Rapid Hydrogenation of Aromatic Nitro Compounds in Supercritical Carbon Dioxide: Mechanistic Implications via Experimental and Theoretical Investigations

An exceptionally rapid hydrogenation of nitrobenzene to aniline [TOF=252,000 h-1] over palladium containing MCM-41 (Pd/MCM-41) with excellent yield of >99% can be achieved in supercritical carbon dioxide at 50 degrees C and a hydrogen pressure of 2.5 MPa. It has been observed that this promising method preferred a single phase between liquid substrate and carbon dioxide-hydrogen system. The ascendancy of the supercritical carbon dioxide medium is established in comparison with the conventional organic solvent and solvent-less conditions. Changes in the reaction parameters such as carbon dioxide and hydrogen pressure, temperature and the reaction time do not affect the selectivity. A combined experimental and theoretical study has elucidated the mechanism under the studied reaction condition because experimental observations revealed a direct conversion of nitrobenzene to aniline. However, density functional theory (DFT) calculation shows that the direct conversion is energetically unfavourable; hence, a stepwise mechanism has been proposed. Theoretical predictions and experimental observations suggested that the rate-limiting step of nitrobenzene conversion is different from that of the liquid phase hydrogenation. This catalytic process can also be successfully extended to the hydrogenation of other aromatic nitro compounds with different substituents. Easy separation of the liquid product from catalyst and the use of an environmentally friendly solvent make this procedure a viable and an attractive green chemical process.