MOF's and Molecular Sieves

Metal-Organic Frameworks, or MOFs, are a promising species of molecules which have enormous internal surface areas. They can be used to selectively grab hold of chemicals such as...
25 January 2009

Interview with 

Professor Joe Hupp, Northwestern University


Chris - We now have Joe Hupp from Northwestern University. Hello Joe. What are these moths and what's their structure if you could zoom in with a very powerful microscope and see them, what would they look like?

Joe - Metal atoms or ions on the corners and then rigid pillars that are made or organic molecules. Each collection of these defines a cage but the cages go on forever and an important property of these is that they're not just isolated cages, they're crystalline materials. If you did zoom in and look at one of these cages you would see a tunnel going on forever with exactly the same size cage all the way through.  

MOF-74 - Hydrogen-Storage CompoundChris - So it's almost like children's building blocks but with a hole in the middle that you can connect together to make enormously voluminous containers but at a molecular level.

Joe - That's right. The materials have surface areas that are gigantic. The largest are more than 6000 square metres per gram. They have the lowest densities of many known crystalline materials. They have gigantic interior volumes. Some of them are more than 80% empty so these tiny walls define lots of miniscule spaces. The important thing is the space is on the size of a molecule. That's what enables them to store molecules like hydrogen or methane for fuel or to, as you mentioned pull some molecules off the mixture.

Chris - Just to add some perspective, 6000m squared is as big as a football pitch so per gram that's incredible. When you say that you actually make these things with metal atoms on the corner - if we imagine a cube - which is the simplest structural and functional unit you've got a metal atom on the corner of the cube and they're connected by these organic linkers. How do you actually make these things?

Joe - Well they turn out to be pretty easy to make. It's just hard to make them well. You simply cook them, the pieces that are sticky for the metal ions, in a mixture for four or five days. If everything goes right you get crystals. What we typically do it try three or four hundred sets of conditions and a few of them work. It's really a very empirical science at the moment.

Chris - how much of these things can you make? Are we talking per gram quantities or are you making a reasonable amount that could hold reasonable amounts of molecules?

Joe - In the laboratory we are designing these and scouting these for very specific purposes so we only need tens or hundreds of milligrams. BASF which is the world's largest chemical company has demonstrated that it's easy with the right materials to make kilogram quantities of these. In fact they've driven a demonstration vehicle across Europe where the fuel tanks are filled with methane. What stores the methane are these tiny cavities?

Chris - Could you just explain to us how these tiny cavities are useful for storing methane? Why is it better to use a complex crystal like this than to just put a large cylinder of gas in the back of the car?

Joe - Well, first of all I don't think you or I would want to drive a car that had a stainless steel cylinder in the back in case of a collision. The methane is much more safely stored if it's in a container that's not much bigger than the methane itself. It turns out that in order to stick to the material it has to be very close to it. In a stainless steel tank most of the molecules never touch the wall. It's better to have tiny channels and have the molecules adhere to them.

Chris - can you tell us, because one of the things I mentioned, that you could scrub out gases from flues and exhaust pipes, for example. Could you make a molecule that has a cavity that's chemically-active enough to grab one species of chemical or waste gas you don't want going into the atmosphere and let less harmful things go past. A kind of molecule sieve, if you like.

Joe - There's lots of interesting work on, for example, pulling carbon dioxide out of methane. One of the companies in the United States, Innovene Chemicals, spends a half billion dollars a year doing this with freezers. They condense carbon dioxide to a liquid to get it out of natural gas and the reason they spend half a billion dollars a year on this is because they can mark up the price of the gas by more than a half billion dollars a year. It's very energy intensive and they would much prefer to have a material that just grabbed all the carbon dioxide and left the natural gas, the methane behind.


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