Quasi-homogeneous Catalysts

Biffis A, Cunial S, Spontoni P, Prati L (2007) Microgel-stabilized gold nanoclusters powerful quasi-homogeneous catalysts for the aerobic oxidation of alcohols in water. J Catal 251 1-6... 

In biocatalysis, hydrolases are the most important class of enzymes for carrying out enzymatic resolutions. Many hydrolases, such as esterases, lipases, epoxide hydrolases, proteases, peptidases, acylases, and amidases, are commercially available a substantial number of them are bulk enzymes [87]. Resting-cell systems, if they are not immobilized, are used in diluted suspensions and could be handled as quasi-homogeneous catalysts. 

Biffis, A., Cunial, S., Spontoni, R, etal. (2007). Microgel-Stabilized Gold Nanoclusters Powerful Quasi-Homogeneous Catalysts for the Aerobic Oxidation of Alcohols in Water, J. CataL, 251, pp. 1-6. 

Catalysts based on colloidal suspensions attracted much attention in recent years, both as supported and as quasi-homogeneous catalysts. These catalysts are prepared by the so-called reduction-deposition method, where a metal is first reduced in solution in the presence of a stabiliser before it is deposited on a heterogeneous support. By using the appropriate reaction conditions, met crystallites of < 10 nm are av able. These catalysts are called nanocatalysts, although the metal crystallite size is not much different from commercial catalysts prepared by traditional methods. However, the metal crystallite size distribution of the nanocatalysts is narrower and they do not contain large metal crystallites that are sometimes observed in traditional catalysts (see right-hand side of Figure 1). 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

Mathematical models of packed bed reactors can be classified into two broad categories (1) one-phase, or pseudohomogeneous, models in which the reactor bed is approximated as a quasi-homogeneous medium and (2) two-phase, or heterogeneous, models in which the catalyst and fluid phases and the heat and mass transfer between phases are treated explicitly. Although the... 

In practice, a solid catalyst is most conveniently modeled as a quasi-homo-geneous phase. Even if the catalyst particle is porous, visualize it as a homogeneous, but permeable solid. Mass transfer in its interior is retarded by two effects obstruction of part of the cross-sectional area by the solid material, and diffusion paths that are longer because molecules have to wind their way around the obstructions (tortuosity effect). In the quasi-homogeneous model, the retardation is accounted for by the use of appropriately smaller "effective mass-transfer or diffusion coefficients. 

Thus, the mathematic models for reactor design are also classified into continuous heat exchange bed and adiabatic one. Usually, the design of reactor adopts one-dimension quasi-homogeneous model which considers that when reactive gas passes the catalyst bed like a plug-flow, there exist no radial and axial return mixture, and microkinetics can be treated in intrinsic kinetics multiplied by an effective factor that involves the effects of transfer processes, and by an active coefficient that involves the effects of reduction, poisoning and declining etc. Macrokinetics can be... 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

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