Enhancing Rate and Selectivity for Hydrodechlorination of Poly(Vinyl Chloride) with Rh Catalysts.

Abstract

Hydrodechlorination of poly(vinyl chloride) (PVC) directly to polyethylene (PE) represents a way to repurpose PVC waste, while avoiding toxic and/or corrosive byproducts that are produced at the end of life. Prior studies identified a rhodium-catalyzed route to hydrodechlorinate PVC to form PE products with sodium formate as a hydrogen source. While all chlorine could be removed to form PE-like polymers, the reaction was slow and side reactions introduced undesirable cross-links in the polymer product. In this work, mechanistic studies are pursued to improve catalytic activity for this method. Xantphos and diphenylphosphinoethane (DPPE) both support Rh(I) to promote this reaction to full conversion, effectively removing all chlorine from PVC samples, with Xantphos support providing the fastest metal catalysis for hydrodechlorination to date. However, side reactions to form cross-links are present for both catalyst systems. Control studies suggest the proposed route for cross-link formation also deactivates the Rh catalyst, indicating the cross-link formation can also be the cause for the reaction to slow over time. Other reaction conditions were found to influence the selectivity between hydrodechlorination and cross-link formation. These results introduce key catalyst design principles to improve methods for hydrodechlorination of PVC, allowing for sustainable repurposing of this toxic polymer waste.