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Study: Solar reforming of plastics using acid-catalyzed depolymerization. Image Credit: Clare Louise Jackson/Shutterstock.com

A recent study by researchers from the University of Cambridge, published in Joule, proposed a new solar-driven strategy that transforms plastic waste into hydrogen fuel and other valuable chemicals.

By combining waste treatment with renewable energy conversion, the approach demonstrates a scalable and economically viable pathway toward circular chemical production and sustainable hydrogen generation.

Global Plastic Waste: The Crisis

Global plastic production has alarmingly exceeded 400 million metric tons annually, posing a serious environmental challenge. Most plastic waste is dumped into natural ecosystems or landfills, and despite awareness, recycling rates remain low.

Conventional methods, such as mechanical recycling, are limited by feedstock purity and material degradation, while chemical recycling methods often require high energy input or generate secondary waste streams. This makes them unsustainable options for addressing the plastic waste crisis.

Photoreforming has emerged as a promising approach for converting organic waste into hydrogen and value-added chemicals using light-driven catalysis. However, existing systems typically operate under neutral or alkaline conditions, which require significant chemical inputs and yield mixed, low-selectivity products. Acid hydrolysis, widely used in industrial processes, offers advantages such as catalytic operation and direct recovery of high-purity monomers, but its integration with solar reforming has remained largely unexplored.

This work introduces a novel strategy that combines acid-catalyzed depolymerization with visible-light photoreforming. The system enables efficient conversion of condensation polymers into hydrogen and useful chemicals using an acid-stable photocatalyst and recycled sulfuric acid. The study establishes a new framework for circular plastic upcycling under mild and scalable conditions.

Methodology and Approach

The team developed a two-step process, which involves acid hydrolysis of plastics followed by photocatalytic reforming. In the first step, plastics such as Polyethylene Terephthalate (PET), Nylon 66, and polyurethane are depolymerized using concentrated sulfuric acid at elevated temperatures.

The second step uses a newly designed photocatalyst composed of cyanamide-functionalized carbon nitride integrated with cobalt-promoted molybdenum sulfide (CoMoS2–CNx). This catalyst is engineered to remain stable under strongly acidic conditions, overcoming a major limitation of previous catalysts. It enables the formation of active metal sulfide sites within the carbon nitride matrix.

The depolymerized solutions are diluted and irradiated in the presence of the catalyst under controlled conditions. Photoreforming experiments are conducted under visible light irradiation using 405 nm LEDs or simulated sunlight. The products obtained are analyzed using gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR).

Additional studies include pH-dependent performance analysis, long-term catalyst stability tests, and scale-up demonstrations, which are also performed to evaluate the feasibility of integrating solar and LED-driven systems for continuous operation.

Results and Discussion

The system demonstrates efficient conversion of plastic-derived intermediates into hydrogen and valuable chemicals. For PET-derived ethylene glycol, hydrogen production reached 1.9 mmol g-1 under LED irradiation, accompanied by high selectivity toward acetic acid, which accounted for approximately 89 % of the liquid products. Nylon 66 and polyurethane also showed strong performance, producing up to 4.2 mmol g-1 hydrogen within 24 hours.

Understanding the critical role of acidic conditions is one of the key findings from this study. Acidic media significantly enhance hydrogen production and enable highly selective formation of acetic acid when compared to neutral or alkaline environments. The product distribution data shows that neutral and alkaline conditions produce complex mixtures, whereas acidic conditions favor a single dominant product.

Mechanistic studies reveal that ethylene glycol undergoes light-driven oxidation to glycolaldehyde, followed by acid-catalyzed rearrangement to acetic acid. This dual pathway, combining photocatalysis and acid-mediated chemistry, improves both efficiency and selectivity. The photocatalyst exhibits strong durability, maintaining activity over 11 days with minimal metal leaching. Structural changes during operation enhance catalytic performance after initial cycles. The system achieves a high apparent quantum yield of up to 9 %, indicating efficient light utilization.

Scalability is demonstrated through larger reactor experiments, achieving sustained hydrogen production over multiple days. Importantly, sulfuric acid recovered from spent car batteries performs comparably to fresh acid, confirming the feasibility of circular resource integration. A technoeconomic analysis indicates that the process can become profitable when co-products, such as terephthalic acid and acetic acid, are considered.

Conclusion

This study establishes acid-catalyzed photoreforming as a powerful, scalable approach for upcycling plastic waste. By integrating acid hydrolysis with photocatalytic reforming, the system overcomes key limitations of existing recycling technologies.

The use of an acid-stable, non-precious metal photocatalyst enables efficient operation under conditions that were previously unsuitable for photoreforming. The approach offers multiple advantages, including high hydrogen yields, strong product selectivity, compatibility with diverse plastics, and the ability to reuse waste sulfuric acid. The demonstrated scalability and favorable economic outlook further support its potential for real-world deployment.

The work provides a framework for integrating waste management with renewable energy technologies. Future research should focus on continuous flow reactor design, improved catalyst optimization, and integration with industrial processes. The concept can also be extended to other waste streams and catalytic systems, supporting the development of sustainable and circular cleantech solutions.

Journal Reference

Kwarteng, P. K., Liu, Y., et al. (2026). Solar reforming of plastics using acid-catalyzed depolymerization. Joule, 102347. DOI: 10.1016/j.joule.2026.102347 https://www.cell.com/joule/fulltext/S2542-4351%2826%2900031-0

By Akshatha Chandrashekar

Reviewed by Laura Thomson