inorganics

Review Synthesis, Characterization and Applications in Catalysis of Polyoxometalate/Zeolite Composites

Frédéric Lefebvre C2P2, Université Lyon 1, 43 Boulevard du 11 Novembre 1918, Villeurbanne Cedex 69626, France; [email protected]; Tel.: +33-472-431-807

Academic Editor: Greta Ricarda Patzke Received: 7 March 2016; Accepted: 26 April 2016; Published: 4 May 2016

Abstract: An overview of the synthesis, characterization and catalytic applications of polyoxometalates/zeolites composites is given. The solids obtained by direct synthesis of the polyoxometalate in the presence of the zeolite are first described with their applications in catalysis. Those obtained by a direct mixing of the two components are then reviewed. In all cases, special care is taken in the localization of the polyoxometalate, inside the zeolite crystal, in mesopores or at the external surface of the crystals, as deduced from the characterization methods.

Keywords: polyoxometalates; zeolites; encapsulation; impregnation; synthesis; catalysis

1. Introduction In chemistry, a polyoxometalate is a polyatomic anion that consists of three or more transition metal atoms linked together by oxygen atoms. The metal atoms are usually from groups 5 (vanadium, niobium, tantalum) or 6 (molybdenum, tungsten) in their higher oxidation states [1]. The polyoxometalates can be divided into isopolyoxometalates (with only one transition metal) and heteropolyoxometalates (with one transition metal and a main group oxyanion such as phosphate, silicate, etc.). These polyoxometalates have interesting properties in a lot of domains [2–9]. Those usually used in catalysis, the subject of this review, have in most cases the so-called “Keggin” structure [10]. There are a lot of reasons for this choice, namely they are: (i) easy to prepare; (ii) more thermally stable than the other; (iii) acidic or redox systems depending on their atoms and (iv) some of them are commercially available. Heteropolyacids of the Keggin type consist of a central ion surrounded symmetrically by 12 MO6 octahedra, which form four groups of three edge-shared octahedra (Figure1). The central atom is usually phosphorus or silicon, while tungsten or molybdenum form MO6 groups. The anion of a polyoxometalate with the Keggin structure is close to a sphere in shape, with a diameter of about 1 nm. These heteropolyoxometalates with the Keggin structure possess acidic and redox properties, which can be easily tailored by changing the composition of the n´ Keggin units [4,5]. Their formula is [XM12O40] with X = P, Si, ... and M = W, Mo. From this point onward, it will be abbreviated as XM12. When the polyacid or an acidic form is referred to, it will be abbreviated as HXM12. Zeolites, the second component of the composites studied here, are crystalline microporous aluminosilicates, which are commonly used as adsorbents and catalysts due to their unique properties. By changing either their nature or the identity of the cations present in their structure (each aluminium atom in the framework will create a negative charge compensated by a cation, some or all of which can be exchanged with a different cation), it is possible to achieve fine separations such as oxygen/nitrogen. When the cation is a proton, the zeolite displays very acidic properties which are used industrially, for example in petroleum chemistry. Zeolites occur naturally but are also produced industrially on a large scale. A lot of zeolite frameworks have been identified, and over 40 naturally occurring zeolite frameworks are known [11].

Inorganics 2016, 4, 13; doi:10.3390/inorganics4020013 www.mdpi.com/journal/inorganics Inorganics 2016, 4, 13 2 of 23

One of these zeolites, faujasite, is very interesting in view of its application to form composites with polyoxometalates as its framework forms large cages (called “supercages”) with a 1.12 nm diameter (Figure 1). These supercages are interconnected by windows of 0.735 nm in diameter. InorganicsTherefore,2016, 4, 13 this zeolite should be an ideal material for the encapsulation of Keggin polyoxometalates, 2 of 24 which should then be sterically trapped in the supercages without any possibility to leave (Figure 2).

Inorganics 2016, 4, 13 2 of 23

One of these zeolites, faujasite, is very interesting in view of its application to form composites with polyoxometalates as its framework forms large cages (called “supercages”) with a 1.12 nm diameter (Figure 1). These supercages are interconnected by windows of 0.735 nm in diameter. Therefore, this zeolite should be an ideal material for the encapsulation of Keggin polyoxometalates, which should then be sterically trapped in the supercages without any possibility to leave (Figure 2).

(a) (b)

Figure 1. Schematic drawings of (a) Keggin heteropolyoxometalate (red: oxygen, blue: tungsten, Figure 1. Schematic drawings of (a) Keggin heteropolyoxometalate (red: oxygen, blue: tungsten, orange: X atom) and (b) supercage of Y zeolite (red: oxygen, yellow: silicon or aluminium). The orange:drawings X atom) are and not (atb )the supercage same scale. of Y zeolite (red: oxygen, yellow: silicon or aluminium). The drawings are not at the same scale.

One of these zeolites, faujasite, is very interesting in view of its application to form composites with polyoxometalates as its framework forms large cages (called “supercages”) with a 1.12 nm diameter (a) (b) (Figure1). These supercages are interconnected by windows of 0.735 nm in diameter. Therefore, this Figure 1. Schematic drawings of (a) Keggin heteropolyoxometalate (red: oxygen, blue: tungsten, zeolite should be an ideal material for the encapsulation of Keggin polyoxometalates, which should orange: X atom) and (b) supercage of Y zeolite (red: oxygen, yellow: silicon or aluminium). The then be stericallydrawings trapped are not at in the the same supercages scale. without any possibility to leave (Figure2).

Figure 2. Keggin polyoxometalate in the supercage of Y zeolite (red: oxygen, yellow: silicon or aluminium).

The main interest of such materials should be that they can combine the advantages of the zeolite (for example, its adsorption capacities) and those of the polyoxometalate (for example, its acidic properties), without polyoxometalates’ main disadvantage, which is a high solubility in polar solvents that prevents its use in heterogeneous catalysis in their presence. As the polyoxometalate cannot pass through the windows of the supercages, it is not possible to obtain encapsulated materials by adding the polyanion to the zeolite, and allowing it to diffuse in the framework. The Figure 2. Keggin polyoxometalate in the supercage of Y zeolite (red: oxygen, yellow: silicon or Figureonly 2.possibilityKeggin is polyoxometalateto synthesize it directly in the inside supercage the zeolite of Y crystal. zeolite Unfortunately, (red: oxygen, the yellow: synthesis silicon of aluminium). or aluminium).Keggin compounds can only be made in very acidic media (pH < 1) and the commercial Y zeolite (with a Si/AlThe ratio main near interest 2.5) is notof suchvery materialsstable in these should conditions. be that theyIt is thencan combinenecessary the to dealuminateadvantages of it, thefor Thezeolite main (for interest example, of such its adsorption materials capacities) should be an thatd those they of canthe combinepolyoxomet thealate advantages (for example, of theits zeolite (for example,acidic properties), its adsorption without capacities) polyoxometalates’ and those main disadvantage, of the polyoxometalate which is a high (for solubility example, in polar its acidic solvents that prevents its use in heterogeneous catalysis in their presence. As the polyoxometalate properties), without polyoxometalates’ main disadvantage, which is a high solubility in polar solvents cannot pass through the windows of the supercages, it is not possible to obtain encapsulated that preventsmaterials its by use adding in heterogeneous the polyanion catalysisto the zeolite, in their and allowing presence. it to As diffuse the polyoxometalate in the framework. cannotThe pass throughonly the possibility windows is of to the synthesize supercages, it directly it is inside not possible the zeolite to crystal. obtain Unfortunately, encapsulated the materials synthesis by of adding the polyanionKeggin compounds to the zeolite, can only and be allowing made in very it toacidic diffuse media in (pH the < 1) framework. and the commercial The only Y zeolite possibility (with is to synthesizea Si/Al it directlyratio near inside2.5) is not the very zeolite stable crystal.in these conditions. Unfortunately, It is then the necessary synthesis to dealuminate of Keggin it, compounds for can only be made in very acidic media (pH < 1) and the commercial Y zeolite (with a Si/Al ratio near 2.5) is not very stable in these conditions. It is then necessary to dealuminate it, for example by a hydrothermal treatment, before use, although some papers have reported syntheses in these Inorganics 2016, 4, 13 3 of 24 conditions (see below). The emphasis of these papers is the catalytic performance, rather than the physical characterization, of the composite materials thereby obtained. The materials prepared by this way can be divided into two groups, depending on the transition metal used, molybdenum or tungsten. We will then describe two recent papers where the zeolite is Inorganics 2016, 4, 13 3 of 23 synthesized around the polyoxometalate. The next part will be devoted to the systems prepared by combiningexample the Keggin by a hydrothermal polyoxometalate treatment, withbefore the use, zeolite although and, some finally, papers wehave will reported describe syntheses the in systems based onthese other conditions polyoxometalates. (see below). The emphasis of these papers is the catalytic performance, rather than the physical characterization, of the composite materials thereby obtained. 2. SynthesisThe of Molybdicmaterials prepared Keggin by Species this way or Theircan be Salts divided in/on into Zeolites two groups, depending on the transition metal used, molybdenum or tungsten. We will then describe two recent papers where the 3´ Thezeolite 12-phosphomolybdate is synthesized around Keggin the polyoxometalate. species ([PMo The12O next40] part, referred will be todevoted as PMo to 12thefrom systems this point forward)prepared can be easilyby combining synthesized the Keggin from polyoxometalate MoO3 and H3 POwith4 :the zeolite and, finally, we will describe the systems based on other polyoxometalates. 12 MoO3 ` H3PO4 Ñ H3PMo12O40 (1) 2. Synthesis of Molybdic Keggin Species or Their Salts in/on Zeolites

This reactionThe 12-phosphomolybdate has been applied toKeggin the preparationspecies ([PMo of12O materials40]3−, referred where to as PMo the polyoxometalate12 from this point was expectedforward) to be located can be ineasily the synthesized supercages from of Y MoO zeolite3 and simply H3PO4: by performing the synthesis in the presence of the zeolite. 12 MoO3 + H3PO4 → H3PMo12O40 (1)

2.1. The PioneeringThis reaction Works has of Mukaibeen applied et al. [ 12to –the15 preparat] ion of materials where the polyoxometalate was expected to be located in the supercages of Y zeolite simply by performing the synthesis in the Mukaipresenceet al. of[12 the–15 zeolite.] synthesized H3PMo12O40 from MoO3 and phosphoric acid in the presence of dealuminated Y-zeolite (US-Y), previously exchanged with NH4Cl and calcined, in order to obtain it 2.1. The Pioneering Works of Mukai et al. [12–15] in its H-form. They studied the resulting solid in the esterification of acetic acid with ethanol. They pointed out thatMukai it et was al. [12–15] necessary synthesized to use H a3PMo dealuminated12O40 from MoO zeolite3 and phosphoric as the usual acid Y in zeolite, the presence with of its high dealuminated Y-zeolite (US-Y), previously exchanged with NH4Cl and calcined, in order to obtain it aluminium content (Si/Al = ca. 2.5), is not stable in the acidic medium needed for the synthesis of the in its H-form. They studied the resulting solid in the esterification of acetic acid with ethanol. They polyoxometalate.pointed out As that the it was intent necessary was to to prepare use a dealuminated an acid catalyst zeolite withas the protons usual Y zeolite, on the with polyoxometalate, its high 3´ the use ofaluminium its H form content was (Si/Al also needed= ca. 2.5), to is not avoid stable formation in the acidic of saltsmedium of [PMoneeded12 forO40 the] synthesis. An indirect of the proof of the presencepolyoxometalate. of Keggin As unitsthe intent in thewas supercages to prepare an obtained acid catalyst from with the protons adsorption on the polyoxometalate, of isobutene, which was lowerthe thanuse of thatits H achievedform was also with needed the to unmodified avoid formation zeolite. of salts The of [PMo isotherms12O40]3−. An were indirect analyzed proof of by the Dubinin–Astakhovthe presence equationof Keggin tounits obtain in the the supercages micropore obtained volumes. from Itthe was adsorption deduced of fromisobutene, these which values that was lower than that achieved with the unmodified zeolite. The isotherms were analyzed by the about one third of the supercages were filled by [PMo O ]3´ molecules. As the polyoxometalate Dubinin–Astakhov equation to obtain the micropore volumes.12 40 It was deduced from these values that can be synthesizedabout one third not of only the superc in theages supercages were filled butby [PMo also12 onO40] the3− molecules. external As surface the polyoxometalate of the crystallites, can the ˝ samplesbe were synthesized immersed not inonly hot in waterthe supercages (at 80 C)but [ 13also]. Afteron the oneexternal hour surface of agitation, of the crystallites, the samples the were removedsamples from hot were water immersed and thein hot amount water (at of 80 dissolved °C) [13]. After molybdenum one hour of was agitation, determined the samples by inductivelywere coupledremoved plasma analysis.from hot water This and washing the amount sequence of dissolved was repeatedmolybdenum five was times. determined The amount by inductively of polyanion coupled plasma analysis. This washing sequence was repeated five times. The amount of polyanion on the support varied from 0.16 g per g of support after the first wash to 0.09 g after the fourth and on the support varied from 0.16 g per g of support after the first wash to 0.09 g after the fourth and fifth onesfifth (Figure ones (Figure3). 3).

20

15

10

5

0 0123456

Figure 3. Amount of PMo12 remaining on the sample (in wt %, Y-axis) as a function of the number of Figure 3. Amount of PMo12 remaining on the sample (in wt %, Y-axis) as a function of the number of washing steps (X-axis). Adapted from [13]. washing steps (X-axis). Adapted from [13].

Inorganics 2016, 4, 13 4 of 24

Clearly, after four washing steps, no further molybdenum can be extracted from the solid in agreement with the formation of the Keggin anion in the supercages of the zeolite. This point is very important for liquid phase catalysis as heteropolyacids are known to be highly soluble in polar solvents, such as water or alcohols. This solid is then a good potential heterogeneous and recyclable catalyst for reactions in polar solvents. The solid was used in the model reaction of esterification of acetic acid with ethanol. The performance was lower than that of the pure heteropolyacid, but better than that of H3PMo12O40 supported on the same zeolite. An increased resistance towards leaching was obtained by replacing, after the synthesis, some protons by cesium cations through a reaction with cesium carbonate [14]. Indeed, cesium salts of polyoxometalates are generally insoluble and so it can be expected that a higher stability could be achieved by this treatment. In order to retain some Bronstead acidity that is necessary for catalytic activity, only partial replacement of protons by cesium cations was conducted, leading to CsxH3´xPMo12O40 formulations with x = 1, 2 and 2.5. The resulting solids were also tested in the esterification of acetic acid with ethanol and showed only a small decrease of the catalytic activity but without leaching (Table1).

Table 1. Conversion after 180 min in the esterification of acetic acid with ethanol catalyzed by various a PMo12 catalysts synthesized in Y zeolite . Adapted from [14].

Catalyst Conversion (%) no catalyst 4.9 H3PMo12O40 9.0 CsH2PMo12O40 8.3 Cs2.5H0.5PMo12O40 7 a ˝ ´3 Reaction conditions: 60 C, 0.5 mol acetic acid and ethanol, PMo12 concentration 1.2 mol¨ m .

Mukai et al. also determined the key factors for the encapsulation of the heteropolyoxometalates in the supercages of Y-zeolite [12]. Encapsulation occurs only when the Si/Al ratio is between 10 and 50. When the amount of aluminium atoms is too high, the zeolite is decomposed during the synthesis of the polyacid. When it is too low, no encapsulation occurs, even if the polyoxometalate is synthesized in the reaction mixture. The presence of cations in the supercage is then needed to ensure the encapsulation. One explanation could be that highly siliceous zeolites are hydrophobic. As the polyoxometalate is hydrophilic, it cannot interact with the framework in these conditions. Another key parameter was the identity of the cation in the zeolite. No encapsulation occurred when it was Na+,K+, 2+ 2+ + + Ca or Ba . The best results were obtained with Cs , while NH4 led only to a partial encapsulation. However, when the synthesis was made in the presence of these two cations, some insoluble salts were also obtained at the surface of the zeolite crystals. Finally, a study of the effect of the degree of ion exchange on encapsulation found that encapsulation occurred for cesium even with low exchange, whereas for ammonium it was necessary to have a high degree of exchange to achieve encapsulation. In another paper [15], they showed that only a conventional synthesis starting from MoO3 and H3PO4 led to encapsulation of the polyoxometalate. Addition of t-butyl alcohol had a beneficial effect on the encapsulation, which could occur even at low synthesis temperatures.

2.2. Other Reports of Syntheses of Encapsulated PMo12 in Y Zeolite

The synthesis was further improved by Tran et al. who prepared encapsulated PMo12 and SiMo12 4´ ([SiMo12O40] ) in US-Y in its H form by a two steps method [16]. They had observed that the method of Mukai did not give reproducible loadings. Their assumption was that this was due to the low solubility of MoO3 in water. Therefore, they applied a hydrothermal technique to introduce the molybdenum oxide in the supercages and then to synthesize the polyoxometalate by introduction of phosphoric acid (for the obtention of encapsulated HPMo12) or nitric acid (for HSiMo12). The resulting materials could have high polyoxometalate loadings ranging from 1.2 to 11.1 wt % in a relatively Inorganics 2016, 4, 13 5 of 24 predictable fashion. The solids were characterized by infrared spectroscopy, X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure (EXAFS) to reveal the local structure around Mo. Tran et al. observed the formation of SiMo12 even in the case of the synthesis in presence of phosphoric acid and attributed it to the fact that the slurry was refluxed during the preparation. However, they claimed that additional investigations were necessary to determine the formation conditions of both SiMo12 and PMo12 on supports in the presence of phosphoric acid. The solids were then used for the partial oxidation of methane to formaldehyde at high temperatures (600 ˝C). Due to the presence of water vapor in the feedstock, the polyoxometalates were stabilized at this temperature and good HCHO yields were achieved. However, high CO amounts were observed, due to the decomposition of the aldehyde inside the supercages. Wang et al. reported a synthesis of PMo12 encapsulated in dealuminated Y zeolite derived from that of Mukai but where the zeolite, in its ammonium form, and various amounts of MoO3 were first mixed, ground uniformly to a fine powder and calcined at 500 ˝C for 4 h [17]. This treatment led to a spontaneous dispersion of MoO3 onto the Y zeolite surface to form an amorphous layer. The desired ˝ amount of H3PO4 was then added and the sample heated at 95 C for 3 h. The slurry was finally filtered and washed three times with hot water in order to remove the polyoxometalate located on the zeolite surface. The resulting solids were characterized by infrared spectroscopy, XRD and nitrogen adsorption. As the zeolite was in its ammonium form, one can expect that the synthesized PMo12 was in the form of its ammonium salt, which is insoluble and so the washing step could not remove it from the external surface of the zeolite. Unfortunately, no indication was given of the PMo12 amount before and after the washing. The solid was then used as catalyst for the oxidative desulfurization of dibenzothiophene by H2O2 in acetonitrile. The best results were obtained with the sample containing 10 wt % PMo12 (83.2% desulfurization efficiency) and the catalyst could be recycled at least three times without loss of activity. However, this system was less active than PMo12 in solution and it must be pointed out that in these conditions the active species is not the Keggin species but the peroxo complex 3´ PO4[MoO(µ-O2)(O2)]2 , which is formed by contacting it with H2O2. Wei et al. proposed a method derived from the studies of Mukai for the encapsulation of phosphomolybdic acid in a non-dealuminated Y zeolite (Si/Al = 2.6) [18]. The protons of the zeolite were first replaced by cesium cations by reaction with the desired amount of cesium carbonate followed by calcination. The polyoxometalate was then synthesized “in situ” by addition of sodium molybdate and phosphoric acid, with the pH adjusted to 2 or 3 by addition of HCl. The solid was separated by centrifugation and washed with hot water. It was characterized by X-ray diffraction, infrared spectroscopy and 31P Magic Angle Solid-state Nuclear Magnetic Resonance (MAS NMR) spectroscopy. Unfortunately, the 31P MAS NMR spectrum showed the presence of an additional broad signal proving the existence of other species. It is therefore difficult to draw conclusions about whether the polyoxometalate is entirely encapsulated within the zeolite or not, especially as the cesium salt of PMo12 is insoluble and could be located on the external surface. In addition, X-ray data showed that, at high loadings, the crystallinity of the zeolite was lost, as expected from the works of Mukai et al. who observed the destruction of the framework in the presence of the highly acidic PMo12. This solid was used for the esterification of acetic acid with n-butanol and did not show any deactivation after five runs.

2.3. Synthesis of PMo12 on ZSM-5

Chen et al. reported the synthesis of H3PMo12O40 encaged in nanocrystalline H-ZSM-5 and its use in combination with Ni for the hydroconversion of n-octane [19]. The zeolite crystals sizes were ca. 18 nm with formation of particles of 70–100 nm. The solids were characterized by a lot of physicochemical methods including solid-state NMR and Electron Spin Resonance (ESR). It was concluded (as expected when looking at the crystallographic data of the zeolite) that PMo12 could not fit the pore channels of H-ZSM-5 but was located in the secondary pore system formed by the nanocrystals. This solid was compared to a system obtained by classical impregnation in the Inorganics 2016, 4, 13 6 of 24

Inorganics 2016, 4, 13 6 of 23 Inorganicshydroconversion 2016, 4, 13 of n-octane. The performances of the encapsulated material were slightly better6 thanof 23 those of that prepared by classical impregnation. 2.4. Applications in Catalysis 2.4. Applications in Catalysis 2.4. Applications in Catalysis Moghadam et al. used the method proposed by Mukai et al. for the synthesis of HPMo12 Moghadam et al. used the method proposed by Mukai et al. for the synthesis of HPMo12 encapsulated in US-Y (Si/Al = 3.8) and its use for the oximation of aldehydes [20]. After synthesis encapsulatedMoghadam in US-Yet al. (Si/Alused = the 3.8) method and its proposeduse for the by oximation Mukai et of al. aldehydesfor the synthesis [20]. After of synthesis HPMo12 and washing with hot water, the remaining protons of the zeolite were exchanged for Na+ by andencapsulated washing inwith US-Y hot (Si/Al water, = 3.8) the and remaining its use for pr theotons oximation of the ofzeolite aldehydes were [20exchanged]. After synthesis for Na+ andby suspending the solid in a solution of sodium chloride for 4 h. The catalyst, which contained+ 5.0 wt % washingsuspending with the hot solid water, in a the solution remaining of sodium protons chlori of thede zeolitefor 4 h. wereThe catalyst, exchanged which for Nacontainedby suspending 5.0 wt % Mo, was characterized by infrared spectroscopy (which showed the characteristic bands of the Mo,the solidwas incharacterized a solution of by sodium infrared chloride spectroscopy for 4 h. The(which catalyst, showed which the containedcharacteristic 5.0 wtbands % Mo, of wasthe polyoxometalate but slightly shifted), X-ray powder diffraction and BET surface area analysis. An polyoxometalatecharacterized by infraredbut slightly spectroscopy shifted), (whichX-ray powder showed diffraction the characteristic and BET bands surface of the area polyoxometalate analysis. An electrochemical characterization of this material was also reported [21]. The electrochemical studies electrochemicalbut slightly shifted), characterization X-ray powder of this diffraction material andwas BETalso surfacereported area [21]. analysis. The electrochemical An electrochemical studies characterizationrevealed that the of encapsulated this material PMo was12 also showed reported electrochemical [21]. The electrochemical responses much studies slower revealed than those that the of revealed that the encapsulated PMo12 showed electrochemical responses much slower than those of impregnated ones, arising from the restraints in the zeolite pores towards diffusion or impregnatedencapsulated PMoones,12 showedarising electrochemicalfrom the restraints responses in muchthe slowerzeolite thanpores those towards of impregnated diffusion ones, or charge-balancing electrolytes. charge-balancingarising from the restraints electrolytes. in the zeolite pores towards diffusion or charge-balancing electrolytes. This materialmaterial waswas used used for for the the oximation oximation of variousof various aldehydes aldehydes with with NH NHOH2¨OH·HClHCl (Scheme (Scheme1)[ 20 1)]. This material was used for the oximation of various aldehydes with 2NH2OH·HCl (Scheme 1) [20]. The reaction was performed under mechanical stirring, ultrasonic irradiation or without any [20].The reactionThe reaction was performed was performed under under mechanical mechanical stirring, stirring, ultrasonic ultrasonic irradiation irradiation or without or without any solvent. any solvent. High yields were achieved in all cases with short reaction times, the best results being solvent.High yields High were yields achieved were inachieved all cases in with all shortcases reaction with short times, reaction the best times, results the being best obtained results underbeing obtained under solvent free conditions. For example, for 2-chlorobenzaldehyde a 100% yield was obtainedsolvent free under conditions. solvent Forfree example, conditions. for 2-chlorobenzaldehydeFor example, for 2-chlorobenzaldehyde a 100% yield was achieveda 100% yield in 10 min.was achieved in 10 min. However, in all cases, some molybdenum was leached, in agreement with the achievedHowever, in in 10 all min. cases, However, some molybdenum in all cases, wassome leached, molybdenum in agreement was leached, with the in results agreement of Mukai withet the al. results of Mukai et al. (see Figure 3). The amount of leached Mo decreased with the number of results(see Figure of Mukai3). The et amount al. (see of Figure leached 3). Mo The decreased amount with of leached the number Mo decreased of recycles with (as also the observed number byof recycles (as also observed by Mukai) and after three runs it was null, while the yield had only recyclesMukai) and(as afteralso observed three runs by it wasMukai) null, and while after the three yield runs had onlyit was slightly null, while decreased. the yield However, had only this slightly decreased. However, this late result must be taken with care because the yield values were slightlylate result decreased. must be takenHowever, with this care late because result the must yield be valuestaken with were care near because 100% and the this yield could values mask were an near 100% and this could mask an activity decrease. nearactivity 100% decrease. and this could mask an activity decrease.

SchemeScheme 1.1. Oximation ofof substitutedsubstituted benzaldehydesbenzaldehydes withwith NHNH22OHOH·HCl.¨ HCl. Scheme 1. Oximation of substituted benzaldehydes with NH2OH·HCl.

The same group used also this catalyst for the synthesis of 14-substituted-14-H-dibenzo [a,j] The same group used also this catalyst for thethe synthesissynthesis ofof 14-substituted-14-14-substituted-14-H-dibenzo [ a,j] xanthenes [22] (Scheme 2). Both thermal and microwave irradiation conditions were studied. High xanthenes [22] [22] (Scheme 22).). BothBoth thermalthermal andand microwavemicrowave irradiationirradiation conditionsconditions werewere studied.studied. HighHigh yields (more than 90%) were obtained by use of microwave irradiation after short reaction times (a yields (more(more thanthan 90%)90%) were were obtained obtained by by use use of of microwave microwave irradiation irradiation after after short short reaction reaction times times (a few (a few minutes). As above, leaching was observed during the first runs, but ceased later, without a fewminutes). minutes). As above, As above, leaching leaching was observed was observed during du thering first the runs, first but runs, ceased but later, ceased without later, a without noticeable a noticeable modification of the catalytic activity. noticeablemodification modification of the catalytic of the activity. catalytic activity.

Scheme 2. Synthesis of 14-substituted-14-H-dibenzo [a,j] xanthenes. Scheme 2. Synthesis of 14-substituted-14- H-dibenzo [ a,j] xanthenes. The same material was also used for the synthesis and deprotection of 1,1-diacetates under The same material was also used for the synthesis and deprotection of 1,1-diacetates under solvent-freeThe same conditions material [23] was (Schem also usede 3). As for above, the synthesis a non-negligible and deprotection amount of of molybdenum 1,1-diacetates leached under solvent-free conditions [23] (Scheme 3). As above, a non-negligible amount of molybdenum leached solvent-freeduring the first conditions runs, leading [23] (Scheme to a decrease3). As of above, the ca atalytic non-negligible activity that amount was initially of molybdenum very good leached (yields during the first runs, leading to a decrease of the catalytic activity that was initially very good (yields duringhigher than the first 90%). runs, leading to a decrease of the catalytic activity that was initially very good (yields higher than 90%). higher than 90%).

Inorganics 2016, 4, 13 7 of 24 Inorganics 2016, 4, 13 7 of 23

SchemeScheme 3.3. Protection-deprotectionProtection-deprotection ofof 1,1-diacetates.1,1-diacetates.

Ferreira et al. synthesized HPMo12 encapsulated in dealuminated NaY zeolite (Si/Al = 30) and Ferreira et al. synthesized HPMo encapsulated in dealuminated NaY zeolite (Si/Al = 30) and used the thereby obtained catalysts12 for the esterification of glycerol with acetic acid [24]. The used the thereby obtained catalysts for the esterification of glycerol with acetic acid [24]. The synthesis synthesis was performed as described by Mukai and the samples were thoroughly washed with hot was performed as described by Mukai and the samples were thoroughly washed with hot water. water. Four different catalysts were prepared with amounts of PMo12 varying from 0.006 to 0.054 g Four different catalysts were prepared with amounts of PMo varying from 0.006 to 0.054 g per g of per g of zeolite. The solids were characterized by nitrogen12 adsorption, infrared spectroscopy and zeolite. The solids were characterized by nitrogen adsorption, infrared spectroscopy and XRD. The XRD. The catalytic activity reached a maximum for a PMo12 loading of 0.019 g/g of zeolite catalytic activity reached a maximum for a PMo loading of 0.019 g/g of zeolite (conversion = 68%) (conversion = 68%) with a high selectivity towards12 mono- (37%) and di-esters (59%). A study of with a high selectivity towards mono- (37%) and di-esters (59%). A study of recycling was also made recycling was also made and no decrease of activity was observed after four consecutive runs (the and no decrease of activity was observed after four consecutive runs (the catalyst was separated by catalyst was separated by centrifugation, washed with water and dried at 120 °C before reuse). centrifugation, washed with water and dried at 120 ˝C before reuse). Vital et al. studied the hydration of α-pinene catalyzed by HPMo12 encapsulated in US-Y [25]. Vital et al. studied the hydration of α-pinene catalyzed by HPMo encapsulated in US-Y [25]. The solid was prepared by the method described by Mukai et al. and was12 characterized by infrared The solid was prepared by the method described by Mukai et al. and was characterized by infrared spectroscopy and nitrogen adsorption. The main reaction product of α-pinene hydration was spectroscopy and nitrogen adsorption. The main reaction product of α-pinene hydration was α-terpineol, along with minor amounts of other monoterpenes. The selectivity was higher than for α-terpineol, along with minor amounts of other monoterpenes. The selectivity was higher than PMo12 supported on silica, whatever the conversion, and could reach 75% at 100% conversion. The for PMo supported on silica, whatever the conversion, and could reach 75% at 100% conversion. catalyst 12could be reused at least four times but it showed a decrease of activity from 1.5 × 10−3 to The catalyst could be reused at least four times but it showed a decrease of activity from 1.5 ˆ 10´3 to 0.75 × 10−3 mol·h−1·gcat−1. ˆ ´3 ¨ ´1¨ ´1 0.75 This10 catalystmol h wasgcat later. dispersed in polydimethylsiloxane (PDMS), leading to an This catalyst was later dispersed in polydimethylsiloxane (PDMS), leading to an HPMo12-US-Y/PDMS membrane, resulting in an increase of the catalytic activity [26]. The activity HPMoincreased12-US-Y/PDMS in the subsequent membrane, uses of resulting the same in me anmbrane, increase probably of the catalytic due to activity the interaction [26]. The between activity increasedretained α in-terpineol the subsequent and the usespolymer of the matrix. same membrane, probably due to the interaction between retained α-terpineol and the polymer matrix. 3. Synthesis of Tungstic Keggin Species or their Salts in/on Zeolites 3. Synthesis of Tungstic Keggin Species or their Salts in/on Zeolites Quite simultaneously with Mukai et al., Sulikowski et al. reported the encapsulation of Quite simultaneously with Mukai et al., Sulikowski et al. reported the encapsulation of 12-tungstophosphoric acid (HPW12) in the supercage of Y faujasite [27]. As above, the zeolite was 12-tungstophosphoric acid (HPW12) in the supercage of Y faujasite [27]. As above, the zeolite was first first dealuminated (by treatment with SiCl4 or EDTA) prior synthesis of the polyoxometalate. dealuminated (by treatment with SiCl or EDTA) prior synthesis of the polyoxometalate. Unfortunately, Unfortunately, no indication was4 given about the method used for the encapsulation of no indication was given about the method used for the encapsulation of 12-tungstophosphoric acid 12-tungstophosphoric acid (the reference is “Patent Application” and there is no patent in the (the reference is “Patent Application” and there is no patent in the literature). The resulting solid was literature). The resulting solid was characterized by infrared spectroscopy and solid-state 31P MAS characterized by infrared spectroscopy and solid-state 31P MAS NMR and used in the isomerization NMR and used in the isomerization and disproportionation of m-xylene. The encapsulation was and disproportionation of m-xylene. The encapsulation was claimed on the basis of the 31P MAS claimed on the basis of the 31P MAS NMR spectrum which gave a broad peak (linewidth ca. 15 ppm, NMR spectrum which gave a broad peak (linewidth ca. 15 ppm, while the sample prepared by while the sample prepared by classical impregnation led to a very sharp signal). This broadening classical impregnation led to a very sharp signal). This broadening was interpreted as due to the was interpreted as due to the interaction between the zeolite and the polyoxometalate. The interaction between the zeolite and the polyoxometalate. The encapsulated material showed a drastic encapsulated material showed a drastic increase of activity (from 8% to more than 31% at 300 °C) and increase of activity (from 8% to more than 31% at 300 ˝C) and selectivity (from 5% to 50%) towards selectivity (from 5% to 50%) towards disproportionation into toluene and trimethylbenzenes disproportionation into toluene and trimethylbenzenes compared to the starting faujasite. compared to the starting faujasite. Jin et al. reported the synthesis and encapsulation of HPW12 in the supercages of ultra-stable Jin et al. reported the synthesis and encapsulation of HPW12 in the supercages of ultra-stable Y Y zeolite under microwave irradiation [28]. The reaction occurred in a few minutes (3–9 min). The solids zeolite under microwave irradiation [28]. The reaction occurred in a few minutes (3–9 min). The were then washed 20 times with hot water to remove HPW12 located on the external surface of the solids were then washed 20 times with hot water to remove HPW12 located on the external surface of crystallites. The resulting material was characterized by various methods such as 31P MAS NMR the crystallites. The resulting material was characterized by various methods such as 31P MAS NMR spectroscopy, High-Resolution Transmission Electron Microscopy (HR-TEM), nitrogen adsorption, spectroscopy, High-Resolution Transmission Electron Microscopy (HR-TEM), nitrogen adsorption, Inductively Coupled Plasma (ICP) analysis and XRD. The 31P MAS NMR spectrum was quite similar Inductively Coupled Plasma (ICP) analysis and XRD. The 31P MAS NMR spectrum was quite similar to that reported by Sulikowski (broad signal at ´15 ppm with a 20 ppm linewidth). XRD data to that reported by Sulikowski (broad signal at −15 ppm with a 20 ppm linewidth). XRD data showed that increasing the microwave irradiation time resulted in a partial destruction of the zeolitic showed that increasing the microwave irradiation time resulted in a partial destruction of the zeolitic framework. HR-TEM micrographs showed clearly the encapsulation of the polyoxometalate into the zeolite. Temperature-Programmed Desorption of NH3 (NH3-TPD) showed that HPW12-US-Y

Inorganics 2016, 4, 13 8 of 24 framework. HR-TEM micrographs showed clearly the encapsulation of the polyoxometalate into the zeolite. Temperature-Programmed Desorption of NH3 (NH3-TPD) showed that HPW12-US-Y exhibited a stronger acidity than that of pure US-Y, and adsorbed pyridine infrared spectroscopy disclosed that the concentration of Brönsted acid sites increased. This hybrid solid acid exhibited an activity higher 1 than those of the zeolite and HPW12 in the synthesis of 4,4 -dimethyldiphenylmethane from toluene and formaldehyde and could be used as a solid acid catalyst in aqueous solutions. Anandan et al. described the synthesis of HPW12 encapsulated in a titanium-exchanged Y zeolite and its photocatalytic properties [29,30]. The zeolite was first dealuminated in a steam atmosphere for 10 h. After obtention of the H-form of the zeolite, titanium was introduced by exchange with an aqueous solution of ammonium titanyl oxalate. HPW12 was finally synthesized by addition, to a suspension of the TiHY zeolite, of sodium tungstate and disodium phosphate and a stoichiometric amount of HCl. Finally, the zeolite was isolated and washed thoroughly with hot water. The system was mainly characterized by solid-state 31P MAS NMR. Five relatively sharp peaks were observed between ´11 and ´14 ppm and attributed to HPW12 in various environments and in the supercages. When looking at the above results it seems more probable that these peaks, which are relatively sharp, correspond to decomposition products of HPW12 and/or HPW12 at the external surface of the crystals. 3´ The infrared spectra showed some characteristic peaks of the [PW12O40] anion, but they were ´1 shifted by ca. 5–8 cm compared to the values of pure HPW12. This shift was much the same as that observed when HPW12 was interacting with TiO2 in a colloidal solution [31]. The system was 20 times more active than the same system without titanium for the photodegradation of methyl orange [29] (see below a detailed explanation of the mechanism of this reaction). It was also studied in the generation of hydrogen and oxygen from aqueous solutions upon illumination through two photon reactions [30]. To our knowledge, the method used for the synthesis of encapsulated HPMo12 (predispersion of the oxide) has not been applied to the synthesis of tungstic heteropolyacids inside the supercages of Y zeolite, even if it has been reported that WO3 could be efficiently introduced into the channels for tungsten loadings below 11.4 wt % [32].

4. Synthesis of the Zeolite around the Polyoxometalate Rather than synthesizing the polyoxometalate inside the supercages of the zeolite, another possibility would be to build the zeolite around it. However, as previously noted, Y zeolite must be selectively synthesized under conditions typically detrimental to the stability of the polyoxometalate, with the available pH range being particularly restrictive. To our knowledge, there are only two reports that utilize this strategy, with claims of polyoxometalate encapsulation poorly evidenced by physical characterization. Zendehdel et al. reported the synthesis of PW12/NaY and PW12/NaY/MCM-41 composites by synthesis of the zeolite in the presence of the polyoxometalate [33]. HPW12 (10 g) was mixed with 16.13 g of sodium silicate and 6 mL of the initiator (with the composition ˝ 16 Na2O:Al2O3:15 SiO2:320 H2O aged at 32 C for 32 h). Then, 13.2 mL of sodium hydroxide (24 wt %) and 5.2 mL of deionized water were added and the mixture was placed in an autoclave at 100 ˝C for 24 h. The solid, called HPW12/NaY, was recovered, washed and dried in air. It was characterized by XRD and infrared spectroscopy and then used for the synthesis of substituted perimidines. When looking at the preparation method, the polyoxometalate cannot be stable in the starting medium which is highly basic and it must decompose. However, the infrared spectrum of the composite displays three characteristic bands of the polyoxometalate at 980, 888 and 790 cm´1. It is then necessary to suppose that the Keggin unit has been rebuilt during the hydrothermal treatment. Unfortunately, this was the only evidence for the presence of the polyoxometalate in the composite and additional experiments, including 31P solid-state NMR, will be necessary before drawing conclusions about the presence of the polyoxometalate and its localization inside the supercages or elsewhere. Inorganics 2016, 4, 13 9 of 24

This solid was used as catalyst for the preparation of 2-(4-nitrophenyl)-2,3-dihydro-1H-perimidine ˝ byInorganics reaction 2016 of, 4, 1,8-naphthalenediamine 13 with 4-nitrobenzaldehyde in ethanol at 80 C (Scheme4). Yields9 of 23 around 70% were obtained and the catalyst could be recycled three times (after filtration and washing withsignificant ethanol) loss without of activity. a significant This reaction loss ofis activity.catalyzed This by reactionBrönsted is acid catalyzed sites, which by Brönsted must arise acid from sites, whichthe polyoxometalate must arise from asthe thepolyoxometalate zeolite is in its sodium as the zeoliteform. is in its sodium form.

Scheme 4. Synthesis of substituted perimidine cataly catalyzedzed by the encapsulatedencapsulated polyoxometalate.

Very recently, Sun reported a “build-bottle-around-ship” synthesis of HPMo12 encapsulated in Very recently, Sun reported a “build-bottle-around-ship” synthesis of HPMo encapsulated in Y zeolite [34]. The preparation involves also the introduction of the polyoxometalate12 in a basic Y zeolite [34]. The preparation involves also the introduction of the polyoxometalate in a basic solution solution (pH = 10) of initiator containing silicon and aluminium (formed by dissolution of a natural (pHclay =in 10)highly of initiator alkaline containingmedium and silicon hydrothermal and aluminium synthesis) (formed followed by dissolutionby a hydrothermal of a natural treatment clay inat ˝ highly130 °C alkalinefor 48 h. mediumThe resulting and hydrothermal solid was collected synthesis) by filtration, followed treated by a hydrothermal with 0.01 M treatmentnitric acid at at 130 60 °CC ˝ for 4810 h.h in The order resulting to remove solid the was unencapsulated collected by filtration, polyoxometalate treated with and 0.01 was M finally nitric washed acid at 60withC hot for 10water. h in The order main to remove difference the unencapsulatedis that, in the present polyoxometalate case, the polyoxometalate and was finally was washed not in with its acid hotwater. form The main difference is that, in the present case, the polyoxometalate was not in its acid form but but as its ammonium salt, (NH4)3[PMo12O40]. Two relatively high PMo loadings (15 and 6 wt %) were asobtained its ammonium by varying salt, the (NH amount4)3[PMo of 12addedO40]. polyoxometalate. Two relatively high The PMo solids loadings were characterized (15 and 6 wt by %) X-ray were obtainedfluorescence, by varying X-ray thediffraction, amount of infrared added polyoxometalate. spectroscopy, Thenitrogen solids adsorption–desorption were characterized by X-rayand fluorescence, X-ray diffraction, infrared spectroscopy, nitrogen adsorption–desorption and solid-state solid-state 27Al and 31P MAS NMR spectroscopy. As in the above case, the polyoxometalate should 27Al and 31P MAS NMR spectroscopy. As in the above case, the polyoxometalate should not be stable not be stable in the synthesis medium, but the 31P MAS NMR spectra provide evidence of the 31 inpresence the synthesis of the medium,Keggin unit but in the the Presulting MAS NMR solid, spectra with provideonly a sharp evidence peak of at the −3.5 presence ppm is of observed. the Keggin A ´ unitreason in for the this resulting result solid,couldwith be that only the a sharpammonium peak atsalt 3.5of 12-phosphomolybdate ppm is observed. A reason is insoluble for this and result so couldits reaction be that with the ammoniumthe very basic salt medium of 12-phosphomolybdate occurs only slowly. is insoluble X-ray diffraction and so its shows reaction only with the the peaks very basicof the mediumY zeolite occurs while onlythe nitrogen slowly. X-raysorption diffraction capacities shows decrease only when the peaks the HPMo of the Yloading zeolite increases. while the nitrogenThe presence sorption of the capacities Keggin decrease units in when the supercag the HPMoes loadingwas concluded increases. from The presencethese results. of the Even Keggin if unitsadditional in the experiments supercages was are concludedneeded to fromconclusively these results. prove Even the encapsulation if additional experiments of the polyoxometalate, are needed to conclusivelythe use of insoluble prove the salts encapsulation could provide of the a good polyoxometalate, approach to theovercome use of insolublethe problem salts of could the instability provide a goodin basic approach media. toUnfortunately, overcome the this problem solid ofwas the only instability used for in adsorption/desorption basic media. Unfortunately, of alkaline this solid cations was onlyand no used catalysis for adsorption/desorption application was evaluated. of alkaline cations and no catalysis application was evaluated. 5. Keggin Polyoxometalates Supported on Zeolites 5. Keggin Polyoxometalates Supported on Zeolites 5.1. Preliminary Studies 5.1. Preliminary Studies Pamin et al. prepared 12-tungstophosphoric acid on Y zeolite by impregnation in water [35]. Pamin et al. prepared 12-tungstophosphoric acid on Y zeolite by impregnation in water [35]. Before encapsulation, the zeolite was first dealuminated by treatment with EDTAH4. The resulting Si/AlBefore ratioencapsulation, of the material the zeolite was 4.24. was This first treatment dealuminated led to by the treatment formation with of a EDTAH secondary4. The pore resulting system withSi/Al poresratio of ranging the material from 12 was to 504.24. Å andThis atreatment mean value led ofto 15the Å. formation The polyoxometalate of a secondary loading pore system varied betweenwith pores 1 ranging and 10 wtfrom %. 12 The to 50 samples Å and a were mean characterized value of 15 Å. by The various polyoxometalate physicochemical loading methods varied includingbetween 1 XRD,and thermogravimetry,10 wt %. The samples infrared were spectroscopy characterized and by solid-state various physicochemical MAS NMR spectroscopy. methods Theincluding main conclusionsXRD, thermogravimetry, were that two typesinfrared of Kegginspectroscopy units were and presentsolid-state on theMAS surface: NMR thosespectroscopy. strongly interactingThe main conclusions with the hydroxyl were that groups two of types the support of Keggin (probably units viawere their present protonation on the [ 36surface:]), prevailing those atstrongly low coverage, interacting and with those the interacting hydroxyl groups weakly of with the thesupport zeolite (probably crystals andvia their corresponding protonation to [36]), bulk prevailing at low coverage, and those interacting weakly with the zeolite crystals and corresponding to bulk heteropolyacid, prevailing at high coverage. These solids were tested in the transformation of m-xylene by isomerization, disproportionation and dealkylation. Deposition of the 12-tungstophosphoric acid dramatically increased the selectivity for disproportionation. The results were comparable to those obtained by the same group with the encapsulated polyoxometalate [27].

Inorganics 2016, 4, 13 10 of 24 heteropolyacid, prevailing at high coverage. These solids were tested in the transformation of m-xylene by isomerization, disproportionation and dealkylation. Deposition of the 12-tungstophosphoric acid dramaticallyInorganics 2016, 4 increased, 13 the selectivity for disproportionation. The results were comparable to10 those of 23 obtained by the same group with the encapsulated polyoxometalate [27]. These solidssolids werewere alsoalso used used for for catalysis catalysis in in liquid liquid phase phase for for the the oxidation oxidation of variousof various substrates substrates by hydrogenby hydrogen peroxide peroxide [37]. [37]. Only Only activated activated molecules molecu (containing,les (containing, for example, for example, hydroxyl hydroxyl groups) groups) could becould oxidized be oxidized but unfortunately but unfortunately no studies no studies were made were onmade leaching on leaching and recycling. and recycling. Sulikowski alsoalso prepared prepared samples samples by impregnationby impregnation of the of same the dealuminatedsame dealuminated zeolite withzeolite HPW with12 inHPW diethyl12 in diethyl ether [38 ether]. The [38]. mixture The mixture was stirred was stirred in a closed in a closed vessel vessel at room at room temperature temperature for 1.5 for h 1.5 and h thenand then the solid the solid was isolatedwas isolated by filtration by filtra andtion washed and washed with diethylwith diethyl ether. ether. PW12 PWloadings12 loadings ranging ranging from 2from to 38 2 wtto 38 % werewt % preparedwere prepared by this by protocol. this protocol. Similar Similar conclusions conclusions to Parmin to Parminet al. were et al. drawn were drawn about theabout presence the presence of two of types two oftypes Keggin of Keggin units, units, even ifeven in the if in present the present case thecase bulk the bulk heteropolyacid heteropolyacid was alsowas also present present at low at low coverage. coverage. These These solids solids were were tested tested in in the the disproportionation disproportionation ofof toluenetoluene and itsits transalkylationtransalkylation withwith 1,2,4-trimethylbenzene.1,2,4-trimethylbenzene. All All hybrid catalysts exhibitedexhibited enhancedenhanced catalyticcatalytic activity in both processes, but the Keggin units located in the mesoporesmesopores were found to be thethe mostmost active species. Haber et al. [39][39] performed a a detailed detailed study study on on HPW HPW1212 supportedsupported on on dealuminated dealuminated zeolite zeolite as as a afunction function of of the the HPW HPW1212 loadingloading (between (between 0.00016 0.00016 and and 46.8 46.8 wt wt %). %). The The samples samples were were prepared by impregnationimpregnation inin waterwater as as in in the the first first paper paper [35 [35].]. These These catalysts catalysts were were fully fully characterized characterized and and studied studied in thein the vapor-phase vapor-phase dehydration dehydration of ethanol. of ethanol. It was concludedIt was concluded that the catalyticthat the behavior catalytic was behavior determined was bydetermined few very by active few very sites, active composed sites, composed of Keggin of anions Keggin and anions lacunary and lacunary Keggin Keggin anions anions located located in the supercagesin the supercages opened opened at the surface,at the surface, which haswhic ah honeycomb has a honeycomb structure. structure. This structure This structure was formed was duringformedthe during zeolite the surface zeolite pretreatment surface pretreatment and dealumination, and dealumination, resulting inresulting a disintegration in a disintegration of the zeolite of crystalsthe zeolite and crystals a breaking and of a thebreaking supercages of the forming supercages “bowls” forming which “bowls” could perfectly which could fit the perfectly Keggin anionsfit the (FigureKeggin4 anions). (Figure 4).

Figure 4.4. Location of Keggin polyoxometalate on the surfacesurface of Y zeolite (colors same as in Figures1 1 and2 2).).

5.2. Applications in RefiningRefining (Hydroisomerization(Hydroisomerization ofof Alkanes,Alkanes, Hydrodesulfurization)Hydrodesulfurization)

There are some reports on the use of catalysts based on PW12 and platinum on zeolites in the There are some reports on the use of catalysts based on PW12 and platinum on zeolites in the hydroisomerization of alkanes. These catalysts are quitequite similar to those prepared on more classicalclassical supports such as silica.silica. Their main interest is to combine three differentdifferent functions:functions: the acidity of the polyacid,polyacid, thatthat ofof thethe zeolitezeolite andand thethe noblenoble metal.metal. The hydroisomerization of n-heptane was studied on PW12/DUS-Y (dealuminated ultra-stable Y The hydroisomerization of n-heptane was studied on PW12/DUS-Y (dealuminated ultra-stable Yzeolite) zeolite) [40,41]. [40,41 ].The The catalysts catalysts were were prepared prepared by by impregnation impregnation of of the the US-Y US-Y or or DUS-Y zeolite withwith 12-tungstophosphoric12-tungstophosphoric acid,acid, followedfollowed byby wetwet impregnationimpregnation withwith aa platinumplatinum salt. salt. AlthoughAlthough Pt/zeolitePt/zeolite catalysts were active for this reaction, an additional synergic effect was observed in the case of the DUS-Y sample in the presence of platinum and PW12. Unfortunately, the selectivity decreased when the temperature was increased, irrespective of sample composition, and around 300 °C the formation of toluene was also observed. This promotion effect was not observed on US-Y or silica. It was assumed that the polyoxometalate was not located in the supercages of faujasite but in the mesopores of the solid, their mean pore diameter being 1.8 nm for US-Y and 2.3 nm for DUS-Y. In

Inorganics 2016, 4, 13 11 of 24 catalysts were active for this reaction, an additional synergic effect was observed in the case of the DUS-Y sample in the presence of platinum and PW12. Unfortunately, the selectivity decreased when the temperature was increased, irrespective of sample composition, and around 300 ˝C the formation of toluene was also observed. This promotion effect was not observed on US-Y or silica. It was assumed that the polyoxometalate was not located in the supercages of faujasite but in the mesopores of the solid, their mean pore diameter being 1.8 nm for US-Y and 2.3 nm for DUS-Y. In the case of US-Y, the low catalytic activity was explained by assuming that the protons of PW12 had been neutralized by extra framework aluminum, resulting in a poorly active system. Wei et al. [42] studied the hydroisomerization of n-heptane on HPW12 supported on Hβ zeolite in presence of platinum. As this zeolite can only accommodate spheres of 0.67 nm in diameter and possesses channels ca. 0.6 nm in diameter [11], the polyoxometalate cannot fill them and is deposited on the external surface of the crystallites. The zeolite was used as received or dealuminated by a hydrothermal treatment followed by HCl washing. The samples were prepared by impregnation as ˝ in [41] with 0.4 wt % Pt and 5 to 15 wt % HPW12. The catalytic experiments were made at 250 C ´1 (0.5 g catalyst, weight hourly space velocity 2 h ,H2 to n-C7 molar ratio 7.9, pretreatment under hydrogen at 300 ˝C). Only the XRD patterns were presented and did not show any peak for Pt or HPW12. Unfortunately, no further characterization of the solids was made. The cracking products were only propane and butane. The catalytic results are given below (Table2) where H β-m(n) is a Hβ zeolite treated hydrothermally at m ˝C during n h.

Table 2. Comparison of the activity and selectivity of various catalysts in the hydroisomerization of n-hexane [42] a.

Catalyst Conversion (%) Selectivity for Isomerization (%) Pt/Hβ 27.9 99.0 Pt-5% PW12/Hβ 32.2 96.1 Pt-10% PW12/Hβ 47.1 83.8 Pt-15% PW12/Hβ 34.5 70.0 Pt/Hβ-650(5) 14.8 100 Pt-10% PW12/Hβ-300(5) 21.1 95.0 Pt-10% PW12/Hβ-550(5) 25.3 91.1 Pt-10% PW12/Hβ-650(5) 39.6 88.5 Pt-10% PW12/Hβ-700(5) 20.1 92.4 Pt-10% PW12/Hβ-650(4) 19.3 96.0 Pt-10% PW12/Hβ-650(6) 21.0 91.1 Pt-10% PW12/Hβ-650(7) 20.3 93.8 a ´1 Reaction conditions: 0.5 g catalyst, weight hourly space velocity 2 h ,H2 to n-C7 molar ratio 7.9, pretreatment under hydrogen at 300 ˝C. Numbers in brackets denote hours at the treatment temperature.

These results are relatively difficult to explain as they depend on many parameters, with some of them being not known. For example, the amount of acidic sites, the Pt dispersion or the porosity of the samples. Some features can still be deduced from the data, such as the ideal PW12 loading for Hβ is 10 wt %. As the polyoxometalate cannot fill the channels of the zeolite, this value is probably related to the optimal amount of isolated Keggin units. A puzzling feature of the data is the large deviation of the conversion observed for zeolite pretreated at 650 ˝C for 5 h, as compared to zeolite pretreated at 650 ˝C for 4 or 6 h. There is probably another unobserved variable responsible for the anomalous result for zeolite pretreated at 650 ˝C for 5 h, with all the other pretreatments yielding conversions comparable to zeolite without hydrothermal treatment. If one accepts this data point, the results show conversion close to 20%, with a selectivity between 90% and 95%, showing that a pretreatment at 300 ˝C is sufficient to roughly halve the conversion. Later, the same group studied the effect of the incorporation of another metal, namely chromium or lanthanum, on the catalytic activity [43]. The catalysts were characterized by X-ray diffraction (absence of peaks arising from crystalline polyoxometalate), infrared spectroscopy (presence of the Inorganics 2016, 4, 13 12 of 24

bands of the polyoxometalate), BET surface area analysis and H2 adsorption (increase of Pt dispersion when the Cr amount increases). The addition of chromium leads to an increase of both the conversion and the selectivity to isomerization. A study was made as a function of both the Cr and PW12 loading and the reaction conditions (temperature and ratio alkane/catalyst). The conversion could reach 75.3% with 91.3% selectivity to isomerization. This effect was attributed to the improvement of the polyoxometalate, the higher dispersion of platinum and the creation of a “desorption-transfer promoting site”, due to the introduction of chromium in the catalyst. Introduction of lanthanum instead of chromium also led to an improvement of the catalytic activity but the effect was less pronounced. This study was also extended to the introduction of Cerium in the Pt-PW12/DUS-Y system with quite the same conclusions, the presence of cerium increasing the dispersion of platinum [44]. As above, the solids were characterized by X-ray diffraction, infrared spectroscopy, BET surface area analysis and hydrogen chemisorption. Finally, the same authors reported the synthesis of cesium salts of PW12 supported on dealuminated US-Y and their use in the hydroisomerization of n-heptane [45]. For that purpose, the desired amount of cesium was first deposited by impregnation of Cs2CO3 on the zeolite, followed ˝ by drying and calcination at 550 C. The desired amount of PW12 and platinum were then introduced by the same methods as above. The solids obtained were characterized by the same techniques as listed above, and additionally NH3 TPD analysis. The key result was the confirmation of the presence of the cesium salt of PW12 by X-ray diffraction. These solids were also active in hydroisomerization of n-heptane with high activity and selectivity (for example, at 310 ˝C 76.2% conversion was observed with a selectivity to isomerization of 92.2%). Kostova et al. prepared Mo-containing β-zeolite by use of PMo12 and a mechanochemical approach [46]. The solid was characterized by infrared spectroscopy and Temperature Programmed Reduction (TPR) analysis. It was then used in the hydrodesulfurization of thiophene. The activity was 50% higher than that of a sample prepared by classical impregnation.

5.3. Applications in Organic Chemistry While the above section was devoted to the transformation of apolar molecules (if one accepts thiophene), where the heteropolyacid is not soluble, the following applications involve polar molecules where it is highly soluble. Some examples which are not from purely organic chemistry but involving polar substrates will also be included here. Patel et al. studied the esterification of oleic acid with methanol catalyzed by HPW12 supported on Hβ zeolite [47]. Heterogeneous acid catalysts comprised of 12-tungstophosphoric acid (10%–40%) and zeolite Hβ were synthesized by classical impregnation from an aqueous solution of HPW12 and subsequent drying at 100 ˝C. The 30 wt % loaded catalyst was characterized by various physicochemical techniques. The use of this catalyst was explored for biodiesel production by esterification of a free fatty acid, oleic acid, with methanol (Scheme5). The effect of various reaction parameters such as catalyst concentration, acid/alcohol molar ratio, and temperature were studied to optimize the conditions for maximum conversion. The catalyst showed high activity in terms of high conversion (84%) and high turnover number, 1048. A kinetic study, as well as a Koros´Nowak test were carried out, and it was found that esterification of oleic acid followed first order kinetics with a calculated activation energy ´1 4 ´1 Ea of 45.2 kJ¨ mol and a pre-exponential factor A of 5.4 ˆ 10 min . As the reaction occurred in methanol, where the heteropolyacid is highly soluble, the question of recycling was very important. The catalyst showed potential for being used as a recyclable catalytic material, with simple regeneration and no significant loss in conversion after recycling observed. As an application, preliminary studies were carried out for biodiesel production from waste cooking oil and using jatropha oil as feedstock. Inorganics 2016, 4, 13 13 of 24 Inorganics 2016, 4, 13 13 of 23 Inorganics 2016, 4, 13 13 of 23

Scheme 5. Esterification of oleic acid by methanol. Scheme 5. EsterificationEsterification of oleic acid by methanol.

Later, the study was extended to HSiW12 supported on the same Hβ zeolite [48]. The catalyst Later, thethe studystudy was was extended extended to to HSiW HSiW12supported supported on on the the same same Hβ Hzeoliteβ zeolite [48 ].[48]. The The catalyst catalyst was was also used for the trans-esterification 12of soybean oil with methanol. Recycling studies were wasalso usedalso forused the fortrans the-esterification trans-esterification of soybean of soybean oil with methanol.oil with methanol. Recycling Recycling studies were studies performed were performed and did not show any deactivation of the catalyst. performedand did not and show did any not deactivation show any deactivation of the catalyst. of the catalyst. Srinivas et al. prepared catalysts by impregnation of 12-tungstophosphoric acid on Y zeolite Srinivas etet al.al. preparedprepared catalysts catalysts by by impregnation impregnation of of 12-tungstophosphoric 12-tungstophosphoric acid acid on Yon zeolite Y zeolite [49]. [49]. Unfortunately, the Si/Al ratio of the zeolite used for these experiments was not given. The PW12 [49].Unfortunately, Unfortunately, theSi/Al the Si/Al ratio ratio of theof the zeolite zeolit usede used for for these these experiments experiments was was not not given. given. TheThe PWPW12 loading varied from 10 to 25 wt %. The solids were characterized by BET analysis, X-ray diffraction,12 loading varied from from 10 to 25 wt %. The The solids solids were were characterized characterized by by BET BET analysis, analysis, X-ray X-ray diffraction, diffraction, infrared spectroscopy and NH3 TP. They were used for the etherification of glycerol with t-butanol. infrared spectroscopy and and NH NH3 TP.TP. They were used for the etherification etherification of of glycerol glycerol with with tt-butanol.-butanol. The highest conversion (84%) and3 highest selectivity to the mono-ether were achieved on the sample The highest conversion (84%) and highest selectivity to the mono-ether were achieved on the sample containing 20 wt % PW12, which had also been shown to be the most acidic by TPD of ammonia. The containing 20 20 wt wt % % PW PW12, which, which had had also also been been shown shown to be to the be themost most acidic acidic by TPD by TPD of ammonia. of ammonia. The reusability was studied 12by recovering the catalyst at the end of the reaction by filtration, washing it reusabilityThe reusability was wasstudied studied by recovering by recovering the thecatalyst catalyst at the at theend end of the of the reaction reaction by byfiltration, filtration, washing washing it with methanol and drying at 100 °C for 2 h. No appreciable variation of activity was detected after withit with methanol methanol and and drying drying at at 100 100 °C˝C for for 2 2h. h. No No appreciable appreciable variation variation of of activity activity was was detected detected after four cycles. Another important point is that the filtrate of the reaction mixture did not contain four cycles.cycles. AnotherAnother important important point point is thatis that the filtratethe filtrate of the of reaction the reaction mixture mixture did not containdid not tungsten, contain tungsten, proving that the polyoxometalate was well anchored on the surface. However, as the Si/Al tungsten,proving that proving the polyoxometalate that the polyoxom wasetalate well was anchored well anchored on the surface. on the However,surface. However, as the Si/Al as the ratio Si/Al of ratio of the starting zeolite was not given, one cannot exclude a partial decomposition of the zeolite ratiothe starting of the starting zeolite waszeolite not was given, not onegiven, cannot one excludecannot exclude a partial a decompositionpartial decomposition of the zeoliteof the zeolite by the by the polyoxometalate, resulting, for example, in an insoluble aluminium salt of PW12. bypolyoxometalate, the polyoxometalate, resulting, resulting, for example, for example, in an insoluble in an insoluble aluminium aluminium salt of PWsalt of. PW12. Zhang et al. prepared a series of catalysts by immobilization of 12-tungstophosphoric12 acid and Zhang et al. prepared aa seriesseriesof of catalysts catalysts by by immobilization immobilization of of 12-tungstophosphoric 12-tungstophosphoric acid acid and and its its cesium salt on ultra-stable Y zeolite and its dealuminated form (DUS-Y) and silica materials. The itscesium cesium salt salt on ultra-stableon ultra-stable Y zeolite Y zeolite and and its dealuminated its dealuminated form form (DUS-Y) (DUS-Y) and and silica silica materials. materials. The acidThe acid was deposited by impregnation while the cesium salts were obtained by a two-step protocol: wasacid depositedwas deposited by impregnation by impregnation while while the cesiumthe cesi saltsum salts were were obtained obtained by a by two-step a two-step protocol: protocol: the the desired amount of cesium was first introduced by impregnation of cesium carbonate followed by thedesired desired amount amount of cesiumof cesium was was first first introduced introduced by by impregnation impregnation of of cesium cesium carbonatecarbonate followed by calcination, after which the polyacid was then introduced by a second impregnation. The different calcination, after which the polyacid was then introduced by a second impregnation. The different materials were characterized by various physicochemical methods including X-ray powder materials werewere characterized characterized by variousby various physicochemical physicochemical methods methods including including X-ray powder X-ray diffraction, powder diffraction, BET surface area analysis, 31P and 29Si MAS NMR spectroscopy and SEM. The early work diffraction,BET surface BET area surface analysis, area31 Panalysis, and 29Si 31 MASP and NMR29Si MAS spectroscopy NMR spectroscopy and SEM. and The SEM. early The work early reported work reported that they were tested in the liquid-phase esterification of acetic acid with butanol [50]. They reportedthat they that were they tested were in thetested liquid-phase in the liquid-phase esterification esterification of acetic acid of acetic with butanolacid with [50 butanol]. They [50]. were They then were then used in the synthesis of fructone by acetalization of ethylacetoacetate with ethylene glycol wereused then in the used synthesis in the synthesis of fructone of byfructone acetalization by acetalization of ethylacetoacetate of ethylacetoacetate with ethylene with ethylene glycol [ 51glycol–53] [51–53] (Scheme 6). [51–53](Scheme (Scheme6). 6).

Scheme 6. Synthesis of fructone by acetalization of ethylacetoacetate with ethylene glycol. Scheme 6. SynthesisSynthesis of of fructone fructone by acetalization of ethylacetoacetate with ethylene glycol.

Various parameters were studied (amount of PW12, of Cs, reaction conditions, etc.). The best Various parameters were studied (amount of PW12, of Cs, reaction conditions, etc.). The best systemsVarious were parameters those based were on the studied Cs salts (amount of PW of12 PWsupported12, of Cs, on reaction dealuminat conditions,ed US-Yetc zeolite.). The and best a systems were those based on the Cs salts of PW12 supported on dealuminated US-Y zeolite and a systemsconversion were as thosehigh basedas 98.7% on thewith Cs a saltsselectivity of PW of12 supported97% to fructone on dealuminated could be achieved US-Y zeolite over and30% conversion as high as 98.7% with a selectivity of 97% to fructone could be achieved over 30% aCs conversion2.5H0.5PW12/DUS-Y. as high A as study 98.7% of with leaching a selectivity by treatment of 97% with to water fructone showed could that be the achieved systems over based 30% on Cs2.5H0.5PW12/DUS-Y. A study of leaching by treatment with water showed that the systems based on the pure heteropolyacid were not stable, with most of the Keggin units being released in water. In the pure heteropolyacid were not stable, with most of the Keggin units being released in water. In contrast, those based on the cesium salt did not show a significant leaching. contrast, those based on the cesium salt did not show a significant leaching.

Inorganics 2016, 4, 13 14 of 24

Cs2.5H0.5PW12/DUS-Y. A study of leaching by treatment with water showed that the systems based onInorganics the pure 2016, heteropolyacid4, 13 were not stable, with most of the Keggin units being released in14 water. of 23 In contrast, those based on the cesium salt did not show a significant leaching. The same authorsauthors also studiedstudied thethe synthesissynthesis ofof fructone-Bfructone-B byby acetalizationacetalization ofof ethylacetoacetateethylacetoacetate withwith 1,2-propanediol1,2-propanediol and and observed observed similar similar results, results, the the Cs Cs salts salts being being active active with with minimal minimal leaching leaching [54]. In[54]. another In another paper paper [55], they[55], usedthey used these these catalysts catalysts for the for liquid-phase the liquid-phase esterification esterification of acetic of acetic acid withacid nwith-butanol. n-butanol. The conversion The conversion was higher was than higher that observedthan that when observed using thewhen zeolite using or the the polyoxometalate zeolite or the alone.polyoxometalate Again, the alone. supported Again, cesium the supported salts exhibited cesium only salts minimal exhibited leaching, only minimal leading leaching, to only leading a small decreaseto only a ofsmall the catalyticdecrease activityof the catalytic upon recycling. activity upon recycling. Moosavifar reported the use of heteropolyacidsheteropolyacids supported on Y zeolite for the synthesissynthesis of dihydropyrimidinones by thethe BiginelliBiginelli reactionreaction [56[56]] (Scheme(Scheme7 ).7). PMo PMo1212,, PWPW1212 and SiWSiW1212 were deposited on a YY zeolitezeolite dealuminateddealuminated byby treatmenttreatment withwith HClOHClO44. Good yields were obtainedobtained forfor allall threethree systemssystems withinwithin a a few few hours hours and and the the catalysts catalysts could could be be recycled recycled after after washing washing with with hot waterhot water and ethanol.and ethanol. Such Such behavior behavior is surprising, is surprising, as the heteropolyacidsas the heteropolyacids are highly are solublehighly insoluble these polarin these solvents. polar However,solvents. However, the catalytic the reaction catalytic involves reaction urea, involves which is urea, probably which protonated is probably by the protonated polyacid, leadingby the topolyacid, the formation leading of to an the insoluble formation salt. of The an synthesis insoluble of salt. such The a species synthesis has of been such reported a species recently has been [57]. Duereported to the recently great size [57]. of Due the molecules,to the great polyacids size of the encapsulated molecules, inpolyacids Y zeolite encapsulated such as those in described Y zeolite insuch [20 as,22 those,23] were described inactive in for[20,22,23] this reaction. were inactive for this reaction.

Scheme 7. Synthesis of dihydropyrimidinones by the Biginelli reaction.reaction.

More recently, Narkhede and Patel studied this reaction over HPW12 and HSiW12 supported on More recently, Narkhede and Patel studied this reaction over HPW and HSiW supported on Hβ zeolite [58]. The catalysts were those reported previously [47,48] 12and led to higher12 yields in Hβ zeolite [58]. The catalysts were those reported previously [47,48] and led to higher yields in smaller smaller times (for example a 98% yield in 15 min instead of a 97% yield in 360 min). The reaction was times (for example a 98% yield in 15 min instead of a 97% yield in 360 min). The reaction was then then extended to other substrates (Table 3). extended to other substrates (Table3). Nandiwale et al. prepared a catalyst based on HPW supported on desilicated H-ZSM-5 and Table 3. Synthesis of various dihydropyrimidones12 on PW12/Hβ and SiW12/Hβ [58]. studied it in the esterification of levulinic acid by ethanol, in view of the synthesis of ethyl levulinate, which can be usedR–C(=O)H, as diesel miscible R′C(=O)CH biofuel2C(=O)CH [59]. The3, desilificationNH2–C(=X)NH was2, performedReaction by treatmentYield Catalyst ˝ with NaOH at various concentrationsR = (fromR′ 0.2= to 1.5 M) duringX 30 = min at 65 TimeC. The (min) resulting (%) solids possessedPW12 an/H increasedβ surfaceC6H5 area with a–OC high2H number5 of mesoporesO where HPW1215could be deposited98 by theSiW incipient12/Hβ wetnessC6H technique.5 These–OC solids2H5 were used in theOesterification of15 levulinic acid95 and various parameters were studied (relative amounts of reactants, of catalyst, temperature, speed of PW12/Hβ C6H5 –OC2H5 S 17 90 agitation, etc.) including the recycling of the catalyst (reused directly after filtration). Unfortunately, no SiW12/Hβ C6H5 –OC2H5 S 20 92 study of leaching was made and even if a kinetic law was proposed, the high conversions (more than PW12/Hvβ C6H5 –OCH3 O 14 92 80%) prevented some conclusions to be made. SiW12/Hβ C6H5 –OCH3 O 12 94

PW12/Hβ C6H5 –OCH3 S 15 97

SiW12/Hβ C6H5 –OCH3 S 15 96

PW12/Hβ 4-ClC6H4 –OC2H5 O 30 89

SiW12/Hβ 4-ClC6H4 –OC2H5 O 28 90

PW12/Hβ 4-NO2C6H4 –OC2H5 O 32 82

SiW12/Hβ 4-NO2C6H4 –OC2H5 O 30 92

Inorganics 2016, 4, 13 15 of 24

Table 3. Synthesis of various dihydropyrimidones on PW12/Hβ and SiW12/Hβ [58].

R–C(=O)H, R1C(=O)CH C(=O)CH , NH –C(=X)NH , Reaction Yield Catalyst 2 3 2 2 R = R1 = X = Time (min) (%)

PW12/Hβ C6H5 –OC2H5 O 15 98 SiW12/Hβ C6H5 –OC2H5 O 15 95 PW12/Hβ C6H5 –OC2H5 S 17 90 SiW12/Hβ C6H5 –OC2H5 S 20 92 PW12/Hvβ C6H5 –OCH3 O 14 92 SiW12/Hβ C6H5 –OCH3 O 12 94 PW12/Hβ C6H5 –OCH3 S 15 97 SiW12/Hβ C6H5 –OCH3 S 15 96 PW12/Hβ 4-ClC6H4 –OC2H5 O 30 89 SiW12/Hβ 4-ClC6H4 –OC2H5 O 28 90 PW12/Hβ 4-NO2C6H4 –OC2H5 O 32 82 SiW12/Hβ 4-NO2C6H4 –OC2H5 O 30 92

5.4. Applications in Photocatalysis

5.4.1. Systems without Titanium

Marchena et al. [60] prepared two series of materials based on HPW12 immobilized on NH4Y and NH4ZSM-5 and prepared by wet impregnation of aqueous solutions. The concentration of HPW12 was varied in order to obtain PW12 contents of 5, 10, 20 and 30 wt % in the solid. The materials were characterized by N2 adsorption–desorption isotherms, X-ray diffraction, infrared spectroscopy, 31P MAS-NMR spectroscopy, thermogravimetry, and Diffuse Reflectance UV–Visible spectroscopy, with the acidic behavior studied by potentiometric titration with n-butylamine. The specific surface area (SBET) decreased as the polyoxometalate content was increased, as a result of zeolite pore blocking. The X-ray diffraction patterns of the solids modified with PW12 showed the characteristic peaks of NH4Y and NH4ZSM-5 zeolites, and an additional set of peaks assigned to the presence of (NH4)3PW12O40 formed by exchange of the protons of the polyacid with the ammonium ions of the zeolite. By interpretation of the infrared and 31P NMR spectra, the main species present in 3´ 6´ the samples was the [PW12O40] anion, which was partially transformed into [P2W21O71] during the synthesis and drying steps. The thermal stability of these materials was similar to that of the parent zeolites. Moreover, the samples with the higher PW12 content had band gap energy values similar to those reported for TiO2. These systems were highly active in the photocatalytic degradation of 4-chlorophenol and they could be reused at least three times with minimal decreases in the degree of degradation of 4-chlorophenol attained. This stability was probably related to the formation of the ammonium salt of PW12, which is insoluble. An increase of the photocatalytic activity was observed when the amount of PW12 increased, showing its direct participation in the degradation of the organic substrate (Equations (2) to (4)) and/or in the production of the ¨ OH reactive species (Equation (5)) that participates in the degradation of the organic substrate S (Equation (6)).

PW12 ` hv ÑPW12˚, (2)

´ + PW12 ˚ ` S Ñ PW12 ` S , (3) ´ ´ PW12 ` O2 Ñ PW12 ` O2 , (4) ´ + PW12 ˚ ` H2O Ñ PW12 ` ¨ OH ` H , (5) ¨ OH ` S Ñ oxidation products. (6) Inorganics 2016, 4, 13 16 of 24

Additionally, the increase in the catalytic activity may be due to the lower band gap values of the samples with higher PW12 contents, which would increase the capacity to absorb higher wavelength radiation. Later, the same authors used the catalysts prepared on Y zeolite for the photodegradation of methyl orange [61] and those prepared on ZSM-5 for the photodegradation of waste water containing azo dyes [62]. The highest photocatalytic activity was obtained for a 30 wt % HPW12 loading. The catalyst separated by filtration and treated at 400 ˝C for 2 h could be reused without significant decrease of the conversion. Ozer et al. studied the photocatalytic oxidation of aqueous solutions of 1,2-dichlorobenzene by polyoxometalates supported on NaY zeolite [63]. Three different polyoxometalates were used, H2NaPW12O40,H4SiW12O40 and H3PMo12O40. The catalysts were prepared in situ by the polyoxometalate, the zeolite and the solution of dichlorobenzene mixing in the reactor. Preliminary experiments had shown that there was adsorption of the polyoxometalate on the zeolite but, unfortunately, no characterization data was presented, with only the catalytic performances being reported. However, even if they did not characterize their catalysts, the authors pointed out their complexity. Very recently, Marchena et al. reported the synthesis and characterization of HSiW12 supported on NH4Y and NH4ZSM-5 and its use in the photodegradation of methyl orange [64]. The catalysts were prepared by wet impregnation at 80 ˝C with polyoxometalate loading varying between 5 and 30 wt %. The Si/Al ratio of the zeolites was 2.47 for NH4Y and 17 for ZSM-5. The samples were characterized by various methods, but the most important results were obtained by XRD, which showed some additional peaks compared to those of the starting zeolite. These peaks were due to the presence of the ammonium salt of SiW12. The formation of this new phase can be easily understood, as in solution there is an exchange between the ammonium ions of the zeolite and the protons of the polyoxometalate. This reaction is in equilibrium, but, as the ammonium salt of SiW12 is insoluble, it drives the equilibrium to yield the ammonium salt. The only limitation could be the ammonium content of the zeolite, but even for ZSM-5, which had the highest Si/Al ratio with the highest SiW12 loading, there are more than two ammonium ions per proton of the polyoxometalate. A kinetic study of the photocatalytic degradation of methyl orange showed a linear dependence of the reaction rate on the amount of SiW12 in the solid, due to its direct participation in the catalytic cycle. As with the previous studies by the same authors with PW12, the catalyst could be reused without any problems.

5.4.2. Systems with Titanium Rayalu and coworkers developed the synthesis of photocatalytic systems based on heteropolyacids, TiO2 and Y zeolite [65–70]. The zeolite used for these studies was the commercial faujasite with a Si/Al ratio of 2.5. TiO2 was first introduced by reaction of titanium isopropoxide with the Y zeolite, followed by calcination at 500 ˝C. In some cases, titanium was introduced by other ways [66]. The HPMo12 heteropolyacid was then introduced by a classical impregnation. In some cases, partial replacement of the zeolite cations by exchange with Co2+ was conducted prior to the reaction with the polyoxometalate. These materials were characterized by infrared and UV–Visible spectroscopies, XRD and chemical analysis. They were first tested in the photocatalytic reduction of methyl orange in aqueous solution with ethanol as a sacrificial electron donor [66,68]. The composite containing cobalt photoreduced methyl orange to an extent of about 4.11 mg/g TiO2 and showed a better photocatalytic activity as compared to the same catalyst without cobalt, showing the role of transition metal ions. A mechanism was proposed based on these considerations:

‚ A charge separation is created in the TiO2 species upon irradiation by the UV component of the light source.

‚ The electron from the conduction band of TiO2 is transferred to the polyoxometalate, which is ´ reduced to a visibly active species PM12 . Inorganics 2016, 4, 13 17 of 24

‚ This reduced species is excited by the visible radiation leading to an excited PM12* species, which is able to transfer the excited electrons to the transition metal in the zeolite. This transfer is not direct but occurs via the Lewis acid sites of the zeolite.

‚ The excited species PM12* can also be formed by direct excitation of the polyoxometalate by the UV radiation followed by a reduction via the sacrificial electron donor. ‚ Finally, the substrate reacts with the transition metal. A study of the effect of the method used for the introduction of phosphomolybdic acid was reported in [69]. Three different methods were used: adsorption, impregnation and encapsulation. For the adsorption procedure, the zeolite was treated with a solution containing PMo12 for one hour and then isolated by filtration. The adsorption cycle was repeated three times. Impregnation was made by the procedure already outlined above. Encapsulation was made by using the method of Mukai et al. but starting from commercial Y zeolite (Si/Al = 2.5). The amounts of Mo observed in these materials were (in mg/g of catalyst) 0.88 for adsorption, 42.42 for impregnation and 4.31 for encapsulation, while the conversion of methyl orange was 12%, 51% and 19%, respectively. It was concluded that the sample prepared by impregnation was the most active and a study as a function of the PMo12 loading showed that at lower and higher loadings the efficiency was lower. However, it is interesting to note that the materials prepared by adsorption and encapsulation were also active, but with Mo loadings which are at least 10 times lower than those achieved by impregnation. Three sets of material were prepared by the impregnation procedure, starting from Y zeolite, dealuminated Y zeolite (Si/Al = 80) and β zeolite (Si/Al = 15), were also tested in the water splitting reaction for the production of hydrogen [65,67]. Only the samples prepared from classical Y zeolite were active for both the methyl orange photoreduction and the water splitting reaction. As above, the introduction of cobalt enhanced not only the visible light absorption of the material but also its photocatalytic efficiency. In the absence of cobalt, the hydrogen yield was about 3000 µmol/h/g TiO2 when illuminated in the UV range, a value quite low as compared to those reported in the literature. In the presence of cobalt, the hydrogen yield was 2300 µmol/h/g TiO2, but in the visible range, a value which is high for these illumination conditions. In other publications, various parameters were studied in order to optimize the hydrogen yield [70,71], for example, the nature of the zeolite (Y zeolite, ultrastable Y zeolite, titanium silicalite, β zeolite) and the method of introduction of the polyoxometalate (impregnation, direct encapsulation by in situ synthesis). In these studies, again only the system obtained on Y zeolite through impregnation of the polyoxometalate led to hydrogen evolution. Najafabadi and Taghipour [72,73] prepared composites by a similar protocol: First of all, TiO2 was synthesized on/in the zeolite by hydrolysis of titanium isopropoxide. Cobalt was then introduced by exchange with the cations of the zeolite and, finally, the polyoxometalate (HPMo12) was introduced by impregnation. The solids were characterized by Energy Dispersive X-ray Spectroscopy (EDX), nitrogen adsorption, Scanning Electron Microscopy (SEM) and UV–Vis reflectance spectroscopy. In the first paper [72], the zeolites studied were NaY (Si/Al = 2.55) and Na-mordenite (Si/Al = 6.5). In the second paper, HY (Si/Al = 15) and Hβ (Si/Al = 12.5) were also used [73]. These materials were used for the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid and the photocatalytic hydrogen evolution. An effect of the remaining anion of the cobalt salt used for the exchange was evidenced, with nitrate leading to a 30% lower hydrogen production than chloride. The hydrogen production was also higher with the sodium-exchanged zeolites. This was explained by the pH dependency of the TiO2 band edges. In these solids, TiO2 was the semiconductor, cobalt ions were the hydrogen evolution sites and the heteropolyacid was the multifunctional solid acid with significant excitability under visible light. This interpretation is quite similar to that of Rayalu.

6. Other Polyoxometalates Supported on Zeolites Other polyoxometalates have also been supported on zeolites. Typically, these compounds were lacunary Keggin species, although a few studies with alternate structures have also been reported. Inorganics 2016, 4, 13 18 of 24

Very recently, Tayebee et al. prepared heteropolyacids on nanoclinoptilolite and studied them in the cyclotrimerization of aryl methyl ketones [74] (Scheme8). Various heteropolyacids were used, HPMo12, HPW12, HPW10V2 and H6P2W18O62. Only the sample with this late polyacid was fully characterized, as it was the most active system. Clinoptilolite is a zeolite containing two types of channels, those composed of eight and 10 membered rings, respectively [75]. The maximum diameter of a sphere that can be included in its framework is 5.97 Å [11], so polyoxometalates cannot be incorporated within it and can, therefore, only be deposited on its external surface. In order to increase the sorbed amount, nanocrystals (size ca. 40 nm) were used, with the cations initially replaced by protonsInorganics through2016, 4, 13 exchange with ammonium and subsequent calcination. A study of the adsorption18 of of23 H6P2W18O62 showed that it follows a Langmuir isotherm with adsorption of 200 mg of polyacid per grampolyacid of clinoptilolite. per gram of This clinoptilolite. value is compatible This value with is ancompatible adsorption with of the an external adsorption surface of ofthe the external zeolite assurface it corresponds of the zeolite to a surfaceas it corresponds of ca. 40 m 2toper a surface gram of of zeolite. ca. 40 Them2 per solid gram was of characterized zeolite. Theby solid various was methodscharacterized including by X-rayvarious diffraction, methods infrared including and UV–Visible X-ray diffraction, spectroscopies infrared and SEM. and It wasUV–Visible used in thespectroscopies cyclotrimerization and SEM. of variousIt was used aryl methylin the cycl ketonesotrimerization and shown of to various be recycled aryl withoutmethyl ketones a noticeable and lossshown of activity.to be recycled without a noticeable loss of activity.

Scheme 8. Cyclotrimerization of aryl methyl ketones.

Narkhede and Patel reported the synthesis of a recyclable catalyst comprising a monolacunary Narkhede and Patel reported the synthesis of a recyclable catalyst comprising a monolacunary silicotungstate [SiW11O39]8− (SiW11) supported on zeolite Hβ [76]. The polyoxometalate was as a silicotungstate [SiW O ]8´ (SiW ) supported on zeolite Hβ [76]. The polyoxometalate was as a sodium salt (acidic or11 not,39 no data 11were given about this point) and it was deposited on Hβ zeolite sodium salt (acidic or not, no data were given about this point) and it was deposited on Hβ zeolite by by impregnation. Various loadings between 10 and 40 wt % were prepared. The impregnated impregnation. Various loadings between 10 and 40 wt % were prepared. The impregnated materials materials were treated with 10% 0.1 M HCl and then washed with distilled water. As SiW11 is not were treated with 10% 0.1 M HCl and then washed with distilled water. As SiW11 is not stable in stable in these conditions, this treatment has probably transformed part of it into SiW12 and/or has these conditions, this treatment has probably transformed part of it into SiW and/or has replaced replaced some sodium ions by protons. An interesting point is the presence12 of mesopores in this some sodium ions by protons. An interesting point is the presence of mesopores in this zeolite zeolite (diameter 2.48 nm), whose diameter decreases after adsorption of SiW11 (to 1.71 nm) proving (diameter 2.48 nm), whose diameter decreases after adsorption of SiW (to 1.71 nm) proving that the that the polyoxometalate is located inside them. The acidity was 11determined by n-butylamine polyoxometalate is located inside them. The acidity was determined by n-butylamine titration and the titration and the solids were used in the esterification of oleic acid by methanol and the solids were used in the esterification of oleic acid by methanol and the transesterification of soybean oil. transesterification of soybean oil. The reaction is purely heterogeneous and the catalyst can be The reaction is purely heterogeneous and the catalyst can be reused without loss of activity or leaching. reused without loss of activity or leaching. As the cations are composed solely of protons and As the cations are composed solely of protons and sodium ions, the absence of leaching is surprising. sodium ions, the absence of leaching is surprising. One explanation should be that the negative One explanation should be that the negative charges of SiW11 are partially compensated by protons of charges of SiW11 are partially compensated by protons of the zeolite, leading to a cation/anion the zeolite, leading to a cation/anion interaction that stabilizes it. Note that this interaction is more interaction that stabilizes it. Note that this interaction is more stable than that occurring when a stable than that occurring when a polyacid is deposited on silica [36]. These authors have recently polyacid is deposited on silica [36]. These authors have recently used these catalysts for the used these catalysts for the valorization of glycerol via acetalization and carboxylation reactions [77]. valorization of glycerol via acetalization and carboxylation reactions [77]. Later, the same group reported the synthesis of PMo11 supported on the same zeolite Hβ and Later, the same group reported the synthesis of PMo11 supported on the same zeolite Hβ and its its use in the aerobic oxidation of alcohols and alkenes [78]. The preparation was quite similar to use in the aerobic oxidation of alcohols and alkenes [78]. The preparation was quite similar to that that reported above but no treatment with HCl was conducted. The catalyst loading varied from reported above but no treatment with HCl was conducted. The catalyst loading varied from 10 to 40 10 to 40 wt %. The system with 30 wt % PMo11, which displayed the highest catalytic activity, wt %. The system with 30 wt % PMo11, which displayed the highest catalytic activity, was fully was fully characterized. Differential Thermal Analysis showed an increase of the thermal stability characterized. Differential Thermal Analysis showed an increase of the thermal stability of the polyoxometalate upon deposition on the zeolite (from ca. 300 to 350 °C). Infrared and Raman spectroscopy, as well as XRD, were in full agreement with the conservation of the structure of the polyoxometalate, but the 31P MAS NMR spectrum showed unambiguously that a partial degradation occurred upon impregnation, with formation of PMo12 (peak at −4.3 ppm) and an unknown species (peak at +2.6 ppm). The catalysts were used in the catalytic oxidation by oxygen of benzyl alcohol or styrene at 80 and 90 °C, respectively. A small amount of t-butyl hydrogen peroxide was added to initiate the reaction. The major reaction product in the two cases was benzaldehyde (the selectivity was 90% when starting from benzyl alcohol). Very high turnover numbers (TON) were achieved (ca. 10,000 for the alcohol and more than 20,000 for styrene). The catalyst could be reused at least two times without any loss of activity and no leaching was observed. Finally, the study was extended to other alcohols and alkenes (see Table 4).

Inorganics 2016, 4, 13 19 of 24

of the polyoxometalate upon deposition on the zeolite (from ca. 300 to 350 ˝C). Infrared and Raman spectroscopy, as well as XRD, were in full agreement with the conservation of the structure of the polyoxometalate, but the 31P MAS NMR spectrum showed unambiguously that a partial degradation occurred upon impregnation, with formation of PMo12 (peak at ´4.3 ppm) and an unknown species (peak at +2.6 ppm). The catalysts were used in the catalytic oxidation by oxygen of benzyl alcohol or styrene at 80 and 90 ˝C, respectively. A small amount of t-butyl hydrogen peroxide was added to initiate the reaction. The major reaction product in the two cases was benzaldehyde (the selectivity was 90% when starting from benzyl alcohol). Very high turnover numbers (TON) were achieved (ca. 10,000 for the alcohol and more than 20,000 for styrene). The catalyst could be reused at least two times without any loss of activity and no leaching was observed. Finally, the study was extended to other alcohols and alkenes (see Table4). Inorganics 2016, 4, 13 19 of 23 Inorganics 2016, 4, 13 19 of 23 InorganicsInorganics 2016 2016Table, 4,, 413, 4.13 Catalytic oxidation of various substrates by oxygen catalyzed with PMo /Hβ [78] a.1919 of of 23 23 InorganicsInorganics 20162016,, 44,, 1313 11 1919 ofof 2323 11 a Table 4. Catalytic oxidation of various substrates by oxygen catalyzed with PMo11/Hβ [78] a. Table 4. Catalytic oxidation of various substrates by oxygen catalyzed with PMo11/Hβ [78] a. Table 4. Catalytic oxidation of various substrates by oxygen catalyzed with PMo /H11 β [78] . a Table 4. Catalytic oxidation of various substrates by oxygen catalyzed with PMo11 /Hβ [78]a . Table 4. Catalytic oxidationSubstrate of various Conversion substrates by (%) oxygen Selectivity catalyzed (%) with PMo TON11/Hβ [78] aa. Table 4. Catalytic Substrateoxidation of variousConversion substrates by (%)Selectivity oxygen catalyzed (%) with TON PMo 11/Hβ [78] . SubstrateSubstrate ConversionConversion (%)Selectivity (%)Selectivity (%) (%) TON TON Substrate Conversion (%)Selectivity (%) TON Substrate Conversion25.525.5 (%)Selectivity90 90 (%) TON9551 9551 25.5 90 9551 25.525.5 9090 95519551 25.525.5 9090 95519551

21 >99 8590 2121 >99 >99 8590 8590 2121 >99>99 85908590 2121 >99>99 85908590

20 >99 8171 202020 >99 >99>99 81718171 8171 20 >99 8171 209 >99>99 37458171 9 >99 3745 99 9 >99 >99>99 37453745 3745 9 >99 3745 579 >9972 20,9753745 57 72 20,975 575757 72 7272 20,97520,975 20,975 5757 7272 20,97520,975

59 70 22,099 59 70 22,099 5959 7070 22,09922,099 595959 7070 70 22,09922,099 22,099

50 67 18,728 5050 6767 18,72818,728 50 67 18,728 a 5050 67 67 18,728 18,728 a Reaction conditions: For alcohols T = 90 °C and t = 24 h; for alkenes T = 80 °C and t = 8 h, catalyst a Reaction conditions: For alcohols T = 90 °C and t = 24 h; for alkenes T = 80 °C and t = 8 h, catalyst Reactiona conditions: For alcohols T = 90 °C and t = 24 h; for alkenes T = 80 °C and t = 8 h, catalyst amounta Reaction ca. 0.2 conditions: wt %. For alcohols T = 90 °C and t = 24 h; for alkenes T = 80 °C and t = 8 h, catalyst amountaa Reaction ca .conditions: 0.2 wt %. For alcohols T = 90 °C and t = 24 h; for alkenes T = 80 °C and t = 8 h, catalyst amount Reactiona Reaction ca. conditions:0.2 conditions:wt %. For For alcohols alcohols TT = 90 ˝°CC andandt t= = 24 24 h; h; for for alkenes alkenesT = 80T =˝ C80 and °C tand= 8 h,t = catalyst 8 h, catalyst amount amountamount ca ca. 0.2. 0.2 wt wt %. %. amountamountca. 0.2 caca wt.. 0.20.2 %. wtwt %.%. 7. Conclusions 7. Conclusions 7.7. Conclusions Conclusions 7.7. Conclusions7.ConclusionsAs Conclusions shown in this review, there are different ways to obtain composites based on As shown in this review, there are different ways to obtain composites based on polyoxometalatesAsAs shownshown inandin this thiszeolites: review,review, (i) The therethere polyoxometalate areare differedifferentnt can waysways be synthesized toto obtainobtain compositesincomposites the presence basedbased of the onon polyoxometalatesAsAsAs shownshown shown ininand thisthisthis zeolites: review, review,review, there(i) Thetherethere are polyoxometalate different areare differediffere waysntnt to can obtainwaysways be compositessynthesizedtoto obtainobtain based compositescompositesin the on presence polyoxometalates basedbased of theonon zeolite;polyoxometalatespolyoxometalates (ii) the zeolite and and can zeolites: zeolites: be synthesized (i )(i )The The polyoxometalate polyoxometalatein the presence canof can the be bepolyoxometalate synthesized synthesized in in the andthe presence (iii)presence the zeoliteof of the the zeolite;polyoxometalatespolyoxometalatesand zeolites: (ii) the zeolite (i) Theandand can polyoxometalatezeolites:zeolites: be synthesized (i(i)) TheThe polyoxometalate canpolyoxometalate in bethe synthesized presence canofcan inthe be thebe polyoxometalate synthesizedsynthesized presence of theinin the the zeolite;and presencepresence (iii) (ii) the the zeolite ofofzeolite thethe andzeolite;zeolite; the (ii)polyoxometalate (ii) the the zeolite zeolite can can can be be synthesizedbe synthesized contacted, in forin the the example, presence presence by of ofimpregnation. the the polyoxometalate polyoxometalate and and (iii) (iii) the the zeolite zeolite andzeolite;zeolite;can the be (ii)(ii)polyoxometalate synthesized thethe zeolitezeolite in cancan the can bebe presence synthesizedbesynthesized contac ofted, the inin polyoxometalatefor thethe example, presencepresence by ofof impregnation. and thethe (iii)polyoxometalatepolyoxometalate the zeolite and andand the (iii)(iii) polyoxometalate thethe zeolitezeolite andand theWhen the polyoxometalate polyoxometalate the polyoxometalate can can be be iscontac contacsynthesizedted,ted, for for example,in example, the presence by by impregnation. impregnation. of the zeolite, encapsulation can occur andandcan thetheWhen be polyoxometalatepolyoxometalate contacted, the polyoxometalate for example, cancan bebe iscontaccontac by synthesized impregnation.ted,ted, forfor example,inexample, the presence byby impregnation.impregnation. of the zeolite, encapsulation can occur only WheninWhen the casethe the polyoxometalate ofpolyoxometalate faujasite. For allis is synthesizedother synthesized zeolites, in in the the prpolyoxometalate presenceesence of of the the zeolite, zeolite,will be encapsulation encapsulationlocated on the can canexterior occur occur only WheninWhen Whenthe casethethe the polyoxometalatepolyoxometalate of polyoxometalate faujasite. For all isis synthesized synthesized isother synthesized zeolites, inin inthethe the prpolyoxometalatepresence presenceesence ofof ofthethe the zeolite,zeolite, will zeolite, be encapsulationencapsulation located encapsulation on the cancan exterior can occuroccur occur ofonlyonly the in crystal.in the the case case The of of faujasite.effect faujasite. of the For For Si/Al all all other otherratio zeolites, ofzeolites, faujasite the the polyoxometalateis polyoxometalate also very important, will will be beas located locatedencapsulation on on the the exterior exteriorseems ofonlyonly onlythe inin crystal. inthethe the casecase caseThe ofof offaujasite.effectfaujasite. faujasite. of the ForFor ForSi/Al allall all otherother ratio other zeolites,zeolites, of zeolites, faujasite thethe the polyoxometalatepolyoxometalateis polyoxometalatealso very important, willwill will bebe as be locatedlocated encapsulation located onon on thethe the exteriorexterior seems exterior toofof proceedthe the crystal. crystal. only The Thefor effect dealuminated effect of of the the Si/Al Si/Al samples, ratio ratio of althoughof fauj faujasiteasite some is is also also recent very very reportsimportant, important, describe as as encapsulation encapsulation a preparation seems withseems toofof ofproceedthethe the crystal.crystal. crystal. only TheThe for The effecteffect dealuminated effect ofof of thethe the Si/AlSi/Al Si/Al samples, ratioratio ratio ofof although of faujfauj faujasiteasiteasite some isis is alsoalso also recent veryvery very reports important,important, important, describe asas as encapsulationencapsulation encapsulation a preparation seems seems with to lowtoto proceed proceedSi/Al ratios, only only forfull for dealuminated characterizationdealuminated samples, samples, of these although although materials some some remains recent recent unpublished. reports reports describe describe Another a apreparation preparation consideration with with lowtoto proceedproceed Si/Al ratios, only only for forfull dealuminateddealuminated characterization samples,samples, samples, of these althoughalthough although materials somesome some remains recentrecent recent unpublished. reportsreports reports describedescribe describe Another aa preparationpreparation a preparation consideration withwith with islowlow the Si/Al Si/Alidentity ratios, ratios, of full thefull characterization cationcharacterization in the zeolite. of of these these As materials ammonium materials remains remains and cesiumunpublished. unpublished. lead to Another insolubleAnother consideration consideration salts of the islowlow thelow Si/AlSi/Al identity Si/Al ratios,ratios, ratios, of fullfull the full characterization characterizationcation characterization in the zeolite. ofof thesethese of theseAs materials materialsammonium materials remainsremains remains and cesium unpublished.unpublished. unpublished. lead to AnotherAnother insoluble Another considerationconsideration salts consideration of the Kegginisis the the identity identitypolyoxometalates, of of the the cation cation their in in the thepresence zeolite. zeolite. favorsAs As ammonium ammonium the formation and and cesium ofcesium catalysts lead lead tothat to insoluble insolublecan be recycledsalts salts of of the inthe Kegginisis theisthe the identityidentity polyoxometalates, identity ofof ofthethe the cationcation cation their inin inthethe presence the zeolite.zeolite. zeolite. favors AsAs As ammoniumammonium ammoniumthe formation andand and cesiumcesiumof cesium catalysts leadlead lead totothat toinsolubleinsoluble can insoluble be recycledsaltssalts salts ofof ofthethe in the polarKegginKeggin solvents.polyoxometalates, polyoxometalates, However, their theirdealuminated presence presence favors favors Y zethe theolite formation formation contains of of catalysts smallcatalysts mesoporesthat that can can be be whererecycled recycled the in in polarKegginKegginKeggin solvents. polyoxometalates,polyoxometalates, polyoxometalates, However, theirtheir theirdealuminated presencepresence presence favorsfavors Y theze theolite formationformationformation contains of ofof catalysts catalystscatalystssmall that mesopores thatthat can cancan be bebe recycled whererecycledrecycled inthe polar inin polyoxometalatepolarpolar solvents.solvents. can However,However, be localized dealuminateddealuminated and it is difficult YY zezeolite oliteto determine containscontains if small thesmall preparation mesoporesmesopores will where wherelead to the thea polyoxometalatepolarpolarsolvents. solvents.solvents. However, canHowever,However, be dealuminated localized dealuminateddealuminated and Y zeolite it is difficult containsYY zezeoliteolite to small determine containscontains mesopores ifsmall smallthe where preparation mesoporesmesopores the polyoxometalate will wherewhere lead tothethe a can purelypolyoxometalatepolyoxometalate encapsulated can can Keggin be be localized localized ion (located and and it in itis isthe difficult difficult supercage) to to determine determine or if it will if if thebe the located preparation preparation in these will will mesopores. lead lead to to a a purelypolyoxometalatepolyoxometalatebe localized encapsulated and cancan it isKeggin bebe difficult localizedlocalized ion to (located determine andand itit in isis the ifdifficultdifficult the supercage) preparation toto determinedetermine or if will it will lead ifif thebethe to located apreparationpreparation purely in encapsulated these willwill mesopores. leadlead Keggintoto aa Thepurelypurely main encapsulated encapsulated conclusion Keggin of Keggin all these ion ion (located studies(located inis in thethat, the supercage) supercage)in the majority or or if ifit of itwill willcases, be be located either located noin in theseencapsulation these mesopores. mesopores. or Thepurelypurelyion main (located encapsulatedencapsulated conclusion in the supercage) Keggin Kegginof all these ionion or (located(located studies if it will ininis be thethat,the located supercage)supercage) in the in these majority oror mesopores. ifif itit of willwill cases, bebe The located locatedeither main no inin conclusion thesetheseencapsulation mesopores.mesopores. of all theseor onlyTheThe mainpartial main conclusion conclusionencapsulation of of all all occurs. these these studies studies is is that, that, in in the the majority majority of of cases, cases, either either no no encapsulation encapsulation or or onlyTheThestudies mainmainpartial isconclusionconclusion encapsulation that, in the ofof majority allall occurs. thesethese of studies studies cases, eitherisis that,that, no inin encapsulation thethe majoritymajority ofof or cases,cases, only partial eithereither encapsulationnono encapsulationencapsulation occurs. oror onlyonly Whenpartial partial the encapsulation encapsulation zeolite is synthesized occurs. occurs. in the presence of the polyoxometalate, the zeolite is always onlyonly Whenpartialpartial the encapsulationencapsulation zeolite is synthesized occurs.occurs. in the presence of the polyoxometalate, the zeolite is always faujasite,WhenWhen the the theaim zeolite zeolite being is is tosynthesized synthesized obtain encapsulation. in in the the presence presence However, of of the the polyoxometalate, thepolyoxometalate, preparation methodthe the zeolite zeolite involves is is always always the faujasite,WhenWhen the thethe aim zeolitezeolite being isis tosynthesizedsynthesized obtain encapsulation. inin thethe presencepresence However, ofof thethe polyoxometalate,polyoxometalate,the preparation methodthethe zeolitezeolite involves isis alwaysalways the introductionfaujasite,faujasite, the the ofaim aimthe beingpolyoxometalate being to to obtain obtain encapsulation. inencapsulation. a very basic However,medium, However, in the whichthe preparation preparation it is not stable. method method Surprisingly, involves involves the atthe introductionfaujasite,faujasite, thethe of aimaim the beingbeingpolyoxometalate toto obtainobtain encapsulation.encapsulation. in a very basic However,medium,However, in thethe which preparationpreparation it is not stable. methodmethod Surprisingly, involvesinvolves thethe at theintroductionintroduction end of the of reaction,of the the polyoxometalate polyoxometalate it has been reformed in in a avery very but basic basic its medium,exact medium, location in in which which remains it itis is notcontroversial. not stable. stable. Surprisingly, Surprisingly, at at theintroductionintroduction end of the ofof reaction, thethe polyoxometalatepolyoxometalate it has been reformed inin aa veryvery but basicbasic its medium,medium,exact location inin whichwhich remains itit isis notnotcontroversial. stable.stable. Surprisingly,Surprisingly, atat thethe end Finally,end of of the the the reaction, reaction, last method, it ithas has whichbeen been reformed isreformed also the but buteasiest, its its exact exact allows location location for the remains remains preparation controversial. controversial. of catalysts that can thethe endendFinally, ofof thethe the reaction,reaction, last method, itit hashas whichbeenbeen reformedreformed is also the butbut easiest, itsits exactexact allows locationlocation for the remainsremains preparation controversial.controversial. of catalysts that can have Finally,similarFinally, theproperties the last last method, method, to those which which prepared is is also also by the the the easiest, easiest, two above allows allows procedures. for for the the preparation preparation The major of of advantagecatalysts catalysts that thatis thatcan can haveFinally, Finally,similar thepropertiesthe lastlast method,method, to those whichwhich prepared isis alsoalso by thethe the easiest,easiest, two above allowsallows procedures. forfor thethe preparationpreparation The major ofof advantagecatalystscatalysts thatthat is that cancan manyhavehave similar parameters similar properties properties can be to variedto those those easily,prepared prepared such by byas the the two polyoxometalatetwo above above procedures. procedures. loading, The The th major emajor nature advantage advantage of the zeolite, is is that that manyhavehave similarsimilar parameters propertiesproperties can be toto varied thosethose easily, preparedprepared such byby as thethe the twotwo polyoxometalate aboveabove procedures.procedures. loading, TheThe th majormajore nature advantageadvantage of the zeolite, isis thatthat itsmanymany cations, parameters parameters etc. Recyclability can can be be varied varied in polar easily, easily, solvents such such as ascan the the bepolyoxometalate polyoxometalate achieved by the loading, loading,introduction th the enature nature of cations of of the the suchzeolite, zeolite, as manyitsmany cations, parametersparameters etc. Recyclability cancan bebe variedvaried in polar easily,easily, solvents suchsuch asas can thethe bepolyoxometalatepolyoxometalate achieved by the loading,loading, introduction ththee naturenature of cations ofof thethe suchzeolite,zeolite, as ammoniumitsits cations, cations, etc. oretc. cesium.Recyclability Recyclability This canin in polar be polar done solvents solvents by using can can abe zeolitebe achieved achieved containing by by the the introductionthese introduction cations ofor of cationsby cations introducing such such as as ammoniumitsits cations,cations, etc.etc. or cesium.RecyclabilityRecyclability This can inin polarpolar be done solventssolvents by using cancan a bebe zeolite achievedachieved containing byby thethe introductiontheseintroduction cations oforof cationsbycations introducing suchsuch asas themammoniumammonium by impregnation, or or cesium. cesium. This after This can thecan be impregnationbe done done by by using using of athe azeolite zeolite polyoxometalate. containing containing these these However, cations cations theseor or by by introducingcations introducing will themammoniumammonium by impregnation, oror cesium.cesium. This Thisafter cancan the bebe impregnation donedone byby usingusing of a athe zeolitezeolite polyoxometalate. containingcontaining thesethese However, cationscations theseoror byby introducingcationsintroducing will precludethemthem by by impregnation, theimpregnation, use of theseafter after thesystems the impregnation impregnation for reactions of of the the polyoxometalate.requiring polyoxometalate. a Brönsted However, However, acid, these thesein whichcations cations casewill will themprecludethem byby impregnation,impregnation,the use of these afterafter thethesystems impregnationimpregnation for reactions ofof thethe polyoxometalate.polyoxometalate.requiring a Brönsted However,However, acid, thesethese in whichcationscations casewillwill encapsulationprecludepreclude thethe remains useuse ofof the thesethese best systems proceedure.systems forfor reactionsreactions requiringrequiring a a BrönstedBrönsted acid,acid, inin whichwhich casecase encapsulationprecludepreclude thethe remainsuseuse ofof thethesethese best systemssystems proceedure. forfor reactionsreactions requiringrequiring aa BrönstedBrönsted acid,acid, inin whichwhich casecase encapsulationencapsulationIn concluding remains remains this thereview, the best best proceedure.we proceedure. wish to report a new method, which is completely different from encapsulationencapsulationIn concluding remainsremains this thethereview, bestbest proceedure.weproceedure. wish to report a new method, which is completely different from those describedIn concluding above this and review, which we is verywish original, to report that a new is the method, synthesis which of HPWis completely12 in the hollowdifferent voids from thoseIn described concluding above this and review, which we is wish very to original, report athat new is method, the synthesis which of is HPW completely12 in the different hollow voids from thoseIn described concluding above this and review, which we is wish very to original, report athat new is method,the synthesis which of is HPW completely12 in the different hollow voids from ofthosethose silicalite described described [79]. above aboveSilicalite-1 and and which whichis first is is verytreated very original, original, by tetrapropylammonium that that is is the the synthesis synthesis of of hydroxide,HPW HPW12 12in in the theleading hollow hollow to voids voidsthe thoseofthose silicalite describeddescribed [79]. aboveabove Silicalite-1 andand whichwhich is first isis veryverytreated original,original, by tetrapropylammonium thatthat isis thethe synthesissynthesis ofof hydroxide,HPWHPW1212 inin thethe leading hollowhollow to voidsvoids the ofof silicalite silicalite [79]. [79]. Silicalite-1 Silicalite-1 is is first first treated treated by by tetrapropylammonium tetrapropylammonium hydroxide, hydroxide, leading leading to to the the of silicalite [79]. Silicalite-1 is first treated by tetrapropylammonium hydroxide, leading to the of silicalite [79]. Silicalite-1 is first treated by tetrapropylammonium hydroxide, leading to the

Inorganics 2016, 4, 13 20 of 24

When the zeolite is synthesized in the presence of the polyoxometalate, the zeolite is always faujasite, the aim being to obtain encapsulation. However, the preparation method involves the introduction of the polyoxometalate in a very basic medium, in which it is not stable. Surprisingly, at the end of the reaction, it has been reformed but its exact location remains controversial. Finally, the last method, which is also the easiest, allows for the preparation of catalysts that can have similar properties to those prepared by the two above procedures. The major advantage is that many parameters can be varied easily, such as the polyoxometalate loading, the nature of the zeolite, its cations, etc. Recyclability in polar solvents can be achieved by the introduction of cations such as ammonium or cesium. This can be done by using a zeolite containing these cations or by introducing them by impregnation, after the impregnation of the polyoxometalate. However, these cations will preclude the use of these systems for reactions requiring a Brönsted acid, in which case encapsulation remains the best proceedure. In concluding this review, we wish to report a new method, which is completely different from those described above and which is very original, that is the synthesis of HPW12 in the hollow voids of silicalite [79]. Silicalite-1 is first treated by tetrapropylammonium hydroxide, leading to the creation of regular hollow voids in the interior of the crystals, the thickness of the shell being about 10–20 nm. HPW12 is then synthesized from Na2WO4, Na2HPO4 and HCl in the suspension of the hollow zeolite in water at pH = 1.0. As Na2WO4 and Na2HPO4 are smaller than the pore size of silicalite-1, they can enter and exit the hollow nanospheres through the micropores of the zeolite. They react then inside the hollow nanospheres, leading to the formation of HPW12, which cannot escape as they are too large to pass through the microporous channels of the shell. After washing with water 6–10 times, the solid is used in a model reaction, the esterification of acetic acid with ethanol. No deactivation is observed even after five recycle runs. This method could be applied to other zeolites—for example, faujasite—and could allow for the obtention of catalysts combining the activity of the polyacid and the separation properties of the zeolite. Due to the law of micro-reversibility, this method could lead to unexpected selectivities.

Conflicts of Interest: The author declares no conflict of interest.

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