Operando Magic Angle Spinning Solid-State NMR Spectroscopy of Methanol Photoreforming over Titania-coated Silica Monoliths

RESEARCH ARTICLE 1 Operando Magic Angle Spinning Solid-State NMR Spectroscopy of Methanol Photoreforming over Titania-coated Silica Monoliths Fei Ding,[a] Karl König,[a,b] Johannes Böhmer,[c] Florian M. Wisser,[a] Erdmann Spiecker,[c] Benjamin Apeleo Zubiri,[c] Dorothea Wisser*[a] [a] Mr. F. Ding, Mr. K. König, Dr. F. M. Wisser, Dr. D. Wisser Erlangen Center for Interface Research and Catalysis (ECRC) Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Egerlandstrasse 3, 91058 Erlangen, Germany E-mail: [email protected] [b] Mr. K. König Faculty for Chemistry and Pharmacy Ludwig-Maximilians-Universität München Würmtalstr. 201, 81375 München, Germany [c] Mr. J. Böhmer, Prof. Dr. E. Spiecker, Dr. B. Apeleo Zubiri Institute of Microand Nanostructure Research (IMN) & Center for Nanoanalysis and Electron Microscopy (CENEM) Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Cauerstrasse 3, 91058 Erlangen, Germany Supporting information for this article is given via a link at the end of the document. Abstract: Solid-state nuclear magnetic resonance spectroscopy under Magic Angle Spinning 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. Magic Angle Spinning 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. Introduction Solid-state NMR under magic angle spinning (MAS) is one of the most versatile and powerful techniques for investigating crystalline and amorphous materials, their surface and bulk structures, reaction mechanisms and side reactions as well as binding sites and adsorption or binding of reactive species under ambient conditions. In complex mixtures, both solid and liquid species can be detected, and discerned by adapted pulse sequences. While in situ and operando NMR spectroscopic techniques under MAS have been developed for classic heterogeneous (catalytic) reactions,[1] and yet remain challenging, observation of ongoing light-irradiated processes inside a spinning NMR rotor is yet scarce. The lag in this field of research is linked to the technical challenge of achieving light irradiation of the sample container in MAS NMR spectroscopy, the so-called MAS rotor. These hollow cylinders, filled with a (usually powdered) solid sample, spin in a contact-free manner around their own axis in a gas flow, with spinning rates of up to > 100 kHz, depending on the rotor size. MAS rotors are usually made from a non-transparent zirconia ceramic, limiting light penetration. RESEARCH ARTICLE 2 In the past, attempts to realize light irradiation MAS NMR spectroscopy have included sealed 5 mm glass tubes,[2– 4] irradiation via with a quartz window at the bottom of a 7 mm MAS rotor,[5] or via transparent rotor caps.[6] MAS rates were limited in these cases to 2-4 kHz. Recently, the group of Sakellariou has shown light irradiation via a glass rod inserted along the axis of a 4 mm zirconia rotor, and could thus observe UV-light induced photopolymerization in 1H and 13C MAS NMR spectra.[7] The advent of UV-Vis transparent rotors opens up new avenues for more efficient light-irradiation into a spinning sample. Using transparent sapphire rotors, photoexcitation and acquisition of spectra under DNP conditions on proteins samples were achieved by combining both microwave irradiation and a light guide.[8,9] Just recently, the group of Emsley obtained a photochemically induced Dynamic Nuclear Polarization (photo-CIDNP) 1H signal enhancement in a frozen solution.[10] Van der Wel and co-workers irradiated sapphire rotors to observe in real time an azobenzene-based photo-switch undergoing cis-trans transition in a light-transparent gel as well as a photopolymerization reaction.[11] This emerging technology of light-irradiation MAS NMR paves the way for multiple applications including photo-switches, photo-driven reactions, photodegradation or heterogeneous photocatalysis, all of which may be observed in real time and with the molecular level information provided by solid-state NMR. Yet, the above-mentioned applications have so far been limited to optically transparent materials including frozen glasses[8–10], liquid monomers,[7] or hydrogels[11], all of which easily enable light penetration. Photocatalytic reactions are an important pathway to shift chemical industry away from energy-intensive, fossildriven processes. To the best of our knowledge, the only observations of heterogeneous photocatalytic studies over solid materials in situ under MAS were made by the group of Raftery, who loaded solid catalysts in 5 mm NMR glass tubes, which were spun at rather low MAS rates of up to 3 kHz. Irradiation was ensured by an optical fiber in the probe, shining through a gap in the radio frequency coil. Oxidation of 13C-labelled ethanol to acetate and CO2 over TiO2-based, shaped or powdered photocatalysts[2–4] and photocatalytic oxidation of chlorinated compounds over zeolite-coated nanorods and TiO2 were observed.[12,13] In heterogeneous photocatalytic assays, a powdered catalyst is usually added to a solution, yielding a slurry. In an MAS rotor, the powdered photocatalyst needs to be wetted with the reaction mixture and densely packed, as a high amount of liquid phase would impede stable rotation. Light penetration into a packed powder bed is however often restricted to several µm: In case of TiO2 particles (dp ~ 30 nm) the light penetration was reported to vary between 1 and 30 µm depending on the density of the bed.[14,15] A large proportion of the sample volume in a 3.2 mm outer diameter (2.2 mm inner diameter) rotor might thus be not irradiated, leading to a loss in photocatalytic activity. In contrast, macroporous catalysts, either in the form of monolithic, honeycomb-type supports or aerogels have been suggested to increase light penetration into the bed in gas phase catalysis. For classical honeycomb structures (channel diameter > 1 mm) coated with a thin TiO2 layer, light penetrates un-shadowed by the monolith walls over a length of at least 2.3 cm.[15] Likewise, for smaller channel/pore diameters in the 50-1000 nm range, mean free paths of up to several micrometers have been reported for various materials including TiO2.[16,17] The optical porosity (i.e., the porosity experienced by photons) of ~3 mm thick Al2O3 discs with macropores in the range of 1 – 5 µm was determined to be about 44% of the porosity obtained by classical mercury intrusion porosimetry (MIP).[18] Niederberger and co-workers demonstrated the advantage of large mesoand macropores (dp >>30 nm) in H2 photo-production from methanol in continuous gas-phase reaction over TiO2-aerogels.[19] The presence of those large pores was crucial for achieving a high activity. After crushing the aerogel, the activity dropped to ~50% of the initial activity. All these works indicate a superior efficiency of light penetration deep into a solid material by macropores larger than the excitation wavelength, as compared to a packed powder. In this paper, we demonstrate efficient light irradiation of a 3.2 mm sapphire rotor via four LEDs at 365 nm, under Magic Angle Spinning up to 11.5 kHz, allowing for solid-state NMR operando observation of photoreforming of 13Clabelled methanol to formaldehyde, with hydrogen as a side product. Following the above-mentioned reasoning, we introduce here a photocatalytic material with a particularly well-adapted topology: a macroporous monolith consisting of amorphous silica, prepared by spinodal decomposition, and coated with titania by Atomic Layer Deposition in solution.[20,21] The titania exhibits mainly anatase structure after calcination. We show that the light penetration by the macroporous network is indeed effective and increases the conversion of methanol per gram of catalyst by a factor of two compared to the same material, ground to a fine powder. These silica monoliths constitute a versatile platform that can be decorated with various photocatalysts for efficient operando photocatalysis followed in real time by MAS NMR spectroscopy. RESEARCH ARTICLE 3 Results and Discussion TiO2@SiO2 monoliths: Synthesis and Structural Characterization To allow for an optimized light penetration into the solid catalyst bed (see below), we used macroporous silica monoliths as catalyst supports (Figure 1a). The silica monoliths were shaped to fit perfectly into transparent 3.2 mm outer diameter sapphire MAS rotors (Figure 1b-d). They were prepared by spinodal phase separation between a growing silica phase and an aqueous polymer solution, here polyethylene glycol. After washing and calcination, free standing monoliths with a connected macropore network were obtained. We noticed that the volume of the monoliths shrinks by 16 % during this procedure (Figure S2). The known degree of shrinkage was used to design suitable molds for rotor-sized monoliths. As a consequence of silica precursor enrichment at the phase boundary during spinodal decomposition, the outer surface of the as-obtained monolith exhibits an almost dense silica layer. This dense layer might hinder efficient light penetration into the monolith as well as wetting with a solution during solution-based Atomic Layer Deposition for TiO2 grafting. The pore structure was therefore opened by an alkaline surface treatment using a paste consisting of NaOH as etching medium, P123 as viscosity regulator, CaCO3 and water for 1 h at room temperature. After washing with water and calcination, completely open monolithic structures were obtained. SEM images (Figure 2a and b) as well as atomic fractions determined by Scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy (EDXS, Figure 2d and e) reveal that also the inner surface of the monolithic macropore network exhibits a thin, dense surface silica layer. Below this layer, the material is highly porous. Ar physisorption isotherms recorded at 87 K show the presence of mesopores of about 20 nm (Figure S3a). Mercury intrusion shows the presence of large macropores with a total pore volume of up to 3.6 cm3/g (Figure S3c). Overall, the monoliths exhibit a high apparent surface area of 400 m2/g. Figure 1 a) silica monoliths, b, c) silica monoliths in sapphire rotors, d) dry silica monolith (right) and silica monolith wetted with methanol (left) on an illuminated screen. The monolith impregnated with methanol shows higher optical transparency. e, f) Scanning Electron Microscope (SEM) images of pristine silica monoliths. Next, a thin layer of TiO2 particles was deposited on the surface of the silica monoliths by Atomic Layer Deposition in solution (sALD). We have recently reported the deposition of TiO2 by sALD using alternating immersion of silica particles in a solution of titanium(IV) isopropoxide and a solution of water in diethyl ether. These two steps make up one “sALD cycle”.[21] A modified procedure was applied here to deposit TiO2 in 5 cycles on the silica monoliths, followed by calcination of the material. The final TiO2@SiO2 monolith contains 7.8 wt-% of Ti, as determined from ICP-OES (Table S1). Electron microscopy measurements were conducted to validate the structure of the TiO2@SiO2 monoliths. Scanning electron microscopy (SEM) images show the homogeneously interconnected and arbitrarily percolated mesoporous structure of the monoliths with some larger spherical macropores in this particular TiO2@SiO2 monolith RESEARCH ARTICLE 4 (Figure 2a and Figure S4). The surface of the monolith is visibly smoother than the mesoporous fracture surface, which may result from the denser silica layer and, additionally, from a layer of TiO2 on the monolith (Figure 2b). Scanning transmission electron microscopy (STEM) images (Figure 2c) and energy dispersive X-ray spectroscopy (EDXS, Figure 2d) performed on a thin focused ion beam (FIB) cross-section lamella (Figure S5) confirm the presence of TiO2 on the surface. Importantly, TiO2 particles are also present deeply inside the mesoporous network of the monolith, with a constant concentration persisting down to micrometer depth (Figure S6). The individual half cycles of TiO2 deposition by sALD are fast reactions, therefore, the nascent layer is most likely amorphous. After calcination of the TiO2@SiO2 monoliths, the phase composition was determined by powder XRay diffraction (pXRD) analysis (Figure S7). After 5 sALD cycles, very weak peaks characteristic of anatase are just visible in the diffractogram. For comparison, a monolith sample was grafted with TiO2 in 20 sALD cycles and calcined. This sample clearly exhibits the characteristic anatase peaks. High-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) pattern clearly show the presence of anatase nanoparticles on the surface and deep inside the SiO2 monolith already after 5 sALD cycles (Figure S8). Scherrer analysis of the (101) and (004) diffraction rings (Figure S9) leads to a mean particle size of 5.95 nm and 5.83 nm, in agreement with the particle size observed in the HRTEM images (Figure S8). To further identify the TiO2 modification the calcined TiO2@SiO2 monoliths, total scattering experiments were performed. The pair distribution functions (PDFs), G(r) provide local structural information, such as inter-atomic distances between atom pairs, even in X-ray amorphous materials (Figure S10). The PDFs of both materials reveal interatomic distances up to approx. 5 Å, indicating short range order. The dominant peaks in both G(r) plots are assigned to the relevant inter-atomic distances of silica.[22,23] To clearly identify the contribution of titania-based pairs of atoms, the ΔG(r) plot is extracted. In the ΔG(r) plot, we attributed interatomic distances at approx. 1.9 Å to Ti-O bonds, at 2.0 Å to Ti···Ti and from 3.6 to 3.9 Å to edge and corner sharing Ti···Ti as well as Ti···O interatomic distances.[24–26] Broad and weak features around 4.4 – 4.8 and 6.2 Å are attributed to multiple contributions of Ti···O/Ti. Those structural features describe the TiO2 particles as a nascent anatase phase.[27,28] As a consequence of TiO2 deposition, both the mesopore and the macropore diameter are shifted to smaller values (Figure S3b, d and SI for further discussion), and the total pore volume decreases from 3.6 to 2.9 cm3/g. The apparent surface area is reduced slightly from 400 to 350 m2/g (Table S1). In summary, the scaffold exhibits a truly hierarchical architecture of connected macroand mesopores, with mesopores being perfectly accessible for TiO2 deposition by sALD from organic precursors. Figure 2 a) Low-magnification scanning electron microscopy (SEM) image of the TiO2@SiO2 monolith after 5 sALD cycles of TiO2 deposition (cf. Figure S4). (b) Higher-magnification image of a fracture surface (bottom-left region) of the same monolith. (c) Scanning transmission electron microscopy image (STEM) of a thin cross-section lamella prepared from the same monolith with (d) energy dispersive X-ray spectroscopy (EDXS) map of the region marked by the yellow box in (c) (cf. area 1 in Figure S5a). Red color indicates silicon from 0 to 30 atomic percent and green titanium from 0 to 10 atomic percent. (e) Line plot of the oxygen, silicon and titanium atomic fractions and the high-angle annular darkfield (HAADF) detector intensity of the white box marked in (d). RESEARCH ARTICLE 5 Quantification of photon flux in MAS NMR rotor Scheme 1. Reaction scheme for the photoisomerization of ortho-nitrobenzaldehyde (o-NBA) into ortho-nitroso benzoic acid (o-NBAH). To quantify the photon flux reaching the interior of the rotor inside the photoprobe, we used a well-established actinometric method, i.e. the photoisomerization of ortho-nitrobenzaldehyde (o-NBA) into ortho-nitroso benzoic acid (o-NBAH, Scheme 1). This reaction is characterized by a quantum yield of 𝛷𝛷 = 0.5 in the 300 – 400 nm region.[29] The conversion of o-NBA into o-NBAH was followed in situ by the disappearance of the aldehyde peak at ~10.5 ppm in the 1H NMR spectrum (Figure S11). From the rate of o-NBA conversion, the number of photons reaching the rotor were calculated according to 𝐼𝐼0=−𝑑𝑑𝑑𝑑(𝑜𝑜-NBA) 𝑑𝑑𝑑𝑑 ·1 Φ·1 1−10𝜀𝜀·𝑐𝑐·𝑙𝑙 (1) with 𝑑𝑑𝑑𝑑(𝑜𝑜-NBA) 𝑑𝑑𝑑𝑑 the change in number of moles of o-NBA over time, 𝐼𝐼0 the photon flux reaching the bottom of the probe, Φ the quantum yield, 𝜀𝜀 the molar absorbance of o-NBA (260 mol-1·cm-1),[30] 𝑐𝑐 the initial concentration of oNBA and 𝑙𝑙 the path length (here the inner rotor radius, 1.1 mm). A rotor filled with either a solution or with a wetted silica monolith was inserted into a prototype HX 3.2 mm MAS NMR probe by NMR Service Erfurt and irradiated continuously at 365 nm by four LEDs, connected to the stator via optic fibers. For a rotor with a 15 mmol·L-1solution of o-NBA, a photon flux of 1.45·10-8 mol·s-1, equal to a light intensity of 483 µmol·L-1·s-1 was obtained when irradiating the rotor with all four LEDs (Table S2). This photon flux is one order of magnitude higher than previously reported values of 23 µmol·L-1·s-1 for a solution NMR system,[30] and 55 µmol·L-1·s-1for a MAS probe with sapphire rotor irradiated by one LED.[11] At a higher o-NBA concentration of 1 mol·L-1, this value decreased slightly to 436 µmol·L-1·s-1. When performing the same experiment in the presence of a silica monolith (without TiO2), in which the pore volume was filled with 1 mol·L-1 o-NBA in DCM, the photon flux was reduced to 6.91·10-9 mol·s-1, approximately half the value of the pure solution. Importantly, after grinding the monolith into a fine powder, leading to a destruction of the macropore network, the photon flux was further reduced by a factor of two to 3.54·10-9 µmol·L-1·s-1 (see Table S2). This observation corroborates literature reports, that by destroying the large macropores of an aerogel, catalytic activity in continuous gas-phase MeOH photoreforming over TiO2-aerogels dropped to ~50% of the initial activity.[19] It shows that indeed the macroporous network strongly improves light penetration throughout the liquid contained in the monolith pore volume. Note that determining the photon flux in a TiO2@SiO2 monolith or with powdered anatase used as a reference material (see below) would not be conclusive, since titania may catalyze the photodegradation of the o-NBA.[31] Operando MAS NMR observation of photoreforming of 13C-methanol For operando MAS NMR observation of photoreforming of methanol, a TiO2@SiO2 monolith (5 cycles of TiO2 deposition, ~8 wt-% Ti) was impregnated with 5 mL 13C-labelled methanol, and inserted into a 3.2 mm sapphire rotor under argon. This volume fills about 25% of the total pore volume. The rotor was spun at 11.5 kHz MAS rate and irradiated continuously at 365 nm by four LEDs. 1H, 13C direct excitation (DE) (Figure 3b and d) and 13C cross polarization (CP) MAS NMR spectra (Figure S12) were recorded alternatingly during 88 h of photocatalysis. As a blank experiment, a TiO2@SiO2 monolith impregnated with 5 mL 13C-labelled methanol was stored for 80 h in the dark, before recording a 13C DE MAS spectrum. No compounds other than methanol were detected (Figure S13), confirming the photocatalytic nature of the experiment. In 13C DE MAS NMR experiments, all carbon atoms in the rotor are detected. The 13C resonances here stem mostly from the liquid methanol phase, as evidenced by their narrow line width. The main product in this photoreforming reaction is formaldehyde, which immediately reacts with methanol to form methoxymethanol (CH3-O-CH2-OH). The latter gives rise to 13C resonances at 54 and 90 ppm (Figure 3b, after 1 h). Thus, for one molecule of RESEARCH ARTICLE 6 methoxymethanol, two molecules of methanol are consumed. After approx. 5 h of continuous photocatalysis, first traces of methylformate can be detected by their characteristic signals at 163 and 50.5 ppm. Methylformate is the ester formed by formic acid, an overoxidation product, and methanol. After ~ 1d of irradiation, longer hemiformals, e.g. (methoxymethoxy)methanol (H3CO-(CH2O)2-H) and ((methoxymethoxy)methoxy)methanol (H3CO-(CH2O)3-H) can be identified by their characteristic signals at 93 and 86 ppm as well as 93, 86 and 83 ppm,[32] respectively (Figure 3b). Likewise, minor amounts of adsorbed formate species on the surfaces are observable with chemical shifts around 170-175 ppm (Figure 3b). However, their contribution remains weak (~1% after 4 d). The signals at 171 ppm can be discerned as formate bound to one Ti atom in a bidentate mode (Ti···η2OOCH),[33] and at 174 ppm to formate binding to two Ti atoms in a bridging mode (Ti···µ2OCHO···Ti) (Figure 3b and d).[34] The occurrence of either methylformate or adsorbed formate does not reduce significantly the catalytic activity. The conversion of methanol and the production yield are nearly linear over the whole reaction time. The slight deviation from linearity, visible mainly above 35 % conversion of methanol (> 60 h of continuous photocatalysis, Figure 3c) may be explained by competitive adsorption between adsorbed formate and adsorbed methanol/methoxy species, changes in the viscosity and/or diffusion limitation. In contrast to 13C DE MAS NMR spectra which show mainly signals of the dissolved species, the 13C CP MAS NMR spectra show with increased sensitivity immobile, surface-bound species (Figure S12). A significant amount of chemically bonded methoxy species (Ti-OMe) at approx. 64 ppm is observed.[35] 13C CP MAS NMR spectra also reveal the presence of physically bonded methanol at 51 ppm, a signal which is hidden by the broad and intense liquid methanol signal at 49 ppm in the 13C DE MAS NMR spectra (Figure 3b). Physically bonded MeOH species are completely consumed after 4 days of photocatalysis, while some chemically bonded methoxy species remain. The evolution of the main products can also be easily followed also by operando 1H MAS NMR spectra (Figure 3d). The doublet (1JC-H =142 Hz) at 3.30 ppm is attributed to the methyl protons,[36] the broad signal at 4.88 ppm to the OH group of 13C-labelled MeOH. The formation of methoxymethanol can be followed by the signal at 4.50 ppm (part of the doublet at 4.67 ppm, 1JC-H =165 Hz) belonging to its methylidene (CH2) protons after about 4 h, while first traces of methylformate can be detected after ~ 24 h by the doublet at 8.05 ppm (1JC-H =225 Hz). All trace products detected by 13C NMR are hidden under those strong signals. The hydrogen formed during the reaction is likely to diffuse through the rotor cap and is not detectable in the 1H MAS NMR spectra. RESEARCH ARTICLE 7 Figure 3: a) Reaction scheme for photoreforming of methanol to methoxymethanol over TiO2.[37] Bold: species detected by operando 13C DE MAS NMR (see b) black by operando 13C CP MAS NMR (Figure S8) and species in gray are not detected. Further condensation products of methoxymethanol with either formaldehyde or methanol are omitted for clarity. b) Operando 13C MAS NMR spectra of photocatalytic methanol over TiO2@SiO2 monolith at 11.5 kHz MAS. The negative peak at 94 ppm is a measurement artifact (“center spike”). 20 Hz line broadening were applied to all spectra. c) Corresponding yield of main products and conversion of methanol (red), trace products with yields < 1% are shown in Figure S15. d) Corresponding operando 1H MAS NMR spectra. To evaluate the effect of the macropore network on the photocatalysis efficiency, a TiO2@SiO2 monolith was then ground to a fine powder and filled in the rotor, and wetted with 13C-methanol. 13C DE MAS NMR spectra were recorded during 95 h (Figure S14). A methanol conversion of 36.5% after 82 h was observed. However, note that to completely fill the rotor with a wetted powder, the ratio between 13C-methanol and TiO2 differed from that in the monolith. Therefore, the conversion of methanol per g of Ti between the intact and the ground TiO2@SiO2 monolith are compared (Table S3 and Figure S16). The conversion of methanol is by a factor of 2 higher for the intact monolith than for the powdered monolith, which constitutes a significant increase in photocatalytic efficiency and RESEARCH ARTICLE 8 shows again that the macropore network of the intact monolith improves light penetration throughout the rotor volume. For comparison, we also recorded the same reaction over a packed crystalline anatase powder, impregnated with 5 µL 13C-labelled methanol (Figure 4a). Interestingly, we observe a splitting of the CH2-resonance into two signals corresponding to methoxymethanol at ~ 90 ppm, and to dimethoxymethane (CH3-O-CH2-O-CH3) at ~98 ppm, product of a second condensation reaction after 5 h, with almost constant contribution. The production rates of both molecules are relatively high in the first 5 h of catalysis, but then decrease strongly, and thus the overall yield remains low with ~2.5% after 2 days (Figure 4b). Note that it is ambiguous to compare absolute yields between TiO2@SiO2 monoliths and anatase powder, since the specific surface areas are different (Table S1 and Figure S17), and the number and density of active Ti sites may intrinsically differ in the two materials, stemming from very different preparation procedures. However, the fact that the conversion of methanol quickly levels off in the packed anatase powder may be an indication for significant light scattering and low light penetration into the slurry. Thus, only methanol in the outer layer of the packed bed close to the rotor wall can undergo photoreforming.[38] This outer layer is assumed to be in the order of a few tens of particle layers.[39] Once methanol has reacted here (t < 5 h), the reaction becomes limited by diffusion of methanol from the bulk inner part of the slurry into the outer, lightirradiated layer. Figure 4. a) Operando 13C DE MAS NMR spectra at 8 kHz MAS rate of methanol photoreforming into methoxymethanol and dimethoxymethane over anatase powder. To all spectra, 20 Hz line broadening were applied. b) Corresponding yield of different products and conversion of methanol (red). Conclusion We have shown here for the first time efficient photoreforming of methanol to formaldehyde inside an MAS NMR rotor – catalyzed by light irradiation in the transparent sapphire rotor and followed in real time, operando, by 1H and 13C DE and CP MAS NMR spectroscopy. This was made possible using a prototype MAS probe irradiating 3.2 mm outer diameter sapphire rotors by four LEDs at 365 nm. By combination of quantitative 13C DE and qualitative 13CP MAS NMR spectroscopy, not only the evolution of the main product can be followed, but complete picture of the time resolved evolution of surface bound species, reaction intermediates and (side) products in this model reaction is obtained. As photocatalyst support, we prepared silica monoliths, shaped to fit perfectly into a MAS NMR rotor. These monoliths exhibit a continuous network of macropores, and inside the macroporous silica scaffold, a network of mesopores, leading overall to a high surface area of ca 400 m2/g. TiO2 anatase particles were deposited in both macroand mesopores by Atomic Layer Deposition in solution. With this catalyst, a nearly constant conversion rate of methanol was observed over four days, with almost 50% conversion after 84 h. The macropores network with pores larger than the wavelength of visible light enhance the penetration of light throughout the rotor volume (inner diameter of 2.2 mm), and improves methanol conversion per g of titanium by a factor of 2 as high as compared to the same material, ground to a powder. Overall, the silica monoliths presented here combine the advantages of high surface area, large a pore volume and optimized light incoupling. They thus constitute an excellent support for grafting photocatalytic complexes or deposition of other photocatalytically active inorganic materials. They can RESEARCH ARTICLE 9 thus serve in future as a versatile platform for an emerging line of research, operando photocatalysis, investigated in real time by light-irradiated MAS NMR spectroscopy. Supporting Information The authors have cited additional references within the Supporting Information.[21–23,40–50] Acknowledgements The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation): Project-ID 503810318 – WI 4934/3-1, project-ID 416229255 – SFB 1411“Design of Particulate Products“, project-ID 538767711 – SFB 1719 „ChemPrint“ and project-ID 539724755 – FPS Core Facility. The authors are grateful to Dr. Thomas Hartmann and Dr. Laura C. Volkers-Karlsson (STOE & Cie GmbH) for providing access to PDF instrumentation, to Ferdinand Günther and Tadahiro Yokosawa for TEM measurements, to Frederic Dietrich for additional SEM measurements and to Susanne Pachaly for ICP OES analysis. Keywords: Operando Magic Angle Spinning NMR • Light-irradiated NMR • Photocatalysis • Methanol Photoreforming • Atomic Layer Deposition in Solution References [1] C. Paula, D. Wisser, M. Rangus, W. Schwieger, M. Hovestadt, M. Kriesten, K. Vanatalu, A. Oss, M.-L. Org, A. Samoson, M. Hartmann, “Phase Transformations in Porous Materials Studied by In Situ Solid-State NMR Spectroscopy and In Situ X-ray Diffraction” J. Phys. Chem. C 2020, 124, 19136–19145. [2] S. Pilkenton, S. J. Hwang, D. Raftery, “Ethanol Photocatalysis on TiO2-Coated Optical Microfiber, Supported Monolayer, and Powdered Catalysts: An in Situ NMR Study” J. Phys. Chem. B 1999, 103, 11152–11160. [3] S. J. Hwang, D. Raftery, “In situ solid-state NMR studies of ethanol photocatalysis: Characterization of surface sites and their reactivities” Catal. Today 1999, 49, 353–361. [4] S. Pilkenton, D. Raftery, “Solid-state NMR studies of the adsorption and photooxidation of ethanol on mixed TiO2-SnO2 photocatalysts” Solid State Nucl. Magn. Reson. 2003, 24, 236–253. [5] M. Hunger, W. Wang, “Formation of cyclic compounds and carbenium ions by conversion of methanol on weakly dealuminated zeolite H-ZSM-5 investigated via a novel in situ CF MAS NMR/UV-Vis technique” Chem. Commun. 2004, 584. [6] J. G. Hu, B. Q. Sun, A. T. Petkova, R. G. Griffin, J. Herzfeld, “The Predischarge Chromophore in Bacteriorhodopsin: A 15 N Solid-State NMR Study of the L Photointermediate” Biochemistry 1997, 36, 9316–9322. [7] T. J. N. Hooper, R. de Oliveira-Silva, D. Sakellariou, “High-resolution in situ photo-irradiation MAS NMR: application to the UV-polymerization of n -butyl acrylate” J. Mater. Chem. A 2025, 13, 933–939. [8] M. L. Mak-Jurkauskas, V. S. Bajaj, M. K. Hornstein, M. Belenky, R. G. Griffin, J. Herzfeld, “Energy transformations early in the bacteriorhodopsin photocycle revealed by DNP-enhanced solid-state NMR” Proc. Natl. Acad. Sci. 2008, 105, 883– 888. [9] J. Becker-Baldus, C. Bamann, K. Saxena, H. Gustmann, L. J. Brown, R. C. D. Brown, C. Reiter, E. Bamberg, J. Wachtveitl, H. Schwalbe, C. Glaubitz, “Enlightening the photoactive site of channelrhodopsin-2 by DNP-enhanced solidstate NMR spectroscopy” Proc. Natl. Acad. Sci. 2015, 112, 9896–9901. [10] F. De Biasi, G. Karthikeyan, M. Visegrádi, M. Levien, M. A. Hope, P. J. Brown, M. R. Wasielewski, O. Ouari, L. Emsley, “Light-Induced 1 H NMR Hyperpolarization in Solids at 9.4 and 21.1 T” J. Am. Chem. Soc. 2024, 146, 19667–19672. [11] A. Lasorsa, P. van der Meulen, E. Naumann, M. M. Lerch, M. R. Marquez-Garcia, X. Lan, K. Loos, B. L. Feringa, W.