Catalysts For Partial Oxidation Research Articles

Structural characterization of high-performance catalysts for partial oxidation—the high-resolution and analytical electron microscopy approach

A set of Mo, V, and W mixed-oxide samples with additives P and Cs showing high performance in selective partial oxidation of propene is studied by means of elemental mapping and crystal structure determination using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) as supplement to XRD measurements which are insufficient for phase determination. The set is divided into two subsets of samples denoted A and B, where subset A samples show the highest performance in selective partial oxidation of propene. These high-performance catalysts are all predominated by structures of the Mo 5O 14 type in contrast to a negligible amount of the orthorhombic MoO 3, which is known to favor total oxidation.

A Study of Hydrophobic Pd/SDB Catalyst for Ethylene Partial Oxidation to Acetic Acid

The performance of 0.2 wt% Pd/SDB catalyst used for partial oxidation of ethylene to acetic acid has been evaluated in a fixed-bed reactor. The catalyst has high activity and selectivity toward acetic acid, but it starts to deactivate after running 84 h at 120 °C and 1200 kPa. BET, XRD, FTIR and XPS have been employed to the characterization of the catalyst. It has been found that the deactivation of the catalyst is mainly due to the partial oxidative degradation of the SDB framework during reaction, particularly, in the presence of oxygen, which results in a decrease in surface area and an increase in hydrophilicity of the catalyst.

Effect of promoters on the catalytic performance of Ni/Al2O3catalyst for partial oxidation of methane to syngas

The effect of promoters such as Ce, La and Ca on catalytic performance of Ni catalyst was measured in a continuous fixed bed reactor. The effect of promoters on Ni/a-Al2O3 catalyst is more significant than on Ni/g-Al2O3 catalyst. Ce was proved to be the best promoter among the three promoters tested and the optimum loading of Ce was 1%. The catalyst was characterized by TG, XPS, TPR and XRD techniques. TPR results showed that Ce can improve the reducibility of the Ni/Al2O3 catalyst. XRD results indicated that Ce was highly dispersed when its loading was low, but at higher loading it was crystallized into bulk CeO2, thus, decreased the catalytic activity.

Effect of calcination temperature on the properties of industrial V2O5-TiO2(anatase) catalysts in methanol oxidation

The properties of the catalysts for partial oxidation of o-xylene depend on the structure of the supported vanadium sites. The structure itself is strongly dependent on the calcination temperature of the catalyst at which thermal deposition of the metal oxide on the oxide support takes place. We have investigated the effect of calcination temperature on the activity and selectivity of industrial V2O5-TiO2 (anatase) supported catalysts designed for partial oxidation of o-xylene in their application to methanol oxidation.

Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Effect of composition

Nickel-calcium phosphate/hydroxyapatite catalysts have recently been reported to exhibit high activity and selectivity in partial oxidation of methane (POM). In this work the optimum composition was determined. The optimum mole ratio of Ca/PO4 was around 10/6 and that of Ni/PO 4 was in a range from 1.0/6 to 3.0/6 with the optimum Ca/PO4, and the activity could be related with the amount of metallic nickel. In a temperature range from ca. 400 to 700 K, an apparent autothermal reaction was observed to occur in some cases. This is due to the fact that the actual catalyst temperature is higher than the measured temperature, which comes from the exothermic nature of the reaction. The mixing sequence of the precursors during the catalyst preparation does little affect the catalyst activity and char- acteristics. Deactivation of the catalyst occurred slowly, but the catalyst could easily be regenerated. Moreover, the nickel-calcium phosphate/hydroxyapatite catalyst showed higher activity than the nickel-strontium phosphate catalyst.

Partial oxidation of methane to syngas over glow discharge plasma treated Ni–Fe/Al 2O 3 catalyst

Ni–Fe/Al 2O 3 catalyst for partial oxidation of methane was prepared first by glow discharge plasma treatment following by calcinations thermally. Such prepared catalyst exhibits better performance over the catalyst prepared without such plasma treatment. The conversion of methane and the selectivity of CO and H 2 over the plasma treated catalyst are 97.44, 100 and 100% at 875 °C, while, at the same temperature, they are 90.09, 97.28 and 97.09%, respectively, over the catalyst prepared without plasma treatment. At the same methane conversion, the reaction temperature with the plasma treated Ni–Fe/Al 2O 3 is at least 80 °C lower. TEM characterization shows that the catalytically active species after plasma treatment are still highly dispersed on the support. In addition, the plasma treated Ni–Fe/Al 2O 3 catalyst also possesses better anti-carbon deposit ability.

In situ investigations of structure-activity relationships in heteropolyoxomolybdates as partial oxidation catalysts.

The structural evolution of Keggin-type heteropolyoxomolybdates (HPOM) during thermal treatment in propene and in propene and oxygen in the temperature range from 300 to 773 K was investigated by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) combined with mass spectrometry. During treatment in propene or hydrogen and at reaction temperatures above 673 K, the initially triclinic H(3)[PMo(12)O(40)].13 H(2)O is transformed quantitatively into a cubic HPOM (Pn$\bar 3$m, a=11.853 A) exhibiting a long-range structure similar to that of the corresponding cesium salts. The treatment described constitutes the first readily available preparation route for a cubic HPOM without alkali metal ions in the structure. For both H(3)[PMo(12)O(40)] and Cs(2)H[PMo(12)O(40)] migration of molybdenum from the Keggin ion onto interstitial sites is proposed to occur in propene or hydrogen at temperatures above about 573 K to give thermally stable, partially reduced lacunary Keggin ions. During activation in propene and oxygen the onset of catalytic activity of H(3)[PMo(12)O(40)] and Cs(2)H[PMo(12)O(40)] at about 573 K correlates with partial reduction of Mo and characteristic changes in the local structure of the Keggin ion. The structural changes observed indicate that, similar to the treatment of the HPOM in propene, migration of molybdenum from the Keggin ions onto interstitial sites and formation of lacunary Keggin ions take place. Moreover, the formation of these partially reduced lacunary Keggin ions appears to be a prerequisite for the material to become an active heterogeneous catalyst. Evidently, the undistorted Keggin ion in the as-prepared HPOM has to be regarded as a precursor of the active catalyst.

Zeolite-encapsulated Co(II), Ni(II) and Cu(II) complexes as catalysts for partial oxidation of benzyl alcohol and ethylbenzene

Co(II), Ni(II) and Cu(II) complexes of dimethylglyoxime and N, N′-ethylenebis(7-methylsalicylideneamine) have been synthesized in situ in Y zeolite by the reaction of ion-exchanged metal ions with the flexible ligand molecules that had diffused into the cavities. The hybrid materials obtained have been characterized by elemental analysis, SEM, XRD, surface area, pore volume, magnetic moment, FTIR, UV-Vis and EPR techniques. Analysis of data indicates the formation of complexes in the pores without affecting the zeolite framework structure, the absence of any extraneous species and the geometry of encapsulated complexes. The catalytic activities for hydrogen peroxide decomposition and oxidation of benzyl alcohol and ethylbenzene of zeolite complexes are reported. Zeolite Cu(II) complexes were found to be more active than the corresponding Co(II) and Ni(II) complexes for oxidation reactions. The catalytic properties of the complexes are influenced by their geometry and by the steric environment of the active sites. Zeolite complexes are stable enough to be reused and are suitable to be utilized as partial oxidation catalysts.

Plasma treatment of Ni catalyst via a corona discharge

In this paper, plasma treatment of Ni catalyst for partial oxidation of methane to syngas was investigated by using plate–plate discharge model via a corona discharge by flowing argon under atmospheric pressure and room temperature. The catalytic activity and stability for partial oxidation of methane (POM) process were measured for the catalyst before and after plasma treatment. It was found that after plasma treatment, methane conversion increased 5–10% and the syngas selectivity kept the same as before treatment. Also, these catalysts were characterized by thermogravimetric analysis and XPS measurement. It was found that the argon plasma treatment can improve the catalytic stability by preventing the carbon deposition on the Ni catalyst.

Nickel–calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: characterization and activation

Nickel–calcium phosphate/hydroxyapatite catalysts exhibited high activity and selectivity in the partial oxidation of methane (POM). Characterizations have been performed in order to understand the characteristics of these catalysts. TEM showed that very fine, needle-shaped nickel particles of a few nanometers were present in the sample after the reaction. TPR showed the presence of three nickel species in the fresh (before-reaction) sample, confirming that the nickel in the calcium hydroxyapatite and phosphate structures comes out as fine particles and is reduced during the reaction. The crystalline phases observed were calcium phosphate, calcium hydroxyapatite, NiO, and metallic Ni. As the Ca/PO 4 ratio increased, the calcium hydroxyapatite phase tended to increase over the calcium phosphate phase. As the Ni/PO 4 ratio increased, the amount of NiO increased in the fresh samples and the amount of metallic Ni also increased in the used (after-reaction) catalysts with the decrease of the NiO. The catalytically active component is certainly the metallic nickel that is produced under reducing environment during the reaction, but its surface is totally reoxidized at 673 K and loses the activity completely. The catalyst could be activated by the reactants only, without the H 2 pretreatment. The first activation could be achieved at 923 K or below, and the subsequent activation at 723 K.

Thermal Chemistry of 2-Propanol and 2-Propyl Iodide on Clean and Oxygen-Pretreated Ni(100) Single-Crystal Surfaces

The thermal chemistry of 2-propanol and 2-propyl iodide on clean and oxygen-pretreated Ni(100) single-crystal surfaces was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and reflection−absorption infrared spectroscopy (RAIRS). 2-Propyl iodide was used as a precursor for the preparation of 2-propyl surface species, in analogy to chemistry reported for other halo hydrocarbons. Those 2-propyl groups were shown to convert to acetone on both oxygen- and hydroxide-covered Ni(100) surfaces by following a two step mechanism, starting with a facile oxygen insertion into the metal−alkyl bond. The resulting 2-propoxide surface moiety, an intermediate that can also be prepared via dehydrogenation of the hydroxyl group in adsorbed 2-propanol, is stable up to about 300 K, at which point it undergoes β-hydride elimination to acetone. Coadsorbed surface hydroxide facilitates the first oxygen insertion reaction, and surface oxygen enhances the latter conversion of 2-propoxide to acetone. Some alcohol dehydration to propene is also observed, presumably by reaction of two hydrogen-bonded alcohol molecules. These results point to the feasibility of designing nickel-based catalysts for the selective partial oxidation of hydrocarbons.

Characterization of a nickel-strontium phosphate catalyst for partial oxidation of methane

A nickel-strontium phosphate catalyst was recently reported to exhibit high activity and selectivity in partial oxidation of methane (POM). Further characterizations have been performed in the present work in order to better understand the characteristics of this catalyst. TEM showed that very fine nickel particles of a few nanometers of the size were present in the needle shape in the sample after the reaction. TPR showed the presence of three nickel species, confirming that the nickel in the strontium hydroxyapatite and phosphate structures comes out as fine particles and is reduced during the reaction. As the calcination time during the preparation increased, the total re-oxidation temperature became higher. This is considered due to larger crystallite size owing to the longer calcination, which results in a smaller amount of nickel coming out of the matrix to the surface.

Vanadium antimonate as a partial oxidation catalyst

The catalytic selective oxidation on vanadium antimonate oxides partially doped with titanium was studied. Solids with nominal composition VSbO 4, V 0.8SbTi 0.2O 4 and VSb 0.8Ti 0.2O 4 were prepared by solid-state reaction. The solids were characterised by sorptometry, X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). Catalytic tests were carried out in a fixed-bed reactor using toluene and air as reactants. It was found that partial substitution of V or Sb by Ti produces changes in the unit-cell composition and the redox properties of the solids. A possible role for V and Sb in the reaction mechanism of partial oxidation was established. Vanadium sites (V 4+) are involved in the incorporation of gaseous oxygen as lattice oxygen and hold the Sb sites in an oxidation state (Sb 5+) high enough to insert the lattice oxygen in the hydrocarbon molecule. A relationship between activity and V 4+/Sb 5+ ratio of the unit-cell was found. The selectivity to benzaldehyde is correlated with the redox properties of the solids while the selectivity to total oxidation products could be interpreted by the site isolation theory.

Supported foam-silver catalysts for alcohol partial oxidation

Structural, mechanical, gas-dynamic and catalytic properties of silver catalysts supported on foam ceramics have been studied. The foam catalysts have high gas permeability, mechanical strength, and low density. Catalytic activity and selectivity of the foam catalysts in the process of oxidation of methanol to formaldehyde exceed the characteristics of the commercial crystalline and granulated catalysts. Different electronic states (ions, charged cluster, crystals) of silver on the catalyst surface have been studied by the method of UV–visible spectroscopy.

In situ Characterization of Catalysts Active in Partial Oxidations: TS‐1 and Fe‐MFI Case Studies.

A concise review of the firmly established knowledge and of the unresolved problems concerning the structure and the reactivity of Ti and Fe sites in TS-1 and Fe-MFI partial oxidation catalysts is given. Some new experimental and theoretical results are also described.

Supported Rh catalysts for the indirect partial oxidation of isooctane

The production of hydrogen from isooctane over three rhodium-based catalysts has been examined. The reaction entailed total oxidation of a proportion of the fuel followed by reforming of isooctane to produce hydrogen. Rhodium (1% wt) was impregnated on three different supports: alumina, ceria-alumina, and ceria-zirconia. No differences in catalytic activity were observed, but reaction yield changed with the support. Ceria-zirconia was found to be the preferred support since methanation did not occur over the catalyst.

MCM-41-supported iron phosphate catalyst for partial oxidation of methane to oxygenates with oxygen and nitrous oxide

Iron phosphate supported on MCM-41 has been studied for partial oxidation of methane with both oxygen and nitrous oxide. Characterizations with XRD, Raman spectroscopy, XPS, and H 2-TPR suggest that the supported iron phosphate species with loading amounts lower than 40 wt% are located and dispersed in the mesopores of MCM-41. Such iron phosphate species can be reduced more readily than the unsupported iron phosphate at lower temperatures. Methane is selectively converted to methanol, formaldehyde, and dimethyl ether over the supported and the unsupported iron phosphate with nitrous oxide at milder temperatures (300–500 °C), while formaldehyde is mainly produced along with carbon oxides with oxygen at relatively higher temperatures (400–600 °C). The supporting of iron phosphate onto MCM-41 with loading amounts of ca. 20–40 wt% increases both methane conversion and overall selectivity to useful oxygenates with either oxygen or nitrous oxide. Kinetic studies indicate that the activation of oxygen occurs rapidly, while the activation of nitrous oxide proceeds at a comparable rate with the conversion of methane by the active oxygen species over both the supported and the unsupported catalysts. The supported catalyst, however, enhances the activation of nitrous oxide and thus remarkably inhibits the carbon deposition occurring over the unsupported iron phosphate.

Nanoengineered catalysts for high-temperature methane partial oxidation

Novel nanoengineered catalysts for the high-temperature direct oxidation of methane to synthesis gas have been synthesized via a microemulsion-mediated sol–gel route. Through the addition of Pt-salts, catalysts with large surface areas, a well-controlled morphology and very homogeneous distributions of highly active Pt-nanoparticles in a hexaaluminate (BHA) matrix are obtained. The catalysts show syngas yields significantly exceeding the yields over conventional catalysts. Furthermore, excellent high-temperature stability of the materials is observed with no measurable deactivation or metal particle sintering over prolonged reactor operation at high-temperature reaction conditions. Overall, the materials seem highly promising candidates for syngas formation and hydrogen production from hydrocarbons.

The Relationship Between Structural and Catalytic Activity of α and γ-Bismuth-Molybdate Catalysts for Partial Oxidation of Propylene to Acrolein

Bismuth-molybdate catalysts are known to be effective for catalytic partial oxidation of propylene to acrolein. Their properties and the kinetics and reaction mechanisms for acrolein production have been extensively studied, especially in their basic forms, such as α, β, and γ-bismuth-molybdate. Although the reaction mechanisms have been reported widely in the literature, a general agreement has not been reached, especially from a catalyst-structure point of view. The present contribution reports an effort to understand the structural changes of α and γ-bismuth-molybdate catalysts at varying temperatures as examined using high temperature XRD and to relate the catalyst performance (activity and selectivity) for propylene partial oxidation to acrolein. The XRD analysis was performed at temperature between 250 and 450°C in ambient atmosphere and the Rietveld refinement method was used to extract unit cell parameters. The results showed a distinct similarity between the shapes of the thermal expansion of the catalysts and their activity and selectivity curves, indicating a significant role that the catalyst interatomic structure plays in the overall reaction mechanism.

Characterization of alumina-supported Ni and Ni-Pd catalysts for partial oxidation and steam reforming of hydrocarbons

Hydrogen production by partial oxidation and steam reforming (POSR) of n-octane has been investigated over alumina-supported Ni and Ni-Pd catalysts. The process was found to occur by a combination of exothermic partial oxidation and endothermic steam reforming. Ni/γ-Al2O3 catalysts had good activity at high temperatures; the activity increased with increasing Ni loadings up to 5.0wt.%. However, the Ni/γ-Al2O3 catalysts rapidly lost activity during the POSR process. XRD results showed that an increase in the particle size was the main reason for the poor stability of the Ni/γ-Al2O3 catalysts. A further important reason was the deposition of carbon produced during the POSR process on the Ni/γ-Al2O3 catalyst surface. The activity and hydrogen selectivity of Ni-Pd/γ-Al2O3 catalyst were significantly higher than the values of the nickel catalyst under the same experimental conditions. Catalytic stability was also enhanced by the presence of palladium. Such improvement indicates that palladium plays an important role in the catalytic action. SEM and TG results did not show any great difference between the fresh and spent Ni-Pd/γ-Al2O3 catalysts. Overall, the bimetallic Ni-Pd/γ-Al2O3 catalyst could be an effective catalyst for the production of hydrogen from hydrocarbons.