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Graphene derivative in magnetically recoverable catalyst determines catalytic properties in transfer hydrogenation of nitroarenes to anilines with 2-propanol Vijay Kumar Das, Sumaira Mazhar, Lennon Gregor, Barry D. Stein, David Gene Morgan, Nicholas Maciulis, Maren Pink, Yaroslav B. Losovyj, and Lyudmila M. Bronstein ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06378 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Materials & Interfaces

Graphene derivative in magnetically recoverable catalyst determines catalytic properties in transfer hydrogenation of nitroarenes to anilines with 2-propanol Vijay Kumar Das1, Sumaira Mazhar1, Lennon Gregor1, Barry D. Stein2, David Gene Morgan1, Nicholas A. Maciulis1, Maren Pink1, Yaroslav Losovyj1*, Lyudmila M. Bronstein 1,3,4* 1

Indiana University, Department of Chemistry, Bloomington, IN 47405, USA 2

3

Indiana University, Department of Biology, Bloomington, IN 47405, USA

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow, 119991 Russia 4

King Abdulaziz University, Faculty of Science, Department of Physics, Jeddah, Saudi Arabia

KEYWORDS. Graphene derivative, magnetite, silver, nanoparticles, transfer hydrogenation.

ABSTRACT. Here, we report transfer hydrogenation of nitroarenes to aminoarenes using 2-propanol as a hydrogen source and Ag-containing magnetically recoverable catalysts based on partially reduced graphene oxide (pRGO) sheets. X-ray diffraction and X-ray photoelectron spectroscopy data

*

To whom correspondence should be addressed: [email protected]; [email protected]

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demonstrated that, during the one-pot catalyst synthesis, formation of magnetite nanoparticles (NPs) is accompanied by the reduction of graphene oxide (GO) to pRGO. The formation of Ag0 NPs on top of magnetite nanoparticles does not change the pRGO structure. At the same time, the catalyst structure is further modified during the transfer hydrogenation, leading to a noticeable increase of sp2 carbons. These carbons are responsible for the adsorption of substrate and intermediates, facilitating a hydrogen transfer from Ag NPs and creating synergy between the components of the catalyst. The nitroarenes with electron withdrawing and electron donating substituents allow for excellent yields of aniline derivatives with high regio- and chemoselectivity, indicating that the reaction is not disfavored by these functionalities. The versatility of the catalyst synthetic protocol was demonstrated by a synthesis of a Ru-containing graphene derivative based catalyst, also allowing for efficient transfer hydrogenation. Easy magnetic separation and stable catalyst performance in the transfer hydrogenation make this catalyst promising for future applications.

Introduction Transfer hydrogenation received considerable attention for the reduction of nitroarenes to anilines because of the high demand for aniline derivatives. They are utilized as intermediates and precursors for syntheses of pharmaceuticals, dyes, polymers, etc.1 A number of homogenous catalysts has been developed over the years based on Ir,2-3 Au,4 Ru,2, 5 etc. complexes. Many of them show high selectivity towards anilines but cannot be separated and reused. Heterogeneous catalysis allows for catalyst separation and reuse, but often suffers from lower selectivity and activity. The heterogeneous catalysts for the transfer hydrogenation of nitroarenes include Co-and Ni-containing molecular sieves,6-7 perovskites,89

Pd/C,10-11free or polymer stabilized Pd nanoparticles (NPs),12-13 Ag NPs stabilized in a mesoporous

polymer,14 graphene derivative (GD) based catalysts,15-16 etc. Considering the importance of anilines for chemical and pharmaceutical industries, the quest for the development of efficient, selective and reusable catalysts for this process is continuing. 2


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A smart choice of a hydrogen donor is especially important for environmentally friendly processes. Among hydrogen donors formic acid,15, 17 hydrazine,18-19 NaBH4,20 and various alcohols21-22 have been reported. We believe isopropyl alcohol (2-propanol) is the most environmentally friendly choice allowing one to avoid expensive or corrosive chemicals.3, 22-23 Another avenue for sustainable processes is the utilization of magnetically recoverable catalysts. The major advantage is an easy separation compared to traditional heterogeneous catalysts, leading to conservation of energy, greener chemical processes, and cheaper target products.24-32 In addition, the presence of a magnetic phase, for example, iron oxide, can enhance the catalytic activity due to electron transfer.33-34 Perovskite-type ferromagnetic BiFeO3 nanopowder has been explored in transfer hydrogenation with 2-propanol.8 The recent development of graphene based catalysts opened a new possibility in the exploration of such catalysts in transfer hydrogenation. NiPd alloy NPs supported on graphene were used as catalysts for the chemoselective transfer hydrogenation reactions of nitro compounds and nitriles using ammonia-borane as the stoichiometric reductant.35 Significant enhancement of the catalyst performance has been achieved when these NPs and a monolayer of nitrogen-doped graphene were organized on a solid surface.16 Iron oxide NPs coated with nitrogen-doped graphene layers showed reasonable selectivity in the transfer hydrogenation of halogenated nitroarenes using formic acid as a hydrogen source.36 FePd NPs assembled on reduced graphene oxide (RGO) catalyzed transfer hydrogenation of nitroarenes to anilines using ammonia borane.37 Magnetically recoverable graphene oxide (GO) decorated with both Pd and Fe3O4 NPs was explored in the transfer hydrogenation of nitro compounds with different hydrogen donors, but promising results were obtained only for NaBH4.38 The assembly of Ag/Pd core/shell NPs on RGO catalyzed the transfer hydrogenation of nitroarenes to anilines using ammonia borane.39 Herein, we report novel, magnetically recoverable catalysts for transfer hydrogenation of nitroarenes to anilines with 2-propanol as a hydrogen donor. These catalysts are prepared in a one-pot reaction and include Fe3O4 and Ag NPs assembled on partially reduced GO (pRGO) sheets via polyethyleneimine 3



(PEI) functionalities. The developed catalyst activates under the catalytic reaction conditions and shows much higher chemo- and regioselectivity in the transfer hydrogenation of nitroarenes compared to the analogous catalyst without pRGO. We discuss the mechanism of the catalytic reaction and the pRGO role. Easy magnetic separation and stable catalyst performance in the sustainable transfer hydrogenation make this catalyst promising for future applications.

Experimental Materials Graphite flakes (99%), potassium permanganate (≥99.9%), triethylene glycol (TEG, 99%), nitrobenzene (≥99.9%), nitrophenol (≥99.9%), 4-chloro nitrobenzene (99%), 4-nitro benzonitrile (99%), 4-iodo nitrobenzene (≥99.9%), and anhydrous isopropyl alcohol (2-propanol, ≥99.9%) were procured from Sigma-Aldrich. Silver (I) nitrate (˃ 99.9%) and branched polyethyleneimine (PEI) with M.W. 1,800 (99%) were purchased from Alfa Aesar. Sulfuric acid (97%) and acetone (99.5%) were purchased from MACRON. Phosphoric acid (85%) and anhydrous diethyl ether (99.8%) were purchased from Fischer Chemicals. Iron (III) acetylacetonate (+99%) was purchased from Acros Organics. Hydrochloric acid (37%) was purchased from Merck. Ethanol (95%) was purchased from Pharmco-AAPER. All reagents were used as received.

GO synthesis GO sheets have been synthesized by a modified Tour’s method.40 In a typical experiment, a mixture of concentrated sulfuric acid (120 mL) and concentrated phosphoric acid (13 mL) in a 9:1 volume ratio were slowly added to a 500 mL round-bottom flask containing mixed graphite flakes (1 g) and KMnO4 (6 g). The reaction was allowed to stir for 16 h at 50 °C. Subsequently, the reaction mixture was cooled to room temperature and poured onto deionized ice (130 mL). After this, 30% H2O2 was added dropwise (~1 mL) under stirring with a glass rod until a bright yellow color was achieved. The reaction solution was passed through a metal U.S standard testing sieve (W.S. Tyler, 300 µm). The filtrate was centrifuged at 4000 rpm for 4 h. The collected solid was washed with 30% HCl (3 × 200 mL), deionized water (8 × 200 mL, 4


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achieving near neutral pH) and finally with ethanol (3 × 200 mL), separating the solid via centrifugation (4000 rpm, 4 h) after each wash and sifting through the U.S standard testing sieve before that. The final paste-like substance was coagulated with ether (60 mL) followed by the filtration of the resultant suspension through a PTFE membrane (0.45 µm pore size). The solid was dried in a vacuum oven at room temperature overnight. The GO sheets obtained (765 mg) were dispersed in ethanol (~1 mg/ mL) by sonication.

Synthesis of GD-IO-Ag Before the synthesis, GO sheets (25 mg) and TEG (5 mL) were placed in a 10 mL test tube and sonicated (Branson 3510 ultrasonic) for about 5 h to achieve a homogeneous dispersion. A 50 mL threeneck round-bottom flask with elongated necks equipped with a magnetic stir bar, a reflux condenser, and two septa, one of which contained an inserted temperature probe protected with a glass shield and the other had a long needle, was loaded with 0.355 g (1 mmol) of Fe(acac)3, 0.7188 g of PEI and the GO dispersion in TEG. The flask was placed in a Glas-Col heating mantle attached to a digital temperature controller, which, in turn, was placed on a magnetic stirrer. The flask was degassed by argon bubbling for 15 min under stirring. Then the temperature was raised to 60 °C at 10°/min and the solution was stirred at this temperature for 30 min to allow proper mixing. Then the temperature was increased with a heating rate of 10°/min until stabilizing around 283-285 °C (boiling point of TEG) and heated for 2 h. Afterwards, the reaction mixture was cooled to 60 °C. Meanwhile, a small vial was charged with 0.0057 g (0.034 mmol, 3.4 mol.% towards Fe(acac)3) of AgNO3 and 2 mL of TEG and sonicated for 1 h. This solution was added dropwise via a syringe into the 60 °C reaction solution while vigorously stirring, and the reaction was continued at this temperature for another two hours. After the completion of the reaction, the flask was cooled to room temperature. The reaction mixture was precipitated using 5 mL of acetone. The solid was collected after centrifugation (Compact II Centrifuge, Model Nos. 420225, 420227) and washed twice with 5 mL of acetone. The GD-IO-Ag product was washed several times with acetone until the supernatant became colorless and dried at room temperature in a vacuum oven overnight to collect 5



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360 mg. To characterize GD-IO (GD stands for a graphene derivative and IO stands for iron oxide), the isolation and purification of the product were carried out before the addition of AgNO3. Gd-IO-Ag and GD-IO-Ag-2 were prepared with 0.7188 g and 0.4228 of AgNO3, respectively. Ru-containing catalysts have been synthesized in a similar way using RuCl3×H2O as precursor and N2H4×H2O as reducing agent. The reaction conditions for the catalysts synthesized are listed in Table 1.

Table 1. Reaction conditions of nanocomposite syntheses.a

a

Nanocomposite

AgNO3, g

RuCl3×H2O, g

N2H4×H2O, g

IO-Agb

0.7188

-

-

GD-IOc

“

-

-

GD-IO-Ag

“

-

-

GD-IO-Ag-2

0.4228

-

-

GD-Ag

0.7188

-

-

IO-Ru

-

0.013

0.002

GD-IO-Ru

-

“

“

Fe(acac)3 (0.355 g), GO (0.025 g), PEI (0.7188 g), TEG (7 mL), 4 h. bPEI (0.1023 g). cPEI (0.4846 g).

Transfer hydrogenation of nitroarenes An oven dried Schlenk tube with anhydrous 2-propanol (5 mL) was charged with a mixture of the catalyst (29 mg, 10 wt.% towards KOH and nitrobenzene) and KOH (168 mg, 3 mmol), which was ground together using an agate mortar and pestle. After the nitrobenzene (123 mg, 1 mmol) addition, the Schlenk tube was sealed and sonicated for 45 min to allow catalyst dispersion. Oxygen removal was achieved using a triple degassing procedure involving freeze-thaw cycles and filling with argon at the end. After that the Schlenk tube was flushed with hydrogen gas to promote the transfer hydrogenation and sealed. Then the Schlenk tube was heated at 100 °C for 24 h. After the completion of the reaction (monitored by TLC), the catalyst was magnetically separated and the reaction mixture was pipetted out of the Schlenk tube. 2-propanol was removed using a rotary evaporator. The remainder was extracted by 6


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ethyl acetate (3 × 20 mL), dried over anhydrous Na2SO4, concentrated in the rotary evaporator, and finally the crude product was purified by column chromatography using 20% ethyl acetate:hexane as eluent. The aniline yield was 96%, 114 mg. This procedure was followed for all of the products listed in Table 2. All products were characterized by EI-MS (Thermo Electron Corporation MAT 95XPTrap manufactured by Thermo Electron Corp.). The catalyst was separated using a rare earth magnet and washed with hot 2-propanol (3 × 10 mL), hot deionized water (3 × 10 mL) to remove alkali and hot ethanol (3 × 10 mL) to remove other impurities. Finally, it was dried overnight in a vacuum oven at room temperature.

Characterization Electron-transparent NP specimens for TEM were prepared by placing a drop of a diluted solution onto a carbon-coated Cu grid. Images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron microscope. Scanning TEM (STEM) energy dispersive X-ray spectra (EDS) were acquired at an accelerating voltage of 300 kV on a JEOL 3200FS transmission electron microscope equipped with an Oxford Instruments INCA EDS system. The same TEM grids were used for all analyses. Images were analyzed with the National Institute of Health developed image-processing package ImageJ. X-ray powder diffraction (XRD) patterns were collected on an Empyrean from PANalytical. X-rays were generated from a copper target with a scattering wavelength Kα of 1.54187 Å. Soller slits, antiscatter slits, divergence slits and a nickel filter were in the beam path. During the measurement in reflection mode the sample was spinning with a revolution time of 2 s. The measurement was performed with a step-size of 0.017 and a counting time of 1800 s/step. X-ray photoelectron spectroscopy (XPS) experiments were performed using PHI Versa Probe II instrument equipped with a focused monochromatic Al Kα source. The X-ray power of 50 W at 15 kV was used for a 200 micron beam size. The instrument work function was calibrated to give a binding energy (BE) of 84.0 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was 7



adjusted to give BEs of 284.8 eV, 932.6 eV and of 368.2 eV for the C 1s line of adventitious (aliphatic) carbon present on the non-sputtered samples, Cu 2p3/2 and Ag 3d5/2 photoemission lines, respectively. The PHI dual beam charge neutralization system was used on all samples. The ultimate Versa Probe II instrumental resolution was determined to be better than 0.125 eV using the Fermi edge of the valence band for metallic silver. XPS spectra with the energy step of 0.1 eV were recorded using PHI SmartSoft– VP v2.6.3.4 at the pass energy of 23.5 eV for HR spectra of Fe 2p, C 1s, O 1s, Ru 3d and valence band regions and at the pass energy of 46.95 eV for Ag 3d yielding 1-2×103 c/s. Further spectra were processed with PHI MultiPack v9.3.0.3 software and fitted using GL line shapes and/or asymmetric shapes (for C sp2 and Ru 3d), i.e., a combination of Gaussians and Lorentzians with 10-40% of Lorentzian contents. A Shirley background was applied for curve-fitting. NP samples for XPS were prepared by drop casting from solution onto the native surface of a Si(111) wafer. BE calibration was done using sp2 carbon peak (284.4 eV) of freshly exfoliated HOPG strip placed in the middle of each drop-cast sample. Atomic adsorption spectrometry (AAS) to determine Ag and Fe contents in the liquid phase after the catalytic reaction was carried out with MGA-915 (Lumex), equipped with hollow cathode lamps for Ag (328.1 nm) and Fe (248.3 nm).

Results and discussion Catalyst synthesis and structure Iron oxide NPs have been formed in the presence of GO sheets and PEI by the thermal decomposition of Fe(acac)3 in boiling TEG. The TEM image displayed in Figure 1a indicates the graphene oxide sheets are well exfoliated and dispersed, showing wrinkles due to partial folding. The role of branched PEI is to encapsulate the GO sheets and to ensure the interaction with iron species via the polymer amino groups. It is noteworthy that PEI efficiently encapsulates GO sheets at the weight ratio of GO:PEI = 1:5 most likely due to hydrogen bonding between the GO oxygen containing functionalities and hydrogen atoms of the PEI amino groups. When IO NPs were formed on the PEI encapsulated GO sheets, more polymer was needed to prevent the IO NP aggregation. The optimal GO:PEI weight ratio was determined to be 1:19. 8


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At this ratio the GO sheets are fully coated with iron oxide NPs after the Fe(acac)3 decomposition (Fig. 1b) and easily magnetically recoverable. No iron oxide NPs were observed between the GO sheets. For the formation of Ag NPs, the reaction solution containing GD-IO was charged with silver nitrate at 60 °C. Again, the polymer amount was increased (PEI:GO = 1:29) to prevent Ag NP aggregation. It is worth noting that no additional reducing agent was needed because PEI serves as both stabilizing and reducing agent.41-42 In the final product (GD-IO-Ag, Fig. 4d) all NPs are located on the GD sheets allowing for the catalyst magnetic separation.

Figure 1. TEM images of GO (a), GD-IO (b), and GD-IO-Ag (c, d) at two different magnifications. Blue arrows in (a) show GO wrinkles. Red arrows in the (b) inset and (d) show the edge of the GD sheet. The STEM dark-field image of GD-IO-Ag (Fig. 2a) shows a distinct contrast between the two types of NPs with a different electron density. Brighter NPs (most likely Ag) are located on top of less bright NPs 9



(most likely IO). EDS mapping (Fig. 2, b-f) indicates that Fe and O maps have the same size and shape, while the silver map is very different (resembling the location of bright NPs in the dark-field image), revealing that Ag NPs most likely do not contain O, while they are fused with IO NPs or located on top of the IO NPs. To assess the crystalline structure of GD-IO-Ag, an XRD study has been carried out. The XRD pattern of parent GO (Fig. 3, red pattern) shows a single sharp reflection at 10° two theta, which is consistent with the literature data for GO.40 The XRD pattern of GD-IO-Ag (Fig. 3, blue pattern) contains a set of reflections notated in blue which are characteristic of spinel. A similar set of reflections is observed in the XRD pattern of IO NPs stabilized by PEI (Fig. 3, green pattern). The positions and intensity of the Bragg reflections characteristic of spinel are typical for those of magnetite,43-44 but the similarity of the XRD patterns of magnetite (Fe3O4) and maghemite (γ-Fe2O3) NPs makes this assignment tentative due to line broadening. This assumption is based on the reaction conditions (argon atmosphere) and the presence of PEI (reducing agent) so the oxidation should be minimized. The XPS data discussed below demonstrate that magnetite NPs are formed. The mean magnetite crystallite size in GD-IO-Ag determined from the Scherrer formula is 10.6 nm.

10


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Figure 2. STEM dark-field image (a) and EDS maps of GD-IO-Ag for Fe (b), O (c), Ag (d), the Fe-Ag mix (e), and the Ag-O mix (f).

300000

GD-IO-Ag GO PEI-IO

a (311)

GO GD-IO-Ag

b 70000

250000

60000

X-ray intensity

X-ray intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200000 150000

(220) Ag (111)

100000

(400)

(440) (511)

(422) 30

40

50

30000 20000

0

0 20

40000

10000

50000

10

50000

60

70

6

80

8

10

12

14

16

2 Theta degrees

2 Theta degrees

Figure 3. XRD patterns of GO, IO NPs stabilized by PEI, and GD-IO-Ag in the full angle range (a) and GO and GD-IO-Ag at low angles (b). 11



Furthermore, in the XRD pattern of GD-IO-Ag a sharp refection at 10° two theta of parent GO transforms into a very broad reflection in the range of 5 to 11° which could be indicative of disordering of GD sheets due to NPs on the sheet surface and/or the formation of pRGO. An additional peak at ~38° annotated by the black label is characteristic of Ag0 NPs (ref. no. 01-08-0715). The mean Ag crystallite size calculated using the Scherrer formula is 9.6 nm, which is consistent with an average size of Ag NPs (10.3 nm) obtained from the EDS map (Fig. 2d). It is noteworthy that TEM images do not allow one to distinguish between IO and Ag NPs so the accurate size analysis is impossible. XPS has been utilized to assess the composition of the catalyst developed to further clarify its structure. The survey XPS spectrum of parent GO (Fig. S1, Supporting Information, SI) shows mainly C and O. The survey XPS spectrum of GD-IO-Ag (Fig. S1, SI) shows C, O, Fe, and Ag, indicating the elemental purity of the catalyst. The HR XPS C1s of GO and GD-IO-Ag are presented in Figure 4a. The HR XPS C1s spectrum of GO was deconvoluted into five peaks belonging to sp2 carbon (C=C, 284.42 eV), sp3 carbon (C-C, 285.13 eV), hydroxyl (C-OH, 286.14 eV), carbonyl/epoxy (C=O/C-O-C, 287.24 eV), and carboxyl (O-C=O, 288.38 eV)40, 45-46 with the total of all oxidized groups of ~51%. In the HR XPS C1s spectrum of GD-IO-Ag, all peaks associated with oxygen containing groups are significantly diminished (the total content drops to 45%). The observed increase in the ratio of sp2/sp3 carbon atoms (~284.4/285.2 eV) to those bound to oxygen indicates that the reduction does restore the C=C and C–C bonding through the removal of oxygen functional groups. The intensity of both the carbonyl (~287.3 eV) and carboxyl acid (~288.4 eV) peaks decreases with the reduction. As the total ratio of oxygen functionalities to sp2/sp3 carbons decreases, the ratio of hydroxyl to carbonyl functionalities significantly increases. It is noteworthy that GD-IO-Ag contains a considerable amount of PEI, so the sp3 peak is increased due to the PEI presence. There is also a new peak at a BE of 285.70 eV for C-N of PEI. These data indicate that during catalyst preparation GO is partially reduced, becoming pRGO. A comparison with RGO (Fig. S2b, SI) synthesized by the N2H4×H2O reduction of GO at 100 °C47 validates

12


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that there is only a partial reduction of GO in GD-IO-Ag. In RGO the total content of the oxidized groups in C 1s region is under 21%. The GD-IO-Ag synthesis consists of two steps: (i) the formation of IO NPs at 285 °C and (ii) the formation of Ag NPs at 60 °C. To understand at what stage the changes in GO occur, we have analyzed the HR XPS C1s spectrum of GD-IO (before the addition of Ag precursor) (Fig. S2c, SI). It displays similar changes as those in the spectrum of GD-IO-Ag, showing a decrease of the intensity of peaks associated with oxygen containing groups (the total content of 45%), while the ratio between sp3 and sp2 carbons (2.5) is different from that in GD-IO-Ag (3.25) because the former was prepared with only ~65% of PEI compared to that of the latter. The HR XPS Fe 2p spectra of GD-IO, GD-IO-Ag and GD-IO-Ag-16 are displayed in Figure S3 (SI). They are identical and show a major peak centred at a BE of 710.5 eV which is typical for iron oxides.48 A satellite feature which could be observed for Fe3+ ions at a BE value of 8-9 eV higher than the major peak, is absent, indicating that there is no excess of the Fe3+ species beyond the Fe3+:Fe2+=2:1 ratio of magnetite.49-51 For Fe3O4, the Fe3+ and Fe2+ satellites are combined, resulting in a plateau between the Fe 2p3/2 and Fe 2p1/2 peaks,52 as is observed in our case. The HR XPS Ag 3d spectrum of GD-IO-Ag is shown in Figure S4 (SI). The position of the major peak at 368.09 eV is characteristic of Ag0, 53-54 which is in a good agreement with the XRD data. The Ag content determined from the sum of all elements (except hydrogen) is 2.45 wt.%.

13



a

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b

c

290

288

286

284

282

Binding energy, eV Figure 4. HR XPS C 1s spectra of GO (a), GD-IO-Ag (b), and GD-IO-Ag-16h (c). The blue line indicates sp2 carbon; the maroon line is for sp3 carbon; the yellow peak indicates a hydroxyl group; the green peak shows carbonyl/epoxy groups; the orange peak is for a carboxyl group. The magenta peak in (b) and (c) is for the C-N group of PEI. The deconvolution data are presented in Tables S1-S3 (SI).

Catalytic behavior in transfer hydrogenation The transfer hydrogenation with the GD-IO-Ag catalyst has been first studied with nitrobenzene to optimize the reaction conditions. It is noteworthy that this is a commonly used model substrate, although aniline obtained is a valuable product. The results of catalytic studies are presented in Table 2. A control experiment (without any catalyst) has been performed under the same reaction conditions for the sake of comparison. Although aniline was detected (Table 2, entry 1), the yield was low and the product also contained a mixture of N-phenyl hydroxylamine, azobenzene, and unreacted nitrobenzene. Similar result was obtained for GD-IO (not shown), indicating that GD-IO is inactive in this reaction. Alternatively, 14


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when 10 wt.% (towards nitrobenzene and KOH) of GD-IO-Ag was used as catalyst (Table 2, entry 2), a single reaction product – aniline - was formed with an isolated yield of 96%. This is comparable to a few best results in the transfer hydrogenation with 2-propanol using Ag-containing catalysts and only in a single case, the catalyst is magnetically recoverable.14, 55 At the 7 wt.% of the catalyst loading, the yield was lower (only 90%) and the reaction product contained unreacted nitrobenzene (Table 2, entry 3). Surprisingly, when the loading was increased to 15 wt.% (Table 2, entry 4), the aniline yield further dropped (to 84%), which could be explained by shielding of catalytic sites on Ag NPs at too high loading. In this case, the reaction product contained aniline as well as unreacted nitrobenzene. The replacement of 2-propanol with ethanol as a hydrogen source (Table 2, entry 5), leads to a very low aniline yield (46%), while in water (Table 2, entry 6) only nitrobenzene has been detected (Table 2). The latter could be due to low solubility of nitrobenzene in water, or H2O molecules occupying the active sites. To understand the role of pRGO in the catalyst, the transfer hydrogenation of nitrobenzene was carried out with a catalyst prepared without GO (Table 2, entry 7) in the reaction conditions analogous to those for entry 2 (Table 2). In this case, the aniline yield was much lower and an intermediate product, azobenzene, was detected in the reaction mixture along with nitrobenzene. This observation indicated that pRGO plays a crucial role in achieving the selectivity and the high yield in the nitrobenzene transfer hydrogenation, allowing for synergy between Ag NPs and pRGO. In order to understand the role of IO, the reaction was performed using GD-Ag which was synthesized in the conditions analogous to those of GD-IO-Ag but without the Fe(acac)3 addition, i.e., without IO NPs in the catalyst. In this case, the aniline yield was only 76% (Table 2, entry 8) and unreacted nitrobenzene was in the reaction product. The TEM image of GD-Ag (Fig. S5, SI) shows much larger Ag NPs than those in GD-IO-Ag (~32 nm vs ~10 nm). Thus, the presence of IO NPs stabilizes the Ag NP growth, most likely allowing for heterogeneous nucleation of the latter on the IO NP surface, leading to the enhancement of their catalytic properties.

15



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Table 2. Transfer hydrogenation of nitrobenzene to aniline.a Entry Catalyst

Solvent

Catalyst Reaction time, Isolated loading, h yield, wt.% %b

1

No catalyst

2-propanol

-

24

37c

2

GD-IO-Ag

“

10

“

96

3

GD-IO-Ag

“

7

“

90

4

GD-IO-Ag

“

15

“

84

5

GD-IO-Ag

EtOH

10

“

46

6

GD-IO-Ag

H2O

10

“

-

7

IO-Ag

2-propanol

10

“

86e

8

GD-Ag

“

10

“

76

9d

GD-IO-Ag

“

10

“

72

10

GD-IO-Ag-2

“

10

“

86

11

GD-IO-Ag

“

10

12

-

12

GD-IO-Ag

“

10

16

34f

13

GD-IO-Ru

“

10

24

92

14

GD-IO-Ru

“

20

“

98

15

IO-Ru

“

10

“

84g

a

Reaction conditions: nitrobenzene (1 mmol, 123 mg), KOH (3 mmol, 168 mg), solvent (5 mL), the catalyst loading is calculated as weight percentage towards the weights of nitrobenzene and KOH (for 10 wt. % catalyst loading, Ag loading is 6.5 mol.% to nitrobenzene), 100 °C, 3.4 mol.% of Ag towards the Fe(acac)3 loading unless indicated otherwise; bisolated yields; cmixture of N-phenyl hydroxylamine and azobenzene. d1 mmol of KOH (otherwise 3 mmol were used). e9%, f16% or g12% of the azobenzene yield. As is well documented, KOH allows for generation of hydrogen from 2-propanol.56 It is noteworthy that the decrease of the KOH loading from 3 mmol to 1 mmol leads to a poor aniline yield (Table 1, Entry 8), indicating that in this case the amount of KOH was insufficient to generate hydrogen. The decrease of the Ag loading in GD-IO-Ag by approximately 40% (GD-IO-Ag-2, Table 2, Entry 9) also leads to a

16


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lower aniline yield, revealing that the chosen Ag amount (3.4 mol.% towards Fe) is optimal for an efficient catalyst performance. It was reported that the reaction time in the nitrobenzene transfer hydrogenation with various catalysts lies between 3 h and 24 h.3, 57-58 To our surprise, no reaction products could be detected after 12 h, while the reaction mixture after 16 h contained unreacted nitrobenzene, aniline (34%), and azobenzene (16%) (Table 2, Entry 11). These data point to an induction period, which is needed for the catalyst activation. A short induction period has been previously reported and attributed to the formation of metal hydride, which catalyzes the reaction.3 Then the question arises, what could cause such a considerable induction period for GD-IO-Ag? To understand how the nanocomposite changes under the reaction conditions, the catalyst after the 16 h reaction (with the incomplete nitrobenzene conversion) was analyzed using TEM, XRD, and XPS. The TEM image of GD-IO-Ag-16h displayed in Figure S6a (SI) indicates that the catalyst morphology does not change. The XRD pattern of this sample (Fig. S6b) shows that the reflections of the magnetite only slightly sharpen, demonstrating the crystallite size increase from 10.6 nm to 12.7 nm (calculated from line broadening using the Scherrer formula). At the same time, more noticeable changes are observed for reflections from pRGO and Ag. The broad reflection between 5 to 11° two thetas becomes less intense, while the Ag(111) reflection becomes significantly sharper. The latter is a reflection of the Ag crystallite size increase from 9.6 nm in the initial GD-IO-Ag catalyst to 23 nm in the catalyst after the 16 h transfer hydrogenation. The XPS C 1s spectrum of GD-IO-Ag (Fig. 4a) dramatically changes after 16 h under the catalytic reaction conditions. The intensity of the sp2 carbon increases from 12% to ~32%, while the sp3 carbon fraction decreases from 32% to 25%, both indicating further reduction of GO moieties to RGO. At the same time the fraction of the C-N carbon does not change, revealing no PEI loss. The fraction of oxidized carbon atoms decreases by 10% (from ~45% to ~35%). No changes have been observed in the HR XPS Fe 2p spectrum (Fig. S3, SI), while the Ag 3d spectrum (Fig. S4, SI) can be deconvoluted into two peaks 17



with a BE of 367.69 eV and 368.1 eV, which can be assigned to Ag+ and Ag0, respectively.59-61 These data reveal that Ag partially oxidizes, most probably forming AgH, while pRGO becomes even more reduced (sp2 carbon intensifies in the GD-IO-Ag-16h sample). Both manifestations are most likely responsible for the catalyst activation. To demonstrate that our approach to the catalyst synthesis is versatile and applicable to other catalytic metals, we synthesized GD-IO-Ru, because Ru NPs are known to catalyze the transfer hydrogenation of nitrobenzene.62 (The TEM and XPS data for GD-IO-Ru are shown in Figures S7-S8, ESI.) Table 2 shows that the aniline yield at 10 wt.% of the GD-IO-Ru loading (Table 2, entry 13) is only 92%, while at 20 wt.% (Table 2, entry 14), a 98% yield was obtained. Again, in the absence of pRGO, the catalyst (IO-Ru, Table 2, Entry 15) allowed for only 84% of aniline and 12% of azobenzene, once more demonstrating the synergistic effect between pRGO and catalytic NPs. For both catalytic metals, the pRGO based catalysts were found to provide a highly regio- as well as chemoselective (100%) transfer hydrogenation with no side products under optimized reaction conditions. However, considering that Ru is about ten times more expensive than Ag and less GD-IO-Ag catalyst loading is needed to achieve a high aniline yield, we conclude the silver based catalyst is more promising for future studies.

Scope of the transfer hydrogenation with GD-IO-Ag and its recycling To analyze the scope of the transfer hydrogenation with GD-IO-Ag, several substituted nitroarenes have been tested (Table 3). GD-IO-Ag was found to be a robust and sustainable catalyst for the transfer hydrogenation of substituted nitro aromatics furnishing aniline derivatives with high yields (Table 3, entries 2-6). Moreover, nitrobenzene containing an electron donating group (Table 3, entry 2) and electron withdrawing moieties (Table 3, entries 3-5) produced aniline derivatives with excellent yields under the same reaction conditions, which indicates that the reaction is not disfavored by these functionalities. Moreover, the hydrogenation was very selective leaving nitrile or carbonyl groups (Table 3, entries 3 and 6) intact. When azobenzene was used as a starting compound, it underwent 100%

18


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conversion with the >99% yield of aniline, indicating the complete transformation of the intermediate formed in the catalytic reaction.

19



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Table 3. GD-IO-Ag catalyzed transfer hydrogenation of nitroarenes to corresponding anilines.a Entry

a

Nitroarenes

Aniline derivatives

1

2

Isolated yield, %

1

96

2

98

3

94

4

96

5

95

6

94

EI-MS data of the reaction products are presented in Figures S9-S14 (SI).

Catalyst recovery and recycling are the other important aspects of green chemistry (from environmental and economics points of view) in both industry and research. The recycling experiments with GD-IO-Ag were carried out for the nitrobenzene transfer hydrogenation. The data presented in Table 4 indicate that the catalyst activity remains unchanged in three consecutive cycles. In the fourth cycle slightly less catalyst is recovered. (It is noteworthy that in every run, more than 94% of the catalyst was magnetically recovered.) In the fifth cycle, the aniline yield drops to 90%, revealing that the catalyst has undergone some changes. To follow the catalyst changes, we analyzed TEM images of the catalysts after first, second, and fifth runs (Fig. S15, SI). The morphology of the catalyst after the first and second cycles remains practically unchanged. After the fifth reaction the NPs are mainly stripped off of the pRGO sheets and are generally aggregated, thus destroying the synergy between the catalyst components. At the 20


Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


same time, the analysis of the liquid phase after each cycle showed only traces of Ag and Fe after the fifth cycle and no leaching before that, indicating excellent stabilization of NPs by PEI and GD. Table 4. Recycling experiments of the nitrobenzene transfer hydrogenation with GD-IO-Ag. No of runs Catalyst recovery, % Isolated yield, % 1st

100

96

2nd

99

96

3rd

99

96

4th

97

95

5th

94

90

Mechanism of the transfer hydrogenation with GD-IO-Ag To understand the mechanistic pathway of the transfer hydrogenation of nitrobenzene with GD-IO-Ag, we considered the reaction products after 16 h when both nitrobenzene and azobenzene were detected and only 34% of aniline were formed. It is worth noting that after 12 h no target or side products were detected. As was discussed above, the catalyst based on pRGO is superior to that without pRGO. Then the question arises what the reaction mechanism is when pRGO is present in the catalyst. Most probably, 2propanol adsorbs on the Ag NP (sitting on top of the magnetite NPs) and a hydride is transferred to the NO group (Scheme 1) with a simultaneous formation of acetone.55 Nitrosobenzene formed by elimination of water is adsorbed on the sp2 carbon atoms at the edges of pRGO. This is supported by DFT calculation reported elsewhere, showing that the sp2 carbon atoms at the zigzag edges of graphene can interact with oxygen atoms of nitrobenzene, thus weakening the N–O bonds and activating the molecule.63-64 This adsorption is followed by a consecutive hydrogenation to form phenylhydroxylamine which, in turn, undergoes fast condensation with previously generated nitrosobenzene to form azobenzene as an intermediate.65

21



Page 22 of 33

Scheme 1. Plausible mechanism for the transfer hydrogenation reaction.

Conclusions We developed novel magnetically recoverable catalysts suitable for regio- and chemoselctive transfer hydrogenation of nitroarenes to aminoarenes using 2-propanol as a hydrogen source. The nanocomposite synthesis, allowing a simultaneous formation of magnetite NPs and partial reduction of GO sheets followed by an Ag NP formation in a one-pot reaction, leads to a catalyst exhibiting excellent catalytic properties. Electron microscopy studies (TEM and STEM EDS) show that Ag NPs are mainly placed on top of Fe3O4 NPs and both types of NPs, fused together, are located on the pRGO sheets enveloped by PEI. We believe PEI allows for the stabilization of all the nanocomposite components. It was revealed, however, that as-prepared nanocomposites


22

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are not immediately active in the transfer hydrogenation. An induction period is needed for (i) further reduction of pRGO to facilitate the adsorption of the substrate and intermediates and (ii) the formation of silver hydride on the Ag NP surface to enable a hydrogen transfer from 2propanol. Easy magnetic separation and environmentally friendly conditions of transfer hydrogenation with high aniline derivative yields make this catalyst suitable for future applications. It is noteworthy that we did not detect any other intermediates besides azobenzene, revealing that the interaction of phenylhydroxylamine with nitrosobenzene occurs very fast, while the conversion of azobenzene to aniline is the rate determining step. Basic conditions of the reaction promote the azobenzene formation.66 Therefore, pRGO most likely allows for the efficient azobenzene formation in the vicinity of the Ag NP, thus, facilitating its further reduction.

ASSOCIATED CONTENT Supporting Information. XPS, TEM, XRD, and EI-MS data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT


23


Page 24 of 33

We thank the Indiana University Nanoscale Characterization Facility for access to the instrumentation as well as NSF grant #CHE-1048613 which funded the Empyrean from PANalytical. L.B. thanks Russian Foundation for Basic Research (project 17-03-00578) and V. K. D. acknowledges the SERB Indo-US Postdoctoral Fellowship 2016/43 for funding.

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