Lignin from bark as a resource for aromatics production by hydrothermal liquefaction

Abstract

Biorefineries, which are using mostly unused side streams of other existing processes like bark or lignin, have a huge potential to open new resources, for example, chemicals. But with new resources new challenges will be met along the way. These challenges must be addressed and discussed to build a solid and far‐sighted process. This work focuses on the formation of monocyclic compounds like catechol as a valuable product during the hydrothermal liquefaction of beech wood bark as well as Kraft lignin from pine wood like Indulin AT. The focus is to get a better knowledge of the behavior of bark during hydrothermal liquefaction for depolymerization aiming at the production of aromatic building blocks for chemicals. Therefore, the influence, for example, of temperature and reaction time, the chemical reaction pathways, and the therefore necessary analytics need to be understood. Several limitations and challenges of common analytical methods are discussed and compared for bark and Kraft lignin, which is relatively well investigated and can act as a reference material to build a common ground and make it possible to build standards for all bioeconomic processes. Hydrothermal conditions increase the yield and selectivity toward bifunctional molecules like catechol. With rising temperatures and longer retention times, the catechol mass yields get lower. At temperatures above 350°C, nearly no catechol could be found any more. Different types of wood deliver different lignin compositions in terms of the monomeric units. However, it can be observed that different lignins show the same trends in regard to the catechol yield concerning temperatures and reaction time dependence, but overall a different product spectrum.

Integrated lignocellulosic value chains in a growing bioeconomy: Status quo and perspectives

“…Value chains analysed in the modular lignocellulosic biorefinery concept of the Baden‐Württemberg Lignocellulose Research Network. The coloured lines indicate selected value chains described in one of the research articles of this special issue: Seibert‐Ludwig et al () (light blue), Rohde et al () (medium blue), Schuler et al () (dark blue), Siebenhaller et al (), Horlamus et al (), (dark green), Dörsam et al (), Lange et al () (brown). Other data are taken from Lange et al () (medium green)…”

“…The same is true for many other biobased chemicals under development today. A kinetic model has been developed by Schuler, Hornung, Dahmen, and Sauer () that predicts the product composition when hydrothermal liquefaction is applied to different types of biomass, at different reaction temperatures and with different reaction times. As with other lignin depolymerization processes, hydrothermal liquefaction leads to several products.…”

Sub- and Supercritical Water Liquefaction of Kraft Lignin and Black Liquor Derived Lignin

“…The properties of water under sub-and supercritical conditions enhance both the dissolution and degradation of lignin. Lignin degradation occurs via different reactions, such as hydrolysis, alkylation and repolymerization reactions [95,96]. It is well documented that the low solubility of salts in (near) supercritical water leads to pipes getting fouled up and blocked [67,78,[81][82][83][84][85][86].…”

“…The properties of water under sub-and supercritical conditions enhance both the dissolution and degradation of lignin. Lignin degradation occurs via different reactions, such as hydrolysis, alkylation and repolymerization reactions [95,96]. The separation of the liquid product occurs in a phase separator, where the non-polar bio-oil fraction separates from the aqueous fraction containing polar organic compounds, i.e., water soluble organics [61].…”

Biobased value chains for a growing bioeconomy

“…Several contributions to this issue describe how these intermediates can be further processed into final products. Schuler, Hornung, Dahmen, and Sauer () illustrate the use of lignin from bark for the production of aromatics by hydrothermal liquefaction. Wang et al () have developed recombinant Pseudomonas putida strains that use hemicellulose‐derived pentoses or wheat straw hydrolysate as their sole carbon source.…”