Penetrant permeability coefficients in high-free-volume, glassy poly(1-trimethylsilyl-1propyne) [PTMSP] increase systematically with increasing concentration of nonporous, nanoscale fumed silica [FS]. For example, the permeability of PTMSP containing 40 wt % FS to methane is 180% higher than that of the unfilled polymer. Gas and vapor solubility in the nanocomposites are unaffected by FS at concentrations of up to 50 wt %. Penetrant diffusion coefficients in PTMSP increase with increasing FS content, and the enhanced permeability in the nanocomposites is due to this rise in diffusivity. These results are qualitatively similar to behavior previously observed when FS was added to another stiffchain polyacetylene, poly(4-methyl-2-pentyne) [PMP]. However, in contrast to PMP, the permeability of PTMSP to relatively small gases increases more upon filling than that of larger penetrants. This results in a reduction in vapor/permanent-gas selectivity for filled PTMSP. In fact, mixed-gas n-butane/methane selectivity is 64% lower in PTMSP containing 50 wt % FS than in pure PTMSP. These results, combined with penetrant diffusion coefficients on the order of 10 -3 cm 2 /s in filled PTMSP, suggest an escalating influence of free phase transport mechanisms such as Knudsen diffusion as FS concentration in the polymer increases.

The addition of nanoscale, nonporous fumed silica [FS] particles to size-selective poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene) [AF2400] systematically increases penetrant permeability coefficients, similar to behavior previously observed in vapor-selective polyacetylenes, but contrary to results in traditional filled polymer systems. Permeability coefficients of large penetrants increase more than those of small molecules in filled AF2400, thereby decreasing the size selectivity of this polymer. AF2400 is readily plasticized by n-butane, whereas AF2400 containing 40 wt % FS exhibits antiplasticization behavior, suggesting that filler addition alters AF2400 to allow n-butane molecules to be accommodated in the polymer without significant swelling and subsequent plasticization of the matrix. Both filled and unfilled AF2400 have essentially the same gas solubility coefficients, so all of the increase in penetrant permeability in filled AF2400 is a result of increased diffusion coefficients. There is reasonable agreement between diffusion coefficients obtained from transient sorption and steady-state data, both of which increase regularly with increasing FS content. Positron annihilation lifetime spectroscopy reveals that FS addition increases the size of free volume elements in AF2400. Thermal analysis of filled AF2400 shows that FS has no detectable effect on the polymer's glass transition temperature, indicating that FS has little impact on long-range chain mobility.

Low natural gas prices are contributing to rapid growth in natural gas combined cycle (NGCC) power production in the United States. CO2 capture from the exhaust gas of these plants is complicated by the relatively low CO2 concentration in this flue gas (3%–4%). A membrane process using incoming combustion air as a sweep stream in a selective exhaust gas recycle configuration can be used to preconcentrate CO2 from 4% to 15%–20% with almost no energy input. Depending on the process configuration, the selective recycle membrane design reduces the minimum energy of a CO2 capture step by up to 40%. An all-membrane design using a capture step in series with a selective recycle membrane can capture 90% of CO2 from an NGCC power plant using less energy and at a lower cost than the base-case amine process analyzed by the U.S. Department of Energy. The current state-of-the-art membranes for use in this process have a CO2 permeance of 2200 gpu and a CO2/N2 selectivity of 50. Higher CO2 permeance will improve the economics and reduce the footprint of a membrane CO2 capture system, while higher CO2/N2 selectivity is of less benefit, because the process is limited by the affordable pressure ratio.

No abstract

Mn2+ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which make it an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest for stable and nonlabile Mn2+ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylate functional groups, H4ENOTA. The protonation constants of the ligand and the stability constants of the complexes formed with some endogenously important metals (Ca2+, Cu2+, Zn2+), as well as with Mn2+ and Ce3+, have been assessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability of the complexes is lower as compared to that of the corresponding NOTA analogues (H3NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn2(ENOTA)(H2O) x 5H2O contains two six-coordinated Mn2+, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylate groups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of the metal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the 17O chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclear complex. A variable-temperature and -pressure 17O NMR study has been performed on [Mn2(ENOTA)(H2O)2] to assess the rate and, for the first time on a Mn2+ chelate, also the mechanism of the water exchange. The inner sphere water is slightly more labile in [Mn2(ENOTA)(H2O)2] (k298ex = 5.5 x 107 s-1) than in the aqua ion (2.1 x 107 s-1, Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limiting associative mechanism, as evidenced by the large negative activation volume (deltaV = -10.7 cm3 mol-1). The proton relaxivities measured on [Mn2(ENOTA)(H2O)2] show a low-field dispersion at approximately 0.1 MHz arising from a contact interaction between the MnII electron spin and the water proton nuclear spins.