Wednesday, January 12, 2011

A word few need

kw: lexicography, chemistry

I learned a new word today: thermomorphic.

Used in the expression "thermomorphic catalysts".

By itself the word thermomorphic just means "temperature-mediated change in form". We all know an example, a piece of ice melting. That is the key to the meaning here, except in the other direction, of water freezing to ice.

Why is it applied to catalysts? There are two great classes of catalysts in chemistry, homogeneous catalysts and heterogeneous catalysts. Each has virtues and each has drawbacks. A homogeneous catalyst is in the same phase as the materials being reacted. Usually this means that everything is in liquid form. This has the great virtue that a bit of stirring will accomplish intimate contact between the catalyst and the reactants (reacting materials). The reaction runs at a maximum rate. But it has a significant drawback, that it is hard to get the catalyst out of the reacted product. Usually, the separation is accomplished by distillation or some kind of absorption separation.

Heteregeneous catalysts are in a different phase from the reactants. Usually, the catalyst is a solid and the reactants and product are liquids (at the temperature of reaction). It is easy to separate the catalyst from the product, just by filtration. But the drawback is that intimate contact during the reaction is harder to maintain, and the reaction is slower and perhaps not uniform everywhere.

Enter the thermomorphic catalyst. It is liquid at the reaction temperature, and turns solid when the completed product cools, but before the product itself freezes. So you run the reaction at a high temperature, cool it partway and filter out the catalyst, then cool the product further if needed. I call that neaty-keen.

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