Polyethylene Terephthalate Deconstruction Catalyzed by a Carbon-Supported Single-Site Molybdenum-Dioxo Complex

Article abstract of DOI:10.1002/anie.202007423

Polyethylene terephthalate (PET) is selectively depolymerized by a carbon-supported single-site molybdenum-dioxo catalyst to terephthalic acid (PTA) and ethylene. The solventless reactions are most efficient under 1 atmosphere of H₂. The catalyst exhibits high stability and can be recycled multiple times without loss of activity. Waste beverage bottle PET or a PET + polypropylene (PP) mixture (simulating the bottle + cap) proceeds at 260 °C with complete PET deconstruction and quantitative PTA isolation. Mechanistic studies with a model diester, 1,2-ethanediol dibenzoate, suggest the reaction proceeds by initial retro-hydroalkoxylation/β-C?O scission and subsequent hydrogenolysis of the vinyl benzoate intermediate.

Full text of DOI:10.1002/anie.202007423

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Accepted Article
Title: Polyethylene Terephthalate Deconstruction Catalyzed by a
Carbon-Supported Single-Site Molybdenum-dioxo Complex.
Authors: Yosi Kratish, Jiaqi Li, Shanfu Liu, Yanshan Gao, and Tobin
Jay Marks
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007423
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Polyethylene Terephthalate Deconstruction Catalyzed by a
Carbon-Supported Single-Site Molybdenum-dioxo Complex.
Yosi Kratish, Jiaqi Li, Shanfu Liu, Yanshan Gao* and Tobin J. Marks*
Abstract: Polyethylene terephthalate (PET) is selectively depolymer-
ized by a carbon-supported single-site molybdenum-dioxo catalyst to
terephthalic acid (PTA), and ethylene. The solventless reactions are
most efficient under 1 atmosphere of H2. The catalyst exhibits high
stability and can be recycled multiple times without loss of activity.
Waste beverage bottle PET or a PET + polypropylene (PP) mixture
(simulating the bottle + cap) proceeds at 260°C with complete PET
deconstruction and quantitative PTA isolation. Mechanistic studies
with a model diester, 1,2-ethanediol dibenzoate, suggest the reaction
proceeds via initial retro-hydroalkoxylation/β-C-O scission, followed
by hydrogenolysis of the vinyl benzoate intermediate.
phosphine ligands, LRu(tmm) (L = triphos, triphos-xyl, tmm=
trimethylenemethane),[15a] were shown to catalyze PET
hydrogenolysis to the corresponding alcohols.[15] Although H2 is a
cost-effective reductant, these processes require high H2
pressures, long reaction times (16-48 h), use of solvents, and
expensive, air-sensitive, potentially toxic Ru catalysts with
complex ligands (Figure 1B).
Polymer-based plastics are among the most widely used
synthetic materials worldwide and are essential for modern life
and the global economy. By 2050, their annual production should
reach ~1.12 billion tons.[1] Since almost all plastics are produced
from fossil feedstocks their impact on finite natural resources is a
concern, as is waste plastic accumulation and the worldwide
environmental consequences.[2] Underlying this accumulation is a
linear economic model whereby most products are discarded
after use. In contrast, a circular economy in which waste plastics
are recycled and repurposed is far more rational.[3]
Due to the great popularity of polyesters, there is a rising need
for their recycling. Current technologies are: 1) Thermo-
mechanical recycling in which sorted polyethylene terephthalate
(PET) waste is remelted and reprocessed. The high temperatures
lead to significant thermal degradation, and lower grade plastics
with inferior optical, thermal, and mechanical properties. 2)
Chemical recycling in which one or more component monomers
is recovered.[4] The attraction here is that valuable monomers can
be repurposed for known or new materials having identical or
higher performance. The most common chemical process for
polyesters such as PET is glycolytic, methanolytic, or hydrolytic
trans-esterification. These are catalyzed by metal acetates[5],
titanium complexes,[6] metal chlorides,[7] metallic,[8] and metal
oxide nanoparticles,[9], ionic liquids,[10] and bases.[11] Hydro-
silylation[12] and microbial agents[13] were recently shown to also
affect PET deconstruction. The principal limitation of such
processes is formation of difficult separated side products and the
requirement of12` large solvent and degradation agent excesses
(Figure 1A).
Figure 1. PET chemical recycling options.
The ideal polyester depolymerization catalyst should be: (a)
Earth-abundant, sustainable, and thermally, moisture, and air-
stable, (b) Inexpensive with simple ligands; (c) Nontoxic since the
recovered monomers will be reused in consumer products. (d)
Selective and indefinitely recyclable to lower the cost; (e) Effective
in diverse C-O hydrogenolysis processes; (f) Active in solvent-
free scenarios.
Recently this Laboratory reported an air-stable, earth-
abundant, recyclable, single-site Mo-dioxo catalyst grafted on
activated carbon, C/MoO2 which is active for trans-
esterification,[16]
hydrosilylation,[17]
and
N-oxide/sulfoxide
reductions.[18] C/MoO2 is also active for alcohol dehydrogenation
to H2 and aldehydes.[19] Also C/MoO2 can be easily recovered by
filtration and reused with negligible Mo leaching or activity
loss.[15,18] These characteristics raise the intriguing question of
whether C/MoO2 might provide a platform for transesterification
and hydrogenation processes useful in polyester recycling. Here
we report an efficient solventless PET depolymerization process
catalyzed by C/MoO2, yielding the starting monomer, terephthalic
acid (PTA) and ethylene as the major products (Figure 1C). Model
substrate 1,2-ethanediol dibenzoate (1) is first used for
mechanistic insight and process optimization. As noted above,
In principal, catalytic hydrogenolysis is attractive for
deconstructing polyesters, however, there has been limited
research. Recently, homogeneous Ru pincer complexes, such as
Milstein`s catalyst,[14] and Ru complexes with tridentate
Dr. Y. Kratish, J. Li, S. Liu, Dr. Y. Gao and Prof. Tobin J. Marks.
Department of Chemistry and the Institute for Catalysis in Energy
Processes (ICEP), Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208-3113, USA.
E-mail: yanshan@northwestern.edu, t-marks@northwestern.edu
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10.1002/anie.202007423
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COMMUNICATION
this air- and moisture-stable catalyst is easily separated and
reused without activity loss.
Since C/MoO2 activates H2 at temperature above 220°C to
yield putative complex C/Mo(=O)(OH)(H) (2) by EXAFS and
TPR,[16] it is not surprising that hydrogenolysis of diester 1 (in
toluene) is sluggish at lower temperatures, e.g., 200°C (Scheme
1, path a; Table 1, entry 1) and proceeds significantly above
220°C/15 atm H2 to afford benzoic acid (3) in 16 % yield plus
ethylene and trace acetaldehyde (Scheme 1, path b; Table 1,
Scheme 2. β-heterolytic scission thermolysis pathway for polyesters.
In the catalytic hydrogenolysis of diester 1 (Scheme 1), the
formation of benzoic acid (3) and ethylene implies possible C-O
scission β to the CO carbon (Scheme 2). According to DFT
analysis, this step is nearly isoergic for diester 1 (ΔG°= 1.2
kcal/mol). Since a C-H bond is cleaved in the transition state a
kinetic isotope effect (KIE) might be expected. Indeed, comparing
the hydrogenolysis kinetics of d4-1,2-ethanediol dibenzoate (1-d4)
and 1 yields KIE = 1.85 (see SI, Figure, 31S), in agreement with
previously reported KIEs of reactions having a similar β-scission
step.[23] In principal a β-scission process should not require H2.
Indeed, heating 1 at 220°C over C/MoO2 under Ar instead of H2,
affords a 46% yield of 3 and, as expected, small quantities of
ethylene (Table 1, entry 4, Figure 15S). In β-scission, vinyl
benzoate (4) is also expected, however is not observed by NMR,
presumably due to its known instability at 220°C.[24] Further
support for a β-scission step comes from additional 1-d4
experiments. Heating 1-d4 over C/MoO2 under Ar affords only d1-
benzoic acid (3-d1) in 30% yield by NMR (Scheme 3, path a; Table
1, entry 5). Note that neat diester 1 is thermally stable at 220°C
for 24h, while heating 1 over neat activated carbon under Ar or H2
yields <5% and 10%, respectively, of 3, arguing that C/MoO2
catalyzes the lower temperature conversion of 1 via β-scission.
entry 2). However, without solvent, 91% of 1 is consumed and an
[20]
84% isolated yield of 3, ethylene and trace acetaldehyde
are
obtained (Scheme 1, path c; Table 1, entry 3). In the presence of
D2, deuterated benzoic acid (3-d1) is obtained, verifying that H2
(D2) participates in the hydrogenolysis reaction (see SI, Figure
3S). DFT-derived thermodynamic parameters reveal that Scheme
1 is overall exoergic (ΔG°) by ~13.2 kcal/mol (see SI, Eq. 21S).
Monitoring neat 1 consumption as a function of time reveals first-
order dependence on ester concentration (see SI, Figure 30S).
Scheme 1. C/MoO2-catalyzed hydrogenolytic deconstruction of diester 1.
Table 1. C/MoO2-catalyzed deconstruction of 1 and PET.
Entry
Polyester
Time
(h)
Temp.
(°C)
Ester:Mo
(mol)
Conv.
(%)
0
Yield
(%)
0
1
2
1 a
1 a
1 b
24
200
40:1
72
19
19
16
16
24
96
24
24
24
24
96
220
220
220
220
220
260
260
260
260
260
260
260
40:1
40:1
40:1
40:1
40:1
40:1
100:1
40:1
40:1
40:1
-
24
91
16
84
46
30
65
87
85
86
87
58
9
3
Scheme 3. C/MoO2-catalyzed cleavage of deuterated diester 1-d4 to benzoic
acid under Ar or H2.
4
1 b, c
50
c
5
1-d4
35
When Scheme 3 is carried out under H2, both 3 and 3-d1 are
obtained as assayed by 1H and 2H NMR, respectively. The total 3
+ 3-d1 yield is 65% (Scheme 3, path b; Table 1, entry 6). The
formation of 3, as well as the higher overall yield under H2
indicates that in addition to β-scission, an additional pathway is
operative. Since it is known that H2 reacts with C/MoO2 to produce
adsorbate 2 (vide supra), it is plausible that 2 mediates the formal
hydrogenolysis of intermediate vinyl benzoate (4) to 3 and
ethylene. DFT analysis indicates this step is exoergic by ~14.5
kcal/mol, which is supported by an independent experiment
(C/MoO2, 220°C, 1 atm H2) in which 4 affords 3 and ethylene in
76% yield.
6
1-d4
PET
71
7
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
8
PET
9
PET d
PET+PP
PET c
PET
10
11
12
13
PET e
160:1
90
C/MoO2 (3.23 wt% Mo), 1 and PET stored in air; solventless, 1 atm H2.
Conversions and acid yields determined by 1H NMR using an internal standard;
products confirmed by 13C NMR. a Reactions in toluene under 15 atm H2, 500
rpm; b Mo catalyst dried at 225°C overnight, diester 1 dried over CaH2; c under
Ar; d Beverage bottle PET. e Fresh PET added every 24h.
From the above information the scenario in Scheme 4 is
proposed for the formal catalytic hydrogenolysis of substrate 1.
Step i: Diester C=O and alkoxy O of 1 bind to Mo, generating a
well-precedented hexacoordinate Mo-dioxo complex (Scheme
4A, step i).[25] Such bidentate coordination/preorganization
appears crucial for the subsequent β-scission, generating benzoic
acid (3) and vinyl benzoate (4) (Step ii), and is supported by the
poor reactivity of ethyl benzoate (Scheme 4B, path a), lacking a
In esters and polyesters with H atoms β-to the C-O bond, the
major thermal degradation pathway (T>300°C) is β-scission
/retro-hydroalkoxylation yielding a carboxylic acid and vinylic
product (Scheme 2).[21] This step is common in diester/polyester
pyrolysis and proceeds via a syn-Ei mechanism involving a six-
membered transition state in which the bridging glycol O-C bond
is cleaved and a β-H atom transferred to the carbonyl O (Scheme
2).[22]
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10.1002/anie.202007423
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second glycol binding site (8% yield of 3 under Ar), and the
greater reactivity of ethylene glycol monobenzoate, yielding 32 %
of 3 under Ar (Scheme 4B, path b). Bidentate coordination likely
assists in pre-organizing the required transition state. Step iii: H2
adds across the Mo=O bond[16] with release of benzoic acid,
yielding a 2←4 complex. In principle, both Mo-OH and Mo-H
moieties in 2 could react with vinyl benzoate 4. Step iv: Hydroxide
and trace acetaldehyde (<5%) [20] are obtained from PET powder
(40:1 ester:Mo ratio) under 1 atm H2/24h (Scheme 5, path a; Table
1 entry 7). As expected, using D2 (instead of H2) yields deuterated
terephthalic acid, p-C6H4(CO2D)2. Decreasing the catalyst loading
to 100:1 ester:Mo and increasing the reaction time to 96h similarly
yields 85% of 5 (Table1, entry 8). Additionally, an overall yield of
5 (86%) is obtained from waste (beverage bottle) PET (Table 1
entry 9), indicating negligible effects of any additives on the
catalytic process. Furthermore, conducting the reaction with a
typical waste plastics mixture, i.e., PET + isotactic polypropylene
(iPP; simulating a bottle cap), 5 sublimes out in 87% yield (Table
1 entry 10), unaffected by the iPP which remains in the reactor.
As in the diester 1 reaction, a 58% yield of 5 is obtained under Ar
(Scheme 5, path b and Table 1, entry 11) suggesting that C/MoO2
catalyzes the low-temperature pathway in Scheme 4. Without
C/MoO2, only 9% yield of 5 is obtained (Table 1, entry 12) -
possibly via partial PET thermolysis,[21, 32] and over neat activated
carbon under H2 affords <5% yield of 5. Recently, several reports
on catalytic PET pyrolysis appeared, however, all require
temperatures >450°C to yield waxes, tars, and complex product
mixtures.[33]
attack at the ester carbonyl generates
3
and
a
C/Mo(=O)H(OCH=CH2) species. This step is plausible
considering the known leaving group tendency of alkoxy-vinyl
substituents.[26] Further support comes from two control
Scheme 5. C/MoO2-catalyzed depolymerization of PET to terephthalic acid (5)
and ethylene.
One additional attraction of catalyst C/MoO2 is that it can be
easily recycled without losing activity. The average yield of 5 in
four successive runs is 90% at an overall ester:Mo ratio of 160:1
(Table 1, entry 13, see SI, Figure 26S), higher than a single run
experiment (87%). XPS analysis of fresh and spent C/MoO2
samples indicates that the surface-bound Mo species remain
unchanged as Mo(VI) (see SI, Figures 27S-29S).
In conclusion, we demonstrate that the earth-abundant, air-
and moisture-stable carbon-supported single-site Mo-dioxo
complex, C/MoO2, catalyzes solvent-free polyethylene
terephthalate (PET) depolymerization under 1 atmosphere of H2
to selectively afford terephthalic acid and ethylene in high yields.
This process is effective for both commercial and waste PET, and
the catalyst exhibits high stability and recyclability. Moreover, the
reaction is unaffected by the presence of polypropylene.
Mechanistic studies with the model 1,2-ethanediol dibenzoate,
suggest that the reaction proceeds via a C–O β-scission step
followed by hydrogenolysis of a vinyl benzoate intermediate.
C/MoO2 is therefore an attractive catalyst for chemical recycling
of polyesters since it is selective, thermally-, moisture- and air-
stable, earth-abundant, non-toxic, and recyclable.
Scheme 4. A. Tentative diester hydrogenolysis reaction pathway. Control
experiments for: B. C-O β-scission step; C. hydroxide nucleophilic attack. 3 =
benzoic acid.
experiments with esters having a poor (ethyl benzoate) and good
(phenyl benzoate) alkoxy leaving groups (Scheme 4C). Under
identical conditions, low ethyl benzoate conversion (15%; path a)
and high phenyl benzoate conversion (76% of 3; path b) is
observed. Note that a 76% yield of 3 is obtained with vinyl
benzoate (4; path c). Step v: ethylene β-elimination from
C/Mo(=O)H(OCH=CH2) completes the catalytic cycle.[27] β-
elimination reactions regenerating C/MoO2 were reported
previously.[16-17, 19] Steps vi-viii: alternatively, the C=C bond[28] of
4 inserts into the Mo-H in complex 2, yielding a metal alkyl which
then undergoes a β-oxygen elimination[29] to produce ethylene
and a C/Mo(=O)OH(OCOPh) species, completing the catalytic
cycle after β-elimination to 3. Mo-H mediation of olefin elimination
from vinyl or allyl esters has precedent.[30]
Next, PET depolymerization/hydrogenolysis was investigated
with C/MoO2 to determine whether diester 1 is a realistic model.
For reactions with neat PET, the temperature was necessarily
raised near the PET Tm (260°C).[31] Importantly, 87% yields of
terephthalic acid (5) subliming from the reaction vessel, ethylene,
Supporting Information
Experimental procedures, kinetic studies, catalyst recycling and
plastic mixture experiments, NMR, GC, XPS, and DFT-optimized
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10.1002/anie.202007423
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COMMUNICATION
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Acknowledgements
Financial support by Office of Basic Energy Sciences,
Department of Energy (DE-FG02-03ER15457) to the Institute for
Catalysis in Energy Processes (ICEP) at Northwestern University
(Y.K., J.L.) is gratefully acknowledged. Purchase of the NMR
instrumentation at IMSERC was supported by NSF (CHE–
1048773). We thank Katia Kratish for expert graphics design.
Keywords: Chemical recycling • PET • Heterogenous catalyst •
hydrogenolysis • molybdenum
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10.1002/anie.202007423
Angewandte Chemie International Edition
COMMUNICATION
Captain Moly to the rescue: A carbon-supported single-site molybdenum-dioxo catalyst, C/MoO2, catalyzes the hydrogenolytic
deconstruction of polyethylene terephthalate (PET) to its monomer, terephthalic acid, and ethylene under 1 atm of H2. C/MoO2 can be
multiply recycled without loss of activity. Mechanistic studies suggest an initial β-scission step, followed by hydrogenolysis of the vinyl
benzoate intermediate.
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