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We report a new class of CO 2 binding organic liquids that chemically capture and release CO 2 much more efficiently than aqueous alkanolamine systems. Mixtures of organic alcohols and amidine/ guanidine bases reversibly bind CO 2 chemically as liquid amidinium/guanidinium alkylcarbonates. The free energy of CO 2 binding in these organic systems is very small and dependent on the choice of base, approximately À9 kJ mol À1 for DBU and Barton's base and +2 kJ mol À1 for 1,1,3,3tetramethylguanidine. These CO 2 capturing agents do not require an added solvent because they are liquid, and therefore have high CO 2 capacities of up to 19% by weight for neat systems, and slightly less when dissolved in acetonitrile. The rate of CO 2 uptake and release by these organic systems is limited by the rate of dissolution of CO 2 into and out of the liquid phase. Gas absorption is selective for CO 2 in both concentrated and dilute gas streams. These organic systems have been shown to bind and release CO 2 for five cycles without losing activity or selectivity.

Supercritical fluid extraction (SEE) of organic compounds has been the subject of many studies in recent years.1•2 A suitable solvent for this purpose is carbon dioxide due to its moderate critical constants (Tc = 31.1 °C, Pc = 72.8 atm), inertness and easy availability in purified form. Little

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The development of condensed-phase hydrogen-storage materials for fuel-cell-powered vehicles that meet the 2015 system target goals of the US Department of Energy of a volumetric density greater than 82 g H 2 per liter and a gravimetric density greater than 90 g H 2 kg À1 has attracted recent interest. [1,2] Ammonia borane, NH 3 BH 3 (AB), which has a volumetric density of 146 g H 2 per liter and a gravimetric density of 194 g H 2 per kilogram, shows promise as a material that could meet these long-term storage targets. [3] The details of the mechanisms for hydrogen release from AB are not completely understood; however, significant progress has been made in furthering our understanding of these mechanisms. Wolf and co-workers showed that the thermal decomposition of AB in the solid state occurs stepwise by a series of exothermic reactions to yield 12 wt % hydrogen. [4,5] The first step yields H 2 and polyaminoborane (PAB); subsequent decomposition of PAB yields another equivalent of H 2 and polyiminoborane (PIB). One important breakthrough for our understanding of the reaction mechanisms for H 2 release from AB was the recent in situ NMR spectroscopic study that showed formation of the diammoniate of diborane, [NH 3 BH 2 NH 3 ], to be an essential first step for hydrogen release from solidstate AB.[6] Once DADB is formed, hydrogen release from AB follows, whereby the release of the first equivalent of H 2 yields acyclic amineborane oligomers.DADB is also observed in the decomposition of AB in ionic liquids; [7] however, no one has reported the observation of DADB in organic solvents. Indeed, Mayer observed that the addition of DADB to a glyme solution results in decomposition and the evolution of hydrogen at À20 8C.[8]Wang and Geanangel followed the decomposition of AB at 80 8C in several solvents by ex situ NMR spectroscopy.[9] They did not report the observation of DADB or linear decomposition products in glyme, but they did observe the formation of cyclic (-BH 2 NH 2 -) n species. Unfortunately, the lower resolution did not permit them to discriminate between the n = 2, n = 3, and n = 4 cyclic products. These results suggest that the decomposition of AB in organic solvents occurs by a different reaction mechanism to that observed in the solid state or ionic liquids. It is important to understand the mechanism of hydrogen release, as this mechanism should provide critical insight into the stability of AB solutions that may be used as hydrogen-storage media. For example, Rassat et al. were able to predict the thermal stability of AB at 60 8C on the basis of the mechanism of hydrogen release in the solid state.[10] The mechanistic details of decomposition permit the evaluation of the long-term thermal stability of the storage compound, whether it is stored in the solid or solution phase.Herein, we revisit the initial thermolysis pathways of AB in the solution phase. In the process, we introduce a new highpressure, spinning NMR cell that enables the in situ observation of the decomposition of AB by 11 B NMR spect...

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also the most extensive desolvation, which virtually dominates the complex stability. This image coincides nicely with the complexation mechanism of a cation with a bis(crown ether), and the present result again supports our proposal that the complexation phenomenon can be understood well in terms of the enthalpyentropy compensation effect.Acknowledgment. This work was supported by the National Science Fund of China and in part by Grant-in-Aid for Scientific Researches 61550645 from the Ministry of Education, Science, and Culture of Japan, which are gratefully acknowledged. Y.L. is grateful for the generous financial support of Ako Chemical Co. for his stay at Himeji Institute of Technology.

fit in this cavity better than in other parts of the lipid bilayer. The proximity of 02 would then enhance the effect of paramagnetic relaxation on the fluorine atoms and raise qF. In the meantime, <7h of the aliphatic protons near the cavity would also increase.However, this would only contribute to a very small fraction of the weighted average of all protons. Thus, we observe qF > qH in the bilayers. Since the cavity may have a better chance to be located near the middle part of the PFD molecule (Figure 4B), the increase in qF of the la, le, and 2a fluorine atoms would be larger than that in qF of the 2e fluorine (Table I). This is in contrast to the situation in bulk PFD liquid, in which qF of the 2e fluorine (2.45 ± 0.05 s™1 atm™1 at 75.38 MHz) is slightly larger than qF of other fluorine atoms (2.17 ± 0.08 for la, 2.15 ± 0.04 for le, and 2.20 ± 0.03 for 2a, all in s™1 atm™1 at 75.38 MHz),10 because the small end of an ellipsoid is sterically more favorable for the approach of 02 in an isotropic liquid. The values of qF at 250 MHz (2.05-2.25 s™1 atm™1) are smaller, and the differences are also less. 19 The qF values of PFD droplets decrease further at 282.23 MHz (Table I). However, the differences in the qF values for the PFD droplets as well as the decane solution of PFD are too small to be significant, possibly due to the change in medium and larger experimental uncertainties. Since the corresponding F-19 signals of m-PFD are very broad and unresolved at 298 K,12,13 the interaction between lipid bilayers cannot be determined by NMR under these conditions. Because of the low solubility of PFD in the lipid bilayers,9 molecules of the two isomers are not likely to either interact or interfere with each other.PFTPA is a flexible molecule. Therefore, a schematic representation like Figure 4 may not be entirely adequate. However, the lipid bilayer would still experience a local perturbation by a dissolved PFTPA molecule. It is well-known that solutions of fluorocarbons in hydrocarbons exhibit nonideal behavior.24™27 The (24) Hildebrand,