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Selective synthesis of monoglycerides from glycerol and oleic acid in the presence of solid catalysts
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.
539
SELECTIVE SYNTHESIS OF MONOGLYCERIDES FROM GLYCEROL AND OLEIC ACID IN THE PRESENCE OF SOLID CATALYSTS S. ABRO, Y. POUILLOUX and J. BARRAULT Laboratoire de Catalyse, URA CNRS 350, ESIP, 40 avenue du Recteur Pineau , 86022 POITIERS CEDEX, FRANCE ABSTRACT : The selective synthesis of glycerol monooleate can be performed in the presence of solid catalysts less corrosive and more easily reusable than homogeneous mineral acids. The study of various acid solids (zeolite, clay, ion-exchange resin) for the esterification of glycerol (coproduct of methyl ester synthesis) with oleic acid has shown that cationic exchange resins were the best catalysts for the selective preparation of monooleyl glyceride in mild experimental conditions. Indeed, a selectivity of about 90% for an oleic acid conversion greater than 50% is obtained. It seems that the activity and the selectivity is influenced by the resin structure ; depending on its crosslinking, the resin acts as a shape selective catalyst. L INTRODUCTION The main objective of this work consists in the synthesis of monoglycerides from glycerol and fatty acids issued from vegetable oils in the presence of solid catalysts. Indeed, the use of natural feedstocks presents several advantages : i) the diversity of the available products, ii) the renewable character of natural compounds. Moreover, in the chemical industry, the use of natural products opens an area of investigation of new processes and of new products some of which are quite different from those accessible by petrochemical paths. In our Laboratory, we are involved in a general programme on the selective transformation of fatty acids (or of methyl esters) and glycerol issued from sunflower oil (1,2). We have studied recently the preparation of esters of glycerol, specially monoglycerides, which are important intermediates for the manufacture of lubricants, emulsifiers, surfactants used in the industries of pharmaceuticals, cosmetics and food,... (3). O II CH2—O—C—R
CH2OH RCOOH + CHOH I CH2OH
•
CHOH I CH2OH
+ H2O
R : 8 to 22 carbon atoms. Monoglycerides are obtained generally from glycerolysis or hydrolysis of triglycerides (4) or from the direct esterification of fatty acids by glycerol.
540 Alcohol esterification is usually catalysed by homogeneous catalysts such as sulfuric acid (5), para-toluenesulfonic acid (6) or bases such as sodium (potassium, ...) hydroxide (carbonate,...) (3,7). Unfortunately, it is well known that these bases favour the production of soaps. Moreover, homogeneous catalysts are corrosive, difficult to separate from the products and lead to excessive wastes (salts). As for the direct esterification of glycerol, previous works have shown that numerous solid and acid oxides could be used as catalysts (6, 8-11). In the presence of: i) tin or zinc chloride which are active at low pressure and at 200°C, glycerol is easily esterified, however dehydration and/or oxidation of glycerol can occur, ii) large pores zeolites, the patent of Aracil and Corma claims that monoglycerides can be obtained with a high selectivity (90%) in the same pressure range but at high temperatures (12). Hoelderich and Siegel also used different types of zeolites for this esterification reaction (13). iii) enzymes (supported on resins), it can be observed that there is either a selective formation of monoglycerides, or a mixture of mono, di and triglycrides (14-16). On the other hand, monoglycerides can be obtained selectively from the reaction of glycidol with fatty acids over an anionic ion-exchange resin (17). The aim of our work is to find a new type of solid catalysts in order to be able to control the selectivity. We present in this paper some results obtained with crosslinked porous polymers. Also, we compare the behaviour of different catalysts, in particular, ion-exchange resins, in the esterification of glycerol with oleic acid. The influence of the nature of the resin as well as its swelling properties are discussed. 2. EXPERIMENTAL 2.1. Catalytic tests The esterification was carried out at atmospheric pressure in a glass reactor equipped with a mechanical stirrer and heated v^th an oil bath. Moreover, we verified that the activity and the selectivity were independent of the stirring rate. The reaction was studied at 90°C during 24 hours. The molar ratio glycerol/oleic acid was 6.3, the weight ratio oleic acid/catalyst 4.5 and the catalyst weight 1 g. At the end of the reaction, the mixture was dissolved into ethanol and analysed with an HPLC equipped with a light scattering detector and an apolar column (Licrospher). The separation of different products was done by a gradient elution. The percentage of each compound was determined by using standardisation methods with methyl laurate as an internal standard. The conversion is expressed as follows ;
Conversion (%) = ^ . o / la
y'Oiki
. c +
iS oleic
Yi: stoichiometric coefficient of the i product ki: response factor of the i product The selectivity of each glycerol ester is the ratio : mono-, di- or triester/(oleic acid)transf-
541 2.2. Catalyst The catalysts used in this study were a zeolite (Zeocat HY 510, Si/Al = 10), a Montmorillonite clay pillared with Titanium species and cationic ion-exchange resins ; a macroporous lER Amberlyst 15 (Rohm & Haas) and resins with a gel structure (K1481 Bayer and Amberlyst 31 - Rohm & Haas). 3. RESULTS AND DISCUSSION 3.1. Comparison of the activity and the selectivity of acid catalysts In the first part of our study, the esterification of glycerol with oleic acid in the presence of different acid solids with a controlled porosity (zeolite, clay, ion-exchange resin) was studied (Table 1). Table 1 Esterification of glycerol with oleic acid. Comparison between various acid solid catalysts. Conversion
Catalysts
Selectivity (%)
(%)
Mono-ester^^^
Di-ester^^^
Tri-ester^^^
Without
1
-
-
-
Zeolite HY510
5
67
27
6
Montmorillonite-Ti
10
71
24
5
K1481 resin
49
78
21
1
ABS*
92
54
46
0
* ABS : benzenesulfonic acid, homogeneous reaction Reaction temperature : 90°C, reaction time : 24 h.
(a)
C17H23COOCH2
.. --
Monoester:
J-
C17H23COOCH2
CHOH Diester: C17H23COOCH CH2OH
C17H23COOCH2
I
I Triester iCizHssCOOCH
CH2OH
C^H23COoiH2
The esterification rate was very low when the reaction was performed without a catalyst. The reaction was much faster in the presence of benzenesulfonic acid. Over a solid catalyst, we observed the selective formation of monooleyl glyceride (mainly of the a form) in the presence of a catalyst since the monoglyceride selectivity was between 60 to 70%, the diester selectivity is of about 25% (mainly a,P €ster), the formation of triester being very small. Moreover, the oleic acid conversion varied significantly with the solid used as a catalyst. Thus, the activity of the HY 510 zeolite and of the titanium-pillared clay was lower than the one obtained over the K1481 cationic resin. The selectivity to monoglyceride in the presence of the K1481 resin was higher than the one observed using benzenesulfonic acid in a homogeneous reaction. These results seem to show that the activity of the catalyst depends mainly on the accessibility of the protonic centres to the oleic acid. Indeed the oleic acid could diffuse
542
slowly inside the bi or the tridimensional structure of the pillared clay or of the zeolite (pore size between 10 and 13 A). Moreover, the hydrophilic character of these materials could favour the glycerol adsorption while inhibiting the oleic acid adsorption and the esterification reaction. By contrast over gel resins, mainly the active centres located on the surface of the microspheres of the resin particles would be involved in the reaction. 3.2. Activity and selectivity of ion-exchange resins As the K 1481 resin for the esterification of glycerol with oleic acid was the most active solid catalyst of the series, we compared different cationic resins whose characteristics are presented in Table 2. Table 2 Characteristics of resins. Resin
K1481
Supplier
Type
Crosslinking
Acidity
Particle size
level^'^ (%)
(meq H'/g)
(mm)
Bayer
gel
8
4.8
powder < 0,05
Amberlyst 31
Rohm & Haas
gel
4
4.8
1.2 to 1.3
AmberlySt 119
Rohm & Haas
gel
8
4.8
1.2 to 1.6
•oporous
12
5.0
1.2 to 1.6
20 "oporous Rohm & Haas macroporous ^^^ % Divinylbenzene (DVB) added to the polymer matrix.
4.8
1.2 to 1.6
Amberlyst 16 Amberlyst 15
The catalytic results (Table 3) show that the catalytic activity varies with the structure of the resin. Indeed, the gel resins are more active than the macroporous resins. The conversion of oleic acid is about 55% in the presence of the Amberlyst 31 catalyst whereas it is only of 35 % with Amberlyst 15 or 16 (macroporous type). Table 3 Esterification of glycerol with oleic acid in the presence of ion exchange resins. Influence of the resin structure. Catalysts
Conversion
Selectivity (%)
(%)
Mono-ester
Di-ester
Tri-ester
K1481
49
78
21
1
Amberlyst 31
54
90
7
3
Amberlyst 16
37
83
12
5
Amberlyst 15
36
75
22
3
Reaction temperature : 90°C, reaction time : 24 h. Formulation, see experimental.
543 Moreover, the selectivity to glycerol monoesters is higher than 90 % in the presence of Amberlyst 31 (4 % of Divinylbenzene (DVB)) and it seems that the crosslinking level influences the selectivity of gel organic polymers. With macroporous resins, the selectivity is much less influenced by the crosslinking density. In macroporous resins, two kinds of active sites can be distinguished : i) the sites on the surface of microparticles or in the macropores which are easily accessible by the reactants, ii) the sites located inside the polymer matrix with a greater acid strength but of limited accessibility. The (ii) sites availability or the diffusion of the reagents to these (ii) sites depends on the crosslinking degree. From the results, it seems that a DVB percentage of about 10 % is the value above which the reaction occurs only over (i) sites. On the other hand, gel resins have only a microporous network, a crosslinking density lower than 10 % and can swell in a polar solvent by solvation of sulfonic acid groups. The swelling of the resin is accompanied by a stretching of the crosslinked hydrocarbon matrix leading to the formation of pseudo-pores (20 to 40 A), the size of which depends on the DVB content. As a result these resins look like shape selective materials and that could explain the significant monoglyceride selectivity obtained with the lightly crosslinked (4 %) Amberlyst 31 sample. 3.3 Influence of reaction conditions on the catalytic properties of gel resins 3.3.1. Change of the activity and of the selectivity with reaction time Figure 1 shows that under standard conditions the oleic acid conversion increases linearly with reaction time and reaches 80 % after 48 hours. Thus, the initial activity is around 0.4mmol.h-i.g-i. 100 80
o
>
e o
60 40 20 4-
10
20
30
40
50
Time (h) Figure 1 : Esterification of glycerol with oleic acid in the presence of the K1481 resin. Conversion of oleic acid versus reaction time. Reaction temperature : 90°C As for as the monoester selectivity is concerned. Figure 2 shows that the formation of monoester decreases with increasing conversion, which is what can be expected from a kinetic point of view. At the same time, the formation of diester increases but the selectivity to glycerol trioleate is still very low.
544 100 80 ^ 60
Monoester
i
I 40 Diester C/2
20
^-4=-r-trir
Triester •
m
—=*t= 40 60 Conversion (%)
20
f
1
80
100
Figure 2 : Esterification of glycerol with oleic acid in the presence of the resin K1481. Selectivity to the esters of glycerol with the conversion of oleic acid. 3.3.2. Effect ofparticle size of the resin Two resins with the same physicochemical characteristics (crosslinking level : 8 % ; exchange capacity : 4.8 meq HVg) were studied. However, Amberlyst 119 has a larger particle size than the K1481 polymer (Table 4). The results show that the K1481 catalyst is more active by a factor of 2 than Amberlyst 119, which indicates that particle size influences the esterification rate. As the K1481 catalyst is a powder, it has a significant outer surface area and acid centres located on this surface are easily accessible. Table 4 Esterification of glycerol with oleic acid in the presence of gel resins. Influence of particle size. Resin
K1481 Amberlyst 119
Particle size
Conversion
Selectivity (%)
(mm)
(%)
Mono-ester
Di-ester
Tri-ester
powder
49
78
21
1
0.65
21
93
3
4
The decrease of the particle size induces a decrease of the osmotic pressure within pores and a faster solvation of the protonic sites. Under these conditions, the properties of the K1481 resin resemble those of a homogeneous catalyst. Thus, the oleic acid is a molecule having a significant lateral chain (CI8) and, when esterified with glycerol, a bulky terminal ester group. Therefore, it cannot or just very slowly diffuse in the micropores of K1481 which has a crosslinking of 8 %. The reaction occurs mainly on the surface of the microspheres. On the other hand, the comparison of the activity of K1481 with that of the Amberlyst 119 resin shows that the sites located on the surface are accessible only by oleic acid since the conversion is much lower with the resin in bead form.
545 The available outer surface on Amberlyst 119 is smaller than that of K1481. It seems that the oleic acid diffusion is inhibited by the Amberlyst 119 catalyst. 3.3,3. Effect of the amount of catalyst The results obtained with the K 1481 resin reported in Table 5 show that the acid conversion normally increases with the increase of the amount of catalyst in the medium. The variations of the activity and the selectivity observed in these experiments are similar to those presented in the previous paragraph concerning the influence of reaction time. It can thus be concluded that there is no significant modification caused by external diffusion phenomena. Table 5 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the amount of catalyst. Selectivity (%)
meq H+/mmol
Conversion
OA
(%)
Mono-ester
Di-ester
Tri-ester
0.15
45
75
22
3
0.30
71
61
35
4
0.60
81
52
47
1
OA : Oleic acid ; Reaction temperature: 90°C ; reaction time : 24 h. 3.3.4. Thermodynamics and influence of the reaction temperature The glycerol esterification vyath oleic acid is exothermic (AH°R = -30 kJ.mol"^) and under our experimental conditions the equilibrium constant is above 270. The conversion decreases as expected with the temperature (see Table 6), leading to an apparent activation energy for the formation of the mono-ester (calculated for low conversion values) around 75 kJ.mor\ Table 6 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the reaction temperature. Temperature
Conversion
(°C)
(%)
Monoester
Diester
Triester
90
5
97
3
0
110
23
97
3
0
140
72
43
57
1
Selectivity (%)
Reaction time : 2 h. The selectivity to monoglyceride is modified when the temperature increases since the diester is formed mainly at 140°C at higher conversion of the acid (Table 6). However, at 140°C, we observed the formation of the by-products resulting from the polycondensation of glycerol as well as from the esterification of polyglycerols. We believe that these new
546 reactions modify significantly the catalytic properties of the resin for the esterification of glycerol. 4. CONCLUSION The present study of the esterification of glycerol by oleic acid over different acid solids showed that the ion-exchange resins are the most active catalysts for the selective preparation of monooleyl glyceride. The zeolite and the clay used in this study are much less active than the sulfonic acid resins at low temperature. A comparison between macroporous resins and gel-type resins shows that the gel resins are the most active. The macroporous resins, which are strongly crosslinked, do not permit the swelling of the matrix and as a result the reactants cannot reach the internal active sites (the strongest acid centres); consequently the reaction occurs on the outer surface of the microspheres or in the macropores. Moreover, the selectivity to mono-oleyl glyceride is higher than 90 % at a conversion of 50 % over the gel resin Amberlyst 31. This selectivity is due to the structure of the gel resin which can easily swell in the glycerol favouring the diffusion of reactants to the internal sites. 5. ACKNOWLEDGEMENTS This study was carried out within the framework of a European Program concerning the valorisation of natural feedstoks (AAIR). The authors from the University of Poitiers are very grateful to the European Communities for their financial support and also to the ADEME and the "Region Poitou-Charentes" for their financial support for this research.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A. Piccirilli, Y. Pouilloux, J. Barrault, J. Mol. Cat., accepted for publication. X. Caillault, Y. Pouilloux, J. Barrault, J. Mol. Cat. A: 103 (1995) 117-123. E. Jungermann, Cosm. Sci. Tech. Serv., 11 (1991) 97-112. K. Holmberg, E. Osterberg, EP Patent 237092. P. Marchal, Rev. Fran?. Corps gras, 32,11-12 (1985) 421-432. G. Devinat, J.L. Coustille, Rev. Frang. Corps gras, 30,11-12 (1983) 463-468. L. Rongsheng, Y. Hua, Z. Wuyang, W. Naixiang, Ind. J. Chem, 31 A (1992) 449 R. Schuch, R. Barrufaldi, L.A. Gioielli, Rev. Farm. Bioquim. Univ. S. Paulo, 20, 1 (1984)51-55. M. Martinez, E. Torrano, J. Aracil, Ind. Eng. Chem. Res., 27 (1988) 2179-2182. A. C. Bhattacharrya, D.K. Bhattacharrya, J. Am. Oil Chem. Soc, 64, (1987) 128-131. R. O. Feuge, E. A. Kraemer, A. E. Bailey; Oil and soap, 22 (1945) 202. J. Aracil, A. Corma, M. Martinez, WO Patent 94 /13617 ; 23 -06 - 94. W.F. Hoelderich, H. Siegel, BASF S. A, EP Patent 0312921; 26 -04 - 89. S. Mert, L. Dandik, Appl. Biochem. Biotech., 50, (1995) 333-342. A. Millquist, P. Aldercreutz, Enzyme Microb. Technol. ;16, 12 (1994) 1042. K. Kitano, Lion Corp ; EP Patent 0407959 ; 16 - 01 - 91. V. Rakotondrazafy, Thesis N° 985, INPT, Toulouse, France, 1994. C. L. Levesque, A. M. Craig, Ind. Eng. Chem., 40,1 (1948) 96-99.
539
SELECTIVE SYNTHESIS OF MONOGLYCERIDES FROM GLYCEROL AND OLEIC ACID IN THE PRESENCE OF SOLID CATALYSTS S. ABRO, Y. POUILLOUX and J. BARRAULT Laboratoire de Catalyse, URA CNRS 350, ESIP, 40 avenue du Recteur Pineau , 86022 POITIERS CEDEX, FRANCE ABSTRACT : The selective synthesis of glycerol monooleate can be performed in the presence of solid catalysts less corrosive and more easily reusable than homogeneous mineral acids. The study of various acid solids (zeolite, clay, ion-exchange resin) for the esterification of glycerol (coproduct of methyl ester synthesis) with oleic acid has shown that cationic exchange resins were the best catalysts for the selective preparation of monooleyl glyceride in mild experimental conditions. Indeed, a selectivity of about 90% for an oleic acid conversion greater than 50% is obtained. It seems that the activity and the selectivity is influenced by the resin structure ; depending on its crosslinking, the resin acts as a shape selective catalyst. L INTRODUCTION The main objective of this work consists in the synthesis of monoglycerides from glycerol and fatty acids issued from vegetable oils in the presence of solid catalysts. Indeed, the use of natural feedstocks presents several advantages : i) the diversity of the available products, ii) the renewable character of natural compounds. Moreover, in the chemical industry, the use of natural products opens an area of investigation of new processes and of new products some of which are quite different from those accessible by petrochemical paths. In our Laboratory, we are involved in a general programme on the selective transformation of fatty acids (or of methyl esters) and glycerol issued from sunflower oil (1,2). We have studied recently the preparation of esters of glycerol, specially monoglycerides, which are important intermediates for the manufacture of lubricants, emulsifiers, surfactants used in the industries of pharmaceuticals, cosmetics and food,... (3). O II CH2—O—C—R
CH2OH RCOOH + CHOH I CH2OH
•
CHOH I CH2OH
+ H2O
R : 8 to 22 carbon atoms. Monoglycerides are obtained generally from glycerolysis or hydrolysis of triglycerides (4) or from the direct esterification of fatty acids by glycerol.
540 Alcohol esterification is usually catalysed by homogeneous catalysts such as sulfuric acid (5), para-toluenesulfonic acid (6) or bases such as sodium (potassium, ...) hydroxide (carbonate,...) (3,7). Unfortunately, it is well known that these bases favour the production of soaps. Moreover, homogeneous catalysts are corrosive, difficult to separate from the products and lead to excessive wastes (salts). As for the direct esterification of glycerol, previous works have shown that numerous solid and acid oxides could be used as catalysts (6, 8-11). In the presence of: i) tin or zinc chloride which are active at low pressure and at 200°C, glycerol is easily esterified, however dehydration and/or oxidation of glycerol can occur, ii) large pores zeolites, the patent of Aracil and Corma claims that monoglycerides can be obtained with a high selectivity (90%) in the same pressure range but at high temperatures (12). Hoelderich and Siegel also used different types of zeolites for this esterification reaction (13). iii) enzymes (supported on resins), it can be observed that there is either a selective formation of monoglycerides, or a mixture of mono, di and triglycrides (14-16). On the other hand, monoglycerides can be obtained selectively from the reaction of glycidol with fatty acids over an anionic ion-exchange resin (17). The aim of our work is to find a new type of solid catalysts in order to be able to control the selectivity. We present in this paper some results obtained with crosslinked porous polymers. Also, we compare the behaviour of different catalysts, in particular, ion-exchange resins, in the esterification of glycerol with oleic acid. The influence of the nature of the resin as well as its swelling properties are discussed. 2. EXPERIMENTAL 2.1. Catalytic tests The esterification was carried out at atmospheric pressure in a glass reactor equipped with a mechanical stirrer and heated v^th an oil bath. Moreover, we verified that the activity and the selectivity were independent of the stirring rate. The reaction was studied at 90°C during 24 hours. The molar ratio glycerol/oleic acid was 6.3, the weight ratio oleic acid/catalyst 4.5 and the catalyst weight 1 g. At the end of the reaction, the mixture was dissolved into ethanol and analysed with an HPLC equipped with a light scattering detector and an apolar column (Licrospher). The separation of different products was done by a gradient elution. The percentage of each compound was determined by using standardisation methods with methyl laurate as an internal standard. The conversion is expressed as follows ;
Conversion (%) = ^ . o / la
y'Oiki
. c +
iS oleic
Yi: stoichiometric coefficient of the i product ki: response factor of the i product The selectivity of each glycerol ester is the ratio : mono-, di- or triester/(oleic acid)transf-
541 2.2. Catalyst The catalysts used in this study were a zeolite (Zeocat HY 510, Si/Al = 10), a Montmorillonite clay pillared with Titanium species and cationic ion-exchange resins ; a macroporous lER Amberlyst 15 (Rohm & Haas) and resins with a gel structure (K1481 Bayer and Amberlyst 31 - Rohm & Haas). 3. RESULTS AND DISCUSSION 3.1. Comparison of the activity and the selectivity of acid catalysts In the first part of our study, the esterification of glycerol with oleic acid in the presence of different acid solids with a controlled porosity (zeolite, clay, ion-exchange resin) was studied (Table 1). Table 1 Esterification of glycerol with oleic acid. Comparison between various acid solid catalysts. Conversion
Catalysts
Selectivity (%)
(%)
Mono-ester^^^
Di-ester^^^
Tri-ester^^^
Without
1
-
-
-
Zeolite HY510
5
67
27
6
Montmorillonite-Ti
10
71
24
5
K1481 resin
49
78
21
1
ABS*
92
54
46
0
* ABS : benzenesulfonic acid, homogeneous reaction Reaction temperature : 90°C, reaction time : 24 h.
(a)
C17H23COOCH2
.. --
Monoester:
J-
C17H23COOCH2
CHOH Diester: C17H23COOCH CH2OH
C17H23COOCH2
I
I Triester iCizHssCOOCH
CH2OH
C^H23COoiH2
The esterification rate was very low when the reaction was performed without a catalyst. The reaction was much faster in the presence of benzenesulfonic acid. Over a solid catalyst, we observed the selective formation of monooleyl glyceride (mainly of the a form) in the presence of a catalyst since the monoglyceride selectivity was between 60 to 70%, the diester selectivity is of about 25% (mainly a,P €ster), the formation of triester being very small. Moreover, the oleic acid conversion varied significantly with the solid used as a catalyst. Thus, the activity of the HY 510 zeolite and of the titanium-pillared clay was lower than the one obtained over the K1481 cationic resin. The selectivity to monoglyceride in the presence of the K1481 resin was higher than the one observed using benzenesulfonic acid in a homogeneous reaction. These results seem to show that the activity of the catalyst depends mainly on the accessibility of the protonic centres to the oleic acid. Indeed the oleic acid could diffuse
542
slowly inside the bi or the tridimensional structure of the pillared clay or of the zeolite (pore size between 10 and 13 A). Moreover, the hydrophilic character of these materials could favour the glycerol adsorption while inhibiting the oleic acid adsorption and the esterification reaction. By contrast over gel resins, mainly the active centres located on the surface of the microspheres of the resin particles would be involved in the reaction. 3.2. Activity and selectivity of ion-exchange resins As the K 1481 resin for the esterification of glycerol with oleic acid was the most active solid catalyst of the series, we compared different cationic resins whose characteristics are presented in Table 2. Table 2 Characteristics of resins. Resin
K1481
Supplier
Type
Crosslinking
Acidity
Particle size
level^'^ (%)
(meq H'/g)
(mm)
Bayer
gel
8
4.8
powder < 0,05
Amberlyst 31
Rohm & Haas
gel
4
4.8
1.2 to 1.3
AmberlySt 119
Rohm & Haas
gel
8
4.8
1.2 to 1.6
•oporous
12
5.0
1.2 to 1.6
20 "oporous Rohm & Haas macroporous ^^^ % Divinylbenzene (DVB) added to the polymer matrix.
4.8
1.2 to 1.6
Amberlyst 16 Amberlyst 15
The catalytic results (Table 3) show that the catalytic activity varies with the structure of the resin. Indeed, the gel resins are more active than the macroporous resins. The conversion of oleic acid is about 55% in the presence of the Amberlyst 31 catalyst whereas it is only of 35 % with Amberlyst 15 or 16 (macroporous type). Table 3 Esterification of glycerol with oleic acid in the presence of ion exchange resins. Influence of the resin structure. Catalysts
Conversion
Selectivity (%)
(%)
Mono-ester
Di-ester
Tri-ester
K1481
49
78
21
1
Amberlyst 31
54
90
7
3
Amberlyst 16
37
83
12
5
Amberlyst 15
36
75
22
3
Reaction temperature : 90°C, reaction time : 24 h. Formulation, see experimental.
543 Moreover, the selectivity to glycerol monoesters is higher than 90 % in the presence of Amberlyst 31 (4 % of Divinylbenzene (DVB)) and it seems that the crosslinking level influences the selectivity of gel organic polymers. With macroporous resins, the selectivity is much less influenced by the crosslinking density. In macroporous resins, two kinds of active sites can be distinguished : i) the sites on the surface of microparticles or in the macropores which are easily accessible by the reactants, ii) the sites located inside the polymer matrix with a greater acid strength but of limited accessibility. The (ii) sites availability or the diffusion of the reagents to these (ii) sites depends on the crosslinking degree. From the results, it seems that a DVB percentage of about 10 % is the value above which the reaction occurs only over (i) sites. On the other hand, gel resins have only a microporous network, a crosslinking density lower than 10 % and can swell in a polar solvent by solvation of sulfonic acid groups. The swelling of the resin is accompanied by a stretching of the crosslinked hydrocarbon matrix leading to the formation of pseudo-pores (20 to 40 A), the size of which depends on the DVB content. As a result these resins look like shape selective materials and that could explain the significant monoglyceride selectivity obtained with the lightly crosslinked (4 %) Amberlyst 31 sample. 3.3 Influence of reaction conditions on the catalytic properties of gel resins 3.3.1. Change of the activity and of the selectivity with reaction time Figure 1 shows that under standard conditions the oleic acid conversion increases linearly with reaction time and reaches 80 % after 48 hours. Thus, the initial activity is around 0.4mmol.h-i.g-i. 100 80
o
>
e o
60 40 20 4-
10
20
30
40
50
Time (h) Figure 1 : Esterification of glycerol with oleic acid in the presence of the K1481 resin. Conversion of oleic acid versus reaction time. Reaction temperature : 90°C As for as the monoester selectivity is concerned. Figure 2 shows that the formation of monoester decreases with increasing conversion, which is what can be expected from a kinetic point of view. At the same time, the formation of diester increases but the selectivity to glycerol trioleate is still very low.
544 100 80 ^ 60
Monoester
i
I 40 Diester C/2
20
^-4=-r-trir
Triester •
m
—=*t= 40 60 Conversion (%)
20
f
1
80
100
Figure 2 : Esterification of glycerol with oleic acid in the presence of the resin K1481. Selectivity to the esters of glycerol with the conversion of oleic acid. 3.3.2. Effect ofparticle size of the resin Two resins with the same physicochemical characteristics (crosslinking level : 8 % ; exchange capacity : 4.8 meq HVg) were studied. However, Amberlyst 119 has a larger particle size than the K1481 polymer (Table 4). The results show that the K1481 catalyst is more active by a factor of 2 than Amberlyst 119, which indicates that particle size influences the esterification rate. As the K1481 catalyst is a powder, it has a significant outer surface area and acid centres located on this surface are easily accessible. Table 4 Esterification of glycerol with oleic acid in the presence of gel resins. Influence of particle size. Resin
K1481 Amberlyst 119
Particle size
Conversion
Selectivity (%)
(mm)
(%)
Mono-ester
Di-ester
Tri-ester
powder
49
78
21
1
0.65
21
93
3
4
The decrease of the particle size induces a decrease of the osmotic pressure within pores and a faster solvation of the protonic sites. Under these conditions, the properties of the K1481 resin resemble those of a homogeneous catalyst. Thus, the oleic acid is a molecule having a significant lateral chain (CI8) and, when esterified with glycerol, a bulky terminal ester group. Therefore, it cannot or just very slowly diffuse in the micropores of K1481 which has a crosslinking of 8 %. The reaction occurs mainly on the surface of the microspheres. On the other hand, the comparison of the activity of K1481 with that of the Amberlyst 119 resin shows that the sites located on the surface are accessible only by oleic acid since the conversion is much lower with the resin in bead form.
545 The available outer surface on Amberlyst 119 is smaller than that of K1481. It seems that the oleic acid diffusion is inhibited by the Amberlyst 119 catalyst. 3.3,3. Effect of the amount of catalyst The results obtained with the K 1481 resin reported in Table 5 show that the acid conversion normally increases with the increase of the amount of catalyst in the medium. The variations of the activity and the selectivity observed in these experiments are similar to those presented in the previous paragraph concerning the influence of reaction time. It can thus be concluded that there is no significant modification caused by external diffusion phenomena. Table 5 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the amount of catalyst. Selectivity (%)
meq H+/mmol
Conversion
OA
(%)
Mono-ester
Di-ester
Tri-ester
0.15
45
75
22
3
0.30
71
61
35
4
0.60
81
52
47
1
OA : Oleic acid ; Reaction temperature: 90°C ; reaction time : 24 h. 3.3.4. Thermodynamics and influence of the reaction temperature The glycerol esterification vyath oleic acid is exothermic (AH°R = -30 kJ.mol"^) and under our experimental conditions the equilibrium constant is above 270. The conversion decreases as expected with the temperature (see Table 6), leading to an apparent activation energy for the formation of the mono-ester (calculated for low conversion values) around 75 kJ.mor\ Table 6 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the reaction temperature. Temperature
Conversion
(°C)
(%)
Monoester
Diester
Triester
90
5
97
3
0
110
23
97
3
0
140
72
43
57
1
Selectivity (%)
Reaction time : 2 h. The selectivity to monoglyceride is modified when the temperature increases since the diester is formed mainly at 140°C at higher conversion of the acid (Table 6). However, at 140°C, we observed the formation of the by-products resulting from the polycondensation of glycerol as well as from the esterification of polyglycerols. We believe that these new
546 reactions modify significantly the catalytic properties of the resin for the esterification of glycerol. 4. CONCLUSION The present study of the esterification of glycerol by oleic acid over different acid solids showed that the ion-exchange resins are the most active catalysts for the selective preparation of monooleyl glyceride. The zeolite and the clay used in this study are much less active than the sulfonic acid resins at low temperature. A comparison between macroporous resins and gel-type resins shows that the gel resins are the most active. The macroporous resins, which are strongly crosslinked, do not permit the swelling of the matrix and as a result the reactants cannot reach the internal active sites (the strongest acid centres); consequently the reaction occurs on the outer surface of the microspheres or in the macropores. Moreover, the selectivity to mono-oleyl glyceride is higher than 90 % at a conversion of 50 % over the gel resin Amberlyst 31. This selectivity is due to the structure of the gel resin which can easily swell in the glycerol favouring the diffusion of reactants to the internal sites. 5. ACKNOWLEDGEMENTS This study was carried out within the framework of a European Program concerning the valorisation of natural feedstoks (AAIR). The authors from the University of Poitiers are very grateful to the European Communities for their financial support and also to the ADEME and the "Region Poitou-Charentes" for their financial support for this research.
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