Examination of catalytic behavior and origin of the initial transient period for Pt nanoclusters modified with cinchonidine

Abstract

Nanoclusters prepared by a novel water-free method are compared directly with nanoclusters prepared by the known aqueous preparation as well as conventional Pt/Al 2 O 3 for the enantioselective hydrogenation of ethyl pyruvate. The catalytic behavior of cinchonidine on colloidal Pt was investigated during ethyl pyruvate hydrogenation in acetic acid under 10 bar of hydrogen at 22°C with (1 mmol L )1 ) and without addition of free cinchonidine. The effect of hydrogen pressure, cinchonidine concentration, ethyl pyruvate and catalyst loading on the enantiomeric excess (EE) with time were also studied. Through these studies, we propose that the nature of the observed initial transient period (ITP) for these ''quasi-homogeneous'' systems may be explained by desorption of the weakly adsorbed tilted ''N lone pair bonded'' cinchonidine species from the Pt surface due to interaction with hydrogen.

Surface-level mechanistic studies of adsorbate–adsorbate interactions in heterogeneous catalysis by metals

“…Furthermore, analysis of the interactions between chiral molecules and reactants at the metal surface is hampered in part by the complexity of chiral modifiers such as CD. CD has been found to adsorb on surfaces such as Pt (111) through multiple coordination with the metal surface, and in a variety of orientations that depend on surface coverage and temperature [62][63][64][65][66][67][68]. At conditions of low (room) temperature, CD has been found to adsorb with its quinoline ring lying more or less flat on the surface at low coverage, though significant tilt becomes obvious at higher temperatures and coverage.…”

“…The flat-lying geometry is generally assumed to be the structure that imparts enantioselectivity, in part because the modifier is most effective in conferring enantioselectivity at moderate coverages. Tilted structures are generally assumed to be spectators [62][63][64][65][66][67][68]. Interestingly, the presence of hydrogen on the surface appears to affect the binding of CD on metal surfaces.…”

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“…This is particularly important when analyzing the hydrogenation of EP, the most widely applied test reaction, in which side reactions play important roles. Minimization of side reactions and the resulting catalyst deactivation can be best achieved by applying a weakly acidic solvent [9,10] and ensuring the appropriate order of addition of the reaction components into the reactor. Studies in nonacidic media [8,11], particularly in dichloromethane [7], cannot give reliable information on the intrinsic rate acceleration induced by the chiral modifier.…”

Ionic Liquid-Supported Pt Nanoparticles as Catalysts for Enantioselective Hydrogenation

“…CD adsorbs strongly on Pt and hence it can also be used as a potent stabilizer for the synthesis of NPs. 25,26 These CD-stabilized NPs where shown to act as efficient catalytic systems for the hydrogenation of ethyl pyruvate with enantioselectivities of around 80%. 25−27 Thus, a second set of IL-supported NPs (denoted as Pt CD @IL) was synthesized where CD was added as a costabilizer not only There is no need for any predrying procedure of the ILs as is often described in the literature for other protocols.…”