Carbon molecular sieves catalysis

In addition to mesostructured metal oxide molecular sieves prepared through supramolecular assembly pathways, clays, carbon molecular sieves, porous polymers, sol-gel and imprinted materials, as well as self-assembled organic and other zeolite-like materials, have captured the attention of materials researchers around the globe. Clays, zeolites and sol-gel materials are still very popular because of their extensive and expanding applications in catalysis and separation science. Novel carbons and polymers of ordered porous structures have been synthesized. There are almost unlimited opportunities in the synthesis of new organic materials of desired structural and surface properties via self-assembly or imprinting procedures. 

In this paper we briefly review the important aspects of carbon molecular sieve materials with special emphasis on their use in catalysis, and our most recent results with composite structures that we have termed inorganic oxide-modified carbon molecular sieves (IOM-CMS). The literature on carbon molecular sieves, particularly patents, is large and growing, with European and Japanese researchers dominating in recent years. 

As is evident relatively little research has been devoted to catalysis with carbon molecular sieves. This is especially surprising in view of the amount of recent interest there has been in novel catalytic materials. The latter two reports dealing with alcohol dehydration over In-CMS and Diels-Alder dimerizations with CMS materials are very interesting. Although carbon-based molecular sieves have had considerable impact upon the science and technology of small molecule separations, they have been much less important in catalysis. 

Carbon molecular sieves can be made with a wide range of physical properties, and from a variety of natural and synthetic materials. The primary use of these materials has been in separation processes, the most notable of which is pressure swing adsorption for the separation of nitrogen from air. Relatively little attention has been given to the catalytic properties of these materials beyond the early work by Walker and Trimm. However, some of the more recent reports in the patent literature are very intriguing, and suggest that the use of carbon molecular sieves in catalysis merits further attention. 

Fig 6 shows the single-stage system, which is referred to as plasma-driven catalysis [77]. In the PDC process, catalysts arc directly placed in the NTP reactor. These catalysts arc activated by NTP at low temperature region, where the thermal catalysis docs not occur. The shape of catalyst is cither of honeycomb, foam or pellet. In contrast to the PEC system, all reactions of gas-phase, surface and their interaction lake place simultaneously. In this sense, it is quite complicate to understand and optimize the chemical reactions in the PDC system. In an early USA patent, Henis proposed a PDC reactor for NO.r removal. Figure 7 shows the schematic diagram of the PDC reactor proposed by Henis [78], which is quite similar to those used in recent studies. The gases arc introduced to the reaction zone through the contact materials for heat transfer purpose. The catalysts listed in the patent are alumina, zirconium silicate, cobalt oxide, Thoria, activated carbon, molecular sieves, silica gel etc. 

Ryoo R, Joo SH, Jun S, Tsubakiyama T, Terasaki O (2001) Ordered mesoporous carbon molecular, sieves by templated synthesis the structural varieties. In Galameau A, Fajula F, Renzo FD, Vedrine J (eds) Studies in surface science and catalysis. Elsevita-, Amsterdam... 

Design and Synthesis of Carbon Molecular Sieves for Separation and Catalysis... 

Carbon molecular sieves (CMS) are highly microporous materials having a preponderance of pores of < 1 nm. Among the various types of carbon, CMS materials represent one member of the family of activated carbons. CMS differ from activated carbons in the actual surface composition and the pore size distribution. Unlike CMS, activated carbons display far better detectable surface functionalities. CMS are finding a number of possible uses for the separation of air or other gases and in catalysis. CMS for use as air separation sorbents are usually made from activated carbons by a post-treatment that narrows the pore-size distribution to produce a material with a biomodal pore distribution having a predominance of pores < 0.6 nm [38]. Key to the performance of these materials is their size specific selectivity. CMS are similar to zeolites in that their porous structures have dimensions sized close to the critical dimensions of small to medium sized molecules, that is, the range between 0.3 and 1 nm. As a result, separations can be made on the basis of differences in molecular sizes and... 

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. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Cooper, B. J. Platinum-Carbon Catalysts with Molecular Sieve Properties. Shape Selectivity in Hydrogenation Catalysis. Platinum Metals Rev, 14, 133 (1970). 

The rewards for being able to understand and control the process of carbonization to give a particular pore structure are potentially enormous, with applications which include catalysis, carbon-in-pulp metal adsorption and separation processing, molecular sieves and bioethical applications. 

Research in ACF has attracted increasing attention in the last few years in terms of their synthesis, and their suitability in different applications that include solvent recovery, molecular sieving, gas storage and catalysis. Activated carbon fibres are usually prepared from precursors of low or intermediate crystallinity such raw materials include polyacrylonitrile (PAN) fibres, cellulose fibres, phenolic resin fibres, pitch fibres, cloth or felts made from them, and viscose rayon cloth. They are first pyrolysed and then activated at a temperature of 700-1000 C in an atmosphere of steam or carbon dioxide. Both the processing costs and the properties of the fibre products are dependent on the nature of the starting material. 

Li, Y., Zhao, X.Q., Wang, Y.J., 2005. Synthesis of dimethyl carbonate from methanol, propylene oxide and carbon dioxide over KOH/4A molecular sieve catalyst. Applied Catalysis... 

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