(436g) Theoretical Prediction of Interpenetrating Metal-Organic Frameworks

Kutay B. Sezginel, Tianyi Feng, Christopher E. Wilmer

Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

Metal-organic frameworks (MOFs) are crystalline materials comprising building blocks of inorganic nodes and organic linkers. MOFs offer high permanent porosities, very high surface areas and versatile framework interactions which makes them promising candidates towards various applications such as gas storage [1-3] and separation [4, 5], electrochemistry [6], sensing [7], catalysis [8], and drug delivery [9]. The structural properties of MOFs such as pore size and shape and the chemical environment of the framework can be precisely designed by finding the right combination of building blocks and suitable reaction conditions. An important consideration for the atomically precise design of MOFs is the interpenetration of the framework. Interpenetration is especially observed in MOFs with large void fraction where multiple frameworks interpenetrate each other to form a more thermodynamically favorable and stable product. [10]. In common cases, two, three, or four identical frameworks interpenetrate each other, however, as many as 54 frameworks have been demonstrated to interpenetrate each other [11].

Interpenetration in MOFs if often considered a negative phenomenon since it decreases the available space within the structure. [12] However, interpenetration also provides a degree of freedom in the design of MOFs which results in many beneficial properties such as enhanced framework stability [13], selective gas adsorption [14], increased heat of adsorption [15], adsorption hysteresis [16], photoluminescence control [17] and guest-responsive porosity [18]. Majority of the interpenetrating structures have been discovered as byproduct crystalline phases resulting from MOF synthesis. Therefore, interpenetration is generally observed within the same framework which is also named as homo-interpenetration. [19] A much rare case is observed when two distinct frameworks interpenetrate with each other also termed as hetero-interpenetration. [20] An exciting prospect in the near future, once control over catenation in MOFs has sufficiently matured, is the selective interpenetration of different frameworks that each confer different functionality to the overall material [21] This however, requires a thorough understanding of experimental methods that facilitate control over interpenetration as well as tools that enable rational design of interpenetrated frameworks. [22] Many hetero-interpenetrated MOFs have been reported, with different dimensionalities, [23, 24] topologies, [25, 26] and chemical compositions. [27] Therefore controlling catenation is an important design criterion which has no established control mechanism and remains quite challenging.

Motivated by the promising properties of interpenetrated MOFs and lack of a design tool that enables systematic engineering of these materials, we developed a computational tool to discover interpenetrating MOFs. As a proof of concept, we initially started investigating interpenetration in MOF-5, which is a widely studied homo-interpenetrating cubic MOF. [12] After accurately identifying interpenetration in this MOF we generalized our algorithm to search for interpenetrating structures for any given MOFs. We then investigated a set of experimentally synthesized MOFs from the database constructed by Chung et al. [28] and discovered several hypothetically hetero-interpenetrating MOFs. Our tool is able to discover interpenetration in any given set of periodic network in a matter of minutes (avg. 3.5 min), therefore this method can be easily extended to be used for any periodic crystalline materials.

The interpenetration algorithm starts with creating a potential grid for one of the MOFs. The potential grid is created using Lennard-Jones potential with UFF parameters and a cutoff radius selected according to the size of the MOF unit cell. The potential grid is used to rapidly check for collisions between periodic frameworks and determine the overall potential energy of the structure. After potential grid is constructed for one of the structures, interpenetration of the other structure is checked by screening all the grid points in the potential grid with random orientations by rotating the structure in 3D space. The framework atoms of the interpenetrating network are placed one by one and energy penalty for inserting each atom is calculated. For cases with energy penalty higher than a previously defined energy limit, the interpenetration trial is interrupted and next trial is performed, otherwise insertion of atoms is continued. If all the framework atoms are added without going over the energy penalty, the orientation of the frameworks is recorded. The recorded orientations are checked for collisions in the periodic unit cells and the atomic positions for cases without collisions are exported and sorted according to their overall potential energy. Our initial trials with a set of 50 MOFs (approx. 1250 combinations) resulted in the discovery of tens of interpenetrating frameworks. We are currently screening hundreds of structures to discover novel interpenetrating MOFs. The discovered structures are being investigated in detail and geometrically optimized to identify thermodynamically achievable structures. These interpenetrating MOFs can then be studied in detail to reveal promising properties of these complex structures.

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