Heterogenization of heteropoly compounds: a review of their structure and synthesis

Abstract

The application of catalysis to reduced toxicity systems and benign and renewable energy systems is a central focus area for green chemistry research. It is possible to prepare heterogeneous analogues of the most commonly used homogeneous soluble catalysts by immobilizing them on various insoluble supports. The use of heterogeneous catalysts in chemical processes would simplify catalyst removal and minimize the amount of waste. Therefore, to maintain economic viability, a suitable heterogeneous system should not only minimize the production of waste, but should also exhibit activities and selectivities comparable or superior to the existing homogeneous route. Accordingly, catalysis by heteropoly (and related) compounds (HPCs) is a field of increasing importance, particularly concerning nanocatalysts. Furthermore, the heterogenization of bulk HPCs is an interesting area of research from an industrial point of view. As a rapidly growing and increasing field, HPC catalysis exhibits three main merits: (1) HPCs not only possess a strong acidic property, but also an oxidative property, which can support fast reversible multi-electron redox transformations under mild conditions. (2) HPCs exhibit fairly high thermal stability in supported or salt forms. (3) Their catalytic properties can be tuned in a wide range by changing their chemical compositions. A number of important heterogenization methods will be discussed in this review. These methods can be classified into two major categories: exchange of the HPC protons with cations (precipitation and hybridization) and immobilization of the HPCs on a suitable solid support (encapsulation, grafting, tethering and dispersion). Although these compounds have been known well for over a century, only in the last few years has scientific interest in these materials begun to increase dramatically. Therefore, in this review, we aim to describe the different methods of heterogenization of these compounds developed and reported over the past 15 years.

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E. Rafiee

Ezzat Rafiee was born in Mashhad, Iran, in 1971. She received her B.Sc., then M.Sc. degree in inorganic chemistry from Shiraz University in 1997 under supervision of Professor M. Rashidi. She received her Ph.D. degree from Isfahan University under the supervision of Professor Shahram Tangestaninejad in 2004. She has worked as a researcher at Liverpool University, UK, on LCIC in 2003 under the supervision of Professor Ivan Kozhevnikov. Her research is focused on the synthesis, catalytic applications, nanocomposite and hybrids of polyoxometalates. She is currently a Professor in the Department of Chemistry and the Institute of Nano Science and Nano Technology at Razi University in Kermanshah, Iran.

S. Eavani

Sara Eavani gained her B.Sc. degree in chemistry from Razi University in 2004 and received her M.Sc. and Ph.D. degrees in inorganic chemistry from Razi University under the supervision of Professor Ezzat Rafiee in 2008 and 2014, respectively. She is currently an Assistant Professor in the Department of Chemistry at Razi University in Kermanshah, Iran. Her interests include polyoxometalate synthesis, the development of polyoxometalate-based heterogeneous catalysts, hybrid materials and inorganic nanocomposites.

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1. General aspect

Heteropoly compounds (HPCs) or heteropoly acids (HPAs) constitute an extremely large and diverse class of polyoxometalates (POMs) containing one or more p-, d- or f-block “heteroatoms”, d⁰ and/or d¹ transition metal cations (addenda atoms) and oxide anions. The building blocks of HPCs comprise a central heteroanion (XO₄, X = P⁵⁺, As⁵⁺, Si⁴⁺, Ge⁴⁺, B³⁺, etc.) surrounded by transition metal oxides with a general formula {MO_x}_n, where M = Mo, W, V and sometimes Nb and x = 4–7.¹ Since 1933, hundreds of HPCs with different structures have been synthesized, characterized and used in many diverse areas. Among these, Keggin (Xⁿ⁺M₁₂O₄₀⁽⁸⁻ⁿ⁾⁻), Anderson–Evans (Xⁿ⁺M₆O₂₄^(12−n)−) and Wells–Dawson (X₂ⁿ⁺M₁₈O₆₂^(16−2n)−) structures have attracted considerable interest as catalysts.^2–8

The catalytic properties of the above-mentioned HPCs can be controlled by suitable selection of the heteropolyanion and its constituents, including heteroatoms, addenda atoms and counter cations. Lacunary species and transition metal-substituted HPCs are other structural derivatives that are exploited in catalysis. Lacunary species (most commonly with one, two or three vacancies) are formed by the removal of one or more addenda atoms from Keggin and Wells–Dawson anions. These anions have been used for preparation of hybrid POM compounds. Transition metal-substituted polyanions are designed by the replacement of one or more addenda atoms and the terminal oxo-group by a transition metal atom cation. The transition metal cations possess five bridging oxygen atoms, while the sixth coordination site has been shown to be occupied by a water molecule and is considered to serve as an entry to the inner-sphere electron transfer pathways, which are usually inaccessible in the unsubstituted anions. Though they are completely inorganic in nature, these compounds have been shown to exhibit catalytic chemistry similar to metalloporphyrins.

The extreme variability of the compositions, specific molecular architectures, strong Brönsted acidities, suitable redox potentials and high thermal stabilities along with low volatilities and low corrosive properties of HPCs make them suitable for green technology applications. Based on this, the synthesis, characterization and application of new HPC-based catalysts constitute exceptionally active and fast-developing fields. Although homogeneous HPC catalysts are remarkably efficient, they share a common drawback: separation and reuse of the catalyst is extremely difficult. For this reason, future practical applications of HPCs will also require methods for “catalyst engineering” to aid in catalyst recovery and recycling. In the last two decades, many research groups have focused on the replacement of homogeneous HPC catalysts with heterogeneous analogues, which can be easily recovered from the reaction mixture, thereby minimizing the amount of waste formed. The present review article highlights the increasing interest in HPCs in heterogeneous catalysis and provides a perspective for researchers in the development of new strategies for the recycling of HPC-based catalysts.

2. Heterogenization of HPC catalysts: why and how?

The development of new strategies for the recycling of catalysts is a task of great economic and environmental importance in the chemical and pharmaceutical industries. Although HPCs are used as heterogeneous catalysts in non-polar solvents, their high solubility in many polar solvents renders them into their homogeneous phase. In this case, heterogenization is necessary to prevent this. Increasing the amount of accessible surface active sites is another goal that could be considered in the preparation of heterogeneous HPC catalysts. Although HPAs are efficient homogeneous catalysts, the amount of active sites on their surface is small because of their low surface area (5–10 m² g⁻¹). In this case, non-polar reactant molecules cannot diffuse in to the pseudo-liquid phase of the HPAs and are adsorbed only on their external surface. This mechanism is called “surface-type catalysis”, where the reactions take place on the two-dimensional (2D) surface (outer surface and pore wall) of the solid catalysts, and the reaction rate is, in principle, proportional to the surface area. Therefore, for many heterogeneous catalytic applications, the immobilization of HPAs onto a high surface area carrier is desirable.

It is possible to prepare heterogeneous analogues of HPCs by the dispersion of HPAs on different insoluble supports. These catalysts can be recycled simply by filtration or centrifugation. However, in some cases, weak interaction between the HPAs and the supports lead to the leaching of HPA from the support surface in polar reaction media. In order to resolve this problem, other strategies, including encapsulation, grafting and tethering, have been developed (Fig. 1). These strategies will be discussed in the following section. Using these methods, the leaching of HPAs is negligible, but, compared to the homogeneous cases, lower activities or selectivities are commonly detected due to steric and diffusion factors. However, when the size of the support material is decreased to the nanometre scale, the surface area of the support increases dramatically, and, as a consequence, nanoparticles have a higher catalyst loading capacity and a higher dispersion than many conventional supports, leading to improved catalytic activity. The strategy of magnetic separation is typically more effective than filtration or centrifugation for catalyst recovery as it prevents loss of the catalyst. Therefore, currently, much attention is focused on the design and synthesis of nano-magnetically recyclable HPC-based catalysts.

Fig. 1 Correlation diagram between heterogenization methods and the recycling of HPC catalysts.

Another alternative approach for the heterogenization of HPCs is to prepare insoluble HPA salts by partially or completely exchanging protons of the parent HPAs with suitable cations (Fig. 1). Traditionally, large alkali metals and/or transition metals have been used for this purpose. Except for a few cases, such as Cs_2.5H_0.5PW₁₂O₄₀, these salts have a low surface area and generally produce a colloidal dispersion, and, as a result, catalyst separation is difficult. In the last decade, many improvements have been developed for the synthesis of organic–inorganic HPA salts through the different combinations of various organic cations and heteropoly anions. Consequently, the development and application of these new hybrids has emerged as one of the most potentially significant fields of investigation in catalysis. Some of these materials have been used as “reaction-induced self-separation” or “thermoregulated phase-separable”, depending on the change in the polarity or temperature of the reaction mixture, respectively. In such cases, the reaction medium switches from a homogeneous system to a heterogeneous one at the end of the reaction. Therefore, the use of these catalysts combines the advantages of a one-phase homogeneous catalysis with a facile method for catalyst separation.

The heterogenization of HPC-based catalysts has been an important trend in catalysis research in recent years, and several ways have been reported so far. A number of important instances of such methods will be discussed in the following sections (Fig. 2). These methods can be classified into two major groups: exchange of the HPA protons with cations (precipitation and hybridization) and immobilization of the HPA on a suitable solid support (encapsulation, grafting, tethering and dispersion). In addition to recyclability, as discussed in the previous section, thermal stability and the amenability to continuous processing, and in some cases multifunctionality, are other factors that are considered in the design of heterogeneous HPC-based catalysts.

Fig. 2 Different methods for the heterogenization of HPCs.

2.1. HPAs precipitation with inorganic metal cations

Water-insoluble HPA salts with certain inorganic cations exhibit dramatic changes in their surface area and pore size in comparison to the parent HPAs. These solids are prepared simply by precipitation from aqueous HPA solutions. If these solids are prepared by the partial substitution of protons, residual quantities of protons will still remain, and it is these that are responsible for the catalytic activity of these salts.⁹ If all the protons of the parent HPAs were substituted by the inorganic cations, the obtained “neutral” polyanion salts could also gain proton sites upon interaction with the reaction media. Two mechanisms for proton generation in neutral salts can be distinguished: acidic dissociation of coordinated water and reduction of the metal cations.¹⁰ Also, metal salts of HPA potentially show Lewis acidity, originating from the metal cations as electron pair accepters.

2.1.1. HPA salts with main-group metal cations. HPA salts with large main-group metal ions, such as Cs⁺, K⁺ and Rb⁺, are insoluble and possess high surface areas.^11–13 In particular, the caesium salts of Keggin-type HPAs have been widely used as catalysts due to their facilely modified surface area, porosity, quantities of protons and hence the number of acid sites on the surface through changing the Cs⁺ content. Misono et al. demonstrated that Cs_xH_3−xPW₁₂O₄₀ salts have tuneable molecular shape selective catalysis.¹³ The surface acidity of Cs_xH_3.0−xPW₁₂O₄₀ (x = 2.0–3.0) shows a maximum value when x is 2.5. The high catalytic activity of Cs_2.5H_0.5PW₁₂O₄₀ is ascribed to the hydrophobicity and bimodal pore structure as well as its strong acid strength.^13–22

Recently, Kamiya et al. synthesized bimodal Cs_xH_4−xSiW₁₂O₄₀ with micropores and interconnected mesopores by treating microporous Cs_xH_4−xSiW₁₂O₄₀ with x = 1.0–2.5 in refluxing ethanol.²³ The presence of three crystallites (including H₄SiW₁₂O₄₀, Cs_2.2H_1.8SiW₁₂O₄₀ and Cs_3.7H_0.3SiW₁₂O₄₀) in the appropriate ratio was crucial for the formation of mesopores with regular shapes in the bimodal caesium salt. In addition, the choice of solvent used for the treatment is another factor for controlling the shape and volume of the mesopores.²³

The Keggin-type HPAs, i.e. H_3+nPMo_12−nV_nO₄₀ (n = 0–2), and their caesium salts are well-known catalysts for the partial oxidation of organic compounds by oxygen.^24,25 Kozhevnikov et al. reported the use of these catalysts in the vapour-phase hydrogenation of propanoic acid.²⁶ They also showed that the catalyst acidity has a crucial effect on the reaction selectivity. Tuning the catalyst acidity by caesium substitution in HPAs allows for the propanal selectivity to be maximized. There is compelling evidence that this reaction occurs via a redox Mars–van Krevelen mechanism, with catalyst acidity (controlled by ion exchange, e.g. with Cs⁺ cations) playing an important role.²⁶ Colloidal Cs₃PW₁₂O₄₀ was used as a photocatalyst in the photo-oxidations of a number of organic molecules.^27–29

The physico-chemical and catalytic properties of caesium partly-substituted salts of Wells–Dawson HPAs, i.e. Cs_xH_6−xP₂W₁₈O₆₂, have also been reported;^30,31 for instance, Guo et al. synthesized and characterized a series of insoluble caesium partly-substituted Wells–Dawson-type HPAs, i.e. Cs_xH_6−xP₂W₁₈O₆₂ with x = 1.5–6.0.³¹ These salts were applied to produce diphenolic acid from levulinic acid. The catalytic behaviour of Cs_1.5H_4.5P₂W₁₈O₆₂ was compared with that of the Keggin-type Cs_2.5H_0.5PW₁₂O₄₀. It was found that both the activity and selectivity of Cs_1.5H_4.5P₂W₁₈O₆₂ exceed those of Cs_2.5H_0.5PW₁₂O₄₀, owing to their different reaction types. For the Cs_1.5H_4.5P₂W₁₈O₆₂ samples, a pseudo-liquid behaviour is considered as the governing factor for its high catalytic activity and selectivity. Also, the catalyst activity did not change significantly after three repetitive runs.³¹

As an interesting example, K₅CoW₁₂O₄₀, with E₀ = 1.07 V (against a normal hydrogen electrode), was used as an oxidant, both for organic and inorganic substances. It is an apparently perfect outer-sphere oxidant because the central Co(III) atom is protected by a sheath of chemically inert oxygen atoms that protect the central ion from undesired inner-sphere substitution reactions. This heterogeneous catalyst has shown excellent reactivity in different organic reactions.^32–41 The possible mechanism for the catalytic addition of cyclic and acyclic diketones to alcohols in the presence of K₅CoW₁₂O₄₀ was reported by Rafiee et al.⁴¹ The alcohol was protonated to generate a stable carbocation after dehydration, which could quickly combine with the employed 1,3-dicarbonyl compound to produce, after the release of H⁺, the final alkylated product. Also, due to the resonance of oxygen with the adjacent π-bonding electrons, the cyclic 1,3-dicarbonyl compound as a nucleophile combined with the stable carbocation to produce β-keto enol ethers (Scheme 1). Also, the catalyst could be used four times with only a little change in its catalytic activity.

Scheme 1 Proposed mechanism for the catalytic addition of different diketones to alcohol.⁴¹

2.1.2. HPA salts with transition metal cations. Stoichiometric and non-stoichiometric salts of HPAs with monovalent transition metal cations have been synthesized with high surface areas and microporous structures. The acidic silver salts of HPAs have been studied in different organic transformations under heterogeneous conditions.^42–45 Pale et al. synthesized a series of silver salts of H₄SiW₁₂O₄₀ (Ag–HPAs).⁴⁵ Characterization data showed that a single crystalline phase of the Ag–HPAs was formed, whatever the amount of Ag⁺ introduced. A well-defined dodecahedral shape was observed for all the Ag–HPAs and is a characteristic feature of the analogous materials. The catalytic activities of the Ag–HPAs were examined in the rearrangement of alkynyloxiranes to furans. Interestingly, purely acidic H₄SiW₁₂O₄₀ furnished approximately 10% furan, while the Ag–HPAs combining Brönsted acidity and silver(I) were effective catalysts and quantitatively led to the furan within 5 h, regardless of the H⁺/Ag⁺ ratio. In contrast, the reaction only proceeded slowly in the presence of the non-acidic Ag₄SiW₁₂O₄₀ and required 24 h to be nearly complete. Therefore, both the acidity and the silver content represent key parameters for performing the reaction. As a result, Ag–HPAs have exhibited both Brönsted acidity and metallic properties, and could be applied in organic transformations as a bifunctional recoverable catalyst towards more sustainable chemistry.

The neutral silver salt Ag₃PW₁₂O₄₀ is characterized by its outstanding acidity, which is about one order of magnitude higher than that for Cs₃PW₁₂O₄₀.^{46 1}H MAS NMR measurements of Ag₃PW₁₂O₄₀ salt have shown protons at 9.3 ppm, resulting from the dissociation of the structural water. The same chemical shift is observed in H₃PW₁₂O₄₀ after preheating at 473 K.⁴⁷ It was observed that the counter cations in the Ag₃PW₁₂O₄₀ salt possess the ability to dissociate molecular hydrogen, leading to the generation of active protons.⁴⁸ The reduction of the silver salt with hydrogen is performed according to the equation:

Ag⁺ + ½H₂ → Ag⁰ + H⁺

Therefore, the catalytic activity of Ag₃PW₁₂O₄₀ is greatly enhanced by the presence of hydrogen in the gas phase. The activity of reduced Ag₃PW₁₂O₄₀ in the presence of hydrogen is much higher than that of H₃PW₁₂O₄₀, in which the catalytic activity is not influenced by hydrogen. Besides, the dynamic properties of the protons in the Ag₃PW₁₂O₄₀ salt play an important role in acid-catalyzed reactions.^49–51

Mucha et al. reported detailed structural studies on Ag₃PW₁₂O₄₀ using XRD measurements and the Rietveld refinement, as well as XPS, EDX and FT-IR techniques.⁵¹ They showed that Ag₃PW₁₂O₄₀ salt forms a cubic structure similar to the structures of Cs₃PW₁₂O₄₀. However, the basic difference between them lies in hydration state of the Ag⁺ ion and in its asymmetric position in the octahedral space between the Keggin anions. This causes the neutral silver salt to form two different structures: the first one exists up to 473 K while the other one appears above 573 K. At 473 K, the [Ag(H₂O)]⁺ complex is centred in the octahedral space between the Keggin anions. Each silver cation is bonded to two terminal oxygens of different Keggin anions, while the water molecule forms weak hydrogen bonds with the two other Keggin anions. Surprisingly, in the structure of the Ag₃PW₁₂O₄₀ salt existing at 573 K, the dehydrated silver cation Ag⁺ is not centred in the octahedral space between Keggin anions, and instead the silver cation occupies statistically one of four 24f Wyckoff positions. This results in a large possibility of dimer Ag–Ag formation, with bond lengths even as short as 2.62 Å.⁴⁹ Hill et al. synthesized Ag₅PMo₁₀V₂O₄₀ salts for the first time and reported its better stability and catalytic activity than Na₅PMo₁₀V₂O₄₀ (soluble salt).⁵²

Many research groups have focused on the bifunctional catalysts based on noble metal salts of Keggin-type HPAs and investigated their catalytic performances in various organic transformation. The application of palladium salts of HPAs (Pd–HPA) is very attractive, as in this case the palladium centre and the redox component are joined in one complex. This ensures direct contact between the palladium site and HPA, thus facilitating the transfer of electrons to the HPA. The application of Pd–HPA also simplifies the preparation of the catalysts, since both the palladium and redox component can now be deposited in one impregnation step. Stobbe-Kreemers et al. described the catalytic cycle of Pd–HPA in 1-butene oxidation (Pd⁰H₂–HPA), representing the reduced palladium salts:⁵³

Pd^II–HPA + C₄H₈ + H₂O → Pd⁰H₂–HPA + C₄H₈O

Pd⁰H₂–HPA + ½O₂ → Pd^II–HPA + H₂O

The oxidation of arenes by Pd(II) has attracted considerable interest. These reactions appear to involve the electrophilic substitution of arene by Pd(II) to form an σ-arylpalladium(II) intermediate.^54,55 The oxidation can be made catalytic by reoxidizing Pd(0) back to Pd(II). Oxygen can deoxidize Pd(0) directly, though it needs elevated temperatures and pressures. The catalytic oxidation occurs easier when mediated by HPCs as redox co-catalysts,^56,57 similarly to the Wacker-type oxidation of alkenes.⁵⁸

Pd-containing compounds are traditional catalysts for C–C and C–N coupling reactions. The combinations of Pd compounds and HPAs have received much attention. HPAs act as co-catalysts for the regeneration of Pd(II), and the reduced HPAs are reoxidized by O₂ in the next step.^59–64

The copper, zinc, cobalt, iron and manganese salts of Keggin-type HPAs have also been used as catalysts for various organic reactions.^65–78 Filek et al. reported the use of reduced copper salts of Wells–Dawson-type HPAs, i.e. Cu₃P₂W₁₈O₆₂, as a bifunctional catalyst for ethanol conversion.⁷⁹ XPS measurements indicated the presence of both Cu(0) and Cu(I) species. In oxygen-free helium, the typical reaction products are formed on Brönsted acid centres (including ethylene, diethyl ether and water) supplied by free HPA. On the other hand, in the presence of air, acetaldehyde was predominantly obtained in the catalytic reaction. Such a reaction occurs on redox centres and is ascribed to the presence of catalytic redox centres formed by the reduced copper.

2.1.3. HPA salts with multivalent cations as Lewis acid catalysts. The protons of the HPAs could be exchanged with different multivalent cations, such as Sn²⁺, Al³⁺, Sn⁴⁺ and Hf⁴⁺, to generate Lewis acidic sites originating from the metal cation as an electron pair acceptor. In this context, Sm³⁺, Ce³⁺ and La³⁺ were also used as counter cations in order to enhance the Lewis acidity.^80–86 Most of these catalysts could be recovered easily and are reusable without any considerable loss in activity.

Shimizu et al. used the metal salts of H₃PW₁₂O₄₀ (including Y³⁺, Al³⁺, Zr⁴⁺, Ti⁴⁺, Hf⁴⁺ and Sn⁴⁺) as catalysts in the alkylation of toluene with iso-propanol.⁸⁰ It was found that the activity increased with the electronegativity of the cation, and that Sn⁴⁺ and Hf⁴⁺ salts showed higher catalytic activities than the others. This suggested that the catalytic efficiency increases with the Lewis acid strength, and the rate correlated well with the number of Lewis acids, indicating that the Lewis acid sites originating from the Sn⁴⁺ cation were responsible for the alkylation reaction.

The use of multivalent metal (Ti⁴⁺, Fe³⁺, Sn⁴⁺, Bi³⁺, Ru³⁺) salts of H₃PW₁₂O₄₀ in the Friedel–Crafts acylation of aromatics with carboxylic acids was also reported by Shimizu's group.⁸¹ The rate of acylation depended strongly on the Lewis acidity originating from the exchanged metal cations.

Jie et al. reported the use of a Ce³⁺ salt of a Dawson-type tungstophosphoric acid (Ce₂P₂W₁₈O₆₂·16H₂O) in the synthesis of n-butyl acetate with acetic acid and n-butanol.⁸⁶ The results of pyridine infrared spectroscopy indicated that the catalyst possessed both Brönsted and Lewis acid sites. The catalysts could be recycled and still exhibited high catalytic activity, with a 90.4% conversion after five cycles of reaction.

2.1.4. Mixed salts of HPAs with different cations. Mixed salts of POMs with different cations have also been explored.^87–93 The most cited of those salts involve Cs⁺ or Ag⁺ and NH₄⁺ as cations in Keggin structures. The mixed salts of other cations have not been significantly explored. Dias et al. reported that the secondary structure of the (NH₄)_xCs_yH_0.5PW₁₂O₄₀ salts was strongly dependent on the order of addition of each cation.⁸⁷ BET results showed that salts formed by proton substitution from the ammonium to caesium had much higher surface areas.

Wang et al. used Ag_x(NH₄)_5−xPMo₁₀V₂O₄₀ as a catalyst in starch oxidation.⁸⁹ Among all the compounds, Ag_3.5(NH₄)_1.5PMo₁₀V₂O₄₀ exhibited the highest activity, with a high oxidative degree of 0.62 mol per 100 g due to its surface area and due to the synergistic effect of the Ag and V cations. It also worked as a heterogeneous catalyst, which allowed it to be easily recycled and reused.

Pd_0.15Cs_2.5H_0.2PW₁₂O₄₀ was applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen.⁹⁰ It showed high catalytic performance even in the absence of the H₂SO₄ additive, indicating that it could act as an efficient catalyst and serve as an alternate acid source in the reaction.

Liu-Cai et al. investigated the catalytic activity and selectivity of the Cs_xH_1−xVO[PMo₁₂O₄₀] and Cs_yH_0.5−yCu_0.25VO[PMo₁₂O₄₀] salts in the oxidation of isobutane as a function of the Cs⁺ content.⁹¹

Millet's group designed a new catalyst by the partial substitution of protons in Cs₂HPMo₁₂O₄₀ by tellurium and vanadium cations.^92,93 The Cs₂Te_xV_yPMo₁₂O₄₀ catalysts were used in the partial oxidation of isobutane into methacrylic acid. Tellurium was selected for its hydrogen abstracting efficiency and showed a strong effect on the selectivity to methacrylic acid. The concomitant substitution of protons by the vanadyl cations allowed increasing the activity of the catalysts without considerable decreasing the selectivity to methacrylic acid.

2.2. Hybridization

Recently, many improvements have been achieved by the synthesis of new hybrid materials through the different combinations of various complexes and organic or organometallic cations with HPA anions. Consequently, these new hybrids have emerged as one of the most potentially significant fields of investigation in catalysis today. The controlled assembly of POM-based building blocks represents a crucial challenge to engineer the POM building blocks in such a way that they can assemble into novel architectures with desired functionality. An important extension to this building block concept is realized by the use of POMs to form organic–inorganic hybrid compounds, which comprise covalently connected cluster and organo fragments, thereby allowing the intermit combination of the properties of the metal–oxo and organic building blocks; this has been used to prepare polymers, dendrimers and macroporous materials. It is apparent now that organic components can dramatically influence the microstructures of inorganic oxides, thus providing a route for the design of novel materials, and this is widely shown in the natural world. Such organic–inorganic hybrid materials not only combine the advantages of organic molecules, such as structural fine tuning, but also the close interaction and synergistic effects of the organic group and inorganic cluster. According to the abovementioned discussion, it could be better to divide these materials into three main categories. Of course there are other different forms of hybrid materials based on POM compounds but these three categories are the most synthetized ones.⁹⁴

2.2.1. Organometallic cations. Organometallic POMs, in particular ruthenium-derived species, are of growing industrial importance due to their unique redox and catalytic activity.^95,96 POMs have been used as the models for the metal oxides commonly used as supports in transition metal catalysis, but generally these hybrid materials operate as soluble catalysts under homogeneous reaction conditions. A novel compound comprising a Ru–POM Keggin anion, of formula [HNEt₃]⁺[(Ru{η⁵-C₅H₅}{PPh₃}₂)₂(PW₁₂O₄₀), was synthesized by L. D. Dingwall et al. and showed high activity and selectivity in alkyne oligomerization (Fig. 3).⁹⁷

Fig. 3 Ru-tethered POM Keggin anion. Reproduced from ref. 97 with permission from Elsevier.

A novel organometallic-POM hybrid was prepared by the anion-exchange of a V Schiff base functionalized ionic liquid (IL) with a V-containing Keggin-type POM (Fig. 4). The hybrid solid, containing two types of catalytic active V components, demonstrated a remarkable capability for the heterogeneous hydroxylation of benzene, with an excellent phenol yield of 19.6% and 100% selectivity. The synergistic effect between the metal Schiff base complex and POM plays an important role in promotion of the catalytic activity.⁹⁸

Fig. 4 Hybrid V-containing Keggin-type POM.

2.2.2. Metal complexes as cations. Some researchers have focused on the rational construction of new organic–inorganic complexes based on POMs and transition metal complexes due to their intriguing structures and potential applications in many areas. In these complexes, POMs can coordinate to “secondary” transition metal atoms with organic moieties using their terminal or bridging oxygen atoms to stabilize the frameworks. Meanwhile, transition metal complexes with diverse structural arrangements not only serve as charge-compensating units but also modify the wide-ranging properties of POMs. The combination of transition metal complexes and POMs could provide new organic–inorganic complexes with novel structural types, as well as new properties arising from the interplay of the two components, which would support their use as effective functional materials.

As an example a new inorganic–organic complex [Cu(phen)₂]₂PVW₁₁O₄₀ was formed by utilizing POMs. The polyanion unit, PVW₁₁O₄₀⁴⁻, in which every heavy-atom position is occupied by 11/12 W and 1/12 V, acts as an inorganic ligand coordinating to two Cu(II) atoms through one terminal and one bridging oxygen atom, respectively. Furthermore, the molecules stack with each other to form a 3D structure due to the C–H⋯O hydrogen bonds and π–π stacking interactions (Fig. 5).⁹⁹

Fig. 5 A new inorganic–organic complex [Cu(phen)₂]₂PVW₁₁O₄₀. Reproduced from ref. 99 with permission from Elsevier.

Three pure inorganic eight-connected self-catenated networks based on the Silverton-type POM [CeMo₁₂O₄₂]⁸⁻ with lanthanide, transition metal and alkali metal cations as linkers: [Li(H₂O)₄]₂Co(H₂O)₄Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O, H_0.5[Li(H₂O)₄]_2.5[Ni(H₂O)₄]_0.5Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O and H[Li(H₂O)₄]₃Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O were successfully synthesized by H. Tan et al. Single-crystal X-ray diffraction analyses revealed that all three compounds were isostructural (Fig. 6).¹⁰⁰

Fig. 6 Inorganic eight-connected self-catenated networks based on the Silverton-type POM. Reproduced from ref. 100 with permission from Elsevier.

Another hybrid compound was hydrothermally synthesized by using a cobalt complex as a new bicapped Keggin-type HPA derivative as [Co(bpy)₃]₂[PMo^VI₈V^V₃V^IVO₄₀(V^IVO)₂] [{Co(bpy)₂(H₂O)}₂{PMo^V₈V^V₃V^IVO₄₀(V^IVO)₂}]·6H₂O. The most interesting feature of this hybrid compound is that a polyoxoanion and a neutral component coexist in the crystal structure. Hydrogen bonding contacts are also formed between the different polyanions through the lattice water molecules (Fig. 7).¹⁰¹

Fig. 7 Bicapped Keggin HPA derivative.

The synthesis of tailored multimetal catalysts is becoming increasingly important for their high efficiency and multifunctionality in various applications. A totally inorganic oxygen-evolving catalyst was synthesized by the template-directed metalation of [γ-SiW₁₀O₃₆]⁸⁻ using tetraruthenium(IV)-oxo-core (Fig. 8).¹⁰²

Fig. 8 Metalation of SiW₁₀ by complementary Lego assembly.¹⁰² Reprinted with permission from J. Am. Chem. Soc., 2008, 130, 5006. Copyright © 2008 American Chemical Society.

A monovacant Keggin-type POM-supported trirhenium carbonyl derivate, [(CH₃)₄N]₅H₂₃[(PW₁₁O₃₉){Re(CO)₃}₃(μ₃-O)(μ₂-OH)]₄·24H₂O, was prepared as an efficient catalyst for cyclic carbonate and epoxide synthesis.¹⁰³

Six novel organic–inorganic hybrid derivatives with two types of organic ligands were hydrothermally synthesized and structurally characterized. Furthermore, the photocatalysis properties of rhodamine-B under 500 W Hg lamp irradiation in the presence of these catalysts were examined (Fig. 9).¹⁰⁴

Fig. 9 Polyhedral/ball-and-stick representation of (a) structural unit and (b) 1D chain built by alternate [Ce(R-PW₁₁O₃₉)₂]¹¹⁻ units, [Cu₃(en)₂]²⁺ and [Cu₂(en)(2,20-bipy)]²⁺ bridges.¹⁰⁴ Reprinted with permission from Cryst. Growth Des., 2011, 11, 3769. Copyright © 2011 American Chemical Society.

Magnetite–POM hybrid nanomaterials, Fe₃O₄/SiO₂/salen/Mn/IL/PW₁₂, were prepared by grafting H₃PW₁₂O₄₀ onto IL-functionalized Fe₃O₄ magnetite nanoparticles (Fig. 10).¹⁰⁵

Fig. 10 Magnetite–POM hybrid nanomaterials. Reproduced from ref. 105 with permission from Springer.

2.2.3. Organic cations. Recent advances in IL research have provided another route for achieving organic–inorganic hybrid salts, which have been constructed by the creation of electrostatic interactions between an organic IL cation and a POM anion. These materials generally form a liquid–liquid biphasic reaction system and the catalyst can be relatively easily separated from the reaction mixture.

Some of these organic ligands use phosphorous as the donating atoms and some use nitrogen. As an example, the pairing of a Keggin or Lindqvist POM anion with an appropriate tetraalkylphosphonium cation yielded the first members of a new family of ILs. Detailed characterization of one of them, an ambient-temperature “liquid POM” comprising the Lindqvist salt of the trihexyl(tetradecyl) phosphonium cation, by voltammetry, viscometry, conductimetry and thermal analysis showed that it exhibited conductivity and viscosity comparable to those of the previously described inorganic–organic POM–IL hybrid but with substantially improved thermal stability.^106,107

There are more reported publications with nitrogen as the coordinating atom. There are also some publications using imidazolium hybrid ILs as photocatalysts. Certain interesting structural and electronic features of the BMIm-based ILs, such as: (i) the ability to provide the reaction system with a radical or cation propagating source (the imidazolium moiety); (ii) as in ILs, the ability to form an extended hydrophobic channel-like compartments, which can stabilize the emerging oligomers,^108,109 led to the choice of BMIm⁺/DMIm⁺ as the second component.^108–110 In short, the design strategy of such catalysts exploits: (i) the photoactivity of POM coupled with the presence of imidazolium to catalyze radical/cationic polymerization initiation; (ii) the advantages of the imidazolium moiety; (iii) the stabilization of the emerging oligomeric chains in the extended hydrophobic compartments by the dangling butyl chains, thus rendering the catalyst functional and (iv) the redox recoverability of POM under the stipulated reaction conditions.¹¹¹

Such complexes combine the flexibility and reactivity of HPAs with the tunable solvency and task-specific property of ILs and, thus, have been used as, depending on the selected reaction media, liquid–liquid biphasic catalysts, reaction-induced self-separation catalysts or heterogeneous catalysts in various organic transformations. More importantly, imidazolium cations have, in some cases, shown the ability of activating HPAs, leading to elevated catalytic efficiency.^112–133

A series of Anderson-type Q₄NiMo_6−xW_xO₂₄H₆ (x = 0, 2, 4, 6) catalysts were synthesized and employed in ILs extraction coupled with catalytic oxidation desulfurization systems for the removal of sulfur compounds in a model diesel fuel and in an actual commercial diesel (Fig. 11).^134,135

Fig. 11 Anderson-type IL–POM for extraction coupled with catalytic oxidation desulfurization.

Some of the POM-based organic–inorganic hybrid materials include amphiphilic catalysts that have been synthesized by combining quaternary ammonium cations and POM anions.^136–140 These compounds have the ability to form emulsion droplets, which perform as homogeneous catalysts in the interface of two immiscible liquids to help achieve high activities during the oxidative process. However, the de-emulsification for catalyst recovery is often challenging in a large-scale unit. Catalyst recovery in an active form suitable for recycling is generally not feasible and the products may be contaminated with the catalyst residues. This situation often leads to the need for expensive purification procedures, which is at odds with the development of more sustainable processes. Recently, to solve this problem, a novel thermoregulated phase-separable catalyst was introduced for the selective oxidation of organic sulfur compounds in aqueous media.^141–144 These temperature-dependent systems not only showed excellent catalytic performance, but also provided a convenient way to reuse and avoid the leaching of catalysts from the solvents.

A new kind of POM–IL, [(CH₃)₃NCH₂CH₂OH]₅PV₂Mo₁₀O₄₀, was synthesized by a precipitation/ion exchange method with choline chloride and H₅PMo₁₀V₂O₄₀ as the precursors. The produced salt turned out to be an original IL catalyst and exhibited a novel switchable property based on temperature variation. Also, in this case, on decreasing the temperature, the catalyst precipitates and becomes in a heterogeneous form, allowing it to be separated automatically from the reaction mixture. The combination of POMs with choline chloride was also proven to be effective in catalyzing the oxidation of starch, which showed a higher or nearly identical performance with traditional catalysts, such as FeSO₄.¹⁴²

Some other thermoregulated IL–POMs were produced by Rafiee et al. and the activity of these catalysts were investigated in biodiesel production and in extractive oxidative desulfurizations and other reactions.^141,143,145

Among these systems, the soluble polymer-based thermoregulated system has been of great interest in recent years. Based on the same concept, J. Chen et al. utilized a polymer-based thermoregulated IL as an efficient and convenient catalyst for the epoxidation of olefins in the presence of H₂O₂ in ethyl acetate, and attempted to solve the problem associated with the separation and recycling of the catalysts (Fig. 12).¹⁴⁶

Fig. 12 Polymer-based thermoregulated ionic liquid.

The structural bonding of some hybrid ILs was investigated by Zonoz and co-workers, who proved that there are two types of hydrogen bonding in compound 1: (i) the hydrogen bonding existing between the Keggin clusters and imidazole ligands; (ii) the hydrogen bonding present between the Keggin structure, the non-coordinated water molecule and the imidazole ligand. And also, there were shown to be π interactions between the imidazole ligands.¹⁴⁷

A novel nano-sized biological active multilayer film composed of a POM anion α-[SiW₁₁O₃₉Co(H₂PO₄)]⁷⁻ (SiW₁₁Co–PO₄) and poly(diallyl dimethylammonium chloride) (PDDA) was fabricated by a layer-by-layer self-assembly (Fig. 13).¹⁴⁸

Fig. 13 A nano-sized biological active multilayer film composed of a POM anion.

Some other kinds of polymer-based IL–POM hybrids are shown in Fig. 14.^149–153

Fig. 14 Some polymer-based IL–POM hybrids.

Mixed organic and metal cation salts of IL–POMs sometimes have excellent catalytic activity.¹⁵⁴ More recently, in order to combine the merits of HPA–onium cation catalysts with supported HPA catalysts, several reports have been published describing the use of immobilized HPA–onium cation complexes as intrinsically heterogeneous catalysts. In comparison with HPA–onium cation complexes, the resultant catalysts show additional advantages, such as a decreased consumption of HPA–onium cation complexes, a facilitation of catalyst separation from the reaction system and a lower contamination of product, just like in the cases of IL heterogenization (Fig. 15a).¹⁵⁵ Another hybrid was reported by B. Zhen et al. (Fig. 15b).¹⁵⁶

Fig. 15 Immobilized POM–IL hybrids.

Magnetic-supported HPA–ILs were also synthesized with different methods. Two of the synthesis procedures are presented in Fig. 16.^157,158

Fig. 16 Synthesis of magnetic-supported HPA–ILs.

Three novel multi-SO₃H–functionalized heteropolyanion-based ionic hybrids were synthesized and characterized, e.g. as heterogeneous catalysts for the Baeyer–Villiger oxidation.¹⁵⁹ There are some other research papers in this subject area (Fig. 17).^160,161

Fig. 17 Multi-SO₃H functionalized heteropolyanion-based ionic hybrids. Reproduced from ref. 159 with permission from Elsevier.

2.3. Dispersion

Dispersion is the immobilization of HPA on supports that do not contain specific anchoring sites for dispersed HPA particles. Virtually any solid that has a surface area of at least 10 m² g⁻¹ can be used as a support. Support materials, such as silica, alumina, titania, activated carbon, magnesia, etc., have been applied with varying levels of success, with new supporting materials and methods continually being actively pursued. Acidic or neutral substances, such as silica and active carbon, are suitable as supports. Basic solids, such as alumina, titania and magnesia, tend to decompose HPA. Some recent publications reporting the application of these supports for HPAs are listed in the following sections.

2.3.1. Silica. Among various solids, silica is the most often used support due to several reasons:

(i) It is widely available, is neutral or mildly acidic and possesses tuneable specific surface areas and porosity.

(ii) According to temperature programmed desorption data, the acid strength of supported H₃PW₁₂O₄₀ decreases in the series of carriers: SiO₂ > α-Al₂O₃ > activated carbon.

(iii) The thermal stability of HPA on SiO₂ seems to be comparable to or even slightly higher than that of the parent HPA.

(iv) The thermally decomposed Keggin structure on the silica surface may be reconstructed on exposure to water vapour.

Silica-supported HPA catalysts were prepared by impregnating Aerosil 300 silica (S_BET, 300 m² g⁻¹) with an aqueous solution of H₃PW₁₂O₄₀ by J. Kaur and I. V. Kozhevnikova, and their catalytic activity were investigated in Friedel–Crafts acylation and in Fries rearrangement of aryl esters, respectively.^162,163 During the reaction, even in non-polar solvents, such as dodecane, H₃PW₁₂O₄₀ leached from the silica support. Significant coking was observed for the highly porous H₃PW₁₂O₄₀/SiO₂ (6–13 wt% carbon). However, the catalyst could be separated by filtration and, after a simple work-up, such as washing with dichloroethane, reused, albeit with reduced activity. The authors tried to control the reusability of the catalyst by Pd doping. They found that 1.5% Pd doping of H₃PW₁₂O₄₀/SiO₂ had little effect on the coke burning: only about a 50 °C temperature shift was observed in the TGA/TPO plot. There may be several reasons for this, such as how the coke was laid down on the highly porous H₃PW₁₂O₄₀/SiO₂. In the case of H₃PW₁₂O₄₀/SiO₂, the massive coke localized in the pores would be more difficult to burn out. In addition, Pd loading on H₃PW₁₂O₄₀/SiO₂ (1.5%) may be insufficient. It has been shown that Pd doping only becomes effective for coke burning at loadings of 2–2.5%. Sometimes with insufficient reusability, it could be better to change the washing solvent or to do the catalytic reaction in a solvent-free system.¹⁶⁴

According to the already-mentioned advantages of silica as a support, there are a lot of research on different HPAs supported on silica by impregnation methods and that are used in various organic synthesis and industrial reactions.^165–174 H₃PW₁₂O₄₀/SiO₂ was chosen by Rafiee et al. as a model system to investigate the effects of different amounts of HPA loading on the support. They proved that in almost all the publications there were differences in catalytic activity among the catalysts with 20, 40 and 60 wt% of H₃PW₁₂O₄₀ on silica and sometimes on other supports. Lowering the loading of the deposited H₃PW₁₂O₄₀ resulted in a reduction of the catalytic activity. Furthermore, no improvements in the reaction rate and yield were observed by increasing the amount of H₃PW₁₂O₄₀ on SiO₂ from 40 to 60 wt%, since 40 wt% of H₃PW₁₂O₄₀/SiO₂ was the best catalyst loading in almost all the cases.¹⁷⁵ Acidity measurements of the catalysts by means of potentiometric titration with n-butylamine were carried out. This method enables the determination of the total number of acid sites and their distribution. The addition of H₃PW₁₂O₄₀ was accompanied by a gradual increase in both the surface acidity and acid strength up to 40 wt% of H₃PW₁₂O₄₀ (E_i = +693 mV), then followed by a decrease in both at a loading of 60 wt% of H₃PW₁₂O₄₀. These results indicate that 40 wt% of H₃PW₁₂O₄₀/SiO₂ contains the strongest acid sites. Here, the increases in the surface area enhance the dispersion of acidic protons up to a monolayer coverage at 40 wt% of H₃PW₁₂O₄₀, while a further increase in the HPA content above the monolayer leads to the aggregation of HPA on SiO₂ surface, which leads to a decrease in the surface acidity.¹⁷⁶ Also, they reported the N₂ adsorption–desorption isotherm of H₃PW₁₂O₄₀/SiO₂. The decrease in the surface area and pore volume from the BET results, i.e. from 311 m² g⁻¹ and 1.7 cm³ g⁻¹ to 115 m² g⁻¹ and 0.36 cm³ g⁻¹, respectively, observed for the SiO₂ sample in comparison with 40 wt% of H₃PW₁₂O₄₀/SiO₂ and the relatively low initial adsorption in the isotherm plot can be ascribed to the partial blocking of the micropores by HPA particles.¹⁷⁷

They reported that the TGA/DSC profile for 40 wt% H₃PW₁₂O₄₀/SiO₂ showed three mass losses. The first weight loss was related to the release of the physically adsorbed water molecules. This water release ranged from 34 °C up to 141 °C, with the maximum at about 80 °C. The temperature range, as well as the maximum, could vary according to the amount of water contained initially in the material. A second weight loss in the temperature range of 141–217 °C centred at about 190 °C was ascribed to the dehydration process of the hydrated water molecules. The third step took place in a very wide range of temperature, with two maxima at 233 and 309 °C. The wide range of the third mass loss was related to more than one event under that condition. The global event was attributed to the dehydration of SiO₂, the loss of all acidic protons or the beginning of decomposition of the Keggin structure.¹⁷⁵

Rafiee et al. prepared nano-silica from rice husk with a high surface area and then fully characterized it.¹⁷⁸ It has also been used as a support for HPAs. This form of support also related to the synthesis procedure, showed a higher catalyst loading capacity and higher dispersion of HPA, leading to higher catalytic activity. The catalyst showed excellent activity in some important organic reactions, including the Biginelli, Hantzsch, Mannich and Claisen–Schmidt reactions, together with good reusability. The catalytic activity of this nanocatalyst is an improvement over the commercially available silica used to support H₃PW₁₂O₄₀.¹⁷⁹ Also the same authors used this support for other HPAs, such as for Mo-containing HPAs and H₅CoW₁₂O₄₀, to catalyze other organic reactions.^180–183 H₅Mo₁₀V₂O₄₀ supported on nano-silica (Mo₁₀V₂@NSiO₂) from rice husk ash was used as a co-catalyst for the C–C coupling reaction as a novel application of HPAs. The authors also developed a convenient catalytic system composed of palladium-deposited poly(N-vinylpyrrolidone) coated-nano zero valent iron (Pd–PVP–Fe) and Mo₁₀V₂@NSiO₂ nanoparticles as a reusable catalytic system for the Heck coupling reaction under ligand- and base-free conditions.¹⁸⁴ However, conventional separation methods may be inefficient for supporting particle sizes below 100 nm. The incorporation of magnetic nanoparticles (MNPs), such as iron oxide, into supports offers a solution to this problem. The strategy of magnetic separation, taking advantage of MNPs, is typically more effective than filtration or centrifugation as it prevents loss of the catalyst. The magnetic separation of MNPs is simple, economical and thus promising for industrial applications. This kind of support is been discussed in the following sections.

G. D. Yadav and co-workers synthesized H₃PW₁₂O₄₀ supported on hexagonal mesoporous silica.¹⁸⁵ This catalyst was found to be very active and also stable without any deactivation in an environmentally benign route for acetoveratrone synthesis. Different forms of silica show different surface areas and pore sizes and accordingly a different surface acidity. H₃PW₁₂O₄₀ was supported on three different carriers by Blasco et al.: a commercial silica, a high surface area amorphous aluminosilicate (MSA) and an all silica-mesoporous MCM-41; and their catalytic properties were determined for the alkylation of 2-butene with isobutene.¹⁸⁶ The high surface area of the MCM-41 and MSA supports showed higher acid dispersions compared with SiO₂, but H₃PW₁₂O₄₀/SiO₂ showed the maximum activity, selectivity to trimethylpentane and stability with the time on-stream. The activity of H₃PW₁₂O₄₀ depends directly on the interaction of H₃PW₁₂O₄₀ with the functional group on the support, and thus, in the H₃PW₁₂O₄₀/MSA samples, which have a stronger interaction of H₃PW₁₂O₄₀ with the surface sites of the aluminosilicate together with a decrease in both the number of Brönsted acid sites and the average acid strength of those sites of the HPA, lower catalytic activity was predicted. In the H₃PW₁₂O₄₀/MCM-41 samples, partial blockage of the monodimensional pores of MCM-41 decreased the accessibility of the reactants to the Brönsted acid sites of the HPA located inside the pores. This pore blockage could be decreased, and the catalytic activity of the H₃PW₁₂O₄₀/MCM-41 catalysts increased, by using a MCM-41 sample with a larger pore diameter.

Although, Keggin-type HPAs have been widely studied as homogeneous and heterogeneous catalysts for the oxidation of organic compounds, only a few Wells–Dawson-type polyoxoanions have demonstrated catalytic activity in a heterogeneous form.¹⁸⁰ Generally, Keggin structures show more catalytic activity among HPAs,^181,182 whereas Wells–Dawson HPAs exhibit greater selectivity and activity in some acid-catalyzed and oxidation reactions.¹⁸⁰ The Keggin anions offer limited hydrolytic stability compared to the other types of HPAs, especially when they are supported on interacting substrates. The silica-supported Co(II)-substituted Wells–Dawson HPA salt, Cs₆H₂P₂W₁₇O₆₁Co·H₂O, show good activity in an organic solvent. This particular salt not only offers the characteristic acidic and oxidative properties of an HPA skeleton, but also, it exhibits the catalytic activity of the transition metal cation.¹⁸⁷

Although impregnation is the widely used method for supporting HPAs, due to the reversible adsorption of HPAs on the supports, HPAs are prone to be dissolved in polar media. Therefore the continuous dissolution of HPAs has become a serious problem leading to some environmental pollution and catalyst waste. In order to overcome the problems of the impregnation method, the sol–gel incorporation method was developed. Here, the silicate structure can trap the HPA molecules, while their acidic property remains to a great extent. Kukovecz et al. compared H₃PW₁₂O₄₀–silica composites developed by conventional impregnation and by a sol–gel incorporation technique, and found that the sol–gel derived composites had less dissolution of H₃PW₁₂O₄₀, and were more suitable for the purposes of heterogeneous catalysis.^188,189

Y. Chen compared the structural and catalytic properties of MCM-41 and SBA-15 as supports for H₃PW₁₂O₄₀. The results revealed that the mesoporous materials retained the typical hexagonal mesopores for the support of H₃PW₁₂O₄₀. It was found that H₃PW₁₂O₄₀ exhibited a higher dispersion within MCM-41 than within SBA-15 and other mesoporous molecular sieves. Moreover, the as-prepared materials were found to be the efficient catalysts for the green synthesis of benzoic acid. In particular, H₃PW₁₂O₄₀/MCM-41 exhibited the best catalytic properties due to its suitable textural and structural characteristics.¹⁹⁰

A series of HPAs–mesoporous silica composites were prepared from Ni₁HPMo₁₂O₄₀ by supporting them on SBA-15 mesoporous silica in different concentrations of the active phase. By impregnating Ni salt on mesoporous silica, the thermal stability of the Keggin structure increases in comparison with its parent bulk HPA. Indeed, the total acidity of the weak and strong acidic sites of Ni₁HPMo₁₂O₄₀/SBA-15 composites were obviously increased in comparison with the bulk Ni salt.¹⁹¹

2.3.2. Titania. TiO₂ is a widely used semiconductor material, which has received intense study for a range of applications due to its interesting chemical and physical properties, such as its wide band gap, wide availability, low cost, nontoxicity and biocompatibility. Titania, particularly its anatase form, is widely known to enhance catalytic activity due to a strong interaction between the active phase and the support; however, the application of titania is often limited due to problems associated with the charge-recombination (electron–hole recombination) phenomenon inherent in semiconductor materials and its large band gap of 3.2 eV, which requires its exposure to ultraviolet radiation for potential photocatalysis applications. Electron–hole recombination is always a critical issue in the efficiency of TiO₂ photocatalyst systems, regardless of the application. The solutions to this problem are:

(i) Use of a sacrificial oxidant, such as dioxygen, to scavenge the photogenerated conduction band electrons.

(ii) Adsorbing the TiO₂ photocatalyst directly onto the surface of an electrode, where the electrons can be collected by applying a suitable voltage.

(iii) Employing an electron scavenger, such as a POM, to transport electrons from the suspended TiO₂ particles.

The first and second solutions are not always efficient, while the third way offers the additional advantage of being able to turn over a larger fraction of the incident photons by using more TiO₂ than could be directly adsorbed onto the surface of the anode. The ability of POMs to accept electrons readily has been known for many years. Upon chemical or electrochemical reduction, many POMs form stable, highly coloured mixed-valence species generically termed “heteropoly blues”.^192,193 Ozer and Ferry investigated the use of POMs such as H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀ and H₄W₁₀O₃₂ to facilitate the transfer of photogenerated TiO₂ conduction band electrons to dioxygen.¹⁹⁴ Zhao and co-workers reported similar findings, but also discovered that different product distributions can result when using TiO₂ alone or in tandem with POMs.¹⁹⁵ POM systems have also been shown to be viable in fuel cell applications. For instance, Dumesic and co-workers demonstrated that it is possible to use H₃PMo₁₂O₄₀ to drive a fuel cell by oxidizing CO to CO₂, thus bypassing the water gas shift reaction.¹⁹⁶ C. Gu, and C. Shannon employed a simple Na₃PW₁₂O₄₀ to scavenge electrons from photoexcited TiO₂ particles suspended in solution. The reduced POMs were subsequently oxidized back to the parent Keggin complex at the Pt anode, and a 50-fold increase in the methanol oxidation photocurrent was observed as compared to the use of TiO₂ alone (Fig. 18).¹⁹⁷

Fig. 18 Photoexcitation of TiO₂ in the presence of POM.

Pearson and his co-workers reported the decoration of titania nanotubes synthesized by electrochemical anodization, with gold, silver, platinum and copper nanoparticles, by using H₃PW₁₂O₄₀ as a UV-switchable reducing agent with inherent photochemical properties (Fig. 19).¹⁹⁸

Fig. 19 Schematic of the formation of TiO₂–H₃PW₁₂O₄₀ co-catalyst materials decorated with metal nanoparticles.

The formation of certain HPA salts of metal ions (Mn⁺) of H₃PW₁₂O₄₀ show Lewis acidity, originating from the metal cation as an electron pair acceptor, together with the characteristic Brönsted acidity of HPAs, generated from the dissociation of coordinated water under the polarizing effect of the cation. One of these metals is aluminium, which was supported on titania and showed excellent activity.^199,200

H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ were deposited on TiO₂ by incipient wetness impregnation and used as catalysts for the production of dimethyl ether from methanol. It was found that for HPA loadings ≤2.3 KU nm⁻², the HPA remains well dispersed on TiO₂, while for higher loadings, 3-dimensional (3D) clusters are formed. At low HPA loadings, an interaction between HPA and the support was observed by the appearance of stronger Lewis acid sites, assigned to coordinately unsaturated Ti⁴⁺ species and by the shift in the ¹H-NMR spectra of the signal assigned to the protons in the HPA.²⁰¹

The photocatalytic activity of HPCs in a homogeneous regime has been extensively reported along with the use of supported HPAs in thermal catalysis for acidic and redox processes. However, the nature of the HPAs as highly soluble compounds in polar solvents hinders their use as heterogeneous photocatalysts. Consequently, appropriate heterogenization has been reported by many researchers so that HPAs can be used in heterogeneous reaction media, in particular to carry out the photocatalytic degradation of several pollutants. These reported research studies were reviewed by G. Marcì.²⁰²

H₃PW₁₂O₄₀ supported on different metal oxides (TiO₂, γ-Al₂O₃, K10 and KSF) and activated carbon were used in the condensation of aniline with ethylacetoacetate to afford the corresponding β-enaminone. The catalytic activity was discussed in relation with the acid strength of the catalysts. The best catalytic activity was obtained with H₃PW₁₂O₄₀/TiO₂. According to the textural properties of the supports and H₃PW₁₂O₄₀/supports data in Rafiee et al.'s report in 2009, in all cases, supported H₃PW₁₂O₄₀ showed a higher reactivity compared to the support only on a unit weight basis, except for K10 and KSF montmorillonite, which can mask the catalytic activity of H₃PW₁₂O₄₀. Carbon as a support has a high pore volume size so it can entrap HPAs firmly in its pores. The HPA fraction occluded in the pores may interact more weakly than the adsorbed fraction with the reactants, thus leading to the low catalytic activity. Among the supported catalysts, 40 wt% of H₃PW₁₂O₄₀/TiO₂ was the best in terms of yield and reaction time in β-enaminone synthesis.²⁰³ It should be noted that the same authors investigated the effect of this factor in other catalytic reactions with these same kinds of support and proved that it was not similar in all reaction media and depended on the solvents, reactants and mechanism of the organic reactions.

Rafiee and co-workers investigated the acidic properties of H₄SiW₁₂O₄₀ on TiO₂, SiO₂ and alumina. All the supported catalysts had stronger acid sites in comparison with the support only, and according to the mentioned scale, all of them presented very strong acid sites. It was observed that H₄SiW₁₂O₄₀/γ-Al₂O₃ is less acidic than the others. This may be due to the interaction of the strongest protons of H₄SiW₁₂O₄₀ with the most basic sites on the alumina surface. This acid–base interaction induces a partial neutralization of the acid sites that leads to a decrease in the acidity and catalytic activity. The acidity of H₄SiW₁₂O₄₀/TiO₂ is comparable with H₄SiW₁₂O₄₀/SiO₂.¹⁷²

There are a number of other new research papers in the literature on the preparation and catalytic activity investigations on HPAs/TiO₂.^{170,173,204–207}

2.3.3. Carbon. Using carbon (C) as a support for catalysts is widely known, as it provides properties such as high surface area and pH stability. Thus, several studies have been carried out using C as supports for POM.^208–211 Some of these have tried to analyze the variation of the acid strength of bulk POM when supported on C. However, as indicated by M. A. Schwegler et al.,²¹¹ it is not even clear whether the mechanism of POM being supported on C is physical or chemical in nature. Some researchers tried to analyze where POMs are adsorbed and how their adsorption modifies the porous texture of carbon. Monge and co-workers proved that the adsorption of POM on activated carbon followed a similar trend to the physical adsorption of N₂. Saturation of the surface happened quickly as the loading increased. The higher the porosity of C, the greater the amount of HPAs adsorbed. The micropore volume of C decreases with the impregnation of HPAs. They also proved that this reduction is located mainly in the supermicroporosity, while the narrow microporosity and mesoporosity show little decrease because the high molecular size of HPAs limits its access to narrow microporosities.²¹² Such investigations have been done by other researchers.^{175,213–216} H₃PW₁₂O₄₀/C has shown higher activity compared to the other supports.²¹⁷

A carbon support with a nanorod structure was synthesized from natural potato as a green and very cheap source via a hydrothermal method. A novel type of core/shell nanorod catalyst was synthesized by the immobilization of H₃PW₁₂O₄₀ on the surface of the as-prepared C (C@PW). Complete characterization of the core/shell nanorod catalyst was carried out. The C@PW core/shell nanorod was found to be a unique, effective and eco-friendly catalyst for the C–N coupling reactions for a broad range of aryl and alkyl amines and alcohols under aerobic conditions at room temperature. The acidity measurements of C and C@PW by means of potentiometric titration with n-butylamine were used to estimate their relative acid strength. Leaching of the H₃PW₁₂O₄₀ from the C was minimized by optimization of the calcination temperature.²¹⁸

In recent years, considerable progress has been made to synthesize POMs in the nano-size range. Starch microspheres were prepared from low-cost raw materials-soluble starch by reverse-phase microemulsion polymerization methods using phosphorous oxychloride as the linking agent. Si₂W₁₈Ti₆-loaded starch nanoparticles were prepared by Wang et al.²¹⁹ Such kinds of support for HPAs were synthesized and characterized by Rafiee et al.²²⁰

Researchers working in the fields of catalysis and adsorption have paid much attention to carbon–carbonaceous composite materials. Catalytic filamentous carbons (CFCs) were produced by the decomposition of gaseous hydrocarbons over the iron-group catalysts at 700–900 K.^221,222 The morphology and structure of growing CFCs are dictated by the catalyst composition.^{221,223–226} The adsorption and desorption of H₃PW₁₂O₄₀, H₆P₂W₁₈O₆₂, H₆P₂W₂₁O₇₁ and H₆As₂W₂₁O₆₉ on CFCs were studied by M. N. Timofeeva.²²⁶ They also studied the influence of the structure of CFCs and their specific surface area on the strength and amount of HPA adsorbed.

2.3.4. Zeolites and clays. Zeolites and clays that give the desired level of activity and selectivity are the best alternative to liquid acids. Several petrochemicals are already manufactured by different reactions using these compounds. Also, the use of these compounds as a support for HPAs has been reported. A number of HPAs supported on K10 or KSF clay were reported as novel catalysts for a number of reactions.^{172,175,216,227–235} Furthermore, also salt-modified HPA (Cs_2.5H_0.5PW₁₂O₄₀/K-10) clay have been reported as nanocatalysts for a number of important industrial reactions.^236,237 HPAs are sometimes impregnated onto the surface of the zeolites and are sometimes encapsulated in the pores. Keggin anions molecularly dispersed at the zeolite surface and in its mesopores become thermally stabilized and remain stable even after calcination at 650 °C in contrast to pure H₃PW₁₂O₄₀, which decomposes on heating at 610 °C. Sometimes Keggin anions interact with the OH groups of the support and hence are chemically modified, while some anions weakly interact with the support and are practically unmodified.²³⁸

In some cases, it is better to modify such kinds of supports to achieve better catalytic activity. As an example, S. K. Bhorodwaj modified the purified clay by refluxing in 4 M HCl acid for various time intervals. The slurry was cooled, filtered and washed thoroughly with water and then dried in an air oven.^186,189 The clay samples were then used as a support for HPAs.²³⁹

Also, H₃PW₁₂O₄₀ supported on dealuminated ultra-stable Y zeolite (DUSY) was prepared by an impregnation method, and then its physico-chemical properties were characterized. The DUSY-supported H₃PW₁₂O₄₀ catalyst exhibited high catalytic activity in the liquid phase acetalization of ethylacetoacetate with ethylene glycol for synthesizing fructone. However, continuous leaching of the HPAs into the reaction medium led to a poor catalytic stability. A very high catalytic activity and stability, however, was found on Cs_2.5H_0.5PW₁₂O₄₀/DUSY catalyst, which could be ascribed to their superacidity, high surface area and water tolerance.²⁴⁰

A facile post-hydrothermal treated process was established for pore size and volume expansion of silica-pillared clay using octadecylamine as the structural agency. The pore size and volume of the starting silica-pillared clay were greatly expanded from 1.9 nm and 0.71 cm³ g⁻¹ to 5.1 nm and 1.21 cm³ g⁻¹, respectively, and the large surface area was successfully conserved. This study provided an effective, controllable approach for pore structure expansion of the silica-pillared clay (Fig. 20).²⁴¹

Fig. 20 Preparation of silica-pillared clay. Reproduced from ref. 241 with permission from Elsevier.

A new type of nanohybrid material H₆P₂W₁₈O₆₂/nanoclinoptilolite was fabricated, and performed as an efficient and reusable catalyst in the mild and one-pot condensation of different acetophenones.²⁴² These types of zeolites contain open tetrahedral cages, generating a system of channels, the sizes of which are determined by the content of silicon. These cages are formed with a network of eight-membered and ten-membered rings. The chemical and thermal stability of CP along with its pore system diameter are the most important parameters in studying the conjunction of HPA into the nanozeolite surface.

2.3.5. Zirconia. In recent years, zirconia has attracted much attention as both a catalyst and as a catalyst support because of its high thermal stability and due to the amphoteric character of its surface hydroxyl groups. Relatively few works report on zirconia modified by HPAs as a catalyst in acid-catalyzed reactions.^243–246 The main factor that is drawing attention to the use of hydrated zirconia as a precursor of a catalyst carrier is the fact that its surface hydroxyl groups are able to undergo a chemical reaction or strong interaction with the incorporated components.^247–249

A series of mesoporous composite frameworks consisting of zirconium oxide and H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ compounds were synthesized in 2014 through a surfactant templating approach. The as-prepared materials possessed ZrO₂–H₃PW₁₂O₄₀ and ZrO₂–H₄SiW₁₂O₄₀ frameworks composed of a mesoporous structure, large internal surface area and narrow-sized mesopores. Unlike the specific surface area, the framework composition of these materials played a dominant role in the catalytic performance.²⁵⁰

A comparison of the catalytic activity of H₄SiW₁₂O₄₀ or H₃PW₁₂O₄₀ supported on ZrO₂ calcined at 750 °C and silica-supported H₄SiW₁₂O₄₀ calcined at 300 °C with similar loading showed that H₄SiW₁₂O₄₀ and H₃PW₁₂O₄₀ supported on zirconia acted as efficient and stable solid acid catalysts, whereas H₄SiW₁₂O₄₀ supported on silica was leached into the reaction medium and thus catalyzed the reaction homogeneously.²⁵¹

Starting with the impregnation of a hydrogel-derived zirconyl hydroxide [ZrO(OH)₂–CP] with H₃PW₁₂O₄₀ in solution, ZrO₂ was recently found to be superior to SiO₂ as a support material for H₃PW₁₂O₄₀ as the high-dispersion of H₃PW₁₂O₄₀ on ZrO₂ could maintain its Keggin structure at high temperatures up to 750 °C.²⁵² The use of alcogel-derived ZrO(OH)₂–AN for the support, compared with conventional hydrogel-derived ZrO(OH)₂–CP, produced H₃PW₁₂O₄₀/ZrO₂ catalysts that showed higher surface areas and better catalytic performance for the selective formation of acrolein from glycerol dehydration. ZrO(OH)₂–AN is an anti-sintering ZrO₂ nanocrystal prepared by the thermal processing of an alcogel-derived ZrO(OH)₂–AN at elevated temperature.^253–255

Sometimes, especially in some industrial processes, the reaction conditions affect the stability of the catalyst, for example, in the dehydration of glycerol. This may be due to the high acidity of the catalysts, which is thought to be responsible for coke formation and ultimately the lack of stability observed.²⁵⁶ The effect of acidity on the stability of catalysts for the dehydration of glycerol was studied by grafting zirconia onto silica.²⁵⁶ A comparison of the grafted and ungrafted catalysts showed that they appeared to differ with respect to stability, with the zirconia-grafted catalyst being more stable. This effect was attributed to a minor distortion in the structure of the HPA and its binding strength to the support, leading to a decrease in acidity and, in turn, to an increase in the stability of the catalysts.²⁵⁷ Supported HPAs catalysts are very acidic, typically with a Hammett acidity of ca. 9, and they have been used in the catalytic dehydration of glycerol.²⁵⁸ Different loadings of H₄SiW₁₂O₄₀, supported on silica or carbon, have been investigated; however, these catalysts were also found to be unstable. Zirconia-supported HPA catalysts were found to be more thermally stable, with an improved dispersion of the HPA active phase, and this was found to be a key factor in tuning the activity and selectivity towards acrolein.^257,258 There are a number of catalytic reactions reported in the literature that were catalyzed by zirconia-supported HPAs.^259–263

2.3.6. Alumina. Similar to zirconia, alumina increases the stability of the HPAs according to the high interaction of the HPAs with the surface functional groups, but sometimes these interactions are attributed to a minor distortion in the structure of the HPAs and its binding strength to the support, leading to a decrease in acidity.²⁶⁴

A bifunctional catalyst with alumina as the support was produced by Liu and co-workers in 2015. They showed that increasing the calcination temperature from 350 to 650 °C can lead to structural evolution of the supported H₄SiW₁₂O₄₀ and a subsequent activity change. A large amount of (V, Mo)O_x heteropolyoxo mixed oxides were also formed above 550 °C. However, the Keggin structure of H₄SiW₁₂O₄₀ began to dissociate around 450 °C, causing the formation of various WO_x species (Fig. 21).^265,266

Fig. 21 Effect of calcination on the structure of a Keggin unit supported on alumina. Reproduced from ref. 265 and 266 with permission from Elsevier.

Different HPAs containing W, Mo, V, Co, etc. with different hetero atoms supported on alumina were used as catalysts in a number of organic reactions in the literature.^{164,175,267–273} in these cases, it seems that the Keggin structure is sustained even after catalyst preparation and catalytic reaction.

2.3.7. Magnetic supports. Nanoparticles offer many advantages as a support due to their unique size and physical properties. Nanoparticles have also come into view as high surface area heterogeneous catalysts and catalyst supports; however, they are frequently difficult to remove from the product mixture. The incorporation of magnetic compounds, such as iron oxide, into the supports offers a solution to this problem. The strategy of magnetic separation, i.e. taking advantage of MNPs, is typically more effective than filtration or centrifugation as it avoids loss of the catalyst. The magnetic separation of MNPs is simple, economical and promising for industrial applications. Recent studies have shown that MNPs are excellent supports for HPAs. Surface functionalized magnetic iron oxide nanoparticles are a kind of novel functional material and have been widely used in catalysis. Moreover, the surface of iron oxide nanoparticles can be modified by organic or inorganic materials, such as polymers, biomolecules, silica, metals, etc.^274–282

A magnetite–titania photocatalyst was used as a support for HPAs to make it easy to recover TiO₂ powder and HPAs, which commonly require subsequent liquid–solid separation, especially when using the catalyst in water treatment. However, it was found that the magnetite–titania magnetic photocatalyst suffered from serious photodissolution during degradation.²⁸³ Researchers have tried adding a silica layer between the magnetic core and photocatalyst to prevent the magnetite core from photodissolution.^284,285 An ideal magnetic photocatalyst should possess the following features: (i) high activity in photocatalytic degradation; (ii) no photodissolution during degradation and (iii) a high magnetization for easy magnetic separation. A novel magnetic photocatalyst was prepared by grafting H₃PW₁₂O₄₀ anions onto Fe₃O₄ nanoparticles via a layer of Ag, as shown in Fig. 22.²⁸⁶

Fig. 22 Grafting of H₃PW₁₂O₄₀ onto Fe₃O₄ nanoparticles via a layer of Ag.

The structural and catalytic properties of a Mo-based HPA generated after the impregnation of MoO_x onto the surface of porous iron phosphate nanotubes were studied. Nano-sized MoO_x–FeP composites with different Mo molar loadings were prepared under acidic conditions.²⁸⁷

Rafiee and Eavani produced a type of magnetically recoverable catalyst by the immobilization of H₃PW₁₂O₄₀ on the surface of silica-encapsulated Fe₂O₃ (Fe₂O₃@SiO₂) nanoparticles. The acidity of the catalyst was measured by potentiometric titration with n-butylamine. The immobilization of H₃PW₁₂O₄₀ on Fe₂O₃@SiO₂ was considerably enhanced by the dispersion of acidic protons to 48 meq. per g catalyst. Therefore, a larger fraction of active sites are exposed to the surface, and thus the catalyst exhibited excellent activity in organic reactions, even with low catalyst loading. After the reaction, the catalyst/product separation could be easily achieved with an external magnetic field and more than 95% of the catalyst could usually be recovered. The potentiometric titration curve of the reused catalyst showed that all the acidic sites were preserved during five cycles. According to the ICP-AES results, H₃PW₁₂O₄₀ leaching was negligible (Fig. 23).²⁸⁸

Fig. 23 Preparation of H₃PW₁₂O₄₀ supported on Fe₂O₃@SiO₂.

In continuation of this work, the preparation, leaching and solid acidity measurements of H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₄SiW₁₂O₄₀ supported on silica-encapsulated Fe₂O₃ nanoparticles were investigated. The impregnating solvent and calcination temperature were optimized using various techniques. The best preparation achieved was to impregnate H₃PW₁₂O₄₀ on Fe₂O₃@SiO₂ in CH₃CN solution, followed by evaporation of the solvent and calcination at 250 °C. The catalysts were mostly spherical in shape and had an average size of approximately 92 nm. Experiments assessing the stability of the supported HPA catalysts indicated their resistance to leaching by methanol as a polar solvent. The acidity of the samples was determined by NH₃-TPD and by the chemisorption of pyridine. The strength and dispersion of the protons on H₃PW₁₂O₄₀/Fe₂O₃@SiO₂ was significantly high and active surface protons became more available for the reactant.^289–292 H₃PW₁₂O₄₀-functionalized silica-coated CoFe₂O₄ nanoparticles were also prepared by Rafiee et al. as a new magnetically separable catalyst.²⁹³

A new strategy by a two-component heterogeneous catalytic system composed of Pd–PVP–Fe and magnetically supported H₅Mo₁₀V₂O₄₀ (Fe@Si–Mo₁₀V₂) as a co-catalyst was reported for the Suzuki coupling reaction.²⁹⁴ In order to study the role of Fe@Si–Mo₁₀V₂ in the Suzuki coupling reaction, the electron transfer property of Fe@Si–Mo₁₀V₂ and Pd–PVP–Fe were investigated by means of cyclic voltammetry measurements. Moreover, this catalytic system could be recovered in a facile manner from the reaction mixture and recycled several times without any significant loss in activity. In this study, the heterogeneity of both components of our catalytic system was investigated and the content of palladium and H₅Mo₁₀V₂O₄₀ into the filtrates was evaluated quantitatively by ICP-AES. According to the obtained results, a small amount of Pd and H₅Mo₁₀V₂O₄₀ were leached.²⁹² This magnetically supported catalyst has been used for the extractive oxidative desulfurization of different model oils.²⁹⁵

Also, the synthesis and characterization of a green nanocomposite of H₃PW₁₂O₄₀ and starch-coated magnetite nanoparticles in Friedel–Crafts alkylation was reported recently (Fig. 24).²⁹⁶

Fig. 24 H₃PW1₂O₄₀ immobilized on starch-coated MNPs. Reproduced from ref. 296 with permission from Elsevier.

Starch, as a green source of carbon, was used for the preparation of a Fe₃O₄@carbon core–shell structure for the immobilization of H₃PW₁₂O_40.²²⁰

2.3.8. Mixed oxide supports. The sulfated zirconia has attracted much attention since its first report by Hino and Arata in 1979.²⁹⁷ However, a major disadvantage associated with sulfated zirconia is its rapid deactivation at high temperatures and under a reducing atmosphere due to the formation of SO_x and H₂S in the hydrodesulfurization process.²⁹⁸ Recently, some authors have reported that sulfated zirconia-based mixed oxides show more stability and enhanced acidity. Three types of mixed oxides, namely SiO₂–ZrO₂,²⁹⁹ Al₂O₃–ZrO₂ (ref. 300) and TiO₂–ZrO₂,³⁰¹ have been mainly investigated for the sulfation. The sulfated silica–zirconia mixed oxide can be considered as sulfated zirconia dispersed on silica, because the sulfate ion selectively interacts with the non-silica component of the mixed oxide.²⁹⁹ Gao et al.³⁰² reported that the addition of small amounts of alumina to sulfated zirconia enhances the activity and stability of the catalyst.

Another mixed support HPA catalyst was prepared by the wet impregnation of H₃PW₁₂O₄₀/TiO₂ nanoparticles into SBA-15 and calcined at different temperatures. The characterization results revealed that the textural properties and the acidity of the prepared catalysts could be finely controlled with the simple adjustment of the calcination temperature, and that the structure of the support, decorated with the TiO₂ and HPA nanoparticles, was intact even after the modification.³⁰³

A series of ZrO₂-based hybrid catalysts functionalized by both organosilica groups and Keggin-type HPA, H₃PW₁₂O₄₀/ZrO₂–Si(Et/Ph)Si and H₃PW₁₂O₄₀/ZrO₂–Si(Ph) with different structural orderings and pore geometries were controllably prepared by a one-pot template-assisted sol–gel co-condensation–hydrothermal treatment route. Different surface areas from 67 to 353 m² g⁻¹ were obtained by using different synthesized catalysts. The pore diameters were approximately similar but the surface acidity changed from 779 to 1972 μeq. per g by titration with NaOH.³⁰⁴

These hybrid catalysts have excellent acid catalytic activity, originating from the combination of the strong Brönsted acidity and Lewis acidity, rationally designed porosity and an enhanced surface hydrophobicity. Moreover, ordered 2D hexagonal mesostructured H₃PW₁₂O₄₀/ZrO₂–Si(Ph)Si-1.0 exhibited a much higher catalytic activity with respect to the 3D interconnected wormhole-like H₃PW₁₂O₄₀/ZrO₂–Si(Et)Si or H₃PW₁₂O₄₀/ZrO₂–Si(Ph), which was attributed to the decreased mass transfer limitations of the reactants and products owing to the ordered mesoporous structure (Fig. 25).

Fig. 25 H₃PW₁₂O₄₀/ZrO₂–Si(Ph)Si (a) and H₃PW₁₂O₄₀/ZrO₂–Si(Ph) (b) hybrid catalyst.

Mesoporous-silica–zirconia-supported H₃PW₁₂O₄₀ with a regulatable surface acidity was synthesized by an evaporation-induced self-assembly method and used as an oxidative desulfurization catalyst.³⁰⁵ Whereas, mesoporous titania–silica composite was successfully prepared by a modified sol–gel process using the less expensive silica precursor and titanium oxychloride as a titania source. H₃PW₁₂O₄₀ was impregnated on the prepared mixed support to produce an acid catalyst.³⁰⁶

In some industrial processes, the catalytic activity of these solid base catalysts are high but it seems difficult to avoid dissolution of these base catalysts. This problem causes an irreversible loss of the catalyst and pollution of the product with an inorganic residue. Using a modified supported catalyst provides the simplest solution to this problem. The catalytic activities of HPAs supported on solid acids such as cation-exchange resins,^307,308 sulfated zirconia,^309,310 W–Zr–Al–O,³¹⁰ Zn–Al–O,³¹¹ acidic zeolite^312,313 and Mn–Ti–O (ref. 314) have been reported. A sulfonated carbon material produced by the treatment of glucose with fuming sulfuric acid gave a high stability,³¹⁵ and a high activity of HPA-promoted Ta₂O₅ was also observed.^316,317

TiO₂ nanoparticles-stabilized H₃PW₁₂O₄₀ in SBA-15 were prepared, and a liquid phase hydroamination of α,β-ethylenic compounds with amines was investigated in the presence of this catalyst. The catalysts were prepared by the wet impregnation of H₃PW₁₂O₄₀/TiO₂ nanoparticles into the SBA-15 and then calcined at different temperatures (Fig. 26).³¹⁸

Fig. 26 Wet impregnation of H₃PW₁₂O₄₀/TiO₂ nanoparticles into the SBA-15. Reproduced from ref. 318 with permission from Elsevier.

H₃PW₁₂O₄₀/TiO₂–SiO₂ was synthesized by the impregnation method, which significantly improved the catalytic activity under simulated natural light.³¹⁹ TiO₂–SiO₂ was prepared by a sol–gel method, and H₃PW₁₂O₄₀/TiO₂–SiO₂ was synthesized by an impregnation method. The degradation of methyl violet was used as a probe reaction to explore the influencing factors on the photodegradation reaction.

2.4. Encapsulation

The encapsulation of HPAs into porous solid supports with a high surface area, large pore diameter and high pore volume was used as a critical method for improving their catalytic performances and recyclabilities in many heterogeneous catalytic applications. Various porous supports have been applied for the encapsulation of HPAs, including zeolites, mesoporous molecular sieves, metal organic frameworks (MOFs) and mesoporous metal oxides.

2.4.1. Zeolites. Zeolites could be ideal materials for the encapsulation of HPCs, and for blocking them in the supercages without any possibility to leave. As the polyanion cannot pass through the windows of the supercages, it is not possible to obtain encapsulated materials by adding the polyanion to the zeolite. Thus, it is necessary to synthesize this system in situ.

Mukai et al. synthesized H₃PMo₁₂O₄₀ encapsulated in a Y-type zeolite by a “ship in the bottle” synthesis, and the resulting solid catalyst was used in the esterification of acetic acid with ethanol.³²⁰ The influences of the Si/Al ratio and the type and amount of counter cations of the zeolite on the amount of encapsulation of H₃PMo₁₂O₄₀ molecules were investigated. Y-type zeolites have supercages the diameters of which are approximately 1.3 nm, and these supercages are interconnected by windows 0.74 nm in diameter. It was found that H₃PMo₁₂O₄₀ can be formed within the supercages of the support when the number of aluminium atoms per unit cell is roughly in the range of 4–20. When the number exceeds this range, the support is likely to be destroyed during catalyst synthesis, since the durability in acidic solutions is low for supports with high aluminium contents. Moreover, it was difficult to encapsulate H₃PMo₁₂O₄₀ using supports with extremely low aluminium contents. These results imply that a moderate number of cation-exchange sites induced by the aluminium atoms included within the support structure are inevitable for the encapsulation of HPA molecules. The role of the counter cations was also investigated using Na⁺, K⁺, Cs⁺, NH₄⁺, Ca²⁺ and Ba²⁺. Among these cations, only Cs⁺ and NH₄⁺ led to a significant amount of H₃PMo₁₂O₄₀ encapsulation. When supports with other cations were used, it was impossible or at least difficult to encapsulate H₃PMo₁₂O₄₀. The differences among the cations may be attributed to the differences in their abilities to promote the formation of polyanions from MoO₄²⁻. When cations such as K⁺ and Na⁺ coexist, the formation of polyanions is not observed. Also in the case of divalent cations, MoO₄²⁻ anions were hardly detected. From these results, it is obvious that cations such as Cs⁺ and NH₄⁺ have a higher ability to promote polyanion formation than other cations. The amount of polyanion on the support varied from 0.17 g per g of support after the first washing to 0.08 g after the fifth washing, showing that some H₃PMo₁₂O₄₀ was lost, probably by decomposition–redeposition phenomena in hot water.

The influences of temperature and solvent addition during synthesis on the amount of HPA encaging and the stability of the resulting encaged catalyst were also investigated by Mukai's group.³²¹ It was found that by adjusting the reaction temperature, the stability of the resulting encaged catalyst could be enhanced to higher levels. Moreover, it was found that by adding t-butyl alcohol to the solution used for the catalyst synthesis, an active and stable encaged catalyst could be prepared, even at low synthesis temperatures. A further improvement of this method was observed by treatment of the modified zeolite with caesium carbonate (Cs–Y zeolite), leading to partially Cs-exchanged salts, which were more stable towards leaching and showed considerable catalytic activity and excellent reusability in the esterification reaction.³²²

Under microwave irradiation, H₃PW₁₂O₄₀ could be successfully synthesized in situ and encapsulated in the supercage of the ultra-stable Y (USY) zeolite in several minutes.³²³ XPS spectra of the sample confirmed that phosphorous and tungsten in the matrix of USY were combined into Keggin anions. These formed H₃PW₁₂O₄₀ could be located in the supercage of the USY (about 13–15 Å in diameter) as the size of a spherical H₃PW₁₂O₄₀ molecule is about 11–15 Å (diameter). H₃PW₁₂O₄₀ molecules encapsulated in the supercages of USY cannot escape from its smaller apertures of 7.4 Å diameters. Based on the ICP results, the calculated amount of formed H₃PW₁₂O₄₀ encapsulated in USY is about 5.6 wt% of the parent USY. This catalyst exhibited high activity in the synthesis of 4,4′-dimethyldiphenylmethane via toluene and formaldehyde and could be utilized as a solid acid catalyst in aqueous solutions.³²³

2.4.2. Mesoporous molecular sieves. Mesoporous molecular sieves, which have a significantly large surface area (∼1000 m² g⁻¹), a large pore size and uniform pore distribution, are another material used for the encapsulation of HPCs. There are several ways that HPCs could be entrapped in the pores of the support. One possible route is the direct impregnation of the support with a HPA solution, followed by evaporation of the solvent. The incorporation of HPA into the mesopores can also be achieved via incipient wetness impregnation. However, a weak interaction between HPA and the support results in its leaching in polar media, which was described in the previous section.

Khanchi et al. prepared an HPA–Zr-modified mesoporous silica composite by an incipient wetness method.³²⁴ This composite were used for the removal of Sm(III) and Dy(III) ions in aqueous solutions. The adsorption capacity of the composite after four cycles indicated a loss in the adsorption capacity of 7.4% for Sm(III) and 8.2% for Dy(III) metal ions compared to the initial cycle.

The incorporation of HPCs into the pores of a mesoporous material can also be achieved by encapsulating HPCs during the synthesis of the silica material itself. Yang et al. synthesized H₃PW₁₂O₄₀ encapsulated in SBA-15 by adding HPA to the SBA-15 synthesis mixture using a sol–gel technique.³²⁵ This catalyst was tested in the esterification of acetic acid and showed a higher leaching stability compared with the same impregnated catalyst sample. Toufaily et al. published a similar approach to incorporate H₃PW₁₂O₄₀ into MSU.³²⁶

Shi et al. obtained SBA-15–encapsulated HPA catalysts by adding P and W (or Mo) sources into the initial sol–gel system during the hydrolysis of tetraethyl orthosilicate to form the Keggin-type HPA in situ.^327,328 However, in some cases the HPA loadings were relatively low since higher loadings resulted in a lower catalytic activity due to HPA agglomeration. These catalysts have potential applications in the chemical industry due to their recyclability and reusability.

Martens et al. prepared SBA-15 containing up to 40 wt% of H₃PW₁₂O₄₀ via an original direct synthesis method.³²⁹ This direct synthesis route was inspired by the proposed supramolecular mechanism of SBA-15 formation. The SBA-15 synthesis is considered to follow a S⁰H⁺X⁻I⁺ mechanism, in which S⁰, H⁺, X⁻ and I⁺ stand for the tri-block copolymer (EO₂₀PO₇₀EO₂₀), hydronium cations, anions (Cl⁻) and the positively charged silica source at pH < 2, respectively.

The HPAs were presented at the interface between the silica and polymer. Washing and drying steps did not remove the HPAs since they are trapped inside the SBA-15 particles. In the calcination step, the polymer template decomposes and is eliminated from the pore system but the H₃PW₁₂O₄₀ molecules stay fixed onto the pore walls. The original direct synthesis method were also used for the synthesis of MCM-41 incorporated with transition metal-substituted HPAs (PW₁₁O₃₉M₁⁵⁻, M₁ = Ni²⁺, Co²⁺ or Cu²⁺)³³⁰ The samples exhibited excellent catalytic performance for the esterification of n-butanol with acetic acid, which was better than that of the supported lacunary POM PW₁₁O₃₉⁷⁻ and the impregnated samples.

Chen et al. used a one-pot hydrothermal method for the preparation of SBA-15-encapsulated H₃PW₁₂O₄₀.³³¹ This catalyst showed the highest photocatalytic efficiency for the degradation of rhodamine-B and also the highest microwave catalytic efficiency in the synthesis of isoamyl acetate. The reusability of the catalyst was also investigated. It was found that 20 wt% of H₃PW₁₂O₄₀/SBA-15 showed excellent reusability after three cycles, but 40 wt% of H₃PW₁₂O₄₀/SBA-15 was not reusable.

In another way, the support surfaces were first functionalized with an example amine ligand and then treated with HPAs, which is categorized as a tethering method (discussed in Section 2.5).

2.4.3. Metal–organic frameworks (MOFs). MOFs are crystalline materials whose porous structure is defined by isolated metal ions or metal clusters interconnected by bi- or tripodal rigid organic ligands. MOFs can be compared with zeolites but these have larger cavities and channels. This led to an increasing interest in these materials in various domains and more specifically in heterogeneous catalysis.

One possible route for the encapsulation of HPAs onto a MOF framework is the introduction of HPA during the synthesis, i.e. in an in situ production. A lot of new structures could be obtained this way, depending on the type of HPA, metal ion and organic linker. However, in some cases, the obtained catalysts are not reusable or thermally stable.

Liu et al. reported the syntheses of a number of remarkable crystalline compounds [Cu₂(BTC)_4/3(H₂O)₂]₆[H_nXM₁₂O₄₀]·(C₄H₁₂N)₂ (X = Si, Ge, P, As; M = W, Mo) the using simple one-step hydrothermal reaction of copper nitrate, benzentricaboxylate (BTC) and different Keggin HPAs.³³² In these compounds, the catalytically active HPAs were alternately arrayed as non-coordinating guests in the cages of a MOF host matrix. This system is stable until ca. 250 °C and no HPA leaching or framework decomposition was observed in this study. The catalytic activity of a compound containing the H₃PW₁₂O₄₀ species was examined through the hydrolysis of esters in excess water, which showed a high catalytic activity and that the catalyst could be used repeatedly without activity loss.

An original synthesis approach to prepare Cu₃(BTC)₂ (BTC = benzene tricarboxylic acid)-encapsulated H₃PW₁₂O₄₀ involving mixing the reagents at room temperature, followed by quenching in liquid nitrogen and freeze drying was reported by Martens' group.³³³ The sample was isostructural in the HKUST-1 framework, accommodating H₃PW₁₂O₄₀ ions in the cavities. The catalytic properties of this catalyst were assessed using a model esterification reaction of acetic acid with 1-propanol. The catalyst was partially dissolved in the presence of acetic acid. At a high molar ratio of acetic acid to 1-propanol (1 : 2), a leaching of Cu²⁺ and H₃PW₁₂O₄₀ was observed. However, at a low molar ratio of acetic acid to 1-propanol (1 : 40), the leaching of Cu²⁺ and H₃PW₁₂O₄₀ could be prevented and the catalyst was stable. However, the BET surface area and catalytic activity was significantly decreased after several cycles.³³³

Rafiee and Nobakht reported the use of Cu₃(BTC)₂-encapsulated Keggin-type HPAs (including H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₄SiW₁₂O₄₀) for the selective oxidation of sulfides and for the deep desulfurization of model fuels.^334,335 It was observed that the catalytic system could be recycled at least four times with little decrease in catalytic activity.

Hill et al. used a combination of Keggin-type [CuPW₁₁O₃₉]⁵⁻ and MOF-199 in air-based oxidations.³³⁶ This sample catalyzed the rapid chemo- and shape-selective oxidation of thiols to disulfides and, more significantly, the rapid and sustained removal of toxic H₂S via the following reaction:

H₂S + ½O₂ (air) → ⅛S₈ + H₂O

The hydrolytic stability of the catalyst was greater than that of either the MOF or the HPA alone. The strong HPA–MOF interactions dramatically increased the rate of the air-based oxidations catalyzed by the HPA guest.³³⁶

H₆PMo₉V₃@Cu₃(BTC)₂ was used as catalyst in the liquid hydroxylation of benzene with O₂.³³⁷ In only 20 min, a high TOF (44.2 h⁻¹) was obtained. No significant decrease in phenol yield was observed for the recycling experiments. During the reaction, the liberation of a fraction of HPAs could lead to a slight deactivation of the system. From the powder XRD patterns, it was found that the crystallinity of the recovered catalyst was changed a little in the three recycles.³³⁷

Gao et al. presented a new hybrid material, i.e. HPA@MOF@SBA-15, in which metal–organic MOF-199-encapsulated HPA catalyst was facilely confined in mesoporous SBA-15.³³⁸ The organic bridged inside liner was stabilized by coating with an outer inorganic rigid SBA-15, allowing the catalyst to exhibit exceptional stabilities in the liquid oxidation reaction. These results confirmed a new method to stabilize the functionalized MOF structure via encapsulating it in SBA-15, which is also a novel strategy for synthesizing micropore mesoporous hybrid materials.

Duan et al. reported the preparation of heterogeneous asymmetric catalysts by incorporating the [BW₁₂O₄₀]⁵⁻ and the chiral group, i.e. L- or D-pyrrolidin-2-ylimidazole (PYI), into one single framework (Fig. 27).³³⁹ This catalyst was used in the asymmetric dihydroxylation of aryl olefins. Excellent stereoselectivity was observed due to the coexistence of both the chiral directors and the oxidants within a confined space. The catalysts could be reused at least three times with only a moderate loss of activity (from 75% to 68% of the yield) and a slight decrease in the selectivity (ee values from >95% to90%).

Fig. 27 Asymmetric POM–MOF catalyst.³³⁹ Reprinted with permission from J. Am. Chem. Soc., 2013, 135, 10186. Copyright © 2013 American Chemical Society.

Recently, a new strategy was proposed to construct the IL, HPA and MOF composite.³⁴⁰ The [SO₃H–(CH₂)₃–HIM]₃PW₁₂O₄₀ was encapsulated within the framework of MIL-100(Fe). It demonstrated a significant increase in acidity and had both Brönsted and Lewis acid sites, compared with the HPA-based MOF. This composite was used as a catalyst for the esterification of oleic acid with ethanol. The catalyst could be easily recovered and reused six times, with no obvious leaching of the active component.

The encapsulation of HPA into the pores of MOF can also be achieved by its adsorption on a preformed MOF. This strategy was developed after the discovery of MIL-101 by Férey and co-workers.³⁴¹ The zeotype cavities of two different sizes, the fully accessible porosity and therefore the outstanding sorption properties, together with a high thermal and chemical stability, make MIL-101 an excellent candidate for supporting HPA species. Various groups have reported the encapsulation of Keggin- and Dawson-type HPAs onto MIL-101 and the use of the resulting materials in heterogeneous catalysis.^342–352

The sizes of the Keggin HPAs (13–14 Å diameters) are smaller than the pentagonal window openings of the large and medium-sized cages of MIL-101, and also bigger than the hexagonal window openings of the large cages. In the MIL-101 encapsulated HPA composites, the HPA guests are only encapsulated into the large cavities using the impregnation approach. Thus, the medium-sized cages, which represent 2/3 of the total number of cavities of MIL-101, remain unfilled (Fig. 28).

Fig. 28 Schematic of the MIL-101 cavities.

It is obvious that the encapsulation of such active species into the medium cavities would offer many advantages, like a better dispersion and utilization of the support. Moreover, if contained in the medium cavities, leaching of the HPAs should never be a problem, since they are bigger than the windows of these cavities. In order to achieve highly dispersed and reusable catalysts, the successful one-pot encapsulation of HPAs in the large and medium mesoporous cavities of MIL-101 (also known as ‘‘bottle around the ship’’ or ‘‘templated synthesis’’) have been explored by various research groups^.353,354 It has been shown that these materials are stable to HPA leaching and can be used repeatedly without any considerable loss of activity or selectivity.

Recently, Ji et al. reported the Friedel–Crafts acylation of anisole with benzoyl chloride over H₃PW₁₂O₄₀ encapsulated into a zeolite imidazolate framework.³⁵⁵ This novel subclass of MOFs has the advantages of both zeolite and conventional MOFs. It was found that 14.6–31.7 wt% of H₃PW₁₂O₄₀ were encapsulated into the zeolite imidazolate framework. This catalyst was recycled and reused five times without any appreciable loss of activity or selectivity.

Wells–Dawson POM-based MOFs were prepared recently by two research groups.^356,357 These catalysts showed remarkable photocatalytic and electrocatalytic activities in aqueous solutions and could be reused several times.

2.4.4. Mesoporous metal oxides. Another possibility pursued recently is the immobilization of HPAs into the metal oxide matrix by means of a sol–gel technique. During hydrolysis of the source, subsequent slow dehydrolysis and calcinations, the HPA clusters were entrapped into the mesoporous network, resulting in an HPA/support composite with microporous structures.

Peng et al. reported the encapsulation of Cs salt of HPAs, Cs_xH_3−xPMo₁₂O₄₀ and Cs_yH_5−yPMo₁₀V₂O₄₀ in silica and the use of the resulting materials in the liquid phase oxidation of benzyl alcohol to benzaldehyde.³⁵⁸ The catalytic activities were strongly dependent on the H⁺/Cs⁺ ratio in the HPAs. The best results were obtained for a value of one, which indicated that not only oxidative properties but also acidity was necessary to achieve a good catalyst. The catalyst could be recycled up to five times without loss of activity, and no leaching of the HPA was observed in the solution.

Wang et al. described the synthesis of XW₁₁O_n−39 (X = P, Si, Ge, B) encapsulated in silica.^359–361 These materials were used in the photocatalytic degradation of malic and formic acids and displayed interesting properties, with no leaching of the HPAs.

Mesoporous-silica-pillared clay materials with different contents of H₃PW₁₂O₄₀ were synthesized by introducing HPA into a clay interlayer template in an acidic suspension using a sol–gel method.³⁶² The catalysts showed a homogeneous dispersion of the H₃PW₁₂O₄₀ molecules, even at 25 wt% loading. The encapsulated materials exhibited better catalytic performance than the impregnated samples in the oxidative desulfurization of dibenzothiophene-containing model oil.

Dou and Zeng reported the targeted synthesis of H₄SiMo₁₂O₄₀ within mesoporous silica hollow spheres.³⁶³ The mesoporous shell was formed by removal of the organic template using thermal treatment of as-synthesized MoO₂@SiO₂ core–shell spheres. The encapsulated MoO₂ was oxidized to Mo⁶⁺ and infused into the mesoporous silica shells, forming heptamolybdate species (Mo₇O₂₄⁶⁻) uniformly dispersed on the mesopore surfaces of silica. After hydration with water, H₄SiMo₁₂O₄₀ was formed by reaction between the surface Mo₇O₂₄⁶⁻ and silica species. The prepared H₄SiMo₁₂O₄₀@mSiO₂ hollow spheres were tested for the Friedel–Crafts alkylation of toluene by benzyl alcohol. This catalyst showed excellent catalytic activity and could be reused after regeneration.

Okada et al. developed a highly active and water-tolerant solid acid catalyst by encapsulating HPA into hydrophobic hollow polymethylsiloxane microspherical particles.³⁶⁴ H₃PW₁₂O₄₀ and Cs_2.5H_0.5PW₁₂O₄₀ were used for this purpose. Their synthesis approach was based on the deposition of an organosilica shell on water droplets in a water-in-oil (W/O) emulsion through a sol–gel reaction of alkylsilyl trichlorides. Droplets of an aqueous solution of H₃PW₁₂O₄₀ or Cs_2.5H_0.5PW₁₂O₄₀ suspension were dispersed in viscous liquid paraffin to create a W/O emulsion. Methyltrichlorosilane was subsequently hydrolyzed on the droplets to form Cs_2.5H_0.5PW₁₂O₄₀–polymethylsilane microcapsules. As a result of water vapourization, fine particles of HPCs were successfully immobilized into the capsules. These catalysts were active in the hydrolysis of ethyl acetate in a large excess of water. The activity of the Cs_2.5H_0.5PW₁₂O₄₀-encapsulated catalyst was maintained, and it showed low leaching. The polymethylsiloxane shell plays several roles, including the shielding of Cs_2.5H_0.5PW₁₂O₄₀ and allowing penetration of the reactants (ethyl acetate and water) and products (ethanol and acetic acid) through the microporous membrane.

Titania-encapsulated HPAs were also synthesized with a high surface area and interesting photocatalytic properties. Titanium tetraisopropoxide was used as the TiO₂ source.^365,366

Guo et al. developed the synthesis of ordered mesoporous PW–TiO₂ composites by a block copolymer surfactant-assisted templating route.^367–369 These solids were used for the photocatalytic degradation of various substrates, such as diethyl phthalate, methyl orange, crystal violet, rhodamine or methylene blue.

Pizzio et al. described the synthesis of HPA–TiO₂ mesoporous nanocomposites by a method derived from that of Guo et al., but with urea as the pore forming agent.^370–373 They suggested that the interaction between HPA and TiO₂ was probably ionic before calcination and then became covalent after calcination at higher temperatures.

Guo et al. reported two efficient heterogeneous photocatalysts, i.e. H₃PW₁₂O₄₀/Ta₂O₅ and H₆P₂W₁₈O₆₂/Ta₂O₅, with an amorphous phase, microporosities or micro-mesoporosities, a narrow band gap and nanometre sizes.³⁷⁴ These catalysts were prepared by a one-step sol–gel process following hydrothermal treatment at a constant heating rate. H₃PW₁₂O₄₀/Ta₂O₅ and H₆P₂W₁₈O₆₂/Ta₂O₅ were successfully used in the visible-light photocatalytic degradation of salicylic acid and rhodamine-B. These catalysts were easily separated and recovered, and little deactivation of the catalysts was observed after three catalytic cycles. Guo's group also synthesized the H₃PW₁₂O₄₀/Ta₂O₅ catalyst with the addition of a tri-block copolymer surfactant as the template.³⁷⁵ The catalytic performance of the sample was evaluated in the esterification of lauric acid and myristic acid and in the transesterification of tripalmitin as well as in the direct use of soybean oil for biodiesel production. Regardless of the presence of free fatty acids, the H₃PW₁₂O₄₀/Ta₂O₅ composite showed high reactivity and selectivity towards simultaneous esterification and transesterification under mild conditions. The catalyst could be recovered, reactivated and reused several times.³⁷⁵

Up to now, there have only been a few reports of the synthesis of HPA entrapped within ZrO₂. In them, zirconium n-butoxide was used as the ZrO₂ source and the samples were used for photocatalysis.^376–380 As with the other metal oxides, HPA@ZrO₂ systems are fully recyclable.

Armatas et al. described the synthesis and catalytic activity of HPAs entrapped within mesoporous Co₃O₄ (ref. 381) and Cr₂O₃.³⁸² Co₃O₄/H₃PW₁₂O₄₀ composites were synthesized using the hard templating method. Cubic mesoporous KIT-6 silica was used as a template. A series of mesoporous materials with different HPA loading was obtained after removing the silica by treatment with HF. The products had a 3D cubic structure with a large internal surface area and narrow distribution of pore sizes, and exhibited outstanding catalytic activity in the direct decomposition of N₂O.³⁸¹

Mesoporous nanocomposite frameworks of Cr₂O₃ and H₃PMo₁₂O₄₀ compounds were prepared via a “nanocasting” method, using mesoporous silica SBA-15 as the template. They demonstrated for the first time, the feasibility of the proposed synthesis method to achieve a high loading of HPA clusters in mesoporous metal oxide frameworks. These materials, with up to 63 wt% of HPA content, retained the crystal structure of the silica template. The composites also showed excellent catalytic activity and stability in the oxidation of 1-phenylethanol using H₂O₂ as the oxidant.³⁸²

2.5. Tethering

In the tethering method, HPA is attached to the support surface via a spacer species. Amino-functionalization is one of the most commonly used modifications, and is often realized by using silane coupling agents, with amino-alkyl silane derivatives used in the tethering method. The interaction between the NH₂ groups in the silanes and the HPA molecules has been demonstrated to be based on first the acid–base interaction between the –NH₂ groups and HPA molecules, and second the interaction between the NH₂ groups and the interior water molecules inside the HPA:³⁸³

Nowadays, the functionalization of mesoporous silica has started to attract particular interest, mainly due to potential applications of these materials in catalysis, drug delivery and adsorption.^384–387 Mesoporous supports became more versatile after the development of procedures to functionalize the surfaces or framework and could be elevated to the next level of utility by surface functionalization. This effort can help fine tune the interactions with guest molecules and optimize the bulk and interfacial properties of the materials. As an example, the outer mesoporous silica shell of the core–shell magnetic mesoporous silica nanocomposite makes it possible for multifunctional surface modification. 3-Aminopropy(l)triethoxysilane (APTES) is generally used to generate terminal amino groups (ANH₂) and form covalent bonds with functional groups on the surface of the modified material. Up to now, much work has been done to incorporate amino groups on the surface of mesoporous silicas, such as HSM, MCM-41, SBA-15 and so forth.^388–392 In 2013, Zhao and co-workers successfully anchored H₃PW₁₂O₄₀ to the surface of amino-functionalized Fe₃O₄@SiO₂@meso-SiO₂ microspheres by means of chemical bonding to the aminosilane groups, with an aim to remove unwanted organic compounds from aqueous media.³⁹³

Transition metal-substituted POM of the type [M^II(H₂O)PW₁₁O₃₉]⁵⁻ (M = Co, Zn) and [SiW₉O₃₇{Co^II(H₂O)}₃]¹⁰⁻ have been chemically anchored to modified macroporous (400 nm pores), mesoporous (2.8 nm pores) and amorphous silica surfaces by B. J. S. Johnson (Fig. 29).³⁹⁴

Fig. 29 Transition metal-substituted POM anchored to modified silica.

Two types of POM-functionalized magnetic nanoparticles catalysts, consisting of H₃PW₁₂O₄₀ supported on surface-modified Fe₃O₄ MNPs, were prepared using an easy synthetic route (Fig. 30).³⁹⁵

Fig. 30 Synthesis pathway for the preparation of H₃PW₁₂O₄₀ supported on surface-modified Fe₃O₄ MNPs.

It has been shown that the presence of water in the atmosphere is necessary for the hydrolysis of silane molecules and is also essential for silanization modification of the silica particles. Efficient silanization modification has enabled the high loading ratio of HPA onto the surface of the silica.³⁹⁶

Palygorskite (also known as attapulgite), a hydrated magnesium aluminium silicate with the theoretical formula of (Mg,Al)₄(Si)₈(O,OH,H₂O)₂₆·nH₂O commonly has a lath or fibrous morphology and is rich in reactive surface OH groups. It is characterized by its parallel pores.^397–401 The above-mentioned spacer arm methodology has also been attempted in modifying palygorskite, in particular, with APTES (Fig. 31).^402–406

Fig. 31 Modification of palygorskite for H₃PW₁₂O₄₀ immobilization.

Different kinds of magnetic POMs were obtained by a simple sonication between functionalized magnetic nanoparticles and POM. These materials could be used not only as a highly active acid catalyst, but also as a catalyst support for chiral amines, with one of them presented in Fig. 32 as an example.⁴⁰⁷

Fig. 32 Magnetic nanoparticle functionalized by a chiral amine. Reproduced from ref. 407 with permission from the Royal Society of Chemistry.

One heterogenization technique is known as supported liquid-aqueous phase catalysis, wherein catalysts, dissolved in solvents, are strongly adsorbed onto supports, such as silica. In such systems, considering the liquid state of the catalyst-containing phase, contact with a reactant in a fluid phase is maximized. Typically, hydrophilic solvents, such as water, ethylene glycol and even polyethylene oxide, have been used. In a new system, a solvent, polyethylene and/or propylene oxide was covalently anchored to a silica surface. This tethering of the polyethylene and/or propylene oxide onto the silica supports has the perceived additional advantages of enabling the anchoring of both hydrophilic and hydrophobic solvents, and of preventing leaching of the solvent phase and the associated catalyst from the heterogeneous support.⁴⁰⁸

HPAs could also be supported on these functionalized surface area, as shown in Fig. 33.

Fig. 33 Imobilization of HPAs in tethered silica. Reproduced from ref. 408 with permission from Elsevier.

A new inorganic–organic nanohybrid material H₄SiW₁₂O₄₀/pyridino-MCM-41 was prepared, and performed as an efficient, eco-friendly and highly recyclable catalyst for the one-pot multi-component synthesis of different substituted 1-amidoalkyl-2-naphthols under solvent-free conditions. The nanohybrid catalyst was prepared through electrostatic anchoring of Keggin H₄SiW₁₂O₄₀ on the surface of MCM-41 nanoparticles modified by N-[3-(triethoxysilyl)propyl]isonicotinamide.⁴⁰⁹

3-Mercaptopropyl-triethoxysilane, with a terminal thiol group (–SH), a grafting method in which the hydroxyl group of the mesoporous silica reacts with the organic silane to form a functional group on the surface layer of mesoporous silica by covalent bonding, is one of the most promising methods for the surface functionalization of mesoporous silica. Mesostructured cellular foam (MCF) silica was prepared via a surfactant templating method. The MCF silica was then modified by grafting APTES to create a positive charge on the surface, and thus, to provide sites for the immobilization of H₃PMo₁₂O₄₀ (Fig. 34).⁴¹⁰

Fig. 34 Surface modification of MCF silica for H₃PMo₁₂O₄₀ immobilization.

2.6. Grafting

Grafting refers to the covalent attachment of HPA to the support surface. Although, this method is a very promising approach for the heterogenization of HPA catalysts, only a few examples of covalent linking have been reported so far. This is due to the fact that achieving covalent immobilization requires carefully designed anchors and a surface modification method to simplify the preparation steps to avoid high costs and to extend the large-scale application potential.

Errington's group reported the use of mono-functional alkoxide HPA derivatives, [(RO)MW₅O₁₈]ⁿ⁻ (M = Ti, Zr, Nb), for the covalent attachment of TiW₅O₁₈⁴⁻ to alkanol-derivatized silicon surfaces using a stepwise method (Fig. 35).⁴¹¹ Here, TiW₅O₁₈⁴⁻ clusters were attached to Si through covalent Ti–O–C bonds by alcoholysis of the MeO–Ti bond with a preassembled alkanol monolayer on silicon surfaces.

Fig. 35 Covalent attachment of TiW₅O₁₈⁴⁻ to alkanol-derivatized silicon surfaces.

Rao et al. synthesized a zirconium phosphate supported HPA catalyst by a surface grafting method (Fig. 36).⁴¹² A series of HPA catalysts with different W loadings (1–25 wt%) were prepared and successfully used in the esterification of palmitic acid with methanol. These catalysts were resistant to leaching of the active HPA, were readily recovered and could be recycled without major activity loss.

Fig. 36 Possible interaction of an in situ constructed Keggin cage with the surface Zr–PO₄.

Yang et al. reported novel HPA-functionalized mesoporous hybrid silica by the formation of Si–O–W covalent bonds.⁴¹³ A hexagonal mesostructure was synthesized directly with TEOS and Keggin-type SiW₁₁O₃₉⁸⁻ (SiW₁₁) as precursors in the presence of EO₂₀PO₇₀EO₂₀ (P123) as a block copolymer. SiW₁₁ reacts with TEOS to yield free SiW₁₁Si₂ molecules at step (I); then SiW₁₁Si₂ penetrates into the preorganized silica framework and reacts with the framework by condensation between SiOH groups on both surfaces to block the terminal of silica at step (II). Thus, perfect Keggin units can be covalently linked on the walls of mesoporous silica after removing the template (Fig. 37). The HPA concentration in the initial mixture, the prehydrolysis time of TEOS and the temperature for ageing are important factors influencing the composition and structure of the hybrid material.

Fig. 37 Incorporation of SiW₁₁ and bonding with the framework.

Tour et al. reported the direct covalent grafting of HPA onto silica surfaces using diazonium chemistry.⁴¹⁴ A diazonium salt-derived organoimido hexamolybdate was synthesized and covalently immobilized on silica surfaces to form both monolayers and multilayers.

Recent progress in “surface organometallic chemistry” has opened a new route to HPC-based materials.^415,416 These strategies constitute an alternative method for the covalent grafting compared to the classical immobilization techniques of HPAs involving sol–gel and classical impregnation techniques. The principle consists in using: (i) a partially dehydroxylated silica support bearing the desired functionality and (ii) the appropriate dehydrated HPA corresponding to this functionality. Thus, it is possible to carefully control the distribution of HPA units on the silica support and to prevent its leaching in polar solvents. Lefebvre et al. used this strategy with dehydrated H₄PVMo₁₁O₄₀ on silica modified by silane surface groups.⁴¹⁵ The use of silica modified by SiH surface groups led indeed to HPA covalently linked to the oxide support. Moreover, the use of partially dehydroxylated silica led to control of the HPA loading. Therefore, this approach is a useful way to obtain well-defined and well-distributed HPA surface species and control of the HPA surface interaction.⁴¹⁷

The heterogenization of HPA catalysts by direct covalent immobilization in polymer matrices has also been reported. Wang et al. reported a simple and effective strategy for covalently immobilizing Wells–Dawson-type HPCs onto a solid macroporous resin.⁴¹⁸ The channel surface of the resin was post-functionalized with alkynyl groups. Then, organic-modified HPA clusters were “clicked” onto the functionalized channel surface of the resin to efficiently generate a solid catalyst (Fig. 38). The catalytic performance of this solid was examined in tetrahydrothiophene oxidation. The solid catalyst was found to be efficient and had a high selectivity. Moreover, it could be reused several times without detectable catalytic activity loss.

Fig. 38 Schematic of the procedure for the covalent immobilization of HPA clusters.

Yin and Liu used a new strategy based on the emulsion polymerization technique to synthesize HPA-latex material. A trivacant Keggin-type HPA, i.e. Na₉PW₉O₃₄, was functionalized with four short organic tails with methyl methacrylate (MMA) as the ending groups. A target hybrid molecule, K₃PW₉O₃₇(SiC₇H₁₁O₂)₄, was used as the surfactant in a typical emulsion polymerization reaction of styrene. Due to the copolymerization of MMA and styrene, the HPA clusters were chemically bound onto the surface of the cross-linked polystyrene latex particles because of the multiple covalent linkages after the polymerization reaction. The obtained latex demonstrated a typical core–shell structure with a polystyrene core and an HPAs close-packed shell.⁴¹⁹ The HPA-latex could be re-collected from the reaction solution through centrifugation or other convenient technologies. The system could be used for the purpose of “quasi-homogeneous catalysis”, which is a promising method for bridging homogeneous and heterogeneous catalysis.

3. Comparisons and the prospects of the heterogenization methods

A number of reasons justify the need for heterogenization of HPCs, one major example being for recovery and recycling of the catalyst. A correlation diagram between the various heterogenization methods and recycling of the HPC catalysts is shown in Fig. 1 and discussed in Section 2. However, as can be seen in Sections 2.1–2.6, numerous examples of heterogenized HPCs have been reported in the literature, and the viable options are too wide to allow the proposal of a comprehensive set of rules. It is thus suggested that the best heterogenization method must be selected for each individual case, considering the structure of the catalyst, the types of reactions, the method of catalyst recovery and recycling, the physical and chemical properties of the products and if any there is by-product, etc. Still, some general considerations can be offered by applying common sense and from considering the literature results.

In a general view, the heterogenization method could be related with the interaction between the HPCs and the supports. Therefore, there are two typical kinds of heterogenized HPCs: supported HPCs (dispersion, encapsulation, tethering and grafting methods) and HPC salts (precipitation and hybridization methods). However, several matters should be considered when choosing the best methods for the heterogenization of HPCs.

In supported HPCs, the choice of the support itself is likely the most relevant. For many catalytic applications, the dispersion of HPC on a high surface area carrier is desirable. The support can either play only a mechanical role or it can be used to modify the catalytic properties of the HPC deposited on the surface by favouring the growth of certain structures or by inducing different types of interaction.

In the dispersion method, the catalytic activity and leaching stability of HPC are affected by its interaction with the support surface and monolayer coverage capacity of the support. Rafiee et al. compared the acidity, activity, leaching and recyclability of H₃PW₁₂O₄₀/aerosol silica and H₃PW₁₂O₄₀/Fe₂O₃@SiO₂ catalysts in the synthesis of biologically active compounds in water.²⁹¹ With similar POM loading, the H₃PW₁₂O₄₀/Fe₂O₃@SiO₂ catalyst showed a higher dispersion of acid sites and a higher catalytic activity compared with H₃PW₁₂O₄₀/aerosol silica. This is due to the high monolayer coverage capacity of the Fe₂O₃@SiO₂ support. H₃PW₁₂O₄₀/aerosol silica showed poor reusability, and a significant loss of catalytic activity was observed after the first run. However, the H₃PW₁₂O₄₀/Fe₂O₃@SiO₂ catalyst could be recovered and reused four times. About 27.3% of the initial POM content though leached into the reaction mixture during four successive runs but the yield of the product was not substantially changed (10% difference). Kholdeeva et al. reported the immobilization of several redox active HPCs on two commercial active carbons: microporous L2701 and a mesoporous Sibunit. The interaction between HPC and carbon appeared to be so strong that even after washing with hot ethanol or acetic acid, the HPC loading remained unchanged. Depending on the nature of the HPC and carbon, irreversible immobilization of 7–17 wt% of HPC occurred.⁴²⁰ However, the activity of carbon-supported HPCs was lower than that of silica-supported HPCs in α-pinene and isobutyraldehyde co-oxidation.

Generally, despite a few cases, in the dispersion method, the support does not contain specific anchoring sites for dispersed HPC particles thus, the leaching of HPC species usually occurs into polar media.

Encapsulation of the HPCs into a porous matrix is one way to increase the leaching stability. But the catalysts tolerance for structurally different substrates is affected by the encapsulation, since the reagents may not be free to interact with the HPC into the pores. For example, Liu et al. synthesized [Cu₁₂(BTC)₈][H₃PW₁₂O₄₀] as a highly stable crystalline catalyst. This catalyst was used in the hydrolysis of ester in excess water.³³² It was found that the catalytic activity and selectivity depended on the size and accessibility of the substrates for the surface pores. The conversion of methyl acetate and ethyl acetate molecules with dimensions of 4.87 × 3.08 Å and 6.11 × 3.11 Å, respectively, reached ∼64% after 5 h. In contrast, the conversion of ethyl benzoate with molecular dimensions of 8.96 × 4.65 Å was reduced to below 20% under similar conditions. The conversion of 4-methyl-phenyl propionate, a larger ester with dimensions of 10.61 × 4.04 Å, was still below 1% after 24 h.

The immobilization of POMs on specially modified supports via the formation of a chemical bond (tethering and grafting methods) is another way to improve the leaching stability of the catalyst. Mahjoub et al. compared the activity and stability of H₃PW₁₂O₄₀ supported on SBA-15 (SBA/W), H₃PW₁₂O₄₀ and the hexamethylphosphoramide (HMPA) hybrid, H₃PW₁₂O₄₀. n[C₆H₁₈N₃OP]NH₂, supported on SBA-15 (SBA/HMPAW) and amine, functionalized SBA-15 ([SBA/NH₂]/HMPAW) in the epoxidation of olefins in CH₂Cl₂ as the solvent.⁴²¹ In the SBA/HMPAW catalyst, the acidic protons of the H₃PW₁₂O₄₀ were engaged in hydrogen bonding with the HMPA. Hence, the acidity and catalytic activity were reduced significantly compared with SBA/W. In the ([SBA/NH₂]/HMPAW) catalyst, amine functionalization improved the sorption properties and guest incorporation within the SBA-15 mesoporous host compared to the non-functionalized SBA-15. Thus, a higher epoxides yield is achieved with the amine functionalized systems ([SBA/NH₂]/HMPAW). Despite the lower catalytic activity, the [SBA/NH₂]/HMPAW catalyst was heterogeneous and reusable and could be recovered and used for at least four runs without any significant loss in activity. Similar results were obtained by Hu's group,⁴²² who synthesized a series of novel water-tolerant POM-containing composite photocatalysts and investigated their activities and stabilities in the photocatalytic degradation of parathion-methyl. As for the stability of the POM on the supporters, selecting lacunary or substituted POM as the inorganic precursors is better than the saturated POM. Therefore, the strong chemical interactions between the lacunary or substituted POM unit and the matrix (i.e. via formation of a covalent or coordination bond) would effectively avoid the leakage of the active component from the matrix. However, the photocatalytic activity of the saturated POM is somewhat higher than that of the lacunary or substituted one.⁴²²

As a result of the tethering and grafting methods, a decrease in chemical activity should be expected due to the interaction of the linker with POM, but exceptions have been reported. For example, the photocatalytic activities of K₆[Ni(H₂O)SiW₁₁O₃₉] supported on amine-modified silica materials (SiW₁₁Ni/NH₂/SiO₂) have been improved compared with the K₆[Ni(H₂O)SiW₁₁O₃₉] in homogeneous systems due to their porous structures.⁴²³ In addition, the leaching of POM from the support was controlled effectively due to the chemical attachment of the cluster onto the support. Therefore, the judgement about where on the catalyst the handle for the connection to the support should be placed, and what kind of linker (tethering and grafting methods) should be inserted between the catalyst active site and the support are two important factors that should be considered. The catalytic activity and stability are also related to the types of interaction of the HPCs with the linker (covalent and non-covalent interactions).

Among various solid supports, silica can be utilized in different heterogenization methods, including dispersion, encapsulation, tethering and grafting. Since Keggin-type H₃PW₁₂O₄₀ was the first commercialized in the industrial research field, a large number of heterogeneous catalysts have been synthesized by the immobilization and/or incorporation of H₃PW₁₂O₄₀ on/in the silica matrix. The Keggin ion interacts with the surface hydroxyl groups of the silica by an exchange reaction, leading to the formation of water and the replacement of the surface hydroxyl anion by a Keggin anion:

or through interaction of a protonated hydroxyl group (OH₂⁺) with the anion:

Most of the HPAs easily interact with the basic SiOH groups on the silica matrix. In the case of the nanoparticles, however, more HPA molecules might be exposed to the outer surface of the silica nanoparticles due to the high surface-to-volume ratio of nanoparticles. This issue has been investigated by various research groups.^288,291,424 The reduction of particle size of the catalyst improves the surface of the nanoparticle. As the particle size of silica is decreased, the exposure of H₃PW₁₂O₄₀ was increased and thus the amount of surface acid sites was also increased. Hence, the catalytic activity of H₃PW₁₂O₄₀/SiO₂ nanoparticles was higher than the activities of pure H₃PW₁₂O₄₀ and bulk H₃PW₁₂O₄₀/SiO₂ catalysts.⁴²⁴

Silica-encapsulated H₃PW₁₂O₄₀ was prepared simply via a sol–gel process. This catalyst was used as a green and recyclable photocatalyst, under O₂ gas as the sole catalyst reoxidant and in acetonitrile as the polar solvent. The catalyst was reused several times without a significant decrease in the yield or reaction rate.⁴²⁵ However, a substantial decrease in the activity and selectivity of the encapsulated HPA may be observed due to the heterogeneous nature of the support materials in specific reaction media, and due to steric and diffusion factors. Hollow nano-structured silica materials are considered as promising nano-supports for the encapsulation of HPAs. In this regard, void spaces within mesoporous silica spheres can function as nano-reactors for catalytic reaction. Compared to mesoporous bulk silica, hollow spheres have shorter pore channels and thus experience less travelling blockage, which is highly desirable for the mass transfer of reactants and products, especially for liquid phase reactions.³⁶³

H₃PW₁₂O₄₀ was tethered on the inner surface of mesoporous MCM-41, fume silica and silica-gel by means of chemical bonding to aminosilane groups.⁴²⁶ MCM-41 showed the largest amine to silica ratio due to its large surface area and the large number of available surface hydroxyl groups. In contrast, functionalized MCM-41 showed the least tendency for anchoring with HPAs in comparison with the functionalized fume silica with a lower surface area. This was due to the smaller accessibility of its inner surface, which is mostly confined within the channels. For all the samples, the strong interaction of H₃PW₁₂O₄₀ with amine groups results in the highest HPA retention on the silica surface when used as a catalyst in polar media.

The covalent attachment of H₃PW₁₂O₄₀ onto the silica surface via a grafting method had not been reported, until now. This is due to the fact that covalent immobilization requires suitable anchors in HPA structure and surface modification of the support. These anchoring sites are provided via removal of one or more M atoms to form “lacunary Keggin molecules” or by replacement of one or more addenda atoms and the terminal oxo-group by a suitable metal atom cation to form metal-substituted polyanions.

Generally, in dispersion, encapsulation, tethering and grafting methods, the choice of the support is crucial, because the properties of the support influence the catalyst behaviour at every level. Relevant features of the support are: its cost, commercial availability and degree of functionalization. Each of these factors should be considered in the design of the catalyst and when choosing the method of heterogenization.

The solubility properties are the other important factors that should be considered in the choice of a suitable counter cation for HPCs (precipitation and hybridization methods). There is no general rule about whether working with a soluble catalyst is better or worse than working with an insoluble one. In principle, one can expect a heterogeneous catalyst to be less reactive than a homogeneous one, but a number of results indicate that definitely this is not always the case. On the other hand, insoluble catalysts can be easily recovered and thus more easily recycled than their homogeneous analogues. Moreover, a number of organic–inorganic POM hybrid catalysts have solubility properties that can be varied by changing the environmental conditions, such as the medium polarity and reaction temperature, thus coupling the advantages of homogeneous and heterogeneous systems. These compounds represent a type of self-supported heterogeneous catalyst with highly tunable structural features of acidity, porosity, charge density and surface properties. The catalytic activity is another factor that should be considered in this matter. As mentioned in Sections 2.1 and 2.2, porosity, redox and acidity of the HPC can be tuned with changing the counter cation and thus, a list of suggestions depends on the reaction type.

From this discussion, the heterogenization of HPCs emerges as a field of research still in its infancy and open to great expansion in the future, provided that inorganic, organic, polymer and material chemists are able to combine their efforts in a multidisciplinary approach.

Conclusions

The heterogenization of different kinds of HPCs via a wide variety methods was reviewed. The main processes for this goal are dispersion, encapsulation, precipitation, tethering and grafting. All of these ways have been greatly improved over the last 15 years, especially when HPCs are produced as nano-size catalysts. Significant progress is being made in several key research areas, such as by using a wide variety of supports for the heterogenizations, including silica, alumina, carbon, zeolite, MOF, etc. Also, improvement of the preparation methods provides a wide subject area for scientists to increase the ability of using such important catalysts in industrial processes, because easy recovery of the catalysts and the reuse possibility with no significant change in activity or selectivity classify these compounds as green catalysts. In this review, all the methods for the heterogenization of HPCs and related compounds have been summarized to support further improvements in this subject area and to facilitate extending these methods to other homogeneous catalysts.

Acknowledgements

The authors thank the Razi University Research Council for support of this work.

Notes and references

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