Dehydrogenation of Methanol by Vanadium Oxide and Hydroxide Cluster Cations in the Gas Phase

Abstract

Bare vanadium oxide and hydroxide cluster cations, V(m)O(n)+ and V(m)O(n-1) (OH)+ (m = 1-4, n = 1-10), generated by electrospray ionization, were investigated with respect to their reactivity toward methanol using mass spectrometric techniques. Several reaction channels were observed, such as abstraction of a hydrogen atom, a methyl radical, or a hydroxymethyl radical, elimination of methane, and adduct formation. Moreover, dehydrogenation of methanol to generate formaldehyde was found to occur via four different pathways. Formaldehyde was released as a free molecule either upon transfer of two hydrogen atoms to the cluster or upon transfer of an oxygen atom from the cluster to the neutral alcohol concomitant with elimination of water. Further, formaldehyde was attached to V(m)O(n)+ upon loss of H2 or neutral water to produce the cation V(m)O(n)(OCH(2))+ or V(m)O(n-1) (OCH(2))+, respectively. A reactivity screening revealed that only high-valent vanadium oxide clusters are reactive with respect to H2 uptake, oxygen transfer, and elimination of H2O, whereas smaller and low-valent cluster cations are capable of dehydrogenating methanol via elimination of H2. For comparison, the reactivity of methanol with the corresponding hydroxide cluster ions, V(m)O(n-1) (OH)+, was studied also, for which dominant pathways lead to both condensation and association products, i.e., generation of the ions V(m)O(n-1) (OCH(3))+ and V(m)O(n-1) (OH)(CH(3)OH)+, respectively.