Magnetically Retrievable Rh(0) Nanocomposite as Relevant Catalyst

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Magnetically retrievable Rh(0) nanocomposite as relevant catalyst for mild hydrogenation of functionalized arenes in water. Carl-Hugo Pélisson, Audrey Denicourt-Nowicki, and Alain Roucoux ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00045 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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ACS Sustainable Chemistry & Engineering

Magnetically retrievable Rh(0) nanocomposite as relevant

catalyst

for

mild

hydrogenation

of

functionalized arenes in water. Carl-Hugo Pélisson, Audrey Denicourt-Nowicki,* and Alain Roucoux* ENSCR, UMR, CNRS 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France [email protected]; KEYWORDS: Maghemite - Rhodium - Nanocomposite - Arenes – Hydrogenation – Magnetic recycling ABSTRACT: A Rh0@γ-Fe2O3 nanocomposite was easily prepared by straightforward deposit of metal nanoparticles on the non-functionalized magnetic support, through a wet impregnation method. This nanomaterial proved to be highly active and magnetically retrievable in room-temperature hydrogenation of various arenes, under atmospheric hydrogen pressure, in neat water. The catalytic applications were extended to the selective reduction of nitroarenes into aniline, a relevant synthon for industrial applications, and to the dechlorination of chloroarenes, which could be of great interest for wastewater treatment. Finally, these Rh0@γ-Fe2O3 nanocomposites were compared to their Pd analogs, thus affording complementary catalytic activities.

ACS Paragon Plus Environment

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INTRODUCTION Arene hydrogenation could be considered as one of the most important organic transformations, finding applications in the synthesis of cyclohexane derivatives as relevant synthons for chemicals and pharmaceuticals,1 and also for the removal of carcinogenic benzene content in gasoline2-3 owing to increasing government limitations.4 Traditionally, this reaction has been carried out using heterogeneous noble metal catalysts, under severe temperature and/or pressure conditions, due to the resonance stabilization of the aromatic ring. However, in the current context of more eco-responsible chemical industrial processes,5 the search of highly active, selective and retrievable catalysts, that could perform the arene reduction under mild reaction conditions, still remains an impelling research area. Among the various catalytic systems, soluble or supported nanometer-sized metallic particles have proved to be reference catalysts for the hydrogenation of arene derivatives, particularly in regards to the mild reaction conditions required for this transformation, as well as to their ease of recovery under adapted conditions (biphasic conditions or heterogeneization).6-12 Among the various approaches for catalyst recovery,13-15 magnetically retrievable particles have appeared as appealing and sustainable supports for the immobilization of catalysts.16-17 Ironcontaining nanoparticles, such as magnetite (Fe3O4) or maghemite (γ-Fe2O3), are particularly relevant, owing to their low cost, availability, non-toxicity and easy functionalization with other metallic species or organocatalysts.18-20 These magnetic particles could be easily and efficiently recovered from the reaction mixture, through the use of an external magnet or by magnetically assisted cross-flow filtration and centrifugation.21


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Although bare magnetic iron-based particles could act as catalysts in some organic reactions as recently reviewed,22 they could also be coated with a polymer, carbon or silica,16, 23-26 to facilitate the catalyst’s anchoring at the magnetic surface. More recently, the straightforward deposition of metal transition nanoparticles on the non-functionalized magnetic surface, without any external organic reagents, has been reported and the obtained catalysts were successfully used in the reduction of olefins,27-28 the carbonylative Sonagashira coupling,29 or asymmetric α-arylations.30 Herein, we describe the use of a magnetically recoverable rhodium(0) nanocomposite, as efficient catalytic system in the hydrogenation of aromatic compounds in neat water, and under very mild conditions (1 bar H2, room temperature). Moreover, the investigation was extended to the dechlorination of halogenoarenes, which could constitute a relevant methodology for the remediation of arene-based endocrine disruptors from aqueous effluents.31 Finally, the selective reduction of nitroarene derivatives into the corresponding aniline, a useful building block for fine chemistry,32-33 has also been investigated. RESULTS AND DISCUSSION Spherical and well-defined magnetic iron oxide nanoparticles, possessing mean diameters around 11 nm as determined by TEM and XRD analyses, were easily synthesized by the classical coprecipitation approach,34-35 using aqueous Fe2+/Fe3+ salt solutions, and fully characterized (Figures S1 to S4, Supporting Information). XPS analyses, corroborated with

57

Fe Mössbauer

spectrometry (Figures S2 and S3, Supporting Information), proved the formation under those conditions of fully-oxidized iron oxide maghemite (γ−Fe2O3) particles instead of magnetite (Fe3O4), as previously described28 and as frequently reported in the literature for the last decade


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in the case of very small nanocatalysts (< 20 nm).36-37 Those particles also present the expected superparamagnetic behavior, a key point for their use as magnetic support. Then, as already described for palladium nanospecies,38 rhodium(0) nanoparticles were directly deposited on the maghemite surface (Rh0@γ-Fe2O3), through an easy and environmentally friendly wet impregnation of metallic salts, followed by their chemical reduction, in neat water and at room temperature. This protocol was performed using sodium borohydride, as usually reported in the literature starting from metal salts,39-40 but dihydrogen could also be used as a clean and green reducing agent, classically applied for organometallic precursors.41-42 From inductively-coupled plasma atomic emission spectrometry (ICP-AES) analyses, the rhodium loading for the obtained nanocomposite material was found around 1.03 wt.%., corresponding to the expected one. In the TEM images (Figure 1a), rhodium(0) nanoparticles, with mean sizes around 2.2 nm (Figure 1b), are well-dispersed on the magnetic core.

(a)

(b)

(c)

Figure 1. (a) TEM picture (Scale Bar = 10 nm), (b) Size distribution of Rh0 NPs, and (c) X-ray photoelectron spectrum of Rh 3d of Rh0@γ-Fe2O3 nanocomposite.


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The oxidation state of the deposited nanospecies was determined by X-ray photoelectron spectroscopy. Figure 1c shows the photopeak related to the Rh 3d core level recorded on the Rh0@γ-Fe2O3 material. Two sets of spin orbit doublets corresponding to different rhodium species were observed after mathematical decomposition according to the procedure described in the Supporting Information. Accordingly, the two signals observed at 309.7 eV (Rh 3d5/2) and 314.4 eV (Rh 3d3/2) are consistent with metallic rhodium, whereas the higher ones (BE of Rh 3d5/2 at 308.8 eV and Rh 3d3/2 at 313.1 eV) in the deconvoluted spectrum could be attributed to oxidic rhodium species, and more specifically to Rh2O3.43-44 Consequently, this result indicates a partial oxidation of the rhodium species at air, as previously reported with rhodium nanoparticles deposited on SiO2 support.45 First, the maghemite-supported Rh0 nanocatalysts have been investigated in arene hydrogenation, as a probe reaction to evaluate their efficiency, since rhodium(0) nanoparticles are known to be reference catalysts for this reaction6,

46

The catalytic experiments have been performed under

very mild conditions (1 bar H2, room temperature) and in neat water as a readily available, safe recommended and environmentally benign solvent.47-48 Before gas chromatography analyses, the catalyst was magnetically removed and the aqueous reaction mixture was extracted with diethylether. For easy comparison with the literature’s data, the Turnover Frequencies (TOFs) were calculated considering an optimized reaction time for a complete conversion of the substrate and based on the total metal amount and not only on the surface metal part, according to Janiak’s approach.49 In fact, as already reported for heterogeneous catalysts,50 the catalytic activities could also result from metallic species under the exposed surface atoms due to surface reconstruction, thus having a strong influence on the fraction of surface atoms. Consequently,


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these TOFs were clearly underestimated but could be sufficient from an economic and practical point of view.51 The results are gathered in Table 1. Table 1. Hydrogenation of arenes and halogenoarenes with Rh0@γ-Fe2O3 in watera Entry

Substrate

Product (%)b

Time (h)

TOFc (h-1)

1

Toluene

Methylcyclohexane (100)

0.5

600

2

p-Xylene

1,4-dimethylcyclohexane cis/trans (74/26)

1

300

3

Anisole

Methoxycyclohexane (93), Cyclohexanone (7)

6

50

4

Chlorobenzene

Cyclohexane (100)

3

130

5

2-Chloroanisole

Anisole (85), Cyclohexanone (3), Methoxycyclohexane (12)

4

n.d.d

6

2-Chloroanisole

Anisole (55), Cyclohexanone (5),Methoxycyclohexane (40)

24

n.d.d

7

2,4-Dichloroanisole

Anisole (77), Cyclohexanone (2), Methoxycyclohexane (21)

6

n.d.d

a

Reaction conditions: Rh0@γ-Fe2O3 (50 mg, 1 wt% Rh, 4.8 x 10-6 mol Rh), substrate (0.48 mmol), 10 mL water, 1 bar H2, room temperature b Percentage of conversion determined by GC analysis c Turnover Frequency defined as number of consumed H2 per mol of introduced Rh per hour d Non determined

From this study, the Rh0@γ-Fe2O3 nanocomposite is active towards the reduction of arenes whereas no reaction occurred with the bare γ-Fe2O3 support, thus clearly proving that Rh(0) particles are the active catalytic species. In fact, this nanocatalyst showed relevant activities towards the hydrogenation of aromatic compounds in water, under very mild temperature and pressure conditions (Table 1, Entries 1-3), with TOFs up to 600 h-1 for toluene (Table 1, Entry 1). The hydrogenation of p-xylene into 1,3-dimethylcyclohexane (Table 1, Entry 2) leads to the major formation of the cis product, as usually observed with heterogeneous catalysts52 or aqueous suspensions of metal nanoparticles.53-54 The reduction of anisole (Table 1, Entry 3), a methoxy-rich lignin model substrate, was achieved with a complete conversion in 6h, affording the expected methoxycyclohexane as major product and cyclohexanone as co-product (7 %). The reaction rate of the disubstituted arene was observed to be lower than the one of the monosubstituted aromatic ring with similar substituents (Table 1, Entry 1 vs. Entry 2), as already


6

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reported in the literature for di- and tri-alkylated benzenes.55 Moreover, to comparable electrodonor effect (methoxy vs. methyl group) the steric effect of the substituent seems preponderant (Table 1, Entry 1 vs. Entry 3).

This study was extended to halogenoarene derivatives, which could react through the tandem dehalogenation-hydrogenation process as an appealing and competitive wastewater treatment to usual oxidation processes.56 In the case of chloroarenes (Table 1, Entries 4-7), a cascade dechlorination-hydrogenation reaction occurred with Rh0@γ-Fe2O3 nanocatalyst. For instance, this catalytic system showed a remarkable TOF of 130 h-1 for the transformation of chlorobenzene into cyclohexane (Table 1, Entry 4), which could not be performed with PVP-Ru nanocatalysts57 or surfactant-protected Rh nanoparticles.58 The hydrodechlorination of 2-chloroanisole (Table 1, Entry 5), as a model substrate of endocrine disrupting compound, yielded anisole from the reductive cleavage of the C-Cl bond, the relatively non-toxic saturated product (methoxycylohexane) and a slight amount of cyclohexanone (< 3 %). Moreover, the kinetics of the reduction of the aromatic ring is very slow, with only 40 % of methylcyclohexane formed after 24 h (Table 1, Entry 6). This result could be attributed to the release during the dehalogenation step of hydrochloric acid in the reaction mixture, which acts as a poison towards the catalytic species. This phenomenon has already been reported for the hydrogenation of 2chloroanisole with Pd@γ-Fe2O3 nanocomposite.28,

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Similarly, in the case of a dichlorinated

substrate (Table 1, Entry 7), the reaction leads to the formation of the same products than those previously observed with the monochlorinated compound, with good conversion in 6 h. The formation of cyclohexanone as co-product has already been reported in the literature in the presence of acid traces, and could be explained from the partially hydrogenated intermediate, which leads in water to the formation of a hemiacetal, as shown on Figure 2.53


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Figure 2. Formation of cyclohexanone through anisole hydrogenation – A plausible pathway Secondly, the reduction of nitroarenes has been studied, using the Rh0@γ-Fe2O3 nanocomposite, under standard mild conditions (1 bar H2, room temperature), in neat water (Table 2). Table 2. Hydrogenation of nitroarenes with Rh0@γ−Fe2O3 nanocomposite in watera Entry

Substrate

Product (%)b

Time (h)

TOFc (h-1)

1

Nitrobenzene

Aniline (98), Cyclohexylamine (2)

2

150d

2

Nitrobenzene


6

n.d.e

3

4-Chloronitrobenzene


24

17d

a

Reaction conditions: Rh0@γ-Fe2O3 (50 mg, 1 wt% Rh, 4.8 x 10-6 mol Rh), substrate (0.48 mmol), 10 mL water, 1 bar H2, room temperature b Percentage of conversion determined by GC analysis c Turnover Frequency defined as number of consumed H2 per mol of introduced Rh per hour d Calculated by omitting the amount of formed cyclohexylamine e Non determined

As previously reported, the bare maghemite support was totally inactive towards the reduction of nitro groups. In opposition, the Rh0@γ-Fe2O3 catalyst was efficient and chemoselective in the hydrogenation of nitroarenes, leading to the sole formation of aniline, a relevant building block for industrial applications (Table 2, Entries 1-3). In fact, no reduction of the aromatic ring was observed in these very mild conditions, even after a longer reaction time (Table 2, Entry 2). The tandem dehalogenation-hydrogenation reaction of 4-chloronitrobenzene was also carried out, with a complete conversion into aniline in 24 h (Table 2, Entry 3). From kinetic investigations (Figure S7, Supporting Information), the preferential pathway for this cascade reaction involves


8

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first the reduction of the nitro group, affording 4-chloroaniline with a 98% selectivity at 80% conversion after 4h, followed by the dechlorination step (Figure 3). Aniline was only produced when most of the 4-chloroaniline is formed. Similar work, very recently reported, showed that ruthenium-magnetite catalysts supported on carbon support leads to the formation of chloroaniline under stronger reaction conditions.60 As often reported in the literature, in the hydrogenation of 4-chloronitrobenzene, Pd nanoparticles-based catalysts almost lead to considerable amounts of dehalogenation product.61 In contrast, higher selectivities towards the sole reduction of the nitro group were achieved using supported Pt, Ru or Ni nanocatalysts, leading to the formation of 4-chloraniline as an interesting building synthon for fine chemistry.32 Finally, kinetic investigations have showed that a good selectivity to 4-chloroaniline, as well as to aniline, could be obtained by controlling the reaction time, using the same Rh0@γ-Fe2O3 nanocatalyst.

Figure 3. Proposed reaction pathway for the tandem hydrogenation-dechlorination reaction of 4-chloronitrobenzene. The durability of the Rh0@γ-Fe2O3 catalyst was also checked, since easy and loss-free catalyst recovery remains of crucial importance for industrial applications.62 For that purpose, a set of experiments were performed in neat water, through 5 successive hydrogenation reactions on


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three various functionalized arenes (toluene, chlorobenzene and nitrobenzene) into respectively methylcyclohexane, cyclohexane and aniline (Figure 4). The catalyst was easily removed from the reaction mixture, using a neodymium magnet, and washed with water before reuse under the same conditions. This methodology prevents from loss of catalyst during the filtration step and from a potential oxidation of active species. For hydrogenation reactions of toluene and nitrobenzene, the catalyst could be magnetically recycled, washed and efficiently reused over 5 successive runs without any significant loss of catalytic activity. These results prove the efficient adsorption of the metallic nanospecies on the magnetic core surface, maintaining a constant catalytic activity during the sequential reuse reactions, as well as the efficiency of the support with superparamagnetic behavior for complete recycling. However, for chlorobenzene dehalogenation, a progressive destabilization of the Rh0@γ-Fe2O3 suspension, followed by its deactivation in the fourth run, was observed, probably owing to the production of secondary species such as hydrochloric acid, as similarly reported for Pd0@γ-Fe2O3 nanocomposite.28

Figure 4. TOF values as a function of the recycle runs with Rh0@γ-Fe2O3 nanocatalyst, in the hydrogenation of toluene, chlorobenzene and nitrobenzene in water (Reaction conditions: S/M = 100, room temperature, 1 bar H2, Reaction time: 0.5 h (Toluene), 2h (Nitrobenzene), 3h (Chlorobenzene))


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Finally, the catalytic efficiencies of the nanocomposite Rh0@γ-Fe2O3 catalyst developed in the present paper were compared in Table 3 to those of the palladium species, previously reported.28 Table 3. Rh0 vs. Pd0 nanocomposite: comparison study in hydrogenation reactions a Entry

Metal

Substrate

Product (%) b

Time (h)

TOF c (h-1)

1

Rh

Toluene

Methylcyclohexane (100)

0.5

600

2

Pd

Toluene

Toluene (100)

-

-

3

Rh

Chlorobenzene

Cyclohexane (100)

3

133

4

Pd

Chlorobenzene

Benzene (100)

1.5

67

5

Rh

2,4-Dichloroanisole

Anisole (77), Cyclohexanone(5), Methoxycyclohexane (21)

6

n.d. d

6

Pd

2,4-Dichloroanisole

Anisole (100)

4.5

22

7

Rh

Nitrobenzene

Aniline (100)

2

150

8

Pd

Nitrobenzene

Aniline (100)

2

150

a

Reaction conditions: catalyst M0 (M = Rh or Pd)@γ-Fe2O3 (50 mg, 1 wt%), Substrate/Metal = 100 (molar ratio), 10 mL H2O, 1 bar H2, room temperature b Percentage of conversion determined by GC analysis c Turnover Frequency defined as number of consumed H2 per mol of introduced Rh per hour d Non determined

As usually reported in the literature,6,

10

compared to their palladium counterparts, rhodium

nanocomposites are active towards the reduction of aromatic rings (Table 3, Entry 1 vs. Entry 2). As previously reported with aqueous suspensions of Rh(0) nanoparticles,31, 63 the nanocomposite Rh0@γ-Fe2O3 catalyst successively leads to the hydrogenolysis of the C-Cl bond followed by the total hydrogenation of the aromatic ring (Table 3, Entry 3), while the palladium one is only limited to the cleavage of the C-Cl bond (Table 3, Entry 4). Finally, the selectivity of the reaction could be controlled according to a judicious preliminary choice of the metal. CONCLUSION


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Rhodium(0) nanoparticles, possessing sizes around 2.2 nm, were easily deposited onto the magnetic surface of maghemite, using a straightforward, scalable and ecologically-sound strategy, at room temperature, in water and without the use of organic modifiers. The obtained nanocatalyst afforded relevant catalytic activities in the hydrogenation of arenes into their saturated analogs in neat water, under very mild reaction conditions (1 bar H2, room temperature), combined with an easy magnetically-driven recycling. Moreover, the magnetic Rh0@γ-Fe2O3 nanocomposites proved also to be efficient in the dechlorination reactions of chloroarenes, affording the totally hydrogenated compound, as well as in the reduction of nitroarenes, providing tunable selectivities according to the reaction time. EXPERIMENTAL SECTION ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures for catalyst preparation, as well as for catalytic hydrogenation experiments, are given. Characterizations of the maghemite core are provided. This material is available free of charge via the Internet at http://pubs.acs.org.”. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. E-mail : [email protected]


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Funding Sources The authors declare no competing financial interest. ACKNOWLEDGMENTS This research project was supported by the Région Bretagne (PhD fellowship MAGFOCAT). The authors are indebted to Patricia Beaunier from Université Pierre et Marie Curie (UMPC) for Transmission Electron Microscopy analyses, and to Cristelle Meriadec for XPS experiments (IPR, UMR UR1 - CNRS 6251). REFERENCES 1. Nishimura, S., Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis. Wiley: New-York, 2001; p 414-496. 2. Harley, R. A.; Hooper, D. S.; Kean, A. J.; Kirchstetter, T. W.; Hesson, J. M.; Balberan, N. T.; Stevenson, E. D.; Kendall, G. R., Effects of Reformulated Gasoline and Motor Vehicle Fleet Turnover on Emissions and Ambient Concentrations of Benzene. Envir. Sci. & Technol. 2006, 40 (16), 5084-5088. 3. Gu, W.; Stalzer, M. M.; Nicholas, C. P.; Bhattacharyya, A.; Motta, A.; Gallagher, J. R.; Zhang, G.; Miller, J. T.; Kobayashi, T.; Pruski, M.; Delferro, M.; Marks, T. J., Benzene Selectivity in Competitive Arene Hydrogenation: Effects of Single-Site Catalyst···Acidic Oxide Surface Binding Geometry. J. Am. Chem. Soc. 2015, 137 (21), 6770-6780. 4. (a) Directive 98/69/EC of the European Parliament and of the Council of 13 October 1998`; Official Journal of the European Communities`, 28.12.98`, L350`, pp 1-56`; (b) EPA Regulatory announcement`, EPA420-F-07-017`, February 2007. 5. Dunn, P. J., The importance of Green Chemistry in Process Research and Development. Chem. Soc. Rev. 2012, 41, 1452-1461. 6. Gual, A.; Godard, C.; Castillón, S.; Claver, C., Soluble transition-metal nanoparticlescatalysed hydrogenation of arenes. Dalton Trans. 2010, 39, 11499-11512 7. Guerrero, M.; Chau, N. T. T.; Noel, S.; Denicourt-Nowicki, A.; Hapiot, F.; Roucoux, A.; Monflier, E.; Philippot, K., About the Use of Rhodium Nanoparticles in Hydrogenation and Hydroformylation Reactions. . Currrent Organic Chemistry 2013, 17, 364-399. 8. Denicourt-Nowicki, A.; Roucoux, A., Ammonium surfactant capped Rh(0) nanoparticles for biphasic hydrogenation. In Metal Nanoparticles for Catalysis: Advances and Applications, Tao, F., Ed. Royal Society of Chemistry: 2014; pp p 99-111. 9. Qi, S.-C.; Wei, X.-Y.; Zong, Z.-M.; Wang, Y.-K., Application of supported metallic catalysts in catalytic hydrogenation of arenes. RSC Adv. 2013, 3 (34), 14219-14232. 10. Yuan, Y.; Yan, N.; Dyson, P. J., Advances in the Rational Design of Rhodium Nanoparticle Catalysts: Control via Manipulation of the Nanoparticle Core and Stabilizer. ACS Catal. 2012, 2 (6), 1057-1069.


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Table of Contents Use only

Synopsis A magnetically retrievable Rh0@γ-Fe2O3 nanocomposite, easily synthesized via a green approach, shows pertinent activities in the mild hydrogenation of arenes in neat water.


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Figure 1. (a) TEM picture (Scale Bar = 10 nm) 388x392mm (72 x 72 DPI)



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Figure 1. (c) X-ray photoelectron spectrum of Rh 3d of Rh0@γ-Fe2O3 nanocomposite. 254x190mm (96 x 96 DPI)


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Figure 4. TOF values as a function of the recycle runs with Rh0@γ-Fe2O3 nanocatalyst, in the hydrogenation of toluene, chlorobenzene and nitrobenzene in water (Reaction conditions: S/M = 100, room temperature, 1 bar H2, Reaction time: 0.5 h (Toluene), 2h (Nitrobenzene), 3h (Chlorobenzene)) 254x190mm (96 x 96 DPI)



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GraphicalAbstract 254x190mm (96 x 96 DPI)


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Magnetically Retrievable Rh(0) Nanocomposite as Relevant Catalyst

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