Restricted pores

Molecular dynamics results for restricted pore average density versus pore width. 

The catalysts are predominantly modified ZSM-5 zeolite. In general, the modifications are intended to restrict pore mouth size to promote the shape selective production of para-xylene within the microporous structure. The same modifications also serve to remove external acid sites and eliminate the consecutive isomerization of para-xylene. Methods used to modify the zeolite pore openings have included silation [50], incorporation of metal oxides such as MgO, ZnO and P2O5 [51, 52], steaming and the combination of steaming and chemical modification [53]. 

Speciation is best carried out directly in the aquatic system, without sampling. This has been possible since the development of Diffusive Equilibrium in Thin Films (DET) and Diffusive Gradient in Thin Films (DGT) probes.31 The DET probe consists of a very thin gel layer that is immersed in the aquatic system and allowed to equilibrate with the bulk solution. The concentration of solutes in the gel is similar to that in the bulk solution for all solutes that can diffuse through the pore openings of the gel (some gels have open pores >5 nm and some gels have restricted pores <1 nm). The DGT... 

With a DGT device, Cr(III) can be bound to the chelex resin because of its cationic nature, whereas Cr(VI) is not bound to the resin (it has an anionic nature) but is present in the diffusive gel layer (as in a DET probe), reaching equilibrium with Cr(VI) in the aquatic system. Hence, Cr(VI) can be measured in the diffusive layer and Cr(III) in the resin layer.44 For Mn the same procedure can be adopted. The oxidized Mn(IV) species form colloids or even larger particles and will not be sampled by the DGT probe, whereas Mn(II) species are free or labile complexes. For Fe speciation, DGTs with open pores and with restricted pores are often used. Since in aquatic systems, Fe(III) is present mostly as a ligand complex or in colloidal form, the restrictive pore size excludes these forms and makes only Fe(II) species available to the restrictive DGT,45 whereas the open-pore DGT allows the passage of Fe(II) and small and labile Fe(III) complexes. In the case of arsenic speciation, As(III) and As(V) diffuse through the diffusive gel layer of the DGT, but only As(III) is immobilized on the chelating resin layer As(V) remains in the diffusive layer as an anionic compound. 

Jacobs has described (249a) these two different approaches in terms of secondary (physical mixtures) and primary (FT function in zeolite matrix) effects. In the former case the results obtained can be quite well understood in terms of the separate behavior of each component. However, in the latter case the results may be different since the primary FT products are formed inside the spatially restricting pores of the zeolite. 

The ship-in-a-bottle catalyst s main feature is the host-guest interaction which is neither covalent nor ionic. The guest is retained in the zeolite matrix by restrictive pore openings and will, in principle, keep all properties of the homogeneous complex in addition to the advantages offered by the heterogeneous system. 

Coke is formed through aromatic condensation reactions (21) which on the molecular scale are spatially quite demanding. Zeolites, with molecular-sized pore systems, tend to be significantly less prone to coke formation than ASA-based catalysts, which do not have such spatially restricted pore structures. Correspondingly, in commercial operation zeolitic catalysts tend to be significantly more stable than ASAs (see Fig. 6.5). 

Electrophoresis protein separation membranes have been prepared by step-growth condensation of water-soluble polyvinyl alcohol with selected water-soluble difunctional crosslinking agents. These membranes have broad pore size ranges, restricted pore size distribution, greater resistance to hydrolysis in an alkaline medium, and improved gel clarity when higher amounts of crosslinkers are used. 

The systematic study of polar permeant permeation served to confirm the existence of a porous permeation pathway through the HEM. It also led to the characterization of important properties of this pathway. The results of this study demonstrated that the diffusion of polar permeants through skin is limited by the low effective porosity of the HEM and by hindrance effects due to restrictive pore dimensions. Effectively enhancing the transport of polar drugs in the MW range of many therapeutic peptides may require increasing the effective Rp of the HEM as well as the effective porosity/tortuosity ratio. Perhaps novel combinations of chemical permeation enhancers and physical means such as an applied electrical field or ultrasound may be necessary to achieve this objective. 

The active components of a present commercial FCC catalyst are zeolites Y (0.74 nm) and ZSM-5 (0.54 nm x 0,56 nm) whose pore sizes give limited access to the active centres for long chain and/or bulky molecules. The new extra-large pore crystalline materials MCM-41 (2.0-20 nm), VPI-5 (1.21 nm) or cloverite (1.32 nm) have significantly increased the restricted pore sizes encountered in zeolites Y and ZSM-5 and opened up interesting perspectives for the conversion of heavier feedstocks [1]. 

Several mechanisms for the migration of macromolecules through polymer networks have been described. The Ogsten model considers the molecule as a nonde-formable sphere. The speed of migration is based on the mobility of the solute in free solution modified by the probability of an encounter with a restricting pore. 

Some catalytic processes are limited by gas phase diffusion, usually those with very high surface area catalysts which in turn means very small diameter, restricted pores within the material. After the gas has diffused to the active... 

The topological distortion across these polymer films can be described by modern percolation theory, and used to define the maximum in the sodium transport versus caustic concentration curve as an optimum restricted pore volume. 

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