Fourier transform infrared spectroscopic study on the adsorption of ethyl pyruvate on Pt/Al₂O₃: Side reactions suppressed by adsorbed hydrogen and cinchonidine

Adsorption and reaction of ethyl pyruvate (EtPy), a model reactant in chiral hydrogenation of α-ketoester on Pt/γ-Al₂O ₃ were studied by IR spectroscopy. In vacuum and Ar flow at room temperature, several side reactions of EtPy are detected by IR spectroscopy: decomposition on Pt, aldol condensation, and hydrolysis on γ-Al ₂O₃. When the quantity of adsorbed EtPy is small, CO formed from EtPy decomposition on Pt can react with the hydroxyl groups of Al₂O₃ to generate formate species. It is very interesting to find out that H₂ not only acts as a reactant for the hydrogenation reaction but also plays an important role in suppressing the side reactions of EtPy on Pt/γ-Al₂O₃ catalysts. The suppressing effect of H₂ is not observed on γ-Al₂O₃ alone. The function of spilt-over H atoms in suppressing these side reactions was confirmed and discussed. Adsorbed cinchonidine (CD) alone cannot suppress the condensation and hydrolysis of EtPy on γ-Al₂O₃, but the decomposition of EtPy on Pt can be suppressed by absorbed CD. The coexistence of CD and H₂ thoroughly inhibits all of these side reactions of EtPy on Pt/γ-Al₂O₃. Adsorbed CD can help hydrogen to avoid site-blocking with side-reaction products then facilitate hydrogenation on Pt, which may be a reason for the rate acceleration effect of the adsorbed CD.