Methanol Steam Reforming Controls the Selective Transfer Hydrogenation vs N-Methylation of Nitrobenzene Using a PVP-Stabilized IrO2 Nanoparticle Catalyst

Polyvinylpyrrolidone-stabilized iridium oxide nanoparticles (PVP_1.7IrO₂) catalyzed the transfer hydrogenation and N-methylation of nitrobenzene, where the water content controlled the selectivity between aniline and N-methylaniline (NMA). Methanol served as the source of hydrogen and carbon in these reactions. When 1:1 methanol/water was used as the solvent, aniline was formed in a ratio of 13:1 over NMA. Without water, the chemoselectivity is reversed using an organic superbase, 1,8-diazabicyclo(5.4.0)undec-7-ene, instead of KOH. Both reactions achieved quantitative substrate conversion at 160 °C in 1 h. In control experiments under otherwise identical conditions, IrO₂ and metallic Ir powder led to ineffective and unselective reactions, respectively. In contrast to PVP_1.7IrO₂, these microcrystalline solids could not form a homogenized colloidal suspension in solution. At the end of the PVP_1.7IrO₂-catalyzed reactions, up to 5,131 ppm of CO₂ was detected in the gas phase by nondispersive infrared analysis. The amount of CO₂ in the solution phase was determined using the precipitation of BaCO₃ from Ba(OH)₂. The BaCO₃ was verified by powder X-ray diffraction and quantified by gravimetric analysis to reveal an 88% yield of CO₂ relative to the substrate for the aniline-selective reaction and an 87% yield for the NMA-selective reaction with KOH. A control experiment without methanol under otherwise identical conditions in pure H₂O solvent gave only a 5% yield of CO₂. When the reaction temperature was lowered to 100 °C, potassium formate was detected in 34% yield at the end of the reaction in methanol, consistent with a pathway of methanol steam reforming. Overall, the formation of C1 products from methanol and the water effect on chemoselectivity suggest that methanol steam reforming is key to the catalytic transfer hydrogenation and N-methylation of nitrobenzene.