Pore dimensions diffusion studies

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

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. 

In view of the current interest in environmental protection, we have studied the possibility of using transition substituted molecular sieves for catalyzing the heterogeneous oxidation of aniline and, more generally arylamines. We have recently reported that TS-l, the titanium-substituted silicalite-1, was an excellent catalyst for the selective oxidation of aniline into AZY [6], provided that the H202/aniline ratio was relatively low (< 1.6). We showed that the oxidation more likely proceeded via the formation of phenylhydroxylamine (PH) and nitrosobenzene (NSB) and that these intermediate compounds could react together to form AZY. Because of the small pore dimensions of the structure, the reaction could not be carried out with TBHP and was limited by diffusion of reagents and/or products in the channels. 

A totally different picture emerges from measurements by using PFG NMR. In this case, the molecular trajectories are traced on a length scale above hundreds of nanometers, notably exceeding the pore dimension. The data of Table 2 show that the diffusivities in PSi with comparable pore diameters, as considered in the QENS study, measured in the direction parallel to the channel axes are found to be few times slower than the bulk diffusivity. This fact indicates that, in addition to... 

Nuclear magnetic resonance (NMR) provides a powerful method for the study of molecular motion. The techniques can distinguish molecular reorientation and translation and have proved particularly valuable for the study of self-diffusion in bulk liquids. The molecular motion of liquids in the confined geometry provided by their containment in porous materials has been of considerable interest for many years. It is of importance both as a fundamental scientific problem and because of its technological importance in such diverse systems as oil recovery from rocks and catalytic agents. The purpose of this paper is to question the reliability of many previous investigations and the validity of their interpretation. Potential sources of error are demonstrated by measurements on mobile liquids adsorbed into porous silicas with different geometrical characteristics. The principles illustrated are equally valid for other porous systems. Preliminary measurements of the diffusion coefficient of n-butane in silica as a fimction of temperature and the effect of pore dimensions are presented. 

To cross the filter, the solute must diffuse through the filter pores. Therefore, pore size, density, and tortuosity must be taken into consideration. Many filter configurations are commercially available and have been employed for these types of studies. In general, the greater the pore size and porosity, the less the potential for the filter to act as a significant diffusion barrier. For small solutes, the filter will probably not present much of a problem. However, as the molecular size of the solute increases and approaches the dimension of the pore, these considerations become more important. In principle, solute diffusion through the... 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

Figure 1. Dimensions of Ring-Shaped Catalyst Particle with one Sealed End for studying Diffusion in Large Pores.

The third distance, perhaps the most relevant to reactions on surfaces, is the actual distance traversed by a diffusing molecule. This is a very complex issue which we only begin to understand. The diffusional distance reflects not only the geometric considerations made above, but also the facts that the surface is energetically heterogeneous, and that the diffusion is some combination of movements which follow closely the surface features, and of jumps from pore-wall to pore-wall and from one tip to the next. Obviously this diffusional distance is also a function of the temperature and of the solvent interfaced with the solid. Furthermore, since different types of connectedness can yield the same D value, this textural characteristic is an additional parameter to be considered (the fracton or spectral dimension (IS)). In view of this complex picture, what is then the practical advise Under the current state of art, the best one can do is to get a preliminary estimate of d from eq s [4]-[6] the direct observation of actual diffusional process in disordered systems, is still in its infancy. For some recent studies see ref. 16,17. 

In the present study, we report our molecular graphics investigations and force field calculations for the diffusion of isomers of IBEB in mordenite, mazzite and faujasite. The dimensions of the pores in these zeolites as well as the molecules are listed in Table 1. Using... 

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