Upcycling Plastic Waste by Using Excess Carbon Dioxide Olivier Coulembier

Highlights

• Using organic catalysts, carbon dioxide could be used to transform plastic waste into new functional materials with superior properties

• Current recycling and upcycling processes are suboptimal, and even those with a “low carbon footprint” produce massive amounts of carbon dioxide

• Establishing a recycling process that has a “zero” or even “negative” carbon footprint could be possible using carbon dioxide-derived organic catalysts and tools already at our disposal

• By adding “anchor points” to plastic molecules, the ability to reuse and sequester carbon dioxide could be enhanced

In a world of excessive plastic waste and carbon dioxide production, innovative solutions that have potential on an industrial scale are sorely needed. An estimated 1 million plastic waste products are thrown away every minute after an average use time of only 15 minutes; globally, around 250 million metric tons of plastic waste is generated each year, only 30% of which is recycled. Adding to this sad reality is the alarming production of carbon dioxide, which we know contributes to global warming and climate change. According to 2021 estimates, 1 million kg of carbon dioxide are emitted every second, and 39 gigatons were emitted that year. These figures are simply dizzying.

The AXA Research Fund is supporting the new AXA Chair in Environment and Climate Research (2022-2027) that aims to address these two major environmental pollution issues in one fell swoop. The Chair is held by Professor Olivier Coulembier, senior researcher at the Fonds de la recherche scientifique (FRS-FNRS) within the Services des Matériaux Polymères et Composites (SMPC) of the University of Mons in Belgium. The official inauguration of the AXA Research Chair was held on 28th November 2022 at Mons University, during which Prof Coulembier introduced the exciting possibilities of his research.

Industrial-Scale Viability Of Plastic Recycling

In 2020, the estimated plastic production rate was 367 million tons worldwide, 55 million tons of which were produced in Europe. Why do we continue to produce such massive quantities of plastic, knowing full well the detrimental effect this has on the environment? Quite simply, producing plastic from fossil fuels costs practically nothing compared to valorization or recycling. 

While a great many people insist that the way forward is producing biodegradable plastics, Prof Coulembier is not one of them. “Most individuals believe that biodegradable plastic is like a banana skin that you can simply throw away – this is not the case,” he explained. Biodegradable plastics require very specific conditions to decompose, and even if such polymers were truly biodegradable, their decomposition would result in the release of carbon dioxide, methane, and water vapor, which are three key gasses involved in the greenhouse effect. “This shows that we need to instead invest our efforts into the recycling of non-degradable plastic materials,” said Prof Coulembier.

His research, therefore, focuses on the most polluting family of plastics: “polyolefins.” Polyolefins such as polyethylene and polypropylene are produced in bulk by the food packaging industry, which produces the most plastics after the automobile and construction industries. Unfortunately, recycling these very long plastic macromolecules, made up of carbon and hydrogen atoms, is particularly difficult.

Industrial Viability Of Carbon Dioxide Valorization

Economically viable carbon dioxide valorization methods are already used in three noteworthy sectors. Namely, carbon dioxide is used in the production of soft drinks, in the simple reaction of ammonia-based fertilizers to produce urea, and in “enhanced oil recovery,” whereby carbon dioxide injected into oilfields facilitates oil extraction by increasing the pressure of oil reservoirs. The continued use of these valorization methods has been estimated to help us reach a carbon dioxide “recovery rate” of up to 272 million tons by 2025. “Alone, this figure is relatively high,” said Prof Coulembier, “but it is still a drop in the ocean compared to those estimated 39 gigatons of carbon dioxide emissions per year.”

One way to offset the enormous excess of carbon dioxide waste is to set aside our concerns about what is “economically viable.” Several experts have agreed that many more gigatons of carbon dioxide could be valorized by targeting sectors that produce fuel, carbon dioxide-based polymers such as polycarbonate plastic, chemical products, and construction materials. Still, the carbon dioxide used to prepare these products and materials will, at some point, be re-released into the atmosphere.

For fuel, this timeframe for carbon dioxide re-release is relatively short, at an estimated 1 year. However, polymers re-release carbon dioxide after around 100 years, and construction materials, such as cement or cement derivatives in which carbon dioxide replaces water, re-release carbon dioxide after an estimated 1 million years. “To have the least possible repercussions for the climate, we, therefore, need to invest efforts into the preparation of carbon dioxide-based plastic and of construction materials,” said Prof Coulembier.

A further problem with existing industrial-scale processes that valorize carbon dioxide, such as the production of the common painkiller aspirin, is that they themselves produce excessive carbon dioxide. The high temperatures and high carbon dioxide pressures needed for these methods require the use of energy-hungry reactors that, in turn, emit massive amounts of carbon dioxide. “So, we have to develop industrially viable methods that transform carbon dioxide, but which do not release carbon dioxide,” said Prof Coulembier.

The Carbon Footprint Of Plastic Recycling

Another motivation of Prof Coulembier’s research is to address head-on the suboptimal, unsustainable, and even questionable recycling techniques currently in use. In 2018, around 17.8 million tons of plastic were treated and recycled, which is an increase of around 19% since 2006. These figures might seem promising, but delve a little deeper, and a different picture emerges. “All the existing treatment and recycling methods release variable yet massive amounts of carbon dioxide into the atmosphere,” explained Prof Coulembier.

Of the reported 17.8 million tons of plastic treated and recycled, 39.5% was incinerated to generate energy—In Europe, we don’t consider this to be recycling—and 42% was recycled through either pyrolysis or mechanical recycling. Pyrolysis involves heating plastic to thermally break down the macromolecules to create smaller molecules that can be recovered and potentially purified for use as, for example, synthetic fuels. Mechanical recycling—the most attractive method of the two—involves separating plastic by type, washing it, heating it, then reusing it.

While mechanical recycling is simple and economically viable, it degrades and changes the properties of polymers over repeated recycling and so cannot be implemented indefinitely. Moreover, while mechanical recycling has what the media calls a “low carbon footprint,” it nonetheless releases massive amounts of carbon dioxide into the atmosphere. “Mechanical recycling does indeed release less carbon dioxide than other waste valorization techniques, but still has a 50% production of carbon dioxide,” explained Prof Coulembier. This means that with 2 tons of plastic, mechanical recycling releases 1 ton of carbon dioxide into the atmosphere.

PLUCO: Developing A Recycling Method With A Negative Carbon Footprint

The purpose of Prof Coulembier’s research program, “preparation of new materials by synergetic treatment of plastic pollutants and atmospheric carbon dioxide” (PLUCO), is to develop a plastic recycling method with a low, zero, or even negative carbon footprint. A negative carbon footprint means that there is less carbon dioxide produced at the end of the cycle than the quantity used at the beginning of the recycling process, and the strategy to achieve this is, therefore, relatively simple: to use carbon dioxide for plastic recycling. 

During the AXA Chair, Prof Coulembier will adopt a stepwise research approach in three complemental work packages, summarized as follows:

1)     Fundamental research on how to transform carbon dioxide into new organic molecules. For any such method to be taken seriously on an industrial scale and to have a real societal impact, it needs to be fast, efficient, economically viable, and with simple reaction conditions.

2)     Research on how to treat plastic materials to create new molecules. The hope is to develop a plastic treatment method that has a “gentler” transformation to avoid molecule degradation and enable “chain functionalizations.” This will require conditions that are low in energy, non-destructive, selective to plastic’s carbon and hydrogen bonds, and tolerant to additives, such as those found in most plastic.

3)     Optimizing the reaction between carbon dioxide-derived molecules and “functionalized” plastics to produce new materials. This process needs to have conditions that render it industrially exploitable and with real societal impacts, such as in the construction materials sector, to have the least possible impact on the climate.

These three branches of the PLUCO project are firmly grounded in polymer chemistry, and aspirations to create a double-pronged solution for the plastic and carbon dioxide waste problems are by no means, said Prof Coulembier, “overly utopian.” For the first arm of the PLUCO project, Prof Coulembier’s lab has been focusing on a very simple reaction through which carbon dioxide is transformed into carbonate salts. This one-step process has been known for some time but is usually carried out in the presence of toxic metal catalysts and requires high temperatures and carbon dioxide pressures. But, with a different kind of catalyst—a non-toxic organic catalyst—the reaction conditions needed are milder, with lower ambient temperatures and “ridiculously low” carbon dioxide pressures, as well as short reaction times.

Speaking about the second step of the PLUCO project, Prof Coulembier explained how adding anchor points could functionalize molecules made up of carbon and hydrogen—the atoms that also make up plastic molecules—through the application of an organic catalyst, and with unprecedented process improvements. “These reactions require only an ambient temperature and atmospheric pressure, have a short reaction time, and a tolerance to a number of reagents,” he said. Work in the laboratory has demonstrated that this transformation was possible for molecules with 4 to 11 carbon atoms, i.e., liquids, as well as for molecules with 20 carbon atoms, i.e., solids. Prof Coulembier and his team will continue to investigate other ways to catalyze this reaction so that anchor points can be added to plastics, enabling them to react with carbon dioxide-derived molecules.

Along with his international academic network, Prof Coulembier will work hand-in-hand with industrial partners to develop possible preparation techniques in the third stage of the PLUCO project. He explained that the proposed process is industrially viable given the tools already available to the SMPC, in particular their expertise and research in “reactive extrusion.” This is a manufacturing method that combines traditionally separated chemical processes, such as polymer synthesis, with extrusions, such as melting and blending, to create a single process. This, as well as “flow chemistry” methods, will be further studied in an inter-university collaboration, as will the feasibility of their application in industrial-scale polymer production.

January 2023

Photo credit: Xavi Cabrera/Unsplash