Epoxidation of cyclooctene over mesoporous Ga,Ga–Nb,and Ga–Mo oxide catalysts

We demonstrate the epoxidation of cyclooctene to epoxycyclooctane over mesoporous Ga and mixed Ga–Nb and Ga–Mo oxides synthesized via self-assembly hydrothermal-assisted approach. These mesoporous catalysts displayed high epoxide selectivities at moderate cyclooctene conversions. Mesoporous Ga oxides with average particle sizes between 2 and 3 μm exhibited higher cyclooctene conversions, while larger particle sizes between ~ 4.5–6.5 μm showed lower conversions. The incorporation of Nb led to an increase in the acidity of the resultant Ga–Nb mixed oxides. These mesoporous Ga–Nb mixed oxides displayed 80–100% selectivity for the epoxide at conversion levels of cyclooctene in the 17 to 30% range. Finally, a mesoporous Ga–Mo oxide with molar composition of 35% Mo displayed the highest cyclooctene conversion of all mesoporous samples at 60 °C. The conversion of this sample was 41% with 100% selectivity.     

Water gas shift reaction over Cu–Mn mixed oxides catalysts: Effects of the third metal

Highly efficient Cu–Mn spinel catalysts for water gas shift (WGS) reaction were achieved by a single step urea-nitrate combustion method. A series of doped Cu–Mn-M catalysts (M = Ce, Zr, Zn, Fe, Al) were prepared by the same method. Effects of dopants on WGS activity and stability of doped Cu–Mn catalysts were investigated. The doped catalysts were characterized by BET, XRD and TPR. XRD results showed that non-doped samples and Zr-doped samples are mainly composed of Cu_1.5Mn_1.5O₄ phase, while CuO, Cu₂O and Cu_1.5Mn_1.5O₄ for other doped samples. It was further found that WGS activities depend strongly on the natures of the dopant employed despite of their lower content, varying in the order of Zr > Fe > non-doped > Ce > Al > Zn. TPR profiles revealed that all dopants shift the reduction peaks to lower temperature region, indicating no direct correlation between WGS activity and the reducibility. In addition, Zr-doped Cu–Mn catalyst with 5 wt.% content showed the best catalytic performance and, optimal stability exposed to oxygen-stream and on-stream operation. It indicates that ZrO₂ is an effective promoter for Cu–Mn catalyst, and the catalytic performances are related to the existence of a Cu_1.5Mn_1.5O₄ phase and ease reducibility of the catalysts.     

The effect of the change of the structure on oxidative dehydrogenation of propane over cerium–zirconium catalysts

A series of pure CeO₂, ZrO₂, and CeZrO_x mixed metal oxide catalysts were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 500°C and 0.1 MPa. The prepared catalysts were characterized by thermal gravimetric analysis (TGA), Brunauer, Emmett, and Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopes (TEM), Raman spectroscopy, and H₂-TPR. It was observed that the zirconium content of the solid solution of the mixed metal oxide catalyst was 5%–25%, while the zirconium content of the material with phase segregation was higher (50%). The addition of zirconium was proven to decrease the oxygen vacancy concentration on the catalyst surface and change the intensity of (111) crystal of cerium oxide in the catalysts. Among the prepared catalysts, the Ce_0.90Zr_0.10O_x catalyst with the maximum strength of the (111) crystal plane of cerium oxide exhibited the better catalytic oxidation performance for the dehydrogenation of propane to propylene. Compared with ZrO₂ in the blank experiment, the average propane conversion and propylene selectivity of the Ce_0.90Zr_0.10O_x catalyst were increased by 10.78% and 17.95%, respectively.     

Promoting effect of carbon dioxide on the dehydrogenation and aromatization of ethane over gallium‐loaded catalysts

Ga₂O₃ and Ga₂O₃/TiO₂ catalysts were found to be effective agents for the dehydrogenation of ethane to ethene in the presence of carbon dioxide at 650 °C. The activity of the Ga₂O₃ and Ga₂O₃/TiO₂ catalysts in the presence of CO₂ was 2–4 times higher than that without CO₂. Ethene yields reached ca. 20–25% and selectivity was ca. 70–90% at 650°C in the 17% ethane and 83% CO₂ feed at an SV of 9,000 ml/(g‐cat h). The presence of CO₂ markedly promoted dehydrogenation of ethane over Ga₂O₃ and Ga₂O₃/TiO₂ catalysts. Furthermore, the promoting effect of CO₂ on the aromatization of ethane and ethene over a Ga₂O₃+H/ZSM‐5 catalyst was also observed above 650 °C. Aromatics yields were higher than those without CO₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the intrinsic activity of vanadium centres in the oxidative dehydrogenation of propane over V–Ca–O and V–Mg–O catalysts

The performance of V–Ca–O catalysts for the oxidative dehydrogenation of propane has been studied for the first time, being compared with that of similar V–Mg–O catalysts, and their differences are interpreted in terms of their physico-chemical properties. The VCaO catalyst showed the same initial selectivity to propene as the most selective VMgO composition, but it decreased faster with increasing propane conversion. Vanadium species present in the catalyst surface were different for the two supports: isolated V⁵⁺ tetrahedra on CaO and magnesium orthovanadate and pyrovanadate on MgO. The intrinsic activity of isolated vanadium centres in the surface of the V–Ca–O catalyst was more than one order of magnitude higher than that of dimeric or polymeric tetahedral species in the magnesium-containing catalysts. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of the Ce content on the oxidative dehydrogenation of propane over CrOy-CeO2/γ-Al2O3 catalysts

A series of CrO_y (17.5 wt%)-CeO₂ (X wt%)/γ-Al₂O₃ catalysts (X=0, 0.5, 2, 5, 8) with various Ce contents were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 550℃ and 0.1 MPa. The prepared catalysts were characterized by BET, H₂-TPR, O₂-TPD, XPS, XRD, SEM-EDS and Raman spectroscopy. Among the prepared catalysts, the 17.5Cr-2Ce/Al catalyst with the largest amount of lattice oxygen exhibited the best catalytic performance for the dehydrogenation of propane to propylene with lattice oxygen. The decreased presence of oxygen defects and reducibility were the factors responsible for the improved dehydrogenation activity of the catalysts. The CeO₂ layer could inhibit the evolution of lattice oxygen (O^2-) to electrophilic oxygen species (O₂^-), and the oxygen defects on the catalyst surface were reduced. The inhibited lattice oxygen evolution prevented the deep oxidation of propane or propylene, the average CO_x selectivity decreased from 24.41% (17.5Cr/Al) to 5.71% (17.5Cr-2Ce/Al), and the average propylene selectivity increased from 60.15% (17.5Cr/Al) to 85.05% (17.5Cr-2Ce/Al).     

The effect of the Ce content on the oxidative dehydrogenation of propane over CrOy-CeO2/γ-Al2O3 catalysts

A series of CrO_y (17.5 wt%)-CeO₂ (X wt%)/γ-Al₂O₃ catalysts (X = 0, 0.5, 2, 5, 8) with various Ce contents were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 550 °C and 0.1 MPa. The prepared catalysts were characterized by BET, H₂-TPR, O₂-TPD, XPS, XRD, SEM-EDS and Raman spectroscopy. Among the prepared catalysts, the 17.5Cr-2Ce/Al catalyst with the largest amount of lattice oxygen exhibited the best catalytic performance for the dehydrogenation of propane to propylene with lattice oxygen. The decreased presence of oxygen defects and reducibility were the factors responsible for the improved dehydrogenation activity of the catalysts. The CeO₂ layer could inhibit the evolution of lattice oxygen (O²⁻) to electrophilic oxygen species (O₂⁻), and the oxygen defects on the catalyst surface were reduced. The inhibited lattice oxygen evolution prevented the deep oxidation of propane or propylene, the average CO_x selectivity decreased from 24.41% (17.5Cr/Al) to 5.71% (17.5Cr-2Ce/Al), and the average propylene selectivity increased from 60.15% (17.5Cr/Al) to 85.05% (17.5Cr-2Ce/Al).     

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.     

Oxidative dehydrogenation of propane to propylene over Al2O3-supported Sr–V–Mo catalysts

This work presents the results of propane oxidative dehydrogenation on alumina-supported V–Mo oxides and Sr–V–Mo oxide catalysts. The reaction was conducted at atmospheric pressure and 500 °C. Catalysts composed of 2:4 vanadium to molybdenum ratio showed the best performance. The presence of strontium in the catalyst's matrix enhanced its performance (increase in conversion and selectivity) and decreased its reducibility by changing the nature of its surface as confirmed by BET, XRD and TGA techniques. Moreover, the presence of strontium improved the stability of the catalyst. Hence, Sr–V–Mo stands as a promising catalyst for oxidative dehydrogenation of propane.     

Oxidation and reduction effects of propane–oxygen on Pd–chlorine/alumina catalysts

Pd–chloride precursor salt was used to prepare Pd/Al₂O₃ catalysts. TPSR measurements showed three distinct reactions for the oxidation of propane on palladium surface under excess of hydrocarbon: complete oxidation, steam reforming and propane hydrogenolysis. Propane oxidation on palladium catalysts was related to the Pd²⁺ sites observed on Pd/Al₂O₃ through infrared of adsorbed carbon monoxide. In fresh catalysts reduced by H₂, the IR spectra showed the linear and bridge adsorbed CO species on the Pd⁰ surface. After propane reaction, a new band at 2130 cm^-1 related to CO adsorption on Pd²⁺ species was noted. Carbon monoxide species adsorbed on Pd⁰ were also observed in all samples after reaction. Our results suggest surface ratios of Pd⁰/PdO during the propane oxidation. On the other hand, time on stream conversions of the complete oxidation of propane were affected by either the water generated during the reaction or added as a reactant at 10 vol%. The water generated by the reaction helped to eliminate chlorine residues in the form of oxychloride species leading to an increasing of the activity. However, the presence of water into the reaction mixture caused a strong decreasing of the activity. The inhibition mechanism of propane oxidation in the presence of water consisted in the dissociative adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd–OH) at the surface, diminishing the number of active surface sites. Dynamic fluctuations into the reaction conditions supported the idea that a pseudo‐equilibrium adsorption–desorption of water was reached. After water removal or increasing in the reaction temperature the equilibrium was shifted to the direction of OH–Pd decomposition. This behavior suggests that the inhibitory effect of water is a reversible phenomenon, being a function of the amount of water and the reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Effect of sodium content on the interaction between Ni and support and catalytic performance for syngas methanation over Ni/Zr–Yb–O catalysts

In this paper, Ni/Zr–Yb–O catalysts with different sodium contents are prepared by a co-precipitation method, using aqueous Na₂CO₃ solution as a precipitant, and the effect of sodium on the catalyst structure and catalytic performance for syngas methanation is extensively investigated using five Ni/Zr–Yb–O catalysts, containing 0, 0.5, 1.5, 4.5 and 13.5 wt% Na⁺, those are denoted as Cat-1, Cat-2, Cat-3, Cat-4 and Cat-5 respectively. It is found that the interaction between Ni and support determines the catalytic performance of Ni/Zr–Yb–O and the residual sodium content negatively affects the interaction between Ni and support. Cat-1 exhibits an excellent catalytic performance. During a long run time of 380 h, no deactivation is observed and both CO conversion and CH₄ selectivity maintain a level above 90%. However, Cat-3 and Cat-5 suffer rapid deactivation under the same reaction condition. The characterization results indicate the strong interaction between Ni and support enables Cat-1 to possess well dispersed Ni species, resistance to sintering and carbon deposition and thus the excellent catalytic performance. However, the presence of sodium ions over Ni/Zr–Yb–O degrades the interaction between Ni and support and the catalytic performance, especially for the stability. The relative weak interaction between Ni and support results in severe sintering of both ZrO₂ and Ni under the reaction condition, carbon deposition and the poor catalytic performance.     

Effects of hafnium sources and hafnium content on the structures and properties of SiBNC–Hf ceramic precursors

The synthesis of SiBNC–M (M is metal element) multinary ceramic precursors has mainly been through chemical modification of the SiBNC ceramic precursors with the introduction of the metal element/compound. This misses the simplicity, efficiency, and combined benefits of single reactor synthesis routes. We herein adopt a one-pot method to synthesize SiBNC–Hf ceramic precursors (PBSHZ) from different Hf sources, and with varying Hf content. The starting materials include hafnium tetrachloride, hafnium dichloride, trichlorosilane, hexamethyldisilazane, and boron trichloride. Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis are employed to characterize the structures and properties of the PBSHZ. The results reveal the successful incorporation of Hf into the precursors. Compared to the hafnium tetrachloride source, the precursor synthesized from hafnium dichloride yields more ceramics and shows better solubility, due to fewer Hf–Cl bonds available for reaction. Meanwhile, there is a positive correlation between the rise in the Hf content of PBSHZ and the ceramic yield. However, the solubility and processability of the precursors decline, due to the multiplication of the cross-linking degree in the molecular structure. Further FT-IR, NMR, XPS, and thermogravimetry–mass spectrometry (TG–MS) analysis indicate a full polymer-to-ceramic transformation at 1000°C pyrolysis temperature. At this point, the SiBNC–Hf ceramics mainly consist of an Si–B–N skeleton, HfN, and free carbon.     

Production of renewable p-xylene from 2,5-dimethylfuran via Diels–Alder cycloaddition and dehydrative aromatization reactions over silica−alumina aerogel catalysts

We report the selective conversion of biomass-derived 2,5-dimethylfuran (DMF) to p-xylene (~ 70% selectivity) through Diels–Alder cycloaddition and subsequent dehydration with silica−alumina aerogel (SAA) catalysts. The high activity of SAA can be attributed to its high surface area, large mesoporous volume, and high acid site concentrations. The conversion of DMF and the yield of p-xylene were strongly dependent on the silica alumina ratio of SAA. A higher aluminum content in SAA led to a progressive increase in the concentration of Brønsted acid sites and a corresponding increase in the p-xylene production rate. The effect of solvent on the production of p-xylene was examined, and it was found that the p-xylene production rate increases significantly in polar aprotic solvents (i.e. 1,4-dioxane).     

Density functional theory and kinetic Monte Carlo simulation study the strong metal–support interaction of dry reforming of methane reaction over Ni based catalysts

Oxide supports modify electronic structures of supported metal nanoparticles,and then affect the catalytic activity associated with the so-called strong metal-support interaction(SMSI).We herein report the strong influence of SMSI employing Ni_4/α-MoC(111) and defective Ni_4/MgO(100) catalysts used for dry reforming of methane(DRM,CO_2+CH_4→2 CO+2 H_2) by using density functional theory(DFT) and kinetic Monte Carlo simulation(KMC).The results show that α-MoC(111) and MgO(100) surface have converse electron and structural effect for Ni_4 cluster.The electrons transfer from a-MoC(111) surface to Ni atoms,but electrons transfer from Ni atoms to MgO(100) surface;an extensive tensile strain is greatly released in the Ni lattice by MgO,but the extensive tensile strain is introduced in the Ni lattice by α-MoC.As a result,although both catalysts show good stability,H_2/CO ratio on Ni_4/α-MoC(111) is obviously larger than that on Ni_4/MgO(100).The result shows that Ni/α-MoC is a good catalyst for DRM reaction comparing with Ni/MgO catalyst.     

Ordered mesoporous Ga2O3 and Ga2O3–Al2O3 prepared by nanocasting as effective catalysts for propane dehydrogenation in the presence of CO2

Thermally stable mesoporous gallium and gallium–aluminum (atomic ratio of Ga/Al = 4/1 and 1/4) oxides with controlled textural and structural properties were prepared by means of the nanocasting approach. All materials have uniform micron-sized particles, with a quite narrow pore-size distribution centered in the range of 6.2–6.5 nm and specific surface areas as high as 231–322 m²·g^− 1. Pure mesoporous gallium and gallium–aluminum (Ga/Al = 4:1) oxides exhibit a promising catalytic performance in the dehydrogenation of propane to propene in the presence of CO₂ (DHP-CO₂). Over the most active materials, during 4 h on stream at 823 K, propene was produced with the yield of 10–18% and high selectivity of 91–95%. Moreover, pure mesoporous gallium oxide exerted a higher resistance on deactivation during the DHP-CO₂ process in comparison with gallium oxide prepared without a hard template.     

Oxidative dehydrogenation of propane over a series of low‐temperature rare earth orthovanadate catalysts prepared by the nitrate method

A series of high‐purity rare earth orthovanadates were prepared by the nitrate method and found to be effective low‐temperature catalysts for the oxydehydrogenation of propane at 320°C, at which no reactions occurred over the catalysts reported in the literature, and, thus, may be of practical significance. The catalytic performances of LnVO₄ (Ln = Y, Ce–Yb) at 500°C were much better than those of rare earth orthovanadate catalysts and also slightly exceeded that of magnesium orthovanadate Mg₃(VO₄)₂ reported in the literature. LnVO₄ (Ln = Y, Ce–Yb) materials were tetragonal active phases which could stabilize the existence of active sites for the oxydehydrogenation of propane. Some catalysts with a certain amount of LnVO₃ reduced from LnVO₄ (Ln = Ho–Yb) under reaction atmosphere exhibited better redox properties and catalytic performances possibly due to the existence of biphasic catalytic synergy. LaVO₄ was a monoclinic unstable active phase, although its bulk structure did not change after reaction. The remarkable deactivation of the LaVO₄ catalyst was probably due to that LaVO₄ could not stabilize the existence of surface active sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of sintering-resistance and large metal–support interface of alumina nanorod-stabilized Pt nanoparticle catalysts on the improved high temperature water gas shift reaction activity

Pt nanoparticles stabilized with alumina nanorods (Pt@Al₂O₃) were synthesized by a simple preparation method using a mixture of Pt nanoparticles, an alumina precursor, an organic surfactant, and a reducing agent. Because the alumina nanorods stabilized Pt nanoparticles, the sintering of Pt nanoparticles was significantly suppressed during a high temperature water gas shift reaction, demonstrating 1.7–13.6 times higher CO conversions or 1.0–8.0 times higher TOF compared to other alumina-supported Pt catalysts. In addition, the increased metal–support interface for Pt@Al₂O₃ significantly improved the water gas shift reaction activity.     

Characteristics of methane combustion over La–Cr–O catalysts

The catalytic activities of methane combustion of La–Cr–O catalysts prepared with and without polyacrylic acid as a template have been compared. The polymer-templated catalyst had a higher BET surface area, 12.3 m² g⁻¹, than that obtained from the conventional precipitation method, 2.9 m² g⁻¹. The results of XRD and SEM experiments suggested that the structural characteristics were almost similar. Surprisingly, the areal rate of methane combustion over the catalyst with a small surface area was ten times larger than that of the catalyst with a large surface area. However, the site time yields (STY) based on the oxygen adsorption capacity were similar, independent of the surface area and preparation conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methanation of carbon dioxide over mesoporous Ni–Fe–Ru–Al2O3 xerogel catalysts: Effect of ruthenium content

Mesoporous nickel (35 wt%)–iron (5 wt%)–ruthenium (x wt%)–alumina xerogel (denoted as 35Ni5FexRuAX) catalysts with different ruthenium contents (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a single-step sol–gel method for use in the methane production from CO₂ and H₂. Conversion of CO₂, yield for CH₄, metal surface area, and the amount of desorbed carbon dioxide of the catalysts showed volcano-shaped trends with respect to ruthenium content. Experimental results revealed that metal surface area and the amount of desorbed carbon dioxide of 35Ni5FexRu catalysts were well correlated with conversion of CO₂ and yield for CH₄.