Alkene selectivity enhancement in the oxidation of propane on calcium-based catalysts

The oxidation of propane has been investigated in the presence and absence of tetrachloromethane (TCM) on calcium hydroxyapatite (CaHAp), Ca₃(PO₄)₂, CaSO₄ and CaO at 723 K. In the absence of TCM, the conversion of C₃H₈ on CaHAp was 7.7–9.2% during 6 h on-stream while that on Ca₃(PO₄)₂, CaSO₄ and CaO was 0.6, 0 and 0.2–0.4%, respectively. The principal products on all catalysts in the absence of TCM were CO and CO₂ with small selectivities to C₃H₆ and C₂H₄ (both 5–6%) observed on CaHAp. Upon addition of TCM, the selectivity to C₃H₆ on all catalysts and the conversion of C₃H₈ on CaSO₄ increased while, with increasing time-on-stream, the changes in the conversion and selectivity were dependent upon the nature of the catalysts. XPS and XRD analyses provide evidence for the presence of chlorine in the surface and/or bulk of three of the catalysts, suggesting that chlorinated species on the solids play a role in the selectivity enhancement, but the absence of chlorine from the sulphate demonstrates the dissimilarities of the catalysts in their abilities to sorb and decompose TCM. This revised version was published online in July 2006 with corrections to the Cover Date.     

A site-isolated mononuclear iridium complex catalyst supported on MgO: Characterization by spectroscopy and aberration-corrected scanning transmission electron microscopy

Supported mononuclear iridium complexes with ethene ligands were prepared by the reaction of Ir(C₂H₄)₂(acac) (acac is CH₃COCHCOCH₃) with highly dehydroxylated MgO. Characterization of the supported species by extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies showed that the resultant supported organometallic species were Ir(C₂H₄)₂, formed by the dissociation of the acac ligand from Ir(C₂H₄)₂(acac) and bonding of the Ir(C₂H₄)₂ species to the MgO surface. Direct evidence of the site-isolation of these mononuclear complexes was obtained by aberration-corrected scanning transmission electron microscopy (STEM); the images demonstrate the presence of the iridium complexes in the absence of any clusters. When the iridium complexes were probed with CO, the resulting IR spectra demonstrated the formation of Ir(CO)₂ complexes on the MgO surface. The breadth of the ν_CO bands demonstrates a substantial variation in the metal–support bonding, consistent with the heterogeneity of the MgO surface; the STEM images are not sufficient to characterize this heterogeneity. The supported iridium complexes catalyzed ethene hydrogenation at room temperature and atmospheric pressure in a flow reactor, and EXAFS spectra indicated that the mononuclear iridium species remained intact. STEM images of the used catalyst confirmed that almost all of the iridium complexes remained intact, but this method was sensitive enough to detect a small degree of aggregation of the iridium on the support.     

Effects of MgO promoter on properties of Ni/Al2O3 catalysts for partial oxidation of methane to syngas

The effects of MgO promoter on the physicochemical properties and catalytic performance of Ni/Al₂O₃ catalysts for the partial oxidation of methane to syngas were studied by means of BET, XRD, H₂-TPR, TEM and performance evaluation. It was found that the MgO promoter benefited from the uniformity of nickel species in the catalysts, inhibited the formation of NiAl₂O₄ spinel and improved the interaction between nickel species and support. These results were related to the formation of NiO-MgO solid solution and MgAl₂O₄ spinel. Moreover, for the catalysts with a proper amount of MgO promoter, the nickel dispersiveness was enhanced, therefore making their catalytic performance in methane partial oxidation improved. However, the excessive MgO promoter exerted a negative effect on the catalytic performance. Meanwhile, the basicity of MgO promoted the reversed water-gas shift reaction, which led to an increase in CO selectivity and a decrease in H₂ selectivity. The suitable content of MgO promoter in Ni/Al₂O₃ catalyst was ∼7 wt-%. Translated from Journal of Fuel Chemistry and Technology, 2006, 34(4): 450–455 [译自: 燃料化学学报]     

A new supported Fe-MnO catalyst for the production of light olefins from syngas. II. Effect of support on the secondary reactions of C2H4

The TPSR technique was used to investigate the effect of the support on the secondary reactions of ethylene formed during CO hydrogenation. Based on the results of CO hydrogenation and CO/H₂-TPSR characterization, it was found that different supports induced different secondary reactions, and thus affected the selectivity to light olefins directly. The Fe-MnO/MgO catalyst (based on basic support) causes disproportionation of C₂H₄, and thus, leads to the formation of C₃H₆. The Fe-MnO/Al₂O₃ catalyst (based on acidic support) showed obvious hydrogenation of C₃H₆. The disproportionation of C₂H₄ was also promoted by the Fe-MnO/Al₂O₃ catalyst, but because of its activity for C₃H₆ hydrogenation, a large amount of C₃H₈ was produced. The different C₂H₄ secondary reactions are relevant to different CO/H₂ reaction pathways over the catalyst surface, so the Fe-MnO/MgO catalyst is a desirable catalyst for the production of light olefins from CO/H₂ while the Fe-MnO / Al₂O₃ catalyst was not so.     

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.     

Oxidative coupling of methane. The effect of alkali chlorides on molybdate based catalyst leading to high selectivity in C3-product formation

LiCl-Na₂MoO₄ was found to be an active catalyst for oxidative coupling of methane at temperatures around 620 °C. In these systems, the selectivity for the formation of C₃-products exceeds the selectivity for the formation of C₂-products. While the homogeneous reaction of CH₄ and O₂ leads to C₃H₆ as C₃-product, the 50% LiCl-50% Na₂MoO₄ catalyst leads to C₃H₈ as the predominant C₃-product, indicating that in the latter case the reaction cannot be purely homogeneous. The dependency of the product distribution on temperature, gas composition, reactor dimensions, flow rate, CH₄/O₂ ratio and type of catalyst has been studied. The reaction was studied by co-feeding CH₄, O₂ and a diluent gas at atmospheric pressure continuously in a conventional flow reactor containing the catalyst. The reaction products observed were: C₂H₄, C₂H₆, C₃H₆, C₃H₈, H₂O and CO + CO₂. The two latter gases were the main oxidation products observed. Characterization of the catalysts used was carried out by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD).     

Oxidative coupling of methane using Li/MgO catalyst: Re-appraisal of the optimum loading of Li

The effect of the level of lithium carbonate doping on MgO, prepared by thermal decomposition of the basic carbonate, is re-examined. A low, sub monolayer, loading, ie. 0.2% Li₂CO₃-MgO is shown to significantly enhance both the specific activity for methane activation and the total C₂ hydrocarbon selectivity. The study indicates that the optimal loading of alkali promoters on MgO prepared in this way is considerably lower than indicated in previous studies.     

Catalytic methane coupling under periodic operation

Composition modulation strategies for improving selectivity to the C₂ products in the oxidative coupling of methane were investigated experimentally with Li₂O/MgO and CeO/Li₂O/MgO catalysts at about 100 kPa pressure and 750°C. Strategies used were 1) “methane cycling” between mixtures with different concentrations of CH₄, 2) “oxygen cycling” between mixtures with different concentrations of oxygen, 3) bang-bang cycling (air-CH₄), and 4) flushing the catalyst with an inert between exposures to reactants. Experimental C₂ yields and ethylene to total C₂ ratios showed that composition forcing is better with the Ce/Li/MgO catalyst. Methane cycling proved to be the best strategy explored.     

Effect of chlorine on the selectivity of a fischer-tropsch catalyst composed of iron/vanadium oxides

Fischer-Tropsch catalysts (Fe/V oxides with ZnO and K₂CO₃ as promoters) were exposed to CHCl₃, thereby producing surface and bulk chlorides. The effect of this exposure on activity and selectivity was studied in a continuous recycle reactor at a total pressure of 10 bar (CO/H₂ in most experiments: ca 1:1) in a temperature range between 200 and 343°C. CHCl₃ was introduced in amounts of up to 1 × 10^?2 mol chlorine per g catalyst. The catalyst samples were characterized by internal surface area, pore-size distribution and adsorption capacities for CO, H₂ and C₂H₄. Prior to synthesis, the catalysts were reduced by H₂. Catalyst exposure to CHCl₃ resulted in a decrease of activity and considerable changes in product distribution. Hydrogenation and isomerization of 1-olefins were partly suppressed; the chain length of the products was slightly increased. Deactivation of the catalysts due to chlorine addition was partly reversible during operation, while olefin formation was not significantly altered with time-on-stream. The effect of chlorine on activity and selectivity is explained by dissociation of CO as the chain initiating step and CO insertion into a carbon/metal bond as a possible chain propagation step. Since adsorption capacity for H₂ decreases on the addition of chlorine, this may also contribute to lower activity and change in selectivity, compared to the unexposed catalyst.     

Methane-Propylene homologation and reactivity of surface carbon on modified Ni-Al2O3 catalysts

Modified Ni catalysts supported on alumina and reduced in CH₄ have been investigated for the synthesis of C₄ hydrocarbons from CH₄ and C₃H₆. Addition of K or P to the Ni/Al₂O₃ catalyst increased C₄ selectivity and C₃H₆ conversion. A maximum selectivity to the desired C₄ product of 9 mol % was obtained at 350°C and 101 kPa with a feed gas composition of 90 mol% CH₄/10 mol% C₃H₆, but the catalyst activity declined rapidly with time-on-stream. Large amounts of unreactive carbon were deposited on the catalyst surface following reduction in CH₄ and reaction in CH₄/C₃H₆. However, the relative amount of a much more reactive species identified from Temperature-Programmed-Surface-Reaction that was formed in the presence of CH₄ and C₃H₆, is shown to correlate with the catalyst C₄ yield. Both the C₄ yield and the relative amount of this low temperature carbonaceous species increased in the order Ni<Ni/P<Ni/K.     

Influence of the presence of chlorine on the selectivity to ethane and ethene during methane coupling over samarium-based catalysts

The oxidative coupling of methane to ethane and ethene has been investigated on chlorine-containing catalysts. The sensitivity of the product distribution to temperature, gas composition, flow-rate, catalyst mass, and reactor dimensions has been demonstrated. It has been found that a long contact time is an important factor in raising the C₂H₄/C₂H₆ ratio, and that reactions within the catalyst bed are important for the conversion of ethane to ethene. Back-mixing of the reagents into the gas space above the catalyst bed tended to lead to combustion, but appeared to have little influence on enhancing the C₂H₄/C₂H₆ ratio. Experiments have been performed with Sm₂O₃, SmOCl and SmCl₃ and with various pairs of these catalysts when separated by a gas space. These experiments have demonstrated the importance, under certain circumstances, of gas phase chlorine species, especially radicals, in the conversion of ethane to ethene. However, the results also show that these chlorine-induced gas phase reactions occur within the voids between the particles in the catalyst bed and not in the free gas space outside the catalyst bed.     

Promotion effect of K2O and MnO additives on the selective production of light alkenes via syngas over Fe/silicalite-2 catalysts

The addition of K₂O and MnO promoters enhances catalyst activity and selectivity to light alkenes during CO hydrogenation over silicate-2 (Si-2) supported Fe catalysts. The results of CO hydrogenation and CO-TPD, CO/H₂-TPSR, C₂H₄/H₂-TPSR and C₂H₄/H₂ pulse reaction over Fe/Si-2 catalysts with and without promoters clearly show that the MnO promoter mainly prohibits the hydrogenation of C₂H₄ and C₃H₆. Therefore, it enhances the selectivity to C₂H₄ and C₃H₄ products. Meanwhile further incorporating the K₂O additive into the FeMn/ Si-2 catalyst leads to a remarkable increase in both the capacity and strength of the strong CO adspecies. These produce much more [C_ad] via their dissociation and disproportionation at higher temperatures. This results in an increase in the CO conversion and the selectivity to light olefins. Moreover, the K₂O additive modifies the hydrogenating reactivity of [C_ad] and suppresses the disproportionation of C₂H₄ that occurs as a side-reaction. Both K₂O and MnO promoters play key roles for enhancing the selective production of light alkenes from CO hydrogenation over Fe/Si-2 catalyst.     

The conversion of methane to ethylene and ethane with near total selectivity by low temperature (< 610 ° C) oxydehydrogenation over a calcium-nickel-potassium oxide catalyst

The catalytic oxidative coupling of methane to C₂, C₃ and C₄ paraffins and olefins has been accomplished with close to 100% selectivity at methane conversions of about 10% per pass. Essentially no carbon oxides are formed and the mechanism appears to be a surface catalyzed reaction. Temperatures of < 600 ° C are used and the presence of steam is important. The catalyst comprises a ternary mixture of calcium, nickel and potassium oxides. Method of preparation and composition of the catalyst are critical for its performance. Presence of a carbidic carbon on the catalyst surface may be important.     

Oxidative coupling of methane over sodium-chloride-added sodium zirconium phosphates

Monosodium zirconium phosphate or disodium zirconium phosphate by itself did not catalyze the oxidative coupling of methane and also the deep oxidation of methane. However, NaCl-added sodium zirconium phosphates showed markedly increased activity and high C₂₊ selectivity in the oxidative coupling of methane, which indicates that chlorine species or NaCl plays an essential role in the catalytic action. The catalytic performance became more stable with increasing content of NaCl. The primary reason for the catalyst deactivation is the loss of chlorine, and a possible secondary reason is the transformation of catalytic substance to the sodium zirconium phosphates having higher Na/Zr ratios or decomposition of sodium zirconium phosphates to zirconium oxide and sodium phosphate. Two kinds of surface chlorine species were observed, and the lower-binding-energy species is considered to be much more active than the higher-binding-energy species in methane activation, although the latter is present in a larger amount than the former.     

Effect of Vanadium Promotion on Activated Carbon-Supported Cobalt Catalysts in Fischer–Tropsch Synthesis

The effect of vanadium promotion on activated carbon (AC)-supported cobalt catalysts in Fischer–Tropsch synthesis has been studied by means of XRD, TPR, CO-TPD, H₂-TPSR of chemisorbed CO and F-T reaction. It was found that the CO conversion could be significantly increased from 38.9 to 87.4% when 4 wt.% V was added into Co/AC catalyst. Small amount of vanadium promoter could improve the selectivity toward C₁₀–C₂₀ fraction and suppress the formation of light hydrocarbon. The results of CO-TPD and H₂-TPSR of adsorbed CO showed that the addition of vanadium increased the concentration of surface-active carbon species by enhancing CO dissociation and further improved the selectivity of long chain hydrocarbons. However, excess of vanadium increased methane selectivity and decreased C₅⁺ selectivity.     

Effect of surface species on activity and selectivity of MoO3/SiO2 catalysts in partial oxidation of methane to formaldehyde

The effect of the nature of surface species on the activity and selectivity of MoO₃/SiO₂ catalysts has been investigated for the partial oxidation of methane to formaldehyde. Characterization techniques including BET surface area, ambient and in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction were used in conjunction with steady-state reaction studies to relate the presence of different surface species to the activity and selectivity of the catalyst. Results of these experiments indicate the presence of a highly dispersed silicomolybdic species with terminal Mo=O sites appearing at lower MoO₃ loadings. As the weight loading increases, these sites are transformed into polymolybdate species, forming more Mo-O-Mo bridging sites at the expense of Mo=O sites. At high weight loadings, crystalline MoO₃ begins to form. The abundance of the Mo=O sites is believed to affect activity and selectivity in the partial oxidation of methane to formaldehyde.     

Influence of Mg Addition on the Catalytic Activity of Alumina Supported Ag for C3H6-SCR of NO

The influence of different magnesium (Mg) weight percentages (1, 2.5, 5, 7.5 and 10) over silver (3 wt%) impregnated alumina (SA) catalyst was investigated for the reduction of NO by C₃H₆. Mg doped SA catalysts were prepared by conventional impregnation method and characterized by XRD, BET-SA, ICP-MS, XPS, SEM, UV-DRS, H₂-TPR and O₂-TPD. The existence of MgO and MgAl₂O₄ phases on Mg doped SA catalysts were observed from XRD and XPS analyses. Existence of high percentage MgAl₂O₄ phase on 5% Mg doped SA catalyst (Mg (5) SA) enhances the dispersion and stabilization of silver phases (Ag₂O). Mg (5) SA catalyst shows a 51% of high selectivity (NO to N₂) in presence of SO₂ (80 ppm) at low temperatures (350 °C) and maintained high selectivity’s with a wide temperature window (350–500 °C). An optimal high surface availability of Ag⁰ and Ag⁺ species were observed from XPS analysis over Mg (5) SA catalyst. H₂-TPR analysis shows high temperature reduction peak over Mg (5) SA compared to SA catalyst. XPS analysis confirms the high percent availability of MgAl₂O₄ species over Mg (5) SA catalyst. DRIFTS study reveals the molecular evidences for the evolution of enolic species during NO reduction over the highly active Mg (5) SA catalyst at low temperatures. It also confirms further transformation of enolic species into –NCO species with NO + O₂ and finally into N₂ and CO₂.     

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.     

Oxy-CO2 reforming of methane to syngas over CoOx/MgO/SA-5205 catalyst

The oxy-CO₂ methane reforming reaction (OCRM) has been investigated over CoO_x supported on a MgO precoated highly macroporous silica–alumina catalyst carrier (SA-5205) at different reaction temperatures (700–900 °C), O₂/CH₄ ratios (0.3–0.45) and space velocites (20,000–100,000 cc/g/h). The reaction temperature had a profound influence on the OCRM performance over the CoO/MgO/SA-5205 catalyst; the methane conversion, CO₂ conversion and H₂ selectivity increased while the H₂/CO ratio decreased markedly with increasing reaction temperature. While the O₂/CH₄ ratio did not strongly affect the CH₄ and CO₂ conversion and H₂ selectivity, it had an intense influence on the H₂/CO ratio. The CH₄ and CO₂ conversion and the H₂ selectivity decreased while the H₂/CO increased with increasing space velocity. The O₂/CH₄ ratio and the reaction temperature could be used to manipulate the heat of the reaction for the OCRM process. Depending on the O₂/CH₄ ratio and temperature the OCRM process could be operated in a mildly exothermic, thermal neutral or mildly endothermic mode. The OCRM reaction became almost thermoneutral at an OCRM reaction temperature of 850 °C, O₂/CH₄ ratio of 0.45 and space velocity of 46,000 cc/g/h. The CH₄ conversion and H₂ selectivity over the CoO/MgO/SA-5205 catalyst corresponding to thermoneutral conditions were excellent: 95% and 97%, respectively with a H₂/CO ratio of 1.8.     

Methane conversion to higher hydrocarbons in a dielectric-barrier discharge reactor with Pt/γ-Al2O3 catalyst

Plasma catalytic reactions were applied to the conversion of methane to C₂, C₃ or higher hydrocarbons in a dielectric-barrier discharge (DBD) reactor at atmospheric pressure. Methane conversion was increased with the increase of Pt loading on γ-Al₂O₃. The highest C₂H₆ selectivity was 50.3% when 3 wt% Pt/γ-Al₂O₃ catalyst was calcined at 573 K. Methane conversion was increased with the increase of the catalyst weight in DBD reactor. The major products were C₂H₆ and C₃H₈, which were independent of catalyst weight in the presence of catalyst.