Membrane mesoporous/macroporous

Membrane characterization means the determination of structural and morphological properties of a given membrane. Because membranes range from porous to nonporous depending on the type of separation problem involved, different characterization techniques are required in each case. For example, in MF or UF membranes, fixed pores are present. MF membranes have macropores (pore diameter > 50 mn), while UF membranes have mesopores (2 mn < pore diameter < 50 nm). The pore size (and size distribution) mainly determines which particles or molecules are retained or pass through. On the other hand, for dense or nonporous membranes, no fixed pores are present and the material chemistry itself mainly determines the performance. 

Zhao D., Yang P., Chmelka B.F., Stucky G.D. Multiphase assembly of mesoporous-macroporous membranes. Chem. Mater. 1999 11 1174-1178 Zhao D., Yang P., Margolese D.I., Chmelka B.F., Stucky G.D. Synthesis ofcontinuous mesoporous silica thinfilms with three-dimensional accessible pore structures. Chem. Commun. 1998b 2499-2500... 

Inorganic membranes employed in reaction/transport studies were either in tubular form (a single membrane tube incorporating an inner tube side and an outer shell side in double pipe configuration or as multichannel monolith) or plate-shaped disks as shown in Figure 7.1 (Shinji et al. 1982, Zaspalis et al. 1990, Cussler 1988). For increased mechanical resistance the thin porous (usually mesoporous) membrane layers are usually supported on top of macroporous supports (pores 1-lS /im), very often via an intermediate porous layer, with pore size 100-1500 nm, (Keizer and Burggraaf 1988). 

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). 

For the description of this flow, the Carman-Kozeny expression [16] can be applied, since the Hagen-Poiseuille equation is not valid, given that usually inorganic macroporous and mesoporous membranes are prepared by the sinterization of packed quasispherical particles, which develop a random pore structure [19]. In this case, the Carman-Kozeny factor for a membrane formulated with pressed spherical particles is [74]... 

The viscous diffusion mechanism is also valid for transport process in the liquid phase. Then, if we have a liquid filtration process through a porous (i.e., macroporous or mesoporous) membrane, the following form of the Carman-Kozeny equation can be used [9]... 

Amorphous microporous silica membranes as discussed here, consist of a macroporous a-alumina support (pore diameter -100 nm) with a mesoporous y-alumina intermediate layer (Kelvin radius of 2.5 nm) and a microporous silica top layer (pore diameter -4 A) [1,2],... 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

The diameter or the radius of the pores is one of the most important geometric characteristic of porous solids. In terms of lUPAC nomenclature, we can have macropores (mean pore size greater than 5 x 10 m), mesopores (between 5 x 10 and 2 x 10 m) and micropores (less than 2 x 10 m). The analysis of species transport inside the porous structure is very important for the detailed description of many unit operations or applications among them we can mention suspension filtration, solid drying and humidification, membrane processes (dialysis, osmosis, gaseous permeation. ), flow in catalytic beds, ion exchange, adsorp-... 

Fig. 3 Scaiming electron microscope images of a mesoporous (A) and a macroporous glass membrane (B)...

In the case of the smallest pores (mesopores and micropores), the developed area is very large and the permeability is very low. Thus the thickness of the separative layer must be thin enough to reach attractive fluxes with experimentally acceptable transmembrane pressure. On the other hand, the mechanical strength of the membrane must be large enough to withstand the applied pressure. These considerations led to the concept of asymmetric structure based on macroporous support and successive layers with decreasing thickness and pore size (Table 25.3 Figure 25.1). 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

In liquid filtration using micro-, ultra-, and nanofiltration porous membranes, the driving force for transport is a pressure gradient. Solvent permeability and separation selectivity are the two main factors characterizing membrane performance. Convective flux is predominant with macroporous and mesoporous membrane strucmres, the selectivity being controlled by a... 

Basic mechanisms involved in gas and vapor separation using ceramic membranes are schematized in Figure 6.14. In general, single gas permeation mechanisms in a porous ceramic membrane of thickness depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. In membranes with large mesopores and macropores the separation selectivity is weak. The number of intermolecular collisions is strongly dominant and gas transport in the porosity is described as a viscous flow that can be quantified by a Hagen-Poiseuille type law ... 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

The flux with a given membrane material(s) and structure can be increased by decreasing the membrane thickness. The thinner the separation layer, however, the larger the risk of forming defects which decrease the separation factor. Mesoporous separation layers of good quality with layer thicknesses down to 5-10 pm on macroporous supports has been realised with reasonably large surface areas. For microporous layers this has been shown only on small plates for silica (layer thickness 0.1 pm) and zeolites (layer thickness 5-10 pm). 

Porosities of membrane components vary widely and values are reported ranging from 20 to 60%. Commonly, values of 30-40% are used. Pore sizes range from macropores (>500 nm) via mesopores (20-500 nm) to micropores (<2 nm). A great problem is the lack of reliable measurement methods to measure the porosity and pore size distribution of supported membranes (see Chapter 4). 

There are relatively few studies dealing with adsorption on microporous inorganic membranes. Except the work by Ma and his co-workers and Burggraaf and his co-workers, few studies on the interrelation between adsorption and permeation have been reported. The extremely thin membrane layer on a relatively thick membrane support makes the adsorption measurement rather difficult. Neither gravimetric nor volumetric technique will provide sufficient accuracy for the measurement due to the extremely small fraction of the membrane layer in a supported membrane. Nevertheless, adsorption measurements can give important information on pore sizes and permeation mechanisms in microporous membranes. This section will examine the adsorption of gases on microporous membranes and of liquids on mesoporous and macroporous membranes. 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

The film-coating process is applied using suspensions on macroporous substrates to produce intermediate films with macropores in order to obtain microfiltration membranes or to obtain composite, asymmetric supports suitable for production of ultra-fine, mesoporous membranes as discussed in Chapter 6. 

Viscous (Poiseuille) flow and molecular diffusion are non-selective. Nevertheless they play an important role in the macroporous substrate(s) supporting the separation layer and can seriously affect the total flow resistance of the membrane system. Mesoporous separation layers or supports are frequently in the transient-regime between Knudsen diffusion (flow) and molecular diffusion, with large effects on the separation factor (selectivity). 

Permeation in Binary Gas Mixtures in Macroporous and Mesoporous Membranes... 

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