Allyl alcohols, adsorption

Figure 5. Comparison of acrolein desorption traces from the Cu2O(100) surface following (a) exposure to propene at 1 atm., (b) allyl alcohol adsorption in UHV and (c) acrolein adsorption in UHV.

Yu. I. Tarasevich, and M. M. Radul, 1967. Study of allyl alcohol adsorption on montmoril-... 

The adsorption of allyl alcohol thus readily gives rise to an allyl alcoholate species at room temperature. At higher temperatures this species first transforms to chemisorbed acrolein and then to an acrylate species. 

Co-adsorption experiments show a complex role of the nature and concentration of chemisorbed ammonia species. Ammonia is not only one of the reactants for the synthesis of acrylonitrile, but also reaction with Br()>nsted sites inhibits their reactivity. In particular, IR experiments show that two pathways of reaction are possible from chemisorbed propylene (i) to acetone via isopropoxylate intermediate or (ii) to acrolein via allyl alcoholate intermediate. The first reaction occurs preferentially at lower temperatures and in the presence of hydroxyl groups. When their reactivity is blocked by the faster reaction with ammonia, the second pathway of reaction becomes preferential. The first pathway of reaction is responsible for a degradative pathway, because acetone further transform to an acetate species with carbon chain breakage. Ammonia as NH4 reacts faster with acrylate species (formed by transformation of the acrolein intermediate) to give an acrylamide intermediate. At higher temperatures the amide may be transformed to acrylonitrile, but when Brreform ammonia and free, weakly bonded, acrylic acid. The latter easily decarboxylate forming carbon oxides. 

Natural compounds are also applied as chiral ligands in enantioselective homogeneous metallo-catalysts. A classical example is the Sharpless epoxidation of primary allylic alcohols with tert-butyl hydroperoxide [37]. Here the diethyl ester of natural (R,R)-(+)-tartaric acid (a by-product of wine manufacture) is used as bi-dentate ligand of the Ti(iv) center. The enantiomeric excess is >90%. The addition of zeolite KA or NaA is essential [38], bringing about adsorption of traces of water and - by cation exchange - some ionization of the hydroperoxide. 

It must be repeated that this argument depends upon the assumption that there is only one way in which the molecules of formic acid can be attached to the surface of the catalyst. There is, however, some evidence against this assumption. Constable finds that the two simultaneous reactions undergone by allyl alcohol when passed over heated copper are differently influenced by the physical state of the catalyst. This points to the conclusion that there are two independent centres of activity on the catalyst surface with two different modes of adsorption, or, at any rate, centres where the energy of adsorption is so different that different reactions are facilitated. Hoover and Rideal f find that the two alternative decompositions of ethyl alcohol by thoria show a different behaviour towards poisons, which points to the same conclusion. 

The formation of such multiply coordinated surface intermediates would be expected to be enhanced by adsorption of multi-functional reagents, e.g., oxygenates with hydrocarbon chains more reactive than saturated alkyl ligands. To test this hypothesis, we have also examined the adsorption and reaction of allyl alcohol (CH2=CH-CH20H) and acrolein (CH2=CH-CHO) on the Rh(lll) surface. While these molecules do exhibit evidence for interaction with the surface via both their oxygen and vinyl functions, and while they appear to preserve the divergence of decarbonylation pathways observed for their aliphatic counterparts, their reactivity patterns add yet another layer of complexity to the puzzle of oxygenate decarbonylation. 

Fig. 2. FT-IR spectra of (a) trans-2-butene reversibly adsorbed as such on MgFe204 (b) products of the reactive adsorption of trans-2-butene on MgFe204 at r.t. (c) products of the irreversible adsorption of but-3-en-2-ol (methyl-allyl alcohol) on MgFc204 at r.t..

The data reported above show that well characterized molecular adsorbed species of the three n-butene isomers are formed on the surface of MgFe204 n-butene oxy-dehydrogenation catalyst. Their vibrational perturbation indicates that a 7c-bonding occurs between the olefinic C=C double bond and Fe surface cationic centers. The results described above show that methyl-allyl alkoxides are also formed. Such species can also be produced by adsorption of but-3-en-2-ol (methyl-allyl alcohol) and can easily decompose to give butadiene gas. 

Carbonyl hydrogenation is generally less facile than olefin hydrogenation, making selective hydrogenation of a, -unsaturated aldehydes to the allyl alcohol a special challenge. Substitution of the carbon atom attached to the carbonyl (i. e. from the aldehyde to the ketone), substantially increases the steric hindrance to carbonyl adsorption, hence the lack of reports in the literature of selective unsaturated ke-... 

With purified resin, alcohol decreases the interphase tension, with the exception of OP-10 (allyl phenol oxyethylated ester) for which it produces some increase (by ImN/m) [12]. These findings can be explained by the fact that the alcohol facilitates desorption of the low-molecular weight resin fractions with surface-active properties from the boundary between the resin and the mercmy by increasing their compatibility with the bulk resin. The free energy advantage for alcohol adsorption on the mercury surface is less than that for low-molecular weight fractions, which is why it results in increase of the interphase tension. 

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