Alumina dehydroxylation

In the case of alumina, the surface is much more complex because A1 atoms have different coordination numbers (Al(iv), Al(v), and Al(vi)), and the hydroxyl can be bridged with a different number and type of A1 centers /r -, /r -OH), so that their acidity and reactivity can be very different. Additionally, the temperature of dehydroxylation can change the crystalline phase of this oxide, changing the proportion of each of the types of A1 atoms, Al(rv) and Al(vi) atoms. Typically, a 7-alumina dehydroxylated at 500 °C contains around dOHnm. ... 

In the case of silica-alumina, dehydroxylated at temperatures >300 °C, the surface is mainly composed of isolated silanols, which can be close to A1 Lewis centers. Other supports such as magnesia or titanium not only contain hydroxyl groups, but also Lewis acid and basic centers. 

Figure 4. Transition alumina dehydroxylation (taken from Ref [15] under permission).

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

Alumina is known to have more ionic character and its surface has a more complex structure than that of silica. Reaction of Bu3SnH with the surface of partially dehydroxylated aluminas was followed and it was found that the extreme sensitivity of tin chemical shifts to the molecular environment constitutes a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides.248... 

Cp CpZrMe2]/Al203 (5oo) (216 h-1) [185]. Typically, highly dehydroxylated alumina (Al203.(iooo)) is a better support, and it has been associated with the formation of cationic surface species. 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

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... 

Scheme 1.5 Silica, alumina and titania surface oxygens behaving as ligands in the M.L.H. Green formalism [9] after reaction of r -tris(allyl)rhodium with a partially dehydroxylated surface [39].

The use of highly dehydroxylated AI2O3 and zerovalent M(CO)6 (M = Cr, Mo, W) led to highly dispersed low-valent, surface-subcarbonyl catalysts, in which the oxidation of M by the hydroxyl groups of alumina was prevented [19, 20] ... 

On an alumina support, independently of the cobalt carbonyl precursor used, complex cobalt sub-carbonyls compounds, [Co(CO)4] and hydrogencarbonate species formed [143, 149]. However, the reactivity of the alumina surface depends on the degree of hydroxylation highly hydroxylated alumina is more reactive against Co2(CO)g and facilitates decarbonylation, whereas dehydroxylated alumina favors the formation of high nuclearity species like [Co6(CO),5] , which would need higher temperatures than the initial Co2(CO)8 to be decarbonylated [149]. 

Furthermore, the removal of these groups by thermal treatment (dehydroxyl-ation) yields coordinatively unsaturated oxygens and anions in which coordina-tively unsaturated aluminum atoms are exposed (Lewis acid sites). In general, the total concentration of OH groups on a alumina support ranges between 10 and... 

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

Importantly, under CVD conditions the temperature can be sufficiently high to induce dehydroxylation of the alumina surface, so that the mechanism could follow reaction routes other than the one observed when GPID is carried out. This phenomenon can be even more pronounced when experiments are performed under dynamic UHV. 

Interaction of [Mo(CO)e] with Partially and Fully Dehydroxylated Alumina... 

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