Hydroxyl on alumina

Similarly, for TiO whose spectrum consists of two band systems, the high-frequency one corresponds to basic, i.e. single-bonded, hydroxyl groups, while the other corresponds to more acidic bridged hydroxyls. Hie anionic character of the type I hydroxyls on alumina and titania is exemplified by the fact that they preferentially exchange with fluoride ions ... 

The features due to adsorbed water and carbonates observed on the boehmite and y-alumina deserve further attention as they differ from results published by previous investigators. Figure 4 shows a series of difference spectra for adsorption on y-alumina. Spectra were taken after drying the y-alumina at 350 C, cooling to room temperature and carrying out room temperature adsorption. The spectra are the difference of the sample before and after adsorption. Spectrum 4e is the spectrum for the as received alumina differenced with the dried alumina. The positive band at 3400 cm" is due to adsorbed water, and the small negative feature at 3740 cm" is due to isolated hydroxyls on the dried surface. Besides the three... 

The nature of hydroxyl species on alumina surface has been comprehensively reviewed also by Morterra and Magnacca [97] and by Lambert and Che [93]. Additional advances in understanding the intricate surface chemistry of y-Al203 have been made by Busca and coworkers [98,99] and Tsyganenko and Mardilovich [100]. 

The steric requirements of the surface during the formation of the adsorption complex or transition state also manifest themselves in the dehydration of rigid alcohols with fixed conformations, e.g. of cyclic alcohols. Cis- and trans-2- and 4-alkylcyclohexanols differ markedly in their rate of dehydration on alumina (see Table 5). Most significant are the data on 4-tert-butylcyclohexanols where the bulky ferf-butyl group is in an equatorial position, and thus the differences in the reactivity of the cis and trans isomers indicate the differences in the reactivity of axial and equatorial hydroxyls. The high reactivity of cis-2-terf-butylcyclohex-anol is caused most probably by steric acceleration of the elimination, which is, however, absent in the case of 2,2-dimethylcyclohexanol. 

The reaction mechanism of amine deamination and disproportionation has been put forward by analogy with other eliminations, namely dehydration and dehydrochlorination [149,155], its characteristic feature being the cooperation of acidic and basic sites. In the deamination, /3-hydrogen and the NR2 group (R is hydrogen or alkyl) are eliminated by an E2-like mechanism on alumina, but by an El-like mechanism on silica-alumina. The nature of the acidic sites is not clear, protons from surface hydroxyls or aluminium ions are possible candidates. However, it seems... 

TLC separation of DNS-amines is usually made on layers of silica gel with solvents covering a range of polarity, e.g., chloroform, ethyl acetate, diisopropyl ether and methanol. Seiler and Wiechmann [97] developed 30 solvent systems for the separation of DNS derivatives of over 100 biogenic amines on TLC plates of silica gel. The selective reaction of DNS-C1 with the amino group of catecholamines has been examined [98]. The drugs dopamine, norepinephrine and epinephrine are adsorbed on alumina which protects their hydroxyl groups from dansylation. The N-dansylated compounds are separated with benzene-dioxane-acetic acid (90 25 4) on layers of silica gel. 

Figure 7. Measured vibrational energies for hydroxyl and benzoate ions on alumina using (9) optical techniques and (O) IETS for three different counterelectrode metals. The energies measured using IETS are always lower than those measured optically, but the shifts are not large enough to interfere with identification of the modes or to indicate large perturbations of the surface species by the presence of the counterelectrode (19).

These results indicated that the hydrogenation reaction used hydrogen ( or deuterium) from the surface hydroxyl groups, that the number of hydroxyl groups available to the reaction depended on the temperature of the substrate, and that the reaction stopped when the available hydrogen was used up. The results of McBride and Hall emphasized the importance of surface hydroxyl groups in this catalytic reaction on alumina. 

As with platinum, the palladium-catalyzed oxidation of anomeric hydroxyl groups in aldoses is a rather selective process.84 The influence of pH in the Pd-catalyzed oxidation of glucose has been studied. It was observed that the gluconic acid formed, in its free form, reversibly inhibits the oxidation process in acidic media.85 The oxidation of D-glucose has been performed with palladium-on-alumina and with bismuth-containing palladium-on-charcoal in water.85 The selectivity in the air oxidation of... 

It must also be pointed out that other basic materials have been synthesized which present no surface oxygens and hydroxyls, but other types of active sites whose exact nature remains controversial. These type of solids are, for example, impregnated imides and nitrides on zeolites and alumina, amorphous oxynitrides obtained by treatment with ammonia or aluminium orthophosphate, zirconium phosphate, aluminium vanadate or galloaluminophosphate, and KF supported on alumina/1,3,41 One of the main advantages of these solids with respect to basic oxides is their resistance to carbon dioxide or water. 

This reaction certainly occurs for less noble metals such as Mo (150), Fe (151), or Ni (152) supported on alumina. In the case of noble or near-noble metals, such an oxidation seems less likely, but its feasibility could be increased by the involvement of very small metal aggregates. It should be mentioned that considerable gain in entropy due to the formation of gaseous H2 makes this reaction more feasible at very high temperatures. Tzou et al. (153) found that Pt atoms located in sodalite cages in Y zeolite are oxidized by the surface hydroxyls. 

IV sites. A protonated pyridine has never been observed, and the Lewis acid sites on titanium oxides cannot be converted into Br0nsted sites by water vapor adsorption (217). Although Jones and Hockey (216) suggest that the chemistry of surface hydroxyl on rutile corresponds more closely to that of the OH" ion rather than that of the hydroxyl group, no surface reactions similar to that observed with alumina [Eq. (14)] have since been reported. 

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