Fine-tuning the thickness of super-thin carbon molecular sieves (CMSs) can enhance the efficiency of separating and purifying different gaseous mixtures, report researchers at the King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia, who have developed a way to tweak the thicknesses.
Creation of CMS membranes involves depositing a layer of carbon-rich polymers onto a suitable support and then applying heat to convert the polymer into a microporous CMS film. The KAUST researchers found that membrane thicknesses of 50–300 nm contain “sweet spots” that the researchers believe can be optimized for separating different gaseous mixtures.
Currently, the team produces the membranes as laboratory-scale single-fiber modules (Figure 1); its immediate target is to develop multi-fiber modules with 10–100 cm2 active membrane areas.
The membranes show permeances after several months of around 500 GPU (gas permeation units) for H2 and CO2, and 100 GPU for O2, with O2/N2, CO2/CH4 selectivities of 6–7 and >40, respectively.
“Importantly, we are developing promising, scalable modification ways to boost membrane selectivities at only a minor expense of permeance,” says group leader and professor of chemical engineering Ingo Pinnau. The challenge in scaling up the membranes predominantly lies in maximizing the achievable performance of the module versus the fabrication reproducibility of individual fibers.
“We need to consider whether we would want to use as small as possible fibers to pack a large membrane surface area into the module at the expense of increased risk of defects,” he adds.
Pinnau notes that the work is continuously guided by the understanding of the underlying physics of thin and ultra-thin separating layers — something that is particularly important when dealing with microporous amorphous materials that undergo an extended process of decelerating collapse, i.e., physical aging.
One conclusion already very clear is that opting for excessively thin layers is strongly counterproductive, points out research scientist Wojciech Ogieglo.
“Such thin layers turn out to collapse at an exponentially accelerated rate, which often renders them practically useless for efficient separations. Going to thicker layers seems much more beneficial as it also helps in dealing with unavoidable defects,” he explains.
“Next to carbon membranes, we are also exploring novel polymers of intrinsic microporosity, which are a relatively new class of very promising membrane materials,” reveals Ogieglo.
Current funding comes from KAUST’s Advanced Membranes and Porous Materials Center, which promotes projects with clear scale-up potential. Both researchers believe that industrial involvement could happen as their work progresses.