Dehydroxylation, homogeneous

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

We first studied group 4 metals (titanium, zirconium and hafnium) supported on a silica dehydroxylated especially at 700 °C (Table 3.8). Following the laboratory-developed strategy, surface-species have been well-characterized by classical techniques (IR, solid-state NMR gas evolvement, reactivity, etc.). Catalysis results show that titanium is the most active even if its activity is far less than that of homogeneous catalysts. In addition, an important amount of metal was lost by lixiviation even if this phenomenon seemed to stop after a certain time. 

Zero-valent metal complexes provide important advantages as precursors to otherwise hardly accessible reactive metal ensembles on dehydrated/ dehydroxylated catalyst supports. Such precursors owe their advantage to the preparation of zero-valent or low-valent metal aggregates with homogeneous (or near-homogeneous) particle size distributions on their initial formation. 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations, (ii) the homogeneous mixture of the cations on an atomic scale, and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group. In the first decomposition step, water is removed from the structure. This transition is followed by dehydroxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co,Al)204). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

Taylor, H. F. W., 1962. Homogeneous and inhomogeneous mechanisms in the dehydroxylation of minerals. Clay Min. Bull. 5 45. 

Interestingly, homoleptic trisamides grafted on partially dehydroxylated meso-porous zeolites exhibited activities higher than that of the trisamides in homogeneous solution. The activity decreases in the row Y > La > Nd and is also dependent on the pore size and particle morphology [105]. 

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