Ionic liquids/ZnO nanoparticles as recyclable catalyst for polycarbonate depolymerization

Article abstract of DOI:10.1016/j.molcata.2016.11.006

A useful protocol for waste bis-phenol A-polycarbonates (BPA-PC) chemical recycling is proposed based on a bifunctional acid/basic catalyst composed by nanostructured zinc oxide and tetrabutylammonium chloride (ZnO-NPs/NBu₄Cl) in quality of Lewis acid and base, respectively. Retro-polymerization reaction proved to be of general application for several nucleophiles, including water, alcohols, amines, polyols, aminols and polyamines, leading to the complete recovery of BPA monomer and enabling the PC polymer to function as a green carbonylating agent (green phosgene alternative) for preparing carbonates, urethanes and ureas. A complete depolymerization can be obtained in seven hours at 100 °C and ZnO nanocatalyst can be recycled several times without sensible loss of activity. Remarkably, when polycarbonate is reacted with glycerol, it is possible to realize in a single process the conversion of two industrial wastes (BPA-PC and glycerol) into two valuable chemicals like BPA monomer and glycerol carbonate (the latter being a useful industrial solvent and fuel additive).

Full text of DOI:10.1016/j.molcata.2016.11.006

Accepted Manuscript
Title: Ionic liquids/ZnO Nanoparticles as Recyclable Catalyst
for Polycarbonate Depolymerization
Author: Francesco Iannone Michele Casiello Antonio
Monopoli Pietro Cotugno Maria Chiara Sportelli Rosaria
Anna Picca Nicola Cioffi Maria M. Dell’Anna Angelo Nacci
PII:
S1381-1169(16)30480-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.11.006
MOLCAA 10103
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
10-8-2016
29-10-2016
4-11-2016
Please cite this article as: Francesco Iannone, Michele Casiello, Antonio Monopoli,
Pietro Cotugno, Maria Chiara Sportelli, Rosaria Anna Picca, Nicola Cioffi, Maria
M.Dell’Anna, Angelo Nacci, Ionic liquids/ZnO Nanoparticles as Recyclable Catalyst
for Polycarbonate Depolymerization, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.11.006
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Ionic liquids/ZnO Nanoparticles as Recyclable Catalyst for
Polycarbonate Depolymerization
Francesco Iannonea, Michele Casielloa,*, Antonio Monopolia , Pietro Cotugnoa,*, Maria Chiara
Sportellia, Rosaria Anna Piccaa, Nicola Cioffia, Maria M. Dell’Annac, and Angelo Naccia,b,*
a Dipartimento di Chimica, Università di Bari, via E. Orabona, 4, 70126 Bari, Italy.
b CNR-ICCOM, UOS Bari, via E. Orabona 4, 70126 Bari, Italy.
c DICATECh, Politecnico di Bari, Via Orabona, 4, 70126 Bari, Italy.
* Corresponding authors: angelo.nacci@uniba.it; pietro.cotugno@uniba.it; michele.casiello@uniba.it; Tel.:
+39-080-544-2499
Graphical abstract
Abstract. A useful protocol for waste bis-phenol A-polycarbonates (BPA-PC) chemical recycling is
proposed based on a bifunctional acid/basic catalyst composed by nanostructured zinc oxide and
tetrabutylammonium chloride (ZnO-NPs/NBu4Cl) in quality of Lewis acid and base, respectively.
Retro-polymerization reaction proved to be of general application for several nucleophiles,
including water, alcohols, amines, polyols, aminols and polyamines, leading to the complete
recovery of BPA monomer and enabling the PC polymer to function as a green carbonylating agent
(green phosgene alternative) for preparing carbonates, urethanes and ureas. A complete
depolymerization can be obtained in seven hours at 100 °C and ZnO nanocatalyst can be recycled
several times without sensible loss of activity. Remarkably, when polycarbonate is reacted with
glycerol, it is possible to realize in a single process the conversion of two industrial wastes

(BPA-PC and glycerol) into two valuable chemicals like BPA monomer and glycerol carbonate (the
latter being a useful industrial solvent and fuel additive).
Keywords: ionic liquids; zinc oxide nanoparticles; polycarbonate; depolymerization.
1. Introduction
Growing of plastic wastes, together with CO2 emission, is another plague of modernity. The
increasing level of plastic wastes in the environment is an urgent problem that needs fast responses.
Actually, politics and researchers are pursuing two different strategies to reduce plastic dispersal: i)
recycling of plastic items, and ii) incineration of plastic wastes [1]. However, in this latter case, a
further drawback has to be considered, connected to the identification of suitable storage sites and
to the minimization of the risk of the industrial plants. For these reasons, in the last years, the
chemical valorisation of plastic wastes is regaining more and more interest as a complementary way
to the thermovalorisation. In fact, some plastics can be considered as a source of CO2 that can be
introduced, as chemical synthon, in important and valuable products such as carbamates, urethanes
and heterocyclic structures, just to cite a few [2]. For example, carbon dioxide is also present into an
important and widespread polymer such as bis-phenol A polycarbonates (BPA-PC), a thermoplastic
material with significant properties as good stability, excellent flexibility and good transparency [3].
BPA-PC recycling is often difficult, then incineration becomes an easy way to produce electricity
and to avoid landfills disposal. However, pyrolysis of BPA-PC results in the release of a large
amount of greenhouse gases released and low-value carbonaceous products.
Alternatively, chemical depolymerization has to be considered the most promising technique
for recycling waste polymers and plastics. Chemical polymer degradation can represent a potential
option for converting plastics into their monomers or other valuable products (depolymerization)
[4]. Alcoholysis, aminolysis and hydrolysis [5] are conventional procedures typically performed at
high pressure and temperatures as well as in the presence of large quantities of concentrated acids or
bases during long time processing [6].
Among these methods, depolymerization of BPA-PC has been done by using hot compressed
water [7], alkaline solutions [8], supercritical ethanol [9], microwave irradiation [10], and metal
catalysts such as Mn(OAc)2 [7] and a variety of metal oxides and salts [11,12].
Recently, besides the chemical recovery of the starting monomer (BPA), particularly attractive
became the use of polycarbonate as carbonylating agent in an economic and environmentally

benign non-phosgene alternative for preparation of many carbonyl derivatives (carbonates,
carbamates, etc.). As an example, methanolysis of polycarbonate can lead simultaneously to the
recovery of the starting monomer BPA and to the synthesis of dimethylcarbonate (DMC), an
industrially important solvent and methylating agent.
During the past decades, we have been focusing our efforts in developing powerful catalytic
methods using ionic liquid (ILs) as both catalysts and reaction media [13]. In particular, we paid
attention to tetraalkylammonium salts that are capable of stabilizing metal nanoparticles catalysts
providing them a protecting shell that impedes aggregation, thus prolonging catalyst lifetime and
facilitating its prompt recycling. In addition, due to the shielded positive charge on cationic
nitrogen, tetraalkylammonium salts anions are good nucleophiles and can behave as basic catalysts
themselves [13b].
Based on our skills in handling metal nanocatalysts [14], and being PC transesterification also
promoted by a variety of Lewis acids, we decided to conjugate the nucleophilic/stabilizing ability of
quaternary tetraalkylammonium ILs with the Lewis acid properties of ZnO, a catalyst successful
employed in depolymerization processes (e.g. on polyethylene terepththalate) [15], developing an
efficient and recyclable bifunctional NBu4Cl/ZnO-NPs catalyst useful for depolymerization of
polycarbonate waste material. In this study, ZnO nanoparticles (ZnO-NPs) were synthesized by a
hybrid electrochemical-thermal process [16] and the influence of calcination temperature on
catalytic performance was evaluated.
2. Experimental
2.1. Materials and general procedure
Unless otherwise stated, all manipulations were carried out in air. Solvents, metal oxides,
nucleophilic reactants (alcohols, amines, phenols, diols, glycerol, diamines and aminols) and
additives (BuNH2, pyridine, ILs), were Aldrich or Fluka products and were used as received.
Bisphenol A-Polycarbonate (BPA-PC) with an average molecular mass of 45,000 was purchased by
Sigma-Aldrich.
1
Reaction products were identified by IR, GLC-MS and H-NMR, by comparison with the
literature data or with authentic samples (spectral data are reported into the supplementary
material). IR spectra were recorded on a Shimadzu IR-Prestige-21 spectrophotometer. GLC-MS
analyses were carried out with a Shimadzu GLC 17-A linked to a Shimadzu GLC/MS QP5050A
selective mass detector (capillary column: HP-5 MS, 30 m).

Depolymerization percentages were determined measuring the ratios of the chromatographic
areas between the BPA and the end capping agent 4-(2-phenylpropan-2-yl)phenol released by the
polymer (BPA-PC from Sigma-Aldrich) during the reaction, according to a procedure described
into the supplementary material.
Catalytic tests were carried out in a 55,6 mL stainless steel autoclave mounted in an electrical
oven having a magnetic stirrer on its base. Catalysts, solvent and reagents were introduced in a glass
vial (~12 mL) placed into the autoclave, in order to avoid any contact with metal walls.
2.2. Preparation of ZnO nanoparticles catalysts
A three-electrode conventional cell was employed to perform the electrochemical synthesis:
highly pure zinc sheets (Goodfellow, 99.99+%) were used as working and counter electrodes, while
an Ag/AgCl (in saturated KCl) was the reference electrode. Electrolytic solution consisted of 1 g/L
PSS [Poly(sodium-4-styrenesulfonate, average Mw ~ 70,000, Sigma] in 30 mM NaHCO3 (purum
p.a., ≥ 99.0%). Electrodes were polished with alumina (Sigma) and sonicated in Milli-Q water
before use. Zinc activation in 1 M HCl (Fluka) was performed for 30 s just before immersion in the
electrolytic bath. Afterwards, the electrosynthesis was carried out under galvanostatic conditions (j
= 10 mA/cm2) for 1 h at room temperature under stirring, using a CH-1140b
potentiostat-galvanostat (CH Instruments, USA). The resulting nanocolloid was then collected and
centrifuged at 5000 rpm for 45 minutes. The obtained precipitate was dried at 70°C in an oven
overnight. Finally, the thermal annealing step for the synthesis of ZnO-NPs was performed either at
300°C or at 600°C for 1 h.
TEM characterization was performed with a FEI Tecnai 12 instrument (HV: 120 kV; LaB6
filament) on aqueous suspensions of calcined ZnO-NPs using Formvar®-coated Cu s (300 mesh,
Agar Scientific).
XPS analysis was carried out using a Theta Probe Spectrometer equipped with AlKα source
and beam spot diameter of 300 μm. All XPS and Auger spectra were registered in constant analyzer
energy (CAE) mode. Survey scans were registered at a pass energy of 150 eV and step size 1.0 eV.
Besides, high-resolution spectra (C1s, O1s, Zn2p, ZnL3M45M45, S2p and Na1s) were acquired at
100 eV with 0.1 eV step size.
XPS quantification was performed with Thermo Avantage software (v. 4.75, © 1999–2010
Thermo Fisher Scientific). A Shirley background removal and a database of Scofield sensitivity
factors optimized for oxides were used for processing. Signal positions were corrected on the basis

of C1s aliphatic component set at 284.8 eV.
FTIR spectra on commercial ZnO (Sigma) and calcined nanopowders were collected on a
BioRad FTS6000 instrument in the 4000 – 400 cm-1 range (resolution 4 cm-1) preparing KBr
pellets.
2.3. Catalytic tests
In a typical experiment, a 50 ml glass vial was charged with THF (5 ml) and 1 mmol of
BPA-Polycarbonate that was left to dissolve under vigorous stirring for 10 minutes at room
temperature. Then, ZnO-NPs, Bu4NCl and nucleophile reagent were added and the vial introduced
into the autoclave, which was sealed and charged with N2 at atmospheric pressure. The autoclave
was heated at 100°C for the proper reaction time.
Next, it was cooled to room temperature, then centrifuged to remove ZnO nanoparticles and
then analyzed by IR and GC-MS for reactants and products. Procedure for isolation of reaction
products was dependent on the nucleophiles employed for depolymerization (water, alcohols or
amines).
In the case of hydrolysis, procedure was more simple, being BPA the sole reaction product
(apart carbonic acid). To isolate BPA starting monomer, reaction mixture was treated with aqueous
HCl under vigorous stirring and extracted more times with Et2O. The organic layer was dried over
anhydrous Na2SO4 and evaporated to dryness. Recrystallization of the residue from chloroform and
1
hexane gave highly pure bisphenol A. H NMR (400 MHz, CDCl3):  1.75 (s, 6H), 7.13-7.15 (d,
13
4H), 7.22-7.24 (d, 4H); C NMR (400 MHz,CDCl3),    GC-MS m/z
(%): 228 (M+, 27), 213 (100), 119 (64), 43 (26), 99 (10), 91 (12), 65 (6) [17].
In the case of alcoholysis and aminolysis, procedures were complicated by the separation of
BPA from the carbonylated products (carbonates, urethanes and ureas). Procedures are reported into
the supplementary material together with spectral data used for identification.
2.4. Procedure for catalyst recycling
Recycling experiments were checked on hydrolysis as model reaction. A 50 ml glass vial was
charged with THF (5 ml) and 1 mmol of BPA-Polycarbonate that was left to dissolve under
vigorous stirring for 10 minutes at room temperature. Then, ZnO-NPs, Bu4NCl (5 mol%) and water

(30 mmol) were added and the vial introduced into the autoclave, which was sealed and charged
with N2 at atmospheric pressure. The autoclave was heated at 100°C for 7 hours.
Being insoluble in THF, ZnO nanoparticles remained as precipitate at the end of the
depolymerization process and were removed from the reaction medium by centrifugation, dried
with a nitrogen stream and then promptly reused by adding fresh reagents and Bu4NCl co-catalyst.
In this way, the Lewis acid catalyst ZnO nanoparticles could be recycled up to five cycles
with negligible loss of activity (fig. 7), whereas its basic counterpart Bu4NCl was only partially
recovered with the following separation procedure. After centrifugation, reaction mixture was
treated with aqueous HCl and extracted with ethyl ether. The organic layer was treated to recovery
BPA, while aqueous solution was evaporated and the residue re-dissolved into chloroform, dried
over Na2SO4 and re-evaporated furnishing almost pure Bu4NCl (yield 39%).
3. Results and discussion
3.1. Synthesis and characterization of ZnO nanocatalyst
ZnO-NPs were prepared according to a two-step process developed in our laboratory [16]. In
the first step, jellified nanocolloids consisting of zinc hydroxide-carbonate were electrochemically
synthesized under mild and green conditions, in the presence of an anionic stabilizer PSS in sodium
bicarbonate aqueous solution. In place of zinc(II) salts used in hydrothermal methods, a sacrificial
anode made of pure Zn (see experimental part) was employed as a source of Zn2+ ions. This process
proved to have a very high yield (> 90%) and low power consumption. The electrochemical
procedure allowed the formation of zinc mixed species of undefined morphology that were easily
converted into spheroidal ZnO nanostructures by thermal treatment at temperatures above 250°C
[16].
Transmission electron microscopy (TEM) characterization presented in Fig. 1 shows that the higher
the calcination temperature, the larger is the nanoparticle diameter, as expected based on previous
literature [18]. On the other hand, the good morphology of NPs confirms that the addition of PSS
allows controlling NP size and shape. Moreover, the surface and bulk chemical compositions of
calcined NPs were investigated by means of X-ray photoelectron (XPS) and Fourier transform
infrared (FTIR) spectroscopies, respectively. Element abundances, expressed in terms of surface
atomic% (surface At%), relevant to nanocatalysts calcined at 300°C and 600°C are reported in
Fig. 2.

Values determined on samples dried at 70°C, not subjected to calcination, are also shown in the
same figure, for comparison purposes. In all the cases, carbon, oxygen, zinc, sodium, and sulphur
are identified and can be ascribed to the chemicals involved in the synthesis (e.g. sodium
bicarbonate, zinc, PSS). Interestingly, sulphur is present at limit of quantification even at 600°C,
suggesting a certain retention of the polymeric stabilizer. In fact, S2p3/2 signal does not change
position (Binding Energy, BE = 168.2 ± 0.2 eV) and line shape being undoubtedly associated to the
sulphonate groups of PSS [19]. Moreover, the ratio between oxygen (as oxide) and zinc
corroborates the hypothesis that stoichiometry of ZnO-NPs improves at higher calcination
temperatures as this value changes from 0.81 ± 0.03 –for calcination at 300°C- to 1.02 ± 0.04, for
powders treated at 600°C. By contrast, samples dried at 70°C did not show a component ascribable
to zinc oxide, as O1s signal fell typically at BE compatible with zinc hydroxides and basic
carbonates. In accordance to XPS results, FTIR investigation of calcined samples (Fig. 3)
demonstrates that the stretching of Zn-O bond at about 460 cm-1 is more intense for 600 °C than for
300 °C-calcined nanocatalysts, indicating that conversion to ZnO-NPs is more effective in the latter
case. Moreover, the band at about 3450 cm-1, associated to hydroxyl groups and adsorbed water, is
slightly visible only for 300 °C treated nanopowders. 300°C and 600 °C calcined ZnO-NPs were
then employed as nanostructured catalysts, as illustrated in the following paragraphs.
3.2. Preliminary catalytic tests
Catalysis conditions were surveyed on the model reaction between PC-BPA and
1,2-propanediol as a nucleophile (1,2-PD). This transesterification leads to the retro-polymerization
process releasing BPA monomer and incorporating carbon monoxide into the glycol according to a
high atom economy reaction (fig. 4). Beside the cyclic carbonate 4-methyl-1,3-dioxolan-2-one, a
number of by-products could be formed (such as phenol, esters, etc.) coming from degradation of
BPA dependently on the reaction conditions.
Reaction parameters were calibrated to process 1 mmol of polymer (0.254 g) in 5 mL of THF
with IL and/or other additives as catalytic system (generally 5 mol%). The screened range of
temperatures was 80 - 140 °C. Higher temperatures resulted in better conversions, but generally in
lower selectivities. Reaction times were carefully regulated to limit BPA degradation (maximum 3 -
7 hours dependently on the temperature conditions). The most representative results of this
preliminary screening are summarized in table 1.
Initially, we paid attention to the sole basic catalysis by checking activity of common organic
bases like pyridine, butylamine and proton sponges. In all cases, despite the high temperatures (140
°C), after 3 hours conversions did not exceeded 53%, while for longer reaction times by-products
formation became the predominant process (table 1, runs 2-4).

Next, we decided to exploit the basic properties of ionic liquids investigating a variety of the
most common tetraalkylammonium- and imidazolium salts. Among them, the former proved to be
the most efficient ones (table 1, entries 5-11), probably due to the lower solvation effect exerted by
the bulkier and shielded tetraalkylammonium cations towards their counter-anions, thus enhancing
basicity of these latter. As expected, reactivity of ILs was profoundly affected by the nature of the
anion. For instance, tetraalkylammonium ILs bearing AcO– and Cl– anions were found to promote
depolymerization much better than the analogous ones with Br– and I– (table 1, entries 5-8).
Correspondingly, [Bmim]AcO displayed better performance than [Bmim]Cl and [Bmim]Br (table 1,
runs 9-11).
Recently, a similar effect was observed by us for the N-alkylation of arylamines in ionic
liquids [20] and interpreted as reflecting the capability of the IL anions to accept a hydrogen bond.
Similarly, these early results on PC depolymerization seem to be in good agreement with a
mechanism connected with the ability of the IL anions to accept a hydrogen bond form propylene
glycol, thus enhancing its nucleophilicity. Such a hydrogen-bond basicity can be quantitatively
–
expressed by Kamlet–Taft β parameters for ILs: 1.16 (AcO–), 1.00 (Cl–), 0.67 (Br–), 0.37 (BF4 ),
–
0.30 (I–), 0.21 (PF6 ) [21] (fig. 5).
In line with this assumption, better performances emerged in the case of tetraalkylammonium
salts bearing acetate or chloride ions as basic catalyst (NBu4OAc or NBu4Cl). In these cases,
retro-polymerization occurred in only 3 hours without metals or any other co-catalysts (table 1,
entries 5 and 6). Unfortunately, selectivities towards BPA proved to be incomplete ( 80%) and
conversions dropped at temperatures below 140 °C (Table 1, run 12).
To overcome this drawback, we turned out attention to Lewis acids in order to enhance
electrophilicity of the polymer carbonyl group and exploit their synergic combination with a good
H-bonding acceptor anion of a tetraalkylammonium salt (fig. 5).
With this in mind, we explored bifunctional basic/acid catalysts composed by NBu4Cl and Cu
or Zn chlorides (or their bulk oxide powders) obtaining the depolymerization process with a
complete selectivity towards the expected products at 100 °C. Notably, chlorides proved to be more
efficient than oxides and ZnCl2 was more reactive than CuCl2 (table 1, runs 13-16).
However, albeit the milder conditions and improved selectivity, conversions proved to be still
unsatisfactory (35% - 70 %). In addition, all attempts to recover and re-use the catalyst system
ZnCl2/NBu4Cl failed, probably due to the good solubility of these salts in THF.
These problems were all circumvented replacing bulk zinc materials with nanostructured

catalysts by virtue of their high surface area. Indeed, while commercial bulk ZnO powder still
provided disappointing results (Table 1, entry 14), the new assembled bifunctional catalyst
ZnO-NPs/NBu4Cl enabled not only the complete PC depolymerization to propylene glycol
carbonate and BPA in 7 hours at 100 °C, but also a whole catalyst recovery and its prompt re-use
(see Table 1, entry 17 and section 2.6).
Of particular concern was the influence of calcination temperature employed to prepare ZnO
colloids. Indeed, the best catalytic performances were displayed by the bigger ZnO nanoparticles
calcined at 600 °C (fig. 1), in contrast to the smaller and less active ZnO nanoparticles treated at
300 °C (Table 1, entries 17-21). This different activity was probably due to the lower sulphur
residue percentages of PSS surfactant present on colloids treated at 600 °C, as emerged by element
abundances obtained by surface chemical composition analyses (fig. 2).
Similar results were obtained both with the assembled bifunctional catalyst composed by
[Bmim]OAc as the base and calcinated ZnO nanoparticles as Lewis acid counterpart (Table 1,
entries 20-21). On the contrary, reactions carried out at lower temperatures (80 °C), as well in the
absence of the IL co-catalyst (NBu4Cl), required longer reaction times, thus emphasizing the
synergistic effect exerted by both metal oxide and IL (Table 1, entries 22-24).
3.3. Effect of nucleophile/PC ratio
As expected, excess amounts of nucleophiles were found to be necessary to reach a high
depolymerization efficiency. The influence of nucleophile/PC molar ratio is elucidated in figure 6 in
the case of BPA-PC hydrolysis (water as the nucleophile). Excess amounts of water were calculated
based on the average number of the polymer carbonyl functionalities. As can be seen,
depolymerization reached the maximum conversion with an excess amount of 30 equivalents of
H2O.
A similar trend was also observed in the case of methanolysis. In the presence of reactants
bearing twice nucleophilic sites such as diols (or glycerol) and diamines, the best Nu/PC ratio was
approximately 14. In all cases, too large excess amounts of nucleophiles proved to be detrimental
for reaction, probably due to the increase of the polarity of the medium and the consequential
decreases of solubility of the low polar polymer.

3.4. Alcoholysis and aminolysis of BPA-PC
With the optimized conditions in hand, we performed a series of alcoholysis and aminolysis
experiments of BPA-PC in which the polymer was used as a carbonylating agent alternative to
phosgene, thus producing relevant fine chemicals like carbonates and ureas in high yields (table 2).
These products are useful compounds that find application in many practical fields. Carbonates
are successfully employed as solvents [22] and as intermediates in the synthesis of agrochemicals,
pesticides, and pharmaceuticals [23]. Ureas are used as dyes for cellulose fibers, antioxidants in
gasoline, corrosion inhibitors, plant growth regulators, pesticides, insecticides, and as tranquilizing
and anticonvulsant agents [24].
ZnO nanocatalyst in combination with NBu4Cl displayed good activity under moderate
conditions. With methanol as the nucleophile, isolation of dimethylcarbonate (DMC) product
required more complex procedures, due to the formation of a THF/CH3OH azeotropic mixture
(Table 2, entry 1).
A remarkable result was obtained in the case of phenol as the nucleophile (table 2, entry 2). In
the literature, BPA-PC transesterification reactions with phenols to form diarylcarbonates have been
extensively studied [25]. However, most of the proposed methods necessitate of a relatively large
amounts of basic catalysts such as hydroxides, and the depolymerization rate was relatively low
resulting in poor yields and limited catalyst recycle.
Diphenylcarbonate (DPC), a valuable starting material useful to produce polycarbonate by
molten transesterification with BPA [26], is typically obtained from DMC by transesterification
with phenol. However, high yields of DPC are rarely obtained being this reaction severely
equilibrium limited. DPC can be also obtained from phenol and CO2 using molecular or
heterogeneous catalysts [27].
With method proposed here, good yields of DPC, as well as of dinaphthylcarbonate, can be obtained
from the waste polycarbonate by using a very simplified procedure and relatively mild conditions (Table 2,
entries 2-3).
Another interesting product such as dibenzylcarbonate (DBnC) can be easily obtained with our
strategy in an economical way (Table 2, entry 4). DBnC, which is used in pharmaceutical and fine
chemical industry as protecting group instead of toxic benzyl halides, is a highly costly product
(with prices ranging from 30 to 350 times more higher than other alkyl or aryl carbonates) [28].
Tundo et al [29] proposed a straightforward catalytic protocol for the synthesis of DBnC via
transesterification of the precious DMC. With our method, DBnC can be directly obtained from a
plastic waste with clear economic advantages.

When N-nucleophiles were used in place of alcohols, PC was smoothly depolymerized
furnishing the corresponding ureas. These compounds can be accessed by reaction of phosgene with
primary or secondary amines, by direct metal-catalyzed oxidative carbonylation of amines [30] and
other protocols [31].
Under the presented conditions, PC was depolymerized giving linear aryl and alkyl ureas in
excellent yields together with BPA as by-product (Table 2, entries 5-7). Reaction proceeded
smoothly even with weaker nucleophiles such as aromatic amines. For instance, in case of aniline,
diphenylurea – a key intermediate in the preparation of 4,4’-diisocyanatephenylmethane (MDI) [32]
– was obtained in 96% of yield (after re-crystallization) without increasing temperature or
prolonging reaction time (Table 2, entry 6).
3.5. Synthesis of cyclic derivatives
When diols, aminols and diamines were used as nucleophiles, precious cyclic carbonyl
derivatives such as 2-dioxolanones, 2-oxazolidinones and cyclic ureas were obtained, respectively
(table 3). These latter are key subunits of bioactive molecules [33] and many six membered rings
ureas possess anticancer [34] or antiviral [35] activities. Typically, cyclic ureas are synthesized by
reaction of diamines with phosgene (or related products) [36], CO, CO2 and CS2 in the presence of
metal catalysts [29, 37].
From the environmental point of view, reaction of scrap polycarbonate with diamines provides
an attractive alternative method. Consequently, by following our proposed procedure, we reacted
propane-1,2-diamine with PC to get five-membered cyclic ureas (Table 3, entry 3). Similarly,
propane-1,3-diamine furnished the corresponding tetrahydropyrimidinone in an almost complete
conversion (Table 3, entry 5).
By replacing diamines with diols or aminols, other high-value carbonyl derivative such as
cyclic carbonates, 2-oxazolidinones and 1,3-dioxan-2-ones were achieved. Particularly attractive is
the transformation of glycerol (the well-known waste product of biodiesel industry) into the
corresponding cyclic carbonate, a raw material that has gained much industrial interest as solvent,
additive and chemical intermediate [38] (table 3, entry 2). A plethora of methods are available in the
literature for the preparation of glycerol carbonate [39], but none of them, to the best of our

knowledge, uses scrap polycarbonate as starting material. This process has a special symbolic value
from the green chemistry point of view as it permits the simultaneous valorization of two industrial
wastes, such as polycarbonates and glycerol, transforming them into valuable raw chemicals.
Under the same conditions, propylene glycol, 2-amino-1-propanol and butane-1,3-diol and
were reacted in only 7 hours to give the corresponding cyclic carbonates and 2-oxazolidinone (table
3, entries 1,4,6). To widen the scope of the present work, a scale-up reaction was also performed.
To this end, 15 grams of PC obtained from a wasted compact disc, were reacted in 7 hours with
1,3-diamine propane. With our delight, after 7 hours, 5.44 grams of the corresponding pure cyclic
urea were isolated.
3.6. Catalyst recycling
Catalyst robustness was evaluated using polycarbonate hydrolysis as the model reaction.
Being insoluble in THF, zinc oxide nanoparticles remained as precipitate at the end of the
depolymerization and could be removed from the reaction medium by centrifugation, dried with a
nitrogen stream and then reused by addition of the co-catalyst Bu4NCl and fresh reagents (see
experimental section).
In this way, the Lewis acid catalyst ZnO-NPs could be recycled up to five cycles with
negligible loss of activity (fig. 7), whereas its basic counterpart Bu4NCl was only partially
recovered during the separation procedure and must be integrated with fresh co-catalyst after each
run (see section 2.4).
4. Conclusions
Chemical depolymerization has to be considered the most promising technique for recycling
waste polymers and plastics, the sole allowing the conversion of plastics into their monomers or
other valuable products. Alcoholysis and aminolysis are conventional procedures for chemical
recycling of polycarbonates. With them it is possible not only the recovery of the starting monomer
(BPA), but also the use of the polymer as carbonylating agent for the synthesis of valuable
compounds, without wasting carbon dioxide.
Typically these processes are performed under hard conditions that require hot compressed
water, alkaline solutions, supercritical ethanol, microwave irradiation, or metal catalysts such as
Mn(OAc)2, MgO and CaO.

The protocol presented here, unprecedented in the literature, is based on the double catalytic
effect obtained by combining a nanostructured metal Lewis acid catalyst (ZnO-NPs) that would
enhance electrophilicity of carbonyl moiety, with a tetraalkylammonium ionic liquids (NBu4Cl), in
which the H-bond accepting ability of anion would enhance nucleophilicity of alcohols and amines,
so favoring depolymerization at milder temperature conditions.
A complete depolymerization of polycarbonate occurs in seven hours at 100 °C and ZnO
nanocatalysts can be recycled several times without sensible lack of activity.
Remarkably, when polycarbonate is reacted with glycerol it is completely converted into BPA
and glycerol carbonate. This means that in a single process is possible to convert two industrial
wastes (polycarbonate and glycerol) into two valuable chemicals like BPA (the monomer ready to
be reused) and glycerol carbonate (a useful industrial solvent and fuel additive) [40].
Due to the simple and mild experimental procedure and the low cost of the easily recyclable
co-catalysts, this protocol can compete with others of industrial importance.
Acknowledgment
We thank University of Bari, Ministry of University of Italy (MIUR) and Regione Puglia
(“PON Ricerca e Competitività” 2007-2013 – Avv. 254/Ric. del 18/05/2011, Project PONa3 00369
“Laboratorio SISTEMA”) for the financial supports.
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Figure 1. TEM images relevant to 300°C (a-a’), and 600°C (b-b’)-calcined ZnO-NPs, electrochemically prepared in the
presence of 1 g/L PSS in 30 mM NaHCO3.

50
45
40
35
30
25
20
15
10
5
70°C
300°C
600°C
0
C
O
Zn
Na
S
Element
Figure 2. Surface chemical compositions (expressed as At%) of dried (black bars), 300°C-calcined (red bars), and
600°C-calcined (blue bars) nanomaterials, as determined by XPS analysis.

ZnO commercial powder
ZnO NPs calcined at 300°C
ZnO NPs calcined at 600°C
4000 3600 3200 2800 2400 2000 1600 1200 800
400
Wavenumber / cm-1
Figure 3. FTIR spectra relevant to ZnO-NPs treated at 300°C (red curve), or at 600°C (blue curve). Spectrum for
commercial ZnO (black curve) is reported for comparison.

Figure 4. Depolymerization of bis-phenol A polycarbonate with 1,2-PD.

Figure 5. Synergic combination of a bifunctional acid/basic catalyst composed by nanostructured ZnO and NBu4Cl.
Performance of IL as basic co-catalyst is related to the H-bond accepting ability of its anions (mainly AcO- and Cl-)
expressed by the Kamlet-Taft parameters.

100
80
60
40
20
0
0
10
20
30
40
50
60
70
80
nucleophile (equiv.)
Figure 6. Reaction of BPA-PC with water promoted by ZnO (nano): influence of Nu / PC ratio.
Reaction conditions: THF, 100°C, 7h, Zn / BPA-PC ≈ 5 mol%, NBu4Cl (5 mol%).

Figure 7. BPA-PC hydrolysis promoted by ZnO-NPs/Bu4NCl: catalyst recycling.
Reaction conditions: THF, 100°C, 7h, Water (30 equiv.) Zn / BPA-PC ≈ 5 mol%, NBu4Cl (5 mol%).

Table 1. Preliminary catalytic tests(a)
Entry
1
Catalyst
T (°C)
140
140
140
140
140
140
140
140
140
140
140
100
100
100
100
100
100
100
100
100
100
80
time
Conversion(b)
0
Selectivity(c)
-
-
3
2
Py
3
30%
50%
53%
99%
98%
95%
80%
90%
40%
30%
99%
35%
38%
50%
70%
98%
60%
38%
98%
93%
98%
33%
98%
75%
100%
100%
100%
80%
3
tBuNH2
3
4
“Proton sponge”(d)
NBu4OAc
NBu4Cl
3
5
3
6
3
82%
NBu4Br
7
3
70%
NBu4I
8
3
99%
9
[Bmim]AcO
3
80%
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
[Bmim]Cl
3
99%
[Bmim]Br
3
97%
NBu4Cl
7 days
100%
100%
100%
100%
100%
100%
100%
100%
99%
NBu4Cl + CuO
NBu4Cl + ZnO(e)
NBu4Cl + CuCl2
NBu4Cl + ZnCl2
NBu4Cl + ZnO-NPs(f)
NBu4Cl + ZnO-NPs(f)
NBu4Cl + ZnO-NPs(g)
7
7
7
7
7
3
3
[Bmim]AcO + ZnO-NPs(f)
NBu4Cl + ZnO-NPs(g)
NBu4Cl + ZnO-NPs(f)
7
7
100%
100%
100%
99%
18
ZnO(e)
100
100
100
7 days
7 days
7
ZnO-NPs(f)
NaOH
99%
(a)
Reaction conditions: BPA-PC (1 mmol, 0.254 g), 1,2-propanediol (14 mmol, 1.06 g), cat (5 mol%); IL (5 mol%),
(b)
THF (5 mL/mmol PC).
Based on the glc areas ratio between BPA monomer and end capper molecule

(c)
(d)
2-phenoxy-cumene (see experimental section);
Based on glc areas of all by-products;
1,8-Bis(dimethylamino)naphthalene. (e) ZnO commercial bulk powder; (f) ZnO nanoparticles calcined at 600°C; (g) ZnO
nanoparticles calcined at 300 °C.

Table 2.
ZnO-NPS/NBu4Cl catalysed alcoholysis/aminolysis of PC:
synthesis of linear carbonates and ureas(a)
Nucleophile
Linear carbonates
Entry
Yields (%)(b)
> 99 (97)(c)
(R-OH or R-NH2)
or ureas products
CH3OH
1
2
> 99% (98%)(d)
3
> 99%
4
5
6
7
> 99%
> 99%
> 99% (96%)(d)
> 99% (97%)(d)
a)
Reaction conditions: THF 5 mL/mmol of PC; Zn / BPA-PC ≈ 5 mol%, NBu4Cl (5 mol%), PC 1 mmol (0.254 g),
b)
c)
nucleophile (28 mmol), 7 hrs, 100°C. GLC yield based on calibration curves (for details see experimental section).
In brackets, isolated yield as azeotrope. d) In brackets, isolated yields.

Table 3. Synthesis of cyclic carbonates and ureasa)
Entry
1
Nucleophile
Time (h)
7
Product
Yields (%)b)
98%
2
3
14
7
> 99 (97)
> 99 (98)
[83 (81)]d)
4
7
> 99 (97)
5
6
7
7
97 (96)
> 99
a) Reaction conditions: THF 5 mL/mmol of PC; Zn / BPA-PC ≈ 5 mol%, NBu4Cl (5 mol%), PC 1 mmol (0.254 g),
nucleophile (14 mmol), 7 hrs, 100°C. b) GLC yield based on calibration curves (for details see experimental section). In
brackets, isolated yields. d) In square brackets yields of reaction performed without catalyst.