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 one of interesting and important applications due to their well-controlled catalytic sites and well-defined porous stuctures. 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 Zr4+, as Lewis acid sites, and the hydroxide ion (OH–) located at the defect sites. The catalytic performances of UiO-66 catalysts synthesized through modulator-free approach (UiO-66) and acetic acid modulator-assisted approach (UiO-66(AA)) are distinct due to the different concentrations of the local defects. The UiO-66 catalyst possessing higher defect concentration exhibits superior LA yield of 1.17 mol from 1 mol of xylose. However, 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 AA removal of UiO-66(AA) resulting in the novel MOF catalyst (UiO-66(AA)*) which provides more accessible catalytic sites with the retained crystalinity. This work highlights that the structural engineering of MOF catalysts is crucial for the fine-tuning of their catalytic properties.