Introduction
Recently, the development of fatty oil-based biodiesel, which is produced from vegetable oils by transesterification with methanol or ethanol, has received much attention because of its great potential as an alternative to fossil fuels [1], [2] and [3]. In the production of biodiesel, glycerol (GL) is formed as an undesired by-product (about 100 g/kg of biodiesel) [2]. The biodiesel production capacity has been increasing annually and results in the accumulation of GL content as well. Thereby, in order to promote the development of the biodiesel industry, it has become a focus of the researchers to convert GL into value-added chemicals. GL can be converted into several important chemicals such as propanediol, acrolein, glyceric acid, esters of glycerol, and glycerol carbonate, etc. Among the derivatives of GL, glycerol carbonate (GC) is a promising one due to its physical properties and potential uses [4]. GC is a nontoxic, readily biodegradable, water-soluble, not flammable (fp 165.9 °C) and viscous liquid with a very low evaporation rate (bp 353.9 °C at 101.325 kPa). All these features make GC a green chemical that can be used as a polar high boiling solvent, a surfactant component, and an intermediate for many kinds of polymers such as polyesters, polycarbonates, polyamides to name but a few [5], [6], [7] and [8].
Several processes for the synthesis of GC have been discovered. GC can be prepared through carboxylation of GL with carbon dioxide under supercritical condition [2], however, the yield obtained with this method is too low so that the industrial synthesis of GC with this route is virtually unpractical. The carbamoylation–carbonation reaction between GL and urea is another method to produce GC [7] and [9], but this reaction must be carried out at vacuum conditions in order to separate ammonia, which is simultaneously generated in this reaction. Furthermore, GC can be alternatively synthesized through transesterification of GL with ethylene carbonate [5]. However, with this method, the high energy consuming will be needed to purifying the product because the boiling points of both ethylene carbonate (bp 261 °C at 101.325 kPa) and the byproduct, ethylene glycol (bp 197 °C at 101.325 kPa) are very high. Transesterification of GL with dimethyl carbonate (DMC) is also thermodynamically possible, which offers an environmentally benign route for synthesizing GC considering the following factors: (i) non-toxic raw material, (ii) mild operation condition, (iii) high yield, and (iv) simple purification of GC (Scheme 1) [10], [11], [12], [13], [14], [15], [16], [17] and [18].
Full-size image (12 K)
Scheme 1.
Synthesis of glycerol carbonate (GC) from glycerol (GL) and dimethyl carbonate (DMC).
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Ochoa-Gómez et al. reported that some homogeneous bases, such as KOH, NaOH, and K2CO3, are valuable catalysts for the transesterification of GL with DMC [9]. However, the separation of the homogeneous catalyst from the products is very difficult, and large amounts of waste water would also be produced. A heterogeneous enzymatic catalyst, lipase Novozym 435, has also been employed in the synthesis of GC from GL and DMC [11]. Unfortunately, as for now this system is suffering from many drawbacks, such as high cost, poor activity and long reaction time (25 h). Recently, mixed metal oxides have been widely applied as heterogeneous basic catalysts for the synthesis of GC from GL and DMC. As the basic property and the surface area of mixed metal oxides can be adjusted easily by changing the composition and calcination temperature, these catalysts present normally higher catalytic activity than that of a single component. Until now, MgO/La2O3, CaO/MgO, MgO/Al2O3, Mg/Al/Zr mixed oxide, and Co3O4/ZnO have been applied to GC synthesis [17], [19], [20], [21] and [22]. However, in order to achieve a high yield of GC, the reactions with these catalysts have to be either performed at high temperature (>100 °C) or conducted with a high molar ratio of DMC/GL (>5). In addition, the activities of some catalysts are still far from satisfactory, such as MgO/La2O3 and MgO/Al2O3[17] and [20].
CaO is one of the catalysts which have been frequently used for the synthesis of GC from GL and DMC [9] and [14]. When the molar ratio of DMC to GL is less than 2.0, CaO operates as a homogeneous catalyst. But in a reaction system that has a higher molar ratio (>2.0), CaO will act as a heterogeneous catalyst [14]. The salient feature of CaO catalyst is less expensive and less toxic. Unfortunately, this catalyst is easy to deactivate. Furthermore, its activity in the transesterification of GL with DMC is still moderate. Ochoa-Gómez et al. found that the GL conversion decreased quickly during its recycling. In the fourth time of reuse, the GL conversion was lower than 24% with the recovered CaO catalyst [9]. A similar phenomenon was also reported by Li and Du [23] and [24]. As a result, a time-consuming and troublesome regeneration step will be needed for reusing the CaO catalyst in this reaction. Consequently, seeking a method to increase the catalytic activity and stability of CaO catalyst is extremely important to the industrial product of GC by transesterification of GL with DMC [3], [25] and [26].
On the other hand, a neutral salt, KNO3, which is also an inexpensive and easily available material, has been widely used as a key guest species to facilitate the generation of strong basic site on various porous materials, such as alumina, zirconia, and zeolite [27] and [28]. Especially, a super basicity (H_ = 27.0) can be formed on alumina by loading KNO3[27]. In addition, KNO3/Al2O3, KNO3/NaX, and KNO3/MCM-48 catalysts have been successfully used as catalysts for the production of biodiesel [1], [29] and [30].
The basic sites on heterogeneous catalyst are the active centers for the transesterification reaction [31] and [32]. Therefore, there is an evident correlation between the basic property (basic strength and amount of basic sites) of the catalysts and their catalytic activity. Previously, in the synthesis of biodiesel via transesterification, Xie et al. found that the biodiesel yield increased monotonically with the increase of the amount of basic sites in the catalyst surface, whereas the basic strength of the catalyst had no obvious effect on the yield of biodiesel [1]. In contrast, Kim et al. reported that the good catalytic performance of Na/NaOH/γ-Al2O3 catalyst in the same reaction could be ascribed to its strong basic strength [33]. Recently, in the synthesis of GC by transesterification of GL with DMC, Liu et al. found that the activity of the Mg–Al mixed oxide catalyst was proportional to the surface density of basic sites [20]. Simanjuntak et al. also discovered that the catalytic activity of MgO/La2O3 catalyst in the synthesis of GC from GL and DMC depended closely on the concentration of the basic sites [17].
The aim of the present work is to develop a new catalytic process for the synthesis of GC under environmentally benign conditions with the aid of highly active, robust and inexpensive heterogeneous catalyst. To the best of our knowledge, there is no report on the synthesis of GC from GL and DMC by means of using KNO3/CaO as catalyst. In this work, KNO3/CaO composite catalysts were prepared and used for the synthesis of GC by transesterification of GL with DMC. The catalytic activities and stabilities of the prepared catalysts were studied in detail. XRD, FT-IR, SEM, CO2-TPD, BET method, and Hammett titration method were used for scrutinizing physicochemical properties of the prepared catalysts. The correlation between the basic property of the catalysts and their catalytic activity was discussed, and a plausible deactivation mechanism about KNO3/CaO catalyst was propose