Catalytic Performance For Oxidative Coupling Research Articles

Al- and Mg-doped SrTiO3 perovskite steps: The catalytic performance for oxidative coupling of methane

One of the most important processes for converting methane into value-added chemical products is the reaction of the oxidative coupling of methane. The catalytic performance of a heterogenous catalyst can be evaluated using the first principles. The simultaneous presence of defects (oxygen vacancies, metal dopants and edges) on perovskites makes the simulations more realistic and closer to the experimental conditions. Results showed that CH3 adsorption and formation energy of the oxygen vacancy are sufficient to verify catalytic activity. The comparison of the steps and the surface simulations shows different properties and that the step is not always the preferred catalytic site.

Atomically dispersed SrOx species on exposed {2 2 2} facets of pyrochlore La2Zr2O7 nanocrystals for boosting low-temperature oxidative coupling of methane

The efficient catalysts of strontium oxide (SrOx) supported on exposed {222} facets of pyrochlore La2Zr2O7 nanocrystals (SrOx/LZO-Y) were elaborately synthesized by incipient wetness impregnation method. The introduction of atomically dispersed SrOx species on La2Zr2O7 surface can increase the amount of surface oxygen vacancies induced by Sr2+ substitution of La3+ cations, and the increasing of moderate surface alkaline sites are beneficial to enhance catalytic performance for oxidative coupling of methane (OCM). The SrOx/LZO-Y catalysts have super catalytic performance during low-temperature OCM reaction. SrOx/LZO-12 catalyst with the highest concentrations of O2– species exhibits the highest activity and the lowest apparent activation energy (84 kJ mol−1) during low-temperature OCM into C2H4 and C2H6 (C2) products, i.e., its CH4 conversion, selectivity and yield of C2 products at 600 °C are 30.5 %, 47.4 % and 14.5 %, respectively, and its ratio of C2H4/C2H6 products is 0.77. Based on the results of multiple experimental characterizations, the mechanism of SrOx/LZO-Y catalysts for OCM reaction is proposed: the surface alkaline active centers (O2–) induced by doping of Sr ions can promote the critical step of CH4 activation significantly during low-temperature OCM reaction. It is a new strategy to develop the low-temperature and high-activity catalysts for OCM in practical application.

Raw Biogas as Feedstock for the OCM Process

The paper presents the research results obtained in the process of oxidative coupling of methane, in which unpurified biogas was used as the feedstock. Biogas obtained from two kinds of biomass materials, i.e., plant materials (potato and beet pulp, Corn-Cob-Mix—biogas 1) and animal waste (waste from fish filleting—biogas 2) was considered. The influence of temperature, the ratio of methane/oxygen and total flows of feedstock on the catalytic performance in oxidative coupling of methane process was investigated. Comparative tests were carried out using pure methane and a mixture of methane-carbon dioxide to simulate the composition of biogas 2. The process was carried out in the presence of an Mn-Na2WO4/SiO2 catalyst. Fresh and used catalysts were characterised by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption techniques. In oxidative coupling of methane, the type of raw material used as the source of methane has a small effect on methane conversion (the differences in methane conversion are below 3%), but a significant effect on the selectivity to C2. Depending on the type of raw material, the differences in selectivity to C2 reach as high as 9%. However, the Mn-Na2WO4/SiO2 catalyst operated steadily in the tested period of time at any feedstock composition. Moreover, it was found that CO2, which is the second main component of biogas in addition to methane, has an effect on catalytic performance. Comparative results of catalytic tests indicate that the CO2 effect varies with temperature. Below 1073 K, CO2 exerts a small poisoning effect on methane conversion, while above this temperature the negative effect of CO2 disappears. In the case of selectivity to C2+, the negative effect of CO2 was observed only at 1023 K. At higher temperatures, CO2 enhances selectivity to C2+. The effect of CO2 was established by correlating the catalytic results with the temperature programmed desorption of CO2 investigation. The poisoning effect of CO2 was connected with the formation of surface Na2CO3, whose concentration depends on temperature.

Morphology Effects of Nanoscale Er2O3 and Sr-Er2O3 Catalysts for Oxidative Coupling of Methane

Er2O3 nanorods were prepared by a hydrothermal method, and Sr-modified Er2O3 nanorods (Sr-Er2O3) were synthesized using an impregnation method. Their catalytic performance for oxidative coupling of methane was investigated. The catalysts were characterized by several techniques such as XRD, N2 adsorption, TEM, XPS, O2-TPD and CO2-TPD. Compared with Er2O3 and Sr-Er2O3 nanoparticles, Er2O3 and Sr-Er2O3 nanorods exhibit higher CH4 conversion and C2–C3 selectivity. This is caused by higher (O− + O2−)/O2− ratio, a higher number of chemisorbed oxygen species and moderate basic sites achieved on the nanorods catalysts. The Sr-Er2O3 nanorods afford a 23.2% conversion of CH4 with 50.3% selectivity to C2–C3 at 650 °C.

Oxygen availability and catalytic performance of NaWMn/SiO2 mixed oxide and its components in oxidative coupling of methane

Relationships between catalytic performance in oxidative coupling of methane (OCM) and properties of lattice oxygen of the mixed NaWMn/SiO2 oxide and its components are studied. It is demonstrated that in hydrogen and methane flows tungsten and manganese can be practically completely reduced from W6+ to W0 and from Mn4+/Mn3+ to Mn2+ states, respectively. Reduction in hydrogen proceeds at substantially lower temperatures. If the system is reduced in methane, the formation of methane oxidation products (C2 hydrocarbons, CO, CO2, water, H2) is observed, and the product distribution substantially changes at increasing degree of reduction. In addition to strongly-bonded oxygen which can be removed from the NaWMn/SiO2 system by reduction, a more weakly-bonded form can be detected using temperature programmed desorption (TPD). This form of oxygen can be reversibly removed at temperatures above 640°C and replenished at much lower temperatures. Its amount (∼16μmol O2/g) is about 4.5% of that removable by reduction in hydrogen and methane, or about 10% of the formal oxygen surface monolayer. Using the sequential O2/CH4 pulse technique, it is also shown that this weakly-bonded form of oxygen can participate in the steady-state catalytic OCM reaction because its presence substantially increases the rate of selective formation of C2 hydrocarbons and its lifetime at typical OCM temperatures (around 800°C) exceeds by far the characteristic time of the catalytic reaction.

Influence of CaO–ZnO supplementation as a secondary catalytic bed on the oxidative coupling of methane

A reactor with dual catalyst beds was proposed for the direct improvement of methane productivity to C2 hydrocarbons. In this bed configuration, methane was converted to ethane and ethylene over lithium supported magnesium oxide as a primary catalytic bed. The reaction tests were performed in the presence of CaO–ZnO catalysts as the secondary bed to convert the methane residue into ethane and ethylene. Both Li/MgO and CaO–ZnO catalysts were prepared by the wet impregnation method, tested in the dual fixed bed quartz reactor and characterized by BET, XRD, scanning electron microscopy and temperature programmed desorption of carbon dioxide. Catalytic activity tests were carried out at different temperatures ranging from 675 to 825 °C. The influence of CaO–ZnO supplementation as a secondary bed and its composition on total catalytic performance for oxidative coupling of methane was studied by changing Zn/Ca ratio. The effects of secondary bed dilution with quartz and mixed bed configuration on reaction productivity were also investigated. The experimental results showed that this supplementation could improve total yield toward C2 production.

A novel particle/monolithic two-stage catalyst bed reactor and their catalytic performance for oxidative coupling of methane

A novel two-stage catalyst bed reactor was constructed comprising of the 5%Na2WO4–2%Mn/SiO2 particle catalyst and the 5%Na3PO4–2%Mn/SiO2/cordierite monolithic catalyst. The reaction performance of the oxidative coupling of methane (OCM) in the two-stage bed reactor system was evaluated. The effects of the bed height and operation mode, as well as the reaction parameters such as reaction temperature, CH4/O2 ratio and flowrate of feed gas on the catalytic performance were investigated. The results indicated that the two-stage bed reactor system exhibited a good performance for the OCM reaction when the feed gases were firstly passed through the particle catalyst bed and then to the monolithic catalyst bed. The CH4 conversion of 32.6% and C2 selectivity of 67.5% could be obtained with a particle catalyst bed height of 10mm and a monolithic catalyst bed height of 50mm in the two-stage bed reactor. Both of the CH4 conversion and C2 selectivity have been increased by 4.8% and 2.5%, respectively, as compared with the 5%Na2WO4–2%Mn/SiO2 particle catalyst in a single-bed reactor and by 7.7% and 16.1%, respectively, as compared with the 5%Na3PO4–2%Mn/SiO2/cordierite monolithic catalyst in a single-bed reactor. The catalytic performance of the OCM in the two-stage bed reactor system has been remarkably improved. The TPR results indicate the high temperature reduction oxygen species in the monolithic catalyst might be favorable to the formation of C2 products.

Ce-promoted Mn/Na 2WO 4/SiO 2 catalyst for oxidative coupling of methane at atmospheric pressure

A series of Ce-promoted Mn-Na 2WO 4/SiO 2 catalysts were prepared by incipient wetness impregnation method, and their catalytic performance for oxidative coupling of methane (OCM) was investigated at atmospheric pressure in a micro-quartz-tube reactor. The catalysts were characterized by X-ray diffraction (XRD), temperature program reduction (TPR) and BET surface area. Ce promoter increased surface area and Na 2WO 4 species dispersion, which enriched the amount of the surface species. In addition, Ce promoter increased the Na/W species reduction, but the reduction peak shifted to higher temperature. Stability test of 5wt%Ce catalyst indicated suitable performance and stability. The selectivity and yield of C 2+ hydrocarbons after 50 h operation reached 65.5% and 19.6%, respectively, at 840 °C over 5wt%Ce-2wt%Mn-5wt%Na 2WO 4/SiO 2 catalyst.

Studies on the structure and catalytic performance of S and P promoted Na-W-Mn-Zr/SiO 2 catalyst for oxidative coupling of methane

A series of Na-W-Mn-Zr/SiO 2 catalysts promoted by different contents of S or/and P were prepared and their catalytic performance for oxidative coupling of methane was investigated to clarify the effect of S and P on the Na-W-Mn-Zr/SiO 2 catalyst. The catalysts were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). From the characterization results, it is found that the addition of S and P to the Na-W-Mn-Zr/SiO 2 catalyst helps the formation of active phases, such as a-cristobalite, Na 2WO 4, ZrO 2, and Na 2SO 4. Moreover, the addition of S and P increases the concentration of surface-active oxygen species by improving the migration of active components from the bulk phase to the surface of the catalyst. According to the activity test, impressive methane conversion and C 2 hydrocarbons yield were obtained at a low temperature of 1023 K over the six-component Na-W-Mn-Zr-S-P/SiO 2 catalyst, which contained 2 wt% S and 0.4 wt% P simultaneously. The deactivation of Na-W-Mn-Zr-S-P/SiO 2 was due to the loss of surface active components.

The catalytic performance in oxidative coupling of methane and the surface basicity of La 2O 3, Nd 2O 3, ZrO 2 and Nb 2O 5☆

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La 2O 3, Nd 2O 3, ZrO 2 and Nb 2O 5 has been investigated under various conditions. The results confirmed that the activity of La 2O 3 and Nd 2O 3 was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La 2O 3, Nd 2O 3, ZrO 2 and Nb 2O 5 was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO 2. Both methods showed that La 2O 3 and Nd 2O 3 had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO 2 had only negligible amount of weak basic sites and Nb 2O 5 was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO 2 it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.

Further evidence of responsibility of impurities in MgO for variability in its basicity and catalytic performance in oxidative coupling of methane

Further evidence was delivered that certain impurities, which could be contained in MgO samples, might be responsible for observed variability in MgO basicity and catalytic performance in oxidative coupling of methane. The surface basicity/base strength distribution of a series of MgO samples containing or not containing Ca and Na impurities was determined by a temperature-programmed desorption of CO 2. It was revealed that samples containing Ca and Na impurities have much more medium, strong and very strong basic sites. The surface basicity of MgO samples containing added alkali or alkaline earth compounds or water was characterized by a test reaction of transformation of 2-butanol. It was confirmed that the introduction of these compounds to a pure MgO enhanced both its basicity and activity in oxidative coupling of methane.

The effect of some impurities on the basicity of MgO tested by the transformation of 2-butanol and on its catalytic performance in oxidative coupling of methane

The properties and the role of MgO in oxidative coupling of methane to C 2+ hydrocarbons (OCM) have been studied. The transformation of 2-butanol over several unpromoted MgO samples was applied as a test reaction for the determination of the surface basicity/acidity. The specific rates of alcohol transformation and of product formation were determined in a flow silica reactor at 608±1 K. All the investigated MgO samples exhibited significant dehydrogenation activity and extremely weak dehydration activity, producing 2-butanone as the main product and only minor or even negligible amounts of butenes. The samples of MgO containing Ca compounds as impurities showed enhanced basicity, as concluded from the higher rates of ketone formation and from the presence of solely 1-butene among butenes. These samples also showed enhanced catalytic performance in the OCM process. The surface basicity of MgO, affected by the content or the absence of some impurities (Ca, Na, etc. oxides), was recognized as the decisive factor responsible for observed diversity in the catalytic performance of MgO in OCM, more significant than other surface properties or precursors.

The effect of gas atmosphere used in the calcination of MgO on its basicity and catalytic performance in oxidative coupling of methane

Influence of gas atmosphere used in the calcination of MgO (flowing oxygen, air, helium, nitrogen) on its basicity has been investigated. Pure MgO of relatively low basicity and MgO of higher basicity, containing Ca and Na compounds as impurities, were examined. The surface basicity/base strength distribution was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO 2. Both methods revealed that the calcination of MgO samples in the stream of oxygen or air gave MgO of lower basicity than obtained by calcination in the stream of helium or nitrogen. This effect may indicate a different character of interactions between various gas-phase molecules and the surface. It was also shown that MgO calcined in oxygen or air when used in the catalytic oxidative coupling of methane, gave lower conversions of methane and oxygen than that calcined in the inert gas atmosphere. The choice of a gas used in the calcination of MgO, may therefore, influence the basicity and catalytic properties of calcined MgO.

Oxidative coupling of methane over Na-Mn-W/SiO2 catalyst at higher pressure

Na-Mn-W/SiO2 catalysts were prepared and their catalytic performance for oxidative coupling of methane (OCM) was evaluated in a stainless-steel microreactor at elevated pressure. The results show that a CH4 conversion of 15.1% with a C2+ selectivity of 71.8% was obtained under 750oC, 1.0×105h-1 GHSV, CH4/O2 ratio of 8 and 1.0 MPa. Moreover, 17.3% CH4 conversion with 51.6% C2 selectivity and 23.6% C3-C4 selectivity was obtained under 750oC, 2.0×105h-1 GHSV, CH4/O2 ratio of 8 and 1.0 MPa.

Oxidative coupling of methane over calcium chloride-promoted calcium chlorophosphate

CaCl 2-promoted calcium chlorophosphate catalysts exhibited high catalytic performance for oxidative coupling of methane (OCM), while unpromoted calcium chlorapatite and calcium chlorophosphate exhibited poor catalytic performance. The presence of CaCl 2 also yielded high ethene selectivity. Partial substitution of the calcium with transition metals, such as zinc, lead and nickel further enhanced the catalytic performance at 973 K; the optimum extent of substitution was about 4%. At 1023 K, however, the effect of substitution appeared to be little. Too excessive substitution gave rise to a decrease of the activity. At 1023 K, the highest C 2 yield of around 22% was obtained with the C 2 selectivity of 56–59%. The C 2H 4 selectivity was also high, around 50%. Production of a significant amount of H 2 was observed over the CaCl 2-promoted calcium chlorophosphate catalysts. We propose that the major pathways for the production of H 2 are the steam reforming and partial oxidation of ethane and ethene which accompany simultaneous production of CO.

Effect of Metallo-organic Precursors on the Synthesis of Sm–Sn Pyrochlore Catalysts: Application to the Oxidative Coupling of Methane

Synthesis of a series of Sm2Sn2O7pyrochlore catalysts by a sol-gel-like method has been developed. The preparation from the oxides gives stoichiometric pyrochlore, unlike the result obtained when starting from chlorinated salts (SmCl3or SnCl2), which form tin-deficient pyrochlores. A spectroscopic study of the behaviour of the starting oxides or chlorides in propionic acid, to generate a series of precursors, has been carried out, particular attention being focused on the precursors generated by SnCl2in propionic acid using119Sn NMR and FTIR spectroscopy. The formation of SnCl2(C2H5COO)2as the main precursor is responsible for the tin deficiency observed in the pyrochlore structure. The catalytic performance in oxidative coupling of methane is directly correlated with the tin deficiency in the bulk of the pyrochlore. The work highlights the importance of controlling precisely the synthesis of the catalysts.