On the deactivation of Mo/HZSM-5 in the methane dehydroaromatization reaction

Tempelman, Chl Christiaan; Hensen, Emiel Emiel

doi:10.1016/j.apcatb.2015.04.052

Cited by 191 publications

(152 citation statements)

References 37 publications

(61 reference statements)

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“…The ethylene yield increases in the order: 6Mo‐0.2Fe/ZSM‐5<6Mo‐1Fe/ZSM‐5<6Mo/ZSM‐5, which is opposite to the trend obtained with the benzene yield in the case of the precarburized catalysts. This phenomenon has been observed previously and has been attributed to the gradual blocking of the zeolite channels by coke deposits on surface Brønsted acid sites. As the reaction proceeds, carbon deposits cover the Brønsted acid sites and reduce the zeolite pore diameters, therefore, methane is activated and converts to C 2 species, such as ethylene, which cannot polymerize to bigger species because of channel constraints.…”

Section: Resultssupporting

confidence: 77%

“…Much research has been dedicated to the improvement of Mo/ZSM‐5 catalysts for methane aromatization. Amongst the different techniques, the addition of promoters, such as Fe, and the use of different pretreatment methods have been studied extensively . The results obtained with Fe as an additive are not conclusive as different groups obtained different results based on the synthesis method and on the Mo and Fe loadings employed.…”

Section: Discussionmentioning

confidence: 99%

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Enhancement of Molybdenum/ZSM‐5 Catalysts in Methane Aromatization by the Addition of Iron Promoters and by Reduction/Carburization Pretreatment

2018

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The influence of an Fe additive and different types of pretreatment were studied on HZSM‐5‐supported molybdenum oxide (MoOx) catalysts for methane aromatization. The catalytic behavior of catalysts that consist of 6 wt % Mo/ZSM‐5 with 0, 0.2, and 1 wt % Fe was tested with two types of pretreatment: 1) heating under He flow and 2) reduction in H2/CH4 and carburization in CH4. Under He pretreatment, the addition of 0.2 wt % Fe improved the benzene yield, but the addition of 1 % Fe decreased it slightly. Precarburization of the catalysts resulted in enhanced catalytic properties for all Fe loadings and improved the catalyst stability. The precarburized 6 wt % Mo–0.2 wt %Fe catalyst presented the highest benzene yield (6.9 %), which was almost stable in the subsequent 10 h test. The fresh and spent catalysts were characterized by using XRD, N2 adsorption, temperature‐programmed reduction, SEM, temperature‐programmed oxidation, and thermogravimetric analysis. The results show that the precarburized catalysts are more stable because of the formation of smaller amounts of carbon deposits and, consequently, less pore blockage. The addition of Fe causes the carbon deposits to be more reactive and easier to burn off. Higher Fe loadings are linked to the formation of carbon nanotubes.

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Section: Resultssupporting

confidence: 77%

Section: Discussionmentioning

confidence: 99%

Enhancement of Molybdenum/ZSM‐5 Catalysts in Methane Aromatization by the Addition of Iron Promoters and by Reduction/Carburization Pretreatment

2018

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“…[2] Based on detailed characterization, we proposed that the coke deposits with an overall stoichiometry of about CH 0.4 are most likely acenes,occluded in the straight channels (Supporting Information, Figure S9). [21] With increasing pressure,h owever, ah igher coke content of the pores is attained. [21] With increasing pressure,h owever, ah igher coke content of the pores is attained.…”

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confidence: 99%

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

et al. 2019

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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.

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“…The MDA reaction is conventionally run at around 700ºC in presence of bifunctional catalysts comprising carbided molybdenum nanoclusters dispersed in acidic shape-selective zeolites such as ZSM-5 and MCM-22 (1). The process suffers from two major hurdles that challenge its further development and industrial implementation: The per-pass conversion to aromatics is limited by thermodynamics, and the catalyst activity rapidly drops with time on stream owing to the accumulation of polyaromatic-type coke on the external zeolite surface that impedes the access to internal active sites (2,3). Attempts to overcome thermodynamic limitations by selective removal of the co-product hydrogen from the MDA reactor using, for instance, Pd-type (4) or ceramic (La 5.5 W 0.6 Mo 0.4 O 11.25-δ ) (5) membranes were not satisfying due to enhanced coke formation that accelerated catalyst decay.…”

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confidence: 99%

Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor

Morejudo

Zanón

Escolástico

et al. 2016

Science

376

242

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Non-oxidative methane dehydroaromatization (MDA:6CH 4 ↔ C 6 H 6 + 9H 2 ) using shapeselective Mo/zeolite catalysts is a technology to exploit stranded natural gas reserves by direct conversion into transportable liquids. The reaction, however, faces two major issues: the onepass conversion/yield is limited by thermodynamics, and the catalyst deactivates fast due to the kinetically-favored formation of coke. Here we show that integration of an electrochemical BaZrO 3 -based membrane exhibiting both proton and oxide ion conductivity into an MDA reactor enables high aromatic yields and outstanding catalyst stability. These effects originate from the simultaneous extraction of hydrogen and distributed injection of oxide ions along the reactor length. Further, we demonstrate that the electrochemical co-ionic membrane reactor enables high carbon efficiencies (up to 80%) significantly improving the techno-economic process viability, and sets the ground for its commercial deployment. One Sentence Summary:The integration of a co-ionic membrane in a MDA reactor remarkably enhances aromatics yield and catalyst lifetime. Main text:

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On the deactivation of Mo/HZSM-5 in the methane dehydroaromatization reaction

Cited by 191 publications

References 37 publications

Enhancement of Molybdenum/ZSM‐5 Catalysts in Methane Aromatization by the Addition of Iron Promoters and by Reduction/Carburization Pretreatment

Enhancement of Molybdenum/ZSM‐5 Catalysts in Methane Aromatization by the Addition of Iron Promoters and by Reduction/Carburization Pretreatment

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor

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