High-Temperature Behavior of Cellulose I

Microwave-assisted depolymerization of chitin and chitosan extracted from crayfish shells waste: A sustainable approach based on graphene oxide catalysis

This study targeted the sustainable utilization of chitin and chitosan from crayfish shell waste, and further depolymerization of the recovered products in one step through synergy between microwaves and graphene oxide, aiming for the monosaccharides, 5-hydroxymethylfurfural and other high-value products. The results indicated that graphene oxide was more effective than graphene in enhancing the microwave absorption properties of the system, which is contrary to the parameters of their dielectric properties. The heating rate was increased by 0.37 K/s and 0.26 K/s when graphene oxide was introduced into the chitin and chitosan depolymerization systems, respectively, at a microwave power of 5 W/g. The mechanism underlying the impact of graphene oxide on chitin and chitosan under a microwave field was proposed by analyzing the variations in the depolymerization products of chitin and chitosan systems under different reaction conditions, including holding time, catalyst content, solvent content, and reaction temperature. Furthermore, the recovered graphene oxide exhibited delamination upon redispersion in water, which was not observed in the initial samples. The infrared spectra and scanning electron microscopy results suggest that the catalytic reaction is associated with oxygen-containing functional groups. This study demonstrated the synergistic effect of microwaves and graphene oxide on the depolymerization of chitin and chitosan, and the ability to achieve rapid one-step depolymerization in an acid/alkali-free solvent, which provides a green and promising development for the degradation of carbohydrate macromolecules in crustacean solid waste.

Cooperative Catalysis Mechanism of Brønsted and Lewis Acids from Al(OTf)3 with Methanol for β-Cellobiose-to-Fructose Conversion: An Experimental and Theoretical Study

Al-containing catalysts, e.g., Al(OTf)3, show good catalytic performance toward the conversion of cellulose to fructose in methanol solution. Here, we report the catalytic isomerization and alcoholysis mechanisms for the conversion of cellobiose to fructose at the PBE0/6-311++G(d,p), aug-cc-pVTZ theoretical level, combining the relevant experimental verifications of electrospray ionization mass spectrometry (ESI-MS), high-performance liquid chromatography (HPLC), and the attenuated total reflection-infrared (ATR-IR) spectra. From the alcoholysis of Al(OTf)3 in methanol solution, the catalytically active species involves both the [CH3OH2]+ Brønsted acid and the [Al(CH3O)(OTf)(CH3OH)4]+ Lewis acid. There are two reaction pathways, i.e., one through glucose (glycosidic bond cleavage followed by isomerization, w-G) and another through cellobiulose (isomerization followed by glycosidic bond cleavage, w-L). The Lewis acid ([Al(CH3O)(OTf)(CH3OH)4]+) is responsible for the aldose-ketose tautomerization, while the Brønsted acid ([CH3OH2]+) is in charge of ring-opening, ring-closure, and glycosidic bond cleavage. For both w-G and w-L, the rate-determining steps are related to the intramolecular [1,2]-H shift between C1-C2 for the aldose-ketose tautomerization catalyzed by the [Al(CH3O)(OTf)(CH3OH)4]+ species. The Lewis acid ([Al(CH3O)(OTf)(CH3OH)4]+) exhibits higher catalytic activity toward the aldose-ketose tautomerization of glycosyl-chain-glucose to glycosyl-chain-fructose than that of chain-glucose to chain-fructose. Besides, the Brønsted acid ([CH3OH2]+) shows higher catalytic activity toward the glycosidic bond cleavage of cellobiulose than that of cellobiose. Kinetically, the w-L pathway is predominant, whereas the w-G pathway is minor. The theoretically proposed mechanism has been experimentally testified. These insights may advance on the novel design of the catalytic system toward the conversion of cellulose to fructose.

Understanding Nanocellulose-Water Interactions: Turning a Detriment into an Asset

Modern technology has enabled the isolation of nanocellulose from plant-based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of sustainable materials applications. Water is generally seen as a detrimental component when in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic materials such as paper or cardboard. However, water is an integral component in plants, and many applications of nanocellulose already accept the presence of water or make use of it. This review gives a comprehensive account of nanocellulose-water interactions and their repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose, materials applications, and analytical methods to map the water interactions and the effect of water on a nanocellulose matrix.

Cellulose Fast Pyrolysis Activated by Intramolecular Hydrogen Bonds

The conversion of inedible biomass by fast pyrolysis is a promising route for sustainable production of renewable fuels and value-added chemicals, but low selectivity toward desired products hampers its economic viability. Understanding the molecular-level reaction pathways of biomass fast pyrolysis could be the key to overcoming this challenge. However, the effects of intramolecular and interchain hydrogen bonds near the reaction center have not been thoroughly explored. In this work, the reaction pathways and kinetics of fast pyrolysis of cellulose, a major component of biomass, were investigated using the density functional theory. A new intramolecular hydroxyl-activated mechanism is presented for cellulose activation. Our calculations incorporating noncovalent interactions accurately captured the activation energy of 50.8 kcal mol^-1, agreeable with the apparent activation energy measured experimentally. The findings of cellulose pyrolysis provide insights into the investigation of interactions during real-life biomass pyrolysis.

Cellulose: Characteristics and applications for rechargeable batteries

Cellulose, an abundant natural polymer, has promising potential to be used for energy storage systems because of its excellent mechanical, structural, and physical characteristics. This review discusses the structural features of cellulose and describes its potential application as an electrode, separator, and binder, in various types of high-performing batteries. Various surface and structural characteristics of cellulose (e.g., fiber size, surface functional groups, the hierarchy of pores, and porosity levels) that contribute to its electrochemical performance are discussed. Cellulose structure/property/processing/function relationships are further focused and elucidated in terms of the latest developments in the emerging field of sustainable materials in Li-Ion, Na-Ion, and LiS batteries.

Potential cosmeceutical lamellar liquid crystals containing black longan (Dimocarpus longan Lour.) seed extract for MMP-1 and hyaluronidase inhibition

The aims of this study were to evaluate the biological activities of black Dimocarpus longan Lour. seed extracts and develop a lamellar liquid crystal (LLC). Different solvents, including petroleum ether, ethyl acetate, and 95% v/v ethanol, were used in the maceration of black D. longan seeds. The inhibitory activities on matrix metalloproteinase-1 (MMP-1) and hyaluronidase were evaluated. The irritating potency of D. longan seed extracts was determined using the hen's egg chorioallantoic membrane test. The extract with the strongest anti-ageing activities and no irritant impact was examined for its chemical contents using high-performance liquid chromatography and incorporated into the LLC. Various factors affecting the LLC formulations, including surfactant types and amounts, thickening agent types and amounts, and various oil types, were investigated. The results demonstrated that the ethyl acetate extract (EtOAc) was the most potent inhibitor of MMP-1 (IC₅₀ = 21.7 ± 5.4 µg/mL) and hyaluronidase (oleanolic acid equivalent = 0.44 ± 0.03 g per g extract). Interestingly, its MMP-1 inhibition was equivalent to that of oleanolic acid, corilagin, and gallic acid. Furthermore, its hyaluronidase inhibition was equivalent to that of gallic acid and ellagic acid. Gallic acid, which was the most abundant compound (15.6% ± 0.06% w/w) was suggested as the compound responsible for the biological activities of EtOAc extract. Considering its potential anti-skin ageing properties with no irritation of EtOAc extract, it was incorporated into the most suitable LLC, which was composed of 5% w/w Lexfeel® D5 oil, 0.5% w/w Carbopol® U21, 2% w/w Liquid Crystal Cream Maker, and 92.5% w/w DI water. The LLC containing EtOAc extract presented birefringence under a polarizing light microscope, indicating its crystallinity. The formulation had good stability after heating-cooling cycles in terms of external appearance, crystallinity, viscosity, and pH (5.5). As a result, it is recommended as a potential cosmeceutical formulation for anti-skin wrinkling. It is proposed that more research be conducted on the safety and efficacy of the treatment on human volunteers.

Liquefaction of Cellulose for Production of Advanced Porous Carbon Materials

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to a liquid state suitable for the preparation of porous carbons are summarized in this review. Hydrothermal liquefaction leading to liquid mixtures of low-molecular-weight organics is described in detail together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in a strong gravitation field). We also focus on dissolution of cellulose without decomposition, with special attention paid to dissolution of nonderivatized cellulose. For this purpose, cold alkalines, hot acids, ionic liquids, or alcohols are commonly used.

Pyrolytic activation of cellulose: Energetics and condensed phase effects

Bottom-up design of lignocellulose pyrolysis to optimize the quality and yield of bio-oil is hindered by the limited knowledge of the underlying condensed phase biomass chemistry. The influence of condensed...

Combining Computational Chemistry and Crystallography for a Better Understanding of the Structure of Cellulose

The approaches in this article seek to enhance understanding of cellulose at the molecular level, independent of the source and the particular crystalline form of cellulose. Four main areas of structure research are reviewed. Initially, the molecular shape is inferred from the crystal structures of many small molecules that have β-(1→4) linkages. Then, conformational analyses with potential energy calculations of cellobiose are covered, followed by the use of Atoms-In-Molecules theory to learn about interactions in experimental and theoretical structures. The last section covers models of cellulose nanoparticles. Controversies addressed include the stability of twofold screw-axis conformations, the influence of different computational methods, the predictability of crystalline conformations by studies of isolated molecules, and the twisting of model cellulose crystals.

Molecular Insight into the Cosolvent Effect on Lignin-Cellulose Adhesion

Understanding and controlling the physical adsorption of lignin compounds on cellulose pulp are key parameters in the successful optimization of organosolv processes. The effect of binary organic-aqueous solvents on the coordination of lignin to cellulose was studied with molecular dynamics simulations, considering ethanol and acetonitrile to be organic cosolvents in aqueous solutions in comparison to their monocomponent counterparts. The structures of the solvation shells around cellulose and lignin and the energetics of lignin-cellulose adhesion indicate a more effective disruption of lignin-cellulose binding by binary solvents. The synergic effect between solvent components is explained by their preferential interactions with lignin-cellulose complexes. In the presence of pure water, long-lasting H-bonds in the lignin-cellulose complex are observed, promoted by the nonfavorable interactions of lignin with water. Ethanol and acetonitrile compete with water and lignin for cellulose oxygen binding sites, causing a nonlinear decrease in the lignin-cellulose interactions with the amount of the organic component. This effect is modulated by the water exclusion from the cellulose solvation shell by the organic solvent component. The amount and rate of water exclusion depend on the type of organic cosolvent and its concentration.

Electric Field Intensity Effects on the Microstructural Characteristics Evolution of Methyl Vinyl Silicone Rubber via Molecular Simulation

During ultra high voltage (UHV) transmission, the discharge caused by high intensity electric fields aggravates the aging process of external insulation materials used for composite insulators. The microstructural characteristics of its base material polymer-methyl vinyl silicone rubber-are the key basis for the performance of insulation materials under electric field exposure. Based on molecular dynamics simulations, a molecular model of methyl vinyl silicone rubber was established. Mechanisms influencing the microstructure evolution under electric fields had been studied at the atomic level. The results showed that the initial reaction characteristics of silicone rubber molecules involve the violent vibrated of all the methyl and vinyl atoms, and shortening of the chemical bonds. The neighboring groups were close to each other and generated different amounts of -Si-Si- bonds. This promoted the helical shrinkage of the molecule, and protrusion of the middle of the molecule which presented an inverted U shape. The high electric field greatly reduced the total energy of molecules, and the potential energy in particular was more severely destroyed, resulting in degradation of its structure. Besides, as the electric field intensity increased, the elastic modulus of the molecule gradually increased. It was shown that high electric fields would make the stiffness of silicone rubber become larger, and the brittleness to become stronger, which reduced the mechanical properties of materials, accelerating its aging. The results provide a theoretical basis for establishing the connection between the micro appearance and macro characteristics of materials, as well as reference values for the optimization of base materials used for making composite insulators.

Acetic Acid Bacteria in the Food Industry: Systematics, Characteristics and Applications

The group of Gram-negative bacteria capable of oxidising ethanol to acetic acid is called acetic acid bacteria (AAB). They are widespread in nature and play an important role in the production of food and beverages, such as vinegar and kombucha. The ability to oxidise ethanol to acetic acid also allows the unwanted growth of AAB in other fermented beverages, such as wine, cider, beer and functional and soft beverages, causing an undesirable sour taste. These bacteria are also used in the production of other metabolic products, for example, gluconic acid, l-sorbose and bacterial cellulose, with potential applications in the food and biomedical industries. The classification of AAB into distinct genera has undergone several modifications over the last years, based on morphological, physiological and genetic characteristics. Therefore, this review focuses on the history of taxonomy, biochemical aspects and methods of isolation, identification and quantification of AAB, mainly related to those with important biotechnological applications.

Hydrogen bonds and twist in cellulose microfibrils

There is increasing experimental and computational evidence that cellulose microfibrils can exist in a stable twisted form. In this study, atomistic molecular dynamics (MD) simulations are performed to investigate the importance of intrachain hydrogen bonds on the twist in cellulose microfibrils. We systematically enforce or block the formation of these intrachain hydrogen bonds by either constraining dihedral angles or manipulating charges. For the majority of simulations a consistent right handed twist is observed. The exceptions are two sets of simulations that block the O2-O6' intrachain hydrogen bond, where no consistent twist is observed in multiple independent simulations suggesting that the O2-O6' hydrogen bond can drive twist. However, in a further simulation where exocyclic group rotation is also blocked, right-handed twist still develops suggesting that intrachain hydrogen bonds are not necessary to drive twist in cellulose microfibrils.

Unraveling cellulose microfibrils: a twisted tale

Molecular dynamics (MD) simulations of cellulose microfibrils are pertinent to the paper, textile, and biofuels industries for their unique capacity to characterize dynamic behavior and atomic-level interactions with solvent molecules and cellulase enzymes. While high-resolution crystallographic data have established a solid basis for computational analysis of cellulose, previous work has demonstrated a tendency for modeled microfibrils to diverge from the linear experimental structure and adopt a twisted conformation. Here, we investigate the dependence of this twisting behavior on computational approximations and establish the theoretical basis for its occurrence. We examine the role of solvent, the effect of nonbonded force field parameters [partial charges and van der Waals (vdW) contributions], and the use of explicitly modeled oxygen lone pairs in both the solute and solvent. Findings suggest that microfibril twisting is favored by vdW interactions, and counteracted by both intrachain hydrogen bonds and solvent effects at the microfibril surface.

Optimizing Solute-Solute Interactions in the GLYCAM06 and CHARMM36 Carbohydrate Force Fields Using Osmotic Pressure Measurements

GLYCAM06 and CHARMM36 are successful force fields for modeling carbohydrates. To correct recently identified deficiencies with both force fields, we adjusted intersolute nonbonded parameters to reproduce the experimental osmotic coefficient of glucose at 1 M. The modified parameters improve behavior of glucose and sucrose up to 4 M and improve modeling of a dextran 55-mer. While the modified parameters may not be applicable to all carbohydrates, they highlight the use of osmotic simulations to optimize force fields.

Multi-scale processes of beech wood disintegration and pretreatment with 1-ethyl-3-methylimidazolium acetate/water mixtures

BACKGROUND

The valorization of biomass for chemicals and fuels requires efficient pretreatment. One effective strategy involves the pretreatment with ionic liquids which enables enzymatic saccharification of wood within a few hours under mild conditions. This pretreatment strategy is, however, limited by water and the ionic liquids are rather expensive. The scarce understanding of the involved effects, however, challenges the design of alternative pretreatment concepts. This work investigates the multi length-scale effects of pretreatment of wood in 1-ethyl-3-methylimidazolium acetate (EMIMAc) in mixtures with water using spectroscopy, X-ray and neutron scattering.

RESULTS

The structure of beech wood is disintegrated in EMIMAc/water mixtures with a water content up to 8.6 wt%. Above 10.7 wt%, the pretreated wood is not disintegrated, but still much better digested enzymatically compared to native wood. In both regimes, component analysis of the solid after pretreatment shows an extraction of few percent of lignin and hemicellulose. In concentrated EMIMAc, xylan is extracted more efficiently and lignin is defunctionalized. Corresponding to the disintegration at macroscopic scale, SANS and XRD show isotropy and a loss of crystallinity in the pretreated wood, but without distinct reflections of type II cellulose. Hence, the microfibril assembly is decrystallized into rather amorphous cellulose within the cell wall.

CONCLUSIONS

The molecular and structural changes elucidate the processes of wood pretreatment in EMIMAc/water mixtures. In the aqueous regime with >10.7 wt% water in EMIMAc, xyloglucan and lignin moieties are extracted, which leads to coalescence of fibrillary cellulose structures. Dilute EMIMAc/water mixtures thus resemble established aqueous pretreatment concepts. In concentrated EMIMAc, the swelling due to decrystallinization of cellulose, dissolution of cross-linking xylan, and defunctionalization of lignin releases the mechanical stress to result in macroscopic disintegration of cells. The remaining cell wall constituents of lignin and hemicellulose, however, limit a recrystallization of the solvated cellulose. These pretreatment mechanisms are beyond common pretreatment concepts and pave the way for a formulation of mechanistic requirements of pretreatment with simpler pretreatment liquors.

Hydrogen-Bonding Network and OH Stretch Vibration of Cellulose: Comparison of Computational Modeling with Polarized IR and SFG Spectra

Hydrogen bonds play critical roles in noncovalent directional interactions determining the crystal structure of cellulose. Although diffraction studies accurately determined the coordinates of carbon and oxygen atoms in crystalline cellulose, the structural information on hydrogen atoms involved in hydrogen-bonding is still elusive. This could be complemented by vibrational spectroscopy; but the assignment of the OH stretch peaks has been controversial. In this study, we performed calculations using density functional theory with dispersion corrections (DFT-D2) for the cellulose Iβ crystal lattices with the experimentally determined carbon and oxygen coordinates. DFT-D2 calculations revealed that the OH stretch vibrations of cellulose are highly coupled and delocalized through intra- and interchain hydrogen bonds involving all OH groups in the crystal. Additionally, molecular dynamics (MD) simulations of a single cellulose microfibril showed that the conformations of OH groups exposed at the microfibril surface are not well-defined. Comparison of the computation results with the experimentally determined IR dichroism of uniaxially aligned cellulose microfibrils and the peak positions of various cellulose crystals allowed unambiguous identification of OH stretch modes observed in the vibrational spectra of cellulose.

Electric Interfacial Layer of Modified Cellulose Nanocrystals in Aqueous Electrolyte Solution: Predictions by the Molecular Theory of Solvation

The X-ray crystal structure-based models of Iα cellulose nanocrystals (CNC), both pristine and containing surface sulfate groups with negative charge 0-0.34 e/nm(2) produced by sulfuric acid hydrolysis of softwood pulp, feature a highly polarized "crystal-like" charge distribution. We perform sampling using molecular dynamics (MD) of the structural relaxation of neutral pristine and negatively charged sulfated CNC of various lengths in explicit water solvent and then employ the statistical mechanical 3D-RISM-KH molecular theory of solvation to evaluate the solvation structure and thermodynamics of the relaxed CNC in ambient aqueous NaCl solution at a concentration of 0.0-0.25 mol/kg. The MD sampling induces a right-hand twist in CNC and rearranges its initially ordered structure with a macrodipole of high-density charges at the opposite faces into small local spots of alternating charge at each face. This surface charge rearrangement observed for both neutral and charged CNC significantly affects the distribution of ions around CNC in aqueous electrolyte solution. The solvation free energy (SFE) of charged sulfated CNC has a minimum at a particular electrolyte concentration depending on the surface charge density, whereas the SFE of neutral CNC increases linearly with NaCl concentration. The SFE contribution from Na(+) counterions exhibits behavior similar to the NaCl concentration dependence of the whole SFE. An analysis of the 3D maps of Na(+) density distributions shows that these model CNC particles exhibit the behavior of charged nanocolloids in aqueous electrolyte solution: an increase in electrolyte concentration shrinks the electric interfacial layer and weakens the effective repulsion between charged CNC particles. The 3D-RISM-KH method readily treats solvent and electrolyte of a given nature and concentration to predict effective interactions between CNC particles in electrolyte solution. We provide CNC structural models and a modeling procedure for studies of effective interactions and the formation of ordered phases of CNC suspensions in electrolyte solution.

Molecular deformation mechanisms in cellulose allomorphs and the role of hydrogen bonds

Differences in tensile properties between cellulose crystal allomorphs cannot be rationalized by simply counting hydrogen bonds. From molecular dynamics computer simulations the cooperative nature of energy contributions to axial cellulose crystal modulus becomes apparent. Using a decomposition of inter and intramolecular forces as a function of tensile strain, the three allomorphs show dramatic differences in terms of how the contributions to elastic energy are distributed between covalent bonds, angles, dihedrals, electrostatic forces, dispersion and steric forces.

Unique aspects of the structure and dynamics of elementary Iβ cellulose microfibrils revealed by computational simulations

The question of how many chains an elementary cellulose microfibril contains is critical to understanding the molecular mechanism(s) of cellulose biosynthesis and regulation. Given the hexagonal nature of the cellulose synthase rosette, it is assumed that the number of chains must be a multiple of six. We present molecular dynamics simulations on three different models of Iβ cellulose microfibrils, 18, 24, and 36 chains, to investigate their structure and dynamics in a hydrated environment. The 36-chain model stays in a conformational space that is very similar to the initial crystalline phase, while the 18- and 24-chain models sample a conformational space different from the crystalline structure yet similar to conformations observed in recent high-temperature molecular dynamics simulations. Major differences in the conformations sampled between the different models result from changes to the tilt of chains in different layers, specifically a second stage of tilt, increased rotation about the O2-C2 dihedral, and a greater sampling of non-TG exocyclic conformations, particularly the GG conformation in center layers and GT conformation in solvent-exposed exocyclic groups. With a reinterpretation of nuclear magnetic resonance data, specifically for contributions made to the C6 peak, data from the simulations suggest that the 18- and 24-chain structures are more viable models for an elementary cellulose microfibril, which also correlates with recent scattering and diffraction experimental data. These data inform biochemical and molecular studies that must explain how a six-particle cellulose synthase complex rosette synthesizes microfibrils likely comprised of either 18 or 24 chains.

Plant nanobionic materials with a giant temperature response mediated by pectin-Ca2+

Conventional approaches to create biomaterials rely on reverse engineering of biological structures, on biomimicking, and on bioinspiration. Plant nanobionics is a recent approach to engineer new materials combining plant organelles with synthetic nanoparticles to enhance, for example, photosynthesis. Biological structures often outperform man-made materials. For example, higher plants sense temperature changes with high responsivity. However, these properties do not persist after cell death. Here, we permanently stabilize the temperature response of isolated plant cells adding carbon nanotubes (CNTs). Interconnecting cells, we create materials with an effective temperature coefficient of electrical resistance (TCR) of -1,730% K(-1), ∼2 orders of magnitude higher than the best available sensors. This extreme temperature response is due to metal ions contained in the egg-box structure of the pectin backbone, lodged between cellulose microfibrils. The presence of a network of CNTs stabilizes the response of cells at high temperatures without decreasing the activation energy of the material. CNTs also increase the background conductivity, making these materials suitable elements for thermal and distance sensors.

Solution Properties of Hemicellulose Polysaccharides with Four Common Carbohydrate Force Fields

Hemicellulose polysaccharides play an important role in the swelling behavior of the primary plant cell wall, and molecular dynamics simulations provide the means of gaining a concise understanding of the interactions of hemicellulose polysaccharides with water. Here, we compare four of the main polysaccharide force fields (CHARMM36 TIP3P, GROMOS56A6(CARBO) SPC, GLYCAM06h TIP3P, and GLYCAM06h TIP5P) for the most abundant hemicellulose backbone components. In particular, we compare aggregation, diffusion coefficients, system density, and investigate the free energy of hydration of saccharides in water. We find that the saccharides show nonphysical aggregation at low concentrations with the GLYCAM06h TIP3P force field, which can be rectified by the use of the TIP5P water model. As a result of the aggregation, GLYCAM06h TIP3P does not lead to reasonable diffusion coefficients whereas the diffusion coefficients, as well as the system density, agrees best with experimental data for the GLYCAM06h TIP5P force field. Overall, GLYCAM06h TIP5P gives good agreement with experimental free energy of hydration data for small saccharides. In addition, the free energy of hydration for short polysaccharides calculated with the GLYCAM06h TIP5P force field is consistent with the radial distribution functions between the polysaccharides and water, the hydration number of the polysaccharides, and the hydrogen bonds formed in the system.

MARTINI coarse-grained model for crystalline cellulose microfibers

Commercial-scale biofuel production requires a deep understanding of the structure and dynamics of its principal target: cellulose. However, an accurate description and modeling of this carbohydrate structure at the mesoscale remains elusive, particularly because of its overwhelming length scale and configurational complexity. We have derived a set of MARTINI coarse-grained force field parameters for the simulation of crystalline cellulose fibers. The model is adapted to reproduce different physicochemical and mechanical properties of native cellulose Iβ. The model is able not only to handle a transition from cellulose Iβ to another cellulose allomorph, cellulose IIII, but also to capture the physical response to temperature and mechanical bending of longer cellulose nanofibers. By developing the MARTINI model of a solid cellulose crystalline fiber from the building blocks of a soluble cellobiose coarse-grained model, we have provided a systematic way to build MARTINI models for other crystalline biopolymers.

Autohydrolysis pretreatment of Arundo donax: a comparison between microwave-assisted batch and fast heating rate flow-through reaction systems

BACKGROUND

Autohydrolysis of lignocellulosic biomass in liquid hot water has been widely studied owing to its high efficiency and relatively low cost. In the perspective of industrial applications, continuous or semi-continuous processes are more interesting than batch systems. Moreover, microwave heating of pretreatment systems has been proposed to intensify the kinetics of the process. In this study, the autohydrolysis of Arundo donax was performed in pure liquid hot water using a microwave-heated batch reactor and a semi-continuous flow-through reaction system with fast heating rate at the same operating conditions with the aim of performing a systematic comparison between the two different experimental apparatuses.

RESULTS

The effect of process temperature and time, biomass to water mass to volume ratio and water flow rate on the concentration and yield of hydrolysis products was investigated. The flow-through set-up allowed us to reach biomass solubilization up to 44.5 wt% on dry basis, while the batch system stopped at 34.5 wt% suggesting that the mass transfer could be the rate-determining step in the solubilization of the constituting biopolymers. For example, in the flow-through layout, using a flow rate of 3.5 mL/min at 200 °C with 20 min of processing time, quantitative recovery of hemicellulose was obtained with limited formation of degradation products. Interestingly, higher cellulose/hemicellulose extraction ratios were found using the microwave-assisted batch reactor. FTIR analyses of the solid residues recovered after the pretreatment offered independent information on the fractions of liquefied biopolymers complementary to those derived from HPLC and UV-Vis spectroscopy.

CONCLUSIONS

Collected experimental results indicated that the flow-through system can be adopted to obtain complete solubilization of the hemicellulose fraction of Arundo donax addressing the product distribution in soluble compounds towards fermentable sugars with limited formation of sugar degradation products and with limited penalty in terms of dilution of the hydrolysate solution. It was also found that microwaves can promote cellulose depolymerization and solubilization, thus allowing a more comprehensive utilization of the biomass and that infrared spectroscopy can be a useful technique to estimate the effect of the pretreatment.

Water structuring over the hydrophobic surface of cellulose

Many important biological solutes possess not only polar and hydrogen-bonding functionalities but also weakly hydrating, or hydrophobic, surfaces. While the aggregation of these hydrophobic surfaces has been shown to play an important role in the aggregation of individual chains of cellulose, it is not known whether the water structuring imposed by these hydrophobic surfaces more closely resembles that associated with small hydrophobic solutes like methane and fats or more closely resembles that associated with extended hydrophobic surfaces like mica or waxy planes. By using molecular dynamics simulations to characterize the water molecule orientations over different regions of the 100 surface of cellulose in contact with water, it was found that the hydrophobic strips of the cellulose crystal are sufficiently narrow that they hydrate like a fatty acid chain, rather than like a more extended surface, suggesting that their aggregation would be dominated by entropy rather than enthalpy.

Effect of microfibril twisting on theoretical powder diffraction patterns of cellulose Iβ

Previous studies of calculated diffraction patterns for cellulose crystallites suggest that distortions that arise once models have been subjected to MD simulation are the result of both microfibril twisting and changes in unit cell dimensions induced by the empirical force field; to date, it has not been possible to separate the individual contributions of these effects. To provide a better understanding of how twisting manifests in diffraction data, the present study demonstrates a method for generating twisted and linear cellulose structures that can be compared without the bias of dimensional changes, allowing assessment of the impact of twisting alone. Analysis of unit cell dimensions, microfibril volume, hydrogen bond patterns, glycosidic torsion angles, and hydroxymethyl group orientations confirmed that the twisted and linear structures collected with this method were internally consistent, and theoretical powder diffraction patterns for the two were shown to be effectively indistinguishable. These results indicate that differences between calculated patterns for the crystal coordinates and twisted structures from MD simulation can result entirely from changes in unit cell dimensions, and not from microfibril twisting alone. Although powder diffraction patterns for models in the 81-chain size regime were shown to be unaffected by twisting, suggesting that a modest degree of twist is not inconsistent with experimental data, it may be that other diffraction techniques are capable of detecting this structural difference. Until such time as definitive experimental evidence comes to light, the results of this study suggest that both twisted and linear microfibrils may represent an appropriate model for cellulose Iβ.

Fast Pyrolysis of Wood for Biofuels: Spatiotemporally Resolved Diffuse Reflectance In situ Spectroscopy of Particles

Fast pyrolysis of woody biomass is a promising process capable of producing renewable transportation fuels to replace gasoline, diesel, and chemicals currently derived from nonrenewable sources. However, biomass pyrolysis is not yet economically viable and requires significant optimization before it can contribute to the existing oil-based transportation system. One method of optimization uses detailed kinetic models for predicting the products of biomass fast pyrolysis, which serve as the basis for the design of pyrolysis reactors capable of producing the highest value products. The goal of this work is to improve upon current pyrolysis models, usually derived from experiments with low heating rates and temperatures, by developing models that account for both transport and pyrolysis decomposition kinetics at high heating rates and high temperatures (>400 °C). A new experimental technique is proposed herein: spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP), which is capable of measuring biomass composition during fast pyrolysis with high spatial (10 μm) and temporal (1 ms) resolution. Compositional data were compared with a comprehensive 2D single-particle model, which incorporated a multistep, semiglobal reaction mechanism, prescribed particle shrinkage, and thermophysical properties that varied with temperature, composition, and orientation. The STR-DRiSP technique can be used to determine the transport-limited kinetic parameters of biomass decomposition for a wide variety of biomass feedstocks.

Levoglucosan formation from crystalline cellulose: importance of a hydrogen bonding network in the reaction

Levoglucosan (1,6-anhydro-β-D-glucopyranose) formation by the thermal degradation of native cellulose was investigated by MP4(SDQ)//DFT(B3LYP) and DFT(M06-2X)//DFT(B3LYP) level computations. The computational results of dimer models lead to the conclusion that the degradation occurs by a concerted mechanism similar to the degradation of methyl β-D-glucoside reported in our previous study. One-chain models of glucose hexamer, in which the interchain hydrogen bonds of real cellulose crystals are absent, do not exhibit the correct reaction behavior of levoglucosan formation; for instance, the activation enthalpy (Ea =≈38 kcal mol(-1) ) is considerably underestimated compared to the experimental value (48-60 kcal mol(-1) ). This problem is solved with the use of two-chain models that contain interchain hydrogen bonds. The theoretical study of this model clearly shows that the degradation of the internal glucosyl residue leads to the formation of a levoglucosan precursor at the chain end and levoglucosan is selectively formed from this levoglucosan end. The calculated Ea (56-62 kcal mol(-1) ) agrees well with the experimental value. The computational results of three-chain models indicate that this degradation occurs selectively on the crystalline surface. All these computational results provide a comprehensive understanding of several experimental facts, the mechanisms of which have not yet been elucidated.

Investigating the thermal decomposition of starch and cellulose in model systems and toasted bread using domino tandem mass spectrometry

Many dietary products containing polysaccharides, mostly starch and cellulose, are processed by thermal treatment. Similarly to the formation of caramel from mono- and disaccharides, the chemical structure of the carbohydrates is dramatically altered by heat treatment. This contribution investigates the products of thermal decomposition of pure starch and cellulose as model systems followed by an investigation of bread obtained at comparable conditions using a combination of modern mass spectrometry techniques. From both starch and cellulose, dehydrated oligomers of glucose and dehydrated glucose have been predominately observed, with oligomers of more than four glucose moieties dominating. Moreover, disproportionation and oligomers with up to six carbohydrates units are formed through unselective glycosidic bond breakage. MALDI-MS data confirm the presence of the majority of products in toasted bread.