This journal is © The Royal Society of Chemistry 2015 Chem. Soc. Rev., 2015, 44, 103--118 | 103 Cite this: Chem. Soc. Rev., 2015, 44, 103 Facilitated transport of small molecules and ions for energy-efficient membranes† Yifan Li, ab Shaofei Wang, ab Guangwei He, ab Hong Wu, ab Fusheng Pan ab and Zhongyi Jiang* ab In nature, the biological membrane can selectively transport essential small molecules/ions through facilitated diffusion via carrier proteins. Intrigued by this phenomenon and principle, membrane researchers have successfully employed synthetic carriers and carrier-mediated reversible reactions to enhance the separation performance of synthetic membranes. However, the existing facilitated transport membranes as well as the relevant facilitated transport theories have scarcely been comprehensively reviewed in the literature. This tutorial review primarily covers the two aspects of facilitated transport theories: carrier-mediated transport mechanisms and facilitated transport chemistries, including the design and fabrication of facilitated transport membranes. The applications of facilitated transport membranes in energy-intensive membrane processes (gas separation, pervaporation, and proton exchange membrane fuel cells) have also been discussed. Hopefully, this review will provide guidelines for the future research and development of facilitated transport membranes with high energy efficiency. Key learning points (1) The panorama of facilitated transport and its important implications. (2) Chemistries and reactions involved in facilitated transport. (3) Approaches to exploring advanced functional materials to facilitate the transport of molecules and ions. (4) Application paradigms of facilitated transport in membrane processes. (5) Design of energy-efficient, high-performance membranes with a facilitated transport feature through biomimetic and bioinspired strategies. 1. Introduction A high-performance membrane which allows fast and selective transmembrane permeation of small molecules/ions is essential for triggering revolutionary changes in many significant chemical processes. 1 As the pore size of the membrane falls below 1 nm, the relevant membrane processes can easily inherit the benefits from equilibrium-governed separation and rate- governed separation, thus acquiring high energy efficiency. The permeation of small molecules through the membrane is usually described by the well-known ‘‘solution–diffusion’’ mechanism, where solubility and diffusivity are governed by thermodynamic and kinetic/transport factors, respectively. Therefore, the ideal membranes should render an appropriate chemical microenvironment to ensure high solubility, and also possess a well-tailored microstructure to ensure high diffusivity. In this regard, molecular sieve membranes seem to be the preferred choice for molecular transport, which have great potential for simultaneous enhancement of permeability and selectivity. However, molecular sieve membranes may not be the best choice for ion transport, which strongly relies on electro- chemical interactions. Also, the difficulties in fabricating mole- cular sieves into defect-free thin membranes impede the broad applications of the molecular sieving membrane. 2 Consequently, interest has been growing in the selective transport mechanisms which allow efficient enrichment of the desired permeant based on broader material chemistries and more specific interactions. If we take a look at nature, we can easily find an ideal model- biological membrane, which can selectively transport essential small molecules/ions through facilitated diffusion via carrier protein. Early evidence of carrier-mediated facilitated diffusion was traced back to half a century ago. 3 As one important type of a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail: zhyjiang@tju.edu.cn b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China † Electronic supplementary information (ESI) available: Additional information and references for the facilitated transport membranes discussed. See DOI: 10.1039/c4cs00215f Received 25th June 2014 DOI: 10.1039/c4cs00215f www.rsc.org/csr Chem Soc Rev TUTORIAL REVIEW Published on 09 October 2014. Downloaded on 05/04/2016 07:24:25. View Article Online View Journal | View Issue