Magnetically Recoverable Heterobimetallic Co2Mn3O8: Selective and

Research Article pubs.acs.org/journal/ascecg

Magnetically Recoverable Heterobimetallic Co2Mn3O8: Selective and Sustainable Oxidation and Reduction Reactions Kasturi Sarmah,† Joyeta Pal,† Tarun K. Maji,† and Sanjay Pratihar*,† †

Department of Chemical Sciences, Tezpur University, Napaam, Asaam-784028, India S Supporting Information *

ABSTRACT: The present work reports an efficient, sustainable, and cost-effective chemical route for gram-scale synthesis of heterobimetallic Co−Mn oxide (Co2Mn3O8) from the redox reaction between in situ-generated zero-valent cobalt nanomaterial and KMnO4 in water. The magnetically recoverable Co2Mn3O8 nanomaterial showed promising catalytic activity and good selectivity (>99%) for the oxidation of alcohols to aldehydes/ ketones both in the presence of air and hydrogen peroxide for a variety of alcohols including benzyl, aliphatic, cinamyl, pyridine, and thiophene moieties. The inexpensive Co2Mn3O8 furnishes excellent catalytic activity and chemoselectivity (>99%) for the reduction of a wide range of aromatic and heteroaromatic nitro substrates to corresponding amines and various dyes under relatively milder reaction conditions with high turnover frequency (TOF). The high catalytic performance and durability in the Co2Mn3O8 nanomaterial for both reactions in comparison to their monometallic oxide (MnO2 and Co3O4) is further attributed to the synergistic effects between each component. Easy synthesis, large-scale application, excellent selectivity, effortless separation of the catalyst using an external magnet, and efficient recycling make the catalyst and the protocol economical and sustainable. KEYWORDS: Cooperativity, Oxidation, Reduction, Benzyl alcohol, Nitro aromatics

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transition-main group metals41−43 for different type of catalytic and material applications are noteworthy. On the other hand, oxidation of alcohols to aldehydes or ketones and reduction of nitro to amine are two important fundamental reactions because their products are essential building blocks for various drugs, agro-chemicals, and fragrances.44,45 Traditionally, stoichiometric oxidizing and reducing agents in the presence of strong mineral acids were used for oxidation and reduction reactions, which generate enormous amounts of poisonous metal salts as waste.46,47 In the past few decades, many efforts have been made to develop environmentally benign oxidation and reduction systems to protect the environment from these wastes.48−51 In this regard, various heterogeneous catalysts have been developed for both methodologies.52−54 However, these methods have one or more of the following limitations, for example, lack of broad functional group tolerance, use of expensive metals, applicable for activated substrates, use of excess quantities of additives such as bases, oxidants, and electron transfer mediators, low turnover frequency, and so on.55,56 To overcome the abovementioned problems, various approaches have been applied by various research groups for both reactions.57,58 As an important

INTRODUCTION

Catalytic reactions are not only at the heart of making most chemicals, including our domestic products, materials, and medicines, but also play an important role in various types of energy and environment applications.1,2 Heterogeneous catalysis, in which the phase is different than that of the reactants, is quite prevalent in the chemical industry and affects our everyday life in many ways.3−6 In this regard, the advent of transition-metal-catalyzed strategies for new carbon−carbon and carbon−heteroatom bond-forming methodologies have revolutionized the field of organic chemistry, allowing the efficient synthesis of ligands, materials, and biologically active molecules.7−11 Over the past few decades, heterobimetallic catalysis,12−17 an important subarea within the broader domain of multimetallic catalysis,18−22 has received much attention since synergistic cooperation of two distinct metal centers can enhance catalytic activity and selectivity, where a single metal fails to promote a selective or efficient transformation. Toward this end, development of various homogeneous catalysts including cooperative homo- or/and hetero-bimetallic,23−29 tandem catalysts,30,31 along with dual metal reagents,32,33 for several types of bond-forming methodologies are noteworthy. At the same time, heterogeneous bimetallic catalysts with various combinations including early−early,34−36 early− late,37,38 and late−late39,40 transition metals as well as various © 2017 American Chemical Society

Received: August 9, 2017 Revised: September 26, 2017 Published: October 19, 2017 11504

DOI: 10.1021/acssuschemeng.7b02739 ACS Sustainable Chem. Eng. 2017, 5, 11504−11515

Research Article

ACS Sustainable Chemistry & Engineering contribution, Beller et al. utilized earth-abundant cobalt- and cobalt oxide-based catalysts with activated carbon support in many chemical processes, including nitroarene reduction and aerobic oxidation of alcohols.59,60 Recently, Li et al. reported a Mott−Schottky catalyst of nitrogen-rich carbon-coated cobalt nanoparticles, which boost the activity of a transition-metal nanocatalyst through electron transfer at the metal/nitrogendoped carbon interface.61 By combining an oxygen-activating species, nitrogen-doped carbon, and a simple metal-oxide catalyst, Rothenberg et al. showed cooperative catalysis for selective aerial oxidation of alcohol to aldehydes/ketones. We are particularly interested in inexpensive and efficient oxygendeficient bimetallic catalysts that could be easily accessible, reusable, and suitable for large-scale applications for various chemical transformations. Herein, we presented an unconventional redox mediated cost-effective and environmently friendly approach for the synthesis of mixed heterobimetallic oxygendeficient Co2Mn3O8 from the redox reaction between in situgenerated zero-valent cobalt nanomaterial and KMnO4 in water. The as-prepared Co2Mn3O8 is found to be useful as a magnetically recoverable catalyst for chemo-selective reduction of nitro to amine and various dyes in the presence of hydrazine hydrate at room temperature and selective oxidation of alcohols to corresponding aldehydes/ketones in the presence of air or hydrogen peroxide. The enhanced reactivity of the Co2Mn3O8 nanomaterial in comparison to its monometallic oxide in both cases justifies the co-operativity of the catalyst (Figure 1).

Figure 2. PXRD pattern of Co-MnOx nanomaterial.

pattern of Co-MnOx corresponded to the orthorhombic phase, and the composition of the material comes out to be Co2Mn3O8 (Figure 2). The average crystallite size (Dcry) of Co-MnOx, determined from the Williamson−Hall equation βcos θ = 0.9λ/Dcry + 4ϵ sin θ, is found to be 26 nm.63 The FTIR spectra of the Co-MnOx show characteristic peaks at 652 and 559 cm−1 for the bending vibrations of Co−O and Mn−O, respectively.64 The XPS survey spectra of Co-MnOx exhibit the presence of Co, Mn, C, and O elements (Figure 3a). All spectra were calibrated using the C1s peak with a fixed value of 286.8 eV. The peak present at 531.2 eV was for oxygen (O1s) in Co2Mn3O8. When we observed the high resolution spectra for Co, the peaks with BEs at 781.4 and 796.3 eV were mainly for Co2p1/2 and Co2p3/2. The presence of a satellite peak at 786.2 eV confirms the oxidation state of Co as Co2+. Further, the peaks at binding energies of 715, 826.2, and 865.3 eV correspond to Co auger peaks, and the peak at 952.6 eV is for Co2s. The XPS study of Mn oxide is of great interest due to the ability of manganese to form various oxides with Mn in different oxidation states (Mn2+ to Mn4+). However, the high resolution spectra of Mn shows peaks for Mn2p3/2 and Mn2p1/2 at 641.8 and 652.2 eV attributes for the Mn4+ oxidation state in the synthesized material. A much weaker satellite peak at 645.6 eV was observed for Mn4+. The peaks at binding energies of 61.8, 49.1, and 903.4 eV were for Mn3s, Mn3p, and Mn auger electrons, respectively.65 From the XPS analysis, it was confirmed that Co and Mn in the Co-MnOx nanomaterial were in their II and IV oxidation states, respectively, which further confirmed the material as Co2Mn3O8.66 The calculated atomic percentage of Co and Mn in Co2Mn3O8 from XPS analysis was found to be 11.4 and 13.7, respectively. The FE-SEM of the synthesized material shows an agglomerated morphology. The energy-dispersive X-ray analysis (EDS) shows the presence of Co, Mn, and O in the nanomaterial (Figure S23, SI). Further, HR-TEM analysis confirms the agglomerated nature of the synthesized material. The fringe spacing shows that the particles grow in 112 and 103 planes with alternate dark and light fringes. The SAED pattern of Co2Mn3O8 suggests the polycrystalline nature of the material. The calculated d-spacing was well matched with the planes observed in PXRD analysis. The particle size distribution has also been analyzed, and the average particle size was found to be ∼27 nm (Figure 4).67,68 Co2Mn3O8 Nanomaterial-Promoted Oxidation of Alcohol. Next, to check the catalytic activity of the Co2Mn3O8

Figure 1. Synthetic route of cooperative Co-MnOx.

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RESULTS AND DISCUSSION Synthesis and Characterization. The reduction of CoCl2.6H2O in water by NaBH4 in the presence of trisodium citrate was done at room temperature. During the reaction, a purple colored solution slowly turned black and finally appeared as a black precipitate, which was further reacted with an aqueous solution of KMnO4 at 120 °C under a nitrogen atmosphere for 24 h. After the completion of the reaction, the black precipitate was collected using a tiny magnet and washed with water several times and dried in oven at 80 °C for 12 h. The PXRD analysis (Figure 2) of the synthesized heterobimetallic Co−Mn oxide nanomaterial (hereafter Co-MnOx) showed peaks at 2θ values of 19.2°, 20.5°, 26.1°, 30.8°, 32.3°, 34.1°, 37.2°, 38.8°, 44.6°, 48.9°, 50.9°, 56.5°, 65.2°, and 74.4° for characteristics diffraction indices (002), (011), (012), (112), (103), (031), (211), (004), (122), (220), (310), (030), (314) and (324) respectively (JCPDS No: 70-0931).62 The PXRD 11505


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ACS Sustainable Chemistry & Engineering

Figure 3. Wide-scan (survey spectrum; a) and narrow-high resolution scan XPS spectra (b−d) of Co2Mn3O8 nanomaterial.

Figure 4. SEM (a−c), HR-TEM (d), fringe spacing (e), SAED pattern (f), and particle size distribution (inset of d) of Co2Mn3O8.

nanomaterial, the aerial oxidation69,70 of benzyl alcohol was chosen as a model reaction. Initially, reaction conditions were optimized from the screening of solvent, temperature, and catalyst loading. Co2Mn3O8 is found to be active to produce the desired benzaldehyde in 92% yield at 130 °C in toluene. Upon decreasing the catalyst loading from 1 to 0.25 mol %, the TOF steadily increases, while the product yield drops slightly at 0.25 mol % loading (Table 1, entries 4−7). Among other oxidants, H2O2 was found to be effective for the reaction and proceeds with higher turnover frequency (TOF). It is noteworthy to

mention that under optimized reaction conditions, all of the tested catalysts inclusive of anhydrous FeCl3, CoCl2, MnCl2, Fe3O4,71 MnO2, Co0, and Fe(ox)-Fe3O472 were found to be less effective on the reaction (Table 1, entries 12−21). Notably, either in the absence of catalyst or in the absence of oxidant, no reaction was observed (Table 1, entries 9 and 10). Further, to check the cooperative effect, the reactivity of the synthesized Co2Mn3O8 nanomaterial was compared with Co3O4 and MnO2 nanomaterials for model reaction.73 Among the three materials, Co3O4 is found to be least reactive and affords corresponding 11506


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ACS Sustainable Chemistry & Engineering Table 1. Screening of Catalysta

No.

Cat.

Oxidant

mol %

T, °C

Solvent

t, h

Yield, %c

TOF, h−1

1 2 3 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 Co2Mn3O8 − Co2Mn3O8 Co3O4 Co3O4 MnO2 MnO2 Co0 Fe3O4 Fe(ox)-Fe3O4 CoCl2 FeCl3 MnCl2

Air Air Air Air Air Air O2 N2 H2O2b H2O2b Air H2O2 Air H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

1 1 1 1 0.5 0.25 0.25 0.25 − 0.05 0.5 0.5 0.5 0.5 1 1 1 1 1 1

80 100 80 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130

MeCN H2O MeOH Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene

12 12 12 12 12 12 12 12 12 10 12 12 12 12 12 12 12 12 12 12

58 50 65 88 92 62 94 20 20 92 10 21 32 52 14 30 40 22 32 36

4.8 4.2 5.4 7.3 15.2 20.6 31.3 6.6 − 184 1.6 3.5 5.3 8.6 1.1 2.5 3.3 1.8 2.6 3

a Model reaction done at 10 mmol scale. b0.7 mL of H2O2 used for the reaction. cYields were calculated from GC analysis with o-xylene as an internal standard.

Figure 5. Substrate scope for Co2Mn3O8-promoted oxidation of a variety of alcohols in the presence of air and hydrogen peroxide. Yields were calculated from (superscript a) GC with o-xylene as an internal standard, (superscript b) isolated yield, and (superscript c) NMR yield, TOF values are in h−1. Aerial oxidation: alcohol (10 mmol), Co2Mn3O8 (0.25 mol %), toluene (30 mL), temperature 130 °C. H2O2-promoted oxidation: alcohol (10 mmol), Co2Mn3O8 (0.05 mol %), toluene (30 mL), H2O2 (0.7 mL), temperature 130 °C.

hand, alcohol, such as p-methoxy benzyl alcohol, reacted slowly to form the corresponding aldehyde in good yields. Furthermore, 3-nitrobenzyl alcohol also produced the corresponding aldehyde in 82% yield. The Co2Mn3O8-promoted aerial oxidation reactions also have been tested for heterocyclic alcohol, like (1H-indol-3-yl)methanol and thiophen-2-ylmethanol, and afforded the corresponding aldehyde selective in 58% and 42% yields, respectively. However, at optimized reaction conditions, pyridin-2-ylmethanol was found to be inactive to produce the corresponding aldehyde even after 18 h.

benzaldehyde in 12% yield even after 24 h. When judged in terms of their progress, the Co2Mn3O8 catalyst is found to be much more superior compared to other two catalysts, which further justifies the cooperativity in the catalyst (Figure S2, SI). Next, to check the generality of the reaction, Co2Mn3O8promoted aerial oxidation reactions were tested with various alcohols. Under optimized reaction conditions, benzyl alcohol substituted with electron-withdrawing groups at both p- and opositions were effective and produced their corresponding aldehyde selectively with a high yield (Figure 5). On the other 11507


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Figure 6. Reduction of p-nitrophenol (a), 2,4-dinitrophenol (b), 2,4,6-trinitrophenol (c) by NaBH4. Percent of reduction vs time plot for Co2Mn3O8-, Co3O4-, and MnO2-promoted reduction of p-nitrophenol to p-aminophenol by NaBH4 (d) and N2H4 (e) and mechanism of nitro to amine reduction by NaBH4 and N2H4 (f).

Furthermore, the aerial oxidation reaction of cinamyl alcohol leads to its corresponding aldehyde with a 44% yield. In the case of 4-(dimethylamino)benzaldehyde, reactions failed to produce any desired product because of its low electrophilicity.74 Further, to check the effect of oxidizing agents on the reactivity of the catalyst, Co2Mn3O8-promoted oxidation reactions were tested with various alcohols using H2O2 as an oxidizing agent. Under optimized reaction conditions, both aliphatic and substituted benzyl alcohols were oxidized to their corresponding aldehydes with good to excellent yields (Figure 5). The alcohols, like 1-phenyl ethanol and diphenylmethanol, also selectively converted into their corresponding ketones 2q and 2r in 95% and 85% yields, respectively, with appreciable TOF. To our delight, Co2Mn3O8-promoted oxidation reactions using H2O2 as an oxidant showed a higher catalytic activity and excellent selectivity (>99%) in the oxidation of alcohols to aldehydes/ketones for a variety of alcohols including cinamyl, pyridine, and thiophene moieties. Co2Mn3O8 Nanomaterial-Promoted Reduction of Nitro Aromatics. Catalytic hydrogenation of nitro compounds is an industrial process that has experienced a renovated interest in the past two decades due to the discovery of highly selective and environmentally friendly solid catalysts.75−78

Particularly, the performance of chemo-selective reduction in the presence of other sensitive functional groups, such as double bonds, carbonyls, etc., is important.79−82 In this regard, various nanoparticles83 and supported nano materials84,85 with other stoichiometric reducing agents have been utilized.86,87 Toward this goal, we wanted to check the catalytic activity of our synthesized Co2Mn3O8 nanomaterial for reduction of nitro aromatics. Initially, the reduction of nitrophenol was chosen as a model reaction. To perform the experiment, 3 mL of solution of 4-nitrophenol (200 μM) was taken in a UV-cuvette and to it 300 μL of 5 × 10−2 (M) NaBH4 and 1 mg of catalyst was added, and the progress of the reaction was monitored with UV−vis spectroscopy (Figure 6). 4-Nitrophenol exhibits an absorption peak at 317 nm in neutral solution, which is shifted to 400 nm after the addition of NaBH4 due to the generation of corresponding 4-nitrophenolate. During the course of reaction, the existing band at 400 nm gradually decreases with the generation of a new small peak at 300 nm. Therefore, the progress of the reaction and its kinetics was determined from the steady decrease of the absorbance at 400 nm. To compare the activity of the Co2Mn3O8 nonmaterial and also to check the cooperative effect of it, Co3O4 and MnO2 nanomaterials were synthesized by following the reported procedure. The activity of all three materials was checked with the above-mentioned 11508


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ACS Sustainable Chemistry & Engineering Table 2. Reaction Kinetics of Co2Mn3O8-Promoted Reduction of Nitro Aromatics to Amines

No. 1 2 3 4 5 6

Substrate 4-Nitrophenol 4-Nitrophenol 2,4-Dinitrophenol 2,4-Dinitrophenol 2,4,6-Trinitrophenol 2,4,6-Trinitrophenol

Reductant

Catalyst (mg) 1 1 1 1 1 1

NaBH4 N2H4 NaBH4 N2H4 NaBH4 N2H4

k (min−1) −3

62 × 10 32 × 10−3 59 × 10−3 38 × 10−3 56 × 10− 33 × 10−3

k′ (s−1mg−1) 1.03 0.53 0.98 0.63 0.93 0.55

× × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3

Co2Mn3O8-promoted reduction of nitrobenzene in the presence of NaBH4, the reaction mixture was collected at different time intervals and was analyzed with GC-MS to identify the different products (Figures S24−27, SI). Initially, we observed the formation of nitrosobenzene, which is converted to aniline via the formation of azobenzene, which clearly suggests that the reaction proceeds through the condensation route (Figure 6). The condensation route is also preferred in our case irrespective of the reducing agent as we observed both nitrosobenzene and azobenzene as transient intermediates for Co2Mn3O8-promoted reduction of nitrobenzene with hydrazine hydrate. Next, to check the generality of the methodology, the substrate scopes of the Co2Mn3O8 nanomaterial-promoted reaction for various nitro aromatics/heteroaromatics using both NaBH4 and N2H4.H2O as reducing agents were done and are illustrated in Tables 3 and 4, respectively. Initially, from a

optimized reaction condition with UV−vis monitoring. Among the three materials, MnO2 is found to be the least reactive, and only 10% conversion of corresponding 4-nitrophenolate was observed even after 2 h. On the other hand, both the percentage of reduction versus time plot as well as the measured pseudo-first-order rate constant (k) and rate activity parameter (k′) for the Co2Mn3O8 nanomaterial suggest higher reduction reactivity compared to Co3O4 and further justifies the cooperative effect of the Co2Mn3O8 catalyst (Figure 6). Further, the Co2Mn3O8 nanomaterial-promoted reduction reaction was checked with 2,4-dintrophenol and 2,4,6trinitrophenol under UV−vis monitoring. In both cases, complete disappearance of the peak at the visible region indicates the reduction of both substrates to the corresponding amino phenol (Figure 6). In a heterogeneous system, it is demonstrated that reduction of aromatic nitro compounds generally involves four steps: (i) absorption of hydrogen, (ii) absorption of aromatic nitro compounds to the metal surfaces, (iii) electron transfer mediated by metal surfaces from BH4 to aromatic nitro compounds, and (iv) desorption of aromatic amino compounds. To check the binding of nitrobenzene in the surface of Co2Mn3O8, FTIR analysis was done. For sample preparation, 100 μL of nitrobenzene was added to 50 mg of the Co2Mn3O8 catalyst in 0.5 mL of methanol and sonicated for 1 h. After that, the material was washed with methanol and then centrifuged, and the process has been repeated 10 times to completely remove the unbound nitrobenzene from the material. Finally, the material was dried before performing the experiment. The presence of nitrobenzene in the material was confirmed from the FT-IR analysis as the peaks are well matched with standard nitrobenzene. This confirms the absorption of nitrobenzene on the surface of the Co2Mn3O8 nanomaterial (for details, see Figure S4 in the SI). Further, to check the effect of the reducing agent on the reaction, hydrazine hydrate was utilized in the Co2Mn3O8promoted reduction reaction, and its kinetics was monitored for 4-nitrophenol, 2,4-dintrophenol, and 2,4,6-trinitrophenol under UV−vis monitoring. To perform the experiment, 3 mL of solution of 4-nitrophenol (200 μM) was taken in a UV-cuvette and to it 100 μL of N2H4 and 1 mg of catalyst was added, and the progress of the reaction was monitored with UV−vis spectroscopy (Figure S5, SI). Gratifyingly, the observed rate activity parameters (k′) in all the three cases are comparable with k′ obtained using NaBH4 as the reducing agent (Table 2). The higher reduction rate of Co2Mn3O8 for 4-nitrophenol in comparison to their monometallic oxide counterpart (MnO2 and Co3O4) further justifies the synergistic effects between each component (Figure S4, SI). Next, to know the mechanism of

Table 3. Substrate Scope for Co2Mn3O8-Promoted Reduction Reaction Using NaBH4a

a Reaction conditions: nitro aromatics (10 mmol), Co2Mn3O8 (0.05 mol %), MeOH/H2O (5/25 mL), NaBH4 (750 mg), room temperature. Yields were calculated from (superscript a) UV−vis, (superscript b) GC-MS, and (superscript c) 1H NMR monitoring of the crude reaction mixture.

screening of solvent, temperature, and catalyst loading, the reaction condition was optimized. Under optimized reaction conditions, Co2Mn3O8-promoted reduction for electron-withdrawing and -releasing groups containing nitro aromatics were found to be effective for the formation of corresponding amino aromatics almost quantitatively with good TOF utilizing both NaBH4 and N2H4·H2O as the hydrogen source. To our delight, bromo-substituted nitroarenes, which can undergo facile dehelogenation, were selectively reduced to the respective haloaromatic amine without showing any sign of dehalogenation. Further, to check the chemoselectivity, the reaction done with a multifunctional substrate containing nitro with other 11509


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dye. The reversible redox reaction of methylene blue (MB) to leuco methylene blue (LMB) is a very interesting reaction as we can visually observe the dramatic reversible color change. To perform the experiment, 3 mL of solution of MB (1 × 10−4 M) was taken in a UV-cuvette and to it 20 μL of N2H4 and 1 mg of catalyst was added, and the progress of the reaction was monitored with UV−vis spectroscopy (Figure 7). On addition of Co2Mn3O8 in aqueous medium, the initial blue color of the MB dye faded away producing colorless leuco-methylene blue (LMB). The steady decrease in the two absorbance maxima at 662 and 290 nm and appearance of a new band at 255 nm due to the formation of leuco methylene blue (LMB) suggests the progress of the reaction. However, no color change of MB was observed in the absence of the catalyst. Interestingly, shaking of the reaction mixture or blowing air through the solution resulted in a color change from colorless to blue. Again on standing, the blue color turned colorless. The kinetics of Co2Mn3O8-promoted MB to LMB reduction reaction was done by monitoring the steady decrease in absorbance at the 662 nm band at different time intervals. Furthermore, a plot of Ln(A) vs time leads to a straight line, further confirming the pseudo-first-order reaction kinetics. To check the effect of the amount of catalyst, dye, and N2H4·H2O on the progress of the reaction, a series of experiments were performed using UV−vis spectroscopy. On increasing the concentration of the catalyst and N2H4, the rate of the reaction increases, while on increasing the concentration of the dye the rate of the reaction decreases with time (Figure S10, SI). Further, to check the substrate scope for the Co2Mn3O8promoted reaction, various cationic, anionic, and neutral dyes were taken as substrates, and their rate kinetics were monitored. It was observed that the reductions of dyes in the presence of Co2Mn3O8 were active for both cation and anionic dyes with appreciable rate constants. But in case of neutral dyes, no change was observed (Figure S8, SI). To our delight, Co2Mn3O8 is found to be active for the reduction mixture of five different cationic and anionic dyes with appreciable rates.

Table 4. Substrate Scope for Co2Mn3O8-Promoted Reduction Reaction Using N2H4a

a

Reaction conditions: nitro aromatics (10 mmol), Co2Mn3O8 (0.05 mol %), MeOH/H2O (5/25 mL), N2H4 (100 uL), room temperature. Yields were calculated from (superscript a) UV−vis, (superscript b) GC-MS, and (superscript c) 1H NMR monitoring of the crude reaction mixture.

reducible functionalities such as CN, styrene, and CO2H. Interestingly, when we used NaBH4 as the hydrogen source, chemo selective reduction of nitro to amine is possible, and other functional groups remained unaffected. The Co2Mn3O8 nanomaterial-promoted reduction reaction is also found to be suitable for 5-nitro indole, and the corresponding product is achieved in 92% yield after 4 h. It should be pointed out that hydrazine-mediated protocols cannot be used for aldehyde or ketone functionalities since these carbonyl derivatives readily form the corresponding hydrazones at room temperature and is in agreement with previous reports.88,89 Similarly, Co2Mn3O8promoted reduction of 3-nitro styrene in the presence of hydrazine hydrate produces corresponding 3-ethyl aniline in 90% yield. Co2Mn3O8 Nanomaterial-Promoted Reduction of Dye. As the synthesized material shows promising reactivity in the presence of N2H4, we wanted to check the reduction of various dyes as they are also known as environmental pollutants.90,91 For this purpose, methylene blue (MB) was chosen as a model

Figure 7. MB clock reaction (a), reduction of crystal violet (b), Congo red (c), methyl red (d), and methyl violet (e). 11510


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Figure 8. Percent of conversion versus no of cycle plot for Co2Mn3O8-promoted reduction of 4-nitrophenol and aerial oxidation of benzyl alcohol (a). Digital images of the magnetically reusable catalyst in three reactions before and after the use (b). XPS (c) and PXRD (d) of used Co2Mn3O8 nanomaterial.

Reusability Test for Co2Mn3O8-Promoted Reduction and Oxidation Reaction. For practical applications of such a heterogeneous nanocatalyst, the lifetime and its reusability is a very important factor. To determine this, a set of experiments for both the reduction and oxidation reactions were done under optimized reaction conditions. The room temperature reduction of 4-nitrophenol (10 mmol) was done with 10 mg of Co2Mn3O8 catalyst in the presence of 750 mg of NaBH4 in 30 mL water. After the completion of the first reaction (via UV− vis monitoring), the catalyst was recovered using a tiny magnet, washed three to four times with MeOH and water, and then used for the next set of reactions with fresh reactants (Figure 8). We have monitored the reaction up to the 10th cycle with no appreciable reduction in its reactivity. After the 10th cycle, AAS analysis of the used Co2Mn3O8 nanomaterial was done after digesting the sample with concentrated HCl (Table 4). Interestingly, no appreciable change in both cobalt and manganese contents was observed even after the 10th cycle, which directly indicates the reusability of the nanomaterial for the reduction reaction. The PXRD and XPS analysis of the used material after the 10th cycle was also analyzed. Both analyses suggest no change in the material, which directly indicates the durability of Co2Mn3O8 even after the 10th cycle. Next, to check the reusability of the Co2Mn3O8 nanocatalyst for the aerial oxidation reaction, benzyl alcohol was chosen as a model substrate. After the completion of the reaction (vide TLC and yield via GC), used material was recovered using a tiny magnet, washed three to four times with MeOH, and used for the next set of reactions. Interestingly, up to the fifth cycle,

no appreciable reduction in its activity was observed. However, a slight drop in the yield was observed in the eighth (64%) and 10th (54%) cycles (Figure 8). After the fifth and 10th cycle, AAS analysis of the used Co2Mn3O8 nanomaterial was done to check the leaching of the catalyst (Table 5). No appreciable leaching was observed after the fifth cycle. However, both the Co and Mn contents in the used Co2Mn3O8 nanomaterial after the 10th cycle was observed. Large-Scale Application of Co2Mn3O8-Promoted Reduction and Oxidation Reaction. Next, we wanted to extend our study for large-scale application, and thus, the activity of the Co2Mn3O8 nanomaterial was checked for both Table 5. Determination of Co and Mn Content in Material Using AAS Analysisa Reduction Material

Co (wt %)

Mn (wt %)

Co-MnOx Co-MnOx (after 10th cycle)

15.65 11.16

15.28 10.67

Material

Co (wt %)

Mn (wt %)

Co-MnOx Co-MnOx (after 5th cycle) Co-MnOx (after 10th cycle)

15.65 10.01 7.86

15.28 9.56 6.4

Oxidation

a

Both the Co and Mn contents were determined using AAS analysis after digesting the sample with HCl.

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ACS Sustainable Chemistry & Engineering Table 6. Large-Scale Application of Co2Mn3O8-Promoted Reduction Reactiona

No.

Scale (mmol)

Cat. (mg)

Cat. (mol %)

NaBH4 (g)

H2O (mL)

t (h)

TOF (h−1)

Yield

1 3 4 5

10 50 100 300

2 20 50 200

0.05 0.1 0.125 0.16

0.75 3.0 6.0 15.0

30 200 300 600

0.33 6 10 18

5939 150 72 25.7

98 90 90 74

a

Reaction conditions: 50 mmol scale, 7.0 g of 4-nitro phenol and 3.0 g of NaBH4 were taken in 200 mL of water and to it 20 mg of Co2Mn3O8 catalyst was added and stirred at room temperature; 300 mmol scale, 42.0 g of 4-nitrophenol and 10.0 g of NaBH4 were taken in 600 mL of water and to it 200 mg of catalyst was added and stirred at room temperature.

Table 7. Large-Scale Application of Co2Mn3O8-Promoted Oxidation Reactiona

No.

R

Scale (mmol)

Catalyst (mg)

Catalyst (mol %)

Oxidant

Toluene (mL)

Time (h)

TOF (h−1)

Yield (%)

1 3 4 5 6 7 8 9

H H H H H OMe Cl Octanol

10 50 100 100 100 100 100 100

10 100 100 200 50 200 200 200

0.25 0.5 0.25 0.5 0.125 0.5 0.5 0.5

Air Air Air Air H2O2 Air Air H2O2

3 100 200 200 200 200 200 200

18 24 36 36 15 48 36 36

18.2 5.2 5.3 3.9 45.9 2.2 4.2 4.5

82 62 48 71 86 54 76 81

a Reaction conditions: 50 mmol scale, 5.2 mL of benzyl alcohol and 100 mg of Co2Mn3O8 nanocatalyst were taken in 100 mL of toluene and refluxed at 130 °C; 100 mmol scale, 10.4 mL of benzyl alcohol and 200 mg of Co-MnOx nanocatalyst were taken in 200 mL of toluene and refluxed at 130 °C. Yields were calculated from (superscript a) GC analysis with o-xylene as an internal standard.

min. Again, a solution of NaBH4 is prepared by adding 1.4 g (40 mM) of NaBH4 in 30 mL of deaerated water. Then, the NaBH4 solution was added slowly to the earlier prepared solution. During the course of the reaction, the purple color of CoCl2·6H2O immediately changes to black indicating the formation of Co nanoparticles. The solution is stirred at room temperature at 900 rpm for 30 min until the evolution of H2 gas ceased. After this, a 40 mM solution of KMnO4 is prepared and poured it into the reaction flask and kept under a refluxing condition for 24 h under a nitrogen atmosphere. After 24 h, black material was collected by centrifugation and thoroughly washed with water five to six times and then dried in an oven at 80 °C for 12 h. The weight of the collected material was 8.2 g. PXRD. PXRD spectra were recorded on a Philips PW1710X-ray difractometer (40 kV, 20 mA) using Cu Kα radiation (k1/4 1.5418°A) in the 2θ range of 10−60° at a scanning rate of 0.5° min−1. UV−Vis Study. Absorption spectra were recorded in a Dynamica Halo DB-30 double beam digital spectrophotometer (Switzerland) attached with a Lab Companion RW-0525G chiller and also a SHIMADZU UV 2550 spectrophotometer with a quartz cuvette. UV− vis spectra of the synthesized complexes were recorded in water. Field Emission Scanning Electron Microscope (FESEM). The morphology of the synthesized Co-MnOx was characterized using a field emission scanning electron microscope (ZEISS EVO 60 with oxford EDS detector) operating at 5−10 kV. FT-IR. All the samples for FTIR study were properly washed and then dried under vacuum. Finally, samples for the FTIR spectra were recorded using an IMPACT 410 Thermo-Nicolet instrument. General Procedure for Co2Mn3O8-Promoted Aerial Oxidation of Alcohols to Aldehydes/Ketones. To perform the experiment of oxidation of alcohol, 10 mmol of corresponding alcohol was dissolved in 30 mL of toluene and to it 10 mg (0.25 mol %) of catalyst was added. The reaction mixture was stirred at 130 °C for a desired time. To confirm the formation of the product, TLC was

the reduction and oxidation reactions for the synthesis of both amino aromatics and aldehyde selectively in gram scale. Initially, the Co2Mn3O8-promoted reduction of 4-nitrophenol to 4-aminophenol was scaled up stepwise from 1 to 300 mmol with a variable amount of catalyst and solvent (Table 6). At a level of 50 mmol, 20 mg of catalyst in the presence of 3.0 g of NaBH4 was sufficient to convert 7.0 g of 4-nitrophenol to a corresponding 4.9 g of amino phenol within 6 h. Further scale up from 50 to 300 mmol with 200 mg of catalyst leads to corresponding amine in 74% yield. Next, the Co2Mn3O8promoted aerial oxidation of benzyl alcohol was also sequentially scaled up from 1 to 100 mmol with a variable amount of catalyst and solvent (Table 7). At 50 mmol scale, 5.2 mL of benzyl alcohol and 100 mg of Co2Mn3O8 were taken in 100 mL of toluene and refluxed at 130 °C for 24 h to afford the benzaldehyde in 62% yield. Further scale up from 50 to 100 mmol with 200 mg of catalyst afforded benzaldehyde selectively in 71% yield. The large-scale (100 mmol) application of Co2Mn3O8-promoted aerial oxidation also has been extended for other substrates, like 4-methoxybenzyl alcohol, 4-chloro benzyl alcohol, and octanol, and afforded the corresponding aldehyde in 54% and 76% and 81% yield, respectively.

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EXPERIMENTAL SECTION

Typical Procedure for Synthesis of Heterobimetallic Co2Mn3O8 Nanomaterial. A solution of CoCl2·6H2O is prepared by adding 4.75g (20 mM) of CoCl2·6H2O in 50 mL of double distilled water. To it, a solution of 5.0 mM (1.29g) trisodium citrate dihydrate is added in the round-bottomed flask and the volume made up with water to 150 mL. The mixture is deaerated with nitrogen gas for 10 11512


ACS Sustainable Chemistry & Engineering monitored at a regular interval of time. The yield of the reaction is then monitored with GC analysis using o-xylene as an internal standard. General Procedure for Co2Mn3O8-Promoted Oxidation of Alcohols to Aldehydes/Ketones Using H2O2. To perform the experiment of oxidation of alcohol, 10 mmol of the corresponding alcohol was dissolved in 30 mL of toluene and to it 10 mg (0.05 mol %) of catalyst and 0.7 mL of H2O2 were added. After that, the reaction mixture was stirred at 130 °C until the completion of the reaction. To confirm the formation of the product, TLC was monitored at a regular interval of time, and the conversion was calculated from the GC analysis utilizing o-xylene as an internal standard. General Procedure for Reduction of Nitro Compound to Amine Compound Using NaBH4. To perform the experiment, 10 mM of the corresponding nitro compound was taken and to it 750 mg of NaBH4 and 0.05 mol % of catalyst were added. The progress of the reaction is then monitored with a UV−vis spectrometer or GC analysis, or 1H NMR monitoring. General Procedure for Reduction of Nitro Ccompound to Amine Compound Using N2H4. To perform the experiment, 10 mM of a corresponding nitro compound was taken in the reaction flask and to it 100 uL of N2H4 and 0.05 mol % of catalyst were added. The progress of the reaction is then monitored by using a UV−vis spectrometer or GC analysis, or 1H NMR. General Procedure for Reduction of Various Dye Molecules. To perform the experiment, 3 mL of solution of dye solutions (2 × 10−4 M) is taken and to it 20 μL of N2H4 and 2 mg of catalyst is added. The reaction is then monitored under UV−vis spectrometer.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSION In summary, a cost-effective chemical route for producing Co2Mn3O8 nanomaterial in gram scale has been reported for the redox-mediated reaction between in situ-generated Co(0) nonmaterial and KMnO 4 in water. The as-synthesized Co2Mn3O8 nanomaterial is found to be useful as a magnetically reusable catalyst for chemoselctive reduction of nitro to amines, various dyes, and aerial oxidation of alcohols to corresponding aldehydes/ketones under relatively milder reaction conditions with high turnover frequency (TOF). The enhanced reactivity in Co2Mn3O8 in comparison to their monometallic oxide nanomaterial (MnO2 and Co3O4) for both the oxidation and reduction reaction further justifies the cooperativity in the catalyst. Aqueous reaction medium, easy and scalable synthesis, large-scale application, efficient recycling, and effortless separation of the catalyst using an external magnet make the protocol economical and sustainable. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02739. FT-IR spectra, UV−vis data, and NMR data. (PDF)

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Financial support of this work by DST-New Delhi (to S.P. for INSPIRE Grant No: IFA/12-CH-39 and K.S. for INSPIRE fellowship) is gratefully acknowledged. The author thanks the esteemed reviewers for their useful suggestions. The INUP, IITB, IITBNF, and IITB (sponsored by DeitY, MCIT, Government of India) is gratefully acknowledged for giving us access of the XPS and HR-TEM facility.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Sanjay Pratihar: 0000-0002-0229-735X Notes

The authors declare no competing financial interest. 11513


Research Article

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Magnetically Recoverable Heterobimetallic Co2Mn3O8: Selective and

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