How to make high-quality renewable lignin

A team of US researchers has found a way to extract high-quality, odourless and valuable lignin from wheat straw.

The lignin could be used to make renewable products to replace plastics and other industrial materials.

Lignin is the name for a class of carbon-based polymers found in the cell walls of plants.

It’s an abundant, fossil-fuel-free source of carbon, and is produced in large amounts from processes like paper making.

But the lignin by-product from paper making is very low quality, having been damaged in the process and thus made from a jumble of different molecules – what chemists call “heterogenous”.

This means it is usually only suitable to burn for fuel or add to low-value materials like cement or animal feed.

“Because of its heterogeneity, lignin can’t be used as a valuable material despite centuries of effort,” says Professor Xiao Zhang, a researcher at Washington State University, US.

“The saying has been that ‘you can make anything out of lignin, except money.’ There’s so much heterogeneity in the molecules that nobody can reliably make things out of it.”

But now Zhang and colleagues have figured out how to get lignin that’s much purer in form.

“This method allows us to extract lignin from plant material in its native form and at a high yield,” says Zhang.

“We were able to demonstrate to industry that it is possible to make colour-neutral and odourless lignin, and we can make quite a bit of the material to begin evaluating its applications.”

Their solvent is a mixture of lactic acid and a compound called pyrazole.

When heated to temperatures between 90°C and 147°C, for between 3 and 12 hours, the solvent could dissolve the lignin in wheat straw, allowing it to be extracted.

The researchers achieved yields as high as 93.7%, with purities up to 98%.

They’ve filed a patent for their technology and are now looking to see if they can speed the process up, or work with less pure ingredients, to help it get commercialised.

The researchers have published their new extraction method in Proceedings of the National Academy of Sciences.

Please login to favourite this article.