RSC Advances REVIEW
Cite this: RSC Adv., 2014, 4, 42092
Recent advances in polyoxometalate-based heterogeneous catalytic materials for liquid-phase organic transformations Yu Zhou, Guojian Chen, Zhouyang Long and Jun Wang* Polyoxometalates (POMs) are a unique class of molecular metal oxides with a tunable structure at the atomic level. They have demonstrated various potential applications in various fields. In particular, POMs have been widely used as catalysts, due to their facilely modified acid–base properties and redox potential through molecular designing. Normally, POMs can be utilized as both homogeneous and heterogeneous catalysts, where the former favors high activity, while the latter benefits facile separation
Received 31st May 2014 Accepted 11th August 2014
of the catalysts. Faced with the requirement for sustainable development, significant effort has been made in the preparation of POM-based heterogeneous catalysts. This review focuses on the recent
DOI: 10.1039/c4ra05175k
developments in heterogeneous strategies of POM-based catalysts and their applications in liquid
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organic reactions.
1
Introduction
Polyoxometalates (POMs), a class of structurally diverse anionic metal-oxygen clusters covering various early transition metals (W, Mo, V, Nb, Ta, Fe, Co, Ni, Cu, Ti, Zn, and Mn), have attracted increasing attention due to their extensively alterable physical and chemical properties through “task-specic” adjusting of their dynamic structure, which ranges in size from the molecular or atomic level to the nano and even to the micrometer scale.1–4 Much effort has been directed toward the synthesis, characterization, and application of POM-based functional materials in numerous elds, such as catalysis, materials science, analytical chemistry, surface and interface science, medicine and life science, and electro-, photo- and magnetic chemistry.5–28 During the past more than two decades, POM-, especially heteropolyanions (HPAs), based catalytic materials have received continuous attention because of the numerous advantages of POMs in catalysis.24,25 First, POMs exhibit very strong Br¨ onsted type acidity, making them suitable for various acidic reactions, such as esterication, transesterication, hydrolysis, Friedel–Cras alkylation and acylation, and Beckmann rearrangement.17,19,21,29 Second, some special POMs also possess basic properties and can be used in base-catalytic reactions.30–33 Third, POMs are well known to have fast and reversible multi-electron redox behaviors under mild conditions, which makes them promising candidate catalysts for the oxidation of alkanes, aromatics, olens, alcohol, etc.6,17,23–26,34–36 State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, NanjingTechUniversity, Nanjing 210009, Jiangsu, P. R. China. E-mail:
[email protected]; Tel: +86 25-83172264
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More important, the chemical properties of POMs, like their acid–base strength, redox potential and solubility in aqueous or organic media, can be facilely and nely altered in a wide range on purpose through smoothly varying their composition and structure. Moreover, compared with common organometallic complexes, POMs are thermally and oxidatively stable toward oxygen donors. Owing to the above unique properties, various POMs have been used extensively as efficient homogeneous catalysts in numerous related reactions. However, POMs are usually soluble in many polar solvents, causing difficulties in the recovery, separation, and recycling of the catalysts, which affects their use in systems that require environmentally friendly efficient transformations and sustainable development. Therefore, it is imperative to develop easily recoverable and recyclable POMbased catalysts for practical application in industry. In order to achieve this, heterogeneous catalysis are preferred because of the advantages of facile catalyst/product separation. Nonetheless, heterogeneous POM-based catalysts usually also entail some disadvantages, such as leaching of the active sites and low activity.17,25 One signicant example is in the reactions catalyzed by the widespread use of salts of PW12O403 in combination with H2O2.37–39 Such oxidations are usually not catalyzed by the plenary Keggin ion (PW12O403), as the plenary Keggin ion reacts with H2O2 to give the well-known Ishii–Venturello complex {PO4[WO(O2)2]4}3, a tungstophosphate with oxide and peroxide ligands. The latter is oen soluble during heterogeneous catalysis, and is nearly always the true catalyst for “heterogeneous” reactions of the plenary Keggin ion (PW12O403). Besides, heterogeneous POM-based catalysts usually exhibit inferior catalytic performance than their homogeneous counterparts, mainly due to the mass transfer
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Review
resistance and the diffusion limitation of the active sites. In order to overcome the above limitations, many approaches have been proposed to improve the stability and catalytic performance. Generally, POM-based heterogeneous catalysts can be prepared mainly by two strategies, namely “immobilization” and “solidication” of the catalytically active POMs.29 As shown in Scheme 1, the former involves supporting the POM active species on various porous materials, while the latter involves preparing insoluble POM salts. Many reviews have covered the preparation and application of POM-based heterogeneous catalysts, but they rarely pay any particular focus on their synthetic strategies. The current review aims to summarize recent advance in these two strategies in preparing POM-based heterogeneous catalysts for liquid organic reactions, although some earlier references are also included for a more comprehensive discussion. Moreover, owing to the special advantage of phase-transfer catalysis,40,41 we also highlight some important phase-transfer catalysts derived from POMs.
2 Immobilization of POMs on porous supports The “immobilization” of catalytically active POMs onto appropriate supports is one efficient and facile strategy for the design of heterogeneous catalysts. For heterogeneous catalysis, the catalytic reaction usually occurs on the solid surface, and the substrates need to diffuse to the active sites through the boundary layer surrounding the solid before intra-particle diffusion and chemical adsorption. The products also need to be desorbed from the solid through a reverse process. Therefore, the pore structure, including the pore size, volume, and distribution, and the surface micro-environmental state, such as the hydrophilic–hydrophobic properties, will signicantly affect the actual catalytic activity and selectivity. Normally, the supports are chosen to be porous materials, in order to highly and uniformly distribute the active sites and decrease the mass transfer resistance. Various supports have been applied for the immobilization of POMs, including mesoporous silica, metal– organic frameworks (MOFs), mesoporous polymers,
Scheme 1 The major strategies for preparing heterogeneous POMbased catalysts.
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RSC Advances
mesoporous transition metal oxides, magnetic nanoparticles, porous carbons, zeolites, and so on.41–45 According to the difference of the host–guest interaction, POM-based active sites can be immobilized through adsorption, ion exchange, covalent linkage, encapsulation, and substitution, etc. Moreover, the porous supports will not only provide a larger surface to highly distribute the active sites, but will also inuence the catalytic activity and selectivity through the strong host–guest interaction that affects the active center atoms. In order to design efficient supported heterogeneous POM-based catalysts, it is thus imperative to choose suitable supports. The rapid development of materials science has been dramatically promoting the emergence of numerous porous materials with tunable chemical composition, pore structure, surface state, and morphology throughout the size ranges, from the nanoscale to the micrometer level, thus enabling the preparation of abundant multi-functional immobilized POMs catalysts for liquid phase organics. Table 1 summarizes the typical used supports and their application as POM immobilizers for liquid organic reactions.
2.1
Mesoporous silica-based materials
Mesoporous silica materials have various excellent textural parameters, such as larger surface area and pore volume, and narrow pore size distribution, as well as special quantum size effects.46,47 Furthermore, the synthesis and modication of mesoporous silica can be facilely tuned in a continuous wide arrangement. For example, the pore size of SBA-15, one typical ordered mesoporous silica, can be adjusted from 5 nm to 30 nm through varying the synthetic conditions.48,49 Up to now, a large number of series mesoporous silica families (M41S, FSM-n, HMS-n, SBA-n, AMS-n, FDU-n, HOM-n, and MSU-n, etc.) have been successfully synthesized, and not only can the pore size and volume be adjusted, but also the symmetries of the obtained materials can be tuned to P6mm, Pm3n, Fd3m, Im3m, Fm3m, Ia3d and Pn3m, etc.50 Therefore, mesoporous silica materials, especially the high ordered mesoporous silicas,47 have been widely used as common supports to immobilize POMs for heterogeneous catalysis reactions. There are many strategies to prepare POM-loaded mesoporous silica catalytic materials, such as impregnation, sol–gel techniques, electrostatic interactions, ion-exchange, and covalent graing.51–58 Many examples deal with heteropolyacids that are bound to silica supports through the protonation of hydroxyl groups of the surface and by the interaction of the resulting ^SiOH2+ species with external oxygen atoms of the POMs. However, these weak interactions cannot provide enough stability for anchoring POMs, and thus it is hard to avoid the loss and leaching of catalytically active POM species during the recycling when used in acid or oxidation reactions. In order to improve the recyclability and stability of POM catalysts, organically modied mesoporous silicas are oen used as carriers for anchoring POMs by stronger chemical bonds (e.g., ionic bonds and covalent bonds).59,60 For example, Inumaru and coworkers reported the construction of water-tolerant solid acid catalysts by immobilizing polyoxometalate H3PW12O40 in the
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42094 | RSC Adv., 2014, 4, 42092–42113 [{W(]O)(O2)2(H2O)}2(m–O)]2 [PMo10V2O40]5 [PW12O40]3 Na7H2LaW10O36$32H2O
IL-modied SiO2 IL-modied SBA-15 IL-modied SBA-15 IL-modied SiO2
Ion-exchange Ion-exchange Ion-exchange Ion-exchange Ion-exchange Ion-exchange Ion-exchange Copolymerization
PW12O403 PW12O403 and PW4O163 PW12O403 PMo10V2O405 [PO4{WO(O2)2}4]3 [PO4{WO(O2)2}4]3 PMo10V2O405 [{CH2]CH–(CH2)6Si}xOySiWwOz]4 (NBu4)6[a2-P2W17O61(SiC6H4CH2N3)2O]
Polymers PIL poly(VMPS) Ionic copolymers AM-BM, DIM-CIM NDMAM-AVIM, AVIM-DVB PIL poly(VMCA) Ordered mesoporous polymeric materials Polymer-immobilized ionic liquid phase
Porous cross-linked ionic copolymer Cross-linked POM polymers
Pent-4-ynoic acid modied commercial D380 macroporous benzylamine resin
Covalent immobilization
One-pot synthesis One-pot synthesis One-pot synthesis One-pot synthesis Self-assembly
H3PW12O40 and others Na3PW12O40$12H2O H5PV2Mo10O40 [CuPW11O39]5 [BW12O40]5
HKUST-1 NENU-11 HKUST-1 HKUST-1 Ni-PYI1 and Ni-PYI2
One-pot synthesis One-pot synthesis One-pot synthesis and impregnation
H3PW12O40 Ru–H3PW12O40 H3PW12O40 H3PW12O40
One-pot synthesis and impregnation
Impregnation method Impregnation method Impregnation method
Electrostatic interactions Two consecutive post-graing steps Covalent graing Inside-out pre-installation–infusion– hydration Ion-exchange Ion-exchange Ion-exchange or one-pot procedure Ion-exchange
Immobilization methods
MIL-101(Cr) MIL-100(Cr) MIL-101(Cr) MIL-101(Cr)
MIL-101(Cr)
[PW11CoO39]5 and [PW11TiO40]5 [PW4O24]3 and [PW12O40]3 [PW11O39]7, [Co4(H2O)2(PW9O34)2]10, [Ln(PW11O39)2]11 and [Tb(PW11O39)2]11 H3PW12O40
H3PW12O40 H3PW12O40 B,a-[AsIIIW9O33{P(O)(CH2CH2CO2H)}2]5 H4SiMo12O40
Mesoporous silicas Octyl and 3-aminopropyl graed SBA-15 3-Aminopropyl functionalized SBA-15 3-Aminopropyl functionalized SBA-15 Mesoporous silica hollow sphere
MOFs MIL-101(Cr) MIL-101(Cr) MIL-101(Cr)
POM units
Esterication reactions Epoxidation of alkenes with H2O2 Oxidation of alcohols with H2O2 Hydroxylation of benzene with H2O2 Epoxidation of olens with H2O2 Epoxidation of allylic alcohols and alkenes with H2O2 Hydroxylation of benzene with H2O2 Oxidation of methyl p-tolyl sulde and the oxidation and removal of dibenzothiophene Tetrahydrothiophene (THT) oxidation
Alkene oxidation with O2 or H2O2 Alkene oxidation with H2O2 Alkene oxidation and oxidative desulfurization with H2O2 Knoevenagel condensation; esterication and dehydration of methanol Carbohydrate dehydration to HMF Conversion of cellobiose and cellulose Baeyer condensation and epoxidation Alcoholysis of styrene oxide to b-alkoxyalcohol Hydrolysis of esters Removal of nerve gas Ultradeep oxidative desulfurization Detoxication of various sulfur compounds Asymmetric dihydroxylation of olens
Epoxidation of olens with H2O2 Aerobic oxidation of alcohols Oxidation of alcohols with H2O2 Oxidative desulfurization
Ester hydrolysis reaction Acid–base tandem reaction — Friedel–Cras Alkylation
Catalytic studies
Immobilization of catalytically active POMs onto various supports and their heterogeneous catalytic studies in different organic transformations
Supports
Table 1
103
101 102
91 92 and 93 94 and 95 96 100 99
85 86 89 87 88
81 82 83 84
80
74 75 76–79
67 68 69 and 70 71
61 62 54 63
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116 H3PW12O40 PIL-coated magnetic Fe3O4
Epoxidation of bio-derived olens with H2O2
Incorporation Ion-exchange Hydrogen bonds or covalent bonds Si–O linkage Ion-exchange (DODA)3PW12O40 WO42 [HDMIM]2[W2O11] and DSPIM-PW11
113 114 115
110–112
Friedel–Cras reactions and aldol reaction of acetone, esterication and transesterication Oxidation of suldes to sulfones Oxidation of alcohols, suldes and olens Epoxidation of olens with H2O2 “Acid–base” and ion-exchange strategy H3PW12O40
Magnetic nanoparticles 3-Aminopropyl and IL modied magnetic nanoparticles SiO2-MNPs Ferromagnetic NCs (Fe3O4) PIL-modied magnetic nanoparticles Silica layer-coated ferrite-based MNP
105 107–109 Oxidation of 1-phenylethanol with H2O2 Synthesis of levulinate esters from a biomassderived platform molecule, levulinic acid H3PMo12O40 (PMA) H3PW12O40 Mesoporous Cr2O3 ZrO2–Si(Et/Ph)Si
104 and 106
Surfactant-assisted sol–gel copolymerization route A “nanocasting” method One-pot template-assisted sol–gel co-condensation-hydrothermal treatment route H3PMo12O40 (PMA), H3PW12O40 (PTA) Mesoporous metal oxides Ordered mesoporous ZrO2
Oxidation of alkenes with H2O2
Immobilization methods POM units (Contd. ) Table 1
RSC Advances
Supports
Catalytic studies
Ref.
Review
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hydrophobic nanospaces of organomodied mesoporous silica SBA-15 (Fig. 1). The obtained catalyst showed high stability and good activity during the ester hydrolysis reaction in water.61 Recently, surface-organized mesoporous silicas with siteisolated amino and phosphotungstic acid groups were synthesized through two consecutive post-graing steps. The catalyst combined two antagonistic active functions on one solid catalyst; in other words, the amino and phosphotungstic species provided the basic and acidic active centers. Such bifunctional catalysts can be used in the acid–base tandem reaction, such as in the hydrolysis of benzaldehyde di-methyl acetal and the consecutive Henry reaction of benzaldehyde with nitromethane, or the aldol condensation of benzaldehyde with malononitrile.62 Zeng and coworkers developed an inside-out pre-installation– infusion–hydration method for the targeted synthesis of Keggin heteropolyacids (H4SiMo12O40) within mesoporous silica hollow spheres with high BET surface areas from 156 to 701 m2 g1 (Fig. 2). The obtained H4SiMo12O40@mSiO2 catalysts presented superior activity than the commercial Amberlyst-15 catalyst during the Friedel–Cras alkylation of toluene with benzyl alcohol.63
Fig. 1 Schematic illustration outlining the preparation and structure of a water-tolerant solid acid catalyst by the immobilization of H3PW12O40 in hydrophobic organomodified mesoporous silica SBA-15. Reprinted with permission from ref. 61. Copyright (2007) Wiley-VCH.
Schematic flowchart of the targeted synthesis of silicomolybdic acid (H4SiMo12O40) inside silica hollow spheres. Reprinted with permission from ref. 63. Copyright (2012) American Chemical Society. Fig. 2
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Ionic liquids (ILs) have attracted increasing attention as reaction media and catalysts in many organic syntheses, and as versatile modiers for improving the surface and electronic properties of various functional porous materials.64–66 In 2005, Mizuno and coworkers rst used an IL-modied silica support to immobilize a homogeneous POM catalyst, producing an efficient heterogeneous catalyst for the epoxidation of olens with H2O2.67 Subsequently, several research groups reported IL-modied mesoporous silicas supported POMs as heterogeneous catalysts for the oxidation of alcohols,68–70 and for oxidative desulfurization.71 Generally, mesoporous silica materials used as supports to immobilize POM active species have attracted various attention, because of their facile synthesis and modication, which makes them especially suitable for fundamental study. However, the stability of the mesostructure for these mesoporous silica-based POM catalysts have not been well investigated and still need to be enhanced.
2.2
POM-based metal–organic frameworks (MOFs)
Metal–organic framework (MOF) materials are a new class of crystalline porous coordination polymers with open frameworks, which are able to act as unique and outstanding candidate carriers to encapsulate various active centers, due to their high surface areas, permanent porosity, and adjustable hydrophilic–hydrophobic channel properties.72 Much effort has also been made to immobilize active POMs species on MOFs to prepare efficient heterogeneous catalysts, through electrostatically attaching POM anions into the interior surface of the MOF matrix via anion exchange. As early as 2005, F´ erey et al. successfully incorporated the lacunary heteropolytungstate K7PW11O39 within the cages of MOF MIL-101, which had a rigid zeotype crystal structure, and large pores and surface area, as well as good stability.73 Later, Kholdeeva and coworkers reported that titanium/cobalt monosubstituted Keggin heteropolyanions ([PW11CoO39]5 and [PW11TiO40]5) and other anions ([PW4O24]3 and [PW12O40]3) could be electrostatically bound to MIL-101(Cr) and used as heterogeneous catalysts for oxidation reactions.74,75 Recently, Balula and coworkers prepared a series of heterogeneous catalysts by supporting various POMs on MIL-101(Cr), and applied them in different oxidation reactions, including in the epoxidation of olens, the oxidations of styrene and cyclooctane, and the oxidative desulfurization.76–79 Moreover, phosphotungstic acid H3PW12O40 is oen encapsulated into MIL-101(Cr) as a recyclable solid acid catalyst for various organic reactions, such as in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate, the esterication reaction of acetic acid with n-butanol,80 carbohydrate dehydration to 5-hydroxymethylfurfural (HMF),81 the conversion of cellobiose and cellulose into sorbitol,82 Baeyer condensation,83 and in the alcoholysis of styrene oxide to b-alkoxyalcohol.84 Moreover, other series of POM-based MOF crystalline materials possessing the features of both POMs and MOFs have also attracted signicant attention in recent years. In 2009, Liu and Su et al. obtained a series of crystalline compounds through a
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one-step hydrothermal reaction of the precursors, where the catalytically active Keggin anions, PW12O403, were alternately arrayed as noncoordinating guests in the cuboctahedral cages of the HKUST-1 host matrix, named NENU-n series.85 The main properties of the framework for the NENU-n series were preserved, with the loading amount of POM being as high as 35– 45 wt%, i.e., exceeding the value in traditional supports. The acid catalytic performance of NENU-3 was assessed through the hydrolysis of esters in excess water, and the catalyst showed high catalytic activity and good reusability. Later in 2011, Liu and Su et al. hydrothermally synthesized a sodalite-type porous POM-based framework, NENU-11, with Keggin anions as the templates. NENU-11 displayed its potential application for the removal of nerve gas, with encapsulated PW12O403 as its catalytically active center.86 In 2011, Hill et al. employed [CuPW11O39]5 to t the pores of HKUST-1, resulting in a new crystalline catalyst, [Cu3(C9H3O6)2]4[{(CH3)4N}4CuPW11O39H]. The catalyst showed efficient catalytic performance in the detoxication of various sulfur compounds, from H2S to S8, using only ambient air, and its activity was found to be higher than POM or the MOF precursor alone, suggesting a synergy between the two structural components (POM and MOF). The catalyst could be isolated and reused, thus providing a heterogeneous catalyst that requires only the ambient environment for some oxidation processes.87 In 2013, Duan et al. synthesized two new enantiomorphs Ni-PYI1 and Ni-PYI2 MOFs amphipathic catalysts for the asymmetric dihydroxylation of aryl olens, through incorporating the oxidation POM species [BW12O40]5 and the chiral group, L-or D-pyrrolidin-2-ylimidazole (PYI), within one single MOF framework (Fig. 3). The hydrophilic–hydrophobic properties of the MOFs Ni-PYIs, consolidated by the organic ligands and the POM anions, was benecial for the amphipathic catalysis of the epoxidation processes and presented excellent stereoselectivity in the asymmetric dihydroxylation of the aryl olens.88 Almost at the same time, catalytically active POMs were incorporated into nonpolar reaction systems by Liu and Zheng et al. through a novel strategy of using organic ligands as
Fig. 3 Synthetic procedure of Ni-PYI1, showing the guest exchange and the potential amphipathic channel for the asymmetric olefin dihydroxylation. Reprinted with permission from ref. 88. Copyright (2013) American Chemical Society.
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hydrophobic groups to encapsulate the inorganic catalyst within the pores of a MOF structure. The catalysts were assessed in both a model diesel environment and with real diesel, wherein dibenzothiophene and the obtained nanocrystalline catalysts prepared by both solution and by mechano-chemical synthesis all showed remarkable activity in catalytic oxidative desulfurization reactions.89
2.3
Polymer-supported POM catalysts
Polymeric materials, especially organic polymers, have been widely studied in basic research and applied in industry due to their tunable abundant structure, functionality, and easy processability.90 Polymers are suitable as the supports for heterogeneous catalysts, because of their advantage of easy production, low weight, and ductile nature. Moreover, the organic framework of polymers enables them to have good compatibility with organic substrates and the ability to tune their hydrophilic–hydrophobic properties continuously from superhydrophilicity to superhydrophobic makes them especially suitable for liquid organic reactions. Therefore, the fabrication of POM/polymer hybrid materials or polymersupported POM catalysts has attracted great interest in the last decade.44 With the rapid emergence of new polymers with enriched ionic centers or porous structures with high BET surface areas, such as poly(ionic liquid)s and porous ionic copolymers, various heterogeneous POM catalysts supported on these polymers have been successively prepared in recent years. Leng and Wang et al. developed a series of POM-based polymeric hybrids by the anion-exchange of various organic groups functionalized poly(ionic liquid)s or ionic copolymers with Keggin heteropolyacids. The obtained catalysts can be used as efficient heterogeneous catalysts in various esterications,91 in the H2O2-mediated epoxidation of alkenes,92,93 in the oxidation of alcohols,94,95 and in the hydroxylation of benzene.96 For example, in 2012, Leng and Wang et al. reported a stimuliresponsive heteropolyanion-based polymeric hybrid catalyst by coupling task-specic synthesized ionic copolymers and Keggin POM H3PW12O40.94 The resulting amino-functionalized polymeric hybrid NDMAM-AVIM-PW behaved as a swelling heterogeneous catalyst in the solvent-free oxidation of benzyl alcohol with aqueous H2O2 and gave a high conversion of 93%, with 99% selectivity, in a short reaction time of two hours. The amino-free polymeric hybrid NDMAM-BVIM-PW was also insoluble in benzyl alcohol before the reaction, forming a stable emulsion at 90 C aer the addition of H2O2; and also, a satisfactory conversion of 96%, with 98%, selectivity was obtained by prolonging the reaction time to four hours (Fig. 4). The organic-functionalized cationic copolymer not only served as supports for immobilizing POMs active sites but also provided a suitable hydrophobicity–hydrophilicity balanced surface microenvironment for interfacial catalysis, which was crucial for the organic reactions.97,98 For instance, Leng and Wang et al. designed an amphiphilic POM-paired ionic copolymer by pairing H3PW4O16 with hydrophobic alkyl chains (–C12H25) and hydrophilic carboxyl groups (–COOH) functionalized ionic copolymer (Fig. 5). The obtained catalyst, DIM-CIM-PW, was capable of catalyzing the
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Fig. 4 Top: synthesis of HPA-based polymeric hybrids. Bottom: photographs of the solvent-free oxidation of benzyl alcohol with H2O2 over (A) NDMAM-AVIM-PW and (B) NDMAM-BVIM-PW: (a) catalyst (light brown solid at bottom) and alcohol (liquid) before mixing; (b) during the reaction after adding H2O2; (c) at the end of the reaction. Taken from ref. 94 with permission from RSC Publications.
epoxidation of alkenes in a liquid–liquid–solid triphase reaction system, showing high catalytic conversion and selectivity. The amphiphilic structure supplied a special microenvironment for the catalyst, involving both the hydrophobic alkene substrates in the oil phase and hydrophilic H2O2 molecules in the aqueous phase, thereby allowing superior accessibility of the catalytic centers PW in the bulk of the catalyst.93 Generally, surface properties, the microenvironment, and porosity oen dramatically inuence the catalyst–surface interactions, the substrate accessibility, and the mass-transfer efficiency.99 Recently, various mesoporous polymers were synthesized and used as the supports for heterogeneous catalysts. Correspondingly, many approaches have been proposed to study the porous effects of various polymeric supports for heterogeneous POM-catalyzed organic reactions. In 2011, Ryoo et al.100 synthesized an ordered mesoporous polymeric material (MPM) by the copolymerization of chloromethylstyrene with divinylbenzene, using mesoporous silica
Schematic illustration outlining the preparation and structure of the catalyst DIM-CIM-PW. Taken from ref. 93 with permission from RSC Publications.
Fig. 5
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KIT-6 as the hard template. The obtained MPM materials were further functionalized with ammonium groups through the reaction between the trimethylamine and chloromethyl groups in the MPM. A polyoxotungstate anion, [PO4{WO(O2)2}4]3, was immobilized through its introduction as a counter ion to the ammonium group. The mesoporous polymeric material supporting the polyoxotungstate anions in this manner showed good catalytic performance in the liquid-phase epoxidation of olens, using an aqueous solution of H2O2 as the oxidant. The high catalytic activity and epoxide selectivity were attributed to an optimized hydrophilicity–hydrophobicity balance in the mesoporous environment, as well as the facile diffusion of the reactants and products due to the existence of a suitable mesostructure. The catalyst can be separated by ltration and recycled without a signicant loss of activity. In 2012, Doherty and coworkers prepared a linear cation-decorated polymeric support with tunable surface properties and a microstructure to immobilize the POMs active center through the interaction of the electrostatic effect. The synthesized peroxophosphotungstate-based polymer-immobilised ionic liquid catalyst had a moderate surface area of 42 m2 g1 and a pore volume of 0.15 cm3 g1. The hybrid acted as an efficient and recyclable catalyst for the epoxidation of allylic alcohols and alkenes. Moreover, the catalyst could be recovered in an operationally straightforward procedure and reused with only a small reduction in performance in successive cycles.99 Our group prepared a heteropolyanion-based cross-linked ionic copolymer by the anion-exchange of heteropolyacid (HPA) H5PMo10V2O40 with a polymeric ionic liquid. Nitrogen adsorption–desorption experiments showed that the catalyst possessed rich mesopores/macropores structures with a relative high BET surface area of 104 m2 g1. The porous HPA-based ionic copolymer catalyst showed high activity, convenient recovery, and steady reuse in heterogeneous catalysis for the hydroxylation of benzene with H2O2 to phenol (yield of 23.7%).101 Carraro and Bonchio et al.102 reported the synthesis of a series of porous POM-based copolymers by the free-radical polymerization of polymerizable isocharged polyanions [{CH2]CH–(CH2)6Si}xOySiWwOz]4 with methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EDM). During the synthesis, the active POM species were immobilized through covalently cross-linking to the organic framework. The obtained hybrids were macroporous resins exhibiting irregular pores with an average diameter of 250 nm, which can swell in CH3CN, DMF and DMSO to exhibit a gel state. The polymeric hybrids can activate hydrogen peroxide for oxygen transfer, such as in the quantitative and selective oxidation of methyl p-tolyl sulde, and in the oxidation and removal of dibenzothiophene. Very recently, Xiao and coworkers reported a heterogenization process for the POM catalyst by a direct covalent immobilization method. The polymer support was the pent-4-ynoic acid modied commercial D380 macroporous benzylamine resin of a styrene-divinylbenzene matrix with uniform macropores and high specic surface areas (Fig. 6). The synthesis was achieved through the surface reaction of two-azido-organically modied POM clusters with the functionalized group on the channel surface of the macroporous resin via click chemistry. The solid
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(a) Schematic illustrations of the procedure for the covalent immobilization of POM clusters and the oxidative desulfurization of tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO); (b) inset showing the homogeneous catalyst (A-POM) and the heterogeneous catalyst (R3). Taken from ref. 103 with permission from RSC Publications.
Fig. 6
catalyst showed high activity and selectivity in the appraisement of the catalytic performance via catalysis on tetrahydrothiophene (THT) oxidation. Owing to the strong covalent bonding between the POM clusters and the macroporous resin surface, the catalyst could be reused several times without any detectable catalytic activity loss.103
2.4 Mesoporous transition metal oxide supported POM catalysts With the development of materials science and synthetic technology, various mesoporous transition metal oxide materials, such as ZrO2, ZnO2, Cr2O3, Fe2O3, and so on have been synthesized and applied as catalyst supports, due to their tunable chemical composition, pore structure, inert framework to resist the corrosion of the guest species, and the capability to provide strong host–guest interactions for higher catalytic performance. Much effort has been made toward the immobilization of POM active sites on mesoporous transition metal oxides to prepare heterogeneous catalysts. Armatas and coworkers synthesized a series of well-ordered mesoporous ZrO2-based 12-phosphomolybdic acid (PMA) and Cr2O3–PMA nanocomposite frameworks,104,105 which exhibited exceptional stability and catalytic activity in the oxidation of alkenes and 1-phenylethanol, using hydrogen peroxide as oxidant. The mesoporous ZrO2–PMA nanocomposite frameworks were prepared through a surfactant-assisted sol–gel copolymerization route. The pore walls of these materials were a mixture of nanocrystalline tetragonal ZrO2 and Keggin-type PMA components, with the PMA loading amount varying from 12% to 37%. The obtained mesoporous ZPMA(w) composites were found to have high BET surface areas in the range of 60– 103 m2 g1 and pore volumes in the range of 0.04–0.07 cm3 g1.104 Mesoporous nanocomposite frameworks of Cr2O3–PMA were prepared via a “nanocasting” method, using mesoporous silica SBA-15 as the hard template, and were found to possess a well-ordered hexagonal mesostructure, with the content of PMA clusters as high as 63 wt%, and a large internal surface area (up to 165 m2 g1).105 Very recently, as a continuation of the above works, Armatas' group reported the application of a surfactantassisted co-polymerization route to prepare ordered mesoporous composite catalysts consisting of nanocrystalline
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Review
tetragonal ZrO2 and heteropolytungstic clusters, including 12-phosphotungstic (PTA) and 12-silicotungstic (STA) acids. The loading amount of POM active centers varied from 2 to 20%, and the mesoscopic order ranged from wormholes to hexagonal pore structures. The resultant materials exhibited a large internal surface area of 126–229 m2 g1 and a narrow pore size distribution, with the most probable pore size being 2.2–2.6 nm. The catalytic performances of the obtained ZrO2-PTA and ZrO2STA hybrids were tested in the hydrogen peroxide mediated oxidation of 1,1-diphenyl-2-methylpropene, and they exhibited surprisingly high activity under mild conditions.106 Moreover, Guo and coworkers also prepared a series of mesoporous H3PW12O40/ZrO2–Si(Et/Ph)Si and H3PW12O40/ZrO2– Si(Ph)Si hybrid catalysts by a one-pot template-assisted sol–gel cocondensation-hydrothermal treatment route.107–109 For example, in 2013, they reported the synthesis of highly ordered mesoporous H3PW12O40/ZrO2–Si(Ph)Si hybrid with a surface area of about 250 m2 g1 through a single co-condensationhydrothermal treatment of benzene-bridged organosilica groups and a Keggin type heteropolyacid.108 The catalyst H3PW12O40/ZrO2–Si(Ph)Si exhibited excellent catalytic activity and good reusability toward the esterication of LA to produce methyl levulinate under mild conditions. The structural orderings and pore geometries of prepared H3PW12O40/ZrO2–Si(Et/Ph) Si or H3PW12O40/ZrO2–Si(Ph) hybrid catalysts can be adjusted through tuning the initial Si/Zr molar ratios and using different organosilica precursors (Fig. 7). Owing to the combination of the strong Br¨ onsted acidity, as well as the Lewis acidity, rational pore structure, and the enhanced surface hydrophobicity, the obtained hybrid catalysts exhibited superior heterogeneous acid catalytic activity toward the conversion from a biomass-derived platform molecule, levulinic acid, to levulinate esters under atmospheric pressure reuxing conditions.109 2.5
Magnetic nanoparticle supported POM catalysts
Magnetic nanoparticles (MNP) available from cheap precursors via simple synthesis that are easily tunable by structural surface
Fig. 7 Route for the preparation of ZrO2-based hybrid catalysts functionalized by both organosilica moieties and a Keggin-type heteropolyacid. Taken from ref. 109 with permission from RSC Publications.
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RSC Advances
modications have attracted signicant attention in the past few decades, due to their vast applications and their feasibility for separation.30 Recently, various magnetic nanoparticles have been used as catalyst supports and have become a hot topic due to their unique properties, like high surface area, good dispersion, and superparamagnetic behavior. The most attractive feature of MNPsupported catalysts is their facile recycling controlled by simple magnetically driven separation, eliminating the requirement for catalyst ltration and centrifugation. Therefore, several research groups have attempted to prepare magnetically recoverable POM catalysts for various reactions, including for acid-catalyzed Friedel– Cras reactions,110 esterication and transesterication,111,112 the oxidation of suldes to sulfones,113 the oxidation of alcohols,114 and the epoxidation of olens.115,116 In 2011, Wang and coworkers reported the concept of nanocone nanoreactors, by nanoscale incorporating the ferromagnetic NCs, (Fe3O4) NCs (6–7 nm), into the assembly of the supramolecular nanobuilding block (DODA)3PW12O40. With the advantages derived from the nanospaces and the increased surfactant alkyl chain density around the POM in the nanocones, the obtained hybrids displayed enhanced catalytic performance for the oxidation of suldes to sulfones (Fig. 8). Moreover, the materials also presented advanced recovery by an external magnetic eld.111 In 2012, Hou et al. reported the preparation of two different magnetically separable catalysts for the epoxidation of olens with hydrogen peroxide. The catalytically active ionic liquid type peroxotungstate was immobilized by hydrogen bonds or by the covalent bonds Si–O linkage. The immobilization by covalent linkage inhibited the metal leaching; but nonetheless, the concept of ionic liquid type compounds immobilization by hydrogen bonds allowed a much easier tuning and a high ionic liquid amount for a given transformation, even showing some benets that outperformed the immobilization by covalent linkage. Both catalysts have been tested in the epoxidation of olens, with the catalysts presenting superior reusability, showing that they can be recycled at least ten times with no loss of catalytic properties.115
Fig. 8 (a) Schematic illustration of a POM nanocone nanoreactor. (b) Oxidation of sulfides to sulfones in the presence of the cones as a catalyst. Taken from ref. 111 with permission from RSC Publications.
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In 2013, Pourjavadi et al. prepared a novel magnetically recoverable oxidation catalyst, MNP@PILW, through the anion exchange of tungstate species with a magnetic poly(ionic liquid) matrix prepared from the in situ polymerization of ionic liquid monomer 3-n-dodecyl-L-vinylimidazolium bromide and the cross-linker 1,4-butanediyl-3,30 -bis-L-vinylimidazolium dibromide (BVD) on the surface of silica and organic group modied magnetic nanoparticles. The obtained materials were assessed in a selective oxidation reaction, using H2O2 as an oxidant for a wide range of substrates, including alcohols, suldes, and olens. The hydrophobic surface and its multi-layered nature improved the catalytic activity, and their magnetic properties enabled easy catalyst separation aer reaction.114 Very recently, Leng and Wang et al. synthesized a core–shell structure amphiphilic hybrid with a magnetic Fe3O4 core and a dodecylamine-modied polyoxometalate-paired poly(ionic liquid) shell. The synthesis was achieved through three steps: (1) preparation of magnetic core, (2) modication of the core, and (3) anion exchange with the POM active species (Fig. 9). Catalytic tests for the H2O2-based epoxidation of bio-derived olens indicated that this newly designed catalyst exhibited high activity and selectivity, due to the unique amphiphilic catalyst structure and the intramolecular charge transfer between the amino groups and heteropolyanions. Convenient magnetic recovery enabled the effective separation and recycling of the catalyst.116
2.6
Review
3 Solidification of POMs with various counter-cations Another strategy to design POM-based heterogeneous catalysts is the “solidication” of POMs by the formation of insoluble ionic materials with appropriate counter-cations. The role of the counter-cations is necessarily associated with the polyanions and is a specic issue throughout POM-based catalysis.123 Normally, POMs salts prepared from small-sized and highlycharged metal cations, such as rst-row transition metal cations, are soluble in water and in (highly) polar organic solvents. However, insoluble solids can be achieved through the complexation of POMs with suitable cations with appropriate composition, charge, size, shape, and hydrophobicity that provides strong ionic interaction between the ionic components. For example, the complexation of POMs with large cations such as Cs+ is able to produce insoluble POMs solids that can be used as heterogeneous catalysts in polar solvents.124 The preparation process of insoluble POMs catalysts usually occurs in a one-step precipitation process, and benets from the simple synthesis. Moreover, POM-based catalysts display a special “pseudo-liquid phase” behavior during the liquid organic reaction, especially for strong polar small size substrates, and thus are able to exhibit good catalytic performance, even with non-porous structures.125 Therefore, various multi-functional POM-based heterogeneous catalysts have been prepared through the solidication method for use in various liquid organic reactions.
Other supported POM catalysts
Besides the above-mentioned supports, various other porous materials have also been used to immobilize POMs. For example, carbon materials including carbon nanotube,117 carbon nitride,118 and graphene oxide119–121 have been employed to support POMs for various reactions, including in water oxidations, the electro-oxidation of nitrite, the one-pot tandem deacetalization-nitroaldol reaction, and in the oxidation of benzyl alcohol. Another example is the intercalation of POM anions into layered double hydroxides (LDHs), which have then been applied in various acid and oxidation reactions, such as in the esterication of acetic acid with n-butanol to yield n-butylacetate and in the oximation of aromatic aldehydes to aldoximes and ketoximes.122
Schematic illustration of the synthetic procedure for the catalyst Fe@PILPW-AM and its application in the epoxidation of bioderived olefins with H2O2. Reprinted with permission from ref. 116. Copyright (2014) American Chemical Society. Fig. 9
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3.1
Inorganic cations
Traditionally inorganic cations (NH4+, Cs+, Na+, K+, etc.) were employed as counter-cations for heterogenizing pure POMs for liquid–solid biphasic catalysis and the obtained catalysts usually presented good thermal stability compared to their organic-modied counterparts. Among them, POMs with the cations NH4+ and Cs+ were widely studied, because of their tunable micro-/mesoporous structures and superior BET surface areas of 85–156 m2 g1.124 For instance, Misono et al. discovered that CsxH3xPW12O40 exhibited tunable, molecular shape selective catalysis.126 The corresponding salt had a microporous structure in the case of x ¼ 2.1–2.2 and a large surface area of 135 m2 g1 that provided a maximized surface amount of acid sites, affording high catalytic activity if x ¼ 2.5.127 The mesocrystal (NH4)3PW12O40 had a microporous dodecahedra shape with BET surface areas of 51–91 m2 g1. The crystallinity and porosity of the all-inorganic (NH4)3PW12O40 could be controlled by changing the synthetic temperatures.128,129 Recently, several groups have continued to explore the pronounced solid acid catalysts of Cs salts of POMs in new energy-oriented applications.130–134 For example, Cs-exchanged silicotungstic acid catalysts CsxH4xSiW12O40 are used as active catalysts for biodiesel production, including for C4 and C8 triglyceride transesterication and for palmitic acid esterication with methanol.130 Cs2.5H0.5PW12O40 is used as a heterogeneous solid acid catalyst for the transesterication of Eruca
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Sativa Gars (ESG) vegetable oil and crude Jatropha oil.131,132 Gusevskaya et al. reported that Cs2.5H0.5PW12O40 showed good catalytic performance in the reaction of monoterpenes limonene/a-pinene/b-pinene with crotonaldehyde.134 Moreover, Liu and coworkers applied a Keggin-type polyoxometalate Cs3PW12O40 to support Ru nanoparticles. The obtained heterogeneous catalyst did not possess strong intrinsic acidity, but can efficiently catalyze the conversions of cellobiose and cellulose into sorbitol by using water as the medium in relatively mild conditions. The existence of H2 promotes the formation of Bronsted acid sites, which play a key role in the formation of sorbitol.133 Besides the common Cs+ counter-cation, other metal ions, inducing Na+,135,136 K+,137–139 Cu+,140 Ag+,141,142 Sn2+,143 Zn2+,144,145 and La3+,146,147 are also used as counter-cations to fabricate POM-based heterogeneous catalysts for acid or base-catalyzed organic reactions. For example, in 2014, Song et al. synthesized two tri-lacunary POMs with Lewis basic sites, and applied them as heterogeneous catalysts for Knoevenagel condensation, and for the cyanosilylation of various aldehydes and ketones, as well as for the production of benzoxazole derivatives at room temperature under mild conditions.136 In 2009, Li and coworkers reported zinc dodecatungstophosphate (Zn1.2H0.6PW12O40, ZnPW) nanotubes with Lewis and Bronsted acid sites. The obtained ZnPW nanocatalyst can be used as a heterogeneous catalyst for biodiesel production, and it exhibits higher catalytic activities for the simultaneous esterication and transesterication of palmitic acid than the parent acid catalyst H3PW12O40. The catalyst can be recycled and reused with negligible loss in activity over ve cycles.144 Later, the similar POM nanotube, Zn1.5PMo12O40, was prepared for catalyzing the wet air oxidation of dye pollutants Safranin-T (ST) under room temperature conditions.145
3.2
Organic compounds
Compared with inorganic cations, organic compounds have the advantage of wide variety due to the versatility of the organic groups, thus they are also used to modify POM anions with multiple functions in a wide adjustable range. Moreover, the organomodied POMs provide good accessibility and compatibility of the organic substrates, and also, varying the organic groups benets the facile adjustment of the surface state, such as the hydrophilic and hydrophobic properties, which makes them especially suitable for liquid organic reactions. Generally, there are mainly three types of organic compounds that have been used to modify the POMs, namely, organic amines, organic surfactants, and ionic liquids. 3.2.1 Organic amines. In the early stages, organic amines were oen employed to modify heteropolyacids for preparing POM-based catalysts with tunable surface area and redox properties. For example, our group attempted to use organic compounds including cyclodextrins (CyDs),148 pyridine (Py),149–151 and long chain aliphatic amines152 for modifying vanadium-substituted heteropolyacids. The resulting catalysts PMoV1-b-CyDs and Py4PMo11V performed as an efficient phase transfer catalyst and a homogeneous catalyst in the direct
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RSC Advances
hydroxylation of benzene to phenol (yields of 13.1% and 9.1%) with molecular oxygen, respectively. However, the abovementioned catalysts were homogeneous. Recently, we prepared a quinine-modied phosphovanadomolybdate Q-V2, which was tested as a heterogeneous catalyst for the H2O2mediated hydroxylation of benzene to phenol, affording a yield of 23.6%.153 In 2014, we utilized 4,40 -bipyridine (Bipy) to modify Keggin-structured molybdovanadophosphoric acid (PMoV1), producing an organic–inorganic hybrid heterogeneous catalyst Bipy2PMoV1 for direct hydroxylation of benzene to phenol by O2 with a phenol yield of 7.8%. More importantly, the obtained catalyst showed a better performance than previous work in the recycling test. The organic modier 4,40 -bipyridine contributed both the heterogeneous process and good structural stability in recycling the catalyst.154 Moreover, in 2013, Wang and Jiang et al. reported an acid–base bifunctional heteropolyacid (HPA) nanocatalyst, Ly2HPW, by the self-assembly of lysine and HPA. The obtained catalyst had the antagonistic properties of an acid and base on one body, allowing acid–base tandem conversions in one-pot reactions. The catalyst was evaluated in the direct conversion from low quality feedstocks to biodiesel, presenting high activity for the simultaneous transesterication of oil and esterication of free fatty acids (FFAs).155 According to the recent research, organic compounds, especially N-containing organic amines, can dramatically inuence the solubility and the electronic states of POMs by H-bonding interactions and the organic p electron delocalization effects. 3.2.2 Organic surfactants. Organic surfactants have been widely used to construct cationic surfactant encapsulated POMs and dendritic POM hybrid catalysts with amphiphilic properties. Several recent reviews have covered the above topic,156–158 thus in this review we only focus on the most recent advances. In 2010, Mizuno's group synthesized the nonporous tetra-nbutylammonium (TBA) salt of [g-SiW10O34(H2O)2]4([TBA]4[gSiW10O34(H2O)2]$H2O) and used it toward the heterogeneous size-selective oxidation of various organic substrates, including olens, suldes, and silanes with aqueous H2O2 in ethyl acetate. The good catalytic performance was ascribed to the high mobility of the catalyst in the solid bulk, probably contributing to the easy cosorption of the substrate olens and oxidant H2O2. The catalyst can be easily separated by ltration and reused four times with no signicant decrease in activity.159 The structural investigations illustrated that the high catalytic activities for the epoxidation of alkenes were derived from the facile sorption of solvent molecules, the exibility of structures, and the high mobility of alkylammonium cations that beneted the distribution of the reactant and oxidant molecules throughout the bulk solid.160 In 2011, Nisar and coworkers constructed amphiphilic mesoporous POM catalysts by the combination of single-alkyl-chain surfactant molecules and the POM anion PW12O403. The catalysts were assessed in the oxidation of suldes with aqueous H2O2, exhibiting enhanced catalytic efficiencies and good reusability. The long alkyl chains on the surface of the POM cluster provided suitable hydrophobic– hydrophobic properties and polarity that promoted the adsorption of the substrate sulde molecules and desorption of the products sulfones (Fig. 10).161 Recently, Zhang et al.
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Fig. 10 Schematic illustrations of: (a) preparation of POM semitube and wire assemblies (top), and (b) the oxidation of sulfides to sulfones (bottom). Center: illustration of the catalytic center (POM) and the hydrophobic traps (alkyl chains) in amphiphilic POM mesostructures. Reprinted with permission from ref. 161. Copyright (2011) Wiley-VCH.
prepared a series of POM-based amphiphilic catalysts via functionalization of the V-containing Keggin POM H4PMo11VO40 with cationic surfactants containing different carbon-chain lengths. The catalytic performance was tested in the selective oxidation of benzyl alcohol with H2O2 under organic solvent-free conditions, with the amphiphilic (ODA)4PMo11VO40 catalyst showing good catalytic efficiency and reusability.162 Moreover, Wang and Huo et al. developed a series of micellar catalytic systems by using surfactant-based micellar POMs for various reaction systems, including for the conversion of cellulose to HMF,163 for the production of glucose from polysaccharides,164 and for the wet peroxide oxidation of phenol.165 In 2012, Rataj and coworkers reported the synthesis of surfactant alkyltrimethylammonium-based POM spherical monodisperse nanoparticles by combining decyl-, dodecyl-, and tetradecyltrimethylammonium cations with [PW12O40]3 anions. The obtained [C12]3[PW12O40] compound formed a Pickering emulsion in the presence of water and organic solvents such as toluene, and performed as a new effective medium for the epoxidation of olens with H2O2.166 Later, Song et al. prepared surfactant encapsulated amphiphilic lanthanidecontaining POMs for the oxidative desulfurization reaction. The results indicated that (DDA)9LaW10/[omim]PF6 catalytic emulsion system with H2O2 as the oxidant was one of the most efficient desulfurization systems.167 In 2013, Li and Wu's group prepared a photoresponsive surfactant-encapsulated POM complex, in which the surface of the POM was electrostatically modied with cationic surfactants bearing azobenzene groups at the hydrophobic ends (Fig. 11). By alternating the irradiations with UV and visible light, recycling of this POM-based catalyst was achieved through a reversible phase-transfer shuttle.168 3.2.3 Ionic liquid cations. In general, ionic liquids (ILs) have been rapidly gained in interest due to their unique physicochemical properties, such as their large liquid state range, low melting temperature, non-volatility/ammability, favorable solvation behavior, high thermal stability, and ionic conductivity.64
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Fig. 11 Chemical structures of cationic surfactant (AzoC6)2N+Br and a POM cluster and preparation of an Azo–SEP complex, as well as reversible phase transfer of the complex between toluene and H2O/ DMF mixed solution. Reprinted with permission from ref. 168. Copyright (2013) American Chemical Society.
Recently, many attempts have been made to utilize the IL cations as adjustable counter-cations to pair with POM cluster anions, generating a family of IL-POM hybrids that can be applied in electrochemistry, catalysis, materials science, and so on.169 To date, there are two series of IL-POM hybrid catalysts, including POM anion-based ILs (POM-ILs) with low melting points around 100 C, and IL cation-based POM ionic solids (IL-POMs) with high melting points of more than 250 C. All of them have shown potential application in heterogeneous catalysis. Beginning around 2004, the rst family of POM-based ionic liquid salts were obtained by partial exchange of the surface protons of the 12-tungstophosphoric acid core cluster by a PEG-containing quaternary ammonium cation.170 Subsequently, a few examples of these IL compounds (e.g., [(n-C4H9)4N]2M6O19, [Cnmim]3PW12O40, [(n-C4H9)4N]4S2M18O62 (M ¼ Mo, W; n ¼ 2, 5)) were described, and they were oen used in electrochemical process.171,172 Furthermore, some examples of IL-POM hybrids, including 1-butyl-3-methylimidazolium[bmim]3[PO4(W(O)(O2)2)4],173 [bmim]4[W10O23],174 175 [bmim]3PW12O40, N-dodecyl pyridinium [Dopy]3PW12O40,176 and guanidinium-based POMs177 have been prepared as homogeneous catalysts for the epoxidation of olens and the oxidation of alcohols with H2O2 using ILs (e.g., [bmim][BF4] and [bmim][PF6]) as solvents. In order to improve the recovery and recycling of the catalysts, various efforts have been made in the preparation of IL-POM hybrid heterogeneous catalysts. To date, recent numerous research studies have demonstrated the construction of recyclable IL-POM phaseseparation or heterogeneous catalysts for various reactions, including for acid-catalyzed esterication and transesterication reactions, the Beckmann rearrangement, the conversion of biomass, and especially for diverse oxidations, such as oxidative desulfurization, epoxidation of olens, oxidation of alcohols and the oxidation of benzene. Typical examples of these catalysts and their applications in liquid organic reactions are listed in Table 2. In 2009, our group178 reported the preparation of a series of nonconventional heteropolyanion-based ILs (HPA-ILs: [MIMPS]3PW12O40, [PyPS]3PW12O40, and [TEAPS]3PW12O40, see Fig. 12) tethered with propane sulfonate functionalized organic cations. These hybrids had high melting points above 100 C.
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[BuPyPS]PW, [PhPyPS]PW and [BzPyPS]PW
Schiff base structured pyridinium POM [PySaIm]3PW HDIm]2[W2O11] and [HHIm]2[W2O11] PEG-2000 chain-functionalized alkylimidazolium H3PW12O40 PEG chain-functionalized N-dodecylimidazolium POMs Nitrile-tethered pyridinium[C3CNpy]4HPMoV2 Amin-attached imidazolium MimAM(H)-PW Amino-functionalized bipyridinium DPyAM(H)-PW Dicationic methylimidazolium POM [Dmim]1.5PW and [Dmim]2.5PMoV V Schiff base functionalized IL with V-containing POM Alkyl-functionalized imidazolium POMs [Cn+2mim]3PM PdII-coordinated IL-POM[(C3CNpy)2Pd(OAc)2]2HPMoV2 [C12mim]5PTiW11O40, [CTA]5PTiW11O40 and [TBA]5PTiW11O40 Q3[PMo12O40] [Q ¼ tetra-n-butylammonium (TBA), n-butylpyridinium (BP), cetylpyridinium (CP)] 1-Hexadecyl-3-methyl-imidazolium cation with peroxomolybdate anion Dihydroxy-tethered guanidinium mesoporous IL-POM [TMGDH]2.3H0.7PW Alcohol amino group-functionalized guanidinium mesoporous IL-POM [TMGHA]2.4H0.6PW
8
9
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24
23
22
21
17 18 19 20
12 13 14 15 16
10 11
2 3 4 5 6 7
[MIMPS]3PW12O40, [PyPS]3PW12O40, and [TEAPS]3PW12O40 [TPSPP]3PW12O40, [MIMPSH]2.0HPW12O40 [MIMPS]3PW12O40, [DPySO3H]1.5PW [PyBS]3PW12O40 [MIMBS]3PW12O40 [TMEDASO3H]1.5PW12O40 [PSPy]3PW
1
Liquid–liquid–solid triphasic systems
Liquid–liquid–solid triphasic systems
Reaction-controlled foam-type POM catalyst
Solvent-free heterogeneous catalytic systems
Liquid-solid heterogeneous catalytic systems Liquid-solid heterogeneous catalytic systems Liquid-solid heterogeneous catalytic systems Liquid-solid heterogeneous catalytic systems
Epoxidation of cis-cyclooctene with H2O2 in the presence of small amounts of solvent CH3OH Oxidation of benzyl alcohol with H2O2 using water medium (on water)
Oxidative desulfurization process
Cyclooctene epoxidation by TBHP and H2O2
Epoxidation of olens with H2O2 Hydroxylation of benzene to phenol with H2O2 Epoxidation of alkenes with H2O2 Oxidation of benzyl alcohol with H2O2 Oxidation of benzyl alcohol with H2O2, Hydroxylation of benzene with H2O2 Hydroxylation of benzene with H2O2 Oxidation of suldes with H2O2 Aerobic oxidation of benzene to biphenyl Epoxidation of olens with H2O2 in ethyl acetate
Reaction-induced phase-separation catalyst Emulsion catalytic systems Self-separation and thermoregulated catalysts Reaction-controlled phase-transfer process Liquid-solid heterogeneous catalytic systems Solvent-free heterogeneous catalytic systems Liquid-solid heterogeneous catalytic systems
Epoxidation of olens with H2O2 Esterication and oxidative esterication
Acid–base bifunctional heterogeneous catalyst
Esterication reactions Beckmann rearrangements Transesterication of trimethylolpropane Conversion of furfuryl alcohol into alkyl levulinates Formation of HMF and EL Deep desulfurization of dibenzothiophene
Esterication reactions
Oxidation of thioethers and thiophenes and deep desulfurization of model fuels Knoevenagel condensation
Phase transfer and heterogeneous systems Liquid-solid heterogeneous systems Self-separation systems Heterogeneous catalysts Solid acids catalysts Combination of extraction and catalytic oxidation systems Thermoregulated phase-separable catalysts
Reaction-induced self separation
Reaction types
IL-POM catalysts
No.
Features of catalytic systems
Summary of the solidification of POMs with IL cations and their phase transfer and heterogeneous catalytic applications
Table 2
211
210
207
206
202 203 205
195 and 196 197 198 199 200 and 201
193 194
192
191
180 and 185 182 and 183 184 187 189 190
178 and 179
Ref.
Review RSC Advances
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Fig. 12 Top: a series of novel heteropolyanion-based ILs. Bottom: photographs of the esterification of citric acid with n-butanol over [MIMPS]3PW12O40. (a) [MIMPS]3PW12O40 (light brown solid at bottom), citric acid (white solid in the middle), and alcohol (liquid in the upper level) before mixing; (b) homogeneous mixture during the reaction; (c) heterogeneous mixture near completion of the reaction; (d) at the end of the reaction; the catalyst has precipitated. Reprinted with permission from ref. 178. Copyright (2009) Wiley-VCH.
When HPA-IL solid catalysts, such as [MIMPS]3PW12O40, are used in the esterication reactions, with one of the reactants being polycarboxylic acid or polyol, they can be completely dissolved in one substrate: the polycarboxylic acids, at reaction temperature or in another substrate, the polyols, at room temperature, but insoluble in the product (ester). Thus, it is homogeneous at the early stage of the esterication reaction. With the consumption of the polycarboxylic acids or polyols, the system becomes heterogeneous, inducing a spontaneous selfseparation of the catalyst (Fig. 12). As a result, these heteropolyanion-IL catalysts were used as “reaction-induced self-separation catalysts” in the esterication reaction, combining the advantages of both homogeneous and heterogeneous catalysis. In our following works, we explored the design concept on the fabrication of various other solid acid catalysts, by combing modied IL-structured cations with heteropolyanions.179–181 Inspired by the above design, many related works emerged for various organic reactions catalyzed by sulfonate functionalized acid POM-IL hybrid solids. Our group further observed that the sulfonated imidazolium salt of phosphotungstate [MIMPS]3PW12O40 can efficiently catalyze the Beckmann rearrangement of various oximes to the corresponding amides in the presence of ZnCl2.182 Very recently, as a continuation of this work, we reported a new dual-sulfonated dipyridinium phosphotungstate [DPySO3H]1.5PW, prepared by pairing Kegginstructured HPA of the phosphotungstate anion with dualsulfonated 4,40 -dipyridinium IL-cation. The catalyst was used as an efficient heterogeneous catalyst for the low-temperature rearranging of cyclohexanone oxime to 3-caprolactam in the absence of the co-catalyst ZnCl2.183 Yan et al. reported a series of
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HPA salts as catalysts for the transesterications of trimethylolpropane with various methyl esters and studied the inuence of organic cations and heteropolyanions on the reaction, indicating that pyridinium with PW12O403 as the anion [PyBS]3PW12O40 showed the best catalytic performance.184 Han et al. synthesized sulfonated IL-POM catalysts [MIM-PSH]xH3xPW12O40 and employed them as efficient and reusable catalysts for the palmitic acid esterication to biodiesel.185 Moreover, Li et al. tried to support heteropolyacid SiW12O40-based ionic liquid (SWIL) on silica to prepare SWIL/SiO2 heterogeneous catalysts, and applied them in the esterication of oleic acid for biodiesel production. The fresh SWIL/SiO2 had a high catalytic activity and could be easily separated through simple ltration; however, the leaching of SWIL in the reaction caused the deactivation of SWIL/SiO2.186 In 2011, Zhang and coworkers prepared the heterogeneous catalyst of methylimidazolebutylsulfate phosphotungstate [MIMBS]3PW12O40 and applied it to catalyze the conversion of furfuryl alcohol into alkyl levulinates.187 Rogers and coworkers reported the utilization of an ionic liquid-compatible form of POM [C2mim]H [PV2Mo10O40] as a heterogeneous catalyst for the dissolution and delignication of wood using the IL [C2mim]OAc (1-ethyl-3methylimidazolium acetate) as the solvent.188 Recently, Chen and coworkers189 prepared a series of phosphotungstic acidderived IL-POMs (Fig. 13), of which the dicationic IL-POM [3$2H]3[PW12O40]2 catalyst showed the highest performance in synthesizing 5-hydroxymethylfurfural (HMF) and ethyl levulinate (EL) with high HMF and EL yields of up to 99% and 82%, respectively. Li and coworkers190 proved that the sulfonated POM-IL [PSPy]3PW could be used as an effective catalyst for the desulfurization of fuels in [omim]PF6 by using aqueous H2O2 as the oxidant. Raee et al. synthesized a series of POM-ILs catalysts, namely, [BuPyPS]PW, [PhPyPS]PW, and [BzPyPS]PW, which performed as efficient thermoregulated phase-separable catalysts for the selective oxidation of organic sulfur compounds in aqueous media. This POM-IL catalyst can be easily separated from the products upon cooling of the reaction solution and reused.191 Besides for the sulfonated group modied ionic liquid, many approaches have also been proposed to explore the preparation
Synthesis of IL-POMs. Taken from ref. 189 with permission from RSC Publications.
Fig. 13
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Fig. 14 The acid–base cooperative mechanism proposed for the heterogeneous Knoevenagel condensation catalyzed by the Schiff base tethered [PySaIm]3PW ionic hybrid. Reprinted with permission from ref. 192. Copyright (2012) Wiley-VCH.
of other types of IL-based hybrid materials containing organic cations with POM anions and their applications in various reactions. For example, the introduction of basic groups in the IL cations will lead to basic properties in the IL-POM hybrids, though reports are rare. In 2012, our group designed a Schiff base structured acid–base bifunctional ionic solid catalyst [PySaIm]3PW by the anion exchange of the IL precursor 1-(2salicylaldimine)pyridinium bromide ([PySaIm]Br) with the Keggin-structured sodium phosphotungstate (Na3PW12O40). This acid–base bifunctional catalyst could efficiently catalyze the Knoevenagel condensation of various substrates in solvent ethanol or even under solvent-free conditions. Fig. 14 shows the acid–base cooperative mechanism in the heterogeneous Knoevenagel condensation. During the reaction, the weakly acidic proton of –OH in salicyl and the lone-electron-pair-bearing nitrogen of an imine group provide the acidic and basic sites to activate the C]O bond of the benzaldehyde and methylene hydrogen, respectively. Moreover, the Schiff base structure enables a proximate position for the acid and base sites with a suitably short distance for the synergistic catalysis.192 Compared with acid and base catalytic reactions, organic oxidation reactions are more versatile yet complicated, and have received various attention. Much effort has been directed toward designing POM based-catalysts for oxidant reactions. The following are two examples of POM-based phase-transfer catalysts. In 2009, Hou's group193 synthesized two protic alkylimidazolium POMs ([HDIm]2[W2O11] and [HHIm]2[W2O11]), together with two corresponding aprotic N-methyl-alkylimidazolium POMs ([HMIm]2[W2O11] and [DMIm]2[W2O11]). Among the above POM-IL catalysts, [HMIm]2[W2O11] was the most efficient reaction-induced phase-separation catalyst for the epoxidation in CH3OH–H2O medium. The present reaction system switched from triphase to biphase during the epoxidation reaction, and the catalyst was visually self-precipitated at the end of reaction. Later, Hou's group194 prepared a new family of POM-ILs by partial exchange of the protons of the H3PW12O40 with a PEG2000 chain-functionalized alkylimidazolium chloride (Fig. 15). This POM-IL was employed as an excellent emulsion catalyst in the esterication of alcohols with acetic acid. The emulsion was stabilized by the amphiphilic characteristic of the ionic liquid catalyst, and facilitated both the catalytic reaction and the products separation. Moreover, the POM-IL catalyst was also extended to the oxidative esterication of aldehydes with methanol. Subsequently, a series of POM-based room
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Fig. 15 Top: the structure of the POM-based ionic liquid. Bottom: different stages of esterification between acetic acid and ethanol: (a) before the reaction; (b) during the reaction; and (c) at the end of the reaction, after adding cyclohexane. Reprinted with permission from ref. 194. Copyright (2010) Wiley-VCH.
temperature ILs with PEG chain-functionalized N-dodecylimidazolium cations were prepared as self-separation and thermoregulated catalysts for the epoxidation of olens with H2O2.195,196 Recently, our group synthesized a nitrile-tethered pyridinium phosphovanadomolybdate [C3CNpy]4HPMoV2 with a high melting point of ca. 230 C, which behaved as a highly efficient catalyst for the H2O2-mediated hydroxylation of benzene to phenol (yield: 31.4%). The catalyst [C3CNpy]4HPMoV2 caused a reaction controlled phase-transfer process, as shown in Fig. 16. In the absence of H2O2, the orange powdered catalyst [C3CNpy]4HPMoV2 was insoluble in the mixture of the reaction solution containing the substrate and solvent, even at a relatively high temperature of 60 C. With the addition of H2O2, the reaction started along with the dissolving of the catalyst in the reaction media, forming a homogeneous liquid phase in the succeeding reaction. At the end of the reaction, the catalyst was self-precipitated as a dark green powder that could be recovered by ltration and recharged in the next run. During the recycling test, the homogeneous solution with an orange color appeared once again with the addition of H2O2, showing the good reusability.197
Fig. 16 Photographs of the [C3CNPy]4HPMoV2-catalyzed phasetransfer hydroxylation of benzene with H2O2; reaction conditions: catalyst 0.1 g, molar ratio of H2O2 to benzene 3 : 1, amount of the mixed solvent acetonitrile and acetic acid (volume ratio 1 : 1) 6 mL, 60 C, 2 h. (a) Reaction mixture without adding H2O2; (b) homogeneous solution after adding H2O2; and (c) precipitation of the catalyst at the end of the reaction; the white solid is the stirring bar. Taken from ref. 197 with permission from RSC Publications.
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Fig. 17 Synthesis of the ionic solid hybrid MimAM(H)-PW. Taken from ref. 198 with permission from RSC Publications.
For oxidation reactions using H2O2 or O2 as the oxygen sources, many POM-based organic–inorganic hybrids paired with “task-specic” IL cations have been designed as the heterogeneous catalysts for various oxidations, and are described in the following. In 2011, our group198 designed POM-based ionic solids, prepared by protonating and anion-exchanging the amino attached ionic liquid cations with Keggin POM-anions (Fig. 17). The typical catalyst MimAM(H)-PW was a semi-amorphous solid with the micro-morphology of nanospheres. The solid was insoluble in almost all the commonly used solvents, except for being sparingly soluble in DMSO, therefore, it can be used as a heterogeneous catalyst in the liquid-solid biphasic epoxidation of alkenes with H2O2 using CH3CN as the solvent. The catalysis results indicated that the catalyst had the advantages of convenient recovery, steady reuse, high conversion, and selectivity. The superior catalytic performance was ascribed to the inuence of the amino groups within the IL cations and the coexistence of terminal W6+/W5+ species in MimAM(H)-PW through the formation of WV–O–O/AM at the interface of the amino-cations and PW anions. Moreover, owing to the above effects, the degradation of the intramolecular W–O–W bonds was inhibited, which may explain the undegradable solid nature of the hybrid catalyst in the H2O2-based reaction.198 Subsequently, our group developed a series of POM-based ionic hybrids by modifying POM anions with various task-specic IL cations toward efficient heterogeneous catalysts for various oxidation reactions, including for the epoxidation of olens, the oxidation of alcohols, the oxidation of suldes, and the oxidation of benzene. For instance, a new amino-functionalized bipyridine-heteropolyacid ionic hybrid, DPyAM(H)-PW, was prepared by protonating and anion-exchanging the aminoattached bipyridine ionic liquid with phosphotungstic acid, and could be used as a highly efficient heterogeneous catalyst for the solvent-free oxidation of benzyl alcohol with a high turnover frequency (TOF) of 350 h1.199 A simple dicationic methylimidazolium IL-POM ionic hybrid [Dmim]1.5PW was also prepared for the heterogeneous oxidation of alcohols with H2O2 in the solvent mixture of CH3CN and H2O.200 By paring the Vcontaining POM anion PMo10V2O405 with the above dicationic IL [Dmim]Br2, the obtained POM-based ionic hybrid [Dmim]2.5PMoV was able to heterogeneously catalyze the hydroxylation of benzene using H2O2 as the oxidant, giving a relatively high phenol yield of 26.5%.201 A series of alkylfunctionalized imidazolium IL-based POMs [Cn+2mim]3PM was also prepared. Among these IL-POMs, the catalyst [C4mim]3PM with the butyl chain was insoluble in the solvent
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and oxidative reaction mixture used in the oxidation of suldes with H2O2 and thus resulted in a liquid-solid heterogeneous process throughout the reaction, offering a very high conversion of 98.8%, with a selectivity to methyl phenyl sulfoxide of 98.4%.202 Furthermore, “task specic” IL cations with suitable functional groups were used to coordinate Pd(OAc)2 or a metal VSchiff base and then paired with the PMo10V2O405 anion for preparing a multifunctional heterogeneous catalyst. The resulting metal-IL–POM ionic hybrids can be applied in the oxidation of benzene. In 2012, we reported a POM-based PdIIcoordinated ionic catalyst, [(C3CNpy)2Pd(OAc)2]2HPMoV2, by pairing Keggin POM-anion HPMo10V2O404 with PdII-coordinated nitrile-tethered pyridinium IL cations (Fig. 18). The catalyst was the rst example toward an efficient heterogeneous system for the aerobic oxidation of benzene to biphenyl, with a high yield of 18.3%.203 Recently, Leng and Wang et al. prepared a new organometallic-polyoxometalate hybrid by anionexchange of the V Schiff base functionalized ionic liquid with the V-containing Keggin-type polyoxometalate. The hybrid solid contained two types of catalytic active V components, thus demonstrating a remarkable capability for the heterogeneous hydroxylation of benzene, giving a superior phenol yield of 19.6%, and a selectivity of 100%. Aer reaction, the catalyst can be simply recovered by ltration and reused at least four times, showing relatively good reusability.204 Other groups have also reported many IL-POM ionic hybrids for heterogeneous catalysts in various oxidation reactions. For instance, Hou et al. prepared three IL-structured Tisubstituted polyoxometalates, including [C12mim]5PTiW11O40, [CTA]5PTiW11O40, and [TBA]5PTiW11O40, by pairing different IL precursors with the Ti containing POM anions. Among these, [C12mim]5PTiW11O40 had a moderate BET surface area of 26.2 m2 g1 and can efficiently catalyze the heterogeneous epoxidation of cis-cyclooctene with H2O2 in the solvent of ethyl acetate.205 Agustin and Poli et al. applied the phosphomolybdate salts of Q3[PMo12O40] [Q ¼ tetra-n-butylammonium (TBA), nbutylpyridinium (BP), cetylpyridinium (CP)] in solvent-free epoxidation by H2O2 or TBHP, and systematically investigated various reaction parameters such as induction times, activity, selectivity, interface, and mass transport. The catalyst (BP)3[PMo12O40] showed a high TOF value of 471 h1 in cyclooctene epoxidation by TBHP, with a conversion of 90.7% and a
Fig. 18 Synthesis of the POM-based PdII-coordinated ionic catalyst [(C3CNpy)2Pd(OAc)2]2HPMoV2. Taken from ref. 203 with permission from RSC Publications.
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selectivity of 71.6%. Aer three runs, the conversion and selectivity changed to 85.5% and 74.8%, respectively.206 Recently, Zhu and Li et al. reported a novel reaction-controlled foam-type POM catalyst by pairing 1-hexadecyl-3-methylimidazolium cation with the peroxomolybdate anion. When applied in the oxidative desulfurization process, the obtained catalyst switched from the powder to the foam-type active species, exhibiting high catalytic activity. Aer the reaction, the foam became brittle and returned to the powder form, and thus could be easily separated and reused (Fig. 19).207 With the development of IL-POM ionic hybrids as heterogeneous oxidative catalysts, it has been realized that some ne structures, such as the surface microenvironment, porous structures, and the hydrophilic and hydrophobic properties, also signicantly inuence the heterogeneous catalytic activities of POM-based catalysts. As is well known, mass-transfer limitations are usually encountered in liquid-phase oxidations, and thus how to enhance the mass-transfer by tuning the morphology and porous structure of POM-based heterogeneous catalysts has attracted various research interests.208,209 Besides the porous carriers supported POM catalysts, recent work from our groups has focused on the self-assembly of mesoporous POM catalysts using suitable organic counter cations.210,211 In 2013, our group successfully synthesized a mesoporous IL-POM hybrid [TMGDH]2.3H0.7PW by the self-assembly of a new dihydroxy-tethered guanidinium-based IL and Keggin-type H3PW12O40 (Fig. 20).210 The mesoporous [TMGDH]2.3H0.7PW possessed an irregular coral-shaped morphology and a nanoscale hollow structure, and had a higher BET surface area of about 30 m2 g1 and a narrow mesopore size centered at 6.2 nm. The hybrid catalyst [TMGDH]2.3H0.7PW was used as an efficient liquid-liquid-solid triphasic catalyst in the epoxidation of ciscyclooctene with H2O2 in the presence of a small amount of polar protic solvent CH3OH. A “substrate-solvent-catalyst” reaction mechanism was proposed in Fig. 20 to understand the superior performance of the catalyst [TMGDH]2.3H0.7PW. The epoxidation process occurred through three steps, involving the formation of the ve-membered ring structure I via the
Fig. 19 Schematic illustration of the oxidation of sulfur compounds by
a reaction-controlled foam-type catalyst (C16mim)2Mo2O11. Reprinted with permission from ref. 207. Copyright (2013) American Chemical Society.
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Top: synthesis of dihydroxy-tethered guanidinium-based polyoxometalate [TMGDH]2.3H0.7PW. Bottom: proposed catalytic mechanism for [TMGDH]2.3H0.7PW catalyzed epoxidation of ciscyclooctene with H2O2 in CH3OH. Reprinted with permission from ref. 210. Copyright (2013) Wiley-VCH. Fig. 20
hydrogen-bonding interaction between H2O2 and the hydroxyl of methanol; the formation of {PO4[WO(O2)2]4}3 containing tungsten-peroxo complex II through the contact of complex I with the Keggin-framework W sites; the formation of the possible tungsten-peroxo-cyclooctene intermediate structure III through the attacking of the substrate cis-cyclooctene with the tungsten-peroxo species in II, giving the corresponding epoxide product. From the above mechanism, it can be seen that the
Fig. 21 (I): synthesis of alcohol amino group-functionalized guanidinium polyoxometalate [TMGHA]2.4H0.6PW. (II): the proposed on-water catalytic reaction route for the [TMGHA]2.4H0.6PW-catalyzed triphase oxidation of benzyl alcohol with H2O2. Reprinted with permission from ref. 211. Copyright (2014) American Chemical Society.
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mass-transfer was one important factor in the hetero-catalytic process, and therefore, the inuences of morphology and pore structure become dominate in the “substrate–solvent–catalyst” interaction. Due to the loosely packed nanoparticles with moderate mesopores providing a suitable situation, the obtained hydroxy-rich mesostructured [TMGDH]2.3H0.7PW hybrid showed high catalytic activity in the epoxidation using protic solvents.210 Later in 2014, as a continuation of the above work, we designed a similar self-assembled mesoporous ILPOM hybrid [TMGHA]2.4H0.6PW, prepared by pairing the alcohol amino group-functionalized guanidinium cation (TMGHA) with PW12O403 anions (PW) (Fig. 21).211 The hybrid catalyst had a uffy coral-shaped morphology and a typical mesoporous structure, with a moderate BET surface area of 25 m2 g1. Moreover, the contact angle test showed that the hybrid [TMGHA]2.4H0.6PW had a hydrophilic–hydrophobic balanced surface that exhibited good wettability for both water and organic substrates like benzyl alcohol. Thus, the hybrid catalyst can efficiently catalyze the liquid-liquid-solid triphasic oxidation of benzyl alcohol with H2O2 using water as the medium. During the reaction, the triphase catalytic system showed a special “on water” promotional effect, mainly due to the suitable mesostructure and surface wettability (Fig. 21).211
4 Other heterogenized POM-based catalysts Besides the above-mentioned POM-based heterogeneous catalysts prepared from the “immobilization” or “solidication” method, there are also some other POM-based heterogeneous catalysts for liquid phase organic reactions, and these are concisely presented below. For instance, several covalently linked metal(salen)–POM complexes (such as [Mo(O)2(salen)POM], [MoO2(acac)-POM] and Pd(salen)-POM, and L1-Mn-SiW) have been reported as heterogeneous catalysts for the oxidation of alkenes,212,213 the Suzuki coupling reaction,214 and for the esterication of phosphoric acid with lauryl alcohol.215 Several POM-based coordination polymers have performed as heterogenous catalysts in various oxidations.216–220 For example, Wu's group reported a unique layered POM-MnIII-metalloporphyrinbased hybrid material via a two-step synthesis strategy. The hybrid solid demonstrated a remarkable capability for scavenging dyes and the heterogeneous selective oxidation of alkylbenzenes with excellent product yields and 100% selectivity.218 Wu's group also reported a 1D anionic POM, [Mn4(H2O)18WZnMn2(H2O)2(ZnW9O34)2]4 by using {Mn2Zn3W19} as a building block and the MnII cation as a connecting node, and applied it in the oxidative aromatization of Hantzsch 1,4-dihydropyridines. The solid material presented interesting catalytic activity that was dependent on the inserted heterogeneous metal cations.219 Recently, Lu and Wang et al. prepared purely inorganic porous frameworks using catalytically active [MnV13O38]7 clusters as nodes and rare earth ions as linkers. The obtained POM-based porous framework was a kind of multifunctional material, exhibiting remarkable catalytic activity for the heterogeneous oxidation of suldes.220
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5 Conclusions and outlook Polyoxometalates have shown their numerous potential as catalysts for liquid phase organic reactions, and many approaches have been proposed to prepare easy recoverable and recycling POM-based heterogeneous catalysts and to apply them in various acid–base and oxidation reactions. This review summarizes the recent advances in the two main synthetic strategies, the immobilization and solidication of POMs active species for heterogeneous POM-based catalysts. The immobilization strategy benets from the highly dispersed active POM center on large surface porous supports through various interactions, such as physical adsorption, Van der Waals forces, hydrogen bondings, and electrostatic and covalent interactions. Development in materials science and synthesis technology provide numerous new functional supports, causing a rapid development of the immobilized POMs catalysts. How to choose suitable active POMs and supports to fabricate favorable micro-environments (pore structure, acidity, hydrophilic and hydrophobic properties, etc.) toward certain reaction becomes one crucial factor to the design of efficient heterogeneous POMs catalysts. The degradation of POMs during the immobilization is another issue that requires attention and should be further studied. The solidication method avoids the utilization of supports, and favors facile synthesis and a high density of active sites, and thus has been receiving ever increasing attention recently. Especially, organic modiers, such as ionic liquid cations, have been revealing their special advantage in adjusting various properties of the POM-based catalysts, such as solubility, surface state, and acidic and basic properties, as well as redox capability, producing various efficient heterogeneous catalysts for different liquid organic reactions. Owing to the “pseudoliquid phase” behavior toward strong polar small substrate molecules, POMs solids can still perform activity even in the nonporous form. Indeed, most current insoluble POMs salts are nonporous materials. Faced with the requirement to deal with bulky substrates and to decrease the mass transfer resistance, one promising attempt is to fabricate porous heterogeneous POMs catalysts through a self-assembly solidication process. In particular, the uniform pore structure may contribute to the shape selective catalysis. Regardless of whether they are immobilized or solidied POM-based heterogeneous catalysts, they usually suffer from leaching of the active sites during liquid phase organic reactions (especially in the case of H2O2-involved oxidation reactions) due to the slight dissolving of the whole POM center or degradation of the POM anions, therefore, limiting the reusability of the POM-based heterogeneous catalysts. Therefore, additional improvements in the stability and recycling properties are still challenges for scientists and are stimulating further numerous research works. Moreover, the majority of the applications of POM-based heterogeneous catalysts have focused on acid or oxidation reactions, rarely are they related to the base reaction. How to develop POM-based heterogeneous base catalysts therefore remains a large research blankness.
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In general, the synthesis and application of POM-based heterogeneous catalysts have been evolving into a very exciting research area. The combination of various interdisciplinary sciences by using different technologies and methodologies will dramatically drive the further development and provide a great number of opportunities toward the facile recovering and recycling of POMs catalysts. Moreover, the versatility and individuality of numerous organic reactions also provide various opportunities and challenges in this eld.
Acknowledgements We thank the National Natural Science Foundation of China (nos 21136005, 21303084 and 21476109), and Jiangsu Provincial Natural Science Foundation for Youths (no. SBK201342704).
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