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Cite this: Dalton Trans., 2014, 43,12077

Received 16th April 2014,Accepted 19th June 2014

DOI: 10.1039/c4dt01123f

www.rsc.org/dalton

Synthesis of carbon embedded MFe2O4

(M = Ni, Zn and Co) nanoparticles asefficient hydrogenation catalysts†

Devaki Nandan,a,b Peta Sreenivasulu,a,b Nagabhatla Viswanadham,*a,b Ken Chiangc

and Jarrod Newnhamc

Successful synthesis of stable MFe2O4 nanoparticles@C has been realized by applying the novel concept

of using levulinic acid possessing carboxyl and carbonyl groups to facilitate the interaction with metal

ions (M2+ and Fe3+) and the carbon source (phloroglucinol) in the sol–gel polymerization method. All the

samples have been characterized by XRD, SEM, FT-IR, TEM, HRTEM, ICP-AES, CHNS, and N2 adsorption–

desorption, and were studied for their performance towards hydrogenation reaction of styrene. Out of

three samples NiFe2O4 gave excellent selective hydrogenation activity of styrene to ethyl benzene (100%

conversion and 100% selectivity). Optimal production of ethyl benzene over NiFe2O4 nanoparticles@C

has been established at 80 °C reaction temperature after 24 h reaction time under 40 bar hydrogen

pressure.

1. Introduction

Recently carbon materials are gaining importance as catalystsupports because of their energy efficient and environmentallyfriendly synthesis process facilitated by simple hydrothermaltreatment of low-cost chemicals such as glucose.1–5 This typeof synthesis process belongs to “green chemistry” because thereactant is safe and the preparative process causes no contami-nation to the environment. Moreover, the material also pos-sesses the properties suitable for functionalization with acidicand metal groups required for catalytic applications. Accordingto the research findings on the synthesis steps of carbon basedmaterials, the carbon source first polymerizes to form smallspheres or agglomerated particles which begin to carbonize toform multi-aromatic carbon sheets that eventually lead to theformation of a well condensed inner dense carbon matrix withan outer layer of a multi aromatic ring during the process ofhydrothermal synthesis and heat treatments.1,6–9 The hightemperature carbonization treatments applied during the

process give the material thermal and chemical stabilities toefficiently protect the metal spheres from being dissolved in aprotic environment. Moreover, the outer multi-carbon layer ofthe material can have many functional groups, such as carb-oxylic, aldehyde and hydroxyl groups on their surface, suit-able for establishing a chemical interaction with the desiredcompounds such as noble metal nanoparticles (NPs) to obtainmetal functionalized catalysts.10,11 Based on the above advan-tages, many researchers have tried to attach metal spheres ormetal nanoparticles onto the carbon support.12–14 Wang et al.used oleic-acid-decorated Fe3O4 NPs as the core of Fe3O4/carbon spheres.15 Zhang et al. reported the fabrication offunctional 1D magnetic NP chains with thin carbon coatingsusing urea as the surfactant.16 However, the size uniformityand the thickness of the carbon layer still need to be bettercontrolled and its application as a catalyst support needs to beinvestigated.

In the present work, we have successfully synthesizedmagnetically separable carbon supported MFe2O4 nanoparticles(MFe2O4@C) where M = Ni2+, Zn2+ and Co2+ by adopting anovel route of using environmentally friendly phloroglucinolas a carbon source and levulinic acid possessing both carbonyland carboxyl functional groups as a connecting agent betweenmetal ions and the carbon source through hydrothermal treat-ment followed by carbonization, where the interaction of carb-oxyl groups with the metal ions is believed to be responsiblefor the formation of MFe2O4 nanoparticles. The synthesizedmaterials are explored for their catalytic application in selec-tive hydrogenation reactions.

†Electronic supplementary information (ESI) available: HRTEM, FT-IR, EDX,N2 adsorption–desorption isotherm, etc. of synthesized samples. See DOI:10.1039/c4dt01123f

aAcademy of Scientific and Innovative Research (AcSIR) at CSIR-Indian Institute of

Petroleum, Dehradun-248005, Uttarakhand, IndiabCatalysis and Conversion Processes Division, Indian Institute of Petroleum,

Council of Scientific and Industrial Research, Dehradun-248005, India.

E-mail: nvish@iip.res.in; Fax: +91-135-2525702; Tel: +91-135-2525856cEarth Science and Resource Engineering, CSIRO, Clayton, VIC 3168, Australia

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The selective hydrogenation of organic molecules is one ofthe most important chemical reactions for the synthesis of newcompounds, but the synthesis of effective catalysts that can cata-lyze hydrogenation of arenes under milder conditions remains agreat challenge.17 The reaction can be catalyzed homogeneouslyor heterogeneously, but the heterogeneous version is consideredby far as more interesting from an industrial point of view,18

offering well-known benefits in terms of waste reduction, easyseparation of the catalysts and recyclability of catalysts.19 Withthe aim of improving efficiency, new catalysts and supports arebeing developed continuously. The catalysts (both homo-geneous and heterogeneous) containing transition metals suchas Pd, Pt, Ru, Rh or Ni are promising materials for this reaction.However, in an effort to develop a more sustainable approach,lowering the cost with simultaneous depletion in toxicity of thematerials urged the development of alternative hydrogenationcatalysts. Subsequently, the low cost iron, cobalt and nickelcomplexes were shown to be active catalysts20 for the hydrogen-ation of olefins,21 and the selective hydrogenation of alkynes toalkenes. Recent developments in nanomaterials providedefficient methods for catalyst development and the use of ironin the form of suspendable nanoparticles for its applications incatalysis is interesting as it also provides magnetic propertiessuitable for easy separation of the catalyst from the reactionmixture. One of the challenging tasks in this regard is achievingthe stability of metal nanoparticles on the catalyst support.Stein et al.22 have overcome this limitation by stabilizing Fe NPsprepared by decomposition of Fe(CO)5 onto graphene sheets.Although the resulting particles were active hydrogenation cata-lysts, they were prone to oxidation in the presence of either theoxygen or the water atmosphere prevailed during the reaction.

In an attempt to address the above mentioned issues, thepresent method deals with the concept of simultaneous carbon-ization and metal dispersion to synthesize MFe2O4 oxide nano-particle embedded carbon supports (MFe2O4@carbon) usefulfor the selective hydrogenation of the double bond present incyclic hydrocarbons (non-aromatic) and side chains. TheNiFe2O4@C catalyst exhibits excellent activity in selective hydro-genation of styrene to ethyl benzene (towards side chain hydro-genation) with as high as 100% selectivity. The catalyst alsoexhibits activity towards hydrogenation of cyclic olefin, cyclo-hexene to produce cyclohexane with ∼75% selectivity. The catalystmaterials show stability in the protic environment of the solventsuch as ethanol that makes the synthesis method of thesematerials advantageous for catalytic applications. Compared tothe reported prior art catalysts, the as-synthesized catalyst of thepresent study exhibits higher or comparable catalytic activityand better recyclability towards the reduction of styrene andcyclohexene in the presence of a protic solvent viz. ethanol.

2. Experimental2.1. Materials

All the reagents were of analytical grade (Merck) and usedwithout further purification including phloroglucinol, glucose,

Fe(NO3)3, Zn(NO3)2, Co(NO3)2 and levulinic acid, while de-ionized water was used for preparing the solutions.

2.2. Synthesis of MFe2O4@C materials

The MFe2O4 nanoparticles were prepared by the hydrothermalmethod. In a typical synthesis procedure a certain amount ofphloroglucinol was dissolved in water to form a clear solution,followed by sequential addition of Fe(NO3)3 solution, bivalentmetal solution (NiCl2 or Zn(NO3)2 or Co(NO3)2) and levulinicacid. The mixture with the molar ratio of 1 Fe(NO3)3 : 1.05phloroglucinol : 4.5 levulinic acid : 1.68 M salt (NiCl2 orZn(NO3)2 or Co(NO3)2) : 73 H2O was stirred vigorously for60 minutes and then sealed in a Teflon-lined stainless-steelautoclave (250 ml capacity). The autoclave was heated andmaintained at 170 °C for 48 h, and then allowed to cool toroom temperature. The black solid product obtained at theend of the synthesis was then carbonised at 500 °C for 4 hunder a nitrogen atmosphere, cooled down to room tempera-ture and washed several times with ample amount of water fol-lowed by ethanol, which was finally dried at 60 °C for 6 h.

2.3. Characterisation

The powder X-ray diffraction patterns of the samples wererecorded on a Regaku Dmax III B equipped with a rotatinganode and CuKα radiations. SEM images and energy dis-persive X-ray spectra (EDX) were recorded for determining particlemorphology and elemental composition respectively on aQuanta 200f instrument, Netherland. Inductively coupledPlasma Atomic Emission Spectroscopic (ICP-AES) analysis(model: PS 3000 uv, (DRE), Leeman Labs, Inc., USA) wascarried out for analyzing the presence of metals in the freshand used catalysts so as to understand the occurrence of anyleaching out of these metal ions from the NiFe2O4@C catalyst.Nitrogen sorption isotherms were obtained using a Micro-meritics ASAP 2010 unit, USA, operated at −196 °C, where thesamples were degassed at 300 °C prior to measurement todetermine the specific BET surface area (SBET) and the porevolume. The pore size was calculated from the desorptionbranch of the adsorption–desorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The IR spectra of the samplewere recorded on a Thermonicolate 8700 instrument, Thermo-scientific Corporation, USA.

2.4. Application of materials for selective hydrogenationreaction

The catalytic performance of all the synthesized materialshas been studied towards the hydrogenation of three types ofreactants namely (1) styrene, (2) cyclohexene and (3) cyclo-hexanone. In a typical reaction procedure, 10 ml ethanol was addedto a mixture of 1 mol styrene/cyclohexene/cyclohexanone and5 mol% of catalyst and the whole mixture was transferred to aParr reactor autoclave of 25 ml volume capacity, sealed tightlyand pressurised by hydrogen up to 40 bar. The reaction wasconducted at 80 °C for 24 h and the product obtained at theend of the run was filtered and analysed by GC/GC-MS. Thequalitative measurement of the product was performed by

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GC-MS, while the quantitative analysis was performed with GCresults. The reaction product is analyzed using a GC equippedwith the DBwax column and the FID detector. After the com-pletion of the reaction, the catalyst was recovered from thereaction mixture via magnetic separation followed by washingwith hot water, ethanol, dried at 100 °C and reused formultiple cycles. The recyclability of the as-synthesized catalystwas determined using the spent catalyst up to 4 cycles. Further tosee the effect on reaction kinetics the 4th time recycled catalystwas used and the reaction product was analyzed at differenttime intervals. The reaction was also conducted homo-geneously under the same reaction conditions so as to checkthe activity of free metal ions where NiCl2 and Fe(NO3)3 saltsolutions were directly used as the source of Ni2+ and Fe3+ ionswith the concentration of ions equivalent to those in theheterogeneous NiFe2O4@C catalyst.

3. Results and discussion3.1. Crystallinity, morphology and porosity properties of thesynthesised materials

The morphology and the structure of the materials were exam-ined by field emission scanning electron microscopy(FE-SEM), transmission electron microscopy (TEM) and highresolution TEM (HRTEM). The FE-SEM images of as-syn-thesized materials shown in Fig. 1 reveal the difference inmorphology of the particles, where well-defined and uniform sizespherical particles of ∼30 nm are observed in a CoFe2O4@Csample. The ZnFe2O4@C material also exhibited similar par-ticle size and morphology but the particles appeared as closeagglomerates in this sample. On the other hand, theNiFe2O4@C material exhibited compact agglomerated particlemorphology without showing any clear defined particles. Theparticle size of the materials is further supported by theaverage crystallite size of the materials estimated from the fullwidth at half maxima of the respective peaks at 2θ values of29–60 (in XRD), using Scherrer’s equation (Table 1, ESI†). TheTEM images of NiFe2O4@C, ZnFe2O4 and CoFe2O4@Cmaterials (Fig. 2) clearly show the presence of metal oxidenanoparticles at carbon with a grain size range of 10–20 nm.The size of metal oxide nanoparticles (indicated with arrows inimages) in the case of NiFe2O4@C is smaller than that ofZnFe2O4 and CoFe2O4@C materials. Further, the HRTEMimages of NiFe2O4 (Fig. 3) reveal the well-resolved latticefringes with an inter plane distance of 0.252 nm (representingthe spinel type of the lattice structure of NiFe2O4) arising fromthe (311) plane of NiFe2O4 material, which is consistent withthe X-ray diffraction results (Fig. 4).The wide angle XRD ana-lysis (Fig. 4) revealed that the positions and relative intensitiesof the diffraction peaks matched well with those of the stan-dard MFe2O4. The peaks at 2θ values of 18.5, 30.28, 35.76,37.2, 43.72, 54.08 and 57.4 are indexed to the (111), (220),(311), (222), (400), (422) and (511) planes of a face-centeredcubic M2+ iron spinel phase respectively, which are consistentwith the standard XRD data of the MFe2O4 phase (JCPDS

no. 10-325). On comparing the intensity of the reflections of threemixed oxides (spinel), the one having NiFe2O4 exhibited sharpand intense reflections than that of ZnFe2O4 and CoFe2O4. TheSEM and TEM images of the samples indicate that the amount

Fig. 1 SEM images of MFe2O4 nanoparticles@C.

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of carbon surrounding the metal oxide particles is less densein the case of NiFe2O4@C. Based on this observation, the highintensity of metal oxide peaks observed in the NiFe2O4@Csample can be ascribed to the presence of less carbon shield-

ing around the metal oxide particles in this sample.23 TheXRD spectra of ZnFe2O4@C exhibited other peaks at 2θ of 31.6,34.4, 36.2 are indexed to the (100), (002) and (101) planes ofthe hexagonal wurtzite structure of ZnO (as impurity) (JCPDSdata no. 36-1451) (ESI Fig S1†),24–26 while such crystallineimpurities are not observed in other two samples, i.e.NiFe2O4@C and CoFe2O4@C.

The Fourier Transmission Infrared (FT-IR) spectra (Fig. S2†)of NiFe2O4@C, ZnFe2O4@C and CoFe2O4@C demonstrate theevidence for the formation of carbon supported MFe2O4,where two bands were observed at 3435 cm−1 and1500–1600 cm−1 related to –OH stretching and CvC in-planevibrations27 respectively. The band at 591–600 cm−1 could beascribed to the typical lattice absorption property ofMFe2O4@C that confirms the existence of the MFe2O4 struc-ture.28 The elemental composition of the sample analyzed byEDX spectra (Fig. S3†) further confirms the presence of

Fig. 2 TEM images of MFe2O4 nanoparticles@C.

Fig. 3 HRTEM images of NiFe2O4@C nanoparticles.

Fig. 4 Wide angle XRD patterns of MFe2O4 nanoparticles@C materials.

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carbon, M2+ metal and iron metal in the materials. The percent-age of metal and carbon is given in Table S2,† where themetal percentage was determined by ICP and percentage ofcarbon was determined by EDX and CHNS analysis. All thethree samples exhibited the comparable carbon content of23–25 wt% and is in accordance with the weight of the carbonsource and levulinic acid taken in the initial gel (similar to thesynthesis mixture). The wt% of divalent metal ions (Ni2+, Zn2+

and Co2+) is observed to be higher than that of the trivalentone (Fe3+) which is again in accordance with the weight ofmetal salts taken during the synthesis.

The porous nature of the materials was confirmed bymeasurement of the nitrogen adsorption–desorption iso-therm (Fig. S4†) that represents the type-IV isotherm with ahysteresis loop in the range of 0.7–1.0 P/P0, suggesting thecapillary condensation of the adsorbed gas in the narrowpores of the material. The pore size distribution of the corres-ponding sample measured by the Barrett–Joyner–Halenda(BJH) method (inset of Fig. S4†) further reveals the hierarchicalnature of the porous MFe2O4@carbon sample where the pres-ence of mesopores of different diameter was observed to co-exist. The BET surface area and total pore volume measure-ments of the hierarchical porous NiFe2O4@C of the presentstudy are 13 m2 g−1, 0.12 cm3 g−1 which are almost similar tothat of the single crystal magnetite hollow spheres of Fe3O4

reported in the literature (13.5 m2 g−1 total pore volume is0.21 cm3 g−1), while the surface area and the total pore volumeof ZnFe2O4@C and CoFe2O4@C are 27 m2 g−1, 0.17 cm3 g−1

and 39 m2 g−1, 0.18 cm3 g−1 respectively show that thesematerials are more porous than that of NiFe2O4@C.

The formation of such a high quality nanoparticles ofMFe2O4@C material obtained in the present study can beexplained by the schematic reaction path of reactants facili-tated during the synthesis (Scheme 1) which is proposed basedon the XRD, TEM and porosimetry properties of the material.It is known from the prior art that the carboxylic group con-taining compounds are used for the stabilization of metalnanoparticles29,30 and the carbonyl group containing com-pounds are used for the formation of polymer by reacting with

phloroglucinol.31,32 Using this information, the novel conceptof establishing the metal–carbon support interaction in themonomer level itself is achieved in the present study, wherethe levulinic acid possessing both carboxyl and carbonylgroups is used to facilitate interaction with M2+ and Fe3+ metalions on the one side and with the carbon source phloroglucinolon the other side respectively. Scheme 1 shows the possibleformation of metal ion interacted polymer species through thereaction among various chemical ingredients when treatedunder autogenous pressure conditions inside the autoclave at170 °C. The material obtained from the autoclave is subjectedto heat treatment at 500 °C for 4 h to facilitate the carboniz-ation that eventually lead to the formation of well dispersedmetal nanoparticles on the carbon support. The advantageand the novelty of the present method are involved in the firststep of achieving metal–carbon source interaction before start-ing any carbonization of the carbon source, which upon sub-sequent carbonization forms the well dispersed metal particleson the carbon support. Here, the carboxyl group interaction ofthe metal ions helps to control any agglomeration of the metalions during the hydrothermal and carbonization steps.

3.2. Catalytic application of materials

The catalytic performance of all the materials synthesized inthe present study has been tested for the hydrogenation ofstyrene having a double bond at the side chain under similarreaction conditions of 80 °C, 40 bar H2 pressure. In a typicalprocedure the reaction is conducted by taking 5 mol% of thecatalyst and 1 mol of styrene/cyclohexene in a high pressureautoclave reactor (Parr 4848) where the reaction mixture wasleft under stirring condition at 500 rpm for 24 h. Out of thethree catalysts NiFe2O4@C gave the highest styrene conversion(100%) while ZnFe2O4@C and CoFe2O4@C gave 85% and 75%styrene conversions respectively (Table 1). A common thingobserved with all three catalysts is the highest product selecti-vity (100%) towards ethyl benzene (Table 1). The reaction wasalso conducted homogeneously under the same reaction con-ditions so as to check the activity of free metal ions whereNiCl2 and Fe(NO3)3 salt solutions were directly used as thesource of Ni2+ and Fe3+ ions with the concentration of ionsequivalent to those in the heterogeneous NiFe2O4@C catalyst.As is given in Table 1, the metal ions tested under homo-geneous conditions could not give any reaction that suggests

Scheme 1 Schematic illustration of the formation of MFe2O4@Cnanoparticles.

Table 1 Hydrogenation of styrene over synthesized materialsa

CatalystConversion(%) Product

Ethyl benzeneselectivity (%)

NiFe2O4@C 100 Ethyl benzene 100ZnFe2O4@C 85 Ethyl benzene 100CoFe2O4@C 75 Ethyl benzene 100Ni2+Fe3+ ionsb 0 — —

a Reaction conditions: reaction temperature = 80 °C, H2 pressure =40 bar, reactant = 1 mmol, catalyst = 5 mol%, reaction time = 24 h.bNi2+ & Fe3+ ions with the same ratio as in NiFe2O4@C.

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the active role of the mixed oxide spinel supported carbon asan efficient catalyst for the selective hydrogenation reaction.

The NiFe2O4@C catalyst stands as the best among the threecatalysts and is further explored for the conversion of otherreactants: (1) cyclohexene, having a double bond in the cyclicring and (2) cyclohexanone, where the double bond position isbetween carbon of the cyclic ring and oxygen. The materialalso exhibited promising catalytic activity in cyclohexenehydrogenation, but the conversion is less (70%) compared tothat of styrene. In contrast, no noticeable conversion isobserved in the cyclohexanone hydrogenation reaction on thismaterial under similar reaction conditions (Table 2). Hence, itis interesting to see that the material exhibited different activi-ties towards the hydrogenation of three different reactants;excellent catalytic activity in the selective hydrogenation ofstyrene to ethyl benzene (as high as 100% conversion and100% selectivity), moderate activity towards cyclohexene tocyclohexane (∼60%) while no activity for cyclohexanone hydro-genation. These results reveal that the material is highly selec-tive for the hydrogenation of the side chain double bond,moderately active for the isolated double bond in the cyclicrings but ineffective for the hydrogenation of carbonyl groups.

This observation clearly emphasizes the selective hydro-genation functionality of the present catalyst system to apply forthe hydrogenation of side chain double bonds with high con-version and selectivity. The reaction parameters such as timeand pressure were varied to see the effect on conversion andselectivity. Fig. 5A shows the effect of pressure on the conver-sion, where increase of reaction pressure enhanced the conver-sion of styrene; at initial 10 bar pressure the styreneconversion was only 35%, which was increased to almost 100%

at 40 bar pressure. A similar trend in increased styrene conver-sion was also observed with the increase of the reaction time(Fig. 5B). The curve shows three regions, an exponentialincrease in conversion up to 3 h, followed by linear increaseup to 24 h reaction time, while the conversion is levelled offup to the studied period of 26 h. We have seen that theoptimum conversion (100%) on the catalyst was achieved after24 h reaction time. At any level of conversion the catalystexhibited as high as 100% selectivity to the ethyl benzeneproduct. The linear increase of conversion with reaction timemay be due to initial inhibition in interaction of the reactantwith the active sites of the catalyst in the presence of thecarbon moiety towards hydrogenation. As the reaction timeprogress, the interaction of molecules with the catalyst will befacilitated due to the porous nature of carbon that results inincrease in conversion values.

3.3. Reusability of the catalyst

The catalyst NiFe2O@C displayed a high leaching resistancecapability. Reuse of the recovered catalyst in 4 consecutiveruns did not lead to any significant decrease in its catalyticactivity in terms of its conversion, yield and selectivity. Re-cycling and reusability of the catalyst were examined by introdu-cing the used catalyst up to four times. The catalyst exhibitedthe magnetic nature that allowed to separate the catalyst fromthe reaction mixture using the magnet (ESI Fig S4†). After eachrun the catalyst was separated by the magnet and washed byhot water followed by ethanol and dried at 100 °C. The catalystwas effective enough to give comparable conversions after eachcycle (Fig. 6), which demonstrates that no significant loss inthe catalytic activity was observed during recycle operation.Further, the used catalyst obtained after the 4th cycle wasstudied for its performance with reaction time of up to 26 hand the performance with time is compared with that of thefresh catalyst in Fig. 5B. It is interesting to see that almostidentical conversion patterns were observed for both the freshand the recycled catalyst at all reaction times studied that con-firms the intact of active sites in the catalyst during recycleoperations and proves the recyclability of the catalyst. TheICP-AES data of the fresh and the spent catalysts along withthe carbon percent given in Table S2† show comparable valuesthat confirm that there is no leaching of the metals as well ascarbon occurred during the reaction and the active sites are

Table 2 NiFe2O4@C catalysed hydrogenation reactions

S. no Reactant ProductConversion(%)

Selectivity(%)

1a Styrene Ethyl benzene 100 1002a Cyclohexene Cyclohexane 70 1003a Cyclohexanone Cyclohexanol 0 —

a Reaction conditions: reaction temperature = 80 °C, H2 pressure =40 bar, reactant = 1 mmol, catalyst = 5 mol%, reaction time = 24 h.

Fig. 5 (A) Effect of pressure on conversion and (B) effect of time onconversion at 80 °C reaction temperature by fresh NiFe2O4@C (■) and4th time recycled NiFe2O4@C (▼) as a catalyst. Fig. 6 Reusability of the NiFe2O4@C catalyst.

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intact in the NiFe2O4@C catalyst. Further we have also usedthe hot infiltration method and analyzed the presence of anymetals in the filtrate by ICP-AES analysis. As given in Table ESIS2,† the data indicate the absence of Fe and Ni in the filtrate(zero values) obtained from NiFe2O4@C confirms the intactmetal active sites in the catalyst during the reaction. A refer-ence experiment was also conducted in the absence of thecatalyst to see the catalytic role of NiFe2O4@C where no conver-sion was obtained. To see the effect of heterogeneous con-ditions a reaction was also conducted homogeneously underthe same reaction conditions by taking same metal ions(Ni and Fe) in the same ratio as that of the heterogeneousNiFe2O4@C catalyst (Table 1).

No reaction progressed on the catalyst under homogeneousconditions, thus supporting the catalytic role of NiFe2O4 activesites in the heterogeneous catalyst.

By virtue of its higher conversion of the double bond con-taining hydrocarbons to produce a side chain hydrogenatedproduct with high selectivity, the catalyst has potential appli-cations in the dye industry, fine chemical synthesis andpetrochemicals.

4. Conclusions

In summary, highly crystalline, uniform size spinel of MFe2O4

nanoparticles@C was obtained in the present study throughthe sol–gel hydrothermal synthesis method followed by car-bonization, adopting a novel approach of establishing an inter-action between the carbon source and metal ions in themonomer level itself. The levulinic acid possessing both carb-oxyl and carbonyl functional groups used in the presentstudy might be responsible for facilitating interaction with thecarbon source on the one hand and the metal ions on theother hand so as to form the carbon embedded metal nano-particles. Further, the –COOH group in levulinic acid might beresponsible for the stabilization of the NiFe2O4 unit againstagglomeration during polymerization/carbonization reactionsof phloroglucinol. The NiFe2O4@C catalyst exhibiting well dis-persed small size nanoparticles of ∼10 to 20 nm obtained inthe present study provides a scope for the synthesis of othermetal nanoparticle supported catalytic systems by adoptingthis novel approach of using bi-functional levulinic acid as abinding molecule for establishing strong metal–support inter-action. Excellent activity in selective hydrogenation of styreneto ethyl benzene exhibited by the present catalyst system envi-sions its scope for industrial applications through the hydro-genation of various non-aromatic double bonds involved inchemical systems related to fine chemicals and drug delivery.

Acknowledgements

We acknowledge the support of CSIR for the research fundingof the project under 12th FYP. Authors are thankful to theDirector, IIP, for his encouragement. DN and PS acknowledge

CSIR, New Delhi, for awarding senior research fellowship. Weare thankful to the groups at IIP for XRD, IR, Porosimetry, andSEM analysis.

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