Date of Award
Open Access Dissertation
Christopher T Williams
The production of optically pure chemicals through enantioselective catalytic reaction plays a key role a variety of industrials (e.g., pharmaceutical, agrochemical). Among several established methods, chiral-modified supported transition metal catalyst has been addressed attentions since past decades due to an environmental concern. Asymmetric hydrogenation of α, β-unsaturated carboxylic acid over Pd catalyst is of particular interest in this area. Nevertheless, the spectroscopic aspect, as well as the kinetic mechanism of such reaction system are still not clearly understood and require further study. In this work, a model catalytic reaction has been investigated from both spectroscopic and kinetic view. To be specific, the hydrogenation of a model aliphatic unsaturated carboxylic acid (2-methyl-2-pentenoic acid) over cinchonidine (chiral modifier) modified Pd/Al2O3 catalyst has been examined. The adsorption behaviors of the acid and the modifier, as well as the acid-modifier interaction complex over metal and oxide surface have been characterized by in-situ attenuated total reflection infrared (ATR-IR) spectroscopy. It has been found that the substrate acids predominantly adsorb as bridged bidentate at the surface. The active intermediate is preferential in a modifier:acid=1:1 ratio, regardless of solvent. The racemic and enantioselective hydrogenation has been carried out in a batch reactor. Kinetic aspects (e.g., H2 pressure, solvent effect, external mass transfer,
modifier effect) have been studied. To summary, the reaction exhibits a strong solvent-dependent behavior, with polar solvent provides higher activity. H2 pressure shows 1st reaction order at low regime (< 8 atm), while become pressure-independent when greater than 20 atm. The substrate shows 1st reaction order at low conversion region (< ca. 60%), but increase to high order (1.5-2) afterward. The presence of chiral modifier (cinchonidine) does not affect reaction order for both reactant and modifier. However, it will dramatically decrease the reaction and lead to e.e. value up to ca. 30%, due to the high occupied percentage of surface metal sites, and rather low TOF of these modified sites. Moreover, the accumulation of product in liquid mixture appears to be the reason for increasing reaction order of acid substrate, although this does not affect the final e.e. value.
Tan, S.(2013). Asymmetric Hydrogenation of α, β-Unsaturated Carboxylic Acid Over Chiral-Modified Palladium Catalyst. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/598