Liquid phase oxidation of p-xylene to terephthalic acid at medium-high temperatures: multiple benefits of CO2-expanded liquids

Abstract

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.

A spray reactor concept for catalytic oxidation of p-xylene to produce high-purity terephthalic acid

“…Zuo et al [3] investigated the use of CO 2 -expanded liquids (CXLs) for pX oxidation to TPA and found that the reaction with a CO 2 /O 2 mixture (with the maximum O 2 content being 50 vol%)…”

“…Figure 1.4: Structures of major impurities formed during pX oxidation to TPA Over-oxidation of hydrocarbon reactant, intermediates and solvent (also termed as "burning" [40]) along with decarboxylation and decarbonylation form the undesired products such as CO, CO 2 , water, benzoic acid, methyl acetate and methyl bromide. The formation of CO x (CO and CO 2 ) provides a measure of burning reactions and the CO 2 /CO molar ratio (the so-called "burning index") is typically around 3 [3] in the TPA process. It is estimated that 70% of acetic acid produced worldwide is used to manufacture TPA.…”

“…Details of this reactor unit and operation are provided elsewhere [3]. Briefly, 25 mL of acetic acid (~97 wt% in water) solution containing the catalyst components, whose concentrations were identical to those in the spray reactor experiments, was initially charged into …”

“…It is estimated that 70% of acetic acid produced worldwide is used to manufacture TPA. Approximately 5% of the acetic acid is burned per pass [3]. This is equivalent to approximately 0.07 lb of acetic acid burned per pound of TPA produced [6].…”

“…As part of the selective oxidation testbed, p-xylene (pX) oxidation to terephthalic acid (TPA) has been investigated extensively at CEBC in an effort to develop greener alternatives to the conventional Mid-Century process. One of the key published findings to date is that the use of a CO 2 /O 2 mixture instead of the traditional air oxidation system effectively reduces the solvent burning rate [3].…”