Abstract
Catalytic fast pyrolysis (CFP) of biomass is a versatile thermochemical process for producing a biogenic oil that can be further upgraded to sustainable transportation fuels, chemicals, and materials. CFP oil exhibits reduced oxygen content and improved thermal stability compared to noncatalytic fast pyrolysis oil. However, some level of reactive oxygenates remain in CFP oils, and reactions between these species can result in molecular weight growth and increased viscosity, leading to the potential for challenges during transportation, storage, and downstream processing. Previous research has provided considerable insight into the reactivity of noncatalytic fast pyrolysis oils, but CFP oils have yet to be studied in a similar fashion. Consequently, the degree of catalytic upgrading that is necessary to effectively stabilize CFP oils has yet to be established, and little is known about the mechanistic details underlying the process. The current study addresses this knowledge gap by controlling the CFP reaction conditions to systematically vary the oxygen content of the resulting oil. Accelerated thermal reactivity studies were then performed, and the CFP oils were analyzed using gas chromatography-mass spectrometry (GC-MS), Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), gel permeation chromatography (GPC), and viscometry to evaluate the impact of heating on their physical and chemical properties. The results revealed that short chain carbonyls, anhydrosugars, and lignin derivatives with conjugated vinyl groups likely play a role in the thermal reactivity of CFP oils. Additionally, experiments performed across a wide variety of feedstocks revealed relatively low thermal reactivity for CFP oils with oxygen contents of <20 wt %. However, above this threshold value, the thermal reactivity grew exponentially as a function of oxygen content, resulting in large increases in viscosity and molecular weight. These results serve to deepen the mechanistic understanding of CFP oil thermal reactivity and help inform the development of quality specifications for catalytic upgrading to effectively stabilize CFP oils.