The catalytic oxidation of biomass to new materials focusing on starch,cellulose and lignin

Biomass is a renewable class of materials of growing interest amongst researchers aiming to achieve global sustainability. This review focuses on the homogeneous catalysis of the oxidation of biomass, in particular starch, cellulose and lignin. Often such catalytic reactions lead to depolymerisation of the material as happens in Nature with for example brown rot fungi. This depolymerisation can be desirable or not, and control in industrial applications is thus important to obtain the desired outcome. The two main oxidants in use are O₂ and H₂O₂ and their use is described as appropriate. Industrial oxidation catalysis is highly significant in the bleaching of cellulose-containing materials due to its high volume application in the paper, pulp and laundry industries. Here, the presence of a ligand on the oxidising metal ion has a significant effect on the catalyst selectivity and stability. In addition to the bleaching of cellulose-containing materials, the oxidation of cellulose, starch, lignin and lignin model compounds are discussed with a focus on generating even more hydrophilic materials which have important applications or materials which may be further modified. Finally developing applications of biomass are described such as new support materials for catalysts, as supports for sensors and nanomaterials for microbial culture.     

Plasma-Assisted Pretreatment of Wheat Straw

O₃ generated in a plasma at atmospheric pressure and room temperature, fed with dried air (or oxygen-enriched dried air), has been used for the degradation of lignin in wheat straw to optimize the enzymatic hydrolysis and to get more fermentable sugars. A fixed bed reactor was used combined with a CO₂ detector and an online technique for O₃ measurement in the fed and exhaust gas allowing continuous measurement of the consumption of O₃. This rendered it possible for us to determine the progress of the pretreatment in real time (online analysis). The process time can be adjusted to produce wheat straw with desired lignin content because of the online analysis. The O₃ consumption of wheat straw and its polymeric components, i.e., cellulose, hemicellulose, and lignin, as well as a mixture of these, dry as well as with 50% water, were studied. Furthermore, the process parameters dry matter content and milled particle size (the extent to which the wheat straw was milled) were investigated and optimized. The developed methodology offered the advantage of a simple and relatively fast (0.5–2 h) pretreatment allowing a dry matter concentration of 45–60%. FTIR measurements did not suggest any structural effects on cellulose and hemicellulose by the O₃ treatment. The cost and the energy consumption for lignin degradation of 100 g of wheat straw were calculated.     

Effect of absorbed dose on the postradiation dry distillation of cellulose and lignin

The postradiation dry distillation of cellulose and lignin has been studied. During the course of irradiation to 3 MGy, the weights of samples remained almost unchanged. The overpoints of lignin and cellulose irradiated at 2.2 MGy decrease by ～80° and ～100°, respectively. A third of the condensate from cellulose and almost a half that from lignin were distilled off at lower temperatures. Thermally unstable compounds are formed in cellulose; these compounds are predominantly converted into furans upon subsequent heating. The distillation of irradiated lignin affords a smaller amount of tar, but it is richer in methoxyphenols. The aqueous organic fraction distilled off has a higher concentration of soluble organic compounds.     

Dielectric properties of lignin and glucomannan as determined by spectroscopic ellipsometry and Lifshitz estimates of non-retarded Hamaker constants

We present in this study a quantitative estimate of the dispersive interactions between lignin, hemicellulose and cellulose, which are the dominating components in wood and also extensively used to produce paper and packaging materials. The dielectric properties in the UV-visible region of spin-coated films of pure lignin and glucomannan were determined by spectroscopic ellipsometry. The non-retarded Hamaker constants were estimated from the determined spectral parameters using Lifshitz theory for lignin and glucomannan interacting with cellulose, titania and calcium carbonate in vacuum, water and hexane. The Hamaker constants for the different combinations of cellulose, lignin and glucomannan fall within a relatively narrow range of 35–58 and 8–17 zJ, for the values in vacuum (air) and water, respectively. The estimated Hamaker constants for the interactions of the wood components with TiO₂ and CaCO₃, common additives in paper, in water range from 3 to 19 zJ, thus being similar in magnitude as the interactions between the wood components themselves. In contrast, the Hamaker constant is essentially zero for glucomannan interacting with calcium carbonate in hexane. The Hamaker constants for lignin, hemicellulose and cellulose determined in this study can provide information regarding the surface interactions important for e.g. adhesion, friction, swelling and wetting in paper processing as well as for the resulting behavior of paper products.     

Microbial Pretreatment of Corn Stovers by Solid-State Cultivation of Phanerochaete chrysosporium for Biogas Production

The microbial pretreatment of corn stover and corn stover silage was achieved via the solid-state cultivation of Phanerochaete chrysosporium; pretreatment effects on the biodegradability and subsequent anaerobic production of biogas were investigated. The peak levels of daily biogas production and CH₄ yield from corn stover silage were approximately twice that of corn stover. Results suggested that ensiling was a potential pretreatment method to stimulate biogas production from corn stover. Surface morphology and Fourier-transform infrared spectroscopy analyses demonstrated that the microbial pretreatment of corn stover silage improved biogas production by 10.5 to 19.7 % and CH₄ yield by 11.7 to 21.2 % because pretreatment could decrease dry mass loss (14.2 %) and increase substrate biodegradability (19.9 % cellulose, 32.4 % hemicellulose, and 22.6 % lignin). By contrast, the higher dry mass loss in corn stover (55.3 %) after microbial pretreatment was accompanied by 54.7 % cellulose, 64.0 % hemicellulose, and 61.1 % lignin degradation but did not significantly influence biogas production.     

Elucidation of interaction among cellulose, lignin and xylan during tar and gas evolution in steam gasification

Time profiles of evolution rates of gas and tar in steam gasification of model biomass samples were examined using a continuous cross-flow moving bed type differential reactor to elucidate the interaction of the major biomass components (cellulose, xylan, lignin) during gas and tar evolution. Two types of model biomass samples (sample A: mixture of cellulose (65 wt%) and lignin (35 wt%); sample B: mixture of cellulose (50 wt%), xylan (23 wt%), and lignin (27 wt%)) were used for the experiment. In steam gasification of sample A, the evolutions of water-soluble tar and gaseous products (CO, H₂, CH₄ and C₂H₄) are significantly suppressed by the interaction between cellulose and lignin. The primary (initial) decomposition of lignin is hindered by the interaction with pyrolysate of cellulose. Then, the CO₂ evolution appreciably enhanced and the evolution of water-soluble tar delays. These results may imply that the volatilization of water-soluble tar derived from cellulose is suppressed by lignin and then the decomposition of char derived from polymerized saccharides and lignin takes place, emitting mainly CO₂. From the results using sample B, it was found that the addition of xylan greatly enhances the evolutions of gases (CO₂, CO, CH₄ and H₂) and accelerates the evolution of water-soluble tar and CO₂, implying that the enhancement of decomposition of water-soluble tar into gases and/or xylan decomposes into gases without significant interaction with cellulose or lignin. In addition, yields of the major tar components (levoglucosan, furfural and 5-methylfurfural) were measured using HPLC. It was observed that the interaction among cellulose, xylan and lignin suppresses the evolution of levoglucosan and significantly increases the evolution rate of 5-methylfurfural. There is an insignificant influence of interaction among cellulose, xylan and lignin for furfural evolution.     

Preparation of Arabinoxylan and its Sorption on Bacterial Cellulose During Cultivation

The sorption of arabinoxylan (AX) on bacterial cellulose was investigated by adding AX to the culture medium of Gluconacetobacter xylinus. The starting AX material was produced by alkaline extraction of oat spelts. To investigate the impact of varying AX quality, the residual lignin was reduced by ClO₂ bleaching. Furthermore, bleached and unbleached xylans were subjected to xylanase hydrolysis in order to produce fractions of varying molar mass. Of all samples only the water soluble fractions were used for sorption experiments. A reduced molar mass resulted in a lower sorption of AX to the cellulose, while the lignin content increased the sorption of AX on bacterial cellulose. The sorption of AX resulted in a reduction of bacterial cellulose crystallinity and cellulose Iα content. In combined treatments of AX with xyloglucan and β-glucan no synergistic effect of those polysaccharides on the AX sorption was found.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

Assessment of pretreatment conditions to obtain fast complete hydrolysis on high substrate concentrations

Steam-heating of aspen wood chips improved the enzymatic digestibility of the cellulose. Scaling up the reaction vessel from 2 to 60 L had virtually no influence on the chemical composition and the accessibility of the lignocellulosic substrate. Over 85% of the cellulose could be hydrolyzed to glucose when an 8% substrate concentration was used. The residual content of alkali-insoluble lignin appeared to control the digestibility of the cellulose. Increased delignification either by prolonged steaming, oxidative posttreatment, or SO₂ catalysis improved the accessibility of the cellulose. The use of SO₂ as a catalyst also increased the recovery yield of the wood after steam-heating, with more than 70% of the original xylan recovered as monomeric xylose. Conversion yields of above 90% were achieved at low levels of filter paper activity after a relatively short incubation time. Removal of alkali-soluble lignin did not influence digestibility when the enzyme concentration was based on the cellulose content of the substrates.     

Sugarcane bagasse cellulose fibres and their hydrous niobium phosphate composites: synthesis and characterization by XPS,XRD and SEM

Cellulose fibres obtained from sugarcane bagasse were submitted to a purification process, which consisted of an acid hydrolysis for elimination of the major part of lignin and hemicellulose. This was followed by a delignification process carried out in two steps to yield crude cellulose (CCell) fibres in the first one and with a subsequent bleaching in order to yield bleached cellulose fibres (BCell). Composites of crude and bleached cellulose fibres with hydrous niobium phosphate, cell/NbOPO₄·nH₂O, were subsequently synthesized. Scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction characterization of the obtained materials showed CCell/NbOPO₄·nH₂O and BCell/NbOPO₄·nH₂O are real composites. The nature of the cellulose (CCell or BCell) has an important role on the composites obtained, namely on the niobium salt composition at the composite surface. The synthesis of membranes of both cellulose and mixed matrix cellulose/NbOPO₄·nH₂O was only possible when the bleached cellulose was used.     

Pretreatment of corn stover using wet oxidation to enhance enzymatic digestibility

Corn stover is an abundant, promising raw material for fuel ethanol production. Although it has a high cellulose content, without pretreatment it resists enzymatic hydrolysis, like most lignocellulosic materials. Wet oxidation (water, oxygen, mild alkali or acid, elevated temperature and pressure) was investigated to enhance the enzymatic digestibility of corn stover. Six different combinations of reaction temperature, time, and pH were applied. The best conditions (60g/L of corn stover, 195°C, 15 min, 12 bar O₂, 2 g/L of Na₂CO₃) increased the enzymatic conversion of corn stover four times, compared to untreated material. Under these conditions 60% of hemicellulose and 30% of lignin were solubilized, whereas 90% of cellulose remained in the solid fraction. After 24-h hydrolysis at 50°C using 25 filter paper units (FPU)/g of dry matter (DM) biomass, the achieved conversion of cellulose to glucose was about 85%. Decreasing the hydrolysis temperature to 40°C increased hydrolysis time from 24 to 72 h. Decreasing the enzyme loading to 5 FPU/g of DM biomass slightly decreased the enzymatic conversion from 83.4 to 71%. Thus, enzyme loading can be reduced without significantly affecting the efficiency of hydrolysis, an important economical aspect.     

Chemical and thermal analysis of the biopolymers in thyme (Thymus vulgaris)

Thyme (Thymus vulgaris) has been known, long time ago, for its aromatic properties. It contains essential oils and polymers such as cellulose (mixture of hemicellulose and cellulose) and lignin. The thyme, studied in this work, was gathered from the same place, in the period from November 1999 to October 2000. The chemical analysis (water, total ash, essential oils, extractive substances, cellulose, holocellulose and lignin) can be used roughly in the characterisation of the four periods that correspond to the four seasons of the year. The cellulose level was found to be more than lignin level in the wet periods (growth of the plant). The opposite was found in the dry periods. The total ash and essential oil levels were found to be high during the period of high pluviometry. The thermal decomposition of cellulose and holocellulose was found to fit well with the first-order kinetics. The activation energy, under air flow, was 185 and 196 kJ mol⁻¹ for cellulose and holocellulose, respectively. The maximum decomposition rate and thermal analysis heating rate of lignin were found to have a direct linear relationship.     

Electrospun recycled PET-based mats: Tuning the properties by addition of cellulose and/or lignin

The aim of this study was to evaluate the influence of cellulose and/or lignin on the properties of mats prepared from dissolution (for 48 h or 72 h, solvent: trifluoroacetic acid) of recycled poly (ethylene terephthalate) (PET). Briefly, the presence of cellulose led to a tendency of higher average fiber diameter and average pore area as well as lower average porosity compared to the neat mat (PET_ref, 242 ± 59 nm, 9.6 ± 1.1 10⁴ nm² and 19.0 ± 1.1%, respectively). The T_g values for electrospun PET combined with cellulose and/or lignin were higher than that of PET_ref (92.5 ± 0.1 °C), and the tensile strength increased with the cellulose and/or lignin loading. In addition, the presence of lignin (72 h of dissolution) led to a mat with an elongation at break of 149 ± 9% compared to 14 ± 2% for PET_ref. The results indicated that the properties of mats based on PET can be tuned by adding cellulose and/or lignin to solutions posteriorly electrospun as well as by varying the dissolution time.     

Application of sequential aqueous steam treatments to the fractionation of softwood

The FIRST (Feedstock Impregnation and Rapid Steam Treatment) approach was used in this study to isolate extractives, hemicellulose, lignin, fibers, and cellulosic fines of softwood. With hydrolysis and fermentation of the hemicellulose and cellulosic fines fractions, this approach produces four co-products: extractives, cellulose, lignin, and ethanol. The first unit operation uses aqueous/alcohol to remove and recover the extractive rich fraction. The second unit operation uses steam treatment to destructure the matrix and solubilize a large fraction of the hemicelluloses. The third unit operation uses alkaline delignification to dissolve a lignin fraction. The fourth unit operation uses the refining process to separate fibers from cellulosic fines. The fibers are bleached. The yields of lignin and bleached cellulose were about 20.0 kg and 38.3 kg out of 100 kg initial dry pine, respectively. The recovered hemicelluloses were 23.3 kg (containing 16.1 kg hexoses and 5.0 kg pentoses) and the cellulose fines derived hexoses amounted to 3.4 kg out of 100 kg initial dry pine. When the two liquors containing the hemicellulose sugars and the cellulose fines-derived hexoses were fermented for ethanol production, an ethanol yield of 6.8 kg was obtained.     

Characterization of Highly Porous Materials from Cellulose Carbamate

Highly porous cellulose was formed by gelation of cellulose carbamate solutions in caustic soda. Two methods for gel preparation were optimized for the formation of beads and bulky materials – the chemical precipitation from dilute sulfuric acid and the thermal gelation by annealing at elevated temperatures. Various methods were used for characterizing of the pores of low density materials: scanning electron microscopy, small angle X-ray scattering, mercury intrusion and nitrogen sorption. These methods were optimized and used for characterizing the complete pore system from micro to macro pores. The effects of different preparation (cellulose carbamate concentration in caustic soda) and processing (precipitation, drying and pyrolysis) on the pore structure were studied by the set of complementary methods. Aerocell samples with a minimum density of 0.06 g/cm³ were prepared from cellulose carbamate. They are characterized by a broad pore size distribution ranging from 0.5 nm to 1 mm, specific internal surfaces of up to 660 m²/g and total pore volumes of up to 18 cm³/g.     

TG-MS analysis for studying the effects of fire retardants on the pyrolysis of pine-needles and their components

Thermogravimetry-mass spectrometry (TG-MS) was used to study the effect of the inorganic salts (NH₄)₂SO₄ and (NH₄)₂HPO₄, active substances of many commercial forest fire retardants, on the pyrolysis of Pinus halepensis needles and their main components (cellulose, lignin and extractives). These salts seemed to affect the pyrolysis of cellulose by increasing significantly the char residue, decreasing the pyrolysis temperature and changing the composition of the evolved gases, that is, increasing levoglucosenone and decreasing oxygen containing volatile products. (NH₄)₂SO₄ seemed to have negligible effect on the pyrolysis of lignin, while (NH₄)₂HPO₄ increased the char residue and decrease the relative contribution of guaiacols in the evolved gases. No effects of the inorganic salts on the extractives were observed. Finally, the inorganic salts seemed to affect the pyrolysis of pine-needles, mainly the cellulose component, but the effects were not as intense as in the pyrolysis of cellulose.     

Electron-beam decomposition of phytogenous substances: Solid-to-liquid conversion

Chain destruction and distillation of various lignocellulose materials were initiated by 8 MeV electron beam at a dose rate of ∼2 kGy/s. Electron-beam distillation transforms plant materials to organic liquid several times more productively than conventional dry distillation (pyrolitic). For example, the yield of an organic condensate from cellulose, wood and straw was ⩾50 wt%. Products of electron-beam distillation differed by composition from products of conventional dry distillation. For example, the tar forming by lignin radiolysis contains very few alkylbenzenes and alkylphenols. High efficiency of electron-beam distillation can be a key to development of wasteless methods for plant materials transformation into the useful products, such as polymerization retarders, monomers or the alternative fuel.     

Experimental and molecular simulating study on promoting electrolyte-immersed mechanical properties of cellulose/lignin separator for lithium-ion battery

Lithium-ion batteries have been developing intensively and earn an unprecedented reputation, yet advanced performance and safety issue still require considerable investigation. Separator is vital to comprehensive properties of batteries, where the mechanical properties are key to breaking through of new-type separator. Unfortunately, electrolyte submersion has caused damage to strength of cellulose separator. Whereupon, in this work, cellulose separator is optimized by introducing lignin particles to promote electrolyte-immersed mechanical strength. Experiments are conducted concerning surface morphology, contact angle, porosity, electrolyte uptake, mechanical properties and electrochemical performance. Molecular simulation is implemented to explore the mechanism of tensile behavior of cellulose and lignin subjected to electrolyte solvents. Experimental results confirm positive effect of lignin addition in improving mechanical properties and simultaneously maintaining impressive electrochemical performance of the cellulose/lignin composites separators. Besides, lignin addition amount of 2.5% and 5% is recommended to achieve promising overall properties. Molecular simulation has successfully unveiled that weakening of cellulose separator submerged in electrolyte is resulted by the deformed cellulose amorphous region and the promoting effect of adding lignin is contributed from the new hydrogen bonds generated between cellulose and lignin molecules. Hopefully, this work provides novel insight on preparing remarkable separator and mechanism of materials behavior.     

Evaluation and Characterization of Forage Sorghum as Feedstock for Fermentable Sugar Production

Sorghum is a tropical grass grown primarily in semiarid and drier parts of the world, especially areas too dry for corn. Sorghum production also leaves about 58 million tons of by-products composed mainly of cellulose, hemicellulose, and lignin. The low lignin content of some forage sorghums such as brown midrib makes them more digestible for ethanol production. Successful use of biomass for biofuel production depends on not only pretreatment methods and efficient processing conditions but also physical and chemical properties of the biomass. In this study, four varieties of forage sorghum (stems and leaves) were characterized and evaluated as feedstock for fermentable sugar production. Fourier transform infrared spectroscopy and X-ray diffraction were used to determine changes in structure and chemical composition of forage sorghum before and after pretreatment and the enzymatic hydrolysis process. Forage sorghums with a low syringyl/guaiacyl ratio in their lignin structure were easy to hydrolyze after pretreatment despite the initial lignin content. Enzymatic hydrolysis was also more effective for forage sorghums with a low crystallinity index and easily transformed crystalline cellulose to amorphous cellulose, despite initial cellulose content. Up to 72% hexose yield and 94% pentose yield were obtained using modified steam explosion with 2% sulfuric acid at 140 °C for 30 min and enzymatic hydrolysis with cellulase (15 filter per unit (FPU)/g cellulose) and β-glucosidase (50 cellobiose units (CBU)/g cellulose).