The solution for the resistance to oxidation of p-toluic acid was solved by the discovery of bromide-controlled air oxidation in 1955 that was led to the implementation of AMOCO process [28-31]. In AMOCO process, the oxidation of para-xylene was conducted using a combination of three ions as a homogeneous catalyst which is cobalt, manganese and bromide ions. Acetic acid and oxygen/air were used as solvent and oxidant, respectively . The common bromide ion sources are hydrobromic acid (HBr) and sodium bromide (NaBr). The oxidation operated at 175-225°C and 15-30 bar of oxygen. The terephthalic acid formed mostly in the form of solid due to the low solubility of terephthalic acid in the acetic acid. AMOCO process successfully gives a promising reaction yield, since more than 98% of para-xylene reacted, while terephthalic acid selectivity yield was about 95% in the reaction time of 8-24 hours (Scheme 3).
Scheme 3. AMOCO commercial process of para-xylene oxidation to terephthalic acid .
As shown in Scheme 2, the consecutive oxidations of methyl group take place for p-toluic acid and 4-CBA. In order to gain terephthalic acid as the end product, the purification from impurities including 4-CBA content in crude terephthalic acid must be conducted. In purification process, the crude terephthalic acid (a solid terephthalic acid that has been undergoing centrifugation and drying) will be dissolved in hot water to reduced 4-CBA to p-toluic acid. This reduction process is through catalytic hydrogenation on palladium catalyst. Then, the purified terephthalic acid is obtained . Apart AMOCO process, the other catalytic process to produce terephthalic acid by direct oxidation was widely studied to make it as a chosen and dominant choice for terephthalic acid production industry.
2. Catalytic oxidation of para-xylene
2.1. Homogeneous catalytic oxidation of para-xylene
As mentioned earlier, AMOCO process was conducted in homogeneous liquid phase oxidation. Even the production yield of this process is promising, unfortunately the utilisation of hydrobromide acid (HBr) or sodium bromide (NaBr) as a bromide ion source, causing hazardous reaction condition in terepthalic acid production. This is due to the corrosive nature of HBr. In addition, the bromide sources also accounted for non-environmentally friendly condition, since it is harmful and dangerous to handle .
Therefore, the effort to apply homogeneous catalyst with the utilisation of bromide ion but in the less corrosion nature was conducted. The study in 2008 used CoBr2 and MnBr2 catalysts with the important role of bromide ion as a free radical sources . The high yield of terephthalic acid was achieved (93%) at temperature of 100°C and the Br/Co atomic ratio of three. However, the utilisation of this less corrosive bromide ion still needs an expensive titanium reactor to avoid the corrosion in the reactor. Thus, this making the oxidation...