Predicting Molecular Weight Characteristics of Reductively Depolymerized Lignins by ATR-FTIR and Chemometrics.

Recent scientific advances in the valorization of lignin, through e.g., (partial-)catalytic depolymerization, require equally state-of-the-art approaches for the analysis of the obtained depolymerized lignins (DLs) or lignin bio-oils. The use of chemometrics in combination with infrared (IR) spectroscopy is one avenue to provide rapid access to pertinent lignin parameters, such as molecular weight (MW) characteristics, which typically require analysis via time-consuming size-exclusion methods, or diffusion-ordered NMR spectroscopy. Importantly, MW serves as a marker for the degree of depolymerization (or recondensation) that the lignin has undergone, and thus probing this parameter is essential for the optimization of depolymerization conditions to achieve DLs with desired properties. Here, we show that our ATR-IR-based chemometrics approach used previously for technical lignin analysis can be extended to analyze these more processed, lignin-derived samples as well. Remarkably, also at this lower end of the MW scale, the use of partial least-squares (PLS) regression models well-predicted the MW parameters for a sample set of 57 depolymerized lignins, with relative errors of 9.9-11.2%. Furthermore, principal component analysis (PCA) showed good correspondence with features in the regression vectors for each of the biomass classes (hardwood, herbaceous/grass, and softwood) obtained from PLS-discriminant analysis (PLS-DA). Overall, we show that the IR spectra of DLs are still amenable to chemometric analysis and specifically to rapid, predictive characterization of their MW, circumventing the time-consuming, tedious, and not generally accessible methods typically employed.

Molecular Design of Lignin-Derived Side-Chain Phenolic Polymers toward Functional Radical Scavenging Materials with Antioxidant and Antistatic Properties.

This article reports a new family of functional side-chain phenolic polymers derived from lignin monomers, displaying a combination of properties that are usually mutually exclusive within a single material. This includes a well-defined molecular structure, transparency, antioxidant activity, and antistatic properties. Our design strategy is based on the lignin-derived bioaromatic monomer dihydroconiferyl alcohol (DCA), a promising and yet largely unexplored asymmetrical diol bearing one aliphatic and one phenolic hydroxyl group. A lipase-catalyzed (meth)acrylation protocol was developed to selectively functionalize the aliphatic hydroxy group of DCA while preserving its phenolic group responsible for its radical scavenging properties. The resulting mono-(meth)acrylated monomers were then directly copolymerized using reversible addition-fragmentation chain-transfer (RAFT) polymerization without any protection of the phenolic side chains. Kinetics studies revealed that, under select conditions, these unprotected phenolic groups surprisingly did not inhibit the radical polymerization and lead to polymers with defined molar masses, low dispersities, and block copolymers. Finally, applications of these new radical scavenging polymers were demonstrated using an antioxidant assay and antistatic experiments. This research opens the door to the direct incorporation of natural antioxidants within the synthetic polymer backbones, increasing the biobased content and limiting the leaching of potentially harmful additives.

Selective Enzymatic Esterification of Lignin-Derived Phenolics for the Synthesis of Lipophilic Antioxidants.

Lignin is an abundant and renewable source of phenolic compounds that can be used as natural antioxidants to substitute synthetic, petroleum-based alternatives. The development of lignin depolymerization techniques has improved the accessibility of low-molecular-weight phenolic fractions with enhanced antioxidant activity compared to native lignin. The selective esterification of the aliphatic OH groups in these compounds is necessary in order to increase their compatibility with hydrophobic product matrixes, while preserving their antioxidant capacity. In the present work, lipase was chosen as a selective catalyst for the esterification of the monolignol dihydroconiferyl alcohol (DCA), in order to target the esterification of aliphatic OHs without modifying the aromatic groups. The reaction was studied under solvent-assisted and solvent-free conditions, using different fatty acids and substrate ratios. A product yield of 97% could be obtained after 24 h in a solvent-assisted reaction with 2 molar equivalents of fatty acid, or after 3 h in a solvent-free reaction with 10 molar equivalents of the fatty acid. The esterified monolignol showed relevant long-term radical scavenging activity, comparable to other commercial, petroleum-based antioxidants. Different lignin fractions were also used as substrates for enzymatic esterification with different fatty acids, resulting in esterification degrees of 20-58% (of the total aliphatic OH), depending on the specific combination of fatty acid-lignin fractions.

Stabilization of Hybrid Adhesives and Sealants by Thermodynamic Tuning of Molecularly Optimized Lignin Bio-Additives: Small Changes, Big Effects.

The use of lignin as a functional additive has long been a promising topic in both industry and academia, but the development of such systems is still limited by the considerable challenges posed by the incompatibility of lignin with common polymers. Herein, we designed modified silicone (MS) sealants with enhanced UV and thermal stability by incorporating molecularly engineered lignin bio-additives while establishing robust design principles to finely adjust the morphology of such blends by tailoring the molecular structures of lignin fractions. To that end, we first constructed a library of lignin fractions with various molecular weights (obtained by fractionating Kraft lignin and by using a lignin model compound) and with several chemical modifications (acetylation, butyrylation, and silylation). The lignin bio-additives were then melt-blended with MS polyethers. The experimental phase diagrams of the resulting blends were established and rationalized with a thermodynamic framework combining Hansen solubility parameters and Flory-Huggins theory, unraveling fascinating insights into the complex solubility behavior of lignin fractions and notably, for the first time, the subtle interplay between molecular weight (entropic effects) and chemical modifications (enthalpic effects). A molecularly optimized lignin additive was then selected to achieve full solubility while providing better thermal stability and UV-blocking properties to the resulting MS material. Overall, this article provides robust design principles for the elaboration of functional biomaterials with optimized morphologies based on rationally engineered lignin fractions.

Enzymatic Production of Xylo-oligosaccharides from Destarched Wheat Bran and the Impact of Their Degree of Polymerization and Substituents on Their Utilization as a Carbon Source by Probiotic Bacteria.

The enzymatic production of xylo-oligosaccharides (XOs) from destarched wheat bran with a GH11 xylanase was studied. Xylo-oligosaccharides (XOs) produced were separated into different fractions according to their degree of polymerization (DP) and the nature of their substituents: arabinoxylo-oligosaccharides (AXOs) with a DP from 2 to 3 and DP from 2 to 6 and feruloylated arabinoxylo-oligosaccharides (FAXOs) esterified by ferulic and p-coumaric acids with a DP from 3 to 6. Both AXOs (short and long DP) and FAXOs stimulated the growth of Bifidobacterium adolescentis, Faecalibacterium prausnitzii, and Prevotella copri similarly but not Lactobacillus rhamnosus. The utilization of AXOs and FAXOs as a carbon source resulted in the increase in turbidity, decrease in pH, and production of short-chain fatty acids (SCFAs) in the culture broth. The highest amount of SCFAs was produced by F. prausnitzii using FAXOs. Results suggest that FAXOs and AXOs have the potential to be considered as prebiotics.

Strategies for the Removal of Polysaccharides from Biorefinery Lignins: Process Optimization and Techno Economic Evaluation.

The utilization of biorefinery lignins as a renewable resource for the production of bio-based chemicals and materials remain a challenge because of the high polysaccharide content of this variety of lignins. This study provides two simple methods; (i) the alkaline hydrolysis-acid precipitation method and (ii) the acid hydrolysis method for the removal of polysaccharides from polymeric biorefinery lignin samples. Both purification strategies are optimized for two different hardwood hydrolysis lignins, HL1 and HL2, containing 15.1% and 10.1% of polysaccharides, respectively. The treated lignins are characterized by polysaccharide content, molecular weight, hydroxyl content, and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). Preliminary techno-economic calculations are also carried out for both purification processes to assess the economic potential of these technologies. The results indicate that both protocols could be used for the purification of HL1 and HL2 hydrolysis lignins because of the minimal polysaccharide content obtained in the treated lignins. Nevertheless, from an industrial and economic perspective the acid hydrolysis technology using low acid concentrations and high temperatures is favored over the alkaline hydrolysis-acid precipitation strategy.

Electrosynthesis of Biobased Chemicals Using Carbohydrates as a Feedstock.

The current climate awareness coupled with increased focus on renewable energy and biobased chemicals have led to an increased demand for such biomass derived products. Electrosynthesis is a relatively new approach that allows a shift from conventional fossil-based chemistry towards a new model of a real sustainable chemistry that allows to use the excess renewable electricity to convert biobased feedstock into base and commodity chemicals. The electrosynthesis approach is expected to increase the production efficiency and minimize negative health for the workers and environmental impact all along the value chain. In this review, we discuss the various electrosynthesis approaches that have been applied on carbohydrate biomass specifically to produce valuable chemicals. The studies on the electro-oxidation of saccharides have mostly targeted the oxidation of the primary alcohol groups to form the corresponding uronic acids, with Au or TEMPO as the active electrocatalysts. The investigations on electroreduction of saccharides focused on the reduction of the aldehyde groups to the corresponding alcohols, using a variety of metal electrodes. Both oxidation and reduction pathways are elaborated here with most recent examples. Further recommendations have been made about the research needs, choice of electrocatalyst and electrolyte as well as upscaling the technology.

Aldehyde-Assisted Lignocellulose Fractionation Provides Unique Lignin Oligomers for the Design of Tunable Polyurethane Bioresins.

Thanks to chemical stabilization, aldehyde-assisted fractionation (AAF) of lignocellulosic biomass has recently emerged as a powerful tool for the production of largely uncondensed lignin. Depolymerization of AAF lignin via ether cleavage provides aromatic monomers at near theoretical yields based on ether cleavage and an oligomeric fraction that remains largely unexploited despite its unique material properties. Here, we present an in-depth analytical characterization of AAF oligomers derived from hardwood and softwood in order to elucidate their molecular structures. These bioaromatic oligomers surpass technical Kraft lignin in terms of purity, solubility, and functionality and thus cannot even be compared to this common feedstock directly for material production. Instead, we performed comparative experiments with Kraft oligomers of similar molecular weight (Mn ∼ 1000) obtained through solvent extraction. These oligomers were then formulated into polyurethane materials. Substantial differences in material properties were observed depending on the amount of lignin, the botanical origin, and the biorefining process (AAF vs Kraft), suggesting new design principles for lignin-derived biopolymers with tailored properties. These results highlight the surprising versatility of AAF oligomers towards the design of new biomaterials and further demonstrate that AAF can enable the conversion of all biomass fractions into value-added products.

Microbial electrosynthesis feasibility evaluation at high bicarbonate concentrations with enriched homoacetogenic biocathode.

An enrichment methodology was developed for a homoacetogenic biocathode that is able to function at high concentrations of bicarbonates for the microbial electrosynthesis (MES) of acetate from carbon dioxide. The study was performed in two stages; enrichment of consortia in serum bottles and the development of a biocathode in MES. A homoacetogenic consortium was sequentially grown under increasing concentrations of bicarbonate, in serum bottles, at room temperature. The acetate production rate was found to increase with the increase in the bicarbonate concentration and evidenced a maximum production rate of 260 mg/L d-1 (15 g HCO3-/L). On the contrary, carbon conversion efficiency decreased with the increase in the bicarbonate concentration, which evidenced a maximum at 2.5 g HCO3-/L (90.16%). Following a further increase in the bicarbonate concentration up to 20 g HCO3-/L, a visible inhibition was registered with respect to the acetate production rate and the carbon conversion efficiency. Well adapted biomass from 15 g HCO3-/L was used to develop biocathodic catalyst for MES. An effective biocathode was developed after 4 cycles of operation, during which acetate production was improved gradually, evidencing a maximum production rate of 24.53 mg acetate L-1 d-1 (carbon conversion efficiency, 47.72%). Compared to the enrichment stage, the carbon conversion efficiency and the rate of acetate production in MES were found to be low. The production of acetate induced a change in the catholyte pH, from neutral conditions towards acidic conditions.

Thermosetting Polymers from Lignin Model Compounds and Depolymerized Lignins.

Lignin is the most abundant source of renewable ready-made aromatic chemicals for making sustainable polymers. However, the structural heterogeneity, high polydispersity, limited chemical functionality and solubility of most technical lignins makes them challenging to use in developing new bio-based polymers. Recently, greater focus has been given to developing polymers from low molecular weight lignin-based building blocks such as lignin monomers or lignin-derived bio-oils that can be obtained by chemical depolymerization of lignins. Lignin monomers or bio-oils have additional hydroxyl functionality, are more homogeneous and can lead to higher levels of lignin substitution for non-renewables in polymer formulations. These potential polymer feed stocks, however, present their own challenges in terms of production (i.e., yields and separation), pre-polymerization reactions and processability. This review provides an overview of recent developments on polymeric materials produced from lignin-based model compounds and depolymerized lignin bio-oils with a focus on thermosetting materials. Particular emphasis is given to epoxy resins, polyurethanes and phenol-formaldehyde resins as this is where the research shows the greatest overlap between the model compounds and bio-oils. The common goal of the research is the development of new economically viable strategies for using lignin as a replacement for petroleum-derived chemicals in aromatic-based polymers.

Bioelectrochemical conversion of CO2 to chemicals: CO2 as a next generation feedstock for electricity-driven bioproduction in batch and continuous modes.

The recent concept of microbial electrosynthesis (MES) has evolved as an electricity-driven production technology for chemicals from low-value carbon dioxide (CO2) using micro-organisms as biocatalysts. MES from CO2 comprises bioelectrochemical reduction of CO2 to multi-carbon organic compounds using the reducing equivalents produced at the electrically-polarized cathode. The use of CO2 as a feedstock for chemicals is gaining much attention, since CO2 is abundantly available and its use is independent of the food supply chain. MES based on CO2 reduction produces acetate as a primary product. In order to elucidate the performance of the bioelectrochemical CO2 reduction process using different operation modes (batch vs. continuous), an investigation was carried out using a MES system with a flow-through biocathode supplied with 20 : 80 (v/v) or 80 : 20 (v/v) CO2 : N2 gas. The highest acetate production rate of 149 mg L-1 d-1 was observed with a 3.1 V applied cell-voltage under batch mode. While running in continuous mode, high acetate production was achieved with a maximum rate of 100 mg L-1 d-1. In the continuous mode, the acetate production was not sustained over long-term operation, likely due to insufficient microbial biocatalyst retention within the biocathode compartment (i.e. suspended micro-organisms were washed out of the system). Restarting batch mode operations resulted in a renewed production of acetate. This showed an apparent domination of suspended biocatalysts over the attached (biofilm forming) biocatalysts. Long term CO2 reduction at the biocathode resulted in the accumulation of acetate, and more reduced compounds like ethanol and butyrate were also formed. Improvements in the production rate and different biomass retention strategies (e.g. selecting for biofilm forming micro-organisms) should be investigated to enable continuous biochemical production from CO2 using MES. Certainly, other process optimizations will be required to establish MES as an innovative sustainable technology for manufacturing biochemicals from CO2 as a next generation feedstock.

Enzymatic Electrosynthesis of Formic Acid through Carbon Dioxide Reduction in a Bioelectrochemical System: Effect of Immobilization and Carbonic Anhydrase Addition.

The enzymatic electrosynthesis of formic acid from the reduction of carbon dioxide (CO2 ) by using formate dehydrogenase (FDH) as a catalyst at the cathode in both its free and immobilized forms was studied in detail in a bioelectrochemical system (BES). The essential role of solubilizing CO2 for its conversion was also studied by adding carbonic anhydrase (CA) to the FDH enzyme in both its free and immobilized forms. FDH alone in the free form showed large variation in the reduction current [(-6.2±3.9) A m-2 ], whereas the immobilized form showed less variation [(-3.8±0.5) A m-2 ] due to increased enzyme stability. The addition of CA with FDH increased the consumption of the current in both forms due to the fact that it allowed rapid dissolution of CO2 , which made it available for the catalytic reaction with FDH. Remarkably, stable consumption of the current was observed throughout the operation if both CA and FDH were immobilized onto the electrode [(-3.9±0.2) A m-2 ]. Product formation by the immobilized enzyme was also continued for three repetitive cycles, which revealed the longevity of the enzyme after immobilization. The recyclability of NADH (NAD=nicotinamide adenine dinucleotide) was also clearly evidenced on the derivative voltammetric signature. Extension of this study for continuous and long-term operation may reveal more possibilities for the rapid capture and conversion of CO2 .

Long-term operation of microbial electrosynthesis cell reducing CO2 to multi-carbon chemicals with a mixed culture avoiding methanogenesis.

In microbial electrosynthesis (MES), CO2 can be reduced preferably to multi-carbon chemicals by a biocathode-based process which uses electrochemically active bacteria as catalysts. A mixed anaerobic consortium from biological origin typically produces methane from CO2 reduction which circumvents production of multi-carbon compounds. This study aimed to develop a stable and robust CO2 reducing biocathode from a mixed culture inoculum avoiding the methane generation. An effective approach was demonstrated based on (i) an enrichment procedure involving inoculum pre-treatment and several culture transfers in H2:CO2 media, (ii) a transfer from heterotrophic to autotrophic growth and (iii) a sequential batch operation. Biomass growth and gradual acclimation to CO2 electro-reduction accomplished a maximum acetate production rate of 400mgLcatholyte-1d-1 at -1V (vs. Ag/AgCl). Methane was never detected in more than 300days of operation. Accumulation of acetate up to 7-10gL-1 was repeatedly attained by supplying (80:20) CO2:N2 mixture at -0.9 to -1V (vs. Ag/AgCl). In addition, ethanol and butyrate were also produced from CO2 reduction. Thus, a robust CO2 reducing biocathode can be developed from a mixed culture avoiding methane generation by adopting the specific culture enrichment and operation procedures without the direct addition of chemical inhibitor.

Application of gas diffusion biocathode in microbial electrosynthesis from carbon dioxide.

Microbial catalysis of carbon dioxide (CO2) reduction to multi-carbon compounds at the cathode is a highly attractive application of microbial electrosynthesis (MES). The microbes reduce CO2 by either taking the electrons or reducing the equivalents produced at the cathode. While using gaseous CO2 as the carbon source, the biological reduction process depends on the dissolution and mass transfer of CO2 in the electrolyte. In order to deal with this issue, a gas diffusion electrode (GDE) was investigated by feeding CO2 through the GDE into the MES reactor for its reduction at the biocathode. A combination of the catalyst layer (porous activated carbon and Teflon binder) and the hydrophobic gas diffusion layer (GDL) creates a three-phase interface at the electrode. So, CO2 and reducing equivalents will be available to the biocatalyst on the cathode surface. An enriched inoculum consisting of acetogenic bacteria, prepared from an anaerobic sludge, was used as a biocatalyst. The cathode potential was maintained at -1.1 V vs Ag/AgCl to facilitate direct and/or hydrogen-mediated CO2 reduction. Bioelectrochemical CO2 reduction mainly produced acetate but also extended the products to ethanol and butyrate. Average acetate production rates of 32 and 61 mg/L/day, respectively, with 20 and 80 % CO2 gas mixture feed were achieved with 10 cm2 of GDE. The maximum acetate production rate remained 238 mg/L/day for 20 % CO2 gas mixture. In conclusion, a gas diffusion biocathode supported bioelectrochemical CO2 reduction with enhanced mass transfer rate at continuous supply of gaseous CO2. Graphical abstract ᅟ.

Electro-Fermentation - Merging Electrochemistry with Fermentation in Industrial Applications.

Electro-fermentation (EF) merges traditional industrial fermentation with electrochemistry. An imposed electrical field influences the fermentation environment and microbial metabolism in either a reductive or oxidative manner. The benefit of this approach is to produce target biochemicals with improved selectivity, increase carbon efficiency, limit the use of additives for redox balance or pH control, enhance microbial growth, or in some cases enhance product recovery. We discuss the principles of electrically driven fermentations and how EF can be used to steer both pure culture and microbiota-based fermentations. An overview is given on which advantages EF may bring to both existing and innovative industrial fermentation processes, and which doors might be opened in waste biomass utilization towards added-value biorefineries.

Gas Diffusion Electrodes Manufactured by Casting Evaluation as Air Cathodes for Microbial Fuel Cells (MFC).

One of the most intriguing renewable energy production methods being explored currently is electrical power generation by microbial fuel cells (MFCs). However, to make MFC technology economically feasible, cost efficient electrode manufacturing processes need to be proposed and demonstrated. In this context, VITO has developed an innovative electrode manufacturing process based on film casting and phase inversion. The screening and selection process of electrode compositions was done based on physicochemical properties of the active layer, which in turn maintained a close relation with their composition A dual hydrophilic-hydrophobic character in the active layer was achieved with values of εhydrophilic up to 10% while εTOTAL remained in the range 65 wt % to 75 wt %. Eventually, selected electrodes were tested as air cathodes for MFC in half cell and full cell modes. Reduction currents, up to -0.14 mA·cm2- at -100 mV (vs. Ag/AgCl) were reached in long term experiments in the cathode half-cell. In full MFC, a maximum power density of 380 mW·m-2 was observed at 100 Ω external load.

An enriched electroactive homoacetogenic biocathode for the microbial electrosynthesis of acetate through carbon dioxide reduction.

In the direction of generating value added chemicals from carbon dioxide (CO2) reduction through microbial electrosynthesis (MES), considering the crucial impact of the electrode material for the biofilm development and electron delivery, an attempt was made in this study to evaluate the efficiency of two different materials as biocathodes and their respective output in terms of electrosynthesis. The electrode material is a key component in the MES process. Several electrodes such as platinum, graphite foil, dimentionally stable anode (DSA) and graphite rod, and VITO-CoRE™ derived electrodes were tested for their suitability for ideal electrode combination in a three electrode cell setup. Bicarbonates (the dissolved form of CO2) was reduced to acetate by a selectively developed biocathode under a mild applied cathodic potential of -400 mV (vs. SHE) in 500 mL of single chamber MES cells operating for more than four months. Among the two electrode combinations evaluated, VITO-CoRE™-PL (VC-IS, plastic inert support) as the cathode and VITO-CoRE™-SS (VC-SS, stainless steel metal support) as the counter electrode showed higher production (4127 mg L(-1)) with a volumetric production rate of 0.569 kg per m(3) per d than the graphite rod (1523 mg L(-1)) with a volumetric production rate of 0.206 kg per m(3) per d. Contrary to the production efficiencies, the coulombic efficiency was higher with the second electrode combination (40.43%) than the first electrode combination (29.91%). Carbon conversion efficiency to acetate was higher for VC-IS (90.6%) than the graphite rod (82.0%).

Carbon dioxide reduction by mixed and pure cultures in microbial electrosynthesis using an assembly of graphite felt and stainless steel as a cathode.

Carbon dioxide (CO2) reduction to multi-carbon compounds at the cathode using chemolithoautotrophs is an emerging application of microbial electrosynthesis (MES). In this study, CO2 reduction in MES was investigated at hydrogen evolving potentials, separately by a mixed culture and Clostridium ljungdahlii, using a graphite felt and stainless steel assembly as cathode. The mixed culture reactor produced acetate at the maximum rate of 1.3 mM d(-1), along with methane and hydrogen at -1.1 V/Ag/AgCl. Over 160 days of run-time in four fed-batches, 26% of bicarbonate was converted to acetate between day 28 and 41, whereas in the late batches, methane production prevailed. Out of 45 days of run-time in the C. ljungdahlii reactor, 2.4 mM d(-1) acetate production was achieved at -0.9 V/Ag/AgCl in Batch 1. Simultaneous product degradation occurred when the mixed culture was not selectively enriched. Hydrogen evolution is potentially the rapid way of transferring electrons to the biocatalysts for higher bioproduction rates.

Enzymatic electrosynthesis of formate through CO2 sequestration/reduction in a bioelectrochemical system (BES).

Bioelectrochemical system (BES) was operated using the enzyme formate dehydrogenase as catalyst at cathode in its free form for the reduction of CO2 into formic acid. Electrosynthesis of formic acid was higher at an operational voltage of -1V vs. Ag/AgCl (9.37mgL(-1) CO2) compared to operation at -0.8V (4.73mgL(-1) CO2) which was strongly supported by the reduction catalytic current. Voltammograms also depicted a reversible redox peak throughout operation at -1V, indicating NAD(+) recycling for proton transfer from the source to CO2. Saturation of the product was observed after 45min of enzyme addition and then reversibility commenced, depicting a lower and stable formic acid concentration throughout the subsequent time of operation. Stability of the enzyme activity after immobilization on the electrode and product yield will be studied further.