Elucidation of the Structure–Activity Relationship for Cu-Erionite in the Direct Conversion of Methane to Methanol: An Operando XAS Study

TL;DR

This study explores how the structure of Cu-erionite zeolites affects their ability to convert methane to methanol, revealing key factors that improve efficiency.

Contribution

The study identifies the critical role of the Cu/Al ratio in controlling copper reducibility and methanol yield in Cu-ERI zeolites.

Findings

The Cu/Al ratio strongly influences the reducibility of copper species, which determines methanol yield per copper atom.

The Si/Al ratio affects copper loading and maximum methanol yield but not the normalized yield.

Higher Al content increases total methanol yield by providing more copper exchange sites at an optimal Cu/Al ratio.

Abstract

The partial oxidation
of methane to methanol over copper-exchanged
zeolites offers a promising avenue for methane valorization. Numerous
zeolites have been demonstrated to be active for the selective oxidation
of methane, with the methanol yield varying significantly depending
on the zeolite framework, Si/Al ratio, and copper loading. Herein,
we present a comprehensive study of one of the most active Cu-erionite
(Cu-ERI) zeolites with different compositions for the stepwise conversion
of methane to methanol, aiming to elucidate the relationship between
the methanol yield and the nature of copper species in Cu-ERI zeolites.
Operando X-ray absorption spectroscopy (XAS), combined with Fourier-transform
infrared spectroscopy (FTIR), allows us to establish a correlation
that reveals the dependence of the methanol yield on the reduction
rate of copper species. Our findings demonstrate that the Cu/Al ratio
plays a crucial role in determining the reducibility of copper species
in Cu-ERI zeolites, which in turn governs methanol yield normalized
to the copper content. While the Si/Al ratio of the parent zeolite
determines the achievable copper loading and the maximal methanol
yield, it does not influence the normalized methanol yield. This work
suggests that controlling the Cu/Al ratio is essential for maximizing
copper efficiency and achieving selective methane partial oxidation.
At a fixed optimal Cu/Al ratio, increasing the Al content enhances
the total methanol yield by providing more copper exchange sites.
The structure–activity relationship of Cu-ERI zeolites in the
direct conversion of methane to methanol offers valuable insights
into the interplay between the zeolite host and copper species, highlighting
the importance of both Cu/Al and Si/Al ratios in designing selective,
high-performance materials for this challenging reaction.

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