From: lessgovt@gmail.com   
      
   How chemists are tackling the plastics problem   
   By Casey Crownhart, Nov. 30, 2022, MIT Tech Review   
      
   Over 400 million metric tons of plastic are produced each year worldwide. Of   
   that, less than 10% is recycled, about 30% remains in use for some time, and   
   the rest either finds its way to landfills or the environment, or is   
   incinerated. Plastics are also    
   a significant driver of climate change: their production accounted for 3.4% of   
   global greenhouse gas emissions in 2019. Not only does recycling keep plastics   
   out of landfills and oceans, new ways to produce building blocks for plastics   
   could help cut    
   emissions as well.    
      
   “What we’re really trying to do is think about ways that we can see these   
   waste plastic materials as a valuable feedstock,” says Julie Rorrer, a   
   postdoctoral fellow in chemical engineering at MIT and one of the lead authors   
   of the recent research.   
      
   A major benefit of the new approach Rorrer and her colleagues developed is   
   that it works on the two most common plastics used today: polyethylene and   
   polypropylene. Into the reactor goes a mixture of the plastics that make   
   bottles and milk jugs, and out    
   comes propane. The approach has high selectivity, with propane making up about   
   80% of the final product gases.   
      
   “This is really exciting because it’s a step toward this idea of   
   circularity,” Rorrer says.   
      
   French startup Carbios just opened a demonstration plant—and hopes to expand   
   the world’s menu of recycling options.   
      
   To lower the energy needed to break down plastics, the process uses a catalyst   
   with two parts: cobalt and porous sand-like material called zeolites.   
   Researchers still aren’t sure exactly how the combination works, but Rorrer   
   says the selectivity likely    
   comes from the pores in the zeolite, which limit where the long molecular   
   chains in plastics react, while the cobalt helps keep the zeolite from being   
   deactivated.   
      
   The process is still far from being ready for industrial use. Right now, the   
   reaction is done in small batches, and it would likely need to be continuous   
   to be economical.   
      
   Rorrer says the researchers are also considering what materials they should   
   use. Cobalt is more common and less expensive than some other catalysts   
   they’ve tried, like ruthenium and platinum, but they are still searching for   
   other options. Better    
   understanding how the catalysts work could allow them to replace cobalt with   
   cheaper, more abundant catalysts, Rorrer says.   
      
   The ultimate goal would be a fully mixed-feed plastic recycling system, Rorrer   
   says, “and that framework is not completely far-fetched.”   
      
   Still, achieving that vision will take some tweaks. Polyethylene and   
   polypropylene are simple chains of carbon and hydrogen, while some other   
   plastics contain other elements, like oxygen and chlorine, that could pose a   
   challenge to chemical recycling    
   methods.    
      
   For example, if polyvinyl chloride (PVC), widely used in bottles and pipes,   
   winds up in this system, it could deactivate or poison the catalyst while   
   producing toxic gas side products, so researchers still need to figure out   
   other ways to handle that    
   plastic.   
      
   Scientists are also pursuing other ways to accomplish mixed-feed plastic   
   recycling. In a study published in Science in October, researchers used a   
   chemical process alongside genetically engineered bacteria to break down a   
   mixture of three common plastics.   
      
   The first step, involving chemical oxidation, cuts up long chains, creating   
   smaller molecules that have oxygen tacked on. The approach is effective   
   because oxidation is “quite promiscuous,” working on a range of materials,   
   explains Shannon Stahl, a    
   lead author of the research and a chemist at the University of Wisconsin.   
      
   Oxidizing the plastics generates products that can then be gobbled up by soil   
   bacteria that have been tweaked to feast on them. By altering the metabolism   
   of the bacteria, researchers could eventually make novel plastics, like new   
   forms of nylon.   
      
   The research is still a work in progress, says Alli Werner, a biologist at the   
   National Renewable Energy Laboratory and one of the authors of the Science   
   study. In particular, the team is working to better understand the metabolic   
   pathways bacteria are    
   using to make the products so that they can speed up the process and produce   
   larger amounts of useful materials.   
      
   This approach could likely be used on a larger scale, as both oxidation and   
   genetically engineered bacteria are already widespread: the petrochemical   
   industry relies on oxidation to make millions of tons of material every year,   
   and microorganisms are    
   used in industries like drug development and food processing.   
      
   As biologists like Werner and chemical engineers like Rorrer turn their   
   attention to new plastic recycling methods, they open up opportunities to   
   rethink how we deal with the vast amounts of plastic waste.    
      
   “This is a challenge that the community is well suited to tackle,” Rorrer   
   says. And she’s noticed a significant influx of new researchers starting to   
   work on plastics: “It seems like everyone and their sister is getting into   
   plastic upcycling.”   
      
   https://www.technologyreview.com/2022/11/30/1063837/how-chemists   
   are-tackling-the-plastics-problem/   
      
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