Catalysts substrate transport influencing

A fundamental Issue In immobilized catalyst studies is the extent to which substrate transport influences the observed reaction rate. The issue can be resolved by making use of the mathematical formalism which has been developed for heterogeneously catalyzed reactions (13,14,15). This formalism will be presented below for both the batch reactor and the fixed-bed reactor systems. The treatment assumes that the concentration of substrate in the bulk phase is equal to the concentration immediately outside a polymer particle and that the substrate s diffusion coefficient is not a function of concentration or position in the particle. The polymer... 

There are a number of factors which may influence the activity or selectivity of a polymer-immobilized catalyst. Substrate diffusion is but one. This article has reviewed the mathematical formalism for interpreting reaction rate data. The same approach that has been employed extensively in heterogeneous systems is applicable to polymer-immobilized systems. The formalism requires an understanding of the extent of substrate partitioning, the appropriate intrinsic kinetic expression and a value for the substrate s diffusion coefficient. A simple method for estimating diffusion coefficients was discussed as were general criteria for establishing the presence of substrate transport limitations. Application of these principles should permit one to identify experimental conditions which will result in the intrinsic reaction rate data needed to probe the catalytic properties of immobilized catalysts. 

We presented a simple theoretical analysis of the steady-state amperometric response of a two-dimensional catalytic microstructure in Section 3.2.1. We considered the processes of electron exchange between the substrate and the surface-bound catalytic center B and examined the influence of substrate transport in the diffusion layer adjacent to the electrode surface. However in many practical situations, the substrate may be adsorbed onto the electrode surface, or alternatively, a discrete substrate catalyst complex may be formed. If this occurs, then the theory has to be modified to a certain extent. We now discuss a simple form of this modified theory as presented by Albery and Bartlett and Sharp and coworkers. ... 

Cyclodextrins are often used as inverse phase transfer catalysts [11-14]. They are able to intercalate hydrophobic substances and to transport them into a polar phase like water, where the reaction takes place. To study the influence of cyclodextrins on the isomerizing hydroformylation of frans-4-octene in the biphasic solvent system propylene carbonate/dodecane, the concentration of methylated /3-cyclodextrin was varied from 0.2 up to 2.0 mol.-% relative to the substrate frans-4-octene [24]. The results are given in Table 7. 

Additional experiments in a loop reactor where a significant mass transport limitation was observed allowed us to investigate the interplay between hydrodynamics and mass transport rates as a function of mixer geometry, the ratio of the volume hold-up of the phases and the flow rate of the catalyst phase. From further kinetic studies on the influence of substrate and catalyst concentrations on the overall reaction rate, the Hatta number was estimated to be 0.3-3, based on film theory. 

Intrinsic kinetic data can only be measured provided that the overall reaction rate is not limited by mass transport. Only then reahstic parameters can be calculated concerning the influence of catalyst and substrate concentrations (reaction order) as well as the temperature dependency (activation... 

As already shown by Wiese et al. [17] mass transport rates in biphasic catalysis can be dramatically influenced by hydrodynamics in a tube reactor with Sulzer packings. Above all, the volume rate of the catalyst phase in which the substrates are transported by diffusion plays a decisive role in accelerating the mass transport rate. This effect was also investigated for citral hydrogenation in the loop reactor. Overall reaction rates and conversions as a function of the catalyst volume rate can be seen in Fig. 15. 

The selective hydrogenation of one alkene in the presence of another is an example of Type I selectivity (Chapter 5). The olefin that is hydrogenated faster will be favored for saturation. However, with this type of selectivity the faster reaction is more influenced by diffusion limitations, so it is best to use a relatively unreactive catalyst. It is particularly important that the migration of the organic substrates to the catalyst is beyond mass transport limitations so the catalyst will have equal access to both alkenes. As the reaction proceeds, the amount of the more readily adsorbed species decreases so reaction selectivity may also decrease. 

The catalyst form is is also decisively influenced by the chosen reactor type. It has been found experimentally that at particle sizes below 0.1 mm pore diffusion is rarely limiting, whereas at particle sizes above 5 mm, pore diffusion is always dominant. This is the reason why shell catalysts are advantageously used in trickle-bed reactors. The diffusion limitation need not always result from the transport of the gas in the pores it can also be due to the substrate. Such effects are found with long-chain organic molecules, for example ... 

In the previous chapters we predominantly considered catalysis as a molecular event, in which substrate molecules are activated by the catalyst. In this chapter and the next we will emphasize catalytic features of dimensions in space much larger than that of single catalytic centers and times much longer than those associated with the individual molecular catalytic cycles. Often mass and heat transport cause reaction cycles, which occur at different sites, to interact. Under particular conditions this gives rise to cooperative phenomena with oscillatory kinetics and temporal spatial organization. As such, interesting surface patterns such as spirals or pulsars may form. Such complex cooperative phenomena are known in physics as appearances of excitable systems. Their characteristic features are easily influenced by small variations in external conditions. Hence these systems have also features that are called adaptive. 

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