SELECTIVE EPOXIDATION OF SOYBEAN OIL WITH ACID ZEOLITES

Epoxidation of vegetable oils is a commercially important reaction, because the epoxides obtained from these renewable raw materials have wide applications as plasticizers and polymer stabilizers, such as PVC. Epoxidized oils are natural, non toxic, non corrosive and biodegradable, for these reasons they are useful as substitutes of phthalates, plasticizers derived from petroleum recently banned by EU in many countries due to their toxicity. Moreover, considering the high reactivity of epoxide groups, the epoxidized plant oils also acts as raw materials for the production of a variety of chemicals such as polyols, alkanolamines, and polymers like polyesters and polyurethane. Today, one of the most important epoxidized oil is soybean oil and its production is about of 200000 tons/year. On industrial scale the vegetable oils epoxidation is currently performed in batch reactors, with peracids, generated in situ through the acid catalyzed oxidation of the respective organic acid with hydrogen peroxide. Soluble mineral acids are usually used as catalyst. However, the latter promotes also the side reactions of oxirane ring opening with a dramatic decrease of the selectivity1,2. The use of a heterogeneous catalyst, in alternative to a soluble mineral acids, could allow the suppression of the side degradation reactions, improving the selectivity. An ideal catalyst would be characterized by the presence of Brønsted acid sites, and of a porous structure having a size smaller than that of the natural unsaturated triglycerides molecules. This catalyst would be a porous acid solid characterized by a surface having a hydrophilic character, and by acid sites located inside the pore, that is, accessible only to the components dissolved in the aqueous phase. In this case, the catalyst would promote only the formation of performic acid; this migrates in the oil phase where it spontaneously reacts giving the desired product, while, the undesired ring opening reactions are prevented. The use of acidic ionic exchange resins (AIER) as heterogeneous catalysts was found to minimize the side reactions and improve the selectivity, in addition of an easier recovery of the catalyst at end of the process. Acidic resins allow the isolation of stronger sites inside the pore structure thus the oxirane ring can be protected from the attack of the acid protons. Several polystyrene sulphonic resins, with different amount of divinylbenzene as cross-linking agent, were tested in the epoxidation of vegetable oils 3. Unfortunately, the use of these resins in continuous packed bed reactors is a difficult operation because their strong osmotic swelling, especially with gel-type resins causing a mechanical stress with the consequence of both the breakage of the resin and the increase of the pressure drop. The use of a more dimensionally stable heterogeneous catalyst, that could improve the selectivity allowing the shift from batch to continuous reactor, represents the key point to increase the productivity of the process. High specific area, strong acidity, remarkable (hydro) thermal stability, and the presence of pores of adequate size, comparable with the molecular dimension, should predict that zeolites could be good catalysts for the epoxidation with percaboxylic acids. The use of this type of catalysts in epoxidation is new, although the use of zeolites in acid catalyzed reaction was consolidated. The aim of this work is to evaluate the possibility of using zeolites as heterogeneous catalysts for promoting vegetable oils epoxidation. For this purpose, a preliminary screening of different zeolites was performed considering the diameter size of principal channels. A very high yield than classical process was reached using zeolite Y as solid catalyst. Therefore, different reaction parameters, such as temperature, catalyst and reagent amounts, were investigated, in order to find the optimal conditions to obtain a product that answer to the market requirements. The stability of the catalyst zeolite Y was also studied with reuses runs with satisfactory results.