Substance over style

Natural gas extracted from gas fields often contains naturally co-existing impurities such as carbon dioxide (CO₂). These contaminants, particularly CO₂, not only reduce the energy content of methane, the predominant combustion substance in natural gas, but also corrode the pipes carrying the gas. Industrially, techniques such as amine scrubbing and cryogenic separation strip CO₂ from natural gas, but they are energy-intensive and operationally demanding. The same is true for biogas.  

Of late, it has been seen that sponge-like materials called covalent organic frameworks (COFs) — crystalline lattices built from organic molecules riddled with tiny pores — trap CO₂ while letting methane pass through. Scientists have shown that when unpurified natural gas passes over a bed that contains such COFs, CO₂ can be captured more efficiently and economically.  

COFs used for such purification typically need crystallinity, porosity, and chemical compounds that trap the gases to be removed. For years, researchers assumed that the best COFs for this job needed to be highly crystalline (perfectly ordered, like a well-built lattice) and highly porous (with ample surface area for gas to stick to). 

In a recent paper published in the Journal of the American Chemical Society (bit.ly/CO2-trap), researchers from the Indian Institute of Science Education and Research (IISER) Kolkata, led by Chemistry Professor Rahul Banerjee, set out to understand whether this assumption was true. Working closely with the IISER team were scientists from the Indian Institute of Technology (IIT) Madras, IIT Patna, and the University of Alberta in Canada. While the IISER researchers designed and synthesised the COFs, the others conducted experiments and simulations. 

For their experiments, the team built four different COFs that spanned a wide range of structural quality. The first two — 'gold standard' COFs — were highly crystalline with large surface areas, but were made from fairly plain non-polar building blocks. The third one was loosely ordered (poorly crystalline) but contained nitrogen-rich chemical groups known to attract CO₂. The fourth COF was non-crystalline and less porous but packed with even more CO₂-attracting chemical groups.  

They tested how well each material adsorbed CO₂, using different methods: simple gas uptake measurements, models that predict how the two gases compete for space, real flow-through "breakthrough" experiments (mimicking how a separation unit would actually work), and computer simulations that show exactly where CO₂ molecules like to sit inside each material. 

"To our surprise, we found that the functionality (to attract CO₂) is more important than crystallinity and porosity when it comes to trapping CO₂ molecules," says Soubhik Khata, Banerjee's research student and the study's first author. The experiments showed that higher surface area and crystallinity did not necessarily translate into better performance. 

Materials with a crystalline order and more pores performed the worst at separating CO₂ and methane. Despite far higher surface areas, they showed only modest selectivity for CO₂. The messier, less-ordered materials performed better. The framework with almost no crystalline order but rich in CO₂-attracting chemistry achieved the best separation performance, even though it had the smallest surface area among the four COFs. The computer simulations showed why: CO₂ molecules clustered densely around chemical 'hot spots' in these materials, such as nitrogen atoms and certain polar groups, while methane showed no such attraction. 

The results flip a common design assumption in materials chemistry. Instead of chasing maximum surface area and perfect crystal structure, the findings suggest that packing a material with the right chemicals was the more important design lever. "The key message is powerful," says Vivek Polshettiwar, Professor of Chemistry at the Tata Institute of Fundamental Research, Mumbai. "The future design strategy should be to combine high accessible porosity with precisely engineered CO₂ binding sites," he says.