Tuning the Pore Structure and Surface Properties of Carbon-Based Acid Catalysts for Liquid-Phase Reactions

A series of sulfonic-acid-containing carbon catalysts were synthesized through the pyrolyzation of resorcinol-formaldehyde resins and subsequent sulfonation to investigate the tunability of their pore structure and surface properties. These catalysts were characterized by nitrogen gas adsorption, water vapor adsorption, elemental analysis, Boehm titration, and IR spectroscopy. Catalytic consequences of these properties were examined using two esterification reactions in which reactants of substantially different sizes (oleic acid vs acetic acid) are involved as well as the condensation of furfural with 2-methylfuran. The esterification of acetic acid with ethanol proceeded at nearly the same activity (TOF similar to 0.02 s(-1)) for all synthesized catalysts regardless of the variation of their surface and pore properties. Poisoning experiments of acid groups in the synthesized catalysts with NaCl indicate that nearly all -SO3H groups are accessible to the reactants. However, the esterification of oleic acid with methanol requires a large volume of mesopores (>0.4 cm(3) g(-1)) to proceed as efficiently as that using p-toluenesuffonic acid (p-TSA) as the catalyst. The condensation reaction requires both hydrophobic surfaces and a large volume of mesopores. A synthesized carbon-based catalyst having a large mesopore volume (1.1 cm(3) g(-1)) and surfaces with a relatively high hydrophobicity catalyzes this reaction at a much higher rate and higher selectivity (>90%) than Amberlyst-15. The results reported here show that the pore structure and surface properties of phenolic-resin-derived carbon catalysts can be easily tuned to maximize their catalytic performance in the reaction for which they are intended.