Oxidative dehydrogenation of propane over V2O5-MgO/TiO2 catalyst: Effect of reactants contact mode

The performance of the active catalyst 5%V₂O₅-1.9%MgO/TiO₂ in propane oxidative dehydrogenation is investigated under various reactant contact modes: co-feed and redox decoupling using fixed bed and co-feed using fluid bed. Using fixed bed reactor under co-feed conditions, propane is activated easily on the catalyst surface with selectivities ranging from 30 to 75% depending on the degree of conversion. Under varying oxygen partial pressures, especially for higher than the stoichiometric ratio O₂/C₃H₈ = 1/2, nor the propane conversion or the selectivities to propene and COx are affected. The performance of the catalyst in the absence of gas phase oxygen was tested at 400 °C. It was confirmed that the catalyst surface oxygen participates to the activation of propane forming propene and oxidation products with similar selectivities as those obtained under co-feed conditions. The ability of the catalyst to fully restore its activity by oxygen treatment was checked in repetitive reduction–oxidation cycles. Fluid bed reactor using premixed propane–oxygen mixtures was also employed in the study. The catalyst was proved to be very active in the temperature range 300–450 °C attaining selectivities comparable to those of fixed bed.     

Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor at high pressures

Oxygen permeation fluxes through dense disk-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm² min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO_x/γ-Al₂O₃ as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm² min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm² min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded.     

Electrochemical modification of the catalytic properties of composite membranes of substituted Bi4V2O11 (BIMEVOX) for propane oxidation in a catalytic dense membrane reactor

Dense BICOVOX membrane, bulk BICOVOX/Au cermet membrane and BICOVOX membrane with a BICOVOX/Au cermet on its surfaces are investigated in the partial oxidation of propane under open circuit voltage (OCV) and under electrical bias at 700 °C. The propane conversion remains in the range 10–12% whatever the conditions of polarisation. Mostly cracking products are observed. Hydrogen is the main product with a selectivity around 55–60%. At OCV, no product of oxidation is detected except water which can not be quantified. On the membrane with BICOVOX/Au cermet electrodes on surface, traces of CO are observed as well as a small increase of propylene content under anodic polarisation. This can be explained by partial oxidation and oxidative dehydrogenation of propane, respectively. An anodic polarisation leads to a decrease of hydrogen due to its oxidation into water. In contrast, an increase of the hydrogen content is observed under cathodic polarisation. The effects on the membrane are modest, but they show the possibility that such a system offers for modifying the catalytic properties of membrane materials in a CDMR.     

SO2 promoted oxidation of ethyl acetate, ethanol and propane

The effect of SO₂ addition on the oxidation of ethyl acetate, ethanol, propane and propene, over Pt/γ-Al₂O₃ and Pt/SiO₂ has been investigated. The reactants (300–800 vol. ppm) were mixed with air and led through the catalyst bed. The conversions below and above light-off were recorded both in the absence and in the presence of 1–100 vol. ppm SO₂. For the alumina-supported catalyst, the conversion of ethyl acetate, ethanol and propane was promoted by the addition of SO₂, while the conversion of propene was inhibited. The effect of SO₂ was reversible, i.e. the conversion of the reactants returned towards the initial values when SO₂ was turned off. However, this recovery was quite slow. The oxidation of propane was inhibited by water, both in absence and presence of SO₂. For the silica-supported catalyst no significant effect of SO₂ could be observed on the conversion of ethyl acetate, ethanol or propane, whereas the conversion of propene was inhibited by the presence of SO₂. In situ FTIR measurements revealed the presence of surface sulphates on the Pt/γ-Al₂O₃ catalyst with and after SO₂ addition. It is proposed that these sulphate groups enhance the oxidation of propane, ethyl acetate and ethanol by creating additional reaction pathways to Pt on the surface of the Pt/γ-Al₂O₃ catalyst.     

Reaction and oxygen permeation studies in Sm0.4Ba0.6Fe0.8Co0.2O3 − δ membrane reactor for partial oxidation of methane to syngas

A disk-type Sm_0.4Ba_0.6Co_0.2Fe_0.8O_3 − δ perovskite-type mixed-conducting membrane was applied to a membrane reactor for the partial oxidation of methane to syngas (CO + H₂). The reaction was carried out using Rh (1 wt%)/MgO catalyst by feeding CH₄ diluted with Ar. While CH₄ conversion increased and CO selectivity slightly decreased with increasing temperature, a high level of CH₄ conversion (90%) and a high selectivity to CO (98%) were observed at 1173 K. The oxygen flux was increased under the conditions for the catalytic partial oxidation of CH₄ compared with that measured when Ar was fed to the permeation side. We investigated the reaction pathways in the membrane reactor using different membrane reactor configurations and different kinds of gas. In the membrane reactor without the catalyst, the oxygen flux was not improved even when CH₄ was fed to the permeation side, whereas the oxygen flux was enhanced when CO or H₂ was fed. It is implied that the oxidation of CO and H₂ with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and that CO₂ and H₂O react with CH₄ by reforming reactions to form syngas.     

Evaluation of V2O5–WO3–TiO2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V₂O₅–WO₃–TiO₂ system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO₂. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V₂O₅–WO₃–TiO₂ catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration.     

Partial oxidation of alkanes to oxygenates in supercritical carbon dioxide

The catalytic partial oxidation of propane in supercritical carbon dioxide has been investigated in a stirred batch reactor. Various metals (oxides) have been used as supported catalysts with respect to their activity and selectivity for the formation of oxygenates. The reactions run with a 1:2.3–2.9:68–108 molar ratio of propane:synthetic air:CO₂ at 453–573 K and 80–100 bar. Using a precipitated 2.4 wt.% Co₃O₄–SiO₂ catalyst at 573 K, a total oxygenate (i.e. acetic acid, acetone, acetaldehyde, methanol) selectivity of 59% and a propene selectivity of 21% were obtained at a propane conversion of 12 mol%. The same catalyst has been used to investigate the influence of the supercritical conditions and initial feed composition on the reaction, varying the density of CO₂ and the concentration of synthetic air, respectively.     

Investigation on the partial oxidation of methane to syngas in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

A perovskite material of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), with both electronic and ionic conductivity, was synthesized by a combined citrate–EDTA complexing method. The dense membrane tube made of BSCF was fabricated using the plastic extrusion method. The partial oxidation of methane (POM) to syngas was performed in the tubular BSCF membrane reactor packed with a LiLaNiO/γ–Al₂O₃ catalyst. The reaction performance of the membrane reactor was investigated as functions of temperature, air flow rate in the shell side and methane concentration in the tube side. The mechanism of POM in the membrane reactor was discussed in detail. It was found that in the tubular membrane reactor, combustion reaction of methane with permeated oxygen took place in the reaction zone close to the surface of the membrane, then followed by steam and CO₂ reforming of methane in the middle zone of the tube side. The membrane tube can be operated steadily for 500 h in pure methane with 94% methane conversion and higher than 95% CO selectivity, and higher than 8.0 ml/cm² min oxygen permeation flux.     

Partial oxidation of propane in compressed carbon dioxide using CoOx/SiO2 catalysts

The application of compressed carbon dioxide as an alternative reaction medium was investigated for the heterogeneously catalysed partial oxidation of propane. The catalytic tests were performed in a stirred batch as well as in a continuous flow reactor at 553–623 K, 2.6–11.3 MPa and a CO₂:synthetic air:propane molar ratio of 94–124:4–7:1 using precipitated CoO_x/SiO₂ (2.4–3.7 wt.% Co) catalysts. In addition, the critical temperature and pressure of the reaction mixture were determined by the opalescence method in a high-pressure optical cell. The catalytic experiments revealed a significantly higher cumulative oxygenate selectivity (i.e. acetic acid, methanol, acrolein, acetone) with increasing pressure. It is supposed that the formed oxygenates were removed more easily from the catalyst surface without being totally oxidised due to the higher solvent power of the dense CO₂ in the supercritical phase.     

Oxidation state of palladium as a factor controlling catalytic activity of Pd/SiO2–Al2O3 in propane combustion

The oxidation state of palladium on SiO₂–Al₂O₃ used for propane combustion was examined by XPS and XRD, and the correlation of the catalytic activity with the oxidation state of palladium was systematically studied. The propane conversion over 5 wt% Pd/SiO₂–Al₂O₃ was measured in the range 1.0≤S≤7.2 (S is defined as [O₂]/5[C₃H₈] based on stoichiometric ratio). The propane conversion strongly depended on the S value and reached the maximum at S=5.5. The oxidation state of palladium also changed with the S value; palladium particles were more oxidized under the reaction mixture of higher S value. On the sample used for the reaction at S=5.5, both of metallic palladium and palladium oxide were found. It is concluded that partially oxidized palladium which has optimum ratio of metallic palladium to palladium oxide shows the highest catalytic activity in propane combustion.     

The effect of ceria content on the performance of Pt/CeO2/Al2O3 catalysts in the partial oxidation of methane

The effect of CeO₂ loading (1–20 wt.%) on the properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pt catalysts on the partial oxidation of methane was studied. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and oxygen storage capacity (OSC). XRD and TPR results showed that the pretreatment temperature of the support influences on the amount of CeO₂ with fluorite structure. The pretreatment temperature of the support and CeO₂ loading influenced the morphology of Pt. OSC analysis showed a significant increase in the oxygen storage capacity per weight of CeO₂ for samples with high CeO₂ loading (12 and 20 wt.%). TPR analyses showed that the addition of Pt promotes the reduction of CeO₂. This effect was more significant for the catalysts with high CeO₂ loading (≥12 wt.%). The dispersion of Pt, measured by the rate of cyclohexane dehydrogenation, increases with increasing of the pretreatment temperature of the support. It was shown that the kind of the support is very important for obtaining of catalysts resistant to carbon formation. The catalysts with high CeO₂ loading (≥12 wt.%) showed the highest catalytic activity and stability in the reaction of partial oxidation of methane due to a higher Pt–CeO₂ interface.     

Development of a transient kinetic model for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst

A transient kinetic model was developed for the CO oxidation by O₂ over a Pt/Rh/CeO₂/γ-Al₂O₃ three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O₂ in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393–433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O₂ on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO₂. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations.     

Oxidation of H2S on activated carbon KAU and influence of the surface state

Properties of the oxidized activated carbon KAU treated at different temperatures in inert atmosphere were studied by means of DTA, Boehm titration, XPS and AFM methods and their catalytic activity in H₂S oxidation by air was determined. XPS analysis has shown the existence of three types of oxygen species on carbon catalysts surface. The content of oxygen containing groups determined by Boehm titration is correlated with their amount obtained by XPS. Catalytic activity of the KAU catalysts in selective oxidation of hydrogen sulfide is connected with chemisorbed charged oxygen species (O_3.1 oxygen type with BE 536.8–537.7 eV) present on the carbons surface.

Formation of dense sulfur layer (islands of sulfur) on the carbons surface and removal of active oxygen species are the reason of the catalysts deactivation in H₂S selective oxidation. The treatment of deactivated catalyst in inert atmosphere at 300 °C gives full regeneration of the catalyst activity at low temperature reaction but only its partial reducing at high reaction temperature. The last case is connected with transformation of chemisorbed charged oxygen species into CO groups.

The KAU samples treated in flow of inert gas at 900–1000 °C were very active in H₂S oxidation to elemental sulfur transforming up to 51–57 mmol H₂S/g catalyst at 180 °C with formation of 1.7–1.9 g S_x/g catalyst.     

Oxidation of propane with oxygen, nitrous oxide and oxygen/nitrous oxide mixture over Co- and Fe-zeolites

Oxygen, nitrous oxide and their mixture was used in oxidation of propane over CoH(Fe)–BEA and parent H(Fe)-zeolites (containing trace concentrations of Fe) of beta and ZSM-5 structures. CoH(Fe)–BEA zeolite exhibited a substantial increase in propene yield (from 2.0 to 6.8%), if together with molecular oxygen, nitrous oxide was used. No other oxygenates were detected. With increasing concentration of molecular oxygen in the propane/nitrous oxide/oxygen mixture, a synergetic effect of oxygen and nitrous oxide resulting in nearly threefold increase in propene yield was observed. At the reaction conditions studied, the highest propane conversion (15.8%) with 61.9% selectivity to propene and propene yield of 9.8%, and the sum of selectivities to oxygenates of 10.3% (foremost propyl alcohol and propanal) was achieved with the steamed H(Fe)–MFI zeolite when a mixture of nitrous oxide and oxygen was used. It is concluded that, the oxidation activity of nitrous oxide originates from specific Fe species, present in CoH(Fe)–BEA and H(Fe)-zeolites, exhibiting extra-ordinary activity in hydroxylation of propane to propyl alcohol (dehydrated to propene) and its further oxidation to propanal, whereas Co ions contribute only to propane oxidation to propene and carbon oxides.     

AgBiVMo oxide catalytic membrane for selective oxidation of propane to acrolein

A metal ions (Ag, Bi, V, Mo) modified sol–gel method was used to prepare a mesoporous Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ catalytic membrane which was used in the selective oxidation of propane to acrolein. By optimizing the preparation parameters, a thin and perfect catalytically active membrane was successfully prepared. SEM results showed that the membrane thickness is 5 μm. XRD results revealed that Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ with a Scheelite structure, which is catalytically active for the selective oxidation of propane to acrolein, was formed in the catalytic membrane only when AgBiVMoO concentrations were higher than 40%. Catalytic reaction results demonstrated that the selective oxidation of propane could be controlled to a certain degree, such as to acrolein, in the catalytic membrane reactor (CMR) compared to the fixed bed reactor (FBR). For example, a selectivity of 54.85% for acrolein in the liquid phase was obtained in the CMR, while only 8.31% was achieved in the FBR.     

Mixed reforming of heptane to syngas in the Ba0.5Sr0.5Co0.8Fe0.2O3 membrane reactor

Fuel cells are recognized as the most promising new power generation technology, but hydrogen supply is still a problem. In our previous work, we have developed a LiLaNiO/γ-Al₂O₃ catalyst, which is excellent not only for partial oxidation of hydrocarbons, but also for steam reforming and autothermal reforming. However, the reaction needs pure oxygen or air as oxidant. We have developed a dense oxygen permeable membrane Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ which has an oxygen permeation flux around 11.5 ml/cm² min at reaction conditions. Therefore, this work is to combine the oxygen permeable membrane with the catalyst LiLaNiO/γ-Al₂O₃ in a membrane reactor for hydrogen production by mixed reforming of heptane. Under optimized reaction conditions, a heptane conversion of 100%, a CO selectivity of 91–93% and a H₂ selectivity of 95–97% have been achieved.     

A correlation between crystal structure and catalytic activity in the solid solutions CdMoxW1−xO4

Two CdMo_xW_1−xO₄ solid solutions with the scheelite and the wolframite structures were synthesised via different techniques and their structures were determined from powder refinements. The limit of the reciprocal solubility of Mo in CdWO₄ has been checked. Several compositions of the solid solutions have been tested in the catalytic mild oxidation of propane or propene and relations between the structures and the catalytic activity are proposed.     

Characterization and tests of planar Co3O4 model catalysts prepared by chemical vapor deposition

The chemical vapor deposition method was used to deposit thin films of cobalt oxide starting with cobalt (II) acetylacetonate and oxygen. The deposition process was investigated and the obtained films were identified as a cubic spinel-type polycrystalline Co₃O₄ with a crystallite size of 30–40 nm. The coating was carbon-free and the surface oxygen concentration was measured to be 66 at.% with AES analysis. Smooth and highly uniform thin films were deposited on planar stainless steel substrates and subjected to TPR and catalysis tests that show positive correlation. The apparent activation energy of Co₃O₄ reduction to CoO was measured to be (33±5) kJ/mol. The catalytic activity of Co₃O₄ was investigated toward the conversion of both propane and ethanol to carbon dioxide. Though the catalytic action was registered at the same temperature, the deactivation process was seen to be different. The catalytic conversion of ethanol induces a fast deactivation process, which was linked to its high ability to reduce Co₃O₄.     

Selective oxidation and ammoxidation of propane on a Mo–V–Te–Nb–O mixed metal oxide catalyst: a comparative study

A comparative study on the selective oxidation and the ammoxidation of propane on a Mo–V–Te–Nb–O mixed oxide catalyst is presented. The catalyst has been prepared hydrothermally at 175 °C and heat-treated in N₂ at 600 °C for 2 h. Catalyst characterization results suggest the presence mainly of the orthorhombic Te₂M₂₀O₅₇ (M = Mo, V and Nb) bronze in samples before and after use in oxidation and ammoxidation, although some little modifications have been observed after its use in ammoxidation reaction. Propane has been selectively oxidized to acrylic acid (AA) in the 340–380 °C temperature range while the ammoxidation of propane to acrylonitrile (ACN) has been carried out in the 360–420 °C temperature interval. The steam/propane and the ammonia/propane molar ratios have an important influence on the activity and the selectivity to acrylic acid and acrylonitrile, respectively. The reaction network in both oxidation and ammoxidation reactions as well as the nature of active and selective sites is also discussed. The catalytic results presented here show that the formation of both ACN and AA goes through the intermediate formation of propene.     

Comparison of different catalysts in the membrane-supported dehydrogenation of propane

Dehydrogenation of propane is studied in a high temperature packed bed catalytic membrane reactor with a hydrogen-selective silica membrane. The silica membrane is prepared by a two-step sol–gel process. The removal of hydrogen in the membrane reactor results in higher propane conversion and higher propene yields in comparison to an equivalent fixed-bed reactor. Unfortunately, as a result of the H₂ removal coking is favoured in the membrane reactor. Therefore, the higher propene yields are found only for the first 100–120 min time on stream. However, the lower selectivity of the membrane reactor due to coking is compensated to some extend by a reduced hydrogenolysis. Two commercial dehydrogenation catalysts of different activity were tested in the membrane reactor: Cr₂O₃/Al₂O₃ and Pt–Sn/Al₂O₃. The two catalysts show a different activity, coking, and regeneration behaviour in the membrane-supported propane dehydrogenation.