One crystal, two phases: Designing phase coexistence in metal-organic frameworks

  1. One crystal, two phases: Designing phase coexistence in metal-organic frameworks

    20NANO08 / Nanoporous materials - catalysis
    Promotor(en): V. Van Speybroeck, S.M.J. Rogge / Begeleider(s): A.E.J. Hoffman

    Probleemstelling:

    As a recent class of nanoporous framework materials composed of inorganic and organic building blocks (see Figure 1a), metal-organic frameworks (MOFs) have attracted widespread attention for a variety of applications including gas adsorption and energy storage. Of specific interest in this proposal are flexible MOFs or soft porous crystals (SPCs) [1], as SPCs are largely crystalline materials that undergo large-amplitude phase transformations between multiple metastable phases under the influence of external stimuli such as temperature, pressure, or adsorption [2, 3]. Given this unique response, SPCs are often envisioned as next-generation nanosensing and –actuating materials. For instance, DUT-49(Cu) has been proposed as the active material in gas-releasing rescue systems given its unique potential for ‘anomalous’ negative gas adsorption (NGA). However, in order to design these promising materials for specific applications, it is crucial to fundamentally understand the origin of this cooperative behavior and rationalize how these macroscopic properties can be tuned by introducing specific building blocks in the material. The NGA phenomenon in DUT-49(Cu), for instance, was experimentally observed to critically depend on the size of the MOF crystal [4], an effect that remains elusive to date and precludes its full exploitation in applications.

    To obtain insight in these elusive but highly attractive size-dependent phase transitions in SPCs, we recently investigated a series of MOFs (see Figure 1a) at length scales going substantially beyond the nanocells that are typically simulated (see Figure 1c) [5]. In this way, we demonstrated that phase transformations in MOFs do not occur collectively, but rather proceed in a stepwise fashion in which part of the MOF crystal undergoes a phase transition first. During the transformation, multiple phases temporarily coexist in the same MOF crystal, leading to interfacial defects (see Figure 1b). We furthermore rationalized that this observed phase coexistence, a new type of spatial disorder in MOFs, could be stabilized in the material through carefully controlling the external stimuli, resulting in a MOF crystal that exhibits the attractive properties of both single phases. Given that phase coexistence was moreover demonstrated to depend on the crystallite size, it forms a possible explanation for the size-dependent structural flexibility in SPCs.

    Doelstelling:

    In order to enable the targeted introduction of phase coexistence in MOFs as a new pathway to design defect-engineered MOFs [6], several key questions need to be answered (see Figure 2):

    1. Can the introduction of small alterations in the MOF building blocks, such as linker functionalization or multivariate inorganic nodes, expand the window of thermodynamic conditions under which phase coexistence can be stabilized?
    2. Can phase coexistence exist in MOFs exhibiting topologies other than the winerack topology, thus providing evidence for a more fundamental mechanism through which phase coexistence nucleates and propagates through the crystal?

    In this thesis, it is the goal to explore these fundamental questions by extending our earlier thermodynamic protocol using flexible and ab initio-derived force fields for mesosized MOF crystals [5]. First, the impact of functionalizing the organic ligands with, e.g., –OH and –Br groups on the emergence of phase coexistence in MIL-53 is rationalized. These groups provide both steric hindrance–favoring the open-pore configuration–as well as attractive interactions that favor the closed-pore configuration; judiciously positioning these groups in the framework may hence culminate in a substantially expanded stability window for phase coexistence. Second, ZIF-4 and ZIF-7 are investigated as MOF architectures that both exhibit a rich polymorphism and are distinctly different from the earlier investigated winerack-type MOFs, allowing for more complicated forms of phase coexistence. The obtained thermodynamic insight will form a crucial step forward to consciously design spatial disorder in MOFs and exploit the phenomenon for practical applications.

    The student will be actively coached to make him/her acquainted with the several advanced simulation and coarse-graining techniques early in the thesis year, and to transfer necessary programming skills needed to perform the research.