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Fig. 131: a) In-situ observed and calculated PDF and their difference for the best fits obtained for the two coppers in the 8-membered channel. b) Refined distance between Cu-Cu pair and their average value weighted on the inverse fit residual. c) XANES spectra of the hydrated, activated and reacted form. d) Fourier Transform EXAFS and (e) the corresponding K-space. Black: experimental data; red: fitting.

The direct conversion of methane to methanol is a promising way to utilise methane from dispersed and remote sources. At petroleum extraction sites, where methane is often co-extracted, it is often not economical to transport the co-extracted methane, so it is flared (i.e., combusted to CO2 and emitted into the atmosphere) instead of utilised. Methanol is a high-value and transportable liquid fuel, and if methane could be easily and directly converted to methanol onsite, significant waste of methane could be avoided. One promising route to convert methane to methanol directly is through a stepwise procedure using copper-exchanged zeolites [1], but the methanol productivity today remains below industrially feasible levels [2]. Therefore, a better understanding of the optimal local environment for highly selective copper active sites in zeolites is needed to make more productive materials.

With over 200 different zeolite structures available, some provide better local geometric environments than others to stabilise the active sites and intermediate methoxy species, and ultimately prevent over-oxidation to CO2. Specifically, zeolite omega is highly selective for the conversion of methane to methanol [3] under a stepwise process that includes activation, reaction and extraction

steps. In order to elucidate the active site, a sample of copper-exchanged zeolite omega was prepared after activation in oxygen and then after reaction with methane. By employing anomalous X-ray powder diffraction (AXPD) below, at, and above the copper K-edge, the active copper and the associated oxygen or carbon species could be distinguished and located in the zeolite structures. Specifically, after activation, two distinct copper sites were observed in the 8-membered channels of zeolite omega, and each had an associated oxygen (Figure 130). With a similar analysis after reaction, a methoxy intermediate species was observed near one of these copper species, thus revealing the exact location of the methoxy intermediate species for methanol formation and the location of the active sites.

In-situ paired distribution function (PDF) and ex-situ extended X-ray absorption fine-structure spectroscopy (EXAFS) were then employed to further examine the Cu-Cu structure (Figure 131). High-energy X-ray diffraction measurements for PDF analysis were carried out at beamline ID15A. Both PDF and EXAFS confirm that these two coppers exist simultaneously in the same channel, and the active site is the paired copper monomeric species. The table in Figure 131 shows the distance