Adsorption two-point

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

The work of Herington and Rideal (H4) showed that a good prediction of the relative rates of cyclization of various paraffins could be made based on considerations of the number of ways in which ring closure could be effected, assuming that cyclization involves two-point adsorption in the manner illustrated in Fig. 8. For example, in the dehydro-cyclization of re-hexane, five two-point contacts with the surface are possible, but only two can lead to formation of a six-membered ring. By assuming the cyclization rate constant to be given by... 

The linearized Eq. 30 is used to verify the Langmuir adsorption mechanism. The constant /3 can be estimated from the slope of regression lines. For a two-point adsorption, when a molecule adsorbs after a former dissociation at the interface, Eq. 30 gets the form... 

Within the framework of the clusters proposed previously, one can visualize three types of water molecule adsorption, as shown in Fig. 11. In Mikheiklin et al. (87) we examined types I and III adsorption for the case of a cluster that does not contain pseudo-atoms B. It was found that the energy of two-point adsorption according to type III actually decreased sharply (from 25 to 9.4 kcal/mol) if the cluster contained the nonplanar fragment... 

Senchenya et al. (96) have treated the adsorption of ethanol on a structural hydroxyl group (Fig. 14) using a CTP scheme and the CNDO/BW method. The separation of a molecule and cluster with respect to the z axis was optimized, the optimal values being r = 1.19 A and R = 1.28 A The adsorption energy was 23.2 kcal/mol, which was close to the experimental value (97). Note that this was essentially the two-point adsorption involving both acid and base sites. This case is quite similar to the above propylene adsorption (90). There is also no definite trend toward proton transfer from the hydroxyl group of a zeolite to the alcohol molecule. The carbocation state is also predicted to be activated. This, in turn, increases relative efficiency of the synchronous mechanism (with the same recommendation for its experimental examination). The estimation (96) of the energetics of the intermediate structures of the synchronous mechanism showed that such a mechanism is quite realistic. 

The multiplot theory deals with a two-point adsorption of molecules which in the extreme case of deformation of bonds brings about their dissociation. The activated adsorption discovered by Taylor 26), which has played such an important part in the development of the theory of catalysis, follows from the multiple theory. Such strong adsorption, however, is not always useful for catalysis (see Section I,C). 

While hydrogenating compounds with a triple bond on Ft and other metals cis-olefins are formed in an overwhelming quantity over trans-olefins. This happens in spite of the fact that the /raws-olefins are more stable thermodynamically. The reason for such behavior of substituted acetylenes, according to the multiplet theory, is that the surface in the case of the two-point adsorption of a triple bond hinders the substituents and makes both of them move off the surface. For the same reason the cis forms of olefins hydrogenate more easily than the trans forms. This phenomenon is well known in catalytic synthesis. 

Many years before reliable information was available concerning the actual structure of any cataij st, attempts were made to consider the disposition of a reacting molecule on the active surface. So long as these conjectures were confined to reactants which were adsorbed by single atoms of the catalyst surface, little progress could be made by such hypotheses. Burk (1), however, studied the implications of two-point adsorption of diatomic molecules and came to the conclusion that some of the known facts concerning catalyst poisons could be accounted for by the assumption that this mechanism was involved. 

Although the original form of this theory has been seriously criticized for several reasons (3), it provided a stimulus to further interest in problems of this type. Subsequent study of the relation between the dimensions of organic molecules, and of the catalysts which are efToctive in their conversions, has proved most interesting and has established the fact that two-point adsorption frequently occurs. It is with reactions of this type that this article is concerned in particular, the importance of the geometrical factor in defining the properties of a catalytic surface is considered. 

These results would be expected from two-point adsorption of the olefin. Thus, with propene the following changes could occur ... 

From the discussion above, it may be concluded that for reactions requiring two-point adsorption there should be a direct relation between the number of pairs of atoms in the catalyst surface at the correct spacing, and the catalytic activity. Confirmation of this effect has been provided by the work of Beeck, Smith, and Wheeler (9) on the hydrogenation of ethylene. As catalysts, they used films of nickel deposited by evaporation... 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

Each plane of molybdenum atoms is held by covalent bonds between two planes of sulfur atoms part of the structure is shown in Fig. 8. The important feature of this lattice is that the molybdenum atoms are arranged triangularly 3.15A. apart. Such planes could readily be exposed in a crystalline M0S2 catalyst from which one layer of outer sulfur atoms had been removed by reaction with hydrogen. On this lattice, with the Mo—C bond 2.14A. it can be seen that two-point adsorption of an olefin would take place with an angle of 112° for the Mo—C—C bonds. This fit is not... 

What has already been said about the disposition of olefins on MoSa can obviously be applied equally to the thiophene molecule, assuming that one double bond is opened by two-point adsorption. This argument, which can explain the known activity of MoSa for this conversion, would also indicate a similar activity for NiS, since this sulfide consists of a hexagonal lattice with alternate layers of nickel and sulfur atoms in which the shortest Ni—Ni distance is 2.65A., and which would, therefore, be suitable for two-point adsorption of thiophene. 

In many instances, as, for example, in the production of ethylene oxide by oxidation of ethylene with silver catalysts (Twigg, 39), the reaction takes place by collision with chemisorbed oxygen. It is, however, quite likely that instances will be found where the mechanism is entirely different and where two-point adsorption takes place. 

Two point adsorption at the double bond. Twigg and Rideal s (33 a) considerations suggest that on the 100 plane neighbor interactions might just be large enough to result in 1 in 4 occupation. 

It seems likely that other physical data, such as contact potentials, will be required to decide the mode of adsorption in cases like ethylene. As we shall see, data on the catalyzed exchange reactions of ethylene are most easily interpreted on the basis of two-point adsorption at the double bond. This is indeed what we should expect from the reactive character of the mobile t electrons. 

Since two-point adsorption is no longer possible if the metal-metal distance is too large, the optimal catalyst for ethylene hydrogenation should have a certain medium interatomic distance. This is the case for rhodium with 0.375 nm, but since energetic aspects (adsorption enthalpies) must also be taken into accoimt, it can not be said that this is solely the result of steric effects. 

FIGURE 8.6 Illustrative representation of adsorption patterns for diastereoisomeric compounds of type I. The full circle denotes the strongest adsorbing groups of the molecule. A = active site comprising the different types of the adsorbent surface hydroxyl groups, (a) Two-point adsorption with X and Y Rp(erythrv) > (b) one-point adsorp-... 

The trans-cis retention sequence depends on the number of adsorbed groups. The conformational factors are more favorable for the /ran -isomer under one-point adsorption and for the cM-isomer under two-point adsorption. The established relations of Rp(trans) > f F(cij) and / f(cu) > EF(trans) have practical importance if the adsorption of the groups X and Y can be reliably predicted for other newly synthesized compounds of the type II without intramolecular hydrogen bonds or with bonds of the types OH... N or OH... O. 

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