Propene, partial oxidation

Synergy Effect of Multicomponent Co, Fe, and Bi Molybdates in Propene Partial Oxidation... 

Catalytic properties were studied for two reactions namely isopropanol conversion and propene partial oxidation. The first reaction is a test reaction which allows to characterize acidic, basic or redox properties of a catalyst. One gets dehydration to propene or di-isopropylether for acid catalyst, acetone for basic catalyst in absence of air and acetone and water for redox type catalyst in presence of air. The experimental results at 100°C clearly show that at low Mo loadings acidic features are favored while redox features are favored at higher loadings. This indicates that monomeric MoOj" species are acidic (presumably as in silicomolybdic acid) while polymeric species exhibit redox properties. 

Iron antimony oxide catalysts enriched with antimony have been prepared. The impregnation of FeSb04 is not very reproducible. The catalyst prepared according to method A showed in the propene partial oxidation results similar to iron antimony oxide catalysts with Sb/Fe = 2. In the preparation of the catalysts according to method B most of the antimony precipitated as a separate phase. 

Small olefins, notably ethylene (ethene), propene, and butene, form the building blocks of the petrochemical industry. These molecules originate among others from the FCC process, but they are also manufactured by the steam cracking of naphtha. A wealth of reactions is based on olefins. As examples, we discuss here the epoxida-tion of ethylene and the partial oxidation of propylene, as well as the polymerization of ethylene and propylene. 

Propene is an intermediate utilized in the chemical and pharmaceutical industries. The partial oxidation of propene on cuprous oxide (CU2O) yields acrolein as a thermodynamically imstable intermediate, and hence has to be performed under kinetically controlled conditions [37]. Thus in principle it is a good test reaction for micro reactors. The aim is to maximize acrolein selectivity while reducing the other by-products CO, CO2 and H2O. Propene may also react directly to give these products. The key to promoting the partial oxidation at the expense of the total oxidation is to use the CU2O phase and avoid having the CuO phase. 

GP 6] [R 5] With a stabilized CU2O catalyst layer, by addition of bromomethane (ppm level), 20% selectivity at 5% conversion was found (0.5 vol.-% propene 0.1 vol.-% oxygen 2.25 ppm promoter 350 °C) [37]. This is far better than with non-conditioned copper oxide catalysts which contain CuO besides CU2O. It is expected that the first species promotes more total oxidation, whereas the latter steers partial oxidation. In the above experiment, selectivity rises from 7 to 30% at slightly reduced conversion after 3 h of promoter conditioning. 

Vanadia catalysts exhibit high activity and selectivity for numerous oxidation reactions. The reactions are partial oxidation of methane and methanol to formaldehyde, and oxidative dehydrogenation of propane to propene and ethane to ethcnc.62 62 The catalytic activity and selectivity of... 

Numerous chemical intermediates are oxygen rich. Methanol, acetic acid and ethylene glycol show a O/C atomic ratio of 1, as does biomass. Other major chemicals intermediates show a lower O/C ratio, typically between 1/3 and 2/3. This holds for instance for propene and butene glycols, ethanol, (meth)acrylic acids, adipic acid and many others. The presence of some oxygen atoms is required to confer the desired physical and chemicals properties to the product. Selective and partial deoxygenation of biomass may represent an attractive and competitive route compared with the selective and partial oxidation of hydrocarbon feedstock. 

More recently, it was demonstrated that 80 is a catalyst for the partial oxidation of olefins using dioxygen (230). For example, dry propene was oxidized to acetone when water vapor was present in the catalyst stream, some propanal could also be detected. Other reactions reported included the conversion of styrene to acetophenone and phenylacetaldehyde in an 80 20 product ratio, and 2-norbornene to 2-norbomanone and cyclohexene-4-carboxyaldehyde in a 70 30 product ratio. 

Potential reactions of hydrocarbons are total oxidation, partial oxidation and steam reforming. These are shown below, using propene as an example, but similar equations would apply for other hydrocarbons. 

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 formation of methacrolein by the partial oxidation of 2-methyl-propene, based on the above modification, can be visualized to take place according to the following scheme. 

Table 2 provides a comparison of the catalytic properties of the two oxides, M0O3 Y and (MoVW)5Oi4 (Dieterle, 2001 Mestl, 2002). M0O3 x was additionally preconditioned in 5 vol% H2/N2 at 450 °C to induce partial reduction. At identical propene conversions and space velocities, M0O3 Y produced much more C02 (Figure 14B) than partial oxidation products compared to (MoVW)5Oi4, which showed a high selectivity to partial oxidation products in accordance with the two-domains model (Petzodt et al., 2001). 

Conversely, reaction conditions that maintained a rapid reoxidation and a small number of Mo5+ centers in the catalyst resulted in an increased selectivity. Hence, it may be concluded that in a process that involves diffusion of oxygen ions in the catalyst bulk and a prolonged lifetime of partially reduced V4+—Mo5+ metal sites, total oxidation of propene dominates. On the other hand, catalytic oxidation of propene proceeding on an oxidized V4+—Mo6+ active site at the surface of the catalyst yields an improved selectivity for partial oxidation products. 

Molybdenum trioxide constitutes an active model catalyst for the oxidation of propene in the presence of gas-phase 02 at temperatures above approximately 600 K (Grzybowska-Swierkosz, 2000). Reduction of M0O3 in propene and oxidation of Mo02 in 02 were investigated by time-resolved XAFS spectroscopy combined with mass spectrometry (Ressler et al., 2002). Reduction and reoxidation of M0O3 x are of particular interest because they constitute the two fundamental transformations of the so-called redox mechanism for partial oxidation of alkenes on molybdenum oxide catalysts. 

In another example, a complex multi-component BiMoCoFeO catalyst used in the partial oxidation of propene to acrolein was characterized by Mossbauer spectroscopy. This example has been chosen because it provides a good demonstration of the high efficiency of Mossbauer spectroscopy for the characterization of working catalysts (181,182). 

Table 4 Acrolein selectivities in partial oxidation of propene...

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