Carbon molecular sieve precursor preparation

Indeed, the compositional flexibility of mesoporous materials is such that it is possible to prepare mesoporous organosilicas with organic groups (e.g., ethene) inside the pore walls. An unusual property of mesoporous silicas is that they can themselves be used as templates for the formation of mesoporous carbon. The mesoscopi-cally ordered nanoporous (or mesoporous) carbon molecular sieves are prepared by carbonizing sucrose (or other carbon precursors) inside the pores of mesoporous silica molecular sieve. The mesoporous carbon is obtained after subsequent removal of the silica template by dissolution in HF (hydrofluoric acid) or NaOH (sodium hydroxide) solution. 

Barsema JN, Van Der Vegt NFA, Koops GH, Wessling M (2002) Carbon molecular sieve membranes prepared from porous fiber precursor. J Membr Sci 205 (1-2) 239-246 Soffer A, Azariah A, Amar A, Cohen H, Golub D, Saguee S, Tobias H (1997) Method of improving the selectivity of carbon membranes by chemical vapor deposition. US Patent... 

Barsema JN, van der Vegt NFA, Koops GH, Wessling M (2002) Carbon molecular sieve membranes prepared from porous fiber precursor. J Membr Sci 205 (1-2) 239-246. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

Jones C.W. and Koros W.J., Carbon molecular sieve gas separation membranes—I. Preparation and characterization based on polyimide precursors. Carbon 52 1419 (1994). 

Figure 4 represents the evolution of the EoWo/EorcfWo ref ratio for the different carbon molecular sieves of the two series, as a function of the molecular size of the immersion liquid, and using CH2CI2 as a reference. A decrease of this ratio as the size of the immersion liquid increases indicates that the accessibility of the porosity is limited. It can be seen that the pore size distributions obtained by this method are comparable to those shown in Figure 3, corresponding to the surface area accessible to the different immersion liquids. In conclusion of this pore-size analysis, a variety of CMS with different pore size distribution, but always smaller than 0.7 nm, have been obtained. CMS with the narrowest pore diameter are prepared from the acid-washed precursors, i.e., without ashes able to catalyse the gasification reaction. 

Carbon molecular sieves have been prepared of active carbon precursors with chemical vapor deposited (CVD) amorphous carbon. Villar-Rodil and co-workers [159] employed STM to visualize the changes brought about by the CVD treatment on the pore mouth structure. They used STM as extremely surface-sensitive technique, since the CVD treatments are supposed to modify only the pore entrances. 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

Walker and coworkers carried out several studies on the preparation of carbon molecular sieves using active carbons and cellular precursors. Schmitt and Walker ... 

The subject area of activated carbon was by now a significant technology in several industries where the applications of carbons, prepared by thermal (as well as chemical activation) were of fundamental importance. This handbook provided a chapter (summary) of aspects which must be considered in discussions of the use of activated carbon. The chapter contains an Introduction, Production Methods, Precursors, Physical (Thermal) Activation, Chemical Activation, Combined Activations, Carbon Molecular Sieves, Activated Carbon Fibers and Cloths, Pelletized Activated Carbons, Washed, Treated and Impregnated Activated Carbon, as well as a section covering industrial production and applications. As such, this chapter is a substantial reference document and will remain so for some considerable time (Rodriguez-Reinoso, 2002). 

Areas in which further developments are expected are related to the optimization of the solution of air and water pollution, gas purification (removal of oxides of sulfur and nitrogen, of hydrogen sulfide, motor vehicle emissions, etc.), gas separation, mineral industries, regeneration, etc. Many of these areas will require the use of new forms of activated carbon such as cloth, felts, fibers, monoliths, etc., and consequently a search for the appropriate precursor and preparation mode is essential. Other areas in continuous progress will be gas storage, carbon molecular sieves and heterogeneous catalysis, all of them requiring considerable research efforts in the next few years. 

The preparation method of flat supported carbon molecular sieve membranes has been investigated by using different polymeric materials by Fuertes and Centeno. They used 3,3 4,4 -biphenyltetracaiboxyhc dianhydride (BPDA)—4,4 -pheitylene diamine (pPDA) [1, 16], phenolic resin [17] as precursor to make flat CMSMs supported on a macroporous carbon substrate. In a later study, they ehose poly-etherimide (PEI) as a precursor to prepare flat supported CMSMs [18]. PEI was chosen because it was one of PI based materials which can be used economically. On the other hand, these PEI carbon membranes showed performance similar to the CMSMs prepared by Hayashi et al. [19], which was obtained from a laboratory-synthesized PI (BPDA-ODA). 

Jones CW, Koros WJ (1994) Carbon molecular sieve gas separation membranes- I. Preparation and characterization based on polyimide precursors. Carbon 32 (8) 1419-1425 Jones CW, Koros WJ (1995) Characterization of ultramicroporous carbon membranes with humidified feeds. Ind Eng Chem Res 34 (1) 158-163... 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

The polymer/zeolite host-guest precursor for preparation of porous carbon can also be obtained through direct contact of monomers in a carrier gas with zeolite molecular sieves followed by polymerization. For example, propylene can enter into zeolite Y under the carriage of N2 and polymerize to form polypropylene. After pyrolysis, the polypropylene undergoes carbonization, and the host zeolite framework of the carbonization product can be removed by dissolution in acids, leaving carbon material with characteristic pores.[93] However, the pore-size distribution of the porous carbons obtained through this approach is not uniform, and hence they are hardly used as molecular sieves for sieving small molecules. 

Highly porous carbons can be produced from a variety of natural and synthetic precursors [11, 12]. Relatively inexpensive activated carbons are useful adsorbents, but generally their surface and pore structures are exceedingly complex [11, 13]. However, it is now possible to prepare a number of more uniform carbonaceous adsorbents. For example, molecular sieve carbons (MSCs) are available with narrow distributions of ultramicropores, which exhibit well-defined molecular selectivity [11], and carbon nanotubes, aerogels, and membranes are also amongst the most interesting advanced materials for research and development [12, 14]. 

Shortly thereafter, it was realized that molecular sieving carbons could be prepared by controlled pyrolysis of polymeric precursors (2.). Early estimates of pore sizes for this carbon were seven to eight angstroms, but progressive activation increased surface area, pore volume, and pore dimensions. 

A boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

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