Preparation of metallic cobalt inside NaY zeolite with high catalytic activity in Fischer–Tropsch synthesis

Metallic cobalt clusters are synthesized inside the cages of NaY zeolite by a method including four steps, i.e.: (1) ion exchange, (2) precipitation of exchanged cobalt cations, (3) calcination and (4) reduction of the calcined oxide nano-particles inside the zeolite. As compared with that prepared by the ion exchange followed directly by calcination and reduction and that by the conventional impregnation method, the sample by this four-step method exhibits higher CO conversion and higher selectivity to n-C₁₀–C₂₀ paraffins in Fischer–Tropsch (FT) synthesis. The small metallic cobalt clusters inside the supercages of NaY zeolite probably account for the high catalytic performances.     

In situ encapsulated subnanometric CoO clusters within silicalite-1 zeolite for efficient propane dehydrogenation

Cobalt-based catalysts are promising alternatives to replace Pt- and Cr-based catalysts for propane dehydrogenation (PDH). However, the sintering and reduction of unstable Co sites cause fast deactivation. Herein, the ultrasmall cobalt oxide clusters encapsulated within silicalite-1 zeolites (CoO@S-1) has been obtained via a ligand assistance in situ crystallization method. This CoO@S-1 catalyst exhibits an attractive propylene formation rate of 13.66 mmol_C3H6·g_cat⁻¹·h⁻¹ with selectivity of >92% and is durable during 120-h PDH reaction with five successive regeneration cycles. The high PDH activity of CoO@S-1 is assigned to the encapsulated CoO clusters are favorable for propane adsorption and can better stabilize the detached H* species from propane, leading to the lower dehydrogenation barriers than framework Co²⁺ cations and Co₃O₄ nanoparticles. Additionally, the π-binding propylene on CoO clusters can prevent the over-dehydrogenation reaction compared with the di-σ binding propylene on metallic Co, leading to the superior propylene selectivity and catalytic stability.     

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co_0.95MoO _x and Mg_0.95MoO _x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.     

Fischer–Tropsch Synthesis on Co/Al2O3 Catalyst: Effect of Pretreatment Procedure

The effect of pretreatment procedure on catalytic performance during Fischer–Tropsch synthesis (FTS) of unpromoted cobalt on alumina (15 %Co/Al₂O₃) catalyst was studied in a fixed-bed reactor. Pure carbon monoxide or a mixture of hydrogen and carbon monoxide were used as reducing agents prior to FTS. Pretreatments with pure CO result in low activity and high selectivity to methane and gaseous hydrocarbons in comparison to standard hydrogen reduction. The use of synthesis gas (H₂/CO = 2/1) as a reducing agent resulted in high initial activity and high selectivity to gaseous hydrocarbons. Pretreatment procedure that utilized synthesis gas after the CO reduction resulted in low activity but high selectivity to high molecular weight hydrocarbons. Catalyst performance is strongly affected by the presence of cobalt carbides, cobalt oxide and/or various types of carbon species on the surface as determined by X-ray diffraction and temperature-programmed hydrogenation and oxidation characterization techniques.     

Metal Exchanged ZSM-5 Zeolite Based Catalysts for Direct Decomposition of N2O

Co+Pt/ZSM-5 and Ag+Pt/ZSM-5 type catalysts were prepared by ion exchange method followed by calcination. These Co and Ag based catalysts, promoted by a small amount of Pt have been studied for their catalytic activity towards N₂O decomposition. Both the catalysts show high catalytic activity, however, cobalt–platinum based catalyst shows relatively better activity at higher temperature. At 550 °C almost 100% conversion of N₂O is achieved over Co+Pt/ZSM-5 with a maximum of 0.08479 mmole of N₂O decomposed per gram of the catalyst per unit time. These catalytic materials have been characterized for their structure, composition, morphology and other details, using XRD, SEM, EDX, ICP, BET techniques. Much improved catalytic activity for the bimetallic zeolite than the mono-metal containing compositions clearly demonstrate the synergistic effect of these transition metals, while high surface area of ZSM-5 is also responsible for the improved N₂O decomposition activity.     

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.     

The influence of the initial acidity of HFER on the status of Co species and catalytic performance of CoFER and InCoFER in CH4-SCR-NO

The role of the initial acidity of ferrierite type zeolite on the status of cobalt and the catalytic activity of CoFER and InCoFER was investigated. Two FER zeolites were used: NH₄FER without any pretreatment (FER-1) and the same zeolite, dehydroxylated at 825 K (FER-2). Dehydroxylation removed most of the Si–OH–Al groups, therefore the resulting zeolite revealed practically no ion exchange capacity. The status of cobalt was followed by IR spectroscopy with probe molecules: CO (a probe for Co²⁺) and NO (a probe for Co³⁺). The introduction of cobalt by solid-state ion exchange produced divalent cobalt in exchange positions and in the form of oxide-like clusters, their respective concentration was determined by quantitative IR experiments of CO sorption. The amount of Co³⁺, present in CoFER-1 and InCoFER-1, was also determined. All these forms of cobalt were practically absent from CoFER-2 and InCoFER-2. The NO conversion and selectivity to N₂ of CoFER-2 in CH₄-SCR-NO was poor, indicating the essential role of the initial acidity of the ferrierite matrix on the formation of catalytically active Co species. The introduction of indium into CoFER only slightly increased the NO conversion and shifted the reaction path from NO₂ towards N₂ formation for FER-1, while greatly improved the catalytic performance of the FER-2 series.     

Selective oxidation of ethane and propane over sulfated zirconia‐supported nickel oxide catalysts

The oxidative dehydrogenations of ethane and propane were investigated over a series of zirconia and nickel‐oxide supported on zirconia catalysts. It was found that zirconia, sulfated zirconia as well as NiO‐based zirconia catalysts showed high catalytic activities for oxidative dehydrogenation of ethane and propane. However, conversion and selectivity differed depending on the nature of the catalysts. Zirconia, sulfated zirconia (SZ) and their supported NiO catalysts showed high ethane conversions but lesser selectivities to olefins while NiO/Li₂ZrO₃ exhibited high selectivities to ethylene and propylene. Addition of an LiCl promoter in the NiO/SZ catalyst increased the catalytic activity and olefin selectivity, thus resulting in a higher olefin yield. In the oxidative dehydrogenations of ethane and propane NiO–LiCl/SZ exhibited 79% ethylene selectivity at 93% ethane conversion at 650 °C and 52% selectivity to propylene at 20% propane conversion at 600 °C, respectively. Characterization showed that the physico‐chemical properties of the catalysts determine the catalytic activity and selectivity. © 2001 Society of Chemical Industry     

Oxidative Dehydrogenation Properties of Novel Nanostructured Polyoxovanadate Based Materials

Abstract  

A comparative study of the catalytic oxidative dehydrogenation of propane by a novel polyoxovanadate based open-framework material (Co-POV)—[Co₃V₁₈O₄₂(H₂O)₁₂(XO₄)]·24H₂O (X = V, S), which is composed of nanometer size vanadium oxide clusters interlinked by cobalt oxide {–O–Co–O–} motifs, showed that Co-POV has superior catalytic property as compared to its individual metal oxide constituents, vanadium oxide and cobalt oxide, and their mixture, with high propylene selectivity.     

Ruthenium promoted cobalt–alumina catalysts for the synthesis of high-molecular-weight solid hydrocarbons from CO and hydrogen

The effect of the ruthenium promotion of Fischer–Tropsch (FT) cobalt–alumina catalysts on the temperature of catalyst activation reduction and catalytic properties in the FT process is studied. The addition of 0.2–1 wt % of ruthenium reduces the temperature of reduction activation from 500 to 330–350°C while preserving the catalytic activity and selectivity toward C₅₊ products in FT synthesis. FT ruthenium-promoted Co–Al catalysts are more selective toward higher hydrocarbons; the experimental value of parameter α_ASF of the distribution of paraffinic products for ruthenium-promoted catalysts is 0.93–0.94, allowing us to estimate the selectivity toward C₂₀₊ synthetic waxes to be 48 wt %, and the selectivity toward C₃₅₊ waxes to be 23 wt %. Ruthenium-promoted catalysts also exhibit high selectivity toward olefins.     

Propane ammoxidation on bulk, diluted and supported VSb oxides

VSb oxides diluted with Mg, Al and Zr displayed substantially higher selectivity for propane ammoxidation to acrylonitrile than their bulk and supported analogues. Diluted catalysts were found to consist of oxide compounds of antimony with diluent element, such as AlSbO₄ or MgSb₂O₆, and small amounts of individual oxides of antimony and diluent element. No VSbO₄ phase was detected in their body in contrast with bulk and supported catalysts. It appears that better isolation of vanadium and antimony entities in the structure of diluted oxides was responsible for their enhanced catalytic behavior. Tungsten loaded to the surface of diluted catalysts further improved their selectivity through tuning the surface acidity.     

Catalytic and electrocatalytic oxidation of propane on V-Mg-O and V-Mg-O (Ag) catalysts

Vanadium magnesium oxide catalysts prepared in this work were found active in selective oxidation of propane to propene. A selectivity as high as 79% was obtained at 10% conversion (813 K). No oxygenated or C₂ products were detected and the catalysts were found to undergo no change in activity over many weeks of operation. Under electrochemical pumping of oxygen (EOP) towards the catalyst (with oxygen present in the feed gas), both conversion and selectivity were found to increase slightly as external current increased, indicating the effect of electrical current can be exhibited by an oxide catalyst. However, in the absence of oxygen in the feed gas, EOP could lead to an even higher selectivity: 84 and 86.9% respectively for a 24 V-Mg-O and a 24 V-Mg-O (Ag) (1/2) catalyst. The overall results obtained suggest that electrochemically supplied oxygen is more selective towards C₃H₆. Mechanisms of both catalytic and electrocatalytic oxidation of propane were tentatively suggested, with surface oxygen ion vacancy identified as active surface species and the rate determining step involving heterolytic splitting of the C₃H₈ molecule to form a surface bonded C₃H ₇ ^– ion and a surface hydroxyl ion. The higher selectivity towards C₃H₆ in case of EOP was explained on this basis. While mixing with Ag powder was found to improve significantly the electrocatalytic performance of vanadium magnesium oxide, its role appears to be non-chemical: it simply gives rise to a larger area of the gas/catalyst/Ag electrode interface.     

The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract  

The effect of Cu loading on the selective catalytic reduction of NO_x by NH₃ was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h⁻¹. Both NO reduction and NH₃ oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH₃ oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO₂ = 1), only a slight improvement in NO_x reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N₂ was observed; only a very small amount of N₂O was produced even in the presence of NO₂. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively.     

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Abstract  

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H₂-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C₅+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst.     

Selective epoxidation of styrene with air catalyzed by CoOx and CoOx/SiO2 without any reductant

Cobalt oxide (CoOx) prepared by a direct calcination of cobalt nitrate was considerably active for the epoxidation of styrene with air in DMF under mild conditions. A substrate conversion of 75.8 mol% with an epoxide selectivity of 82.1% was achieved at 353 K over 10 mg of cobalt oxide catalyst. Once CoOx was loaded on the support SiO₂ through a simple procedure consisting of wet impregnation, drying and calcination, the as-prepared catalyst presented higher catalytic activity and epoxide selectivity than cobalt oxide itself. Over the optimized catalyst CoOx/SiO₂ (1.0 wt% Co), 85.7 mol% of styrene was effectively converted at 363 K within 4 h, with a high epoxide selectivity up to 86.0%. The results showed that many factors influenced the performance of the catalyst, such as the Co loading, the support, the temperature and the atmosphere, etc. The leaching of cobalt from the catalyst CoOx/SiO₂ was negligible, indicating the applicability of the catalyst CoOx/SiO₂ as a true heterogeneous catalyst. The control test and UV–vis spectra revealed a synergic interaction among solvent, oxygen and substrate over CoOx/SiO₂.     

Novel nitrogen containing heterogeneous catalysts for oxidative dehydrogenation of light paraffins

Novel nitrogen contained catalyst CoN_x/Al₂O₃ yielded high performance in the oxidative dehydrogenation of propane and n-butane. 47.6 and 37.4 wt% yield of olefins at 82% butane and 76.7% propane conversion were measured at 600 °C. Ethylene and propylene were mainly formed at >400 °C via oxidative cracking of paraffins. XRD and XPS studies of the novel catalytic system indicate an essential modification of cobalt by nitrogen.     

Promoted Fe/ZSM-5 catalysts prepared by sublimation: de-NOx activity and durability in H2O-rich streams

Fe/ZSM-5 catalysts with an Fe/Al ratio 1:0, were prepared by sublimation of FeCl3 into H/ZSM-5. They display high activity and durability for the selective catalytic reduction of NOx to N2, both in dry and wet gas flows. These catalysts have now been modified by exchanging a second cation into the zeolite. Mere neutralization of zeolite protons by Na⁺ lowers the selectivity for NOx reduction to N2, but the cations Ce₃ ⁺ and La₃ ⁺ act as true catalyst promoters. With isobutane as the reductant in a simulated vehicular emission gas, almost 90% of NOx is reduced to N2 at 350°C over the La-promoted catalyst. The presence of 10% H2O in the feed does not impair the catalyst performance at high temperature; in the temperature region below 350°C it even increases the N2 yield. The beneficial effect of La is due to its lowering of the catalyst activity for the undesired combustion of the hydrocarbon. No signs of zeolite destruction are evident after 100 h TOS in a wet gas flow at 350°C. Carbonaceous deposits causing a slight deactivation are easily removed in an O2/He flow at 500°C; this in situ regeneration fully restores the original activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation of bifunctional catalysts by solid-state ion exchange in zeolites

Bifunctional catalysts containing an acidic as well as a hydrogenation/dehydrogenation function were prepared by solid-state ion exchange. Preparation and properties of Pd-loaded H-ZSM-5 are described in detail. The catalytic behavior of the reduced catalysts was investigated using hydrogenation and hydroisomerisation of ethylbenzene as a test reaction. The catalytic performance of the catalysts could be significantly improved by concomitant incorporation of Ca²⁺. This was advantageously achieved in a two-step procedure, where first a solid-state ion exchange with CaCl₂ and subsequently a second solid-state ion exchange with PdCl₂ was carried out. A Ca, H-ZSM-5 with 1.5 wt-% Pd obtained via this two-step procedure and subsequent reduction in H₂ exhibited high activity, long life-time and good selectivity with respect to hydrogenation and hydroisomerisation of ethylbenzene to ethylcyclohexane and dimethylcyclohexanes, respectively. Electron micrographs of the reduced catalyst showed finely dispersed palladium with maximum size of about 2.0 nm.On leave from Central Research Institute of Chemistry of the Hungarian Academy of Sciences.     

Oxidative Dehydrogenation of Propane on Calcium Hydroxyapatites Partially Substituted with Vanadate

Solid solutions of phosphate and vanadate calcium hydroxyapatites were synthesized and the catalytic activities for the oxidative dehydrogenation of propane to propylene on those catalysts were examined. Although the conversion of propane and the selectivity to propylene were 7.6 and 3.5% on calcium hydroxyapatite (CaHAp), the incorporation of vanadate to CaHAp by V/P=0.05 (atomic ratio) resulted in the enhancement of the conversion and the selectivity to 17.2 and 52.4%, respectively, corresponding to those on Mg₂V₂O₇ under the same reaction conditions (14.0 and 50.9%, respectively).     

Estimation of the intracrystalline diffusion coefficient of the reactant during selective catalytic reduction of nitrogen oxide by propane on Co-ZSM-5

In order to estimate the effect of diffusion during the selective catalytic reduction of nitrogen oxides (NOx) using propane on Co-ion-exchanged ZSM-5, the catalytic activity was measured for the catalysts prepared from zeolites having different crystal sizes. The conversions of NOx and propane on a catalyst having a large crystal size (1.3 μm) were much less than those on a catalyst having a smaller crystal size (0.10 μm). Based on the experimental data and certain assumptions, the effective intracrystalline diffusion coefficient of NO during the reaction in the presence of water vapor at 673 K was estimated to be (6-9)× 10^-15 m²/s. This revised version was published online in July 2006 with corrections to the Cover Date.