Chemists from France have learned to convert silicone polymers into monomeric chlorosilanes. They used boron chloride as a source of chlorine and gallium chloride as a catalyst. As the researchers write in Science, they were able to depolymerize silicone oils, baking molds, and recycled silicone-containing waste.
The chemical industry produces more than two and a half million tons of silicone polymers per year. To do this, chemists take natural quartz (SiO2) and obtain elemental silicon (Si) from it. This silicon is introduced into the Mueller-Rochow process, that is, heated with methyl chloride (CH3Cl) to 300 degrees Celsius in the presence of a copper catalyst - the output is a mixture of chlorosilanes (CH3)nSiClm. The mixture is separated, and its components are used to obtain silicone polymers.
Silicone polymers are used to produce sealants, pipes, oils and lubricants, cosmetics, various rubber products and much more. At the end of their life, these products either fill landfills or are burned. And scientists have not yet come up with an effective way to recycle silicones.
But recently, chemists led by Jean Raynaud at the first Claude Bernard University in Lyon have come up with a convenient method for depolymerizing silicones. They used a combination of two Lewis acids, boron chloride BCl3 and gallium chloride GaCl3, to convert silicones into dimethylchlorosilane (CH3)2SiCl2.
The researchers first tested this combination of reagents on silicone oil. They mixed it with excess BCl3 and a catalytic amount of GaCl3 in toluene. After two hours of heating at 40 degrees Celsius, the scientists isolated dimethylchlorosilane from the reaction mixture in 95 percent yield, and the boron chloride was converted to B2O3. The reaction did not work if the chemists used either BCl3 or GaCl3 separately, rather than a combination of the two.
The method was then tested on commercial silicone samples. In both silicone oils with varying degrees of polymerization and more complex rubber products—breast pads, baking pans, silicone tubes and sheets—the yield of dimethyldichlorosilane exceeded 90 percent.
Thus, chemists have developed a method for obtaining the main precursor of siloxane polymers — dimethylchlorosilane — from the polymers themselves. Perhaps this method will help preserve natural quartz reserves and reduce energy consumption for the Mueller-Rochow process.
We recently talked about how chemists learned to recycle Teflon and plexiglass.