Efficient and Selective Chemical Recycling of CO₂‐Based Alicyclic Polycarbonates via Catalytic Pyrolysis

Abstract

Chemical recycling of polymers to their constituent monomers is the foremost challenge in building a sustainable circular plastics economy. Here, we report a strategy for highly efficient depolymerization of various CO2‐based alicyclic polycarbonates to epoxide monomers in solvent‐free conditions by a simple CrIII‐Salen complex mediated catalytic pyrolysis process. The chemical recycling of the widely studied poly(cyclohexene carbonate) exhibits excellent reactivity (TOF up to 3000 h−1, 0.1 mol % catalyst loading) and high epoxide monomer selectivity (>99 %). Mechanistic investigation reveals that the process proceeds in a sequential fashion via a trans‐carbonate intermediate.

Solid-State Chemical Recycling of Polycarbonates to Epoxides and Carbon Dioxide Using a Heterodinuclear Mg(II)Co(II) Catalyst

“…Average error taken from repeat runs (<5%). d Mass loss rate = mass PCHC consumed (20–80% conversion)/catalyst mass/time. e k obs = gradient of linear fitting of the logarithm of %polymer mass vs time (Figures S8–18). f Polymer:catalyst mixed by pestle and mortar. g Values calculated from ref . h Values calculated from ref . …”

“…Thus, achieving its chemical recycling to monomer is important; earlier this year, we reported a Mg­(II)­Mg­(II) catalyst, applied in para -xylene solution, which delivered very high CHO selectivity (98 %) and a turn-over-frequency (TOF) of 150 h –1 (0.33 mol % catalyst, 120 °C, [PCHC] 0 = 1 M) (Figure ). As this manuscript was submitted, Lu and co-workers reported PCHC pyrolysis using a Cr­(III)­(salen)­Cl/PPNN 3 catalyst system, which showed really excellent rates, albeit at high temperature (Figure , 0.2 mol % catalyst, TOF = 3000 h –1 , 200 °C) . A decomposed di-Zn­(II) catalyst was also reported for PCHC depolymerization .…”

“…26 As this manuscript was submitted, Lu and co-workers reported PCHC pyrolysis using a Cr(III)(salen)Cl/PPNN 3 catalyst system, which showed really excellent rates, albeit at high temperature (Figure 1, 0.2 mol % catalyst, TOF = 3000 h −1 , 200 °C). 27 A decomposed di-Zn(II) catalyst was also reported for PCHC depolymerization. 33 Our focus is on improving catalytic activity, selectivity, and applicability in PCHC depolymerization.…”

“…At the lowest loadings, depolymerization still occurred although with slightly lower activity, (1:10000, TOF = 12400 h −1 ), which is attributed to some catalyst decomposition. Compared with the next best other depolymerization catalyst (Cr(salen)/PPNCl) under as close to equivalent conditions as possible, 27 the Mg(II)Co(II) catalyst shows >5000× greater activity at one tenth the catalyst loading (Table 1, entry 14). Successful depolymerization catalysis was achieved at temperatures above 100 °C, yielding CHO as the sole product in all cases (Figure 3a).…”

Recycling and Upgrading Utilization of Polymer Plastics†

“…[ 35 ] For instance, a simple Cr III ‐Salen catalyst is adopted for the catalytic pyrolysis of various CO 2 ‐based polycarbonates without solvent, which can make CO 2 ‐based polycarbonates depolymerize back to an epoxy monomer (Figure 2a). [ 42 ] More importantly, this method can be extended to other aliphatic carbonates returned to epoxy monomers with high selectivity (Figure 2b).…”