## 3 Purification Routes Applied to Lignocellulosic Acid Hydrolysates
### Low Pressure Chromatography  
Several eleuents and a process set-up with numerous steps are used for adsorption: feed loading, rinsing, desorption, regeneration, and equilibration; chromatography requires only one eluent and an easier process set-up--feed loading and elution--which generally leads to lower economic and environmental cost [82]. If two types of particles differ in their adsorption rate, separation of them may be accomplished by selectively adsorbing one species on the sorbent. The reversibility of adsorption allows chromatographic separation of particles contained in a solution [83; 84]. After acid fractionation on lignocellulosic biomass, the chromatographic resin process can be used to separate acid and sugars on a first step as demonstrated by the Arkenol patents for example [20; 66]. Gel type PS-DVB strong acid cation-exchange resin with H\({}^{+}\) as counter-ion was used during this step under chromatographic conditions with water as the eluent [65; 85]. The water flow rate on the resin bed was about 2 to 5 m/h, and the temperature was kept between 40 and 60 \({}^{\circ}\)C. Sugars were slowed down by the resin while sulfuric acid was not retained. On batch elution, depending on the different conditions tested, it was possible to reach 90-93% sugar purity and 90-96% sugar recovery; 95-96% acid purity and 97-99% acid recovery [66]. A plant using this technology is run by BlueFire Renewable in Izumi, Japan, with a capacity of 80,000 L/year of ethanol 99.5% (_v/v_) ([[https://bfreinc.com/](https://bfreinc.com/)]([https://bfreinc.com/](https://bfreinc.com/)), accessed on 25 November 2017). The performance of the chromatographic separation was found to decrease with increasing concentration of sulfuric acid from 20% to 70% (_w/w_) [86]. Besides, as explained previously, lignocellulosic biomass concentrated acid extraction is often followed by a more diluted acid hydrolysis in order to optimize the recovery of the monosaccharides. Therefore, lignocellulosic acid hydrolysates containing 20% (_w/w_) sulfuric acid were often studied as the feed of the chromatographic step for the monosaccharides/acid separation. A process was developed to recycle the acid after the chromatographic separation and send it back to the beginning of the process for the lignocellulosic biomass hydrolysis, and 92% of the acid needed for initial hydrolysis was obtained through recycling [86]. Moreover, the water removed from the acid can be reused as the eluent for the chromatographic separation, decreasing the water consumption of the whole process by 60% [86]. Some fermentation inhibitors (acetic acid, furans) can also be removed from lignocellulosic acid hydrolysates along with sulfuric acid by the chromatographic process. The use of a cation-exchange resin with a PS-DVB matrix induced the elution of sulfuric acid first, then the monomeric sugars, and finally, acetic acid and furans, whereas the use of an anion-exchange resin with a polyvinylpyrrolidone matrix led to the elution of monomeric sugars first, followed by the fermentation inhibitors, and finally, sulfuric acid [87; 88]. After fermentation of the resulting purified sugar mixtures, the ethanol yields were as good as on a synthetic sugar solution and better than the acid hydrolysates purified by overlining [88]. Resins were also studied for the separation of the monomeric sugars constitutive of lignocellulosic acid extracts after acid removal. The adsorption behavior of glucose, xylose, and arabines has been tested on several strong acid cationic exchanger with various cross linking degree (DVB content of 4, 6, and 8%) and different counter-ions (K\({}^{+}\), Ca\({}^{2+}\), Fe\({}^{3+}\)) for the separation of glucose, xylose, and arabines [89]. Resins with Ca\({}^{2+}\) as counter-ions were the most suitable for the separation of the monomeric sugars after adsorption isotherm determination tests; 6% DVB resin being more efficient for arabines/xylose separation and 8% DVB resin being more efficient for xylose/glucose separation [89].