Catalyst amount, partial oxidation

Oxidation of ethylene with the objective of obtaining ethylene oxide is conducted on the surface of silver. To the best of our knowledge, on no other heterogeneous catalyst can ethylene oxide be obtained in more than trace amounts. Partial oxidation of ethylene into ethylene oxide... 

After panial oxidation of a coked catalyst, the first peak of the TPO profile disappears, and the activity for n-buiane dehydrogenation is completely recovered. A signiricaiu amount of graphitic carbon is detected by TEM and SAED examination of the residual carbon on Pt-Sn catalysts after partial oxidation. This implies that the first peak of the 1 PO profile corresponds to carbon deposits located mainly on the metal surface, wbile the second one derives from the more graphitC Iikc carbon located on the support. 

A special but important case where the SCR reacting system includes NO2 in signiflcant amounts is represented by the new generation of urea-SCR converters for diesel vehicles integrated with an upstream preoxidation catalyst, which partially oxidizes NO to NO2 (10,11). In such devices, the so-called fast SCR reaction 3, which may be over ten times faster than the standard SCR reaction 1 at low temperatures, plays a critical role in boosting the DeNOx activity at 180 -300°C (12). However, large NO2 feed contents result also in the occurrence of two additional unselective reactions, not observed in the presence of NO-NH3 only, namely the formation of ammonium nitrate, which is critical below 180°C,... 

Oxidation Step. A review of mechanistic studies of partial oxidation of propylene has appeared (58). The oxidation process flow sheet (Fig. 2) shows equipment and typical operating conditions. The reactors are of the fixed-bed shell-and-tube type (about 3—5 mlong and 2.5 cm in diameter) with a molten salt coolant on the shell side. The tubes are packed with catalyst, a small amount of inert material at the top serving as a preheater section for the feed gases. Vaporized propylene is mixed with steam and ak and fed to the first-stage reactor. The feed composition is typically 5—7% propylene, 10—30%... 

Emissions from methanol vehicles are expected to produce lower HC and CO emissions than equivalent gasoline engines. However, methanol combustion produces significant amounts of formaldehyde (qv), a partial oxidation product of methanol. Eormaldehyde is classified as an air toxic and its emissions should be minimized. Eormaldehyde is also very reactive in the atmosphere and contributes to the formation of ozone. Emissions of NO may also pose a problem, especiaHy if the engine mns lean, a regime in which the standard three-way catalyst is not effective for NO reduction. 

As a chemical compound, methane is not very reactive. It does not react with acids or bases under normal conditions. It reacts, however, with a limited number of reagents such as oxygen and chlorine under specific conditions. For example, it is partially oxidized with a limited amount of oxygen to a carbon monoxide-hydrogen mixture at high temperatures in presence of a catalyst. The mixture (synthesis gas) is an important building block for many chemicals. (Chapter 5). 

This process includes two main sections the burner section with a reaction chamber that does not have a catalyst, and a Claus reactor section. In the burner section, part of the feed containing hydrogen sulfide and some hydrocarbons is burned with a limited amount of air. The two main reactions that occur in this section are the complete oxidation of part of the hydrogen sulfide (feed) to sulfur dioxide and water and the partial oxidation of another part of the hydrogen sulfide to sulfur. The two reactions are exothermic ... 

The present work demonstrates that the mixed oxide catalyst with inhomogeneous nanocrystalline MosOu-type oxide with minor amount of M0O3- and Mo02-type material. Thermal treatment of the catalyst shows a better performance in the formation of the crystals and the catalytic activity. The structural analysis suggests that the catalytic performance of the MoVW- mixed oxide catalyst in the partial oxidation of methanol is related to the formation of the M05O14 t3 e mixed oxide. 

Steam reforming needs a secondary fuel to provide the energy supply necessary for the reaction that occurs and a catalysts to improve the kinetic of this process. In Equation (3), the primary fuel is partially oxidised by a limited amount of oxygen. Partial oxidation produces less H2 per fuel unit than stream reforming, but the kinetic reaction is faster, it requires smaller reactors and neither catalyst nor energy supply from a secondary fuel. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

H2 production from ethanol (as well as methanol) employs these methodologies either as such or after slight modifications, especially in the ATR process, wherein a separate combustion zone is usually not present (Scheme 3). A mixture of ethanol, steam and 02 with an appropriate ethanol steam 02 ratio directly enters on the catalyst bed to produce syngas at higher temperature, around 700 °C.18,22 The authors of this review believe that under the experimental conditions employed, both steam reforming and partial oxidation could occur on the same catalyst surface exchanging heats between them to produce H2 and carbon oxides. The amount of 02 may be different from what is required to achieve the thermally neutral operation. Consequently the reaction has been referred to as an oxidative steam reforming... 

The amount of the products formed over the studied catalysts, in the presence and absence of molecular O2, are listed in Table III. It is evident that the formation of the oxidation products is associated with the gas phase oxygen supply. Then, as the reaction rates in the mixture of reactant and in separate steps differ (19), these data exclude the participation of lattice oxygen in the partial oxidation of methane via a two step redox mechanism as main reaction pathway proving the occurrence of a "concerted mechanism". 

With decrease of the reaction temperature, no change was experienced in the selectivity for ethylene, but there was an increase in the selectivity of acetaldehyde formation. The oxidation of ethane has been also investigated on Mo03/Si02. Under these conditions, this catalyst was found to be very active for the total oxidation of ethane. At 510 K, the conversion of ethane was 21%, the products of partial oxidation were formed only in trace amount. 

As the mass transfer rate is increased, reactions producing H2O and CO2, whether in the boundary layer or on the catalyst surface, are reduced. Thus, because the direct oxidation of CH4 is so fast, the mass transfer rate must be high or H2 and CO will react with O2 to form the total oxidation products, reducing the partial oxidation selectivity and decreasing the amount of O2 available to react with CH4. 

The current two-step industrial route for the synthesis of methanol, from coal or methane to synthesis gas and then from synthesis gas to methanol, has certain drawbacks. The economic viability of the whole process depends on the first step, which is highly endothermic. Thus a substantial amount of the carbon source is burned to provide the heat for the reaction. It would be highly desirable, therefore, to replace this technology with a technically simpler, single-step process. This could be the direct partial oxidation of methane to methanol, allowing an excellent way to utilize the vast natural-gas resources. Although various catalysts, some with reasonable selectivity, have been found to catalyze this reaction (see Sections 9.1.1 and 9.6.1), the very low methane conversion does not make this process economically feasible at present. 

Studying the effect of palladium as a promotor in silver catalysts, Cor-mack et al. [90] found that increasing amounts of Pd alloyed with silver drastically decreased the selectivity. No other partial oxidation products were found. 

The properties of Mn—Mo—O catalysts strongly depend on the Mn/Mo ratio. Machek and Tichy [193] report that at Mn/Mo > 1, the catalyst consists of MnMo04 and Mn203, and is already very active at 300°C. However, combustion is the only process. At Mn/Mo < 1, the catalyst consists of MnMo04 and M0O3, and may produce substantial amounts of partial oxidation products at 400—500° C. Machek and Tichy report, for example, that at 479°C and Mo/Mn = 7/1, a selectivity of 78% is obtained. Hov/ever, formaldehyde is also formed and amounts to 20—50% of the total aldehyde yield. Viswanathan et al. [342] have prepared a catalyst with a Mn/Mo ratio of 1/1 which has catalytic qualities very similar to that of bismuth molybdates. At 400—450°C, a selectivity is 70% is obtained at... 

Kinetic and mechanistic investigations on the o-xylene oxidation over V205—Ti02 catalysts were carried out by Vanhove and Blanchard [335, 336] using a flow reactor at 450°C. Possible intermediates like o-methyl-benzyl alcohol, o-xylene-a,a -diol, toluic acid and phthalaldehyde were studied by comparing their oxidation product distribution with that of toluene. Moreover, a competitive oxidation of o-methylbenzyl alcohol and l4C-labelled o-xylene was carried out. The compounds investigated are all very rapidly oxidized, compared with o-xylene, and essentially yield the same products. It is concluded, therefore, that these compounds, or their adsorbed forms may very well be intermediates in the oxidation of o-xylene to phthalic anhydride. The ratio in which the partial oxidation products are formed appears to depend on the nature of the oxidized compounds, i.e. o-methylbenzyl alcohol yields relatively more phthalide, whereas o-xylene-diol produces detectable amounts of phthalan. This... 

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