Kinetic Investigations of p-Xylene Oxidation to Terephthalic Acid with a Co/Mn/Br Catalyst in a Homogeneous Liquid Phase

Abstract

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.

Aerobic oxidation of benzyl alcohol in a slug flow microreactor: Influence of liquid film wetting on mass transfer

“…section S1), this was also not affected by the inlet benzyl alcohol concentration. Thus, the reaction is considered first order in benzyl alcohol, which is consistent with the reported kinetic studies on the oxidation of p ‐xylene to terephthalic acid using metal bromide catalysts in acetic acid 52 …”

“…The benzaldehyde yield at a given residence time was practically equal in both cases under otherwise the same reaction conditions (Figure 6c), indicating that the reaction rate is not affected by and thus zero order in oxygen. This zero order has been reported for the aerobic Co/Mn/Br catalyzed oxidation of p ‐xylene 52 . Furthermore, this is an additional proof that the reaction rate under the involved conditions was not affected by mass transfer of oxygen, given a considerable increase of when using pure O 2 .…”

Mass transfer and reaction characteristics of homogeneously catalyzed aerobic oxidation of 5-hydroxymethylfurfural in slug flow microreactors

“…This illustrates the absence of mass transfer limitations and furthermore supports that the formation of DFF from HMF is zero order in oxygen. This zero order dependency is consistent with the previous reports on the Co/Mn/Br catalyzed oxidation of benzyl alcohol to benzaldehyde [68], and the oxidation of p-xylene to terephthalic acid using similar metal bromide catalysts in the acetic acid solvent [87]. Also in the previous kinetic studies on the oxidation of HMF, the formation rates of DFF, FFCA and FDCA were assumed zero order in oxygen under elevated pressures (p O2 = 2.8-7.6 bar) [49].…”

Manganese and Cobalt Doped Hierarchical Mesoporous Halloysite-Based Catalysts for Selective Oxidation of p-Xylene to Terephthalic Acid

“…The TPA product is easily isolated from the reaction medium as white needle-like glittering crystals [2]. The best conditions for p-xylene oxidation, found out in academic studies and widely used in industry (AMOCO process) are as follows: temperature of 180-225 • C, oxygen pressure of 15-30 atm, p-xylene to acetic acid molar ratio of 0.08-0.16, and molar ratios Mn/Co and Co:Br being 1:8-10 and 3-4 respectively [2,7,8,[48][49][50]. We have used a temperature of 200 • C, oxygen pressure of 20 atm and molar ratios p-xylene/acetic acid and Mn/Co of 0.08 and 1:10 respectively, both in homogeneous and heterogeneous processes.…”