Metal nanoparticles assisted amine catalyzed transesterification under ambient conditions

Journal of Molecular Catalysis A: Chemical 377 (2013) 129–136

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Metal nanoparticles assisted amine catalyzed transesterification under ambient conditions Puran Singh Rathore a , Jacky Advani a , Sonika Rathore b , Sonal Thakore a,∗ a b

Department of Chemistry, Faculty of Science, The M.S. University of Baroda, Vadodara 390 002, India Mangalam Drugs and Organics Ltd, Vapi 396 195, India

a r t i c l e

i n f o

Article history: Received 31 December 2012 Received in revised form 1 May 2013 Accepted 6 May 2013 Available online xxx Keywords: Transesterification Nanoparticles Catalysis Ethylenediamine

a b s t r a c t Ethylenediamine provides excellent activity under ambient conditions when assisted by metal nanoparticles for the transesterification of higher to lower esters and vice versa. This is the first report on room temperature conversion of diethylmalonate to dimethylmalonate using this novel catalytic system. The nanoparticles were successfully recycled up to 24 cycles (although with an increase in reaction time) with no compromise in the yield. Various primary alcohols were used for transesterification. The scope was extended to aliphatic, heterocyclic and aromatic esters with various functional groups. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Catalytic transesterification of carboxylic esters with alcohols is an important organic synthesis [1] and an important tool in the synthesis of biologically active compounds and drugs [1a,2,3]. Esters play an important role in the multiple-step synthesis or as a protecting group of several natural products [4]. The large scale applications of transesterification are in polymerization processes [5], paint industry [6] and in conversion of fats (triglycerides) into biodiesel [7]. Carboxylic acids are sparingly soluble in organic solvents, whereas esters are normally soluble in most of organic solvents [8]. Transesterification being an equilibrium reaction, high conversions are difficult to attain. Hence, reactions are catalyzed by acids [6b,9] and bases [10] under homogeneous as well as heterogeneous conditions [11]. However, acid catalysts [12] exhibit low substrate selectivity and can cleave sensitive functional groups. They may also lead to formation of side products [13]. Strongly basic catalysts lead to high conversions, but fail for base-sensitive substrates [12]. Therefore, with either acidic or basic conditions, such transesterification reactions do not prove to proceed efficiently under mild reaction conditions [11c]. A number of organic amines have shown good catalytic activity for transesterification [14]. Although the recovery process is simpler the process has narrow range of applicability due to longer reaction time and low conversions.

On the other hand, nanoparticles (NPs) are core base materials for implementing nanotechnology and have promising applications in organic synthesis [15]. It has been proved that Ni and CuNPs as catalysts offer great opportunities for a wide range of applications in organic synthesis [16]. The high efficiency of NPs system relies mainly on the approach to the metal core, size, and surface modification. Various functional groups can be introduced on the surface of the NPs by using appropriate capping agent. In order to identify an ideal catalyst for transesterification, we decided to study the catalytic activity of some organic amines in presence of metal NPs. In the past, we have synthesized metal NPs with various capping agents which were used for biological applications [17]. In this paper, we report first protocol for metal NPs (nickel and copper) assisted amine catalyzed synthesis of higher to lower esters and vice versa under ambient conditions. Initially diethylmalonate (DEM) was used as the model system to simplify the analysis and to accelerate the screening speed for suitable amine-NPs catalytic system. Consequently the optimized conditions were used for the synthesis of various esters. 2. Experimental Details of synthesis and characterization of nanoparticles are mentioned in supporting information. 2.1. General procedure for room temperature transesterification

∗ Corresponding author. Tel.: +91 0265 2795552; fax: +91 0265 2429814. E-mail address: [email protected] (S. Thakore). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.05.003

In a typical reaction methanol (20 mmol), amines (0.12 mmol) and starch capped nickel nanoparticles (NiNPs) (80 mg,) were

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Table 1 Transesterification of diethylmalonate.a Entry

Amines

Metal NPs

Temperature of reaction (◦ C)

Reaction time (h)

GC yield (±1%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

– – TEA DEA EDA TEA DEA EDA TEA DEA EDA TEA DEA EDA EDA EDA

– Cu/Ni – – – – – – Ni Ni Ni Ni Ni Ni Cu Cu

Reflux Reflux rt rt rt Reflux Reflux Reflux rt rt rt Reflux Reflux Reflux Reflux rt

72 72 72 72 72 48 48 48 24 8.0 0.5 8.0 8.0 0.25 0.25 0.50

3 8 42 47 50 45 49 56 52 80 99 56 86 99 99 99

a

Reaction condition: Substrate-diethylmalonate (1 mmol), methanol (20 mmol), amines (0.12 mmol) (TEA/DEA/EDA) and NiNPs/CuNPs-80 mg (60 wt%).

initially sonicated at room temperature (rt) for 30 min in a round bottom flask (RBF) prior to addition of the substrate. After sonication diethylmalonate (1 mmol) was added and the reaction mixture was stirred at rt (25–30 ◦ C). In cases where the reaction did not proceed to completion the mixture was gradually heated to reflux (at 70 ◦ C) with an aim to achieve maximum conversion. Reaction monitoring was done by thin-layer chromatography (TLC) and gas chromatography (GC), (supporting information). After the completion of reaction, NPs were separated by centrifuge and alcohol (methanol or ethanol) and amines were distilled out. Isolated NPs were washed by acetone and they were dried at 60 ◦ C under vacuum and used for the next reaction. The yield and purity of final products were analyzed by GC. Short-path silica gel chromatography (5:95, ethyl acetate in hexane, v/v) was used to obtain an analytically pure dimethylmalonate (DMM). Finally, the product DMM was also analyzed by nuclear magnetic resonance (NMR) (BRUKER 400 MHz, supporting information). Transesterification of other esters was carried out in a similar manner by using alcohol and esters molar ration 10:1. The products purified by short-path silica gel chromatography (0–25% ethyl acetate in hexane, v/v) were analyzed by GC and NMR (supporting information). All experiments have been repeated three times and the reproducibility confirmed. 3. Results and discussion 3.1. Transesterification of diethylmalonate In order to assess the efficiency of different amines as base catalyst the transesterification of diethylmalonate was carried out using each of the three amines (triethylamine (TEA), diethylamine (DEA), ethylenediamine (EDA)) in the absence of NPs (Table 1). It was observed that the reaction yields were very low (∼50–55%) even after 72 h at rt (Table 1, entries 3–5, respectively) and after 48 h under reflux (Table 1, entries 6–8). The order of the catalytic activity of amines was observed to be EDA (pKa -11.09 at 25 ◦ C) > DEA (pKa -11.01 at 25 ◦ C) > TEA (pKa -10.9 at 25 ◦ C) respectively which is the same as the order of basicity.

Fig. 1. FT-IR spectra of (a) NiNPs recovered after 1st run showing traces of bound EDA (b) EDA alone.

Hence for establishment of optimum conditions a preliminary study was carried out using EDA as the catalyst (supporting information). After several experiments with different concentrations of EDA, methanol and various quantities of NiNPs, the optimum conditions for transesterification of DEM to dimethylmalonate (DMM) were established (Scheme 1). Interestingly, when the same reactions were carried in presence of NiNPs a significant enhancement in conversion was observed. The effect was more pronounced in EDA and DEA catalyzed reactions, which were completed within 30 min and 8 h with 99% (entry 11) and 80% (entry 10) conversion, respectively. On the other hand, TEA catalyzed reactions were less affected by NiNPs (entry 12). To understand the role of metal, the reaction under optimized conditions was also carried out with copper nanoparticles (CuNPs) instead of NiNPs to yield similar results. (Table 1, entry 15). Hence to get better insight into the mechanism of the reaction, the FTIR spectrum (Fig. 1) of the recovered NiNPs provided the evidence for surface-functionalization of NiNPs with EDA. The FTIR spectrum of the NiNPs recovered (Fig. 1a) after the first cycle of transesterification showed peaks at 3354 cm−1 and 1611 cm−1 which can be assigned to the N H stretching and bending respectively. The peaks at 2979 and 2883 cm−1 are assigned to the C H stretching vibrations and the one at 1482 cm−1 is due to C H bending of alkanes. Based on these observations we propose that role of metal NPs is to provide surface binding sites for the reactants (methanol) and milder catalyst (amine) during sonication. This facilitates the generation of nucleophile, the methoxide ion, with the help of a weak base [18], like EDA which then attacks DEM as shown in Scheme 2. The higher efficiency of metal NPs under mild conditions is due to their higher dispersion in solvent so that the reactant reaches the catalytic site by diffusion. EDA being a diamine gives highest catalytic activity because of greater basicity, followed by DEA. TEA being a tertiary amine was probably too weak to generate nucleophile due to steric hindrance. Sequence of addition of reactants was also observed to be important. If substrate DEM is added before amine, the reaction slows down probably due to partial adsorption of DEM which affects the generation of nucleophile. Hence it is advantageous that amine and methanol come in contact with NiNPs before DEM. A similar

Scheme 1. Optimum conditions for transesterification of diethylmalonate.

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131

Scheme 2. A proposed reaction mechanism for Starch-NiNPs supported organic amine catalyzed transesterification of diethylmalonate systems.

Table 2 Transesterification of diethylmalonate at rt (25–30 ◦ C) with methanol using NiNPs and EDA (Recycling experiments). Cycle

Time (min)

Yielda (%)

IYb (%)

Cycle

Time (min)

Yielda (%)

IYb (%)

1 2 3 4 5 6 7 8 9 10 11 12

30 30 30 30 30 30 30 40 40 60 60 60

99 99 99 99 99 99 99 99 99 99 99 99

96 96 96 96 96 96 96 96 96 96 96 96

13 14 15 16 17 18 19 20 21 22 23 24

60 60 60 60 60 60 60 60 60 60 90 120

99 99 99 99 99 99 99 99 99 99 99 99

96 96 96 96 96 96 96 96 96 96 96 96

a b

GC yield. Isolated yield.

observation has been reported by Liu et al. [19] for hydrotalcite based catalyst. At reflux temperature of the methanol, much better conversion was 99% and the reaction was complete within 15 min (Table 1, entry 14). The NPs were recovered by centrifugation with insignificant loss and recycling experiments were carried out under optimized conditions. Interestingly, it was observed that the NPs could be reused directly without further purification for 24 consecutive runs as can be seen in Table 2. Up to seven cycles there was no loss in activity. After that, however, the reaction time increased in each successive recycling experiment reaching from 30 min to 2 h finally. This may be due to gradual loss of the catalytic activity of NiNPs with number of runs which may be due to various reasons. One of the reasons may be surface modification due to deposition of matter during reaction [20,21]. The IR spectra of the NiNPs recorded after 24th run displays a number of peaks of deposited matter (supporting

information). Nevertheless, the reaction does proceed to completion at the same temperature at the end of 24 cycles giving the same yield. Further the used NiNPs were also characterized by XRD in order to confirm the metallic state of nickel or the possibility of surface oxidation. The formation of oxide is less as evident as seen from the XRD analysis (supporting information) which does not show any peak corresponding to NiO at 37◦ . This can be attributed to the presence of starch coating which makes the NPs stable to water and air and maintains their metallic state [17a,22]. This increases their utility and enhances recycling efficiency. Further, it is also possible that hydrogen-bonding between hydroxyl groups of capping agent starch and methanol also assists the generation of methoxide ion on the surface of NiNPs [23]. Thus the metal surface, capping agent and amine simultaneously make the amine-NiNPs system an effective catalyst. Starch capped CuNPs were observed to be much less stable and more prone to oxidation than NiNPs. Hence there was considerable loss due to leaching out and recycling experiments were difficult to perform. Under the optimized conditions, we investigated the scope of converting various methyl esters to ethyl esters and vice versa by using ethanol and methanol, respectively (Scheme 3). Both NiNPs and CuNPs were applied for all substrate and they almost gave same yield in similar reaction time (Table 3). The results were satisfactory for aliphatic, aromatic, and heterocyclic substrates. Conversion from ethyl to methyl esters and vice versa occurs almost in comparable time. Reaction conversion time was less in aliphatic diesters (entries 4, 5, 15 and 16) compared to long chain monoesters (entries 6 and 17) but, reaction yield were comparable in both cases. Heterocyclic esters undergo smooth conversion as any other ester (entries 9, 10 and 19). The transesterification of triglyceride like triacetin (entries 11 and 21) with ethanol resulted in 96–97% HPLC yield (data not shown) in 3 h. This opens up new possibilities for ecofriendly synthesis biodiesel since organic amines can be recovered by distillation [24,25], and the NPs can be recycled.

Scheme 3. General scheme for transesterification of various esters.

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Table 3 Transesterification of various esters at rt (25–30 ◦ C) with methanol/ethanol using NPs and EDA. Entry

Substrate

Product

Time (h)

Yielda (±1%) NiNPs

CuNPs

Without NPsd

1

2.5(2.5)b

96

96

40

2

2.0(2.0)b

96

95

50

3

5.0(4.5)b

96.5

97

34

4

3.0(3.5)b

95

95

50

5

0.30(0.40)b

96

95

50

6

5.0(5.30)b

94

95

40

7

5.30(5.30)b

96

96

15

8

4.0(4.0)

95

95

23

9

4.0(4.0)

95

95

20

10

3.0 (3.0)

94

93

35

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133

Table 3 (Continued) Entry

Substrate

Product

Time (h)

Yielda (±1%) NiNPs

CuNPs

Without NPsd

11

3.0 (3.0)b

97c

97c

45c

12

1.30 (1.0)b

92

93

10

13

1.0 (1.0)b

96

95

20

14

6.0 (5.0)b

90

91

30

15

0.30 (0.30)b

95

96

40

16

3.5 (3.5)b

80

81

2

17

6.5 (7.0)b

96

96

20

18

6.5 (6.5)b

90

92

5

19

3.0 (3.5)b

95

95

40

21

3.0 (3.0)b

96c

96c

40c

a b c

Isolated yield after column chromatography. Reaction time for CuNPs catalyzed reaction. HPLC yield (data not shown) and (d) reaction time after 24 h.

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Table 4 Transesterification with other alcohols using NPs and EDA at rt (25–30 ◦ C). Entry

Substrate

Alcohol

Product

Yielda (±1%) NiNPs

CuNPs

Without NPs

1

n-Propanol

94

94

23

2

n-Butanol

89

91

15

3

n-Propanol

94

93

24

4

n-Butanol

88

90

10

5

Methanol

95

94

34

6

Ethanol

95

95

33

7

n-Propanol

92

93

30

8

n-Butanol

92

91

15

9

n-Pentanol

92

91

10

n-Hexanol

90

91

17

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135

Table 4 (Continued) Entry

Substrate

11

a

Alcohol

Product

Allyl alcohol

Yielda (±1%) NiNPs

CuNPs

Without NPs

93

93

32

GC yield after 24 h.

3.2. Transesterification with other alcohols In order to validate the process further, the transesterification of DEM and DMM was attempted with other alcohols. The results in Table 4 show that with primary alcohols (C3–C6) transesterification successfully occurred although a progressive decrease in yield was observed due to increasing steric hindrance. However with secondary, tertiary and benzylic alcohols as well as phenol the reaction either did not initiate or conversions were poor (data not shown). The reactions were also tried with a different substrate (Table 4, entries 5–11) having aromatic groups instead of ethyl/methyl side chains. But similar results were obtained. 4. Conclusion The study leads to a conclusion that the present catalytic system of EDA–NiNPs, provides smooth transesterification over a wide range of structurally diverse esters without disturbing other functional groups. The reaction proceeds to offer high yields under ambient conditions and starch capped NPs exhibited excellent recyclability. In future, the scope of this catalytic system may be extended to green synthesis of biodiesel. Further attempts to explore the precise mechanism and role of metal NPs are in progress. Acknowledgements The authors are grateful to the Solvay group, new Research, Development and Technology Centre at Savli, Gujarat, India, for financial assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.molcata.2013.05.003. References [1] (a) J. Otera, Esterification, Wiley-VCH GmbH: Weinheim, Germany, 2003; (b) J. Otera, Chem. Rev. 93 (1993) 1449–1470; (c) G.A. Grasa, R. Singh, S.P. Nolan, Synthesis 7 (2004) 971–985; (d) D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5606–5655. [2] S. Wilson, S. Benetti, R. Romagnoli, C. De Risi, G. Spalluto, V. Zanirato, Chem. Rev. 95 (1995) 1065–1114. [3] (a) R. Larock, In Comprehensive Organic Transformations, second ed., WileyVCH, New York, 1996; (b) J. Mulzer, in: B.M. Trost, I. Fleming (Eds.), Comprehensive Organic Synthesis, Pergamon Press, New York, 1992. [4] T.W. Green, P.G.M. Wuts, Protective Groups in Organic Synthesis, third ed., Wiley, New York, 1999. [5] W. Riemenschneider, H.M. Bolt, Esters Organic Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

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