Carbon molecular sieves materials

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. 

Carbon Molecular Sieves. Excellent carbon molecular sieve materials have been developed in industry. On these sorbents, the O2, N2, and Ar isotherms are approximately equal because they all adsorb by van der Waals interactions, and their polarizabilities are approximately the same. But the diffusivily ratio for 02(Ar)/N2 is approximately 30 due to differences in molecular size (Chen et al., 1994). 

New Adsorbent Materials. SihcaUte and other hydrophobic molecular sieves, the new family of AlPO molecular sieves, and steadily increasing families of other new molecular sieves (including stmctures with much larger pores than those now commercially available), as well as new carbon molecular sieves and pillared interlayer clays (PILCS), will become more available for commercial appHcations, including adsorption. Adsorbents with enhanced performance, both highly selective physical adsorbents and easily regenerated, weak chemisorbents will be developed, as will new rate-selective adsorbents. 

Adsorbents are natural or synthetic materials of amorphous or microcrystalhne structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

Advanced Materials Experimental membranes have shown remarkable separations between gas pairs such as O9/N9 whose kinetic dian ieters (see Table 22-23) are quite close. Most prominent is the carbon molecular sieve membrane, which operates by ultran iicro-porous molecular sieving (see Fig. 22-48c). Preparation of large-scale permeators based on ultran iicroporous membranes has proven to be a major challenge. 

By carefully choosing the starting material and the activating process, it has been possible in recent years to generate in carbon a pore system with a narrow span of pore sizes. With a mean pore diameter of perhaps 0.6 nm, such products are known as carbon molecular sieves. 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Catalytic Formation of Carbon Nanotubes on Fe-loading Molecular Sieves Materials An XPS Study... 

The preparation of the used Fe-loading molecular sieves materials and the catalytic synthesis of carbon nanotubes have been described in detail in our previous report [22]. The textural properties and compositions of catalysts are shown in Table 1. XPS spectra for samples were recorded on a PHI-5300 ESCA system. The pass energy was 71.550 eV. Before the XPS measurement, all the samples were ground and then dried at 393 K for 2 h. For these samples, the C(ls) level (284.4 eV) was taken as the reference binding energy (B.E.). 

The development of new porous materials that could be used as adsorbents, catalysts, catalyst supports, molecular sieves, etc. [1], are very well discussed by several authors [2-9], describing interesting properties and characteristics of materials such as MCM-41, MCM-48, M41S, FSM16, lamellar phases, intercalation products, special CMS (carbon molecular sieves), fullerenes, carbon nanotubes, etc. being some of them silica based materials, and carbon based the others. 

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. 

Carbon has been used in several forms. Activated charcoal found limited usage because of significant batch-to-batch variations. Kaiser (16) developed a carbonaceous material with pore structure similar to zeolites and referred to it as carbon molecular sieve. It is useful only for gases and very short chain compounds. It is unique in that it can be used for the analysis of oxygen, nitrogen, and carbon dioxide as shown in Figure 3.7. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

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. 

Carbon molecular sieves are produced by controlled pyrolysis and subsequent oxidation of coal, anthracite, or organic polymer materials. They differ from zeolites in that the micropores are not determined by the crystal structure and there is therefore always some distribution of micropore size. However, by careful control of the manufacturing process the micropore size distribution can be kept surprisingly narrow, so that efficient size-selective adsorption separations are possible with such adsorbents. Carbon molecular sieves also have a well-defined bi-modal (macropore-micropore) size distribution, so there are many similarities between the adsorption kinetic behavior of zeolitic and carbon molecular sieve systems. 

Besides polymers and inorganic materials, commercially available porous carbons can be employed to immobilize Mo. For instance, the utility of Mo-impregnated carbon molecular sieves as epoxidation catalyst has been demonstrated by Gaffney et al. (249). For example, in a patent 1-octene... 

The above examples of shape selective reactions show the complexity of such systems and that several factors need to be considered before shape selective control can be realized. The use of other porous supports besides zeolites such as carbon molecular sieves, clays, pillared clays and related materials to catalyze shape selective reactions appears to be growing. Molecular modeling of the spatial constraints of various pores is also an area of increased research effort. 

In gas-solid chromatography, the stationary phase is an active solid. These solids may be inorganic materials, e.g. synthetic zeolite molecular sieve, carbon molecular sieve, silica gel, or graphitised carbon, or they may be oiganic polymers. They are generally used for the separation of low molecular weight materials, i.e. gases and liquids. 

Porous carbonaceous materials are important in many application areas because of their remarkable properties, such as high surface areas, chemical inertness, and good mechanical stability. Carbon molecular sieves that are amorphous and microporous are commercially important for the separation of nitrogen from air, and activated carbons with a wide pore size distribution are also useful adsorbents for various applications. 

This work focuses on the application of water adsorption to the characterisation of narrow microporosity (uitramicroporosity) on carbon molecular sieves (CMS). In addition, the mechanism of water adsorption in carbonaceous solids is analysed. Despite of the presence of surface groups in these materials, they present low adsorption at low relative pressures (P/Po <0.2), indicating the significant role of micropore size in the starting of water adsorption. Interestingly, it has been seen that water adsorbs even on samples, which do not present CO2 adsorption. 

Similarly, there is a great potential in the use of water vapour for the analysis of the porous texture, because it has considerable potential due to both the easy experimental conditions (at room temperature the whole range of relative pressures can be covered) and the characteristics of the molecule itself (polar molecule and small kinetic diameter-0.28 nm). This vapour is widely used in the characterisation of inorganic porous solids, such as zeolites, silicas, and clays. However, its interaction with carbon materials (microporous carbons coals, activated carbon fibres, carbon molecular sieves and porous carbons activated carbons), is more complex than the interaction of non-polar molecules [8]. 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

Materials. Three different carbon molecular sieves, CMS-Kl, CMS-K2, CMS-R, used originally for separation of gaseous mixtures were investigated. For the first two adsorbents the size of pores was comparable to the size of the separated molecules and the process was... 

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