Encapsulating Metal Nanocatalysts within Porous Organic Hosts

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Review

Encapsulating Metal Nanocatalysts within Porous Organic Hosts Xinchun Yang1 and Qiang Xu1,2,* The encapsulation of metal nanoparticles (MNPs) inside porous organic hosts (POHs), such as metal-organic frameworks, covalent organic frameworks, organic molecular cages, and amorphous porous organic polymers has attracted significant attention because this procedure can generate highly catalytically active MNPs. This short review describes important recent progress in the fabrication of active MNP catalysts through confinement inside POH cavities, including various innovative synthetic strategies and catalytic applications. In particular, several representative reports of MNP@POH hybrids are chosen to showcase why such POHs are so unique for confining MNPs and why the confined MNPs possess superior catalytic activity, selectivity, and stability. Finally, the challenges of employing MNP@POH catalysts for future practical catalytic applications are addressed.

Why Use Porous Organic Hosts (POHs) to Encapsulate Metal Nanoparticles (MNPs)? Heterogeneous catalysts based on ultrafine (see Glossary) MNPs have received significant attention because of their superior catalytic activities and advanced catalytic applications in industry, energy, and the environment [1,2]. The controllable synthesis of MNPs remains challenging because primary MNPs are thermodynamically unstable and naturally prone to aggregation due to their high surface energy. An effective way to lower the primary MNPs’ surface energy is via covering MNP surfaces with excessive organic capping agents (OCAs) [3,4]. However, OCAs usually limit the surface accessibility of MNPs by unintentionally blocking metal active sites and interacting with intermediates, causing loss of catalytic performance. High surface area supports such as porous carbon, graphene, silica, and metal oxides have been explored for stabilizing MNPs and designing more efficient catalysts with OCA-free surfaces (i.e., allowing most surface atoms to be exposed as accessible active sites) [5–9]. However, the resulting MNPs often suffer from irregular particle distribution, regrowth, and constant leaching during catalytic processes due to the weak interactions of MNPs and the supports. Newly emerging porous organic materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), organic molecular cages (OMCs), and amorphous porous organic polymers (POPs), have been widely targeted for molecule storage and separation [10–13]. Recently, such materials have been employed as POHs for loading MNPs. However, MNPs loaded on the external surfaces of POHs are often easy to aggregate and leach during catalytic reactions. Considering their unique features, such as high inner surface areas and porosities, uniform but tunable cavities, open channels, and tailorable chemistry [14,15], encapsulation of MNPs within POHs is more promising for the development of highly catalytically active MNPs because: (i) the spatial confinements provided by the cavities of POHs can allow the generation of ultrafine MNPs with narrow size distribution, controllable shape, high dispersibility, and high stability; (ii) the high porosities and open channels of POHs can allow most MNP surface atoms to be exposed as accessible active sites and facilitate mass transfer; (iii) the tunable inner pore environments of POHs can modulate the electronic structures and surface atom coordination of MNPs, leading to strong metal-support interactions; and (iv) the separation and recycling of ultrafine MNPs can be easily achieved by filtration, centrifugation, or recrystallization. In the past decade, significant progress has been made towards introducing highly active, selective, and stable MNPs inside POHs for a wide range of catalytic applications. Here, we present a critical review that focuses on state-of-the-art methodologies for the encapsulation of ultrafine MNPs within POHs and highlight the advantages of their small size effects and synergistic interactions for a series

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https://doi.org/10.1016/j.trechm.2019.12.001

Highlights The encapsulation of metal nanoparticles (MNPs) within porous organic hosts (POHs), such as metal-organic frameworks, covalent organic frameworks (COFs), and organic molecular cages, can allow the generation of ultrafine, highly dispersed, homogeneously distributed, and structurally stable MNPs by confining them within cavities of POHs. Such materials make use of synergistic effects between the MNP cores and POH shells. Various innovative synthetic strategies have been developed to encapsulate MNPs inside the small pores of POHs. The encapsulated MNPs have been applied as heterogeneous catalysts in many industrially and environmentally important catalytic reactions, which exhibit superior catalytic activity, selectivity, recyclability, and stability.

1Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan 2AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

*Correspondence: [email protected]

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Key Figure

Schematic Illustration of Encapsulation of MNPs Inside POHs, with Emphasis on Their Advantages for Advanced Catalytic Applications Advantages of POHs:

Advantages of

High inner surface areas

encapsulated MNPs:

High porosities

Ultrafine particle size

Uniform but tunable cavities

High dispersibility

Open channels

Homogeneous distribution

Tailorable chemistry

Stable structure

Good solubility (OMCs only)

Strong synergistic effect Easy separation

MNP@MOF Synthetic strategies:

Chemical vapor decomposition Solid grinding Solution impregnation

MNP@COF Catalytic applications:

MNP@OMC

Double solvent method Partial thermal decomposition Reverse double solvent approach

Coupling reactions Oxidation reactions Reduction reactions Chemical hydrogen storage

One-step synthesis Trends in Chemistry

Figure 1. Abbreviations: @, Inside; COF, covalent organic framework; MNP, metal nanoparticle; MOF, metalorganic framework; OMC, organic molecular cage; POH, porous organic host.

of important catalytic reactions, involving couplings, oxidations, reductions, and hydrogen generation (Figure 1, Key Figure). Finally, we address current challenges with respect to the synthesis of MNP@POH catalysts to meet the demands of future practical catalytic applications.

Glossary @: inside. For example, MNP@MOF means that MNPs are encapsulated inside the pores of MOFs. Capping agents: the organic complex or polymers that are used to stabilize MNPs and control their sizes. Typically, polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and cetyltrimethylammonium bromide (CTAB). Chemical vapor deposition (CVD): a method that can be used to introduce the metal precursors into the pores of porous organic hosts (POHs). In this method, the desolvated POHs and the highly volatile organometallic precursors are first kept in two separate glass vials in a high vacuum tube. The desolvated POHs are subsequently exposed to the vapor of the volatile organometallic precursors. The organometallic precursors will diffuse into POH pores under a suitable temperature and vapor pressure. Solid grinding: a method that can be used to introduce the metal precursors into the POH pores. In this method, the solid-state organometallic precursors and porous organic hosts are firstly mixed in a porcelain by grinding. The sublimated vapor of the organometallic precursors will diffuse into the POH cavities during the grinding. Ultrafine: although the term ‘ultrafine’ has not been formally defined, in this review, this refers to metal nanoparticles less than 5 nm in size.

Strategies to Encapsulate MNPs Inside MOFs MOFs, an emerging class of porous crystalline materials, are constructed from organic ligands and metal-based nodes through coordinative bonds [16,17]. The synthetic strategies for incorporating MNPs inside MOFs are generally classified into two types: (i) the introduction of the metal precursors within the presynthesized MOFs first and subsequent reduction of the metal precursors to MNPs, and (ii) the self-assembly of MNP@MOF hybrids by mixing the metal precursors/MNPs and MOF starting chemicals together. With MOF crystals in hand, chemical vapor deposition (CVD), solid grinding, and solution impregnation are facile methods for introducing the metal precursors into MOFs [18–24]. Generally, highly volatile organometallic precursors are critical for CVD and solid grinding because the vapor can diffuse more easily into the pores of activated MOFs at suitable temperatures and pressures (Figure 2A–E). Capillary forces are critical for solution impregnation because they drive the metal

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(Figure legend at the bottom of the next page.)

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precursor-containing solutions into the MOF crystals. In these processes, strong interactions of the MOF inner surfaces to the metal precursors can facilitate the introduction of the metal precursors while the encapsulated metal precursors can be reduced to MNPs by thermal/chemical processes, UV irradiation, or reductive gases. MOFs having interconnected three-dimensional cavities accessible through small pore sizes can provide powerful confinement of MNPs: the metal precursors penetrate the cavities through small accessible pores but are trapped in the cavities following reduction due to size limitations. For these processes, however, it is difficult to avoid MNP aggregation because the MOF outer surfaces are usually completely exposed to the vapor or solution of the metal precursors. To overcome this issue, the double-solvent method has been developed [25], which involves a small amount of hydrophilic solvent (water) containing metal precursors (with a volume less than the pore volume of MOFs) and a large amount of hydrophobic solvent (hexane) for covering the MOF outer surface (Figure 2F–I). Since the inner surface area of MOFs is much larger than the outer surface area, by capillary force, the small amount of aqueous precursor solution is absorbed into the hydrophilic pores. Finally, by reduction with H2 or reducing agents, MNPs can be formed inside MOF cavities without any aggregation on the outer surfaces. The double-solvent method is now very popular for the preparation of various mono- and bimetallic nanoparticles (NPs) within MOFs [26,27]. MNPs are also be incorporated into MOFs by the partial thermal decomposition of MOFs themselves [28,29]. The strong advantage of this strategy is that MNPs can be encapsulated within the pores of MOFs without the use of additional metal precursors and capping agents. The biggest key of this strategy is the pyrolysis conditions because MOFs undergo collapse at high temperatures and MNPs cannot be produced at low temperatures. In this regard, nickel (II) 2,5-dihydroxyterephthalate (Ni-MOF-74) is an ideal platform. Highly dispersed Ni NPs inside Ni-MOF-74 have been successfully synthesized by heating Ni-MOF-74 in vacuum at 623 K for 12 h [28]. The rich substituent hydroquinone in Ni-MOF-74 played an important role in reducing Ni2+ ions and generating the Ni NPs. However, when the heating temperature further increased to 673 K, the structures of Ni-MOF-74 were completely eliminated and only large Ni particles were formed. MNP@MOF composites are also fabricated by one-step synthesis [30–34], involving two major approaches: (i) the self-assembly of MNP@MOF hybrids by mixing the metal precursors and MOF starting chemicals together, and (ii) the preparation of MNPs first and subsequent addition of suitable chemicals to form MNP@MOF hybrids during crystallization. In the former, functional groups on the organic linkers are utilized for trapping the metal precursors while dimethylformamide (DMF) or H2 are used as reducing agents. In the latter, surfactants, such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), mercaptoacetic acid, and polydopamine (PDA), are often required as binders (Figure 2J). For example, Au@MOF-5 composites have been synthesized by mixing both the MOF starting chemicals [Zn(NO3)2 and 1,4-benzendicarboxylic acid] and the metal precursor (HAuCl4) in a reaction solution containing PVP, DMF, and ethanol. Here, HAuCl4 was first reduced to Au NPs by DMF and then MOF-5 was constructed around the Au NPs with the help of PVP [33]. Pd@UiO-67 has been synthesized by using 2,2-bipyride-5,5ʹ-dicarboxylic acid to trap Pd2+ cations and DMF as a reducing agent [34]. In addition, the introduction of PVP-capped Pt, Ag, Au, and CdTe NPs into ZIF-8 starting chemicals has produced a series of MNP@ZIF-8 hybrids (Figure 2J) [30].

Figure 2. Synthesis of MNP@MOF Hybrids by Solid Grinding, the Double-Solvent Method, and One-Step Synthesis, Respectively. (A) Schematic illustration of the encapsulation of Au clusters inside MOF by solid grinding. (B,C) TEM images of the obtained Au/CPL-2 and Au/MIL-53. (D,E) Size distributions of the obtained Au/CPL-2 and Au/MIL-53. Adapted, with permission, from [20]. (F) Schematic illustration of the encapsulation of Pt NPs inside MIL-101 by the double-solvents method. (G,H) HAADF-STEM images and (I) TEM image of Pt@MIL-101. Adapted, with permission, from [25]. (J) Schematic illustration of the controlled encapsulation of PVP-capped MNPs with various size, shapes and compositions inside ZIF-8 crystals. Adapted, with permission, from [30]. Abbreviations: HAADF-STEM, High-angle annular dark-field scanning TEM; MOF, metal-organic framework; PVP, polyvinylpyrrolidone; TEM, transmission electron microscopy.

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Strategies to Encapsulate MNPs Inside COFs COFs represent another class of porous crystalline materials that are solely assembled by organic building blocks through reversible covalent bonds [35]. COFs, having high stability to thermal treatment, water, and most organic solvents, are ideal candidates to confine MNPs. The synthetic protocols for the incorporation of MNPs inside COFs usually involve two approaches: (i) the introduction of the metal precursors inside COF pores first and subsequent reduction of the metal precursors to MNPs, and (ii) the in situ formation of MNP@COF hybrids by mixing MNPs and COF starting chemicals together. As mentioned above, CVD is an effective method to construct MNP@MOF hybrids. This method is also useful for the encapsulation of MNPs inside COFs [36]. The highly volatile and light-sensitive organometallic precursors can spontaneously diffuse into the cavities of COFs through gas-phase infiltration and the encapsulated organometallic precursors can be reduced to MNPs by photodecomposition. In addition, solution impregnation has also been extended to incorporate MNPs inside COFs [37–40]. A significant volume of large MNPs are often deposited on the COF outer surfaces owing to their straight channels and weak interactions with the MNPs. Recent studies indicate that the presence of strong binding groups inside the pores of COFs can facilitate the incorporation of the metal precursors and formation of nucleation sites inside the cavities during the solution impregnation, thus allowing confined growth of MNPs with controllable particle size (Figure 3A) [39,40]. Self-assembly processes are also useful for the encapsulation of MNPs within COFs [41]. Such bottom-up strategies are based on the idea that COFs can spontaneously form around the surfaces of MNPs during crystallization. In this context, MNPs must usually be modified with surfactants (e.g., PVP and CTAB) in order to interact with the initially formed COF polymeric precursors. It is well known that surfactants play negative roles in catalysis as they can block the MNPs’ surface active sites. Very recently, by mixing MNP@ZIF-8 composites and COF starting chemicals in a mildly acidic solution, multiple ligand-free MNPs have been confined inside COFs [42]. During this process, ZIF-8 was etched in situ by acid, while COF shells were formed through an amorphous-to-crystalline transformation of polyimine shells in the acidic condition.

Strategies to Encapsulate MNPs Inside OMCs MOFs and COFs are extended frameworks in which molecular building blocks are linked together by strong coordinative or covalent bonds. Unlike MOFs and COFs, the distinguishing features of OMCs are that they consist of discrete molecular frameworks with intrinsic and guest-accessible cavities and can be dissolved in several common solvents [12,43]. Therefore, encapsulation of MNPs inside OMCs may allow the production of soluble MNPs with confined particle size and without any aggregation. Moreover, the soluble MNPs inside discrete and open cavities can achieve extremely high dispersibility in solution with more accessible metal active sites for liquid-phase catalytic reactions, comparable with homogeneous catalysts. Self-assembly processes are widely used for incorporating MNPs within MOFs and COFs. However, it is difficult to apply this method for preparing MNP@OMC hybrids because OMCs possess ultrafine pore sizes (<2 nm) and weaker interactions to the metal precursors or MNPs. In this context, to the best of our knowledge, there is only one example, but also the first report of MNP@OMC composites [44]. In 2003, Konishi and coworkers reported the controllable encapsulation of Au55 clusters inside hexaporphyrin cages by directly mixing zinc meso-tetraarylporphyrin, Grubbs’s second-generation ruthenium catalysts, and thiolate-protected Au55 clusters in CH2Cl2 solution (Figure 3B). The Au55 cluster cores can be accessible to small substrates through the pores of the cages. Solution impregnation is promising for encapsulating MNPs in OMCs. In this regard, various OMCs functionalized with interior thioether or amine groups have been prepared as host matrices to encapsulate Ag, Au, Pd, Ir, Ru, and Pd NPs [45–51]. The strong interactions between MNPs and the inner thioether/amine groups are responsible for the loading of MNPs inside cage cavities rather than on their outer surfaces. In addition, highly charged OMCs are also ideal candidates for encapsulating

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Figure 3. Synthesis of MNP@COF and MNP@OMC Composites. (A) Synthesis of thio-COFs and the schematic illustration of the encapsulation of Pt and Pd NPs inside thio-COFs. Adapted, with permission, from [39]. (B) Schematic illustration of a hexaporphyrin cage confining an Au55 cluster. Adapted, with permission, from [44]. (C) Schematic illustration of the encapsulation of Pd clusters inside the RCC3 cages using the reverse double-solvent approach. (D) Scanning electron microscopy image, (E,F) HAADF-STEM images, and (G) particle size distribution of the obtained Pd@RCC3 hybrids. Adapted, with permission, from [56]. Abbreviations: COF, Covalent organic framework; HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; MNP, metal nanoparticle; NP, nanoparticle; OMC, organic molecular cages; V, volume.

MNPs as the metal precursors can be trapped by electrostatic attraction while the MNP aggregation can be avoided by electrostatic repulsion. For example, negatively charged coordination cages (PCC2) can adsorb Ru3+ and Co2+ cations into cage cavities by a cation-exchange process [52–54]. After reduction with NaBH4, the encapsulated Ru3+ and Co2+ cations can form Ru and Co NPs, respectively. Similarly, a series of Au, Pd, and Pt clusters have been successfully confined inside the ionic organic cages [55]. As mentioned above, the double-solvent method is useful for encapsulating metal precursors inside the hydrophilic pores of MOFs. Considering the hydrophobic cavities of OMCs, a reverse double-solvent approach may be useful for encapsulating MNPs inside OMCs (Figure 3C) [56]. To verify this idea, hydrophobic RCC3 cages have been dispersed into a relatively large quantity of hydrophilic solvent, H2O, with the subsequent addition of a small amount of hydrophobic solvent, CH2Cl2, containing the metal precursor Pd(OAc)2. Owing to the immiscibility of H2O and CH2Cl2, the Pd(OAc)2/CH2Cl2 could be retained as stable droplets in H2O and could be adsorbed into the hydrophobic cavities of OMCs

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through the strong interactions between the hydrophobic molecules and the inner hydrophobic surfaces of cages. After the reduction with a highly concentrated aqueous solution of NaBH4, ultrafine Pd clusters have been formed inside the cavities of the isolated OMCs without large MNPs aggregated on the outer surfaces (Figure 3D–G). More recently, MNP-decorated OMCs with excellent crystal structures have been synthesized by using a recrystallization method [57].

Strategies to Encapsulate MNPs Inside Amorphous POPs POPs, a class of amorphous porous materials possessing highly crosslinked structures and disordered interpenetrations, have emerged as a versatile host for the encapsulation of MNPs in recent years [58–61]. To date, synthetic strategies for incorporating MNPs inside POPs usually involve two steps: the introduction of metal precursors inside the cavities of a presynthesized POP followed by reduction of the metal precursors to MNPs using chemical reductants or hydrogen. In this regard, solution impregnation is one of the simplest and most popular methods to introduce metal precursors inside the cavities of POPs. For example, ultrafine Pd clusters (1.5 nm) have been encapsulated inside a 1,3,5-tris(2-thienyl)benzene-derived conjugated microporous polymer (CMP) by exposing the CMP to an ethanol solution of PdCl2 first and subsequent reduction with hydrogen [58]. Au NPs have been encapsulated within the channels of the nanoporous ionic organic networks (PIONs) by soaking the PIONs in an aqueous solution of HAuCl4 first and subsequent reduction with NaBH4 [59]. The functional groups and surface charges of the POP cavities are responsible for the strong interaction between the metal precursors and POPs as they can capture the metal ions from aqueous/organic solutions. Theoretically, most of the methods in synthesizing MNP@MOF, MNP@COF, and MNP@OMC hybrids can be employed to synthesize MNP@POP hybrids as POPs have the similar pore structure and surface wettability with MOFs, COFs, and OMCs. More recently, the reverse double-solvent approach, which has been used for encapsulating Pd clusters inside OMCs, has been successfully employed to encapsulate Pd clusters inside POPs [60].

Characterization of MNPs Encapsulated Inside POHs The physicochemical properties of MNPs encapsulated inside POHs have been investigated by a series of advanced analytical techniques. Here, we discuss several characterization techniques that can provide key information on the unique features of encapsulated MNPs as well as the spatial location of the guest MNPs inside POH cavities. Most POHs are not robust and their structures collapse easily during the preparation of MNP@POH hybrids and catalytic reaction under harsh conditions. Powder X-ray diffraction (XRD) is of critical importance to characterize POH phase purity, crystallinity, and structural integrity, while N2 sorption is of critical importance to characterize pore size, pore volume, and specific surface area before and after both loading MNPs and catalytic reaction [25–27]. Interestingly, compared with XRD, N2 sorption is more sensitive to POH structure; the former usually shows no change when a small percentage of the POH has decomposed. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy measurements are informative to probe the oxidation state, coordination environment, and chemical form of MNPs within the cavities of POHs [62]. Currently, the location of the guest MNPs relative to POHs is usually determined by the size difference between MNPs and POHs pores; MNPs possessing smaller sizes than that of POH pores are deemed to be encapsulated within POHs. Electron microscopy techniques including transmission electron microscopy (TEM), high-resolution TEM, high-angle annular dark-field scanning TEM, and electron tomography are effective ways to directly visualize the size, distribution, and morphology of the encapsulated MNPs. Meanwhile, by slicing the crystals, the tomographic reconstruction can visualize the three-dimensional distribution of MNPs within the whole skeletons of POHs. Until now, based on these advanced electron microscopy techniques, the homogeneous distribution of various MNPs such as Pt, Au, Pd, and AuNi NPs within MOFs have been confirmed [25–27]. However, with the current electron microscopy techniques, it is difficult to obtain high-quality images for the sub-2-nm MNPs because of their charging behavior under the electron beam. Such problems can be overcome by NMR spectroscopy, which can provide the key information of the spatial relationship between the

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guest MNPs and POHs. Generally, MNP@MOF hybrids show the clear chemical shifts of 129Xe NMR spectrum [63]. MNP@OMC hybrids show the broadening in the widths of all peaks in 1H NMR spectra owing to the local restricted motion and heterogeneity, and their two-dimensional diffusion ordered spectroscopy show the similar diffusion coefficients with OMCs owing to the similar sizes and shapes [56]. More recently, an investigation indicates that the positron annihilation technique can evaluate the location of MNPs relative to MOFs, as positrons are very sensitive to the pores and/or defects on the atomic scale [63].

MNP@POH Hybrids in Heterogeneous Catalysis Catalysis is at the core of sustainable chemistry. MNP@POH composites have been demonstrated as excellent heterogeneous catalysts for many important catalytic processes, including: coupling, oxidation, reduction, and chemical hydrogen storage. Here, we present some representative examples of MNP@POH composites for advanced catalytic applications.

Coupling Reactions Coupling reactions are a class of organic synthetic reactions in which two hydrocarbon fragments are coupled with the help of a catalyst. Supported palladium NPs are widely recognized as promising heterogeneous catalysts. In this context, ultrafine and well-dispersed palladium NPs have been immobilized into a series of MOFs [including MIL-101, MOF-3, ZIF-8, ethylenediamine (ED)-MIL-101, and 3aminopropyltrialkoxysilane (APS)-MIL-101], COFs (including COF-LZU1 and TaPa-1), and OMCs [47,64–68], which all exhibited considerably high catalytic activities toward the Suzuki-Miyaura and Heck cross-coupling reactions.

Oxidation Reactions The oxidation of CO to produce CO2 is of great importance in automotive, industrial, and environmental pollution control. POH-encapsulated MNPs are also efficient catalysts for CO oxidation. For example, Au@ZIF-8 and Au@HKUST-1 have shown excellent catalytic activities for high-temperature CO oxidation (>473 K) [21,69]. More recently, Au@quasi-MIL-101 composites have been prepared by controllable thermal transformation of Au@MIL-101 samples [70], which not only retained the porous structure but also achieved a strong interaction between Au atoms and the inorganic Cr-O nodes, leading to dramatically enhanced catalytic performance in the low-temperature oxidation of CO (Figure 4A). The selective oxidation of alcohols to carbonyl compounds using molecular oxygen is of great importance in chemical synthesis. Unsurprisingly, POH-encapsulated MNPs are useful for the selective oxidation of alcohols. In early reports, Au@ZIF-8 and Au@ZIF-90 have been used to oxidize benzyl alcohol (Figure 4B) [71]. Pt@MOF-177 and Ru@MOF-5 have been used for the solvent- and basefree room temperature oxidation of cinnamyl alcohol, benzyl alcohol, and their derivatives with air as an oxidant [72,73]. It is important to note that some MOFs, such as MOF-177 and MOF-5, are easily destroyed in the alcohol oxidation reactions as the formation of H2O as by-product. Therefore, waterstable MOFs (e.g., UiO-66, NH2-UiO-66, and UiO-67) have been developed to encapsulate Au, Pt, and Pd NPs [34,74–76]. The resultant MNPs all exhibited exceedingly high activity and stability in the aerobic oxidations of alcohols owing to small size and molecular-sieving effects. In addition, Au@PION hybrids showed exceptional activity for the selective oxidation of cyclohexanol to cyclohexanone [59]. POH-encapsulated MNPs are also used as the heterogeneous catalysts for the selective oxidation of cyclohexane to cyclohexanone and cyclohexanol, which are two important chemicals in the synthesis of nylon-6 and nylon-66 polymers. It has been found that Au@MIL-101 was a stable and reusable catalyst for the aerobic oxidation of cyclohexane at 403 K, giving 30.5% conversion and 87.7% selectivity, respectively [77]. Considering the strong synergetic effects between two different metals, Au-Pd alloy NPs have been incorporated inside MIL-101(Cr) [78], which exhibited higher activity and selectivity for the oxidation of cyclohexane and cyclohexene with molecular oxygen compared with their monometallic and physically mixed counterparts.

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Figure 4. Schematic Illustration of the Use of MNP@MOF Hybrids for Catalytic Reactions. (A) Au@quasi-MIL-101 showed high activity for CO oxidation because the deligandation-induced accessible Cr-O sites can uptake and activate O2 molecules and then release active oxygen species to the CO molecules adsorbed on the Au nanoparticles near the Cr-O sites. Adapted, with permission, from [70]. (B) Schematic view of liquid phase alcohol oxidation with Au@ZIF-8: both benzyl alcohol and methyl benzoate were able to access the pores and can diffuse through the network. Adapted, with permission, from [71]. Abbreviations: MNP, Metal nanoparticle; MOF, metal-organic framework.

Reduction Reactions MNPs encapsulated inside POHs have been applied for the reduction of various unsaturated functional groups towards synthesis of useful chemicals and organic building blocks. In the hydrogenation of styrene to ethylbenzene, highly dispersed Pd NPs loaded inside MOF-5 and UiO-67 exhibited superior catalytic activity and selectivity at room temperature [22,79]. In the selective hydrogenation of phenylacetylene to styrene, Pd@COF-SO3H showed the outstanding catalytic performance with 97% conversion and 93% selectivity [40]. With a narrow size distribution, Pd@MIL-101 showed high catalytic activity for the hydrogenation of aryl alkyl ketones to the corresponding alcohols [80]. Pd@UiO-66, Pt@MIL-101, Pt@UiO-66, and Pd@UiO-67 showed high catalytic activities for the shape-selective hydrogenation of olefins, benzonitrile, linoleic acid, acetophenone, and benzophenone [34,81,82]. As for the synergetic effects, bimetallic PdNi@MIL-101 composites showed higher catalytic activities than their monometallic counterparts in the hydrogenation of phenol, cyclic ketones, and dialkyl ketones [83]. Interestingly, by etching Co element of PtCo@MIL-101, the obtained defective Pt NPs inside MIL-101 showed an improved catalytic activity towards the

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hydrogenation of nitroarenes under mild conditions [84]. The sandwich MIL-101@Pt@MIL-101 showed high activity for the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol [85]. Pd@TP-POP, Au@MIL-101(Fe), Pt@UiO-66, and Pt@ZIF-8 were applied for the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 or other reducing agents [60,86,87] and Pd@UiO-67 was used for the reduction of nitrobenzene [34]. Compared with insoluble MOFs and COFs, soluble OMCs can serve as stabilizers and homogenizers toward the homogenization of heterogeneous MNP catalysts with enhanced catalytic performance. As expected, OMC-encapsulated Au, Ag, and Pd nanoclusters showed superior catalytic activities in a series of liquid-phase reduction reactions of nitrobenzene, nitroarenes, and nitrophenol [46,50–53].

Chemical Hydrogen Storage Formic acid, a major product of biomass processing, is considered a sustainable liquid carrier for hydrogen storage and delivery [88]. Depending on the catalyst, decomposition of formic acid occurs through two pathways: dehydrogenation (HCOOH / H2 + CO2) and dehydration (HCOOH / H2O + CO). Dehydration should be avoided as CO can poison fuel cell catalysts and H2O can reduce the H2 production efficiency. In order to obtain high-performance catalysts, the acid-stable MOFs, such as MIL-101, ED-MIL-101, NH2-MIL-125, MIL-100(Fe), and NH2-UiO-66, have been selected as host matrices to confine various MNPs, including Pd, AuPd, and AgPd NPs [89–91]. Owing to the small size effects of MNPs and the strong interactions between MNP cores and MOF shells, the as-prepared MNP catalysts showed outstanding catalytic activities for H2 generation from formic acid at low temperatures. Ammonia borane (AB) is another promising hydrogen-storage chemical owing to its high stability and hydrogen content [88,92]. Hydrogen can be released from AB by thermolysis, hydrolysis, and methanolysis. In this context, by using MIL-101 as host matrices, a series of MNPs, such as Pt, AuNi, AuCo, RuNi, and Ni NPs, have been prepared as highly active catalysts for the hydrolysis of AB [25,26,93–95]. Interestingly, the soluble OMC-encapsulated Rh, Pt, Ru, and Pd NPs could homogenize the heterogeneous methanolysis of AB, achieving a superior catalytic activity for the hydrogen generation from AB [52–56].

Concluding Remarks The encapsulation of ultrafine MNPs inside the cavities of MOFs, COFs, OMCs, and amorphous POPs has become a fascinating research topic in chemistry. The recent important developments of this field are described in the present review. In particular, the synthetic strategies of MNP@POH hybrid materials and their catalytic applications are briefly discussed. It is without doubt that the construction of MNP@POH hybrids by confining MNPs inside cavities of POHs can allow the generation of ultrafine, highly dispersed, homogeneous distributed, and structurally stable MNPs while also enhancing the synergistic effects between MNP cores and POH shells. However, this field is still in its infancy and faces many challenges. Here, we have discussed the challenges which we believe are the most important (see Outstanding Questions). The industrially catalytic applications of MNP@POH hybrids are limited by the low thermal and chemical stability of MOF, COFs, OMCs, and POPs. Many catalytic reactions are performed at harsh conditions, such as high temperature, acidity or alkalinity, in which POHs usually undergo collapse or decomposition, thus resulting in the regrowth or re-aggregation of MNPs. It is highly desirable to develop more robust POHs that can be synthesized by facile methods with high yields while retaining the frameworks in harsh catalytic conditions. The confinements of POH cavities can endow the encapsulated MNPs with a very small size. However, in some cases, the accessibility of the encapsulated MNPs can be hindered by the high diffusion resistance because of the small windows and long channels. The preparation of nanoscale MOFs and COFs with large voids may reduce the diffusion length, facilitating the mass transfer of reactants and products. Considering that the soluble and discrete OMCs can endow the encapsulated MNPs with high solubility and high dispersibility in many solvents comparable with homogeneous

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catalysts, the construction of MOFs and COFs with good solubility in common solvents for the encapsulation of MNPs may open a new avenue to highly active catalysts. Although the strong synergistic effects between POHs and MNPs can be obtained by adjusting the unsaturated metal centers or functional groups in the POH cavities for boosting the catalytic performance of MNPs, the catalytic mechanisms associated with the synergistic effects remain unclearly revealed. To this end, in-depth studies of the synergistic effects through in situ, operando, or other innovative characterization technologies are needed. The in-depth understanding of the synergistic effects will guide the rational design/synthesis of highly active MNP@POH composites. In conclusion, thanks to the great efforts of many researchers, MNP@POH hybrids have made significant progress toward boosting various advanced catalytic applications. Sustainable research in this exciting area can be expected to enable advanced catalytic applications in industry, energy, and the environment.

Acknowledgments The authors thank METI and AIST for the financial support.

Disclaimer Statement The authors declare no conflicts of interest.

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Outstanding Questions How can highly stable porous organic hosts (e.g., MOFs, COFs, OMCs, and amorphous POPs) be synthesized for catalytic reactions under harsh conditions by a facile method with a high yield? Is it possible to synthesize nanoscale MOFs and COFs with large voids? Such materials may facilitate the mass transfer of reactants and products during catalytic reactions. Is it possible to synthesize MOFs and COFs with good solubility in organic solvents or water? Such materials may dramatically improve the dispersibility of the encapsulated metal nanoparticles in solvents and therefore boost their catalytic activities. How can the synergistic effects between metal nanoparticle cores and porous organic host shells be characterized in situ? Is it possible to develop a new technique to characterize such synergistic effects?

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