Zeolites close packing

The threshold value derived from a plot of the residual amount of crystalline NaCl versus the total amount of NaCl in each sample, as shown in Fig. 15, is 0.39 g NaCl/g NaY. This value corresponds to a dispersion capacity of 10.5 NaCl molecules Vsodalite cage in the NaY zeolite. In view of the fact that a sodalite cage can accommodate only one NaCl molecule, most of the NaCl molecules are dispersed on the wall of the supercages. On the basis of the close-packed monolayer capacity of NaCl (0.085 g/100 m2) taken from Table II and the BET surface area of NaY zeolite (800 m2/g), we estimate the utmost monolayer capacity at 0.68 g/g NaY, which is reasonably higher than the experimental value of 0.39 g NaCl/g NaY, because in our calculation we have neglected heterogeneity of the internal surface of the zeolite. 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

Lee JS, Ha K, Lee YJ, and Yoon KB. Ultrasound-aided remarkably fast assembly of monolayers of zeolite crystals on glass with very high degree of lateral close packing. Adv Mater 2005 17 837-841. 

The simplest analytic model for an isolated proton in a lattice assumes that it is situated in a potential well centred on an interstitial site. This model is particularly appropriate to protons in a transition metal lattice, where the electron from the hydrogen atom can be accommodated in the d-band of the metal, but is also applicable to many other cases as well - e.g. to molecular hydrogen trapped in ion-exchanged zeolites (see Section 6.8.2 below). The model assumes that there are no interactions between neighbouring hydrogen atoms and that there is little coupling with the lattice modes. This implies that M/wjp 1 where m is the mass of the proton and M is the mass of the lattice atom. In transition metals, with face-centred cubic (FCC), body-centred cubic (BCC) or hexagonal close packed (HCP) lattices, the proton normally sits on either octahedral or tetrahedral sites. In more complex intermetallic... 

The work was later extended to the study of adsorption of n-butanol from water and it was found that, again, as in the case of the adsorption from n-heptane, n-butanol forms closed packed monolayers but this time exclusively on the hydrophobic sites, which for graphitic carbons are basal planes. The work on carbons was reported by the author in 1987 [18], Subsequently the work on carbons was continued with other partly hydrophobic solids, such as heat treated silica gels, ZSM-5 zeolites, coals, and carbon blacks [28]. All these solids possessed various proportions of hydrophilic sites, the surface areas of which could be independently estimated from the integral heats of adsorption of n—butanol from water. 

The geometrical structure of pores is of great concern, but the three-dimensional description of pores is not established in less-crystalline porous solids. Only intrinsic crystalline intra-particle pores offer a good description of the structure. The hysteresis analysis of molecular adsorption isotherms and electron microscopic observation estimate the pore geometry such as cylinder (cylinder closed at one end or cylinder open at both ends), cone shape, slit shape, interstice between closed-packing spheres and inkbottle. However, these models concern with only the unit structures. The higher order structure of these unit pores such as the network structure should be taken into accoimt. The simplest classification of the higher order structures is one-, two- and three-dimensional pores. Some zeolites and aluminophosphates have one-dimensional pores and activated carbons have basically two-dimensional slit-shaped pores with complicated network structures [95]. 

Chen and Sholl presented a detailed model for the permeation of CH4/H2 mixtures through membranes constructed from closely packed bundles of single walled carbon nanotubes [13]. Combination of atomically detailed and continuum models that has proven effective in previous treatments of mixture permeation through zeolite membranes was apphed. 

In addition, from Table 6.9 it can be observed that majority of pure zeolite crystals are orthorhombic, rhombohedral, and spherical in shapes, which are observed to have resemblance with the shapes noticed in the micrographs of the zeolitized residues (refer Fig. 6.24). Further, with reference to their less values of d-spacing of the crystal lattice (dioo = 2.85-3.26 A, refer Table 6.9), these zeolites are inferred to have more closely packed atomic planes, which indicates their better stmctural stability, as compared to monoclinic and cubic shapes, formed in the residues of Step-1 treatment. 

The density of molecules arrangement of various nature and geometry in pentasil zeolites varies. For example, n.alkanes in silicalite are arranged very dense (end to end) and occupy all adsorption space (9). N.alcohols fill O.b of sorption volume of their channels while benzene only O.b of the volume. (Thus, in one case intermolecular interaction of alcohols with each other through hydrogen bonds prevent their close packing in the channels while in the other case (when benzene is adsorbed) molecular conformation prevent it from penetration into more narrow zig-zag channels. 

For instance, the activation energy for butene formation from n-butyl alcohol is 140 lOkJmol-1 on HZSM-5 and only 95 lOkJmol-1 on AAS. At 378 K, 94% ether plus 6% butene are formed over HZSM-5, whereas 43% ether and 57% butene are formed over AAS. Bearing in mind that butyl alcohol molecules, as well as those of intermediates and products of their dehydration, have dimensions closely similar to the diameter of the zeolite channels, we infer that a liquid-like packing of butyl alcohol molecules and other reaction participants occurs in the channels (as schematized in Fig. 5). We opine that some specific ordering of the adsorbed species in the catalyst channels may be induced by hydrogen bonding and hydrophobic interactions between them. 

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