Oxidation Of Isobutane Research Articles

Evaluation of the role played by praseodymium molybdates in Pr 6O 11–MoO 3 catalysts for the selective oxidation of isobutene to methacrolein

The catalytic role played by praseodymium molybdate phases (Pr 6MoO 12, Pr 2MoO 6, Pr 2Mo 3O 12) generated in Pr 6O 11–MoO 3 catalysts during their use in the selective oxidation of isobutene to methacrolein is evaluated in order to determine whether the observed synergetic effects have to be assigned to cooperation between binary praseodymium and molybdenum oxides, or to the occurrence of praseodymium molybdate phases, that would be catalytically active themselves or would act synergetically with MoO 3 or Pr 6O 11. X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (ToF SIMS) are used to determine the composition of the used Pr 6O 11–MoO 3 catalysts. Pr 6MoO 12 appears to be the major ternary phase present in the bulk and Pr 2Mo 3O 12 is predominant at the surface. To understand the catalytic behaviour of the Pr 6O 11–MoO 3 mixtures, the catalytic performances of the pure praseodymium molybdate phases and those of their mixtures with MoO 3, Sb 2O 4 and Pr 6O 11 were also examined. It is shown that the cooperative effects previously observed in the MoO 3–Pr 6O 11 mixtures most probably cannot be assigned exclusively to the intrinsic catalytic properties of praseodymium molybdates. A cooperation is also observed between other couples of oxide phases, i.e. Pr 6MoO 12 displays donor properties with respect to MoO 3. It appears also that the evolution of the catalytic properties of Pr 6MoO 12–MoO 3 mixtures with respect to the Pr/Mo content is completely different from the one observed in MoO 3–Pr 6O 11 mixtures. In addition, it does not seem that the observed synergetic effects could be assigned to the simultaneous presence of ternary Pr x Mo y O z phases and Pr 6O 11. The generation of praseodymium molybdate phases during the catalytic tests seems therefore to play no significant role in the catalytic behaviour of MoO 3–Pr 6O 11 mixtures.

ChemInform Abstract: An Efficient Aerobic Oxidation of Isobutane to t‐Butyl Alcohol by N‐Hydroxyphthalimide Combined with Co(II) Species.

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Selective Oxidation of Ethane, Propane, and Isobutane Catalyzed by Copper-Containing Cs2.5H1.5PVMo11O40under Oxygen-Poor Conditions

Effects of the addition of various transition metal additives to Cs2.5H1.5PVMo11O40on the catalytic performance for oxidation of ethane were compared under oxygen-poor conditions and copper was found to most enhance the catalytic performance. Similar enhancements by the addition of copper were observed for the oxidation of propane and isobutane under oxygen-poor conditions. It is suggested that the enhancement by the copper addition is due to the promotion of the reoxidation of the catalyst.

Isobutene Oxidation and Ignition: Experimental and Detailed Kinetic Modeling Study

The oxidation of isobutene has been investigated for the first time in a jet-stirred reactor at high temperature (∼800-l230K) and at l,5and 10 atm. Molecular species concentration profiles of O2, H2, CO, CO2, CH2O, CH4, C2H2, C2H4, C2H6, C3H4 (allene and propyne), C3H6, acetone, acrolein, methacrolein, I-C4H8, i-C4H8, 1,3-C4H6, l-butyne, isoprene, 2-methyl-l-butene, 2-methyl-2-butene, and benzene were obtained by probe sampling and GC analysis. The oxidation of isobutene in these conditions and the ignition of isobutene-oxygen-argon mixtures in a shock-tube were modeled using a detailed kinetic reaction mechanism (l 10 species and 743 reactions, most of them reversible). The proposed mechanism, also validated for the oxidation of CH4, C2H2, C2H4, C2H4, C3H6, acetaldehyde, ethylene oxide, and natural gas blends in the same conditions, is able to predict experimental results obtained in our high-pressure jet stirred reactor and the ignition delays measured behind a reflected shock wave. Two routes to benzene formation have been delineated: (a) the addition of propargyl radicals to allene, and (b) the recombination of propargyl radicals. Sensitivity analyses and reaction path analyses are used to interpret the present results.

Cooperation effects towards partial oxidation of isobutene in multiphasic catalysts based on bismuth pyrostannate

The catalytic behaviour of α-Bi 2Sn 2O 7 was investigated within the framework of the remote control theory. Mixtures of bismuth pyrostannate with typical oxygen donors (Bi 2O 3, Sb 2O 4) or acceptors (MoO 3, WO 3) were therefore prepared and tested in the isobutene–methacrolein conversion at 648 and 673 K. The catalysts were characterized before and after their use by X-ray diffractometry, X-ray photoelectron spectroscopy, scanning electron microscopy and BET specific surface area measurements. The experiments indicate that cooperation for partial oxidation occurs when Bi 2Sn 2O 7 is mixed with acceptor phases. Several mechanisms allowing to explain the synergetic effects observed in the Bi 2Sn 2O 7–WO 3 and the Bi 2Sn 2O 7–MoO 3 mixtures are discussed. In the Bi 2Sn 2O 7–WO 3 catalysts, arguments are presented showing that surface enrichment by one oxide or the in situ formation of a bismuth tungstate phase cannot be reasonably considered as responsible for the cooperation effects. Bismuth stannate is consequently concluded to behave as an oxygen donor with respect to WO 3. Bi 2Sn 2O 7 could “a priori” display the same behaviour in the Bi 2Sn 2O 7–MoO 3 catalysts and cooperate with MoO 3 via a remote control mechanism. But in the latter case, as α-Bi 2Mo 3O 12 is generated in situ, the intrinsic catalytic activity of this bismuth molybdate must probably be taken into account as an additional explanation for the synergy observed in these mixtures. On the other hand, the hypothesis of coverage of one oxide by the other one seems to be ruled out in these Bi 2Sn 2O 7–MoO 3 catalysts.

An Efficient Aerobic Oxidation of Isobutane to t-Butyl Alcohol by N-Hydroxyphthalimide Combined with Co(II) Species

Abstract Highly selective aerobic oxidation of isobutane to t-butyl alcohol was successfully achieved by the use of a radical catalyst, N-hydroxyphthalimide (NHPI) in the presence of Co(II) salt under relatively mild conditions. The oxidation of isobutane by NHPI combined with Co(acac)2 under a pressure of air (10 atm) in benzonitrile at 100 °C gave t-butyl alcohol in high yield (84%) along with acetone (13%). The reaction is thought to proceed via hydrogen abstraction from isobutane by the phthalimidooxyl radical (PINO), which seems to be a key active species. The formation of acetone can be explained by a partial β-scission of the t-butoxy radical, generated from the redox decomposition of t-butyl hydroperoxide by cobalt ion. Alkyl-substituted butanes and pentanes were difficult to be oxidized selectively under these conditions because of easy degradation to smaller fragments of the resulting alkoxyl radical intermediates.

Alkane oxidation by dinuclear iron complexes in hexagonal mesoporous solids

Biomimetic oxygen activation on binuclear iron active sites occluded within the voids of mesoporous oxides such as HMS (hexagonal mesoporous silica) is an interesting new field of research. As guests, binuclear iron(III) complexes were chosen. Heptapodate N,N,N′,N′-tetrakis(2-R methyl)-2-hydroxy-1,3-diaminopropane ligands in which R is either a pyridyl or a benzimidazole group allow metal coordination as in methane mono-oxygenase active sites. The physicochemical characterization with SEM, TGA and sorption analysis shows the interactions of the complexes with the support. IR, Raman and diffuse reflectance-UV-Vis-NIR spectroscopy is used to monitor interaction of oxygen ( 16O 2 and 18O 2) or peroxides with the supported active sites. The cyclohexane, cyclohexylhydroperoxide and isobutane oxidation is described and the mono- and binuclear mechanisms that are occurring in nucleophilic and electrophilic activation pathways are discussed.

Isobutane oxidation in the liquid and supercritical phases: comparison of features

The liquid phase oxidation (LPO) of isobutane is normally carried out in industry at conditions close to critical, and much data is available under these conditions. This reaction therefore affords an opportunity for a comparative study between supercritical oxidation (SCO) and LPOs, under very similar conditions of temperature and pressure. While studies in the literature report higher rates and yields for the SCO, it is not clear whether the reported effects have to do with the supercriticality of the reaction mixture or merely the different conditions employed to cause the mixture to become supercritical. This paper aims to clarify this point through a careful analysis of the published data on supercritical and liquid phase oxidations. The analysis demonstrates that the features of SCO extrapolate directly from those of LPO. A kinetic model previously proposed for the oxidation of cyclohexane [1] has been extended to isobutane oxidation and shown to fit the data well, not only in the liquid phase region, but in the supercritical region as well. The conclusion that emerges is that the oxidation under supercritical condition offers higher rates and productivities than conventional LPO, because it makes it possible to obtain liquid phase-like reaction behaviour at temperatures not accessible to LPO. These findings are of significance in view of claims in the patent literature on the advantages of SCO, and make it possible to predict the rate and yield obtainable under supercritical conditions.

Kinetic study and modeling of the hetero-homogeneous pyrolysis and oxidation of isobutane around 800 K. Part I. Pyrolysis in an unpacked pyrex reactor

Kinetic study and modeling of the hetero-homogeneous pyrolysis and oxidation of isobutane around 800 K. Part I. Pyrolysis in an unpacked pyrex reactor

Kinetic study and modeling of the hetero-homogeneous pyrolysis and oxidation of isobutane around 800 K. Part II. Pyrolysis in pyrex reactors packed with platinum foils or PbO-treated pyrex rods

Isobutane pyrolysis has been studied at 20–200 torr initial pressures and 773–793 K, in a packed reactor treated with PbO and in a reactor packed with platinum foils. These packings strongly inhibit product formation and this effect is explained by the occurrence of the heterogeneous termination step: at the reactor walls. The reaction has been modeled in the temperature and pressure range on the basis of a kinetic scheme which has been proposed for the homogeneous reaction and step (w) with the following values of kw: for both types of packing. The corresponding sticking coefficients of hydrogen atoms are: © 1998 John Wiley & Sons, Inc. Int J Chem. Kinet 30: 439–450,1998

Kinetic study and modeling of the hetero‐homogeneous pyrolysis and oxidation of isobutane around 800 K. Part I. Pyrolysis in an unpacked pyrex reactor

Isobutane pyrolysis is studied in an unpacked Pyrex reactor at 20–100 torr initial pressures and 750–793 K. Results are interpreted in terms of a long chain radical mechanism and the reaction is modeled. The reaction selectivity or ratio of the initial production rate of isobutene (or hydrogen) to that of propene (or methane) is practically given by the ratio of the rate constant of abstraction of a tertiary hydrogen atom of isobutane to that of a primary one. A sensitivity analysis clearly shows that self-inhibition is essentially due to methylallyl radicals produced by hydrogen abstraction from isobutene. The model has been manually adjusted to experimental results and most of the adjusted rate constants are in agreement with literature data. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 425–437, 1998

Kinetic study and modeling of the hetero‐homogeneous pyrolysis and oxidation of isobutane around 800 K. Part II. Pyrolysis in pyrex reactors packed with platinum foils or PbO‐treated pyrex rods

Isobutane pyrolysis has been studied at 20–200 torr initial pressures and 773–793 K, in a packed reactor treated with PbO and in a reactor packed with platinum foils. These packings strongly inhibit product formation and this effect is explained by the occurrence of the heterogeneous termination step: at the reactor walls. The reaction has been modeled in the temperature and pressure range on the basis of a kinetic scheme which has been proposed for the homogeneous reaction and step (w) with the following values of kw: for both types of packing. The corresponding sticking coefficients of hydrogen atoms are: © 1998 John Wiley & Sons, Inc. Int J Chem. Kinet 30: 439–450,1998

Kinetic study and modeling of the hetero-homo-geneous pyrolysis and oxidation of isobutane around 800 K. Part III. Pyrolysis-oxidation in unpacked and in pbO-coated packed Pyrex reactors

Kinetic study and modeling of the hetero-homo-geneous pyrolysis and oxidation of isobutane around 800 K. Part III. Pyrolysis-oxidation in unpacked and in pbO-coated packed Pyrex reactors

Experimental and modeling study of the oxidation of isobutene

Experimental and modeling study of the oxidation of isobutene

Time-of-flight SIMS study of heterogeneous catalysts based on praseodymium and molybdenum oxides

Time-of-flight secondary ion mass spectrometry (TOF SIMS) analyses have been performed on MoO3–Pr6O11 mixtures before and after their use in the selective oxidation of isobutene to methacrolein at 673 K. The three pure molybdate phases (Pr2MoO6, Pr2Mo3O12, Pr6MoO12) obtained independently by the citrate route have also been examined to determine their respective SIMS fragmentation patterns. Although the Pr2MoO6 and Pr2Mo3O12 phases were found to give the same molecular fragments, these phases could be distinguished by the relative intensity ratios of some particular species. Identification of Pr6MoO12 was easier owing to the presence of characteristic high-mass mixed Pr–Mo fragments. A linear relationship was observed when plotting the experimental intensity ratios of several molecular fragments against the Pr/Mo bulk composition. The same behaviour was also noticed for a series of Pr6O11–MoO3 mixtures. Moreover, the examination of the 3-D crystal structure of the pure molybdate phases indicates that the fragmentation patterns of these phases were directly related to their structure. As far as the used catalysts were concerned, the SIMS results showed that: (i) the experimental intensity ratios of given fragments were much smaller in the used catalysts than in the fresh ones; (ii) fragments characteristic of the Pr6MoO12 phase were absent; and (iii) differences in the relative SIMS intensities of certain fragments showed the dominant presence of the Pr2Mo3O12 phase.

Catalysis by heteropoly compounds Part 39 The structure and redox behaviour of vanadium species in molybdovanadophosphoric acid catalysts during partial oxidation of isobutane

The states and roles of vanadium of 11-molybdo-1-vanadophosphoric acid (H4PMo11VO40, PMo11V) catalyst in the partial oxidation of isobutane (2-methylpropane) were analysed by EPR, 51V and 31P NMR, IR spectroscopy, and redox titration, and compared with dodecamolybdophosphoric acid (H3PMo12O40, PMo12) catalyst. Thermal treatment of PMo11V at 623 K in O2 caused the elimination of V from the Keggin anion and formed undefined polymeric and a monomeric V species. It was shown based on the redox titration and EPR spectra that the average valency of V in used catalysts significantly changed with the oxygen partial pressure, while that of Mo remained almost unchanged for both PMo11V and PMo12. The selectivities to methacrylic acid (MAA) and methacrolein (MAL) extrapolated to zero conversion were similar for both catalysts, but PMo11V showed a significantly higher selectivity as the conversion increased. This is because the secondary reaction, that is, oxidative decomposition of MAL and MAA, was slower over PMo11V than over PMo12. This was consistent with the marked difference in the reaction order in oxygen pressure, and corresponded to the differences in the above redox properties.

Oxidation of Isobutane Catalyzed by Partially Salified Cesium Molybdovanadophosphoric Acids

The catalytic activity for the oxidation of isobutane catalyzed by 12-molybdophosphoric acid was much changed by the V5+- and Cs+-substitution for Mo6+ and H+, respectively. The best yield of methacrylic acid was 9.0% and was obtained for the heteropoly catalyst with V and Cs contents of 1 and 2.5, respectively. The high catalytic activity obtained for Cs2.5Ni0.08H1.34PVMo11O40 is presumably due to the high surface area. The activities of 12-molybdovanadophosphoric acids as well as those of 12-molybdophosphoric acid partially salified with Cs+ were controlled by the oxidizing ability of catalysts. The conversion vs selectivity relationships, the simulation of the data, and kinetic results for Cs2.5Ni0.08H1.34PVMo11O40 and Cs2.5Ni0.08H0.34PMo12O40 catalysts show that the first steps, i.e., selective and complete oxidation reactions of isobutane, are rate-determining and mainly catalyzed by molybdenum ion and that vanadium ion efficiently accelerates the selective oxidation of methacrolein to methacrylic ac...

Recent topics in heterogeneous catalysis of heteropolyacids

First, the state of the art of the catalyst design based on heteropoly compounds is briefly overviewed. Then, selected topics of solid heteropoly catalysts, that is, the selective oxidation of isobutyric acid and isobutane over molybdovanadophosphoric acids, the methyl t-butyl ether synthesis in pseudoliquid of heteropolyacids, and the shape-selective and bifunctional (combined with noble metals) acid catalysis of cesium and ammonium salts of tungstophosphoric acid are described based on the recent studies from our laboratory.

Characterization of Pt/SbOxCatalysts Active for Selective Oxidation of Isobutane by Means of XRD, TEM, and XAFS

New Pt/SbOxcatalysts active for the selective oxidation ofi-C4H10andi-C4H8were characterized by means of XRD, TEM, and XAFS as well as H2and CO adsorption. Pt was present as metallic particles, while SbOxexisted as Sb6O13phase. It was suggested that the surface layer of the Pt particles was modified with SbOy(y<x) under thei-C4H10andi-C4H8selective oxidation reaction conditions at 773 K. This modification of Pt particles by SbOyinduces high selectivity to MAL in bothi-C4H10andi-C4H8oxidation reactions. Activation ofi-C4H10(i-C4H10dehydrogenation) is suggested to occur on the Pt–SbOysites in the presence of oxygen, while the Sb6O13phase is responsible for selective oxidation of dehydrogenated intermediates. The degree of the Sb modification depends on the reaction conditions. When the Pt/SbOxcatalyst was reduced with H2at 473 K, Pt–Sb alloy particles (Pt–Pt=0.274 nm and Pt–Sb=0.260 nm) were formed, and the Sb6O13was partly reduced to α-Sb2O4. This heavily reduced state of the surface was nonselective for thei-C4H10oxidation.

Reaction phase effect on tertiary butyl alcohol synthesis by air oxidation of isobutane

Direct introduction of air to supercritical phase isobutane exhibited high activity for selective synthesis of tertiary butyl alcohol, especially on SiO 2 TiO 2 and Pd/carbon catalysts. Remarkable enhancement of activity around critical point was observed if the reaction phase changed to supercritical state.