Robust Fluorine-Functionalized {Ln5}-Organic Frameworks for Excellent Catalytic Performance on Cycloaddition of CO2
Chong Li1, Hongxiao Lv1, Kun Yang2
1Shanxi Key Laboratory of Advanced Carbon Based Electrode Materials, School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, P. R. China.
Abstract:
Lanthanide-organic frameworks (LnOFs) are a class of promising catalysts on a large number of organic reactions because of the higher coordination number of Ln3+ ions, inspired by which exploratory preparation of cluster-based LnOFs was carried out by us. Herein, the exquisite combination of spindly [Ln5(μ3-OH)6(CO2)6(H2O)6] clusters (abbreviated as {Ln5}) and fluorine-functionalized tetratopic ligand of 2',3'-difluoro-[p-terphenyl]-3,3″,5,5″-tetracarboxylic acid (F-H4PTTA) engendered two highly robust isomorphic nanoporous frameworks of {[Ln5(FPTTA)2(μ3-OH)6(H2O)6](NO3)} (NUC-61, Ln = Ho and Dy). NUC-61 compounds are rarely reported {Ln5}-based 3D frameworks with nano-caged voids (19 Å × 17 Å), which are shaped by twelve [Ln5(μ3-OH)6(COO)8] clusters and eight completely deprotonated F-PTTA4- ligands. Activated NUC-61a compounds are characterized by plentiful coexisted Lewis acid-base sites of open LnIII sites, capped μ3-OH, and -F. Judged by the ideal adsorbed solution theory (IAST), activated NUC-61Ho-a had a high CO2/CH4 adsorptive selectivity with the value of 12.7 (CO2/CH4 = 50/50) and 9.1 (CO2/CH4 = 5/95) at 298 K, which could lead to high-purity CH4 (≥99.9996%). Furthermore, catalytic experiments exhibited that NUC-61Ho-a, as a representative, could efficiently catalyze the cycloaddition reactions of CO2 with epoxides as well as the Knoevenagel condensation reactions of aldehydes and malononitrile. This work proves that the {Ln5}-based skeletons of NUC-61 with chemical stability, heterogeneity, and recyclability are an excellent acid-base bifunctional catalyst for some organic reactions.
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