Introducing alkyl chains to realize the construction of superhydrophobic/superoleophilic MOFs and the transformation from three-dimensional to two-dimensional structure

Abstract

Superhydrophobic metal–organic frameworks (MOFs) exhibit excellent application prospects in many fields, such as catalysis, water pollution treatment, self-cleaning, and so on. In this study, two stable salen-based three-dimensional (3D) MOFs, i.e., ([Cd(CuL^s)(L)]_n (1) and [Zn(CuL^s)(L)DMF]_n (2) [L^s = N,N′-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-cyclohexanediamine copper^II, L = [1,1′-biphenyl]-4,4′-dicarboxylic acid]), have been synthesized. To build superhydrophobic MOFs, the low-surface-energy alkyl chains were introduced, and the water contact angles (WCAs) gradually increased from 136.9° to 155.0°. Interestingly, the structures are different from 1 when the alkyl chains are n-butyl, n-hexyl, or n-octyl. In particular, the 3D structure changes to the 2D structure by introducing the n-octyl, and the 2D stacking mode with a large number of aromatic rings is also conducive to improving hydrophobicity. As evidenced by scanning electron microscopy (SEM), the introduction of alkyl chains also enhances the overall roughness of MOFs, which is also pivotal to improving hydrophobicity. In addition, the superhydrophobic/superoleophilic 1-Oct was loaded into a sponge to achieve selective oil–water separation. Another application of this MOF-functionalized sponge was the highly selective separation of water-in-oil emulsions. It was found that the pore environment of 1-Et was improved with the introduction of the ethyl side chain, which forms the basis for the other potential applications.