Robust Fluorine-Functionalized {Ln<sub>5</sub>}-Organic Frameworks for Excellent Catalytic Performance on Cycloaddition of CO<sub>2</sub> with Epoxides and Knoevenagel Condensation
Chong Li, Hongxiao Lv, Kun Yang, Xiutang Zhang
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 Ln 3+ ions, inspired by which exploratory preparation of cluster-based LnOFs was carried out by us. Herein, the exquisite combination of spindly [Ln 5 (μ 3 -OH) 6 (CO 2 ) 6 (H 2 O) 6 ] clusters (abbreviated as {Ln 5 }) and fluorine-functionalized tetratopic ligand of 2′,3′-difluoro-[ p -terphenyl]-3,3″,5,5″-tetracarboxylic acid (F-H 4 PTTA) engendered two highly robust isomorphic nanoporous frameworks of {[Ln 5 (FPTTA) 2 (μ 3 -OH) 6 (H 2 O) 6 ](NO 3 )} n ( NUC-61, Ln = Ho and Dy ). NUC-61 compounds are rarely reported {Ln 5 }-based 3D frameworks with nano-caged voids (19 Å × 17 Å), which are shaped by twelve [Ln 5 (μ 3 -OH) 6 (COO) 8 ] clusters and eight completely deprotonated F-PTTA 4– ligands. Activated NUC-61a compounds are characterized by plentiful coexisted Lewis acid–base sites of open Ln III sites, capped μ 3 -OH, and -F. Judged by the ideal adsorbed solution theory (IAST), activated NUC-61Ho-a had a high CO 2 /CH 4 adsorptive selectivity with the value of 12.7 (CO 2 /CH 4 = 50/50) and 9.1 (CO 2 /CH 4 = 5/95) at 298 K, which could lead to high-purity CH 4 (≥99.9996%). Furthermore, catalytic experiments exhibited that NUC-61Ho-a, as a representative, could efficiently catalyze the cycloaddition reactions of CO 2 with epoxides as well as the Knoevenagel condensation reactions of aldehydes and malononitrile. This work proves that the {Ln 5 }-based skeletons of NUC-61 with chemical stability, heterogeneity, and recyclability are an excellent acid–base bifunctional catalyst for some organic reactions.