Berkley Scientist Vaporize Plastic Bags And Bottles To Monomers To Make New, Recycled Plastics

Recycle Vapourize PP PE Bottles Bags to Monomers

03-Sep-24

The catalytic process, discovered by researchers at UC Berkeley, efficiently reduces polymers to chemical precursors, bringing a circular economy for plastics one step closer to reality

The new chemical process can essentially vaporize plastics and turn them into hydrocarbon building blocks for new plastics.

The catalytic process, developed at the University of California, Berkeley, works equally well with the two dominant types of post-consumer plastic waste: polyethylene, the component of most single-use plastic bags; and polypropylene, from microwavable dishes to luggage. It also efficiently degrades a mix of these types of plastics.

John Hartwig, a UC Berkeley professor of chemistry who led the research says “What we can now do, in principle, is take those objects and bring them back to the starting monomer by chemical reactions we’ve devised that cleave the typically stable carbon-carbon bonds. By doing so, we’ve come closer than anyone to give the same kind of circularity to polyethylene and polypropylene that you have for polyesters in water bottles.”

Two years ago, Hartwig and his UC Berkeley team came up with a process for breaking down polyethylene plastic bags into the monomer propylene — also called propene — that could then be reused to make polypropylene plastics. This chemical process employed three different bespoke heavy metal catalysts: one to add a carbon-carbon double bond to the polyethylene polymer 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. But the catalysts were dissolved in the liquid reaction and short-lived, making it hard to recover them in an active form.

In the new process, the expensive, soluble metal catalysts have been replaced by cheaper solid ones commonly used in the chemical industry for continuous flow processes that reuse the catalyst.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.

Synthesizing a catalyst of sodium on alumina, Conk found that it 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 polypropylene 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.

Surprisingly, the tungsten catalyst was even more effective than the sodium catalyst in breaking polypropylene chains.

One key advantage of the new catalysts is that they avoid the need to remove hydrogen to form a breakable carbon-carbon double bond in the polymer, which was a feature of the researchers’ earlier process to deconstruct polyethylene. Such double bonds are an Achilles heel of a polymer, in the same way that the reactive carbon-oxygen bonds in polyester or PET make the plastic easier to recycle. Polyethylene and polypropylene don’t have this Achilles heel — their long chains of single carbon bonds are very strong.

“Think of the polyolefin polymer like a string of pearls,” Hartwig said. “The locks at the end prevent them from falling out. But if you clip the string in the middle, now you can remove one pearl at a time.”

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%. For polyethylene or polypropylene alone, the yield was even higher.

Conk added plastic additives and different types of plastics to the reaction chamber to see how the catalytic reactions were affected by contaminants. Small amounts of these impurities barely affected the conversion efficiency, but small amounts of PET and polyvinyl chloride — PVC — significantly reduced the efficiency. This may not be a problem, however, because recycling methods already separate plastics by type.

Source: U C Berkley:- Prof. Hartwig, graduate student Richard J. Conk, chemical engineer Alexis Bell, who is a UC Berkeley Professor of the Graduate School, and their colleagues have published the details of the catalytic process on Aug. 29 in the journal Science.