Carbon dioxide chemistry during oxidative coupling of methane on a Li/MgO catalyst

Reactor tests, temperature-programmed reaction/desorption (TPR/D) carried out in a thermogravimetric balance and FT-IR were employed to investigate the course of CO₂ formed during oxidative coupling of methane (OCM) on a 7-wt.-% Li/MgO catalyst between 600–800°C. Initially, the carbonate free Li/MgO catalyst showed good OCM activity even at 600°C. However, its activity diminished considerably after about 20 min on stream. This coincided with the appearance of CO₂ in the OCM products and the disappearance of ethylene. During OCM, a substantial amount of the available lithium was converted to a stable carbonate which did not decompose easily even at 800°C. FT-IR and TPR revealed that carbonate formation began at 400°C and that the lithium carbonate existed in the form of monodentate (LiOCO₂) and perhaps mixed bridged [Mg(Li)O₂CO] carbonates. Simultaneous existence of a commonly accepted Li₂CO₃ cannot be excluded. The role of CO₂ produced during OCM in modifying the catalytic performance of Li/MgO is discussed.     

The effect of addition of a third component on the behaviour of the lithium doped magnesium catalysts for the oxidative dehydrogenation of ethane

The oxidative dehydrogenation of ethane was studied with the use of promoted Li/MgO catalysts at temperatures of 600–650°C. The addition of known promoters, cobalt and tin, gave a slight Increase In activity but a strong decrease in selectivity to ethylene under the conditions used. The addition of sodium improved the selectivity to ethylene and suppressed the formation of carbon monoxide. Using a feed of 12 vol% ethane and 6 vol% oxygen, the U/Na/MgO catalyst with 3.2wt% sodium showed a selectivity of 86 % to ethylene at 38 % conversion of ethane; the Li/MgO catalyst showed a selectivity of 80 % at similar conversions Thermal Investigations of the Li/Na/MgO catalyst showed that an eutectic melt of LINaCO₃ is formed at 490°C; the existence of this molten phase is probably the cause of the Increased selectivity.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH₄/O₂ feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm³ g⁻¹ h⁻¹) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry     

The influence of various parameters on the oxidative coupling of methane reaction over lithium doped lanthanum oxide

The influence of reaction temperature, space velocity and methane and oxygen partial pressures were studied for the oxidative coupling of methane reaction (OCM) over a lithium doped lanthanum oxide catalyst. The kinetic data obtained with this catalyst, at temperatures between 650 and 750°C, indicate that oxygen is adsorbed non-dissociatively and non-competitively. Product selectivities extrapolated to zero percent methane conversion are similar to those obtained with Li/MgO, suggesting that both rare earth and alkaline earth based catalysts involve similar mechanisms. Carbon monoxide and ethylene were found to be secondary products exclusively.     

Investigations on a Ce/Li/MgO-Catalyst for the oxidative coupling of methane

The heterogeneously catalyzed oxidative coupling of methane into ethane and ethylene has been investigated using a Ce/Li/MgO catalyst. A tentative catalytic mechanism, which explains the role of cerium, is proposed and confirmed by pulsing experiments. The loss of lithium during pretreatment and under reaction conditions was determined. A correlation between the lithium loading of the catalyst and its C₂ production rate was found. The reaction scheme has been partly elucidated using C₂H₄/O₂N₂ as feed gas. It is shown that both homogeneous and heterogeneously catalyzed reaction steps are of great importance.     

Oxidative dehydrogenation of propane over vanadium oxide based catalysts Effect of support and alkali promoter

The oxidative dehydrogenation of propane was investigated using vanadia type catalysts supported on Al₂O₃, TiO₂, ZrO₂ and MgO. The promotion of V₂O₅/Al₂O₃ catalyst with alkali metals (Li, Na, K) was also attempted. Evaluation of temperature programmed reduction patterns showed that the reducibility of V species is affected by the support acid–base character. The catalytic activity is favored by the V reducibility of the catalyst as it was confirmed from runs conducted at 450–550°C. V₂O₅/TiO₂ catalyst exhibits the highest activity in oxydehydrogenation of propane. The support’s nature also affects the selectivity to propene; V₂O₅ supported on Al₂O₃ catalyst exhibits the highest selectivity. Reaction studies showed that addition of alkali metals decreases the catalytic activity in the order non-doped>Li>Na>K. Propene selectivity significantly increases in the presence of doped catalysts.     

In situ IR and pulse reaction studies on the active oxygen species over SrF2/Nd2O3 catalyst for oxidative coupling of methane

Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF₂/Nd₂O₃ catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C₂ hydrocarbons. IR band of superoxide species (O₂⁻) was detected on the O₂-preadsorbed SrF₂/Nd₂O₃. The substitution of ¹⁸O₂ isotope for ¹⁶O₂ caused the IR band of O₂⁻ at 1128 cm⁻¹ to shift to lower wavenumbers (1094 and 1062 cm⁻¹), consistent with the assignment of the spectra to the O₂⁻ species. A good correlation between the rate of disappearance of surface O₂⁻ and the rate of formation of gas phase C₂H₄ was observed upon interaction of CH₄ with O₂-preadsorbed catalyst at 700 °C. The O₂⁻ species was also observed on the catalyst under working condition. These results suggest that O₂⁻ species is the active oxygen species for OCM reaction on SrF₂/Nd₂O₃ catalyst.     

A comparative study of coprecipitated BaCO3/La2On(CO3)rn catalysts for the oxidative coupling of methane

Coprecipitated catalyst systems containing BaCO₃ and La₂O_n(CO₃)_m (n≥1.5) with La/Ba = 0.05-10 were tested for catalytic activity, selectivity and stability in the oxidative coupling of methane reaction (OCM). Maximum C₂⁺ selectivities of 78% and C₂⁺ yields of 11% were obtained. The results show that La is the more active cation component of the system. The presence of BaCO₃ in the system leads to decreasing crystal size of the La phases, and to higher C₂⁺ selectivities at equal methane conversions. Life time tests showed that the Ba- La-containing catalysts were quite stable. Na impurities in the system lead to larger crystals in the La phases, and to less selective and less stable catalysts for the OCM reaction. Na is lost during reaction.     

Lithium-doped sulfated-zirconia catalysts for oxidative coupling of methane to give ethylene and ethane

Li-doped sulfated-zirconia catalysts were found to be effective for oxidative coupling of methane (OCM). The catalyst performances depend on the sulfate content and calcination temperature. A maximum C₂ yield is attained over the catalysts, which contain 6 wt.% sulfate and calcined at 923–973 K, being closely related to the preparation conditions of sulfated-ZrO₂ as solid super-acids. When the performances of the Li-doped sulfated-ZrO₂ (Li/SZ) catalysts were tested at 1023 K as a function of reaction time, both the C₂ and CO_x selectivities remained constant over the range of 8 h, but the CH₄ conversion decreased from 17.5% to 11.9%. The nature of Li/SZ catalysts for the OCM was investigated by X-ray diffraction, XPS, and NH₃ and CO₂ TPD measurements. It could be postulated that the sulfated-ZrO₂ surface could play an important role in the formation of a catalytically active structure by Li-doping.     

Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts

Reforming of methane with carbon dioxide to synthesis gas (CO/H₂) has been investigated over rhodium supported on SiO₂, TiO₂, γ-Al₂O₃, MgO, CeO₂, and YSZ (ZrO₂ (8 mol% Y₂O₃)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using ¹³CH₄) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al₂O₃ catalyst during reforming reaction at 750°C are primarily derived from the CO₂ molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH₄ molecular route is found to be very small.     

Comparison of behaviour of rare earth containing catalysts in the oxidative dehydrogenation of ethane

Catalyst promotion by addition of either La and Sm to MgO or Na aluminate to Sm₂O₃ and La₂O₃ has been investigated for the oxidative dehydrogenation of ethane in the temperature range 550–700°C. With all unpromoted and promoted catalysts, the selectivity to ethylene is strongly enhanced by the temperature, the highest values being obtained at 700°C. Sm₂O₃ is the most active among the bulk oxides, while samarium addition to MgO results in higher surface area, but does not enhance the catalytic activity. Ethylene productivity on La₂O₃ promoted MgO samples is higher than with pure La₂O₃, Sm₂O₃ and MgO, not only due to the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk oxides and rare earth promoted MgO, the selectivity to ethylene strongly increases by decreasing the O₂/C₂H₆ feed ratio, while it is quite unaffected by ethane conversion and catalyst composition, in agreement with the hypothesis that the main role of catalyst in the experimental conditions investigated is to produce ethyl radicals which are converted in the gas phase to CO and C₂H₄. When La₂O₃ is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, likely due to a different, mostly heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover, only CO₂ is produced as by-product, the formation of CO being quite negligible.     

Oxidative coupling of methane over doped Li/MgO catalysts

A series of zirconia doped Li/MgO catalysts with a fixed amount of zirconia and varying concentrations of lithium was used for the oxidative coupling of methane. It was found that an increase in lithium concentration resulted in a decrease in initial activity, while the selectivity was not affected. The life-time of Zr doped Li/MgO catalysts with a fixed concentration of ZrO₂ is a function of the lithium concentration. Previous results have shown that Li₂Mg₃ZrO₆ is active and selective but it is now shown to be instable under reaction conditions.     

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.     

An investigation of the comparative reactivities of ethane and ethylene in the presence of oxygen over Li/MgO and Ca/Sm2O3 catalysts in relation to the oxidative coupling of methane.

In order to examine the importance of the further oxidation of the desired C₂ products in the oxidative coupling of methane, ethylene and ethane have been added to the feed (containing methane and oxygen) to a Li/MgO or Ca/Sm₂O₃ catalyst. The results of these measurements show that neither of these C₂ molecules is stable under these conditions with either of the catalysts. Additionally, the rates of the oxidation of ethane and of ethylene alone have been measured using a gradientless reactor for both catalysts as well as for a quartz bed. It was found that the Ca/Sm₂O₃ material had higher activities for the oxidation of C₂H₆ and C₂H₄ (and also of CH₄) than had the Li/MgO material. These higher activities result in a lower optimal reaction temperature for the oxidative coupling of methane and are (at least partially) responsible for the lower selectivity to C₂ products observed with the Ca/Sm₂O₃ catalyst compared to that with the Li/MgO catalyst.     

Oxidative coupling of methane over K/Ni/Ca oxide and K/Ni/Mg oxide catalysts

The oxidative coupling behaviour of a series of K/Ni/Ca oxide catalysts with low nickel-to-calcium ratios has been examined and the results are compared with those for a magnesium-based catalyst. The effect of gas composition and the stability of ethylene under reaction conditions have also been studied. The catalysts were calcined at 1200°C unless otherwise stated. Potassium was added after the calcination stage. It is found that a high calcination temperature of 1200°C is necessary to give a Ca-based catalyst with high activity and selectivity. The catalysts based on MgO were less selective. Substitution of K for Li in the MgO based catalyst gave a slight improvement in the selectivity. A series of experiments was carried out with the K_0.1Ni_0.012 Ca material with the aim of optimising the yield. It was found that the selectivity could be improved by increasing the concentration of CH₄ or by adding CO₂ to the feed. However the addition of CO₂ decreased the activity of the catalyst. The activity could be increased by increasing the H₂O concentration. An increase of the O₂ concentration in the feed from 10.85 to 13% with 31% of CH₄ and 21% H₂O increased the C₂ yield from 15.1% to 17.8%. In a series of experiments in which different concentrations of C₂H₄ were added to the feed, it was found that the main oxidation product of ethylene was CO₂. The formation of ethane was unaffected by the addition of ethylene. It is therefore proposed that two different sites are required for the oxidation of ethylene and the activation of methane to form ethane.     

Rh-based catalysts for catalytic dechlorination of aromatic chloride at ambient temperature

Catalytic dechlorination of chlorotoluene to toluene was carried out using several supported Rh-based catalysts in a 2-propanol solution of NaOH at ambient temperature (27°C). A carbon-supported Rh catalyst (Rh/C) showed high catalytic activity, although an induction period was involved in the reaction and the activity of the catalyst reduced during storage in air. The existence of Pt on the Rh catalyst was effective in overcoming the activity reduction by exposure to air and gave the reaction without any induction period. The composite Rh–Pt catalyst supported on TiO₂ as well as on carbon was much more active for the reaction than the catalysts supported on SiO₂, MgO and Al₂O₃.     

A study on the alkali metal-promoted magnesium oxide system used as a catalyst for acrylonitrile synthesis from methanol and acetonitrile

Alkali metal-promoted MgO catalysts such as Li/MgO, Na/MgO, K/MgO, and Cs/MgO were studied for the acrylonitrile synthesis from methanol and acetonitrile in the range 300–500°C. The doping of alkali metal with larger cationic radius than that of Mg²⁺ increased the basic strength of MgO and facilitated the formation of acetonitrile carbanion, a reaction intermediate for acrylonitrile synthesis. Thermodynamic analysis showed that oxidative conditions were necessary to suppress the production of propionitrile, the main product under non-oxidative conditions. The doping of alkali metal also decreased the Lewis acid strength of MgO and consequently inhibited complete oxidation of anionic intermediates to CO and CO₂. The catalytic activity for acrylonitrile production under oxidative conditions was in the order of K/MgO>Na/MgO>Cs/MgO>Li/MgO.     

Cooperative action of cobalt and MgO for the catalysed reforming of CH4 with CO2

The reforming of CH₄ with CO₂ over activated carbon- or silica-supported cobalt catalysts with and without added MgO as promoter has been studied over a range of temperatures (500–700°C). A significant effect of the MgO on catalyst efficiency was observed. The presence of MgO markedly reduces the carbon deposition on the surface of the catalyst and therefore, contributes to the stability of the catalyst. Based on temperature-programmed surface reaction experiments of chemisorbed CO₂, the role of MgO may be ascribed to the formation of strongly adsorbed CO₂ species over its surface. These CO₂ species can easily react with the surface carbon deposits under CO₂-reforming reaction conditions, preventing in this way catalyst deactivation.     

Numerical investigation of complex chemistry performing in Ptcatalyzed oxidative dehydrogenation of ethane fixed-bed reactors

Ethylene is one of the most important basic chemicals in the modern chemical industry. Thermal or catalytic cracking of hydrocarbons is the main industrial technologies nowadays, which suffer from equilibriumlimitation and rapid coke formation. The oxidative dehydrogenation of ethane (ODHE) is considered to be a promising alternative process since it overcomes equilibrium-limitations, avoids catalyst deactivation by coke formation, and decreases the number of side reactions. In this study, particle-resolved 2D CFD simulations of fixed-beds filled with eggshell catalysts coupled with micro-kinetics of Pt-catalyzed ODHE were performed to understand the effect of operation conditions and catalyst properties on ethylene selectivity. The catalyst bed was created by discrete element method (DEM) and the central longitudinal section of the reactor tube was defined as the 2D simulation region. Both of the homogeneous and catalytic heterogeneous chemical reactions were described by detailed micro-kinetics within the particle-resolved CFD simulation. At first, the established model of monolith reactors was verified by comparing the simulated results with experimental results reported in literature. Then, the effects of operation conditions and catalyst concentration on the ethylene selectivity in randomly packed beds were explored. The specific variation of certain operation conditions including inlet flow rate, inlet temperature, pressure, inlet C₂H₆/O₂ ratio and N₂ dilution ratio can effectively increase ethylene selectivity. And the reduction of ratio of catalytic active area to geometric area F_cat/geo representing catalyst properties from 140 to 30 increases the selectivity from 42.2% to 59.3%. This research can provide reference for the industrialization of ODHE process in the future.     

Characterization and activity in dry reforming of methane on NiMg/Al and Ni/MgO catalysts

Dry reforming of methane has been investigated on two series of catalysts either prepared by co-precipitation: n(Ni_xMg_y)/Al, Ni_xMg_y and Ni_xAl_y or prepared by impregnation: Ni/MgO (mol% Ni = 5, 10). The catalysts, calcined at 600–900 °C, were characterized by different techniques: BET, H₂-TPR, TPO, XRD, IR, and TEM-EDX analysis. The surface BET (30–182 m² g⁻¹) decreased with increasing the temperature of calcination, after reduction and in the presence of Mg element. The XRD analysis showed, for n(Ni_xMg_y)/Al catalysts, the presence of NiAl₂O₄ and NiO–MgO solid solutions. The catalyst reducibility decreased with increasing the temperature of pretreatment. The n(Ni_xMg_y)/Al catalysts were active for dry reforming of methane with a good resistance to coke formation. The bimetallic catalyst Ni_0.05Mg_0.95 (calcined at 750 °C and tested at 800 °C) presents a poor activity. In contrast, the 5% Ni/MgO catalyst, having the same composition but prepared by impregnation, presents a high activity for the same calcination and reaction conditions. For all the catalysts the activity decreased with increasing the temperature of calcination and a previous H₂-reduction of the catalyst improves the performances. The TPO profiles and TEM-EDX analysis showed mainly four types of coke: CH_x species, surface carbon, nickel carbide and carbon nanotubes.