Molybdate propene activation

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catdysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect to acrolein. Acetone is the chief product at low temperature (200°C), whereas carbon oxides, besides some acrolein, predominate at higher temperatures [182,257], Firsova et al. [112,113] report that adsorption of propene on iron molybdate (Fe/Me = 1/2) at 80—120°C causes cation reduction (Fe3+ -> Fe2+) as revealed by 7-resonance spectroscopy. Treatment with oxygen at 400°C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related to the low selectivity of iron molybdate. 

The interaction of propene and oxygen with sodium and potassium molybdates was studied by Burlamacchi et al. [66]. ESR measurements reveal that Mos+ is formed (at 380°C) although the catalytic activity is zero. The reoxidation appears to be very difficult. The authors conclude that a cation like Bi3+ or Fe3+ is required to facilitate reoxidation. 

Villa et al. [340] have shown that the bismuth tungstates are comparable with bismuth molybdates with respect to dehydrogenation catalysis, although activities and selectivities are somewhat lower. Although the phase structures are different, interesting catalysts are formed in a similar composition range Bi/W = 2/3 to 2/1. (Note that, in case of propene (amm)oxidation, tungstates are definitely inferior to molybdates.)... 

Bismuth molybdate and other binary compositions (Fe—Mo, Sn—Sb and others) were tested by Germain and Perez [128] using a pulsed reactor. The authors demonstrate that a qualitative analogy may exist between ammonia and propene oxidation but if activities are compared, different sequences of catalytic efficiency arise. It must be noted, however, that these conclusions are based only on pulse experiments. These can be quite different from results in flow reactors, depending mainly on the nature of the steady state. 

Oxo-metal complexes also intervene as active species in the heterogeneous gas-phase oxidation of hydrocarbons over metal oxide or mixed metal oxide catalysts at high temperatures. Characteristic examples are the bismuth molybdate-catalyzed oxidation of propene to acrolein and the V205-catalyzed oxidation of benzene to maleic anhydride (equations 17 and 18).SJ... 

FIGURE 13 Raman-GC recorded during propene oxidation on bismuth molybdate with simultaneous activity measurement (A) Raman spectra (Reprinted from J. Catal. 132,536 (1991), Snyder T.P., Hill C.G., Stability of bismuth molbydate catalysts at elevated temperatures in air and under reaction conditions, copyright (1991) with permission from Elsevier) (Snyder and Hill, 1991) (B) simultaneous conversion and selectivity (based on Snyder and Hill, 1991). 

Bismuth molybdates having a Bi/Mo ratio in the range of 0.67 2.0 catalyze the selective oxidation of propene to acrolein, and the ammoxidation of propene to acrylonitrile (equations 5 and 6). Both reactions proceed through an aUyhc intermediate. Three typical active phases o -Bi2Mo30i2,... 

The catalytic activity of bismuth-lead molybdate catalysts is related to the defect site densities. Olefin oxidations take place most rapidly when ( ) = 0.06. Acrylonitrile is formed by the ammoxidation of propene in about 75% yield over these mixed oxides when <(> is between 0.04 and 0.08.38... 

Propene oxidation to acrolein is carried out commercially over a range of bismuth molybdate catalysts to which are added 3-4 additional metal oxides to boost the activity. The final catalysts are mixtures of binary and ternary oxides and some solid solutions. One feature is the ability of lattice oxygen to transfer readily at the reaction temperature between the multiple phases that make up this catalyst and to the reacting propene. Another key feature is that the initial point of activation of the propene is one of the methyl C-H bonds with the production of a surface allyl intermediate, hence the term allylic oxidation. 

Scheme 5.5. Schematic of the active site for acrolein formation from propene over bismuth molybdate...

The series of nickel molybdate catalysts show a decrease in the selectivity towards propene with an increase in conversion. Figure 2 shows the selectivity towards propene at a propane conversion level of 20%. As the Ni/Mo ratio increases above 1, the selectivity towards propene decreases almost linearly as carbon oxides become the dominant products. It is interesting to note, however, that the selectivity towards propene is essentially independent of the Ni/Mo ratio at values less than and equal to 1. This suggests that it is the increase in activity with decreasing Ni/Mo values that accounts for the increase in yield noted above. 

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions. ... 

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