Impact of Macroporosity on Catalytic Upgrading of Fast Pyrolysis Bio-Oil by Esterification over Silica Sulfonic Acids

Fast pyrolysis bio-oils possess unfavorable physicochemical properties and poor stability, in large part, owing to the presence of carboxylic acids, which hinders their use as biofuels. Catalytic esterification offers an atom- and energy-efficient route to upgrade pyrolysis bio-oils. Propyl sulfonic acid (PrSO3H) silicas are active for carboxylic acid esterification but suffer mass-transport limitations for bulky substrates. The incorporation of macropores (200nm) enhances the activity of mesoporous SBA-15 architectures (post-functionalized by hydrothermal saline-promoted grafting) for the esterification of linear carboxylic acids, with the magnitude of the turnover frequency (TOF) enhancement increasing with carboxylic acid chain length from 5% (C-3) to 110% (C-12). Macroporous-mesoporous PrSO3H/SBA-15 also provides a two-fold TOF enhancement over its mesoporous analogue for the esterification of a real, thermal fast-pyrolysis bio-oil derived from woodchips. The total acid number was reduced by 57%, as determined by GCxGC-time-of-flight mass spectrometry (GCxGC-ToFMS), which indicated ester and ether formation accompanying the loss of acid, phenolic, aldehyde, and ketone components.