Pore glass diffusion studies

A number of recent developments in 129Xe NMR spectroscopy are presented with direct applications to the study of mesopore space in solids. This includes the establishment of a relationship between pore size and chemical shifts for a number of controlled pore glasses and the exploration of hyperpolarized (HP) xenon for a number of NMR and microimaging applications to porous solids. With HP xenon, the increase in experimental sensitivity is remarkable. Experiments illustrated include the rapid characterization of the void space in porous solids, including the in-situ study of processes such as diffusion and dehydration, and imaging with chemical shift resolution. 

This section describes both dynamics studies at the molecular level and also cooperative bulk macroscopic properties as sensed by diffusion studies. The measurements of the proton spin-lattice relaxation time of liquid crystal 4- -octyl-4 -cyanobiphenyl (8CB) confined in randomly oriented untreated porous glass have been presented. The studies are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. The local translational diffusion of molecules within the cavities is found to be nearly as fast as in bulk. Orientational relaxation of a model discotic liquid crystal, consisting of dislike molecules... 

Surface diffusion has been extensively studied in literature. An overview of experimental data is given in Table 6.1. Okazaki, Tamon and Toei (1981), for example, measured the transport of propane through Vycor glass with a pore radius of 3.5 nm at 303 K and variable pressure (see Table 6.1). The corrected gas phase permeability was 0.69 m -m/m -h-bar, while the surface permeability was 0.55 m -m/m -h-bar, and so almost as large as the gas phase permeability (Table 6.1). It is clear from Table 6.1, that the effects of surface diffusion, especially at moderate temperatures, can be pronounced. At higher temperatures, adsorption decreases and it can be expected that surface diffusion will become less pronounced. 

Kameyama, Fukuda and Dokiya (1980, 1981) reported separation experiments of H2-H2S mixtures with porous glass (mean pore diameter 4.5 nm) and an alumina membrane (mean pore diameter 100 nm) up to temperatures of 800°C. They used the separation of H2 from the mixture to shift the equilibrium in the decomposition reaction of H2S. H2S, however, diffused partly along the surface, especially at the lower temperature range, thereby decreasing the separation efficiency. No attempt was made to describe the transport. Shindo et al. (1983, 1984) reported data on the separation of He-C02 mixtures by means of a porous glass membrane over the temperature range 296-947 K. A theoretical analysis was made, studying the... 

On the contrary, silica membranes with pore diameters in the 0.3-0.8 nm on porous glass supports have been claimed to exhibit an H2/CO separation factor as high as 30. This is significantly higher than what Knudsen diffusion alone predicts. It is possible that some molecular sieving takes place in the reportedly small pores in the silica membranes. This postulation appears to be further reinforced by another study using hollow fiber silica... 

While glass has proven to be the most popular conventional platform for protein microarrays, a number of other physical substrates have also been evaluated. These have included gold, silica, and polyvinylidene fluoride and nitrocellulose membranes. Nitrocellulose has been employed in array development as it can retain large quantities of antibody, is inexpensive, and disposable. Flowever, it is not suitable for kinetic studies due to the slow diffusion of proteins through its pores. It is also not compatible... 

The association of several techniques to study the same system is always advantageous. For the specific case of surfaces, techniques like XPS, contact angle, diffuse reflectance infrared with Fourier transform, surface area and pore size measurements, scanning electron microscopy appear frequently associated. This is the case for the study of carbon or glass fibers [ 105-109], activated carbons [110], and polymeric films obtained by plasma polymerization [111]. In Section 5, the specific combination of diffuse reflectance spectroscopies in the region of UVA is and XPS will be emphasized. 

Dvoyashkin et al. (2009) studied surface diffusion of n-heptane in two mesoporous adsorbents with different morphologies of the pore network (Vycor random porous glass and porous silicon with linear pores) using PFG-NMR (Figure 1.238). The obtained diffusivities revealed increasing... 

To evaluate the appropriate form of Equation (7.18) we must employ the design equation used to obtain the kinetic data. We studied the kinetics of adsorption using a batch reactor with recycle operation in the differential mode. The reactor consists of a packed column with the adsorbent between two layers of glass beads. Pore diffusion and mass transfer resistances were minimized by using small particle sizes (180 to 120 pm) and high flow rates. The design equation written for the aforanentioned metal cation extraction is... 

Porous glasses in the shape of beads and ultrathin membranes with comparable texture properties are an ideal model system to investigate the transport characteristics of the mesopores inside the primary particles of silica supports. The hydrogenation of benzene over a nickel catalyst based on the porous glass beads is a suitable test reaction to estimate the effective diffusivities. The tortuosity factors can be obtcdned from measurements of the permeability of membranes with comparable texture properties. Now, the pore diffusivity of benzene under reaction conditions can be calculated. The low absolute values of the pore diffusivities obtained in this study indicate that interactions between the difiusing reactant and the surface of the support or pore roughness effects have to be considered. 

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