Alumina framework structures

The use of framework structures to minimize AH for alkali-ion electrolytes has been demonstrated to provide a means of opening up the bottlenecks to cation motion in a number of oxides (Goodenough, Hong and Kafalas, 1976). Framework structures may provide one-dimensional tunnels as in hollandite, two-dimensional transport in planes as in the )S-aluminas, or three-dimensional transport as in NASICON and LISICON. Since one-dimensional tunnels are readily blocked, the two-and three-dimensional conductors are the more interesting. 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

The structural framework of silica [5] is based on interconnected Si(-0->4 tetrahedra, with some OH terminations at the surface. Aluminas are built primarily of A1(-0-)4 tetrahedra and AK-O-) octahedra, with some surface-OH terminations [6]. The fundamental framework structure of aluminosilicates include primarily Si(-0-)4 and AK-O-) units plus charge compensating bridging or framework hydroxyl groups, namely structure I, which shows formal charges on oxygen and aluminum [7]. On the basis of these kinds of structures and various so-called defect structures that may include octahedral and three-coordinate aluminum sites, the investigation by NMR of a wide variety of nuclei can be anticipated. Numerous NMR studies based on Si, Al, H, H, and O have been reported on silica, alumina, and silica-alumina systems. 

Zeolites are crystalline alumina-silica-based open anionic framework structures with uniformly sized, rigid pores and channels. Incorporation of porphyrins into zeolites has been explored as means to biomimetic oxidation catalysts. " The initial efforts to combine porphyrins with zeolites resulted in materials in which the porphyrins were supported on the external zeolite surfaces rather than encapsulated inside the pores. The first of this type of material was reported by Bedioui and coworkers which the... 

The synthesis of silicalite-1 as described earher does not yield an active catalyst per se, but rather a framework structure aldn to the well-known zeolite ZSM-5, which is heavily applied as solid add in, for example, petrochemical industries for the production of transportation fuels and bulk chemicals. Other examples of support oxides are, for example, amorphous silica and alumina. These are most regularly functionalized in a set of steps leading to the deposition of metal or metal oxide nanoparticles. This procedure is schematically shown in Figure 12.4, and explained later, with direct links to possible in situ characterization studies. 

In addition to changes to the cationic structure, the Si/Al ratio can be varied during manufacture from unity to well over 1000. Thus zeolites with widely different adsorptive properties may be tailored by the appropriate choice of framework structure, cationic form and silica to alumina ratio in order to achieve the selectivity required for a given separation. Many zeolites are extremely polar and therefore separations may be effected using both molecular sieving and internal surface property effects. The kinetic selectivity is determined from the free diameters of the windows in the intra-crystalline channel structure. Examples of such diameters, together with the principal properties and main uses of zeolites, are given in Table 2.4. 

The elementary building block of the zeolite crystal is a unit cell. The unit cell size (UCS) is the distance between the repeating cells in the zeolite structure. One unit cell in a typical fresh Y-zeolite lathee contains 192 framework atomic positions 55 atoms of aluminum and 1atoms of silicon. This corresponds to a silica (SiOj) to alumina (AI.O,) molal ratio (SAR) of 5. The UCS is an important parameter in characterizing the zeolite structure. 

The conductivity, due to Na ions, passes through a maximum at intermediate x. It is optimised at x 2, where the values approach those of Na / "-alumina, especially at high temperature, >300°C, Fig. 2.11. At the solid solution limits, x = 0 and 3, the conductivity is very low, for the same reasons given in the discussion of Fig. 2.3. The crystal structure of NASICON is a framework, built of (Si, P)04 tetrahedra and ZrOg octahedra which link up in such a way as to provide a relatively open, three-dimensional network of sites and conduction pathways for the Na ions, Fig. 2.12(a). Two Na sites are available, Nal and Na2. The former is a six-coordinate site while the latter is an irregular eight-coordinate site. These sites are partially occupied at intermediate x. 

Tschortnerite (TSC) surely is the most remarkable novel zeohte mineral discovered [67]. Its unique framework topology contains five different cages D-6Rs, D-8Rs, sodahte cages, truncated cubo-octahedra and a unique 96-membered cage. Cu-containmg clusters are encapsulated within the truncated cubo-octahedra. The pore structure is three-dimensional with 8R charmels, and the framework density of 12.2 is among the lowest known for zeolites. The framework is alumina-rich with Si/Al = 1, unusual for zeohte minerals. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Framework (skeleton) structures of oxides have been identified for fast ion conduction of Na" and other ions (Goodenough et al., 1976). One-, two- or three-dimensional space is interconnected by large bottlenecks in these oxide hosts. While the tungsten bronze and j8-alumina structures contain one- and two-dimensional interstitial space, the hexagonal framework of NaZr2(P04)3 has a three-dimensional... 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

Zeolite is a crystalline, porous aluminosilicate mineral with a unique interconnecting lattice structure. This lattice structure is arranged to form a honeycomb framework of consistent diameter interconnecting channels and pores. Negatively charged alumina and neutrally charged silica tetrahedral building blocks are stacked to produce the open three-dimensional honeycomb framework. 

---