Highly efficient catalytic transfer hydrogenation for the conversion of nitrobenzene to aniline over PdO/TiO: The key role of in situ switching from PdO to Pd.

The reduction of nitrobenzene to aniline is very important for both pollution control and chemical synthesis. Nevertheless, difficulties still remain in developing a catalytic system having high efficiency and selectivity for the production of aniline. Herein, it was found that PdO nanoparticles highly dispersed on TiO support (PdO/TiO) functioned as a highly efficient catalyst for the reduction of nitrobenzene in the presence of NaBH. Under favorable conditions, 95% of the added nitrobenzene (1 mmol/L) was reduced within 1 min with an ultra-low apparent activation energy of 10.8 kJ/mol by using 0.5%PdO/TiO as catalysts and 2 mmol/L of NaBH as reductants, and the selectivity to aniline even reached up to 98%. The active hydrogen species were perceived as dominant species during the hydrogenation of nitrobenzene by the results of isotope labeling experiments and ESR spectroscopic. A mechanism was proposed as follows: PdO activates the nitro groups and leads to in-situ generation of Pd, and the generated Pd acts as the reduction sites to produce active hydrogen species. In this catalytic system, nitrobenzene prefers to be adsorbed on the PdO nanoparticles of the PdO/TiO composite. Subsequently, the addition of NaBH results in in-situ generation of a Pd/PdO/TiO composite from the PdO/TiO composite, and the Pd nanoclusters would activate NaBH to generate active hydrogen species to attack the adsorbed nitro groups. This work will open up a new approach for the catalytic transfer hydrogenation of nitrobenzene to aniline in green chemistry.