Methane to Methanol Conversion over Cu-Zeolites

Methane to methanol conversion over Cu-zeolites

A process allowing energy-effective methane to methanol (MTM) conversion would represent a major breakthrough for the chemical industry. Cu-exchanged zeolites have been shown to possess active Cu sites able to cleave the C−H bond of methane at temperatures ≤ 200 °C, enabling the stoichiometric transformation of methane into methanol [1]. The conversion is performed through a stepwise process, involving high-temperature activation in O₂ to generate the active sites, methane loading at 200 °C, and steam-assisted methanol extraction.

We have combined laboratory performance testing with in situ/operando Cu K-edge XAS to establish structure-activity relationships for the MTM conversion over Cu-CHA [2] and Cu-MOR [3] zeolites. By operando XAS, we tracked the oxidation state and average coordination of Cu ions during each step of the process and explored the impact of different pretreatments and compositional characteristics by in situ XAS.

High-temperature reaction with O₂ is evidenced as a key requirement to form the active Cu(II) sites, which then undergo reversible redox chemistry during the CH₄-loading and CH₃OH extraction steps. For Cu-CHA, we identified a positive linear correlation between the methanol productivity and the composition-dependent self-reducibility under high-temperature treatment in He, allowing us to rationalize the impact of the sample composition on the productivity for the MTM conversion. The most recent research efforts on Cu-MOR have highlighted the crucial role of operating at consistent conditions for both spectroscopy and performance testing. Having fulfilled this requirement, aided by high-energy resolution XANES and multivariate analysis [4], we unambiguously assessed the nuclearity of the active Cu site in the investigated series of Cu-MOR materials. These studies highlights the potential of the combination between XAS and testing at consistent conditions and paves the way for rationalized material synthesis to improve the reaction performance.

[1] E. Borfecchia et al., Chem. Soc. Rev. (2018) in press, doi: 10.1039/c8cs00373d.
[2] D. K. Pappas et al., J. Am. Chem. Soc., 139 (2017) 14961.
[3] D. K. Pappas et al., J. Am. Chem. Soc., under review.
[4] A. Martini et al., Chem. Sci. 8 (2017) 6836.