Ionically-bonded catalyst

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29],... 

Organisms deal with this situation by speeding up reactions with catalysts called enzymes. A catalyst affects the rate of a reaction but does not otherwise participate, so it is not chemically altered. Enzymes are usually proteins that temporarily bind the reactants in such a way as to bring them together in the correct position. This binding is not done with strong bonds such as covalent or ionic bonds, but with weaker attractions that are more easily broken. An enzyme usually catalyzes only one specific reaction since its shape and composition are generally such that it binds only a specific set of reactants. 

The immobilization by ionic bonding on Al-MCM-41 was carried out similarly to the immobilization of rhodium-diphosphine complexes as described above. This catalyst was named MCMIHC. For immobihzation via the metal center and covalent bonding of salen, the all-silica MCM-41 was modified with (3-aminopro pyl)triethoxysilane (APTES) [52]. The catalyst obtained by the metal center immobilization was denoted MCM2HC [53], whereas the material obtained by covalent bonding of the salen ligand was named MCM3HC [54]. Detailed procedures are described extensively elsewhere [55]. 

An example of a noncovalent attachment of a metal-phosphine complex to a solid support is presented in Figure 31, as reported by Bianchini et al. (120). The complex is attached via a sulfonated variant of the "triphos" ligand, which is known for its successful application in several catalytic reactions. The ligand is attached to the silica by an ionic bond, which is stable in the absence of water. The catalyst was used for the hydroformylation of styrene and of hex-1-ene in batch mode and showed moderate activity. The triply coordinated rhodium atom is strongly boimd although the conditions were rather harsh (120 °C, 30 bar) the concentration of leached metal measured by atomic emission spectroscopy was at most at the parts per million level. However, for commercial applications, for example, in a process such as hydroformylation of bulk products, these concentrations should be less than 10 ppb 111,121). 

The second theme comes from examining the properties of zeolites. Here, the three dimensional nature of these catalysts has long been exploited by the petroleum industry, mostly because they function as molecular sieves. Zeolites, like all of the mineral silicates of which they are a special class, are constructed with highly ionic bonds and this introduces two severe limitations. First, the number of possible structures made out of ionic bonds is much less diverse than those possible from covalent bonding, and... 

The linear/branched ratio was 7.5 1, which was greater than that found with the control catalyst in solution. Rhodium(III) chloride has been immobilized on a support made by polymerization of vinylpyridine and divinylben-zene in the presence of silica. The best activity for the conversion of methanol to acetic acid by carbon monoxide was obtained after 20% of the pyridine groups were quaternized with methyl iodide. This suggests ionic bonding of a tetra-halorhodate ion to the polymer.211... 

Two basic mechanisms of fixation of a homogeneous catalyst onto a polymeric support have been cited. The first involves different means of catalyst component precipitation, impregnation, inclusion in gel, microencapsulation, etc. The second concerns fixation of catalytic sites by valence, coordination and ionic bonds, immobilization, ion exchange, etc. 

Oxidation catalysts often have a large proportion of ionic bonding, mostly with simple a bonding of hard ligands (H2O, ROH, RNH2, OH, COO ) to the metal ion. An example is the selective epoxidation of olefins with organic hydroperoxides (Eq. 2-86). The key step of this process is the nondissociative coordination of the hydroperoxide molecule by a hard-hard interaction of the type ... 

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