Síntese de ácido adípico via catálise heterogênea: aplicação de polioxometalatos

Abstract

The aim of this work was to study the production of adipic acid from the oxidation of cyclohexene using H2O2 as oxidizing agent and heterogeneous Keggin type polyoxometalate catalysts. KPW-600, KPMo-600, CsPW-600 and CsPMo-600, were synthesized by ion exchange from the respective heteropolyacids (HPMo-200 and HPW-200) and cesium and potassium carbonates. During the synthesis and in the study of the properties of the catalysts, techniques such as SEM-EDS, XRF, XRD, FTIR, Raman, textural analysis, acid-base titration, TGA and XPS were used. The catalytic tests were performed in a batch type reactor under autogenous pressure at 75°C and magnetic stirring with a catalyst/cyclohexene mass ratio of 0.105 w/w. The Raman and XRD results showed that the Keggin structure of the polyoxometalates can be successfully obtained for both heteropolyacids and heteropolysalts. The XRF results confirmed the insertion of the cations (Cs and K) into the heteropolysalt structure. The acid-base titration proved to be adequate for determining the density of total acid sites, since it was performed in an aqueous environment similar to that of the reaction system. All catalysts completely converted cyclohexene within 24 h of reaction, and all catalysts were selective to adipic acid, yielding the following order of adipic acid yield: KPW-600>KPMo-600>CsPW-600>CsPMo-600. The KPW-600 reuse was performed twice consecutively without affecting the adipic acid yield, even though it was calcined at 600°C in muffle after each test. The heteropolysal KPW-600 presented higher acidity and higher concentration of surface species (W-O-W)/W, fundamental parameters for the oxidation reaction of cyclohexene to adipic acid under the conditions of the catalytic tests performed in this work. A reaction scheme was proposed based on the observations of this work and the literature, considering the main path of adipic acid formation from the cyclohexene epoxide, as well as the undesirable parallel reactions of diol rearrangement and allylic oxidation.