Designer MOFs: Algorithm calculates likely frameworks

Making the MOF of it

US researchers have developed a computational algorithm to construct all conceivable metal-organic frameworks (MOFs) from a library of building blocks and to rapidly screen them for optimal methane storage capacity. X-ray diffraction was used to characterize one of the most promising of the materials. The approach could have applications in developing gas storage materials, catalysts and sensors.

At standard temperature and pressure the fuel tank in your car will hold only about a thousandth the energy equivalent of a combustible gas compared to liquid petrol (gasoline). Your mileage may vary, but by my reckoning you would be able to drive for about 100 meters on a full tank of methane. Methane, like all gases, is, of course, attracted to surfaces and herein lies a clue as to how to build a gas-storage tank with a fuel density high enough to get you from A to B even if A to B is more than a hundred kilometers, rather than meters. The advantage of opting for gaseous fuels is they can be generated readily from sustainable resources rather than fossil fuels (i.e., hydrogen gas from solar energy), and/or their combustion is much cleaner than the mixed hydrocarbons of petrol (i.e. methane gas).

Christopher Wilmer and colleagues from the Snurr and Hupp groups at Northwestern University (NU) have now devised an algorithm that allows them to search hundreds of thousands of possible metal-organic frameworks (MOFs) that could be assembled from molecular "building blocks." Such porous materials could be put into a fuel gas tank to create a high surface area ready to adsorb methane or another gas. Their prototype material, MOF NU-100, has a surface area of 50 million square metres per fuel tank, but Wilmer says that "computational screening could greatly accelerate the search for even better materials." The NU team generated a library of 137,953 hypothetical MOF from 102 building blocks and they predict that many of them have very high capacity to store methane.

A prediction full of holes

"We can predict how well a crystal can store methane by connecting it to an imaginary methane reservoir," explains Wilmer. "We then fill the crystal with the gas until the system reaches thermodynamic equilibrium in the simulation." The configurations of methane molecules within this equilibrium system are then statistically averaged to mimic the real life fluctuations of the physical system. Hundreds of thousands of MOFs can be simulated at the same time on supercomputers. The simulations to test all these MOFs can be done in less time than it normally takes chemists to synthesize a single MOF in the laboratory.

The team identified 300 or so optimal MOF structures with a theoretical capacity greater than any known porous material. Through a process of structure-property relationship analysis, the team was then able to determine the most effective methane adsorber in the library, which ironically turned out to be a MOF functionalised with methyl groups within its pores. They thus synthesized one of the top candidates and tested it to demonstrate how well the predicted adsorption capacity matched experimental results. X-rays crystallography of this novel MOF could also then confirm structural details and mesh with the activity observed.

Omar Farha, a Research Associate Professor, describes the building approach as being akin to the children's plaything "The Tinkertoy Construction Set," which has inspired several generations of chemists looking for self-assembly approaches to making novel compounds. Tinkertoys essentially involve clicking together rods and bobbins to make different structures. "Tinkertoys were invented here in Evanston, Illinois, in 1914 by Pajeau & Petit and are now inspiring some of our chemistry research," Farha told SpectroscopyNOW. He added that, "The NU team is actively pursuing the synthesis of the top candidates."

Porous synthetic

"When our understanding of materials synthesis approaches the point that we are able to synthesize any material, the new problem becomes which materials should we synthesize?" ask Wilmer and Farha, "We have now shown how a systematic approach to testing hypothetical MOFs can focus our attention on the most promising and interesting targets."

Farha adds that, "Although forecasting the direction of technology growth is always a risky business, there are many fewer roadblocks to implementing high-density methane fuel tanks than, for example, new drugs or semiconductor technologies. If the price of petrol (gasoline) tripled tomorrow, we could very well be driving road vehicles with MOFs in the fuel tanks one or two years from now."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.