Molecular sieve table, pore diameter

Table 12 Pore Diameters, dp, of Common Molecular Sieves and Critical Diameters, d, of Selected Molecules...

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

If the water content is driven off (usually by heating to 350 °C in a vacuum), the dehydrated zeolite becomes an avid absorber of small molecules, especially water. The size of the molecules that can be absorbed is limited by the zeolite pore diameter, which is different for different zeolites (Table 7.1) a given zeolite (e.g., zeolite 3A) can be a highly selective absorber of, say, small amounts of water from dimethyl sulfoxide (DMSO) solvent. For this reason, dehydrated zeolites are often called molecular sieves. 

Powdered, particulate MCM-41 molecular sieves (Si/Al = 37) with varied pore diameters (1.80, 2.18, 2.54 and 3.04 nm) were synthesized following the conventional procedure using sodium silicate, sodium aluminate and C TMAB (n = 12, 14, 16 and 18) as the source materials for Si, A1 and quaternary ammonium surfactants, respectively [13]. Each sample was subjected to calcination in air at 560 °C for 6 h to remove the organic templates. The structure of the synthesized material was confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. Their average pore sizes were deduced from the adsorption curve of the N2 adsorption-desorption isotherm obtained at 77 K by means of the BJH method (Table 1). 

On the other hand, actual binary mixture tests using porous alumina and glass membranes show separation factor values for helium recovery from oxygen that are lower than what Knudsen diffusion provides, as indicated in Table 7.15. Only Koresh and Soffer [1983a 1983b] show an ideal separation factor of 20 to 40 with a low permeability of 1.2x10 barrers when molecular sieve membranes with a reported pore diameter of 0.3 to 0.5 nm are used. 

That is the reason why lower pore size membranes working by molecular-sieving mechanism have been tested with a view to perform continuous operations. The use of a zeolite membrane (pore diameter estimated from the Dubinin-Astakhov analysis [20] 1.1 nm) provided an interesting rejection of 0.98 [10] (see Table 7.1). In that case, caffeine adsorption was weak and zeolite membrane could not be easily fouled with the solute. Transport was mainly controlled by molecular sieving, as indicated by the good rejection rate also obtained with other molecules (e.g., lauric acid) having molecular weight close to caffeine. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Table 4-3 gives an overview of technical application of adsorption used for gas drying, separation of gas mixtures, gas generation, gas enrichment, gas cleaning, and wastewater treatment by means of selected examples. Table 4-4 gives the effective pore size of different molecular sieves and adsorbate molecule diameters. 

Table 4-4. Effective pore diameter of different molecular sieves and corresponding adsorbate with smaller critical molecular diameter [4.11].

Some characteristics of channels (windows) are of the same order as the dimensions of small molecules. The diameter of the entry window determines the size of the molecules which can enter the pores of 4 A, 5 A, 8.0 A and 9.0 A (see Table 4-7). A molecular sieve of 5 A-type is the most commonly used adsorbent for separation of permanent gases, for example, nitrogen and oxygen. The elution order at room temperature is follows neon, hydrogen, argon/oxygen, nitrogen, methane, carbon monoxide etc. 

This extremely fine-pore structure gives vitreous carbon the characteristics of a molecular sieve and allows the absorption of some very small molecules as shown in Table 6.1.01 This table shows a minimal absorption for water (in spite of the small diameter of its molecule). This contradicts the results of Fitzert l who found the absorption to be similar to that of methanol. 

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