Computer simulation of molecular mazes
Establishing the potential of metal organic frameworks for refrigerant separation and chilling technology
Novel materials may pave the way towards more efficient chillers and towards new separation methods for fluorocarbon refrigerant molecules. Professor Rajamani Krishna of the Van 't Hoff Institute for Molecular Sciences (HIMS) contributed to an American study of the adsorption characteristics of these so-called metal organic framework (MOF) materials. Nature Communications has published the results.
Over the last few decades chemists have developed a promising class of microporous materials. The adsorption characteristics of these materials can be tuned by clever molecular design. These metal organic frameworks (MOFs) consist of metal ions combined with organic molecules to form highly structured porous molecular architectures.
Professor Krishna of the Computational Chemistry research group at HIMS has performed numerous simulations of the performance of MOFs in a broad range of potential applications. He now contributed to research at Pacific Northwest National Laboratory (PNNL) in Richland, Washington (United States) on the interaction of MOFs with common fluorocarbon refrigeration molecules.
Two types of MOF
The research aimed at establishing how the internal structure of two different MOFs affected the material's ability to adsorb a range of six fluorocarbon refrigerant molecules, varying in the number of chlorine and fluorine atoms. The range started with methane (one carbon and four hydrogen atoms) and ended with R12 (two chlorine and two fluorine atoms surrounding a single carbon atom).
The study was performed with two types of MOF, both well known for their extremely high adsorption capacity at room temperature and both relatively easy to synthesize. One was a nickel-based MOF with uniform pores, highly active sorbent sites and internal channels, known as MDOBDC. Here, M stands for nickel and cobalt and DOBDC for 2,5-dioxido-1,4-benzenedicarboxylate. The other type of MOF was the so-called MIL-101, chosen for its hierarchical pore structure, meaning it has small and large pores connected in a specific arrangement.
The interaction of the MOFs with the gaseous refrigerants was studied through gas adsorption measurements combined with solid-state 19F nuclear magnetic resonance spectrometry. Computer simulations were performed to obtain further insights in the adsorption behaviour.
The researchers found that while MIL-101 with its hierarchical pore structure was slower to pick up the refrigerants, it had far more capacity because it has a higher surface area than the nickel-based MOFs. MIL-101 also turned out to host refrigerant molecules more effectively as the fluorocarbon's boiling point rises. This effect can be exploited in systems for the separation of fluorocarbons, as was shown in breakthrough measurements of an experimental separation column packed with MIL-101. Professor Krishna contributed with computer simulations of these breakthrough experiments. It was clearly shown that MIL-101 effectively separates a mixture of fluorocarbons into individual fractions. The research thus provides the proof of concept for fluorocarbon separation.
The PNNL research also confirmed the potential of the MOFs for application in chilling technologies. The majority of large cooling systems is based on an adsorption and resorption cycle of coolant molecules in an adsorbent. The efficiency of this cooling process depends to a large extent on the interaction between the coolant and the adsorbent. Now that it has been shown that MOFs can adsorb fluorocarbons at relatively low pressures, they can potentially replace the commonly silicon dioxide (silica gel) adsorbent, thus reducing the energy consumption.
In close cooperation with a manufacturer of fluorocarbons, the PNNL team is now focusing on fluorocarbons with a very low global warming potential. They expect to move the MOFs to the air conditioning marketplace in just a few years, improving the performance of large chillers used for office spaces, hospitals and other large buildings.
Motkuri RK, HVR Annapureddy, M Vijaykumar, HT Schaef, PF Martin, BP McGrail, LX Dang, R Krishna, and PK Thallapally. Fluorocarbon Adsorption in Hierarchical Porous Frameworks. Nature Communications 2014 Jul 9; 5:4368. DOI: 10.1038/ncomms5368