Methane activation by V3PO10•+ and V4O10•+ clusters: A comparative study

A series of vanadium and phosphorus heteronuclear oxide cluster cations (VxPyOz+) are prepared by laser ablation and the reactions of V3PO10 center dot+ and V4O10 center dot+ with methane in a fast flow reactor under the same conditions are studied. A time of flight mass spectrometer is used to detect the cluster distribution before and after reactions. In addition to previously identified reaction of V4O10 center dot+ + CH4 -> V4O10H+ + CH3 center dot, the observation of hydrogen atom pickup cluster V3PO10H+ suggests the reaction: V3PO10 center dot+ + CH4 -> V3PO10H+ + CH3 center dot. The rate of the reaction of V4O10 center dot+ with CH4 is approximately 2.5 times faster than that of V3PO10 center dot+ with CH4. Density functional theory (DFT) calculations predict that structure of V3PO10 center dot+ is topologically similar to that of V4O10 center dot+, as well as that of P4O10 center dot+, which is very similar to V4O10 center dot+ in terms of methane activation in previous studies. The facile methane activation by the homo-and hetero-nuclear oxide clusters can all be attributed to the presence of an oxygen-centered radical (O-center dot) in these clusters. Further theoretical study indicates that the O-center dot radical (or spin density of the cluster) can transfer within the high symmetry V4O10 center dot+ and P4O10 center dot+ clusters quite easily, and CH4 molecule further enhances the rate of intra-cluster spin density transfer. In contrast, the intra-cluster spin density transfer within low symmetry V3PO10 center dot+ is thermodynamically forbidden. The experimentally observed reactivity difference is consistent with the theoretical consideration of the intra-cluster spin density transfer.