Researchers at the University of California, Berkeley have developed a catalytic process that can essentially vaporize common plastics such as polyethylene and polypropylene into hydrocarbon building blocks. This advancement could contribute to a circular economy by converting plastic waste into reusable monomers for new polymers, reducing reliance on fossil fuels and the environmental impact of plastic waste.
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Polyethylene and polypropylene constitute the majority of post-consumer plastic waste with products ranging from single-use plastic bags to durable goods like microwavable dishes and luggage.
Globally these plastics make up about two-thirds of plastic waste with 80% ending up in landfills, being incinerated or polluting oceans as microplastics.
Current recycling methods often degrade the quality of plastics limiting their reuse to low-value products like decking materials or sporks.
Led by UC Berkeley Professor John Hartwig, the research team developed a process that can break down polyethylene and polypropylene into their monomer components using inexpensive and available catalysts.
The process addresses the limitations of previous methods, which relied on expensive and short-lived metal catalysts like iridium, ruthenium and palladium.
The new process utilizes a combination of sodium on alumina and tungsten oxide on silica as catalysts, which are cheaper. Sodium, one of the cheapest and most abundant elements and tungsten, an earth-abundant metal used in the chemical industry, make the process more viable for large-scale application.
The catalytic process works by breaking the strong carbon-carbon bonds in polyethylene and polypropylene polymers, which are typically resistant to degradation.
The sodium on alumina catalyst efficiently cracks the polymer chains creating reactive carbon-carbon double bonds at the ends of the broken chains.
Once the polymer chains are broken, the tungsten oxide on silica catalyst facilitates a process called olefin metathesis, where the reactive ends of the chains are combined with ethylene gas to form propylene molecules.
This process continues until the entire polymer chain is converted into propylene, which can then be reused to create new polypropylene plastics.
For polypropylene, the process also produces isobutylene, a valuable hydrocarbon used in various industries from making polymers for cosmetics to high-octane gasoline additives.
The new catalytic process has showed an efficiency of nearly 90% in converting a mixture of polyethylene and polypropylene into valuable monomers like propylene and isobutylene.
The efficiency is even higher when processing either polyethylene or polypropylene alone, making the process effective for different types of plastic waste.
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Every year the world generates hundreds of millions of tons of plastic waste. Traditional recycling methods have struggled to keep pace leading to vast amounts of plastic ending up in landfills or the environment.
Most plastic waste is recycled through mechanical processes, where plastics are shredded and melted down to create new products.
With each recycling cycle, the quality of the plastic diminishes limiting its usability. Not all plastics can be recycled mechanically and the process often leads to downcycling, where the recycled product is of lower quality and value than the original.
Chemical recycling provides a more sustainable solution by breaking down plastic polymers into their basic building blocks. This allows for the creation of high-quality plastics that can be reused indefinitely without loss of quality.
The process involves breaking down long-chain plastic molecules into shorter chains, which can be reassembled into new, high-quality plastics or even converted into fuels. The breakdown of plastic polymers can yield a range of valuable products, including:
- Liquid Fuels such as petrol and jet fuel.
- Lubricants: High-grade lubricants that can be used in various industrial applications.
Scientists at ETH Zurich led by Professor Javier Pérez-Ramírez have made advancements in chemical recycling by developing a new mathematical formula that optimizes the process.
The research focuses on the use of powdered catalysts containing metals like ruthenium to enhance the efficiency of the chemical reactions. Gaseous hydrogen is introduced into the molten plastic.
The molten plastic is extremely viscous making proper mixing a critical factor. Through experiments and simulations, the team found that using an impeller with blades parallel to the axis provides the best results.
The ideal stirring speed was identified to be close to 1,000 revolutions per minute, balancing the need for thorough mixing without causing excessive turbulence.
The development of a mathematical model to describe the process allows for precise control over the recycling parameters enabling more effective experimentation and scaling up.
The process converts a nearly equal mixture of polyethylene and polypropylene into valuable hydrocarbon building blocks with approximately 90% efficiency.
These catalysts can be regenerated and reused through multiple reaction cycles, improving the process’s sustainability. Unlike previous methods the new catalysts do not require the removal of hydrogen to create breakable points in the plastic’s structure.
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