Carbon capture: Porous trap for greenhouse gas

Synergistic sorption

Scientists at the King Abdullah University of Science and Technology (KAUST) and the University of South Florida (USF), USA, have developed a unique, efficient, cost-effective as well reusable metal-organic framework (MOF) material, for trapping and separating carbon dioxide from various gas streams. These crystalline materials could lead to clean-air and energy-saving technologies.

"Reducing carbon dioxide emissions in the atmosphere is absolutely critical," said Mohamed Eddaoudi, adding that, "I strongly believe that as scientists it is our responsibility to take a crack at this challenge." He and his colleagues have found what they consider to be a cost-effective and sorption-efficient made-to-order porous solid that can selectively remove carbon dioxide from a mixed gas stream. Writing in the journal Nature, Eddaoudi and KAUST colleagues - Youssef Belmabkhout, Amy Cairns and Ryan Luebke - working with USF's Patrick Nugent, Katherine Forrest, Tony Pham, Shengqian Ma, Brian Space, Lukasz Wojtas and Michael Zaworotko describe their tests on a previously underused material and reveal a novel mechanism for trapping the greenhouse gas.

Metal centres

The researchers point out that there are already numerous porous materials containing unsaturated metal centres (UMC) or organic amines that can selectively sorb carbon dioxide even if other gases, such as nitrogen, methane or hydrogen, are present. However, these require high-energy input to activate, regenerated and recycle them, thus additional cost penalty. In addition the driving force for selectivity is purely equilibrium based and water vapour competes strongly with CO2 carbon dioxide in case of UMC. The researchers confirmed that large surface area, up to 7000 square meters per gram of material, which is one of the unique features of MOFs is not the key parameter for the design of highly efficient CO2 carbon dioxide capture MOFs.

The team has worked with two-dimensional nets based on linked metal nodes that are pillared using hexafluorosilicate anions -'SIFSIX' - to make them three dimensional with primitive cubic topology. These porous inorganic-organic hybrid materials can be engineered by judicious choice of metal and ligands to have pore sizes and affinities suitable for selective trapping of carbon dioxide molecules only. The team points out that the original material on which their structures are based was actually first described more than 15 years ago but deducing its exceptional propensity for CO2 carbon dioxide capture from single gas adsorption analysis was not a straightforward task and had languished on a shelf, figuratively speaking in Zaworotko's laboratory. The team has now applied for a US patent for the adapted material and its applications.

Pillared pores

The rediscovery of this material emerged from an undergraduate research project conducted at USF supervised by Zaworotko and taken up by professor Eddaoudi and his team at KAUST who have carried out unique experiments that allowed them to reveal the unprecedented sorption properties of the material. Fundamentally, the team has found and manipulated a synergy between adsorption thermodynamics and kinetics. The researchers used X-ray diffraction data (powder and single crystal) to confirm the bulk purity and to obtain pore sizes in a series of related compounds with ores range from ultra-microporous to nanoporous.

"This unparalleled synergistic effect is a paradigm shift in sorption-based separation that offers the potential to address many enduring challenges related to energy and environmental sustainability," Eddaoudi explains. "Most importantly, I believe now more than ever that MOF materials will provide the needed sorbents, and it is only a matter of time before MOFs find their way to key industrial separation applications. The route is paved to access unique, highly selective and economical MOFs for carbon dioxide capture, exclusively based on physical adsorption."

The next step is to collaborate with process engineers to determine how the materials can be manufactured on the industrial scale and put to use to save energy, reduce carbon emissions by capturing carbon dioxide from flue streams before it enters the atmosphere. There are numerous additional applications that would be useful to the oil and gas industries as well as in reducing carbon emissions including the use of MOFs to separate and purify important gas streams, such as natural gas, shifted syngas, biogas and stack gas.