Methane and ethane activation without adding oxygen: promotional effect of W in Mo-W/HZSM-5

The conversions of methane and ethane over Mo/HZSM-5 and W/HZSM-5 catalysts are compared. A reaction model for hydrocarbon formation over Mo/HZSM-5 catalysts is proposed, which involves heterolytic splitting of methane and a molybdenum-carbene intermediate. Ethene is shown to be the initial product of methane conversion, and it undergoes further reaction to form aromatics in a solid acid environment. The promotional effect of addition of tungsten in the Mo-W/HZSM-5 catalyst in methane conversion reaction suggests the formation of Mo-W mixed oxide. The product selectivity patterns of Mo/HZSM-5 and W/HZSM-5 catalysts in ethane conversion reaction are consistent with a dual-path model involving dehydrogenation and cracking (or hydrogenolysis) of ethane. The rates of both these reactions over Mo/HZSM-5 are higher than over W/HZSM-5.     

Methane dehydroaromatization over Mo/HZSM-5: A study of catalytic process

The deactivation process of methane dehydroaromatization (MDA) reaction has been followed by various physical chemistry methods.     

Ru-Mo/HZSM-5 Catalyzed Methane Aromatization in Membrane Reactors

Oxygen-free methane conversion into benzene was carried out in a catalytic membrane reactor over 0.5%Ru-3%Mo/HZSM-5 in the temperature range 873-973 K following three reaction protocols: (i) straight-run catalytic reactor without hydrogen permeation (OFF), (ii) cycled OFF/ON hydrogen permeation sequences, and (iii) cycled OFF/ON hydrogen permeation sequences intertwined with CH₄/H₂ regenerative steps. X-ray photoelectron spectroscopy analysis of fresh and spent catalysts identified, in all cases, three types of carbon species that formed during aromatization, including carbide formation. The presence of a permeating membrane did not give rise to different chemical states of carbon and molybdenum on the catalyst from those known to form in straight runs under no hydrogen permeation. The ON mode, i.e., during permeation, led to the accumulation of graphite-like and aromatic-aliphatic (coke) species on the catalyst. However, both types of carbon were reduced during the OFF step either by autogenous hydrogen or via an external source of hydrogen under CH₄/H₂ regenerative steps.     

Methane Aromatization over 2 wt% Mo/HZSM-5 in the Presence of O2 and NO

In the non-oxidative aromatization reaction (temperature = 770 C, flow rate = 34 ml min^-1), 2 wt% Mo/HZSM-5 deactivated after 4 h due to severe coking. We observed that with a suitable amount of O₂ (5.3 vol%) in the methane feed, the catalyst could last for more than 6 h with a ca. 4% yield of aromatics at 770 °C. Depending on the concentration of O₂ or the reaction temperature, there are three reaction zones in the catalyst bed: (i) methane oxidation; (ii) methane reforming; and (iii) methane aromatization. CO and H₂ produced in the first two zones are accountable for stability amelioration of the catalyst. The addition of NO exhibited similar effects on the reaction. Further increase in O₂ (8.4 vol%) or NO (14.2 vol%) concentration would result in CO and CO₂ being the predominant carbon-containing products; C₂H₄ and C₂H₆ were generated in small amounts and no aromatics were detected.     

Hydrothermal Post-synthesis of HZSM-5 Zeolite to Enhance the Coke-resistance of Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Hydrothermal post-synthesis was used to modify the micropores and acidity of commercially available HZSM-5 zeolites. The recrystallization and the dynamic incorporation and extraction of the framework Al not only stabilized the framework with high crystallinity, but also inhibited the creation of extra-pores during the post-synthesis in NaOH aqueous solution. The resulted Mo/HZSM-5 catalyst showed rather high catalytic stability and greatly enhanced selectivity towards aromatics for methane dehydroaromatization reaction by effectively inhibiting the coke formation.     

Catalytic conversion of methane to benzene over Mo/ZSM-5

Dehydroaromatization of methane to benzene occurs over a 2 wt% Mo/ZSM-5 catalyst at 700C under non-oxidizing conditions. Following an initial induction period, during which CH₄ reactant reduces the original Mo⁶⁺ ions in the zeolite to Mo₂C and deposition of coke occurs, a benzene selectivity of 70% at a CH₄ conversion of 8–10% could be sustained for more than 16 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements indicate that the reduced Mo is highly dispersed in the channels of the zeolite. Initial activation of CH₄ reactant occurs on Mo₂C sites, leading to the formation of C₂H₄ as the primary product. The latter then undergoes subsequent oligomerization reactions on acidic sites of the zeolite to form aromatic products.     

钇对甲烷芳构化催化剂Mo／HZSM—5性能影响的研究

研究了稀土钇的含量对Ｍｏ／ＨＺＳＭ－５催化剂的活性和选择性的影响，发现稀土钇的加入，不同程度上提高了Ｍｏ／ＨＺＳＭ－５的活性和选择性。特别是，当Ｙ／Ｍｏ＝０．０４时，活性最佳。甲烷在１０２３Ｋ芳构化反应，转化率达１９．６％，苯的选择性达９６．５％，且活性较稳定。     

Interaction between ammonium heptamolybdate and NH4ZSM-5 zeolite: the location of Mo species and the acidity of Mo/HZSM-5

Mo/HZSM-5 catalysts show high reactivity and selectivity in the activation of methane without using oxidants. Mo/HZSM-5 catalysts with Mo loading ranging from 0 to 10% were prepared by impregnation with an aqueous solution of ammonium heptamolybdate (AHM). The samples were dried at 393 K, and then calcined at different temperatures for 4 h. The interaction between Mo species and NH₄ZSM-5 zeolite was characterized by FT-IR spectroscopy, differential thermal analysis (DTA) and temperature programmed decomposition (TPDE) and NH₃-TPD at different stages of catalyst preparation. The results showed that if Mo/HZSM-5 catalysts were calcined at a proper temperature, the Mo species will interact with acid sites (mainly with BrØnsted acid sites) and part of the Mo species will move into the channel. The Mo species in the form of small MoO₃ crystallites residing on the external surface and/or in the channel, and interacting with BrØnsted acid sites may be responsible for the methane activation. Strong interaction between Mo species and the skeleton of HZSM-5 will occur if the catalyst is calcined at 973 K. This may lead to the formation of MoO ₄ ^2– species, which is detrimental to methane activation.     

Methane aromatization over Mo/H-ZSM-5: on the reaction pathway

Rates of benzene formation on Mo/HZSM-5, H-ZSM-5 and Mo/SiO₂ were measured with different reactants: methane, mixture of C₂H₄/H₂/N₂ and mixture of C₂H₂/H₂/N₂. Since the rate of benzene formation starting from C₂H₄/H₂/N₂ is higher on Mo/H-ZSM-5 compared to H-ZSM-5 it is concluded that the aromatization of methane on Mo/H-ZSM-5 is not going via ethylene which is aromatized over acid sites. Another reaction pathway is proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dehydrogenation of Methane over Mo/ZSM-5. Effects of Additives in the Methane Stream

The dehydrogenation of methane was carried out over a Mo/ZSM-5 catalyst. It was revealed that the purity of the methane was very critical for the evaluation of the catalyst activity. In order to study the phenomenon, the effects of the addition of O₂, CO₂, CO or H₂ to the feed were investigated. A small amount of O₂ increased the amounts of aromatic compounds and CO produced. The addition of H₂ scarcely affected the conversion of methane, but it prevented the deactivation of the catalyst, i.e., benzene production remained constant during a 6 h test.     

Mo/HZSM-5上甲烷无氧芳构化催化剂制备因素的考察

采用Mo/HZSM-5作为甲烷无氧芳构化催化剂,考察了催化剂制备部分影响因素.结果表明,采用较低的nSi:nAl时(25)载体制备的催化剂活性和稳定性较好;浸渍法优于固相反应法;Mo担载的质量分数为4%时,催化剂表现出最高活性;分子筛预先经碱处理,能够明显改善催化剂稳定性.     

A Transient Behaviour in the Initial Production of Aromatic Compounds from Methane Catalyzed by Mo/HZSM-5

The production of aromatic compounds directly from methane using catalysts based on zeolite HZSM-5 impregnated with molybdenum was investigated using a flow reactor at 750 °C. Main products were benzene and naphthalene as well as toluene and others like azulene in smaller amounts. Naphthalene appeared after benzene following an induction period suggesting a consecutive reaction path. A transient behavior in the production of naphthalene was observed along the time on stream, being benzene practically the only product at the steady state. The transient behavior disappeared when increasing contact time, which produced more naphthalene at the expense of benzene.     

Improvement of methane activation using n-hexane as co-reactant over Zn/HZSM-11 zeolite

Significance of effective conversion of methane (C₁) to more valuable compounds such as aromatics is studied using n-hexane (C₆) as co-reactant. AH yield was as high as 30 mol% C using the following reaction conditions: temperature, 500 °C; contact time w/f=30 g h mol⁻¹ and a C₁ molar fraction, X_C1:=(C₁/C₁ + C₆)=0.60, was achieved. The effect of the contact time, molar fraction, and time on stream was analyzed in order to obtain more information about different species evolution. The C₁ conversion reached at 50 mol% C using Zn/HZSM-11 with 2.13 mol Zn per cell unit.     

Methane Transformation using Light Gasoline as Co-Reactant over Zn/H-ZSM11

The catalytic conversion of methane (C1) to aromatic hydrocarbons (AH) such as benzene, toluene and xylenes (BTX) over a Zn/H-ZSM-11 catalyst using Light Gasoline (C5+C6) as co-reactant was studied. AH yields were as high as 30 %mol at 500 °C, w/f = 40 g h mol⁻¹ at C1 molar fraction = 0.20. The contact time and time-on-stream effects on the product distribution, were analyzed in detail in order to obtain information about the evolution of different species. The C1 conversion reached 36 mol% C using Zn/HZSM-11 with content of 2.13 mol of Zn²⁺ per cell unit.     

Methane oxidative coupling over promoted Pb/Ca catalysts

A series of Pb/Ca catalysts, promoted with Li, K and La, and prepared by different procedures, have been examined for the oxidative coupling of methane. The Li promoted Pb/Ca had the highest C₂ selectivity and yield of the catalysts studied, including the unpromoted Pb/Ca. For the Li/Pb/Ca catalyst, CO₂ added to the feed resulted in a decreased CH₄ conversion and an increased C₂ selectivity. Addition of H₂O to the feed with the same catalyst, increased C₂ yield. These effects are discussed in terms of deactivation and regeneration of the active sites by CO₂ and H₂O, respectively.     

Methane dehydrogenation and aromatization over 4 wt% Mn/HZSM-5 in the absence of an oxidant

Methane dehydrogenation and aromatization over 4 wt% Mn/HZSM-5 in the absence of an oxidant (GHSV = 1600 mL h⁻¹ g⁻¹) was investigated. Mn/HZSM-5 was prepared by impregnation of HZSM-5 with manganese acetate tetrahydrate solution; Mn₃O₄ formed was the precursor of active phase for methane activation. The induction period over Mn/HZSM-5 catalyst before aromatic products appear was long at 700 °C. This period shortened with a rise of the reaction temperature to 800 °C. XPS and TPO results showed that the partly carburized or carburized Mn species formed are probably responsible for methane activation.     

Shape-selective methylation of 2-methylnaphthalene with methanol over hydrothermal treated HZSM-5 zeolite catalysts

Shape-selective methylation of 2-methylnaphthalene (2-MN) was carried out over hydrothermally treated HZSM-5 (SiO₂/Al₂O₃=83) zeolite catalysts under a fixed-bed down-flow reactor, methanol and 1,3,5-trimethylbenzene (TMB) were used as methylating agent and solvent, respectively. The results show that hydrothermal treatment improves the selectivity to 2,6-dimethylnaphthalene (2,6-DMN) and the stability. HZSM-5 catalyst hydrothermally treated at 550 °C followed by acid leaching exhibits 7.1% of 2,6-DMN yield after 5 h time on stream (TOS), and 13.4% of 2-MN conversion with the 2,6-/2,7-DMN ratio of 1.8 after 8 h of TOS.     

Conversion of methane to benzene over Mo2C and Mo2C/ZSM-5 catalysts

The activation and dehydrogenation of CH₂ on Mo₂C and MO₂C/ZSM-5 have been investigated under non-oxidizing conditions. Unsupported Mo₂C exhibited very little activity towards methane decomposition at 973 K. The main reaction pathway was the decomposition of methane to give hydrogen and carbon with a trace amount of ethane. Mixing Mo₂C with ZSM-5 support somewhat enhanced its catalytic activity, but did not change the products of the reaction. A dramatic change in the product formation occurred on partially oxidized Mo2C/ZSM-5 catalyst; besides some hydrocarbons benzene was produced with a selectivity of 70–80% at a conversion of 5–7%. Carburization of highly dispersed MoO₃ on ZSM-5 also led to a very active catalyst: the conversion of methane at the steady state was 5–6% and the selectivity of benzene formation was 85%.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Highly Stable Performance of Methane Dehydroaromatization on Mo/HZSM-5 Catalyst with a Small Amount of H2 Addition into Methane Feed

With a small amount of H₂ (3 6%) addition into methane feed, coke formation on 6 wt% Mo catalyst during the methane dehydroaromatization reaction was effectively suppressed and the catalyst stability was increased evidently under the reaction conditions of 1023K, 0.3MPa and 2520 mL g-MFI^-1 h^-1 of methane space velocity.     

Carbon monoxide hydrogenation over Fe/HZSM-5 catalysts. Effect of SiO2/Al2O3 zeolite ratio

Iron catalysts supported on ZSM-5 zeolites of a wide range of silica-to-alumina ratios (29-) have been prepared and tested in carbon monoxide hydrogenation. The crystalline phases of the catalysts were characterized by X-ray diffraction and their acidity by infrared spectroscopy of adsorbed pyridine. The catalytic tests were conducted at 533 K, an overall pressure of 21 bar and a feed ratio CO/H₂ close to 1. It was found that the selectivity to light olefins (C₂–C₄) increases in parallel with the increase of the Si/Al ratio of the zeolite. This was explained in terms of the decrease in Brønsted acidity of the catalysts. As a consequence, very high olefin selectivities can be achieved by decreasing the number of strong acid sites in the zeolite structure, but at the expense of high oxygenate formation.