# Recent Advances in Muconic Acid Extraction Process
## 1 Introduction  
Muconic acid (MA), 2,4-hexadienediotic acid, is an important ingredient in the manufacturing of adipic acid, which serves as a starting point for the synthesis of many other polymers, including bio-plastics, coatings, pharmaceuticals, new resins, agrochemicals, nylon, and food additives [1; 2; 3]. Based on the geometry around the double bonds (positions 2 and 4), three isomers of MA are known: cis,cis-MA, cis,trans-MA, and trans,trans-MA, presented in Figure 1[4].  
Cis_cis-muconic acid is a well-known platform chemical derived from the catechol catabolic pathway that can be produced from aromatic molecules derived from lignin, carbohydrates, and waste plastics [5]. It can be obtained through chemical synthesis, starting from pricey catechol as raw material, but this method requires environmentally sensitive, non-renewable petroleum-based feedstock and large concentrations of heavy metal catalysts, and the final product is a mixture of two isomers (cis,cis-MA and cis,trans-MA), [1]. Numerous attempts have been made to develop a biotechnological technique for the synthesis of muconic acid that is economical, environmentally benign, and sustainable [1,6]. Studies using the strains of _E. coli_[3], _Saccharomyces cerevisiae_[7; 8; 9; 10], _Pseudomonas_[5; 11], and _Pichia occidentalis_[1] have been reported on MA production from different kinds of carbon sources. Phosphoenolpyruvate and erythrose 4-phosphate, precursors of MA  
Figure 1: Representation of muconic acid stereoisomers.  
biosynthesis through the shikimate pathway, were produced in greater quantities by one of the earliest characterized _E. coli_ strains (final titer of 36.8 g/L in the fed-batch fermentation) [5]. Although it has low productivity and needs further development, more research has been carried out regarding the production of MA through an alternative pathway, from chorismate to anthranilate [10].  
In industrial biotechnology, recovery techniques are just as crucial as fermentation (the development of microorganisms that create target chemicals), as they guarantee both efficiency and profitability (downstream processing is responsible for more than half of the expense of generating carboxylic acids through fermentation) of such a process [12]. For the recovery of carboxylic acids from aqueous solutions, a variety of separation techniques are used, including membrane separation, precipitation, adsorption, electrodialysis, liquid-liquid extraction, ion exchange, and reverse osmosis [13]. Each of these approaches has distinct drawbacks, such as low efficiency, high complexity, high expense, high energy consumption, and the production of wastewater and by-products [14].  
The reactive extraction approach has recently drawn more attention as a viable and efficient way to remove carboxylic acids from the fermentation broth and is frequently carried out utilizing a combination of extractants and solvents [13; 14; 15; 16; 17; 18; 19]. The classic systems, which include amines or organophosphorus extractants dissolved in organic solvents, have been improved in a substantial number of research articles over the past few decades in terms of sustainability and efficiency [20; 21; 22; 23; 24; 25; 26].  
The development of alternative production methods based on the use of biomass is necessary to support the present transition from an economy based on fossil fuels to one that is more sustainable and biobased. Because it can be converted into chemicals with broad industrial applications, MA is a desirable platform chemical for the bio-based economy. However, its current production through biosynthesis (using second-generation abundant and inexpensive substrates) requires an effective downstream process. The current analysis offers a thorough and organized summary of recent developments and difficulties in the extraction of MA. A variety of solvents and extractants are presented, along with any limitations and potential solutions. Finally, the possibilities for this field in light of its state, difficulties, and tendencies are explored.