Ruthenium silica nanoreactors with varied metal–wall distance for efficient control of hydrocarbon distribution in Fischer–Tropsch synthesis

The Impact of Oxygen Surface Coverage and Carbidic Carbon on the Activity and Selectivity of Two-Dimensional Molybdenum Carbide (2D-Mo2C) in Fischer-Tropsch Synthesis

Transformations of oxygenates (CO2, CO, H2O, etc.) via Mo2C-based catalysts are facilitated by the high oxophilicity of the material; however, this can lead to the formation of oxycarbides and complicate the identification of the (most) active catalyst state and active sites. In this context, the two-dimensional (2D) MXene molybdenum carbide Mo2CTx (Tx are passivating surface groups) contains only surface Mo sites and is therefore a highly suitable model catalyst for structure-activity studies. Here, we report that the catalytic activity of Mo2CTx in Fischer-Tropsch (FT) synthesis increases with a decreasing coverage of surface passivating groups (mostly O*). The in situ removal of Tx species and its consequence on CO conversion is highlighted by the observation of a very pronounced activation of Mo2CTx (pretreated in H2 at 400 °C) under FT conditions. This activation process is ascribed to the in situ reductive defunctionalization of Tx groups reaching a catalyst state that is close to 2D-Mo2C (i.e., a material containing no passivating surface groups). Under steady-state FT conditions, 2D-Mo2C yields higher hydrocarbons (C5+ alkanes) with 55% selectivity. Alkanes up to the kerosine range form, with value of α = 0.87, which is ca. twice higher than the α value reported for 3D-Mo2C catalysts. The steady-state productivity of 2D-Mo2C to C5+ hydrocarbons is ca. 2 orders of magnitude higher relative to a reference β-Μo2C catalyst that shows no in situ activation under identical FT conditions. The passivating Tx groups of Mo2CTx can be reductively defunctionalized also by using a higher H2 pretreatment temperature of 500 °C. Yet, this approach leads to a removal of carbidic carbon (as methane), resulting in a 2D-Mo2C1-x catalyst that converts CO to CH4 with 61% selectivity in preference to C5+ hydrocarbons that are formed with only 2% selectivity. Density functional theory (DFT) results attribute the observed selectivity of 2D-Mo2C to C5+ alkanes to a higher energy barrier for the hydrogenation of surface alkyl species relative to the energy barriers for C-C coupling. The removal of O* is the rate-determining step in the FT reaction over 2D-Mo2C, and O* is favorably removed in the form of CO2 relative to H2O, consistent with the observation of a high CO2 selectivity (ca. 50%). The absence of other carbon oxygenates is explained by the energetic favoring of the direct over the hydrogen-assisted dissociative adsorption of CO.

Precisely Engineering Architectures of Co/C Sub-Microreactors for Selective Syngas Conversion

Fischer-Tropsch synthesis (FTS) is an effective route to produce olefins, gasoline, diesel, and oxygenates from syngas (CO + H₂ ). However, it still remains a challenge for regulating the product distribution of FTS. Here, a series of Co/C sub-microreactors with precise designed nanoarchitectures are synthesized for selective syngas conversion. Through a combination of surface protection-assisted etching and following carbonization process, Co/C sub-microreactors with solid cube, double-shelled hollow box, and hollow box architectures, namely, Co/C-Cube, Co/C-DBox, Co/C-Box can be obtained. In FTS, comparing with solid Co/C-Cube, double-shelled hollow structured Co/C-DBox is inclined to grow long-chain hydrocarbon products, whereas hollow structured Co/C-Box avails the formation of short-chain hydrocarbon chemicals. Therefore, shape selective catalysis and controlled product distribution of FTS are realized by tuning the architectures of Co/C sub-microreactors. It is expected to fundamentally unravel the heterogeneous catalytic process via upfront designing and precisely regulating the architectures of micro/nanoreactors.

Carbon-based catalysts for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) is an essential approach to convert coal, biomass, and shale gas into fuels and chemicals, such as lower olefins, gasoline, diesel, and so on. In recent years, there has been increasing motivation to deploy FTS at commercial scales which has been boosting the discovery of high performance catalysts. In particular, the importance of support in modulating the activity of metals has been recognized and carbonaceous materials have attracted attention as supports for FTS. In this review, we summarised the substantial progress in the preparation of carbon-based catalysts for FTS by applying activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon spheres (CSs), and metal-organic frameworks (MOFs) derived carbonaceous materials as supports. A general assessment of carbon-based catalysts for FTS, concerning the support and metal properties, activity and products selectivity, and their interactions is systematically discussed. Finally, current challenges and future trends in the development of carbon-based catalysts for commercial utilization in FTS are proposed.

Controlled Nanostructure of Zeolite Crystal Encapsulating FeMnK Catalysts Targeting Light Olefins from Syngas

Light olefins (C₂⁼-C₄⁼) are important basic raw materials in chemical industries. Direct production of light olefins from syngas using zeolite encapsulation catalysts shows great potential due to their regulation of product distribution in the Fischer-Tropsch process. Herein, we report a series of silicalite-1 zeolite-encapsulated FeMnK catalysts with distinct nanostructures, including FeMnK@S-1, FeMnK@Hol-S-1, and FeMnK@HM-S-1. It was found that the FeMnK@HM-S-1 catalyst (encapsulation of FeMnK oxide in hollow mesoporous silicalite-1 crystal) had an enhanced C₂⁼-C₄⁼ selectivity of 49% at a CO conversion of 12%. Our results revealed that superior light olefins selectivity of the FeMnK@HM-S-1 catalyst was achieved by the synergic effect between the inherent silicalite-1 micropores and the hollow mesoporous structure, which is responsible for restricting heavy hydrocarbon (C₅⁺) formation, maximizing C₂-C₄ hydrocarbons selectivity, quickly removing the primary light olefin products, and increasing the O/P ratio. We demonstrated that the enhanced CO adsorption and the declined H₂ adsorption (lower [H*]/[C*] ratio) over the FeMnK@HM-S-1 catalyst could also facilitate the olefin synthesis. This work provides guidance for reasonable designing of F-T catalysts to tailor product selectivity.

Lower olefins from methane: recent advances

Modern methods for methane conversion to lower olefins having from 2 to 4 carbon atoms per molecule are generalized. Multistage processing of methane into ethylene and propylene via syngas or methyl chloride and methods for direct conversion of CH4 to ethylene are described. Direct conversion of syngas to olefins as well as indirect routes of the process via methanol or dimethyl ether are considered. Particular attention is paid to innovative methods of olefin synthesis. Recent achievements in the design of catalysts and development of new techniques for efficient implementation of oxidative coupling of methane and methanol conversion to olefins are analyzed and systematized. Advances in commercializing these processes are pointed out. Novel catalysts for Fischer – Tropsch synthesis of lower olefins from syngas and for innovative technique using oxide – zeolite hybrid catalytic systems are described. The promise of a new route to lower olefins by methane conversion via dimethyl ether is shown. Prospects for the synthesis of lower olefins via methyl chloride and using non-oxidative coupling of methane are discussed. The most efficient processes used for processing of methane to lower olefins are compared on the basis of degree of conversion of carbonaceous feed, possibility to integrate with available full-scale production, number of reaction stages and thermal load distribution.

The bibliography includes 346 references.