The global pursuit of effective climate change mitigation has placed a spotlight on carbon capture and storage technologies, as well as the need for high-capacity storage for clean-burning fuels like hydrogen and methane. Among the most promising materials for these applications are metal-organic frameworks, specifically the variant MIL-101(Cr). This material is celebrated for its large surface area and stability, yet its traditional synthesis has long been hindered by the use of hydrofluoric acid, a highly corrosive and toxic reagent.
In a significant step forward for sustainable chemistry, researchers from BITS-Pilani have published a study detailing a facile or simplified synthesis of MIL-101(Cr) that replaces hazardous chemicals with more benign alternatives while actually improving the material's performance.
The researchers found that by employing acetic acid and nitric acid as modulators, substances that control the rate and quality of crystal growth, they could produce MIL-101(Cr) with superior structural characteristics. They achieved this by controlling the delicate balance of nucleation and crystal expansion. During the solvothermal process, where metal ions and organic ligands are heated in a solvent, the modulator competes with the organic linker to bond with the metal. This competition slows down the formation process, leading to more ordered, crystalline structures rather than a messy, amorphous solid. The researchers discovered that the nitric acid modulator, being a strong acid, produced larger crystals and more open metal sites. These sites act like molecular magnets, creating strong electrostatic interactions that pull gas molecules out of the air and hold them within the framework's pores.
Historically, researchers relied on non-eco-friendly solvents like N,N-dimethylformamide for cleaning the synthesised material, but this team successfully utilised dimethyl sulfoxide, which has a much lower environmental impact. Furthermore, while past research often looked at the adsorption of different gases in isolation or at extreme cryogenic temperatures, this study collectively examined carbon dioxide, methane, hydrogen, and water vapor under realistic room-temperature conditions (25°C) and high pressures up to 50 bar. The results showed that the nitric acid-based sample exhibited remarkable adsorption capacities, particularly for carbon dioxide and water vapor, which the authors attributed to its higher chromium content and a smaller pore diameter that intensifies the confinement effect, trapping gas molecules more efficiently.
However, the authors note that this work still needs to be extended to determine how these materials perform across a wider range of temperatures and pressures, ensuring they are viable for various industrial environments. Additionally, while the yield and characteristics are improved, the precision required in balancing pKa values of different modulators remains a complex chemical hurdle for mass production.
By making it safer and more efficient to produce gas-capturing frameworks, this research paves the way for more affordable carbon scrubbers at power plants, better hydrogen fuel tanks for clean transport, and even advanced atmospheric water harvesting systems that can pull clean drinking water from thin air.
