Atomic-scale explorations of stimulus-responsive framework properties in an ultramicroporous gas sorbent
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Date
2017-12-03
Journal Title
Journal ISSN
Volume Title
Publisher
Society of Crystallographers in Australia and New Zealand
Abstract
Functional microporous materials capable of efficiently separating and/or storing gases at noncryogenic temperatures are sought for a wide variety of important industrial applications, including pre- and post-combustion carbon capture, hydrogen fuel storage, and the purification of component gases from air. Understanding the atomic-scale interactions between the host material and guest species under variable operating conditions is essential for obtaining information about adsorption and separation mechanisms, which can in turn be used to design better sorbents targeted at specific applications.
The ultramicroporous metal-organic framework [Cu3(cdm)4] (cdm = C(CN)2CONH2 -) was recently reported to exhibit moderately selective adsorption of CO2 over CH4, along with excellent exclusion of elemental gases such as H2 and N2 [1]. Although the very small pore diameter (3–4 Å) results in unpromisingly slow diffusion dynamics, its close similarity to the kinetic diameters of many small gas molecules [2] also raises the prospect of altering the gas sorption and selectivity characteristics of the material via minor structural modifications, such as might be introduced by changing the temperature and/or guest concentration during sorbent operation under industrially relevant conditions. Using a combination of in situ neutron scattering experiments and density functional theory-based calculations, we examine in detail the interplay between lattice shape, pore size, temperature, and CO2 concentration in [Cu3(cdm)4]. The rare and interesting fundamental property of areal negative thermal expansion (NTE) in [Cu3(cdm)4] is attributed to a new variation of a well-known NTE mechanism, and is triggered by dynamic motions of the rigid cdm ligand within the constraints of the complicated framework topology. Although the thermal response of the pore diameter is surprisingly insignificant due to competition between multiple effects, the potential for similar materials to exhibit temperature induced changes in adsorption properties is clearly demonstrated. This study illustrates the breadth and depth of information that can be obtained by combining the power of experimental and theoretical characterisation in an approach that is generally applicable to crystalline sorbent systems.
Description
Keywords
Adsorption, Separation processes, Gases, Diffusion, Neutron diffraction, Thermal expansion
Citation
Auckett, J. E., Duyker, S. G., Izgorodina, E. I., Hawes, C. S., Turner, D. R., Batten, S. S., Peterson, V. K. (2017). Atomic-scale explorations of stimulus-responsive framework properties in an ultramicroporous gas sorbent. Paper presented at CRYSTAL 31, the 31st Biennial Conference of the Society of Crystallographers in Australia and New Zealand, Pullman Bunker Bay, Western Australia, 3 – 7 December 2017. Retrieved from: https://crystal31.com/wp-content/uploads/2017/11/Final-Program-CRYSTAL-31-20171123.pdf