CatalysisScience &Technology

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PAPER View Article OnlineView Journal

This journal is © The Royal Society of Chemistry 2014

a Chemical Engineering & Process Development Division, National Chemical

Laboratory, Dr. Homi Bhabha Road, Pune 411008, India. E-mail: cv.rode@ncl.res.in;

Fax: +91 20 2590 2621; Tel: +91 20 2590 2349bCentre of Excellence in Solar Energy, National Chemical Laboratory (CSIR-NCL),

Dr. Homi Bhabha Road, Pune 411008, India. E-mail: sb.ogale@ncl.res.in;

Fax: +91 20 2590 2636; Tel: +91 20 2590 2260

† Electronic supplementary information (ESI) available: Results of Williamson–Hall analysis of RGO–MnCoO and XRD, XPS and HRTEM analyses of thereused catalyst. See DOI: 10.1039/c3cy01025b

Cite this: DOI: 10.1039/c3cy01025b

Received 7th December 2013,Accepted 27th February 2014

DOI: 10.1039/c3cy01025b

www.rsc.org/catalysis

Triple nanocomposites of CoMn2O4, Co3O4

and reduced graphene oxide for oxidation ofaromatic alcohols†

Ajay Jha,a Dattakumar Mhamane,ab Anil Suryawanshi,b Sameer M. Joshi,a

Parvez Shaikh,b Narayan Biradar,a Satishchandra Ogale*b

and Chandrashekhar V. Rode*a

A composite of reduced graphene oxide (RGO) with oxides of manganese and cobalt together was

prepared by a solvothermal method. During synthesis, both the reduction of graphene oxide as well

as the growth of nanorod shaped CoMn2O4 and Co3O4 occurred simultaneously having a crystallite

size of ~8 nm calculated from X-ray diffraction (XRD). The as-obtained triple nanocomposite material

designated as RGO–MnCoO exhibited excellent activity for the liquid phase aerobic oxidation of aromatic

alcohols under base-free conditions selectively giving the corresponding aldehydes (>85%). RGO loading

was varied in the range of 1–10%, among which 1% RGO–MnCoO showed maximum catalytic activity

enhancement of 24% as compared to the bare mixed oxide (MnCo-MO) for the oxidation of vanillyl

alcohol. HR-TEM of RGO–MnCoO revealed that it was a composite material having uniform nanotubes

of ~25 nm length and 6 nm diameter with a fringe pattern showing the (103) and (004) planes and lattice

spaces of 0.26 nm and 0.22 nm, respectively, for the spinel CoMn2O4. The detailed studies on the

morphology, size and composition of the as-prepared RGO–MnCoO nanocomposite by XRD, XPS,

N2-adsorption/desorption and O2-TPD techniques were used to understand the role of RGO in the

enhancement of catalytic activity for oxidation reaction.

1. Introduction

Selective oxidation of aromatic alcohols to the correspondingcarbonyl compounds, e.g. aldehydes or ketones by removingelectrons and/or adding oxygen atoms (oxidation) has beenpervasive in academic as well as in industrial research andcritical to the pharmaceutical, petrochemical, agriculturaland other industries.1 Traditional oxidation processes employstoichiometric amounts of metallic oxidants notably perman-ganates, ruthenium(VIII) oxide and chromium(VI) reagents thatproduce environmentally unsuitable heavy metal wastes andtoxic or unacceptable by-products2 with low yields of thedesired products. Various catalytic systems involving transi-tion metal salts or organometallic complexes have beendeveloped3,4 as homogeneous catalysts which suffer a major

problem of separation from the reaction mixture and theirsubsequent recyclability. In addition, the metal functions ina homogeneous catalyst system tend to precipitate fromwater as catalytically inactive and insoluble metal oxides.These problems can be overcome by designing a suitableheterogeneous catalyst which can offer major advantages ofeasy handling, separation and reusability. A major challengein the oxidation with heterogeneous catalysts is to maintaintheir consistent activity. In the reaction mixture, sometimesthe reactants or carboxylic acid-type (by)products being goodchelating agents, form complexes with the active metal cen-tres of the catalyst, leading to its dissolution in the reactionmedium.5 Thus, considerable efforts have been directedtowards the development of new heterogeneous catalystswhich possess stability during reactions. Until now, numer-ous catalysts systems have been explored and noble metalbased catalysts have received special attention due to theirhigh activity and selectivity6–8 but these systems becomecostly, minimizing their potential for commercial applica-tions. Apart from cost considerations, noble metal residues(e.g. Pt and Pd) in pharmaceutical and nutritional productsare highly problematic and thus should be avoided.9 Inrecent years, we have been focusing on the development ofnon-noble metal heterogeneous catalysts for liquid phase

Catal. Sci. Technol.

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aerobic oxidations of aromatic alcohols, in which Co3O4 wasfound to be an efficient catalyst for the oxidation of vanillylalcohol to vanillin in the presence of NaOH.10 Further, wealso developed a mixed metal oxide catalyst of manganeseand cobalt oxide (MnCo-MO) that led to the formation of thetetragonal CoMn2O4 phase, which was found to be an activespecies for oxidation reactions and catalyzed base-free oxida-tion of vanillyl alcohol to vanillin.11 In continuation of ourefforts for the development of base-free oxidation catalysts,we report here the composite material of reduced grapheneoxide with cobalt–manganese mixed oxide (RGO–MnCoO) forliquid phase oxidation of a wide range of aromatic alcoholsunder base-free conditions using air as an ideal oxidant. Theuse of graphene based composite materials for catalysts wasdue to their unique structure and extraordinary properties forvarious applications such as electrocatalysts, electrodes, sen-sors and more specifically, dispersion of metal oxide nano-particles (NP) on graphene oxide sheets potentially provides anovel way to develop catalysts for oxidation reactions.12,13

The reduced graphene oxide (RGO) used in this work wasprepared by chemical oxidation of graphite powder accordingto the modified Hummer's method14,15 followed by its reduc-tion using ethylene glycol. The surface of reduced grapheneoxide still has some oxygen containing functional groups likehydroxyl (–OH) and epoxide located on the basal planes andcarbonyl (>CO) and carboxyl groups (–COOH) located atthe edges of the RGO.16 These functional groups on the sur-face of RGO can provide anchoring sites for semiconductorsand metal nanoparticles and realize redox reactions, whichopen up new opportunities for designing novel catalysts.17,18

2. Experimental2.1 Synthesis and characterization of the RGO–MnCoOnanocomposite

The RGO–MnCoO nanocomposite was synthesized by asolvothermal method. In a typical synthesis, GO was dis-persed in 60 mL of ethylene glycol and sonicated for 1 h,followed by subsequent addition of 1.6 mL of 30% aqueousNH3 and stirred again for 1 h. To this, manganese and cobaltacetates were added in a 1 : 2 molar ratio and the solutionwas gradually heated from room temperature to 160 °C. Asthe temperature reached 160 °C, 200 mL of aqueous 0.3 MNa2CO3 solution was added dropwise and the slurry was fur-ther aged for 2 h under nitrogen atmosphere. The resultingproduct was filtered and thoroughly washed with distilledwater until neutralization. The solid material was dried for6 h at 100 °C and then calcined at 350 °C for 4 h.

X-ray diffractograms (XRD) of the catalysts were recordedin the 2θ range of 10–80° (scan rate of 5.3° min−1) using aPANalytical PXRD Model X-Pert PRO-1712, using Ni filteredCu Kα radiation (λ = 0.154 nm) as the source (current inten-sity, 30 mA; voltage, 40 kV) and an Xcelerator detector. Tem-perature programmed oxidation (TPO) experiments werecarried out using a Micromeritics ChemiSorb 2720 instru-ment. For the TPO experiments, 50 mg of the catalyst was

Catal. Sci. Technol.

placed in a quartz tube and treated with He gas (25 mL min−1)at 200 °C for 1 h. Thereafter, the sample was cooled at roomtemperature and then a gas mixture of 5% oxygen in heliumwas passed through the quartz reactor at 50 °C for 1 h. Datarecording was started from 50 °C to 900 °C at a heating rateof 10 °C min−1. X-ray photoelectron spectroscopy was studiedusing an ESCA-3000 (VG Scientific Ltd. England) with a 9channeltron CLAM4 analyzer under vacuum (1 × 10−8 Torr)using Al Kα radiation (1486.6 eV) and a constant pass energyof 50 eV. The carbonaceous C 1s line (284.6 eV) was used as areference to calibrate the binding energies. The BET surfacearea and the type of isotherm were determined by the nitro-gen gas adsorption–desorption method at 77 K using anAutosorb 1, Quantachrome instrument. HR-TEM was carriedout using a HR-TEM FEI Tecnai 300.

2.2 Catalytic activity measurement

All of the catalytic oxidation reactions were carried out in a300 mL capacity high-pressure reactor supplied by ParrInstruments Co. U.S.A. A Thermo Scientific HPLC modelAS3000 liquid chromatograph equipped with an ultravioletdetector was used for the analysis of liquid samples. HPLCanalysis was performed on a 25 cm RP-18 column. The prod-ucts and reactants were detected using a UV detector at λmax

= 254 nm. Aqueous methanol (50%) was used as a mobilephase at a column temperature of 35 °C and a flow rate of0.7 mL min−1.

In a typical experiment, 3.24 mmol of aromatic alcohol,0.1 g of the catalyst and 70 mL of acetonitrile were chargedinto a 300 mL Parr reactor. The reaction mixture was heatedto 140 °C. After the desired temperature was attained, thereactor was pressurized with 21 bar air pressure and the reac-tion was started by agitating at 1000 rpm continued up to2 h. After 2 h, the reactor was cooled to room temperatureand the unabsorbed air was vented out. Then the contentsof the reactor were discharged and the final volume wasnoted down.

3. Results and discussion3.1 Structural properties of the RGO–MnCoO nanocomposite

The XRD analysis is divided in two sections as Fig. 1a and b.XRD patterns of graphite and GO are shown in Fig. 1a.

The graphite powder showed a sharp diffraction line at2θ = 26.5° corresponding to a d-spacing of 3.34 Å. However,graphene oxide (GO) showed a diffraction peak at 2θ = 11.4°This shift led to an increase in the d-spacing to 7.5 Å. Thisis because the oxidation of graphite involves introductionof oxygen containing functionalities between the layers ofgraphite.12,19 The diffraction line at 2θ = 11.4° in GO was notobserved in the XRD pattern of RGO–MnCoO (Fig. 1b), whichclearly confirmed the complete reduction of GO. Fig. 1(b)shows the XRD pattern of the RGO–MnCoO nanocomposite.The spectrum undoubtedly revealed the formation of thebiphasic metal oxide i.e. CoMn2O4 with a tetragonal crystalstructure (JCPDF card no. 77-0471) and Co3O4 with a cubic

This journal is © The Royal Society of Chemistry 2014

Fig. 1 (a) XRD patterns of graphite powder and GO. (b) XRD patternsof the RGO–MnCoO nanocomposite.

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crystal structure (JCPDF card no. 74-1657). Williamson–Hallanalysis was applied for the accurate measurement of crystal-lite sizes of the material under study. For this purpose, theW–H equation used is shown below.20

cos sin KD

where β is the full width at half maxima, K is the shape

Fig. 2 XPS plots of (a) C 1s of GO, (b) C 1s of RGO–MnCoO, (c) Co 2p3/2,and (d) Mn 2p3/2 of the RGO–MnCoO nanocomposite.

factor, λ is the wavelength, D is the average crystallite sizeand ε is the strain. β cos θ vs. ε sin θ was plotted by consider-ing six peaks of both phases (CoMn2O4 and Co3O4) presentin the material separately (ESI† Fig. S1a, b). The y-axis inter-cept of this plot provided the values of Kλ/D which were usedfor the determination of the average crystallite size, D. Theaverage crystallite size for both phases calculated from theseplots was ~8 nm.

This journal is © The Royal Society of Chemistry 2014

The X-ray photoelectron spectrum of RGO–MnCoO wasused for studying the electronic properties of the metal oxidecatalysts prepared in combination with GO. The C 1s XPSspectrum of GO showed a broad carbon peak that could befitted satisfactorily to four major peaks after deconvolutionas shown in Fig. 2a. The peaks at 284.4, 286, 287.5 and288.7 eV were due to C–C, C–O, CO and O–CO species,respectively.19 The XPS spectrum of C 1s in RGO–MnCoO(Fig. 2b) showed reduction in the intensity of the peaks corre-sponding to C–O, CO and O–CO species as compared toGO, due to the removal of the oxygen containing groups andthe recovery of the sp2 carbon network.21 The percentagedistributions of different carbon species in GO and RGO–MnCoO are shown in Table 1. In the RGO–MnCoO nano-composite, Co 2p3/2 showed a broad peak that could be fittedsatisfactorily to 3 principle peaks and one satellite peak afterdeconvolution as shown in Fig. 2c. The peak at 780 eV wasassigned to Co3+ of Co2O3 (as Co3O4 may exist as a mixture ofCoO and Co2O3), while the peak at 781 eV was assigned toCo2+ of CoMn2O4. Formation of CoMn2O4 was also confirmedby XRD and HR-TEM results. Another peak at 782.1 eV wasalso present due to Co2+ but from spinel Co3O4. A satellitepeak at 784 eV was indicative of Co2+ in both spinel Co3O4

and CoMn2O4. All of the peaks showed a positive shifttowards a higher binding energy (BE) as compared to thebare mixed metal oxide (MnCo-MO) (Fig. S2a†), attributed tothe stronger metal–RGO interactions that were induced by anincrease in the number of defects in RGO. This also corre-sponds to a decrease in the electron density and a change inthe electronic states of metals leading to higher valencestates. This trend could be interpreted as delocalization ofd and p-orbital electrons of metals and carbons of RGO,respectively. The RGO–MnCoO catalyst also showed a broadsignal of Mn 2p3/2, which could be fitted satisfactorily to2 major peaks and a satellite peak after deconvolution, as

Catal. Sci. Technol.

Table 1 Percentage distribution of different carbon species

Carbon species in GO Carbon species in RGO–MnCoO

Chemical species Binding energy (eV) Area (%) Binding energy (eV) Area (%)

C–C 284.4 30 284.6 74C–O 286 33 286 12CO 287.5 21 287 7O–CO 288.7 16 288.2 7

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shown in Fig. 2d. The peaks at 641.8 and 643.7 eVcorresponded to Mn3+ and Mn4+, respectively, while the smallpeak at 646 eV corresponded to the satellite peak of Mn4+. Itis reasonable therefore to conclude that Mn exists as Mn3+ inCoMn2O4 spinel oxides. The presence of the spinel CoMn2O4

in the RGO–MnCoO catalyst was also confirmed by XRD dif-fraction peaks at 2θ = 33.4° (103). However, the presence ofMn4+ indicates the possibility of MnO2, which was notobserved in the XRD pattern due to peak broadening. Theshift in the BE of Mn towards higher values was alsoobserved as compared to bare mixed metal oxides (Fig. S2b†)that might be due to the extremely effective delocalizeddonor–acceptor interactions with the π-system of the RGOand the metal oxide.22 The manner in which oxygen wasbonded to metals in the RGO–MnCoO nanocomposite couldbe determined through XPS of oxygen. Deconvolution of theO1s spectrum of RGO–MnCoO produced three main peaksaround 530.3, 532, and 533 eV as shown in Fig. 3. The peakat lower binding energy (530.3 eV) could be assigned to theoxygen species involved in metal oxide chemical bonds in thecomposite and must therefore correspond to Co–O and Mn–O bonds.23 However, the peaks at higher binding energies(532 and 533 eV) were assigned to C–O (oxygen singly bondedto aliphatic carbon) and phenolic (oxygen singly bonded toaromatic carbon) groups, respectively.24

Nitrogen adsorption–desorption isotherms and the corre-sponding pore size distributions of MnCo-MO and RGO–MnCoO catalysts obtained from analysis of the adsorptionbranch using the BJH (Barett–Joyner–Halenda) method areshown in Fig. 4. MnCo-MO and RGO–MnCoO catalystsexhibited type IV isotherm, with hysteresis loops at a relativepressure of 0.7 and 0.5, respectively, with different pore size

Catal. Sci. Technol.

Fig. 3 XPS plot of O 1s of the RGO–MnCoO nanocomposite.

distributions. BJH calculations disclosed that the pore sizedistribution was in the range of 3–20 nm in the case ofMnCo-MO whereas uniform pore size distribution (2–5 nm)was observed for RGO–MnCoO catalysts. The RGO–MnCoOcatalysts showed a BET surface area of 111 m2 g−1 with porevolume of 0.393 cm3 g−1; however, the MnCo-MO sampleshowed a surface area and a pore volume of 105 m2 g−1 and0.161 cm3 g−1, respectively.

HR-TEM provides insight into the morphology anddetailed structure of the prepared catalysts. Fig. 5a revealedthat RGO–MnCoO was a composite material of reducedgraphene oxide and mixed metal oxide (MnCo) having uni-form nanotubes of ~25 nm in length and 6 nm in diameter.The morphology of the mixed oxide did not change aftermaking a composite with RGO, as the bare mixed metal oxidealso showed the rod like morphology (Fig. 5b). The latticefringe pattern of the RGO–MnCoO nanocomposite is shownin Fig. 5c. The dominant exposed planes of RGO–MnCoO

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Fig. 4 N2 adsorption/desorption isotherms of (a) MnCo-MO (inset poresize distribution) and (b) RGO–MnCoO (inset pore size distribution).

Fig. 6 FE-SEM images of (A) MnCo-MO and (B) RGO–MnCoO.

Fig. 7 O2-TPD profiles of bare MnCo-MO and RGO–MnCoO.

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were (103) and (004) with a lattice spacing of 0.26 nm and0.22 nm, respectively, that could be related by the spinelCoMn2O4. The other exposed planes were (111) and (400)with a lattice spacing of 0.46 and 0.20 nm, respectively, dueto spinel Co3O4. Fig. 5d showing the SAED pattern of theRGO–MnCoO nanocomposite was used to determine the lat-tice constants which were in excellent agreement with thoseobtained from XRD analysis.

The FE-SEM images of the bare mixed oxide andRGO–MnCoO also supported the HR-TEM results. The bareMnCo-MO showed uniform nanorod like structures (Fig. 6A)while RGO–MnCoO showed the composite nature of the mate-rial in which metal oxides were attached on the surface ofreduced graphene oxide (Fig. 6B). It was demonstrated thatboth vacancies and edges in reduced graphene oxide wereeffective trapping centres for metal oxide nanoparticles.

To investigate the types of active oxygen species, O2-TPDmeasurements of the RGO–MnCoO nanocomposite andMnCo-MO were performed and the results are shown inFig. 7. Four peaks appearing in the TPD profile, which weredesignated as peaks α, β, γ and δ, indicated four kinds of oxy-gen species present in the oxides. Generally, the adsorbedoxygen undergoes the following transformations:25

O2(ad) → O2−(ad) → O−(ad) → O2−(lattice)

The physically adsorbed oxygen and O2− (ad) species weredesorbed at relatively low temperature (<300 °C). In Fig. 7,peak α corresponds to O2− (ad) species. The O− speciesdesorbed at higher temperature (>400 °C), therefore it wasmore reasonable to assign the β and γ peaks to the desorp-tion of the O− species. Besides the three peaks, the lattice

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Fig. 5 HR-TEM images of (a) the RGO–MnCoO nanocomposite,(b) MnCo-MO; (c) lattice fringe patterns of RGO–MnCoO; and (d) theSAED pattern of RGO–MnCoO.

oxygen desorbs at a very high temperature (>800 °C), hencethe δ peak could be assigned to the presence of lattice oxy-gen. In the case of the RGO–MnCoO nanocomposite, the β, γand δ peaks shifted to lower temperatures compared to thosein bare MnCo-Mo. This suggests that the addition of GO intothe mixed oxide (MnCo) increased the surface oxygen mobil-ity and facilitated the oxygen desorption from the catalystsurface at lower temperatures. This in turn increased the rateof reaction as observed in the case of the RGO–MnCoO nano-composite (Table 2).

3.2 Catalytic performance of the RGO–MnCoOnanocomposite

The activity of the RGO–MnCoO catalyst was attributed to theco-existence of Co2+/Co3+ and Mn3+/Mn4+ ion pairs anchoredon RGO because of a facile one-electron transfer betweenthese ions during O2 activation. As discussed already,the metal superoxo intermediates (M–O–O˙) are the reactive

Catal. Sci. Technol.

Table 2 Effect of GO loading on the catalytic activity of mixed oxide

CatalystsConv.(%)

Selectivity (%)

Vanillin Vanillic acid Quinone

1% RGO–MnCoO 74 83 8 103% RGO–MnCoO 73 81 6 135% RGO–MnCoO 73 83 6 1110% RGO–MnCoO 63 83 6 11MnCo-MO 50 79 8 13GO — — — —

Fig. 8 Catalyst screening for vanillyl alcohol oxidation. Reactionconditions: vanillyl alcohol (3.24 mmol), catalyst (0.1 g), 140 °C, acetonitrile(70 mL), air pressure (21 bar), time (2 h).

Scheme 1 Schematic representation of the interaction of the aromaticsubstrate with the delocalized π-system of the reduced graphene oxide.

Fig. 9 Effect of time on conversion and selectivity. Reaction conditions:vanillyl alcohol (3.24 mmol), RGO–MnCoO catalyst (0.1 g), 140 °C,acetonitrile (70 mL), air pressure (21 bar).

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species for the selective oxidation of alcohol and theseare the products of one electron reduction of oxygen on thecatalyst.26 The activity of the composite of single metaloxides (Co and Mn) with RGO was compared with that of theRGO–MnCoO catalyst for liquid phase air oxidation of vanillylalcohol in base-free medium. Fig. 8 shows that among thesecatalysts, 1% RGO–CoO and 1% RGO–MnO showed lowerconversions (42 and 58%, respectively) than 1% RGO–MnCoO(74%) with almost similar selectivity (~83%) to vanillin,suggesting that doping of manganese into the cobalt oxideled to an increase in catalytic activity. The higher activity ofthe mixed metal oxides was a cooperative effect due to anenhancement in the mobility of the oxygen as well as stabiliz-ing the more active species and favouring the redox cycleswhich also allowed the reactivation of the catalyst. The 1%RGO–MnCoO catalyst showed higher catalytic activity thatcould be also possible because of the formation of spinelCoMn2O4 along with Mn species of varying oxidation states,as formation of these species was also confirmed throughXRD and XPS characterization.

The results of activity screening of RGO–MnCoO catalystswith varying GO loadings for the oxidation of vanillyl alcoholin terms of conversion (%) and product selectivity (%) areshown in Table 2. GO without any metal oxide did not showany activity for the oxidation of vanillyl alcohol. The mixedoxide (MnCo) composite with RGO exhibited higher conver-sion (74%) and selectivity to vanillin (83%) than bare mixedoxide (MnCo-MO) (50% conv. and 79% selectivity). This couldbe explained on the basis of N2 adsorption/desorption iso-therm results. Since the RGO–MnCoO catalyst exhibit highersurface area (111 m2 g−1) and pore volume (0.393 cm3 g−1)than the bare oxide, this caused faster diffusion and reduc-tion of mass transfer limitation which would facilitate theoxidation reaction over the RGO–MnCoO catalyst.

The higher activity of the RGO–MnCoO nanocompositewas due to the chemical adsorption of the aromatic compoundsonto the RGO surface through π–π interaction and reaction

Catal. Sci. Technol.

with the nearest metal redox center (MnCo) anchored to thereduced graphene oxide (Scheme 1).27

These results suggest a synergistic effect between theRGO and MnCo mixed oxide. In spite of an increase in RGOcontent in the mixed oxide composite (3% and 5% RGO),the activity of the catalyst remained almost the same.However, the catalytic activity started to decrease when theRGO content exceeded >5% in the composite (Table 2, conv.dropped down from 74 to 63%). The reason could be the aggre-gation of composite metal oxides in which the active phase ofthe mixed oxide gets hidden by excess graphene oxide. There-fore, an appropriate content of GO was crucial for optimumcatalytic activity of the RGO–MnCoO nanocomposite.

The effect of reaction time on vanillyl alcohol conversionand selectivity to vanillin was also studied for the RGO–MnCoOcatalyst and the results are shown in Fig. 9. The conversion ofvanillyl alcohol increased from 41 to 74% with almost constantselectivity (83%) to vanillin, with an increase in the reactiontime from 30 min to 2 h, thereafter the conversion remainedalmost constant (76%). The decrease in conversion after 2 hwas due to the formation of 2-methoxy benzoquinone, which is

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Scheme 2 Proposed mechanism of the oxidation of para-substitutedphenolics to quinones.

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a well-known radical trapping agent and can decrease the rateof reaction.

The RGO–MnCoO catalyst was also evaluated for liquidphase oxidation of a wide range of aromatic alcohols underbase-free conditions, and the results are summarized inTable 3. The substituents on the aromatic alcohols showed asignificant influence on the oxidation activity depending ontheir ability to donate electrons. Among these aromatic alco-hols, secondary alcohol (4-hydroxy-3-methoxy-α-methyl benzylalcohol) showed the highest conversion (92%) and lowestselectivity to ketone (57%) compared to primary alcohol. Itwas clear from Table 3 that among the primary aromatic alco-hols, those with a hydroxyl group at the para position (e.g.vanillyl alcohol and 4-hydroxy benzyl alcohol) were relativelyless reactive than those which did not have a hydroxyl groupat the para position (e.g. anisyl alcohol and veratryl alcohol).This is because of the formation of benzoquinone in thehydroxy substituted aromatic alcohols at the para position,which is a well known radical trapping agent. The plausible

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Table 3 Oxidation of aromatic alcoholsa

Entry Substrate Conv. (%)

Selectivity

Aldehyde Acid Quinone

1. 74 84 6 10

2. 45 88 5 7

3. 92 57b 15 28

4. 82 100 — —

5. 91 99 1 —

6. 78 100 — —

a Reaction conditions: aromatic alcohol (3.24 mmol), RGO–MnCoOcatalyst (0.1 g), 140 °C temperature, acetonitrile (70 mL), air pressure(21 bar), time (2 h). b 4-Hydroxy-3-methoxyacetophenone.

mechanism of quinone formation from para-substituted aro-matic alcohols is shown in Scheme 2.

The stability of the 1% RGO–MnCoO catalyst was investi-gated by a recycle study for vanillyl alcohol oxidation in thefollowing way. After the first oxidation run with the freshRGO–MnCoO catalyst, the reaction crude was allowed to set-tle and the clear supernatant product mixture was removedfrom the reactor. The catalyst that remained in the reactorwas washed with methanol two times and then dried at100 °C for 4 h. The subsequent run was continued by addinga fresh charge to this catalyst. Fig. 10 shows that the activitydecreased if only the dried reused catalyst was used (recycle 1),but the original activity could be regained (72%) when thereused catalyst (recycle 2 and 3) was calcined again at 350 °Cfor 1 h. A small loss in the activity was observed, which wasdue to handling of the catalyst.

To re-establish the catalyst stability further, we performedXRD, XPS and HR-TEM analyses of the recovered catalystafter the 3rd recycle experiment. XRD results of the used cata-lyst (ESI,† Fig. S3) showed an increase in particle size from8 to 10 nm that could result from the agglomeration of thecatalyst as also evidenced from the HR-TEM image of thereused catalyst (ESI,† Fig. S4). The XPS spectrum of the recov-ered catalyst showed that the surface composition and oxida-tion state of the metals (ESI,† Fig. S5) did not change after

Catal. Sci. Technol.

Fig. 10 Recycle study of vanillyl alcohol oxidation.

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the reaction and also the catalyst showed mixed compositionof Co3O4 and CoMn2O4 with RGO.

Conclusions

In this work, the RGO–MnCoO nanocomposite was preparedby a solvothermal method and its activity was evaluated forliquid phase aerobic oxidation of aromatic alcohols underbase-free conditions. The 1% RGO–MnCoO nanocompositegave higher conversion (74% conversion) than the bare mixedoxide (MnCo-MO) for vanillyl alcohol oxidation. Reducedgraphene oxide provided anchoring sites for the metal oxidesand realized the formation of efficient redox couples. The higheractivity of the prepared RGO–MnCoO nanocomposite was dueto the interaction of π-electrons of RGO which made availablethe anchoring sites for the aromatic alcohols on the catalyst sur-face by π–π stacking near the metal redox center. The preparedmaterial showed nanorod shaped morphology attached to RGO,which was confirmed by HR-TEM and FE-SEM images. Theas-prepared RGO–MnCoO nanocomposite was highly activefor a wide range of aromatic alcohols and was successfullyrecycled three times for vanillyl alcohol oxidation.

Acknowledgements

One of the authors, Ajay Jha gratefully acknowledges the Coun-cil of Scientific and Industrial Research (CSIR, New Delhi) forthe award of senior research fellowship to him. Thanks are alsodue to Dr. Sambhaji Warule for help in FE-SEM analysis.

Notes and references

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