Roles of acidic sites in alumina catalysts for efficient d-xylose conversion to lactic acid†
This work investigated the conversion of D-xylose to lactic acid over heterogeneous Al2O3 and ZSM-5 catalysts in aqueous solution. γ-Al2O3, an abundant low cost catalyst, exhibited superior lactic acid yield (63 mol%) at 170 °C, compared to α-Al2O3, ZSM-5 catalysts and the reported data in the literature so far. Our experiment suggested that the outstanding lactic acid yield could be attributed to the large specific surface area and the abundance of Lewis acid sites in γ-Al2O3. In contrast, furfural selectivity was significantly promoted in the catalyst free system and the ZSM-5 catalyst, which has abundant Brønsted acid sites, but lacks Lewis acid sites. The theoretical part revealed that reactive Lewis acid sites in γ-Al2O3 and the solvent play important roles in the C–C bond activation and thermodynamic stability of the D-xylose to lactic acid pathway. The green catalytic system proposed in this work shows great potential as an alternative method for lactic acid production for future industrial application.