Chemical Composition, Aging Reactions, and Stabilization of Bio-Oils

The search for renewable energy sources is driven by the limited supply of non renewable fossil fuels and their adverse environmental effects. The oil crisis in the mid 1970’s motivated development of several processes for the production of liquid fuels from lignocellulosic biomass. Fast pyrolysis is one of the highest energy yielding thermal biomass conversion processes to generate bio-oils. The use of biomass for energy provides significant environmental advantages over fossil fuels. Pyrolysis oil requires further chemical upgrading to be suitable as transportation fuel. Bio-oil is a complex mixture of hundreds of organic compounds that result from depolymerization of three building blocks of biomass (cellulose, hemicelluloses and lignin). Because the composition depends on both the biomass source and process conditions, functional group information is needed rapidly for process development. Nuclear Magnetic Resonance (NMR) Spectroscopy has advantages over chromatographic methods because the whole bio oil can be dissolved in deuterated DMSO and the relative amount of different functionalities can be estimated. Quantitative analysis using 13C NMR is time consuming because of long T1 relaxation times and low natural abundance of 13C, and depends on accurate chemical shift regions. Here, we propose revised chemical shift regions and T1 correction factors to provide rapid, semi quantitative analysis of bio-oil. Pyrolysis oils are viscous, unstable and need to be upgraded to use as transportation fuels. Previous studies of bio oil stability show that polymerization occurs, and a few of the chemical compounds that react during aging are known, but the classes of reactive compounds, types of reactions and products are not known. Understanding the reactions that cause aging is crucial to prevent aging processes. Our chemical analysis identified aldehydes, trihydroxy benzenes and guaiacols with conjugated propylene side chains that react during accelerated aging. We suggest that an acid catalyzed oligomerization mechanism, through a quinone methide pathway, leads to the reactivity of conjugated aromatic molecules. As an approach to stabilize bio-oil, methanol addition has proven to increase the stability of the bio-oil. But very little is known about the reaction of bio-oil components with methanol. Here we employ isotope labeling to study the reactions using NMR, gas chromatography/mass spectrometry (GC/MS) and gel permeation chromatography (GPC). Labeling studies provide direct evidence of esterification and acetal formation