Gas Adsorption DOI: 10.1002/ange.200900479 Amorphous Molecular Organic Solids for Gas Adsorption** Jian Tian, Praveen K. Thallapally, Scott J. Dalgarno, Peter B. McGrail, and Jerry L. Atwood* There has been intense interest in developing new porous materials for the storage and separation of important gases such as carbon dioxide and hydrogen. In this regard, crystalline materials such as zeolites, [1] metal–organic frame- works (MOFs) or coordination polymers (CPs), [2] and cova- lent organic frameworks [3] have been the subject of significant research. These classes of compounds generally possess nanochannel structures suitable for trapping guests and have concomitantly large surface areas. Amorphous materials such as activated carbon, [4] carbon nanotubes, [5] and cross- linked organic polymers [6] are also being investigated. Amor- phous materials are typically difficult to characterize and display lower degrees of gas adsorption, [7] but their avail- ability and scalability renders them industrially applicable. In the gas sorption arena, molecular organic solids have been largely neglected because of the close packing. Voids larger than 25  3 are rarely seen in molecular organic crystals, but some exceptions have been reported. [8] One such example is the low-density phase of p-tert-butylcalix[4]arene (TBC4) that adopts a dimeric capsule-like arrangement. [8b] This phase is obtained by sublimation under reduced pressure, and the capsule exists with a slightly offset head-to-head arrangement of tBu groups, and combination of the two cavities results in the generation of a void space of approximately 235  3 . Remarkably, it has been shown that various gases can penetrate crystalline low-density TBC4, coming to rest in the void space present within the capsule. [8d] Selective gas storage within amorphous coordination polymer microparticles has recently been demonstrated by Mirkin and co-workers. [9] However, such a phenomenon has yet to be observed for amorphous molecular organic solids, which have generally been considered inactive for gas sorption and release. [7, 10] Organic molecules typically pack so as to maximize attractive intermolecular interactions, and in doing so generally preclude the formation of large voids and channels. [11] Given this feature, the packing efficiency (PI) in organic crystals often falls between 0.59 to 0.69, and it is very difficult to calculate this value in the amorphous solid state owing to the general lack of structural characterization. We reasoned that molecules possessing large internal cavities in a rigid molecular structure should dramatically alter the close-packing and PI. Herein we show adsorption properties of amorphous molecular organic solids produced from noria (a waterwheel-like cyclic oligomer, 1), a Boc-protected derivative (2), [12, 13] and the related multivinylmonomer (3), [14] all of which are shown in Figure 1A. Molecule 1 was synthesized according to a one-pot procedure reported by Nishikubo and co-workers. [12, 13] The molecular structure of noria 1 was not previously elucidated but was postulated from the structure of 2. [12] In our hands, single platelike crystals of 1 were obtained by diffusion of methanol into a dimethylsulfoxide solution. Single crystal X- ray structural analysis showed that 1 is the expected double- cyclic ladder-type oligomer structure akin to 2, possessing 24 hydroxy groups, six shallow cavities (around the periphery), and a large hydrophobic central cavity (Figure 1 A–C). [15, 16] From examination of the crystal structure, the molecule has a flattened oval shape, with a central cavity and a portal diameter of approximately 7 and 5 , respectively. The internal volume of the cavity in 1 is calculated to be about 160  3 . [17] Compound 1 was precipitated as a light yellow powder during synthesis. [13] Solvent-free 1 was isolated by drying under reduced pressure at 80 8C, as confirmed by thermog- ravimetric analysis (TGA, Figure S1 in the Supporting Information). X-ray powder diffraction (XRPD) and selected area electron diffraction (SAED) studies on dried samples of 1 showed the materials to be amorphous with minor crystallinity (Figure S2 in the Supporting Information). Grinding also affords totally amorphous 1 (Figure S2 in the Supporting Information). Unexpectedly, amorphous 1 rapidly gained weight upon cooling in air and when subsequently suspended in water produced copious air bubbles. This observation implied that the material contains significant porosity and readily adsorbs surrounding gases. Given this finding, we studied the porosity of the bulk sample of 1. The adsorption capacity of bulk 1 was measured volu- metrically at room temperature for N 2 ,H 2 , and CO 2 under isothermal conditions. The affinity of 1 for CO 2 is high and is significantly greater than for N 2 and H 2 . The isotherm for CO 2 at room temperature exhibits a type I curve (Figure 1 D and Figure S4 in the Supporting Information), which is typically observed for microporous materials, and little hysteresis was observed within measurement accuracy. The rate of adsorp- tion is rapid and equilibration was established within approx- imately 10 min. At the plateau region, approximately 4 mole [*] J. Tian, Prof. J. L. Atwood Department of Chemistry, University of Missouri-Columbia Columbia, MO 65211 (USA) E-mail: atwoodj@missouri.edu Dr. P. K. Thallapally, Dr. P. B. McGrail 902 Battelle Blvd., Pacific Northwest National Laboratory Richland, WA 99352 (USA) Dr. S. J. Dalgarno School of Engineering and Physical Sciences—Chemistry Heriot– Watt University, Riccarton, Edinburgh, EH14 4AS (UK) [**] We acknowledge the NSF and DOE for financial support of this work. P.K.T. thanks the Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE) for work performed in the Environ- mental Sciences Laboratory. PNNL is a multi-program laboratory operated by Battelle Memorial Institute for the DOE under contract DE-AC05-76L01830. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200900479. Zuschriften 5600  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2009, 121, 5600 –5603