Propene oxide formation

The propene oxidation process is globally conducted on a 5 million t/a scale. A new gas-phase oxidation process of propene to propene oxide using hydrogen peroxide would avoid use of solvents and is expected to have higher selectivity. 

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FIGURE 12.6 Deactivation versus the amount of propene oxide produced during the propene epoxidation over 1 wt.% Au/Ti02. 0 propene oxide formation rate A water formation rate/10. [0.3 g of catalyst, 50 Nml/min gas feedrate (10% H2, O2, and propene), total pressure 1.1 bar(a).]... 

Gorokhovatskii and Rubanik (119) came to the conclusion that the catalytic combustion of propene over silver does not proceed via the propene oxide formation step, since the rate of propene oxide conversion at 240° is considerably lower than that for propene. 

Figure 14.7 Rate constant for propene oxide formation over Au/Ti-Si02 (Ti-impregnated Si02) catalyst as a function of reactant concentrations o, C3H6/O2/ H2 = 5/10/10-40 (393K) , C3H6/O2/H2 = 5/5/10-90 (350K) A, C3H6/O2/H2 = 5/2-20/40 (393 K) O, C3H6/...

FIGURE 7.8 Computationally determined structure of a Au, cluster on an alumina support. This and related clusters are highly active for propene oxide formation via direct propene epoxidation. Reprinted with permission from Ref. [91]. John Wiley Sons. (See insert for... 

Catalysis is a special type of closed-sequence reaction mechanism (Chapter 7). In this sense, a catalyst is a species which is involved in steps in the reaction mechanism, but which is regenerated after product formation to participate in another catalytic cycle. The nature of the catalytic cycle is illustrated in Figure 8.1 for the catalytic reaction used commercially to make propene oxide (with Mo as the catalyst), cited above. 

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

The CsHe desorption was essentially inhibited in the presence of SO2 because sulfur species can react with Fe O radical to form a relatively stable Fe SOs Fe (see Eq. 23), resulting in a significant decline in the density of available adsorption sites for CsH . Simultaneously, the scarcity of a-02 surface species (Fe 02") due to a competitive SO2 adsorption (Eq. 22) leads to a decrease in both rates of propene oxidation and carbonaceous species (CO and CO2) formation. 

The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [95,96] demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [84] are understandable, as the wall of the empty reactor may have been sufficiently active to initiate the reaction. 

Unlike propene oxidation to acrolein or butene oxidation to maleic anhydride, oxygen is not incorporated into the selective oxidation product butadiene. However, water is formed together with butadiene, and it could conceivably be formed with lattice oxygen. There have been no isotopelabeling experiments to elucidate this. Similarly, it is not known whether the formation of any of the combustion products involves lattice oxygen. 

The molecular mechanism of the selective oxidation pathway is believed to be the one shown in Scheme 2 (Section I). Adsorbed butene forms adsorbed 7r-allyl by H abstraction in much the same way as xc-allyl is formed from propene in propene oxidation (28-31). A second H abstraction results in adsorbed butadiene. Indeed, IR spectroscopy has identified adsorbed 71-complexes of butene and 7t-allyl on MgFe204 (32,33). On heating, the 7r-complex band at 1505 cm 1 disappears between 100-200°C, and the 7t-allyl band at 1480 cm-1 disappears between 200-300°C. The formation of butadiene shows a deuterium isotope effect. The ratio of the rate constants for normal and deuterated butenes, kH/kD, is 3.9 at 300°C and 2.6 at 400°C for MgFe204 (75), 2.4 at 435°C for CoFe204, and 1.8 at 435°C for CuFe204 (25). The large isotope effects indicate that the breaking of C—H (C—D) bonds is involved in the slow reaction step. 

A similar Te02/HBr catalyst has been used in acetic acid for the oxidation of propene at ca. 120 °C to propene oxide and propene glycol via 1,2-diacetoxypropane.361,362 However, this reaction gives lower yields because of side allylic oxidation reactions. Oxidation of toluene under similar conditions (Te02/LiBr, 160 °C) results in the formation of methylbenzyl acetate mixtures rather than benzyl acetate as observed with Se02/LiBr catalyst (equation 133).363... 

Wienold et al. (2003) XAS, TGA Ammonium heptamolybdate Phase formation ligand removal, sub-oxides + + + Propene oxidation, oxidation and reduction of oxide... 

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