Search

Non-oxidative dehydroaromatization of methane over Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM-5 catalysts and a wide range of reaction temperatures and space velocities. High-pressure operando X-ray absorption spectroscopy demonstrated that the structure of the active Mo-phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass-spectrometry and 13 C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space.

In recent years, noble metals atomically dispersed on solid oxide supports have become a frontier of heterogeneous catalysis. In pursuit of an ultimate atom efficiency, the stability of single-atom catalysts is pivotal. Here we compare two Pd/CeO2 single-atom catalysts that are active in low-temperature CO oxidation and display drastically different structural dynamics under the reaction conditions. These catalysts were obtained by conventional impregnation on hydrothermally synthesized CeO2 and one-step flame spray pyrolysis. The oxidized Pd atoms in the impregnated catalyst were prone to reduction and sintering during CO oxidation, whereas they remained intact on the surface of the Pd-doped CeO2 derived by flame spray pyrolysis. A detailed in situ characterization linked the stability of the Pd single atoms to the reducibility of the Pd–CeO2 interface and the extent of reverse oxygen spillover. To understand the chemical phenomena that underlie the metal–support interactions is crucial to the rational design of stable single-atom catalysts. Single-atom catalysts have become a frontier of heterogeneous catalysis, but to achieve a high stability under turnover is often a challenge. Now, a Pd/CeO2 single-atom catalyst prepared using flame spray pyrolysis is able to stabilize the isolated Pd species during CO oxidation due to a high mobility of surface lattice oxygen.

Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

Coke deposition is one of the main challenges in the commercialisation of dry reforming of methane over supported Ni catalysts. Besides the coke quantity, the structure of the deposits is also essential for the catalyst lifetime. Accordingly, in this study, we analysed the effect of Na, K, and Cs promoters on both these variables over Ni/ ZrO2 catalysts. Besides blocking the most active coke-forming sites already at low loading, the promoting effect of the alkali metals is also contributed by their coke gasification activity. To evaluate the additional impact of the latter, the behaviour of alkali-doped catalysts was compared to that for Mn-doped catalysts, exclusively featuring the site-blocking promotion mechanism. While the conversion is barely affected by the type of promoter, it has a profound effect on the amount and the composition of carbon deposits formed during the reaction. Promoting with K or Mn reduces the coke content to a similar degree but with less carbon fibres observed in the case of K. The promotion by Cs and Na results in the lowest coke content. The superior performance of Cs and Na-doped Ni/ZrO2 catalysts is attributed to the enhanced coke gasification via carbonate species on top of the site blocking effects.

The viability of the Power-to-Gas (PtG) concept is strongly dependent on the development of highly active and stable methanation catalysts obtained from cheap and abundant elements. In this paper, the promotional effect of MnO on Ni catalysts supported on silica-modified γ-Al2O3 (SA) was investigated in CO2 and CO methanation on catalysts with Mn/Ni atomic ratios between 0 and 0.25. Significantly higher methanation rates and CH4 selectivities were obtained for Mn-promoted compositions compared to Ni-only catalysts. The optimal NiMn/SA (Mn/Ni = 0.25) catalyst exhibited improved stability compared with unpromoted Ni/SA at 20 bar. The nature of the catalyst precursor and active catalyst was studied with STEM-EDX, XPS, and X-ray absorption spectroscopy (XAS). Evidence of a mixed Ni-Mn oxide in the catalyst precursor was obtained by EXAFS. EXAFS measurements revealed that the reduced catalyst consisted of metallic Ni particles and small oxidic Mn2+ species. Moreover, Mn addition improved the Ni dispersion and enhanced the Ni2+ reducibility by weakening the interaction between the Ni-oxide precursor and the support. A mechanistic study involving IR spectroscopy and steady-state isotopic (13CO2) transient kinetic analysis (SSITKA) showed that the presence of Mn enhanced CO2 adsorption and activation.

Liu et al. recently reported their results on coconversion of methane and methanol at 973 K over a typical methane dehydroaromatization (MDA) catalysts, Mo/HZSM-5.1 In this work, the authors claimed that adding a small amount of methanol to a methane feed led to more than two times higher methane conversion, substantially higher xylene and toluene selectivities (i.e., combined ca. 80%, nearly an order of magnitude increase as compared to experiments without methanol), and improved catalyst stability to such an extent that no deactivation was observed during 60 h on stream. If reproducible, this result would be a significant achievement, because formation of coke in the MDA reaction has been considered inevitable hitherto. To support their experimental data, Liu et al. carried out a thermodynamic analysis, whose results were in good agreement with their experimental findings.

The facile preparation of structured catalysts featuring highly-dispersed non-precious metal (Fe, Ni, Co) oxide nanoparticles stabilized within a super-microporous network of activated carbon fibres (ACF) is described. The catalyst morphology was controlled over multiple levels from the nano-designed active phase up to the macro-structure of the ACF support. A range of physicochemical techniques was used to characterize the catalyst and rationalize the catalytic data during chemoselective transfer hydrogenation of nitroarenes. The challenging reduction of m-nitrostyrene containing an easily reducible vinyl-group was obtained under mild conditions with a near-quantitative yield of m-vinylaniline using hydrazine hydrate as the reducing agent over CoOx/ACF as a structured catalyst. (C) 2016 Elsevier B.V. All rights reserved.