Plastics are ubiquitous in the modern world, with annual production reaching 45 million metric tons. Waste polyolefins are a major source of global pollution, posing a serious and escalating concern. Research efforts have developed several chemical processes to convert waste polyolefins into valuable products, but their long carbon chain and high viscosity in the melt lead to severe heat and mass transfer limitations. This work investigated the potential of subcritical CO2 as a tunable medium to enhance the catalytic hydrocracking of polyethylene (PE) into hydrocarbon fuels. The addition of subcritical CO2 (CO2 + H2 = 6 MPa total pressure) to the hydrocracking of PE over 5 wt% Ru/H-β at 200°C, 3 MPa of initial H2 initial pressure for 4 h led to a 27% increase in PE conversion and 66% increase in liquid yield when compared to an H2-only control reaction (no CO2, 3 MPa of initial H2 pressure). In addition to this, the presence of subcritical CO2 resulted in a narrower carbon number distribution of liquid products, increasing selectivity toward gasoline-range hydrocarbons (C5–C10) from ~30% to 84%. These tunable properties are of interest for applications such as higher-octane values for fuel. Moreover, product isomerization increased with the addition of subcritical CO2. Similar distribution and isomerization results were achieved using a 5 wt% Ru/H-Y catalyst in the presence of subcritical CO2. To further assess the impact of subcritical CO2, hydrocracking performance was evaluated in a system with reduced mass transfer limitations (i.e., a reaction with a low-viscosity substrate, n-hexadecane) with 5 wt% Ru/H-Y at 200°C for 1 h. Unlike the enhanced performance of the PE reactions, it was found that subcritical CO2 interfered with catalysis in the n-hexadecane system. These results showcase the potential of subcritical CO2 to reduce transport limitations during the conversion of high viscous molten plastics.
