Ultrasonication-driven downscaling and formation of Lewis acid–base sites in hydrogen-bonded Co metal–organic frameworks trigger exceptional catalytic performance for the Knoevenagel condensation reaction

Abstract

Ultrasonication has been employed as an effective approach to break down bulk materials into nanoscale size. This approach seems more adaptable to hydrogen-bonded crystals due to the weak hydrogen bonding between building units, and its expected effect might be beyond size reduction. Here, we report the facile fabrication of Co-MOF nanorods via ultrasonic cleavage of the hydrogen bonds in their corresponding hydrogen-bonded bulk crystals and their exceptional catalytic performance for the Knoevenagel reaction which occurs in the presence of abundant Lewis acid–base sites due to ultrasonication-induced removal of coordinated water molecules. The as-prepared novel Co-MOF structure is formed by stacking of one-dimensional chains mainly via interchain hydrogen bonding between the H atom of coordinated H₂O and the O atom of coordinated HCOO⁻. Due to the weak inter-chain force of the as-prepared hydrogen-bonded Co-MOF, blocky Co-MOF powders (CoMOF-B) could be disassembled into Co-MOF nanorods (CoMOF-NR) via ultrasonication under very mild conditions (20 min in deionized water at room temperature). Meanwhile, a large number of unsaturated Co sites were generated on the surface after ultrasonic treatment, which led to the formation of active Lewis acid–base sites with the original Lewis basic sites in the ligands. The abundant Lewis acid–base sites endow the CoMOF-NR with excellent performance in the Knoevenagel condensation reaction of benzaldehyde (81.42% yield after 5 min and over 99% yield within 30 min at room temperature) and its homologs, outperforming other common catalysts. No appreciable degradation in catalytic performance can be observed within five test cycles.