February 1, 2009
After more than 50 years, there may be a better process for terephthalic acid. One of the fastest growing chemicals in the petrochemical industry is terephthalic acid, the key monomer for producing poly(ethylene terephthalate). The most common process for making terephthalic acid is the liquid-phase oxidation of p-xylene using soluble cobalt and manganese salts as catalysts in conjunction with bromide promoters in acetic acid solvent. This process was developed by Scientific Design in the 1950s; it was licensed to Amoco (now owned by BP) and is used in almost all terephthalic acid plants.
The process has relatively high capital costs because it is necessary to use corrosion-resistant materials of construction in the reactor. The mixture of the bromide promoters and acetic acid is particularly corrosive.
S. A. Hashmi and S. Al-Luhaidan disclose a method for oxidizing p-xylene to terephthalic acid in good yield without the need for a highly corrosive promoter mixture. The promoter they use is an ionic liquid that consists of an organic cation and a bromide or iodide anion. In one example, p-xylene is oxidized with air as the oxidant, acetic acid as the solvent, a combination of Co(OAc)2∙4H2O and Mn(OAc)2∙4H2O as the catalyst, and 1-ethyl-3-methylimidazolium bromide as the promoter. The reaction is run at 215–220 °C for 2 h.
p-Xylene conversion is 100%, with 95.6% selectivity to terephthalic acid. Importantly, no bromine-containing byproducts are detected, nor is any MeBr found in the exhaust gases. The inventors conclude that the bromide remains bound in the ionic liquid and therefore cannot cause corrosion. Unfortunately, no metallurgical corrosion data are given in the patent to validate this supposition. Nevertheless, this is an intriguing approach for improving a process that has been in large-scale use for more than 50 years. (Saudi Basic Industries Corp. [Riyadh]. Eur. Patent 2125686, Dec. 2, 2009; Jeffrey S. Plotkin)
View patent information from CAS.
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