Interplay of Metal Node and Amine Functionality in NH 2 ‑MIL-53: Modulating Breathing Behavior through Intra-framework Interactions Pablo Serra-Crespo, † Elena Gobechiya, ‡ Enrique V. Ramos-Fernandez, † Jana Juan-Alcañ iz, † Alberto Martinez-Joaristi, † Eli Stavitski, § Christine E. A. Kirschhock, ‡ Johan A. Martens, ‡ Freek Kapteijn, † and Jorge Gascon* ,† † Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands ‡ Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, University of Leuven, 3001 Leuven, Belgium § National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States * S Supporting Information ABSTRACT: A series of amino-functionalized MIL-53 with different metals as nodes has been synthesized. By determining adsorption properties and spectroscopic characterization, we unequivocally show that the interaction between the amines of the organic linker and bridging μ 2 -OH of the inorganic scaffold modulates metal organic framework (MOF) flexibility. The strength of the interaction has been found to correlate with the electropositivity of the metal. ■ INTRODUCTION A special class of metal organic frameworks (MOFs) is those whose pore dimensions change without breaking chemical bonds within the framework. This results in special properties, such as the breathing effect 1-3 and the gate phenomenon, 4-6 where pores contract or open during molecule adsorption. An example of a breathing-type material is the MIL-53 series, first reported and rigorously characterized by Ferey, Serre, and co- workers (MIL stands for Material Institute Lavoisier). 3 MIL-53 is built from MO 6 octahedra (where M can be Fe 3+ , Cr 3+ , Al 3+ , Ga 3+ , In 3+ , or Sc 3+ ) formed from trans-bridging OH ions and the oxygens of the coordinate, bridging 1,4-benzene- dicarboxylate linkers. In this way, a crystalline material with one-dimensional (1D) diamond-shaped pores is formed. Upon adsorption of guest molecules, e.g., CO 2 or H 2 O, or by changing operating conditions, the framework structure reversibly changes. 2,7 For the Cr- or Al-containing forms of MIL-53, MIL-53(Cr) or MIL-53(Al), the structure in which the pores are in the “open” form is the most stable form after thermal activation. These two forms of MIL-53 show a transition from a large pore form (lp form, the initial dehydrated form) to a narrow pore form (np form) during adsorption of certain molecules. When the driving force (the partial pressure of the adsorbing molecule) is large enough, the pores reopen to their original lp form. Thermodynamical, stress-based, and molecular models have been developed to describe and analyze these breathing transitions in MIL-53. 8-11 The iron and gallium forms of MIL-53, MIL-53(Fe) and MIL-53(Ga), display a more complex and different behavior. In the case of gallium, the stability domain of the narrow pore structure MIL-53(Ga) np is larger (up to 160 °C, instead of 20-30 °C for Al, for instance), while the iron form of MIL-53 is in a “very narrow pore” form (vnp form) when it is initially dehydrated. The latter can hardly accommodate guest molecules. With increasing pressure, this structure passes via intermediate forms to the lp form. 12-14 Amino-MIL-53, hereafter NH 2 -MIL-53 (see Figure 1), is a material with the topology of MIL-53. During the synthesis of NH 2 -MIL-53, 2-amino-terephthalic acid is used as the linker molecule, instead of terephthalic acid. The isoreticular material obtained has the formula X(OH)[O 2 C-C 6 H 3 NH 2 -CO 2 ], with X denoting a trivalent metal at the nodes. 15-17 During the past few years, we have extensively investigated the Al version of the amine-functionalized MIL-53 framework. 18-21 NH 2 -MIL- 53(Al) appears to be an excellent candidate for CO 2 capture from multicomponent mixtures, displaying an almost infinite selectivity for CO 2 over other gases, e.g., CH 4 ,H 2 O, N 2 , and CO, based on shape selectivity. 22,23 Although we initially assumed amino-MIL-53(Al) to behave similar to MIL-53(Al or Cr), i.e., starting in the open pore configuration after thermal activation, followed by a transition to the narrow pore form after adsorption of CO 2 molecules at relatively low pressures, more detailed combined spectroscopic (IR), in situ diffraction (XRD), and theoretical (DFT) studies Received: July 12, 2012 Revised: August 12, 2012 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A dx.doi.org/10.1021/la302824j | Langmuir XXXX, XXX, XXX-XXX