(513bv) Tuning Electronic Properties of Active Sites for Alkene Epoxidations with H2O2
AIChE Annual Meeting
2020
2020 Virtual AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Poster Session: Catalysis and Reaction Engineering (CRE) Division
Friday, November 20, 2020 - 8:00am to 9:00am
We have synthesized groups 4â6 metals (Ti, Nb, W) substituted into the zeolite BEA framework through post-synthetic modification to study 1-hexene epoxidation with H2O2. Epoxidation selectivities at comparable conditions are greatest on Ti-BEA (93%), followed by Nb- (38%) and W-BEA (20%) and rates are 250- and 60-fold lower on W-BEA and Nb-BEA than Ti-BEA, respectively. These large kinetic differences are not due to differences in the epoxidation mechanism; rather, the disparity in rates and selectivities reflect differences in the electrophilicity of the metal-hydroperoxo and peroxo intermediates. Electrophilicities of these surface species are inferred through measurements of apparent activation enthalpies (âHâ¡) and determination of the relative ligand-to-metal charge transfer energies measured via in situ UV-Vis spectroscopy. The heats of 1,2-epoxyhexane adsorption (âHAds) onto active sites, measured by isothermal titration calorimetry, show W-BEA binds 1,2-epoxyhexane less strongly than Ti- and Nb-BEA. âHâ¡ values decrease linearly with âHAds, showing that 1-hexene epoxidation exhibits a linear free energy relationship. These findings suggest rates and selectivities can be simultaneously improved through design principles that increase functional Lewis acidity of active sites. Preliminary results for a similar study reveal Ti catalysts supported on SiO2 (SBA-15) have higher rates and lower âHâ¡ than Ti supported on Al2O3 for 1-hexene epoxidation. Ongoing work will expand on tuning electronic properties of active metal sites by investigating these metal-support interactions. We gratefully acknowledge support from the Army Research Office (W911NF-16-1-0100).