The University of California, Berkeley developed new catalytic process to recycle polyethylene (PE) and polypropylene (PP), two dominant types of post-consumer plastic waste. This new chemical process can essentially vaporize the plastics and turn them into hydrocarbon building blocks for new plastics.

The work was funded by the Department of Energy (DE-AC02-05CH11231). Details of the catalytic process were published in the journal Science.

The authors include John Hartwig, a UC Berkeley professor of chemistry who led the research, graduate student Richard J. “RJ” Conk, chemical engineer Alexis Bell, who is a UC Berkeley Professor of the Graduate School, and their colleagues.

John Hartwig (left), a UC Berkeley professor of chemistry who led the research, and graduate student Richard J. “RJ” Conk. (Source: UC Berkeley)

Previous chemical process with challenges

Two years ago, Hartwig and his team came up with a process for breaking down PE plastic bags into the monomer propylene, also called propene, that could then be reused to make PP plastics.

This chemical process employed three different bespoke heavy metal catalysts. One is to add a carbon-carbon double bond to the PE and the other two to break the chain at this double bond and repeatedly snip off a carbon atom and, with ethylene, make propylene (C3H6) molecules until the polymer disappeared.

However, the catalysts were dissolved in the liquid reaction and short-lived, making it hard to recover them in an active form.

New process to overcome

In the new process, the expensive, soluble metal catalysts have been replaced by cheaper solid ones commonly used in the chemical industry for continuous flow processes that reuse the catalyst. Continuous flow processes can be scaled up to handle large volumes of material.

Conk adjusts a reaction chamber in which mixed plastics are degraded into the reusable building blocks of new polymers. (Source: UC Berkeley)

The synthesized catalyst of sodium on alumina is found efficiently broke or cracked various kinds of polyolefin polymer chains, leaving one of the two pieces with a reactive carbon-carbon double bond at the end.

A second catalyst, tungsten oxide on silica, added the carbon atom at the end of the chain to ethylene gas, which is constantly streamed through the reaction chamber, to form a propylene molecule.

The latter process, called olefin metathesis, leaves behind a double bond that the catalyst can access again and again until the entire chain has been converted to propylene.

The same reaction occurs with PP to form a combination of propene and a hydrocarbon called isobutylene. Isobutylene is used in the chemical industry to make polymers for products ranging from footballs to cosmetics and to make high-octane gasoline additives.

In addition, the tungsten catalyst was even more effective than the sodium catalyst in breaking PP chains.

Effective even with mixed polymer

One key advantage of the new catalysts is that they do not require removing hydrogen to form a breakable carbon-carbon double bond in the polymer.

Examples of the types of plastics the new process can handle. (Source: UC Berkeley)

The two catalysts together turned a nearly equal mixture of polyethylene and polypropylene into propylene and isobutylene, both gases at room temperature, with an efficiency of nearly 90%. If processing polyethylene or polypropylene alone, the yield will be even higher.