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Complete catalytic oxidation of methane in the presence of steam at low temperatures (T < 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C–H bond of methane at low temperature is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technological applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure–property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from commercially available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepared. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T 50 of 355 °C and T 90 of 375 °C (where T 50 and T 90 are temperature values at which the samples reach 50% and 90% methane conversion, respectively) in steam-containing reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications.

Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, ²⁷Al and ¹⁹F MAS-NMR, Rietveld structure refinement and N₂- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst

The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. < 40 kJ mol −1 ) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si−(OD)−Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O−(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O−D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique.

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Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks.T herefore,d irect imaging of all atoms at the atomic scale,b asic units (Si, Al, and O), heteroatoms in the framework, and extra-framework cations,i sn eeded. TEM provides local information at the atomic level, but the serious problem of electron-beam damage needs to be overcome.H erein, all framework atoms,i ncluding oxygen and most of the extraframework Na cations,are successfully observed in one of the most electron-beam-sensitive and lowest framework density zeolites,N a-LTA.Z eolite performance,f or instance in catalysis,i sh ighly dependent on the location of incorporated heteroatoms.Fesingle atomic sites in the MFI framework have been imaged for the first time.T he approach presented here, combining image analysis,e lectron diffraction, and DFT calculations,c an provide essential structural keys for tuning catalytically active sites at the atomic level.

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It is well-known that water has a detrimental effect on the low-temperature methane combustion activity of palladium catalysts. However, when the transient activity (i.e., light-off or ignition–extinction experiments) of methane combustion catalysts is compared, the effects of water adsorption–desorption phenomena are seldom directly considered. While these effects are important to keep in mind when studying support-dependent methane combustion activity, they are crucial when selecting a catalyst diluent. In many cases, the water adsorption–desorption properties of “inert” reactor diluents may dominate the transient methane combustion activity of a Pd catalyst. In this contribution, we show how different catalyst pretreatments on various Pd catalysts can influence the presence of water and hydroxyl groups on the surface of catalyst supports, reactor diluents, and active phase, and how this process dramatically affects observed methane combustion activity. Transiently, alumina (both support and diluent), which strongly binds water that is produced in the reaction, can keep the PdO phase active and water-free. However, after alumina surfaces become saturated with water, the PdO surface also becomes hydroxylated, which decreases the catalyst’s methane combustion activity. Due to this time-dependent surface titration, care must be taken when comparing transient experiments between catalysts on different supports; comparable data for methane combustion must be collected while carefully checking for water adsorption on the surface of the catalyst and diluent. Finally, we propose that a channel for sustainable high combustion rates is possible if water is prevented from adsorbing on the highly active PdO surfaces.