Catalysis is very much like the loch ness monster or Area 51; everyone has heard something about it, but it seems like nobody knows what's going on. You might know that your car needs them to protect the environment. Perhaps you heard they are used during the making of some foods like margarine and that catalysts will poison you if you end up eating them. Okay, so catalysts are not good for you, right?
Well, not so fast! Vitamin C is a catalyst that your body uses to keep you healthy. Better yet, if the enzymes (also catalysts) stopped doing catalysis in your cells, you would die instantly. Catalysis is everywhere, from recent chemistry Nobel prizes to your car, food, and even the inside of your brain cells. Our world and our future revolve around catalysis and doing it better, greener, and cheaper! Care to explore?
This article is about catalysis.
First, we are going to build up an intuition for what catalysis is before we get to learn the official definition.
Then we will take a look at energy diagrams describing catalysis from a more physical point of view.
After this, we jump into the various examples of catalysis and the classification of catalysts.
Throughout our journey in the magical land of catalysts, we will also take some deep dives into how catalysts actually work for concrete examples.
How does catalysis work?
At the end of this section, I will give you the official definition of catalysis. However, it is somewhat tricky to understand. So first, we will be building up some intuition about what catalysis actually is. Let's start with what chemistry is all about, changing compounds into each other. Hopefully, at this point in your studies, this is not really surprising to you, but in chemistry, we are creating new compounds from existing ones. Catalysts essentially make this process easier.
Think about it this way. If you buy a bed and you have to put it together you can do it in a day or so. If I give you the guidelines on how to put this thing together you can get the same job done in 2 hrs. In this case, the blueprint would be the catalyst, and the bed is the chemical you are synthesizing.
Catalysts are basically molecules that make it easier to do chemical reactions. Now let's take a look at how this is done in practice. But before that, I promised you an official definition that you have to learn for your AP Chemistry exam.
Catalysis is defined as a process in which the rate of a reaction increases without modifying the overall standard Gibbs energy change in the system.
Energy Diagram and Catalysis
One way for the catalyst to make chemical reactions easier is to open up a new way for the reaction. For example, first, the reactants react with the catalyst forming an intermediate, then the intermediate decays to form a product, and it gives back the catalyst. You can see below an example of this, drawn in terms of energy and reaction coordinate.
Fig.1-Catalysis schematic representation
In this case, there is only one intermediate step in either case. Catalytic reactions typically have multiple intermediates. Since they will always end up reproducing the catalysts these reactions are called catalytic cycles. Catalytic cycles can be extremely complicated or super trivial, but in the end, they must reproduce the catalyst, and somewhere along the way they will produce your desired product.
Another way to improve reaction rates is to concentrate the reactants on the surface of the catalysts. This works because reaction rates are not only dependent on the activational energy but also the concentrations of the reactants.
If you want to learn more about this second type of catalysis check out "Reaction Rates" and "Catalysts".
Types of Catalysis:
It is very hard to find a box for every type of catalysis, they can be quite weird. The easiest way to organize them is by the phases of the reactant and the catalysts. If they are in the same phase it's referred to as Homogeneous catalysis if the catalysts and the reactants are in different phases we will call it Heterogeneous catalysis. Now enzymes are kind of weird and since they are hard to categorize we just gave them their own category the so-called Enzymatic catalysis. Let us take an in-depth look at each of these categories with the help of some examples.
If you find these distinctions complicated, you might want to take a look at "Solids, Liquids, and Gases" before continuing with catalysis.
Homogeneous catalysis
Every reaction where the catalyst is in the same phase as the reactants is classified as homogeneous catalysis.
Homogeneous catalysis almost always happens in solution, but this is not a requirement you could even do gas-gas catalysis. Here is a simple example.
Have you ever made or seen an elephant's toothpaste experiment? If yes, congratulations, you experienced homogeneous catalysis firsthand. This is a catalytic reaction because the catalyst (yeast or potassium iodide) does not change the reaction only speeds it up. In reality with or without the catalyst hydrogen peroxide would transform into water and oxygen. This oxygen then forms bubbles with the detergent (soap) to produce foam.
Fig.2-How to for elephant toothpaste
Now, this is pretty tame normally and looks like a can of soda sitting in a cup. if you add a catalyst you can speed up the reaction and make a lot of foam or in some cases make it explode. That one is dubbed devil's toothpaste for some reason.
If homogeneous meant it was in the same phase, can you guess what heterogeneous would mean?
Heterogeneous catalysis
Every reaction where the catalyst is in a different phase than the reactants is called heterogeneous catalysis.
For heterogeneous catalysis, we need at least two phases. For example, we could have two gases and react them on the surface of a solid catalyst.
Other arrangements are also possible. For example, you could have gases reacting in a liquid catalyst or whatever else you can come up with.
Now on paper, everything goes, but to make it practical for heterogeneous catalysis we almost always aim for a solid catalyst.
There is one catalytic reaction without which we could only feed 2/3 as many people as today. This one is the core of artificial fertilizer production and takes up around 1.3% of the whole energy consumption of the human race.
$$3H_2 + N_2 \rightleftharpoons 2NH_3$$
This one is called the Haber-Bosch process and to put the impact in perspective; producing ammonia uses half as much energy as Canada. With the help of a better catalyst for this reaction, we could cut this in half, and then in half again, and so on. It would not only be cheaper but also could decrease our CO2 emissions by the same amount as stopping all air traffic around the world.
What is the role of adsorption in heterogeneous catalysis?Adsorption can have many different effects in heterogeneous catalytic reactions. First, it often increases the reactant concentration on the surface, thereby speeding up the reaction. Secondly, the absorbed molecules can also react with the catalyst they are absorbed on to produce more reactive intermediates.
But this begs the question, how do you go about making better catalysts? Well, a very prominent approach is to just copy what nature has already developed for us!
Catalysis in Nature
Did you know that enzymes are also used in industrial processes? In practice when we are using an organism to produce a specific chemical from another one we are relying on its enzymes to do the job. After that, we destroy the organism and extract the product. A good example of this is using yeast to create ethanol from sugars.You could ask, why do we go the extra step with fungi and bacteria when we could just use a gallon of enzyme right? Well, enzymes are very prone to a process called denaturation. This basically means that if you look a little suspiciously at an enzyme it will fall apart and stop functioning. Even if it survives till you can start the reaction outside of the typical environment ( inside a cell) for that specific enzyme it will stop functioning very soon.
Hence developing artificial "enzymes" that can survive in our industrial environment would be something akin to the holy grail of chemistry. Making life immensely greener and cheaper for everyone! Let us look into the real deal now, time to learn about enzyme catalysis!
Enzyme catalysis
Every reaction where the catalyst is an enzyme is called enzymatic catalysis.
Enzymes are catalysts produced by natural sources such as bacteria, animals, or humans. These enzymes are made up of amino acids and usually have metal atoms in their active center.
The active center is the part of the catalysts system (enzyme) that actually does the catalysis.
Enzymes are huge and to our current knowledge, they cannot be produced in a laboratory. That would be way too complex a task for today's chemistry. What we do instead is isolate enzymes and study them. After this, you can try to recreate the active center without all the surrounding amino acids.
- A very famous example of an enzyme is alcohol dehydrogenase which converts alcohols to aldehydes, to do this it removes hydrogens hence the name. This guy is the reason we can sober up at all!
- Another quite famous group of enzymes is the kinases, they help your cell create oxygen-phosphorus bonds in a process called phosphorylation.
- Lastly, the enabler of all complex life are Photosystems 1 and 2. Without them, there is no photosynthesis and consequently, no advanced life can be sustained. Also, these two are notoriously hard to replicate in a laboratory even though many scientists are trying their hardest to do it!
How does coffee work; Competitive inhibition
You might have heard that caffeine, the main active ingredient in coffee works as a competitive inhibitor of adenine which is a chemical that helps signal to your brain that you need sleep. We will now take a look at what competitive inhibition actually means. By the end of this section, you will not only understand why you are not sleepy after drinking coffee in the morning but you will also know how you get addicted to that magical black liquid.
A competitive inhibitor is essentially a molecule that can block the enzyme's active center thus stopping it from working.
Well, this is the big idea with competitive inhibition, the two molecules are very similar and the enzyme can't differentiate between them, it will bind to both of them. Think about how annoying it is when you just can't stick the USB stick to your laptop. Both sides are similar enough that it is hard to differentiate just by looking at it, but trying to use the stick wrong side up will just not work. This happens to the enzymes they get bounded to by the competitive inhibitor but no reaction can occur.
In the case of caffeine, this means that your brain will feel like there is barely any adenine there (as long as it has caffeine around) and will thus not know that it is tired. To combat this it will start creating new sites for adenine to bind and be more sensitive to it. You naturally will just plug up those sites with the next coffee, but as soon as you stop drinking it you will feel immensely tired all the time. This is the result of your competitive inhibition!
We have seen already how we can characterize catalysis based on the phases of reactants and products.
But this is only one way to characterize catalysts and catalytic processes there is an alternative and often- used categorization. As you could see so far we discriminated based on the phase of reactants and the origin of the catalyst (enzyme/ not enzyme) but it is also possible to look at the chemical properties of said catalysts and decided the type based on what makes it a good catalyst.
Types of Catalyst
There are many different kinds of catalysts out there. For now, we will only look at the three most commonly used ones. I'm sure you have already heard of them but now we are taking a deep dive into each category. All of these categories are overlapping to some extent and no clear division can be made between them.
Take a look at Mn2+ this is a metal-ion so clearly, it will do metal-ion catalysis, right? Well, yes but it is also a Lewis acid so it can do Acid-base catalysis too. I hope at this point it is not surprising that during an electrocatalytic cycle of CO2 reduction Mn2+ can form covalent bonds with the substrate so it can also do covalent catalysis! These classifications mostly help you as a rule of thumb when considering possible reactions, they are not separate and fundamentally different categories like the first 3 were! With this out of the way, let us look at some classification.
Acid-base Catalysis
Acids can be defined in many ways, by Arhennius, Bronsted, or by Lewis. In catalysis, the two most important ones are the Bronsted acids (H+ donors) and Lewis acids (electron acceptors). Bases work the same way just with the abstraction of H+ (Bronsted bases) from the molecule or by donating electrons into the molecule (Lewis base).
You might want to check out "Acids and Bases" if this is something new to you!
Bronsted acid catalysis
The Fischer esterification is the perfect example of an acid-catalyzed reaction. Here we simply react an alcohol with an acid in the presence of a stronger acid ( catalyst) and we form a so-called ester.
Fig.3-The mechanism of the Fisher esterification
As you can see you can combine a molecule of an acid and an alcohol to form an ester with the help of an H3O+ while liberating water and an H+ from the system. Since at the end you got back the oxonium ion and it made the reaction work this oxonium ion suspiciously looks like a catalyst to me!
Lewis acid catalysis
Lewis acid catalysis is a kind of a hard topic at the AP level but let me give one simple example for you without the actual mechanism.
For the same esterification reaction starting with alcohols and acid-chloride you could use AlCl3 a Lewis acid to form an ester and HCl whilst you would be getting back the AlCl3. Now, this mechanism is a bit more complicated than the ones so far, but the actual catalysis step is the one where we steal a Cl- right out of the acid chloride.
Here from benzoic chloride, you would abstract the blue Chlorine + 1 electron from the carbon to form your active species:
Fig.4-Structure of benzoic chloride
The produced species will react hastily with any alcohol it can find to form a so-called benzoic ester.
Metal-ion Catalysis
Metal-ion catalysis is any reaction where the catalyst is a metal ion.
Metal-ion catalysis is dominantly just lewis acid catalysis from another perspective. But no worries I have a really easy one to demonstrate the concept here:
If you put MnO2 into hydrogen peroxide it will produce a lot of oxygen, water, and heat. This will end up evaporating the water and producing a curtain of water vapor. You see hydrogen peroxide really doesn't want to exist, it has too much energy and would really love to be water and oxygen. Manganese dioxide (MnO2) is able to help liberate the oxygen thus reducing the energy of the compound. Of course, with all catalysis, this could occur without MnO2 and it does occur just very slowly.
And now we are arriving at our last category, for now, the so-called covalent catalysis. Can you guess why is it named covalent catalysis?
Covalent Catalysis
Yes, you guessed it right! Unsurprisingly just with the last two categories, this is a super important research area of current chemistry. I would bet you there is not a single university where chemistry is done and some form of covalent catalysis is not researched as of today.
Covalent catalysis is any reaction wherein the catalyst forms a covalent bond with the substrate.
A good example of covalent catalysis is enzymatic catalysis, almost all enzymes form covalent bonds during their reaction cycles. If you want a concrete example here is a deep dive for you with the famous Heck reaction. (Nobel prize 2010) Naturally, this is not part of the AP curriculum just for you to see what covalent catalysis is about.
The so-called Heck reaction is a key industrial process many of the medications you have taken during your life were partially made by a heck reaction. What is it used for? Making carbon-carbon bonds!
The main catalytic step is the addition of a double bond to Pd forming two palladium carbon single bonds. This step makes it covalent catalysis. Of course, later on in the catalytic cycle, these bonds break and we get back the catalyst in its starting form.
I know catalysis is a very hard topic and it requires hard work and dedication to understand it. However, I hope I managed to show you how interesting it can be and how essential it is to our everyday life. I guarantee you any product you may buy has seen some catalyst along the way!
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