Date of Award
Doctor of Philosophy (PhD)
Wayne S. Seames
The goal of this research was to explore selected non-biological lignin decomposition reactions to determine if these reactions have the potential to generate fuel and chemical intermediates in a commercially feasible manner. Two different strategies were employed: 1) metal doped silica-alumina and γ-alumina catalytic decomposition reactions, and 2) base catalyzed sub- and supercritical water liquefaction.
The first strategy was built upon previous research to explore metal doped silica-alumina and γ-alumina catalytic lignin decomposition reactions in a batch reactor system. Commercially available silica-alumina and γ-alumina catalysts were individually doped with 5 and 10 wt % of molybdenum or copper via a wet impregnation method. All catalysts were characterized with SEM, XRD and EDS analyses. Twelve runs in a Plackett-Burman design were used in a screening study of the significance of seven factors: catalyst support type, catalyst dopant type, dopant concentration, lignin concentration in water, catalyst-to-lignin ratio, reactor stirring rate, and reaction time. Aqueous products were extracted in DCM and analyzed in GC-MS. Solid residues from the reactor were analyzed via TGA and SEM. Screening study results showed that 5 wt% Cu on silica-alumina with 3 g of catalyst and 3 g of lignin in 250 ml of deionized water was the preferred condition to degrade lignin to monomers.
Next, the effect of varying the reaction temperature between 300 and 350 ℃ was investigated at the best reaction conditions from the screening studies. The optimum temperature was found to be around 320 ℃. Lower reaction temperatures (300 ℃) result in more unreacted lignin while higher temperatures (350 ℃) lead to increased formation of char and gaseous products. However, the quantity of monomers produced is still below the commercialization threshold.
The base catalyzed decomposition of lignin to monomeric compounds was studied in a novel continuous flow reactor. In these experiments, 10 wt % lignin was dissolved in a 5 wt % sodium hydroxide in water solution at either sub or supercritical conditions and then fed to a heated tubular reactor. The products from these reactions were collected as gas phase and water-soluble liquid compounds. The gas was quantified by weight difference while the water soluble compounds were acidified and extracted in DCM and analyzed with GC-FID/MS. The solid residues from the acidification treatment were filtered and analyzed with TGA. The morphology of solid residue particles was studied with SEM.
The concentration of monomers was found to increase with increasing temperature in supercritical condition experiments (6 wt %) , all of which were higher than those from the subcritical experiments (4 wt %) where the results showed that the maximum concentration of monomers (mostly creosols) was obtained at 340 ℃ in subcritical water (4.7 wt%). Analysis of solid residues showed that the concentration of partially decomposed lignin was lower in residue from supercritical condition experiments and the solid residues were larger in size compared to the char that was formed at subcritical conditions.
These initial experiments did not result in monomer production at desired levels, but they were comparable to metal-doped experiments results. However, the novel reactor design substantially minimizes concerns due to tar or char formation. Future work is recommended to explore additional reaction strategies using this approach.
Pourjafar, Sara, "An Investigation Of The Thermal Degradation Of Lignin" (2017). Theses and Dissertations. 2316.