Catalytic dehydration of 2-propanol by size-selected (WO₃)_n and (MoO₃)_n metal oxide clusters

Here, we report the catalytic dehydration of 2-propanol by metal oxide clusters, (WO₃)_n and (MoO₃)_n (n = 1, 2, 3, 5, 30), prepared by mass selecting and soft-landing metal oxide cluster anions created in the gas phase. Temperature-programmed reaction (TPR) was used to characterize the catalytic activity of the deposited clusters by measuring the production of propene from 2-propanol. The nature of the support, thermal history, size of the cluster, and cluster composition were all found to play important roles in influencing catalytic activity. (WO₃)₃ clusters deposited on HOPG (highly ordered pyrolytic graphite) and oxide supports exhibited catalytic activity, although (WO₃)₁ monomers deposited on HOPG did not catalyze 2-propanol dehydration effectively, an effect ascribed to their coalescence into large aggregates on HOPG. For tungsten oxide clusters deposited on annealed oxide films, catalytic activity was observed for all cluster sizes and was linearly correlated with the size of the deposited clusters. Two different mechanisms, linear-scaling of active sites and cluster ripening, could account for this linear dependence. However, even on oxide supports, deposited tungsten oxide clusters lost catalytic activity after annealing to 400 °C. The effect is consistent with the loss of dioxo groups rather than any cluster aggregation. Compared to tungsten oxide clusters, molybdenum oxide clusters exhibited little or no catalytic activity toward the dehydration of 2-propanol, rationalized by the decrease in Lewis acidity of molybdenum-oxygen bonds.