(617bn) On the Influence of Mass Transport in Sulfonated Carbon Catalysts Derived from Various Carbon Sources on Liquid-Phase Esterification Reactions

Sulfonated carbon catalysts have been widely investigated for their potential applications such as biomass conversion. Various carbons and carbon precursors have been used to synthesize such catalysts, leading to a wide range of properties and performance of catalysts. Our goal is to gain some insight into the structure and performance relationship of these catalysts and use them to design a new catalyst. In this presentation, we will report results from synthesis and characterization of sulfonated carbon catalysts derived from various sources: pyrolyzed phenolic resin (sulfonated carbon gel, SCG) [1], pyrolyzed cellulose (SCel) and glucose (SGlu), graphite oxide (SGO), activated carbon (SAC), carbon derived by hydrothermal treatment of glucose (SHTGlu), and multi-walled carbon nanotube (SCNT). Catalytic performance of these catalysts was tested in three liquid-phase esterification reactions at 333 K with substantially different size of reactants (acetic acid with ethanol, oleic acid with methanol, and levulinic acid with ethanol), and the results are correlated with catalyst properties characterized by Boehm titration, elemental analysis, Raman and IR spectroscopies, and N₂ and water vapor adsorption conducted at 77 and 298 K, respectively. A combined analysis of N₂ and water vapor adsorption data indicates that the catalysts derived from carbohydrates (SCel, SGlu, and SHTGlu) as well as graphite oxide (SGO) swell significantly in the presence of water vapor. Thus, although they show low surface areas in dry form, this property appears to allow easy access of small and polar reactants to their â??SO₃H sites, and the esterification reaction of acetic acid with ethanol proceeded at essentially the same initial turnover frequency per â??SO₃H group (â??0.02 s^â??1) for all catalysts regardless of significantly different properties of the catalysts. However, the reaction of oleic acid with methanol proceeded efficiently only on catalysts that have a large mesopore volume in dry form presumably because the catalysts derived from carbohydrates and graphite oxide do not swell efficiently in the reaction mixture, which limits the access of reactants to their â??SO₃H sites. In contrast, the esterification reaction of levulinic acid with ethanol proceeded slightly faster on catalysts that can swell or have high concentrations of surface oxygen-containing functional groups than other catalysts. These results indicate that both mass transfer and surface properties influence their catalytic property for this reaction. Collectively, our results show the performance of active sites (â??SO₃H groups) in sulfonated carbon catalysts, a role of surface oxygen-containing functional groups, and the influence of porous and swelling properties of the catalysts on mass transport of reactants for the liquid-phase esterification reactions.

[1] Ogino, I., Suzuki, Y., Mukai, S.R. ACS Catal. 5, 4951â??4958 (2015)