Highlights

• University of Pennsylvania researchers introduce SES-Q (Separations by Excited State Quenching), a photochemical approach that exploits light-driven electronic differences between rare earths rather than relying on conventional size-based solvent extraction.
• The method could disrupt China's rare earth processing monopoly by using low-intensity LEDs to trigger element-specific reactions, potentially separating difficult pairs like Dy/Y or Dy/Ho that are nearly impossible to separate today.
• While promising, SES-Q remains early-stage science requiring scalability, faster kinetics, materials robustness, and integration with existing industrial processes before commercial viability can be achieved.

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Light Over Liquids: New Preprint (opens in a new tab) Proposes a Photochemical Path to Break Rare Earth Processing Bottlenecks

A new preprint led by Kevin P. Ruoff and Professor Eric J. Schelter of the University of Pennsylvania—spanning the Vagelos Laboratory for Energy Science and Technology (opens in a new tab) and UPenn’s Departments of Chemistry, Earth & Environmental Science, and Chemical & Biomolecular Engineering (opens in a new tab)—introduces a provocative idea: rare earth elements might be separated using light-driven chemistry, not brute-force solvent extraction.

Titled “Separations by Excited State Quenching (SES-Q): An Emerging Concept for Reactive Rare Earth Separations,” the ChemRxiv manuscript synthesizes years of experimental evidence to argue that electronic and photophysical differences between rare earths could be harnessed to selectively sort them, potentially challenging the entrenched, China-dominated processing paradigm.

In simple terms, instead of separating rare earths by tiny size differences—an approach that requires hundreds or thousands of extraction cycles—the authors propose using how different rare earths interact with light to make some react faster than others. Those reaction differences could then be tied to physical separations such as precipitation, solubility changes, or chromatography.

Why This Matters: A Chemical Root of a Geopolitical Problem

China’s dominance in rare earth processing is not just a mining story; it is a chemistry story. Conventional solvent extraction works, but it is slow, capital-intensive, chemically wasteful, and difficult to replicate outside China’s established infrastructure. The UPenn team argues that this technological lock-in exists because industry has focused almost exclusively on ionic size, ignoring other intrinsic properties—especially excited-state behavior.

Rare earths differ in their 4f-electron configurations. Some, like europium or terbium, readily accept or quench excited energy states; others, like yttrium, cannot. The SES-Q concept proposes exploiting those differences so that, under light irradiation, certain rare earth complexes undergo chemical changes while others remain inert. Over time, mixtures could be biased toward separable products.

What the Study Shows

Rather than reporting a single new experiment, the paper systematically reviews and re-interprets dozens of photochemical systems—including photoisomerization, photocycloaddition, and light-driven redox reactions—where reaction rates depend on the identity of the rare earth. Key findings include:

• Excited-state quenching is element-specific, driven by electronic structure rather than size.
• In several systems, neighboring rare earths (e.g., Dy vs. Y or Dy vs. Ho), which are notoriously difficult to separate today, show dramatically different photochemical behaviors.
• Light-driven processes could, in principle, use low-intensity LEDs rather than lasers, making them more practical than earlier photochemical separation attempts.

Taken together, these observations support SES-Q as a conceptual separation strategy, not yet a commercial process.

What It Would Take to Commercialize SES-Q

The authors are explicit—and Rare Earth Exchanges agrees—that SES-Q is early-stage science. To move from preprint to plant, several hurdles must be cleared:

1. Reaction Discovery to Engineering Translation

Laboratory photochemical reactions must be coupled to scalable physical separations (e.g., selective precipitation or phase transfer).

2. Throughput and Kinetics

Reaction rates must be fast enough to compete with industrial solvent extraction, not just demonstrate selectivity.

3. Materials Robustness

Many candidate systems involve air- or moisture-sensitive compounds. Industrial systems must tolerate real-world feedstocks and impurities.

4. Recyclability and Cost

Ligands and photoactive materials must be reusable, inexpensive, and compatible with circular processing.

5. Integration with Existing Flowsheets

SES-Q is more likely to augment current processes—reducing stages or targeting specific separations—than replace solvent extraction outright.

Limitations and Controversial Points

This work is a ChemRxiv preprint, meaning it has not yet undergone peer review. No full rare earth separation has been demonstrated at scale, and many examples discussed rely on solid-state or idealized systems. Skeptics may question whether photochemical selectivity can survive the complexity of real ores and recycled materials.

Still, dismissing SES-Q as impractical would repeat a familiar mistake: assuming that rare earth separation chemistry is “solved” simply because it exists.

REEx Takeaway

This study does not end China’s rare earth processing monopoly—but it challenges the chemical assumptions that sustain it. If even part of SES-Q proves scalable, it could lower barriers for non-Chinese processors and introduce entirely new design space for rare earth separation. For investors, policymakers, and technologists, that alone makes this work worth watching.

Source: Ruoff, K. P.; Schelter, E. J. Separations by Excited State Quenching (SES-Q): An Emerging Concept for Reactive Rare Earth Separations. ChemRxiv Preprint, 2025

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