Accessible metal sites

Noble metals are preferred when non chlorinated VOCs have to be destroyed. For reasons of accessibility, FAU type zeolites are the most frequently employed catalysts and a parameter of utmost importance is the number of accessible metal sites and the reductibility. This was recently put in evidence for the o-xylene oxidation on Pd-FAU catalysts (19). Pd° was proposed as an active site, and the specific activity increased with the ratio (nPd)/nacid. A lower acidity (high Si/Al ratio) favors the Pd2 and PdO reductibility, and decreases the extent of coke formation. 

Figure 3.13 The crystal structure of [Tb2(BDC)3-(H20)4] shown approximately down the crystallographic b-axis, where aqua ligands are found to point toward the center of the ID-channels (Tb, black O, grey C, white H, omitted). (Redrawn from the GIF file of T.M. Reineke et al, From condensed lanthanide coordination solids to microporous frameworks having accessible metal sites, Journal of the American Chemical Society, 121, 1651-1657, 1999 [72].)...

The kinetics is an important tool to investigate the nature of interactions between accessible metal sites and adsorbed molecules, providing useful informations on the type of mechanism as well as on the role of the specific active phases. The following mechanism has... 

On the other hand, lead poisoning deactivation curves have non-selective characteristics (Type A). These result when the poison is less strongly chemisorbed, and it tends to suffer many collisions with the alumina washcoat structure before chemisorption. Consequently, lead is found deep inside the washcoat structure, as is demonstrated by electron probe microanalysis, and the more accessible metal sites are left active to gaseous reactants by the faster bulk transport processes. 

However, the activity of these metal-loaded pillared clays is more than 10 times lower that for carbon-supported catalysts (15) for both unsaturated nitriles. This loss of activity could be explained in two ways the first supposes a lower accessibility of the metallic surface in the case of the pillared clays catalyst as compared with carbon based catalysts. We may suppose some strong diffusional effect. This would indicate that, assuming a good repartition of the metal into the layers of the pillared clay, the most active accessible metallic sites would just be those near the outer edge of the clay. 

Metal-assisted enantioselective catalytic reactions are one of the most important areas in organic chemistry [1-3]. They require the appropriate design and the preparation of chiral transition metal complexes, a field also of major importance in modern synthetic chemistry. These complexes are selected on both their ability to catalyze a given reaction and their potential as asymmetric inducers. To fulfill the first function, it is absolutely required that the catalysts display accessible metal coordination sites where reactants can bind since activation would result from a direct interaction between the metal ion... 

The nature of such sites seems consistent with the behavior shown In the pyridine and lutldlne poisoning experiments. The acidic nature of the reduced metal sites which hold the nitrogen bases seems established. Difference In the extent to which the exposed metal cation Is accessible to the nitrogen atom of the organic nitrogen base could explain the selective poisoning seen with different substituted pyrldlnes. Presumably the cations In an active pair are somewhat less accessible than most exposed Co and Mo cations, which, because they normally hold two MO molecules, are probably exposed In Incomplete tetrahedral sites. 

Bimetallic activation of acetyl and alkoxyacetyl ligands — through formation of cationic P2 acyl complexes — to reaction with nucleophilic hydride donors was established. Cationic transition metal compounds possessing an accessible coordination site bind a neutral T -acyl ligand on another complex as a cationic P2 acyl system. These i2 3icyl systems activate the acyl ligand to reduction analogous to carbocation activation. Several examples of i2-acyl complexation have been reported previously. 

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

The absence of a second cyclopentadienyl ring coupled with the short bridge gives a very open environment at the metal site. This allows easier access for bulky monomers, including 1-alkenes and norbomene, compared to polymerization with metallocenes. CpA initiators yield ethylene copolymers not easily available with metallocenes. Copolymers containing significant amounts of comonomers such as styrene, norbomene, and a-olefins from 1-hexene to 1-octadecene are easily obtained with CpA, but not with metallocene or traditional Ziegler-Natta initiators. 

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