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.

Figure 1

Electron microscopy of NiAl-T catalysts. (a) and (b) HAADF-STEM imaging with elemental mapping via EDS and EELS of NiAl-350 and NiAl-550, respectively. (c) ETEM of in-situ reduction of the calcined NiAl catalyst.

Figure 2

Catalyst characterization of the NiAl-T catalysts and reference standards: (a) XANES and (b) EXAFS analysis of XAS spectra, respectively. (c) ²⁷Al MAS NMR. (d) Air-free XPS. (e) IR of CO adsorption at 500 mTorr. (f) H₂ TPR with the temperature ranges highlighted for the various Ni²⁺ cations. (g) Conceptual schematic of the structural transformation and Ni nanoparticle generation in the near surface layers.

Figure 3

LDPE hydrogenolysis results using the NiAl-T catalysts. (a) C₁–C₃₅ carbon selectivities and yield of solid for each catalyst. (b) DSC curves of the solid residues and the neat LDPE. (c) Carbon product distribution for three active catalysts. The inset shows a magnification from C5 to C35 carbons. Reaction conditions: 2.0 g LDPE (M_w ∼ 4 kDa), 200 mg NiAl-T, 300 °C, and 30 bar H₂ for 2 h.

Figure 4

Characterization and analysis of the NiAl-T and reference catalysts after LDPE hydrogenolysis. (a) Air-free XPS spectra of the Ni 2p_3/2 region from the NiAl-T catalysts. (b) Methane selectivity as a function of the tetrahedral and octahedral cationic Ni ratio. (c) Methane selectivity as a function of Ni nanoparticle size. (d) Air-free XPS spectra of the Ni 2p_3/2 region from the Ni/NiAl₂O₄ and Ni/γ-Al₂O₃ reference catalysts. (e) LAS density from the spent NiAl-T catalysts as a function of the tetrahedral cationic Ni fraction. (f) Methane selectivity as a function of the spent NiAl-T catalyst LAS density. (g) Conceptual picture of the proposed mode by which Ni_Td²⁺promotes methane production. Selectivity comparisons are made at 25–45% LDPE deconstruction.