On-Line Analysis of the Heterogeneous Pd-Catalyzed Transfer Hydrogenation of p-Nitrophenol in Water with Formic Acid in a Flow Reactor

An versatile experimental setup composed of a fixed-bed reactor and an HPLC apparatus has been developed for the study of heterogeneous catalytic reactions under transient conditions. The transfer hydrogen reaction of formic acid in water to give p-nitrophenol catalyzed by Pd@Al2O3 support was chosen as the model reaction in a flow reactor. On one hand, this setup allowed the first experimental on-line measurements of p-aminophenol in an acidic medium, indicating that it is the only product of the reaction at steady state. On the other hand, the setup evidenced a quite long transient period before stationary conditions were reached, involving a transient component that is quite instable and can transform to p-aminophenol without a catalyst. Results of numerical simulations indicate that only a change in the reaction mechanism, due to a change in the catalyst activity after few minutes of reaction, can explain the transient production of this intermediate and that this occurs when the active hydrogen cover...