Oxygen Transmission in Solid Phase——MnO2 promoted Phase-Pure M1 Movnbte Oxide for Ethane Oxidative Dehydrogenation

MoVNbTe oxide is among the most promising catalysts for oxidative dehydrogenation of ethane (ODHE) process. Vanadium ions come in two valence states in the catalyst system: V⁵⁺ and V⁴⁺. The V⁵⁺ ions in M1 phase are considered as the paraffin activating sites. Therefore, increasing the V⁵⁺ concentration is the most effective way to improve the catalytic performance.

High temperature oxygen treatment is the traditional way to oxidize the elements in metal oxide. But considering the complex structure of the MoVNbTe catalyst, high temperature oxygen treatment would destroy the catalyst crystal structure, while contacting oxygen molecule under 400 ^oC has limited effectiveness.

To break the limit of gas-solid oxygen transmission, we have used oxygen plasma at room temperature to insert oxygen into the catalyst surface through a gas-solid reaction by raising the activity of oxygen donor ¹. In another work, we have introduced CeO₂ to the catalyst system to form a nanocomposite with the M1 catalyst, thus oxidizing the vanadium on the M1 catalyst surface through a self-redox solid-state reaction ².

In this work, to break the limit of gas-solid oxygen transmission, manganese oxide (MnOx) is introduced into the catalyst system. Mn is an element whose valence state is variable, and is easy to react with oxygen molecule. By mixing MnOx xerogel with M1 catalyst and calcinating the mixture at 400 ^oC in air for 6 h, the oxygen molecule in air is inserted into MnOx. Meanwhile, the oxygen transmission happens from MnO_x to M1 phase, increasing the abundance of V⁵⁺ on the catalyst surface by a self-redox solid-state reaction. The SEM and TEM characterization show the composite of MnO₂ on M1 catalyst surface. The increase of V⁵⁺ concentration is proved by XPS characterization. Corresponding to the valence state increment, the catalyst promoted by MnO₂ gives an improved catalytic performance, e.g., making a 10% increase of ethane conversion in ODHE process at 400 ^oC. As a comparison, without the introduction of MnOx, calcinating the the M1 catalyst alone in air can only give a 3% increase of ethane conversion under the same condition. In this work, manganese oxide plays a role of ‘oxygen bridge’, allowing the oxygen molecule in gas phase to be transmitted to the catalyst phase more easily through a gas-solid-solid path during calcination.

It is anticipated that this insight into oxygen transmission in phase-pure M1 MoVNbTe oxide catalyst can give more inspiration in redox catalysts.

1. X. Chen, B. Chu, Q. Yang, H. An, Y. Cheng, Valence variation of phase-pure M1 MoVNbTe oxide by plasma treatment for improved catalytic performance in oxidative dehydrogenation of ethane. RSC Adv. 2015; 5: 91295-91301

2. B. Chu, H. An, T.A. Nijhuis, J. C. Schouten, Y. Cheng, A self-redox pure-phase M1 MoVNbTeOx/CeO2 nanocomposite as a highly active catalyst for oxidative dehydrogenation of ethane. J Catal. 2015; 329: 471–478.