The paper is a characterization study of commercial prototype carbon nanotube (CNT) membranes consisting of sub–1.27-nm-diameter CNTs traversing a large-area nonporous polysulfone film. The membranes show rejection of NaCl and MgSO₄ at higher ionic strengths than have previously been reported in CNT membranes, and specific size selectivity for analytes with diameters below 1.24 nm.

The CNTs used in the membranes were arc discharge nanotubes with inner diameters of 0.67 to 1.27 nm. Water flow through the membranes was 1000 times higher than predicted by Hagen-Poiseuille flow, in agreement with previous CNT membrane studies. Ideal gas selectivity was found to deviate significantly from that predicted by both viscous and Knudsen flow, suggesting that surface diffusion effects may begin to dominate gas selectivity at this size scale.

Images of the membranes tested in the study. (A) SEM image of the CNT membrane surface, showing CNT tips emerging from the polymer. Inset shows the membrane sample without magnification. (B) SEM image of the surface of the CNT membrane using an imaging technique that uses high voltage without sputter coating to reveal the CNTs below the membrane surface. (C) TEM image of the CNT membrane in planar view, showing CNT pore openings, indicated by red circles, emerging from CNTs below the membrane surface. McGinnis et al. Click to enlarge.

Background. For 20 years, researchers have shown that CNT membranes offer tremendous promise for a wide variety of uses including the low-cost production of ethanol fuel, precision drug delivery, low-energy desalination of seawater, purification of pharmaceutical compounds, and high-performance catalysis for the production of fuels.

The difficulty and high cost of making CNT membranes has confined them to university laboratories and has been frequently cited as the limiting factor in their widespread use. Mattershift’s ability to mass-produce CNT membranes could unleash the potential of this technology.

Achieving large scale production of carbon nanotube membranes is a breakthrough in the membrane field. It’s a huge challenge to take novel materials like these and produce them at a commercial level, so we’re really excited to see what Mattershift has done here. There’s such a large, unexplored potential for carbon nanotubes in molecular separations, and this technology is just scratching the surface of what’s possible.

—Dr. Freeman, Professor of Chemical Engineering at UT Austin

The company has already booked its first sales and will ship products later this year for use in a seawater desalination process that uses the least amount of energy ever demonstrated at pilot scale.

Three significant advances made this breakthrough possible:

1. There has been a 100-fold reduction in the cost of carbon nanotubes in the last 10 years, with a corresponding increase in their quality.

2. A growing understanding of how matter behaves in nano-confined environments such as in the interior of sub-nanometer CNTs, in which molecules move single file at high rates and act differently than they do in bulk fluids.

3. The increase in funding for tough tech startups, which enabled Mattershift to spend 5 years of intense R&D developing its technology.

This technology gives us a level of control over the material world that we’ve never had before. We can choose which molecules can pass through our membranes and what happens to them when they do. For example, right now we’re working to remove CO₂ from the air and turn it into fuels. This has already been done using conventional technology, but it’s been too expensive to be practical. Using our tech, I think we’ll be able to produce carbon-zero gasoline, diesel, and jet fuels that are cheaper than fossil fuels.

—Mattershift Founder and CEO, Dr. Rob McGinnis

Using CNT membranes to produce fuels is just one example of a technology predicted by Nobelist Richard Feynman in the 1950s, known as Molecular Factories. Molecular Factories work by combining processes such as catalysis, separation, purification, and molecular-scale manipulation by nanoelectromechanical systems (NEMS) to make things from molecular building blocks. Each nanotube acts as a conveyor belt that performs functions on molecules as they pass through, single file, analogous to how factories function at the macro scale.

It should be possible to combine different types of our CNT membranes in a machine that does what molecular factories have long been predicted to do: to make anything we need from basic molecular building blocks. I mean, we’re talking about printing matter from the air. Imagine having one of these devices with you on Mars. You could print food, fuels, building materials, and medicines from the atmosphere and soil or recycled parts without having to transport them from Earth.

—Rob McGinnis

Mattershift technology for fuels production. The basic building block of a Mattershift Molecular Factory is the Programmable Molecular Gateway. It consists of a carbon nanotube fixed within a flexible polymer sheet and aligned so that both of its ends are open. The gateways are called “programmable” because a great variety of gates can be added to their openings, allowing them to manipulate molecules in specific ways.

A catalyst gate—a gateway with a catalyst attached to the opening of the nanotube—would force all molecules passing through the gateway to interact with the catalyst, which may be active or passive, removing or adding electrons, combining or splitting molecular parts. A great many types of gates are possible, and many have already been demonstrated in laboratories around the world.

An example of a simplified schematic for converting atmospheric carbon dioxide, water, and solar or wind electricity into ethanol, gasoline, diesel, and jet fuel. Source: Mattershift. Click to enlarge.

Mattershift. Mattershift was founded in 2013 to realize the potential of molecular factories, with the ultimate goal of printing matter from the air.

Founder Rob McGinnis was previously Co-founder and CTO of venture-backed startup Oasys Water, where his forward osmosis desalination technology cut the energy and cost of removing salt from water by 50%. McGinnis has authored over 30 patents and peer-reviewed articles in the fields of membranes, energy, desalination, and nanotechnology. He has a PhD in Environmental Engineering from Yale University.

Benny Freeman is the Richard B. Curran Centennial Chair in Engineering at The University of Texas at Austin in the McKetta Department of Chemical Engineering in the Cockrell School of Engineering. Dr. Freeman’s research is in polymer science and engineering specifically in mass transport of small molecules in solid polymers. His laboratory focuses on gas and liquid separations using polymer and polymer-based membranes, developing and characterizing new materials for hydrogen separation, natural gas purification, carbon capture, water/ion separation, desalination, and fouling resistant membranes. His research is described in 395 publications and 22 patents/patent applications. He has co-edited 5 books on these topics.

He has won numerous awards, including the PMSE Distinguished Service Award (2016), AIChE Clarence (Larry) G. Gerhold Award (2013), Society of Plastics Engineers International Award (2013), the ACS Award in Applied Polymer Science (2009), and the AIChE Institute Award for Excellence in Industrial Gases Technology (2008).

Jeffrey McCutcheon is an Associate Professor at the University of Connecticut in the Department of Chemical & Biomolecular Engineering. Dr. McCutcheon’s research is in membrane separations with a focus on membrane fabrication and characterization. His group focuses on applications in liquid separations, including forward osmosis, membrane distillation, nanofiltration, and organic solvent separations. He has written 65 refereed publications, 3 patents, and 2 book chapters. He has won numerous awards including the FRI/John G. Kunesh Award from the AIChE Separations Division (2014), The DuPont Young Professor Award (2013), the 3M Nontenured Faculty Award (2011), and the Solvay Specialty Polymers Young Faculty Award (2011). He is President-elect of the North American Membrane Society and recently finished his term as Area Chair of Area 2D of the AIChE Separations Division (2015-2017).

Mattershift has been a member of the Grand Central Tech and The Hub incubators in NYC, and the Advanced Technology Laboratories Technology Incubation Program at UConn.

The company has received support from the US Department of Energy, Office of Basic Energy Sciences, and the National Science Foundation.

Resources

• Robert L. McGinnis, Kevin Reimund, Jian Ren, Lingling Xia, Maqsud R. Chowdhury, Xuanhao Sun, Maritza Abril, Joshua D. Moon, Melanie M. Merrick, Jaesung Park, Kevin A. Stevens, Jeffrey R. McCutcheon and Benny D. Freeman (2018) “Large-scale polymeric carbon nanotube membranes with sub–1.27-nm pores” Science Advances Vol. 4, no. 3, e1700938 doi: 10.1126/sciadv.1700938