Depolymerization of Technical-Grade Polyamide 66 and Polyurethane Materials through Hydrogenation

Article abstract of DOI:10.1002/cssc.202002465

Chemical recycling provides a promising solution to utilize plastic waste. Here, a catalytic hydrogenative depolymerization of polyamide 66 (PA 66) and polyurethane (PU) was developed. The system employed Ru pincer complexes at high temperature (200 °C) in THF solution, and even technical-grade polymers could be hydrogenated with satisfactory yields under these conditions. A comparison of the system with some known heterogeneous catalysts as well as catalyst poisoning tests supported the homogeneity of the system. These results demonstrate the potential of chemical recycling to regain building blocks for polymers and will be interesting for the further development of polymer hydrogenation.

Full text of DOI:10.1002/cssc.202002465

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Depolymerization of Technical Grade Polyamide 66 and
Polyurethane Materials via Hydrogenation
Authors: Wei Zhou, Paul Neumann, Mona Al Batal, Frank Rominger, A.
Stephen K. Hashmi, and Thomas Schaub
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemSusChem 10.1002/cssc.202002465
Link to VoR: https://doi.org/10.1002/cssc.202002465
01/2020

10.1002/cssc.202002465
ChemSusChem
COMMUNICATION
Depolymerization of Technical Grade Polyamide 66 and
Polyurethane Materials via Hydrogenation
Wei Zhou,[a] Paul Neumann,[b] MonaAl Batal,[b] Frank Rominger,[c] A. Stephen K. Hashmi[a],[c] and
Thomas Schaub*[a],[b]
[a]
Dr. Wei Zhou, Prof. Dr. A. Stephen K. Hashmi, Dr. ThomasSchaub
Catalysis ResearchLaboratory (CaRLa)
Universityof Heidelberg
Im Neuenheimer Feld 584, 69120 Heidelberg (Germany)
E-mail: thomas.schaub@basf.com
[b]
[c]
Dr. Paul Neumann, Dr. Mona Al Batal, Dr. ThomasSchaub
BASF SE
Carl-Bosch-Straße38, 67056 Ludwigshafen (Germany)
Dr. Frank Rominger, Prof. Dr. A. Stephen K. Hashmi
Organisch-Chemisches Institut
Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Supporting information for this articleis given via a link at theend of the document.
Abstract: Chemical recycling provides a promising solution to utilize
plastic waste. Here, we present catalytic hydrogenative
waste compared to traditional reduction process. Over the past
decades, catalytic reductions of carboxylic and carbonic acid
derivatives using molecular hydrogen have been investigated
a
depolymerization of polyamide 66 (PA 66) and polyurethane (PU).
The system features employing Ru pincer complexes at high
temperature (200 °C) in THF solution, even technical gradepolymers
can be hydrogenated with satisfactory yields under these conditions.
A comparison of the system with some known heterogeneous
catalysts as well as catalyst poisoning tests support homogeneity of
the system. These results demonstrate the potential of chemical
recycling to regain building blocks for polymers and will be interesting
for the further development of polymer hydrogenation
[6]
extensively.[5] Homogeneous catalysts based on ruthenium,
iron[7] manganese[8] and molybdenum[9] were reported.
Hydrogenolysis of polyesters and polycarbonates were also
realized by several groups.[10] In comparison, hydrogenative
depolymerization of polyamides and polyurethanes are muchless
developed, presumably due to the highly resistant property of
these materials. Researchers from Invista described a two-step
process where hydrogenation was applied after ammonolysis of
polyamides,[11] however, the direct hydrogenation of polyamides
is not known until quite recently. During our preparation of this
manuscript, Milstein and coworkerspublished the first example of
hydrogenative depolymerization of polyamides (including a
polyurethane) with ruthenium pincer complex.[12] Their strategy
uses dimethyl sulfoxide (DMSO), which can dissolve polyamide
at elevated temperature, as hydrogenation solvent. Due to the
deactivating effect of DMSO on Ru catalyst,[13] the turnover
number of catalyst was quite limited, thus a two-step approach
was adopted in order to improve the overall conversion and yield
of the reaction. In addition, decomposition of DMSO at elevated
temperature raises potential safety issues.[14] Moreover, in their
report, hydrogenation of technical grade polymers was not
demonstrated. Here, we disclose our results on ruthenium-
catalyzed hydrogenation of polyamides and polyurethanes via
cleavage of the C-N bond of amide and carbamate groups.
Improved activity, as well as the depolymerization of authentic
technicalgrade samples are achieved.
The widespread and increasing use of plasticsposes a great
challenge for the sustainability of human society. Until now, the
majority of these post-consumer polymers are discarded, being
either landfilled, incinerated or illegally dumped in the oceans,
which causes serious environmental problems.[1] Thus, it is
imperative to establish efficient recycling process to avoid the
pollution with plastic waste and also to save fossil feedstocks in
the monomer synthesis. Although mechanical recycling process
(melting and remolding) can prolong the lifetime of polymers, it is
still quite limited, for instance, due to high sorting requirements
and decreasing materialqualityin each cycle.[2]On the other hand,
chemical recycling, namely decomposing polymers into their
monomers, offers a sustainable loop of plastic production, since
the monomers obtained by this means can be used to prepare
polymerswithout lossin properties, thuscreating an ideal, circular
polymer economy.[3] In this regards, pyrolysis, hydrolysis,
alcoholysis and aminolysis have been used for the chemical
[4]
recycling of polyesters, polyamides and polyurethanes.
However, theseprocessessuffer fromdisadvantages such as not
being cost competitive, lowefficiencyof monomer regeneration or
generating additional chemical wastes. Therefore, more efficient
and sustainable depolymerization process for polymer recycling
are required.
Catalytic hydrogenationisa green and sustainable methodfor
functional group transformation, it doesn’t produce stoichiometric
1
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10.1002/cssc.202002465
ChemSusChem
COMMUNICATION
Furthermore, we examined the hydrogenation of N,N'-
dicaproylhexamethylenediamine using complexes 1, 2 and 3,
similar results to the hydrogenation of adipic diamide were
observed with complex2 which showed the highest activity(Table
S1, supporting information).
Next, we tested the hydrogenation of a technical grade
polyamide 66 sample, Ultramid® A 27[17], using complex 2.
However, no activity was observed at 150 °C under 70 bar of H2
with different organic solvents (THF, toluene and anisole). We
speculated the lackof solubilityof polyamide in these solvents at
150°C hampered its hydrogenosis, although, polar solvents like
formic acid, m-cresol can dissolve polyamide 66 via hydrogen
bonding effect; unfortunately, the acidic nature is incompatible
with the used catalysts.[18] Besides, for the reasons mentioned
above, we also avoided to use DMSO as solvent. Then we turned
to a low molecular weight polyamide sample, PA-1. It is
noteworthy to mention that this polyamide has excess of amine
Figure 1. Rutheniumcomplexes studied in this report
We initiated our study by evaluating a series of ruthenium
catalysts (Figure 1), using N,N'-dihexyladipamide as a model
substrate as it has very similar structure motif to polyamide 66.
When the Milstein’s amidehydrogenation catalyst 1[6] wasapplied, end group overcarboxylicend group, making it a “basic” polymer
moderate yields of amine and diol were obtained (Table 1, Entry
(Table S2, supporting information). When this polyamide 66
1). Changingthe substituent on phosphine in complex1 from tert- sample was subjected to a hydrogenation pressure of 70 bar, only
butyl to cyclohexylincreased the yield slightly (Table 1, Entry 2).
The commercially available ruthenium catalyst 3, which can be
operated under base-free conditions, gave a moderate yield
(Table 1, Entry 3), while the activated form of Ru-MACHO[15] was
inactive (Table 1, Entry 4). A ruthenium complex bearing
tetradentate ligand,[6] developed by Saito and coworkers, was
tested and satisfactory activity was observed (Table 1, Entry 5).
In addition, a ruthenium complex with bis-aminophosphine ligand
failed to give the desired products (Table 1, Entry 6).[16] Finally,
the Ru-triphoscatalyst with acid as additive[6],[10] turned out to be
inactive in this hydrogenation (Table 1, Entry 7).
very low yields of diamine and diolwas observed at 150 °C after
20 hours. Gratifyingly, with increased temperature to 200 °C and
pressure to 100 bar, satisfactory yields of diamine and diol were
obtained. Decreasing the temperature or hydrogen pressure led
to reduced activity (Table 2, Entries 3, 4), and the influence of
temperature seems to be more significant. Other solvents
including toluene, anisole and dimethoxyethane all resulted in
inferior yields (Table 2, Entries 5 - 7). In the absence of Ru
catalyst, KtOBu or hydrogen, productswere not observed, which
confirmed the necessity of each component.
Table 2. Hydrogenation of a low molecular weight polyamide 66 (Mw = 8240
g/mol; AEG = 1748 mmol/kg) sample with Ru complex 2[a]
Table 1. Hydrogenation of N,N'-dihexyladipamide with Ru complexes[a]
Entry
Temp
(°C)
P (H2)
(bar)
Yield / (TONs)[b]
(diamine)
Yield / (TONs)[b]
(diol)
Entry
cat.
(2 mol%)
additive
(4 mol%)
yield[b,c]
(amine)
yield[b,c]
(diol)
1
150
200
180
200
200
200
200
70
100
100
80
12% (15)
78% (98)
60% (75)
70% (88)
59% (74)
69% (86)
70% (88)
< 5% (< 7)
62% (78)
35% (44)
47% (59)
32% (40)
49% (61)
46% (58)
KOtBu
KOtBu
---
77%
82%
63%
18%
75%
0%
64%
75%
47%
< 5%
53%
0%
2
1
1
2
3
4
5
6
7
3
2
4
3
5[c]
6[d]
7[e]
100
100
100
4
---
5[d]
6
NaH
KOtBu
HNTf2
7
0%
0%
[a] Conditions: PA-1 (0.3 g, 1.25 mmol according to the repeating unit of
polyamide 66), cat. 2 (0.01 mmol), KOtBu(0.04 mmol), THF (5 mL). [b] Yields
were determined by GC analysis using mesitylene as internal standard. [c]
Toluene as solvent. [d] Anisole as solvent. [e] Dimethoxyethane as solvent.
TONs, turnover numbers.
[a] Conditions: adipamide (0.5 mmol), cat. (0.01 mmol), additive (0.02 mmol),
H2 (50 bar), THF (3 mL), 120 °C, 24 h. [b] Yields were determined by GC
analysis using mesitylene as internal standard. [c] due to the low solubility of
N,N´-dihexyladipamide in THF, we were not able to quantifythe conversion of
the starting material in an accurate manner, as in some reactionssolid martial
remained. Therefore, only the yield of the products according the amount of
used starting material could be quantified accurately via calibrated GC.[d] 0.2
mmol NaH wasused
To further investigate the effect of the carboxylic acid end
group in polyamide 66, three other different samples were tested
(Table 3, see Table S2 in supporting information for details). For
polyamide PA-2 (Mw = 8750, Table 3, Entry 1), a sample has
2
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10.1002/cssc.202002465
ChemSusChem
COMMUNICATION
excess of amine end group as PA-1, moderate yields of diol and
diamine were obtained. While polyamide PA-3 (Mw =2470, Table
3, Entry 2) and PA-4 (Mw =4090, Table 3, Entry 3), both with an
excess of acidic end groups, didn’t give any desired products,
even though the latter two samples have a lowermolecular weight
than the first one. The results here clearly demonstrated
deactivating effect of excessacidic groups. Fortunately, thisisnot
an important issue for technical grade polyamide hydrogenation,
since these materials usually possess a similar amount of
carboxylicend groupsand amine end groups.[19]
feasibility to hydrogenate the main linking group in the
polyurethanes. Self-prepared toluene diisocyanate (TDI) and
methylene diphenyl diisocyanate (MDI) based polyurethanes
were depolymerized smoothly (Scheme 1a). Due to the soluble
nature of these two polyurethane samples, high yields of
monomerswere obtained under much milder conditionsthan the
polyamide hydrogenation. This corroborates the results by
Milstein and coworkers,[12] indicating that the solubility of
substrates has a great influence on reactivity. In addition, two
polyurethane-related compounds, a urea and an isocyanurate
were tested, which are also present in polyurethanes as linking
groups.[22] Both were hydrogenated to give the corresponding
aromatic amineswith moderate and high yields(Scheme 1b). To
the best of our knowledge, this is the first time an isocyanurate
was hydrogenated to give correspondingamine. Thisshowed the
feasibility that all isocyanate-derived connecting groups in
technical polyurethanes can be hydrogenated to the
corresponding amines.
As the hydrogenation was operated under much higher
temperature compared to most homogeneous ruthenium
catalyzed reactions, it is conceivable that the Ru-pincer complex
may decompose and form heterogeneous particles which are
responsible forthe observed activities.[20] To probe thispossibility,
several heterogeneous catalysts were examined (Table S3,
supporting information). With Ru/C, Raney Ni or Raney Co,
desired products were not observed, while Ru nanoparticles on
silica, afforded diamine as the only product. Further control
experimentsrevealed that 1,6-hexanediol can be fully converted
by Ru nanoparticles on alumina, possible to hexane via
deoxygenation. In contrast, the diol remained intact when Ru
catalyst 2 was used under the same conditions (Scheme S3,
supporting information). Moreover, mercury test disclosed the
activitywas not inhibited (Scheme S4, supporting information). All
these results suggested the system is a homogeneous
hydrogenation ratherthan a heterogeneousone.[21]
Finally, we tested the hydrogenative depolymerization of
technical grade polyamide 66 and polyurethane at the high
temperature and pressure conditions. To our delight, with
Ultramid® A27 the desired productswere observed after50 hours,
albeit in limited amount. Regarding polyurethane, many additives
like antioxidants, stabilizersand plasticizersare added during the
manufacturing process,[22] which maycomplicate the system, thus
we started with a well-defined polyurethane sample. Satisfyingly,
1 g of additive-free polyurethane was fully converted after 24
hours and 0.2 g of toluenediamine (TDA, as a mixture of 2,4 and
2,6 isomers) was isolated. The reaction can be upscaled to 8.5 g
without decrease of activity and 1.63 g of TDAand 4.84 gram of
the polyol were obtained respectively (Figure S2, supporting
information), corresponding a turnover number of 668.
Remarkably, when a commercial household kitchen sponge
made of a TDI-based polyurethane was applied, almost full
conversion was observed on 10-g scale, and 2.37 g of TDAwas
isolated, corresponding to a turnover numberof 970 according the
formed TDA (Figure S8, supporting information). In addition,
polyetherols were formed as the other hydrogenation products.
As the composition of the polyetherols used for the synthesis of
this foam was not known to the authors, we were not able to
quantifythe yield of the polyetherol unlike in the hydrogenation of
the defined foam described above. We could onlysee by NMR of
the crude mixture, that a mixture of polyetherolsis formed (use of
polyetherols mixtures for technical PU foams is common).
Besides TDA, no other products detectable by GC were formed.
To the best of our knowledge, this is the first example of
hydrogenation of real-lifepolyurethane materialswith such a high
efficiency.
Table 3. Hydrogenation of different polyamide 66 [a]
Entry
Polyamide
66
Mol Wt
(g/mol)
AEG / CEG[b]
(mmol/kg)
Yield[c]
(diamine)
Yield[c]
(diol)
1
2
3
PA-2
PA-3
PA-4
8750
2470
4090
1482 / 7.5
23.3 / 1683
13.6 / 1286
62%
0%
37%
0%
0%
0%
[a] Conditions as showed, polyamide 66 (0.3 g, 1.25 mmol according to the
repeating unit of polyamide 66) [b] AEG, amine endgroup;CEG, carboxylicend
group. [c] Yields were determined by GC analysis using mesityleneas internal
standard.
After the hydrogenation of polyamides substrates, we also
investigated polyurethanes models. Dimethyl toluene-2,4-
dicarbamate can be readily hydrogenated (Scheme S1,
supporting information), which demonstrated the general
3
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10.1002/cssc.202002465
ChemSusChem
COMMUNICATION
Scheme 1. a) Hydrogenation of polyurethane models, 1 mmol of substrates was used (accordingto the repeating unit of polyurethanes), yields were determined
by GC analysis , TONsweregiven in parentheses. b) Hydrogenation oftwo polyurethane related compounds, yieldswere determined by GCanalysis.
CCDC 2042476 (2) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/structures/.
Procedure forpolyamide and polyurethane hydrogenation
Inside an Ar glove box, the polymer sample, Ru catalyst,
KOtBu and solvent were put into a Premex autoclave (60 or 200
mL depending on solvent amount) equipped with a Teflon insert
and a magnetic stirring bar. The autoclave was sealed and taken
out of the glove box, charged with H2, and was put into a
preheated aluminum block. The reaction wasstirred forindicated
time and cooled to room temperature, pressure was carefully
released, mesitylene was added as an internal standard and an
aliquot amount of crude product was submitted for GC analysis.
Scheme 2. Hydrogenation oftechnical grade PA 66 and PU samples.
In conclusion, we report here an improved system for
hydrogenative depolymerization of polyamide 66 and
polyurethane. Key to successis the employment of high reaction
temperature to enhance the solubility of polymers. For the first
time, technical grade polyamide 66 and polyurethane were
hydrogenated. Although the overall efficiency of the system,
especially for the hydrogenation of polyamide 66 still need to be
improved for practical application, as well as the recycling ofthe
catalyst needs further investigation, we believe these results will
be relevant for the furtherdevelopment of plasticrecycling based
on hydrogenation.
In case of the largerscale hydrogenationsgiven in scheme2, the
products were isolated bycolumn chromatography(fordetailssee
the supporting information).
Acknowledgements
CaRLa (Catalysis Research Laboratory) is co-financed by the
Ruprechts-Karls-University Heidelberg (Heidelberg University)
and BASF SE.
Keywords: depolymerization • polyamide 66 • polyurethane•
hydrogenation • Ru pincercomplexes
Experimental Section
All reactions were carried out under an Ar atmosphere using
standard Schlenkand high-vacuum-line techniquesorin a Braun
inert atmosphere gloveboxfilled with Ar.
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Complex2 was prepared via a similar procedure reported for
the synthesisof complex1:[6c] RuHCl(CO)(PPh3)3 (476.2 mg, 0.5
mmol) and the PNN ligand (201.6 mg, 0.55 mmol) wereplaced in
a 25 mL Schlenk tube equipped with a magnetic bar, 12 mL of
THF was added. The tube wasput into a 70 °C oilbath and stirred
for 7 h. After cooled to room temperature, solvent was removed
via filtered cannula, solid was washed by THF (3 x 5 mL), dried
under vacuum. Orange solid, 240.0 mg, 91% yield
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10.1002/cssc.202002465
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A Ru-pincercomplex isdeveloped for the hydrogenative depolymerization of polyamide66 and polyurethane. The system features
high reaction temperature with THF as solvent, technicalgrade polymerswere hydrogenation to give substantial amount of products.
Remarkably, turnover numbersclose to 1000 is obtained for thehydrogenation of a household kitchen sponge.
Institute and/orresearcher Twitter usernames: @LaboratoryCaRLa
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