Guest sorption

Very recently, Fedin, Kim and colleagues synthesized a homochiral metalorganic polymeric material, [Zn2(bdc)(L-lac)(dmf)] (DMF) (36), by using a one-pot solvothermal reaction of Zn(NO3)2, L-lactic acid (L-H2lac) and 1,4-benzenedicarbox-ylic acid (H2bdc) in DMF [53]. This 3-D homochiral microporous framework exhibited permanent porosity and enantioselective host-guest sorption properties towards several substituted thioether oxides. Although 36 could catalyze the oxidation of thioethers to sulfoxides with size and chemoselectivity, no asymmetric induction was observed. 

Coordination polymer Type of adsorption isotherm Guest sorption... 

Zhang Z, Xiang S, Rao X et al (2010) A rod packing microporous metal-organic framework with open metal sites for selective guest sorption and sensing of nitrobenzene. Chem Commun 46 7205... 

All 8 clathrate and intercalate co-crystals can be obtained by guest sorption into SPS samples presenting the 8 phase [49-52] or by guest exchange in cocrystals [93-96], In fact, for some guests, (mainly volatile, like carbon dioxide [CO2] [97], butadiene [97], or ethylene [98] but also bulkier guests like acetone, limonene, or carvone) the two latter are the only available procedures to obtain the corresponding SPS co-crystals. 

As for the e clathrates, they can be only obtained by guest sorption in into SPS samples presenting the e phase, which in turn can be obtained by chloroform sorption and desorption in y form samples [53-56],... 

Guest Sorption The sorption behavior of low-molecular-mass compounds in semicrystalline polymeric materials is usually assumed to occur only in the amorphous domains in fact, molecules are not likely to penetrate... 

Sorption studies from liquid and gas phases have shown that nanoporous phases of SPS are able to absorb suitable guest molecules, even when present at very low concentrations, and that guests sorption eventually leads to the formation of co-crystalline phases [172-174]. 

Guest Diffusivity and Crystaiiine Phase Orientation Guest sorption studies from dilute aqueous solutions and from gas phases as well as desorption studies have been conducted for SPS hlms presenting the three different kinds of uniplanar orientation of the nanoporous 8 phase (a,/ Cu, an Cl, and fli C//). These investigations have been affected mainly by FTIR measurements combined with gravimetric measurements. 

As for nanoporous SPS films and aerogels, a relevant objective will also be the modification, with different kinds of functional groups, of the amorphous phase. The functionalization of the sole amorphous phase [193, 194] could bring several advantages, like increase of rates of guest sorption from the nanoporous crystalline phases. 

Scheme 6 Schematic representation of fundamental motifs observed for dynamic guest sorption involving 2D coordination frameworks (outside circle) and a model of combined motifs (inside circle)...

FIGURE 3.8 Eight metal-organic supramolecular isomers formed with the flexible ligand TA with transition metals Zn, Cd, Co, Cu, and Mg. All the isomers showed the rigid framework while structure A (TetZB) showed flexible framework towards guest sorption [21, 22, 47], Py = pyrazine, DB = l,4-diazabicyclo[2.2.2]octane, and BP = bipyridine. (The stmctures reproduced from Motkuri et al. in referenced RSC and ACS journals, with permission). 

A wide variety of guest molecules may be trapped by the Wemer-type crystalline host lattice, ranging, eg, from noble gases to condensed aromatic hydrocarbons. These clathrates may be formed from solution or by sorption. Kinetics of sorption—desorption have been studied (83). 

The study of sorption of guests within zeolite hosts is complementary to the study of diffusion in zeolites. Having discussed the pathways and trajectories of molecules through micropores, we now consider the favored sorption locations, conformations of sorbates, and sorption energetics. Indeed, so close are the two subjects that they are frequently considered within the same paper. 

Zeolitic sorption, whereby guest molecules move freely in and out of permanent host crystals. Thus windows and intracrystalline cavities are relatively large. This is exhibited by zeolites, expanded (alkylammonium) layer silicates, and Ni(4-methylpyridyl)4(NCS)2. 

Clathration is the third sorption situation. Guests are incorporated during the growth of the host lattice. Their liberation occurs on heating or lowering the pressure with lattice breakdown. This situation is found for water, phenol, quinol, cresol, urea, and Dianin s compound. 

The standard heat of sorption, Afl°, can then be interpreted in terms of appropriate energy contributions and the corresponding entropy, AS°, in terms of various degrees of freedom of the guest molecules relative to the host lattice (Table II) (10). An example of the use of the above method is given elsewhere in this volume (11). In comparing observed standard entropies with those based on the models of Table II, only empirical methods (12) are available for a priori estimates of the frequencies v. Nevertheless, comparisons have been of considerable interest (10). 

In the ethylenediammonium forms of these two preparations there was a sensitive dependence of uptake upon molecular dimensions under the experimental conditions used (72) and distinctions between the crystals of the two exchange capacities. The free distance between pairs of siliceous layers of the sorbate free fluorhectorites was 2.8 A. When guest molecules are taken up, this distance within limits increases enough to accommodate the guest. Thus, among the paraffins isobutane and 2,3-dimethylbutane were taken up. However, paraffins with quaternary carbons (neo-Cs, 2,2-di-CH3-C4, 2,2,4-tri-CH.3-C5) were not. The important dimension is the difference in height of these species (4.65 and 5.9 A). The sorption of n-paraffins (minimum thickness 4.0 A) occurred more rapidly than that of isoparaffins. Isotherms and kinetics of uptake are illustrated for n-octane in Figure 12 (72). 

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