Alcohols, unsaturated adsorption

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Fig. 14. 8. Proposed adsorption modes for unsaturated alcohols on nickel and palladium catalysts.

Similar results were reported (77), where it was found that the adsorption coefficients of various unsaturated compounds related to 1-hexene may be correlated using Eq. (23) only in the case of unsaturated alcohols, and hence in systems similar to the one chosen by definition for the determination of 0. 

It was found (70) that the character of functional groups strongly affected the adsorptivity of substrates in various solvents. The effect of solvents on the relative adsorptivity of unsaturated compounds of a similar type (cyclohexene-1-hexene or 2-methyl-3-butene-2-ol-2-propene-l-ol) was less pronounced than in systems of compounds more differing in their structure (pair olefin-unsaturated alcohol). 

The most pronounced change in relative adsorptivity with a change in solvent was observed (77) for the pair of substrates unsaturated alcohol-unsaturated hydrocarbon. The sorption of olefins was preferred in polar solvents (low values of the parameter ), while in nonpolar solvents (high values of the same parameter) unsaturated alcohols were preferably sorbed. An explanation may be seen in an interaction between the solvent and the... 

The results obtained in the hydrogenation of l-hexene-3-ol and 1-hexene-4-01 were in accord with the conclusions in Cerveny et al. (71) concerning the effect of structure of unsaturated alcohols on their reactivity and adsorptivity. In most of the seven solvents used, l-hexene-3-ol was hydrogenated more rapidly, but differences in the reaction rates were unpronounced, similarly to differences in the adsorptivity. 

Of the cases described so far, the presence of a third compound had the strongest effect on the selectivity of hydrogenation of two olefinic substrates in the pair olefin-unsaturated alcohol. This influence appeared both in cases where the third compound was unsaturated, was adsorbed competitively and reacted on the catalyst surface, and in cases where the third compound was represented by an inert solvent not undergoing competitive adsorption (was not entering equations of the Langmuir-Hinshelwood type, which were degraded to pseudo-zero order) and obviously operated through interactions of molecules from the bulk phase with adsorbed molecules of the substrate. 

The influence of reactant pressures upon selectivity can be easily explained by the above mechanism the changes of selectivity essentially concern the complementary formations of unsaturated alcohol and hydrocarbons which i the re uk of competitive 1,2 and 1,4 additions of hydrogen. An increase of the partial pressure of prenal on the initially saturated surface, is likely to induce a change of the adsorption configuration of the molecule in favor of a 1,2 di-o adsorption and consequently of a selective hydrogenation of the C=0 bond. 

In a previous study on the selective hydrogenation of aliphatic a,fi imsaturated aldehydes (R,R2C=CR3-CR4=0) to the unsaturated alcohols [6] we foimd that the main problem is how to suppress adsorption on the C=C bond and simultaneously enhance that on C=0 bond. A literature search and the results of our earlier study revealed that very little can be won by varying the metal, but that the use of a proper promoter can enhance the selectivity considerably. 

In this work we have shown that Ziegler type cobalt catalysts are rather selective for the hydrogenation of unsaturated carbonyl compounds into unsaturated alcohols. Nevertheless the selectivity was more important with cinnamaldehyde than with other aldehydes, which showed the eflfect of the aromatic cycle on the stabilization and the adsorption of the conjugated double bond. 

It should also be mentioned that in addition to the presence of Lewis-acid type surface entities there is another requirement for the preparation of selective catalysts used in the hydrogenation of a,p-unsaturated aldehydes. In this reaction both the adsorption of the substrate via its olefinic double bond and the re-adsorption of the formed crotylalcohol should be suppressed in order to increase the selectivity for the formation of the unsaturated alcohol. 

Sn-Pt/Si02 catalysts below monolayer coverage show only a slight increase in the selectivity of unsaturated alcohol, however the activity of these catalysts is much higher than that of the parent Pt/Si02 catalyst. These results already show the positive effect of tin in carbonyl activation. However, due to the relatively low tin content of these catalysts, the adsorption of the substrate molecule by its olefinic double bond is still possible, consequently the formation of butiral-dehyde is very pronounced. 

Transition States in Acid-Cataiyzed Reactions on Zeolites. A considerable number of experimental attempts were carried out to elucidate the structure of intermediates in the acid-catalyzed reactions. This information may be used to deduce the nature of elusive transition states, which are present on the catalytic surface in concentrations far below detection limit. For example, adsorption of alcohols and unsaturated hydrocarbons on acidic zeolites gives rise to the MAS NMR signal attributed to alkoxy species (135). Transition of these surface complexes to the corresponding carbenium ions, an important step in their further transformations, was shown to have low activation energy. In the synthesis of methyl-tetrabuthyl ether on zeolite Beta, features due to secondary and tertiary carbon atoms in alkoxy species have been observed. This observation along... 

Unavailable because experimental methods for estimation of this parameter for unsaturated alicyclic alcohols are lacking in the documented literature. However, its high solubility in water suggests its adsorption to soil will be nominal (Lyman et al., 1982). 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

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