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To cite this version: Kong, Pei San and Aroua, Mohamed Kheireddine and Daud, Wan Mohd Ashri Wan and Lee, Hwei Voon and Cognet, Patrick and Peres-Lucchese, Yolande Catalytic role of solid acid catalysts in glycerol acetylation for the production of bio-additives: a review. (2016) RSC Advances, 6 (73). 68885-68905. ISSN 2046-2069

Official URL: https://doi.org/10.1039/c6ra10686b

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RSC Advances

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Catalytic role of

PcMEleEEoiicd

ests include catalysis (heterogeneouProcess engineering (reaction enginEnergy & Fuels (biodiesel, biolubricproduct developments (polyol ester

aDepartment of Chemical Engineering, Facu

50603 Kuala Lumpur, Malaysia. E-ma

379675319; Tel: +60 379674615bNanotechnology and Catalysis Research C

Level 3, IPS Building, 50603 Kuala LumpurcLaboratoire de Genie Chimique (Labege), B

Emile Monso, 31432 Toulouse Cedex 4, Fra

Cite this: RSC Adv., 2016, 6, 68885

Received 25th April 2016Accepted 23rd June 2016

DOI: 10.1039/c6ra10686b

www.rsc.org/advances

This journal is © The Royal Society of C

solid acid catalysts in glycerolacetylation for the production of bio-additives:a review

Pei San Kong,ac Mohamed Kheireddine Aroua,*a Wan Mohd Ashri Wan Daud,a

Hwei Voon Lee,b Patrick Cognetc and Yolande Peresc

Bio-additives obtained from the acetylation of biodiesel-derived glycerol have been extensively synthesized

because of their nature as value-added products and their contribution to environmental sustainability.

Glycerol acetylation with acetic acid produces commercially important fuel additives. Considering that the

recovery of individual monoacetin, diacetin (DA), and triacetin (TA) is complicated, many endeavours have

enhanced the selectivity and total conversion of glycerol using acetic acid during catalytic acetylation. In this

work, we extensively review the catalytic activity of different heterogeneous acid catalysts and their important

roles in glycerol acetylation and product selectivity. In addition, the most influential operating conditions to

attain high yields of combined DA and TA are achieved by closely examining the process. This review also

highlights the prospective market, research gaps, and future direction of catalytic glycerol acetylation.

1. Introduction

Catalytic acetylation of glycerol using acetylating agents, suchas acetic acid and acetic anhydride, has been extensively

ei San Kong is a joint-PhDandidate of University ofalaya (Malaysia) and INP-NSIACET, University of Tou-ouse (France). She holds a bach-lor degree in Chemicalngineering and a Master ofngineering Science. She previ-usly worked as an R&D engineern a Malaysia oleochemicalndustry and mainly involved inatalysis, process and technologyevelopment. Her research inter-s/homogeneous acid catalysts),eering, microwave processing),ant, biofuels) and oleochemicals, glycerol derivatives).

lty of Engineering, University of Malaya,

il: mk_aroua@um.edu.my; Fax: +60

entre (NANOCAT), University of Malaya,

, Malaysia

P84234 Campus INP-ENSIACET, 4 allee

nce

hemistry 2016

investigated recently, which is driven by the intention to searchfor new economic applications of glycerol. Acetic acid is nor-mally used as an acetylating agent in producing biofuel addi-tives via acetylation of glycerol; the use of acetic acid isattributed to the its lower price (0.5 USD per kg) compared withthat of acetic anhydride (about 0.98 USD per kg).1 S. Sandeshet al. reported that the acetylation of glycerol with acetic anhy-dride required a lower temperature (30 �C) than glycerol acety-lation with acetic acid (85 �C).2 Despite the fact that glycerolacetylation with acetic anhydride can be conducted at roomtemperature with low energy utilization, the high potential of

Dr Mohamed Kheireddine Arouais a senior Professor at theDepartment of Chemical Engi-neering and the Deputy Dean atthe Institute of GraduateStudies, University of Malaya,Malaysia. He is also heading theCenter for Separation Scienceand Technology (CSST). Hisresearch interests include CO2

capture, membrane processes,electrochemical processes usingactivated carbon, biodiesel

production and conversion of bioglycerol to value added chem-icals. He published more than 130 papers in ISI ranked journalswith more than 3500 citations. His h-index is 30.

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explosion for acetic anhydride is not suitable for manufacturingespecially for large-scale production (as above 49 �C explosivevapour/air mixtures may be formed).

On the contrary, the sudden decline in crude oil prices hassignicantly reduced the prices of biodiesel during the secondhalf of 2014. Fig. 1(a) shows the biodiesel prices declinedstrongly from 112 USD per hL (2013) to less than 80 USD per hL(2014); the ten year forecast for biodiesel prices are expected torecover in nominal terms close to those in 2014 level (prices varyfrom 85–90 USD per hL). Fig. 1(b) indicates that the globalbiodiesel production is expected to reach almost 39 billion litersby 2024; moreover, the projected production volume of bio-diesel is stable and is mostly policy driven.3 Nevertheless,conversion of biodiesel-derived glycerol into value-addedproducts is necessary to support long-term growth of the oleo-chemical market. The price reported for 80% pure crude glyc-erol is $0.24 kg�1 and that for United States Pharmacopeia(USP)-grade glycerol is $0.9 kg�1 in mid-2014.4

Various studies on transforming glycerol into different value-added derivatives, such as propylene glycerol, polyglycerols,

Dr Wan Mohd Ashri Wan Daudis a Professor at the departmentof Chemical EngineeringUniversity Malaya since 2008.His research interests includeactivated carbon, pyrolysisprocess, second generation bio-diesel and hydrogen production.He has published more than 160ISI papers that have garneredmore than 3000 citations. His h-index is 27.

Dr Hwei Voon Lee is a seniorlecturer at Nanotechnology andCatalysis Research Center(NANOCAT), University ofMalaya, Malaysia. She receivedher PhD in Catalysis (2013) andBSc (Hons) in Industrial Chem-istry (2008) from UniversitiPutra Malaysia. Her majorresearch interests are Energy &Fuels (biodiesel, renewablediesel, biofuels); BiomassConversion Technology (cata-

lytic conversion of biomass); Oleochemical Technology (methylester, polyol ester), Catalysis (heterogeneous catalyst, mixed metaloxides, acid–base catalyst) and Nano-Materials (biomass derivednanocrystalline cellulose and application).

68886 | RSC Adv., 2016, 6, 68885–68905

succinic acid, gaseous hydrogen, glycerol carbonate, acrolein,fuel additives, ethanol, glycerol esters, and lubricant additive,were conducted.5–12 This review aims to study the role ofheterogeneous acid catalysts in glycerol acetylation using aceticacid given that the low selectivity of the desired products(diacetin (DA) and triacetin (TA)) remains the greatest challengein catalytic acetylation. In addition, recovery of key derivatives isa very complicated work because the mono-, di-, and tri-substituted derivatives that constitute a mixture exhibit indis-tinguishable boiling points.1 This review then focuses on theimportant features of solid acid catalysts and on the inuenceof operating parameters in enhancing the product selectivity ofglycerol acetylation. To our best knowledge, this work is the rstcritical review focusing on the important role of acid hetero-geneous catalysts in producing DA and TA as bio-additives.

2. Glycerol derived bio-additives

Glycerol-based additives is suitable for efficient engine perfor-mance and is environment friendly.13 Table 1 shows four

Dr Patrick Cognet received hisChemical Engineering Diplomafrom ENSIC (Ecole NationaleSuperieure des Industries Chi-miques de Nancy) and PhD(Electrochemical Engineering) atChemical Engineering Labora-tory, Toulouse. He joins ENSIA-CET (University of Toulouse) asAssistant Professor in 1994 andis a Professor since 2010. Hiswork is focused on Green ProcessEngineering, more precisely on

reactor design, activation techniques (ultrasound, electrochem-istry), intensication and processes involving new media. Hecreated the Green Process Engineering Congress (GPE) in 2007.

Dr Yolande Peres received herPhD in coordination chemistryin 1985 from the University PaulSabatier (Toulouse, France). Shejoined the research group ofProfessor Hoberg at the Max-Planck-Institut fur Kohlenfor-schung (Mulheim an der Ruhr,Germany) as a postdoctoralfellow in 1985. Her research isfocused on the transformation ofCO2 and olens to carboxylicacids catalyzed by transition

metal complexes. Aer one year in industry, she obtained a posi-tion as assistant professor at ENSIACET (University of Toulouse).At present she carries out her research at the Chemical EngineeringLaboratory on the topics of catalytic and phytoextraction process.

This journal is © The Royal Society of Chemistry 2016

Fig. 1 (a) Evolution of biodiesel world prices. (b) Development of the world biodiesel market (OECD 2015 market report).

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important glycerol-derived bio-additives formed by differentreactants via esterication or acetylation routes. Monoacetin(MA), DA, and TA are produced via glycerol acetylation usingacetic acid. The specic industrial applications of MA, DA, andTA are summarized. Given that MA is water soluble, DA and TAare the preferred components for bio-additives. MA is proven tobe completely soluble in water. By contrast, TA is completely

Table 1 General product applications of glycerol derived bio-additives

Bio-additives Starting materials Product ap

(i) MA Acetic acid; glycerol � Excellent� Plasticizecellulose a

(ii) DA � Plasticize� Raw mat

(iii) TA � As an an� Improve

(i) GTBE, GDBE Isobutylene (gas phase);glycerol or tert-butyl alcohol(liquid phase); glycerol

Used in di� Oxygena� Decreasi� To reduccarbonyl c

(i) Di-, tri-GEE Ethanol; glycerol (i) Used fo(ii) Mono-GEE (ii) ImportPolyglycerols Glycerol Excellent l

This journal is © The Royal Society of Chemistry 2016

soluble in ethyl acetate.14 DA and TA are utilized as fuel bio-additives because they effectively improve cold and viscosityproperties, they enhance octane rating, and they can reduce fuelcloud point. Furthermore, TA and DA are alternative for tertiaryalkyl ether that causes greenhouse gas emissions.

Glycerol tertiary butyl ether (GTBE) is another potential bio-additive and can be produced via glycerol etherication using

plications Ref.

solvency 22 and 23r for cellulose acetate, nitrocellulose, ethylnd vinylidene polymersr for cellulosic polymers and cigarette lter 13 and 24erial for the production of biodegradable polyesterstiknock additive for gasoline 25cold ow and viscosity properties of biodieselesel and biodiesel reformulation 16 and 17ted additives for diesel fuelng cloud point of biodiesel fuele fumes, particulate matters, carbon oxides andompounds in exhaustsr fuel formulation 18ant intermediate for various chemicalsubricity and used as additive in lubricant 20 and 21

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gas-phase isobutylene or via glycerol etherication using tert-butyl alcohol (TBA). The operation cost for gas-phase isobutyleneis higher than that for liquid TBA.15 Isobutylene is obtained fromreneries and thus requires additional purication steps toseparate isobutylene from C4 mixtures through sulfuric acidextraction or molecular sieve separation. On the contrary, TBA isa biomass-derived compound and is easier to handle duringmanufacture. Thus, high fraction of glycerol di-tert butyl ether(GDBE) is a reliable oxygenated additive for diesel and biodieselfuels.16,17 In addition, a fuel with 10–25 v/v% of oxygenate reducesparticulate emission.13 Similar to DA and TA, GDBE andGTBE aredesired components over glycerol mono-tert butyl ether (GMBE)given the low solubility of GMBE in diesel.

Exploitation of sugar-fermented bioethanol for glycerol ethylether (GEE) synthesis facilitates production of a completelybiomass-derived product.18 Bioethanol is formerly used as bio-fuel in gasoline engines. However, di- and tri-GEE derivativesare more suitable for diesel and biodiesel formulation becausethey are water insoluble. Moreover, mono-GEE (dioxolane),a water-soluble secondary compound, is an important inter-mediate in producing various chemicals.19 Oligomer diglycerolsare synthesized from single-molecule glycerol via etherication.Polyglycerols, such as diglycerols, are also widely applied in thepharmaceutical, microbiology, food, and automotive indus-tries,20 as well as used as additives in lubricants.21

3. Mechanism acetylation of glycerol

Fig. 2 illustrates the reaction mechanism in MA, DA, and TAproduction. MA is rst produced via glycerol acetylation usingacetic acid. DA is then synthesized by reacting MA with aceticacid followed by reacting DA with acetic acid to produce TA.Water is produced as by-product during glycerol acetylation.

3.1 Market and demand

China market analysis report revealed that the price of TAremains rigid despite the speculated weakening of the marketof TA.26 Fig. 3 demonstrates TA constitutes 10% of the world-wide glycerol market among other uses, which is redrawn on thebasis of a previous study.27,28 The current global demand for TAis approximately 110 000 T per annum, whereas 35% of demandcomes from China. The TA production capacity of China isapproximately 55 000 T per annum, 38 500 T of which isintended for domestic consumption and 16 500 T is exported.The price of TA ranges from RM4273–5560 per ton (equivalentto 1097–1428 USD per ton). In addition, the demand for TA isrecently growing by 5–10% yearly. The demand for TA is ex-pected to remain strong.29 Dr Kongkrapan Intarajang, GroupChief Executive Officer of Emery Oleochemicals, mentioned in2012 that the increase in demand for plastic has led to a steadyannual growth of 4–5% in plastic additive consumptionworldwide.30

3.2 Conventional TA production method

The main manufacturers of TA include Croda, BASF (Cognis),Daicel, Lanxess, ReactChem, Yixing YongJia Chemical,

68888 | RSC Adv., 2016, 6, 68885–68905

Yunnan Huanteng, Klkoleo, and numerous manufacturersfrom China. The conventional TA manufacturing processinvolves two steps.23 Fig. 4 shows the diagram of theconventional TA production. First, glycerol is esteried usingacetic acid in the absence of catalyst, where conversion intoMA occurs. Water is formed and is removed using an azeo-tropic distillation system. Second, the produced MAs arefurther esteried using acetic anhydride under exothermicconditions; TA and acetic acid are formed in this step. Aceticacid is simultaneously recycled as reactant into the rstreactor.31 Table 2 shows the product specication of food-grade TA.

4. Mechanism of Brønsted and Lewisacid-catalyzed esterification4.1 General mechanism of glycerol acetylation

Glycerol esterication using acetic anhydride or acetic acid toproduce MA, DA, and TA can be extensively explained in thepresence of three hydroxyl groups (–OH) that are attached to theglycerol backbone. Acetic acid will selectively attach to any –OHof glycerol or any –OH from partially reacted glycerides; thisphenomenon is related to the steric hindrance effect. Thus, theproduced MA and DA normally present isomer forms depend-ing on the position of acetylation in the glycerol molecule(Fig. 2).13

Among the obtained products, DA and TA are the mostinteresting products that can be applied as fuel additive. MAis an unfavorable product owing to its relatively high solu-bility in water. However, direct transformation of the highlyselective DA and TA is impossible as the reaction involvesa series of consecutive esterication steps, forming variousintermediates (glycerides); moreover, each step is driven bychemical equilibrium because of the formation of water as by-product.33 The selectivity of MA, DA, and TA also dependsmostly on the catalyst features (surface acidity, pore structure,and catalyst stability)34 and esterication parameters (glycerolto acetic acid molar ratio, temperature, catalyst amount, andreaction time).35 Furthermore, the acid-catalyzed glycerolacetylation involves two plausible reaction mechanismsbased on the types of acid catalyst used: (i) Bronsted acid-catalyzed esterication and (ii) Lewis acid-catalyzedesterication.

4.2 Brønsted acid-catalyzed esterication

The Brønsted acid-catalyzed esterication is also named asFischer esterication. Fig. 5 shows a conventional reactionmechanism of the esterication reaction. This reaction mech-anism involves addition of nucleophile (the glycerol) into aceticacid followed by an elimination step, as follows:36

(i) The acetic acid is initially protonated by the Brønsted-typeacid catalyst.

(ii) In the second step, the oxygen atom (two lone pairs) fromthe –OH of glycerol acts as a nucleophile and attaches to the sp2

carbon, leading to the loss of proton from the –OH.

This journal is © The Royal Society of Chemistry 2016

Fig. 2 Reaction mechanism of glycerol acetylation synthesis into MA, DA and TA.

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(iii) A series of fast equilibrium proton exchanges occurs ineither of the –OH of acetic acid. In this step, a new ester bondforms between the carboxyl group carbon and the oxygen inglycerol.

(iv) Water is then eliminated in either site.(v) In the nal step, the excess proton leaves, regenerating

a Brønsted acid catalyst.(vi) This process continues until all three strands of the

glycerol backbone are converted into esters.

This journal is © The Royal Society of Chemistry 2016

4.3 Lewis acid-catalyzed esterication

Theoretically, Lewis acid-based esterication involves a reactionmechanism similar to that in Brønsted acid-based reaction. Inaddition, Lewis acid-based esterication involves the attack ofglycerol in a nucleophilic addition reaction. A slight differencebetween these two processes is that the Brønsted-catalyzedreaction uses a proton generated from the acid catalyst. Bycontrast, the Lewis-based reaction involves a metal cation (Mn+)

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Fig. 3 Distribution of TA in the worldwide glycerol market.

Table 2 Product specification of TA32

Property (unit) Specication

Appearance Clear liquid free from suspended matterOdour Essentially odourlessPurity (%) >99.5Colour (Hazen) <15Acidity (%) <0.005Moisture <0.05Arsenic (mg kg�1) <3Heavy metals (mg kg�1) <5Viscosity (cP) 21–30Density 25 �C 1.154–1.158Refractive index 1.429–1.431

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as an electrophile to facilitate the interaction between thecarbonyl oxygen from acetic acid and the Lewis acidic site (L+) ofthe catalyst to form carbocation. The nucleophile from glycerolattacks the carbon cation and produces tetrahedral intermedi-ates (Fig. 6). During esterication, the tetrahedral intermediateeliminates water molecule to form an ester product.37

5. Heterogeneous acid catalysts forglycerol acetylation

Solid acid catalysts play a crucial role in esterication reactionduring ester production. In particular, solid acids have largelyreplaced the traditional homogeneous acid catalyst because ofenvironmental, technological, and economic reasons. A varietyof solid acid catalysts have been studied for glycerol acetylation.Their catalytic efficiency are also categorized into differentgroups (Table 3).38 The key role of solid acid catalyst in high rateof glycerol conversion and selective formation of DA and TAproducts include: (i) acidity of catalyst (especially the Brønstedacid sites), (ii) texture, and (iii) surface morphology.

Although many studies have demonstrated the high reac-tivity of glycerol acetylation, most catalysts exhibit low thermalstability and unsatisfactory selectivity to DA and TA produc-tion.39 Furthermore, the hydrophilic character of catalystsurface is a challenge in active site deactivation resulting fromthe inevitable water formation during esterication, leading toleaching of active components into the reaction medium. Thewater-tolerant property of solid acid catalyst exhibitinga hydrophobic-enhanced surface is thus necessary to excellently

Fig. 4 Process flow diagram of conventional TA production.

68890 | RSC Adv., 2016, 6, 68885–68905

perform glycerol acetylation. Another reason of catalyst deacti-vation is the partial blockage of the catalyst's active sites by thereaction medium, such as glycerol and/or partial glycerideswithin the pore structure of catalysts, thereby reducing thenumber of acid sites for continuous esterication until thedesirable end-products are achieved.40

5.1 Ion exchange catalyst

Rezayat et al. synthesized MA and TA under supercritical carbondioxide (CO2) conditions by using the commercially availableAmberlyst 15 catalyst.41 The results showed that the use ofcatalyst under supercritical CO2, as well as the molar ratio of thereactants, determine the yield and selectivity of the product. A100% selectivity of TA was obtained using the parameters suchas 200 bar, 110 �C, and an acetic acid to glycerol molar ratio of24 for a 2 hour reaction time. However, the selectivity of TAdecreased at a reaction time of more than 2 h. The Lewis acidityof CO2 also inuences the reaction.

Dosuna recently investigated glycerol acetylation by usingve different ion exchange resins: Amberlyst 15, Amberlyst36, Dowex 50Wx2, Dowex 50Wx4, and Dowex 50Wx8.42

Amberlyst 36 and Dowex 50Wx2 were both outperformed bythe other catalysts when reaction was performed at 105 �C, ata glycerol to acetic acid molar ratio of 8 : 1, under atmo-spheric pressure, at a 6.25 g of dry catalyst/L of glycerol ratio,

This journal is © The Royal Society of Chemistry 2016

Fig. 5 Brønsted-acid reaction mechanism.

Fig. 6 Lewis acid reaction mechanism.

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and at a 10 hour reaction time. Unlike the previous studies,this work was aimed to produce high selectivity of MA. Theuse of Amberlyst 36 resulted in 70.3% MA selectivity, whereasDowex 50Wx2 produced 80.8% selectivity of MA with excess ofglycerol. Our result showed that high selectivity towardformation of small MA molecule was caused by cross-linkageof the catalyst.

Zhou demonstrated that the Amberlyst 15-catalyzed glycerolacetylation produced high selectivity of DA and TA.35 Highamount of acid sites and sufficient pore spaces are the vitalfactors in formation of large molecular DA and TA derivatives.However, Amberlyst 15 exhibits weak acid strength, leading tolow glycerol conversion. In addition, the leaching of catalyst

This journal is © The Royal Society of Chemistry 2016

Amberlyst 15, resulting from the loss of functional groupsthrough hydrolysis at operating temperatures above the poly-mer thermal stability limit (>120 �C) should be considered inacetylation process.12 As A. Ignatyev Igor et al.43 observed thephenomenon of protons leaching from the Amberlyst 15 in thesynthesis of glucose esters from cellulose via hydrolysis–acety-lation steps. Therefore, Wang developed an improved swelling-changeful polymer catalyst by using sodium 5,50-sulfonylbis(2-chlorobenzenesulfonate) and bis(4-chlorphenyl)sulfone toincrease selectivity and conversion of glycerol acetylation.44 Theimproved polymer-based acid (PES) catalyst exhibits strongeracid strength (two times stronger than Amberlyst 15) and betterswelling property, resulting in glycerol conversion of 98.4% with94.9% total selectivity of DA and TA. The high acid strength ofPES catalyst is attributed to its electron-withdrawing –SO2

�

group.

5.2 Zeolite-based acid catalyst

Zeolites are microporous crystalline solids that offer wideapplication in oil rening industry, petrochemistry, and nechemical production. Zeolites are generally categorized asaluminosilicate minerals, which are applied as catalyst supportfor active species owing to their unique pore system, highsurface area, and high stability. Different zeolite systems,

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Table 3 Different groups of solid acid catalysts for glycerol acetylation

Solid acid catalysts Properties

Ion exchange resin � Ion exchange resins are synthesized from polymers that are capable of exchanging particular ions. The drawback of theion exchange resin catalyst is its low temperature stability

Zeolites � Crystalline solids composed of silicon and aluminum oxides arranged in a three-dimensional network of uniformlyshaped micropores (<2 nm) of tuneable topology and composition� Brønsted acid sites in zeolites are commonly generated when protons balance the negatively charged frameworkinduced by the presence of tetrahedrally coordinated aluminum (Al) atoms

Heteropolyacids � A class of metal salts wherein the oxo-anions are balanced by a wide range of cations with varying acid strengthMetal oxides � The Brønsted acid sites in metal oxides originate from highly polarized hydroxyl groups, acting as proton donors

� The Lewis acid sites generated from coordinatively unsaturated cationic sites, which leave M+ exposed to interact withguest molecules as an acceptor of pairs of electrons

Mesoporous silica � Mesoporous silica is a mesoporous form of silicate that consists of unique features: high surface area, chemical,thermal, and mechanical stability, highly uniform pore distribution and tunable pore size, high adsorption capacity, andan ordered porous network� This material is potentially used as solid supported catalyst due to its recyclability, enhanced catalytic reactivity, andselectivity

Carbon � Porous carbon is an attractive catalytic material as it can be prepared from various low-cost waste carbon materials� This material consist of suitable characteristics that can be used as a catalyst support, such as heat resistance, stabilityin both acidic and basic media, the possibility of easy recovery of precious metals supported on it and the possibility oftailoring both its textural and surface chemical properties

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including ZSM-5, zeolite-beta, and USY, can be applied aspotential catalysts in glycerol esterication/acetylation.45

The catalytic esterication reaction over zeolite-based cata-lysts depends on their different crystal structure, Si/Al ratio, andproton exchange level; these properties allows the catalyticproperties, such as pore size, hydrophobicity/hydrophilicity,Brønsted/Lewis acidity, and acid strength distribution, to bedesigned. For esterication, surface acidity is the most vitalcharacteristic in designing a zeolite-based catalyst. The acidity ofzeolite can be tuned by altering their chemical composition (Si/Alratio) and ion-exchange abilities. Theoretically, protonic zeoliteconsisting of bridging –OH groups (Al–(OH)–Si) is an active acidsite that favors Brønsted acid-catalyzed esterication reactions(Fig. 7 (ref. 46)). Zeolites exhibiting low Al framework are themosthydrophobic types. In addition, acidity measurements of zeolitesnormally comprise both Brønsted and Lewis acid sites, acidstrength distribution, and precise location of the acid sites.47

Gonçalves et al. investigated the production of MA, DA, andTA during glycerol acetylation using acetic acid and catalyzed bythe zeolites HZSM-5 and HUSY. The reactivity of the zeolitecatalyst was also compared with that of the traditional acid

Fig. 7 Brønsted acidity of zeolite in esterification reaction.

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catalysts, such as Amberlyst-15 acid resin, clay K-10 montmo-rillonite, and niobic acid. Although the zeolites HZSM-5 andHUSY exhibit high surface area (374 and 566 m2 g�1, respec-tively), their acidity were lower than that of Amberlyst-15(HZSM-5 zeolite ¼ 1.2 mmol g�1; HUSY zeolite ¼ 1.9 mmolg�1; and Amberlyst-15 ¼ 4.2 mmol g�1). Given that the esteri-cation rate mostly depends on the catalyst's acidity, thedecreasing order of acetylation is as follows: Amberlyst-15 > K-10 clay > niobic acid � HZSM-5 > HUSY. The poor perfor-mance of zeolites is possibly related to difficulty of diffusion ofacetylated esters within the catalyst's cavities. This phenom-enon resulted in low selectivity of DA and TA as both moleculesare space demanding and their formation and diffusion withinthe zeolite pores are difficult.48

Ferreira et al. attempted to improve the catalytic activity ofesterication by enhancing the acidity of zeolite-based acidcatalysts. Dodeca-molybdophosphoric acid (H3PMo12O40) wasencaged in USY zeolite for glycerol acetylation. H3PMo12O40 isknown as heteropolyacid with strong Brønsted acidity and iswidely applied as acid catalyst in esterication. Nonetheless,H3PMo12O40 exhibits low specic surface area (1–10m2 g�1) and

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low thermal stability. Therefore, encapsulation of H3PMo12O40

in acid supercages of NaUSY support increased the number ofaccessible acid sites at a surface area of 713 m2 g�1. Moreover,the 1.9 wt% dosing of PMo zeolite catalyst demonstrated thehighest catalytic activity, with 59% selectivity of DA and 68%total glycerol conversion.24

5.3 Heteropolyacids (HPAs)-based acid catalyst

HPAs, such as silicotungstic acid (HSiW), phosphotungstic acid(HPW), and phosphomolybdic acid (HPMo), are typicalBrønsted acids containing a superacid region that displaysoutstanding catalytic esterication activity both in homoge-neous and heterogeneous phases. HPAs are complex protonacids that incorporate the Keggin-type polyoxometalate anions(heteropolyanions) containing metal–oxygen octahedra witha formula XM12O40

x�8, where X is the central atom (Si4+/P5+), x isits oxidation state, and M is the metal ion (Mo6+ or W6+).49 Theacid strength of crystalline HPAs generally decreases in thefollowing order: PW > SiW $ PMo > SiMo, which is identical tothe dissociation constants presented in Table 4. Moreover,HPAs in solution are stronger than the usual mineral acids,such as H2SO4, HCl, and HNO3.50 However, bulk HPAs exhibitlow thermal stability, low surface area (1–10 m2 g�1), and arehighly soluble in polar media (water, lower alcohols, ketones,ethers, esters, etc.), which restricts their application as solidacid catalyst in esterication reaction.

Researchers have improved the thermal stability, surfacearea, and solubility of HPAs in polar media by exchanging theprotons of HPA with metal/alkali metal ions and by supportingbulk HPA with a suitable acidic or neutral carrier (such as SiO2,active carbon, acidic ion-exchange resin, or metal oxide). Zhuutilized zirconia as a support material to develop HPA-basedcatalyst.51 HPAs display excellent catalytic performance overa wide variety of acid-catalyzed reactions owing to their lowcorrosiveness, well-built structure, as well as adjustable acidity.Nevertheless, HSiW is deemed more active than the otheravailable HPAs, such as HPW and HPMo. HSiW consists of fourKeggin protons and advanced Brønsted acid sites, as well asexhibits stronger hydrothermal stability, making HSiWconsiderably superior over the two other HPAs. Therefore, theactivity of HSiW supported with ZrO2 (H4SiW12O40/ZrO2) inglycerol acetylation at 120 �C, 0.8 wt% of catalyst, 1 : 10 molarratio of glycerol : acetic acid for 4 h reaction time, was investi-gated. Three vital ndings were revealed: (i) H4SiW12O40/ZrO2 is

Table 4 Dissociation constants of HPAs in acetone at 25 �C (ref. 50)

Acids pK1 pK2 pK3

H3PW12O40 1.6 3.0 4.0H4PW11VO40 1.8 3.2 4.4H4SiW12O40 2.0 3.6 5.3H3PMo12O40 2.0 3.6 5.3H4SiMo12O40 2.1 3.9 5.9H2SO4 6.6HCl 4.3HNO3 9.4

This journal is © The Royal Society of Chemistry 2016

a predominant glycerol esterication catalyst as it can be reusedup to four continuous runs without showing any deactivation;(ii) H4SiW12O40/ZrO2 can be directly used to catalyze crudeglycerol material; and (iii) a 93.6% combined selectivity of DAand TA was achieved.

Glycerol acetylation over 12-tungstophosphoric acid (TPA)supported on Cs-containing zirconia (TPA/Csx–ZrO2) catalystwas investigated to improve the selectivity of MA, DA, and TAformation.52 The comparative study of different Cs amountdoped on ZrO2 support was insight studied to evaluate thecatalyst activity towards glycerol acetylation. Given that thepartial substitution of H+ by Cs+ has altered the total number ofavailable surface acid sites, the TPA–Cs catalyst demonstratedbetter esterication activities than the TPA parent acid duringthe reaction. In addition, this work revealed that the TPA/Cs2–ZrO2 catalyst (with Cs amount equal to 2-protons of TPA)demonstrated the highest acidity and catalytic activitycompared to zero or excess Cs content of catalysts. By contrast,the zero Cs-content catalyst yielded the lowest conversion, whilethe catalyst with Cs amount equal to 3-protons of TPA alsoshowed low activity owing to the absence of residual protons.This work concluded that the catalytic activity, acidity andtextual properties of the catalyst are varied with the amount ofCs present on support. Where, the presence of exchangeable Cscontent has improved the surface acidic sites resulting from theexistence of residual protons. Conversion of more than 90% canthen be achieved within 2 h. When the reaction time was pro-longed to 4 h at 120 �C, 1.5 acetic acid to glycerol molar ratio,and 0.2 wt% of catalyst concentration, the selectivity towardsDA and TA is 55% and 5%, respectively.

Further, S. Sandesh et al. reported glycerol acetylation withacetic acid under mild reaction condition in the presence ofcesium phosphotungstate catalyst (CsPW).2 CsPW catalyst wasprepared by precipitation of 0.75 M HPW solution with 0.47 Maqueous cesium carbonate. This study revealed that theconversion and selectivity of DA and TA were greatly inuencedby the total acidity of catalyst (1.87 mmol g�1) and Brønstedacidic sites of CsPW catalyst. The comparative characterizationresults for original HPW and Cs-precipitated CsPW catalysthave conrmed that CsPW catalyst exhibits signicant centeredcubic assembly and unaltered primary Keggin structure. Inaddition, CsPW catalyst has the highest Brønsted to Lewis acidratio compared to Amberlyst 15, sulfated zirconia, H-beta and K-10 catalysts. Apparently the Cs content in CsPW catalyst hasaltered the textural properties (with 110 m2 g�1 specic surfacearea), acidity as well as its catalytic activity towards acetylation.The acetylation of glycerol with acetic acid resulted in conver-sion of more than 98% and total 75% selectivity of DA and TA at7 wt% of CsPW catalyst, 85 �C reaction temperature, 8 : 1 molarratio of acetic acid to glycerol for 120 min reaction time.

Patel et al. recently investigated the use of TPA anchored ontwo types of support, namely, zirconia and MCM-41, for glycerolacetylation. The use of catalyst support successfully improvedthe mechanical stability of TPA and enabled catalytic activitymodication. These results conrmed that the MCM-41-supported TPA rendered high esterication activity (conver-sion of 87% and 60% selectivity of DA). By contrast, the ZrO2-

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supported TPA resulted in low conversion (80%) and 36%selectivity of DA. The high-acidity prole and specic hexagonalchannels of MCM-41 also facilitated the diffusion of functionalglycerol molecule compared with those of ZrO2.53

Given that the acidity and esterication activity of HPA-basedcatalyst is improved by exchanging the protons of HPA withdifferent cations, another group researcher designed a silver-modied HPW catalyst (HPW/Ag), which was prepared throughthe ion-exchange method. The HPW/Ag catalyst consists of highnumber of Brønsted acid and Lewis acid sites and exhibitsexcellent water tolerance, strong stability in polar reaction envi-ronment, and a unique Keggin structure (these properties areresponsible for the tremendous esterication reaction). Thereaction was performed at 120 �C, 10 wt% of catalyst, and 4 : 1acetic acid to glycerol molar ratio under vigorous mixing at 1200rpm for 4 h. A total of 96.8% conversion and 90.7% selectivitytowards DA and TA was attained. The advantage of the HPW/Agcatalyst is the existence of identical symmetry like parent HPWtogether with a bonded unit cell in the form of dihydronium ionsH2O2

+, where it is formed by protons exchange in the secondaryKeggin structure. Neither leaching nor deactivation was detectedin ve consecutive reaction cycles.54

In addition, niobic acid-supported tungstophosphoric acidcatalyst (TPA/Nb2O5) for glycerol acetylation was studied.55 TPAKeggin ion (25 wt%) was well-dispersed in the niobic acidsupport (25 wt% TPA/Nb2O5). The catalyst demonstrated hightotal conversion (90%) and 76% selectivity towards DA and TA atoperating parameters of 120 �C, 1 : 5 glycerol to acetic acidmolar ratio, and catalyst weight of 200 mg for a 4 hour reactiontime. The ndings of this work suggested that glycerol acetyla-tion conversion and selectivity depend on catalyst acidity, whichis highly related to the content of niobic acid-supported TPA.

5.4 Metal oxide-based acid catalyst

The use of metal oxide-based catalysts for esterication reactionhas attracted attention owing to their strong surface acidity andhigh activity at low operating temperatures. The presence ofLewis acid (cations) and Brønsted acid (OH� group)/Brønstedbase (O2� group) (anions) of metal oxides provided the requiredcatalytic sites for esterication. Fig. 8 illustrates the existence ofLewis and Brønsted sites in the metal oxide catalyst.38 Moreover,functionalization of metal oxide via sulphonation (sulfatedmetal oxides, such as sulfated-zirconia, -tin oxide (SnO2), -tita-nium oxide, and -mixed oxides) is a convenient means ofenhancing the surface area and acidity of a catalyst.

Mallesham reported that SnO2 is one of the potential metaloxide solid acid catalysts prepared through the wet impregna-tion method, where SO4

2�, MoO3, and WO3 were incorporatedinto the SnO2 support. Compared when metal oxide is usedalone, incorporation of promoters into SnO2 can enhance the

Fig. 8 Lewis and Brønsted site of metal oxide catalyst.

68894 | RSC Adv., 2016, 6, 68885–68905

thermal stability and catalytic performance of SnO2. Theperformance of three catalysts in esterication of acetic acidusing glycerol was comparatively investigated. The resultsshowed that all of these catalysts consisted of both Brønstedand Lewis acidic sites. The ascending order of the activities ofthe catalysts is as follows: SnO2 < WO3/SnO2 < MoO3/SnO2 <SO4

2�/SnO2. The sulfated SnO2 (SO42�/SnO2) showed the high-

est performance mainly because of the presence of largenumber of acidic sites associated with super acidic sites.56

Mixed metal oxide system offers interesting and enhancingproperties, especially when each component differs remark-ably from each other. Mixed metal oxide catalysts can gener-ally be prepared via co-precipitation, impregnation, or sol–gelmethods from its bulk oxide or metal salt as precursors.57 Thebinary metal oxide system oen establishes new acid sites ormodulates the acid properties of the bulk oxides, which areactive during esterication.58 For instance, sulfated binaryoxide solid superacids (SO4

2�/TiO2–SiO2) synthesized by Yanget al. was utilized as catalyst in glycerol esterication usingacetic acid under toluene solvent-reaction system.59 The TiO2–

SiO2 showing the highest catalytic quality was calcined at 450�C and consists of 13.8 wt% of TiO2. Yang reported that thecoupling of two oxides (TiO2–SiO2) can generate stronger acidsites and higher acidity compared with bulk metal oxide owingto the larger specic surface area of the coupling. Whereby, thepresence of strong acidity of SO4

2�/TiO2–SiO2 is initiated by anexcess of a negative or positive charge in a binary oxide. TheTiO2 content, special surface area and unique structure ofmodied SO4

2�/TiO2–SiO2 catalyst resulted in 91.4%conversion.

Reddy et al. performed similar glycerol esterication usingacetic acid catalyzed by various types of zirconia-based cata-lysts: ZrO2, TiO2–ZrO2, WO3/TiO2–ZrO2, and MoO3/TiO2–ZrO2.MoO3/TiO2–ZrO2 showed the highest conversion among theinvestigated catalysts. A 100% conversion with 80% selectivitytowards TA was achieved at 120 �C, acetic acid to glycerolmolar ratio of 6 : 1, and 5 wt% of catalyst concentration for 60h reaction time.25 The results revealed that the MoO3

promoted the number of acidic sites of TiO2–ZrO2 support andthe strong interaction between the dispersed MoO3 withsupport have increased catalytic activity for glycerol acetyla-tion. Despite its high conversion and selectivity, long opera-tion time is required for TA production, which is not cost-effective toward upscaling for industrial uses.

Reddy et al. further improved the activity and selectivity ofproducts (DA and TA) by using a modied sulfonated zirconia-basedmixed metal oxide; they used a better metal oxide support(SO4

2�/CeO2–ZrO2) for acetylation. The wet impregnated sulfateions on CeO2–ZrO2 mixed oxides has increased the initial metaloxide surface area resulting from the formation of poroussurface sulfate compounds between the sulfate groups and thesupports. The reaction time was successfully shortened to 1 h,and a high glycerol conversion (100%) and 74.2% selectivitytowards DA and TA was achieved. Nevertheless, a longer reac-tion time (40 h) was required for the steric structure of partialglycerides to esterify into 90% highly selective TA.60

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5.5 Mesoporous silica-based catalysts

Mesoporous silica materials, such as MCM-41 and SBA-15, haveattracted much attention as a catalyst support in heterogeneouscatalysis owing to their high specic surface area ($1000 m2

g�1), well-ordered mesoporous structure, and large pore sizes (2nm # size # 20 nm).61 The preparation of sulfonated silica isshown in Fig. 9, as redrawn on the basis of previous study.62

Given that they display a high accessibility to large organicmolecules, such as fatty acids and esters, the mesoporous silica-based catalysts were chosen as acid catalysts for esterication ofglycerol with fatty acids, especially when these catalysts arefunctionalized with R–SO3H groups.63 The plausible mecha-nism of esterication of acetic acid using methanol and cata-lyzed by acid-functionalized SBA-15 was reported by Miao.64 Thefunctionalized SBA-15 obeyed dual-site mechanism (Langmuir–Hinshelwood type), which involved adsorption of both aceticacid and alcohol molecules. This phenomenon demonstratedthat the reaction occurred at a high rate. Fig. 10 illustrates thedual-site esterication mechanism catalyzed by functionalizedSBA-15, which is modied on the basis of a previous study.64

The SBA-15 silica-based catalysts are oen modied as novelsolid acid catalysts for TA and DA production. SBA-15 possessesunidirectional channels arranged in hexagonal symmetry andinterconnected by micropores. The cross-sectional area of thiscatalyst is speculated to be much larger than that of MCM-41.65,66 SBA-15 exhibits a large surface area (700–900 m2 g�1),large pore size (5–9 nm), and thick walls (3.5–5.3 nm).67 Gon-çalves et al. used ve different solid acids (Amberlyst-15, K-10montmorillonite, niobic acid, HZSM-5, and HUSY) to produceDA and TA. Among the tested catalysts, Amberlyst-15 (acidity ¼4.2 mmol g�1) was the most active, showing 97% conversionand 67% selectivity of DA and TA, followed by K-10 clay (acidity¼ 0.5 mmol g�1) with 96% conversion but a lower combinedselectivity of DA and TA (54%).48 Despite of K-10 clay does notpossess high acidity, the predominant of medium-weak

Fig. 9 Preparation of sulfonated silica.

This journal is © The Royal Society of Chemistry 2016

Brønsted acid sites of K-10 clay are mainly responsible forcatalytic activity as K-10 clay is type of acetic acid-treated cata-lyst.68,69 As shown by the recent studies, the use of SBA-15 assolid acid catalyst has demonstrated enormous improvement interms of conversion and selectivity. For instance, a hybrid SBA-15 catalyst functionalized with molybdophosphoric acid (MPA/SBA-15) can achieve 100% glycerol conversion with a corre-sponding 86% combined selectivity of DA and TA. The thermaldecomposition of 15% MPA over SBA-15 support has shownimproved catalytic activity in glycerol acetylation using aceticacid.70

Trejda et al. also modied the SBA-15 catalyst. They modiedthe mesoporous niobiosilicate (nb-SBA-15) with 3-mercapto-propyl trimethoxysilane (MPTMS) followed by hydrogenperoxide oxidation of the thiol species of the catalyst (denotedas MP-nb-SBA-15 catalyst). The work revealed that, comparedwith silica material, the presence of niobium in the matriximproved the transformation of thiols into sulphonic speciesvia hydrogen peroxide oxidation. The optimum Si/MPTMS ratiois 1 : 1. The elemental analysis of the catalysts revealed that theinclusion of MPTMS into nb-SBA-15 is effective and has reducedthe surface area, pore volume, and pore diameter. Thus, themodied catalyst used in this work does not only change theconversion but also strongly affects the selectivity. The domi-nant product is DA (approximately 50%), and a considerablyhigh selectivity of TA of up to nearly 40% was attained. Never-theless, weak stability of catalyst was detected during recycla-bility test.71

Catalytic esterication of yttrium supported on silicateframework (Y/SBA-3) during glycerol acetylation was alsostudied. Graing of yttrium on silicate framework increased thesurface area of Y/SBA-3 (1568 m2 g�1) compared with that of theblank silicate (SBA-3 at 1462 m2 g�1), conrming the inhibitoryeffect of nano-sized yttrium agglomeration. The Y/SBA-3 used tocatalyze glycerol acetylation, resulting in 100% glycerolconversion and 89% combined selectivity of DA and TA. The

RSC Adv., 2016, 6, 68885–68905 | 68895

Fig. 10 Dual-site esterification mechanism (Langmuir–Hinshelwood type, L–H) through functionalized SBA-15 catalyst.

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catalyst activity was greatly inuenced by two factors: (i) acidityof the catalyst and (ii) the combination of high surface area andlarge pore size, which facilitated the diffusion of bulky glyceroland the nal product. This work also compared the catalyticactivities of different SBA-15-based catalysts and found that thedecreasing order of the catalytic activity is as follows: 3%Y/SBA-3 > SBAH-15 (15) (hybrid SBA-15 functionalized with molybdo-phosphoric acid) > AC-SA5 (sulfated activated carbon).40

Although mesoporous silica has displayed good perfor-mance in esterication, Stawicka further improved the meso-porous support by creating more open structures, whichimprove the performance of the catalyst during estericationusing glycerol and acetic acid. Mesostructured cellular foams(MCF) containing acidic sites derived from oxidized MPTMSwere developed. Fig. 11 shows the simplied model of the MCFstructure. MCF consists of uniform spherical cells approxi-mately 20–40 nm in diameter and possesses surface areas up toapproximately 900 m2 g�1. The cells are interconnected byuniform windows (7–20 nm in diameter), forming a continuous3-D pore system. In addition, the walls of the MCFs are formedfrom silica; the silanols present on the surface can thus be usedto modify the material. The presence of niobium (Nb/MCF) andtantalum (Ta/MCF) doped on the MCF also improves theBrønsted acidity of the catalyst. The acidic sites of Nb/MCF and

Fig. 11 Simplified model of MCF structure.

68896 | RSC Adv., 2016, 6, 68885–68905

Ta/MCF catalysts were derived from the oxidized MPTMS andwere further enhanced by modication with Nb and Ta. Inter-estingly, the sulphonic species on silica surfaces showed highstability and strength aer being modied by the presence ofpromoters. Both Nb/MCF and Ta/MCF showed high conversionand selectivity of DA and TA even though the acidity of Ta/MCF(0.57 mequiv. g�1) was higher than that of Nb/MCF (0.32mequiv. g�1).72

5.6 Mesoporous carbon-based acid catalyst

Mesoporous carbon has been actively studied generally asa catalyst support and/or acid-functionalized carbon for esteri-cation reaction. The presence of surface oxide group in mes-oporous carbon enables it to provide anchoring sites for activemetals, which can tune the properties of carbon as a catalystsupport material. Furthermore, the existence of unique prop-erties, such as high thermal–mechanical stability with lowmetal leaching, as well as controllable textural and surfacechemical properties, makes carbon a suitable catalyst support.Compared with mesoporous silica, mesoporous carbon is moreresistant to structural changes caused by hydrolytic effects inaqueous environments. Acid-functionalized carbon, such assulfonated-carbon via sulfonation by concentrated H2SO4

(formation of high density sulfonic acid group (–SO3H)), hasbeen extensively studied in esterication. Fig. 12 shows thepreparation of sulfonated carbon.73,74

Sulfonated activated carbon (AC) was investigated in glycerolesterication using acetic acid to produce DA and TA. Theporosity of the AC was measured, and the results revealed that27% of the AC exhibits mesoporous structure. A blank AC nor-mally exhibits large specic surface area of up to 780 m2 g�1.This work found that the acid treatment has slightly reducedthe surface area of AC to 742 m2 g�1. The acid-functionalized ACwas prepared through hydrothermal treatment with sulfuricacid at 85 �C for 4 h. Although the sulfonated AC catalyst (AC-SA5) demonstrated good stability during recycling (up to four

This journal is © The Royal Society of Chemistry 2016

Fig. 12 Preparation of SO3H–carbon.

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consecutive batch runs), the conversion and selectivity resultswere not encouraging, that is 91% conversion and a 62%combined selectivity of DA and TA (slightly lower than those ofother mentioned catalysts). Nevertheless, the density ofBrønsted acid sites and mass transfer rate in the mesoporouschannels of AC are the governing factors leading to TAsynthesis.75

A new class of sulfonated carbon catalysts termed “sugarcatalyst” was recently reported. They were prepared via incom-plete carbonization of simple carbohydrates (starch, cellulose,glucose, and sucrose) followed by sulphonation. Sanchez61

studied the effect of the porous system of a sulphonated sucrose-derived carbon prepared via silica-template carbonization forglycerol acetylation reaction. The presence of interconnectedmicro- and mesopores (45% in 0–2 nmmicropores and 51% in 2–50 nm mesopores) in sucrose-derived carbon allowed effectivesurface sulfonation (C–SO3H). High glycerol conversion (>99%)with high selectivity (50%) of TA was achieved during esterica-tion (at 9 : 1 molar ratio of acetic acid to glycerol, 5 wt% catalyst,180 �C optimum temperature, and 4 h reaction time).

Another study reported on the preparation of sulfonatedcarbon catalyst, which is derived from low-cost biomass (willowcatkins), to be used in producing carbon support. The aciddensity of the sulphonated carbon is greatly inuenced by sul-phonation condition. The sulphonation temperature of 90 �Crendered the highest acidity of sulfonated carbon catalystcompared with that obtained at 100 �C, 80 �C, and 70 �C.Furthermore, the study indicated that ester selectivity is mainlycorrelated with catalyst acidity, where high acidity of sulfonatedcarbon yields 67.2% of DA and TA products. These types ofcatalysts showed good heat stability and high water tolerance atlow production cost.76

The acid-functionalized carbon was further improved usinggraphene oxide (GO) as carbon material. GO consists of oxygen-rich functional groups, such as SO3H, carboxyl, hydroxyl, andepoxide groups, which provide moderate acidic site for glycerolesterication. The unique layered structure of graphene alsoimproves the accessibility of reactant for adsorption duringreaction. Moreover, the acid-functionalized GO enhanced the

This journal is © The Royal Society of Chemistry 2016

acid density of the catalyst containing acidic SO3H groups. Theglycerol acetylation showed a good glycerol conversion of 98.5%with high selectivity of DA (60%) and TA (24.5%). In addition,GO has displayed high reusability without showing reduction incatalytic activity and changes in product distribution.77

5.7 Others

A new type of catalyst, hydroxylated magnesium uoride(MgF2�x(OH)x, where x < 0.1), which contains both Lewis andBrønsted acid sites, was synthesized for glycerol acetylation.The hydroxylated magnesium uoride exhibits the followingstructural and chemical features: (i) high surface area with porediameters at the mesopore range; (ii) very low solubility instrong polar solvents; (iii) hydrolysis resistance; (iv) medium-strength Lewis and Brønsted acid sites; (v) possibility of easytuning of surface acidity; and (vi) nanoscopic particle dimen-sion. MgF2�x(OH)x showed a majority of Lewis sites that aregenerated from Mg2+ sites of Mg–F bond, while Brønsted acidsites character from the Mg–OH group. The surface acidity ofhydroxylated magnesium uoride exhibiting high Lewis/Brønsted ratio sites is suggested to favor DA and TA formationduring glycerol acetylation. Troncea36 reported that themedium-strength Lewis acid sites of catalyst (such as incom-pletely coordinated Mg2+) exert a major effect on acetylationcompared with Brønsted acid sites (M–OH groups) that favorMA formation. The increased selectivity to DA and TA withloading of Lewis sites may be explained by considering thedouble role of these sites: rst, as active catalytic sites involvedin the formation of the reactive electrophilic intermediate, andsecond, as dehydrating sites coordinating the water moleculesformed during the reaction.

Recent studies have applied a magnetic solid acid catalyst(Fe–Sn–Ti(SO4

2�))-400 in glycerol acetylation and successfullyproduced 99% selectivity of TA with 100% total conversion.78 Allof the reactions were conducted at 80 �C, 2.5 wt% of catalyst, 5.6mass ratio of acetic acid to glycerol, and 0.5 h reaction time.This magnetic-based catalyst consists of iron (Fe), tin (Sn), andtitanium (Ti), sulfated by (NH4)2SO4 solution, and calcined at

RSC Adv., 2016, 6, 68885–68905 | 68897

Tab

le5

Differentkindofheterogeneousca

talystsusedforglyce

rola

cetylationwithac

eticac

id

Catalyst

Catalystprep

aration

Catalystch

aracterization

Ope

ratingpa

rameters

Performan

ceRef.

Ionexch

ange

resincatalyst

Ambe

rlyst15

Com

mercial

available

SSA¼

37.6

m2g�

1T¼

110

� CY¼

41%

41Acidity

¼4.7mmol

g�1

P¼

200ba

rS¼

100%

TA

PD¼

30nm

t¼

2h

Tstab

ility¼

120

� CMolar

ratioacetic

acid

toglycerol

¼24

PV¼

0.4cm

3g�

1Flow

rate

ofscCO2¼

1.2mL

min

�1

Moisturecontent¼

48%

Cross-linka

ge¼

20%

Acidstrength,H

0¼

�3to

�2.2

(i)Ambe

rlyst36

Com

mercial

available

Amberlyst36

:T¼

105

� CAm

berlyst36

:42

SSA¼

33m

2g�

1t¼

10h

C¼

95.6%

Acidity

¼5.4mmol

g�1

Molar

ratioglycerol

toacetic

acid

¼8:1

S¼

70.3%

MA;4

.5%

DA

PD¼

24nm

Tstab

ility¼

140

� CPV

¼0.2cm

3g�

1

Moisturecontent¼

52.4%

(ii)Dow

ex50

Wx2

Cross-linka

ge¼

12%

Dow

ex50

Wx2

:Dow

ex50

Wx2

:Acidity

¼4.8mmol

g�1

C¼

95.2%

Tstab

ility¼

140

� CS¼

80.8%

MA;5

.1%

DA

Moisturecontent¼

76.7%

Cross-linka

ge¼

2%Gel-type

Ambe

rlyst15

Com

mercial

available

Sameas

above

T¼

110

� CC¼

97.3%

35t¼

270min

S¼

47.7%

DA;

44.5%

TA

Molar

ratioacetic

acid

:glycerol¼

9:1

Improved

polymer-

basedsolidacid

catalyst

(PES)

Precipitationan

dion-exchan

gemethod

Acidity:2

.1mmol

g�1

T¼

110

� CC¼

98.4%

44Thepo

lymer

withSS

BCBSan

dBCPS

molar

ratioof

5:5

(PES-50

)was

synthesized

inratioof:0

.05mol

of4,40-biphen

ol(BP),0

.025

mol

ofBCPS

,0.025

mol

ofSS

BCBSan

d0.05

8mol

ofK2CO3.T

hepo

lymer

PES-Pwas

precipitated

,followed

bybe

ingwashed

thorou

ghly

with

hot

deionized

water

forthreetimes.T

hepo

lymer

was

driedin

oven

at10

0� C

for12

h.A

s-synthesized

PES-Ppo

lymer

was

exch

angedin

0.5

MH

2SO4at

25� C

for6h,a

ndthen

ltrated

Acidstrength,H

0¼

�5.6

to�3

.0t¼

3h

S¼

45%

DA;4

9.5%

TA

Catalystload

ing¼

0.15

gMolar

ratioacetic

acid

:glycerol¼

8:1

68898 | RSC Adv., 2016, 6, 68885–68905 This journal is © The Royal Society of Chemistry 2016

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Tab

le5

(Contd.)

Catalyst

Catalystprep

aration

Catalystch

aracterization

Ope

ratingpa

rameters

Performan

ceRef.

Zeolite-ba

sedacid

catalyst

(i)Ze

oliteHZS

M-5

Zeolites

HZS

M-5

andHUSY

werepu

rchased

from

Petrob

ras

HZS

M-5:

T¼

110

� CHZS

M-5:

48(ii)Ze

oliteHUSY

SSA¼

374m

2g�

1t¼

30min

C¼

30%

Acidity

¼1.2mmol

g�1

Molar

ratioacetic

acid

:glycerol¼

3:1

S¼

10%

DA&TA

Tstab

ility¼

386

� CSi/Al¼

28HUSY

:HUSY

:SS

A¼

566m

2g�

1C¼

14%

Acidity

¼1.9mmol

g�1

S¼

14%

DA&TA

Tstab

ility¼

397

� CSi/Al¼

2.6

PMo/NaU

SYzeolite

Preparationof

NaU

SYzeolite

Acidity

¼0.01

9g H

PAg c

at�1

T¼

not

repo

rted

C¼

68%

24HUSY

zeolitewas

neu

tralized

by3repe

ating2M

NaC

lat8

0� C

.Itw

aswashed

withdistille

dwater

andthen

driedat

120

� CSS

A¼

713m

2g�

1t¼

3h

S¼

61%

DA&TA

Micropo

revolume,

mPV

¼0.17

cm3g�

1Catalyst¼

10wt%

ofglycerol

Hydrothermal

impregna

tion

forPM

o/NaU

SYThecatalystswereprep

ared

byhyd

rothermal

impregnationmethod

:by

encagingmolyb

denum(V

I)oxide

onNaU

SY,followed

byneu

tralizationan

ddryingat

110

� C

HPA

s-ba

sedacid

catalyst

H4SiW

12O

40/ZrO

2Wetness

impregna

tion

metho

dAcidity

¼0.69

mmol

g�1

T¼

120

� CC¼

100%

80HSiW/ZrO

2catalyst

was

prep

ared

throug

hincipien

twetness

impregnationmethod

.Zirconia

supp

ortwas

impregnated

with13

9mmol/H

SiW

solution

for8h,followed

bydrying(110

� C)an

dcalcination(250

� Cin

static

air,4h)

SSA¼

48.7

m2g�

1t¼

4h

S¼

93.6%

DA&TA

PD¼

10.7

nm

Molar

ratioacetic

acid

:glycerol¼

10:1

PV¼

0.17

cm3g�

1Catalyst¼

0.8wt%

TPA

/Cs 2–Z

rO2

Precipitationan

dim

pregna

tion

metho

dAcidity

¼not

repo

rted

T¼

120

� CC¼

90%

52Zircon

iasu

pportwas

prep

ared

byprecipitationmethod

anddriedat

120

� Cfor36

h.Z

irconia

supp

ortis

then

load

edby

CsN

O3solution

,calcined

at50

0� C

for2h

t¼

4h

S¼

55%

DA;5

%TA

Later,TPA

supp

ortedCs–zircon

iawas

prep

ared

byim

pregnationof

20wt%

ofTPA

solution

,calcined

at35

0� C

for4h

Molar

ratioacetic

acid/glycerol

¼1.5

Catalyst¼

0.2wt%

(i)TPA

/MCM-41

Impregna

tion

metho

dTPA

/MCM-41:

T¼

100

� CTPA

/MCM-41:

53BothTPA

/MCM-41an

dTPA

/ZrO

2catalystswereprep

ared

byincipien

tim

pregnation.M

CM-41was

impregnated

with1%

ofTPA

solution

;while,

ZrO2with10

–40%

ofTPA

,bothdriedat

100

� Cfor10

h

SSA¼

360m

2g�

1t¼

360min

C¼

87%

Acidity

¼0.85

5mmol

g�1

(majorityof

strongacid

strength)

Moleratioaceticacid

:glycerol

¼6:1

S¼

75%

DA&TA

Catalyst¼

0.15

g(ii)TPA

/ZrO

2TPA

/ZrO

2:

TPA

/ZrO

2:

SSA¼

146m

2g�

1C¼

80%

Acidity

¼0.84

0mmol

g�1(fully

weakacid

strength)

S¼

40%

DA&TA

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 68885–68905 | 68899

Review RSC Advances

Publ

ishe

d on

27

June

201

6. D

ownl

oade

d by

IN

P T

oulo

use

on 7

/11/

2018

3:4

2:17

PM

. View Article Online

Tab

le5

(Contd.)

Catalyst

Catalystprep

aration

Catalystch

aracterization

Ope

ratingpa

rameters

Performan

ceRef.

HPW

/Ag

Ion-exchan

gedmetho

dAcidity

¼1.92

meq

uiv.

g�1

T¼

120

� CC¼

96.8%

54HPW

/Agwas

prep

ared

byion-exchan

gedmethod

.A0.1mol

L�1

AgN

O3solution

was

adde

ddrop

-wiseinto

HPW

solution

,aged2h,

evap

orated

,dried

at80

� C(12h)an

dcalcined

at25

0� C

for4h

t¼

4h

S¼

90.7%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

4:1

Catalyst¼

10wt%

ofglycerol

Speed¼

1200

rpm

TPA

/Nb 2O5

Impregna

tion

metho

dAcidity

¼1.14

9mmol

g�1

T¼

120

� CC¼

90%

55TPA

/Nb 2O5was

prep

ared

byim

pregnationmethod

.Nb 2O5su

pport

was

impregnated

with5–30

wt%

ofTPA

inmethan

ol,d

ried

at12

0� C

(12h)an

dcalcined

inairat

300

� Cfor2h

SSA¼

66m

2g�

1t¼

4h

S¼

76%

DA&TA

PV¼

0.36

cm3g�

1Molar

ratioacetic

acid

:glycerol¼

5:1

Catalyst¼

4wt%

ofglycerol

CsP

WA(cesium

phosph

otun

gstate)

catalyst

CsP

WAwas

prep

ared

inmolar

ratioCs 2

.5H

0.5PW

12O40,by

adding

drop

wiseaq

ueou

scesium

carbon

ate(0.47M)toH

3PW

12O

40(0.75M)

atroom

tempe

rature.T

heprecipitatewas

aged

inaq

ueou

smixture

for48

hat

room

tempe

rature,d

ried

inarotary

evap

orator

at45

� C,

then

inan

oven

at15

0� C

for1.5h

Acidity

¼1.87

mmol

g�1

T¼

85� C

C¼

98.1%

2SS

A¼

110m

2g�

1t¼

120min

S¼

75%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

8:1

Catalyst¼

7wt%

Metal

oxide-ba

sedacid

catalyst

SO42�/SnO2

Wet-impregna

tion

metho

dAcidity

¼0.18

6mmol

g�1

T¼

70� C

C¼

99%

56Thecatalyst

was

prep

ared

bywet-im

pregnationmethod

,whereSn

O2

was

adde

dinto

0.5M

H2SO4solution

(massratioof

10wt%

ofSO

42�),

driedat

120

� C(12h)an

dcalcined

inairat

650

� Cfor5h

SSA¼

41m

2g�

1t¼

2h

PD¼

15.79nm

Molar

ratioacetic

acid

:glycerol¼

1:1

PV¼

0.16

23cm

3g�

1Catalyst¼

5wt%

ofglycerol

Speed¼

800rpm

Performed

insolven

tsystem

SO42�/TiO

2–SiO

2Precipitationan

dim

pregna

tion

metho

dSS

A¼

550m

2g�

1T¼

120

� CC¼

91.4%

59Thecatalystwas

prep

ared

byco-precipitation

method

,whereTi(OH) 4

sol,Si(O

C2H

5),C2H

5OH

andwater

weremixed

toform

Ti(OH) 4an

dSi(O

H) 4mixture

sola

t80

� C.Itw

asthen

driedat

100

� Can

dcalcined

for1hat

500

� C

Catalyst¼

5wt%

ofglycerol

TiO

2–SiO2was

sulfated

with1.0M

sulfuric

acid

for1h,d

ried

unde

rinfrared

lampan

dcalcined

at45

0� C

for3h

Performed

insolven

treaction

system

MoO

3/TiO

2–Z

rO2

Co-precipitationan

dim

pregna

tion

metho

dAcidity

¼0.61

mmol

g�1

T¼

120

� CC¼

100%

25MoO

3/TiO

2–Z

rO2was

prep

ared

byco-precipitation

andim

pregnation

method

.Thesolids

weredriedat

120

� C(12h)a

ndcalcined

inairat

650

� C(5

h),wheretheratioof

TiO

2–Z

rO2is

1to

1.Meanwhile,

the

ZrO2was

prep

ared

inad

vance

byprecipitationof

ZrOCl 2withdrop

-wiseof

NH

4OH

solution

SSA¼

7m

2g�

1t¼

60h

S¼

80%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

6:1

Catalyst¼

5wt%

ofglycerol

SO42�/CeO

2–Z

rO2

Impregna

tion

metho

dSS

A¼

92m

2g�

1T¼

120

� CC¼

100%

60Thecatalyst

was

prep

ared

bywet

sulfon

ation-im

pregnationmethod

,where0.5M

H2SO

4solution

was

mixed

into

1:1

mol

ratioof

CeO

2–

ZrO2(based

onoxides),stirredfor1h,d

ried

at12

0� C

(3h)an

dcalcined

for5hat

500

� C

t¼

1h

S¼

74.2%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

6:1

Catalyst¼

5wt%

68900 | RSC Adv., 2016, 6, 68885–68905 This journal is © The Royal Society of Chemistry 2016

RSC Advances Review

Publ

ishe

d on

27

June

201

6. D

ownl

oade

d by

IN

P T

oulo

use

on 7

/11/

2018

3:4

2:17

PM

. View Article Online

Tab

le5

(Contd.)

Catalyst

Catalystprep

aration

Catalystch

aracterization

Ope

ratingpa

rameters

Performan

ceRef.

Mesop

orou

ssilica-based

acid

catalyst

MPA

/SBA-15

SBA-15

preparation

Acidity

¼0.86

mmol

g�1

T¼

110

� CC¼

100%

70SS

A¼

754m

2g�

1t¼

3h

S¼

92.3%

DA&TA

Pluron

icP1

23was

dissolvedin

1.8M

HCl,follo

wed

byad

dition

ofBrijS1

00.A

er

18h,d

roplet

ofTEOSwas

adde

dat

35–40

� C.T

he

form

edSB

A-15was

driedat

90� C

(24h),follo

wed

byDIwater

washingan

dre-dried

atroom

tempe

rature

PD¼

6.53

nm

Molar

ratioacetic

acid

:glycerol¼

6:1

PV¼

1.24

cm3g�

1Catalyst¼

2wt%

ofglycerol

Impregna

tion

metho

dMPA

/SBA-15catalyst

was

impregnated

bymolyb

doph

osph

oric

acid

solution

atroom

tempe

rature

for24

h.Itwas

driedan

dcalcined

inairfor6hat

560

� CMP-nb-SB

A-15

nb-SBA-15

silica

preparation:

Acidity

¼0.50

meq

uiv.

g�1

T¼

150

� CC¼

94%

71Molar

ratioof

mixture

was

prep

ared

atch

emical

ratioof:

1SiO

2:0

.005

Pluron

icP1

23:1

.45H

Cl:

124H

2O.A

mmon

ium

nioba

te( V)oxalate

was

then

adde

dinto

mixture

andstirredfor8hat

55� C

.Thermal

treatingof

solution

was

condu

cted

at80

� Cwithou

tstirringfor16

h.T

hesolidsamplewas

ltered

,washed

anddriedat

60� C

(12h)an

dcalcined

at55

0� C

(8h)

SSA¼

565m

2g�

1t¼

4h

S¼

89%

DA&TA

PD¼

6.8nm

(51%

)Molar

ratioacetic

acid

:glycerol¼

9:1

S TA¼

40%

PV¼

0.38

cm3g�

1Catalyst¼

0.6wt%

Post

synthesisof

nb-SBA-15

:SB

A-15wereheated(350

� C,4

h)priorto

addition

ofMPT

MSin

toluen

esolution

.Themixtureswereheatedat

100

� Cfor20

h,

washed

,dried

at10

0� C

(4h).Oxidationof

mod

ier

was

carriedou

tby

usingH

2O2an

dH

2SO

4solution

Y/SBA-3

Hydrothermal

metho

dAcidity

¼1.34

mmol

g�1

T¼

110

� CC¼

100%

40Thecatalyst

was

prep

ared

bydissolvingof

CTMABrin

0.4V%

HCl

(37%

)solution

s.Y(N

O3) 3$6H

2O

was

drop

wisead

dedto

TEOS

solution

andthen

aged

at12

h(roo

mtempe

rature).Itwas

washed

,ltered

,dried

(12hat

100

� C)a

ndthen

calcined

inairat

560

� Cfor8

h(heatingrate

of2

� Cmin

�1)

SSA¼

1568

m2g�

1t¼

2.5h

S¼

89%

DA&TA

PD¼

2.54

nm

Molar

ratioacetic

acid

:glycerol¼

4:1

PV¼

0.81

cm3g�

1Catalyst¼

5wt%

ofglycerol

Speed¼

350rpm

(i)Nb/MCF

Hydrothermal

metho

dNb/MCF:

T¼

150

� CNb/MCF:

72BothMCFsu

pportof

thecatalystswereprep

ared

bydissolvingof

P123

into

0.7M

HClsolution

,add

edby

1,3,5-trim

ethylbe

nzene,

NH

4F,T

EOS(aer

1h).Next,(i)am

mon

ium

nioba

te(V)oxalate

hyd

rate;(ii)tan

talum(V)ethoxide,was

adde

dto

form

Nb/MCFan

dTa/

MCFresp

ectively.T

hesolution

was

mixed

(20h),hyd

rothermal-

treated(100

� Cfor24

h),driedan

dcalcined

inairat

500

� Cfor8h

Acidity

¼0.32

meq

uiv.

g�1

t¼

4h

C¼

89%

Ta/MCF:

Molar

ratioacetic

acid

:glycerol¼

9:1

S¼

89%

DA&TA

Acidity

¼0.57

meq

uiv.

g�1

Catalyst¼

4wt%

ofglycerol

Ta/MCF:

C¼

91%

(ii)Ta/MCF

S¼

87%

DA&TA

Mesop

orou

scarbon

-based

acid

catalyst

AC-SA5

Hydrothermal

metho

dBlank

ACT¼

120to

135

� CC¼

91%

75Thecatalystwas

prep

ared

byhyd

rothermalmethod

.Activated

carbon

was

treatedin

5mol

L�1of

H2SO

4solution

at85

� Cfor4h

SSA¼

780m

2g�

1t¼

3h

S¼

62%

DA&TA

PV¼

0.52

cm3g�

1Molar

ratioacetic

acid

:glycerol¼

8:1

PSD

¼50

0–71

0mm

Catalyst¼

4wt%

ofglycerol

AC-SA5

Acidity

¼0.89

mmol

g�1

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 68885–68905 | 68901

Review RSC Advances

Publ

ishe

d on

27

June

201

6. D

ownl

oade

d by

IN

P T

oulo

use

on 7

/11/

2018

3:4

2:17

PM

. View Article Online

Tab

le5

(Contd.)

Catalyst

Catalystprep

aration

Catalystch

aracterization

Ope

ratingpa

rameters

Performan

ceRef.

SSA¼

742m

2g�

1

PV¼

0.47

cm3g�

1

PSD

¼50

0–71

0mm

Mesop

oreSS

A¼

208m

2g�

1

Sulfated

silica

template

carbon

ized

acid

catalyst

Hydrothermal

metho

dAcidity

¼1.35

mmol

g�1

T¼

180

� CC¼

99.6%

61Na 2SiO3an

dH

2O

weremixed

at70

� Cto

prep

aresilica

template.

Sucrosean

dHClw

erethen

adde

dinto

mixture.T

hesolution

was

le

for24

hforpo

lymerization.C

arbo

nized

ofsilica

was

condu

cted

at40

0� C

inN2(5

h),follo

wed

byim

merse

with3M

NaO

Hsolution

(12h

and12

0� C

)an

dhot

water

washing

SSA¼

556m

2g�

1t¼

4h

S¼

50%

TA

PD¼

2–50

nm

(51%

)Molar

ratioacetic

acid

:glycerol¼

9:1

PV¼

0.51

cm3g�

1Catalyst¼

5wt%

Carbo

nized

solids

werelein

contact

overnightw

ithfumingsu

lfuric

acid

(7%

ofSO

3)

Sulfon

ated

willow

catkins-ba

sedcarbon

Hydrothermal

metho

dAcidity

¼5.14

mmol

g�1

T¼

120

� CC¼

98.4%

76Thewillowcatkin

(10g)

was

carbon

ized

inN2at

450

� Cfor5h.T

he

blackpo

wde

r(1

g)was

then

heatedin

5mLof

concentrated

H2SO

4

(95–98

%)for3hat

differen

ttempe

ratures.

Itwas

cooled

down,

ltered

,dried

at80

� C(5

h).Thedriedpo

wde

r(1

g)was

then

treated

in14

mLof

fumingsu

lfuricacid

(15wt%

SO3)a

tto10

0,90

,80,an

d70

� C,tem

peraturesfor2han

dthen

cooled

toroom

tempe

rature

Con

tentof

–SO3H

¼2.85

mmol

g�1

t¼

2h

S¼

67.2%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

5:1

Catalyst¼

5wt%

GO

Hydrothermal

andoxidationmetho

dNot

repo

rted

T¼

120

� CC¼

98.5%

77t¼

1h

S¼

94.5%

DA&TA

Molar

ratioacetic

acid

:glycerol¼

10:1

S DA¼

60%

Graph

ite(5

g)an

dNaN

O3(2.5g)

wereplaced

into

115mLH

2SO

4in

aniceba

thun

dervigo

rousstirring.

Aer

addingof

15gKMnO4,itwas

heatedto

35� C

andstirredforextra30

min.T

hemixture

was

diluted

withwater

andre-heatedto

98� C

.Sub

sequ

ently,

50mLH

2O2(30

wt%

)was

adde

d,follo

wed

byltered

,washed

,dried

(50

� C).Then

,thedisp

ersedGO

was

kept

inwater

forsonication(1

h),centrifug

edan

ddriedat

ambien

ttempe

rature

Catalyst¼

0.1g

Others

MgF

2�x(OH) x,x

<0.1

Hydrothermal

metho

dAcidity

¼0.33

mmol

g�1

T¼

100

� CC¼

>99%

36MetallicMgwas

dissolvedin

methan

ol(50mL)

atroom

tempe

rature

overnight.Aer

heatingunder

reux

condition

sfor3h,H

Fsolution

was

adde

dto

theform

edMg(OCH

3) 2solution

,then

aged

for12

han

ddried

unde

rvacu

umat

room

tempe

rature.T

hesolidprod

uctthus

obtained

was

then

further

driedunde

rvacu

umat

70� C

for5h

SSA¼

424m

2g�

1t¼

22h

S¼

85%

DA&TA

PV¼

0.25

cm3g�

1Molar

ratioacetic

acid

:glycerol¼

3:1

PD¼

2.2nm

Fe–Sn–T

i(SO

42�)-40

0Precipitationan

dhydrotherm

almetho

dSS

A¼

18.88m

2g�

1T¼

80� C

C¼

100%

78Themag

neticmatrixwas

mad

eby

mixture

of[Fe 2SO

4/Fe

2(SO4) 3]a

t45

� C.T

hematrix(0.1

mol

L�1)was

then

treatedwithstan

nic

chloride

pentahyd

rate

(17.5g),tetrabu

tyltitan

ate(10mL)

andNH

3$H

2O.T

he

form

edsolidsampleis

dried(100

� C)an

dfurther

sulfated

with

(NH

4) 2SO

4solution

for24

h,w

ashed

,ltered

,dried

andcalcined

atdifferen

ttempe

ratures

PV¼

0.15

cm3g�

1t¼

30min

S¼

99%

TA;1

%DA

PD¼

3.8nm

Massratioacetic

acid

toglycerol

¼5.6

Catalyst¼

2.5wt%

SSA¼

specicsu

rfacearea

ofcatalyst;P

D¼

pore

diam

eter

ofapa

rticle;P

S¼

particle

size

ofcatalyst;P

V¼

pore

volumeof

catalyst;P

SD¼

particle

size

distribu

tion

ofcatalyst.C

¼conversion;S

¼selectivity;

Y¼

yield.

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400 �C. The calcination temperature is a crucial factor todevelop a good acid strength of the catalyst given that a catalystcalcined at 400 �C displays weak acid sites. The catalysts calcinedat 500 �C, 600 �C, and 700 �C promoted the formation ofa superacid structure. Furthermore, the acidity of magnetic cata-lyst is mainly contributed by Lewis acid (from Sn and Ti metalcations) and Brønsted acid (from proton of –OH groups). Both theLewis and Brønsted acid sites play key role in activating thecarbonyl group from acetic anhydride, which is further attackedby glycerol molecule for esterication process. The (Fe–Sn–Ti(SO4

2�))-400 catalyst exhibits good stability during catalystrecyclability test. Unfortunately, (Fe–Sn–Ti(SO4

2�))-400 demon-strated a water intolerance behavior as selectivity of TA is reducedto 26% when small amount of water was added during the reac-tion. This work also concluded that the catalytic activity is highlydependent on the acid strength but not on the total acid amountof the catalyst. Similar fact was also supported by Huang et al.,79

where the presence of both Lewis and Brønsted acid sites areobserved from sulfated Sr–Fe oxide and sulfated Ca–Fe oxidecatalysts, respectively. The study summarized that Brønsted acidsites are able to catalyze the esterication of fatty acids via theprotonation of the acid group (–COOH) to give oxonium ions,while the Lewis acid sites catalyze the esterication of fatty acidsthrough the coordination of acid groups on the active sites.Table 5 shows the different types of heterogeneous catalysts usedin glycerol acetylation using acetic acid.

6. Conclusions

Glycerol acetylation using acetic acid allows the cost-effectiveproduction of MA, DA, or TA compared with the use of aceticanhydride. Despite the spontaneous reaction and formationof electrophilic intermediates in catalytic acetylation of glyc-erol, the combined high conversion and selectivity of DA andTA can now be attained under mild reaction environment byusing a well-designed heterogeneous acid catalyst. For TA, themagnetic solid acid catalyst (Fe–Sn–Ti(SO4

2�))-400 iscurrently the most competent catalyst because 99% selectivityof TA with a 100% total conversion was attained. However, TAformation is strongly affected by the acidity of the catalyst(more specically by weak acid strength), by pore aperture(sufficient pore space to facilitate formation of large mole-cule), as well as by the correct shape–structure (high cross-linkage) at high molar ratio of acetic acid to glycerol (9 : 1).By contrast, low-pore catalyst should be used to generate highselectivity of small MA under excessive glycerol concentration(1 : 8 molar ratio of acetic acid to glycerol). The vital roles ofthe catalyst to increase product selectivity include controllingthe acid sites, pore diameter, hydrolysis resistance, andhydrophobicity, whereas the molar ratio of acetic acid toglycerol is the more inuential factor that improves thecombined selectivity of DA and TA. Developing a hydro-phobic-enhanced magnetic solid acid catalyst to overcomethe problem on water deactivation and subsequently devel-oping a scalable high-conversion-selectivity catalyst isstrongly recommended.

This journal is © The Royal Society of Chemistry 2016

Abbreviations

MA

Monoacetin DA Diacetin TA Triacetin TBA tert-Butyl alcohol GMBE Mono-tert butyl ether GDBE Glycerol di-tert butyl ether GTBE Glycerol tertiary butyl ether GEE Glycerol ethyl ether mono-GEE Glycerol mono-ethyl ether di-GEE Glycerol di-ethyl ether tri-GEE Glycerol tri-ethyl ether TBA tert-Butyl alcohol PES Polymer-based acid H3PMo12O40 Dodeca-molybdophosphoric acid HPAs Heteropolyacids HSiW Silicotungstic acid HPW Phosphotungstic acid HPMo Phosphomolybdic acid TPA 12-Tungstophosphoric acid SnO2 Tin oxide MPTMS 3-Mercaptopropyl trimethoxysilane MCF Mesostructured cellular foams AC Activated carbon GO Graphene oxide

Acknowledgements

The authors thank University of Malaya for supportingthis research under High Impact Research grant (GrantNumber: UM.C/625/1/HIR/MOHE/ENG/59). The Laboratoir-ede Genie Chimique of Campus INP-ENSIACET, SBUMscholarship and French government scholarship are grate-fully acknowledged.

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