Transformation of biomass to value-added chemicals through non-thermal plasma
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This study explored non-thermal plasma (NTP) conversions of lignocellulosic biomass for producing biobased chemicals. Non-thermal plasma is emerging as a promising green, low-energy technology for biomass conversion, benefited by its ability to produce chemically reactive environment at mild conditions by using external electric field. In this PhD study, novel NTP assisted pyrolysis and plasma electrolysis were developed to convert cellulose and biomass to obtain several important chemicals. In the first study, cellulose was NTP pretreated using a dielectric barrier discharge reactor followed by pyrolysis. It showed that the maximum levoglucosan yield from the NTP pretreated cellulose was 78.6 wt%, compared to 58.2 wt% by pyrolyzing untreated cellulose. Comprehensive analysis of the NTP pretreated cellulose showed that homolytic cleavage of glycosidic bonds occur during the NTP treatment. The resulting free radicals with relative long lifetime were trapped within the cellulose structure, allowing subsequent pyrolysis of the NTP-pretreated cellulose to proceed through a radical-based depolymerization mechanism. The present results also revealed that although the radical-based mechanism is highly selective to levoglucosan formation, this pathway is usually discouraged when cellulose pyrolysis without plasma pretreatment due to the high energy barrier for homolytic cleavage. Initiating homolytic cleavage during the NTP pretreatment also helped the pretreated cellulose to produce higher yields of levoglucosan using lowered pyrolysis temperatures. In this study, 77.6 wt% of levoglucosan was produced by pyrolyzing NTP pretreated cellulose at 375 °C. In the second study, plasma electrolysis of cellulose was investigated using polar aprotic solvents and diluted sulfuric acid in the absence of external heating. A levoglucosenone (LGO) yield of 43 mol% could be obtained after 15 min of conversion in γ-valerolactone (GVL) by applying a voltage of 6 kV and frequency of 6 kHz. With sulfolane as the solvent, a 38 mol% yield of LGO was obtained within 3 min by applying a voltage of 4 kV. A comparison study showed that plasma electrolysis could produce much greater LGO yields using significantly less energy than conventional liquefaction in the same solvents. The study also revealed that the plasma electrolysis of cellulose proceeds through novel radical-based mechanisms involving in situ-generated hydrogen radicals, by which cellulose depolymerizes and dehydrates much more efficiently than it does during conventional liquefaction. During the plasma electrolysis process, cellulose conversion was significantly enhanced by synergistic effects between the Joule heating and plasma chemistry. In the third study, plasma electrolysis of red oak in GVL was performed in the presence of diluted sulfuric acid. Biomass was completely liquified within 10 min to produce carbohydrate-derived monomers including furfural (FF), LGO, levulinic acid (LA), and 1,4:3,6-dianhydro-α-D-glucopyranose (DGP). With 10.5 mM acid and 4 wt% biomass loading, LGO yield up to 40.6 mol% per cellulose fraction and furfural yield of 98.1 mol% per hemicellulose fraction were obtained. During the plasma electrolysis, rapid dissolution of lignin occurred prior to direct dehydrations of polysaccharides assisted by plasma. It showed that β-O-4 linkages were nearly completely disappeared in the plasma electrolysis-derived lignin. However, the plasma electrolysis extracted lignin could produce 20.91 wt% phenolic monomers, significantly higher than 8.6 wt% obtained from pyrolysis of natural lignin. Further investigation revealed that OH at Cɑ position of lignin side chain was either oxidized to Cɑ=O or removed to form Cɑ=Cβ bonds, thus greatly limiting condensation reactions of lignin.