Recycling Nylon-6 to ε-Caprolactam
Nylon-6 is a tough, non-biodegradable plastic that cannot
be recycled by conventional methods. A new way has now been introduced
by a team from the USA in the journal Angewandte Chemie: with
an easily accessible lanthanum trisamido catalyst, Nylon-6 can be
depolymerized highly selectively, nearly quantitatively, with no
solvent, and at moderate temperatures to recover the monomer,
ε-caprolactam. The monomers are removed sequentially from one polymer
end, just like unthreading pearls from a chain.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Nylon is the fabric from which stockings are made. It
is also the material of choice for many applications in areas
including automobile manufacturing, packaging, infrastructure,
textiles, and fishing. Its advantageous properties, such as
elasticity, chemical resistance, high tensile strength, and high
abrasion resistance get in the way of its biodegradability. Abandoned
nylon fishing nets, for example, account for about 10 % of the plastic
waste in the oceans.
Industrially, the variant Nylon-6 is made through a
ring-opening polymerization of ε-caprolactam on a scale of 5 million
tons a year. The market volume is projected to reach 21.5 billion
dollars by 2026. The heaps of trash are growing correspondingly,
increasing the danger to the environment and our health. In addition,
the production of Nylon-6 has a large CO2 footprint. The
monomer, ε-caprolactam, is made from fossil-based raw materials in a
costly, multistep process. Its reclamation would conserve resources
and save on production costs and energy. There is thus a high demand
for a circular economy for Nylon-6.
While recycling for some other plastics is slowly ramping
up, Nylon-6 is very difficult to recycle. Melting it to give it a new
form is not possible because it partially decomposes at the high
temperatures required. Burning it for energy production is also not
possible because it forms toxic compounds like hydrogen cyanide.
Previous methods of chemical recycling have proven to be too complex
and ineffective or require problematic chemicals.
A team led by Yosi Kratish and Tobin J. Marks at
Northwestern University (Evanston, USA) and the National Renewable
Energy Laboratory (Golden, USA) has now developed a new, efficient
catalytic process for recycling Nylon-6. Nylon-6 is depolymerized to
ε-caprolactam with over 95 % selectivity and over 90 % yield—without
solvents or toxic chemicals and at the comparably mild temperature of
240 °C. Admixtures of polyethylene, polypropylene, or polyethylene
terephthalate do not interfere.
The team's success rests on a catalyst based on
commercially available trisamido complexes of the rare earth metals. A
lanthanum complex demonstrated the highest catalytic activity.
Experimental data and computations suggest a novel mechanism for the
reaction. In the first step, a hydrogen ion is removed from a terminal
amide N–H bond and the catalyst is covalently bound to the polymer.
Subsequently, the ε-caprolactam units are split off the end of the
chain one by one in a backbiting process.
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About the Authors
Dr. Tobin Marks is Vladimir Ipatieff Professor of Catalytic
Chemistry at Northwestern University with interests in unconventional
catalysts and catalytic transformations, soft matter opto-electronics,
and sustainability. Dr. Yosi Kratish is a Research Assistant Professor
in the Chemistry Department at Northwestern University. He is an
expert in the field of catalytic plastic recycling.
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