Abstract

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.

Highlights

  • Biofuels have an important role to play in mitigating anthropogenic climate change arising from the combustion of fossil fuels.[1]

  • 3-Propylsulfonic acid (PrSO3H)/SBA-15 has been reported as an efficient catalyst for acetic acid esterification with methanol[2a,25] and other alcohols in simulated bio-oils,[26] and the most widely used sulfonic acid in solid acid catalyzed esterification.[27]. Such catalysts exhibit improved water tolerance during esterification when the sulfonated silica surface is co-functionalized with alkyl chains.[2a,5,25b] We recently reported a postmodification hydrothermal saline-promoted grafting (HSPG)

  • The BET surface areas decreased after sulfonic acid grafting over both silicas owing to micropore blockage, which was apparent as a dramatic drop in the micropore area and pore volume

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Summary

Introduction

Biofuels have an important role to play in mitigating anthropogenic climate change arising from the combustion of fossil fuels.[1]. The catalytic performance of mesoporous and macroporous– mesoporous sulfonic acid silicas was evaluated in the esterification of propanoic (C3), hexanoic (C6), and lauric acids (C12) with methanol to explore the influence of the macropores on the reactivity under previously optimized conditions.[2a] Because both catalysts possessed similar acid loadings and strength, any differences in activity must arise from their pore architecture.

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