Can an ingenious utilization of waste carbon dioxide enable a fully circular plastics economy? Image credit: Marek Piwnicki, via Unsplash.
Plastics have, without a doubt, revolutionised our daily lives. Their adaptability, as well as their lightweight and durable nature, mean that plastic products are everywhere: from packaging to piping to furniture and toys. However, the production and disposal of this “wonder material” has quickly become the source of serious environmental concerns.
What is the “plastics problem”?
According to a 2021 report by the United Nations Environment Program (UNEP), approximately 9.2 billion tonnes of plastic was made between 1950 and 2017—equivalent to 189,000 times the combined weight of all the books in the Bodleian libraries! Virtually all (99 %) of these plastics originate from fossil fuels (coal, oil, or natural gas). Greenhouse gases are emitted during the extraction, transport, and refining of these fuels, all of which are processes required for plastic production. In fact, an estimated 850 million metric tonnes of greenhouse gases were released into the Earth’s atmosphere in 2019 from the production and disposal of plastic. These numbers are concerning as the release of greenhouse gases into the environment is causing climate change, which results in hotter temperatures, more severe storms, and a rising ocean, among many other worrying effects.
According to a 2021 report, approximately 9.2 billion tonnes of plastic was made between 1950 and 2017—equivalent to 189,000 times the combined weight of all the books in the Bodleian libraries.
And the trouble does not end there. Scientists estimate that less than 10% of all the plastic waste generated globally is recycled. The rest is incinerated, ends up in landfills, or leaks into the environment, where it is negatively impacting ecosystems. For example, plastic waste in the oceans can disrupt the movement of organisms (e.g., turtle hatchlings) and inhibit the growth of seagrass meadows.
To call it a “problem” puts it mildly. Rapidly growing production and consumption of plastic and the lack of circular approaches—dedicated to keeping plastic in the economy and out of the environment—constitute a global emergency.
What exactly are plastics?
Plastics refer to a family of synthetic or semi-synthetic materials that use polymers as a main ingredient. Polymers comprise several smaller units, called “monomers”, which are covalently bonded together to form a chain. The chemical structure of the polymer backbone and side chains determines its properties, and the diversity of available monomers enables the synthesis of polymers with a wide range of characteristics for highly varied applications, such as in packaging, pipes, clothing, adhesives, and toys.
Polyethylene and polypropylene are the most common polymers used in plastic packaging. The monomers required for this polymer synthesis—ethylene and propylene—are derived from fossil fuels. Furthermore, these polymer chains contain very strong carbon-carbon and carbon-hydrogen bonds, making these materials very hard to degrade. As a result, plastic waste can persist in the environment for tens to hundreds of years.
Making plastics from CO2
Carbon dioxide (CO2) is an inexpensive, colourless gas of low toxicity and high abundance—factories belt out tonnes of it as waste gas. The release of CO2 into the atmosphere is also a significant contributor to global warming. Therefore, it would be desirable to capture waste CO2 produced from industrial manufacture before releasing it into the atmosphere and make use of it in some way. In 1969, Shohei Inoue first reported on how to use CO2 as a raw material to make polymers. The polymerisation reaction involves two monomers: CO2 and an epoxide. The polymer produced is called a polycarbonate.
Many different kinds of epoxides can be used in this polymerisation reaction, producing polycarbonates with varying properties. However, a catalyst (a substance that increases the rate of a chemical reaction by lowering its activation energy barrier) is needed for this polymerisation to occur on an appreciable timescale. These catalysts can take many different forms but are often complexes containing one or two metals and an organic molecule. The search for more effective catalysts that are applicable in industry (i.e., tolerant to common impurities in CO2 waste gas, robust to high temperatures, and easy to prepare from sustainable reagents) for this polymerisation is a topic of ongoing research.
The Williams Group, based in Oxford, was early in recognising the power of bimetallic catalysts (i.e., catalysts containing two metals) for this reaction and was the first to report on “intermetallic synergy” between two different metals. This refers to the phenomenon where catalysts with certain metal combinations show enhanced activity and are said to be “synergic”. Hence, partnering up the right metals is key in designing an effective catalyst, which is essential for this polymerisation to become widely applicable in industry.
What are the benefits of this process?
This method of producing polymers has the potential to valorise waste CO2 and thereby provide an economic incentive for carbon capture. To prove the potential, the Williams group tested this polymerisation with CO2 captured from a carbon capture demonstrator plant at Ferrybridge Power Station (UK). A promising finding was that the catalysts tested were tolerant to the impurities found in industrial flue gas, showing nearly equivalent activity to when purified CO2 was used.
Furthermore, the polymers produced are classed as “polyols”—meaning hydroxyl (-OH) groups are located at the chain ends. These “polycarbonate polyols” can then be used to manufacture polyurethanes, which are found in rigid and flexible foams, footwear, coatings, and adhesives, to give a few examples.
Traditionally, polyurethanes are manufactured from polyether or polyester polyols. These polyols are made from petrochemicals produced from emissions-intensive oil refining processes. However, for the reaction of epoxides and CO2 to generate polycarbonate polyols, only the epoxide component is sourced from oil refinery. CO2 provides a partial substitute, meaning less of the petrochemical component (the epoxide) is needed in the polymerisation. According to a 2014 industrial case study, up to three tonnes of CO2 emissions may be avoided for every tonne of CO2 used to make polycarbonate polyols, primarily due to the lower epoxide demand.
Another benefit of polycarbonates is that they show high potential for a future circular material economy.
In addition, it is possible to prepare fully renewable polycarbonates by applying epoxides derived from plants. Two commonly used epoxides in these polymerisations, propylene oxide (PO) and cyclohexene oxide (CHO), can be derived from bio-glycerol and triglycerides (both obtained from plant oils), respectively. Limonene Oxide, derived from the peel of citrus fruits, can also be used.
Another benefit of polycarbonates is that they show high potential for a future circular material economy. The carbonate linkages enable the polymers to be enzymatically degraded or chemically recycled back to monomers at low temperatures. This would allow for the synthesis of new polymers from the recycled monomers, avoiding the generation of waste that would otherwise end up in the environment.
So, can waste CO2 solve our plastics problem?
Society needs improved methods to transform CO2 into useful products, both to reduce greenhouse gas emissions by substituting petrochemical feedstocks and as a method of locking away this pesky molecule, which would otherwise contribute to global warming. The polymerisation of epoxides and CO2 is a promising method to achieve both of these aims while also producing a polymer that has the potential to be chemically recycled.
However, it is important to note that this is only one part of the solution. The scale of the problem of plastic production and waste is monumental, and action is needed at many levels to develop a fully circular plastics economy and more sustainable processes for plastic production.
From innovative scientific research to government policy and increasing public awareness, the solution to our plastics problem must be multi-pronged. Using waste CO2 to make polymers will not solve the entire plastics problem but is a step in the right direction. Great outcomes are made up of many small actions, and with cooperation between several different sectors, this problem is surmountable. Nevertheless, the urgency with which we need to find solutions must not be underestimated.
