Date of Award
Doctor of Philosophy
In recent decades, thermochemical conversion of biomass, such as pyrolysis, has gained popularity as a source for renewable materials. Pyrolysis generates three products: biochar that has shown promise as a soil amendment and carbon sequestration agent, bio-oil that could supplement petroleum-based products and transportation fuel, and syngas that is useful for ammonia, methanol, or hydrocarbon/aromatic production. At the molecular level, these samples are complex and difficult to analyze, which creates a bottleneck for thorough understanding. This dissertation utilizes high-resolution mass spectrometry (HRMS) to overcome the sample complexity and improve understanding at the molecular level.
HRMS was applied to understanding organic molecules entrapped in biochar during pyrolysis and gasification of switchgrass. Extraction of organic molecules used toluene and a mixture of water/methanol for hydrophobic aromatic compounds and hydrophilic polar compounds, respectively. Orbitrap mass spectrometric data acquisition revealed that molecular compounds previously known in bio-oils were observed for fast pyrolysis biochar, whereas polycyclic aromatic hydrocarbons (PAHs) with various ring sizes were observed for gasification and slow pyrolysis biochars.
Bio-oils from fast pyrolysis of switchgrass harvested at various times throughout the year were studied using high-resolution mass spectrometry. Nearly three hundred total nitrogen-containing species were detected through efficient ionization and accurate mass information. Nitrogen-containing species, particularly N2 compounds, were highly abundant for early summer bio-oils, but decrease significantly in later harvest times. Contour plots of double bond equivalent (DBE) versus carbon number and tandem mass spectrometric analysis were utilized to determine the major structural motif for N1 and NO class compounds as pyridine and N2 class compounds as imidazole. The dramatic decrease in nitrogen compounds correlates to the decomposition of proteins as the perennial plant senesces.
Catalytic deoxygenation of cellulose pyrolysis was evaluated using micropyrolyzer-gas chromatography (µPy-GC) coupled to dopant-assisted atmospheric pressure chemical ionization (dAPCI) time-of-flight mass spectrometry (TOF MS). A vast majority of compounds produced via catalysis and/or pyrolysis cannot be found in the database. However, dAPCI-TOF MS produces soft ionization and accurate mass measurement for direct chemical composition analysis of GC-separated molecules. This analytical technique demonstrated the ability to evaluate catalytic efficiency and monitor the change in reaction products. A total of 142 compounds could be analyzed with this approach compared to 38 compounds in traditional Py-GC-EI-MS analysis.
Finally, HRMS is utilized for the real-time monitoring of fast pyrolysis products of glucose-based carbohydrates. The soft ionization and rapid-scanning capabilities provided new insights into molecular-level understanding of pyrolysis chemistry. Comparing time evolution profiles and yields for individual products revealed that hydrogen bonding may play a larger role in degradation of cellulose and that cyclodextrin does not appear to be a good surrogate for understanding cellulose pyrolysis. More work is necessary to piece together all the information, but the first steps have been taken toward unraveling the complex network of elementary reactions.
Daniel Paul Cole
Cole, Daniel Paul, "High resolution mass spectrometry for molecular characterization of pyrolysis products and kinetics" (2015). Graduate Theses and Dissertations. 14342.