Knitting N-doped Hierarchical Porous Polymers to Stabilize Ultra-small Pd Nanoparticles for Solvent-Free Catalysis

Hierarchical porous polymers with more than one pore size distributions can effectively support noble metal catalysts and circumvent the limitation of the diffusion of the reactants, and thus exhibit both excellent catalytic activity and superior diffusive properties. Herein, we report a simple one-step Friedel–Crafts reaction to knit a series of benzene heterocycle monomers, such as indane, indoline, indole and tetrahydronaphthalene to obtain hierarchical porous polymers with high surface areas and/or abundant N sites. These polymers can be directly used to immobilize Pd ions, and stabilize Pd nanoparticles during the thermal reduction process to obtain Pd/polymer catalysts. In particular, indoline- and indole-based polymers with high N content up to 7 wt % exhibit outstanding ability to stabilize uniform ultrasmall Pd nanoparticles. The obtained Pd-polymers exhibit excellent catalytic activity in the solvent-free oxidation of benzyl alcohol with O₂.