This catalyst is a golden egg

By infusing eggshells with gold nanoparticles, researchers have made a low-cost, sustainable catalyst that can be reused and eventually recycled (ACS Appl. Mater. Interfaces 2022, DOI: 10.1021/acsami.2c13564). They used the robust “megacatalyst” to detoxify dye waste and run other organic reactions by dropping the eggshell catalyst into reaction solutions, and they could simply remove it by hand when done.

Typically, to make nanoparticle catalysts easy to reuse, chemists anchor them to porous materials such as hydrogels and metal-organic frameworks. These impregnated materials are easy to separate from a reaction mixture, but making them is laborious and expensive. Also, recovering the metal from the support material usually requires time- or energy-intensive methods like magnetic separation and centrifugation.

Anil K. Suresh of SRM University, Andhra Pradesh, and colleagues chose eggshells as supports because of their high porosity and large surface area with a protein layer rich in amino acids. The team dipped waste eggshells collected from restaurants in a suspension containing gold chloride for 6 h, and amino acids on the shells reduced the gold ions to form crystalline gold nanoparticles. Suresh claims this is the simplest synthesis of a supported catalyst ever reported.

The researchers tested the recyclability of the eggshell catalyst by using it to detoxify dye waste from local textile operations. They reused the eggshell 14 times to break down a total of 2.8 L of waste. They also used a gold-infused eggshell to make several grams of a precursor to acetaminophen by hydrogenating 4-nitrophenol, with a conversion efficiency of 99%.

Dipping the eggshell in nitric acid and hydrochloric acid dissolved the protein layer on its surface, releasing the nanoparticles and allowing the team to recoup about 77% of the gold. Suresh says he and his colleagues have also have grafted catalytic silver and copper oxide on eggshells and are now working on methods to do this for platinum and palladium.