ABSTRACT:The development of fundamentally new approaches for lignin depolymerization is challenged by the complexity of this aromatic biopolymer. While overly simplified model compounds often lack relevance to the chemistry of lignin, the use of lignin streams directly, poses significant analytical challenges to methodology development. Ideally, new methods should be tested on model compounds that are complex enough to mirror the structural diversity in lignin, but still of sufficiently low molecular weight to enable facile analysis. In this contribution we present a new class of advanced (β-O-4)-(β-5) dilinkage models that are highly realistic representations of a lignin fragment. Together with selected β-O-4, β-5 and β-β structures, these compounds provide a detailed understanding of the reactivity of various types of lignin linkages in acid catalysis in conjunction with stabilization of reactive intermediates using ethylene glycol. The use of these new models has allowed for identification of novel reaction pathways and intermediates and led to the characterization of new dimeric products in subsequent lignin depolymerization studies. The excellent correlation between model and lignin experiments highlights the relevance of this new class of model compounds for broader use in catalysis studies. Only by understanding the reactivity of the linkages in lignin at this level of detail can fully optimized lignin depolymerization strategies be developed.

Lignin holds the key for maximizing value extraction from lignocellulosic biomass. This is currently hindered by the application of fractionation methods that significantly alter the lignin structure to give highly recalcitrant materials. For this reason, it can be highly beneficial to use less-severe fractionation conditions that allow for efficient extraction of lignin with retention of the β-aryl ether (β-O-4) content. Here, we present a detailed study on mild alcohol-based organosolv fractionation with the aim of understanding how to achieve a balance between efficiency of lignin extraction and the structure of the resulting lignin polymers, using walnut shells as model biomass. Monitoring different extraction conditions reveals how the structure of the extracted lignin changes depending on the extraction conditions in terms of molecular weight, alcohol incorporation, and H/G/S ratios. Moving from ethanol to n-pentanol, it was revealed that, in particular, alcohol incorporation at the benzylic α-position of β-aryl ether units not only plays a key role in protecting the β-O-4 linking motif but more importantly increases the solubility of larger lignin fragments under extraction conditions. This study shows that α-substitution already occurs prior to extraction and is essential for reaching improved extraction efficiencies. Furthermore, αsubstitution with not only bulky secondary alcohols and tertiary alcohols but also chloride was revealed for the first time and the latter could be involved in facilitating α-alkoxylation. Overall, this study demonstrates how by tuning the fractionation setup and conditions, the resulting lignin characteristics can be influenced and potentially tailored to suit downstream demands.

A mild lignin‐first acidolysis process (140 °C, 40 min) was developed using the benign solvent dimethyl carbonate (DMC) and ethylene glycol (EG) as a stabilization agent/solvent to produce a high yield of aromatic monophenols directly from softwood lignocellulose (pine, spruce, cedar, and Douglas fir) with a depolymerization efficiency of 77–98 %. Under the optimized conditions (140 °C, 40 min, 400 wt % EG and 2 wt % H2SO4 to pinewood), up to 9 wt % of the aromatic monophenol was produced, reaching a degree of delignification in pinewood of 77 %. Cellulose was also preserved, as evidenced by a 85 % glucose yield after enzymatic digestion. An in‐depth analysis of the depolymerization oil was conducted by using GC‐MS, HPLC, 2 D‐NMR, and size‐exclusion chromatography, which provided structural insights into lignin‐derived dimers and oligomers and the composition of the sugars and derived molecules. Mass balance evaluation was performed.

Abstract:The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields, under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy to handle metal triflates. Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2 acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol, or methyl aromatics obtained by catalytic decarbonylation. Notably, when the former method was ultimately tested on lignin, especially Fe(OTf)3 proved very effective and the phenolic C2 acetal products were obtained in an excellent, 19.3 ±3.2 Wt% yield.

The development of fundamentally new valorization strategies for lignin plays a vital role in unlocking the true potential of lignocellulosic biomass as sustainable and economically compatible renewable carbon feedstock. In particular, new catalytic modification and depolymerization strategies are required. Progress in this field, past and future, relies for a large part on the application of synthetic model compounds that reduce the complexity of working with the lignin biopolymer. This aids the development of catalytic methodologies and in‐depth mechanistic studies and guides structural characterization studies in the lignin field. However, due to the volume of literature and the piecemeal publication of methodology, the choice of suitable lignin model compounds is far from straight forward, especially for those outside the field and lacking a background in organic synthesis. For example, in catalytic depolymerization studies, a balance between synthetic effort and fidelity compared to the actual lignin of interest needs to be found. In this Review, we provide a broad overview of the model compounds available to study the chemistry of the main native linking motifs typically found in lignins from woody biomass, the synthetic routes and effort required to access them, and discuss to what extent these represent actual lignin structures. This overview can aid researchers in their selection of the most suitable lignin model systems for the development of emerging lignin modification and depolymerization technologies, maximizing their chances of successfully developing novel lignin valorization strategies.