The catalytic activation of primary alcohols on niobium oxide surfaces unraveled:: the gas phase reactions of NbxOy- clusters with methanol and ethanol

The reactions of oligomeric niobium oxide anions (up to Nb6O15-), generated by laser ablation and studied using a Fourier transform ICR mass spectrometer, have been used to deduce the roles of(i) Nb(III,IV,V) centers, (ii) Nb/O double bonds and (iii) proximal Nb centers, in the catalytic activation of methanol and ethanol. The most important recurring mechanism involves initial alcohol condensation at a cluster metal-oxygen double bond to yield Nb(OH)(OCH3). There is no change in the oxidation state of the cluster during this step. The so-formed niobium-hydroxyI bond is the new reactive site in the cluster, and undergoes ligand switching in a follow-up collision to yield a bis-methoxy cluster and neutral water. Dehydrogenation is only observed to occur with clusters possessing two Nb/O double bonds at a single metal center, and involves reduction of the participating Nb(V) center to Nb(III). An ion ejection/selection step was used to monitor the activity of a number of the ionic reaction products towards the alcohols, and in most instances spontaneous or kinetically-activated decomposition resulted in regeneration of the parent cluster from the substituted species. (C) 2000 Elsevier Science B.V. All rights reserved.