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 (200 nm) 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 % (C3) to 110 % (C12). 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 GC×GC–time‐of‐flight mass spectrometry (GC×GC–ToFMS), which indicated ester and ether formation accompanying the loss of acid, phenolic, aldehyde, and ketone components. The hole truth: The incorporation of macropores in mesoporous SBA‐15 enhances its activity for the esterification of linear carboxylic acids. The turnover frequency (TOF) increased from 5 % to 110 % with increasing alkyl chain length of the carboxylic acids from C3 to C12, respectively. The macroporous–mesoporous 3‐propylsulfonic acid (PrSO3H)/SBA‐15 also provided a twofold increase of TOF compared with the mesoporous analogue for the esterification of a real, thermal fast‐pyrolysis bio‐oil derived from woodchips.