Reaction-induced porosity

The fact that different types of cell wall respond differently to the decay conditions is clear and extremely interesting. These differentiations no doubt arise partly from differences in the chemical nature of the substances in the walls and lumina. But could they not also arise in part from differences in physical structure—e.g., a porosity or surface accessibility to enzymes or other reaction-inducing substances ... 

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

The secondary reactions result in a H2 production in the absence of gaseous diborane. The secondary reactions are induced by temperature. Although the primary and secondary reactions seem fairly compatible regarding the mechanism, their effect on porosity may differ significantly. Gillis-D Hamers investigated also the porosity changes induced by these secondary reactions. 

This is presented schematically in Fig. 6.3, which also shows that the kinetics of these processes is described by the transport rate of A from the wall to the adjacent media. Using Fig. 6.3, we can establish that two elementary processes are presented in this system. The first is the flow induced by the concentration gradient and the second is the mass transfer sustained by the processes on the surface (a chemical reaction in the case of the metal placket immersed in a specifically formulated liquid and the transport through the porosity in the case of the drying wall). The case presented here corresponds to the situation when, in respect of the bulk density, the fluid density begins to decrease near the wall. This generates the displacement of the media and the specific ascension force, which is equivalent to the density difference. This phenomenon depends on the concentration difference in fluid A Aca=(cap - c ). From Fig. 6.3 we can write a list of process variables ... 

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