Sustainable method naturally divides wood’s parts.

Synopsis by Wim Thielemans and Wendy Hessler, Jul 22, 2011

vom Stein,T, PM Grande, H Kayser, F Sibilla, W Leitner and P Domınguez de Marıa. 2011. From biomass to feedstock: one-step fractionation of lignocellulose components by the selective organic acid-catalyzed depolymerization of hemicellulose in a biphasic system. Green Chemistry http://dx.doi.org/10.1039/c1gc00002k.

 


2011-0717poplartrees
timabbott/flickr

A single-step process using only natural materials may transform the way lignocellulosic biomass is separated into its prized parts of cellulose, lignin and hemicellullose.

All three are valued raw materials for a wide variety of products and processes, including the cellulose for biofuels. The sustainable procedure can run continuously and is being considered for industrial use.

Current methods to separate the trio use harsh conditions that damage part of the end products and produce too much waste.

 

Context

Plants are a complex mix of materials that differ among various vegetative groups. Corn, wheat, trees and other woody plants contain lignocellulosic biomass. This is made up of three major components: cellulose, lignin and hemicellulose.

Cellulose is a water friendly part of these plants. It provides strength and structure and helps transport water from the roots to the leaves. Lignin is like a water tight glue that binds the cellulose together. It also protects cellulose from degrading under the sun’s ultraviolet rays and decomposing from microbes, bugs and fungi. Hemicellulose binds the cellulose and lignin, which are not very compatible.

Plants naturally make cellulose in staggering quantities, with some estimates topping several hundred billion tons per year. Since cellulose is abundant, it is an obvious target as a source for renewable feedstocks – the raw materials used by industry to produce products. Once processed, cellulose – more commonly known as pulp – is used widely in paper products, cellophane, as food additives and, more recently, in the production of bioethanol.

Lignin currently has only minor industrial uses, for example as dispersants and a feed additive. The majority of lignin is burned to recover the energy that os used in separating it from the biomass. However, it is the only natural source of large amounts of aromatic compounds, which are vital for making plastics – such as polystyrene – and drugs. Therefore,  a large amount of research is focusing on breaking down pure lignin into these aromatic compounds.

Industrial use of lignocellulosic biomass to make chemicals and fuel is hindered by its complex mix of cellulose, lignin and hemicellulose, which are difficult to separate. Harsh processes – high temperatures and caustic chemicals – that degrade the raw materials and make waste are commonly used.

What did they do?

In this study, researchers from Germany describe a very clever way to separate cellulose, lignin and hemicellulose in a single process.

They mixed lignocellulosic biomass – which can be wood, corn or wheat stalks or other cellulose and lignin containing materials – with three natural, benign additives. The additives were: water; oxalic acid – an acid obtained from the simple sugar glucose and found in many plants, including black tea; and 2-methyltetrahydrofuran (2-MTHF), a solvent that can be obtained from biomass.

The concoction was heated to an industrial low temperature of 140 degrees Celsius.

What did they find?

The enzymes and low temperatures work together to separate the lignocellulosic biomass into its component parts. During the process, the oxalic acid breaks down the hemicellulose and turns it into water-soluble sugars. The lignin and cellulose are not affected.

Since lignin and cellulose are very different, they spontaneously separate when the hemicellulose the glue that holds them together is degraded. The cellulose remains as an insoluble pulp that can be filtered off and used as a feed to make bioethanol or other chemicals. The lignin is extracted from the water-oxalic acid mixture using 2-MTHF.

Lignin is recovered by boiling off the 2-MTHF via distillation. This also allows recovery of the 2-MTHF, which is recycled in the process of extracting the lignin.

Equally, the oxalic acid can easily be recovered from the water-sugar mixture. It is recycled back into the process again to break down hemicellulose anew.

So in the end, the process yeilds an inlet stream of lignocellulosic materials and three outlet streams: lignin, cellulose and a mixture of water and sugars.

What does it mean?

Natural additives and lower temperatures offer a more sustainable way to divide certain types of plants into three main components. These lone parts are valuable starting products for many industrial applications.

This process is a very important development since it shows that it is possible to separate lignocellulosic materials using benign conditions and in a straightforward manner. It solves many of problems that plague current methods, such as high temperature, waste generation and product damage.

The researchers also designed the process so it can run continuously. This is a major advantage for industrial use and may make the process very attractive to industry.

However, this is a small scale study and further work will be needed to investigate whether it works as well on a very large scale and for a wide variety of different materials.

Resources

Taherzadeh, MJ and K Karimi. 2009. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production:  review. International Journal of Molecular Sciences, http://dx.doi.org/10.3390/ijms9091621.