Ni/SiO2 catalysts for polyolefin deconstruction via the divergent hydrogenolysis mechanism
Graphical Abstract
Introduction
The current linear (take, make, and waste) economy is harmful to natural environments. Plastics are mainly derived from fossil-fuels [1] with ∼1 % from renewable bio-sources [2]. In the USA, 90 % of the plastics' carbon ends in landfills, incineration, or the environment [1], [3], [4], threatening the ecosystem [5], [6], [7] and human health [5], [8], [9]. The remaining is mechanically recycled, which degrades their physical properties [10]. Plastics waste chemical upcycling and deconstruction technologies are urgently needed.
Polyolefins (POs), such as polyethylene (PE) and polypropylene (PP), and polystyrene (PS) comprise ∼66 % of plastics production [1] and are difficult to deconstruct due to the C-C bond stability. Several methodologies, such as hydrogenolysis [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], hydrocracking [23], [24], [25], pyrolysis [26], acid cracking [27], alkane metathesis [28], [29], and non-conventional plasma [30] or microwave-assisted [31] chemistries, have been recently developed to deconstruct POs and PS to valuable lubricants, waxes, fuels and aromatics. Catalytic hydrogenolysis is particularly promising as it operates at mild temperatures (200–300 °C), requiring significantly lower energy than high-temperature pyrolysis and acid cracking, and has tunable product distributions. Noble metals (NMs), e.g., Ru, Pt, and Rh, have been predominately used and shown tremendous potential. However, NMs are highly susceptible to heteroatom impurities (S, N, O, etc.) and contaminants of commercial plastic additives and waste streams [22], respectively. Their global reserves (∼0.069 million metric tons (Mt) [32]) are tiny compared to the PO production (>215 Mt/yr[1]). Considering all NMs, a 1:10,000 catalyst-to-polymer ratio (far below the industrial minimum), and 0.1 wt% metal loading (all conservative estimates) reveals the scale of the problem: > 350 times the NM reserves are needed to handle the annual production scale of POs. Earth-abundant metals (EAMs), such as Ni, Co, Mo, etc., are 3–4 orders of magnitude cheaper [33], 10,000 times more abundant [34], and can handle impurities in hydrotreating petroleum feedstocks [35]. EAMs are active in small alkane hydrogenolysis [36], [37], [38], [39] and plastics hydrocracking [40], [41], [42], [43], but are ineffective for plastics hydrogenolysis [14], [15], [16]. Overcoming this scale barrier could advance the potential for commercialization.
Here, we demonstrate simple nickel on silica (Ni/SiO2) catalysts to be comparably active to NMs for hydrogenolysis of low-density polyethylene (LDPE) with high yields of diesel and lubricant ranged hydrocarbons and low methane yields. They are regenerable and applicable even to commercial plastics. We introduce a mechanism incorporating single and multiple cracking events sensitive to the adsorbate location and alkane size, supported by stochastic simulations, and reconcile the disparity with the small alkanes. We showcase the applicability of the catalyst to various everyday plastics, producing a slate of valuable products, and the promise of additional EAMs.
Section snippets
Catalyst synthesis
Nickel nitrate hexahydrate (Aldrich, Ni(NO3)2•6 H2O, ≥ 99.999 % trace metal basis) was used as received and silica-gel (Sigma-Aldrich, SiO2, Davisil grade 646, 35–60 mesh) was calcined in static air at 650 °C for 3 h (2 °C/min ramp) prior to synthesis. The xNi/SiO2 catalysts, where x corresponds to the wt% Ni loading, were prepared by wetness impregnation of a nickel nitrate hexahydrate solution, dried in air at 110 °C, and then calcined at 550 °C for 3 h (2 °C/min ramp).
Catalyst characterization
X-Ray diffraction (XRD)
Effect of nickel loading and reaction conditions on catalyst performance
The metal loading and dispersion strongly influence small alkane hydrogenolysis [48], [49] and PO hydrogenolysis [13], [21]. Fig. 1a shows a typical carbon distribution from 4LDPE (Sigma, Mw ∼4 kDa) deconstruction at 300 °C for 2 h over 15Ni/SiO2 (the number in front of Ni indicates the wt%). Excitingly and in stark contrast to previous reports, moderate methane and a broad range of liquid products form, consisting chiefly of n-alkanes (liquid and gas) with 25 % of isomers, and a maximum at C22
Conclusions
We have discovered a simple Ni/SiO2 catalyst with comparable performance to Ru- and better than Pt-based catalysts, expanding the viable catalysts to earth-abundant metals. Unexpectedly, hydrogenolysis proceeds via single and multiple cracking events mainly on internal polymer bonds and terminal liquid product bonds. This transition in pathways with molecular weight stems from the stronger adhesion of macromolecules, the much higher catalyst activity for terminal scissions, and the ratio of the
CRediT authorship contribution statement
Brandon C. Vance: Conceptualization, Methodology, Investigation, Software, Validation, Formal analysis, Writing – original draft, Writing – review & editing. Pavel A. Kots: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing. Cong Wang: Formal analysis. Jack E. Granite: Investigation. Dionisios G. Vlachos: Conceptualization, Writing – review & editing, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dionisios Vlachos, Brandon Vance, Pavel Kots, and Cong Wang has patent #PCT/US2022/019667 pending to University of Delaware.
Acknowledgements
This work was supported as part of the Center for Plastics Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Grant Number DE-SC0021166. B.C.V. acknowledges a Graduate Research Fellowship through the National Science Foundation under Grant Number 1940700. This research used instruments in the Advanced Materials Characterization Lab (AMCL) at the University of Delaware. The authors used the NMR facilities at the
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