Ceramic membranes nanopores

Ceramic membrane is the nanoporous membrane which has the comparatively higher permeability and lower separation fector. And in the case of mixed gases, separation mechanism is mainly concerned with the permeate velocity. The velocity properties of gas flow in nanoporous membranes depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collision. The Knudsen number Kn Xydp is characteristic parameter defining different permeate mechanisms. The value of the mean free path depends on the length of the gas molecule and the characteristic pore diameter. The diffusion of inert and adsorbable gases through porous membrane is concerned with the contributions of gas phase diffusion and sur u e diffusion. 

Nanoporous inorganic ceramic membranes show significant promise for hydrogen separation and purification, primarily due to the high selectivity that is afforded by this class of membranes. Development work has focused on zeolites although... 

In contrast, if the membrane is an inorganic composition (e.g., a dense metal membrane or a nanoporous ceramic membrane), the membrane module may be operated at the elevated temperature of 450 °C. In this case, there is no need for optional HEX 2 as the fuel gas stream will exit the membrane module at 450 °C and pass to the burner without further cooling. In addition to a net increase in overall process energy efficiency, the elimination of HEX 2 also represents a reduction in capital cost for the system. 

Likewise, a thin nanoporous alumina membrane deposited onto a porous stainless steel support will likely fail unless efforts are taken to match the CTE of these diverse layers. This may be done by building up layers on the support such that a gradient in CTE is achieved. Also, thin metal membranes deposited on porous ceramic membranes have historically exhibited poor durability with respect to cracking and general membrane failure due, at least in part, to mismatched CTE. 

This chapter focuses on the chemical processing of ceramic membranes, which has to date constituted the major part of inorganic membrane development. Before going further into the ceramic aspect, it is important to understand the requirements for ceramic membrane materials in terms of porous structure, chemical composition, and shape. In separation technologies based on permselective membranes, the difference in filtered species ranges from micrometer-sized particles to nanometer-sized species, such as molecular solutes or gas molecules. One can see that the connected porosity of the membrane must be adapted to the class of products to be separated. For this reason, ceramic membrane manufacture is concerned with macropores above 0.1 pm in diameter for microfiltration, mesopores ranging from 0.1 pm to 2 nm for ultrafiltration, and nanopores less than 2 nm in diameter for nanofiltration, per-vaporation, or gas separation. Dense membranes are also of interest for gas... 

Recently much attention has been paid to ceramic membranes exhibiting a nanoporous structure with the aim of new membrane processes for the nanofiltration of liquids [26], pervaporation [27], gas separation [27,28], or catalysis... 

S. Sircar and M. B. Rao, "Nanoporous carbon membranes for gas sepmation," in Recent advances on gas separation microporous ceramic membranes," N. Kanellopoulos (ed.), Elsevier, New York, p. 473 (2000). 

Recently, boron carbide nanostructures have attracted much attention as they have certain advantages over their bulk counterparts [147]. Nanoscale ceramic fibers, nanocylinders and nanoporous structures - as do their well-known carbon counterparts - have a tremendous number of potential applications, including uses as quantum electronic materials, structural reinforcements, and ceramic membranes for use as catalyst supports or in gas separations [148]. 

Janowska, I. Hajiesmaili, S. Begin, D. Keller, V. Keller, N. Ledoux, M.-J. Pham-Huu, C., Macronized aligned carbon nanotubes for use as catalyst support and ceramic nanoporous membrane template. Catal. Today 2009,145 76-84. 

Particularly promising is the development of nanoporous ceramic semiconductor membranes [692-695], They not only possess all of the advantages of ceramic materials, but they may also be efficient light harvesters having large surface areas which could provide sites for sensitizers [108, 711-714]. Indeed, FeS2 particles, deposited into (and onto) a porous Ti02 electrode, sensitized photoelectron conversion well (Fig. 118) [714]. 

When, to satisfy ultrafiltration, nanofiltration, or gas separation requirements, the required pore size is under 0.1 pm, the ceramic powder approach is no longer viable. Indeed, individual particles yielding pore diameters smaller than 0.1 pm cannot be handled by powder processing. In fact, particles of this type enter the category of colloids and must be maintained as a stable suspension during the process. As indicated in the introduction, the sol-gel method is a very suitable way to produce mesoporous and nanoporous membranes. The latter is elaborated in the next section. 

These membranes are achievable using the concept of nanophase ceramics. According to literature, this new class of materials can result from the emphasis of some new ceramic processes, such as the condensation of gaseous atomic clusters [30] or the sol-gel process [31]. This last method, which has been successfully applied to ultrafiltration membranes, was used recently to prepare ceramic nanofilters. Nanophase materials deal both with the nanometer-sized particle and with the nanometer pore size aspects. The nanopore aspect is central to membrane technologies because of the need for selective separation processes at the molecular level. 

Concerning membranes, new separation capabilities are expected for these materials. The molecular sieving effect caused by connected nanopores can be applied to the separation of molecules with molecular weights smaller than 1000. The key properties of such membranes are based on the preponderant effect of activated diffusion in nanopores, however. This phase transport phenomenon derives from the nanophased ceramic concept and classes these membranes among those materials expected to be crucial in the areas of modem technology, such as environmental protection, biotechnology, and the production of effect chemical. 

Recent advances in sol-gel science allowed us to go further in controlling the individual particle size at the sol stage, resulting in individual nanoscale grains in the ceramic. This new development is based on the chemical modification of both metal salt and metal alkoxide precursors to modify their reactivity and to achieve a nanopore structure for the membranes [34]. Some ex-... 

Lin, 2(X)1). However, the sol-gel process presents inherent advantages for the preparation of membrane materials made of nanophase and/or nanoporous ceramic oxides or hybrid materials. In particular the nanostructure of sol-gel derived materials can be controlled together with their porous structmre (e.g., formation ofmesopores or micropores). Other quoted advantages of sol-gel processed ceramics are compositional homogeneity and the ability to prepare shaped-materials such as spherical particles, fibers and thin films (Guizard, 1999). 

Kawasaki T, Tokuhiro M., Kimizuka N., Kunitake T. Hierarchical self-assembly of chiral complementary hydrogen-bond networks in water. J. Am. Chem. Soc. 2001 123 6792-6800 Kharton V.V., Marques F.M.B. Mixed ionic-electronic conductors effects of ceramic microstructure on transport properties. Curr. Opin. Solid State Mater. Sci. 2002 6(3) 261-269 Kikkinides E.S., Stoitsas K.A., Zaspalis V.T. Correlation of structural and permeation properties in sol-gel-made nanoporous membranes. J. Colloid Interface Sci. 2003 259 322-330 Kilner J., Benson S., Lane J., Waller D. Ceramic ion conducting membranes for oxygen separation. Chem. Ind. November 1997 907-911... 

Seol JH, Won JH, Yoon KS, Hong YT, Lee SY (2012) Si02 ceramic nanoporous substrate-reinforced sulfonated poly(arylene ether sulfone) composite membranes for proton exchange membrane fiiel ceUs. Int J Hydrog Energy 37(7) 6189-6198... 

Novel nanoporous membranes have recently been developed by Peled and coauthors [156]. Such membranes consisted of PTFE as the backbone with a nanosized ceramic powder (Aerosil 200 or Aerosil 130) dispersed in it. An aqueous solution of sulfuric acid adsorbed inside the pores of such membranes acted as an ionic conductor. Thus prepared membranes consisted of 50-200 nm spherical particles with nanovoids between them. They have been found to be quite elastic. Preliminary tests conducted using FCs with 250 pm thick nanoporous membrane, with electrodes on which 4 mg Pt/cm was dispersed on both the anode and the cathode, and fueled by 1 M methanol (in 3 M aq. H2SO4) flowing at the rate of 180 ml/min, against 3 atm of dry air, yielded 50 and 130 mW/cm at 80 °C and at 130°C, respectively. The crossover of methanol in these relatively inexpensive membranes was 0.27 and 0.56 A/cm at 80 and 130°C, respectively. Its selectivity to methanol was estimated to be in the same range as PVDF-ceramic powder hybrid [157]. 

The preparation of a novel catalytic membrane system to be used in multiphase H2O2 production has also been discussed in detail by Tennison et al. in 2007. In their review, it was shown that it is possible to produce a membrane system that is potentially suitable for use in both multiphase and gas phase membrane reactor systems based on a 2-layer ceramic substrate. Moreover, the performance is sensitive to the degree of perfection of the support. The carbon membrane deposited within the nanoporous layer of the substrate has the structure and surface area to enable high dispersions of catalyst metals to be achieved when oxidized in carbon dioxide that have shown good performance in the direct synthesis of H2O2. When prepared under nitrogen, despite the simple production route, the carbon membrane shows excellent gas separation characteristics. 

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