According to new research from MIT and elsewhere 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.
chemical process
Propane can then be used as a fuel for stoves, heaters, and vehicles, or as a feedstock to produce a wide variety of products, including new plastics, thus potentially providing at least a partial closed-loop recycling system.
The finding is described 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 difficult problem, according to Román-Leshkov. 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 refining methods to separate out into usable specific compounds. Leshkov added: adds: “there’s no way to control where in the carbon chain you break the molecule.”
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. 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.
Román-Leshkov continued: “Once you have this one compound, propane, you lessen the burden on downstream separations, That’s the essence of why we think this is quite important. We’re not only breaking the bonds, but we’re generating mainly a single product” that can be used for many different products and processes.
The materials needed for the process, zeolites and cobalt, “are both quite cheap” and widely available”. Although today most cobalt comes from in the Democratic Republic of Congo. Some new production is being developed in Canada, Cuba, and other places. The other material needed for the process is hydrogen.
The researchers tested their system on a real example of mixed recycled plastic, producing ‘promising’ results. But more testing will be needed on a greater variety of mixed waste streams to determine how much fouling takes place from various contaminants in the material.