Methane Aromatization Reaction Research Articles

A Convenient Synthesis of Core‐shell ZSM‐5@MoO3 Catalyst for Enhanced Catalytic Properties in Methane Aromatization

AbstractA novel core‐shell ZSM‐5@MoO3 catalyst was successfully synthesized by the hydrothermal coating strategy, and its physicochemical properties and catalytic properties in methane aromatization were investigated using a physically‐blended ZSM‐5/MoO3 catalyst as a comparison. Multitechniques including XRD, SEM, TEM, EDS, N2 adsorption, ICP, IR and IG were employed to investigate the characteristics of methane aromatization reaction and the deactivation of carbon deposits. At the reaction temperature of 700 °C and a space velocity of 2400 mL g−1 h−1, methane conversion and aromatics selectivity of the core‐shell ZSM‐5@MoO3 catalyst were 11.3 % and 88.1 %, respectively, which were higher than those of the physically‐blended ZSM‐5/MoO3 catalyst by 3.1 % and 14.8 %, respectively. The special reaction pathway of core (ZSM‐5, acid sites)‐shell (MoO3, Mo sites) and the relatively close proximity between MoO3 and ZSM‐5 were mainly responsible for the excellent properties in the methane aromatization reaction. At the same time, the formation of the micro‐mesoporous property accelerated the mass‐transfer rate and inhibited the formation of carbonaceous deposition, greatly improving the stability of methane aromatization. Thanks to its core‐shell structure, ZSM‐5@MoO3 catalyst showed the excellent properties in methane aromatization.

Coupling Oil Increase by Coal Liquefaction Residue Pyrolysis and Coal Pyrolysis Depolymerization Based on Big Data

With the continuous improvement of big data technology, my country’s coal liquefaction technology has also continued to mature, maintaining a stable industrial development. Traditional coal pyrolysis technology for tar production with the purpose of increasing tar production, such as coal hydropyrolysis, has problems such as high cost of pure hydrogen atmosphere and complex process and equipment operations, which severely restrict its industrial operation process. Based on this, this paper proposes a new technology of coal pyrolysis and depolymerization coupled with oil increase by using hydrogen precipitated by the condensation polymerization reaction at relatively high temperature under big data technology to study the effect of this process on coal pyrolysis for oil production. Experiments show that at 700°C, the tar yield reaches 21.5wt.%, which is 6% and 7% higher than the pyrolysis tar yield under the same conditions under hydrogen and nitrogen atmospheres. At 600°C, the methane aromatization reaction is relatively weak, and it can be seen that the tar yield is only slightly higher than that under hydrogen and nitrogen atmospheres. As the temperature of the methane anaerobic aromatization reaction increases, the equilibrium conversion rate increases accordingly. Therefore, as the reaction temperature increases, the tar yield also begins to increase.

Methane dehydroaromatization over Mo and W catalysts supported on ZSM-5

Methane aromatization reaction to produce benzene using tungsten and molybdenum catalysts supported on ZSM-5 was investigated at 800 oC. Catalysts were prepared by impregnation of tungsten and molybdenum salts on ZSM-5 zeolite with various metal loadings in the range of 2-10 wt. %. In order to obtain the catalytic structures before and after the reaction, catalysts were characterized by XRD and FTIR analysis. It was indicated from reactor tests that increase of metal loading on the catalyst surface leads to increase methane conversion (1.1% and 3.2% for 2W/ZSM-5 and 6 W/ZSM-5, and 2.4% and 4.8% for 2W/ZSM-5 and 6 W/ZSM-5, respectively, at time of stream equals 120 min). It was also concluded that Mo catalysts show higher activity and stability than W (methane conversion of 3.2 and 9 % using 10Mo/ ZSM-5 and 10 W/ZSM-5 catalysts respectively, at time of stream equals 100 min), and increase of Mo loading leads to enhancement of catalytic activity and methane conversion, which indicate initial activation of methane is occurred on metal sites of catalysts. This activation leads to occur the later reactions and production of final benzene. These conditions confirm the two-factor mechanism which include two stages: i) hemolysis break of C-H bond and CH3 radical formation and then ethylene formation, and ii) cyclization of ethylene species in the presence of acidic sites within the zeolite channels. Investigations on mesoporous HMS support showed no aromatic production, which show increase of support channel diameter leads to reduce the possibility of ring formation.

Elucidating the nature of Mo species on ZSM-5 and its role in the methane aromatization reaction

The valorization of methane is one of the most important goals during the transition period to the general use of renewable energies.

Enhancing Methane Aromatization Performance by Reducing the Particle Size of Molybdenum Oxide.

Efficient use of natural gas to produce aromatics is an attractive subject; the process requires catalysts that possess high-performance active sites to activate stable C–H bonds. Here, we report a facile synthetic strategy to modify HMCM-49 with small molybdenum oxide nanoparticles. Due to the higher sublimability of nano-MoO3 particles than commercial MoO3, they more easily enter into the channels of HMCM-49 and associate with Brønsted acid sites to form active MoCx-type species under calcination and reaction conditions. Compared with commercial MoO3 modified MCM-49, nano-MoO3 modified MCM-49 exhibits higher methane conversion (13.2%), higher aromatics yield (9.1%), and better stability for the methane aromatization reaction.

New Data on the Ability of Alumina–Platinum Systems to Catalyze the Methane Aromatization Reaction under Nonoxidizing Conditions

The state and size of the metal on the surface of aluminum oxide and the acidic properties of the support depending on the concentration of supported platinum were studied in this work. The effect of the Pt content of the alumina–platinum catalyst on the activation (chemisorption) of methane was investigated, and the composition of hydrocarbon fragments formed in this case was calculated. The sample most active in a reaction of the joint conversion of methane with n-pentane, which was performed for the production of aromatic hydrocarbons under nonoxidizing conditions, was established. The effect of the temperature of n-pentane supply to the reaction atmosphere was studied for the 0.6%Pt/Al2O3 catalyst. The degree of enrichment of the resulting aromatic hydrocarbons and the quantity of incorporated methane activated on the catalyst surface were determined with the use of isotope mass spectrometry.

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.

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.

Preparation of Mo/HZSM-5/Bentonite Catalyst for Methane Aromatization in a Fluidized Bed Reactor

Abstract Methane aromatization is a promising technology for the transformation of natural gas to added-value products. The main objective of this work was to obtain a catalyst with suitable performance and good mechanical stability for methane aromatization reaction in fluidized bed reactors. The selected catalyst was Mo/H-ZSM-5/bentonite mixture. Mo/ZSM-5 was chosen as the active material, since it provides good selectivity to aromatics but the particle size of the zeolite was too small for operation in a fluidized bed and a binder was needed. We prepared two series of catalysts with two different zeolites. We tested several heating velocities (1, 7 and 10 °C min‒1) in the different stages of catalyst synthesis. Methane conversion and selectivity to aromatic products improved when using gentle thermal treatments, increasing 2% and 10%, respectively, for the best catalyst tested.

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.

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 addition of a second metal in Mo/ZSM-5 catalyst for methane aromatization reaction under elevated pressures

Non-oxidative methane dehydro-aromatization (MDA) over transitional metal-doped zeolites (i.e. ZSM-5) has a potential to be an alternative for methane transformation processes to higher hydrocarbons. Despite of systematic studies of the effects of the catalyst composition, preparation procedure, pretreatment, and the reaction conditions systematically followed since the pioneering works of the Chinese group in 1993, the suppression of fast deactivation of catalyst by coke deposition was still not solved successfully.In the present work the long-term MDA experiments at 973K and elevated pressures up to 600kPa were carried out with metal-doped Mo/ZSM-5 catalysts. Several Mo-based catalysts doped with various amount of Co, Mn, and Ce were prepared and tested in the MDA reaction. Experimental results are presented in terms of profiles of methane conversion as well as ethylene, benzene, and toluene concentrations as a function of time on stream, accompanied by the TGA analysis of the coke residue accumulated on the spent catalysts. The amount and nature of coke deposits which were influenced by introduction of different metals were also investigated. The Co content of 0.8wt% in Co-Mo/ZSM-5 catalyst was evaluated as optimal for total benzene production. Prolonged experiments showed the evident positive effect of higher reaction pressure on catalyst stability for all tested catalysts. Moreover, the Co–Mo/ZSM-5 catalyst showed better resistance to the deactivation by coke formation in comparison to Mo/ZSM-5 and M–Mo/ZSM-5 (where M=Mn, Ce) catalysts. In addition, the Ce-modified catalyst showed comparable higher production of toluene at elevated pressures.

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.

Catalytic performance of zeolite ITQ-13 with 9- and 10-member rings for methane dehydroaromation

ITQ-13, a tridirectional medium-pore zeolite containing 9- and 10-member-ring pores, has been successfully synthesized by a hydrothermal method. The catalytic performance of Mo-ITQ-13 is worse than with Mo-ZSM-5 in the methane aromatization reaction. It has been shown that 9-member-ring channels favor coke and are a disadvantage in methane non-oxidative aromatization.

Methane Aromatization over Cobalt and Gallium -Impregnated HZSM-5 Catalysts

The influence of the catalyst acidity, the ratio of cobalt in the catalyst on the conversion of methane and the stability were evaluated using a fixed-bed microreactor at atmospheric pressure and at a flow rate of 1500 mL/g h (GHSV 600 h−1). The reaction was conducted at 973 K and 1023 K over gallium and cobalt -impregnated HZSM-5 catalysts. The 2%Ga–2% Co/HZSM-5 catalyst exhibited remarkable stability with no significant deactivation for 100 h on stream, and yielded a maximum conversion of methane to benzene equal to 9.9%. These catalysts were thoroughly characterized using XRD, N2 adsorption measurements, TPD of NH3 and FT-IR. The acidity changes severely affected aromatization, and resulted in drastic modifications in product distribution. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be decreased by adding Ga and Co, are shown to be related with the aromatic carbonaceous deposits on the catalysts. After adding Ga and Co the strength of Lewis acid sites of the catalyst increased. But the total amount of the acidity on the catalyst decreased.

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

The acid properties of Mo/HMCM-22 catalyst, which is the precursor form of the working catalyst for methane aromatization reaction, and the synergic effect between Mo species and acid sites were studied and characterized by various characterization techniques. It is concluded that Brønsted and Lewis acidities of HMCM-22 are modified due to the introduction of molybdenum. We suggest a monomer of Mo species is formed by the exchange of Mo species with the Brønsted acid sites. On the other hand, coordinate unsaturated sites (CUS) are suggested to be responsible for the formation of newly detected Lewis acid sites. Computer modelling is established and coupling with experimental results, it is then speculated that the effective activation of methane is properly accomplished on Mo species accommodated in the 12 MR supercages of MCM-22 zeolite whereas the Brønsted acid sites in the same channel system play a key role for the formation of benzene. A much more pronounced volcano-typed reactivity curve of the Mo/HMCM-22 catalysts, as compared with that of the Mo/HZSM-5, with respect to Mo loading is found and this can be well understood due to the unique channel structure of MCM-22 zeolite and synergic effect between Mo species and acid sites.

Effects of Co-fed O 2 and CO 2 on the deactivation of Mo/HZSM-5 for methane aromatization

The aromatization of methane in the presence of H 2, CO and CO 2 over a 2 wt.% Mo/HZSM-5 catalyst was studied. The results were compared with those for the aromatization reaction in pure methane and with added O 2. The addition of O 2 up to 5.3% or CO 2 up to 12.8% reduces deactivation so that, at the reaction temperature of 770 °C, an aromatic yield of ca. 4% can be maintained for 6 h; in the absence of the gaseous additive, however, the catalyst would have been completely deactivated for aromatic formation within 4 h. XPS analysis revealed that, in the presence of O 2 or CO 2, the molybdenum oxide supported on the HZSM-5 located at the reactor inlet was not converted to molybdenum carbide, whereas in the zone away from the reactor inlet, Mo 2C was found. Investigation by temperature-programmed surface reaction showed that the production of aromatic compounds was always preceded by the reaction of molybdenum oxide with methane to form Mo 2C and CO. The beneficial effect of adding CO 2 and O 2 in low concentration was mainly attributed to the formation of CO and H 2 by oxidation and the reforming of methane in the zone closed to the reactor inlet. H 2 enhances the stability of the catalyst by suppressing the excessive dehydrogenation of the reaction intermediates into inactive entities. When the concentration of CO 2 and O 2 was too high, the entire catalyst bed remained oxidized and the methane aromatization reaction could not occur.

Remarkable Improvement on the Methane Aromatization Reaction: A Highly Selective and Coking-Resistant Catalyst

An effective way (without addition of any oxidative gases to the reactant) to suppress coke formation (by modification of the Mo/HZSM-5 catalyst) and thus to significantly improve the aromatic selectivity in the methane aromatization reaction is presented in this study. A greater yield of benzene and an enhanced durability of the catalyst when compared with the conventional Mo/HZSM-5 catalysts are observed. Multinuclear solid-state NMR, TPD, TPO, TG, and UV-Raman methods are applied to characterize and rationalize the structure-property relationship of the improved catalytic performance of the modified catalysts. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be removed upon steaming treatment, are shown to be related with the aromatic carbonaceous deposits on the catalysts.

Synthesis and characterization of galloaluminosilicate/gallosilicalite (MFI) and their evaluation in methane dehydro-aromatization

MFI-type gallosilicalite and galloaluminosilicate were synthesized by hydro-thermal method. For all the as-synthesized Ga-containing samples, 71Ga MAS NMR spectra confirmed that the Ga 3+ cations are located in the zeolite framework. As for the case in galloaluminosilicate, Ga 3+ and Al 3+ cations can enter the zeolitic site at the same time, but there is a competition between the two cations to incorporate into the zeolite framework. Quantitative MAS NMR results suggest that the incorporation of the Ga 3+ cations is seriously inhibited by that of the Al 3+ cations if they existed together, while the reversed process is not observed. The catalytic performances of non-oxidative aromatization of methane on these zeolite catalysts, with or without loading of MoO 3, have been investigated. The results show that zeolites containing framework Ga species, which are excellent catalysts for propane aromatization, are poor catalyst supports for methane dehydro-aromatization. The catalytic performance of the molybdenum-loaded H-gallosilicate (MFI) catalysts present an activity for methane activation and subsequent aromatization, however, it is not as good as that of the Mo-loaded H-galloaluminosilicate (MFI) catalysts. Bearing this in mind, it was suggested that zeolitic acidity that originated from the heteroatom substitution is essential for catalyzing methane aromatization. The weaker acidity of framework GaO 4 − tetrahedral species as compared to framework AlO 4 − tetrahedral species is suggested to be responsible for the inferior activity for methane aromatization reaction.