Oriented Superhydrophobic Bimetallic MOF Composite Membrane for Efficient Ethanol-Water Separation

Extracting ethanol from aqueous solutions is important but challenging in industry. Pervaporation membranes show great promise for separating ethanol from water, with the design of their structure being crucial for enhancing performance. In this study, we developed an oriented bimetallic metal-organic framework (MOF) membrane, designated as ZIF-CoZn, for the pervaporation separation of ethanol from water. During crystal growth, bimetallic salts provide specific nucleation sites, and the competitive coordination between Co and Zn ions shifts the energetically favorable (100) plane to the (211) plane. This orientation enables precise molecular-level control over hydrophobic ligand arrangement, effectively repelling water molecules. Meanwhile, bimetallic competition enlarges pore sizes, facilitating ethanol permeation. When compared to single-metal MOF membranes made of cobalt or zinc, the separation factor of the ZIF-CoZn membrane for ethanol/water mixtures increased by 127% and 160%, respectively. Benefiting from the high roughness and increased exposure of hydrophobic ligands due to the preferential (211) orientation, ZIF-CoZn exhibits superhydrophobicity after vinyl-polydimethylsiloxane coating. The oriented ZIF-CoZn membrane was also scaled up to an area of 1 m². This work provides valuable insights into optimizing MOF membrane structure and lays the foundation for its promotion and application in the industry.