Heterogeneously catalyzed reactions with vegetable oils : epoxidation and nucleophilic epoxide ring-opening with alcohols

摘要

Chemical modification of vegetable oils, via epoxidation and epoxide ring-opening with alcohols was studied, using environmentally acceptable methods. Epoxidation of vegetable oils with peracids produced in the presence of heterogeneous acidic catalysts requires low contact between the epoxide and the acidic centers to avoid consecutive reactions. The main by-product is the ketone, obtained from the epoxide rearrangement reaction. Ketone formation is favored by strongly acidic catalysts and by high temperatures. Apolar solvents decrease the ketone formation, polar-protic solvents react with the epoxide and polar-aprotic solvents suppress this reaction. It is proposed that the epoxide rearrangement occurs via an intra-molecular hydride migration. Molecular sieves, as titanium support for the epoxidation of vegetable oils, do not bring any other advantage than the high surface area. Amorphous Ti-SiO2/TBHP (TBHP=tert-butyl hydroperoxide) has similar activity to Ti-MCM-41/TBHP. A high titanium dispersion renders the catalysts Ti-SiO2 more active and apolar-aprotic solvents improve the activity. Ti-SiO2 lacks of Ti-leaching, and it is reusable. Catalysts based on tungstenate anion/hydrotalcite-like materials showed a very high peroxide decomposition rate and very low epoxide yield. A new general reaction system for the epoxide ringûopening reactions with alcohols, under very mild reaction conditions and using heterogeneous catalysts, was developed. It allows the addition of less reactive alcohols such as those with the neo structure. Highly acidic catalysts are advantageous only when small and linear alcohols are added. For bulkier alcohols, highly acid catalysts decrease the selectivity because the quickly protonated epoxide rearranges faster than it reacts with the alcohols. Crystalline aluminosilicates, like Y zeolite, showed very low activity for the epoxide alcoholysis because most of the acidic sites are not accessible to the bulky epoxidized oil. Lamellar aluminosilicates of the montmorillonite type, especially those with a higher amount of Brönsted sites in the silicate layers, showed to be the best catalysts for the epoxide alcoholysis thanks to the enhanced Brönsted-acid site accessibility that the open structure offers. Regarding the effect of the alcohol structure, increasing the alcohol branching degree and/or its molecular size leads to lower reaction rates and lower selectivities.