Effect of Macro- and Microstructures on Catalytic Hydrogenolysis of Polyolefins

Polyethylenes of varying molecular weight and branch density, as well as polypropylenes of varying molecular weight and tacticity, were catalytically converted to lowermolecular-weight liquid products to showcase how these various properties in a mixed waste plastic stream could affect the final product. A Pt nanoparticle on a strontium titanate nanocuboid (Pt/STO) catalyst was used under solvent-free conditions in the presence of 170 psi of H-2 at 300 degrees C for hydrogenolysis. The initial molecular weight of polyethylene was found to have a moderate effect on the yield to the final product (ranging from 55 wt% for M-n similar to 7600 Da to 67 wt% for M-n similar to 50,950 Da). The microstructure, defined as the length and density of branches in a polymer, of higher-molecular-weight polymers was the dominant factor in determining the yield (ranging from 67 wt% for M-n 50,950 Da for linear low-density polyethylene (LLDPE) with C-2 branches to 97 wt% for M-n similar to 38,850 Da for LLDPE with C-6 branches). The same products (M-n = C-29-C-46, D = 1.1-1.6) and distribution of undesired light gases (C-1-C-4 approximate to 90 mol%, C-5-C-8 approximate to 10 mol%) are obtained from conversions of PE of varying molecular weight. The tacticity of polypropylene at a given molecular weight had a significant effect on the molecular weight of the final product, while not strongly affecting conversion. Hydrogenolysis of isotactic polypropylene (iPP) produced approximate to C-18 with a wider polydispersity (D similar to 1.4) compared to the narrow approximate to C-64 (D similar to 1.0) and approximate to C-54 (D similar to 1.0) products from atactic (aPP) and syndiotactic (sPP) polypropylene, respectively. The stereochemistry of the methyl groups dictates the shape and structure of the polymer in the melt, which in turn affects how the hydrocarbon chain interacts with the catalyst surface, thereby impacting the number of C-C scissions. These results show how various characteristics such as the molecular weight and structure of a waste plastic stream could affect the final product.

Automated summary

This study demonstrated how the characteristics of waste plastic stream, such as molecular weight and structure, can impact the final products obtained by catalytically converting polyethylenes and polypropylenes. The molecular weight and microstructure were found to have significant effects on product yield and molecular weight, while the tacticity of polypropylene mainly influenced the molecular weight and distribution of the products.