UC Riverside

Improve the enantioselectivity to obtain chiral product has been the major concern in pharmaceutical and biochemical engineering. In most cases, separation of racemic product is extremely time consuming and requires special equipment, thus it usually becomes a less efficient and economic option than chiral synthesis even though such synthesis often involves modified noble metal catalyst. In this work, chiral modification of platinum has been studied to gain better understanding of adsorption mechanism of surface modifiers on Pt surface as well as it affects the conversion and enantioselectivity of our model reaction—hydrogenation of activated keto-esters.

Majority of this work has been focused on exploring the factors that may play an important role in the adsorption of chiral modifier onto a variety of Pt surfaces including polycrystalline Pt and Pt nanoparticles supported on oxide. Several in-situ FT-IR (Fourier Transform- Infrared) spectroscopy setup has been designed and built based on a Bruker Tensor IR spectrometer. In-situ IR spectroscopy has been proved itself to be overwhelmingly powerful and efficient to study the adsorption geometry of modifiers at solid/liquid interfaces. Pt surface is probed with advanced IR characterization methods such as RAIRS (Reflection-Absorption Infrared Spectroscopy) and Multi-bounce ATR-IR (Attenuated Total Reflection) methods due to their ability to distinguish adsorbed species from dissolved molecules in solution phase.

A series of modifiers are involved in this project, not only limited to the well-studied classic cinchona-alkaloid such as cinchonidine and cinchonine, but also naphthyl-based chiral compounds including (R)- or (S)-(−)-1-(1-naphthyl) ethylamine (NEA), naphthylmethyl amine, and dimethyl naphthyl ethylamine. The adsorption strength of the different modifier molecules was found to be quite different among those compounds, which is illustrated by the fact that quinoline can displace s-NEA from Pt but not vice versa, for instance, and by the observation that when Pt is exposed to a solution containing both quinoline and s-NEA only the quinoline’s signature peaks can be detected by ATR-IR spectroscopy. The ordering of the modifiers studied in terms of adsorption strength was found to correlate with their ability to chirally modify the Pt catalyst during the hydrogenation of unsaturated aldehydes.

In addition, adsorption geometry of s-NEA/r-NEA shows that these modifiers adsorb on Pt surfaces through the nitrogen atom of the primary amine moiety not aromatic ring as commonly believed in the past.

Future follow-up work of this project might include: optimizing the IR instrumentation to minimize the interference from polar solvent and efficiently run and monitor hydrogenation reactions using in-situ IR spectroscopy.