Glycerol transesterification with methyl stearate over solid basic catalysts

Applied Catalysis A: General 218 (2001) 1–11

Glycerol transesterification with methyl stearate over solid basic catalysts I. Relationship between activity and basicity Sébastien Bancquart, Céline Vanhove, Yannick Pouilloux, Joël Barrault∗ Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, ESIP 40, avenue de Recteur Pineau, 86022 Poitiers Cedex, France Received 14 December 2000; received in revised form 26 February 2001; accepted 5 March 2001

Abstract The preparation of monoglycerides from fatty acids or fatty methyl esters and glycerol can be carried out in the presence of acid or basic catalysts. The use of solid basic catalysts could limit secondary reactions leading to product degradation. A comparison of several basic solids (MgO, CeO2 , La2 O3 and ZnO) has shown that the more significant the intrinsic basicity is, the more active the catalyst is. In order to increase the performance of these solids, several methods have been used for the preparation of MgO and CeO2 . MgO prepared by hydration followed by calcination of a commercial raw material is the most active catalyst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Glycerol transesterification; Monoglyceride; Magnesium oxide; Cerium oxide; Methyl stearate

1. Introduction Numerous compounds may be produced by using raw materials, such as vegetable oils. The specific properties of these compounds and the renewability of the raw material are of growing interest for the industry. The first main reactions used in oleochemistry are the hydrolysis or the methanolysis of triglycerides, leading to glycerol and fatty acids or fatty methyl esters (CH2 OC(O)R–CHOC(O)RR–CH2 OC(O)R + 3CH3 OH → CH2 OHR–CHOHR–CH2 OH + 3RCO2 CH3 ). However, as these reactions lead to the formation of glycerol, we have to find valuable applications for this side-product. The glycerol transformation into glycerol monoesters has significant applications in food, ∗ Corresponding author. E-mail address: [email protected] (J. Barrault).

pharmaceutical, cosmetics, or in detergent chemistry [1]. The monoglycerides are generally obtained by: (1) the glycerolysis of triglycerides, (2) the hydrolysis of triglycerides, (3) the direct esterification of glycerol with fatty acids, or (4) the transesterification of glycerol with fatty methyl esters [2]. We decided to study this fourth method (reaction: CH2 OH–CHOH– CH2 OH + RCO2 CH3 → CH2 OC(O)R–CHOH–CH2 OH+CH3 OH). The industrial process generally involves homogeneous acid or basic catalysts, leading to a mixture of mono-, di-, and tri-esters (respectively 40, 50, and 10%), and a molecular distillation is necessary to obtain high purity products. Moreover, the use of homogeneous catalysts leads to the formation of a high amount of salts. At last, these catalysts favour side-reactions leading to the degradation of the fatty oils. The use of solid catalysts could avoid these drawbacks.

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 7 9 - 8

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Our objective is the transesterification of glycerol with basic solid catalysts in the absence of solvent in order to reduce the waste formation and extraction steps and allow an easier removal of the catalyst [3]. In this paper, we present the catalytic activity of several heterogeneous basic catalysts (MgO, ZnO, La2 O3 , and CeO2 ) and the effect of the preparation method on their physicochemical and catalytic properties. Because it has been shown [4] that porous materials are able to induce shape selectivity in such reactions, the grafting of our catalysts on porous supports will be explained in a further paper. 2. Experimental 2.1. Adsorption–desorption experiments 2.1.1. CO2 temperature programmed adsorption–desorption About 65 mg of catalyst are degassed under helium (30 ml min−1 ) during 8 h at 450◦ C (10◦ C min−1 ). Then, the catalyst is regularly saturated with CO2 pulses at ambient temperature (about 25◦ C). The adsorption is followed by a thermal conductivity detector. After 20 min of helium flowing, the catalyst is saturated again with CO2 in order to measure the physisorbed CO2 amount. Then, the catalyst is heated until 600◦ C (3◦ C min−1 ) in order to determine the CO2 desorption and the strength of the basic sites. 2.1.2. NH3 temperature programmed desorption About 100 mg of catalyst are heated at 500◦ C (10◦ C min−1 ) under helium (40 ml min−1 ). After cooling at 50◦ C, the catalyst is saturated with NH3 . Then, under helium (40 ml min−1 ), NH3 is desorbed by heating, using 50◦ C steps (4◦ C min−1 ). Desorption is followed by a thermal conductivity detector. The gas is neutralized with a 10−2 N chlorhydric acid to get the amount of desorbed NH3 . 2.2. Preparations 2.2.1. Preparation of MgO by precipitation The precipitation of magnesium hydroxide from Mg(NO3 )2 and NH4 OH is based on the method described by Berkani [5]: 100 g of Mg(NO3 )2 ·6H2 O are dissolved into 250 ml distilled water. 100 ml of NH4 OH solution (about 30%) are added under stirring.

The precipitate is washed several times with distilled water, then dried in an oven during 16 h (100◦ C). Finally, the solid is calcinated under air (40 ml min−1 ) at 450◦ C (5◦ C min−1 ) during 12 h. 2.2.2. Preparation of MgO by hydration 250 ml of distilled water is slowly added to 25 g MgO (Prolabo) and the solution is heated (1) at 80◦ C under stirring in order to eliminate the excess water, (2) at 85◦ C on a sand-bath during 16 h. The solid obtained is calcinated under air (30 ml min−1 ) at 350◦ C (3◦ C min−1 ) during 2 h and at 500◦ C during 8 h. 2.2.3. Preparation of CeO2 by the citrate method CeO2 was prepared with the method developed by Marcilly et al. [6] and described by Sato et al. [7]. Ce(NO3 )2 ·6H2 O is heated at 70◦ C and it becomes liquid. Citric acid is then added up to a molar ratio of 1. The water is eliminated from the mixture by heating at 80◦ C using a rotavapor. The solid is then dried under nitrogen (150 ml min−1 at 170◦ C during 2 h (4◦ C min−1 ) and calcinated under air (60 ml min−1 ) during 2 h at 550◦ C. 2.3. Catalytic test In a 100 ml Pyrex tricol 4.5 g of glycerol (48 mmol) and 14.02 g of methyl stearate (48 mmol) are mixed and heated to 220◦ C under stirring (500 rpm) without solvent and under nitrogen atmosphere. When the temperature reaches 220◦ C, 0.5 g catalyst (2.7 wt.%) are added, which corresponds to the starting time of the reaction which takes about 6 h. 2.4. Analysis The products are analyzed with a Varian 3300 GPC equipped with a FID detector, an on-column injector, and a BPX5 (SGE) column. Before analysis, the products are silylated to prevent their degradation following the method described by Sahasrabuhde [8]. The reagent conversion to mono-, di-, and tri-glycerides is given by the following equation Molar conversion (%) Smono /fmono + 2Sdi /fdi + 3Stri /ftri = Smono /fmono + 2Sdi /fdi + 3Stri /ftri + Sms /fms ×100

S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

where Smono , Sdi , Stri and Sms are the respective areas of the peaks corresponding to monoglyceride, diglyceride, triglyceride and methyl stearate, and fmono , fdi , ftri and fms their respective scaling factors. The selectivities to monoglycerides, diglycerides and triglycerides are given by the following equations:

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be observed on alkali doped oxide, but not on the pure oxide. The basicity is measured by adsorption–desorption of CO2 . These two methods give convenient relative scales to compare the properties of our solids. Some characteristics of ZnO (from calcination of Zn(CO3 )2 ), MgO (Prolabo), La2 O3 (Prolabo) and CeO2 (Rhône-Poulenc) are reported in Table 1.

Monoglycerides selectivity (%) Smono /fmono = × 100 Smono /fmono + 2Sdi /fdi + 3Stri /ftri

1. If we compare the specific acidity (acidity per unit weight), La2 O3 and CeO2 are the most acidic, while MgO has a very low specific acidity. If we compare the intrinsic acidity (acidity per unit area), La2 O3 is much more acidic than CeO2 , ZnO and MgO. 2. If we consider the specific basicity, MgO is by far the most basic solid, followed by La2 O3 , ZnO and CeO2 . If we compare the intrinsic basicity, the most basic is La2 O3 , followed by MgO, CeO2 and ZnO, the basicities of the last two oxides being very low.

Diglycerides selectivity (%) 2Sdi /fdi × 100 = Smono /fmono + 2Sdi /fdi + 3Stri /ftri Triglycerides selectivity (%) 3Stri /ftri × 100 = Smono /fmono + 2Sdi /fdi + 3Stri /ftri 3. Results

The main trend is that these oxides have basic properties [11,12], even if the results can be discussed for La2 O3 . Indeed, La2 O3 is able to trap the atmospheric CO2 to give carbonates, hydroxycarbonates and oxyhydroxycarbonates [13–15]. The CO2 desorbed at high temperature could be due to the decomposition of these carbonates. However, it is interesting to notice that the acid character of La2 O3 , CeO2 and ZnO is not negligible (A/B ratio > 1). Moreover, the nature and the strength of the acid sites differ and depend on the oxide, as shown in Fig. 1. The acid sites of La2 O3 are stronger

3.1. Characterization of basic oxides 3.1.1. Comparison between several basic oxides The acidity of the solid oxides is measured by adsorption–desorption of NH3 . Indeed, ammonia is not able to adsorb on basic sites, except in the case of “superbasic” solids, as described by Tanabe [9]. We postulate that the effect of superbasic sites on the measure of the acidity is negligible and will not change the order of the results. For example, it is well known [10] that for MgO, the superbasic sites could Table 1 Characterizations of the basic oxides used by CO2 TPD and NH3 TPD Catalyst

ZnOa MgOb La2 O3 b CeO2 c

SBET (m2 g−1 )

33 13 4.6 19

Acidity ␮mol g−1

␮mol m−2

␮mol g−1

␮mol m−2

455 110 565 546

13.9 8.5 120 29

21 202 80.6 16.3

0.6 15.5 18 0.9

Made from zinc carbonate by calcinating under air during 4 h at 350◦ C. Prolabo. c Rhône-Poulenc. d Acidity/basicity ratio. a

b

A/Bd

Basicity

23.2 0.5 6.7 32.2

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Fig. 1. Temperature programmed desorption of NH3 : repartition of acid sites on the different metal oxides.

than that observed on ZnO, while the acid sites of MgO and CeO2 are rather weak. The strength of the basic sites was also compared, and as shown in Fig. 2, the results lead to the following order: La2 O3 > MgO > ZnO.

It clearly appears that there are only strong basic sites over lanthanum oxide. However, as mentioned above, the CO2 desorbed at highest temperature could be due to the presence of the carbonates, oxycarbonates and hydroxycarbonates.

Fig. 2. Temperature programmed desorption of CO2 : repartition of basic sites on the different metal oxides.

S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11 Table 2 Conversion and selectivity to mono-, di- and tri-glycerides in the reaction of glycerol transesterification with methyl stearate in the presence of metal oxides after 6 h at 220◦ Ca Catalyst

Without ZnO MgO La2 O3 CeO2 Homogeneous catalyst

Conversion (%)b

Selectivity (%) Mono

Di

Tri

2.5 18 83 97 4 90

100 80 42 28 100 40

0 20 52 61 0 50

0 0 6 11 0 10

a Experimental conditions: T = 220◦ C, gly/ms ratio = 1, t = 6 h, W catalyst = 0.5 g, atmospheric pressure, the reaction is performed under nitrogen. b Methyl stearate conversion.

3.2. Catalytic properties 3.2.1. General survey of basic solids Activity: These oxides have been tested in the glycerol transesterification with methyl stearate to selectively prepare the glycerol monostearate, in the following experimental conditions: equimolar ratio of glycerol and methyl ester, reaction under nitrogen atmosphere, at 220◦ C. The results reported in Table 2 show that even at 220◦ C, the transesterification reaction is very slow in

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the absence of catalyst, and the presence of a metallic oxide really increases the reaction rate. The activity order is as follows: La2 O3 > MgO ZnO > CeO2 , and it seems that there is a relationship between the transesterification rate and the intrinsic basicity of the catalyst (Fig. 3). However, from results reported in Fig. 2, it is more difficult to conclude about the influence of the sites strength of the catalysts since their CO2 desorption spectra are greatly different. Selectivity: A comparison of the formation of mono- and di-glycerides obtained in the presence of MgO and La2 O3 shows that the nature of the oxide has a low effect on the selectivity of the reaction (Fig. 4). Indeed, at 80% conversion, the selectivity to mono-, di-, or tri-esters are similar to that obtained in using homogenous basic catalysts (i.e. 40% monoester, 50% diester and 10% triester). This could be explained by the fact that those catalysts cannot induce what we call in heterogenous catalysis “a shape selectivity” coming from the use of a material with a controlled pore size. The selectivity to monoglyceride could be easily increased by using an excess of glycerol. However, the excess of glycerol should be removed at the end of the reaction, needing a separation step, which could be difficult (due to the tensioactive properties of the

Fig. 3. Relationship between the activity (calculated at 5% conversion) and the intrinsic basicity of the metal oxides.

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S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

Fig. 4. Selectivity to mono and diglycerides in the reaction of the glycerol transesterification with methyl stearate in the presence of MgO or La2 O3 at 220◦ C.

monoglyceride, it is impossible to wash the mixture) so that there is no real gain (in a real industrial synthesis, an excess ratio of only 1.2 is used for that reason). In the presence of La2 O3 , the formation of acrolein is also observed which results of a glycerol dehydration over the acid centers of the catalyst. From that results, we may conclude that some solid and basic materials (La2 O3 , MgO) could replace the homogeneous systems in the glycerol transesterification with methyl stearate without change of the selectivity, La2 O3 being the most active. Moreover, there are some relationships between the activity and the total basicity, but as it is difficult to obtain a lanthanum oxide with a high surface area, MgO and CeO2 were first selected in order to improve their activity.

3.3. MgO and CeO2 catalysts 3.3.1. Influence of the preparation method of MgO In order to improve the surface area of the magnesium oxide, the influence of the preparation method on the physicochemical and the catalytic properties was studied. Three different magnesium oxides have been tested: MgO(I), a commercial oxide from Prolabo, MgO(II), prepared by precipitation from Mg(NO3 )2 with NH4 OH, and MgO(III), obtained by hydration and calcination of MgO(I). Their physicochemical properties are reported in Table 3. It appears that the preparation method influences the surface area of the catalyst, the third method (hydrolysis and calcination) giving the best result. The NH3

Table 3 Physicochemical properties of magnesium oxides Catalyst

MgO(I)a MgO(II) MgO(III) a

Prolabo.

SBET (m2 g−1 )

13 102 151

X-ray structure

– MgO MgO

Acidity

Basicity

A/B

␮mol g−1

␮mol m−2

␮mol g−1

␮mol m−2

110 – 290

8.5 – 1.9

202 138 345

15.5 1.3 2.3

0.5 – 0.8

S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

Fig. 5. Influence of the preparation method of MgO on the repartition of acid sites.

Fig. 6. Influence of the preparation method of MgO on repartition of basic sites.

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Table 4 Activities (at 20% conversion) in the reaction of glycerol transesterification with methyl stearate in the presence of MgOa Catalyst

Specific activity (mmol h−1 g−1 )

SBET (m2 g−1 )

Intrinsic activity (mmol h−1 m−2 )

Intrinsic basicity (␮mol m−2 )

MgO(I) MgO(II) MgO(III)

10.7 14.9 20.2

13 102 151

0.88 0.15 0.13

15.5 1.3 2.3

a

Influence of the preparation method.

and CO2 TPD show that the global acidity and basicity depends also on the preparation method. However, whatever the preparation method, the basicity is always the dominant character (the A/B ratio is lower than 1) even if there are some differences between the results. Moreover, the preparation method does not influence the NH3 or the CO2 desorption spectra so that the strength of the acid or basic sites are more or less similar, as shown in Figs. 5 and 6. The activities of these solids calculated at a methyl stearate conversion of 20% are presented in the Table 4. If the specific activity of MgO increases with the specific surface area, the intrinsic activity increases with the intrinsic basicity as observed in previous experiments. 3.3.2. Influence of the preparation method of CeO2 Three different cerium oxides have been characterized and tested in the same reaction: CeO2 (I) and CeO2 (II) were supplied by Rhone-Poulenc, and CeO2 (III) was prepared via a citrate salt. The characterizations are given in Table 5. The preparation method of CeO2 has a direct influence on its specific surface area and on its acidobasic properties. It appears that the specific acidity

(per weight unit) orders as: CeO2 (II) > CeO2 (I) > CeO2 (III), while for the intrinsic acidity (per surface unit) the order is: CeO2 (III) CeO2 (I) > CeO2 (II). For the basicity, quite similar orders are observed: as far as the specific basicity is concerned: CeO2 (II) > CeO2 (I) ∼ CeO2 (III), while for the intrinsic basicities, CeO2 (III) is the most basic sample: CeO2 (III) > CeO2 (II) > CeO2 (I). In the three cases, a strong acidic character is obtained (A/B > 1). But it appears that the preparation method has no significant influence on the repartition of the basic sites (Fig. 7) or of the acidic sites (Fig. 8), even if it seems that there are more “weak acid” sites (Tdes : 50–100◦ C) at the surface of CeO2 (II). The catalytic results reported in Table 6 indicate that the catalytic activity of CeO2 is dependant on the surface area of the catalyst since CeO2 (II) is most active. 3.3.3. Comparison between cerium and magnesium oxides In this part, the most active cerium oxide (CeO2 (II)) and magnesium oxide (MgO(III)) are compared. It appears that these two catalysts have similar specific surface area and basicity (Table 7). However,

Table 5 Physicochemical properties of the different cerium oxides Catalyst

CeO2 (I)a CeO2 (II)a CeO2 (III)b a

SBET (m2 g−1 )

19 135 1.2

Rhône-Poulenc. Made via the citrate method. c Acidity/basicity ratio. b

Acidity

A/Bc

Basicity

␮mol g−1

␮mol m−2

␮mol g−1

␮mol m−2

546 805 264

29 5.9 220

16.3 345 16

0.9 2.6 13

32.2 2.3 16.9

S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

Fig. 7. Influence of the preparation method of CeO2 on the repartition of basic sites.

Fig. 8. Influence of the preparation method of CeO2 on the repartition of acid sites.

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S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

the NH3 desorption occuring mainly between 50 and 150◦ C for CeO2 (II) and between 150 and 250◦ C for MgO(III). The repartitions of the weak basic sites are similar for the two solids, but it is noticeable that for temperature greater than 500◦ C, there is a more important CO2 desorption from MgO(III) than from CeO2 (II). The comparison of the catalytic results between CeO2 (II) and MgO(III) show that even if they have similar intrinsic basicity and surface area, their initial activity in our reaction are different. The magnesium oxide MgO(III) is the most active solid which could be due to the presence of stronger basic sites. But, as shown in Fig. 9, the selectivities to the monoglycerides are similar and only depend on the reagent conversion.

Table 6 Conversion and selectivity to mono-, di- and tri-glycerides in the glycerola Catalyst

Conversion (%)

Selectivity (%) Mono

Di

Tri

CeO2 (I) CeO2 (II) CeO2 (III)

4.1 82 3.9

100 42 100

0 52 0

0 6 0

a Experimental conditions: T = 220◦ C, gly/ms molar ratio = 1, t = 6 h, W catalyst = 0.5 g, atmospheric pressure, the experiments are performed under nitrogen.

CeO2 (II) has a stronger acidic character than MgO(III). The repartition of the acidic and basic sites on these two solids are different, as precedently shown. MgO(III) has stronger acid sites than CeO2 (II),

Table 7 Physicochemical properties of CeO2 (II) and MgO(III) Catalyst

CeO2 (II) MgO(III)

SBET (m2 g−1 )

135 151

Acidity

Basicity

A/B

␮mol g−1

␮mol m−2

␮mol g−1

␮mol m−2

805 290

5.9 1.9

345 345

2.6 2.3

Fig. 9. Glycerol transesterification with methyl stearate in the presence of MgO(III) and CeO2 (II): selectivity to monoglycerides.

2.3 0.8

S. Bancquart et al. / Applied Catalysis A: General 218 (2001) 1–11

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4. Conclusion

Acknowledgements

Even if all the materials studied have a strong basic character, some of them have also strong acid sites depending of the preparation procedure. Such acidity favours side reactions like glycerol dehydration to acrolein which must be avoided. On the other hand, it is also demonstrated that the reaction rate of the transesterification directly depends of the basicity of the oxide, especially of the strong basic sites. From the comparison of the catalytic properties, lanthanum oxide appears as one of the best catalysts for such reactions. Nevertheless, it was rather difficult to control the surface area and the basic properties of such oxide due to the easy formation of carbonate or oxycarbonate species from the reaction of CO2 with the oxide. Magnesium oxide, easily obtained from a commercial product, is finally proposed for the reaction. Even if the activity of this solid catalyst is lower than that of a general homogeneous catalyst, the glyceride selectivities and yields are similar. Moreover, no catalyst leaching was observed and the solid is easily extracted from the reaction medium and reusable without formation of waste. Such oxides could easily replace homogeneous catalysts and avoid the formation of wastes and washing steps of a usual process using soluble alkaline hydroxides. Now, in our laboratory, different ways of increasing both the activity and the yield to monoglycerides are under investigation, i.e. the grafting of basic species inside a porous structure leading to the same basicity as that of materials presented in this paper and the results will be submitted further.

This work is part of the European project no. AIR 3CT 94 2218: “Reactivity of fatty esters and glycerol: new methods”. The authors want to thank the Stearinerie Dubois and ADEME for fruitful discussions and financial support of one of the authors (C. Vanhove, convention no. BOU 9640).

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