A key problem is that plastics come in so many different varieties, and chemical processes for breaking them down into a form that can be reused in some way tend to be very specific to each type of plastic. Sorting the hodgepodge of waste material, from soda bottles to detergent jugs to plastic toys, is impractical at large scale. Today, much of the plastic material gathered through recycling programs ends up in landfills anyway. Surely there’s a better way.
According to new research from MIT and elsewhere, it appears there may indeed be a much better way. A chemical process using a catalyst based on cobalt has been found to be very effective at breaking down a variety of plastics, such as polyethylene (PET) and polypropylene (PP), the two most widely produced forms of plastic, into a single product, propane. Propane can then be used as a fuel for stoves, heaters, and vehicles, or as a feedstock for the production of a wide variety of products — including new plastics, thus potentially providing at least a partial closed-loop recycling system.
The finding is described today in the open access journal JACS Au, in a paper by MIT professor of chemical engineering Yuriy Román-Leshkov, postdoc Guido Zichitella, and seven others at MIT, the SLAC National Accelerator Laboratory, and the National Renewable Energy Laboratory.
Recycling plastics has been a thorny problem, Román-Leshkov explains, because the long-chain molecules in plastics are held together by carbon bonds, which are “very stable and difficult to break apart.” Existing techniques for breaking these bonds tend to produce a random mix of different molecules, which would then require complex refining methods to separate out into usable specific compounds. “The problem is,” he says, “there’s no way to control where in the carbon chain you break the molecule.”
But to the surprise of the researchers, a catalyst made of a microporous material called a zeolite that contains cobalt nanoparticles can selectively break down various plastic polymer molecules and turn more than 80 percent of them into propane.
Although zeolites are riddled with tiny pores less than a nanometer wide (corresponding to the width of the polymer chains), a logical assumption had been that there would be little interaction at all between the zeolite and the polymers. Surprisingly, however, the opposite turned out to be the case: Not only do the polymer chains enter the pores, but the synergistic work between cobalt and the acid sites in the zeolite can break the chain at the same point. That cleavage site turned out to correspond to chopping off exactly one propane molecule without generating unwanted methane, leaving the rest of the longer hydrocarbons ready to undergo the process, again and again.