title: Extra adsorption and adsorbate superlattice formation in metal-organic frameworks.
abstract: Metal-organic frameworks (MOFs) have a high internal surface area and widely tunable composition, which make them useful for applications involving adsorption, such as hydrogen, methane or carbon dioxide storage. The selectivity and uptake capacity of the adsorption process are determined by interactions involving the adsorbates and their porous host materials. But, although the interactions of adsorbate molecules with the internal MOF surface and also amongst themselves within individual pores have been extensively studied, adsorbate-adsorbate interactions across pore walls have not been explored. Here we show that local strain in the MOF, induced by pore filling, can give rise to collective and long-range adsorbate-adsorbate interactions and the formation of adsorbate superlattices that extend beyond an original MOF unit cell. Specifically, we use in situ small-angle X-ray scattering to track and map the distribution and ordering of adsorbate molecules in five members of the mesoporous MOF-74 series along entire adsorption-desorption isotherms. We find in all cases that the capillary condensation that fills the pores gives rise to the formation of 'extra adsorption domains'-that is, domains spanning several neighbouring pores, which have a higher adsorbate density than non-domain pores. In the case of one MOF, IRMOF-74-V-hex, these domains form a superlattice structure that is difficult to reconcile with the prevailing view of pore-filling as a stochastic process. The visualization of the adsorption process provided by our data, with clear evidence for initial adsorbate aggregation in distinct domains and ordering before an even distribution is finally reached, should help to improve our understanding of this process and may thereby improve our ability to exploit it practically.
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.
authors.forename: Hae
authors.initials: H
authors.lastname: Sung Cho
authors.affiliation: Key Laboratory of Biomedical Polymers-Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Luojiashan, Wuhan 430072, China.|The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China.
authors.forename: Hexiang
authors.initials: H
authors.lastname: Deng
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Miyasaka
authors.affiliation: Key Laboratory of Biomedical Polymers-Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Luojiashan, Wuhan 430072, China.
authors.forename: Zhiyue
authors.initials: Z
authors.lastname: Dong
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.
authors.forename: Minhyung
authors.initials: M
authors.lastname: Cho
authors.affiliation: Department of Chemical and Biochemical Engineering, Rutgers University, 98 Brett Road, Piscataway, New Jersey 08854, USA.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Neimark
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.
authors.forename: Jeung
authors.initials: J
authors.lastname: Ku Kang
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.|Department of Chemistry, University of California, Materials Sciences Division at Lawrence Berkeley National Laboratory, and Kavli Energy NanoSciences Institute, Berkeley, California 94720, USA.|King Fahd University of Petroleum and Minerals, Dhahran 34464, Saudi Arabia.
authors.forename: Omar M
authors.initials: OM
authors.lastname: Yaghi
authors.affiliation: Graduate School of Energy, Environment, Water and Sustainability, WCU/BK21Plus, KAIST, Daejeon 305-701, South Korea.|Department of Materials and Environmental Chemistry, Berzelii Centre EXSELENT on Porous Materials, Stockholm University, Stockholm SE-10691, Sweden.
authors.forename: Osamu
authors.initials: O
authors.lastname: Terasaki