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Shape-selective enantioselective hydrogenation on Pt nanoparticles.
Erik Schmidt1, Angelo Vargas, Tamas Mallat
1Department of Chemistry and Applied Biosciences, ETH Zürich, HCI, CH-8093, Zürich, Switzerland.
Journal of the American Chemical Society
|August 1, 2009
Summary
Platinum nanoparticle shape influences enantioselective hydrogenation rates and selectivity. Higher {111} face ratios on platinum catalysts enhance catalytic activity and enantiomeric excess for chiral modifier adsorption.
Area of Science:
- Heterogeneous catalysis
- Chiral chemistry
- Surface science
Background:
- Enantioselective hydrogenation is crucial for synthesizing chiral molecules.
- The influence of metal nanoparticle shape on catalytic performance is not fully understood.
- Chiral modifiers are essential for inducing enantioselectivity in hydrogenation reactions.
Purpose of the Study:
- To investigate the structure sensitivity of enantioselective hydrogenation on chirally modified platinum (Pt) nanoparticles.
- To determine the effect of Pt nanoparticle shape and surface facet exposure ({100} vs. {111}) on reaction rate and enantioselectivity.
- To elucidate the role of chiral modifier adsorption in structure-dependent catalytic outcomes.
Main Methods:
- Synthesis of Pt nanoparticles with controlled shapes (cubic, cubooctahedral, octahedral) and average size of 10 nm.
- Enantioselective hydrogenation of ethyl pyruvate and ketopantolactone using cinchonidine and quinine as chiral modifiers.
- Analysis of reaction rates and enantiomeric excess (ee) as a function of Pt nanoparticle morphology.
- Density Functional Theory (DFT) calculations to study the adsorption behavior of cinchonidine on Pt(100) and Pt(111) surfaces.
Main Results:
- In the absence of chiral modifiers, Pt nanoparticle shape did not affect hydrogenation rates.
- Addition of cinchonidine or quinine significantly enhanced reaction rates (4-15x) and achieved high enantioselectivities (72-92% ee).
- Both reaction rate and enantioselectivity increased with a higher ratio of Pt{111} to Pt{100} faces, indicating shape selectivity.
- DFT studies revealed stronger adsorption of cinchonidine on Pt(100) compared to Pt{111}, explaining the observed structure sensitivity.
Conclusions:
- The shape of Pt nanoparticles critically influences enantioselective hydrogenation when chiral modifiers are present.
- Pt nanoparticles with a higher proportion of {111} facets are more active and selective due to favorable chiral modifier adsorption.
- Optimizing Pt nanoparticle morphology, specifically maximizing {111} terraces, is key for efficient and highly enantioselective hydrogenation catalysts.
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