Kinetics selective oxidation catalysts

Kelkar and McCarthy (1995) proposed another method to use the feedforward experiments to develop a kinetic model in a CSTR. An initial experimental design is augmented in a stepwise manner with additional experiments until a satisfactory model is developed. For augmenting data, experiments are selected in a way to increase the determinant of the correlation matrix. The method is demonstrated on kinetic model development for the aldol condensation of acetone over a mixed oxide catalyst. 

Ethylene is selectively oxidized to ethylene oxide using a silver-based catalyst in a fixed-bed reactor. Ethylene and oxygen are supplied from the gas phase and ethylene oxide is removed by it. The catalyst is stationary. Undesired, kinetically determined by-products include carbon monoxide and water. Ideally, a pure reactant is converted to one product with no by-products. 

It is noteworthy that, as early as 1929, Shibata and Tsuchida reported a kinetic resolution of rac-3,4-dihydroxyphenylalanine by selective oxidation of one enantiomer using a chiral cobalt complex [Co(en)3NH3Cl]Br2 as a catalyst [46,47]. Figure 12 shows a highly enantioselective addition of diisopropy-Izinc to 2-(ferf-butylethynyl)pyrimidine-5-carbaldehyde via an autocatalytic process in the presence of a chiral octahedral cobalt complex with ethylenedi-... 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

It is tempting to associate directly the absence of ethylene oxide over catalysts with more than 40% Pd with the appearance of holes in the d-band. It could be assumed that ethylene is chemisorbed directly on Pd-rich alloys and rapidly decomposed, whereas on Ag-rich alloys ethylene is only adsorbed on top of an oxygen-covered surface leading to selective oxidation. However, the general conclusion from earlier kinetic studies (143) is that the rate-determining step over pure palladium also involves the latter mode of ethylene chemisorption. 

The multi-functionality of metal oxides1,13 is one of the key aspects which allow realizing selectively on metal oxide catalysts complex multi-step transformations, such as w-butane or n-pentane selective oxidation.14,15 This multi-functionality of metal oxides is also the key aspect to implement a new sustainable industrial chemical production.16 The challenge to realize complex multi-step reactions over solid catalysts and ideally achieve 100% selectivity requires an understanding of the surface micro-kinetic and the relationship with the multi-functionality of the catalytic surface.17 However, the control of the catalyst multi-functionality requires the ability also to control their nano-architecture, e.g. the spatial arrangement of the active sites around the first centre of chemisorption of the incoming molecule.1... 

In the results presented in Table 13.5, the addition of tin affects the kinetic selectivity r differently, depending on the catalyst preparation method. When compared to the monometallic PdO catalyst, r slightly decreases for the coimpregnated PdSn catalyst, but it sharply increases for the PdOSn catalyst prepared via the colloidal oxide synthesis. As the intrinsic kinetic constant rates k do not show significant discrepancies between the different catalysts, the main contribution of the variation of the kinetic selectivity is ascribed to the adsorption constant ratio fBo/ Butenes- In the case of the PdOSn catalyst, formation of but-l-ene is favored compared to its consumption because the X Bo/ Butenes ratio increases, indicating that olefin adsorption is much more destabilized than diene adsorption. Thus, the olefin easily desorbs before being hydrogenated into butane. 

Complex (1) is a catalyst for selective oxidation of benzylic, allylic alcohols to aldehydes, and secondary alcohols to ketones using r-butyl hydroperoxide. Primary aliphatic alcohol oxidation failed. The use of cumyl hydroperoxide as radical probe discounted the involvement of i-BuO /t-BuOO. Hammett studies p = -0.47) and kinetic isotope effects kn/ku = 4.8) have been interpreted as suggesting an Ru—OO—Bu-i intermediate oxidant. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

The gas-phase selective oxidation of o-xylene to phthalic anhydride is performed industrially over vanadia-titania-based catalysts ("7-5). The process operates in the temperature range 620-670 K with 60-70 g/Nm of xylene in air and 0.15 to 0.6 sec. contact times. It allows near 80 % yield in phthalic anhydride. The main by-products are maleic anhydride, that is recovered with yields near 4 %, and carbon oxides. Minor by-products are o-tolualdehyde, o-toluic acid, phthalide, benzoic acid, toluene, benzene, citraconic anhydride. The kinetics and the mechanism of this reaction have been theobjectof a number of studies ( 2-7). Reaction schemes have been proposed for the selective pathways, but much less is known about by-product formation. 

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catdysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure Hquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat kinetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more dilEcult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment... 

Propene to acrolein. Hildenbrand and Lintz87,88 have used solid electrolyte potentiometry to study the effect of the phase composition of a copper oxide catalyst on the selectivity and yield of acrolein during the partial oxidation of propene in the temperature range of 420-510°C. Potentiometric techniques were used to determine the catalyst oxygen activity, and hence the stable copper phase, under working conditions. Hildenbrand and Lintz used kinetic measurements to confirm that the thermodynamically stable phase had been formed (it is known that propene is totally oxidised over CuO but partially oxidised over ). 

The ammoxidation of isobutene has not received much attention. The only contribution in this field is by Onsan and Trimm [2.44] for a rather unusual catalyst, a mixture of the oxides of Sn, V and P (ratio 1/9/3) supported on silica. At 520 C, a maximum selectivity to methacrylonitrile + methacrolein of 80% was reached with a Sn—V—P oxide catalyst (ratio 1/9/3), an isobutene/ammonia/oxygen ratio of 1/1.2/2.5 and a contact time of 120 g sec l ]. The kinetics are very similar to those for the pro-pene ammoxidation. Again, the data are initially analysed by means of (parallel) power rate equations, for which the parameters were calculated, while a more detailed analysis proves that a Langmuir—Hinshelwood model with surface reaction as the rate-controlling step provides the best fit with regard to the two main products. At 520° C, the equation which applies for the production of methacrolein plus methacrylonitrile is... 

Aromatic hydrocarbons which have methyl side chains mainly behave like toluene and form aldehydes, while combustion is stimulated and selective oxidation of the nucleus is repressed. The oxidation of methyl-naphthalene, for example, exhibits a low selectivity with respect to phtha-lic anhydride formation, combustion and maleic acid formation being the dominating reactions. Durene is a special case because it resembles o-xy-lene. The oxidation of durene over a V—W—O catalyst at 420° C is reported to produce pyromellitic dianhydride, phthalic and maleic anhydride, although combustion dominates (Geiman et al. [122]). 1,2,4-Trimethyl-benzene yields dimethylbenzene and trimellitic acid if oxidized on a Sn— V—O catalyst. Kinetic data have been measured by Balsubramanian and Viswanath [37]. 

The use of isotopic tracers has demonstrated that the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Probably the most convincing evidence is presented by the isotopic tracer studies utilizing, 4C-labeled propylene and deuterated propylene. Adams and Jennings 14, 15) studied the oxidation of propylene at 450°C over bismuth molybdate and cuprous oxide catalysts. The reactant propylene was labeled with deuterium in various positions. They analyzed their results in terms of a kinetic isotope effect, which is defined by the probability of a deuterium atom being abstracted relative to that of a hydrogen atom. Letting z = kD/kH represent this relative discrimination probability, the reaction paths shown in Fig. 1 were found to be applicable to the oxidation of 1—C3He—3d and 1—QH —1 d. 

In the previous sections of this review, it has been shown that most effective catalysts for the selective oxidation of propylene contain at least two types of metal oxides—an amphoteric or low-valence oxide, such as bismuth, tin, iron, or cobalt, and an oxide of a high valence metal, such as molybdenum or antimony. Moreover, it has been suggested several times that each of these metal oxide components may give rise to an active site for example, propylene may adsorb on an active site associated with one of the metal oxide components, and oxygen may adsorb on an active site associated with another metal oxide component. This problem has been studied using spectroscopic, adsorption, and kinetic techniques. It now seems appropriate to consider some of these studies in detail, attempting to relate the solid structure of the catalyst to the active sites wherever possible. 

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