Researchers at Michigan State University (MSU) have shown that sodium chloride (NaCl), or table salt, outperforms expensive chemicals as a catalyst in pyrolysis of polyolefins. They have shared their findings in “Revolutionizing Plastics Chemical Recycling with Table Salt,” recently published in Advanced Sustainable Systems.
The team used table salt to facilitate the low temperature pyrolysis of polyolefins comprised of high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and polypropylene (PP) at the ratio of 4:2:2:3, respectively. They compared the results with the performance of platinum on carbon or aluminium as a catalyst. Platinum has attractive catalytic properties but comes at a premium price of around $29,000/kg whereas table salt is inexpensive at $0.04/kg.
The output of the pyrolysis of polyolefins using salt as a catalyst was 86% in liquid form and 14% gas, as measured by weight. The process did not produce any undesirable solid wax, in contrast with using platinum on carbon or aluminium as a catalyst, which produced 54% and 62% of solid wax, respectively. Wax obtained from the pyrolysis of waste plastics is considered a low-value product and usually undergoes further treatment in a refinery, such as cracking, to get short-chain hydrocarbons for producing useful chemicals, diesel, petroleum, and gasoline.
“This is really exciting,” said associate professor at MSU’s school of packaging Muhammad Rabnawaz. “We need simple, low-cost solutions to take on a big problem like plastics recycling.”
While the process shows promise to help close the plastic waste loop, the pyrolysis oil is not suitable for producing new polymers. Rabnawaz and his team noted that the oil produced when using table salt as a catalyst mostly contained hydrocarbon molecules similar to what’s found in diesel fuel. They believe the approach can be improved ‘so that the liquid products contain chemicals with more valuable applications than being burned as fuel’, according to a MSU statement. The team also noted that further work is needed to fully characterise the gas outputs.
The researchers also showed that metallised plastic can be successfully pyrolysed with table salt with excellent conversion efficiencies. Table salt didn’t outperform a platinum-alumina catalyst but showed similar results, at a fraction of the cost.
Based on a preliminary economic analysis supported by the U.S. Department of Agriculture and MSU AgBioResearch, the team estimated a commercial pyrolysis reactor could see a 3.5-fold increase in revenues by using the new method as compared to pyrolysis without any salt.
The research was partially supported by Conagra Brands, a U.S.-based consumer packaged goods company.