From Science Advances, 2016

"Recycling plastic can be difficult, but maybe we could squeeze something else out. by Scott K. Johnson - Jun 19, 2016

Plastics are great. They can take any shape and serve an endless variety of roles. But... the beginning and end of a plastic’s life are problematic. While some plastics are made from renewable agricultural products, most are derived from petroleum. Plastics are not as easy to recycle as we'd like, and a huge percentage ends up in landfills (or the ocean) where they can be virtually immortal.

The easy way to recycle plastic is to just rip it up, melt it down, and pour a new mold. But that only works when the plastic is all the same chemical type, which is a level of purity you rarely find in a recycling bin. Without separating plastics precisely into different types, you get a mixture that is much less useful than pure plastics. We’re limited in what we can make out of it. Other methods for recycling plastics require serious energy input, like high pressure and temperatures over 400°C. That can produce a variety of hydrocarbon compounds, but they can be difficult to work with.

Recently, a team led by Xiangqing Jia of the Shanghai Institute of Organic Chemistry decided to try some chemical tricks to turn some of these plastics into something useful, even if it’s not more plastic. They worked with Polyethylene, which makes up the majority of the plastic we use. Polyethylenes are essentially long chains made of repeating links of carbon, with hydrogen hanging off the side. The challenge is to break that resilient chain into shorter pieces so we can use the pieces to make other compounds.

The new process involves two steps, each run by a catalyst. The first catalyst is a molecule including an atom of iridium. This catalyst pulls some of the hydrogens off the carbon backbone of a polyethylene. With the loss of these hydrogens, some of the single-electron-pair bonds between carbons become double bonds. That opens up vulnerability for the second catalyst.

That second catalyst, which can be based on atoms of rhenium and aluminum, teams up with some short chain petroleum compounds that the researchers added in. The long chain plastic is sliced at the double bond, and pieces of the short chain petroleum molecules are glued to either side. Where there was once a single, very long chain, there are now two chains.

But the whole process is cyclical and doesn't stop there. The first catalyst releases some hydrogens as it pulls them off the plastic, which can be used to convert any double bonds back to single bonds. The same series of reactions can play out again. Repeat this for a few hours, and only shorter chain compounds remain. Heat does still have to be added to fuel this process, but temperatures around 150°C are sufficient.

The end result is three basic types of compounds. There are very short chain compounds (things like butane) that can be used to get the reaction started for the next batch of plastic. (The catalysts can also be separated out and reused.) There are some longer chain wax compounds that are useful inputs for the plastics-making process. And in between, you get diesel fuel.

By tuning different parts of the process, the researchers were able to control the proportion of wax vs. fuel that came out, as well as the range of wax compounds. Most of the plastic can easily be turned into fuel. Some of the chemicals that are added to plastics to modify their properties should be recoverable, too, so they can be used again.

Of course, this isn’t as good as recycling plastics into further generations of plastics, particularly when the first generation was born of petroleum. But imagine if all the packaging your food came in could fuel the next shipment instead of clogging up landfills for centuries. And if we grew our plastics instead of pumping them from oil fields, we could get two for the renewable price of one.