Recent work by a team of researchers from Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) and the University of Tennessee (Knoxville; www.utk.edu) has brought cost-effective, fluorinated carbon molecular-sieve membranes within closer reach. To make the membrane, the researchers used a sol-gel polymerization process in which a fluorine-containing substrate that also contains nitrile and ether groups is polymerized using strong Brønsted acid (CF3SO3H) as a promoter/catalyst. The reaction is performed in a way that allows the scientists to regulate the size and thickness of the obtained polymer membrane. Subsequently, the polymer film is carbonized in a tubular oven at 500°C under a slow stream of nitrogen gas to generate the carbon molecular-sieve-based membrane, explains Ilja Popovs, ORNL Nanomaterials Chemistry Group scientist. The high fluorine content in the polymer improves its ability to adsorb and separate CO2 from nitrogen and water vapor, which are prominent components of exhaust gas, Popovs adds.
By carefully controlling the carbonation temperature in the oven, the researchers were able to control the porosity of the membrane, which leads to greater permeability, but at the expense of selectivity. “We were able to identify a ‘sweet spot’ in the temperature and the thickness of membrane that provides the best combination of transport properties (permeability) and separation (selectivity),” Popovs reports.
The ORNL-led group demonstrated the effectiveness of their membrane on a gas stream of nitrogen, carbon dioxide and water, simulating industrial fluegas. The experimental results suggest the materials are “promising candidates for CO 2 separation.”