With global plastic production reaching 413.8 million metric tons in 2024, finding scalable chemical recycling methods is critical. Polyolefins—mainly polyethylene (PE) and polypropylene (PP)—make up approximately 55% of global plastic waste and remain notoriously difficult to transform in a controlled way chemically.
A new perspective article published in the journal Engineering by the Institute for Cooperative Upcycling of Plastics (iCOUP)—an Energy Frontier Research Center based at Ames National Laboratory and Iowa State University—highlights the differences in reaction mechanisms of hydrocracking and hydrogenolysis and their rapid advancements in polyolefin upcycling. The article outlines the “make-or-break” challenges the field must overcome to transition from lab-scale batch reactions to real-world deployment.
The authors center the discussion on two hydrogen-based approaches, hydrocracking and hydrogenolysis, which are often grouped, even though they rely on fundamentally different chemistries.

• Hydrocracking: A metal and acid teamwork approach, where metal sites handle hydrogenation/dehydrogenation and acid sites drive cracking and isomerization. This route tends to favor branched products, which can be useful for fuel-range blends.
• Hydrogenolysis: A metal-site catalyzed process, where C–C bonds are cleaved on the metal surface, offering a cleaner path to more linear n-alkanes and narrower product distributions.

The authors note that hydrocracking and hydrogenolysis are not in direct competition; each offers unique advantages, and the optimal choice depends on the desired product and the process’s capabilities.
The perspective does not gloss over what is holding the field back. While proof-of-concept results for both routes have advanced rapidly, scale-up is bottlenecked by issues often underestimated in ideal lab settings. The researchers highlight three critical hurdles:

1. Catalyst deactivation caused by impurities in real plastic feeds.
2. Severe mass- and heat-transfer issues in highly viscous molten plastics within reactors.
3. Analytical blind spots that make closing the carbon balance difficult.

To push the field toward real deployment, the team defines five interlocking research priorities:

1. Impurity-robust catalysts: Developing affordable catalysts that can tolerate contaminants in real plastic waste to avoid costly reactor shutdowns.
2. Continuous operation with real feeds: Moving to continuous flow reactors capable of handling thick, melted plastics without clogging or overheating.
3. Lower-pressure hydrogen integration: Designing catalysts that operate at lower pressures and pairing them with green hydrogen sources to cut costs and carbon emissions.
4. Unified, high-resolution analytics: Standardizing how products are measured and reporting data consistently so different labs can easily compare results.
5. Data-driven process design: Using machine learning and advanced modeling to accelerate catalyst optimization.

“If these priorities are met, hydrocracking could reliably provide branched alkanes, while hydrogenolysis could feed linear-alkane supply chains, turning plastic waste from an environmental liability into a versatile carbon resource,” the authors conclude.
The paper “Hydrogenolysis Versus Hydrocracking for Polyolefin Upcycling,” is authored by Ruoxi Zhang, Aaron D. Sadow, Wenyu Huang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.12.041. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.