Adsorbed states of olefins

It is now generally accepted that the chemisorption of an olefin precedes its hydrogenation. Since any mechanism will depend upon the nature of the adsorbed state of the olefin, it is pertinent to examine the 

It is well established that when ethylene is admitted to a freshly prepared evaporated metal film, self-hydrogenation resulting in the rapid production of ethane is observed [50—52]. A similar phenomenon is observed when ethylene is adsorbed on supported metal catalysts [49,53] (see Fig. 5). These observations have been interpreted as indicating that ethylene is first chemisorbed dissociatively, viz. 

The hydrogen atoms thus liberated to the surface may then react with either gaseous ethylene [50,51], or associatively adsorbed ethylene [53] or with the surface C2H4. complex [52]. Volumetric [52,54] and magnetic susceptibility measurements [55] suggest that the extent of dissociation is dependent upon the temperature and varies from metal to metal. 

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process 

Further increase in temperature results in carbon—carbon bond fission and the formation of a surface carbide containing single carbon units. 

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For information on the adsorbed state of olefins during their hydrogenation, we are driven to rely almost entirely on the logical interpretation of the kinetics and product distributions observed. We shall, in fact, carry out our discussion in terms of two basic kinds of adsorbed olefin, namely (1) <7-diadsorbed olefin and (2) 7r-adsorbed olefin. A o--diadsorbed olefin is supposed to be formed by rehybridization of the carbon atoms of the olefinic bond to sp hybridization followed by the... 

In Section III, A, 2, thermodynamic and mechanistic factors were said to determine selectivity. The effectiveness of the thermodynamic factor depends partly upon the strength of ethylene adsorption. Heats of adsorption of ethylene on supported metals are not available, but it is known, however, (see Section IV, B, la) that the stability of ethylene-metal and other olefin-metal complexes increases with increasing atomic number down each triad of Group VIII. If it is assumed that the normal adsorbed state of olefin is that of a 7r-complex with the surface, it would... 

The existence of several adsorbed states of an olefin on metal surfaces is shown by infrared spectroscopic studies [68]. This technique has the advantage that it yields direct information regarding the chemical identity of the various adsorbed species, although there are limitations to its use. One of the main limitations is that the presence of surface intermediates may not be revealed if the appropriate band intensities are too weak [69]. In this context, it has been suggested [70] that the C—H bands associated with carbon atoms which are multiply bonded to the surface are too weak to be observed. Pearce and Sheppard [71] have also proposed the operation of an optical selection rule, similar to that found with bulk metals [72], in determining the bands observed with adsorbed species on supported metal catalysts. In spite of these limitations, however, the infrared approach has contributed significantly to the understanding of the nature and reactivity of adsorbed hydrocarbons. 

In the ensuing discussion, it will be assumed that structure L is the relevant species in alkyne hydrogenation, and that the catalytically active adsorbed state of an alkadiene can be represented as a jr-olefin complex in which either one or both olefinic bonds interact with the surface. 

The hydrogenations of dialkylacetylenes are of particular interest from a stereochemical viewpoint since, as noted in Sect. 4.3, the adsorbed state of the acetylene is expected to adopt a c/s-configuration and, consequently, upon hydrogenation to yield the cis-olefin. Wide use of this fact has been made in preparative organic chemistry as noted by Burwell [192] and by Campbell and Campbell [193]. Although early studies of the catalytic hydrogenation of disubstituted acetylenes [194—196] revealed the formation of trans- as well as c/s-olefins, it was generally assumed that the trans-isomer was formed by isomerisation of the cis-ole-fin. However, more recent studies have shown that this view may have... 

In developing the mechanism, it was assumed that the adsorbed state of buta-1 2-diene active in hydrogenation was the di-7r-adsorbed species C in Fig. 32. The close similarity in the ZV-profiles for each butene formed in a given reaction suggests that each was formed as a primary product and the general mechanism shown in Fig. 33 was proposed. The difference in behaviour of the type A and B catalysts was explained by proposing that, at the type A surface, a-bonded and 7r-olefinic species are of importance as... 

Traces of octalins were always present during the hydrogenation of tetralin or naphthalene (3). Observed concentrations of octalins are listed in Table IV. The amounts were particularly small with palladium catalysts. One of the unique characteristics of palladium is its ability to adsorb and saturate olefins in the presence of aromatics or, conversely, its relative inability to adsorb and saturate aromatics in the presence of olefins. By way of contrast, some other metals, particularly ruthenium and rhodium, are more able to adsorb and saturate aromatics in the presence of olefins. Whatever the nature of the adsorbed state of naphthalene that leads to hydrogenation, one could imagine the possibility of two isomeric forms—one of which behaved more like an adsorbed olefin and the other more like an adsorbed aromatic ... 

It is now firmly believed that the hydrogenation of aromatic substances has certain steps in common with the hydrogenation of cyclic olefins 16, 23), and hence a word concerning the adsorbed state of the aromatic ring and of possible partially hydrogenated intermediates is in... 

Similarly, the adsorbed state of 1,2-dienes may be represented in two ways, as shown in Structures (IV) and (V). Since the two 7r-electron systems in a 1,2-diene are mutally at right angles it follows that the two metal-olefin bonds in Structure (V) must also be at right angles to each other. The fission of one olefinic linkage and the subsequent... 

Reaction energy scheme of isomerization of an olefin by zeolitic proton (kJ mol ). The zr-adsorbed state of the olefin has not been... 

Apparently the geometry of the transition state for adsorption is approximately that of a ir-complexed olefin in that its structure seems to be only slightly distorted from that of the isolated alkene. However, this does not necessarily mean that the adsorbed state which is formed in the elementary reaction to which the stereochemistry refers is a tt complex, because the same geometry also represents a stage in the progression of olefin to the eclipsed 1,2-diadsorbed alkane. Hopefully other experi-... 

In contrast to olefin-forming dehydrations, the transformation of alcohols to ethers very probably includes surface alkoxides as intermediates. It is assumed that one molecule of the alcohol forms the alkoxide which is then attacked by the second alcohol molecule either from the gas phase or from a weakly adsorbed state. Again, cooperation of acidic and basic sites seems to be necessary [116,142,143]. The important step of ether forma-... 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

The "liquid state" in-situ NMR studies made here probe the reactions of physisorbed, and possibly in part chemisorbed, species. NMR studies of olefins adsorbed on acidic catalysts have difficulty evidencing the olefinic nature of the initial chemisorbed species [2,6,7]. This is probably because they are short-lived (e g. carbenium ions) or have poor spectroscopic properties (e g. broad lines). These problems are not resolved by magic angle spinning and solid state methods. 

Olefins.—Experimental results have been explained by assuming the existence of a number of chemisorbed states from complete dissociation through bound methylene and vinyl groups to 77-complexes. The wealth of material cannot be detailed here. In summary it can be seen that for all adsorptives the adsorbate may take a number of forms and each may occur over a range of adsorption energies. The possibilities for the spillover of dissociated species are more readily visualized than for the associated types. It must be supposed that for the switch to take place the valency state of the adsorbate on the good adsorber (as found by photoelectron microscopy, for instance) should be particularly appropriate. 

A short comment is necessary on the formation of an alkyl radical from a cyclic olefin. If it is adsorbed as Structure (A) (u-diadsorbed species), it has already assumed the configuration of the product unless some unusual processes follow 16). If, however, it is adsorbed as Structure (B) (17-016(10), the structure of the product is not assumed until it becomes an alkyl radical, for the configuration-determining step is that in which the sp -hybridized carbon atoms change their state of hybridization to that of the final product. 

A second source of evidence of the adsorbed state lies in the manner in which adsorbed 1,3-dienes react. This will be discussed fully in Section III, F, 6 it is sufficient for present purposes to state that, at the surfaces of most metals, one olefinic linkage appears to hydrogenate independently of the other to give adsorbed 1-butene. 1,2-Addition is thus generally preferred to 1,4-addition. The olefinic linkages retain their identity to a larger degree in Structure (II) than in Structure (I) and thus preferential 1,2-addition is more easily understood if the second structure is accepted. On the other hand, 1,4-addition would be expected to be at least half as important as 1,2-addition if Structure (I) was correct. It appears, therefore, that Structure (II) represents a preferable notation, based on the evidence at present available. 

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