Highly Stable Performance of Methane Dehydroaromatization on Mo/HZSM-5 Catalyst with a Small Amount of H2 Addition into Methane Feed

With a small amount of H₂ (3 6%) addition into methane feed, coke formation on 6 wt% Mo catalyst during the methane dehydroaromatization reaction was effectively suppressed and the catalyst stability was increased evidently under the reaction conditions of 1023K, 0.3MPa and 2520 mL g-MFI^-1 h^-1 of methane space velocity.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

Synthesis and photocatalytic properties of H2La2Ti3O10/TiO2 intercalated nanomaterial

H₂La₂Ti₃O₁₀/ TiO₂ intercalated nanomaterial was fabricated by successive intercalation reactions of H₂La₂Ti₃O₁₀ with n-C₆H₁₃NH₂/C₂H₅OH mixed solution and acid TiO₂ sol, followed by irradiating with a high-pressure mercury lamp. The intercalated materials possess a gallery height of 0.46 nm and a specific surface area of 31.58 m²·g⁻¹, which indicate the formation of a porous material. H₂La₂Ti₃O₁₀/TiO₂ shows photocatalytic activity for the decomposition of organic dye under irradiation with visible light and the activity of TiO₂ intercalated material was superior to the unsupported one.     

Autothermal reforming of methane to syngas with palladium catalysts and an electric metal monolith heater

The autothermal reforming of methane to syngas for use in the Fischer-Tropsch synthesis was studied in this work over PdO containing various combinations of CeO₂, BaO or SrO in a washcoated form on a metallic monolith at atmospheric pressure. This study focused on the autothermal operation of the system, in which an electric heater inside the reactor was used only to reach the ignition temperature, and thereafter the autothermal reaction successfully sustained itself without any external heat source. It was concluded from the experiments that the PdO/Al₂O₃ catalyst was better than the others, except for PdO-CeO₂-BaO-SrO/Al₂O₃, which showed similar performance in terms of the CH₄ conversion and H₂+CO selectivity, while affording a higher H₂/CO ratio (close to ca. 3) than the PdO/Al₂O₃ catalyst did (close to ca. 2). The gas hourly space velocity and O₂/CH₄ ratio governed the methane conversion, while the H₂O/CH₄ ratio controlled the H₂/CO ratio. A methane conversion of ∼87%, H₂+CO selectivity of ∼94%, H₂/CO ratio of ∼2.9, and M factor ∼2.15 were obtained under the conditions of a gas hourly space velocity (GHSV) of 120,000 h⁻¹, O₂/CH₄=0.6 and H₂O/CH₄=0.5.     

Electrogeneration of Hydrogen Peroxide in Acid Medium using Pyrolyzed Cobalt-based Catalysts: Influence of the Cobalt Content on the Electrode Performance

Cobalt tetramethoxyphenyl porphyrin (CoTMPP) adsorbed on a high area carbon support (Vulcan XC72-R) and heat-treated at 900 °C under inert atmosphere was studied as electrocatalyst for the reduction of O₂ to H₂O₂ in acid medium. Experiments performed on rotating ring-disc electrode (RRDE) and gas diffusion electrode (GDE) show that the catalyst performance depends on the cobalt loading, going through a maximum at 0.2 wt. % Co. For higher cobalt loadings, a growing part of oxygen is reduced into water, decreasing therefore the selectivity of the catalyst. These results are interpreted in terms of a further reduction of H₂O₂ on Co-based catalytic sites before leaving the catalytic layer. For a GDE polarized at −150 mV vs. saturated calomel electrode (SCE) and loaded with 0.9 μg cm⁻² of 0.2 wt. % Co-based catalyst, a H₂O₂ production rate of 300 μmol h⁻¹ cm⁻² was obtained which is five times higher than the H₂O₂ production rate measured with Vulcan. In these conditions, the selectivity of the Co-based catalyst for H₂O₂ production is 92%. The good agreement observed between RRDE and GDE results confirms the relevance of using RRDE experiment for screening these non-precious metal catalysts for further GDE applications.     

Oxidative dehydrogenation of ethane over Co–BaCO3 catalysts using CO2 as oxidant: effects of Co promoter

Co–BaCO₃ catalysts exhibited high catalytic performance for oxidative dehydrogenation of ethane (ODE) using CO₂ as oxidant. The maximal formation rate of C₂H₄ was 0.264 mmol · min⁻¹ · (g · cat.)⁻¹ (48.0% C₂H₆ conversion, 92.2% C₂H₄ selectivity, 44.3% C₂H₄ yield) on 7 wt% Co–BaCO₃ catalyst at 650 °C and 6000 ml. (g · cat.)⁻¹. h⁻¹. Co–BaCO₃ catalysts were comparatively characterized by XRF, N₂ isotherm adsorption-desorption, XRD, H₂-TPR and LRs. It was found that Co⁴⁺–O species were active sites on these catalysts in ODE with CO₂. The redox cycle of Co–O species played an important role on the catalytic performance of Co–BaCO₃ catalysts. On the other hand, the co-operation of BaCO₃ and BaCoO₃ was considered to be one of possible reasons for the high catalytic activity of these catalysts.     

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H₂SO₄/HZSM−5 catalyst prepared from a H₂SO₄‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH₄)₂WO₄ showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO₆)^n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H₂‐TPR results showed that the reduction of precursor of the 3% W–H₂SO₄/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W⁵⁺ and W⁴⁺, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W⁵⁺. The results seem to imply that the observed high DHAM activity on the W–H₂SO₄/HZSM‐5 catalyst was closely correlated with (WO₆)^n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H₂SO₄/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H₂SO₄/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH₄) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

The issue of selectivity in the direct synthesis of H2O2 from H2 and O2: the role of the catalyst in relation to the kinetics of reaction

The kinetic behavior in the direct synthesis of H₂O₂ with Pd–Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation (DP) methods on α-Al₂O₃ asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br⁻, pH) determine the selectivity in H₂O₂ formation, influencing the rate of the consecutive reduction of H₂O₂ (which is faster with respect to H₂O₂ decomposition on the metal surface) and/or of direct H₂ + O₂ conversion to H₂O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel and consecutive to H₂O₂ formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics. Increasing the noble metal loading on the membrane not only increases the productivity to H₂O₂, but also the selectivity, due to the formation of larger, less defective, Pd particles.     

Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts: Effect of the presence of O2 and H2O and the addition of Pt

The complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O has been achieved on ultrafine powdered TiO₂ photocatalysts and the addition of H₂O was found to enhance the reaction. The photocatalytic reaction has been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C₂H₄ with O₂ into CO₂, CO, and H₂O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C₂H₄. The photoformed OH species as well as O ₂ ⁻ and O ₃ ⁻ anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O. Interestingly, small amount of Pt addition to the TiO₂ photocatalyst increased the amount of selective formation of CO₂ which was the oxidation product of C₂H₄ and O₂.     

Methane conversion over sulfated zirconia

In a continuous-flow differential microreactor, sulfated zirconia (SZ), deliberately activated in situ by water, has converted methane at 673 K to a C₂–C₆ hydrocarbon mixture of which 65–70% was ethene and isobutane. Maximum conversion activity of ∼4.6%, corresponding to 4 × 10⁴ mole methane reacted per mole sulfate per second, was attainable at S/(added H₂O) molar ratio of 3.0 and methane flow rate of 5.6 × 10⁶ mol (g-SZ)⁻¹ s⁻¹. This methane conversion could be catalytic and may involve superacidic sites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Cocatalytic activities of methylaluminoxane prepared by direct water addition method in ethylene polymerization over Cp2ZrCl2

The characterization of ethylene polymerization behaviors catalyzed over Cp₂ZrCl₂/MAO homogeneous system using methylaluminoxanes prepared by the direct hydrolysis of AlMe₃ (Me=methy1) were reported. The MAO was prepared at the ratio of [H₂O]/[A1]=1 and 0.5 and at three different temperatures, i.e., −40, −60 and −80 °C. The polymerization rate was not decreased with polymerization time when the MAO prepared at the ratio of [H₂O]/[AlMe₃]=l at −60 °C was used as a cocatalyst regardless of the ratio of Al/Zr and the polymerization temperature. The polymerization rate drastically decreased with polymerization time above 60 °C in case of using MAO prepared at the ratio of [H₂O]/[AlMe₃]=l at −80 °C. However, in case of the MAO prepared at the ratio of [H₂O]/ [AlMe₃]=0.5 at −80 °C, the rate continuously increased with polymerization time at the polymerization temperature of 70 °C and 80 °C. The amount of MAO needed to activate Cp₂ZrC1₂ was larger than that of MAO prepared at the ratio of [H₂O]/[A1]=1. The viscosity molecular weight of polyethylene (PE) cocatalyzed with MAO prepared at the ratio of [H₂O]/[Al]=0.5 was lower than that of polyethylene obtained with MAO prepared at the ratio of [H₂O]/[A1]=1.     

Effect of A-site metal on methane combustion on 2% Pd / AMn1-x Fe x O3 (A = Ba, La, Pr; x = 0.4, 0.6, 1) perovskites

Barium, lanthanum, and praseodymium perovskites were prepared by malic acid complexation. Surface areas of the La and Pr perovskites were between 17.1 and 21.6 m² g⁻¹. The moderate low surface areas (5.7 m² g⁻¹) observed for corresponding barium perovskites were due to the high calcination temperatures. The calcination temperature also affected the shapes and sizes of the perovskite particles. According to SEM images the nanoparticles of the La and Pr perovskites were spherical, whereas those of barium perovskites were flakes. The conversion of methane increased in the order of A-site metal Ba < Pr < La. The CH₄ conversion after SO₂ treatment correlated with size of the perovskite particles: the smaller the particles the better the activity. The highest methane conversion after SO₂ treatment was achieved with lanthanum perovskite with B-site metal combination Mn_0.4Fe_0.6.     

Low Temperature and H2 Selective Catalysts for Ethanol Steam Reforming

Supported Rh catalysts have been developed for selective H₂ production at low temperatures. Ethanol dehydration is favorable over either acidic or basic supports such as γ-Al₂O₃ and MgAl₂O₄, while ethanol dehydrogenation is more favorable over neutral supports. CeO₂–ZrO₂-supported Rh catalysts were found to be especially effective for hydrogen production. We focused on a support prepared by a co-precipitation method having composition Ce_0.8Zr_0.2O₂. A 2%Rh/Ce_0.8Zr_0.2O₂ catalyst, prepared via impregnation without pre-calcination of support, exhibited the highest H₂ yield at 450 °C among various supported Rh catalysts evaluated in this study. This may be due to both the strong interaction between Rh and Ce_0.8Zr_0.2O₂ and the high oxygen transfer rate favoring reforming of acetaldehyde instead of methane production.     

A study of the Influence of the Synthesis Conditions upon the Catalytic Properties of LaMnO3.15 in Methane Combustion in the Absence and Presence of H2S

We report here on the activity and stability of LaMnO_3.15 for the methane combustion, in the absence and presence of H₂S, in a temperature interval of 250–750 °C. Two powders with different specific surface area were prepared by coprecipitation method using ammonia. Precursors calcined at high temperature, in air, for 10 h have led to LaMn-C solid with S_BET = 11 m²/g, while those previously aged in solution (hydrothermal treatment at 200 °C under 20 atm. for 24 h) then calcined at high temperature led to LaMn-HydC with S_BET = 31 m²/g. Temperature programmed reduction (TPR) profile of both samples showed two main peaks; surface and weakly bound oxygen named α-oxygen species and lattice oxygen β-oxygen species. While for LaMn-C the maximum reduction temperature peak corresponding to α-oxygen species was found to be ca. 600 °C, for LaMn-HydC samples this peak was shifted to lower temperature ca. 430 °C. Indeed, LaMn-HydC samples showed higher depletion of surface and weakly bound oxygen species compared to LaMn-C. The superior catalytic performance of LaMn-HydC in methane combustion was attributed to its high BET surface area and to both the high amount of α-oxygen species and their mobility. In the presence of 100 ppm H₂S in the feed this catalyst showed a higher propensity to poisoning by sulphur compounds than LaMn-C. This was attributed to the rapid formation of stable sulphate/sulphite species, the decomposition of which occurs above 800 °C.     

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H₂O₂ selectivity (or yield) in direct oxidation of H₂-to-H₂O₂ (using 1.7 mol% H₂ in O₂ as a feed) and also the H₂O₂ decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H- ) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H₂SO₄, HCl, HNO₃, H₃PO₄ and HClO₄), and also by the form of palladium (Pd⁰ or PdO). The oxidized (PdO-containing) catalysts are active for the H₂-to-H₂O₂ conversion and show very poor activity for the H₂O₂ decomposition. However, the reduced (Pd⁰-containing) catalysts show higher H₂ conversion activity but with no selectivity for H₂O₂, and also show much higher H₂O₂ decomposition activity. No direct correlation is observed between the H₂-to-H₂O₂ conversion activity (or H₂O₂ selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H₂O₂ yield and lower H₂O₂ decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.     

Simultaneous Production of Syngas and Ethylene from Methane by Combining its Catalytic Oxidative Coupling over Mn/Na2WO4/SiO2 with Gas Phase Partial Oxidation

A new route of methane utilization is presented, in which methane is converted to H₂, CO and C₂H₄ simultaneously with equal mole ratio, in order that the produced mixture could be used in the synthesis of propanal via hydroformylation. Kinetically controlled free radical gas phase methane oxidation was combined with its catalytic oxidative coupling over Mn/Na₂WO₄/SiO₂ to concomitantly acquire ethylene and syngas with close concentration. Under the optimal reaction condition, a mole ratio of CO:H₂:C₂H₄=1.0:1:0.9 was obtained with a yield of 11.6% and a selectivity of 68% to the target products based on C, while the selectivity to CO₂ is as low as 18.1%.     

Characterization of metal (Ba, Al, Si, V, and W)-incorporated TiO2 and toluene photodecomposition in the presence of H2O

This study focused on toluene photodecomposition in the presence of H₂O over metal (Ba, Al, Si, V, and W)-incorporated TiO₂. The nanometer-sized, metal-TiO₂ photocatalyst samples, including Ba²⁺, Al³⁺, Si⁴⁺, V⁵⁺, and W⁶⁺ ions, were prepared by using the solvothermal method. The X-ray photoelectron spectroscopy (XPS) results showed that the Ti-OH peak, which indicates hydrophilicity, increased with increasing Al and Si ion components but decreased with increasing Ba, V, and W ion components. The contact angles were distributed over the range of 0–10° on almost all films (200-nm thick) after irradiation for 2 h, and in particular approached 0° on the Al-TiO₂ and Si-TiO₂ nanometer-sized films after just 30 min. The toluene (100 ppm) photodecomposition in the continuous system increased in the order of Al-TiO₂>Si-TiO₂>pure TiO₂>W-TiO₂>Ba-TiO₂>V-TiO₂, and the maximum toluene conversion rate achieved was 45% over Al-TiO₂ film after 120 min. The toluene conversion remarkably increased; however, over all photocatalysts, with H₂O addition during the toluene photo-decomposed reaction, and in particular, the conversion reached up to 90% after 120 min over Al-TiO₂ and Si-TiO₂ with increased hydrophilicity. After photoreaction for 24 h, minimal carbon was deposited on the photocatalyst under both reaction conditions, with and without H₂O addition, although the deposited carbon amounts were smaller for the former. These results confirmed that the hydrophilicity of the photocatalyst had a greater effect on toluene decomposition, while the photocatalytic deactivation could be retarded by H₂O supplementation during toluene decomposition.     

Catalytic Reduction of Nitric Oxide by Hydrogen Sulfide Over γ-alumina

The ability of H₂S to reduce NO in a fixed bed reactor using a γ-alumina catalyst was studied with the objective of generating new methods for conversion of NO to N₂. Compared to the homogenous reaction of NO with H₂S, the catalyzed reaction showed improved conversions of NO to N₂. Using a gas space velocity of 1000 h⁻¹ and a feed of 1% NO and 1% H₂S in argon, it was found that the conversion of NO to N₂ was complete at 800 °C. This result compared to a 38% conversion of NO to N₂ for the homogeneous gas phase reaction at 800 °C. At temperatures below 800 °C, a short fall in the nitrogen balance was discovered when the γ-alumina was employed as a catalyst. This discrepancy was explained by conversion of NO to NH₃ and subsequent reaction of the NH₃ with any SO₂ in the system to form ammonium sulfur oxy-anion salts. This suggestion is supported by the finding that when larger amounts of H₂S were used relative to NO, more NH₃ was formed together in tandem with lower N₂ mass balances. Several reaction pathways have been proposed for the catalytic reduction of NO by H₂S.     

Kinetics of the formation of symmetrical wax esters from the corresponding alcohols with the use of hydrobromic acid and hydrogen peroxide

The primary aliphatic alcohols n-octanol, n-decanol, and n-dodecanol have been converted to their corresponding symmetrical esters by using HBr and H₂O₂ in the absence of a solvent. The reaction was carried out at 30, 40, and 50°C and at mole ratios of alcohol to HBr of 1∶0.1, 1∶0.2, 1∶0.3, and 1∶0.5. The rate of the reaction was found to increase with increase in the reaction temperature and concentration of HBr. The maximal conversion of n-octanol was 72% at 40°C and a mole ratio of n-octanol to HBr of 1∶0.5. The kinetics of the reaction have been established, and the reaction was found to be first-order with respect to alcohol and bromine concentration in the organic phase, and second-order with respect to both. The second-order rate constants for n-octanol, n-decanol, and n-dodecanol are 27.08, 32.58, and 37.42 mL mol⁻¹ min⁻¹, respectively, at 40°C. The activation energy for the esterification reaction of n-octanol was found to be 16.32 kcal mol⁻¹.