Engineering Atomically Precise Nanochannels Using Layered 2D Sheets to Enable Chemical Separation Membranes with Exceptional Permeance and Size-Selectivity

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The separation of air into its components, oxygen, nitrogen, carbon dioxide, water vapor, and other trace gases such as helium, is a billion dollar industry. Examples include: use of purified nitrogen in high performance tires and infusion into specialty coffee; use of purified oxygen for healthcare and the launch space shuttles; use of helium for balloons. Separation of any mixture requires both energy and a strategy to isolate one component. Size selective membranes provide one such strategy, allowing molecules to permeate through internal channels at rates that are dictated by their ability to fit within a membrane channel, their diffusivity, and the strength by which they interact with the surface. Decreasing this surface interaction increases permeation, which translates to an increased production rate at lower energy consumption. One candidate for frictionless transport are membrane channels comprised of carbon atoms, such as cylindrical carbon nanotubes or stacked planar graphene sheets. As weak surface interactions provide little impediment to flow, the permeation of water through carbon nanotubes has been experimentally shown to be 1000 times greater than that predicted from classical models. Yet, homogenous channels of closely packed carbon nanotubes have been difficult to synthesize in large quantities. Propped graphene sheets show more promise for large scale synthesis, but are currently derived from graphite via a highly oxidative delamination process, which imparts significant residual oxygen atoms that invalidate frictionless transport. This project will utilize a bottoms-up nonoxidative approach to create propped graphene membranes with controlled channels optimized for size selective transport of small molecules, such as oxygen, nitrogen, hydrogen, helium, and water. This project will use molecular spacers as proppants to synthesize controlled nanochannels between pristine unoxidized parallel graphene sheets. Robust chemistries will be developed for precisely fabricating nanochannels with sub-nanometer gaps that range from 2-8 Angstroms to impart size selectivity. Candidate spacer molecules include para substituted benzene derivatives, functional groups grafted via [2+2] cycloaddition, and non-covalently adsorbed planar and non-planar aromatic hydrocarbons. Both isolated bilayer channels and multilayered laminate membranes will be fabricated. The isolated channels will afford fundamental surface science measurements of structure and properties whereas the multilayered membranes will enable macroscopic measurements of transport to validate the theoretical prediction of frictionless, ultrahigh permeance transport. Microstructural characterization data, in conjunction with transport measurements, will guide design of increasingly effective membrane materials. Both graduate and undergraduate students will perform laboratory research for this project, with an effort to target underrepresented groups. The research will inform interactive lessons targeted at high school level students, accompanied by dissemination of training videos to teachers.