Influence of Heterogeneous Catalysts and Reaction Parameters

Home , Carboxylation, Lauric acid, Triacetin

applied sciences

Review Inﬂuence of Heterogeneous Catalysts and Reaction Parameters on the Acetylation of Glycerol to Acetin: A Review

Usman Idris Nda-Umar 1,2,*, Irmawati Binti Ramli 1,3,4,* , Ernee Noryana Muhamad 1,3, Norsahida Azri 1,3 , Uchenna Fidelis Amadi 1 and Yun Hin Tauﬁq-Yap 1,3

1 Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia; [email protected] (E.N.M.); [email protected] (N.A.); ﬁ[email protected] (U.F.A.); tauﬁ[email protected] (Y.H.T.-Y.) 2 Department of Chemical Sciences, Federal Polytechnic, P.M.B. 55, Bida 912001, Nigeria 3 Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia 4 Laboratory of Processing and Product Development, Institute of Plantation Studies, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia * Correspondence: [email protected] (U.I.N.-U.); [email protected] (I.B.R.)

Received: 8 September 2020; Accepted: 21 September 2020; Published: 14 October 2020

Abstract: Glycerol, a polyhydric alcohol, is currently receiving greater attention worldwide in view of its glut in the market occasioned by the recent upsurge in biodiesel production. The acetylation of glycerol to acetin (acetyl glycerol) is one of the many pathways of upgrading glycerol to fine chemicals. Acetin, which could be mono, di, and or triacetin, has versatile applications in the cosmetics, medicines, food, polymer, and fuel industries as a humectant, emulsifier, plasticizer, and fuel additive and so it is of high economic value. Given the critical role of catalysts in green chemistry, this paper reports the influence of the different heterogeneous catalysts used in glycerol acetylation. It also reviewed the influence of catalyst load, temperature, molar ratio, and the time on the reaction.

Keywords: glycerol; acetylation; heterogeneous catalyst; reaction parameters; acetin

1. Introduction Glycerol, also known as propane-1,2,3-triol, is a polyhydric alcohol with three hydrophilic hydroxyl groups, each attached to the three carbon atoms. The hydroxyl groups are mainly responsible for the properties and applications of glycerol [1,2]. Glycerol is soluble in water and other polar solvents. It is a clear, colourless, odourless, viscous, non-toxic, and hygroscopic liquid with a high boiling point (290 ◦C) and low melting point (18 ◦C) [3–5]. Glycerol has versatile applications in medical, pharmaceutical, personal care, and food industries as sweetener, flavour, preservative, emulsifier, humectant, antioxidant, and additive. It is also used in the automotive and chemical industries as a plasticizer, lubricant, and cryoprotectant [6–8]. The distribution of some of these applications varies in different literature reports. The percentage distribution, as found in various literature articles, is shown in Figure1[ 9]. It has been reported that glycerol is among the 12 most important bio-based chemicals worldwide [10], with the global market expected to reach USD 2.52 billion by 2020 [11].

Appl. Sci. 2020, 10, 7155; doi:10.3390/app10207155 www.mdpi.com/journal/applsci Appl.Appl. Sci. 2020 Sci.,2020 10, x, 10FOR, 7155 PEER REVIEW 2 of 332 of 34

Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 33

Cosmetics 7% 5% 5% FoodCosmetics 8% 7% 37% AlkydFood resin 8% 37% 9% TobaccoAlkyd resin 9% PharmaceuticalTobacco 9% PolyurethanPharmaceutical 9% 25% OthersPolyurethan 25% Others

Figure 1. Distribution of major applications of glycerol in various literature reports [9]. Figure 1. Distribution of major applications of glycerol in various literature reports [9]. GlycerolFigure can be 1. obtainedDistribution via of syntheticmajor applications or natural of glycerol means. in Thevarious synthetic literature route reports is usually[9]. through propyleneGlycerol can oxidation be obtained or chlorination, via synthetic while or natura the naturall means. route The involves synthetic the route hydrolysis is usually ofvegetable through oil Glycerol can be obtained via synthetic or natural means. The synthetic route is usually through propyleneand or theoxidation transesterification or chlorination, of fatty while acids the or natural vegetable route oil involves as well as the the hydrolysis fermentation of vegetable of yeast [4 ,oil12 ,13]. propylene oxidation or chlorination, while the natural route involves the hydrolysis of vegetable oil and Theor the synthesis transesterification of glycerol of via fatty propylene acids or is ve basedgetable on oil petroleum as well as feedstock the fermentation (Scheme1 )of and yeast is no and or the transesterification of fatty acids or vegetable oil as well as the fermentation of yeast [4,12,13].longer The popular synthesis due of to glycer the globalol via shiftpropylene to less is carbon based on emission. petroleum The feedstock hydrolysis (Scheme of oil, 1) also and known is [4,12,13]. The synthesis of glycerol via propylene is based on petroleum feedstock (Scheme 1) and is no longeras saponification, popular due involvesto the global the reactionshift to less of triglyceridecarbon emission. with The alkaline hydrolysis hydroxide of oil, (Scheme also known2). It is no longer popular due to the global shift to less carbon emission. The hydrolysis of oil, also known as saponification, involves the reaction of triglyceride with alkaline hydroxide (Scheme 2). It is an old anas old saponification, method that involves dates back the reaction to 2800 of BC triglyceri and accountedde with alkaline for most hydroxide of the commercial(Scheme 2). It needs is an old over method that dates back to 2800 BC and accounted for most of the commercial needs over the years themethod years [that14,15 dates]. However, back to 2800 production BC and throughaccounted this for method most of isthe no commercial longer meeting needs over the global the years need. [14,15]. However, production through this method is no longer meeting the global need. The The[14,15]. fermentation However, method production is limited through in industrial this meth applications.od is no longer The transesterification meeting the global method need. involves The fermentationthefermentation reaction method of method triglyceride is limited is limited in (oil) industrial in with industrial alcoholapplic applications. inations. the The presence The transesterification transesterification of a catalyst, method method which involves involves results the in the the reactionproductionreaction of triglyceride of of triglyceride biodiesel (oil) with(fatty (oil) alcoholwith acid alcohol esters) in the in and presence the glycerol presence of a (Scheme ofcatalyst, a catalyst,3). which The which transesterification results results in inthe the production production method is of biodieselnowof biodiesel the (fatty most acid(fatty popular esters) acid becauseesters) and glycerol and of glycerol the (Scheme large (Scheme quantity 3). The 3). of Thetransesterification biodiesel transesterification produced. method method Biodiesel is nowis now is the consideredthe most most popularto bepopular because an environmentally because of the of large the benign largequan quantity renewable oftity biodiesel of energybiodiesel produced. that produced. is biodegradable, Biod Biodieseliesel is non-toxic,isconsidered considered and to to renewablebe be an an environmentallywithenvironmentally less obnoxious benign benign renewable emissions renewable [energy11,16 energy]. that The thatis transesterification bi odegradable,is biodegradable, non-toxic, method non-toxic, and also and renewable produced renewable glycerolwith with less less as a obnoxiousby-product,obnoxious emissions whichemissions [11,16]. is known [11,16]. The transesterification asThe crude transesterification glycerol duemethod method to its also low-grade also produced produced purity. glycerol glycerol The as recentasa by-product,a by-product, increase in whichbiodiesel whichis known is production known as crude as hascrude glycerol led glycerol to adue proportional dueto its to low-gradeits increaselow-grade purity. in purity. the The crude The recent glycerol.recent increase increase It has in been inbiodiesel biodiesel reported productionthatproduction for has every led has 100to leda kgproportional to of a proportional biodiesel increase produced, increase in the in 10 crude the kg crude (10%) glycerol. glycerol. of glycerol It has It has been is been obtained reported reported as that thethat for by-productfor every every 100 kgleading 100of biodiesel kg to of surplus biodiesel produced, in produced, the market 10 kg 10 (10%) [12 kg,17 (10%) of,18 glycerol]. of Recent glycerol is literature obtained is obtained indicatesas the as theby-product thatby-product by the leading yearleading 2020, to tosurplus thesurplus global in theproduction inmarket the market [12,17,18]. of glycerol [12,17,18]. Recent will Recent moveliterature literature up to indicates 41.9 indicates billion that litresthatby theby [19 theyear]. Thisyear 2020, number2020, the the global isglobal expected production production to increase of of glycerolwithglycerol will the recentmove will move predictionup to up41.9 to thatbillion41.9 biodieselbillion litres litres [19]. will [19].Th accountis Th numberis fornumber over is expected 70%is expected of the to globalincreaseto increase transportation with with the the recent recent fuel by prediction that biodiesel will account for over 70% of the global transportation fuel by 2040 [20]. This prediction2040 [20 that]. This biodiesel forecast will is supportedaccount for by over many 70% deliberate of the global policies transportation put in place fuel by di byfferent 2040 countries[20]. This and forecast is supported by many deliberate policies put in place by different countries and forecastorganizations is supported to encourage by many biodiesel deliberate production policies and put its in utilization, place by such different as the implementationcountries and of organizations to encourage biodiesel production and its utilization, such as the implementation of organizationsmandatory to biodiesel encourage blending biodiese targets,l production tax exemptions, and its utilization, government such support, as the implementation investment subsidies, of mandatory biodiesel blending targets, tax exemptions, government support, investment subsidies, mandatoryand research biodiesel and blending development targets, programs tax exemptions, [20]. Figure government2 shows the support, progressive investment increase subsidies, in biodiesel and research and development programs [20]. Figure 2 shows the progressive increase in biodiesel and research and development programs [20]. Figure 2 shows the progressive increase in biodiesel productionproduction for for the the last last ten ten years years [ 21[21].]. TheThe aboveabove policies have have led led to to the the diversification diversification of ofbiodiesel biodiesel production for the last ten years [21]. The above policies have led to the diversification of biodiesel feedstocksfeedstocks depending depending on on the the country, country, as as indicated indicated inin Table 11.. The impact impact of of excess excess glycerol glycerol produced produced feedstocks depending on the country, as indicated in Table 1. The impact of excess glycerol produced fromfrom the the transesterification transesterification process process is is already already feltfelt inin thethe marketmarket [22]. [22]. from the transesterification process is already felt in the market [22].

SchemeScheme 1. 1.Synthesis Synthesis ofof glycerolglycerol via prop propyleneylene chlorination/oxidation. chlorination/oxidation. Scheme 1. Synthesis of glycerol via propylene chlorination/oxidation. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 33 Appl. Sci. 2020, 10, 7155 3 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 33 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 33

Scheme 2. Alkaline hydrolysis of oil to produce glycerol (saponification reaction). Y = Alkali metal. SchemeScheme 2. 2.Alkaline Alkaline hydrolysis hydrolysis of of oil oil toto produceproducproduce glycerol (saponification (saponiﬁcation(saponification reaction). reaction).reaction). Y Y= =Alkali Alkali metal. metal.

Scheme 3. Transesterification of triglyceride to biodiesel fatty acid esters and glycerol. SchemeScheme 3. 3.TransesterificationTransesteriﬁcation Transesterification of of triglyceride triglyceride to biodiesel fatty fatty acid acid esters esters and and glycerol. glycerol.

35 3535 3030 30 2525 25 2020 20 1515 15 1010 10 5 5 Annual Total Biodiesel Total Annual Annual Total Biodiesel Total Annual 5 Production (billion litres) (billion Production Production (billion litres) (billion Production 0 0 Annual Total Biodiesel Total Annual Production (billion litres) (billion Production 0 20082008 2009 2009 2010 2010 2011 2011 2012 2012 2013 2013 2014 2015 2015 2016 2016 2017 2017 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 YearYear Year

Figure 2. Annual total biodiesel production for the last ten (10) years. Data obtained from REN21 [21]. FigureFigure 2. Annual 2. Annual total total biodiesel biodiesel production production for for the the last last tetenn (10) years. DataData obtained obtained from from REN21 REN21 [21]. [21]. Figure 2.Table Annual 1. totalCommon biodiesel feedstocks production of biodiesel for the productionlast ten (10) byyears. diﬀ erentData obtained countries from [23–26 REN21]. [21]. Table 1. Common feedstocks of biodiesel production by different countries [23–26]. Table 1. CommonFeedstocks feedstocks of biodiesel production by different Countries countries [23–26]. Table 1. Common feedstocks of biodiesel production by different countries [23–26]. Feedstocks Countries SoybeanFeedstocks oil (Glycine max) America, Brazil,Countries Argentina SoybeanFeedstocks oil (Glycine max) America, CountriesBrazil, Argentina RapeseedSoybean oiloil ( Brassica(Glycine napus max) L. ) Germany, Australia, America, Argentina, Brazil, Argentina France, Canada RapeseedSoybean oil ((BrassicaGlycine napusmax) L.) Germany, Australia, America, Argentina,Brazil, Argentina France, Canada LinseedRapeseed oil (oilLinum (Brassica usitatissimum napus )L. and) Germany, Australia, Argentina, France, Canada LinseedRapeseed oil (oilLinum (Brassica usitatissimum napus L.)) and Germany, Australia,Spain Argentina, France, Canada Linseed oilOlive (Linum oil (Olea usitatissimum europaea) ) and Spain Linseed oilOlive (Linum oil (Olea usitatissimum europaea) ) and Spain SunﬂowerOlive oil oil (Olea (Helianthus europaea annuus) ) France,Spain Italy SunflowerOlive oil oil (Olea (Helianthus europaea annuus) ) France, Italy Sunflower oil (Helianthus annuus) France, Italy SunflowerCastorCastor oiloil ( Ricinus(RicinusHelianthus communis communis annuus)) ) France, BrazilBrazil Italy Castor oil (Ricinus communis) Brazil CastorGuangGuang oil pipi ( (Ricinus Cornus(Cornus wilsonianacommunis wilsoniana))) ChinaChinaBrazil GuangPalm oilpi (CornusElaeis guineensis wilsoniana) and) China GuangPalm oil pi ( Elaeis(Cornus guineensis wilsoniana) and) Indonesia, Malaysia,China Colombia, Thailand Palmcoconut oil (Elaeis oil (guineensisCocus nucifera) and) Indonesia, Malaysia, Colombia, Thailand Palmcoconut oil (Elaeis oil (Cocus guineensis nucifera) and) Indonesia, Malaysia, Colombia, Thailand coconutAnimals oil ( Cocusfat/Beef nucifera tallow) Indonesia, Australia, Malaysia, Ireland, Colombia, Canada Thailand coconutAnimals oil fat(Cocus/Beef tallownucifera) Australia, Ireland, Canada JojobaAnimals oil ( Simmondsiafat/Beef tallow chinensis ) Mexico, Australia, Southwest Ireland, and Central Canada America Animals fat/Beef tallow Australia, Ireland, Canada JojobaJojoba *oil Jatropha oil (Simmondsia (Simmondsia oil (J. curcas chinensis), )) Mexico, Mexico, Southwest Southwest and and Central Central America America Jojoba oil (Simmondsia chinensis) Mexico, Southwest and Central America * Karanja* Jatropha* Jatropha seed oiloil oil ((Pongamia (J.J. curcas curcas),), pinnata ), * Karanja* Karanja* Neem* Jatropha seed seed oil oil oil(Azadirachta oil(Pongamia (Pongamia (J. curcas indica pinnatapinnata), ),), ), * Neem oil (Azadirachta indica), Countries cut across Asia, Europe, and Africa * Karanja* Rubber seed seed oil oil (Pongamia (Hevea brasiliensis pinnata), Countries cut across Asia, Europe, and Africa ** RubberNeem oil seed (Azadirachta oil (Hevea brasiliensis indica), ), * Neem* Mahua oil (oilAzadirachta (Madhuca indica), Countries cut across Asia, Europe, and Africa * Rubber* Mahua seed oiloil ( Madhuca(Hevea brasiliensis indica), ), Countries cut across Asia, Europe, and Africa * Rubber* Mahua** AlgaeAlgae seed oil oil oiloil (Madhuca ( Cyanobacteria(HeveaCyanobacteria brasiliensis indica) ),) ), * Mahua oil (Madhuca indica), * Algae oil (Cyanobacteria* Non-edible* Non-edible) oil oil feedstocks feedstocks recentlyrecently receiving receiving attention. attention. * Algae oil (Cyanobacteria) * Non-edible oil feedstocks recently receiving attention. * Non-edible oil feedstocks recently receiving attention. Appl. Sci. 2020, 10, 7155 4 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 33

CrudeCrude glycerol,glycerol, due due to to its its impurities impurities (methanol, (methanol, inorganic inorganic salts, salts, fatty acids,fatty andacids, water), and water), has limited has applicationslimited applications and is usually and is targetedusually attargeted non-sensitive at non-sensitive areas such areas as polymer such as andpolymer energy, and including energy, fuelincluding [11,12 ,27fuel,28 ],[11,12,27,28], unlike the refined unlike glycerol the refined that has glycerol applications that in has cosmetics, applications pharmaceuticals, in cosmetics, and food,pharmaceuticals, as stated previously. and food, However, as stated crudepreviously. glycerol However, can be purified crude glycerol to improve can be its purified quality to to over improve 95% viaits quality different to processes over 95% outlined via different in Figure processes3. The purificationoutlined in processFigure 3. is The not economicalpurification due process to the is cost not involved.economical The due lowest to the cost cost was involved. put at 0.149The lo USDwest/Kg cost [13 was]. put at 0.149 USD/Kg [13].

FigureFigure 3.3. Crude glycerol puriﬁcationpurification process.process. * Matter organic non-glycerol.non-glycerol.

GivenGiven thethe low low commercial commercial value value of the producedof the produced glycerol, itsglycerol, surplus, andits surplus, possible environmentaland possible issues,environmental efforts are issues, ongoing efforts to are convert ongoing it to to high-value convert it to products, high-value which products, is expected which to is improveexpected itsto commercialimprove its viability,commercial biodiesel viability, economics, biodiesel and econom eliminateics, and the perceivedeliminate environmentalthe perceived environmental concern of the surplus.concern of The the conversion surplus. The of glycerol conversi toon the of high glycerol valued to productsthe high valued is usually products done through is usually various done catalytic through reactionsvarious catalytic such as hydrogenolysisreactions such [29as ,30hydrogenolysis], reforming [31 [29,30],,32], dehydration reforming [33[31,32],,34], carboxylationdehydration [[33,34],35,36], oxidationcarboxylation [37, 38[35,36],], oligomerization oxidation [37,38], [39,40 ],oligomerizat esterificationion or[39,40], acetylation esterification [41,42], or etherification acetylation [[41,42],43,44], acetalizationetherification [43,44], [45,46], acetalization pyrolysis and [45,46], gasification pyrolysis [ 47an,d48 gasification]. A good [47,48]. number A ofgood literature number reviews of literature are availablereviews are to available show the to research show the activities research in activities most of in the most reactions of the abovereactions [7, 9above,34,49 [7,9,34,49–52].–52]. OneOne ofof the the high high valued valued products products receiving receiving attention attention by researchersby researchers and industrialistsand industrialists in recent in recent times istimes oxygenated is oxygenated fuel additives. fuel additives. These additives These additive are usuallys are blendedusually blended with gasoline, with gasoline, diesel, and diesel, or biodiesel and or tobiodiesel improve to their improve cold flow their properties, cold flow viscosity,properties, octane viscosity, number, octane thermal number, stability, thermal reduce stability, gum formation reduce andgum particulate formation emissionand particulate [53–55 emission]. The oxygenated [53–55]. The fuel oxygenated additives from fuel additives glycerol include from glycerol glycerol include esters orglycerol acetyl esters glycerol or (acetin),acetyl glycerol glycerol (acetin), ethers, andglycerol glycerol ethers, formal and (solketalsglycerol formal and acetals) (solketals [3,55 and–58 ].acetals) [3,55–58].Scientific literature articles have revealed few reviews on the synthesis of acetin in recent yearsScientific [59–61], unlikeliterature other articles oxygenated have revealed fuel additives, few reviews namely on ethers, the synthesis solketals, of and acetin acetals in recent [22,62 years–69]. Thus,[59–61], this unlike paper other reviews oxygenated the recent fuel additives, research findings namely ethers, on acetin solketals, production and acetals from [22,62–69]. glycerol with Thus, a particularthis paper focusreviews on the recent heterogeneous research catalystsfindings on used acetin and production how they influenced from glycerol the reaction.with a particular Unlike earlierfocus on reviews, the heterogeneous this review furthercatalysts emphasized used and how the conditions they influenced in which the thereaction. catalysts Unlike were earlier used withreviews, detailed this review findings further documented emphasized to serve the conditio as an appropriatens in which guidethe catalysts for subsequent were used studies with detailed in this area.findings This documented review also to presents serve as some an appropriate of the findings guid ine for pictorial subsequent form forstudies easy in understanding, this area. This review unlike earlieralso presents reviews. some of the findings in pictorial form for easy understanding, unlike earlier reviews.

2. Acetin (Glycerol Esters or Acetyl Glycerol) Acetin is obtained via the reaction of glycerol with acetic acid (or any carboxylic acids), acetic anhydride, or through the transesteriﬁcation of either triglycerides or glycerol with methyl Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 33 acetate [3,54,70]. There are three types of acetin depending on the number of the hydroxyl group fromsubstituted the glycerol from theatom glycerol with the atom acetyl with or the acyl acetyl group. or Hence, acyl group. the reaction Hence, theis referred reaction to is as referred acetylation. to as Theacetylation. acetin includes The acetin monoacetin includes monoacetin (monoacetyl (monoacetyl glycerol), diacetin glycerol), (diacetyl diacetin glycerol) (diacetyl glycerol)and triacetin and (triacetyltriacetin (triacetyl glycerol) glycerol) [71–73]. [Their71–73 ].structures Their structures are shown are shownin Scheme in Scheme 4. All 4types. All typesof acetin of acetin have havea wide a widerange rangeof applications. of applications.

Scheme 4. Acetylation of glycerol with carboxylic acid/acetic anhydride. R = alkyl or alkenyl group. Scheme 4. Acetylation of glycerol with carboxylic acid/acetic anhydride. R = alkyl or alkenyl group. Monoacetin andand diacetin diacetin are are used used in the in cosmetics, the cosm medicines,etics, medicines, and food and industries food asindustries humectants as humectantsand emulsifiers and emulsifiers [12,74]. Monoacetin [12,74]. Monoacetin is also used is also as used a tanning as a tanning agent agent and inand the in the production production of ofexplosives explosives [75 [75].]. Both Both monoacetin monoacetin and and diacetin diacetin are are monomers monomers forfor thethe productionproduction ofof biodegradable polyesters [76[76].]. TriacetinTriacetin is is used used as aas solvent a solvent in various in various reactions, reactions, as a plasticizer as a plasticizer of cellulosic of cellulosic polymers polymersand copolymers and copolymers [59,77]. Triacetin [59,77]. and Triacetin diacetin and are, diacetin most importantly, are, most importantly, also used as also biofuel used additives as biofuel to additivesimprove viscosityto improve and viscosity cold flow and properties cold flow asproperti well ases toas reducewell as greenhouseto reduce greenhouse gas emissions gas emissions [3,78–82]. [3,78–82].In view of In the view excellent of the applicationsexcellent applications of triacetin, of itstriacetin, price is its high price and is high remains and stableremains with stable a growing with a growingdemand ofdemand 5–10% of yearly 5–10% [59 yearly]. Hence, [59]. e Hence,ffort in effort the diversification in the diversification of triacetin of triacetin production production is apt and is aptshould and beshould welcomed. be welcomed.

2.1. Mechanism Mechanism of Acetylation of Glycerol Various articles report the plausible reaction mechanism in the acetylation of glycerol with acetic acid [[58,83–87].58,83–87]. TheThe mechanismmechanism proceedsproceeds via via the the activation activation (protonation) (protonation) of of the the carbonyl carbonyl group group of the of theacetic acetic acid acid by by a strong a strong acid acid catalyst catalyst (usually (usually Bronsted-type Bronsted-type acid). acid). The The resultant resultant acyliumacylium ionion makes it more susceptible to nucleophilic attack by the oxygen atom of the hydroxylhydroxyl molecule attached to glycerol, thereby forming a tetrahedral intermediate and leading to the transfer of hydrogen to the next hydroxyl moleculemolecule (attached(attached to to the the carbonyl carbonyl carbon) carbon) to to form form a watera water molecule. molecule. On On the the loss loss of the of waterthe water molecule, molecule, a monoacetin a monoacetin (monoacetyl (monoacetyl glycerol) glycerol is formed,) is formed, and and on further on further reaction reaction with with acetic acetic acid acidfollowing following a similar a similar mechanism, mechanism, diacetin diacetin (diacetyl (dia glycerol)cetyl glycerol) is formed, is formed, and ﬁnally, and triacetin finally, (triacetyltriacetin (triacetylglycerol) isglycerol) formed is as formed shown as in shown Scheme in5. Scheme 5. Appl. Sci. 2020, 10, 7155 6 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 33

SchemeScheme 5. 5Plausible. Plausible reaction reaction mechanismmechanism showing the the formation formation of of mono mono-,-, di di-,-, and and tri triacetinacetin via via the the acetylationacetylation of of glycerol glycerol with with a a carboxylic carboxylic acid.acid.

TheThe acetylation acetylation of of glycerol glycerol withwith aceticacetic anhydride in in a a strong strong acidic acidic medium medium proceeds proceeds via via two two possiblepossible mechanisms mechanisms [ 88[88––9090].]. TheThe firstfirst plausible mechanism, mechanism, which which is isless less energetic, energetic, involves involves the the protonationprotonation of of the the carbonyl carbonyl oxygen atom, atom, which which provide providess room room for nucleophilic for nucleophilic attack attack leading leading to the to theformation formation of of a a tetrahedral tetrahedral intermediate intermediate (quaternary (quaternary carbon carbon atom) atom) which which require requiress space. space. This This is is exemplifiedexemplified in thein the reaction reaction Scheme Scheme6. While 6. While in the in second the second plausible plausible mechanism, mechanism, the protonation the protonation occurs onoccurs the oxygen on the atom oxygen attached atom toattac thehed carbonyl to the carbonyl group leading group toleading the formation to the formation of acylium of acylium ion (acylation) ion due(acylation) to the presence due to the ofa presence bulky group of a bulky as shown group in as Scheme shown7 in. Scheme 7. The conversion of glycerol to acetin can be influenced by several experimental parameters, just like most reactions. The main parameters include the nature of the catalyst and the catalyst amount (load), temperature, reactants and their mole ratios, as well as the time of the reaction. These parameters are reviewed below. Appl. Sci. 2020, 10, 7155 7 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 33

Scheme 6 6.. PlausiblePlausible reaction reaction mechanism mechanism for the the acetylation acetylation of of glycerol glycerol with acetic acetic anhydride anhydride showing the nucleophilic attack leading to the formation of a tetrahedraltetrahedral intermediate intermediate and the subsequent formation of mono-,mono-, di-,di-, and triacetin. Appl. Sci. 2020, 10, 7155 8 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 33

SchemeScheme 7. 7Plausible. Plausible reaction reaction mechanism mechanism for the acetylation acetylation of of glycerol glycerol with with acetic acetic anhydride anhydride showing showing thethe protonation protonation on on the the oxygen oxygen atom atom leadingleading toto the formation of of an an acylium acylium ion ion intermediate intermediate and and the the subsequentsubsequent formation formation of of mono-, mono-, di-, di-, and and triacetin.triacetin.

2.2. InfluenceThe conversion of Reaction of Parameters glycerol to onacetin the Acetylationcan be influenced of Glycerol by several experimental parameters, just like most reactions. The main parameters include the nature of the catalyst and the catalyst amount 2.2.1. Catalyst and Catalyst Load (load), temperature, reactants and their mole ratios, as well as the time of the reaction. These parametersThe presence are reviewed of a catalyst below. in the reaction medium is to lower the activation energy and to form the desired product. In acetylation, the nature of the catalyst plays an important role, just like other2.2. reactions.Influence of GlycerolReaction Parameters acetylation on isthe mostly Acetylation catalysed of Glycerol with acid catalysts [80,91]. These catalysts could be homogeneous or heterogeneous [92]. Although some studies have reported a high yield 2.2.1. Catalyst and Catalyst Load of acetylated products using homogeneous acid catalysts such as sulphuric, nitric, hydrofluoric, hydrochloric,The presence p-toluenesulfonic of a catalyst acids,in the asreaction well as medium phosphoric is to lower acids the [19 ,activation76,93], they energy are bedevilled and to form with manythe desired environmental product. andIn acetylation, technical the drawbacks. nature of the Some catalyst of these plays include an important difficulty role, in just separating like other the homogeneousreactions. Glycerol catalyst acetylation from the product,is mostly the catalysed corrosion with of acid equipment, catalysts di [80,91].fficulty These in waste catalysts disposal, could and toxicitybe homogeneous of the reagents or heterogeneous [22,58,73]. With [92]. the Alt abovehough disadvantages, some studies haveefforts reported have generally a high yield shifted of to theacetylated use of heterogeneous products using catalysts homogeneous that are less acid toxic, catalysts highly such selective, as sulph moreuric, stable, nitric, easy hydr toofluoric, separate, hydrochloric, p-toluenesulfonic acids, as well as phosphoric acids [19,76,93], they are bedevilled with greener, and more sustainable [78,94,95]. The major advantage of heterogeneous catalysts is that it many environmental and technical drawbacks. Some of these include difficulty in separating the affords scientists the ability to manipulate the surface area and the acid density to suit a particular homogeneous catalyst from the product, the corrosion of equipment, difficulty in waste disposal, and reaction. The reusability of heterogeneous catalysts aids their industrial applications irrespective of toxicity of the reagents [22,58,73]. With the above disadvantages, efforts have generally shifted to the the type of reactor used [96]. A lot of heterogeneous catalysts have been used in glycerol acetylation, use of heterogeneous catalysts that are less toxic, highly selective, more stable, easy to separate, somegreener, of which and includemore sustainable ion-exchange [78,94,95]. and functionalized The major advantage resins [ 82of, 97heterogeneous–99], zeolites catalysts and functionalized is that it zeolitesaffords [73 scientists,100,101 the], heteropoly ability to manipulate acids and the supported surface area heteropoly and the acidsacid density [93,102 –to104 suit]], a metal particular oxides includingreaction. mixedThe reusability oxides and of heterogeneous supported mixed catalysts oxides aids [105their–108 industrial], montmorillonite applications irrespective (K-10 clay) of and modifiedthe type montmorillonite of reactor used [96]. [87,91 A, 109lot of], mesoporousheterogeneous silica catalysts and functionalized have been used silica in glycerol [94,110, 111acetylation], activated, carbonsome andof which functionalized include ion- biomass-derivedexchange and functionalized carbon [112 resins–114 ].[82,97 However,–99], zeolites it is important and functional to indicateized thatzeolites the focus [73,100,101], of most ofheteropol the studiesy acids has beenand supported the effect ofheteropoly the type ofacids catalyst, [93,102 loading,–104]], metal and reusability. oxides However,including other mixed salient oxides features and supported that also aidmixed the oxides efficiency [105 of–108], catalysts montmorillonite in the acetylation (K-10 clay) of glycerol and includemodified the poremontmorillonite size and volume, [87,91,109], surface mesoporous acidity, and silica preparation and functionalized techniques silica [41, 87 [94,110,111],,105,115,116 ]. Appl. Sci. 2020, 10, 7155 9 of 34

The ﬁndings of various researchers in glycerol acetylation using diﬀerent catalysts are hereby reviewed in a cluster of catalyst type.

Montmorillonite (K-10 Clay) and Modiﬁed Montmorillonite

Montmorillonite is an aluminohydroxysilicate (an octahedral AlO6 layer sandwiched between two tetrahedral SiO4 layers), which is the main constituent of most clay minerals. Most clay minerals are swellable (and also reversible) and have high ion-exchange capacity, and hence are widely used in catalysis generally [117]. The representative structure of montmorillonite clay is shown in Figure4. Recent advances have shown various modifications of montmorillonite, thereby improving its Bronsted and Lewis acids and pore size distribution [118]. These modifications might have informed their deployment as a catalyst and catalyst support in glycerol acetylation. Kakasaheb et al. [91] reported the synthesis of a series of sulphuric acid-modified Montmorillonite (K-10) catalysts (10, 20, and 30% (w/w)H2SO4/K10). They were used for the acetylation of bio-glycerol with acetic acid at reaction conditions of 120 ◦C, 0.4 g catalyst loading, the glycerol-to-acetic acid molar ratio of 1:12, and for 5 h. Furthermore, 20% (w/w)H2SO4/K10 performs better with 99% glycerol conversion and 23%, 59%, 15%, and 2% yield towards monoacetin, diacetin, triacetin, and diglycerol triacetate. The obtained results were found to correlate with the acidity and other textural properties of the catalysts. The reusability of 20% (w/w)H2SO4/K10 catalyst was fairly good because, after three reaction cycles, only the partial deactivation of the catalyst was noticed, which was partly attributed to coke deposition and a loss of active acid sites. In a related study, the organic acids-treated montmorillonite clay performed better as a catalyst in acetylation than the untreated [87]. The organic acids used include methanesulfonic acid (MSA), p-toluenesulfonic acid (pTSA), and phenoldisulfonic acid (PDSA), and over 90% glycerol conversion was obtained in each case when compared with the untreated montmorillonite clay with only about 60% glycerol conversion. The selectivity towards triacetin was favoured with the organic acids-treated clay in the following order: PDSA > MSA > p-TSA > untreated clay. The performance was attributed to the enhancement of the catalyst’s acidity and pore characteristics (increase in pore volume, especially around the acid sites) arisen from the organic acid’s treatment. In another study, the modification of montmorillonite with metals enhanced the selectivity towards diacetin. The lanthanum cation-exchanged montmorillonite (La3+-mont), was found to be efficient with 98% glycerol conversion and selectivity of > 99% towards diacetin only. However, when the La3+ was substituted with Ce4+, Sc3+, Ti4+ and Fe3+, selectivity towards monoacetin and triacetin improved at 120 ◦C, glycerol-to-acetic acid molar ratio of 1:2, and reaction time of 24 h [109] which is not cost effective. Appl. Sci. 2020, 10, 7155 10 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 33

FigureFigure 4. 4.Representative Representative structurestructure of montmorillonite montmorillonite clay clay [119]. [119 ].

MetalsMetals and and Metal Metal Oxides Oxides MetalsMetals and and metal metal oxides oxides mostly mostly used used inin catalysiscatalysis in in recent recent times times are are multi-metals multi-metals or multiphase or multiphase oxidesoxides and and are are of transitionof transition metal metal origin. origin. BecauseBecause of the the synergy synergy of of various various components, components, the theresultant resultant metalsmetals and metaland metal oxides oxides catalysts catalysts usually usually exhibit exhibit Bronsted Bronsted and Lewis and acidLewis sites, acid stability, sites, regenerationstability, capacity,regeneration and were capacity, found toand be were active found over to a widebe acti rangeve over of temperaturesa wide range [of120 temperatures,121]. Hence, [120,121]. researchers Hence, researchers have also deployed these catalysts in glycerol acetylation with different findings. have also deployed these catalysts in glycerol acetylation with different findings. The use of Cu and Ni The use of Cu and Ni metals and Cu–Ni bimetals supported on γ-Al2O3 were compared with metals and Cu–Ni bimetals supported on γ-Al2O3 were compared with sulphuric acid-treated alumina sulphuric acid-treated alumina (H2SO4/γ-Al2O3) catalysts in the esterification of glycerol with acetic γ (H2SOacid4/ at-Al 1102O °C,3) atmospheric catalysts in pressure, the esterification the molar ofratio glycerol of 1:9, and with the acetic catalyst acid loading at 110 of◦ C,0.25 atmospheric g [122]. pressure, the molar ratio of 1:9, and the catalyst loading of 0.25 g [122]. The results revealed that the 2 M The results revealed that the 2 M H2SO4/γ-Al2O3 catalyst showed the best activity with 97% glycerol H2SOconversion4/γ-Al2O 3andcatalyst a selectivity showed of the 27%, best 49.9%, activity and with 23.1% 97% mono-, glycerol di-, conversionand triacetin and after a selectivitya 5 h reaction of 27%, 49.9%,time. and The 23.1% catalyst mono-, was di-, also and reported triacetin not after to alose 5 h reactionactivity even time. after The catalystthree consecutive was also reported runs. The not to loseperformance activity even of after the 2 three M H consecutive2SO4/γ-Al2O3 runs.catalyst The was performance attributed to of the the high 2 M acid H2SO strength4/γ-Al 2ofO 2.513 catalyst mmol was −1 1 attributedg when to thecompared high acid to the strength other ofcatalysts 2.51 mmol tested. g− Inwhen a similar compared use of tobimetallic-supported the other catalysts catalyst, tested. In a similarRamalingam use of bimetallic-supported et al. [90] reported the catalyst, use of RamalingamAg–Cu-doped et rice al. husk [90] reportedsilica–alumina the use (RHS–Al)-based of Ag–Cu-doped ricebiomass husk silica–alumina catalyst under (RHS–Al)-basedsimilar reaction conditions biomass catalystto the glycerol-to-acetic under similar ac reactionid molar conditions ratio of 1:10, to the 110 °C and a catalyst load of 0.80 g. The results showed high catalytic activity with 97.5% glycerol glycerol-to-acetic acid molar ratio of 1:10, 110 ◦C and a catalyst load of 0.80 g. The results showed highconversion catalytic activity and selectivity with 97.5% towards glycerol mono-, conversion di-, and and triacetin selectivity (3.7, towards 58.4, and mono-, 37.9%) di-, over and 1Ag- triacetin 10Cu/RHS–Al catalyst indicating that the bimetallic catalyst performed better than the single metal (3.7, 58.4, and 37.9%) over 1Ag-10Cu/RHS–Al catalyst indicating that the bimetallic catalyst performed catalyst and was attributed to the synergistic effect between the metals. The use of several metal better than the single metal catalyst and was attributed to the synergistic effect between the metals. The oxides (Bi2O3, Sb2O3, SnO2, TiO2, Nb2O5 and Sb2O5) was also investigated at the glycerol-to-acetic acid use of several metal oxides (Bi O , Sb O , SnO , TiO , Nb O and Sb O ) was also investigated at the molar ratio of 1:6 and a temperature2 3 2 of3 120 °C.2 Only2 Sb2O2 5 5(antimony2 pentoxide)5 resulted in good glycerol-to-aceticactivity and selectivity acid molar towards ratio of diacetin 1:6 and aafter temperature 1 h [123]. of Glycerol 120 ◦C. Only conversion Sb2O5 (antimonywas 96.8% pentoxide)while resultedselectivity in good towards activity mono-, and selectivity di- and triacetin towards were diacetin 33.2, 54.2 after and 1 h 12.6%, [123]. Glycerolrespectively. conversion Incidentally, was 96.8%it while selectivity towards mono-, di- and triacetin were 33.2, 54.2 and 12.6%, respectively. Incidentally, it was Sb2O5 that showed strong Bronsted acid sites was able to protonate adsorbed pyridine which might have influenced selectivity towards diacetin. The Sb2O5 catalyst was found to be active even Appl. Sci. 2020, 10, 7155 11 of 34

after six reaction cycles. More recently, Zhang et al. [107] synthesized a diatomite-loaded H2SO4/TiO2 catalyst and was used in the esterification reaction of glycerol with oleic acid. In a 6 h reaction time, 59.6% diacetin was obtained at a much higher temperature of 210 ◦C, low catalyst load (0.1%), and low molar ratio of glycerol-to-oleic acid (1:2). The reusability of the catalyst was tested five times. The diacetin obtained was about 50% in the first three cycles but decreased in the fourth cycle by 12.5%, and 10% in the fifth cycle. The author attributed the results to the blockage of active sites by the bulky triacetin and the leaching of active sites from the catalyst. Recently, Kulkarni et al. [108] deployed unsulphated and sulphated CeO2–ZrO2 mixed oxide catalysts in glycerol acetylation with acetic acid and reported that the sulphated CeO2–ZrO2 mixed oxide catalyst exhibited better performance when compared with the unsulphated catalyst. Within 3 h reaction time, 99.1% of glycerol conversion was achieved with 57.28% and 21.26% selectivity towards diacetin and triacetin at 100 ◦C. The catalyst was reused in three consecutive cycles without a significant loss of activity.

Mesoporous Silica and Functionalized Silica

Mesoporous silica is an ordered porous network material with SiO4 tetrahedral as the building block, illustrated in Figure5. It is made of weak Bronsted acid oxides and hence has a low catalytic value. However, because the surface is amenable to functionalization and other modifications, resulting in a highly stable material with a high surface area and large pore size, it is currently receiving significant attention as catalyst support in various reactions to enhance the reactivity and selectivity of products, especially of large molecules like fatty acids and their esters [59,117,124]. Ghoreishi and Yarmo [110] reported the influence of a sulphated silica catalyst, prepared with the sol-gel technique with different sulphate loading percentages (5, 10, 15, and 20%) in the esterification of glycerol with acetic acid at a molar ratio of 1:6, at a temperature of 50 ◦C and with 0.2 g catalyst load in 6 h. The results revealed that the catalytic activity increased with an increase in the acidic properties, which in turn was found to increase with the increased sulphuric acid loading. The 20% sulphated silica exhibited the highest catalytic activity with 96.88% glycerol conversion and a selectivity of 51.9, 45.27, and 2.11% towards mono-, di-, and triacetin. The authors further reported a strong correlation between the textural properties (surface area and pore volume) and the sulphur content of the catalysts. The decrease in both the surface area and pore volume was found to correlate with an increase in sulphur content, attributable to the blockage of pores by the active non-porous species. The authors further studied the effect of catalyst loading on the reaction, and the results revealed a slight increase in glycerol conversion with an increase in sulphated silica catalyst loading from 0.2 to 0.8 g. The selectivity to monoacetin decreased with an increase in catalyst loading, while the selectivity to diacetin and triacetin improved marginally. Khayoon et al. [82] reported a much higher selectivity towards diacetin and triacetin with 34 and 55%, when a 3% yttrium supported on mesoporous silicate material (SBA-3) prepared via hydrothermal method was deployed as a catalyst at a glycerol-to-acetic acid molar ratio of 1:4 and a temperature of 110 ◦C. A 100% glycerol conversion was achieved in 2.5 h. The catalyst was found to be stable after being reused in three reaction cycles. The high triacetin produced was attributed to the strong acidity, and the high surface area with the large pore size, which makes the diffusion of both the substrates and products possible. However, in the fourth cycle, the glycerol conversion and triacetin were reduced to 80% and 50, respectively, due to mass transfer limitations. The authors further reported that only 44% glycerol conversion was achieved with a catalyst load of 0.05 g. However, when the catalyst load was increased to 0.20 g, the glycerol conversion increased to 100%. The subsequent increase in the catalyst load did not improve the conversion, an indication of reaching the optimum limit. In a similar vein, Costa et al. [73] also reported the excellent performance of SBA-15-functionalized with propylsulfonic group (Pr-SO3H–SBA-15) as a catalyst but at a higher temperature of 120 ◦C and a glycerol-to-acetic acid mole ratio of 1:6. In comparison with zeolites (H-ZSM-5 and H-Beta), the Pr-SO3H–SBA-15 catalyst exhibited a 96% glycerol conversion and a selectivity of 13, 55, and 32% mono-, di- and triacetin in 2.5 h reaction time. The good performance of the Pr-SO3H–SBA-15 catalyst was attributed to adequate balance between the amount of acid site and Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 33 Appl. Sci. 2020, 10, 7155 12 of 34 between the amount of acid site and its distribution on the surface of the mesoporous structure.

Unlikeits distribution earlier studies, on the surface the Pr–SO of the3H–SBA-15 mesoporous catalyst structure. could Unlike not maintain earlier studies, the activity the Pr-SO in consecutive3H–SBA-15 cycles,catalyst even could with not maintaintreatment the in activitybetween. in consecutiveIn a comparable cycles, study, even withTesta treatment et al. [92] in deployed between. Inthe a synthesizedcomparable solid study, acid Testa catalysts, et al. [92 namely] deployed propyl-SO the synthesized3H functionalized solid acid amorphou catalysts,s namelysilica (SAS), propyl-SO propyl–3H SOfunctionalized3H-functionalized amorphous mesoporous silica (SAS), silica propyl-SO SBA-15 (SSBA),3H-functionalized titania-doped mesoporous silica (hexagonal silica SBA-15 mesoporous (SSBA), silica,titania-doped HMS) (Ti10HMS), silica (hexagonal sulphated mesoporous zirconia (SZ silica, and HMS)calcined (Ti10HMS), SZ-470) and sulphated niobic acid zirconia nanoparticles (SZ and (Nbcalcined2O5·nH SZ-470)2O) in andthe esterification niobic acid nanoparticles reaction of glycerol (Nb O nHwithO) acetic in the acid esteriﬁcation in a molar reaction ratio of of1:3 glycerol at 105 2 5· 2 °C.with The acetic SAS acid and in SSBA a molar catalysts ratioof showed 1:3 at 105 the◦ bestC. The resu SASlts. andThough SSBA the catalysts complete showed glycerol the bestconversion results. (100%)Though was the achieved complete glycerolwithin 1 conversionh, the triacetin (100%) of 49% was achievedand 33% withinwere achieved 1 h, the triacetinafter 3 h of with 49% SAS and and 33% SSBA,were achieved respectively. after 3Both h with catalysts SAS and were SSBA, found respectively. to be very Both stable, catalysts with wereno leaching found to of be the very sulfonic stable, groups.with no The leaching excellent of the activities sulfonic of groups. the two The catalysts excellent were activities attributed of the to twotheir catalysts acid capacities, were attributed including to thetheir strength acid capacities, and the surface including density the strength of the acid and site. the surface density of the acid site.

Figure 5. TheThe tetrahedral tetrahedral network network structure of SiO 4. Carbon and Functionalized Carbon-Based Materials Carbon and Functionalized Carbon-Based Materials The use of carbon-based catalysts in acetylation has been reported as well. Several literature articles The use of carbon-based catalysts in acetylation has been reported as well. Several literature have indicated the synthesis of a functionalized catalyst from biomass materials via carbonization and articles have indicated the synthesis of a functionalized catalyst from biomass materials via sulfonation, but only few reported their application in glycerol acetylation. Okoye et al. [114] reported carbonization and sulfonation, but only few reported their application in glycerol acetylation. Okoye the use of crude glycerol as the precursor material in carbon-based catalyst synthesis and was found to et al. [114] reported the use of crude glycerol as the precursor material in carbon-based catalyst be active in glycerol acetylation. They also reported that an increase in catalyst load from 1 to 2 wt% synthesis and was found to be active in glycerol acetylation. They also reported that an increase in increased glycerol conversion (>90%) and remained relatively constant even when the catalyst load catalyst load from 1 to 2 wt% increased glycerol conversion (>90%) and remained relatively constant was increased up to 4 wt%. This observation was attributed to active site saturation. An increased even when the catalyst load was increased up to 4 wt%. This observation was attributed to active site catalyst load favours the selectivity to di- and triacetin due to increased active sites, while a lower saturation. An increased catalyst load favours the selectivity to di- and triacetin due to increased catalyst load favours monoacetin with decreased glycerol conversion. The authors also observed that active sites, while a lower catalyst load favours monoacetin with decreased glycerol conversion. The the synthesized carbon-based catalyst was reused seven times, and the glycerol conversion, as well authors also observed that the synthesized carbon-based catalyst was reused seven times, and the as the product selectivity, remained constant, indicating high stability. Similarly, sulfonated carbon glycerol conversion, as well as the product selectivity, remained constant, indicating high stability. material derived from Pongamia pinnata de-oiled seed cake was synthesized and used for the same Similarly, sulfonated carbon material derived from Pongamia pinnata de-oiled seed cake was reaction but with different acetylating agents (lauric and oleic acids) [101]. The catalyst was found synthesized and used for the same reaction but with different acetylating agents (lauric and oleic to perform better when compared to zeolites H–Y, H-ZSM-5, and liquid H2SO4. Glycerol conversion acids) [101]. The catalyst was found to perform better when compared to zeolites H–Y, H-ZSM-5, and ranged from 80 to 95% with monoacetin as the major product (70–80%) within 24 h. The authors also liquid H2SO4. Glycerol conversion ranged from 80 to 95% with monoacetin as the major product (70– reported that the sulfonated catalyst increased the conversion of glycerol, which was subsequently 80%) within 24 h. The authors also reported that the sulfonated catalyst increased the conversion of found to depend on the catalyst load. Up to 96% glycerol conversion was obtained at 7.5 wt% catalyst glycerol, which was subsequently found to depend on the catalyst load. Up to 96% glycerol load. However, 5 wt% was considered to be the optimum load because it was not significantly different conversion was obtained at 7.5 wt% catalyst load. However, 5 wt% was considered to be the optimum from the former. The performance of the sulfonated catalyst was found to correlate with its acid load because it was not significantly different from the former. The performance of the sulfonated density, pore size, and the dimensions of the reacting molecules. The authors also confirmed the catalyst was found to correlate with its acid density, pore size, and the dimensions of the reacting reusability of the sulfonated carbon catalyst in three successive esterification cycles with oleic acid. molecules. The authors also confirmed the reusability of the sulfonated carbon catalyst in three The results indicated that the catalyst was deactivated by only 14% but retained the active site (-SO3H) successive esterification cycles with oleic acid. The results indicated that the catalyst was deactivated by almost 90% after the third reaction cycle with a slight improvement in the selectivity of monoacetin, by only 14% but retained the active site (-SO3H) by almost 90% after the third reaction cycle with a which was the desired product. The slight loss was attributed to the possible restricted access to slight improvement in the selectivity of monoacetin, which was the desired product. The slight loss the active sites due to adsorbed carbonaceous impurities on the surface of the catalyst, and the high was attributed to the possible restricted access2 to the active sites due to adsorbed carbonaceous stability was due to the stability of the C(sp )–SO3H bond. Similarly, Konwar et al. [115] deployed impurities on the surface of the catalyst, and the high stability was due to the stability of the C(sp2)– the same catalyst but with acetic anhydride. Additionally, 100% triacetin selectivity was achieved in SO3H bond. Similarly, Konwar et al. [115] deployed the same catalyst but with acetic anhydride. Appl.Appl. Sci. Sci. 20202020, ,1010, ,x 7155 FOR PEER REVIEW 1313 of of 33 34

Additionally, 100% triacetin selectivity was achieved in 20–50 min with both mesoporous sulfonated carbon20–50 minand withzeolite both H–Y mesoporous catalysts. The sulfonated authors carbon further and revealed zeolite that H–Y the catalysts. selectivity The to authors triacetin further was influencedrevealed that mainly the selectivity by the pore to triacetinstructure was and influenced surface acid mainly site bydensity the pore of structurethe catalysts. and surfaceSulfonated acid carbonsite density catalyst of themade catalysts. from the Sulfonated mixture carbonof carbonized catalyst rice made husk from and the sulphuric mixture ofacid carbonized was reported rice husk by Carvalhoand sulphuric et al. acid[75]. wasAfter reported 5 h of reaction, by Carvalho 90% etglycerol al. [75]. conversion After 5 h of and reaction, selectivity 90% glycerolof 11, 52, conversion and 37% towardsand selectivity mono, di, of 11,and 52, triacetin and 37% were towards obtained, mono, respectively. di, and triacetin The improved were obtained, selectivity respectively. towards Thedi- 1 improved selectivity towards di- and triacetin was attributed to− the1 high total acidity (5.8 mmol g ). and triacetin was attributed to the high total acidity (5.8 mmol·g ). Surprisingly, non-functionalized· − biocharSurprisingly, (Karanja non-functionalized seed shells) was biochar also able (Karanja to catalyse seed the shells) esterification was also able of glycerol to catalyse with the acetic esterification acid at 120of glycerol°C, at a withmolar acetic ratio acid of 1:5, at 120 a catalyst◦C, at a load molar of ratio 0.2 g, of and 1:5, a>80% catalyst glycerol load ofconversion 0.2 g, and was>80% reported glycerol afterconversion 1 h. However, was reported the major after product’s 1 h. However, selectivity the was major monoacetin. product’s selectivityThe reaction was was monoacetin. also attributed The toreaction moderate was to also strong attributed acidic sites to moderate exhibited to by strong the biochar acidic sites despite exhibited the non-functionalization by the biochar despite [125]. the Thenon-functionalization use of a two-step [125reaction]. The system use of awith two-step a su reactionlfonic acid-functionalized system with a sulfonic glycerol-based acid-functionalized carbon catalystglycerol-based produced carbon 100% catalysttriacetin produced [126]. In the 100% first triacetin step, acetic [126 ].acid In thewas first used, step, and acetic in the acid second was step, used, aceticand in anhydride the second was step, added acetic to anhydride complete the was reaction added. to The complete developed the reaction.catalyst was The found developed to be catalysthighly stablewas found and recyclable. to be highly The stable authors and further recyclable. studied The the authors effect of further the catalyst studied loading the e ff(5–20ect of wt%) thecatalyst on the reactionloading and (5–20 revealed wt%) on that the the reaction catalyst and load revealed of 5 wt% that led the to catalyst100% glycerol load of conversion 5 wt% led towith 100% 22, glycerol67, and 11%conversion mono-, withdi- and 22, triacetin 67, and 11%selectivity mono-, after di- and1 h, triacetinrespectively, selectivity and when after compared 1 h, respectively, with the and control, when thecompared selectivity with was the 42, control, 54 and the 4%, selectivity for mono-, was di- 42, and 54 triacetin, and 4%, for respectively, mono-, di- which and triacetin, indicates respectively, the slow pacewhich of the indicates esterification the slow reaction pace of without the esterification the catalyst. reaction However, without when the the catalyst. catalyst However,load was increased when the tocatalyst 20 wt%, load the was glycerol increased conversi to 20on wt%, increased the glycerol to 100%, conversion and the increased selectivity to 100%,to monoacetin and the selectivity decreased to whilemonoacetin the di- decreased and triacetin while increased the di- and simultaneously triacetin increased. The trend simultaneously. was attributed The trendto catalyst was attributed acidity, whichto catalyst enhanced acidity, the whichsurface enhanced interactions the of surface the glycer interactionsol with acetic of the acid. glycerol The reusability with acetic and acid. stability The ofreusability the catalyst and were stability found of to the be catalystexcellent were even found after five to be reaction excellent cycles even at after a 1:4 fiveglycerol-to-acetic reaction cycles acid at a molar1:4 glycerol-to-acetic ratio, at 115 °C and acid with molar a 1 ratio, h reaction at 115 time◦C and. No with deactivation a 1 h reaction and leaching time. No under deactivation the reaction and conditionsleaching under were theobserved. reaction Synthesized conditions wereordered observed. mesoporous Synthesized carbon orderedvia hard mesoporous template method carbon and via itshard modification template method with concentrated and its modification sulphuric withor 4-am concentratedinobenzenesulfonic sulphuric acid or 4-aminobenzenesulfonic were tested in glycerol acetylationacid were tested[116]. inBoth glycerol showed acetylation good activity [116]. Both with showed the material good activity modified with via the diazonium material modified cation exhibitingvia diazonium high glycerol cation exhibiting conversion high and glycerol selectivity conversion to triacetin and under selectivity similar to conditions. triacetin under Figure similar 6 is anconditions. illustration Figure of a sulfonated6 is an illustration carbon-based of a sulfonated catalyst. carbon-based catalyst.

FigureFigure 6. 6. SchematicSchematic structure structure of of a asulfonat sulfonateded carbon-based carbon-based catalyst catalyst [127]. [127]. Ion-Exchange and Functionalized Resins Ion-Exchange and Functionalized Resins Ion-exchange resins are usually co-polymers of polystyrene and divinylbenzene with exchangeable Ion-exchange resins are usually co-polymers of polystyrene and divinylbenzene with ions or functional groups. They exist commonly as Amberylsts, Nafion, Dowex-50W, and sulfonic acid exchangeable ions or functional groups. They exist commonly as Amberylsts, Nafion, Dowex-50W, resin. Figure7 shows the structure of some resin catalysts. Several reports indicate the deployment and sulfonic acid resin. Figure 7 shows the structure of some resin catalysts. Several reports indicate of ion exchange resins in its original and functionalized forms as well as catalyst support in glycerol the deployment of ion exchange resins in its original and functionalized forms as well as catalyst acetylation. Lacerda et al. [99] compared the use of acidic anhydride and acetic acid in glycerol support in glycerol acetylation. Lacerda et al. [99] compared the use of acidic anhydride and acetic acetylation over amberlyst-15. A 100% glycerol conversion was achieved with both acetylating agents acid in glycerol acetylation over amberlyst-15. A 100% glycerol conversion was achieved with both but with varying selectivity to mono-, di-, and triacetin. The conditions of the reaction with the acetic acetylating agents but with varying selectivity to mono-, di-, and triacetin. The conditions of the anhydride were much milder when compared with that of acetic acid. The reaction of glycerol with reaction with the acetic anhydride were much milder when compared with that of acetic acid. The acetic anhydride was at a molar ratio of 1:3, a temperature of 60 C, and a catalyst load of 5 wt% reaction of glycerol with acetic anhydride was at a molar ratio of 1:3,◦ a temperature of 60 °C, and a Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 33 Appl. Sci. 2020, 10, 7155 14 of 34 catalyst load of 5 wt% glycerol, which led to 100% glycerol conversion with a selectivity of 0.7, 1.3, and 98.1% mono, di, and triacetin in 2 h. While the reaction of glycerol with acetic acid produced 100%glycerol, glycerol which conversion led to 100% with glycerol a selectivity conversion of with 3.5, a selectivity8.7, and 87.8% of 0.7, for 1.3, mono, and 98.1% di, mono,and triacetin, di, and respectively,triacetin in 2 this h. While was at the a higher reaction molar of glycerol ratio of with1:6 and acetic a higher acid producedtemperature 100% of 120 glycerol °C with conversion the same withcatalyst a selectivity load and oftime. 3.5, Similar 8.7, and results 87.8% were for mono, reported di, andby Silver triacetin, et al. respectively, [88] using acetic this was anhydride at a higher but atmolar a lower ratio reaction of 1:6 and time a higherof just 20 temperature min. Kim et of al. 120 [41]◦C also with used the sameamberlyst-15 catalyst loadcatalyst and in time. comparison Similar withresults wereseveral reported other by Silversolid et al.catalysts [88] using including acetic anhydride silica–alumina, but at a lower zeolite reaction (HUSY), time of dodecamolybdophosphoricjust 20 min. Kim et al. [41] alsoacids used supported amberlyst-15 on Nb catalyst2O5 (HPMo/Nb in comparison2O5) and with mesoporous several other SBA-15 solid (HPMo/SBA-15),catalysts including sulphated silica–alumina, ceria–zirconia zeolite (HUSY), (SCZ), dodecamolybdophosphoric propylsulfonic acid functionalized acids supported SBA-15 on (PrSONb2O35H–SBA-15),(HPMo/Nb sulfonic2O5) and acid mesoporous functionalized SBA-15 SBA-15 (HPMo (SO/3SBA-15),H–SBA-15) sulphated and microcrystalline ceria–zirconia cellulose (SCZ), propylsulfonic(SO3H–Cell) catalysts acid functionalized in glycerol acetylation SBA-15 (PrSOwith acetic3H–SBA-15), acid at a sulfonic molar ratio acid of functionalized 1:6, a temperature SBA-15 of (SO80 °C3H–SBA-15) and the catalyst and microcrystalline load of 5 wt% with cellulose respect (SO to3 glycerol.H–Cell) catalysts After a long in glycerol reaction acetylation time of 8 h, with PrSO acetic3H– SBA-15,acid at a amberlyst-15, molar ratio of and 1:6, aSO temperature3H–SBA-15 ofcatalysts 80 ◦C and exhibited the catalyst supe loadrior performance of 5 wt% with in respect glycerol to conversionglycerol. After and a selectivity long reaction to higher time of acetins. 8 h, PrSO However,3H–SBA-15, the SO amberlyst-15,3H–SBA-15 andcatalyst SO3 H–SBA-15showed significant catalysts deactivationexhibited superior due to performance the loss of inacidity. glycerol The conversion excellent andperformance selectivity by to higherthe PrSO acetins.3H–SBA-15 However, and amberlyst-15the SO3H–SBA-15 catalysts catalyst could showed not be significant entirely deactivationdue to the strength due to the of lossBronsted of acidity. acid Thebecause excellent the performancesulphated ceria–zirconia by the PrSO (SCZ)3H–SBA-15 exhibited and the amberlyst-15 higher Bronsted catalysts acid strength could not (2.9 be mmol·g entirely−1) duecompared to the tostrength PrSO3H–SBA-15 of Bronsted (1.2 acid mmol·g because−1). the However, sulphated its ceria–zirconia catalytic activity (SCZ) in exhibitedacetylation the reaction higher Bronstedwas inferior. acid strength (2.9 mmol g 1) compared to PrSO H–SBA-15 (1.2 mmol g 1). However, its catalytic activity The authors suggested· − that in addition to3 PrSO3H–SBA-15 and· −amberlyst-15 catalysts exhibiting moderatein acetylation to high reaction Bronsted was acid inferior. strength The (1.2 authors and suggested4.9 mmol·g that−1), they in addition also have to the PrSO configuration3H–SBA-15 and amberlyst-15 catalysts exhibiting moderate to high Bronsted acid strength (1.2 and 4.9 mmol g 1), they spatial arrangement of their surface acid moieties that contributed to their excellent activities,· − unlike thealso other have catalysts. the configuration The two-step and spatial acetylation arrangement of glycerol of their with surface acetic acidacid moietiesand acidic that anhydride contributed over to amberlyst-35their excellent resin activities, catalyst unlike was the compared other catalysts. with Thezeolite two-step (HY acetylationand HZSM-5) of glycerol catalysts with [80]. acetic At acidthe optimumand acidic conditions anhydride of over 105 amberlyst-35°C, the glycerol-to-acetic resin catalyst acid was molar compared ratio of with 1:9, 0.5 zeolite g catalyst (HY and loading, HZSM-5) and 4catalysts h reaction [80 ].time At in the the optimum first step, conditions the glycerol of 105 conv◦C,ersion the glycerol-to-acetic was almost 100% acid with molar a selectivity ratio of 1:9,of 25.9% 0.5 g towardscatalystloading, triacetin and with 4 hthe reaction amberlyst-35 time in catalyst. the first step,The catalyst the glycerol performance conversion was was attributed almost 100% to its with high a acidselectivity capacity of 25.9%when compared towards triacetin to the rest with of thethe amberlyst-35catalysts used. catalyst. With the The addition catalyst of performance acetic anhydride was inattributed the second to its step, high the acid selectivity capacity of when triacetin compared improved to the to rest almost of the 100% catalysts within used. 15 min. With Amberlyst-35 the addition wasof acetic found anhydride to be the most in the excellent second step,catalyst the for selectivity the conversion of triacetin as no improved significant to deactivation almost 100% occurred within 15after min. being Amberlyst-35 reused five wastimes. found Even to when be the the most reaction excellent was catalystextended for to the 12 conversionh, the catalyst as nowas significant found to bedeactivation very stable. occurred The same after authors being reusedalso investigated five times. the Even influence when the of reactionthe catalyst was load extended at a fixed to 12 molar h, the ratiocatalyst of glycerol-to-acetic was found to be veryacid of stable. 1:6, 105 The °C, same and authors4 h reaction also time. investigated The glycerol the influence conversion of thewas catalyst almost 100%load at with a fixed the presence molar ratio of 0.25 of glycerol-to-acetic g of the catalyst. acidWhile of without 1:6, 105 ◦theC, andcatalyst, 4 h reaction the conversion time. The was glycerol 73.6%. Theconversion selectivity was to almost monoacetin 100% with decreased the presence with an of increa 0.25 gse of in the catalyst catalyst. amount While up without to 0.5 theg, while catalyst, the thedi- andconversion triacetin was increased. 73.6%. TheHowever, selectivity no appreciable to monoacetin change decreased was noticed with anin the increase selectivity in catalyst of mono-, amount di- ,up and to triacetin 0.5 g, while when the the di- catalyst and triacetin load was increased. further in However,creased to no 1.0 appreciable g. The authors change concluded was noticed that much in the ofselectivity the catalyst of mono-, added di-, did and not triacetin substantially when theimprove catalyst its load activity. was furtherHowever, increased it has tobeen 1.0 established g. The authors by severalconcluded researchers that much that of ion the exchange catalyst added resins did are notnot substantiallythermally stable. improve They its become activity. easily However, deactivated it has abovebeen established 150 °C [59,98]. by several researchers that ion exchange resins are not thermally stable. They become easily deactivated above 150 ◦C[59,98].

(b) -(CH2CH2)n-

SO3H

Figure 7. StructureStructure of of ion ion exchange exchange resin resin catalysts: catalysts: (a) (perfluorinateda) perﬂuorinated resinsul resinsulfonicfonic acid acid (Nafion) (Naﬁon) (b) b (sulfonated) sulfonated polystyrene polystyrene (Amberlyst) (Amberlyst) [128,129]. [128,129 ]. Appl. Sci. 2020, 10, 7155 15 of 34

Heteropoly Acids and Supported Heteropoly Acids Heteropoly acids (HPA) are complex compounds that exhibit very strong Bronsted acidity but with low specific surface area and low thermal stability [103,130]. They also exhibit homogeneous behaviour (highly soluble in polar media) in most reactions. However, with intense research, the protons of HPA can now be substituted by incorporating metal ions, especially transition metals. The Keggin structure of heteropoly acids (HPA) is illustrated in Figure8. Other materials such as silica, carbon, zeolite, resin, metal oxides, etc., have also been used to immobilized HPA as support, thereby improving their surface area, thermal stability, as well as reduce their solubility and leaching in reaction medium [59,71,102,118]. The modifications have greatly improved the catalytic activity of HPA and have been used in glycerol acetylation. Zhu et al. [131] reported the complete crude glycerol conversion (100%) and selectivity of 6.4%, 61.3%, and 32.3% towards mono-, di-, and triacetin, respectively, at 120 ◦C in 4 h over a zirconia-supported H4SiW12O40 (HSiW/ZrO2) catalyst. There was no catalyst deactivation noticed even after four reaction cycles and was attributed to good surface Bronsted acid site and hydrothermal stability. Similarly, 100% glycerol conversion was achieved within situ-generated supported silicomolybdic heteropolyanions from the sol–gel synthesized MoO3/SiO2 catalyst. As the mol% of MoO3 supported on SiO2 increased, the conversion remained the same, but the selectivity varies. 1 mol% MoO3 gave selectivity of 56, 28, and 17% mono-, di-, and triacetin. A 10 mol% MoO3 produced a selectivity of 36, 31, and 33% mono-, di-, and triacetin, respectively. While 20 mol% MoO3 produced 17, 33, and 50% mono-, di-, and triacetin, respectively, in 8 h reaction time, at 100 ◦C, with a glycerol-to-acetic acid molar ratio of 1:10 and a catalyst load of 10 wt% with respect to glycerol. However, 76% selectivity towards triacetin was obtained when the temperature and reaction time were increased to 118 ◦C and 20 h, respectively [132]. Sandesh et al. [89] studied various solid catalysts, namely cesium phosphotungstate (CsPWA), amberlyst-15, H-beta, sulphated zirconia (SZ), and montmorillonite K-10, in glycerol acetylation or esterification with acidic anhydride and acetic acid. The reaction of glycerol with acidic anhydride was carried out under a milder condition of 1:3 mole ratio, 30 ◦C and a catalyst load of 1 wt% with respect to glycerol when compared with that of acetic acid, a 1:8 mole ratio, 85 ◦C and catalyst load of 7 wt%. The activity of the catalysts in both reactions is in the order SZ < H-beta < K10 < A-15 < CsPWA. The CsPWA catalyst exhibited the highest glycerol conversion of >98% in both reactions with combined di- and triacetin selectivity of 99.1% with acetic anhydride and 75% with acetic acid. The high glycerol conversion was attributed to the presence of high Bronsted acidity while that of selectivity was found to depend on the nature of the active sites on the catalyst. The authors went further to study the effect of the CsPWA catalyst load on the glycerol esterification with acetic acid. The result showed increased glycerol conversion from 56 to 98% when the catalyst load was increased from 3 to 7 wt% in a reaction time of 2 h. The selectivity to di- and triacetin also increased from 31 and 0 to 59 and 16%, respectively. This observation was attributed to the presence of more active sites with a high catalyst load. However, when the catalyst amount was increased further to 9 wt%, no appreciable change was observed, and the authors concluded that the maximum glycerol conversion and product selectivity were attained with the catalyst load of 7 wt%. The CsPWA catalyst exhibited good reusability with minimal decreased activity after three reaction cycles. The activity of CsPWA in glycerol esterification was also confirmed by Veluturla et al. [121], but the maximum glycerol conversion and esters yield were obtained at a 5 wt% catalyst load. Most of the earlier works prefer the use of lower catalyst loads irrespective of the catalyst used, and the pattern of the results was similar [98,102,133]. Polymeric material and polyvinylpyrrolidone (PVP) were separately impregnated with phosphotungstic, silicotungstic, and phosphomolybdic acids, and each used as a catalyst in the conversion of glycerol to acetin using acetic acid as the acetylating reagent [104]. It was observed that the PVP impregnated with phosphotungstic acid (PVP–DTP) exhibited the highest selectivity to triacetin (34%), and 100% glycerol conversion was also achieved at the optimum conditions. The performance was attributed to its high acidity. However, the catalyst was found to be highly hydrophobic. Even when the authors compared the PVP–DTP catalyst with the commercial montmorillonite KSFO catalyst under the same conditions, the former performed better. Appl. Sci. 2020, 10, 7155 16 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 33

n− n FigureFigure 8. Keggin 8. Keggin structure structure of of heteropoly heteropoly acidsacids (HPA) (HPA) catalyst catalyst [XM [XM12O4012] O [118].40] − [118].

Others Others 2 The useThe of use a magnetic of a magnetic solid solid acid acid catalyst catalyst (Fe–Sn–Ti(SO (Fe–Sn–Ti(SO42-4)-400)−)-400) in glycerol in glycerol acetylation acetylation with acetic with acetic anhydrideanhydride was reportedwas reported by Sunby Sun et et al. al. [134 [134].]. AA 100 100%% glycerol glycerol conversion conversion and 99.0% and 99.0% selectivity selectivity to to triacetin were achieved at a molar ratio of 1:6, at 80 °C, and within 30 min. The catalyst was reused triacetin were achieved at a molar ratio of 1:6, at 80 ◦C, and within 30 min. The catalyst was reused three times,three andtimes, the and activity the activity remained remained unchanged. unchanged. The The authorsauthors attributed attributed the the activity activity of the of catalyst the catalyst to to the high surface area and pore volume. However, when the acetic acid was used as the acetylating the highagent surface under area similar and reaction pore volume.conditions, However, the glycerol when conversion the acetic reduced acid drastically was used to 32.3%, as the while acetylating agent underthe only similar product reaction was monoacetin conditions, with athe selectivity glycerol of 100%. conversion However, reduced when the drastically reaction temperature to 32.3%, while the onlyincreased product to was 140 monoacetin°C, the glycerol with conversion a selectivity improv ofed 100%. up to However,75.7% with when monoacetin the reaction selectivity temperature of increased90.3%. to 140Meanwhile,◦C, the Sandesh glycerol et conversional. [135] showed improved that acetin up could to 75.7% also be with produced monoacetin simultaneously selectivity of 90.3%. Meanwhile,with biodiesel Sandesh by reacting et al.glycerol [135] showedwith methyl that acetate acetin over could Ca also and be Sn-mixed produced metal simultaneously hydroxides with catalysts. The CaSn(OH)6 catalyst performed much better when compared to other hydroxy stannates biodiesel by reacting glycerol with methyl acetate over Ca and Sn-mixed metal hydroxides catalysts. and metal oxides like MgSn(OH)6, ZnSn(OH)6, SrSn(OH)6, Ca(OH)2, CaO and MgO. The high glycerol The CaSn(OH) catalyst performed much better when compared to other hydroxy stannates and conversion6 (78.2%) was attributed to the high basicity exhibited by the CaSn(OH)6 catalyst contrary metal oxidesto the expectation. like MgSn(OH) The authors6, ZnSn(OH) also reported6, SrSn(OH) that this transesterification6, Ca(OH)2, CaO method and is MgO. cost-effective. The high The glycerol conversionuse of (78.2%) acid-functionalized was attributed ionic toliquids the highas a catalyst basicity in exhibitedglycerol acetylation by the CaSn(OH) has been reported6 catalyst to be contrary of to the expectation.high catalytic The activity authors [136–138]. also reported A >95% thatyield this of triacetin transesterification was obtained methodin glycerol is cost-eacetylationffective. with The use of acid-functionalizedacetic acid over 1-methyl-3-(3-sulfopropy ionic liquids as a catalystl)-imidazolium in glycerol hydrogen acetylation sulphate has ([HSO been reported3-pmim][HSO to be4]) of high as the catalyst after 6 h of reaction time at 120 °C [136]. The catalyst was reused ten times, and yet the catalytic activity [136–138]. A >95% yield of triacetin was obtained in glycerol acetylation with acetic activity remained at >91%. A similar performance was reported by Liu et al. [137], but the selectivity acid overto diacetin 1-methyl-3-(3-sulfopropyl)-imidazolium (51.4%) was the main product after 0.5 h hydrogen at 100 °C and sulphate 0.5 wt% ([HSO catalyst3-pmim][HSO load. Similarly,4]) as the catalystKeogh after 6et h al. of [138] reaction reported time a range at 120 of◦ nitrogen-basC[136]. Theed catalystBrønsted-acidic was reused ionic liquids ten times, (alkyl-pyrrolidone and yet the activity remainedand at alkylamine>91%. A similarcations) with performance reasonable was activity reported. N-methyl-2-pyrrolidinium by Liu et al. [137],but hydrogen the selectivity sulphate [H- to diacetin NMP][HSO4] was found to be the most active and cost-effective catalyst with >99% glycerol (51.4%) was the main product after 0.5 h at 100 ◦C and 0.5 wt% catalyst load. Similarly, Keogh et al. [138] reportedconversion a range and of nitrogen-based 42.3% selectivity towards Brønsted-acidic triacetin, and ionic >95% liquids of combined (alkyl-pyrrolidone di- and triacetin selectivity and alkylamine within 1 h. Non-porous materials such as hydroxylated magnesium fluoride were deployed in cations)glycerol with reasonableacetylation with activity. acetic acid N-methyl-2-pyrrolidinium (1:3) at 100 °C, and the result indicates hydrogen >90% sulphate glycerol [H-NMP][HSOconversion 4] was foundwith todiacetin be the as most the major active product and cost-e(≈60%)ff followedective catalystby triacetin with (≈30%)>99% after glycerol 22 h reaction conversion time, unlike and 42.3% selectivitythe towardsuncatalyzed triacetin, reaction and that> produced95% of combined only 30% gl di-ycerol and conversion triacetin selectivitywith monoacetin within as 1the h. major Non-porous materialsproduct such (>90%). as hydroxylated However, the magnesium reaction time fluoride was reduced were from deployed 4 h to in0.5 glycerolh when non-conventional acetylation with acetic activation methods, namely microwave or ultrasound irradiation, were used [139]. The authors acid (1:3) at 100 ◦C, and the result indicates >90% glycerol conversion with diacetin as the major attributed the excellent performance of the catalyst to the density of acid sites on the external surface product ( 60%) followed by triacetin ( 30%) after 22 h reaction time, unlike the uncatalyzed reaction of ≈the catalyst, while the selectivity was≈ influenced by the nature of acid sites (Lewis and Brønsted). that producedFindings only from 30% the glycerol above catalytic conversion studies with showed monoacetin that an increase as the in major catalyst product loading ( >to90%). a certain However, the reactionlevel (amount) time was improved reduced glycerol from conversion, 4 h to 0.5 hespecial whenly non-conventional to diacetin and triacetin, activation due to the methods, increased namely microwavenumber or ultrasound of available irradiation,and accessible were active used acid [site139 ].in Thethe reaction authors mixture. attributed Additional the excellent findings performance also of the catalystrevealed tothat the the density presence of acidof the sites Bronsted on the acid external site onsurface the catalyst of the promotes catalyst, optimal while glycerol the selectivity was influencedacetylation. by The the effect nature on ofmonoacetin acid sites production (Lewis and is minimal, Brønsted). and this can be seen in the pictorial Findings from the above catalytic studies showed that an increase in catalyst loading to a certain level (amount) improved glycerol conversion, especially to diacetin and triacetin, due to the increased number of available and accessible active acid site in the reaction mixture. Additional findings also revealed that the presence of the Bronsted acid site on the catalyst promotes optimal glycerol acetylation. The effect on monoacetin production is minimal, and this can be seen in the pictorial representation in Figure9. The performance of some selected catalysts in recent studies is summarized in Table2. Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 33

Appl.representation Sci. 2020, 10, 7155 in Figure 9. The performance of some selected catalysts in recent studies 17is of 34 summarized in Table 2.

100 90 GC 80 MA 70 60 DA 50 TA 40 30 20 10

% GC and selectivity to acetins % GC and 0 5 7 9 0.5 0.2 0.3 0.4 0.05 0.92 1.38 1.84 0.75 0.075 E (g) 0.1 E (g) B (g) 0.46 D (g ) 0.25 A (g) 0.025 C (wt%G) 3 Catalysts at various loads

FigureFigure 9. Influence 9. Influence of of various various catalyst catalyst loads loads onon glycerol conversion conversion (GC) (GC) and and selectivity selectivity to monoacetin to monoacetin (MA),(MA), diacetin diacetin (DA) (DA) and and triacetin triacetin (TA) (TA) using using diff erentdifferent catalysts catalysts at various at various reaction reaction conditions conditions (A (A= SSBA = , SSBA, glycerol (G):acetic acid (AAc) 1:3, T = 105 °C, t = 3 h [92]; B = glycerol-based carbon, G:AAc 1:4, glycerol (G):acetic acid (AAc) 1:3, T = 105 ◦C, t = 3 h [92]; B = glycerol-based carbon, G:AAc 1:4, T = 115 °C, t = 1 h [126]; C = cesium phosphotungstate (CsPWA), G:AAc 1:8, T = 85 °C, t = 2 h [89]; D = T = 115 ◦C, t = 1 h [126]; C = cesium phosphotungstate (CsPWA), G:AAc 1:8, T = 85 ◦C, t = 2 h [89]; 2 M H2SO4/Al2O3, G:AAc 1:9, T = 110 °C, t = 5 h [106]; E = glycerol-based carbon, G:AAc 1:3, T = 110 D = 2 M H SO /Al O , G:AAc 1:9, T = 110 C, t = 5 h [106]; E = glycerol-based carbon, G:AAc 1:3, °C, t = 32 h [114].4 2 3 ◦ T = 110 ◦C, t = 3 h [114]. 2.2.2.2.2.2. Temperature Temperature TemperatureTemperature is an is importantan important factor factor that that influences influences the the equilibrium equilibrium of of the the reaction, reaction, whichwhich also has a significanthas a significant impact onimpact the rateon the of reaction,rate of reaction, glycerol gl conversion,ycerol conversion, and product and product selectivity. selectivity. It is generally It is reportedgenerally that reported the acetylation that the acetylation of glycerol of with glycerol acetic with acid acetic is mainly acid is mainly an endothermic an endothermic reaction reaction process, process, and hence requires a large amount of heat [61]. However, since the reaction is catalyst driven, and hence requires a large amount of heat [61]. However, since the reaction is catalyst driven, care is care is taken to avoid the deactivation of thermally unstable catalysts by using moderate taken to avoid the deactivation of thermally unstable catalysts by using moderate temperatures [80]. temperatures [80]. A good number of researchers have reported the influence of temperature in A good number of researchers have reported the influence of temperature in glycerol acetylation, and glycerol acetylation, and their findings are reviewed below. their findingsGhoreishi are reviewedand Yarmo below. [110] varied the temperature from 50 °C to 110 °C in the esterification of glycerolGhoreishi with and acetic Yarmo acid [in110 a] stirred varied batch the temperature reactor equipped from with 50 ◦ Ca toreflux 110 system,◦C in the and esterification the results of glycerolshowed with a slight acetic increase acid in in a glycerol stirred conversion batch reactor with equipped an increase with in temperature. a reflux system, The authors and thefurther results showedreported a slight that increasediacetin and in glycerol triacetin conversion were the majo withr products an increase at high in temperature.temperature while The authorsmonoacetin further reportedwas the that predominant diacetin and product triacetin at a lower were thetemperature. major products In a similar at high study, temperature using a carbon-synthesized while monoacetin wascatalyst the predominant from crude product glycerol, at Okoye a lower et al. temperature. [114] reported In that a similar at 100 study,°C, the using glycerol a carbon-synthesized conversion was almost 100% with the corresponding selectivity of 50.97%, 45.98%, and 3% towards mono-, di-, and catalyst from crude glycerol, Okoye et al. [114] reported that at 100 ◦C, the glycerol conversion wastriacetin, almost 100% respectively. with the However, corresponding when the selectivity temperature of 50.97%, increased 45.98%, to 105, and110, 3%115, towards and 120 mono-,°C, the di-, monoacetin selectivity decreased, while there was a corresponding increase in diacetin at 105 and 110 and triacetin, respectively. However, when the temperature increased to 105, 110, 115, and 120 ◦C, °C, respectively. However, at 115 and 120 °C, the triacetin selectivity increased while diacetin and the monoacetin selectivity decreased, while there was a corresponding increase in diacetin at 105 and monoacetin continued to decline. The highest combined selectivity of the di and triacetin of 88% was 110 C, respectively. However, at 115 and 120 C, the triacetin selectivity increased while diacetin and ◦obtained at 110 °C, with a 2 wt% catalyst load,◦ the glycerol-to-acetic acid molar ratio of 1:3, and in 3 monoacetinh. The authors continued explained to decline. that the The increased highest temp combinederature selectivityenhanced the of theendothermic di and triacetin process, of which 88% was obtainedfurther at promoted 110 ◦C, with the deprotonation a 2 wt% catalyst of the load, second the and glycerol-to-acetic third hydroxyl acidgroups molar of glycerol ratio of over 1:3, the and in 3 h.active The authors sites of explainedthe catalyst, that hence the promoting increased temperaturethe formation enhancedof di- and thetriacetin. endothermic Similar results process, were which furtherreported promoted by Carvalho the deprotonation et al. [75]. The ofdi- the and second triacetin and selectivity third hydroxyl of 54 and groups16% were of obtained glycerol when over the activethe sites temperature of the catalyst, was increased hence up promoting to 120 °C over the a formation sulfonated ofcarbonized di- and triacetin.rice husk catalyst. Similar However, results were reportedthe maximum by Carvalho glycerol et al. conversion [75]. The di- was and achieved triacetin at selectivity 150 °C and of subsequently 54 and 16% weredecreased obtained when when the the temperature was increased up to 120 ◦C over a sulfonated carbonized rice husk catalyst. However, the maximum glycerol conversion was achieved at 150 ◦C and subsequently decreased when the temperature was further increased to 180 ◦C. This observation was attributed to the non-availability of the acetic acid > 150 ◦C due to vaporization. It was concluded that the suitable temperature to improve the esterification of glycerol with acetic acid to di- and triacetin was 120 ◦C. Testa et al. [92] Appl. Sci. 2020, 10, 7155 18 of 34

reported that at 60 ◦C, the glycerol conversion was 44% in 3 h with monoacetin as the main product. However, when the temperature was increased to 105 ◦C, the glycerol conversion was complete with 33% triacetin after 3 h of the reaction. Further increase in the temperature up to 120 ◦C did not change both the glycerol conversion and selectivity towards triacetin, contrary to the findings of Carvalho et al. [75] as indicated above, which may be due to the catalyst used (sulfonic acid-functionalized amorphous silica (SAS) and mesoporous silica (SSBA)). Konwar et al. [101] reported the effect of temperature on the esterification of glycerol with lauric acid and oleic acid. Results revealed that as the temperature increased from 100 to 150 ◦C, the rate of esterification increased for both agents with decreased selectivity towards monoacetin and increased selectivity towards di- and triacetin due to a secondary reaction. By optimizing the esterification reaction of glycerol with acetic acid, the influence of temperature was investigated by Liao et al. [80] over the amberlyst-35 catalyst. The temperature range investigated was from 95 to 115 ◦C at a fixed acetic acid to glycerol molar ratio of 6, with a catalyst weight of 0.5 g. Almost 100% glycerol conversion was achieved after 4 h. The selectivity to mono- and diacetin decreased (55.5 to 47.3% and 28.0 to 24.3%) while triacetin increased (16.5 to 28.3%) with an increase in temperature. This performance was attributed to a consecutive esterification reaction. Similarly, Mufrodi et al. [140] reported an increased glycerol conversion with increased temperature (100–120 ◦C) over a sulphuric acid catalyst, however, above 118 ◦C, the conversion reduced due to the evaporation of the acetylating agent (acetic acid). The increased temperature also increased the selectivity to mono-, di-, and triacetin. The authors further reported that for every 5 ◦C rise in temperature, the triacetin selectivity increased. The increase from 1.96% to a maximum of 13.69% at 115 ◦C was recorded. However, a further increase in the temperature reduced triacetin to 12.93%, which was attributed to the evaporation of acetic acid. The use of iron oxide supported on mesoporous aluminosilicate (Fe/Al–SBA-15) catalyst at 100 ◦C did not show any glycerol conversion even after 8 h of reaction until at 120 ◦C when the quantitative conversion of glycerol was achieved (>99%) with a selectivity of 71% diacetin and 28% triacetin. Further increase in temperature to 140 ◦C led to a slight improvement of diacetin (80%), and a reduction of triacetin (20%) which was attributed to the low thermal stability of the triacetin. However, the acetylating agent used was levulinic acid (1:4) in a continuous stirring (1200 rpm) reactor. The formation of diacetin as the major product could also be explained based on the lower activity of the secondary hydroxyl group, thereby preventing further reaction to form triacetin. Meanwhile, monoacetin was not observed in the product due to the equal activity of the two terminal hydroxyl groups. In another study, Khayoon et al. [82] also reported a substantial increase in glycerol conversion from 65 to 100% with increased temperature from 90 to 110 ◦C. Further increase in temperature did not improve the conversion. The selectivity to triacetin was found to depend on temperature. At lower temperatures (<90 ◦C), only about 7% of triacetin was produced, but when it was increased to 110 ◦C the triacetin increased to 55%. Further increase in temperature to 120 ◦C does not affect the triacetin selectivity. Similar findings have been reported by Goncalves et al. [141] and Dosuna-Rodriguez et al. [142]. However, contrary to the above, Rafi et al. [125] revealed that increasing the temperature from 80 to 120 ◦C increased the glycerol conversion from 25 to 85%. However, further increase to 140 ◦C did not show any appreciable enhancement. The selectivity to mono-, di-, and triacetin followed the same pattern as reported previously. Di- and triacetin increased continuously with an increase in temperature, while monoacetin decreased, and this observation was related to the increased activity of the biochar. However, the use of lauric acid in glycerol esterification over layered double hydroxide (LDH, Mg-Al–CO3) catalyst accommodated the use of high temperatures. At 100 ◦C, only 25% of glycerol conversion was achieved in 2 h, but when the temperature was increased to 180 ◦C, glycerol conversion increased to 99%. Nevertheless, the selectivity towards monoacetin (>50%) was largely favoured [143]. Sandesh et al. [89] also investigated glycerol esterification at lower temperatures (65–95 ◦C) over the CsPWA catalyst. Results indicate that in 2 h, molar ratio of 1:8, and temperature of 65 ◦C, the glycerol conversion was low (65%). However, as temperature increased to 75 ◦C, glycerol conversion increased to 92%. While at 85 ◦C, the conversion reached 98%, which appears to be the maximum because no further improvement was observed even Appl. Sci. 2020, 10, 7155 19 of 34 Appl. Sci. 2020, 10, x FOR PEER REVIEW 19 of 33

conversion increased to 92%. While at 85 °C, the conversion reached 98%, which appears to be the when the temperature was further increased to 95 ◦C. The selectivity of di- and triacetin also improved withmaximum increased because temperature no further while improvement the monoacetin was decreasedobserved even substantially. when the temperature The authors was attributed further the increased to 95 °C. The selectivity of di- and triacetin also improved with increased temperature while low performance of the reaction at low temperature to a low-level formation of acylium ion by the the monoacetin decreased substantially. The authors attributed the low performance of the reaction acetic acid, and as the temperature increased, the acylium ion formation also improved leading to high at low temperature to a low-level formation of acylium ion by the acetic acid, and as the temperature performance.increased, Athe similar acylium finding ion formation was reported also improved using the leading same typeto high of catalystperformance. but with A similar a temperature finding of 110 ◦wasC as reported the most using desirable the same [121 type]. The of influencecatalyst but of wi temperatureth a temperature in glycerol of 110 acetylation°C as the most has desirable been earlier reported[121]. inThe several influence literature of temperature articles [ 102in ,glycerol133,144 ,145acetylation], and the has results been earlier obtained reported were similarin several to the findingsliterature reported articles above. [102,133,144,145], and the results obtained were similar to the findings reported above.The findings from various researchers are in line with the Arrhenius equation (Equation (1)), which establishesThe findings a relationshipfrom various betweenresearchers the are temperature, in line with the activation Arrhenius energy, equation and (Equation rate of reaction.(1)), Increasedwhich temperatureestablishes a relationship directly increased between the the numbertemperature, of collisions activation of energy, the reactant, and rate which of reaction. is directly proportionalIncreased temperature to the rate ofdirectly reaction increased [60,146 the]. number The increased of collisions temperature of the reactant, promotes which rapid is directly glycerol conversion,proportional with to athe corresponding rate of reaction increase [60,146]. in The diacetin increased and temperature triacetin selectivity promotes atrapid the glycerol expense of conversion, with a corresponding increase in diacetin and triacetin selectivity at the expense of monoacetin. This observation is exemplified in the pictorial representation in Figure 10: monoacetin. This observation is exemplified in the pictorial representation in Figure 10: Ea/RT K = Ae− (1) =/ (1) wherewhereK is K the is the reaction reaction rate rate constant, constant,A Ais is the the Arrhenius Arrhenius constant constant relatedrelated to to the the frequency frequency factor, factor, EaEa is the activationis the activation energy, energy,R is theR is gas the constant,gas constant, and andT is T theis the temperature. temperature.

100 90 GC 80 MA 70 DA 60 50 TA 40 30 20

% GC and selectivity to acetins % GC and 10 0 95 80 75 85 95 80 110 105 120 100 100 110 120 E 60 C 60 B 60 A 80 D 65 Catalsyst at various temperatures (℃)

FigureFigure 10. 10.Influence Influence ofof temperature on on glycerol glycerol conversion conversion (GC) (GC)and selectivity and selectivity to monoacetin to monoacetin (MA), (MA),diacetin diacetin (DA) (DA)and triacetin and triacetin(TA) using (TA) different using catalysts different at various catalysts reaction at conditions various reaction(A = amberlyst-15 conditions (Dry), glycerol (G):acetic acid (AAc) 1:9, catalyst load (CL) = 2.645 g, t = 5 h [83]; B = propyl–SO3H- (A = amberlyst-15 (Dry), glycerol (G):acetic acid (AAc) 1:9, catalyst load (CL) = 2.645 g, t = 5 h [83]; functionalized mesoporous silica SBA-15 (SSBA), G:AAc 1:3, CL = 50 mg, t = 3 h [92]; C = SO42−/12ZrKIL- B = propyl-SO3H-functionalized mesoporous silica SBA-15 (SSBA), G:AAc 1:3, CL = 50 mg, t = 3 h [92]; 2, G:AAc2 1:10, CL = 0.1 g, t = 2 h [105]; D = CsPWA, G:AAc 1:8, CL = 7 wt%G, t = 2 h [89]; E = 1Ag-10Cu-rice C = SO4 −/12ZrKIL-2, G:AAc 1:10, CL = 0.1 g, t = 2 h [105]; D = CsPWA, G:AAc 1:8, CL = 7 wt%G, husk silica–Al2O3, G:AAc 1:10, CL = 80 mg, t = 5 h [90]. t = 2 h [89]; E = 1Ag-10Cu-rice husk silica–Al2O3, G:AAc 1:10, CL = 80 mg, t = 5 h [90]. 2.2.3.2.2.3. Reaction Reaction Time Time ForFor any any reaction reaction to reachto reach equilibrium, equilibrium, the the period period of contactof contact of theof the reactants reactants with with the the catalyst catalyst plays an importantplays an important role. Reports role. inReports the literature in the literature also showed also theshowed influence the influence of time inof glyceroltime in glycerol acetylation. Ghoreishiacetylation. and YarmoGhoreishi [110 and] reported Yarmo that[110] in reported glycerol that acetylation in glycerol with acetylation acetic acid with at aacetic molar acid ratio at ofa 1:3, molar ratio of 1:3, 50 °C, and 200 mg catalyst load (10% sulphated silica), monoacetin was favoured 50 ◦C, and 200 mg catalyst load (10% sulphated silica), monoacetin was favoured at a lower reaction at a lower reaction time (2 h). However, higher acetins were favoured as the reaction proceeds with time (2 h). However, higher acetins were favoured as the reaction proceeds with time, attributable to time, attributable to the continuous conversion of monoacetin to diacetin and triacetin. The glycerol the continuous conversion of monoacetin to diacetin and triacetin. The glycerol conversion increased conversion increased from about 60% in 2 h to almost 100% in 6 h. The effect of time in glycerol from about 60% in 2 h to almost 100% in 6 h. The effect of time in glycerol conversion was also reported by Okoye et al. [114] over glycerol-based carbon catalyst and concluded that the glycerol conversion increased with an increase in reaction time from 1 to 3 h. In 1 h, about 70% glycerol Appl. Sci. 2020, 10, 7155 20 of 34

Appl. Sci. 2020, 10, x FOR PEER REVIEW 20 of 33 conversionconversion was was achieved also reported with high by Ok monoacetinoye et al. [114] selectivity, over glycerol-bas but in 3 h,ed thecarbon glycerol catalyst conversion and concluded increased to almostthat the 100% glycerol with conversion higher di- increased and triacetin with selectivityan increase butin reaction low monoacetin time from 1 selectivity. to 3 h. In 1 Chandrakalah, about et al.70% [126 glycerol] also reported conversion the was effect achieved of the with reaction high timemonoacetin on the selectivity, esterification but ofin 3 glycerol h, the glycerol with acetic acid (1:4)conversion in the increased presence to of almost 5 wt % 100% glycerol-based with higher carbon di- and catalyst triacetin at selectivity 115 ◦C for but various low monoacetin time of 1 to 4 h. Resultsselectivity. indicate Chandrakala a complete et al. glycerol [126] also conversion reported the within effect 1of h the with reaction a selectivity time on the of 22%esterification monoacetin, 67% diacetin,of glycerol and with 11% acetic triacetin, acid (1:4) respectively.in the presence When of 5 wt the % glycerol-based reaction time carbon was increased catalyst at 115 up to°C 4for h, the various time of 1 to 4 h. Results indicate a complete glycerol conversion within 1 h with a selectivity triacetin increased to 25%, with a corresponding slight decrease in mono- and diacetin. Similar findings of 22% monoacetin, 67% diacetin, and 11% triacetin, respectively. When the reaction time was were reported by Khayoon et al. [82] over the 3% yttrium on mesoporous silicate material support increased up to 4 h, the triacetin increased to 25%, with a corresponding slight decrease in mono- and (3%/SBA-3),diacetin. whereSimilar the findings glycerol were conversion reported by was Khayoon 62% afteret al. [82] 0.5 h,over and the the 3% conversion yttrium on mesoporous was completed after 1silicate h. The material selectivity support to monoacetin (3%/SBA-3), decreased,where the gl whileycerol the conversion diacetin increasedwas 62% after with 0.5 time. h, and Additional the findingsconversion showed was that completed triacetin after commenced 1 h. The afterselectivity 0.5 h to into monoacetin the reaction, decreased, which while was attributedthe diacetin to the directincreased acetylation with of time. monoacetin, Additional and findings after 1.5showed h, some that oftriacetin the produced commenced diacetin after 0.5 also h transformedinto the into triacetin,reaction, which hence was the attributed observed to decrease the direct inacetylat diacetin.ion of The monoacetin, authors and concluded after 1.5 thath, some the of complete the glycerolproduced conversion diacetin with also combined transformed selectivity into triaceti of di-n, andhence triacetin the observed of 89% decrease was achieved. in diacetin. The The biochar catalystauthors also showedconcluded excellent that the activity complete with glycerol the increased conversion reaction with timecombined to a maximumselectivity ofof 4di- h, whichand in triacetin of 89% was achieved. The biochar catalyst also showed excellent activity with the increased turn improved the selectivity towards di- and triacetin with a concurrent decrease in the monoacetin reaction time to a maximum of 4 h, which in turn improved the selectivity towards di- and triacetin selectivity [125]. The use of sulfonated carbonized willow catkins by Tao et al. [113] also showed a with a concurrent decrease in the monoacetin selectivity [125]. The use of sulfonated carbonized similarwillow trend. catkins The by glycerol Tao et conversional. [113] also increased showed a rapidlysimilar trend. to 100% The withglycerol the conversion increased reactionincreased time fromrapidly 0.25 to to 2 h.100% The with extension the increased of time reaction up to 7time h led from to 0.25 a slight to 2 decreaseh. The extension in glycerol of time conversion, up to 7 h led which the authorsto a slight attributed decrease toin theglycerol drop conversion, in glycerol whic concentrationh the authors and attributed the accumulation to the drop ofin waterglycerol as the by-product.concentration However, and the the accumulation selectivity toof higherwater as acetin the by-product. improved However, significantly the selectivity with time. to higher In 7 h, the combinedacetin selectivity improved significantly of di- and triacetin with time. reached In 7 h, 95.3%.the combined The catalyst selectivity performance of di- and wastriacetin attributed reached to the acid property95.3%. The and catalyst the nature performance of the was surface attributed arisen to from the acid the sulfonation,property and whichthe nature might of the have surface provided goodarisen access from to the the active sulfonation, sites for which the hydrophilicmight have pr glycerol.ovided good Similar access findings to the haveactive been sites reportedfor the by hydrophilic glycerol. Similar findings have been reported by several researchers despite the use of several researchers despite the use of different catalysts [89,102,133,144]. Figure 11 shows the pictorial different catalysts [89,102,133,144]. Figure 11 shows the pictorial representation of some selected representationfindings of of the some influence selected of reaction findings time of on the glycerol influence conversation of reaction and time selectivity on glycerol towards conversation mono-, di- and selectivityand triacetin towards as mono-,reported di- in and some triacetin recent research as reported articles. in some The reviewed recent research literature articles. showed The a linear reviewed literaturerelationship showed between a linear reaction relationship time betweenand increased reaction acetin time formation. and increased Increased acetin time formation. favours higher Increased time favoursselectivity higher towards selectivity diacetin and towards triacetin, diacetin respectively. and triacetin, respectively.

100 90 GC 80 MA 70 DA 60 50 TA 40 30 20

%GC and selectivity to acetins 10 0 2 2 3 4 1 2 3 3 24 E 1 1.5 B 1 A 1 C 0.5 D 0.5 Catalysts at various reaction time (h)

FigureFigure 11. 11.Inﬂuence Influence of of the the reaction reaction time timeon glycerol on glycerol conversion conversion (GC) and the (GC) selectivity and theto monoacetin selectivity to (MA), diacetin (DA) and triacetin (TA), using different catalysts at various reaction conditions (A = monoacetin (MA), diacetin (DA) and triacetin (TA), using diﬀerent catalysts at various reaction HSiW/ZrO2, glycerol (G):acetic acid (AAc) 1:10, T = 120 °C, CL = 0.3 g [131]; B = glycerol-based carbon, conditions (A = HSiW/ZrO , glycerol (G):acetic acid (AAc) 1:10, T = 120 C, CL = 0.3 g [131]; G:AAc 1:4, T = 115 °C, CL 2= 5 wt%G [126]; C = CsPWA, G:AAc 1:8, T = 85 °C, CL ◦= 7 wt%G [89]; D = B = glycerol-based carbon, G:AAc 1:4, T = 115 ◦C, CL = 5 wt%G [126]; C = CsPWA, G:AAc 1:8, T = 85 ◦C, CL = 7 wt%G [89]; D = arenesulfonic acid-functionalized bentonite, G:AAc:toluene 1:7:1.4, T = 100 ◦C, CL = 0.074 wt%G [147]; E = SBA–propyl sulfamic acid, G:AAc 1:3, T = 105 ◦C, CL = 50 mg [94]. Appl. Sci. 2020, 10, 7155 21 of 34

2.2.4. The Reactants and Their Molar Ratios The most common reactants used in the conversion of glycerol to acetin include acetic acid and acetic anhydride [126,134,135]. However, it has been reported that acetic acid is, thermodynamically, resistant to acetylation due to the formation of a tetrahedral intermediate. The formation of water (in situ), may also deactivate the catalyst and reverse the reaction [41,110]. Nevertheless, acetic acid is still preferred because it is cheaply available and less toxic despite the positive Gibbs free energy exhibited. Acetylation with acetic anhydride proceeds quickly to the product, attributed to the ease of formation of acylium ion [80]. However, the use of acetic anhydride is of grave concern because of its application in the production of narcotics. Currently, it is banned in some countries [114]. It was also reported that the rate of reaction of glycerol with acetic anhydride is much higher than with acetic acid, and because of its speed, it is much more difficult to handle [67,80]. Several journal articles also reported the use of other carboxylic acids, such as oleic, levulinic, lauric, propionic, and butanoic acids in acetylation [58,84,143,148]. Most of these carboxylic acids have shown good glycerol conversion and selectivity to monoacetin but limit the selectivity to higher acetins (diacetin and triacetin). The limitation was attributed to their bulky structure. The performance of these acetylating agents in the reaction can also be influenced by their molar ratios. In terms of the stoichiometric equation, 3 mol of acetic acid is needed to react completely with 1 mol of glycerol to produce monoacetin and subsequently to diacetin and triacetin as the final product [60]. However, excess acetic acid shifts the equilibrium towards higher diacetin and triacetin, respectively. An increase in the molar ratio of acetic acid to glycerol led to an increase in glycerol conversion [110]. In addition, 96.88% glycerol conversion and selectivity of 51.90, 45.27, and 2.11% monoacetin, diacetin, and triacetin were obtained at an acetic acid to glycerol molar ratio of 6 over a 20% sulphated silica catalyst. When the molar ratio of 3 was used over the same catalyst, the glycerol conversion reduced to 90.12% with a selectivity of 58.83, 37.10, and 3.24% monoacetin, diacetin, and triacetin, respectively. However, when 10% sulphated silica catalyst was used, monoacetin selectivity was favoured with 81.12%, while diacetin was 18.32%, and only traces of triacetin were observed at a molar ratio of 3. The difference in the catalyst performance was attributed to the acid density and the availability of the acetylating agent. That is, a catalyst with a higher acid density favours higher acetin (di- and triacetin) formation. A similar observation was reported with a crude glycerol-derived carbon catalyst [114]. At an acetic acid to glycerol molar ratio of 1, over 70% of monoacetin was obtained with 55% glycerol conversion. However, when the molar ratio was varied from 3 to 9, glycerol conversion increased to 100% with a high production of di- and triacetin (46%, 43%) and low production of monoacetin (11%). When the molar ratio was > 4, the triacetin production decreased, and it was attributed to the likely effect of excess acetic acid leading to mass transfer resistance. This resistance limits the sufficient interaction or contact between the glycerol and the catalyst. Isahak et al. [13] reported the esterification of glycerol with oleic acid over a mixture of silicotungstic acid–ionic liquid catalyst at 100 ◦C for 8 h. High glycerol conversion (96.4%) was achieved with 96% selectivity towards monoacetin. Only 4% of diacetin was reported. A good yield of glycerol ester was also reported by Sari et al. [148] using the same reactants (glycerol and oleic acid) but with a different catalyst made of methyl ester sulfonic acid. Liao et al. [80] also reported the influence of acetic acid to glycerol molar ratio (2:1, 3:1, 6:1, 9:1) at fixed conditions of 105 ◦C, 0.5 g catalyst load and in 4 h over amberlyst-35 catalyst. The results indicate a higher conversion of glycerol and a higher selectivity towards triacetin with an increased amount of acetic acid. This observation was attributed to the improvement in the acid strength with the availability of more acetic acid, which subsequently improved the catalytic activity. In the work of Mufrodi et al. [140], it was reported that based on the stoichiometric calculation, three moles of acetic acid requires one mole glycerol to produce one mole of triacetin. However, in the experimental work, the increase in the molar ratio of acetic acid to glycerol (3, 4, 5, and 6) increased the glycerol conversion by 0.29% for each addition of 1 mol acetic acid. The maximum glycerol conversion of 98.50% was achieved. The increase in molar ratio also affected the selectivity as indicated by the decreased monoacetin and increased di- and triacetin, respectively. The influence of the acetic acid Appl. Sci. 2020, 10, 7155 22 of 34 to glycerol molar ratio in esterification was also evaluated by Carvalho et al. [75] over a sulfonated carbonized rice husk catalyst. The results indicate an increase in the formation of di- and triacetin with a simultaneous decrease in monoacetin as the molar ratio increases. The authors opined that the observation was due to the continuous conversion of monoacetin to the higher acetins; however, the glycerol conversion was not significantly affected until at a molar ratio of 5. The glycerol conversion was 90%, and a further increase in the molar ratio to 9 did not significantly improve the conversion. However, the selectivity to di- and triacetin improved greatly, but for economic and environmental reasons, the authors felt that there was no need to use a high volume of acetic acid given the amount of products produced. Similarly, a report by Chandrakala et al. [126] showed a marginal improvement of diacetin from 67 to 73% and triacetin from 11 to 20% when the molar ratio of acetic acid to glycerol was varied from 4:1 to 10:1 over 5 wt% of a glycerol-based carbon catalyst at 115 ◦C for 1 h. However, when the reaction time was increased to 4 h, the triacetin was increased to 40% at a molar ratio of 10:1. This finding is different from the findings of Rafi et al. [125], who used unfunctionalized biochar as a catalyst. About 75% glycerol conversion was obtained at a mole ratio of 3:1. The conversion increased up to 89% when the molar ratio was varied to 5:1 but at a higher temperature of 120 ◦C. The improvement was also observed in the combined di- and triacetin selectivity (60%). Further variation in the molar ratio did not produce any appreciable change in the glycerol conversion. The difference in the above performance may be due to the nature of the catalyst and the different operating temperatures used. Similarly, the higher conversion of glycerol (99%) was obtained when glycerol was esterified with lauric acid (3:1) but at a higher temperature of 180 ◦C in 2 h over layered double hydroxide (LDH, Mg–Al–CO3) catalyst with a selectivity of approximately 58, 30 and 12% for mono-, di- and tri-acetins, respectively [143]. The use of different carboxylic acids, namely acetic acid, propionic acid, and 1-butanoic acid as esterification agents in glycerol acetylation over silver-exchanged phosphotungstic acid (Ag1PW) catalyst (1 wt% catalyst load) at 120 ◦C, with a 1:10 molar ratio (glycerol to carboxylic acids), was reported by Zhu et al. [84]. In less than 20 min, acetic acid showed 96.8% glycerol conversion with 48.4, 46.4, and 5.2% selectivity towards mono-, di-, and triacetin, propionic acid achieved 70.9% glycerol conversion with 55, 43.1, and 1.9% selectivity towards mono-, di- and triacetin, respectively, while 1-butanoic acid produced 64.3% glycerol conversion with 53.9, 46 and 0.1% selectivity towards mono-, di-, and triacetin, respectively. The very low selectivity towards triacetin could be attributed to steric hindrance arising from the structures of the carboxylic acids (acetic acid, propionic acid, and 1-butanoic acid) because the trend indicates decreased glycerol esterification with increasing chain length. The influence of the acetic acid to glycerol molar ratio (4:1, 6:1, and 8:1) was investigated at a fixed reaction temperature of 110 ◦C over 3% Y/SBA-3 by Khayoon et al. [82]. The best result was obtained with a molar ratio of 4:1, which resulted in the complete conversion of glycerol with the corresponding selectivity of 11% monoacetin, 34% diacetin, and 55% triacetin, respectively. The good selectivity towards triacetin at a 4:1 molar ratio was also attributed to the excellent textural characteristics, the crystal phase stability of the catalyst, and the availability of the acetylating agent. It was a general expectation that excess acetylating agents could shorten the time required to reach the reaction equilibrium and enhance the formation of higher acetins due to consecutive acetylation reactions, but that was not the case. The authors attributed it to the hydrolysis effect of the co-produced water. Contrary to the above finding, Kotbagi et al. [132] showed higher glycerol conversion at a higher molar ratio of acetic acid to glycerol. The molar ratio of 3, 5, and 10 resulted in a glycerol conversion of 67, 91, and 100%, respectively, after 8 h of reaction using a supported silicomolybdic heteropolyanions catalyst at 10 wt% with respect to glycerol. The selectivity to mono-, di-, and triacetin also varies with the molar ratio. At a molar ratio of 3, monoacetin was favoured with 81% selectivity, while the selectivity to di and triacetin was 16 and 3%, respectively. However, at a molar ratio of 10, triacetin was favoured with 50% selectivity, followed by diacetin with 33% while the monoacetin decreased to 17%. This result was similar to the earlier work reported by Chandrakala et al. [126], confirming the conversion of monoacetin to the higher acetins (di- and triacetin) in line with the Appl. Sci. 2020, 10, 7155 23 of 34

Appl. Sci. 2020, 10, x FOR PEER REVIEW 23 of 33 earlier stated mechanism of the reaction. Sandesh et al. [89] also reported that the molar ratio of acetic that the molar ratio of acetic acid-to-glycerol of 8 produced the highest glycerol conversion of 98% acid-to-glycerol of 8 produced the highest glycerol conversion of 98% with the CsPWA catalyst. When with the CsPWA catalyst. When a molar ratio of 10 was used, no noticeable change in the conversion a molar ratio of 10 was used, no noticeable change in the conversion was reported. The selectivity of was reported. The selectivity of the desired product also followed the same pattern. The molar ratio of 4 the desired product also followed the same pattern. The molar ratio of 4 and 6 did not produce much and 6 did not produce much of di- (35% each) and triacetin (0 and 6%) until at a molar ratio of 8 (59% of di- (35% each) and triacetin (0 and 6%) until at a molar ratio of 8 (59% diacetin and 16% triacetin), diacetin and 16% triacetin), which showed that high quantity of acetic acid increased its accessibility to which showed that high quantity of acetic acid increased its accessibility to glycerol. This finding was glycerol. This finding was corroborated by Veluturla et al. [121] with a maximum glycerol conversion of corroborated by Veluturla et al. [121] with a maximum glycerol conversion of 98.2% at an acetic acid to 98.2% at an acetic acid to glycerol molar ratio of 9 over a similar catalyst. Several literature articles glycerol molar ratio of 9 over a similar catalyst. Several literature articles previously reported a similar previously reported a similar pattern of increased glycerol conversion with a variant molar ratio pattern of increased glycerol conversion with a variant molar ratio [34,83,102,133,145]. Figure 12 shows [34,83,102,133,145]. Figure 12 shows the pictorial representation of some selected findings of the influence the pictorial representation of some selected findings of the influence of molar ratios of the reactants of molar ratios of the reactants on the glycerol conversion and selectivity towards mono-, di-, and triacetin on the glycerol conversion and selectivity towards mono-, di-, and triacetin as reported by some as reported by some recent studies. recent studies.

100 90 80 GC 70 MA 60 DA 50 TA 40 30 20

% GC and selectivity to acetins % GC and 10 0 1:6 1:8 1:5 1:6 1:8 1:2 1:3 1:10 1:10 1:10 1:15 1:20 1:25 E 1:5 C 1:4 B 1:3 A 1:4 D 1:1 Catalysts at various G:AA molar ratios

Figure 12. Inﬂuence of glycerol to acetylating agent (G:AA) molar ratios on glycerol conversion Figure(GC) and 12. Influence selectivity of to glycerol monoacetin to acetylating (MA), diacetin agent (G: (DA)AA) and molar triacetin ratios (TA)on glycerol using di conversionﬀerent catalysts (GC)

andat variousselectivity reaction to monoacetin conditions (MA), (A = diacetin3% Y/ SBA,(DA) AAcand triacetin= acetic (TA) acid, using T = different100 ◦C, CLcatalysts= 0.2 at g, varioust = 3 h [reaction82]; B = 20conditions mol%MnO (A3 =/SiO 3%2 ,Y/SBA, AAc, T AAc= 100 = ◦acC,etic CL acid,= 10 T wt%G, = 100 t°C,= 8CL h [=132 0.2]; g, C =t =CsPWA, 3 h [82]; AAc, B = 20T =mol%MnO85 ◦C, CL3/SiO= 7 wt%Gly,2, AAc, T t == 1002 h °C, [89 ];CL D = 10CsPWA, wt%G, AAh t = 8 =h acetic[132]; C anhydride, = CsPWA, T AAc,= 30 ◦TC, = CL85 °C,= 4 CL wt%G, = 7 wt%Gly,t = 2 h [89 t ];= E2 h= [89];1Ag-10 D = Cu-rice CsPWA, husk AAh silica–Al = acetic2O anhy3, AAc,dride, T = T110 = 30◦C, °C, CL CL= 80= 4 mg, wt%G, t = 5 t h= [290 h]. [89]; E = 1Ag-10 Cu-rice husk silica–Al2O3, AAc, T = 110 °C, CL = 80 mg, t = 5 h [90].

Appl. Sci. 2020, 10, 7155 24 of 34

Table 2. High performing catalysts in glycerol acetylation as reported by recent studies.

SA Acidity Catalyst Conditions for the Reaction Performance Reference (m2/g) (mmol/g) Acetylating MR Stirring GC MA DA TA T( C) CL (g) t (h) Agent (AA) ◦ (G:AA) (rpm) (%) (%) (%) (%) 2 SO4 −/CeO2-ZrO2 22.07 NA Acetic acid 100 1:10 5 wt%G 3 Stirred 99.12 21.46 57.28 21.26 [108] PVP–DTP NA NA Acetic acid 110 1:20 0.25 6 NA 100 13 54 34 [104] 0.40g [H-NEP][HSO4] Ionic liquid NA NaOH/g Acetic acid 100 1:6 2 mol% 0.30 800 99 23.4 55.2 21.3 [138] IL HCl-activated zeolite NA NA Acetic acid 90 1:9 3% AA 130 400 95 43.0 48.6 8.3 [100] HZSM-5/MCM-41 micro/mesoporous molecular 501 NA Acetic acid 125 1:8 1.5 24 Stirred 100 0.20 17.9 81.9 [149] sieve Sucrose–SBA-15–BDS 547 0.96 Acetic acid 110 1:9 0.7 6 Stirred 95 19 56 22 [116] 20%SO4/K10 187 0.19 Acetic acid 120 1:12 0.4 5 Stirred 99 23 59 15 [91] Arenesulfonic acid Acetic acid/ 5.4 1.7 100 1:7:1.4 1 3 na 100 0 26 74 [147] functionalized bentonite * Toluene Propyl-SO3H SBA-15 366 1.80 Acetic acid 120 1:6 4 wt%G 2.5 na 96 13 55 32 [73] SBA-Pr NH–SO3H na 1.02 Acetic acid 105 1:3 0.05 3 na 100 16 57 28 [94] 2 Diatomite-loaded SO4 −/TiO2 na na Oleic acid 210 1:2 0.1 wt% 6 200 95.1 20.2 59.6 15.3 [107] Sulfonated glycerol-based carbon na 3.6 Acetic acid 110 1:3 0.2 3 stirred 99 12 45 43 [114] Sulfonated activated carbon 469 5.41 Oleic acid 125 1:1 0.5 24 650 90 77 na na [101] (ACSO3H) ACSO3H 469 5.41 Lauric acid 100 1:1 0.5 24 650 90 70 na na [101] Acetic Fe–Sn–Ti(SO 2 )-400 18.88 na 80 1:6 0.05 0.5 Stirred 100 0 1 99 [134] 4 − anhydride Methyl CaSn(OH) 3.95 na 30 1:10 7 wt%G 2 Stirred 78.2 67.3 32.6 na [135] 6 acetate 2 2M SO4 −/γ-Al2O3 8.4 2.51 Acetic acid 110 1:9 0.25 2 700 97 27 49.9 23.1 [122] 1Ag–10Cu-doped rice husk silica 108 na Acetic acid 110 1:10 0.08 5 700 83.4 68.3 18.5 13.2 [90] Fe4(SiW12O40)3 na na Acetic acid 60 1:3 0.06 mmol 8 Stirred 100 24 69 7 [146] PDSA-treated montmorillonite 276 0.258 Acetic acid 120 1:3 1 1 500 96 14 26 56 [87] MSA-treated montmorillonite 204 0.254 Acetic acid 120 1:3 1 1 500 94 22 31 41 [87] p-TSA-treated montmorillonite 141 0.256 Acetic acid 120 1:3 1 1 500 94 30 35 29 [87] 2 2M SO4 −/γ-Al2O3 8.4 2.51 Acetic acid 95 1:12 0.75 5 na 99 22.6 48.2 29.2 [106] 20 mol% MnO3/SiO2 217 0.94 Acetic acid 100 1:10 10 wt%G 8 na 100 17 33 50 [132] Acetic 5% per mol Amberlyst-15 na na 60 1:3 2 na 100 0.7 1.3 98.1 [99] anhydride G Appl. Sci. 2020, 10, 7155 25 of 34

Table 2. Cont.

SA Acidity Catalyst Conditions for the Reaction Performance Reference (m2/g) (mmol/g) Acetylating MR Stirring GC MA DA TA T( C) CL (g) t (h) Agent (AA) ◦ (G:AA) (rpm) (%) (%) (%) (%) 5% per mol Amberlyst-15 na na Acetic acid 120 1:6 2 na 100 3.5 8.7 87.8 [99] G Sulphated willow na 5.14 Acetic acid 120 1:5 5 wt%G 2 stirred 98.4 32.8 54.5 12.7 [113] catkins-based carbon Biochar-400 14 6.328 Acetic acid 120 1:5 0.2 1 na 88.5 56 40 4 [125] Graphene oxide na na Acetic acid 120 1:10 0.1 1 Stirred 98.5 15.5 60 24.5 [42] CsPWA 110 1.87 Acetic acid 85 1:8 7 wt%R 2 Stirred 98.1 25 59 16 [89] Acetic CsPWA 110 1.87 30 1:3 4 wt%R 2 Stirred 100 1.2 21.5 77.3 [89] anhydride Sb2O5 53 0.094 Acetic acid 120 1:6 0.1 1 Stirred 96.8 33.2 54.2 12.6 [123] Amberlyst-36 na na Acetic acid 100 1:7 3 2 na 100 43 44 13 [150] 2 SO4 −/12ZrKIL-2 306 na Acetic acid 100 1:10 0.1 2 Stirred 100 2.7 77.1 20.1 [105] 1:10 SO 2 /12ZrKIL-2 306 na Acetic acid 100 0.1 2 Stirred 91.1 63.7 26.4 20.5 [105] 4 − Crude G Levulinic Fe-SBA-15 688 0.104 140 1:4 0.05 8 1200 >99 0 80 20 [111] acid 3%Y-SBA-15 1568 1.342 Acetic acid 110 1:4 0.2 3 350 100 11 34 55 [82] Sulphated glycerol-based carbon na na Acetic acid 115 1:10 0.46 1 Stirred 100 8.4 71.8 19.8 [126] PrSO3H SBA-15 701 1.2 Acetic acid 80 1:6 5 wt%G 8 Stirred 100 15.8 64.6 19.6 [41] Amberlyst-15 53 4.9 Acetic acid 80 1:6 5 wt%G 8 Stirred 100 21.1 63.8 15.1 [41] SO3H-SBA15 515 0.8 Acetic acid 80 1:6 5 wt%G 8 Stirred 100 11.1 61.9 27.0 [41] Acetic acid/ La3+-montmorillonite na na 120 1:2 0.02 mmol 24 na 98 0 >99 0 [109] * Toluene 30%TPA/MCM-41 360 0.855 Acetic acid 100 1:6 0.15 6 Stirred 87 na 60 15 [102] Layer double 106 na Lauric acid 180 1:3 2 wt%G 1 500 >99 58 30 10 [143] hydroxide–hydrotalcite ≈ ≈ 1.41/** Amberlyst-15 (1.6 wt) 37.6 Acetic acid 110 1:9 2.645 4.5 1100 97.1 <20 47.7 44.5 [34] 4.7 Appl. Sci. 2020, 10, 7155 26 of 34

Table 2. Cont.

SA Acidity Catalyst Conditions for the Reaction Performance Reference (m2/g) (mmol/g) Acetylating MR Stirring GC MA DA TA T( C) CL (g) t (h) Agent (AA) ◦ (G:AA) (rpm) (%) (%) (%) (%)

H4SiW12O40/ZrO2 48.7 0.69 Acetic acid 120 1:10 0.3 4 250 100 6.4 61.3 32.3 [131] Silver-exchanged phosphotungstic na 1.92 Acetic acid 120 1:10 1 wt%G 0.25 Stirred 96.8 48.4 46.4 5.2 [84] acid (Ag1PW) Propanoic Ag PW na 1.92 120 1:10 1 wt%G 0.25 na 70.9 55.0 43.1 1.9 [84] 1 acid 1-Butanoic Ag PW na 1.92 120 1:10 1 wt%G 0.25 na 64.3 53.5 46.4 0.1 [84] 1 acid 20% sulphated silica na 479 Acetic acid 50 1:6 0.2 2 Stirred 96.88 51.90 45.27 2.11 [110] PrSO3H SAS 640 1.7 Acetic acid 105 1:3 0.05 3 Stirred 100 0 51 49 [92] PrSO3H SBA-15 489 1.2 Acetic acid 105 1:3 0.05 3 Stirred 100 5 62 33 [92] Sulfonated carbonized rice husk <80 5.8 Acetic acid 150 1:4 5 wt%G 5 Stirred 90 11 52 37 [75] Acetic acid/ Amberlyst-70 36 2.55 105 1:6 5 wt%G 10 na 100 0 7.5 87.6 [151] * Toluene Acetic acid/ Amberlyst-15 53 4.7 105 1:6 5 wt%G 10 na 100 0 12.3 83.9 [151] * Toluene H3PW12O40 na na Acetic acid 60 1:3 0.03 mmol 8 Stirred 96 66 34 0 [93] Amberlyst-15 na ** 4.7 Acetic acid 110 1:9 2.645 5 1100 97.13 7.59 46.29 43.23 [83] SA = surface area, T = temperature, MR = mole ratio, G = glycerol, AA = acetylating agent, CL = catalyst load, t = time, GC = glycerol conversion, MA = monoacetin, DA = diacetin, TA = triacetin, R = reactants mixture, NA = not available, PVP–DTP = polyvinylpyrrolidone impregnated with phospotungustic, [H-NEP][HSO4] = N-Ethyl-2-pyrrolidinium hydrogen sulphate, SBA = functionalized ordered mesoporous silica, BDS = 4-aminobenzenesulfonic acid, PDSA = phenoldisulfonic acid, MSA = methane sulfonic acid, p-TSA = para-toluenesulfonic acid, SAS = functionalized amorphous silica, * entrainer (azeotropic agent), ** commercial. Appl. Sci. 2020, 10, 7155 27 of 34

3. Conclusions From the above review, it is evident that there is an increased interest in glycerol acetylation to acetin, which may not be unconnected with the surplus production of glycerol and the drive to improve the economics of biodiesel production. It may also be attributed to the high-value application of acetin in the cosmetic, pharmaceutical, medical, food, and polymer industries as a humectant, emulsifier, plasticizer, and most importantly, as a biofuel additive to improve the viscosity and cold flow properties of gasoline, diesel, and bio-diesel. This review paper summarizes the different heterogeneous catalysts deployed in acetylation reaction. This review indicates that apart from the popularly used acetylating agents (acetic acid and acetic anhydride), other agents include oleic, levulinic, lauric, propionic, and butanoic acids. This paper also clearly indicates that the nature of the catalyst, catalyst loading, temperature, reactants molar ratio, and time of the reaction influences the glycerol conversion to mono-, di-, and triacetin with the temperature and reactants molar ratio been the most influential. Finally, there is the need for continual research in this area to identify new inexpensive, reusable, and environmentally friendly catalytic materials and milder conditions of reaction to favour high selectivity to triacetin. The need for further studies on catalyst deactivation and the kinetic of acetylation is recommended to facilitate the understanding of the process for industrial applications.

Author Contributions: U.I.N.-U., I.B.R., E.N.M., N.A., U.F.A., Y.H.T.-Y. contributed immensely to the success of the research from the conception to data analysis. The authors were also involved in the writing, reviewing and editing of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Universiti Putra Malaysia (UPM) under the IPS research fund, grant number GP-IPS/2018/9619500 and the article processing charge (APC) was funded by Research Management Committee (RMC), UPM. Acknowledgments: The authors appreciate the University Putra Malaysia for the IPS research grant. We equally appreciate the RMC for approving and funding the APC. The sponsorship received from the Tertiary Education Trust Fund (TETFund) through the Federal Polytechnic Bida, Nigeria, is also deeply appreciated. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Johnson, D.T.; Taconi, K.A. The Glycerin Glut: Options for the Value-Added Conversion of Crude Glycerol Resulting from Biodiesel Production. Environ. Prog. 2009, 26, 338–348. [CrossRef] 2. Ayoub, M.; Abdullah, A.Z. Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy industry. Renew. Sustain. Energy Rev. 2012, 16, 2671–2686. [CrossRef] 3. Rahmat, N.; Abdullah, A.Z.; Mohamed, A.R. Recent progress on innovative and potential technologies for glycerol transformation into fuel additives: A critical review. Renew. Sustain. Energy Rev. 2010, 14, 987–1000. [CrossRef] 4. Gupta, M.; Kumar, N. Scope and opportunities of using glycerol as an energy source. Renew. Sustain. Energy Rev. 2012, 16, 4551–4556. [CrossRef] 5. Babajide, O. Sustaining Biodiesel Production via Value-Added Applications of Glycerol. J. Energy 2013, 2013, 1–7. [CrossRef] 6. Sakthivel, A.; Nakamura, R.; Komura, K.; Sugi, Y. Esterification of glycerol by lauric acid over aluminium and zirconium containing mesoporous molecular sieves in supercritical carbon dioxide medium. J. Supercrit. Fluids 2007, 42, 219–225. [CrossRef] 7. Bagheri, S.; Julkapli, M.N.; Yehye, W.A. Catalytic conversion of biodiesel derived raw glycerol to value added products. Renew. Sustain. Energy Rev. 2015, 41, 113–127. [CrossRef] 8. Konstantinović,S.S.; Danilović,B.R.; Cirić,J.T.;´ Ilić,S.B.; Savić,D.S.; Veljković,V.B. Valorizacija sirovog glicerola iz proizvodnje biodizela. Chem. Ind. Chem. Eng. Q. 2016, 22, 461–489. 9. Bagnato, G.; Iulianelli, A.; Sanna, A.; Basile, A. Glycerol production and transformation: A critical review with particular emphasis on glycerol reforming reaction for producing hydrogen in conventional and membrane reactors. Membranes 2017, 7, 17. [CrossRef] Appl. Sci. 2020, 10, 7155 28 of 34

10. Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass Volume I—Results of Screening for Potential Candidates from Sugars and Synthesis Gas; BiblioGov: Columbus, OH, USA, 2004. 11. He, Q.S.; Mcnutt, J.; Yang, J. Utilization of the residual glycerol from biodiesel production for renewable energy generation. Renew. Sustain. Energy Rev. 2017, 71, 63–76. [CrossRef] 12. Quispe, C.A.G.; Coronado, C.J.R.; Carvalho, J.A. Glycerol: Production, consumption, prices, characterization and new trends in combustion. Renew. Sustain. Energy Rev. 2013, 27, 475–493. [CrossRef] 13. Isahak, W.N.R.W.; Ramli, Z.A.C.; Ismail, M.; Jahim, J.M.; Yarmo, M.A. Recovery and Purification of Crude Glycerol from Vegetable Oil Transesterification Recovery and Purification of Crude Glycerol from Vegetable Oil Transesterification. Sep. Purif. Rev. 2015, 44, 250–267. [CrossRef] 14. Christoph, R.; Schmidt, B.; Steinberner, U.; Dilla, W.; Karinen, R. Glycerol; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2012; ISBN 9780471238966. 15. Marchetti, J.M.; Miguel, V.U.; Errazu, A.F. Possible methods for biodiesel production. Renew. Sustain. Energy Rev. 2007, 11, 1300–1311. [CrossRef] 16. Knothe, G.; Razon, L.F. Biodiesel fuels. Prog. Energy Combust. Sci. 2017, 58, 36–59. [CrossRef] 17. Bauer, F.; Hulteberg, C. Is there a future in glycerol as a feedstock in the production of biofuels and biochemicals? Biofuels Bioprod. Biorefining 2013, 7, 43–51. [CrossRef] 18. Anuar, M.R.; Zuhairi, A. Challenges in biodiesel industry with regards to feedstock, environmental, social and sustainability issues: A critical review. Renew. Sustain. Energy Rev. 2016, 58, 208–223. [CrossRef] 19. Nanda, M.R.; Yuan, Z.; Qin, W.; Ghaziaskar, H.S.; Poirier, M.; Xu, C.C. Thermodynamic and kinetic studies of a catalytic process to convert glycerol into solketal as an oxygenated fuel additive. Fuel 2014, 117, 470–477. [CrossRef] 20. Naylor, R.L.; Higgins, M.M. The political economy of biodiesel in an era of low oil prices. Renew. Sustain. Energy Rev. 2017, 77, 695–705. [CrossRef] 21. REN21. Renewables 2018-Global Status Report. Available online: https://cleanenergysolutions.org/sites/ default/files/documents/180605_gsr_2018_presentation_webinar_rad.pdf (accessed on 5 June 2018). 22. Trifoi, A.R.; Agachi, P.¸S.;Pap, T. Glycerol acetals and ketals as possible diesel additives. A review of their synthesis protocols. Renew. Sustain. Energy Rev. 2016, 62, 804–814. [CrossRef] 23. Bajpai, D.; Tyagi, V.K. Biodiesel: Source, Production, Composition, Properties and Its Benefits. J. Oleo Sci. 2006, 55, 487–502. [CrossRef] 24. Singh, S.P.; Singh, D. Biodiesel production through the use of different sources and characterization of oils and their esters as the substitute of diesel: A review. Renew. Sustain. Energy Rev. 2010, 14, 200–216. [CrossRef] 25. Takase, M.; Zhao, T.; Zhang, M.; Chen, Y.; Liu, H.; Yang, L.; Wu, X. An expatiate review of neem, jatropha, rubber and karanja as multipurpose non-edible biodiesel resources and comparison of their fuel, engine and emission properties. Renew. Sustain. Energy Rev. 2015, 43, 495–520. [CrossRef] 26. Shah, M.; Dai, J.J.; Guo, Q.X.; Fu, Y. Products and production routes for the catalytic conversion of seed oil into fuel and chemicals: A comprehensive review. Sci. China Chem. 2015, 58, 1110–1121. [CrossRef] 27. Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Pina, C. Della Review Article Recent advances in the conversion of bioglycerol into value-added products. Eur. J. Lipid Sci. Technol. 2009, 111, 788–799. [CrossRef] 28. Coronado, C.R.; Carvalho, J.A.; Quispe, C.A.; Sotomonte, C.R. Ecological efficiency in glycerol combustion. Appl. Therm. Eng. 2014, 63, 97–104. [CrossRef] 29. Deng, C.; Duan, X.; Zhou, J.; Chen, D.; Zhou, X.; Yuan, W. Size effects of Pt-Re bimetallic catalysts for glycerol hydrogenolysis. Catal. Today 2014, 234, 208–214. [CrossRef] 30. Kant, A.; He, Y.; Jawad, A.; Li, X.; Rezaei, F.; Smith, J.D.; Rownaghi, A.A. Hydrogenolysis of glycerol over Ni, Cu, Zn, and Zr supported on H-beta. Chem. Eng. J. 2017, 317, 1–8. [CrossRef] 31. Wang, C.; Dou, B.; Chen, H.; Song, Y.; Xu, Y.; Du, X.; Zhang, L.; Luo, T.; Tan, C. Renewable hydrogen production from steam reforming of glycerol by Ni-Cu-Al, Ni-Cu-Mg, Ni-Mg catalysts. Int. J. Hydrogen Energy 2013, 38, 3562–3571. [CrossRef] 32. Bepari, S.; Pradhan, N.C.; Dalai, A.K. Selective production of hydrogen by steam reforming of glycerol over Ni/Fly ash catalyst. Catal. Today 2017, 291, 36–46. [CrossRef] 33. Ma, T.; Ding, J.; Shao, R.; Xu, W.; Yun, Z. Dehydration of glycerol to acrolein over Wells–Dawson and Keggin type phosphotungstic acids supported on MCM-41 catalysts. Chem. Eng. J. 2017, 316, 797–806. [CrossRef] 34. Zhou, L.; Al-zaini, E.; Adesina, A.A. Catalytic characteristics and parameters optimization of the glycerol acetylation over solid acid catalysts. Fuel 2013, 103, 617–625. [CrossRef] Appl. Sci. 2020, 10, 7155 29 of 34

35. Indran, V.P.; Syuhada Zuhaimi, N.A.; Deraman, M.A.; Maniam, G.P.; Yusoff, M.M.; Yun Hin, T.-Y.; Rahim, M.H.A. An accelerated route of glycerol carbonate formation from glycerol using waste boiler ash as catalyst. RSC Adv. 2014, 4, 25257–25267. [CrossRef] 36. Ishak, Z.I.; Sairi, N.A.; Alias, Y.; Aroua, M.K.T.; Yusoff, R. Production of glycerol carbonate from glycerol with aid of ionic liquid as catalyst. Chem. Eng. J. 2016, 297, 128–138. [CrossRef] 37. Skrzynska, E.; Zaid, S.; Girardon, J.-S.; Capron, M.; Dumeignil, F. Catalytic behaviour of four different supported noble metals in the crude glycerol oxidation. Appl. Catal. A Gen. 2015, 499, 89–100. [CrossRef] 38. Ning, X.; Li, Y.; Yu, H.; Peng, F.; Wang, H.; Yang, Y. Promoting role of bismuth and antimony on Pt catalysts for the selective oxidation of glycerol to dihydroxyacetone. J. Catal. 2016, 335, 95–104. [CrossRef] 39. Bookong, P.; Ruchirawat, S.; Boonyarattanakalin, S. Optimization of microwave-assisted etherification of glycerol to polyglycerols by sodium carbonate as catalyst. Chem. Eng. J. 2015, 275, 253–261. [CrossRef] 40. Galy, N.; Nguyen, R.; Blach, P.; Sambou, S.; Luart, D.; Len, C. Journal of Industrial and Engineering Chemistry Glycerol oligomerization in continuous fl ow reactor. J. Ind. Eng. Chem. 2017, 51, 312–318. [CrossRef] 41. Kim, I.; Kim, J.; Lee, D. A comparative study on catalytic properties of solid acid catalysts for glycerol acetylation at low temperatures. Appl. Catal. B Environ. 2014, 148–149, 295–303. [CrossRef] 42. Gao, X.; Zhu, S.; Li, Y. Graphene oxide as a facile solid acid catalyst for the production of bioadditives from glycerol esteri fi cation. Catal. Commun. 2015, 62, 48–51. [CrossRef] 43. Ayoub, M.; Khayoon, M.S.; Abdullah, A.Z. Synthesis of oxygenated fuel additives via the solventless etherification of glycerol. Bioresour. Technol. 2012, 112, 308–312. [CrossRef] 44. Pinto, B.P.; De Lyra, J.T.; Nascimento, J.A.C.; Mota, C.J.A. Ethers of glycerol and ethanol as bioadditives for biodiesel. Fuel 2016, 168, 76–80. [CrossRef] 45. Gadamsetti, S.; Rajan, N.P.; Rao, G.S.; Chary, K.V.R. Acetalization of glycerol with acetone to bio fuel additives over supported molybdenum phosphate catalysts. J. Mol. Catal. A Chem. 2015, 410, 49–57. [CrossRef] 46. Timofeeva, M.N.; Panchenko, V.N.; Krupskaya, V.V.; Gil, A.; Vicente, M.A. Effect of nitric acid modification of montmorillonite clay on synthesis of solketal from glycerol and acetone. Catal. Commun. 2017, 90, 65–69. [CrossRef] 47. Valliyappan, T.; Bakhshi, N.N.; Dalai, A.K. Pyrolysis of glycerol for the production of hydrogen or syn gas. Bioresour. Technol. 2008, 99, 4476–4483. [CrossRef][PubMed] 48. Dianningrum, L.W.; Choi, H.; Kim, Y.; Jung, K.; Susanti, R.F.; Kim, J.; Sang, B. ScienceDirect Hydrothermal gasification of pure and crude glycerol in supercritical water: A comparative study. Int. J. Hydrogen Energy 2014, 39, 1262–1273. [CrossRef] 49. Fan, X.; Burton, R.; Zhou, Y. Glycerol (Byproduct of Biodiesel Production) as a Source for Fuels and Chemicals—Mini Review. Open Fuels Energy Sci. J. 2010, 3, 17–22. [CrossRef] 50. Leoneti, B.A.; Aragão-leoneti, V.; de Oliveira, S.V.W.B. Glycerol as a by-product of biodiesel production in Brazil: Alternatives for the use of unre fi ned glycerol. Renew. Energy 2012, 45, 138–145. [CrossRef] 51. Len, C.; Luque, R. Continuous flow transformations of glycerol to valuable products: An overview. Sustain. Chem. Process. 2014, 2, 1–10. [CrossRef] 52. Luo, X.; Ge, X.; Cui, S.; Li, Y. Value-added processing of crude glycerol into chemicals and polymers. Bioresour. Technol. 2016, 215, 144–154. [CrossRef] 53. Mota, C.J.A.; Silva, C.X.A.; Rosenbach, N.; Costa, J.; da Silva, F. Glycerin Derivatives as Fuel Additives: The Addition of Glycerol/Acetone Ketal (Solketal) in Gasolines. Energy Fuels 2010, 24, 2733–2736. [CrossRef] 54. García, J.I.; García-Marín, H.; Pires, E. Glycerol based solvents: Synthesis, properties and applications. Green Chem. 2014, 16, 1007–1033. [CrossRef] 55. Cornejo, A.; Barrio, I.; Campoy, M.; Lázaro, J.; Navarrete, B. Oxygenated fuel additives from glycerol valorization. Main production pathways and effects on fuel properties and engine performance: A critical review. Renew. Sustain. Energy Rev. 2017, 79, 1400–1413. [CrossRef] 56. Farinha, J.; Caiado, M.; Castanheiro, J.E. Valorisation of glycerol into biofuel additives over heterogeneous catalysts. In Materials and Processes for Energy: Communicating Current Research and Technological Developments; Mendez-Vilas, A., Ed.; Formatex Research Center: Badajoz, Spain, 2013; pp. 422–429. 57. Samoilov, V.O.; Ramazanov, D.N.; Nekhaev, A.I.; Maximov, A.L.; Bagdasarov, L.N. Heterogeneous catalytic conversion of glycerol to oxygenated fuel additives. Fuel 2016, 172, 310–319. [CrossRef] 58. Mallesham, B.; Rao, B.G.; Reddy, B.M. Production of biofuel additives by esteri fi cation and acetalization of bioglycerol. Comptes Rendus Chim. 2016, 19, 1194–1202. [CrossRef] Appl. Sci. 2020, 10, 7155 30 of 34

59. Kong, P.S.; Aroua, M.K.; Daud, W.M.A.W.; Lee, H.V.; Cognet, P.; Pérès, Y. Catalytic role of solid acid catalysts in glycerol acetylation for the production of bio-additives: A review. RSC Adv. 2016, 6, 68885–68905. [CrossRef] 60. Okoye, P.U.; Hameed, B.H. Review on recent progress in catalytic carboxylation and acetylation of glycerol as a byproduct of biodiesel production. Renew. Sustain. Energy Rev. 2016, 53, 558–574. [CrossRef] 61. Okoye, P.U.; Abdullah, A.Z.; Hameed, B.H. A review on recent developments and progress in the kinetics and deactivation of catalytic acetylation of glycerol—A byproduct of biodiesel. Renew. Sustain. Energy Rev. 2017, 74, 387–401. [CrossRef] 62. Maximov, A.L.; Nekhaev, A.I.; Ramazanov, D.N. Ethers and Acetals, Promising Petrochemicals from Renewable Sources. Pet. Chem. 2015, 55, 3–24. [CrossRef] 63. De Torres, M.; Jiménez-osés, G.; Mayoral, J.A.; Pires, E.; Santos, M.D.L. Glycerol ketals: Synthesis and profits in biodiesel blends. Fuel 2012, 94, 614–616. [CrossRef] 64. Izquierdo, J.F.; Montiel, M.; Pales, I.; Outo, P.R.; Galan, M.; Jutglar, L.; Villarrubia, M.; Izquierdo, M.; Hermo, M.P.; Ariza, X. Fuel additives from glycerol etherification with light olefins: State of the art. Renew. Sustain. Energy Rev. 2012, 16, 6717–6724. [CrossRef] 65. Agirre, I.; Garcia, I.; Requies, J.; Barrio, V.L.; Guemez, M.B.; Cambra, J.F.; Arias, P.L. Glycerol acetals, kinetic study of the reaction between glycerol and formaldehyde. Biomass Bioenergy 2011, 35, 3636–3642. [CrossRef] 66. Di Serio, M.; Casale, L.; Tesser, R.; Santacesaria, E. New Process for the Production of Glycerol tert-Butyl Ethers. Energy Fuels 2010, 24, 4668–4672. [CrossRef] 67. Garcı, E.; Laca, M.; Perez, E.; Garrido, A.; Bilbao, J. New Class of Acetal Derived from Glycerin as a Biodiesel Fuel Component. Energy Fuels 2008, 22, 4274–4280. [CrossRef] 68. Deutsch, J.; Martin, A.; Lieske, H. Investigations on heterogeneously catalysed condensations of glycerol to cyclic acetals. J. Catal. 2007, 245, 428–435. [CrossRef] 69. Nanda, M.R.; Zhang, Y.; Yuan, Z.; Qin, W.; Ghaziaskar, H.S.; Charles, C. Catalytic conversion of glycerol for sustainable production of solketal as a fuel additive: A review. Renew. Sustain. Energy Rev. 2016, 56, 1022–1031. [CrossRef] 70. Moraes, D.S.; Angélica, R.S.; Costa, C.E.F.; Rocha-Filho, G.N.; Zamian, J.R. Bentonite functionalized with propyl sulfonic acid groups used as catalyst in esteri fi cation reactions. Appl. Clay Sci. 2011, 51, 209–213. [CrossRef] 71. Khayoon, M.S.; Hameed, B.H. Synthesis of hybrid SBA-15 functionalized with molybdophosphoric acid as efficient catalyst for glycerol esterification to fuel additives. Appl. Catal. A Gen. 2012, 433–434, 152–161. [CrossRef] 72. Kong, P.S.; Aroua, M.K.; Daud, W.M.A.W.; Wan Daud, W.M.A. Catalytic esterification of bioglycerol to value-added products. Rev. Chem. Eng. 2015, 31, 437–451. [CrossRef] 73. Dalla Costa, B.O.; Decolatti, H.P.; Legnoverde, M.S.; Querini, C.A. Influence of acidic properties of different solid acid catalysts for glycerol acetylation. Catal. Today 2017, 289, 222–230. [CrossRef] 74. Pinazo, A.; Lozano, N.; Perez, L.; Moran, M.C.; Infante, M.R.; Pons, R. Arginine diacyl-glycerolipid conjugates as multifunctional biocompatible surfactants. Comptes Rendus Chim. 2011, 14, 726–735. [CrossRef] 75. Carvalho, W.A.; Galhardo, T.S.; Simone, N.; Goncalves, M.; Figueiredo, F.C.A.; Mandelli, D.; Gonçalves, M.; Figueiredo, F.C.A.; Mandelli, D.; Carvalho, W.A. Preparation of sulfonated carbons from rice husk and their application in catalytic conversion of glycerol. ACS Sustain. Chem. Eng. 2013, 1, 1381–1389. 76. Zhou, C.H.; Beltramini, J.N.; Fan, Y.X.; Lu, G.Q. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527–549. [CrossRef] [PubMed] 77. Setyaningsih, L.; Siddiq, F.; Pramezy, A. Esterification of glycerol with acetic acid over Lewatit catalyst. MATEC Web Conf. 2018, 154, 2–5. [CrossRef] 78. Goncalves, V.L.C.; Pinto, B.P.; Silva, J.C.; Mota, C.J.A. Acetylation of glycerol catalyzed by different solid acids. Catal. Today 2008, 135, 673–677. [CrossRef] 79. Melero, J.A.; Vicente, G.; Morales, G.; Paniagua, M.; Moreno, J.M.; Roldan, R.; Ezquerro, A.; Perez, C. Acid-catalyzed etherification of bio-glycerol and isobutylene over sulfonic mesostructured silicas. Appl. Catal. A Gen. 2008, 346, 44–51. [CrossRef] 80. Liao, X.; Zhu, Y.; Wang, S.; Li, Y. Producing triacetylglycerol with glycerol by two steps: Esterification and acetylation. Fuel Process. Technol. 2009, 90, 988–993. [CrossRef] Appl. Sci. 2020, 10, 7155 31 of 34

81. Rashedul, H.K.; Masjuki, H.H.; Kalam, M.A.; Ashraful, A.M.; Rahman, S.M.A.; Shahir, S.A. The effect of additives on properties, performance and emission of biodiesel fuelled compression ignition engine. Energy Convers. Manag. 2014, 88, 348–364. [CrossRef] 82. Khayoon, M.S.; Triwahyono, S.; Hameed, B.H.; Jalil, A.A. Improved production of fuel oxygenates via glycerol acetylation with acetic acid. Chem. Eng. J. 2014, 243, 473–484. [CrossRef] 83. Zhou, L.; Nguyen, T.; Adesina, A.A. The acetylation of glycerol over amberlyst-15: Kinetic and product distribution. Fuel Process. Technol. 2012, 104, 310–318. [CrossRef] 84. Zhu, S.; Gao, X.; Dong, F.; Zhu, Y.; Zheng, H.; Li, Y. Design of a highly active silver-exchanged phosphotungstic acid catalyst for glycerol esterification with acetic acid. J. Catal. 2013, 306, 155–163. [CrossRef] 85. Sutter, M.; Da Silva, E.; Duguet, N.; Raoul, Y.; Métay, E.; Lemaire, M. Glycerol Ether Synthesis: A Bench Test for Green Chemistry Concepts and Technologies. Chem. Rev. 2015, 115, 8609–8651. [CrossRef][PubMed] 86. Alegría, A.; Cuellar, J. Esterification of oleic acid for biodiesel production catalyzed by 4-dodecylbenzenesulfonic acid. Appl. Catal. B Environ. 2015, 179, 530–541. [CrossRef] 87. Venkatesha, N.J.; Bhat, Y.S.; Prakash, B.S.J. Volume accessibility of acid sites in modified montmorillonite and triacetin selectivity in acetylation of glycerol. RSC Adv. 2016, 6, 45819–45828. [CrossRef] 88. Silva, L.N.; Gonçalves, V.L.C.; Mota, C.J.A. Catalytic acetylation of glycerol with acetic anhydride. Catal. Commun. 2010, 11, 1036–1039. [CrossRef] 89. Sandesh, S.; Manjunathan, P.; Halgeri, A.B.; Shanbhag, G.V. Glycerol acetins: Fuel additive synthesis by acetylation and esterification of glycerol using cesium phosphotungstate catalyst. RSC Adv. 2015, 5, 104354–104362. [CrossRef] 90. Ramalingam, R.J.; Radhika, T.; Adam, F.; Dolla, H.T. Acetylation of glycerol over bimetallic Ag-Cu doped rice husk silica based biomass catalyst for bio-fuel additives application. Int. J. Ind. Chem. 2016, 7, 187–194. [CrossRef] 91. Kakasaheb, Y.N.; Prashant, S.N.; Vijay, V.B. Synthesis of Oxygenated Fuel Additives via Acetylation of Bio-Glycerol over H 2 SO 4 Modified Montmorillonite K10 Catalyst. Prog. Petrochem. Sci. 2018, 1, 1–5. 92. Testa, M.L.; La Parola, V.; Liotta, L.F.; Venezia, A.M. Screening of different solid acid catalysts for glycerol acetylation. J. Mol. Catal. A Chem. 2013, 367, 69–76. [CrossRef] 93. Gonçalves, C.E.; Laier, L.O.; Cardoso, A.L.; Silva, M.J. Da Bioadditive synthesis from H3PW12O40 -catalyzed glycerol esteri fi cation with HOAc under mild reaction conditions. Fuel Process. Technol. 2012, 102, 46–52. [CrossRef] 94. Beejapur, H.A.; La Parola, V.; Liotta, L.F.; Testa, M.L. Glycerol Acetylation over Organic-Inorganic Sulfonic or Phosphonic Silica Catalysts. ChemistrySelect 2017, 2, 4934–4941. [CrossRef] 95. Churipard, S.R.; Manjunathan, P.; Chandra, P.; Shanbhag, G.V.; Ravishankar, R.; Rao, P.V.C.; Sri Ganesh, G.; Halgeri, A.B.; Maradur, S.P. Remarkable catalytic activity of a sulfonated mesoporous polymer (MP-SO3H) for the synthesis of solketal at room temperature. New J. Chem. 2017, 41, 5745–5751. [CrossRef]

96. Rao, B.G.; Sudarsanam, P.; Rangaswamy, A.; Reddy, B.M. Highly Efficient CeO2–MoO3/SiO2 Catalyst for Solvent-Free Oxidative Coupling of Benzylamines into N-Benzylbenzaldimines with O2 as the Oxidant. Catal. Lett. 2015, 145, 1436–1445. 97. Fukumura, T.; Toda, T.; Seki, Y.; Kubo, M.; Shibasaki-kitakawa, N.; Yonemoto, T. Catalytic Synthesis of Glycerol Monoacetate Using a Continuous Expanded Bed Column Reactor Packed with Cation-Exchange Resin. Ind. Eng. Chem. Res. 2009, 48, 1816–1823. [CrossRef] 98. Dosuna-Rodríguez, I.; Gaigneaux, E.M. Glycerol acetylation catalysed by ion exchange resins. Catal. Today 2012, 195, 14–21. [CrossRef] 99. Lacerda, C.V.; Carvalho, M.J.S.; Ratton, A.R.; Soares, I.P.; Borges, L.E.P. Synthesis of Triacetin and Evaluation on Motor. J. Braz. Chem. Soc. 2015, 26, 1625–1631. 100. Marwan, M.; Indarti, E.; Darmadi, D.; Rinaldi, W.; Hamzah, D.; Rinaldi, T. Production of triacetin by microwave assisted esterification of glycerol using activated natural zeolite. Bull. Chem. React. Eng. Catal. 2019, 14, 672–677. [CrossRef] 101. Konwar, L.J.; Mäki-Arvela, P.; Kumar, N.; Mikkola, J.P.; Sarma, A.K.; Deka, D. Selective esterification of fatty acids with glycerol to monoglycerides over—SO3H functionalized carbon catalysts. React. Kinet. Mech. Catal. 2016, 119, 121–138. [CrossRef] 102. Patel, A.; Singh, S. A green and sustainable approach for esterification of glycerol using 12-tungstophosphoric acid anchored to different supports: Kinetics and effect of support. Fuel 2014, 118, 358–364. [CrossRef] Appl. Sci. 2020, 10, 7155 32 of 34

103. Ferreira, P.; Fonseca, I.M.; Ramos, A.M.; Vital, J.; Castanheiro, J.E. Acetylation of glycerol over heteropolyacids supported on activated carbon. Catal. Commun. 2011, 12, 573–576. [CrossRef] 104. Magar, S.; Mohanraj, G.T.; Jana, S.K.; Rode, C.V. Synthesis and characterization of supported heteropoly acid: Efficient solid acid catalyst for glycerol esterification to produce biofuel additives. Inorg. Nano-Metal Chem. 2020, 50, 1157–1165. [CrossRef] 105. Popova, M.; Szegedi, Á.; Ristic, A.; Tusar, N.N. Catalysis Science & Technology supported sulphated zirconia catalysts. Catal. Sci. Technol. Esterification 2014, 4, 3993–4000. 106. Arun, P.; Pudi, S.M.; Biswas, P. Acetylation of Glycerol over Sulfated Alumina: Reaction Parameter Study and Optimization Using Response Surface Methodology. Energy Fuels 2016, 30, 584–593. 107. Zhang, Z.; Huang, H.; Ma, X.; Li, G.; Wang, Y.; Sun, G.; Teng, Y.; Yan, R.; Zhang, N.; Li, A.J. Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO /TiO . J. Ind. 42− 2 Eng. Chem. 2017, 53, 307–316. [CrossRef] 108. Kulkarni, R.M.; Britto, P.J.; Narula, A.; Saqline, S.; Anand, D.; Bhagyalakshmi, C.; Herle, R.N. Kinetic studies

on the synthesis of fuel additives from glycerol using CeO2-ZrO2 metal oxide catalyst. Biofuel Res. J. 2020, 7, 1100–1108. [CrossRef] 109. Mizugaki, T.; Arundhathi, R.; Mitsudome, T.; Jitsukawa, K.; Kaneda, K. Highly efficient and selective transformations of glycerol using reusable heterogeneous catalysts. ACS Sustain. Chem. Eng. 2014, 2, 574–578. [CrossRef] 110. Ghoreishi, K.B.; Yarmo, M.A. Sol-gel Sulfated Silica as a Catalyst for Glycerol Acetylation with Acetic Acid. J. Sci. Technol. 2013, 1, 65–78. 111. Gonzalez-Arellano, C.; De, S.; Luque, R. Selective glycerol transformations to high value-added products catalysed by aluminosilicate-supported iron oxide nanoparticles. Catal. Sci. Technol. 2014, 4, 4242–4249. [CrossRef] 112. De La Calle, C.; Fraile, J.M.; García-bordejé, E.; Pires, E.; Roldán, L.; De Calle, C.; Fraile, J.M.; García-bordejé, E.; Pires, E.; Roldán, L. Biobased catalyst in biorefinery processes: Sulphonated hydrothermal carbon for glycerol esterification. Catal. Sci. Technol. 2015, 5, 2897–2903. [CrossRef] 113. Tao, M.L.; Guan, H.Y.; Wang, X.H.; Liu, Y.C.; Louh, R.F. Fabrication of sulfonated carbon catalyst from biomass waste and its use for glycerol esterification. Fuel Process. Technol. 2015, 138, 355–360. [CrossRef] 114. Okoye, P.U.; Abdullah, A.Z.; Hameed, B.H. Synthesis of oxygenated fuel additives via glycerol esterification with acetic acid over bio-derived carbon catalyst. Fuel 2017, 209, 538–544. [CrossRef] 115. Konwar, L.J.; Mäki-Arvela, P.; Begum, P.; Kumar, N.; Jyoti, T.A.; Mikkola, J.; Deka, C.R.; Deka, D. Shape selectivity and acidity effects in glycerol acetylation with acetic anhydride: Selective synthesis of triacetin over Y-zeolite and sulfonated mesoporous carbons. J. Catal. 2015, 329, 237–247. [CrossRef] 116. Goscianska, J.; Malaika, A. A facile post-synthetic modification of ordered mesoporous carbon to get efficient catalysts for the formation of acetins. Catal. Today 2019, 1–10. [CrossRef] 117. Deutschman, O.; Knozinger, H.; Kochloefl, K. Heterogeneous Catalysis and Solid Catalysts. Available online: https://onlinelibrary.wiley.com/doi/abs/10.1002/14356007.a05_313.pub2 (accessed on 15 April 2019). 118. Mansir, N.; Taufiq-Yap, Y.H.; Rashid, U.; Lokman, I.M. Investigation of heterogeneous solid acid catalyst performance on low grade feedstocks for biodiesel production: A review. Energy Convers. Manag. 2017, 141, 171–182. [CrossRef] 119. Mukasa-Tebandeke, I.Z.; Ssebuwufu, P.J.M.; Nyanzi, S.A.; Schumann, A.; Nyakairu, G.W.A.; Ntale, M.; Lugolobi, F. The Elemental, Mineralogical, IR, DTA and XRD Analyses Characterized Clays and Clay Minerals of Central and Eastern Uganda. Adv. Mater. Phys. Chem. 2015, 05, 67–86. [CrossRef] 120. Ray, C.; Pal, T. Recent advances of metal-metal oxide nanocomposites and their tailored nanostructures in numerous catalytic applications. J. Mater. Chem. A 2017, 5, 9465–9487. [CrossRef] 121. Veluturla, S.; Narula, A.; Rao, D.S.; Shetty, S.P. Kinetic study of synthesis of bio-fuel additives from glycerol using a hetropolyacid. Resour. Technol. 2017, 3, 337–341. [CrossRef] 122. Rane, S.A.; Pudi, S.M.; Biswa, P. Esterification of Glycerol with Acetic Acid over Highly Active and Stable Alumina-based Catalysts: A Reaction Kinetics Study. Chem. Biochem. Eng. Q. J. 2016, 30, 33–45. [CrossRef] 123. Hu, W.; Zhang, Y.; Huang, Y.; Wang, J.; Gao, J.; Xu, J. Selective esterification of glycerol with acetic acid to diacetin using antimony pentoxide as reusable catalyst. J. Energy Chem. 2015, 24, 632–636. [CrossRef] Appl. Sci. 2020, 10, 7155 33 of 34

124. Sánchez, J.A.; Hernández, D.L.; Moreno, J.A.; Mondragón, F.; Fernández, J.J. Alternative carbon based acid catalyst for selective esterification of glycerol to acetylglycerols. Appl. Catal. A Gen. 2011, 405, 55–60. [CrossRef] 125. Rafi, J.M.; Rajashekar, A.; Srinivas, M.; Rao, B.V.S.K.; Prasad, R.B.N.; Lingaiah, N. Esterification of glycerol over a solid acid biochar catalyst derived from waste biomass. RSC Adv. 2015, 5, 44550–44556. [CrossRef] 126. Chandrakala, U.; Prasad, R.B.N.; Prabhavathi Devi, B.L.A. Glycerol valorization as biofuel additives by employing a carbon-based solid acid catalyst derived from glycerol. Ind. Eng. Chem. Res. 2014, 53, 16164–16169. [CrossRef] 127. Prabhavathi Devi, B.L.A.; Vijaya Lakshmi, K.; Gangadhar, K.N.; Prasad, R.B.N.; Sai Prasad, P.S.; Jagannadh, B.; Kundu, P.P.; Kumari, G.; Narayana, C. Novel Heterogeneous SO3Na-Carbon Transesterification Catalyst for the Production of Biodiesel. ChemistrySelect 2017, 2, 1925–1931. [CrossRef] 128. Tang, H.; Li, N.; Chen, F.; Li, G.; Wang, A.; Cong, Y.; Wang, X.; Zhang, T. Highly efficient synthesis of 5-hydroxymethylfurfural with carbohydrates over renewable cyclopentanone-based acidic resin. Green Chem. 2017, 19, 1855–1860. [CrossRef] 129. Hara, M.; Nakajima, K.; Kamata, K. Recent progress in the development of solid catalysts for biomass conversion into high value-added chemicals. Sci. Technol. Adv. Mater. 2015, 16, 1–22. [CrossRef] 130. Hoo, P.; Abdullah, A.Z. Direct synthesis of mesoporous 12-tungstophosphoric acid SBA-15 catalyst for selective esterification of glycerol and lauric acid to monolaurate. Chem. Eng. J. 2014, 250, 274–287. [CrossRef] 131. Zhu, S.; Zhu, Y.; Gao, X.; Mo, T.; Zhu, Y.; Li, Y. Production of bioadditives from glycerol esterification over zirconia supported heteropolyacids. Bioresour. Technol. 2013, 130, 45–51. [CrossRef] 132. Kotbagi, T.V.; Pandhare, S.L.; Dongare, M.K.; Umbarkar, S.B. In situ Formed Supported Silicomolybdic Heteropolyanions: Efficient Solid Catalyst for Acetylation of Glycerol. Environ. Anal. Chem. 2015, 2, 1–5. 133. Khayoon, M.S.; Hameed, B.H. Acetylation of glycerol to biofuel additives over sulfated activated carbon catalyst. Bioresour. Technol. 2011, 102, 9229–9235. [CrossRef] 134. Sun, J.; Tong, X.; Yu, L.; Wan, J. An efficient and sustainable production of triacetin from the acetylation of glycerol using magnetic solid acid catalysts under mild conditions. Catal. Today 2015, 264, 115–122. [CrossRef] 135. Sandesh, S.; Kristachar, P.K.R.; Manjunathan, P.; Halgeri, A.B.; Shanbhag, G.V. Synthesis of biodiesel and acetins by transesterification reactions using novel CaSn(OH)6heterogeneous base catalyst. Appl. Catal. A Gen. 2016, 523, 1–11. [CrossRef] 136. Li, L.; Yu, S.; Xie, C.; Liu, F.; Li, H. Synthesis of glycerol triacetate using functionalized ionic liquid as catalyst. J. Chem. Technol. Biotechnol. 2009, 84, 1649–1652. [CrossRef] 137. Liu, X.; Ma, H.; Wu, Y.; Wang, C.; Yang, M.; Yan, P.; Welz-biermann, U. Esterification of glycerol with acetic acid using double SO 3 H-functionalized ionic liquids as recoverable catalysts. Green Chem. 2011, 13, 697–701. [CrossRef] 138. Keogh, J.; Tiwari, M.S.; Manyar, H. Esterification of Glycerol with Acetic Acid Using Nitrogen-Based Brønsted-Acidic Ionic Liquids. Ind. Eng. Chem. Res. 2019, 58, 17235–17243. [CrossRef] 139. Troncea, S.B.; Wuttke, S.; Kemnitz, E.; Coman, S.M.; Parvulescu, V.I. Hydroxylated magnesium fluorides as environmentally friendly catalysts for glycerol acetylation. Appl. Catal. B Environ. 2011, 107, 260–267. [CrossRef] 140. Mufrodi, Z.; Rochmadi, R.; Sutijan, S.; Budiman, A. Synthesis Acetylation of Glycerol Using Batch Reactor and Continuous Reactive Distillation Column. Eng. J. 2014, 18, 29–40. [CrossRef] 141. Goncalves, C.E.; Laier, L.O.; da Silva, M.J. Novel Esterification of Glycerol Catalysed by Tin Chloride (II): A Recyclable and Less Corrosive Process for Production of Bio-Additives. Catal. Lett. 2011, 14, 1111–1117. [CrossRef] 142. Dosuna-Rodríguez, I.; Adriany, C.; Gaigneaux, E.M. Glycerol acetylation on sulphated zirconia in mild conditions. Catal. Today 2011, 167, 56–63. [CrossRef] 143. Hamerski, F.; Corazza, M.L. General LDH-catalyzed esterification of lauric acid with glycerol in solvent-free system. Appl. Catal. A Gen. 2014, 475, 242–248. [CrossRef]

144. Reddy, P.S.; Sudarsanam, P.; Raju, G.; Reddy, B.M. Selective acetylation of glycerol over CeO2-M and SO42-/CeO2-M (M = ZrO2 and Al2O3) catalysts for synthesis of bioadditives. J. Ind. Eng. Chem. 2012, 18, 648–654. [CrossRef] Appl. Sci. 2020, 10, 7155 34 of 34

145. Reddy, P.S.; Sudarsanam, P.; Raju, G.; Reddy, B.M. Synthesis of bio-additives: Acetylation of glycerol over zirconia-based solid acid catalysts. Catal. Commun. 2010, 11, 1224–1228. [CrossRef] 146. Da Silva, M.J.; Liberto, N.A.; De Andrade Leles, L.C.; Pereira, U.A. Fe4(SiW12O40)3-catalyzed glycerol acetylation: Synthesis of bioadditives by using highly active Lewis acid catalyst. J. Mol. Catal. A Chem. 2016, 422, 69–83. [CrossRef] 147. Tangestanifard, M.; Ghaziaskar, H.S. Arenesulfonic Acid-Functionalized Bentonite as Catalyst in Glycerol Esterification with Acetic Acid. Catalysts 2017, 7, 211. [CrossRef] 148. Sari, V.I.; Hambali, E.; Suryani, A.; Permadi, P. Esterification Reaction of Glycerol and Palm Oil Oleic Acid using Methyl Ester Sulfonate Acid Catalyst as Drilling Fluid Formation. In Proceedings of the Materials Science and Engineering, Purwokerto, Indonesia, 15–16 September 2016; IOP Publishing: Bristol, UK, 2017; Volume 172. 149. Liu, J.; Wang, Z.; Sun, Y.; Jian, R.; Jian, P.; Wang, D. Selective synthesis of triacetin from glycerol catalyzed by HZSM-5/MCM-41 micro/mesoporous molecular sieve. Chin. J. Chem. Eng. 2019, 27, 1073–1078. [CrossRef] 150. Rastegari, H.; Ghaziaskar, H.S.; Yalpani, M. Valorization of biodiesel derived glycerol to acetins by continuous esterification in acetic acid: Focusing on high selectivity to diacetin and triacetin with no byproducts. Ind. Eng. Chem. Res. 2015, 54, 3279–3284. [CrossRef] 151. Kale, S.; Armbruster, U.; Umbarkar, S.; Dongare, M.; Martin, A. Esterification of glycerol with acetic acid for improved production of triacetin using toluene as an entrainer. In Proceedings of the 10th Green Chemistry Conference. An International Event, Barcelona, Spain, 15 November 2013; pp. 70–71.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).