Two-dimensional materials: an emerging platform for gas separation membranes Farhad Moghadam and Ho Bum Park Graphene and its derivatives are of particular interest as emerging platforms for molecular separation. However, there are abundant 2D nanomaterials other than graphene that have unique and intriguing structural features that can be exploited for membrane-based separation that have not been extensively studied. Furthermore, from a materials synthesis standpoint, realizing high yield and scalable methods for massive production of 2D nanomaterials is critical. Liquid-based exfoliation to produce 2D nanosheets from layered bulk crystals has recently drawn considerable attention. Such single-layered or few-layered 2D nanosheets can be formed into laminate membranes with exceptional molecular-sieving properties or more preferably utilized as high aspect ratio nanofillers to improve the separation performance of polymer membranes. Most remarkably, 2D nanosheets can mitigate the well-known plasticization phenomenon in polymer membranes by interacting strongly with the polymer chains. There is no doubt that these emerging 2D nanomaterials will facilitate fabrication of next-generation membranes. Address Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea Corresponding author: Park, Ho Bum (badtzhb@hanyang.ac.kr) Current Opinion in Chemical Engineering 2018, 20:28–38 This review comes from a themed issue on Separation engineering Edited by Wei Zhang, WS Ho and Kang Li https://doi.org/10.1016/j.coche.2018.02.004 2211-3398/ã 2018 Elsevier Ltd. All rights reserved. Introduction Since the discovery of graphene by the Geim group in 2004 [1], there has been a resurgence of interest in various 2D nanomaterials. Hexagonal boron nitride (h-BN), tran- sition metal dichalcogenide (TMD), layered double hydroxide (LHD), and metal–organic framework (MOF) nanosheets are promising 2D materials for devel- opment of high-performance membranes [2–4,5  ]. Unique structural properties of 2D nanomaterials, such as their atomic size thickness, high aspect ratio, outstand- ing mechanical strength, and excellent chemical stability make them ideal to realize a new generation of gas separation membranes. Despite being atomically thick, most 2D nanomaterials are inherently impermeable to ions and even small gases such as helium [6]. Numerous studies have been under- taken to create pores with a well-defined size on planar 2D nanosheets to afford atomically thin nanoporous membranes. In this context, various perforation techni- ques have been proposed to generate atomically thin nanoporous graphene membranes [7–10]. However, engi- neering of large-scale nanoporous graphene containing high-density, uniform pores using perforation techniques remains a critical challenge. Therefore, the practical realization of atomically thin nanoporous membranes seems unlikely at the current stage. Laminated membranes assembled from stacked 2D nanosheets have been recognized as a feasible alternative to nanoporous graphene [6,11  ,12  ,13]. Molecular siev- ing through interlayer galleries of stacked nanosheets with sub-nanometer thickness minimizes gas transport resistance and thus maximizes gas flux. Incorporation of 2D nanosheets as nanofillers into a polymer matrix, resulting in so-called mixed matrix membranes (MMMs), is considered another promising membrane development strategy as it combines the film processing ability of polymers with the unique structural and physiochemical properties of 2D materials [14,15  ]. From a materials synthesis point of view, massive pro- duction of 2D nanosheets by facile, scalable, and eco- nomically viable methods is another challenge. Most of the developed techniques are high cost, require a signifi- cant amount of time, and suffer from scalability issues. In this regard, liquid-based exfoliation holds great promise as a versatile and scalable technique that can be used to produce a wide variety of 2D nanosheets [16  ]. This short review aims to briefly introduce 2D nano- materials that have enormous potential in gas separa- tion membranes and then highlight the latest break- through achievements and technical challenges in the synthesis of 2D materials. We also discuss the current status of 2D materials-based membranes with particu- lar emphasis on MMMs, and the associated challenges. Furthermore, we underline the unique potential of 2D materials to improve gas separation performance and mitigate the plasticization behavior of polymer membranes. Available online at www.sciencedirect.com ScienceDirect Current Opinion in Chemical Engineering 2018, 20:28–38 www.sciencedirect.com