In designing effective catalysts, one must consider how to control the accessibility and activity of the active sites. Inspired by nature, we have leveraged the chemistry of thermoresponsive poly(N-isopropylacrylamide) (p-NIPAM) to tailor the extent of solvation of the transition state key surface reaction intermediates during the hydrogenation of nitrobenzene to aniline on Pd/SiO2. Detailed reaction kinetics, catalyst characterization, and NMR diffusion-ordered spectroscopy (DOSY)/nuclear Overhauser effect spectroscopy (NOESY) experiments indicate that nitrobenzene reduction is co-limited by both the formation and the hydrodeoxygenation of phenylhydroxylamine (PHA) to aniline (AN) precursor. Transition-state treatment of the kinetic data revealed that when the temperature is below the lower critical solution temperature (LCST) of p-NIPAM (32 °C), the apparent enthalpy of activation decreases 3-fold. This change was attributed to the drop in the apparent enthalpy of activation when the polymer was in a swollen state. A concomitant reduction in the apparent entropy of activation was obtained at these conditions, indicative of losses in the degree of freedom of the kinetically relevant intermediate (i.e., surface hydrogen). At temperatures above the LCST, it was possible to reverse these effects, leading to similar apparent activation energy as that observed in the Pd/SiO2 catalyst. These results establish the foundational work on the development of materials capable of taming the intrinsic activity of the active site in a fast, reversible manner. We envision that these results will facilitate the development of catalysts that can mimic the homeostatic behavior of enzymes, allowing more stable operation even when complex feedstocks are employed (e.g., biomass conversion and pollution control).
- nitrobenzene hydrogenation mechanism
- polymer-coated catalyst
- solvation effect
- transition states