Effect of Macro- and Microstructures on Catalytic Hydrogenolysis of Polyolefins

Abstract

Polyethylenes of varying molecular weight and branch density, as well as polypropylenes of varying molecular weight and tacticity, were catalytically converted to lower-molecular-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 H2 at 300 °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 Mn ∼ 7600 Da to 67 wt% for Mn ∼ 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 Mn ∼ 50,950 Da for linear low-density polyethylene (LLDPE) with C2 branches to 97 wt% for Mn ∼ 38,850 Da for LLDPE with C6 branches). The same products (Mn = C29–C46, Đ = 1.1–1.6) and distribution of undesired light gases (C1–C4 ≈ 90 mol%, C5–C8 ≈ 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 ≈C18 with a wider polydispersity (Đ ∼ 1.4) compared to the narrow ≈C64 (Đ ∼ 1.0) and ≈C54 (Đ ∼ 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.