Oxidação do cicloexeno a ácido adípico com catalisador polioxometalato do tipo Keggin: influência da temperatura de calcinação

Abstract

Hexanodioic acid, also known as adipic acid (AA), has several applications, such as: plasticizing agent, food additive, input for the production of Nylon 66, among others. Currently, the commercial production of AA is mainly focused on three distinct catalytic steps in a homogeneous system. The last step of the process occurs by oxidizing a mixture of cyclohexanol and cyclohexanone (called KA oil) with HNO3 as the oxidizing agent. From this stage, two critical points can be highlighted: the difficulty of separating AA from the reaction medium and the detachment of N2O, a potent greenhouse gas, as a by-product of HNO3. In this context, alternative routes for the synthesis of AA are sought, employing a "green" oxidizing agent in a heterogeneous catalytic system. The oxidative cleavage of cyclohexene with polyoxometalate catalyst and hydrogen peroxide as oxidizing agent has been shown to be efficient for the production of AA. Compounds with Keggin-type structure, [PW12O40]3-, are an interesting option as a catalyst, because they have bifunctionality, acidic and oxidizing, in order to obtain good yield in AA in the oxidation reaction of cyclohexene in one step. However, due to its high solubility in reaction medium (aqueous phase), there is the challenge of preparing effectively heterogeneous catalysts with this configuration. Thus, the strategy used to decrease their solubility in aqueous medium was to use higher calcination temperatures in order to synthesize new K3PW12O40 saline polyoxometalates and test them in the oxidation of cyclohexene to AA. The catalysts were synthesized from the heteropolyacid H3PW12O40 by precipitation with K2CO3, resulting in the heteropolysal K3PW12O40. The solid obtained was calcined at 600, 700, 800 and 900°C. The catalysts were characterized by TG, DRX, FTIR, textural analysis, leaching test, acid-base titration and SEM-EDS. It was verified, with the aid of characterizations, that the material KPW900 had the Keggin structure degraded due to the high calcination temperature. The XRD and FTIR results showed that the prepared heteropolysals kept the Keggin structure intact, even when calcined at 800oC. However, a decrease in the acid sites of these catalysts was observed after calcination at 600°C or at higher temperatures. In the catalytic tests, the decrease in the acidity of these materials directly affected the performance in the production of adipic acid, considering that the catalysts with lower acidity presented lower yield to AA in relation to the material that was not calcined.