Diffusion control from excess catalyst

This is investigated by manipulating the ratio between Ca, surface and Cb, surface, which is defined by Tb, surf- Effectiveness factors are not very sensitive to a fourfold change in this parameter (i.e., from 0.5 to 2.0), as illustrated in Table 19-4. An excess of reactant B (i.e., 4/3. surf = 2) causes A2 to be depleted at a faster rate relative to the situation when Tb, surf = 0.5. This is reasonable because the probability of finding B on an active site inside the catalyst increases (probably by a factor of 4 in this example) when B is present in excess. Since the rate of the forward reaction depends linearly on active-site surface coverage by reactant B, larger values of Tb, surf could produce diffusion-controlled conditions. This is marginally obvious in Table 19-4 at A = 100, because the effectiveness factor is smaller (i.e., 0.26 vs. 0.32) when reactant B is present in excess. 

Nanostructured microporous catalysts or catalyst supports offer intensified catalysis as they provide enhanced surface area accessible to the reactants and products. In nonstructured catalysts, although the surface area may be large, they are often inaccessible as a result of surface fouling and diffusion resistance can slow down the rate of reaction. In a recent development, microporous materials were used as templates for the solution deposition of metals, which were subsequently heat treated to obtain porous metallic structures, where the size of the pores ranged from 10 pm to lOnm. " The relative phase volume of these two regions can be controlled and the overall porosity can be in excess of 50%. Fig. 7 illustrates the size scale of structures ranging from 10 pm to 10 nm. 

The influence of equilibrium on the ethylene/propylene product ratio obtained with SAPO-34 catalyst over a range of temperatures and pressures can be seen from (Fig. 35) [189]. The relationship is linear with a slope that is significantly greater than 1, indicating that ethylene is in excess of equilibrium on the small-pore zeolites. An explanation for this relationship is that ethylene and propylene are in equilibrium within the pore structure of the SAPO-34 molecular sieve. However, because the 0.43 nm pore diameter of the SAPO-34 is too small to allow propylene to take a straight-line path through the pore mouth, its diffusion is hindered relative to ethylene. This would imply that diffusion limitations are the key factor controlling the C " / C," ratio [189]. 

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