Esterification of levulinic acid with ethanol over sulfated Si-doped ZrO2 solid acid catalyst: Study of the structure–activity relationships

Abstract

Esterification of levulinic acid with ethanol to produce ethyl levulinate was examined by using sulfated Si-doped ZrO2 solid acid catalysts with enlarged surface areas and the relationships between the structural properties and catalytic performances were investigated. Structures of the catalysts were verified by XRD, nitrogen physisorption, FE-SEM, UV–vis and FTIR measurements. Acidity of the catalysts that substantially affect the catalytic activity was evaluated by NH3-TPD measurement. Incorporation of Si atom into the lattice structure of ZrO2 (up to 30mol% Si per Zr atom) afforded high-surface-area SiO2-ZrO2 mixed oxides, and their sulfated forms provided increased numbers of sulfate anions and the associated acid sites. Several distinct correlations were found between the structural properties/acidities and catalytic activities, which suggested that (i) the number of accessible active acid sites and (ii) the accessibility of the organic reactants to the active sites play crucial roles in determining the overall activity. Among the catalysts tested, sulfated Si-doped ZrO2 with optimum Si content (5.0–10mol% Si per Zr) was found to be the best catalyst, the activity of which was far superior to that of the conventional sulfated ZrO2. In addition, direct conversion of cellulosic sugars (glucose and fructose) into levulinate esters was also examined, in view of their practical applications in acid-catalyzed biomass conversion processes.