NO x -catalyzed partial oxidation of methane and ethane to formaldehyde by dioxygen

At 600 °C, NO_x catalyzes the partial oxidation of both methane and ethane by dioxygen to form formaldehyde. The yield of oxygenates from methane is over 11. The yield increases to over 16 when 0.7% of ethane is added to the gas mixture. The yield of oxygenates from ethane is over 24. A catalytic cycle involving NO₂ as the C–H activating species is proposed.     

A kinetic model for partial oxidation of ethane to acetic acid on promoted VPO catalyst

The partial oxidation of ethane to acetic acid on promoted VPO with Mo, using an Mo/V ratio of 0.2, has been investigated experimentally and theoretically. The reaction was carried out in a differential reactor at 1360 kPa, in the temperature range 548–623 K, with space times of 1.2–3.6 s and oxygen concentrations of 5–20%. The rate of oxidation of ethane was found to be approximately first order in ethane and zero order in oxygen at 548 K. At 623 K, the order of reaction with respect to ethane decreased to about 0.5, while that for oxygen increased to about 0.27. A kinetic model has been developed, which assumes that adsorbed oxygen reacts with ethane to form ethene, acetic acid, CO and CO₂. Ethene is further oxidized to acetic acid, CO and CO₂ through a redox mechanism. The model exhibits good agreement with the experimental data. © 2000 Society of Chemical Industry     

Catalytic behavior of YBa2Cu3O7-x in the partial oxidation of ethanol to acetaldehyde

The partial oxidation of ethanol to acetaldehyde was performed over YBa₂Cu₃O_7-x in a flow reactor. The catalytic characteristics of YB₂Cu₃O_7-x were compared with those of an individual oxide comprising the YBa₂Cu₃O_7-x . The structural change of YBa₂Cu₃O_7-x and the other catalysts after the reaction was measured by means of XRD and XPS, and it was found that the high oxidation state of copper in YBa₂Cu₃O_7-x was responsible for the higher activity and the higher selectivity for acetaldehyde.     

Potassium-modified molybdenum-containing SBA-15 catalysts for highly efficient production of acetaldehyde and ethylene by the selective oxidation of ethane

Mo-incorporated SBA-15 (Mo-SBA-15) catalysts with different Mo:Si molar ratios were synthesized by a direct hydrothermal method, and SBA-15-supported Mo catalysts (Mo/SBA-15 and K-Mo/SBA-15) were prepared by an incipient-wetness impregnation method. The structures of the catalysts were characterized by means of N₂ adsorption–desorption, XRD, TEM, FT-IR, and UV-Raman, and their catalytic performance for the oxidation of ethane was tested. Turnover frequency and product selectivity are strongly dependent on the molybdenum content and catalyst preparation method. Furthermore, the addition of potassium promotes the formation of isolated tetra-coordination molybdenum species and potassium molybdate. The K/Mo-SBA-15 catalysts possess much higher catalytic selectivity to acetaldehyde in the selective oxidation of ethane than the supported molybdenum catalysts (Mo/SBA-15 or K-Mo/SBA-15). The highest selectivity of CH₃CHO + C₂H₄ 68.3% is obtained over the K/Mo-SBA-15 catalyst. Analysis of kinetic results supports the conclusion that ethane oxidation is the first-order reaction and ethane activation may be the rate-determining step for the oxidation of ethane. Ethylene is a possible intermediate for acetaldehyde formation.     

Partial oxidation of ethane to synthesis gas over Co-loaded catalysts

Attention has been increasingly paid to the partial oxidation of lower alkanes to synthesis gas, due to its intrinsic energy saving process. We studied the partial oxidation of ethane (POE) on Co loaded on various supports. The POE performance varied as follows: Y₂O₃, CeO₂, ZrO₂, La₂O₃  SiO₂, Al₂O₃, TiO₂ > MgO. Comparing Y₂O₃ and CeO₂, the carbon deposition during the POE was negligible on CeO₂ and therefore CeO₂ was the most preferable support. By changing space velocity and O₂ partial pressure, reaction mechanism of POE was studied and it was revealed that two-step mechanism was prevailing; combustion of ethane to H₂O and CO₂ and subsequent reforming of ethane with H₂O and CO₂ to synthesis gas. Co/CeO₂ catalyst exhibited high and stable catalytic activity for 10 h; high ethane conversion of 18% (maximum ethane conversion 20% at O₂/C₂H₆ = 0.2) with H₂ and CO selectivities of 93 and 84%, respectively.     

Oxidation of propane to acetone and of ethane to acetaldehyde by O2 in zeolites with complete selectivity

Oxidation of propane by O₂ to acetone was observed in solvent-free BaY and CaY under irradiation with visible light as well as under dark thermal conditions. The reaction was monitored in situ by FT-infrared spectroscopy. In the case of the photochemical oxidation, isopropyl hydroperoxide was detected as reaction intermediate. No byproduct was observed even upon > 50% conversion of the alkane. Ethane was oxidized completely selectively to acetaldehyde in CaY under irradiation with blue light. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photocatalytic oxidation of ethane over TiO2

The photocatalytic oxidation of ethane has been studied at room temperature using a specially designed stirred reactor. The effect on the rate of ethane oxidation, of the partial pressure of reactants, the UV light intensity, the amount of catalyst and the residence time has been evaluated. Running the experiments according to a factorial plan permitted to deduce from a limited amount of experimental data the complete kinetic model corresponding to the steady state adsorption mechanism. On the basis of the experimental critical mass of the catalyst (1.5 g), an expression permitting calculation of the critical mass of the catalyst (TiO₂) for any type of photocatalytic reactor has been derived.     

Methane partial oxidation to methanol-solid initiated homogeneous methane oxidation

The initiation temperature of methane partial oxidation was markedly lowered by platinum wire placed upstream of a high pressure reactor. Added hydrogen in the reactant gas promoted the methanol selectivity. The radicals formed on the platinum surface were desorbed from it and initiated the reaction.     

The selective oxidation of methane to ethane and ethylene in a solid oxide electrolyte reactor

A study of the selective oxidative coupling of methane (OCM) to C₂ hydrocarbons (ethane and ethylene) in a solid-state electrochemical reactor made from yttria-stabilized zirconia (YSZ) has been made. Three different catalyst–electrode systems based on silver and two trimetallic formulations of Mn modified alkali (Na and K) tungstates supported on silica were used. A comparison is made between co-fed and electrochemically-supplied oxygen. The electrochemically-supplied oxygen gave higher overall C₂ selectivities than the co-fed method under low current conditions, which was attributed to differences in local methane to oxygen ratios at the catalyst surface. The potassium tungstate supported catalyst gave the best overall C₂ selectivity (86% at 4% C₂ yield).     

Catalytic partial oxidation of ethane to ethylene and syngas over Rh and Pt coated monoliths: Spatial profiles of temperature and composition

The catalytic partial oxidation of C₂H₆ over Pt and Rh coated monolithic supports (4.7 wt% M/α-Al₂O₃ 45 PPI) was investigated with a capillary sampling technique for a range of C₂H₆/air ratios at constant inlet flow (～8 ms contact time), with and without H₂ addition. Effluent data clearly indicate the differences in product distribution between catalysts and equilibrium. Rh effectively converts the reactant mixtures to syngas with ～80% selectivity, whereas Pt produces C₂H₄ with ～55% C-atom selectivity, while neither produces thermodynamically favored C. Spatially resolved measurements provide direct evidence of the multi-zone nature of the reactors. With Rh, complete conversion of O₂ occurs to produce mostly CO, H₂ and H₂O within the first 3 mm of catalyst, followed by a reforming zone to produce additional syngas. Pt consumes O₂ more slowly, which results in a steady increase in temperature along the reactor. Ethylene formation correlates to reactor temperatures >750 °C, regardless of C/O, in line with the onset of homogeneous reactions. Hydrogen addition tests (C₂H₆/O₂/H₂=2/1/2) clearly exhibit preferential oxidation of H₂ with O₂ over Pt, which shifts the maximum in temperature upstream while preserving a portion of the C₂H₆ for C₂H₄ production. H₂ addition modifies the concentration and temperature profiles minimally on Rh. The main differences between catalysts are the high reforming and O₂ consumption activity with Rh compared to Pt, which are likely responsible for differences in C₂H₄ yields.     

Electrocatalytic oxidation of acetaldehyde on Pt alloy electrodes

Reaction intermediates occurring during the oxidation of acetaldehyde were investigated by in situ infrared reflectance spectroscopy (SPAIRS and SNIFTIRS techniques). These measurements, showing that acetaldehyde was transformed into CO and CO₂ successively, allowed us to choose a suitable potential program to oxidize electrocatalytically the reactant on binary and ternary platinum alloy electrodes with two compositions (Pt/Os and Pt/Ru/Os). IR results were useful to set a potential pulse program which allowed to adsorb dissociatively the organic compound. The analysis of the electrolysis products was performed by high performance liquid chromatography (HPLC). Acetic acid, formic acid and carbon dioxide were determined as the main oxidation products of acetaldehyde.     

The selective oxidation of methane to ethane and ethylene over doped and un-doped rare earth oxides

A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb₄O₇ (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm₂O₃ was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm₂O₃ gives a higher optimum C₂ yield than that of unpromoted Sm₂O₃. Sm₂O₃ and Ca/Sm₂O₃ catalysts are more stable than Li/MgO, Li/Sm₂O₃ or Na/Sm₂O₃. The addition of Li or Na to Sm₂O₃ causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm₂O₃ catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm₂O₃.     

Skeleton kinetic model of acetaldehyde oxidation from comprehensive models

This work elucidated the chemical reaction scheme developed by Wang and Mou (Wang-Mou scheme) in 1985 for describing the dynamic behavior of acetaldehyde oxidized in a continuous-flow stirred tank reactor (CSTR). We deduced the Wang-Mou scheme from the more comprehensive chemical reaction schemes proposed by previous literature works (detailed scheme), and the correlation between the detailed scheme and the Wang-Mou scheme was established. Numerical simulation was adopted to explore the dynamic characteristics of the Wang-Mou scheme and compared with the experiments, revealing a reasonable agreement with the experimental findings. Finally, mechanisms responsible for various dynamic regimes were discussed. The link of the skeleton, Wang-Mou scheme, with the more comprehensive schemes was established.     

Influence of the operating conditions on yield and selectivity for the partial oxidation of ethane in a catalytic membrane reactor

In this study, simulation results are presented for the partial oxidation of ethane to ethylene in a Catalytic Membrane Reactor (CMR) under isothermal and non-isothermal conditions. Considering the importance of the transport processes, a 2D model was developed and implemented in FLUENT^® using self-designed program modules for reaction kinetics, transport properties and post-processing. An analysis of significance of the influencing variables is carried out on the basis of a reference case. The number of parameters were minimized by the dimensionless formulation of the model. One of the most important variables is the oxygen dosage through the membrane. Both velocity and oxygen concentration of the trans-membrane stream were varied with the aim of attaining maximum ethylene yield. The results of the different simulations clearly show the advantages of the CMR compared to the Catalytic Wall Reactor (CWR). The numerical simulations are essential in order to reduce the experimental costs and to evaluate different reactor concepts.     

Effect of carbon dioxide on the selectivities obtained during the partial oxidation of methane and ethane over Li+/MgO catalysts

AtT 650 °C carbon dioxide either formed during reaction or added to the system increases the selectivity for the desired hydrocarbon products during the oxidative coupling of methane and the oxidative dehydrogenation of ethane reaction over Li⁺/MgO catalysts. Similarly, CO₂ inhibits secondary reactions of CH₃-radicals with the surface of the Li⁺/MgO. The improved selectivities are attributed to the poisoning effect that CO₂ has on the secondary reactions of alkyl radicals with the surface.     

Propane partial oxidation to acrolein over combined catalysts

A novel approach for the partial oxidation of propane to acrolein, based on the use of layers of combined catalysts in a single reactor, provides good yields of acrolein with selectivity above 62%. The results depend strongly on the layer configuration, and reveal new mechanistic features for the process. This revised version was published online in August 2006 with corrections to the Cover Date.     

K改性Mo-SBA-16催化剂催化乙烷选择氧化制乙烯和醛类

采用超声辅助等体积浸渍法制备了K改性Mo掺杂SBA-16催化剂x K/Mo-SBA-16,评价其对乙烷选择氧化反应的催化性能,并利用XRD、TEM、N_2吸附-脱附、UV-Vis、UV-Raman和FT-IR等对催化剂物化性质进行表征,同时考察反应温度和K含量等因素对催化剂性能的影响。结果表明,K的添加提高了产物中醛类选择性。在V(C_2H_6)∶V(O_2)=3∶1和反应温度550℃条件下,0. 1K/5. 0Mo-SBA-16催化剂上乙醛选择性为17. 1%,总醛(包括乙醛和丙烯醛)产率7. 4%。碱金属K的添加改变了催化剂中活性组分的结构和分散度,孤立和高分散的MoO_4四面体逐渐转变为低聚扭曲的MoO_x四面体,可能形成了K_2Mo_2O_7新相。     

Stabilization of the ethane oxidation catalytic activity of Cu-ZSM-5

The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Treatment by steam at 630°C for 17 h or high-temperature dry calcination at 850°C induce an irreversible change in coordination for practically all square-planar Cu²⁺ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu²⁺ ions remain accessible to gas-phase molecules, but the catalytic reactivity of these altered copper sites decreases drastically. A stabilizing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part of the copper ions (20–30%) preserves the parent square-planar Cu²⁺ state even after calcination at 850°C for 0.5 h. The effect of ca. 1% La or Ce is much less pronounced. The catalytic activity in the complete oxidation of ethane correlates well with the number of square-planar cupric cations retained by the samples after different treatments. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500–650°C.     

天然气甲烷部分氧化制合成气的研究进展

甲烷部分氧化制合成气是高转化率、高选择性、高空速、低H2/CO、温和的放热反应,综述了近几年来甲烷部分氧化制合成气的催化剂、反应机理及活性中心的研究进展及反应中的存在问题。