Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene

Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C₂ products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C₂ products for all the MnO₂-based catalysts and for lithium-doped MgO and Sm₂O₃ catalysts. The effect is greatest in MnO₂-based catalysts. When dichloromethane is added to a pure, unpromoted MnO₂ catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C₂H₄ increases from virtually zero to 56.3%. The highest C₂ selectivity observed after promotion of pure MnO₂ by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.     

Oxidative coupling of methane and oxidative dehydrogenation of ethane over strontium-promoted rare earth oxide catalysts

Sr-promoted rare earth (viz. La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxide catalysts (Sr/rare earth ratio = 0·1) are compared for their performance in the oxidative coupling of methane (OCM) to C₂ hydrocarbons and oxidative dehydrogenation of ethane (ODE) to ethylene at different temperatures (700 and 800°C) and CH₄ (or C₂H₆)/O₂ ratios (4–8), at low contact time (space velocity = 102000 cm³ g⁻¹ h⁻¹). For the OCM process, the Sr–La₂O₃ catalyst shows the best performance. The Sr-promoted Nd₂O₃, Sm₂O₃, Eu₂O₃ and Er₂O₃ catalysts also show good methane conversion and selectivity for C₂ hydrocarbons but the Sr–CeO₂ and Sr–Dy₂O₃ catalysts show very poor performance. However, for the ODE process, the best performance is shown by the Sr–Nd₂O₃ catalyst. The other catalysts also show good ethane conversion and selectivity for ethylene; their performance is comparable at higher temperatures (≥800°C), but at lower temperature (700°C) the Sr–CeO₂ and Sr–Pr₆O₁₁ catalysts show poor selectivity. © 1998 SCI.     

Oxidative coupling of methane to C2-hydrocarbons over lithium–cerium-promoted MgO and MgO–CaO catalysts

The oxidative coupling of methane to ethylene and ethane was studied over lithium–cerium-promoted MgO and MgO–CaO catalysts in the presence of molecular oxygen at 730°C and at atmospheric pressure in a continuous flow, fixed bed quartz reactor. The catalysts were prepared by an impregnation method and finally calcined at 900°C. The surface area, pore size distribution and pore volume of the catalysts were determined. The feed consisted of only methane and oxygen in the molar ratio of 2:1. The results obtained over the catalyst systems, viz. (i) lithium–cerium-promoted MgO and (ii) lithium–cerium-promoted MgO–CaO, have been compared. A relatively high C₂-selectivity has been obtained with Li–Ce-promoted MgO–CaO catalysts. The optimum yield and selectivity for C₂-hydrocarbons were found to be 21·5% and 76·8% respectively at a methane conversion of 28% over Li (7 wt%)–Ce (2 wt%)-doped MgO–CaO (3:1 wt ratio) catalyst. The various factors governing the activity and the selectivity of the catalyst systems have been discussed.     

Investigation of reactivity and selectivity of methane coupling catalysts using isotope exchange techniques

The relationship between the activity and selectivity of four metal oxide catalysts for the oxidative coupling of methane has been compared with their basicity and their ability to effect the scission of C-H and O-O bonds. The rates of isotope exchange reactions (CH₄-D₂ and¹⁶O₂–¹⁸O₂) were used as a measure of catalyst's ability to activate both reactants. It is demonstrated that catalysts showing high C₂ selectivity in the oxidative coupling reaction activate methane strongly and oxygen weakly. The lack of direct correlation between the rates of methane conversion and bonds' activation indicates that the formation of the methyl radical cannot be explained by a simple, one-step mechanism.Visiting scientist, on leave from the Department of Chemistry, University of Zhejiang, Hangzhou, Zhejiang, 310027, Peoples Republic of China.     

Catalytic and electrocatalytic oxidation of propane on V-Mg-O and V-Mg-O (Ag) catalysts

Vanadium magnesium oxide catalysts prepared in this work were found active in selective oxidation of propane to propene. A selectivity as high as 79% was obtained at 10% conversion (813 K). No oxygenated or C₂ products were detected and the catalysts were found to undergo no change in activity over many weeks of operation. Under electrochemical pumping of oxygen (EOP) towards the catalyst (with oxygen present in the feed gas), both conversion and selectivity were found to increase slightly as external current increased, indicating the effect of electrical current can be exhibited by an oxide catalyst. However, in the absence of oxygen in the feed gas, EOP could lead to an even higher selectivity: 84 and 86.9% respectively for a 24 V-Mg-O and a 24 V-Mg-O (Ag) (1/2) catalyst. The overall results obtained suggest that electrochemically supplied oxygen is more selective towards C₃H₆. Mechanisms of both catalytic and electrocatalytic oxidation of propane were tentatively suggested, with surface oxygen ion vacancy identified as active surface species and the rate determining step involving heterolytic splitting of the C₃H₈ molecule to form a surface bonded C₃H ₇ ^– ion and a surface hydroxyl ion. The higher selectivity towards C₃H₆ in case of EOP was explained on this basis. While mixing with Ag powder was found to improve significantly the electrocatalytic performance of vanadium magnesium oxide, its role appears to be non-chemical: it simply gives rise to a larger area of the gas/catalyst/Ag electrode interface.     

An apparent ensemble effect in the oxidative coupling of methane on hydroxyapatites with incorporated lead

The introduction of small quantities of lead into calcium hydroxyapatite catalysts produces marked increases in the selectivity to C₂₊ hydrocarbons, while the conversion of methane remains relatively constant. Small surface concentrations of lead are sufficient to achieve C₂₊ selectivities of 80 and 90%, with oxygen and nitrous oxide, respectively, in contrast with 18 and 46%, respectively, obtained in the absence of lead. Since surface concentration of lead species sufficient to stabilize pairs of methyl radicals in close proximity to each other would be expected to facilitate the formation of C₂ hydrocarbons, an ensemble effect appears to be extant.     

Preparation and effect of Mo-V-Cr-Bi-Si oxide catalysts on controlled oxidation of methane to methanol and formaldehyde

Mo-Cr-V-Bi-Si multi-component oxide catalysts were synthesized by three different coprecipitation methods and used in the controlled oxidation of methane to methanol and formaldehyde. It was shown that Mo content in Mo-V-Cr-Bi-Si oxides and the performance of these catalysts were strongly influenced by different coprecipitation methods. The highest methanol and formaldehyde selectivity of 80.2% could be achieved at a methane conversion of 10 % for the catalyst prepared by a particular method. The results of XRD indicated that the crystalline phase structures of catalysts were sensitive to Mo, V and Bi loadings. Bi(III) could combine with V(V) and Mo(VI) to form BiVO₄ and γ-Bi₂MoO₆, whereas Cr seemed to form a single Cr₂O₂ crystalline phase in the presence of Bi. The effects of Mo and Cr loading on controlled methane oxidation were also investigated. Mo(VI) oxide appears to favor the formation of partial oxidation products and Cr(III) oxide seems to enhance the conversion of methane.     

Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C₂ products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C₂ hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C₂ hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 (NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO₂ yields were observed under these reaction conditions, and the selectivities to formaldehyde and C₂ hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C₂ hydrocarbons).     

Rh负载的整体型催化剂甲烷催化部分氧化过程

采用负载Rh的泡沫独石整体型催化剂研究了甲烷催化部分氧化过程,考察了外界温度、空速和甲烷与氧气进料比例对反应转化率和选择性的影响,并对过程控制条件和调控参数进行了分析。研究结果表明该过程为毫秒级超短接触过程,反应可以在自热条件下进行,高空速条件下（8×10⁵ h^-1）,甲烷与氧气进料比（体积比）为1.8,甲烷转过率超过90%,CO选择性接近95%,H₂选择性超过90%。外界加热对过程有利,可获得更高的转化率和选择性。     

Comparison of the surface and catalytic properties of rare earth‐promoted CaO catalysts in the oxidative coupling of methane

Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650–800 °C, CH₄/O₂ ratios, 2.0–8.0 and space velocity, 51 360 cm³ g^?1 h^?1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm‐promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below ?700 °C, whereas at higher temperature no further effect is seen. The La₂O₃? CaO, Nd₂O₃? CaO and Yb₂O₃? CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd‐promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH₄ conversion with 70.8% C₂₊ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) and the C₂₊ selectivity and/or C₂₊ yield has been observed. Copyright © 2005 Society of Chemical Industry     

Role of defect structure of active oxides in oxidative coupling of methane

A new approach to the choice of catalysts for oxidative coupling of methane based on the determination of the defect degree is suggested. For the most active catalysts (oxides of rare earth elements (REE), alkaline earth metals (AEM), manganese, lead), it is shown that oxygen defectness is of great importance in this process.Based on the structural approach Bi₂O₃-containing catalytic systems have been suggested. They have C₂-hydrocarbon formation activity and selectivity to be comparable with those ones for most active catalysts as REE oxides.     

Oxidative coupling of methane over NbO (p-type) and Nb2O5 (n-type) semiconductor materials

Oxidative coupling of methane to higher hydrocarbons was investigated using two types of semiconductor catalysts, NbO (p-type) and Nb₂O₅ (n-type) at 1 atm pressure. The ratio of methane partial pressure to oxygen partial pressure was changed from 2 to 112 and the temperature was kept at 1023 K in the experiments conducted in a cofeed mode. The results indicated a strong correlation between C₂₊ selectivity performance and the electronic properties of the catalyst in terms of p-vs. n-type conductivity. The p-type semiconductor catalyst, NbO, had a larger selectivity (e.g. 95.92%) over the n-type Nb₂O₅ catalyst (23.08%) both at the same methane conversion of 0.64%. Catalyst characterization via X-ray diffraction, TGA and reaction studies indicated that NbO was transformed to Nb₂O₅ during the course of the reaction which limits catalyst life.     

Activities of γ-Al2O3-Supported Metal Oxide Catalysts in Propane Oxidative Dehydrogenation

The activities of metal oxide catalysts in propane oxidative dehydrogenation to propene have been studied. The catalysts are M/-Al₂O₃ (where M is an oxide of Cr, Mn, Zr, Ni, Ba, Y, Dy, Tb, Yb, Ce, Tm, Ho or Pr). Both transition metal oxides (TMO) and rare-earth metal oxides (REO) are found to catalyze the reaction at 350-450 °C, 1 atm and a feed rate of 75 cm³/min of a mixture of C₃H₈, O₂ and He in a molar ratio of 4:1:10. Among the catalysts, Cr-Al-O is found to exhibit the best performance. The selectivity to propene is 41.1% at 350 °C while it is 54.1% at 450 °C. Dy-Al-O has the highest C₃H₆ selectivity among the REO. At 450 °C, the other catalysts show C₃H₆ selectivity ranging from 16.2 to 37.7%. In general TMO show higher C₃H₆ selectivity than REO, which, however, show higher C₂H₄ selectivity. An attempt is made to correlate propane conversion and selectivity to C₃H₆ with metal-oxygen bond strength in the catalysts. For the TMO a linear correlation is found between the standard aqueous reduction potential of the metal cation of the respective catalyst and its selectivity to propane at 11% conversion. No such correlation has been found in the case of REO. Analyses of the product distributions suggest that for TMO propane activation the redox mechanism seems to prevail while the REO activate it by adsorbed oxygen.     

Oxidative conversion of methane on lanthanum-containing hydroxychloride catalysts

The activity of lanthanum-containing oxide and hydroxychloride catalysts in oxidative condensation of methane has been studied. It has been established that modification of La₂O₃ with sodium (NaCl) and application on Al₂O₃ leads to high selectivity of the process. Selectivity with respect to C₂ hydrocarbons at 760°C amounts to 84% with 20·4% methane conversion. Catalyst NaClLaO/Al₂O₃ is multiphase, and NaCl, LaOCl and LaAlO₃ phases coexist in reaction conditions. As the reaction progresses there is a change in the phase composition and selectivity of the catalyst.     

Vanadium oxide catalysts on structured microfiber supports for the selective oxidation of hydrogen sulfide

Vanadium(V) oxide catalysts for the selective oxidation of hydrogen sulfide to sulfur on a nonporous glass-fiber support with a surface layer of a porous secondary support (SiO₂) are studied. The catalysts are obtained by means of pulsed surface thermosynthesis. Such catalysts are shown to have high activity and acceptable selectivity in the industrially important region of temperatures below 200°C. A glass-fiber catalyst containing vanadium oxide (10.3 wt % of vanadium) in particular ensures the complete conversion of H₂S at a temperature of 175°C and a reaction mixture hourly space velocity (RMHSV) of 1 cm³/(g_cat s) with a sulfur yield of 67%; this is at least 1.35 times higher than for the traditional iron oxide catalyst. Using a structured glass-fiber woven support effectively minimizes diffusion resistance and greatly simplifies the scaleup of processes based on such catalysts. Such catalysts can be used for the cleansing of tail gases from Claus units and in other processes based on the selective oxidation of H₂S.     

Composite resins for 3D printing of corundum and corundum-kaolin based monoliths for catalytic applications

Several composite organic-inorganic resins dedicated to 3D printing using Digital Light Processing (DLP) technology containing nano- and micro-structured corundum as well as corundum/kaolin mixtures were prepared and characterised in terms of their rheology and stability. Using these resins, short monoliths were printed using the DLP technology at a resolution of x:y:z = 30/30/50 μm. After thermal pre-treatment, the printed materials were studied by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Based on the selected monoliths, MnO_x- and Na₂WO₄-containing catalysts were prepared by two-step impregnation and tested in the oxidative coupling of methane (OCM) at 820 °C using a ratio of methane/oxygen of 3.8/1. The maximum conversion of methane (28%) and total conversion of oxygen (100%), as well as stable selectivity to ethane (22%) and ethylene (44%), were achieved using three short monoliths (GHSV = 7676 h^?1). A further enhancement of the number of monoliths influenced only the CO_x and C₃ selectivity. Moreover, a comparative study of monolithic and powder samples with identical compositions revealed that for the monolithic catalyst the yield of C₂–C₃ hydrocarbons is slightly lower (1–2%) but at a 3–4 times lower pressure drop.     

Low-Temperature SCR of NO with NH3 over USY-Supported Manganese Oxide-Based Catalysts

A series of catalysts of manganese oxide, manganese–cerium and iron–manganese oxide supported on USY (ultra-stable Y zeolite) were studied for the low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. It was found that MnO_x/USY have high activity and high selectivity to N₂ in the temperature range 80-180 °C. The addition of iron and cerium oxide increased NO conversion significantly although the single-component Fe/USY and Ce/USY catalysts had low activities. Among the catalysts studied in this work, the 14% Ce-6% Mn/USY showed the highest activity. The results showed that this catalyst yielded nearly 100% NO conversion at 180 °C at a space velocity of 30 000 cm³ g^-1 h^-1. The only product is N₂ (with no N₂O) below 150 °C. The effects of the concentration of oxygen, NO and NH₃ were studied and the steady-state kinetics were also investigated. The reaction order is 1 with respect to NO and zero with respect to NH₃ on the 14% Ce-6% Mn/USY catalyst at 150 °C.     

Methane-water redox reaction on A2SnO4 (A=Mg, Ca, Sr, Ba) oxide to produce C2 hydrocarbons

A₂BO₄ type oxides consisting of an alkali earth metal and tin showed high selectivity (>99%) and activity for the oxidative coupling of methane at 1023 K in a methane-water redox system where active oxygen species were regenerated by water. The products were C₂ hydrocarbons and hydrogen. Repeated reaction-oxidation cycles showed that the oxide is stable under both oxidative and reductive atmosphere. Doping of Bi to the oxide was found to enhance the activity for the oxidative coupling of methane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Confined Iron Nanoparticles on Mesoporous Ordered Silica for Fischer–Tropsch Synthesis

Iron oxide particles were deposited in an ordered mesoporous material (SBA-15) with the aim of studying its behavior in the catalytic hydrogenation of CO (Fischer–Tropsch Synthesis). Bulk iron oxide, and iron supported on porous silica with different textural properties (Aerosil^®-200) were used for comparison. The characterization of the materials showed that in the Fe@SBA-15 material, iron nanoparticles were confined inside the mesopores of the SBA-15 support (pore diameter ~?8 nm), and Fe@Aerosil^®-200 material also presented iron oxide nanoparticles highly dispersed on the material. In situ Synchrotron radiation XRD studies were performed in order to study the evolution of iron phases in the Fe@SBA-15 and the bulk iron oxide under hydrogen and hydrogen/carbon monoxide conditions. DFT calculations were performed on bare Fe(100) and a Fe₁₆ cluster in CO activation and C_xH_y hydrogenation. Catalytic microactivity tests, performed at conversions of ~?6–8%, showed important differences in the selectivity of the materials. Higher selectivity to methane and light hydrocarbons were observed in the supported catalysts (Fe@SBA-15 and Fe@Aerosil^®-200) than in bulk Fe catalyst. Moreover, the supported catalysts showed selectivity to ethylene (Fe@SBA-15) and propylene (Fe@Aerosil^®-200), products that were not observed in the bulk iron catalyst. On the other hand, bulk iron showed a major selectivity to higher hydrocarbons (C₅–C₉) and oxygenates.