catalysis by oxometalates and their microheterogeneous media
TRANSCRIPT
CHAPTER 9
Catalysis by Oxometalates andTheir MicroheterogeneousMedia
J. LODH AND S. ROY*
EFAML, Materials Science Center, Department of Chemical Sciences,Indian Institute of Science Education & Research, Mohanpur Campus,Kolkata, India*Email: [email protected]
9.1 IntroductionCatalytic processes are of great interest, can have a long-lasting effectand can even change the dynamics and demographics of global popu-lation.1 Catalytic processes, thus, have been of immense economic im-portance, and include the Haber–Bosch process for activation of nitrogen,2
oxidation of sulphur-dioxide for synthesis of sulphuric acid,3 epoxidationcatalysis,4 Ziegler–Natta catalysis,5 olefin metathesis6–8 and hydrogenationreactions.9,10 Using supramolecular interactions, supramolecular catalysishas also led to several possibilities including, but not limited to, self-organisation and complex matter.11 Using the multiplicity of weakerinteractions and entities with diffuse interfaces, soft-matter systems be-came important in micellar catalysis and related processes.12 All suchcatalytic reactions had an immediate effect on the human condition byaddressing hunger alleviation and significant life-style changes. In many ofthese catalytic processes, metals and especially transition metals and theiroxides play a significant role. Thus, transition metals and their oxides have
Catalysis Series No. 41Vanadium CatalysisEdited by Manas Sutradhar, Jose Armando L. da Silva and Armando J. L. Pombeiror Royal Society of Chemistry 2021Published by the Royal Society of Chemistry, www.rsc.org
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been an agent of choice for carrying out important catalytic processes.In recent examples of catalytically important reactions ranging from C–Xactivation,13 C–C coupling,14 OER,15 ORR16 to small molecule activation,17
transition metals and, at times, their oxides, continue to play an importantrole as catalysts. In this chapter, a closer look on transition metal-basedmodel systems, oxometalates in both molecular state of oxometalates andin their colloidal state of soft-oxometalates in the context of catalysis isundertaken. In doing so, we explore the effects of the physical state ofoxometalates (molecular and microheterogeneous or soft-matter) oncatalysis.
Polyoxometalates (POMs) can be viewed as a crystalline state of discretemolecular anionic clusters constituted of early transition metals (M¼V, Nb,Ta, Mo, W) in their highest oxidation state, which might also include certainmain group heteroatoms (X) such as B,18 P,19 Al20 and Ge.21 The incorpor-ation of these heteroatoms in the molecular cluster renders these ox-ometalates [XaMbOc]n� highly stable, but can also, at times, be highlyreactive. Some of these early transition metals (M) can be substituted withother metals like Cr,22 Mn,23 Fe,24 Co,25 Ni26 and Cu27 and affects theirproperties and reactivity.28 This stability coupled with reactivity of suchsystems renders the field of oxometalates as an area of immense importanceand has been the subject of several books and reviews.29–34 We now give abrief overview of recent design strategies of oxometalates.
The design of oxometalates was performed by Patzke from a molecularbuilding block to an extended structure.35 Understanding the formationof molecular POMs from sub-molecular precursors has been studied byCronin and Poblet36,37 and also by the authors of this chapter.38 Usingsimple systems like Keggins, Bera and Antonio have demonstrated theformation of liquid like microheterogeneous media of oxometalates (soft-oxometalate or SOM phases)39–42 and their formation in great detail.43,44
Going a step further, crystallisation of POMs in isopoly Mo7 and Mo8 enroute to a micro-heterogeneous medium of the soft-oxometalates have beenstudied.45,46 More specifically, in the context of compartmentalisation andcatalysis, the Crans group has developed a very elegant method for effectivereactions and studying their pathways in great detail.47,48 Exploring theSOMs, the Wu group has very elegantly demonstrated peptide-based ad-hesive SOM coacervate for environment-responsive underwater adhesion.49
SOM-based water cleaning systems employed as active matter exploringtheir photo-redox behaviour have been explored.50 From the ongoing de-velopments in the literature, such oxometalate-based entities seem to beimportant in various catalytic processes. The reason for this importancecan be attributed to: (i) their nature as acids, (ii) their capacity as an oxi-dation and reduction catalyst, and (iii) the possibility of catalysis in thesoft-matter state, thus acting as a model system to understand molecularand heterogeneous catalysis. In this chapter, we explore these facets inmore detail. We first describe the rationale of using POMs and their SOMstates as catalysts.
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9.2 The Rationale of Using Oxometalates as CatalystsOver the years, oxometalates (OMs) have been used as catalysts for a widerange of reactions such as oxidation, reduction, electrocatalysis and photo-catalysis. The redox properties of these oxometalates ensure great catalyticefficacy, as well as their stability over a pH range of 2–12 and their resistanceto oxidants. Oxometalates are considered green catalysts due to their high-oxygen content and high-atom efficiency.51,52 OMs carry multiple protonsites or metal centres with Lewis acidity. These sites work as catalytic centresin acid catalysis. Owing to this reason, they have been used to catalyse majoracid catalysed reactions such as Friedel–Crafts acylation and Fries re-arrangement for the synthesis of aromatic ketones. Instead of using strongmineral acids or traditional Lewis acids, the oxometalates can be consideredas attractive alternatives, being more acidic than solid acids such as zeoliteor mixed metal oxide,53 which makes these polyoxometalate moieties evenmore favourable for acid-catalysed reactions. Over the years, major attentionhas been given to their acidic as well as redox properties. In fact, in theabsence of protons they act as efficient base catalysts since electron-richoxygen atoms are present on the surface of oxometalates and carry a high-negative charge. In this regard, lacunary oxometalates need a special men-tion since the lacunary sites carry a high-negative charge, making it possibleto extract protons from organic reactants. In order to act as base catalysts,oxometalates should satisfy the following criteria: (1) The thermal and oxi-dative stabilities of the oxometalates need to be higher than the organicbases; (2) the metal-oxo moieties should be able to activate the nucleophilicsites; and (3) the design of basic sites should be electrically and structurallycontrolled.
POMs can be used as oxidation catalysts instead of conventional oxidationcatalysts such as H2SO4, HNO3 and other peroxy acids, as the latter producemore side products. Additionally, the focus has shifted towards greeneroxidants. Oxometalates are an attractive choice for versatile catalysis owingto their thermal and hydrolytic stability compared to organometallic cata-lysts. POMs are responsive to light and can be excited from their groundstate, which facilitates the charge transfer from oxygen to the transitionmetal d-orbitals.54 This forms the basis of several photocatalytic reactionssuch as CO2 reduction and water oxidation. These polyanions (POMs) havewell-separated HOMO and LUMO band gaps that are able to resist theelectron and hole recombination while being irradiated with light higher orequal to their band-gap energy. The photogenerated electrons and holes arecapable to initiate chemical reactions due to the strong photo-reductive andphoto-oxidative ability of electrons and holes, respectively. The metals inthese clusters in higher oxidation states easily undergo one- or two-electrontransfer. The potential of these redox couples is dependent on pH value. It ispossible to tune the catalytic activity of POMs by tuning the pH and redoxstates and engineering the band gaps. Several reviews have highlightedcatalysis by POMs exploring these facets or related aspects in the context of
Catalysis by Oxometalates and Their Microheterogeneous Media 167
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catalysis by POMs. (See Table 9.1 for an overview of various reactions cata-lysed by oxometalates.)
Catalysis by POMs refers to the molecular state (homogenous) of the ox-ometalates and the colloidal state (heterogeneous) of oxometalates. The latterstate, referred to as soft-oxometalates (SOMs) has certain advantages in thecontext of catalysis. These advantages are: (1) the presence of a large numberof MQO centres; (2) these states can be kinetically trapped active states incatalysis; and (3) they can act as a model system to study the transition fromhomogenous (molecular) to heterogeneous (soft-matter state) in catalysis.Keeping such advantages of SOMs in mind, our group has been exploring thecatalytic activity of these states in detail. We now describe the catalysis byPOMs and SOMS (the types of reactions are illustrated in Table 9.1).
9.3 POMs in Acid Catalysis
9.3.1 Friedel–Crafts Reactions and Fries Rearrangement
Oxometalates have always been well-known for their acidic properties due tothe presence of one or more acidic sites or protons. Early studies used ox-ometalates as solid catalysts for liquid-phase reactions.55,56 Using Keggin-typeheteropolyphosphotungstic acid in water, Michael-type Friedel–Crafts alkyla-tion of N,N-dialkylanilines with nitroalkanes57 has been shown. Further,studies on electron-rich arenes and different nitroalkanes showed thatN,N-dimethylanilines and N,N-diethylanilines gave maximum yield. 12-phosphotungstic acid, a Keggin, and its composite have been used for cata-lysing Friedel–Crafts reactions. Another Keggin-type heteropolyanion,12-phosphotungstic acid based ionic liquid-functionalized mesoporous co-polymer P(VB–VMS)PW, was developed and shown to lead to excellent yieldfor Friedel–Crafts benzylation of arenes with benzyl alcohol.58 Owing to itsmesoporous copolymeric structure it could be easily recovered, as well as thefunctionalization of heteropolyanion with the copolymer enhancing thestructural stability and the acidity, thus promoting the acid catalysis. Alongsimilar lines, works have been done on functionalizing heteropolypho-sphotungstic acid with acid leached porous kaolin for Friedel–Crafts acylationof anisole.59 There have also been works featuring Fe-based hetero-polytungstate supported on activated carbon, whose activated carbon porousstructure played a significant role on benzylation reaction.60 The activatedcarbon structure not only facilitated the dispersion of heteropolytungstateduring benzylation due to high-surface area, but also ensured easy recoveryfrom the reaction mixture. Later works encapsulated phosphotungstic acid inMIL-100, mesoporous metal organic framework and showed its application onacid-catalysed acetalization.61 Zeolite frameworks have also been popular as asupport to HPAs for efficient Friedel–Crafts acylation.62 Going a step furtherfrom the extended porosity of framework materially, in molecular Kepleratecapsules the porosity was explored in the context of catalysis. In 2016, usingthe confined space in {Mo132} Keplerate the encapsulation of benzene, phenol,
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Table 9.1 Different organic reactions catalysed by oxometalates.
Category Representative catalyst Reaction References
Acid Catalysed Reactions
H3PW12O40
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(NH4)42[MoVI72MoV
60O372(CH3COOH)30(H2O)72].ca300 H2O
63
Cs2[Cr3O(OOCC2H5)6(H2O)3]2[a-SiW12O40]
71
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Table 9.1 (Continued)
Category Representative catalyst Reaction References
Base Catalysed Reactions
(TMA)6-[Nb10O28]�6H2O
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[RE(H2O)7]3� P2W15Ta3O62�nH2O
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[Sm(H2O)5][Sm(H2O)7][Co2Mo10H4O38].6H2O
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Oxidation Reactions
THA3[SbW9O33(tBuSiO)3Ti(OiPr)]
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Na2H2[(CH3)4N]6[Te2W20O70{Re(CO)3}2]�20H2O
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(3-aminopropyl)triethoxysilane linked toanthracene and a monovacant polyoxometalate,(SiW11O39)
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Table 9.1 (Continued)
Category Representative catalyst Reaction References
CO2 reduction coupled withhydration of
phenylacetylene
(NH4)42[MoVI72MoV
60O372(CH3COOH)30(H2O)72].ca300 H2O
CO2þ 2e�þ 2H1-HCOOH
84
Water Oxidation coupledwith click reaction
{[K6.5Cu(OH)8.5(H2O)7.5]0.5[K3PW12O40]}
2H2O-O2þ 4H1þ 4e�
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halobenzenes, phenols have been studied, as well as for chloroarenes of theFriedel–Crafts arylation.63 Keggin, phosphomolybdic acid (H3PMo12O40), hasbeen identified as a bifunctional catalyst for Friedel–Crafts type dehydrativecoupling, i.e., C–C bond formation via the dehydrative reaction of dia-rylmethanols with nuclepohiles such as indoles, benzothiophene, benzo-furan, etc.64 The protons in the oxometalates help in the activation of alcohol,whereas polyanion helps in stabilizing the carbocation species. Reports re-lated to Fries rearrangement catalysed by several POMs/HPAs are found in theliterature.65–67 In this context, we now know that Keggin of the typeH3PW12O40 with or without a supported network has been a catalyst of choicefor Friedel–Crafts reactions for the reason that it is a stronger acid thanconventional mineral acids in homogenous media.
9.3.2 Biomass Utilisation
Biomass has always generated great interest in energy research and5-(hydroxymethyl)furfural has been the most important molecule in the util-isation of biomass. Here again, Keggins like H3PW12O40 (PW12)-, H3PMo12O40
(PMo12)- and H4SiW12O40 (SiW12)-based ionic liquids have shown great cata-lytic activity for conversion of fructose to 5-hydroxymethylfurfural and alkyllevulinate.68 Reports on the synthesis of levulinic acid and formic acid fromfructose using various ionic liquid POM-based salt are reported in the litera-ture.69 The effect of a counter-ion substituent has also been studied by partiallysubstituting H1 in H3PMo12O40 with Cs1 for one pot, one-step synthesis of2,5-diformylfuran from carbohydrates.70 Along similar lines, Cr-based het-eropolyacid crystal Cs2[Cr3O(OOCC2H5)6(H2O)3]2[a-SiW12O40] has been provenas an efficient catalyst for the same reaction. Cr is responsible for the Lewisacidity of the catalyst whereas dissociation of the polyoxometalate unit fromthe coordinated water under the polarising effect of cation generates showsBronsted acidity.71 The presence of dual acidic sites in the catalyst ensuresisomerization and dehydration of glucose to hydroxymethylfurfural (HMF).
The strategy of encapsulation of an oxometalates framework material hasbeen employed to enhance the acidity of the catalyst. Encapsulatingphosphomolybdic acid in MIL-101, leading to high Bronsted acidity andmoderate redox potential of the catalyst, facilitates conversion of fructose to2,5-diformylfuran.72 Several acid site-carrying catalysts have been popular inthis regard.73 Further, various works have also demonstrated bifunctionalacid–base catalysts using amino acids and HPAs such as (C6H15O2N2)3–x
HxPW12O40 to catalyse the selective conversion of glucose to levulinic acid.74
The amino acid in these cases, like lysine, helped the polyanions in formingmicellar assemblies in eutectic solvents.75 Such works can be taken as earlyexamples of micellar catalysis. Oxide- or silica matrix-supported POMs havebeen used as support to solid acid catalysts for selective conversion of glu-cose to 5-HMF as well as oxidation of biomass.76,77 In the matter of biomassutilisation, Keggin-type POMs seem to be the chosen catalysts.
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9.3.3 Isomerization of Alkanes
POMs also have an important role in catalysing the isomerization ofalkanes.78,79 H3PW12O40 (PMo12) and H4SiW12O40 (SiW12) had been im-pregnated into SBA-15 type silica and the resultant monofunctional hybridmaterial based on Keggin-type oxometalates supported on silica was used forisomerization of n-hexane.80 The polyoxometalate used has a broader re-action temperature window and higher di-branching selectivity than thezeolites.81 H4SiW12O40 is preferred over H3PW12O40 owing to its greater se-lectivity. Metals such as Pt were also impregnated in HPW/SBA-15 and it hasbeen observed that carbocations in this case hydrogenated faster on Ptparticles, thus making bifunctional Pt/HPA/SBA-15 more efficient catalyststhan monofunctional HPA/SBA-15.82 Recently, isomerization of cyclohexaneusing bifunctional metal-acid catalysts based on Cs salt of H3PW12O40
with Pt and Au as metallic components have been developed. Here also,Keggin-type POMs seem to be the preferred catalysts. Similar results wereobtained with PtAu/CsPW that exhibited two-fold higher activityfor n-hexane isomerization.83
9.3.4 Carbon Dioxide Reduction Coupled with ProtonFacilitated Hydration Reaction
Combining the intrinsic acidity of oxometalates and their nature of photo-redox catalysis, an unusual activity can be observed. For instance, our grouphas reported coupling of CO2 reduction with hydration of phenylacetylene.Keplerate {Mo132} catalysed reduction of CO2 to formic acid with organictransformation from phenylacetylene to acetophenone. The protons gener-ated in the process are taken up for oxidation of organic substrates. Since thereduction of carbon dioxide takes up protons in the medium released bywater oxidation, we can conclude that with decreasing pH, the concentrationof the reduced product would increase. CO2 reduction can thus be con-sidered as a proton-coupled process. Further, during hydration of phenyla-cetylene, the triple bonds act as a Lewis basic site and are attacked by theprotons in the medium and generate a vinylic carbocation. This addition ofprotons is Markovnikov’s addition. The water then reacts with the carbo-cation and tautomerization gives acetophenone. The whole reaction is thusfavoured by protons generated in the medium due to the water oxidationcatalysed by the {Mo132–CO2} unit under visible light irradiation.84 Thus, inshort, this reaction demonstrates the acidity and photo-redox capacity ofOMs as a key factor in driving catalytic reactions (Figure 9.1).
9.4 Base Catalysis and OxometalatesThe base catalysis by oxometalates has been less explored compared toother catalytic reactions. However, there have been some noteworthy basecatalytic reactions facilitated by oxometalates that will be discussed here.
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Figure 9.1 (a) A probable pathway for coupling carbon dioxide reduction with hydration of phenylacetylene catalysed by {Mo132}.[Inset: Cyclic voltammogram for CO2 adsorbed species (bottom-left corner) and NMR spectra for deuterium-substitutedproduct (top-right corner).] (b) Effect of substitution on the reaction yield is shown in the table.
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Na8H[A–PW9O34]�7H2O and Na8H[B–PW9O34]�19H2O and tri-lacunary POMshave been reported as efficient Lewis base catalysts for Knoevenagel con-densation and cyanosilation of aldehydes and ketones.85 These lacunaryPOMs have greater efficiency compared with the Keggins such asNa3[PW12O40]. From a molecular point of view, Na8H[A–PW9O34]�7H2O andNa8H[B–PW9O34]�19H2O carry three lacunary sites and nine negative char-ges, compared to monolacunary Keggin. Thus, the former can abstractprotons from ethyl cyanoacetate more easily than the latter in case ofKnoevenagel reaction. An alkaline, highly negatively charged, Keggin-typeoxometalate Na16[SiNb12O40]�xH2O for catalysing CO2 cycloaddition andKnoevenagel condensation reaction has been reported.86 Usually, Keggin-type POMs such as [H3PW12O40] are acidic and the sodium or potassiumcounterparts [e.g., Na3PW12O40] are weakly basic, thus making them un-suitable for base-catalysed reactions. The basicity of [SiNb12O40]16� has beensupported theoretically, which shows that NBO (natural bond orbital, whichdepicts the natural atomic orbitals – a complete orthonormal set to describethe constituents within the molecular environment) changes for two crys-tallographically distinct [SiNb12O40]16� a-Keggin ions from �0.9 to �1,which is an index of high basicity.86 Further, the decaniobate (TMA)6-[Nb10O28]�6H2O was found to catalyse base-promoted, aldol-type conden-sation.87 Lindqvist-type polyoxoniobate K7HNb6O19�13H2O has beenobserved to successfully catalyse Knoevenagel condensation of benzaldehydeand ethyl cyanoacetate to ethyl (E)-acyanocinnamate experimentallyand theoretically.88 Mizuno and co-workers have reported a g-Keggingermanodecatungstate [g-HGeW10O36]7� synthesised via the reaction of[g-H2GeW10O36]6� with one equivalent of [(n-C4H9)4N]OH under non-aqueous conditions.89 The catalytic efficiency was found to increasedramatically for base-catalysed reactions as the negative charge increasedfrom �6 to �7.
Ester, being a valuable chemical, makes the base-catalysed acylation ofalcohols with acyl chlorides, acid anhydrides and esters as acylating agentimportant and promising. Using [g-HGeW10O36]7� as the catalyst for acylat-ion of benzyl alcohol with vinyl acetate, the yield of the product increased upto 86% (from 5%) compared to [g-H2GeW10O36]6� as catalyst. Metal (M¼Ti,Ln) substituted Keggin polyoxometalate-based hybrid dimers have alsoshown considerable reactivity towards catalysing cyanosilylation.90 In an-other work, the synthesis of the Sm-based catalyst [Sm(H2O)5][Sm(H2O)7][Co2Mo10H4O38]�6H2O for catalysing cyanosilylation of benzaldehyde hasbeen reported.91 The higher yield of the product has been obtained due tothe synergistic effect of Lewis acid (Sm31) and Lewis base, and the surfaceoxygen atoms ([Co2Mo10H4O38]6�) lead to activation of aldehyde and trialk-ylsilyl cyanides (TMSCN). Similar reports of catalysing a cyanosilylation re-action have been obtained for rare earth-POMs with chemical formulae[RE(H2O)7]3–P2W15Ta3O62�nH2O [RE¼Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb andLu].92 Knoevengael condensation and CO2 fixation to epoxide was studied inorder to understand the basicity of POMs with group 5 [M(V)6O19]8� and
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group 6 [M(VI)6O19]2� metals.93 The base-catalysed activity increased in theorder of [Mo6O19]2�, [W6O19]2�{[H4Nb6O19]4�o[H4Ta6O19]4�, which is inaccordance with the NBO charges on the surface of oxygen atoms. It is to benoted that in the context of base-catalysed reactions, in addition to substi-tuted Keggins, hexadecaoxometalates seem to be the chosen catalysts. Itseems reasonable to state that charges on the surface oxygen controlling thebasicity of the catalysts play a crucial role in the choice of these oxometalatesas base catalysts.
9.5 Oxidation Catalysis and OxometalatesOxometalates are used as oxidation catalysts instead of traditional oxidationcatalysts such as H2SO4, HNO3 and peroxy acids, as the latter catalystsproduce more side products. With changing times, the focus has shiftedmore towards green oxidants. POMs are an attractive choice as metal cata-lysts owing to their thermal and hydrolytic stability compared to organo-metallic catalysts.
9.5.1 Epoxidation Catalysis
The oxidation of alkenes to epoxides using POMs and H2O2 as oxidant wasachieved in the early 1980s.94 A large body of work exists on the epoxidationof alkenes.95–116 Over the past five years, there has been a significantdevelopment in the field of the epoxidation of alkenes. Using Kepleratenano-sphere of the type {Mo72(M)30} as an efficient catalyst, epoxidatonof olefins in ethanol with hydrogen peroxide has been achieved (M¼Mo, V,Cr, Fe).117,118 The process showed excellent selectivity. Stereoselectivity wasobserved for norborene and other cis- and trans-olefins. Later, incorporatingpolyoxometalate with an organotriphosphonate ligand K4H6[H4{(As-W9O33)Zn(H2O)W5O11(N–(CH2PO3)3)}2(m2-O)2]�27H2O type catalyst has beenused to investigate its efficacy for alkene epoxidation in acetonitrile usingH2O2 as the oxidant.119 A green method for epoxidation of olefins usingpolyoxometalate over gold nanoparticle support, wherein the reaction can becarried out with or without using solvent and by using a stoichiometricamount of oxidant, in presence of t-butyl hydroperoxide as an initiator,120
has been developed. Similar works121–126 have also been reported in thisregard, using amphiphillic polyoxometalate nanoparticles as catalysts.Composites of [g-H2PV2W10O40]3� and [a-SiW12O40]4� supported on Fe2O3
acted as catalysts for selective oxidation using hydrogen peroxide.127 En-capsulated Zn substituted polyoxometalate [PW11Zn(H2O)O39]5� (PW11Zn) insilica nanoparticles as a model catalyst for oxidation of alkene128 led to acomplete conversion within 24 h. The integration of POM into silica leads toenhancement of the catalysis. Layered double hydroxides (LDHs) pillaredwith POMs have been used for epoxidation for a long time.129–134 Organic–inorganic hybrid polyoxometalate-based materials act as effective catalystsfor epoxidation of alkenes. Several such organic–inorganic polyoxometalate
Catalysis by Oxometalates and Their Microheterogeneous Media 177
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based frameworks HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF135
show excellent chemoselectivity. The hydroxyl group sensitive to oxidationremained completely intact in the presence of the catalyst. Cyclo-octene wasconverted to 1, 2-epoxycyclooctane with a high yield and selectivity usingKH[(CH3)4N]3–{[Re(CO)3]4[(m2-OH)(m3-O)(W5O18)]}�6H2O.136 Phase-transferagents such as alkylaminophosphazenes (RPN) can be used for oxidationusing H2O2 catalysed by polyoxometalates.137 These phosphazene cationsrender the oxometalate soluble in organic media. The activity of the OMsin decreasing order in the medium is found to be: PMo12O40
3�4PW12O40
3�cSiW12O40
4�. Keggin type transition-metal-substituted phospho-tungstates [PW11M(L)O39]m�(PW11M, M¼Co, Cu, Fe, Ni and Mn) have beenused with H2O2 in acetonitrile as efficient catalysts for oxidation of cyclo-hexene.138 Lanthanide-substituted oxometalates have also been a particularlypopular choice for epoxidation.139,140 Aromatic sulfonium octamolybdate hy-brids using aromatic sulfonium counterions and sodium molybdate havebeen used as self-separating catalysts for olefin epoxidation.141 Aromaticsulfonium octamolybdates are known to combine homogenous and hetero-geneous properties in a single moiety. A novel polymer-supported decatung-state catalyst bearing a poly(isopropylacrylamidealkylammonium) matrix hasbeen reported as a catalyst for aerobic oxidation of alkene to epoxide orchlorohydrins with moderate to excellent yield.142 Phosphomolybdic acid-based hybrid material was synthesised using polyamine-mediated bioinspiredassembly via noncovalent interactions of PMA with PAH–citrate aggregates forselectivity towards epoxidation.143 A series of seven compounds based on[P2W18O62]6� and transition metal mixed organic ligand complexes was syn-thesised and characterised for high catalytic performance for styrene epox-idation.144 Also, HPA/POM encapsulated in a metal organic framework givesexcellent yield due to the uniform distribution of HPA/POM clusters withinthe MOF coupled with multiple interactions.145 Sandwich-type oxometalatesare familiar catalysts for epoxidation of olefins.146 Similar observations forcatalysing sulphoxidation and epoxidation were made for Mg3Al–ILs–La(PW11)2.147 Further, there have been reports of functionalized catalystssynthesised by reacting silanol functionalized polyoxometalate [XW9O34�x
(tBuSiOH)3]3� and [Ti(iPrO)4] acting as a model catalyst for olefin epoxida-tion.148 Cyclooctene epoxidation has also been known to be catalysed byvanadium-substituted, Lindqvist-type polyoxotungstate149 [VW5O19]3�. It notonly catalyses epoxidation, but also facilitates decomposition of H2O2, as asignificant amount of O2 evolves. Even the isolated monomeric tell-urotungstate(IV)-supported rhenium carbonyl derivative Na2H2[(CH3)4N]6[Te2-W20O70{Re(CO)3}2]�20H2O has been exploited for epoxidation. Selectiveepoxidation of various alkenes including different cycloalkenes, styrene deriva-tives and internal and long-chain alkenes were carried out.150 There have beenmany significant reports151,152 of epoxidation of olefin using polyoxometalate orpolyoxometalate-supported catalysts. Using optically patterned trails, our grouphas been successful in carrying out site-specific epoxidation catalysis using 1-octadecene and styrene as substrates.153
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9.5.2 Alkene and Alcohol Oxidation Catalysis
In addition to epoxidation, alkenes have also been oxidised to aldehydes orketones using POMs as catalysts. Here we give a few examples. Oxometalatesshow significant activity towards selective oxidation of alkene to aldehyde. Thegroup of Neumann and Poblet employed a Cu-substituted polyoxometalate,such as {a2-Cu(L)P2W17O61}8� or {[(Cu–(L)]2WZn(ZnW9O34)2}12� (L¼NO2) as acatalyst for C–C bond cleavage reaction in alkenes to yield the correspondingaldehydes or ketones.154 In another work, the synthesis of the sandwich-typepolyoxometalate Na10H4[BiIIIBiVO[Zn2(ZnW9O34)2]�37H2O was reported whoseterminal position was substituted with a Lewis basic centre BiIII, leading toa change in chemoselectivity of catalytic oxidation reactions of alkenes anda,b-unsaturated alcohols.155 The high reactivity towards oxidation of allylicalcohol is due to the fact that in the base catalysed reaction, the Bi(III) siteleads to the generation of an alkoxide intermediate, which is oxidised by aperoxo species via oxidative b-elimination to yield the carbonyl product. Dis-persing oxo-metalates into metal–organic frameworks to catalyse selectiveoxidation of alkenes to aldehydes has been achieved.156
The selective oxidation of alcohol to carboxyl is of utmost importance tothe industry due to the significance of carboxyl compounds in our daily life,starting from perfumes to medicine. A hybrid catalyst [TMGHA]2.4H0.6PW byassembling alcohol, H3PW12O40 and guanidinium ionic liquid [TMGHA]Cl[N00-(3-amino-2-hydroxypropyl)-N,N,N0,N0-tetramethylguanidinium chloride]as a triphasic catalyst, effectively mediates the oxidation of benzyl alcohol.157
Keggin-type oxometalates158–162 have been found to be extremely effective ascatalysts for the selective oxidation of alcohols, and have therefore beenpolyoxometalate-supported on various matrixes.163–170 Another catalyst wassynthesised by attaching (3-aminopropyl)triethoxysilane linker to the an-thracene and a monovacant polyoxometalate, (SiW11O39) for catalysing theoxidation of different benzylic and aliphatic alcohols to the correspondingcarbonyl compounds using O2 as an oxidant under visible light.171 Roomtemperature-catalytic oxidation using oxovanadate, i.e., Cs5(V14As8O42Cl), asa catalyst for conversion of secondary alcohol to ketones172 has beenachieved. In another example, the sandwich-type POM {Zn3Cu2W19} withhighly oxophilic Gd31 ions was used to form a 2D inorganic porous POMframework material CZJ-11, consisting of redox copper(II) sites inside thepore space wherein CZJ-11 was simply transformed into a stable 3D porousPOM framework CZJ-12 by partial dehydration to realise the biomimeticactivation of molecular oxygen for aerobic oxidation of aliphatic alcohols.173
Furthermore, incorporation of double catalytic sites into a single moleculehas been shown to result in an intramolecular, cooperative reaction path-way, thus generating an enhanced reaction rate and higher selectivity in theselective oxidation of benzyl alcohol with H2O2 in water. The di-imidazoliumIL cations have been shown to induce a controlled phase transfer functionto P-based hybrids.174 A nanocomposite made of PdNi/RGO/oxometalatehas been used as a promising electrocatalyst for electrochemical alcohol
Catalysis by Oxometalates and Their Microheterogeneous Media 179
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oxidation. The long-term stability and excellent catalytic activity of thecatalyst can be compared to the commercial Pd/C catalyst.175 Many suchnanocomposites using oxometalates have been able to successfully catalysethe oxidation of alcohols.176 A hybrid surfactant with catalytic activityprepared by integrating [PW11O39]7� anion and an alkoxysilane by a covalentbond was reported for catalysing selective oxidation of alcohol.177
The epoxidation of olefin and oxidation of alcohol was catalysed by thepraseodymium(III)-containing arsenotungstate K16H15Li7[Pr2(H2O)3(pzdc)As3W29O103]2�38H2O.178 Oxidation of 1-phenylethanol, in particular, attains aturnover frequency of 10 170 h�1. There also have been studies on the effectof mono- and multi-valent cations on the acidity of a catalyst, whichultimately influences reactions such as the oxidation of alcohols.179 EvenOM encapsulated in MOF has been a popular choice for catalysing theoxidation of alcohols.180
9.5.3 Olefin Oxidation and Polymerisation Reaction
Photoactivity of oxometalates (OM) can be explored in the context of organicoxidation and polymerisation catalysis. Exciting a Keggin-type unit photo-chemically and generating a radical in situ has been a strategy that we used forthe polymerisation of a series of industrially important monomers. In doingso, we incorporated BMIm (1-butyl-3-methylimidazolium) type IL (ionicliquid)-based counterions that enable and facilitate a HAT type reactionand thereby induce a polymerisation cascade181 (Figure 9.2). In another re-action, we have homo-coupled the generated radicals with concomitantoxidation to synthesise di-one from an aromatic olefin182 (Figure 9.2).
9.5.4 Site-specific Oxidation Catalysis on an OpticallyPatterned Catalyst Chip
By using patterned microchips of POMs, synthesised by directed self-assembly induced in the microheterogeneous medium or SOM state ofMo7 type units, epoxidation catalysis was carried out.153 Such catalysis givesthe possibility of spatially localising the catalytic site (Figure 9.3). We showed
Figure 9.2 (a) Schematic representation of synthesis of di-one from styrene usingdifferent POMs. (b) Mechanistic pathway for polymerisation reaction.
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that epoxidation catalysis only takes place on the specific catalytic site of theheterogenized substrate (i.e., glass slide). Likewise, in another work,exploiting the same concept of spatial localisation, site-specific oxidation ofbenzaldehyde to benzoic acid was carried out using a supramolecularcomplex of a porous organic framework based on phosphonium (JUC-Z4)with Mo7 (Figure 9.3).44 In another work, polymerisation of pyrrole, anilinewith concomitant doping by the Mo7 based SOM pre-catalyst, led to thedesign of plastic conductors.183
9.5.5 Water Oxidation Catalysis
There have been ample examples of oxometalates as water oxidationcatalysts.184–188 Co-based POMs have been a popular choice as water
Figure 9.3 (a) Incorporation of ammonium heptamolybdate on JUCZ-4 (porousorganic framework). (b) Schematic representation of epoxidation ofalkenes ammonium heptamolybdate patterned trail.
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oxidation catalysts.189–196 In 2014, the Wang group reported four highnuclearity POM-based cobalt–phosphate (Co–Pi) molecular catalysts forwater oxidation.197 The oxidatively and thermally stable catalyst Na10[Co4
(H2O)2(VW9O34)2]�35H2O synthesised by the Hill group was found to catalysewater oxidation in visible and dark conditions.198 Under mild conditions, itsturnover frequency was reported to be greater than 1�103 s�1. Along similarlines, cobalt(II) substituted-silicotungstate199 was used as a visible light-driven water oxidation catalyst (WOC) where [Ru(bpy)3]21 acted as photo-sensitizer. The Wang group further synthesised three polyoxometalate basednickel clusters for visible light-driven water oxidation experiments, thehighest reported TON among the three clusters was 204.5. The photo-catalytic behaviour can be explained by virtue of structural motif of OM,band-gap structure and number of active sites.200 Nickel-based OMs havebeen one of the choices for water oxidation catalysts.201 The groups ofScandola, Kortz and Bonchio reported tetramanganese-substituted tung-stosilicate [MnIII
3MnIVO3(CH3COO)3(A-a-SiW9O34)]6� (Mn4POM) to mimic thefunctional core of oxygenic photosynthesis for water oxidation. It is known tobe the first Mn POM to be catalytically active towards photoinduced wateroxidation.202 Since Mn-based WOCs generate a low turnover number, anactive WOC based on a Mn platform was designed by using the complex[MnV(N)(CN)4]2� and employing (NH4)2[Ce(NO3)6] as the terminal oxidant.203
Further, Mn substituted POMs have been known to catalyse visiblelight-driven water oxidation reaction effectively.204 In another work, the 11iron-containing nanoscale inorganic polyanionic oxide cluster [Fe11
(H2O)14(OH)2(W3O10)2–(a-SbW9O33)6]27� (Fe11) was reported as an ex-ceptional water-oxidation catalyst with a remarkable TON of 1815� 50.24
Along similar lines, coupling the iron-containing polyoxometalate (Fe11) witha semiconductor BiVO4, photocatalytic water oxidation under acidic con-ditions was reported where the semiconductor acts as the light-harvestingunit.205 The first efforts on designing WOC-containing vanadium as a cata-lyst [(VIV
5VV1)O7(OCH3)12]� were by Santoni, Scandola, Campagna and others.
The reaction occurred in the presence of Ru(bpy)321 (bpy¼ 2,20-bipyridine)
and Na2S2O8, in an acetonitrile–aqueous phosphate buffer generating aquantum yield of 0.20.206 Plasmonic gold nanoparticles (AuNPs) when de-rivatized with polyoxometalate (H3PMo12O40) and incorporated into WO3
resulted in a large increase in water photo-oxidation currents.207 Carbonquantum dots (CQDs)/Ag/Ag3PW12O40 nanocomposites are also known to beeffective WOCs.208 There are reports of photocatalytic water oxidation usingnanomaterial doped with oxometalates confined in a metal–organic frame-work. The presence of nanocrystals allows the catalyst to be fine-tuned toincrease the water-oxidation efficiency.209 The group of Bonchio and Prato in2019 reported a system where self-assembly of multi-perylene-bisimidechromophores (PBI) were designed to function by interacting with a poly-oxometalate water-oxidation catalyst (Ru4POM) resulting in the formation of[PBI]5Ru4POM to participate in photo-assisted water oxidation.210 In anotherwork, a tetra Ru-substituted polyoxometalate, Na10[{Ru4O4(OH)2(H2O)4}
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(g-SiW10O36)2] (Ru4POM), was immobilised on glassy carbon electrodes andindium tin oxide (ITO) coated glass slides by employing a conducting poly-pyrrole matrix assembled by the layer-by-layer (LBL) technique and exhibitedelectrochemical water oxidation at high pH.211 Cobalt-based POMs have alsobeen particularly popular as electrocatalysts.212–217 A multilayer covalentcobalt porphyrin framework on multi-walled carbon nanotubes ((CoP)n-MWCNTs) was reported as an excellent electrocatalyst for water oxidation.In addition to great durability, the Faradaic efficiency for oxygen productionwas greater than 86%.218 Hexaniobate Lindqvist [Nb6O19]8� was used todevelop a method219 for efficient electrodeposition of cobalt and nickelnanostructures to study catalytic activity in water oxidation. Another workdemonstrated that anionic POM WOCs and cationic polyelectrolytes (PEs)can be efficiently incorporated on the surface of various photoelectrodeswith complex geometries such as Fe2O3, BiVO4 and TiO2, and their propertiescan be tuned to increase the efficacy of photoanodes towards water split-ting.220 Photoanodes have been designed and fabricated using cobalt POMand graphene oxide into a nacre-like, multilayer architecture on a haematitephotoanode. The polymeric base layer improves the catalytic activity by fa-cilitating the transfer of photogenerated holes for water oxidation throughmodification of the flat-band potential of the underlying photoelectrode.221
Thus, modified and heterogeneous photoanodes are becoming popularfor photo-electrochemical water oxidation or splitting.222,223 Thepolyoxometalate-based Ni coordination compound [NiII(2,2 0-bpy)3]3[{NiII(2,2 0-bpy)2(H2O)}{HCoIIWVI
12O40}]2�3H2O has been reported as an excellent elec-trocatalyst for water oxidation reaction,224 where Ni(II) acts as the catalyticcentre and the reaction follows a proton-coupled electron transfer pathwaygenerating a high turnover frequency of 18.49 s�1. Electrochemical wateroxidation has been tested using Cu-substituted oxometalates; in this regard,work has been done wherein [(a-SbW9O33)2Cu3(H2O)3]12� acts as an elec-trochemical WOC at neutral pH.225 The Das group rendered the Kegginanion [CoW12O40]6� active for electrochemical water oxidation by confiningit in the well-defined void space of ZIF-8, a metal organic framework(MOF).226 In a significant development, the Mascaros group recently re-ported a WOC-based on earth-abundant metals,227 i.e., barium salt of acobalt-phosphotungstate polyanion, which is highly effective in acidic con-ditions (pHo1) with an overpotential of 189 mV at 1 mA cm�2.
The fields of photocatalytic228 and electrocatalytic water oxidation229 haveundergone significant development over the past five years. In the matter oflow overpotential, POMs seem to be good electrocatalysts. In the context ofphoto- and electro-catalysis, clearly oxometalates and their heterogenizedmedia emerge as candidates of choice. This raises possibilities for exploringfurther coupled processes. For instance, we have reported light driven-wateroxidation coupled with C–N coupling using a Cu-based SOM, {[K6.5Cu–(OH)8.5(H2O)7.5]0.5[K3PW12O40]}. The electrons generated from photoch-emical water oxidation reduced Cu(II) to Cu(I). Cu(I) further acts as the activecatalyst for click reaction230 (Figure 9.4). Thus, oxometalates hold promise as
Catalysis by Oxometalates and Their Microheterogeneous Media 183
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catalysts for coupled processes not just limited to photo-redox. Havinghighlighted the oxometalates as oxidation catalysts we now archive theirroles in reduction catalysis.
9.6 Reduction Catalysis and OxometalatesOMs have been reported majorly for oxidation catalysis. Reductions cata-lysed by them have been less explored. In this section, we highlight the
Figure 9.4 A pathway for water oxidation reaction coupled with C–N coupling.[Inset: EAS and EPR for probing the catalyst.] See ref. 230 for moredetails.
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development in POM-catalysed reductive reactions over the past five years.Other leading efforts using different important catalyst types will betouched upon.
9.6.1 CO2 Reduction Catalysis
Over the past three decades there has been a significant increase in theemission of greenhouse gases (GHG), which has created a great challenge forthe scientific community to reduce GHG, primarily CO2 and high-energyfuels, using solar energy. CO2 has very low Gibbs free energy owing to thedouble bond between carbon and oxygen atoms, thus making it thermo-dynamically stable. The real challenge to activate an unreactive moleculesuch as CO2 can be overcome by the introduction of a binding site for CO2 inthe cluster since the interaction between CO2 moiety and the negativelycharged polyoxometalate is unfavourable. The reduction of CO2 viatwo-, four-, six-, eight- or twelve-electron processes yields HCOOH, HCHO,CH3OH, CH3COOH and CH3CH2OH,231 respectively. The reductioncan proceed photochemically, electrochemically or photo-electrochemically.232–234 Lehn and Sauvage, during 1980s, pioneered thefield of photoelectrochemical CO2 reduction using {Re-(2,2 0-bipyridine)},235
Ni-cyclams236 and polyporphyrins. The electrochemical reduction of CO2 hasbeen popular over a long period of time. Different aspects of electrochemicalCO2 reduction have been studied, such as electrodes, catalysts and methodsgenerating product selectivity, such as formic acid, methanol and ethanol.POMs, owing to their tuneable size, reversible redox process, robust struc-ture and charge distribution make them efficient catalysts for the electro-chemical and photochemical reduction of CO2. The Isaacs group reportedelectrochemical and photoelectrochemical CO2 reduction using anITO/multilayer modified electrode where the multilayer formation was byLBL using Mn(III), Ni(II) and Zn(II) in the central cavity coordinated to m-(meso-5,10,15,20-tetra(pirydil)porphyrin)tetrakis{bis(bipyridine)chloride ru-thenium(II)} and anionic [SiW12O40]4� polyoxometalate moiety.237 Thesemultilayer modified electrodes [MTRP]n1/[SiW12O40]4� act as efficient elec-trocatalysts and their activity was found to be increased by light and appliedpotential where light produces excited state sites on the multilayer which aremore sensitive towards carbon dioxide reduction. Electrochemical reductionof CO2 by (TOA)6[a-SiW11O39Co(_)] (TOA¼ tetraoctyl ammonium; _¼ vacantposition in the coordination sphere of Co) resulted in the formation of COand formaldehyde.238 It has been observed that the shape of cyclic voltam-mogram of (TOA)6[SiW11O39Co(_)] changed with a six-fold increase in thereduction peak intensity in the presence of CH2Cl2. In the first step, the POMgets reduced, followed by a transfer of electrons from POM to CO2 leadingto two- or four-electron process giving Co and formaldehyde. Anotherinteresting work by the Zhang group features bioinspired electrocatalyticreduction of CO2 by bovine serum albumin capped silver nanoclusters239
mediated by [a-SiW12O40]4�, where AgNC@BSA in conjunction with
Catalysis by Oxometalates and Their Microheterogeneous Media 185
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[a-SiW12O40]4� displayed major selectivity towards CO with a faradic effi-ciency greater than 75%. A catalyst with polyoxometalate and metallopor-phyrin framework for electrochemical CO2 reduction has also been designedby the Lan group.240 The integration of metalloporphyrin and the {e-PMo8
V
Mo4VIO40Zn4} polyoxometalate moiety lends them a greater efficiency due to
electron donating and accepting capabilities and electron migration. This isbecause: (a) metalloporphyrins, which are p-electrons conjugated macro-cycles, facilitate electron mobility; (b) reduced POMs consisting of low valentmetal ions such as {e-PMo8
VMo4VIO40Zn4} are majorly electron-rich aggre-
gates and easily donate electrons when acted upon by a stimulus; and (c) theintegration of metalloporphyrin with POMs creates a directed electrontransport system under an applied electric field which is extremely feasiblefor accomplishing a multi-electron transfer process for electrochemical re-duction of CO2 to CO. Co-PMOF displays an excellent Faradic efficiency(greater than 94%) over a broad potential range of �0.8 to �1.0 V.The system shows a turnover frequency of 1656 h�1 and the catalyst is stablefor more than 36 h. It is worth noting that without the more oxidisingoxometalate component, just employing a more reducing Co-corrole andemploying covalent and non-covalent interactions on the corrole with PEGside chains we were able to achieve 12 electron reduction of CO2 to ethanolwith a high Faradaic efficiency.241 Therefore, to obtain more reduced prod-ucts from CO2 (like ethanol), alone oxometalates may not suffice.
The groups of Blanchard and Proust had designed a catalyst[a-H2PW11O39{RhIIICp*(OH2)}]3� which closely resembles [Cp*RhIII(bpy)Cl]1
by substituting the mono-vacant [PW11O39]7� Keggin-based POM for thebipyridine ligand.242 The electrochemical behaviour suggested deviation inthe presence of CO2, indicating interaction with the POM derivative in spiteof the presence of the solvent. In the presence of water, the electrochemicalreduction of CO2 favoured the catalytic formation of dihydrogen, with theformate being the minor product. To ensure the atom economy of the pro-cess, coupled low-energy electroreduction of CO2 with glycerol oxidationusing chemical regenerative phosphomolybdic acid (PMo12), which acts aselectron coupled proton carrier, was achieved.243 Glycerol, a by-product frombiodiesel synthesis, acts as a reducing agent to provide electrons to phos-phomolybdic acid under light irradiation. The novelty of the process involvesa low-electrolysis potential and the use of a noble metal free anode owinglow overpotential of H–PMo12(red) oxidation on the anode.
The Sun and Wang groups introduced noble metal free polyoxometalate(Co4) into g-C3N4 resulting into a hybrid material (Co4@g-C3N4) for visiblelight-driven photochemical CO2 reduction.244 The material showed greateractivity towards CO2 reduction than bare g-C3N4. The polyoxometalate Co4
facilitates charge transfer to g-C3N4 and increases the surface catalyticoxidative ability. H5PW2
VW10O40, an acidic reduced polyoxometalate, wasused to act as a photoactive electron and proton donor with visible light viaexcitation of intervalence charge-transfer band for light-driven reductionof CO2 to CO.245 This polyoxometalate was integrated with dirhenium
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molecular catalyst, resulting in a cascade of transformation where poly-oxometalate is reduced electrochemically at the low potential of 1.3 V vs.Ag/AgNO3. Visible light was used to transfer electrons from polyoxometalateto dirhenium catalyst for selectively reducing CO2 to CO. CO2 reductionusing a catalyst based on Ti substituted polyoxometalate [PTi2W10O40]7�, Aunanoparticles and HKUST-1, where PTiW serves as the reactive centre and areservoir for electrons and protons placed in HKUST-1, was achieved.246
Here the incorporated Au nanoparticles help in harvesting visible light andthe HKUST-1 cage functions as a microreactor to concentrate CO2 molecules.A porous material containing reductive polyoxometalate has been found tocatalyse photochemical CO2 reduction successfully. The group of Lan andHuang further synthesised TBA5[P2Mo16
VMo8VIO71(OH)9Zn8(L)4] (NNU 29), a
polyoxometalate-based metal organic framework (MOF) where L is ahydrophobic organic ligand ensuring chemical stability and restrain inhydrogen generation for NNU 29.247 Formic acid produced has a yield of35.2 mmol with selectivity of 97.9% after 16 h. Two cobalt-basedpolyoxometalates, i.e., a binuclear cobalt cluster [Co2.67(SiW12O40)(H2O)4
(Htrz)4]�Cl1.33 (Htrz¼ 1,2,4-triazole) and a trinuclear cobalt cluster [Co3(Si-W12O40)(H2O)3(Htrz)6Cl]�Cl�6H2O, were used for visible light-driven CO2 re-duction to CO.248 The trinuclear cluster was more catalytically active in thiscase. Thus, from the above studies we note a pattern in the matter of electro-catalytic CO2 reduction efforts with POMs and porphyrins: (1) Presence of Coseems to play a role; (2) POMs and porphyrins appear to be effective as lig-ands; and (3) for obtaining less reduced products like CO, formic acid andformaldehyde, POMs seem to be the catalyst of choice. On the other hand,for more reduced products like ethanol and acetic acid, porphyrins andcorroles seem to be the efficient catalyst. We now explore oxygen reductionreactions.
9.6.2 Oxygen Reduction Reaction Catalysis
The oxygen reduction reaction (ORR) has an important role as cathode inenergy-converting systems such as fuel cells that contribute to harvestingsustainable energy. {[FeTPyP(Crphen2Cl)4]81/SiW12O40
4�}ni (1) and {[NiT-PyP(Crphen2Cl)4]81/SiW12O40
4�}ni (2), where TPyP¼ 5,10,15,20-tetra(4-pyr-idyl)porphyrin, phen¼ 1,10-phenanthroline, SiW12O40
4�¼ silicontungstateand ni¼number of multilayers were assembled onto different electrode sur-faces for oxygen reduction.249 Both systems reduce oxygen differently withrespect to the applied potential, {[FeTPyP(Crphen2Cl)4]81/SiW12O40
4�}ni/GC(where ni¼ 1, 2) reduced oxygen by [2e�þ 2e� ] steps and direct 4e-reduction.On the other hand, for the ni¼ 3 system, direct transfer of 4 electronsoccurs.
Green, facile electrochemical reduction of oxygen has been reported usingAg nanoparticles, oxometalates and graphene oxide (GO).250,251 Here, poly-oxometalate serves not only as an electrocatalyst but also as bridging cata-lyst. The oxygen reduction followed a four-electron pathway due to the
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synergistic effect of Ag nanoparticles and reduced graphene oxide. TheWei group reported a novel reduced POM on Pd-catalyst substrate, i.e.,Pd/HnMon
VIMo12�nVO40, for oxygen reduction reaction.252 The high efficiency
of the catalyst is observed due to electron delocalisation between the Pd andthe reduced POM, causing a down-shift in the d-band structure of the PdNPs. The reduced POM centre acts as an assisting catalyst, thus supportingthe decomposition of the peroxide intermediate. Further, a Co7Zn/C cathodemade of noble metal-free polyoxometalate Co7(AlePyZn)2 and Vulcan XC-72,a low-cost carbon material, has also been reported as a highly efficientoxygen reduction catalyst.253 The group of Freire, Mbomekalle and Fer-nandes reported vanadium-substituted phosphomolybdate [PMo11VO40]4�
incorporated into carbon-based materials [Graphene (GF), SWCNTs, CBV] asORR catalysts.254 All of them showed promising catalytic activity in acidicmaterial in a 2-electron mechanism. Zhang, He and co-workers synthesisedlarge-scale, carbonaceous-supported, precious metal-free POM-based com-posites for highly efficient oxygen reduction in a neutral or basic media viathe four-electron reduction mechanism.255 Along similar lines Fernandesand co-workers synthesised (TBA)7H3[Co4(H2O)2(PW9O34)2] (TBA–Co4(PW9)2)polytungstate-based, sandwich-type nanocomposites immobilised on car-bonaceous material as an ORR catalyst.256 Similarly, there have been reportsof [PW11MO39]5� decorated on Ru-reduced graphene oxide nanosheets asORR catalysts where M¼Co, Ni and Cu.257 [SiW11O39]8� (SiW11) covalentlycombined with graphitic carbon nitride (g-C3N4) through an organic linkerhas been demonstrated as an excellent catalyst for promoting photocatalyticH2O2 production via two-electron reduction of O2 to H2O2.258 Next, we de-scribe a few examples of HERs with POMs.
9.6.3 Hydrogen Evolution Reaction
The copper cation and hexa-capped Keggin oxoniobates based 3D framework[Cu(en)2]4[PNb12O40(VO)6](OH)5�8H2O and [Cu(enMe)2]4[PNb12O40(VO)6](OH)5�6H2O is reported as an efficient catalyst for photocatalytic water re-duction leading to hydrogen evolution.259 Likewise, a gigantic Co14 con-taining 36 niobates, Na12K8[Co14(OH)16(H2O)8Nb36O106]�71H2O showsphotocatalytic H2 evolution activity using Pt loading in methanol solutionunder irradiation using a 300 W Xe lamp.260 Copper-based polyoxometalatecomplexes, such as Na3K7[Cu4(H2O)2(B-a-PW9O34)2]�30H2O (Na3K7–Cu4P2),work as efficient visible light-driven hydrogen evolution catalysts whencoupled with (4,4 0-di-tert-butyl-2,20-dipyridyl)-bis(2-phenylpyridine(1H))-ir-idium(III) hexafluorophosphate ([Ir(ppy)2(dtbbpy)][PF6]) as a light absorberand triethanolamine (TEOA) as sacrificial electron donor.261 A tetra-nickelcontaining polyoxometalate Na6K4[Ni4(H2O)2(PW9O34)2]�32H2O (Na6K4–Ni4P2) has also been known to be an efficient catalyst for photocatalytic H2
evolution with a turnover number as high as 6500.262 The Patzke group re-ported nickel containing polyoxometalate ([Ni(H2O)XW11O39]n� for photo-catalytic H2 evolution.263 The turnover number for the reaction depended on
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the heteroatom X and the highest turnover number in the series was re-corded for [Ni(H2O)GeW11O39]6�. An assembly of noble metal free POM in-side a metal organic framework [P2W18O62]6�@UiO has been found tobe capable of visible light-driven proton reduction.264 It is one of the firstreported examples of light driven-proton reduction by Wells Dawson typePOM through synergistic photoexcitation of MOF framework and multi-electron injection from the excited states of the [Ru(bpy)3]21-based bridgingligands to the encapsulated POMs. The Lin group encapsulated[Ni4(H2O)2(PW9O34)2]10� into porous and phosphorescent MOFs.265 Thevicinity of Ni4P2 to the photosensitizers in Ni4P2@MOF facilitates the multi-electron transfer to facilitate visible light-driven HER. The Ta/W mixed ad-dendum POMs Cs5K2[(Si2W18Ta6O78)Cr(H2O)4]�8H2O and Cs3K4H2[(Si2W18
Ta6O78)FeCl2(H2O)2]�15H2O were shown to increase the rate of H2
evolution.266
Hill and co-workers found the self-assembly of oxometalates (POM), metalorganic framework (MOF) and Pt nanoparticles (NP) to a hybrid material thatshows synergistic activity for visible light-driven catalytic H2 evolution due tobinding of POM-stabilised Pt nanoparticles with NH2-MIL-53.267 The POMsplay four roles here, namely: (1) reduction of H2PtCl6 to Pt NPs, (2) stabilisePt NPs, (3) association of Pt NPs with cationic support, and (4) facilitating H2
evolution by electron transfer from NH2-MIL-53 to Pt-NPs. Oxometalateshave also been integrated with a porphyrin system such as zinc(II) mesote-trakis(N-methyl-pyridinium-4-yl)porphyrin (ZnTMPyP41)268 or organosilicaframework269 to increase its efficiency as a photocatalytic HER catalyst. Thegroups of Liu, Bao and Lan synthesised a novel polyoxometalatedecorated g-C3N4-wrapping snowflake-like CdS nanocrystal photocatalyst,which displays high activity towards hydrogen production.270 The intro-duction of Ni4(PW9)2 POM not only serves as redox electron modulatorbut also as co-catalyst. It has been found that the synergistic effect fromthe multicomponent system increases the activity towards HER.A polyoxometalate consisting of tetradecanuclear nickel biphosphonatecluster capping a {SiW9} displays a high hydrogen evolution reactionunder visible light via a reductive quenching mechanism.271 K12.5Na1.5
[NaP5W30O110] (P5W30) type Preyssler anion and graphene oxide was inte-grated to form a polyoxometalate–graphene nanocomposite multilayer alongwith protonated polyethylenimine through LBL assembly for catalysingelectrochemical hydrogen evolution.272 A highly active hydrogen evolutionreaction electrode with low Pt loading on glassy carbon prepared using co-deposition of polyoxometalate and anodic platinum dissolution273 was re-ported. It shows an overpotential of 65 mV at 20 mA cm�2, comparable tocommercial working Pt electrodes. Furthermore, a porous Mo-based com-posite derived from POMOFs (polyoxometalate-based metal–organic frame-work) and graphene oxide exhibits low Tafel slope of 41 mV dec�1, which is avery positive onset potential close to that of 20% Pt/C and high exchangecurrent density of 4.8�10�4 A cm�2, making it one of the best performingelectrocatalysts for HER.274 In another effort, a catalyst comprising a POM
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anion encapsulated MOF was coated with carbon nitride polymer on thesurface and was annealed.275 Its hollow structure and unique compositionmake it a remarkable catalyst for electrochemical HER. Fe3C/Mo2C-containing N, P co-doped graphitic carbon derived from POM@MOF-100(Fe) (denoted as Fe3C/Mo2C@NPGC) was reported and displays great elec-trocatalytic ability towards HER with a low onset overpotential of 18 mV(vs. RHE), small Tafel slope of 45.2 mV dec�1, as well as long-term durabilityfor 10 h.276 [H7P8W48O184]33� (P8W48) fixed in a 3D configuration on reducedgraphene oxide sheets (rGO) was found to display excellent electrocatalyticH2 evolution with a quantitative Faradic yield and high turnover frequencyof 11 s�1 at 295 mV overpotential.277 In another report, a catalyst consistingof Mo2C NPs, NPC and NPRGO, was obtained by carbonizing a ternaryPMo12–PPy/RGO nanocomposite.278 It displays outstanding electrocatalyticactivity for the hydrogen evolution reaction and excellent stability in acidicmedia, the best among the reported non-noble-metal catalysts. Anotherstable electrocatalyst for HER by encapsulating P-modified tungsten car-bide in N-doped carbon (P-W2C@NC) has been prepared by annealingH3PW12O40 and dicyandiamide.279 There have been many such hybridsreported to be efficient catalyst for photochemical and electrochemicalhydrogen evolution reactions. We now outline CO2 reduction coupled withwater oxidation using microheterogeneous media of oxometalates, alsocalled soft-oxometalates.
9.6.4 Carbon Dioxide Reduction Coupled with WaterOxidation
By exploring the intrinsic, optical, semiconductor nature of oxometalates, itis possible to oxidise water and generate reducing equivalents. Such re-ducing equivalents can, in turn, be transiently stored and injected into asuitable substrate (Figure 9.5a). Our efforts have centred around using waterand CO2 as substrates. In 2016, we reported simultaneous CO2 reductioncoupled with water oxidation using three soft-oxometalates catalysts basedon {Mn6P3W24}, {Mo154} and {Mo132} to efficiently catalyse the reduction ofCO2 to formaldehyde and formic acid.280 These metal oxide-based clustersare photoactive towards UV light and, hence, do not need an external pho-tosensitizer. The water used in the reaction medium gets oxidised to gen-erate oxygen, protons and electrons. These generated protons and electronsfurther assist in reducing CO2 with a maximum turnover number andturnover frequency as high as 1.4�106 and 610 s�1, respectively. The realnature of the active catalyst in these cases is yet unknown. It is reasonable tostate that perhaps SOMs or the microheterogenized state is the pre-catalyststate, which upon activation carries out catalysis.281 The exact pathway ofactivation of these and other related SOM-based catalysts are now beinginvestigated in our laboratory. The question whether these catalysts are instate of kinetic arrest has to be explored. Further, we have reported other
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soft-oxometalate catalysts based on {[K6.5Cu–(OH)8.5(H2O)7.5]0.5[K3PW12O40]}for photocatalytic reduction of CO2 to formic acid282 (Figure 9.5b). Onpurging CO2 in the medium, it gets adsorbed on the catalyst species. Onphotoexcitation, the CO2 adsorbed catalyst species get excited due to the{PW12O40} core going into {PW12O40}* state. It oxidises water to O2 and theelectrons released during water oxidisation reduce the {W12O40} core. Thus,WVI centres get reduced to WV, leading to the emergence of an inter-valencecharge transfer from WV to WVI. Photochemical CO2 reduction takes placeon the Cu centre wherein CuII is reduced to Cu0 by reduced tungsten, which
Figure 9.5 (a) Representative pathway for water oxidation coupled carbon dioxidereduction reaction. (b) Pictorial representation of [Cu(H2O)5]21 moietycoordinated to Keggin ion. (c) Mechanistic route for photochemicalcarbon dioxide reduction using {[K6.5Cu-(OH)8.5(H2O)7.5]0.5[K3PW12O40]}as a catalyst. [Insets: A (CV), B, C (EAS) and D(XRD) represent differentstates for probing the catalytic pathway.] For details refer to ref. 282.
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is oxidised to (þVI) oxidation state (Figure 9.5c). In turn, Cu0 likely goesback to CuII after carrying out the reduction. We synthesised a reducedOM, [{(CH3)NH2}4{(Mo2
VO4)6(m2-OH)10(m2-O)2(m3-OH)2(m3-O)2(MoVIO3H)4}]0.94
[{(Mo2VO4)6(m2-OH)10(m2- O)2(MoVIO3H)4}{MoVI(m3-O)4}]0.06¼ {Mo16} and have
shown that by tuning the reduced metal centre of the synthesised OM, onecan achieve selectivity for formaldehyde in CO2 reduction over formic acid.{Mo16} is sensitive to light and, hence, no external photosensitizer is used andwater acts as the electron donor, making the reaction environment-friendly.283
9.7 Conclusions and PerspectivesTo summarise, in this chapter recent endeavours in the context of catalysiswith oxometalates centres concerning certain key features of these catalystsare provided. These are: (i) acid–base properties, (ii) redox properties, and(iii) the existence of these entities in heterogenized state. Exploration of thefirst two properties of the oxometalates has led to an extremely rich lit-erature. Interest is now increasing to understand and exploit the richnessof the phases and states of the oxometalates, especially in the hetero-genized (liquid or soft-matter) state. In this chapter, setting out to overviewthe literature on reactions as diverse as Friedel–Crafts to Fries rearrange-ment and to alkene isomerization, we have presented the efforts on pro-cesses for energy harvesting and catalysis. All the examples are stitchedtogether on the two aspects of oxometalates: acid-base properties and redoxproperties.
Recent endeavours raise the question of possibility and challenge oftransferring oxometalates from the crystalline (polyoxometalate) or homo-geneous phase of molecular liquid to heterogeneous (liquid or soft-matter)phase. Open and challenging questions emerge. What is the nature of thesecatalysts in general? How can one delineate and thus exploit and understandtheir behaviour in the heterogeneous phase? Are the catalysts in hetero-geneous phase responsive, dynamic and self-healing? Are the probed statesof these catalysts in heterogeneous phase pre-catalysts? What is the nature ofthe active catalyst? What are the active sites? Finally, the what is the effi-ciency and atom economy of catalytic processes involving oxometalates inheterogenized media? By asking these questions, one faces the typical con-undrum on facets of heterogeneous catalysis. How can these processes beunderstood down to every step and at the same time their efficiency befurther improved so that a scalable, rapidly converting catalytic system canbe designed? Perhaps the answer to these challenges lies in designing ahybrid catalyst system where homogeneous, heterogeneous and patternedcatalysts can co-operate and act in unison. For such a realisation to becomereality, the first step would be to understand the processes that activate apre-catalyst and initiate the catalysis, giving the system the active catalyst –the true nature of which remains elusive and challenging to delineate,especially in the context of oxometalates.
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