Catalysts For Methane Dehydroaromatization Research Articles

Rational Tailoring of Metal Precursor Interactions with the Zeolite Support in the Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Rational Tailoring of Metal Precursor Interactions with the Zeolite Support in the Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Methyl radical chemistry in non-oxidative methane activation over metal single sites

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Forced periodic operation via pulsing and feed switching to address methane dehydroaromatization catalyst deactivation challenge

Methane Dehydroaromatization is a one-step catalytic reaction that converts methane into aromatics and hydrogen with zero CO2 emission from the reaction stoichiometry. This work represents an engineering approach to address the rapid catalyst deactivation challenge by implementing forced periodic operation with shorter times at the scale of minutes. A systematic study was carried out by investigating pulse and periodic feed switching on a typical Mo/HZSM-5 catalyst. A fixed operating temperature and space velocity was maintained in both reaction and regeneration, while the operational time and type of regenerating molecules using reductive (H2) and oxidative (CO2) were investigated. For both modes of regeneration, CO2 was not an efficient oxidative molecule. Pulse feeding with hydrogen improved the catalyst stability, but it minimized the rate of aromatics production. The best performance was observed for the periodic switch with H2. Even a very short regenerating during 1 min H2 after 15 min reaction, was sufficient to achieve stable operation in the tested range. The global benzene production doubled compared to the base case which is a significant improvement reached by optimizing the mode of regeneration and conditions. A mathematical model was constructed and successfully used to simulate and explain the repeated reaction-regeneration cycles for different conditions. The result here is aiming to shed light on the opportunity of the forced periodic operation as an engineering strategy to mitigate the catalyst deactivation challenge and develop a reliable and stable MDA reactor operation.

Isomorphously Substituted [Fe,Al]ZSM-5 Catalysts for Methane Dehydroaromatization

Dehydroaromatization of methane (MDA) under non-oxidative conditions is a promising reaction for direct valorization of natural gas and biogas. Typically, Fe-modified ZSM-5 catalysts display low aromatic productivity and high coke selectivity in the MDA reaction. Herein, we show the benefit of starting from isomorphously substituted Fe-sites in [Fe,Al]ZSM-5 zeolites prepared by direct hydrothermal synthesis. Upon calcination, these samples contain predominantly isolated Fe3+ species, either atomically dispersed within the zeolite framework or anchored at exchange sites inside zeolite channels. In terms of the integral hydrocarbon productivity, [Fe,Al]ZSM-5 catalysts outperform Fe/ZSM-5, prepared by impregnation, as well as Mo/ZSM-5 catalysts with the same Si/Al ratio and molar metal loading. Operando X-ray absorption spectroscopy coupled with mass spectrometry (XANES-MS) demonstrates that the initial tetrahedral Fe3+ within the zeolite framework or at exchange sites are transformed into octahedral extraframework Fe2+ active sites during the MDA reaction and form small Fe2O3 clusters during oxidative regeneration. Combining activity measurements and operando thermogravimetry shows that the duration of the induction period, related to the formation of active hydrocarbon pool intermediates, strongly depends on the Fe dispersion and loading and can be used as a suitable descriptor for the MDA activity of [Fe,Al]ZSM-5. The shorter induction period of [Fe,Al]ZSM-5 in comparison to impregnated Fe/ZSM-5 can be linked to the higher methane conversion rate over highly dispersed Fe-sites and faster formation of active hydrocarbon pool intermediates.

Catalytic conversion of methane into benzene over ceria–zirconia‐promoted molybdenum‐supported zeolite for methane dehydroaromatization

The promotional effect of mixed ceria–zirconia oxides (CZO) over the Mo/HZSM‐5 catalyst for methane dehydroaromatization (MDA) reaction was studied. The surface and structural properties of the synthesized catalyst were thoroughly exemplified using various microscopic and spectroscopic tools, and the correlation between catalytic properties and its performance for MDA reaction is discussed. The impregnation of CZO solid solution on Mo/HZSM‐5 was monitored to give an excellent catalytic performance and improved benzene production rate (4.5 μmol/gcat.s) related to the conventionally prepared Mo/HZSM‐5 (3.1 μmol/gcat.s) catalyst. In addition, a significant reduction in coke formation was examined in the CZO‐modified Mo/HZSM‐5 catalyst. The prevailing comprehension for higher catalytic activity could be because of the redox properties of CZO‐deposited Mo/HZSM‐5, which acts as a selective oxygen supplier and performs hydrogen combustion during the reaction, which is indirectly probed by residual gas analysis‐mass spectroscopy, oxygen‐temperature programmed desorption, and hydrogen‐temperature programmed reduction analysis. The selective hydrogen combustion prevents the over‐oxidation of aromatic species formed during the reaction, whereas the generated steam helps in reducing the coke content deposition in the MDA reaction. Thus, the advantage of CZO‐incorporated Mo/HZSM‐5 is manifested as it promotes the reaction equilibrium to shift towards the formation of benzene which is favourable for MDA reaction.

Coke relocation and Mo immobilization in donut-shaped Mo/HZSM-5 catalysts for methane dehydroaromatization

Molybdenum-modified HZSM-5 catalysts are widely used for methane dehydroaromatization (MDA) but suffer from rapid deactivation due to coke formation.

Transition-Metal Catalysts for Methane Dehydroaromatization (Mo, Re, Fe): Activity, Stability, Active Sites, and Carbon Deposits

The most studied catalysts for methane dehydroaromatization (MDA)─Mo/ZSM-5─are not commercialized yet due to the rapid deactivation and insufficient activity. Catalytic systems based on Fe and Re are potential alternatives to Mo-containing zeolites. Here, we compare the catalytic performance of these catalysts as a function of metal type and its loading in ZSM-5 zeolite. The results show that the catalytic activity decreases in the order of Re/ZSM-5 > Mo/ZSM-5 > Fe/ZSM-5, while the catalyst stability decreases in the opposite order: Fe/ZSM-5 > Mo/ZSM-5 > Re/ZSM-5. The active metal species in the working catalysts were determined by operando X-ray absorption near-edge structure spectroscopy combined with mass spectrometry. We found that Re0 and Fe2+ species are the most likely active species for the catalytic dehydroaromatization of CH4 to aromatics in respective catalysts. Combining the pulse reaction technique with operando thermogravimetry analysis–mass spectrometry experiments, we demonstrate that the length of the induction period strongly correlates to the activity of the catalyst. The longer induction period of the Fe/ZSM-5 catalyst indicates the slow growth of hydrocarbon pool intermediates inside the zeolite pores and thus explains its poor catalytic performance. Finally, both the formation of hydrocarbon pool species and the activity of Fe/ZSM-5 can be improved by increasing the Fe loading, reaction pressure, and space velocity.

Acidity regulation for improved activity of Mo/HZSM-5 catalyst in methane dehydroaromatization

The Mo/HZSM-5 is a widely studied bifunctional methane dehydroaromatization (MDA) catalyst, and the acid site of zeolite is essential to the reaction process. In this study, the relationship between acid site control and MDA reaction performance is investigated. The Brønsted/Lewis acid sites have been toned by introducing promoters to the ZSM-5 zeolite. It shows that proper strong Brønsted acid sites are conducive to aromatic formation, while excessive Brønsted acid sites will lead to serious coke deposition. Thus, the MDA catalytic stability is further improved by passivation of partial Brønsted acid sites of Mo/HZSM-5 via coating Silicalite-1 on the catalyst surface. The optimized catalyst shows 95 % aromatic selectivity at initial with only 0.06 h−1 deactivation rate constant. The surface of HZSM-5 is reconstructed after hydrothermal treatment and the surface aluminum content is reduced. Moreover, the Silicalite-1 layer is beneficial to Mo species dispersion for preventing its aggregation and stabilizing the Mo-C active sites.

Are hierarchical zeolites good catalysts for Methane Dehydroaromatization? A critical analysis

The socioeconomic dependence on natural gas must be reduced to comply with the stricter carbon emissions requirements. However, avoiding natural gas from the future energy mix is easier said than done. To bridge this gap, lignocellulosic biomass-derived biomethane (aka. “renewable natural gas”) represents an elegant solution to this contemporary problem. Owing to the increased industrial interest in hierarchically structured zeolites for biomethane valorization, through this work, we explore the technical feasibility and challenges associated with methane dehydroaromatization over Mo-loaded on both microporous and hierarchical zeolite ZSM-5. Hierarchical zeolites were prepared using inexpensive and environmentally benign glucose as a secondary organic structure directing agent, leading to two shapes (coffins and hexagonal bars) with comparable physicochemical properties. Although a similar catalytic performance was obtained over (nano-sized) microporous and hexagonal bar-derived zeolites, coffin-shaped zeolite catalysts led to lower activity and slower deactivation. Herein, catalyst deactivation was governed by inter- and intra-particle diffusional properties.

Reaction regeneration cycle of Mo/HZSM-5 catalyst in methane dehydroaromatization with the addition of oxygen-containing components

Reaction regeneration cycle of methane dehydroaromatization (MDA) with oxygen-containing gas components was studied. XRD, BET, Raman, NH3-TPD, O2-TG/DTG, TEM and 27Al-NMR were used to monitor textural structure, Mo species, and acid sites of catalyst before and after regeneration. Addition of oxygen components can extend reaction time per pass but cannot prevent the deactivation of regenerated catalyst. MoO3 species, acidity, and texture properties of Mo/HZSM-5 was identical after first regeneration, which may account for complete recovery of methane conversion and aromatic formation. During regeneration, the inevitable increase in Mo species size disrupts carbon deposition and significantly reduces methane dissociation. The crystallinity and number of strong acid sites of Mo/HZSM-5 catalyst decreased to a stable level before the collapse of zeolite framework without hindering aromatic formation. The recovered large-sized Mo species may accelerate the collapse of zeolite framework, irreversibly destroying Mo/HZSM-5 catalyst.

Understanding the Preparation and Reactivity of Mo/ZSM-5 Methane Dehydroaromatization Catalysts.

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 Mo‐species 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.

Direct Conversion of Natural Gases to Aromatics in Solid-Oxide Membrane Reactor

Non-oxidative conversion of natural gases within a solid-oxide ceramic membrane reactor to aromatics is a promising electrochemical process that can synthesize aromatics and hydrogen through methane dehydroaromatization (MDA) reaction with increased process efficiency. The introduction of the electrochemical current density significantly promotes the reaction equilibrium and avoids product separation procedure. The catalyst development and integration into reactor are two important aspects to achieve the enhanced conversion rate and yield. In this work, a new MDA catalyst has been developed to enhance the conversion and selectivity towards aromatics and other useful olefins with enhance coke resistance. The integration of this catalyst into the membrane reactor is also evaluated to study the optimal catalyst/electrode combination method and to investigate the role of hydrogen extraction on improving the aromatics and olefin yielding. The integration work will further demonstrate the capability of this technology in upgrading natural gases to chemical intermediates and aromatics.

Si–OH Nest and Al Distribution of Silicalite-1 Core/Al-ZSM-5 Shell Zeolites for Methane Dehydroaromatization Reaction

Silicalite-1 core/Al-ZSM-5 shell zeolite crystals were prepared in various sizes for use as catalysts in methane dehydroaromatization (MDA), and the growth kinetics and corresponding physicochemical properties of this core–shell zeolite were investigated. Al-ZSM-5 was grown on silicalite-1 seeds at various Si/Al ratios. Core–shell catalysts of all size variations exhibited similar deactivation trends in the MDA reaction, with minor changes in aromatic yields despite clear differences in reaction channel lengths and acid-site properties. This outcome was shown to originate from the unique growth kinetics of the Al-ZSM-5 layer on silicalite-1 seeds, in which the Al species in the sol used in the synthesis were consumed quickly during the early aggregative growth period. This led to an interesting spatial distribution of Al in the Al-ZSM-5 layer, in that the inner layer was relatively Al-rich. This distribution is advantageous because it can inhibit coke deactivation, which often occurs at the catalyst surface during MDA. However, a substantial quantity of Si–OH nests, which inhibit the effective loading of Mo species at the acid sites of the crystals, were detected in the microstructural analysis of large crystals. Therefore, this study shows that silicalite-1 core/Al-ZSM-5 shell zeolites can be prepared for use as coke-resistant catalysts for the MDA reaction. Further work is required, however, to design a synthesis method which reduces the number of Si–OH nests formed.

Direct observation of the active sites in methane dehydroaromatization by NMR

Direct observation of the active sites in methane dehydroaromatization by NMR

Active Sites and Induction Period of Fe/ZSM-5 Catalyst in Methane Dehydroaromatization

Active Sites and Induction Period of Fe/ZSM-5 Catalyst in Methane Dehydroaromatization

Enhanced reactivity and stability in methane dehydro-aromatization over Mo/HZSM-5 physically mixed with NiO

While Mo/HZSM-5 is known as a catalyst for methane dehydro-aromatization (MDA), low activity and rapid deactivation have been considered issues to be overcome. In order to improve catalytic performance, NiO was physically mixed with Mo/HZSM-5, which significantly enhanced the yield for aromatic compounds (benzene and toluene), and improved catalytic stability. Combined XRD, CH4–TPSR, CO chemisorption, TEM-EDS, NH3–TPD, pyridine FT–IR, TPO, visible Raman, and N2 physisorption analysis confirmed that NiO promoted the dispersion of MoCx active sites and suppressed coke formation on MoCx, by converting coke precursors to carbon nanotube (CNT) on itself. As a result, after the reaction, the pore size and pore volume did not change much, which is attributed to the improved stability. This study demonstrates that physically mixed NiO can act as an excellent catalyst promotor for MDA.

Study on attrition of spherical-shaped Mo/HZSM-5 catalyst for methane dehydro-aromatization in a gas–solid fluidized bed

As a potential methane efficient conversion process, non-oxidative aromatization of methane in fluidized bed requires a catalyst with good attrition resistance, especially in the states of high temperature, long-time rapid movement and chemical reaction. Existing evaluation methods for attrition resistance, such as ASTM D5757 and Jet Cup test, are targeted for fresh catalysts at ambient temperature, which cannot well reflect the real process. In this study, spherical-shaped Mo/HZSM-5 catalyst prepared by dipping and spray drying was placed in a self-made apparatus for attrition testing, in which the catalyst attrition under different system temperatures, running time and process factors was investigated with percent mass loss (PML), particle size-mass distribution (PSMD) and scanning electron microscope (SEM). Carbon deposition on the catalyst before and after activation, aromatization and regeneration was analyzed by thermogravimetry (TG), and the attrited catalysts were evaluated for methane dehydro-aromatization (MDA). The results show that the surface abrasion and body breakage of catalyst particles occur continuously, with the increase of system temperature and running time, and make the PML rise gradually. The process factors of activation, aromatization and regeneration can cause the catalyst attrition and carbon deposits, which broaden the PSMD in varying degrees, and the carbon-substances on catalysts greatly improve their attrition resistance at high temperature. Catalyst attrition has a certain influence on its catalytic performance, and the main reasons point to particle breakage and fine powder escape.

Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane Dehydroaromatization: Insights from Solid-State NMR Spectroscopy.

Methane dehydroaromatization (MDA) on Mo/ZSM-5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM-5 is a challenge for applications. Herein, using 1 H{95 Mo} double-resonance solid-state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM-5 catalyst by direct observation of internuclear spatial interaction between Brønsted acid site and Mo species in zeolite channels. The acidic proton-Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM-5 channels is monitored by detecting their host-guest interactions with both active Mo sites and Brønsted acid sites via 1 H{95 Mo} double-resonance and two-dimensional 1 H-1 H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Dual Active Sites on Molybdenum/ZSM‐5 Catalyst for Methane Dehydroaromatization: Insights from Solid‐State NMR Spectroscopy

AbstractMethane dehydroaromatization (MDA) on Mo/ZSM‐5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM‐5 is a challenge for applications. Herein, using1H{95Mo} double‐resonance solid‐state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM‐5 catalyst by direct observation of internuclear spatial interaction between Brønsted acid site and Mo species in zeolite channels. The acidic proton–Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM‐5 channels is monitored by detecting their host–guest interactions with both active Mo sites and Brønsted acid sites via1H{95Mo} double‐resonance and two‐dimensional1H–1H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Coking mechanism of Mo/ZSM-5 catalyst in methane dehydroaromatization

Methane dehydroaromatization is a potential technique in converting natural gas into value-added aromatics and clean hydrogen. However, severe coking hinders industrial application of conventionally prepared Mo/ZSM-5 catalyst for methane dehydroaromatization. Usually, Bronsted acid sites (BAS) were considered the key for coking. To gain deeper insight into the coke, here we present a simple post-impregnation of Na+ to Mo/HZSM-5 to regulate its surface acidity. Influences of BAS on catalytic performance and coking behavior were thus exclusively studied. Na+ modified catalysts exhibit lower naphthalene and higher C2 selectivity, while selectivity for benzene is not obviously changed. These results contradict conventional bi-functional pathway that methane is dehydrogenated on Mo while BAS act as the center for intermediates cyclization. Characterizations and calculations indicate restricted growth of hydrocarbon pools confined in zeolite channels of Na+ modified catalysts. Hence, it is proposed that aromatization is the intrinsic property of Mo, and the Mo-associated carbonaceous species act as precursors for both coke and aromatics production.