Scientists discover cheap catalyst to upcycle hard-to-recycle plastics into biodegradables

22 Feb 2023 --- Ames National Laboratory researchers in the US have created a catalyst that transforms aliphatic hydrocarbons, commonly found in plastics, into chemicals that are easier to recycle, biodegradable and of higher value than their original form. 

The catalyst converts materials such as motor oil, soft plastics, water or milk bottles, caps and natural gas into useful materials repurposed into biodegradable packaging. 

Aliphatic hydrocarbons make up a lot of petroleum and refined petroleum products, such as plastics and motor oils. Currently, precious metal catalysts and aggressive reagents are used to transform aliphatic hydrocarbons into valuable materials with high costs and limited selectivity. 

“Previous catalysts break and shorten hydrocarbon chains. This reaction mostly keeps chains intact and introduces aluminum,” Aaron Sadow, lead researcher and scientist at Ames National Laboratory tells PackagingInsights.

The hydrocarbons “don’t have other functional groups, which means they are not easy to biodegrade. It has long been a goal in the field of catalysis to take these materials and add other atoms, such as oxygen, or build new structures from these simple chemicals.” 

Aliphatic hydrocarbons
Sadow uses zirconium- and aluminum-based reagents, which synthesizes the aluminum without creating waste byproducts. Aluminum is the most abundant metal on earth, which lowers the cost and accessibility of breaking down hard-to-recycle plastics. 

“The reaction is new, so there really aren’t comparable catalysts based on rate, or turnover, or yield.” says Sadow.

Chart from the study explaining how the catalyst turns aliphatic hydrocarbons from plastics into new biodegradable textiles. Aliphatic hydrocarbons do not contain functional groups, which the catalyst is designed to introduce. Adding functional groups to these hydrocarbon chains can drastically affect their properties and make the materials recyclable.

“Methane in natural gas is the simplest of hydrocarbons with nothing but carbon-hydrogen bonds. Oils and polymers have chains of carbon atoms, linked by carbon-carbon (CC) bonds,” Sadow explains. 

Developing the catalysis 
The zirconium alkoxide-based catalyst precursor is air-stable, readily available and activated in the reactor. “So unlike a lot of early organometallic chemistry that’s extremely air sensitive, this catalyst precursor is easy to handle,” asserts Sadow. 

According to the study, the catalyst and reactant are advantageous in terms of environmental sustainability and cost. The discovery can allow various commercially beneficial products to be created from previously unuseful and environmentally damaging commodities. 

“The other way to make the molecules we are making is via hydroalumination of alkenes, whereas we are using saturated hydrocarbons,” Sadow adds.

The researchers state this discovery is a step toward affecting the physical properties of a variety of plastics, such as making them stronger and easier to color. 

“As we develop the catalysis more, we expect that we’ll be able to incorporate more and more functional groups to affect the physical properties of the polymers,” they write. 

Usual process
The conventional way to add atoms to hydrocarbon chains requires considerable energy inputs. 

First, petroleum is “cracked” with heat and pressure into small building blocks. Next, those building blocks are used to grow chains. Finally, the desired atoms are added at the end of the chains. 

This new approach converts existing aliphatic hydrocarbons directly without cracking and at low temperatures. 

Sadow’s team previously used a catalyst to break the CC bonds in these hydrocarbon chains and simultaneously attached aluminum to the ends of the smaller chains. Next, they inserted oxygen or other atoms to introduce functional groups. To develop a complementary process, the team found a way to avoid the CC bond-breaking step.

“Depending on the starting material’s chain length and the desired properties of the product, we might want to shorten chains or simply add the oxygen functional group,” Sadow continues. 

By avoiding the “cracking” process, the scientists could transfer the chains from the catalyst to aluminum and then add air to install the functional group and create the reusable biodegradable material. 

Edited by Sabine Waldeck