Counting Active Sites on Titanium Oxide–Silica Catalysts for Hydrogen Peroxide Activation through In Situ Poisoning with Phenylphosphonic Acid

Abstract

Quantifying specific active sites in supported catalysts improves our understanding and assists in rational design. Supported oxides can undergo significant structural changes as surface densities increase from site‐isolated cations to monolayers and crystallites, which changes the number of kinetically relevant sites. Herein, TiOx domains are titrated on TiOx–SiO2 selectively with phenylphosphonic acid (PPA). An ex situ method quantifies all fluid‐accessible TiOx, whereas an in situ titration during cis‐cyclooctene epoxidation provides previously unavailable values for the number of tetrahedral Ti sites on which H2O2 activation occurs. We use this method to determine the active site densities of 22 different catalysts with different synthesis methods, loadings, and characteristic spectra and find a single intrinsic turnover frequency for cis‐cyclooctene epoxidation of (40±7) h−1. This simple method gives molecular‐level insight into catalyst structure that is otherwise hidden when bulk techniques are used.

Rate and Selectivity Control in Thioether and Alkene Oxidation with H2O2 over Phosphonate‐Modified Niobium(V)–Silica Catalysts

“…Further, two Nb‐SiO 2 samples were treated with 0.26 and 1.0 equivalents of PPA (Scheme ), recovered, washed with acetonitrile, and then used as catalysts (labeled 0.25‐PPA‐Nb‐SiO 2 and PPA‐Nb‐SiO 2 , respectively). Consistent with prior observations over related materials, essentially all of the added PPA is captured by the surface, which gives an increase in the optical edge energy (Supporting Information, Figure S1) and apparent coordination number from X‐ray absorption (Supporting Information, Figure S2). Finally, 31 P CP‐MAS NMR shows only one type of surface phosphonate regardless of the Nb:P ratio (Supporting Information, Figure S3) .…”

“…The small shoulder at about 340 nm for Nb‐SiO 2 suggests a small extent of aggregation . Further, all three materials have optical edge energies of 3.7–3.9 eV, as calculated from a separate Tauc plot, in agreement with previous edge values for highly dispersed oxides reported by us and by others for similar catalysts. We briefly note that if the calixarene ligand is unavailable, catalysts with moderately diminished performance can often be synthesized from metallocene chloride precursors, although they are typically more expensive than if the metal alkoxide/chloride and calixarene ligand were obtained separately …”

“…To address these questions for the high‐performing niobium case, we revisit a site titration technique previously established by our group for epoxidation and photooxidation . The titrant phenylphosphonic acid (PPA) selectively poisons undercoordinated, Lewis acidic Group 4 and 5 oxide sites in situ during cyclooctene epoxidation with H 2 O 2 ; this behavior is due to electron donation by the titrant that strongly inhibits rate‐limiting peroxygen transfer to the alkene . Using this method, it was shown that large families of Ti‐SiO 2 and Nb‐SiO 2 materials differ in the numbers of active sites, but not their intrinsic activity.…”

“…From this study, we demonstrated that niobia–silica (Nb‐SiO 2 ) is a high performer for epoxidation, proceeding with two‐fold faster rate and higher selectivity for direct, heterolytic (that is, non‐radical) H 2 O 2 pathways in cyclohexene epoxidation relative to benchmark Ti‐SiO 2 catalysts. We followed this investigation with a detailed study of synthesis–structure–function relationships for a broad series of Nb‐SiO 2 catalysts, and we applied an in situ poisoning experiment using phenylphosphonic acid (PPA) to count the number of Lewis acid sites active for cis ‐cyclooctene epoxidation . This study also demonstrated mechanistic differences between the Nb V and benchmark Ti IV cyclooctene epoxidation catalysts …”

“…In the past, we have used cyclooctene epoxidation as a probe reaction during PPA titration and to investigate different supported oxides because this reaction is intrinsically selective to the epoxide . Herein, we expand our prior studies to examine thioanisole and other thioether oxidations with H 2 O 2 over three representative, highly dispersed supported oxides, Nb‐SiO 2 , Ti‐SiO 2 , and Zr‐SiO 2 , each made through a molecular precursor method that yields highly dispersed sites.…”