Georgia Tech researchers used these carbon membrane materials to separate para-xylene from ortho-xylene. Photo: Christopher Moore, Georgia Tech.
Georgia Tech researchers used these carbon membrane materials to separate para-xylene from ortho-xylene. Photo: Christopher Moore, Georgia Tech.

The petrochemical industry recognizes the importance of para-xylene, given its many uses in everyday products, from plastic soda bottles to polyester fiber.

The challenge is that xylenes tend to travel in threes and are virtually identical, making it extremely difficult to efficiently separate and purify para-xylene from its less useful siblings such as ortho-xylene. The size of these molecules differs by just one-tenth of a nanometer. However, membranes with tiny pores engineered to differentiate these molecules can potentially achieve this important separation.

Building on long-term research with ExxonMobil, researchers at the Georgia Institute of Technology have uncovered new insights into the fabrication of carbon membranes with the potential to drive significant cost savings once this solution for xylene separation is scaled for industrial use. The researchers report their findings in a paper in the Proceedings of the National Academy of Sciences.

Their work focuses on 'carbon-based molecular sieves', made by heating thin layers of polymer material in such a way as to drive off all the atoms other than carbon, resulting in a charcoal-like substance that has molecule-sized holes. In 2016, researchers at Georgia Tech and Exxon Mobil first demonstrated that a new carbon-based molecular sieve membrane could successfully separate xylene molecules and extract the super-useful para-xylene from the pack.

Now, the Georgia Tech researchers have advanced this work, devising improved carbon barriers that allow the skinnier p-xylene to slip through more rapidly, while rejecting the larger molecules. Importantly, the team discovered a powerful relationship between the bonding chemistry of the carbons and the mobility of xylenes through the carbon membranes.

The performance of these carbon membranes – if realized at industrial scales – could significantly lower energy costs compared with existing refining processes such as the crystallization method or the adsorption-based method. The former approach involves freezing the xylene molecules such that only the para-xylene forms crystals, making them easy to isolate, but this method is energy intensive. The latter approach utilizes less energy than crystallization but requires expensive and complex equipment to operate. According to the Georgia Tech researchers, the issue with membranes is that, so far, they have only worked well in the lab environment, not in an industrial setting.

“We have made more stable materials by changing the polymer precursor we use,” explained Ryan Lively, an associate professor in Georgia Tech’s School of Chemical & Biomolecular Engineering and the paper’s corresponding author. “Then by changing how we transform the polymer into the carbon, we’ve made the membranes more productive.”

Just how much more productive? The team has shown the new carbon membranes can lead to purification systems that are estimated to be “three to six times lower cost than other state-of-the-art methods”, Lively said.

He estimates that separation and purification currently account for around half the energy consumed in producing commodity chemicals and fuels. Globally, the amount of energy used in conventional separation processes for aromatics such as benzene toluene is equal to that produced by about 20 average-sized power plants.

This advance could thus have a big impact on petrochemical energy consumption. The research was funded by ExxonMobil and builds on more than 15 years of collaborative research effort between Georgia Tech and the global oil and gas leader.

“Through collaboration with strong academic institutions like Georgia Tech, we are constantly exploring new, more efficient ways to produce the energy, chemicals and other products consumers around the world rely on every day," said Vijay Swarup, vice president of research and development at ExxonMobil Research and Engineering Company.

The Georgia Tech researchers also uncovered new insights regarding the carbon structure itself. The team observed that subtle changes in the ratio of three dimensional to two-dimensional carbon centers in the membrane led to impressively large changes in the mobility of xylene isotherms within the material. They observed that a change in this ratio (the sp3/sp2 carbon ratio) from 0.2 to 0.7 led to a factor of 1000 increase in the productivity of the membrane. Surprisingly, the membrane largely maintained its selectivity, or its ability to perform the xylene isomer separation, despite these changes in carbon structure.

“The more three-dimensional carbons are in there, the higher the productivity,” said M.G. Finn, professor and chair of Georgia Tech’s School of Chemistry and Biochemistry and co-corresponding author of the paper. “The more you crank up productivity, while maintaining the same selectivity, the less membrane you need to handle the same amount of xylene feed. From a design perspective, it shows that you have this enormous control over how the membrane works by making very small changes in the carbon chemistry.”

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.