Microwave-assisted transesterification of industrial grade crude glycerol for the production of glycerol carbonate

Accepted Manuscript Microwave-assisted transesterification of industrial grade crude glycerol for the production of glycerol carbonate Wai Keng Teng, Gek Cheng Ngoh, Rozita Yusoff, Mohamed Kheireddine Aroua PII: DOI: Reference:

S1385-8947(15)01191-2 http://dx.doi.org/10.1016/j.cej.2015.08.108 CEJ 14101

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 April 2015 13 August 2015 23 August 2015

Please cite this article as: W.K. Teng, G.C. Ngoh, R. Yusoff, M.K. Aroua, Microwave-assisted transesterification of industrial grade crude glycerol for the production of glycerol carbonate, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.08.108

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Microwave-assisted transesterification of industrial grade crude glycerol for the production of glycerol carbonate Wai Keng Teng, Gek Cheng Ngoh*, Rozita Yusoff, Mohamed Kheireddine Aroua

Centre for Separation Science & Technology (CSST), Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

* Corresponding author: Gek Cheng Ngoh; Phone no.: +603-79675301; Fax: +603 79675371 E-mail addresses: [email protected] (W.K. Teng), [email protected] (G.C.Ngoh), [email protected] (RozitaYusoff), [email protected] (M.K. Aroua)

Postal address: Centre for Separation Science & Technology (CSST), Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

1

Table of Contents Abstract 1. Introduction

2. Materials & Methods 2.1. Materials 2.2. Characterization of Crude Glycerol 2.3. Microwave-assisted Transesterification 2.4. Conventional Transesterification 2.5. Process Evaluation of Glycerol Carbonate Synthesis Using Crude Glycerol of Different Purity 2.6. Energy Consumption

3. Results and Discussions 3.1. Characterization of Pure and Crude Glycerol 3.2. Evaluation of Process Parameters 3.2.1. Effect of glycerol with different impurity on glycerol carbonate synthesis 3.2.2. Effect of reaction time on glycerol carbonate synthesis 3.2.3. Effect of reaction temperature on glycerol carbonate synthesis 3.2.4. Effect of catalyst loading on glycerol carbonate synthesis 3.2.5. Effect of molar ratio of dimethyl carbonate:glycerol on glycerol carbonate synthesis 3.2.6. Effect of impurities on glycerol carbonate synthesis 3.3. Comparison

of

Microwave-assisted

Transesterification

and

Conventional

transesterification 3.4. Energy Requirements

4. Conclusions Acknowledgement References

2

Abstract This study is the first of its kind to report on the transesterification of industrial grade crude glycerol (GLY) to glycerol carbonate (GC) using microwave processing. Pure and two grades of crude GLY at 70% and 86% purity were transesterified with dimethyl carbonate (DMC) using calcium oxide (CaO) as catalyst. A comparison study was made of the conventional and microwave-assisted transesterification process. It was found that 70% purity of crude GLY gave higher yield in both conventional and microwave-assisted transesterification processes with the latter technique showing better energy efficiency. The highest GC yield of 93.4% was obtained from GLY with purity of 70% under microwave irradiation system with 1 wt% of catalyst, 2:1 molar ratio of DMC:GLY at 65 °C in five minutes reaction time. The yield of GC was observed to increase with temperature, time and molar ratio but not the catalyst loading. The impurities such as methanol (MeOH) and sodium methylate (NaOMe) in the crude GLY were found to increase GC yield by four times. This indicates that the impurities which might not be desirable for the transesterification process under normal circumstances have unconventionally demonstrated positive effects on the performance of transesterification under microwave irradiation.

Keywords: Transesterification; crude glycerol; biodiesel; glycerol carbonate; impurities; microwave irradiation

3

1. Introduction Biodiesel industries have generated vast amounts of glycerol (GLY). To manage the abundance of the GLY produced, many researchers and industries have either purified it [1] or converted it into valuable chemicals such as glycerol carbonate (GC) via carbonylation [2, 3], transesterification [4-12], glycerolysis [13-15], etc. Catalytic transesterification of GLY is a simplistic route to produce high GC yield [16]. The commonly used catalyst in the process is metal oxide based catalyst which is cheap and has high catalytic activity [17, 18]. For instance, calcium oxide (CaO) has been reported as a very efficient catalyst in transesterification, moreover it can easily be separated and easily available at industrial scale [18]. Production of GC from pure GLY as reported in many studies involves the high purification cost of crude GLY, which directly increases the production cost of GC. The crude GLY obtained from the biodiesel plant is normally 40-70% purity before acid treatment and attain 80% purity after acid treatment [16]. The impurities present in the crude GLY and residue catalyst are generally undesirable for the conventional transesterification process. However, some impurities such as methanol (MeOH), soap, salt, fatty acids (FA), glycerides in the crude glycerol had reported to demonstrate positive effect [19-22]. To justify the direct use of crude GLY in the production of GC, its economic advantage must be substantiated with acceptable GC yield in a feasible process. Thus, investigation into the effect of impurities of crude GLY on the performance of GC synthesis is essential. To ensure the energy efficiency of the synthesis process, various process intensification technologies based on alternate energy sources such as ultrasound and microwave irradiation have been attempted [5, 23]. Microwave-assisted organic synthesis has been reported to have shorter reaction time, lower operating temperature, and is effective in chemical transformations and thus it has been widely accepted as a non4

conventional method for organic synthesis [24]. Although microwave technology has its limitation to penetrate through larger sample volumes [25], the scalability might be achieved via kinetic modelling study as reported in the work by Chan et al [26]. Another possible commercialization of microwave application is to employ continuous flow mode of microwave reactor to have larger scale production [27]. Considering the benefits of microwave technology such as shorter reaction time and lower energy requirement as compared with the conventional heating [28], transesterification of glycerol in GC synthesis using microwave irradiation is an economical process worthy of research. The reaction rate involved GLY as reactant with the aid of microwave irradiation can be greatly sped up due to the high dielectric properties of GLY [29]. Research on industrial grade crude GLY conversions is still scare in the open literature and this is the first article reporting microwave-assisted conversion of crude GLY to GC. In this study, microwave-irradiated transesterification of GLY with DMC to produce GC is illustrated in Scheme 1. The pure and the crude GLY subjected to microwave irradiation in solventfree conditions were compared with that of the conventional transesterification in this study. This was carried out to ascertain the feasibility of utilizing crude GLY in the production of GC via microwave incorporated transesterification process. The effects of the process operating conditions and the impurities of crude GLY such as MeOH and sodium methylate (NaOMe) on the microwave assisted transesterification process were investigated. To examine the possible alleviation of the production cost, energy consumption for the microwave assisted transesterification was determined and compared with that of the conventional transesterification. [Scheme 1]

5

2. Materials and Methods 2.1 Materials Same batch of crude GLY with purity of 70% and 86% was obtained from a local biodiesel plant in Malaysia, while pure GLY, DMC, GC, NaOMe and MeOH were supplied by Sigma Aldrich (USA). The metal oxide catalyst CaO was purchased from Merck (Germany).

2.2 Characterization of Crude GLY The moisture content of crude GLY was determined by Karl Fisher titration following the standard method EN ISO 12937, while the contents of GLY and MeOH were obtained by High Performance Liquid Chromatography (HPLC). Soap content was analysed by standard titration method based on American Oil Chemists Society (AOCS) Cc 17-95 and salt content was determined using the precipitation method, while the ash content was analysed according to the standard method ISO 2098-1972 with sample crude GLY burned in muffle furnace at 750°C for 3 h.

2.3 Microwave-assisted Transesterification Microwave-assisted transesterification was performed in a microwave reactor (Milestones, 1200 W, 2450 MHz) equipped with an infrared temperature sensor, temperature probe, a three-neck round-bottomed flask, electromagnetic stirrer and a condenser. A quantity of 0.125 mol GLY was placed into a 100 ml three-neck reaction flask and mixed with catalyst CaO and DMC in predetermined ratio. The prepared samples were subjected to a maximum microwave power of 500 W and heated to the

6

desired temperature that was measured by two in situ temperature probes at different locations. The experimental design shown in Fig. 1 lists the CaO-catalyzed process using three grades of GLY with different purity levels. Three runs were conducted to examine the reproducibility of the GC production. The sample was taken and the catalyst was removed by filtration. Recovery and reusability of the catalyst are not essential as low cost of CaO and only small amount of CaO was used in the reaction. The sample was then analysed for GC and the unreacted GLY using Waters HPLC apparatus equipped with a PL Aquagel-OH column (Agilent) and a refractive index (RI) detector (Waters 410). The mobile phase used was water and the flow rate was set at 1 mL/min.

[Fig. 1]

2.4 Conventional Transesterification The transesterification reaction using CaO catalyst was carried out in a jacketed reactor connected to a reflux condenser at 650 rpm. A quantity of 0.125 mol GLY was added to the 100 ml jacketed reactor mixed with catalyst and DMC in a fixed ratio and heated to 65°C for 5 h to ensure the completion of the reaction. Samples were first taken at 5th min and every 20 min in the first 1 h followed by every hourly thereafter for 4 h. The taken sample was analysed for GC and unreacted GLY as in the microwave-assisted transesterification. The yields of GC and conversions of GLY were determined using Eq. (1) and Eq. (2), respectively. ,  

 ೔೙೔೟೔ೌ೗

,  

100 %

೔೙೔೟೔ೌ೗  ೝ೐ೞ೔೏ೠೌ೗ ೔೙೔೟೔ೌ೗

(1)

100%

(2) 7

2.5 Process Evaluation of Synthesis Using Crude GLY of Different Purity

The effects of impurities in the crude GLY such as MeOH and NaOMe on the transesterification with DMC were studied. The synthetic crude GLY made up of the pure GLY with either MeOH or NaOMe or both impurities was used to evaluate the individual effects as well as their combined effects on the process performance. Reaction with pure GLY was served as control, Run 1. The synthetic crude GLY mixture was made up using pure GLY to mimic the % composition of MeOH found in the 70% purity crude GLY obtained from the biodiesel plant were represented by Run 2-4. In addition, the crude GLY which served as a control in Run 5 was compared with the crude GLY mixture with added impurities shown in Run 6-8 to investigate probable enhancement in GC yield with added MeOH and NaOMe.

2.6 Energy Consumption Estimation of the energy requirement for the transesterification system in this study was calculated using Eq. (3). The energy consumed in the microwave heating system and conventional heating were calculated based on the actual power applied to the system throughout the course of reaction. Milestones power measurement software (Milestones, Italy) was used to measure the actual power used throughout the microwave assisted reaction. As for the conventional heating reaction, the actual power consumption was measured by power meter (Brennenstuhl, Germany).   ,    ,    !, 

(3)

3. Results and Discussions 8

3.1 Characterization of Pure and Crude GLY Two grades of crude GLY obtained from the biodiesel plant at 70% and 86% purity were characterized and the impurities identified were moisture, MeOH, fatty acid ester (FAE), ash, soap and salt as shown in Table 1. The pH of the two types of crude GLY investigated varied greatly; with the 70% purity crude GLY exhibiting an alkaline medium and the 86% purity possessing acidic pH. It was also observed that the MeOH content in the alkaline crude GLY was much higher. The % composition of other impurities such as FAE and ash was rather similar for the two grades of crude GLY. However, salt was detected in the 86% purity crude GLY which also showed much higher moisture content than the 70% purity crude GLY. On the other hand, soap concentration in the 86% purity crude GLY was much lower than that in the 70% purity crude GLY. This can be explained by the hydrolysis of soap to FA during the acid treatment of the 70% purity crude GLY carried out in the biodiesel processing plant.

[Table 1]

3.2 Evaluation of Process Parameters GLY of different purity levels that had been used in the synthesis of GC via catalytic transesterification processes possess different properties which would impart different effects on the process performance. To ensure the findings obtained are sound and reliable, preliminary study on the evaluation of process parameters within a specific range was performed. The parametric ranges investigated were obtained from the preliminary study on pure glycerol (which is not reported here) and the best GC yield was obtained with reaction conditions of DMC:GLY = 2:1 (i.e. 0.125 mol GLY, 0.25 mol DMC), 6 9

wt% catalyst, reaction temperature of 65°C and a reaction time of 45 min. The conditions were then applied to all three types of glycerol samples according to Fig. 1(i) to determine the suitable grade of GLY with specific purity for GC synthesis.

3.2.1 Effect of GLY with different impurity on GC synthesis The performance of GLY, with varying impurities, on GC production is presented in Fig. 2. The result shows that the best yield and conversion above 75% were achieved by using the basic 70% purity crude GLY. This grade of crude GLY contained around 15 wt% MeOH and residue catalyst (NaOMe); the MeOH present in the GLY exerted a positive effect by resolving the immiscibility between reactants DMC and GLY [30]. Besides, crude GLY of 70% purity contained a greater amount of NaOMe than the 86% purity crude GLY. The latter was obtained as a result of neutralization with acid in the 70% purity crude GLY and the cationic sodium was neutralized to salt during the process. This implies that the higher amount of residue catalyst (NaOMe) in the 70% purity crude GLY prior to the neutralization favoured the GC yield and GLY conversion. Thus, the acidic crude GLY of 86% purity merely gave a 6.15% yield and a conversion of 20% as concurred by the preferred base-catalysed transesterification reaction [7]. Further evidence on the desirable basic reaction can be witnessed from the moderate GLY conversion of 45.16% and 37.79% GC yield obtained from the pure GLY with pH 7. Also, in both the 99% purity and the 86% purity GLY-DMC reaction mixture, immiscibility between reactants was obvious. The mass transfer barrier in the liquid-liquid solid systems likely could have caused the low yield GC and GLY conversion. With the exception of basicity of pH, MeOH and catalyst NaOMe, some other impurities might inhibit the transesterification

10

process. In view of that, 70% purity crude GLY is considered as the most suitable for GC synthesis.

[Fig. 2]

3.2.2 Effect of reaction time on GC synthesis When DMC was added to GLY, two layers were formed as the reactants were immiscible. To overcome this mass transfer resistance, longer reaction time might be required for the reactants to eliminate the inter- and intra-molecular forces between them. In the conventional transesterification reaction using CaO as catalyst, yield increased with time and the optimum reaction time was reported between 30 and 120 min [7, 10, 31]. Nevertheless, the transesterification reaction can be accelerated with microwave heating with shorter reaction time. It can be evidenced from the 45 min microwave transesterification which gave 75% GC yield and 78% GLY conversion compared with the relatively more effective 90.88% GC yield and 84.48% GLY conversion achieved in reduced reaction time of 5 min as shown in Fig. 3. The high efficiency was due to microwave energy that had accelerated the reaction rate via the enhancement of catalyst’s interaction with the reactant [32]. Energy was transferred from microwaves to the reactants through either resonance or relaxation and this created more molecular friction and collisions in the reaction medium and gave rise to intense localized heating. Consequently, the internal rapid heating provided by microwave heating had successfully reduced the reaction time to a modest 5 min compared with 90 min required by conventional heating. The yield decreased with the further increase of microwave reaction time beyond 5 min could 11

be due to the further decomposition of cyclic carbonate and formation of by-product [33]. An additional peak was observed in the chromatogram but it had not been identified in this work. Furthermore, GC yield obtained was higher than conversion. This is because the glycerol was not completely converted in the reaction. Also, the high GC yield could have attributed by the impurities in crude glycerol such as fatty acid esters which react with DMC to produce GC [34-36].

[Fig. 3]

3.2.3 Effect of reaction temperature on GC synthesis Temperature can affect a reaction significantly. In GC synthesis, high yield i.e. 78.31% - 90.88% of GC and GLY conversion between 68.42% and 84.48% were achieved when transesterification reaction was carried out between 45°C and 75°C as shown in Fig. 4. Beyond 65ºC a slight drop in GC yield was likely due to the presence of 15 wt% MeOH in the crude GLY that evaporated above its boiling point at 64.7°C. Evaporation reduced the MeOH concentration in the reaction mixture which in turn hindered the immiscibility between GLY and DMC leading to a lower GC yield. To prevent evaporation of MeOH, it is advisable to carry out transesterification reaction at 65°C.

[Fig. 4]

12

3.2.4 Effect of catalyst loading on GC synthesis The catalyst plays an important role to speed up chemical reaction by lowering the activation energy of the reaction. It was reported that the highest yield was attained in the CaO-catalyzed conventional transesterification reaction with 1.8 wt% [10] and 6.1 wt% [7] catalyst loading in their respective studies. Based on the findings, the range of CaO catalyst applied in this study was set between 0 and 8 wt%. Interestingly, it was observed that CaO catalyst did not enhance the GC yield in the transesterification of crude GLY and there was no set trend in the performance of the catalyst loading as far as the conversion of GLY and GC yields were concerned as shown in Fig. 5. The values fluctuated in a randomised manner. Even without the addition of CaO to the reaction, 81.35% of GC yield could still be achieved. This probably was due to the residue catalyst (NaOMe) remaining in the crude GLY, which catalysed the reaction. At 1 wt% catalyst loading, 93.40% yield of GC was obtained from the transesterification reaction. As shown in Fig. 5, further increase in catalyst loading beyond 1 wt% had caused a decrease in the yield. For example, the GC yield is the lowest while the catalyst loading was 2 wt%. The GC yield fluctuated in a randomised manner was likely caused by the presence of NaOMe in the reaction system as well as particle agglomeration of CaO occurred at high catalyst loading. The reaction catalysed by both strong base catalyst CaO and NaOMe is very effective to give high GC yield and conversion of GLY. This signifies the importance of using crude GLY with positive effect impurities in GC synthesis via microwave irradiation to reduce production cost.

[Fig. 5]

13

3.2.5 Effect of molar ratio of DMC:GLY on GC synthesis Stoichiometrically, equal molar of DMC and GLY is required to produce the theoretical amount of GC as shown in Scheme 1. However, the miscibility of the reactants and chemical equilibrium shifts also play crucial roles in transesterification. Thus, molar ratio of DMC to GLY would affect GC synthesis via transesterification. Since transesterification of GLY is a reversible reaction and an excess amount of DMC can shift the equilibrium towards the formation of GC. The GC yield of greater than 90% was achieved with molar ratio of 2 [10] and 3.5 [7]. Hence, the molar ratio of DMC:GLY investigated in this work ranged from 1 to 4. As shown in Fig. 6, highest GC yield of 93.40% was obtained at DMC:GLY molar ratio of 2 as an additional amount of DMC facilitated better miscibility between the reactants. Further increase in the molar ratio to 3 and 4 decreased the GC yield to 88.26% and 84.73%, respectively. This implies that the excess carbonate with increased DMC to GLY molar ratio might have triggered the reaction of GC further to form other by-product [8]. Conversely, the lowest GC yield and GLY conversion respectively at 77.17% and 57.22% were obtained at DMC:GLY molar ratio of 1 as backward reaction might have occurred.

[Fig. 6]

14

3.2.6

Effect of impurities on GC synthesis The two main impurities namely MeOH and NaOMe in the 70% purity crude GLY were investigated closely in this study. MeOH is proven as the solvent that could enhance the miscibility of GLY and DMC in transesterification of GLY [37]. The fluctuating trend of GC yield influenced by various catalyst loadings as shown in Fig. 5 previously could probably due to the presence of NaOMe in the crude GLY. Microwave-assisted transesterification of the synthetic crude GLY mixtures respectively prepared from 70% purity GLY and pure GLY with added known amount of MeOH and NaOMe were carried out under optimum conditions obtained previously at 65°C, 5 min, DMC:GLY = 2:1 with 1 wt% CaO. The results of the findings are presented in Table 2. Crude GLY and the crude GLY with added impurities (Run 5-8) performed much better than the pure GLY and the synthetic crude GLY mixtures made up of pure GLY and impurities (Run 1-4) as shown in Table 2. When the optimum reaction conditions obtained in transesterification of crude GLY are applied to the reaction involving pure GLY, only 7.62% yield was achieved as shown in Run 1. The yield is much lower when compared with 37.79% which was achieved in 45 min with 6 wt% catalyst loading previously discussed in Section 3.2.1. This could be due to the absence of the positive-effect impurities in pure GLY. It also indicates that the optimum operating condition for crude GLY at shorter reaction time and lower catalyst loading is not applicable to pure GLY. Both MeOH and NaOMe affect GC synthesis positively via microwave-assisted transesterification of either pure or crude GLY to produce higher GC yield. When both impurities are added to the reaction mixture, great improvement in yield was

15

observed as shown in Run 1 to Run 4 in which the yield increased from 7.62% to 30.56% for the combination of pure GLY and the added impurities. The GC yield in Run 4 is comparable to 37.79% that was previously mentioned. This confirms the positive effect of impurities of MeOH and NaOMe as witnessed in cases with no or inadequate impurities which had been compensated by longer reaction time and higher catalyst loading. However, when these impurities are added separately to the reaction mixture, the GC yield increases slightly as shown in Runs 2 and 3. The presence of methanol in the reaction mixture might reverse the reaction backward. However, methanol is a good microwave radiation absorption solvent that enhances the miscibility between glycerol and DMC to yield high GC [37]. Also, the impurities i.e. sodium methylate could have played a catalyst role in the transesterifcation reaction [38, 39]. Thus, greater improvement can be achieved as shown in Run 4 when NaOMe with strong basicity is incorporated with MeOH to facilitate the alkaline condition that is favourable for GC synthesis. On the other hand, greater than 90% GC yield was achieved in the transesterification of crude GLY with or without added impurities. When extra amounts of MeOH and NaOMe were added to crude GLY that already contained the impurities, GC yield showed a slight increase from 93.40 to 96.11% as shown in Runs 5-8. The synthetic crude GLY in Run 4 gave 30.56% of GC yield compared with 93.40% GC yield achieved by 70% purity crude GLY in Run 5. The vast difference in the GC yield implies that other impurities like soap, FA, FAE, ash and water also might have played important roles in GC synthesis.

[Table 2]

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3.3 Comparison of Microwave-assisted Transesterification

Transesterification

and

Conventional

Both conventional heating and microwave-assisted transesterification of crude GLY of 70% purity and pure GLY were carried out under optimum conditions at 65°C, DMC:GLY molar ratio of 2 and 1 wt% CaO. As shown in Fig. 7, microwave-assisted transesterification of crude GLY gave the best performance with 93.40% yield of GC achieved within 5 min at the optimum operating conditions. Crude GLY gave good yield in microwave heating system and also in conventional heating with longer reaction time. For instance, 84.30% of GC yield was achieved in 2 h in conventional heating system. Unlike the crude GLY, in the reaction using pure GLY, the performance was not as good in either the conventional heating or in microwave heating system at the predetermined optimum conditions. In the conventional system, the maximum GC yield was only 10.66% for 40 min reaction duration. Even when the reaction was extended for a further 5 h, no increase in yield was noticed. The same happened to microwave heating as very low yields of 5.35% (Table 3) and 7.66% were obtained at molar ratio of 2 and 5 after 1.5 h and 3 h reaction, respectively. This shows that higher molar ratio of DMC:GLY does not seem to improve the GC yield. Nevertheless, the process performance can be conciliated with an increase in the catalyst loading. By increasing the catalyst loading to 6 wt%, the yield increased 8 fold to 55.31% in a much shorter time of 1 h. Prolonging the reaction time to 3 h did not increase the yield but instead resulted in a decrease in the yield to 29.51%. It is obvious that under mild reaction conditions (65°C, DMC:GLY = 2:1), pure GLY is more suitable to be used in microwave heating system with higher catalyst loading.

17

Under mild reaction conditions, crude GLY gives better yield in the microwave heating system than the conventional system. The remarkable performance of crude GLY could once again be attributed to the presence of MeOH and NaOMe as both impurities provided alkaline conditions and possess good catalytic ability. MeOH enhances the miscibility between GLY and DMC and also plays an important role in speeding up the reaction due to its good adsorption nature in microwave heating [40]. In view of the findings obtained thus far, microwave-assisted transesterification of crude GLY is more efficient than the conventional transesterification of either pure or crude GLY as far as the reaction time and GC yield achieved are concerned. High GC yield can be achieved in conventional transesterification only with longer reaction time as shown in Table 3. The GC yield obtained from the conventional transesterification of 70% purity crude GLY is comparable to those achieved from the conventional transesterification of pure GLY [7, 10] as listed in Table 4. The reaction involving crude GLY has several advantages such as the exemption from energy intensive purification of crude GLY, additional cost of catalyst and milder operating conditions. When comparing the conventional transesterification using crude GLY with that of microwave heating, the latter required shorter reaction time under the same operating conditions to attain higher GC yield.

[Fig. 7] [Table 3]

[Table 4]

18

3.4 Energy Requirements Considering the economic aspects of the reaction process is vital in determining the viability of a process, and the energy requirements for both the conventional and microwave assisted transesterification are estimated. The average power dissipated during the actual use for microwave heating system and conventional heating system were 60 W and 75 W, respectively. 93.40% yield of GC was obtained within 5 min for microwave heating system, whereas 84.30% yield of GC was obtained even with longer reaction time, i.e. 120 min. This indicate that microwave-assisted transesterification needs much lower energy of 18 kJ compared to conventional heating of 540 kJ to achieve a comparable yield of GC. This indicates that the mechanism of conventional heating based on the conversion of electrical energy to thermal energy is less efficient than the microwave dielectric heating. Also, the presence of MeOH in crude GLY increases the dipolar polarization [40] leading to a better interaction with reactants and a reduction in activation energy. Moreover, the OH-group in both MeOH and GLY rendered them as strong microwave absorption media to facilitate localised rotations. The localised superheating initiated subsequently speeded up the reaction. As a consequence, microwave irradiation transesterification of crude GLY has shown encouraging and positive GC yield.

4. Conclusions The GC yield obtained by the 70% purity crude GLY outshone other grades of GLY via the employment of microwave-assisted transesterification. GC yield increases with the values of the operating parameters investigated except for the catalyst loading. The optimum GC yield of 93.40% was achieved in 5 minutes reaction time, 1 wt% CaO, 2:1 19

molar ratio of DMC:GLY and 65°C. Adding MeOH and NaOMe to synthetic crude GLY greatly enhances the GC yield. Without the exertion of positive effect by MeOH and NaOMe, the yield performance in the transesterification reaction can be compromised with longer reaction time and higher catalyst loading. Crude GLY performs better than pure GLY in both microwave and conventional heating with the microwave heating demonstrating greater energy efficiency. Thus, direct utilisation of crude GLY from the biodiesel plant to produce GC via microwave irradiation transesterification is viable and more economical.

Acknowledgement The authors thank University of Malaya for supporting this research under the High Impact Research Grant (HIR) with Project no. UM.C/625/1/HIR/MOHE/ENG/59.

References [1] M. Hájek, F. Skopal, Treatment of glycerol phase formed by biodiesel production, Bioresour. Technol., 101 (2010) 3242-3245. [2] J. Hu, J. Li, Y. Gu, Z. Guan, W. Mo, Y. Ni, T. Li, G. Li, Oxidative carbonylation of glycerol to glycerol carbonate catalyzed by PdCl2(phen)/KI, Appl Catal A-Gen, 386 (2010) 188-193. [3] M. Casiello, A. Monopoli, P. Cotugno, A. Milella, M.M. Dell’Anna, F. Ciminale, A. Nacci, Copper(II) chloride-catalyzed oxidative carbonylation of glycerol to glycerol carbonate, J Mol Catal A-Chem, 381 (2014) 99-106. [4] L. Zheng, S. Xia, Z. Hou, M. Zhang, Z. Hou, Transesterification of glycerol with dimethyl carbonate over Mg-Al hydrotalcites, Chinese J Catal, 35 (2014) 310-318.

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[5] G.V. Waghmare, M.D. Vetal, V.K. Rathod, Ultrasound assisted enzyme catalyzed synthesis of glycerol carbonate from glycerol and dimethyl carbonate, Ultrason. Sonochem., 22 (2015) 311-316. [6] Y.T. Algoufi, B.H. Hameed, Synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate over K-zeolite derived from coal fly ash, Fuel Process Technol, 126 (2014) 5-11. [7] J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, B. Maestro-Madurga, A. PesqueraRodríguez, C. Ramírez-López, L. Lorenzo-Ibarreta, J. Torrecilla-Soria, M.C. VillaránVelasco, Synthesis of glycerol carbonate from glycerol and dimethyl carbonate by transesterification: Catalyst screening and reaction optimization, Appl Catal A-Gen, 366 (2009) 315-324. [8] J.R. Ochoa-Gomez, O. Gomez-Jimenez-Aberasturi, C. Ramirez-Lopez, B. MaestroMadurga, Synthesis of glycerol 1,2-carbonate by transesterification of glycerol with dimethyl carbonate using triethylamine as a facile separable homogeneous catalyst, Green Chem, 14 (2012) 3368-3376. [9] F.S.H. Simanjuntak, V.T. Widyaya, C.S. Kim, B.S. Ahn, Y.J. Kim, H. Lee, Synthesis of glycerol carbonate from glycerol and dimethyl carbonate using magnesium–lanthanum mixed oxide catalyst, Chem Eng Sci, 94 (2013) 265-270. [10] F.S.H. Simanjuntak, T.K. Kim, S.D. Lee, B.S. Ahn, H.S. Kim, H. Lee, CaO-catalyzed synthesis of glycerol carbonate from glycerol and dimethyl carbonate: Isolation and characterization of an active Ca species, Appl Catal A-Gen, 401 (2011) 220-225. [11] M.G. Alvarez, A.M. Segarra, S. Contreras, J.E. Sueiras, F. Medina, F. Figueras, Enhanced use of renewable resources: Transesterification of glycerol catalyzed by hydrotalcite-like compounds, Chem Eng J, 161 (2010) 340-345. [12] J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, C.A. Ramírez-López, J. NietoMestre, B. Maestro-Madurga, M. Belsué, Synthesis of glycerol carbonate from 3-chloro-1,2propanediol and carbon dioxide using triethylamine as both solvent and CO2 fixation– activation agent, Chem Eng J, 175 (2011) 505-511.

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[13] N. Lertlukkanasuk, S. Phiyanalinmat, W. Kiatkittipong, A. Arpornwichanop, F. Aiouache, S. Assabumrungrat, Reactive distillation for synthesis of glycerol carbonate via glycerolysis of urea, Chem Eng Process Process Intensif, 70 (2013) 103-109. [14] D.-W. Kim, K.-A. Park, M.-J. Kim, D.-H. Kang, J.-G. Yang, D.-W. Park, Synthesis of glycerol carbonate from urea and glycerol using polymer-supported metal containing ionic liquid catalysts, Appl Catal A-Gen, 473 (2014) 31-40. [15] C. Hammond, J.A. Lopez-Sanchez, M.H. Ab Rahim, N. Dimitratos, R.L. Jenkins, A.F. Carley, Q. He, C.J. Kiely, D.W. Knight, G.J. Hutchings, Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts, Dalton Trans, 40 (2011) 3927-3937. [16] W.K. Teng, G.C. Ngoh, R. Yusoff, M.K. Aroua, A review on the performance of glycerol carbonate production via catalytic transesterification: Effects of influencing parameters, Energy Convers Manage, 88 (2014) 484-497. [17] P. Lu, H. Wang, K. Hu, Synthesis of glycerol carbonate from glycerol and dimethyl carbonate over the extruded CaO-based catalyst, Chem Eng J, 228 (2013) 147-154. [18] P.-L. Boey, G.P. Maniam, S.A. Hamid, Performance of calcium oxide as a heterogeneous catalyst in biodiesel production: A review, Chem Eng J, 168 (2011) 15-22. [19] S. Hu, Y. Li, Polyols and polyurethane foams from base-catalyzed liquefaction of lignocellulosic biomass by crude glycerol: Effects of crude glycerol impurities, Industrial Crops and Products, 57 (2014) 188-194. [20] J. Xu, X. Zhao, W. Wang, W. Du, D. Liu, Microbial conversion of biodiesel byproduct glycerol to triacylglycerols by oleaginous yeast Rhodosporidium toruloides and the individual effect of some impurities on lipid production, Biochem. Eng. J., 65 (2012) 30-36. [21] S.J. Sarma, G.S. Dhillon, S.K. Brar, Y. Le Bihan, G. Buelna, M. Verma, Investigation of the effect of different crude glycerol components on hydrogen production by Enterobacter aerogenes NRRL B-407, Renew Energ, 60 (2013) 566-571. [22] C.X.A. da Silva, C.J.A. Mota, The influence of impurities on the acid-catalyzed reaction of glycerol with acetone, Biomass Bioenergy, 35 (2011) 3547-3551.

22

[23] P.D. Patil, V.G. Gude, L.M. Camacho, S. Deng, Microwave-Assisted Catalytic Transesterification of Camelina Sativa Oil, Energy Fuels, 24 (2009) 1298-1304. [24] R.R. Pawar, S.V. Jadhav, H.C. Bajaj, Microwave-assisted rapid valorization of glycerol towards acetals and ketals, Chem Eng J, 235 (2014) 61-66. [25] G.S.J. Sturm, M.D. Verweij, A.I. Stankiewicz, G.D. Stefanidis, Microwaves and microreactors: Design challenges and remedies, Chem Eng J, 243 (2014) 147-158. [26] C.-H. Chan, R. Yusoff, G.-C. Ngoh, Assessment of Scale-Up Parameters of MicrowaveAssisted Extraction via the Extraction of Flavonoids from Cocoa Leaves, Chemical Engineering & Technology, 38 (2015) 489-496. [27] A. Diaz-Ortiz, A. de la Hoz, J. Alcázar, J.R. Carrillo, M.A. Herrero, J. de M. Muñoz, P. Prieto, A. de Cózar, Reproducibility and scalability of microwave-assisted reactions, in: Microwave Heating, InTech Europe, 2011, pp. 138-161. [28] J. Liu, R. Takada, S. Karita, T. Watanabe, Y. Honda, T. Watanabe, Microwave-assisted pretreatment of recalcitrant softwood in aqueous glycerol, Bioresour. Technol., 101 (2010) 9355-9360. [29] P. Bookong, S. Ruchirawat, S. Boonyarattanakalin, Optimization of microwave-assisted etherification of glycerol to polyglycerols by sodium carbonate as catalyst, Chem Eng J, 275 (2015) 253-261. [30] J. Esteban, M. Ladero, L. Molinero, F. García-Ochoa, Liquid–liquid equilibria for the ternary systems DMC–methanol–glycerol, DMC–glycerol carbonate–glycerol and the quaternary system DMC–methanol–glycerol carbonate–glycerol at catalytic reacting temperatures, Chem. Eng. Res. Des., 92 (2014) 2797-2805. [31] J. Li, T. Wang, On the deactivation of alkali solid catalysts for the synthesis of glycerol carbonate from glycerol and dimethyl carbonate, Reac Kinet Mech Cat, 102 (2011) 113-126. [32] A. Islam, Y.H. Taufiq-Yap, E.-S. Chan, M. Moniruzzaman, S. Islam, M.N. Nabi, Advances in solid-catalytic and non-catalytic technologies for biodiesel production, Energy Convers Manage, 88 (2014) 1200-1218.

23

[33] M.J. Climent, A. Corma, P. De Frutos, S. Iborra, M. Noy, A. Velty, P. Concepción, Chemicals from biomass: Synthesis of glycerol carbonate by transesterification and carbonylation with urea with hydrotalcite catalysts. The role of acid–base pairs, J Catal, 269 (2010) 140-149. [34] J.Y. Min, E.Y. Lee, Lipase-catalyzed simultaneous biosynthesis of biodiesel and glycerol carbonate from corn oil in dimethyl carbonate, Biotechnol. Lett., 33 (2011) 17891796. [35] Y.M. Kurle, M.R. Islam, T.J. Benson, Process development and simulation of glycerolfree biofuel from canola oil and dimethyl carbonate, Fuel Process Technol, 114 (2013) 49-57. [36] L. Zhang, B. Sheng, Z. Xin, Q. Liu, S. Sun, Kinetics of transesterification of palm oil and dimethyl carbonate for biodiesel production at the catalysis of heterogeneous base catalyst, Bioresour. Technol., 101 (2010) 8144-8150. [37] S. Pan, L. Zheng, R. Nie, S. Xia, P. Chen, Z. Hou, Transesterification of Glycerol with Dimethyl Carbonate to Glycerol Carbonate over Na–based Zeolites, Chinese J Catal, 33 (2012) 1772-1777. [38] U. Rashid, F. Anwar, R. Yunus, A.H. Al-Muhtaseb, Transesterification for Biodiesel Production Using Thespesia Populnea Seed Oil: An Optimization Study, International Journal of Green Energy, 12 (2013) 479-484. [39] M.L. Pisarello, C.A. Querini, Catalyst consumption during one and two steps transesterification of crude soybean oils, Chem Eng J, 234 (2013) 276-283. [40] V. Gude, P. Patil, E. Martinez-Guerra, S. Deng, N. Nirmalakhandan, Microwave energy potential for biodiesel production, Sustain Chem Process, 1 (2013) 5-36.

List of Scheme Scheme 1. Microwave-irradiated transesterification of GLY with DMC

24

Lisst of o Figu F uress Figg. 1: Fllow w chhartt off expperrimeentaal desi d ign of CaaO-ccataalyzzed d traanseesteerifficaationn off G GLY Y Figg. 2: Peerfoorm mance of o diff d fereent ggraadess off GL LY in GC C syynthhesiis via v mic m crow wavve assi a isteed trranssestterificaatioon Figg. 3: In nfluuencce of o reac r ctioon tiimee onn gllyceeroll caarboonaate ssyn ntheesis viaa micro m owaavee assistted trranssestterificaatioon nfluuencce of o reac r ctioon teempperatuure on o G GC C syynthhesiis via mic m crow wavve Figg. 4: In asssisstedd traansesteerifficaation Figg. 5: In nfluuencce of o CaO C O caatallystt loaadinng on GC C syynthhesiis via v mic m crow wavve asssisstedd traansesteerifficaation Figg. 6: In nfluuencce of o mol m lar rati r io of o D DMC C:G GLY Y on o GC G synntheesiss viia micr m row wavee asssissted d traanseesteerifficaationn M row wavve assissted d annd con c nvenntio onall traanssesteerifficaation with w h cruudee GLY G Y annd Figg. 7: Micr pu uree GL LY.

S hem Sch me 1. Mic M crow wavve-iirraddiatedd traanseesteerifi ficattionn off GL LY Y wiith DM MC 25 2

Fig. 1 Flow chart of experimental design of CaO-catalyzed transesterification of GLY

26

90

Yield or Conversion (%)

80 70 60

Yield

Reaction Temp: 65 °C Reaction Time: 45 min Catalyst loading: 6 wt% Catalyst : CaO DMC: Gly: 2:1

Conversion

50 40 30 20 10 0

99

86

70

Purity of Glycerol (%)

Fig. 2: Performance of different grades of GLY in GC synthesis via microwave assisted transesterification

27

100

95

Reaction Temp: 65 °C Catalyst loading: 6 wt% Catalyst : CaO DMC: Gly: 2:1 Gly : Crude gly (70% purity)

Yield

Conversion

Yield or Conversion %

90

85

80

75

70

65

60 3

5

15

30

45

Reaction time (min)

Fig. 3: Influence of reaction time on GC synthesis via microwave assisted transesterification

28

Yield or Conversion %

95

Reaction Time: 5 min Catalyst loading: 6 wt% Catalyst : CaO DMC: Gly: 2:1 Gly : Crude gly (70% purity)

Yield

Conversion

85

75

65

55

45 45

55

65

75

Reaction temperature (°C) Fig. 4: Influence of reaction temperature on GC synthesis via microwave assisted transesterification

29

100

Yield or Conversion (%)

95

Reaction Time: 5 min Reaction Temp: 65 °C Catalyst : CaO DMC: Gly: 2:1 Gly : Crude gly (70% purity)

Yield

Conversion

90 85 80 75 70 65 60 0

0.5

1

2

4

6

8

Catalyst loading (wt%)

Fig. 5: Influence of CaO catalyst loading on GC synthesis via microwave assisted transesterification

30

100

Yield

Conversion

Yield or Conversion %

90

Reaction Time: 5 min Reaction Temp: 65 °C Catalyst loading: 1 wt% Catalyst : CaO Gly : Crude gly (70% purity)

80

70

60

50

40 1:1

2:1

3:1

4:1

Molar ratio of DMC: Glycerol

Fig. 6: Influence of molar ratio of DMC:GLY on GC synthesis via microwave assisted transesterification

31

100 90 80

Yield (%)

70 60 50 40 30 20 10 0 0

50

100

150

200

250

300

Reaction time (min)

Microwave -Crude gly -1wt% CaO Microwave -Pure gly - 6wt% CaO Microwave -Crude gly -6wt% CaO

Conventional- Crude gly -1wt% CaO Conventional- Pure gly -1wt% CaO Microwave -Pure gly -1wt% CaO (DMC:gly=5:1)

Fig. 7: Microwave assisted and conventional transesterification with crude GLY and pure GLY.

32

List of Tables Table 1: Characteristics of different grade of GLY Table 2: Effect of MeOH and NaOMe addition on the GC yield Table 3: The comparison of best GC yield obtained in microwave assisted transesterification and conventional transesterification of 70% purity crude GLY and pure GLY Table 4: Comparison of microwave irradiation with conventional heating method using CaO as catalyst in GC synthesis

33

Table 1: Characteristics of different grade of GLY Properties

Unita

Crude

GLY Crude GLY with Pure GLY with

with purity 70% purity 86 % Appearance

-

Brownish viscous Brownish

purity 99 %

viscous Colorless,

liquid

liquid

viscous liquid

Moisture

wt/wt %

0.34

8.22

1.0

GLY Assay

wt/wt %

69.9

86.21

99.0

MeOH

wt/wt %

15

0.14

-

12.69

5.85

7

30,030

6,659

-

1.50

-

pH

-

Soap

ppm

FAE

mL

0.5N 1.95

clear

NaOH/50g Ash content

wt/wt %

2.02

2.50

-

M.O.N.G b

wt/wt %

27.74

2.08

-

Salt

wt/wt %

-

3.45

-

a

Unit of wt/wt% is presented based on total GLY solution

b

. . . ,          100         %

34

Table 2: Effect of MeOH and NaOMe addition on the GC yield Run

GLY + Impurities (wt%)a

Yield (%)

1

Pure GLY (Control)

7.62

2

Pure GLY + 20 wt% MeOH

8.38

3

Pure GLY + 1 wt% NaOMeb

7.34

4

Pure GLY + 20 wt% MeOH + 1 wt% NaOMeb

30.56

5

Crude GLY (Control)

93.40

6

Crude GLY + 5 wt% MeOH

93.83

7

Crude GLY + 1 wt% NaOMeb

94.35

8

Crude GLY + 5 wt% MeOH + 1 wt% NaOMeb

96.11

a

wt% of impurities is described based on total GLY content

b

1wt% NaOMe was used as the exact quantity cannot be accurately detected from the crude GLY.

35

Table 3: The comparison of best GC yield obtained in microwave assisted transesterification and conventional transesterification of 70% purity crude GLY and pure GLY Method

Grade

CaO

Reaction

Best

yield

of GLY

(wt%)

Time

obtained (%)

of

GC,

(min) Microwave irradiation Crude

1

5

93.40

Microwave irradiation Pure

1

90

5.35

Conventional

Crude

1

120

84.30

Conventional

Pure

1

40

10.66

36

Table 4: Comparison of microwave irradiation with conventional heating method using CaO as catalyst in GC synthesis Heating Methods

Microwave heating

References. GLY purity Catalyst used Pretreatment catalyst

Conventional heating

This study 70% purity CaO of Untreated

Heating method Reaction time (min) Temperature (°C) Pressure DMC:GLY Molar ratio Catalyst loadinga (wt %)

70% purity CaO Untreated

Microwave 5

Water bath 120

Pure CaO Uncalcined, dried at 110°C, overnight Water bath 90

65 atmosphere 2:1

65 atmosphere 2:1

75 atmosphere 5:1

75 atmosphere 5:1

1

1

10 mol % (= 6.1 wt %)

10 mol % 6 mol % (= 6.1 wt (=3.66 wt %) %) 91.1 95.3

Yield of GC, 93.4 84.3 Y (%)    a     %   100%

Simanjuntak et. al.[10]

Ochoa Gomez et. al.[7]

64.1

Pure CaO Calcined at 900°C, overnight Water bath 90

Pure CaO Uncalcined, dried at 110°C, overnight Autoclave 90

Pure CaO Calcined 900°C, 3h

Pure CaO Untreated

Water bath 30

Water bath 30

95 6 bar 3.5:1

75 atmosphere 2:1

75 atmosphere 2:1

3 mol% (= 1.8 wt %)

3 mol% (= 1.8 wt %)

94

90.2

   

37

Highlights •

This is the first report on microwave-assisted conversion of industrial grade crude glycerol to GC.

•

Impurities of crude glycerol demonstrated positive effect on GC synthesis.

•

More energy efficient in microwave irradiation transesterification compared to conventional transesterification.

38