Surface characterization of pretreated and microbial-treated populus cross-sections

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Tolbert, Allison K.
Ragauskas, Arthur J.
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With the finite supply of fossil fuels and the contribution they have made to greenhouse gas emissions, one of the most promising alternative renewable energy sources for fuel is bioethanol. Lignocellulosic biomasses (e.g. woody biomass, grasses, and agriculture/forestry residues) are ideal feedstock since they avoid the “food versus fuel” debate of the first-generation biofuels from food-based crops. However, the challenge of utilizing the lignocellulosic biomass lies in overcoming its natural recalcitrance in order to ferment the sugars into ethanol. Various methods have been developed to increase cellulose accessibility by altering the physical and chemical structure of the plant cell wall. Optimizing chemical or biological treatments requires enhanced characterization techniques to analyze biomass changes. In this dissertation, time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to analyze the surface chemistry of chemically pretreated or microbial treated juvenile poplar stem sections. The first objective of this thesis is to illustrate the advantages of surface characterization in biomass utilization studies. Untreated, ammonia-treated, and organosolv-treated poplar samples were analyzed for their chemical composition, lignin syringyl-to-guaiacyl ratio, and sugar release using surface and bulk characterization techniques. The second objective is to gain insight into the workings of potential consolidated bioprocessing microorganisms on the surface of poplar samples. Surface characterization of biomass in microbial studies is often neglected despite the common knowledge that enzymes first interact and bind to the sample’s surface. Scanning electron microscopy and ToF-SIMS analysis provide insight into how Caldicellulosiruptor bescii and its enzymes alter the surface of the poplar cross-sections. The ToF-SIMS along with confocal laser scanning microscopy determines how Clostridium thermocellum hydrolysis impacts the surface chemistry of the poplar cross-sections. The third objective is to determine the impact biomass recalcitrance has on enzymatic hydrolysis and microbial fermentation in relation to the surface chemistry. The influence lignin content, lignin S/G ratio, and cellulose content has on the glucose release of pretreated biomass was determined. Potential causes for the limited hydrolysis of poplar by C. thermocellum was addressed through chemical analysis, enzymatic hydrolysis, and surface characterization. Overall, the lignocellulosic content on the biomass surface does impact enzymatic and microbial hydrolysis.
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