Modifying reaction rates and stimulus-responsive behavior of polymer-coated catalysts using aprotic solvents

Abstract

The impact of solvent composition on the reaction rates and apparent activation barriers for the reduction of nitrobenzene on Pd has been investigated by changing the solvent from pure water to mixtures with increasing concentrations of 1-methyl-2-pyrrolidone (NMP). When using pure NMP as the solvent, the activity was negligible and a high activation energy barrier was observed. Surprisingly, switching to water led to faster reaction rates and lower apparent barriers. Considering that previous research has demonstrated that water molecules near the catalyst surface facilitate the hydrogen insertion on R-NO* and R-HNO* surface species via proton-electron transfer, it is possible to link the herein observed trends in activity for the nitrobenzene hydrogenation to the ability of the reaction media to shuttle protons during the reaction. Furthermore, the polymer-induced solvation effects were investigated using thermo-responsive Pd/SiO2-p-NIPAM catalyst. Here, we observed that the utilization of NMP inhibits the thermo-responsive behaviour of poly N-isopropylacrylamide (p-NIPAM). This explains the constant particle size of Pd/SiO2-p-NIPAM catalyst observed at different temperatures during dynamic light scatting characterization (DLS). We speculate that this non-responsive behaviour of the p-NIPAM in the presence of NMP is the cause of the constant activation energy barrier at temperatures above and below the lower critical solution temperature (LCST) of the polymer (32 °C). When the reaction was conducted in pure water, the polymer-coated catalyst showed significant changes in both the apparent enthalpy and entropy of activation for temperatures below and above the LCST. This suggests that the microenvironment induced by the polymer can significantly influence the reaction rate.