The kinetics and mechanism of carbon monoxide hydrogenation over alumina-supported ruthenium

A study was conducted of hydrocarbon synthesis from CO and H₂ over an alumina-supported Ru catalyst. Rate data for the formation of methane and C₂ through C₁₀ olefins and paraffins were fitted by power law rate expressions. The kinetics observed experimentally can be interpreted in terms of a comprehensive mechanism for CO hydrogenation, in which CH_x(x = 0–3) species play a primary role. Expressions for the kinetics of methane synthesis, the kinetics and distribution of C₂₊ olefins and paraffins, and the probability of hydrocarbon chain growth derived from this mechanism are found to be in good agreement with the experimental results. The observed deviations from theory can be ascribed to secondary processes such as olefin hydrogenation and paraffin hydrogenolysis.     

Bifunctional Catalysis of Mo/HZSM-5 in the Dehydroaromatization of Methane to Benzene and Naphthalene XAFS/TG/DTA/MASS/FTIR Characterization and Supporting Effects

The direct conversion of methane to aromatics such as benzene and naphthalene has been studied on a series of Mo-supported catalysts using HZSM-5, FSM-16, mordenite, USY, SiO₂, and Al₂O₃as the supporting materials. Among all the supports used, the HZSM-5-supported Mo catalysts exhibit the highest yield of aromatic products, achieving over 70% total selectivity of the hydrocarbons on a carbon basis at 5–12% methane conversion at 973 K and 1 atm. By contrast, less than 20% of the converted methane is transformed to hydrocarbon products on the other Mo-supported catalysts, which are drastically deactivated, owing to serious coke formation. The XANES/EXAFS and TG/DTA/mass studies reveal that the zeolite-supported Mo oxide is endothermally converted with methane around 955 K to molybdenum carbide (Mo₂C) cluster (Mo-C, C.N.=1,R=2.09 Å; Mo-Mo, C.N.=2.3–3.5;R=2.98 Å), which initiates the methane aromatization yielding benzene and naphthalene at 873–1023 K. Although both Mo₂C and HZSM-5 support alone have a very low activity for the reaction, physically mixed hybrid catalysts consisting of 3 wt% Mo/SiO₂+HZSM-5 and Mo₂C+HZSM-5 exhibited a remarkable promotion to enhance the yields of benzene and naphthalene over 100–300 times more than either component alone. On the other hand, it was demonstrated by the IR measurement in pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO₂/Al₂O₃ratios, around 40, show maximum Brönsted acidity among the catalysts with SiO₂/Al₂O₃ratios of 20–1900. There is a close correlation between the activity of benzene formation in methane aromatization and the Brönsted acidity of Mo/HZSM-5, but not Lewis aciditiy. It was found that maximum benzene formation was obtained on the Moz/HZSM-5 having SiO₂/Al₂O₃ratios of 20–49, but substantially poor activities on those with SiO₂/Al₂O₃ratios smaller and higher than 40. The results suggest that methane is dissociated on the molybdenum carbide cluster supported on HZSM-5 having optimum Brönsted acidity to form CH_x(x>1) and C₂-species as the primary intermediates which are oligomerized subsequently to aromatics such as benzene and naphthalene at the interface of Mo₂C and HZSM-5 zeolite having the optimum Brönsted acidity. The bifunctional catalysis of Mo/HZSM for methane conversion towards aromatics is discussed by analogy with the promotion mechanism on the Pt/Al₂O₃catalyst for the dehydro-aromatization of alkanes.     

In Situ Delamination of Ferrierite Zeolite and its Performance in the Catalytic Cracking of C4 Hydrocarbons

Ferrierite (FER) zeolites were synthesized by solid transformation at different alkalinities (OH⁻/Al₂O₃ molar ratios). The in situ delamination of FER zeolites were achieved and their catalytic performances in the catalytic cracking of C₄ hydrocarbons were examined. The relationships among the OH⁻/Al₂O₃ molar ratio, FER structure, composition, surface acidity and catalytic performance in C₄ hydrocarbon cracking were investigated. The results of X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, N₂ adsorption, NH₃ temperature-programmed desorption and catalytic cracking showed that with increasing OH⁻/Al₂O₃ molar ratio in the synthesis gel, the SiO₂/Al₂O₃ molar ratio of the as-synthesized FER zeolite decreased, the amount of acid sites in the corresponding H-FER increased, and the acid strength weakened. Additionally, the FER zeolite was delaminated at the mesoscale. H-FER5 synthesized at the highest alkalinity had the largest number of acid sites and exhibited the highest catalytic activity in C₄ hydrocarbon catalytic cracking among three of the prepared catalysts. H-FER3 synthesized at the second-highest alkalinity showed that the highest yield of benzene and toluene because of the secondary pores resulted from the gaps between the layers, which were beneficial to the diffusion and formation of large molecules.     

Low-Temperature Combustion of CH4 over CeO2–MO x Solid Solution (M = Zr4+, La3+, Ca2+, or Mg2+) Promoted Pd/γ–Al2O3 Catalysts

The catalytic behavior of Pd (2 wt%) catalysts supported on γ-Al₂O₃ and promoted with CeO₂ ? MO _x (M = Zr⁴⁺, La³⁺, Ca²⁺, or Mg²⁺) solid solution was investigated for methane combustion. The results demonstrated that Pd/γ-Al₂O₃CeO₂ MO _x catalysts can be effective for the low-temperature catalytic combustion of methane and are comparable in activity to other conventional catalysts for this reaction. The XPS and XRD results indicated that an enhanced mobility of lattice oxygen induced by the perturbation of Ce–O lattice was responsible for an increased catalytic performance during oxidation reaction. The most active sites in the catalyst system involve contacts between Pd and the CeO₂–MO _x mixed oxide component. Meanwhile, pre-treatment conditions have significant effect on the catalytic activity in methane combustion.     

Aromatization of methane in the absence of oxygen over Mo-based catalysts supported on different types of zeolites

The catalytic performance of Mo-based catalysts supported on various zeolites has been studied for methane aromatization in the absence of oxygen in a fixed-bed continuous-flow quartz reactor, and their catalytic properties are correlated with features of zeolite structure. It was found that H-type silica–alumina zeolites, such as ZSM-5, ZSM-8, ZSM-11 and β possessing two-dimensional structure and pore diameter equaling the dynamic diameter of a benzene molecule (about 6 Å) simultaneously, are fine supports for methane activation and aromatization catalysts. Among them, MoO₃/H-ZSM-11 has the best activity and stability; for instance, a methane conversion of 8.0% and selectivity higher than 90% was obtained at 973 K. The catalytic performance of MoO₃/H-ZSM-8 is somewhat lower than that of MoO₃/H-ZSM-5, while activity of MoO₃/H-β is lower than that of MoO₃/H-ZSM-8. Catalysts supported on H-MCM-41 and H-SAPO-34 exhibit low activity for methane aromatization and those supported on H-MOR, H-X and H-Y give only a little amount of ethylene. Over MoO₃/H-SAPO-5 and MoO₃/H-SAPO-11 no hydrocarbons were detected.     

Low-temperature combustion of methane using PdO/Al2O3 catalyst: Influence of crystalline phase of Al2O3 support

Palladium oxide (PdO) catalysts supported on various alumina supports (θ-, δ-, κ-, η-, and γ-Al₂O₃) were prepared and the effect of the crystalline phases of the supports on the catalytic performance in methane combustion was investigated. Among the catalysts examined, the PdO/θ-Al₂O₃ catalyst, which showed the lowest reduction temperature in the temperature-programmed reduction of methane and the highest oxygen capacity in methane pulse experiments, exhibited the highest CH₄ conversion and stability for methane combustion. However, all the catalysts were deactivated because of the growth of PdO particles and strong adsorption of water vapor.     

The Effect of Support on Sulphur Tolerance of Rh Based Catalysts for Methane Partial Oxidation

The presence of sulphur containing compounds, naturally occurring in natural gas or added as odorants, can adversely affect the performance of noble metals based catalysts for the partial oxidation of methane to syngas. In this paper the effect of SO₂ addition on the catalytic partial oxidation of methane was investigated on Rh (1 wt.%) catalysts prepared by incipient wetness impregnation method on two different commercially available high temperature γ-alumina supports stabilized either with 10% SiO₂ or 3% La₂O₃. Based on the results of catalytic activity measurements and in-situ FT-IR spectroscopic characterisation, as well as TPR/TPD studies, it has been shown that the presence of sulphur can severely suppress the formation of synthesis gas by inhibiting the reforming reactions during the catalytic partial oxidation of methane. The results also demonstrated that the support plays a crucial role in the partial oxidation reaction. In the presence of a sulphating support such as La₂O₃–Al₂O₃ the partial oxidation reaction was much less inhibited than a less sulphating support such as SiO₂–Al₂O₃. The sulphating support acts as a sulphur storage reservoir, which minimises the poison from adsorbing on or near the active Rh sites where reactions take place.     

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     

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C₂-hydrocarbons at different temperatures (700–750°C), CH₄/O₂ ratios (4·0 and 8·0) in feed, and space velocities (10320 cm³ g⁻¹ h⁻¹). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C₂-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI.     

Production of middle distillate through hydrocracking of paraffin wax over Pd/SiO2–Al2O3 catalysts

Pd/SiO₂–Al₂O₃ catalysts (Pd/SA-X) with different SiO₂ contents (X, wt%) were prepared for use in the production of middle distillate (C₁₀–C₂₀) through hydrocracking of paraffin wax. The effect of SiO₂ content of Pd/SA-X catalysts on their physicochemical properties and catalytic performance in the hydrocracking of paraffin wax was investigated. High surface area and well-developed mesopores of Pd/SA-X catalysts improved the dispersion of Pd species on the SiO₂–Al₂O₃ support. Acidity of Pd/SA-X catalysts determined by NH₃-TPD experiments showed a volcano-shaped trend with respect to SiO₂ content. Conversion of paraffin wax increased with increasing acidity of the catalyst, while selectivity for middle distillate decreased with increasing acidity of the catalyst. Yield for middle distillate showed a volcano-shaped curve with respect to acidity of the catalyst. This indicates that acidity of Pd/SA-X catalysts played an important role in determining the catalytic performance in the hydrocraking of paraffin wax. Among the catalyst tested, Pd/SA-69 with moderate acidity showed the highest yield for middle distillate.     

Total Oxidation of Methane and Chlorinated Hydrocarbons on Zirconia Supported A1–xSrxMnO3 Catalysts

The catalytic behavior of ZrO₂ and ZrO₂ containing 8 mol‐% Y₂O₃ supported A_1–xSr_xMnO₃ (A = La, didymium) perovskites was studied in the total oxidation of methane, chloromethane and dichloromethane considering catalyst deactivation and byproduct formation. The perovskites are dispersed on the support surface; clusters with a perovskite‐like structure were formed. The supported catalysts are characterized by higher specific surface areas compared with the unsupported ones. Partial substitution of A‐site cations by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane, but not in the total oxidation of chlorinated hydrocarbons (CHC). The catalytic activity of supported and unsupported catalysts is comparable in the total oxidation of methane in spite of the significantly lower perovskite content of the supported catalysts. In the CHC conversion the catalytic activity of the supported catalysts is higher than that of the unsupported ones.     

Combination of flame synthesis and high-throughput experimentation: The preparation of alumina-supported noble metal particles and their application in the partial oxidation of methane

Mono and multi-noble metal particles on Al₂O₃ were prepared in one step by flame spray pyrolysis (FSP) of the corresponding noble metal precursors dissolved in methanol and acetic acid (v/v 1:1) or xylene. The noble metal loading of the catalysts was close to the theoretical composition as determined by WD-XRF and LA-ICP-MS. The preparation method was combined with high-throughput testing using an experimental setup consisting of eight parallel fixed-bed reactors. Samples containing 0.1–5 wt% noble metals (Ru, Rh, Pt, Pd) on Al₂O₃ were tested in the catalytic partial oxidation of methane. The ignition of the reaction towards carbon monoxide and hydrogen depended on the loading and the noble metal constituents. The selectivity of these noble metal catalysts towards CO and H₂ was similar under the conditions used (methane: oxygen ratio 2:1, temperature from 300 to 500 °C) and exceeded significantly those of gold and silver containing catalysts.Selected catalysts were further analysed using XPS, BET, STEM-EDXS and XANES/EXAFS. The catalysts exhibited generally a specific surface area of more than 100 m²/g, and were made up of ca. 10 nm alumina particles on which the smaller noble metal particles (1–2 nm, partially oxidized state) were discernible. XPS investigation revealed an enrichment of noble metals on the alumina surface of all samples. The question of alloy formation was addressed by STEM-EDXS and EXAFS analysis. In some cases, particularly for Pt–Pd and Pt–Rh, alloying close to the bulk alloys was found, in contrast to Pt–Ru being only partially alloyed. In situ X-ray absorption spectroscopy on selected samples was used to gain insight into the oxidation state during ignition and extinction of the catalytic partial oxidation of methane to hydrogen and carbon monoxide.     

Transition metal-doped TiO2 nanowire catalysts for the oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) on transition metal-doped TiO₂ nanowire catalysts was performed and the effects of metal dopants were studied. With transition metal doping, the electric and optical properties of nanowires were adjusted, which seemed to improve the catalytic activity and selectivity of the OCM reaction. A Mn-doped TiO₂ nanowire catalyst exhibited the highest C₂ yield with the highest (ethylene)/(ethane) ratio because of its moderate oxidation activity, while a highly active Rh-doped TiO₂ nanowire catalyst converted methane into fully oxidized CO and CO₂. The electric conductivity assessed by UV–vis absorption represented the oxidation activity of the nanowire catalysts.     

Conversion of natural gas to C2 product,hydrogen and carbon black using a catalytic plasma reaction

In the microwave and RF plasma catalytic reaction at room temperature, the decomposition of natural gas over Pd–NiO/γ-Al₂O₃ was carried out. The decomposition of methane is caused by collision by excitation of unstable electronic state. Measuring the flow rate and plasma power can represent kinetic data and mechanism. The conversion of C₂ hydrocarbons was increased from 47% to 63.7% in the microwave plasma catalytic reaction within electric field. Comparing the activities of catalysts, Pd–NiO/γ-Al₂O₃ bimetallic catalyst was more active than Pt–Sn/γ-Al₂O₃ catalyst because of modifying the surface of catalysts by carbon formation. In RF plasma catalytic reaction, we obtained high C₂ yield of 72%, in which the conversion and selectivity of C₂ hydrocarbons were related to the applied power and feed rate of natural gas.     

The modification of SiO2 by various organic groups and its influence on the properties of cobalt-based catalysts for Fischer-Tropsch synthesis

SiO₂ was modified by various organic groups before the impregnation of cobalt precursor. These modified supports and the corresponding catalysts were characterized by BET, ²⁹Si CP MAS NMR, XRD, Raman, XPS and H₂-TPR. These characterizations clearly show the changes of morphology as well as reducibility of the catalysts. The organic modification of SiO₂ remarkably influences the reducibility and catalytic properties of Co catalysts. Co catalyst supported on (CH₃)₃-modified SiO₂ exhibits high activity and C₅⁺ hydrocarbon selectivity. However, COOH-, NH₂-, and NH₂(CH₂)₂NH-modified SiO₂ distinctly suppress the catalytic activity of Co catalysts.     

Studying the role of the state of platinum in Pt/SO4/ZrO2/Al2O3 catalysts in the isomerization of n-hexane

Samples of SO₄/ZrO₂/Al₂O₃ and Pt/Al₂O₃ Pt/Al₂O₃ catalysts and their physical mixtures are prepared, and the catalytic properties of the samples in n-hexane isomerization are studied. The considerable effect of the state of platinum on the catalytic performance of the samples is revealed. IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration show that the reduced catalysts contain ionic forms of platinum capable of adsorbing up to three hydrogen atoms per each surface atom of platinum. By means of H/D isotopic exchange, it is found that specific properties of ionic platinum are apparent in the formation of the hydride form of adsorbed hydrogen. It is speculated that the activity and stability of catalysts based on sulfated zirconia in n-hexane isomerization can be attributed to the involvement of ionic and metallic platinum in the activation of hydrogen. The results can be used to develop effective catalysts for the isomerization of C₅–C₆ gasoline fractions in order to obtain the isomerizate as a high-octane additive for modern gasolines.     

Hydrogenation of carbon dioxide over alumina supported Fe-K catalysts

The hydrogenation of CO₂ has been studied over Fe/alumina and Fe-K/alumina catalysts. The addition of potassium increases the chemisorption ability of CO₂ but decreases that of H₂. The catalytic activity test at high pressure (20 atm) reveals that remarkably high activity and selectivity toward light olefins and C₂₊ hydrocarbons can be achieved with Fe-K/alumina catalysts containing high concentration of K (K/Fe molar ratio = 0.5, 1.0). In the reaction at atmospheric pressure, the highly K-promoted catalysts give much higher CO formation rate than the unpromoted catalyst. It is deduced that the remarkable catalytic properties in the presence of K are attributable to the increase in the ability of CO₂ chemisorption and the enhanced activity for CO formation, which is the preceding step of C₂₊ hydrocarbon formation.     

Synthesis of light olefins from syngas over Fe–Mn–V–K catalysts in the slurry phase

Fe–Mn–V–K catalysts for the synthesis of light olefins from CO hydrogenation were prepared by a specially controlled degradation method. The effect of the V content on the structure and the catalytic performance of the catalysts were investigated in a continuously stirred tank slurry reactor. Mössbauer spectra (MES) results show that the incorporation of V with appropriate contents can improve the dispersion of the α-Fe₂O₃ phase. CO hydrogenation results indicate that a small addition of V can improve the product distribution. The addition can also increase the selectivity to light olefins by inhibiting the secondary hydrogenation reaction of the initial olefin. The best catalytic performance was obtained at the Fe/Mn/V molar ratio of 3/1/0.2. The total C₂–C₄ content in all hydrocarbons and O/P in the C₂–C₄ fraction were 49.15 wt% and 3.95, respectively.     

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.     

Plasma catalytic methane conversion over sol–gel derived Ru/TiO2 catalyst in a dielectric-barrier discharge reactor

Plasma catalytic methane conversion was carried out in the presence of sol–gel derived Ru/TiO₂ catalysts within a dielectric-barrier discharge (DBD) reactor. Plasma-assisted reduction (PAR) was applied to reduce the prepared Ru/TiO₂ catalysts in DBD reactor, and most of the catalysts were successively reduced by PAR within 15 min. The highest methane conversion was obtained when 5 wt% Ru/TiO₂ catalysts were used after calcination at 400 °C. The selectivities of light alkanes (C₂H₆, C₃H₈, C₄H₁₀) were highly increased when Ru/TiO₂ catalysts were used in DBD reactor.