Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase Catalyst for Carbenes and Nucleophilic Substitution Reactions

Article abstract of DOI:10.1007/s10562-019-02849-w

Abstract: Separation and reusability had been main problems for the using of polyethylene glycol (PEG) as phase transfer catalysts (PTCs). To solve these problems, PEG was firstly and successfully grafted on graphene oxide (GO) using BF₃·C₂H₅OC₂H₅ as Lewis acid catalyst. The solid GO-PEG composites were systemically investigated by characterization techniques (TG, FT-IR, XPS, ICP-AES etc.) and then applied to some carbenes and nucleophilic substitution reactions as novel triphase catalysts. As the results, GO-PEG showed not only equally excellent catalytic activity (≥ 93% yield of 7,7-dichlorobicyclo[4.1.0]heptane and iodooctane) but also incomparable reusability (≥ 85% yield of iodooctane after using for four times) in comparison with traditional PTCs (PEG). Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02849-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02849-w
Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase
Catalyst for Carbenes and Nucleophilic Substitution Reactions
Xiaohai Yang1 · Jie Zhai1 · Tongchun Xu1 · Bing Xue1 · Jie Zhu1 · Yongxin Li1
Received: 4 April 2019 / Accepted: 20 May 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Separation and reusability had been main problems for the using of polyethylene glycol (PEG) as phase transfer catalysts
(PTCs). To solve these problems, PEG was frstly and successfully grafted on graphene oxide (GO) using BF3·C2H5OC2H5
as Lewis acid catalyst. The solid GO-PEG composites were systemically investigated by characterization techniques (TG,
FT-IR, XPS, ICP-AES etc.) and then applied to some carbenes and nucleophilic substitution reactions as novel triphase
catalysts. As the results, GO-PEG showed not only equally excellent catalytic activity (≥ 93% yield of 7,7-dichlorobicy-
clo[4.1.0]heptane and iodooctane) but also incomparable reusability (≥85% yield of iodooctane after using for four times)
in comparison with traditional PTCs (PEG).
Graphic Abstract
Keywords Polyethylene glycol · Graphene oxide · Triphase catalyst · Carbenes reaction · Nucleophilic substitution
* Yongxin Li
liyxluck@163.com
Extended author information available on the last page of the article
Vol.:(0123456789)
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X. Yang et al.
traditional PTCs). What’s more, the good amphiphilicity
of GO might make it to be a superior support for PTCs.
However, as far as we know, the immobilization of PEG on
GO as a triphase catalyst had not been reported.
1 Introduction
The phase-transfer catalysts (PTCs) had been widely
applied to heterogeneous (e.g. aqueous–organic) catalysis
reactions because they could efciently promote difusion
of the aqueous and organic substrates benefting from
their excellent amphiphilicity [1, 2]. Polyethylene glycol
(PEG) was one of the mostly preferred PTCs depending on
its good environmental friendliness, thermostability and
low commercial price and had been applied to various
chemical processes [3–5]. Nevertheless, separation and
reusability of the catalyst were limited due to that PEG
could be dissolved in reactants and products. Therefore,
the introduction of solid catalysts by immobilization of
PEG on supports via covalent bonds should be an efcient
approach to eliminate or alleviate the leaching of active
PEG sites and improve durability of the catalysts.
Carbenes and nucleophilic substitution reactions were
both with great signifcance and widely applied to organic
synthesis. In many cases, these reactions should be con-
ducted in heterogeneous catalytic systems (immiscible
liquid–liquid or liquid–liquid–solid phases) catalyzing by
PTCs. As demonstrated before, PEG was a kind of efcient
PTCs for many carbenes and nucleophilic substitution reac-
tions [19–21]. In this contribution, PEG was frstly and suc-
cessfully grafted on GO via the construction of PEG–O–GO
covalent bonds using BF3·C2H5OC2H5 (strong lewis acid) as
catalyst. The stable structure of the grafted PEG with difer-
ent polymeric level had been systematically demonstrated
via techniques such as ICP-AES, XPS, FT-IR and TG. Fur-
thermore, catalytic performance for typical carbenes and
nucleophilic substitutions reactions over the PEG–O–GO
composites was evaluated.
For all we know, appropriate supports for the immobili-
zation of PEG was barely reported. This might be ascribed
to that PEG could hardly be directly grafted on common
supports such as metallic oxides, silica and molecu-
lar sieves. Terminal hydroxyls were the only available
chemical-groups in PEG for the immobilization reaction.
Generally, hydroxyls could react with halogens, carbox-
yls, epoxys or sulfonics etc. under appropriate conditions.
However, there were scarcely any available chemical
groups in these common supports. As reported before, the
terminal hydroxyls could efciently react with epoxy and
sulfonic groups in some polymers to form stable covalent
bonds [6, 7]. Unfortunately, the using of polymers as the
support would result in swelling of the catalyst in organic
substrates and badly hinder its practical application. Thus,
it should be urgent to fnd adaptive supports for the immo-
bilization of PEG.
2 Experimental
2.1 Catalysts Preparation
2.1.1 Preparation of GO
GO was prepared by modifed Hummers method mentioned
before [22]. In brief, graphite (2 g) and NaNO3 (1 g) were
mixed with 50 mL concentrated sulfuric acid in a three-
necked fask under continuous stirring. The reaction temper-
ature was 0 °C and controlled by an ice bath. Subsequently,
KMnO4 (9 g) was injected in the fask within 30 min. The
aeruginous mixture was then stirred for 30 min (tempera-
ture<1 °C). After that, the mixture was heated to 35 °C and
held for 4 h. 50 mL deionized water was gradually injected
into the brown mixture (temperature<60 °C) and the mix-
ture was then cooled to 35 °C by stirring for 40 min. Where-
after, the brown mixture was heated to 90 °C for 1 h and
the color was turned into earthy yellow. The earthy yellow
mixture was transferred to a breaker (1 L) and diluted to
450 mL by deionized water. The mixture was cooled to room
temperature by stirring. After that, 20 mL hydrogen peroxide
(30%) was injected and the fnal suspension was then stirring
for 15 min and fltered, washed by hydrochloric acid (15%)
for 3 times. The flter cake was dried at 80 °C overnight and
then washed by deionized water until pH of the fltrate was
7. The dried flter cake was GO.
In recent years, graphene oxide (GO) had attracted
much attention due to its excellent physicochemical prop-
erties and had been applied to diferent felds [8–11].
For instance, the super electric conductivities, outstand-
ing mechanical and electrical properties had made it to
be commendable support for metal nanocatalysts applied
to fuel cells [12–14]. Besides, the abundant chemical
groups in GO (e.g. carboxyl, carbonyl, hydroxyl and epoxy
groups) could selectively react with specifc groups in
some supported species and made it to be a versatile immo-
bilization support. Many GO-immobilized catalysts could
exhibit remarkable performance for diferent reactions
[15–18]. Likewise, the abundant epoxys in GO should be
active and available to react with the terminal hydroxyls
in PEG. Thus, the grafting of PEG on GO should also be
possible. The using of this solid GO-PEG composites as
triphase catalysts might be an efcient approach to solve
the problems stated above (separation and reusability of
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Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase Catalyst for Carbenes and…
2.1.2 Preparation of GO‑PEG
spent catalyst was also evaluated after washing and drying
processes.
Nucleophilic substitution reactions: The reactants includ-
The as-prepared GO (0.2 g) and PEG (2 g) were injected
into a double-necked fask with 100 mL toluol. The mixture
was then ultrasonic processed for 3 times (30 min/time). The
well dispersed mixture was stirred and heated to 60 °C and
purged by Ar fow for 15 min. After that, BF3·C2H5OC2H5
(2 mL) was injected under the Ar fow. The dispersed sus-
pension was refuxed at 60 °C for 20 h and then fltered,
washed by ethanol for several times and followed drying at
80 °C over night. The dried sample was signed as GO-Px
(x=400, 600 and 1000) and x was polymeric level of PEG.
In brief, the mechanism for the grafting of PEG on GO could
be described in Scheme 1.
ing bromooctane, NaI, chlorobenzene and GO-Px catalyst
were mixed in a fask equipped with magnetic stirrer and oil
bath. The reactions were conducted at diferent temperature
and the analysis method was similar to carbenes reactions
mentioned above.
3 Results and Discussion
3.1 Catalyst Characterization
Structural information of the as-prepared GO-Px and GO
samples was investigated by FT-IR. As displayed in Fig. 1a,
the absorption peaks located at 1725 cm−1, 1628 cm−1 and
1050 cm−1 could be attributed to stretching vibration of
C=O, C=C and C–OH bonds and 1250 cm−1 and 840 cm−1
could be ascribed to vibration of C–O–C bonds (epoxy
groups) in single GO, respectively [23, 24]. However, the
absence of absorption peak at 840 cm−1 might imply the
disappearance of epoxy groups in the GO-Px samples. The
characteristic peak appeared at 940 cm−1 should be assigned
to typical –CH2 groups in PEG [25]. Additionally, the
absorption peaks in the GO-Px samples at 1628 cm−1 (C=C
bond) and 1250 cm−1 (C–O–C bond) shifted to lower wave-
numbers. These should demonstrate that PEG was success-
fully grafted on GO via the new covalent bonds (C–O–C, as
shown in Scheme 1) catalyzing by BF3·C2H5OC2H5.
2.2 Characterization
Thermogravimetric (TG) experiments were performed on
a TGA 7 instrument with a heating rate of 10 °C min−1 (Ar
atmosphere). Elemental analysis was obtained by using a
FLASH 2000 CHNS/O organic elemental analysis appa-
ratus. A TENSOR27 FT-IR spectrometer in the range of
400–4000 cm−1 with a resolution of 4 cm−1 was used to
obtain the FT-IR spectra of the samples. The X-ray photo-
electron spectroscopy (XPS) experiments were performed
on a VG MiltiLab 2000 spectrometer with Mg Kα radiation.
2.3 Activity Test
Carbenes reactions: A setting amount of cinnamene, chlo-
roform, NaOH and GO-Px catalyst were mixed in a dou-
ble-necked fask (50 mL) equipped with mechanical stirrer
and refux condenser. The reactions were conducted under
continuous stirring (500 rpm) at diferent temperature con-
trolling by an oil bath. After reactions, the products were
isolated from the catalysts by centrifugation and fltration.
The products were collected and analyzed by using a GC
instrument with a FID. Furthermore, reusability of the
Thermostability of the catalysts was investigated via TG
analysis. As exhibited in Fig. 1b, the weight loss around
100 °C should be attributed to adsorbed water in surface
of the samples. The evident decomposition between 200
and 250 °C of the samples might be ascribed to losses of
oxygen-containing groups (hydroxyl, epoxy group etc.)
[10]. Additionally, the decomposition of GO-Px samples
around 400 °C might be assigned to loss of the adsorbed
Scheme 1 The grafting of PEG on GO catalyzed by BF3·C2H5OC2H5
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X. Yang et al.
Fig. 1 FT-IR spectroscopy (a) and TG analysis (b) of GO-Px catalysts and GO
Table 1 Elemental analysis of GO-Px and GO samples
Sample
C (wt%)
H (wt%)
N (wt%)
GO-P1000
GO-P600
GO
54.74
49.96
45.54
3.4
3.64
2.93
0.08
0.04
0.11
As shown in Fig. 2, the less ID/IG of GO-P600 than GO (1.01
vs. 1.29) should imply that the amount of defects and oxy-
gen functional groups in GO were evidently decreased after
the immobilization of PEG. This might be attributed to the
cleavage of epoxys via the reaction with terminal hydroxys
of PEG. This could further demonstrate that PEG was suc-
cessfully grafted on GO.
Elemental components of the catalysts were measured and
exhibited in Table 1. As shown in the table, the introduction
of PEG could evidently decrease content of oxygen-contain-
ing groups and increase that of C and H. For instance, the
content of C and H in GO-P1000 and GO was 58.14% and
48.47%, respectively. Besides, the superior content of C in
GO-P1000 in comparison with GO-P600 could be assigned
to their diferent polymeric level of PEG. Additionally, the
residual nitrates originating from the preparation procedures
of GO (as describe above) should be responsible for the N
content in the samples. What’s more, molar contents of PEG
in GO-P600 and GO-P1000 had been calculated on basis
of the elemental analysis and they were about 0.14 mmol/g
and 0.15 mmol/g.
Fig. 2 Raman spectra of GO and GO-P600
and/or grafted PEG. The further weight loss of GO above
600 °C should belong to decomposition of skeleton carbons
[23]. On the other hand, the GO-Px samples showed less
weight loss above 600 °C. According to the results, the total
weight losses of GO-P600 and GO-P1000 were obviously
less than GO (around 40% vs. 75%). These should further
demonstrated that PEG was successfully grafted on GO and
improved thermostability of GO.
Structure of the functionalized GO could be further
identifed by Raman spectra. The peaks located at around
1353 cm−1 and 1592 cm−1 could be assigned to the vibra-
tions of sp3 carbon atoms of defects and disorder (D band)
and the vibrations of sp2 carbon atoms in a graphitic 2D
hexagonal lattice (G band), respectively [13, 26, 27]. The
intensity-ratio of these peaks (ID/IG) could refect the amount
of defects and oxygen functional groups in the samples [13].
XPS analysis of GO-Px and GO was further investi-
gated and included in Fig. 3. The full spectrums (Fig. 3a–c)
showed that surface of all the samples should be mainly
composed of C and O. The stronger peaks of GO-Px
might imply that their superfcial content of C and O was
1 3

Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase Catalyst for Carbenes and…
Fig. 3 XPS analysis of GO (a, d), GO-P600 (b, e) and GO-P1000 (c, f)
promoted by PEG. As displayed in Fig. 2d–f, there should
Table 2 Catalytic performance of GO-Px catalysts for carbenes reac-
be four peaks located at 284.6 eV, 286.5 eV, 287.4 eV and
289.1 eV after ftting and deconvolution of C 1s XPS peaks
of the samples and could be attributed to sp2 hybrid C in
C=C, C–OH, C–O–C and C=O groups, respectively [10,
28]. However, the new peaks located at 285.5 eV in GO-Px
samples compared with GO should belong to C in cova-
lent bonded C–O–C between PEG and GO (as depicted in
Scheme 1). Additionally, the depressed peak intensity of C
in C–O–C (287.4 eV, epoxy groups in GO) should further
demonstrate the grafting of PEG on GO. This coincided well
with the results of FT-IR.
tions
Entry Substrates Products
Catalyst
Blank
Conv. (%) Sel. (%)
1
10
100
2a
3
4
GO
38
100
100
100
100
100
100
GO-P400 83
GO-P600 96
GO-P1000 91
PEG-600 96
GO-P600 96
5
6b
7c
3.2 Catalyst Performance
Conditions: catalysts 0.5 g, chloroform 10 mL, cyclohexene 1 mL,
NaOH 10 g, 55 °C, 10 h, stirring rate 500 rpm
Synthesis of 7,7-dichlorobicyclo[4.1.0]heptane from chlo-
roform and cyclohexene was conducted to investigate per-
formance of the catalysts for carbenes reactions. As shown
in Table 2, the grafted GO-Px catalysts exhibited evidently
aCatalyst: GO 0.45 g
bCatalyst: PEG-600 0.05 g
cSubstrate: cinnamene 1.2 mL
1 3

X. Yang et al.
superior activity than pure GO in spite of that they showed
similar selectivity to target product (nearly 100%). The best
conversion (96%) could be attributed to GO-P600 among
the GO-Px and this implied that chain length of PEG could
obviously infuence activity of the catalysts. Noteworthily,
the single PEG-600 (corresponded to amount of PEG in
GO-P600) exhibited a similar conversion to GO-P600. This
should demonstrate that the novel triphase catalysts could
exhibit equally excellent catalytic activity in comparison
with the traditional phase-transfer catalyst (single PEG).
Besides, the substrate (cyclohexene) was replaced by cin-
namene to further investigate its catalytic activity for other
carbenes reaction. As expected, a high yield of target prod-
uct for carbenes reaction could be also obtained over the
optimized GO-P600 (96%, shown in Table 2). These fur-
ther demonstrated the excellent catalytic performance of the
GO-P600 for diferent carbenes reactions.
The correlations between reaction conditions and conver-
sion of the substrate were also revealed in Fig. 4. As shown
in Fig. 4a, the conversion of cyclohexene was increased evi-
dently along with the improving of reaction time from 3 to
10 h. However, the conversion could not be further improved
when reaction time was above 10 h. On the other hand, the
conversion of cyclohexene displayed a volcanic trend when
the reaction temperature was ranged from 35 to 70 °C. The
distinct decrease of conversion above 60 °C might be attrib-
uted to the low boil point of chloroform (61.3 °C). As the
result, the best conversion of cyclohexene should be ascribed
to 96% and the optimized conditions should be 60 °C and
10 h.
Likewise, synthesis of iodine octane from chlorobenzene
and NaI had been also investigated to evaluate performance
of the catalysts for nucleophilic substitution reactions. As
displayed in Table 3, all the catalysts showed a nearly com-
plete selectivity of target product. The best activity should
be also attributed to GO-P600 among the GO and GO-Px
catalysts. The infuences of the chain length of PEG on cata-
lytic activity of GO-Px could be further demonstrated in
nucleophilic substitution reactions. Noteworthily, activity
of the grafted GO-P600 and single PEG-600 (corresponded
to amount of PEG in GO-P600) was very similar (conver-
sions: 93% vs. 95%). This implied that the novel triphase
catalyst could also exhibit equivalent catalytic activity for
nucleophilic substitution reactions in comparison with the
traditional phase transfer catalyst. Furthermore, the reac-
tant was replaced by potassium acetate and NaCN and high
yields of target products (85% and 96%, shown in Table 3)
could be obtained as well. These could further demonstrate
the excellent catalytic activity of the GO-P600 catalyst for
diferent nucleophilic substitution reactions.
Fig. 4 Conversions of substrate (cyclohexene) as the function of reac-
tion time (a, temperature: 55 °C) and temperature (b, time: 10 h) for
carbenes reactions over GO-P600 catalyst
The infuences of reaction time and temperature on cata-
lytic performance of GO-P600 were also investigated and
Table 3 Catalytic performance
Entry
Substrates
NaI
Products
C8H17I
Catalyst
Conv. (%)
Sel. (%)
of GO-Px catalysts for
nucleophilic substitution
reactions
1
Blank
GO
6
23
77
93
79
95
85
96
100
100
100
100
100
100
100
100
2a
3
GO-P400
GO-P600
GO-P1000
PEG-600
GO-P600
GO-P600
4
5
6b
7c
8d
CH3COOK
NaCN
CH3COOC8H17
C8H17CN
Conditions: catalysts 0.5 g, NaI 1.6 g, chlorobenzene 4 mL, bromooctane 1.3 mL, 100 °C, 10 h, magnetic
stirring
aCatalyst: GO 0.45 g
bCatalyst: PEG-600 0.05 g
cSubstrates: potassium acetate 4.0 g, bromooctane 2.9 mL
dSubstrates: NaCN 0.7 g, bromooctane 1.6 mL, DMSO 4 mL. Reaction time: 8 h
1 3

Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase Catalyst for Carbenes and…
amphiphilicity of PEG and GO could provide with
their afnity to both aqueous and organic reactants. It’s
acknowledged that PEG chains were crooked and saw-
tooth-confgurated in aqueous and oil phase respectively.
The crooked PEG in aqueous phase could coordinate with
metal ions (M+) similarly with the function of crown ether.
The conjugated anions (Y−) could be transfered from aque-
ous to organic phase via oil solubility of the coordinated
PEG. Afterwards, the coordinated PEG could turn into
sawtooth confguration in oil phase and activated Y− (I−,
CN− CH3COO− for nucleophilic substitution and OH− for
carbenes reactions) could be released. Thus, the organic
reactions could be smoothly conducted. The excellent per-
formance of the triphase catalysts should mainly depend
on their super transfer capacity of Y− from aqueous to oil
phase.
Fig. 5 Conversions of substrate (bromooctane) as the function of
reaction time (a, temperature: 100 °C) and temperature (b, time:
10 h) for nucleophilic substitution reactions over GO-P600 catalyst
Reusability of the spent catalyst (GO-P600) was
also evaluated via synthesis reaction of iodine octane
after some simple treatments (isolation and washing, as
described above). As displayed in Fig. 6a, catalytic activ-
ity of the catalyst did not show obvious decrease after
using for four times and the yield of target product could
be steadily more than 85%. Additionally, structural infor-
mation of the spent catalyst was also investigated via
FT-IR. As depicted in Fig. 6b, the spent catalyst exhibited
almost the same FT-IR pattern to the fresh one, imply-
ing their similar molecular structure. These demonstrated
that the grafted GO-PEG should be both efcient and sta-
ble as a kind of novel triphase catalyst applied to these
organic–solid–aqueous reaction systems.
included in Fig. 5. As shown in Fig. 5a, the conversion of
bromooctane could be dramatically increased from 47% to
93% as the reaction time was improved from 4 to 10 h. How-
ever, the conversion of bromooctane was almost steady when
the reaction time was above 10 h. Similarly, as demonstrated
in Fig. 5b, the improvement of reaction temperature above
100 °C would not further promote its catalytic performance
for the nucleophilic substitution reaction. According to these
results, the best conversion of bromooctane should be attrib-
uted to 93% and the optimized reaction conditions should be
around 100 °C and 10 h.
The efficient performance of the triphase catalysts
could be illustrated more clearly in Scheme 2. The good
Scheme 2 Probable schematic description of the triphase catalytic system
1 3

X. Yang et al.
Fig. 6 Reusability of GO-P600 (a) and FT-IR of the fresh and spent catalysts (b)
8. Zhang W, Zhao Q, Liu T, Gao Y, Li Y, Zhang G, Zhang F, Fan X
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4 Conclusion
9. Dhakshinamoorthy A, Alvaro M, Concepción P, Fornés V, Garcia
H (2014) Chem Commun 48:5443–5445
PEG was frstly and successfully grafted on GO via the
construction of covalent bond between terminal hydrox-
yls in PEG and epoxies in GO catalyzing by Lewis acid
(BF3·C2H5OC2H5). Polymeric level of PEG could infu-
ence catalytic performance of the GO-Px catalysts for
both carbenes and nucleophilic substitution reactions. As
the result, GO-P600 (polymeric level = 600) could exhibit
equivalent catalytic activity in comparison with single
PEG for both of the model reactions. However, the GO-
PEG composites could provide with superior advantages in
both catalyst-products separation and reusability. Catalytic
performance of the GO-PEG was maintained well after
using for four times. These results could also demonstrated
that GO was an excellent PEGylated support. This novel
GO-PEG would show potential applications in organic
synthesis such as carbenes and nucleophilic substitution
reactions.
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Grafted Polyethylene Glycol–Graphene Oxide as a Novel Triphase Catalyst for Carbenes and…
Afliations
Xiaohai Yang1 · Jie Zhai1 · Tongchun Xu1 · Bing Xue1 · Jie Zhu1 · Yongxin Li1
1
Jiangsu Key Laboratory of Advanced Catalytic Materials
and Technology, School of Petrochemical Engineering,
Changzhou University, Changzhou 213164, China
1 3