Membranes template leaching

Figure 3.6 Flow schematic of a melt extruder system used to make polypropylene membranes by template leaching [13]...

Template leaching is another methodology applied for the production of membranes it can be applied to produce porous glass membranes [180], The method consists in the formation of a structure with the help of a homogeneous melt of a three-component system, for example, NazO—B203 — Si02 when... 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

J. Sa-nguanruksa, R. Rujiravanit, P. Supaphol, and S. Tokura, Porous polyethylene membranes by template-leaching technique preparation and characterization. Polymer Testing 23, 91-99 (2004). 

The template leaching technique is suitable for preparing porous membranes from polymers, which do not dissolve in common organic solvents [9], or from glass, metal alloys, and ceramics [5]. 

When the membrane synthesis requires a template, it has to be removed from the zeolite pores. Chemical leaching or ion exchange with ammonium ions is only possible for big pore sizes (e.g. MOR, Beta, MCM). In most cases, and namely for the MFI structure, the template is removed by thermal treatment in strictly controlled conditions (atmosphere, heating rate, temperature and duration). We have to note that an efficient and gentle low temperature ozone treatment was recently proposed for organic template removal from MFI zeolite membranes [117]. 

Sihcate composites can provide an efficient size-exclnsion membrane. The pore size distribntion can be controlled, at least to some extent, by incorporation of water-solnble polymers such as poly(ethylene glycol) and their leaching after sol-gel formation or by pH control. On the low-pore-size side, low-pH silicate processing may yield silicate membranes that exhibit Knudsen flow and can separate nitrogen from air [245]. The npper limit for porous silicates probably lies in living cell templated materials [246,247] or latex sphere templates [248], and in sol-gel-derived ruthenium and vanadinm oxide aerogels for supercapacitor applications [249,250]. 

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