Investigation into the Reaction Pathways and Catalyst Deactivation for Polyethylene Hydrogenolysis over Silica-Supported Cobalt Catalysts

Abstract

Chemical repurposing has emerged as a promising route to valorize "end-of-use" plastic waste and mitigate its release to the environment. In this work, we applied silica-supported cobalt (5 wt % Co/SiO 2 ) catalysts to produce liquid-range hydrocarbons (C 5 −C 30 ) in the batch phase at 200−300 °C, 20−40 bar H 2 , and 2−36 h with high selectivity and investigated the reaction pathways, the influence of catalyst phase on the product yields and selectivity, and the catalyst deactivation mechanisms. Reaction conditions were optimized for improving liquid product yields at 275 °C, 30 bar H 2 , and 8 h reaction time, giving a 55% liquid product yield (C-mole basis), comprising 75% of nonsolid products, with gas yields limited to ∼19%. By tracking product evolution over time and with varying cobalt surface density, we propose a multipathway mechanism, including a dominant, nonterminal C−C cleavage route on the polymer chain over the catalyst, which drives the high liquid product selectivity. The catalyst also showed recyclability over four reactions with reduced activity and a shift in yield toward liquid products after the first reaction. It was effectively regenerated by calcination under air at 450 °C. We combined the reactivity data with powder X-ray diffraction (PXRD), thermogravimetric analysis coupled with mass spectrometry (TGA-MS), and catalyst surface areas via N 2 physisorption of various fresh, spent, recycled, and regenerated catalysts to attribute the reduced activity and selectivity shift mainly to the presence of a recalcitrant polymer species embedded on the catalyst, comprising 10.5−18.5 wt % of the spent catalyst, which obstructs access to active sites and increases liquid selectivity and overshadows the influence of carbonaceous coke or catalyst phase reduction to Co. Moreover, we successfully applied the catalyst to various postconsumer polyethylene (HDPE and LDPE) samples. These results move the field toward more sustainable and economically viable catalysts for the chemical upcycling of waste plastics.

Precise activation of C–C bonds for recycling and upcycling of plastics

“…commonly favor the C aliph -C aliph bonds at the terminal position. 76,84,94,96 84 The interaction between the support and the metal sites can adjust the electronic state of the active center to affect the cleavage of C aliph -C aliph bonds, which will be discussed in detail in the subsequent section. Nonetheless, the inherent activation properties of metal sites are undeniably crucial.…”

Converting Waste Plastic to Liquid Organic Hydrogen Carriers

“…Chemical recycling offers an opportunity to keep waste plastics in the economy by converting them to value‐added products. Previous studies have utilized heterogeneous catalysis to target products including monomers such as terephthalic acid, aromatic compounds, and fuel‐ and lubricant‐range alkanes from oxygenated aromatic waste and polyolefins [2] . In a recent article published in Angewandte Chemie , J. Wei and colleagues demonstrated that oxygen‐containing aromatic plastic waste can be selectively converted into cycloalkanes over a physical mixture of Ru−ReO x supported on SiO 2 combined with zeolite HZSM‐5 as a co‐catalyst (Ru−ReO x /SiO 2 +HZSM‐5) under relatively mild conditions (180–210 °C, 3 MPa H 2 ) [3] .…”

“…Previous studies have utilized heterogeneous catalysis to target products including monomers such as terephthalic acid, aromatic compounds, and fuel-and lubricant-range alkanes from oxygenated aromatic waste and polyolefins. [2] In a recent article published in Angewandte Chemie, J. Wei and colleagues demonstrated that oxygen-containing aromatic plastic waste can be selectively converted into cycloalkanes over a physical mixture of RuÀ ReO x supported on SiO 2 combined with zeolite HZSM-5 as a co-catalyst (RuÀ ReO x /SiO 2 + HZSM-5) under rela-tively mild conditions (180-210 °C, 3 MPa H 2 ). [3] Cycloalkanes can be blended with isoalkanes to meet density requirements for fuels, and potentially provide comparable seal-swelling capacity to aromatic compounds for applications in sustainable aviation fuel.…”

Quantifying synergy for mixed end-scission and random-scission catalysts in polymer upcycling

“…Current research in catalytic polymer upcycling is mainly empirical, 27–40 with only a small fraction of studies attempting theoretical analysis of the underlying reaction mechanisms and reaction progress. 41–49 Based on an extrapolation from other branches of catalysis, 50–62 theoretical models and calculations will likely have a major impact on polymer upcycling, from understanding the chemistry, to developing new catalysts, to industrial process design.…”