On the Meaning and Origins of Lignin Recalcitrance: A Critical Analysis of the Catalytic Upgrading of Lignins Obtained from Mechanocatalytic Biorefining and Organosolv Pulping

In the broad context of catalysis for lignin valorization, the term “recalcitrance” is often used to describe the resistance of lignin to undergo chemical transformations (generally, reductive processes) rendering small molecules soluble in the reaction medium. Unfortunately, the current usage of the term “recalcitrance” often remains vague in meaning, hindering the search for better catalysts for lignin valorization. In the quest to address the research question—What is lignin recalcitrance?—we present our search for the factors responsible for the resistance of lignin to reductive catalytic processes, from various perspectives. In this study, lignins isolated as a precipitate obtained from the saccharification of water‐soluble lignocelluloses (produced by solvent‐free mechanocatalytic depolymerization of beechwood, pinewood, or sugarcane bagasse) and their counterparts isolated by solvent extraction (organosolv pulping) are investigated. The critical analysis of structure and bonding, in addition to the in‐depth understanding of results from the catalytic upgrading of lignin streams, in the presence of Raney Ni and H2 pressure under mild and extreme conditions, reveals that the simple evaluation of the total yield of liquid products provides no quantitative measure of the lignin recalcitrance. Our results shed light on the real meaning, origins and implications of “lignin recalcitrance” for catalysis research. The results demonstrate that lignin recalcitrance is associated not only with its intrinsic properties (i.e. molecular weight, the occurrence of native linkages, and their bond dissociation enthalpies) but also with its extrinsic properties (e.g. residual polysaccharides and solubility). Overall, this study presents a detailed evaluation of recalcitrance of lignin through the critical analysis of the product mixture properties (e.g. H/C and O/C ratios, molecular weight distribution, yield of key individual products, and several others)

Utilization of mechanocatalytic oligosaccharides by ethanologenic Escherichia coli as a model microbial cell factory

Mechanocatalysis is a promising method for depolymerization of lignocellulosic biomass. Microbial utilization of the resulting oligosaccharides is one potential route of adding value to the depolymerized biomass. However, it is unclear how readily these oligosaccharides are utilized by standard cell factories. Here, we investigate utilization of cellulose subjected to mechanocatalytic depolymerization, using ethanologenic Escherichia coli as a model fermentation organism. The mechanocatalytic oligosaccharides supported ethanol titers similar to those observed when glucose was provided at comparable concentrations. Tracking of the various oligomers, using maltose (alpha-1,4) and cellobiose (beta-1,4) oligomers as representative standards of the orientation, but not linkage, of the glycosidic bond, suggests that the malto-like-oligomers are more readily utilized than cello-like-oligomers, consistent with poor growth with cellotetraose or cellopentaose as sole carbon source. Thus, mechanocatalytic oligosaccharides are a promising substrate for cell factories, and microbial utilization of these sugars could possibly be improved by addressing utilization of cello-like oligomers

Sulfur containing organic compounds in the raw producer gas of wood and grass gasification

The detailed description of gas streams in biomass gasification plants is necessary for the correct design and operation of these units. Sulfur containing compounds are usually present in biomass gasification gas, since sulfur is typically found in the feedstock. Sulfur compounds are important contaminants present in the gas streams, since even at concentrations as low as a few ppm they poison catalysts causing significant technical challenges to the production process. The determination of contaminants is often challenging due to their low concentration and the presence of steam and tars in the gas streams. Here a method is presented, which allows the qualitative and quantitative analysis of an extensive number of organic sulfur compounds found in low concentration in biomass gasification gas. The method is a combination of an adequate sampling technique (based on the liquid quench of the sampled gas) and a sensitive analytical equipment (gas chromatograph coupled with a sulfur chemiluminescence detector, CG/SCD). This work shows that several organic sulfur species are found in biomass gasification gas, which are usually not reported, but have to be considered for the design of biomass-based gasification plants. The presence of these compounds is discussed considering the feedstock used, gasification conditions and the sampling technique. Moreover, the results presented here evidence that only measuring thiophene, benzo[b]thiophene and dibenzothiophene in the producer gas can be misleading, since the sum of concentrations of all other organic sulfur compounds could be above the tolerable limits for total sulfur in gasification-based processes. (C) 2014 Elsevier Ltd. All rights reserved

Mechanocatalytic depolymerization of cellulose and raw biomass and downstream processing of the products

The utilization of lignocelluloses (e.g. wood, grass, crops residues and several others) shows great potential as part of the solution for decreasing the dependence of modern societies on fossil resources. In spite of this, the catalytic conversion of these renewable carbon resources via chemical and biotechnological processes is hindered by their complex polymeric nature. For this reason, chemical or enzymatic processes for hydrolysis of cellulose suffer from low efficacy due to harsh reaction conditions and high byproduct formation in case of the chemical methods, or high costs and long reaction times for the enzymatic methods. There is thus an urgent need for processes able to convert the whole plant biomass, which allow the formation of fermentable sugars and technical sulfur-free lignins. Recently, we demonstrated the combination of acid-catalysis with mechanical forces to be an efficient approach to fully overcome the recalcitrance of lignocellulose. As a result, the solvent-free depolymerization of lignocellulose (in solid-state) forms ‘water-soluble lignocellulose’ in quantitative yield. In this article, we present an overview of the mechanocatalytic depolymerization of lignocellulose and downstream processing of the ‘water-soluble’ lignocellulose’ to sugar alcohols and furfurals. The water-soluble products appear to be the ideal platform at the beginning of advanced value chains of biorefining, starting with ‘real’ lignocellulosic substrates

Utilization of mechanocatalytic oligosaccharides by ethanologenic Escherichia coli as a model microbial cell factory

Mechanocatalysis is a promising method for depolymerization of lignocellulosic biomass. Microbial utilization of the resulting oligosaccharides is one potential route of adding value to the depolymerized biomass. However, it is unclear how readily these oligosaccharides are utilized by standard cell factories. Here, we investigate utilization of cellulose subjected to mechanocatalytic depolymerization, using ethanologenic Escherichia coli as a model fermentation organism. The mechanocatalytic oligosaccharides supported ethanol titers similar to those observed when glucose was provided at comparable concentrations. Tracking of the various oligomers, using maltose (alpha-1,4) and cellobiose (beta-1,4) oligomers as representative standards of the orientation, but not linkage, of the glycosidic bond, suggests that the malto-like-oligomers are more readily utilized than cello-like-oligomers, consistent with poor growth with cellotetraose or cellopentaose as sole carbon source. Thus, mechanocatalytic oligosaccharides are a promising substrate for cell factories, and microbial utilization of these sugars could possibly be improved by addressing utilization of cello-like oligomers.This article is published as Jin, Tao, Mats Käldström, Adriana Benavides, Marcelo D. Kaufman Rechulski, and Laura R. Jarboe. "Utilization of mechanocatalytic oligosaccharides by ethanologenic Escherichia coli as a model microbial cell factory." AMB Express 10 (2020): 28. DOI: 10.1186/s13568-020-0965-4. Posted with permission.</p

Utilization of mechanocatalytic oligosaccharides by ethanologenic Escherichia coli as a model microbial cell factory

Mechanocatalysis is a promising method for depolymerization of lignocellulosic biomass. Microbial utilization of the resulting oligosaccharides is one potential route of adding value to the depolymerized biomass. However, it is unclear how readily these oligosaccharides are utilized by standard cell factories. Here, we investigate utilization of cellulose subjected to mechanocatalytic depolymerization, using ethanologenic Escherichia coli as a model fermentation organism. The mechanocatalytic oligosaccharides supported ethanol titers similar to those observed when glucose was provided at comparable concentrations. Tracking of the various oligomers, using maltose (alpha-1,4) and cellobiose (beta-1,4) oligomers as representative standards of the orientation, but not linkage, of the glycosidic bond, suggests that the malto-like-oligomers are more readily utilized than cello-like-oligomers, consistent with poor growth with cellotetraose or cellopentaose as sole carbon source. Thus, mechanocatalytic oligosaccharides are a promising substrate for cell factories, and microbial utilization of these sugars could possibly be improved by addressing utilization of cello-like oligomers

Mechanocatalytic Depolymerization of Lignocellulose Performed on Hectogram and Kilogram Scales

Mechanocatalytic depolymerization of lignocellulose constitutes a new frontier in biorefining. In a one-pot process, the combination of mechanical forces and acid catalysis leads to the full conversion of (dried) lignocellulose into water-soluble products (oligosaccharides and lignin fragments). In solution, these products undergo hydrolysis and other reactions, rendering high yields of monosaccharides along with precipitation of a sulfur-free lignin. Therefore, the water-soluble oligosaccharides may serve as a unique replacement for glucose and xylose for the production of platform chemicals. In this work, we report the results obtained from mechanocatalytic depolymerization of α-cellulose, beechwood, and poplar wood performed in Simoloyer mills operating on hectogram and kilogram scales. Irrespective of the process scale, full conversion of the substrate into “water-soluble (ligno)­celluloses”, within milling durations of 1–3 h, is achieved. A phenomenological analysis of parameters for the process upscaling is provided. Moreover, the energy consumption of the process on different scales is assessed. Remarkably, energy consumption significantly decreases (from ca. 200 MWh·t^–1 to 9.6 MWh·t^–1) with upscaling of the experiment from 1 g (planetary mill) to 1 kg. This increase in energy efficiency constitutes key evidence for the feasibility of the mechanocatalytic depolymerization on a large scale

Liquid-Quench Sampling System for the Analysis of Gas Streams from Biomass Gasification Processes. Part 1: Sampling Noncondensable Compounds

In biomass gasification plants and related research units, on-line analysis of noncondensable compounds (H-2, CO, CO2, CH4, and light hydrocarbons) and condensable compounds (water, tars, etc.) is desired. Long-duration on-line measurements are often a challenging task, because blocking of the sampling lines has to be avoided and because some analytical equipment do not tolerate certain components of these streams. A continuous liquid-quench sampling system has been successfully employed to sample these gas streams and prepare them for on-line measurement. In comparison to other systems, it promotes long-duration measurements with high time resolution over a wide concentration range with little man-hour requirements. In this work, the sampling system is described in detail for the first time and its performance is investigated regarding the sampling of noncondensable compounds. On the basis of experimental observations, optimal operational parameters are suggested to minimize the measurement bias