Tyre chemical 6PPD faces replacement hunt as MOFs turn up in nature

Researchers have identified the first known naturally produced metal–organic frameworks (MOFs, crystalline networks of metal ions and organic linkers) inside cable bacteria on the sea floor — structures that conduct electricity as efficiently as some of the best synthetic polymers and MOFs in use today. The same week, tyre manufacturers in the US have narrowed a replacement search for 6PPD, a preservative used in virtually every tyre for 60 years, to roughly 40 candidate compounds after its breakdown product was linked to mass salmon deaths.

Chemistry World's Chemical Breakdown podcast, hosted by science media producer Mariana Kneppers with business editor Phillip Broadwith and news editor Patrick Walter, examined both stories. Each carries direct consequences for R&D portfolios: one for materials design, one for a regulated chemical facing substitution across three jurisdictions.

Why is 6PPD so hard to replace?

6PPD protects tyre rubber from ozone. The rubber contains carbon–carbon double bonds that ozone attacks, causing cracking and loss of grip. 6PPD preferentially reacts with ozone at the tyre surface and must also diffuse through the solid rubber matrix, replenishing the surface as deeper molecules follow a concentration gradient outward.

That mobility requirement constrains any replacement, Broadwith explained. A substitute must react with ozone, protect the rubber, and diffuse through the solid tyre. It cannot simply be covalently bonded in place.

The problem emerged over roughly two decades of unexplained salmon deaths. Investigators did not identify 6PPD-quinone, the toxic breakdown product, until about 2020. Regulators in the US, Canada and the EU are now moving to restrict the chemical.

What do we know about the toxicity?

The toxicity profile is narrow and poorly understood. 6PPD-quinone kills coho salmon, found mostly in the northwestern US, and rainbow trout. Other fish species show far lower sensitivity, and researchers do not yet know the mechanism behind the species selectivity.

6PPD itself degrades quickly in air through oxidation — one reason it was long assumed harmless. But in anaerobic river sediments it persists far longer, though not at PFAS levels of persistence. When sediment is stirred up, the chemical re-enters the water, oxidises back to the quinone, and the quinone is toxic at very small concentrations.

The ozone-reactive core of 6PPD is a paraphenylenediamine (PPD), an aromatic ring with alkyl groups attached through amine groups. The family includes variants already used in aircraft tyres and other rubber applications. Some family members show little toxicity to coho salmon or rainbow trout, but Broadwith cautioned that toxicology on other fish species remains incomplete.

Who is searching for alternatives?

A group of US tyre manufacturers has screened candidate replacements first for tyre performance, ruling out many compounds and leaving about 40 under investigation — some PPDs, some from entirely different molecular families. The most promising candidates are now entering detailed toxicology work.

The complication is regulatory scope. Europe and Canada are considering restrictions on the PPD family as a whole. If that happens, even the PPDs that appear safer would become unsuitable as tyre additives, forcing development away from the path of least resistance.

News editor Patrick Walter framed the wider risk as regrettable substitution — replacing a problem chemical with a structurally similar one that causes comparable harms, as when bisphenol A was replaced by BPS, which showed similar toxicity in human and animal models.

How do bacteria build MOFs?

The second story rewrites a central assumption about MOFs. Until now, these materials were considered a purely human invention — the 2024 Nobel Prize in Chemistry went to three researchers for their discovery and development. Cable bacteria, filamentous microorganisms living in sea-floor sediment, biosynthesise nickel-organic frameworks from nickel and dithiolene ligands, linking them into long conductive strings twined into nanoribbons.

The scale is notable. The filaments are 1–2 micrometres thick but reach 2–3 centimetres in length — roughly 10,000 times the bacteria's own width. The bacteria embed their cables in sediment, metabolise sulfides there, and transport the resulting electrons up the conductive filaments, like power lines, into the cells to drive metabolism. The process is chemosynthesis: energy drawn from chemical reactions rather than light.

The conductivity finding matters because synthetic MOF research has concentrated on three-dimensional structures and on gas storage — a gram of some MOFs has a surface area comparable to a football field — rather than on electrical conduction. The bacterial ribbons, by contrast, rival the best conductive polymers and even some metals. Broadwith noted the parallel with synthetic conductive polymers that use similar sulfur-based dithiolene motifs, a case of convergent design arriving independently at the same chemical arrangement.

The evolutionary origin, Walter suggested, is likely repurposing. Nickel-dithiolene complexes already exist in other bacteria as simple cofactors shuttling electrons between proteins; cable bacteria appear to have chained that existing motif into macroscopic wires, much as evolution adapted the protein actin into cellular filaments for division and structure.

What comes next?

For 6PPD, the near-term decisions are toxicological: whether the surviving 40 candidates clear fish-safety testing before regulators restrict the entire PPD family. For the natural MOFs, the research team is already exploring sustainable and biologically grown electronics, though scaling the nanoribbon structure will require either persuading bacteria to produce it or finding a chemical route, since synthetic MOFs so far form as crystals rather than ribbons.

In the same roundup, Chemistry World reported warnings that England's student loan system could push science graduates out of research, Chinese work using covalent organic frameworks to improve nitrate-to-ammonia reduction, X-ray pulse tracking of fleeting intermediates in natural penicillin synthesis, and the removal of an AI-generated entry from the Small World Microscopy Competition's video category.