Episode 59: Dr. Paul Chirik on Modern Alchemy

Introduction

0:00Hi, you're listening to Let's Talk Chemistry, a podcast by Chemtalk. On today's episode, we interview Dr. Paul Chirik, the Edwards S. Sanford Professor of Chemistry and the Chair of the Department of Chemistry at Princeton University. He researches sustainable chemistry with a focus on making catalysts more sustainable in what he terms modern alchemy. Dr. Chirik then goes on to share how his work has been applied outside the lab in his many

0:33collaborations with industry. We hope you enjoy.

0:39Hello, everyone. Welcome back to another episode of Let's Talk Chemistry. My name is Mehreen, and I'm one of your co-hosts.

Sustainable Chemistry

0:46And I'm Elizabeth. Today, we're going to be talking about sustainable chemistry, particularly the importance of catalysts and the chemically recyclable plastics. We're so excited to have the incredible Dr. Chirik with us on this episode. Yes, Dr. Chirik is a professor and chair of the Department of Chemistry at Princeton University. He is an expert on using modern alchemy to make new catalysts, which can then be utilized for drug synthesis and plastic production. Alchemy may sound like some sort of impossible magic or witchcraft, but the idea behind it

1:16is actually pretty straightforward. Even better, the catalysts it creates help make chemistry more sustainable. Let's hear more about Dr. Chirik's background and how he got interested in chemistry. My name is Paul Chirik. I'm a professor of chemistry at Princeton University. I earned my Bachelor of Science at Virginia Tech in 1995, and then my PhD at Caltech in 2000. The interest in chemistry started in high school. I think that's pretty common among students. And, you know, I had a really inspiring chemistry teacher in the sense that it started with me

1:49a fascination of making new things, and I think that drew me to chemical synthesis. I definitely agree that teachers can be pretty influential in getting a student interested in a subject. Before I took my first chemistry class last school year, I thought chemistry was really complicated and I wouldn't be able to do it. But I was fortunate to have a great teacher that explained everything clearly and gave us fun laughs. So the class was a lot more enjoyable than I initially expected. Absolutely. I am curious, though. Chemistry is a pretty big subject. What made Dr. Chirik pursue organometallic chemistry specifically?

2:22And why is it so important? Let's hear more from him. And that interest was sparked by someone named Joe Morola, who was my undergraduate research advisor. And that led to my interest in the field that I'm in now, which is organometallic chemistry. And what was really enticing about the field was transition metals seem to be able to promote reactivity that you couldn't do with traditional organic molecules. And that's really what sparked my interest in what I do now. It seems like Dr. Chirik has benefited a lot from great teachers.

2:53I can't wait to learn more about his research on catalysts and different applications of it today. I bet a lot of our listeners are eager to hear too.

Current Research

3:01Let's hear from Dr. Chirik about his current research. What we do is sustainable chemistry. Just that's a very, very simple way to look at it. It's a very broad definition. And I think the thing I like to talk to students about, especially, is this concept of not carbon footprint, but elemental footprint. And what that means is, as you go through your life, how many elements do you need to live your quality of life? And the one that gets a lot of attention, and we don't work in this area, is your mobile phone, right? Your mobile phone has anywhere from 50 to 70 elements in it, right?

3:35So if you think, okay, what do we work on? Well, we work in this area of catalysis. And so just about everything you interact with today was probably synthesized by some kind of catalytic reaction. And so if you're sick and you take a medicine, chances are a carbon-carbon bond was made in that medicine by a palladium cross-coupling. And so the fundamental question is, is palladium as sustainable as it can be? Well, no, because palladium is really rare on Earth. And so wouldn't it be much better if you could make that catalyst out of, say, iron instead

4:06of palladium? And so that's the general motivation of what we do. And so it's an idea of looking at important chemical transformations and asking, are they as sustainable as they can be? You may be wondering, what are catalysts? How do they contribute to chemical reactions? Good question, Maharin. Catalysts are substances that increase reaction rates without getting consumed. For example, there are countless catalysts in the human body in the form of enzymes, and enzymes are essential for breathing, digestion, muscle movement, and more.

4:38Catalysts are useful because they speed up chemical reactions by lowering the activation energy needed to start a reaction, like changing the transition state of the reaction or facilitating collisions with the right orientations, so it's easier for reactants to form products. Even though we can't see it, catalysis occurs all around us, all the time. Without it, life wouldn't be sustainable. But not all catalysts are the same, and different catalysts can behave differently in a reaction. Here's more from our expert. What chemists think about it and maybe divide catalysis up in a bit of an artificial way,

5:11meaning how is the catalyst used in the reaction it's being used for? So if it's in a different phase of matter, so let's say the catalyst is a solid and your reactants are gases, that's how we make ammonia, for example, that's called heterogeneous catalysis. So we don't study that. We study what's called homogeneous catalysis or molecular catalysis. And what that means is that our catalysts are soluble. They're made from well-defined metals surrounded by organic ligands.

5:43And we can control lots of things about the catalyst through manipulating the structure. This process of controlling and manipulating catalysts is known as modern alchemy, which takes the function of base metals to mimic or exceed that of precious metal catalysts. This is extremely important because many catalysts are rare, expensive, or have a significant negative environmental impact with regards to its extraction and isolation. So this method gives an effective alternative. Modern alchemy helps scientists make catalysts that are cheaper and ideally more active, so

6:15less of the catalysts would be needed to speed up a reaction. That would also mean less catalysts would need to be thrown away. So you wouldn't need to worry as much about disposing them in an environmentally friendly way if there's barely any to begin with. Exactly. It's a common goal of chemists like Dr. Cherick. Take the biggest successes so far, like efficient catalysts, and then make them even better. So those catalysts would be even more active and lead to even less waste. While it is important to be researching topics such as catalysis, I wonder how catalysts can

6:46actually be used in real-life applications. Dr. Cherick can explain that for us as well. So some of the real-world applications, one is in this area known as asymmetric hydrogenation. That's a big, long, scary-sounding term maybe to some. But what it means is to take a carbon-carbon double bond, add hydrogen to it, and make a carbon-carbon single bond that then has handedness. So you can make a molecule that's left-handed or right-handed.

7:16And if in drug synthesis, you have to make one or the other because your proteins and the targets are chiral, right? And there's different biological properties for a left-handed molecule and a right. So this is a challenge for chemists. How do you make left-handed versus right-handed molecules? And Nobel Prizes have been awarded for this, for learning how to do it. And so one of the most common reactions and catalysts used to do this asymmetric hydrogenation reaction is rhodium. And so one of the things we recently discovered, and we published a paper on this earlier on

7:482023, where you can use a cobalt catalyst to hydrogenate a molecule that rhodium won't. And so in the synthesis of the drug molecule, the rhodium route actually takes four more steps. And so what the cobalt does is you can drop it in and hydrogenate a different molecule and shorten the synthesis. Wow, that's amazing. Shortening a synthesis can actually make a big difference when synthesizing complicated molecules. Yes, you're right. Each step in a synthesis generates waste.

8:19So we want to use as few steps as possible to minimize waste generation. If we can eliminate four whole steps with the use of cobalt, then we can conduct the synthesis in a much more efficient way. Rhodium is also very rare and expensive. So in addition to being more eco-friendly, using cobalt is much cheaper. So it sounds like Dr. Cherik has definitely made some significant contributions to industry with his work. But what companies has he collaborated with specifically? Let's take a listen. I've been very fortunate to have some absolutely terrific collaborations with industry.

8:51I've had maybe seven to 10 different companies work with us over the years and on all sorts of different problems, which I think has been really terrific because it gives us as academics a lot of perspective as to how the chemistry is actually used and what matters to translate it from a little tiny flask in our lab to maybe a much bigger reactor on scale. So a recent one that comes to mind, the one that I just told you about in that asymmetric hydrogenation space, that's actually been with a couple of companies. The chemistry was developed with Merck, a pharmaceutical company.

9:24So we've worked with them for about 11, 12 years through the National Science Foundation has a special funding mechanism to partner academia and industry. And they've been very supportive of our work with Merck over the years. And that's where these chiral catalysts came from. So that was one part of it. A recent one that we're working on now is with another drug company called Bristol-Myers Squibb, which I'm sure you've heard of. They're a very big one, and they're interested in making carbon-carbon bonds. So you know organic molecules have lots of carbon-carbon bonds in them.

9:57And so what we've found is that we can make iron, cobalt, and nickel catalysts do what palladium won't. So palladium is the thing that normally joins carbon-carbon bonds together in what's called cross-coupling. And we've developed a new variant of that reaction with Bristol-Myers Squibb working on iron, cobalt, and nickel. And so what's been really useful in that collaboration first is if you come to me as a chemist and say, okay, we want new methods to make carbon-carbon bonds, that's a vast

10:29problem because carbon-carbon bonds are everywhere. So which one do you make? So BMS came to us and said, well, we have a specific target and a specific type of bond that would really impact our program. So let's work on that. And so that's been great. And then we get some initial data and discoveries and hits, and then they come back and say, well, it would be great if we didn't need to use air-free techniques to handle the catalyst. It would be great if this part of the catalyst was less expensive. And so then the chemistry evolves and has been impacted in a very positive way.

11:03And they also offer all kinds of great scientific advice too, right? They're amazing chemists as well. And they say, well, maybe the reaction is working this way or that way. It's always great to look at the application of research in industry because we can really

Industry Applications

11:16see how our ideas get transferred from a lab to the products we use every day. You can also see this application of research in emerging new inventions, like one of Dr. Cherik's amazing projects with ExxonMobil, chemically recyclable plastics. With physical recycling, we take the plastic, chop it up, and then reuse the same plastic in a different way. But being chemically recyclable means that the polymer can be converted back into the monomer that made it. We can literally run the reaction backwards and end up with our original starting material, which we can now use in any way we want.

11:47That was pretty exciting, right? Let's see what Dr. Cherik elaborates. One of the biggest applications of catalysis is to make plastic. So the common plastics that you use every day are polyethylene. You probably know that one. Polypropylene. So polypropylene, you know, polyethylene is like the infamous plastic bag, any kind of wrapper, you know, like saran wrap, that kind of thing. Polypropylene is a harder plastic. So like a yogurt cup, even a car bumper can be made from polypropylene. And so those are amazing plastics because what you do is you take high energy molecules like

12:22ethylene or propylene, right? These are commodity organic molecules, some of the most produced molecules on the planet. And they take these high energy molecules with a catalyst and then the catalyst makes the plastic out of them. So take a high energy molecule to a low energy molecule. And then you get your plastic bottle or your plastic bag and it's incredibly stable, right? So what we've worked on is we take a molecule called butadiene. And so what polybutadiene is, is like the sole of your shoe. It's like a rubbery material, okay? And that's when the molecules are not in rings.

12:56And so what we learned how to do was to make an iron catalyst that puts butadiene into squares. And no one had ever made a structure like that before. And so that's really amazing. And that goes back to the first question you asked me about, why did I like chemistry? It's exactly that reason. I can put carbons and squares in a chain and no one has ever done that before. And we can study it and see what amazing properties it has. Well, one amazing property that it has is you can take that material and put it back with

13:28the iron catalyst and pull vacuum on it. And the plastic goes back to the butadiene that it was made from. So you can't do that with polyethylene and polypropylene because you went from really high energy molecules to low energy molecules. But with butadiene going to these squares, the two molecules are almost energetically equal. And so that's how you can run the reaction in the forward or the reverse. But the cool thing is, is you need a catalyst to do either one. So if the plastic is there and the catalyst isn't around, it's very, very stable.

14:00It's fine. You can heat it to 400 degrees C. You can beat on it with a hammer. It's really crystalline, really tough stuff. And then you put it back with the iron catalyst and you pull vacuum and then it goes back to where it started. So you would hopefully not have it accumulate in a landfill or in the ocean or all the other places that plastic ends up. So that's what the idea of a chemically recyclable plastic is. I love his explanation of the science behind it. I hope one day we'll be able to use chemically recyclable plastics as much as we use our current

14:30plastic. It sounds like a much greener option that could lead us toward a better future. Absolutely. Now, our timeless Dr. Charek is coming to an end. So let's hear his advice for future scientists. First is, I think you have to do what you really enjoy. That's the science you need to find, right? The science that does that to you, that makes you just smile and get excited. And, you know, you're walking down the street and you just want to think about it because it's just so exciting to you. And I think once you find that, then when you have failure, it's not that big of a deal

15:02because you're doing something that you're just so passionate about that, you know, you know, you're going to have these setbacks, but you know, the long game is going to be the right way to go. And you're going to just do amazing things when your heart's into it like that. As we try to be fast to fail when it comes to ideas, right? Try to do an experiment. Ideally, you'd like to do the most perfect, informative experiment, but sometimes you can't do that or that experiment takes six months. So do something that you can do in six hours that'll teach you a lot.

15:35And you might say, oh, this is just not going to work. And then you realize, okay, we have to go down another road. And, you know, everyone needs to, I think, realize this is that you read papers in scientific journals or you talk to me and I talk to you all about these amazing discoveries and things like that. But for everyone you see, there's some number of other ones you never see, right? And those are the failures. And yeah, it's tough to deal with. I mean, emotionally, right? You think, oh, I finally got this figured out or I'm going to revolutionize science and

16:05change the way everyone thinks about it. And then it just doesn't work and it's frustrating. But you have to realize that that's just, you have to learn something from every experiment, right? Even the failed ones and say, okay, I tried that. It's not going to work. So now where do I go next? And never take it personally, right? That's the most important thing. Try to do it with a smile on your face. Wow. That's great advice. It reminds me of a quote by Thomas Edison. I have not failed 10,000 times. I've just found 10,000 ways that will not work.

16:35If you pursue a field that you truly enjoy, whatever that may be, then that passion will motivate you to continue. So when you hit roadblocks, instead of giving up, you'll learn from those past results and keep pursuing your goal. I agree. If you're able to learn from past unsuccessful experiments, then those experiments weren't failures. They were stepping stones to success. You can even say they're catalysts that speed up the process of getting valuable results. Nice one. Plus, there's always room for new ideas and improvement, especially in a field like catalytic chemistry.

17:05As Dr. Cherik mentioned earlier, the beauty of this field is taking the biggest successes and making them even better. Absolutely. Thank you so much to Dr. Cherik for sharing your knowledge and advice with us today. And thank you to all of you for tuning in. Until next time. Bye.

17:22Thank you for listening to Let's Talk Chemistry, a podcast by Chemtalk. We hope you enjoyed it. For more information on today's episode and countless chemistry resources, please visit our website at www.chemistrytalk.org.

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