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Names 2007, 3rd France-Russia Seminar
“Vegetable acids” for selective precipitation of metallic cations :
a green chemistry application
Rup S.a,b, Zimmermann F.a,b, Meux E.b*, Sindt M.a, Oget N.a* a Laboratoire de Chimie et de Méthodologies pour l'Environnement (LCME) EA 4164 b Laboratoire d'Electrochimie des Matériaux (LEM) UMR CNRS 7555 Université Paul Verlaine-Metz - 1 boulevard Arago, 57078 METZ Cedex 3, France [email protected] ; [email protected] ; [email protected] The aim of this paper is to propose a green chemistry approach for the synthesis of carboxylic acids from biomass and for the selective precipitation of metallic cations contained in industrial wastewaters. This project allows a new way of non-food valorization of vegetable oils and a metal recovery. Introduction
Green chemistry1 is the universally accepted term to describe the movement towards more environment friendly chemical processes replying to sustainable development. It encourages the
design of products and processes that reduce or eliminate the use and the generation of hazardous
substances. The ideal synthesis should be realized in one step with a simple separation, leads to a
quantitative yield and no waste. Moreover it must be safe, environmentally acceptable and based
on “atom efficiency”. Nowadays, for chemical industry, the use of renewable raw materials
becomes increasingly important not only for economical reasons but also for ecological effects.
In oleochemistry, the use of vegetable oils can lead to the synthesis of competitive products2,3.
The fatty acid composition determines the use of the oil. For instance, rapeseed and sunflower
oils are composed by many long chain fatty acids (C18, saturated and unsaturated acids), which
are commonly used for polymers and lubricants. The oxidation of fatty acids leads to mono
and/or di-carboxylic acids which can be easily transform to carboxylates, besides we have
showed that carboxylates can be used for the precipitation of metallic cations4. Surface treatment
and hydrometallurgical leaching processes produce industrial wastewaters containing heavy
metals, the conventional treatment to remove these metals involve a chemical precipitation. The
hydroxide precipitation using lime or caustic is the most commonly used method. This process
leads to sludges without any commercial value, indeed they are stored in specialized landfills
after solidification/stabilization process using hydraulic binders. Contrary to this method, the
selective precipitation of metallic cations by carboxylates5 allows the metal recovery. As a
consequence, the production of carboxylic acids has been studied in accordance with a green
chemistry approach.
1. Fatty acid oxidation
The oxidation of oleic acid 1, from rapeseed and sunflower oils, leads to pelargonic 2 and azelaic 3 acids (Scheme 1). These carboxylic and dicarboxylic acids are industrially produced by Scheme 1 : Oxidative cleavage of oleic acid Names 2007, 3rd France-Russia Seminar
as oxidation by permangate6 or metallic-based catalysts with hydrogen peroxide, peracetic acid, sodium hypochlorite7. Because periodates can be electrochemically regenerated8, the oxidative catalytic system developed by Sharpless9 using 2.2% RuCl3 / 4.1eq NaIO4 with H2O/CH3CN/CCl4 (3/2/2) can be considered to be the most adapted for the oleic acid cleavage. Nevertheless, the toxic and carcinogen carbon tetrachloride must be substituted. Various co-solvents were studied to replace CCl4 for the oleic acid oxidation10. The scission of the carbon double bond was complete after 4h30 and leads to 70% azelaic acid (Table 1, Entry 1). When the Table 1 : Influence of cosolvents on oleic acid oxidation 48h 71% role and acetonitrile prevents the 2h 73% ruthenium in the catalytic cycle8. cyclohexane, acetone or ethyl acetate (Entries 4-6). Table 1 shows clearly that the best result was obtained for the mixture H2O/CH3CN/AcOEt in a 3/2/2 ratio. In order to optimize the reaction and the mixture of solvents H2O/CH3CN/AcOEt, a Design Of Experiment (DOE) was applied. The mixture design was constrained with experimental factors : a minimum of 50 percent of water for the mix of solvent and the reaction was carried out with 2% Aliquat 336® and ultrasounds (20Hz)11. Sonication in liquids is based on acoustic cavitation and associated with an emulsifier, it can increase final product yields and decrease the reaction time12. Table 2 gives some results of this DOE. The oxidation of oleic acid Table 2 : Results of the DOE for the oleic acid oxidation diketone (9,10-dioxostearic acid ) was produced up to 20% yields. The entries 4 and 5, with the binary mixture H2O/CH3CN, show that no diketone has been observed. In the ratio 1/1, azelaic and pelargonic acids were obtained in 81% and 97% yields respectively for a reaction time of 30min (Entry 5). So the best results were obtained for the oxidative system 2,2% RuCl3 / 4,1eq NaIO4 with 2% Aliquat 336® in H2O/CH3CN (1/1) under ultrasounds 20 Hz. This catalytic oxidative system has been applied to other fatty acids. For example, with palmitoleic acid, this reaction led to the heptanoic acid (80% yield) and azelaic acid (83% yield). Palmitoleic acid
Scheme 2 : Oxidation of palmitoleic acid by 2.2%RuCl3 / 4.1eq NaIO4 Names 2007, 3rd France-Russia Seminar
The general procedure for the oleic acid cleavage is the following : a flask is charged with 7mL of solvents, 282mg (1mmol) of oleic acid, 5mg (2.2%) RuCl3 and 877mg (4.1eq) NaIO4. To this mixture, 2% of Aliquat 336® is added and the reaction is carried out under ultrasounds (20 Hz). The final products are extracted by 3 x 20mL of AcOEt, the combined organic extracts are dried with MgSO4 and concentrated. The resulting pelargonic and azelaic acids are purified in warm water. After a second purification azelaic acid can be obtained with a purity of 99%13. 2. Selective precipitation of metallic cations using sodium azelates
In order to predict the feasibility of the separation of metallic cations in mixture by sodium azelate, a theoretical study of the metallic azelate precipitation has been realised. The solubility of Ca, Cu, Ni, Pb and Zn azelates has been determined in pure water at 20°C4. Fig. 1 : Conditional solubility of metallic azelates The solubility of divalent metallic azelates in water depends of pH. Therefore we solubility of metallic azelates (Fig. 1) using diagram, it is possible to predict theoretically the separation of metallic cations in a M mixture. When log([M2]/[M1]) > 2, the separation leads to the precipitation of 99.9% Table 3 shows the calculation results for M1- M2 mixtures. Log([M2]/[M1]) is determined by researching the highest difference between Table 3 : Theoretical selective separation from a theoretical point of view, sodium precipitation of lead for the mixtures Pb2+- 1 Pb2+-Ca2+ 2,49 6,4 2 Pb2+-Ni2+ 2,79 6,4 Ca2+, Pb2+-Ni2+ and copper for Cu2+-Ni2+. Azelate can precipitate selectively not only divalent metallic cations but also trivalent cations such as FeIII, CrIII. Previous studies have showed the possible formation of hydroxycarboxylate14. In this case, the precipitation of iron (III) by azelate leads to different species : Fe2Az3, FeOHAz, Fe(OH)2Az1/2 or a mixture of all these compounds. The stoechiometry of the synthetized compounds depends of the precipitation pH, that make impossible the calculation of conditionnal solubility. For instance at pH 4,5 the solubility of iron-azelate is equal to 1.58x10-6 mol.L-1. Names 2007, 3rd France-Russia Seminar
2.2. Selective precipitation of iron in a FeIII-ZnII mixture We have chosen to experiment the selective precipitation on the FeIII-ZnII mixture corresponding to sulphate leach liquors in zinc hydrometallurgy15. The aim is to remove iron without embedding zinc. Figure 1 shows that this separation is possible (log([Zn]/[Fe]) = 3.55). Different factors have been investigated by using a Design Of Experiment : the pH, the zinc concentration of the solution, the addition speed of the reactant and the precipitate stirring time. The results clearly show that pH is the most influent factor for iron removal. Zinc concentration has a significant effect too. In the best condition (pH = 2), it is possible to remove 99.9% of iron without co-precipitating more than 2.1% of zinc. This process leads to a filtrate with a zinc concentration more than 3.5 g.L-1 and an iron concentration less than 7.3 mg.L-1. From this solution zinc can be recover by electrodeposition. Conclusion
In conclusion, the oxidation of fatty acids has been performed with a ruthenium/periodates catalytic system : the optimisation of co-solvents has been realised by eliminating CCl4 which is carcinogen. The oxidative system, 2.2% RuCl3 / 4.1eq NaIO4 with 2% Aliquat 336® in H2O/CH3CN in ratio 1/1 under ultrasounds 20 Hz, is the most adapted for fatty acid cleavage. The oxidation can be carried out in water with emulsifier and sonication. The final products have been purified in water. The produced carboxylic acids can perform the selective separation of metallic cations. Azelate could be use for the precipitation of lead in Pb2+-Ca2+, Pb2+-Ni2+ and copper in Cu2+-Ni2+. The selective precipitation was realized for FeIII-ZnII mixture. As a result, this process using “vegetable acids” could lead to a selective separation of metallic cations contained in industrial wastewaters: this comes within the context of green chemistry. The oxidation of other reactants such as olefins, cyclo-olefines etc by the oxidative catalytic system RuCl3 / NaIO4 are actually in progress in our laboratory, as well as studies for the selective separation of metallic cations by sodium nonanoate. Acknowledgments : We are grateful to ADEME (the French Agency for Environment and
Energy Management) and the Regional council of Lorraine for their financial support.
References :
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