Thermodynamic Methods and Models to Study Flexible Metal–Organic Frameworks

Abstract

Much attention has recently been focused on a fascinating subclass of metal-organic frameworks that behave in a remarkable stimuli-responsive fashion. These soft porous crystals feature dynamic crystalline frameworks displaying reversible, large-amplitude structural deformations under external physical constraints such as temperature, electric field or gas exposure. The number of reported syntheses of such materials is rapidly growing and they are promising for practical applications, such as gas capture, purification and fluid separation. Herein, we summarize the recently developed thermodynamic tools that can help understand the process of fluid adsorption and fluid mixture coadsorption in these flexible nanoporous materials. These tools, which include both molecular simulation methods and analytical models, can help rationalize experimental results and predict adsorption properties over a wide range of thermodynamic conditions. A particular focus is given on how these methods can guide the experimental exploration of a large number of materials and working conditions (temperature, pressure, composition) to help design efficient processes relying on fluid adsorption in soft porous crystals.

Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity

“…Mass losses on activation correspond well to those expected for solvent removal. Solid-state 13 C NMR and IR spectroscopy confirmed complete removal of DMF from SHF-61-DMF (see Supplementary Figure 3). …”

“…3 . The MOF was characterised by 13 C CP-MAS NMR spectroscopy and both single crystal and powder X-ray diffraction, the latter confirming phase purity. The framework structure (Figure 1) is almost identical for both solvated forms, SHF-61-DMF and SHF-61-CHCl 3 .…”

“…[8][9][10] The dynamic behaviour of these materials can occur through a variety of mechanisms including a rotation of the organic linkers, a hinge motion of the carboxylate groups that commonly connect ligands to the metal nodes or a displacement of subnetworks relative to each other. [11][12][13] These motions lead to opening or closing of pores and are associated with loss/uptake of guest molecules. The structural changes occur predominantly through defined transitions between different crystalline states, a process commonly referred to as "breathing."…”

Molecular Simulation of Adsorption in Microporous Materials

“…Swelling of polymeric systems upon adsorption [76,77], coal swelling induced by gas adsorption [72], behavior of clay minerals [78,79], or cement paste [80] are some examples of deformations of porous materials on which performing reliable simulations remains challenging in the perspective of industrial applications. There has been recent progress in this field, by introducing the Osmotic Framework Adsorbed Solution Theory (OFAST) [81] and by performing simulations in the osmotic ensemble [82]. There is still a promising development to be done, in order to establish an efficient process of dealing with a wide range of systems, combining existing methodologies and expanding them to treat complex systems.…”

Computational characterization and prediction of metal–organic framework properties

“…Since its emergence in the 1950s, molecular simulation has seen an ever-growing use in research in the fields of physical, chemical, and materials sciences, where it offers an additional arXiv:1506.08219v3 [cond-mat.mtrl-sci] 24 Aug 2015 [2] 2014 high-throughput computational screening [3] 2014 first-principles force fields for guest molecules [4] 2013 gas separation [5] 2012 screening for adsorption and separation [6] 2012 methane, hydrogen, and acetylene storage [7] 2011 adsorption in flexible MOFs [8] 2011 energy, environmental and pharmaceutical applications [9] 2011 screening for separation applications [10] 2009 hydrogen storage [11] 2009 adsorption [12] 2008 adsorption and transport [13] 2007 adsorption of small molecules [14] Table 1: List of reviews published on computational characterization of metal-organic frameworks.…”