Operando Magic Angle Spinning Solid-State NMR Spectroscopy of Methanol Photoreforming Over Titania-Coated Silica...

Solid-state nuclear magnetic resonance spectroscopy under Magic Angle Spinning (MAS) is one of the most powerful analytic techniques and in principle the method of choice to elucidate with molecular detail all components of complex solid or solid-liquid samples. MAS NMR under light irradiation is yet little developed, with technical solutions and first applications just emerging. We present the first operando observation of photoreforming of methanol to formaldehyde in a transparent rotor, irradiated at 365 nm by four LEDs, at spinning rates up to 11.5 kHz. The photon flux inside the rotor is quantified by an actinometric reaction. Efficient light penetration into the entire rotor volume is crucial. Therefore, we introduce silica monoliths with a continuous network of macropores, coated with TiO₂ (anatase) as supported heterogeneous catalyst. These monoliths provide a large pore volume to accommodate the liquid substrate and a high surface area. Centrally, the network of pores larger than the visible light enhances light penetration into the material by a factor of two compared to a powder. Silica monoliths may be easily decorated with various photocatalysts and thus provide a versatile platform for observing in real time photocatalytic reactions by solid-state NMR.