Adsorbates butene

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 ... 

Figure 22 shows the spectrum in the OH region for zinc oxide after admission of butene-1 at a pressure of about 8 mm (14). Spectrum (a), taken after 8 min exposures, shows two features (1) the strong surface hydroxyl band at 3615 cm-1 is shifted about 5 cm-1 to lower frequencies (2) a new band appears at 3587 cm-1. This new band, clearly an OH, appears to arise from dissociation of the adsorbed butene. Spectrum (b) shows the same region after exposure to the gas phase for 1 hr. It is clear that the OH band formed from butene grows with time detailed studies, however, reveal that there is little change after the first 20 min. Spectrum (c) was taken after 20 min evacuation. Two features are evident (1) in the absence of the gas phase the hydroxyl band of the zinc oxide has shifted back to its previous position (2) the OH band formed from butene is reduced somewhat in intensity. Spectrum (d) was taken after degassing for 90 min ... 

These results are similar to those with propylene insofar as they indicate dissociative adsorption of the olefin. The hydrogen that yields the hydroxyl has not been identified but it seems reasonable to suppose that, once again, the allylic hydrogen is lost. Results with butene, however, do differ from those with propylene in two respects first, the dissociation (as evidenced by the OH band) is rapid but not instantaneous as found for propylene second, dissociatively adsorbed butene is more easily removed by room temperature evacuation than dissociatively adsorbed propylene. These facts suggests that steric effects are present hence, the kinetic behavior of these two species may be quite different. 

The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

While preadsorbed oxygen has no effect, the presence of gaseous oxygen drastically changes the product distribution in the thermal desorption and pulse reaction of butene on a-Fe203, as can be seen from results shown in Table VI (6). On this oxide, thermal desorption in an 02 instead of an He carrier results in a much lower yield of hydrocarbons and a much higher yield of C02, The same is observed in pulse reactions. Thus, on a-Fe203, adsorbed butene, adsorbed butadiene, and/or butadiene precursors must be very... 

The production of butadiene from butene involves at least three surface intermediates adsorbed butene, 7t-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on a-Fe203. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion... 

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. 

Thus, reaction intermediate r/// for dehydration of butyl alcohols can exist in two forms, i.e., butyl silyl ether (BSE) and adsorbed butyl carbenium ion (BC1). Our NMR and kinetic data imply the existence of reversible transformations between BSE, BCI, and adsorbed butene (BuadJ that ar shown in Scheme... 

The spectra of the adsorbed butenes, pentenes, and hexenes discussed above were obtained by chemisorbing on a hydrogen-covered surface at 35° C. The results show that some dehydrogenation must occur under these conditions, since it is impossible to get four-point adsorption without having some dissociation. When higher-molecular-weight olefins (or paraffins) are chemisorbed on a bare nickel surface, spectra similar to A of Fig. 3 are obtained, and no distinguishing characteristics are observed. 

Table II shows the results of butenes hydrogenation reactions. An interesting feature of the power function model for the hydrogenation reaction is the pressure dependencies obtained. As shown in Table II some experimental results indicated consistently that the hydrogenation rate is proportional to the 0.5 power of hydrogen and butenes partial pressure. This may imply, as discussed by Bond (I), that hydrogen is dissociated upon adsorption and that butenes are adsorbed weakly on a portion of active sites owing to the steric hindrance of adsorbing butenes molecules.

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