Synthesis of Lattice Mismatched MOF-on-MOF System

Abstract

Metal organic frameworks (MOFs) are compounds composed of metal ions or clusters coordinated to organic ligands, creating 1,2, or 3-dimensional structures. They are also known for their highly porosity and large surface area. To enhance their original functionalities, various strategies have been utilized, including the incorporation of different metal cations and organic ligands or post-synthetic modification. These methods have expanded the practical applications of MOFs to various fields. Recently, introducing heterogeneity in MOFs has become a fascinating strategy for developing their functionality. Modifying the interior architecture of MOFs through metal/linker substitution, defect engineering, or multivariate MOF synthesis is one of the straightforward approaches. Another interesting method to enhance functionalities involved combining MOFs with other functional substances. Additionally, MOF-on-MOF composites have also garnered significant attention due to their enhanced functionality and well-defined structure. Another characteristic of MOF-on-MOF is that it can exhibit new properties distinct from both the core and shell MOFs, arising from phenomena at the interface between the core and shell MOFs. Hence, a thorough understanding of the properties of MOF-on-MOF structures necessitates analysis and observation of the phenomena occurring at the interface. Finding pairs of two different MOFs that can be epitaxially grown is a challenging task. In our previous research, we reported on the epitaxial growth of a different type of MOF on a core MOF. We established a strategy to identify synthesizable MOF-on-MOF pairs through two approaches: cell parameter matching and chemical point matching. Through these two approaches, we developed an algorithm to identify synthesizable MOF-on-MOF pairs. By utilizing this algorithm, we were able to calculate and obtain a variety of MOF pairs and successfully synthesized some of these pairs, providing strong evidence for the validity of our approach. However, synthesizing a variety of MOF-on-MOF pairs remains challenging due to factors such as the instability of the core MOF during the synthesis stage, which can lead to synthesis failures. In this study, we propose two advancements for the MOF-on-MOF systems. First one is the construction of a double-shell MOF system to enable more complex functional performance. Specifically, we aim to synthesize UiO-67@HKUST-1@MOF-5 at the single-crystal level and conduct an analysis of the interface. The second proposal involves expanding the range of synthesizable MOF- on-MOF pairs. By using flexible MOFs to relax the conditions for cell parameter matching condition and adjusting the cell parameters of flexible MOFs through changes in the solvent system, we can synthesize MOF-on-MOF structures with various rigid MOFs.