Reviving iron catalysts for green chemistry

When the French emperor Napoleon III offered funds for a new butter substitute to feed his army and subjects in 1866, he unknowingly spurred development of the margarine industry, now valued in the billions of dollars. The first margarine consisted of churned beef tallow and milk, but French and German chemists soon found a way to turn inexpensive liquid vegetable oils into solid saturated fats through a process called catalytic alkene hydrogenation—a reaction that turns carbon double bonds into single bonds with the help of hydrogen gas and a metal catalyst (Fig. 1). Such hydrogenation processes also have wide applications beyond the food industry and are now one of the key technologies used in petrochemical refining, pharmaceutical synthesis and biofuel production.

The Achilles’ heel of hydrogenation chemistry remains its reliance on expensive platinum-series catalysts. The limited supply of these metals and their volatile pricing, as well as increasing awareness of their possible environmental toxicity, has prompted an intense search for alternative ways to add hydrogen to unsaturated chemical bonds. Yoichi Yamada and Yasuhiro Uozumi from the RIKEN Center for Sustainable Resource Science, in collaboration with researchers from McGill University in Canada and Japan’s Institute for Molecular Science, have now developed an efficient iron-based catalyst that promises to fundamentally change industrial hydrogenation processes¹.

Iron (Fe) is the second most abundant metal in the Earth’s crust. It has an essential role in a multitude of biological processes and is used on large scales as a catalyst for ammonia fertilizer production. Iron can also catalyze hydrogenation reactions—but only under the right conditions. Iron catalysts require far higher gas pressures in comparison to the conventional metal catalysts, notes Uozumi, and the iron must remain in its pure metallic Fe(0) state. In fact, any contact with water or air during hydrogenation causes the iron catalyst to oxidize into iron oxide, which is not catalytic.

Because such conditions run counter to the demands of large-scale food manufacturing and other industries, iron has generally been ruled out as a hydrogenation catalyst. However, new research into iron-based nanoparticles has prompted chemists to take a second look at the potential of iron catalysts and their economic and environmental advantages. Such nanoparticles expose a significantly higher proportion of the active iron surfaces to chemical reactants than other structures and can also be attached to permanent substrates to boost catalyst recycling. Unfortunately, iron nanoparticles remain highly susceptible to oxidation and can even become explosive, necessitating special handling.

Yamada and Uozumi are specialists in the preparation of polymer resin coatings that allow metal particles to perform as catalysts in water. The pair met Chao-Jun Li and Audrey Moores, both experts in nanoscale catalysts, at the 2012 RIKEN–McGill University Scientific Workshop, and the group decided to combine their expertise to tackle the challenge of stabilizing iron nanoparticles with polymers.

According to Uozumi, optimal polymers for iron nanoparticles must be amphiphilic, containing both water-repelling and attracting components to protect the metal and at the same time ensure compatibility with aqueous mixtures. To achieve this goal, the researchers developed a synthetic procedure using polystyrene beads just 100 micrometers in diameter coated with polyethylene glycol. Heating an iron precursor in the presence of these beads results in thermal decomposition of the iron and the formation of iron nanoparticles coated with polyethylene glycol, which inhibits oxidation (Fig. 2). As part of the process, the protected nanoparticles and polystyrene beads are embedded in a resin that is sturdy and recyclable.