Selective Partial Oxidation Research Articles

Maleic Anhydride from Mixtures of n-Butenes and n-Butane: Simulation of a Production-Scale Nonisothermal Fixed-Bed Reactor

The partial selective oxidation of mixtures of n-butenes and n-butane in an industrial-scale fixed-bed reactor has been simulated to find suitable operation conditions and to give a rough estimate of the performance of such a reactor. A two-dimensional homogeneous reactor model has been used in combination with a kinetic model based on experimental results. Since the pressure drop along the reactor tubes was assumed to be 1−1.5 bar, the gas phase in the reactor tubes had to be treated as a compressible fluid. The operation conditions of the reactor were found by repeated simulation and continuous improvement. The simulations resulted in a conversion level of 90% and a yield of maleic anhydride of 48% for a synthetic raffinate II mixture containing only n-butenes and n-butane. A typical real raffinate II mixture which also contains isobutane would result in a yield of maleic anhydride of ≈42%. Production capacities of 17 100 and 22 5000 t/y where achieved with allowed pressure drops of 1.0 and 1.5 bar, respectively.

Partial oxidation of higher olefins on Ag(1 1 1): Conversion of styrene to styrene oxide, benzene, and benzoic acid

Temperature programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and reflection absorption infrared spectroscopy (RAIRS) were used to study the partial oxidation of styrene on Ag(1 1 1). Styrene oxide, benzene, and benzoic acid were identified as partially oxidized reaction products. XPS and RAIRS provide evidence for the formation of a styrene-derived surface oxametallacycle that either forms styrene oxide or is further oxidized in a branching reaction to form benzoate, which is the intermediate responsible for the formation of both benzene and benzoic acid. The strong dependence of the product distribution on oxygen coverage suggests that O monomers adsorbed on Ag(1 1 1) provide a higher selectivity for partial oxidation than oxygen from the Ag(1 1 1)–p(4 × 4)-O reconstruction.

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in γ-Bi4V2O11 by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO2 in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H2), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic versus oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations.

Promotion effects of ceria in partial oxidation of methane over Ni-calcium hydroxyapatite

Effects of ceria added as a promoter to a nickel-calcium hydroxyapatite catalyst, which has recently been reported to exhibit high activity and selectivity in partial oxidation of methane, were investigated. The ceria-promoted catalyst exhibited higher activity and stability than the unpromoted one. This is considered due to the oxygen storage capacity of ceria, which promotes easier removal of the deposited carbon. The optimum content of ceria was determined to lie in the range of the Ce/Ni ratio from 0.1/2.5 to 0.2/2.5.

Selective Combinatorial Studies on Ce and Zr Promoted Cu/Zn/Pd Catalysts for Hydrogen Productio via Methanol Oxidative Reforming

The activity and selectivity for partial oxidation of methanol to H2 and CO2 on Zr, Ce, promoted Cu/Zn/Pd catalysts, have been studied using a high-throughput method of screening and analysis. In this work, infrared thermography was first used as a descriptor of overall catalytic activity. Then, activity and selectivity of samples with high infrared signal were measured in a flow reactor and characterized by BET, XRD, and XPS. Catalysts promoted with 10% Zr and showed H2 selectivity >95% with methanol conversion approaching 100% at ~200 °C.

RECENT ADVANCES IN CATALYTIC OXIDATION IN SUPERCRITICAL WATER

ABSTRACT This article summarizes recent research in homogeneous and heterogeneous catalytic oxidation in supercritical water. We consider both selective partial oxidation for chemical synthesis and complete oxidation for waste destruction. Recent advances in selective catalytic oxidation in supercritical water center around the conversion of p-xylene to terephthalic acid catalyzed homogeneously by MnBr2. Terephthalic acid yields of >90 mol% can be achieved from reactions at 400°C. Using water as the reaction medium provides genuine opportunities for both a more economical and more environmentally benign terephthalic acid production process. Recent advances in complete oxidation via catalysis in supercritical water include the demonstration of heteropolyacids as effective homogeneous oxidation catalysts, and alkali carbonates and carbons as effective heterogeneous catalysts. Additionally, progress has been made in screening transition metal oxide catalysts and determining the reaction-induced chemical and physical changes that take place in the hydrothermal environment. Bulk MnO2 is a good catalyst for complete oxidation because it combines high activity, hydrothermal stability, activity maintenance, and resistance to metal leaching under reaction conditions.

Partial oxidation of n-butane in a solid electrolyte membrane reactor: Periodic and steady-state operations

An electrochemical membrane reactor using yttria-stabilized zirconia as a solid electrolyte membrane was applied to the selective partial oxidation of n-butane to maleic anhydride over a vanadium phosphorus oxide (VPO) catalyst. In this reactor, the oxygen required for anodic butane oxidation was generated by electrical oxygen pumping, i.e. by imposing an external current to the membrane reactor from anode to cathode. Periodic redox experiments, in which electrical oxygen pumping and butane oxidation were carried out sequentially, confirmed that the reduced activity of the VPO catalyst in the butane oxidation could be regenerated by electrical oxygen pumping. A series of steady-state experiments were carried out to assess the influence of operating conditions such as imposing current, reaction temperature, butane concentration and flow rate on the catalytic performance, where the electrochemical oxygen pumping and butane oxidation were performed simultaneously. The studied membrane reactor gave maleic anhydride yield of 10% with maleic anhydride selectivity up to 53% at reaction temperature of 753 K.

Oxidation of C 2–C 4 hydrocarbons over MoO 3 and V 2O 5 supported on a YSZ-aided membrane reactor

An electrochemical reactor, using yttria-stabilized zirconia (YSZ) as a solid electrolyte and gold and silver as the anode and cathode, respectively, has been employed for the selective partial oxidation of C 2–C 4 hydrocarbons at 775 K. MoO 3 or V 2O 5 was deposited as the catalyst film on the Au anode of the electrochemical cell. The reaction was carried out over the catalyst film under oxygen pumping through the cell; the results are compared with those obtained by the reaction with gaseous oxygen. Under oxygen pumping, no alkane was oxidized over MoO 3, while isobutane and propane were partially oxidized to methacrolein and propene, respectively, and ethane was oxidized a little over V 2O 5. It is supposed that CH cleavage is the rate determining step of the alkane oxidation over V 2O 5. Alkenes were partially oxidized over both MoO 3 and V 2O 5, and the selectivities to the oxygenated products, i.e., acetaldehyde, acrolein and methacrolein from ethene, propene, and isobutene, respectively, were higher on the former than on the latter. Over both MoO 3 and V 2O 5, the selectivity to aldehyde was higher in the reaction under oxygen pumping than in that by gaseous oxygen. Over MoO 3, the other oxygenated compounds than aldehyde were observed in the reaction by gaseous oxygen, while this is not the case under oxygen pumping. Under oxygen pumping, the highest selectivity was observed with methacrolein, followed by acrolein and acetaldehyde, among which acetaldehyde was further oxidized to carbon oxides. Over V 2O 5, alkenes were far more rapidly oxidized by gaseous oxygen than under oxygen pumping, and the selectivity to the oxygenated products were still higher under oxygen pumping than by gaseous oxygen. The highest selectivity was observed with methacrolein, and those of both acrolein and acetaldehyde were low. No further oxidation of acetaldehyde was observed over V 2O 5.

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.

Visible-light-sensitive photocatalytic reaction on chromium-containing mesoporous silica: selective partial oxidation of propane with molecular oxygen

The photocatalytic reactivities of chromium-containing mesoporous silica molecular sieves (Cr-HMS) under visible light irradiation have been investigated. Cr-HMS involves tetrahedral chromium oxide (Cr-oxide) moieties which are highly dispersed and incorporated in the framework of molecular sieve with two terminal Cr=O groups. In the presence of propane with molecular oxygen, a partial oxidation proceeded under visible light irradiation to produce acetone and acrolein, with high selectivity, while a complete oxidation proceeded under UV light irradiation mainly to produce CO 2 . The charge-transfer excited state of the tetrahedral Cr-oxide moieties plays a significant role in the photocatalytic reactions.

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.

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.

The synthesis and structure of a single-phase, nanocrystalline MoVW mixed-oxide catalyst of the Mo 5O 14 type

The different preparation steps are characterized for the single-phase, crystalline, ternary oxide (MoVW) 5O 14, which is important for catalytic, mild selective oxidation reactions. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed at each preparation step, starting from the precursor to the final product, were studied using scanning and transmission electron microscopy, X-ray powder diffraction, thermal analysis, and Raman spectroscopy. Raman spectroscopy was also applied to shed some light into the aqueous chemistry of the mixed precursor solutions. Raman data indicate that a molecular structure which seems to be closely related to that of the final crystalline Mo 5O 14-type oxide is already formed in solution. X-ray diffraction revealed that the thermal treatment steps strongly affect the degree of crystallinity of the ternary Mo 5O 14 oxide. Transmission electron microscopy with energy-dispersive microanalysis confirmed the presence of V and W in the molybdenum oxide particles and gave evidence for the (010) plane as the most developed face of the crystals of this phase. Details of the structural transformation of this system at the different preparation and calcination steps are discussed in relation to their performance in the selective partial oxidation of acrolein to acrylic acid.

Study of the Preparation and Catalytic Performance of Molybdenum Carbide Catalysts Prepared with C2H2/H2 Carburizing Mixture

A series of molybdenum carbide catalysts were prepared using 10% C2H2/H2 to carburize molybdenum trioxide at different temperatures. Their catalytic performance was evaluated using methane partial oxidation to synthesis gas as the model reaction. The structure of the catalysts treated under various conditions was characterized using X-ray diffraction, laser Raman spectroscopy, and scanning electron microscopy. The results show that MoO3 is first converted into MoO2 and MoC1−x. With the rise of the carburization temperature, MoO2 is converted into MoC1−x and the catalyst crystallites tend to be very small. However, the catalyst surface remains coated by MoO3 or MoO2. When the carbides are heated in N2 to 850°C, they are converted into MoC1−x and β-Mo2C. Interestingly, all the catalyst samples show high CH4 conversion and CO selectivity in partial oxidation of methane, although the pretreatment has some effect on the preliminary activity and selectivity. The catalyst is partly oxidized and there is a small amount of carbon deposition after the activity test. Acetylene is therefore a good carburizing agent for preparing molybdenum carbide catalyst at low temperature for the partial oxidation of methane.

Study of the Preparation and Catalytic Performance of Molybdenum Carbide Catalysts Prepared with C 2H 2/H 2 Carburizing Mixture

A series of molybdenum carbide catalysts were prepared using 10% C 2H 2/H 2 to carburize molybdenum trioxide at different temperatures. Their catalytic performance was evaluated using methane partial oxidation to synthesis gas as the model reaction. The structure of the catalysts treated under various conditions was characterized using X-ray diffraction, laser Raman spectroscopy, and scanning electron microscopy. The results show that MoO 3 is first converted into MoO 2 and MoC 1− x . With the rise of the carburization temperature, MoO 2 is converted into MoC 1− x and the catalyst crystallites tend to be very small. However, the catalyst surface remains coated by MoO 3 or MoO 2. When the carbides are heated in N 2 to 850°C, they are converted into MoC 1− x and β-Mo 2C. Interestingly, all the catalyst samples show high CH 4 conversion and CO selectivity in partial oxidation of methane, although the pretreatment has some effect on the preliminary activity and selectivity. The catalyst is partly oxidized and there is a small amount of carbon deposition after the activity test. Acetylene is therefore a good carburizing agent for preparing molybdenum carbide catalyst at low temperature for the partial oxidation of methane.

Evolution of Extraframework Iron Species in Fe Silicalite: 2. Effect of Steaming

In the first part of this investigation, we studied in detail the structure, the oxidation state, and the mutual interconversion of extraframework iron species formed in Fe silicalite samples upon migration of iron from framework to extraframework positions induced by thermal activation and redox treatments. In this second part, we used temperature-dependent EPR, UV–vis, and IR spectroscopies to investigate the nature of the extraframework iron species present in Fe silicalite after steaming. Relatively large (≅30 nm) particles of iron oxide were detected. These large clusters are located outside the zeolite pores and are believed not to participate in partial oxidation reactions. It is shown that the Fe species which contribute to the formation of Fe2O3 extralattice particles are preferentially clustered grafted Fe2+ species. In contrast, isolated Fe2+ species are less affected by the steaming treatment. This suggests that the oxygen species active in the selective partial oxidation of benzene are adsorbed on isolated extraframework grafted Fe2+ centers. This result is of primary importance for the understanding of the structure of the oxygen species active in partial selective oxidation using N2O as oxidant.

In situ Raman spectroscopy for the characterization of MoVW mixed oxide catalysts

AbstractIn situ Raman spectroscopy was used to characterize the synthesis and activation steps of mixed metal oxide catalysts starting from the mixed solutions, through subsequent drying and activation procedures, to the catalytic propene partial oxidation reaction. Comparison with Raman spectra recorded for defined, well‐crystallized reference oxides allowed the assignment of the spectra obtained during the catalyst preparation to certain oxides, such as Mo5O14. Especially the latter oxide phase is relevant for selective partial oxidation catalysis. The selectivity for partial oxidation products could be improved between two‐ and three‐fold when the amount of Mo5O14‐type oxide increased. This phase forms from mixtures of molybdenum and tungsten and from mixtures of molybdenum, tungsten and vanadium, but not from binary mixtures of molybdenum and vanadium with high vanadium concentrations. Tungsten and vanadium play an important role as structural promoters in the formation and stabilization of this oxide and for high catalytic activity. A resonance Raman effect was proved for reduced molybdenum oxides due to resonant coupling of the exciting laser to the IVCT transitions between fivefold‐coordinated Mo5+ and sixfold‐coordinated Mo6+. This resonance enhancement renders possible the in situ characterization of operating, reduced molybdenum oxide catalysts. In contrast to the Mo5O14‐type phase, MoO3−x exhibits a high selectivity to total oxidation, and it even becomes re‐oxidized during propene partial oxidation. These different catalytic properties of MoO3−x and the Mo5O14‐type oxide led to the development of a structure–activity relationship which explains the behavior of industrial catalysts. A model is proposed on the basis of a bond order–Raman wavenumber relationship, which explains the different selectivities of these two oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

Selective partial oxidation in supercritical water: the continuous generation of terephthalic acid from para-xylene in high yield

The continuous selective oxidation of p-xylene (p-X) by O2 (generated by thermal decomposition of aqueous H2O2) catalyzed by MnBr2 in supercritical H2O at ca. 400 oC is reported for the first time. The selectivity for terephthalic acid (TA) is >90%. Compared to existing industrial processes, the reaction has the potential for a significant increase in energy efficiency and a substantial reduction in waste. This reaction is significant because the presence of H2O lowers the catalytic activity of MnBr2 in the conventional route to TA via oxidation of p-X in CH3COOH.

TiO 2/SiO 2 photocatalysts at low levels of loading: preparation, structure and photocatalysis

The effect of precursor on the dispersion and photocatalytic performance of titanium oxide supported on silica has been investigated. The catalysts were prepared by a simple impregnation method with three kinds of titanium complexes of different ligands (dipyvaroylmethanato (DPM), acetylacetonato (acac), isopropoxide (O- iPr)) abbreviated to D-TS, A-TS, and I-TS. The UV/Vis diffuse reflectance, Raman, XAFS spectroscopic study showed that surface titanate of D-TS is isolated TiO 4 species, and that of I-TS is a mixture of rutile and anatase crystallites. The surface state of A-TS is the intermediate between D-TS and I-TS. TiO 2/SiO 2 samples were more active than bulk TiO 2 and partial oxidation selectivity of TiO 2/SiO 2 samples are higher than that of TiO 2 in photo-oxidation of propane. Products distribution over TiO 2/SiO 2 was drastically varied with the surface titanates. The activity for photo-assisted selective catalytic reduction (SCR) of NO with ammonia was found for D-TS.