Bismuth molybdate rates

Using the pulse microreactor method( ), the general rate expression for reoxldatlon of bismuth molybdate catalysts was found to be ... 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

The results are presented in Table II and the variations of the rates of formation of acrolein and the selectivity to acrolein observed at 380°C on these catalysts as a function of their bismuth molybdate content are plotted in Fig. 3 and 4. The activity increased slowly to reach a maximum for a mass content of bismuth molybdate of... 

The kinetics of the parallel formation of by-products is generally similar to the kinetics of the main reaction (i.e. rate independent of the oxygen pressure and first order with respect to propene), which presumes identical reaction steps. A study of the origin of acetaldehyde and formaldehyde carried out by Gorshkov et al. [144] is of interest in this connection. 14C-Labelled propene was oxidized over a bismuth molybdate catalyst at 460° C the results showed that acrolein, acetaldehyde and formaldehyde are formed via a symmetrical intermediate, presumably one and the same intermediate. The study, moreover, shows that acetaldehyde is exclusively formed from this intermediate, while formaldehyde may also be formed from the aldehyde group of acrolein and the methyl group of acetaldehyde. 

A commercial iron-promoted catalyst (Sn/Sb/Fe = 1/4/0.25) was studied by Germain et al. [92,93,135,137]. Iron is reported to improve the ammoxidation qualities of the catalyst although it has no effect on the oxidation [93], The kinetics, determined in a flow reactor at 445°C and with a feed ratio C3H6/NH3/air = 1/1.2/10, are essentially similar for this catalyst and bismuth molybdate. The initial selectivity is 80% and the maximum yield is 65% (at 445°C). The initial selectivity markedly depends on the temperature (e.g. 91% at 415°C and 72% at 507°C). The effect of water is hardly significant for this catalyst the acrylonitrile formation is slightly inhibited, while some more acrolein is formed. Presumably, water and ammonia compete in the interaction with the catalyst, which is much less reactive with respect to ammonia than bismuth molybdate. The acrolein ammoxidation is very rapid (about six times the propene ammoxidation rate) and selective (86%). A comparison of the Sn—Sb—Fe—O catalyst with bismuth molybdate is presented in Table 14. 

Ray and Chanda [261] studied bismuth molybdates (prepared by the method of Peacock [250,251]) in an integral flow reactor. At constant W/F = 8 g h mol-1 and a feed ratio isobutene/oxygen = 1/6, a maximum selectivity of 75% was found at 400—450°C. As with propene, the reaction is first order with respect to isobutene and the rate is independent of the oxygen pressure. The reoxidation of the catalyst is very fast. Assuming that the kinetics can be described by three parallel first-order reactions for the production of methacrolein, carbon monoxide and carbon dioxide, rate coefficients were calculated (Table 18). 

Mann and Ko [202] likewise examined the selective oxidation of isobutene on bismuth molybdate. With an integral flow reactor, the highest selectivity was obtained at over 30% conversions for an oxygen/olefin ratio of 2/1 and a W/F = 2.5 g h mol-1 (390°C). The data were correlated with a rather complicated Langmuir—Hinshelwood expression inconsistent with a redox mechanism. This was based on a rate-controlling step between adsorbed isobutene and adsorbed oxygen, and included an inhibiting effect of methacrolein by competitive adsorption with isobutene, viz. 

Ammosov and Sazonov [21,22,24] demonstrated that for iron antimonates the initial selectivities are lower than for bismuth molybdates due to a higher rate of the parallel combustion reaction. It is proved that both selective oxidation and combustion occur by a redox mechanism. In another publication [23], the same authors report the kinetics of the butene and butadiene combustion reactions. 

Wise [350] investigated the parallel between ammoxidation and oxidation of ammonia over bismuth molybdates. It was shown that the rate of conversion to nitrogen is first order in NH3 and independent of oxygen concentration, analogous to the selective oxidation of propene. Under conditions in which propene combusts, NH3 is converted to nitrogen oxides. 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

Modem day bismuth molybdate catalysts, in addition to A, —3.V i2.v hio04 phases, contain other compounds such as Bi2Mo06 and Bi2M02O9 as well as small amounts transition metal molybdates that not only increase conversion rates and selectivity but also increase catalyst hfetime and allow operation at lower temperatures. ... 

The effect of the additional components has been illustrated by Sleight et al. [117], who prepared a series of Scheelite type (derived from the mineral CaW04) bismuth molybdate phases to which Pb was added to give a series of solid solutions of composition Pbi 3xBi2xx(Mo04). For every two bismuth ions in the structure a cation vacancy (())) was generated. When this series of catalysts was tested a relationship was observed between vacancy concentration and the rate of propene oxidation, as shown in Fig. 5.26. 

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. 

Although catalytic oxidation of propylene has been found to be first order with respect to the olefin (5), a dependence on oxygen has also been reported (9, 54). Investigations of the participation of lattice oxygen in the oxidation process over mixed oxides, which were thought to contain antimony(V), antimony(III), and tin(IV), reported no support for the redox mechanism observed with bismuth molybdate. The matter of oxygen participation has also been considered by Christie et al. (53), who reported that the rates of... 

As discussed previously, the relative rates of reduction of several bismuth molybdate-based catalyst systems using propylene decrease in the order multicomponent system > Bi2Mo2O9(j6) Bi2Mo3O,2(a) > Bi3FeMo2Oj2... 

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