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General Fabrication of CMs

The production process of CMs mainly includes precursor selection, membrane formation, pyrolysis and modification.

Precursor Selection

CMs can be made from a large variety of carbonaceous materials, either natural or synthetic. Natural precursors include coal, pitch and biomass (e.g., lignin). In contrast to synthetic materials, the use of cheap mineral materials (e.g., coal, pitch), and in particular renewable biomass, has the advantage of low fabrication cost. However, in some cases, it is hard to control the microstructure and properties of the as-prepared CMs, to satisfactorily meet the separation requirement, because of the complicated and variable components in the starting products.

Synthetic precursors featuring uniform texture and stable properties are preferential for the fabrication of CMs without defects. Numerous synthetic materials have been reported to successfully form CMs, such as polyimide, phenolic resin, polyacrylonitrile, polyffurfuryl alcohol), polyvinylidene chloride, polyphenylene oxide, polyfphthalazinone ether sulfone ketone), poly- pyrrolone and so forth [12,13].

Owing to differences in property, cost, availability and processability of raw materials, synthetic and natural materials are commonly applied to prepare CMs for gas separation and liquid separation, respectively.

Membrane Formation

Before pyrolysis, a defect-free thin organic layer is usually deposited on a permanent or temporary support to make supported or self-standing membranes, respectively. The supported CMs can be categorized into plate, tube, hollow fiber and spiral wound, whereas unsupported (or self-standing) CMs include plate, hollow fiber and capillary.

Unsupported plate CMs, which are facilely produced via casting or solvent evaporation, are restricted to the laboratory for academic study. Hollow fiber and capillary CMs have gained considerable attention owing to their huge specific volumetric membrane surface areas. Therefore, hollow fiber CMs have been commercially available for several years from Carbon Membranes Ltd (Israel) [14].

In practice, supported CMs are more promising for their robust mechanics to tolerate harsh environmental conditions in operation. To date, various coating technologies have been used in the preparation of CMs, such as brush coating, spray coating, dip-coating, drop-coating, ultrasonic spray coating, chemical vapor deposition, physical vapor deposition and so forth [13]. Supported plate CMs are commercially available from Blue Membranes GmbH (Germany) [15].

Prior to coating, porous supports should be carefully selected and pretreated; they include graphitic plate, carbon, ceramic, hollow fiber, stainless steel and polymeric substrates [16,17]. In addition, nanoporous wafers made from glass, silicone and poly(tetrafluoroethylene) are frequently utilized as substrate to fabricate unsupported CMs during the solvent evaporation of the casting solution. The support should be selected to match the viscosity and nature of the membrane solution. Usually, supports with extremely large pore openings intake large amounts of membrane solution, which can devastate the formation of the surface layer. It is also detrimental for the surface layer to tightly adhere to the support, and for permeation of desired species if the pores are too small.

In some cases, CMs can be formed by directly compressing paste precursors into a membrane shape. This simple method is common for the production of CMs with a mesoporous structure for microfiltration and ultrafiltration, as will be elaborated in later sections.

 
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