Molecular diffusion sieves

Figure 2. Fluorine NMR relaxation times for a sample of Linde molecular sieve 13X containing about 6.6 molecules of SFg per cage O, spin lattice relaxation time , spin-spin relaxation time T2 characterized by exponential decay V and A, T2 characterizedby two relaxation times ticked O, decay as r2. Solid lines are theory to the left of 10Z/T = 6 based on molecular diffusion to the right of 10Z/T controlled by Tu. For dashed lines see text (20)...

Porosity, pore size >1 ml/g, 2-50 nm Resistance to diffusion, molecular weight sieving, flow properties, enzyme retention... 

Figure 4 provides a comparison of the nmr data or, molecular diffusion with the break-through capacity of a granulated Linde HacaA molecular sieve after different times cn stream in a petroleum refinery The increasing deterioration oi the dynamic... 

Principal processes transport by diffusion the material of the membrane may affect transport properties separation according to molecular size sieve effect the material of the membrane has no influence, the important characteristic being the pore size... 

In Chapter 1, Fyfe, Mueller, and Kokotailo describe the applications of solid-state NMR to the study of zeolite molecular sieve catalysts and related systems. Zeolites provide an apt arena in which to demonstrate the capabilities of modern techniques such as sample spinning, cross-polarization, and multidimensional correlation spectroscopy. In Chapter 2, Karger, and Pfeifer consider the question of molecular diffusion in catalyst systems. Applications of NMR techniques such as imaging, lineshape analysis, relaxation, pulsed field gradient echo spectroscopy, and NMR tracer exchange are described and compared with other, more traditional techniques such as radioactive tracing. In Chapter 3, Haw discusses the use of NMR to probe catalytic processes, showing how the combination of temperature control with novel NMR probes makes it possible to elucidate reaction mechanisms in situ. 

The most straightforward cause of shape selectivity is the discrimination between molecules on the basis of their diffusion rates through the channels or cage windows. Microporous solids act as true molecular sieves, because the well-defined pores are able to select molecules on the basis of differences in dimensions of 0.1 A or less. Examples of strong molecular sieving effects include the selection of normal alkanes over branched ones by small-pore solids and the selection of para-substituted over ortho- and meta-substituted aromatics over medium-pore zeolites. This type of selectivity according to molecular diffusion rate may act on both reactant and product molecules. The much faster dehydration of n-butanol compared to isobutanol over Ca-A demonstrated by Frilette and Weisz is the classic example of reactant diffusion... 

These expressions provide a simple and convenient model for the analysis of experimental uptake curves when the equilibrium isotherm is highly favorable and micropore diffusion is rapid. These conditions are amply fulfilled for the adsorption of water at or near ambient temperature in molecular sieve adsorbents. Experimental uptake curves for this system measured by Kyte are shown in Figure 6.9. The experimental conditions and the effective diffusivity calculated according to Eq. (6.27) are giveii in Tables 6.4 and 6.5. Under the experimental conditions the estimated value of the Knudsen diffusivity [from Eq. (5.17)] is much larger than the molecular diffusivity... 

FIGURE 6.20. Test of the spinning basket adsorber. Overall rate coefficients for adsorption of propane on Linde 5A at 54 C were measured at several speeds of rotation in the presence of 500 Torr of either iQH,) or SFj. These species are both too large to penetrate the sieve. The uptake rate correlates with the gas phase molecular diffusivity indicating external mass transfer control. (Data of Taylor.< °>)... 

Post, M.F.M., "Diffusion in Molecular Zeolite Sieves", Studies in Surface Science and Catalysis, 58, Chpt 11, van Bekkum et al. (Eds.), Elsevier, Amsterdam, 1991. 

In principle, the separation properties of a multilayer porous ceramic membrane, such as permselectivity, should be dependent only on the pore size distribution of the top separation layer. However, they can be compromised if resistances in the intermediate layers and the macroporous support become significant. For transport through macro- and meso-pores, molecular diffusion, Knudsen diffusion and viscous flow all contribute to the total transport, while the activated surface flow of the adsorbed phase will affect microporous transport. Therefore, any theoretical models used in analysing the transport data of gases through a porous ceramic membrane with a distributed pore size must take the following contributions into consideration (1) viscous flow, (2) Knudsen flow, (3) surface flow and (4) molecular sieving... 

Fig. 8. Variation of activation energy with kinetic molecular diameter for diffusion in 4A 2eohte (A), 5A 2eohte (0)> carbon molecular sieve (MSC-5A) (A). Kinetic diameters are estimated from the van der Waals co-volumes. From ref. 7. To convert kj to kcal divide by 4.184.

Fig. 9. Uptake curves for N2 in two samples of carbon molecular sieve showing conformity with diffusion model (eq. 24) for sample 1 (A), and with surface resistance model (eq. 26) for example 2 (0)j LDF = linear driving force. Data from ref. 18.

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Porin channels are impHcated in the transport of cephalosporins because ceds deficient in porins are much more impermeable than are ceds that are rich in porins. The porins appear to function as a molecular sieve, adowing molecules of relatively low molecular weight to gain access to the periplasmic space by passive diffusion. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to permeabdity. However, such a relationship is not clearly defined for Pseudomonas aeruginosa where additional barriers may be involved (139,144,146). 

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