Quantifying Kinetically Relevant Species on Zr-SiO₂ Materials for MPV Reduction

On supported metal catalysts such as Zr-SiO₂, it can be challenging to isolate characteristics that result from intrinsic properties of the active site from those that result from the environment surrounding the active site. In this report, we utilize in situ titration of Lewis acid sites with phosphonic acid to accurately and quantitatively describe kinetically relevant Zr species on Zr-SiO₂ materials for the MPV reduction of cyclohexanone. We find that rate of MPV reduction on Zr-SiO₂ materials can be described as a combination of rate over titratable Zr, that is likely well dispersed Zr, and rate over non-titratable Zr, that is likely supported ZrO_x. The fraction of Zr that is well dispersed on the SiO₂ is dependent on the surface density at which Zr is grafted but not the choice of Zr precursor. We demonstrate that phosphonic acid titration can offer a more relevant, quantitative description of Zr dispersion than UV-vis and can be used to quantitatively describe changes that occur to the material during regeneration.