(255c) Catalyst and Media Alternatives in the Oxidation of P-Xylene to Terepthalic Acid
Selective catalytic oxidation reactions produce multi-billion dollar polymer intermediates (commodity, bulk, specialty, fine chemicals). They typically utilize organic solvents and air as the oxidant. Pure O2 is avoided due to safety concerns. However, the use of air produces enormous volumes of polluted vent gas. Further, the overall conversion may be limited by O2 solubility in the liquid phase.
The oxidation of p-xylene to terephthalic acid (shown above) is a large-scale industrial application, generating high purity fiber-grade product1. Limitations in the commercial process include: solvent (HOAc) loss via burning, safety issues and environmental issues arising from the use of halide and polluted vent gas.
The unique properties of CO2-based media provide opportunities to explore cleaner, safer and economically competitive alternatives to the existing systems by facilitating process intensification advantages with the use of highly enriched oxygen. It has been previously demonstrated2 that CO2 based media can increase O2 solubility, enhance solute transport in liquid phases and improve protection from fire and explosion. This study will explore the use of CO2 -expanded liquids (CXLs) for two catalytic systems- Shell-patented3 Co/Zr and the commercially used MC catalyst comprised of Co/Mn/Br4. The effects of media engineering via addition of CO2 will be discussed in terms of induction period, reaction performance parameters (conversion, selectivity)and product quality, as well as safety. Relevant economic and environmental impact assessment will also be presented.
Reference: 1. Chang-Man P. and Sheehan, R.J., Kirk-Othmer Encyclopedia of Chemical Technology.1996,John Wiley and Sons, Inc. 2. Wei, M. et al., J. Am. Chem. Soc., 124:11 (2002) 2513. 3. Raymond L. J., et al.,US patent #6153790,1998. 4. Chester, A.W. et al., J. Cat, 46, 308-319. Partenheimer W., Cat. Today., 23(1995) 69.