Plastic processed into carbon dioxide sorbent

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Rice University researchers have developed a chemical technique that turns waste plastic into an effective carbon dioxide (CO2) sorbent for industry.

sorbent
A Rice University chemist prepares to heat plastic powder combined with potassium acetate to turn it into porous particles that absorb carbon dioxide. (Credit: Jeff Fitlow/Rice University)

Rice chemist James Tour and co-lead authors Rice alumnus Wala Algozeeb, graduate student Paul Savas and postdoctoral researcher Zhe Yuan reported in ACS Nanothat heating plastic waste in the presence of potassium acetate produced particles with nanometre-scale pores that trap carbon dioxide molecules.

These particles can be used to remove CO2 from flue gas streams, they reported.

“Point sources of CO2 emissions like power plant exhaust stacks can be fitted with this waste-plastic-derived material to remove enormous amounts of CO2 that would normally fill the atmosphere,” Tour said in a statement. “It is a great way to have one problem, plastic waste, address another problem, CO2 emissions.”

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A current process to pyrolyze plastic produces oils, gases and waxes, but the carbon by-product is nearly useless, Tour said, whereas pyrolyzing plastic in the presence of potassium acetate produces porous particles that hold up to 18 per cent of their own weight in CO2 at room temperature.

In addition, while typical chemical recycling does not work for polymer wastes with low fixed carbon content in order to generate CO2 sorbent, including polypropylene and high-and low-density polyethylene (the main constituents in municipal waste), those plastics work especially well for capturing CO2 when treated with potassium acetate.

The lab estimated the cost of carbon dioxide capture from a point source like post-combustion flue gas would be $21 a ton, which is far less expensive than the amine-based process used to extract carbon dioxide from natural gas feeds, which costs $80-$160 a ton.

Like amine-based materials, the sorbent can be reused. Heating it to about 75oC releases trapped carbon dioxide from the pores, regenerating about 90 per cent of the material’s binding sites.

Because it cycles at 75oC, polyvinyl chloride vessels are sufficient to replace the expensive metal vessels that are normally required. The researchers noted the sorbent is expected to have a longer lifetime than liquid amines, cutting downtime due to corrosion and sludge formation.

To make the material, waste plastic is turned into powder, mixed with potassium acetate and heated at 600oC for 45 minutes to optimise the pores, most of which are about 0.7nm wide. Higher temperatures led to wider pores. The process also produces a wax by-product that can be recycled into detergents or lubricants, the researchers said.