Concept

Cellulose

Summary
Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fiber is 90%, that of wood is 40–50%, and that of dried hemp is approximately 57%. Cellulose is mainly used to produce paperboard and paper. Smaller quantities are converted into a wide variety of derivative products such as cellophane and rayon. Conversion of cellulose from energy crops into biofuels such as cellulosic ethanol is under development as a renewable fuel source. Cellulose for industrial use is mainly obtained from wood pulp and cotton. Some animals, particularly ruminants and termites, can digest cellulose wit
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Related publications (65)

Developing structure-oriented models for enzymatic hydrolysis of lignocellulosic biomass

Jessica Charlène Rohrbach

With the unsustainable use of fossil fuels increasing strains on human institutions and ecosystems, the development of a renewable energy alternative is of paramount importance. Second generation biorefineries, based on the production of fuels and chemicals from lignocellulosic biomass, appear as attractive alternative to their non-renewable counterparts. Within the multiplicity of existing valorisation routes, enzymatic hydrolysis of lignocellulosic biomass into its constituent sugars has generated considerable interest, notably in the context of biofuels production. However, major hurdles stemming from the intricate structure of the lignocellulosic feedstocks still impede large-scale deployment of this process. The multiplicity of highly intertwined spatiotemporal factors impacting the enzymatic hydrolysis of lignocellulosic biomass represents a challenge to rationally design efficient hydrolysis process. Here, we propose a theory-based modelling framework relying on pore-diffusion and surface reaction to explore the effects of recognised bottlenecks on the enzymatic hydrolysis of lignocellulosic substrates. The model is based on a set of partial differential equations describing the evolution of the substrate morphology to investigate the interplay between experimental conditions and the physical characteristics of biomass particles as the reaction proceeds. The overall quantity of cellulase present in the hydrolysis mixture is carefully considered to investigate its interplay with the available accessible cellulose surface. Also, non-uniformity in terms of cellulose accessibility and cellulose digestibility are introduced in the model to weight their influence on observed hydrolysis rates. Finally, deactivation mechanisms are considered through unproductive adsorption of cellulases on both cellulose and lignin fractions, with the existence of such phenomena alleged to be critical in the efficiency of the hydrolysis process. Based on predictions of our model, we were able to confirm the critical role of cellulose accessibility, as defined by the combination of particle size, porosity and accessible cellulose surface, in dictating early reaction rates for a range of pretreated beech wood substrates. While high biomass loading should be favoured to improve enzyme penetration in the substrates, high enzyme loadings going beyond the initial number of cellulose adsorption appeared beneficial in notably two cases: (i) to promote internal diffusion in large particles and (ii) counteract undesired enzyme adsorption on lignin. For the latter, a relatively low increase in enzyme loading was sufficient to offset the resulting slowdown. We also showed that the existence of structural heterogeneities, and in particular non-uniform pore volume distribution within the lignocellulosic samples, contribute to the rate slowdown observed at later stage of the hydrolysis, while not explaining it in its entirety. Unproductive adsorption to cellulose, coupled to decrease in the cellulase efficiency at the cellulose surface, appeared as major contributor to the rate slowdown. Overall, we show how the use of a theory-based model can help decouple and evaluate the effects of key factors in the enzymatic hydrolysis of lignocellulosic biomass. As such, our model can help pave the way towards efficient integrated rational design strategies for enzyme process engineering for biomass conversion.
EPFL2021

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.
EPFL2012
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