Important steps in regeneration of Deep Eutectic Solvents

Efficient regeneration of the Deep Eutectic Solvent used for the delignification of wood, is important to achieve a cost-effective and sustainable new pulping process. Recently, results have been published providing new opportunities to valorize the lignin and hemicellulose based side products.

Deep Eutectic Solvents (DES) are composed of two or more components interacting with each other by self-association forming a eutectic mixture with a melting temperature far below than each of its constituents. This new class of ‘designer solvents’, as discovered only this century, opens a whole new world of opportunities for separation challenges. The many different ‘hydrogen bond donor’ and ‘hydrogen bond acceptor’ combinations possible provide a high variety of DES-systems with different functionalities.

Application

The ISPT DES cluster studies the application of DES for the fractionation of wood into high quality cellulose, lignin and hemicellulose based compounds.

After the DES delignification step, the cellulose fibers for paper making are separated from the DES dark liquor by filtration. The DES dark liquor contains all other components released from the wood being dissolved or stabilized in the DES phase. Before recycling the DES back to delignify new wood, it is key to remove most of the lignin and other wood constituents.

Cellulose and more

Since 2014 the DES cluster developed this mild DES-based wood pulping process – providing cellulose for papermaking – as well as a lignin recovery strategy based on liquid-liquid extraction (LLX). The total process achieved a cellulose fibre yield of about 50%, which is less than the to so-called kraft pulping, the current industrial standard. This lower yield appeared due to the dissolution and break-down of a large part of the hemicellulosic sugars in the DES.

Sugar recovery approaches

Therefore, two years ago the DES cluster started additional research on the recovery of small molecule sugars (monomeric and oligomeric). This research aimed for isolation and valorization of these sugars as well as to improve the regeneration of the DES for reuse.

In addition, the recovery of lignin was performed on larger scale in order to be able to characterize and evaluate its valorization potential. The larger scale lignin recovery studies have been done together with Suster BV. based on earlier work in the PROVIDES consortium. It was known that lignin can be removed by liquid-liquid extraction (LLX) with the biobased solvent 2-methyl tetrahydrofuran (2-MTHF).

The lignin recovery by LLX is based on the more apolar character of the lignin, through which it prefers the apolar 2-MTHF phase over the polar DES phase. Sugars in contrary are highly polar themselves, and have not been detected in 2-MTHF.

As a result, these sugar compounds stay in the DES phase, and when not separated otherwise, they return to the pulping stage.

Technological concepts

In the DES cluster two technological concepts have been investigated: One using membranes for separation of the sugars from the DES, and as an alternative approach, to use in situ conversion of sugars into furans or derivatives thereof. Since furans are molecularly similar to 2-MTHF, these compounds are extracted with the lignin via the LLX or in a separate stage.

Based on the results, the DES cluster concluded that high fructose conversions can be achieved in short times, and that recovery of HMF as valuable byproduct is certainly possible.

Read the full report here.