Water Adsorption in Flexible Gallium-Based MIL-53 Metal−Organic Framework

Understanding the adsorption of water in metal−organic frameworks (MOF), and particularly in soft porous crystals, is a crucial prerequisite before considering MOFs for industrial applications. We report here a joint experimental and theoretical study on the behavior of a gallium-based breathing MOF, Ga-MIL-53, upon water adsorption. By looking at the energetics and thermodynamics of Ga-MIL-53, we demonstrate why it behaves differently from its sibling Al-MIL-53, showing a different phase at room temperature (a nonporous phase) and the presence of a hydrated narrow-pore structure at gas saturation pressure. Moreover, we present a complete water vapor pressure vs temperature phase diagram of Ga-MIL-53 upon water adsorption. ■ INTRODUCTION Soft porous crystals (SPC) are a fascinating subclass of metal− organic frameworks (MOFs) that exhibit structural transformations of large amplitude triggered by external stimuli such as temperature variations, mechanical pressure or adsorption of guest molecules (in gas or liquid phase). The changes in the framework of SPCs in response to the external constraint are reversible and maintain the crystalline character of the solid. A growing number of SPC structures have been synthesized and reported in the literature, and although none of them have yet been directly used at the industrial scale, they have been proposed for a large range of potential practical applications. In addition to the general application of MOFs, flexible frameworks are expected to present an intrinsic interest due to their large-scale stimuli-responsive transformations, which could leverage novel nanobiotechnologies, such as sensing for detecting traces of organic molecules, slow release of drugs for long-release single-injection therapies, and specific gas separations. While the existing soft porous crystals display a large gamut of flexibility mechanisms, one of the most studied systems in this subclass is the family of MIL-53 materials, in which the adsorption of increasing amounts of guest molecules (carbon dioxide, xenon, alkanes, water, etc.) may induce a double structural transition with volume changes of up to 40%, a phenomenon called “breathing”. Various aspects of gas and liquid adsorption in these materials have been studied extensively in the literature: structure, energetics, thermodynamics, dynamics, and transport properties. However, despite the critical role that water is known to play in phase stability and gas separation properties in MOFs in general, only a few studies have dealt with the impact of water on the MIL-53 frameworks and their properties. Most dealt with the chromium-based Cr-MIL-53. In 2010, Bourrelly and Devautour-Vinot published experimental studies of adsorption and desorption of water, among other polar vapors, along with quantum chemistry calculations of the preferential arrangements of the molecules within the pores. A later experimental study of Cr-MIL-53 immersed in liquid water used high resolution powder X-ray diffraction to solve the structure of the pores of both the large-pore (lp) and narrowpore (np) phases, fully filled with water. Molecular simulations were further used to characterize the structure, diffusion and adsorption thermodynamics of water in the lp and np phases of Cr-MIL-53. A similar molecular simulation Received: December 19, 2013 Revised: February 14, 2014 Published: February 21, 2014 Article