Alumina, surface groups

A brief exposure of diacetate derivatives of aromatic aldehydes to MW irradiation on neutral alumina surface rapidly regenerates aldehydes (Scheme 6.5) [36], The selectivity in these deprotection reactions is achievable by merely adjusting the time of irradiation. As an example, for molecules bearing acetoxy functionality (R = OAc), the aldehyde diacetate is selectively removed in 30 s, whereas an extended period of 2 min is required to cleave both the diacetate and ester groups. The yields obtained are better than those possible by conventional heating methods and the procedure is applicable to compounds bearing olefmic moieties such as cinnamaldehyde diacetate [36],... 

A variety of alcohols, protected as t-butyldimethylsilyl (TBDMS) ether derivatives, can be rapidly regenerated to the corresponding hydroxy compounds on alumina surface using MW irradiation (Scheme 6.8) [42], This approach prevents the use of corrosive fluoride ions that are normally employed for cleaving the silyl protecting groups. 

In a volume otherwise heavily concerned with mechanistic questions, the editors have included a contribution (H. P. Boehm) from another sphere of investigative interest which struck us as having the quality of the imported spice to stimulate new interests within the accustomed sphere of catalytic gastronomy. It concerns the chemical nature of the surface groups on a series of materials (e.g. silica, alumina) which happen to be so frequently and universally used by the catalytic researcher. 

Recently, Scott, Brookhart, Goldman and coworkers have heterogenized (tB 4pcp)ij. and ( B " POCOP)lr derivatives by supporting them on 7-AI2O3. This was most successful with derivatives in which the pincer aryl group had a Lewis-basic para-substituent, e.g., NH2, or a group that could be hydrolyzed and thus react with the alumina surface, e.g., -C(0)(0Me) or -OP( Bu)2 [50]. 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

Figure 9.5 Surface organometallic chemistry of iron pentacarbonyl with alumina hydroxyl groups.

The model of Knozinger and Ratnasamy [60] is widely accepted as the most comprehensive way to rationalize the reactivity of the Al-OH groups on the alumina surface. This empirical model proposes that y-Al203 has a defective spinel structure, whose (111), (110) and (100) faces are covered by hydroxyl groups. Five configurations for the hydroxyl groups can be present (Scheme 11.3). 

Aluminas are amphoteric, hence, they possess acidic and basic properties that are controlled by the surface groups or ions which terminate the microcrystallites [91]. The acidic and basic properties of these materials can be modified by the heat treatment conditions and by incorporating additives, such as halogen or alkali. 

The nature of both the anchored species and of the deposit do strongly depend upon the concentration of - OH groups on the alumina surface. The early stage of the interaction between vapours of [Mo(CO)e] and the support was identified as physisorption when using y-alumina, in which almost all of the surface aluminium atoms are hydroxylated. Indeed, in this case physisorption can be ascribed to the interaction between an oxygen atom of a surface - OH group and the carbon atom of one CO ligand, which causes... 

Fig. 3 Surface-reactivity of the molybdenum hexacarbonyl complex as a function of the area density of - OH groups on an alumina surface...

The influence of the surface polarity of powders on their adsorption and dispersion properties can be profound, as is discussed in Sec. VIII,A. The values of F are likely to be put to many uses as more of them are measured. The electrostatic surface fields are doubtless involved in the phenomena of chemisorption and catalysis, capable of inducing polarization or electron shift of adsorbing molecules. For silica-alumina catalysts, the production of active M-O-M surface groups must be considered the most important factor responsible for chemisorption and catalj ic activity. 

The reaction of water sensitive compounds with alumina surfaces is complicated not only by the multiplicity of the surface hydroxyl groups but also by the Lewis acid centers which can give a strong physical adsorption of the compound without chemical reaction. 

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