Catalytic Hydrogenolysis of Polyethylene-Based Plastics into Fuels Using Ruthenium Catalysts Supported on Micro/Mesoporous Carbons.

Catalytic hydrogenolysis is considered as a promising process for the chemical upcycling of polyethylene under mild conditions providing a homogeneous mixture of n-alkanes. This work highlights the potential of hydrogenolysis to effectively convert high molecular weight polyethylene-based polymers (LDPE and HDPE) and real plastic consumer products under mild reaction conditions using highly dispersed ruthenium catalysts supported on high surface area micro/mesoporous activated carbons (AC). The effect of the reaction conditions (temperature, time, hydrogen gas pressure, catalyst-to-feed ratio) and catalyst (metal typeRu, Pt, Reand loading, carbon support) was investigated using n-octane as a solvent/substrate and a high molecular weight (M w ∼ 110000 g/mol) low-density polyethylene (LDPE). Under the optimized conditions of 250 °C, 3 h, 50 bar H2, and a catalyst-to-feed ratio (C/F) = 0.1 with 5%Ru/AC, complete conversion of pure LDPE was achieved toward 78 wt % liquid alkanes with C5-C30 carbon number distribution. More intense conditions (i.e., higher temperature or prolonged reaction time) favored further hydrogenolysis of initially formed higher molecular weight alkanes toward smaller alkanes. The hydrogenolysis of high-density polyethylene (HDPE, M w ∼ 95000 g/mol) exhibited higher liquid alkane yields compared to LDPE (up to 91 wt % at 67-99 wt % conversion), owing to its higher melt flow index that facilitated solubility and depolymerization. The composition of the liquid products in terms of carbon number and established refinery fractions (i.e., naphtha, diesel, kerosene, heavy/lubricants) from both LDPE and HDPE was determined and correlated to reaction conditions, feedstock type, and catalyst properties. When real consumer LDPE- or HDPE-based plastics were used as feedstocks, the conversion was 67-98 wt % and the liquid alkanes yielded up to 91 wt %, similar to pristine polymers. The negative effect of inorganic additives in the real plastics was also identified, with CaCO3 and TiO2 reducing the hydrogenolysis reactivity and shifting selectivity toward larger alkane fractions. Overall, the ruthenium-based catalysts supported on high surface area micro/mesoporous activated carbons exhibited high activity in the hydrogenolysis of high molecular weight polyethylene-based plastics, exhibiting also high stability and reusability for at least three successive catalytic cycles without intermediate regeneration.