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(116b) Selective Hydrogenolysis of Polyethylene and Polypropylene to Liquid Alkanes over Tunable Ruthenium-Based Heterogeneous Catalysts

Rorrer, J. - Presenter, Massachusetts Institute of Technology
Beckham, G. T., National Renewable Energy Laboratory
Roman, Y., MIT
The massive consumption of single-use plastics has led to a rapid accumulation of waste plastics in landfills and the environment, causing a global environmental and health crisis. Chemical upcycling has emerged as a method of producing higher value products from waste plastic feedstocks. The strong C-C bonds in common plastics such as polyolefins are particularly recalcitrant to degradation. Heterogeneous thermal catalysis enables targeted cleavage of bonds that can depolymerize polyolefins under milder conditions relative to other thermal deconstruction methods such as pyrolysis and gasification. We previously identified Ruthenium nanoparticles supported on carbon as an active and selective catalyst for the deconstruction of polyethylene via hydrogenolysis under relatively mild conditions (200-225°C, 20 bar H2) to produce high yields of liquid n-alkanes. In this study, the differences in C-C bond activation of straight-chain (polyethylene) versus branched (polypropylene) hydrocarbons were investigated to understand the effect of branches on the mechanism and kinetics of C-C bond hydrogenolysis. Further, we hypothesized that nanoparticle-support interactions play a role in dictating hydrogenolysis selectivity, and that undesired methane formation could be suppressed by tuning the support acidity, nanoparticle size, and nanoparticle distribution. To this end, a series of Ruthenium-based catalysts on varying supports with different nanoparticle size and distribution were synthesized and characterized with TEM, XRD, BET, NH3-TPD, H2-TPD, and CO2-TPD to identify the Ru dispersion, support structure, and acid-base properties. These catalysts were employed for the hydrogenolysis of polyethylene and polypropylene feedstocks to produce liquid n-alkanes and iso-alkanes, respectively. By tuning the support structure, acidity, confinement, and nanoparticle size, a liquid iso-alkane yield of 80% (C8-C18 hydrocarbons) from polypropylene (Mw ~340,000 Da) was achieved. Further investigation into bifunctional Ru-based catalysts on acidic supports, confinement effects, and earth-abundant materials are currently underway, and may enable economical routes to produce high quality processible liquids from waste plastic feedstocks.