Transporter selectivity

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

The copper(II) transport rate increases, as a rule, as Cu + initial concentration in the feed solution increases. The increase of the caiiier s concentration from 10 to 30 vol.% results in a decrease of both metal fluxes and in an increase of Cu transport selectivity. The increase of TOA concentration in the liquid membrane up to 0.1 M leads to a reduction of the copper(II) flux, and the platinum(IV) flux increases at > 0.2 M. Composition of the strip solution (HCl, H,SO, HNO, HCIO, H,0)does not exert significant influence on the transport of extracted components through the liquid membranes at electrodialysis. 

Metal nanotube membranes with electrochemically suitable ion-transport selectivity, which can be reversibly switched between cation-permeable and anion-permselective states, have been reported. These membranes can be viewed as universal ion-exchange membranes. Gold nanotube molecular filtration membranes have been made for the separation of small molecules (< 400 Da) on the basis of molecular size, eg. separation of pyridine from quinine (Jirage and Martin, 1999). 

Time required to transport selected radionuclides added into marine waters at surface from the upper mixed layer by biological transport... 

Table 32.12 Time Required to Transport Selected Radionuclides Added into Marine Waters at Surface out of the Upper Mixed Layer by Biological Transport (Processes include diurnal vertical migration, fecal pellets, and sinking of dead matter.)...

Gold Nanotubule Membranes with Electrochemically Switchable Ion-Transport Selectivity 24... 

Molecular Eiltration and Chemical Transport Selectivity in the Au Nanotubule Membranes 30... 

B. Chemical transport selectivity 42 Nanomaterials in Secondary Battery Research and Development 48... 

IV. GOLD NANOTUBULE MEMBRANES WITH ELECTROCHEMICALLY SWITCHABLE ION-TRANSPORT SELECTIVITY... 

V. MOLECULAR FILTRATION AND CHEMICAL TRANSPORT SELECTIVITY IN THE AU NANOTUBULE MEMBRANES... 

The work discussed above shows that the Au nanotubule membranes can have one important type of transport selectivity—charge-based selectivity. It occurred to us that because the Au nanotubules can be of molecular dimensions, these membranes might show molecular size-based transport selectivity as well [72]. Finally, the thiol chemisorption chemistry introduced above provides a route for introducing chemically based transport selectivity [85]. Hence, the Au nanotubule membranes should, in principle, be able to show all three of the important transport selectivity paradigms—... 

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

We demonstrated above that these Au nanotubule membranes can show charge-based transport selectivity, and we have now shown that these membranes can also have molecular size-based selectivity. The next step was to attempt to introduce chemical transport selectivity. 

In addition to the transport selectivities based on molecular charge or size described above, chemical interactions between the membrane material and the molecule to be transported can also strongly influence the rate and selectivity of transport. The introduction of chemically based transport selectivity was accomplished by chemisorbing thiols (RSH) to the Au tubule surfaces [113]. Membranes derivatized with two different R groups—the hydrophobic R = -CigHjj and the more hydrophilic (2)R = -C2H4-OH— were prepared. The rate and selectivity of transport in these membranes is dramatically altered by the chemical identity of the R group. 

Fig. 4. Transport selectivity Kjj and potentiometric selectivity Kj j of a Na+-selective neutral carrier membrane using ligand 11. Experimental coefficients fCNaM obtained with (2) and (11) respectively given for different cations M. Membrane composition 32wt.% polyvinyl chloride, 65 wt.% dibutyl sebacate, 3wt.% carrier //. Thickness of membrane = 100 p.m. Current density approx. 0.1 p.Amm 2.

The selectivity factors determined in potentiometric studies (K[,ot) should therefore be identical to the ones (Kjj) determined in transport experiments. In Fig. 4 selectivities obtained potentiometrically on a membrane containing ligand 11 (3 wt.% carrier 11, 65 wt.% dibutyl sebacate 32 wt.% polyvinyl chloride, thickness =100 /xm) are compared with those obtained in electrodialytic transport experiments.55 Although widely different methods have been used to determine the ion selectivity, the agreement between the two sets of data is evident and corroborates the model presented. The deviation for CsH may possibly be due to kinetic limitations suggesting a loss in transport selectivity (see Section IV.D). 

Fig. 6. Transport selectivity and potentiometric selectivity of a Ca2 selective neutral carrier membrane (3 wt.% carrier 10, 65 wt.% o-nitrophenyl-octyl ether, 32wt.% polyvinyl chloride). Experimental selectivity coefficients KCaNa obtained with (16) and (18), respectively, as a function of the cationic concentration m (in moles/liter).

The values given in column 3 of Table IV were obtained from the data in column 2 ((2) and (6)]. A comparison of the results for 14 and 15 indicates that the introduction of methyl groups at sites 4 and 5 (see 12), leading to central chirality R at both carbon atoms, is the main cause of enantiomer selectivity. This is in agreement with the only slightly different enantiomer selectivity of 16 relative to 15. As expected, the effect of 17 is reversed by 18. Although valinomycin 1 is chiral, no enantiomer selectivity was detectable (see Table IV). The potentiometrically determined enantiomer selectivity AEMF is correlated to the transport selectivity [(2), (6), (10), and (11)]... 

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