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Selective Oxidation of Alcohols Using Photoactive VO@g-C3N4 Sanny Verma, R. B. Nasir Baig, Mallikarjuna N Nadagouda, and Rajender S. Varma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01163 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 2, 2016

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ACS Sustainable Chemistry & Engineering

Selective Oxidation of Alcohols Using Photoactive VO@g-C3N4 Sanny Vermaa†, R. B. Nasir Baiga†, Mallikarjuna N. Nadagoudab and Rajender S. Varmaa*

a

Sustainable Technology Division, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA. Fax: 513- 569-7677; Tel: 513-487-2701. E-mail: [email protected] b WQMB,WSWRD, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA † Equal contribution Abstract A photoactive VO@g-C3N4 catalyst has been developed for the selective oxidation of alcohols to the corresponding aldehydes and ketones. The visible light mediated activity of the catalyst could be attributed to photoactive graphitic carbon nitrides surface. Keywords Photo-catalyst; Graphitic carbon nitride; Selective oxidation; Vanadium oxide; Visible light Introduction Substantial progress has been made towards the use of solar energy in automobile-related industries and efforts are ongoing to generalize it in chemical synthesis. Despite being the most ecological source of energy, the visible light mediated organic transformations have been rather limited because most of the organic molecules absorb poorly in visible region, which contributes about 40% of solar radiation.1-3 Photocatalysis has a great potential to combat environmental pollution. The scarcity of visible light active catalysts is the main impediment in the development of benign protocols for the synthesis of pharmaceuticals and industrially important products. Thus the design and synthesis of photoactive catalysts which can participate in organic reaction under visible light has gained significant interest.4-5 The controlled oxidation of alcohols to the corresponding aldehydes is a very important reaction in organic synthesis.6-7 Progress have been made to accomplish this transformation using homogenous reagents. Most of these reagents become inactive after the reaction resulting in the generation and accumulation of toxic waste.8 Heterogenized catalyst have been developed to

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circumvent the problem of waste generation.9-11 Most of the reported catalysts employed for such reactions involve precious metals such as palladium, gold, silver etc.12-14 Recently, scientists have focused on the use of organic material such graphene oxide, 15 N-doped graphene,

16

thiourea dioxide,

17

graphitic carbon nitride

18-23

in oxidation chemistry. These

materials have not been realized in any practical application in industries, mostly because of prolonged reaction time, use of stoichiometric reagents15,16 and sophisticated experimental setup which involve the use of high pressure reactor along with complication involved to irradiate the reaction mixture with visible light in close pressured environment. 18 As part of our quest for discovery and design of benign reaction conditions and catalysts,

24-27

herein, we report VO@g-C3N4 as a photoactive heterogeneous catalyst and demonstrate its application in the selective oxidation of series of alcohols. Result and discussion The graphitic carbon nitride support (g-C3N4) was synthesized by calcinations of urea at 500 oC and dispersed in aqueous methanol (50%) using sonication.28,

29

A methanolic solution of

vanadyl acetylacetonate [VO(acac)2] was added to a dispersed solution of g-C3N4 and stirred for 4 hours at room temperature. The reaction mixture was centrifuged, washed with methanol and dried under vacuum at 50 oC to afford VO@g-C3N4 catalyst (Scheme 1).

Scheme 1. Synthesis of VO@C3N4


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The catalyst was characterized using scanning electron microscopy (SEM), energy-dispersive Xray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), XPS analysis and inductive-coupled plasma atomic emission spectroscopy (ICP-AES). The SEM image of the g-C3N4 support and the catalyst, VO@g-C3N4, show different morphologies suggesting the immobilization of vanadium oxide over the graphitic carbon nitride surface (Figure 1a and 1b). The EDX spectra of the g-C3N4 support and VO@g-C3N4 (Figure 1c and 1d) confirm the presence of vanadium element in VO@g-C3N4. The TEM images of VO@g-C3N4 (Figure 2a and 2b) and g-C3N4 support (ESI; S3) do not show morphological difference. EDX obtained from TEM (Figure 2c) and ICP-AES analysis confirms the presence of elemental vanadium in the VO@g-C3N4 catalyst. The broad peaks in the XRD pattern (Figure 3) of VO@g-C3N4 and the support (g-C3N4) do not give any additional information about vanadium oxide presumably due to its amorphous nature. The weight percentage of vanadium element was found to be 4.91% by ICP-AES and the oxidation state of vanadium is confirmed by XPS analysis (ESI; Figure S1).

Figure 1. (a) SEM image of g-C3N4, (b) SEM image of VO@g-C3N4, (c) EDX spectra g-C3N4, (d) EDX spectra of VO@g-C3N4



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Figure 2. TEM images of VO@g-C3N4 (2a and 2b), EDX spectra of VO@g-C3N4 (2c)

Figure 3. XRD spectra of (a) g-C3N4 and (b) VO@gC3N4


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Our main objective was to design and develop an active photo catalyst that can facilitate the oxidation of alcohols to ketones and aldehydes selectively using the most sustainable energy source. This would not only render the procedure greener but adaptable for general use in industries and academia. Graphitic carbon nitride is known to have affinity towards visible light. Harnessing this energy to the reactants could help assist them to cross the energy barrier to generate the desired products. This instigated us to combine g-C3N4 with vanadium acetylacetonate [VO(acac)2] which would provide active catalyst for the oxidation reaction. VO(acac)2 was immobilized over graphitic carbon nitride surface via non-covalent interaction with g-C3N4.30 An important next step was to find active stoichiometric ratio of oxo-vanadium and graphitic carbon nitride surface which could be optimum for this transformation. Accordingly, VO@g-C3N4 was prepared with different weight percentage of elemental vanadium and screened towards the photo chemical oxidation of benzyl alcohol at room temperature (Table 1). The reactions were performed using 25 mg of VO@g-C3N4 in acetonitrile and hydrogen peroxide as a co-oxidant under visible light. The percentage of vanadium loading was very important in the development of photoactive VO@g-C3N4 catalyst. The catalyst with 1% V (Table 1; entry 1) required 8 hours to give 49% of oxidation product. The increase of vanadium loading to 2% improves the yield to 71% (Table 1; entry 2). The VO@g-C3N4 catalyst with 5% loading was very effective as it afforded the product in quantitative yield (98%, Table 1; entry 3) within 1 hour of exposure to visible light. Further increase in the elemental vanadium loading did not show any positive impact. There was a clear decline in the yield of the product with the increase of vanadium oxide percentage (Table 1; entries 4-5). The reduced yield of the product may be attributed to over oxidation to the corresponding carboxylic acid. A control experiment with bare VO(acac)2 under similar condition gave 21% of desired product (Table1; entry 6) along with traces of corresponding carboxylic acid and benzyl ester. Oxidation using pure gC3N4 support gave 10% conversion after 24 hours stirring (Table 1; entry 7), thus indicating that immobilization of vanadium acetylacetonate over g-C3N4 surface not only increases the activity of the catalyst but also provide the energy via visible light absorption. The fast reaction is possibly due to the synergic effect of oxo-vanadium and g-C3N4 combination. After identifying the ideal active catalyst, it was imperative to find optimum amount of catalyst loading for the oxidation reaction. Accordingly, the reactions were performed using 20 mg, 15 mg, 10 mg, and 5 mg of the catalyst and the reactions were stopped after 1 hour and the products isolated. There



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was nearly a quantitative conversion when 20 mg, 15 mg, and 10 mg catalyst was used affording over 95% of isolated yield. However, with 5 mg of the catalyst there was decrease in the rate of reaction as it gave 45% of yield after 1 hour of the reaction. The general reactivity and scope of this reaction was demonstrated using optimized reaction conditions. Table 1 Catalyst screening and reaction optimization O OH

Catalyst, H2O2

H

Visible light

Entry h

Yielda, b

Catalyst

Time

1

VO@g-C3N4 (1% V)

8h

49%

2

VO@g-C3N4 (2% V)

8h

71%

3

VO@g-C3N4 (5% V)

1h

98%

4

VO@g-C3N4 (10% V)

1h

85%

5

VO@g-C3N4 (20% V)

1h

81%

6

VO(acac)2 ( 5 mol%)

12 h

21%

7

g-C3N4 ( 25 mg)

24 h

10%

8g

VO@g-C3N4 (5% V)

1h

(>99%) 98% C

9g

VO@g-C3N4 (5% V)

1h

(>99%) 98% d

10g

VO@g-C3N4 (5% V)

1h

(>99%) 99% e

11

VO@g-C3N4 (5% V)

1h

45% f

Reaction conditions a) Benzyl Alcohol (1mmol); VO@g-C3N4 ( 25 mg); H2O2 ( 1.5 mmol); CH3CN (2 mL); 40 watt domestic bulb, room temperature; b) Isolated yield; c) Reaction performed using 20 mg of VO@g-C3N4; d) Reaction performed using 15 mg of VO@g-C3N4 ; e) Reaction performed using 10 mg of VO@g-C3N4; f) Reaction performed using 5 mg of VO@g-C3N4; g) The % in paranthesis indicate the level of selectivity in oxidation reaction; h) The isolated yield is based on the conversion or completion of the reaction in given time.


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A wide range of primary and secondary alcohols were subjected to selective oxidation using VO@g-C3N4; oxidation of benzyl alcohol and its derivatives proceeded smoothly leading to the formation of corresponding aldehydes (Table 2; entries 1-4). The electron withdrawing and electron releasing substituents on the benzene ring did not show any difference in the product outcome and reaction rate (Table 2; entries 2-4). 2-Thiophenemethanol and furfuryl alcohol were efficiently converted into corresponding aldehydes in excellent yield (Table 2; entries 5 and 6). The general applicability of this catalyst has been demonstrated by oxidation of secondary alcohols to the corresponding ketones (Table 2; entries 7-11). The aliphatic dihydroxy compounds (Table 2; entry 12) and α-hydroxy ketones (Table 2; entry 13) selectively gave the formation of corresponding di-ketones when subjected to VO@g-C3N4 oxidation.



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Table 2 VO@g-C3N4 photocatalyzed oxidation of alcohols Entry c,d

Substrate

Product

Time

Yielda, b

H

1h

(>99%) 98%

H

1h

(>99%) 95%

H

1h

(>99%) 98%

H

1h

(>99%) 97%

H

1h

(>99%) 96%

H

1h

(>99%) 96%

1.5 h

(>99%) 97%

2h

(>99%) 93%

1.5 h

(>99%) 97%

1.5 h

(>99%) 95%

2h

(>99%) 87%

O

2.5 h

(>99%) 85%

Ph

1.5 h

(>99%) 96%

O OH

1

O OH

2 O2 N

O2 N

O

OH

3 O

O

O OH

4

OH

5

S

S

O OH

6

O

O

O O

OH 7 OH 8

O

Ph

Ph

OH

O

9 I

OH

I

O

10 I

I

OH

O

11

OH

O OH

12

OH

O Ph

13 O

O

Reaction conditions a) Alcohol (1mmol); VO@g-C3N4 ( 10 mg); H2O2 ( 1.5 mmol); CH3CN (2 mL); 40 watt domestic bulb, room temperature; b) Isolated yield; c) The % in paranthesis indicate the level of selectivity in oxidation reaction; d) The isolated yield is based on the conversion or completion of the reaction in given time.


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To establish the activity and recyclability of the catalyst, a set of experiments were performed for the oxidation of benzyl alcohol using VO@g-C3N4 under visible light. After the completion of reaction the catalyst was recovered using a centrifuge, washed with methanol and reused for the oxidation of fresh batch of benzyl alcohol; VO@g-C3N4 catalyst could be recycled and reused over 10 times without losing its activity. The absence of metal leaching was confirmed using ICP-AES analysis of the catalyst before and after completion of the reaction. The concentration of elemental vanadium was found to remain constant throughout the recycling experiments. The negligible difference in the concentration of vanadium element before (4.91%) and after the 10th cycle (4.89%) was discerned. The SEM image (ESI; S4) and XRD pattern (ESI; S2) of recycled catalyst did not show any significant change in the morphology. The ICP-AES of reaction solvent did not show the presence of vanadium, confirming the fact that the g-C3N4 was held tightly to the oxo-vanadium complex thus facilitating the recycling of the catalyst. Conclusions An efficient protocol for the selective oxidation of alcohols to the corresponding carbonyl compounds has been developed using photoactive VO@g-C3N4 catalyst. A wide range of alcohols and diols were selectively converted to corresponding aldehydes and ketones. The expeditious reaction proceeds at room temperature under visible light. The salient features of the catalyst is its activity under photochemical conditions, stability and recyclability over several reactions cycles. ASSOCIATED CONTENT Supporting Information Synthesis of the catalyst, VO@g-C3N4, general procedure for the oxidation of alcohols, including the XPS, XRD, SEM, TEM and 1H NMR of the products. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]



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Notes The authors declare no competing financial interest. Acknowledgements SV and RBNB were supported by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency. Disclaimer The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. References (1) Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. Electron-transfer photoredox catalysis: development of a tin-free reductive dehalogenation reaction. J. Am. Chem. Soc. 2009, 131, 8756-8757. (2) Nicewicz, D. A.; MacMillan, D. W. C. Merging photoredox catalysis with organocatalysis: the direct asymmetric alkylation of aldehydes. Science 2008, 322, 77-80. (3) Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. Efficient visible light photocatalysis of [2+2] enone cycloadditions. J. Am. Chem. Soc. 2008, 130, 12886-12887. (4) Hernandez-Perez, A. C.; Collins, S. K. A visible-light-mediated synthesis of carbazoles. Angew. Chem. Int. Ed. 2013, 52, 12696-12700. (5) Kumar, P.; Verma, S.; Jain, S. L. A TiO2 immobilized Ru(II) polyazine complex: a visible-light active photoredox catalyst for oxidative cyanation of tertiary amines. J. Mat. Chem. A 2014, 2, 4514-4519. (6) Liu, X.; Xia, Q.; Zhang, Y.; Chen, C.; Chen, W. Cu-NHC-TEMPO catalyzed aerobic oxidation of primary alcohols to aldehydes. J. Org. Chem. 2013, 78, 8531-8536. (7) Zhu, Y.; Zhao, B.; Shi, Y. Highly efficient Cu(I)-catalyzed oxidation of alcohols to ketones and aldehydes with diaziridinone. Org. Lett. 2013, 15, 992-995.


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(8) Shi, Z.; Zhang, C.; Tang C.; Jiao, N. Recent advances in transition-metal catalyzed reactions using molecular oxygen as the oxidant. Chem. Soc. Rev. 2012, 41, 3381-3430. (9) Verma, S.; Nandi, M.; Modak, A.; Jain, S. L.; Bhaumik, A. Novel organic-inorganic hybrid mesoporous silica supported oxo-vanadium schiff base for selective oxidation of alcohols. Adv. Synth. Catal. 2011, 353, 1897-1902. (10) Mungse, H. P.; Verma, S.; Kumar, N.; Sain, B.; Khatri, O. P. Grafting of oxo-vanadium schiff base on graphene nanosheets and its catalytic activity for the oxidation of alcohols J. Mat. Chem. 2012, 22, 5427-5433. (11) Verma, S.; Bras, J. L.; Jain, S. L.; Muzart, J. Thiol-yne click on nano-starch: An expedient approach for grafting of oxo-vanadium Schiff base catalyst and its use in the oxidation of alcohols. Applied Catalysis A: General 2013, 468, 334-340. (12) Su, F. Z.; Chen, M.; Wang, L. C.; Huang, X. S.; Liu, Y. M.; Cao, Y.; He, H. Y.; Fan, K. N. Aerobic oxidation of alcohols catalyzed by gold nanoparticles supported on gallia polymorphs. Catal. Commun. 2008, 9, 1027-1030. (13) Verma, S.; Tripathi, D.; Gupta, P.; Singh, R.; Bahuguna, G. M.; Shivakumar K, L. N.; Chauhan, R. K.; Saran S.; Jain, S. L. Highly dispersed palladium nanoparticles grafted onto nanocrystalline starch for the oxidation of alcohols using molecular oxygen as an oxidant. Dalton Trans. 2013, 42, 11522-11527. (14) Beier, M. J.; Hansen, T. W.; Grunwaldt, J. D. Selective liquid-phase oxidation of alcohols catalyzed by a silver-based catalyst promoted by the presence of ceria. Journal of Catalysis 2009, 266, 320-330. (15) Dreyer, D. R.; Jia, H. P.; Bielawski, C. W. Graphene oxide: a convenient carbocatalyst for facilitating oxidation and hydration reactions. Angew. Chem. Int. Ed. 2010, 49, 68136816. (16) Sundar, J. V.; Subramanian, V. Novel chemistry for the selective oxidation of benzyl alcohol by graphene oxide and N-doped graphene. Org. Lett. 2013, 15, 5920-5923. (17) Verma, S.; Singh, R.; Tripathi, D.; Gupta, P.; Bahuguna, G. M.; Jain, S. L. Thiourea dioxide with TBHP: a fruitful and greener recipe for the catalytic oxidation of alcohols. RSC Adv. 2013, 3, 4184-4188.



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(18) Su, F.; Mathew, S. C.; Lipner, G.; Fu, X.; Antonietti, M.; Blechert, S.; Wang, X. mpgC3N4-catalyzed selective oxidation of alcohols using O2 and visible light. J. Am. Chem. Soc. 2010, 132, 16299-16301. (19) Zhang, P.; Wang, Y.; Yao, J.; Wang, C.; Yan, C.; Antonietti, M.; Li, H. Visible-lightinduced metal-free allylic oxidation utilizing a coupled photocatalytic system of g-C3N4 and N-hydroxy compounds. Adv. Synth. Catal. 2011, 353, 1447-1451. (20) Zhang, P.; Deng, J.; Mao, J.; Li, H.; Wang, Y. Selective aerobic oxidation of alcohols by a mesoporous graphitic carbon nitride/N-hydroxyphthalimide system under visible-light illumination at room temperature. Chinese J. Catal. 2015, 36, 1580-1586. (21) Gong, Y.; Li, M.; Li, H.; Wang, Y. Graphitic carbon nitride polymers: promising catalysts or catalyst supports for heterogeneous oxidation and hydrogenation. Green Chem. 2015, 17, 715-736. (22) Zhang, P.; Gong, Y.; Li, H.; Chen, Z.; Wang, Y. Solvent-free aerobic oxidation of hydrocarbons and alcohols with Pd@N-doped carbon from glucose. Nature Commun. 2013, 4, 1593. (23) Wang, Y.; Wang, X.; Antonietti, M. Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry. Angew. Chem. Int. Ed. 2012, 51, 68-89. (24) Varma, R. S. Journey on greener pathways: from the use of alternate energy inputs and benign reaction media to sustainable applications of nano-catalysts in synthesis and environmental remediation. Green Chem. 2014, 16, 2027-2041. (25) Nasir Baig, R. B.; Varma, R. S. Magnetically retrievable catalysts for organic synthesis. Chem. Commun. 2013, 49, 752-770. (26) Verma, S.; Mungse, H. P.; Kumar, N.; Choudhary, S.; Jain, S. L.; Sain, B.; Khatri, O. P. Graphene oxide: an efficient and reusable carbocatalyst for aza-michael addition of amines to activated alkenes. Chem. Commun. 2011, 47, 12673-12675. (27) Nasir Baig, R. B.; Nadagouda M. N.; Varma, R. S. Carbon-coated magnetic palladium: applications in partial oxidation of alcohols and coupling reactions. Green Chem. 2014, 16, 4333-4338.


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(28) Verma, S.;

Nasir Baig, R. B.; Han, C.; Nadagouda, M. N.; Varma, R. S. Magnetic

graphitic carbon nitride: its application in the C-H activation of amines. Chem. Commun. 2015, 51, 15554-15557. (29) Verma, S.; esterification

Nasir Baig, R. B.; Han, C.; Nadagouda, M. N.; Varma, R. S. Oxidative via

photocatalytic

C-H

activation.

Green

Chem.

2015,

DOI:

10.1039/C5GC02025E. (30) Borah, P.; Ma, X.; Nguyen, K. T.; Zhao Y. A vanadyl complex grafted to periodic mesoporous organosilica: a green catalyst for selective hydroxylation of benzene to phenol. Angew. Chem. Int. Ed. 2012, 51, 7756-7761.



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For Table of Contents Use Only

Selective Oxidation of Alcohol Using Photoactive VO@g-C3N4 Sanny Vermaa†, R. B. Nasir Baiga†, Mallikarjuna N. Nadagoudab and Rajender S. Varmaa*

A photoactive VO@g-C3N4 catalyst has been developed for the selective oxidation of alcohols to the corresponding aldehydes and ketones; visible light mediated activity of the catalyst could be attributed to photoactive graphitic carbon nitrides surface.


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For Table of Contents Use Only

Selective Oxidation of Alcohol Using Photoactive VO@g-C3N4 Sanny Vermaa†, R. B. Nasir Baiga†, Mallikarjuna N. Nadagoudab and Rajender S. Varmaa*

A photoactive VO@g-C3N4 catalyst has been developed for the selective oxidation of alcohols to the corresponding aldehydes and ketones; visible light mediated activity of the catalyst could be attributed to photoactive graphitic carbon nitrides surface.


Selective Oxidation of Alcohols Using Photoactive ... - ACS Publications

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