Deciphering positional effects in sulfonated carbon catalysts: Bridging DFT-predicted acidity and catalytic performance in sorbitol dehydration

Intrinsic acidity vs. accessibility in biomass dehydration is investigated. • TOF diverges from macroscopic yield in sorbitol dehydration. • The para-isomer gives the highest yield (∼74%) despite its lower site-normalized TOF. • Highly accessible acid-site density and balanced adsorption behavior contribute to the para-isomer's performance. This study investigates how intrinsic acid-site activity and spatial accessibility jointly regulate biomass dehydration. Using position-isomeric sulfonated carbon catalysts, namely 2-, 3-, and 4-CBC-PhSO₃H, we demonstrate a divergence between microscopic site activity and macroscopic catalytic performance in sorbitol dehydration. Kinetic analysis shows that the ortho-isomer exhibits the highest site-normalized turnover frequency (TOF), whereas the para-isomer achieves the highest overall isosorbide yield of approximately 74%. This difference arises from a synergistic balance between intrinsic reactivity and accessible acid-site density: the lower steric hindrance of the para-configuration favors a higher density of effective acid sites, which compensates for its lower single-site TOF. Theoretical calculations further suggest that substitution position affects proton dissociation, charge distribution, and adsorption behavior. In particular, the para-isomer maintains favorable interactions with organic reactants/intermediates while showing relatively weak water binding, which may help reduce competitive water adsorption during dehydration. These findings highlight that the catalytic performance of carbon-based solid acids is governed by the combined effects of intrinsic acidity, spatial accessibility, acid-site density, and interfacial adsorption behavior, guiding the rational design of robust solid acid catalysts for biomass conversion. Download: Download high-res image (210KB) Download: Download full-size image