Catalyst aggregation

It must be concluded from the results that reactions take place which change the number of active sites present, due to the different behavior of the polymers in solution. With polyethylene, crystalline insoluble globules cause break-up of the catalyst aggregates, and even crystal scission with poly( 1-butene) the chains are highly solubilized. 

The addition of achiral additives lacking chiral conformations to asymmetric catalysts can enhance catalysts enantioselectivity. The exact nature of the interaction between the additive and the chiral catalyst, however, is often not clear. The additives usually contain Lewis basic groups, and can behave as ligands by coordinating to catalysts and changing the metal geometry and/or catalyst aggregation state. 

Figure 6.11 shows the activity of an artificial enzyme can be controlled based on the phase behavior of a lipid bilayer. The catalytic site for hydrolysis was supplied by a monoalkyl azobenzene compound with a histidine residue which was buried in the hydrophobic environment of a hpid bilayer matrix formed using a dialkyl ammonium salt. Azobenzene compound association depended on the state of the matrix bilayer. The azobenzene catalyst aggregated into clusters when the bilayer matrix was in a gel state. In contrast, the azobenzene derivative can be dispersed into the liquid crystalhne phase of the bilayer matrix above its phase transition temperature. This bilayer-type artificial enzyme catalyzed the hydrolysis of a Z-phenylalanine p-nitrophenyl ester. The activation energy for this reaction in the gel state is twice as large as that observed in the hquid crystalline state. The clustering of the catalysts upon phase separation suppress their catalytic activity, probably due to the disadvantageous electrostatic environment around the catalysts and the suppressed substrate diffusion. This activity control is unique to such molecular assembhes. 

Carbon atoms released by the support upon heating contaminate the surface of Pd particles to make them inaccessible to adsorbates [31-33,35]. At temperatures as high as 600-730 °C, carbon ad-atoms aggregate into extended graphitelike crystallites, which build up capsules around metal particles and prevent further sintering of the catalyst. Aggregation of carbon ad-atoms into graphite clusters at the Pt surface is observed at 630-930 °C in [73] and at 1100°C in [30]. 

TEM images of Ir/TiOz prepared by DP at 3 and 5 pH are also shown in Fig.5 (c) and (d). In the case of these catalysts, aggregates of clusters and small clusters were respectively deposited as the major species on the TiOz surfaces. This result and that of ICP analysis produce good evidence that the amount of loading and the structure of the Ir can be controlled by changing the pH of the precursor solution while using the DP method. 

A detailed analytical study of fhe acfivity of some solid acid catalysts, including mesoporous silica-supported Nation, in the acylation of anisole with AAN allows the conclusion that catalyst deactivation is caused by the primary ketone product and/or multiple acetylated products in the micropores of Nation catalyst aggregates. i Experiments were performed with a commercially available silica-supported Nation catalyst in a continuous-mode slurry operation by using carbon-dioxide-expanded liquids (nitromethane or nitrobenzene) as solvents. At 90°C, 80% AAN conversion is observed with a TOS of 2 h, but the catalyst rapidly deactivates, and 27% conversion is evaluated after 6 h TOS with a TON value of about 400. The catalyst can, however, be completely regenerated upon nitric acid treatment. These results confirm that silica-supported Nation catalysts are promising alternatives for the traditional aluminum chloride homogeneous Lewis acid catalyst. 

The importance of microstructural optimization of hydrophobic gas diffusion electrodes was emphasized in an article by Tantram and Tseung (1969). Tantram and Tseung considered porous electrodes that consisted of mixtures of finely dispersed Pt black particles, bonded by polytetrafluoroethylene (PTFE). Hydrophobic and hydrophilic parts formed interconnected networks of porous PTFE and porous catalyst aggregates. The two authors recognized the importance of agglomeration and dual hydrophobic/hydrophilic porosity. 

Scheme 21.9 Reaction pathways yielding to the different linear, cyclic and bicyclic structures during t e en to-en eye ization of an ABC linear triblock copolymer, bearing A and C sequences with antagonist reactive nctions followed by the magnification of the obtained structures. The two stars schematize catalyst aggregates with several acidic sites.

From the experimental tests a dependence of the phenol degradation rate liom the catalyst loading in the membrane was observed (Fig. 27.3). The better results were obtained at 25.0 wt% catalyst loading a further increase of catalyst loading (33.3 wt%) reduces the reaction rate probably because of catalyst aggregation phenomena. Also the transmembrane... 

Mechanistic investigation of the Au -catalysed SiO)"-exo-trig hydroalkoxylation of allene (240) revealed a rapid and reversible C-0 bond formation to generate (241), followed by the turnover-limiting protodeauration producing the vinyl tetrahydrofuran (242). This pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex (243). ... 

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