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.

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The active site requirements for methane dehydroaromatization by Mo/HZSM-5 were investigated by employing as catalysts physical mixtures of Mo-bearing supports (HZSM-5, SiO 2 , γ-Al 2 O 3 , and activated carbon) and HZSM-5. Separation of the two catalyst components after activation or reaction was possible by using two different sieve fractions. Our comparison demonstrates that migration of volatile Mo oxides into the micropores of HZSM-5 is at the origin of the observed catalytic synergy in methane dehydroaromatization for physical mixtures. The propensity of Mo migration depends on the activation method and the Mo−support interaction. Migration is most pronounced for Mo/SiO 2 . Prolonged exposure of HZSM-5 zeolite to Mo oxide vapors results in partial destruction of the zeolite framework. Mo carbide dispersed on nonzeolitic supports afforded predominantly coke with only very small amounts of benzene. The main function of the zeolite is to provide a shape-selective environment for the conversion of methane to benzene. A comparison of Mo/HZSM-5 and Mo/Silicalite-1 demonstrates that aromatization of methane is an intrinsic ability of molybdenum carbides dispersed in the 10-membered-ring micropores of MFI zeolite. Thus, one important role of the Brønsted acid sites is to promote the dispersion of the Mo oxide precursor and, accordingly, the active Mo carbide phase in the micropores of HZSM-5.

Surface carbon (coke, carbonaceous deposits) is an integral aspect of methane dehydroaromatization catalyzed by Mo/zeolites. We investigated the evolution of surface carbon species from the beginning of the induction period until the complete catalyst deactivation by the pulse reaction technique, TGA, 13C NMR, TEM, and XPS. Isotope labeling was performed to confirm the catalytic role of confined carbon species during MDA. It was found that “hard” and “soft” coke distinction is mainly related to the location of coke species inside the pores and on the external surface, respectively. In addition, MoO3 species act as an active oxidation catalyst, reducing the combustion temperature of a certain fraction of coke. Furthermore, after dissolving the zeolite framework by HF, we found that coke formed during the MDA reaction inside the zeolite pores is essentially a zeolite-templated carbon material. The possibility of preparing zeolite-templated carbons from the most available hydrocarbon feedstock is important for the development of these interesting materials.

In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.

Non-oxidative dehydroaromatization of methane (MDA) is apromising catalytic process for direct valorization of natural gas to liquid hydrocarbons.T he application of this reaction in practical technology is hindered by al acko f understanding about the mechanism and nature of the active sites in benchmark zeolite-based Mo/ZSM-5 catalysts,w hich precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopya nd 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 ap ulse reaction techniquew ith isotope labeling of methane,M DA is shown to be governed by ah ydrocarbon pool mechanism in whichb enzenei sd erived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.The abundance of natural gas reserves calls for the development of an efficient conversion technology to upgrade its principal component, methane,i nto easily transportable chemicals.[1] Several catalytic technologies,w hich could replace the current indirect route involving an expensive synthesis gas generation step,a re being considered. Broadly, we can distinguish between oxidative and non-oxidative direct routes.[2] Among the non-oxidative approaches,c atalytic methane dehydroaromatization (MDA) is one of the most promising methods.A fter the initial reports on MDA almost three decades ago, [3] as ubstantial body of literature has appeared.[4] Thei ndustrial implementation of the MDA process is mainly hindered by rapid catalyst deactivation caused by the deposition of ac arbonaceous material that blocks the catalytically active sites. [5] Although there have been remarkable achievements in regeneration procedures, [6] developing astable MDAcatalyst is still required to arrive at acommercial process.Aprogress in this direction is seriously hampered by limited understanding of the active sites in the benchmark Mo/ZSM-5 catalyst and the mechanism of methane conversion to benzene and hydrogen. Despite considerable debate on the nature of the active phase,t here is ag rowing consensus that the active sites are confined as highly dispersed Mo species by the zeolite pores in working Mo/ZSM-5 catalysts and that Mo 2 Cn anoparticles on the external surface are inactive. [7] Concerning the reaction mechanism, most reports support ab ifunctional pathway in which methane is activated and coupled to ethylene over Mocarbide species,followed by ethylene aromatization over the zeolite Brønsted acid sites. [8] Important challenges in gaining insight into these aspects are the high reaction temperature at which the MDAreaction takes place and its transient nature,w hich involves rapid activation and deactivation stages when the fresh Mo/ZSM-5 catalyst is exposed to am ethane feed. These factors complicate operando spectroscopy and kinetic investigations.A valuable approach in this regard is to increase the temporal resolution by pulsing the reactant over the catalyst an...

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X-ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM-5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mospecies proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM-5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

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.