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

The catalyst\u27s 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

Acidity as Descriptor for Methanol Desorption in B-, Ga- and Ti-MFI Zeotypes

The isomorphous substitution of Si with metals other than Al in zeotype frameworks allows for tuning the acidity of the zeotype and, therefore, to tailor the catalyst\u27s properties as a function of the desired catalytic reaction. In this study, B, Ga, and Ti are incorporated in the MFI framework of silicalite samples and the following series of increasing acidity is observed: Ti-silicalite < B-silicalite < Ga-silicalite. It is also observed that the lower the acidity of the sample, the easier the methanol desorption from the zeotype surface. In the target reaction, namely the direct conversion of methane to methanol, methanol extraction is affected by the zeotype acidity. Therefore, the results shown in this study contribute to a more enriched knowledge of this reaction

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (rho) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm(-1) are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm(-1) is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm(-1) can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm(-1) is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Bronsted acid sites. The intensity of the characteristic mode at 3611 cm(-1) reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure

Porosity measurements in suspension plasma sprayed YSZ coatings using NMR cryoporometry and X-ray microscopy

A large variety of coatings are used to protect structural engineering materials from corrosion, wear, and erosion, and to provide thermal insulation. In this work, yttria-stabilized zirconia coatings produced by suspension plasma spraying were investigated with respect to their microstructure and especially their porosity, as the porosity affects the thermal insulation of the underlying component. To determine porosity, pore size distribution, and pore shape, the coatings were investigated using novel advanced characterization techniques like NMR cryoporometry and X-ray microscopy. In general, the porosity is inhomogeneously distributed and the coatings showed a large variety of pore sizes ranging from a few nanometers to micrometers

Porosity measurements on heat treated suspension plasma sprayed YSZ coatings using NMR cryoporometry

Suspension plasma sprayed (SPS) coatings can be produced with fine powder particles and tailor-made porosity.This allows to achieve low thermal conductivity which makes the coatings attractive as e.g. topcoats in thermalbarrier coatings (TBCs). Used in gas turbine applications, the TBCs are exposed to high temperatures which leadsto alterations of the microstructure. To obtain coatings with optimized properties, possible microstructurealterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBCtopcoats consisting of 8 wt.% yttria-stabilized zirconia (YSZ) were heat treated in air at 1150\ub0C and thereafterinvestigated using scanning electron microscopy, X-ray diffraction, and nuclear magnetic resonance (NMR)cryoporometry. For all investigated samples, the porosity decreased as a result of the heat treatment. The finerpores and cracks disappeared and the larger pores grew slightly and achieved a more distinct shape as thematerial seemed to become more compact

Porosity measurements in suspension plasma sprayed YSZ coatings

A large variety of coatings are used to protect structural engineering materials from corrosion, wear, and erosion, and to provide thermal insulation. In this work yttria-stabilized zirconia (YSZ) coatings produced by suspension plasma spraying (SPS) were investigated with respect to their microstructure and especially their porosity, as the porosity affects the thermal insulation of the underlying component. To determine porosity, pore size distribution and pore shape, the coatings were investigated using novel advanced characterization techniques like NMR cryoporometry and X-ray microscopy. In general, the porosity is inhomogeneously distributed and the coatings showed a large variety of pore sizes ranging from a few nanometers to micrometers

The Influence of Heat Treatments on the Porosity of Suspension Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed coatings are produced using fine-grained feedstock. This allows to control the porosity and to achieve low thermal conductivity which makes the coatings attractive as topcoats in thermal barrier coatings (TBCs). Used in gas turbine applications, TBCs are exposed to high temperature exhaust gases which lead to microstructure alterations. In order to obtain coatings with optimized thermomechanical properties, microstructure alterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBC topcoats consisting of 4 mol.% yttria-stabilized zirconia were heat-treated in air at 1150 \ub0C and thereafter the coating porosity was investigated using image analysis (IA) and nuclear magnetic resonance (NMR) cryoporometry. Both IA and NMR cryoporometry showed that the porosity changed as a result of the heat treatment for all investigated coatings. In fact, both techniques showed that the fine porosity decreased as a result of the heat treatment, while IA also showed an increase in the coarse porosity. When studying the coatings using scanning electron microscopy, it was noticed that finer pores and cracks disappeared and larger pores grew slightly and achieved a more distinct shape as the material seemed to become more compact

Methoxy ad-species in MFI zeotypes during methane exposure and methanol desorption followed by in situ IR spectroscopy

The formation and evolution of methoxy ad-species in MFI zeotypes after CH4 exposure, and during temperature programmed desorption of CH3OH have been investigated in situ with diffuse reflectance Fourier transform infrared spectroscopy. Fe and/or Al atoms have been incorporated in framework position prior to crystallization and the influence of the resulting acidity on CH4 activation and CH3OH desorption has been examined. The results show that the presence of Fe promotes CH4 activation and that methanol is more strongly bound to the zeotype in the presence of Al. Because CH4 activation and CH3OH extraction are two of the key steps in the direct conversion of methane to methanol, our results indicate that Al-free zeotypes containing Fe atoms pinpoint important catalyst design parameters needed for this reaction