Tuning the Surface Wettability of Pd/COFs for Selective Hydrogenation

Covalent organic frameworks (COFs) with imine linkage are promising solid supports to disperse ultra-fine metal nanoparticles (NPs), but the modulation of their surface wettability, an important parameter affecting the catalytic performance of metal NPs, is still very challenging. Here, we report the preparation of two imine-linked COFs with moderate and strong surface hydrophobicity by changing the carbon chain length of side oxyalkyl groups. Ultra-small Pd NPs with an average size of similar to 2.0 nm was successfully loaded on the two COFs, attributed to the strong interactions of Pd NPs and imine linkage as evidenced by the electron donating from imine linkage to Pd NPs by XPS characterizations. The TOF of Pd/COF is much higher than commercial Pd/C (714 h(-1) vs 489 h(-1)) in the hydrogenation of nitrobenzene, which is attributed to the electron rich Pd, the hydrophobic surface and crystalline structure of COFs. Pd NPs on COFs with strong hydrophobicity are more active than those on COFs with moderate hydrophilicity in the hydrogenation of moderate lipophilic nitro compounds such as nitrobenzene, methyl nitrobenzene and nitronaphthalene. Reversed tendency was observed for the hydrogenation of moderate hydrophilic p-amino-nitrobenzene. The modulation of the surface wettability of COFs provides an efficient strategy to improve the catalytic performance of supported metal NPs.