Researchers at MIT have invented a new membrane to separate gases, reducing the energy required and reducing emissions by roughly 10%. The study was published in Science journal.
The world’s energy needs are accounted for by around 15% of industrial chemical separations. They also contribute to the world’s greenhouse gases emissions. The use of membranes to separate chemicals is more efficient than using distillation or absorption. However, there has always been a tradeoff between permeability — how quickly gases can penetrate through the material –and selectivity — whether the membrane allows the desired molecules to pass while blocking all others.
Researchers claim that the new family, which is based on “hydrocarbon- ladder” polymers, overcomes this tradeoff by providing high permeability and very good selectivity. Yan Xia is an associate professor of chemistry from Stanford; Zachary Smith is an assistant professor of chemical Engineering at MIT; Ingo Pineau, a professor at King Abdullah University of Science and Technology; and five other researchers.
Gas separation is an important industrial process. It removes impurities and unwanted compounds from natural gas and biogas, produces hydrogen for use as a non-carbon-free transportation fuel, and separates carbon dioxide from other gases. The performance of these separation processes can be dramatically improved by the use of ladder polymer membranes. These new membranes are capable of separating carbon dioxide and methane with five times more selectivity than existing cellulosic ones. They are also 100x more permeable and three-times as selective for separating hydrogen from methane.
The Xia lab developed a new type of polymers over the past several years. They are called ladder polymers because they are made from double strands linked by rung-like bonding. These linkages provide a high level of rigidity, stability, and flexibility to the polymer material. These ladder polymers are made using a selective and efficient chemistry that the Xia laboratory developed called CANAL. This acronym stands for catalyticarene-norbornene cancelation. It stitches readily available chemicals into ladder structures that have hundreds, or even thousands, of rungs.
The polymers were created in a solution. They form rigid and kinked ribbon-like filaments that can be easily rolled into thin sheets with sub-nanometre pores using industrially available polymer casting techniques. The hydrocarbon starting compound can be adjusted to control the size of the pores. Xia stated that this chemistry and the choice of chemical building blocks allowed for very rigid ladder polymers in different configurations.
The collaboration used Xia’s expertise and Smith’s specialization to apply the CANAL polymers to selective membranes. Holden Lai, a Stanford doctoral student, was responsible for much of the research and development of their structures’ effects on gas permeation properties. “It took eight years to develop the new chemistry and find the right polymer structures that bestow high separation performance,” Xia stated.
The Xia laboratory spent several years studying the structure of CANAL polymers in order to understand how they affect separation performance. Surprisingly they discovered that adding kinks in their original CANAL CANAL polymers significantly increased the mechanical robustness of their membranes. This also increased their selectivity to molecules of similar sizes such as oxygen or nitrogen gases without losing the permeability. As the material ages, the material’s selectivity actually increases. Researchers found that these materials have a high selectivity and high permeability combination, which makes them superior to all other polymer materials in gas separations.
Today, 15% of global energy use is for chemical separations. These separation processes are often “often based upon century-old technology,” Smith said. They are efficient, but they also have a large carbon footprint and consume huge amounts of energy. Today’s key challenge is to replace these non-sustainable methods. He said that most of these processes require high temperatures to boil and reboil solutions. These are often the most difficult processes to electrify.
The difference in size of the two molecules is 0.18 angstroms, or ten-billionthsof a meter, for separation of nitrogen and oxygen from the air. It’s incredibly difficult to create a filter capable of efficiently separating them without decreasing throughput. He said that the new ladder polymers when made into membranes create tiny pores that achieve high selectivity.
“In some cases 10 oxygen molecules permeate with every nitrogen, despite a razor-thin sieve necessary to access this type o size selectivity. Smith stated that the new membrane materials have “the highest combination of selectivity and permeability of all polymeric material for many applications.” Smith said that CANAL polymers are strong, ductile, and soluble in certain solvents. They could be scaled up for industrial deployment in a few years.
Osmoses, a MIT spinoff company, was founded by the authors of this study. It won the MIT $100K entrepreneurship contest and has been partially funded by The Engine to commercialize its technology. Smith says that these materials could be used in a variety of applications in the chemical processing industry. This includes the separation of carbon dioxide and other gas mixtures to reduce emissions.
Another option is the purification from agricultural waste products of biogas fuel to make carbon-free transport fuel. Hydrogen separation could be used to produce a fuel or chemical feedstock. This would aid in the transition to a hydrogen-based economy. The close-knit research team is working to improve the process and understand how the macromolecular structure and packing results in the ultrahigh selectiveity.
Smith stated that he expects this platform technology to play an important role in multiple decarbonization pathways. He started with hydrogen separation and carbon trap because there is such urgent need for these technologies to transition to a climate-free economy. (ANI)
(This story is not edited by Devdiscourse staff.
