Estudo da otimização do processo de hidrogenação catalítica do ácido fumárico

Abstract

The selective production of valuable products for the chemical industry such, as γ-butyrolactone (GBL), tetrahydrofuran (THF) and 1,4-butanediol (BDO) that results from the catalytic hydrogenation of fumaric acid (Ac.FUM) is a quite interesting subject in the field of biomass valorization. In fact, these products can be considered bioplatforms of molecular construction, functioning as a starting point for countless synthesis processes. From the point of view of green chemistry, water and CO2 are considered ideal solvents to drive this type of chemical transformation. Although achieving good conversion rates and selectivity in these reaction media, the catalytic performance of this hydrogenation reaction can be a challenge, even when a suitable catalyst is employed. In this context, this process was studied and optimized, firstly through experimental design using a palladium-rhenium silica-supported catalyst (Pd-Re/SiO2), and a iridium-rhenium silica-supported catalyst (Ir -Re/SiO2), as second choice, barely used in the literature for selective hydrogenation of Ac.FUM in GBL an aqueous medium. The catalysts were synthesized by sequential wet impregnation, and characterized by XRD, RTP, ICP, TEM, surface area and pore volume analysis. Reaction products were identified by GC-FID and HPLC-RI. In a second moment, supercritical CO2 was used as a reaction medium to optimize the process, varying pressure between 105-300 bar under 60-250 ºC. Finally, solvent evaluation was studied in the reaction selectivity, where methanol was also used under 60-250 ºC and 10-300 bar. The catalysts showed metal percentage of 3 % for Pd and Ir, and 6 % for Re, with a surface area of 257 and 219 m2gcat-1, pore volume of 1.31 and 1.20 cm3gcat-1 and average particle size of 2 and 1.5 nm, for Pd-Re and Ir-Re, respectively. TPR, DRX and TEM analyzes suggest that the metals are in close contact, with interaction between them. The results of Ac.FUM hydrogenation using catalysts based on palladium-rhenium and iridium-rhenium showed that GBL was produced with 91 and 90% selectivity, respectively, under the optimized conditions of pressure and temperature predicted by the empirical model of 188 °C and 41 bar of H2 for Pd-Re/SiO2 and 210 °C and 25 bar of H2 for Ir-Re/SiO2. Thus, the experimental design allowed a high selectivity to be obtained in an aqueous medium, providing better rationalization of the combined effect of the temperature and pressure variables. Furthermore, statistical analysis in experimental design showed that temperature has more signifcant effect on GBL selectivity concerning H2 pressure, being non-linear for both catalysts. In studies using supercritical CO2, it was possible to model the catalytic activity, suppressing the formation of by-products and obtaining 89 % and 64% selectivity for GBL using PdRe/SiO2 and Ir-Re/SiO2, respectively, under optimized operating conditions. The addition of methanol as solvent changed the composition of products, obtaining THF as the major product, with a maximum selectivity of 97 % under the conditions of 175 °C, 50 bar of H2 and 150 bar of total pressure for Pd-Re/SiO2, and 89 % under conditions of 175 °C, 50 bar H2 and 300 bar total pressure for Ir-Re/SiO2.