Product diffusion selectivity

Molecular sieving by controlling access of molecules to the internal surfaces and by restricting molecular dilfusivity is particularly important in processes that require the separation of branched from linear alkanes or in resolution of the different isomers of xylene relevant dimensions of some important hydrocarbons of these types are given in Table 7.2. Small-pore zeolites such as Na-A are particularly important for the separation of n-alkanes and n-alkanols from their branched isomers, whereas medium-pore zeolites such as ZSM-5 show adsorption of p-xylene but very slow (or no) adsorption of o-xylene. Molecular sieving is also important in restricting the size of molecules that leave the pores of zeolites after catalytic reaction within them. This product diffusivity selectivity is described in detail for specific examples in the next chapter, but the intra-zeolitic isomerisation of xylenes in ZSM-5 to give predominantly p-xylene product is an excellent example. 

The work of Thiele (1939) and Zeldovich (1939) called attention to the fact that reaction rates can be influenced by diffusion in the pores of particulate catalysts. For industrial, high-performance catalysts, where reaction rates are high, the pore diffusion limitation can reduce both productivity and selectivity. The latter problem emerges because 80% of the processes for the production of basic intermediates are oxidations and hydrogenations. In these processes the reactive intermediates are the valuable products, but because of their reactivity are subject to secondary degradations. In addition both oxidations and hydrogenation are exothermic processes and inside temperature gradients further complicate secondary processes inside the pores. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

It follows from Equation 8.13 that aA/B can be expressed as the product of the diffusivity selectivity, DA/DB, and the solubility selectivity, SA/SB. Diffusion (or mobility) selectivity is governed primarily by the size difference between gas molecules and always favors smaller gas molecules. Solubility selectivity is controlled by the relative condensability of the gases in the polymer and their relative affinity for the polymer. Solubility selectivity typically favors larger, more condensable molecules. From Equation 8.13, it is seen that the product of gas mobility and solubility selectivity determines the overall membrane selectivity. It is clear that for a membrane to be C02 selective, it must have high diffusivity selectivity based on the affinity for C02 but it should be flexible enough to permeate larger molecules such... 

Mass transfer effects are very important for the selectivity in the Fischer-Tropsch synthesis. Even though the reactants are in the gas phase, the catalyst pores will be filled with liquid products. Diffusion in the liquid phase is about 3 orders of magnitude slower than in the gas phase and even slow reactions may become diffusion limited. Diffusion limitations may occur through limitation on the arrival of CO to the active points or through the limited removal of reactive products.8,9... 

We interprete the above effects as conventional product shape selectivity inside the pore system of zeolite ZSM-5 or ZSM-11, and part of our arguments were presented earlier, in a preliminary note [28]. While the catalyst is on stream, coke is gradually formed and deposits, in part, inside the channel system. As a consequence, the diffusion pathways for product molecules increase. Slim molecules, such as 2,6-dimethylnaphthalene are less affected than... 

Derouane and Gabelica16 proposed molecular traffic control as another type of shape-selectivity that could occur in zeolites having more than one type of intersecting pore system. Here, reactant molecules may preferentially enter into the catalyst through one pore system while the products diffuse out through the other, thereby minimizing counter diffusion and increasing the reaction rate. 

Catalyst selection involves two features productivity and selectivity. The process rate is a subtle combination of four limiting steps adsorption/desorption of reac-tants/product, surface reaction between species, diffusion through pores and diffusion through external film. Pore structure, surface area, nature and distribution of active sites play a crucial role in forming the process rate at the level of catalyst... 

The operation principle is based on the selective extraction of substrates from the feed stream and/or products from the reaction mixture by specific membranes. The enzyme-substrate contact occurs once the substrate has diffused through the membrane to the enzymatic region [98]. The generated products diffuse again through the membrane to reach the outlet stream. The main difficulty of these reactors is that the diffusion is the main mechanism of substrate and product transport. Therefore, the kinetics of the process tends to decrease [109]. 

Basically, reactant and product selectivities are mass transfer effects, where the diffusivities of the various species in practice frequently do not differ that extremely as indicated above. Instead, in most cases only a preferred diffusion of certain species is observed, a fact which often hinders a clear understanding of product shape selectivity. This is because the various products, during their way through the pore system, may be reacted when contacting the catalytically active surface of the wall. This combined effect of diffusion and reaction will be discussed in detail in the following, as it is of great importance for the product distribution in zeolite-catalyzed reactions. 

Olson and Haag [80] in 1983 showed that the yield of p-xylene observed during the disproportionation of toluene on various modified and unmodified ZSM-5 catalysts is actually influenced by product shape selectivity. The authors attributed the observed effects to an interaction of diffusion and reaction, characterized by means of a dimensionless modulus similar to the classical Thiele modulus . The mathematical treatment of shape selectivity in zeolite catalysts, which will be applied in this section, is largely based on the theory of Olson and Haag [80], although some modifications and extensions to this are given. 

The maximum gain of the para selectivity, which gives the size of the product shape selective effect, naturally increases as the ratio of the diffusivities Ro becomes larger, although not to 100%, but instead to a limiting value which is approached at Ro > 100. This is controlled by several other factors, namely the absolute values of the diffusivities. 

Separation of molecules with different sizes can be achieved by a proper choice of zeolites (nature of the zeolite and adjustment of the pore architecture, especially the pore size). The simplest forms of shape selectivity come from the impossibility of certain molecules in a reactant mixture entering the zeolite pores (reactant shape selectivity) or of certain product molecules (formed inside the pore network) exiting from these pores (product shape selectivity). In practice, reactant and product shape selectivities are observed not only when the size of molecules is larger than the size of the pore apertures (size exclusion) but also when their diffusion rate is significantly lower than that of the other molecules. Differences of diffiisivities by 2 orders of magnitude are required to produce significant selectivities between reactant species (35). 

Figure 6 illustrates product shape selectivity, i.e., the capacity of favouring the formation, among all possible products, of those that diffuse faster out of the pores. While the entire range of products can be present inside zeolite channels and cavities, the effluent stream is mainly composed of the less hindered... 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

Since the shape selectivity is clearly related to reactant and/or product diffusion through the pores and cavities of the zeolite, the selectivity should increase as the size of the zeolite particles and, thus, the extent of diffusion, increases. This has been established using the H-ZSM-5 alkylation of toluene with methanol as the probe reaction. Three catalysts with particles ranging in size from 0.025 j.m to 4.5 im were used. The results listed in Table 10.1 for reactions run at various temperatures show that with the largest catalyst particles selectivity toward p-xylene formation was 100% at all temperatures. As the particle size decreased so did the reaction selectivity. Increasing the temperature increased the reaction selectivity with the smaller particle sized catalysts. 3... 

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