Ligand Polarity Regulated Metal–Support Charge Transfer over Pd@MOFs for Direct H2O2 Synthesis: A Structure–Property Relationship Study

Precise regulation of metal–support interactions to tune the activity and selectivity of catalytic reactions and build structure–property relationships is highly desired in heterogeneous catalysis. Herein, a series of isostructural aluminum-based metal–organic frameworks (MOFs) based on different ligands with tunable polarities were investigated as supports for Pd nanoparticles (NPs), affording five Pd@Al-MOF composites. We find that the polarity of ligands in MOFs plays a critical role on catalytic activity and selectivity in direct hydrogen peroxide (H2O2) synthesis from H2 and O2, which is caused by the charge transfer between the Pd NPs and the MOF supports, as determined by X-ray photoelectron spectroscopy (XPS) measurements. Consequently, the H2O2 yield of Pd@Al-MOFs follows the order of Pd@A520 > Pd@MIL-53(Al)-TDC > Pd@MOF-303 > Pd@MIL-160 > Pd@Al-NDC, which is in line with the order of the polarity of ligands in MOFs. Among them, Pd@A520 possesses the highest H2O2 yield of 1931 mol kg–1Pd h–1, which is approximately 4.9 times higher than that of commercial Pd/C. Moreover, we established multiple quantitative relationships between the ligand polarity of MOFs and H2O2 yield, H2 conversion, H2O2 selectivity, and the H2O2 degradation rate. This work not only provides a new perspective to optimize the activity and selectivity of catalytic reactions and regulate the charge transfer between metals and supports via adjusting the ligand polarity of MOFs, but also builds the quantitative structure–property relationships for the direct synthesis of H2O2.