Waste Polyethylene?terephthalate Derived Carbon Dots for Separable Production of 5-Hydroxymethylfurfural at Low Temperature

Article abstract of DOI:10.1007/s10562-020-03484-6

Abstract: Waste Polyethylene terephthalate plastic bottles were used as precursors to fabricate carbon dots (CDs) via air oxidation and sulfuric acid hydrothermal treatment. Owing to the nano-size effect and abundant acidic groups, CDs allowed their application as a novel solid acid for quasi-homogeneous catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF). Under the optimal condition, an ultra-high yield of HMF was 97.4% along with 100% fructose conversion at low temperature of 50?°C when [BMIM]Cl/ethanol was used as solvent. The CDs catalyst also displayed a prominent recyclability and was used for six recycles without massive loss in catalytic activity. Furthermore, 98.6% of HMF could be obtained from final products. The findings provide a novel sustainable route using recyclable plastics to synthesize carbon dots with a superior catalytic performance for conversion of biomass to important bio-based platform chemicals. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03484-6

Catalysis Letters
https://doi.org/10.1007/s10562-020-03484-6
Waste Polyethylene terephthalate Derived Carbon Dots for Separable
Production of 5‑Hydroxymethylfurfural at Low Temperature
Yaoping Hu1,2 · Mingfu Li1,4 · Zhijin Gao1,4 · Lei Wang1,4 · Jian Zhang1,3,4
Received: 6 October 2020 / Accepted: 27 November 2020
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
Waste Polyethylene terephthalate plastic bottles were used as precursors to fabricate carbon dots (CDs) via air oxidation and
sulfuric acid hydrothermal treatment. Owing to the nano-size efect and abundant acidic groups, CDs allowed their application
as a novel solid acid for quasi-homogeneous catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF). Under the
optimal condition, an ultra-high yield of HMF was 97.4% along with 100% fructose conversion at low temperature of 50 °C
when [BMIM]Cl/ethanol was used as solvent. The CDs catalyst also displayed a prominent recyclability and was used for
six recycles without massive loss in catalytic activity. Furthermore, 98.6% of HMF could be obtained from fnal products.
The fndings provide a novel sustainable route using recyclable plastics to synthesize carbon dots with a superior catalytic
performance for conversion of biomass to important bio-based platform chemicals.
Graphic Abstract
Keyword Carbon dots · PET · 5-Hydroxymethylfurfural · Ionic liquid · Recyclability
* Mingfu Li
1 Introduction
limingfu@nimte.ac.cn
* Jian Zhang
The production of renewable fuels and chemicals from abun-
dant biomass has attracted considerable attention in order to
relieve the dependence of fossil resources towards sustain-
ability [1]. 5-hydroxymethylfurfural (HMF) derived from
lignocellulosic carbohydrates is a versatile and key platform
chemical that can be converted into a broad range of value-
added derivatives such as 2,5-dimethylfuran, 5-ethoxymeth-
ylfuran, 2,5-diformylfuran, and 2,5-furandicarboxylic acid
with a high potential in fuel, pharmaceutical or polymer
applications [2–4].
jzhang@nimte.ac.cn
1
2
3
Ningbo Institute of Materials Technology & Engineering,
Chinese Academy of Sciences, Ningbo 315201, China
School of Materials Science and Chemical Engineering,
Ningbo University, Ningbo 315211, China
Key Laboratory of Bio-Based Polymeric Materials
Technology and Application of Zhejiang Province,
Ningbo 315201, China
4
University of Chinese Academy of Sciences, Beijing 100049,
China
Vol.:(0123456789)
1 3

Y. Hu et al.
The past several decades has seen the emergency of
numerous studies for dehydration of fructose to HMF in
water [5, 6], organic solvents [7, 8], ionic liquids (ILs) [9,
10], and mixed solvents [11, 12]. Water is a green solvent.
Unfortunately, the reaction temperatures required in aqueous
media are generally beyond 120 °C, resulting in partial deg-
radation of HMF to undesired byproducts including levulinic
and formic acid. Organic solvents can suppress the rehydra-
tion of HMF. Nevertheless, the high temperatures, typically
ranging from 120 to 180 °C, are still needed, which favors
the generation of humins, thereby reducing HMF yield
and selectivity [13]. IL-mediated fructose conversions are
mostly carried out at the temperature above 80 °C. With the
aim of preventing high-temperature induced side reactions
and further decreasing energy consumption, a few reports
described that the fructose-to-HMF transformation in ILs
could even occur at room temperature to 50 °C with appro-
priate catalysts. Zhang et al. demonstrated WCl6-catalyzed
conversion of fructose to HMF in butylmethyl imidazolium
chloride ([BMIM]Cl), getting a HMF yield of 63% at 50 °C
for 4 h [14]. Qi et al. added cosolvents such as acetone,
dimethyl sulfoxide (DMSO), methanol, ethanol, and ethyl
acetate (EtOAc) into [BMIM]Cl, and with strong acidic ion-
exchange resin as the catalyst, the yield of obtained HMF
was 78–82% with 89–95% fructose conversion at room tem-
perature for 6 h [15]. Despite some impressive advances, the
research area of low-temperature production of HMF from
fructose is still in its infancy, and a number of issues remain
to be resolved. One of the most crucial issues is how to
increase the HMF yield in such mild conditions. Rationally
design and carefully select a good catalyst may overcome
the problem.
theoretically be converted into carbon materials by carboni-
zation, but it could not be turned into CDs by the classical
H2O2-assisted hydrothermal technique even at high tempera-
ture [25]. In order to destroy the robust structure of PET,
thermal oxidation method was used.
Herein, a novel nanoscale carbocatalyst of carbon dots
(CDs) was synthesized by the air oxidation and sulfuric acid
sulfonation of PET. The nanoparticles solid acid contained
functional group of –SO3H, –COOH and –OH, presented
a high selectivity and activity for HMF production from
fructose in green solvents [BMIM]Cl/ethanol at low tem-
perature. The CDs catalyst displayed a prominent recycla-
bility and was used for six recycles without massive loss in
catalytic activity. Furthermore, the produced HMF could be
directly extracted from the catalytic system by ethyl acetate,
realizing the readily isolation, making the whole process of
HMF production from fructose green and sustainable.
2 Experimental
2.1 Materials and Chemicals
Polyethylene terephthalate of waste Plastic bottles were
obtained from Nongfu Spring Co., Ltd. (Hangzhou, China).
Fructose was provided by Aladdin Chemical Reagent Co.,
Ltd. (Shanghai, China). 1-butyl-3-methyl imidazolium chlo-
ride ([BMIM]Cl) was purchased from Lanzhou AoKe Chem.
Co., Ltd. (Lanzhou, China). Sulfuric acid (98%), ethanol
(EtOH), N-methyl-2-pyrrolidone (NMP), dimethylsulfox-
ide (DMSO), dimethylformamide (DMF), and ethyl acetate
(EtOAc) were purchased from Sinopharm Chemical Rea-
gent Co., Ltd. (Shanghai, China). Deionized water was used
throughout the experiments.
To date, various homogeneous and heterogeneous cata-
lysts have been utilized for conversion of fructose to HMF.
Compared with homogeneous catalysts such as H2SO4, HCl,
and organic acids which are difcult to recycle and seriously
corrode the equipment [16]. Nevertheless, the heterogeneous
catalysts including zeolites [17], metal oxides [18, 19], ion-
exchange resins [20], and sulfonated carbonaceous materials
[21, 22] share the advantages of easy separation and less
corrosion. Among them, sulfonated carbonaceous materials
consisting of small polycyclic aromatic carbon sheets with
attached SO3H groups have emerged as a promising class
of cheap and active solid acids [23]. However, the conven-
tional carbon materials show poor dispersion ability, which
is unbenefcial for sufcient contact between active sites and
reactants, limiting their catalytic performance. Moreover, the
commonly use of carbon materials at high temperature may
cause the leaching of sulfur-containing moieties, resulting
in the loss of activity. Plastic products are widely used in our
daily life, compared with its counterparts such as polyeth-
ylene, polypropylene, and polyvinyl chloride, polyethylene
terephthalate (PET) was much more stable [24]. PET can
2.2 Synthesis of Catalysts
CDs were fabricated by air oxidation of waste plastic bottles
made of PET followed by the immersion in concentrated
H2SO4 (Scheme 1). In a typical procedure, waste plastic
bottles were cut into pieces, and then 3.0 g of the raw mate-
rials were put into an open ceramic crucible and heated
at 300 °C for 2 h under air. The brown oxidized product
was then immersed into 10 mL H2SO4 (98%), and heated
at 120 °C for 6 h. After that, a black solution containing
CDs was obtained. The solution was carefully diluted with
200 mL deionized water and fltrated through a Buchner
funnel to remove the residue. Further purifcation of CDs
was conducted through a dialysis tube (molecular weight
cut of: 1000 Da) for 3 days in the dark (changed deionized
water every 8 h) until sulfate ions were no longer detected.
The CDs powder was obtained by freeze-drying treatment
of such a purifed solution.
1 3

Waste Polyethylene terephthalate Derived Carbon Dots for Separable Production of…
Scheme 1 A diagram about the synthesis of CDs from waste plastic bottles made of PET
Q0F − QMF
2.3 Characterization of Catalysts
FC=
QF0
Transmission electron microscopy (TEM) analysis of CDs
were taken on a JEM-2100 electron microscope operat-
ing at an accelerated voltage of 200 kV. X-ray difraction
(XRD) patterns were recorded on a Bruker D8 advance
X-ray difractometer with Cu Ká radiation (1.54056 Å).
Raman spectra of the as-prepared samples were obtained
on a Renishaw in Via-refex spectroscopy with a laser at
an excitation wavelength of 532 nm. X-ray photoelectron
spectroscopy (XPS) measurements were performed with
an AXIS UL TRA DLD spectrograph with Al/Kα as the
source. Fourier transformed infrared (FTIR) spectra were
collected in the KBr medium on a Nicolet 6700 FTIR
spectrometer.
QHMF
YHMF
=
QF0
here, Q0 and QM are the initial quantity of fructose (g) and
the quaFntity ofFfructose (g) in the reaction solution; Q
is the actual quantity of HMF (g) in the reaction solutioHnM. F
2.5 Recyclability of SCDs Catalyst
When the reaction reached completion, 5 mL EtOAc and
1 mL H2O were added into the resultant mixture to form a
biphasic solution after distillation of ethanol (Scheme 2),
the HMF was efectively extracted by EtOAc under stirring
at 300 rpm, while the solubilities of [BMIM]Cl and CDs
in EtOAc were negligible, so they remained in the aque-
ous layer. The liquor was separated by separatory funnel
to obtain the EtOAc/HMF and CDs/IL/H2O components.
The EtOAc/HMF liquor was distilled by reduced pressure to
remove EtOAc and obtain HMF. The EtOAc can be recycled
to extract HMF in this system. CDs/IL/H2O liquor was dis-
tilled by reduced pressure to remove water and obtain CDs/
IL component for reusability. Five repeated runs of extrac-
tion were carried out for sufcient HMF separation (total
25 mL EtOAc was used).
2.4 Catalytic Procedure and Products Analysis
In a typical reaction, 50 mg fructose was dissolved in the
[BMIM]Cl/ethanol solvent (1 g [BMIM]Cl mixed with
1 mL ethanol) before the addition of CDs (50 mg). The
resulted suspension was stirred at 500 rpm for a certain
period of time at fxed low temperatures. After the reaction
reached completion, the liquor was diluted with deionized
water for the product analysis by high performance liquid
chromatography (HPLC).
The fructose concentration was detected on an Agilent
1260 HPLC system equipped with an Aminex HPX-87H
column and a RID detector. H2SO4 solution (5 mM) was
used as the mobile phase and the column temperature
was set at 30 °C [26]. Quantitative analysis of HMF was
performed by HPLC equipped with a ZORBAX SB-C18
column and a UVW detector (278 nm). Water–methanol
[95:5 (v/v)] solution was used as the mobile phase. The
fructose conversion (FC) and the HMF yield (YHMF) were
calculated according to the following equations:
3 Results and Discussion
3.1 Characterization of Catalysts
The transmission electron microscopy (TEM) analysis
(Fig. 1a) showed that the CDs had a circular shape and were
well dispersed. Their diameters ranged from 1 to 6 nm with a
1 3

Y. Hu et al.
Scheme 2 A recycle process of the CDs-[BMIM]Cl/ethanol system for HMF production from fructose and the procedure of HMF separation
Fig. 1 a TEM image, b size
distribution, c XRD pattern,
and d Raman spectrum of CDs.
Inset in a: high-resolution TEM
image of CDs
mean size of approximately 3 nm by counting more than 200
nanoparticles (Fig. 1b). Clear Lattice fringes with a spacing
of 0.21 nm were observed in the high-resolution TEM image
(Fig. 1a, inset), which were corresponding to the (100) in-
plane lattice of graphite, indicating the good crystallinity
of CDs [27]. The X-ray difraction (XRD) pattern (Fig. 1c)
exhibited a broad peak at 2θ angle of 21.6°, being associ-
ated with the disordered graphitic structure [28]. The Raman
spectrum (Fig. 1d) displayed the characteristics of D and G
bands at 1351 and 1580 cm−1, respectively, suggesting the
coexistence of both sp3 and sp2 carbons inside CDs [29].
X-ray photoelectron spectroscopy (XPS) measurement
was carried out to determine the elemental composition and
chemical bonding of the CDs samples. The signals of C1s,
O1s, S2s, and S2p peaks were found in the XPS spectrum
(Fig. 2a), the elements ratio of C, O and S were 70.7%
25.7% and 3.6%, respectively, revealing the CDs mainly
contained carbon and oxygen element as well as a small
amount of sulfur element. The C1s spectrum (Fig. 2b) could
be resolved into three peaks of C–C/C=C (284.5 eV),
C–O/C–S (286.1 eV), and C=O (288.8 eV) [30], indi-
cated that the percentage of C–C/C=C was higher than
the C–O/C–S and C=O, due to the air oxidation and sul-
furic acid sulfonation of waste PET. In the XPS spectrum
of S2p (Fig. 2c), the 2p3/2 and 2p1/2 peaks of SO3H groups
were assigned to 168.5 and 169.6 eV, respectively, indicat-
ing the sulfonate was successful attached on the surface of
CDs [31, 32]. The Fourier transform infrared (FTIR) spec-
trum of CDs was shown in Fig. 2d, the stretching vibra-
tions of O=S=O and SO3– were assigned to 1232 cm−1 and
1 3

Waste Polyethylene terephthalate Derived Carbon Dots for Separable Production of…
Fig. 2 a XPS survey, b C1s, c
S2p, and d FTIR spectra of CDs
1039 cm−1, respectively, demonstrating the CDs possessed
SO3H groups [33, 34]. The absorption bands of O–H, C–H,
C=O, and C=C bonds were also illustrated in the FTIR spec-
trum. These results are consistent with the XPS analysis. As
a result, it was concluded that the as-synthesized materials
were carbon-rich, nanosized dots, which had graphite-like
structure and bore abundant acid groups on the surface. Such
a nanocarbon was then employed as a solid acid for catalytic
dehydration of fructose to HMF.
94.6%, due to the degradation of HMF and the formation
of humins at elevated temperatures [21, 35]. The efect of
catalyst dosage on the conversion of fructose to HMF was
evaluated (Fig. 3c). The conversion of fructose was negli-
gible and no HMF was detected when the reaction system
lack of CDs. In contrast, by adding a small amount of CDs
(10 mg), a satisfactory yield of HMF was 80.0% with 84.1%
fructose conversion, confrming the essential catalytic role
of CDs. With increasing the catalyst dosage to 50 mg, the
yield of HMF reached the maximum value of 97.4%. An
extra increase of catalyst dosage, however, the yield of HMF
slightly decreased, implying too much acid sites provided
by excess CDs favored side reactions like humin formation.
Thus, the optimum conditions for converting fructose to
HMF in the presence of CDs were obtained, the reaction
temperature and time were 50 °C and 2 h, respectively, the
catalyst dosage was 50 mg.
3.2 Optimization Condition of Fructose Conversion
The CD catalysts could thoroughly distribute in the [BMIM]
Cl/ethanol solution containing fructose, forming a pseudo-
homogeneous state. Excitingly, the conversion of fructose
to HMF proceeded smoothly at room temperature (25 °C).
The yield of HMF was 13.4% after 2 h and then gradu-
ally increased to approach the plateau value of 79.4% with
fructose conversion of 98% for 24 h (Fig. 3a and b, insets).
Encouraged by these observations, the catalytic system was
examined at temperatures ranging from 40 to 70 °C (Fig. 3a
and b). Notably, an ultra-high yield of HMF was 97.4% with
100% fructose conversion when the reaction temperature and
time were 50 °C and 2 h, respectively. The reaction rates
increased with the increase of temperature, but the yield of
HMF decreased. For example, when the temperature was
70 °C, the reaction rate increased, but the yield of HMF was
3.3 Reaction Kinetics and Activation Energy
Analysis
Figure 4a illustrated a nice linear relationship between
− ln(1 − Conversionfructose) and time, revealing the fruc-
tose dehydration followed the frst-order model of kinetics
model [7, 36]. The plot of (ln k) versus (1/T × 103) based
on the Arrhenius equation was given in Fig. 4b, reporting
a low activation energy of fructose conversion to HMF
1 3

Y. Hu et al.
Fig. 3 Efects of time and temperature on a fructose conversion and b HMF yield of the catalytic reaction. c Efect of catalyst dosage on fructose
conversion and HMF yield
Fig. 4 a Plots of − ln(1 −
Conversionfructose) versus time
at diferent temperatures. b Plot
of (ln k) versus (1/T×103) for
fructose dehydration
was 73.8 kJ mol−1 and a large pre-exponential factor was
3.4 Efect of Co‑solvent and Catalyst on Fructose
1.4×1012, which was much lower compared to the sulfuric
Conversion
acid (99 kJ mol−1), ion-exchange resin (103.4 kJ mol−1)
[37], and mesoporous silica nanoparticles (80.05 kJ mol−1)
The feasibilities of low-temperature HMF production
[38], this result accounted for the superior catalytic per-
formance of the CD-based system.
from fructose in [BMIM]Cl through addition of various
co-solvents were examined (Fig. 5a). Both the fructose
1 3

Waste Polyethylene terephthalate Derived Carbon Dots for Separable Production of…
Fig. 5 a Efect of co-solvent on
fructose conversion and HMF
yield. b Efect of amount of
ethanol on fructose conversion
and HMF yield
Table 1 Comparison of the
No.
Catalyst
Solvent
T (°C)
Time (h)
Yield (%)
Ref.
dehydration of fructose to HMF
using diferent catalytic system
at low temperatures
1
2
3
4
5
6
7
GeCl4
WCl6
Amberlyst 15
HCl
P2O5
Amberlyst 15
–
[BMIM]Cl/DMSO
[BMIM]Cl
[BMIM]Cl/acetone
[BMIM]Cl
[BMIM]Cl
[BMIM]Cl/[BMIM][BF4]
25
50
25
23
50
25
25
12
4
6
24
1
3
70.0
63.0
78.1
72.0
81.2
56.0
89.2
[11]
[14]
[15]
[41]
[42]
[43]
[12]
[BMIM]Cl/[HNMP]
6
[CH3SO3]/ethanol
8
Carbon dots
[BMIM]Cl/ethanol
50
2
97.4
This work
conversion and HMF yield were below 6% in the pres-
ence of water, indicating the water was an unsuitable co-
solvent, due to the water inhibited the equilibrium shift
in fructose dehydration [39]. The co-solvents of NMP,
DMSO, and DMF offered complete fructose conver-
sion, but the selectivity and yield of HMF were not high
enough (67.4–87.7%), due to the generation of certain
amounts of humins as by-products [40]. Clearly, ethanol
was a preferred co-solvent, because of it not only gave an
extra-high HMF yield (97.4%), but also was an environ-
mentally friendly, cost-efcient, and easy-to-use reaction
media. The efect of the amount of ethanol on the fructose
conversion was shown in Fig. 5b. Insufcient addition of
ethanol (< 1 mL) reduced the catalytic efciency owing to
the limited mass transfer in the viscous reaction mixture
[11], whereas excessive ethanol addition (>1 mL) lowered
the dielectric constant of the reaction media which also
brought about a negative efect [15].
Fig. 6 Catalytic ability of the CDs catalyst recycled from [BMIM]Cl/
ethanol system
Compared to the reported catalyst and solvent for con-
version fructose to HMF, the highest yield of HMF was
obtained when using the CDs as catalyst (Table 1). Such
a good results had never been reported before under mild
conditions, demonstrating the remarkable catalytic efciency
of this reaction system, which benefted from the appropriate
acid strength and excellent interfacial compatibility of the
CD catalysts in the [BMIM]Cl/ethanol solution.
3.5 Recyclability of Catalysts
This unique phase behavior was really benefcial for HMF
separation without cross contamination (Scheme 2). Suf-
fcient extraction allowed a high HMF yield of 96.1% in
EtOAc with an extraction efciency of 98.6%. [BMIM]Cl
1 3

Y. Hu et al.
8. Ren L, Zhu L, Qi T et al (2017) Performance of dimethyl sul-
foxide and brønsted acid catalysts in fructose conversion to
5-hydroxymethylfurfural. ACS Catal 7:2199–2212
and CDs could be easily reused after removed the EtOAc
and H2O. As shown in Fig. 6, the yield of HMF still retained
91.4% in EtOAc even the six recycles of the CDs catalyst,
confrming that the catalytic system was recyclable without
signifcant deactivation of CDs.
9. Chinnappan A, Jadhav AH, Kim H et al (2014) Ionic liquid
with metal complexes: an efcient catalyst for selective dehy-
dration of fructose to 5-hydroxymethylfurfural. Chem Eng J
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11. Zhang Z, Liu B, Zhao ZK (2012) Conversion of fructose into
5-HMF catalyzed by GeCl4 in DMSO and [Bmim]Cl system at
room temperature. Carbohyd Polym 88:891–895
In conclusion, nanosized CDs possessing abundant SO3H,
COOH, and OH groups were economically fabricated
through air oxidation of waste plastic bottles followed by
the sulfonation of H2SO4. Such nanomaterials as a novel
solid acid could well dispersed in [BMIM]Cl/ethanol sol-
vent to form a quasi-homogeneous catalytic state, that ena-
bled the high-yield production of HMF from fructose at
low-temperature. Easy separation of HMF was realized by
EtOAc extraction, and the CDs based catalytic system was
actually recyclable. This work provides new opportunities
for large-scale synthesis of HMF under mild conditions for
application on conversion biomass to biofuels.
12. Zhang J, Yu X, Zou F et al (2015) Room-temperature ionic liquid
system converting fructose into 5-hydroxymethylfurfural in high
efciency. ACS Sustain Chem Eng 3:3338–3345
13. Sun S, Zhao L, Yang J et al (2020) Eco-friendly synthesis of SO3
H-containing solid acid via mechanochemistry for the conversion
of carbohydrates to 5-hydroxymethylfurfural. ACS Sustain Chem
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14. Chan JYG, Zhang Y (2009) Selective conversion of fructose to
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Acknowledgements This project was funded by the fnancial support
from Chinese Academy of Sciences (QYZDB-SSW-JSC037), Ningbo
Science and Technology Bureau (2018B10056, 2019B10096), and
Fujian Institute of Innovation (FJCXY18020202).
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Compliance with Ethical Standards
Conflicts of interest The authors declare no competing fnancial in-
terests.
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