In propene partial oxidation

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

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). 

Contrary evidence, that adsorbed oxygen is involved in the partial oxidation of propene, has been obtained using labelling." When a pulse containing propene and 0-enriched oxygen was passed over a Cu2 0 catalyst at 623 K the ratio of in the acrolein produced was identical to the 0 ... 

Use of Halide Ions to Improve Selectivity. Earlier work has claimed that enhanced selectivities for alkene oxidation can be achieved by the inclusion of electronegative elements such as S, Se, or halogens. This has been reviewed elsewhere. " More recent work has demonstrated substantial improvements in selectivity for propene (25—70%) and isobutene (35—80%) oxidation when either chloride or bromide is present. Both elements are added to the catalyst in the form of trace levels of organo-halide in the process gas stream. The selectivity increase is the result of a decrease in the rate of complete oxidation rather than an increase in the partial oxidation rate. Since the reaction is first order in oxygen pressure and zero order with respect to alkene in the presence and absence of halide, the reaction mechanism is probably similar in both cases. In the light of Anshits recent work, the effect of the halide is presumably to reduce the relative number and/or reactivity of surface lattice oxygen species and thus reduce the amount of irreversibly adsorbed alkene. 

The selectivity depends on the anion present, but can be as high as 70—80%. The reaction is a co-oxidation of copper and propene rather than a catalytic system and the propene oxide production rate rapidly falls as the copper catalyst is re-oxidized. This effect demonstrates that the oxygen chemistry occurring on copper(i) oxide is closer to that occurring on silver, rather than on mixed oxides such as bismuth molybdate, despite the difference in the partial oxidation product. 

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. 

Mixed metal oxides are used quite often in industrial partial oxidation reactions, examples being Bi Oj-MoOj for the oxidation of propene to acrolein and V O -MoOj for oxidation of benzene to maleic anhydride. Some mixed oxides also are quite active deep oxidation catalysts, a good example being MnOj-CuO. The difficulties in understanding mixed oxides are of course more formidable than they are for single metal oxides. It is a well-established empirical fact that mixed oxides behave quite differently than as individual oxides in most catalytic reactions. This situation is further complicated by the often dramatic effect of promoters, such as alkali metal oxides that are added to the catalyst intentionally. 

A Pd-MOF reported by Corma [60] was also found to be active in the partial oxidation of alcohols using air to oxidize 3-phenyl-2-propen-l-ol (cinnamyl alcohol). Ciimamyl alcohol is a suitable substrate to probe the activity and chemoselectivity of a catalyst for the aerobic alcohol oxidation. With Pd-MOF as catalyst and ambient pressure air as the oxidant, total conversion of ciimamyl alcohol was observed after... 

However, in the same temperature range and O2 partial pressure total oxidation of acrolein and propene largely predominates. This can be taken as a further support that on transition metal oxide catalysts the same oxygen species (lattice oxygen) are involved in both partial and total oxidation. 

Propyhdyne formed from propene on lr4 supported on y-Al203 was observed by IR and NMR spectroscopies [38]. When ethene or propene was brought in contact with oxide-supported lr4 [39,40], Ire [39,40], or Rhe (A.M. Argo and B.C. Gates BC, impubhshed results) in the presence of H2, hydrocarbon hgands were formed (namely, alkyls and /r-bonded alkenes), which have been inferred from IR spectra to be intermediates in hydrogenation to make alkanes, as discussed later. The population of these hydrocarbon ligands on the supported clusters depends sensitively on the conditions, such as reactant partial pressures and temperature. 

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. 

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. 

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 ). 

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. 

---