At the University of California, Berkeley, a new chemical process claims to ‘essentially vaporize’ polyethylene, polypropylene, and mixed-plastic waste into building blocks for repolymerization into new plastics.
If scaled up, this catalytic process is set to reduce the fossil fuels required to make new plastics and unlock circularity for various single-use plastics – including clear PET water bottles, which the university says were designed for circular recycling processes.
Around two-thirds of post-consumer plastic generated worldwide is thought to be made of polyethylene and polypropylene. Approximately 80% of it is believed to end up in landfills, incineration, or the natural environment, leading to the leakage of microplastics into streams and oceans. The remainder is downcycled into low-value plastic, the university says.
“We have an enormous amount of polyethylene and polypropylene in everyday objects, from lunch bags to laundry soap bottles to milk jugs — so much of what’s around us is made of these polyolefins,” said research leader and UC Berkeley professor of chemistry John Hartwig. “What we can now do, in principle, is take those objects and bring them back to the starting monomer by chemical reactions we’ve devised that cleave the typically stable carbon-carbon bonds.
“By doing so, we’ve come closer than anyone to give the same kind of circularity to polyethylene and polypropylene that you have for polyesters in water bottles.”
Hartwig and his team previously conceptualized a depolymerization process for plastic bags made of polyethylene. This broke them down into propylene (or propene) monomers, which could then be reconstructed into polypropylene plastics.
Three different heavy metal catalysts were utilized. Onen added a carbon-carbon double bond to the polyethylene monomer, at which point the other two would break the chain and cut off a carbon atom. This atom would react with ethylene to produce propylene in a repeated process that continued until the polymer disappeared.
However, because the catalysts were soluble and would dissolve in the liquid reaction, it was difficult to recover them in an active form. The new process implements cheaper solid catalysts that are commonly used in the chemical industry for continuous flow processes; this means they can be reused, with the processes able to be scaled up for large material volumes.
Graduate student Richard J. “RJ” Conk initially consulted with Alexis Bell, chemical engineer and heterogeneous catalyst expert in the Department of Chemical and Biomolecular Engineering, to experiment with these catalysts. By synthesizing a catalyst of sodium on alumina, he was able to break down various polyolefin polymer chains, with only one of two pieces with a reactive carbon-carbon double bond left behind.
Tungsten oxide on silica was then able to add the carbon atom at the end of the chain to the ethylene gas constantly streamed through the reaction chamber. In a process known as olefin metathesis, this created a polypropylene molecule and left behind a double bond that the catalyst could repeatedly access until the whole chain was converted into propylene.
The reaction can be replicated with polypropylene to form a combination of propylene and the hydrocarbon isobutylene, which is used by the chemical industry to produce polymers for high-octane gasoline additives and various consumer products, including cosmetics and footballs.
“You can’t get much cheaper than sodium,” Hartwig continued. “And tungsten is an earth-abundant metal used in the chemical industry in large scale, as opposed to our ruthenium metal catalysts that were more sensitive and more expensive.
“This combination of tungsten oxide on silica and sodium on alumina is like taking two different types of dirt and having them together disassemble the whole polymer chain into even higher yields of propene from ethylene and a combination of propene and isobutylene from polypropylene than we did with those more complex, expensive catalysts.”
Reportedly, the new catalysts negate the need to remove hydrogen to create a breakable carbon-carbon double bond. These are described as a polymer’s ‘Achilles heel’, just as the reactive carbon-oxygen bonds in polyester or PET theoretically make the plastic easier to recycle.
Polyethylene and polypropylene do not have these breakable bonds, instead consisting of strong single-carbon bonds. Nevertheless, the combination of the sodium and tungsten catalysts turns a ‘nearly equal’ mixture of polyethylene and polypropylene into propylene and isobutylene with a reported efficiency of almost 90% – and the yield is thought to increase for polyethylene and polypropylene individually.
“Think of the polyolefin polymer like a string of pearls,” said Hartwig. “The locks at the end prevent them from falling out. But if you clip the string in the middle, now you can remove one pearl at a time.”
Conk experimented with adding plastic additives and different plastic types to the reaction chamber to test whether contaminants would affect the catalytic reactions. Apparently, its efficiency was significantly reduced by small amounts of PET and polyvinyl chloride, in particular.
However, since different plastic types are separated into different streams during existing recycling processes, the researchers do not anticipate that contamination will be a significant problem in practice.
Even so, Hartwig warns that the hard-to-recycle plastics in circulation today may still be a problem for years to come.
“One can argue that we should do away with all polyethylene and polypropylene and use only new circular materials,” he said. “But the world’s not going to do that for decades and decades.
“Polyolefins are cheap, and they have good properties, so everybody uses them. People say if we could figure out a way to make them circular, it would be a big deal, and that’s what we’ve done. One can begin to imagine a commercial plant that would do this.”
Hartwig, Conk, Bell, and their colleagues – graduate students Jules Stahler, Jake Shi, Natalie Lefton and John Brunn of UC Berkeley and Ji Yang of Lawrence Berkeley National Laboratory – published the details of the process in the Science journal on 29th August.
A previous study from Brunel University London – published in the Biofilms and Microbiomes journal – revealed two new enzymes that could apparently dissolve plastic bottles faster than existing recycling methods, creating the raw material needed to construct new ones. The researchers genetically engineered plastic-degrading bacteria to attach to the waste plastic and form biofilms on it, which then sped up the degradation process.
More recently, GR3N has joined forces with Schneider Electric to the ‘first’ open automation system for advanced plastic recycling; this will be applied to its Microwave Assisted Depolymerization (MADE) technology, with plans to reduce human error by 40% and engineering costs by 30%.
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