Hartwig pushes plastics to propene

Link to talk: https://www.youtube.com/watch?v=ee9PYHtiaiE

I've wanted to highlight this video series for a while, and thought this recent talk by John Hartwig was a great opportunity. 

Did you know that ACS hosts seminars by researchers around the world on their youtube channel? These get depressingly low views, and I think they're one of the best resources out there to (a) see some chemistry around the world or (b) reference how to put together a presentation. There's a half hour video interview with Roald Hoffmann, my chemistry hero! (The interviewer is Erick Carreira, btw: https://www.youtube.com/watch?v=XsQy-2etChw) So I highly recommend giving the channel a look, as there's almost certainly something of interest to any chemist who reads this blog. 

Some of the relevant papers for this work are listed below:

 Conk et al., "Catalytic deconstruction of waste polyethylene with ethylene to form propylene." Science 2022, 377, 1561-1566. https://www.science.org/doi/10.1126/science.add1088

Conk et al., "Polyolefin waste to light olefins with ethylene and base-metal heterogenous catalysts." Science 2024, 385, 1322-1327.  https://www.science.org/doi/10.1126/science.add1088

Summary Figure:

Background:

 Obviously, plastic waste is a major problem in our current society. While it is very cheap and easy to make polymers from small molecule oil products, it is not cheap or easy to break them back down into small molecule oil products. This means that things like polyethylene, polypropylene, and other common plastics end up sticking around indefinitely in landfills. 

There are some known ways to break down polymers into smaller units, which can then be broken down further. However, most of them have significant flaws. 

One method is pyrolysis- just burn the polymer (at >400 C), which breaks the C-C bonds and ends up forming a complex mixture of smaller hydrocarbons, many of which are unsaturated. 

Another method is hydrogenolysis, where you use a metal catalyst and hydrogen gas to form a complex mixture of smaller alkanes. The problem with these methods is that it doesn't produce a single, useful material- you end up with a mix that's basically crude oil that would need to be separated out again. 

However, this is a really hard problem. You need to activate C-C bonds that have absolutely no functional groups to give selectivity. 

How it works:

The idea here is to do a multistep process using alkene handles to slowly break down the polymer. 

First, using existing dehydrogenation reactions, the high density polyethylene becomes partially unsaturated. This is still producing statistical mixtures, because there isn't selectivity. 

Then, using olefin metathesis reactions, the polymer gets broken apart at those alkenes. Now you end up with a bunch of smaller chains of hydrocarbons, with a terminal alkene. 

Next, the terminal alkene is isomerized using another known reaction, which isomerizes terminal alkenes into internal alkenes one carbon over.

Finally, you do olefin metathesis again with ethylene, which breaks off a 3 carbon unit (propylene). It also makes a (slightly smaller) terminal alkene with a carbon chain at the end, which can undergo the previous 2 steps to keep making propylene over and over again.  

You can see how after just the metathesis, the size of the fragments decreases significantly: 

 

 Once they combined the last 2 steps so you have the catalyst for olefin metathesis with the isomerization catalyst in the same pot, you can start breaking down the alkenes all the way to polypropylene. They even did this on some collected waste plastic, showing that this hopefully is going to work in the "real world":

 

Initial Questions and Key findings:

1. Can these known reactions be used on polymers and polymer byproducts?

A: Yes, polymers are competent substrates for all of these reactions. 

2. Can you combine multiple of these steps together? One problem with this method is that each cycle only produces 1 polypropylene unit, so if you had to do the isomerization and then metathesis in two separate pots, then this would be infeasible. 

A: The isomerization and metathesis can be done in the same pot, so once you have a terminal alkene, the entire chain gets eaten up like a fruit roll up. 

 

3. Can you do this with actual waste plastics from commercial materials?

A: Yes, although some of the other junk in the plastics do cause some problems. They note that the black beer can top has a bunch of carbon black, and impurities like this do end up reducing the yield. This is currently a major issue in the recycling industry, so it's really important to clean out your recycling before you toss it!

4. Can you use heterogenous catalysts instead of homogenous catalysts to help scale the reaction?

A: Yes, they collaborated to design some tungsten/silcon oxide catalysts plus some sodium/aluminum catalysts that are able to replace the precious metal catalysts to do the reaction, which is super cool. 

 

They do end up getting some different products, for example 2-butene, but the dominant product remains propylene.  

Takeaways:

1. Existing reactions can be great starting points for new projects. None of the reactions developed here are new, they are just being applied to new systems. 

2. The use for reactions may not be clear upon first discovery, but may make a major impact eventually. I don't think the isomerization of a terminal alkene to an internal alkene is an obviously valuable reaction, but when combined with other discoveries, it plays a critical role in this process. 

3. Homogenous precious metal catalysis can help lay the foundations for future cheaper and/or heterogenous catalysts. We understand homogenous catalysis very well, so it helps to establish the feasibility of the reaction. Once we know the reaction is possible and what drives it, then we can work on improving it via cheaper metals or making it heterogenous. 

4. It's awesome to see organometallic catalysis have the potential to solve a major issue in the world.