Catalytic Upcycling of Polyolefins

Abstract

The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C-C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.

Figure 1

Comparison of the number of catalysis papers published from 2000 to 2023 on “alkene polymerization” (grey; search terms: catalytic polymerization AND polyethylene OR polypropylene OR polystyrene OR poly(vinyl chloride)) vs. “polyolefin depolymerization” (green; search terms: catalytic depolymerization AND polyethylene OR polypropylene OR polystyrene OR poly(vinyl chloride)). Data from Web of Science.

Scheme 1. Comparison of Chemical Recycling and Upcycling of Polyolefins

Figure 2

Time-dependence of PP hydrocracking: (a) increase in PP conversion with time; and (b) variations in the yields of various hydrocarbon product groups with conversion. Dual data points at 100 % PP conversion were measured at two reaction times (13 and 20 h), where the colored arrows represent increasing reaction time. Reproduced with permission from ref (48). Copyright 2023, Elsevier.

Scheme 2. Each C–C Bond Scission Event Generates One New Hydrocarbon Chain, Making the Number of Events Equal to the Number of New Chains

Figure 3

Activity comparison of three Pt-based catalysts for PE depolymerization: (a) by mass fractions of hydrocarbons recovered from a batch reactor: green, gases; orange, liquids soluble in hot CH₂Cl₂; black, organic residues insoluble in hot CH₂Cl₂; and (b) by average rate of C–C bond scission (defined as n_{C–Cscission}/t). Adapted with permission from ref (50). Copyright 2023, Elsevier.

Figure 4

Model of the mass distribution of hydrocarbon product groups resulting from the random hydrogenolysis of a collection of 2,000 linear and atomically-precise PE chains C₃₅₇₁H₇₁₄₄ (M = 50,000 g/mol) as a function of the degree of scission (DS_n). Assuming that all C–C bonds have an equal chance of undergoing hydrogenolysis implies that the likelihood of a chain being cut is proportional to its length. The mass of H₂ incorporated was not included in the product mass yields.

Figure 5

Predicted evolution of the number-averaged molecular weight (M_n) as a function of the average degree of scission (DS_n), starting from 2,000 chains of a monodisperse PE with a precise M value of 50,000 g/mol (i.e., C₃₅₇₁H₇₁₄₄). The mass of H₂ incorporated via hydrogenolysis is not included in the product mass.

Scheme 3. Principal Steps in PE and PP Hydrogenolysis Catalyzed by (a) Late Transition Metal Catalysts; and (b) Early Transition Metal Hydride Catalysts

Scheme 4. Elementary Steps and Intermediates in Alkane Hydrogenolysis Catalyzed by Metal Surfaces

All steps except for (d) are considered to be quasi-equilibrated. * denotes a vacant surface site; l* indicates that the adsorbate occupies l surface sites. K_x and k_y are equilibrium and rate constants, respectively, for individual steps; a/b indicate numbers of C/H atoms, respectively, in the cleaved fragments. Reproduced with permission from ref (75). Copyright 2016, American Chemical Society.

Figure 6

Results of squalane hydrogenolysis: (a) product selectivity; and (b) proposed C–C bond cleavage positions, based on product analysis. Reaction conditions: 4.23 g squalane, 50 mg Ru/CeO₂, 60 bar H₂, 240 °C, 6 h, batch reactor. In part (a), gray and white bars represent yields of dominant products and other isomers, respectively. Products with 4, 5, 9, 10, 14, 16, 20, 21, 25, or 26 carbons result from C(2°)–C(2°) bond cleavage, while products with 1, 3, 6, 8, 11, 13, 17, 19, 22, 24, 27, or 29 carbons result from C(3°)–C(x°) bond cleavage. Reproduced with permission from ref (91). Copyright 2016, Wiley.

Scheme 5. General Mechanism for PO Hydrogenolysis Catalyzed by a Metal Surface, with Types of C–C Bond Scissions Classified As: (1) Internal Scission (Type 1); (2) Terminal Scission (Type 2); and (3) Successive Scission (Type 3, Referring to Multiple Scission Events for a Particular Adsorbed Species)

“Bonds” between the metal surface and polymer fragments (indicated by dashed lines) are shown for illustration purposes, and are not intended to be chemically precise.

Scheme 6. Location-Dependent Kinetics of PE Hydrogenolysis, Catalyzed by Ni/SiO₂: (a) Cleavage of a Terminal C–C Bond, with Rate-Determining Desorption; and (b) Cleavage of an Internal C–C Bond, with Rate-Determining C–C Bond Cleavage

Adapted with permission from ref (90). Copyright 2023, Elsevier.

Figure 7

GPC molecular weight distributions for various polymers (solid lines) and their hydrogenolysis products (dashed lines) derived from atactic (aPP), isotactic (iPP), and syndiotactic polypropylene (sPP). Reaction conditions: Pt/SrTiO₃ (10 wt% Pt), 300 °C, 170 psi H₂, 72 h. Reprinted with permission from ref (112). Copyright 2022, American Chemical Society.

Figure 8

Progress of iPP hydrogenolysis, catalyzed by Ru/TiO₂: (a) initial polymer; (b) heavy oil, with lower stereoregularity than the initial polymer; (c) light oil, with even lower stereoregularity; and (d) light oil, with loss of CH₃ branches due to demethylation. Reprinted with permission from ref (113). Copyright 2021, American Chemical Society.

Figure 9

(a) Workflow to evaluate scission preferences in catalytic PP hydrogenolysis. Bottom left: Relationship between the ratios of different level carbon atoms and cleavage mechanisms. Bottom right: scission preference for demethylation vs. backbone scission. P: primary, S: secondary, T: tertiary. (b) ¹H NMR spectrum of a depolymerization product mixture obtained with Ru/TiO₂ (5 wt% Ru). (c) Dependence of the light alkane yield on scission preference for various Ru-based catalysts. (d) Comparison of scission preferences for various Ru-based catalysts (C_n>45 was classified as unreacted solid residue). Reaction conditions: 240 °C, 20 bar H₂, 4 h. Adapted with permission from ref (115). Copyright 2023, Elsevier.

Figure 10

Effect on yields in PE hydrogenolysis of (a) temperature; and (b) P_H2. Reaction conditions: 16 h, 30 bar H₂, 100 mg PE (M_w = 4,000 g/mol, M_n = 1,700 g/mol), 25 mg Ru/C (5 wt % Ru). (c) Dependence of the carbon number distribution in liquid products on P_H2. Reaction conditions: 200 °C, 16 h, 700 mg PE, 25 mg Ru/C. Reprinted with permission from ref (46). Copyright 2021, American Chemical Society.

Figure 11

(a) Time-dependence of PE conversion and product yields in hydrogenolysis catalyzed by Ru/CeO₂. Reaction conditions: 500 mg Ru/CeO₂ (5 wt% Ru), 3.4 g PE (M_n = 1,700 g/mol, M_w = 4,000 g/mol), 20 bar H₂, 200 °C. (b) Hydrogenolysis of post-consumer plastics catalyzed by Ru/CeO₂. Adapted with permission from ref (110). Copyright 2021, Elsevier.

Figure 12

Temperature dependence of the hydrogenolysis of iPP (M_w = 250,000 g/mol, M_n = 67,000 g/mol) and PE (M_w = 4,000 g/mol, M_n = 1,300 g/mol) catalyzed by Ru/C: (a, b) selectivity for various product classes; and (c, d) carbon number distributions in the range C₁–C₃₈ (gas product amounts are multiplied by 0.2, except where noted). Reaction conditions: 30 bar H₂, 18 h, 1 g iPP or PE, 100 mg Ru/C (5 wt% Ru). Adapted with permission from ref (127). Copyright 2022, Royal Society of Chemistry.

Figure 13

(a) Electron micrographs of 5c-Pt/SrTiO₃ (5 wt% Pt), including the Pt nanoparticle size distribution. Time-dependent analyses of products from catalytic PE hydrogenolysis: (b) molecular weight distribution; (c) number-average molecular weight (M_n); and (d) dispersity (Đ). Reaction conditions: 12 bar H₂, 300 °C, PE (3 g, M_n = 8,150 g/mol, Đ= 2.7), 5c-Pt/SrTiO₃ (5 wt% Pt, 160 mg). (e) Characterization of ¹³C-enriched PE adsorbed on SrTiO₃ and Pt/SrTiO₃ by ¹³C MAS NMR (red) and CP-MAS NMR (black). Adapted with permission from ref (54). Copyright 2019, American Chemical Society.

Figure 14

Comparison of hydrogenolysis performance for mSiO₂/Pt/SiO₂ and Pt/SiO₂: (a) product distributions; (b) GPC analysis of starting HDPE (black) and insoluble residues remaining after 24 h; and (c) carbon number distributions of light hydrocarbons after 6 h. Reaction conditions: 250 °C, 14 bar H₂. Reproduced with permission from ref (130). Copyright 2020, Springer Nature.

Figure 15

Comparison of PE hydrogenolysis by various supported Ni catalysts: (a) Product yields for various broad product categories; and (b) compositional analysis of the light gases. Reaction conditions: 1.5 g PE (M_w = ∼4,000 g/mol, M_n = ∼1,700 g/mol), 0.3 g catalyst (5 wt% Ni), 280 °C, 30 bar H₂, 4 h. Reproduced with permission from ref (141). Copyright 2022, Elsevier.

Figure 16

PE hydrogenolysis catalyzed by Ni/SiO₂: (a) distribution of light alkanes, for 15 wt% Ni; and (b) broad categories of product yields at 2 h, as a function of Ni loading. Time dependence observed with 15 wt% Ni catalyst, for: (c) yields in broad product categories; and (d) DSC analyses of solid residues. (e) Independence of alkane selectivities until high PE conversion. Reaction conditions: 2.0 g PE (M_w ∼ 4,000 g/mol, Đ unspecified), 200 mg Ni/SiO₂, 300 °C, 30 bar H₂. (f) Proposed hydrogenolysis pathways. Adapted with permission from ref (90). Copyright 2022, Elsevier.

Figure 17

For PE hydrogenolysis catalyzed by Co/SiO₂, the effect on product distribution of (a) temperature (all at 4 h reaction time); and (b) reaction time (all at 275 °C). (c) Comparison of distributions for liquid-phase products from fresh and reactivated catalysts, in reactions conducted at 300 °C for 4 h. The reactivated catalyst was calcined at 450 °C for 5 h in air between run 0 and run 1. Reaction conditions: 1.0 g PE (M_w = 4,000 g/mol, Đ unspecified), 0.10 g Co/SiO₂ (5 wt% Co), 30 bar H₂, 200 rpm. Adapted with permission from ref (143). Copyright 2023, American Chemical Society.

Figure 18

PE hydrogenolysis catalyzed by various ZrO₂-based catalysts: (a) time-dependent mass yields for various product categories and H₂ consumption in the reaction catalyzed by L-ZrO₂@mSiO₂, and (b) distribution of liquid products for the same catalyst. (c) Comparison of activities for several ZrO₂-based catalysts in C–C bond cleavage (darker bars, derived from the rate of H₂ consumption) and PE conversion (lighter bars) after 6 h at 300 °C. Reaction conditions: PE: M_n = 20,000 g/mol, M_w = 90,000 g/mol, Đ = 4.8, 300 °C. (d) Comparison of C–C bond cleavage activities (at 300 °C, averaged over 6 h) for different PEs. (e) Distribution of liquid products produced by ZrO₂-based catalysts with various architectures, at constant H₂ consumption. Reproduced with permission from ref (146). Copyright 2023, Springer Nature.

Scheme 7. Comparison of Proposed Mechanisms for Alkane Hydrogenolysis Catalyzed by Zr(IV) Hydrides (Involving β-Alkyl Transfer, Left) and Ta(V) Hydrides (Involving α-Alkyl Transfer, Right)

Reprinted with permission from ref (151). Copyright 2016, American Chemical Society.

Figure 19

Results of PE hydrogenolysis catalyzed by ZrNp₂/sulfated alumina (0.03 mol% Zr) at 150 °C under 2.0 bar H₂: (a) time-dependent product distributions; (b) GC/FID analysis of volatile products after 30 min; and (c) GC/MS analysis of soluble products extracted into CH₂Cl₂, after various reaction times. Linear and branched alkanes are indicated with circles and triangles, respectively. Reprinted with permission from ref (148). Copyright 2022, Springer Nature.

Scheme 8. Proposed Mechanism for iPP Hydrogenolysis Catalyzed by a Cationic Tantalum Hydride Supported on Sulfated Alumina

Reprinted with permission from ref (158). Copyright 2023, American Chemical Society.

Figure 20

Dependence on Ru loading of iPP hydrogenolysis catalyzed by Ru/CeO₂: (a) overall product phase selectivity; (b) rate of conversion, normalized by surface Ru atoms; and (c) selectivity to CH₄ and C₁₀ isomers. Reaction conditions: 1 g iPP (M_w ∼250,000 g/mol, M_n = ∼67,000 g/mol), 0.5 mg Ru, 260 °C, 30 bar H₂, 18 h. Reproduced with permission from ref (106). Copyright 2022, American Chemical Society.

Figure 21

Comparison of PE hydrogenolysis catalyzed by Ru/CeO₂ with various Ru loadings: (a) conversion and product phase yields; and (b) carbon number distribution. Reaction conditions: 250 °C, 6 h, 20 bar H₂, PE (M_w= 4,000 g/mol, Đ unspecified), constant PE/Ru mass ratio of 2000. Adapted with permission from ref (117). Copyright 2023, AAAS.

Figure 22

(a) Post-reaction air-free XPS spectra of reduced nickel aluminate (NiAl-T) catalysts, in the Ni 2p region. (b) Correlation between methane selectivity and the ratio of tetrahedral to octahedral Ni sites. (c) Schematic illustration showing methane production promoted by Ni_Td²⁺. Reaction conditions: 2.0 g PE (M_w = 4,000 g/mol, Đ unspecified), 200 mg catalyst, 300 °C, 30 bar H₂. Adapted with permission from ref (142). Copyright 2023, American Chemical Society.

Scheme 9. Polyolefin Hydrogenolysis Promoted by Hydrogen Spillover: Comparison of Reaction Pathways for (a) WO_x-Modified Ru/ZrO₂; and (b) Ru/ZrO₂. (c) Hydrogen Spillover; and (d) H₂ Dissociation, Both for Ru/TiO₂

Panels a and b: Adapted with permission from ref (103). Copyright 2021, American Chemical Society. Panels c and d: Adapted with permission from ref (116). Copyright 2022, Springer Nature.

Figure 23

(a) Schematic of the proposed “processive” mechanism for PE hydrogenolysis catalyzed by a mSiO₂/Pt/SiO₂, with Pt nanoparticles (orange) located at the bottom of nanopores in the mesoporous SiO₂ (mSiO₂) shell. ¹³C MAS NMR spectra of ¹³C-enriched PE (M_n = 130,000 g/mol, Đ= 3.2) adsorbed onto (b) silica gel; or (c) mSiO₂. (d) ¹³C MAS NMR spectra of unlabeled PE (M_n = 7,000 g/mol, Đ unspecified) on mSiO₂. Proposed polymer conformations are indicated to the right of the spectra. Adapted with permission from ref (130). Copyright 2020, Springer Nature. (e) Product distributions for wax (C_<45) recovered at times corresponding to similar LDPE conversions (ca. 80 %) from reactions catalyzed by mSiO₂/Pt/SiO₂ with various Pt nanoparticle sizes. Reaction conditions: LDPE (M_n = 20,000 g/mol, M_w = 90,000 g/mol), 300 °C, 8.9 bar H₂. Adapted with permission from ref (131). Copyright 2022, American Chemical Society. (f) Dependence of the average molecular weight of the wax product on the mesopore length of the mSiO₂/Pt/SiO₂ catalyst. Reaction conditions: PE (M_n = 2,800 g/mol, M_w = 5,000 g/mol), 300 °C, 8.9 bar H₂, 6 h. Adapted with permission from ref (171). Copyright 2023, American Chemical Society.

Figure 24

Polymer-support interactions: (a) ¹³C MAS NMR spectra of C₂₀ adsorbed in various catalysts (A-D: Ru@SBA-15, with different catalyst/C₂₀ ratios as indicated; E: Ru/SBA-15; F: Ru/SiO₂; G: pure C₂₀. Schematic representations of (b) conventional polymer chain adsorption on Ru/SBA-15; and (c) spatial confinement of adsorbed polymer chains and the corresponding entropy reduction for Ru@SBA-15. Reproduced with permission from ref (124). Copyright 2023, Wiley.

Figure 25

(a) Structures of UiO-6X-RuH₂ catalysts. Distributions of (b) gas; and (c) liquid products, generated by hydrogenolysis of commercial PE catalyzed by UiO-6X-RuH₂ catalysts with different pore sizes. Distributions of (e) gas; and (f) liquid products, generated by hydrogenolysis of (d) post-consumer single-use LDPE plastic bags. (g) Selectivity comparison for liquid products in several alkane ranges. Reaction conditions: polymer (600 mg), Ru (4 mmol), 200 °C, 35 bar H₂, 72 h for commercial PE (M_w = 4,000 g/mol, M_n = 1,700 g/mol) and 20 h for the LDPE bag. Reproduced with permission from ref (175). Copyright 2023, American Chemical Society.

Figure 26

(a) Product yields from HDPE hydrogenolysis in various solvents. Reaction conditions: 0.1 g HDPE (post-consumer water jugs), 25 mL solvent, 220 °C, 20 bar H₂, 1 h. (b) NVT (substance, volume, and temperature) simulations of PE conformations in decalin (left), and hexane (right). Adapted with permission from ref (125). Copyright 2021, Elsevier.

Figure 27

(a) Schematic of the continuous flow reactor system used to study intrinsic PO hydrogenolysis kinetics. Reproduced with permission from ref (89). Copyright 2023 Elsevier. (b) Schematic of reactor configuration for reactive separation during PO hydrogenolysis. Reproduced with permission from ref (177). Copyright 2024, American Chemical Society.

Scheme 10. Depiction of Overall Processes for (a) Catalytic Cracking; (b) Hydrocracking; and (c) Hydrogen Redistribution, with their Component Catalytic Reactions

Scheme 11. Two Mechanisms for Carbenium Ion Formation Induced by Brønsted Acid Sites: (a) Protonation of 3-Methyl-2-pentene to give a Tri-coordinate Carbenium Ion Directly; and (b) Protolytic Cracking of 3-Methylpentane and n-Hexane, via a Penta-coordinate Carbonium Ion

Scheme 12. Two Types of Carbenium Ion Isomerization

The red circles indicate the carbon atoms that undergo skeletal isomerization.

Scheme 13. Classification of β-Scission Reactions for Carbenium Ions (the Value of i is the Minimum Number of Carbon Atoms Required)

Reproduced with permission from ref (69). Copyright 2012, Wiley.

Scheme 14. Classical Mechanism for Hydrocracking of n-Alkanes on a Bifunctional Catalyst Whose Metal Sites Catalyze Alkane Dehydrogenation/Hydrogenation and Whose Bro̷nsted Acid Sites Catalyze Alkene Skeletal Isomerization, Leading to C–C Bond Cleavage

Reproduced with permission from ref (69). Copyright 2012, Wiley.

Figure 28

MAB dependence of the initial activity (A₀, normalized by total catalyst mass) for n-decane hydrocracking catalyzed by Pt/HY zeolite, at 200 °C. MAB is represented by the ratio of the number of accessible Pt atoms to the number of acid sites (n_Pt/n_A). Data labeled PtHY3 were recorded for a constant number of acid sites. Acidity decreases in the order HY3 > HY9 > HY35. Reproduced with permission from ref (195). Copyright 1996, Elsevier.

Figure 29

Molar distribution of hydrocarbon products from the catalytic cracking or hydrocracking of n-hexadecane. Typical results are shown for catalysts with no hydrogenation component (SiO₂–Al₂O₃–ZrO₂), a weak hydrogenation component (sulfided Co-Mo-S/SiO₂-Al₂O₃), or a strong hydrogenation component (Pt/Ca-Y zeolite), at similar yields of cracked products (Y_cr. ≈ 50 %). S_j^* is the modified cracking selectivity, defined as moles of hydrocarbons with j carbon atoms per mol n-hexadecane converted. Reproduced with permission from ref (69). Copyright 2012, Wiley.

Figure 30

(a) Linear dependence of the rate constants for the conversion of n-alkanes catalyzed by Pt/H-Y at 233 °C on the hydrocarbon chain length: k_MB, k_MTB, and k_CR are rate constants for the conversion of linear alkanes to mono-branched alkanes, mono-branched to multi-branched alkanes, and multibranched to cracked alkanes, respectively. Adapted with permission from ref (196). Copyright 1997, American Chemical Society. (b) Comparison of n-alkane chain length dependence of Langmuir adsorption constants (red circles) on Pt/H-Y (adapted with permission from ref (196), copyright 1997, American Chemical Society), with activities for the catalytic cracking of n-alkanes (blue diamonds) over FSS-1 at 350 °C, normalized to the reactivity of 2-methylnonane (adapted with permission from ref (201), copyright 1990, Elsevier).

Figure 31

Effect of chain length on lumped rate constants for hydrocracking of n-alkanes, catalyzed by Pt/amorphous silica-alumina (0.3 wt% Pt) at 380 °C, where k₁, k₂, and k₃ are pseudo-first-order rate constants for the isomerization of n-alkanes to mono-branched isomers, isomerization of n-alkanes to multi-branched isomers, and hydrocracking, respectively. Reproduced with permission from ref (203). Copyright 2004, American Chemical Society.

Scheme 15. Possible Carbenium Ion Generation in [nC₄Py]Cl-AlCl₃

Adapted with permission from ref (210). Copyright 2023, AAAS.

Figure 32

(a) ¹³C MAS NMR spectra of virgin LDPE and solid residues recovered from its depolymerization catalyzed by xPt–yWZr. Dependence on n_metal/n_BAS of the signal intensities for (b) tertiary carbons; and (c) methyl groups, relative to the total number of methylenes. (d) Fractions of linear, mono-branched, and di-branched isomers in the extractable hydrocarbons. Conditions: 250 °C, 30 bar H₂, 2 h. Reproduced with permission from ref (219). Copyright 2021, Elsevier.

Figure 33

Effect of Ni coverage on PP hydrocracking catalyzed by Ni/TiO₂-A-SG. The red dots/dashed line represent PP conversion (right ordinate), while stacked bars represent carbon selectivities. The Ni loadings (left to right) are 0.04, 0.25, 0.5, 1, 2, 5, 10, and 20 wt%. Reaction conditions: 260 °C, 30 bar H₂, 1 g PP, 30 mg catalyst, 3 h. Reproduced with permission from ref (48). Copyright 2023, Elsevier.

Scheme 16. Expected Products for Various Pentadecyl Cations Undergoing of β-Scission of (a) Type B₁; and (b) Type B₂ the Positive Charge Is Located at Either of Carbon Atom Marked *

Figure 34

(a) Schematic illustration of the diffusion of 1-hexene through zeolite crystals with empty micropores and with micropores containing propylene. (b) Mean-square displacement (MSD) of propylene molecules diffusing across an s-ZSM-5 zeolite with 1, 2, 4, or 8 unit layers. Inset: model showing propylene diffusion in s-ZSM-5 micropores with 4 unit layers. (c) Propylene diffusion coefficients, in (d) s-ZSM-5 zeolite micropores with different unit layer thicknesses. (d) Schematic illustration of cracking on zeolite external surfaces for n-ZSM-5 (left) and within zeolite micropores for s-ZSM-5 (right). (e) C₁–C₇ yields from PE cracking catalyzed by ZSM-5 with different b-axis thicknesses, as well as in mesoporous ZSM-5. Reproduced with permission from ref (209). Copyright 2022, American Chemical Society.

Figure 35

(a) TEM images; and (b) illustrations of pore geometries, for microporous mordenite (MOR), as well as hydrothermally-treated and recrystallized MOR (HyMOR), and desilicated and dealuminated MOR (DDMOR6). Reproduced with permission from ref (245). Copyright 2023, American Chemical Society.

Figure 36

TEM images and particle size distributions for (a) Pt@S-1; and (b) Pt/S-1. Comparison of depolymerization products generated by (c) various Pt-zeolite combinations; and (d) Pt@S-1 and Pt/S-1 catalysts. The reaction involved 2.0 g LDPE and 0.2 g catalyst (where two catalysts were used, the mass was 0.1 g each, as a physical mixture), and was performed at 250 °C for 2 h under 30 bar H₂. Reproduced with permission from ref (241). Copyright 2023, American Chemical Society.

Figure 37

DFT calculations and molecular dynamics simulations of alkene adsorption, hydrogenation, and alkane desorption in (a) Pt@S-1; and (b) Pt/S-1 (Si in orange; O in red; Pt in cyan). (c) Energies of adsorption for alkenes and desorption for alkanes in S-1, Pt@S-1, and Pt/S-1. (d) Mean-square displacements (MSD) of alkenes diffusing in Pt@S-1 as a function of chain length and branching. Reproduced with permission from ref (241). Copyright 2023, American Chemical Society.

Scheme 17. Proposed Mechanism for PE Hydrocracking Catalyzed by Various Pt/USY Materials, Based on Metal Location and Metal–Acid Proximity

Reproduced with permission from ref (488). Copyright 2024, Royal Society of Chemistry.

Figure 38

(a) Time-dependent yields of the solids and extractables recovered during LDPE hydrocracking catalyzed by 0.5Pt–15WZr. Optimum performance envelope (OPE) curves for hydrocarbon yields obtained in LDPE hydrocracking: (b) theoretical curves for various product types: 1-stable primary product; 2-stable primary plus secondary product; 3-unstable primary product; 4-unstable primary plus secondary product; 5-stable secondary product; 6-unstable secondary product. Reproduced with permission from ref (251). Copyright 2004, Wiley. (c–e) Experimental time-dependent data, obtained at time intervals from 1–24 h. Reaction conditions: LDPE (2.0 g), 0.5Pt–15WZr catalyst (200 mg), 250 °C, and 30 bar H₂. Reproduced with permission from ref (219). Copyright 2021, Elsevier.

Figure 39

One-pot catalytic upcycling of LDPE with i-C₅H₁₂ into liquid alkanes in a Lewis acidic chloroaluminate ionic liquid. The graph shows time-resolved profiles for LDPE conversion and the cumulative yield of alkanes (C₄, green diamonds; C₆–C₁₀, orange triangles; C₁₁–C₃₆, red squares,). Reaction conditions: LDPE, 200 mg; i-C₅H₁₂, 800 mg; [C₄Py]Cl, 1 mmol; AlCl₃ 2 mmol; ^tBuCl, 0.05 mmol; CH₂Cl₂, 3 mL; 70 °C. Curves represent an “optimal” fit (details not described) to the data. Reproduced with permission from ref (210). Copyright 2023, AAAS.

Scheme 18. Proposed Mechanism for Tandem Polyolefin Cracking-Alkylation with i-C₅H₁₂

The 2-methyl-2-butene in the cracking cycle is a representative intermediate, based on the finding that 2,3,4,4-dimethylhexane is a representative product. Reproduced with permission from ref (210). Copyright 2023, AAAS.

Scheme 19. A Four-Lump Model Describing LDPE Hydrocracking to Heavy Liquids (HL), Then to Naphtha (N), and Finally to Light Gases (G)

Reproduced with permission from ref (253). Copyright 2021 American Chemical Society.

Figure 40

(a) Schematic of the stirred batch reactor. Evolution of mass concentrations for various hydrocarbon groups: (b) PE; (c) heavy liquids (HL); (d) naphtha (N); and (e) gases (G), as a function of time and temperature in LDPE hydrocracking catalyzed by Pt/HBeta (1 wt% Pt). Points represent experimental data, while lines are predicted by the model. Reproduced with permission from ref (253). Copyright 2021, American Chemical Society.

Figure 41

Time evolution of the molecular weight distribution (MWD) of a hydrocarbon mixture undergoing hydrocracking by (a) chain-end scission (note: the monomer concentration is not shown); and (b) random chain scission. Each line denotes the MWD at a point evenly spaced in t, with the initial distribution at t = 0 (purple). Adapted with permission from ref (254). Copyright 2021, Royal Society of Chemistry. The illustration of chain-end scission and random scission is reproduced with permission from ref (255). Copyright 2023, Royal Society of Chemistry.

Figure 42

Effect of H₂ pretreatment on Brønsted acid strength of a Pt/WO₃/ZrO₂ catalyst: (a) IR spectra of adsorbed pyridine on catalysts pre-treated at 250 °C in a flow of either He or H₂ (20 % in He). Weakly-adsorbed pyridine was desorbed at 150 °C in flowing He prior to recording the spectra. (b) Decrease in the area of the band due to chemisorbed pyridinium with desorption temperature. While both catalysts have similar total numbers of Brønsted acid sites, the lower slope for the H₂-pretreated catalyst indicates its Brønsted acid sites are stronger. Reproduced with permission from ref (212). Copyright 2020, AAAS.

Figure 43

Light hydrocarbon product distributions present after 2 h hydrocracking catalyzed by 0.5Pt–15WZr at 250 °C and 30 bar H₂. The feed was (a) LDPE alone; (b) n-hexacosane alone (compared to results from LDPE alone, after 24 h reaction); and (c) a mixture of LDPE (90 wt%) and n-hexacosane (10 wt%). Reproduced with permission from ref (219). Copyright 2021, Elsevier.

Scheme 20. Comparison of Products Obtained from PS via Different Chemical Degradation Mechanisms

Reproduced with permission from ref (265). Copyright 2023, Royal Society of Chemistry.

Figure 44

Thermogravimetric analysis of major POs as well as PET, recorded at 10 K min^-1 under N₂. Reproduced with permission from ref (25). Copyright 2016, Elsevier.

Scheme 21. Proposed Mechanism for Electrochemical Dechlorination of PVC, Coupled to Chloroarene Synthesis

Reproduced with permission from ref (283). Copyright 2023, Springer Nature.

Scheme 22. Tandem Catalytic Dechlorination of Chlorine-Containing Polymers, Accompanied by Formation of Chloroarenes

Reproduced from ref (284). Copyright 2024, Springer Nature.

Scheme 23. Tandem Hydrodechlorination/Friedel–Crafts Alkylation of PVC,Initiated by [Ph₃C][B(C₆F₅)₄]

Scheme 24. Proposed Catalytic Cycle for Tandem Hydrodechlorination/Friedel–Crafts Alkylation of PVC Initiated by [Ph₃C][B(C₆F₅)₄], Using Et₃SiH as a Stoichiometric Reductant and an Aromatic to Trap the Carbocation; Adapted from Ref (286). Copyright 2023, Royal Society of Chemistry.

Reproduced with permission from ref (286). Copyright 2023, Royal Society of Chemistry.

Scheme 25. Chemical Structures of Three Common Polymer Additives: Antioxidant BPA, Antioxidant Irgafos 168, and Slip Agent Erucylamide

Figure 45

(a) Chemical structures of three common antioxidants; and (b) overall yields of solid, liquid, and gas products for all antioxidant-containing HDPE samples (at 0.5 and 2 wt% loadings of antioxidant) and for HDPE depleted of additives. Reaction conditions: 250 °C, 30 bar H₂, 2 g HDPE, 50 mg Pt/WO₃/ZrO₂, 2 h. Reproduced with permission from ref (290). Copyright 2022, Royal Society of Chemistry.

Figure 46

DRIFTS characterization of Pt/WO₃/ZrO₂ catalysts with and without adsorbed antioxidants: (a) CO adsorption at 35 °C shows a reduction in Pt coordination number; and (b) pyridine chemisorption on acid sites at 150 °C, showing changes in the relative concentrations of Lewis acid (LA) and Brønsted acid (BA) sites. Reproduced with permission from ref (290). Copyright 2022, Royal Society of Chemistry.

Figure 47

Hydrocracking of post-consumer plastic waste. (a) Photographs of the post-consumer plastics used in (b). The disposable cup and tray were not cleaned prior to reaction. (b) Non-solid yields from catalytic hydrocracking of plastic mixture (1.0 g) with MoS_x-HBeta (0.100 g) at 250 °C under 25 bar H₂, after 6 and 12 h. (c) Hydrocracking of a mixture of virgin polyolefins (1.0 g) with MoS_x-HBeta (0.100 g) at 250 °C under 25 barH₂, after 6 h. (d) Non-solid yields from catalytic hydrocracking of LDPE (1 g, M_w = 300,000 g/mol, Đ unspecified) in the presence of MoS_x-HBeta (0.100 g) and various impurities (mixed salt 1 is 50 mg each of CaCO₃, FeCl₃, MgSO₄, and KBr; mixed salt 2 is 50 mg each of Na₂CO₃, PbSO₄, ZnCl₂, and CuBr₂), at 250 °C under 25 bar H₂ for 6 h. All values were calculated on a carbon basis. Reproduced with permission from ref (213). Copyright 2023, AAAS.

Scheme 26. Two Main Reactions in Naphtha Catalytic Reforming

Scheme 27. Proposed Mechanism for Bifunctional Catalytic Reforming, Involving Alkane (De)Hydrogenation, Isomerization, Dehydrocyclization and Aromatization

Horizontal reactions represent H₂ transfers; vertical reactions are skeletal rearrangements. Redrawn with permission from ref (230). Copyright 2010, Wiley.

Scheme 28. Hydrocarbon Species Observed on (a) Pt(100); and (b) Pt (111), After Exposure to 1.5 Torr n-Hexane in the Presence or Absence of H₂

Reproduced with permission from ref (308). Copyright 2007 American Chemical Society.

Scheme 29. Some Proposed Reaction Pathways in PE Conversion to Alkylaromatics

The colored circles represent alkyl chains of varying lengths.

Figure 48

Top: (a) Product selectivity in PE aromatization after 4 h at 400 °C with various ZSM-5-based catalysts, configured in either single or dual beds. For the single-bed systems (first four bars), a mixture of PE (500 mg) and s-ZSM-5 (100 mg), ZSM-5 (100 mg), Zn/ZSM-5 (400 mg), or a mixture of s-ZSM-5 (100 mg) and Zn/ZSM-5 (400 mg), were combined in one bed. For the dual-bed systems (last three bars), a mixture of PE (500 mg) with s-ZSM-5 (100 mg) was placed in the first bed, while the Zn-containing aromatization catalyst (400 mg) was placed in the second bed. H₂ (3.3 %) balanced with Ar/N₂ flowed through both beds at 3 mL min^-1. A small amount of C₅₊ alkene products is included with the alkane products. Bottom: TEM images of (b) ZSM-5, and (c) meso-ZSM-5. (d) Schematic diagram of hydrocarbon transport through the dual catalyst beds. Reproduced with permission from ref (42). Copyright 2023, Royal Society of Chemistry.

Scheme 30. Comparison of Routes to Linear Alkylbenzenes: (a) Current Route, i.e., “Assembly” Synthesis; and (b) One-Pot Tandem Process from PE, i.e., “Deconstruction” Synthesis

Figure 49

Strategy for converting PE to aromatics in the presence of CO₂: (a) Schematic of the reactor and product fractions; (b) recovered product distributions; and (c) distribution of hydrocarbons in the volatile and low-boiling liquid products. Reaction conditions: physical mixture of Zn/ZSM-5 5 (0.2 g, Si/Al = 42.5, 3 wt% Zn, Zn particle size and size distribution unspecified), Cu-Fe₃O₄ (0.2 g, 10 wt% Cu, Cu particle size and size distribution unspecified), and PE (2.0 g, M_w = 3.5 × 10³ g/mol, Đ unspecified) at 360 °C under 20 bar Ar or CO₂, 1.0 h. Reproduced with permission from ref (330). Copyright 2023, Chinese Chemical Society.

Figure 50

Analysis of the liquid hydrocarbons recovered from the solvent-free depolymerization of PE (M_w = 3.5 × 10³ g/mol, Đ = 1.9) catalyzed by Pt/γ-Al₂O₃ (0.200 g, 1.5 wt% Pt), after 24 h at 280 °C in an unstirred mini-autoclave reactor: (a) GPC analysis, conducted using both refractive index (RI) and UV detectors; (b) ¹H and ¹³C NMR spectra, recorded in 1,1,2,2-tetrachloroethane-d₂; and (c) FD-MS analysis. Reproduced with permission from ref (49). Copyright 2020, AAAS.

Figure 51

(a) Time course of solvent-free depolymerization of PE (M_w = 3.52 × 10³ g/mol, Đ = 1.9) catalyzed by Pt/γ-Al₂O₃ (0.200 g, 1.5 wt% Pt) in an unstirred mini-autoclave reactor at 280 °C. Average molecular weight (M_n, blue) and dispersity (Đ, red) are shown for all non-gas hydrocarbons. The red dashed line is present only to guide the eye. The shaded region indicates 95 % confidence bands for the model fit. Reproduced with permission from ref (49). Copyright 2020, AAAS. (b) Time-dependent GPC analysis of solid residue recovered from depolymerization of HDPE (5 g, M_w = 2.8 × 10⁵ g/mol, Đ = 3.8) catalyzed by Ru/HZSM-5 (0.5 g, Si/Al = 300) at 280 °C. Reproduced with permission from ref (329). Copyright 2023, Springer Nature.

Scheme 31. Gibbs’ Energies for Aromatization and C–C Bond Hydrogenolysis at 280 °C, Estimated Using Benson Group Contributions

Figure 52

Statistical analysis of the reaction network for n-decane aromatization coupled with hydrogenolysis. The abscissa represents the number of carbon atoms in each product molecule, while the blue line is the number of possible molecules with carbon number C_n for n = 1 to 10 (right-hand ordinate). Species in the reaction network are classified as alkanes (orange), acyclic alkenes (including alkadienes, gray), and cyclic hydrocarbons (including cycloalkanes, cycloalkenes, cycloalkadienes, and aromatics, yellow). Reproduced with permission from ref (333). Copyright 2023, American Chemical Society.

Figure 53

(a) The number of strong BAS correlates with the yield of aromatics from triacontane. Reaction conditions: n-C₃₀H₆₂ (0.120 g), Pt catalyst (0.100 g, 1.5 wt% Pt supported on either SiO₂ or γ-Al₂O₃) alone or physically mixed with various amounts of either F-Al₂O₃ (1.3 wt% F) or Cl-Al₂O₃ (1.8 wt% Cl), 12 h at 280 °C. Abscissa error bars represent the standard deviation in the measurement of the sum of the number of strong BAS contributed by each catalyst support. Ordinate error bars represent the standard deviation of duplicate experiments. (b) Proposed bifunctional mechanism for aromatization of PE fragments. Reproduced with permission from ref (50). Copyright 2023, Elsevier.

Figure 54

(a) Acid site characterization of Ru/HZSM-5 with various Si/Al ratios. The effect of various Ru-based catalysts on HDPE upcycling at 280 °C for 24 h on (b and d) the yields of volatiles/gases, liquid-phase products and insoluble hydrocarbons; (c and e) the selectivity for volatiles/gases and liquid-phase products; and (f) GPC analysis of the solid residues. Adapted with permission from ref (329). Copyright 2023, Springer Nature.

Scheme 32. Proposed Mechanism for the 1,5- and 1,6-Cyclization of Alkadienes during HDPE Upcycling Catalyzed by Ru/HZSM-5

Scheme 33. Semiquantitative Depiction of Reaction Conditions and Catalyst Architectures That Determine Selectivity in Polyolefin Cracking to Different Hydrocarbon Types (Filled Circles) and Reaction Regimes (Dashed Circles), for Monofunctional Acid Catalysts and Bifunctional Metal–Acid Catalysts

Scheme 34. Overall Process of Tandem Alkane Metathesis to Transform PE into Intermediate Chain-Length Alkanes

Figure 55

Tandem alkane cross-metathesis of n-eicosane (5 wt%, 1 g) with n-pentane (ca. 19 g), catalyzed by Re₂O₇/γ-Al₂O₃ (8 wt% Re, 500 mg) and either PtSn/γ-Al₂O₃ (0.8 wt% Pt, 1.7 wt% Sn, 500 mg) or Pt/γ-Al₂O₃ (5 wt% Pt, 500 mg) in a batch reactor at 200 °C: (a) distribution of linear alkane products formed after 15 h; and (b) time-course of the product distribution in the tandem reaction catalyzed by PtSn/γ-Al₂O₃ and Re₂O₇/γ-Al₂O₃. Reproduced with permission from ref (345). Copyright 2021, American Chemical Society.

Scheme 35. Structures of Homogeneous Dehydrogenation Catalysts (1 and 2) Compatible with a Schrock Catalyst (3) for Tandem Alkane Metathesis

Figure 56

(a) Hydrocarbons produced during the self-metatheses of ethane, propane, n-butane, and n-pentane (all at 3% conversion), catalyzed by silica-supported [Ta]-H at 150 °C (ethane, propane, or n-butane/Ta ∼ 800, P = 1 bar; or n-pentane/Ta ∼ 400, P = 0.5 bar), and (b) proposed mechanism for ethane self-metathesis. Reproduced with permission from ref (340). Copyright 1997, AAAS. (c) Proposed mechanism for propane metathesis catalyzed by silica-supported [Ta-H]. Adapted with permission from ref (364). Copyright 2005, American Chemical Society.

Figure 57

(a) Preparation of a series of bimetallic supported W-Zr hydride catalysts; and (b) kinetic profiles for the self-metathesis of n-decane, catalyzed by the bimetallic catalysts. Adapted with permission from ref (366). Copyright 2016, American Chemical Society.

Figure 58

(a) Overall scheme for cross-metathesis between propane and n-decane, and (b) influence of the C₃/C₁₀ molar ratio on the product distribution. Reaction conditions: propane (0.5 mL/min), fully deuterated n-decane (in Ar carrier gas, 10 mL/min), molar ratio C₃/C₁₀ = 2-100, [(≡SiO)W(CH₃)₂(H)₃] catalyst (150 mg, 1.5 wt% W), 150 °C. Adapted with permission from ref (372). Copyright 2019, American Chemical Society.

Scheme 36. Proposed Mechanism for Cross-Metathesis Between n-Decane and Propane, Catalyzed by Silica-Supported “Single-Site” W

Including competing self-metathesis reactions. Reproduced with permission from ref (372). Copyright 2019, American Chemical Society.

Figure 59

Selectivity for liquid alkane products as a function of time, in alkane cross-metathesis between n-C₁₆H₃₄ and either (a) n-C₇H₁₆; or (b) n-C₁₀H₂₂. (c) Differential molecular weight distributions (as measured by high temperature GPC) of the post-reaction solids from C₇/C₁₆ cross-metathesis after 2 h; and from C₁₀/C₁₆ cross-metathesis after 3 h. Adapted with permission from ref (346). Copyright 2022, Elsevier.

Scheme 37. Tandem Ethenolysis/Isomerization of an Unsaturated Alkene to Propylene

Reproduced with permission from ref (344). Copyright 2022, American Chemical Society.

Figure 60

(a) Scheme for C₆₊ alkene conversion to propylene via tandem ethenolysis-isomerization; and (b–c) propylene yields from tandem ethenolysis-isomerization of 1-hexene, catalyzed by a physical mixture of activated (and reactivated) HBEA and MoO₃/γ-Al₂O₃ (12 wt% Mo) at various temperatures (A and A′ correspond to a space time of 6 g_cat g_hexene^–1 h; B and B′ to a space time of 12 g_cat g_hexene^–1 h). (d) Evolution of propylene yield in tandem ethenolysis-isomerization of FCC light and crude FCC, catalyzed by a physical mixture of activated (and reactivated) HBEA and MoO₃/γ-Al₂O₃ (12 wt% Mo), as a function of time on-stream at 75 °C under 3 bar ethylene, with a space-time of 6 g_cat g_alkene^–1 h. (e) Propylene yields in tandem ethenolysis-isomerization of 1-hexene using activated and reactivated 12MoO₃/Al₂O₃-HBEA catalyst mixtures, as a function of catalyst pretreatment. Reaction conditions: 3 bar ethylene, ethylene/1-hexene molar ratio of 10, 75 °C, 1 h time on-stream, space time 6 g_cat g_hexene^–1 h. Adapted with permission from ref (383). Copyright 2021, Royal Society of Chemistry.

Scheme 38. Structures of Homogeneous Catalysts Used in Tandem Isomerization-Metathesis of 1-Octadecene: (a) Bicyclic (Alkyl)(amino)Ru Carbene Catalyst (for Alkene Metathesis), and (b) Ru Hydride Catalyst (for Alkene Isomerization)

Adapted with permission from ref (386). Copyright 2022, Wiley-VCH GmbH.

Scheme 39. (a) Structures of Two PCP-Pincer Ir Catalysts; and (b) Transfer Dehydrogenation of Poly(1-Hexene), Accompanied by C=C Bond Isomerization

Adapted with permission from ref (391). Copyright 2005, Royal Society of Chemistry.

Scheme 40. Mechanism of Transfer Dehydrogenation between Poly(1-hexene) and Norbornene, a Sacrificial Hydrogen Acceptor, Catalyzed by a Pincer-PCP Ir Complex

Reproduced with permission from ref (392). Copyright 2006 Elsevier.

Figure 61

(a) Mechanism of cross-metathesis between PE and a light alkane; and (b) structures of the homogeneous alkane dehydrogenation/alkene hydrogenation catalysts, and formula for the heterogeneous alkene metathesis catalyst. Reproduced with permission from ref (343). Copyright 2016, the authors.

Figure 62

Product characterization from tandem dehydrogenation/alkene cross-metathesis of PE (130 mg, M_w = 5,200 g/mol, Đ = 3.0) with n-pentane (30 mL), catalyzed by Re₂O₇/γ-Al₂O₃ (500 mg) and PtSn/γ-Al₂O₃ (500 mg) at 200 °C: (a) molecular weight distributions for PE and post-reaction solids; and (b) analysis of light alkane products. Reproduced with permission from ref (345). Copyright 2021, American Chemical Society.

Figure 63

(a) Selectivity in the alkane cross-metathesis of LDPE with n-decane. Reaction conditions: LDPE (M_w = 75,000 g/mol, M_n = 9,100 g/mol, 1.55 g), n-C₁₀H₂₂/LDPE = 6.3 (g/g), Pt/γ-Al₂O₃ (1 wt% Pt, 500 mg), WO_x/SiO₂ (2 wt% W, 800 mg), zeolite 4A (1.0 g), 30 bar N₂, 300 °C, 3 h. (b) Comparison of differential molecular weight distributions for the solid products, measured by high temperature GPC.^, Reproduced with permission from ref (346). Copyright 2022, Elsevier.

Figure 64

(a) Catalysts used in PE conversion to propylene by dehydrogenation and tandem ethenolysis/isomerization. Reproduced with permission from ref (396). Copyright 2022, AAAS. (b) Setup of the flow reactor for semi-continuous tandem ethenolysis/isomerization at atmospheric pressure. Reproduced with permission from ref (344). Copyright 2022, American Chemical Society. (c) Proposed pathway for forming doubly-¹³C-labeled propylene via tandem ethenolysis/isomerization of a hydrocarbon chain using doubly-¹³C-labeled ethylene. Reproduced with permission from ref (344). Copyright 2022, American Chemical Society. (d) Isotopic labeling of propylene made by dehydrogenation/ethenolysis/isomerization of labeled PE (99.9 % ¹³C) with unlabeled ethylene. Adapted with permission from ref (396). Copyright 2022, AAAS.

Scheme 41. Strategy for Free Radical Bromination of PE, Followed by Dehydrobromination and Ethenolysis

Reproduced with permission from ref (399). Copyright 2021, American Chemical Society.

Scheme 42. Two Strategies for Depolymerization of Chlorinated Polyolefins: (a) Partial Dehydrochlorination via Base-Catalyzed Elimination of HCl, Followed by Alkene Cross-Metathesis with Z-1,4-Diacetoxy-2-butene; and (b) Complete Dehydrochlorination in an Ionic Liquid, Followed by Ethenolysis

(a) Adapted with permission from ref (400). Copyright 2023, Wiley-VCH GmbH. (b) Adapted with permission from ref (281). Copyright 2014, Wiley.

Figure 65

(a) Evolution of polymer molecular weight (blue) and dispersity (red) during PE self-metathesis catalyzed by [Ta-H]⁺ supported on sulfated alumina; and (b) proposed single-site mechanism. Adapted with permission from ref (156). Copyright 2023, American Chemical Society.

Scheme 43. Strategy for Converting Post-Consumer Polyethylene to a Chemically Recyclable PE-Like Polymer: (a) Ir-Catalyzed Dehydrogenation; and (b) Reaction Conditions and Key Intermediates

DH1–DH4: products from dehydrogenation of HDPE at various times (24–96 h); P1: telechelic macromonomer; RP1: repolymerization product; [P1 + P2]: macromonomers; RP2: products from repolymerization of [P1 + P2]. Adapted with permission from ref (401). Copyright 2022, American Chemical Society.

Scheme 44. Two Strategies for Oxidative Upcycling of Polyolefins: Oxidative Cleavage to Small Molecule Oxygenates, and Polyolefin Functionalization without Chain Cleavage

Scheme 45. Proposed Radical Chain Mechanism for Thermal Catalytic Oxidation of Polyolefins: (a) Initiation, Starting with the Formation of Reactive Oxygen Species (ROS); (b) Propagation, Including β-Scission Resulting in Polymer Chain Cleavage; (c) Termination, Showing Several Possible Fates of the Alkyperoxy Radical ROO^•; and (d) Peroxide Decomposition

Scheme 46. Norrish-Type Photochemical Reactions, Shown in the Context of Polyolefin Photooxidation Where a Prior Reaction Has Installed a Ketone or Aldehyde on the Polymer Backbone

Scheme 47. Fe-Catalyzed Formation of HO^• and HOO^• Radicals via the Fenton Reaction

Scheme 48. Two Possible Outcomes of the Stoichiometric Oxidation of an Alkene by Permanganate

Figure 66

Quantitative analysis of LLDPE after Co(acac)₂-catalyzed oxidation for 3 h at various temperatures: (a) oxygen content of the solid residue; (b) weight loss relative to the starting amount of polymer; (c) number- and weight-averaged molecular weights of the solid residue; and (d) “oxygen demand”. Adapted with permission from ref (421). Copyright 1993, Elsevier.

Figure 67

Liquid yields from the oxidation of LDPE at various temperatures and air flow rates: (a) without a catalyst; (b) in the presence of a MoO₃/SiO₂ catalyst; (c) AN and PN of liquid products formed at 400 °C with and without the catalyst, as a function of air flow rate; and (d) functional group distribution with and without the catalyst at 400 °C, in flowing air (600 mL/min). Adapted with permission from ref (422). Copyright 2006, Elsevier.

Figure 68

Effect of metal acetylacetonate catalysts on PE oxidation, as judged by the acid numbers (AN) of the hydrocarbon products: (1) without catalyst or dicumyl peroxide (DCP) initiator; (2) without catalyst but with DCP; (3–9) with DCP and a transition metal acetylacetonate, containing (3) Fe(III), (4) Mn(II) and (5) Co(II), (6) V(III), (7) Cu(II), (8) Ni(II), or (9) Al(III). Reproduced with permission from ref (423). Copyright 2010 Wiley.

Scheme 49. Simplified Mechanism for the Amoco Mid-Century Process for Hydrocarbon Oxidation

Scheme 50. Hydrolysis of Chloroacetic Acid and Subsequent Condensation with Acetic Acid

Scheme 51. Two-Step Chemo-biocatalytic Strategy for Upcycling Mixed Plastic Waste via Initial Chemical Oxidation, Followed by Bioconversion of the Oxygenated Molecular Fragments

Reproduced with permission from ref (434). Copyright 2022, AAAS.

Scheme 52. (a) Catalytic Oxidation of PE to Small-Molecule Dicarboxylic Acids; and (b) Catalytic Oxidation of PS to Benzoic Acid, Followed by Upgrading via Fungal Metabolism

(a) Reproduced with permission from ref (435). Copyright 2022 Wiley. (b) Reproduced with permission from ref (436). Copyright 2023 American Chemical Society.

Scheme 53. Proposed Initiation Step in PS Oxidation Catalyzed by THICA

Figure 69

(a) Schematic of the two-step upcycling of commercial POs to long-chain organic acids. (b) Overall reactions, showing the conversion of PE and PP to long-chain organic acids; (c) product yields from PE “pyrolysis” in various atmospheres; (d) product yields from PP “pyrolysis” in various atmospheres, as well as from a waste PE/PP mixture; and (e) proposed mechanism for polymer oxidation. Adapted with permission from ref (439). Copyright 2023, AAAS.

Figure 70

Comparison of 2D GC analyses and ¹³C NMR spectra for (a, d) HDPE pyrolysis oil (light cut); (b, e) after hydroformylation of the pyrolysis oil; and (c, f) after hydrogenation of the hydroformylated oil, as well as (g) ¹H-¹³C HSQC NMR of the hydrogenated oil (blue and red represent signals for primary and secondary carbons, respectively). Reproduced with permission from ref (440). Copyright 2023, AAAS.

Figure 71

Comparison of proposed pyrolysis oil upgrading processes: (a) without, and (b) with separation prior to hydroformylation; (c) CAPEX and product MSPs analysis; (d) comparison of GHG emissions from (I) the proposed technology for converting 1 kg plastic to chemicals, (II) producing the same amount of chemicals using conventional technologies, (III) converting 1 kg plastic to alkenes by pyrolysis and steam cracking, and (IV) incineration of 1 kg plastic. Reproduced with permission from ref (440). Copyright 2023, AAAS.

Scheme 54. (a) Photooxidation of PS to Benzoic Acid; and (b) Proposed Mechanism

Adapted with permission from ref (441). Copyright 2021, Wiley.

Figure 72

(a) IR spectra of PS recorded during 6 h of Fe-catalyzed photooxidation; and (b) GPC analysis of the oxidized PS recovered during the first 5 h of reaction. Reproduced with permission from ref (445). Copyright 2021, Wiley.

Figure 73

Schematic of PS photooxidation in flow. Reproduced with permission from ref (446). Copyright 2022, American Chemical Society.

Figure 74

Product distributions from PS photooxidation catalyzed by (a) FeCl₃; and (b) FeBr₃. Reproduced with permission from ref (447). Copyright 2023, American Chemical Society.

Figure 75

(a) Schematic for flow setup used in PS photooxidation; (b) scale-up of PS photooxidation; and (c) UV–vis spectra of PS-acid solutions. Adapted with permission from ref (448). Copyright 2022, American Chemical Society.

Scheme 55. Proposed Mechanism for PS Photooxidation Catalyzed by Fluorenone

Reproduced with permission from ref (449). Copyright 2022, American Chemical Society

Scheme 56. Proposed Mechanism for PS Oxidation Catalyzed by N-Bromosuccinamide

Reproduced with permission from ref (451). Copyright 2022, Elsevier.

Figure 76

(a) PS oxidation to aromatic oxygenates, catalyzed by g-C₃N₄; and (b) evolution of the product distribution with time. Adapted with permission from ref (452). Copyright 2022, Springer Nature.

Figure 77

Outcomes for photooxidation of various types of plastics, catalyzed by V(O)(acac)₂. Carbon recovery refers to the mol fraction of carbon present in the carboxylic acid products and/or soluble oligomeric products, relative to mol carbon in the original polymer. Reproduced with permission from ref (456). Copyright 2023, Elsevier.

Figure 78

(a) Schematic of photo-oxidative conversion of PE, PP, and PVC to acetic acid; (b) CO₂ yields; and (c) CH₃COOH yields. Adapted with permission from ref (457). Copyright 2020, Wiley.

Figure 79

Proposed band structure of the VPOM/CNNS hybrid catalyst. Reproduced with permission from ref (458). Copyright 2022 Elsevier.

Figure 80

(a) Overall process for PE degradation to organic acids via an unsaturated sulfonated polymer, PESO₃-Fe; and (b) proposed pathway for PESO₃-Fe oxidation. Adapted with permission from ref (460). Copyright 2016, Wiley.

Scheme 57. Conversion of a Sulfonated PE to Acetic and Butanoic Acids

Reproduced with permission from ref (462). Copyright 2021, Elsevier.

Scheme 58. C–H Borylation and Subsequent Oxidation of Various Branched Polyolefins⁻

Scheme 59. (a) Catalytic Conversion of PE to Functionalized Alkane Fragments, via Alkylaluminum Intermediates, and (b) Proposed Mechanism for PE Alumination, Catalyzed by a Supported Organozirconium

Adapted with permission from ref (473). Copyright 2021, Elsevier.

Figure 81

Targets of PE upcycling vary in their enthalpy cost. Coreagent-free strategies (complete depolymerization to ethylene, and partial depolymerization to other alkenes/alkadienes, as well as aromatization) are endothermic, while many strategies that involve a coreagent are exothermic.

Figure 82

Catalytic strategies for PE upcycling, organized by C/H ratio: coreagent-free (upper grey arrows) and reagent-assisted (lower colored arrows; colored lines indicate coreagents). C/H ratios are shown for PE and for each product category. For products where the ratio varies depending on the carbon number C_m, the limit for the smallest possible carbon number is shown in blue; the limit as C_m approaches infinity is shown in red.