Mechanistic classification and benchmarking of polyolefin depolymerization over silica-alumina-based catalysts

Abstract

Carbon-carbon bond cleavage mechanisms play a key role in the selective deconstruction of alkanes and polyolefins. Here, we show that the product distribution, which encompasses carbon range and formation of unsaturated and isomerization products, serves as a distinctive feature that allows the reaction pathways of different catalysts to be classified. Co, Ni, or Ru nanoparticles immobilized on amorphous silica-alumina, Zeo-Y and ZSM-5, were evaluated as catalysts in the deconstruction of n-hexadecane model substrate with hydrogen to delineate between different mechanisms, i.e., monofunctional- (acid site dominated) or bifunctional-hydrocracking (acid site & metal site) versus hydrogenolysis (metal site dominated), established from the product distributions. The ZSM-5-based catalysts were further studied in the depolymerization of polyethylene. Based on these studies, the catalysts are plotted on an activity-mechanism map that functions as an expandable basis to benchmark catalytic activity and to identify optimal catalysts that afford specific product distributions. The systematic approach reported here should facilitate the acceleration of catalyst discovery for polyolefin depolymerization.

Catalytic hydroconversion processes for upcycling plastic waste to fuels and chemicals

“…52 Therefore, development of single-atom and subnanometric noble metal catalysts as well as alloy catalysts with low metal loading is encouraged for practical applications. Although some non-noble catalysts, such as Ni–SiO 2 61 and Ni/HZSM-5, 37 have been prepared for the hydrogenolysis and hydrocracking of polyolefins, the performances are still inferior to noble-metal catalysts. Based on the site requirements for C–C, C–O and C–N bond activation, selecting the optimal combination of non-noble metals, promoters, and supports, and carefully modulating the catalyst specific structure could be the key criteria for the development of non-noble metal-based hydroconversion catalysts with high efficiency.…”

“…Bifunctional catalysts combining metal catalysts and solid acids have been widely used for this process. The metal catalysts include Pt-, [28][29][30][31][32][33][34] Pd-, Ni- [35][36][37] and Co- 36,37 based catalysts, among which Pt-based catalysts are the most efficient ones. The solid acids used in the hydrocracking process contain zeolites, [31][32][33][34][35][36][37] silica-alumina materials, 35,37 acidic mixed oxides (WO 3 /ZrO 2 ) [29][30][31] and so on.…”

Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

“…149 Recently, Dyson et al compared the hydrocracking of LDPE over Co/-, Ni/-and Ru/zeolite catalysts and constructed an activity−mechanism map. 183 As shown in Figure 10f, Ni deposited on ZSM-5 and HY showed better activity than Co/zeolite due to a better hydrogenation/dehydrogenation ability. They also revealed the influence of metal type on the hydrocracking mechanism.…”

“…It has been reported that the introduced metal type greatly influences polyolefin hydrocracking . Recently, Dyson et al compared the hydrocracking of LDPE over Co/-, Ni/- and Ru/zeolite catalysts and constructed an activity–mechanism map . As shown in Figure f, Ni deposited on ZSM-5 and HY showed better activity than Co/zeolite due to a better hydrogenation/dehydrogenation ability.…”

“…(f) Activity–mechanism map via types of metal modifications. Reproduced with permission from ref . Copyright 2022 Springer Nature.…”