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Synthesis of carbon embedded MFe2O4 (M = Ni, Zn and Co) nanoparticles as efficient 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–
Received 16th April 2014, Accepted 19th June 2014 DOI: 10.1039/c4dt01123f www.rsc.org/dalton
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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.
Introduction
Recently carbon materials are gaining importance as catalyst supports because of their energy efficient and environmentally friendly synthesis process facilitated by simple hydrothermal treatment of low-cost chemicals such as glucose.1–5 This type of synthesis process belongs to “green chemistry” because the reactant is safe and the preparative process causes no contamination to the environment. Moreover, the material also possesses the properties suitable for functionalization with acidic and metal groups required for catalytic applications. According to the research findings on the synthesis steps of carbon based materials, the carbon source first polymerizes to form small spheres or agglomerated particles which begin to carbonize to form multi-aromatic carbon sheets that eventually lead to the formation of a well condensed inner dense carbon matrix with an outer layer of a multi aromatic ring during the process of hydrothermal synthesis and heat treatments.1,6–9 The high temperature carbonization treatments applied during the
a Academy of Scientific and Innovative Research (AcSIR) at CSIR-Indian Institute of Petroleum, Dehradun-248005, Uttarakhand, India b Catalysis and Conversion Processes Division, Indian Institute of Petroleum, Council of Scientific and Industrial Research, Dehradun-248005, India. E-mail: [email protected]; Fax: +91-135-2525702; Tel: +91-135-2525856 c Earth Science and Resource Engineering, CSIRO, Clayton, VIC 3168, Australia † Electronic supplementary information (ESI) available: HRTEM, FT-IR, EDX, N2 adsorption–desorption isotherm, etc. of synthesized samples. See DOI: 10.1039/c4dt01123f
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process give the material thermal and chemical stabilities to efficiently protect the metal spheres from being dissolved in a protic environment. Moreover, the outer multi-carbon layer of the material can have many functional groups, such as carboxylic, aldehyde and hydroxyl groups on their surface, suitable for establishing a chemical interaction with the desired compounds such as noble metal nanoparticles (NPs) to obtain metal functionalized catalysts.10,11 Based on the above advantages, many researchers have tried to attach metal spheres or metal 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 of functional 1D magnetic NP chains with thin carbon coatings using urea as the surfactant.16 However, the size uniformity and the thickness of the carbon layer still need to be better controlled and its application as a catalyst support needs to be investigated. In the present work, we have successfully synthesized magnetically separable carbon supported MFe2O4 nanoparticles (MFe2O4@C) where M = Ni2+, Zn2+ and Co2+ by adopting a novel route of using environmentally friendly phloroglucinol as a carbon source and levulinic acid possessing both carbonyl and carboxyl functional groups as a connecting agent between metal ions and the carbon source through hydrothermal treatment followed by carbonization, where the interaction of carboxyl groups with the metal ions is believed to be responsible for the formation of MFe2O4 nanoparticles. The synthesized materials are explored for their catalytic application in selective hydrogenation reactions.
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The selective hydrogenation of organic molecules is one of the most important chemical reactions for the synthesis of new compounds, but the synthesis of effective catalysts that can catalyze hydrogenation of arenes under milder conditions remains a great challenge.17 The reaction can be catalyzed homogeneously or heterogeneously, but the heterogeneous version is considered by far as more interesting from an industrial point of view,18 offering well-known benefits in terms of waste reduction, easy separation of the catalysts and recyclability of catalysts.19 With the aim of improving efficiency, new catalysts and supports are being developed continuously. The catalysts (both homogeneous and heterogeneous) containing transition metals such as 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 the materials urged the development of alternative hydrogenation catalysts. Subsequently, the low cost iron, cobalt and nickel complexes were shown to be active catalysts20 for the hydrogenation of olefins,21 and the selective hydrogenation of alkynes to alkenes. Recent developments in nanomaterials provided efficient methods for catalyst development and the use of iron in the form of suspendable nanoparticles for its applications in catalysis is interesting as it also provides magnetic properties suitable for easy separation of the catalyst from the reaction mixture. One of the challenging tasks in this regard is achieving the stability of metal nanoparticles on the catalyst support. Stein et al.22 have overcome this limitation by stabilizing Fe NPs prepared by decomposition of Fe(CO)5 onto graphene sheets. Although the resulting particles were active hydrogenation catalysts, they were prone to oxidation in the presence of either the oxygen or the water atmosphere prevailed during the reaction. In an attempt to address the above mentioned issues, the present method deals with the concept of simultaneous carbonization and metal dispersion to synthesize MFe2O4 oxide nanoparticle embedded carbon supports (MFe2O4@carbon) useful for the selective hydrogenation of the double bond present in cyclic hydrocarbons (non-aromatic) and side chains. The NiFe2O4@C catalyst exhibits excellent activity in selective hydrogenation of styrene to ethyl benzene (towards side chain hydrogenation) with as high as 100% selectivity. The catalyst also exhibits activity towards hydrogenation of cyclic olefin, cyclohexene to produce cyclohexane with ∼75% selectivity. The catalyst materials show stability in the protic environment of the solvent such as ethanol that makes the synthesis method of these materials advantageous for catalytic applications. Compared to the reported prior art catalysts, the as-synthesized catalyst of the present study exhibits higher or comparable catalytic activity and better recyclability towards the reduction of styrene and cyclohexene in the presence of a protic solvent viz. ethanol.
2. Experimental 2.1.
Materials
All the reagents were of analytical grade (Merck) and used without further purification including phloroglucinol, glucose,
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Fe(NO3)3, Zn(NO3)2, Co(NO3)2 and levulinic acid, while deionized water was used for preparing the solutions. 2.2.
Synthesis of MFe2O4@C materials
The MFe2O4 nanoparticles were prepared by the hydrothermal method. In a typical synthesis procedure a certain amount of phloroglucinol was dissolved in water to form a clear solution, followed by sequential addition of Fe(NO3)3 solution, bivalent metal solution (NiCl2 or Zn(NO3)2 or Co(NO3)2) and levulinic acid. The mixture with the molar ratio of 1 Fe(NO3)3 : 1.05 phloroglucinol : 4.5 levulinic acid : 1.68 M salt (NiCl2 or Zn(NO3)2 or Co(NO3)2) : 73 H2O was stirred vigorously for 60 minutes and then sealed in a Teflon-lined stainless-steel autoclave (250 ml capacity). The autoclave was heated and maintained at 170 °C for 48 h, and then allowed to cool to room temperature. The black solid product obtained at the end of the synthesis was then carbonised at 500 °C for 4 h under a nitrogen atmosphere, cooled down to room temperature and washed several times with ample amount of water followed by ethanol, which was finally dried at 60 °C for 6 h. 2.3.
Characterisation
The powder X-ray diffraction patterns of the samples were recorded on a Regaku Dmax III B equipped with a rotating anode and CuKα radiations. SEM images and energy dispersive X-ray spectra (EDX) were recorded for determining particle morphology and elemental composition respectively on a Quanta 200f instrument, Netherland. Inductively coupled Plasma Atomic Emission Spectroscopic (ICP-AES) analysis (model: PS 3000 uv, (DRE), Leeman Labs, Inc., USA) was carried out for analyzing the presence of metals in the fresh and used catalysts so as to understand the occurrence of any leaching out of these metal ions from the NiFe2O4@C catalyst. Nitrogen sorption isotherms were obtained using a Micromeritics ASAP 2010 unit, USA, operated at −196 °C, where the samples were degassed at 300 °C prior to measurement to determine the specific BET surface area (SBET) and the pore volume. The pore size was calculated from the desorption branch of the adsorption–desorption isotherms by the Barrett– Joyner–Halenda (BJH) method. The IR spectra of the sample were recorded on a Thermonicolate 8700 instrument, Thermoscientific Corporation, USA. 2.4. Application of materials for selective hydrogenation reaction The catalytic performance of all the synthesized materials has been studied towards the hydrogenation of three types of reactants namely (1) styrene, (2) cyclohexene and (3) cyclohexanone. In a typical reaction procedure, 10 ml ethanol was added to a mixture of 1 mol styrene/cyclohexene/cyclohexanone and 5 mol% of catalyst and the whole mixture was transferred to a Parr reactor autoclave of 25 ml volume capacity, sealed tightly and pressurised by hydrogen up to 40 bar. The reaction was conducted at 80 °C for 24 h and the product obtained at the end of the run was filtered and analysed by GC/GC-MS. The qualitative measurement of the product was performed by
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GC-MS, while the quantitative analysis was performed with GC results. The reaction product is analyzed using a GC equipped with the DBwax column and the FID detector. After the completion of the reaction, the catalyst was recovered from the reaction mixture via magnetic separation followed by washing with hot water, ethanol, dried at 100 °C and reused for multiple cycles. The recyclability of the as-synthesized catalyst was determined using the spent catalyst up to 4 cycles. Further to see the effect on reaction kinetics the 4th time recycled catalyst was used and the reaction product was analyzed at different time intervals. The reaction was also conducted homogeneously under the same reaction conditions so as to check the activity of free metal ions where NiCl2 and Fe(NO3)3 salt solutions were directly used as the source of Ni2+ and Fe3+ ions with the concentration of ions equivalent to those in the heterogeneous NiFe2O4@C catalyst.
3. Results and discussion 3.1. Crystallinity, morphology and porosity properties of the synthesised materials The morphology and the structure of the materials were examined by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The FE-SEM images of as-synthesized materials shown in Fig. 1 reveal the difference in morphology of the particles, where well-defined and uniform size spherical particles of ∼30 nm are observed in a CoFe2O4@C sample. The ZnFe2O4@C material also exhibited similar particle size and morphology but the particles appeared as close agglomerates in this sample. On the other hand, the NiFe2O4@C material exhibited compact agglomerated particle morphology without showing any clear defined particles. The particle size of the materials is further supported by the average crystallite size of the materials estimated from the full width at half maxima of the respective peaks at 2θ values of 29–60 (in XRD), using Scherrer’s equation (Table 1, ESI†). The TEM images of NiFe2O4@C, ZnFe2O4 and CoFe2O4@C materials (Fig. 2) clearly show the presence of metal oxide nanoparticles at carbon with a grain size range of 10–20 nm. The size of metal oxide nanoparticles (indicated with arrows in images) in the case of NiFe2O4@C is smaller than that of ZnFe2O4 and CoFe2O4@C materials. Further, the HRTEM images of NiFe2O4 (Fig. 3) reveal the well-resolved lattice fringes with an inter plane distance of 0.252 nm (representing the spinel type of the lattice structure of NiFe2O4) arising from the (311) plane of NiFe2O4 material, which is consistent with the X-ray diffraction results (Fig. 4).The wide angle XRD analysis (Fig. 4) revealed that the positions and relative intensities of the diffraction peaks matched well with those of the standard 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-centered cubic M2+ iron spinel phase respectively, which are consistent with the standard XRD data of the MFe2O4 phase (JCPDS
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Fig. 1
SEM images of MFe2O4 nanoparticles@C.
no. 10-325). On comparing the intensity of the reflections of three mixed oxides (spinel), the one having NiFe2O4 exhibited sharp and intense reflections than that of ZnFe2O4 and CoFe2O4. The SEM and TEM images of the samples indicate that the amount
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Fig. 2
TEM images of MFe2O4 nanoparticles@C.
of carbon surrounding the metal oxide particles is less dense in the case of NiFe2O4@C. Based on this observation, the high intensity of metal oxide peaks observed in the NiFe2O4@C sample can be ascribed to the presence of less carbon shield-
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Fig. 3
HRTEM images of NiFe2O4@C nanoparticles.
Fig. 4
Wide angle XRD patterns of MFe2O4 nanoparticles@C materials.
ing around the metal oxide particles in this sample.23 The XRD 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 of the hexagonal wurtzite structure of ZnO (as impurity) (JCPDS data no. 36-1451) (ESI Fig S1†),24–26 while such crystalline impurities 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 the evidence for the formation of carbon supported MFe2O4, where two bands were observed at 3435 cm−1 and 1500–1600 cm−1 related to –OH stretching and CvC in-plane vibrations27 respectively. The band at 591–600 cm−1 could be ascribed to the typical lattice absorption property of MFe2O4@C that confirms the existence of the MFe2O4 structure.28 The elemental composition of the sample analyzed by EDX spectra (Fig. S3†) further confirms the presence of
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carbon, M2+ metal and iron metal in the materials. The percentage of metal and carbon is given in Table S2,† where the metal percentage was determined by ICP and percentage of carbon was determined by EDX and CHNS analysis. All the three samples exhibited the comparable carbon content of 23–25 wt% and is in accordance with the weight of the carbon source and levulinic acid taken in the initial gel (similar to the synthesis mixture). The wt% of divalent metal ions (Ni2+, Zn2+ and Co2+) is observed to be higher than that of the trivalent one (Fe3+) which is again in accordance with the weight of metal salts taken during the synthesis. The porous nature of the materials was confirmed by measurement of the nitrogen adsorption–desorption isotherm (Fig. S4†) that represents the type-IV isotherm with a hysteresis loop in the range of 0.7–1.0 P/P0, suggesting the capillary condensation of the adsorbed gas in the narrow pores of the material. The pore size distribution of the corresponding sample measured by the Barrett–Joyner–Halenda (BJH) method (inset of Fig. S4†) further reveals the hierarchical nature of the porous MFe2O4@carbon sample where the presence of mesopores of different diameter was observed to coexist. The BET surface area and total pore volume measurements of the hierarchical porous NiFe2O4@C of the present study are 13 m2 g−1, 0.12 cm3 g−1 which are almost similar to that of the single crystal magnetite hollow spheres of Fe3O4 reported in the literature (13.5 m2 g−1 total pore volume is 0.21 cm3 g−1), while the surface area and the total pore volume of 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 these materials are more porous than that of NiFe2O4@C. The formation of such a high quality nanoparticles of MFe2O4@C material obtained in the present study can be explained by the schematic reaction path of reactants facilitated during the synthesis (Scheme 1) which is proposed based on the XRD, TEM and porosimetry properties of the material. It is known from the prior art that the carboxylic group containing compounds are used for the stabilization of metal nanoparticles29,30 and the carbonyl group containing compounds are used for the formation of polymer by reacting with
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phloroglucinol.31,32 Using this information, the novel concept of establishing the metal–carbon support interaction in the monomer level itself is achieved in the present study, where the levulinic acid possessing both carboxyl and carbonyl groups is used to facilitate interaction with M2+ and Fe3+ metal ions on the one side and with the carbon source phloroglucinol on the other side respectively. Scheme 1 shows the possible formation of metal ion interacted polymer species through the reaction among various chemical ingredients when treated under autogenous pressure conditions inside the autoclave at 170 °C. The material obtained from the autoclave is subjected to heat treatment at 500 °C for 4 h to facilitate the carbonization that eventually lead to the formation of well dispersed metal nanoparticles on the carbon support. The advantage and the novelty of the present method are involved in the first step of achieving metal–carbon source interaction before starting any carbonization of the carbon source, which upon subsequent carbonization forms the well dispersed metal particles on the carbon support. Here, the carboxyl group interaction of the metal ions helps to control any agglomeration of the metal ions during the hydrothermal and carbonization steps. 3.2.
Catalytic application of materials
The catalytic performance of all the materials synthesized in the present study has been tested for the hydrogenation of styrene having a double bond at the side chain under similar reaction conditions of 80 °C, 40 bar H2 pressure. In a typical procedure the reaction is conducted by taking 5 mol% of the catalyst and 1 mol of styrene/cyclohexene in a high pressure autoclave reactor (Parr 4848) where the reaction mixture was left under stirring condition at 500 rpm for 24 h. Out of the three 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 thing observed with all three catalysts is the highest product selectivity (100%) towards ethyl benzene (Table 1). The reaction was also conducted homogeneously under the same reaction conditions so as to check the activity of free metal ions where NiCl2 and Fe(NO3)3 salt solutions were directly used as the source of Ni2+ and Fe3+ ions with the concentration of ions equivalent to those in the heterogeneous NiFe2O4@C catalyst. As is given in Table 1, the metal ions tested under homogeneous conditions could not give any reaction that suggests
Table 1
Hydrogenation of styrene over synthesized materialsa
Catalyst
Conversion (%)
Product
Ethyl benzene selectivity (%)
NiFe2O4@C ZnFe2O4@C CoFe2O4@C Ni2+Fe3+ ionsb
100 85 75 0
Ethyl benzene Ethyl benzene Ethyl benzene —
100 100 100 —
a
Scheme 1 Schematic illustration of the formation of MFe2O4@C nanoparticles.
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Reaction conditions: reaction temperature = 80 °C, H2 pressure = 40 bar, reactant = 1 mmol, catalyst = 5 mol%, reaction time = 24 h. b Ni2+ & Fe3+ ions with the same ratio as in NiFe2O4@C.
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the active role of the mixed oxide spinel supported carbon as an efficient catalyst for the selective hydrogenation reaction. The NiFe2O4@C catalyst stands as the best among the three catalysts and is further explored for the conversion of other reactants: (1) cyclohexene, having a double bond in the cyclic ring and (2) cyclohexanone, where the double bond position is between carbon of the cyclic ring and oxygen. The material also exhibited promising catalytic activity in cyclohexene hydrogenation, but the conversion is less (70%) compared to that of styrene. In contrast, no noticeable conversion is observed in the cyclohexanone hydrogenation reaction on this material under similar reaction conditions (Table 2). Hence, it is interesting to see that the material exhibited different activities towards the hydrogenation of three different reactants; excellent catalytic activity in the selective hydrogenation of styrene to ethyl benzene (as high as 100% conversion and 100% selectivity), moderate activity towards cyclohexene to cyclohexane (∼60%) while no activity for cyclohexanone hydrogenation. These results reveal that the material is highly selective for the hydrogenation of the side chain double bond, moderately active for the isolated double bond in the cyclic rings but ineffective for the hydrogenation of carbonyl groups. This observation clearly emphasizes the selective hydrogenation functionality of the present catalyst system to apply for the hydrogenation of side chain double bonds with high conversion and selectivity. The reaction parameters such as time and pressure were varied to see the effect on conversion and selectivity. Fig. 5A shows the effect of pressure on the conversion, where increase of reaction pressure enhanced the conversion of styrene; at initial 10 bar pressure the styrene conversion was only 35%, which was increased to almost 100%
Table 2
NiFe2O4@C catalysed hydrogenation reactions
S. no
Reactant
Product
Conversion (%)
Selectivity (%)
1a 2a 3a
Styrene Cyclohexene Cyclohexanone
Ethyl benzene Cyclohexane Cyclohexanol
100 70 0
100 100 —
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 on conversion at 80 °C reaction temperature by fresh NiFe2O4@C (■) and 4th time recycled NiFe2O4@C (▼) as a catalyst.
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at 40 bar pressure. A similar trend in increased styrene conversion was also observed with the increase of the reaction time (Fig. 5B). The curve shows three regions, an exponential increase in conversion up to 3 h, followed by linear increase up to 24 h reaction time, while the conversion is levelled off up to the studied period of 26 h. We have seen that the optimum conversion (100%) on the catalyst was achieved after 24 h reaction time. At any level of conversion the catalyst exhibited as high as 100% selectivity to the ethyl benzene product. The linear increase of conversion with reaction time may be due to initial inhibition in interaction of the reactant with the active sites of the catalyst in the presence of the carbon moiety towards hydrogenation. As the reaction time progress, the interaction of molecules with the catalyst will be facilitated due to the porous nature of carbon that results in increase in conversion values. 3.3.
Reusability of the catalyst
The catalyst NiFe2O@C displayed a high leaching resistance capability. Reuse of the recovered catalyst in 4 consecutive runs did not lead to any significant decrease in its catalytic activity in terms of its conversion, yield and selectivity. Recycling and reusability of the catalyst were examined by introducing the used catalyst up to four times. The catalyst exhibited the magnetic nature that allowed to separate the catalyst from the reaction mixture using the magnet (ESI Fig S4†). After each run the catalyst was separated by the magnet and washed by hot water followed by ethanol and dried at 100 °C. The catalyst was effective enough to give comparable conversions after each cycle (Fig. 6), which demonstrates that no significant loss in the catalytic activity was observed during recycle operation. Further, the used catalyst obtained after the 4th cycle was studied for its performance with reaction time of up to 26 h and the performance with time is compared with that of the fresh catalyst in Fig. 5B. It is interesting to see that almost identical conversion patterns were observed for both the fresh and the recycled catalyst at all reaction times studied that confirms the intact of active sites in the catalyst during recycle operations and proves the recyclability of the catalyst. The ICP-AES data of the fresh and the spent catalysts along with the carbon percent given in Table S2† show comparable values that confirm that there is no leaching of the metals as well as carbon occurred during the reaction and the active sites are
Fig. 6
Reusability of the NiFe2O4@C catalyst.
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intact in the NiFe2O4@C catalyst. Further we have also used the hot infiltration method and analyzed the presence of any metals in the filtrate by ICP-AES analysis. As given in Table ESI S2,† the data indicate the absence of Fe and Ni in the filtrate (zero values) obtained from NiFe2O4@C confirms the intact metal active sites in the catalyst during the reaction. A reference experiment was also conducted in the absence of the catalyst to see the catalytic role of NiFe2O4@C where no conversion was obtained. To see the effect of heterogeneous conditions a reaction was also conducted homogeneously under the same reaction conditions by taking same metal ions (Ni and Fe) in the same ratio as that of the heterogeneous NiFe2O4@C catalyst (Table 1). No reaction progressed on the catalyst under homogeneous conditions, thus supporting the catalytic role of NiFe2O4 active sites in the heterogeneous catalyst. By virtue of its higher conversion of the double bond containing hydrocarbons to produce a side chain hydrogenated product with high selectivity, the catalyst has potential applications in the dye industry, fine chemical synthesis and petrochemicals.
4.
Conclusions
In summary, highly crystalline, uniform size spinel of MFe2O4 nanoparticles@C was obtained in the present study through the sol–gel hydrothermal synthesis method followed by carbonization, adopting a novel approach of establishing an interaction between the carbon source and metal ions in the monomer level itself. The levulinic acid possessing both carboxyl and carbonyl functional groups used in the present study might be responsible for facilitating interaction with the carbon source on the one hand and the metal ions on the other hand so as to form the carbon embedded metal nanoparticles. Further, the –COOH group in levulinic acid might be responsible for the stabilization of the NiFe2O4 unit against agglomeration during polymerization/carbonization reactions of phloroglucinol. The NiFe2O4@C catalyst exhibiting well dispersed small size nanoparticles of ∼10 to 20 nm obtained in the present study provides a scope for the synthesis of other metal nanoparticle supported catalytic systems by adopting this novel approach of using bi-functional levulinic acid as a binding molecule for establishing strong metal–support interaction. Excellent activity in selective hydrogenation of styrene to ethyl benzene exhibited by the present catalyst system envisions its scope for industrial applications through the hydrogenation of various non-aromatic double bonds involved in chemical systems related to fine chemicals and drug delivery.
Acknowledgements We acknowledge the support of CSIR for the research funding of the project under 12th FYP. Authors are thankful to the Director, IIP, for his encouragement. DN and PS acknowledge
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CSIR, New Delhi, for awarding senior research fellowship. We are thankful to the groups at IIP for XRD, IR, Porosimetry, and SEM analysis.
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30 P. R. Selvakannan, S. Mandal, S. Phadtare, R. Pasricha and M. Sastry, Langmuir, 2003, 19, 3545. 31 R. T. Mayes, C. Tsouris, J. O. Kiggans Jr., S. M. Mahurin, D. W. DePaoli and S. Dai, J. Mater. Chem., 2010, 20, 8674. 32 C. Liang and S. Dai, J. Am. Chem. Soc., 2006, 128, 5316.
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27 M. Acik, G. Lee, C. Mattevi, M. Chhowalla, K. Cho and Y. J. Chabal, Nat. Mater., 2010, 9, 840. 28 M. Fu, Q. Jiao and Y. Zhao, J. Mater. Chem. A, 2013, 1, 5517. 29 Y. Wang, J. F. Wong, X. Teng, X. Z. Lin and H. Yang, Nano Lett., 2003, 3, 1555.
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12084 | Dalton Trans., 2014, 43, 12077–12084
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Synthesis of carbon embedded MFe2O4 (M = Ni, Zn and Co) nanoparticles as efficient 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–
Received 16th April 2014, Accepted 19th June 2014 DOI: 10.1039/c4dt01123f www.rsc.org/dalton
1.
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.
Introduction
Recently carbon materials are gaining importance as catalyst supports because of their energy efficient and environmentally friendly synthesis process facilitated by simple hydrothermal treatment of low-cost chemicals such as glucose.1–5 This type of synthesis process belongs to “green chemistry” because the reactant is safe and the preparative process causes no contamination to the environment. Moreover, the material also possesses the properties suitable for functionalization with acidic and metal groups required for catalytic applications. According to the research findings on the synthesis steps of carbon based materials, the carbon source first polymerizes to form small spheres or agglomerated particles which begin to carbonize to form multi-aromatic carbon sheets that eventually lead to the formation of a well condensed inner dense carbon matrix with an outer layer of a multi aromatic ring during the process of hydrothermal synthesis and heat treatments.1,6–9 The high temperature carbonization treatments applied during the
a Academy of Scientific and Innovative Research (AcSIR) at CSIR-Indian Institute of Petroleum, Dehradun-248005, Uttarakhand, India b Catalysis and Conversion Processes Division, Indian Institute of Petroleum, Council of Scientific and Industrial Research, Dehradun-248005, India. E-mail: [email protected]; Fax: +91-135-2525702; Tel: +91-135-2525856 c Earth Science and Resource Engineering, CSIRO, Clayton, VIC 3168, Australia † Electronic supplementary information (ESI) available: HRTEM, FT-IR, EDX, N2 adsorption–desorption isotherm, etc. of synthesized samples. See DOI: 10.1039/c4dt01123f
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process give the material thermal and chemical stabilities to efficiently protect the metal spheres from being dissolved in a protic environment. Moreover, the outer multi-carbon layer of the material can have many functional groups, such as carboxylic, aldehyde and hydroxyl groups on their surface, suitable for establishing a chemical interaction with the desired compounds such as noble metal nanoparticles (NPs) to obtain metal functionalized catalysts.10,11 Based on the above advantages, many researchers have tried to attach metal spheres or metal 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 of functional 1D magnetic NP chains with thin carbon coatings using urea as the surfactant.16 However, the size uniformity and the thickness of the carbon layer still need to be better controlled and its application as a catalyst support needs to be investigated. In the present work, we have successfully synthesized magnetically separable carbon supported MFe2O4 nanoparticles (MFe2O4@C) where M = Ni2+, Zn2+ and Co2+ by adopting a novel route of using environmentally friendly phloroglucinol as a carbon source and levulinic acid possessing both carbonyl and carboxyl functional groups as a connecting agent between metal ions and the carbon source through hydrothermal treatment followed by carbonization, where the interaction of carboxyl groups with the metal ions is believed to be responsible for the formation of MFe2O4 nanoparticles. The synthesized materials are explored for their catalytic application in selective hydrogenation reactions.
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The selective hydrogenation of organic molecules is one of the most important chemical reactions for the synthesis of new compounds, but the synthesis of effective catalysts that can catalyze hydrogenation of arenes under milder conditions remains a great challenge.17 The reaction can be catalyzed homogeneously or heterogeneously, but the heterogeneous version is considered by far as more interesting from an industrial point of view,18 offering well-known benefits in terms of waste reduction, easy separation of the catalysts and recyclability of catalysts.19 With the aim of improving efficiency, new catalysts and supports are being developed continuously. The catalysts (both homogeneous and heterogeneous) containing transition metals such as 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 the materials urged the development of alternative hydrogenation catalysts. Subsequently, the low cost iron, cobalt and nickel complexes were shown to be active catalysts20 for the hydrogenation of olefins,21 and the selective hydrogenation of alkynes to alkenes. Recent developments in nanomaterials provided efficient methods for catalyst development and the use of iron in the form of suspendable nanoparticles for its applications in catalysis is interesting as it also provides magnetic properties suitable for easy separation of the catalyst from the reaction mixture. One of the challenging tasks in this regard is achieving the stability of metal nanoparticles on the catalyst support. Stein et al.22 have overcome this limitation by stabilizing Fe NPs prepared by decomposition of Fe(CO)5 onto graphene sheets. Although the resulting particles were active hydrogenation catalysts, they were prone to oxidation in the presence of either the oxygen or the water atmosphere prevailed during the reaction. In an attempt to address the above mentioned issues, the present method deals with the concept of simultaneous carbonization and metal dispersion to synthesize MFe2O4 oxide nanoparticle embedded carbon supports (MFe2O4@carbon) useful for the selective hydrogenation of the double bond present in cyclic hydrocarbons (non-aromatic) and side chains. The NiFe2O4@C catalyst exhibits excellent activity in selective hydrogenation of styrene to ethyl benzene (towards side chain hydrogenation) with as high as 100% selectivity. The catalyst also exhibits activity towards hydrogenation of cyclic olefin, cyclohexene to produce cyclohexane with ∼75% selectivity. The catalyst materials show stability in the protic environment of the solvent such as ethanol that makes the synthesis method of these materials advantageous for catalytic applications. Compared to the reported prior art catalysts, the as-synthesized catalyst of the present study exhibits higher or comparable catalytic activity and better recyclability towards the reduction of styrene and cyclohexene in the presence of a protic solvent viz. ethanol.
2. Experimental 2.1.
Materials
All the reagents were of analytical grade (Merck) and used without further purification including phloroglucinol, glucose,
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Fe(NO3)3, Zn(NO3)2, Co(NO3)2 and levulinic acid, while deionized water was used for preparing the solutions. 2.2.
Synthesis of MFe2O4@C materials
The MFe2O4 nanoparticles were prepared by the hydrothermal method. In a typical synthesis procedure a certain amount of phloroglucinol was dissolved in water to form a clear solution, followed by sequential addition of Fe(NO3)3 solution, bivalent metal solution (NiCl2 or Zn(NO3)2 or Co(NO3)2) and levulinic acid. The mixture with the molar ratio of 1 Fe(NO3)3 : 1.05 phloroglucinol : 4.5 levulinic acid : 1.68 M salt (NiCl2 or Zn(NO3)2 or Co(NO3)2) : 73 H2O was stirred vigorously for 60 minutes and then sealed in a Teflon-lined stainless-steel autoclave (250 ml capacity). The autoclave was heated and maintained at 170 °C for 48 h, and then allowed to cool to room temperature. The black solid product obtained at the end of the synthesis was then carbonised at 500 °C for 4 h under a nitrogen atmosphere, cooled down to room temperature and washed several times with ample amount of water followed by ethanol, which was finally dried at 60 °C for 6 h. 2.3.
Characterisation
The powder X-ray diffraction patterns of the samples were recorded on a Regaku Dmax III B equipped with a rotating anode and CuKα radiations. SEM images and energy dispersive X-ray spectra (EDX) were recorded for determining particle morphology and elemental composition respectively on a Quanta 200f instrument, Netherland. Inductively coupled Plasma Atomic Emission Spectroscopic (ICP-AES) analysis (model: PS 3000 uv, (DRE), Leeman Labs, Inc., USA) was carried out for analyzing the presence of metals in the fresh and used catalysts so as to understand the occurrence of any leaching out of these metal ions from the NiFe2O4@C catalyst. Nitrogen sorption isotherms were obtained using a Micromeritics ASAP 2010 unit, USA, operated at −196 °C, where the samples were degassed at 300 °C prior to measurement to determine the specific BET surface area (SBET) and the pore volume. The pore size was calculated from the desorption branch of the adsorption–desorption isotherms by the Barrett– Joyner–Halenda (BJH) method. The IR spectra of the sample were recorded on a Thermonicolate 8700 instrument, Thermoscientific Corporation, USA. 2.4. Application of materials for selective hydrogenation reaction The catalytic performance of all the synthesized materials has been studied towards the hydrogenation of three types of reactants namely (1) styrene, (2) cyclohexene and (3) cyclohexanone. In a typical reaction procedure, 10 ml ethanol was added to a mixture of 1 mol styrene/cyclohexene/cyclohexanone and 5 mol% of catalyst and the whole mixture was transferred to a Parr reactor autoclave of 25 ml volume capacity, sealed tightly and pressurised by hydrogen up to 40 bar. The reaction was conducted at 80 °C for 24 h and the product obtained at the end of the run was filtered and analysed by GC/GC-MS. The qualitative measurement of the product was performed by
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GC-MS, while the quantitative analysis was performed with GC results. The reaction product is analyzed using a GC equipped with the DBwax column and the FID detector. After the completion of the reaction, the catalyst was recovered from the reaction mixture via magnetic separation followed by washing with hot water, ethanol, dried at 100 °C and reused for multiple cycles. The recyclability of the as-synthesized catalyst was determined using the spent catalyst up to 4 cycles. Further to see the effect on reaction kinetics the 4th time recycled catalyst was used and the reaction product was analyzed at different time intervals. The reaction was also conducted homogeneously under the same reaction conditions so as to check the activity of free metal ions where NiCl2 and Fe(NO3)3 salt solutions were directly used as the source of Ni2+ and Fe3+ ions with the concentration of ions equivalent to those in the heterogeneous NiFe2O4@C catalyst.
3. Results and discussion 3.1. Crystallinity, morphology and porosity properties of the synthesised materials The morphology and the structure of the materials were examined by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The FE-SEM images of as-synthesized materials shown in Fig. 1 reveal the difference in morphology of the particles, where well-defined and uniform size spherical particles of ∼30 nm are observed in a CoFe2O4@C sample. The ZnFe2O4@C material also exhibited similar particle size and morphology but the particles appeared as close agglomerates in this sample. On the other hand, the NiFe2O4@C material exhibited compact agglomerated particle morphology without showing any clear defined particles. The particle size of the materials is further supported by the average crystallite size of the materials estimated from the full width at half maxima of the respective peaks at 2θ values of 29–60 (in XRD), using Scherrer’s equation (Table 1, ESI†). The TEM images of NiFe2O4@C, ZnFe2O4 and CoFe2O4@C materials (Fig. 2) clearly show the presence of metal oxide nanoparticles at carbon with a grain size range of 10–20 nm. The size of metal oxide nanoparticles (indicated with arrows in images) in the case of NiFe2O4@C is smaller than that of ZnFe2O4 and CoFe2O4@C materials. Further, the HRTEM images of NiFe2O4 (Fig. 3) reveal the well-resolved lattice fringes with an inter plane distance of 0.252 nm (representing the spinel type of the lattice structure of NiFe2O4) arising from the (311) plane of NiFe2O4 material, which is consistent with the X-ray diffraction results (Fig. 4).The wide angle XRD analysis (Fig. 4) revealed that the positions and relative intensities of the diffraction peaks matched well with those of the standard 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-centered cubic M2+ iron spinel phase respectively, which are consistent with the standard XRD data of the MFe2O4 phase (JCPDS
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Fig. 1
SEM images of MFe2O4 nanoparticles@C.
no. 10-325). On comparing the intensity of the reflections of three mixed oxides (spinel), the one having NiFe2O4 exhibited sharp and intense reflections than that of ZnFe2O4 and CoFe2O4. The SEM and TEM images of the samples indicate that the amount
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Fig. 2
TEM images of MFe2O4 nanoparticles@C.
of carbon surrounding the metal oxide particles is less dense in the case of NiFe2O4@C. Based on this observation, the high intensity of metal oxide peaks observed in the NiFe2O4@C sample can be ascribed to the presence of less carbon shield-
12080 | Dalton Trans., 2014, 43, 12077–12084
Fig. 3
HRTEM images of NiFe2O4@C nanoparticles.
Fig. 4
Wide angle XRD patterns of MFe2O4 nanoparticles@C materials.
ing around the metal oxide particles in this sample.23 The XRD 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 of the hexagonal wurtzite structure of ZnO (as impurity) (JCPDS data no. 36-1451) (ESI Fig S1†),24–26 while such crystalline impurities 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 the evidence for the formation of carbon supported MFe2O4, where two bands were observed at 3435 cm−1 and 1500–1600 cm−1 related to –OH stretching and CvC in-plane vibrations27 respectively. The band at 591–600 cm−1 could be ascribed to the typical lattice absorption property of MFe2O4@C that confirms the existence of the MFe2O4 structure.28 The elemental composition of the sample analyzed by EDX spectra (Fig. S3†) further confirms the presence of
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carbon, M2+ metal and iron metal in the materials. The percentage of metal and carbon is given in Table S2,† where the metal percentage was determined by ICP and percentage of carbon was determined by EDX and CHNS analysis. All the three samples exhibited the comparable carbon content of 23–25 wt% and is in accordance with the weight of the carbon source and levulinic acid taken in the initial gel (similar to the synthesis mixture). The wt% of divalent metal ions (Ni2+, Zn2+ and Co2+) is observed to be higher than that of the trivalent one (Fe3+) which is again in accordance with the weight of metal salts taken during the synthesis. The porous nature of the materials was confirmed by measurement of the nitrogen adsorption–desorption isotherm (Fig. S4†) that represents the type-IV isotherm with a hysteresis loop in the range of 0.7–1.0 P/P0, suggesting the capillary condensation of the adsorbed gas in the narrow pores of the material. The pore size distribution of the corresponding sample measured by the Barrett–Joyner–Halenda (BJH) method (inset of Fig. S4†) further reveals the hierarchical nature of the porous MFe2O4@carbon sample where the presence of mesopores of different diameter was observed to coexist. The BET surface area and total pore volume measurements of the hierarchical porous NiFe2O4@C of the present study are 13 m2 g−1, 0.12 cm3 g−1 which are almost similar to that of the single crystal magnetite hollow spheres of Fe3O4 reported in the literature (13.5 m2 g−1 total pore volume is 0.21 cm3 g−1), while the surface area and the total pore volume of 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 these materials are more porous than that of NiFe2O4@C. The formation of such a high quality nanoparticles of MFe2O4@C material obtained in the present study can be explained by the schematic reaction path of reactants facilitated during the synthesis (Scheme 1) which is proposed based on the XRD, TEM and porosimetry properties of the material. It is known from the prior art that the carboxylic group containing compounds are used for the stabilization of metal nanoparticles29,30 and the carbonyl group containing compounds are used for the formation of polymer by reacting with
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phloroglucinol.31,32 Using this information, the novel concept of establishing the metal–carbon support interaction in the monomer level itself is achieved in the present study, where the levulinic acid possessing both carboxyl and carbonyl groups is used to facilitate interaction with M2+ and Fe3+ metal ions on the one side and with the carbon source phloroglucinol on the other side respectively. Scheme 1 shows the possible formation of metal ion interacted polymer species through the reaction among various chemical ingredients when treated under autogenous pressure conditions inside the autoclave at 170 °C. The material obtained from the autoclave is subjected to heat treatment at 500 °C for 4 h to facilitate the carbonization that eventually lead to the formation of well dispersed metal nanoparticles on the carbon support. The advantage and the novelty of the present method are involved in the first step of achieving metal–carbon source interaction before starting any carbonization of the carbon source, which upon subsequent carbonization forms the well dispersed metal particles on the carbon support. Here, the carboxyl group interaction of the metal ions helps to control any agglomeration of the metal ions during the hydrothermal and carbonization steps. 3.2.
Catalytic application of materials
The catalytic performance of all the materials synthesized in the present study has been tested for the hydrogenation of styrene having a double bond at the side chain under similar reaction conditions of 80 °C, 40 bar H2 pressure. In a typical procedure the reaction is conducted by taking 5 mol% of the catalyst and 1 mol of styrene/cyclohexene in a high pressure autoclave reactor (Parr 4848) where the reaction mixture was left under stirring condition at 500 rpm for 24 h. Out of the three 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 thing observed with all three catalysts is the highest product selectivity (100%) towards ethyl benzene (Table 1). The reaction was also conducted homogeneously under the same reaction conditions so as to check the activity of free metal ions where NiCl2 and Fe(NO3)3 salt solutions were directly used as the source of Ni2+ and Fe3+ ions with the concentration of ions equivalent to those in the heterogeneous NiFe2O4@C catalyst. As is given in Table 1, the metal ions tested under homogeneous conditions could not give any reaction that suggests
Table 1
Hydrogenation of styrene over synthesized materialsa
Catalyst
Conversion (%)
Product
Ethyl benzene selectivity (%)
NiFe2O4@C ZnFe2O4@C CoFe2O4@C Ni2+Fe3+ ionsb
100 85 75 0
Ethyl benzene Ethyl benzene Ethyl benzene —
100 100 100 —
a
Scheme 1 Schematic illustration of the formation of MFe2O4@C nanoparticles.
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Reaction conditions: reaction temperature = 80 °C, H2 pressure = 40 bar, reactant = 1 mmol, catalyst = 5 mol%, reaction time = 24 h. b Ni2+ & Fe3+ ions with the same ratio as in NiFe2O4@C.
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the active role of the mixed oxide spinel supported carbon as an efficient catalyst for the selective hydrogenation reaction. The NiFe2O4@C catalyst stands as the best among the three catalysts and is further explored for the conversion of other reactants: (1) cyclohexene, having a double bond in the cyclic ring and (2) cyclohexanone, where the double bond position is between carbon of the cyclic ring and oxygen. The material also exhibited promising catalytic activity in cyclohexene hydrogenation, but the conversion is less (70%) compared to that of styrene. In contrast, no noticeable conversion is observed in the cyclohexanone hydrogenation reaction on this material under similar reaction conditions (Table 2). Hence, it is interesting to see that the material exhibited different activities towards the hydrogenation of three different reactants; excellent catalytic activity in the selective hydrogenation of styrene to ethyl benzene (as high as 100% conversion and 100% selectivity), moderate activity towards cyclohexene to cyclohexane (∼60%) while no activity for cyclohexanone hydrogenation. These results reveal that the material is highly selective for the hydrogenation of the side chain double bond, moderately active for the isolated double bond in the cyclic rings but ineffective for the hydrogenation of carbonyl groups. This observation clearly emphasizes the selective hydrogenation functionality of the present catalyst system to apply for the hydrogenation of side chain double bonds with high conversion and selectivity. The reaction parameters such as time and pressure were varied to see the effect on conversion and selectivity. Fig. 5A shows the effect of pressure on the conversion, where increase of reaction pressure enhanced the conversion of styrene; at initial 10 bar pressure the styrene conversion was only 35%, which was increased to almost 100%
Table 2
NiFe2O4@C catalysed hydrogenation reactions
S. no
Reactant
Product
Conversion (%)
Selectivity (%)
1a 2a 3a
Styrene Cyclohexene Cyclohexanone
Ethyl benzene Cyclohexane Cyclohexanol
100 70 0
100 100 —
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 on conversion at 80 °C reaction temperature by fresh NiFe2O4@C (■) and 4th time recycled NiFe2O4@C (▼) as a catalyst.
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at 40 bar pressure. A similar trend in increased styrene conversion was also observed with the increase of the reaction time (Fig. 5B). The curve shows three regions, an exponential increase in conversion up to 3 h, followed by linear increase up to 24 h reaction time, while the conversion is levelled off up to the studied period of 26 h. We have seen that the optimum conversion (100%) on the catalyst was achieved after 24 h reaction time. At any level of conversion the catalyst exhibited as high as 100% selectivity to the ethyl benzene product. The linear increase of conversion with reaction time may be due to initial inhibition in interaction of the reactant with the active sites of the catalyst in the presence of the carbon moiety towards hydrogenation. As the reaction time progress, the interaction of molecules with the catalyst will be facilitated due to the porous nature of carbon that results in increase in conversion values. 3.3.
Reusability of the catalyst
The catalyst NiFe2O@C displayed a high leaching resistance capability. Reuse of the recovered catalyst in 4 consecutive runs did not lead to any significant decrease in its catalytic activity in terms of its conversion, yield and selectivity. Recycling and reusability of the catalyst were examined by introducing the used catalyst up to four times. The catalyst exhibited the magnetic nature that allowed to separate the catalyst from the reaction mixture using the magnet (ESI Fig S4†). After each run the catalyst was separated by the magnet and washed by hot water followed by ethanol and dried at 100 °C. The catalyst was effective enough to give comparable conversions after each cycle (Fig. 6), which demonstrates that no significant loss in the catalytic activity was observed during recycle operation. Further, the used catalyst obtained after the 4th cycle was studied for its performance with reaction time of up to 26 h and the performance with time is compared with that of the fresh catalyst in Fig. 5B. It is interesting to see that almost identical conversion patterns were observed for both the fresh and the recycled catalyst at all reaction times studied that confirms the intact of active sites in the catalyst during recycle operations and proves the recyclability of the catalyst. The ICP-AES data of the fresh and the spent catalysts along with the carbon percent given in Table S2† show comparable values that confirm that there is no leaching of the metals as well as carbon occurred during the reaction and the active sites are
Fig. 6
Reusability of the NiFe2O4@C catalyst.
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intact in the NiFe2O4@C catalyst. Further we have also used the hot infiltration method and analyzed the presence of any metals in the filtrate by ICP-AES analysis. As given in Table ESI S2,† the data indicate the absence of Fe and Ni in the filtrate (zero values) obtained from NiFe2O4@C confirms the intact metal active sites in the catalyst during the reaction. A reference experiment was also conducted in the absence of the catalyst to see the catalytic role of NiFe2O4@C where no conversion was obtained. To see the effect of heterogeneous conditions a reaction was also conducted homogeneously under the same reaction conditions by taking same metal ions (Ni and Fe) in the same ratio as that of the heterogeneous NiFe2O4@C catalyst (Table 1). No reaction progressed on the catalyst under homogeneous conditions, thus supporting the catalytic role of NiFe2O4 active sites in the heterogeneous catalyst. By virtue of its higher conversion of the double bond containing hydrocarbons to produce a side chain hydrogenated product with high selectivity, the catalyst has potential applications in the dye industry, fine chemical synthesis and petrochemicals.
4.
Conclusions
In summary, highly crystalline, uniform size spinel of MFe2O4 nanoparticles@C was obtained in the present study through the sol–gel hydrothermal synthesis method followed by carbonization, adopting a novel approach of establishing an interaction between the carbon source and metal ions in the monomer level itself. The levulinic acid possessing both carboxyl and carbonyl functional groups used in the present study might be responsible for facilitating interaction with the carbon source on the one hand and the metal ions on the other hand so as to form the carbon embedded metal nanoparticles. Further, the –COOH group in levulinic acid might be responsible for the stabilization of the NiFe2O4 unit against agglomeration during polymerization/carbonization reactions of phloroglucinol. The NiFe2O4@C catalyst exhibiting well dispersed small size nanoparticles of ∼10 to 20 nm obtained in the present study provides a scope for the synthesis of other metal nanoparticle supported catalytic systems by adopting this novel approach of using bi-functional levulinic acid as a binding molecule for establishing strong metal–support interaction. Excellent activity in selective hydrogenation of styrene to ethyl benzene exhibited by the present catalyst system envisions its scope for industrial applications through the hydrogenation of various non-aromatic double bonds involved in chemical systems related to fine chemicals and drug delivery.
Acknowledgements We acknowledge the support of CSIR for the research funding of the project under 12th FYP. Authors are thankful to the Director, IIP, for his encouragement. DN and PS acknowledge
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CSIR, New Delhi, for awarding senior research fellowship. We are thankful to the groups at IIP for XRD, IR, Porosimetry, and SEM analysis.
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