Comparative study of the two-zone fluidized-bed reactor and the fluidized-bed reactor for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst

In this work, oxidative coupling of methane over Mn/Na₂WO₄/SiO₂ catalyst is studied in a two-zone fluidized-bed reactor (TZFBR) and its performance is compared with a fluidized-bed reactor (FBR). Diluted oxygen in argon was introduced into the bottom of the TZFBR through a quartz ferrite and methane was entered at higher positions along the fluidized bed. The catalyst circulated between the oxygen-rich and methane-rich zones in the TZFBR reactor. The effects of the main operating variables including bed temperature, the methane/oxygen ratio (R_mo), and the height at which methane was introduced into the reactor (H_m) were investigated. It is found that under some operating conditions the TZFBR gives a higher C₂ selectivity than that obtained in the FBR reactor. Reaction of methane with lattice oxygen of the Mn/Na₂WO₄/SiO₂ redox catalyst in the methane-rich zone may have led to the higher selectivity.     

Effect of promoter in the oxidative coupling of methane over synthesized Mn/SiO2 nanocatalysts via incipient wetness impregnation

The M–Na–Mn/SiO₂ nanocatalysts (M = W, Mo, Nb, V, Cr) were synthesized with the size of 12–92 nm by incipient wetness impregnation method to study the effect of different promoters on the catalytic performance in the oxidative coupling of methane. The results at 1 atm, 1048 K, 2500 ml h^?1 g^?1, and CH₄/O₂/N₂ = 2/2/1 revealed that C₂ selectivity was significantly increased (31.6%) in the order of W > Mo > Nb > Cr > V whereas moderate enhancement (12.6%) was observed in the CH₄ conversion in the order of W > Cr > Nb > Mo > V. The results of the characterization techniques (Raman, FT-IR, BET, TGA/DTA and XRD) demonstrated that Mn₂O₃ and α-cristobalite were the predominant species and active sites in the nanocatalyst surface and Na₂MoO₄, Na₂WO₄ and Mn₂O₃ crystalline phases contributed to achieving high selectivity of C₂ products. The redox mechanism involving two metal sites such as Mn^3+/2+ and W^6+/5+ or Mn^3+/2+ and Mo^6+/5+ was found to be the most compatible route with the OCM reaction path in which CH₄ and O₂ adsorption was the controlling step.     

C2 selectivity enhancement in oxidative coupling of methane over microwave-irradiated catalysts

The oxidative coupling of methane over Li/MgO and BaBiO_{3 - x} catalysts irradiated by microwaves and classically heated is reported. Enhanced selectivities in C₂⁺ products are observed at lower temperatures under microwave conditions, especially with the Li/MgO catalyst.

The complex permittivity measurements of BaBiO_{3 - x} show that the regeneration of the active oxygen species on the surface is lower under microwave irradiation than classical heating. X-ray diffraction analyses of the catalyst before and after catalytic reaction, when it is classically heated and when it is heated by microwave irradiation, corroborate these results. Therefore, the CH₃⁻ carbanions are less oxidated at the catalyst surface under microwave irradiation.

On the other hand, the quenching of the output gas probably decreases the oxidation of CH°₃ radicals in the gas phase when the Li/MgO catalyst is irradiated by microwaves. The quenching of the output gas is a unique consequence of microwave irradiation which heats the catalyst without heating the wall of the reactor.     

Numerical simulation of fixed bed reactor for oxidative coupling of methane over monolithic catalyst

A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na₃PO₄-Mn/SiO₂/cordierite monolithic catalyst, and an improved Stansch kinetic model was established to calculate the OCMreactions using the computational fluid dynamicsmethod and Fluent software. The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min^-1 under standard state, the CH₄/O₂ ratio is 3 and the temperature and pressure is 800 ℃ and 1 atm, respectively. The contour of the characteristic parameters in the catalyst bed was analyzed, such as the species mass fractions, temperature, the heat flux on side wall surface, pressure, fluid density and velocity. The results showed that the calculated valuesmatchedwell with the experimental values on the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO,CO₂ and H₂) in the reactor outlet with an error range of ±4%. The mass fractions of CH₄ and O₂ decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and 0.120 at the outlet, where the mass fractions of C₂H₆, C₂H₄, CO and CO₂ were 0.0245, 0.0460, 0.0537 and 0.116, respectively. Due to the existence of laminar boundary layer, the mass fraction contours of each species bent upwards in the vicinity of the boundary layer. The volume of OCM reaction was changing with the proceeding of reaction, and the total moles of products were greater than reactants. The flow field in the catalyst bed maintained constant temperature and pressure. The fluid density decreased gradually from 2.28 kg·m^-3 at the inlet of the catalyst bed to 2.18 kg·m^-3 at the outlet of the catalyst bed, while the average velocity magnitude increased from 0.108 m·s^-1 to 0.120 m·s^-1.     

On the oxidative coupling of methane with carbon dioxide over CeO2/ZnO nanocatalysts

The nano-CeO₂/ZnO catalysts were prepared using a novel combination of homogeneous precipitation with micro-emulsion for oxidative coupling of methane with CO₂ as an oxidant. The prepared catalysts were compared with those prepared using the conventional impregnation. The catalysts prepared in two ways were characterized with FTIR, TEM, XRD and CO₂-TPD. The effects of the reaction temperature, the amount of ZnO doped in the catalysts and the average size were investigated. The experimental investigation demonstrated that methane conversion over the nano-CeO₂/ZnO catalysts prepared by the combined technique was higher than that obtained over catalysts prepared by the conventional impregnation. A better low-temperature activity has also been achieved over the nanocatalysts. There was no clear trend between the average size of nano-CeO₂/ZnO catalysts and their catalytic performance but methane conversion increased with increasing fractal dimension of nanocatalysts.     

Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst

Nanocomposite Ni/ZrO₂-AN catalyst consisting of comparably sized Ni metal and ZrO₂ nanoparticles is studied in comparison with zirconia- and alumina-supported Ni catalysts (Ni/ZrO₂-CP and commercial Ni/Al₂O₃-C) for steam reforming of methane (SRM) and for combined steam and CO₂ reforming of methane (CSCRM). The reactions are performed under atmospheric pressure with stoichiometric amounts of H₂O and CH₄ or (H₂O + CO₂) and CH₄ at 1073 K. Under a wide range of methane space velocity (gas hourly space velocity of methane GHSV_CH4 = 12,000–96,000 ml/(h g_cat.), the nanocomposite Ni/ZrO₂-AN catalyst always shows higher activity and stability for both SRM and CSCRM reactions. The two supported Ni catalysts (Ni/ZrO₂-CP and Ni/Al₂O₃-C) exhibit fairly stable catalysis under low GHSV_CH4 but they are easily deactivated under high GHSV_CH4 and become completely inactive when they are reacted for ca.100 h at GHSV_CH4 = 48,000 ml/(h g_cat.). The CSCRM reaction is carried out with different H₂O/CO₂ ratios in the reaction feed while keeping the molar ratio (H₂O + CO₂)/CH₄ = 1.0, the results prove that the nanocomposite Ni/ZrO₂-AN catalyst can be highly promising in enabling a catalytic technology for the production of syngas with flexible H₂/CO ratios (ca. H₂/CO = 1.0–3.0) to meet the requirements of various downstream chemical syntheses.     

Catalytic properties of various MgO catalysts for oxidative coupling of methane

A series of MgO catalysts for the oxidative coupling of methane prepared by different methods have been investigated. Specific surface area, XRD and XPS measurement results reveal that at lower temperatures catalysts with larger specific surface area, larger lattice distortion, smaller crystal dimension, and higher amount of unsaturated coordinated surface oxygen give higher catalytic activity. However, if we compare the catalytic properties of the samples in terms of unit surface area, the dependence of catalytic properties of the samples will be different.     

Multi-objective optimization of simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane

A comprehensive optimization study on a simulated countercurrent moving bed chromatographic reactor (SCMCR) is reported in this article for oxidative coupling of methane (OCM) reaction. The selection of the operating parameters such as the switching time, make-up feed rate, methane to oxygen ratio in feed, length of columns and flow rates in different sections are not straightforward in an SCMCR. In most cases, conflicting requirements and constraints govern the optimal choice of the decision (operating or design) variables. An experimentally verified mathematical model was selected to optimize the performance of the SCMCR for OCM. A few multi-objective optimization problems were solved for both existing setup and at design stage. The optimization was performed using a state-of-the-art AI-based non-traditional optimization technique, non-dominated sorting genetic algorithm with jumping genes (NSGA-II-JG), which resulted in Pareto optimal solutions. It was found that the performance of the SCMCR could be improved significantly under optimal operating conditions.     

Oxidative coupling of methane to C2-hydrocarbons over La-promoted CaO catalysts

La₂O₃ promoted CaO [La/Ca (mol/mol) = 0.05] catalyst shows very high activity and selectivity (methane conversion: 25%, C₂-selectivity: 66% and C₂-space-time-yield: 864 mmol ·g^–1 (cat.)·h^–1) with no catalyst deactivation in oxidative coupling of methane to C₂-hydrocarbons at 800 ° C.     

Direct conversion of methane into methanol over MoO3/SiO2 catalyst in an excess amount of water vapor

Well-dispersed MoO₃ on SiO₂ showed a high activity for partial oxidation of methane (mixed with oxygen in a molar ratio of 9:1) into methanol and formaldehyde at 873 K in an excess amount of water vapor, which is attributed to the formation of silicomolybdic acid (SMA) on the catalyst surface during reaction. One of the roles of SMA for the partial oxidation of methane is proved to depress the successive oxidation of methanol and formaldehyde into carbon oxides.     

Effect of the method of preparation of a Nd2O3-MgO catalyst on its efficiency in the reaction of oxidative coupling of methane

It is shown that the decomposition of a mixture of nitrates or coprecipitated carbonates or hydroxides of Mg and Nd form catalysts manifesting similar catalytic properties, while the catalyst obtained by impregnation is more active but much less selective. The mechanism of formation of the catalytically active phase and the nature of the active sites are discussed.     

Oxidative coupling of methane catalyzed by Li, Na and Mg doped BaSrTiO3

The XBaSrTiO₃ (X = Li, Na, Mg) with perovskite structure was prepared by impregnation of LiCl, NaCl or MgCl₂ solution on BaSrTiO₃ (BST) surface. The products were characterized with XRD, SEM, UV, FT-IR and Raman spectroscopy. Determination of band gap, conduction and basicity of the prepared catalysts revealed that NaBST exhibits the lowest band gap and the most conduction and basicity. The catalytic performances of XBSTs for the oxidative coupling of methane (OCM) in a tubular continuous flow reactor were evaluated. The NaBST showed the maximum catalytic effect on methane conversion (47%), C₂₊ selectivity (51%) and ethylene yield (24%) at the temperature of 800 °C.     

Catalyst-modified perovskite hollow fiber membrane for oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4) represents one of the most effective ways to convert natural gas to more useful products,which can be performed effec-tively using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite hollow fiber membrane microreactor.In this work,the effects of adding a thin BaCe0.8Gd0.2O3-δ (BCG) catalyst film onto the inner LSCF fiber surface as the OCM catalyst and a porous Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite layer onto the outer LSCF surface to improve the oxygen permeation were evaluated.Between 700 ℃ and 1000 ℃,methane conversion increased in the order of uncoated,BCG and BSCF-coated,and BCG-coated LSCF hollow fiber while C2-selectivity and C2-yield increased in the order of BCG and BSCF-coated,uncoated,and BCG-coated LSCF hollow fiber.Oxygen permeation flux at the same temperature range,on the other hand,was enhanced in the order of uncoated,BCG-coated,and BCG and BSCF-coated LSCF hollow fiber,This finding demon-strates the complex interplay between oxygen permeation and OCM performance.The BCG and BSCF-coated hollow fiber was also subjected to thermal cycles between 850 ℃ and 900 ℃ for up to 175 hours during which the fiber showed minor degradation in oxygen permeation fluxes and relatively stable OCM performance.     

Catalytic conversion of methane and CO2 to synthesis gas over a La2O3-modified SiO2 supported Ni catalyst in fluidized-bed reactor

In this contribution, a commercial spherical SiO₂ was modified with different amounts of La₂O₃, and used as the support of Ni catalysts for autothermal reforming of methane in a fluidized-bed reactor. Nitrogen adsorption, XRD and H₂-TPR analysis indicated that La₂O₃-modified SiO₂ had higher surface area, strengthened interaction between Ni and support, and improved dispersion of Ni. CO₂-TPD found that La₂O₃ increased the alkalescence of SiO₂ and improved the activation of CO₂. Coking reaction (via both temperature-programmed surface reaction of CH₄ (CH₄-TPSR) and pulse decomposition of CH₄) disclosed that La₂O₃ reduced the dehydrogenation ability of Ni. CO₂-TPO, O₂-TPO (followed after CH₄-TPSR) confirmed that only part amount of carbon species derived from methane decomposition could be removed by CO₂, and O₂ in feed played a crucial role for the gasification of the inactive surface carbons. Ni/xLa₂O₃-SiO₂ (x = 10, 15, 30) possessed high activity and excellent stability for autothermal reforming of methane in a fluidized-bed reactor.     

Numerical simulation of particle/monolithic two-stage catalyst bed reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane

A three-dimensional geometry model of the particle/monolithic two-stage reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane (OCM) was set up. The improved Stansch kinetic model adapting different operating temperatures was established to calculate the OCM reactor performance using computational fluid dynamics (CFD) and FLUENT software. The results showed that the calculated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO₂, CO) in the OCM reactor. The distributed dioxygen feeding with the percentage of 5–20% based oxygen flow rate of top inlet promoted the OCM reaction in monolithic catalyst bed and led to the conversion of CH₄ and the selectivity and yield of C₂ (C₂H₆ and C₂H₄) increase obviously. The distributed dioxygen feeding was 15%, the conversion of CH₄, the selectivity and the yield of C₂ reached 34.1%, 68.2% and 23.3%, respectively.     

Diffusion and electromigration in solid Na2WO4

The differential equation for the case of one-dimensional diffusion/electromigration of silver ions in solid Na₂WO₄ is solved for the case of galvanostatic electrolysis of the cell system Ag/Na₂WO₄/Ag. Galvanostatic pulse measurements and silver ion concentration profile measurements (by microscan analysis) of sectioned electrolyzed Ag/Na₂WO₄/Ag cell arrangements were performed. The theory is not in quantitative accordance with the experiments, the reasons of which are discussed. According to the present study, the diffusion of Ag⁺ in ∝-Na₂ WO₄ (high temperature phase, 589–694°C) can be given by:

     

Methane oxidative coupling over Na2WO4/SiO2

Methane oxidative coupling (MOC) was studied over Na₂WO₄/SiO₂. The effect of Na₂WO₄ loading and reaction conditions on the catalytic behaviour was investigated. XRD, SEM, LRS and XPS have been used to study the catalyst morphology, Na₂WO₄ dispersity and surface oxygen species. These results were correlated with the catalytic activity and selectivity.     

Selective oxidative dehydrogenation of propane over surface molybdenum-enriched MgMoO4 catalyst

Drastic activity increases were observed by the treatments of the magnesium-rich MgMo_0.99O_y catalysts, which are poorly active for the oxidative dehydrogenation of propane, with inorganic or organic acid to remove excess magnesium on the surface. MoO₃ loading on magnesium-rich MgMo_0.99O_y catalysts also resulted in drastic activity increases. The activity increases followed non-effective loadings of MoO₃ in the range 0–2 wt%, because it is necessary to neutralize the surface magnesium with MoO₃ before the formation of molybdenum-rich surface. The pH of the aqueous (NH₄)₆Mo₇O₂₄ solution for the MoO₃ loading apparently influenced the activity. Under the acidic conditions the MoO₃ loading resulted in the drastic activity increase but under the basic conditions the effect of the MoO₃ loading was poor, suggesting that a cluster-type MoO₃ on MgMoO₄ surface is responsible for the activity of propane oxidative dehydrogenation.