Effective pore diameter

Zeohte type Designation Cation Effective pore diameter, E Unit cell parameter, E... 

In the presence of polyethylene oxide MW 300,000 at a concentration of 0.025 g liter , variations in pH and ionic strength have no effect on elution volumes and a single calibration curve is obtained as shown in Figure 4 and Table II. This behavior presumably also results from modification of the glass surface by the polyethylene oxide surfactant, but in this case charge effects appear to be completely suppressed and the effective pore diameter and volume reduced. Such an interpretation is also in accord with the fact that the elution voliomes are lower with polyethylene oxide than with Tergitol, since Tergitol is a much smaller molecule than the polyethylene oxide. 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

Figure 10.1 Kinetic diameter of common industrial molecules shown relative to the pore sizes of common zeolite structures shaded areas represent the range of effective pore diameters for each group of zeolites.

Pore diameter (dp) for channel-like PMSs according to the BJH pore size distribution, which, however, underestimates the effective pore diameter by circa 1.0 nm, as shown by theoretical and geometrical calculations [48, 49] pore diameter of cage-like PMSs according to Ravikovitch and Neimark [50],... 

Derive an expression for the effectiveness factor of a porous catalyst slab of thickness 2L that has an effective pore diameter = 4e/pcS with a first-order reaction r" = VCa- Can you justity 4 = 4ep Sl... 

An important general feature of SEC is its limited separation selectivity. The separation itself proceeds in the pores of the column packing and its extent depends on both the effective pore diameter and the pore volume. The pore volume of a column is limited by the column size and by... 

Knudsen Diffusion Only Is Occurring. For a very fine pore material in which the effective pore diameter is less than the mean free path of the molecules, bulk diffusion and Poiseuille flow do not occur. For this case, the change in volume given when C + CO2 —> 2CO has no influence on the rate of diffusion of carbon dioxide into the rod, and is not dependent on the total pressure in the pores. Considering a wedge of carbon (Fig. Al),... 

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

Molecular sieve, 5A 225 Air and light gas analysis H2, 02, N2, (CH4, CO, NO, SFe co-elute) Synthetic calcium aluminosilicate (zeolite), having an effective pore diameter of 0.5 nm, C02 is adsorbed strongly 5A usually gives the best results of all synthetic zeolites should be activated before use and used above critical adsorption temperature 21.6% (mass/mass) water capacity... 

The physicochemical characteristics of the HZSM5 samples studied are summarized in Table 1. In none of the samples studied extra lattice alumina was detected by 27AL-MAS-NMR, The parent zeolites were modified by treatment with SifOEt). These silylated samples were prepared by suspending the zeolite powder in dry n-hexane in which a calculated amount of tetraethyl orthosilicate was added [15]. The concentration of acid sites hardly changed by this modification (only the sites at the external surface were blocked), but the rate of transport of the molecules in the pores was drastically lower than with the untreated samples indicating a decrease in the effective pore diameter... 

The above values of pore diameter are the distances between the centres of the carbon atoms in the pore walls. From a practical standpoint (as pointed out in previous chapters) it has been customary to refer to the effective pore diameter (or width). In the case of a cylindrical buckytube, an approximate evaluation of the effective diameter is arrived at by allowing for the radius of the carbon atom (c. 0.17 nm). Thus, for the two model diameters considered by Maddox and Gubbins (i.e. 1.02 and 4.78 nm), the corresponding effective pore diameters are c. 0.7 and 4.4 nm. These are the values used in the following discussion. 

Most of these theories have one significant drawback They are all derived for the flat open adsorbent surface, but HPLC adsorbents are porous materials with average pore size on the level of 100 A for bare material. After chemical modification of the original silica surface, the effective pore diameter decreases and the properties of electric double layer in the confined space of small pores are significantly different from that on the flat surface. 

In smaller pores where the majority of the molecules are in the proximity of the walls, the presence of an immobile superficial layer limits even more the diffusion coefficient by reducing the effective pore diameter as a result in saturated Vycor (pore size 29 A) the movement of particles is about two times slower than the one in bulk phase, while in MCM-41-S (pore diameter 25 A) and MCM-48-S (pore diameter 22 A) the movement of particles is about four to ten times slower. The limit case is an in-file diffusion process, where particles are unable to pass each other in a channel. 

The acidity of zeolites can be reduced by the incorporation of boron in the zeolite framework [162,163] and therefore B-substitut ZSM-5, ZSM-11 and Beta were teaed [158,164]. Al " " free boron zeolites are inactive, but these zeolites with low levels of Al " " ions which can be obtained by adding AI2O3 binder to the Al free boron zeohte have weak acidity and are moderately active at 500 - 600°C and isobutene selectivities of up to 50 % have been reported. At these conditions the observed activity and selectivity of B/Al-ZSM-5, B/Al-ZSM-11 and B/Al-Beta were. similar and therefore it was concluded that the pore aructure did not play a decisive role in the converaon of n-butene into isobutene [164]. However, A1 which migrates into the pores not only modifies the acidity but also modifies the effective pore diameter. 

One of the problems which existed when this study began was that of suitable membranes. Regenerated cellulose membranes [Visking Co., Chicago, 111.] having an effective pore diameter of 24 A were easily available in both sheet and tube form. The problem with such material is the rate of decomposition. It was at first assumed that a run could be made before the membrane was decomposed. However, this objective proved to be impossible to attain. Usually in about 6 to 10 days the membrane would fail. 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

Filtration Model. A model based on deep-bed filtration principles was proposed by Soo and Radke (12), who suggested that the emulsion droplets are not only retarded, but they are also captured in the pore constrictions. These droplets are captured in the porous medium by two types of capture mechanisms straining and interception. These were discussed earlier and are shown schematically in Figure 22. Straining capture occurs when an emulsion droplet gets trapped in a pore constriction of size smaller than its own diameter. Emulsion droplets can also attach themselves onto the rock surface and pore walls due to van der Waals, electrical, gravitational, and hydrodynamic forces. This mode of capture is denoted as interception. Capture of emulsion droplets reduces the effective pore diameter, diverts flow to the larger pores, and thereby effectively reduces permeability. 

Two factors make the use of this direct method problematic. As the interactions in micropores are strong, relaxations of the lattice (and consequently changes in the effective pore diameter of the material) are observed upon adsorption. The accessibility is strongly temperature dependent. For example it is impossible for m-xylene to enter the pores of H-ZSM5 at ambient temperatures, while at 573 K the whole pore volume is accessed [32]. This results from the fact that, in the region of tight fit between the molecules and the lattice, the van der Waals radii are important. While this poses a significant constraint for the molecule to enter at 300 K, the barrier can be overcome at elevated temperatures. 

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