Structure–Property Relationships for Nickel Aluminate Catalysts in Polyethylene Hydrogenolysis with Low Methane Selectivity

Abstract

Earth-abundant metals have recently been demonstrated as cheap catalyst alternatives to scarce noble metals for polyethylene hydrogenolysis. However, high methane selectivities hinder industrial feasibility. Herein, we demonstrate that low-temperature ex-situ reduction (350 °C) of coprecipitated nickel aluminate catalysts yields a methane selectivity of <5% at moderate polymer deconstruction (25–45%). A reduction temperature up to 550 °C increases the methane selectivity nearly sevenfold. Catalyst characterization (XRD, XAS, 27Al MAS NMR, H2 TPR, XPS, and CO-IR) elucidates the complex process of Ni nanoparticle formation, and air-free XPS directly after reaction reveals tetrahedrally coordinated Ni2+ cations promote methane production. Metallic and the specific cationic Ni appear responsible for hydrogenolysis of internal and terminal C–C scissions, respectively. A structure-methane selectivity relationship is discovered to guide the design of Ni-based catalysts with low methane generation. It paves the way for discovering other structure–property relations in plastics hydrogenolysis. These catalysts are also effective for polypropylene hydrogenolysis.

Polystyrene Hydrogenolysis to High-Quality Lubricants Using Ni/SiO2

“…Catalyst characterization has been reported previously . Excitingly, methane and other gaseous hydrocarbon production is negligible for PS hydrogenolysis, whereas methane is a major byproduct in PE and PP hydrogenolysis. ,,− Minor yields of liquid alkanes, aromatics, and oligomers (<10% each) were detected in the methanol wash. The alkane and aromatic hydrocarbons all contained alkyl chains with 1–4 carbons, while oligomers consisted of fully and partially hydrogenated derivatives of the PS dimer and trimer, per GCMS (Figure c).…”

“…Chemical recycling is promising for the utilization of these wasted thermoplastics. Polyethylene and polypropylene feedstocks have curated the most attention, with demonstrations of pyrolysis − and hydroconversion − producing light olefins, alkylaromatics, fuel-range hydrocarbons, naphtha, lubricant base oils, and more. Polystyrene (PS) feedstocks have attracted considerably less attention.…”

Conversion of Polypropylene into Light Hydrocarbons and Aromatics by Metal Exchanged Zeolite Catalysts

“…In our work, high Ni loading (above 0.01 M Ni-exchange) was associated with higher methane yields (>7 wt %) compared to the parent and other metal exchanged zeolites. Methane selectivity in polyolefin hydrogenolysis has recently been found to depend on the concentration of tetrahedral Ni 2+ sites of Lewis acidic character on a Ni-aluminate catalyst . We speculate our cationic Ni sites to act in terminal C–C bond cleavage toward methane production from PP or its cracking products.…”

Review on Catalytic Depolymerization of Polyolefin Waste by Hydrogenolysis: State-of-the-Art and Outlook

“…Their work was targeted on low methane selectivity (<5%), since too much of it can hinder the industrial viability of this technology. 118 The Vance research team also conducted another upcycling of polyolefin via a new mechanism termed divergence hydrogenolysis. That was achieved via the coprecipitation of the nickel alumina catalyst in an ex situ reduction process with the metal in both cationic and atomic states as evidenced by the characterization techniques at a considerable destruction rate of the polymer (25−45%) (Table 2, entry 10).…”

“…Aside from silica, the Vance group used alumina as a supporting material and nickel as the catalyst for the hydrogenolysis of polyethylene. Their work was targeted on low methane selectivity (<5%), since too much of it can hinder the industrial viability of this technology . The Vance research team also conducted another upcycling of polyolefin via a new mechanism termed divergence hydrogenolysis.…”