Solvent driven pore engineering in coffee-derived activated hydrochar: implications for post-combustion CO2 capture

Engineering the pore structure of biomass-derived activated carbons is critical for optimizing their performance in adsorption-based applications. This study demonstrates for the first time that washing hydrochars in solvents of different polarity before activation is a simple yet powerful strategy to tailor pore size distribution. Hydrochar is produced from spent coffee grounds via hydrothermal carbonization, followed by washing in various solvents and activation in KOH. This results in carbons with a very large surface area (∼2700 m2/g), and washing is demonstrated to significantly increase product yield. Furthermore, washing in non-polar or mixed-polarity solvents removes long-chain carboxylic acids and esters from the hydrochar, promoting the development of narrow micropores while suppressing mesopore formation. To illustrate the impact of this structural control of porous carbons, post-combustion CO2 capture is investigated as a case study. Narrower pore size distribution enhances CO2 uptake, significantly improving capacity from 2.8 mmol/g for unwashed samples to 3.8 mmol/g for acetone-washed samples. Interestingly, moderate pore size (9–12 Å) is shown to be optimal for CO2:N2 selectivity, while smaller pores result in lower selectivity due to stronger interactions between N2 and the pore walls. These findings highlight the potential role of solvent washing in directing pore architecture of hydrochars for adsorption-based carbon capture technologies and beyond.