A new process could make recycling plastic bags much easier.
Polyethylene plastics, especially the ubiquitous plastic bags that spoil the landscape, are notoriously difficult to recycle.
They are tough and difficult to break down, and if recycled, they are melted down into a polymer stew useful primarily for decking and other low-value products.
The new process uses catalysts to break the long polyethylene (PE) polymers into uniform pieces – the three-carbon-molecule propylene – which are the raw materials for making other high-value plastic types, such as polypropylene.
The process, admittedly in the early stages of development, would transform a waste product – not just plastic bags and packaging, but all types of PE plastic bottles – into a major product in high demand. Previous methods for breaking polyethylene chains required high temperatures and yielded much less demanded component mixtures. The new process could not only reduce the need for the production of fossil fuel propylene, often called propene, but also help fill a currently unmet need by the plastics industry for more propylene.
“To the extent that they are recycled, many polyethylene plastics are transformed into inferior materials. You can’t take a plastic bag and make another one with the same properties,” says John Hartwig, who holds a chair in organic chemistry at the University of California, Berkeley. “But if you can take that polymer bag back to its monomers, break it down into little pieces, and repolymerize it, then instead of pulling more carbon out of the ground, you’re using it as a carbon source to make other things, for example, polypropylene We would use less shale gas for this purpose, or for the other uses of propene, and to fill the so-called propylene gap.
Polyethylene plastics account for about a third of the entire global plastics market, with more than 100 million tonnes produced each year from fossil fuels, including natural gas obtained by hydraulic fracturing, often referred to as shale gas.
Despite recycling programs – recyclable PE products are referred to as plastic numbers 2 and 4 – only around 14% of all polyethylene plastic products are recycled. Due to their stability, polyethylene polymers are difficult to break down into their component parts or depolymerize, so most recycling involves melting it down and molding it into other products, such as garden furniture. , or to burn it as fuel.
The depolymerization of polyethylene and its transformation into propylene is a means of recycling, that is, of producing higher value products from essentially zero-value waste, while reducing the use of fossil fuels.
Hartwig specializes in using metal catalysts to insert unusual, reactive bonds into chains of hydrocarbons, most of which are petroleum-based. New chemical groups can then be added to these reactive bonds to form new materials. The hydrocarbon polyethylene, which typically occurs as a polymer chain of perhaps 1,000 ethylene molecules – each ethylene is made up of two carbon atoms and four hydrogen atoms – posed a challenge to its team due to their general unresponsiveness.
Hartwig and graduate students Steven Hanna and Richard J. “RJ” Conk came up with the idea of breaking two carbon-hydrogen bonds on polyethylene with a catalyst – initially, an iridium catalyst and, later, with platinum-tin and platinum-zinc catalysts. —to create a reactive carbon-carbon double bond, which would serve as an Achilles’ heel. With this flaw in the armor of the carbon-hydrogen bonds of the polymer, they could then unravel the polymer chain by reaction with ethylene and two additional catalysts that react cooperatively.
“We take a saturated hydrocarbon – all carbon-carbon single bonds – and remove a few hydrogen molecules from the polymer to create carbon-carbon double bonds, which are more reactive than carbon-carbon single bonds. A few people had looked at this process, but no one had done it on a real polymer,” says Hartwig.
“Once you have that carbon-carbon double bond, you use a reaction called olefin metathesis, which was the subject of a Nobel Prize in 2005, with ethylene to cleave the carbon-carbon double bond. Now you’ve taken this long chain polymer and broken it into smaller pieces that contain a carbon-carbon double bond at the end.
Adding a second catalyst, consisting of palladium, repeatedly cleaved the propylene molecules (three-carbon molecules) from the reactive end. Result: 80% of the polyethylene was reduced to propylene.
“Once we have a long chain with a carbon-carbon double bond at the end, our catalyst takes that carbon-carbon double bond and isomerizes it, a carbon in it. Ethylene reacts with that initial isomerized product to make propylene and an almost identical product, just shorter, polymer with a double bond at the end. And then it does the same thing over and over. It steps, splits; between, splits; between and cleaves until until all the polymer is cut into three-carbon pieces.Throughout the chain, it just eats away at the chain and spits out propylenes until there’s no more chain.
The reactions were carried out in a liquid solution with soluble or “homogeneous” catalysts. The researchers are currently working on a process using non-soluble, or “heterogeneous” catalysts to achieve the same result, since solid catalysts can be reused more easily.
The group demonstrated that the process works with a variety of PE plastics, including translucent milk bottles, opaque shampoo bottles, PE packaging and the hard black plastic caps that connect the four-pack aluminum cans. All were efficiently reduced to propylene, with only the dyes needing to be removed.
Hartwig’s lab also recently used innovative catalysis to create a process that turns polythene bags into adhesives, another valuable product. Together, these new processes could dent the proliferating piles of plastic that end up in landfills, rivers, and ultimately the oceans.
“Both are far from commercialized,” he says. “But it’s easy to see how this new process would convert the bulk of plastic waste into a huge chemical feedstock – with a lot more development, of course.”
New research appears in review Science. Additional coauthors are from UC Berkeley and the Berkeley Lab.
A US Department of Energy grant to study new catalytic reactions funded the work.
Source: UC Berkeley