Activation and deactivation characteristics of sulfonated carbon catalysts

Recently, there have been reports in the literature on sulfonated carbon catalysts prepared from sugars exhibiting interesting acid catalytic properties. However, to date, little has been reported about their activation/deactivation characteristics. For this paper, the activation/deactivation behaviors of such sulfonated carbon catalysts were investigated through the esterification of free fatty acids (acetic acid and caprylic acid) and transesterification of triglycerides (triacetin, tricaprylin and soybean oil) with methanol. The catalytic properties of the sulfonated catalysts were compared with those of Nafion® SAC-13 and sulfuric acid, other similar Brønsted acid catalysts. For the liquid-phase esterification of acetic acid and transesterification of triacetin with methanol, the sulfonated carbon catalysts exhibited catalytic site activities comparable to those for sulfuric acid, along with higher (by weight) specific catalytic activities but lower initial TOFs compared with Nafion® SAC-13. Catalyst swelling was critical for catalytic activity of the sulfonated carbon catalysts. Poor swelling in the gas phase of the catalysts led to the appearance of an induction period and inferior catalytic performance in the gas-phase esterification of acetic acid. In the liquid-phase esterification of acetic acid, catalytic activity decreased during the initial reaction batch cycles but reached a plateau after several catalytic cycles at 60 °C. Elemental analysis, 1H NMR results of the liquid reaction mixtures after reaction, and the decreased activity of the catalysts after extensive solvent washing suggest that the initial catalyst deactivation was caused by the leaching of polycyclic aromatic hydrocarbons containing SO3H groups. Cycling experiments involving large triglycerides actually showed a cumulative effect of slower catalyst swelling in the less-polar reaction mixtures moderated by deactivation of the catalyst due to leaching, resulting in increased activity in the early cycles followed by a loss of activity in later cycles.