Hydrogenolysis of Polypropylene and Mixed Polyolefin Plastic Waste over Ru/C to Produce Liquid Alkanes

Abstract

Catalytic depolymerization of polyolefins is a promising chemical recycling strategy to create value-added products from waste plastics, which are accumulating in landfills and the natural environment at unsustainable rates. The cleavage of strong C−C bonds in polyolefins can be performed using a noble metal and hydrogen via a hydrogenolysis mechanism. Previously, we identified ruthenium nanoparticles supported on carbon (Ru/C) as a highly active heterogeneous catalyst for the conversion of polyethylene into liquid and gaseous n-alkanes under mild conditions. In the present study, we investigated the catalytic depolymerization of polypropylene (PP) under mild conditions (200−250 °C, 20−50 bar H 2 ). We demonstrate that Ru/C produces C 5 −C 32 iso-alkane yields above 68% in the absence of solvent and identify trade-offs between product yield and temperature, hydrogen pressure, and reaction time. We apply a rigorous analytical method to quantify all liquid and gaseous alkane products. The characterized catalyst was found to be recyclable after the complete depolymerization of high molecular weight PP (M w ∼ 340,000 Da) to liquid and gaseous hydrocarbons and after depolymerization of a postconsumer PP centrifuge tube. Further, the catalyst was shown to be effective in depolymerizing a mixture of high-density polyethylene and PP to produce a mixture of linear and branched liquid alkanes, demonstrating feasibility for the depolymerization of streams of mixed polyolefin waste.

Sustainable chemical upcycling of waste polyolefins by heterogeneous catalysis

“…Thus, the reported reaction temperatures for Pt catalysts on polyolefin hydrogenolysis are relatively high (250−300°C) and the degradation time is fairly long 59,60 . As a cheaper active metal for hydrogenolysis, Ru‐based catalysts have drawn much attention in recent studies to catalyze polyolefins (including HDPE, LDPE, PP, waste polyolefins) to liquid fuels under mild conditions, with a reaction temperature of 200−250°C, H 2 pressure of 0−6 MPa, and reaction time from 0−12 h 56,61–64 . However, the following challenges still exist in this reaction: (1) high yield of low‐value gases (predominantly methane), generally above 10%, and (2) the long reaction time compared to the hydrocracking reaction.…”

Expanding Plastics Recycling Technologies: Chemical Aspects, Technology Status and Challenges

“…Ruthenium supported on metal oxides or carbon has been reported to catalyze the production of alkanes, aromatics, and liquid fuels from polyolefins (i.e., LDPE, HDPE, PP), PS, and PC. [881][882][883][884][885][886][887] LDPE, HDPE, and PP can be converted to liquid fuel (C5-C21) and lubricants/waxes (C22-C45) at low temperature and low H2 pressure (i.e., 200-250 °C, 20-30 bar) on metal oxides-supported Ru (e.g., Ru/TiO2, Ru/CeO2). [881][882] Over multifunctional Ru/Nb2O5, monocyclic aromatics can be selectively produced from single or mixed aromatic plastics (i.e., PET, PC, PS, polyphenylene oxide) at 200-320 °C in the presence of hydrogen and solvent (e.g., water, octane etc.).…”

“…The support can be adjusted to treat different plastics and obtain varied products. [884][885][886][887] Over Ru/WO3/ ZrO2, LDPE can be converted into higher molecular weight fuels and wax/lubricant base-oils at 250 °C and 50 bar H2. 887 While on Ru/FAU, methane (>97%) can be produced from PE, PP, and PS under 50 bar H2 at 300-350 °C.…”

“…886 Ru nanoparticles can also be supported on carbon (Ru/C) to catalyze the hydrogenolysis of LDPE, HDPE, and PP towards liquid n-alkanes and alkane gases (C1-C6) under mild conditions (200-250 °C, 20-50 bar H2). [884][885] In the liquid phase (water, nhexane etc. ), around 90 wt% of HDPE can be converted to liquid hydrocarbons (C8-C38) within 1 h on Ru/C catalysts at 220 °C in the presence of H2 (30 bar).…”

Electrochemical Activation of C−C Bonds through Mediated Hydrogen Atom Transfer Reactions

“…For example, thermochemical pathways used for plastic depolymerization, such as pyrolysis and thermal cracking, operate at temperatures >400 °C and suffer from low product selectivity [8] . Reductive catalytic depolymerization strategies, such as hydrogenolysis and metathesis, improve the energy efficiency by ameliorating reactions conditions (temperatures>200 °C), but require reductants such as high‐pressure H 2 and/or high‐cost noble metal catalysts [9,17–21] . Oxidative C−C bond cleavage generates oxygenated products as valuable chemicals, [3] but the traditional thermal catalysis approach requires bromine as a co‐catalyst, [22] impacting on the environment.…”