Non-oxidative Methane Dehydroaromatization Research Articles

Direct conversion of methane to aromatics and hydrogen via a heterogeneous trimetallic synergistic catalyst

Non-oxidative methane dehydro-aromatization reaction can co-produce hydrogen and benzene effectively on a molybdenum-zeolite based thermochemical catalyst, which is a very promising approach for natural-gas upgrading. However, the low methane conversion and aromatics selectivity and weak durability restrain the realistic application for industry. Here, a mechanism for enhancing catalysis activity on methane activation and carbon-carbon bond coupling has been found to promote conversion and selectivity simultaneously by adding platinum–bismuth alloy cluster to form a trimetallic catalyst on zeolite (Pt-Bi/Mo/ZSM-5). This bimetallic alloy cluster has synergistic interaction with molybdenum: the formed CH3* from Mo2C on the external surface of zeolite can efficiently move on for C-C coupling on the surface of Pt-Bi particle to produce C2 compounds, which are the key intermediates of oligomerization. This pathway is parallel with the catalysis on Mo inside the cage. This catalyst demonstrated 18.7% methane conversion and 69.4% benzene selectivity at 710 °C. With 95% methane/5% nitrogen feedstock, it exhibited robust stability with slow deactivation rate of 9.3% after 2 h and instant recovery of 98.6% activity after regeneration in hydrogen. The enhanced catalytic activity is strongly associated with synergistic interaction with Mo and ligand effects of alloys by extensive mechanism studies and DFT calculation.

Methane dehydroaromatization over Mo/H-ZSM-5: A comparative study of the regeneration methods

The catalytic activity and deactivation characteristic of spherically shaped Mo/H-ZSM-5 particles in the non-oxidative dehydroaromatization of methane was evaluated in a fluidized bed reactor under different cyclic reaction-regeneration operation modes. The aim is to verify the superiority of the H2 regeneration approach over that oxidative approach in restoring the catalytic performance of coke-deactivated Mo/H-ZSM-5 catalyst. Compared with the fresh catalyst, the catalyst samples oxidatively regenerated at 973 and 1073 K exhibited at 1073 K a largely reduced initial activity and an obviously accelerated deactivation rate. By contrast, the catalyst sample regenerated in a H2 stream at 1073 K maintained basically the initial activity level and deactivation characteristic of the fresh catalyst over cycles. Oxidatively regenerated at a lower temperature of 773 K, the catalyst sample was allowed to maintain the initial high activity of the fresh catalyst but its deactivation rate estimated over a 20 min reaction period showed an obvious tendency to increase with increasing the number of regeneration cycles. Characterization of all spent catalyst samples using TG, XRD and NMR techniques revealed that a significant loss of active Mo sites via sublimation and/or formation of inactive Al2(MoO4)3 and local collapse of zeolite framework occurred to the two samples once oxidatively regenerated at 973 and 1073 K, while neither Mo loss nor framework breakdown was detected with the H2-regenerated sample. Additionally, the formation of disordered non-framework Al was found to occur to the two samples once oxidatively regenerated at 773 and 873 K. All these suggest that applying the H2 approach to regenerate coke-deactivated Mo/H-ZSM-5 enables the catalyst to avoid losing any of its active Mo sites in its cyclic regeneration process and therefore must be helpful for maintenance of its catalytic performance over the reaction-regeneration cycles.

Elucidating the role of Fe-Mo interactions in the metal oxide precursors for Fe promoted Mo/ZSM-5 catalysts in non-oxidative methane dehydroaromatization

Literature shows that adding Fe as a separate phase to MoO3/ZSM-5 catalysts can improve benzene selectivity in methane dehydroaromatization (MDA), but only when added in small quantities, making it difficult to characterize the state of Fe in the catalyst and understand the role of Fe-Mo interactions on the catalytic properties. Here, we explore how the nature of the Mo-Fe interactions in the catalyst precursor can influence the stability and product selectivity in MDA, by employing for the first time Fe2(MoO4)3/ZSM-5 as a catalyst precursor in MDA. We have compared the activity of Fe2(MoO4)3/ZSM-5 with monometallic MoO3/ZSM-5 and mixed MoO3 + Fe2O3/ZSM-5 containing equivalent Mo and Fe loadings and found that Fe2(MoO4)3/ZSM-5 shows higher benzene selectivity than the mixed MoO3 + Fe2O3/ZSM-5 catalyst and exhibits higher stability in reaction compared to the monometallic MoO3/ZSM-5 catalyst. Structural characterization suggests that Fe2(MoO4)3 partially segregates to Fe2O3 and amorphous MoOx during thermal pretreatment. The MoOx species migrate into the zeolite channels during pretreatment, while Fe oxides remain on the external surface of the zeolite. Gas adsorption/desorption techniques and density functional theory calculations demonstrate that the preexisting Fe2O3 phases on the external surface of the zeolite in the mixed MoO3 + Fe2O3/ZSM-5 precursor trap (MoO3)3 clusters preventing them from migrating into the zeolite channels during pretreatment, whereas gradual formation of amorphous MoOx together with the segregation of the Fe2O3 phase when using the Fe2(MoO4)3 precursor diminishes trapping of (MoO3)3 and consequently enhances migration and anchoring of the MoOx species in the zeolite channels, boosting selectivity to benzene. Characterization of used catalysts suggests that the presence of Fe promotes formation of structured carbon nanofibers which reduce the rate of catalyst deactivation.

Plant-wide modeling and techno-economic analysis of a direct non-oxidative methane dehydroaromatization process via conventional and microwave-assisted catalysis

Direct non-oxidative methane dehydroaromatization (DHA) process via conventional and microwave (MW)-assisted thermo-catalytic catalysis is studied. Rate models for methane DHA reactions, including the effect of catalyst deactivation, are developed by using the in-house experimental data. Model results for gas concentration profile and catalyst deactivation are in good agreement with the experimental data. This rate model is then used for the development of dynamic multi-scale, multi-physics commercial-scale reactor models. Total number of fixed bed reactors desired for a cyclic steady state process is estimated. Plant-wide models are then developed for conventional and MW-assisted processes for producing products of desired specifications. Techno-economic analysis of the methane DHA process is undertaken. Economics of these methane DHA processes are compared with the typical multi-step natural gas to aromatics production process via methanol synthesis. Sensitivity of internal rate of return (IRR) and net present value (NPV) to various economic and process parameters such as plant scale, desired rate of return, reactor cost, feedstock and utility cost, catalyst variable cost, and MW reactor cost is studied. Electric equivalent efficiency of the conventional methane DHA process is found to be 69.2 % and 67.3 % at 750 °C and 800 °C, respectively, while the MW-assisted methane DHA process has the electric equivalent efficiency of 48.9 % at 800 °C. IRRs of the conventional methane DHA process at 750 °C and 800 °C, and MW-assisted process are 15.2 %, 17.5 %, and 18.8 %, respectively for a methane feed flowrate of 19,782 kg/h, while the IRR of the multi-step natural gas to aromatics production process is estimated to be 0 % for the same plant scale. Impact of change in the methane price, electricity price, and catalyst cost is found to be considerable on the process economics, while the cost of the MW reactor is found to have negligible impact.

Catalytic Routes for Direct Methane Conversion to Hydrocarbons and Hydrogen: Current State and Opportunities

Natural gas, the cleanest fossil fuel, is an abundant source of methane and expected to play an increasingly important role in powering the world’s economic growth over the energy transition of the coming decades. Methane has the potential to be a CO2-free feedstock to cogenerate hydrogen (H2) and added value “building-blocks” chemicals (e.g., olefins and aromatics) for petrochemistry. In this review, the two processes (i) the oxidative coupling of methane (OCM) for production of ethylene and (ii) the nonoxidative methane dehydroaromatization (MDA) producing hydrogen and benzene are discussed. Both routes convert methane directly into valuable products, an advantage over the several-steps syngas route. The performances of various a variety of catalysts reported during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures, M/Zr(SO4)2 with M = Mo, Fe, Pt) under similar reaction conditions are compared. The major drawbacks and the strategies used to mitigate the main challenges related with the performance of the catalysts in both OCM and MDA reactions are critically revealed. For instance, the overoxidation in the OCM is mitigated by optimizing of the operating conditions, using alternative oxidants, and the application of membrane reactor technology are discussed. In the MDA reaction, the major issue is the catalyst deactivation by coke formation and migration and sintering of metallic active phases. Strategies for robust catalysts, methods for mild coke removal, pretreatment under reductive atmosphere are presented. Approaches to improve aromatics yields over coke production by addition of promoters or co-feed reactants to the MDA catalysts are also discussed.

Multiscale Modeling of a Direct Nonoxidative Methane Dehydroaromatization Reactor with a Validated Model for Catalyst Deactivation

Due to the recent boom in shale gas production, aromatics production using direct nonoxidative methane dehydroaromatization (DHA) is being investigated extensively. However, due to rapid coke forma...

Identification of atomically dispersed Fe-oxo species as new active sites in HZSM-5 for efficient non-oxidative methane dehydroaromatization

Non-oxidative methane dehydroaromatization (MDA) provides a promising and efficient technology for the valorization of huge amount of CH4 resources. Fe-based HZSM-5 zeolite as the second most researched catalyst system for MDA, however, commonly showed a much lower activity or/and selectivity for aromatics’ formation, probably due to an unsuitable Fe phase in the zeolite. Different from the reported Fe/HZSM-5 catalysts by the traditional impregnation method with Fe(NO3)3, a Fe/HZSM-5 catalyst was specially designed for the title reaction which was prepared by ion-exchange method with a stable, soluble and divalent Fe salt, i.e., FeSO4. Interestingly, the Fe2+-exchanged HZSM-5 catalyst can show an extremely higher aromatics formation activity with a maximum selectivity of 75–80% at a CH4 conversion of about 15% than any other Fe3+-exchanged and Fe-impregnated HZSM-5 zeolite catalysts at 1073 K. With various characterizations (UV-vis, UV-Raman, CO-DRFIT, HADDF-STEM and, especially, 57Fe Mössbauer spectroscopy), atomically dispersed Fe-oxo species anchored at framework Al sites are recognized to be responsible for selectively activating CH4 into CHx species available for aromatics’ formation. By comparison, the common Fe/HZSM-5 catalysts prepared by impregnation method with Fe(NO3)3 had a large amount of reducible Fe3O4 clusters on zeolite surface, and as they were reduced and carburized, the resulted metallic Fe and Fe carbide were both favorable for CH4 decomposition only, probably due to their poor dispersion. Fortunately, the limited S residuals (about 0.05 wt%) in the Fe2+-exchanged HZSM-5 catalyst from FeSO4 did not show its negative side on strongly promoting C–H cleavage. By comparison, the two impregnated Fe/HZSM-5 catalysts from FeSO4 and Fe2(SO4)3 both showed a complete CH4 decomposition due to considerable SO42- remained. Thanks to the application of the ion-exchange method and FeSO4 as Fe precursor, a highly active and selective Fe-based HZSM-5 catalyst has been successfully modeled and Fe-oxo species are recognized as new active sites for MDA, which may give us a new insight into constructing a catalyst with better catalytic performance for MDA and other reactions.

Ultrasound-Assisted Preparation of Mo/ZSM-5 Zeolite Catalyst for Non-Oxidative Methane Dehydroaromatization

The activity and selectivity of Mo/ZSM-5, benchmarking catalyst for the non-oxidative dehydroaromatization of methane, strongly depend on the cluster size, spatial distribution, and chemical environment of the Mo-based active sites. This study discloses the use of an ultrasound-assisted ion-exchange (US-IE) technique as an alternative Mo/ZSM-5 synthesis procedure in order to promote metal dispersion along the zeolite framework. For this purpose, a plate transducer (91.8 kHz) is employed to transmit the ultrasonic irradiation (US) into the ion-exchange reactor. The physico-chemical properties and catalytic activity of samples prepared under the said irradiation procedure and traditional impregnation (IWI) method are critically evaluated. Characterization results suggest that US neither affects the crystalline structure nor the particle size of the parent zeolite. However, US-IE promotes molybdenum species dispersion, avoids clustering at the external fresh zeolite surface and enhances molybdate species anchoring to the zeolite framework with respect to IWI. Despite the improved metal dispersion, the catalytic activity between catalysts synthesized by US-IE and IWI is comparable. This suggests that the sole initial dispersion enhancement does not suffice to boost the catalyst productivity and further actions such ZSM-5 support and catalyst pre-conditioning are required. Nevertheless, the successful implementation of US-IE and the resulting metal dispersion enhancement pave the way toward the application of this technique to the synthesis of other dispersed catalysts and materials of interest.

Unravelling the reactivity of metastable molybdenum carbide nanoclusters in the C-H bond activation of methane, ethane and ethylene.

C-H bond activation steps in non-oxidative methane dehydroaromatization (MDA), constitute a key functionalization of the reactant and adsorbed species to form aromatics. Previous studies have focused on studying the energetics of these steps at the most stable active sites involving molybdenum carbide species. Herein, a different paradigm is presented via studying the reactivity of a metastable molybdenum carbide (Mo2C6) nanocluster for the C-H bond activation of methane, ethane, and ethylene and comparing it with the reactivity of the lowest energy Mo2C6 nanocluster. Interestingly, the metastable nanocluster is observed to result in a consistent reduction (by half) in the C-H bond activation barrier of the respective alkane and alkene molecules compared to the global minimum isomer. This specific metastable form of the nanocluster is identified from a cascade genetic algorithm search, which facilitated a rigorous scan of the potential energy surface. We attribute this significant lowering of the C-H bond activation barrier to unique co-planar orbital overlap between the reactant molecule and active centers on the metastable nanocluster. Based on geometrical and orbital analysis of the transition states arising during the C-H bond activation of methane, ethane, and ethylene, a proton-coupled electron transfer mechanism is proposed that facilitated C-H bond cleavage. Motivated by the high reactivity for C-H bond activation observed on the metastable species, a contrasting framework to analyze the elementary-step rate contributions is presented. This is based on the statistical ensemble analysis of nanocluster isomers, where the calculated rates on respective isomers are normalized with respect to the Boltzmann probability distribution. From this framework, the metastable isomer is observed to provide significant contributions to the ensemble average representations of the rate constants calculated for C-H bond activation during the MDA reaction.

Reactivity, Selectivity, and Stability of Zeolite‐Based Catalysts for Methane Dehydroaromatization

Non-oxidative dehydroaromatization is arguably the most promising process for the direct upgrading of cheap and abundant methane to liquid hydrocarbons. This reaction has not been commercialized yet because of the suboptimal activity and swift deactivation of benchmark Mo-zeolite catalysts. This progress report represents an elaboration on the recent developments in understanding of zeolite-based catalytic materials for high-temperature non-oxidative dehydroaromatization of methane. It is specifically focused on recent studies, relevant to the materials chemistry and elucidating i) the structure of active species in working catalysts; ii) the complex molecular pathways underlying the mechanism of selective conversion of methane to benzene; iii) structure, evolution and role of coke species; and iv) process intensification strategies to improve the deactivation resistance and overall performance of the catalysts. Finally, unsolved challenges in this field of research are outlined and an outlook is provided on promising directions toward improving the activity, stability, and selectivity of methane dehydroaromatization catalysts.

Boric acid treated HZSM-5 for improved catalyst activity in non-oxidative methane dehydroaromatization

The dehydroaromatization (DHA) reaction of methane under non-oxidative conditions is carried out over a molybdenum catalyst supported on HZSM-5 and boric acid (BA) treated HZSM-5 at 700 °C and atmospheric pressure.

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

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 13C 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.

Techno-economic evaluation of a process for direct conversion of methane to aromatics

Natural gas is widely abundant and relatively inexpensive. As such, large quantities are flared every year. This presents an opportunity to use natural gas as a chemical feedstock. Non-oxidative dehydroaromatization of methane, the major component of natural gas is proposed as an option to monetize natural gas. This study evaluates the economics of the methane to benzene process. A catalytic membrane reactor model written in MATLAB has been directly implemented in an Aspen Plus V9 process model using CAPE-OPEN interoperability. Minimum utility requirements and heat exchanger designs were identified in Aspen Energy Analyzer, and process economics for a variety of feed and product prices were assessed using Aspen Process Economic Analyzer. The results indicate that the venture is profitable (profitability index of 1.17, IRR of 35.5%, NRR of 18.2%). The capital cost was found to be $35,500 per daily standard barrel of benzene. Additionally, the process remains economically attractive so long as either the price of benzene remains above $470/ton, or the price of hydrogen remains above $0.8/kg.

Non-oxidative dehydroaromatization of methane over Mo/H-ZSM-5 catalysts: A detailed analysis of the reaction-regeneration cycle

Of several Mo/H-ZSM-5 catalysts (Mo loading = 1, 3, 5, 7 wt%), 5Mo/H-ZSM-5 (5 wt%) exhibited the best catalytic performance for methane conversion and benzene yield for methane dehydroaromatization at 700 °C. It was observed that deactivation of zeolite acid sites precedes deactivation of the Mo2C sites. Increasing the number of accessible Mo carbide sites with minimizing isolated surface acid sites is required for high benzene selectivity and stability. Under oxidative conditions, bifunctional metal-acid sites were reversibly regenerated at 450 °C; at higher temperatures of 550–850 °C, irreversible deactivation was observed. The selective recovery of Brønsted acid sites near Mo sites other than isolated acid sites is sufficient to restore the catalytic activity in terms of benzene formation. Spectroscopic studies revealed that high-temperature oxidative regeneration induced (MoO42−)n oligomerization and subsequent carburization of bulk Mo carbide cluster. The 5Mo/H-ZSM-5 underwent MoO3 sublimation and loss of Brønsted acidity during oxidative regeneration at 850 °C. Reductive regeneration required a temperature higher than 700 °C for coke removal, but resulted in thermal degradation of the catalyst. Theoretical calculations indicated that partial oxidation of coke precursor (i.e. naphthalene) at 450 °C was more favorable than its partial hydrogenation at 850 °C on the Mo clusters in ZSM-5 channel. Overall, oxidative regeneration at 450 °C can maintain high Mo2C dispersion and efficient coke removal during 60 h of methane reaction.

Non-oxidative methane dehydroaromatization reaction over highly active $${\upalpha }$$ α - $$\hbox {MoC}_{1{-}\mathrm{x} }$$ MoC 1 - x ZSM-5 derived from pretreatment

The catalytic active-phase of reduced Mo species plays a vital role in non-oxidative methane dehydroaromatization (MDA) reaction. Pretreatment effect of one of the gases containing $$\hbox {N}_{2}$$ , $$\hbox {H}_{2}$$ and (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ over 15% Mo-loaded HZSM-5 catalyst has been investigated in the present work. Various spectroscopic investigations viz., XRD, TPR, TPO, XPS, etc., show that the pretreatment of 15% Mo-HZSM-5 catalyst with (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ gas stream exclusively leads to the formation of $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}\hbox {H}_{\mathrm{z}} $$ which acts as precursor moieties for the formation of highly active metastable fcc ( $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{X}})$$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ phases during the induction period. Comparatively, $$\hbox {H}_{2}$$ and $$\hbox {N}_{2}$$ -pretreated catalysts showed major formation of hcp ( $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ ) species that are found to be a less-active phase in MDA reaction. The active fcc ( $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{X}})$$ phases are immune to inert coking and assist primary ethylene products formed on carbonized Mo associated Brønsted acid sites to travel in the zeolite channels which is further aromatized over Brønsted acid sites deep inside the channels. XPS analysis of the catalyst shows that $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}}$$ and $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ are major catalytic phases that are covered with graphitic carbon and amorphous carbon present on the surface. The active phases $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}}$$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ , associated with Brønsted acid sites along with the vacant Brønsted acid sites in catalyst pretreated with (90 vol%) $$\hbox {CH}_{4}+\hbox {H}_{2}$$ mixture are responsible for high activity in methane conversion ( $$\sim $$ 13%), excellent aromatic selectivity (38%), and high stability of the catalyst. The catalytically active $${\upalpha }$$ -MoC $$_{1{-}}$$ x phase over 15% Mo-HZSM-5-A is more immune to coking than $$\upbeta $$ - $$\hbox {Mo}_{2}\hbox {C}$$ phase over 15% Mo-HZSM-5-B and 15% Mo-HZSM-5-C. The active phases, $${\upalpha }$$ - $$\hbox {MoC}_{1{-}\mathrm{x}} $$ and $$\hbox {MoO}_{\mathrm{x}}\hbox {C}_{\mathrm{y}}$$ , associated with Brønsted acid sites in 15% Mo-HZSM-5-A is responsible for high activity in methane conversion ( $$\sim $$ 13%), excellent aromatic selectivity (38%), and high stability of the catalyst.

Progress in Nonoxidative Dehydroaromatization of Methane in the Last 6 Years

Revolutionary shale gas production has resulted in increasing interest in the use of methane for producing various types of high-value chemicals and among them is aromatics via methane dehydroaromatization (MDA). Progresses achieved in the area of MDA during the last 6 years are significant. However, a review in this area is lacking. This review is designed to fill the gap. The review not only discusses he development of MDA catalysts, including conventional molybdenum (Mo)-based catalyst and non-Mo-based catalysts and novel catalysts. Also, the insights for the associated reaction mechanisms and catalyst regeneration methods and applications of various types of reactors are provided.

Plasma-Assisted Non-Oxidative Conversion of Methane over Mo/HZSM-5 Catalyst in DBD Reactor

Methane utilization has been recognized as a challenging research topic because methane is very stable with strong C–H bond. Many efforts have been devoted to find an effective method for the methane activation. In this work, we examined the feasibility of decreasing the temperature of a non-oxidative methane dehydroaromatization by using DBD plasma-catalyst hybrid system. Mo/HZSM-5 catalyst was prepared and then it was used for the DBD plasma-assisted methane dehydroaromatization reaction. When the DBD plasma was applied to the Mo/HZSM-5 catalytic reactor, methane was converted at much lower temperatures than the conventional methane dehydroaromatization process. It was also confirmed that benzene selectivity could be improved by enhancing the interaction between plasma and catalyst. These results imply that the non-oxidative methane dehydroaromatization could occur effectively at milder conditions than the conventional process by using the optimized DBD plasma-catalyst hybrid system. Moreover, it was observed that the catalyst deactivation was effectively prevented when H2 gas was mixed with the reaction feed.

Coke accumulation and deactivation behavior of microzeolite-based Mo/HZSM-5 in the non-oxidative methane aromatization under cyclic CH4-H2 feed switch mode

The lifetime deactivation patterns of a microzeolite-based 6wt%Mo/HZSM-5 in the non-oxidative dehydroaromatization of methane were measured first at 1073K and a space velocity of 12,000ml/g-Cat/h in continuous CH4-feeding, cyclic 5min CH4–5min He feed switch and cyclic 5min CH4–5min H2 feed switch modes. Appearance of an obvious inflection point in the course of benzene selectivity under the cyclic CH4-H2 feed switch operation mode revealed the two stages deactivation feature of the catalyst: the deactivation was first slow and then accelerated. TG and TPO measurements of the spent samples were performed and the results exhibited strong similarity in the total coke content and the intensity and area of the low temperature TPO-COx peak. Then, a set of tests under the cyclic CH4-H2 feed switch mode over different time frames, followed by the TPO measurements of the spent samples, was performed to pursue the coke accumulation behavior of the catalyst over its lifetime. The low-temperature burning coke defined by the low-temperature TPO-COx peak was confirmed to be the dominant type of coke, reside on the external surface of the zeolite and in the inner mouth region of the zeolite channels, and accumulate over the catalyst lifetime at a constant rate. In parallel, a constant rate was also observed for the decrease of the benzene formation rate itself over the slow deactivation period. While these observations suggest the possible existence of a cause and effect relationship between the low-temperature burning coke and catalyst deactivation, a detailed discussion was made on the dominant effect of accumulation of graphite-like coke on the slow deactivation of catalyst.

Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM-5-Catalyzed Methane Dehydroaromatization.

Non‐oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H2/CO ratio close to two is the main side‐product of coke combustion. Using 13C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one‐fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.

A binder-free fluidizable Mo/HZSM-5 catalyst for non-oxidative methane dehydroaromatization in a dual circulating fluidized bed reactor system

This work highlights the need for a two beds type of circulating fluidized bed reactor system and a binder-free fluidizable Mo/HZSM-5 catalyst for processing non-oxidative methane dehydroaromatization. Also discussed is the process optimization by varying temperature, CH4 space velocity, the height of catalyst bed and the mean catalyst residence times in a CH4 convertor and the paired catalyst regenerator for the binder-free Mo/HZSM-5. Typical profiles of CH4 conversion and benzene yield versus time obtained under continuous catalyst regeneration mode are discussed to show the high activity and stability of the binder-free catalyst as well as its high resistance towards attrition at 1073K and continuous circulation conditions. Recommendations to obtain an advanced binder-free Mo/HZAM-5 catalyst are made in improving the coking resistance, hydrogen regeneration activity and benzene formation activity of the binder-free catalyst.