Allylic specie formation, propylene

When propylene chemisorbs to form this symmetric allylic species, the double-bond frequency occurs at 1545 cm-1, a value 107 cm-1 lower than that found for gaseous propylene hence, by the usual criteria, the propylene is 7r-bonded to the surface. For such a surface ir-allyl there should be gross similarities to known ir-allyl complexes of transition metals. Data for allyl complexes of manganese carbonyls (SI) show that for the cr-allyl species the double-bond frequency occurs at about 1620 cm-1 formation of the x-allyl species causes a much larger double-bond frequency shift to 1505 cm-1. The shift observed for adsorbed propylene is far too large to involve a simple o--complex, but is somewhat less than that observed for transition metal r-allyls. Since simple -complexes show a correlation of bond strength to double-bond frequency shift, it seems reasonable to suppose that the smaller shift observed for surface x-allyls implies a weaker bonding than that found for transition metal complexes. 

Temperature Programmed Reaction. Examination of another redox system, propylene oxidation on M0O3, provides further insight. It is well accepted that propylene oxidation on molybdenum-based catalysts proceeds through formation of allylic intermediates. From isotopic studies it has been demonstrated that formation of the allylic intermediate is rate-determining (H/D effect), and that a symmetric allylic species is formed ( C labelling). 

The use of isotopic tracers has demonstrated that the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Probably the most convincing evidence is presented by the isotopic tracer studies utilizing, 4C-labeled propylene and deuterated propylene. Adams and Jennings 14, 15) studied the oxidation of propylene at 450°C over bismuth molybdate and cuprous oxide catalysts. The reactant propylene was labeled with deuterium in various positions. They analyzed their results in terms of a kinetic isotope effect, which is defined by the probability of a deuterium atom being abstracted relative to that of a hydrogen atom. Letting z = kD/kH represent this relative discrimination probability, the reaction paths shown in Fig. 1 were found to be applicable to the oxidation of 1—C3He—3d and 1—QH —1 d. 

Keulks et al. (32) have also concluded from the oxidation of 14C-labeled and unlabeled acrolein that carbon dioxide is formed almost exclusively from the further oxidation of acrolein. Thus, it can be seen that the initial step in the formation of carbon dioxide and the other side products of propylene oxidation is the formation of a symmetrical 7r-allyl intermediate. This 7r-allylic intermediate is responsible for both the selective and nonse-lective oxidation of propylene, the course of the overall reaction depending on the subsequent reaction pathway of the allylic species. 

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. 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

Considerable evidence exists that indicates the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Subsequent steps may vary as a function of the catalyst. Some catalyst systems may abstract a second hydrogen atom before the insertion of oxygen. Others may add molecular oxygen, forming a hydroperoxide intermediate, which may then subsequently decompose into acrolein and water. 

Reaction Schemes and Networks. Within the last few years a series of review articles have appeared concerning the oxidation of propylene to acrolein (10-16). It is generally assumed that the first reaction step, the formation of an adsorbed allylic species, is rate-determining for the formation of acro-... 

Keulks et al. (7) discussed evidence for a model in which the selective oxidation of propylene proceeded via the formation of a symmetric allylic species with subsequent steps depending on the nature of the catalyst. In some catalytic systems the abstraction of a second hydrogen atom seemed to precede the insertion of oxygen, while others appeared to add molecular oxygen to form a hydroperoxide intermediate which subsequently decomposed into acrolein and water. 

N-allyl species and two hydrogen abstractions account for acrylonitrile formation. Thus, although allyl radicals are probably not the selective intermediate in propylene oxidation and ammoxidation, they can form acrolein or acrylonitrile via these selective O- or N-allyl intermediates. 

Similar to the basic surface studies discussed above, promoters often show markedly different behaviors depending on the alkene species used. Lambert and co-workers (68) reported a study of ethene and propene epoxidation with different promoters that showed no real correlation based on the promoter used. In the case of NOx species as promoters, there was no effect for the formation of propylene oxide, which is interesting considering the high activity of NO in formation of ethylene oxide. Also, addition of potassium ions into the NO promoter feed decreased both activity and selectivity for propylene oxide formation, again completely opposite to the behavior seen for EO. As in the other surface studies, the authors postulate a chemical effect from the presence of allylic hydrogens. 

The rhodium-catalyzed addition of ethylene to 1,3-butadiene to yield 1,4-hexadiene (5a, 151) proceeds via a similar mechanism (151) with the exception that, upon formation of the alkylrhodium(III) species, the hexadiene synthesis proceeds without further change in the oxidation state of the metal. In these reactions with butadiene the coordinated alkyl groups are either chelate or 7r-allyl structures which appear to stabilize Rh(III) (151). The addition of propylene to butadiene and isoprene to produce [Pg.297]

The rate of conversion of propane is practically the same in the presence and in the absence of ammonia. The oxidation yields propylene and carbon oxides, which are the prevailing products. However, when ammonia is added to the feedstock, the yield to propylene remains unchanged, while the yield to carbon oxides is remarkably decreased in favour of the formation of acrylonitrile. This suggests that in the absence of ammonia the propylene is oxidized to a compound or to an intermediate which under these conditions is burnt to carbon oxides. The addition of ammonia allows this intermediate compound (which might be acrolein or an allyl radical species) to be quickly transformed to the stable... 

A complete analysis of the products reported in Fig. 1 requires some more comments on cyclopentadiene and benzene. Both are typical secondary products, and are mainly the result of successive addition and condensation reactions of alkenes and unsaturated radicals. Once a significant amount of ethylene and propylene is formed, vinyl and allyl radicals are present in the reacting system and form butadiene, via butenyl radicals. Successive addition reactions of vinyl and allyl-like radicals on alkenes and dialkenes sequentially explain the formation of cyclopentadiene and benzene. These reactions are discussed in-depth in the literature and will be also analysed in the coming paragraphs (Dente et al., 1979). It seems worthwhile mentioning that these successive reactions and interactions of small unsaturated radicals and species constitute the critical sub-mechanism for the correct evaluation of ethylene selectivity. In fact, once the primary decomposition of the hydrocarbon feed has largely completed, the primary products and mainly small alkenes can be... 

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