Environmentally benign aerial oxidation of benzoin over copper containing hydrotalcite

Catalysis Communications 11 (2010) 684–688

Contents lists available at ScienceDirect

Catalysis Communications journal homepage: www.elsevier.com/locate/catcom

Environmentally benign aerial oxidation of benzoin over copper containing hydrotalcite Divya Sachdev a, Mallari A. Naik a, Amit Dubey a,*, Braj Gopal Mishra b a b

Chemistry Group, Birla Institute of Technology and Science-Pilani, Rajasthan 333031, India Chemistry Department, National Institute of Technology, Rourkela, India

a r t i c l e

i n f o

Article history: Received 27 October 2009 Received in revised form 20 January 2010 Accepted 28 January 2010 Available online 1 February 2010

a b s t r a c t Liquid phase aerial oxidation of benzoin was carried out on copper containing hydrotalcites under milder reaction conditions. Very high activity (80–90% yield) and selectivity (100%) of the product was obtained depending upon the different metal ion combination in hydrotalcites. Various reaction parameters were studied in detail and the possible mechanism of the reaction was proposed. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Hydrotalcites Benzoin Heterogeneous catalysis

1. Introduction Benzil, an alpha diketone, is one of the important organic intermediates, has received an enormous attention because of its practical applications in organic and pharmaceutical industry such as photosensitive and synthetic reagents. Benzil is extensively used as substrate in benzylic rearrangements and also acts as a starting material for the synthesis of many heterocyclic compounds [1,2] exhibiting biological activity such as anticonvulsant derivative dilantin [3]. Additionally, benzil and its 1,2-substituted dione derivatives are found to be non-toxic and selective carboxylesterases inhibitors involved in the metabolism of esterified drugs including cocaine, heroin and xenobiotics [4]. Furthermore, benzil plays an important role in the synthesis of pesticides and acts as a photosensitizer in UV resin. Benzil exhibits high absorption in far UV region and therefore they are widely used as photo initiators for the radical polymerization of vinyl monomers [5,6]. The use of homogeneous reagents such as nitric acid, thallium nitrate ammonium nitrate–copper acetate [7–9] in stoichiometric amounts for the oxidation of benzoin limits their practical utilizations over heterogeneous catalysts in terms of product recovery, product selectivity and environmental concerns. Many heterogeneous catalysts such as zeolite [10], vanadium modified heteropolyacids H3+nPMo12 nVnO40 [11], ferric oxide–aluminum oxide [12], VOCl3 [13], chromium trioxide on Kieselghur [14] and Co–

* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Dubey). 1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.01.020

MCM-41 mesoporous material [15] were exploited to overcome these difficulties but the desired product selectivity still remains a challenge for this interesting reaction. Hydrotalcites (HTs), otherwise referred as layered double hydroxides, are receiving a tremendous attention due to their potential use in catalysts, ion exchange, adsorption and petrochemical applications. They are represented by the general formula [M(II)1 xM(III)x(OH)2] [Ax/n]n
mH2O where M(II) is a bivalent metal ion, M(III) is a trivalent metal ion, A is the interlayer anion, and x can have values between 0.2 and 0.4 [16,17]. Synthesis of these materials having transition metal ions in the sheets are of particular interest, owing to their selective oxidation behavior, efficiently employed for various redox-mediated catalytic transformations [18–21]. Previously Ni–Al hydrotalcites were reported for the oxidation of various alcohols using molecular oxygen [22] but the detailed investigation for the oxidation of benzoin has not been reported so far on hydrotalcites. Therefore we report for the first time the detailed use of the transition metal containing hydrotalcites with the possible mechanism for this one step conversion under environmental friendly conditions. 2. Experimental M(II)Al–HTs and CuM(II)Al–HTs, where M(II)–Mg, Ni, Co, Mn, Zn and Cu, were synthesized by coprecipitation method under low supersaturation. Two solutions; solution (A) containing the desired amount of metal nitrates and solution (B) having precipitating agents (i.e., NaOH and Na2CO3), were added simultaneously, while maintaining the pH around 9–10 under stirring at room temperature. The addition took around 100 min and the final pH of the

685

D. Sachdev et al. / Catalysis Communications 11 (2010) 684–688

C Ph

Table 1 Variation of different M(II)Al–HTs on the yield of product.

O

O OH CH

Hydrotalcite

+ Air (O 2)

MeOH

C Ph

O C

Ph

Ph

Scheme 1. Aerial oxidation of benzoin in presence of CuMgAl-3 catalyst.

solution was adjusted to 10. The samples were aged at 338 K for 18 h, filtered, washed (until total absence of nitrates and sodium in the washing liquids) and dried in an air oven at 353 K for 12 h. The samples obtained were powdered and denoted as CuM(II)Al– X, where X stands for Cu/M(II) atomic ratio at fixed composition of (Cu + M(II))/Al = 3. In all cases the atomic ratio of (Cu + M(II))/ Al was maintained at 3, while Cu/M(II) was varied from 5:1 to 1:5. Oxidation of benzoin was carried out in liquid phase conditions using air as an oxidant (Scheme 1). Typically, 212 mg (1 mmol) of the substrate (benzoin) and 10 ml of solvent was added in a glass reactor at desired temperature (298–363 K) under stirring conditions. Different amount of the catalyst weights (5–500 mg) was added at once to the reaction mixture. The products of the reaction mixture were analyzed through TLC and later confirmed by GC after considering the response factors of the authentic samples. The product was extracted from the reaction mixture using petroleum ether and finally the yield of the pure product was reported as isolated yield. 3. Results and discussion Elemental composition of the samples was in agreement with the theoretical calculated values. Powder X-ray diffraction pattern, carried out in a Philips X’Pert MPD system with Cu-Ka radiation (k = 1.54056 Å), showed the presence of pure hydrotalcites phase without co-crystallization of any detectable impurities (Fig. 1). The details of the characterization can be seen in our earlier reports [18,19] and presently the emphases are devoted to the catalytic studies only. 3.1. Catalytic studies 3.1.1. Effect of different metal ion concentration Table 1 showed the catalytic activity results on various binary hydrotalcites. In all, only benzil was formed as a major product with 100% product selectivity. No conversion was obtained on blank reaction (i.e. without catalyst) infers the necessity of the catalyst for this transformation. Based on our earlier studies on hydrotalcites, it was also concluded that the binary hydrotalcites are generally inactive for liquid phase oxidation of aromatics but are

Fig. 1. PXRD pattern of the samples. Top (a), bottom (e).

Catalysta M(II)/ Al = 3

Conversionb (%)

Isolated yieldc (%)

Product selectivity (%)

NiAl CoAl MnAl ZnAl CuAl MgAl

10 18 20 14 15 0

7 14 15 10 12 0

100 100 100 100 100 –

a Reaction conditions: (benzoin – 214 mg, air – 1 atm, catalyst – 10 mg, solvent – methanol (15 ml), temperature – 325 K, time – 24 h). b % Based on the product. c % Based on product isolated.

significantly active when combined with other bivalent metal ions in ternary hydrotalcites [18,19]. Therefore, the interest emerged to see the effect of Cu (having known its redox properties) as co-bivalent metal ion in ternary CuM(II)Al–HTs with CuM(II)/Al-3 and different compositions of Cu/M(II). The catalytic activity results (Table 2) showed remarkably high conversion and the product yield (10–90%), with 100% selectivity of the product corroborating our endeavor and hence the subsequent detailed study was carried out on CuM(II)Al-ternary hydrotalcites. Among the screening of different co-bivalent metal, maximum conversion and yield was obtained on CuMgAl-3 catalyst. This difference in the activity on various CuM(II)Al catalysts is attributed to the different electronic environment coupled with surface area on the catalyst surface (Table 3) [18,19]. It is further mentioned that the surface area of catalyst CuMgAl-31 (77 m2/g) was found higher compared to the catalysts CuNiAl-31 (48 m2/g), CuCoAl-31 (38 m2/g) respectively, hence responsible for higher conversion. Owing to the inherent properties of the calcined hydrotalcites (mixed oxides); the catalytic activity was also tested on calcined hydrotalcites but the catalytic activity was found to be lower than the fresh hydrotalcites clearly demonstrating the necessity of hydrotalcite phase [19]. These results have advantages over some of the Cu–MCM-41 mesoporous materials; although having the same Cu metal as active center [15]. These results further indicate that the desired activity and selectivity does not essentially depend on the active centers but also on the overall geometric and electronic environment around the catalyst. In all, the best catalyst CuMgAl-3 catalyst with optimum catalyst weight and temperature (Supporting information, Tables S1 and S2) was selected for further studies.

3.1.2. Effect of the oxidant and solvent Among the various oxidants (H2O2, m-perchlorobenzoic acid and air), it was concluded that the best activity and selectivity can be achieved by air itself under optimized reaction conditions (Table S3). We further observed that air present inside the reaction mixture itself is sufficient enough to derive this reaction and hence no additional supply of the air was pumped into the reaction mixture. Moreover, in order to see the effect of the air present in the reaction mixture, different reactions with increased amount of the substrate (Table 4) were carried out under similar experimental conditions. The results showed that the conversion and the product yield decreased with increase in the amount of the reactants but no difference in the product selectivity was observed. This decrease in the conversion and yield is obvious because the air present in the reaction mixture is not sufficient for the conversion of increased amount of reactants. Furthermore higher conversion (100%) and the yield (96%) were obtained using higher oxygen concentration (0.2 atm) under our reaction conditions. Here we have not used the external supply of the air and optimum concentration of the reactant was chosen for further study.

686

D. Sachdev et al. / Catalysis Communications 11 (2010) 684–688

Table 2 Effect of different CuM(II)Al-HTs on the yield of product.

a b

Cu/M(II)a

CuMgAl yieldb (%)

CuNiAl yieldb (%)

CuCoAl yieldb (%)

CuZnAl yieldb (%)

CuMnAl yieldb (%)

Product selectivity (%)

5 3 1 0.5 0.2

51 90 66 44 13

28 62 70 55 32

67 57 69 35 31

46 70 82 52 25.7

76 74 64 37 11

100 100 100 100 100

Conditions as in Table 1 (Cu + M(II))/Al = 3. % Based on product isolated.

Table 3 Variation of different Cu/Mg composition on the conversion of benzil (reaction conditions as in Table 1). Catalyst CuMgAl CuMgAl CuMgAl CuMgAl CuMgAl

51 31 11 0.5 0.2

Conversiona (%)

Isolated yieldb (%)

SBETc (m2/g)

TONd (w.r.t. Cu as active site)

60 98 72 51 20

51 90 66 44 13

65 77 110 120 132

1324 2513 2441 2443 1203

a

% Based on the product. % Based on product isolated. c SBET of CuMgAl catalyst. d TON = no of moles of reactant/no of moles of active center (Cu) times the % yield of the product. b

Table 4 Variation of reactant weight on the product yield over CuMgAl-3 catalyst. Reactant wt (g)

Isolated yield (%)

TONa (w.r.t. Cu as active site)

Product selectivity (%)

0.106 0.212 0.424 1.060 2.120

96 90 77 22 18

1340 2513 4301 3072 5027

100 100 100 100 100

Conditions as in Table 1. a TON = no of moles of reactant/no of moles of active catalyst times the % yield of the product.

Fig. 2 showed the conversion of benzoin on different solvents and maximum conversion and yield was obtained using methanol as a solvent. It seems that the conversion of benzil depends directly on the polarity and the nature (protic or aprotic) of the solvent because benzoin is more polar and hence making the hydride transfer easier in protic solvent (Scheme 2).

3.1.3. Time on stream studies (TOS) TOS studies (Fig. 3) showed that the catalytic activity of the reactant increased continuously up to 18 h and then became constant with further increase in the reaction time. No difference in the selectivity of the product was noted during the course of time. Further, to calculate the yield of the product with time, separate eight batches (to take the samples at different times) of the reactions were put under similar reaction conditions. The yield of the products was calculated periodically up to 48 h to see the formation of secondary or interconverted products. However no difference in the conversion, yield and selectivity of the products was observed indicating the product formed is quite stable and may be beneficial industrially. 3.1.4. Effect of the pH on the reaction and plausible mechanism In order to see the influence of the pH on the reaction, different experiments were performed at different pH (6–13) range (Supporting information, 1.2). Fig. 4 showed that the reaction is feasible both at lower as well as in the high pH range than the original (without adjustment) pH  6.8 of the reaction. However some interesting trend is observed at relatively higher pH range wherein the conversion decreased initially with the pH (7–10) and then again increased with pH  12. These observations were repeated and found reproducible under our reaction conditions. Mechanistically, we believe this peculiar behavior may be due to the better stabilization of the reactive intermediate (A) and (B) either in radical or ionic form (Scheme 2) responsible for the formation of product [23–25]. In order to investigate the presence of radical or ionic mechanism, the reactions were carried out using ethanol (well known radical scavenger) as a co-solvent at both higher pH  13 and lower pH  4 (Supporting information, Table S4). Interestingly the conversion as well as the yield of the products decreased at lower pH but no significant difference in the activity was observed at higher pH indicating the influence of 120

Conversion Isolated Yield

100 100

80

80

(%)

Isolated Yield (%)

90 70

60

60 50

40

40 30

20

20 10

0

0

MeOH

EtOH

MeCN

THF

MeCOMe

CHCl3

Solvents Fig. 2. Variation of the product yield with different solvents on CuMgAl-3 catalyst.

0

10

20

30

40

50

Time (h) Fig. 3. Variation of the product yield with time over CuMgAl-3 catalyst.

687

(%) Isolated Yield

D. Sachdev et al. / Catalysis Communications 11 (2010) 684–688

stabilized semiquinone species under different reaction conditions leads to the formation of the products [26].

100 90 80 70 60 50 40 30 20 10 0 3

5

7

9 pH

11

13

3.1.5. Reusability of the catalysts In order to see the reusability of the catalysts, the catalyst after the reaction is filtered, washed thoroughly first with methanol, acetone, water and then dried in air and finally subjected to fresh reaction. The catalytic yield is slightly reduced to 85% with 100% selectivity of the product. This small reduction in the activity may be attributed to slight decrease in the crystallinity of the catalyst after the first cycle (Fig. S1, Supporting information). Subsequently the catalyst was repeated for three cycles without significant loss in the activity. Furthermore, no leaching of active metal ions from the catalyst during the course of reaction was observed that was confirmed by several experiments according to the earlier reports [18,27].

15

Fig. 4. Variation of the product yield with pH of the reaction over CuMgAl-3 catalyst.

radical mechanism at lower pH and possibility of the ionic mechanism at higher pH. The presence of the intermediates was chemically identified with the formation of purple color (semiquinone species) under alkaline conditions [23]. Therefore, we believe, the interaction of the oxidant with the catalyst surface generates the peroxy compounds which via proton abstraction leads to the formation of the resonance stabilized intermediates (A) and (B). Finally, the competitive transformation of these intermediates into the

4. Conclusions In conclusion, liquid phase aerial oxidation of benzoin was carried out on copper containing ternary hydrotalcites. Among all the catalysts screened, CuMgAl-3 catalyst was found to be most efficient under milder reaction conditions. The catalysts can be recycled without significant loss in the activity. The results based on

OH HO Cu

2+

HO

OH

+

Air (O 2)

OH OH

O O HO

O

+

Cu HO

OH

Ph

Ph

328 K

O

Cu

HO O

HO

Ph

HO HO

Cu O

HO

OH

Cu

O H

Ph

-

C

O

HO Ph

O

2+

HO

Cu

HO

OH +

OH

Ph

Ph HO

+ O

O

OH

HO

Benzil Scheme 2. Plausible mechanism.

Ph

O H

C

O

C

Ph

B radical intermediate

ionic intermediate

OH

Ph

OH

O

A

Cu

C

proton abstraction

O

OH

Ph

OH

Benzoin

HO

H

O

CH

Methanol

OH

O

Ph

OH HO

CH C HO

O

Cu

+

O H O

Ph

C C Ph

688

D. Sachdev et al. / Catalysis Communications 11 (2010) 684–688

the yield and selectivity is quite beneficial industrially and may be used for further scale. Acknowledgements A.D. and B.G.M. thank DST for financial support. D. Sachdev and M. Naik thank CSIR for senior research fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.01.020. References [1] [2] [3] [4] [5] [6]

H. Wynberg, H.J. Kooreman, J. Am. Chem. Soc. 87 (1965) 1739. W.W. Paudler, J.M. Barton, J. Org. Chem. 31 (1966) 1720. W.R. Dunnavant, F.L. James, J. Am. Chem. Soc. 78 (1956) 2740. R.M. Wadkins, K. Damodaran, P. Beroza, J. Med. Chem. 48 (2005) 2906. J.V. Crivello, M. Sangermano, J. Polym. Sci., Part A: Polym. Chem. 39 (2001) 343. R. Kuhlmann, W. Schnabel, Polymer 18 (1977) 1163.

[7] J.S. Buck, S.S. Jenkins, J. Am. Chem. Soc. 51 (1929) 2163. [8] A. McKillop, B.P. Swann, M.E. Ford, E.C. Taylor, J. Am. Chem. Soc. 95 (1973) 3641. [9] M. Weiss, M. Appel, J. Am. Chem. Soc. 70 (1948) 3666. [10] S. Balalaie, M. Golizeh, M.S. Hashtroudi, Green Chem. 2 (2000) 277. [11] B. El Ali, A.M. El-Ghanam, M. Fettouhi, J. Mol. Catal. A: Chem. 165 (2001) 283. [12] C. Zhebin, S. Zhengui, Chin. J. Org. Chem. 22 (2002) 446. [13] M. Kirihara, H. Nemoto, Chem. Commun. (1999) 1387. [14] J.D. Lou, X.L. Lu, X. Yu, Synth. React. Inorg., Met.-Org., Nano-Met. Chem. 38 (2008) 373. [15] B. Li, J. Wang, J. Fu, J. Wang, C. Zou, Catal. Commun. 9 (2008) 2000. [16] F. Cavani, F. Trifiro, Catal. Today 24 (1995) 307. [17] V. Rives, Layered Double Hydroxides: Present and Future, Nova Publishers, New York, 2001. [18] D. Sachdev, A. Dubey, B.G. Mishra, S. Kannan, Catal. Commun. 9 (2008) 391. and references there in. [19] S. Kannan, A. Dubey, H. Knozinger, J. Catal. 231 (2005) 381. [20] V. Choudhary, P. Chaudhari, V. Narkhede, Catal. Commun. 4 (2003) 171. [21] D. Kishore, E. Alirio, Rodrigues Catal. Commun. 10 (2009) 212. [22] B.M. Choudary, C.V. Reddy, K.K. Rao, Angew. Chem., Int. Ed. 40 (2001) 763. [23] L. Michaelis, E.S. Fetcher Jr, J. Am. Chem. Soc. 59 (1937) 1246. [24] J.L. Ihrig, R.G. Caldwell, J. Am. Chem. Soc. 78 (1956) 2097. [25] A. Weissberger, J.E. LuValle, D.S. Thomas Jr, J. Am. Chem. Soc. 65 (1943) 1934. [26] G.S. Hammond, C.-H.S. Wu, J. Am. Chem. Soc. 95 (1973) 8215. [27] R. Sheldon, M. Wallau, I. Arends, U. Schuchardt, Acc. Chem. Res. 31 (1998) 485.