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Cis–trans isomerization of methyl cis-9-octadecenoate in the presence of cobalt tin catalysts
Journal of Molecular Catalysis A: Chemical 306 (2009) 102–106
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Cis–trans isomerization of methyl cis-9-octadecenoate in the presence of cobalt tin catalysts K. De Oliveira Vigier, J. Barrault, Y. Pouilloux ∗ Laboratoire de Catalyse en Chimie Organique, UMR6503 CNRS, Université de Poitiers, ESIP, 40 avenue du recteur Pineau, 86022 Poitiers, France
a r t i c l e
i n f o
Article history: Received 12 September 2008 Received in revised form 20 January 2009 Accepted 21 February 2009 Available online 6 March 2009 Keywords: Cis–trans isomerization Fatty methyl esters Cobalt–tin catalysts Zinc oxide
a b s t r a c t The isomerization of methyl cis-9-octadecenoate (cis isomer) to methyl trans-9-octadecenoate (trans isomer) was studied in the presence of CoSn/ZnO catalysts under hydrogen pressure. The cis–trans isomerization of methyl cis-9-octadecenoate over a CoSn/ZnO catalysts prepared by co-impregnation, was strongly influenced by the nature of the catalytic sites. The support ZnO did not favour this reaction but the hydrogen played an important role since it influenced the relative amount of different cobalt species. In fact cobalt-containing sites (zerovalent species as well as cobalt oxides) predominated over these reactions. The tin species, which acted as a cobalt modifier for the preferential adsorption of the carbonyl group, inhibited the isomerization of the olefinic bond. Such changes could have also been due to a significant diminution of the cobalt content at the catalyst surface especially for a bulk Sn/Co > 1. Consequently, the active sites for the cis–trans isomerization reaction should be cobalt species. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Simultaneous reactions occur during hydrogenation of edible fats, containing carbon–carbon bonds: double bonds can be saturated, migrated or isomerized from cis to trans. The aim, in fat hydrogenation, is to avoid the formation of trans and totally saturated products. In fact, the health effects of trans fats have received increasing attention [1,2] and they are considered to be detrimental like saturated fats [3]. Some results indicate that the TFA (trans fatty acid) can favour cardio-vascular diseases [4,5]. Recently, dietary studies have shown the potentially harmful effects of trans acids on the human LDL/HDL-cholesterol ratio [6,7]. trans isomers usually appear in oil submitted to a thermal treatment at 190 ◦ C. For this reason, there were various attempts to reduce the trans fatty acid content in food products, especially in hydrogenated edible fats [8–10]. For example, Jacobs [11] showed that the formation of harmful trans unsaturated fatty acid can be avoided over zeolites using “their shape selectivity” property. The carbonyl group of the unsaturated fatty ester is specifically adsorbed over the active sites located at the pore aperture of the zeolites to inhibit the adsorption and the consecutive isomerization of olefinic bonds. In the study of the octadecatrienoic acid (C18:3–9c, 12c, 15c) reactivity, Wolf [12] have shown that the cis/trans isomers, formed during the partial hydrogenation of oils, had a ratio varying with the initial oil composition, the catalyst nature and the temperature. The isomerization also occurred over metal catalysts (e.g., Ni, Pd, Pt, Ru, Rh), due to
∗ Corresponding author. Tel.: +33 5 49 45 40 52; fax: +33 5 49 45 33 49. E-mail address: [email protected] (Y. Pouilloux). 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.02.033
the formation of an intermediate compound allowing a free rotation of the C C bond [13–18]. In addition, this transition state led to a shift of double bonds when the hydrogenation rate was slow. In another study, hydrogenation reaction was proven to be responsible for the presence of trans fatty acid isomers in food oils and fats. It was pointed out that hydrogen was required for the isomerization process to occur but was not consumed and the reaction rate was only slightly dependent on the H2 pressure [19]. Similar isomers were found in other lipids too [20]. In this paper, we study the isomerization of methyl cis-9octadecenoate (C18:1c) into methyl trans-9-octadecenoate (C18:1t) occurring during the hydrogenation of fatty esters into the corresponding fatty alcohols (Scheme 1). Cousins and Feuge [21] observed that, in the presence of different solvents, the hydrogenation of methyl cis-9-octadecenoate at 30 ◦ C led to a trans/cis ratio greater than 2:1. Moreover, Mendes et al. [22,23] observed the formation of trans-9-octadecenoic acid (elaidic acid) during the hydrogenation of methyl cis-9-octadecenoate (oleic acid) over RuSn catalysts. Heertje et al. [24] have studied the mechanism of heterogeneous catalytic cis–trans isomerization and double bond migration of methyl cis-9-octadecenoate. They have shown that, in the presence of silica supported Ni catalyst, cis–trans isomerization occurred over different sites on the catalyst surface (NiH2 ). Recently, Moulijn and co-workers [18] have demonstrated that the hydrogenation of fatty methyl esters over palladium on carbon-based monoliths led to the formation of trans isomers due to hydrogen diffusion limitations. The aim of this study is to obtain further information on the active sites responsible for the cis–trans isomerization reaction of methyl cis-9-octadecenoate in hydrogenating conditions over CoSn
K. De Oliveira Vigier et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 102–106
103
Scheme 1. Reaction scheme of hydrogenation of methyl cis-9-octadecenoate into cis-9-octadecen-1-ol.
supported on zinc oxide. The effects of hydrogen pressure in the reaction media and the influence of cobalt and tin were investigated.
The cis/trans isomerization was calculated according to: trans (%) = trans + cis
0 (%X t ) C18:1t 0 0 +%X t ) (%X t C18:1c C18:1t
× 100
2. Experimental
2.2. Catalyst preparation and characterizations
2.1. Catalytic test
2.2.1. Co-impregnation method The catalysts were prepared by co-impregnation of the support zinc oxide (44 m2 /g, Union Minière) with CoCl2 , 6H2 O (5 × 10−4 mol/gcata , Merck) and/or SnCl2 , 2H2 O (5 × 10−4 mol/gcata , Fluka). After the impregnation (24 h), the solids were placed in an oven for 12 h at 80 ◦ C and dried under a nitrogen flow at 120 ◦ C for 2 h. Before characterization and catalytic tests, all the catalysts were reduced with hydrogen at 300 ◦ C for 6 h and passivated with air at room temperature.
The reaction was carried out at 270 ◦ C in a stainless steel batch reactor (300 ml). The reactant (100 ml) and the catalyst (2.2 g) were placed into the reactor in ambient conditions. The reactor was then purged (four times) with nitrogen at 5 MPa and the mixture was stirred continuously. The temperature was slowly increased up to 270 ◦ C at constant pressure (5 MPa). When this temperature was reached, nitrogen was replaced by hydrogen and the pressure was regulated to the desired pressure (0.1 or 8 MPa). The methyl cis-9-octadecenoate sample (Stearinerie Dubois: 80% purity) contains also other methyl esters: C14 = 1.5%, C16 = 15%, C18 = 80%.Liquid samples were mixed with dodecane and analyzed with a GC equipped with a FID and a Chrompack CP Sil-5 column (Length: 25 m; ID: 0.25 mm; film thickness: 0.1 m). The carrier gas was nitrogen. All methyl esters (methyl cis-9-octadecenoate C18:1c, methyl trans-9-octadecenoate C18:1t, methyl cis,cis-9,12octadecadienoate C18:2), heavy esters (mainly C18–C18 or C16–C16 or C16–C18) and saturated esters (methyl octadecanoate) were separated. Heavy esters were synthesised from transesterification reactions (e.g., reaction between methyl cis-9-octadecenoate and cis-9-octadecenol to produce oleic acid oleyl ester). Using a calibration method the weight percentage (Xx) of reactants and products was determined as follows: Xx(%) =
KxAx i
(KiAi)
× 100
where Ki is the constant and Ai the surface area of the compound i. The conversion was defined as follows: conversion(%) =
0 0 0 0 0 +%X t +%X t )−(%X t +%X t ) C18:1c C18:1t C18:2 C18:1c C18:2 0 0 0 t t t (%X +%X +%X ) C18:1c C18:1t C18:2
(%X t
0
× 100
t : weight percentage of C18 ester At time t, the conversion is: %XC18 t at t = 0; %XC18 : weight percentage of C18 ester at t. The activity of the catalyst was defined as the rate of ester transformation, in molester h−1 g−1 .
2.2.2. XPS analysis X-ray photoelectron spectroscopy (XPS) analysis was performed with an SSI model 301 spectrometer with a focused (diameter of irradiated area (600 m)) monochromatic AlK␣ radiation (10 kV, 10 mA) and coupled with a glove-box, which was used for the transfer of samples that were first reduced and then passivated in air for 10 min. The residual pressure inside the analysis chamber was about 5 × 10−8 Pa. The XPS peaks were deconvoluted into subcomponents using a Gaussian (80%)–Lorentzian (20%) curve fitting program with a nonlinear background [25]. The quantitative analysis was completed using the sensitivity factors given by Scofield [26]. An example of the spectra obtained for cobalt and tin species are represented in Fig. 1. 2.2.3. NH3 temperature programmed desorption The powder catalyst was first outgassed at 500 ◦ C under helium during 1 h. After cooling at room temperature, ammonia was first adsorbed and then desorbed by heating the catalyst from 100 to 500 ◦ C at a temperature rate of 4 ◦ C/min. 3. Results and discussion 3.1. Effect of hydrogen pressure on the cis–trans isomerization reaction The isomerization reaction of methyl cis-9-octadecenoate was carried out under 0.1 and 8 MPa of hydrogen in order to study the
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Fig. 3. Cis–trans isomerization of methyl cis-9-octadecenoate in the presence of Co/ZnO, Sn/ZnO and ZnO catalyst.
Fig. 1. XPS Spectra of Co 2p3/2 and Sn 3d5/2 of the CoSn/ZnO (Sn/Coatom = 0.95) catalyst.
influence of hydrogen pressure in the presence of a CoSn/ZnO catalyst having a Sn/Co molar ratio of 1. As shown in Fig. 2, the isomerization reaction is dependent on the hydrogen pressure over the CoSn/ZnO catalyst. The reaction is enhanced with the increase in the pressure. The percentage of trans isomer reached 75% after 15 h under 8 MPa of hydrogen and only 25% at 0.1 MPa. This result corroborates previous results mentioned in the literature [27]. Litchfield et al. [28] have determined an equilibrium mixture containing 75–80% elaidic acid and 20–25% oleic acid. The transformation of methyl cis-9-octadecenoate into methyl trans-9-octadecenoate was lower when a hydrogen pressure of 0.1 MPa was applied. 3.2. Cis–trans isomerization over Co/ZnO, Sn/ZnO and ZnO catalysts In order to determine the nature of the catalytic sites involved in the methyl cis-9-octadecenoate cis–trans isomerization, three catalysts (ZnO, Sn/ZnO, Co/ZnO) were prepared and tested. The ZnO catalyst is slightly active in the cis–trans isomerization of the olefinic bond of methyl cis-9-octadecenoate (Fig. 3).
Fig. 2. Cis–trans isomerization of methyl cis-9-octadecenoate over a CoSn/ZnO catalyst. Effect of hydrogen pressure.
First, the isomerization levels off at 10% until 50% conversion of methyl cis-9-octadecenoate. This is followed by a sharp increase in the [trans/(trans + cis)] ratio, which reached 45% at a conversion of 80%. It is suggested that at the beginning of the reaction, the selective adsorption of methyl cis-9-octadecenoate via the carbonyl group at the zinc oxide surface is preventing the adsorption of C C bond and consequently is limiting the cis–trans isomerization. It is well known that ZnO activates the carbonyl bond due to amphoteric character [29,30] leading to a preferential adsorption of the carbonyl bond during the competition between C O and C C adsorption. The addition of tin to zinc oxide has inhibited the isomerization reaction and the trans isomer appears only at 40% methyl cis-9-octadecenoate conversion instead of 10% over ZnO. At 80% conversion, the [trans/(trans + cis)] ratio reaches only 30% instead of 45% over ZnO. The by-products are mostly heavy esters resulting from the transesterification reaction between methyl oleate and the alcohol formed [31]. No positional isomerization was observed. The isomerization reaction rate was much slower over Sn/ZnO than over zinc oxide. This result confirms that tin promotes the adsorption of the carbonyl bond without detriment to the C C bond, as mentioned by Narasimhan and co-workers [32]. Moreover, due to a decrease in hydrogen adsorption at the catalyst surface, no hydrogenated compounds were produced (Table 1). We suggest that the tin precursor as well as the final tin species modifies the active sites of the support involved in the isomerization step. When cobalt was impregnated over zinc oxide, the rate of the cis–trans isomerization increased significantly by a factor of 10 (Fig. 3). This result indicates that cobalt species promote the C C bond adsorption. Under these conditions, the formation of methyl trans-9-octadecenoate and saturated esters is favoured (Table 1). The chemisorption of the C C bond is in competition with the hydrogen adsorption and this allows a free rotation of the double bond leading to the formation of trans isomer. These results clearly show that tin inhibits the cis–trans isomerization of methyl cis-9octadecenoate, decreasing the relative coverage of the catalyst with the olefinic bond of methyl esters rather than with the carbonyl bond. This assumption was confirmed with the product distribution listed in Table 1. The selectivity to compounds issued from the C C bond hydrogenation (0% of saturated esters) and from C O bond reduction (20% alcohols and heavy esters) allowed us to confirm that over ZnO and Sn/ZnO the main products were the heavy esters (resulting from C O bond reduction) and over Co/ZnO a mixture of methyl trans-9-octadecenoate (42%) and saturated esters (21%) was yielded. The presence of cobalt species favours the adsorption of the C C bond, enhancing the methyl cis-9-octadecenoate cis–trans isomerization. In fact, the main products were the trans isomer and saturated compounds.
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105
Table 1 Products repartition during the isomerization of methyl cis-9-octadecenoate in the presence of ZnO, Sn/ZnO, Co/ZnO and CoSn/ZnO at 25% of conversion. Catalysts
ZnO Sn/ZnO Co2.9 /ZnO Co3.0 Sn2.1 /ZnO Co2.7 Sn5.3 /ZnO Co2.6 Sn10.8 /ZnO
Bulk Sn/Co ratio
– – – 0.35 0.95 2.10
Products repartition (%) C18:1cis esters
C18:1trans esters
C C hydrogenated compoundsa
C O reduced compoundsb
Others
66 57 16 19 51 56
5 4 42 44 12 7
0 0 21 7 0 0
20 22 12 28 26 28
15 17 9 2 11 9
T = 270 ◦ C; PH2 = 8 MPa; 100 ml of methyl cis-9-octadecenoate; 2.2 g of catalyst. a Saturated esters (methyl octadecenoate); b Alcohols (octadecen-1-ol) and heavy esters (octadecenyl octadecenoate).
The same results were observed in the cis–trans isomerization of the unsaturated alcohols (the reduced compounds) in the presence of ZnO, Sn/ZnO and Co/ZnO catalysts. In fact, 100% of trans unsaturated alcohols were produced over Co/ZnO, only 10% in the presence of Sn/ZnO and no trans isomers were observed on ZnO at 60% of conversion. The unsaturated alcohols and the esters seem to be similar in the distribution of the cis and trans isomers. 3.3. Cis–trans isomerization over CoSn/ZnO catalysts – influence of the tin loading The effect of the tin content on the isomerization properties of CoSn supported over zinc oxide was evaluated (Fig. 4) in order to confirm the previous results. Three kinds of Sn/Co catalysts were examined. 3.3.1. CoSn/ZnO catalysts with a low tin content (0 < Sn/Co < 1) In the presence of CoSn/ZnO with a low tin loading, the main reaction was the cis–trans isomerization since the double bond was preferentially adsorbed over cobalt particles as mentioned above. At high conversion, a cis–trans equilibrium was reached with a majority of trans isomers (thermodynamically stable product). The addition of a small amount of tin (Sn/Co = 0.35) did not significantly change the isomerization reaction rate since the [trans/(trans + cis)] ratio was similar to those observed in the presence of the Co/ZnO support (Fig. 4). This is evident from Table 2 that shows the isomerization activity of the catalysts with different Sn/Co molar ratios. Both catalysts had an activity of 20 molester h−1 g−1 . The XPS data in Table 3 shows that on the surface of pure Co/ZnO catalysts the cobalt content is two times as high as in the bulk. When tin is added, the cobalt content at the surface is significantly reduced (0.019) compared to the bulk (0.047) of the catalyst, while bulk and
surface tin contents are rather similar. Nevertheless, the presence of cobalt at the catalyst surface still allows the adsorption of the olefinic bond. The structural changes of cobalt atoms (or clusters) around and in contact with the tin species leads to an inhibition of the hydrogen adsorption and thus a decrease in the hydrogenation reaction rate. The C C bond reaction became then more difficult and the main product was methyl trans-9-octadecenoate. In fact, the chemisorption of the unsaturated carbon bond may proceed through the abstraction of a hydrogen atom leading to the formation of a half-hydrogenated state. A free rotation of the C C bond was then possible and the removal of a hydrogen atom resulted in a complete cis–trans isomerization [33]. This hypothesis was confirmed by the selectivity to saturated compounds displayed in Table 1. The amount of saturated products decreased from 21% to 7% when a small amount of tin was added indicating the poisoning effect of tin on the adsorption of the C C bond. 3.3.2. CoSn/ZnO catalyst with a Sn/Co bulk ratio near 1 When the catalyst was loaded with an equimolar ratio of tin and cobalt, the initial isomerization reaction rate was low. When the bulk Sn/Co ratio varied from 0.35 to 0.95, the isomerization reaction rate strongly decreased as it was divided by a factor 5 as shown in Table 2. XPS results (Table 3) showed a relative increase in the tin content at the catalyst surface which could be due to a partial coverage of cobalt particles with tin species. As reported before, both the isomerization and the olefinic bond hydrogenation were strongly inhibited by tin and saturated products were not obtained (Table 1). The predominance (26%) of alcohols and by-products (heavy esters produced from the reduction of the carbonyl group) indicated that the main reaction was the reduction of C O bond. 3.3.3. CoSn/ZnO catalysts with a Sn/Co bulk ratio higher than 1 With an increase in both bulk and surface tin contents the cis–trans isomerization reaction rate strongly decreased (Fig. 4, Table 2) and the maximum percentage of [trans/(trans + cis)] reached only 40%. The XPS results indicate a significant increase in the Sn/Co ratio from 2.2 to 8.4 at the support surface (Table 3). Moreover, the limitation of hydrogen adsorption is the result of the modification of the electronic properties of the Co species by the tin species. The cis–trans isomerization of octadecenol, the reduced products, was also investigated and Fig. 5 shows curves with a similar trend to those obtained in the isomerization of methyl oleate. 100% Table 2 Cis/trans isomerization reaction activity in the presence of CoSn/ZnO catalysts. Influence of tin content. Sn/Co ratio
Fig. 4. Cis–trans isomerization of methyl cis-9-octadecenoate over CoSn/ZnO catalysts Effect of tin content.
Isomerization reaction activity (molester h−1 g−1 )
0
0.35
22
22
0.95
2.10
5
1
106
K. De Oliveira Vigier et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 102–106
Table 3 Bulk analysis and XPS analysis of CoSn/ZnO catalysts. Effect of tin content. Catalysts CoSn/ZnO
Co2.9 Co3.0 Sn2.1 Co2.7 Sn5.3 Co2.6 Sn10.8
Bulk analysis
XPS analysis
Co/Zn
Sn/Zn
Sn/Co
Co/Zn
Sn/Zn
Sn/Co
Co0 (%)
Sn0 (%)
0.046 0.047 0.046 0.047
0 0.018 0.044 0.100
0 0.35 0.95 2.10
0.092 0.019 0.015 0.009
0 0.021 0.033 0.076
0 1.1 2.2 8.4
0 10 20 10
0 40 35 15
tin species, which acts as a cobalt modifier leading to the preferential adsorption of the carbonyl group, inhibits the isomerization of the olefinic bond. When the tin content is increased (bulk ratio Sn/Co > 1), the cis–trans isomerization rate decreases due to a strong surface enrichment of tin and a decrease in the cobalt surface content. The active sites for the cis–trans isomerization reaction are cobalt species. The presence of ZnO does not improve this reaction as it favours the adsorption of the carbonyl bond. The cis–trans isomerization of the unsaturated alcohols resulting from the reduction of the esters is not observed since trans- and cis-9-octadecen-1ol are directly formed from the reduction of methyl elaidate and methyl oleate, respectively. Acknowledgement Fig. 5. Distribution of trans and cis isomers of 9-octadecen-1-ol produced from the hydrogenation of the esters in the presence of CoSn/ZnO catalysts. Effect of tin content. Table 4 NH3 temperature programmed desorption analysis. Effect of tin content.
References
Sn/Co ratio
NH3 consumption (mol H+ /g)
The authors from the University of Poitiers are very grateful to the French Ministry of Education and Research for a grant to K. De Oliveira-Vigier.
0
0.38
0.95
2.12
630
810
820
760
of trans alcohol is formed over the catalyst with a Sn/Co bulk ratio lower than 1. As for the ester isomerization, a decrease in the trans alcohols was observed when the Sn/Co bulk ratio was increased up to 0.35. Consequently, the hydrogenation rate of trans and cis esters appeared to be similar since the trans and cis distribution of the unsaturated alcohols also followed that of the esters. From these results, the cis–trans isomerization of the unsaturated alcohols formed was not achieved in the presence of these catalysts. In order to gain information on the influence of acidity–basicity of cobalt and tin species on the adsorption of the C O bond, NH3 TPD measurements of the ZnO catalysts were performed. Table 4 shows that the acidic properties (NH3 consumption) did not significantly change with the relative amount of tin. These results suggest that the acidity of the catalyst is not a significant factor in this reaction since a change in cis–trans isomerization activity is observed with the change of the tin loading. 4. Conclusions In this paper, the cis–trans isomerization of methyl cis-9octadecenoate is studied over a series of CoSn/ZnO catalysts prepared by co-impregnation. The activity is found to be strongly dependent on the amount of cobalt and tin at the catalyst surface. Hydrogen has a key role since it influences the relative amount of cobalt species and is necessary to isomerize methyl cis-9octadecenoate. In fact cobalt-containing sites (zerovalent species as well as cobalt oxides) were predominant in these reactions. The
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Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Cis–trans isomerization of methyl cis-9-octadecenoate in the presence of cobalt tin catalysts K. De Oliveira Vigier, J. Barrault, Y. Pouilloux ∗ Laboratoire de Catalyse en Chimie Organique, UMR6503 CNRS, Université de Poitiers, ESIP, 40 avenue du recteur Pineau, 86022 Poitiers, France
a r t i c l e
i n f o
Article history: Received 12 September 2008 Received in revised form 20 January 2009 Accepted 21 February 2009 Available online 6 March 2009 Keywords: Cis–trans isomerization Fatty methyl esters Cobalt–tin catalysts Zinc oxide
a b s t r a c t The isomerization of methyl cis-9-octadecenoate (cis isomer) to methyl trans-9-octadecenoate (trans isomer) was studied in the presence of CoSn/ZnO catalysts under hydrogen pressure. The cis–trans isomerization of methyl cis-9-octadecenoate over a CoSn/ZnO catalysts prepared by co-impregnation, was strongly influenced by the nature of the catalytic sites. The support ZnO did not favour this reaction but the hydrogen played an important role since it influenced the relative amount of different cobalt species. In fact cobalt-containing sites (zerovalent species as well as cobalt oxides) predominated over these reactions. The tin species, which acted as a cobalt modifier for the preferential adsorption of the carbonyl group, inhibited the isomerization of the olefinic bond. Such changes could have also been due to a significant diminution of the cobalt content at the catalyst surface especially for a bulk Sn/Co > 1. Consequently, the active sites for the cis–trans isomerization reaction should be cobalt species. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Simultaneous reactions occur during hydrogenation of edible fats, containing carbon–carbon bonds: double bonds can be saturated, migrated or isomerized from cis to trans. The aim, in fat hydrogenation, is to avoid the formation of trans and totally saturated products. In fact, the health effects of trans fats have received increasing attention [1,2] and they are considered to be detrimental like saturated fats [3]. Some results indicate that the TFA (trans fatty acid) can favour cardio-vascular diseases [4,5]. Recently, dietary studies have shown the potentially harmful effects of trans acids on the human LDL/HDL-cholesterol ratio [6,7]. trans isomers usually appear in oil submitted to a thermal treatment at 190 ◦ C. For this reason, there were various attempts to reduce the trans fatty acid content in food products, especially in hydrogenated edible fats [8–10]. For example, Jacobs [11] showed that the formation of harmful trans unsaturated fatty acid can be avoided over zeolites using “their shape selectivity” property. The carbonyl group of the unsaturated fatty ester is specifically adsorbed over the active sites located at the pore aperture of the zeolites to inhibit the adsorption and the consecutive isomerization of olefinic bonds. In the study of the octadecatrienoic acid (C18:3–9c, 12c, 15c) reactivity, Wolf [12] have shown that the cis/trans isomers, formed during the partial hydrogenation of oils, had a ratio varying with the initial oil composition, the catalyst nature and the temperature. The isomerization also occurred over metal catalysts (e.g., Ni, Pd, Pt, Ru, Rh), due to
∗ Corresponding author. Tel.: +33 5 49 45 40 52; fax: +33 5 49 45 33 49. E-mail address: [email protected] (Y. Pouilloux). 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.02.033
the formation of an intermediate compound allowing a free rotation of the C C bond [13–18]. In addition, this transition state led to a shift of double bonds when the hydrogenation rate was slow. In another study, hydrogenation reaction was proven to be responsible for the presence of trans fatty acid isomers in food oils and fats. It was pointed out that hydrogen was required for the isomerization process to occur but was not consumed and the reaction rate was only slightly dependent on the H2 pressure [19]. Similar isomers were found in other lipids too [20]. In this paper, we study the isomerization of methyl cis-9octadecenoate (C18:1c) into methyl trans-9-octadecenoate (C18:1t) occurring during the hydrogenation of fatty esters into the corresponding fatty alcohols (Scheme 1). Cousins and Feuge [21] observed that, in the presence of different solvents, the hydrogenation of methyl cis-9-octadecenoate at 30 ◦ C led to a trans/cis ratio greater than 2:1. Moreover, Mendes et al. [22,23] observed the formation of trans-9-octadecenoic acid (elaidic acid) during the hydrogenation of methyl cis-9-octadecenoate (oleic acid) over RuSn catalysts. Heertje et al. [24] have studied the mechanism of heterogeneous catalytic cis–trans isomerization and double bond migration of methyl cis-9-octadecenoate. They have shown that, in the presence of silica supported Ni catalyst, cis–trans isomerization occurred over different sites on the catalyst surface (NiH2 ). Recently, Moulijn and co-workers [18] have demonstrated that the hydrogenation of fatty methyl esters over palladium on carbon-based monoliths led to the formation of trans isomers due to hydrogen diffusion limitations. The aim of this study is to obtain further information on the active sites responsible for the cis–trans isomerization reaction of methyl cis-9-octadecenoate in hydrogenating conditions over CoSn
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103
Scheme 1. Reaction scheme of hydrogenation of methyl cis-9-octadecenoate into cis-9-octadecen-1-ol.
supported on zinc oxide. The effects of hydrogen pressure in the reaction media and the influence of cobalt and tin were investigated.
The cis/trans isomerization was calculated according to: trans (%) = trans + cis
0 (%X t ) C18:1t 0 0 +%X t ) (%X t C18:1c C18:1t
× 100
2. Experimental
2.2. Catalyst preparation and characterizations
2.1. Catalytic test
2.2.1. Co-impregnation method The catalysts were prepared by co-impregnation of the support zinc oxide (44 m2 /g, Union Minière) with CoCl2 , 6H2 O (5 × 10−4 mol/gcata , Merck) and/or SnCl2 , 2H2 O (5 × 10−4 mol/gcata , Fluka). After the impregnation (24 h), the solids were placed in an oven for 12 h at 80 ◦ C and dried under a nitrogen flow at 120 ◦ C for 2 h. Before characterization and catalytic tests, all the catalysts were reduced with hydrogen at 300 ◦ C for 6 h and passivated with air at room temperature.
The reaction was carried out at 270 ◦ C in a stainless steel batch reactor (300 ml). The reactant (100 ml) and the catalyst (2.2 g) were placed into the reactor in ambient conditions. The reactor was then purged (four times) with nitrogen at 5 MPa and the mixture was stirred continuously. The temperature was slowly increased up to 270 ◦ C at constant pressure (5 MPa). When this temperature was reached, nitrogen was replaced by hydrogen and the pressure was regulated to the desired pressure (0.1 or 8 MPa). The methyl cis-9-octadecenoate sample (Stearinerie Dubois: 80% purity) contains also other methyl esters: C14 = 1.5%, C16 = 15%, C18 = 80%.Liquid samples were mixed with dodecane and analyzed with a GC equipped with a FID and a Chrompack CP Sil-5 column (Length: 25 m; ID: 0.25 mm; film thickness: 0.1 m). The carrier gas was nitrogen. All methyl esters (methyl cis-9-octadecenoate C18:1c, methyl trans-9-octadecenoate C18:1t, methyl cis,cis-9,12octadecadienoate C18:2), heavy esters (mainly C18–C18 or C16–C16 or C16–C18) and saturated esters (methyl octadecanoate) were separated. Heavy esters were synthesised from transesterification reactions (e.g., reaction between methyl cis-9-octadecenoate and cis-9-octadecenol to produce oleic acid oleyl ester). Using a calibration method the weight percentage (Xx) of reactants and products was determined as follows: Xx(%) =
KxAx i
(KiAi)
× 100
where Ki is the constant and Ai the surface area of the compound i. The conversion was defined as follows: conversion(%) =
0 0 0 0 0 +%X t +%X t )−(%X t +%X t ) C18:1c C18:1t C18:2 C18:1c C18:2 0 0 0 t t t (%X +%X +%X ) C18:1c C18:1t C18:2
(%X t
0
× 100
t : weight percentage of C18 ester At time t, the conversion is: %XC18 t at t = 0; %XC18 : weight percentage of C18 ester at t. The activity of the catalyst was defined as the rate of ester transformation, in molester h−1 g−1 .
2.2.2. XPS analysis X-ray photoelectron spectroscopy (XPS) analysis was performed with an SSI model 301 spectrometer with a focused (diameter of irradiated area (600 m)) monochromatic AlK␣ radiation (10 kV, 10 mA) and coupled with a glove-box, which was used for the transfer of samples that were first reduced and then passivated in air for 10 min. The residual pressure inside the analysis chamber was about 5 × 10−8 Pa. The XPS peaks were deconvoluted into subcomponents using a Gaussian (80%)–Lorentzian (20%) curve fitting program with a nonlinear background [25]. The quantitative analysis was completed using the sensitivity factors given by Scofield [26]. An example of the spectra obtained for cobalt and tin species are represented in Fig. 1. 2.2.3. NH3 temperature programmed desorption The powder catalyst was first outgassed at 500 ◦ C under helium during 1 h. After cooling at room temperature, ammonia was first adsorbed and then desorbed by heating the catalyst from 100 to 500 ◦ C at a temperature rate of 4 ◦ C/min. 3. Results and discussion 3.1. Effect of hydrogen pressure on the cis–trans isomerization reaction The isomerization reaction of methyl cis-9-octadecenoate was carried out under 0.1 and 8 MPa of hydrogen in order to study the
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Fig. 3. Cis–trans isomerization of methyl cis-9-octadecenoate in the presence of Co/ZnO, Sn/ZnO and ZnO catalyst.
Fig. 1. XPS Spectra of Co 2p3/2 and Sn 3d5/2 of the CoSn/ZnO (Sn/Coatom = 0.95) catalyst.
influence of hydrogen pressure in the presence of a CoSn/ZnO catalyst having a Sn/Co molar ratio of 1. As shown in Fig. 2, the isomerization reaction is dependent on the hydrogen pressure over the CoSn/ZnO catalyst. The reaction is enhanced with the increase in the pressure. The percentage of trans isomer reached 75% after 15 h under 8 MPa of hydrogen and only 25% at 0.1 MPa. This result corroborates previous results mentioned in the literature [27]. Litchfield et al. [28] have determined an equilibrium mixture containing 75–80% elaidic acid and 20–25% oleic acid. The transformation of methyl cis-9-octadecenoate into methyl trans-9-octadecenoate was lower when a hydrogen pressure of 0.1 MPa was applied. 3.2. Cis–trans isomerization over Co/ZnO, Sn/ZnO and ZnO catalysts In order to determine the nature of the catalytic sites involved in the methyl cis-9-octadecenoate cis–trans isomerization, three catalysts (ZnO, Sn/ZnO, Co/ZnO) were prepared and tested. The ZnO catalyst is slightly active in the cis–trans isomerization of the olefinic bond of methyl cis-9-octadecenoate (Fig. 3).
Fig. 2. Cis–trans isomerization of methyl cis-9-octadecenoate over a CoSn/ZnO catalyst. Effect of hydrogen pressure.
First, the isomerization levels off at 10% until 50% conversion of methyl cis-9-octadecenoate. This is followed by a sharp increase in the [trans/(trans + cis)] ratio, which reached 45% at a conversion of 80%. It is suggested that at the beginning of the reaction, the selective adsorption of methyl cis-9-octadecenoate via the carbonyl group at the zinc oxide surface is preventing the adsorption of C C bond and consequently is limiting the cis–trans isomerization. It is well known that ZnO activates the carbonyl bond due to amphoteric character [29,30] leading to a preferential adsorption of the carbonyl bond during the competition between C O and C C adsorption. The addition of tin to zinc oxide has inhibited the isomerization reaction and the trans isomer appears only at 40% methyl cis-9-octadecenoate conversion instead of 10% over ZnO. At 80% conversion, the [trans/(trans + cis)] ratio reaches only 30% instead of 45% over ZnO. The by-products are mostly heavy esters resulting from the transesterification reaction between methyl oleate and the alcohol formed [31]. No positional isomerization was observed. The isomerization reaction rate was much slower over Sn/ZnO than over zinc oxide. This result confirms that tin promotes the adsorption of the carbonyl bond without detriment to the C C bond, as mentioned by Narasimhan and co-workers [32]. Moreover, due to a decrease in hydrogen adsorption at the catalyst surface, no hydrogenated compounds were produced (Table 1). We suggest that the tin precursor as well as the final tin species modifies the active sites of the support involved in the isomerization step. When cobalt was impregnated over zinc oxide, the rate of the cis–trans isomerization increased significantly by a factor of 10 (Fig. 3). This result indicates that cobalt species promote the C C bond adsorption. Under these conditions, the formation of methyl trans-9-octadecenoate and saturated esters is favoured (Table 1). The chemisorption of the C C bond is in competition with the hydrogen adsorption and this allows a free rotation of the double bond leading to the formation of trans isomer. These results clearly show that tin inhibits the cis–trans isomerization of methyl cis-9octadecenoate, decreasing the relative coverage of the catalyst with the olefinic bond of methyl esters rather than with the carbonyl bond. This assumption was confirmed with the product distribution listed in Table 1. The selectivity to compounds issued from the C C bond hydrogenation (0% of saturated esters) and from C O bond reduction (20% alcohols and heavy esters) allowed us to confirm that over ZnO and Sn/ZnO the main products were the heavy esters (resulting from C O bond reduction) and over Co/ZnO a mixture of methyl trans-9-octadecenoate (42%) and saturated esters (21%) was yielded. The presence of cobalt species favours the adsorption of the C C bond, enhancing the methyl cis-9-octadecenoate cis–trans isomerization. In fact, the main products were the trans isomer and saturated compounds.
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105
Table 1 Products repartition during the isomerization of methyl cis-9-octadecenoate in the presence of ZnO, Sn/ZnO, Co/ZnO and CoSn/ZnO at 25% of conversion. Catalysts
ZnO Sn/ZnO Co2.9 /ZnO Co3.0 Sn2.1 /ZnO Co2.7 Sn5.3 /ZnO Co2.6 Sn10.8 /ZnO
Bulk Sn/Co ratio
– – – 0.35 0.95 2.10
Products repartition (%) C18:1cis esters
C18:1trans esters
C C hydrogenated compoundsa
C O reduced compoundsb
Others
66 57 16 19 51 56
5 4 42 44 12 7
0 0 21 7 0 0
20 22 12 28 26 28
15 17 9 2 11 9
T = 270 ◦ C; PH2 = 8 MPa; 100 ml of methyl cis-9-octadecenoate; 2.2 g of catalyst. a Saturated esters (methyl octadecenoate); b Alcohols (octadecen-1-ol) and heavy esters (octadecenyl octadecenoate).
The same results were observed in the cis–trans isomerization of the unsaturated alcohols (the reduced compounds) in the presence of ZnO, Sn/ZnO and Co/ZnO catalysts. In fact, 100% of trans unsaturated alcohols were produced over Co/ZnO, only 10% in the presence of Sn/ZnO and no trans isomers were observed on ZnO at 60% of conversion. The unsaturated alcohols and the esters seem to be similar in the distribution of the cis and trans isomers. 3.3. Cis–trans isomerization over CoSn/ZnO catalysts – influence of the tin loading The effect of the tin content on the isomerization properties of CoSn supported over zinc oxide was evaluated (Fig. 4) in order to confirm the previous results. Three kinds of Sn/Co catalysts were examined. 3.3.1. CoSn/ZnO catalysts with a low tin content (0 < Sn/Co < 1) In the presence of CoSn/ZnO with a low tin loading, the main reaction was the cis–trans isomerization since the double bond was preferentially adsorbed over cobalt particles as mentioned above. At high conversion, a cis–trans equilibrium was reached with a majority of trans isomers (thermodynamically stable product). The addition of a small amount of tin (Sn/Co = 0.35) did not significantly change the isomerization reaction rate since the [trans/(trans + cis)] ratio was similar to those observed in the presence of the Co/ZnO support (Fig. 4). This is evident from Table 2 that shows the isomerization activity of the catalysts with different Sn/Co molar ratios. Both catalysts had an activity of 20 molester h−1 g−1 . The XPS data in Table 3 shows that on the surface of pure Co/ZnO catalysts the cobalt content is two times as high as in the bulk. When tin is added, the cobalt content at the surface is significantly reduced (0.019) compared to the bulk (0.047) of the catalyst, while bulk and
surface tin contents are rather similar. Nevertheless, the presence of cobalt at the catalyst surface still allows the adsorption of the olefinic bond. The structural changes of cobalt atoms (or clusters) around and in contact with the tin species leads to an inhibition of the hydrogen adsorption and thus a decrease in the hydrogenation reaction rate. The C C bond reaction became then more difficult and the main product was methyl trans-9-octadecenoate. In fact, the chemisorption of the unsaturated carbon bond may proceed through the abstraction of a hydrogen atom leading to the formation of a half-hydrogenated state. A free rotation of the C C bond was then possible and the removal of a hydrogen atom resulted in a complete cis–trans isomerization [33]. This hypothesis was confirmed by the selectivity to saturated compounds displayed in Table 1. The amount of saturated products decreased from 21% to 7% when a small amount of tin was added indicating the poisoning effect of tin on the adsorption of the C C bond. 3.3.2. CoSn/ZnO catalyst with a Sn/Co bulk ratio near 1 When the catalyst was loaded with an equimolar ratio of tin and cobalt, the initial isomerization reaction rate was low. When the bulk Sn/Co ratio varied from 0.35 to 0.95, the isomerization reaction rate strongly decreased as it was divided by a factor 5 as shown in Table 2. XPS results (Table 3) showed a relative increase in the tin content at the catalyst surface which could be due to a partial coverage of cobalt particles with tin species. As reported before, both the isomerization and the olefinic bond hydrogenation were strongly inhibited by tin and saturated products were not obtained (Table 1). The predominance (26%) of alcohols and by-products (heavy esters produced from the reduction of the carbonyl group) indicated that the main reaction was the reduction of C O bond. 3.3.3. CoSn/ZnO catalysts with a Sn/Co bulk ratio higher than 1 With an increase in both bulk and surface tin contents the cis–trans isomerization reaction rate strongly decreased (Fig. 4, Table 2) and the maximum percentage of [trans/(trans + cis)] reached only 40%. The XPS results indicate a significant increase in the Sn/Co ratio from 2.2 to 8.4 at the support surface (Table 3). Moreover, the limitation of hydrogen adsorption is the result of the modification of the electronic properties of the Co species by the tin species. The cis–trans isomerization of octadecenol, the reduced products, was also investigated and Fig. 5 shows curves with a similar trend to those obtained in the isomerization of methyl oleate. 100% Table 2 Cis/trans isomerization reaction activity in the presence of CoSn/ZnO catalysts. Influence of tin content. Sn/Co ratio
Fig. 4. Cis–trans isomerization of methyl cis-9-octadecenoate over CoSn/ZnO catalysts Effect of tin content.
Isomerization reaction activity (molester h−1 g−1 )
0
0.35
22
22
0.95
2.10
5
1
106
K. De Oliveira Vigier et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 102–106
Table 3 Bulk analysis and XPS analysis of CoSn/ZnO catalysts. Effect of tin content. Catalysts CoSn/ZnO
Co2.9 Co3.0 Sn2.1 Co2.7 Sn5.3 Co2.6 Sn10.8
Bulk analysis
XPS analysis
Co/Zn
Sn/Zn
Sn/Co
Co/Zn
Sn/Zn
Sn/Co
Co0 (%)
Sn0 (%)
0.046 0.047 0.046 0.047
0 0.018 0.044 0.100
0 0.35 0.95 2.10
0.092 0.019 0.015 0.009
0 0.021 0.033 0.076
0 1.1 2.2 8.4
0 10 20 10
0 40 35 15
tin species, which acts as a cobalt modifier leading to the preferential adsorption of the carbonyl group, inhibits the isomerization of the olefinic bond. When the tin content is increased (bulk ratio Sn/Co > 1), the cis–trans isomerization rate decreases due to a strong surface enrichment of tin and a decrease in the cobalt surface content. The active sites for the cis–trans isomerization reaction are cobalt species. The presence of ZnO does not improve this reaction as it favours the adsorption of the carbonyl bond. The cis–trans isomerization of the unsaturated alcohols resulting from the reduction of the esters is not observed since trans- and cis-9-octadecen-1ol are directly formed from the reduction of methyl elaidate and methyl oleate, respectively. Acknowledgement Fig. 5. Distribution of trans and cis isomers of 9-octadecen-1-ol produced from the hydrogenation of the esters in the presence of CoSn/ZnO catalysts. Effect of tin content. Table 4 NH3 temperature programmed desorption analysis. Effect of tin content.
References
Sn/Co ratio
NH3 consumption (mol H+ /g)
The authors from the University of Poitiers are very grateful to the French Ministry of Education and Research for a grant to K. De Oliveira-Vigier.
0
0.38
0.95
2.12
630
810
820
760
of trans alcohol is formed over the catalyst with a Sn/Co bulk ratio lower than 1. As for the ester isomerization, a decrease in the trans alcohols was observed when the Sn/Co bulk ratio was increased up to 0.35. Consequently, the hydrogenation rate of trans and cis esters appeared to be similar since the trans and cis distribution of the unsaturated alcohols also followed that of the esters. From these results, the cis–trans isomerization of the unsaturated alcohols formed was not achieved in the presence of these catalysts. In order to gain information on the influence of acidity–basicity of cobalt and tin species on the adsorption of the C O bond, NH3 TPD measurements of the ZnO catalysts were performed. Table 4 shows that the acidic properties (NH3 consumption) did not significantly change with the relative amount of tin. These results suggest that the acidity of the catalyst is not a significant factor in this reaction since a change in cis–trans isomerization activity is observed with the change of the tin loading. 4. Conclusions In this paper, the cis–trans isomerization of methyl cis-9octadecenoate is studied over a series of CoSn/ZnO catalysts prepared by co-impregnation. The activity is found to be strongly dependent on the amount of cobalt and tin at the catalyst surface. Hydrogen has a key role since it influences the relative amount of cobalt species and is necessary to isomerize methyl cis-9octadecenoate. In fact cobalt-containing sites (zerovalent species as well as cobalt oxides) were predominant in these reactions. The
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