Magnetically Recoverable Heterobimetallic Co2Mn3O8: Selective and

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Magnetically recoverable heterobimetallic Co2Mn3O8: selective and sustainable oxidation and reduction reaction Kasturi Sarmah, Joyeta Pal, Tarun Kumar Maji, and Sanjay Pratihar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02739 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Sustainable Chemistry & Engineering

1

Magnetically Recoverable Heterobimetallic Co2Mn3O8:

2

Selective and Sustainable Oxidation and Reduction

3

Reaction

4

Kasturi Sarmah,a Joyeta Pal,a Tarun K. Maji,a and Sanjay Pratihara,*

5 6

a

Department of Chemical Sciences, Tezpur University, Napaam, Asaam-784028, India Email: [email protected] or [email protected]

7 8 9

Abstract: The present work reports an efficient, sustainable, and cost effective chemical

10

route for gram scale synthesis of heterobimetallic Co-Mn oxide (Co2Mn3O8) from the redox

11

reaction between in-situ generated zero-valent cobalt nanomaterial and KMnO4 in water. The

12

magnetically recoverable Co2Mn3O8 nanomaterial showed promising catalytic activity and

13

good selectivity (>99%) for the oxidation of alcohols to aldehydes/ketones both in presence

14

of air and hydrogen peroxide for a variety of alcohols including benzyl, aliphatic, cinamyl,

15

pyridine and thiophene moieties. The inexpensive Co2Mn3O8 furnish excellent catalytic

16

activity and chemoselectivity (>99%) for the reduction of wide range of aromatic and

17

heteroaromatic nitro substrates to corresponding amines and various dyes under relatively

18

milder reaction condition with high turnover frequency (TOF). The high catalytic

19

performance and durability in Co2Mn3O8 nanomaterial for both the reaction in comparison to

20

their monometallic oxide (MnO2 and Co3O4) is further attributed to the synergistic effects

21

between each component. Easy synthesis, large scale application, excellent selectivity,

22

effortless separation of the catalyst using an external magnet and efficient recycling make the

23

catalyst and the protocol economical and sustainable.

24

Keywords. Cooperativity, Oxidation, Reduction, Benzyl Alcohol, Nitro Aromatics.

1 ACS Paragon Plus Environment


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Introduction

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Catalytic reactions are not only at the heart of making of most chemicals, including

3

our domestic products, materials, medicines, but also play an important role in various

4

type of energy and environment application.1,2 Heterogeneous catalysis, in which

5

phase is different that of reactants, is quite prevalent in the chemical industry, and

6

affects our everyday life in many ways.3-6 In this regard, the advent of transition-metal

7

catalysed strategies for new carbon-carbon and carbon-heteroatom bond forming

8

methodologies have revolutionized the field of organic chemistry, allowing the

9

efficient synthesis of ligands, materials, and biologically active molecules.7-11 Over the

10

past few decades, heterobimetallic catalysis,12-17 an important subarea within the

11

broader domain of multimetallic catalysis18-22 has received much attention since

12

synergistic cooperation of two distinct metal centres can enhance the catalytic activity

13

and selectivity, where a single metal fails to promote a selective or efficient

14

transformation. Towards this, development of various homogeneous catalyst including

15

cooperative homo or/and hetero bimetallic,23-29 tandem catalyst,

16

metal reagents32,33 for several types of bond forming methodologies are noteworthy.

17

At the same time, heterogeneous bimetallic catalyst with various combinations

18

including early-early,34-36 early-late,37,38 late-late39,40 transition metal as well as various

19

transition-main group metal41-43 for different type of catalytic and material applications

20

are noteworthy.

21

On the other hand, oxidation of alcohols to aldehydes or ketones and reduction of nitro

22

to amine are two important fundamental reactions because their products are essential

23

building blocks for various drugs, agro-chemicals, and fragrances.44,45 Traditionally,

24

stoichiometric oxidizing and reducing agents in presence of strong mineral acids were

25

used for oxidation and reduction reactions, which generate enormous amounts of

30,31

along with dual


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1

poisonous metal salts as waste.46,47 In past few decades, many efforts have been made

2

to develop environmentally benign oxidation and reduction systems to protect the

3

environment from these wastes.48-51 In this regard, various heterogeneous catalysts

4

have been developed for both the methodology.52-54 However, these methods have one

5

or more of the following limitations, for example, lack of broad functional group

6

tolerance, use of expensive metals, applicable for the activated substrates, use of

7

excess quantities of additives such as bases, oxidant, and electron transfer mediators,

8

low turnover frequency and so on.55,56 To overcome the above mentioned problems,

9

various approaches have been applied by various research groups for both the

10

reactions.57,58 As an important contribution, Beller et al. utilized earth-abundant cobalt

11

and cobalt oxides based catalyst with activated carbon support in many chemical

12

processes, including nitroarene reduction and aerobic oxidation of alcohols.59,60

13

Recently, Li et al. reported Mott−Schottky catalyst of nitrogen-rich carbon-coated

14

cobalt nanoparticles, which boost the activity of a transition-metal nanocatalyst

15

through electron transfer at the metal/nitrogen-doped carbon interface.61 By combining

16

an oxygen-activating species, nitrogen-doped carbon, and a simple metal-oxide

17

catalyst, Rothenberg et al. showed cooperative catalysis for selective aerial oxidation

18

of alcohol to aldehydes/ketones. We are particularly interested in inexpensive and

19

efficient oxygen deficient bimetallic catalysts that could be easily accessible, reusable,

20

and suitable for large scale applications for various chemical transformations. Herein,

21

we presented an unconventional redox mediated cost effective, and environment

22

friendly approach for the synthesis of mixed hetero-bimetallic oxygen deficient

23

Co2Mn3O8 from the redox reaction between in-situ generated zero valent cobalt

24

nanomaterial and KMnO4 in water. The as prepared Co2Mn3O8 is found to be useful as

25

a magnetically recoverable catalyst for chemo-selective reduction of nitro to amine



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and various dyes in presence of hydrazine hydrate at room temperature, and selective

2

oxidation of alcohols to corresponding aldehydes/ketones in presence of air or

3

hydrogen peroxide. The enhanced reactivity of Co2Mn3O8 nanomaterial in comparison

4

to its mono-metallic oxide in both the cases justifies the co-operativity in the catalyst

5

(Figure 1).

6 7

Figure 1. Synthetic Route of Cooperative Co-MnOx

8

Results and Discussion

9

Synthesis and Characterization

10

The reduction of CoCl2.6H2O in water by NaBH4 in presence of trisodium citrate was

11

done at room temperature. During the reaction, purple colour solution slowly turned

12

into black and finally appeared as black precipitate, which was further reacted with

13

aqueous solution of KMnO4 at 120 °C under nitrogen atmosphere for 24 h. After the

14

completion of the reaction, black precipitate was collected using a tiny magnet and

15

washed with water for several times and dried in oven at 80°C for 12 h. The PXRD


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1

analysis (Figure 2) of synthesized heterobimetallic Co-Mn oxide nanomaterial

2

(hereafter Co-MnOx) showed peaks at 2θ values of 19.2, 20.5, 26.1, 30.8, 32.3, 34.1,

3

37.2, 38.8, 44.6, 48.9, 50.9, 56.5, 65.2 and 74.4 for characteristics diffraction indices

4

(002), (011), (012), (112), (103), (031), (211), (004), (122), (220), (310), (030), (314)

5

and (324) respectively (JCPDS No: 70-0931).62 The PXRD pattern of Co-MnOx

6

correspond to orthorhombic phase and the composition of the material comes out to be

7

Co2Mn3O8 (Figure 2). The average crystallite size (Dcry) of Co-MnOx, determined

8

from Williamson-Hall equation βcosθ = 0.9λ/Dcry + 4ϵsinθ, is found to be 26 nm.63

9

The FTIR spectra of the Co-MnOx show characteristic peaks at 652 and 559 cm-1 for

10

the bending vibrations of Co-O and Mn-O respectively.64

11 12

Figure 2. PXRD pattern of Co-MnOx nanomaterial.

13

The XPS survey spectra of Co-MnOx exhibit the presence of Co, Mn, C and O elements

14

(Figure 3a). All spectra were calibrated using the C1s peak with a ﬁxed value of 286.8

15

eV. The peak present at 531.2 eV was for oxygen (O1s) in the Co2Mn3O8. When we

16

observed the high resolution spectra for Co, the peaks with BEs at 781.4 eV and 796.3 eV

17

were mainly for Co2p1/2 and Co2p3/2. The presence of satellite peak at 786.2 eV confirms the

18

oxidation state of Co as Co2+. Further, the peaks at binding energies of 715 eV, 826.2 eV and

19

865.3eV correspond to Co auger peaks and peak at 952.6 eV is for Co2s. The XPS study of 5 ACS Paragon Plus Environment


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Mn oxide is of great interest due to the ability of manganese to form various oxides with Mn

2

in different oxidation states (Mn2+ to Mn4+). However, the high resolution spectra of Mn

3

shows peaks for Mn2p3/2 and Mn2p1/2 at 641.8 and 652.2 eV attributes for the Mn4+ oxidation

4

state in the synthesised material. A much weaker satellite peak at 645.6 eV was observed for

5

Mn4+. The peaks at binding energies of 61.8, 49.1 and 903.4eV were for Mn3s, Mn3p and Mn

6

auger electrons respectively.65 From the XPS analysis it was confirmed that Co and Mn in

7

Co-MnOx nanomaterial were in their II and IV oxidation states respectively, which further

8

confirmed the material as Co2Mn3O8.66 The calculated atomic percentage of Co and Mn in

9

Co2Mn3O8 from XPS analysis was found to be 11.4 and 13.7 respectively.

10 11

Figure 3. Wide-scan (survey spectrum; a) and narrow-high resolution scan XPS

12

spectra (b-d) of Co2Mn3O8 nanomaterial.

13


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1

The FE-SEM of the synthesized material shows agglomerated morphology. The

2

energy-dispersive X-ray analysis (EDS) shows the presence of Co, Mn and oxygen in

3

the nanomaterial (Please see Figure S23 in SI-1). Further, HR-TEM analysis confirms

4

the agglomerated nature of the synthesized material. The fringe spacing shows that the

5

particles grow in 112 and 103 planes with alternate dark and light fringes. The SAED

6

pattern of Co2Mn3O8 suggests the polycrystalline nature of the material. The calculated

7

d-spacing was well matched with the planes observed in PXRD analysis. The particle

8

size distribution has also been analyzed and the average particle size was found to be

9

∼27 nm.67-68

10 11

Figure 4. SEM (a-c), HR-TEM (d), Fringe spacing (e), SAED pattern (f) and particle

12

size distribution (inset of d) of Co2Mn3O8.

13 14

Co2Mn3O8 nanomaterial promoted oxidation of Alcohol

15

Next, to check the catalytic activity of Co2Mn3O8 nanomaterial, the aerial oxidation69-

16

70

17

optimization from the screening of solvent, temperature, and catalyst loading. The

of benzyl alcohol was chosen as a model reaction. Initially, reaction condition was



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Co2Mn3O8 is found to be active to produce desired benzaldehyde in 92% yield at 130

2

˚C in toluene. Upon decreasing the catalyst loading from 1 to 0.25 mol% the TOF

3

steadily increases, while the product yield drops down slightly at 0.25 mol% loading

4

(Table 1, entry 4-7). Amongst other oxidants, H2O2 was found to be effective for the

5

reaction and proceeds with higher turnover frequency (TOF). It is noteworthy to

6

mention that under optimized reaction condition, all of the tested catalysts inclusive of

7

anhydrous FeCl3, CoCl2, MnCl2, Fe3O4,71 MnO2, Co0, Fe(ox)-Fe3O472 were found to

8

be less effective on the reaction (Table 1, entry 12-21). Notably, either in the absence

9

of catalyst or in the absence of oxidant, no reaction was observed (Table 1, entry 9-

10

10). Further, to check the cooperative effect, the reactivity of synthesized Co2Mn3O8

11

nanomaterial was compared with Co3O4 and MnO2 nanomaterial for model reaction.73

12

Amongst the three materials, Co3O4 is found to be least reactive and affords

13

corresponding benzaldehyde in 12% yield even after 24h. When judged in terms of

14

their progress, Co2Mn3O8 catalyst is found to be much more superior compare to other

15

two catalysts, which further justifies the cooperativity in the catalyst (Figure S2 in

16

ESI). Table 1. Screening of Catalysta

c

TOF (h-1)

#

Cat.

Oxidant

mol%

T, ˚C

Solvent

t, h

Yield,%

1

Co2Mn3O8

Air

1

80

MeCN

12

58

4.8

2

Co2Mn3O8

Air

1

100

H2O

12

50

4.2

3

Co2Mn3O8

Air

1

80

MeOH

12

65

5.4

4

Co2Mn3O8

Air

1

130

Toluene

12

88

7.3

6

Co2Mn3O8

Air

0.5

130

Toluene

12

92

15.2

7

Co2Mn3O8

Air

0.25

130

Toluene

12

62

20.6

8

Co2Mn3O8

O2

0.25

130

Toluene

12

94

31.3

9

Co2Mn3O8

N2

0.25

130

Toluene

12

20

6.6


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b

-

130

Toluene

12

20

-

b

0.05

130

Toluene

10

92

184

Air

0.5

130

Toluene

12

10

1.6

Co3O4

H2O2

0.5

130

Toluene

12

21

3.5

14

MnO2

Air

0.5

130

Toluene

12

32

5.3

15

MnO2

H2O2

0.5

130

Toluene

12

52

8.6

10

-

H2O2

11

Co2Mn3O8

H2O2

12

Co3O4

13

0

16

Co

H2O2

1

130

Toluene

12

14

1.1

17

Fe3O4

H2O2

1

130

Toluene

12

30

2.5

18

Fe(ox)-Fe3O4

H2O2

1

130

Toluene

12

40

3.3

19

CoCl2

H2O2

1

130

Toluene

12

22

1.8

20

FeCl3

H2O2

1

130

Toluene

12

32

2.6

21

MnCl2

H2O2

1

130

Toluene

12

36

3

a

b

c

Model reaction done at 10 mmol scale, 0.7 mL of H2O2 used for the reaction, Yields were calculated from GC analysis with o-xylene as an internal standard.

1 2

Next, to check the generality of the reaction, Co2Mn3O8 promoted aerial oxidation

3

reaction were tested with various alcohols. Under optimized reaction conditions,

4

benzyl alcohol substituted with electron-withdrawing groups at both p- and o-position

5

were effective and produced their corresponding aldehyde selectively with a high yield

6

(Figure 5). On the other hand, alcohol such as p-methoxy benzyl alcohol, p-methyl

7

benzyl alcohol reacted slowly to form the corresponding aldehyde in good yields.

8

Furthermore, 3-nitro benzyl alcohol also produced the corresponding aldehyde in 82%

9

yield. The Co2Mn3O8 promoted aerial oxidation reactions also have been tested for

10

heterocyclic alcohol like; (1H-indol-3-yl)methanol and thiophen-2-ylmethanol and

11

afforded the corresponding aldehyde selective in 58% and 42% yield, respectively.

12

However, at optimized reaction condition pyridin-2-ylmethanol found to be inactive to

13

produce the corresponding aldehyde even after 18h. Furthermore, the aerial oxidation

14

reaction of cinamyl alcohol leads to its corresponding aldehyde with a 44% yield. In

15

case of octanol or 4-(dimethylamino)benzaldehyde, reactions failed to produce any

16

desired products because of their low electrophilicity.74



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Further, to check the effect of oxidizing agent on the reactivity of the catalyst,

2

Co2Mn3O8 promoted oxidation reaction were tested with various alcohols using H2O2

3

as an oxidizing agent. Under optimized reaction conditions both aliphatic and

4

substituted benzyl alcohols were oxidized to their corresponding aldehydes with good

5

to

6

diphenylmethanol also selectively converted into their corresponding ketones 2q and

7

2r in 95% and 85% yield with appreciable TOF. To our delight, Co2Mn3O8 promoted

8

oxidation reactions using H2O2 as an oxidant showed a higher catalytic activity and

9

excellent selectivity (>99%) in the oxidation of alcohols to aldehydes/ketones for a

10

excellent

yields

(Figure

5).

The

alcohols

like;

1-phenyl

ethanol

and

variety of alcohols including cinamyl, pyridine and thiophene moieties.

11 12

Figure 5. Substrate scope for Co2Mn3O8 promoted oxidation of variety of alcohols in presence of air

13

and hydrogen peroxide. Yields were calculated from aGC with o-xylene as an internal standard,

14

b

15

(0.25 mol%), Toluene (30 mL), temperature 130 ˚C. H2 O2 promoted Oxidation: alcohol (10 mmol),

16

Co2Mn3O8 (0.05 mol%), Toluene (30 mL), H2 O2 (0.7 mL), temperature 130 ˚C.

isolated yield, cNMR yield, TOF values are in h-1, Aerial Oxidation: alcohol (10 mmol), Co2Mn3O8

17 18

Co2Mn3O8 nanomaterial promoted reduction of nitro aromatics

19

Catalytic hydrogenation of nitro compounds is an industrial process that has

20

experienced a renovated interest in the past two decades due to the discovery of highly


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and

environmentally

friendly

solid

catalysts.75-78

1

selective

Particularly,

the

2

performance of chemo-selective reduction in presence of other sensitive functional

3

groups such as double bond, carbonyl etc is important.79-82 In this regard, various

4

nanoparticles,83 supported nano materials84,85 with other stoichiometric reducing agents

5

have been utilized.86,87 Towards this goal, we wanted to check the catalytic activity of

6

our synthesized Co2Mn3O8 nanomaterial for reduction of nitro-aromatics. Initially, the

7

reduction of nitro phenol was chosen as a model reaction. To perform the experiment,

8

3 ml solution of 4-nitrophenol (200 µM) was taken in a UV-cuvette and to it 300µl of

9

5×10-2 (M) NaBH4 and 1 mg of catalyst was added and the progress of the reaction

10

was monitored with UV-vis spectroscopy (Figure 6). 4-nitrophenol exhibits an

11

absorption peak at 317 nm in neutral solution, which is shifted to 400 nm after the

12

addition of NaBH4 due to the generation of corresponding 4-nitro phenolate. During

13

the course of reaction, the existing band at 400 nm gradually decreases with the

14

generation of a new small peak at 300 nm. Therefore, the progress of the reaction and

15

its kinetic was determined from the steady decrease of the absorbance at 400 nm. To

16

compare the activity of Co2Mn3O8 nonmaterial and also to check the cooperative effect

17

in it, Co3O4 and MnO2 nanomaterial was synthesized by following the reported

18

procedure. The activity of all the three material was checked with above mentioned

19

optimized reaction condition with UV-vis monitoring. Amongst the three materials,

20

MnO2 is found to be least reactive and only 10% conversion of corresponding 4-

21

nitrophenolate observed even after 2h. On the other hand, both the percentage of

22

reduction versus time plot as well as measured pseudo first order rate constant (k) and

23

rate activity parameter (k’) for Co2Mn3O8 nanomaterial suggest higher reduction

24

reactivity compare to Co3O4, further justifies the cooperative effect in Co2Mn3O8

25

catalyst (Figure 6). Further, Co2Mn3O8 nanomaterial promoted reduction reaction was



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checked with 2,4-dintrophenol and 2,4,6-trinitro phenol under UV-vis monitoring. In

2

both the cases complete disappearance of peak at visible region is indicates the

3

reduction of both the substrate to corresponding amino phenol (Figure 6).

Figure 6. Reduction of p-Nitrophenol (a), 2,4-Dinitrophenol (b), 2,4,6-Trinitrophenol (c) by NaBH4; % 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).

4 5

In a heterogeneous system, it is demonstrated that reduction of aromatic nitro

6

compounds generally involves four steps; (i) absorption of hydrogen, (ii) absorption of

7

aromatic nitro compounds to the metal surfaces, (iii) electron transfer mediated by

8

metal surfaces from BH4 to aromatic nitro compounds, and (iv) desorption of aromatic

9

amino compounds. To check the binding of nitro benzene in the surface of Co2Mn3O8, 12 ACS Paragon Plus Environment

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1

FTIR analysis was done. For sample preparation, 100 µL nitrobenzene was added to

2

50 mg of Co2Mn3O8 catalyst in 0.5 mL methanol and sonicated for 1 h. After that, the

3

material was washed with methanol and then centrifuged and the process has been

4

repeated for 10 times to completely remove the unbound nitrobenzene from the

5

material. Finally, the material was dried before performing the experiment. The

6

presence of nitrobenzene in the material was confirmed from the FT-IR analysis as the

7

peaks are well matched with standard nitrobenzene. This confirms the absorption of

8

nitro benzene on the surface of the Co2Mn3O8 nano material.88

9

Further, to check the effect of reducing agent on the reaction, hydrazine hydrate was

10

utilized in Co2Mn3O8 promoted reduction reaction and its kinetics was monitored for

11

4-nitrophenol, 2,4-dintrophenol, and 2,4,6-trinitro phenol under UV-vis monitoring.

12

To perform the experiment, 3 ml solution of 4-nitrophenol (200 µM) was taken in a

13

UV-cuvette and to it 100µl of N2H4 and 1 mg of catalyst was added and the progress

14

of the reaction was monitored with UV-vis spectroscopy (Figure S5 in ESI).

15

Gratifyingly, the observed rate activity parameters (k′) in all the three cases are

16

comparable with k′ obtained using NaBH4 as reducing agent (Table 2). The higher

17

reduction rate of Co2Mn3O8 for 4-nitrophenol in comparison to their monometallic

18

oxide counterpart (MnO2 and Co3O4) is further justifies the synergistic effects between

19

each component (Figure S4 in SI-1). Next, to know the mechanism of Co2Mn3O8

20

promoted reduction of nitrobenzene in presence of NaBH4, the reaction mixture was

21

collected at different time interval and was analyzed with GC-MS to identify the

22

different products (Figure S24-27 in ESI). Initially, we observed the formation of

23

nitrosobenzene, which is converted to aniline via the formation of azobenzene, which

24

clearly suggest that the reaction proceeds through condensation route (Figure 6). The

25

condensation route is also preferred in our case irrespective of reducing agent as we



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1

observed both nitrosobenzene and azobenzene as transient intermediates for

2

Co2Mn3O8 promoted reduction of nitrobenzene with hydrazine hydrate. Table 2. Reaction kinetics of Co2Mn3O8 promoted reduction of nitro aromatics to amines.

# 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 NaBH4 N2H4 NaBH4 N2H4 NaBH4 N2H4

Catalyst (mg) 1 1 1 1 1 1

k (min-1) 62×10-3 32×10-3 59×10-3 38×10-3 56×1033×10-3

k’(s-1mg-1) 1.03×10-3 0.53×10-3 0.98×10-3 0.63×10-3 0.93×10-3 0.55×10-3

3 4

Next, to check the generality of the methodology, the substrate scope of Co2Mn3O8

5

nanomaterial promoted reaction for various nitro aromatics/heteroaromatics using both

6

NaBH4 and N2H4.H2O as reducing agent were done and illustrated in Table 3 & 4

7

respectively. Initially, from a screening of solvent, temperature, and catalyst loading,

8

the reaction condition was optimized. Under optimized reaction condition, Co2Mn3O8

9

promoted reduction for electron withdrawing and releasing group containing nitro

10

aromatics were found to be effective for the formation of corresponding amino

11

aromatics almost quantitatively with good TOF utilizing both NaBH4 and N2H4.H2O

12

as hydrogen source. To our delight, bromo-substituted nitroarenes, which can undergo

13

facile dehelogenation, were selectively reduced to the respective haloaromatic amine

14

without showing any sign of dehalogenation. Further to check the chemoselectivity,

15

the reaction done with multi functional substrate containing nitro with other reducible

16

functionalities such as CN, styrene, CO2H. Interestingly, when we used NaBH4 as

17

hydrogen source, chemo selective reduction of nitro to amine is possible and other

18

functional group remained unaffected. The Co2Mn3O8 nanomaterial promoted

19

reduction reaction is also found to be suitable for 5-nitro indole and corresponding 14 ACS Paragon Plus Environment

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1

product achieved in 92% yield after 4h. It should be pointed out that hydrazine-

2

mediated protocols cannot be used for aldehyde or ketone functionalities, since these

3

carbonyl derivatives readily form the corresponding hydrazones at room temperature

4

and is in agreement with previous reports.89-90 Similarly, Co2Mn3O8 promoted

5

reduction of 3-nitro styrene in presence of hydrazine hydrate produces corresponding

6

3-ethyl aniline in 90% yield. Table 3. Substrate scope for Co2Mn3O8 promoted reduction reaction using NaBH4.

Reaction Condition: nitro aromatics (10 mmol), Co2 Mn3O8 (0.05 mol%), MeOH/H2O (5/25 mL), NaBH4 (750 mg), room temperature. Yields were calculated from aUV-vis, bGC-MS, and c1H NMR monitoring of the crude reaction mixture.

Table 4. Substrate scope for Co2Mn3O8 promoted reduction reaction using N2H4.

Reaction Condition: nitro aromatics (10mmol), Co2Mn3O8 (0.05 mol%), MeOH/H2O (5/25 15 ACS Paragon Plus Environment


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Page 16 of 32

mL), N2H4 (100 uL), room temperature. Yields were calculated from aUV-vis, bGC-MS, and c1 H NMR monitoring of the crude reaction mixture. 1 2

Co2Mn3O8 nanomaterial promoted reduction of dye

3

As the synthesised material shows promising reactivity in presence of N2H4, we

4

wanted to check the reduction of various dyes as they are also known as environmental

5

pollutants.91-92 For this purpose, Methylene Blue (MB) was chosen as a model dye.

6

The reversible redox reaction of methylene blue (MB) to leuco methylene blue (LMB)

7

is a very interesting reaction as we can visually observe the dramatic reversible color

8

change. To perform the experiment, 3 ml solution of MB (1×10-4 M) was taken in a

9

UV-cuvette and to it 20µl of N2H4 and 1 mg of catalyst was added and the progress of

10

the reaction was monitored with UV-vis spectroscopy (Figure 7). On addition of

11

Co2Mn3O8 in aqueous medium, the initial blue colour of the MB dye faded away

12

producing colourless leuco-methylene blue (LMB). The steady decrease of the two

13

absorbance maxima at 662 and 290 nm and appearance of a new band at 255 nm due

14

to the formation of leuco methylene blue (LMB) suggests the progress of the reaction.

15

However, no colour change of MB was observed in the absence of catalyst.

16

Interestingly, on shaking of the reaction mixture or blowing air through the solution

17

result in a colour change from colourless to blue. Again on standing, the blue colour

18

turned into colourless. The kinetics of Co2Mn3O8 promoted MB to LMB reduction

19

reaction was done by monitoring the steady decrease of absorbance at 662 nm band at

20

different time interval.


Page 17 of 32

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


Dye

k (min-1)

k′′(k/m, s-1mg-1

Methylene Blue Methyl orange Methyl red Congo red Methyl violet Synthetic Dye

17.5×10-2 4.8×10-2 3.0×10-2 2.4×10-2 10.1×10-2 9.0×10-2

1.45×10-3 4×10-4 2.5×10-4 2×10-4 8.3×10-4 7.5×10-4

Reaction Condition: 3ml of 10-4 (M) solution of dye and 20 µL of hydrazine hydrate was taken and to it 2 mg Co2 Mn3O8 catalyst was added at room temperature.

Figure 7. MB Clock reaction (a), Reduction of Crystal Violet (b), Congo Red (c), Methyl Red (d) and Methyl Violet (e). 1 2

Furthermore, a plot of Ln(A) vs. time leads to a straight line, further confirms the

3

pseudo-first order reaction kinetics. To check the effect of amount of catalyst, dye, and

4

N2H4.H2O on the progress of the reaction, a series of experiment were performed

5

using UV-vis spectroscopy. On increasing the concentration of the catalyst and N2H4,

6

the rate of the reaction increases, while on increasing the concentration of the dye, the

7

rate of the reaction decreases with time (please see Figure S10 in ESI). Further, to

8

check the substrate scope for Co2Mn3O8 promoted reaction, various cationic, anionic

9

and neutral dyes were taken as substrate and their rate kinetics were monitored. It was

10

observed that the reductions of dyes in presence of Co2Mn3O8 were active for both

11

cation and anionic dyes with appreciable rate constant. But in case of neutral dyes, no

12

change was observed (please see Figure S8 in ESI). To our delight, Co2Mn3O8 is 17 ACS Paragon Plus Environment


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Page 18 of 32

1

found to be active for the reduction of mixture of five different cationic and anionic

2

dyes with appreciable rate.

3

Reusability test for Co2Mn3O8 promoted reduction and oxidation reaction

4

For practical applications of such heterogeneous nanocatalyst, the lifetime and its

5

reusability are very important factor. To determine this, a set of experiments for both

6

the reduction and oxidation reactions were done under optimized reaction condition.

7

The room temperature reduction of 4-nitro phenol (10 mmol) was done with 10 mg of

8

Co2Mn3O8 catalyst in presence of 750 mg of NaBH4 in 30 mL water. After the

9

completion of the first reaction (via UV-vis monitoring), the catalyst was recovered

10

using a tiny magnet, washed 3–4 times with MeOH and water and then used for the

11

next set of reactions with fresh reactants (Figure 8). We have monitored the reaction

12

up to 10th cycle with no appreciable reduction in its reactivity. After the 10th cycle,

13

AAS analysis of the used Co2Mn3O8 nanomaterial was done after digesting the sample

14

with conc. HCl (Table 4). Interestingly, no appreciable change in both cobalt and

15

manganese content was observed even after 10th cycle, which directly indicates the

16

reusability of the nanomaterial for reduction reaction. The PXRD and XPS analysis of

17

the used material after 10th cycle was also analyzed. Both the analysis suggests no

18

change in the material, which directly indicates the durability of the Co2Mn3O8 even

19

after 10th cycle.


Page 19 of 32

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1 2

Figure 8. % of conversion versus no of cycle plot for Co2Mn3O8 promoted reduction of 4-nitro phenol

3

and 4-nitrophenol and aerial oxidation of benzyl alcohol (a), digital images of the magnetically reusable

4

catalyst in three reactions before and after the use (b), XPS (c) and PXRD (d) of used Co2Mn3O8

5

nanomaterial.

6 7

Next, to check the reusability of the Co2Mn3O8 nanocatalyst for aerial oxidation

8

reaction, benzyl alcohol was chosen as a model substrate. After the completion of

9

reaction (vide TLC and yield via GC), used material was recovered using a tiny

10

magnet, washed for 3–4 times with MeOH and used for next set of reaction.

11

Interestingly, up to 5th cycle, no appreciable reduction in its activity was observed.

12

However, a slight drop down in the yield was observed in 8th (64%) and 10th (54%)

13

cycle (Figure 8). After the 5th and 10th cycle, AAS analysis of the used Co2Mn3O8

14

nanomaterial was done to check the leaching of the catalyst (Table 5). No appreciable



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Page 20 of 32

1

leaching was observed after the 5th cycle. However, both the Co and Mn content in the

2

used Co2Mn3O8 nanomaterial after 10th cycle was observed. Table 5. Determination of Co and Mn content in the material using AAS analysis Reduction Material Co (wt%) Mn (wt%) Co-MnOx 15.65 15.28 Co-MnOx (after 10th cycle) 11.16 10.67 Oxidation Material Co (wt%) Mn (wt%) Co-MnOx 15.65 15.28 Co-MnOx (after 5th cycle) 10.01 9.56 th 7.86 6.4 Co-MnOx (after 10 cycle) Both the Co and Mn content were determined using AAS analysis after digesting the sample with HCl.

3 4

Large scale application of Co2Mn3O8 promoted reduction and oxidation reaction

5

Next, we wanted to extend our study for large scale application and thus the activity of

6

Co2Mn3O8 nanomaterial was checked for both the reduction and oxidation reaction for

7

the synthesis of both amino aromatics and aldehyde selectively in gram scale. Initially,

8

the Co2Mn3O8 promoted reduction of 4-nitro phenol to 4-amino phenol was scaled up

9

stepwise from 1 mmol to 300 mmol with variable amount of catalyst and solvent

10

(Table 6). Table 6. Large scale application of Co2Mn3O8 promoted reduction reaction.

#

Scale Cat. Cat. NaBH4 H2 O t (h) TOF Yield (mmol) (mg) (mol%) (g) (mL) (h-1) 1 10 2 0.05 0.75 30 0.33 5939 98 3 50 20 0.1 3.0 200 6 150 90 4 100 50 0.125 6.0 300 10 72 90 5 300 200 0.16 15.0 600 18 25.7 74 Reaction Condition: 50 mmol scale, 7.0 g 4-nitro phenol and 3.0 g NaBH4 was taken in 200 ml of water and to it 20 mg Co2Mn3O8 catalyst was added and stirred at room temperature. 300 mmol scale, 42.0 g 4-nitro phenol and 10.0 g NaBH4 was taken in 600 ml of water and to it 200 mg catalyst was added and stirred at room temperature. 11


Page 21 of 32

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1

At a level of 50 mmol, 20 mg catalyst in presence of 3.0 g NaBH4 was sufficient to

2

convert 7.0 g of 4-nitro phenol to corresponding 4.9 g of amino phenol within 6h.

3

Further scale up from 50 to 300 mmol with 200 mg catalyst leads to corresponding

4

amine in 74% yield. Next, the Co2Mn3O8 promoted aerial oxidation of benzyl alcohol

5

was also sequentially scaled up from 1 to 100 mmol with variable amount of catalyst

6

and solvent (Table 7). At 50 mmol scale, 5.2 mL benzyl alcohol and 100 mg

7

Co2Mn3O8 was taken in 100 mL toluene and refluxed at 130 ˚C for 24 h to afford the

8

benzaldehyde in 62% yield. Further scale up from 50 to 100 mmol with 200 mg

9

catalyst afforded benzaldehyde selectively in 71% yield. The large scale (100 mmol)

10

application of Co2Mn3O8 promoted aerial oxidation also has been extended for other

11

substrate like; 4-methoxybenzyl alcohol, 4-chloro benzyl alcohol, and octanol and

12

afforded the corresponding aldehyde in 54% and 76% and 81% yield, respectively. Table 7. Large scale application of Co2Mn3O8 promoted oxidation reaction.

#

R

Scale Catalyst Catalyst Oxidant Toluene Time TOF Yield (mmol) (mg) (mol%) (mL) (h) (h-1) (%) 1 H 10 10 0.25 Air 3 18 18.2 82 3 H 50 100 0.5 Air 100 24 5.2 62 4 H 100 100 0.25 Air 200 36 5.3 48 5 H 100 200 0.5 Air 200 36 3.9 71 6 H 100 50 0.125 H2 O2 200 15 45.9 86 7 OMe 100 200 0.5 Air 200 48 2.2 54 8 Cl 100 200 0.5 Air 200 36 4.2 76 9 Octanol 100 200 0.5 H2 O2 200 36 4.5 81 Reaction Condition: 50 mmol scale, 5.2 mL benzyl alcohol and 100 mg Co2Mn3O8 nanocatalyst was taken in 100 mL toluene and refluxed at 130 ˚C. 100 mmol scale, 10.4 mL benzyl alcohol and 200 mg Co-MnOx nanocatalyst was taken in 200 mL toluene and refluxed at 130 ˚C. Yields were calculated from aGC analysis with o-xylene as an internal standard. 13 14 15



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Page 22 of 32

1

Experimental Section

2

Typical procedure for synthesis of heterobimetallic Co2Mn3O8 Nanomaterial: A

3

solution of CoCl2.6H2O is prepared by adding 4.75g (20 mM) of CoCl2.6H2O in 50ml

4

of double distilled water. To it, a solution of 5.0 mM (1.29g) Trisodium Citrate

5

dihydrate is added in the round bottom flask and make up the volume of water to

6

150ml. The mixture is deaerated with Nitrogen gas for 10min. Again a solution of

7

NaBH4 is prepared by adding 1.4g (40 mM) of NaBH4 in 30ml deaerated water. Then

8

NaBH4 solution was added slowly to the earlier prepared solution. During the course

9

of the reaction the purple colour of CoCl2.6H2O immediately changes to black

10

indicating the formation of Co nanoparticles. The solution is stirred at room

11

temperature at 900 rpm for 30 min till the evolution of H2 gas ceased. After this 40mM

12

solution of KMnO4 is prepared and poured it into the reaction flask and kept it under

13

refluxing condition for 24 hours under nitrogen atmosphere. After 24 hours, black

14

material was collected by centrifugation and thoroughly washed with water for 5-6

15

times and then dried in oven at 80°C for 12 hours. The weight of the collected material

16

was 8.2g.

17 18

PXRD: The PXRD were recorded on a Philips PW1710X-ray difractometer (40kV, 20mA)

19

using CuKα radiation (k¼ 1.5418˚A) in the 2θ range of 10-60° at a scanning rate of 0.5°min-

20

1

.

21 22

UV-vis study: Absorption spectra were recorded in a Dynamica Halo DB-30 double beam

23

digital spectrophotometer (Switzerland) attached with a Lab Companion RW-0525G chiller

24

and also in SHIMADZU UV 2550 spectrophotometer with quartz cuvette. UV-vis spectra of

25

the synthesized complexes were recorded in water.


Page 23 of 32

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1

Field Emission Scanning Electron Microscope (FESEM): The morphology of the

2

synthesized Co-MnOx was characterized using a field emission scanning electron microscope

3

(ZEISS EVO 60 with oxford EDS detector) operating at 5−10 kV.

4 5

FT-IR: All the samples for FTIR study were properly washed and then dried under vacuum.

6

Finally, samples for the FTIR spectra were recorded using IMPACT 410 Thermo-Nicolet

7

instrument.

8 9

General procedure for Co2Mn3O8 promoted aerial oxidation of alcohols to

10

aldehydes/ketones

11

To perform the experiment of oxidation of alcohol, 10 mmol corresponding alcohol

12

was dissolved in 30 mL of toluene and to it 10 mg (0.25 mol%) catalyst was added.

13

The reaction mixture was stirred at 130 °C for desired time. To confirm the formation

14

of the product, TLC was monitored at a regular interval of time. The yield of the

15

reaction is then monitored with GC analysis using o-xylene as an internal standard.

16 17

General

procedure

for

Co2Mn3O8 promoted

oxidation

of

alcohols

to

18

aldehydes/ketones using H2O2

19

To perform the experiment of oxidation of alcohol, 10 mmol corresponding alcohol

20

was dissolved in 30 mL of toluene and to it 10 mg (0.05 mol%) catalyst and 0.7 mL

21

H2O2 were added. After that, reaction mixture was stirred at 130 °C until the

22

completion of the reaction. To confirm the formation of the product, TLC was

23

monitored at a regular interval of time and the conversion was calculated from the GC-

24

analysis utilizing o-xylene as an internal standard.

25



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Page 24 of 32

1

General procedure for reduction of Nitro-compound to amine-compound using NaBH4

2

To perform the experiment, 10 mM of corresponding Nitro-compound was taken and

3

to it 750 mg of NaBH4 and 0.05 mol% of catalyst were added. The progress of the

4

reaction is then monitored with UV-vis spectrometer or GC analysis, or 1H NMR

5

monitoring.

6 7

General procedure for reduction of Nitro-compound to amine-compound using N2H4

8

To perform the experiment, 10 mM of corresponding nitro compound was taken in

9

reaction flask and to it 100 uL of N2H4 and 0.05 mol% of catalyst were added. The

10

progress of the reaction is then monitored by using UV-vis spectrometer or GC

11

analysis, or 1H NMR.

12 13

General procedure for reduction of various dye molecules

14

To perform the experiment, 3 mL solution of dye solutions (2 x 10-4M) is taken and to

15

it 20µl of N2H4 and 2mg of catalyst is added. The reaction is then monitored under

16

UV-vis spectrometer.

17 18

Conclusion

19

In summary, a cost effective chemical route for producing Co2Mn3O8 nanomaterial in

20

gram scale has been reported from the redox mediated reaction between in-situ

21

generated Co(0) nonmaterial and KMnO4 in water. The as synthesized Co2Mn3O8

22

nanomaterial is found to be useful as a magnetically reusable catalyst for

23

chemoselctive reduction of nitro to amines, various dyes and aerial oxidation of

24

alcohols to corresponding aldehydes/ketones under relatively milder reaction condition

25

with high turnover frequency (TOF). The enhanced reactivity in Co2Mn3O8 in

26

comparison to their monometallic oxide nanomaterial (MnO2 and Co3O4) for both the 24 ACS Paragon Plus Environment

Page 25 of 32

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1

oxidation and reduction reaction further justifies the cooperativity in the catalyst.

2

Aqueous reaction medium, easy and scalable synthesis, large scale application,

3

efficient recycling, and effortless separation of the catalyst using an external magnet

4

make the protocol economical and sustainable.

5 6

Acknowledgements

7

Financial support of this work by DST-New Delhi (to SP for INSPIRE grant no:

8

IFA/12-CH-39 and KS for INSPIRE fellowship) is gratefully acknowledged. The

9

author thanks the esteemed reviewers for their useful suggestions. The INUP, IITB

10

and IITBNF, IITB (sponsored by DeitY, MCIT, Government of India) is gratefully

11

acknowledged for giving us the access of XPS and HR-TEM facility.

12 13

Notes and references

14

a

15

Email: [email protected] or [email protected]

16

Electronic Supplementary Information (ESI) available: [FT-IR spectra, UV-vis data, and

17

NMR data was provided in ESI]

Department of Chemical Sciences, Tezpur University, Asaam-784028, India

18 19

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For Table of Contents Use Only

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Synopsis: An efficient, sustainable, and cost effective synthesis was reported for magnetically recoverable Co2Mn3O8 nanomaterial for selective oxidation and reduction reactions.

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Magnetically Recoverable Heterobimetallic Co2Mn3O8: Selective and

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