Insight into the Mechanism of Water-Promoted Hydrogenation of Maleic Acid to Succinic Acid on Pd/C Catalyst

Solvent provides additional degrees of freedom to regulate catalyst reactivity in liquid-phase heterogeneous catalysis, but it is still a challenge to have insight into the multifaceted solvent effects. Herein, a remarkable promotional effect of water in maleic acid (MAc) hydrogenation to succinic acid (SAc) was observed. Kinetic studies showed that the apparent activation energy in water was much lower than in organic solvents. A series of isotope-labeling experiments were designed, and the products were analyzed by NMR (1H, 13C, 2H, and DEPT135 spectra). The results showed that D2O participated in MAc C═C hydrogenation and 34.7% of SAc was deuterated. The structures of these deuterated compounds were further confirmed by electrospray mass spectrometry (ESI-MS). The detailed mechanism of water participating in MAc C═C hydrogenation was studied by quasi-in situ mass spectrometry experiments. The results showed that H2 exchanged with D2O and formed the HD2O* transition state over the active site of Pd. Quantitative 13C NMR demonstrated that 46.2% of SAc was generated through the HD2O* transition state pathway. Based on these results, a rational mechanism of MAc hydrogenation in aqueous solution was proposed. Finally, a recyclability experiment showed that Pd/C had much better stability in water than in organic solvents.