Revisiting glycerol esterification with acetic acid over Amberlyst-35 via statistically designed experiments: Overcoming transport limitations

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The market is overflooded by unrefined glycerol with the production of biodiesel from vegetable oils. A recurrently studied alternative for its valorization is the production of acetins via esterification with acetic acid over acid catalysts in liquid phase. However, most of the published literature uses experimental methodologies that prevent a proper kinetic analysis of the reaction. This work revisits glycerol esterification with acetic acid catalyzed by Amberlyst-35 applying sequential statistically designed experiments. Following such approach, statistically meaningful data were collected providing insight into the catalytic reaction. Experiments were designed to test the influence of the quantity of active sites; assumed herein as the number of surface SO3H+ sites on Amberlyst-35, temperature, and the acetic acid to glycerol molar ratio over the turnover frequency of the catalyst. The collected evidence shows that, under the typical conditions employed in most open literature, heat and mass transport artifacts should not be neglected. In the present case, these limitations were overcome by reducing the particle size of the solid. Under such conditions, a straightforward relationship between the concentration of surface SO3H+ active sites and the glycerol turnover rates was established. Meanwhile, it was shown that the reaction temperature has a rather weak effect on the catalytic turnover. This trend concurred with the thermodynamic equilibrium of the reaction. Likewise, the already established effect of the acetic acid to glycerol molar ratio was also confirmed. Overall, the performed study serves a double purpose. First, it is a methodological guide to collect kinetically relevant information in liquid phase. Second, it reports intrinsic kinetic rates for the esterification reaction over the studied catalyst hence paving the way for a rigorous kinetic modeling.