Oxidation of cyclohexanone and/or cyclohexanol catalyzed by Dawson-type polyoxometalates using hydrogen peroxide

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Oxidation of cyclohexanone and/or cyclohexanol catalyzed by Dawson-type polyoxometalates using hydrogen peroxide

MOHAMMED MOUDJAHED^a, LEILA DERMECHE^a,b,* , YASMINA IDRISSOU^a,c, TASSADIT MAZARI^a,band CHERIFA RABIA^a

aLaboratoire de Chimie du Gaz Naturel, Faculte´ de Chimie, Universite´ des Sciences et de la Technologie Houari Boumedie`ne (USTHB), BP 32, 16111 El Alia, Bab Ezzouar, Alger, Algeria

bLaboratoire de Chimie Applique´e et de Ge´nie Chimique, Hasnaoua I, Universite´ Mouloud Mammeri, B.P.17 RP, 15000 Tizi-Ouzou, Algeria

cEcole Normale Supe´rieure Kouba (ENS), Alger, Algeria E-mail: leila.dermeche@umto.dz; der_lila@yahoo.fr

MS received 3 July 2021; revised 4 December 2021; accepted 19 December 2021

Abstract. The oxidation of cyclohexanone, cyclohexanol or cyclohexanone/cyclohexanol mixture using as catalyst, Dawson-type polyoxometalates (POMs) of formula,a- andb-K6P₂W₁₈O₆₂,a-K6P₂Mo₆W₁₂O₆₂and a1-K7P₂Mo₅VW₁₂O₆₂and hydrogen peroxide, carried out at 90°C with a reaction time of 20 h, led to a high number of mono- and di-acids which were identified by GC-MS. Levulinic, 6-hydroxyhexanoic, adipic, glutaric and succinic acids, major products were evaluated by HPLC. Regardless of the substrate nature, all POMs exhibited high catalytic activity with 94–99% of conversion, whereas the formation of the different products is sensitively related to both the composition and symmetry of the POMs and the substrate nature.

The main products are adipic acid in the presence ofa-K₆P₂Mo₆W₁₂O₆₂anda1-K₇P₂Mo₅VW₁₂O₆₂, levulinic acid in the presence of a1-K₇P₂Mo₅VW₁₂O₆₂andb-K₆P₂W₁₈O₆₂and 6-hydroxyhexanoic acid in the pres- ence ofa- andb-K₆P₂W₁₈O₆₂.

Keywords. Adipic acid; Levulinic acid; Hydroxyhexanoic acid; Cyclohexanone; Dawson polyoxometalate.

1. Introduction

The use of transition metal peroxocomplexes based on V(V), Mo(VI) and W(VI) in the olefin epoxidation and the oxidation of ketones and that of alcohols in the homogeneous phase, has attracted much attention. The peroxocomplexes formed in situ in the reaction mix- ture were considered to be catalytically active by several authors. Their formation has already been reported by the Venturello team, in the epoxidation of olefins by hydrogen peroxide, using tungstic and phosphoric acids in the presence of an organic solvent such as 1,2-dichloroethane or benzene and phase- transfer agents.^1–3 The peroxotungstophosphate spe- cies were isolated from an aqueous solution consti- tuted of H₂O₂, H₂WO₄and H₃PO₄, in the presence of a quaternary ammonium salt. The formula of the peroxo species determined by XRD corresponds to the

[W2O3(O2)4(H2O)2]^2-anion.⁴It has been reported that each metal center of the peroxocomplex, Mo(VI) or W(VI), is coordinated by two peroxo groups, while the V(V) metal center may be coordinated by one or two peroxide groups, structures highlighted by multinu- clear NMR spectroscopy, ¹⁷O, ⁵¹V,⁹⁵Mo and¹⁸³W.⁵

The formation of several peroxo species was men- tioned in the case of the Keggin-type heteroplyacid, H₃PW₁₂O₄₀, in the presence of excess hydrogen per- oxide. The peroxo species, polyoxoperoxometalates, were identified by³¹P and¹⁸³W NMR, UV-visible, IR and Raman spectroscopy, such as [PO4(WO(O2)2)4]^3-, [HPO4(WO(O2)2}2]^2- and [W2O3(O2)4(H2O)2]^2-, [{WO(O₂)₂(H₂O)}₂(l-O)]^2-, {W₂O₂(-O₂)(O₂)₂}, etc.^6–9 They catalyze both the epoxidation of olefins and the oxidation of alcohols, in the presence of cetylpyri- dinium chloride as phase-transfer catalyst and organic solvent. It was therefore proposed that for the

*For correspondence

Supplementary Information: The online version contains supplementary material available athttps://doi.org/10.1007/s12039-022- 02028-2.

https://doi.org/10.1007/s12039-022-02028-2Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

H3PW12O40/H2O2 system, the peroxo species are cat- alytically active species. Among all the peroxo species that may be formed, [PO4(WO(O2)2)4]^3- is the most commonly cited for the selective epoxidation of ter- minal olefins,¹⁰ oxidation of benzyl alcohol to ben- zaldehyde,¹¹and epoxidation of oleic acid.¹²Recently, the [PO₄(WO(O₂)₂)₄]^3- anion, isolated as a tetra- butylammonium salt, [(nC4H9)4N]3{PO4(WO(O2)2)4} has been directly used as a catalyst, and H₂O₂ as an oxidant, for the oxidative removal of organosulfur compounds.¹³It was therefore recognised that in oxi- dation reactions, polyoxometalates (POMs) act as catalyst precursors, in the presence of hydrogen per- oxide and it was observed that peroxometal complexes and polyoxoperoxometalates have higher oxidizing power than hydrogen peroxide and could be used as stoichiometric oxidants.

In previous works, we have tested Keggin-type phosphomolybdic POMs substituted by transition metals (Ni, Co, Fe)^14–19 and non-metallic elements (Sn, Sb and Bi)^20,21 and Dawson phosphotungstic POMs substituted with molybdenum and/or vana- dium and tin^22,23 for the oxidation of cyclohexanone and/or cyclohexanol in the presence of hydrogen peroxide. Contrary to the protocols cited in the lit- erature where the reaction mixture is constituted of the substrate (olefin, alcohol...), POM, organic sol- vent, charge transfer agent and hydrogen peroxide, in the Nomiya one,²⁴ hydrogen peroxide is added, in a second step, after oxidation of organic substrate by the POM (first step). Moreover, it was demonstrated that in the presence of a reducing agent (cyclohex- anone), a-[PMo₁₂O₄₀]^3- (yellow colour, character- istic of the oxidized form of POM) undergoes successive reductions leading to a and b [PMo_12- O40]^5- then a and b [PMo12O40]^7- (blue colour, characteristic of the reduced form of POM).²⁵ Fur- thermore, tungsten atoms with VI and V oxidation states were detected by XPS analysis on the HWO4/ TS-1 catalyst which was tested in the direct oxida- tion of cyclohexane to adipic acid in the presence of hydrogen peroxide.²⁶ The hydrogen peroxide addi- tion, in the second step of the Nomiya method, allowed the oxidation of the POMs previously reduced by the substrate. In our previous works, we showed by ³¹P NMR that after 20 h of reaction, the POM had a chemical shift different from that of the POM in its oxidized form. It was attributed to the formation of polyoxoperoxometalate species which would be responsible for the formation of carboxylic acids. It should be noted that the latter were not observed if the reaction mixture is made up of all the reagents in one pot.^22,23 In these different

studies, only the adipic acid formation was taken into account. It can be easily separated from the other products of the oxidation reaction by cold crystallization.

In this work, we were interested in the formation of all the products of the oxidation reaction of cyclohexanone (one), cyclohexanol (ol) or cyclohexanone/cyclohex- anol (one/ol) mixture using as catalyst, Dawson-type POMs of formula,a- and b-K₆P₂W₁₈O₆₂,a-K₆P₂Mo₆ W₁₂O₆₂anda1-K₇P₂Mo₅VW₁₂O₆₂and hydrogen per- oxide as oxidant, following the Nomiya protocol. The obtained reaction products from the cyclohexanone oxidation were identified by GC-MS and the most important ones were quantified by HPLC. The effects of the composition and symmetry of the POM and the substrate nature on the conversion of cyclohexanone, cyclohexanol and the cyclohexanone/cyclohexanol mixture and the selectivities of the different products were examined. The preparation and characterization of these compounds and reaction conditions have been the subject of previous work.²⁷

2. Experimental

2.1 Catalyst preparation

Dawson-type polyoxometalates were prepared according to the procedures described in the literature and in our previous work.^27–29

2.2 Catalytic test

The synthesis method based on the one described in the literature²⁴ has already been used in our previous work.^22,23 The liquid phase oxidation of cyclohex- anone (-one) or the mixture of cyclohexanone (-one) and cyclohexanol (-ol) was carried out at 90°C, using a 100 mL round-bottom flask equipped with a mag- netic stirring bar and a reflux condenser. The reaction mixture, consisting of a calculated amount of catalyst and substrate, was stirred at 800 rpm; after the reduction of POM, which is manifested by a colour change from yellow or light green to blue, hydrogen peroxide (30%) was then added dropwise until the initial colour of POM reappeared. The POM, which is in its oxidized form (yellow or light green colour), continues to oxidize the substrate and after its reduc- tion (blue colour), peroxide is then added dropwise and so on. The end of the reaction was estimated when the catalyst was no longer being reduced. The reaction time was found to be 20 h. It should be noted that

above 90 °C, hydrogen peroxide can rapidly decompose.

The oxidation products of cyclohexanone were identified by GC-MS (GC 6890 plus, MSD5973, Hewlett Packard-5MS) with one HP-INNOWAX col- umn (30 m 9 0.25 mm). The mass analyzer is a quadrupole type (150 °C). Pure helium was used as a carrier gas with a flow rate of 1 ml/min. The samples were diluted in hexane. The initial oven temperature was set at 70 °C for 4 min, then raised to 280 °C at 15 °C/ min and kept at the highest temperature for 5 min. The temperature and the volume of the injector were 250°C and 0.2 l, respectively. The split ratio was of 20:1. The temperature of the ionization source (electronic impact) was maintained at 230 °C and the temperature of the interface at 280°C.

The analysis of the reaction products was per- formed by HPLC using a Young Lin YL9100 HPLC system with a quaternary pump and a UV/Vis detector. The injections were carried out on a C18 reverse-phase column (25 cm) with a mobile phase consisting of 30% methanol in a 5 mM ammonium acetate buffer solution of pH 3.3 and a temperature of 30 °C. UV data were collected at a wavelength of 210 nm.

3. Results and Discussion

a- andb-K₆P2W18O62 (noteda-P₂W18and b-P₂W18), a-K₆P₂Mo₆W₁₂O₆₂ (noted P₂Mo₆W₁₂) and a1-K₇ P2Mo5VW12O62 (noted P2Mo5 VW12) were tested in the oxidation reaction of cyclohexanone (one), cyclo- hexanol (ol) or cyclohexanone/cyclohexanol (one/ol) mixture using hydrogen peroxide. In a previous study, the operating conditions leading to the highest adipic acid yield of this series of POMs were determined as follows: catalyst mass:125 mg, n(substrate):15 mmol, H2O2(30%): 0.5mL/h, T: 90°C, stirring rate: 800 rpm, reaction time: 20 h.²²

Figures 1 and 2, taken as examples, show the GC- MS and HPLC chromatograms of the observed prod- ucts from the oxidation of cyclohexanone catalyzed by P2Mo6W12, the most selective toward the adipic acid formation, and P₂Mo₅VW₁₂, leading to the largest number of products.

Table1presents the products identified by GC-MS, obtained from the oxidation of the different substrates (one, ol and ol/one) catalyzed by the POMs and Table 2 presents the main products quantified by HPLC, such as adipic (noted AA), glutaric (noted GA), succinic (noted SA), levulinic (noted LA),

6-hydroxyhexanoic (noted HHA) and 6-methoxy-6- oxohexanoic (noted MOH) acids. Although the Baeyer-Villiger reaction^30–32 corresponds to the cyclohexanone oxidation to e-caprolactone, in the presence of hydrogen peroxide, this product was not observed in this study. The other non-quantified out- puts were noted as X. As examples, in the presence of P2Mo6W12 catalyst, the X products are: 1,1-dime- thoxy-cyclohexane, cyclodecane, cyclotetradecane, cyclopentadecane, dimethyl hexanedioate, diethyl pentanedioate, methyl hexanedioate, ethyl hexane- dioate, bis(2-ethylhexyl) hexanedioate and hexadec- 9-enoic and hexadecanoic acids, and in the case of P₂Mo₅VW₁₂ catalyst, 4-methoxy-4-oxobutyric, 6-ethoxy-6-oxohexanoic and 6-methoxy-6-oxohex- anoic acids, ethyl-5-oxohexanoate, diethylbutane- dioate, dimethylbutanedioate, dimethylhexanedioate, ethyl-4-oxopentanoate, bis (1-methylpropyl)-2,2- dimethylpentanedioate, bis (2-ethylhexyl) hexane- dioate and diisopropyladipate. The formation of the observed products (alkanes, monoacids, diacids and esters) resulting from isomerization, ring-opening, cracking and insertion of several oxygen atoms can be explained by the following observed different steps, based on the catalytic test: (1) direct oxidation of the substrate, by the POM acting as an oxidant, a step evidenced by the colour change from yellow-green to blue, a characteristic colour of the reduced POM, as evidenced by the literature,^26,27 (2) oxidation of the catalyst by hydrogen peroxide, as evidenced by the colour change from blue to the initial colour of the catalyst. It was reported that in the presence of hydrogen peroxide, peroxo species such as [PO4(WO(O2)2)4]^3-, [HPO4(WO(O2)2}2]^2- and [W₂O₃(O₂)₄(H₂O)₂]^2-, [{WO(O₂)₂(H₂O)}₂(l-O)]^2-, {W2O2(–O2)(O2)2}, are formed, as evidenced by multinuclear NMR spectroscopy.^6–9 These peroxo species have been reported to have higher oxidizing power than hydrogen peroxide, so they would be the active species that can function as oxygen donors with ring-opening and insertion of oxygen atoms, a behaviour similar to that observed in a Mars and Van Krevelen type mechanism, in which the oxygen atoms of the catalyst participate in the oxidation process.

Table 3 summarizes the catalytic performance as a function of both the composition of the POM and the substrate nature. The results obtained highlighted the high catalytic activity of POMs with an almost total conversion of the substrate, one, ol and one/ol (94–99%). All the catalysts cause a break in the ring and an insertion of oxygen atoms leading to the formation of monoacids (levulinic and 6-hydroxyhexanoic), and

diacids (adipic, glutaric and succinic). The formation of these products is sensitively related to both the composition and symmetry of the POM and the nature of the substrate.

Thus, a-P₂W18 mainly promotes the 6-hydroxy- hexanoic acid (HHA) formation with 44 and 63% of selectivity from the oxidation of the alcohol/ketone mixture and that of alcohol respectively, and that of adipic acid (45% selectivity) from the oxidation of the ketone.

Unlike a-P₂W₁₈ catalyst, where levulinic acid has not been observed, b-P₂W18 leads mainly to the for- mation of this product, with 55 and 67% of selectivity, respectively, from the oxidation of the alcohol/ketone mixture and that of the alcohol. 6-hydroxyhexanoic acid is also obtained with high selectivity (74%) from the ketone oxidation. The difference in the catalytic behaviour between these 2 isomers would lie in the position of the 3 octahedrons in the framework as shown in Figure 3. The b isomer results from the Figure 1. GC-MS (a) and HPLC (b) chromatograms of the products from cyclohexanone oxidation catalyzed by P₂Mo₆W₁₂.

rotation of one of the two M3O13groups by an angle of p/3. It would seem that the position a- would favor more the oxidizing power which would explain the higher adipic acid selectivity (45 against 14%) obtained from the oxidation of the ketone. This observation would be confirmed by the 6-hydroxy- hexanoic acid formation that is more favoured in theb isomer presence (74 against 18% of selectivity). The active sites seem to be related to the position of the octahedrons of the trimetallic entity. It should be noted that glutaric and succinic acids were not observed in

the presence of b-P₂W18, regardless of substrate nat- ure, and in the presence ofa-P2W18, the selectivities of these two products remain very low (2–7%) and the SA formation was not also observed in the case of ol/

one mixture.

The substitution of the tungsten atoms of a-P₂W18

by those of molybdenum (P₂Mo₆W₁₂) mainly facili- tates the adipic acid formation with selectivities of 59, 59 and 70% from the oxidation of alcohol, ketone and alcohol/ketone mixture, respectively. The presence of Mo, promoting adipic acid formation, has already been Figure 2. GC-MS (a) and HPLC (b) chromatograms of the products from cyclohexanone oxidation catalyzed by P₂Mo₅VW₁₂.

reported in previous works.^22,23 The other identified products were obtained with selectivities varying between 0 and 20%.

The substitution of a molybdenum atom of P2Mo6W12, by that of vanadium (P2Mo5VW12) makes the catalyst more sensitive to the substrate nature.

Indeed, adipic acid formation is favoured (54%

selectivity) from cyclohexanol oxidation and levulinic

acid (54% selectivity) from cyclohexanone oxidation.

It should be noted that levulinic acid is favoured only in the presence of P2Mo5VW12 and b-P₂W18 with selectivities varying between 31 and 67%. Unlike other catalysts, P2Mo5VW12 led to all identified products by GC-MS from the oxidation of the ol/one mixture, with selectivities ranging from 6 to 31%. As might be expected, vanadium has the property of Table 1. Products obtained from cyclohexanone oxidation catalyzed by P₂Mo₅VW₁₂ ana-

lyzed by GC-MS.

Acids Esters

4-Oxopentanoic acid (levulinic acid) 4-Methoxy-4-oxobutyric acid 5-Oxohexanoic acid

Pentane-1,5-dioic acid (glutaric acid) Hexane-1,6-dioic acid (adipic acid) 6-Ethoxy-6-oxohexanoic acid 6-Methoxy-6-oxohexanoic acid

Ethyl-5-oxohexanoate Diethylbutanedioate Dimethylbutanedioate

Dimethylhexanedioate (diethyl adipate) Ethyl-4-oxopentanoate (ethyl levulinate) Bis (1-methylpropyl)-2,2-dimethylpentanedioate Bis (2-ethylhexyl)hexanedioate

Diisopropyladipate

Conditions:Treact: 90°C,mcat: 125 mg,nsubstrate: 15 mmol, Agitation rate: 800 rpm, reaction time: 20 h

Table 2. HPLC identification of reaction products from oxidation of substrate (cyclohexanol, cyclohexanone, cyclo- hexanol (50%)/cyclohexanone (50%) over Dawson-type POMs.

Structure Nomenclature Notation Retention time (min)

6-hydroxyhexanoı¨c acid HHA 2.73

HO

O

OH

O butane-1.4-dioic acid (succinic acid) GA 3.38

pentane-1.5-dioic acid (glutaric acid) SA 3.86

hexane-1.6-dioic acid (adipic acid) AA 5

4-oxopentanoı¨c acid (levulinic acid) LA 9.42

O HO

O

O 6-methoxy-6-oxohexanoic acid MOH 9.64

Conditions:Treact: 90°C,mcat: 125 mg,nsubstrat: 15 mmol, Agitation rate: 800 rpm, reaction time: 20 h

increasing the oxidizing power of the POM (V[Mo[W), leading to the formation of several oxi- dation products.

Table 4 presents the turnover numbers calculated (TON= mol of product/mol of catalyst) in relation to the products obtained with the highest selectivity, whatever the nature of the substrate in the presence of the different catalysts. The obtained results show that the TONs are sensitive to both the nature of the sub- strate and the catalyst, varying between 150 and 384.

The highest TONs were obtained in the presence ofa- P2W18 (327) and in the presence of b-P₂W18 (384) from the oxidation of alcohol and ketone, respectively.

This indicates that these two reagents are tolerant of

the reaction conditions compared to the ol/one mix- ture, facilitating thus, the formation of 6-hydroxy- hexanoic acid by ring-opening and the introduction of acid and alcohol functions. While the highest TON was observed with P2Mo6W12 (335) using the ol(50%)/one(50%) mixture which led to adipic acid (presence of two acid functions) and with P2Mo5VW12

(261) using ol which led to adipic acid or one to levulinic acid (presence of acid and ketone functions).

Both catalysts promote deep oxidation which is sup- ported by the substrate whatever its nature. When ol/

one mixture was used as a substrate, P2Mo5VW12

catalyst very reactive leads to a multitude of products which explained the very low value obtained (150).

The oxidation process involves several steps: (1) oxidation of the substrate and reduction of the POM, (2) oxidation of the reduced POM (W^V?W^VIand/or Mo^V?Mo^VIand /or V^IV?V^V) by H₂O₂, then the first step (1) returns and so on. The first two steps are reversible redox processes, knowing that the POM undergoes successive reductions and oxidations. Once the catalyst is no longer reduced (it stays yellow, characteristic colour of POM in its oxidized form), reflecting the almost total conversion of the substrate as shown by the results in Table2, H₂O₂intervenes in a third stage to decompose POM into peroxo species, peroxometalates and oxoperoxometalates, catalytically active intermediates, in the conversion of the reaction intermediates to the final products, acids and esters.

The increased number of products observed in the presence of P2Mo5VW12 can be attributed to the fact that with vanadium, the resulting peroxocomplexes Table 3. Catalytic performance as a function of the POM composition and the substrate

nature.

POMs Substrate nature Con. (%)

Selectivity (%)

AA GA SA LA HHA MOH X

ol 94 12 5 7 0 63 0 13

a-P2W₁₈ one 94 45 5 2 0 18 10 20

ol(50%)/one(50%) 94 26 2 0 0 44 9 19

ol 94 6 0 0 67 23 0 4

b-P2W₁₈ one 94 14 0 0 0 74 3 9

ol(50%)/one(50%) 94 5 0 0 55 30 0 10

ol 98 59 5 0 0 5 0 31

P₂Mo₆W₁₂ one 98 59 4 3 0 9 20 5

ol(50%)/one(50%) 98 70 5 0 0 10 12 3

ol 99 54 7 10 0 4 14 11

P₂Mo₅VW₁₂ one 99 14 7 10 54 0 6 9

ol(50%)/one(50%) 99 22 6 10 31 10 11 10

Catalyst weight: 125 mg, n(substrat): 15 mmol, H2O2(30%): 0.5 mL/h, T: 90°C, stirring rate:

800 rpm, reaction time: 20 h

/3 rotation of trimetallic groups, M₃O₁₃ S

Figure 3. Polyhedral representation of the isomersaand b of the heteropolyanion [X₂M₁₈O₆₂]^6-.

have a greater structural diversity than those of molybdenum and tungsten. Thus, it has been reported that in the peroxocomplexes, Mo(VI) and W(VI) are coordinated by two peroxo groups, while the metal center V(V) may be coordinated by one or two per- oxide groups.⁵ Therefore, in the presence of P₂Mo_5- VW₁₂, the number of active peroxo species is much greater.

From these findings, it can be deduced that hydro- gen peroxide plays two important roles in the process.

In the first, it acts as an oxidant and in the second role, it forms peroxo species. Therefore, the peroxocom- plexes are much more effective oxidants than both hydrogen peroxide and the POM, taken alone or separately.

The high catalytic activity of the studied POMs could be attributed to the large number of metal atoms (18 atoms per Dawson’s unit) bonded to the peroxo (O2)^2-and O^2-oxygen atoms. The peroxo-complex is supposed to give oxygen atoms to the substrate stoi- chiometrically and is then regenerated on contact with hydrogen peroxide.

4. Conclusions

Dawson POM series, a- and b-K₆P₂W₁₈O₆₂, a-K₆P_2- Mo6W12O62 and a1-K₇P2Mo5VW12O62 tested in the oxidation reaction of cyclohexanone, cyclohexanol or cyclohexanone/cyclohexanol mixture, in the hydrogen peroxide presence showed high catalytic activity with 94–99% of substrate conversion, causing a break in the ring and an insertion of oxygen atoms leading to the formation of a high number of products revealed by GC-MS analysis.

The formation of levulinic, 6-hydroxyhexanoic, adipic, glutaric and succinic acids, products quantified by HPLC is sensitively related to both the composition

and the symmetry of the POM and the substrate nature.

a-K₆P₂Mo₆W₁₂O₆₂, selective to adipic acid (59–70%) from all substrates oxidation, a1-K₇P2Mo5VW12O62

and b-K₆P₂W₁₈O₆₂, selective to levulinic acid (55%) from cyclohexanone and (67%) from cyclohexanol, respectively and a- and b- K₆P₂W₁₈O₆₂ to 6-hydrox- yhexanoic acid (63%) from cyclohexanol and (74%) from cyclohexaone oxidation, respectively.

The important role of hydrogen peroxide is to oxi- dize the POM reduced by the substrate at the same time leading to peroxo- species with a multitude of active sites resulting in the presence of molybdenum, tungsten and vanadium.

Supplementary Information (SI)

Supplementary information is available at https://

www.ias.ac.in/chemsci.

Acknowledgment

Ministry of Higher Education and Scientific Research and Directorate General of Scientific Research and Technolog- ical Development for financial support.

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