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Sustainable Biomass Conversions by Highly Efficient Catalytic Processes

Final Report Summary - SUBICAT (Sustainable Biomass Conversions by Highly Efficient Catalytic Processes)

Our society still faces the grand challenge of transitioning from a fossil fuel based economy to one based on clean and sustainable conversions of renewable feedstocks. To address this significant societal problem, we believe the next generation of European researchers will need skills at the interface of biology, chemistry and engineering, thus defining the central goals of the SuBiCat training network. Indeed, we aim to provide an interdisciplinary training platform that will deliver the next generation of European researchers, able to make the quantum leap to a fully sustainable chemical industry based solely on renewable resources. From the very start, we have endeavoured to integrate innovative research in the use of renewable resources with industrial and commercial exploitation in mind.
One of the most abundant renewable sources of carbon is lignocellulose, which represents a huge potential source of valuable fine and bulk chemicals. The degradation of lignin into useful chemical building blocks, under mild conditions, is a major undertaking, which has yet to be achieved. The development of efficient catalytic processes that allow for selective lignin degradation continues to be the long-term aim of the SuBiCat ITN. Lignin is a complex biopolymer which contains several different building blocks connected by numerous types of linkages, which differ in reactivity. Therefore, lignin degradation requires both the development of selective catalysts targeting each linkage and the combination of many catalysts to deliver an integrated process. Thus, we have chosen a multidisciplinary approach, investigating the reactivity of different model compounds and real lignins to produce valuable chemicals from this renewable feedstock.
The scientific component of SuBiCat remained focussed around four research-goal oriented work packages, all addressing crucial components of the lignin challenge.
WP1 dealt with the selective chemical and biological depolymerization of lignin
A novel set of model compounds was synthesised (ESR8, USTAN), which expands the model toolkit of available when studying lignin chemistry. These model compounds were used to gain insight into the acid-catalysed depolymerization process developed at RUG (ER1), including the formation of the organic solvent, 1,3-dioxolane from lignin. This work shows the importance of process development to balance lignin isolation yields in the pre-treatment phase with the lignin’s suitability for higher value applications. This knowledge could help guide future biorefinery design and therefore allow for more efficient preparation of renewable chemicals from lignocellulose for the chemical industry. Lignin depolymerisation was also achieved through base-catalysis by employing catalytic amounts of alkali metal bases in dimethyl carbonate (ESR3, RWTH). This approach proved to be applicable to minimally processed substrates such as wood chips. Additionally, chemoselective γ-alcohol oxidation of the β-O-4 linkage followed by a retro-aldol step was successfully shown to yield a series of substituted benzaldehyde and phenol products (ESR3, RWTH). In a similar approach, the oxidation of model compounds under mild alkaline conditions was achieved, avoiding complex catalytic systems and high pressures and temperatures (ESR4, UU). High yields of aromatics were obtained from model compounds and the approach was successfully translated to an oxidised poplar lignin. Another strategy investigated involved the development of novel homogeneous transition metal catalysts for the defunctionalisation of lignin via tandem dehydrogenation/ decarbonylation reactions (ESR11, RWTH). As a result, a series of new catalysts based on a novel class of Di(iPr)bis(Ar) ligands were developed, which proved to be highly active and selective in diol C-C bond cleavage reactions, with potential applications in lignin defunctionalisation. Similarly, a series of xantphos ligands were synthesised and used in ruthenium-catalysed cleavage reactions of β-O-4 model compounds in ionic liquids (ESR1, USTAN). Alternatively, titanium and aluminium were successfully complexed with lignin, using simple, efficient and reproducible approaches (ESR2, Hybrid). The titanate lignin was shown to be an active heterogeneous catalyst for alkene epoxidation demonstrating that lignin could be used as a catalyst support. In a parallel biological approach, individual lignin-degrading enzymes were heterologously expressed in P. fluorescens in high yields (ESR7, RWTH) and showed good activity towards advanced lignin model compounds and Kraft lignin. In addition, genome mining of the white-rot fungus, D. squalens, revealed several putative β-etherases, some of which showed the ability to selectively cleave the β-O-4 bond in lignin model compounds (ESR6, UH). Genes of A. niger that are putatively involved in aromatic metabolism, were identified and the possibility to improve the tolerance of A. niger towards aromatic compounds was demonstrated (ESR5, CBS-KNAW). These results demonstrate that existing industrial fungi, such as A. niger, can be modified to become cell factories for producing renewable aromatic building blocks, which could in part replace current processes in the petrochemical industry. The identified genes (and other candidates) could be key tools in developing conversion processes for specific aromatic compounds, as needed in the sustainable industrial production of aromatics. Another approach involved the design and production of artificial metalloenzyme (ArM) with improved redox potential to achieve lignin oxidation (ER2, USTAN). The produced ArM mutants were successfully tested in oxidation and hydroformylation reactions.
WP2 dealt with the analysis of lignin and model lignin systems
An on-line monitoring method was developed to follow the catalytic depolymerisation of lignin and its model compounds at elevated temperatures and pressures using Operando ATR-IR spectroscopy combined with chemometrics approaches, namely principal component analysis (PCA) and multicurve resolution analysis (MCR) (ESR9, UU). This involved the acquisition of high quality spectra and building a model, using external calibration data, that can now be applied to other conversion processes.
WP3 dealt with the use of advanced fluids in lignin processing
Electrochemistry represents a green valorisation technique which does not utilise expensive catalysts or harsh pressure/temperature conditions and therefore represents an important achievement towards an economically sustainable biorefinery concept. Hence the development of an electrochemical membrane reactor for lignin depolymerisation and in-situ recovery of products was successfully achieved as well as the use of innovative solvents and emulsion systems for the electrochemical depolymerisation of lignin (ESR10, RWTH).
WP4 dealt with the fine chemicals from lignin-derived feedstock
Tandem palladium and isothiourea relay catalysis for the enantioselective [2,3]-rearrangement of (monolignol derived) allylic ammonium ylides was developed (ESR12, USTAN). The synthetic utility of which was showcased by synthesising enantioenriched alcohols and piperidines. An alternative strategy explored the biocatalytic conversion of depolymerized lignin fragments into useful products. Here, an O-demethylation system was developed, which allows the production of aromatic phenols (ER3, RUG). This easy one pot reaction is of great interest for and applicable to the conversion of highly methoxylated lignin-derived product e.g. in the production of lignin-derived L-DOPA. Progress was also made in engineering a transaminase for the reductive amination of Hibbert ketones.
The work carried out within the SuBiCat network have been disseminated to the scientific community through poster and oral presentations at national and international conferences and the publication of 22 scientific papers in high impact journals. To further facilitate dissemination, a SuBiCat website ( has been used to inform the public about SuBiCat’s training network, our goals and our challenges. It provides a great platform to promote our events and the E(S)Rs works including their outreach activities. For example, the E(S)Rs have been encouraged to submit publicly available newsfeed items on our website and have used this opportunity to write about their experiences when attending conferences as well as when visiting partner’s institutions during their secondments. All E(S)Rs have also been involved in network wide training events, in addition to the local training programs of the host institutes. As a result of the training network offered by the SuBiCat ITN, ER1 Peter Deuss has been appointed as a tenure-track assistant professor in the Dept. of Chemical Engineering at RUG and ESR3 Saumya Dabral has just obtained her PhD and taken up a postdoctoral position at the Catalysis Research Laboratory in Heidelberg. ESR7 Selin Ece and ESR8 Ciaran Lahive have also submitted their PhD thesis to their respective institutes.