Membrane permeable porous silicon

Figure 1.5 Microreactor with permeable porous silicon membrane (squares of 350-750pm, thickness 70pm) coated with Pd (via immersion) with heating filament and temperature sensor for CO measurements up to 140°C. Reprinted from [37], Copyright 2002, with permission from Elsevier.

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Various analytical techniques make use of both porous and nonporous (semipermeable) membranes. For porous membranes, components are separated as a result of a sieving effect (size exclusion), that is, the membrane is permeable to molecules with diameters smaller than the membrane pore diameter. The selectivity of such a membrane is thus dependent on its pore diameter. The operation of nonporous membranes is based on differences in solubility and the diffusion coefficients of individual analytes in the membrane material. A porous membrane impregnated with a liquid or a membrane made of a monolithic material, such as silicone rubber, can be used as nonporous membranes. 

Dense homogeneous polymer membranes are usually prepared (i) from solution by solvent evaporation only or (ii) by extrusion of the melted polymer. However, dense homogeneous membranes only have a practical meaning when made of highly permeable polymers such as silicone. Usually the permeant flow across the membrane is quite low, since a minimal thickness is required to give the membrane mechanical stability. Most of the presently available membranes are porous or consist of a dense top layer on a porous structure. The preparation of membrane structures with controlled pore size involves several techniques with relatively simple principles, but which are quite tricky. 

Table 34.3 H2 Permeation Rates and Permeability Ratio of H2/CH4 for Plasma Polymers of Butyronitrile Deposited on Silicone-Carbonate-Coated Polysulfone Porous Membranes...

VOCs can also be removed by applying vacuum and using composite membranes as, for example, in the VaporSep process commercialized by the MTR, where a porous support is used for a silicone membrane coating in a spiral wound configuration. Hydrophobic polypropylene hollow fibers with an ultrathin and highly VQC-permeable plasma-polymerized nonporous silicone skin on the outer surface can be also effective [31-33]. 

Membranes for vapor removal from air have a structure similar to the prism membrane, but they are prepared on a different principle.Aromatic PEI is used to produce a porous substrate membrane by the dry-wet phase inversion method. This polymer was chosen over PS/PES because of the higher durability of PEI to organic vapors. Unlike an asymmetric PS substrate for the prism membrane, the top layer of asymmetric PEI membrane has a large number of pores, the size of which is equivalent to those of UF membranes. When a layer of silicone rubber is coated on the top layer of the porous substrate membrane, the silicone rubber layer will govern the selectivity and the porous support will provide only mechanical strength to the composite membrane. Because the permeabilities of water and organic vapors through the silicone... 

Polybutadiene/polycarbonate membranes with a pp-ethylenediamine layer had an increased gas permeability (in comparison with the unmodified one) due to surface etching. Their selectivity was closely connected with the chemical composition of the top layer. A high nitrogen content was required for high O2 selectivity (Ruaan et al. 1998). The presence of the amine groups on the membrane surface also enhanced the capacity for CO2/CH4 separation. The plasma-polymerized diisopropylamine on the surface of the composite membrane—porous polyimide (support)/ silicone (skin)— made the separation coefficient as high as 17 for a permeation rate of 4.5 X cmVcm sec cmHg (Matsuyama et al. 1994). 

Agrawal AA, Nehilla BJ, Reisig KV, Gaborski TR, Fang DZ, Striemer CC, Fauchet PM, McGrrath JL (2010) Porous nanocrystalline silicon membranes as highly permeable and molecularly thin substrates for cell culture. Biomaterials 31 5408-5417... 

Note that the ratio of the permeability coefficient of H2 and CO by the poly-sulfone polymer matrix is 4.019 x 10 Vl0.047xlO = 40 therefore, the selectivity of the composite membrane is close to that of the polysulfone polymer matrix. It should be also noted that two important assumptions were made in the above calculation. They are (1) the permeability coefficient of the void space was assumed to be the same as that of silicone rubber, and (2) the effective thickness of the void space was assumed to be the same as that of the polymer network. This implies that the void space was completely filled with silicone rubber, and its effective thickness is the same as that of the polymer network. Furthermore, we can calculate the loss in permeability by coating. The resistance without coating is that of the porous substrate, and therefore, it is designated as / ub- With respect to hydrogen gas, it becomes... 

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