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[Display omitted] • On methane dearomatization reaction, two main deactivation modes occurs. • Irreversible damage by zeolite amorphization during the isothermal pre-treatment. • Reversible deactivation by coke deposition ("soft", "hard" and carbide species). • 3 descriptors : coke level, micropore volume loss, monoclinic/orthorhombic change. We report a detailed kinetic study of methane dehydroaromatization on three bifunctional Mo/HZSM-5 catalysts (0.9 wt%, 2.7 wt%, and 17.2 wt% Mo). Two main deactivation modes are present: (i) irreversible damage by zeolite amorphization during the isothermal pre-treatment in an inert atmosphere and (ii) reversible deactivation by coke deposition ("soft", "hard" and carbide species). The former occurs at high Mo loading (>4.0 w%) and provides low activity catalysts. Catalysts with a lower and well-balanced molybdenum loading (<4 w%) suffer mainly from the second deactivation mode. The catalyst decay is modelled by semi-empirical laws including time on stream and coke level. The latter initiates textural and structural catalyst modifications leading to an activity loss. Three deactivation descriptors, 1 : coke level on the spent samples, 2: loss of micropore volume, and 3: monoclinic/orthorhombic phase transition of the zeolite are identified. Further characterization indicates that both molybdenum and carbon species are responsible for the catalytic deactivation of well-balanced catalysts. At least 50% of the initially dispersed molybdenum migrates to the external surface of the zeolite to form large clusters of molybdenum carbide. The coke extracted from the zeolite micropores are confirmed to be mainly unsubstituted polyaromatics. [ABSTRACT FROM AUTHOR]

• Rapid copper sintering occurring under realistic conditions of direct dimethyl ether synthesis. • No copper oxidation under a wide range of dimethyl ether synthesis conditions. • Strong effect of water on copper sintering and crystallite size. Dimethyl ether is one of the most promising environmentally optimized alternatives to the conventional fossil fuels and an important platform molecule for chemical industry. Catalyst deactivation is one of the most important challenges of the single-step dimethyl ether synthesis from syngas. Because of the lack of direct characterization techniques working under harsh reaction conditions, the information about deactivation mechanisms of bifunctional Cu/ZSM-5 catalysts is rather contradictory. In this paper, a combination of synchrotron-based in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy operating under high pressure and temperature alongside with the conventional ex-situ characterization uncovered very rapid copper sintering occurring under realistic conditions of direct dimethyl ether synthesis. Copper sintering was strongly affected by the presence of water either produced by the reaction or co-fed to the reactor. No copper oxidation was observed under a wide range of experimental conditions. [ABSTRACT FROM AUTHOR]

The zeolite community spent 50 years in pursuing the synthesis of high-silica LTA -type zeolites, and ultimately succeeded with the pure-silica polymorph by applying specific structure-directing agents (SDA). The high-silica zeolite A has exhibited unique properties and strong potential to be applied as an adsorbent and catalyst in several novel processes. In the present study, the high-silica polymorphs of Si/Al ratios from 15 to ∞ were synthesized and subjected to thorough characterizations using multiple physicochemical methods. It has been revealed that there exists strong interactions between the organic SDAs and the framework. Therefore, removing the SDAs by calcination in air is difficult, and can only be done above 700 °C with a certain degree of framework dealumination. The retained framework structure shows connectivity defects and a unique combination of Brønsted and Lewis acidity. Using methanol-to-olefins (MTO) as a probe reaction, the catalytic performance of high-silica LTA -type zeolites has been evaluated. Compared to the well-studied CHA and MFI -type catalysts, high-silica zeolite A exhibited a shorter life-time and higher selectivity towards C 4 olefins, while methane selectivity increased with the deactivation. [Display omitted] • The crystallization of high-silica LTA -type zeolites requires specific structure-directing agents (SDAs). • Strong SDA/framework interactions are evidenced by XRD, NMR and TG. • Complete removal of SDA by calcination accompanied by framework dealumination, while the framework structure retained. • High-silica LTA-type material possesses unique acidity that renders different catalytic performances in MTO reactions. [ABSTRACT FROM AUTHOR]