Engineering zirconium-based UiO-66 for effective chemical conversion of d-xylose to lactic acid in aqueous condition.

Utilizing metal-organic frameworks (MOFs) as heterogeneous catalysts is an interesting and important application due to their well-controlled catalytic sites and well-defined porous structures. In this study we apply, for the first time, Zr-based UiO-66 for the catalytic hydrothermal conversion of d-xylose to lactic acid (LA). The reactions are catalyzed by the coordinatively unsaturated Zr, as Lewis acid sites, and the hydroxide ion (OH) located at the defect sites. The catalytic performances of UiO-66 catalysts synthesized through a modulator-free approach (UiO-66) and an acetic acid modulator-assisted approach (UiO-66(AA)) are distinct due to the different concentrations of local defects. The UiO-66 catalyst possessing a higher defect concentration exhibits a superior LA yield of 1.17 mol from 1 mol of xylose. However, the UiO-66(AA) catalyst with higher crystallinity shows better selectivity for LA over furfural, a side product from the competitive pathway. The enhanced LA yield and excellent selectivity can be achieved by the removal of AA from UiO-66(AA) resulting in a novel MOF catalyst (UiO-66(AA)*) which provides more accessible catalytic sites with retained crystallinity. This work highlights that the structural engineering of MOF catalysts is crucial for the fine-tuning of their catalytic properties.