Renewable and Sustainable Energy Reviews 53 (2016) 558–574

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Renewable and Sustainable Energy Reviews

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n CorrE-m

journal homepage: www.elsevier.com/locate/rser

Review on recent progress in catalytic carboxylation and acetylation ofglycerol as a byproduct of biodiesel production

P.U. Okoye, B.H. Hameed n

School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

a r t i c l e i n f o

Article history:Received 17 May 2015Received in revised form14 August 2015Accepted 29 August 2015

Keywords:GlycerolBiodieselAcetylationCarboxylationCatalystTransesterification

x.doi.org/10.1016/j.rser.2015.08.06421/& 2015 Elsevier Ltd. All rights reserved.

esponding author. Fax: þ60 4 5941013.ail address: chbassim@usm.my (B.H. Hameed)

a b s t r a c t

Biodiesel (BD) is an alternative energy source to conventional diesel derived from fossil materials, which areunsustainable and non-renewable and contribute to global warming. BD production via transesterificationwith methanol leads to the synthesis of glycerol; this process accounts for 10% (w/w) of the total BDproduced worldwide. The increasing demand for environmentally harmless BD has created a glycerol glut,which must be utilized to increase BD profitability. Glycerol is a stable and multifunctional compound usedas a building block in fine chemical synthesis. Acetylation and carboxylation pathways have been studied toutilize and/or upgrade glycerol into fine chemicals. The use of catalysts, especially heterogeneous catalysts,remains the green approach for tailoring carboxylation and acetylation routes to achieve the desiredproducts, namely, glycerol carbonate and glycerol acetyl esters, respectively. However, side-product for-mation, poorly structured channels of some catalysts, and catalyst deactivation or reusability hinder theeffective utilization of heterogeneous catalysts and must be further studied. Moreover, introduction ofvariations to optimize reaction-influencing parameters is a potential green method that must be explored.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. GC synthesis via transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Catalyst applications to glycerol transesterification with DMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1. Homogenous base catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2. Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3. Heterogeneous base catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Reaction pathway for glycerol transesterification with DMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1. Effects of reaction parameters: catalyst loading, temperature, and molar ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.1. Catalyst loading and textural properties of heterogenous solid base catalyst. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2. Reaction temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3. Effects of glycerol/DMC molar ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Glycerol acetylation via esterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.1. Reaction pathway for glycerol esterification with acetic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.2. Effects of reaction parameters: catalyst loading, reaction time, molar ratio, and reaction time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2.1. Effects of reaction temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2.2. Effect of catalyst loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2.3. Effect of molar ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2.4. Effect of reaction time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5. Catalyst deactivation and reusability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156. Conclusion and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

.

P.U. Okoye, B.H. Hameed / Renewable and Sustainable Energy Reviews 53 (2016) 558–574 559

1. Introduction

In search for alternative sources of energy to fossil-derived fuel,researchers have focused on producing biodiesel (BD) fuel frombiomass feedstock [1,2]. These feedstocks, including fatty acid-containing materials (triglycerides or fatty acid methyl esters(FAME) in free acid state or linked to other molecules), Jatrophaseed, and algae, can be utilized to produce BD [3]. Fatty acid-containing materials comprise vegetable oils, animal fats, wastegreases, and non-edible oils from seeds (e.g., Jatropha seed). Theestimated global production of BD for 2015 is 3.1�1010 L andprojected to increase by 11�1010 L by the year 2020 [4]. Withincreasing demand for BD, price increase in food-grade materials,such as vegetable oils, threatens the global food security [5]. Assuch, a low-cost alternative route, that is, using low-cost feedstockin BD production, has been proposed to reduce or avoid the uti-lization of food-grade materials [2]. These low-cost feedstocks canbe sourced from non-edible oil sources, waste cooking oil, grease,Jatropha seed oil, and algae [1,6]. Nevertheless, the quality offeedstock utilized in BD production must be assessed for impu-rities, which may reduce the quality and affect the market value ofthe synthesized BD [7].

Triglyceride transesterification is a widely studied, applied, andindustrially utilized synthesis pathway for BD production; thisprocess involves three methanol molecules, a base or acid catalyst,and one- or two-phase reaction systems at ambient conditions orhigh temperatures and pressures to produce BD (FAME) and gly-cerol as byproducts [8]. The United States Department of Energyreported that in clean cities, the average cost per gallon of 100% BDis approximately USD 0.89 higher than that of fossil diesel fuel [9].The high cost of BD could be due to high production cost, which isprimarily driven by high energy requirement. Therefore, the pricedifference between BD and fossil diesel must be eliminated tomake BD production attractive.

Glycerol (1,2,3-propanetriol) [10–15] is the major byproduct ofBD production, that is, 10 kg of glycerol is produced for every100 kg of BD [16–18]. As such, glycerol supply linearly increaseswith increasing BD demand. The global production of glycerolincreased from 200,000 t in 2003 to 600,000 t in 2006; an addi-tional amount of more than 2�106 t of glycerol was produced in2011, and an increment of more than 2�106 t in glycerol supply,including the fractions obtained from the production of oleo-chemicals, was observed in 2012 [19]. Glycerol production isestimated to reach 4.2�109 L by 2020, provided that all market

Table 1Conducted studies on glycerol upgrading, purification steps and applications.

No Conducted studies on glycerol

1. Conducted studies on several reaction pathways such as selective oxidation, seleterification, carboxylation for glycerol upgrading to commodity chemicals.

2. Recent challenges in optimizing glycerol steam reforming process to obtain hydrcurrent catalytic application prospects and kinetics was reviewed with critical ou

3. Studies on glycerol dehydration to acrolein in both petroleum and bio-based procand gaseous phases, techniques employed in acrolein production and industrial cand yield.

4. Teng et al. elucidated various catalyst applications and methods utilized in synthglycerol and carbonate sources. Insights on catalysts performance, reaction influeterification process were critically studied.

5. Tan et al. conducted studies in details about various glycerol purification steps anstill facing some of the purification steps as a barrier for commercialization.

6. Conducted separate reviews on glycerol oligomerization via etherification to prodroutes, application and catalysis of the process with a view of revealing pathway

7. Studied theoretical evaluation utilizing density functional theory calculation foranhydride to produce glycerol acetates. They concluded that acetylation route usanhydride is thermodynamically preferable as the reaction is exothermic. Unfortunarcotics and a contraband chemical.

8. Conducted separate studies on carboxylation and acetylation with listed applicati

forces are constant [4]. The increasing global demand for BD hascreated a glycerol glut, which affects the glycerol market. Forexample, in 2007, the refined glycerol is as cheap as approximatelyUSD 0.30/lb compared with the price (USD 0.7/lb) prior to theexpansion of BD production in the United States. Accordingly, theprice of crude glycerol sharply declined from about USD 0.25/lb toUSD 0.05/lb [20]. In 2013, the cost of refined crude glycerol wasapproximately USD 900–965/t (depending on the raw materialused in BD production), whereas that of crude or unrefined gly-cerol (approximately 80% purity) was nearly four times lower(approximately USD 240/t) in mid-2014. Therefore, the decreasingprice of unrefined glycerol and the high cost of BD productionrequire novel approaches of glycerol utilization to reduce the netenergy requirement for BD production, compensate for the costdifference between BD and fossil diesel, and increase the marketviability of BD. Meanwhile, the sustained low market value ofglycerol may increase its potential for numerous applications.

Glycerol is a stable compound that consists of three hydroxylfunctional groups, which render this compound with hydrophilicand hygroscopic properties. The molecular structure and physi-cochemical properties of glycerol confer the multi-functionality ofthis compound [11]. In an attempt to utilize glycerol, animal feedaggregates were produced using unmodified glycerol in the past[21]. However, the lack of understanding on impurity concentra-tion levels and aggregate ratios of glycerol poses a barrier foreffective utilization [21]. The upgrading of glycerol into value-added products has been recently studied. Table 1 summarizes thefindings of several studies on purification techniques, upgradingroutes, and applications of glycerol.

With continuous vast research on glycerol upgrading, recentstudies have focused on the shift toward the production ofglycerol-free BD. In this concept, glycerol derivatives, such asmonoglycerides or glycerol carbonate (GC), are produced by uti-lizing lipids and various acyl acceptors, instead of methanol [30].Studies indicated that triglyceride transesterification with ethylacetate [31], methyl acetate [32], and dimethyl carbonate (DMC)[33] as acyl acceptors produces three molecules of fatty acidmethyl or ethyl esters and one molecule of GC or glycerol triace-tate (triacetin, TAG). These compounds exhibit the physicochem-ical properties of BD-like fuel and can be utilized in diesel engineswithout modification. Calero et al. [34] studied recent technologiesto produce glycerol-free BD or glycerol-blended BD (Ecodiesels

and Gliperols). In this study, the atom efficiency of these tech-nologies is 100% because no byproduct or glycerol is generated

Ref.

ctive hydrogenolysis, selective dehydration, pyrolysis, selective transes- [22]

ogen fuel with emphasis on thermodynamic behavior of the process,tlook.

[23]

esses and textural properties of the applied solid catalysts for both liquidost feasibility of the process as factors that can affect acrolein selectivity

[10]

esis of glycerol carbonate via catalytic transesterification reaction ofncing parameters and energy effectiveness of the underlying transes-

[14]

d technique with suggestions on industrial applications and challenges [24]

uce hyper branched ether structures; they studied different productions that are more environmentally benign to obtaining glycerol oligomers.

[25,26]

acetylation via esterification of glycerol with acetic acid and aceticing acetic acid is thermodynamically resisted while that of aceticnately, the later has been documented as substance used for synthesis of

[27]

ons of fine chemicals obtained for glycerol upgrading via the two routes. [28,29]

Polymers

Surfactants( Sorbitan glycerol, Polyglycerol,

Oligoglycerol, High purity monoglycerol,

Hyperbranched aliphatic polyether

Chemical intermediates

Solvents (Biobased, protic, Polar)

Electrolyte Liquid Carriers in Lithium batteries

Aggregates in Cement and Concretes

Plant Bolster

Cosmetics (plasticizer and Nail Lacquer

remover)

Detergents

Blower Agents

GLYCEROL CARBONATE

•

•

•

•

•

•

•

•

•

•

P.U. Okoye, B.H. Hameed / Renewable and Sustainable Energy Reviews 53 (2016) 558–574560

in situ and purification steps employed in conventional BD pro-duction that incorporates glycerol is avoided. However, this area ofresearch remains at initial stages, and most studies conductedindicated that enzymes, which are characterized by slow reactionand high costs, can be utilized as catalyst to drive the reaction. Theformed glycerol glut can be utilized as a building block innumerous reaction pathways to synthesize fine chemicals. Ayouband Abdullah [11] reported that fine chemicals derived from low-cost glycerol raw materials can increase profitability and expandthe BD market.

This study presents the progress on the use of two vastly stu-died pathways, namely, catalytic carboxylation and acetylation, forupgrading glycerol into fine chemicals by using DMC and aceticacid to synthesize GC [35] and glycerol acetyl esters (mono-, di-,and tri-acetyl glycerol) [29]. The current trends in catalyst appli-cations; catalyst performance, reusability, and deactivation; andinfluence of reaction parameters on glycerol upgrading arereviewed. Future outlook on potential research gap for furtherstudies are also presented.

Fig. 2. Key applications of glycerol carbonate [14,46].

2. GC synthesis via transesterification

Researchers have recently focused on upgrading glycerol intofine chemicals [6,7,10,12,36–41] through reaction pathways, suchas selective oxidation, dehydration, acetylation, transesterification,steam reforming, hydrogenolysis, and etherification (Fig. 1) [1,42–45]. Fine chemicals, such as acrolein, GC, acetyl esters, 1,3-propa-nediols, and glycolic acids, have been synthesized via these reac-tion pathways by utilizing glycerol and organic reactants at mildreaction conditions. The use of catalyst during glycerol upgradingremains a green approach for chemical activation and fast reactionto obtain a high yield of fine chemicals. This approach is beneficialand important because catalysts reduce energy requirementsduring conversion, increase the yield of the desired products, andcan be synthesized from relatively low-cost materials.

The carboxylation route via the catalytic transesterification ofglycerol has been vastly studied [16,46]. Transesterification utilizesalcohol and carbonate sources in carbonate exchange reaction tosynthesize GC [47]. Several carbonate sources utilized in GCsynthesis include DMC [48],CH4N2O [49–51], CO/O2[52], organic

Fig. 1. Routes for glycerol upgrade to fine chemicals [10, 21, 48].

carbonate source [43], and CO2 [53]. GC is a chemical compoundwith numerous potential applications in industries as an inter-mediate compound, cement composite aggregate, solvent, sur-factant, hyper-branched aliphatic polyether, plant bolster, cos-metics, and electrolytic carrier in lithium-ion batteries (Fig. 2)[14,39].

Two synthesis techniques, namely, direct and indirect synth-eses, have been employed to produce GC [44]; the synthesis routesfor both techniques are summarized in the literature [14]. Tradi-tionally, GC synthesis via transesterification involves the chemicalreaction of glycerol and carbon monoxides or phosgene with themetal catalyst. Phosgene is a poisonous gas and hazardous tohealth upon exposure to a certain concentration [54]. As such,researchers have shifted to a safe and non-health hazardousmethods for GC synthesis via glycerol transesterification withalkylene carbonates. However, the use of alkylene carbonates ascarbonate source in glycerol transesterification comprises a multi-step chemical process, which requires several separation proce-dures, as well as high energy demand, loss of materials, andwastage of time [22]. This limitation of alkylene carbonates leadsto the use of dialkyl carbonates, which are cost effective andenvironmentally harmless [45,55,56]. The utilization of DMC, adialkyl carbonate source, and glycerol to synthesize GC via trans-esterification is a noteworthy endeavor. Glycerol transesterifica-tion with DMC is a promising route to produce GC from a low costand renewable glycerol substrate [57], and this transesterificationprocess is a reversible chemical reaction. Excessive DMC isemployed to positively shift the reaction equilibrium towardobtaining the desired product [57]. The stoichiometric equation ofthe glycerol/DMC reaction to produce GC indicates that methanolcan be produced as a byproduct (Fig. 3).

Glycerol is a thermally stable compound with physicochemicalproperties that require high energy for bond breaking and productformation; thus, a catalyst must be utilized to reduce the energyrequirement for bond breaking and formation [13,46,58]. GCsynthesis via the transesterification of glycerol with DMC largelydepends on the utilization of an efficient and active catalyst underthe optimal reaction conditions [13,46,58].

2.1. Catalyst applications to glycerol transesterification with DMC

Various organic and inorganic base catalysts have been utilizedto synthesize GC from glycerol; these catalysts include metal

Fig. 3. Stoichiometric equation of GC synthesis via glycerol esterification with DMC.

P.U. Okoye, B.H. Hameed / Renewable and Sustainable Energy Reviews 53 (2016) 558–574 561

oxides [59], ionic liquids (ILs) [60], enzymes [61,62], hydrotalcites[63], and mixed metal oxides [49]. These catalysts, which arecategorized as homogenous, heterogeneous, and enzyme catalysts,depending on their role and application in catalysis, have beenutilized to synthesize GC with yields ranging from 50% to 98%[59,64]. Catalysts with basic active centers capable of activatingglycerol molecule through deprotonation are employed in glyceroltransesterification with DMC to synthesize GC. Table 2 summarizesthe advantages and disadvantages of these three groups ofcatalysts.

2.1.1. Homogenous base catalystsHomogeneous catalysts, which contain potassium or sodium

carbonates, alkoxides, hydroxides, and organic ILs, have been uti-lized to synthesize GC. These bases are strong and readily dis-solved in the reaction medium; the dissolved compound forms ahomogeneous mixture with the medium in situ while con-currently activating the functional groups of the reactants, therebyforming the desired products [58]. Ochoa et al. [59] utilizedsodium hydroxide (NaOH), potassium hydroxide (KOH), andpotassium carbonates as strong homogeneous base catalysts forglycerol transesterification with DMC to synthesize GC. The reac-tions were conducted at 75 °C for 90 min at a DMC/glycerol molarratio of 5, and the GC yield was approximately 100%. Pan et al. [65]also used sodium hydroxide under different reaction conditions(70 °C for 2 h at a DMC/glycerol molar ratio of 4) and obtained 77%glycerol conversion and 23% GC selectivity (Table 3). The differencein glycerol conversion and GC selectivity is caused by the variedreaction conditions coupled with the basic strength of thehomogenous catalysts used; such discrepancies can induce GC toreadily decompose in the strong base medium forming glycidol[58,66]. Ochoa et al. [59] demonstrated that the use of homo-geneous catalysis is an effective method because 100% selectivityof glycerol and GC can be attained even at reduced reaction timescompared with the method of Pan et al. [65]. However, alkalihydroxides or carbonate homogeneous catalysts pose serious

Table 2Advantages and disadvantages of homogeneous base- and heterogeneous base-catalyze

Catalyst Advantage Disadvantage

Homogeneous � High conversion and high selectivitytowards glycerol and GC respectively.

� GC obtained undergoes� Separation at the end of

Heterogeneous � Catalysts are easily separated fromthe reaction mixtures.

� The catalyst sources are often rela-tively low-cost materials derivedfrom clay minerals or waste biomass.

� They are tunable meaning that theircomposition and active sites can bevaried to achieve high efficiency.

� They can be easily reused, regener-ated or recycled and hence morestable.

� Depending on the cataleaching of the catalyst

� Catalysts are usually calintensive.

� CaO catalyst deactivates

Enzyme � Mild reaction conditions arerequired.

� Simple enzyme purification fromfinal products required.

� It can be reused.

� They are costly and can� Too slow in activating re

not industrially feasible.� Molecular sieves, deter

towards glycerol and gly

limitations in the separation of the catalyst from the final reactionmixture. For BD, water is utilized in large quantities to removedissolved catalyst from the product because oil and water areimmiscible. However, in the case of GC, water cannot be used; assuch, the removal of the homogeneous catalysts dissolved in situbecomes challenging and requires high energy. Furthermore,green and environmentally harmless organic ILs have beenemployed in glycerol transesterification with DMC to synthesizeGC. The IL 1-n-butyl-3-methylimidazolium-2-carboxylate has beenutilized in GC synthesis, with refined and crude glycerol obtainedfrom a BD plant [67]. The remarkable conversion of the organicliquid (100% and 93% conversion of GC from the refined and crudeglycerol substrates, respectively) demonstrated the promisingpotential of this IL for industrial applications. However, informa-tion on catalyst reusability and stability was not presented.Chiappie and Rajamani [45] used four basic ILs for glyceroltransesterification with DMC at 120 °C at a DMC/glycerol molarratio of 3:1 and for 13 h; the results showed that N-methyl-N-butylmorpholinium dicyanamide IL is thermally stable and can bereused up to four times without significant leaching or decrease incatalytic activity. However, longer reaction time renders inefficientindustrial use.

2.1.2. EnzymesEnzymes are renewable, non-toxic substances with dual (acid–

base function) catalytic properties [68] and have been utilized asbasic catalysts for glycerol transesterification with DMC to syn-thesize GC. The Aspergillus niger lipase immobilized on magneticnanoparticles and the Candida antarctica lipase immobilized onresins NOVOZYM have been employed to synthesize GC at 60 °Cfor 6 h under a DMC/glycerol molar ratio of 10:1 [69]. The catalystwas stable and reused for 15 times, and no leaching or decrease incatalytic activity was detected. Gycerol conversion was low (30–40%, Table 3), whereas DMC/glycerol molar ratio was humongous.The application of enzymes poses some limitations (Table 2), andenzymes are expensive; thus, industrial applications of enzymesare limited.

2.1.3. Heterogeneous base catalystAn alternative to homogeneous catalysts is heterogeneous

catalysts, which have been the recent focus of several researchers.Compared with homogeneous catalyst, heterogeneous catalystsshow similar activity potentials for tailoring the transesterificationreaction to produce the desired high yields under similar reactionconditions. Although some heterogeneous catalysts, such as

d transesterification and enzymes.

Ref

decomposition at longer reaction time to form glycidol.reaction is difficult, expensive and leads to waste.

[41,50]

lyst preparation method, products can be contaminated throughactive sites.cined at high temperature (450–900 oC) for 3–5 h, which is energy

in the presence of water.

[47,48,58]

be easily deactivated by methanol or water.actants to achieve desired products and hence energy intensive and

gents or solvents are usually required to achieve high conversioncerol carbonate.

[55,56]

Table 3Remarks on catalyst performance and reaction conditions in glycerol transesterification with DMC.

Catalyst Reaction parameters Selectivity Solvent Remarks Ref.

Temp. oC R.T. (h) MR. (DMC:GL)

C.L XGL XGC

LiNO3/Mg4AlO5.5b 80 1.5 3:1(DMC) 4 w% 100 96.28 – Doping Lithium with the hydrotalcites formed strong basic LiAlO2 ion that promoted the high

GC yield. Beyond the optimum Li loading, 5 w%, GC formed decomposes to glycidol showingthat basicity limit (H_418) is exceeded.

[64]

CHT-HMS (Al:Mg¼1:2) 170 3 3:1(DMC) 15%w/w 84.7 84.3 DMF Boiling point of DMC is 90 °C and uncontrolled evaporation or splashing can be initiated at thisreaction temperature. High catalyst loading undermines the cost effectiveness as separationand recovery can lead to waste.

[18]

MgO–Sb 90 0.5 2:1(DMC) 5 w% 76.3 98.8 – Addition of surfactant (triblock copolymer F127) increased the MgO surface area and a cor-responding increase in basic site concentration. However, MgO is weakest of basic group IIoxides and its glycerol conversion is low even with excess DMC and surfactant.

[70]

ZnO/La2O3 mixed oxidesb 150 2 4:1 0.5 w% 98.5 97.2 – Glycidol formation was observed on La2O3 sites because of its high basicity and the reactiontemperature requires condenser to control evaporation of DMC.

[71]

K-Zeoliteb 75 1.5 3:1(DMC) 4 w% 100 96 Methanol Zeolites suffer knocking and deactivation due to its pore structure. Also solvents are mostlyrequired to drive the conversion process. Glycidol formation 4 w% was reported because ofdecarboxylation of GC.

[16]

BaCO3/Cb 140 2 5:1(DMC) 18.5 w% 97.8 98.5 – Again temperature is too high, catalyst loading and the molar ratio of glycerol/DMC are highand not cost effective. Carbon deposition on the catalyst surface was observed.

[72]

Mg/Al hydrotalcite 70 3 3:1(DMC) 0.45 g 66.9 97.1 Methanol Glycerol conversion below 90%, primarily caused by solubility limitations of hydrotalcites. Thisinforms the conventional use of high temperatures when using hydrotalcites. Addition ofmethanol as a solvent, increases the methanol/DMC azeotropic ratio (usually 30:70 w/w) andno azeotropic distillation is incorporated in this process.

[17]

Mg1þxCa1�xO2 70 1.5 2:1(DMC) 3 w% 100 100 – Combined catalytic ability of Mg and Ca oxides was explored with the x suggesting anincrement or a decrease in Mg and Ca ions respectively. Their reason was that they observedthat MgO had favored high glycerol conversion with low GC yield while CaO is vice versa.Therefore a balance was struck and With continuous removal of methanol, 100% yield of GCwas achieved.

[73]

NaOH/γ-Al2O3b 78 1 2:1(DMC) 3 w% 97.9 99 – 80 w% loading of NaOH on Al2O3 gave a high GC selectivity. High NaOH loading which is not

cost effective is required to increase the basicity of the catalyst. Leaching was observed as thealumina support do not have strong bond interaction with the NaOH thereby making finalproduct separation difficult.

[56]

Mg/Al hydrotalcite 100 2 3:1(DMC) 10 w% 66 100 – High catalyst loading, calcination temperature and higher reaction temperature was a strongcontributing factor of the catalysis without solvent. However, glycerol conversion below 90%and low.

[63]

NAY Zeoliteb 70 4 4:1(DMC) 0.5 g 80 100 Methanol Methanol solvent was used to drive the process. The performance could be attributed to thecatalyst textural properties. However, reaction time is longer than others and glycerol con-version is below 90%

[65]

MgOb 75 0.5 2:1(DMC) 3 mol% 20.5 49.9 – Again MgO is the weakest of group II metal oxides, and has low solubility in glycerol/DMCmedium thus active sites are too slow to generate.

[55]

CaO 75 0.5 2:1(DMC) 3 mol% 91.2 98.9 – CaO has high basicity (basicity H_418). However may suffer knocking or deactivates inexposure to air or water. Do not readily dissolve in water because it is uncalcined as in thiscase yet selectivity for glycerol and GC is above 90%.

[55]

CaOb 75 0.5 2:1(DMC) 3 mol% 94.3 99.7 – Similar in activity to CaO described above but with increased surface area and more soluble inreaction medium due to its calcination. Therefore, active sites are generated faster forincreased selectivity.

[55]

Na2O 75 0.5 2:1(DMC) 3 mol% 95.5 96.9 – Na2O has strong basic sites and reaction proceeds fast with high selectivity but with a high riskof glycidol formation due to strong basicity as reported.

[55]

CaOa,b 80 2 1:1(DMC) 2 w% 98 95 – Reactive coupling and azeotropic distillation method using benzene as the azeotropic agentwas employed to constantly remove methanol. Therefore, at stoichiometric ratio of glycerol/DMC (1:1), all the DMC reacted and methanol removal favored more glycerol conversion andGC selectivity. However, benzene is carcinogenic and not ecofriendly.

[74]

Homogenous applicationsNaOH 70 2 4:1(DMC) 0.2 g 77 23 Methanol It was reported that GC decomposes readily on homogenous base catalyst thereby forming

glycidol as a byproduct in large quantity. Separation is very difficult and energy intensive.[65]

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Homogeneous application continuesK2CO3 73–75 3 3:1(DMC) 4.5 w% 100 97 – Although high glycerol conversion and selectivity of GC is recorded and cannot be reused for

many runs hence not industrially feasible.[47]

K2CO3 73-75d 48 10:1(DMC) 4.5 w% 100 18 – High glycerol conversion with a poor selectivity towards GC, 18%. Unwanted glycidol formationis high because of the excessive dimethyl carbonate and long reaction time of 2 days.

[47]

Enzyme applicationA.N Lipase (immobilized on magnetic Nano-particles)

60 6 10:1(DMC) 5 w% 45 92 – Low glycerol conversion observed. DMC: GLY is high and deemed cost ineffective. Also, catalystdenatures at a higher temperature above 80 oC. The catalyst is robust and stable up to fifteentimes reuse without leaching.

[69]

A.N Lipase (immobilized on magnetic Nano-particles)c

60 6 10:1(DMC) 5 w% 35.3 90 – Crude glycerol from sunflower oil was used. Glycerol performance was low 35.3% and blow90%. Also, high DMC/glycerol molar ratio characterized the process. Impurities in the crudeglycerol may likely contribute to the poor performance of the catalyst.

[69]

Enzyme application continuesA.N Lipase (immobilized on magnetic Nano-particles)

60 6 10:1(DMC) 28.6 w% 61 90 – High enzyme loading and hence not industrially feasible because of high cost of enzymes. Theglycerol and DMC was of analytical grade with high purity therefore contributing to a betterconversion and selectivity of glycerol and GC respectively when compared with the resultsobtained using crude glycerol above.

[75]

C.A lipase immobilized on NOVOZYM 60 48 2:1(DMC) 75 g/L – C¼96.25e – The reaction took 2 days to reach 96.25% yield and catalyst loading is high. Therefore, it ishighly energy intensive and not feasible industrially.

[76]

Ionic and organic catalystsIonic liquid (DBU) 100 7.5 3:1(DMC) 0.1 mol% 98 96 – Selectivity and conversion was above 90% with a very low catalyst loading. However this was

achieved with a longer reaction time that is energy intensive. There is no report on reusabilityor stability of this catalyst.

[77]

Ionic liquid (BMIM-2-CO2) 74 1.33 3.2:1(DMC) 1 mol% – C¼100% – Again, the stability of this catalyst is not highlighted especially as regards leaching of the ionsinto the reacting mixture and its reusability. No side product was formed and GC conversionwas high.

[67]

Ionic and organic catalystsIonic liquid (BMIM-2-CO2

f) 74 5 3.2:1(DMC) 5 mol% – C¼93% – Crude glycerol was used and this catalyst showed good potential for utilizing crude glyceroland overcoming impurity barriers that hinders high conversion and yield of glycerol and GC.

[67]

Note: C.L¼Catalyst loading, MR¼Molar ratio, XGL and XGC¼selectivity of glycerol and glycerol carbonate respectively, R.T is the reaction time and HT is Hydrotalcitea Coupling reaction and Azeotropic distillation system involved using benzene.b Calcination of catalyst at 450 °C, 900 °C for 3 h to overnight.c Crude glycerol obtained from transesterification of residual sun-flower oil was used.d Immobilized on magnetic-Nano particles with crude glycerol obtained from transesterification of crude sun-flower oil was used.e C ¼GC conversionð%Þ ¼ mol of GC

Mol of glycerol � 100%.f Crude glycerol from biodiesel plant.

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zeolites, suffers knocking and deactivation because of poorlystructured channels and other drawbacks (Table 1), the reactionmedium can be separated by ordinary centrifugation, therebyreducing high energy demands for separation, which are usuallyencountered when using homogeneous catalysts. Heterogeneouscatalysts, such as alkaline metal oxides and mixed oxides (Mg/CaO,NaOH/γAl2O3, CaO, Na2O) [54,55,64], hydrotalcites (Mg/Al) andmodified hydrotalcite materials (LiNO3/MgAl), zeolites, and silica-based materials functionalized by metal oxides have beenemployed to synthesize GC via glycerol transesterification withDMC. These three categories of catalysts are summarized inTable 3.

Apart from synthesizing potentially efficient catalysts, under-standing the reaction pathway of glycerol transesterification withDMC and the effects of reaction-influencing parameters arenecessary to tailor the reaction to produce the desired product.These reaction-influencing parameters can be tuned appropriatelyto obtain the optimum yield. The plausible reaction pathway andreaction conditions, such as temperature, time, molar ratio, andcatalyst loading, are discussed in Section 3.

3. Reaction pathway for glycerol transesterification with DMC

GC synthesis via glycerol transesterification with DMC cannotproceed without adding a catalyst [65]. Studies on heterogeneoussolid base catalysts, such as alkali metal oxide or alkali-mixedmetal oxide catalysts, alkali-metal-functionalized zeolite catalysts,and hydrotalcite catalysts, provide insights into the catalysis andreaction mechanisms of base-catalyzed transesterification of gly-cerol with DMC to produce GC [18,55,59,64,65,78,79]. A three-stepreaction mechanism has been identified in the transesterificationreaction [18]:

1. The glycerol molecule is activated on the active sites of thecatalyst by hydrogen abstraction to form glyceroxide anion inthe presence of base catalysts.

2. The glyceroxide anion then reacts with DMC to form a complexintermediate (hydroxyl alkyl carbonate) and a methoxide anion.

3. Further intramolecular transesterification in the presence of abase catalyst generates cyclic carbonate (i.e., GC) and methanol.

Fig. 4. Base-catalyzed reaction mechanism

When a highly basic catalyst (basicity H_418) is involved intransesterification, GC formed in situ in the reaction mediumdecomposes and forms glycidol, as reported by Pan et al. [65]. Thevarious reaction pathways proposed are conceptual and based onthe peculiarity of applications. Studies showed that in a hetero-geneous solid base-catalyzed transesterification of glycerol withDMC to synthesize GC and methanol, a unified reaction pathwaycan be adopted as the fundamental principle of the reactionmechanism (Fig. 4) [15,45,54–55,59].

3.1. Effects of reaction parameters: catalyst loading, temperature,and molar ratio

The transesterification reaction must be tailored to obtain highconversion and selectivity (selectivity and conversion are mathe-matically defined in the supplementary notes in Tables 3 and 4) ofglycerol and GC, respectively. Specific reaction-influencingparamters such as temperature, which affects energy demand intransesterification; catalyst loading; and stoichiometric ratio ofgycerol/DMC must be optimized to determine the conditionscontrolling the rate of adsorption, desorption, and surface reactionof transesterification of glycerol with DMC. Yadav et al. [18]reported that transesterification with a hydrotalcite catalyst for GCsynthesis are mainly controlled in three steps: adsorption of gly-cerol and DMC into the catalyst pores, surface reaction in theactive sites between the basic sites and glycerol/DMC, and deso-rption of the produced GC. A pausible and popular methodadopted by most researchers to optimize these parameters is byvarying one variable or parameter while keeping the other para-meters constant.

3.1.1. Catalyst loading and textural properties of heterogenous solidbase catalyst

Heterogeneous catalysts, such as alkali-metal oxides or alkalinemixed metal oxides, basic zeolites, mixed metal oxides, andhydrotalcites [44,65,70], have been applied in transesterification.Understanding the reactivities of these catalysts, especially alka-line earth metals and alkali metals, is crucial. These metals aresuper bases, and their initial loading depends on the weight ofglycerol used in transesterification. For alkali-metal or alkalinemixed oxide metals, 0.5–5 wt% catalyst loading is the optimal

of glycerol transesterification [64].

Table 4Heterogeneous catalytic applications in glycerol esterification with acetic acid with remarks.

Catalyst Acidity Reaction parameters Selectivity (%) Remarks Ref.

C.L MR. (AA:GL) Temp. (oC) R. T (h) MAG DAG TAG Xg

TPA3/MCM-41 0.627 mmol/gs 0.15 g 6:1 100 6 25 60 15 87 Exhibited more selectivity to DAG because of the strong bronsted acid sites, longerreaction time and a stable kegging ion support. Has higher conversion of glycerol thanTPA3/ZrO2 discussed below, however could be affected by elevated temperature due toblockage of acid sites.

[87]

TPA3/ZrO2 0.840 mmol/gw 0.15 g 6:1 100 6 60 36 4 80 Combined selectivity favored MAG and DAG than TAG. This is ascribed to the weakacidity of the support. However, it exhibited higher surface area due to monolayercoverage on the support than MCM-41 on impregnation with different TPA loadings.

[87]

Amberlyst-15 4.9 mmol/g 0.64 g 6:1 80 8 21.1 63.8 15.1 100 All catalysts are prepared as fine powder (o180 mm) to overcome mass transfer lim-itations. Varying agitation speed was found not to have significant effect on conversionsuggesting that liquid film resistance is negligible.

[97]

Silica–alumina 3.2 mmol/g 0.64 g 6:1 80 8 88.5 11.2 0.3 71 The conversion of glycerol and selectivity towards the preferred DAG and TAG washigher in PrSO3H-SBA-15 and Amberylst-15 compared to other tested catalysts. Theorder of catalysts activity in terms of conversion and selectivity was PrSO3H-SBA154Amberlyst-154 HPMo/Nb2O5Z HPMo/SBA154HUSY4SCZ4Silica-Alumina. Sul-fonic functional group hydrolysis was observed in SO3H-SBA-15 and SO3H-cell cat-alyst and their high performance was ascribed to homogeneous behavior of thecatalyst. Turnover rate (TOR) conversion of glycerol and selectivity towards DAG andTAG was investigated. Increase in temperature from 80 °C–100 °C, has a significantinfluence on selectivity of DAG and TAG and glycerol TOR, is not linearly proportionalto acid strength of the catalyst.

HUSY 0.9 mmol/g 0.64 g 6:1 80 8 72.7 25.7 1.6 94PrSO3H-SBA15 1.2 mmol/g 0.64 g 6:1 80 8 15.8 64.6 19.6 100HPMo/SBA15 0.6 mmol/g 0.64 g 6:1 80 8 77.1 22.0 0.9 96

HPMo/Nb2O5 0.7 mmol/g 0.64 g 6:1 80 8 81.8 17.5 0.7 87 The turnover rates were in the order:PrSO3H-SBA-154Amberlyst-154HPMo/SBA-15EHPMo/Nb2O54HUSY4SCZ4SiO2–

Al2O3. Also, the turnover frequency shows no dependency in pore size as catalysts withhigher pore diameter (SCZ pore diameter¼4.3 nm), showed lower activity than smallerpore size catalyst (HUSY pore diameter 0.9¼nm). However, Amberlyst-15 is thermallyunstable.

[97]SCZ 4.1 mmol/g 0.64 g 6:1 80 8 84.7 14.8 0.5 81SO3H-SBA15 0.8 mmol/g 0.64 g 6:1 80 8 11.1 61.9 27.0 100SO3H-Cell 1.3 mmol/g 0.64 g 6:1 80 8 37.6 55.0 13.4 100

Amberlyst-15 4.7 eq/kg 5 w% 6:1 105 10 0 12.3 83.9 100 Higher TAG achieved was as a result of azeotropic distillation system with liquidentrainer (toluene) incorporated in the process for continuous water removal formedin situ during esterification reaction. Amberlyst-15 and 70 had the highest yield oftriacetin. Although Amberlyst-15 had more surface area and higher bronsted acidity,amberlyst-70, slightly better performance was observed in amberlyst-70 because it ismore thermally stable with higher cross-linkages. Heteropoly acids (STA and STP),immobilized on the silica showed inferior selectivity towards TAG even with largesurface area as compared to the resins because of internal diffusion limitations.

[35]Amberlyst-70 2.55 eq/kg 5 w% 6:1 105 10 0 7.5 87.6 100STA/S11 274 mmol/g 5 w% 6:1 105 10 1 55.5 35.8 100STP/S11 154.9 mmol/g 5 w% 6:1 105 10 4.9 71.3 21.8 100

A-36 5.4 0.25 g 1:8 105 10 70.3 4.5 – 95.6a To revaluate glycerol, glycerol in excess was used instead of acetic acid and acetic acidconversion is determined as opposed to other studies that determine glycerol con-version. Resins of gel-type with varying cross linkages and macro reticular type wereemployed. No TAG was detected probably because of excess glycerol and completesubstitution of the hydroxyl groups in glycerol by the acetic acid group cannot beachieved to form TAG even after 10 h of reaction. Hence this method cannot beemployed when TAG is desired. Leaching is not detected or homogeneous catalysis.

[90]A-15 4.7 0.25 g 1:8 105 10 70.3 2.5 – 95.3a

D-2 4.8 0.25 g 1:8 105 10 80.8 5.1 – 95.2a

D-4 – 0.25 g 1:8 105 10 71.6 4.2 – 94.8a

D-8 – 0.25 g 1:8 105 10 72.9 4.7 – 94.7a

Amberlyst-15(dried) 4.7 2.65 g 9:1 110 5 7.8 47.7 44.5 97.1 It was reported that complete conversion of glycerol necessarily do not lead to higherDAG and TAG selectivity even at a higher temperature because longer time is requiredto form higher esters (Dag and TAG) in three consecutive step reaction process. Themolar ratio was predominantly a factor that influences the distribution of the esters.They found out that at low molar ratio of 3:1, higher temperature did not have sig-nificant effect on conversion of glycerol. The K values were dependent on molar ratioand temperature. Also, the catalyst is thermally unstable at temperature (120 °C) andlonger time exposure. In the absence of catalyst, MAG formation was predominant witha yield 460% and about 5% of DAG even at lower reaction time70 minutes meaningthat catalyst is only necessary for DAG and TAG formation.

[46]Amberlyst-15(wet, moisture 3.2%) – 2.65 g 9:1 110 5 18.5 43.2 38.3 93.5Blank (without catalyst) – – 9:1 110 4.20 84.7 13.8 1.5 73.5

SBAH-15(15)j 0.86 mmol/g 0.4 g 6:1 110 3 14 67 19 100 Surfactant (pluronic F127) used enhances the mesoporous silica catalyst texturalproperties (pore volume and diameter increase). The incorporated MPA moieties of the

[96]S–Mo(15) 0.69 mmol/g 0.4 g 6:1 110 3 38 53 9 74

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Table 4 (continued )

Catalyst Acidity Reaction parameters Selectivity (%) Remarks Ref.

C.L MR. (AA:GL) Temp. (oC) R. T (h) MAG DAG TAG Xg

same loading (15 w%) via thermal decomposition in the uncalcined SBA-15 gave highersurface acidity compared to a similar calcined silica material (S–Mo) with a decrease intextural physical quantities (pore volume and diameter). Beyond optimum reportedmolar ratio (AA/glycerol¼6:1), hydrolysis of DAG and TAG to MAG was observed.However, increasing the temperature from 100-110 increases the endothermic ester-ification reaction promoting 100% glycerol conversion and more DAG and TAGformation.

PW2_AC – 0.2 g 16:1 120 3 25 63 11 86 Increase in HPA loading above 4.9 wt% reduces the surface area of the activated carbon,AC. internal diffusion limitations is likely to be a barrier since AC is microporous.Calcination4450 °C saw the kegging structure oxidized to β-MoO3 but can be restoredto kegging structure at exposure in polar environment. Finally, higher molar ratio andtemperature is energy intensive and likely to lead to undesired side products.

[88]

AC-SA5 890 mmol/g 0.8 g 8:1 120 3 38 28 34 91 Compared to the already discussed activation carbon catalyst functionalized with HPA,sulfonic acid functionalized AC in this case gave higher TAG. This is because of thestrong acid sites and large surface area as reported. SO4

2- ions can be said to be lighterthan HPA that decreases AC surface area at higher loading ( BET surface area decreasedsignificantly on adding the HPA acid (836 m2/g AC-389 m2/g PW2_AC) compared to(780 m2/g AC-742 m2/g AC-SA5)). The catalyst was stable with slight decrease inactivity ascribed to mass transfer limitations or carbon deposition on the surface andpores of the catalyst active centers.

[105]

PMo3_NaUSY – 0.2 g 10.5:1 – 3 37 59 2 68 Above optimum PMo loading (1.9 w%), lower catalytic activity was reported due tointernal diffusion limitations ascribed to reduced pore volume, surface and pore size ofthe catalyst. Low conversion of glycerol and TAG may be attributed to low acid strengthand temperature used was not reported in the article. However, the catalyst can bereused up to four times without significant decrease in activity.

[92]

Amberlyst-15 4.2 mol base/g 0.47b g 3:1 110 .5 31 54 13 97 Strong bronsted acidity of amberlyst-15 resulted in better performance compared toother screened catalyst. Zeolites showed a better acidity than K-10; however, its per-formance was low because of knocking and diffusion limitations brought about by poresize. The HZSM-5 is reported to be hydrophobic (Si/Al ratio is higher and HUSY-zeoliteis highly hydrophilic and can be easily deactivated by in situ polar environment. Thisproperty makes the former had better performance in spite of its lower acidity than thelater. In all cases, α-hydroxy-acetone (Acetol) was produced due to dehydration ofglycerol occurring at the third hydroxyl functional group of glycerol. This was attrib-uted to the strong bronsted acid sites on the catalyst surface. However, the reactiontime is not enough to reach equilibrium since the formation of DAG and TAG occursthrough consecutive reaction steps that requires longer time.

[42]K-10 0.5 mol base/g 4.0b g 3:1 110 .5 44 49 5 96Niobic acid 0.3 mol base/g 6.3b g 3:1 110 .5 83 – – 30HZSM-5 1.2 mol base/g 1.6b g 3:1 110 .5 83 10 – 30

Ar-SBA-15 1.15 meq/g 0.2 g 9:1 125 4 15e 47e 38e 96e Optimization using factorial design indicated that molar ratio, of acetic acid/glycerol ismore dominant than other reaction parameters. Comparing the three sulfonic acidmoieties activities, it was observed that the acid strength or activity per active centerpromotes the reaction Catalysis. The acid strength is in increasing order fromProAroF and the acid capacity is FoAroPr. Therefore, Ar-SBA-15 was identified tohave higher selectivity towards DAG and TAG because of its combined optimum acidcapacity and acid strength in 2 h of reaction time (Ar-64%4F-42%4Pr-17%). At 4 h ofreaction time, it was observed that the reaction regime is equilibrium controlled and nosignificant changes in selectivity irrespective of the solid catalyst type used.

[89]F-SBA-15 1.04 meq/g 0.2 g 9:1 125 4 14e 50e 36e 90e

Pr-SBA-15 0.30 meq/g 0.2 g 9:1 125 4 17e 44e 39e 80e

Note: TPA is tungstophosphoric acid, Zr is Zirconium oxide, MCM-41 is synthesized silica based support, HPA is heteropolyacids, PW is dodecatungstophosphoric acid, SCZ is sulfated ceria–zirconia, HPMo is dodecamolybdo-phosphoric acid, Nb is Nobium, HUSY and HZSM-5 are Zeolites, STA is silicotungstic acid, S11 is silica, A is amberlyst grades (15,36), D is the Dowex resina with their cross linkages from 2%, 4% and 8%, SBAH-15(15) is hybrid Silicabased catalyst with 15 wt% MPA loading, MPA is molybdophosphoric acid.1. s ¼ strong acidity (bronsted acidity) measure using NH3-TPD above 500 °C ammonia desorption temperature, W¼ weak acidity detected using NH3-to under 200 °C.2. Xg¼ conversion ¼(initial mol%�final mol%/ initial mol%), Selectivity (%) ¼moles of product formed/moles of substrate consumed x 100%.3. b ¼ catalytic loading to achieve 2 mmol of acid sites in each experiment.4. e ¼ the estimated values obtained from the bar plot.5. a ¼ is the conversion of acetic acid.6. j¼ modified with P123 and Brij S100 surfactant.

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amount. Hydrotalcites containing magnesium and aluminummixed oxide catalyst require high loading to obtain high output.Yadav et al. [18] observed that an increment from 0.001 g/cm3 to0.004 g/cm3 of the hydrotalcite catalyst quantity promoted thecatalytic activity of glycerol/DMC transesterification, which isdriven by the increased number of available basic sites. However,quantities higher than 0.002 g/cm3 are insignificant, indicating thesaturation of active sites in the reaction medium. Similar resultswere observed in other studies [64,78]. The high catalyst loadingrequirement and high temperature (up to 18 w/w% i.e., w/w¼catalyst/glycerol weight) required to activate the glycerolmolecules for the transesterification reaction may be attributed tothe weak basic sites (e.g., Mg ions) of hydrotalcites. Among groupII metal oxides, MgO has been identified as the weakest and haspoor solubility in hydroscopic medium [80,81]. Hydrotalcites aredouble-layered hydroxides with anion exchange capabilities andpromising potential for stable catalysis [82]. Structural modifica-tion can enhance the basic strength of hydrotalcites, which couldbe due to the form of doping with strong active bases. Liu et al.[64] reported that doping Mg/Al hydrotalcite with a strong basicalkali metal (LiNO3) modified the structural activity of the hydro-talcite material and achieved 100% glycerol conversion and 96.3%GC selectivity. This modification converts the weak basic sites ofthe hydrotalcites into strong basic sites by incorporating strongbases in their active centers, resulting in higher conversion andyield compared with the use of hydrotalcite material alone. Fur-thermore, low catalyst loading could result in low conversion ofglycerol and poor selectivity of GC because of the limited numberof active sites required to abstract proton from the glycerolmolecule.

3.1.1.1. Basicity and pore structure of catalysts. The basicity or thebasic strength of a catalyst for transesterification reaction are keycharacteristics of a catalytic material and can be optimized ortuned to achieve satisfactory performance. The basicity of anycatalyst can be determined using the Hammett indicators orthermal adsorption–desorption techniques. Hammett indicatorsinclude bromthymol blue, (H_¼7.2), phenolphthalein (H_49.8),2,4-dinitroaniline (H_415.0), and 4-nitroaniline (H_418.4),whereas the basicity of a base catalyst increases in the followingorder: H_¼7.24H_¼9.8 H_¼15.04H_¼18.4 [64]. Severalreviewed studies have adopted a basicity range of 9.0oH_o18.4to obtain glycerol conversion and GC selectivity of over 50% byusing heterogeous solid base catalysts [16,56,64]. Beyond thisbasicity range (basicity H_418.4), the GC produced is likely todecompose to form glycidol during transesterification.

Apart from catalyst bascitiy, the catalyst pore structure (volumeand diameter) containing the active pore centers play a key role inreaction species activation, mass transfer, and diffusion of reactingmolecules. The mesoporous (pore diameter 2–50 nm) materialswith 12-numbered ring-structured pore channels promote higheradsorption and desorption of reactants and products, resulting inhigher GC and glycerol conversion compared with 8- or 10-numbered ring-structured pore channels [65]. Therefore, thepore diameter of a catalytic material can impose a barrier tomolecule diffusion. This phenomenon was observed in zeolite typematerials with microporous pore diameter and poorly structuredpore channels. Pan et al. [65] investigated the pore channels ofzeolite types 3A, 4A, NaZSM-5, Naβ, and NaY and found that 3A,4A, and NaZSM-5 zeolites possess 8-or 10-numbered ring-structured channel and perform poorly in transesterificationcompared with the 12-numbered ring-structured channel of Naβand NaY zeolites. Further investigation on the relationship of dif-fusion and accessibility of reactants and products to the porechannels of the catalyst were conducted; the geometrical para-meters of the glycerol molecules (length¼0.52 nm,

diameter¼0.47 nm), DMC (length¼0.45 nm, diameter¼0.37 nm),and GC (length¼0.65 nm, diameter¼0.33 nm) were obtainedfrom the length and diameters of the reactants and transester-ification products [65]. The results evidently showed that a cata-lytic material with a pore diameter or length smaller than those ofthe commercial-grade reactants and product can impose limita-tions in either adsorption, active pore channel reactions, or deso-rption, leading to low GC conversion and selectivity. Therefore,mesoporous and macroporous catalysts are prefered so that a highyield of GC will be achieved.

3.1.2. Reaction temperatureTransesterification is a reversible reaction, and temperature is

related to the equlibrum constant thermodynamically obtainedfrom the Arrehenius equation [50]. As such, reaction temperatureinvariably affects the reaction rates of glycerol transesterificationwith DMC [59]. Collision theory explains the role of temperature inchemical reactions, thereby providing better insights into theeffects of temperature on glycerol transesterification. For non-spontaneous chemical reactions, that is, sufficient energy isrequired to overcome the barrier imposed by the viscosity of thereactants, the increase in reaction temperature results in rapidmolecular movement and subsequent vigorous collision amongthe reacting species; consequently, the misciblity of DMC andglycerol is improved and the chances of bond cleavage and rear-rangement increase, leading to product formation.

The synthesis routes of GC have been vastly studied usingcarbonate sources and catalysts at 35–140 °C under atmosphericpressure for glycerol transesterification. The optimum transester-ification temperature is dependent on carbonates sensitive to heat[14]. Wai et al. [14] reported in their review that a high GC yield of87% can be achieved at 35 °C by using ethylene carbonate; how-ever, increasing the temperature to higher than 35 °C reduced theGC yield.

DMC or diethyl carbonate as carbonate sources presents a dif-ferent case, in which temperatures ranging from 40 °C to 80 °C arerequired to shift the equilibrum consant and increase the reactionrates to ensure high GC yields. The use of heterogeneous catalystrequires temperatures within the range of 70–150 °C. Similarly,most heterogeneous solid metal oxides and mixed metal oxidecatalysts (except MgO) require temperatures within the range of70–100 °C, resulting in glycerol conversion and GC selectivity ofmore than 90% during glycerol transesterification with DMC[55,56,64,70,73,74]. Utilizing hydrotalcites as catalysts demandshigh reaction temperatures (100–150 °C) to achieve more than80% glycerol conversion and GC yield because of solubilitylimitations.

In addition to the heat sensitivities of various carbonate sourcesfor transesterification, physical properties, such as DMC and gly-cerol boiling points, is necessary to guide appropriate reactiontemperature selection and to know the limits to avoid hazardousconditions. Glycerol and DMC are highly stable hydrophilic andhydrophobic compounds with boiling points of 290 °C and 90 °C,respectively. Therefore, using temperatures higher than theseboiling points during transesterification may lead to uncontrol-lable evaporation of the reactant molecules, thereby reducing GCyield or promoting glycidol formation through GC decarboxylation(Eq. 6, Fig. 4).

3.1.3. Effects of glycerol/DMC molar ratioGC synthesis via glycerol transesterification with DMC is a

reversible reaction [16]; thus, a molar ratio higher than the stio-chiometric ratio of DMC is usually required to shift the equilibrumconstant and the reaction toward the production of high GCyeields. Table 3 shows that a DMC/glycerol molar ratio of 2 to 5:1is ideal and can increase the conversion and selectivity of glycerol

Fig. 5. Acid-catalyzed reaction mechanism of glycerol acetylation with acetic acid [91].

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and GC by using heterogeneous solid-base catalysts in a shortreaction time (e.g., 30 min to 2 h). Malyaadri et al. [48] reportedthat an increase in DMC/glycerol molar ratio from 2 to 5 increasedGC yield; at molar ratios beyond the optimal DMC/glycerol ratiofor heterogeneous solid base-catalyzed transesterification reac-tion, glycerol conversion decreased with a corresponding decreasein GC yield.

Alogufi and Hameed [16] reported that equal molar ratio ofDMC and glycerol resulted in a low glycerol conversion ofapproximately 50% by using K-zeolite derived from coal fly ash. Bycontrast, Li and Wang [74] utilized equimolar amounts of DMC/glycerol with the CaO catalyst for transesterification; this methodincorporated an azeotropic distillation system and yielded glycerolconversion and GC yield of more than 50%. The azeotropic dis-tillation system was designed to continuously remove themethanol byproduct, thereby shifting the equilibrium and tailor-ing the reaction to increase GC yields.

Methanol is a byproduct of glycerol transesterification withDMC (Fig. 5); methanol–DMC azeotropic mixture exists in trans-esterification, as indicated by their common narrow boiling points[74]. Therefore, solvents that break the binary azeotrope areessential to easily separate final products and achieve high GCyields [83]. The relative volatility of the binary azeotrope formedby methanol–DMC can be altered by using a suitable azeotropicagent; as such, eliminating the azeotrope facilitates the separationof methanol from DMC [84]. These azeotropic agents also aid inreducing the amount of excess DMC/glycerol molar ratio, which isrequired to positively shift the equilibrium toward high GC yields.Azeotropic agents, also known as organic solvents, enhance thesolubility or miscibility of hydrophilic glycerol and hydrophobicDMC [14]; these agents include dimethylformamide (DMF),dimethyl sulfoxide (DMSO), acetonitrile, tetrahydrofuran, tert-butanol, and ethanol or methanol [18,65]. Teng et al. [11] reportedthat the addition of the optimal amount of solvents in glycerol

transesterification with DMC increased the GC yield from 17% to99% in uncalcined Mg/Al hydrotalcite catalyst. In addition, themiscibility of glycerol with DMC increased upon the addition ofvarious organic solvents, in a decreasing order of DMSO, DMF, andDMA [85].

Benzene, cyclohexane, dichloroethane, n-hexane, ethyl acetate,isooctane, n-heptane, and cyclohexene are potential azeotropicagents for transesterification reaction systems incorporated withazeotropic distillation [74]. These entrainers can also form anazeotrope with methanol. Benzene is the most active and suitableazeotropic agent and generates 98% GC yield, even at a stoichio-metric molar ratio of 1:1 DMC/glycerol. The binary mixture ofmethanol and benzene formed during transesterification wasdistilled, whereas all unreacted DMC was retained in the reactor[74]. However, azeotropic distillation are usually energy intensive,and benzene is carcinogenic and thus is not ecofriendly [86].

4. Glycerol acetylation via esterification

Glycerol acetylation via esterification is another vastly studiedroute for glycerol conversion into value-added chemicals [14] byusing acetyl sources to synthesize mono-, di-, and tri-acetyl esters[87–91] (or often referred to as monoacetin (MAG), diacetin (DAG),and TAG, respectively) in numerous industrial applications [88].MAG is used in explosive production, solvents for dye, and treat-ment of animal skin for leather manufacturing, whereas DAG canbe employed as a solvent, plasticizer, and softening agent [87].MAG and DAG can be utilized in cryogenics (where studies areconducted at low temperatures) and as raw materials for theproduction of biodegradable polyesters [92,93]. TAG can be usedfor improving diesel combustion speed improver to reduce nitro-gen oxide emission in exhaust gas [94]. The blend of TAG with fuelcontributes to complete combustion because it reduces carbon

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molecule content in the fuel mixture and serves as a good anti-knocking agent [94]. Furthermore, blending BD fuel with 10% TAGconsiderably improves direct-injection diesel engine performance[95]. Apart from its fuel additive properties, TAG is also utilized asan antimicrobial and emulsifying agent in pharmaceuticals andcigarette filters [87].

Esterification is traditionally performed using mineral acids ashomogenous catalysts to obtain glycerol esters (MAG, DAG, andTAG) [42,89]. These homogeneous catalysts include sulfuric acidand hydrofluoric acid, which are highly toxic, corrosive, andhazardous. Moreover, separation and recovery requires sophisti-cated techniques and energy-intensive multi-step processes [46].Similarly, the effluent disposal of these acids poses challenges thatmay lead to environment, technical, and economic problems[46,96]. Therefore, the use of environmentally harmless hetero-geneous solid catalysts has been proposed [89,97] because theirstrong Bronsted acidity [96] can be used to tailor glycerol acet-ylation and achieve high yield and easy regeneration. Hetero-geneous catalysts used in glycerol esterification with acetylationagents include sulfonic acid-functionalized zeolite support [87],ion exchange resin [90], mixed oxide catalyst [91], and hetero-polyacids [88].

Carboxylic acids are environmentally harmless and mostlyutilized as the acetyl source for glycerol esterification to produceacetyl esters [98]. The two carboxylic acids conventionally used forglycerol esterification are acetic acid [27] and acetic anhydride[99]; the latter is synthesized by dehydration of acetic acid and hasbeen used to produce high amounts of acetyl esters, such as DAGand mainly TAG [100]. Zeolites including Amberlyst-15 acid resinand k10-montmorillonite catalysts have been applied as acid cat-alysts in laboratory-scale glycerol esterification with acetic anhy-dride at 60 °C to attain 100% TAG selectivity [99]. Acetic anhydrideperformance for 100% TAG selectivity is remarkable because TAG isa fuel additive necessary for pollution reduction. However, aceticanhydride, which is used in morphine synthesis, is considered acontraband substance; the use of this compound is punishable bylaw in many countries and is highly restricted in laboratories forresearch, thereby making acetic anhydride as an expensive and ascarce commodity [100]. Moreover, the versatility of the value-added products or the vast range of products obtained from gly-cerol esterification adds more value and induces the dynamic useof glycerol esterification, instead of synthesizing only one product(TAG) from the process.

The use of safe acetic acids (vinegar) is another method used toobtain various products from glycerol esterification [101]. Severalstudies were conducted using acetic acid, with different hetero-geneous catalysts under the optimal reaction conditions to syn-thesize acetyl glycerol esters (MAG, DAG, and TAG)[46,94,96,102,103]. TAG exhibits lower selectivity [97] comparedwith MAG and DAG because of slow conversion that is likelycaused by water molecules formed in situ in the three-step reac-tion. To increase TAG selectivity, water formed in situ is constantlyremoved, or otherwise deactivates the catalyst, to positively shiftthe chemical equilibrium [29,104]. Apart from constant waterremoval to promote acetylation to TAG, 100% TAG selectivity hasbeen achieved through glycerol esterification with acetic acid andacetic anhydride in a two-step process using Amberlyst-35 (detailscan be found in [104]). The details of several catalysts utilized inglycerol esterification with acetic acid are summarized in Table 4.

4.1. Reaction pathway for glycerol esterification with acetic acid

Glycerol esterification with acetic acid for the synthesis ofacetylated esters is a reversible equilibrium reaction that proceedsin three consecutive steps [98]:

(1) Protons in the catalyst attach to the oxygen lone pair electronof the acetic acid, followed by a nucleophilic attack of theglycerol hydroxyl group by electrophiles, thereby releasingwater molecules to produce deprotonated MAG (Eq. 7) [46].Depending on the glycerol molar ratio and reaction conditions,different MAG isomers can be formed in situ during esterifica-tion [46,90]. MAG formation does not strictly depend on thepresence of a catalyst; however, high selectivity towards diand tri acetyl esters and high yields largely depend on thecatalyst type and reaction conditions [46]. The water moleculeis also formed as a byproduct.

(2) Further acetylation via a second nucleophilic attack on thehydroxyl group of the formed MAG in Eq. (7) generates DAGand therefore forms more water as a byproduct of thereaction.

(3) Finally, a third lone pair electron on the oxygen of the lasthydroxyl group in DAG undergoes a third nucleophilic attackto produce TAG and more water (Eq. 9 in Fig. 5). In general,three moles of the water molecule is formed in the consecu-tive steps of esterification to form TAG.

4.2. Effects of reaction parameters: catalyst loading, reaction time,molar ratio, and reaction time

Glycerol acetylation with acetic acid in the presence of an acidcatalyst involves a three-step reversible equilibrium reaction fromMAG to TAG. The reaction behavior under optimal conditions hasbeen studied by researchers to develop kinetic and thermo-dynamic models through acetylation. These models were devel-oped using experimental or mechanistic data to describe theprocess, demonstrate the effects of reaction parameters on reac-tion rate, and elucidate principles governing reaction rates andselectivity [98]. Thermodynamic parameters, such as Gibbs freeenergy and activation energy of esterification, were calculatedunder different reaction conditions [46,87]. Glycerol esterificationwith acetic acid follows a first-order rate law and can be expressedmathematically using the Langmuir–Hinshelwood–Hougen–Was-ton model, with surface reaction as the rate-determining step [46].

Mufrodi et al. [101] and Liao et al. [27] calculated the Gibbs freeenergy and activation energy (Arrhenius Equation) of the trans-esterification reaction of glycerol with DMC; the results showedthat each of the three reaction steps evaluated exhibits positiveGibbs free energy and activation energy values, indicating thatglycerol acetylation with acetic acid is a sequence of consecutivethermodynamically resisted reactions [27,101]. Further investiga-tion showed that the Gibbs free energies of MAG and DAG for-mation were lower than that of TAG, which explains the lowselectivity of TAG. However, high acetic acid to glycerol molar ratiocan be used to shift the equilibrium reaction to obtain a satisfac-tory TAG value, although this approach is not cost effective andindustrially feasible.

Although a balanced equation for the three consecutive steps ofglycerol esterification with acetic acid describes the quantitativerelationships between the amounts of reactants used and theamounts of products formed, the equation provides no informa-tion on the rate of a given reaction. Reaction rate is obtained bystudying the chemical kinetics of a reaction and is dependent onvarious factors, such as reactant concentrations (molar ratio ofglycerol to acetic acid), reaction temperature, and catalyst loading.The obtained information provides insights into the appropriateparameter that should be modified to achieve the highest yield.

4.2.1. Effects of reaction temperatureGhoreishi et al. [102] observed that increasing the temperature

from 50 °C to 110 °C promoted glycerol conversion and high acetylester (DAG and TAG) selectivity at the expense of MAG; they

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concluded that high temperatures promote glycerol conversionand DAG and TAG selectivity, whereas low temperatures enhancesMAG formation. A similar study conducted using zirconia-supported silicotungstic acids revealed that increasing the tem-perature from 60 °C to 120 °C linearly increased the glycerol con-version from 54.4% to 100%, with a corresponding increase in DAGand TAG at the expense of MAG, under an endothermic reaction[29]. Khayoon et al. [106] utilized yttrium-functionalized SBA-3catalyst at temperatures from 90 °C to 110 °C; they found thatglycerol conversion increased from 65% to 100% at 110 °C, althoughfurther temperature increase showed no significant effect on TAGyield. This finding indicates that the optimum temperature for thereaction is achieved and the equilibrium is attained. Therefore,TAG selectivity in glycerol esterification with acetic acid is tem-perature dependent because only 7% of TAG was produced attemperatures below 90 °C compared with 55% selectivity at 110 °C.Similarly, Mufrodi et al. [107] reported that increase in reactiontemperature from 50 °C to 110 °C increased glycerol conversionfrom 58.8% to 98% and TAG selectivity from 5% to 70%; however,MAG selectivity decreased from 75% to 10% by using hetero-geneous tungstic oxide catalyst WO3 supported on polypyrrole(polymer material). The optimum temperature should be selectedbased on the material used for catalyst synthesis. For instance, theconventional and industrially preferred ion-exchange resins arethermally unstable at temperatures higher than 120 °C and arelikely to be deactivated, resulting in reduced catalytic activity[104]. Basing on these studies, we can conclude that as the numberof collisions between the mobile atoms increases with increasingreaction temperature, the reaction rates and constants alsoincrease; this increase can promote rapid glycerol conversion, witha corresponding increase in DAG and TAG selectivity at theexpense of MAG.

4.2.2. Effect of catalyst loadingA catalyst reduces the activation energy of a reaction and

promotes fast formation of the desired products. Studies showedthat increase in catalyst loading promotes the acetylation of MAGand DAG to TAG. This finding possibly resulted from the increasednumber of available and accessible active acid sites in the reactingmixture. Khayoon and Hameed [105] investigated glycerol ester-ification with acetic acid by using the sulfated activation carboncatalyst; increase in catalyst loading (0.2–0.8 g) linearly increasesglycerol conversion and TAG selectivity. However, further increasebeyond 0.8 g yielded no significant conversion or selectivity,indicating the occurrence of saturation. Similarly, the influence ofcatalyst loading from 0.1 g to 0.4 g was investigated using thesilicotungtic acid catalyst supported on zirconia and showed that aslight increase in catalyst loading from 0.1 g to 0.2 g slightlyincreased glycerol conversion from 97.7% to 100%; further increasein catalyst loading up to 0.3 g promoted higher selectivity of DAGand TAG, although catalyst loading over 0.3 g showed no sig-nificant effect on selectivity or conversion [29]. Ghoreishi et al.[107] investigated the influence of catalyst loading by usingpolypyrrole-supported WO3 under catalyst loading from 0.1 g to0.5 g while all other reaction conditions constant (temperature of110 °C, molar ratio of acetic acid: glycerol¼6, and reaction time of10 h); the results showed that the selectivity of MAG decreasedfrom 45% to 5%, whereas that of TAG increased from 15% to 70%with a slight increase in glycerol conversion. The slight increase inglycerol conversion reported by Zhu et al. [29] and Ghoreishi et al.[107] suggests that the conversion during acetylation is not largelydependent on catalyst; hence, MAG production can equally pro-ceed without a catalyst. This finding was supported by a blankexperiment without catalyst performed by Zhou et al. [46]; in thisstudy, 73.5% glycerol and 84.7% MAG were obtained. Therefore, anincrease in catalyst generally promotes higher DAG and TAG

selectivity but minimally affects MAG formation, thereby con-firming the occurrence of a three-step glycerol esterification ofMAG to TAG.

4.2.3. Effect of molar ratioThe influence of molar ratios of reacting components on

esterification is also elucidated. The stoichiometric equation forMAG, DAG, and TAG production indicated that 3 mol of acetic acidis required to react completely with 1 mol of glycerol to produceTAG. Predictably, DAG and TAG can only be produced when excessacetic acid is used during esterification. The excessive acetic acidpositively shifts the equilibrium toward higher DAG and TAGselectivity.

Popova et al. [91] observed that an excess amount of acetic acidrelative to glycerol (10:1) increased the selectivity of DAG and TAGand reduced the time for the reaction to reach equilibrum. Pateland Singh [87] reported that varying the molar ratio of glycerol toacetic acid from 1:1 to 1:9 increased glycerol conversion; theoptimal conversion was reached at a glycerol/acetic acid ratio of1:6 but did not significantly increase at molar ratios higher than1:6. In contrast to Popova et al. [91] Zhou et al. [46] reported thathigher acetic acid to glycerol ratio delays the product and reactionequilibrum. In their study, the molar ratio of acetic acid was variedto 3:1, 6:1, and 9:1 by using the Amberlyst resin catalyst duringesterification. The results showed that the mixture of the reactingspecies showed greater influence on the equilibrum distribution ofthe reaction. However, high acetic acid ratios increase glycerolconversion and selectivity toward DAG and TAG at the optimalacetic acid/glycerol ratio of 9:1 within 300 min. Finally, Mufrodiet al. [94] reported that the glycerol conversion was 0.28% at amolar ratio of acetic acid to glycerol of 1:1 and increased to 98.5%as the molar ratio was increased to 6:1; accordinly, DAG and TAGselectivity increased, whereas that of MAG decreased. This findingsuggests that increase in molar ratio increases glycerol conversion,as well as acetyl ester formation, and that the stoichiometricvalues of glycerol and acetic acid can effectively produce MAG.

4.2.4. Effect of reaction timeTo investigate the speed of acetylation to reach the equilibrum,

we reviewed studies on the influence of reaction time. Ghoreishiet al. [107] reported a linear relationship between the increase intime and glycerol conversion rate in a 10-h reaction; they observed70% TAG selectivity within 10 h and a corresponding reduction inMAG formation. Consequently, DAG selectivity increased from 25%to 50% within 6 h of reaction and then decreased. Hence, a part ofthe formed DAG was further acetylated into TAG. Similarly, Ghor-eishi et al. [102] observed a linear relationship between reactiontime and increased acetyl ester formation, which could be ascribedto the consecutive reaction from esterification to MAG formationand then transesterification or further acetylation to form DAG andTAG. Zhu et al. [29] also found that glycerol conversion reached93% within 30 min and 100% in 2 h by using zirconia-supportedsilicotungtic catalyst; in addition, MAG selectivity at 30 min washigher than that of TAG (o7%). As time progressed, higherselectivity increased DAG and TAG yields.

Khayoon et al. [106] reported that increase in reaction timeincreased glycerol and acetyl ester conversion and selectivity. Incontrast to Zhu et al. [29], who observed slight TAG formation(o7%) within 30 min of reaction, Khayoon et al. [106] reported noTAG formation within the first 30 min of reaction. This disparity islikely caused by the differences in experimental conditions and theuse of different catalyts during esterification.

Table 5Deactivation and leaching of heterogeneous catalysts.

Catalysts Process Reaction conditions No. ofreuse

Report/reasons for catalyst deactivation Ref.

MgO-S5 Transesterification ofglycerol with DMC

DMC:Gly¼2, 90 °C, t¼90 min, C.L¼5 w% 5 Direct use without treatment indicated 17% for first reuse and subsequent ones due to strong adsorption ofglycerol carbonate on the catalyst active sites. Heat treatment and calcination at 500 °C in air for 2 hours gave68.6% of glycerol carbonate compared to 83.6% of the initial use but was maintained for the five runs. Surfacearea decreases from 40 m2/g to 34 m2/g due to agglomeration.

[65]

CHT-HMS Transesterification ofglycerol with DMC

DMC:Gly¼3:1, 170 °C, t¼3 h, C.L¼0.002 g/cm3 5 Observed slight decrease in catalyst activity about 2.2% and 2.3% for glycerol and glycerol carbonate respectively.It is caused by catalyst handling during experiment. The catalyst activity was maintained by washing withmethanol and oven drying at 120 °C for each run.

[17]

BaCO3/C-2 Transesterification ofglycerol with DMC

DMC:Gly¼5:1, 140 °C, t¼2 h, C.L¼18 w% 4 Catalytic activity declined (70.1% glycerol conversion and 94.4% glycerol carbonate selectivity) due to deacti-vation caused by carbon deposition in the mesopores of the catalyst after the fourth reuse. Also, the bindingenergy of Ba (II) was altered and surface area decreased.

[72]

Mg1.8Ca0.8O2 Transesterification ofglycerol with DMC

DMC:Gly¼2:1, 70 °C, t¼90 min, C.L¼0.3 g 4 Leaching of metallic species detected about 7.4%. Catalyst activity reduced insignificantly and the reduction wasattributed to unreacted glycerol not properly removed during washing with methanol.

[70]

3Mg/La1 Transesterification ofglycerol with DMC

DMC:Gly¼2:1, 85 °C, t¼90 min, C.L¼5 w% 4 Direct reuse without treatment after each run showed catalyst activity decline from 81% to 35%. However, only69.1% glycerol carbonate yield can be attained after calcination. The activity decline is caused by the drasticallyreduced surface area (32.5–0.28 m2/g) after experiment.

[109]

ZnO/La2O3 Transesterification ofglycerol with DMC

DMC:Gly¼6:1, 150 °C, t¼2 h, C.L¼0.5 wt% 4 Catalyst activity drops from 97.3% to 63.96% due to deposition of reaction products on the active sites. But can befully regenerated by calcination at 500 °C for 3 h.

[71]

K-zeolite Transesterification ofglycerol with DMC

DMC:Gly¼3:1, 75 °C, t¼90 min, C.L¼4 w% 6% reduction from original conversion and selectivity of glycerol and glycerol carbonate (100% and 96%respectively) was observed. Decrease in catalytic activity is attributed to handling that decreased availablesurface area.

[16]

Amberlyst Acetylation usingacetic acid and aceticanhydride

AA:Gly¼0.3:0.1, 105 °C, t¼4 h, C.L¼0.5 g; after15 min add acetic anhydride.

5 No significant deactivation recorded even when reaction time is extended to 12 hours. However, likely deacti-vation may occur via thermal degradation when reaction is performed at temperature limits of the catalyst(120 °C).

[93]

3%Y/SBA-3 Acetylation AA:Gly¼4:1, 110 °C, t¼2.5 h, C.L¼0.20 g 4 Catalytic activity decreased after 3 run from 100% to 80% for glycerol conversion and 55% to 50% for TAGselectivity. The decrease was ascribed to mass transfer limitations and reaction conditions.

[106]

SAS, SSBA,Amberlyst-15

Acetylation AA:Gly¼3:1, 105 °C, t¼1 h, C.L¼50 mg 5 SAS catalytic activity declined significantly due to leaching of the sulfonic functionalities from the catalyst.Amberlyst-15, was stable up to 3rd cycle then declined to 60 °C at the end of the fifth cycle. SSBA catalyticactivity was stable with no significant decrease in activity and no leaching detected.

[92]

HSiW/ZrO2,HPW/ZrO2

Acetylation AA:Gly¼10:1, 120 °C, t¼4 h, C.L¼0.3 g 4 Constant catalytic activity was observed with HSiW/ZrO2 catalyst for all the cycles while HPW/ZrO2 catalystactivity decreased slightly for cycle 1 and 2 then the glycerol conversion was remained constant. The constantperformance of HSiW/ZrO2 catalyst was ascribed to its hydrolytic stability in the in situ polar environment andstrong bond interaction between HSiW and ZrO2.

[27]

SBAH-15(15) Acetylation AA:Gly¼6:1, 110 °C, t¼3 h, C.L¼0.38 4 Slight decrease of glycerol conversion and combined DAG and TAG was observed. However, catalytic activity ofthe catalyst stabilized for the 3 and 4 cycle indicating that active sites cannot be leached out. The stabilization in3 and 4 cycle is attributed to the restoration of the kegging structure on exposure of beta-MoO3 species to thein situ byproduct water environment.

[84]

Note. C.L¼catalyst loading, t¼reaction time in hours (h) or minutes (min), Gly¼glycerol.

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5. Catalyst deactivation and reusability

Catalytic deactivation is the loss of catalyst activity over time,and variations in deactivation time relatively depend on the typesof applications and contaminants. The deactivation mechanismreported by Argyle and Bartholomew can be generally classified asthermal degradation, fouling, poisoning, leaching accompanied bytransport from catalyst surface or particle, vapor–solid and/orsolid–solid reaction, and crushing/attrition [108]. Thermal degra-dation is usually observed in polymers, resins, or catalyst used forcracking of petroleum fractions. Catalyst poisoning, which is eitherreversible or irreversible, is characterized by strong adsorption of areactant, an intermediate or a product in the active sites of thecatalyst, thereby preventing access of other reacting species to thecatalytic sites. Details on heterogeneous catalyst deactivation andregeneration can be found in a previous study [108]. However,catalyst decay is inevitable, regardless of the use of acid or basecatalyst. Studies on carboxylation and acetylation with the use ofheterogeneous catalysts revealed that leaching and contaminants,such as water, methanol, carbon deposition in active sites, andhigh temperature, evidently reduce catalytic activities [92,93].Reusability test is generally conducted by repeated use of the samecatalyst with or without treatment to evaluate the deactivationtime of a given catalyst. In addition, leaching test is performed todetermine the stability of the catalyst by adopting the hot filtrationmethod [72]. Various heterogeneous solid catalysts for carbox-ylation and acetylation can be used up to four cycles, and deacti-vation in these routes is characterized by loss of their active cen-ters. This loss could be due to reduced surface area, which isusually ascribed to problems on mechanical handling, carbondepositions in the active centers of the catalyst, leaching of activefunctional groups that are loosely bound on the catalyst surface,and catalyst deactivation caused by in situ polar environment,especially in glycerol esterification with acetic acid. Withoutregeneration/treatment before each reuse, catalytic activity andperformance decline rapidly [65]. Therefore, various methods havebeen adopted by researchers in regenerating catalysts after eachcycle to achieve a relatively stable and constant activity [71,109].Techniques such as washing with solvents (ethanol, methanol,water, and acetone), drying (from 4 h to overnight and even days),and calcination have been employed to achieve stable outputwithout significant decrease in catalytic activity [17,27,71,84].However, recycling often results in wastage of time and energy;thus far, no study has reported the reuse of a catalyst for morethan six cycles. Table 5 summarizes several heterogeneous catalystused for deactivation and/or leaching studies on carboxylation andacetylation of glycerol with DMC and acetic acid.

6. Conclusion and future trends

By reviewing several studies on glycerol transesterification, wefound that homogeneous and heterogeneous catalysts can be usedto tailor carboxylation and acetylation of glycerol with DMC andacetic acid to obtain desired products, namely, GC and acetylesters. Although enzymes have been utilized for glycerol transes-terification with DMC to synthesize GC, enzyme properties (longerreaction time, vulnerability to poisoning, etc.) and low productyield seriously limit their industrial application. The focus ofresearch thus shifted to homogeneous catalysts because they canbe utilized under mild reaction conditions to tailor both carbox-ylation and acetylation of glycerol to achieve appreciably highproduct yields. However, the limitations caused by the difficultiesin separating the catalyst from the reaction medium, disposalproblems, recycling, and corrosion of reactors and pipes impera-tively increase the net energy of the upgrading routes, which adds

to the production cost. These limitations encountered in usingenzymes and homogeneous catalysts lead to the recent shift ofresearch focus on heterogeneous catalysis. Studies evidentlyshowed that high yields and mild reaction conditions comparablewith those when using homogeneous catalysis can be achieved byutilizing heterogeneous catalysts; moreover, heterogeneous cata-lysts can be easily separated from the reacting mixtures for recy-cling. However, side product formation, knocking resulting frompoorly structured pore channels, catalyst deactivation or thermaldegradation, and leaching of functional groups require furtherstudies. Tuning of reaction parameters have also been conductedbased on trial and error methods and are therefore subject tovariations, which may increase the energy requirement of theupgrading routes. Finally, the limitations of heterogeneous cata-lysts open avenues and new frontiers for researchers to enhancethe properties of heterogenous catalysts and develop methodsdevoid of variations to optimize reaction parameters. The opti-mized parameters can then be used to alter the carboxylation andacetylation of glycerol with DMC and acetic acid to obtain thedesired products, namely, GC and glycerol acetyl esters.

Acknowledgment

The authors acknowledge the financial support provided byMinistry of Education, Malaysia under the TransdisciplinaryResearch Grant Scheme (TRGS) Phase 2/2014 (203/PJKIMIA/6762002).

References

[1] Serrano-Ruiz JC, Luque R, Sepúlveda-Escribano A. Transformations ofbiomass-derived platform molecules: from high added-value chemicals tofuels via aqueous-phase processing. Chem Soc Rev 2011;40:5266.

[2] Xiu S, Shahbazi A. Bio-oil production and upgrading research: a review.Renew Sustain Energy Rev 2012;16:4406–14.

[3] Leoneti AB, Aragão-Leoneti V, de Oliveira SVWB. Glycerol as a by-product ofbiodiesel production in Brazil: alternatives for the use of unrefined glycerol.Renew Energy 2012;45:138–45.

[4] OECD, FAO. Biofuels. Agric Outlook 2011–2020 2011:77–93.[5] Alka D, Neetu M, Vishakha B. Biofuels : impact on food productivity, land use

environment and agriculture. Int J Environ Res Dev, 4; 2014. p. 9–16.[6] Fan X, Burton R. Recent development of biodiesel feedstocks and the appli-

cations of glycerol: a review. Open Fuels Energy Sci J 2009;2:100–9.[7] Zheng Y, Chen X, Shen Y. (Chemical Reviews (2008) 108 (5253)). Chem Rev

2010;110:1807.[8] Leung DYC, Wu X, Leung MKH. A review on biodiesel production using cat-

alyzed transesterification. Appl Energy 2010;87:1083–95.[9] BourbonE. Clean Cities Alternative Fuel Price Report. Washington DC: 2015.[10] Talebian-Kiakalaieh A, Amin NAS, Hezaveh H. Glycerol for renewable acrolein

production by catalytic dehydration. Renew Sustain Energy Rev 2014;40:28–59.

[11] Ayoub M, Abdullah AZ. Critical review on the current scenario and sig-nificance of crude glycerol resulting from biodiesel industry towards moresustainable renewable energy industry. Renew Sustain Energy Rev2012;16:2671–86.

[12] Bagheri S, Julkapli NM, Yehye W a. Catalytic conversion of biodiesel derivedraw glycerol to value added products. Renew Sustain Energy Rev2015;41:113–27.

[13] Fan X, Burton R, Zhou Y. Glycerol (Byproduct of Biodiesel Production) as asource for fuels and chemicals-mini review�!2009-08-26�!2010-02-02�!2010-04-09�!. Open Fuels Energy Sci J 2010;3:17–22.

[14] Teng WK, Ngoh GC, Yusoff R, Aroua MK. A review on the performance ofglycerol carbonate production via catalytic transesterification: effects ofinfluencing parameters. Energy Convers Manag 2014;88:484–97.

[15] Nguyen NT, Demirel Y. A novel biodiesel and glycerol carbonate productionplant. Int J Chem React Eng 2011;9.

[16] Algoufi YT, Hameed BH. Synthesis of glycerol carbonate by transesterificationof glycerol with dimethyl carbonate over K-zeolite derived from coal fly ash.Fuel Process Technol 2014;126:5–11.

[17] Zheng L, Xia S, Hou Z, Zhang M, Hou Z. Transesterification of glycerol withdimethyl carbonate over Mg–Al hydrotalcites. Chin J Catal 2014;35:310–8.

[18] Yadav GD, Chandan P a. A green process for glycerol valorization to glycerolcarbonate over heterogeneous hydrotalcite catalyst. Catal Today2014;237:47–53.

P.U. Okoye, B.H. Hameed / Renewable and Sustainable Energy Reviews 53 (2016) 558–574 573

[19] Ciriminna R, Della Pina C, Rossi M, Pagliaro M. Understanding the glycerolmarket. Eur J Lipid Sci Technol 2014:1432–9.

[20] KerrBJ, DozierWA, IIIBK. Nutritional Value of Glycerin for Nonruminants. In:Proceedings of the 23rd Annual Carolina Swine Nutrition ConferenceRaleigh, NC; 2007, p. 6–18.

[21] Yang F, Hanna M a, Sun R. Value-added uses for crude glycerol – a byproductof biodiesel production. Biotechnol Biofuels 2012;5:13.

[22] Zhou C-HC, Beltramini JN, Fan Y-X, Lu GQM. Chemoselective catalytic con-version of glycerol as a biorenewable source to valuable commodity che-micals. Chem Soc Rev 2008;37:527–49.

[23] Silva JM, Soria MA, Madeira LM. Challenges and strategies for optimization ofglycerol steam reforming process. Renew Sustain Energy Rev 2015;42:1187–213.

[24] Ardi MS, Aroua MK, Hashim NA. Progress, prospect and challenges in gly-cerol puri fi cation process : A review 2015;42:1164–73.

[25] Martin A, Richter M. Oligomerization of glycerol-a critical review. Eur J LipidSci Technol 2011;113:100–17.

[26] Gholami Z, Abdullah AZ, Lee KT. Dealing with the surplus of glycerolproduction from biodiesel industry through catalytic upgrading to poly-glycerols and other value-added products. Renew Sustain Energy Rev2014;39:327–41.

[27] Liao Xiaoyuan, Zhu Yulei, Wang Sheng-Guang, Hongmei Chen YL. Theoreticalelucidation of acetylating glycerol with acetic acid and acetic anhydride.Appl Catal B Environ 2010;94:64–70.

[28] Lanjekar K, Rathod VK. Utilization of glycerol for the production of glycerolcarbonate through greener route. J Environ Chem Eng 2013;1:1231–6.

[29] Zhu S, Zhu Y, Gao X, Mo T, Zhu Y, Li Y. Production of bioadditives fromglycerol esterification over zirconia supported heteropolyacids. BioresourTechnol 2013;130:45–51.

[30] Luna C, Verdugo C, Sancho ED, Luna D, Calero J, Posadillo A, et al. Productionof a biodiesel-like biofuel without glycerol generation, by using Novozym435, an immobilized Candida antarctica lipase. Bioresour Bioprocess2014;1:11.

[31] Kim Sok-Joong, Jung Sang-Min, Park Yong-Cheol, Park K. Lipase catalyzedtransesterification of soybean oil using ethyl acetate, an alternative acylacceptor. Biotechnol Bioprocess Eng 2007;12:441–5.

[32] Tan KT, Lee KT, Mohamed AR. A glycerol-free process to produce biodiesel bysupercritical methyl acetate technology: an optimization study via ResponseSurface Methodology. Bioresour Technol 2010;101:965–9.

[33] Ilham Z, Saka S. Two-step supercritical dimethyl carbonate method for bio-diesel production from Jatropha curcas oil. Bioresour Technol2010;101:2735–40.

[34] Calero J, Luna D, Sancho ED, Luna C, Bautista FM, Romero AA, et al. Anoverview on glycerol-free processes for the production of renewable liquidbiofuels, applicable in diesel engines. Renew Sustain Energy Rev2015;42:1437–52.

[35] S Kale U Armbruster S Umbarkar M Dongare A Martin. Esterification ofglycerol with acetic acid for improved production of triacetin using tolueneas an entrainer In: Proceedings of the 10th Green Chemistry Conference 201370 71.

[36] Dasari M. Crude glycerol potential described. Feedstuffs 2007 2006–8.[37] Rahmat N, Abdullah AZ, Mohamed AR. Recent progress on innovative and

potential technologies for glycerol transformation into fuel additives: a cri-tical review. Renew Sustain Energy Rev 2010;14:987–1000.

[38] Pagliaro M, Ciriminna R, Kimura H, Rossi M, Della Pina C. Recent advances inthe conversion of bioglycerol into value-added products. Eur J Lipid SciTechnol 2009;111:788–99.

[39] Pagliaro M, Ciriminna R, Kimura H, Rossi M, Della Pina C. From glycerol tovalue-added products. Angew Chemie-Int Ed, 46; 2007. p. 4434–40.

[40] Katryniok B, Paul S, Dumeignil F. Recent developments in the field of cata-lytic dehydration of glycerol to acrolein. ACS Catal 2013;3:1819–34.

[41] Da Silva GP, Mack M, Contiero J. Glycerol: A promising and abundant carbonsource for industrial microbiology. Biotechnol Adv 2009;27:30–9.

[42] Gonçalves VLC, Pinto BP, Silva JC, Mota CJ a. Acetylation of glycerol catalyzedby different solid acids. Catal Today 2008;133-135:673–7.

[43] Hervert B, McCarthy PD, Palencia H. Room temperature synthesis of glycerolcarbonate catalyzed by N-heterocyclic carbenes. Tetrahedron Lett2014;55:133–6.

[44] Ochoa-Gómez JR, Gómez-Jiménez-Aberasturi O, Ramírez-López C, Belsué M.A brief review on industrial alternatives for the manufacturing of glycerolcarbonate, a green chemical. Org Process Res Dev 2012;16:389–99.

[45] Chiappe C, Rajamani S. Synthesis of glycerol carbonate from glyceroland dimethyl carbonate in basic ionic liquids. Pure Appl Chem 2011;84:755–62.

[46] Zhou L, Nguyen TH, Adesina A a. The acetylation of glycerol over amberlyst-15: kinetic and product distribution. Fuel Process Technol 2012;104:310–8.

[47] Rokicki G, Rakoczy P, Parzuchowski P, Sobiecki M. Hyperbranched aliphaticpolyethers obtained from environmentally benign monomer: glycerol car-bonate. Green Chem 2005;7:529.

[48] Malyaadri M, Jagadeeswaraiah K, Sai Prasad PS, Lingaiah N. Synthesis ofglycerol carbonate by transesterification of glycerol with dimethyl carbonateover Mg/Al/Zr catalysts. Appl Catal A Gen 2011;401:153–7.

[49] Rubio-Marcos F, Calvino-Casilda V, Bañares M a, Fernandez JF. Novel hier-archical Co3O4/ZnO mixtures by dry nanodispersion and their catalyticapplication in the carbonylation of glycerol. J Catal 2010;275:288–93.

[50] Li J, Wang T. Chemical equilibrium of glycerol carbonate synthesis fromglycerol. J Chem Thermodyn 2011;43:731–6.

[51] Indran VP, Syuhada Zuhaimi NA, Deraman MA, Maniam GP, Yusoff MM, YunHin T-Y, et al. An accelerated route of glycerol carbonate formation fromglycerol using waste boiler ash as catalyst. RSC Adv 2014;4:25257.

[52] Hu J, Li J, Gu Y, Guan Z, Mo W, Ni Y, et al. Oxidative carbonylation of glycerolto glycerol carbonate catalyzed by PdCl2(phen)/KI. Appl Catal A Gen2010;386:188–93.

[53] Aresta M, Dibenedetto A, Nocito F, Pastore C. A study on the carboxylation ofglycerol to glycerol carbonate with carbon dioxide: the role of the catalyst,solvent and reaction conditions. Atmos Environ 2007;41:407–16.

[54] Li J, Wang T. Coupling reaction and azeotropic distillation for the synthesis ofglycerol carbonate from glycerol and dimethyl carbonate. Chem Eng ProcessProcess Intensif 2010;49:530–5.

[55] Simanjuntak FSH, Kim TK, Lee SD, Ahn BS, Kim HS, Lee H. CaO-catalyzedsynthesis of glycerol carbonate from glycerol and dimethyl carbonate: iso-lation and characterization of an active Ca species. Appl Catal A Gen2011;401:220–5.

[56] Bai R, Wang Y, Wang S, Mei F, Li T, Li G. Synthesis of glycerol carbonate fromglycerol and dimethyl carbonate catalyzed by NaOH/??-Al2O3 Fuel ProcessTechnol 2013;106:209–14.

[57] Lu P, Wang H, Hu K. Synthesis of glycerol carbonate from glycerol anddimethyl carbonate over the extruded CaO-based catalyst. Chem Eng J2013;228:147–54.

[58] Climent MJ, Corma A, De Frutos P, Iborra S, Noy M, Velty A, et al. Chemicalsfrom biomass: Synthesis of glycerol carbonate by transesterification andcarbonylation with urea with hydrotalcite catalysts. The role of acid–basepairs. J Catal 2010;269:140–9.

[59] Ochoa-Gómez JR, Gómez-Jiménez-Aberasturi O, Maestro-Madurga B,Pesquera-Rodríguez A, Ramírez-López C, Lorenzo-Ibarreta L, et al. Synthesisof glycerol carbonate from glycerol and dimethyl carbonate by transester-ification: catalyst screening and reaction optimization. Appl Catal A Gen2009;366:315–24.

[60] Chen J, Wang C, Dong B, Leng W, Huang J, Ge R, et al. Ionic liquids as eco-friendly catalysts for converting glycerol and urea into high value-addedglycerol carbonate. Chin J Catal 2015;36:336–43.

[61] Waghmare GV, Vetal MD, Rathod VK. Ultrasound assisted enzyme catalyzedsynthesis of glycerol carbonate from glycerol and dimethyl carbonate.Ultrason Sonochem 2014;22:311–6.

[62] Kim SC, Kim YH, Lee H, Yoon DY, Song BK. Lipase-catalyzed synthesis ofglycerol carbonate from renewable glycerol and dimethyl carbonate throughtransesterification. J Mol Catal B Enzym 2007;49:75–8.

[63] Liu P, Derchi M, Hensen EJM. Synthesis of glycerol carbonate by transester-ification of glycerol with dimethyl carbonate over MgAl mixed oxide cata-lysts. Appl Catal A Gen 2013;467:124–31.

[64] Liu Z, Wang J, Kang M, Yin N, Wang X, Tan Y, et al. Structure-activity cor-relations of LiNO3/Mg4AlO5.5 catalysts for glycerol carbonate synthesis fromglycerol and dimethyl carbonate. J Ind Eng Chem 2015;21:394–9.

[65] Pan S, Zheng L, Nie R, Xia S, Chen P, Hou Z. Transesterification of glycerolwith dimethyl carbonate to glycerol carbonate over Na–based zeolites. Chin JCatal 2012;33:1772–7.

[66] Alvarez MG, Segarra AM, Contreras S, Sueiras JE, Medina F, Figueras F.Enhanced use of renewable resources: Transesterification of glycerol cata-lyzed by hydrotalcite-like compounds. Chem Eng J 2010;161:340–5.

[67] Naik PU, Petitjean L, Refes K, Picquet M, Plasseraud L. Imidazolium-2-carboxylate as an efficient, expeditious and ecofriendly organocatalyst forglycerol carbonate synthesis. Adv Synth Catal 2009;351:1753–6.

[68] Hruby SL, Shanks BH. Acid-base cooperativity in condensation reactions withfunctionalized mesoporous silica catalysts. J Catal 2009;263:181–8.

[69] Tudorache M, Negoi A, Protesescu L, Parvulescu VI. Biocatalytic alternativefor bio-glycerol conversion with alkyl carbonates via a lipase-linked mag-netic nano-particles assisted process. Appl Catal B Environ 2014;145:120–5.

[70] Simanjuntak FSH, Lim SR, Ahn BS, Kim HS, Lee H. Surfactant-assistedsynthesis of MgO: characterization and catalytic activity on the transester-ification of dimethyl carbonate with glycerol. Appl Catal A Gen 2014;484:33–8.

[71] Singh D, Reddy B, Ganesh A, Mahajani S. Oxide catalyst for the synthesis ofglycerol carbonate by transesterification of glycerol. Zinc/Lanthanum Mixed2014 140605140950009Ind Eng Chem Res 2014 140605140950009.

[72] Wang Y, Liu C, Sun J, Yang R, Dong W. Ordered mesoporous BaCO3/C-cata-lyzed synthesis of glycerol carbonate from glycerol and dimethyl carbonate.Sci China Chem 2014;58:708–15.

[73] Khayoon MS, Hameed BH. Mg1þxCa1�xO2 as reusable and efficient hetero-geneous catalyst for the synthesis of glycerol carbonate via the transester-ification of glycerol with dimethyl carbonate. Appl Catal A Gen2013;466:272–81.

[74] Li J, Wang T. Coupling reaction and azeotropic distillation for the synthesis ofglycerol carbonate from glycerol and dimethyl carbonate. Chem Eng ProcessProcess Intensif 2010;49:530–5.

[75] Tudorache M, Negoi A, Tudora B, Parvulescu VI. Environmental-friendlystrategy for biocatalytic conversion of waste glycerol to glycerol carbonate.Appl Catal B Environ 2014. http://dx.doi.org/10.1016/j.apcatb.2013.02.049274–8.

[76] Jung H, Lee Y, Kim D, Han SO, Kim SW, Lee J, et al. Enzymatic production ofglycerol carbonate from by-product after biodiesel manufacturing process.Enzyme Microb Technol 2012;51:143–7.

P.U. Okoye, B.H. Hameed / Renewable and Sustainable Energy Reviews 53 (2016) 558–574574

[77] Munshi MK, Gade SM, Mane MV, Mishra D, Pal S, Vanka K, et al. 1,8-Dia-zabicyclo[5.4.0]undec-7-ene (DBU): a highly efficient catalyst in glycerolcarbonate synthesis. J Mol Catal A Chem 2014;391:144–9.

[78] Bai R, Wang Y, Wang S, Mei F, Li T, Li G. synthesis of glycerol carbonate fromglycerol and dimethyl carbonate catalyzed by KF modified hydroxyapatite. JInd Eng Chem 2011;17:777–81.

[79] Guo C, Ma R, He L. Metal-promoted Synthesis of Cyclic Carbonates from 1, 2-diols and carbon dioxide 2014:6–14.

[80] Lam MK, Lee KT, Mohamed AR. Homogeneous, heterogeneous and enzy-matic catalysis for transesterification of high free fatty acid oil (wastecooking oil) to biodiesel: a review. Biotechnol Adv 2010;28:500–18.

[81] Kouzu M, Kasuno T, Tajika M, Sugimoto Y, Yamanaka S, Hidaka J. Calciumoxide as a solid base catalyst for transesterification of soybean oil and itsapplication to biodiesel production. Fuel 2008;87:2798–806.

[82] MeyerPDTJ, SauvageHWRJ. Layered Double Hydroxides. In: X. Duan; D.G.Evans, editor. Struct. Bond., 2005, p. vol. 119.

[83] Holtbruegge J, Wierschem M, Steinruecken S, Voss D, Parhomenko L, Lutze P.Experimental investigation, modeling and scale-up of hydrophilic vaporpermeation membranes: separation of azeotropic dimethyl carbonate/methanol mixtures. Sep Purif Technol 2013;118:862–78.

[84] Phase Equilibrium Engineering, vol. 3. Elsevier; http://dx.doi.org/10.1016/B978-0-444-56364-4.00008-X.

[85] Álvarez MG, Plíšková M, Segarra AM, Medina F, Figueras F. Synthesis ofglycerol carbonates by transesterification of glycerol in a continuous systemusing supported hydrotalcites as catalysts. Appl Catal B Environ 2012;113-114:212–20.

[86] Kunnakorn D, Rirksomboon T, Siemanond K, Aungkavattana P, Kuan-chertchoo N, Chuntanalerg P, et al. Techno-economic comparison of energyusage between azeotropic distillation and hybrid system for water–ethanolseparation. Renew Energy 2013;51:310–6.

[87] Patel A, Singh S. A green and sustainable approach for esterification of gly-cerol using 12-tungstophosphoric acid anchored to different supports:kinetics and effect of support. Fuel 2014;118:358–64.

[88] Ferreira P, Fonseca IM, Ramos a M, Vital J, Castanheiro JE. Acetylation ofglycerol over heteropolyacids supported on activated carbon. Catal Commun2011;12:573–6.

[89] Melero J a, van Grieken R, Morales G, Paniagua M. Acidic mesoporous silicafor the acetylation of glycerol: synthesis of bioadditives to petrol fuel. EnergyFuels 2007;21:1782–91.

[90] Dosuna-Rodríguez I, Gaigneaux EM. Glycerol acetylation catalysed by ionexchange resins. Catal Today 2012;195:14–21.

[91] Popova M, Szegedi Á, Ristić A, Tušar NN. Glycerol acetylation on mesoporousKIL-2 supported sulphated zirconia catalysts. Catal Sci Technol 2014;4:3993–4000.

[92] Ferreira P, Fonseca IM, Ramos a M, Vital J, Castanheiro JE. Esterification ofglycerol with acetic acid over dodecamolybdophosphoric acid encaged inUSY zeolite. Catal Commun 2009;10:481–4.

[93] Farinha J, Caiado M, Castanheiro JE. Valorisation of glycerol into biofueladditives over heterogeneous catalysts 2013:422–9.

[94] Mufrodi Z, Rochmadi R, Sutijan S, Budiman A. Synthesis acetylation of gly-cerol using batch reactor and continuous reactive distillation column. Eng J2014;18:29–40.

[95] Rao PV, Rao BVA. Effect of adding Triacetin additive with Coconut oil methylester ( COME ) in performance and emission characteristics of DI dieselengine 2011;1.

[96] Khayoon MS, Hameed BH. Synthesis of hybrid SBA-15 functionalized withmolybdophosphoric acid as efficient catalyst for glycerol esterification to fueladditives. Appl Catal A Gen 2012;433–434:152–61.

[97] Kim I, Kim J, Lee D. A comparative study on catalytic properties of solid acidcatalysts for glycerol acetylation at low temperatures. Appl Catal B Environ2014;148–149:295–303.

[98] Zhou L, Al-Zaini E, Adesina A a. Catalytic characteristics and parametersoptimization of the glycerol acetylation over solid acid catalysts. Fuel2013;103:617–25.

[99] Silva LN, Gonçalves VLC, Mota CJ a. Catalytic acetylation of glycerol withacetic anhydride. Catal Commun 2010;11:1036–9.

[100] Yadav GD, Joshi a V. A green route for the acylation of resorcinol with aceticacid 2002;4:157–64.

[101] Mufrodi Z, Rochmadi S, Budiman A. Chemical kinetics for synthesis of tria-cetin from biodiesel byproduct. Int J Chem 2012;4:101–7.

[102] Ghoreishi KB, Yarmo MA. Sol-gel sulfated silica as a catalyst for glycerolacetylation with acetic acid. J Sci Technol 2013;5:65–78.

[103] Testa ML, La Parola V, Liotta LF, Venezia AM. Screening of different solid acidcatalysts for glycerol acetylation. J Mol Catal A Chem 2013;367:69–76.

[104] Liao X, Zhu Y, Wang SG, Li Y. Producing triacetylglycerol with glycerol bytwo steps: esterification and acetylation. Fuel Process Technol 2009;90:988–93.

[105] Khayoon MS, Hameed BH. Acetylation of glycerol to biofuel additives oversulfated activated carbon catalyst. Bioresour Technol 2011;102:9229–35.

[106] Khayoon MS, Triwahyono S, Hameed BH, Jalil a a. Improved production offuel oxygenates via glycerol acetylation with acetic acid. Chem Eng J2014;243:473–84.

[107] Ghoreishi KB, Yarmo MA, Nordin NM, Samsudin MW. Enhanced catalystactivity of WO3 using polypyrrole as support for acidic esterification of gly-cerol with acetic acid. J Chem 2013;2013.

[108] Argyle M, Bartholomew C. Heterogeneous catalyst deactivation and regen-eration: a review. Catalysts 2015;5:145–269.

[109] Simanjuntak FSH, Widyaya VT, Kim CS, Ahn BS, Kim YJ, Lee H. Synthesis ofglycerol carbonate from glycerol and dimethyl carbonate using magnesium-lanthanum mixed oxide catalyst. Chem Eng Sci 2013;94:265–70.