Carbon molecular sieves results

The abovementioned rate acceleration and selectivity enhancement brought about by catalysts are particularly marked when unactivated dienes and dienophiles are involved. Two molecules of 1,3-butadiene can react in a Diels-Alder reaction, one acting as diene and the other as a dienophile to produce 4-vinylcyclohexene (in 0.1% yield at 250°C in the absence of a catalyst). Cs+, Cu,+ and trivalent transition-metal exchanged montmorillonites534 as well as large-pore sodium zeolites (Na ZSM-20, NaY) and carbon molecular sieves,535 result in 20-35% yields with 95% selectivity. Large rate enhancement was observed when 1,3-cyclohexadiene underwent a similar cycloaddition536 in the presence of K10 montmorillonite doped with Fe3+ ... 

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. 

The carbon/molecular sieve bed was temperature-controlled at 293 K. The volumetric flow rate was Q = 5.19 dm3 min-1 (1.0 dm3 min 1 cm2), resulting in the linear flow rate vL = 1000 cm min-1. The gas flow was controlled by several flow meters. The outlet concentrations were analyzed in cycles of 3 min with a CP 9001 CHROMPACK gas chromatograph with a flame ionization detector. The breakthrough time was determined at the outlet by a TBB concentration (behind the carbon bed) cx = 10 5 mg dm-3 (cx/c0 = 10 5). To study the water influence on TBB breakthrough, water vapour was added to reach 50% relative humidity (RH) of the air flow. The measurements were performed with dry carbon/molecular sieve beds using dry or wet air. 

Two kinetic (CMS-Kl, CMS-K2) and one equilibrium (CMS-R) carbon molecular sieves, used originally for separation of gaseous mixtures, were investigated. The adsorption Nj isotherms at 77 K, in static conditions where obtained. In the case of the two first sieves the adsorption was so low that the calculation of parameters characterizing the texture was impossible. The volume of nitrogen adsorbed on the sieve CMS-R is remarkable From obtained results parameters characterizing micropore structure according to Dubinin -Radushkevich equation and Horvath - Kawazoe method were determined. 

The diameter and volume of the micropores were also determined by the measurement of the density using as displacement molecules with different sizes, e g., helium, water, benzene, decaline. It was found that in the case of CMS-Kl and CMS-K2 sieves, the micropores with the pore size within the range 0.255-0.528 are dominated and that the used measurements enable characterisation of the structure of carbon molecular sieves. For equilibrium sieve the analysis of the micropores volume with the use of the pycnometric technique does not give proper results. 

Such result is consistent with the literature [1]. In this way it was found that the measurements based on displacement of fluids with different molecular sizes enable characterization of the micropore structure of carbon molecular sieves. However for CMS- R sieves a some amount of micropores with the size of 0.255 -s- 0.528 nm was found, but its volume is smaller than in the case of CMSs-K. 

The pycnometric technique makes possible the calculation of the pore volume with any size (Tab. 2), but remarkable values were obtained only for kinetic sieves. Hence, this analysis does not give proper results for equilibrium carbon molecular sieves. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Vu DQ, Koros WJ, and Miller SJ. Mixed matrix membranes using carbon molecular sieves I. Preparation and experimental results. J. Membr. Sci. 2003 211 311-334. 

Two series of carbon molecular sieves have been prepared from coconut shells, with different pore size distribution. They have been characterised by carbon dioxide adsorption at 273 K and immersion calorimetry into liquids of different molecular sizes. The results have been related with the abihty of the CMS to separate the components of O2/N2, CO2/CH4 and n-C4H4/i-C4H4 gas mixtures. 

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. 

Based on these and other results it was concluded that molecular sieving carbons have slit-shaped pores (9-11). This view of the pore geometry of carbon molecular sieves persists and is still consistent with the bulk of experimental evidence. 

For the carbon molecular sieving catalysts, thoe is clear evidence that the global rate is intonally transport limited. Consistent with this, a reduction of particle size of the Fe/SiOi CMS ca yst increased the global reaction rale by decreasing the diffusion length to the active metal sites. Using first-wder kinetics in the Thiele analysis, to consider the conversion ratios of the intemal-transport limited CMS catalyst particles at modest convositxis, leads to the result ... 

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