Defect engineering of Metal-Organic Framework for highly efficient hydrodeoxygenation of lignin derivates in water

Designing highly active catalysts for selective hydrodeoxygenation (HDO) of lignin-derived oxygenates into well-defined compounds is of great importance for realizing high-value utilization of lignin. Defect engineering in metal-organic frameworks (MOFs) is an emerging concept for tailoring MOF properties, which might be a promising strategy in the synthesis of high-performance catalysts. Herein, we developed a facile synthetic strategy by adding trifluoroacetic acid as the modulator to prepare a bifunctional catalyst (Pd/NH2-MIL-53-d) consisting of Pd nanoparticles highly dispersed on defective NH2-MIL-53(Al). Characterizations, including TEM, XRD, IR, XPS and NH3-TPD, were employed to reveal the morphology and physicochemical properties of the catalysts. Results indicated that the introduction of defects leads to the generation of coordinatively unsaturated metal sites, which can act as anchor points for high-dispersion of Pd nanoparticles and as additional active acid sites for catalytic reactions. 100% conversion of vanillin and other 8 representative derivates of lignin into their decarbonyl products with yields over 99% has been achieved under mild conditions in the water. Density functional theory calculations revealed that the HDO of vanillin into 2-methoxy-4-methylphenol mainly un-derwent a cascade of hydrogenation-hydrogenolysis routes, and the synergistic interaction between Pd and defective NH2-MIL-53(Al) facilitated the novel bifunctional catalyst to exhibit outstanding catalytic performance. No apparent deactivation of the catalyst was noticed after it was reused 5 times. Therefore, engineering of defects in MOFs holds enormous potential in designing novel bifunctional catalysts for lignin valorization.