Organic chemistry is very dependant on catalysts. A catalyst is a substance that accelerates a chemical reaction (allowing it to be done faster and at lower temperature), but which isn’t itself consumed during the reaction. Theoretically catalysts can be reused hundreds of times, because they’re not used up and the only concern is that they might degrade over their lifetime. One of the largest classes of catalysts are those which are in the form of a powder. It could be finely divided metal shavings, or a catalyst with a spongy consistency. These solid-state catalysts are a separate class from catalysts which freely dissolve in a liquid. One of the most important attributes of a successful solid catalyst is the surface area.
The solid catalysts aren’t consumed by the chemical reaction, but they are still active participants. The actual reactants come into contact with the solid catalyst particle, often being absorbed onto the surface; the reaction takes place, and the products are released along with the catalyst material. Because the catalyst needs to come into close proximity of the reactants, the most active catalysts are those with a very large surface area. Imagine a paper clip; it weighs about a gram, and if you added up the entire surface area of the clip, it wouldn’t be much – maybe a few square centimeters. A paper clip – maybe made out of platinum, or some other catalytic material – would be terrible at accelerating a chemical reaction. Only a very limited amount of molecules can absorb onto the surface and react at any one time; the reaction is going to be very slow.
I’ve developed several patented technologies for increasing the usefulness of heterogeneous (solid state) catalyst systems; organic chemistry relies on catalysis to perform many key reactions, and I’ve been a practicing synthetic organic chemist for over ten years, post-graduate. One of the strategies I’ve used is to increase the surface area of the catalyst, and I have a pretty good grasp of the various limitations of current technology. I’m always pleased to read of new developments in the field, which is why I was intrigued to read a recent article in the scientific journal Nature Communications (one of the most prestigious publications in the world).
The authors of the article describe how they combined rigid organic molecular components along with large metal carboxylate vertrices to form a hollow three-dimensional framework. Unlike normal metal-organic frameworks (which have tried simply increasing the length of the organic components), the authors of these recent study use metal “joints” of increased thickness. The result is a scaffolding of large metal clusters with connecting linker rods of organic material. Most importantly, because most of the structure is hollow (where the metal atoms are held at set distances from each other, and not allowed to clump up), the accessible surface area of the metal is very high. The compounds produced measured an impressive surface area of 4000 square meters per gram. Compare this to the paper clip surface area of perhaps a few square centimeters per gram, and it’s easy to see how this advance can be useful for catalysis. A catalytic material produced from this metal-organic framework would be incredibly active, because starting materials would have a huge playing surface with which to work.
It’s not often that I come across something basic in the field of catalysis which surprises me; I’ve been studying organic catalysis for close to seventeen years. However, this article really opened my eyes to the possibilities that can arise when organic materials are combined with metals.
The source of this article can be found at:
An, J., et al. “Metal Metal-adeninate vertices for the construction of an exceptionally porous metal-organic framework”. Nature Communications 2012, 3, 604.