(112g) Tuning Zr-SiO2 Surface Environment for MPV Reduction

The Meerwein-Ponndorf-Verley (MPV) reduction allows for hydrogenation under mild conditions without the use of hydrogen gas and is used for production of pharmaceuticals and biomass derivatives . Because of their acid-base properties, Zr-containing materials, such as silica-supported Zr, are common catalysts for MPV reduction. Activity of Zr-based materials for MPV reduction is heavily dependent upon Zr coordination environment. Additionally, MPV reduction is sensitive to steric confinement around the active site due to its bulky transition state. Separating the effects of Zr coordination from effects of the steric environment can be quite challenging.

In this work, we consider how the Zr coordination environment of silica-supported Zr can be tuned by changing the Zr surface density and the choice of Zr precursor used. We find that the rate constant per Zr varies with Zr surface density and choice of precursor. However, using in situ titration of Lewis acid sites with phosphonic acids to quantify the number of Lewis acid sites on each material synthesized, we find that the rate constant per kinetically relevant Zr site is constant across all materials in this study. Therefore, we conclude that the Zr active sites on all materials studied are intrinsically the same.

Additionally, we consider how the local environment changes when we employ molecularly templated atomic layer deposition (ALD) of an inert oxide around the Zr active site to create a sterically constrained local environment. Because we demonstrate that the rate per active site is intrinsically the same across all materials in this study prior to ALD, we can isolate effects on rate resulting from the changes in the local environment created by overcoating from instrinic differences in Zr properties. We show how steric confinement by molecularly templated ALD can be tuned by changing the choice of template and the extent of overcoat deposition.