POM@MOF Hybrids: Synthesis and Applications

Abstract

The hybrid materials that are created by supporting or incorporating polyoxometalates (POMs) into/onto metal–organic frameworks (MOFs) have a unique set of properties. They combine the strong acidity, oxygen-rich surface, and redox capability of POMs, while overcoming their drawbacks, such as difficult handling, a low surface area, and a high solubility. MOFs are ideal hosts because of their high surface area, long-range ordered structure, and high tunability in terms of the pore size and channels. In some cases, MOFs add an extra dimension to the functionality of hybrids. This review summarizes the recent developments in the field of POM@MOF hybrids. The most common applied synthesis strategies are discussed, together with major applications, such as their use in catalysis (organocatalysis, electrocatalysis, and photocatalysis). The more than 100 papers on this topic have been systematically summarized in a handy table, which covers almost all of the work conducted in this field up to now.

1. Introduction

Polyoxometalates (POMs), a class of metal oxide clustered anions, have already been investigated for more than 200 years. Their history dates back to 1826, when Berzelius reported the discovery of the first POM cluster (NH₄)₃[PMo₁₂O₄₀] nH₂O [1]. However, due to difficulties achieving insights into the POM structure, no significant progress was made until Keggin determined the structure of H₃PW₁₂O₄₀ in 1934 [2]. Since then, the interest of scientists in POMs has increased drastically, not only in the development of new POM structures, but also towards their use in various applications, such as catalysis, optics, magnetism, biological medicine, environmental science, life science, and technology [3,4,5,6,7]. In particular, their use in catalysis is one of the most examined fields because of their strong acidity, oxygen-rich surface, photoactivity, and redox capability. Despite these interesting characteristics, POMs still exhibit some drawbacks for their use in catalysis. First, POMs possess a low surface area (<10 m² g⁻¹), which consequently hinders the accessibility of reactants and secondly, their high solubility in aqueous solutions and polar organic solvents results in a low recyclability [4]. The immobilization of POMs into/onto porous solids has been proposed to overcome these shortcomings and to achieve catalysts with a high catalytic performance. In the past few decades, many porous materials have been examined as supports for the immobilization of POMs, e.g., silica, ion-exchange resin, zeolites, and activated carbon [5,6,7,8]. Since the discovery of metal–organic frameworks (MOFs), much effort has been dedicated to use these porous materials as potential supports for POMs. MOFs are inorganic–organic hybrid crystalline materials that are constructed from metal ions or clusters and organic linkers through coordination bonds. These materials have attracted considerable interest in recent years due to their large surface areas, tunable pore size, and designable functionalities. So far, MOFs have shown great potential in gas storage and separation, catalysis, sensing, drug delivery, proton conductivity, solar cells, supercapacitors, and biomedicine [8,9,10,11,12,13]. Moreover, MOFs are regarded as an outstanding platform for introducing guest molecules because of the high accessibility of their internal surface area and long-range ordered channels. So far, several active sites have been successfully embedded in the pores or cages of MOFs, such as noble metals, metal oxides, enzymes, and POMs [14,15,16,17].

The first report of a POM@MOF hybrid was reported in 2005 by Férey and co-workers [18]. In this seminal work, the POM, K₇PW₁₁O₃₉ (van der Waals radius, 13.1 Å), was successfully encapsulated into the big cages of the highly stable Cr-based MOF, MIL-101, by using an impregnation method. To date, several other thermal and chemical stable MOFs have been applied as supports to host POMs for their use in catalysis, including MIL, UiO, ZIF series, NU-1000, and Cu-BTC frameworks (see

Table 1. Physical properties of representative metal–organic frameworks (MOFs) used to encapsulate polyoxometalates (POMs) for catalysis.

MOFs	Chemical Formula	Window	Porosity	BET/cm3 g−1	Ref.
UiO-66	Zr6O4(OH)4(BDC)6, BDC = 1,4-benzenedicarboxylate	6 Å triangular	12 Å (octahedral cages) and 7.5 Å (tetrahedral cages)	1100–2000	[22]
UiO-67	Zr6O4(OH)4(BPDC)6, BPDC = biphenyl-4,4′-dicarboxylate	8 Å triangular	16 Å (octahedral cages) and 12 Å (tetrahedral cages)	2100–2900	[23]
NU-1000	Zr6O4(OH)4(H2O)4(OH)4(TBAPy)2, TBAPy = 1,3,6,8- tetrakis(p-benzoate)pyrene)	10 Å orthogonal	31 Å (hexagonal channels) and 12 Å (triangular channels)	900–2000	[24]
MOF-545	Zr6O8(H2O)8(TCPP-H2)2, TCPP = tetrakis (4-carboxyphenyl)porphyrin	-	~16 Å and 36 Å	1900–2500	[25]
ZIF-8/67	Zn(MeIM)2/Co(MeIM)2 MeIM = imidazolate-2-methyl	3.4 Å hexagonal	~12 Å	1000–1800	[26]
MIL-101	X3(F)O(BDC)3(H2O)2, (X = Cr, Al, Fe) BDC = 1,4-benzenedicarboxylate)	12 Å pentagonal and 16 Å hexagonal	~29 Å and 34 Å	2500–4500	[18]
MIL-100	Fe3FO(H2O)2(BTC)3 BTC = 1,3,5-benzenetricarboxylat	~5.5 Å and 8.6 Å	25 Å and 29 Å	1500–3000	[27]
Cu-BTC	Cu3(BTC)2,	~9 Å and 4.6 Å	10–13 Å	1000–1500	[28]

). The most examined POMs that have been encapsulated into MOFs are the well-known Keggin [XM₁₂O₄₀]ⁿ⁻ and Dawson [X₂M₁₈O₆₂]ⁿ⁻ (X = Si, P, V, Bi, etc.; M = V, Mo, W, etc.) POMs and their derivatives. These POMs are of significant interest because their structure and properties can be easily varied by removing one or more MO⁴⁺ units, leading to lacunary POMs such as [PW₉O₃₄]⁹⁻, or by the substitution of X and M by different metals or a combination of two fragments of the Keggin structure, leading to sandwich-type POMs such as [Tb(PW₁₁O₃₉)₂]¹¹⁻.

There are several advantages of using MOFs as a host matrix to encapsulate POMs. First of all, their exceptionally high surface areas and confined cages/channels make it possible to ensure a homogeneous distribution of the POM in the MOF host. This not only prevents the agglomeration of POMs, but also improves their stability and recyclability and ensures a fast diffusion of substrates and products. Secondly, the highly regular cages and windows of MOFs ensure a high substrate selectivity, or, in other words, only specific substrates/products are able to reach the active POM sites. Thirdly, owing to the good interaction and electron transfer between the MOF and POM, an increased synergistic catalytic performance is typically observed. Finally, the chemical environment of POMs can easily be adjusted through modification or functionalization of MOFs. Therefore, POM@MOF hybrids not only combine the interesting properties of POMs and MOFs, but also allow the aforementioned disadvantages of POMs to be tackled to afford synergistic catalysis. This review is focused on the synthetic aspects of POM@MOF hybrids, as well as their use in catalysis (organocatalysis, electrocatalysis, and photocatalysis). Alongside the POM@MOF systems discussed here, where the POMs are encapsulated inside a MOF host, POM-based MOFs have also been investigated. In these MOFs, the POMs form the actual metal nodes that are interconnected by organic linkers [19,20,21]. However, these fall outside the scope of this review.

2. Synthesis and Design of POM@MOF

To date, several well-known highly stable MOFs have been used to encapsulate POMs, including MIL, UiO, and ZIF series, as well as NU-1000 and Cu-BTC frameworks. One of the most commonly applied methods to embed POMs in MOFs is impregnation. Wet impregnation is a simple and straightforward method, since most of the POMs are well-soluble in polar solvents. Typically, the activated MOF powder is immersed in the POM solution to obtain the composite material. Several POM@MOF hybrids have been successfully synthesized through this wet impregnation method, such as POM@MIL, POM@ZIF, and POM@UN-1000. An important aspect allowing the use of this method is that the size of the POM must be smaller than the windows of the MOF. Moreover, for some POMs and MOFs, it was observed that the POM loading could not be enhanced by increasing the concentration of POM in aqueous solution when a certain POM loading was achieved. For example, for POMs whose size is bigger than the pentagonal windows (12 Å) of MIL-101(Cr), the POMs were only encapsulated into the large cages, while the other cages, which represent 2/3 of the total number of cages of MIL-101(Cr), were unfilled. Naseri et al. demonstrated that the loading of a sandwich-type POM [(HOSnOH)₃(PW₉O₃₄)₂]¹²⁻ (15.2 Å × 10.4 Å) could not be enhanced by increasing the concentration of POM in the aqueous solution [29].

The impregnation method cannot be used for MOFs whose window size is smaller than the POMs, e.g., Cu-BTC, UiO, and ZIF. Therefore, for these MOFs, the one-pot (also known as bottle-around-the-ship) synthesis method has been applied to obtain POM@MOF hybrids. The one-pot method is also often used to obtain POM-encapsulated MOFs in which the anionic form of the POM acts as a structure directing agent to ensure deprotonation of the organic carboxylate ligand. For the preparation of POM@MOF hybrids, typically, the synthesis parameters employed to obtain the parent MOF are used upon addition of the POM. The one-pot method can not only be used to synthesize POM@MOFs that cannot be obtained by impregnation, but can also confine the POMs in the MOF cages to prevent leaching if the size of the POMs is bigger than the windows of the MOFs.

Therefore, in conclusion, the synthesis approaches commonly used to incorporate POMs into MOFs are impregnation and one-pot synthesis. To choose, however, the “correct” methodology, two questions need to be addressed in advance: does the size of the POM fit into the MOF cages and can the pore window of the MOF confine the POM? If both criteria are met, one can expect that the obtained catalyst will work efficiently at a molecular level.

As was mentioned before, the first report on the embedding of a POM into the cages of an MOF was reported by Férey’s group. They showed that a Keggin-type POM, K₇PW₁₁O₄₀ (van der Waals radius, 13.1 Å), can be confined in MIL-101(Cr) by simple impregnation. The resulting MIL-101-Keggin solid was characterized by XRD, TGA, and N₂ sorption, as well as IR and ³¹P solid state NMR, which confirmed the presence of Keggin ions within the pores [18]. As summarized in Table 1, MIL-101 has two types of mesoporous cages: a smaller one with an inner size of ~29 Å and pentagonal windows of ~12 Å, and a larger one with an inner size of ~34 Å and hexagonal windows of ~15 Å. Based on the size of the cage windows and the size of the POM, one can conclude that the POM can only diffuse into the largest cages.

In 2010, Gascon and co-workers prepared PW₁₂@MIL-101 (PW₁₂ = [PW₁₂O₄₀]³⁻) composites by using a one-pot and wet impregnation method [30]. The authors observed a homogeneous distribution of PW₁₂ when the one-pot synthesis was applied under stirring conditions. By using the wet impregnation method, high loadings of PW₁₂ in MIL-101 resulted in a drastic decrease in the surface area and pore volume. However, this decrease in surface area and pore volume was smaller for the one-pot synthesis method in comparison to the impregnation method using the same PW₁₂ loading. The authors stated that in the one-pot synthesis, both the large- and medium-sized cavities were occupied, while, when using the impregnation method, only the larger cavities were accessible.

Canioni and co-workers compared different synthesis methods for encapsulating POMs in MIL-100(Fe) [31]. The authors observed a good agreement between the experimentally obtained POM loading and the maximum theoretical loading for the PMo₁₂@MIL-100 (PMo₁₂ = [PMo₁₂O₄₀]³⁻) obtained by a one-pot solvothermal synthesis. In addition to this, the solvothermally obtained PMo₁₂@MIL-100 showed a good stability in aqueous solution and no POM leaching was observed after 2 months. On the contrary, the PMo₁₂@MIL-100 material prepared through impregnation exhibited significant POM leaching after 2 months.

Based on the above examples, POM leaching was observed for the POM@MIL-101 and POM@MIL-100 obtained by impregnation, since immobilization is based on an adsorption equilibrium. One way to circumvent this leaching is to use amino-functionalized MOF structures, e.g., UiO-66-NH₂ and MIL-53-NH₂, which can ensure a better interaction with the polyanions [32,33,34]. The formation of complexes such as −NH₃⁺[H₂PW₁₂O₄₀]⁻ between primary amines, ammonia, or pyridine and PW₁₂ is well-documented [35]. In 2012, Gascon and co-workers used a microwave-assisted one-pot synthesis to obtain PW₁₂@MIL-101-NH₂(Al) as their attempts to synthesize MIL-101-NH₂(Al) containing PW₁₂ by one-pot solvothermal synthesis were not successful [36]. One year later, Bromberg et al. examined the encapsulation of POMs in amino-functionalized MOFs (NH₂-MIL-101(Al) and NH₂-MIL-53(Al)) by immobilization. They concluded that POMs electrostatically interact with the MOF surface to form a stable composite. The thermal stability of the composites PW₁₂@NH₂-MIL-53(Al) and PW₁₂@NH₂-MIL-101(Al) was similar to the stability of the parent MOFs [37].

Besides MIL-101, Cu-BTC (namely HKUST-1 or MOF-199) has also been used as a host material to encapsulate Keggin- and Dawson-type POMs [38,39]. As shown in Table 1, the larger cages of Cu-BTC have an inner diameter of 1.3 nm and a pore window of 0.9 nm, which perfectly ensures the stable entrapment of POMs. For example, PW₁₂ with a diameter of approximately 1.06 nm was used as a structure directing agent for the self-assembly of Cu-BTC at room temperature [38]. The authors observed an enhanced chemical and thermal stability after the embedding of POM and no POM leaching was noted during catalysis in several studies [40,41]. In one of these studies, Shuxia Liu’s group prepared a series of Keggin-type POMs in Cu-BTC, denoted as NENU-n, n = 1~10, and formulated as [Cu₂(BTC)_4/3(H₂O)₂]₆ [H_nXM₁₂O₄₀]·(C₄H₁₂N)₂ (X = Si, Ge, P, As, V, Ti; M = W, Mo), by using a one-pot hydrothermal synthesis [42,43]. The templating effect of the POMs resulted in highly crystalline composite materials which showed an enhanced thermal stability. Moreover, as large crystals were obtained, the structures were elucidated by means of single-crystal X-ray diffraction, demonstrating that the Keggin polyanions were confined in the larger cuboctahedral cages (inner diameter of 1.3 nm) [43].

Besides MIL-101 and Cu-BTC, isostructural imidazolate frameworks, namely ZIF-8 and ZIF-67, have also been frequently used as the host matrix. The sodalite-type cavities of ZIF-8 have a size of approximately 1.1 nm, but the accessible window of the cavity is rather small (0.34 nm). Keggin-based POMs possess a relatively larger particle size up to 1.3 ~ 1.4 nm in comparison to the cavities of ZIF-8, but can fit perfectly in their anionic form (1 nm diameter of PW₁₂) [33]. Therefore, the bottle-around-the-ship method is an ideal approach for confining POMs inside ZIF-8 or ZIF-67 [44]. For instance, Malka et al. reported a POM encapsulated in ZIF-8 for its use as an esterification catalyst. The authors were able to obtain a PW₁₂ loading of 18 wt% by using a one-pot synthesis strategy at room temperature in aqueous solution. However, after three catalytic cycles, degradation of the MOF occurred and 9% of the POM leached out [45]. A way to overcome the POM leaching in ZIF-8 was demonstrated in the work of Jeon et al. In this study, an impregnation method was used to functionalize the surface of the ZIF-8 nanoparticles with a Keggin-type PW₁₂, in order to obtain a core–shell MOF–POM composite. Interestingly, due to the strong interaction, the POM-decorated MOF became insoluble in hydrophilic solvents [46].

Lin and co-workers constructed a POM@MOF molecular catalytic system with a Ni-containing POM [Ni₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ (namely Ni₄P₂) into an [Ir(ppy)₂(bpy)]⁺-derived MOF by one-pot synthesis, and the MOF was isostructural to UiO-66, with extended ligands. Ni₄P₂ POMs can be encapsulated in the octahedral cages with an inner dimension of 2.2 nm [47].

In the studies mentioned above, the one-pot synthesis and wet impregnation methodology both give a high chance of success in the synthesis of POM-encapsulated MOFs. Although the impregnation method is straightforward, it is only applicable for MOF pore windows larger than the POMs. However, leaching of the POMs might happen unless precautions are taken in advance to ensure a good interaction between POM and MOF supports. The use of POMs as a template might enhance the crystallinity of the MOF framework, which makes a one-pot synthesis very attractive. However, it is important to note here that the size of the POM needs to be larger than the pore window of the MOFs to prevent leaching. In addition to this, the one-pot synthesis method is not applicable to all MOF structures. In most cases, the POM@MOFs materials obtained through a one-pot synthesis or impregnation method have a positive influence on the thermal stability in comparison to the parent non-functionalized MOF.

Besides the commonly used one-pot and impregnation method, there are some other efficient methods for constructing POM-encapsulated MOFs. In 2018, Zhong et al. synthesized NENU-3 (PW₁₂@HKUST-1) by a liquid-assisted grinding method [48]. By using a two-step synthesis, PW₁₂ and the Cu salt were first dissolved and evaporated to obtain the copper salt of PW₁₂. Hereafter, the H₃BTC ligand was added in the presence of a small amount of alcohol (MeOH and EtOH), which was used as the grinding liquid. The mixture was ground for 5 min and the color gradually changed to blue. After washing and drying at 60 °C for 24 h, the obtained nanocrystalline, NENU-3, showed a high crystallinity, and the surface area was slightly higher compared to NENU-3 obtained in one-pot solvothermal synthesis. In 2019, G. Li et al. employed an in-situ hot-pressing approach to encapsulate the Keggin-type PW₁₂ into an indium-based MOF (MFM-300(In)) [49]. As shown in Figure 1, all the ingredients, including the POMs, were ground in the absence of a solvent, after which they were packed with an aluminum foil and heated on a plate at 80 °C for only 10 min to obtain PW₁₂@MFM-300(In) composites. The resulting materials exhibited a high crystallinity and stability and no PW₁₂ aggregates were observed on the MOF surface.

3. Catalytic Applications

3.1. Organocatalysis

3.1.1. Oxidation Reaction

Oxidation reactions are one of the most elementary reactions, and have already been extensively investigated by various catalytic systems. POM@MOF hybrid materials are considered as potential oxidation catalysts due to the presence of acidic sites within MOFs, along with the strong acidity and redox performance of POMs. Accordingly, some well-known MOFs have been reported to encapsulate POMs for their use in oxidation reactions, including MIL(Cr, Fe, or Al), UiO(Zr), and ZIF series, as well as Cu-BTC and NU-1000 frameworks. Among the different oxidation reactions, oxidative desulfurization (ODS) and the selective oxidation of alcohols and alkenes are the most studied reactions using POM@MOF catalysts.

ODS, as one of the promising methods for removing sulfur-containing compounds from fuels, has significant importance in both academic research and industrial chemistry. Since 2012, several Keggin- and sandwich-type POMs, including [A-PW₉O₃₄]⁹⁻ [50], [PW₁₂O₄₀]³⁻ [51,52,53,54], [PW₁₁Zn(H₂O)O₃₉]⁵⁻ [55,56], [Tb(PW₁₁O₃₉)₂]¹¹⁻ [57], and [Eu(PW₁₁O₃₉)₂]¹¹⁻ [58], have been incorporated into the cavities of MIL(Cr, Fe, or Al) for the ODS reaction, using H₂O₂ as the oxidant. The heterogeneous POM@MIL catalysts could not only be easily recycled and reused, but also showed a higher catalytic activity compared to the homogeneous POM counterparts. For example, Balula’s group reported that the heterogeneous Tb(PW₁₁)₂@MIL-101 (Tb(PW₁₁)₂ = [Tb(PW₁₁O₃₉)₂] ¹¹⁻) catalyst exhibits 95% conversion of benzothiophene (BT) at 50 °C after 2 h, whereas the homogeneous Tb(PW₁₁)₂ catalyst affords a conversion of only 32% under the same reaction conditions [57]. Although the synthesized Tb(PW₁₁)₂@MIL-101 catalyst showed POM leaching, its structure and morphology remained intact after three consecutive ODS runs. Another study proved that the chemical and thermal stability of POM@MIL-101(Cr) systems could be enhanced compared to individual POMs and MOFs. More specifically, Silva’s group demonstrated the high stability of the PW₁₁@MIL-101 (PW₁₁ = [PW₁₁O₃₉]⁷⁻) catalyst in aqueous H₂O₂, while PW₁₁ decomposed into peroxo-complexes in the presence of H₂O₂ [59]. In addition, Naseri and co-workers observed that the thermal stability of the synthesized P₂W₁₈Ce₃@MIL-101 (P₂W₁₈Ce₃ = [(OCe^IVO)₃(PW₉O₃₄)₂]¹²⁻) and P₂W₁₈Sn₃@MIL-101 (P₂W₁₈Sn₃ = [(HOSn^IVOH)₃(PW₉O₃₄)₂]¹²⁻) materials improved in comparison to the single MIL-101(Cr) framework. The thermally stable POM@MOF materials exhibited >95% conversion of diphenyl sulfide after five cycles [29].

In an interesting study by Cao and co-workers, the effect of the window size within MOFs on the catalytic ODS performance of different POM@MOF materials was investigated [60]. In this work, PW₁₂ was encapsulated into three robust MOFs with different window sizes, namely MIL-100(Fe) (8.6 and 5.8 Å), UiO-66 (6 Å), and ZIF-8 (3.4 Å) (see Table 1). Among them, PW₁₂@MIL-100(Fe) exhibited the highest activity (92.8%) for the oxidation of 4,6-dimethyldibenzothiophene (3.62 × 6.17 × 7.86 ų) compared to UiO-66 (39.1%) and ZIF-8 (9.1%). The observed higher activity was attributed to the large window size of MIL-100(Fe), which enabled a fast diffusion of the substrate into the cages. Another important parameter is the influence of the POM loading on the catalytic performance. The conversion of dibenzothiophene (DBT) was at least two times higher when 16%-PW₁₂@MIL-100(Fe) was used as a catalyst in comparison to the 7%-PW₁₂@MIL-100(Fe) catalyst, owing to the higher POM loading. However, when the loading was increased to 35%, the conversion of DBT decreased a lot due to partial pore blocking, which limited the diffusion of reactants to the active sites.

To further enhance the reactivity and recyclability of POM@MOF materials in ODS reactions, amine-functionalized MOFs were employed for encapsulating POMs owing to the strong electrostatic interaction between amine groups and POM anions, including NH₂-MIL-101(Cr) [33], NH₂-MIL-101(Al) [36,56], and NH₂-MIL-53(Al) [58]. For instance, Cao and co-workers reported the incorporation of PW₁₂ into NH₂-MIL-101(Cr) as a catalyst for the ODS reaction. The obtained material gave a full conversion of DBT at 50 °C after 1 h [33]. Notably, the reusability tests indicated that the conversion of DBT remained unchanged during six consecutive catalytic cycles using PW₁₂@NH₂-MIL-101(Cr) as a catalyst, due to the strong electrostatic interactions between PW₁₂ and the amine groups. Another report by Su and co-workers showed that the PW₁₂@MIL-101(Cr)-diatomite gave 98.6% conversion of DBT at 60 °C for 2 h after three consecutive cycles, which was attributed to the high dispersion of POMs [54].

In addition to ODS reactions, the selective oxidation of alkenes [61,62,63,64,65,66] and alcohols [34] was evaluated using POM@MIL catalysts. For example, Bo and co-workers synthesized H_3+xPMo_12−xV_xO₄₀@MIL-100(Fe) (x = 0, 1, 2) materials and their catalytic performance were assessed in the oxidation of cyclohexene, using H₂O₂ as the oxidant [66]. Among them, the H₄PMo₁₁VO₄₀@MIL-100(Fe) material exhibited 83% conversion of cyclohexene, with an excellent selectivity for 2-cyclohexene-1-one (90%) after five successive catalytic cycles. In 2007, our group developed a new POM@MIL-101 catalyst based on dual amino-functionalized ionic liquid (DAIL) [34]. Firstly, DAIL was introduced onto the coordinatively unsaturated chromium sites of MIL-101(Cr) by a post-synthetic strategy, followed by immobilization of the Keggin-type PW₁₂ onto the DAIL-modified MIL-101 through anion exchange (see Figure 2). The PW₁₂/DAIL/MIL-101 catalyst exhibited a very high turnover number (TON: 1900) for the selective oxidation of benzyl alcohol towards benzaldehyde at 100 °C for 6 h. The PW₁₂/DAIL/MIL-101 catalyst demonstrated a higher catalytic activity compared to the PW₁₂/MIL-101 catalyst without DAIL functionalities (TON: 1400). The higher activity was due to the presence of remaining free amino groups anchored on the imidazolium moieties of DAIL, which play a crucial role in enhancing the accessibility of TBHP as the oxidant. Moreover, the PW₁₂/DAIL/MIL-101 catalyst was reused for at least five cycles, with no significant leaching of the tungsten species.

Another type of MOF, namely Cu-BTC, has also been employed to encapsulate POMs. In 2008, six kind of Keggin-type POMs were encapsulated into Cu-BTC (named NENU-n, NENU = Northeast Normal University) using a one-pot hydrothermal method and their crystal structures were determined [43]. Subsequently, various POMs were encapsulated into the Cu-BTC framework and their catalytic performance was examined in ODS reactions [67,68], the oxidation of alcohols [69,70], olefins [39,71,72,73], benzene, and H₂S [41,74]. For example, Zheng et al. prepared different sizes of nanocrystal-based catalysts, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₅PV₂Mo₁₀O₄₀] (NENU-9N), by using various copper salts and adjusting the pH of the solution for the ODS reaction (see Figure 3) [75]. They proposed that the reaction kinetics can be facilitated by decreasing the size of the nanocrystals. The 550 nm NENU-9 showed a significantly higher conversion of DBT (~90%) in 60 min compared to 300 μm NENU-9 (41%) and the homogeneous POM (2%) in 90 min. To further improve the stability of POM@MOF materials and their catalytic ODS performance, POM@MOF compounds were confined in other porous materials, e.g., MCM-41 [76,77], carbon nanotubes [78], mesoporous SBA-15 [79], and hollow ZSM-5 zeolite [80]. For example, POM@Cu-BTC was confined in the pores of MCM-41 to prevent deactivation of the catalyst [76]. The POM@Cu-BTC@MCM-41 (POM = Cs₂HPMo₆W₆O₄₀) exhibited almost full conversion (99.6%) of DBT in 180 min under optimal reaction conditions and could be reused more than 15 times without a significant loss of activity. Lu and co-workers prepared a series of POM@Cu-BTC (POM = PW₁₂, [PMo_{12 − x}V_xO₄₀]^{(3 + x)−} (x = 0, 1, 2, 3)) catalysts and investigated their performance for the oxidation of benzyl alcohol to benzaldehyde, with H₂O₂ as the oxidant (Figure 4) [70]. The authors observed that the vanadium-containing POMs improved the conversion of benzyl alcohol because of the high redox ability of the POMs. However, when increasing the vanadium content in the POMs, overoxidation to benzoic acid resulted in a lower selectivity towards benzaldehyde. The PMo₁₂@Cu-BTC showed approximately 75% conversion of benzyl alcohol with ~90% selectivity towards benzaldehyde, whereas the PMo₉V₃@Cu-BTC showed ~98% conversion of benzyl alcohol with ~65% selectivity using the same reaction conditions. In other words, the product distribution could be controlled by adjusting the redox capability of the POMs.

Interestingly, in a few studies, a synergistic effect between the POM and Cu-BTC was observed [41,74,81]. For example, Hill prepared CuPW₁₁@Cu-BTC (CuPW₁₁ = [CuPW₁₁O₃₉]⁵⁻) for the oxidation of several sulfur compounds and proposed synergistic effects between PW₁₁Cu and Cu-BTC [41]. Not only the hydrolytic stability of the hybrid POM@MOF was improved, but also the TON (12), as the oxidation of H₂S under ambient conditions increased significantly compared to the individual Cu-BTC (0.02) and POM (no production).

The robust Zr-based MOFs have also attracted much attention for hosting POMs for oxidation reactions. The earliest study on the introduction of POMs into a Zr-based MOF was reported by Dolbecq and co-workers in 2015 [82]. Three tungstate POMs ([PW₁₂O₄₀]³⁻ (12 Å), [PW₁₁O₃₉]⁷⁻, and [P₂W₁₈O₆₂]⁶⁻ (14 Å)) were encapsulated into the pores of UiO-67. Subsequently, Dai and co-workers examined the catalytic performance of 35%-PW₁₂@UiO-66 for the selective oxidation of cyclopentene (CPE) to glutaraldehyde (GA) [83]. The catalyst showed ~95% conversion of CPE, with a ~78% yield for GA at 35 °C after 24 h of reaction. Unfortunately, the catalyst showed PW₁₂ leaching (~3 wt%) after three catalytic cycles. To address this POM leaching issue, Yu and co-workers used UiO-bpy (bpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) to encapsulate polyoxomolybdic cobalt (CoPMA) [84]. The bpy sites of the UiO-bpy framework provided an extra interaction with the POM compared to the UiO-67 without bpy moieties. The catalytic activities of CoPMA@UiO-bpy and CoPMA@UiO-67 were assessed in the oxidation of styrene, using O₂ as the oxidant. The CoPMA@UiO-bpy exhibited the highest catalytic performance, with 80% conversion of styrene and 59% selectivity towards styrene epoxide.

Another Zr-based MOF, denoted as NU-1000, with small triangular (12 Å) and larger hexagonal (31 Å) channels, has been used to support POMs such as [PW₁₂O₄₀]³⁻ and [PMo₁₀V₂O₄₀]⁵⁻ [85,86,87]. For example, Farha’s group prepared PW₁₂@NU-1000 through an impregnation method for the oxidation of 2-chloroethyl ethyl sulfide (CEES), using H₂O₂ as the oxidant. The authors demonstrated that the most likely location for PW₁₂ clusters is in the small triangular channels, which was further confirmed by means of powder X-ray diffraction, scanning transmission electron microscopy, and difference envelope density analysis. At the same time, PW₁₂@NU-1000 showed a higher conversion of CEES (98% after 20 min) compared to the pristine NU-1000 (77% after 90 min) and homogeneous POM (98% after 90 min). However, the PW₁₂@NU-1000 exhibited only 57% selectivity towards 2-chloroethyl ethyl sulfoxide (CEESO). In a subsequent work, the authors demonstrated that the PW₁₂ could migrate from the micropores to the mesopores of NU-1000 under mild thermal activation (see Figure 5). Moreover, the PW₁₂@NU-1000 showed a full conversion of CEES after 5 min, with ~95% selectivity towards CEESO. Recently, this group also prepared the PV₂Mo₁₀@NU-1000 catalyst by using the same method and the synthesized material showed a full conversion of CEES, with O₂ as the oxidant.

In addition to the well-known MOFs, several other POM@MOF hybrid materials, including [Co(BBPTZ)₃][HPMo₁₂O₄₀]·24H₂O and [Cu^I₆(trz)₆(PW₁₂O₄₀)₂], have been synthesized and applied for ODS [88], the oxidation of aryl alkenes [89,90], alkylbenzenes [91], and alcohols [92] (see

Table 2. Application of POM@MOF materials in heterogeneous catalysis.

Entry	MOF	POM	Synthesis Approach	Catalytic Reaction	Ref.
Organocatalysis
1	MIL-101(Cr)	TBA4.2H0.8[PW11Zn(H2O)O39]	Impregnation	ODS	[55]
2	MIL-101(Cr)	H3PW12O40	One-pot	ODS	[52,54]
3	MIL-101(Cr)	[Tb(PW11O39)2]11−	Impregnation	ODS	[57]
4	MIL-101(Cr)	TBA3PW12O40	Impregnation	ODS	[51]
5	MIL-100(Fe) UiO-66 ZIF-8	H3PW12O40	One-pot	ODS	[60]
6	MIL-101(Cr) NH2-MIL-53(Al)	[Eu(PW11O39)2]11−	Impregnation	ODS	[58]
7	NH2-MIL-101(Cr)	H3PW12O40	Impregnation	ODS	[33]
8	NH2-MIL-101(Al)	[PW11Zn(H2O)O39]5−	One-pot and Impregnation	ODS	[56]
9	MIL-100(Fe)	H3+xPMo12−xVxO40 (x = 0, 1, 2)	One-pot	Oxidation of cyclohexene	[66]
10	MIL-101(Cr)	H5PV2Mo10O40	Impregnation	Oxidation of 2-chloroethyl ethyl sulfide	[94]
11	MIL-101(Cr)	(TBA)7H3[Co4(H2O)2(PW9O34)2]	Impregnation	Oxidation of alkenes and cyclooctane	[65]
12	MIL-101(Cr)	[PW11CoO39]5− [PW11TiO40]5−	Impregnation	Oxidation of alkenes	[62]
13	MIL-101(Cr)	[(HOSnIVOH)3(PW9O34)2]12−, [(OCeIVO)3(PW9O34)2]12−	Impregnation	Selective oxidation of various sulfides to sulfones	[29]
14	MIL-101(Cr)	[PW4O24]3− [PW12O40]3−	Impregnation	Epoxidation of various alkenes	[64]
15	MIL-101(Cr)	H3PW12O40	One-pot and Impregnation	Selective oxidation of sulfides to sulfoxides and sulfones	[53]
16	MIL-101(Cr)	[PZnMo2W9O39]5−	Impregnation	Oxidation of alkenes	[63]
17	NH2-MIL-101(Al)	H3PW12O40	One-pot	CO oxidation	[36]
18	MIL-101(Cr)	H3PMo12O40	One-pot	Epoxidation of propylene	[61]
19	MIL-101(Cr)	H3PW12O40	Impregnation	Oxidation of various alcohols	[34]
20	MIL-101(Cr)	[A-PW9O34]9−	Impregnation	ODS Oxidation of geraniol and R-(+)-limonene	[50]
21	MIL-101(Cr)	[PW11O39]7− [SiW11O39]8−	Impregnation	Oxidation of alkenes	[59]
22	Cu-BTC	H3+xPMo12−xVxO40 (x = 1, 2, 3)	One-pot	Synthesis of phenol from benzene	[74]
23	Cu-BTC	H3PW12O40	Liquid-assisted grinding method	Degradation of phenol	[48]
24	Cu-BTC	H6+nP2Mo18-nVnO62·mH2O (n=1-5)	One-pot	ODS	[78]
25	Cu-BTC	Cs+ ion modified H3PMo6W6O40	One-pot	ODS	[77]
26	Cu-BTC	H3+xPMo12−xVxO40 (x = 0, 1, 2, 3) H3PW12O40	One-pot	Selective oxidation of alcohols	[70]
27	Cu-BTC	H3PW12O40 H3PMo12O40 H7[P(W2O7)(Mo2O7)6] H4SiW12O40	One-pot	ODS	[67,68,80]
28	Cu-BTC	H3PW12O40	One-pot	Oxidation of cyclopentene to glutaraldehyde	[72]
29	Cu-BTC	[CuPW11O39]5−	One-pot	Oxidation of thiols and H2S	[41]
30	Cu-BTC	H5PMo10V2O40	One-pot	ODS	[75]
31	Cu-BTC	H6PMo9V3O40	One-pot	Oxidation of benzene to phenol	[79]
32	Cu-BTC	H3PMo6W6O40	One-pot	ODS	[76]
33	UiO-66	H3PW12O40	One-pot	Selective oxidation of cyclopentene to glutaraldehyde	[83]
34	UiO-67 UiO-bpy	H3PMo12O40	One-pot	Olefins epoxidation	[84]
35	NU-1000	H5PV2Mo10O40H3PW12O40	Impregnation	Oxidation of 2-chloroethyl ethyl sulfide	[85,86,87]
36	[Co(BBPTZ)3][HP Mo12O40]·24H2O	H3PMo12O40	One-pot	ODS	[88]
37	[Co(BBTZ)2][H3BW12O40]·10H2O [Co3(H2O)6(BBTZ)4][BW12O40]·NO3·4H2O	[BW12O40]5−	One-pot	Oxidation of styrene to benzaldehyde	[90]
38	[CuI6(trz)6(PW12O40)2] [CuI3(trz)3(PMo12O40)]	H3PMo12O40 H3PW12O40	One-pot	Oxidation of alkylbenzenes to aldehydes	[91]
39	[Cu3(4,4′-bpy)3][HSiW12O40](imi) [Cu3(4,4′-bpy)3][PMo12O40](ampyd), [Cu2(4,4′-bpy)2][HPMo12O40] (ampyd) [Cu(Phen)(4,4′-bpy)(H2O)]2[PW12O40](4,4′-bpy)	H3PW12O40 H3PMo12O40 H4SiW12O40	One-pot	Oxidation of various alcohols	[69]
40	Ni(4,4′-bpy)2]2 [V7IVV9VO38Cl] (4,4′-bpy) 6H2O	[V7IVV9VO38Cl]4−	One-pot	Oxidation of alkenes	[71]
41	[H(bpy)Cu2][PW12O40] [H(bpy)Cu2][PMo12O40]	H3PMo12O40 H3PW12O40	One-pot	Oxidation of ethylbenzene, alcohol, and cyclooctene	[89]
42	H[CuI5CuΙΙ(pzc)2(pz)4.5(P2W18O62)]·6H2O	[P2W18O62]6−	One-pot	Oxidation of alcohols	[92]
43	[CuI(bbi)]2([CuI(bbi)]2VIV2VV8O26)·2H2O	[V10O26]4−	One-pot	Oxidative cleavage of β-O-4 lignin	[81]
44	ZIF-8	Mo132	Impregnation	ODS	[93]
45	ZIF-8	H3PW12O40	Impregnation	Oxidation of benzyl alcohol	[46]
Condensation reaction
46	ZIF-8	Al0.66-DTP Horic	One-pot	Aldol condensation of 5-hydroxymethylfurfural (HMF) with acetone	[95]
47	MIL-101(Cr)	H3PW12O40	Impregnation	Biginelli condensation reaction	[96]
48	MIL-101(Cr)	H3PW12O40	One-pot	Cyclopentanone self-condensation	[97]
49	MIL-101(Cr)	H3PW12O40	One-pot and Impregnation	Baeyer condensation	[98,99]
50	MIL-101(Cr)	H3PW12O40	One-pot and Impregnation	Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate	[30]
51	NH2-MIL-101(Al) NH2-MIL-53(Al)	H3PW12O40	Impregnation and Joint Heating	Aldehyde condensation and polymerization	[37]
52	MIL-100(Fe) MIL-101(Cr)	H3PW12O40	One-pot	Hydroxyalkylation of phenol with formaldehyde	[100]
Esterification reaction
53	MIL-100(Fe) Cu-BTC	H3PW12O40	One-pot	Enzymatic esterification of cinnamic acid	[101]
54	Cu-BTC	H3PW12O40	One-pot	Esterification of acetic acid with 1-propanol	[102]
55	Cu-BTC	H3PMo12O40	One-pot	Esterification of levulinic acid (LA) and ethanol	[103]
56	UiO-66	H4SiW12O40	One-pot	Esterification of lauric acid with methanol	[104]
57	MIL-101(Cr)	K5[CoW12O40]	One-pot	Esterification of acetic acid with various alcohols, and cycloaddition of CO2 with epoxides	[105]
58	Cu-BTC	H3PW12O40 H3PMo12O40, H4PVMo11O40, H5PV2Mo10O40, H6PV3Mo9O40	One-pot	Oxidative esterification of glycerol	[106]
59	ZIF-8	H3PW12O40	One-pot	Esterification of benzoic anhydride with cinnamyl alcohol	[45]
60	Fe-BTC	H3PMo12O40	One-pot	Esterification of free fatty acids to biodiesel	[107]
61	MIL-100(Fe)	H3PW12O40	One-pot	Esterification of acetic acid with monohydric alcohols, and acetalization of benzaldehyde with ethanediol	[108]
62	Cu-BTC	H3PW12O40	One-pot	Esterification of acetic acid with 1-propanol and salicylic acid with ethanol	[40,109]
63	MIL-101(Cr)	H3PW12O40	One-pot	Esterification of acetic acid with n-hexanol, and hydrolysis of ethyl acetate	[110]
64	MIL-53	H3PW12O40	One-pot by ultrasound irradiation	Esterification of oleic acid by various alcohols	[111]
65	MIL-100(Fe)	H3PW12O40	One-pot	Esterification of oleic acid with ethanol	[112]
66	NENU-3a	H3PW12O40	One-pot	Esterification of long-chain fatty acids	[113]
67	UiO-66-2COOH	H3PW12O40	One-pot	Ransesterification-esterification of acidic vegetable oils	[114]
68	MOF-74	H3PW12O40	One-pot	Hydrogenation–esterification tandem reactions	[115]
Other organic transformations
69	MIL-101(Cr)	H3PW12O40	One-pot	Dehydration of fructose to 5-hydroxymethylfurfural	[116]
70	Cu-BTC	H3PMo12O40	One-pot	Transesterification of 5-hydroxymethylfurfural with ethanol	[117]
71	ZIF-8	H3PW12O40	Impregnation	Transesterification of rapeseed oil with methanol	[46]
72	MIL-100(Fe)	H3PMo12O40	One-pot	Transesterification of soybean oil with methanol and esterification of free fatty acids	[118]
73	UiO-66	H4SiW12O40	One-pot	Hydrogenation of methyl levulinate/transesterification of methyl-3-hydroxyvalerate	[119]
74	MIL-100(Fe)	H3PW12O40	One-pot	Hydrogenation of cellobiose/hydrolysis of cellulose	[120]
75	MIL-101(Cr)	K5[CoW12O40]	One-pot	Methanolysis of epoxides	[32]
76	COK-15 MIL-101(Cr)	H3PW12O40	One-pot	Methanolysis of styrene oxide	[121,122]
77	NENU-11	[PW12O40]3−	One-pot	Hydrolysis of dimethyl methylphosphonate	[123]
78	NENU-15	[SiW12O40]4−	One-pot	Reduction of NO	[124]
79	NENU-1a	H4SiW12O40	One-pot	Dehydration of methanol	[125]
80	Basolite F300	H3PW12O40	Impregnation	Dehydration of ethanol	[126]
81	NENU-3a	H3PW12O40	One-pot	Hydrolysis of esters	[43]
82	NH2-MIL-101(Fe) MIL-101(Cr)	TBA4[PW11Fe(H2O)O39]	Impregnation	Ring opening of styrene oxide with aniline	[127]
83	MIL-100(Fe)	[PMo11Mn(H2O)O39]5−	One-pot	Reduction of p-nitrophenol	[128]
84	MOF-808	[H3PW12O40]	One-pot	Friedel-Crafts acylation of anisole with benzoyl chloride	[129]
85	MIL-101(Cr)	H3PW12O40	Impregnation	Pechmann, esterification, and Friedel-Crafts acylation	[130]
86	UiO-66	Cs2.5H0.5PW12O40	One-pot	Acidolysis of soybean oil	[131]
87	Cu-based MOF	H4SiW12O40.xH2O	One-pot	Azide-alkyne click reaction	[132]
88	NU-1000	H3PW12O40	Impregnation	Isomerization/disproportionation of o-xylene	[133]
89	MIL-101(Cr)	H3PW12O40	One-pot	Hydroformylation of 1-octene	[134]
90	MIL-101(Cr)	H3PW12O40	One-pot	Cycloaddition of CO2 to styrene oxide	[135]
Electrocatalysis
91	MIL-101(Cr)	[Co(H2O)2(PW9O34)2]10−	Ion exchange	H2O oxidation	[136]
92	ZIF-8	[CoW12O40]6− H4SiW12O40	One-pot	H2O oxidation	[137,138]
93	ZIF-67	H3PW12O40	Core–shell coating of POM	H2O oxidation	[139]
94	Ag-based metal-organic nanotubes	H3PW12O40 H4SiW12O40	One-pot	H2 evolution	[140]
95	Cu-based metal-organic nanotubes	K6P2W18O62 H6As2W18O62	One-pot	H2 evolution	[141]
96	MIL-101(Cr)	[PMo10V2O40]5−	Impregnation	Oxidation of ascorbic acid	[142]
Photocatalysis
97	NH2-MIL-101(Al)	K6[a-AgPW11O39]	Impregnation	Degradation of Rhodamine B	[143]
98	Cu-based MOF	H4SiMo12O40	One-pot	Degradation of Rhodamine B	[144]
99	MFM-300(In)	H3PW12O40	In situ hot-pressing	Degradation of sulfamethazine	[49]
100	MOF-545	[(PW9O34)2Co4(H2O)2]10−	Impregnation	Water oxidation	[145,146]
101	MIL-100(Fe)	[CoIICoIIIW11O39(H2O)]7− [Co4(PW9O34)2(H2O)2]10−	One-pot	Water oxidation	[147]
102	MIL-100(Fe)	H3PMo12O40	One-pot	Oxidation of alcohols and reduction of Cr (VI)	[148]
103	Zn-based MOF	[BW12O40]5−	One-pot	Coupling of amines and epoxidation of olefins	[149]
104	UiO-66-NH2	H3PW12O40	One-pot	H2 evolution/degradation of Rhodamine B	[150]
105	NH2-MIL-53	H3PW12O40	Impregnation	H2 evolution	[151]
106	UiO-67	[P2W18O62]6− H4SiW12O40	One-pot	H2 evolution	[152,153]
107	UiO derived structure	[Ni4(H2O)2(PW9O34)2]10−	One-pot	H2 evolution	[47]
108	MIL-101(Cr)	α-PW15V3, α-P2W17Ni, α-P2W17Co	One-pot	H2 evolution	[154]
109	SMOF-1	[P2W18O62]6−	Impregnation	H2 evolution	[155]
110	Cu-BTC	[PTi2W10O4]7−	One-pot	CO2 reduction	[42]

BBPTZ = 4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl]; BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene; trz = 1,2,4-triazole; imi = imidazole; ampyd = 2-aminopyridine; bpy = bipyridine; Phen = 1,10-phenanthroline; bipy = 4,4′-bipyridine; Hpzc = pyrazine-2-carboxylic acid, pz = pyrazine; bbi = 1,1′-(1,4-butanediyl)bis(imidazole); DTP = dodecatungstophosp; TBA = tetrabutylammonium.

).

Besides the use of POMs encapsulated in the cages of MOFs, some POMs have been covered on the surface of MOFs to achieve core–shell structured hybrid materials for oxidation reactions [46,93]. For example, PW₁₂ was loaded onto the ZIF-8 surface to obtain a core–shell catalyst for the oxidation of benzyl alcohol. Notably, strong O-N bonding between PW₁₂ and the imidazole group of the ZIF-8 was detected through X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements. Accordingly, the ZIF-8@PW₁₂ material was insoluble in hydrophilic solvents. The ZIF-8@PW₁₂ material exhibited a high conversion of benzyl alcohol (>95%), with 90% selectivity towards benzaldehyde, and outperformed the activity of pure PW₁₂ (51%) and ZIF-8 (30%).

3.1.2. Condensation Reaction

POM@MOF has revealed potential applications in a range of condensation reactions for producing value-added cyclic organic compounds. Recently, Malkar et al. compared the catalytic performance of three different catalysts, namely 20%-Cs-DTP-K10, 18%-DTP@ZIF-8, and Al_0.66-DTP@ZIF-8 (DTP = dodecatungstophosp), for the aldol condensation of HMF (5-hydroxymethylfurfural), as shown in Figure 6 [95]. It has been proved that the substitution of protons of heteropolyacids with metal ions increases the mobility of protons, which results in an enhancement of the acidity. Based on NH₃-TPD analysis, Cs-DTP-K10 displays the highest acidity (1.51 mmol g⁻¹), whereas DTP@ZIF-8 and Al-DTP@ZIF-8 possess 0.44 and 0.54 mmol g⁻¹ of acidic sites, respectively. Cs-DTP-K10, with the highest acidity, showed the highest activity for the aldol condensation of HMF and acetone to selectively produce the desired C9 product (71.6% after 6 h of reaction), while the selectivity was only 43.1%. Although the total number of acidic sites was much lower in the case of the Al-DTP@ZIF-8 catalyst, a good conversion of 63.1% was still obtained after 6 h of reaction, which is comparable to the former value. Notably, the Al-DTP@ZIF-8 catalyst displayed a much higher selectivity (~92%) towards the C9 product compared to C15. The lowest conversion of HMF was achieved in the case of the 18%-DTP@ZIF-8 material with the lowest acidity. However, a high selectivity towards the C9 product was observed. The higher selectivity towards the C9 adduct, as the desired product in the presence of the DTP@ZIF-8 and Al-DTP@ZIF-8 catalysts, confirms the shape selectivity supplied by the small pore diameter of ZIF-8, which can prevent the production of the C15 adduct.

Another example of the use of MOFs in condensation reactions is the well-known MIL-101. For this purpose, PW₁₂@MIL-101(Cr) composites were synthesized through the direct hydrothermal procedure or post-synthesis modification route [98]. The acidic sites within the MIL-101 and PW₁₂@MIL-101(Cr) materials are desirable for catalyzing the Baeyer condensation of benzaldehyde and 2-naphthol, in the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, as depicted in Figure 7. While no product was produced in the absence of catalysts, a high yield of around 95% was observed for the formation of 1-amidoalkyl-2-naphthol at 130 °C using microwave heating for 5 min. Moreover, no leaching of the active sites was observed, and the catalyst could be reused for four cycles without a notable loss in the product yield.

In addition, PW₁₂ clusters were uniformly encapsulated in the cages of MIL-101 as a selective heterogeneous catalyst for the self-condensation of cyclic ketones [97]. As can be observed in Figure 8, the self-condensation of cyclopentanone can result in three different products based on the active sites in the applied catalysts. By using PW₁₂ as the catalyst, a conversion of around 78% could be obtained after 24 h reaction to trindane as the main product. However, PW₁₂@MIL-101 exhibits a considerably higher selectivity (>98%) towards the mono-condensed component (2-cyclopentylidenecyclopentanone) as the desired product due to the possibility of shape-selective catalysis. The PW₁₂@MIL-101 catalyst could be recycled up to five cycles, with no obvious reduction in the conversion and selectivity.

3.1.3. Esterification Reaction

Modified MOFs with POMs can be employed as active catalysts for a wide range of esterification reactions. Biodiesel, as a secure and sustainable energy source, is a promising alternative for fossil fuel-based energy systems [156]. Among the various methods for biodiesel production, transesterification is the most common procedure. Recently, Xie et al. investigated the catalytic one-pot transesterification-esterification of acidic vegetable oil transesterification reaction over a functionalized UiO-66-2COOH with a Keggin-type POM, namely, AILs/POM/UiO-66-2COOH (AIL = sulfonated acidic ionic liquid) (see Figure 9) [114]. The prepared catalyst displayed synergistic benefits arising from the introduction of AIL as Brønsted acid sites. The presence of both Brønsted acid sites of ILs and Lewis acid sites of POM promoted the catalytic reaction for green biodiesel production. The control experiments showed that all of the applied POMs (PW₁₂, SiW₁₂, and PMo₁₂) could convert soybean oil to biodiesel with a high catalytic performance (conversion of ~100%). However, challenges associated with the work-up and recyclability of these homogeneous catalysts limit their potential application. The pristine UiO-66-2COOH material presented a poor activity, with an oil conversion of around 8% because of its insufficient acidic properties. In addition, the PW₁₂@UiO-66-2COOH, SiW₁₂@UiO-66-2COOH, and PMo₁₂@UiO-66-2COOH composites suffered from sluggish reaction kinetics with conversions below 30% towards biodiesel production, which could have been due to the lack of enough acidic sites required to advance the catalytic reaction. Another control experiment was performed by using the sulfonic acid-functionalized IL as the homogeneous catalyst, affording a high catalytic activity of around 99% conversion. It is interesting to note that AILs/POM/UiO-66-2COOH catalysts can combine the advantages of POMs, AILs, and porous MOFs and therefore present the highest catalytic performance in the mentioned reaction (conversion > 90%). Furthermore, the strong interaction between the POMs and AILs was able to hinder the leaching of active components into the reaction media, which further resulted in no notable loss in the catalytic conversion of oil to biodiesel after five consecutive catalytic cycles.

In 2015, Liu et al. described an effective procedure for designing NENU-3a with different crystal morphologies (cubic and octahedral) comprised of a Cu-BTC skeleton and encapsulated phosphotungstic acid catalyst [113]. The morphology of this framework was generated by applying the method of coordination modulation, using P-toluic acid as the modulator. The NENU-3a with cubic crystals ((100) facets) could effectively promote the conversion of long-chain (C12-C22) fatty acids into corresponding monoalkyl esters (>90% yield) compared to the octahedral counterpart (<22% yield). Moreover, the cubic NENU-3a catalyst was highly robust and could be reused for five reaction runs with a preserved structure and catalytic activity. This report confirms the vital impact of morphological control on MOFs for improving the facet exposure of catalytic sites, which accordingly results in an enhancement of the catalytic performance, especially for bulky substrates with limited access to the catalytic active sites within the pores of MOF catalysts. Another important feature of MOFs is the possibility to control the product selectivity arising from the pore size effect of MOFs. Within this context, Zhu et al. studied the selective esterification of glycerol using a MOF-supported POM catalyst [106]. The catalytic performance of the obtained POM@Cu-BTC catalyst was compared to the metal oxide-supported POMs as the reference materials. Since there was no pore limitation impact using the POM@metal-oxide catalyst, the conversion of glycerol stopped at the acid stage without further reaction and was free to be released from the reaction site (Figure 10). However, when the POM@Cu-BTC catalyst was applied, diffusion of the acid product within the MOF pores was limited and further reaction of the acid product produced the corresponding ester compound.

3.1.4. Other Organic Transformations

POMs exhibit great potential as solid acid catalysts because of their strong Brønsted acidity. The first report on a well-defined MOF-supported POM compound, which behaved as a true heterogeneous acid catalyst, was reported by Su et al. [43]. In this work, a series of POM@MOF catalysts were synthesized using a one-pot method. The POM@MOF compound, which contained the strongest Keggin Brønsted acid PW₁₂, was examined in the hydrolysis of ethyl acetate in the presence of an excess amount of water. This catalyst, denoted as NENU-3a, exhibited almost full conversion (>95%) after approximately 7 h of reaction, which is far more superior than most inorganic solid acids and comparable to organic solid acids. More specifically, when the activity was reported per unit of acid, NENU-3a was 3-7 times more active than H₂SO₄, PW₁₂, nafion-H, and Amberlyst-15. In addition to this, no deactivation of the acid sites by water was observed and no leaching of the POM was noted up to at least 15 cycles. Later on, the same group reported the use of POMs as templates for the construction of novel hybrid compounds, for which the properties of the POM could be tailored towards a specific application [123,124,125]. One of these targeted applications was the adsorption and subsequent hydrolysis of the nerve gas dimethyl methylphosphate to methyl alcohol, for which the conversion increased up to 93% when the temperature was raised to 50 °C [123]. Recycling tests demonstrated that the structural integrity was preserved up until at least 10 cycles. However, it is important to note here that a stabilizing effect of the POM on the MOF will only be obtained when the shape, size, and symmetry of the POM match the MOF host [135]. This stabilizing effect even allowed the application of POM@MOF catalysts in aggressive reactions, as was demonstrated in the very nice work of Hupp, Farha, and Notestein [133]. In this study, the Zr-based MOF, NU-1000, was loaded with H₃PW₁₂O₄₀ for its use in the strong acid-catalyzed reaction of o-xylene isomerization/disproportionation at 250 °C (see Figure 11). At low POM loadings (0.3 to 0.7 POM per Zr₆ node), no activity was observed, which was due to the collapse of the POM and/or MOF structure upon activation or at the start of the reaction. However, when the loading was increased to its maximum, with 1 Keggin unit per unit cell of NU-1000, the hybrid catalyst exhibited an initial reactivity in the examined C-C skeletal rearrangement reaction which was even higher than that of the reference WO_x-ZrO₂ catalyst.

While, in the previously discussed studies, the Keggin ion acted as a template to stabilize the microporous/mesoporous structure of the MOF, the group of Martens et al. used this templating mechanism to introduce mesopores separated by uniform microporous walls in a single crystal structure [121] (see Figure 12). More specifically, a hierarchical variant of the Cu-based MOF, Cu-BTC, was synthesized using a dual templating approach in which the Keggin ions served as a molecular template for the structural motif of the MOF, while the surfactant cetyltrimethylammonium bromide was used to introduce mesoporosity. The resulting mesoporous MOF, denoted as COK-15, was investigated in the alcoholysis of styrene oxide, which often suffers from a low selectivity. The COK-15 catalyst not only exhibited a remarkable activity (100% conversion), but also achieved 100% selectivity for 2 methoxy-2 phenylethanol after 3 h of reaction at 40 °C. For comparison, the microporous POM@Cu-BTC and Cu-BTC material only showed 40% and 2% conversion, respectively. The authors addressed the good activity of the COK-15 to the mesoporous feature, which allowed efficient mass transport. Moreover, the catalyst could be recycled for at least four runs, with a negligible loss in activity and selectivity.

Besides this increase in stability after the embedding of the POM in a MOF support, several groups have demonstrated the mutual activation of the POM guest and MOF support [32,119,127]. A very special and extreme example of such a synergism was demonstrated in the work of Kögerler et al. [128]. In this work, a POM@MOF composite was prepared through a hydrothermal reaction in which an Mn-based POM was added to the reaction mixture to synthesize MIL-100. The obtained 30 wt% loaded Mn-POM@MIL-100 was evaluated for its catalytic performance in the reduction of p-nitrophenol to p-aminophenol in the presence of NaBH₄. While both the individual compounds exhibited no catalytic activity, the composite showed an excellent performance (the activity and rate constant at 50 °C were 683 L g⁻¹ s⁻¹ and 0.23 min⁻¹, respectively), which was even comparable to those observed for noble metal-based catalysts. The authors stated that the high catalytic activity originated from the fact that the Mn-POM facilitated the electron transfer from BH₄⁻ to the Fe³⁺ Lewis acid sites of the MOF, as they assumed that the MIL-100 alone could not accept electrons directly from BH₄⁻. Additionally, the group of Shul observed a distinct acid-base synergy upon examination of the core–shell structured heteropoly acid-functionalized ZIF-8 in the transesterification of rapeseed oil with methanol to produce biodiesel [46]. More than 95% of the rapeseed oil was converted to biodiesel due to the simultaneous presence of the acid functionalities of the POM and the basicity of the imidazolate groups of the MOF, whereas the pure POM and ZIF-8 catalysts showed a catalytic performance of 61% and 32%, respectively. Moreover, the strong chemical O-N bonding between the Keggin and the imidazole units ensured a good recyclability, with no noticeable decrease in the catalytic performance after five cycles and no POM leaching.

3.2. Electrocatalysis

Besides the use of POM@MOF hybrids in organocatalysis, POMs also exhibit interesting electrocatalytic properties as they can undergo fast and reversible multi-electron transfers [157]. Within this context, POMs have already shown great potential in the electrochemical oxygen evolution reaction (OER) in a homogenous manner [158]. Despite the remarkable progress in this field, there are only a few reports on the encapsulation of POMs in the cages of MOFs for electrocatalytic water oxidation, as can be seen in Table 2. This is probably due to the fact that the majority of MOFs possess a low electrical conductivity and high hydrophobicity. The first report on the encapsulation of an unsubstituted Keggin POM in a MOF to perform electrocatalytic water oxidation was reported by Das et al. [137]. More specifically, a one-pot synthesis was performed to include the [CoW₁₂O₄₀]⁶⁻ anion in the size matching cage of ZIF-8 (see Figure 13). During the electrochemical measurements, performed at pH 1.9, the authors observed a clear shift to a less anodic potential for the redox Co^III/Co^II couple in the cyclic voltammogram of POM@MOF with respect to that of the uncapsulated POM (from 1.14 V for the Keggin POM to 0.97 V for the composite material). In addition to this, the POM@MOF catalyst exhibited an excellent stability as only a very small drop in the catalytic current was observed after 1000 catalytic cycles and no leaching of Co species was observed. It is, however, important to note here that, although the catalyst exhibited a high turnover frequency (TOF = 12.5 s⁻¹ based on the quantitative oxygen evolution) and an excellent faradaic efficiency of 95.7%, a rather high overpotential was required (784.19 mV at a current density of 1 mA cm⁻²).

In a very recent report of the same group, a redox inactive SiW₁₂ POM was used to lower the required overpotential [138]. The co-encapsulation of this POM together with the true catalyst, an Fe(salen) complex, within ZIF-8 resulted in a decrease of the overpotential of more than 150 mV. In the absence of the encapsulated POM, the Fe-salen@ZIF-8 required an overpotential of 672.9 mV to attain a current density of 1 mA cm⁻², while in the presence of the POM, the overpotential decreased to 516 mV. The authors attributed this observation to the fact that the POM not only increased the hydrophilicity of the catalyst and facilitated the charge conduction in ZIF-8, but also ensured a higher loading of the Fe-salen complex within ZIF-8. Another way to decrease this overpotential for oxygen evolution and thus improve the hydrogen production efficiency was reported by Pang and co-workers [139]. In this work, the authors demonstrated the successful coating of ZIF-67 with a catalytically active Keggin POM, H₃PW₁₂O₄₀. The unique yolk/shell structure of the ZIF-67@POM catalyst ensured a high electrical conductivity and fast charge transfer, which resulted in a significant reduction of the overpotential. From all of the examined ZIF-67@POM hybrids, the 6-ZIF-67@POM catalyst exhibited the smallest Tafel slope (58 mV dec⁻¹) and lowest overpotential values (287, 313, and 338 mV at current densities of 10, 20, and 30 mA cm⁻², respectively), which are even comparable to those observed for RuO₂, which is one of the most efficient and well-known electrocatalysts for the OER reaction [159].

Besides these few examples on OER, POMs have also shown great potential in the second half reaction for water-splitting, namely the hydrogen evolution reaction (HER) [160]. However, to solve their shortcomings, particularly the limited stability of POMs in the required highly acidic pH for HER, Zhang et al. encapsulated POMs in metal–organic nanotubes (MONTs), which can be considered as a special kind of MOF [140,141]. Upon encapsulation of the POMs, using a one-pot synthesis method, the chemical stability of both the POM and MONTs increased. The POMs served as a kind of template to construct the MONTs, while the MONTs ensured a sort of a shield to increase the chemical stability of the POM. The best POM@MONTs electrocatalyst displayed an overpotential of 131 mV (at a current density of 10 mA cm⁻²), which is far more superior than other POM-based MOFs (which showed overpotentials above 200 mV) [161].

3.3. Photocatalysis

Given the extraordinarily large-scale utilization of solar energy, POM@MOF materials have become particularly attractive for their use in visible-light-driven photocatalytic reactions. In particular, their use as catalysts for proton reduction has received considerable attention in recent years. Within this context, Lin’s group reported on the integration of the two required components, namely the photosensitizer [Ru(bpy)₃]²⁺ or [Ir(ppy)₂(bpy)]⁺ and hydrogen evolution catalyst, into Zr-based MOFs to perform proton reduction [47,152]. For example, by using a one-pot self-assembly synthesis strategy, a transition metal Ni-based anionic POM was embedded into the highly cationic MOF by using a pre-functionalized [Ir(ppy)₂(bpy)]⁺-derived dicarboxylate ligand [47]. In contrast to the homogeneous mixture of POM and the Ir-functionalized ligand, which only produced trace amounts of H₂ (TON = 2), a TON of 1476 was observed for the hierarchically-organized POM@MOF assembly, which allowed facile electron transfer due to the proximity of the Ni₄P₂ to multiple photosensitizers in Ni₄P₂@MOF. Another simple strategy for accommodating antenna molecules into MOFs was demonstrated in the work of Wang et al., in which several transition metal-substituted (V, Ni, and Co) Wells-Dawson-type POM@MIL-101(Cr) were prepared using one-pot synthesis [154]. Prior to the photocatalytic evaluation, the cationic photosensitizer [Ru(bpy)₃]²⁺ was adsorbed onto the POM@MOF, for which they observed that the adsorption ability was significantly enhanced upon increasing the POM loading. The photocatalytic performance of the three POM@MOF frameworks was significantly higher in comparison to their homogeneous counterpart and remained nearly unchanged after three additional cycles. Another very nice work in which [Ru(bpy)₃]²⁺ was also used as a photosensitizer was reported by Li et al. [155]. They reported the first water-soluble supramolecular MOF, denoted as SMOF-1, which was built by a self-assembly process from the hexaarmed [Ru(bpy)₃]²⁺-based precursor and cucurbit uril (CB) (see Figure 14). The resulting polycationic SMOF-1 exhibited only a weak gas adsorption ability, but was able to accommodate the bulky redox active [P₂W₁₈O₆₂]⁶⁻ anion. The hydrogen production of the resulting WD-POM@SMOF-1 was about four times higher than that of its heterogeneous system. More specifically, in acidic media and using methanol as a sacrificial electron donor, the TON and H₂ production rate was 392 and 3.553 µmol g⁻¹ h⁻¹, respectively. The authors attributed this high activity to the unique one-cage-one-guest encapsulation pattern, which allowed (i) a quick diffusion and close contact of the hydronium and methanol molecules and (ii) facile electron transfer from the excited [Ru(bpy)₃]²⁺ to the WD-POM. In addition to this, the catalyst could be recovered by evaporation of the solvent and could be reused at least six times without a significant decrease in TON.

It is, however, important to note here that in the previous studies, the high-cost noble [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺ have been used as photosensitizers. The first noble metal-free photoactive POM@MOF catalyst was reported by Dolbecq et al. [145]. In this work, a redox active Co-based POM was embedded in a light-harvesting porphyrinic MOF, denoted as MOF-545, for the visible-light-driven oxidation of water (Figure 15) or, in other words, both the photosensitizer and the catalyst were incorporated into the same porous material. The authors observed that the O₂ production started upon exposure to light and increased linearly over time, until a plateau was reached after 1 h of catalysis. The authors stated that the unique activity of this “three in one” photoactive catalyst was the result of (i) the immobilization of the porphyrin ligand in the MOF, which increased its oxidizing power, and (ii) the confinement of the Co-POM in the pores of MOF-545, which resulted in an increased stabilization of the POM catalytic sites. Nevertheless, the reuse of this POM@MOF catalyst was hampered due to a partial loss of the powder during centrifugation. To overcome this issue, the authors deposited a thin film of the latter POM@MOF on indium tin oxide, which served as a conducting support to allow better electronic transport, but also permitted easier reuse [146]. The films obtained through drop casting not only exhibited a significantly better performance in photocatalytic water oxidation (TON = 1600 and TOF = 0.467 s⁻¹) in comparison to the POM@MOF in suspension (TON = 70 and TOF = 0.04 s⁻¹), but also outperformed the previously homogeneous P₂W₁₈Co₄-based photosystems (TON = 75) [162].

However, it is important to note here that in the previously presented studies, a sacrificial donor or acceptor was required for the photocatalytic process. In a very recent work by Niu and co-workers, the assembling of a photosensitizer, electron donor, and acceptor into one single framework was reported [149]. For the synthesis of this Zn-based framework, the photosensitizer N,N′-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNDI) was used as the organic ligand, while pyrrolidine-2-yl-imidazole and the [BW₁₂O₄₀]⁵⁻ anion were introduced, respectively, as an electron donor and electron acceptor (see Figure 16). The resulting Zn-DPNDI-PYI catalyst was examined in the oxidative coupling of benzylamine, exhibiting a conversion of 99% after 16 h of reaction. This high activity was not only the result of the consecutive photo-induced electron transfer (conPET) process, but was also assigned to the long-lived charge separated state.

4. Summary and Outlook

Metal–organic frameworks exhibiting well-defined cages, large surface areas, and a high thermal and chemical stability are excellent hosts for encapsulating polyoxometalates. More than 100 studies on such POM@MOF hybrids have appeared in the last decade. In this review, we mainly focused on the common synthetic aspects and catalytic applications of POM@MOF hybrids in organocatalysis, electrocatalysis, and photocatalysis. More specifically, the activity, recyclability, stability, and interesting synergetic functions of POM@MOF were discussed.

The size of the pores and the aperture of the pore windows are very critical parameters in the design of a POM@MOF. The embedding of POMs into MOFs not only allows the shortcomings of POMs to be overcome, but also ensures the use of the unique advantages of MOFs. The rise of POM@MOF systems is mainly attributed to the excellent dispersion and subsequent stability of the POM in the MOF host. The unique cages and windows and the tunable chemical environment of MOFs enable interesting interactions and synergic effects between POM and MOFs, thus creating excellent novel heterogeneous catalysts.

Although POM@MOF hybrid materials have made tremendous progress in recent years, many challenges still need to be addressed. First of all, the interaction between POMs and MOFs is often limited to weak electrostatic interactions, which can result in POM leaching during the catalysis. To this end, stronger interactions, such as covalent bonds between the MOF host and the encapsulated POM, would allow a further increase of the POM@MOF reusability in catalysis. Secondly, at this moment, there is still too much ‘trial and error’ involved to obtain a good control on the position and distribution of POMs inside MOF cages/channels. Thirdly, very little is known about the synergetic effects and electron transfer mechanism in catalytic reactions. To address this problem, theoretical calculations combined with in-situ and ex-situ characterization techniques would provide a better understanding of the synergetic effects and electron transfer mechanism. Finally, up until now, only some well-known archetypical POMs have been encapsulated in MOFs. New and innovative types of POMs (such as V-centered POMs) with a proven excellent performance in oxidation and photocatalytic reactions should be combined with MOFs to further enhance the application range of these hybrids. We have no doubt that several exciting new (catalytic) applications will be reported in the next months and years in this strongly growing field of research.