Lignin,
a major component of lignocellulose, is the largest source
of aromatic building blocks on the planet and harbors great potential
to serve as starting material for the production of biobased products.
Despite the initial challenges associated with the robust and irregular
structure of lignin, the valorization of this intriguing aromatic
biopolymer has come a long way: recently, many creative strategies
emerged that deliver defined products via catalytic or biocatalytic
depolymerization in good yields. The purpose of this review is to
provide insight into these novel approaches and the potential application
of such emerging new structures for the synthesis of biobased polymers
or pharmacologically active molecules. Existing strategies for functionalization
or defunctionalization of lignin-based compounds are also summarized.
Following the whole value chain from raw lignocellulose through depolymerization
to application whenever possible, specific lignin-based compounds
emerge that could be in the future considered as potential lignin-derived
platform chemicals.
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.
Inherently complex,
lignin-derived aromatic monomers comprising
valuable structural moieties present in many pharmaceuticals would
serve as ideal substrates for the construction of biologically active
molecules. Here, we describe a strategy that incorporates all intrinsic
functional groups present in platform chemicals obtained by lignin
depolymerization into value-added amines, using sustainable catalytic
methods and benign solvents. Our strikingly efficient protocol provides
access to libraries of aminoalkyl-phenol derivatives and seven-membered
N-heterocycles directly from wood in two, respectively three, waste-free
steps. Several molecules in these libraries have shown promising antibacterial
or anticancer activities, emphasizing the advantage of this modular
synthetic strategy and the potential for drug discovery. The sustainable
catalytic pathways presented here can lead to significant benefits
for the pharmaceutical industry where reduction of hazardous waste
is a prime concern, and the described strategies that lead to high-value
products from non-edible biomass waste streams also markedly increase
the economic feasibility of lignocellulosic biorefineries.
The direct conversion of ethanol to higher value 1-butanol is a catalytic transformation of great interest in light of the expected wide availability of bioethanol originating from the fermentation of renewable resources. In this contribution we describe several novel compositions of porous metal oxides (PMO) as highly active and selective catalysts for the Guerbet coupling of ethanol to 1-butanol in the temperature range 180−320 °C. The novel PMO catalysts that do not contain any noble metals are obtained by calcination of a series of hydrotalcite precursors synthesized through modular procedures. In particular, catalyst compositions simultaneously containing Cu and Ni dopants have shown excellent catalytic activities. Up to 22% 1-butanol yield at 56% ethanol conversion was reached in a batch mode; recycling and leaching tests showed excellent robustness of the new catalysts. An extensive characterization by means of several techniques such as powder XRD, SEM, TEM, BET, and NH 3 -and CO 2 -TPD was performed in order to understand structure−activity trends.
Lignin is the most abundant natural aromatic feedstock, and the conversion of lignin to value-added chemicals has drawn immense attention in biorefineries. Deep eutectic solvents (DESs) have been applied to...
The temperature and pressure of the hydrothermal process occurring in a batch reactor are typically coupled. Herein, we develop a decoupled temperature and pressure hydrothermal system that can heat the cellulose at a constant pressure, thus lowering the degradation temperature of cellulose significantly and enabling the fast production of carbon sub-micron spheres. Carbon sub-micron spheres can be produced without any isothermal time, much faster compared to the conventional hydrothermal process. High-pressure water can help to cleave the hydrogen bonds in cellulose and facilitate dehydration reactions, thus promoting cellulose carbonization at low temperatures. A life cycle assessment based on a conceptual biorefinery design reveals that this technology leads to a substantial reduction in carbon emissions when hydrochar replacing fuel or used for soil amendment. Overall, the decoupled temperature and pressure hydrothermal treatment in this study provides a promising method to produce sustainable carbon materials from cellulose with a carbon-negative effect.
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