DRIFTS Studies on Zeotypes: Towards Fundamental Understanding of the Direct Conversion of Methane to Methanol

The direct conversion of methane to methanol, DCMM, represents the “holy grail” for catalysis of the last two decades ¹. The oxidation of CH₄ to value added products, e.g. CH₃OH, via non-syngas based techniques is desirable to obtain higher energy storage capacity and more sustainable fuels. The breakage of the strong C-H bond in the CH₄ molecule and the easy over-oxidation of CH₃OH represent the main thermodynamic challenges in the DCMM ². In nature, some enzymes, particularly the methane monooxygenases (MMO), are known for selectively convert methane to methanol, although with low conversion rates ³. Metals containing zeolites seem to show sites similar to the active sites of these enzymes and thus have been intensively investigated for the DCMM ⁴. The DCMM using zeolites as catalysts involves commonly four steps: addiction of metals to the catalyst, formation of the active site by oxidation at high temperature, reaction with methane at low temperature and extraction of methanol by solvents, e.g. water. It has been shown that most of the reaction products remain adsorbed on adsorption sites, making the extraction of methanol the limiting step ².

Zeolites are microporous aluminosilicates and their crystal lattice is based on the SiO₄^4- tetrahedron. The net negative charge due to substitution of Si⁴⁺ with Al³⁺ allows to host a number of elements in extra-framework positions. Nevertheless, Al-free zeotypes can be synthesized with elements other than Al in framework positions ⁵. In 2012, Hammond et al. obtained remarkable CH₄ conversion and selectivity towards CH₃OH using a mixture of Al-free zeotypes as catalysts and liquid H₂O₂ as oxidant. In particular, the catalysts consisted of Fe and Cu containing silicalite, the Al-free parent of the commercial ZSM-5 zeolite with MFI type of framework structure. Fe and Cu were added prior to and post crystallization, respectively, resulting in framework and extra-framework metal species ⁶. The lower acidity of the Fe-silicalite zeotype compared to Fe-exchanged ZSM-5 zeolite is believed to facilitate CH₃OH desorption due to the higher hydrophobicity of the Al-free zeotype.

This study aims to give a fundamental understanding of metal containing zeotypes with MFI type of framework structure suitable for the direct conversion of methane to methanol. The zeotypes are hydrothermally synthesized and characterized with diffraction, physisorption and spectroscopy techniques. Furthermore, the tendency of CH₃OH to desorb from the internal surfaces of the catalyst is detected by temperature programmed desorption of methanol, CH₃OH TPD, and the evolution of surface species is followed using DRIFTS. In a standard synthesis procedure, the silica precursor is mixed with the structure directing agent, SDA, and the metal precursor in aqueous medium. The so obtained gel is crystallized in autoclave and the products are washed, filtered and dried. The SDA is then removed by calcination and the H⁺-form of the zeotype is obtained by ammonium exchange and further calcination. The MFI type of framework structure is confirmed by PXRD and more insights about the incorporation of metals in the framework are given by N₂ physisorption and IR spectroscopy.

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