Acid functionalized coal fly ashes: New solid catalysts for levulinic acid esterification

Abstract

Coal fly ash (CFA) from different sources (CFA 5, 10, 13) and different size fractions (> or < 75 μm) were characterized and used as inorganic supports for sulfonic acid functionalization by two methodologies including one-pot organosilylation (CFA_CSPTMS) and two-step benzyl alcohol condensation followed by sulfonation (CFA_BnSO3H). The as-prepared organosulfonic CFAs were characterized by solid-state 1H, 13C and 29Si NMR spectroscopy, XPS, S content was quantified by elemental analysis and the intrinsic acidity was determined by potentiometric titration; the highest –SO3H content was obtained for CFA13_BnSO3H. All the functionalized CFAs were tested for the first time as acid solid catalysts in the esterification of levulinic acid (LA) a biomass platform molecule, in the presence of n-butanol. n-Butyl levulinate was the only product with conversions of 100% after 40 min. reaction and using the CFA13_BnSO3H as catalyst. This catalyst also exhibited the highest TOFs (447 h−1) and showed to be the most stable and reusable for 5 consecutive catalytic cycles.