KEY CONCEPTS
• A potential sustainable approach for upcycling used plastics is to convert them into base oils.
• A heterogeneous catalyst based on mesoporous silica has been found to convert high-density polyethylene (HDPE) to hydrocarbons with comparable molecular weights to base oils. 
• The catalyst acts in a similar manner to enzymes by cleaving specifically sized chains from HDPE. 

Concern about pollution has led the plastics industry to find ways to be more sustainable. This has involved developing processes for reusing plastics or finding new applications for plastics (see Figure 2).


Figure 2. Researchers used a process known as upcycling to convert the plastic resin HDPE into hydrocarbons with comparable molecular weights to base oils. Figure courtesy of Ames Laboratory.

Mechanical recycling has been the most commonly used approach—but it has limitations. Aaron Sadow, senior scientist at the Ames Laboratory and professor of chemistry at Iowa State University in Ames, Iowa, says, “Recycling of plastics such as high-density polyethylene (HDPE) often produces resins that do not have comparable physical properties to virgin material. Recycled resins are much less robust. The cost of recycling also is higher than the production of resins such as HDPE from their respective hydrocarbon monomers.”

A new approach to reusing plastics is known as chemical upcycling. Frédéric Perras, associate scientist at the Ames Laboratory, says, “Recycling has been restricted because lower quality and lower-cost plastic resins are produced that do not have the properties of the origin material. Upcycling is a strategy to make something else out of used plastics and find a market where the derivative has value.”

Theoretically, one product of value that can be produced from HDPE is base oil. They are both hydrocarbons, though base oils have lower molecular weights than HDPE.

Efforts to produce base oils from alternative sources are not new. In a previous TLT article,¹ research was discussed about how a biofeedstock that is not part of the food supply, switchgrass, can be pyrolyzed to synthesize a bio-oil. Analysis showed that the bio-oil is a complex mixture that the researchers hoped could be converted to a usable alternative such as diesel or gasoline.

In evaluating options for upcycling of HDPE, Sadow and his colleagues examined how nature is able to deconstruct macromolecules such as proteins and cellulose. He says, “In nature, enzymes that break down macromolecules operate by a processive process. In a processive mechanism, the enzyme binds the macromolecular chain in a catalytic channel, which positions the chain close to an active site and facilitates a cleavage reaction, releasing a smaller segment. This is followed by the remaining macromolecular chain moving to the enzyme’s active site to repeat the process and release the same size segment.”

Sadow points out that by adsorbing onto the substrate, the enzyme reduces the kinetic penalty, making the process more efficient. Enzymes also operate in this fashion to fix mistakes made on DNA chains.

The challenge for researchers is to determine if an inorganic catalyst can be designed to perform a similar processive polymer deconstruction on a plastic resin. This objective has now been achieved.

Mesoporous silica
Wenyu Huang, lab scientist at Ames Laboratory and associate professor of chemistry at Iowa State University, developed a heterogeneous catalyst based on mesoporous silica that can convert HDPE to hydrocarbons with molecular weights similar to mineral base oils used in lubricants using a processive polymer deconstruction reaction. Huang says, “Conventional industry catalysts have been using porous materials for a long time. However, control of the location of active sites in porous materials is extremely challenging. In working with porous silica, we took a nanoscience approach and built the catalyst from the bottom up by starting with platinum particles supported on a solid silica sphere and then adding a mesoporous shell to it. Our objective was to mimic an enzyme active site.”

The catalyst has 2.4 nanometer diameter pores that are placed radially from the silica sphere. Each of the pores is 110 nanometers in length. Platinum nanoparticles are placed at the terminal end of linear channels to catalyze a hydrogenolysis reaction, which leads to cleavage of specifically sized chains from HDPE.

The researchers used ¹³C solid-state nuclear magnetic resonance (SSNMR) to monitor the processive polymer deconstruction process. Perras says, “We determined that the orientation of the carbon chains present in HDPE formed conformers that enabled the polymer to thread through the linear channels of the mesoporous silica.”

The researchers evaluated the mesoporous silica catalyst at 250 C and 300 C under 1.38 megapascals of hydrogen gas for 24 hours. Gas chromatography and gel permeation chromatography were used to evaluate the reaction products.

A bell-like distribution of hydrocarbons was found with the 250 C temperature that ranged from C9 to C35 with a peak at C14. In an interesting experiment run at 300 C, the carbon distribution was narrower and was centered around C16.

Sadow says, “The pore size of the catalyst had a substantial effect on the carbon chain distribution produced from the deconstruction of HDPE. A pore size of 1.7 nanometers yielded shorter carbon chain distributions, while larger pore sizes (2.5 and 3.4 nanometers) afforded higher carbon chain distributions. Catalyst activity was better at the higher temperature where more conversion to hydrocarbons was obtained.”

The researchers will be working to find the right parameters that can be used to control the carbon distribution of the hydrocarbons produced by the processive polymer deconstruction reaction. Sadow says, “We will be correlating the architecture of the catalyst, pore diameter and pore length to determine how these parameters can be optimized to produce a specific hydrocarbon distribution.”

This discovery has the potential for lubricant suppliers to consider sourcing renewable base oils from plastic resins such as HDPE. Additional information can be found in a recent paper² or by contacting Sadow at sadow@iastate.edu.

REFERENCES
1. Canter, N. (2013), “Production of bio-oil,” TLT, 69 (4), pp. 10-11.
2. Tennakoon, A., Wu, X., Paterson, A., Patnaik, S., Pei, Y., LaPointe, A., Ammal, S., Hackler, R., Heyden, A., Slowing, I., Coates, G., Delferro, M., Peters, B., Huang, W., Sadow, A. and Perras, F. (2020), “Catalytic upcycling of high-density polyethylene via a processive mechanism,” Nature Catalysis, 3, pp. 893-901.