Role of a Modulator in the Synthesis of Phase-Pure NU-1000

5 years ago

Role of a Modulator in the Synthesis of Phase-Pure NU-1000

Thomas E. Webber, Connie C. Lu, R. Lee Penn, Wei-Guang Liu, Donald G. Truhlar, Sai Puneet Desai

NU-1000 is a robust, mesoporous metal–organic framework (MOF) with hexazirconium nodes ([Zr₆O₁₆H₁₆]⁸⁺, referred to as oxo–Zr₆ nodes) that can be synthesized by combining a solution of ZrOCl₂·8H₂O and a benzoic acid modulator in N,N-dimethylformamide with a solution of linker (1,3,6,8-tetrakis(p-benzoic acid)pyrene, referred to as H₄TBAPy) and by aging at an elevated temperature. Typically, the resulting crystals are primarily composed of NU-1000 domains that crystallize with a more dense phase that shares structural similarity with NU-901, which is an MOF composed of the same linker molecules and nodes. Density differences between the two polymorphs arise from the differences in the node orientation: in NU-1000, the oxo–Zr₆ nodes rotate 120° from node to node, whereas in NU-901, all nodes are aligned in parallel. Considering this structural difference leads to the hypothesis that changing the modulator from benzoic acid to a larger and more rigid biphenyl-4-carboxylic acid might lead to a stronger steric interaction between the modulator coordinating on the oxo–Zr₆ node and misaligned nodes or linkers in the large pore and inhibit the growth of the more dense NU-901-like material, resulting in phase-pure NU-1000. Side-by-side reactions comparing the products of synthesis using benzoic acid or biphenyl-4-carboxylic acid as a modulator produce structurally heterogeneous crystals and phase-pure NU-1000 crystals. It can be concluded that the larger and more rigid biphenyl-4-carboxylate inhibits the incorporation of nodes with an alignment parallel to the neighboring nodes already residing in the crystal.

Publisher URL: http://dx.doi.org/10.1021/acsami.7b11348

DOI: 10.1021/acsami.7b11348