Non-oxidative Aromatization Of Methane Research Articles

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.

Experimental and DFT analysis of the acid and reduction properties of Fe-Cu/ZSM-5

Methane is a well-established feedstock in the chemical industry with a wide range of sources from natural gas to methane hydrates. In previous works, Fe based catalysts impregnated with Cu or Zr in ZSM-5 were tested for the non-oxidative aromatization of methane to benzene, the observed results regarding the induction period and coke deposition were promising. In this work, the interactions between Fe and Cu that could promote these properties were studied by synthesizing Fe-Cu catalysts supported in ZSM-5 with varying Cu/Fe ratios and carrying characterizations by experimental methods (XRD, BET, TPR-H2, TPD-NH3, Raman and TEM) and computational methods (DFT calculations). The TPR results show a promotive effect between Cu and Fe which increases the H2 consumption. Results from TPD tests show that the acid strength increases with Cu loading, and a new peak is observed above 600 °C. The DFT calculations were carried out for a system consisting of a [M(μ−O)2M]2+ (M = Fe, Cu) cation deposited in two Brønsted acid sites in the γ site of the zeolite. Computational results indicate an increased stability of the reduced form of Fe when bonded with Cu, corroborating with the TPR results. Moreover, results for charge calculation show that the presence of Cu leads to a charge depletion in Fe when bonded in the same site, which explains the increased acid strength. The acid and reduction properties of the catalyst can be controlled using different Cu/Fe ratios and tuned for use in methane non-oxidative aromatization and other reactions.

Ammonia-basified 10 wt% Mo/HZSM-5 material with enhanced dispersion of Mo and performance for catalytic aromatization of methane

Mo/HZSM-5 is an excellent catalyst for nonoxidative aromatization of methane, but it tends to deactivate rapidly due to severe coking. To date much effort has been devoted to enhancing activity and stability of Mo-impregnated HZSM-5 zeolite with Mo loading of 6 wt% or lower in methane aromatization. For this aromatization reaction, this work first reports a significant improvement in the catalytic performance of 10 wt% Mo/HZSM-5 material after preparation via impregnating HZSM-5 zeolite with ammonia-basified molybdate aqueous solution and pretreatment with a CH4/Ar/He (9:1:10, v/v) mixture. After 38 h of continuous reaction at 700 °C and 1500 mL g−1 h−1 (90 vol% CH4/Ar), this 10 wt% Mo-impregnated catalyst is still able to give an aromatic yield of 7% and a CH4 conversion of 9.1%. This enhanced catalytic performance of this 10 wt% Mo sample is attributed to the ammonia basification of impregnating solution in catalyst preparation, which leads to a larger amount of highly dispersed Mo species in the zeolitic channels and a less amount of Brønsted acid sites remained on the Mo/HZSM-5 catalyst.

Impact of the presence of Mo carbide species prepared ex situ in Mo/HZSM-5 on the catalytic properties in methane aromatization

The catalytic activity of HZSM-5 supported Mo-oxide (MoOx) and Mo-carbide (MoCy) for methane aromatization was studied using a packed-bed microreactor. MoOx/HZSM-5 catalysts with 3, 6, 10, and 12 wt.% Mo loading were prepared by incipient wetness impregnation method followed by calcination at 500 °C. The MoCy/HZSM-5 catalysts were prepared ex situ by treating the oxide catalysts by temperature-programmed reduction and carburization. The MoCy/HZSM-5 catalysts show significantly higher activities and stability compared to those of the MoOx/HZSM-5 catalysts. Unlike the oxide catalysts, not only methane conversion and benzene yield improve with higher Mo loading but also the deactivation rate becomes much slower for the carbide catalysts. The optimum carbide catalyst has a Mo loading of 10 wt% The catalysts were characterized by XRD, N2 adsorption, TPR, NH3-TPD, TPO,TGA and 27Al-NMR. The results show that the oxide catalysts at a high loading face pore blockage after few hours of reaction, which makes the active sites inaccessible to CH4. Carbide catalysts, on the other hand face no pore blockage even after a long period of reaction, making them much better catalysts than the oxides for nonoxidative methane aromatization reaction.

Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3- under High-Temperature Conditions.

The underlying mechanism for non-oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C-C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC3- ) under high-temperature conditions to produce C-C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well-rationalized by quantum chemistry calculations. This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.

Non-oxidative methane aromatization by bimetallic (La, Co, Pd and Pt) M-Zn/HZSM-5: Impact of propane addition

The non-oxidative aromatization of methane was studied over M-Zn/HZSM-5 (M=La, Co, Pd, and Pt) catalysts using propane as a co-reactant. The catalysts were characterized by BET, SEM, NH3-TPD and XRD techniques. Catalytic tests were performed in a fixed-bed reactor at 823 K, using a mixture of methane, propane, and nitrogen with ratio of 6/1/1.3, respectively. Propane conversion was above50% and remained stable in the first 12 hours on stream; however, the methane conversion rapidly dropped from a high of about 10% to zero within 4 hours, implying the absence of a stoichiometric reaction between the reactants. Both zinc and the second metal (M) had a beneficial effect on aromatic selectivity and Pt-Zn/HZSM-5 exhibited the highest aromatic yields and catalyst stability. The results of the present study showed that co-feeding of methane with propane cannot successfully induce methane to participate in aromatization reactions.

Direct non-oxidative methane aromatization over gallium nitride catalyst in a continuous flow reactor

The direct non-oxidative methane aromatization over commercially available gallium nitride powder has been investigated for the first time in a fixed bed reactor under continuous operation. Gallium nitride catalyst is highly active. High reaction temperatures (650–710°C) are needed to initiate benzene formation due to the small surface area (8m2g−1) of the gallium nitride material, thermodynamic constraints and the low residence time (1–4s). Besides benzene and toluene, C2 species and coke were formed.

Overgrowth of lamellar silicalite-1 on MFI and BEA zeolites and its consequences on non-oxidative methane aromatization reaction

The combination of two compositionally and/or structurally different zeolites into one single zeolite composite particle is a potential approach to integrate advantages of different zeolite structures for desirable properties and applications. In the present study, we report the overgrowth of lamellar mesoporous silicalite-1 on the commercial microporous MFI and BEA zeolites, respectively, to render hierarchical meso-/microporous lamellar silicalite-1/MFI (L-Si/MFI) and lamellar silicalite-1/BEA (L-Si/BEA) zeolite composites via hydrothermal crystallization of lamellar silicalite-1 with the assistance of a diquaternary ammonium template. Epitaxial growth of lamellar silicalite-1 on commercial bulk MFI was observed, resulting in the L-Si/MFI zeolite composite as porcupine sensory message ball with nerve-stimulating silicalite-1 bumps extended from the MFI particle. In the L-Si/BEA zeolite composite, the lamellar silicalite-1 was laid over the surface of or partially interdigitated into the commercial bulk BEA particle, forming a BEA nanosponge structure connected to lamellar silicalite-1 nanosheets. The resultant interconnected micro- and mesoporosity in the L-Si/MFI and L-Si/BEA composite zeolites allowed facile mass transport of bulky molecules. The acid sites sitting on the external surface of commercial MFI and BEA zeolites were partially passivated by the lamellar silicalite-1. The consequences of improved mass transport and passivation of external acid sites on the catalytic performance of these zeolite composites were tested in 2 wt% molybdenum (Mo) loaded L-Si/MFI and L-Si/BEA for direct non-oxidative methane aromatization reaction, which showed higher methane conversion and hydrocarbon product formation as well as higher selectivity to naphthalene and coke in comparison with 2 wt% Mo-loaded commercial MFI and BEA catalysts.

Improved performance of hierarchical porous Mo/H-IM-5 catalyst in methane non-oxidative aromatization

Two novel hierarchical porous IM-5-S and IM-5-M materials were synthesized using mesoporous silica SBA-15 and MCM-48 as the silica sources, and for comparison conventional IM-5-C zeolite was also synthesized with the same synthesis composition. IM-5-S and IM-5-M samples exhibited larger crystal sizes and different textural properties than the conventional IM-5-C zeolite. Moreover, Mo-modified catalysts, Mo-IM-5-S, Mo-IM-5-M, and Mo-IM-5-C, were prepared for the non-oxidative aromatization of methane. The physical properties and acidities of the samples were characterized by XRD, SEM, TEM, BET, and NH3-TPD. Compared with Mo-IM-5-C, the Mo-IM-5-S and Mo-IM-5-M catalysts showed higher yields of aromatics and better stabilities. The preferable catalytic behaviors of Mo-modified mesoporous IM-5 catalysts may be attributed to the generation of secondary mesoporous systems within the zeolite crystals, which may influence the location and state of the active Mo species, simultaneously lead to easier access to the active sites for reactants and be favorable for the diffusion of products formed in the microporous channels during the methane aromatization reaction.

Facile preparation of hierarchically porous IM-5 zeolite with enhanced catalytic performance in methane aromatization

A novel hierarchically porous IM-5-H zeolite material was prepared through one-step crystallization route by means of adjusting the synthesis parameters without introducing any secondary template. The hierarchical IM-5-H zeolite is quite different from the conventional IM-5-C in morphology, textural and acidic properties. After loading Mo, the Mo-IM-5-H catalyst exhibits high activity and stability in non-oxidative aromatization of methane, with a methane conversion of 13.1% and aromatics yield of 7.5%, owing to the mesopores in the IM-5-H zeolite crystals. This work provides a simple way to synthesize hierarchical IM-5 zeolite and expands the application of micro-mesoporous composite material in methane dehydroaromatization.

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.

Active phase, catalytic activity, and induction period of Fe/zeolite material in nonoxidative aromatization of methane

Catalytic dehydroaromatization of methane has been investigated over 2–6wt.% Fe-modified HZSM-5 or HMCM-22 zeolite in the absence of an added oxidant. After pretreatment in a CH4/Ar/He mixture, Fe/HMCM-22, and more significantly, Fe/HZSM-5 exhibited high catalytic activity in nonoxidative conversion of methane to aromatic hydrocarbons. This conversion actually takes part in three consecutive steps, oxidation of methane by iron oxides, decomposition, and aromatization of methane. During this conversion, XPS and XRD results showed the gradual reduction of Fe2O3 to metallic Fe, followed by the carburization of the metallic species. The fact that no aromatic products were detected until iron carbide was formed demonstrated that carburized Fe was the active species for the aromatization of methane. Under the same reaction conditions, both metallic Fe and carburized Fe were highly effective for activation of methane, but they catalyzed dehydrogenation of methane to carbonaceous materials and to aromatics, respectively. Difference in the catalytic properties of those two Fe species suggested that carburized Fe could stabilize carbene, the primary intermediate of activated methane, possibly via formation of a metal–carbene complex, CFeCH2, so that carbene was able to dimerize into ethylene, which was then oligomerized to aromatics on the acid sites of the zeolite.

NGU: Development of a two‐bed circulating fluidized bed reactor system for nonoxidative aromatization of methane over Mo/HZSM‐5 catalyst

A laboratory‐scale two‐bed circulating fluidized bed reactor system was developed for processing the Mo/HZSM‐5 catalyzed methane dehydroaromatization reaction in continuous catalyst‐regeneration mode. A binder‐free, spherical‐shaped fluidizable Mo/HZSM‐5 catalyst was tested in the system to verify its high operativity and industrial applicability. The main operation parameters of the catalyst including temperature, space velocity, the height of catalyst bed, and the mean residence time were optimized in micro fixed and fluidized bed reactors in either continuous CH4 feed or cyclic CH4‐H2 switching feed mode. Its long‐term activity and stability was evaluated in the two‐bed circulating fluidized bed reactor system under continuous regeneration mode. At 1073 K and a space velocity of ∼3000 mL/g/h the catalyst has proven to be capable of providing a stable benzene yield of 12% over a cumulative period of 1600 min. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 325–333, 2016

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

The deactivation of Mo/HZSM-5 during the non-oxidative methane aromatization (MDA) reaction that yields benzene and hydrogen was investigated. Catalysts were recovered from the reactor after pre-activation and after increasing time on stream in methane. The physico-chemical properties of the spent catalysts were characterized in detail by Ar physisorption, 27Al MAS NMR and X-ray photoelectron spectroscopy. The nature of the carbon deposits was determined by UV Raman spectroscopy and TGA, and the size and location of the Mo-carbide particles by TEM and STEM-HAADF. The results show that the main cause for catalyst deactivation is the formation of a carbonaceous layer at the external zeolite surface. This layer is made up from polyaromatic hydrocarbons and decreases the accessibility of the Brønsted acid sites in the micropores. At the same time, the decreased interaction of the Mo-carbide particles with the external zeolite surface results in their sintering. The lower Mo-carbide dispersion decreases methane conversion rates. The decreased accessibility of the Brønsted acid sites shifts the selectivity from benzene to unsaturated intermediates formed on the Mo-carbide particles. Silylation of the external surface mainly results in lower rate of coke formation at the external surface, slowing down catalyst deactivation.

Effect of the particle size of MoO3 on the catalytic activity of Mo/ZSM-5 in methane non-oxidative aromatization

The catalytic results confirmed that nano MoO3 modified ZSM-5 zeolites showed higher methane conversion and aromatics yield (14.1% vs. 9.5%) than micron MoO3 modified ZSM-5 (11.2% vs. 7.3%).

Methane Activation by Heterogeneous Catalysis

Methane activation by heterogeneous catalysis will play a key role to secure the supply of energy, chemicals and fuels in the future. Methane is the main constituent of natural gas and biogas and it is also found in crystalline hydrates at the continental slopes of many oceans and in permafrost areas. In view of this vast reserves and resources, the use of methane as chemical feedstock has to be intensified. The present review presents recent results and developments in heterogeneous catalytic methane conversion to synthesis gas, hydrogen cyanide, ethylene, methanol, formaldehyde, methyl chloride, methyl bromide and aromatics. After presenting recent estimates of methane reserves and resources the physico-chemical challenges of methane activation are discussed. Subsequent to this recent results in methane conversion to synthesis gas by steam reforming, dry reforming, autothermal reforming and catalytic partial oxidation are presented. The high temperature methane conversion to hydrogen cyanide via the BMA-process and the Andrussow-process is considered as well. The second part of this review focuses on one-step conversion of methane into chemicals. This includes the oxidative coupling of methane to ethylene mediated by oxygen and sulfur, the direct oxidation of methane to formaldehyde and methanol, the halogenation and oxy-halogenation of methane to methyl chloride and methyl bromide and finally the non-oxidative methane aromatization to benzene and related aromates. Opportunities and limits of the various activation strategies are discussed. .

Effect of mesopore structure of TNU-9 on methane dehydroaromatization

A series of micro-mesoporous molecular sieves TNU-9-x (TNU-9 Taejon National University no. 9) were prepared by a hydrothermal reaction method for non-oxidative aromatization of methane. The physical properties of the synthesized samples were characterized by XRD, SEM, TEM, BET, NH3-TPD and FT-IR spectroscopy. Characterization results suggested that the micro-mesoporous molecular sieves TNU-9-x exhibited similar geometrical shapes, but different pore properties with conventional microporous molecular sieve TNU-9. Catalytic tests indicated that Mo–TNU-9-20 showed excellent catalytic performance with a 14.9% conversion of methane and a 9.9% yield of aromatics. In addition, the catalytic stability of Mo–TNU-9-20 was better than the conventional microporous material Mo–TNU-9. The improved catalytic behaviour might be attributed to the generation of a secondary mesoporous system within the hierarchical pore materials.

Synthesis of zeolite IM-5 under rotating and static conditions and the catalytic performance of Mo/H-IM-5 catalyst in methane non-oxidative aromatization

The hydrothermal crystallization of zeolite IM-5 was investigated under rotating (denoted as IM-5-R) and static (denoted as IM-5-S) synthesis conditions. Mo-modified catalysts (Mo-IM-5-R and Mo-IM-5-S) were prepared for the methane non-oxidative aromatization. The physical properties and acidities of the samples were characterized by XRD, SEM, BET and IR spectroscopy. Compared with Mo-IM-5-S, the Mo-IM-5-R catalyst showed both a higher conversion of methane and higher selectivity to benzene in methane aromatization. A higher catalytic activity of Mo-IM-5-R may be attributed to the preferable textural properties and acidities of zeolite IM-5-R. Moreover, the catalyst prepared by the physical mixing method exhibited lower initial activity, but better stability for methane aromatization than that prepared by the impregnation method.

Synthesis of Zeolite IM-5 under Rotating and Static Conditions and the Catalytic Performance of Mo/H-IM-5 Catalyst in Methane Non-Oxidative Aromatization

Synthesis of Zeolite IM-5 under Rotating and Static Conditions and the Catalytic Performance of Mo/H-IM-5 Catalyst in Methane Non-Oxidative Aromatization

A comparison study of mesoporous Mo/H-ZSM-5 and conventional Mo/H-ZSM-5 catalysts in methane non-oxidative aromatization

The mesoporous ZSM-5-S and ZSM-5-M samples were synthesized by using ordered mesoporous carbon (CMK-3) and disordered carbon rods (C-MCM-41) as the hard template, respectively, and for comparison, conventional ZSM-5-C was also synthesized with the same synthesis composition and procedure except for the presence of carbon template. The mesoporous ZSM-5 samples exhibited similar geometrical shape, but different pore properties than the conventional ZSM-5. Moreover, Mo-modified catalysts, Mo-ZSM-5-C, Mo-ZSM-5-S and Mo-ZSM-5-M, were prepared for the non-oxidative aromatization of methane. The physical properties and acidities of the samples were characterized by XRD, SEM, TEM, BET and IR spectroscopy. Compared with Mo-ZSM-5-C, the Mo-ZSM-5-S and Mo-ZSM-5-M catalysts showed similar conversions of methane, but higher yields of aromatics. In addition, Mo-ZSM-5-S and Mo-ZSM-5-M were more stable than Mo-ZSM-5-C. The exceptional catalytic behavior of Mo-modified mesoporous ZSM-5 catalysts may be attributed to the generation of secondary mesoporous systems within the zeolite crystal, which may lead to easier access to the active sites for reactants and be favorable for the diffusion of larger molecule product formed in the microporous channels during the methane aromatization reaction.