Plastic is great. They can take on any shape and fill 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 come from petroleum. Plastics aren’t as easy to recycle as we’d like, and a huge percentage ends up in landfills (or in the ocean), where they can be virtually immortal.
The easiest way to recycle plastic is to simply tear it up, melt it down and mold it into a new shape. But that only works if the plastic is all of the same chemical type, which is a level of purity rarely found in a recycling bin. Without precisely separating plastics into different types, you end up with a mixture that is much less usable than pure plastics. We are limited in what we can make of it. Other methods of recycling plastics require serious energy inputs, such as high pressures and temperatures above 400°C. That can yield 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 usable, even if they are no longer plastic. They worked with polyethylene, which makes up most of the plastic we use. Polyethylenes are essentially long chains made of repeating links of carbon, with hydrogen on the side. The challenge is to break that springy chain into shorter pieces so that we can use the pieces to make other connections.
The new process consists of two steps, each carried out by a catalyst. The first catalyst is a molecule containing an iridium atom. This catalyst pulls part of the hydrogen atoms from the carbon backbone of a polyethylene. With the loss of these hydrogen atoms, some of the single electron pair bonds between carbon atoms become double bonds. That opens a vulnerability for the second catalyst.
That second catalyst, which may be based on atoms of rhenium and aluminum, interacts with some short-chain petroleum compounds that the researchers added. The long-chain plastic is cut at the double bond and pieces of the short-chain petroleum molecules are glued together. on both sides. Where once there was a single, very long chain, there are now two chains.
But the whole process is cyclical and does not stop there. The first catalyst releases some hydrogen atoms when pulled from the plastic, which can be used to convert any double bonds back into single bonds. The same sequence of reactions can play out again. Repeat for a few hours and only shorter-chain connections remain. Heat still needs to be added to boost this process, but temperatures around 150°C are sufficient.
The end result is three basic types of connections. There are very short chain compounds (things like butane) that can be used to start the reaction for the next batch of plastic. (The catalysts can also be separated and reused.) There are longer-chain wax compounds that are useful inputs for making plastics. And in between you get diesel.
By tuning different parts of the process, the researchers were able to determine the amount of wax vs. fuel coming out, as well as the range of wax compounds. Most plastic can easily be turned into fuel. Some chemicals added to plastics to modify their properties must also be recoverable so that they can be reused.
Of course, this is not as good as recycling plastics into subsequent generations of plastics, especially when the first generation was born from petroleum. But imagine if all the packaging your food came in could feed the next shipment instead of clogging landfills for centuries. And if we grew our plastic instead of pumping it out of oil fields, we could get two for the renewable price of one.
Scientific progress2016. DOI: 10.1126/sciadv.1501591 (About DOIs).