Abstract
Fractionation of lignocellulose is a fundamental step in the valorization of cellulose, hemicelluloses, and lignin to produce various sustainable fuels, materials and chemicals. Strong alkaline fractionation is one of the most applied processes since the paper industry has been using it for more than a century, and the mineral acid fractionation process is currently the most applied for the production of cellulosic ethanol. However, in the last decade, mild alkaline fractionation has been becoming increasingly widespread in the frame of cellulosic ethanol biorefineries. It leads to the solubilization of hemicelluloses and lignin at various extent depending on the conditions of the extraction, whereas the cellulose remains insoluble. Some studies showed that the cellulose saccharification and fermentation into ethanol gave higher yields than the mineral acid fractionation process. Besides, contrary to the acid fractionation process, the mild alkaline fractionation process does not hydrolyze the sugar polymers, which can be of interest for different applications. Lignocellulosic mild alkaline extracts contain hemicelluloses, lignin oligomers, phenolic monomers, acetic acid, and inorganic salts. In order to optimize the economic efficiency of the biorefineries using a mild alkaline fractionation process, the purification of the alkaline extract to valorize its different components is of major importance. This review details the conditions used for the mild alkaline fractionation process and the purification techniques that have been carried out on the obtained hydrolysates, with a focus on the yields and purities of the different compounds.
Highlights
Second generation biorefineries essentially aim at valorizing sugar polymers—cellulose and hemicelluloses—from lignocellulosic biomass
Lignin and hemicelluloses were not separated from a mild alkaline extract of sugarcane bagasse by filtration on 1 to 50 kDa organic and inorganic membranes, both compounds being retained over 90% at 20 ◦C on a 10 kDa polysulfone hollow fiber membrane whereas inorganic salts, acetic acid and phenolic monomers passed through the membrane [108]
Both hemicelluloses and lignin are solubilized under the oligomeric form, which opens up different valorization pathways, than monomeric sugars obtained in the acid fractionation process for example
Summary
Second generation biorefineries essentially aim at valorizing sugar polymers—cellulose and hemicelluloses—from lignocellulosic biomass. Kraft and soda processes rely on alkaline chemicals, the former being mainly used for wood hydrolysis, while the latter is commonly applied to non-wood biomass, such as bagasse, straw, grass or bamboo [9] In both processes, lignin, low molecular weight hemicelluloses and other extractives from the wood are dissolved in what is called the black liquor [10]. With the expansion of lignocellulosic biorefineries to produce other materials than paper - energy (liquid fuels like ethanol) and chemical intermediates - and the constraint of limiting the cost of the fractionation step, mild alkaline fractionation gained importance [16,17] It challenges the acid fractionation process, currently used in most second-generation ethanol biorefineries, since improved overall ethanol yields were obtained with mild alkaline fractionation followed by enzymatic saccharification [18] and it can reduce the need for expensive equipment to deal with corrosion and severe reaction conditions [17]. The cost and environmental impact of the different processes are not detailed in this review, only general trends are provided
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
