Ying Qing,Houfang Lu,Yingying Liu*,Changjun Liu,Bin Liang,Wei Jiang

1School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2Institute of New Energy and Low Carbon Technology,Sichuan University,Chengdu 610065,China

Keywords:Glycerol DBU Glycerol carbonate Apparent kinetics

A B S T R A C T The biodiesel production technology catalyzed by 1,8-diazabicycloundec-7-ene(DBU)is developed in this work.Crude glycerol containing DBU and DBU/glycerol/CO2(DGC)ionic compounds reacts directly with dimethyl carbonate(DMC)to produce high value-added glycerol carbonate(GC)catalyzed by DBU and DGC.The catalytic performance of DBU and DGC,as well as the kinetics of the reaction catalyzed by DBU,were investigated.The results show that DGC has a weak catalytic effect on the transesterification of glycerol and DMC.When the temperature is higher than 60°C,DGC catalyzes the reaction jointly with DBU,which is produced from the decomposition of DGC.DBU has a good catalytic effect on the reaction between glycerol and DMC,with 90%conversion of glycerol and 84%selectivity to GC under the following conditions:DMC-to-glycerol molar ratio of 3:1,4.0%DBU(based on glycerol mass),reaction time of 60 min,and reaction temperature of 40°C.The apparent kinetics results show that the activation energies are 30.95 kJ·mol-1and 55.16 kJ·mol-1for the forward and reverse GC generation reactions,respectively,and the activation energy of the decomposition reaction of GC to glycidol(GD)is 26.58 kJ·mol-1.

1.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[2–4].

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 hexanemethanol mixture[5,6].Secondly,DBU can be used as a catalyst in the transesterification for biodiesel production[5–7].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 CO2to form a polar DBU/glycerol/CO2ionic 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 CO2or CO/O2,glycerolysis with urea,phosgenation with phosgene,and transesterification with dimethyl carbonate(DMC)or ethylene carbonate(EC).Among these methods,the method with CO2or CO/O2is 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].

Fig.1.The process flow chart of DBU-catalyzed biodiesel production and the conversion of crude glycerol.(Where:FAME is fatty acid methyl ester;GLY is glycerol;MeOH is methanol;DGC is DBU/glycerol/CO2ionic compound;DMC is dimethyl carbonate;GC is glycerol carbonate.)

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.

2.Materials and Methods

2.1.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 further purification.

2.2.Preparation of DGC

DBU and glycerol with a molar ratio of 3:1 were added to a round bottom flask and dry CO2gas was continuously bubbled into the mixtureat50°Cundermagnetic stirring.After2 h,the mixture was washed with acetone to obtain a white ionic compound solid[15].

2.3.Synthesis of GC

Transesterification of DMC and glycerol was performed in a 50 ml round bottom flask with a reflux condenser.Typically,0.20 g(4.0 wt%of glycerol)of catalyst was charged to the flask containing 5.00 g of glycerol(54.29 mmol)and 14.67gof DMC(162.88 mmol).The reaction was performed at 40°C in a water bath under strong magnetic stirring.After the reaction,the mixture was immediately cooled to room temperature and removed from the flask for analysis.Every experiment was repeated in triplicate to ensure reproducibility.

2.4.Product analysis

The reaction mixture was analyzed by gas chromatography(Agilent GC7890A with capillary column AT-WAX,30 m × 0.25 mm×0.25 μm)using tetraethylene glycol as an internal standard.The injector and detector temperatures were 250 °C and 280 °C,respectively.The conversion of glycerol(xGLY),the selectivity of GC(SGC),and the selectivity of glycidol(SGD)are expressed as follows.

When DGC was used as a catalyst,the content of DBU in the system after the transesterification was determined by automatic potentiometric titration with a 0.1 mol·L-1HCl solution.

3.Results and Discussion

3.1.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 biodieselrich phase can be removed by introducing CO2into 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 form crude glycerol;thus,the crude glycerol contains some DBU and DGC.According to the literature[18],DGC can effectively catalyze the transesterification of glycerol and DMC at 100°C with 1 mol%of DGC(based on glycerol),with the glycerol conversion being as high as 96%.However,it has been reported that the decomposition temperature of DGC is～60 °C[19],and the decomposition rate of DGC will increase with temperature.At 100°C,the DBU produced from DGC may take a primary catalytic role,as it is known to act as a catalyst for this reaction[14].Therefore,at 100°C,DGC catalyzes the transesterification between glycerol and DMC jointly with DBU,which is produced by the decomposition of DGC.

Fig.2.The reaction scheme of DBU-glycerol-CO2.

Since the separation of glycerol-rich phase is performed at～40 °C during biodiesel production,it is possible to simplify the process and reduce its energy consumption if DGC and DBU can be used to catalyze the transesterification near this temperature.Hence,the catalytic effect of DGC on the reaction of glycerol and DMC at 40–80 °C was studied(Fig.3).After the transesterification,the content of free DBU in the system was determined(Table 1).Meanwhile,the catalytic effect of the same content of free DBU on the reaction was also studied under the same conditions(Fig.3)to investigate the effect of DGC and DBU on the reaction.

Blank experiments indicate that DMC and glycerol do not react without catalyst at 40–80 °C.As seen in Fig.3,the glycerol conversion is 23%at 40°C when the amount of DGC is 4.0 wt%(based on glycerol mass),and the conversion increases dramatically as the temperature increases.According to Table 1, there is no free DBU in the reaction system at 40°C,which indicates that DGC does indeed elicit a catalytic effect on the reaction since it does not decompose at this temperature.However,the DGC will decompose when the temperature is higher than 50°C,and higher temperatures lead to larger DGC decomposition ratios.According to Fig.3,DBU can also catalyze the reaction,which suggests that the reaction would be catalyzed by both DGC and DBU(produced from DGC decomposition).The catalytic effect of DBU is even higher than that of DGC at 80°C.Compared with DGC,DBU has a stronger catalytic effect on the reaction.

Fig.3.The glycerol conversions with DGC or free DBU as catalyst at different temperatures.Reaction conditions:GLY-to-DMC molar ratio of 1:3;4.0 wt%DGC(based on GLY mass);60 min.

Table 1The amount of free DBU after the reaction

3.2.Catalytic activity of DBU on the reaction

As seen in Fig.3,DBU acts as a catalyst,though it is weak at low temperatures.In the DBU-catalyzed production of biodiesel,the final content of DBU in glycerol-rich phase is 5.15 wt%at 45°C when the amount of DBU is 1 wt%(based on oil)[16],which is much higher than the DBU content produced from the decomposition of DGC(Table 1).Thus,the effect of the DBU content(2 wt%–6 wt%based on glycerol mass)on the reaction of glycerol and DMC was investigated(Fig.4).

With alkaline catalyst,glycerol reacts with DMC as follows[20].

Fig.4.Effect of DBU amount on the reaction. Reaction conditions: GLY-to-DMC molar ratio of 1:3,40°C,60 min.

Glycerol and DMC react to form GC and methanol reversibly,and the decarboxylation of GC under alkaline conditions will occur to generate glycidol(GD).As shown in Fig.4,the conversion of glycerol increases gradually as the amount of DBU is increased.Concomitantly,the selectivity of GC gradually decreases.If the DBU amount exceeds 4.0 wt%,Reaction(2)becomes more dominant than Reaction(1),with the alkalinity of the reaction system increasing as the amount of DBU increases[21].Therefore,in order to obtain high GC selectivity,the amount of DBU must be optimized while maintaining a high conversion of glycerol.The conversion of glycerol reaches 90%when the amount of DBU is 4.0 wt%,and there is little improvement in the conversion of glycerol as the amount of DBU is increased further.Therefore,the optimum amount of DBU is 4.0 wt%.As for the recovery of the catalyst DBU,according to the differences of DBU and GC in boiling points(261°C for DBU and 350°C for GC)and polarity,it is possible to separate DBU from GC by vacuum distillation or by adsorption.

The influence of the reaction temperature was also investigated in the range of 30–80 °C(Fig.5).As seen,the conversion of glycerol increases slightly while the selectivity of GC decreases gradually as the reaction temperature is increased.At 80°C,the selectivity of GC decreases to 60%while that of GD rises to 39%.To obtain a higher GC yield,the reaction should be performed at a lower temperature.Indeed,the glycerol-rich phase is usually separated at～40 °C in biodiesel production processes.Thus,in order to use the glycerol-rich phase directly for GC production,a temperature of 40°C is considered to be optimum without additional energy consumption.

Fig.5.Effect of reaction temperature on the reaction.Reaction conditions:GLY-to-DMC molar ratio of 1:3,4.0 wt%DBU(based on GLY mass),60 min.

Although the transesterification between glycerol and DMC is reversible,the main objective of this work concerns glycerol.Compared to glycerol,DMC has a low boiling point and low viscosity;thus,recycling DMC is straightforward.To convert as much glycerol as possible,it is necessary to increase the amount of DMC to drive the equilibrium to the right. Therefore, it is necessary to determine the optimum amount of DMC required for this purpose.The effect of the DMC-toglycerol molar ratio on the transesterification is shown in Fig.6.

Fig.6.Effect of molar ratio of DMC-to-GLY on the reaction.Reaction conditions:40°C;4.0 wt%DBU(based on GLY mass);60 min.

According to Reaction(1),the stoichiometric ratio of DMC to glycerol is 1:1.As seen in Fig.6,the conversion of glycerol increases from 84%to 90%when the DMC-to-glycerol molar ratio increases from 2:1 to 3:1.Note that the glycerol conversion remains almost identical when the molar ratio increases further.On the one hand,increasing the DMC-to-glycerol molar ratio promotes the forward reaction to improve the conversion of glycerol.On the other hand, DBU and glycerol become diluted with more DMC,which may decrease the number of effective collisions between DBU and glycerol,leading to a consequent decrease of glycerol conversion.Overall,the conversion of glycerol remains constant.Therefore,the optimummolar ratio of DMC to glycerol is 3:1.

The effect of reaction time on the glycerol conversion and selectivity is shown in Fig.7.

Fig.7.Effect of reaction time on the reaction. Reaction conditions: GLY-to-DMC molar ratio 1:3;4.0 wt% DBU(based on GLY mass);40°C.

Fig.8.Fitting curves and experimental data at different temperatures:(a)30 °C;(b)40 °C;(c)50 °C;(d)60 °C;Key:(■)GLY,()GC,()GD,GLY-to-DMC molar ratio 1:3;4.0 wt%DBU(based on GLY mass),Solid lines are the fitting curves.

According to Fig.7,the conversion of glycerol increases significantly and the selectivity of GC decreases clearly as the reaction time increases,reaching as high as 90%and 84%in 60 min,respectively.Increasing the reaction time further has little effect on the glycerol conversion whereas the selectivity of GC significantly decreases.As a result,the optimum reaction time is 60 min.

3.3.Apparent kinetics of glycerol and DMC reaction catalyzed by DBU

To investigate the apparent kinetics of glycerol and DMC,for simplification,it is assumed that the reaction system is pseudo-homogeneous.The reasons are as follows.Firstly,although DMC and glycerol are immiscible,it is assumed that the effect of mass transfer in the initial stage is negligible as the reaction is performed under strong agitation(Further increasing in the stirring speed has little effect on the glycerol conversion.).In addition,both DBU and the product methanol can promote the inter-solubility of glycerol and DMC.In the experiments,one phenomenon should be noticed that even the glycerol conversion was as low as about 28%,the reaction system can become homogeneous.Due to the rapid reaction rate,the heterogeneous state only lasts for a brief period which can be neglected considering the high conversion range in the kinetics study.Moreover,high conversion ofglycerol is beneficial for practical batch reaction,and kinetic behavior at high glycerol conversion is more important compared the initial low conversion stage and it can be treated as pseudo-homogeneous.As a result,the apparent reaction rates for Reactions(1)and(2)can be expressed in terms of reversible and irreversible reactions,respectively,as given by the generally accepted expressions[22–25].

Table 2The apparent reaction rate constants at different temperatures

Fig.9.Arrhenius plot of reaction rate versus temperature.

Table 3Apparent reaction activation energies

Fig.10.Fitting curves and experimental data at different DBU dosage:(a)4.0 wt%;(b)2.8 wt%;(c)1.6 wt%;(d)0.4 wt%(based on GLY mass);GLY-to-DMC molar ratio 1:3;60 °C.Key:(■)GLY,()GC,()GD,Solid lines are the fitting curves.

where k1,k-1,and k2are the apparent reaction rate constants for the forward and reverse Reaction(1)and the reaction rate constant of Reaction(2),respectively.[GLY],[DMC],[GC],[MeOH],and[GD]are the concentrations of glycerol,DMC,GC,methanol,and GD.Therefore,

Table 4The apparent reaction rate constants at different DBU dosages

Fig.11.Apparent rate constants as a function of DBU dosage.

Substituting Eqs.(3)and(4)into Eqs.(5)–(9)gives the following:

Fig.12.Comparison of concentrations between the predicted results and experimental results. Reaction conditions:(a)70 °C,DBU0.4wt%;(b)75 °C,DBU0.4 wt%;(c)80 °C,DBU0.4wt%;(d)85 °C,DBU 0.4 wt%;(e)85 °C,DBU 4.0 wt%;(f)30 °C,DBU 0.4 wt%(based on GLY mass).GLY-to-DMC molar ratio 1:3.Key:(■)GLY,()GC,()GD.Solid line=model prediction.

According to the above rate equations, the experimental data obtained at 30–60 °C and 4.0 wt%DBU(relative to glycerol mass)were fitted.The rate constants were calculated by integration of the differential equations.The ODexLims function in Microsoft Excel 2013 was used to solve these differential equations by the least squares method[26].The fitting curves are shown in Fig.8,and the apparent rate constants for each reaction are given in Table 2.

As seen in Table 2,k1is much larger than k–1.Thus,Reaction(1)can be regarded as an irreversible reaction.All apparent reaction rate constants increase with temperature.The formation rate of GC is much higher than that of GD,although k1and k2have the same order of magnitude,which results from the high initial concentrations of DMC and glycerol.The concentration of GC increases with the consumption of DMC and glycerol.Meanwhile,the concentrations of DMC and glycerol gradually decrease.As a result,the formation rate of GC will diminish and the formation rate of the by-product GD will gradually increase as the reaction proceeds.An optimal reaction temperature of 40°C and reaction time of 60 min are required to obtain higher GC yields.The temperature dependency of the reaction rate constant was studied in terms of Arrhenius's law(Fig.9),and the activation energies for each reaction step were calculated(Table 3).

The activation energy of reaction between glycerol and DMC catalyzed by DBU at 30–60 °C is 30.95 kJ·mol-1,which is similar to the result(of 28.4 kJ·mol-1at 50–70 °C)determined by Esteban et al.with CH3OK as the catalyst[25].The activation energy of the forward reaction in Reaction(1)is close to that of Reaction(2),indicating that the effect of temperature on the two reactions is similar.

The amount of DBU catalyst has an important effect on the reaction of glycerol and DMC. In this work, the effect of the amount of DBU(4.0%,2.8%,1.6%,0.4%based on glycerol mass)on the reaction at 60°C was studied.The fitting curves are shown in Fig.10,and the apparent rate constant for each reaction is given in Table 4.

As seen in Fig.11,the reaction rate constant increases as the amount of DBU increases.Further,the increase of k1is the most significant,indicating that the increase ofDBU has the greatest influence on the forward reaction of Reaction(1).Additionally,the three reaction rate constants show good linear relationship with the amount of DBU,demonstrating that all reaction rates are proportional to the amount of this catalyst.The effect of DBU is not as pronounced for the backward reaction of Reaction(1),since the value of k-1is much smaller than k1and k2as the amount of DBU is increased.

Based on the influence of temperature on the reaction rate constants(Fig.9)and the proportional relationship between these and the amount of catalyst(Fig.11),a kinetic model was used to simulate the reaction at 30–85 °C with 0.4 wt%–4.0 wt%DBU.The results are shown in Fig.12.

The parity plot(Fig.13)shows good agreement between the experimental data and the model prediction,indicating that the kinetic model is suitable for the glycerol and DMC reaction system catalyzed by DBU at 30–85 °C with 0.4 wt%–4.0 wt%DBU.

Fig.13.Parity plot comparing the predicted and experimental concentration. Short dash=±10%error line.

4.Conclusions

The synthesis of glycerol carbonate(GC)via the transesterification of glycerol with dimethyl carbonate(DMC)using DBU/glycerol/CO2ionic 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 on glycerol mass),reaction time of 60 min,and reaction temperature of 40°C.The apparent kinetics study indicates that the selected kinetic model is well suited to the reaction;the transesterification is considered to be a second-order irreversible reaction.Meanwhile,the decomposition of GC is a first-order reaction.The predictions agree well with the experimental results.The activation energies of the forward and backward reactions of GC are estimated to be 30.95 kJ·mol-1and 55.16 kJ·mol-1,respectively,and the activation energy for GC decomposition is 26.58 kJ·mol-1.