Tuned acidity in zeotypes: A descriptor to unravel the direct conversion of methane to methanol

Sammanfattning: The catalyst's acidity is crucial in countless chemical reactions, and thus to tune this parameter means to take the lead on the desired catalytic reaction. Therefore, it is not surprising that zeolites have been used since decades as catalysts for their outstanding properties of solid acids. Alongside, zeotypes are zeolite-like materials whose chemical composition is altered to obtain ad hoc acidity, and are therefore of outmost interest for many catalytic reactions. Here, the chosen reaction affected by the zeotype acidity is the direct conversion of methane to methanol. Indeed, a lower acidity of the zeotype promotes the extraction of methanol, whose precursors tend to remain adsorbed on the acidic sites of the zeotype. In this study, Al, B, Fe, Ga, and Ti are incorporated in the MFI framework of silicalite zeotypes. The zeotype crystallites are imaged with scanning and transmission electron microscopy, and the MFI framework structure is characterized with X-ray diffraction, nitrogen sorption and Raman spectroscopy. The process from the as synthesized samples to the corresponding materials in the acid form is examined with in situ infrared spectroscopy, with and without ammonia and nitric oxide as probe molecules. Furthermore, the following series of increasing acidity is observed by means of infrared spectroscopy: 0 = pure silicalite = Ti-silicalite < B-silicalite < Fe-silicalite < Ga-silicalite < Al-silicalite. The influence of the zeotype acidity during methane exposure and temperature programmed desorption of methanol has been investigated in situ with infrared spectroscopy. The results show that the presence of iron promotes methane activation and that methanol is more strongly bound to the zeotype in the presence of stronger acid sites. Because methane activation and methanol extraction are two of the key steps in the direct conversion of methane to methanol, our results indicate that Al-free zeotypes with tuned acidity pinpoint important catalyst design parameters needed for this reaction.