Towards lignin as a feasible feedstock

Lignin is cheap and abundant. At the same time it is molecularly so heterogeneous, that researchers tend to regard conversion into useful intermediate products as a mission impossible. “Don't look at lignin in the plant, look at the supply of pretreated material”, says professor Joseph Bozell, lecturer at the CatchBio Open symposium. Some of the problems are already solved after industrial pretreatment. Bozell sees light at the end of the tunnel towards feasible conversion.

Lignocellulosic biomass consists of (homogenous) cellulose, (slightly heterogeneous) hemicellulose and (heterogeneous) lignin. “Its abundance and its low price make it an attractive sustainable feedstock for the chemical industry”, says professor Joseph Bozell of the University of Tenessee. His research in the C3Bio program is one of the 46 programs involved in the US Government DOE Energy Frontier Research Program.

Branched structure
But there also is a huge drawback. Chemically, lignocellulosic biomass is a tough nut to crack to convert it into chemical raw materials. Efficient cellulose to glucose conversion as a first step towards bio-ethanol production is still highly complex and expensive. Bozell's research group is active in screening a group of metal catalysts that remain stable in the aqueous environment that is typical for biomass conversion.
But Bozell is also very much interested in an even harder problem: the environmentally benign, highly selective conversion of lignin. Lignin is a by-product of the pulp and paper industry and lignocellulosic pretreatment processes, notorious for its branched molecular structure into many substructural units. This industry now simply burns this particular ligning source to generate steam and heat. As feedstock for chemicals and fuel it would have much higher added value applications, but getting there is difficult.

Lignin is what glues together the two carbohydrate polymers cellulose and hemicellulose, and gives the plants its structural strength. In the plant it contains a lot of substructural units around causing problems for catalyst processes. Here, the part 'in the plant' is important. Researchers have until now mainly looked at lignin in this way. But that is hardly relevant, as there is a large supply of lignin available that has already been pretreated. Several substructural units have disappeared in pretreatment. Bozel: “Lignin users need to recognize that the lignin in the biorefinery will not have the same substructural profile as lignin in the plants.”

Not bad at all
Working with pretreated lignin is what Bozell does. His approach involves oxidizing lignin with oxygen using cobalt salen as a catalyst in a process that produces benzoquinones, a raw material for dyes and pharmaceutical intermediates. “Up to recently”, he stresses, “conversion only succeeded with so-called S-units, 25% of the total amount. The challenge is to create improved conditions during catalysis and to find catalysts that convert the G-units as well as the S-units in the feedstock. We have made good progress at that, reaching above 50% conversion of compounds that model G-units present in lignin.”
With actual lignin, this percentage is still unattainable. So far Bozell and his researchers managed to get 6 to 8% of real feedstock converted into benzoquinones. “This reflects the gap between models and reality. Still, it is not bad at all, given the heterogeneity of the feedstock and the fact that what's left over might also be converted.”