A study on selective oxidation of hydrogen sulfide over zeolite-NaX and-KX catalysts

Selective oxidation of hydrogen sulfide (H₂S) was studied on zeolite-NaX and zeolite-KX. Elemental sulfur yield over zeolite-NaX was achieved about 90% at 225 °C for the first 4 hours, but it gradually decreased to 55% at 40 hours after the reaction started. However, yield of elemental sulfur on zeolite-KX was obtained within the range of 86% at 250 °C after 40 hours. The deactivation of the zeolite-NaX and -KX catalysts was caused by the coverage of a sulfur compound, produced by the selective oxidation of H₂S over the catalysts. The coverage of a sulfur compound over the zeolite-NaX and -KX was confirmed by the TPD (temperature-programmed desorption) tests utilizing thermogravimetric analysis and FT-IR analysis. Even though high temperature was required to prevent the deactivation of zeolite-NaX, the temperature cannot be raised to 250 °C or above due to the SO₂ production and the decrease of thermodynamic equilibrium constant. Zeolite-KX was superior to the zeolite-NaX for both its selectivity to elemental sulfur and its resistance to deactivation in the selective oxidation of H₂S.     

Cobalt molybdenum oxide catalysts for selective oxidation of cyclohexane

Oxidation of cyclohexane has been carried out using molecular oxygen over cobalt molybdenum oxide (CoMoO₄) catalysts in solvent free conditions. The catalysts were prepared using citrate method with three different molar ratios of Co:Mo, 1:1, 1:2, and 2:1 along with individual oxides for comparative studies. While all the catalysts showed significant activity and selectivity, CoMoO₄ with 1:1 ratio showed the best performance compared to the others with a conversion of 7.38%, with selectivity to cyclohexanol and cyclohexanone (KA oil) of 94.3%, in 1 h. The performance of the catalyst, has been studied as a function of oxygen pressure, reaction temperature, and catalyst loading. It was observed that the catalyst deactivates during the course of the reaction. The reasons for deactivation and methods for restoring the activity have been studied. A kinetic model is presented that captures the complex kinetics and matches well with the experimental data. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4384–4402, 2016     

Silver catalysts supported over activated carbons for the selective oxidation of CO in excess hydrogen: effects of different treatments on the supports

Commercial activated carbon (AC) was used as a support either in its original form or after various modifications, giving diverse textural and surface chemical characteristics. The changes of these properties were monitored by N₂ adsorption–desorption isotherms, temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Ag catalysts were prepared over the AC supports by a conventional wet impregnation method. The catalytic performances of supported Ag/AC catalysts in the selective oxidation of CO in excess H₂ were tested. The results indicate the textural and surface chemical characteristics are responsible for the different catalytic performances.     

Testing molten metal oxide catalysts over structured ceramic substrates for diesel soot oxidation

Onboard tests of an oxide (CoO_x) and a molten mixed oxide catalyst (CoPbO_x) supported on cordierite wall flow filters (Corning Durotrap CO EX80) were performed on rollers with a 1.9-l light duty vehicle for three different driving conditions including two constant speeds with different loadings and a standard European cycle (NEDC). The balance point temperatures obtained in these tests were used to compare the catalytic activity under onboard conditions with laboratory measurements. Onboard tests showed similar catalytic activities with the previous micro-reactor experiments performed in “loose contact” mode [D. Uner, M.K. Demirkol, B. Dernaika, Appl. Catal. B: Environ., in press]. The concentration profiles of the active layer determined by SEM-EDX analysis after use and after aging revealed that the mobile component, PbO_x, migrated in the flow direction and accumulated at the closed end of the channel. There is no direct evidence for the evaporative loss of PbO_x. Regions of high activity indicated by low carbon amounts with unique Pb–Co ratios were determined by SEM analysis.     

Carbon catalyst supports for oxidative decomposition of hydrogen sulfide

Carbon supports having various porous structures and compositions of the mineral matter were obtained. On the basis of the carbon supports, a number of catalyst samples were prepared via the decomposition of an iron complex adsorbed in support pores. The effect of the treatment mode during catalyst preparation on the formation of iron clusters in the porous volume of the support has been studied by means of Mössbauer spectroscopy. The catalysts deposited on carbon supports were tested in the decomposition of hydrogen sulfide.     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.     

Lignocellulosic-derived catalysts for the selective oxidation of propane

The formation of dispersed VPO phases on a lignocellulosic-based activated carbon results in a catalyst that is selective for the partial oxidation of propane, and stable under oxidizing conditions. The use of a stable activated carbon as a catalytic support for active VOx species during the partial oxidation of propane is described for the first time.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over Bi-V-Sb-O catalysts

We investigated the selective oxidation of hydrogen sulfide to elemental sulfur and ammonium thiosulfate by using Bi₄V_2-xSb_xO_11-y catalysts. The catalysts were prepared by the calcination of a homogeneous mixture of Bi₂O₃, V₂O₅, and Sb₂O₃ obtained by ball-milling adequate amounts of the three oxides. The main phases detected by XRD analysis were Bi₄V₂O₁₁, Bi_1.33V₂O₆, BiSbO₄ and BiVO₄. They showed good H₂S conversion with less than 2% of SO₂ selectivity with a feed composition of H₂S/O₂/NH₃/H₂O/He=5/2.5/5/60/27.5 and GHSV=12,000 h^-1 in the temperature ranges of 220–260 ‡C. The highest H₂S conversion was obtained for x=0.2 in Bi₄V_2-xSb_xO_11-y catalyst. TPR/TPO results showed that this catalyst had the highest amount of oxygen consumption. XPS analysis before and after reaction confirmed the least reduction of vanadium oxide phase for this catalyst during the reaction. It means that the catalyst with x=0.2 had the highest reoxidation capacity among the Bi₄V_2-xSb_xO_11-y catalysts.     

Novel supports for nickel-based catalysts for the partial oxidation of methane

Zirconia-supported nickel catalysts with different amounts of aluminum (Al/Zr = 0.2, 1 and 2) were studied in this work in order to find alternative supports for nickel-based catalysts for the partial oxidation of methane. This reaction is a promising route for producing hydrogen and syngas for different applications. Samples were prepared by precipitation and impregnation techniques, characterized by several techniques and evaluated in the partial oxidation of methane in the range of 450–750 °C and 1 atm. It was found that aluminum affects the textural and catalytic properties of zirconia-supported nickel catalysts. The tetragonal phase of zirconia was stabilized by aluminum and gamma-alumina was also found in the aluminum-richest samples. Aluminum increased the porosity and the specific surface area of the solids. The catalytic activity also increased with the amount of aluminum in solids probably due to the stronger interaction of nickel with the support, which slowly generates active sites during the reduction step. The methane conversion and hydrogen selectivity increased with temperature, indicating no deactivation. The hydrogen to carbon monoxide molar ratio decreased due to aluminum but was not significantly affected by temperature. The coke produced was not harmful to the catalysts and aluminum affected its amount, although no simple relationship was found between these parameters. The most promising catalyst was the sample with aluminum to zirconium molar ratio of 2, which showed high activity and hydrogen selectivity and was stable under the reaction condition.     

Iron-chromium-molybdenum oxide catalysts for methanol oxidation

The activity and selectivity of a precipitated iron-chromium-molybdenum oxide catalyst (Mo/(Fe + Cr) = 2.5/(0.5 + 0.5)) towards methanol mild oxidation have been studied by a flow-circulation method. Commensurable activity and selectivity with those of the industrial Fe₂ (MoO₄)₃-MoO₃ catalysts as well as an enhanced stability have been found. The Mössbauer spectra of fresh and tested catalysts show that during the catalytic reaction a partial reduction occurs and a steady state composition differing from the initial one is formed.     

Vanadium species in new catalysts for the selective oxidation of methane to formaldehyde: Activation of the catalytic sites

New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. In situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)₂VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 °C under vacuum, leading to a site giving a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state:

(V⁴⁺–O⁻)^*  (V⁵⁺O²⁻). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed.     

Vanadium peroxocomplexes as oxidation catalysts of sulfur organic compounds by hydrogen peroxide in bi-phase systems

New vanadium oxodiperoxocomplexes Bu₄N⁺[VO(O2)₂ L₂]⁻ were synthesized, where L=pyridine (1), 2-methylpyridine (2), 4-methylpyridine (3), 2-oxymethylpyridine (4). All complexes were characterized by NMR (¹H, ⁵¹V) and IR spectroscopy. The oxidation of sulfur organic compounds and diesel fuel desulfurization catalyzed by vanadium peroxocomplexes in bi-phase system was investigated in various solvents. The complexes manifested high catalytic activity and selectivity in oxidation of sulfides.     

环己烷选择性氧化催化剂的研究进展

由环己烷直接氧化生产环己酮、环己醇等含氧化合物的选择性氧化反应虽早已应用于工业生产中 ,但由于产物产率很低 ,能耗大及污染严重等方面的严重不足 ,一直以来 ,科学家们都在对该反应体系加以改进 ,尤其是对催化剂的改进更成为研究的重点。本文主要阐述了近年来该领域的最新研究与进展     

纳米金催化剂在CO低温氧化和选择性氧化中的研究进展

介绍了纳米金催化剂在CO低温氧化和丙烯直接环氧化反应中的研究进展。在CO低温氧化反应中,催化剂的活性相和载体都具有明显的尺寸效应,纳米金颗粒和载体之间的相互作用主要表现载体不仅可以改变纳米金颗粒的大小和形状,而且也影响了氧的活化,从而提高反应活性;在丙烯直接环氧化反应中,由H2和O2在金颗粒表面反应生成的过氧化物种是反应中间体;在选择性氧化和选择性加氢反应中,金催化剂表现出优良的活性和稳定性。     

Copper-vanadium-cerium oxide catalysts for carbon black oxidation

Series of V-Ce and Cu-V-Ce oxides with different atomic ratios were prepared by the incipient wetness impregnation of a ceria support. These solids were studied by TG–DTA in the oxidation of a model soot (carbon black (CB) Degussa N330). V-Ce oxides are active in the carbon black oxidation and an increase in the vanadium content enhances their catalytic activity. This observation is correlated with the presence of a V₂O₅ phase. However, this catalytic system (V-Ce) produces CO during the CB oxidation. Adding copper to the V-Ce oxide to form a ternary system (Cu-V-Ce) has a beneficial effect on the CO₂ selectivity. A synergistic effect between copper and vanadium species on ceria is evidenced. The presence of the V₂O₅ phase on the catalyst enhances the activity for the CB oxidation while copper (II) species decrease the CO production significantly.     

Preparation of structured egg-shell catalysts for selective oxidations by the ANOF technique

Shell-type catalysts supported on metal wires were prepared by anodic oxidation with spark discharge (the “ANOF” technique) in the presence of precursors of the catalytically active phase. Aluminium, magnesium and titanium oxide were employed as carriers, whereas nickel (also doped with lithium), chromium or molybdenum were the active components. The carrier oxides displayed regular pore structures determined by the preparation condition and influenced by the precursor of the active component, which was incorporated in the pore system of the carrier during the anodisation process. The nickel-containing catalysts were found to yield 60–90% cyclohexene selectivity in the oxidation of cyclohexane.     

甲苯液相选择氧化反应催化剂研究进展

以过渡金属氧化物及其负载型催化剂、杂多酸及其负载型催化剂、过渡金属络合物及其负载型催化剂、沸石分子筛基催化剂、超临界CO2介质催化反应体系为线索,评述了各类甲苯液相选择氧化反应催化剂的特性和近年来的研究进展,展望了甲苯液相选择氧化反应催化剂的发展方向,提出以反应对催化剂的结构和性质为指导来研制高效、绿色的催化剂仍是甲苯液相选择氧化反应的关键。     

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO₄, Mo₆V₉O₄₀, MoO₃, and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo_0.61–0.77V_0.31–0.19Nb_0.08–0.04O_x, as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal M=O bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO₄ and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.