​​​​​​​Cobalt Catalyst Converts Mixed Recycled Plastics into Propane

A cobalt-based catalyst has been shown to break down mixed recycled plastics, including polyethylene (PE) and polypropylene (PP), into propane that can be used as fuel or even as feedstock for new plastics.

Research published Oct. 5 in the open-access journal JACS Au by researchers from MIT, the SLAC National Accelerator Laboratory, and the National Renewable Energy Laboratory detailed how 5 wt % cobalt supported on ZSM-5 zeolite catalyzed the solvent-free hydrogenolysis of commonly used plastics into propane.

The catalyst significantly reduced the formation of methane (CH₄) that appears with bulk cobalt oxide or cobalt nanoparticles on other carriers, the paper noted, with zeolite providing stabilization of the dispersed oxidic cobalt nanoparticles.

With the metal loading optimized, it was also found that 10 wt % Co/ZSM-5 catalyzed the hydrogenolysis of low-density (LD) PE, combinations of LDPE and PP, and post-consumer PE. Hydrogenolysis utilizes H₂ to cleave polymers into shorter alkanes, the study explained.

The long-chain molecules in plastics are held together by highly stable carbon bonds, making recycling a challenge. While current methods can break those bonds into various molecules, sorting out specific usable compounds requires complex refining. But by using the cobalt-zeolite catalyst, researchers were able to selectively break down plastic molecules and turn more than 80% of them into propane.

“Once you have this one compound, propane, you lessen the burden on downstream separations,” explained Yuriy Román-Leshkov, MIT professor of chemical engineering, in an article published Oct. 6 on MIT’s website.

The cobalt-zeolite method is also promising because both materials are inexpensive and plentiful, he added.

Researchers are exploring how this method might fit into existing plastic recycling systems, as well as doing further testing on waste streams including contaminants like inks, glues, labels, and other nonplastic materials.