A Review on the Performance of Glycerol Carbonate Production via Catalytic Transesterification...

7/24/2019 A Review on the Performance of Glycerol Carbonate Production via Catalytic Transesterification Effects of Influencing Parameters

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A review on the performance of glycerol carbonate production via

catalytic transesterification: Effects of influencing parameters

Wai Keng Teng, Gek Cheng Ngoh , Rozita Yusoff, Mohamed Kheireddine Aroua

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:

Received 6 June 2014Accepted 15 August 2014Available online 18 September 2014

Keywords:

GlycerolCrude glycerolBiodieselGlycerol carbonateCatalytic transesterificationHydrotalcite

a b s t r a c t

Driven by high energy demand and environmental concerns, biodiesel as a substitute for fossil fuels isrecognized to be promising renewable and clean energy. The increase in the biodiesel plant dramaticallyleads to the oversupply of its by-product glycerol in the biodiesel industries. Developing new industrialuses for glycerol is essential to increase the net energy and sustainability of biodiesel. Moreover, glycerolhas great potential to be converted into marketable and valuable chemicals. The conversion of glycerol toglycerol carbonate (GC) has been extensively studied and transesterification of glycerol to GC has beenproven to be the most promising route. Aimed to reveal the underlying mechanism of this successful con-version path, this paper reviews the chemo- and biocatalytic transesterification of glycerol with differentcarbonates sources. Also, a detail elucidation of the influence of the catalysts and operating conditions onthe GC yield is included to provide an insight into the process. In addition, the future direction of glycerolcarbonate production via catalytic transesterification is provided in this review.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel industry is booming attributing to the threat frompetroleum depletion since the last decade[1,2]. Glycerol is gener-ated at the proportion of 10% (w/w) of the total biodiesel produc-tion[3] and the rapid growing industry has led to a surplus ofglycerol. For instance, the forecasted world biodiesel supply by2016 will be reaching 37 billion gallons with 4 billion gallons ofcrude glycerol production[3]. The abundance of crude glycerol isexpected to exert a great impact on the refined glycerol market.As a consequence, the price of glycerol has been dropped dramat-ically since 2006[4]. In late 2013, the price of refined glycerol wasaround 900965 US$/ton depending on the biodiesel feedstockwhile crude glycerol with purity 80% obtained directly from bio-

diesel plant had been reported at lower value of 240 US$/ton inmid 2014[5]. This has prompted the conversion of the low costglycerol to value-added products.

Inevitably, the paradigm shift has attracted much attention ofresearchers to explore the possibilities of converting glycerol tovalue-added chemicals such as fuel, chemical intermediate andchemicals. The attempts made include the transformation of glyc-erol to 1,3-propanediol, epichlorohydrin, acrolein, fuel additive,

glycerol carbonate (GC) and glycidol [6]. One of the most cele-brated products reported in the last 5 years is GC. It is a highvalue-added product with market price greater than 8141 US$/ton[7]. Due to the high cost, it is still not widely used in commer-cial application[8,9]. A limited usage of only a few kt per year wasreported[7].On the other hand, GC can be produced from biogenicglycerol[10]and has great potential to be used as substitution forpetro-derivative compounds[11]. The potential industrial uses ofGC are presented inFig. 1[8,1114].

GC can be synthesized from different routes by using glycerol asalcohol OH-source and chemicals such as CO/O2, organic carbonate,urea or carbon dioxide as carbonate source [8,12]. Among theroutes for GC synthesis, transesterification of glycerol withdimethyl carbonate (DMC) is one of the most direct and industrial

feasible pathways to produce high GC yield [12]. This synthesisroute will be systematically discussed.

2. Transesterification of glycerol

Transesterification is the carbonate exchange reaction betweenalcohols and carbonate sources[8]. Glycerol, also known as glyc-erin, glycerin, or 1,2,3-propanetriol is the simplest trihedric alco-hol. It is produced conventionally through saponification, fattyalcohol plant and hydrolysis[15]. Utilization of glycerol to synthe-size GC is possible either through direct or indirect synthetic routeas summarized in Fig. 2 [8,12]. Among the routes, the indirect

http://dx.doi.org/10.1016/j.enconman.2014.08.036

0196-8904/2014 Elsevier Ltd. All rights reserved.

Corresponding author. Tel.: +60 3 79675301; fax: +60 3 79675371.E-mail addresses:[email protected](W.K. Teng),[email protected](G.C.

Ngoh),[email protected](R. Yusoff),[email protected](M.K. Aroua).

Energy Conversion and Management 88 (2014) 484497

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route which involved phosgenation and transesterification of glyc-erol with CO2 derivatives such as alkylene carbonate and dialkylcarbonate have favored high yield of GC. The carbon atom of thecarbonate group is nucleophilic which can be attacked by the oxy-gen atom of the glycerols hydroxyl group[8].

The earliest GC production method reported[16]used carbonmonoxide or phosgene with metallic catalysts. The process is haz-ardous due to toxicity of phosgene. A safer method involves amuch lower reaction temperature is the transesterification of glyc-erol with alkylene carbonate. Currently, GC is produced by chemi-

cal companies such as Huntsman from propylene carbonate in the

presence of catalyst at 100150 C and 35 mmHg[17]. The moststudied route for the synthesis of GC is via the environmentallybenign transesterification of glycerol with dialkyl carbonate. Thiscarbonate source can be prepared from methanol and urea and iswidely applied in the production of GC[12].

Catalyst plays a crucial role in transesterification of glycerol toGC. A wide spectrum of catalysts with different property combina-tion i.e. alkaline or acid, homogeneous or heterogeneous can beemployed. Table 1summarizes the reaction conditions for varioustypes of catalysts and their advantages and limitations are

presented in Table 2 to exhibit the influence of catalyst on GC

Fig. 1. The potential uses of glycerol carbonate in various industries [8,1114].

Fig. 2. Various glycerol carbonate synthesis routes [8,12].

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Table 1

Comparison of reaction conditions and performance of various catalysts in glycerol transesterification.

Type of catalysts Reaction conditions

Temperature (C) Molar ratio (carbonate:glycerol)

Reaction time(h)

Catalyst loadinga (wt ormol%)

Sol

Homogeneous base catalyst

K2CO3 7375 3:1 (DMC) 3 4.5 wt% K2CO3 7176 3:1 (DMC) 5 4.5 wt% K2CO3 75 5:1 (DMC) 1.5 15 wt% K2CO3 7375 10:1 (DMC) 48 4.5 wt% K2CO3 7375 3:1 (DEC) 4 3 mol% KOH 75 5:1 (DMC) 1.5 6 wt% NaOH 75 5:1 (DMC) 1.5 4 wt% Triethylamine 6888 4:1 (DMC) 2.5 10 mol% N-heterocyclic carbenes Room

temperature3.5:1 (DMC) 0.33 2.6 mol%

1,3-Dichlorodistannoxanes 100 5:1 (DEC) 2 0.5 mol% Ionic liquidtetra-n-butylammonium 120 2:1 (DMC) 6 3.3 mol% Ionic liquid (BMIM-2-CO2) 74 3.2:1 (DMC) 1.33 1 mol% Ionic liquid (BMIM-2-CO2) 74 3.2:1 (DMC)d 5 5 mol% Ionic liquid [Mor1,4][N(CN)2] 120 3:1 (DMC) 13 17 mol% Ionic liquid ([TMA][OH]) 80 3:1 (DMC) 1.5 1 mol% Calcium complexCa(C3H7O3)(OCO2CH3) 75 2:1 (DMC) 0.5 0.01 wt% Calcium complexCa(C3H7O3)(OCO2CH3) 75 2:1 (DMC) 0.5 0.01 wt%

Homogeneous acid catalyst

H2SO4 75 5:1 (DMC) 1.5 10 mol% p-Toluenesulfonic acid 75 5:1 (DMC) 1.5 10 mol%

Heterogeneous base catalyst

CaOf 75 5:1 (DMC) 1.5 6 wt% CaO 60 1:1 (DMC) 2 2 mol% CaO 75 2:1 (DMC) 0.5 3 mol% CaOf 75 2:1 (DMC) 0.5 3 mol% CaO 35 2:1 (EC) 0.25 0.5 wt% CaO 35 2:1 (EC) 1 0.5 wt% Calcium complex Ca(C3H7O3)2 60 2.5:1 (DMC) 3 8 mol% CaCO3

f 75 5:1 (DMC) 1.5 10 mol% Na2O 75 2:1 (DMC) 0.5 3 mol%

ZnO 75 2:1 (DMC) 0.5 3 mol% MgOf 75 2:1 (DMC) 3 3 mol% MgO 50 2:1 (EC) 5 7 wt% n-Bu2Sn(OMe)2 180 1:1 (DMC) 15 6 mol% Mg/La mixed oxides 85 2:1 (DMC) 1 5 wt% Mg1 + xCa1 xO2mixed oxides 70 2:1 (DMC) 1.5 3 wt% Mg/Zr/Sr mixed oxides 90 5:1 (DMC) 1.5 15 wt% Mg/Al/Zr mixed oxides 75 5:1 (DMC) 1.5 0.1 wt%

Al/Mg hydrotalcitef

50 2:1 (EC) 5 7 wt% Al/Mg hydrotalcitef (rehydrated) 50 2:1 (EC) 5 7 wt% Al/Mg hydrotalcitef 35 2:1 (EC) 1 0.5 wt% Al/Li hydrotalcitef 35 2:1 (EC) 1 0.5 wt% Al/Ca hydrotalcitef 35 2:1 (EC) 1 0.5 wt% Mg/Al hydrotalcite 100 5:1 (DMC) 1 54 wt% DMMg/Al hydrotalcite 100 5:1 (DMC) 2 54 wt% DMMg/Al hydrotalcite 100 5:1 (DMC) 3 54 wt% Mg/Al hydrotalcite 100 5:1 (DMC) 9g 11 wt% Mg/Al hydrotalcite-hydromagnesium 0.1 g 100 5:1 (DMC) 1.16 54 wt% DMMg/Al hydrotalcite-hydromagnesium 0.5 g 100 5:1 (DMC) 9g 27 wt% DMMg/Al hydrotalcitef 100 3:1 (DMC) 2 10 wt%


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Mg/Al hydrotalcitef doped with nickel 100 3:1 (DMC) 2 10 wt% Mg/Al hydrotalcite (rehydrated) 130 17:1 (DEC) 10 16 wt% Mg/Al hydrotalcite supported ona-Al2O3

f 130 21:1 (DEC) 8 2.16 g/g (continuoussystem)

DM

Mg/Al hydrotalcite supported on carbon nanofiberf 130 17:1 (DEC) 2 16 wt% Mg/Al hydrotalcite 140 5:1 (DEC) 3 54 wt% DMZeolite (NaY)f 70 3:1 (DMC) 4 10 wt% MeKF/hydroxyapatite 78 2:1 (DMC) 0.83 3 wt% NaOH/c-Al2O3 78 2:1 (DMC) 1 3 wt% K2CO3/MgO 80 2.5:1 (DMC) 2 1 wt% Extruded CaO-based/Al2O3 80 3:1 (DMC) 5 15 mol% Ionic liquids (Tri-n-butylamine immobilized on mesoporous

MCM41)

80 2:1 (EC) 1.5 9 wt%

Heterogeneous acid catalyst

Amberlyst 131wet 75 5:1 (DMC) 1.5 10 mol% Amberlyst 39wet 75 5:1 (DMC) 1.5 10 mol%

Enzymatic catalyst

C.A. lipase B immobilized on Novozym 435 60 1:1 (DMC) 30 54 wt% THC.A. lipase B immobilized on Novozym 435 70 10:1 (DMC) 48 5 wt% C.A. lipase B immobilized on Novozym 435 60 2:1 (DMC) 48 75 g/L AceC.A. lipase B immobilized on Novozym 435 50 3:1 (DMC) 12 5 wt% t-bC.A. lipase B immobilized on Novozym 435 60 1.5:1 (DMC) 14 54 wt% t-b

A.N. lipase immobilized on magnetic nano particles 60 10:1 (DMC) 6 28 wt%

A.N. lipase (free enzyme) 60 10:1 (DMC) 4 12 wt%

A.N. lipase (cross-linked enzyme aggregates on magneticparticles)

60 10:1 (DMC) 6 28.6 wt%

A.N. lipase immobilized on magnetic nano particles 60 10:1 (DMC)

i

6 5 wt%

A.N. lipase immobilized on magnetic nano particles 60 10:1 (DMC) 6 5 wt%

A.N. lipase immobilized on magnetic nano particles 60 10:1 (DMC)j 6 5 wt%

Note: Y= yield of glycerol carbonate, C= conversion of glycerol, S= selectivity of glycerol carbonate, DMC = dimethyl carbonate, DEC = diethyl carbonate, EC = ethylene carbonata (Amount of catalyst/amount of glycerol) 100%.b Y= (g glycerol carbonate produced/g glycerolinitial) 100%,C= (glycerolinitialglycerolresidual)/glycerolinitial 100%,S=Y/C.c By-product diglycerol tricarbonate was formed.d Crude glycerol obtained from industrial biodiesel plant was used.e By-product glycidol was formed.f Calcination of catalyst at 450 C 6 T6 900 C for 3 h to overnight.g Reaction was scaled-up.h Yield is calculated fromY=C*S.i Crude glycerol obtained from transesterification of residual sun-flower oil was used.j Crude glycerol obtained from transesterification of crude sun-flower oil was used.


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synthesis process. It has been reported that the acidic catalysts inboth homogeneous and heterogeneous forms are not suitable forGC synthesis as their catalytic activities are extremely low [12].GC yields achieved by acidic catalysts were less than 5% even with

the employment of strong acid such as sulfuric and p-Toluenesul-fonic acids or Amberlyst ion exchange resins in the catalytic reac-tion. Acidic heterogeneous catalysts give poor reactionperformance due to the mass transport limitation. This could beexplained by the hydrophobic surface of the resins that hinderedthe diffusion of hydrophilic reactants like DMC towards the cata-lytic sites inside the resins. On the other hand, acidic homogeneouscatalysts do not encounter the diffusion problem and they canachieve a much improved GC yield of 50% after 24 h. The rate-con-trolling step which cyclizes the methyl glyceryl carbonate interme-diate to GC has led to a slow GC formation rate [12]. Unlike theacidic catalysts, both the homogeneous and heterogeneous strongbasic catalysts successfully produce more than 90% GC yield.

Apparently, the key criteria to ensure a viable transesterifica-

tion reaction of glycerol lies in the efficiency of the catalystsapplied. A vast range of base catalysts of either homogeneous orheterogeneous form has proven to be effective in GC synthesisdespite individually possessing certain merits and demerits. Thefollowing sections discuss the performance of various catalysts intransesterification of glycerol in terms of product yield, catalystreusability and etc.

2.1. Homogeneous base catalyzed transesterification

Transesterification of glycerol had been successfully carried outusing homogeneous basic catalysts such as Na or K hydroxides,carbonates or alkoxides with high catalytic activity. Similar tothe transesterification of oil, strong bases such as KOH, NaOH,

K2CO3 and ionic liquids were employed in the transesterificationof glycerol[1821,2528].

In transesterification of glycerol, the reaction equilibrium canbe shifted towards the product either by removing the by-productmethanol continuously or by using excess DMC in the reaction. Theformer is not recommended as methanol forms a minimum boiling

azeotrop with DMC at a weight ratio of 30:70 for DMC: methanolcomposition. Moreover, DMC could also be removed together withthe methanol[21,60]. Alternatively, methanol could be removed byadding molecular sieves [50,51,54] at agitation speed below600 rpm to avoid breaking of molecular sieves and concurrentlyto create extra separation between catalyst and molecular sieves.The other method which can shift the reaction equilibrium wasdeployed by Ochoa et al. [21]. The researchers proved that whenexcess DMC used at DMC/glycerol molar ratio of 5 in the reaction,nearly 100% of glycerol was converted to yield 100% of GC inshorter reaction time and lower catalyst loading of KOH, NaOHand K2CO3. A pioneering work on using K2CO3 reported a 97% ofGC under mild reaction conditions [1820]. Another findings onhigh glycerol conversions using NaOH and K2CO3 [44] reported

the formation of large amounts of byproduct glycidol, which hadunfavorably affected the GC yield.

Generally, the base catalyzed transesterification gave remark-able GC yield. However, the emergence of green solvent such asionic liquid with unique properties had gradually gaining its versa-tile application as catalyst and solvent in process synthesis[61]. Asshown in Table 1, ILs such as imidazolium-based ILs [26,27],ammonium-based ILs[25,28]and dicyanamide-based ILs[27]havebeen applied in the transesterification of glycerol. Interesting tonote that 1-n-butyl-3-methylimidazolium-2-carboxylate (BMIM-2-CO2) achieved 100% GC yield within 80 min [26] which hasoutshone its base counterpart K2CO3 as far as reaction time isconcerned [21]. Furthermore, the chain length of ILs with thepresence of ion halide, hydroxide and bicarbonate have altered

the catalytic activity of ILs [26,28]. The glycerol conversionhad increased from 30% to 100% when ionic liquid was tested on

Table 2

Merits and demerits of various types of catalyst used in transesterification of glycerol.

Types of catalyst Merits Demerits

Homogeneousbase catalyst

K2CO3 High catalytic activity Catalyst difficult to be separ ated from product

KOHNaOHIonic liquid

Heterogeneousbase catalyst

CaO Simple separ ation method of catalyst fromproduct

Sensitive to water, CaO can deactivate in presence of water

Mixed metal oxides Catalyst can be easily recovered and recycled Leaching of catalyst active sites could be happened, lead to productcontamination

Hydrotalcite Catalyst are economical Energy intensive process such as calcination of catalyst required toachieve high yield

Zeolite High possibility to create in-house unique catalystby varying the composition

Catalyst with support

Homogeneousacid catalyst

H2SO4 Catalyst are widely available and economical Very low catalytic activity

p-Toluenesulfonicacid

No diffusion problem in reaction mixture Catalyst difficult to be separated from product

Corrosion on reactor and pipelines could be happened

Heterogeneousacid catalyst

Amberlyst ionexchange resins

Catalyst can be regenerated and reused Extremely low catalytic activity due to mass transport limitation

Cause corrosion on reactor and pipelinesEnzyme Candida Antarctica

(Novozym 435)Mild reaction temperature is sufficient to carryout the reaction

Very slow reaction rate, thus energy intensive

Aspergillus Niger Simple purification step of enzyme from product High costEnzyme can be reused Enzyme can be easily deactivated methanol inhibited the enzyme

performance[59]Additional chemicals like molecular sieves, detergent or solvent areneeded to obtain high yield[5052,54]

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1-n-dodecyl-imidazoles to 1-n-butyl-imidazoles. The catalytic per-formance of 1-n-butylimidazole was better than higher alkyl deriv-atives as it promotes the 2-carboxylate formation with DMC[26]. Ithas been reported that ionic liquids having hydroxide and bicar-bonate counter ions are effective to convert 7795% of glycerol at80 C, 90 min reaction time and molar ratio (DMC: glycerol) of 3[28]as presented inTable 1.

It is economically viable for ILs to be reused. Inspite of dicyan-amide-based ILs i.e. N-methyl-N-butylmorpholinium dicyanamide[Mor1,4][N(CN)2] showed good recyclability without any signifi-cant reduction in the conversion yield after 4 recycles [27], thereaction conditions undermine the feasibility as higher reactiontemperature and longer duration are required moreover, it is diffi-cult to separate the product from ILs.

Though basic homogeneous catalyst has high catalytic activity,problem associated with the separation of the dissolved catalystfrom the product is undesirable. Unlike biodiesel reaction, watercould not be used to wash the dissolved catalyst off the productin the reaction as GC and water are miscible. As a result, this mightincur additional separation cost. For instance, cation-exchangingresin such as Amberlit IR 120 that was used to remove K2CO3fromthe reaction mixtures[18]. In view of that, heterogenenous catalystwith high activity that can ease the separation process should beconsidered.

2.2. Heterogeneous base catalyzed transesterification

Many solid base catalytic systems have been investigated forthe synthesis of GC via transesterification of glycerol. Catalystssuch as alkaline earth metal oxides, basic zeolites, mixed metaloxides derived from hydrotalcites are suitable candidates. Thetop most sought after catalysts are alkaline earth metal oxides likeCaO. These catalysts have relatively strong basicity and are avail-able from cheap sources such as calcium carbonate and calciumhydroxide[62]. GC synthesis via CaO catalyzed transesterificationhad been reported by several groups of researchers [21,29

31,60]. The high catalytic activity of CaO was attributed to theeffect of its calcination at high temperature [21]. Better perfor-mance of calcined CaO than uncalcined CaO had also been provenby Simanjuntak et al.[29]. The small deviation in the performanceof uncalcined catalyst was simply caused by the impurities such asCa(OH)2and CaCO3when water was reacted with CO2in the atmo-sphere. Subsequently, the appearance of these impurities led to areduction in the basicity of catalyst [29].

In addition, Li and Wang[30]pointed out that high activity ofCaO might be caused by the soluble species of CaO that formedthrough the interaction with glycerol and DMC. Simanjuntaket al.[29]further identified the soluble substance as calcium com-plex Ca(C3H7O3)(OCO2CH3). Other metal oxides such as Na2O, MgOand ZnO were also investigated by the same authors[29]. The cat-

alytic activities of MgO and ZnO produced only 10.2% and 0.5%yield of GC, whereas Na2O produced 92.6% GC with high basicity.The low activity of MgO was expected as it has the weakest basicstrength among group II oxides [63,64]. Nevertheless, MgO hadbeen used as efficient catalyst support for K2CO3and together theydemonstrated 99% yield in the catalyzed transesterification reac-tion[47]. Noticeably, the basicity of Mg-contained catalyst couldbe improved by combining Mg with other types of metals suchas Ca, Al, Li and Zn through co-precipitation method. Manyresearchers have prepared mixed magnesium-alumina (MgAl)oxide by using hydrotalcites (HT) (Mg6Al2(OH)16CO34H2O) as pre-cursor and calcined at high temperature [31,3743,65].

Hydrotalcite or Layered Double Hydroxide (LDH), [M2+(1x)M3+-

x(OH)2]x+(Ax/n)

nyH2O are anionic and basic clay minerals [66].

The metal cations M2+

and M3+

and anion An

reside in the inter-layer space of the hydroxides as shown inFig. 3[66,67]. The most

common hydrotalcite is Mg6Al2(OH)16CO34H2O and its conven-tional preparation method is co-precipitation[68]. Hydrotalcite isa powerful catalyst as many of its physical and chemical propertiesresemble those of clay minerals. The acid/basic properties can beeasily controlled by varying their composition making it widelyused in base-catalyzed or -assisted reactions such as alkylation,Michael addition, ClaisenSchmidt condensation, Knoevenagel

condensation, aldol condensation, hydrogenation, olefin epoxida-tion, alcohol oxidation and transesterification[39,43].Eshuis and co-workers had first patented a process of convert-

ing glycerol to a mixture of oligomers by using commercial Mg/Al-hydrotalcite Macrosorb CT100 hydrotalcite[69]. The heteroge-neous catalysts applied only managed to obtain 23% GC. The simplepreparation method for HT and its adjustable basicity encouragedresearchers to opt for catalyst preparation in house. The basicproperties of HT could be tuned via pretreatment of HT such as cal-cination [31,3943,65], rehydration [4143,65], changing theanion composition in HT[65]and doping transition metal cationson the calcined hydrotalcite [39,40]. After pretreatment at hightemperature, catalytic activity would be enhanced. As in the caseof mixed oxides (Al/Mg, Al/Li, Al/Ca) derived from hydrotalcitesby calcinations which would contain higher Lewis basic sites thanthe uncalcined HT making the catalyst more effective during GCsynthesis[31].

Further increase in the catalytic activity of HT is possible viarehydration of calcined HT with the presence of Bronsted basicsites[41]. The rehydrated calcined HT though almost had 4 timeslower surface area, presented a 99% glycerol conversion comparedwith the 76% obtained using the calcined HT despite both of themhave similar total number of basic sites. This implies that theaccessibility and the number of basic sites are not as importantas the basicity of the solid. This shows that the Brnsted basic sitesare better in extracting proton from glycerol (which presentshigher acidity compared with DEC) and thus stabilized the alkox-ide anion on the surface of the solid[41].

The promotional effects on the basicity of calcined hydrotalcites

with transition metals doping was confirmed by Liu et al. [39,40].They discovered that nickel doped hydrotalcites (HTC-Ni) exhib-ited 10 times higher catalytic activity than the uncalcined hydrota-cites precursor in the transesterification reaction as theirreconstructed HT possess more open structure and higher basicity.Furthermore, it is economically viable as it could be recycled andreused at least five times without significant loss of performance,giving 100% GC selectivity even after repetitive use.

The effectiveness of hydrotalcite does not limit to the calcinatedhydrotalcite in GC synthesis. The use of uncalcined Mg/Al hydrotal-cite at 25 ratio in the hydromagnesite phase for the transesterifi-cation of glycerol with DMC increases the yield of GC[37,38]. Thehigh catalysts activity of HTHM (hydrotalciteshydromagnesite)was attributed to the increased HT surface area and adsorption

sites for glycerol during the reaction. Besides coupling calcinationsand rehydration with methods such as ultrasound, microwave hasalso been applied to alter the structure of HT [4143,65,70,71].Regardless of its effectiveness in the catalytic transesterificationreaction, HT has limited industrial application largely due to thedifficulty in obtaining desirable small particle sizes for continuousflow reactors. However, this drawback could be overcome byimpregnating Mg/Al HT precursor salts onto the nano-scale mate-rials supports such as a-Al2O3 and c-Al2O3 in a continuous flowreactor[42].

A number of studies employed mixed oxides have high catalyticactivity as shown by Mg-containing bimetallic [33,34] and trimetallic[35,36]inTable 1. Mixed metal oxides are generally pre-ferred to the single metal oxides owing to their stronger basic

property and larger surface area. In the transesterification ofDMC with glycerol using MgLa mixed oxide[33], it was suggested

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that the modification of catalyst preparation condition i.e. themetal oxide molar ratio, the presence of precipitation agent andcalcination temperature alter the catalyst composition, structure,surface area and basic site concentration and which had signifi-cantly affected the catalytic activity. To achieve high yield of GC,the best conditions determined were at mixed oxide molar ratioof 3 with KOH/K2CO3and calcined between 650 C and 720 C. Inthe Mg1+xCa1xO2 catalyzed glycerol transesterification process

[34], 100% GC yield was achieved after 90 min of reaction time at70 C and 2:1 DMC/glycerol molar ratio. The remarkable findingscould be explained by the synergistic interaction between Ca andMg species in the catalyst structure that contributing to the highcatalytic activity. Furthermore, the researchers[3334]agreed thatthe nature of active sites play an influential role in the catalyticactivity.

Evidences of mixed oxides molar ratio and pretreatment tem-perature that influenced the catalysts activity are demonstratedby the tri metallic mixed oxides Mg/Al/Zr [35]and Mg/Zr/Sr[36].These base catalysts performed well in GC synthesis and thewell-dispersed mixed oxides in stabilized tetragonal phase of zir-conia at calcination temperature of 650 C with Mg/Al/Zr moleratio of 3:1:1 showed higher activity of 94% yield than the bimetal

oxides (Mg/Al and Mg/Zr) which are only 15% and 25% yield,respectively[35].

The basic site clearly has certain influence on the transesterifi-cation yield. From the investigation of Na based zeolite and hydro-talcite in the transesterification process[44], the reaction showshigher dependency on the catalyst structure than the basic sites.The activity of the zeolite is not proportional to the number ofbasic sites but the pore structure of zeolites. With small pore diam-eter such as 3A, 4A, and NaZSM-5, the catalyst was inactivewhereas those NaY and Nabwith pore diameter containing a 12-numbered ring-structured pore channel under mild conditionsyielded excellent GC. As for recycle possibility, NaY indicates a neg-ligible 1% difference in the first three cycles, and remained stablethereafter [44].

Supplementary information on other high performancecatalysts with support is listed in Table 1. Examples shown are

KF/hydroxyapatite (HAP) [45], NaOH/c-Al2O3 [46] and K2CO3/MgO[47]that have their acid/base properties modified by loadingwith other compounds according to the transesterification reactions.

2.3. Enzymatic transesterification

Catalytic transesterifications by enzymes such as lipases arecommonly found in biodiesel industry. Through the application

of enzymatic transesterification, many downstream processingproblems are simplified and eventually the production cost canbe reduced. As listed inTable 2,enzyme under mild reaction con-dition without the generation of by-products not only eases therecovery of the desired product but also enables the reusabilityof the enzyme. This might make enzyme replacing chemical cata-lyst worth to be investigated in the context of a greener GC produc-tion route. However, high cost of enzymes, their comparativelyslow reaction rates and the likelihood of enzyme deactivation haveoften undermined the choice of enzyme for industrial scale pro-duction. Thus, this review only discusses the enzymatic transeste-rification of glycerol at lab scale.

GC synthesis via transesterification can be catalyzed withenzymes such as Candida Antarcticalipase[5054]and Aspergillus

nigerlipase[5559]. Kim et al. [50]successfully synthesized 99%GC from glycerol and DMC using Candida Antarcticalipase B immo-bilized on resins Novozym 435 in the presence of solvent THF. Stoi-chiometric molar ratio of reactants (DMC: Glycerol) at 1:1 wasapplied but the reaction took 30 h to complete and moreover theorganic solvent THF is toxic. To overcome the toxicity problem, sol-vent-free system was proposed[51]at 10:1 DMC: glycerol molarratio with the excess DMC played the dual role as reactant and sol-vent. To prevent two phases from forming due to the poor solubil-ity of glycerol in hydrophobic DMC, glycerol was coated with silicagel in equal amount though this had little effect on the reactionrate that involving free enzyme. The problem associated withthe formation of glycerol coating could one hand be resolvedby the addition of surfactant Tween 80 to enhance mixing but

on the other hand this induces downstream processing prob-lems. Another exploration on the use of both hydrophilic and

Fig. 3. Structure of hydrotalcite (adapted from[67]).



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hydrophobic solvents by Jung et al.[52]attained 96.25% glycerolconversion at DMC:Glycerol molar ratio of 2:1 using acetonitrileas solvent and with the addition of detergent. The 48 h reactiontime is as lengthy as for the solvent free system [51]. Thus, tert-butanol was used to reduce the reaction time to 12 h [53,54]intheir respective reactions.

Besides Candida Antarctica lipase, Aspergillus niger lipase had

been extensively studied in GC synthesis via transesterification ofglycerol with DMC[5559]. GC was successfully produced undermild reaction condition at shorter time but the yield of GC obtainedusingAspergillus nigerlipase was only between 25% and 60%. It isimperative therefore to develop an improved enzymatic processscheme besides searching for suitable and effective catalysts tosynthesize GC feasibly via examining the operational factors criti-cally. The factors influencing the process efficiency will be dis-cussed next.

3. Factors influencing transesterification reaction

The conversion of glycerol and the yield of GC in transesterifica-tion reaction are significantly affected by the reaction conditions

and the parameters such as reaction temperature, reaction time,molar ratio of reactants and solvent, as well as impurity particu-larly water content and methanol in the reaction mixture. Mostof the studies evaluate these factors one at a time and limited work[21] has been done on evaluating their synergistic effects on GCsynthesis. Therefore, these pertinent factors affecting the perfor-mance of GC synthesis will be elucidated individually.

3.1. Effect of temperature

Glycerol Carbonate (GC) synthesis via transesterification ofglycerol can produce high GC yield at mild reaction temperatureand atmosphere pressure. The synthesis route has been widelystudied using different carbonates and catalysts at varying temper-

ature range between 35 C and 140 C at atmosphere pressure.Conventionally at temperature above 140 C, a reduced pressureof 103 MPa is required in the glycerolysis reaction whereas highpressure of 5 MPa requires the temperature range of 80180 Cin the carboxylation process[8]. The carboxylation reaction condi-tion can achieve only 65% conversion of glycerol after 15 h[32].

The reaction temperature is obviously a critical parameter forthe transesterification of glycerol as this process is reversible andfavorable in producing GC which is thermodynamically related tochemical equilibrium constant[72]. According to Arrhenius equa-tion, an increase in reaction temperature can increase the collisionrate between the reactants and thus leading to a higher reactionrate and product yield [73]. Nevertheless, the optimum reactiontemperature in transesterification is closely dependent on the heat

sensitive carbonates and catalysts used. In the transesterificationof glycerol with ethylene carbonate (EC), lower reaction tempera-ture is favorable to achieve a better GC yield. As reported, 87% yieldof GC at 35 C was achieved within 1 h as compared to a loweryield of GC at 50 C even with higher catalyst loading and pro-longed reaction time[31]. This finding reported by Li and Wang[72] through their calculation of chemical equilibrium constantsuggesting that chemical equilibrium constant decreased whenthe temperature increased within the range of 2580 C. The reac-tion shifted to backward reaction at higher temperature causing areduction in GC formation. However, this trend does not apply tothe transesterification reaction of glycerol with DMC or diethyl car-bonate (DEC). Different from the case of EC, temperature increaseswithin the range of 4080 C increases the chemical equilibrium

constant which in turn has increased the reaction rate of glyceroland DMC [72]. Furthermore, the effect of temperature on the

conversion and yield had been witnessed by recent works whichapplied metal oxides [29,3336] and ionic liquid [28,49] ascatalyst. The works involved metal oxides[29,3336]have similartrend of temperature effect whereby less than 40% GC yield at50 C and further increased in temperature increased the yieldup to the optimum temperature. In most of the studies, GC synthe-sis reaction performed well between 70 C and 90 C

[18,21,22,26,29,3336,4448] using chemical catalysts and 60

Cfor biocatalysts [5059]. Higher temperature of 100120 C areessential for the reaction system that involved hydrotalcite [3740]and ionic liquid[2527].

To have a better understanding on the types of catalysts andtheir suitable temperature range for GC synthesis, the physicalproperties of the reactants should be taken into account whendetermining the reaction temperature. DMC and glycerol are ther-mally stable and do not decompose below 390 C[74]and 150 C[75], respectively. This implies that the suitable temperatureapplied to DMC and glycerol should not be exceeded the men-tioned values. This contradicted to the observation on the solidbase-catalyzed decarboxylation[76]whereby, partial decomposi-tion of DMC to dimethyl ether and CO2 occurred at temperature>200 C. Furthermore, it is worth noting that the dialkyl carbonatesare volatile organic compounds and can easily be evaporated. Toavoid excessive and unnecessary evaporation, the boundary ofreaction temperature should be set below their boiling points. Thiswas evidenced from the successful cases of transesterification ofglycerol with DMC or DEC carried out below 140 C. The tempera-ture range required for transesterification of glycerol with DEC byhydrotalcite was 130140 C [37,4143,65]. Even though thetransesterification of glycerol with DEC could withstand harsherreaction conditions, there is a trade-off between the improvedyield and the heat requirement to carry out the reaction at highertemperature.

On the other hand, enzymatic catalyzed transesterification canbe carried out at relatively low temperature range between 40 Cand 70 C[5059]. Too low a temperature i.e. 40 C, cannot ade-

quately dissolve the enzyme and reactants [52]. In such case, anincrease in temperature is needed to reduce the viscosity of reac-tion mixture. The less viscous and fast moving reactant moleculeswould promote the effective collisions with enzyme to increase thereaction rate. At temperature greater than 70 C, the active confor-mation of enzyme might be disrupted and resulted in activity lossand hence the conversion rate and the yield [54]are decreased.Hence, the enzyme activity should increase with increasing tem-perature provided the stability of enzyme pertained.

Increase the operating temperature beyond the optimum tem-perature might not improve the conversion of glycerol but insteadit would probably decrease the yield. When the reaction tempera-ture exceeds 100 C, side-reactions involved dehydrogenation andcondensation of the by-product methanol might have occurred

on the basic sites[39,40]. This was witnessed in the decarbonyla-tion of GC to glycidol as shown inFig. 4[28,35]. The temperatureincrement promoted the formation of glycidol and maximizedthe glycerol conversion from 90 C to 110 C[35]which was man-ifested by the similar trend when temperature increased from70 C to 80 C [28]. The selectivity of GC decreased as it wasconsumed to form glycidol, it is therefore crucial to control the

Fig. 4. Decarbonylation of glycerol carbonate to glycidol[28,35].

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optimum reaction temperature properly in achieving maximumyield of GC.

Most of the GC synthesis through transesterification are inbatch modes rather than in the continuous modes. In the batchoperational mode, round bottom glass flasks or jacketed glass reac-tors were not sufficient to withstand the high reaction temperature.High pressure reactor was used for reactions above 75 C[29,34]and

above 90

C[21]. In the continuous system, a tubular quartz packedbed reactor was applied at 130 C [42]. It can be seen that higher tem-perature tends to favor the continuous mode of operation. Forinstance, continuous operation is recommended to replace batch pro-cess at reaction temperature of 175 C due to the decomposition ofproduct [77]. To prevent the undesired consumption of DMC orDEC and the extensive decomposition of GC, the operating tempera-ture limit should be established case by case for individual catalyticsystem. Moreover, safety aspect should also be highlighted to elimi-nate the formation of highly flammable ethers as a result of the unde-sired decomposition of DMC at high temperature[78].

The research findings so far suggest that chemical catalyzedtransesterification in general required higher temperature thanenzymatic transesterification. Except the case of using EC as car-bonate source, more glycerol converted with the increasing tem-perature. For the reaction carried out beyond the optimumtemperature, the yield of GC was decreased due to the formationof undesired products. However, the optimum temperature rangeis also very much dependent on the carbonates used. Since mostof the researchers focused using different type of catalysts onone carbonate source in the synthesis, more effort should thereforebe devoted to apply different carbonates using same catalyst[18,53]. With that, thorough evaluation on the trend of tempera-ture with different carbonates can be performed. Furthermore,the reaction temperature is also related to the other reaction con-ditions, such as molar ratio of DMC/glycerol and reaction pressure[21]. When transesterification of glycerol takes place under atmo-sphere pressure, DMC and the by-product methanol could simulta-neously be recovered and flowed back to the reactor, this would

significantly affect the reaction temperature [21]. Hence, simpledistillation is normally applied in the transesterification reactionto enable the reaction to be carried out either under total refluxcondition [1821,2831,33,3944,48,65]or with removal of a sub-stantial portion of unreacted DMC and the produced methanol[45,46,79].

3.2. Effect of reaction time

Reaction time has effect in the synthesis of GC. Short reactiontime is predictably attractive for industrial manufacturing of GC.In the absence of catalyst, transesterification was slow and only5% GC yield was achieved after 5 h[31]. The shortest reaction timerequired to yield 81% of GC from transesterification with EC using

catalyst CaO was around 15 min [31,49]. Generally, the reactionrate of the base catalyzed transesterification is much faster thanthat of acid catalysis due to higher catalytic activity[21]. Many dif-ferent types of base catalysts as previously mentioned are suitablefor transesterification of glycerol as the reactions are rapid with noapparent induction period[39]. Furthermore, the conversion rateof glycerol, selectivity and yield of GC increase with reaction time[28,29,31,3335,39,40] in the catalyzed-transesterification reac-tion. The yield of GC improved from 75% to 99% when the reactiontime had increased from 1 h to 2 h[37].

When immobilized ionic liquid[49]was used in the transeste-rification of glycerol with EC, 86% glycerol conversion in 5 min wasaccompanied with the by-product of ethylene glycol of greaterthan 75% selectivity. This has unfavorably lowered the GC yield.

To have higher GC yield of more than 90%, optimum reaction timeranges from 30 min to 3 h were applied in the homogeneous cata-

lyzed transesterifications [18,21,22,26,29]. For instances, CaOachieved 94% GC yield in 30 min and K2CO3and KOH both yielded100% GC in 1.5 h [21]. With the employment of heterogeneouscatalyst, the reaction time were mostly between 1 and 2 h[21,30,3340,4547,49,60,72] . One of the highly recommendedheterogeneous catalysts is Mg1+xCa1xO2 [34] which resemblesthe organometallic catalysts such as Layered Double Hydroxide

(LDH) based heterogeneous catalyst are efficient and reusable forthe transesterification reaction. The highest yield of GC synthesisby LDH catalyst achieved so far is 99% by using uncalcined Mg/Alhydrotalcite in 2 h [37] and 87% GC yield in 1 h using calcinedAl/Ca hydrotalcite[31]. This suggests that carbonate source usedin transesterification affects the reaction time as reflected fromthe slighltly longer time required for hydrotalcite in the transeste-rification with DMC regardless of it is well acclaimed catalyticactivity. When hydrotalcite was involved in the transesterificationof glycerol with DEC, 65% GC yield was achieved in 10 h [41]. Thelow GC yield could have been caused by the long reaction timewhich prolonged the exposure of reaction to high temperatureand resulted in the forming of 33% of side product, glycerol dicar-bonate[77]. The formation of glycerol dicarbonate was confirmedby another study on hydrotalcite-catalyzed transesterification[39,40]. The side product formation decreases the GC selectivitywhereas, glycerol conversion was enhanced as the reaction timewas extended to 5 h. This can be explained by the strong adsorp-tion of side products methanol and its derivatives on the basic sitesduring the reaction[39,40].

Prolonging reaction time does not benefit the conversion andyield in the solid base- catalyzed transesterification with theoccurrence of undesirable reactions such as decomposition of cyc-lic carbonates [31] and the decarbonylation of GC to glycidol[28,35]. In the transesterification of GC with carbonate sourcesusing strong bases catalysts, glycerol dicarbonate or diglycerol tri-carbonate could be produced in 48 h[18]. A much reduced reactiontime of 8 h in the Mg/Zr/Sr mixed oxide base catalyzed-transeste-rification showed an increased selectivity of glycidol and a

decrease in GC selectivity[35]. Also it has been reported that forthe reaction beyond 90 min, a slight decrease in GC yield wasreported which could have been caused by the effect of solubilityof by-product methanol on the reaction mixture[34].

A few studies on supported catalyst employed in the transeste-rification of glycerol have been attempted to replace homogeneouscatalyst by adding support to the catalyst[4547]. When the polar-ity and structure of a catalyst is altered, the catalytic activity wouldbe influenced [43]. For instance, by applying carbon nanofiber(CNF) supported Mg/Al hydrotalcite (HT) in the transesterificationreaction, the reaction time had been reduced from 10 h to 2 h. Thisis because the surface area and polarity of the HTCNF catalystwere reduced which had enhanced the adsorption of reactantsand resulted in higher reaction rate[43].

A different yet interesting approach using glycerol-coated silicagel in the synthesis of GC from glycerol applying lipase in DMC wasadopted to get rid of large glycerol droplet formation [51]. Thisapproach enhanced the accessibility of glycerol and DMC to theenzyme and it significantly has increased the transesterificationrate by tenfold than that of free glycerol. The reaction rate can alsobe sped up by preheating the viscous glycerol before mixing withDMC to increase the miscibility of both reactants. Similar conceptwas applied by Leung and Guo[80]to increase the rate of biodieselreaction and shorten the reaction time through heating oil prior tothe mixing. Thus, proper mixing between reactants and catalystcan determine the completion time of a reaction. To ensure theoperational feasibility, molar ratio of reactants must be adjustedaccordingly and also the problems of hydrophilic glycerol and

hydrophobic DMC need to be resolved by suitable choice ofsolvents.



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While taking short reaction time and high GC yield as selectioncriteria for catalyst in the transesterification processes, reusabilityand greenness of the catalytic reaction had also been considered bymany. This can be witnessed from the work done on enzymaticcatalyzed transesterification to synthesize GC as well as to over-come the drawback of lengthy reaction time (3048 h) associatedwith the process. In particular, Cushing and Peretti[53]and Lanje-

kar et al.[54]have greatly reduced their respective reaction timeby 2-fold to 12 h and 14 h using catalyst Candida AntarcticalipaseB (Novozym 435). A more efficient enzyme Aspergillus nigerlipasethat took only 46 h was employed in the biosynthesis of GC butthe end result showed a conversion of glycerol less than 75% anda GC yield below 60%[5559].

Another fluctuating trend between the reaction time and thecatalyst applied in the GC synthesis can be seen from the use ofthe unconventional catalysts ionic liquids. This type of catalystshowed a vast difference in the transeterification time range from1.33 to 13 h [2528,49]. The inconsistent findings probably arecaused by the interaction between the different anion and cationspecies in the ionic liquid with the reaction mixture.

Higher reaction time could be advantageous as it allows moretime for the glycerol to react in both the chemical catalyzed andenzymatic transesterification. Similar to the temperature effect,prolonging reaction time increases the operating cost but reducesGC yield due to undesired decomposition reaction involved. Otherthan the environmental friendliness demonstrated by the enzy-matic transesterification, the production route is not beneficial asit gives extremely low reaction rate and its performance is noncomparable with those of chemical catalysts. In general, produc-tion route be it chemical or enzymatically catalyzed, properscreening for the compatibility of different carbonates and cata-lysts in GC synthesis is required.

3.3. Effect of molar ratio and solvent

Theoretically, the ratio for transesterification reaction requires

only 1 mol of carbonates and 1 mol of glycerol to produce 1 molof GC and 1 or 2 mol of relevant by-product as shown in Fig. 5.The by-products could be ethylene glycol, propylene glycol, meth-anol or ethanol depending on the carbonate source used. In thetransesterification between the hydrophilic glycerol and hydro-phobic carbonate source, the reactants are not miscible and thereaction is reversible which is in need of an excess carbonatesource to give positive effect on the conversion and yield [27].The prevention of the two-phase formation between the reactantscan be achieved either by applying excessive amount of carbonatesource over the glycerol moiety to act as reactant and solvent or byadding organic solvent.

As reported in the literature, reactions normally carried out atmolar ratio of carbonate source to glycerol in the range of 25 to

shift the chemical equilibrium towards GC formation for greaterglycerol conversion in a shorter time as shown in Table 1. LowGC yield was observed when equimolar of reactants was used inthe transesterification[60]. The yield of GC was increased whenthe carbonate glycerol ratio is raised beyond 2 and reached a max-imum molar ratio of 5[36]. Further increase in the amount of car-bonate beyond the optimal ratio has adverse effect on the yield andwould incur additional cost for the reactants. The yield would dropwhen GC reacted with excess carbonate to form by-product i.e.GDC when the DMC/glycerol ratio increased[22].

Also, molar ratio of the reactants is influenced by the type ofcatalyst used. When homogeneous catalyst was applied, molarratio between 2 and 5 produces 90100% GC yield [18,21,22,2527,29]without the need of adding solvent. Furthermore, homoge-

neous catalyst such as ionic liquid could also serve as solvent in thereaction[27]. In most of the heterogeneous bases catalyst reaction,

DMC mixed well with glycerol at low molar ratio of 2[29,33,34,45,46]. With exceptional cases, organic solvent is addedto compensate for the low molar ratio of carbonate/glycerol[37,38,42,44]. The reaction between the strongly polarized glyceroland aprotic DMC or DEC allows the polar aprotic solvents such asdimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetoni-trile and hydrophilic solvents such as tetrahydrofuran (THF), tert-

butanol, ethanol or methanol to enhance their solubility in thetransesterification reaction. Adding suitable polar solvents DMF[37,38]or DMSO[42]greatly improved the GC yield from 17% to99% in the uncalcined Mg/Al hydrotalcite-catalyzed transesterifica-tion[37]. The glycerol conversion increased with the polarity of thesolvent in the descending order of DMSO, DMF and DMA respec-tively at 50%, 30% and 20%[42]. However, incorrect solvent choicesmay lead to the formation of by-products such as glycidol andother unknown compounds when ethanol was applied [44]. Thissignifies the effect of solvent polarity on the glycerol conversionand selectivity[42,44].

In the enzymatic catalyzed transesterification reaction, highconversion of glycerol could be achieved either by using high molarratio or addition of solvent. For solvent free system, high molarratio of 10 was suggested [51,53,5559]. Other options such asco-solvent THF[50]or acetonitrile[52]can also be used. Regard-less of the methods applied, 93% glycerol conversion was achievedwhen Candida Antarctica lipase B was employed and a compara-tively lower conversion of less than 75% was obtained whenAsper-gillus nigerlipase was used. The performances between these twobiocatalysts varied largely[53]due to the un-similar reaction con-dition applied.

The product yield is critical to the chemical synthesis as theselection of solvent is to the design of experiment [42]. To be intandem with green chemistry, non-toxic solvent must be consid-ered for the enzymatic catalyzed reaction. Therefore, it is not sur-prise that green solvent i.e. tert-butanol had been selected forcatalytic transesterification of glycerol and promising conversionof 94.85%[54]and nearly 100%[53]were reported. These findings

have placed tert-butanol, a non-toxic and chemically inert solvent,favorable for enzymatic reaction to promote singe phasic system[53,54]. Other advantages of tert-butanol are its ability to increaseenzyme flexibility, as it allows the enzyme to be more easily boundwith substrates and alter their conformation to facilitate reactionfor improving the catalytic activity[53].

Solvent could also play an essential role as azeotropic agent onthe synthesis of GC. In such case, azeotropic agent was required forthe rupture of methanol-DMC azeotropic mixture formed due totheir narrow boiling points[81]. Addition of a suitable solvent tothe reaction mixture can alter the relative volatility of the originalbinary mixture to eliminate the azeotrope, thus facilitate theseparation of methanol from DMC[82]. In azeotropic distillationcoupled with glycerol transesterification reaction, the suitable aze-

otropic agent or entrainer are cyclohexane, n-hexane, n-heptane,isooctane, ethyl acetate, cyclohexene, benzene, dichloroethane asthey could form the azeotrope with methanol [60]. Benzene wasfound as the most effective azeotropic agent which gave a 98%GC yield even at the stoichiometric molar ratio of DMC/glycerol[60]. All of the DMC was retained in the reactor for reactionwhereas the resulting mixture of methanol and benzene wasremoved as a distillate in the distillation column [60]. The highyield of GC was attributed to the positively shift of the reversibletransesterification reaction by continuously removal of the pro-duced methanol [60]. In short, azeotropic distillation is capableto overcome the limitation caused by the thermodynamic phaseequilibrium with high yield of GC achieved without excess DMCused. The drawback of this process are energy intensive and

environmental unfriendly due to the carcinogenic nature of theentrainer used[83].

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From economic and environment viewpoint, low molar ratio ofcarbonate/glycerol and solvent free are preferable in the transeste-

rification reaction. Nonetheless, high molar ratio or addition ofsolvent is essential to increase the miscibility of glycerol withDMC or DEC. If high molar ratio of DMC/glycerol is adopted, excessDMC could be easily separated from the reaction product togetherwith methanol via distillation. If solvent or azeotropic agent wereto be applied to the system, more efficient separation and purifica-tion methods are needed. In brief, carbonates should present inexcess of stoichiometric proportion for the effective transesterifi-cation of GC.

3.4. Effect of impurities

Impurities are undesirable for reaction generally. They can beappeared in both reactants carbonate source and crude glycerol

in the transesterification reaction. Crude glycerol obtained frombiodiesel plant contain many impurities such as methanol, waterand soaps, fatty acids, salts like phosphates and sulfates, andmetals such as Na, K, Ca, Mg and Mn depending on the type of oilsand catalysts used in the production[15]. These impurities may below in concentration but is sufficient to affect the conversion ofglycerol and the GC yield.

Pure (99%) glycerol was used in most of the transesterificationof glycerol with very few using crude glycerol at content varyingfrom 40% to 90% [26,58,59,84]. Crude glycerol can be obtainedeither before neutralization/acid treatment (4070%) or after acidtreatment (above 80%). A 88 wt% (59 mol%) crude glycerol gave aGC yield of 93% and 100% yield of GC when pure glycerol was used[26]. A slightly lower yield of GC from the crude glycerol could be

caused by the water and salt content present in the crude glycerol.The crude glycerol used by Ilham et al. [84]for GC synthesis with

supercritical DMC have the same impurities as reported by Naiket al.[26], whereby the glycerol obtained from alkaline-catalyzed

biodiesel contained 70 wt% glycerol, 10 wt% of water and 20 wt%of sodium salt. When these impurities reacted with supercriticalDMC, the GC produced further decomposed to form glycidol asshown in Fig. 4. This further demonstrated that water and salttoo could lower the GC yield under supercritical condition.

The presence of water and methanol in crude glycerol alsoaffected the GC yield significantly in biosynthesis of GC with lipase[59]. It was reported that a 17 wt% of water in crude glycerol couldlead to a decrease of 11% in the GC yield. The negative effect ofwater on GC yield is more obvious for higher water content incrude glycerol. The reduction of yield was caused by the inhibitoryeffect of lipase activity by water as the enzyme required prepon-derantly a hydrophobic environment. The presence of water

jeopardized the stability of GC as well as the short-chain alcohols

like methanol and subsequently destabilized the protein[59]. Con-versely, trace amount of water (hydrophilic phase) can conservethe spatial enzyme structure. Besides, impurities like soap haveno effect on the biocatalytic reaction and do not influence thetransesterification reaction[59].

Crude glycerol produced from different oils contains differentimpurities and thus can result in different GC yield under identicalreaction conditions. Glycerol content varies from 40% to 75% incrude glycerol derived from different oils. The nature of the fattyester precursor determines the yield, which is further dependenton the percentage of methanol and water present in the crudeglycerol produced. Apart from that, lipase activity could be affectedby metal ions like sodium, potassium and magnesium but theynormally were not detected in most of the crude glycerol. Among

the metals and salts, only manganese which was found in crudeglycerol derived corn oil can affect the GC yield [59].

Fig. 5. Transesterification of glycerol with different carbonate sources[8].



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As a main contaminant, water presents in crude glycerol andcommercial pure glycerol [18] as well as in the reagent gradeDMC containing impurities such as water and methanol [53].Water showed negative effect to both the chemical catalyzed syn-thesis [18,28,48] and enzymatic reactions [52,53]. When ionicliquid was used as catalyst, the glycerol conversion decreasedsignificantly from 95% to 35% with the increase amount of water

in the reactants from 0.059 g to 0.238 g [28]. Extruded CaO/Al2O3has certain water-resistant performance, 92.1% glycerol conver-sion and 97.7% GC yield were obtained within 0.4 wt% of waterin glycerol. As the water content in glycerol increased continuallyto 2 wt%, the glycerol conversion and GC yield drop to half [48]as the catalyst was probably deactivated with large quantity ofwater in glycerol. For enzymatic reaction, small amount of waterimproves the enzymatic activity by providing more interfacial area[52]but beyond 0.25% (v/v) of water an inhibitory effect on theenzyme activity would be induced. In addition, excess watershifted the reaction equilibrium in favor of hydrolysis and thuslimiting the overall glycerol conversion[53]. Thus, it is essentialto reduce the water content to an acceptable level in the reactantsprior to be used in transeterification reaction. Water could beremoved to less than 2% of water content in glycerol via pretreat-ing the commercial pure glycerol by azeotropic[18]or via mixingthe reagent grade DMC or crude glycerol with molecular sievesovernight at room temperature[53].

The known positive aspect of impurities as far as transesterifi-cation of crude glycerol is concerned is associated with the promo-tion of the recycle capacity of the biocatalyst. The GC yield wasenhanced by 2050% after the first 23 cycles, and the catalyticcapacity of the biocatalyst was preserved within the next 10 reac-tion cycles [58]. Also, biocatalyst can perform better with theremoval of the superficial enzyme molecules kept by proteinprotein interactions on the biocatalyst surface making the catalyticsites accessible. The soap impurities which act as surfactants canserve the purpose to breakdown the interactions[58].

Weighing the pros and cons of crude glycerol, the energy inten-

sive distillation that currently applied in the industry for removingthe impurities from crude glycerol to obtain pure glycerol is unde-sirable. One promising report suggests that KF/HAP[45]is stable inthe presence of water and soap[45]which signifies the possibilityof producing high GC yield via transesterification of crude glycerol.Thus, further exploration to ascertain the feasibility of transesteri-fication of crude glycerol in GC production is encouraged.

4. Future direction of glycerol carbonate production via

catalytic transesterification

Catalytic transesterification of glycerol is a simple and efficientroute to produce GC. Most of the studies focused on developing

new catalysts to produce high yield of GC and very few studiesare on purification of GC. To have feasible GC production at indus-trial scale, process optimization of the production route andimproved product isolation techniques are essential. Furthermore,the study of the influencing factors on the performance of GC pro-duction that confined in a single factor designed in most of theworks reported has raised the need to conduct design of experi-ment more thoroughly to examine the synergistic interactions ofthe parameters investigated. Also, it is prudent to point out thatthough chemical catalyzed transesterification of glycerol is a prom-ising route to produce GC, the plausible yields had been derivedmainly from the use of pure glycerol. Due to the fact that pure glyc-erol obtained from crude glycerol that contains impurities incurshigh purification cost, future studies should gear towards using

crude glycerol directly obtained from the biodiesel plant to pro-duce GC. This would greatly reduce the production cost of GC

and broaden its industrial usages. Along with that, this shall openup an avenue for making biodiesel production a truly economicviable and integrated process.

5. Conclusions

High yield of GC can be achieved by transesterification of glyc-

erol using different type of catalysts including pure and mixedmetal oxides, ionic liquids, hydrotalcites and lipase. It is worth not-ing that the effects of the operating parameters on the GC produc-tion are closely dependent on the type of catalyst applied. Inchemical catalyzed transesterification, homogeneous based cata-lyst gave good yield in relatively short time meanwhile the useof heterogeneous based catalysts have also become popular dueto their high activities, simple recovery methods and their abilitiesto be recycled. Enzymatic transesterification with milder operatingcondition could circumvent the disadvantages imposed by thechemical catalyzed reaction. The proper control of the operatingparameters in transesterification is important to ensure the suc-cess in GC production as well as to minimize the formation ofundesired intermediates and side products which would compli-

cate the subsequent downstream processing. Organic solvent,molecular sieves, surfactant and silica gel could be used in GCenhancement provided the cost for product purification involvedis justifiable.

Acknowledgement

The authors thank University of Malaya for supporting thisresearch under the Grants of HIR (High Impact Research) with Pro-

ject no. UM.C/625/1/HIR/MOHE/ENG/59 and University of MalayaResearch Grant (UMRG)RP002B-13AET.

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