Coadsorption and Interaction of Quinolines and Hydrogen on Platinum Group Metals and Gold: A First-Principles Analysis

The coadsorption of quinoline and hydrogen, a crucial step in the hydrogenation of quinoline, has been examined by first-principles DFT simulations using the (111) face of the catalytically essential face-centered cubic metals rhodium, palla-dium, platinum, and gold employed for this reaction. Special attention was given to the energetics of coadsorbed hydrogen and quinoline, the configuration of quinoline, and changes in these properties with hydrogen surface coverage and hydrogen transfer from the metal surface to quinoline. In the absence of hydrogen, quinoline is found to be chemisorbed nearly flat-lying on the surface of platinum group metals via the pi-bonding system of the aromatic moiety. In contrast, the favored adsorption mode of quinoline on Au is tilted to the metal surface, and sigma-bonding via the N-lone pair prevails. These binding modes are in line with the calculated Lowdin charges and projected density of states of the quinoline-metal systems. The adsorption strengths of quinoline on the pristine (111) surfaces of the metals follow the order Rh > Pd > Pt >> Au. The stability of the coadsorbed configurations has been analyzed in terms of the hydrogen-binding energy, showing a preference of H to bind to the N atom of quinoline on Au and Pt, while on Pd and Rh, adsorption of H atoms to the metal surface is favored. On Au, hydrogen adsorption is energetically disfavored compared to the platinum group (PG) metals. Insertion of a bulky substituent at the C4 atom of quinoline has only little influence on its energetics and adsorption configuration, as exemplified by the adsorption of cinchonidine for imparting chirality to PG metal surfaces in catalytic enantioselective hydrogenations. Possible impacts of these adsorption behaviors on the hydrogenation of quinoline and the use of the quinoline derivative cinchonidine as a chiral modifier in enantioselective hydrogenation are discussed.

Automated summary

The coadsorption of quinoline and hydrogen plays a crucial role in the hydrogenation of quinoline. This study used first-principles DFT simulations to investigate the coadsorption of quinoline and hydrogen on different metal surfaces. The adsorption modes of quinoline were found to vary on different metals, with pi-bonding on platinum group metals and sigma-bonding on gold. The adsorption strengths of quinoline also differed among the metals, with Rh, Pd, and Pt showing stronger adsorption than Au. The study also discussed the potential impacts of these adsorption behaviors on quinoline hydrogenation and the use of cinchonidine as a chiral modifier in enantioselective hydrogenation.