(734b) Characterizing Active Sites on Chirally-Modified Supported Noble Metal Catalysts for Asymmetric Hydrogenation through Selective Poisoning

The chemical and structural surface heterogeneity of supported noble metal catalysts impedes a detailed description of the active sites. The catalytic performance of a supported noble metal catalyst in terms of the activity and the selectivity towards a specific surface reaction is affected by the adsorption of substrate and product molecules at the catalytically active surface sites. Heterogeneous hydrogenations in the liquid phase exhibit a complex adsorption behavior and the molecular structure of the substrate, the surface properties of the catalyst and the nature of the solvent primarily control the surface processes at the liquid-solid interface. A true in situ approach to study active sites on a supported noble metal catalyst consists in selectively poisoning of the noble metal surface using small amounts of strongly adsorbing substances such as thiols.^1,2 In this work, we made an attempt to study active sites in heterogeneous asymmetric hydrogenations on chirally-modified supported Pd catalyst. We examined the effect of active site poisoning on the asymmetric hydrogenation of 4-methoxy-6-methyl-2-pyrones on cinchonine-modified Pd/TiO₂using different organosulfur compounds.

In heterogeneous asymmetric hydrogenations, enantioselectivity is only induced at chirally-modified sites, while remaining unmodified surface sites lead to racemic product.³The highly specific interplay between co-adsorbed chiral modifier and prochiral substrate further increases the complexity of such poisoning experiments, and various organosulfur compounds with different adsorption behavior are needed to shed light on the complex surface processes. However, such studies have the potential to provide valuable insight into the distribution and intrinsic catalytic performance of racemic and chirally modified sites. Furthermore, they might pave the way to optimization strategies for asymmetric heterogeneous hydrogenations.

The effect of a specific thiol on the catalytic performance was investigated by adding the thiol and the chiral modifier cinchonine simultaneously to the reaction slurry, prior to the substrate. Using operandoattenuated total reflection infrared spectroscopy, that is combining catalytic performance and spectroscopic measurement of the same catalyst sample, we were able to correlate the adsorption geometry and number of strongly bound organosulfur compounds at the active sites to the measured activity and enantioselectivity of the catalytic system. Our current results indicate that thiol adsorption has a strong impact on the catalytic activity, while leaving enantioselectivity unaffected. In contrast, the heterocycle thiophene disrupts the catalytic activity and the enantioselectivity. In general, the molecular structure of the organosulfur compound was found to play a crucial role for the poisoning behavior.

(1) Panthi, B.; Mukhopadhyay, A.; Tibbitts, L.; Saavedra, J.; Pursell, C. J.; Rioux, R. M.; Chandler, B. D. ACS Catalysis 2015, 5, 2232.

(2) Marshall, S. T.; O'Brien, M.; Oetter, B.; Corpuz, A.; Richards, R. M.; Schwartz, D. K.; Medlin, J. W. Nature Materials 2010, 9, 853.

(3) Mallat, T.; Orglmeister, E.; Baiker, A. Chem. Rev. 2007, 107, 4863.