2,5‐furandicarboxylic acid (FDCA) is a potential non‐phthalate based bio‐renewable substitute for terephthalic acid‐based plastics. Herein, we present an investigation of the oxidation rate of 5‐hydroxymethylfurfural (HMF) to FDCA in acetic acid medium using Co/Mn/Br catalyst. Transient concentration profiles of the reactant (HMF), intermediates [2,5‐diformylfuran (DFF), 5‐formyl‐2‐furancarboxylic acid (FFCA)], and the desired product (FDCA) were obtained for this relatively fast reaction in a stirred semi‐batch reactor using rapid in‐line sampling. Comparison of the effective rate constants for the series oxidation steps with predicted gas–liquid mass transfer coefficients reveals that except for the FFCA → FDCA step, the first two oxidation steps are subject to gas–liquid mass transfer limitations even at high stirrer speeds. Novel reactor configurations, such as a reactor in which the reaction mixture is dispersed as fine droplets into a gas phase containing oxygen, are required to overcome oxygen starvation in the liquid phase and further intensify FDCA production. © 2016 American Institute of Chemical Engineers AIChE J, 63: 162–171, 2017

The Co/Mn/Br catalyzed oxidation of p-xylene to terephthalic acid (TPA) is demonstrated in CO 2 -expanded solvents at temperatures lower than those of the traditional Mid-Century (MC) process. As compared with the traditional air (N 2 /O 2 ) oxidation system, the reaction with CO 2 /O 2 mixture at 160• C and using an additional inert gas (N 2 or CO 2 ) pressure of 100 bar increases both the yield of TPA and the purity of solid TPA via a more efficient conversion of the intermediates, 4-carboxybenzaldehyde and p-toluic acid. At the same time, the amount of yellow colored by-products in the solid TPA product is also lessened, as determined by spectroscopic analysis. Equally important, the decomposition or burning of the solvent, acetic acid, monitored in terms of the yield of the gaseous products, CO and CO 2 , is reduced by ca. 20% based on labeled CO 2 experiments. These findings broaden the versatility of this new class of reaction media in homogeneous catalytic oxidations by maximizing the utilization of feedstock carbon for desired products while simultaneously reducing carbon emissions.

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Kinetic investigations of the liquid phase oxidation of p-xylene (pX) to terephthalic acid (TPA) with Co/Mn/Br catalyst were performed in a stirred 50 mL Parr reactor at 200°C and 15 bar pressure under conditions wherein product precipitation is avoided. The oxidant (O 2 ) was introduced by sparging into the liquid phase at constant gas-phase O 2 partial pressure. Apparent kinetic rate constants, estimated by regressing experimental conversion data to a pseudo-first order lumped kinetic model, are at least an order of magnitude greater than those reported in the literature for similar catalytic reactions. We attribute this difference to the presence of gas−solid and liquid−solid mass transfer resistances in the previous studies wherein the TPA product precipitates as it forms, trapping intermediate products and slowing down their oxidation rates. Our results also indicate that it is not possible to completely eliminate the gas−liquid mass transfer limitations associated with the fast intermediate oxidation steps, even when operating without solids formation and at high stirrer speeds. Other types of reactor configurations are therefore needed to better overcome gas−liquid mass transfer limitations. Systematic studies of bromide concentration effects show that the observed reaction rates become zero order in bromide concentration at sufficiently high bromide levels where the elimination of intermediate 4-(bromomethyl)benzoic acid by oxidation is favored. Further, the rate constants do not show any statistically significant dependence on pX concentration as suggested in other reports involving the traditional three-phase gas−liquid−solid reaction system. This again confirms that the formation of a solid phase hinders the overall oxidation rate, resulting in much smaller apparent rate constants.