Pore size distribution inorganic membranes

Fain. D. E. 1990. A dynamic flow-weighted pore size distribution. Proc. 1st Inti Conf. Inorganic Membranes, 1-5 July 1989, 199-205, Montpellier. 

Microfiltration membranes usually have a nominal pore diameter in the range of 0.1-10 pm. However, the membrane specification is not an absolute parameter. The membranes usually present a pore size distribution around the nominal value and the shape of the bioparticles can determine whether they are retained or pass through the membrane. The membranes are manufactured from polymers, such as Teflon, polyester, PVC (polyvinyl chloride), Nylon, polypropylene, polyethersulfone, and cellulose, or from inorganic materials, such as ceramic and sinterized stainless steel. 

Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM... 

This method has been employed to commercialize polycarbonate membranes with extremely narrow pore size distributions and a wide range of pore size. The pore length (depth) is limited due to the energy constraints of the charged particles. No inorganic membranes are commercially produced this way but mica membranes with pore diameters of 6 nm to 1.2 mm have been prepared in the laboratory [Quinn et al., 1972 Riedel and Spohr, 1980]. Membranes prepared by this technique are good candidates for fundamental transport studies due to their very uniform pore shape. 

The pore size distribution or its mean value of a porous inorganic membrane can be assessed by a number of physical methods. These include microscopic techniques, bubble pressure and gas transport methods, mercury porosimetry, liquid-vapor equilibrium methods (such as nitrogen adsorption/desorption), gas-liquid equilibrium methods (such as permporometry), liquid-solid equilibrium methods (thcrmoporometry) and molecular probe methods. These methods will be briefly surveyed as follows. 

Although this technique has been frequently used to characterize microporous organic membranes, it has not been applied to microporous inorganic membranes. Moreover, this method provides only the average pore size but not the pore size distribution. 

Besides higher unit costs, in both initial investment and replacement, another factor limiting inorganic membranes from wider usage is the control of their pore size distributions. Recent advances, however, such as the sol-gel process and anodic oxidation have started to be implemented in commercial production and have made significant impacts on the current and future market shares of the inorganic membranes. 

The transport properties (i.e. permeation and separation efficiency) of inorganic membrane systems depend, to a Icirge extent, on the microstructural features of the membrane and the architecture of membranes and modules. The microstructural features, such as pore shape and morphology, pore size (distribution), interconnectivity/tortuosity, as well as the architecture of the membrane and membrane-support combinations will be briefly described. Here, architecture means the way the different parts of the membrane system or module are shaped and combined. 

In this chapter, different aspects of the sol-gel process have been described which were applied to the synthesis of inorganic membrane materials. Macro-, meso- or microporous as well as almost dense materials can be obtained depending on the preparation method. Some limitations in the control of pore size and pore size distribution are attached to conventional sol-gel methods, namely the colloidal and the polymeric routes, frequently used for the S5mthesis of inorganic membrane materials. Thanks to recent advances in sol-gel processing, a number of these limitations have been overcome introducing new concepts in the preparation of membranes with tailor-made porous textures. 

A majority of commonly used inorganic membranes are composites consisting of a thin separation barrier on porous support (e.g., Membralox zirconia and alumina membrane products). Inorganic MF and UF membranes are characterized by their narrow pore size distributions. This allows the description of their separative performance in terms of their true pore diameter rather than MW CO value which can vary with operating conditions. This can be advantageous in comparing the relative separation performance of two different membranes independent of the operating conditions. MF membranes, in addition, can be characterized by their bubble point pressures. Due to their superior mechanical resistance bubble point measurements can be extended to smaller diameter MF membranes (0.1 or 0.2 pm) which may have bubble point pressure in excess of 10 bar with water. 

Frequently, inorganic membranes are used instead of polymeric membranes because of their outstanding chemical and thermal resistances. In addition, the pore size in these membranes can be better controlled and as a consequence the pore size distribution is generally very narrow (see also chapter IV). Various techniques can be used to prepare ceramic membranes with some important ones being ... 

Figure 3.6. Pore size distribution of inorganic membranes. (Reproduced from [34] with permission.)...

The micropores method is frequently used to determine pore size distributions below 20 A in inorganic membranes. For example, Larbot et al. [77] analyzed the pore size distribution of a Ti02 membrane with a mean pore size of 8 A. They also studied how pore size distribution width changes due to small variations of the sol— gel synthesis method. Other methods to analyze micropores are sometimes used too for instance, Kumar et al. [116] used the Horvath—Kawazoe method to select the appropriate sinterizing (calcination temperature) technique to prepare zeolite membranes. 

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