Effects of Composition on the Reactivity of Protons in Polyoxometalate Clusters
Josef Macht, Cindy Yin, and Enrique Iglesia. Chemical Engineering, UC Berkeley, 201 Gilman Hall, Berkeley, CA 94720
Polyoxometalate clusters provide well-defined atomic connectivity, cluster size and compositional flexibility, thus permitting studies of structure-function relations in oxide catalysis. These studies require that we determine the accessibility of active protons or metal-oxo species during catalysis and the kinetic relevance of steps involved in catalysis. Here, we report the effects of central atom (X), addenda atom (M), charge-balancing cations (Me), and secondary structure on the reactivity of protons in Mem+yH8-n-myXn+M12O40 clusters supported on SiO2, for the specific case of 2-butanol dehydration catalyzed by Brønsted acids. This reaction proceeds via carbenium ion pathways limited by cleavage of C-O bonds to form butoxides with concurrent quasi-equilibrated formation of unreactive dimers. The number of protons accessible to reactants is measured by titration with 2,6-tert-butylpyridine during catalysis. The rate constant for butoxide formation and the equilibrium constant for dimer formation increased as proton acidity increased with increasing central atom valence, which led to a decrease in the number of charge-balancing protons, and also when H+ replaced more electropositive Cs+ cations. These parameters were similar for internal and external H+ protons in secondary structures and decreased with increasing charge on the anion.