Membrane Materials and Preparation

In general, photocatalytic membranes could be divided into polymeric and inorganic ones. The photocatalyst could be supported on a membrane surface or entrapped in a membrane structure. The method of photocatalyst incorporation in the membrane structure depends mainly on the membrane material. In Fig. 6.17 a simplified division of photocatalytic membranes with reference to the appUcation, membrane material and preparation method is shown. 

In addition to the transport selectivities based on molecular charge or size described above, chemical interactions between the membrane material and the molecule to be transported can also strongly influence the rate and selectivity of transport. The introduction of chemically based transport selectivity was accomplished by chemisorbing thiols (RSH) to the Au tubule surfaces [113]. Membranes derivatized with two different R groups—the hydrophobic R = -CigHjj and the more hydrophilic (2)R = -C2H4-OH— were prepared. The rate and selectivity of transport in these membranes is dramatically altered by the chemical identity of the R group. 

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

A number of membrane materials and membrane preparation techniques have been used to make reverse osmosis membranes. The target of much of the early work was seawater desalination (approximately 3.5 wt% salt), which requires membranes with salt rejections of greater than 99.3 % to produce an acceptable permeate containing less than 500 ppm salt. Early membranes could only meet... 

In this paper an approach has been presented that will facilitate selection and evaluation of possible membrane materials. Having selected potential membrane materials for desired separations, steric considerations are taken Into account In membrane preparation. This approach avoids starting with and subsequently modifying aqueous-separation membranes, allows a vast spectrum of polymers to be considered as potential membrane materials and focuses the selection process. A brief review of membrane material evaluation procedures have been discussed with emphasis on those techniques which do not require the fabrication of membranes. Finally, the survey of materials evaluated for possible membrane use Indicates both the Interest In this field and the need for appropriate material selection and evaluation procedures. The ideas presented here will continue to grow In value In the future as membranes are called upon to achieve more difficult separations in an energy efficient fashion. 

The potential of membrane reactors has been widely verified and documented for a large number of reactions. However, all the studies made are stUl confined to the laboratory scale, and their implementation in industrial systems has yet to occur. Research into new membrane materials and improvement in the properties of currently available membranes (permselectivity, resistance to poisoning, stability, reduction of palladium thickness, etc.) are always in progress. The development of procedures to deposit the catalyst within the membrane structure without changing its initial permeability and selectivity is an example of the ongoing research for the preparation of catalytic membranes. 

Cellulose acetate membranes developed by Loeb and Sourirajan for the purpose of seawater desalination continue to be useful in various membrane applications, despite the development of new membrane materials and new membrane preparation techniques. Because of its historical importance, the casting method of the first successful reverse osmosis membrane is described below in detail. 

In our case, these membranes were made of the same material and prepared by phase inversion process [72, 73]. The dense selective skin layer was possible because of the evaporation of solvent during the initial period and the macroporous layer sticking to the skin layer was formed due to the exchange between the solvent and nonsolvent systems inside the precipitation bath [74]. 

Abstract This review is intended to provide the recent status in the development of polymeric-electrolyte (proton-exchange) membranes for the improvement of fuel cell performance based primarily on the preceding chapters of this book. Special attention is paid to the modification of present membranes, recent novel strategies for preparation of membranes, conceptual design of new membrane materials, and also promising approaches to overcome issues that severely restrict commercialization. The critical role of the materials and membranes and also relevant infrastructure of electrode is addressed. The new possibihties to improve technologies for implementation, and future trends are briefly examined. 

The separation layer, either porous or dense, can be formed using different methods such as sol-gel and template routes, hydrothermal synthesis, chemical vapor deposition (CVD), or physical sputtering, depending on the membrane material and its application. These membrane preparation methods will be described in the following chapters of this book for different membranes and membrane reactors. We note that the preparation of inorganic membranes involves a multi-step high-temperature treatment process. Therefore, inorganic membranes are much more expensive than polymeric ones. 

CMRs. It is well established that MIECs are inherently catalytic to oxidation reactions. Therefore, MIEC-derived membranes may serve as both catalyst and oxygen separator, and no other catalysts are used in the dense ceramic MRs. Since chemical reactions take place on the membrane surface, it is required to have a much more porous membrane surface so as to contain a sufficient quantity of active sites. This can be achieved in the membrane preparation process, or by coating a porous membrane material after preparation. The main potential problems for this configuration are that the membrane may not have sufficient catalytic activity and the catalytic selectivity cannot he modulated with respect to the reactions considered. 

Based on Table 5.3, technical trends of MF/UF membranes will be reviewed from the following two viewpoints (1) membrane materials and (2) membrane preparation technology. 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

---