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Transesterification of ethylacetoacetate catalysed by metal free mesoporous carbon nitride
Catalysis Today 204 (2013) 164–169
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Transesterification of ethylacetoacetate catalysed by metal free mesoporous carbon nitride Chokkalingam Anand a,∗ , Subramaniam Vishnu Priya a , Geoffrey Lawrence a , Gurudas P. Mane c , Dattatray S. Dhawale a , Kumaresapillai S. Prasad a , Veerappan V. Balasubramanian b , Mohammad A. Wahab a , Ajayan Vinu a,∗ a
Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, #75 Corner College and Cooper Roads, Brisbane, 4072 QLD, Australia Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, United States c MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan b
a r t i c l e
i n f o
Article history: Received 2 June 2012 Received in revised form 27 July 2012 Accepted 31 July 2012 Available online 30 August 2012 Keywords: Mesoporous Carbon nitride -Keto esters Ethylenediamine SBA-15
a b s t r a c t The basic catalytic performance of the mesoporous carbon nitride (MCN) for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents was demonstrated. The catalyst was prepared by using a nano-hard templating approach through a simple polymerisation reaction between ethylenediamine (EDA) and carbon tetrachloride (CTC) in the mesochannels of the SBA-15 followed by the carbonisation and the silica removal by HF. The material was thoroughly analysed by sophisticated characterisation techniques such as X-ray diffraction (XRD), N2 adsorption studies, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy and CHN analysis. Structural investigation of the MCN by XRD, HRTEM and N2 adsorption revealed that the prepared catalyst exhibits highly ordered two-dimensional (2D) porous arrays with a high surface area and a large pore volume. The catalytic results revealed that the MCN was found to be an efficient catalyst in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding -keto esters in high yields. More importantly, the catalyst was highly active when 1-butanol was used. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product was studied in detail. In addition, the activity of the catalyst was also compared with pure mesoporous carbon and the results were discussed. The recyclability studies revealed that the MCN catalyst was highly stable under the rigorous reaction conditions and can be reused several times without any significant loss of catalytic activity. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Transesterification is one of the significant processes involved in the manufacture of industrially important products such as bio-diesel, lactones, acrylates, ethylene glycol and polyethylene terephthalate (PET) [1]. Furthermore, -keto esters form the basis for the manufacture of numerous natural products, pheromones and additives for paints. They are also of huge interest as chemical intermediates in the polymer and agriculture industries and widely employed as flavour and fragrance in food and beverage industry [2]. Owing to their importance in various applications, several synthetic procedures which involve both homogeneous and heterogeneous catalysts including strong acids, soluble base catalysts, metal salts, amine, zeolites, titanosilicates and enzymes have been
∗ Corresponding authors. Tel.: +61 7 3346 4122; fax: +61 7 3346 3973. E-mail addresses: [email protected] (C. Anand), [email protected] (A. Vinu).
reported for the preparation of -keto esters [3–9]. Since the industrial usage of homogeneous catalysts has unleashed irreversible damage to the environment because of their corrosive and toxic properties, much attention has been given to the heterogeneous catalytic methods because they have been considered as alternates to overcome the woes caused by their homogeneous counterparts [10–12]. Among the heterogeneous catalysts, solid catalytic materials with highly ordered mesoporous structure have gained much attention owing to their excellent textural characteristics including high surface areas, large pore volume and uniform pore size distribution [13–16]. In addition, they offer remarkable hydrothermal, mechanical and chemical stabilities which are needed to withstand vigorous reaction conditions. These properties make them excellent catalysts for the organic transformations like transesterification and aromatic alkylation [17–21]. In most of the catalysed reactions, much focus has been given to mostly the mesoporous acid catalysts with well-ordered porous structure. Although
0920-5861/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.07.025
C. Anand et al. / Catalysis Today 204 (2013) 164–169
mesoporous catalysts with basic characters are highly attractive, the reports on porous solid bases are scarce as they always require post-synthetic modification with organic or inorganic bases which make them to be more vulnerable for leaching and give negative impact on the recyclability. Recently, there has been increased usage of porous carbon materials as catalysts, catalyst supports, adsorbents, sensors, and battery electrodes. One of the most attractive carbon based materials is mesoporous carbon nitride with well-ordered porous structure and excellent textural parameters, reported by Vinu and co-workers [22]. These materials can be prepared by a single step strategy with a simple polymerisation of ethylenediamine (EDA) and carbon tetrachloride in the mesochannels of SBA-15 followed by the carbonisation and the subsequent silica removal by HF [23]. These mesoporous carbon nitrides (MCN) are quite attractive owing to their unique properties such as semi-conductivity, biocompatibility, low density, intercalation ability, hardness but also exhibit basic active sites generated by the presence of free NH and amino groups on the carbon wall structure [24–28]. Despite numerous literature reports discussing the synthesis of mesoporous carbon nitrides using different synthetic strategies and precursors [23,29,20,30–33], the reports on utilising the basic active sites for base catalysed organic transformation are quite limited [20]. Herein we report on the basic catalytic performance of the mesoporous carbon nitride (MCN) prepared from SBA-15 inorganic template for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents. The materials thus prepared have been subjected to thorough characterisation employing high end techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and CHN analysis. It has been found that MCN is highly active in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding -keto esters in high yields. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product has also been demonstrated. 2. Experimental 2.1. Materials copolymer poly-(ethylene glycol)-block-polyTriblock (propylene glycol)-block-poly-(ethylene glycol) (Pluronic P123, molecular weight = 5800, EO20 PO70 EO20 ) as surfactant and tetraethyl orthosilicate (TEOS) as silica source required for the SBA-15 synthesis were purchased from Sigma–Aldrich. Similarly the precursors ethylenediamine (EDA) and carbon tetrachloride (CTC) required for MCN synthesis were also obtained from Sigma–Aldrich. All the materials were used as received without further purification.
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100 ◦ C. Finally the as-prepared SBA-15 white powder was calcined at 540 ◦ C under air flow to get rid of triblock copolymer. Using the calcined SBA-15 as template along with ethylenediamine (EDA) and carbon tetrachloride (CTC) as precursors, mesoporous carbon nitride (MCN) was prepared as follows: SBA15 (0.5 g) was well-stirred with a mixture containing EDA (1.35 g) and CTC (3 g). Further the mixture was refluxed under stirring at 90 ◦ C for 6 h. The resultant dark brown coloured composite material was dried at 100 ◦ C for 12 h. The material thus obtained was subjected to carbonisation at 600 ◦ C (heating rate 3.0 ◦ C min−1 ) under nitrogen flow (50 mL min−1 ) for 5 h. The carbonised material was treated with 5 wt% hydrofluoric acid to dissolve the silica framework and the mesoporous carbon nitride was recovered by filtration, followed by several washings with ethanol and dried at 100 ◦ C. 2.3. Characterisation of the catalysts A Rigaku diffractometer with a Cu K␣ radiation of 0.154 nm was used to collect the powder X-ray diffraction (CRD) pattern of the mesoporous carbon nitride (MCN). The diffractogram was recorded in a 2 range from 0.8◦ to 10◦ with a 2 step size of 0.01 and a step time of 1 s. N2 adsorption–desorption isotherms were measured at −196 ◦ C on a Quantachrome Autosorb 1 sorption analyzer. Before the adsorption measurements, all samples were out-gassed at 250 ◦ C for 3 h in the de-gassing port of the adsorption analyzer. The Brunauer–Emmett–Teller (BET) specific surface area was obtained from adsorption branch of the isotherm. Barrett–Joyner–Halenda (BJH) method was used for the calculation of the pore size distribution. Morphology and topology of MCN were studied using JEOL-3000F and a JEOL-3100FEF field emission high resolution transmission electron microscopes operating at an accelerating voltage of 200 kV. Elemental analysis was done using Yanaco MT-5 CHN analyzer. 2.4. Transesterification of ethylacetoacetate The catalytic activity of the MCN material was probed in the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol. The reaction was carried out in a 20 mL glass reaction tubes fitted to Carousel parallel synthesis reactor set-up equipped with stirring, refluxing and hot plate for uniform heating with inert gas controls. In a typical run, the transesterification reaction was performed with a molar ethylacetoacetate to 1-butanol ratio of 1.4:1 using 2 wt% of MCN-1 catalyst (2 wt% based on the total reaction mixture) at a reaction temperature of 110 ◦ C for 6 h. Samples withdrawn periodically were analysed using a Shimadzu Gas Chromatograph GC-2010 equipped with auto sampler and thermal conductivity detector (TCD) and a DB-5 capillary column. The conversion was calculated on the basis of alcohol consumption in the reaction mixture. 3. Results and discussion
2.2. Catalyst synthesis
3.1. Characterisation of the catalysts
The preparation of mesoporous SBA-15 template was carried out employing the procedure mentioned elsewhere [34]. A typical procedure involving the synthesis of SBA-15 is as follows: 4 g amphiphilic triblock copolymer P123 was well dispersed by stirring with distilled water (30 g) and HCl solution (120 mL, 2 M) for 5 h. To the resultant homogenous mixture tetraethyl orthosilicate (TEOS, 9 g) was added and continuously stirred for 24 h at 40 ◦ C. Further, the solution was aged at 100 ◦ C for 48 h and the resultant gel was filtered hot, washed twice with water and dried overnight at
The detailed characterisation of the hexagonally ordered mesoporous carbon prepared from SBA-15-100 template was reported previously [23,29]. The prepared material exhibits well ordered porous structure with a hexagonal arrangement of linear arrays of pores that arranged in regular intervals [23]. The textural parameters such as specific surface area, specific pore volume and pore diameter of MCN were analysed by nitrogen adsorption desorption measurements. MCN-1 catalyst possessed a specific Brunauer–Emmett–Teller (BET) surface area of 505 m2 g−1 with a
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C. Anand et al. / Catalysis Today 204 (2013) 164–169
Table 1 Textural parameters of mesoporous carbon nitride (MCN-1). Catalyst
a0 (nm)
ABET (m2 g−1 )
Pore volume (cm3 g−1 )
Pore diameter (nm)
MCN-1
9.52
505
0.55
4.2
Fig. 1. HRTEM images of MCN-1: (a) Captured along mesopores and (b) captured across mesopores [29].
Table 2 Elemental composition of MCN-1 measured at different stages of its preparation from CHN analysis. MCN-1
C (wt%)
N (wt%)
H (wt%)
Other elements (Si, O, Cl and F) (wt%)
After carbonisation After silica removal
37.5 69.7
8.8 16.0
1.9 2.2
51.8 12.1
pore volume of 0.55 cm3 g−1 . The pore diameter of the sample is highly uniform and was found to be 4.20 nm (Table 1). HRTEM images of the sample also display well-ordered honeycomb-like hexagonal mesopore arrangement, which can clearly be seen from the cross-sectional HRTEM image of MCN-1 (Fig. 1b), whereas only a stripe-like pattern was displayed when MCN was imaged along the mesopores (Fig. 1a) [29]. The elemental composition of MCN has been listed in Table 2. As it can be noticed from Table 2, the sample contains 2.2% H, confirming the presence of NH or NH2 groups on the surface of the samples. These groups might have originated from the incomplete polymerisation of the carbon tetrachloride and ethylenediamine precursors and offered basic character to the MCN-1.
1-octanol) compared to cyclic alcohols (cyclohexanol and furfuryl alcohol) and aryl (benzyl alcohol) alcohols, revealing that linear aliphatic alcohols are more reactive than cyclic and aromatic alcohols. It should also be noted that the catalyst was active for long chain alcohols such as 1-octanol although the activity of MCN-1 is lower than that obtained for 1-butanol. These results confirm that the difference in the chain length, electron delocalisation and bulkiness of the alcohols play a major role in deciding the activity of the catalyst. In addition, the physico-chemical traits of the catalyst should also be taken into consideration, which influence the alcohols reactivity in a way entirely different from one reaction to another.
3.2. Catalytic activity The basic catalytic performance of the mesoporous carbon nitride was studied in the transesterification of ethylacetoacetate with 1-butanol, 1-octanol, furfuryl alcohol, cyclohexanol and benzyl alcohol. Initially, the reaction was carried out over MCN-1 at an ethylacetoacetate to 1-butanol molar ratio of 1.4:1, 2 wt% catalyst (wt% based on the total weight of reaction mixture), reaction temperature of 110 ◦ C and the reaction time of 6 h under solvent free condition and the results are displayed in Fig. 2. It was found that the catalyst was active, affording a high conversion within a short reaction time. As can be seen in Fig. 2, the conversion increases with increasing the reaction time and reaches the maximum of 69.4% for the reaction time of 6 h. The effect of the nature of alcohols affecting the conversion of the MCN-1 was also investigated. Among all the alcohols studied, MCN-1 was found to be highly performing when 1-butanol was involved, which is clearly evident from Fig. 2. Interestingly, MCN-1 showed much higher performance when the reaction was carried out with aliphatic alcohols (1-butanol and
Fig. 2. Transesterification of ethylacetoacetate with different alcohols using MCN-1 catalyst (䊉) 1-butanol, () 1-octanol, () furfuryl alcohol, () cyclohexanol and () benzyl alcohol. Reaction conditions: ester:alcohol ratio – 1.4, reaction temperature – 110 ◦ C, reaction time – 6 h and catalyst weight – 2 wt% MCN-1.
C. Anand et al. / Catalysis Today 204 (2013) 164–169
Fig. 3. Effect of varying catalyst weight in the reaction mixture over the catalytic performance of MCN-1 in transesterification of ethylacetoacetate with 1-butanol and comparison with CMK-3 and catalyst free reaction system () 1 wt%, (䊉) 2 wt%, () 4 wt%, () 6 wt%, () 8 wt%, () 10 wt%, () CMK-3 and (♦) catalyst free reaction system. Reaction conditions: ester:alcohol ratio – 1.4, reaction temperature – 110 ◦ C and reaction time – 6 h.
A general schematic representation of transesterification reaction is given in Scheme 1. The basic sites in MCN-1, that are originated from the free NH2 groups found on the walls of MCN-1 and the uncondensed terminal NH2 from the ethylenediamine, are the source of its catalytic activity, which stimulate the transesterification of ethylacetoacetate. In previous study, Vinu and co-workers clearly explained the formation of basic sites, their strength and concentration by TPD of CO2 measurements of MCN1 samples [35]. It has been envisaged that the transesterification of ethylacetoacetate catalysed by MCN involves an intermediate product formed as a result of EtO− elimination, ␣-keto ketene that is highly reactive and further converted into transester with the corresponding alcohol [36,37]. Furthermore, other reaction parameters such as effect of catalyst weight taken in the reaction mixture, temperature of the reaction and ester to alcohol molar ratio influencing the catalytic performance of MCN-1 using 1-butanol was studied. At first, the effect of the catalyst concentration on 1-butanol conversion was investigated by varying the concentration from 1 to 10 wt% and the results are given in Fig. 3. As evident from Fig. 3, the catalytic activity of MCN-1 increased with increasing its quantity from 1 to 2 wt% in the reaction mixture, but started to decline with a further increase of the catalyst concentration. This could be due to the poisoning effect caused when more and more MCN was used in the system. Therefore, 2 wt% was chosen to study the influence of other reaction parameters. The catalytic activity of MCN-1 was also studied at different the ester:alcohol molar ratio at a reaction temperature of 110 ◦ C for different time and the results are given in Fig. 4. 1Butanol conversion increased from 69% to 80% with increasing the molar ester:alcohol ratio from 1.4 to 5. This could be mainly due to the preferential adsorption of ethylacetoacetate over 1-butanol, which in turn influences the rate of formation of the intermediate ketene on the MCN surface quite rapidly resulting in higher conversion with increasing the molar ratio of ester to alcohol. The effect of reaction temperature over the catalytic ability of MCN-1 in transesterification of ethylacetoacetate with 1-butanol was studied as a function of time and the results are shown in Fig. 5. As can be seen from Fig. 5, the temperature certainly has a pronounced effect over the catalytic activity of MCN. The 1butanol conversion increased from 32% to 85% with increasing the reaction temperature from 90 to 150 ◦ C. Although, the conversion increases with increasing the reaction temperature, the selectivity of the products takes a beating leading to many side products other
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Fig. 4. Influence of ethylacetoacetate to 1-butanol ratio on the catalytic performance of MCN-1 in transesterification of ethylacetoacetate (䊉) 1.4, () 3 and () 5. Reaction conditions: reaction temperature – 110 ◦ C, reaction time – 6 h and catalyst weight – 2 wt% MCN.
than the desired one. This could possibly be explained in terms of increasing reaction temperature, wherein the hike in temperature not only influences the rate of the reaction but also induces parallel reactions including cracking or oligomerisation that initiates the formation of undesired products. The stability of the MCN-1 catalyst even under vigorous reaction conditions especially, higher temperature is clearly evident from the above study. The recyclability of MCN-1 was tested in the transesterification reaction. The study comprised of three cycles involving fresh MCN-1 catalyst in the first cycle, wherein the transesterifcation of ethylacetoacetate with 1-butanol reaction was continued for 6 h at 110 ◦ C with an ethylacetoacetate to 1-butanol ratio of 1.4. After completion of each cycle the MCN-1, catalyst was separated by filtration, washed with ethanol to remove possible traces of organic compounds and dried in a hot air oven. Subsequently, the dried MCN-1 catalyst was regenerated by calcination at 500 ◦ C under nitrogen atmosphere for 5 h prior to each cycle. The catalytic activity of MCN-1 from each cycle is compared in Table 3. We observed only a slight reduction in the conversion of 1-butanol revealing that the MCN-1 sample is catalytically stable and can be efficiently reused for several cycles (Table 3). In order to confirm whether the transesterification of ethylacetoacetate with 1-butanol caused by the presence of the basic sites in the MCN-1, the reaction was studied with mesoporous CMK-3
Fig. 5. Effect of reaction temperature over the catalytic performance of MCN-1 in transesterification of ethylacetoacetate with 1-butanol (䊉) 90 ◦ C, () 110 ◦ C, () 130 ◦ C and () 150 ◦ C. Reaction conditions: ethylacetoacetate:1-butanol ratio – 1.4, reaction time – 6 h and catalyst weight – 2 wt% MCN.
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Scheme 1. General reaction mechanism involved in the transesterification reaction. Table 3 Recovery and reuse results of MCN-1 for transesterification of ethylacetoacetate with 1-butanol. Catalyst
Usage cycle
1-Butanol conversion
Butyl acetoacetate selectivity (%)
MCN-1 MCN-1 MCN-1
Fresh 1 2
69 67 65
100 100 100
Acknowledgements One of the authors A. Vinu is grateful to ARC for the award of the future fellowship and the AIBN and the University of Queensland for the start-up grant. The authors also acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, University of Queensland.
Reaction conditions: reaction temperature – 110 ◦ C, reaction time – 6 h, ester to alcohol ratio – 1.4 and catalyst weight 2 wt% of total reaction mixture.
References
carbon and without any catalyst and the results are given in Fig. 2. It is clear from Fig. 2 that MCN-1 is overwhelmingly active when compared to CMK-3 for the transesterification of ethylacetoacetate with 1-butanol. We believe that the activity of MCN-1 solid base catalyst in the transesterification of ethylacetoacetate is in fact enhanced due to the presence of NH or NH2 groups that offer strong Lewis basic sites. Moreover the MCN-1 catalyst is free from any metals and nowadays the demand for metal free catalysts is growing rapidly among the industries, especially when it inherits all those advantages of a metal incorporated catalyst. In addition, the regeneration and reusability of the MCN catalysts does not seem to affect its catalytic activity at all, which is a significant behaviour expected from industrial view point.
4. Conclusions In conclusion, we demonstrated the basic catalytic performance of the mesoporous carbon nitride for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol without any solvents. The catalyst was found to be highly active, affording a high conversion of long and short chain alcohols, cyclic and aromatic alcohols and the desired product in good to excellent yield. We also demonstrated that the reaction parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream play a significant role in controlling the activity of the catalyst. The catalyst was found to be highly stable even at high temperature and can be recovered easily and reused efficiently at least three times without any significant loss of the catalytic activity. We strongly believe that this novel basic mesoporous carbon nitride catalyst could make an excellent platform for various basic catalysed organic reactions for the synthesis of industrially important and pharmaceutical products and replace the existing toxic and environmentally unfriendly catalysts that are being currently used in the industry.
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Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Transesterification of ethylacetoacetate catalysed by metal free mesoporous carbon nitride Chokkalingam Anand a,∗ , Subramaniam Vishnu Priya a , Geoffrey Lawrence a , Gurudas P. Mane c , Dattatray S. Dhawale a , Kumaresapillai S. Prasad a , Veerappan V. Balasubramanian b , Mohammad A. Wahab a , Ajayan Vinu a,∗ a
Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, #75 Corner College and Cooper Roads, Brisbane, 4072 QLD, Australia Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, United States c MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan b
a r t i c l e
i n f o
Article history: Received 2 June 2012 Received in revised form 27 July 2012 Accepted 31 July 2012 Available online 30 August 2012 Keywords: Mesoporous Carbon nitride -Keto esters Ethylenediamine SBA-15
a b s t r a c t The basic catalytic performance of the mesoporous carbon nitride (MCN) for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents was demonstrated. The catalyst was prepared by using a nano-hard templating approach through a simple polymerisation reaction between ethylenediamine (EDA) and carbon tetrachloride (CTC) in the mesochannels of the SBA-15 followed by the carbonisation and the silica removal by HF. The material was thoroughly analysed by sophisticated characterisation techniques such as X-ray diffraction (XRD), N2 adsorption studies, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy and CHN analysis. Structural investigation of the MCN by XRD, HRTEM and N2 adsorption revealed that the prepared catalyst exhibits highly ordered two-dimensional (2D) porous arrays with a high surface area and a large pore volume. The catalytic results revealed that the MCN was found to be an efficient catalyst in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding -keto esters in high yields. More importantly, the catalyst was highly active when 1-butanol was used. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product was studied in detail. In addition, the activity of the catalyst was also compared with pure mesoporous carbon and the results were discussed. The recyclability studies revealed that the MCN catalyst was highly stable under the rigorous reaction conditions and can be reused several times without any significant loss of catalytic activity. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Transesterification is one of the significant processes involved in the manufacture of industrially important products such as bio-diesel, lactones, acrylates, ethylene glycol and polyethylene terephthalate (PET) [1]. Furthermore, -keto esters form the basis for the manufacture of numerous natural products, pheromones and additives for paints. They are also of huge interest as chemical intermediates in the polymer and agriculture industries and widely employed as flavour and fragrance in food and beverage industry [2]. Owing to their importance in various applications, several synthetic procedures which involve both homogeneous and heterogeneous catalysts including strong acids, soluble base catalysts, metal salts, amine, zeolites, titanosilicates and enzymes have been
∗ Corresponding authors. Tel.: +61 7 3346 4122; fax: +61 7 3346 3973. E-mail addresses: [email protected] (C. Anand), [email protected] (A. Vinu).
reported for the preparation of -keto esters [3–9]. Since the industrial usage of homogeneous catalysts has unleashed irreversible damage to the environment because of their corrosive and toxic properties, much attention has been given to the heterogeneous catalytic methods because they have been considered as alternates to overcome the woes caused by their homogeneous counterparts [10–12]. Among the heterogeneous catalysts, solid catalytic materials with highly ordered mesoporous structure have gained much attention owing to their excellent textural characteristics including high surface areas, large pore volume and uniform pore size distribution [13–16]. In addition, they offer remarkable hydrothermal, mechanical and chemical stabilities which are needed to withstand vigorous reaction conditions. These properties make them excellent catalysts for the organic transformations like transesterification and aromatic alkylation [17–21]. In most of the catalysed reactions, much focus has been given to mostly the mesoporous acid catalysts with well-ordered porous structure. Although
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C. Anand et al. / Catalysis Today 204 (2013) 164–169
mesoporous catalysts with basic characters are highly attractive, the reports on porous solid bases are scarce as they always require post-synthetic modification with organic or inorganic bases which make them to be more vulnerable for leaching and give negative impact on the recyclability. Recently, there has been increased usage of porous carbon materials as catalysts, catalyst supports, adsorbents, sensors, and battery electrodes. One of the most attractive carbon based materials is mesoporous carbon nitride with well-ordered porous structure and excellent textural parameters, reported by Vinu and co-workers [22]. These materials can be prepared by a single step strategy with a simple polymerisation of ethylenediamine (EDA) and carbon tetrachloride in the mesochannels of SBA-15 followed by the carbonisation and the subsequent silica removal by HF [23]. These mesoporous carbon nitrides (MCN) are quite attractive owing to their unique properties such as semi-conductivity, biocompatibility, low density, intercalation ability, hardness but also exhibit basic active sites generated by the presence of free NH and amino groups on the carbon wall structure [24–28]. Despite numerous literature reports discussing the synthesis of mesoporous carbon nitrides using different synthetic strategies and precursors [23,29,20,30–33], the reports on utilising the basic active sites for base catalysed organic transformation are quite limited [20]. Herein we report on the basic catalytic performance of the mesoporous carbon nitride (MCN) prepared from SBA-15 inorganic template for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents. The materials thus prepared have been subjected to thorough characterisation employing high end techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and CHN analysis. It has been found that MCN is highly active in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding -keto esters in high yields. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product has also been demonstrated. 2. Experimental 2.1. Materials copolymer poly-(ethylene glycol)-block-polyTriblock (propylene glycol)-block-poly-(ethylene glycol) (Pluronic P123, molecular weight = 5800, EO20 PO70 EO20 ) as surfactant and tetraethyl orthosilicate (TEOS) as silica source required for the SBA-15 synthesis were purchased from Sigma–Aldrich. Similarly the precursors ethylenediamine (EDA) and carbon tetrachloride (CTC) required for MCN synthesis were also obtained from Sigma–Aldrich. All the materials were used as received without further purification.
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100 ◦ C. Finally the as-prepared SBA-15 white powder was calcined at 540 ◦ C under air flow to get rid of triblock copolymer. Using the calcined SBA-15 as template along with ethylenediamine (EDA) and carbon tetrachloride (CTC) as precursors, mesoporous carbon nitride (MCN) was prepared as follows: SBA15 (0.5 g) was well-stirred with a mixture containing EDA (1.35 g) and CTC (3 g). Further the mixture was refluxed under stirring at 90 ◦ C for 6 h. The resultant dark brown coloured composite material was dried at 100 ◦ C for 12 h. The material thus obtained was subjected to carbonisation at 600 ◦ C (heating rate 3.0 ◦ C min−1 ) under nitrogen flow (50 mL min−1 ) for 5 h. The carbonised material was treated with 5 wt% hydrofluoric acid to dissolve the silica framework and the mesoporous carbon nitride was recovered by filtration, followed by several washings with ethanol and dried at 100 ◦ C. 2.3. Characterisation of the catalysts A Rigaku diffractometer with a Cu K␣ radiation of 0.154 nm was used to collect the powder X-ray diffraction (CRD) pattern of the mesoporous carbon nitride (MCN). The diffractogram was recorded in a 2 range from 0.8◦ to 10◦ with a 2 step size of 0.01 and a step time of 1 s. N2 adsorption–desorption isotherms were measured at −196 ◦ C on a Quantachrome Autosorb 1 sorption analyzer. Before the adsorption measurements, all samples were out-gassed at 250 ◦ C for 3 h in the de-gassing port of the adsorption analyzer. The Brunauer–Emmett–Teller (BET) specific surface area was obtained from adsorption branch of the isotherm. Barrett–Joyner–Halenda (BJH) method was used for the calculation of the pore size distribution. Morphology and topology of MCN were studied using JEOL-3000F and a JEOL-3100FEF field emission high resolution transmission electron microscopes operating at an accelerating voltage of 200 kV. Elemental analysis was done using Yanaco MT-5 CHN analyzer. 2.4. Transesterification of ethylacetoacetate The catalytic activity of the MCN material was probed in the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol. The reaction was carried out in a 20 mL glass reaction tubes fitted to Carousel parallel synthesis reactor set-up equipped with stirring, refluxing and hot plate for uniform heating with inert gas controls. In a typical run, the transesterification reaction was performed with a molar ethylacetoacetate to 1-butanol ratio of 1.4:1 using 2 wt% of MCN-1 catalyst (2 wt% based on the total reaction mixture) at a reaction temperature of 110 ◦ C for 6 h. Samples withdrawn periodically were analysed using a Shimadzu Gas Chromatograph GC-2010 equipped with auto sampler and thermal conductivity detector (TCD) and a DB-5 capillary column. The conversion was calculated on the basis of alcohol consumption in the reaction mixture. 3. Results and discussion
2.2. Catalyst synthesis
3.1. Characterisation of the catalysts
The preparation of mesoporous SBA-15 template was carried out employing the procedure mentioned elsewhere [34]. A typical procedure involving the synthesis of SBA-15 is as follows: 4 g amphiphilic triblock copolymer P123 was well dispersed by stirring with distilled water (30 g) and HCl solution (120 mL, 2 M) for 5 h. To the resultant homogenous mixture tetraethyl orthosilicate (TEOS, 9 g) was added and continuously stirred for 24 h at 40 ◦ C. Further, the solution was aged at 100 ◦ C for 48 h and the resultant gel was filtered hot, washed twice with water and dried overnight at
The detailed characterisation of the hexagonally ordered mesoporous carbon prepared from SBA-15-100 template was reported previously [23,29]. The prepared material exhibits well ordered porous structure with a hexagonal arrangement of linear arrays of pores that arranged in regular intervals [23]. The textural parameters such as specific surface area, specific pore volume and pore diameter of MCN were analysed by nitrogen adsorption desorption measurements. MCN-1 catalyst possessed a specific Brunauer–Emmett–Teller (BET) surface area of 505 m2 g−1 with a
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Table 1 Textural parameters of mesoporous carbon nitride (MCN-1). Catalyst
a0 (nm)
ABET (m2 g−1 )
Pore volume (cm3 g−1 )
Pore diameter (nm)
MCN-1
9.52
505
0.55
4.2
Fig. 1. HRTEM images of MCN-1: (a) Captured along mesopores and (b) captured across mesopores [29].
Table 2 Elemental composition of MCN-1 measured at different stages of its preparation from CHN analysis. MCN-1
C (wt%)
N (wt%)
H (wt%)
Other elements (Si, O, Cl and F) (wt%)
After carbonisation After silica removal
37.5 69.7
8.8 16.0
1.9 2.2
51.8 12.1
pore volume of 0.55 cm3 g−1 . The pore diameter of the sample is highly uniform and was found to be 4.20 nm (Table 1). HRTEM images of the sample also display well-ordered honeycomb-like hexagonal mesopore arrangement, which can clearly be seen from the cross-sectional HRTEM image of MCN-1 (Fig. 1b), whereas only a stripe-like pattern was displayed when MCN was imaged along the mesopores (Fig. 1a) [29]. The elemental composition of MCN has been listed in Table 2. As it can be noticed from Table 2, the sample contains 2.2% H, confirming the presence of NH or NH2 groups on the surface of the samples. These groups might have originated from the incomplete polymerisation of the carbon tetrachloride and ethylenediamine precursors and offered basic character to the MCN-1.
1-octanol) compared to cyclic alcohols (cyclohexanol and furfuryl alcohol) and aryl (benzyl alcohol) alcohols, revealing that linear aliphatic alcohols are more reactive than cyclic and aromatic alcohols. It should also be noted that the catalyst was active for long chain alcohols such as 1-octanol although the activity of MCN-1 is lower than that obtained for 1-butanol. These results confirm that the difference in the chain length, electron delocalisation and bulkiness of the alcohols play a major role in deciding the activity of the catalyst. In addition, the physico-chemical traits of the catalyst should also be taken into consideration, which influence the alcohols reactivity in a way entirely different from one reaction to another.
3.2. Catalytic activity The basic catalytic performance of the mesoporous carbon nitride was studied in the transesterification of ethylacetoacetate with 1-butanol, 1-octanol, furfuryl alcohol, cyclohexanol and benzyl alcohol. Initially, the reaction was carried out over MCN-1 at an ethylacetoacetate to 1-butanol molar ratio of 1.4:1, 2 wt% catalyst (wt% based on the total weight of reaction mixture), reaction temperature of 110 ◦ C and the reaction time of 6 h under solvent free condition and the results are displayed in Fig. 2. It was found that the catalyst was active, affording a high conversion within a short reaction time. As can be seen in Fig. 2, the conversion increases with increasing the reaction time and reaches the maximum of 69.4% for the reaction time of 6 h. The effect of the nature of alcohols affecting the conversion of the MCN-1 was also investigated. Among all the alcohols studied, MCN-1 was found to be highly performing when 1-butanol was involved, which is clearly evident from Fig. 2. Interestingly, MCN-1 showed much higher performance when the reaction was carried out with aliphatic alcohols (1-butanol and
Fig. 2. Transesterification of ethylacetoacetate with different alcohols using MCN-1 catalyst (䊉) 1-butanol, () 1-octanol, () furfuryl alcohol, () cyclohexanol and () benzyl alcohol. Reaction conditions: ester:alcohol ratio – 1.4, reaction temperature – 110 ◦ C, reaction time – 6 h and catalyst weight – 2 wt% MCN-1.
C. Anand et al. / Catalysis Today 204 (2013) 164–169
Fig. 3. Effect of varying catalyst weight in the reaction mixture over the catalytic performance of MCN-1 in transesterification of ethylacetoacetate with 1-butanol and comparison with CMK-3 and catalyst free reaction system () 1 wt%, (䊉) 2 wt%, () 4 wt%, () 6 wt%, () 8 wt%, () 10 wt%, () CMK-3 and (♦) catalyst free reaction system. Reaction conditions: ester:alcohol ratio – 1.4, reaction temperature – 110 ◦ C and reaction time – 6 h.
A general schematic representation of transesterification reaction is given in Scheme 1. The basic sites in MCN-1, that are originated from the free NH2 groups found on the walls of MCN-1 and the uncondensed terminal NH2 from the ethylenediamine, are the source of its catalytic activity, which stimulate the transesterification of ethylacetoacetate. In previous study, Vinu and co-workers clearly explained the formation of basic sites, their strength and concentration by TPD of CO2 measurements of MCN1 samples [35]. It has been envisaged that the transesterification of ethylacetoacetate catalysed by MCN involves an intermediate product formed as a result of EtO− elimination, ␣-keto ketene that is highly reactive and further converted into transester with the corresponding alcohol [36,37]. Furthermore, other reaction parameters such as effect of catalyst weight taken in the reaction mixture, temperature of the reaction and ester to alcohol molar ratio influencing the catalytic performance of MCN-1 using 1-butanol was studied. At first, the effect of the catalyst concentration on 1-butanol conversion was investigated by varying the concentration from 1 to 10 wt% and the results are given in Fig. 3. As evident from Fig. 3, the catalytic activity of MCN-1 increased with increasing its quantity from 1 to 2 wt% in the reaction mixture, but started to decline with a further increase of the catalyst concentration. This could be due to the poisoning effect caused when more and more MCN was used in the system. Therefore, 2 wt% was chosen to study the influence of other reaction parameters. The catalytic activity of MCN-1 was also studied at different the ester:alcohol molar ratio at a reaction temperature of 110 ◦ C for different time and the results are given in Fig. 4. 1Butanol conversion increased from 69% to 80% with increasing the molar ester:alcohol ratio from 1.4 to 5. This could be mainly due to the preferential adsorption of ethylacetoacetate over 1-butanol, which in turn influences the rate of formation of the intermediate ketene on the MCN surface quite rapidly resulting in higher conversion with increasing the molar ratio of ester to alcohol. The effect of reaction temperature over the catalytic ability of MCN-1 in transesterification of ethylacetoacetate with 1-butanol was studied as a function of time and the results are shown in Fig. 5. As can be seen from Fig. 5, the temperature certainly has a pronounced effect over the catalytic activity of MCN. The 1butanol conversion increased from 32% to 85% with increasing the reaction temperature from 90 to 150 ◦ C. Although, the conversion increases with increasing the reaction temperature, the selectivity of the products takes a beating leading to many side products other
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Fig. 4. Influence of ethylacetoacetate to 1-butanol ratio on the catalytic performance of MCN-1 in transesterification of ethylacetoacetate (䊉) 1.4, () 3 and () 5. Reaction conditions: reaction temperature – 110 ◦ C, reaction time – 6 h and catalyst weight – 2 wt% MCN.
than the desired one. This could possibly be explained in terms of increasing reaction temperature, wherein the hike in temperature not only influences the rate of the reaction but also induces parallel reactions including cracking or oligomerisation that initiates the formation of undesired products. The stability of the MCN-1 catalyst even under vigorous reaction conditions especially, higher temperature is clearly evident from the above study. The recyclability of MCN-1 was tested in the transesterification reaction. The study comprised of three cycles involving fresh MCN-1 catalyst in the first cycle, wherein the transesterifcation of ethylacetoacetate with 1-butanol reaction was continued for 6 h at 110 ◦ C with an ethylacetoacetate to 1-butanol ratio of 1.4. After completion of each cycle the MCN-1, catalyst was separated by filtration, washed with ethanol to remove possible traces of organic compounds and dried in a hot air oven. Subsequently, the dried MCN-1 catalyst was regenerated by calcination at 500 ◦ C under nitrogen atmosphere for 5 h prior to each cycle. The catalytic activity of MCN-1 from each cycle is compared in Table 3. We observed only a slight reduction in the conversion of 1-butanol revealing that the MCN-1 sample is catalytically stable and can be efficiently reused for several cycles (Table 3). In order to confirm whether the transesterification of ethylacetoacetate with 1-butanol caused by the presence of the basic sites in the MCN-1, the reaction was studied with mesoporous CMK-3
Fig. 5. Effect of reaction temperature over the catalytic performance of MCN-1 in transesterification of ethylacetoacetate with 1-butanol (䊉) 90 ◦ C, () 110 ◦ C, () 130 ◦ C and () 150 ◦ C. Reaction conditions: ethylacetoacetate:1-butanol ratio – 1.4, reaction time – 6 h and catalyst weight – 2 wt% MCN.
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Scheme 1. General reaction mechanism involved in the transesterification reaction. Table 3 Recovery and reuse results of MCN-1 for transesterification of ethylacetoacetate with 1-butanol. Catalyst
Usage cycle
1-Butanol conversion
Butyl acetoacetate selectivity (%)
MCN-1 MCN-1 MCN-1
Fresh 1 2
69 67 65
100 100 100
Acknowledgements One of the authors A. Vinu is grateful to ARC for the award of the future fellowship and the AIBN and the University of Queensland for the start-up grant. The authors also acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, University of Queensland.
Reaction conditions: reaction temperature – 110 ◦ C, reaction time – 6 h, ester to alcohol ratio – 1.4 and catalyst weight 2 wt% of total reaction mixture.
References
carbon and without any catalyst and the results are given in Fig. 2. It is clear from Fig. 2 that MCN-1 is overwhelmingly active when compared to CMK-3 for the transesterification of ethylacetoacetate with 1-butanol. We believe that the activity of MCN-1 solid base catalyst in the transesterification of ethylacetoacetate is in fact enhanced due to the presence of NH or NH2 groups that offer strong Lewis basic sites. Moreover the MCN-1 catalyst is free from any metals and nowadays the demand for metal free catalysts is growing rapidly among the industries, especially when it inherits all those advantages of a metal incorporated catalyst. In addition, the regeneration and reusability of the MCN catalysts does not seem to affect its catalytic activity at all, which is a significant behaviour expected from industrial view point.
4. Conclusions In conclusion, we demonstrated the basic catalytic performance of the mesoporous carbon nitride for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol without any solvents. The catalyst was found to be highly active, affording a high conversion of long and short chain alcohols, cyclic and aromatic alcohols and the desired product in good to excellent yield. We also demonstrated that the reaction parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream play a significant role in controlling the activity of the catalyst. The catalyst was found to be highly stable even at high temperature and can be recovered easily and reused efficiently at least three times without any significant loss of the catalytic activity. We strongly believe that this novel basic mesoporous carbon nitride catalyst could make an excellent platform for various basic catalysed organic reactions for the synthesis of industrially important and pharmaceutical products and replace the existing toxic and environmentally unfriendly catalysts that are being currently used in the industry.
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