Rational design of metal-based nanocomposite catalysts for enhancing their stability in solid acid catalysis

Zhenyu Lei and Mingjun Jia*
Department of Physical Chemistry, College of Chemistry, Jilin University, Changchun 130012, China. E-mail: Jiamj@jlu.edu.cn

First published on 5th September 2024

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1. Introduction

Heterogeneous solid acid catalysts constitute one of the most important components in the modern chemical industry, and play a decisive role in the high-quality production of various key intermediates and fine chemicals in pharmaceuticals, agriculture, petroleum and other fields.^1–3 Compared with homogeneous acid catalysts, solid acid catalysts have the advantages of facile separation and high recyclability, which can better meet the demands of green chemistry and sustainable development.⁴ Various metal-based solid acid catalysts, including porous material supported metal oxides, metal oxide complexes, and metal–organic frameworks (MOFs), have shown unusual catalytic activity for a variety of industrially important acid catalysis processes, such as cycloaddition of CO₂, olefin epoxidation, Friedel–Crafts acylation, transesterification and carbonylation reactions, and so on.^4–7 However, rapid deactivation of these metal-based solid acid catalysts commonly occurs during the catalytic reaction processes, which is usually caused by poisoning, sintering and leaching of active metal species, or structural collapse of porous materials.^8,9

For enhancing the stability of metal-based solid catalysts, different strategies in catalyst design have been proposed, including the addition of a promoter or the use of a support that can interact strongly with metal components.^10,11 An interesting approach is to encapsulate metal nanoparticles/clusters with thin shells or to confine them in a porous structure.^8,12,13 Consequently, a variety of metal-based nanocomposite catalysts featuring yolk–/core–shell or other confined structures have been developed for application in various catalytic processes.^14–16 In many cases, the uniform external shells with well-controlled thickness and porosity can protect the inner metal component from leaching or aggregating during the catalytic reaction process, and may also provide a suitable environment for mass diffusion, adsorption/desorption and electron transfer, thus leading to enhanced stability in the premise of maintaining high activity and selectivity for a given catalytic reaction.¹⁷ For instance, Zheng and Zeng reported that the mechanical properties of MOFs (e.g., ZIF-8, ZIF-7, HKUST-1, and UiO-66) could be enhanced by coating them with a harder mesoporous silica shell (mSiO₂) through solution-based chemical processes in the presence of a soft template like CTAC (cetyltrimethylammonium chloride).¹⁸ The mSiO₂ armored MOF nanocomposites exhibited excellent accessibility for chemical reactants, which can be demonstrated by the effective catalytic reduction of 4-nitrophenol by NaBH₄ in aqueous solution. Beller and coworkers reported that graphitic shell encapsulated cobalt nanoparticles, which are derived from the pyrolysis of carbon supported Co-MOFs, exhibited excellent catalytic activity and stability for reductive amination.¹⁹ Lu and coauthors proposed that synergizing metal–support interactions and spatial confinement through Cu atomic grippers could boost the dynamics of graphitic carbon nitride supported atomic nickel for catalytic hydrogenations.²⁰ This synergetic effect enables exceptionally high stability against sintering and coke formation, due mainly to the generation of highly adaptable active sites towards both reactant adsorption and product desorption.

With the continuous effort of researchers, significant progress has already been made on the preparation and catalytic application of various metal-based nanocomposite catalysts, demonstrating their great potential in industrially important catalytic processes.^21–23 Several recent review articles have presented the prevalent preparation methods, advantages and catalytic properties of nanocomposite catalysts with unique core–shell, yolk–shell or other types of confined structures, and analyzed their promoting role in enhancing the stability against deactivation in various catalytic processes on the basis of their structure features and metal–support interaction.^8,11,13,24 In addition, it has also been acknowledged that the structural modification of metal-based nanocomposite catalysts could usually lead to the change in the coordinative environment of active metal species.^23,25 In this regard, optimizing the coordinative environment of the metal sites could also play a determinative role in improving the catalytic stability of the nanocomposite catalysts. In spite of the progress, rational design and construction of stable metal-based nanocomposite catalysts in a simple and efficient way remain highly attractive, yet challenging for a given acid catalysis reaction. In addition, much effort is still required to explore how the microstructure or the microenvironment of the active metal sites can influence the catalytic efficiency and the stability of the metal-based nanocomposite catalysts during a given acid catalytic process.

In this feature article, we mainly summarize our recent work on the design of some metal-based nanocomposites with a core/yolk shell structure or other confined structures for enhanced catalytic application in several important acid-catalyzed reactions, including the cycloaddition of CO₂, epoxidation of olefins, acylation of aromatic compounds with acyl chlorides, transesterification of dialkyl carbonate with alcohols/esters and the carbonylation of glycerol with urea. More attention has been paid to the design concept and preparation strategy of these nanocomposite catalysts, which are highly dependent on the types of catalytic reactions and the reason for the deactivation of the conventional solid acid catalysts. By adopting suitable preparation methods and conditions, various nanocomposite catalysts with a unique structure and stable metal sites might be constructed that can meet the tough requirements of a specific catalytic reaction. Through this article, we hope that we can provide some guidance on the rational design of efficient and stable metal-based nanocomposite catalysts for the industrially important acid catalysis processes, and also try to discuss the existing challenges and the future directions with respect to their preparation strategies and application prospects.

2. Inorganic shell confined MOFs for the cycloaddition of CO₂

During the past two decades, metal–organic framework (MOF) materials, which are constructed by the coordinative interaction between functional organic linkers and various metal nodes, have attracted huge attention in the field of heterogeneous catalysis, adsorption and degradation, drug delivery, and energy storage. In many cases, Lewis acid sites can be generated in the crystal lattice of MOFs by removing the solvent molecules coordinated to metal sites.²⁶ These kinds of MOFs, which possess a certain amount of Lewis acid sites in a nanoscale matrix, have shown great potential for solid acid catalysis. For example, several kinds of MOFs including a Cr-based MOF, MIL-101, have been reported to be catalytically active for the cycloaddition of CO₂ to styrene oxide, which is an interesting method for CO₂ fixation and utilization.^26,27 However, due to their relatively low structure stability, these MOF-based Lewis acid catalysts are liable to collapse or decompose during the catalytic reaction process, especially under harsh conditions like high temperature and pressure, leading to rapid deactivation.²⁸ In this regard, it is of great significance to construct structurally stable MOF-based catalysts for solid acid catalysis reactions.

Previous literature studies have shown that encapsulation of MOF nanocrystals (e.g., Fe-MIL-101, HKUST-1) into an inorganic oxide shell could be an effective way to improve the structural stability of MOFs.^29,30 However, these core–shell nanocomposite materials showed some disadvantages upon application in catalysis. A major drawback is that the active sites on the MOF core could not be effectively utilized due to the blockage of the outer shell or the mismatch of the channels in the core and shell. Besides, the diffusion of reactant/product molecules might be hindered when the catalytic reaction is performed in the shell channels.^31,32 Alternatively, the construction of yolk–shell nanocomposites may overcome the drawbacks.^32,33 A few reference studies have demonstrated that yolk–shell nanocomposites like Au@SiO₂³⁴ and Ni@SiO₂³⁵ exhibit enhanced catalytic activity and stability for the reduction of p-nitrophenol and for reforming of methane with carbon dioxide, respectively. In general, yolk–shell structures with a silica shell could be obtained by a selective etching approach, commonly involving the usage of unfavorable etching agents, like cycanide, cation surfactants, or basic etchants.^35–37 Therefore, the exploration of an appropriate green method for the fabrication of yolk–shell MOF-based nanocomposites has promising potential for catalytic applications.

In this regard, we and collaborators proposed a green and economical selective water-etching approach for the construction of MOF@mesoporous SiO₂ with a yolk–shell structure (MOF@mSiO₂-YS).³⁸ As shown in Fig. 1A, the core–shell structured nanoparticles MOF@mSiO₂-CS were firstly prepared by coating mSiO₂ on the hydrothermally synthesized MIL-101(Cr) nanocrystals in the presence of a hexadecyl trimethyl ammonium bromide (CTAB) template. After that the CTAB template in the channel of mSiO₂ was extracted with NH₄NO₃ solution at 80 °C. The yolk–shell composite named MOF@mSiO₂-YS was finally obtained by etching MOF@mSiO₂-CS in pure water at 100 °C for one day. Fig. 1B–H reveal the morphology evolution process starting from MOFs to MOF@mSiO₂-CS and then to MOF@mSiO₂-YS. A variety of characterization results demonstrated that MOF@mSiO₂-YS possesses a relatively higher thermal stability and larger pore size than the pristine MOF due to the formation of the yolk–shell nanostructure.

In combination with some control experiments, it was found that etching of the MOF pores could not further proceed when extending the etching time up to 11 days. The core size in MOF@mSiO₂-YS shows negligible change at a lower etching temperature of 70 °C, and decreases significantly with the increase in temperature from 100 °C to 120 °C. A possible explanation is that the water-etching process is a dynamic equilibrium process between the decomposition and the regrowth of MOF under the operating conditions. Besides, the core–shell structure of MOF@mSiO₂-CS remained and no yolk–shell was formed when DMF (N,N-dimethylformamide) and ethanol were applied as etchants, implying that MOF@mSiO₂-CS was more stable in organic solvents. In addition, it was also found that without the previous removal of the template CTAB, the mSiO₂ shell was etched instead of the MIL-101 core. These results suggest that a yolk–shell nanocomposite based on MIL-101 and mSiO₂ could be generated through carefully controlling the preparation factors, such as etchants, solvents, etching temperature and time.

By using cycloaddition of CO₂ to styrene oxide (SO) as a model reaction, the catalytic property of MOF@mSiO₂-YS was examined. As shown in Fig. 1I and J, both the fresh samples of MOF (MIL-101) and the nanocomposite of MOF@mSiO₂-YS exhibit excellent catalytic activity and styrene carbonate (SC) yield under the mild conditions (0.8 MPa CO₂, 298 K for 48 h). After three cycles, the SO conversion and SC yield decreased rapidly from 95% to 66% over bare MOFs, while no obvious loss in catalytic activity and SC yield could be detected over MOF@mSiO₂-YS. These results demonstrated that the formation of yolk–shell structured nanocomposite could efficiently enhance the catalytic stability of MOF crystals.

Some additional characterization results revealed that the yolk–shell nanostructure of the recycled MOF@mSiO₂-YS is well maintained after the catalytic tests, and the decline in crystallinity and specific surface area is slower for the nanocomposite than the MOF, further confirming the enhanced stabilizing effect of the external mSiO₂ shell. According to the related literature studies, it was known that the coordination interaction between the Lewis acid active sites (Cr³⁺ ions in the MOF framework) and the oxygen in the SO reagent or SC product may weaken the Cr–O bonds, thus destroying the framework of MIL-101 during the catalytic process.^26,27 For the MOF@mSiO₂-YS reaction system, the concentration of SO and SC inside the nanocomposite should be different from that outside, while a part of the carbonaceous species could be adsorbed on the mSiO₂ shell, hence, the Cr–O bonds in MIL-101 would break down more slowly, finally leading to the improvement in the catalytic stability.

Following the above “silica-protected” pathway, we also synthesized a core–shell bifunctional nanocomposite named MOF@TiO₂-A via amorphous TiO₂ coating followed by water-assisted crystallization under mild conditions.³⁹ Firstly, amorphous TiO₂-coated MIL-101 precursors (MOF@TiO₂-P) were prepared by the hydrolysis and condensation of tetraisopropyl titanate in the presence of aqueous ammonia. Then the water-assisted crystallization method was subsequently employed to achieve phase transformation of amorphous anodized TiO₂ to anatase in hot water (100 °C) for 24 h, resulting in the formation of a crystalline TiO₂ (anatase)-coated MOF composite with a core–shell structure. The resultant MOF@TiO₂-A composite exhibits an excellent adsorption ability, photodegradation efficiency and recyclability for anionic dye Congo red. The existence of the specific core–shell structure may effectively combine the advantages of the MOF's high adsorption capability for dye and TiO₂ shell's photodegradation activity. Meanwhile, the stability of MIL-101 could be considerably improved due to the protection of the TiO₂ shells.

Based on these research results, it can be seen that coating MOFs with an inorganic oxide shell might be performed by utilizing a simple water-assisted etching/crystallization strategy, which would generate novel MOF-based nanocomposites with the desirable core–shell or yolk–shell structure. Considering the diversity of MOF materials and inorganic oxides, it can be expected that the family of these MOF-based nanocomposites would be considerably enlarged by adopting the inorganic oxide-coating and water-assisted treating approach through adequately optimizing the synthetic parameters and improving the uniformity of the composites, thus may greatly enlarge their application potential in diverse fields, including various solid-acid catalysis processes. It should be mentioned here that, aside from being protected by inorganic oxide shells, the stability of MOF-based materials can also be improved through other strategies. For instance, a series of Zr-based MOFs (Zr-MOFs) with improved chemical/thermal stability have been built up from the assembly of Zr₆O₄(OH)₄ oxocluster nodes and organic linkers.⁴ In this case, Zr-MOFs like UiO-66, UiO-67 and NU-1000 could also be used as a host for encapsulating active metal (or oxide) ingredients to generate highly active and stable hybrid composite catalysts. In the following section, we describe the progress in the synthesis of several Zr-MOF encapsulated polyoxometalate clusters and their enhanced catalytic application in olefin epoxidation.

3. MOF cage encapsulated polyoxometalates for epoxidation of olefins

The epoxidation of olefins is an important reaction for the production of various epoxides in the modern industry field. Among the various investigated catalyst systems, polyoxometalate (POM)-based heterogeneous epoxidation catalysts have drawn great attention mainly due to their integrated advantages such as remarkable redox and acid properties, and environmental compatibility.^4,40 However, the application of POM-based heterogeneous catalysts is often confronted with poor stability due to the leaching of POM components resulting from strongly complexing with solvent or oxidants (like H₂O₂).^40–42 For improving the stability of the POM-based catalysts, different approaches have been adopted. An interesting strategy is to encapsulate POM clusters within porous materials like MOFs to generate hybrid composite catalysts (e.g. POMs@MOFs), which has been demonstrated to be an effective way to stabilize POMs and further modify their catalytic performance.^40,43 In particular, the appearance of structurally stable Zr-based MOFs (e.g., NU-1000, UiO-66, and UiO-67) brings about a novel opportunity to develop more stable POMs@MOF composite catalysts for application in various catalysis processes.^4,44 For example, Buru et al. synthesized a composite catalyst by impregnating H₃PW₁₂O₄₀ into NU-1000, which showed enhanced catalytic activity and stability for selective oxidation of 2-chloroethyl ethylsulfide.⁴⁵ In another work reported by Li and co-workers, they demonstrated that UiO-67 encapsulated phosphotungstic acid could be obtained by a direct solvothermal synthesis method, and the resultant hybrid composite exhibits excellent catalytic efficiency for oxidative desulfurization with H₂O₂.⁴⁶ For constructing POMs@MOF composite catalysts, the direct solvothermal method has shown some advantages, such as increasing POM loading and improving their dispersity.⁴³ Additionally, POM leaching may be considerably inhibited due mainly to the confinement of the MOF cages, and the existence of strong interactions between POM units and the framework of MOFs.⁴ Considering the fact that POMs and MOFs are two large families, which have diverse compositions and structures, there should be great potential to design and develop more efficient POMs@MOF composite catalysts for the epoxidation of olefins.

In order to achieve this purpose, we and collaborators have carried out some studies on the syntheses of some POMs@MOF composite catalysts for the epoxidation of olefins with H₂O₂, tert-butyl hydroperoxide (t-BuOOH) or O₂ as the oxidant. Firstly, we incorporated polyoxomolybic cobalt (CoPMA) and polyoxomolybdic acid (PMA) clusters into the Zr-MOFs of UiO-bpy (linked by 2,2′-bipyridine-5,5′-dicarboxylic acid) and UiO-67 (linked by 4,4′-biphenyldicarboxylic acid) using a one-pot solvothermal approach. The self-assembly process of the CoPMA@UiO-bpy composite is shown in Fig. 2A.⁴⁷ A variety of characterization results including the ³¹P NMR spectra (Fig. 2B) demonstrated that the PMA and CoPMA clusters were encapsulated inside the cages of Zr-MOFs, and the resultant composites of POM@UiO-bpy and POM@UiO-67 still maintained the original crystallinity of the corresponding Zr-MOFs. The XPS spectra showed that the Mo 3d in CoPMA@UiO-bpy and PMA@UiO-bpy shifted towards lower values compared with that of CoPMA, and the N 1s binding energies exhibited an obvious upshift as compared to the UiO-bpy support (Fig. 2C and D). These results suggest that relatively strong interaction should be presented between the bipyridine ligands and the POM clusters. In addition, the ³¹P NMR signal of CoPMA@UiO-bpy (Fig. 2B) presented a negative shift relative to pure CoPMA clusters, further confirming the existence of interaction between the CoPMA guest and UiO-bpy host.

The catalytic properties of CoPMA@MOF and PMA@MOF composites were evaluated by the epoxidation of cyclooctene with H₂O₂ as the oxidant. Among them, the sample of CoPMA@UiO-bpy exhibited the highest activity, showing 91% conversion after 6 h of reaction (Table 1). Additional experimental results demonstrated that CoPMA@UiO-bpy had excellent stability against leaching of active Co PMA species, and could be recycled 4 times without an obvious decrease in catalytic activity, much better than that of CoPMA@UiO-67. Moreover, CoPMA@UiO-bpy could also efficiently catalyze the epoxidation of styrene using O₂ as the oxidant and t-BuOOH as the initiator under mild conditions. Under the test conditions (80 °C), 80% styrene conversion and 59% styrene oxide selectivity could be achieved, which are better than those of other POM-based catalysts. The XRD results showed that the used CoPMA@UiO-bpy had the same characteristic diffraction peaks as those of the fresh one, further confirming the excellent structural stability of the composite catalyst.

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The excellent catalytic performance of CoPMA@UiO-bpy should be mainly related to its unique composition and structure features. The encapsulation of CoPMA clusters within the size-matched cages of UiO-bpy could offer an appropriate environment for stabilizing the POM clusters, meanwhile, the open cage of the Zr-MOF allows the effective diffusion of reactants to the active sites. In addition, the multiple interactions existing between the functional groups in UiO-bpy and the CoPMA clusters, such as coordination bonds, hydrogen bonds and electrostatic interaction, could also play a critical role in improving the stability of the composite catalysts.

To extend the family of POMs@MOFs, we also adopted a direct solvothermal method to encapsulate phosphomolybdate acid (PMo₁₂) and Co-substituted phosphomolybdate acid (PM₁₁Co) into UiO-66 MOFs to obtain the corresponding composite catalysts of PMo₁₂@UiO-66 and PMo₁₁Co@UiO-66 (Fig. 3A).⁵⁰ After introducing POM clusters into MOFs, the original morphology of UiO-66 was still maintained, while the Co, P and Mo species were uniformly distributed in the UiO-66 frameworks (Fig. 3B–J). The resultant PMo₁₁Co@UiO-66 exhibits much higher catalytic activity than PMo₁₁Co/UiO-66-imp and PMo₁₂@UiO-66 in the epoxidation of cyclooctene with t-BuOOH as the oxidant, showing a comparable TOF value with other POM-based catalysts reported in the literature.^51–53 Interestingly, PMo₁₁Co@UiO-66 has excellent stability and recyclability, and its catalytic activity remained almost unchanged after the tenth cycling (Fig. 3K and L), much better than the reference catalyst prepared by the conventional impregnation method. In addition, PMo₁₁Co@UiO-66 could also effectively convert other cyclic olefins like limonene to epoxides, and a 91% selectivity to 1,2-limonene oxide is obtained when using hydroquinone as the radical scavenger (Fig. 3M and N). The Co-containing species like Co–O–Mo should be mainly responsible for the high catalytic activity of PMo₁₁Co@UiO-66, and can directly activate t-BuOOH to produce highly active hydroperoxo species. The enhanced stability of the composite catalyst should be mainly assigned to the uniform distribution of PMo₁₁Co clusters within the size-matched cages of UiO-66, and to the strong interface-interactions between the POM and the MOF framework.

In addition, the UiO-66 encapsulated transition metal monosubstituted heteropolyacid compounds (TM-HPA@UiO-66, TM = Fe, Co, Ni or Cu) could also act as activity catalysts for aerobic epoxidation of styrene in the presence of co-reductants of aldehydes.⁵⁴ Among the various TM-based composite catalysts, Co-HPA@UiO-66 exhibited higher catalytic activity and stability under the test conditions (ambient pressure, 313 K), showing a 96% conversion of styrene and an 82% selectivity of styrene oxide with oxygen as the oxidant and pivalaldehyde as the co-reductant. The monosubstituted TM species like Co atoms are the main active sites for the aerobic epoxidation reaction, which can effectively activate the radical intermediate of acylperoxy under mild conditions.

The above results reveal that the encapsulation of POM into MOF cages could be an efficient way to fabricate highly active and stable POM-based hybrid composite catalysts for application in olefin epoxidation reactions. The superior catalytic performance of POM@MOF is mainly attributed to the fact that the abundant BrØnsted or Lewis acid centers in POM enable them to facilitate the activation of reactants, and meanwhile, MOF as host materials can be modified by diverse linkers to provide a suitable chemical environment for stabilizing the incorporated POM units. Depending on the types of POMs and MOFs, it is extremely essential to rationally choose synthetic routes for meeting the size matching requirements between the POM clusters and the cages of the MOFs, which may generate more POM@MOF composite catalysts with unique morphology, porosity and acidity for applications in various industrially important catalytic processes. Despite the progress, much effort is still needed in order to fabricate more stable metal-based composite catalysts which could be used under harsh reaction conditions (i.e., higher temperature or stronger acid environment). Involving the further application of MOF materials, an interesting approach developed recently is to construct carbon-based metal nanocomposites through high-temperature pyrolysis of various MOFs.⁵⁵ In the following section, we would like to introduce the recent advancement in designing MOF-derived graphitic carbon-encapsulated metal nanoparticles for the catalytic application in Friedel–Crafts acylation reactions.

4. Graphitic carbon-encapsulated metal nanoparticles for Friedel–Crafts acylation

Aromatic ketone compounds, which are used as the key intermediates for the production of dyes, perfumes and pharmaceuticals, are typically synthesized by the Friedel–Crafts acylation (FCA) of aromatic compounds with acyl chlorides or acetic anhydrides. For overcoming the drawbacks caused by using the homogeneous Lewis acid catalysts like FeCl₃ and AlCl₃, continued efforts have been made to develop effective solid acid catalysts, including zeolites, supported heteropolyacids, MOFs and metal oxides.^56,57 However, those solid acid catalysts commonly tend to suffer from rapid deactivation due to the leaching or blocking/poisoning of active metal sites. Therefore, it is a very significant and challenging subject to design and prepare efficient heterogeneous FCA catalysts with enhanced stability/recyclability.

Recently, high-temperature pyrolysis of MOFs has become an effective way to prepare stable carbon-based metal nanocomposite for different catalytic reactions.⁵⁸ For instance, Zhu et al. constructed Fe/N doped hierarchically porous carbon frameworks from the pyrolysis of an N-rich MOF precursor, and found that the resultant composites could act as highly efficient catalysts for the electrochemical oxygen reduction reaction.⁵⁹ Zhang et al. reported that a carbon layer coated iron core hybrid (Fe@C), which was fabricated by two-step calcination of a MIL-101(Fe) precursor, showed improved catalytic properties in solar-driven CO₂ reduction by H₂. The presence of carbon layers in the hybrid composite could produce a confinement effect, which can deliver a more stable CO₂ conversion speed and higher selectivity to CO than the naked nanoparticles.⁶⁰ Notably, by using the Fe-based MOFs as precursors, carbon-encapsulated metal nanocomposites with an elegant core–shell structure might be easily obtained through rationally optimizing the high-temperature pyrolysis conditions. In this case, the graphitic carbon shell could provide effective protection to the metal core against leaching or aggregation in harsh environments, meanwhile, it can also allow transfer of electrons from the inside core to the outer carbon layers, thus may fabricate an efficient “chainmail catalyst” for electrocatalysis and catalytic hydrogenation.⁶¹ Additionally, it was reported that the carbon encapsulated Fe-based bimetallic nanoparticles, such as FeNi,⁶² FeCo,⁶³ and FeCu,⁶⁴ tend to have better electrocatalytic properties or degradation competence relative to the monometallic counterparts, due mainly to the fact that more new active sites would be generated through the intermetallic interactions. More significantly, the formed bimetallic nanoparticles often possess higher structural and chemical stability, thus leading to the improved catalytic stability/recyclability during heterogeneous catalytic reactions.

In order to develop more stable heterogeneous catalysts for acylation reactions, we and coworkers made an attempt to prepare a series of carbon-encapsulated Fe-based nanoparticles through pyrolyzing Fe-containing MOFs like Fe-diamine-dicarboxylic acid (Fe-DABCO-TPA, DABCO = 1,4-diazabicyclo[2,2,2]octane, TPA = terephthalic acid) and Fe-terephthalic acid (Fe-TPA) at different temperatures.⁶⁵ As shown in Fig. 4A, the representative composite catalyst of Fe@NC-800 was obtained by pyrolyzing the as-synthesized Fe-DABCO-TPA at a temperature of 800 °C for 5 h under a nitrogen atmosphere. The XRD and Raman results revealed that the composite catalyst of Fe@NC-800 is composed of aggregated Fe₃C particles and carbon architectures with graphitic characteristics. TEM and HR-TEM measurements confirmed the existence of smaller Fe-based nanoparticles (30–70 nm), which are all coated by a uniform carbon shell with a few graphitic layers (Fig. 4B–D). By comparing with the reference sample of Fe@C-800, which was derived from Fe-TPA, it could be deduced that the introduction of the DABCO ligand in the Fe-MOF precursors should play a very positive role in generating smaller Fe-based nanoparticles covered by uniform carbon layers. The XPS spectra showed that both C–N and Fe–N bonds are present in the as-prepared Fe@NC-800 catalyst, confirming the N-doping in the carbon architecture, which is derived from the N-containing ligand of DABCO. Notably, the surface Fe content in Fe@NC-800 determined by XPS analysis is much lower than that of the bulky Fe content detected by ICP-AES, which can be explained by the fact that the generated Fe₃C nanoparticles are mainly entrapped inside the graphitic carbon layers.

The catalytic properties of the carbon-encapsulated Fe-based nanoparticles were studied for the Friedel–Crafts acylation of aromatic compounds with acyl chlorides to produce aromatic ketones. It was found that their catalytic activity and stability are highly dependent on the pyrolysis temperatures and the types of Fe-MOFs. Among them, the catalyst of Fe@NC-800, which was pyrolyzed at 800 °C, exhibited relatively high catalytic activity and excellent stability for the acylation reactions. Under the test conditions, Fe@NC-800 can be recycled and reused at least four times without obvious decline in activity, showing much higher stability than the reference catalyst of Fe@C-800 (Fig. 4E–H). Notably, after treating Fe@NC-800 in 1 mol L⁻¹ HCl solution, it still maintained high catalytic activity for acylation reaction, further confirming the intensive stability of the catalyst. Additional characterization results demonstrated that the basic structure and composition of the used Fe@NC-800 were retained well in comparison with that of the fresh catalyst. Some slight changes in the coordinative environments and chemical states of the Fe species could be detected by XPS measurements, showing that the Fe 2p signals assigned to the Fe_xC species disappeared, while the signals related to Fe³⁺ could be well-identified. Besides, the new signals belonging to Cl 2p_1/2 could be detected on the XPS spectrum of the used catalyst. These results suggest that some Fe_xC and Fe–O species presented on the surface of the fresh catalyst may react with the in situ formed HCl to produce Fe–Cl bonds, thus leading to the formation of more Lewis Fe³⁺ sites during the acylation process. This should be the main reason for why the catalytic activity of the used Fe@NC-800 catalyst increases slightly with increasing the cycling numbers. Combined with the morphology and structure characteristics of the Fe@NC-800 catalyst, it can be proposed that the encapsulation of Fe-based nanoparticles in the graphitic carbon shell might provide a suitable chemical environment for stabilizing the active iron species through the geometric and electrostatic effects derived from the external graphitic layers.

To further enhance the catalytic performance for acylation reactions, we also tried to prepare another series of N-doped carbon-encapsulated iron carbide nanoparticles by pyrolysis of the Fe–Zn bimetallic diamine-dicarboxylic acid MOFs, which are denoted as BMOFs-ZnFe_n (n represents the molar ratio of Fe/Zn).⁶⁶ It was found that changing the molar ratios of Fe/Zn in the BMOFs precursor may significantly affect the particle size of the Fe-based nanoparticles and the thickness of the carbon shell. The optimized catalyst of Fe_xC/NC-0.05 with an Fe/Zn ratio of 0.05 showed improved catalytic activity and stability for the acylation reactions. The introduction of an appropriate amount of Zn species in the BMOFs-ZnFe_n precursors could be beneficial for the formation of highly dispersed Fe_xC nanoparticles (ca. 30 nm), which are encapsulated in a thin graphitic carbon shell. Compared with the previous reported Fe@NC-800, Fe_xC/NC-0.05 has a smaller particle size, higher specific surface (885 m² g⁻¹) and larger mesopores, which are mainly attributed to the evaporation of Zn during the high-temperature pyrolysis process. In this case, Fe_xC/NC-0.05 should possess more catalytically active metal nanoparticles, which are well-protected by graphitic carbon layers through space confinement and electronic effects, finally leading to the enhanced catalytic activity and stability for acylation reaction.

As mentioned above, the addition of a second metal element into the Fe-MOF precursor may generate carbon-encapsulated bimetallic nanoparticles with enhanced catalytic performance. We therefore prepared a series of Fe–Ni alloy nanoparticles confined inside N-doped carbon materials by pyrolysis of the Fe–Ni bimetallic MOFs (BMOFs-Fe_xNi_1−x) with different Fe/Ni molar ratios.⁶⁷ As shown in Fig. 5A, the BMOFs-Fe_xNi_1−x precursors were firstly solvothermally synthesized by self-assembly of Fe²⁺/Ni²⁺ cations with TPA and DABCO, and then the precursors were pyrolyzed at 800 °C for 3 h to obtain the corresponding Fe_xNi_1−x@NC composite catalysts. The formation of uniform FeNi alloy nanoparticles (around 25 nm) coated by a thin carbon shell (ca. 5 nm) was demonstrated by a variety of characterization results.

Among the investigated catalysts, the Fe_0.8Ni_0.2@NC catalyst showed very high catalytic activity and prominent stability for the acylation of aromatic compounds with acyl chlorides, better than other bimetallic FeNi catalysts and the corresponding monometallic catalyst of Fe@NC and Ni@NC (Fig. 5B and C). The addition of an appropriate amount of Ni species could be helpful to generate more catalytically active sites (like Fe³⁺ and Ni²⁺), and may also change the chemical environment of N species to a certain extent, possibly related to the different coordinative ability between Fe and Ni species. These results also suggest that relatively strong interaction could be established between metal nanoparticles and interstitial C, N and O atoms existing in the N-doped carbon materials.

After four consecutive reactions, the catalytic activity of Fe_0.8Ni_0.2@NC still remained good, and no obvious leaching of active metal species could be observed through the hot-filtrating experiment, demonstrating the excellent recyclability and stability of the catalyst. Additional characterization results like XPS showed that more oxidized metal species (Fe^2+/3+or Ni²⁺) are present in the used catalyst of Fe_0.8Ni_0.2@NC, accompanied with the disappearance of the metallic Fe and Ni. Meanwhile, the formation of Fe–Cl, Ni–Cl and C–Cl species could be confirmed, which are derived from the reaction between the encapsulated Fe–Ni nanoparticles and HCl produced from the acylation process.

In order to elucidate the nature of the active sites, DFT (density functional theory) calculation was used to analyze the charge variation of the N-doped carbon encapsulated Fe-based nanoparticles before and after the catalytic tests. As shown in Fig. 5D–F, two simplified models with ion channels were established to simulate the basic structure of the fresh and the used Fe@NC catalysts on the basis of the above characterization results. The calculated results showed that, for the fresh catalyst, a small amount of charge could be transferred from the outermost layer to the inner, leading to a weak distribution of positive charge on the external surface. As for the used catalyst, the replacement of some O atoms with Cl atoms could considerably promote the electron transfer and increase the positive charge distribution on the external surface (bottom). This feature could be verified by the measurement of the zeta potentials on the fresh and used Fe@NC and Fe_0.8Ni_0.2@NC catalysts, showing that the zeta potentials increased from +6.1 mV and +18.0 mV (fresh) to +22.5 mV and +28.5 mV (used), respectively, after catalytic tests. Hence, it can be proposed that the Fe³⁺ and Ni²⁺ cations presented on the inner core may exert a positive effect on the external carbon shell, resulting in the existence of weak positive charge on the external surface of the graphitic carbon-encapsulated Fe-based nanoparticles. Here, the external carbon shell may play a ‘chainmail’ role in transferring the positive charge and protecting the inner metal core from destruction, just as proposed by Deng et al.⁶¹ During the catalytic reaction process, the external carbon shell with weak positive charge should be the location where the acylation reaction occurs initially. The unique core–shell structure of the composite catalysts allows the in situ produced HCl to react with the inner bimetallic core through ion channels, thus generating more oxidized metal active centers that can accelerate the acylation reaction. Meanwhile, the uniform graphitic carbon layers may also provide effective protection on the inner metal cations against leaching or poisoning by HCl and aromatic ketone.

For further optimizing the preparation methods and decreasing the cost of the N-doped carbon encapsulated Fe-based nanocomposite catalysts, we also attempted to use the easily available reagent melamine (Mel) as an assistant ligand for obtaining the Fe-TPA-Mel MOF precursor,⁶⁸ or as an additional carbon/nitrogen resource for obtaining a mixed precursor composed of FeCo-based Prussian blue analogues (PBA-FeCo) and Mel.⁶⁹ The resultant Fe/C-600 and FeCo-5m/NC-800 catalysts, which were obtained through optimizing the preparation parameters and pyrolysis temperature/procedure showed improved catalytic activity and stability for the acylation reactions. Taking the PBA-derived catalyst as an example, it was found that graphitic carbon encapsulated FeCo bimetallic nanoparticles confined in N-doped carbon nanotubes (FeCo-5m/NC-800) could be obtained by pyrolyzing the mixture of PBA-FeCo and Mel (Mel/PBA = 5) via a two-stage programmed pyrolysis method (550 °C, 2 h/800 °C, 3 h) (Fig. 6A).⁶⁹ Compared with other catalysts derived from the pyrolysis of a mixed precursor with different Mel/PBA ratios, the catalyst of FeCo-5m/NC-800 showed higher catalytic activity and improved stability/recyclability for the acylation of m-xylene with benzoyl chloride (Fig. 6B), and no obvious leaching of active metal species could be detected as proved by the hot-filtration test. Additional characterization results demonstrated that the used catalyst FeCo-5m/NC-800# shows similar structure, composition and morphology characteristics as the fresh one, confirming the excellent stability of the composite catalyst. Notably, the surface element concentrations of Fe, Co and O of FeCo-5m/NC-800#, which are determined by XPS measurements, are slightly higher than that of the fresh FeCo-5m/NC-800 catalyst. In addition, a small amount of Cl⁻ species, which are present in the form of Fe–Cl or Co–Cl bonds, could also be detected on the used catalyst. Based on these characterization results, it should be reasonable to propose that a small amount of metal cations (formed by H⁺ and metal) should have migrated from the inner core toward the outside along the defective sites of the carbon shell driven by the interaction with the in situ generated HCl. In this case, more accessible Lewis acid centers could be formed, leading to the improvement of the catalytic activity for the used catalyst. Hence, a possible acylation reaction process catalyzed by the FeCo-5m/NC-800 catalyst may be proposed, just as depicted in Fig. 6C. The excellent stability of FeCo-5m/NC-800 could be mainly assigned to the multiple forms of protection from the graphitic carbon shell and external carbon nanotube on the encapsulated inner bimetallic core through the confinement and coordination interactions.

In short, our recent studies demonstrated that carbon-encapsulated Fe-based nanoparticle catalysts derived from the pyrolysis of various Fe-containing MOFs have shown excellent catalytic performance for the acylation of aromatic compounds with acyl chlorides. The unique metal core–graphitic carbon shell structure makes these nanocomposites function as effective “chainmail” catalysts. In principle, the graphitic carbon shell is capable of maintaining the high distribution of active metal species and preventing them from leaching into liquid-phase medium, while the internal metal cations may lead to the generation of weak Lewis acid sites on the external surface of the spherical core–shell particles through a positive inductive effect. These nice features could finally result in the formation of structurally stable nanocomposite catalysts, which should have great potential for developing commercially applied catalysts for Friedel–Crafts acylation processes. In spite of the progress, more research studies are still required to further improve the catalytic activity of the nanocomposite catalysts since they are not effective for the acylation of the aromatic compounds bearing electron withdrawing groups (e.g., –Cl and –NO₂), and are also inactive for the activation of other acylation agents like acetic anhydride.

In addition, the appearance of various novel carbon-based materials with abundant oxygen- and/or nitrogen-functional groups has also shown great potential in anchoring or confining metal (oxide) ingredients to generate stable carbon supported metal-based nanocomposite catalysts. In the following part, the preparation and catalytic application of a series of N-doped porous carbon confined metal oxides are described.

5. N-doped porous carbon confined metal oxides for the synthesis of organic carbonates

Organic carbonates, especially the unsymmetrical organic carbonates, are important chemical intermediates for functional polymers, lubricant, pharmaceutical syntheses, and have also been widely used as green solvent, additives to fuel, and electrolytes in lithium batteries.⁷⁰ Traditionally, the synthesis of organic carbonates required the usage of toxic reagents like dimethyl sulfate and phosgene. Recent works have been focused on the development of more simple and eco-friendly routes to synthesize various valuable organic carbonates, mainly involving the design and preparation of highly efficient and recyclable heterogeneous catalysts for the transesterification of dialkyl carbonates with alcohols or carbonic esters to synthesize unsymmetrical carbonates like ethyl methyl carbonate (EMC),^71,72 and for the carbonylation of glycerol with urea or carbon dioxide to produce glycerol carbonate (GC).⁷³

So far, a variety of metal oxides including supported metal oxides and complex oxides have been tested as solid acid/base catalysts for the synthesis of organic carbonates. Among them, some metal oxides with nanostructures, such as CaO,⁷⁴ MgO,⁷⁵ ZnO,⁷⁶ Mg–La⁷⁷ and Zn–Al⁷⁸ and Co–Zn metal oxides,⁷⁹ exhibited relatively high catalytic activity. However, these nano-sized metal oxides are usually unstable, and leaching of active metal species commonly occurs during the catalytic reaction processes, especially when a high reaction temperature is operated. For example, it has been proved that almost all the liquid-phase carbonylation or transesterification reactions catalyzed by the solid Zn-base catalysts are essentially homogeneous rather than heterogeneous.^76,78,80

For solving these problems, considerable effort has been devoted to improving the stability of the metal oxide-based catalysts, and some significant progress has already been made by choosing suitable support materials and effective preparation strategies. For instance, Mufsir et al. reported that by using N-doped graphene (NDG) as the support, the resultant ZnCuO/(30%)NDG nanocomposites showed the improved catalytic activity and stability in the transesterification of triglyceride with methanol to produce biodiesel.⁸¹ Xiao and co-authors reported that Zn–Co@N-doped carbon materials, which are prepared by pyrolysis of a zeolitic imidazolate framework of Co/Zn-ZIF with a zeolitic imidazolate framework, exhibited high activity and enhanced stability for the transesterification synthesis of EMC.⁸² In fact, various porous carbon materials containing different N- and O-functional groups have received considerable attention recently due mainly to their great advantages in anchoring or confining various nano-sized or even atomically dispersed metal-based catalysts, which may generate structurally stable nanocomposite catalysts for the application in various important heterogeneous catalytic processes.^25,83 In many cases, it has been revealed that the incorporated O-/N-groups could improve the dispersion degree of metal (oxide) through building metal–support interactions, which may also modulate the electron density or the chemical environment of the confined metal (oxide) centers, finally leading to enhancement in the catalytic efficiency and stability for a given catalytic reaction.^84–86 Therefore, it can be expected that both the catalytic activity and stability of the carbon supported metal oxide catalysts might be further improved for the liquid-phase synthesis of organic carbonates through precisely controlling the structure and coordination environment of the active metal oxides.

For the purpose of obtaining more efficient and stable heterogeneous metal oxide-based catalysts for application in the catalytic synthesis of organic carbonates, we and coauthors also tried to prepare a series of N-doped carbon material (NCM) supported ZnO catalysts (ZnO/NCM-T, where T represents the thermal treated temperature, 600–900 °C) with weak Lewis acidity.⁸⁷ The self-made NCM support was obtained by a sol-gel method using sucrose, citric acid and hexamethylenetetramine as the carbon/nitrogen resources. In the XRD patterns of ZnO/NCM-T catalysts (Fig. 7A), the diffraction signals belonging to the ZnO crystalline phase are not presented, indicating that the introduced ZnO species are evenly dispersed on the NCM support. With increasing the calcination temperature, the XRD peaks related to graphitic carbon (at around 24° and 43°) are downshifted somewhat, implying that a higher temperature would result in stronger interaction between the highly dispersed ZnO and the graphitic carbon layer in NCM. This point could be further confirmed by the Raman measurement, showing that the defect sites in the NCM support decreases gradually with the increase in thermally treated temperatures. Among them, ZnO/NCM-800 possesses the highest specific surface area (1222 m² g⁻¹) and largest pore volume (0.98 cm³ g⁻¹) with abundant mesopores. These nice features might be an indication that highly dispersed ZnO may act as an activating agent to create more pores during the high temperature (e.g., 800 °C) treatment process.

The XPS results demonstrate the existence of C, O, N and Zn species, revealing the formation of N-doped carbon-based ZnO composite materials. The surface contents of N and O species decline somewhat with increasing the treatment temperatures (Fig. 7B). The N 1s spectra show that nitrogen species are mainly present in the form of pyridinic-N, pyrrole-N, and quaternary N, and the O 1s spectra exhibit the existence of various oxygen-containing groups, including carbonyls, hydroxyl (or ethers), and lactone (or anhydride) groups. These N/O-containing functional groups presented on the surface of NCM supports should be helpful for achieving high dispersion of metal oxides. The Zn 2p spectra show that the chemical environment of the Zn atoms changed somewhat after being supported on the NCM support, confirming further the presence of relatively strong interaction between ZnO and the NCM support (Fig. 7C). The NH₃-TPD results shown in Fig. 7D indicate that a certain amount of weak acidic centers are present on NCM supported ZnO catalysts. The two samples of ZnO/NCM-700 and ZnO/NCM-800 possess relatively larger amounts of acidic sites than other samples. Notably, no obvious NH₃-desorption signals could be detected on the desorption profile of the ZnO/AC sample, a commercial active carbon (AC) supported ZnO catalyst, implying that the acidic sites presented on ZnO/NCM-T should mainly originate from the interaction between ZnO and the functional groups of the NCM support.

The catalytic tests showed that the four ZnO/NCM-T catalysts exhibit relatively high catalytic activity for the transesterification of dimethyl carbonate (DMC) with diethyl carbonate (DEC) to produce EMC, much higher than ZnO, NCM and ZnO/AC (Fig. 7E). Among them, the ZnO/NCM-800 catalyst has the highest catalytic activity, giving a 44% conversion of DEC (near from the equilibrium conversion) after 0.5 h reaction. More significantly, ZnO/NCM-800 could be easily recycled several times without an obvious decrease in catalytic activity (Fig. 7F), and no detectable leaching of zinc species occurs during the catalytic reaction process, suggesting the excellent stability of ZnO/NCM-T catalysts.

In combination with the above characterization results, it can be proposed that the existing weak acid sites, which are formed through the interaction between ZnO and the surface N-/O-containing groups of the NCM support, should be the main catalytic active centers for the transesterification reaction. Moreover, the surface N-containing species like pyridine-N may also serve as basic sites to synergistically activate the reactants, thus leading to the formation of a highly efficient ZnO/NCM catalyst for the synthesis of EMC through transesterification reaction.

By further optimizing the preparation parameters of the N-doped porous carbon (NC) support, the contents of impregnated zinc salts, and the thermally treated temperature (i.e., 600 °C), a novel carbon-based single-Zn-atom catalyst named Zn₁/NC could also be obtained, and its catalytic performance was investigated for the carbonylation of glycerol with urea to produce glycerol carbonate (GC), which is thought to be a potential route for transferring industrially surplus biomass-based glycerol to valuable products.⁸⁸ The support of the NC material was prepared by a similar method as the preparation of a NCM support, just without the addition of hexamethylenetetramine. The Zn₁/NC catalyst was obtained by impregnation of Zn(NO₃)₂ solution on the NC support, following a heating treatment at 600 °C. The formation of atomically distributed Zn species on the NC support can be demonstrated by XRD, TEM, XPS and XAS techniques. For the Zn₁/NC catalyst, no obvious characteristic peaks indexed to metallic nanoclusters can be detected and the majority of Zn species are atomically distributed on the whole carbon matrix. The oxidized zinc species and some pyridinic N, carbonyl and acyloxy groups were also present on the surface of the NC support. Meanwhile, it can be inferred that the isolated Zn²⁺ species should be coordinated with surrounding N- and/ or O-containing functional groups and atomically anchored on the surface of NC support to form a Zn₁–N_4−xO_x(H₂O)₂ complex with a hexagonal coordination environment, as verified by XANES and EXAFS measurements (Fig. 8A–D).

Catalytic results demonstrated that the resultant Zn₁/NC catalyst has excellent catalytic property and stability for glycerol carbonylation with urea to produce GC, much better than the reference catalyst of NC supported ZnO nanoparticles (Zn-NPs/NC) and the individual Zn(NO₃)₂ (Fig. 8F). Previous literature results revealed that both Zn(NO₃)₂ and the supported nanosized ZnO could effectively catalyze the carbonylation reaction after in situ forming a homogeneous zinc isocyanate (Zn NCO) complex, which is the truly catalytic active species for the reaction.⁷³ In the present work, the homogeneous essence of Zn(NO₃)₂ and Zn-NPs/NC could also be verified by hot-filtration experiments. As for Zn₁/NC, it exhibited very high catalytic efficiency under relatively mild reaction conditions (120 °C), showing a 94.8% conversion of glycerol and 95.0% selectivity of GC within a 6 h reaction time, thus resulting in a much higher GC yield (above 90%) than those of the reference catalysts.^7,73 In addition, after recycling 5 times, the catalytic activity and selectivity of the used Zn₁/NC catalyst still well remained, suggesting the excellent recyclability and stability of the catalyst. The enhanced catalytic activity and stability of the Zn₁/NC catalyst should be mainly attributed to the formation of structurally stable single-Zn-atom sites, which are coordinated with the N-/O containing groups existing in the NC support.

By using the time-dependent in situ IR spectra to monitor the catalytic reaction process (Fig. 8E and G), it was found that different from the homogeneous Zn(NO₃)₂ catalyst system, the Zn₁/NC system required a much shorter reaction time (without an induction period) to generate the main GC product with less accumulation of intermediate I during the whole catalytic course, which means that the Zn–N/O complex existing on the Zn₁/NC catalyst possesses a strong ability to directly adsorb and activate urea, then to react with glycerol to produce GC via an intermediate state. Moreover, the signal-Zn sites should also have excellent structure/chemical stability against the formation of the unfavorable Zn–glycerol complexes, which are often formed over homogeneous zinc salts like Zn(NO₃)₂ or supported ZnO nanoparticle catalysts.^7,73,78

DFT calculations were further performed to infer the most favorable coordination structure of the Zn–N/O complex on the catalyst of Zn₁/NC. This was achieved by calculating the adsorption energies of glycerol and urea molecules at the Zn sites with seven possible models (Zn₁–N_4−xO_x, x = 0–4) (Fig. 8H). As shown in Fig. 8I, the adsorption energy of urea was more significantly lower than that of glycerol over the state of Zn₁–N₂O₂-1 in the broad temperature range of 300–1000 K, implying that the configuration of Zn₁–N₂O₂-1 should be the main catalytically active site of Zn₁/NC, which can play a dominant role in preferentially activating urea and preventing the formation of ZnGly. Hence, it can be proposed here that the outstanding catalytic activity and GC selectivity of the Zn₁/NC catalyst should mainly benefit from the formation of an appropriate coordination environment for Zn²⁺ sites provided by the neighboring O- and N-containing groups existing on the NC support.

The above results suggest that rationally optimizing the coordination environment of metal oxides or a single atom within functionalized carbon supports may have great potential to develop more efficient and stable nanocomposite catalysts for the catalytic syntheses of unsymmetrical organic carbonates under the milder conditions. Additionally, this work could also provide some meaningful guidance on how to reveal the nature of the catalytically active ZnO nanoparticles or single-Zn sites, which are confined by various O- and N-containing groups on the surface of N-doped porous carbon supports.

6. Conclusions and perspectives

In this feature article, we summarize our recent studies on the fabrication of some metal-based nanocomposites with a core/yolk shell structure or other confined structures for catalytic application in several industrially important acid-catalyzed systems, including the cycloaddition of CO₂, the epoxidation of olefins, the acylation of aromatic compounds with acyl chlorides, and the transesterification/carbonylation synthesis of organic carbonates. By adopting appropriate preparation methods/conditions to optimize the structure, morphology and composition of the nanocomposite catalysts, highly efficient heterogeneous solid acid catalysts with enhanced recyclability and stability have been obtained, including yolk–shell MOF@mSiO₂-YS, MOF encapsulated POMs, graphitic carbon-encapsulated Fe-based NPs, and N-doped carbon confined metal oxides. Their improved capabilities in shielding against deactivation under harsh reaction conditions could be mainly attributed to the effective protection of the metal core exerted by external shell (like SiO₂ or graphitic carbon shell), the confinement of metal oxide clusters within the MOF framework, and/or the multiple interactions between the metal (oxide) species and the N/O-containing groups of the N-doped carbon materials. By combining a variety of characterization results and DFT calculation, the physicochemical properties and the nature of the catalytically active sites of the nanocomposite catalysts have also been explored, showing that the rational design of metal-based nanocomposites may bring fascinating properties such as diverse compositions and confined structures (like core–shell and yolk–shell), controllable morphology, large specific surface area and adjustable acidity, finally leading to the formation of structurally stable active centers for the given solid-acid catalyzed reactions.

Despite the encouraging progress, there are still some issues and challenges in the preparation and catalytic application of nanocomposite catalysts. First, in addition to precise structural design in various metal-based nanocomposites, more facile and green synthetic routes with low-cost and high yield should be developed for better meeting the requirements of large-scale production and industrial application. Second, there is more sufficient room to further modulate the Lewis/Brønsted acidity, electron distribution, surface properties of the nanocomposite catalysts, to generate more stable catalytically active sites with improved capability for the adsorption/activation of reactants. In addition, it is extremely essential to exploit more advanced characterization techniques like operando operations and DFT calculation to determine the coordination environment of active metal species and the adsorption/activation modes of substrates, thus being helpful on revealing the catalytic reaction mechanism and providing strong guidance for designing more efficient nanocomposite catalysts. Hence, it can be imagined that the design of novel metal-based nanocomposites with a unique structure, morphology and multifunctional active sites would have a bright future in broadening their applications in developing sustainable and green catalytic processes including some multi-component cascade reactions.

Author contributions

Z. L. wrote the review. M. J. wrote and edited the review.

Data availability

No primary research results, software or code have been included, and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 22172058).

Notes and references

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