Binuclear Transition Metal Ion Centers in Zeolites for Selective Oxidation of Methane

The recent development of the technology of shale gas production results in the dramatic increase of the interest on the utilization of natural gas. One of promising ways represents selective oxidation of methane to methanol and other valuable liquid products. Methane steam reforming with breaking the C-H bonds to syngas followed by the re-formation of C-H bonds into hydrocarbons is energetically and technically highly demanding and costly. Thus, a direct route of methane selective oxidation to methanol would be a dramatic step ahead in the methane processing. Nevertheless, this process requires development of highly effective catalysts. Iron containing zeolites represent important model catalysts for this reaction.

Interaction of Fe(II) containing zeolites with N₂O results in the formation of active oxygen form (so called α-oxygen), which can result in the formation of molecular products (N₂ and O₂) of N₂O decomposition process or can be used as oxidant in the selective oxidation of methane to methanol.^1,2 However, Fe(II) species active in the formation of α-oxygen in zeolite matrices are difficult to be prepared and they are unstable both under conditions of catalyst preparation and of N₂O decomposition / methane oxidation. Recently, it has been shown that the special arrangement of Fe(II) ions in zeolite matrix in the form of binuclear Fe(II) ion species results in significant facilitation both of N₂O decomposition and α-oxygen stabilization. This begs the question if also other transition metal cations embedded in the zeolite matrix in the form of binuclear M(II) structure are able to effectively decompose N₂O and form α-oxygen formation and subsequently oxidize methane to methanol.

This study aimed in targeted preparation of the binuclear Co(II), Ni(II), Mn(II) structures in zeolite matrix and testing their activity in N₂O decomposition, formation of active α-oxygen and selective oxidation of methane.

The catalysts with high loading of binuclear structures (Me = Mn, Co, and Ni) were prepared using ion exchanged method (Mn and Co) or impregnation (Ni). The presence of divalent cations (Mn, Co, and Ni) in cationic position of the zeolite matrix was confirmed by FTIR spectroscopy in the region of anti-symmetric T-O-T vibrations (1000-860 cm^-1) of the zeolite framework. FTIR results showed that in all prepared Me-zeolites, the majority of metal cations has to be present as binuclear M(II) cations due to the dominant presence of bare M(II) ions and number of possible sites for divalent M(II) ions. In-situ FTIR studies confirmed the interaction of M(II)-zeolites with N₂O at 250°C, reflected in the shift of the skeletal vibrations, which was assigned to atomic oxygen ([M(III)-O^-]²⁺ ) stabilized on metal cations (Co, Mn or Ni). The introduction of CH₄ at 250 °C to oxidized M-zeolites containing active oxygen species [M(III)-O^-]²⁺ resulted in the significant shift of the band of T-O-T vibrations reflecting presence of M(II) ions with adsorbed guest molecule. This result clearly confirms the complete reduction of the [M(III)-O^-]²⁺ species by CH₄. Moreover, the FTIR spectra of M-zeolites ofter oxidation and methane treatment showed new bands at 2920 and 2832 cm^-1, which are typical for vibrations of methoxyl group, and the bands at 2979 and 1371cm^-1, characteristic for methanol vibrations.

The obtained results clearly show that binuclear transition metal species (Mn, Ni and Co) stabilized in zeolite matrix can split N₂O. This interaction results in the stabilization of active oxygen on the metal cation and N₂ formation. This finding is in sound agreement with the results of ab initio calculations which suggest dramatically lower energy barrier of N₂O decomposition over binuclear M(II)structures. The active oxygen formed then easily oxidizes methane to methanol at 250 °C.

Refences:

¹ Snyder, B. E. R.; Vanelderen, P.; Bols, M. L.; Hallaert, S. D.; Böttger, L. H.; Ungur, L.; Pierloot, K.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I., Nature 2016, 536 (7616), 317-321,.

² Sklenak, S.; Andrikopoulos, P. C.; Boekfa, B.; Jansang, B.; Novakova, J.; Benco, L.; Bucko, T.; Hafner, J.; Dedecek, J.; Sobalik, Z., J. Catal. 2010, 272 (2), 262-274.