Striking Doping Effects on Thermal Methane Activation Mediated by the Heteronuclear Metal Oxides [XAlO4].+ (X=V, Nb, and Ta)

• Published in: Chemistry : a European journal 2017-01, Vol.23 (4), p.788-792
• Main Authors: Wu, Xiao‐Nan, Li, Jilai, Schlangen, Maria, Zhou, Shaodong, González‐Navarrete, Patricio, Schwarz, Helmut
• Format: Article
• Language: English
• Subjects: Chemistry; doping; gas-phase reactions; heteronuclear metal oxide cluster; Methane; methane conversion; reaction mechanisms
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201605226

The thermal reactivity of the heteronuclear metal‐oxide cluster cations [XAlO4].+ (X=V, Nb, and Ta) towards methane has been studied by using mass spectrometry in conjunction with quantum mechanical calculations. Experimentally, a hydrogen‐atom transfer (HAT) from methane is mediated by all the three oxide clusters at ambient conditions. However, [VAlO4].+ is unique in that this cluster directly transforms methane into formaldehyde. The absence of this reaction for the Nb and Ta analogues demonstrates a striking doping effect on the chemoselectivity in the conversion of methane. Mechanistic aspects of the two reactions have been elucidated by quantum‐chemical calculations. The HAT reactivity can be attributed to the significant spin density localized at the terminal oxygen atom (Ot.−) of the cluster ions, while the ionic/covalent character of the Lewis acid–base unit [X−Ob] plays a crucial role for the generation of formaldehyde. The mechanistic insight derived from this combined experimental/computational investigation may provide guidance for a more rational design of catalysts. The doping effect strikes back! Mechanistic aspects of the thermal methane conversion mediated by the heteronuclear oxide clusters [XAlO4].+ (X=V, Nb, and Ta) have been revealed by a combined experimental/computational approach. Similar hydrogen‐atom transfer reactions for all these three systems can be attributed to the high spin density on the H‐atom or, whereas for only X=V, the ionic/covalent character of the Lewis acid–base unit [X‐Ob] plays a crucial role in the generation of formaldehyde.