Joël Barrault

Sébastien Bancquart

Yannick Pouilloux

Comptes Rendus Chimie

Preliminary communication / CommunicationSelective glycerol transesterification over mesoporousbasic catalystsJoël Barrault *, Sébastien Bancquart, Yannick PouillouxLaboratoire de catalyse en chimie organique, UMR CNRS 6503, ESIP, 40, av. du Recteur-Pineau, 86022 Poitiers cedex, FranceReceived 1 August 2003; accepted after revision 5 December 2003AbstractThe preparation of monoglycerides from fatty methyl esters and glycerol can be carried out in the presence of acid or basiccatalysts. Moreover, the use of porous and basic solid catalysts could induce a shape selectivity modifying the monoglycerideselectivity. Lauric (C12), myristic (C14) and stearic (C18) acid methyl esters were used in the reaction with several MCM-41magnesium-containing catalysts. Over an Mg-MCM-41 catalyst (interreticular distance d100 of 3.4 nm), glycerol monolauratewas obtained with selectivity and yield of about 80%. To cite this article: J. Barrault et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.RésuméLa préparation de monoglycérides à partir d’esters méthyliques d’acides gras et de glycérol peut être effectuée en présence decatalyseurs acides ou basiques. De plus, l’utilisation de catalyseurs solides poreux et basiques pourrait induire une sélectivité deforme, modifiant la sélectivité en monoglycérides. Les laurate (C12), myristate (C14) et stéarate (C18) de méthyle ont été utilisésdans cette réaction avec des catalyseurs de type MCM-41 contenant du magnésium. Une sélectivité et un rendement proches de80% en monolaurate de glycérol ont été obtenus en utilisant un catalyseur de type Mg-MCM-41, de distance interréticulaire d100égale à 3,4 nm. Pour citer cet article : J. Barrault et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.Keywords: Glycerol transesterification; Monoglyceride; Magnesium oxide; MCM-41; Heterogeneous catalysisMots clés : Transestérification du glycérol ; Monoglycéride ; Oxyde de magnésium ; MCM-41 ; Catalyse hétérogène1. IntroductionNumerous products may be produced by using rawmaterials such as vegetable oils. The renewability ofthe raw materials and the specific properties of thesecompounds are of growing interest for the industry [1].The main reactions used first in oleochemistry werethe hydrolysis or the methanolysis of triglyceridesleading to glycerol and fatty acids or fatty methylesters.* Corresponding author.E-mail address: joel.barrault@univ-poitiers.fr (J. Barrault).C. R. Chimie 7 (2004) 593–599© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.crci.2003.12.006However, as these reactions also lead to the forma-tion of glycerol, we have to find valuable applicationsfor this side-product. The glycerol transformation intoglycerol monoesters has significant applications infood, pharmaceutical, cosmetics or in detergent chem-istry [2].In a previous paper [3], we have shown that basicsolids such as magnesium or cerium oxide could beused as efficient catalysts for this reaction. In varyingthe preparation method of these oxides, we were ableto increase the activity so that the reaction rate wassimilar to that obtained in the presence of homoge-neous catalysts. However, the monoglycerides selec-tivity was rather similar and a distribution ofmono/di/tri glycerides close to 50%/40%/10% was ob-served.In order to increase the monoglycerides selectivity,we suggested a dispersion of active oxides at the sur-face of an MCM-41 type mesoporous structure.MCM-41 are silica or silico-alumina characterizedby a regular hexagonal arrangement of cylindricalpores. The pore diameter is higher than in the case ofzeolites (typically 3 nm or more) with a specific sur-face area of more than 1000 m2 g–1 [4,5]. Moreover,several works gave a strong indication of a quite sig-nificant thermal stability [6].Several attempts were already done to obtainmonoglycerides in using mesoporous catalysts. Forexample, Jacobs et al. used –SO3H groups grafted overMCM-41 to catalyse glycerol esterification [7]. Thismethod was recently improved by grafting alkyl andsulfonates groups in order to tune the hydrophilicity ofthe catalyst [8,9]. However, that processes involvedacid catalysts that are well known to induce glyceroldehydration and thus side-products generation. Brunelet al. used MCM-41 functionalised with amines toperform glycidol esterification. As that reagent hasonly one hydroxyl function, monoglycerides, whichare the only products that can be formed during esteri-fication, are obtained with a high yield [10].In this paper, several acid methyl esters [lauric(dodecanoic, C11H23CO2CH3), myristic (tetrade-canoic, C13H27CO2CH3) and stearic (octadecanoic,C17H35CO2CH3) acid methyl esters] were used in theglycerol transesterification reaction over several mag-nesium containing MCM-41 and the influence of somestructural properties of the solids over the shape selec-tivity will be discussed.2. Experimental2.1. Synthesis of the catalyst2.1.1. Synthesis of the MCM-41 supportThis synthesis is the result of several works done inthe laboratory [11,12]. A mixture of 1.47 g NaOH(37 mmol) and 13.04 g hexadecyltrimethylammoniumbromide (9.1 mmol) in 200 ml water was heated at40 °C in order to dissolve the surfactant. For silico-alumina MCM-41, 2.43 g Al(NO3)3·9 H2O (8 mmol)were added. A little amount of antifoaming agent (An-tifoam A, Sigma) was also added in order to improvethe synthesis. The pH was stabilized at 10.5 with nitricacid (1 M), and 32.04 g of a 30% solution of Na2Si3O7(40 mmol) were slowly added, while maintaining thepH to 10.5. The resulting gel was heated in a sealedTeflon reactor during 16 h at 100 °C without stirring.The content was then filtrated, dried and finally calci-nated at 550 °C (after a temperature increase of 1 °Cmin–1) during 16 h. The support is called AlMCM-41 ifcontaining alumina or MCM-41 without alumina.2.1.2. Synthesis of the Mg-containing catalystThe method is given for Si/Mg = 4, and for anotherMg stoichiometry, the content of precursors must beadjusted.A mixture containing 3 g of MCM-41 freshly calci-nated, 2.96 g Mg(CH3CO2)2·4 H2O and 50 g MeOHwas stirred at room temperature during 2 h. The solventO OOOROROR +    CH3OH OH OHOHROO3 +594 J. Barrault et al. / C. R. Chimie 7 (2004) 593–599was then evaporated. The solid was then calcinated at450 °C (after a temperature increase of 1 °C min–1)during 16 h.These solids exhibit a specific surface area of about800 m2 g–1 and are characterized by an hexagonal arrayof cylindrical pores, as shown by X-ray diffraction andTransmission Electronic Microscopy (Figs. 1 and 2).The solids obtained are called Mg-AlMCM-41 orMg-MCM-41, whether they contain alumina or not.2.2. Catalytic test2.2.1. Without preliminary impregnation of the esterIn a 100-ml three-neck flask, the glycerol and themethyl ester were mixed and heated to 220 °C understirring (500 rpm) without solvent and under nitrogenatmosphere. When the temperature reached 220 °C, thecatalyst was added, which corresponded to the startingtime of the reaction. The amounts of reactants aregiven in Table 1.2.2.2. With preliminary impregnation of the esterIn the same flask and experimental conditions, themethyl ester and the catalyst were mixed. After 2 h, theglycerol was added, which corresponded to the startingtime of the reaction. The amounts of reactants are alsogiven in Table 1.2.3. Analysis of the productsThe products were analysed with a Varian3300 GPC equipped with a FID detector, an on-columninjector and a BPX5 (SGE) column. Before analysis,the products are silylated to prevent their degradationaccording to the method described by Sahasrabuhde[13].The reagent conversion to mono-, di-, and triglycer-ides is given by the following equation:Molar conversion  % =Smono / fmono + 2 Sdi / fdi + 3 Stri / f triSmono / fmono + 2 Sdi / fdi + 3 Stri / f tri + Sms / fms× 100where Smono, Sdi, Stri, and Sms are the respective areas ofthe peaks corresponding to monoglyceride, diglycer-ide, triglyceride and methyl stearate (when ms is theester reagent), and fmono, fdi, ftri and fms their respectivescaling factors.The selectivities to monoglycerides, diglyceridesand triglycerides are given by the following equations:Monoglycerides selectivity  % =Smono /fmonoSmono /fmono + 2 Sdi /fdi + 3 Stri /f tri× 100Fig. 1. XRD of Mg-AlMCM-41.Fig. 2. TEM of Mg-AlMCM-41.Table 1Amount of reactants for the preparation of glycerol esters over mesoporous solidsGly/ester m C12 (g) m C14 (g) m C18 (g) m catalyst (g) m glycerol (g)[mol gly/mol ester]1 9.9 11.2 14.0 0.5 4.53 7.5 8.46 10.38 0.55 10595J. Barrault et al. / C. R. Chimie 7 (2004) 593–599Diglycerides selectivity  % =2 Sdi /fdiSmono /fmono + 2 Sdi /fdi + 3 Stri /f tri× 100Triglycerides selectivity  % =3 Stri /f triSmono /fmono + 2 Sdi /fdi + 3 Stri /f tri× 1003. Results3.1. Reactivity of glycerol with various methyl estersover Mg-AlMCM-41 type catalystsThe reaction of lauric (C12), myristic (C14) andstearic (C18) acid methyl esters with glycerol in thepresence of magnesium-containing silico-aluminateMg-AlMCM-41 (glycerol / methyl ester ratio equal to1 or 3) were performed without or with the pre-impregnation of the methyl ester.3.1.1. Without methyl ester pre-impregnationThe results are completely different from that ob-tained with homogeneous catalysts [2] for which amonoglyceride selectivity of 40% was obtained at aconversion of 80% with a glycerol/methyl ester ratio of1. In this study, the monoglyceride selectivity seems tobe very low (respectively 13, 8 and 1%) at a conversionof 80%. In fact, this catalyst mainly gives glyceroldiesters, which could be due to the strong hydrophilic-ity of the catalyst surface. Glycerol was strongly ad-sorbed and filled in the pores of the MCM-41 structure,whereas the hydrophobic methyl ester was mainly out-side the pores where the reaction occurred. As theMCM-41 pore volume is important (about 1 cm3 g–1),a significant amount of glycerol is unavailable and thenthe apparent glycerol/methyl ester ratio is far less than1. Indeed, if we assume a pore volume of 1.2 cm3 g–1, aglycerol density of 1.216, a quick calculation gives a‘loss’ of glycerol of about 18%.3.1.2. With methyl ester pre-impregnationIn order to confirm such hypothesis, we did thesame experiments with a preliminary impregnation ofthe methyl ester and the results shown in Table 2 aretotally different. Indeed, from the two shortest methylesters (C12 and C14), the monoglycerides selectivity ishigher than that obtained with homogeneous catalysts,which clearly suggests a strong adsorption of glycerol.By doing first an impregnation of the catalyst withthe methyl ester, the surface coverage of the catalystwith the ester was increased. At the beginning of thereaction, the glycerol, which is more labile and farmore hydrophilic than the methyl esters, migrated intothe pores where the reaction took place with the methylesters. When the glycerol fully replaced the methylester in the pores, the reaction ending rate and theselectivity were similar to that obtained in the previouscase.The comparison of the results obtained with anexcess of glycerol (Tables 2 and 3) indicated as ex-pected an increase of glycerol monoesters selectivity,particularly with methyl dodecanoate or tetrade-canoate. It was a first difference suggesting that theporous structure was more or less without effect whenmethyl octadecanoate was the reagent instead of‘shortest’ esters. The reaction rates also increasedwhen a glycerol excess was used in a normal experi-ment (Table 3). On the other hand, there was no changewhen the catalyst was pre-impregnated with the methylester, which was a strong indication of a low coverageof the catalyst surface with the methyl ester.Table 2Transesterification of glycerol over Mg-AlMCM-41 after a preliminary ester impregnationEster Gl/ester Time to 50% (h)a Selectivity at 50%b Time to 80% (h)a Selectivity at80%bInitial activity(mmol h–1 g–1)C12 1 2 65 4 45 2.17C14 1 7 60 13 55 0.87C18 1 14 28 26 15 0.64C12 3 2 80 3 70 2.18C14 3 8 75 10 68 0.94C18 3 7 70 10 40 0.55a,b See Table 2.596 J. Barrault et al. / C. R. Chimie 7 (2004) 593–5993.2. Mg-MCM-41 type catalystsIn order to improve the monoglyceride selectivity,less hydrophilic supports like pure silica MCM-41were synthesized. The Mg-containing pure silica Mg-MCM-41 was then prepared and dehydrated (700 °C,16 h at reduced pressure) in order to transform surfaceSi–OH groups into more hydrophobic Si–O–Si groups.Then the catalytic reaction was performed with methyloctadodecanoate in the presence of both catalysts andthe results were compared to that of Mg-AlMCM41.An increase in both selectivity and activity with adecrease in hydrophilicity was observed (Fig. 3), themost important changes being consecutive to the ab-sence of alumina (the hydroxyls groups eliminatedfrom dehydration of the solid seems out of interest forthe adsorption and/or the reaction). The glycerol is lessadsorbed over the surface compared to the methyl esterand the selectivity remains more important than thatobserved with Mg-AlMCM-41 (Fig. 3).3.3. Pore-size effectThe results presented in the previous tables andfigures indicated that the highest selectivities to glyc-erol monoesters are obtained either with the lauric orthe myristic acid methyl ester. Such selectivity modifi-cations could be due to changes in the hydrophobicityof the catalyst or/and of the hydrocarbon chain lengthof the ester. Indeed, the hydrophilicity of the methylester decreases as the chain length increases, so that adecrease in the monoglycerides selectivity with thechain length of the methyl ester should be expected. InTables 2 and 3, such variation is observed, but mainlywith the C18-ester.In order to check the effect of the pore size of thecatalysts, Mg-MCM41 materials with a smaller poresize were synthesized in using smaller surfactantssuch as tetradecyltrimethylammonium bromide(C14H29N(CH3)3Br) and dodecyltrimethylammoniumbromide (C12H25N(CH3)3Br) instead of the hexadecyl-Table 3Catalytic transesterification of glycerol over Mg-AlMCM-41 without preliminary impregnation of the esterEster Gly/ester Time to 50% (h)a Selectivity at 50%b Time to 80% (h)a Selectivity at80%bInitial activity(mmol h–1 g–1)C12 1 11 45 20 13 0.54C14 1 19 17.5 33 8 0.44C18 1 18 10.5 40 1 0.31C12 3 2 75 3 70 2.12C14 3 9 65 14 62 0.92C18 3 10 55 13 50 0.54a Time needed to reach a 50% (or 80%) methyl ester conversion.b Monoglycerides selectivity at 50% (or 80%) methyl ester conversion.01020304050607080901000 10 20 30Time (hours)Conversion (%)01020304050607080901000 20 40 60 80 100Conversion (%)Selectivity (%)Mg-AlMCM-41 (Si/Al = 20) Mg-MCM-41 dehydrated MCM-41Fig. 3. Transesterification of glycerol with methyl octadocanoate (gly/C18 = 1) in the presence of Mg-MCM-41 and Mg-AlMCM41 catalysts.597J. Barrault et al. / C. R. Chimie 7 (2004) 593–599trimethylammonium bromide (C16H33N(CH3)3Br).The interreticular distance d100 of the mesophases var-ied from 3.4 to 4.1 nm and the surface area was higherthan 1000 m2 g–1 (Table 4).These catalysts were tested in the glycerol transes-terification with the methyl laurate, in the followingexperimental conditions; a temperature of 220 °C and aratio glycerol/ester of 2 (more representative of realconditions).It appears (Fig. 4) that the decrease in the pore sizedid not change the initial activity. A sharper increase ofthe conversion was only observed over the catalystwith the largest pores after 3 h of reaction, which couldbe the result of an increasing effect of the diffusionprocesses with the diminution of the pores size. In-deed, as the pores aperture was reduced, moleculardiffusion increases while Knudsen diffusion is alsomore important. However, if the monoglyceride selec-tivities obtained with ‘C14 and C16 catalysts’ were verysimilar, the selectivity obtained in the presence of ‘C12catalysts’ was higher. This result clearly shows that thepore size reduction has a greater impact over the mo-lecular diffusion control inside the pores of the cata-lyst. Decreasing the pore size avoids polyesterificationof glycerol, so that selectivity and yield to monoestersof 75% are obtained even if the ratioglycerol/methylester is much lower than in the previ-ous cases (Tables 2 and 3).At this stage, it must be also added that the activityand the selectivity to monoesterification are not soimportant if the mesoporous structure of the catalyst isnot as well defined and organized as above. Indeed, ifthe preparation procedure is modified so as to obtain amore amorphous solid with a larger pore size distribu-tion (even centred around the same mean value of3.5 nm), there appears an increase in both diesters andtriesters formation. Then besides the hydrophilicityeffect, there is also a strong influence of the pore size ofthe catalyst over the selectivity.4. ConclusionThis study clearly shows that porous solids such asMCM-41 promoted with magnesium species can beefficient catalysts for clean synthesis processes, par-ticularly when the synthesis of the porous material andthe experimental conditions are finely tuned to thereagents. Here glycerol monoesters were synthesizedwith a yield up to 80%, which is quite higher than the40% monoglycerides obtained with homogeneouscatalysts. In our knowledge, it is the first time that sucha shape selectivity is observed with mesoporous cata-lysts for such reactions.AcknowledgementsThe authors want to thank the Stearinerie Duboisand ADEME–AGRICE for fruitful discussions andfinancial support.Table 4d100 of the ‘smaller pore size’ mesophasesSurfactant used C12 C14 C16d100 (nm) 3.43 3.65 4.1201020304050607080901000 2 4 6 8 10conversion (%)16-MCM14-MCM12-MCM01020304050607080901000 20 40 60 80 0conversion (%)Time (hours) Selectivity (%)10Fig. 4. Glycerol transesterification with methyl laurate over of ‘smaller pore size’ mesoporous Mg-MCM41 catalysts. T = 220 °C,glycerol/ester = 2.598 J. Barrault et al. / C. R. Chimie 7 (2004) 593–599References[1] U. Biermann, W. Friedt, S. Lang, W. Luhs, G. Machmuller,J.O. Metzger, M.R. Klass, H.J. Schafer, M.P. Schneider,Angew. Chem. 105 (2000) 22064.[2] M.M. Rieger, Cosmetics Toiletries 105 (1990) 51.[3] S. Bancquart, C. Vanhove, Y. Pouilloux, J. Barrault, Appl.Catal. 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Chimie 7 (2004) 593–599

Selective glycerol transesterification over mesoporous basic catalysts

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Selective glycerol transesterification over mesoporous basic catalysts

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