Page 74 - Plastics News October 2024
P. 74
PRODUCT NEWS
can be reused, with the processes able to be polypropylene than we did with those more
scaled up for large material volumes. complex, expensive catalysts.”
Graduate student Richard J. “RJ” Conk initially Reportedly, the new catalysts negate the need
consulted with Alexis Bell, chemical engineer to remove hydrogen to create a breakable car-
and heterogeneous catalyst expert in the De- bon-carbon double bond. These are described
partment of Chemical and Biomolecular Engi- as a polymer’s ‘Achilles heel’, just as the reactive
neering, to experiment with these catalysts. By carbon-oxygen bonds in polyester or PET theo-
synthesizing a catalyst of sodium on alumina, he retically make the plastic easier to recycle.
was able to break down various polyolefin poly-
mer chains, with only one of two pieces with a Polyethylene and polypropylene do not have
reactive carbon-carbon double bond left behind. these breakable bonds, instead consisting of
strong single-carbon bonds. Nevertheless, the
Tungsten oxide on silica was then able to add combination of the sodium and tungsten cata-
the carbon atom at the end of the chain to the lysts turns a ‘nearly equal’ mixture of polyeth-
ethylene gas constantly streamed through the ylene and polypropylene into propylene and
reaction chamber. In a process known as olefin isobutylene with a reported efficiency of almost
metathesis, this created a polypropylene mol- 90% – and the yield is thought to increase for
ecule and left behind a double bond that the polyethylene and polypropylene individually.
catalyst could repeatedly access until the whole
chain was converted into propylene. “Think of the polyolefin polymer like a string
of pearls,” said Hartwig. “The locks at the end
The reaction can be replicated with polypropyl- prevent them from falling out. But if you clip the
ene to form a combination of propylene and the string in the middle, now you can remove one
hydrocarbon isobutylene, which is used by the pearl at a time.”
chemical industry to produce polymers for high-
octane gasoline additives and various consumer Conk experimented with adding plastic additives
products, including cosmetics and footballs. and different plastic types to the reaction cham-
ber to test whether contaminants would affect
“You can’t get much cheaper than sodium,” the catalytic reactions. Apparently, its efficiency
Hartwig continued. “And tungsten is an earth- was significantly reduced by small amounts of
abundant metal used in the chemical industry in PET and polyvinyl chloride, in particular.
large scale, as opposed to our ruthenium metal
catalysts that were more sensitive and more ex- However, since different plastic types are sepa-
pensive. rated into different streams during existing re-
cycling processes, the researchers do not an-
“This combination of tungsten oxide on silica ticipate that contamination will be a significant
and sodium on alumina is like taking two dif- problem in practice.
ferent types of dirt and having them together
disassemble the whole polymer chain into even Even so, Hartwig warns that the hard-to-recycle
plastics in circulation today may still be a prob-
higher yields of propene from ethylene and a
combination of propene and isobutylene from lem for years to come.
74 PLASTICS NEWS October 2024
