(513a) Uncovering Active Sites on Oxide Surfaces Using Chemical Treatments and Their Unique Reactivity in C-H Activation Reactions in Anhydrous and Anaerobic Environments

Lewis acid-base pairs at earth-abundant oxide surfaces (ZrO₂, Y₂O₃) activate C–H bonds via heterolytic routes during non-oxidative alkane dehydrogenation reactions. The pairs which stabilize most effectively the anion-proton pairs at C–H activation transition states also bind H₂O and CO₂ most strongly, thus rendering active sites inaccessible for catalysis. Titrant desorption requires thermal treatments at temperatures that cause loss of surface area and the annealing of coordinatively-unsaturated surface structures, leading to lower active site densities. Chemical treatments uncover these sites via dehydroxylation (or decarboxylation) reactions mediated by chemical reagents (dimethyl ether; methanol; alkenes) without concomitant loss of stie density. Gravimetric rates of C₂–C₄ alkane dehydrogenation (and C₂-C₄ alkene hydrogenation) are ~100-fold higher after chemical treatments than thermal treatments at the same temperatures. These rates are proportional to the number of accessible active sites (measured by their titration with H₂O or CO₂ during catalysis); they exceed those measured on Cr and Pt catalysts in current commercial practice. Kinetic and isotopic data show that rates are limited by C-H activation steps on essentially bare surfaces; deactivation occurs predominantly by titration from trace impurities (H₂O; CO₂; O₂) present in inlet streams, thus requiring strictly anhydrous environments to exploit their unique reactivity. DFT-derived energies show that C–H activation occurs heterolytically with the first C–H activation (at -CH₃ positions of weakly bound alkanes) as the sole kinetically-relevant step at the experimental conditions, matching measured barriers and demonstrating the need for low-coordination Zr and O atoms to stabilize both H₂O molecules and C-H activation transition states. The relative barriers for C–C and C–H cleavage from such carbanions are consistent with observed cracking selectivities, which are significantly lower on chemically-activated ZrO₂ surfaces than on metal or oxide catalysts that activate C–H and C–C bonds via homolytic or carbocation-mediated routes.