The ability to precisely control the molecular structure of polymer membranes could expand their use in challenging separations, such as the purification of xylene isomers. Using highly selective membranes that can be operated at ambient conditions to separate such hydrocarbons has been proposed as an alternative to energy-intensive distillation, but membrane productivity has limited their use in such large-scale hydrocarbon separations. Building on previous work from the Georgia Institute of Technology (Georgia Tech; Atlanta; www.gatech.edu) and Exxon Mobil Corp. (Irving, Tex.; www.exxonmobil.com) focused on separating xylene isomers using hollow-fiber membranes, researchers have recently demonstrated an enormous increase in membrane productivity while still maintaining a high selectivity.
In this project, polymer membranes undergo pyrolysis treatment, where they are degraded in a controlled manner to yield the desired final structure. “What we discovered is that you can expose the polymer to different gaseous atmospheres while it is degrading. In our case, we used hydrogen, so while the polymer is degrading, it’s being exposed to low levels of H2. The H2 changes the degradation reaction chemistry, resulting in dramatic increases in the productivity of the membrane while retaining efficiency and selectivity,” explains Ryan Lively, associate professor in Georgia Tech’s School of Chemical & Biomolecular Engineering. By doping the pyrolysis atmosphere with H2 gas, the researchers were able to realize a very subtle change to the membrane by altering the ratio of the types of carbon in its structure. “The membranes contain a ratio of diamond-like versus graphite-like carbons. We found that we can precisely control the ratio of these two types of carbons, and these small changes in carbon chemistry yield positively enormous changes in membrane productivity,” says Lively.
The team’s experimental work demonstrated that increasing the ratio of the three-dimensional, diamond-like carbon by 33% could increase membrane productivity 30- to 50-fold.