Two-step carbothermal welding to access atomically dispersed pd₁ on three-dimensional zirconia nanonet for direct indole synthesis

Herein, we report a novel carbothermal welding strategy to prepare atomically dispersed Pd sites anchored on a three-dimensional (3D) ZrO₂ nanonet (Pd₁@ZrO₂) via two-step pyrolysis, which were evolved from isolated Pd sites anchored on linker-derived nitrogen-doped carbon (Pd₁@NC/ZrO₂). First, the NH₂-H₂BDC linkers and Zr₆-based [Zr⁶(μ₃-O)₄(μ₃-OH)₄]¹²⁺ nodes of UiO-66-NH₂ were transformed into amorphous N-doped carbon skeletons (NC) and ZrO₂ nanoclusters under an argon atmosphere, respectively. The NC supports can simultaneously reduce and anchor the Pd sites, forming isolated Pd₁-N/C sites. Then, switching the argon to air, the carbonaceous skeletons are gasified and the ZrO₂ nanoclusters are welded into a rigid and porous nanonet. Moreover, the reductive carbon will result in abundant oxygen (O) defects, which could help to capture the migratory Pd₁ species, leaving a sintering-resistant Pd₁@ZrO₂ catalyst via atom trapping. This Pd₁@ZrO₂ nanonet can act as a semi-homogeneous catalyst to boost the direct synthesis of indole through hydrogenation and intramolecular condensation processes, with an excellent turnover frequency (1109.2 h^-1) and 94% selectivity.