International audienceSelective hydrogenation of ethyl esters of traditional sunflower oil was carried out at low temperature (40 degreesC) in the presence of supported monometallic catalysts containing noble metals Pd, Pt or Ru. The palladium catalysts prove to be the most active, but isomerisation is quite important. The selectivity in cis C18:1 ester was improved by the modification of the palladium catalysts by copper and lead. or by addition of amines into the reaction medium

Barbier, J.

Especel, C.

Hoang, L.C.

Marecot, P.

Montassier, C.

Nohair, B.

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Selective hydrogenation of sunflower oil over supported precious metals

Bendaoud Nohair

Catherine Especel

Patrice Marécot

Claude Montassier

Lê Chiên Hoang

Jacques Barbier

Comptes Rendus Chimie

Preliminary communication / CommunicationSelective hydrogenation of sunflower oil oversupported precious metalsBendaoud Nohair a, Catherine Especel a,*, Patrice Marécot a, Claude Montassier a,Lê Chiên Hoang b, Jacques Barbier a,ba LACCO, UMR 6503, université de Poitiers, 40, av. du Recteur-Pineau, 86022 Poitiers cedex, Franceb Valorisation des Agroressources (Valagro), 40, av. du Recteur-Pineau, 86022 Poitiers cedex, FranceReceived 1 August 2003; accepted after revision 30 October 2003AbstractSelective hydrogenation of ethyl esters of traditional sunflower oil was carried out at low temperature (40 °C) in the presenceof supported monometallic catalysts containing noble metals Pd, Pt or Ru. The palladium catalysts prove to be the most active,but isomerisation is quite important. The selectivity in cis C18:1 ester was improved by the modification of the palladiumcatalysts by copper and lead, or by addition of amines into the reaction medium. To cite this article: B. Nohair et al., C. R.Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.RésuméL’hydrogénation sélective d’esters éthyliques d’huile de tournesol classique est réalisée à faible température (40 °C) enprésence de catalyseurs supportés à base de métaux précieux Pd, Pt ou Ru. Les catalyseurs monométalliques à base de palladiums’avèrent être les plus actifs, mais ils sont également très isomérisants. La sélectivité en ester C18:1 cis peut être améliorée enmodifiant les catalyseurs à base de palladium par des ajouts de cuivre et de plomb, ainsi que par des amines introduites dans lemilieu réactionnel. Pour citer cet article : B. Nohair et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.Keywords: Vegetable oils; Selective hydrogenation; Palladium; AminesMots clés : Huiles végétales ; Hydrogénation sélective ; Palladium ; Amines* Corresponding author.E-mail address: catherine.especel@univ-poitiers.fr (C. Especel).C. R. Chimie 7 (2004) 113–118© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.crci.2003.10.0121. IntroductionThe valorisation of vegetable product represents agood alternative as opposed to the ineluctable exhaus-tion of the petroleum reserves. Principal qualities ofthe natural substances are their low ecotoxicity, theirgreat biodegradability, and moreover these resourcesare renewable and very diverse. In this field, vegetableoils are largely used into agro-alimentary industry, butstill little at the industrial level. Indeed, in spite ofefforts of research, these oils still contain too manyunsaturated fatty acids (C18:2 and C18:3), very sensi-tive to oxidation, which limits their uses in the non-food sector. The oxygen absorption rate of the linolenicacid (C18:3), the linoleic acid (C18:2) and the oleicacid (C18:1) is 800:100:1, respectively. The oleic acidC18:1 in the ‘cis’ isomer form has not only the advan-tage of being stable under oxygen atmosphere to avoidpolymerisation, but moreover it remains liquid at lowtemperatures. So, the applications are as varied aspharmacy, cosmetics, plastics, detergents, and lubri-cants... But these industrial needs require a minimumof 80% in oleic acid, while traditional sunflower oilcontains only 25 to 30%. Currently, the catalysts usedat the industrial level for the hydrogenation of the fattyacids mainly contain nickel. Studies on the hydrogena-tion of different vegetable oils have been also under-taken on catalysts containing copper [1] or copperchromites [2]. These various systems have good selec-tivity, but remain however not very active even at thehigh reaction temperatures used. The aim of thepresent work is to develop catalysts of hydrogenationcontaining low loadings of noble metals.We studied the selective hydrogenation of a C=Cdouble bond of the C18:2 molecules of sunflower oilethyl esters, while trying to avoid formation of C18:1trans isomer and total hydrogenation towards stearicC18:0. Selective hydrogenation was carried out at lowtemperature (40 °C) on ethyl esters of traditional sun-flower oil in the presence of supported catalysts con-taining noble metals Pd, Pt or Ru. In order to improvethe selectivity in cis oleic derivatives C18:1, thesecatalysts were modified by inactive metals or by addi-tion of amines into the reaction medium.2. Experimental2.1. Catalyst preparationThe support (Degussa Aerosil silica, surface area200 m2 g–1) was ground and then sieved to retainparticles with sizes between 0.10–0.04 mm. The prepa-ration of the monometallic Pd/SiO2, Pt/SiO2 andRu/SiO2 catalysts was carried out by impregnation ofthe support by the precursor salts (PdCl2, H2PtCl6 andRuCl3, respectively) in basic medium (pH = 11, cat-ionic exchange). Monometallic catalysts were calcinedin flowing air for 4 h at 300 °C, then reduced in flowingpure hydrogen for 4 h at 500 °C (Pd and Pt catalysts) or400 °C (Ru catalyst). For the three monometallic cata-lysts, the size of most metal particles is around 2.0 nmaccording transmission electron microscopy (TEM)studies.Bimetallic Pd–Cu catalysts were prepared by sur-face redox reaction between hydrogen activated onpalladium particles and the copper salt(Cu(CH3COO)2) dissolved in water (‘catalytic reduc-tion’ method [3]). This preparation method of bimetal-lic catalysts is well known to induce a strong interac-tion between the two metals whatever the preparedbimetallic system [4–6]. A known amount of the pre-reduced palladium catalyst was introduced into a reac-tor under nitrogen and was activated at 300 °C for 1 hunder hydrogen. Then the degassed solution of themodifier precursor in water was introduced onto thecatalyst at room temperature. After 90-min reactiontime under hydrogen bubbling, the solution was fil-tered out and the catalyst was dried overnight at100 °C. Bimetallic Pd–Pb catalysts were prepared alsoby catalytic reduction with Pb(CH3COO)2 as precursorsalt, in methanol as solvent and under hydrogen bub-bling during 1 h. Finally, all the bimetallic catalystswere reduced under hydrogen flow at 300 °C or 500 °Cfor 2 h (2 °C min–1 heating rate). Blank palladiumcatalysts were prepared following the same procedure,but the solution containing the modifier salt was re-placed by a methanol or an aqueous solution.The elemental analysis of metal contents in thecatalysts was determined by the atomic absorptionmethod.2.2. Hydrogenation of sunflower oilClassical refined sunflower oil (from Valagro) wastransformed in presence of ethanol and sodium ethano-late, under 60 °C, into glycerol and ethyl esters. Afterthe separation of the reaction products by decantation,the sunflower oil ethyl esters (SOEE) were washed anddried at 110 °C for 3 h under vacuum. They were then114 B. Nohair et al. / C. R. Chimie 7 (2004) 113–118purified on a molecular distillation apparatus at 120 °C(P < 10–3 mbar) until a high purity near 98%. Thehydrogenation of SOEE was performed in liquid phasein a stirred autoclave, at constant pressure of 10 bar andat low temperature (40 °C) according to an experimen-tal procedure previously implemented in the labora-tory. A known amount of pre-reduced catalyst wasimmersed into 50 ml of solvent (ethanol) without expo-sure to air and introduced into the autoclave. Then amixture of 75 ml of SOEE in 25 ml of solvent waspoured into the autoclave and flushed with nitrogen.When the desired temperature was raised, a 10-barhydrogen pressure was introduced and the stirring wasswitched on (zero time for the reaction). Some experi-ments were also carried out with introduction of aknown amount of amine in the reaction medium. Liq-uid samples were analysed by VPC on a Varian3400CX chromatograph equipped with a flame ionisa-tion detector and a capillary column DB-23 (J&W,30 m, 0.25 mm i.d.), using nitrogen as carrier gas.3. Results and discussionAll the prepared catalysts and the results obtained inSOEE hydrogenation are reported in Table 1.3.1. Monometallic catalystsFirst, the three monometallic catalysts Pd, Pt and Ruwere compared (entries 1–3). For similar particle size(around 2.0 nm), the Pd catalyst shows the best activ-ity, since a 95% conversion is reached after a 13-minreaction time, while Pt and Ru need longer durationsand higher catalyst weights for the same conversion tobe obtained. The order of the activities Pd > Pt > Ruobserved here is the same one as that described else-where [7,8]. On the other hand, the three metals lead tothe formation of significant amounts of trans oleicC18:1 and stearic C18:0 derivatives. However the Pdcatalyst is the most selective to oleic molecules.3.2. Bimetallic catalystsIn order to inhibit the formation of stearic deriva-tives and to improve the selectivity to cis oleic C18:1,the 1-wt% Pd catalyst was modified by addition ofcopper or lead according to the protocol described inthe experimental part. The results obtained on bimetal-lic catalysts are systematically compared to those ob-tained with the corresponding blank test.For the blank test carried out in aqueous mediumunder hydrogen bubbling (entry 4), we notice a signifi-cant drop of the activity in comparison to that of theparent monometallic Pd catalyst. Moreover, the pro-portions in stearic C18:0 and trans oleic C18:1 deriva-tives notably increase. This loss of activity and selec-tivity to cis oleic isomers is probably explained by anenlargement of the metal particle size during the treat-ment in aqueous medium under hydrogen. This sinter-ing phenomenon was already observed under the sameconditions (hydrogen bubbling in aqueous solution) ona Pt/SiO2 catalyst [9]. The authors explained this resultby the instability of the metallic particles onto thesupport, due to a Me–Hads binding energy that prevailsover the Me–support interaction. The migration of theunstable metal particles onto the support would lead tometal growth.The addition of copper to palladium inhibits thehydrogenating activity of palladium (entries 5 and 6),this effect being all the more obvious as the amount ofcopper is high. This result is in agreement with the lowhydrogenating properties of copper at low temperature[1]. However, the addition of copper allows only aslight decrease in the isomerisation toward trans oleicisomers.In the case of lead addition, the main effect observedis a significant drop of the isomerisation toward transoleic derivatives, while the hydrogenating activity ofpalladium is only slightly inhibited (entries 7–9).3.3. Addition of amines in the reaction mediumVarious amounts of amines were introduced into theethanol solvent during the hydrogenation of SOEE onthe 1 wt% Pd catalyst (entries 10–23, the introducedamount of amine is given by the amine volume and the(amine/Pd) molar ratio).The addition of methylamine and cyclic amines(piperidine, pyrrolidine, quinoline) lowers more or lessthe activity of the Pd monometallic catalyst. In the caseof the methylamine (entry 10), for a (amine/Pd) molarratio comparable to other entries one, the 95% conver-sion is not reached even for a 2-h reaction time. Thisphenomenon does not depend on the pKa value of theamine, since for similar (amine/Pd) molar ratios, thepresence of methylamine and quinoline induces the115B. Nohair et al. / C. R. Chimie 7 (2004) 113–118same deactivation, in spite of pKa values equal to10.6 and 4.9 respectively (entries 10 and 16). The threeother studied amines (butylamine, triethylamine andtripopylamine), with pKa values near that of methy-lamine, lead to activities comparable (or even slightlyhigher) to that of the non-modified Pd catalyst, exceptTable 1Hydrogenation of SOEE in the presence of different catalysts supported on silica (T = 40 °C, PH2 = 10 bar, metallic concentration in the reactionmedium = 0.077 × 10–3 mol l–1 [* 0.230 × 10–3 mol l–1])a Treatment in aqueous solvent under hydrogen bubbling.b Treatment in methanol solvent under hydrogen bubbling.116 B. Nohair et al. / C. R. Chimie 7 (2004) 113–118in the case of butylamine for the largest introducedamount (entry 18).The different behaviour of these two series of amineevidences a non-negligible geometric effect of amineaddition in the reaction medium. In fact, the effect ofamines on palladium activity can be considered asresulting of an electronic effect (positive) and a geo-metric one (negative). The geometric effect would pre-vail in the case of the first series of amine (methy-lamine and cyclic amines). Then, a high coverage ofthe catalyst surface by these modifiers, preventing theadsorption of the C18:2 molecules and their subse-quent hydrogenation, can explain the observed deacti-vation. On the other hand, for comparable (amine/Pd)molar ratios, the electronic effect prevails for the sec-ond series of amine (butylamine, triethylamine andtripopylamine). These linear amines with different car-bon chains seem thus to be able to adsorb on thesurface of catalyst by leaving sufficient accessiblemetal sites for the hydrogenation of the linoleic mol-ecules. The positive electronic effect is related to theelectron donor character of amines, which wouldweaken the adsorption of the linoleic ester on palla-dium surface. Indeed, kinetic studies have shown azero order for the hydrogenation of this ester, meaninga strong adsorption on the metal.Whatever the nature of the amine, the selectivity inC18:1 is improved compared to the non-modifiedmonometallic Pd catalyst, since the formation ofstearic derivatives is inhibited. This beneficial effect ofthe addition of amines on the selectivity can be alsoexplained by an electronic effect: the amines by elec-tron donor effect would increase the electron density ofthe metal particles, and thus would weaken the adsorp-tion of the C18:1 molecules on the catalyst surface.Beneficial effects of nitrogen-containing compoundsaddition were already observed during the selectiveliquid phase hydrogenation of alkynes, explained inthe same way by a decomplexation or predominantligand effect [10–13]. Moreover, the addition ofamines in the reaction medium limits the cis–transisomerisation, but also the position isomerisation ofthe C=C double bond of the oleic molecules afterhydrogenation of the C18:2. Indeed, the distribution ofthe C18:1 isomers obtained for 100% conversion of theC18:2 molecules on the 1 wt%Pd catalyst shows thepresence of oleic isomers with a C=C double bond inthe original 9 and 12 positions, but also in 10, 11 and13 positions (Fig. 1a). On the example displayed inFig. 1b, the addition of triethylamine in the reactionmedium ((amine/Pd)molar = 250) lowers the amount ofC18:1 molecules with the C=C in 10 and 11 positions.In conclusion, the coordination of Pd with amine intro-duced in the reaction medium inhibits isomerisation toconjugated dienes and hence the formation of posi-tional isomers, without affecting catalytic activity insome cases.4. ConclusionFrom the above results concerning the hydrogena-tion of SOEE over silica-supported precious metals,the following conclusions can be put forward:• the activity decreases in the order Pd > Pt > Ru;the three metals are not selective in cis oleic iso-mers since trans oleic and stearic derivatives wereproduced;Fig. 1. Distribution of the C18:1 isomers (position of the C=C double bond) for 100% conversion of the C18:2 molecules on: (a) 1 wt%Pdcatalyst; (b) 1 wt%Pd catalyst + triethylamine ((amine/Pd)molar = 250)117B. Nohair et al. / C. R. Chimie 7 (2004) 113–118• the addition of copper or lead to palladium al-lowed us to promote the selectivity toward the cisoleic isomers, but the catalyst is less active thanthe monometallic palladium;• the best results were obtained by addition ofamines in the reaction medium; in some cases, theselectivity in cis oleic derivatives is significantlyimproved without loss of activity.References[1] N. Ravasio, F. Zaccheria, M. Gargano, S. Recchia, A. Fusi,N. Poli, R. Psaro, Appl. Catal. A 233 (2002) 1.[2] C. Fragale, M. Gargano, N. Ravasio, M. Rossi, I. Santo, Inorg.Chim. Acta 82 (1984) 157.[3] J. Barbier, Advances in Catalyst Preparation, Study Number4191CP, Catalytica Studies Division, Mountain View, 1992.[4] C.L. Pieck, P. Marécot, J. Barbier, Appl. Catal. A 145 (1996)323.[5] G. Lafaye, C. Micheaud-Especel, C. Montassier, P. Marécot,Appl. Catal. A 230 (2002) 19.[6] F. Epron, F. Gauthard, J. Barbier, Appl. Catal. A 237 (2002)253.[7] G. Cecchi, J. Castano, E. Ucciani, Rev. Fr. Corps Gras 26 (10)(1979) 391.[8] G. Cecchi, J. Castano, E. Ucciani, Rev. Fr. Corps Gras 27 (8/9)(1980) 387.[9] A. Douidah, P. Marécot, J. Barbier, Appl. Catal. A 225 (2002)11.[10] J.-P. Boitiaux, J. Cosyns, S. Vasudevan, Appl. Catal. 15 (1985)317.[11] J.-P. Boitiaux, J. Cosyns, E. Robert, Appl. Catal. 49 (1989)219.[12] J.-P. Boitiaux, J. Cosyns, E. Robert, Appl. Catal. 49 (1989)235.[13] T. A. Nijhuis, G. Van Koten, J. A. Moulijn, Appl. Catal. A 238(2003) 259.118 B. Nohair et al. / C. R. Chimie 7 (2004) 113–118

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Selective hydrogenation of sunflower oil over supported precious metals

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