Stabilized Rh0-nanoparticles-Montmorillonite clay composite: Synthesis and catalytic transfer hydrogenation reaction

Applied Catalysis A: General 470 (2014) 355–360

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Stabilized Rh0 -nanoparticles-Montmorillonite clay composite: Synthesis and catalytic transfer hydrogenation reaction Podma Pollov Sarmah, Dipak Kumar Dutta ∗ Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India

a r t i c l e

i n f o

Article history: Received 31 May 2013 Received in revised form 24 October 2013 Accepted 26 October 2013 Available online 5 November 2013 Keywords: Rh0 -nanoparticles Acid activated Montmorillonite Nanopores Transfer hydrogenation Reduction

a b s t r a c t Rh0 -nanoparticles of around 5 nm size distributed homogeneously into the nanopores of acid activated Montmorillonite clay were generated by incipient wetness impregnation of RhCl3 , followed by reduction with ethylene glycol. Acid activation of the Montmorillonite clay was carried out by treating with H2 SO4 under controlled condition to increase the surface area by generating nanopores upto about 10 nm sizes, which act as a host and stabilize nanoparticles into the pores. The synthesized Rh0 -nanoparticles-clay composites characterized by PXRD, SEM-EDX, HRTEM, XPS and N2 -adsorption confirm generation of Rh0 nanoparticle below 5 nm size and fully reduced to metallic state. The supported metal nanoparticles serve as efficient heterogeneous catalyst for reduction of some important aromatic carbonyl compounds leading to corresponding alcohols through transfer hydrogenation up to 100% conversion (GC) and selectivity, where isopropanol was used as both solvent and reductant. The catalyst remained active for several runs without significant loss of its catalytic activities. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Metal nanoparticles show enormous applications in different fields for their intrinsic physical and chemical properties like high reactivity, quantum size effect, low melting point, size dependent colour, etc. that are lacked in the bulk metal [1–6]. These properties are observed due to the high surface to volume ratio, i.e. most of the atoms in nanoparticles are exposed on the surface [1–6], and for that the nanoparticles are very reactive and tend to agglomerate to form bulk metal and thus reduces its activity [1–10]. Therefore, it is very challenging task for researchers to generate metal nanoparticles of uniform size, shape and to stabilize them in the nano domain. Various supports/stabilizers like mesoporous solid, organic ligand, polymer, carbon materials, etc. are used for stabilization of nanoparticles [1–3,11–19]. Montmorillonite clay is one of the suitable supports where metal nanoparticles can be stabilized inside the interlayer spacing or into the pores on the surface, which can be generated by activating with mineral acids under controlled condition [9,10,18–22]. Pores are generated due to leaching out of aluminium ion by mineral acid from the clay matrix. Thus, by controlling the acid concentration, activation time or temperature, the pore size and pore volume can be customized to desired dimension. Among the different metals, platinum group metal nanoparticles have aroused much interest for their greater

∗ Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011. E-mail addresses: [email protected], dutta [email protected] (D.K. Dutta). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.10.049

stability and wide spread applications as industrial catalyst. Stabilized and dispersed Rh0 -nanoparticles are found good catalyst for a wide range of organic reactions like hydrogenation, oxidation, hydroformylation reactions, etc. [23–31]. Reduction of the carbonyl group of organic compounds to the corresponding alcohols is an important organic reaction. These compounds are very vital components in different synthesis of fine chemicals, pharmaceuticals, dyes, biologically active compounds, etc. [32]. Various techniques like catalytic hydrogenation, electrolytic reduction, metal mediated reductions [33], etc. are utilized and among them, transfer hydrogenation is one of the ‘Clean’ and ‘Green’ approaches. Transfer hydrogenation is advantageous over the other hydrogenation technique in which an alcohol like isopropanol, considered as one of the green solvents, acts as solvent as well as the source of hydrogen and the reaction takes place at atmospheric pressure and relatively low temperature. Moreover, the co-produced acetone in the reaction is also a useful chemical and can easily be separated from the reaction mixture by simple distillation. Wide varieties of homogeneous complexes have been reported as transfer hydrogenation catalyst with very high turnover frequency and selectivity [34,35]. However, very few heterogeneous catalysts have been reported though it has several advantages over homogeneous process in respect of catalyst separation, recyclability, higher stability, economic viability etc. With stringent and growing environmental norms, supported metal nanoparticles may be advantageously used as heterogeneous catalyst in different organic transformations. Recently, different metal and metal oxide nanoparticles like Ni, Co, Ru, Rh, MgO–ZrO2 , etc. were reported [36–40], but there is still lot of scope to enhance the

P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360

catalytic activity by improving the reaction time, catalyst quantity, etc. As a part of our continuous research activity [9,10,18,19,41–43], here we have reported the greener synthetic procedure for generation of Rh0 -nanoporticles using environmentally benign activated Montmorillonite (AT-Mont) clay as support and ethylene glycol as reductant. Naturally occurring Montmorillonite clay (Mont) is processed and activated using mineral acid under controlled condition to generate micro and mesoporous matrix with high surface area and mean pore diameter of about 5 nm on the surface. These pores act as a host for the Rh0 -nanoparticles and restrict their size to grow further. Literature survey reveals that for the first time, we are going to report the use of this supported Rh0 -nanoparticle as excellent heterogeneous catalyst for transfer hydrogenation of some important organic carbonyl compounds to corresponding alcohols. The catalysts were recycled several times and found active without significant loss of catalytic efficiency.

1600

40.9

1400

Intensity (counts/s)

356

1200 1000 47.7 800 35.4 600 69.7 400 200 30

40

50

60

70

80

Two theta (Degree)

2. Results and discussion

Fig. 2. The powder XRD pattern of Rh0 -nanoparticles supported on AT-Mont.

2.1. Characterization of the modified support The powder XRD of the parent Mont shows an intense reflection at 2 = 7.06◦ corresponding to a basal spacing (d0 0 1 ) of 12.5 A˚ while in AT-Mont, the reflection almost disappears indicating depletion of the lamellar structure [44]. The cation exchange capacity (CEC) of the purified natural Mont decreases from 127 to 40.8 milli equivalent (meq)/100 g of clay upon acid activation for 1 h. The surface area and pore size distribution of the AT-Mont determined by N2 -adsorbtion study (Fig. 1) reveals that AT-Mont contain both micro and mesoporous with average pore diameters of 5 nm and exhibits high specific surface area of about 417 m2 /g with large specific pore volumes of 0.57 cm3 /g. Acid activation modifies the layered structure of Mont by leaching out aluminium from octahedral sites, thereby creating porous matrix with a high surface area. The adsorption–desorption isotherms were of the type IV with a H3 hysteresis loop at P/P0 ∼0.4–0.9, indicating mesoporous solids [45,46]. The BJH pore size distribution plot indicates relatively a narrow pore size distribution with a peak pore diameter centred within 4–6 nm. IR study reveals that the parent Mont exhibits an intense absorption band at 1034 cm−1 for Si O stretching vibrations of tetrahedral

sheet and shows broad absorption bands at 3633 cm−1 due to stretching vibrations of OH groups of Al OH [47]. The bands at 522 and 460 cm−1 are due to Si O Al and Si O Si bending vibrations mode, respectively. Upon acid activation, the Si O stretching vibration band shifts from 1034 to 1083 cm−1 indicating the change in bonding environment surrounding the tetrahedral sheet. 2.2. Characterization of supported Rh0 -nanoparticles Evidence for the formation of Rh0 -nanoparticles was obtained from powder XRD analysis (Fig. 2), wherein peak centred at 2 = 40.9 and 47.7◦ can be assigned to the (1 0 0) and (1 1 1) reflections from face centred cubic (fcc) Rh0 nanoclusters respectively, which are consistent with the standard rhodium metal data [48]. The size of Rh nanoparticles calculated for the (1 1 1) peak using Scherrer equation was found 5.9 nm. The binding energies of the supported Rh0 -nanoparticles shown in the XPS spectrum peaks (Fig. 3) at 306.7 and 311.4 eV, are assigned to Rh0 3 d5/2 and 3 d3/2 electronic state which are slightly higher by 0.3 and 0.4 respectively than those of the free rhodium metal [49]. These higher values may be attributed to the interaction of Rh0 -nanoparticles with the framework oxygen of the clay matrix, which is expected to induce a positive charge on the metal surface and thereby increasing the binding energies of Rh0 -nanoparticles [10,50].

3d5/2

3d3/2

Intensity

70000

Rh(0) 65000

60000 305

310

315

Binding Energy (eV) Fig. 1. N2 adsorption–desorption isotherm and BJH pore size distribution curve of AT-Mont.

Fig. 3. XPS pattern of Rh0 -nanoparticles supported on AT-Mont.

320

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Fig. 4. EDX patterns of AT-Mont (a) and Rh0 -AT-Mont (b).

The energy dispersive X-ray spectroscopy (EDX) [corresponding SEM images are given in the supplementary information] patterns of AT-Mont and Rh0 -AT-Mont (Fig. 4) indicate the presence of only alumino silicates in acid activated Montmorillonite clay (Fig. 4a) and Rh metal on the clay surface of the nanocomposite (Fig. 4b). The morphological investigation of the supported Rh0 nanoparticles by transmission electron microscopy (TEM) analysis (Fig. 5) reveals that the average sizes of the Rh0 -nanoparticles were below 5 nm, which is in good agreement with value calculated from Scherrer equation. The particle size histogram (Fig. 5(inset)) shows a narrow size distribution range 2–5 nm of the synthesized nanoparticles. High resolution transmission electron microscopy (HRTEM) image of a single Rh0 -nanoparticle (Fig. 6) shows the reticular lattice planes inside the nanoparticles. The lattice planes continuously extended throughout the whole particles without stacking faults or twins, indicating the single crystalline nature. The measured inter-planar lattice fringe spacing is about 0.21 nm, which corresponds to the (1 1 1) plane of fcc Rh0 crystals. The corresponding selected area electron diffraction (SAED) pattern of Rh0 -nanoparticles was obtained by focusing the electron beam on the nanoparticle lying on the TEM grid (Fig. 6(inset)). The formation of hexagonal symmetrical diffraction spot patterns indicates that the generated Rh0 -nanoparticles are mono-crystalline nature. The specific surface area measurement of the Rh0 -nanoparticles supported on AT-Mont by N2 -adsorption (Fig. 7) reveals that the value significantly decreases from 417 to 376 m2 /g, which indicates the generation of the Rh0 -nanoparticles inside the pores on the clay matrix, resulting in decrease of surface area. The total pore volume

Fig. 5. TEM images of Rh0 -nanoparticles and particle size histograms with a Gaussian curve fitting (inset) supported on AT-Mont.

Fig. 6. HRTEM images of Rh0 -nanoparticles supported on activated clay.

of the nanocomposite decreases to 0.43 cm3 /g from 0.57 cm3 /g (ATMont) after metal nanoparticles generation and the BJH pore size distribution shows shifting of pore diameter towards lower value compared to the support, which indicate that the nanoparticles are generated inside the pores resulting in decrease of pore volume and pore sizes. The Rh content in the Rh0 -AT-Mont was estimated using ICP-AES, which shows the presence of 0.047 mol of Rh per 100 g of the nanocomposite.

Fig. 7. N2 -adsorption isotherm and BJH pore size distribution curve of Rh0 nanoparticles supported on activated clay.

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Scheme 1. Transfer hydrogenation of aromatic carbonyl compounds to the corresponding alcohols catalyzed by Rh0 -nanoparticles supported on AT-Mont.

2.3. Catalytic transfer hydrogenation The synthesized Rh0 -nanoparticles were evaluated as a heterogeneous catalyst in transfer hydrogenation of some substituted aromatic carbonyl compounds to corresponding alcohols (Scheme 1). The yields (Table 1) of conversion of the carbonyl compounds catalyzed by Rh0 -nanoparticles are found up to 100% with 100% selectivity. However, a control reaction with AT-Mont only (in the absence of supported Rh) did not form any product, which indicates that acid activated clay has no catalytic effect. To investigate the effect of base on catalysis, reactions were carried out without addition of any base. Although reactions were progressing, very low rate of conversion was observed. Thus, the reaction rate increases upon addition of a base. However, no products were observed in presence of NaOH without addition of catalyst. Probably, the reaction proceeds by adsorption of H-donor and H-acceptor on the

surface of the Rh0 -nanoparticles in close proximity which, facilitate the hydrogen transfer to the carbonyl group along with generation of one molecule of acetone [39]. The presence of base further facilitates the adsorption of isopropoxide ion on the nanoparticles surface [33]. A large number of homogeneous catalysts have been reported in transfer hydrogenation reactions with very high turnover frequency and selectivity [34,35]. However, only a few heterogeneous catalysts are found for such reaction. Kantam et al. [36] have reported an efficient Ru based heterogeneous catalytic system with large number of substrates, however, a long reaction time from 20 to 48 h are required. Subramanian et al. [38] have reported a Cu based catalyst supported on zeolite framework with high conversions rate, however, the high conversions were found only when the reaction was carried out in closed vessel. Literature survey reveals that, for the first time, we have reported Rh metal based heterogeneous catalyst in transfer hydrogenation reaction with high conversion rate and high stability compared to the existing reports. The reduction of carbonyl group is greatly influenced by electronic property of the substituent’s as well as the steric effect. The increase in electrophilicity of the carbonyl group facilitates the reaction, as transfer hydrogenation proceeds through hydride ion transfer. But, a reverse trend is observed in entry 1, 2 and 3, which may be due to the steric hindrance of the bulkier group present near the carbonyl group in entry 2 and 3. In case of para substituted acetophenone, presence of electron withdrawing group increases the electrophilicity and hence shows better conversion than the

Table 1 Results of the transfer hydrogenation reaction of aromatic carbonyl compounds to the corresponding alcohols catalyzed by Rh0 -nanoparticles supported on AT-Mont. Entry

Substratea

Product

O

Yield (%)

Selectivity (%)

H

H

100 (1st run)b

53.2 100

2 98 (2nd run)b 94 (3rd run)b 94 (4th run)b

CH3

CH3

3

O CH3 HO

CH3

CH3

CH3

6

94b

100

16.7

6

91c

100

16.1

12

83c

100

7.4

12

56c

100

4.9

H3C O

OH CH3

H3CO

c

52.7

OH

H3C

a

100

HO

O

b

99b

OH

4

6

2

OH

O

5

52.1 50.0 50.0

OH

O 2

TOF (h−1 )

OH H

1

Reaction time (h)

Initial substrate concentration: 1 mmol. GC yield. Isolated yield.

CH3 H3CO

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catalyst after first run may be due to the destruction of some of the nanopores of the support¸ but during subsequent run, it shows almost steady value indicating high stability of the catalyst. The BJH pore size distribution curve (presented in the supplementary information) of the recovered catalyst shows a slight broadening of the distribution pattern compared to fresh catalyst indicating breakdown of some pore walls and forming larger pores. The Rh content in the recovered catalyst after the fourth run was estimated using ICP-AES, which showed the presence of 0.046 mol per 100 g of the catalyst, which indicates that only very small amounts of Rh metal (∼2%) are leaching out from the clay matrix after fourth run.

3. Experimental 3.1. Materials and methods Fig. 8. Conversion of acetophenone to 1-phenylethanol versus time catalyzed by fresh and recovered catalyst.

1000

Intensity (counts/s)

40.94 750

500

47.8 69.8 250

20

30

40

50

60

70

80

Two theta (Degree) Fig. 9. The powder XRD pattern of recovered catalyst (Rh0 -nanoparticles supported on activated clay).

electron donation group. The electron withdrawing group at the para position enhanced the reaction up to 91% (TOF: 16.1, entry 4, Table 1). On the other hand, electron donors at the para position showed as low as 56% (TOF: 4.9, entry 6, Table 1). In order to investigate the recyclability, the catalyst was reused up to fourth run in the conversion of benzaldehyde to benzyl alcohol (Table 1, entry 1). The used catalyst was recovered by simple filtration and dried under vacuum before using in the next run. The reactions were carried out by maintaining the stoichiometry of the reactants and recovered catalyst. Results show that the catalyst remains active for several runs without significant loss in efficiency. The conversion of acetophenone to 1-phenylethanol catalyzed by fresh catalyst as well as recovered catalyst after first set of reactions are shown in Fig. 8. The recovered catalyst shows almost the same trend of conversion compared to that of the fresh catalyst, indicating that the recovered catalyst remains active after reaction. The morphology of the recovered catalyst was further investigated through TEM, powder XRD and N2 -adsorption analysis. TEM image shows that the Rh0 -nanoparticles were still inside the clay matrix retaining its sizes as that of the fresh catalyst, which indicates no significant morphological changes occurred after the reaction (TEM images are presented in the supplementary information). Further, the recovered catalyst showed almost the same powder XRD pattern like the fresh catalyst (Fig. 9). The specific surface area of the recovered catalysts decrease from the fresh catalyst 417 m2 /g to 356, 341 and 339 for second, third and fourth run, respectively. The initial decrease of the surface area of the

Bentonite (procured from Gujarat, India) is clay, rich in Montmorillonite clay mineral along with other accessory materials like quartz, silt, etc. and was purified by sedimentation technique to collect the <2 ␮m fraction of pure Montmorillonite clay before use. The ˚ The basal spacing (d0 0 1 ) of the air dried samples was about 12.5 A. specific surface area determined by N2 adsorption was 101 m2 /g. The analytical oxide composition of the Bentonite determined was SiO2 : 49.42%; Al2 O3 : 20.02%; Fe2 O3 : 7.49%; MgO: 2.82%; CaO: 0.69%; loss on ignition (LOI): 17.51%; and others (Na2 O, K2 O and TiO2 ): 2.05%. Mont was converted to the homoionic Na-exchanged form by stirring in 2 M NaCl solution for about 48 h, washed and dialyzed using deionized distilled water until the conductivity of the water approached that of distilled water. CEC was 127 meq per 100 g of clay (sample dried at 120 ◦ C). RhCl3 , ethylene glycol, isopropanol and substrates were purchased from Sigma–Aldrich, USA. All reagents were used as supplied. IR spectra (4000–400 cm−1 ) were recorded on KBr discs in a Shimadzu IR Affinity-1 spectrophotometer. Powder XRD spectra were recorded on a Rigaku Ultima-IV from 2 to 80◦ 2 using CuK␣ source ˚ Specific surface area, pore volume, average pore diam( = 1.54 A). eter were measured with the Autosorb-1 (Quantachrome, USA). Specific surface area of the samples was measured by adsorption of nitrogen gas at 77 K and applying the Brunauer–Emmett–Teller (BET) calculation. Prior to experiment, the samples were degassed at 250 ◦ C for 3 h. Pore size distributions were derived from desorption isotherms using the Barrett–Joyner–Halenda (BJH) method [39,41]. The 1 H NMR spectra were recorded at room temperature in CDCl3 solution on a Bruker DPX-300 spectrometer and chemical shifts were reported relative to SiMe4 . Mass spectra were recorded on ESQUIRE 3000 Mass spectrometer. SEM images and EDX patterns were obtained by JEOL JSM-6390 LV operated at 15 kV on a gold coated sample. TEM and HR-TEM images were recorded on a JEOL JEM-2011 electron microscope and the specimens were prepared by dispersing powdered samples in isopropyl alcohol, placing them on a carbon coated copper grid and allowing to dry. X-ray Photoelectron Spectra were recorded on Kratos ESCA model Axis 165 spectrophotometer having a position sensitive detector and hemispherical energy analyzer in an ion pumped chamber.

3.2. Support preparation Purified Mont (10 g) was dispersed in 200 mL 4 M sulfuric acid and refluxed (around 100 ◦ C) for 1 h. After cooling, the supernatant liquid was discarded and the acid activated Mont was repeatedly redispersed in deionized water until no SO4 2− ion could be detected by the BaCl2 test. The modified Mont was recovered, dried in air at 50 ± 5 ◦ C overnight to obtain the solid product. The acid activated Mont was designated as AT-Mont.

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3.3. Rh0 -nanoparticles preparation

References

1 g AT-Mont was impregnated by 20 mL aqueous solution of RhCl3 (0.50 mmol) under vigorous stirring condition. Stirring was continued for 12 h followed by evaporation to dryness in a rotavapour. 0.5 g of the dry composite was dispersed in 20 mL ethylene glycol solution and refluxed at 197 ◦ C for 5–6 h. The colour of the solid changed from reddish brown to black. The solid was allowed to settle and washed with distilled water several times. Finally the solution was filtered through a sintered crucible and washed with methanol and then with distilled water. The composite was collected and dried at 60 ◦ C for 12 h and stored in airtight bottle. The sample thus prepared was designated as Rh0 -AT-Mont.

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3.4. Catalytic transfer hydrogenation reaction 1 mmol of the reactant was dissolved in 20 mL isopropanol followed by addition of 20 mg of Rh0 -AT-Mont and 4 mg NaOH (0.1 mmol) into a 100 mL round bottom flask. The reaction mixture (susceptible to oxidation in air) was refluxed at 82 ◦ C under nitrogen atmosphere for a period of 2–12 h depending upon the nature of the substrates. To investigate the recyclability, the used catalyst was filtered after the reaction and washed with methanol followed by water and finally vacuum-dried at 110 ◦ C before the next reaction run. The product mass along with the reactant was separated from the reaction mixture by evaporating the solvent in a rotavapor. The mixture of solvents (acetone and isopropanol) thus obtained was fractionally distilled to get the pure solvents. The yields of the products were calculated by separating the individual components by column chromatography and the products were identified using mass spectrometry and 1 H NMR spectroscopy. 4. Conclusion Rh0 -nanoparticles below 5 nm size were generated into nanopores of modified Montmorillonite clay by incipient wetness impregnation of RhCl3 , followed by reduction with ethylene glycol. Nanopores in the modified Montmorillonite clay were created by acid activation under controlled condition. Electron microscopy as well as other analytical techniques confirm the formation of Rh0 -nanoparticles. The synthesized Rh0 -nanoparticles show high activity and selectivity in the transfer hydrogenation of some important aromatic carbonyl compounds to the corresponding alcohols. The catalyst is also found to remain active for several runs without significant loss in activity. Acknowledgements The authors are grateful to Dr. R.C. Boruah, Acting Director, CSIRNorth East Institute of Science and Technology, Jorhat, Assam, India, for his kind permission to publish the work. The authors thank Dr. P. Sengupta, Head, Materials Science Division, CSIR-NEIST, Jorhat, for his constant encouragement. Thanks are also given to CSIR, New Delhi for a financial support (XII FYP Project: MLP-6000/01, BSC-0112, CSC-0125 and CSC-0135). Thanks are also due to Dr. S.K. Dolui, Professor, Tezpur University, Assam for arranging the SEMEDX experiment. The author PPS is grateful to CSIR for providing the Senior Research Fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.10.049.