Christopher S. Lancefield

Isolation of Functionalized Phenolic Monomers through Selective Oxidation and CO Bond Cleavage of the β‐O‐4 Linkages in Lignin

Functionalized phenolic monomers have been generated and isolated from an organosolv lignin through a two-step depolymerization process. Chemoselective catalytic oxidation of β-O-4 linkages promoted by the DDQ/tBuONO/O2 system was achieved in model compounds, including polymeric models and in real lignin. The oxidized β-O-4 linkages were then cleaved on reaction with zinc. Compared to many existing methods, this protocol, which can be achieved in one pot, is highly selective, giving rise to a simple mixture of products that can be readily purified to give pure compounds. The functionality present in these products makes them potentially valuable building blocks.

Advanced Model Compounds for Understanding Acid-Catalyzed Lignin Depolymerization: Identification of Renewable Aromatics and a Lignin-Derived Solvent

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 direct use of lignin streams 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.

Selective modification of the β–β linkage in DDQ-treated Kraft lignin analysed by 2D NMR spectroscopy

The depolymerisation of the biopolymer lignin has the potential to provide access to a range of high value and commodity chemicals. However, research in this increasingly important area of green chemistry is hindered by the lack of analytical methods. The key challenge in using NMR is the throughput that can be achieved without the need for high field spectrometers fitted with cryoprobes. Here, we report the use of a relatively fast 2D HSQC NMR experiment performed on a 500 MHz spectrometer fitted with a BBFO+ probe to obtain high quality spectra. The use of the developed protocol to study the selective modification of the β–β linkage in Kraft lignin is also reported.

Pre-treatment of lignocellulosic feedstocks using biorenewable alcohols : towards complete biomass valorisation

Here, we report on the ability of the biomass derived solvents ethanol and, in particular, n-butanol to fractionate lignocellulose into its main components. An organosolv system consisting of n-butanol containing 5% water and 0.2 M HCl at reflux was found to remove effectively the lignin and hemicellulose components of lignocellulosic biomass leaving a cellulose pulp suitable for enzymatic hydrolysis to simple sugars. Using a hardwood beech pulp as an example, essentially complete conversion of the cellulose component to reducing sugars was achieved with a cellulase loading of 22 FPU per g. Analysis of the solubilised hemicellulose fractions revealed that they consist almost exclusively of alkyl xylosides and mannosides which could serve as valuable synthetic building blocks. Additionally, the mild conditions (<120 °C) and high alcohol content of the pre-treatment solvent suppressed lignin degradation reactions and allowed for the isolation of high quality lignins in good yields. Detailed HSQC NMR analysis of the isolated lignins revealed that they still contained large amounts of β-aryl ether units, especially α-ethoxylated and α-butoxylated β-O-4 units, making them particularly suitable for depolymerisation to mono-aromatic chemicals. This was demonstrated using a recently reported acidolysis method utilizing ethylene glycol which gave monomer yields of between 7.4 and 18 wt%. The yields for n-butanol lignins were at least four fold higher than those obtained from a current generation technical organosolv lignin under comparable conditions.

Tandem catalytic depolymerization of lignin by water-tolerant Lewis acids and rhodium complexes

Abstract Lignin is an attractive renewable feedstock for aromatic bulk and fine chemicals production, provided that suitable depolymerization procedures are developed. Here, we describe a tandem catalysis strategy for ether linkage cleavage within lignin, involving ether hydrolysis by water‐tolerant Lewis acids followed by aldehyde decarbonylation by a Rh complex. In situ decarbonylation of the reactive aldehydes limits loss of monomers by recondensation, a major issue in acid‐catalyzed lignin depolymerization. Rate of hydrolysis and decarbonylation were matched using lignin model compounds, allowing the method to be successfully applied to softwood, hardwood, and herbaceous dioxasolv lignins, as well as poplar sawdust, to give the anticipated decarbonylation products and, rather surprisingly, 4‐(1‐propenyl)phenols. Promisingly, product selectivity can be tuned by variation of the Lewis‐acid strength and lignin source.

Phenolic acetals from lignins of varying compositions via iron(III) triflate catalysed depolymerisation

Lignin is a highly abundant, renewable aromatic polymer that can potentially be obtained in large quantities from lignocellulosic biorefineries. Thus the valorisation of this renewable resource by the production of aromatic chemicals would be highly desirable and is especially important for achieving high yields of these products. In this regard, not only the catalytic method used should be highly selective, but also we must better understand the possible correlations between the structure of the lignin used and the yield of useful products. Here, we demonstrate that lignins obtained from a range of different biomass sources and pretreatment methods can be successfully depolymerized using iron(III) triflate in the presence of ethylene glycol to give p-(1,3-dioxolan-2-yl)methyl substituted phenols. 27 lignins, obtained from 13 different pretreatment methods, were examined in this study. A combined yield of up to 35.5 wt% of acetal products was obtained from a β-aryl ether rich organosolv lignin and the best yield of a single component (16.5 wt%) was achieved starting from pine lignin. Much lower yields were obtained from technical lignins which were low in β-aryl ether content, whilst a range of organosolv lignins of intermediate β-aryl ether content gave intermediate yields of acetal products. Overall, correlations were found between the product distributions and yields and structural data of the parent lignins obtained from 2D HSQC NMR analysis.

Metal Triflates for the Production of Aromatics from Lignin

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 (M(OTf)x ). 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 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 synthesis and analysis of advanced lignin model polymers

If the lignin-first biorefinery concept becomes a reality, high quality lignins close in structure to native lignins will become available in large quantities. One potential way to utilise this renewable material is through depolymerisation to aromatic chemicals. This will require the development of new chemical methods. Here, we report the synthesis and characterisation of advanced lignin model polymers to be used as tools to develop these methods. The controlled incorporation of the major linkages in lignin is demonstrated to give complex hardwood and softwood lignin model polymers. These polymers have been characterised by 2D HSQC NMR and GPC analysis and have been compared to isolated lignins.

The synthesis and analysis of lignin-bound Hibbert ketone structures in technical lignins

Understanding the structure of technical lignins resulting from acid-catalysed treatment of lignocellulosic biomass is important for their future applications. Here we report an investigation into the fate of lignin under acidic aqueous organosolv conditions. In particular we examine in detail the formation and reactivity of non-native Hibbert ketone structures found in isolated organosolv lignins from both Douglas fir and beech woods. Through the use of model compounds combined with HSQC, HMBC and HSQC-TOCSY NMR experiments we demonstrate that, depending on the lignin source, both S and G lignin-bound Hibbert ketone units can be present. We also show that these units can serve as a source of novel mono-aromatic compounds following an additional lignin depolymerisation reaction.

The Synthesis of Melohenine B and a Related Natural Product

A concise synthesis of melohenine B and O-ethyl-14-epimelohenine B, from eburnamonine, was achieved via a biomimetic diastereoselective singlet oxygen-mediated oxidative cleavage of the indole C2-C7 bond. These studies enabled the assignment of the absolute configuration of the natural products. In line with a proposed biosynthetic pathway, the resulting nine-membered ring containing products could be converted to the corresponding quinolones.