Active site distribution

Increases the active site distribution becomes more heterogeneous. Simulations show that for a unlmodal distribution 0j reached an... 

Computer simulations have been useful for validating a kinetic model that Is not easily tested. The model was equally capable of describing multi-site polymerizations which can undergo either first or second order deactivation. The model parameters provided reasonably accurate kinetic information about the Initial active site distribution. Simulation results were also used as aids for Interpretation of experimental data with encouraging results. 

These methods produce active artificial membranes, but the active site distribution is not well defined and the theoretical treatment is difficult. 

Catalyst Structural Characteristics. Structural features of AFS and USY materials have been characterized in this work in terms of unit cell size, presence of extraframework material, active-site distributions, and pore-size distributions. These features are similar for both sets of USY and AFS samples which indicates that structural characteristics are not related to the source of Y zeolite. 

Q. To proceed further at this point one has to specify a pore model for the catalyst, and a model for the active site distribution. Froment and co-workers have examined a variety of cases such as single pore models (single-ended pores and pores open on both sides) with both deterministic and stochastic active site distributions, the bundle of parallel pores model and various tree-like models of the porous structure, which were earlier used by Pismen (40) to describe transport and reaction in porous systems. Such treelike models contain interconnected pores but lack any closed loops and are usually called Bethe networks or lattices. They are completely characterized by their coordination number Z, which is the number of pores connected to the same site of the network. 

The olefin readsorption and CO transport models suggest the types of catalyst structures and active site distributions that lead to optimum values of C5+ selectivity in FT synthesis reactions (Fig. 20). The value of the structural parameter x required to give this optimum selectivity depends on the... 

Interphase contiguous systems an active "skin" around an inert core of the same crystal structure but different chemical composition (e g., SAPO around an ALPO4 core) and Active site distributions homogeneous A1 distribution vs. A1 gradients. 

Cocatalysts can also influence polymer properties [155-158,690-694]. This influence can be explained primarily by the change in the active site distribution. Some sites become active more rapidly in the presence, rather than in the absence, of cocatalyst, thus increasing their contribution to the final MW distribution of the polymer. Still other sites become activated only in the presence of cocatalyst, probably because it enhances reduction or alkylation or removes ligands. Usually these sites (the most unreactive sites) tend to produce higher-MW polymer. Therefore, the addition of cocatalyst often tends to introduce a high-MW tail in the MW distribution. This tail accounts for the rise in average MW shown in Figures 204-206 as cocatalyst was added to the reactor. 

Another method used to dampen the catalyst activity in the fluidized-bed process is to deliberately add poisons to the reactor, such as 02 in small amounts. It was discovered that these poisons sometimes cause a broadening or narrowing of the MW distribution of the polymer, because they also affect the active site distribution on the catalyst. 02, for example, tends to increase polymer MI and broaden the MW distribution, which makes the polymer more shear-thinning. Consequently, poisons are sometimes intentionally added to manipulate polymer properties in this process. 

Part of grafting strategies can also be used in association with photoinduced polymerization. As previously stated, grafting from methodology consists of polymerization of monomers initiated from active sites distributed along a polymer backbone or a surface. Photoinitiated polymerizations can well be adapted to this... 

Note that a near to similar ratio of rapidly and slowly exchangeable sulfur was determined when the catalyst was sulfided originally with thiophene instead of elemental irrespective of the 2.5 times lower S content of the sample, sulfided by thiophene. This indicates the similarity of active site distribution for catalysts of equal chemical composition. 

Site heterogeneity was investigated using a deconvolution technique based on inverse Laplace transform (ILT) method. As could be seen from the active site distribution shown in Fig. 11, the unpromoted catalyst had essentially only one kind of site (3). K-promotion increased the average... 

Fig. 11. Active site distribution for unpromoted and K promoted Ru/Si02 catalyst for ammonia synthesis. I ]...

Diffusion of adsorbate molecules throughout the pore space of microporous solids is an essential step in many applications of microporous solids and determines their utility and selectivity in applications. Whereas the thermodynamics of the adsorption determines the equilibrium situation, the kinetics of an adsorptive or catalytic process is controlled by the diffusion rates. This is exemplified in their use in shape-selective catalysis, where molecules must reach and leave active sites distributed through the crystallites and therefore products that diffuse faster will be enriched in the molecular mix leaving the solid. 

There are at least two possible explanations for the higher activity of the Ti-TUD-1 its three-dimensional pore structure and active site distribution. Figure 13... 

The solution of the inverse problem of the formation of the MWD in the case of cis-l,4-polyisoprene made it possible to obtain curves of the active site distribution over kinetic heterogeneity (Figs. 3.3-3.6). As a result of averaging of the positions of all maxima three types of polymerization site that produce isoprene macromolecules with different molecular masses were found ... 

FIGURE 3.3 Active site distributions over kinetic heterogeneity during isoprene polymerization on fractions C-1. Method 1. Here and in Figs. 3.4 and 3.5 numbers next to the curves are conversions (%). 

FIGURE 3.4 Active site distributions over kinetic heterogeneity during isoprene polymerization on fractions C-2, Method 1. 

The result above is obtained under strictly controlled conditions in laboratory, but the single surface of a-iron and fully exposed a-iron (111) surface are hard to get during the preparation and reduction of catalysts. Even more important, because of the existence of promoter, 50% or more surface of a-iron is covered by K2O or KOH and the structure of active phase changes because of the addition of the promoter, and even the active site distribution or active order on the crystal plane of a-Fe are greatly changed. As a result, the impact of microstructure changes on the mechanism of activity is worth exploring. 

The propane reaction is very exothermic and its enthalpy is even higher than both enthalpy steps added from the propylene process, since alkane dehydrogenation must be included in the former process. The propane reaction takes place via an eight-electron transfer, requiring a specific catalyst structure on which an adequate isolated active site distribution exists, in order to carry out the coordinated steps. Moreover, an appropriate element redox balance must be present to complete the catal3dic oxidation-reduction cycle, including in situ catalyst regeneration. 

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