Enlarge /. Workers sort plastic waste while a forklift transports plastic waste at the Yongin Recycling Center in Yongin, South Korea.
A few years ago it looked like plastics recycling would become an integral part of a sustainable future. Then the price of fossil fuels fell, making new plastics cheaper to make. Then China essentially stopped importing recycled plastics for manufacturing. With that, the floor fell out of plastic recycling, and the best thing that can be said for most plastics is that they locked up the carbon they were made from.
However, the lack of a market for recycled plastics also inspired researchers to look for other uses. Two articles this week looked at processes that enable "upcycling" or converting plastics into materials that can be more valuable than the freshly made plastics themselves.
Make me some nanotubes
The first paper created by an international collaboration actually got the plastics it tested from a supermarket chain, so we know it works with relevant materials. The upcycling described also has the advantage of working with very cheap iron-based catalysts. To break down plastics, catalysts and plastics are usually heated together. In this case, the researchers simply mixed the catalyst and grind up plastics and microwaved the iron.
Iron, like water, absorbs microwave radiation and converts it into heat. This causes the heat to be concentrated on the place where catalytic activity is taking place, rather than being evenly distributed throughout the reaction.
The difference is noticeable. Compared to conventional heating, microwave heating released more than ten times the hydrogen from the plastic, leaving only pure carbon and some iron carbide. Better still, the carbon was almost entirely in the form of carbon nanotubes, a product of significant value. And it all happened extremely quickly, with hydrogen being released less than a minute after the microwaves were applied. The process was completed in less than two minutes.
Although some of the iron was bound to carbon, it did not inactivate the catalyst. The researchers found that they could mix in more ground plastic and start the process again, repeating it up to ten times in their tests, even though hydrogen production decreased significantly by cycle 10. On the plus side, the later cycles produced almost pure hydrogen, as impurities such as oxygen and water had been removed by the earlier cycles. And at the end of 10 cycles, 92 percent by weight of the carbon-rich material was nanotubes.
The only thing missing from the work is an indication of how easy it would be to convert the iron to iron oxide, the catalytic form of the material.
We're going to take this hydrogen
If you were at all worried about what to do with this hydrogen, a US-based group has a possible answer. The group was also concerned about the problems the other researchers saw when they simply heated a catalyst and plastic together: the result was an intricate mix of chemicals, rather than the two clean products that resulted from rapid microwave heating. However, this team looked for possible solutions in biology.