Sulfonic acid-functionalized silica with controlled hydrophobicity as an effective catalyst for esterification of levulinic acid

As one of the essential additives in biodiesel and flavoring agents, solvents, and plasticizers in industry, ethyl levulinate (EL) has gained attention in the last two decades, and the global production of the compound is rapidly increased over the years. So far, esterification of biomass-derived levulinic acid (LA) with ethanol (EtOH) in the presence of highly corrosive mineral acid catalysts is the most common technique for the reaction. To avoid the harsh environment during the reaction and to ease the separation of the catalyst, the research has been recently focused on developing highly active solid acid catalysts. In this research, we have systematically investigated the catalytic activity of sulfonic acid-functionalized silica (SiO2-SO3H) with different hydrophobicity for esterification of LA with EtOH. The hydrophobicity of SiO2-SO3H was tuned by incorporating three different alkylsilanes, e.g., methyl (Me), octyl (Oct), and hexadecyl (HD) silanes on the surface of the material. We found that the longer the alkyl group, the lower the acidity and thus resulted in the lower the catalytic activity. On the other hand, the particle size and hydrophobicity increase with the length in the alkyl group. SiO2-SO3H modified with the methyl group (SiO2-SO3H/Me) showed the highest acidity (0.76 mmol g(-1)) and the highest catalytic performance (70.6% conversion). Moreover, while all alkyl-modified catalysts followed pseudo-first-order, the pseudo-first-order rate constant (k1p) for the reaction over SiO2-SO3H/Me was 5.4 x 10(-3) min(-1). The kinetic results of this study could be useful for reactor modeling and simulation in the future.