Palladium Single Atom-supported Covalent Organic Frameworks for Aqueous-phase Hydrogenative Hydrogenolysis of Aromatic Aldehydes via Hydrogen Heterolysis.

Developing a method for the tandem hydrogenative hydrogenolysis of bio-based furfuryl aldehydes to methylfurans is crucial for synthesizing sustainable biofuels and chemicals; however, it poses a challenge due to the easy hydrogenation of the C=C bond and difficult cleavage of the C-O bond. Herein, a palladium (Pd) single-atom-supported covalent organic framework was fabricated and showed a unique 2,5-dimethylfuran yield of up to 98.2 % when reacted with 5-methyl furfuryl aldehyde in an unprecedented water solvent at 30 °C. Furthermore, it exhibited excellent catalytic universality while converting various furfuryl-, benzyl-, and heterocyclic aldehydes at room temperature. The analysis of the catalytic mechanism confirmed that H₂ was heterolytically activated on the Pd-N pair and triggered the keto-enol tautomerism of the covalent organic frameworks (COFs) host, resulting in H^--Pd⋅⋅⋅O-H⁺ sites. These sites served as novel asymmetric hydrogenation sites for the C=O group and hydrogenolysis sites for the C-OH group through a scarce SN₂ mechanism. This study demonstrated remarkable bifunctional catalysis through the H₂-induced keto-enol tautomerism of COF catalysts for the atypical preparation of methyl aromatics in a water solvent at room temperature.