Dense/microporous materials

We have discussed previously diffusion in dense crystalline materials. Now, we study the transport of molecules in porous media. According to the classification scheme proposed by the International Union of Applied Chemistry (IUPAC), pores are divided into three categories on the basis of size macropores (more than 50 nm), mesopores (from 2 to 50 nm), and micropores (less than 2 nm) [74,75],... 

The kind of network affects the textural properties of final products highly branched network favor the formation of open structures (i.e. mesoporous materials), while weakly branched networks tend to collapse forming more densely packed structures (i.e. microporous materials). 

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

When Dense or Microporous Materials Control the Overall Process Performance... 

For membrane operations in which dense or microporous materials control the overall process performance there is no doubt that process intensification will follow directly from improvement of material properties. With the development of new materials having properties controlled at the nanoscale level, operations of this kind seem promised a really bright future. 

The increasingly indispensable role of atomistic and quantum mechanical simulations in inorganic crystallography is perhaps no more strikingly illustrated than in the field of silicates and zeolitic materials. The two classes of material with which we shall be concerned in this chapter, namely microporous zeolites (including both aluminosilicate-based materials and their sister compounds, the aluminophosphates (ALPOs)), and the dense silicate materials and related oxides which constitute the bulk of the Earth s mantle, have in common the fact that their structural properties may be difficult to determine by conventional experimental means. Yet highly detailed and accurate structural information is critical in understanding the properties of both these important types of material. 

The most active areas of development for membrane materials are currently synthesis of supported thin Aims, and pore modification. The complete selectivity to one species provided by dense membranes is very attractive, but is accompanied by low permeation rates if the membrane is composed entirely of the dense material. To maintain structural stability, thinner dense films must be supported by materials that are strong but that offer no additional resistance to permeation. Similar principles apply to the use of microporous materials with high permselectivities or molecular sieving effects. Some examples of these developments are supported Pd films on porous aluminas,or on porous stainless steel, and supported zeolite films. Pore modification has been used to deposit materials inside mesoporous materials, an example being the deposition of SiOa films in porous glass. 

It was not equally obvious that dense ceramic hydrogen-permeable membranes would be of similar interest. There are clearly needs for hydrogen purification membranes, but polymers and microporous materials as well as metals such as palladium and its alloys appeared to fill these needs. In addition, possible candidates for dense ceramic hydrogen-permeable materials were not as appealing as the oxygen-permeable ones in terms of performance and stability. 

Similar methods have been used in the study of the self assembly processes by which both dense phase and microporous materials form. Often these studies have revealed detailed and sometimes surprising insight into... 

Figure 8.35 A schematic representation of the structural changes observed in microporous polytetrafluoroethylene undergoing tensile loading in the x direction (a) initial dense microstructure, (b) tension in fibrils causing transverse displacement of anisotropic nodal particles with lateral expansion, (c) rotation of nodes producing further lateral expansion and (d) fully expanded structure prior to further, plastic deformation due to node break-up. (Reproduced from Evans, K.E. and Caddock, B.D. (1989) Microporous materials with negative Poisson s ratios. II. Mechanisms and interpretation. J. Phys. D. Appl. Phys., 22, 1883. Copyright (1989).)...

Structures at all relevant length scales, as described in Sections 34.2.1-34.2.4, can be classified further into organized and random packing stmetures. For dense materials, structural organization is expressed in the presence (or absence) of a periodic crystal lattice. For microporous materials there is a clear distinction between crystalline zeohtes and amorphous sihea with a very short range order. Zeohte membranes may consist of a three-dimensional mosaic of crystaUites that may be either randomly orientated with respect to each other or possess a certain preferred orientation or texture (Lai et al., 2003). The polycrystaUine nature of and presence of texture in zeohte membranes can have important consequences for flux and separation behavior. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

Not all frameworks built from tetrahedra as described above are considered to be zeolites. Dense phases are not considered to be zeolites, only those phases with some porosity. Generally, materials with pores accessible by windows defined by six T-atoms or less (six-rings) are not considered to be zeolites. In fact, the boundary between zeolites and dense phases is somewhat nebulous. lUPAC defines [1] zeolites as a subset of microporous or mesoporous materials containing voids arranged in an ordered manner and with a free volume larger than a 0.25 nm diameter sphere. The Structure Commission of the International Zeolite Association uses the criterion of framework density (T-atoms per lOOOA ) with the maximum framework density for zeolites ranging from 19 to 21. 

In this respect the dense nonporous ion-exchange material of a membrane may be viewed as a one-phase medium. In contrast to this a porous bulk ion-exchanger (e.g., an ion-exchange bed or a single microporous ion-exchange bead) is a two-phase medium with the possibility for each ion to be in either one of the two phases—in the ion-exchange matrix proper or in the aqueous pore. 

In this sense, the supports are covered with films of microporous (pores from 0.3-2nm) or dense materials, where the support gives mechanical strength while the coating is intended to carry out selective separations [180],... 

Considering the microstructure of membranes, they can be categorized as porous, which allow transport through their pores, or dense, which permit transport through the bulk of the material [19]. Porous membranes are classified as microporous, mesoporous, and macroporous (see Section 6.2). 

Different methods have been used to deposit microporous thin films, including solgel, pyrolysis, and deposition techniques [20], Porous inorganic membranes are made of alumina, silica, carbon, zeolites, and other materials [8], They are generally prepared by the slip coating method, the ceramic technique, or the solgel method (Section 3.7). In addition, dense membranes are prepared with metals, oxides, and other materials (Chapter 2). 

The methods of preparing inorganic membranes with tortuous pores vary enormously. Some use rigid dense solids as the templates for creating porous structures while many others involve the deposition of one or more layers of smaller pores on a premanufactured microporous support with larger pores. Since ceramic membranes have been studied, produced and commercialized more extensively than any other inorganic membrane materials, more references will be made to the ceramic systems. 

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