The_Catalyst_Review_December_2023 - 14

Experimental Abstracts
Highly Efficient Depolymerization of Waste Polyesters Enabled by Transesterification/
Hydrogenation Relay Under Mild Conditions
Because of the major environmental challenges involved with traditional plastic waste remediation processes, efforts are underway to
create more effective and ecologically acceptable plastic recycling methods. Herein, the authors describe a novel transesterification/
hydrogenation relay strategy in the presence of CH3
OH and a new Ru catalyst. Specifically, a Ru(II)-pincer complex with a quinaldinebased
skeleton was found to promote the high-yield degradation of various polyethylene terephthalate (PET) sources and other types
of polyesters under very mild conditions.
These workers began by determining whether
macromolecular PET could be converted
into low-weight and more soluble oligomeric
fragments via transesterification with CH3
OH
by measuring the inherent viscosity of reaction
residues obtained under different conditions.
Compared with the inherent viscosity of
undegraded PET granulates, the inherent
viscosity was dramatically diminished to
0.08 when CH3
Scheme 1. Synthesis of newly designed quinaldine-based Ru(II) catalysts. Conditions:
a) 1,4-dibromobutane, K2
1 h, then Cy2PCl, - 78°C-40°C, 12 h. (2) BH3
DABCO, toluene, 50°C, 12 h; d) RuHCl(CO)(PPh3
e) tBuOK, toluene, r.t., 1 h, 94% yield.
CO3, DMF, 125°C, 8 h, 83% yield; b) (1) nBuLi, - 78°C-r.t.,
·THF, 0°C-40°C, 12 h, 75% yield; c)
)3, THF, 70°C, 12 h, 93% yield;
OH and t-BuOK were both
present in the reaction system. These results
suggest that the PET was transformed into the
lower molecular weight fragments through
transesterification with CH3
OH. Next, the
authors prepared quinaldine-based Ru(II)
catalysts (Scheme 1). The Ru(II)-pincer complex
[Ru]-1 containing a quinaldine-based skeleton
was obtained in 93% yield. The molecular
structure of [Ru]-1 was unambiguously
confirmed by NMR, HRMS, and X-ray
crystallography analysis. A dearomatized
species [Ru]-1' was then prepared from [Ru]-1 in
94% yield with the aid of t-BuOK.
The reaction of [Ru]-1' in the presence of H2
After investigating the reactivity of Ru
complexes, their catalytic performance in
polyester hydrogenolysis was evaluated. A
61% yield of degradation products(diols) was
observed when 0.5 mol% of [Ru]-1'-H2
and 3
mol% t-BuOK were employed as the catalysts
for the relay degradation of PET in the mixture
of CH3
OH and toluene under 5 bar of H2
at 80°C for 24 h. After optimized reaction
conditions were identified, the performance
of this protocol in digesting a diverse array of
commercially available or post-consumer PET
was evaluated. All the tested PET plastics,
including beverage, water, mouthwash,
handwash bottles, and soundproof board, were
successfully transformed to diols and ethylene
glycol in good to excellent yields (78-89%)
with outstanding selectivity. Based on these
experimental results and literature precedents,
the authors propose a plausible catalytic cycle
for this Ru-catalyzed hydrogenolysis of polyesters shown in Scheme 2. Yue H, Shiyun Z, Juanfang X, et al. (2023) Angew. Chem. Int. Ed., doi.
org/10.1002/anie.202312564
14
The Catalyst Review
December 2023
was then explored, resulting in the formation of the complex [Ru]-1'-H2
, which can activate the
H-H bond via the cooperation of the enamine group and Ru. The H- H bond activation and formation were found to be reversible in the
reaction mixture.
Scheme 2. Proposed mechanism of polyester hydrogenative depolymerization

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