Gallium’s quirky liquid-state properties have pushed that element into the scientific spotlight recently, as researchers have tapped the liquid metal for applications in stretchable electronics and three-dimensional printing. Now gallium is back in the news, this time as a catalyst.
Researchers in Germany report that liquid droplets of Ga-Pd alloys function as active and durable catalysts for alkane dehydrogenation. That industrial-scale reaction converts low-value alkanes to higher-value olefins, compounds with C=C bonds that are used to make polymers and chemicals (Nat. Chem. 2017, DOI: 10.1038/nchem.2822).
Gallium and some of its alloys exhibit a handful of unique properties, such as a tendency to remain liquid over an enormous temperature range—about 2,000 °C. The metal also has a knack for spontaneously forming an ultrathin oxide skin that stabilizes liquid droplets but easily breaks, allowing the metal to flow momentarily until the skin re-forms around the liquid.
A team including Nicola Taccardi and Peter Wasserscheid of Friedrich-Alexander University, Erlangen-Nürnberg, took advantage of those properties of gallium and its ability to dissolve numerous metals, generating alloys with various concentrations of palladium, a catalytically active metal. Then they deposited the liquid metals onto porous glass, forming supported liquid metal catalysts, and used them in a test reaction: butane dehydrogenation.
Homogeneous, solution-phase catalysts have the advantage of possessing clearly defined active sites and mechanisms. The aim of the new work was to create a hybrid catalyst with these advantages that can also be easily separated from reaction products and reused, a task that’s currently difficult to carry out with homogeneous versions. Various researchers have attempted this feat previously. But the stability of their liquid-phase catalysts typically limited reactions to roughly 200 °C and below, far lower than temperatures required in many industrial catalytic processes.
The Friedrich-Alexander team ran test reactions at roughly 450 °C and found that gallium-rich catalysts, for example, ones with a Ga-to-Pd ratio of 10:1, had high activity for butane dehydrogenation, produced butene with high selectivity (85%), and remained in the liquid state even after 20 hours of reaction. In addition, they did not accumulate the layer of carbon (coke) that gunks up and deactivates commercial Pt-Al2O3 and Cr2O3-Al2O3 dehydrogenation catalysts.
“Supported liquid metal catalysis is an interesting concept,” says Arizona State University’s Jingyue (Jimmy) Liu, a catalysis specialist. He is particularly intrigued by the researchers’ atomic-level description of their catalyst as individual isolated Pd atoms supported on the surface of a Ga-Pd liquid metal. He adds, “Systematic investigations are needed to better understand reaction processes in such a system and to provide deeper insights into the nature of the newly synthesized liquid metal.”