Adsorbed allylic species

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-... 

These spectra aid the interpretation of the mode of binding of adsorbed allyl species formed in the selective oxidation of hydrocarbons over metal oxide catalysts. The INS spectra of allyl iodide adsorbed by an iron antimonate catalyst at 293 K, and after heating to 353 K, were different from the spectrum of allylpalladium chloride and consistent with the allyl binding to the catalyst through the double bond there was no evidence for a ri -allyl (3) [96]. 

The next step is the insertion of a lattice oxygen into the allylic species. This creates oxide-deficient sites on the catalyst surface accompanied hy a reduction of the metal. The reduced catalyst is then reoxidized hy adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system. ... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

Four kinds of adsorbed species are postulated to result from the adsorption of alkenes on noble metals, Jt-com plcxcd double bonds, Jt-complexed jt-allyl species, mono-O-bonded alkanes, and di-o-bonded alkanes. Distinguishing among these is difficult and has been the subject of much speculation. For example, alkenes may adsorb either by Jt-complexing or by di-C-adsorption, and 7t-allyl species are thought to occur readily on Pd but not so readily on Pt.55... 

In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

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. 

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz ... 

The addition of deuterium to 1,3-butadiene yields mainly 1-butene and isotopic distribution in these products is nearly identical and 70% of the initial product corresponds to simple 1,2 or 1,4 addition. Meyer and Burwell suggest that 1,3-butadiene is adsorbed on the surface in the trans conformation. Addition of deuterium to a terminal earbon atom produces an allylic species which is a common intermediate for the formation of both major products, I-butene and trans-2-butene. 

The decomposition of allyl chloride on Ag(110) (206) at 180 K has yielded a VEEL spectrum reasonably assigned to a 77-allyl species at 300 K, adsorbed, 1,5-hexadiene has been identified. Similar results have been reported for Ag(110)/O (207) together with other oxygenated products. 

C-Propene adsorption on platinum—alumina and platinum—silica [66] differs from ethylene adsorption insofar as a fraction of the initially retained 14C-propene is relatively easily exchanged or removed by hydrogen treatment. This suggests less extensive dissociation of the adsorbed propene and a 7T-allyl species (structure F) has been proposed in this case, viz. 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

Hydrogenation of alkenes on ZnO was studied by means of IR spectroscopy17. The interaction of -adsorbed ethylene and ZnH species was concluded to yield adsorbed ethyl, which reacts with ZnOH to form the product ethane. Higher alkenes adsorb as allylic species. Active sites for hydrogenation and those for exchange and isomerization are independent of ZnO. As a result, the main product in the deuteration of alkenes is the d2 isotopomer. 

The two types of addition were proposed to occur through different surface absorbed forms240,243 (Scheme 6). The 24 -adsorbed intermediate is suggested to result in 1,2 addition (Type A metals). The 25 and 26 jr-allyl species, in contrast, ensure 1,4 addition. The selectivity of the formation of stereoisomeric 2-butenes, in turn, depends on the interconversion of the possible half hydrogenated syn and anti surface jr-allyl complexes... 

This, however, is still a simplified approach in that it does not take into consideration the formation of the primary metalalkyl nor the possible intermediacy of an adsorbed n-allyl species as occurs over palladium and, to some extent, nickel catalysts. ... 

The extent of double bond isomerization also varies with the nature of the catalyst. The degree of isomerization over metal catalysts usually decreases in the order Pd > Ni > Rh > Ru > Os =Ir =Pt.5.6 The extensive double bond isomerization observed with palladium and, to some extent, with nickel catalysts can be attributed to the formation of the adsorbed 7t-allyl species with these catalysts. While double bond isomerization may not be important in a routine alkene hydrogenation, it may influence a selective hydrogenation because the isomerized olefin can have different adsorption characteristics from those of the... 

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