Reusable Catalyst for Transfer Hydrogenation of Aldehydes and

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Reusable Catalyst for Transfer Hydrogenation of Aldehydes and Ketones Designed by Anchoring Palladium as NanoParticles on Graphene Oxide Functionalized with Selenated Amine Renu Bhaskar, Hemant Joshi, Alpesh Kumar Sharma, and Ajai K. Singh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10457 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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ACS Applied Materials & Interfaces

Reusable Catalyst for Transfer Hydrogenation of Aldehydes and Ketones Designed by Anchoring Palladium as Nano-Particles on Graphene Oxide Functionalized with Selenated Amine Renu Bhaskar, Hemant Joshi, Alpesh K. Sharma and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi – 110016, India

ABSTRACT The

treatment

of

graphene

oxide

with

ClCH2COOH,

thionyl

chloride

and

2-

(phenylselenyl)ethylamine successively has resulted in functionalization of its surface with selenated ethylamine molecules which may act as chelating (Se, N) ligands. The graphene oxide grafted with (Se, N) donor sites (GO-Se) on treatment with Na2PdCl4 and NaOH gave GO-Se anchored with Pd(0) nano-particles (NPs) (GO-Se-Pd). The X-ray diffraction(powder), FT-IR, XPS, Raman spectroscopy, thermogravimetric analysis (TGA), electron microscopic techniques (SEM and HR-TEM) authenticated the formation of GO-Se-Pd. The distribution of Pd(0) NPs of size ~1-3 nm on GO-Se was found nearly uniform. The transfer hydrogenation of carbonyl compounds (aldehydes / ketones) with 2-propanol was catalyzed with GO-Se-Pd. The catalyst equivalent to 0.25 mol% of Pd was sufficient to convert aldehydes and ketones to alcohols in good yield (nearly quantitative for some substrates) and found somewhat more efficient for aldehydes than ketones. The reusability of GO-Se-Pd studied for transfer hydrogenation of 4anisaldehyde to corresponding alcohol can be understood by ~96% conversion even in the sixth catalytic run. Flame AAS analysis of GO-Se-Pd revealed negligible leaching of Pd even after sixth catalytic reaction cycle. Hot filtration experiment suggested the heterogeneous nature of catalyst.

KEYWORDS: Palladium, Nanoparticle, Graphene oxide, Transfer Hydrogenation, Aldehyde, Ketone

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INTRODUCTION The catalytic activity of metal nanoparticles (NPs) is high and consequently they are required in low concentration, which makes them important in catalysis.1,2 High conversion rate with selectivity3 under mild reaction conditions in conjunction with recyclability can make them further attractive. Palladium containing species are important in catalysis and therefore among the NPs of various metals, palladium NPs are of special interest as catalyst.4 The higher catalytic activity of Pd in nano-formulation than those of other forms is due to increased fraction of Pd as surface atoms.5 Thus by maximizing population of small NPs of uniform size and bringing undesirable agglomeration to minimum the catalytic activity of NPs may be enhanced. To reduce agglomeration, Pd NPs may be stabilized with ligand but metal leaching problem6 makes such protected NPs less attractive as a catalyst. The immobilization of the NPs on an appropriate inert support is envisaged better to control agglomeration. The right support material is essential to ensure stabilization and uniform distribution of NPs. A variety of materials investigated as a support, include oxide of metals,7,9 silica,8 double hydroxides of layered structure10 and graphene/graphene oxide.11 The unique electronic structure of graphene makes it mechanically strong and chemically stable in most reaction media.12 It has a place among outstanding support materials for metal NPs, to be used as a catalyst. The NPs anchored on it have enhanced catalytic activity13 contributed by low agglomeration, high dispersibility and large surface area. Graphene oxide (GO) is oxidized graphene obtained by the chemical oxidation of graphite.14 Its structure may be described as a graphene sheet which has epoxy and hydroxyl groups at the basal planes and carboxylic groups at the edges. The carbon having sp3 hybridization disrupts the graphite type structure (sp2 carbon based) of graphene15 causing defects. The functional groups and the presence of the defects make grafting on graphene oxide of various moieties including metal NPs easier. The materials of enormous potential have been prepared by grafting on GO.16 Graphene/GO anchored with palladium NPs is reported efficient to catalyze several reactions viz. C-C cross coupling,17 ammonia borane dehydrogenation,18 reduction of nitro compounds19 and CO oxidation.20 Recently we have prepared graphene oxide grafted with Pd17Se15 NPs and found that the composite is efficient in catalyzing C-O coupling resulting ethers at room temperature.21 Palladium NPs of high activity with good recyclability are of current interest. The well dispersed Pd NPs of small size bound on a solid support strongly, may be a recyclable 2


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catalyst of high activity. It was, therefore, thought worthwhile to generate combination of chelating donor groups on GO, as it would reduce agglomeration and leaching problems (due to strong binding with Pd NPs). This would also result in good recyclability of the catalyst. A pair of Se and N donor groups, i.e. a combination of soft and hard donor sites capable to chelate, is worth exploring. Thus selenated amine grafted graphene oxide (GO-Se) decorated with Pd NPs is reported herein. It is found efficient to catalyze transfer hydrogenation (TH) of carbonyl compounds (aldehydes /ketones), as catalyst loading required is low (0.25 mol% of Pd). Most probably palladium NPs immobilized on GO functionalized with selenated ethyl amine and their use in TH of carbonyl compounds using propan-2-ol as a hydrogen source are reported for first time, as no other similar report is in our knowledge. TH method is free from hazardous pressurized H2 gas and sophisticated experimental setup associated with it. It is based only on a hydrogen source, 2-propanol and a catalyst.22 In addition to 2-propanol, glycerol, cyclopentanol and formic acid, are promising hydrogen donors22 for TH. Such hydrogen donors are easily available, economical, and easy to handle. The popular catalysts reported for TH are based on Ru(II), Rh(III), Ir(III) and Ir(I).22 Palladium is scantily reported for TH of carbonyl compounds, as only few examples are in our knowledge.23,24 However, for hydrogenation of C=C or C≡C bond it is found promising.22 Therefore investigations on TH of carbonyl compounds catalyzed with a very stable GO-Se-Pd may be rewarding, as these compounds and their reduction to corresponding alcohols is considered as an essential transformation in agrochemical, pharmaceutical and perfume industry.25 Kim and coworkers have reported ruthenium NPs decorated on graphene nano-sheets for transfer hydrogenation of ketones.26 RESULTS AND DISCUSSION To prepare grapheme oxide, graphite was oxidized with KMnO4 and H2SO4 following Hummer’s method14 with some modifications. On both sides of a GO sheet oxygen containing functional groups are present. Their easy accessibility is responsible for its potential as a support material for designing various catalysts. The epoxy and hydroxyl groups of GO were treated with chloroacetic acid, so that on its surface, carboxylic groups were present mainly. The GO functionalized with COOH was further treated with thionyl chloride. It resulted in a GO sheet having COCl groups required to bind selenated ethyl amine on its surface. Further treatment of this modified GO sheet with PhSeCH2CH2NH2 resulted in GO-Se (i.e. GO grafted with selenated 3



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ethyl amine on to its surface). The GO-Se having (Se, N) chelating groups on its surface was treated with Na2PdCl4 and a base to obtain GO-Se-Pd. The total process of synthesis of graphene oxide functionalized with selenated ethyl amine and its decoration with palladium(0) NPs is depicted in Scheme 1.

Scheme 1 Designing of Pd(0) nano-particle (NP) anchored GO-Se (GO-Se-Pd)

Characterization of GO, GO-COOH, GO-COCl, GO-Se and GO-Se-Pd. The GO was characterized by HR-TEM, FT-IR, Raman, TGA, solid-state

13

C CP-MAS

and PXRD. The HR-TEM image of GO is shown is Figure 1a. The peak centered at 2θ = 11.8° reported in X-ray diffraction pattern of powdered GO (SI: Figure S1) is for [001] plane having interlayer spacing of 7.46 Ǻ.27 In Figure 1(b) HR-TEM image of GO-Se is shown and TEM images of GO-COOH, GO-COCl are shown in Figure S8 (SI). These images support that there is no anchoring of any particle on them 4


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Figure 1 HR-TEM images: (a) GO and (b) GO-Se The powder XRD, HR-TEM, SEM-EDX, FT-IR, thermogravimetric analysis (TGA), Raman spectroscopy and X- ray photoelectron spectroscopy (XPS) authenticated the formation of GOSe-Pd including the presence of grafted Pd NPs. The powder X-ray diffraction pattern of the GOSe-Pd is shown in Figure 2. The peak of GO centered at 2θ = 11.8° has disappeared. The peaks at (h, k, l) 111, 200, and 220 correspond to Pd(0) NPs (JCPDS 87-0643). The peaks at 2θ = 22, 32 and 42 are due to Se and in agreement with literature reports.28

Figure 2 PXRD of GO-Se-Pd In HR-TEM images of GO-Se-Pd, NPs of Pd appear as dark spherical dots of ~1-3 nm size decorated uniformly on the support (Figure 3). Neither aggregation of Pd NPs nor a large vacant spot on the surface of GO-Se-Pd was revealed by the HR-TEM image. The size distribution of Pd NPs present in GO-Se-Pd is shown in Figure 4.

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Figure 3 HR-TEM of GO-Se-Pd at (a-c) 50 nm (d) 20 nm.

Figure 4 Size variation of Pd NPs present in GO-Se-Pd

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The morphology of GO-COOH, GO-COCl, GO-Se and GO-Se-Pd investigated by SEM is shown in Figures S6 and S7 (SI). In SEM-EDX (SI: Figure S2) of GO-COOH the peaks of only carbon and oxygen are present, whereas chlorine, carbon and oxygen peaks (SI: Figure S3) are present in SEM-EDX of GO-COCl. The presence of selenium in GO-Se is shown by TEM-EDX pattern (SI: Figure S4), authenticating the formation of GO-Se. The TEM-EDX of GO-Se-Pd indicates the presence of Pd and Se both (SI: Figure S5) in this material. All these results support the sequence of reactions shown in Scheme 1. The formation of graphene oxide from graphite flakes and its conversion to GO-COOH is supported by solid state

13

C CP-MAS spectrum. The disordered structure of GO has made the

signals broad. The spectrum of graphene oxide exhibits three broad resonances at 61, 71 and 130 ppm (SI: Figure S46).29. The first two signals at 61 and 71 ppm originate from epoxide and COH groups respectively, which generally align perpendicular to basal-plane of carbon atoms. The signal at 130 ppm may be assigned to graphitic sp2 carbon atoms.30 In solid state 13C CP-MAS spectrum of GO-COOH, appearance of small signal around 20 ppm indicates the presence of alkyl carbon i.e. of methylene.31

Figure 5 FT-IR: (a) GO and (b) GO-Se-Pd For pure GO and GO-Se-Pd, FT-IR spectra are shown in Figure 5. The spectrum of pure graphene oxide has bands of various functional groups. The band around 3430 cm-1 is broad and 7



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may be ascribed to O-H. The intercalated water shows O-H stretching at 3368 cm-1. The band due to stretching vibrations32 of both –CH2 and C-(CH3) (caused by defects in graphene) appears around 2359 cm-1. The C-O (epoxy) and C=O (carboxylic) stretching vibrations appear at 1224 and 1725 cm-1 respectively. These bands authenticate the formation of GO as reported earlier.33 The FT-IR spectrum of GO-COOH is given in Figure S9 (SI). The decrease in C-O band intensity of epoxy group, rise in intensity of band around 1725 cm-1 compared to that of graphene oxide and emergence of a new band at ~2920 cm-1 corresponding to -CH2 group, support the carboxylation of GO.34 Moreover, the band due to OH became intense due to carboxylation of GO. The FT-IR spectrum of GO-COCl (SI: Figure S10) has a band of oxygen containing functional group around 1725 cm-1 but its intensity relative to corresponding band of GO-COOH is reduced due to partial conversion of –COOH to –COCl. The C-Cl stretching in FT-IR spectrum of GO-COCl appears at ~620 cm-1. All these results authenticate successful functionalization of GO as shown in Scheme 1. The FT-IR spectrum of GO-Se is shown in Figure S11 (SI). The broad band at ~3400 cm-1 corresponds to N-H stretching of amide bond. The band at ~1130 cm-1 can be ascribed to C=N stretching. The bands noticed at 1561 and 1174 cm-1 are due to N-H bending and C-N stretching of amide (C(O)NH-) group present in GO-Se-Pd respectively (Figure 5b). The intensity of band at 3430 cm-1 (O-H) is significantly reduced on formation of GO-Se-Pd as expected. Raman spectroscopy is useful for the structural and electronic characterization of graphene/GO based composites as it provides information on defects (D band) and in plane vibrations of sp2 carbon atoms bonded in a two dimensional hexagonal lattice (G Band). In Figure 6 Raman spectra of pure GO and GO-Se-Pd are shown. In GO two prominent bands at 1363 cm-1 and 1600 cm-1 are due to disorder induced D band and in phase vibration of the graphene lattice (G Band) respectively. In Raman spectrum of GO-Se-Pd these bands show red shift and appear at 1357 and 1589 cm-1 respectively. This may be attributed to the damage to extended conjugation of carboncarbon bonds during chemical treatment of GO. The intensity ratio of the D- and G-band, I(D)/I(G) is a measure of the quality of graphitic structure and approaches zero in highly ordered pyrolyzed graphite.35 In Raman spectrum of GO the value of this ratio is ~1.06, due to increase in the level of disorder on oxidation of graphite. The intensity ratio becomes 1.24 in the case of GO-Se-Pd, indicating the abundance of surface defects and disorders in graphene sheets caused

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due to the presence of palladium nanoparticles. The structural defects are believed to enhance the catalytic activity,36 although they are undesirable for electronic applications.

Figure 6 Raman spectra: (a) GO (b) GO-Se-Pd. The TGA curves of GO and GO-Se-Pd are shown in Figure 7. The GO shows significant weight loss before 300 0C. The weight loss around 100 0C is due to loss of intercalated water in lattice of GO. The high weight loss around 200 0C may be attributed to CO, CO2 and water37 resulting from oxygen (labile) containing functional groups. In contrast GO-Se-Pd shows up to 190 0C small weight loss of the order of 4% due to loss of adsorbed water. The weight loss below 200 °C shown by GO-Se-Pd, is much lower than that of GO, indicating that oxygen-containing functional groups vulnerable to thermal decomposition present on GO are converted to thermally more stable species. The weight loss is significant (~12 %) after 200 0C and appears to result from pyrolysis of oxygen containing functional groups. The further weight loss around 400 0C is due to pyrolysis of carbon skeleton.38 Up to 800 0C overall weight loss shown by GO-Se-Pd is ~30%. On the basis of TGA results GO-Se-Pd appears to be thermally more stable than GO. Thus its thermal stability at 80 0C, optimum temperature for transfer hydrogenation reaction catalyzed with GO-Se-Pd is assured.

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Figure 7 TGA: (a) GO (b) GO-Se-Pd The oxidation state of palladium in GO-Se-Pd was ascertained with XPS shown in Figure 8. The two intense 3d peaks of Pd are due to spin-orbit splitting which results in 3d5/2 and 3d3/2 states. The binding energies of 3d5/2 and 3d3/2 states of Pd were found 335.8 and 341.2 eV respectively, consistent with zero oxidation state of Pd.38 The binding energies of 3d5/2 and 3d3/2 states of Pd(0) reported in the literature39-41 follow the order 3d5/2 < 3d3/2 and are dependent on size in the case of NPs.42 Our values are close to literature report. However, this order in a recent work Pd(0) immobilized in resin is shown opposite erroneously43

Figure 8 X-ray photoelectron spectra for GO-Se-Pd 10


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Transfer Hydrogenation of Carbonyl Compounds Catalyzed with GO-Se-Pd The catalytic potential of GO-Se-Pd was explored for transfer hydrogenation (TH). The reaction conditions were optimized using 4-anisaldehyde as a substrate. As shown in Table 1, the presence of base is essential as its absence gives very poor conversion (Table 1, Entry 9). The best base for the reaction was KOH as reported earlier.44,45 The 80 0C temperature was optimum for the reaction as the conversion was maximum at this temperature. At room temperature the conversion was poor (Table 1, Entry 3). The optimized amount of catalyst needed for maximum conversion was 10 mg (0.25 mol % of Pd). The 5 mg of catalyst gave relatively lower conversion (Table 1, Entry 6, 10). The GO and GO-Se were tested for catalytic activity (if any) for TH but they gave very poor conversion compared to that of GO-Se-Pd (Table 1, Entries 1 and 2). The catalytic activity of Na2PdCl4 was tested for TH of 4-anisaldehyde using standardized conditions, but conversion to the desired product was poor (Table 1, Entry 11). Time profile for catalytic TH of 4-anisaldehyde is shown in Figure 9. The applicability of GO-Se-Pd as catalyst was studied for various substrates (Table 2). The electron rich (Table 2, Entry 2e) and deficient aldehydes, both (Table 2, Entry 2f) gave corresponding alcohols in good yield. Moreover, substitution at ortho, meta and para position (Table 2, Entries 2g and 2j) of the aromatic ring of substrate did not affect the efficiency of the catalyst much. The heteroaromatic aldehydes also showed good conversion (Table 2, Entries 2i and 2k). Halogens present on benzaldehyde (Table 2, Entries 2b2d and 2j) do not affect the TH displaying chemoselectivity. The catalyst was somewhat less efficient for ketones compared to aldehydes (Table 2, Entries 2l-2o). It was also efficient for aliphatic carbonyl compounds (Table 2, Entries 2l, 2p and 2q). The earlier reported catalysts, having Pd grafted on solid support for transfer hydrogenation, are required in large amount (optimum loading 5-10 mol % of Pd). The required reaction time for good conversion is also of the order 12-48 h.46,24 On comparing the reaction conditions of these catalysts with those of GOSe-Pd, the present one emerges more efficient in terms of both, catalyst loading and reaction time. Compared to commercially available 10 wt% Pd/C,23 GO-Se-Pd performs better for TH, as optimum catalyst amount and reaction time both are relatively low for it. The conversion profiles for TH of carbonyl compounds are shown in Figure 10.

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Table 1. Standardization of Reaction Conditions for Catalytic Transfer Hydrogenationa

Entry Catalyst

Base

Catalyst

Temp. ( 0C)

(0.2 mmol) (mg)

Time

Yield

(h)

(%)b

1

GO

KOH

10

80

3

18

2

GO-Se

KOH

10

80

3

21

3

GO-Se-Pd

KOH

10

RT

3

17

4

GO-Se-Pd

KOH

10

80

1

72

5

GO-Se-Pd

KOH

10

80

2

87

6

GO-Se-Pd

KOH

10

80

3

99

7

GO-Se-Pd

KOH

10

80

5

99

8

GO-Se-Pd

KOH

10

80

9

99

9

GO-Se-Pd

None

10

80

3

nd c

10

GO-Se-Pd

KOH

5

80

3

62

11

Na2PdCl4

KOH

10

80

6

14

a

Reaction conditions: 4-Anisaldehyde, 1 mmol; catalyst, 10 mg (0.25 mol %); 2propanol, 5 mL; time, 3 h. b1H NMR % conversions, cnot determined.

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Figure 9 Time profile of catalytic transfer hydrogenation of 4-anisaldehyde (1 mmol): KOH, 0.2 mmol; 2-propanol, 5 mL; bath temperature, 80 °C; conversion monitored with 1H NMR. Table 2. Products with Yield of Catalytic TH of Aldehydes/Ketones in 2- Propanola

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a

Reaction Conditions : Catalyst, 10 mg; aldehyde/ketone, 1 mmol; KOH, 0.2 mmol; 2-propanol, 5 mL; bath temperature, 80 °C; time, 3h (aldehydes) / 12h (ketones). Isolated Yield in %.

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Figure 10: Conversion profiles for TH of carbonyl compounds The reusability of a catalyst considered important was explored for GO-Se-Pd. For this purpose TH of 4-anisaldehyde was carried out under optimized conditions. After carrying out the catalytic TH, the catalyst was centrifuged, washed with absolute ethanol (2-3 times) and dried in vacuo. It was reused for another run and found not to lose efficiency even after 6 runs. The results are given in Table 3. Table 3. Recycling of GO-Se-Pd

Runa

1

Conversion% 99

2

3

4

5

6

97

97

96

96

96

a

Reaction Conditions: Catalyst, 10 mg; 4-Anisaldehyde, 1 mmol; KOH, 0.2 mmol; 2-propanol, 5 mL; bath temperature 80 °C, 1H NMR % conversions.

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Figure 11 HR-TEM images of Go-Se-Pd after the 6th run at (a) 100 nm (b,d) 50 nm (c) 20 nm The good recyclability gives GO-Se-Pd a significant edge over other immobilized / protected Pd NPs.23,24,46 The HR-TEM (Figure 11) of recovered catalyst after the sixth run indicates high dispersibilty of palladium NPs without detectable agglomeration. The palladium content of GOSe-Pd was studied using flame AAS before and after the six reaction cycles. It was 0.25% in the fresh sample and 0.23% after six catalytic cycles. The change in amount of Pd present in the catalyst is insignificant and rules out leaching of the metal. These results explain the good recyclability of GO-Se-Pd and the insignificant leaching of Pd is probably due to its strong binding with Se. Hot filtration test was carried out. The catalytic reaction mixture was filtered after 1 h when conversion was ~ 72%. The hot filtrate was transferred to another flask and heated further. The conversion monitored with 1H NMR was almost static at pre-filtration value (72%) even after carrying out reaction for another 2 h after filtration. This supports the heterogeneous nature of the present catalytic transfer hydrogenation.

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The possible mechanistic pathway for the present catalytic TH is shown in Scheme 2. The base promotes oxidative addition of 2-propanol on palladium(0) NPs. The carbonyl group of ketone or aldehyde coordinates weakly with Pd. Thereafter release of acetone molecule and formation of C-OH bond47 resulting alcohol occurs. The acetone signal was noticed in the 1H NMR spectrum of the reaction mixture. Thus Pd on surface becomes available again for a new substrate.

Scheme 2 Proposed reaction mechanism

CONCLUSION In summary, selenated ethylamine grafted graphene oxide (GO-Se) has been synthesized for the first time. This on treatment with Na2PdCl4 and NaOH gave Pd(0) nano-particle (NP) anchored GO-Se-Pd. The powder X-ray diffraction (PXRD), FT-IR, XPS, Raman spectroscopy, TGA, SEM, and HR-TEM authenticated GO-Se-Pd. Its surface was found decorated uniformly with Pd(0) NPs (size ~1-3 nm). The GO-Se-Pd as a catalyst was found efficient for transfer hydrogenation of aldehydes/ketones, with 2-propanol (hydrogen source). The catalyst equivalent to 0.25 mol% of Pd was sufficient to convert aldehydes and ketones to their precursor alcohols in good to almost quantitative yield. The GO-Se-Pd as a catalyst was found somewhat more 17



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efficient for aldehydes than ketones. The distinct advantage of GO-Se-Pd as a catalyst is its reusability. The heterogeneous nature of catalysis is suggested by hot filtration. EXPERIMENTAL SECTION Chemicals and Reagents The Na2PdCl4, diphenyldiselenide, sodium borohydride, 2-chloroethylamine hydrochloride and NaOH were obtained from Sigma-Aldrich (USA) and graphite flakes, chloroacetic acid, thionyl chloride, KMnO4, NaNO3 and H2O2 from Fisher Scientific International, USA. 1H NMR spectroscopy was used to monitor progress of the catalytic reactions. The products obtained in transfer hydrogenation were identified by matching their NMR spectra with those reported in the literature. The solvents (AR grade) were dried using the standard procedures.48 Selenated ethyl amine was prepared by following the procedure reported in the literature.49 Physical Measurements NMR and IR (4000−400 cm-1) spectra were recorded as reported earlier.21,45 All reactions were carried out in a round bottom flask dried in an oven under ambient conditions. Nitrogen (purified by reported method21,45) atmosphere was created with Schlenk techniques. SEM, EDX, powder X-ray diffraction (PXRD), TEM and HR-TEM studies were made as reported earlier.21 AA700 series atomic absorption spectrometer (Lab India) was used to estimate palladium in NPs. Raman spectra were recorded with Renishaw (INVIA) confocal micro dispersive Raman spectrometer using 532 nm solid state laser in a regular mode. X-ray photoelectron spectroscopic (XPS) studies were carried out by the reported method.50 Thermogravimetric analyses (TGA) (temperature 40 0C to 900 0C; heating rate 10 0C min-1 and nitrogen flow rate 20 mL min-1) were carried out with a Perkin Elmer - S II pyris diamond thermal analyzer.

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C cross polarization

magic-angle spinning (CP-MAS) NMR spectra were recorded using a JEOL JNM-ECX400II spectrometer at a contact time of 6 ms. Synthesis of Graphene Oxide (GO): The graphite flakes were oxidized by modified Hummers’ method14 to prepare graphene oxide (GO). Sodium nitrate (0.75 g) and graphite flakes (1 g) were stirred for 1 h in a flask containing concentrated H2SO4 (34 mL, 98%), maintaining temperature of the mixture at ~5°C 18


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with an ice bath. The KMnO4 (4.5 g) was added (0.1 g at a time) maintaining temperature below 5°C (in ice bath). Further stirring of the mixture at 25 °C was carried out for five days. Thereafter it was heated for 2h at 900C with the addition of dilute H2SO4 (50 mL, 5 wt%) and constant stirring. The H2O2 (30 wt %; 2.7 mL) was mixed and the mixture stirred further for 2 h at room temperature. The bright yellow colored suspension was centrifuged for 15 min (6000 rpm) and washed successively with 25 mL of H2SO4 (3 wt%), 10 mL of H2O2 (0.5 wt%), and 400 mL of HCl (3 wt%). After final washing with 200 mL of deionized water for several times (till the filtrate became neutral) the precipitate was dried in vacuum desiccators for 5 days before further use.

Carboxylation of GO: Synthesis of GO-COOH The GO prepared as above has carboxyl, epoxy and hydroxyl groups. To facilitate chemical binding of selenated ethyl amine to GO by amide linkage COOH groups are required. The population of COOH was enhanced by reacting epoxy and hydroxyl groups with ClCH2COOH. For this purpose GO (200 mg) was dispersed in distilled water (~100 mL) by ultra-sonication for 1 h. The 1.0 g of ClCH2COOH and 1.2 g of NaOH were mixed with the slurry of GO. The suspension was further sonicated for 3 h at 25 °C. The resulting GO−COOH was made neutral with dilute HCl, washed with deionized water till HCl removed and dried in vacuo. Immobilization of Selenated Ethyl Amine on to GO-COOH (GO-Se) The GO−COOH (200 mg) was suspended in DMF (2 mL) by sonication for 1 h and reacted with thionyl chloride, taken in excess, for 12 h at 65 °C to convert it into acyl chloride derivative. The thionyl chloride left unreacted was removed by vacuum distillation. The resulting GO-COCl was washed with THF (3 x 30 mL) using centrifugation and dried in vacuo. Its 100 mg was taken in DMF and refluxed with 2-(phenylselanyl)ethylamine (100 mg) under nitrogen atmosphere. The GO anchored with selenated ethyl amine was centrifuged with ethanol (3–4 times) and dried in vacuo. Synthesis of GO-Se-Pd The GO-Se (50 mg) was suspended in deionised water (10 mL) by sonication for 1 h. Thereafter, solution of Na2PdCl4 (30 mg) made in water (10 mL) was mixed with constant 19



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stirring and the mixture further stirred for overnight. Its pH was brought to ~13 by adding 1M solution of sodium hydroxide made in water. The resulting mixture was subjected to further stirring for 5 h at 50°C, resulting in a precipitate of GO-Se-Pd, which was freed from alkali by washing it with deionised water. After further washing with ethanol it was dried in vacuo. Procedure for Catalysis of Transfer Hydrogenation Aldehyde / ketone (1 mmol), KOH (0.2 mmol), and catalyst (10 mg, 0.25 mol%) were refluxed in 5 mL of 2-propanol for 3 h. After cooling to 25 °C the mixture was filtered through a G-4 crucible. The residue on the crucible was washed with 5 mL of 2-propanol. The 50 mL of water was poured to the mixture of filtrate and washings. The extraction of the aqueous mixture with 50 mL of ethyl acetate was carried out. From the organic extract solvent was removed with a rotary evaporator. The product obtained as a residue (solid or oil) was purified chromatographically, using a silica gel column and mixture of hexane and ethyl acetate as an eluent. The 1H NMR and

13

C{1H} NMR spectra of the purified product were recorded (See

Supplementary information Figures S12-S45). Hot Filtration Test In round bottom flask dried in an oven, catalyst GO-Se-Pd (10 mg), 4-anisaldehyde (0.136 g, 1 mmol) and KOH (0.2 mmol) were heated at 80 °C in 2-propanol (5 mL) with stirring. After 1 h, the reaction was stopped and the catalyst centrifuged. The clear reaction mixture was divided in two equal parts. One part was taken in a flask. In the other part taken in a separate flask, NPs centrifuged earlier, were added. The reaction in both flasks was carried out for 5 h and conversion in each case estimated with 1H NMR spectroscopy. ASSOCIATED CONTENT Supporting Information PXRD of GO, SEM-EDX of GO-COOH, GO-COCl, TEM-EDX of GO-Se and GO-Se-Pd, SEM of GO-COOH, GO-COCl, GO-Se and GO-Se-Pd, TEM images of GO-COOH and GO-COCl, FT-IR spectra of GO-COOH, GO-COCl and GO-Se, 1H and 13C NMR spectra of products of TH reactions and 13C CP-MAS NMR spectrum of GO and GO-COOH. The material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author 20


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*A.K.S.: e-mail, [email protected], [email protected]; fax, +91 11 26581102; tel, +91 11 26591379. ORCID Ajai Kumar Singh 0000-0003-1650-6316 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors thank Council of Scientific and Industrial Research New Delhi, India (Grant no. 01(2784)14/EMR II 25.06.2014), Department of Atomic Energy (BRNS), India (Grant no. 37(2)/14/08/2015/BRNS/10288) and Nano-mission, Department of Science and Technology, India [SR/NM/NS-1246/2013(G)] for research projects. A. S. thanks CSIR of India for JRF/SRF, RB and HJ thank University Grants Commission (India) for JRF/SRF. The authors also thank Physics Department, IIT Delhi for XPS facility. REFERENCES 1. Wu, B.; Kuang, Y.; Zhang, X.; Chen, J. Noble Metal Nanoparticles/Carbon Nanotubes Nanohybrids: Synthesis and Applications. Nano Today 2011, 6, 75-90. 2. Cuenya, B. R. Synthesis and Catalytic Properties of Metal Nanoparticles: Size, Shape, Support, Composition and Oxidation State Effects. Thin Solid Films 2010, 518, 31273150 3.

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TOC

Reusable Cataly yst for Trransfer Hydrogenat H tion of Aldehydes A and Ketones Designeed by An nchoring Palladium m as Nano-Particless on Graaphene Oxide O Functioonalized wiith Selenatted Aminee Renu Bhaskar, Hemaant Joshi, Allpesh K. Shaarma and Ajaai K. Singh** mistry, Indiann Institute off Technologyy Delhi, New w Delhi – 110016, India Department of Chem

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Reusable Catalyst for Transfer Hydrogenation of Aldehydes and

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