched pathways and delivering linear-to-branched ratios above 400. In contrast, MEL, *BEA, and MWW, despite having comparable channel dimensions, lack the same curvature and confinement-induced ligand field, so the Rh clusters behave more like conventional supported nanoparticles. These topology-dependent trends demonstrate that zeolite framework topology can be treated as an “inorganic ligand” whose three-dimensional connectivity tunes transition-state geometry, electron density, and mass transport, offering a design lever for heterogeneous hydroformylation that rivals homogeneous systems without the need for expensive organic phosphine ligands.

Figure 8. (A) Arrhenius plots of NO conversion over three zeolites with different topologies (a) and standard SCR with soot loaded condition (c). Turnover frequencies calculated using standard SCR rates in the absence (b) and presence (d) of soot. Reproduced with permission from [59], copyright 2024, American Chemical Society. (B) The influence of acidity, and the catalytic performance. Reproduced with permission from [67], copyright 2024, Elsevier.

Figure 8. (A) Arrhenius plots of NO conversion over three zeolites with different topologies (a) and standard SCR with soot loaded condition (c). Turnover frequencies calculated using standard SCR rates in the absence (b) and presence (d) of soot. Reproduced with permission from [59], copyright 2024, American Chemical Society. (B) The influence of acidity, and the catalytic performance. Reproduced with permission from [67], copyright 2024, Elsevier.

Additionally, the pore size of zeolites can significantly influence product distribution and catalyst stability. In the catalytic conversion of low-density polyethylene (LDPE), Wang et al. [68] prepared a hollow-structured Pt-based TS-1 catalyst (Pt@Hie-TS-1) through post-treatment recrystallization; the shape selectivity of zeolite pores can assist to convert LDPE into liquid alkanes. By optimizing the micro-mesoporous structure of the zeolite through controlled post-treatment conditions, they achieved high selectivity (84.8%) and yield (94.0%) for C5–C7 light alkanes. In situ FT-IR revealed that only appropriately sized olefins could diffuse to the internal Pt sites for hydrogenation, resulting in an ultra-narrow product distribution. Furthermore, the optimized pore structure significantly enhanced catalytic efficiency by facilitating rapid diffusion of large reactant and product molecules. The catalyst also demonstrated excellent anti-coking properties and stability, making it suitable for efficient cracking of various commercial polyethylene plastics and offering new insights for the chemical recycling of plastic waste. Liu et al. [69] synthesized Pt@KIT-6 via a one-pot method and further achieved in situ encapsulation to prepare Pt@KIT-6/SAPO-11 catalysts for the decarboxylation of oleic acid to C8–C17 alkanes. Under reaction conditions of 340 °C for 5 h, the yield of C8–C17 alkanes reached 85.1%. This high performance was attributed to the ordered mesoporous structure, high surface area, suitable acidity, and abundant Pt active sites of the as-prepared catalyst.

3.2.2. Acidity of Zeolites

The acid sites of the zeolite can synergize with the encapsulated metal sites, and the spatial location, molar ratio, interaction between metal and zeolite would be significantly affect the overall activity of the catalyst [65]. For example, using a one-pot synthesis strategy by incorporating different organic structure-directing agents (OSDAs) into the synthesis gel, Zhang et al. [67] directly integrated metal precursors into zeolite crystals, thereby controlling the size and spatial distribution of Pt nanoparticles in ZSM-5 zeolite, revealing the significant influence of the spatial relationship between metal and acid sites on catalytic reactions. The results showed that OSDAs primarily affected the encapsulation of Pt precursors within ZSM-5 zeolite during hydrothermal crystallization, leading to differences in the dispersion and spatial distribution of Pt sites in the as-formed encapsulated Pt-zeolite catalyst. When TPAOH was used as the OSDA, subnanometer Pt clusters formed in close proximity to Brønsted acid sites within the zeolite, effectively promoting the ethane dehydrogenation and benzene alkylation, thereby increasing the ethylbenzene production rate (Figure 8B).

Meanwhile, the acid sites of zeolites can directly influence product distribution and reaction pathways. Yang et al. [70] employed a hydrothermal synthesis method to coat Cu/ZnO/Al₂O₃ catalysts with an outer shell of HZSM-5 zeolite (Cu/ZnO/Al₂O₃@HZSM-5) for CO₂ hydrogenation to dimethyl ether (DME). This core–shell structure forces the as-generated methanol intermediates to diffuse through the acid-rich zeolite shell, significantly increasing the probability of contact with acid sites for dehydration to DME. As a result, the DME selectivity dramatically improved from 34.0% for physically mixed catalysts to 77.0% for core–shell catalyst, while effectively suppressing methanol desorption and CO byproduct formation, thereby optimizing the product distribution. In the study of selective catalytic reduction of NOx with C₃H₆, Neylon et al. [71] constructed a CeO₂ coating on Cu-ZSM-5 zeolite via a nano-CeO₂ sol–gel method to prepare a core–shell-structured catalyst (CeO₂/Cu-ZSM-5). This coating not only lowered the peak activity temperature by 150 °C but also increased NO conversion from 36% under dry conditions to 41% (at 250 °C) in the presence of water vapor, whereas conventional Cu-ZSM-5 exhibited reduced activity upon water exposure. The synergistic effect between the acid sites of zeolite and CeO₂ altered the reaction pathway, and CeO₂ promoted NO oxidation to NO₂ intermediates, which were more readily reduced by C₃H₆ at the acid sites. Additionally, the coating may optimize water molecule adsorption at active sites, thereby reversing the inhibitory effect of water into a promotional one.

It should be noted that the presence of acid sites may lead to decreased catalyst stability under certain reaction conditions. For example, under high temperatures or strongly alkaline conditions, acid sites may be neutralized or destroyed, thereby affecting the long-term performance of the catalyst. However, in some cases, acid sites can enhance the structural stability of the catalyst through interactions with metal sites [72]. Therefore, precisely regulating the synergistic matching between metal sites and acid sites is particularly crucial for optimizing catalyst performance [73].

In summary, precise regulation of zeolite topology and acid-site distribution enables synergistic optimization in diffusion, metal anchoring, and reaction pathway, markedly enhancing catalytic activity, selectivity, and stability, and providing clear structural guidelines for designing efficient confined catalysts tailored to specific reactions.

3.3. Effect of the Interaction Between Metal and Zeolite on Performance

The interactions between metal and zeolite serve as the link between the preceding sections. Electronic transfer and spatial confinement jointly determine the electronic state, morphology, and stability of metal species, thereby influencing overall catalytic performance. This section analyzes these interactions and their specific roles in the catalytic field.

3.3.1. Electronic Interaction

The electronic transfer between metal nanoparticles and zeolite supports, such as the strong metal–support interaction (SMSI), can significantly enhance the activity and stability of catalysts. For example, Cao et al. [74] constructed a Na-Cu@TS-1 catalyst via an in situ encapsulation method, where the unique core–shell structure not only confined ~1.8 nm Cu nanoparticles within the zeolite channels but also induced significant electron transfer between Cu species and the zeolite framework through Na+ ion exchange. Characterizations revealed that Na⁺ introduction enriched the electron density of Cu nanoparticles, enhancing H₂ activation and selectivity toward C=O bond hydrogenation (furfuryl alcohol selectivity reached 98.1%). Furthermore, XAS confirmed a strong electronic interaction between Cu and framework Ti species, which stabilized the catalyst structure during cycling tests (Cu particle size increased only from 1.5 nm to 1.6 nm). This study exemplified the precise regulation of metal electronic states via zeolite microenvironments, highlighting the dual role of electron transfer in improving heterogeneous catalyst performance.

By anchoring sub-2 nm Pt clusters inside TS-1 through a pre-anchoring protocol, Tian et al. [75] deliberately situated Pt adjacent to framework Ti-OH nests. This positioning establishes a pronounced Pt→O(H)→Ti electronic metal–support interaction (EMSI) that simultaneously narrows the band gap, elevates the Pt d-band center, and opens a metal-to-ligand-to-metal charge transfer (MLMCT) channel (Figure 9a–c). The resulting synergy suppresses water splitting and COx formation, steering methanol photoreforming toward selective dehydrogenation that yields 63 mmol g⁻¹ h⁻¹ of H₂ with 96.9% formaldehyde selectivity. Spectroscopic and computational evidence revealed electron flow from Pt to Ti upon illumination, which not only stabilizes Pt⁰ against leaching but also enriches Ti sites with a negative charge, thereby favoring methanol adsorption and C-H activation.

Similarly, encapsulated Ni-based catalysts (Ni@Beta) significantly enhanced the activity and stability of CO₂ methanation by confining Ni nanoparticles within the zeolite framework. Compared to Al-free Ni@Silicalite-1, the Al-containing Beta zeolite framework extracted more electrons from active Ni sites, strengthening metal–support interactions, stabilizing the Ni⁰ phase, and suppressing the formation of volatile Ni(CO)x intermediates, thereby preventing Ni sintering and loss. In situ techniques (e.g., DRIFTS and NAP-XPS) confirmed that the reaction followed a formate pathway, with Ni@Beta exhibiting superior CO₂ adsorption and conversion, achieving 74.2% CO₂ conversion and 93.5% CH₄ selectivity [77].

Zeolite supports can also modulate the electronic states of metal nanoparticles through electronic interactions. In the case of Cu@Zr-Beta [78], the framework Zr species induced electron transfer from Cu to Zr, enriching Cu⁺ active sites on the catalyst surface. These Cu⁺ species not only facilitated methanol dehydrogenation but also synergized with Lewis acid sites to promote the formation of key intermediates (e.g., CH₂O and CH₃CHO) in methyl acetate (MA) production. XPS and CO-FT-IR confirmed that the Cu⁺ content in Cu@Zr-Beta (46.1%) was significantly higher than that in Zr-free Cu@Si-Beta (21.0%), while confined Cu⁺-(CO)₂ species exhibited enhanced reactivity. Additionally, Zr incorporation inhibited Cu nanoparticle sintering, maintaining stable MA production (2.84 mol·kg⁻¹·h⁻¹) over 100 h.

3.3.2. Geometric Constraint Effect

The geometric configuration of the pore channels can influence the distribution and morphology of metal nanoparticles. In addition, the pore structure of zeolites can exert spatial confinement effects on metal nanoparticles, thereby enhancing catalyst stability and/or product selectivity (Figure 9d,e) [76]. For instance, in the Pt@MOR-PMOs-x@MSNs catalyst [79], the hierarchical micro-mesoporous structure of MOR zeolite effectively restricted the growth of Pt nanoparticles, maintaining their average size at 3.5–5.2 nm and significantly improving metal dispersion (CO chemisorption reveals a dispersion of 48.4%). This confinement effect, combined with the synergistic role of acid sites, not only increased the number of active sites but also enhanced catalytic activity and sulfur resistance through the formation of electron-deficient Ptδ+ species. The representative catalyst achieved nearly 100% naphthalene hydrogenation conversion at 280 °C, with a TOF value of 81.5 h⁻¹, demonstrating outstanding catalytic performance. Similarly, Lu et al. [80] successfully encapsulated Ni nanoparticles within Silicalite-2 zeolite (Ni@S2-T) using a combined microemulsion and solvent-free crystallization method. On one hand, the microporous structure (0.5–1.0 nm) of the zeolite confines Ni nanoparticles, maintaining an average size of 4.35 nm, which was far smaller than the 20.75 nm observed in traditional impregnation methods, thereby significantly increasing active site density. On the other hand, the nickel phyllosilicate structure formed during preparation strengthened metal–support interactions, enabling the catalyst to retain structural stability even at 800 °C. This synergistic effect endowed Ni@S2-T with exceptional activity and anti-coking performance in dry methane reforming (only 1.1 wt% coke deposition after 70 h of testing) and a TOF value of 10.2 s⁻¹, surpassing most reported Ni-based catalysts.

The synergistic effects of electronic transfer and spatial confinement concurrently enhance activity, selectivity, and durability, offering key handles for the structural design of zeolite-encapsulated metal catalysts.

Thus, the size, shape, and alloy composition of metal nanoparticles; the structure and properties of zeolites; and the interaction between metals and zeolites are interrelated and mutually influential, and they jointly determine the catalytic performance of zeolite-encapsulated metal catalysts (as illustrated in

Table 2. Effect of structural parameters on catalytic performance.

Parameter	Effect on Catalytic Performance
Metal nanoparticle size	Smaller size increases dispersion and surface energy, enhancing activity. Quantum effects alter electronic structure, influencing selectivity.
Metal nanoparticle shape	Determines exposed crystal planes and edge sites, affecting reactant adsorption and activation. Influences activity and selectivity.
Alloy composition	Synergistic effects between metals change activity and selectivity. Bimetallic/multimetallic systems combine advantages of different metals.
Zeolite pore structure	Micropores confine metal nanoparticles, preventing sintering. Hierarchical porosity improves mass transfer and accessibility.
Zeolite acidity	Acid sites synergize with metal sites. Spatial arrangement and acid strength affect activity and product distribution.

). A deep understanding of the relationship between these structures and properties will help us optimize and improve the catalytic performance through the rational design and regulation of the catalyst structure [81,82,83].

4. Application of Zeolite-Encapsulated Metal Catalysts

Zeolite-encapsulated metal catalysts have been widely used because they limit the aggregation of metal species and enhance the interaction between metal and support, which greatly avoids the problem of metal sintering and precipitation at high temperatures, making the catalysts have high activity and stability. In this section, we briefly introduce the excellent catalytic performance of these encapsulated metal-based catalysts in various reactions, including hydrogenation, oxidation, reforming reactions, and biomass conversion.

4.1. Hydrogenation Reaction

Zeolite-encapsulated metal catalysts are rapidly becoming the platform of choice for hydrogenation reactions because the microporous shell acts simultaneously as a nanoscale reactor. By locking monometallic or bimetallic species inside zeolite channels or hollow cavities, the support prevents particle migration and sintering under reaction conditions while exploiting size- and shape-selective diffusion to steer the hydrogenation pathway. This confinement not only boosts intrinsic activity; it also suppresses over-hydrogenation and coke deposition, enabling long-term operation at elevated temperatures.

As mentioned in the preparation method section, the encapsulated catalyst with metal oxides (Co₃O₄, NiO, and CeO₂) confined in SBA-15 prepared by a solvent-free method showed high thermal stability and excellent performance in the hydrodechlorination reaction of chlorobenzene [35]. Compared with the NiO/SBA-15 catalyst prepared by the impregnation method (60%), the chlorobenzene conversion over the encapsulated NiO@SBA-15 (80%) was much higher. And the yield of benzene, as the only organic product, remained at the initial level for 2 h without any change, indicating that the catalyst showed good stability.

A series of metal-encapsulated catalysts in Y-type zeolite (M-ISAS@Y, M = Pt, Pd, Ru, Rh, Co, Ni, and Cu) prepared by hydrothermal crystallization [43] can be applied to catalytic dehydrogenation/hydrogenation, isomerization, and catalytic reforming reactions with ultra-high activity and selectivity. The quantitative structural configuration of Pt-ISAS encapsulated in the Y zeolite framework was further obtained by density functional theory (DFT) calculation and EXAFS fitting. It was found that the isolated Pt atom was in the most stable position at the six-membered ring plane of the zeolite framework. In this structure, Pt was coordinated with the two oxygen atoms of the Al-O-Si bridge. The analysis confirmed their atomic dispersion, which effectively avoided the agglomeration that was can easily occur in the traditional impregnation method. In addition, the catalytic interaction between Pt-ISAS and the reaction substrate can be enhanced over the encapsulated one. The TOF value of Pt-ISAS is five times that of Pt nanoparticles, reaching 727 h⁻¹, and the total isomer selectivity exceeds 98%.

Gu et al. [84] encapsulated metallic Pt nanoparticles inside MFI zeolite crystals using an alkali treatment and dry gel transformation method. Due to the shape selectivity of MFI zeolite, the as-prepared Pt@MFI catalyst had high activity for nitrobenzene hydrogenation, with a final conversion close to 100%, but it is inert to the hydrogenation of 2,3-dimethylnitrobenzene (about 10% of conversion). In addition, the Pt@MFI catalyst had high selectivity to the hydrogenation of 4-nitrostyrene (>80%), while the impregnated Pt/ZSM-5 has no selectivity at all under the same reaction conditions. In addition, the Pt nanoparticles encapsulated in the MFI zeolite had good thermal stability and did not aggregate after 10 h of reaction at a high temperature.

Palladium tungsten phosphate (PdTPA) was synthesized by Patel et al. [85] and encapsulated in silica to prepared PdTPA/SiO₂. After introducing it into cyclohexene hydrogenation, the results showed that a high conversion (97%) and high turnover number (4245) were obtained, and it could be reused for multiple cycles without any degradation or leaching. High-resolution TEM (HRTEM) images confirmed that isolated Pd⁰ nanoclusters (Pd NCs; ~2 nm or smaller) were uniformly dispersed in the pores of the zeolite without any aggregation before and after reaction.

Li et al. [86] encapsulated different metal particles in zeolite (Metal@S-1 or Metal@ZSM-5), and the confinement effect of the zeolite prevented the aggregation of metal particles. The as-prepared encapsulated samples showed excellent shape selectivity, and they had high small-molecule (toluene) hydrogenation activity and extremely low large-molecule (1,3,5-trimethylbenzene) hydrogenation activity. TOF calculations showed that the hydrogenation rate of 1,3,5-trimethylbenzene on Pt@S-1 was at least two orders of magnitude lower than that on impregnated Pt/SiO₂. In addition, the authors encapsulated transition metals (Co, Ni, and Cu) in ZSM-5 hollow cores to prepare a series of bifunctional core–shell catalysts [87], which showed excellent metal nanoparticle dispersion and controllable particle size. In the catalytic hydrogenation reaction of aromatics, the conversion of mesitylene on the Ni@S-1 catalyst was basically negligible, but toluene could still be converted, while the conversion rates of toluene and 1,3,5-trimethylbenzene on the supported nickel catalyst were both very high. This indicates that the transport of mesitylene through the zeolite in the encapsulated catalyst is inhibited, the kinetic diameter of mesitylene is significantly larger than the pore size of S-1, and toluene can easily diffuse into the micropores. The difference in hydrogenation conversion between toluene and mesitylene proved the encapsulating effect of the zeolite.

Fischer–Tropsch synthesis (FTS) is a typical hydrogenation reaction that has attracted widespread attention by converting synthesis gas (a mixture of carbon monoxide and hydrogen) into clean fuels and chemicals [88,89]. Liu et al. successfully confined Co in the pore structure of HZSM-5 by adjusting different hydrothermal conditions with Co/SiO₂ as the precursor (CoZ-xN) [90]. It was observed that the as-prepared encapsulated CoZ-xN catalyst significantly improved the selectivity to gasoline in FTS, among which the CoZ-200N catalyst performed best and showed good catalytic activity and excellent stability. In order to better elucidate the encapsulating structure of the catalyst, the precursor Co/SiO₂ catalyst and the impregnated Co/ZSM-5 catalyst were compared under the same FTS reaction conditions. Among them, the Co/SiO₂ catalyst showed a very high CO conversion, reaching the expected 95.7%. However, the CO conversion over the Co/ZSM-5 was only 18.1%, attributing to its poor reducibility, which meant that there are fewer Co⁰ active sites available in the FTS process. In addition, diffusion effects also played an important role in the process. For impregnated Co/ZSM-5 catalysts, external CO and H₂ molecules must diffuse along the narrow and long zeolite channels to contact the small cobalt particles dispersed in the zeolite crystals, but the pore blockage caused by the tiny cobalt particles in the Co/ZSM-5 catalyst will increase the difficulty of syngas diffusion. The confined reaction environment and diffusion paths formed by the encapsulating structure significantly promoted the selectivity to hydrocarbons and inhibited secondary reactions on the acidic sites of ZSM-5.

Chen et al. [91] prepared an encapsulated Ru-based SBA-16 catalyst with Ru loading of 4%, but in different metal particle sizes. The size of the Ru nanoparticles was controlled by the amount of citric acid added during the preparation process. After applying it to the FTS reaction, the results showed that the selectivity to C5+ increased, while the selectivity to methane and C2–C4 light hydrocarbons decreased with the increase in the average Ru particle size, and the catalyst showed the best performance when the average particle size was 5.3 nm.

In hydrogenation reactions, encapsulated metal-based zeolite catalysts utilize the confinement effects of the zeolite structure to suppress metal sintering and regulate diffusion paths, significantly enhancing catalyst activity, selectivity, and long-term stability. This provides an efficient and stable platform for hydrogenation reactions.

4.2. Oxidation Reaction

In oxidation catalysis, confining metal nanoparticles within zeolite pores transforms the zeolite from a mere support into a molecular-scale oxidation reactor that enforces both geometric and electronic control. The micropores not only prevent high-valent metal species from sintering or leaching under aggressive O₂-rich conditions; they also channel substrate molecules toward coordinatively unsaturated sites, accelerating electron transfer while excluding bulky poisons. This architecture has enabled encapsulated metal species to achieve exceptional activity and selectivity in reactions ranging from aerobic alcohol and aldehyde oxidations to the direct conversion of biomass-derived furans into dicarboxylic acids, often operating under mild, solvent-free conditions with sustained recyclability that far surpasses conventional supported analogues.

Zhang et al. [34] prepared an encapsulated bimetallic AuPd-based Silicalite-1 catalyst. It was found that the size of the nanoparticles was almost unchanged before and after high-temperature calcination. This phenomenon can be attributed to the fact that the AuPd nanoparticles were confined within the S-1 framework, preventing the aggregation of metals. After 11 h of bioethanol oxidation, Au_0.4Pd_0.6@S-1 showed an extremely high conversion (96.0%) and benzaldehyde selectivity of more than 99.5%, and after calcination in oxygen, its catalytic performance was completely restored by removing the coke.

Choi et al. [92] encapsulated metal clusters (Pt, Pd, Ir, Rh, and Ag clusters) in NaA zeolite by hydrothermal method. During the zeolite crystallization process, the metal cluster precursors were stabilized by trimethoxysilane ligands. The thiol (-SH) groups in the ligands interacted with the metal cations, while the alkoxysilane groups formed covalent Si-O-Si or Si-O-Al bonds and successfully encapsulated metals in NaA zeolite. The hydrogenation rates of ethylene and isobutylene were much higher on the metal clusters encapsulated in NaA. These confirmed the high activity achieved by encapsulation. The encapsulation strategy prevented thermal sintering of the metal clusters and inhibited the poisoning of the active sites by organic sulfur species, resulting in much higher catalytic performance of methanol and isobutanol oxidation than those of catalysts impregnated on SiO₂.

A platinum species was successfully encapsulated in S-1 zeolite by Shi et al. [93] (denoted as Pt@S1), and high conversion (92%) and selectivity (70%) can be achieved after applying it to the cyclohexanol oxidation reaction. Due to the protective effect of the zeolite, the catalyst had good anti-leaching and anti-poisoning properties, further giving the catalyst excellent reusability. This was mainly because the zeolite shell not only protected the internal active substances from interacting with toxic substances in large molecular size but also facilitated the dispersion of platinum species.

Trace amounts (0.01 wt%) of precious metal nanoparticles (Au, Pt, and Pd) were reported to encapsulate in nanocapsules (MCM-22 and ZSM-5) zeolites for the oxidation of hydroxymethylfurfural (HMF) by Saxena1 et al. [94]. These catalysts exhibited excellent HMF conversion (>95%) and FDCA yield (>90%) under mild conditions (60 °C, 0.3 MPa oxygen), and they had good selectivity, with no detected by-products produced by cracking. The authors speculated that, on the one hand, the mesoporous structure of the zeolite improved the diffusion of HMF and FDCA, providing a hydrophobic environment to protect HMF from decomposition. On the other hand, the encapsulation structure effectively inhibited the agglomeration or leaching of nanoparticles, thereby maintaining excellent catalytic activity.

In oxidation reactions, encapsulated metal catalysts leverage the protective effect of zeolite to effectively avoid metal sintering and poisoning under high-temperature or strongly oxidizing conditions. This significantly improves the activity, selectivity, and recyclability of reactions ranging from aerobic alcohol/aldehyde oxidation to direct conversion of biomass-derived furans into dicarboxylic acids, showcasing superior comprehensive performance compared to conventional supported catalysts.

4.3. Reforming Reaction

Reforming reactions operate at the harshest catalytic extremes with high temperature, strong oxidants or reductants, and rapid coke formation. Encapsulating metal species inside zeolite channels or hollow shells transforms the support into a nanoreactor that suppresses metal sintering, blocks carbon filament growth, and sieves reactants/products to suppress secondary reactions. This architecture has enabled activities, tunable syngas ratios, and unprecedented long-term stability in methane dry reforming, steam reforming of higher alkanes, and biomass tar cracking, positioning zeolite-encapsulated catalysts as the next-generation platform for clean hydrogen and syngas production.

Dai et al. [48] encapsulated Ni-Pt metal components in Silicalite-1 and used the resulting combination for the dry reforming of methane with carbon dioxide. Since the encapsulating process enhanced the interaction between Ni, Pt, and the support, the formation of coke was effectively inhibited, and the dispersion of the metal was increased. Thus, excellent sintering-resistance performance and catalytic activity were obtained.

Assistance by ligands, subnanoscale NiPt bimetallic clusters (about 1.7 nm) were encapsulated in Silicalite-1 by Zhang et al. [95]. The NiPt metal clusters were confined in Silicalite-1’s microporous channels, which can prevent metals from sintering. Through the shape selective steam reforming of n-dodecane and m-xylene, it was found that, due to the large molecular size of m-xylene, it was difficult to diffuse into the zeolite pores, which further prevented the potential deactivation causing from tar during the reforming process. Therefore, in the steam-reforming experiment of n-dodecane, m-xylene was hardly formed over the encapsulated sample, and the as-prepared catalyst showed good stability and activity, and had high hydrogen selectivity.

Zhang et al. [96] studied the catalytic performance of Silicalite-1-encapsulated Ni-based catalysts in the steam reforming (SRM) of methane. The Silicalite-1 zeolite acted as diffusion channel to prevent alkali poisoning [97,98]. Compared with the core–shell catalyst prepared by the traditional method, the catalytic activity of the catalyst for the SRM reaction was increased by about 10 percentage. Even when the as-prepared catalyst was exposed to alkali vapor during the reaction, it still maintained a high reaction activity and showed high tolerance to alkali poisoning.

Liu et al. [99] used a hydrothermal synthesis method to prepare nickel-based bimetallic catalysts supported by MCM-41 (Ni-Zr-MCM-41). During the synthesis process, ethylene glycol was added to coordinate with Ni²⁺, so that the uniformly distributed nickel nanoparticles were fixed in the mesoporous channels. The as-prepared Ni-Zr-MCM-41 catalysts showed better initial catalytic activity and significantly improved the long-term stability of the catalyst.

Xie et al. [100] used polyol-assisted method to encapsulate Ni nanoparticles with controllable size and position in the mesoporous SBA-15 to obtain highly dispersed Ni catalysts. The confinement effect of the zeolite played an important role in preventing the sintering of Ni nanoparticles. Thus, the catalysts showed excellent resistance to coking and sintering in the dry-reforming reaction of methane (DRM). The high dispersion of Ni nanoparticles in the catalyst allowed more active centers to contact the reactants, thus possessing higher catalytic activity.

For reforming reactions under high-temperature and harsh reactive conditions with rapid coke formation, the encapsulation structure suppresses metal sintering, carbon filament growth, and secondary reactions via confinement effects. This notably enhances the activity, reaction-path controllability, and long-term stability of methane dry reforming, steam reforming, and other such reactions, offering a reliable strategy for the efficient preparation of clean hydrogen and syngas.

4.4. Dehydrogenation Reaction

Dehydrogenation sits at the critical junction between C-H bond and C-C bond activation, yet conventional catalysts succumb rapidly to sintering and coke at the 500–700 °C temperatures required [101,102,103]. By locking ultrasmall metal species within the matrix of MFI, BEA, or other zeolites, the support acts as a nanoscale reactor. It suppresses particle agglomeration, moderates the electronic state of surface metal atoms, and selectively allows alkane in while expelling olefin products before secondary cracking can occur. This confinement strategy has recently delivered high propylene selectivity (>99%) in propane dehydrogenation, near-zero deactivation over multiple redox cycles, and spontaneous coke removal via shape-selective combustion, demonstrating that zeolite-encapsulated metals are poised to redefine high-temperature dehydrogenation processes.

Ultrasmall and highly dispersed bimetallic GaPt nanocatalysts were encapsulated in Silicalite-1 by Wang et al. [104]. The confined GaPt nanoclusters can provide high electron density and promote the desorption of products. In the propane dehydrogenation reaction, it showed excellent catalytic performance and high stability (propylene selectivity up to 92.1%; propylene production rate of 20.5 mol g_Pt⁻¹ h⁻¹), and no obvious deactivation was observed during the whole process, which was much better than that of traditional Ga-based catalysts. In addition, the confinement of the zeolite can improve the regeneration stability of the catalyst, and the catalytic activity remained unchanged after four cycles.

Zhang et al. [105] encapsulated bimetallic PtGa nanoparticles in MFI zeolite and used it in the propane dehydrogenation (PDH) reaction. At 600 °C, the propylene selectivity over the encapsulated PtGa catalyst was as high as 98%. The characterizations found that the encapsulation can inhibit the sintering and coke deposition, thereby improving the catalytic stability. Theoretical simulations showed that subsurface regulation increased the electron density of the surface Pt active center, weakened the adsorption of propylene, and thus improved the selectivity to the target product. In addition, this design strategy can also be applied to some other high-temperature reactions, such as reforming reactions.

Bimetallic catalysts can significantly improve the catalytic performance due to the synergistic effect between metals. Taking advantage of this feature, Wang et al. [106] encapsulated well-dispersed ultrasmall PtZn bimetallic nanoclusters in Silicalite-1 (S-1) (named PtZn@S-1) and then applied these encapsulated nanoclusters to propane dehydrogenation (PDH). The as-prepared catalyst 0.3Pt0.5Zn@S-1 showed excellent catalytic performance in PDH, with propane conversion of 45.3% and propylene selectivity of >99%. In the long-term propane dehydrogenation reaction at 550 °C, no obvious sintering of the PtZn bimetallic nanoclusters was observed. Due to the confinement effect in the S-1 zeolite, the catalyst showed excellent catalytic activity, good stability, and good shape selectivity during PDH.

In the propane dehydrogenation reaction, Zhang et al. [107] encapsulated highly dispersed Pt-Cu alloy nanoparticles in Silicalite-1. The as-synthesized catalyst had better anti-coking ability compared with the catalyst prepared by the impregnation method. It was found that encapsulation improved the anti-coking ability of Pt-Cu alloy. The author deduced that when encapsulated with S-1, the electron density of Pt-Cu alloy can be changed, which improved anti-coking ability.

Ding et al. [108] encapsulated PdAu nanoparticles (NPs) in diamine-containing UiO-66 (UiO-66-(NH₂)₂) and then applied these encapsulated NPs to the dehydrogenation reaction of formic acid. Various characterization results showed that the size of the PdAu nanoparticles was successfully controlled to be less than 1.1 nm. The amine groups in the support can increase the hydrophilicity of the UiO-66 framework, making it easier for the metal precursor to diffuse in the cavity during the preparation process. In addition, UiO-66-(NH₂)₂ can stabilize the metal precursor, and the enhanced coordination ability can better prevent the aggregation of metal species. The as-prepared Pd_0.8Au_0.2/UiO-66-(NH₂)₂ catalyst had excellent catalytic activity, and the turnover frequency (TOF) value could reach 3660 h⁻¹, which was much higher than the palladium-based catalysts reported in the literature. In addition, the catalyst had excellent anti-aggregation stability and high regeneration stability, and almost no deactivation occurred within seven cycles.

In dehydrogenation reactions, ultrasmall metal species encapsulated within zeolite are locked in the framework, suppressing sintering and coking at high temperatures. Through shape selectivity, they promote the removal of target products before secondary cracking, achieving high selectivity (>99%), low deactivation, and spontaneous coke removal in reactions like propane dehydrogenation. This provides an innovative solution for high-temperature dehydrogenation processes.

Encapsulated metal-based zeolite catalysts, through the confinement effects, exhibit unique advantages across diverse reactions (hydrogenation, oxidation, reforming, and dehydrogenation). They not only suppress high-temperature sintering and aggregation of metal species but also optimize reaction paths via diffusion regulation, significantly enhancing catalyst activity, selectivity, and long-term stability. From hydrodechlorination of chlorobenzene to propane dehydrogenation, and from ethanol oxidation to Fischer–Tropsch synthesis, these catalysts outperform their conventionally supported counterparts in energy conversion, fine chemical synthesis, and biomass utilization. They provide a versatile platform for efficient and stable catalytic operations under high-temperature and harsh conditions, thereby expanding the application boundaries of metal-based catalysts.

5. Summary and Outlook

5.1. Current Challenges

In this review, we have summarized the preparation techniques of zeolite-encapsulated metal catalysts and provided a brief overview of their applications in catalytic fields. The main synthetic methods for these catalysts include impregnation, ion exchange, ship in a bottle, solvent-free crystallization, hydrothermal crystallization, dissolution–recrystallization, etc. [4,109,110]. These encapsulated catalysts have demonstrated excellent activity, selectivity, and stability in various reactions, such as hydrogenation, oxidation, reforming, and biomass conversion. Although significant progress has been made in zeolite-encapsulated metal catalysts, it remains challenging to simultaneously achieve both high activity and stability in certain cases. Therefore, developing novel synthetic strategies for encapsulating metal nanoparticles continues to be a key research focus.

Precise control over the size, morphology, and dispersion of metal species still faces technical challenges. Conventional preparation methods like impregnation and ion exchange often struggle to prevent nanoparticle aggregation, particularly during high-temperature treatments where metal atoms tend to migrate and coalesce, leading to non-uniform particle size distribution. Achieving atomic-level dispersion while maintaining catalytic activity is extremely difficult because isolated metal atoms typically exhibit poor stability and tend to migrate and aggregate under reaction conditions [111,112,113]. Furthermore, for bimetallic or multimetallic systems, the differences in distribution and reduction behavior of various metal precursors within zeolites make precise control over alloy composition and structure exceptionally complex.

The structural design and performance regulation of zeolite also present multiple challenges. When constructing hierarchical pore structures, traditional post-treatment methods like alkaline treatment can introduce mesopores, but often at the expense of micropore structure and acid sites. How to precisely regulate pore connectivity and surface chemistry without compromising the structural integrity of zeolite crystals has become a key limiting factor for improving catalyst performance [114,115]. Moreover, effective methods are still lacking for differentially regulating the chemical environments of internal and external surfaces, particularly for selective modification techniques targeting external acid sites. When these modified zeolites are combined with metal components, new challenges emerge in balancing the metal–support interaction strength with catalytic activity: excessively strong interactions may lead to deactivation through excessive modification of metal electronic states, while overly weak interactions cannot effectively stabilize metal nanoparticles.

5.2. Future Development Trends and Prospects

The future development of zeolite-encapsulated metal catalysts will witness an innovative convergence of multiple technologies. Advanced nanofabrication techniques such as atomic layer deposition (ALD) and plasma-assisted synthesis will enable atomic-level precision control of metal nanoparticles, while bio-templating methods offer novel approaches for constructing zeolite carriers with tailored morphologies. Particularly noteworthy is the emerging “dynamic encapsulation” technology, which utilizes responsive polymer materials to achieve in situ regulation of catalytic active sites, thereby overcoming the performance limitations of conventional immobilized catalysts. The integration of theoretical calculations and artificial intelligence will accelerate the establishment of precise structure–activity relationship models for metal–zeolite systems, guiding the design of novel composite catalysts with specific pore architectures and active sites.

Zeolite-encapsulated metal catalysts demonstrate broad application prospects in energy and environmental catalysis fields. For renewable energy conversion, they can be applied in solar-driven CO₂ hydrogenation to produce high-value chemicals. In environmental remediation, their exceptional shape-selective catalytic properties show great potential for efficient VOC degradation. In green chemical processes, they will serve as novel catalytic platforms for biomass valorization. It must be emphasized that future development should adhere to sustainable principles by developing low-energy preparation processes and utilizing renewable raw materials to minimize the environmental footprint throughout the catalyst’s lifecycle. We recommend establishing industry–academia–research collaborative innovation systems to overcome key engineering bottlenecks in scale-up production, facilitating the transition of these high-performance catalysts from laboratory research to industrial applications.

In conclusion, zeolite-encapsulated metal catalysts have achieved remarkable progress in recent years. Continued efforts should focus on optimizing existing technologies to develop more active and stable metal catalysts. Future research should prioritize the development of simple, robust, and scalable synthesis methods—a critical requirement for industrial application. This represents the prevailing trend for advancing this field toward practical implementation.