Microgel organocatalysts: Modulation of reaction rates at liquid-liquid interfaces

Abstract

Controlling the catalytic activity via the use of smart and responsive carriers is a major challenge in research concerning supported catalysis. In homogeneous reactions, responsive polymeric carriers are often used to switch on/off reagent supply by controlled shielding of embedded catalysts. However, reactions on liquid interfaces are rare subjects of these kinds of studies. As an example of colloidal microgel-catalysts based on poly(N-isopropylacrylamide) (PNIPAM) containing covalently bound l-proline, we herein present how temperature and the effect of cononsolvency can be used as triggers to modulate reaction rates in a homogenous phase and on liquid-liquid interfaces. In particular, the aldol reaction of cyclohexanone with 4-nitrobenzaldehyde in water, methanol and water-methanol mixtures was the focus of our study. The swelling degree of the microgels evaluated by dynamic and static light scattering (DLS and SLS) was adjusted to demonstrate the effect of swelling on the rate of the aldol reaction. Combining our experimental results with computer simulations, based on dissipative particle dynamics (DPD), we could relate significant differences in reaction rates to the temperature-responsive swelling of the microgels. The simulations show that in aqueous reaction mixtures, the microgel-catalysts are adsorbed at the liquid-liquid interface between water and the hydrophobic reagents. Increasing the temperature causes the microgel-catalysts to immerse more into the reagent phase due to the temperature-responsiveness of PNIPAM. As a result, the average number of contacts between the l-proline catalyst and the reagents increases drastically resulting in a more than 5-fold increase of the catalytic rate observed in the experiments. On the contrary, the simulations of the reaction in methanol confirm that reagents form a homogeneous mixture in which the defined average number of contacts changes negligibly with the increasing temperature as PNIPAM does not possess a temperature-responsive behaviour in this solvent.