Surface carboxylates, formation

Aldehydes and alcohols can be oxidized to their corresponding carboxylic acids on certain oxide surfaces. Carboxylate formation can also take place by the breakdown of certain adsorbates, such as an ester to carboxylate and alkoxy, or by hydride transfer from one adsorbate to another, as takes place in the Cannizzaro disproportionation reaction of aldehydes to alkoxy and carboxylate. The oxidation reactions are less likely on hydroxylated surfaces, because the surface is less nucleophilic. Hydrogenation of an acid to an aldehyde can take place on oxides of intermediate M-O bond strength at temperatures of 623-723 K, and at lower temperatures on the Group VIII metals with low M-O bond strength. Obviously, once the carboxylates are formed, condensation to ketones could occur according to one of the mechanisms given above. 

Organic material can strongly adsorb metal ions the functional groups on their surfaces act as ligands (carboxyl, amino groups etc.) for metal ions. All these functional groups favor the surface complex formation with metals the adsorption reactions are favored at higher pH (Fig. 11.11). 

These reactions lead to the formation of surface carboxylate and carbonatelike species and to electrons that can reduce transition metal ions located in... 

Ketones and nitriles are rather soft bases their coordination onto electron-deficient sites on oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 245-247), titanium dioxide (194), and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH ion may, therefore, be envisaged as an alternative or competing reaction path ... 

In the case of isobutanol dehydration, a promotional effect is observed (47). Isobutanol forms a surface carboxylate under reaction conditions (340), and this surface species gives rise to a typical symmetric COO -stretching vibration at 1567 cm 1. The CH-stretching vibration of the methylene group of isobutanol at 2870 cm-1 disappears on formation of the oxidized species. Consequently, the intensity of the 1567-cm 1 band can be taken as a measure of the surface concentration of the carboxylate species, whereas the intensity of the 2870-cm 1 band represents the surface concentration of molecular alcohol. The concentra-... 

One important chemical result from the ESCA studies involves the formation of surface carboxylates. Upon transfer of the monolayer to silver, the formation of a carboxylate could be detected by the production of a single peak in the oxygen Is line due to equivalent oxygens in the carboxylate structure. Since the oxygen spectrum of the monolayer yields near stoichiometric C-0/C==0 ratio of 1, the carboxylate is not detectable. Studies with monolayer films of cadmium arachidate indicated detectability of the carboxylate as a single carbon Is peak. The lack of the formation of this interfacial carboxylate indicates the strength (or weakness) of surface chemistry which occurs on this silver surface as prepared. 

The population of surface defects and coordination vacancies drives alkoxide and carboxylate formation and decomposition. When cations have at least two coordination vacancies, bimolecular reactions are possible (e.g., acetone from acetate ions, dimethylether from methoxy groups). 

As mentioned in Section IV.A.2, Leng and Pinto [3861 specifically addressed the effect of surface properties on the oxic and anoxic adsorption behavior of phenol, benzoic acid, and o-cresol. Commercial carbons were oxidized in air at 350°C, which is known [37] to introduce both CO- and C02-yielding surface groups nevertheless, from FTIR spectra, they concluded that the main differences are due to relative quantities of surface carboxylic groups. Presumably because the experiments were performed at pFl = 7.0, i.e., below the pK, of phenol, their explanation for the decrease in uptake with increasing surface oxygen was not the electrostatic repulsion but increased water cluster formation as well as increased removal of n electrons from the basal planes [450,674], which results in weaker dispersion interactions with phenol. ... 

Although attempts have been made to observe surface intermediates directly by infrared spectroscopy, only surface carboxylate species have been identified. These were assumed to be intermediates in CO2 formation. 

More recently, Terzyk [32] also suggested that the irreversibility of phenol adsorption is due to the creation of strong complexes between phenol and surface carbonyl and lactone groups and to phenol polymerization. Salame and Bandosz [33] studied phenol adsorption at 30 and 60°C on oxidized and nonoxidized activated carbons. They concluded, from analyses of the isotherms by the FreundUch equation and the surface acidity of the carbons, that phenol was physisorbed by tt—tt dispersion interactions, whereas it was chemisorbed via ester formation between the OH group of phenol and surface carboxyl groups. 

Aldehyde is formed by reaction of the surface carboxylate and dissociated hydrogen atoms (Figure 2). On the other hand, acid is formed by the reaction of the surface carboxylate and water vapor, with the consequent formation of hydrogen this is the reverse of the reaction of aldehyde formation. The activation mechanism of molecular hydrogen is not yet clear, however. Kondo et al. reported that the main role of the Cr is considered to be related to the activation of hydrogen... 

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