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New Perspectives in Gas Separations

We are all familiar with the way that uncooked spaghetti is tightly packed in a box, as well as the more irregular clustering of a bundle of twigs, even when closely bound together. Imagine shrinking these arrangements down to the molecular level, as strands of polymer chains that form a gas separation membrane, and you enter the world where Ruilan Guo, assistant professor of chemical and biomolecular engineering, works to create more effective membranes for energy and environmental applications. Guo and her team of researchers synthesize and characterize functional polymer materials for gas separation membranes and other purposes. The “molecular cavity” approach she uses allows her to manipulate the packing and assembly of polymer chains and, thus, finely control the permeability and selectivity of the membrane they create. Changing the molecular shape within the membrane’s individual strands through the addition of functional groups affects the packing formation, which changes the architecture of molecular cavities and the space between macromolecules. Gaining control over the creation of these spaces, or free volume, allows membranes to be synthesized with the particular selectivity needed for specific gas separations.
 
Just as holes in a kitchen strainer need to be large enough for water to pass through freely and rapidly, yet small enough to prevent the food from slipping through, the goal in creating an effective gas separation membrane is to have both high permeability and high selectivity. Guo’s group uses a unique method to create gas separation membranes capable of discriminating between gas molecules by their size, where the size differential is much less than an angstrom. The team chemically influences the structure of the polymer chains by judiciously introducing specific functional groups to produce the desired configuration, where the twists and turns of the long chains induced by specific functional groups prevent them from packing tightly. They fit together like twigs rather than smooth spaghetti stands — producing miniscule spaces between the molecules. The “molecular cavities” play the same role as the holes in a sieve and can be tuned to handle specific gas separations, such as CO2 removal from methane.

Guo’s polymer membranes are well suited to many practical gas separation applications, such as purifying natural gas. Before natural gas can be used or put into a pipeline, contaminants like CO2, must be removed to prevent corrosion of the pipes by the acidic gases, as well as to fully recover the heat capacity of the gas. Conventional methods involve scrubbing the gas with an amine solution — at a high capital cost, high energy cost, and high environmental cost. Guo’s polymeric membranes offer an effective solution that lowers costs. For example, using membrane technologies eliminates the need for using corrosive chemical solutions, so the expenses of obtaining, storing, and recovering them are avoided. The environment benefits from lower risks, and the consumer may benefits from the lower costs of production due to lower energy consumption and operating costs.

Another research area for Guo’s team focuses on the development of new polymer electrolyte membranes (PEMs) for electrochemical energy devices, such as fuel cells that provide a clean form of energy. Using the concept of self-assembly of multiblock copolymers, Guo is working to create new PEMs as alternatives to the state-of-the-art polymer electrolyte, which does not perform well in low-humidity. Guo is working to generate a more regularized placement of ion transport pathways in the membrane — controlling the chemistry to develop the morphology and block copolymers that are needed — to produce an improved polymer electrolyte that maintains high proton conductivity and increases stability.

What it boils down to for this 2013 recipient of a Department of Energy (DOE) Early Career Research Program award, is that sometimes looking at an existing challenge through a new perspective — twigs versus spaghetti — can offer a novel materials solution, especially when applied to creating a sustainable energy future via the development of novel polymeric materials.