Bio: Musa Mohammed is a PhD researcher in Chemical Engineering at The Pennsylvania State University, USA. His research focuses o...
Bio:
Musa Mohammed is a PhD researcher in Chemical Engineering at The Pennsylvania State University, USA. His research focuses on innovative reaction engineering technologies, particularly in the areas of chemical reactor design, catalyst development, and chemical kinetics for polyolefin catalytic cracking. Passionate about public education, he writes to raise awareness of emerging reaction engineering solutions that address critical challenges of our time.
Unlocking Waste Plastic's Potential: A New Era of Recycling Technology
Our civilization uses plastic materials in a variety of ways. According to the American Chemistry Council, the US plastics industry supports over 25% of US manufacturing, creates over 1 million jobs, and was projected to earn over $370 billion in revenue in 2023. Plastics are simply central to modern manufacturing. However, plastic waste accumulation is one of the biggest environmental challenges of our time. Global post-consumer plastic waste exceeds 400 million tonnes annually and is projected to reach 1 billion tonnes by 2050.
Plastics have significant chemical complexity and embedded energy, making them promising feedstocks for new manufacturing processes and key components of a circular chemical economy. Most plastics are made from materials known as polyolefins—for example, polyethylene, which is used in plastic bags and bottles, and polypropylene, commonly found in food containers. Together, these two polyolefins account for 60–70% of all plastics. Unfortunately, they are also among the most difficult to recycle, contributing substantially to the growing plastic waste crisis.
To capture the embedded energy in waste plastics, researchers have been developing chemical degradation processes to treat plastics. One such method is polyolefin catalytic hydrocracking. This process, similar to what oil refineries use (for decades) on heavy crude oil, breaks down difficult-to-recycle plastic polymers into smaller, useful molecules like waxes and fuel-range hydrocarbons. A useful analogy is to imagine a polymer as a long freight train. Hydrocracking cuts the train into shorter cars (smaller hydrocarbon molecules) and caps the ends with hydrogen thereby stabilizing them. This process requires the presence of hydrogen and a special material called a catalyst. The catalysts speed up and reduce the energy barrier needed for the cracking process without being consumed at the end.
Catalytic reactions occurs on specific sites on the surface of the catalysts that are referred to as "active sites."
Hydrocracking of polyolefins uses bifunctional catalysts that have two types of active sites: metal sites and an acid sites. The metal sites act to “strip” the polymer chain of its hydrogen (a process called dehydrogenation). Dehydrogenation activates the chain which moves next to an acid site where the acid sites act like scissors, snipping the long polymer chains into smaller pieces. Lastly, the metal sites immediately attach hydrogen to these pieces, stabilizing them and preventing them from rejoining or forming unwanted byproducts. Common catalyst designs pair metals such as platinum, nickel, or ruthenium with acidic supports like zeolites that provide an acid site.
Catalysts commonly pair metals like platinum, nickel, or ruthenium with acidic supports such as zeolites or alumina. For example, the Sarazen Group at Princeton University synthesized a hydrocracking catalyst by encapsulating platinum metals inside the tiny pockets of ZSM-5 zeolite. The ZSM-5 provides acid sites that cut up plastic molecules, and the Pt helps add hydrogen to form stable end-products. By engineering the catalyst formulation – say, using nickel instead of platinum, using materials with different “pocket” sizes to host guest molecules, or changing the density of acid sites on the support – they demonstrate that the products obtained can be rationally controlled.
You may ask, why should we care about hydrocracking? Well, plastics inherently contain energy stored energy from their primary carbon-carbon and carbon-hydrogen bonds. Hydrocracking gives new life to plastic waste which would have otherwise ended up as waste. Instead of plastics sitting for centuries in landfills or fragmenting into microplastics in the ocean, they can be readily turned into valuable resources. Some of the common feedstock we use today that can be obtained from plastics include diesel and gasoline range fuels, benzene, toluene, and xylene (BTX). These could be utilized independently or used as platform chemicals. Another advantage is hydrocracking’s ability to process mixed and contaminated plastic waste streams, which mechanical recycling struggles with. Mechanical recycling requires clean, sorted plastic and often degrades material quality. Hydrocracking is less selective about feedstock purity, making it more suitable for real-world waste.
As mentioned above, hydrocracking process requires the use of novel materials called catalysts. Researchers have been working, for decades on materials that can do hydrocracking faster and more efficiently. For instance, researchers from the Vlachos group at the University of Delaware developed multi-component catalysts that combine a zeolite and a mixed-metal oxide that achieved remarkable results. Interestingly, each component performs poorly when tested separately compared to when they are collocated – elucidating the need for careful engineering of materials to be used for this transformation. This new catalyst was able to break down common single-use plastics at just ~250 °C in a couple of hours – a much lower temperature and shorter time than traditional methods – using about 50% less energy than previous technologies. The output was ready-to-use molecules suitable for jet fuel, diesel, or lubricants.
Recently, the Bo Lin group from Shanghai Tech. University in China reported a breakthrough that address the need for impurity-tolerant catalysts. This discovery was inspired by impurity tolerant traditional MoSx catalysts used in hydrodesulfurization unit in chemical refineries. It’s important to note that the active sites of typical hydrocracking catalysts can be poisoned by contaminants like chlorine or sulfur often present in waste plastics. Particularly, expensive noble metals such as Pt, Ru, and Ni that are used as metals functions in hydrocracking catalysts tend to be susceptible to these poisons – making them incapable of handling mixed plastics feeds.
The Lin group synthesized MoSâ‚“ incapsulated in large pore H-beta zeolite (this avoids the use of noble metals entirely) that can convert 96% of dirty, mixed plastic waste into small alkanes at just 180 °C. This innovation reduces costs and eliminates the need for pre-cleaning or frequent catalyst replacement.
Going forward, polyolefin hydrocracking remains a transformative technology that can be harness to addresses plastic waste accumulation crises while generating valuable products. This process underscores a key principle of the emerging circular economy where today’s trash can be tomorrow’s treasure. With ongoing efforts by researchers and engineers on improvements in catalyst technology and process design, hydrocracking is poised to play a significant role in solving the plastic waste puzzle. It's an indispensable technology for making the world both cleaner and providing energy sources that can be readily recycled.
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