Innovative membrane materials reduce energy consumption of industrial separation processes
Industrial chemical separations are essential for everything from producing oxygen for medical applications, processing fuel for power generation, to purifying feedstock for chemical manufacturing. Altogether, these separation processes account for 15% of the world’s energy consumption. Membranes have the potential to reduce the energy consumption of separation processes by up to 90% compared to traditional methods such as absorption and distillation. But the moderate performance of current membranes limit their widespread deployment, especially in the areas of gas separations. The Xia lab has now developed a new type of membrane materials based on ladder-shaped polymers that significant boost the gas separation performance as they recently reported in the journal of Science “Hydrocarbon Ladder Polymers with Ultrahigh Permselectivity for Membrane Gas Separations”
Traditional polymers are coil-like with single bonds connecting the repeat units, resulting in a high degree of flexibility in the polymer chains. But the Xia lab has been interested in very unusual ladder-type of polymers, where the polymer chains consist of a continuous series of rigid fused rings forming long molecular ladders. The original interest in ladder polymers began in 1960’s due to the expected improved stability over traditional linear polymers, but their synthesis has been challenging. As a result, there have been few types of ladder polymers reported. In 2014, the Xia group reported a novel catalytic method to synthesize rigid and kinked ladder polymers from common chemical building blocks in their first research article at Stanford. They also found that these polymers form abundant tiny pores in the solid state, which are voids between rigid and kinked ladder chains, and the pore sizes are similar to those of small gas molecules. In the past few years, the Xia group has been trying to develop their polymers into membrane materials with good separation properties.
The initial ladder polymers that the Xia lab developed only formed membranes with poor mechanical properties and disappointing separation performance. But after a few more years of exploration his former graduate student, Holden Lai, discovered that by introducing additional appropriate twists in the macromolecular ladders, both the mechanical properties and gas separations performance of these ladder polymer membranes can be dramatically improved. Selectivity and permeability are two most important performance parameters in membrane separations, and there is typically a tradeoff between these two parameters. But with the new molecular design, both parameters are improved, leading to record-setting separation performance for many industrially important gas separations, including CO2/CH4, H2/CO2, H2/N2, and N2/O2 separations.
“We still don’t understand exactly how introducing some additional molecular twists in the ladder polymers significantly boosts the separation performance. It requires investigation of the structure and dynamics at different length scales, particularly how the molecules sit on top of each other and how they are packed.” said Prof. Xia. “It’s fortunate that we have excellent students and postdocs who can tackle hard problems.” The set of new ladder polymer membranes have the potential to significantly improve the energy efficiency and lower the environmental impact of chemical separations, including natural gas purification, carbon capture, hydrogen separation, and the production of oxygen and nitrogen, with high product recovery and purity.