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Developing synthesis-structure- activity relationships for Cu- mordenite systems
The synthesis of materials such as copper zeolites for methane-to-methanol oxidation is influenced to a large degree by the nature of its constituents. X-ray absorption spectroscopy revealed that different Al sources influence the location of Al incorporation in the zeolite system, resulting in a Cu-zeolite system of varying propensity towards the selective oxidation of methane to methanol.
To mitigate the release of large amounts of methane and other small alkanes, the challenge of activating the strong C-H bond of these molecules needs to be addressed. While facilitated on a large scale in steam reforming facilities, Cu-zeolites show promise to provide a pathway towards more decentralised point-source mitigation strategies. Using inexpensive molecular oxygen to form active sites, Cu-zeolites can readily activate the C-H bond. Hereby, the methane molecule forms a methoxy intermediate and is subsequently extracted in the presence of water, yielding methanol. Among the various Cu-zeolites studied, Cu-mordenite exhibits the highest methanol productivity to date and various research has investigated the properties of this specific zeolite.
This work reports the successful use of different Al sources, namely aluminum sulfate and sodium aluminate, to crystallise a material that is in both cases mordenite zeolites MOR-Al2(SO4)3 and MOR-NaAlO2 respectively but that show distinct differences in the
crystal morphology as well as the acid site distribution. Several techniques were used to investigate the Brønsted acidity of the material. Propylamine temperature- programmed desorption showcased that MOR-Al2(SO4)3 has more acid sites compared to MOR-NaAlO2, however, when using a smaller probe molecule such as NH3, the same number of acid sites is obtained. Additional techniques such as titration of acid sites with CO followed by infrared spectroscopy, as well as using deuterated acetonitrile in combination with solid-state 1H NMR present a similar trend (Figure 134).
These results suggest differences in the location of acid sites, with MOR-NaAlO2 containing more acid sites in constrained spaces, such as the 8-ring side pocket, less accessible to larger molecules (e.g. propylamine). It was found that this Al distribution is influenced by the pH of the gel with a more alkaline gel (MOR-NaAlO2) crystallising faster and likely incorporating Al in different locations compared to the slightly less alkaline gel of MOR-Al2(SO4)3. Subsequently, Cu-mordenite samples were prepared based on these two archetype materials and it was found that the performance in the methane- to-methanol reaction varies amongst them; the MOR-Al2(SO4)3 giving a higher methanol productivity compared to the MOR-NaAlO2 sample. This, for the first time, showed that for the same material, a different propensity could be imparted in activating the C-H bond of methane.
Previous work focused on studying the active Cu species in mordenite using synchrotron radiation at the SNBL beamline, BM31 . Analysing the X-ray absorption near-edge structure (XANES) spectra of the Cu k-edge, it
Fig. 134: Acid site distributions for MOR zeolites discussed in this study as assessed with several different techniques. Colour-coding is used to illustrate the respective techniques. The solid bars correspond to acid sites in the 12-ring (AI) with those in 8-rings making up the difference (patterned bars).