ArticleProduction of glycerol carbonate using crude glycerol from biodiesel production with DBU as a catalyst☆
Introduction
As an environmentally friendly renewable energy, biodiesel has been widely studied and developed in the last two decades [1]. However, although the commercial technology of biodiesel has matured, biodiesel production continues to suffer from problems associated with large energy consumption and wastewater pollution. On-going efforts are developing greener and more economical technology, with new catalysts, separation technologies, and glycerol conversion routes being the focus of several investigations [2], [3], [4].
1,8-Diazabicyclo-[5.4.0]-undec-7-ene (DBU) is a non-polar organic strong base that can be used in biodiesel production industry. Firstly, DBU and DBU based ILs can be used to extract lipid with high efficiency which is similar to conventional solvents such as hexane-methanol mixture [5], [6]. Secondly, DBU can be used as a catalyst in the transesterification for biodiesel production [5], [6], [7]. The process flow chart of DBU-catalyzed biodiesel production is shown in Fig. 1. Two advantages of this catalyst are particularly noteworthy. First, it can improve the reaction efficiency by promoting the mutual solubility of oil and methanol. Second, after the reaction, residual glycerol and DBU in the biodiesel-rich phase can be easily removed by bubbling CO2 to form a polar DBU/glycerol/CO2 ionic compound (DGC), which is insoluble in biodiesel. In this way, biodiesel can become purified [5]. This process will therefore avoid both the large amount of waste water during the washing process and the high energy consumption in the distillation process for the purification of biodiesel when conventional homogeneous alkali catalysts are used [3].
However, with the large-scale development of the global biodiesel industry, the amount of crude glycerol has increased rapidly. In 2016, the global production of biodiesel was ~ 32.8 million tons, which resulted in approximately 3.28 million tons of crude glycerol [8]. This amount greatly exceeds the market demand, which has resulted in a steep decrease of the price of glycerol and placed further downward pressure on the economic benefits of biodiesel production. To address this situation, converting crude glycerol into value-added glycerol derivatives (e.g., glycerol carbonate (GC); 1,3-propanediol; 2,3-butanediol; butanol; monoglycerides; and citric acid) is desirable [4], [9], [10]. GC derived from glycerol by carboxylation is an important chemical intermediate and solvent with many potential applications in the chemical industry. GC can be synthesized from glycerol through several routes, including direct carboxylation with CO2 or CO/O2, glycerolysis with urea, phosgenation with phosgene, and transesterification with dimethyl carbonate (DMC) or ethylene carbonate (EC). Among these methods, the method with CO2 or CO/O2 is performed under supercritical conditions that require large capital investment; meanwhile, the yield of GC is low. The approach involving urea requires vacuum conditions to separate the byproduct ammonia, which also requires a non-trivial investment in equipment. Further, the high toxicity of phosgene hinders its utilization in GC synthesis, and the high boiling point of EC makes product separation and purification difficult. Thus, transesterification of glycerol with DMC is the most attractive route for GC synthesis; it utilizes cheap and environmentally friendly materials under mild conditions and the boiling point of DMC is low, resulting in easy product separation [11], [12]. However, selection of the catalyst is crucial for success with this process. Base catalysts such as KOH, K2CO3, and CaO can effectively catalyze the reaction [13]. DBU as a strong base can also effectively catalyze the transesterification of glycerol and DMC. Indeed, it was previously reported that 98% of glycerol can be converted at 100 °C in 7.5 h when the amount of DBU was 0.1 mol% of glycerol [14].
This paper mainly studies the utilization of crude glycerol which contains impurities of DBU and DGC showed in Fig. 1. In order to develop the biodiesel production technology catalyzed by DBU, we propose the utilization of the residual DBU and DGC in crude glycerol as catalysts to convert glycerol and DMC into GC under mild conditions. As this process couples the productions of biodiesel and GC, the market price of GC is 3000–3400 USD·t− 1, much higher than the price of DMC (1050–1150 USD·t− 1) and crude glycerol (300–400 USD·t− 1), therefore, the conversion of crude glycerol to GC can significantly improve the economic benefits of the biodiesel industry. Although the market price of DBU is 8700–16000 USD·t− 1, considering the recovery of DBU, it is acceptable to be used as catalyst in both biodiesel production and GC production. In conventional biodiesel production, biodiesel and crude glycerol are usually separated at ~ 40 °C due to their immiscibility. This work investigates the catalytic performance of DGC and DBU with glycerol and DMC near the phase separation temperature, since low temperatures are favorable for phase separation. In addition, the process conditions of transesterification between glycerol and DMC catalyzed by DBU were optimized, and the apparent kinetics of this reaction was studied.
Section snippets
Materials
Analytical grade glycerol and DMC were purchased from Chengdu Kelong Chemical Co., Ltd., China. DBU (> 99%) was purchased from Shanghai Beihe Chemicals Co., Ltd., China. GC (> 90%) was purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd. Glycidol (96%) was purchased from Sigma Aldrich, America. And tetraethylene glycol (99%) was purchased from Aladdin. CO2 (≥ 99.99%, volume fraction) was purchased from Chengdu Dongfeng Gas Co., Ltd. All the chemicals and gases were used without
Catalytic effect of DGC on the transesterification of glycerol and DMC at different temperatures
In the DBU-catalyzed biodiesel production, DBU distributed into the biodiesel-rich phase and glycerol-rich phase with a certain proportion after the reaction [16]. The residual DBU and glycerol in the biodiesel-rich phase can be removed by introducing CO2 into the mixture to form DGC (Fig. 2), which can be separated from biodiesel due to its low solubility in biodiesel [17], resulting in purification of the biodiesel. Meanwhile, the separated DGC can be dissolved in the glycerol-rich phase to
Conclusions
The synthesis of glycerol carbonate (GC) via the transesterification of glycerol with dimethyl carbonate (DMC) using DBU/glycerol/CO2 ionic compound or DBU as the catalyst at 30–80 °C was investigated. It is found that the ionic compound does indeed elicit a catalytic effect on the reaction, but DBU demonstrates superior catalytic activity. A glycerol conversion of 90% and a selectivity to GC of 84% were obtained under the following conditions: DMC-to-glycerol molar ratio of 3:1, 4.0% DBU (based
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