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Fabrication of Multifunctional Core⁃Shell Structured Nanoreactors and Their Catalytic Performances

Hao Chen , Xu Xu , Chaonan Jiao , Hao Yang , Jing Wang , Yinxian Peng

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 1911 -1934.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 1911-1934 DOI: 10.7536/PC211101
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Fabrication of Multifunctional Core⁃Shell Structured Nanoreactors and Their Catalytic Performances
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Abstract

With the development of nanotechnology, it has been possible to easily adjust the compositions, morphologies, and sizes of nanomaterials to control their properties. In order to endow nanomaterials with new functions and expand their applications in the fields of materials, chemistry, biology and medicine, it is very meaningful to develop new types of nanomaterials that can achieve multiple functions at the same time. One of the methods for obtaining multifunctional nanomaterials is achieved by coating the surface of simple nanoparticles with functional materials, and the resulting composite structure is called a core⁃shell structure. The core and shell of the core⁃shell structure can be composed of the same or different materials. By changing the compositions, structures and surface properties of the core and shell materials, the nanomaterials can be endowed with special optical, electrical, magnetic, catalytic, adsorption and biological activities. The hollow and yolk⁃shell structure can be formed by the controllable transformation of the core and the shell, in which the inner cavity can be used as a high⁃performance nanoreactor in various fields of catalysis. In this review, the design and application of core⁃shell structured nanoreactors with different structures in the field of catalysis are discussed, with emphases on the applications in catalytic degradation of dye pollutants, catalytic hydrogenation, catalytic oxidation, and catalytic cascade reactions. Finally, some prospects are put forward for the future research and development of multifunctional core⁃shell structured nanoreactors.

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nanoreactor / catalysis / mesoporous silica / noble metal nanoparticles / transition metal oxide

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Hao Chen, Xu Xu, Chaonan Jiao, Hao Yang, Jing Wang, Yinxian Peng. Fabrication of Multifunctional Core⁃Shell Structured Nanoreactors and Their Catalytic Performances. Progress in Chemistry, 2022, 34(9): 1911-1934 DOI:10.7536/PC211101

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Contents

1 Introduction
2 Classification of core⁃shell structured nanoreactors
2.1 Traditional core⁃shell structure
2.2 Hollow core⁃shell structure
3 Application of core⁃shell nanoreactors in catalytic reactions
3.1 Catalytic degradation of dye pollutants
3.2 Catalytic hydrogenation
3.3 Catalytic oxidation
3.4 Catalytic cascade reaction
4 Conclusion and outlook

1 Introduction

In the past decade, nanotechnology has revolutionized catalysis, energy, environment, biomedicine, and many other fields. As the cornerstone of nanotechnology, nanoparticles have attracted more and more attention because of their unique nano-size characteristics. Nanoparticles are defined as particles having a size of less than 100 nm in at least one of the three dimensions. Before the word "nanotechnology" was pointed out, people in the field of catalysis had been able to synthesize particles with sizes as small as nanometer. Nanoparticles have high specific surface area, so they have high catalytic activity. With the continuous development of nanotechnology, various methods have been used to design the structural details of nanocatalysts (including the composition, size, shape, and surface properties of the catalyst) to achieve the purpose of regulating the catalytic activity of nanocatalysts[1,2]. Due to the high surface energy of nanoparticles, they tend to spontaneously aggregate into larger aggregates in the catalytic reaction process, resulting in the reduction of the active surface area of nanocatalysts, thus reducing the catalytic performance. In order to improve the stability of the nanocatalyst and endow the material with new functions, a functional shell can be coated on the surface of the nanoparticle to form a core-shell structure[3]. In practical applications, the outer shell can play a role in protecting the inner nanoparticles, reducing the impact of harsh environmental conditions (chemical and thermal) on nanoparticles, and effectively preventing the aggregation of nanoparticles. In addition, the core-shell structure formed after the shell modification makes the nanocomposite have new properties, and there is a synergistic effect between the core and the shell, showing different properties from the single core material. The conventional core-shell structure is shown in Figure 1A and consists of at least two materials with different functions. By changing the properties of the shell and core materials, the formed core-shell structure can have more specific functions, thus providing unlimited possibilities for applications in many fields. In addition, the outer shell can be monolayer or multilayer, and its function can be further expanded by regulating its composition. However, because the traditional core-shell structure can not provide effective internal space, in order to obtain materials with more comprehensive functions, more advanced design and geometric modifications are needed on the basis of traditional core-shell structure materials. After continuous research, it has been found that a core-shell structure with an internal space can be formed by partially removing the shell or core, which is called a yolk shell or a rattle type core-shell structure, and can be described as a structure with a core @ void @ shell, as shown in Figure 1b and C[4,5]. The internal space can be used for cargo storage, which helps to provide more active sites and some confined reactions. This new structure combines the high storage capacity of the hollow structure with the advantages of the core-shell structure. The yolk shell structure can be obtained by removing the sacrificial layer from the core @ sacrificial layer @ shell structure. The yolk-shell structure has a movable solid core, and the core can select nanomaterials with different functions (such as magnetism, metal, or quantum dots). The special structure of yolk-shell nanomaterials can increase the contact area between reactants and core and shell materials, thus making full use of the respective functions of core and shell materials[6]. Through the rational design and precise regulation of core-shell materials with structural diversity, it is expected to prepare multifunctional core-shell composite nanomaterials. In recent years, multifunctional core-shell composite nanomaterials have been widely used in catalysis as nanoreactors, which contain catalyst nanoparticles[7⇓⇓⇓~11]. Nanoreactors are ultra-small reaction vessels that provide a nanoscale space within which the reaction is confined[12]. In the yolk-shell nanoreactor, catalyst nanoparticles can be introduced onto the core and/or shell, and this kind of nanoreactor can realize the precise positioning of various active components. As a typical nanoreactor, core-shell composite nanomaterials show high activity, high stability and selectivity in catalytic reactions. This paper will focus on the unique design of the core-shell composite nanoreactor and its application in the field of catalysis. Firstly, we introduce the design of core-shell composite nanoreactor and its advantages. Then, a series of important applications of nanoreactors with different structures in the field of nanocatalysis were focused on. Finally, some thoughts and prospects for the future research and development of multifunctional core-shell nanoreactors are put forward.

2 Types of core-shell nanoreactors

2.1 Conventional core-shell structure

Core-shell structure is a kind of composite material, and its shell can be materials with different properties. The materials of the core-shell structure can have different combinations, such as the combination of organic materials and inorganic materials, that is, the organic core can be coated by the inorganic shell, and vice versa[13]. In this case, considering the components of the core and shell, roughly four combinations are possible, inorganic core/inorganic shell (e.g. SiO2 core/metal oxide shell), inorganic core/organic shell (e.g. SiO2 core/polymer shell), organic core/inorganic shell (e.g. polymer core /SiO2 shell) and organic core/organic shell (e.g. polymer core/covalent organic framework shell). The properties of the material can be regulated by changing the composition and the core-shell ratio of the material[14]. Among the existing inorganic matrices, silica as a shell material has greater application potential than other inorganic materials[15]. Silica is non-toxic, highly biocompatible, thermally and mechanically stable, and easily functionalized.
Among them, silica materials with mesoporous structure have good applications in various fields because of their high specific surface area, uniform and adjustable pore size, good biocompatibility and other advantages[16]. The properties of core-shell nanomaterials are mainly related to the selection of core and shell materials, as well as the morphology and structure. This unique plasticity provides great opportunities and possibilities for regulating performance by controlling the chemical composition and relative size of core-shell materials[17,18].
The choice of core and shell materials is related to the type of target application, and their design often depends on the final application requirements. In order to make the prepared material easy to separate, the magnetic core can be selected[19]. According to the properties of core and shell materials, various types of spherical core-shell nanostructures can be constructed. The nucleus can be dense or porous (Figure 2A). It can be designed by choosing different shapes of core materials, such as rod-shaped, polyhedral, etc. (Figure 2b). In addition, different types of shells can be used, such as continuous/discontinuous solid shells, continuous porous shells, and multiple shells (Figure 2 C)[20]. In addition to the usual spherical core-shell nanomaterials, there are many other possible shapes for core-shell nanostructures, including nanorods, nanotubes, nanopolyhedrons, and nanorings, among others (Fig. 3)[13,21⇓~23].

2.2 Hollow core-shell structure

Hollow structures arise when there is a need to reduce the density of nanomaterials or to construct nanomaterials with huge storage capacity and enhanced capture ability. The concept of hollow core design was first proposed by removing different types of nuclear materials[24]. The adjustment of the cavity size can be done by precise selection of the removed nucleus size before the nucleus removal step.
Hollow spheres, hollow nanopolyhedrons, hollow fibers and hollow nanoplates were obtained by selecting different shapes of core templates for shape adjustment[25⇓⇓~28]. According to the composition of shell materials, they can be divided into organic and inorganic hollow nanostructures. According to the number of layers, it can be divided into single-shell, double-shell and multi-shell hollow structures. Hollow nanostructures have the characteristics of uniform size, large specific surface area and low density. These properties have greatly promoted the application of hollow nanostructures as carrier materials in the fields of catalysis, drug delivery, selective adsorption, etc. The function of hollow nanostructures is determined by the intrinsic properties of different components, which has a wide range of applications in water treatment and environmental remediation. In addition to providing excellent storage space, the hollow structure has multiple locations for further functionalization, such as the outer surface of the core, the inner/outer surface of the shell, and the pores on the outer shell. Table 1 shows the structure of some core-shell nanoreactors and their main characteristics.
The hollow nanomaterial designed using the functional optimization strategy is more like an efficient nanoreactor than a pure carrier[29]. It can not only provide a localized environment for reactions, but also integrate many functionalized fragments, thus realizing complex chemical reactions and multiple uses. The strategy of internal space functionalization involves the introduction of various functional species into the cavity to optimize and diversify the function of hollow nanoreactors. For example, a nanoparticle catalyst is introduced into the inner wall of a porous cavity to form a reverse bumpy ball structure, and the porous shell is beneficial to the dispersion of the catalyst particles to prevent the aggregation of the catalyst particles, so that the catalyst particles can maintain higher catalytic activity,But also allows reactants and products to pass in and out, which are advantages that traditional nanoparticle catalysts do not have, and the nanomaterial with the structure is expected to have a good application prospect in the industrial field as a nanoreactor[30,31]. The construction of hollow core-shell nanostructures can occur before cavitation, or the hollow shell can be formed first and then the functionalized core can be introduced. The former strategy is based on the core-shell nanostructure, by removing a part of the intermediate layer (such as part of the core or shell), to get the nanocore-shell materials with hollow structure. The latter strategy is to introduce single or multiple nanonuclei on the basis of the formed hollow shell by methods such as boat-in-a-bottle.
The constructed hollow core-shell nanostructures can exhibit a variety of configurations, and Figure 4 shows some common hollow core-shell structures. In the reverse bumpy ball structure (Figure 4A), the nanoparticles are fixed on the inner wall of the hollow shell, which can effectively prevent the agglomeration of nanoparticles and make the catalyst show good stability. The introduction of functional particles into the hollow shell can produce the structure shown in Figure 4B. The particles are fixed on the inner wall of the shell and the outer wall of the inner core respectively. The functional particles can be of the same composition or of different compositions. Magnetic particles can be selected as the inner core, which can be separated by external magnetic field and recycled. If there is only one particle in the hollow shell, the yolk shell structure shown in Figure 4C is formed. If there are more than one particles in the shell, the structure formed is called multi-core yolk shell structure (Figure 4D). Fig. 4E shows a nanoreactor with a double-shell structure. More complex structures can also be formed, for example, as shown in Figure 4F ~ H, which contain single or multiple fixed or movable nanoparticles in or on the surface of the double shell, in which the nanoparticles can be of the same or different chemical compositions[32].
A novel hollow core-shell nanostructure system is formed by combining the functional characteristics of the core-shell structure with the huge storage capacity of the hollow structure, which embodies the advantages of the two structures, not only has an internal space for storing reactants, catalysts and the like, but also can protect the catalysts from being affected by the environment due to the existence of the shell. The yolk-shell structure is one of the typical representatives, which has a core @ void @ shell structure. The most widely used yolk-shell nanostructure is composed of yolk/core nanoparticles embedded in a porous shell, and these nanocores can move freely within the porous shell[33]. These mobile nuclei possess more exposeable active sites in the reaction than the nuclei fixed on the inner wall of the shell[10⇓~12,34]. The yolk-shell nanocomposite with porous shell structure has the following characteristics: (I) a mobile core, (ii) an internal space to accommodate guest molecules, (iii) a mesoporous shell that can provide the diffusion of reactants and products, (iv) a high specific surface area and (V) an additional pore space of the mesoporous shell, which provides convenience for a variety of chemical reactions. In addition, it is possible that there is a synergistic effect between the nano-core and shell in the yolk-shell structure, resulting in stronger performance in applications[35]. Compared with core-shell materials, the presence of voids in the yolk-shell structure not only facilitates molecular storage, separation, and confinement during catalysis, but also provides a homogeneous environment for heterogeneous catalysis[36]. The strong customization ability and diversified modification functions of the core and hollow shell also make yolk-shell nanomaterials show broad application prospects in a series of fields such as catalysis, biosensors and surface-enhanced Raman scattering.At present, the research on these functionalized yolk-shell nanomaterials is mainly focused on the design and preparation of nanostructures with different morphologies in order to obtain ideal properties[8,9]. For example, the design and synthesis of some special nanocomposite structures provide a platform for the study of the synergistic effect of the two catalysts in the rattle structure.

3 Application of Core-Shell Nanoreactor in Catalytic Reaction

Multifunctional hollow core-shell composite structures have many unique and interesting properties as nanoreactors, such as protective shell, movable core, void space between core and shell, and diverse combinatorial properties of core and shell. In view of these advantages, people have tried to apply the hollow core-shell composite structure as a nanoreactor in the fields of adsorption degradation, lithium-ion batteries, nanomedicine, and sensors[37⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~56]. Here, we will highlight some of the existing and potential applications of hollow core-shell composite structures as multifunctional nanoreactors in the field of catalysis.
Hollow core-shell nanocomposites with functionalized internal space provide an ideal platform for the development of catalysts with high activity, selectivity, and recyclability, which can solve many problems of traditional nanoparticle catalysts[9,11,57⇓~59]. Most of the multifunctional nanoreactors used in catalysis introduced in this paper belong to yolk-shell structure, in which single or multiple catalyst nanoparticles are loaded in the cavity surrounded by hollow porous nanoshells or on the inner surface of the shell. Catalyst nanoparticles include metals and their alloys, such as Au, Ag, Pd, Pt, Rh, Ni, Co, metal oxides (such as Fe3O4, MgO, TiO2, MnO2), and organic group functionalized SiO2 or their composites, etc. Their porous shells consist of oxide materials (e.g., SiO2, TiO2, silicates, zeolites), and a few consist of carbon-based materials (e.g., amorphous carbon and graphene oxide) and organic polymers. On the one hand, the permeable porous shell of nanoreactors can be designed according to the size, shape, and compatibility of the molecules involved in the reaction, allowing only specific substrate molecules to enter the inner cavity of the nanoreactor, thus enriching their potential in substrate-selective catalysis, which is rarely achieved by traditional nanocatalysts[60⇓⇓⇓⇓⇓~66]. On the other hand, catalyst nanoparticles supported on functional shell materials or between layers can produce cascade catalytic effects, and their application scope may be extended to composite catalyst systems, thus realizing one-pot continuous multi-catalytic reaction processes in a single nanoreactor[67⇓~69]. In this part, we will give an overview of the applications of the hollow core-shell structures that have been designed and synthesized as multifunctional nanoreactors in the field of catalysis. The main catalytic reactions involved can be roughly divided into four categories: (I) catalytic degradation of dye pollutants; (ii) catalytic hydrogenation; (iii) catalytic oxidation reaction; (iv) catalytic cascade reaction.

3.1 Catalytic degradation of dye pollutant

Wastewater treatment is one of the research hotspots in the field of environment[70,71]. The dye pollutants in the wastewater discharged from paper industry, printing industry, textile industry and other industries are highly colored substances. Some dyes can make the water body produce obvious color when the concentration is low, and a small amount of dyes can also pollute a large area of water[72]. Therefore, it is necessary to continuously develop new dye degradation technologies to protect the natural environment and human health. At present, a series of methods have been developed to remove dye pollutants, such as adsorption, wet air oxidation, electrochemical oxidation, Fenton oxidation and so on[73][74][75]. However, these strategies also have some drawbacks, such as the release of toxic metal ions during Fenton oxidation[76]. The wet air oxidation process requires high temperature and high pressure conditions, which has high energy consumption and safety problems, and the harsh reaction environment also shortens the service life of the catalyst[77]. The nanoreactor with hollow core-shell composite structure as the carrier of catalyst has unique advantages in the catalytic degradation of dye pollutants. Li et al. Prepared nanoreactors by loading transition metal oxides MxOy(M=Mn, Co, Ni, Cu, Zn) and rare earth metal oxides MxOy(M=La, Ce, Eu, Gd) nanoparticles onto the inner surface of a hollow porous silica shell for the degradation of dye molecules[78,79]. The design idea of the nanoreactor is to firstly use the micelle formed by charge-driven self-assembly (formed by the electrostatic attraction between the negatively charged coordination complex chain and the positively charged block copolymer) as a template to deposit silica on the micelle,Then the template was removed by calcination to form a hollow structure, during which the metal ions were oxidized to oxides and deposited on the inner surface of the porous silicon shell, and finally the reverse bumpy ball nanoreactors (HMSNs) loaded with small metal oxide nanoparticles were obtained. In the presence of H2O2, this kind of nanoreactor shows excellent performance in the catalytic degradation of dye molecules. Due to the strong coordination interaction between metal ions and bidentate ligands in the micelle, the size of metal oxide nanoparticles formed on the inner wall of HMSNs can be controlled by simply changing the number of metal ions in the micelle, and metal oxide nanoparticles of various sizes including ultra-small (< 2 nm) can be loaded. In addition, the abundant microporous structure on HMSNs is beneficial to the mass transfer of small molecule H2O2, while the existence of mesopores makes it easy for larger dye molecules to reach the catalytic active sites inside, thus increasing the catalytic reaction rate. Compared with the Fenton oxidation method, the nanoreactor avoids the use of a large amount of Fe2+ and effectively realizes the recycling of the catalyst. Yao et al. Successfully prepared a bimetallic encapsulated N-doped carbon nanotube reactor Fe ￠ Cu @ N ￠ C by pyrolyzing zeolite imidazole framework material ZIF ￠ 8 coordinated by Fe2+ and Cu2+, and the metal embedded in Fe ￠ Cu @ N ￠ C was Cu nanoparticles and atomically dispersed Fe[80]. The nanotube reactor showed outstanding catalytic activity in the catalytic degradation of Orange II, which was attributed to the introduction of bimetallic active sites, high N-doping level, large surface area and conductive porous carbon structure, which was conducive to electron transport.
Yolk-shell nanoreactors with unique structure and tunable function have attracted great attention in the field of catalysis, and people have turned their attention to the preparation of nanoreactors with higher catalytic efficiency and better recyclability. Mao et al. Prepared a yolk-shell-structured nanoreactor with a core satellite structure, which was formed by a single large silver core and many tiny silver nanoparticles wrapped in a mesoporous silica shell[81]. The synthesis route is to prepare Ag @ C with core-shell structure by hydrothermal method, and the C shell is doped with small-sized Ag nanoparticles, then the surface of Ag @ C is coated with PVP and the SiO2 shell is deposited, and the carbon shell and organic matter are removed by calcination to form the yolk-shell nanoreactor with core-satellite structure. The nanoreactor with nuclear satellite structure showed good catalytic activity in the reduction of rhodamine B (Rh B) with NaBH4 as reductant, and the conversion rate of Rh B could still reach 92% after five cycles.
Photocatalytic technology is a new efficient and energy-saving modern wastewater treatment technology, which is widely used in the degradation of organic dyes and the reduction of pollutants. Photocatalytic oxidation has the advantages of mild reaction conditions, strong oxidation ability, complete destruction of organic matter into CO2, and higher treatment depth than other methods. Photocatalytic oxidation can be divided into homogeneous photocatalysis and heterogeneous photocatalysis. As a kind of heterogeneous photocatalyst, nano-semiconductor catalyst has the characteristics of good activity, high thermal stability and low price, and is the most commonly used photocatalyst. For semiconductor nanocatalysts, the recombination of photoexcited electron-hole pairs is extremely unfavorable to the catalytic performance, and the development of catalysts with high electron-hole separation efficiency is the main direction of photocatalytic research at present. It is found that reducing the particle size is one of the effective strategies to improve the photocatalytic efficiency, mainly because if the particle size of semiconductor materials is small enough, a larger energy gap will be formed, thus increasing the difficulty of recombination of photoexcited electron-hole pairs. Li et al. Successfully synthesized a novel nanoreactor H⁃mSiO2⁃BiOBr for visible light photocatalysis by a three-step method, in which hollow mesoporous silica spheres were functionalized first, and then amino groups were modified on the surface of the silica shell by introducing APTES.Then the Bi3+ was anchored to the silica shell by the guiding effect of amino groups, and finally the BiOBr semiconductor nano-photocatalyst with a particle size of 8 ~ 15 nm anchored on the surface of the silica shell was obtained by solvothermal method in the presence of KBr[82]. The nanoscale BiOBr can increase the band gap width (1. 47 eV) and inhibit the recombination of photogenerated electron-hole pairs. In addition, the cavity of the reactor is conducive to the realization of multiple reflections of light in the cavity, which effectively prolongs the action time of light and improves the photocatalytic efficiency. The ordered mesoporous structure is also beneficial to the transfer of reactant molecules. In the photocatalytic degradation of Rh B under visible light, the H⁃mSiO2⁃BiOBr nanoreactor showed good photocatalytic activity and stability. Jiang et al. Used natural halloysite nanotubes (HNT) as nanoreactors for photocatalytic degradation of Rh B, etc[83]. The inner surface of HNT is mainly composed of aluminum-oxygen octahedral Al-OH layer, while the outer surface is composed of silicon-oxygen tetrahedral layer. This unique asymmetry of the inner and outer surfaces leads to an obvious spatial confinement effect in the inner cavity of HNT. Compared with the outer surface, the inner surface of HNT with iron doping shows better photochemical activity and stronger adsorption capacity in the photochemical reaction process. The efficiency of photocatalytic degradation of Rh B, malachite green and ciprofloxacin in the halloysite nanotube reactor was 1.3, 1.8 and 5.1 times higher than that of kaolin nanosheets, respectively.
Photocatalytic degradation of organic pollutants is one of the important ways to solve the problem of antibiotic pollution in the environment. Liu et al. Synthesized rhombic dodecahedron nanoreactor ZnS@In2S3RD with core-shell structure by solvothermal vulcanization method using rhombic dodecahedron ZIF8 as precursor. The fluffy and porous In2S3 shell was wrapped on the ZnS core of rhombicohedron, which increased the specific surface area and adsorption activity of the material.The core-shell nanoreactor has a Z-shaped heterojunction structure, can effectively promote the separation of electrons and holes through In-S-Zn bond contact, and shows efficient photocatalytic activity In the photocatalytic degradation experiment of tetracycline, and the apparent reaction rate constant reaches 0.284 min-1, which is far higher than that of most previously reported sulfide photocatalysts[84]. Liu et al. Prepared three kinds of nanoreactor Cu2O@TiO2 with core-shell structure by coating TiO2 on the surface of Cu2O cube, Cu2O cuboctahedron and Cu2O octahedron.The core-shell Cu2O@TiO2 polyhedra showed higher photocatalytic activity than the single-component Cu2O polyhedra in the degradation of methylene blue under visible light irradiation.The photocatalytic performance of the three morphologies of nanoreactors was different, among which the octahedral Cu2O@TiO2 had the best photocatalytic activity, followed by the cuboctahedral one[85]. The coating of the TiO2 will form a p-n heterojunction structure on the surface of the Cu2O, which is helpful to enhance the separation of charge carriers.

3.2 Catalytic hydrogenation

Hydrogenation is one of the most challenging catalytic reactions, which is widely used in organic chemical production. Catalytic hydrogenation reaction refers to the addition of one or more unsaturated functional groups in organic compounds with hydrogen under the action of catalyst. Enes, alkynes, aldehydes, ketones, aromatics, and nitro compounds can all be catalytically hydrogenated to produce valuable products. The synthesis of methanol and hydrocarbons from low carbon molecules (carbon monoxide, carbon dioxide) and H2 is also a hydrogenation reaction. Hydrogenation reaction in most chemical production requires high hydrogen pressure, which will lead to a large amount of energy consumption, and a large amount of high-pressure H2 needs to be recovered after the reaction, which requires high-cost industrial equipment. Secondly, because most hydrogenation reactions are continuous, the selectivity of products is difficult to control, which greatly hinders the application of hydrogenation technology in industry. Nanoreactors can solve these problems. The hollow structure of nanoreactors can effectively adsorb H2 small molecules, which helps to reduce the consumption of hydrogen and increase the concentration of hydrogen around the catalytic active sites, thus enhancing the catalytic activity[86]. The unique confinement effect of nanoreactors helps to improve the selectivity of the reaction.

3.2.1 CO hydrogenation

Catalytic hydrogenation process has been widely studied in Fischer-Tropsch synthesis. Fischer-Tropsch synthesis is a chemical reaction in which CO is heterogeneously hydrogenated to produce hydrocarbon products with a wide molecular weight distribution. Fischer-Tropsch synthesis technology is an important development direction of the current C1 chemical industry. Although the raw materials of Fischer-Tropsch synthesis reaction are only simple CO and H2, the diversification of products can be realized by coupling with different processes, and the reaction products can reach more than 100 kinds. The raw materials of Fischer-Tropsch synthesis can be coal, natural gas and biomass. This technology is to convert non-oil-based carbonaceous resources such as coal, natural gas and biomass into clean liquid fuels and high value-added chemicals through syngas (CO and H2), which can replace oil and attract the attention of all countries[87]. The Fischer-Tropsch synthesis process is divided into high temperature Fischer-Tropsch synthesis and low temperature Fischer Tropsch synthesis. High temperature Fischer-Tropsch synthesis process uses Fe-based catalyst at 300 ~ 350 ℃, mainly producing gasoline and low molecular weight olefins. Low-temperature Fischer-Tropsch synthesis uses Ru, Fe-based or Co-based catalysts at 200-240 ℃, and mainly produces high molecular weight paraffins[88]. Table 2 lists the catalytic performance results of some of the core-shell nanoreactors for Fischer-Tropsch synthesis reactions.
Lan et al. Constructed a Ru⁃PVP@SiO2YSNs (Fig. 5) for low-temperature Fischer-Tropsch synthesis by encapsulating Ru-PVP in a yolk-shell nanoreactor. Considering that the yolk-shell nanoreactor can avoid the strong interaction between Ru and the carrier, it provides a nanospace for the movement of Ru-PVP and the enrichment of reactants and intermediates[89]. The nanoreactor is an efficient nanoreactor for low-temperature liquid-phase Fischer-Tropsch synthesis. Under similar reaction conditions, the Ru PVP nanowires encapsulated in the nanoreactor exhibited higher catalytic activity (6.35 vs 5.96 mol CO mol-1·Ru h-1) and selectivity to oxygenated products (41.3% vs 21.6%) than the free state. Due to the removal of a part of PVP, which is weakly bound to Ru, during the encapsulation of Ru PVP nanowires, the molar ratio of PVP/Ru in the final nanoreactor is about 2, which is much lower than that of free Ru PVP (PVP/Ru ratio is 20). The catalytic activity and selectivity are enhanced due to the increased exposure of the active material Ru in the nanoreactor.
Chen et al. Prepared yolk-shell-structured Ru@SiO2 nanoreactors with water-in-oil microemulsion technology, and proposed a new strategy to control the hydrocarbon distribution in Fischer-Tropsch synthesis by changing the distance between Ru nanoparticles and the silica surface in the nanoreactor[90]. The internal volume of the nanoreactor can effectively control the selectivity of Fischer-Tropsch synthesis. The internal volume of the nanoreactor can be controlled by adjusting the water content in the W/O microemulsion. The Fischer-Tropsch synthesis reaction shows a carbon chain propagation limitation in nanoreactors with minimal volume, producing only compounds with a maximum of 25 carbons. Nanoreactors with moderate volumes, which can produce compounds with longer carbon chains. While the growth of the hydrocarbon located in the large volume nanoreactor is not limited. The growth of carbon chain in the limited space of nanoreactor will lead to the decrease of entropy and the increase of Gibbs free energy, thus inhibiting the further growth of carbon chain and facilitating the formation of short-chain hydrocarbons. This type of nanoreactor shows good potential for selective control in low-temperature Fischer-Tropsch synthesis, and can be extended to the selective catalytic synthesis of many macromolecular substances.
Subramanian et al. Prepared a silica nanoreactor encapsulated with cobalt nanoparticles, and the porous silica shell played a role in protecting the catalyst, preventing the sintering of cobalt nanoparticles and keeping the nanoparticles in a dispersed state, thereby improving the stability of the catalyst[91]. Silica nanoreactors can have a good control effect on the chain length, and the maximum length of hydrocarbons is limited by the diameter of the nanoreactor, which will lead to a narrower distribution of hydrocarbons.
Xu et al. Prepared hollow mesoporous ZSM-5 molecular sieves, and the mesopore size was adjustable between 7.9 and 20.5 nm[92]. The shell of hollow zeolite plays a crucial role in enhancing the C5-11 selectivity and suppressing the C2-4 selectivity. It avoids the selectivity of C2-4 by accelerating mass transport. ZSM-5 zeolite with a mesopore size of 13. 7 nm was mixed with Fe3O4@MnO2 to form a bifunctional catalyst, which showed a high selectivity for C5-11 (80. 4%) at a CO conversion of 69. 3%, and a high selectivity for C5-11 aromatics of 68. 7%. Hollow mesoporous zeolites exhibit excellent catalytic stability, showing great potential for industrial applications.

3.2.2 Hydrogenation of unsaturated hydrocarbon

Liquid phase hydrogenation has been widely used in the hydrogenation of unsaturated hydrocarbons such as olefins, alkynes and aromatics. Hydrogenation of olefins is one of the most commonly used organic transformations and is usually carried out using nanoparticle catalysts such as Pt, Ru, Pd and their alloys. In order to maintain the stability and high catalytic activity of the nanocatalyst, it is necessary to load the nanocatalyst on a certain carrier. The yolk/core-shell nanoreactor, which has a unique structure and a wide range of applications, has unique advantages in the catalytic hydrogenation of unsaturated hydrocarbons. Compared with the conventional supported catalyst, the yolk/core-shell nanoreactor catalyst provides a unique confined environment for the enrichment of reactants in the catalytic reaction due to the gap between the core and the shell, and the porous shell can protect the catalyst particles from aggregation. In addition, it has a variety of structures, controllable pore size and adjustable surface properties. These characteristics will help to improve the stability, activity and selectivity of the catalyst.
Some novel core-shell nanomaterials have been developed for the hydrogenation of alkenes and alkynes. In order to solve the limitations of traditional hydrophobic catalysts or hydrophilic catalysts, Hao et al., inspired by the structure of natural enzymes, prepared a core-shell structure nanoreactor with palladium-loaded fluorine-modified silica microspheres as the hydrophobic core and mesoporous silicon as the hydrophilic shell.The catalytic active center (metal nanoparticle) is located at the hydrophobic interface and is wrapped by a hydrophilic nanoporous shell,Because of that hydrophilicity of the shell, the nanoreactor can be well disper in water, and the hydrophobic reactant can be adsorbed from the water to the catalytic active center at the hydrophobic interface, so that the catalyst has high activity in the olefin hydrogenation reaction than the traditional catalyst Pd/SiO2(Pd the nano particles are positioned on the surface of the silicon spheres)[104]. At the end of the reaction, the system is automatically divided into two layers, the organic product in the upper layer can be removed by simple phase separation, and the catalyst in the lower water phase can be directly used for subsequent reaction cycles. This catalytic system is expected to achieve high catalytic efficiency and in situ recycling of the catalyst, making the hydrogenation reaction greener and more sustainable.
Hollow nanoreactors have been widely used in the hydrogenation of alkenes and alkynes due to their hole confinement effect. Dong et al. Constructed two types of nanoreactors by introducing PdCu nanoparticles into the outer surface of hollow carbon spheres (PdCu/HCS) and encapsulating them into the cavity of hollow carbon spheres (PdCu @ HCS), respectively, and studied the catalytic performance of these two types of nanoreactors in the liquid phase hydrogenation process[105]. It was found that PdCu @ HCS showed shape-selective catalysis in the hydrogenation process. The hydrogenation of styrene is promoted by the accumulation of reactants, which slows down the hydrogenation of 2 vinylnaphthalene due to the diffusion-limited effect of the shell wall, and hinders the hydrogenation of 9 vinylanthracene due to the molecular sieve effect of the shell wall. Moreover, the hollow space of the nanoreactor provides a unique microenvironment for the reaction, which can change the hydrodynamics of the intermediate in the alkyne hydrogenation reaction and correspondingly change the selectivity of the catalytic reaction. Ren et al. Used a nanoreactor modified by a phosphine ligand to achieve efficient hydrogenation of benzoic acid in organic solvents under mild conditions, while the unmodified nanoreactor did not have such excellent catalytic activity[106]. The phosphine ligand around the Ru nanoparticles induces the preferential adsorption of the aromatic ring of benzoic acid on the Ru nanoparticles, which ensures the efficient hydrogenation of benzoic acid in n-hexane.
Yang et al. Designed and synthesized a spindle-shaped yolk/shell magnetic nanoreactor as a heterogeneous catalyst for olefin hydrogenation[107]. The interior of the nanoreactor contains a magnetic iron core, and the surface of the iron core is modified with a small size of noble metal nanoparticles Pd (~ 2.0 nm). The nanoreactor has shown superior catalytic activity to commercial Pd/C in the hydrogenation of olefins, with a conversion of up to 99%. The nanoreactor showed high stability in use, and the catalytic activity did not decrease significantly after 20 times of recycling. On the one hand, the good catalytic activity and stability of the nanoreactor are due to the confinement effect of the yolk/shell structure, which increases the concentration of reactants around the internal Pd nanoparticles and accelerates the catalytic reaction. Moreover, the special structure of the nanoreactor can make the Pd nanoparticles stay in the shell stably, and the loss of catalyst particles will not occur in the recycling process. In addition, due to the magnetism of the iron core, the nanoreactor can be easily separated from the reaction solution, which simplifies the recovery procedure and avoids the loss of the catalyst during the recovery process.
Yan et al. Prepared hollow mesoporous silica nanoreactors encapsulating rhodium nanoclusters (~ 0.8 nm) for selective hydrogenation of phenol[108]. The Rh3+ ion is combined with the ligand to form a positively charged coordination polymer network structure, and then the polymer micelle combined with the Rh3+ ion is obtained through electrostatic interaction with a negatively charged diblock polymer,The hollow mesoporous silica nanoreactor loaded with Rh nanoclusters can be obtained by depositing a silica shell on the surface of the micelle, calcining and reducing with hydrogen. Compared with the Rh catalyst supported on the silica surface, the nanoreactor can significantly enhance the activity of catalytic hydrogenation and the selectivity of cyclohexanol formation. The small rhodium nanoclusters in the nanoreactor can inhibit the dissociation of phenolic hydroxyl groups and greatly promote the formation of cyclohexanol. The inhibition of cyclohexanone formation improves the selectivity of cyclohexanol, while the anchoring of rhodium nanoclusters near the cavity enhances its thermal and catalytic stability. On the one hand, the improvement of cyclohexanol selectivity may be due to the fact that the ultra-small Rh nanoclusters have low coordination sites, which are conducive to the interaction with reactants to improve the catalytic performance. On the other hand, considering that the Rh nanocluster is close to the cavity, it is beneficial for the Rh nanocluster to fully react with cyclohexanone to promote the deep reaction to generate cyclohexanol.
Song et al. Constructed a Pd@Ag@CeO2 nanoreactor with a core-shell structure using Pd @ Ag with an asymmetric silver-coated palladium structure as the core and CeO2 as the shell[109]. The silver shell in the silver-coated palladium structure is deposited onto the palladium nanocubes by a spontaneous redox reaction between AgNO3 and Ce(OH)3. The catalytic activity in the selective hydrogenation of phenylacetylene in liquid phase was investigated using a Pd@Ag@CeO2 nanoreactor and compared with two conventional core-shell catalysts, Ag@CeO2 and Pd@CeO2. It was found that the Ag@CeO2 had no catalytic activity, while the Pd@CeO2 showed a high conversion of 94%, but the yield of styrene was very low, up to 4%. With Pd@Ag@CeO2 as catalyst, the yield of styrene was as high as 74%, and the selectivity was more than 99%. In addition, it was found that the catalytic activity could be enhanced by reducing the content of Ag in a certain range, and the conversion rate could be as high as 97% under the optimal conditions, while maintaining ultra-high selectivity. Pd nanoparticles have high hydrogenation catalytic activity, which can directly over-hydrogenate alkynes to alkanes, resulting in low selectivity to olefins. Compared with Pd nanoparticles, Ag nanoparticles have poor catalytic performance, but the bimetallic nanoparticles formed by the combination of Ag and Pd nanoparticles have higher catalytic activity and selectivity, which is due to the fact that the Ag shell with appropriate thickness can ensure higher olefin selectivity, while the Pd core can improve the catalytic activity of the Ag shell by forming PdH as a hydrogen source. The unique structure of Pd@Ag@CeO2 nanoreactor ensures both activity and selectivity in the semi-hydrogenation of alkynes.

3.2.3 Hydrogenation of nitroaromatic compounds

Aromatic amine is an important organic chemical raw material and an intermediate for the synthesis of many fine chemicals, which is widely used in the fields of dyes, pharmaceuticals, pesticides, surfactants and polymer materials. Reduction of aromatic nitro compounds to prepare aromatic amines is a common method for fine chemical production because of its simple operation and easy availability of raw materials. The reduction of nitroaromatic compounds by catalytic hydrogenation is mostly carried out by using Pt, Pd, Ni, Cu and other metals or their oxides as catalysts, and loading the catalysts on SiO2, Al2O3, activated carbon and other carriers. Nanoreactors provide the possibility for the efficient, economical and green synthesis of aromatic amines due to their unique structural advantages.
Lan et al. Used ZIF 67 as a template to coat the mesoporous silica shell on its surface, and obtained a nanoreactor with yolk-shell or hollow structure doped with Co⁃N⁃C@SiO2 after pyrolysis treatment in nitrogen atmosphere, in which the active layer Co N C was derived from the ZIF 67 template (Figure 5A)[110]. The hollow and yolk-shell nanoreactors showed high activity in the catalytic hydrogenation of nitrobenzene to aniline, which was about 3.3 times higher than that of the conventional Co/SiO2 catalyst (Fig. 5B). The excellent performance is attributed to the doping of Co nanoparticles into the N C framework to form Co⁃Nx species, and the Co⁃Nx has higher activity for the hydrogenation of nitroaromatics. At the same time, the Co⁃N⁃C@SiO2 catalyst has high aniline selectivity, which can achieve a catalytic selectivity of more than 99%.
Nitroaromatics hydrogenation is divided into homogeneous hydrogenation and heterogeneous hydrogenation. In homogeneous hydrogenation, the catalyst is dissolved in the reaction medium, which has challenges such as difficult separation and recovery. In addition, the residual metal ions after the reaction will pollute the environment. However, when some conventional heterogeneous catalysts are used, there will be some problems [111], such as low catalyst activity, poor selectivity, easy dissolution of catalyst and so on. Wang et al. Designed a heterogeneous synergetic nanoreactor with hollow structure Rh⁃N⁃C@SiO2 doped with ultra-small Rh NPs for the hydrogenation reduction of nitrobenzene[112]. Ultra-small Rh nanoparticles (2.2 nm) were loaded into a hollow sphere with two shells, in which the outer shell was mesoporous silicon and the inner shell was nitrogen-doped carbon, and the Rh nanoparticles were confined within the double shell and tightly combined with the nitrogen-doped functionalized support. The coordination or electron transfer interaction between the Rh NPs and the nitrogen-doped support can reduce the mobility of the metal particles, which can effectively prevent the agglomeration or sintering of the particles, thereby improving the stability of the catalyst material. The inner nitrogen-doped carbon shell is beneficial to the adsorption of reactant molecules, while the outer silicon shell is beneficial to the stable dispersion of nanoreactors in water. The nanoreactor exhibited high catalytic activity and selectivity (100%) at low temperature (80 ° C) and low pressure (1 atm). This excellent catalytic performance is due to the preferential adsorption of nitrogen atoms by nitrogen-doped carbon and the synergistic effect of active centers of Rh nanoparticles. The amphiphilic nanoreactor shows excellent hydrothermal stability in water at 100 ℃, which is superior to other catalysts, and can effectively prevent the aggregation of catalyst particles and leaching during the reaction. This work provides a new strategy for the design of multifunctional nanoreactors, and also shows us the potential of nanoreactors in the green synthesis of aromatic amines.
Peng et al. Synthesized three kinds of yolk-shell nanoreactors. These nanoreactors used silica loaded with ultra-small platinum nanoparticles (2 nm) in the mesoporous channels as the core, and the surface of the silica core in some of the nanoreactors was Hydrophobically modified in advance[113]. Then the surface of the silicon core loaded with platinum particles was coated with organic mesoporous silicon, and the nanoreactor with yolk-shell structure was obtained by selective etching during the coating process. The difference of hydrophilicity and hydrophobicity on the surface of the silicon core resulted in the different positions of Pt NPs in the yolk-shell nanoreactor. In the nanoreactor prepared by the silicon core with the hydrophilic surface, the platinum nanoparticles are basically positioned in the pore channels of the silicon dioxide core; and in the nanoreactor prepared by the silicon core with the hydrophobic surface, the platinum nanoparticles are partially positioned in the pore channels of the silicon dioxide core, and the other platinum nanoparticles are partially positioned in a cavity of the nanoreactors. In the nanoreactor prepared by the silicon core with superhydrophobic surface, most of the platinum nanoparticles are located in the cavity of the reactor. This type of nanoreactor has the weakest interaction between the catalyst and the support and the highest activity in the hydrogenation of nitrobenzene. Manipulating the interaction mode between catalyst particles and support can be used as a means to improve the catalytic effect of nanoreactors.
Metal-organic frameworks (MOFs) are a class of materials with periodic porous network structure formed by metal ions and organic ligands connected by coordination bonds.It has high porosity and adjustable chemical microenvironment, which can achieve the effect of selective sieving by controlling the pore size and shape of MOFs, and can provide an irreplaceable selective and stable microenvironment for chemical reactions. The core-shell structured Gd2O3@Pt@ZIF⁃8 nanoreactor (Fig. 6) designed and constructed by Ning et al. Using the self-assembly strategy showed good performance in terms of catalytic hydrogenation and selectivity, with the hydrogenation conversion rate of nitrobenzene compounds greater than 99.9% and the selectivity greater than 99.9%[114]. The improvement of catalytic activity and selectivity is closely related to the MOF shell. In addition to stabilizing the Pt catalyst, the network channel of the MOF shell can play a sieving effect, which is conducive to the improvement of selectivity. The ternary nanohybrid materials of metal nanoclusters doped with MOF @ COP have the advantages of rich porosity, high stability and large surface area, which can be used as unique multifunctional heterogeneous catalysts for efficient catalytic hydrogenation. By confining ultrasmall palladium nanoclusters (0.8 nm) in UiO⁃66⁃NH2@COP, Zhu et al. Prepared nanoreactor UiO⁃66⁃NH2@COP@Pd, which exhibited excellent catalytic activity and stability for the hydrogenation of various nitroaromatics[115]. The reason should be attributed to the synergistic effect of the ternary components in the nanoreactor, which can :UiO⁃66⁃NH2@COP limit the growth of Pd nanoclusters and improve the utilization of Pd active sites, the COP shell stabilizes and prevents the leaching of Pd nanoparticles, and the UiO⁃66⁃NH2@COP of the hierarchical porous structure is conducive to efficient mass transfer.
Kumar et al. Prepared a C@SiO2 with core-shell structure by dehydration, polymerization and carbonization of D-glucose and self-assembly of silica nanoparticles under hydrothermal conditions, and obtained a hollow silica capsule after removing the C core by ignition treatment[116]. The SiO2 shell is composed of about 15 nm SiO2 nanoparticles with a thickness of 150 – 200 nm. Pd NPs were loaded into hollow silica capsules at different positions by two different impregnation methods, one of which resulted in nanoreactors (Pd@SiO2NACs(SSM)) in which palladium nanoparticles were located inside the nanopores of the silica shell, and the other resulted in nanoreactors (Pd@SiO2NACs(DSM)) in which palladium nanoparticles were mostly located on the inner wall of the silica shell. The two nanoreactors were used as heterogeneous catalysts to study the hydrogenation of nitroaromatics and other compounds, and it was found that the two nanoreactors showed great differences in the hydrogenation of some nitroaromatics with hydrophilic groups. When (Pd@SiO2NACs(SSM)) was used as catalyst to hydrogenate p-nitrophenol, the yield of p-aminophenol was as high as 99%, while the yield of p-aminophenol was only 35% when (Pd@SiO2NACs(DSM)) was used as catalyst. It is well known that silica has a strong interaction with some molecules with hydrophilic groups (-Cl and -OH, etc.), which hinders these molecules from passing through the silica shell to the Pd NP catalyst located on the inner wall, resulting in the decrease of the activity of Pd@SiO2NACs(DSM) in the hydrogenation of p-nitrophenol. However, the two catalysts showed the same excellent catalytic activity (> 99%) in the catalytic hydrogenation of some hydrophobic nitroaromatics. The activity of the Pd@SiO2NACs nanoreactor was compared with that of the commercial Pd/C catalyst (Pd content 10 wt%) in the hydrogenation of nitrophenol. Under similar reaction conditions, the commercial Pd/C catalyst could achieve 70% conversion (TON = 37) in 10 H, while the Pd@SiO2NAC with relatively low Pd content (0. 3 mol%) could achieve 99% conversion (TON = 332). The advantages of this catalytic system are that the synthesis of the catalyst is greener, the Pd content in the catalyst is lower, and the catalyst can be reused.
4-nitrophenol is an important raw material for organic synthesis, which is used as an intermediate of fine chemicals such as pharmaceuticals, pesticides, dyes and so on. At the same time, 4-nitrophenol is a highly toxic organic pollutant, which has a great toxic effect on human body and aquatic organisms. 4-nitrophenol is highly stable in the natural environment, and its reduction product 4-aminophenol is also an important chemical and pharmaceutical intermediate, but its toxicity is much lower, and it is easily degraded in the environment. The reduction of 4-nitrophenol to 4-aminophenol is not only a key step in industrial production, but also an important method for the treatment of p-nitrophenol pollution. The chemical reduction method uses sodium borohydride as the reducing agent and noble metal nanoparticles as the catalyst, which has the advantages of mild reaction conditions and fast reaction rate. Noble metal core-shell nanoreactors have been widely studied in the catalytic reduction of 4-nitrophenol due to their advantages of environmental friendliness, high catalytic activity and easy recycling. Zhao et al. Designed a novel visible light-driven Pd/N⁃Cs@SnO2 nanoreactor with yolk-shell structure, in which movable nitrogen-doped carbon spheres (NCs) were used as the core, mesoporous tin oxide (SnO2) was used as the shell, and Pd nanoparticles were uniformly dispersed on the inner surface of the SnO2 shell and the outer surface of the NCs core[117]. SnO2, a non-toxic n-type semiconductor, has been widely used as a photocatalyst in the field of photocatalysis. The mesoporous tin dioxide shell is favorable for collecting visible light and serving as a carrier of palladium nanoparticles, and the internal space is favorable for improving the absorption efficiency of light through multiple scattering of the light on one hand,On the other hand, it enhances the contact between the active site and the reactant in the photocatalytic reaction. These structural advantages and enhanced synergy make it an efficient and stable photocatalytic reduction catalyst for 4 nitrophenol.
It was found that the Au@SiO2 core-shell nanoreactor had good catalytic activity for the reduction of 4-nitrophenol by sodium borohydride. Nanoparticle catalysts with different morphologies have different effects on the catalytic activity. Shaik et al. Reported a new method to encapsulate gold nanoparticles with different morphologies in a hollow mesoporous silica shell[118]. In the growth process of the gold nanoparticles, no coating agent is introduced, and the shape of the gold nanoparticles grown in the silicon shell is adjusted by the Ag+, so that the gold nanoparticles grown in the silicon shell are sphere-like without the Ag+, and the gold particles grown in the presence of the Ag+ are triangular sheets. The two morphologies of gold nanoparticles showed different catalytic activities in the hydrogenation of 4-nitrophenol, and the gold nanosheets had stronger catalytic activity. The catalytic effect of these two nanoreactors is better than that of some Au@SiO2 core-shell nanoreactors reported in the literature. The enhancement of catalytic activity is related to the absence of coating agent on the surface of gold nanoparticles in these two nanoreactors. The coating agent present on the surface of the nanoparticle may block some active sites and prevent the reactant from reaching the surface of the catalyst particle.
The traditional core-shell catalyst for the hydrogenation of 4-nitrophenol generally uses noble metal nanoparticles as the core and mesoporous silica as the shell. Kang et al. Constructed a nanoreactor different from the traditional core-shell structure. They used Fe3O4@SiO2 core-shell composite particles as the core and coated gold nanoparticles on its surface[119]. The nanoreactor has an obvious photothermal effect due to the gold shell with a thickness of 40 nm on the surface. In the reduction experiment of 4-nitrophenol, the catalytic performance of the nanoreactor was improved by 4. 5 times due to the photothermal effect after irradiation with near-infrared light (808 nm). Photothermal conversion nanostructures, which can absorb incident photons and generate heat, have also gained much attention in the field of catalysis. Mei et al. Designed and synthesized a photothermal responsive nanoreactor PTC Au @ PDA for the catalytic reduction of 4-nitrophenol[120]. The nanoreactor was prepared using PS⁃b⁃P2VP nanospheres as a template and has a unique multi-chamber structure (Fig. 7A). Under near-infrared radiation, the surface temperature of the gold nanoparticles increases, and the generated heat can be preserved by the PDA layer to significantly accelerate the reduction reaction (Fig. 7B). Poly (N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer that undergoes a conformational transition at the lower critical solution temperature (LCST), and this PNIPAM-coated composite has potential applications in smart catalysts and smart nanoreactors[121⇓~123]. Chen et al. Modified the surface of Au@SiO2 with a reverse bumpy ball structure by coating its surface with a temperature-responsive PNIPAM shell (Fig. 7 C)[124]. These nanoreactors showed excellent catalytic activity at 30 ° C (below the LCST) and very low activity at 50 ° C (above the LCST) in the reduction of 4-nitrophenol (Fig. 7d). The catalytic reduction reaction is controlled by the diffusion rate of the reactants through the mesopores into the nanoreactor. When the temperature is lower than the LCST, the PNIPAM chain adopts an extended conformation, which makes it easy for 4 nitrobenzene to diffuse through the polymer coating to the surface of the gold nanoparticles located on the inner wall of the mesoporous silica shell for reaction. However, at 50 ℃, the PNIPAM chains folded into a dense layer covering the surface of mesoporous silicon, which prevented the diffusion of 4 nitrobenzene into the nanoreactor, thus greatly inhibiting the reaction. These photo- and thermo-responsive nanoreactors highlight their potential as a new class of catalytic nanoreactors.
Compared with the simple yolk-shell nanostructure containing only one core, the pomegranate-like nanostructure contains multiple cores, which can provide more exposed active sites[125⇓⇓~128]. Therefore, pomegranate-like nanostructures with multiple cores may show better catalytic activity as nanoreactors, and have broader application prospects in the field of catalysis. Wang et al. Synthesized a pomegranate-like nanoreactor with multi-palladium nanoparticles in the shell of SiO2 modified by 3-mercaptopropyl functional group, which showed good catalytic activity and stability in the hydrogenation reduction of 4-methoxyphenyl-nitrobenzene[129]. The yield of 4-methoxyaniline was 98%, and the activity of the nanoreactor did not change significantly after five cycles. TEM images showed that the structure of the nanoreactor and the dispersion of palladium nanoparticles remained stable after five cycles. Yang et al. Reported a pomegranate-like nanoreactor Ag@mSiO2, where they encapsulated multiple silver nanoparticles (17.6 nm) inside hollow mesoporous silica spheres[130]. Compared with the Ag@SiO2 containing only one silver nanoparticle, the Ag@mSiO2 containing multiple silver nanoparticles showed greater superiority in catalyzing the reduction rate of 4 NP. This is because the presence of multiple silver nanoparticles greatly increases the chance of contact between the reactant molecules and the silver core. Liu et al. Generated a pomegranate-like nanoreactor containing multiple Au nanoparticles (10 nm) in situ using residual amino groups in the cavity of yolk-shell-structured sSiO2@mSiO2, which had good catalytic performance for the reduction of 4-naphthol[131]. In order to recycle the catalyst more conveniently, Shao et al. Prepared a yolk-shell magnetic nanoreactor with mesoporous carbon as the shell and Fe3O4@SiO2 as the core loaded with gold nanoparticles. The Fe3O4@SiO2 core not only provided a place for the immobilization of gold nanoparticles, but also enabled the nanoreactor to be easily recycled from the liquid medium through an external magnetic field[19].
In addition to increasing the number of cores in core-shell nanoreactors to improve catalytic performance, multi-shell mesoporous silica nanoreactors with a large number of internal pores and mesoporous shells also have unique advantages. Wu et al. Prepared multi-shell mesoporous silica nanoreactors loaded with gold nanoparticles using a simple dual-template method,The gold nanoparticles were bound to the outer surface of the amino-functionalized nanoreactor through a ligand exchange reaction, and the nanoreactor showed good catalytic activity and good cycle stability for the reduction of 4-nitrophenol[132].
Most of the core-shell reactors loaded with noble metal nanoparticles have poor stability at high temperature and are only suitable for low-temperature reactions. Dong et al. Prepared a highly active and stable yolk-shell Au@MgSiO3 nanoreactor through a simple self-etching process, and the nanoreactor showed good catalytic activity in the reduction of 4-nitrophenol by NaBH4.The size and shape of the gold nanoparticles are almost unchanged after calcination at up to 500 ℃, and the magnesium silicate shell remains intact[133].
Mass transfer directly affects the catalytic reaction performance in the heterogeneous catalytic process, and the catalytic reaction can occur only when the reactant diffuses to the catalytic active site. Nanoreactors with amphiphilic shells can be used to efficiently enrich hydrophobic reactants in water. Zou et al. Prepared amphiphilic organic hollow silica (PMO) nanospheres by using organosilane oriented growth-induced etching strategy, and loaded gold nanoparticles in the amphiphilic nanospheres as nanoreactors.It was found that the nanoreactor with an amphiphilic shell had better catalytic activity for the reduction of 4-nitrophenol than the nanoreactor with a hydrophilic shell, mainly due to the faster diffusion of organic reactants into the amphiphilic shell nanoreactor[134].
In addition to the traditional spherical carrier, other shapes of carriers also have unique properties. The hollow doughnut-shaped mesoporous silica has a mesoporous structure and a hollow internal space, and can be used as a nanoreactor for catalysis by introducing catalyst nanoparticles. Vo et al. Prepared a circular nanoreactor loaded with silver nanoparticles (4. 2 nm) by amino functionalization of hollow doughnut-shaped mesoporous silica nanoparticles through amino reduction Ag+, which showed good reusability and high efficiency in the catalytic reduction of 4 nitrophenol[135]. Kottappara et al. effectively deposited Ag nanoparticles on the surface of Cu nanowires to form a linear core-shell structure nanoreactor. Compared with monometallic copper nanowires, the catalytic reduction performance of Cu Ag core-shell nanowires for 4-nitrophenol was enhanced by 3.5 times[136]. Table 3 summarizes the application of some core-shell nanoreactors in the catalytic hydrogenation of nitroaromatic compounds, and it can be seen that this kind of nanoreactor has the advantages of strong structural stability and high catalytic efficiency.

3.3 Catalytic oxidation

3.3.1 Oxidation of alcohol

Selective oxidation of alcohols to carbonyl compounds such as aldehydes and ketones is an important organic chemical reaction, and the products are widely used in pharmaceutical and chemical industries. How to effectively oxidize alcohols to the corresponding carbonyl compounds under green and mild conditions is a research hotspot. Most alcohol oxidation reactions still proceed at high temperatures, which often leads to sintering or agglomeration of the catalyst, thereby reducing the activity of the catalysis. The hollow nanoreactor has a high specific surface area, and the catalyst is supported inside it, which can maintain its exposed catalytic active sites at high temperature, and is considered to be a recyclable and efficient catalyst for alcohol selective oxidation reaction[137⇓~139].
Janus particle is a kind of particle with unique properties developed in recent years. It is an asymmetric particle with two different chemical compositions or different polarities in its structure. It has attracted much attention in the fields of interfacial catalysis, drug carriers, functional surfactants, high-performance composite materials and so on[140]. Dai et al. Constructed a novel Janus-structured hollow multifunctional amphiphilic nanoreactor (NxC@mSiO2) containing ultra-small Pt metal nanoparticles (Fig. 8 a) by a confined pyrolysis method, in which nitrogen-doped carbon was partially embedded in a mesoporous silica shell, and the outer surface of the nanoreactor was hydrophilic and the inner surface was hydrophobic[141]. On the one hand, the nitrogen atoms doped in the carbon matrix play a role in stabilizing the ultra-small Pt metal nanoparticles (2.1 nm), and on the other hand, they act as active centers to accelerate deprotonation in the oxidation reaction. This amphiphilic nanoreactor shows more significant activity and selectivity than other hydrophilic and hydrophobic catalysts for the alkali-free oxidation of various alcohols in water (Fig. 8B). Dai et al. Also synthesized an amphiphilic nanoreactor Fe3O4@SiO2@Pd/HS⁃PMO(Et) with a novel yolk-shell structure, which showed high conversion (~ 100%) and good selectivity (~ 99%) for the selective oxidation of various alcohol substrates[142]. Due to the presence of the magnetic core Fe3O4, the nanoreactor can be easily separated and recycled, and 100% conversion and 90% – 95% selectivity can be maintained after 10 cycles of recycling. Selective oxidation of benzyl alcohol to benzaldehyde is a key reaction for the synthesis of high value-added fine chemicals such as pharmaceuticals, pesticides and spices. Yang et al. Prepared a yolk-shell structure amphiphilic nanoreactor PIL@SiO2⁃Pd with poly (ionic liquid) (PIL) nanospheres as hydrophobic core and mesoporous silica as hydrophilic shell, respectively (Fig. 8C)[143]. Polyionic liquids (PILs) are a new type of polymers with anionic and cationic groups in the repeating unit, which are synthesized by the polymerization of ionic liquid monomers. They have the excellent properties of both ionic liquids and polymers. The PIL hydrophobic core of this nanoreactor can adsorb Pd ions, and highly dispersed Pd nanoparticles (2 nm) were obtained after NaBH4 reduction. In the oxidation of benzyl alcohol to benzaldehyde with water as solvent, the nanoreactor showed high activity (TOF up to 414 h-1, 12.9 times higher than that of MCM41 Pd catalyst) and selectivity (100%). Chen et al. Synthesized a polynuclear yolk-shell nanoreactor Co @ C N in the shape of a rhombic dodecahedron by pyrolyzing a hollow Zn/Co ZIF precursor in an inert atmosphere[144]. The shell of the nanoreactor is composed of a nitrogen-doped porous carbon shell, and the internal cavity is composed of nitrogen-doped carbon nanosheets anchored with Co nanoparticles, and the nitrogen-doped carbon nanosheets can effectively stabilize the Co nanoparticles. Due to the unique structure of the Co @ C/N nanoreactor, it shows enhanced catalytic activity in the oxidation of various alcohols such as 1-phenylethanol, with a conversion of up to 99% in the atmosphere and in the absence of alkali. The enhanced catalytic activity is attributed to the unique polynuclear yolk-shell structure, which can facilitate the transport of reactants/products as well as the excellent electronic synergy between Co nanoparticles and N-doped carbon nanosheets.
Tian et al. Constructed a photothermal nanoreactor AuPt @ HMZS loaded with bimetallic catalyst in hollow mesoporous silica[145]. The bimetallic gold and platinum based catalyst loaded in the reactor is a kind of catalyst with excellent photocatalytic activity. The reactor was used in the oxidation of cinnamyl alcohol to cinnamic acid, and excellent catalytic conversion and selectivity (99%) were obtained. The results show that the nanoreactor has multiple advantages in photocatalytic oxidation reaction: the multiple light scattering of the cavity enhances the light capture and promotes the photothermal effect; The synergetic effect of plasma hot electron injection and electron capture improves the utilization of visible light and the electron-hole separation of photocatalyst. The strong metal-metal interaction at the alloy interface can adjust the reaction performance of the gold-platinum catalyst, so that the photocatalytic activity of the gold-platinum catalyst is much higher than that of the monometallic catalyst. Ziarati et al. Prepared yolk-shell-structured Y@HWS⁃TiO2 nanoreactors through three steps of solvothermal reaction, partial etching, and hydrogen treatment (Fig. 8D)[146]. Ti3+ species appeared on the surface of the nanoreactor obtained by hydrogen treatment, which reduced the band gap of the titanium dioxide shell into the visible light region. The difference of the band gap between the TiO2 shell and the inner TiO2 core is beneficial to the separation of the photogenerated electrons and holes. The nanoreactor showed good catalytic activity and selectivity (93% ~ 97%) in the visible light photocatalytic oxidation of aromatic alcohols (Fig. 8e). Ziarati et al. Also modified yolk-shell TiO2 prepared by solvothermal reaction with AAPTES to enhance the interaction with graphene oxide (GO) sheets by introducing diamine groups.The GO sheet was tightly wrapped on the surface of the modified yolk-shell structured TiO2 by electrostatic attraction, and then the graphene-wrapped yolk-shell structured G⁃HW⁃Y@S⁃TiO2 nanoreactor was obtained by hydrothermal treatment and calcination[147]. The selectivity of the nanoreactor can reach 99% when the aromatic alcohol is oxidized to the corresponding aldehyde by visible light, and the selectivity does not depend on the reaction time, and the time-dependent photocatalytic oxidation selectivity is attributed to the special structure of the reactor,The hydrophobic aldehyde generated by the reaction is forced to diffuse out of the internal gap due to the hydrophilic/hydrophobic repulsion between the hydrophobic aldehyde and the hydrophilic TiO2 core, which prevents further over-oxidation. When the reaction product aldehyde diffuses out, the shell completely covered by graphene prevents the aldehyde from further contacting with the photogenerated holes in the TiO2 shell, thus ensuring that the overoxidation reaction does not occur even under the condition of prolonged reaction time.

3.3.2 Epoxidation

Epoxidation reaction refers to the oxidation reaction in which an oxygen atom is added between the carbon atoms at both ends of the double bond of a compound to form a three-membered ring. Epoxide is a very important intermediate in organic synthesis. Epoxide can be converted into valuable compounds through ring-opening reaction, which are widely used in the fields of medicine, dyes, spices and resin additives. The traditional process suffers from problems such as environmental concerns, many by-products, difficulty in homogeneous catalyst separation, and cost efficiency. The nanoreactor has the characteristics of green, energy saving, high efficiency, stability and good selectivity[148,149]. The dispersion of catalyst nanoparticles can be improved by loading catalyst nanoparticles into porous carriers with hollow structure, so that the catalyst nanoparticles can maintain high catalytic activity. Li et al. Prepared a unique yolk-shell structure nanoreactor containing AuPt alloy nanoparticles by embedding multiple AuPt alloy nanoparticles into a mesoporous silica microsphere via an aerosol method (Fig. 9 a), which exhibited good catalytic activity and selectivity for styrene epoxidation (Fig. 9 B)[150]. Noble metal nanoparticles (Au, Pt, Ru, Pd and their alloys) are widely used as heterogeneous catalysts for olefin epoxidation. Precious metal-based catalysts have good activity in olefin epoxidation, but they are expensive. It is of great significance to develop efficient and low-cost non-precious metal-based olefin epoxidation catalysts. Because of the confinement effect of nanoreactors, reactant molecules are locally enriched in the nanoreactor, which can significantly improve the reaction rate, which can be used to compensate for the low catalytic performance of non-noble metals. Among the non-noble metal-based catalysts, molybdenum-based and cobalt-based catalysts have shown superior catalytic activity. Shen et al. Prepared yolk-shell structured nanoreactor MoO2@SiO2 by encapsulating molybdenum dioxide (MoO2) nanoparticles in hollow mesoporous silica lumen[151]. The MoO2 nanoparticles encapsulated in the nanoreactor were ultrafine (3.8 nm) and highly dispersed, which greatly improved the catalytic activity of the catalyst. The prepared MoO2@SiO2 was used in the epoxidation of olefins, and the conversion and selectivity of the reaction were over 95% after 10 H under the optimum conditions. Li et al. Designed a cobalt-based nanoreactor with Co3O4@meso⁃SiO2,Co3O4 nanoparticles loaded on the inner wall of hollow mesoporous silica. The nanoreactor showed high catalytic activity and selectivity in the epoxidation of cis-cyclooctene. The reaction was completed in about 60 min with 90% conversion and 100% selectivity[152].

3.3.3 Oxidation of VOCs

In recent years, annual emissions of volatile organic compounds (VOCs) have increased dramatically with the acceleration of industrialization and economic growth. As one of the most harmful air pollutants, volatile organic compounds (VOCs) pose a serious threat to human body and atmospheric environment[153]. From the point of view of various treatment technologies to remove VOCs, catalytic oxidation is considered to be one of the most effective methods because of no secondary pollution and low energy consumption. Catalytic combustion can efficiently and economically convert low concentrations of VOCs into harmless water and carbon dioxide. Due to the unique advantages of catalytic oxidation, people have been committed to developing effective catalysts for catalytic oxidation of VOCs[154]. The performance of supported bimetallic catalyst is higher than that of supported monometallic catalyst, which is mainly related to the synergistic effect of bimetallic. However, most bimetallic catalysts are irregular in size and shape, and the synergy between them cannot be optimized to the greatest extent. Therefore, the catalytic activity of bimetallic catalysts can be further improved by constructing new nanostructures. In recent years, core-shell structures have attracted much attention and have shown great potential applications in various fields, especially in catalysis. The nanoparticle catalyst can be coated in a porous shell as a core, and the special structure endows the nanoparticle catalyst with excellent stability and high temperature resistance, thereby solving the sintering problem of the nanoparticle catalyst at a higher temperature[155]. Among the catalysts used for catalytic oxidation of VOCs, noble metal catalysts (such as gold, palladium, platinum, ruthenium) are more active than transition metal oxide catalysts, but the latter are more resistant to toxicity and cheaper. Among various transition metal oxides, cobalt-based catalysts have been demonstrated to have excellent catalytic activity for the deep oxidation of light alkanes. The catalytic performance of cobalt-based catalysts can be further improved by compounding with other metal oxides. Fang et al. Synthesized a catalyst with a core-shell structure using Co-Ce ZIFs as a precursor, CeO2@Co3O4 (Fig. 10B), which showed better catalytic activity than the monometallic catalysts CeO2 and Co3O4 in the catalytic oxidation reaction of toluene (Fig. 10d), and the improvement of catalytic performance was inseparable from the synergistic effect between CeO2 and Co3O4 in the core-shell bimetallic catalyst[156]. Wu et al. Constructed a new type of porous core-shell structure Mn-Co-O catalyst, which showed excellent thermal stability, water resistance and CO2 resistance, and showed excellent catalytic activity in the low-temperature (T90=225℃) catalytic oxidation of propane[157]. The catalytic activity of transition metal oxide catalysts can be further improved by introducing noble metal nanoparticles such as platinum and gold, and transition metal oxides can be used as supports for gold, platinum and palladium-based catalysts. Qu et al. prepared two forms of nanocatalysts (hollow nanospheres and nanosheets) by hard template method, and loaded Pd nanoparticles on the surface of the two catalysts (Fig. 10e), and the obtained composite catalyst was used for catalytic oxidation of toluene.The hollow nanosphere h⁃NiCoOx was found to be more active than the nanosheet NiCoOx, and the loading of palladium nanoparticles further improved the catalytic activity of the material, with a toluene conversion of about 100% at 190 ° C (Figure 10 f)[158]. Wang et al. Prepared zirconia nanotube arrays using SBA 15 OH as a hard template, and supported the metal platinum-ruthenium catalyst into the nanotubes by impregnation to obtain a PtxRuy/ZrO2 nanocomposite catalyst (Fig. 10g)[159]. Although the catalytic effect of zirconia is low, its excellent structural stability plays a role. Due to the bimetallic effect, the catalyst has good catalytic activity (Fig. 10h), and the application is more economical.

3.4 Catalytic cascade reaction

With the progress of society and the development of science and technology, people begin to pay attention to environmental awareness, and the concept of green chemistry has become increasingly popular. How to simplify the synthesis route and realize the green synthesis of products has become an important research direction in the field of synthetic chemistry. Cascade reaction refers to the synthesis of target compounds under the combined action of catalysts and reagents without separation of intermediate products. Cascade reaction can effectively avoid the separation and purification process of intermediate products, simplify the reaction steps, improve the atom economy of chemical reactions, and has the characteristics of green and high efficiency. Cascade reactions have received much attention in the field of chemical synthesis. However, how to choose a suitable catalytic system and control the selectivity of the reaction is still a great challenge for chemists. The yolk-shell catalyst with a unique core-hole-shell structure is expected to become an ideal nanoreactor for cascade reactions due to the functionalization of the core and the shell, the high density of exposed active sites conferred by the mobile core, and the protection conferred by the shell. Wang et al. fabricated yolk-shell structured nanoreactors by encapsulating organo-silica (PMO) nanospheres with periodic mesopores in a mesoporous silica shell,The PMO core of the nanoreactor is modified with a basic group through functionalization treatment, the (—NH2),SiO2 shell is modified with an acidic group (—SO3H), and the nanoreactor has high catalytic efficiency for the catalytic deacetylation Henry cascade reaction, and the yield of a target product is not less than 99%[160]. The acidic group on the shell and the basic group on the core of the nanoreactor are effectively separated by the unique yolk shell structure, so that neutralization does not occur, and the synergistic effect of the acidic and basic functional groups in the nanospace can greatly shorten the reaction path and contribute to the improvement of reaction efficiency and selectivity. You et al. Prepared hollow mesoporous silica nanospheres (HMS⁃Al@MS⁃NH2) with a double shell as an acid-base bifunctional catalyst[161]. The acid group (— Al) is located on the inner shell, and the basic group (—NH2) is located on the outer shell. This spatially separated acid-base bifunctional catalyst shows high catalytic performance and selectivity in the one-pot deacetylation Knoevenagel cascade reaction. The conversion of reactants reaches 100%, and the yield of the target product reaches 100%.
Biological enzymes are catalytic organic substances produced by living cells, most of which are proteins, and a very small part of which is RNA. Biological enzyme is a very good catalyst, which has the advantages of high selectivity, mild reaction conditions and high reaction efficiency. However, biological enzymes also have some disadvantages, such as easy to be inactivated by the environment and can not be recovered. Coating biological enzymes into some porous carriers is one of the ways to prevent their inactivation by environmental influences. Rodriguez et al. Designed an integrated chemoenzymatic core-shell nanoreactor, in which the biocatalyst glucose oxidase was used as the core and the polymer network structure was used as the shell, and the heme molecule was coordinated to the polymer skeleton through the imidazole group[162]. The heme complex has peroxidase-like activity. The polymer shell of the nanoreactor can well protect the two catalysts from degradation and deactivation in the presence of organic solvents. The nanoreactor is used to degrade aromatic compounds using glucose as a harmless fuel. Under the catalysis of glucose oxidase, glucose is oxidized and hydrogen peroxide is generated, and the generated hydrogen peroxide oxidizes organic pollutants in situ under the catalysis of a chemical catalyst heme complex. The degradation efficiency of catechol and pyrogallol in the nanoreactor is as high as 100%. Compared with the free enzyme system, the nanoreactor shows higher affinity, activity, and stability. In organisms, biocatalytic transformations, represented by enzymatic cascades, involve a series of complex reactions, which take place in a confined microenvironment. Chen et al. Prepared a nanoreactor by wrapping 2 ~ 3 enzyme catalysts with a metal-organic framework compound ZIF _ 8, and realized biomimetic confined cascade biocatalysis[163]. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were integrated in the nanoreactor. Glucose was oxidized to gluconic acid under the action of GOx, and H2O2 was produced. As a substrate of HRP, the produced H2O2 could catalyze the oxidation of Amplex Red to fluorescent resorufin. Compared with the catalytic reaction in bulk solution, the MOF confinement can increase the activity of the whole catalytic cascade reaction by 7. 5 times.
Nanoenzyme is a kind of mimetic enzyme which not only has the unique properties of nanomaterials, but also has catalytic function. It has similar characteristics of biological enzymes in substrate recognition, catalytic mechanism, reaction kinetics and so on. Since the discovery of ferroferric oxide nanoenzyme, many metal oxide nanoenzymes and non-metal nanoenzymes have been found. The nanoenzyme has that advantage of low price, easy preparation, good stability, and good activity at high temperature, and is an effective substitute for natural enzymes. Xu et al. Constructed a microreactor for oxidative phosphorylation in vitro. Gold nanoparticles (2 ~ 4 nm) loaded on hollow silica spheres have both glucose oxidase and catalase activities, which can convert glucose into gluconic acid, reduce the surrounding pH value, and promote ATP synthetase to convert ADP and inorganic phosphate into ATP[164]. The assembled oxidative phosphorylation microreactor has very high biological activity, and the conversion efficiency can be comparable to the catalytic efficiency of the enzyme in the natural system. Qiao et al. Constructed a core-shell FeSe⁃Pt@SiO2 as a nanoreactor for the detection of glucose and hydrogen peroxide[165]. Ultrafine Pt nanoparticles (7 nm) are distributed on the surface of the FeSe core. FeSe: Pt particles were encapsulated in mesoporous SiO2 shells to prevent aggregation, allowing them to remain stable even under harsh conditions. The nanoreactor has catalytic activity similar to that of glucose oxidase and peroxidase, and can be used as an efficient catalyst for cascade reaction; glucose is oxidized into gluconic acid under the catalysis of Pt nanoparticles in the nanoreactor, and H2O2 is generated; and then the generated H2O2 is directly used to oxidize TMB under the action of FeSe to generate a blue oxidation product. Kang et al. Prepared hollow Mn2O3 as a nanoreactor for glucose detection[166]. As an efficient nanoenzyme, the nanoreactor also has the catalytic activity of peroxidase and glucose oxidase.
Many efficient biochemical reactions in organisms take place in a limited isolation space, and the efficiency of the reaction can be improved by designing an artificial nanoreactor to provide a limited isolation space. Therapeutic nanoreactors are emerging as very promising nanoplatforms that can transform inert drugs into active drugs in situ. Therapeutic nanoreactors have attracted great attention in the field of nanomedicine. Mukerabigwi et al. Designed a polymer nanoreactor GOD ProCPT @ Bz MPE loaded with enzyme and prodrug with adjustable membrane permeability, which can achieve prodrug delivery and tumor-specific activation[167]. The nanoreactor remains stable in the blood circulation and normal tissues, and the permeability of the membrane is greatly increased in the mild acidic microenvironment of the tumor site, thereby allowing small molecules (glucose and oxygen) to diffuse through the membrane and undergo the oxidation reaction of glucose catalyzed by GOD, thereby beginning to produce H2O2 in large quantities and the conversion of prodrug to active drug triggered by H2O2. Ke et al. Developed a pH-responsive polymer nanoreactor, which loaded ultra-small Fe3O4 nanoparticles and glucose oxidase on a hydrophobic polymer membrane and in a hydrophilic lumen, respectively (Fig. 11A)[168]. Under the stimulation of tumor slight acid, the permeability of the polymer membrane increases, triggering a series of cascade reactions, including the consumption of glucose and the production of H2O2 (Fig. 11b). The slightly acidic environment accelerates the release of iron ions from Fe3O4 nanoparticles (Fig. 11c). The Fenton reaction between iron ions and H2O2 produces hydroxyl radicals (· OH), which triggers the rapid release of drugs. Under the cascade reaction, a series of cancer treatment strategies, including starvation therapy (consuming glucose), chemotherapy-driven therapy (producing · OH) and chemotherapy (chemotherapeutic drugs), have achieved efficient tumor suppression.
Xu et al. Constructed a pH/redox dual-responsive tumor-targeting supramolecular catalytic nanoreactor HA/CD/Fc/CA by supramolecular self-assembly of hydrophobic prodrug ferrocene modified cinnamaldehyde Fc/CA and hydrophilic cyclodextrin modified hyaluronic acid HA/CD[169]. This novel supramolecular nanoreactor simultaneously enables tumor-specific oxidative stress amplification and cascade-enhanced Fenton reaction for synergistic chemo/chemokinetic therapy with minimal side effects. In the slightly acidic environment of the tumor, the nanoreactor will dissociate and release CA to increase intracellular oxidative stress and H2O2 levels, and then react with Fc through a cascade of Fenton reactions to produce highly cytotoxic OH. Fenton reactions lead to the decomposition of the nanoreactor structure and further accelerate the release of CA, thus realizing the amplification of ROS and enhancing the killing effect on tumor cells. The nanoreactor has excellent cascade antitumor performance and good biocompatibility.

4 Conclusion and prospect

The core-shell nanostructure is divided into a traditional core-shell structure and a novel hollow yolk-shell structure, and the yolk-shell structure can be formed by removing part of the shell or part of the core of the traditional core-shell structure. Core-shell nanocomposites have unique structures, such as protective shell, fixed or movable core, and void space between core and shell. The catalyst nanoparticles can be modified on the inner wall of the hollow shell to form a reverse bumpy ball structure, which can effectively prevent the nanoparticles from coalescence during high temperature treatment, so that the catalyst shows good stability. The nanocatalyst particles may also be modified to the outer wall of the inner core or introduced into the cavity. The nanoscale confined space (pore or cavity) of the core-shell nanocomposite can be used as a nanoreactor for various catalytic reactions. In nature, all kinds of efficient chemical transformations are often completed in a limited space (enzyme or cell). The nanoscale confinement space is conducive to improving the reaction rate and selectivity, and the porous shell is also conducive to the loading and stability of the nanoparticle catalyst and reducing the impact of the external environment on the core. The unique structure of the core-shell nanoreactor can ensure the stability and repeatability of the catalytic reaction and the recyclability of the catalyst, especially for reactions carried out under harsh conditions (such as high temperature reactions). In addition, by selecting different core and shell materials for combination, the properties of various materials can be integrated, and the physical and chemical properties of the materials can be adjusted to achieve complementary advantages. For example, core-shell nanoreactors made of biocompatible materials can protect the biological activity of biological enzymes, DNA and other bioactive substances. Using functional polymers as core-shell materials, many new active reaction sites can be introduced on the surface. These unique structures are not available in traditional catalysts, and they are a class of multifunctional nanocatalysts with high efficiency, high activity, high selectivity, and recyclability. In recent years, there have been more and more reports on the application of core-shell nanocomposites as efficient nanoreactors in the field of catalysis.Some core-shell nanoreactors have shown excellent catalytic activity, stability and selectivity in catalytic degradation of dye pollutants, catalytic hydrogenation, catalytic oxidation, catalytic cascade reactions and other types of reactions. Despite the excellent performance of core-shell nanoreactors in various branches of catalysis, there are still many problems to be solved. How to precisely control the structural parameters of nanoreactors, such as shell number, shell thickness, shell spacing, shell wall pore size and pore structure, and core size and number, when designing nanoreactors; How to precisely and selectively functionalize the surface of the shell to make it have completely different functions (such as hydrophilic/hydrophobic, positive/negative charge, acid/alkali, etc.), so as to achieve efficient, highly selective catalysis and complex cascade reactions; How to realize the directional and controllable introduction of chiral catalytic functional groups and effectively utilize the spatial confinement effect of nanoreactors to achieve enantioselective catalysis; How to realize the coupling of tandem catalytic reactions in space and time? To solve these problems, it is not easy to solve overnight, and it needs reasonable design and precise construction of nanoreactors. Core-shell nanoreactors have large specific surface area and internal cavity, so it is still a challenge to locate multiple active components uniformly and accurately in different regions of the nanoreactor, so that the multiple active components can play a synergistic role and improve the catalytic activity and selectivity. In addition, the research on the relationship between structure and performance of core-shell nanoreactor is not deep enough, which limits the further development of this kind of nanocomposite structure and the further improvement of its application performance. In addition, in order to meet the needs of industrial applications, many key challenges need to be overcome in the future, such as large-scale industrial low-cost production, material safety and environmental impact. Although the focus of this paper is limited to a few basic reactions in catalytic applications, multifunctional core-shell composite nanoreactors also have important applications in other more fields. It is believed that with the in-depth study of multifunctional core-shell composite nanomaterials in the world, this kind of materials will show a very broad prospect in the future practical application.

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Funding

Discharge Control of Ship Painting and Hazardous Waste Treatment Technology and Equipment Program of the Ministry of Industry and Information Technology of China(MC⁃202003⁃Z01⁃07)

Postgraduate Research & Practice Innovation Program of Jiangsu Province(KYCX20_3151)

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