A new cobalt-based catalyst can break down various plastics into usable propane, a possible alternative to effectively recycling different types of plastics.
FREMONT, CA: One of the major pollution issues of modern times is the accumulation of plastic waste in the oceans, soil, and even human bodies. Despite efforts to recycle plastic products, optimising their various material mixes is challenging.
A primary problem concerns the miscellaneous forms of plastic and the chemical processes for breaking them down into a reused form that is specific to each plastic type. Sorting the waste material mix is impractical on a large scale. Today, much plastic material amassed through recycling programmes ends up in landfills.
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Research indicates alternative ways to curb this issue. A chemical process applying a catalyst based on cobalt is effective at breaking down a variety of plastics, such as polyethene (PET) and polypropylene (PP), the most widely produced plastic forms, into a single product, propane. Propane can also be used as fuel for stoves, heaters, and vehicles or as a feedstock for various products' production, such as new plastics. This provides a partial closed-loop recycling system.
Recycling plastics has been challenging as the long-chain molecules in plastics are combined by carbon bonds, which are stable and difficult to break apart.
Existing techniques for breaking these bonds will generate a random mix of different molecules, thereby requiring complex refining methods to separate them out into usable specific compounds. Fortunately, a catalyst made of a microporous material called zeolite containing cobalt nanoparticles can selectively break down various plastic polymer molecules and translate them into propane.
Although zeolites are riddled with tiny pores less than a nanometer wide, a logical assumption was that there would be little interaction between the zeolite and the polymers. However, it turned out that not only do the polymer chains enter the pores, but the synergetic work between cobalt and the acid sites in the zeolite can break the chain at the same time. That split site corresponds to chopping off one propane molecule without generating unwanted methane, leaving the rest of the longer hydrocarbons ready to undergo the process repeatedly.
The materials required for the process, zeolites and cobalt, are cheap and widely available. Another material needed is hydrogen, which is mostly produced from fossil fuels but can easily be made in other ways, including electrolysis of water using carbon-free electricity such as solar or wind power.
Researchers tested their system on a real example of mixed recycled plastic, observing promising results. However, more testing is essential on a greater variety of mixed waste streams to determine the amount of fouling occurring from various contaminants on the materials, including inks, glues, and labels attached to the plastic containers or other nonplastic materials getting mixed in with the waste. It also needs to determine how that affects the long-term stability of the process.
Collaborative studies are taking place to understand the economics of the systems and analyse how they can fit into today’s systems for handling plastic and mixed waste streams.
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