# Is the effectiveness of chemical catalysts affected by temperature the same way enzymes are?

• Enzymes do not need high temperatures to function while metal catalysts tend to need a bit of an energy kick to get going.

• Enzymes will denature, or break, if heated up too far beyond their optimum temperature (for most around 40 degrees, although some bacterial enzymes can work at 100 degrees).

Do chemical catalysts also have an ideal operating temperature?

I am not asking if the rate of the reaction itself speeds up when a chemical catalyst is used. Rather, if the chemical catalyst becomes more effective at certain temperatures.

• One small puntualization: we are not talking about "optimal temperatures" in terms of efficiency for the enzymes, but of "limit-temperatures": since enzymes are complex (and fragile) molecular structures, high temperatures tend to break them. By the way, there are, indeed, optimal temperatures for chemical catalysts. Just think about the simple reaction of (water), sulphuric acid and ethanol. If you want diethyl ether, you should keep the temperature at around 130°C. If you want ethene, you should increase the temperature until 160°C. Below 100°C, the catalyst has small or no effect. – The_Vinz Jan 4 '17 at 6:05
• Ah right. The way the Rate vs Temp curve (i.imgur.com/9BiTJk7.png) decreased after a peak made me think it was just an optimal point. So is the gradual decrease of the curve actually showing that more and more enzymes breaking/denaturing after a certain limit? And thanks for the info about chemical catalysts. – K-Feldspar Jan 4 '17 at 8:13
• Also, is there a reason why chemical catalysts would decrease efficiency at high temperatures? It kind of makes sense that biological ones would. But I am not too sure why a metal catalysts would decrease after a certain temperature. Or are you just saying that metal catalysts have a minimum, not maximum temperature? – K-Feldspar Jan 4 '17 at 8:15

This is a somewhat complex question to answer because even your ‘simple’ enzyme example isn’t as simple as it looks. Let me discuss enzyme optimum temperatures first.

For any reaction that is exergonic — and that explicitly includes enzyme catalysis — an increase in temperature raises the reaction rate. This is due to at least two microscopic features:

• higher temperature means more and faster movement thus a higher probability of collisions
• each collision has a greater chance of being energetic enough to supply the required activation energy

This is condensed in van ’t Hoff’s postulate that an increase of the reaction temperature by $10~\mathrm{^\circ C}$ leads to an increase of the reaction rate by a factor of $2$ to $4$. Therefore, we would initially expect all enzymatic processes to be faster with increased temperatures.

However, the enzyme itself is a highly fragile system that is held together a good deal by noncovalent interactions. These can break apart under elevated temperatures causing the well-balanced enzymatic structure to break down (‘denature’) into a seemingly random protein chain. This also breaks the catalytic centre meaning that the enzyme can no longer catalyse efficiently. Taking these two effects together, we arrive at the optimum curve of enzyme catalysis.

Chemical catalysts come in a number of shapes and varieties. Consider for example:

• the catalyst (mainly platinum as I heard at school) in your car’s exhaust
• the complex catalytic oxide system in the Haber-Bosch process
• transition metal catalysts such as $\ce{[Pd(PPh3)4]}$
• acid catalysis, i.e. dissolved $\ce{H+}$ is the catalyst
• organic catalysis

Most of these will have some equivalent of a denaturation temperature; e.g. once platinum metal (or palladium, if on charcoal for typical organic hydrogenations) melts, it can no longer act as a catalyst for the reaction in the way it could in solid state. Thus, there should be a ‘denaturation equivalent temperature’ for most chemical catalyst systems at some point. Some compounds — the Haber-Bosch catalysts jump to mind — will, however, probably melt at such high temperatures that you would need to live on the sun’s surface for that to happen. Therefore, we could say that no limit exists for all intents and purposes.

Catalysts such as tetrakis(triphenylphosphane)palladium(0) need to dissociate at least one ligand to be catalytically active. An increase in temperature should enhance that dissociation. Therefore, many homogenously palladium-catalysed reactions are indeed sped up by a moderate increase in temperature. However, side reactions also speed up.

Organocatalysts and simple proton catalysis should be among the most temperature-insensitive — the former because all bonds should be covalent, the latter because it is just a reactive, activating species that cannot be poisoned. However, these two require a medium in which they act; the boiling point of the solvent is the highest possible temperature for their reactions.

Thus basically: yes, all chemical catalysts have some maximum useable temperature somewhere, but for most it does not matter for practical intents and purposes. Also, there are no simple models to explain this like in the enzyme case, hence it isn’t typically taught.