Gas sorption

GAS SORPTION - SURFACE AREA AND PORE SIZE DETERMINATION 

nm are the adsorption pressure, the saturation vapor pressure, a constant, the amount adsorbed (moles per gram of adsorbent) at the relative pressure P/Po, and the monolayer capacity (moles of molecules needed to make a monolayer coverage on the surface of one gram of adsorbent). 

Na is Avogadro s number. The specific surface area that can be determined by gas sorption ranges from 0.01 to over 2000 mVg. Determination of pore size and pore size distribution of porous materials can be made from the adsorption/desorption isotherm using an assessment model, such as the t-plot, the MP method, the Dubinin-Radushkevich method and the BJH model, etc. [42], suitable for the shape and structure of the pores. The range of pore sizes that can be measured using gas sorption is from a few Angstroms up to about half a micron. 

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The gas sorption and transport properties also depend on the bisphenol connector groups [210]. The permeability coefficients for all gases rank in the order ... 

If the solvent concentration is very small, as in the case of gas sorption, the polymer volume fraction is near to unity and Eq. (1) becomes ... 

For application to gas sorption in polymers, we have modified the Prigogine-Flory formalism to apply to low- and high-density fluids and their mixtures (12). The modified equation of state has the form... 

To illustrate the use of the gas sorption mode , we show in Figure 7 results of the supercritical ethylene sorption in low-density polyethylene (12,16). As seen in Figure 7, the theory is capable of fitting the ethylene sorption data. In this instance, the data at three temperatures can be fit within experimental precision using interaction parameters (p o) of 3235 atm, 3178 atm, or 3101 atm at 126°C, 140 0, and 155 C, respectively. 

The algorithm we used for solvent/polydisperse polymer equilibria calls for only one solvent/polymer interaction parameter. The interaction parameter (pto) i ed in the algorithm can be determined from essentially any type of ethylene/polyethylene phase equilibrium data. Cloud-point data have been used (18). while Cheng (16) and Harmony ( ) have done so from gas sorption data. 

S. Surble, F. Millange, C. Serre, T. Duren, M. Latroche, S. Bourrelly, P. L. Llewellyn, and G. Ferey, Synthesis of MIL-102, a chromium carboxylate metal-organic framework, with gas sorption analysis, J. Am. Chem. Soc., 128, 14889-14896(2006). 

Classical characterization methods (gas sorption, TEM, SEM, FTIR, XPS and elemental analysis) were used to describe the resulting porous carbon structures. Temperature-dependent experiments have shown that all the various materials kept the nitrogen content almost unchanged up to 950 °C, while the thermal and oxidation stability was found to be significantly increased with N-doping as compared to all pure carbons. Last but not least, it should be emphasized that the whole material synthesis occurs in a remarkably energy and atom-efficient fashion from cheap and sustainable resources. 

The method most widely used to determine gas solubilities in ILs is gravimetric. As we first showed in 2001 [11] gravimetric techniques are unique, suited to measure the solubility of gases and vapors in ILs because the liquid does not evaporate into the vapor, complicating the measurements. Thus, apparatuses designed to measure gas sorption on solids can be used readily for ILs. 

Our current views, with some elaboration, are summarized below. We do not consider it meaningful with coals, or with microporous solids in general, to deduce specific surface values from sorption data, nor even to apply the concept of specific surface to these materials. We believe that sorption uptake (moles per unit weight or volume of a given adsorbent) under defined conditions is the correct parameter that should be used to describe the sorptive properties of such materials. Thus, whenever the sorption uptake by an active carbon, for a particular sorbate, is required in a practical application such as solvent recovery, purification, or gas sorption, what should be determined in the laboratory is the uptake under the conditions for which the value is to be used it is not possible to predetermine unequivocally the value for one... 

Vapour and gas sorption measurements can be performed with static or dynamic methods, either of which can provide information on equilibrium behaviour. Furthermore, the measurements can be performed using gravimetric or volumetric based instrumentation. The most common flow methods are inverse gas chromatography (IGC) [1] for volumetric studies and dynamic gravimetric instrumentation [2]. 

Dual-Mode Gas Sorption and Diffusion in Glassy Polymer Membranes. 97... 

Fig. 7. Mixed gas sorption isotherms for PMMA-C02> C2H4 in terms of fugacities f, at fC02 = 1.50 + 0.05 atm, T = 35 °C. Experimental data ( ) in comparison with lines calculated from pure...

The general conclusion to be drawn from the above discussion is that the dual gas sorption and mobility model has very useful predictive potential in the field of membrane gas separation. At the present stage, no comparable predictive capability is discernible in the alternative treatments so far published 33 >34 63). 

Sefcik M. D., Raucher D. The Matrix Model of Gas Sorption and Diffusion in Glassy Polymers, to be published... 

Sanders, E. S., Koros, W. J., Hopfenberg, H. B., Stannett, V. T. Pure and Mixed Gas Sorption of Carbon Dioxide and Ethylene in Poly(methyl methacrylate), to be pubhshed... 

Three questions concerning ultrastabilization remain outstanding. They regard the precise mechanism of A1 removal, the nature of the intermediate defect structure (both are depicted schematically in Fig. 38), and the origin of the silicon needed for framework reconstruction. Gas sorption studies (172) indicate that materials prepared in a manner similar to that for sample 4 in ref. 163 (see above) contain a secondary mesopore system with pore radii in the range IS-19 A, suggesting that tetrahedral sites are reconstituted with silicon that, contrary to earlier speculations, does not come only from the surface or from amorphous parts of the sample, but also from its bulk, which may involve the elimination of the entire sodalite cages. 

In the present symposium, Dr. Raucher argued that the observed curvature in gas sorption isotherms does not arise from a site saturation mechanism such as we have described in the preceding discussion. The form of the sorption isotherm and local concentration dependent diffusion coefficient proposed by Raucher and Sefcik are given below ... 

In the following chapter we present the matrix model of gas sorption and diffusion in glassy polymers which is based on the observation that gas molecules interact with the polymer, thereby altering the solubility and diffusion coefficients of the polymer matrix. 

In Section I we introduce the gas-polymer-matrix model for gas sorption and transport in polymers (10, LI), which is based on the experimental evidence that even permanent gases interact with the polymeric chains, resulting in changes in the solubility and diffusion coefficients. Just as the dynamic properties of the matrix depend on gas-polymer-matrix composition, the matrix model predicts that the solubility and diffusion coefficients depend on gas concentration in the polymer. We present a mathematical description of the sorption and transport of gases in polymers (10, 11) that is based on the thermodynamic analysis of solubility (12), on the statistical mechanical model of diffusion (13), and on the theory of corresponding states (14). In Section II we use the matrix model to analyze the sorption, permeability and time-lag data for carbon dioxide in polycarbonate, and compare this analysis with the dual-mode model analysis (15). In Section III we comment on the physical implication of the gas-polymer-matrix model. 

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