Hemicellulose is one of the most important fractions in lignocellulose, representing 25-30 wt % of vegetable biomass. It is present in the cell walls of plants and, among others, its most important biological function is to serve as a cohesive medium, interacting with cellulose and lignin, the other two major components of lignocellulose, thus providing strength to the cell walls.
Hemicellulose is an outstanding source of different sugar monosaccharides
Being the second most abundant polysaccharide on earth, after cellulose, hemicellulose shows completely different properties. Unlike cellulose, which is composed solely of glucose units, hemicellulose is a heteropolysaccharide composed of a mixture of different types of monosaccharides. Both hexoses, like glucose, mannose or galactose, and pentoses, like xylose or arabinose, can be found in hemicelluloses, though the latter are usually more abundant in earth plants. However, the composition of hemicelluloses largely varies between plant species, and even from one part to another in the same plant. For instance, hardwoods contain a high amount of glucuronoxylans (xylane and glucuronic acid, a glucose derivative), softwoods usually display a higher amount of galactoglucomannan (polysaccharides formed of galactose, glucose and mannose), and grass plants contain hemicelluloses with a high population of arabinoxylan (arabinose and xylose). Anyhow, hemicellulose is an outstanding source of different sugar monosaccharides, including rare sugars, which are interesting feedstocks to produce a wide variety of biochemicals and bioproducts. Another important difference between cellulose and hemicellulose is their structure. Cellulose is composed of glucose linear polymer chains which form bundles, thus creating a sort of beams for the plant structure. On the contrary, monosaccharides in hemicellulose are linked through different glycosidic bonds, leading to branched polymers barely ordered. This different configuration makes hemicellulose bonds between sugar units readily accessible and easy to be hydrolyzed by both acids and bases, so that its reactivity is much higher than that of cellulose.
Use of Hemicellulose will increase in the future
The worldwide annual production of hemicellulose in the paper pulp industry, one of the most important industries using lignocellulosic biomass as raw material, is about 150 million tons, quite a significant amount. Nevertheless, taking into account emerging industries related to the use of lignocellulosic biomass and its future projection, the amount of available hemicellulose will be, in the close future, about 30 times higher, making hemicellulose one of the most abundant feedstock for industrial purposes. Currently, the use of hemicellulose is limited and relatively small, since it is usually valorized together with lignin through the combustion of what is known as black liquor in the paper pulp industry. Despite this, this energy recovery is not even much interesting, since the high oxygen content of the monosaccharides that make up hemicellulose leads to a low heat power, around half that of lignin. In this context, a chemical use, based on the recovery of hemicellulose, is revealed as a very interesting option for obtaining bioproducts derived from sugars.
Organosolv fractionation is an excellent recover for hemicellulose
Recovering sugar monosaccharides from hemicellulose is much easier than from cellulose because of the much higher reactivity of the former, due to the accessibility of the glycosidic bonds in hemicellulose. Several methods have been developed aiming to recover this fraction, including extraction with simultaneous acid (or alkaline) hydrolysis, extraction in hot compressed water or after steam treatment, use of microwave radiation, etc. Most of these treatments require severe conditions to be effective, and as a consequence of that, a fraction of the starting carbohydrates is degraded. As an alternative to these methods, organosolv fractionation has revealed to be an excellent option to recover hemicellulose.
FRACTION project is devoted to separate the three most abundant fractions of lignocellulose with high purity using dilute acidic GVL/H2O mixtures, which have proved as outstanding media for hemicellulose recovery with minimum sugar degradation. Moreover, the advances of FRACTION in the purification of the recovered hemicellulose stream through solvent extraction allows producing hemicellulose derived monosaccharides suitable for their chemical valorization. These monosaccharides, depending on the starting feedstock and the natural abundance of hexoses and pentoses, can be used to provide C6- or C5 backbone products, including highly versatile platform molecules like hydroxymethyl furan or furfural or several diols and diacids. From this point, a wide variety of industrially interesting bioproducts can be obtained, such as biopolymers (several polyester formulations), or highly biodegradable, renewable, and non toxic green solvents (such as methyl lactate).
Prof. Jose Iglesias, Ph.D.
Full professor of Chemical Engineering
Department of Chemical, Energy and Mechanical Technology
GIQA. ESCET. Universidad Rey Juan Carlos. www.urjc.es
URJC is leader of FRACTION’s WP2 “C6 Hemicellulose valorization”