We will be using a bio-based terephthalic acid produced from bacterial compounds. During our research we found three main pathways to produce a bio-based terephthalic acid. Terephthalic acid can potentially be synthesized from limonenes, D-fructose, or green crude . We will be exploring all three options, but our end choice will be the limonenes based terephthalic acid for long term production, as it is cheaper than D-fructose production and significantly greener than green crude production.
Terephthalic acid can be produced from a group of chemicals called limonenes, which are extracted from the peels of citrus fruits. Limonenes are cheap and numerous as there are few uses and their annual production is around 50,000 tons . Limonene can be dehydrogenated into para-cymene using a lithium catalyst and a suitable solvent . This reaction requires high amounts of lithium, but palladium can be substituted at the expense of yield . Para-cymene can then be oxidized using chromium trioxide, producing terephthalic acid . The only major downside of this process is the toxicity of chromium trioxide, but as of yet, no other effective catalysts have been discovered for the reaction .
Fructose can be used to produce a monomer very close to terephthalic acid and can potentially create terephthalic acid with the right series of reactions. The monomer produced is called 2,5-furandicarboxylic acid and should behave nearly the same as terephthalic acid, with a lower degree of crystallinity. Fructose is dissolved in an acetone-water solution and mixed with a catalytic amount of sulfuric acid . Heat and pressure is applied and the fructose dehydrogenates down to 5-hydroxymethylfurfural . 5-hydroxymethylfurfural can then be oxidized to 2,5-furandicarboxylic acid .These reactions are not without problems, the reagents are more expensive than the oil based alternatives, the reactions have lower yields, and if terephthalic acid itself is desired additional reactions have to take place.
To produce terephthalic acid from green crude, we can run it through existing infrastructure. The production would consist of two key steps: the oxidation of para-xylene and the production of para-xylene from green crude.
Para-xylene can be produced at existing refineries by using green crude. Para-xylene is one of the key products of the BTX (benzene-toluene-xylene) process. Benzene and its related compounds can be produced via steam cracking of the green crude oil, since it's similar to light sweet crude and has a high paraffin content . Steam cracking is essentially the controlled thermal decomposition and reformation of hydrocarbons and under the right conditions can give high yields of BTX products . These products are then separated via the BTX process (a series of highly controlled distillations) and para-xylene is one of the predominate separated products.
The oxidation of para-xylene occurs at terephthalic acid production plants. Around 75% of the world wide production of terephthalic acid is done through the American Amoco process . Para-xylene goes through a liquid-phase air oxidation in the presence of cobalt acetate, manganese acetate, and bromine . This reaction takes place in an acetic acid solution and produces significant quantities of 4-formylbenzoic acid . The solution is then exposed to a palladium catalyst under heat and pressure to hydrogenate most of the 4-formylbenzoic acid to terephthalic acid . While this is not the greenest method of production, it is very cheap and could be used immediately rather than waiting of years of research like the alternatives.
This alternate production process for terephthalic acid would likely make a good stop-gap process to reduce dependency on fossil fuels while bacteria-based methods were developed.
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