Reactions Responsible for Aging in Wood-Based Pyrolysis Oil: Synthesis and Characterization of a Coniferyl Alcohol Dimer

The negative environmental impact and the diminishing supply of fossil fuels demand a renewable alternative. Pyrolysis oils produced from the decomposition of biomass, like wood, are a potential fuel substitute for energy production and a feedstock alternative for manufacturing value-added chemicals. The possibilities offered by pyrolysis oils are offset by oil instability. The oils contain reactive compounds, such as small aldehydes, conjugated aromatics, and acids that over time react and produce higher molecular mass products. This instability manifests as an increase in viscosity by a process referred to as aging. One chemical component, coniferyl alcohol, is proposed to react with formaldehyde under the acidic oil conditions to produce a dimer. In our lab, researchers have detected the coniferyl alcohol dimer in authentic oil samples and have simulated the reaction under conditions that removes the complexity of the pyrolysis oil matrix. This study focused on the synthesis, isolation, and characterization of the dimer structure by employing NMR analysis. GC/MS analysis of a successful synthesis of the dimer showed multiple dimers were produced, but there was one principal product. The NMR analysis of this dimer was used to elucidate the geometry, providing evidence that the product has E stereochemistry for the double bond and trans stereochemistry in the acetal ring. Confirmation of the principal structure provides support for the dimerization mechanism and will allow for future research to address instability of pyrolysis oils.