Selective Ethylbenzene Dehydrogenation to Styrene at Lewis Acid–Base Site Pairs on Zirconia Surfaces

The chemical dehydroxylation of monoclinic zirconia (m-ZrO2) surfaces uncovers low-coordination Lewis acid–base (LAB) Zr–O pairs with very high reactivity and selectivity in nonoxidative ethylbenzene dehydrogenation to styrene and its reverse reactions (styrene hydrogenation). Ethylbenzene dehydrogenation rates (per mass) on dehydroxylated m-ZrO2 significantly exceed those on promoted Fe-based catalysts, with selectivities above 95%. Kinetic trends for dehydrogenation and hydrogenation reactions reflect their respective stoichiometries, indicating that both reactions are mediated by the same kinetically relevant step on essentially bare LAB pairs. These mechanistic features lead to ratios of their respective rate constants given by the equilibrium constant for the overall reaction. The titration of LAB pairs by H2O impurities in reactant streams leads to rates that decrease with time during the reaction; such rate losses can be reversed by periodic chemical dehydroxylation treatments and minimized by thorough purification of inlet streams. Dehydrogenation rates ultimately reached asymptotic values set by the relative rates of titration by H2O and its removal by its reaction with ethylbenzene; such inherent chemical dehydroxylation reactions lead to essentially constant asymptotic rates. The uncovering and reactivity of LAB pairs at ZrO2 surfaces render such solids (and related oxides) an attractive and unrecognized alternative to catalysts in current practice.