Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes Epoxidation with H2O2

Article abstract of DOI:10.1007/s10562-019-02895-4

Abstract: Diamine-decorated graphene oxide (NH₂-GO) is successfully synthesized by covalently intercalated ethanediamines into the flat planes and edge of the GO sheet. The diamines could be act as buffer layer, thereby avoiding the stacking of graphene sheets, and offer abundant metal-chelating sites to subsequently obtain well-dispersed metal nanoparticles of small size. After the in situ reduction and deposition of AuNPs, it was found that the Au particle size of Au_0.9%@NH₂-GO estimated to be around 2–5?nm. The as-prepared GO-supported small AuNPs exhibited good catalytic performance for the heterogeneous epoxidation of alkenes with H₂O₂ under mild conditions. Notably, they presented good expansibility for either bulky or less bulky alkenes in this reaction, owing to the facilitated efficiency of mass transfer of substrates and products resulted from the supporting effect of intercalated diamines and high surface area of GO. Moreover, these heterogeneous catalysts could be readily recovered and recycled for five times with the remained activity and selectivity. Graphic Abstract: Gold nanoparticles of small size immobilized on the diamine-decorated graphene oxide has been developed, which employed as an efficient, universal and reusable catalyst for the epoxidation of alkenes using H₂O₂ as an oxidant.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02895-4

Catalysis Letters
https://doi.org/10.1007/s10562-019-02895-4
Diamine‑Decorated Graphene Oxide with Immobilized Gold
Nanoparticles of Small Size for Alkenes Epoxidation with H2O2
Weiguo Zheng1 · Haining Hu · Yaju Chen2 · Rong Tan · Donghong Yin
1
3
3
Received: 2 May 2019 / Accepted: 8 July 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Diamine-decorated graphene oxide (NH -GO) is successfully synthesized by covalently intercalated ethanediamines into the
2
ꢀ
at planes and edge of the GO sheet. The diamines could be act as buꢁer layer, thereby avoiding the stacking of graphene
sheets, and oꢁer abundant metal-chelating sites to subsequently obtain well-dispersed metal nanoparticles of small size.
After the in situ reduction and deposition of AuNPs, it was found that the Au particle size of Au @NH -GO estimated to
0
.9%
2
be around 2–5 nm. The as-prepared GO-supported small AuNPs exhibited good catalytic performance for the heterogeneous
epoxidation of alkenes with H O under mild conditions. Notably, they presented good expansibility for either bulky or less
2
2
bulky alkenes in this reaction, owing to the facilitated eꢂciency of mass transfer of substrates and products resulted from
the supporting eꢁect of intercalated diamines and high surface area of GO. Moreover, these heterogeneous catalysts could
be readily recovered and recycled for ꢃve times with the remained activity and selectivity.
Graphic Abstract
Gold nanoparticles of small size immobilized on the diamine-decorated graphene oxide has been developed, which employed
as an eꢂcient, universal and reusable catalyst for the epoxidation of alkenes using H O as an oxidant.
2
2
Keywords Graphene oxide · Gold nanoparticles · Heterogeneous catalysis · Epoxidation
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W. Zheng et al.
1
Introduction
(AuNPs with a diameter of ∼ 13 nm) nanocomposites
by using sodium citrate as a reductant [34]. In general,
the AuNPs that served as high-eꢂciency catalytic cent-
ers under mild conditions are < 5 nm in diameter [13].
Therefore, the development of eꢁective protocols for the
synthesis of well-dispersed and ultraꢃne GO-supported
AuNPs of small size is highly desirable.
As key intermediates for chemicals, medicine, pesticides,
food additives and dyes, epoxides have tremendously
attracted attention from the industry and scientiꢃc com-
munity [1, 2]. The epoxidation of alkenes has been widely
regarded as one of the most eꢂcient and common methods
to obtain epoxides [3, 4]. To date, various homogeneous
and heterogeneous catalysts for this reaction have been
intensively reported [4–12]. Among them, gold nanopar-
ticles (AuNPs) have a large surface-to-volume ratio and
aꢁord plenty available active centers per unit area, leading
to the highly catalytic activity [13–18]. Moreover, it is
commonly believed that NPs-based catalysts could bridge
the gap between homogeneous and heterogeneous catalysis
due to their high activities and excellent separation char-
acteristics [19–21]. However, AuNPs generally have an
inherent lack of thermodynamic stability because of their
high surface energy, resulting in aggregation of NPs and
the loss of activity during catalytic reactions [22, 23]. Over
the last decade, a great deal of eꢁorts have been devoted to
circumventing this problem. It is well known that the sup-
port materials not only play important roles in enhancing
the stability and improving the catalytic performance, but
also facilitate catalyst recovery. Therefore, loading onto
appropriate supporters becomes one of the key technol-
ogy solutions that can signiꢃcantly reduce agglomeration
to obtain well-dispersed AuNPs heterogeneous catalysts.
One of attractive strategies is the utilization of high-
surface-area materials for the immobilization of AuNPs.
Interestingly, a promising research avenue to meet
this burgeoning demand relates to a two-step process,
where the anchored metal precursors on the functional-
ized GO are subsequently reduced. In this method, it could
not only be favorable for metal precursors dispersing on
GO and NPs nucleation, but also avoiding the stacking
of graphene sheets by rational modiꢃcation of GO sur-
face. Moreover, such modiꢃcation may tailor the acid-
ity-alkalinity and electronic property of GO, leading to
enhanced catalytic performance [35]. Besides, it has been
found that the coordination eꢁects between amine groups
and the metal ions could eꢂciently downsize the result-
ant metal NPs [35]. Based on the above considerations,
we presented the immobilization of AuNPs on diamine-
alkalized GO through the in situ reduction and deposition
method (Scheme 1). The physicochemical properties of
as-prepared Au@NH2-GO were characterized by various
techniques, such as infrared spectroscopy (IR), transmis-
sion electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), inductively coupled plasma-atomic
optical emission spectroscopy (ICP-OES), and powder
X-ray diꢁraction (PXRD). Among them, it was found that
the small Au particle size of Au0.9%@NH2-GO estimated
to be around 2–5 nm. The experimental results suggested
that Au0.9%@NH2-GO was employed as superior hetero-
geneous catalyst for the epoxidation of alkenes using H2O2
as an oxidant under mild conditions. Moreover, Au0.9%@
NH2-GO could be easily recycled for stable reusing more
than ꢃve times.
Interestingly, graphene broadly known as one-atom-thick
2
2
D layer of sp carbons has recently been noticed for its
2
−1
tremendous theoretical surface area (2600 m g ) and
outstanding physicochemical properties [24, 25]. Conse-
quently, graphene and its derivatives including graphene
oxide (GO) and reduced GO (rGO), have gained consid-
erable interest for supporting metal NPs [26–30]. Par-
ticularly, the intrinsic abundant oxygen-containing func-
tional groups (carbonyl, carboxyl, epoxide and hydroxyl)
can endow GO excellent dispersibility in water or polar
organic solvents, anchoring ability for the metal ions and
NPs, and easy modiꢃcation through chemical reactions
2 Experimental
2.1 Materials
[
31]. So far, the most convenient preparation procedure to
obtain GO supported metal NPs is reduction of GO and
metal precursors in solvent using reducing agents by one-
step method. However, owing to the reduction of oxidized
groups of GO, the insuꢂcient binding sites and the stack-
ing of graphene sheets increase the diꢂculty of aꢁording
well-dispersed metal NPs [32]. Most of the reported GO-
supported AuNPs are also subject to limitation of large
particle size (5–100 nm) [33]. For example, Gawande
and co-workers have reported the synthesis of Au/rGO
Styrene, styrene oxide, indene, cyclohexene, β-pinene and
1,2-dihydronaphthalene were purchased from TCI. Gold(III)
chloride, ethylenediamine, α-methylstyrene and graphite
(99.999% purity, 200 mesh) were obtained from Alfa Aesar.
Styrene and indene were puriꢃed by passed through a pad
of neutral alumina before use. Other commercially avail-
able chemicals were of analytical purity available from local
suppliers. GO was synthesized according to the procedures
reported in the literature [36] with minor modiꢃcation (see
Sect. 2 in the ESI†).
1
3

Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes…
Scheme 1 Schematic descrip-
tion of synthesis route to Au@
NH -GO
2
2
.2 Synthesis of Catalysts
NH -GO, Au
@NH -GO, and Au
@NH -GO,
7.8% 2
2
3.1%
2
respectively (the Au loading amount was revealed by the
2.2.1 Synthesis of NH ‑GO
ICP analysis). For comparison, the synthesis of Au
@
1.8%
2
GO was very similar to that of the Au
@NH -GO,
2
2
.1%
Typically, a mixture of prepared GO (0.5 g) and SOCl
except for the use of GO as the supporter.
2
(
60 mL) were added into a 150 mL round-bottom ꢀask. The
reaction mixture was stirred at 80 °C for 24 h. After cool-
ing to room temperature, the suspension was ꢃltered. The
precipitate was washed with THF for several times and then
dried at 80 °C under vacuum for 24 h to obtain ClOC-GO
as a grey powder. After that, the ethylenediamine (60 mL)
was dropwise added into the above ClOC-GO and stirred at
2.3 Catalyst Testing
The typical procedures for the epoxidation of alkenes with
H O : a mixture of prepared Au @NH -GO (25 mg),
2
2
0.9%
2
styrene (5 mmol) and acetonitrile (5 mL) was added into
a 25 mL two-neck ꢀask. The reaction mixture was stirred
at 60 °C, followed by dropwise added a methanol solution
of H O (1.5 mL 30 wt% H O in 5 mL methanol). In the
4
0 °C for 24 h under N atmosphere. Sequentially, the target
2
solid was collected by ꢃltration, washed with excess ethanol,
followed by drying at 50 °C under vacuum for 24 h. Finally,
2
2
2
2
the amino-functionalized GO (NH -GO) was obtained as a
dropping process, 1 mol/L of NaOH solution was added duly
in order to adjust the pH value (9.0–9.5). After reaction, the
catalyst was recovered by centrifugation. The liquid phase
was extracted with dichloromethane several times and then
quantitatively analyzed by GC analysis (6890 N) equipped
with a ꢀame ionization detection and a capillary column
(HP19091G-B213, 30 m×0.32 mm×0.25 mm) to determine
the conversion and selectivity. The purity and structure of
2
ꢀ
uꢁy and dark powder.
2
.2.2 Synthesis of Au@NH ‑GO
2
Au@NH -GO was prepared with a two-step synthetic
2
method. Speciꢃcally, NH -GO (0.5 g) was dispersed in
2
5
mL deionized water by sonication for 15 min. To this
1
13
suspension, a solution of 0.01 mmol/L HAuCl (1 mL) was
products were also conꢃrmed by H NMR and C NMR
analysis.
4
added dropwise and the mixture was sonicated for 5 min.
Then, 50 mL ethylene glycol was added to the above mix-
ture and stirred at 100 °C for 1 h. After cooling to room
temperature, the target solid was collected by ꢃltration,
washed with excess deionized water, and dried at 50 °C
overnight under vacuum to yield a dark powder deꢃned
as Au@ NH -GO. The synthesis of Au@NH -GO with
2.4 Recyclability Test for Au0.9%@NH ‑GO
2
After the epoxidation of alkene with H O was completed,
2
2
the catalyst was recovered by centrifugation, washed with
ethanol and deionized water, dried at 50 °C overnight under
vacuum, and then used for the next run. For each recyclabil-
ity test, three parallel experiments were carried out, and the
conversion and selectivity were taken as the average.
2
2
diꢁerent Au loading amount were carried out by follow-
ing the above protocol by using diꢁerent concentration
of HAuCl solution. These NH -GO supported AuNPs
4
2
catalysts were denoted as Au
@NH -GO, Au
@
2.1%
0
.9%
2
1
3

W. Zheng et al.
3
Results and Discussion
immobilization of ethylenediamines led to the changes of
carboxylic and epoxy groups, as shown in Fig. 1b. Com-
pared with GO, in the spectrum of NH2-GO, the charac-
teristic band of C=O (–C=O–NH–) stretching vibrations
3.1 Catalysts Synthesis
−
1
Following the synthetic route outlined in Scheme 1, Au@
shifted from 1730 to 1650 cm along with the introduc-
−
1
NH -GO catalysts were facilely prepared through the in situ
tion of a new strong peak at 3430 cm , corresponding
to the amine group (–NH2) [40]. The peaks at 2940 and
2
reduction and deposition method. The resultant ꢀuꢁy pow-
dery product was ꢃnally obtained in nearly quantitative
yield. This method oꢁers several distinct beneꢃts for the syn-
thesis of eꢂcient, stable and well-dispersed GO-supported
AuNPs and the epoxidation reaction of alkene with H O :
−
1
2830 cm are assigned to the C–H stretching vibration
of the diamine groups. This observation conꢃrmed that
ethylenediamines were successfully anchored on the GO
sheet through an amidation reaction. Additionally, the dis-
2
2
−
1
(
1) the intercalated diamines act as buꢁer layer between GO
appearance of the signal at 1050 cm (C–O–C, epoxy)
suggests the formation of O–NH bonds through the ring-
opening reaction of epoxides with amines, which is sup-
ported by the appearance of a new strong absorption peak
sheets, thereby avoiding the stacking of graphene sheets and
facilitating mass transfer of substrates and products; (2) as
electron-rich functional groups, diamines could ꢃxed-point
anchor and disperse the gold precursors; (3) amine groups
could downsize the resultant AuNPs to some extent by the
coordination eꢁects; (4) the alkaline amine groups could
stabilize H O to avoid its rapid decomposition during the
−
1
at 1150 cm (O–N) [41]. Accordingly, these vibrational
characteristics demonstrate that ethylenediamines have
been successfully intercalated in the GO supporter.
Figure S2A shows the full survey XPS spectra of GO
and Au0.9%@NH2-GO, displaying the element species of
C, N, O and Au. Compared with GO, the XPS spectrum
of the Au0.9%@NH2-GO exhibits two new peaks of Au
2
2
epoxidation of alkene with H O . Additionally, H O is a
2
2
2
2
preferred oxidant due to the lower environmental impact for
the use of this oxidant and its only by-product is H O during
2
7
/2
5/2
epoxidation reaction.
4f and Au 4f at 84.1 and 87.7 eV, respectively, which
is related to the contribution of the immobilized AuNPs
(Fig. 2a) [42]. As shown in Fig. 2b–d, the C 1s spectra in
GO show the values of C–C/C=C, C–OH, and C–O–C
at 284.7, 285.9 and 286.7 eV, respectively, which is well
consistent with those in the NH2-GO and Au0.9%@NH2-
GO, giving the values at 284.6, 285.7 and 286.7 eV; 284.6,
285.6 and 286.6 eV, respectively [43, 44]. The values of
C=O at 289.0 eV in GO is quite diꢁerent from those
(287.8 and 287.9 eV) in the NH2-GO (C), and Au0.9%@
NH2-GO, respectively, due to the difference between
COOH and CO–NH (Fig. 2c) [45]. This diꢁerence indi-
cates the successful immobilization of ethane diamines
onto the GO through an amidation reaction. Additionally,
3.2 Catalysts Characterization
Figure 1 shows the FT-IR spectra of GO (a) and NH -GO
2
(
b). The spectrum of GO shows a wide peak at around
−1
ṽ = 3300 cm , which assigned to the phenolic hydroxy
(
–COOH or –OH) stretching vibration [37]. A series of
−
1
peaks at ṽ = 1730, 1220 and 1050 cm are attributable
to the C=O (–COOH), C–O (alkoxy) and C–O–C (epoxy)
stretching vibrations, respectively [38]. The peaks appear-
−
1
ing at ṽ = 1620 cm could be assigned to the skeletal
vibrations of unoxidized graphitic domains [39]. The
the peak at 285.4 eV in the C 1s XPS spectrum of NH -
2
GO is related to the contribution of C atom originated
from C–N bond. Moreover, after supported with AuNPs,
the Au @NH -GO gives a new C 1s peaks at 286.0 eV,
0
.9%
2
which is related to the contribution of C atom derived from
the C–N–Au bond (Fig. 2d) [46]. As an electron-deꢃcient
…
whole, the –CH –NH Au group presented the electron-
2
2
withdrawing eꢁect, and the electrons transfer from C to N
atoms, which resulted in the decrease of electron density
of C atom, thereby causing the positive shift of C 1s bind-
ing energy. Moreover, the N 1s spectrum of Au @NH -
0
.9%
2
GO is ꢃtted into two asymmetric broad peaks at 399.6 and
4
00.3 eV, which can be assigned to amides or free amines
on GO, and the Au-coordinated amines, respectively (Fig.
S2B) [20, 47]. The existence of the new peak at 400.3 eV
further suggests that the Au NPs had been successfully
Fig. 1 FT-IR spectra of GO (a) and NH -GO (b)
2
1
3

Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes…
Fig. 2 Au 4f XPS spectrum of
Au @NH -GO (a); C 1s
0
.9%
2
spectra of GO (b), NH -GO
2
(
c), and Au0 @NH -GO (d)
.9% 2
incorporated onto GO by coordination with the substrate
amines.
peaks at 2θ=38.2, 44.4, 64.5 and 77.5, which can be indexed
to the (111), (200), (220) and (311) crystallographic planes
of cubic AuNPs, respectively (JCPDS card No. 004-0784)
[38]. The intensity of diffraction peaks increases with
increasing the Au loading amount in the samples (Fig. 4c–f).
TEM analysis was conducted to observe the morphology
of the Au@NH -GO with diꢁerent Au loading amount. As
2
shown in Fig. 3, with the reduction of Au loading amount,
the aggregation of AuNPs remedies gradually. According
to the statistics of particle size distribution, when the Au
amount is large, the diameters of the Au nanoparticles in
Au @NH -GO and Au @NH -GO are mainly in the
3.3 Catalytic Evaluation
Initially, the epoxidation of styrene with H O in CH CN
7
.8%
2
3.9%
2
2
2
3
wide range of 10–80 nm and 5–40 nm, respectively. Fur-
ther reducing the Au loading amount results in the forma-
tion of the smaller AuNPs (4–12 nm for Au @NH -GO
was selected as the model reaction to investigate the cata-
lytic performance of the various catalysts and the results
were summarized in the Table 1. As the control experiment,
this transformation did not occur without any catalysts at
2
.1%
2
and 2–5 nm for Au @NH -GO). From the Fig. 3d, it is
0
.9%
2
clear that small Au NPs are well dispersed on the amino
functionalized GO with a narrow size distribution (2–5 nm)
60 °C in CH CN (entry 1, Table 1). Almost no products
3
were detected by the compared catalyst NH -GO (entry
2
(
Fig. 3d), probably due to the presence of amine function-
2, Table 1). Whereas once the NH -GO supported AuNPs
2
ality attached to the GO sheets that stabilizes the ion pair
(Au@NH -GO) was used, the conversion of styrene was
2
+
−
(
NH ) AuCl , acting as stabilizer for the AuNPs formed
distinctly observed under the same reaction conditions.
3
4
by reduction with ethylene glycol [48].
Therefore, the support NH -GO showed no catalytic activ-
2
Furthermore, PXRD analysis was conducted to reveal the
crystallinity of the samples (Fig. 4). From the curves of a
and b, a characteristic sharp diꢁraction peak was appeared
at 2θ=10.8° (curve a), which could be assigned to the (002)
plane of GO sheets, while disappeared after immobilization
with ethylenediamines (curve b). This disappearance dem-
ity and the AuNPs plays key role and are indispensable to
this epoxidation. The catalytic activity of these NH -GO
2
supported AuNPs catalysts enhanced gradually with the
decline of the Au loading amount from 7.8 to 0.9 wt%
(entries 3–7, Table 1). To our delight, the selectivity of
styrene oxide remained at > 90.0% for all the Au@NH -
2
onstrated the conventional amorphous nature of NH -GO.
GO catalysts. When the Au @NH -GO was employed,
2
7.8%
2
The PXRD pattern of the Au@NH -GO exhibits diꢁraction
a poor conversion of styrene (21.0%) was obtained with a
2
1
3

W. Zheng et al.
Fig. 3 TEM images and particle
size distribution of Au
@
.8%
7
NH -GO (a), Au @NH -GO
2
3.9%
2
(
b), Au @NH -GO (c), and
2.1% 2
Au @NH -GO (d)
0
.9%
2
1
3

Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes…
Table 2 Results of epoxidation of styrene with H O at diꢁerent pH
2
2
1
11
2
values over catalyst Au @NH -GO
0
.9%
2
00
220
311
g
a
a
Entry pH value
Conv. (%) Select. (%)
Styrene oxide Benzaldehyde
f
e
d
c
b
a
1
2
3
4
5
6.5
n.d.
n.d.
n.d.
12.0
5.0
3.7
3.4
7.0–7.5
8.0–8.5
9.0–9.5
16.0
67.0
97.0
83.0
92.0
93.0
92.0
Au JCPDS 04-0784
10.0–10.5 98.0
1
0
20
30
40
theta (degree)
50 60 70 80
Reaction conditions: styrene (5 mmol), catalyst (25 mg), CH CN
2
3
(
5 mL), H O /styrene=3/1(molar ratio), 60 °C, 5 h
2 2
a
Determined by GC using biphenyl as an internal standard
Fig. 4 PXRD spectra of GO (a), NH -GO (b), Au @NH -GO (c),
2
2
0.9%
2
Au @NH -GO (d), Au @NH -GO (e), Au @NH -GO (f)
2
.1%
2
3.1%
3.9%
2
and Au @NH -GO (g)
7.8%
2
This inspiring result is completely comparable with or even
superior to that of the previously reported Au NPs-based
catalysts (Table S2). The interlayer space was increased by
the pillaring of ꢀexible ethylenediamine molecules [49],
thus enhanced mass transfer ability in the epoxidation. As
a control test, the reaction was performed with the cata-
Table 1 Results of the epoxidation reaction of alkenes with H O
2
2
over various catalysts
O
catalyst
H O , NaOH, CH CN
2
2
3
lyst Au @GO and the decreased yield of styrene oxide
1
a
2a
1.8%
was obtained with an increased yield of byproduct (entry 8,
Table 1). Combined with the Au particle size analysis, the
activity was strongly aꢁected by the Au loading amount,
which could be due to the decrease of Au loading amount
reducing the aggregation probability to obtain smaller
AuNPs with high activity. Unfortunately, a clearly decreased
conversion of styrene (81.0 and 55.0%, respectively) was
obtained as the catalyst loading reduced from 25 to 20 and
a
a
Entry Catalyst
Conv. (%) Select. (%)
Styrene oxide Benzaldehyde
1
2
3
Blank
n.d.
<1.0
97.0
n.d.
<1.0
93.0
n.d.
<1.0
3.7
NH -GO
2
Au .9%@NH2-
0
GO
4
5
6
7
Au .1%@NH2-
84.0
76.0
57.0
21.0
93.0
93.0
92.0
90.0
3.4
4.2
4.1
4.6
2
1
0 mg (entries 9 and 10, Table 1).
GO
Proverbially, with the introduction of Payne Reaction in
Au .1%@NH2-
3
GO
this epoxidation process, the alkaline condition plays a vital
role in transforming H O /CH CN into the peroxycarbox-
Au .9%@NH2-
2
2
3
3
GO
imidic acid intermediate [50]. Hence, we investigated the pH
value of this reaction by adjusting with NaOH solution. As
expect, epoxidation did not occur in the absence of NaOH
Au .8%@NH2-
7
GO
8
9
Au1.8%@GO
48.0
81.0
77.0
94.0
9.2
2.9
(
entry 1, Table 2), Then, the pH values were adjusted at the
b
Au .9%@NH2-
0
range of 7.0–7.5, 8.0–8.5, 9.0–9.5 and 10.0–10.5, enhanced
styrene conversions of 16.0, 67.0, 97.0 and 98.0% were
obtained, verifying the importance of hydroxyl ion. This
is also supported by the reaction mechanism of this H O /
GO
1
0c
Au .9%@NH2-
55.0
93.0
3.5
0
GO
2
2
Reaction conditions: styrene (5 mmol), catalyst (25 mg), CH CN
3
CH CN-epoxidation system [51]. Importantly, the alkaline
3
(
5 mL), pH 9.0–9.5, H O /styrene=3/1(molar ratio), 60 °C, 5 h
2 2
condition is also one of the critical factors for the formation
of well-dispersed and smaller size AuNPs, as well as stabi-
lizing the resultant AuNPs [52] during the reaction. Addi-
tionally, diꢁerent solvents, such as n-hexane, 1,2-dichloro-
ethane, N,N-dimethylformamide, acetonitrile, ethanol and
water, were tested in the presence of Au @NH -GO
a
Determined by GC using biphenyl as an internal standard
Catalyst (20 mg)
b
c
Catalyst (10 mg)
0
.9%
2
9
0.0% styrene oxide selectivity (entry 7, Table 1). However,
and H O at 60 °C. According to the results listed in the
2
2
the catalyst was replaced with Au @NH -GO, the con-
0
.9%
2
Table S1, CH CN remained eꢁectively the best solvent
3
version of styrene was dramatically increased up to 97.0%
with a 93.0% styrene oxide selectivity (entry 3, Table 1).
in this reaction system because it gave the highest styrene
1
3

W. Zheng et al.
conversion. This observation is consistent with the men-
tioned reaction mechanism.
Furthermore, other eꢁects of reaction parameters such
as the H O amount, reaction time and reaction temperature
2
2
were also detailly studied (Fig. 5). As illustrated in Fig. 5a,
when the molar ratio of H O to styrene was drastically
2
2
increased from 1:1 to 3:1, the styrene conversion increased
from 43 to 97% with a nearly constant styrene oxide selectiv-
ity (93%). Nevertheless, the further rise in the use of H O
2
2
led to low conversion and selectivity, which was similar to
the previous reported [51]. From Fig. 5b, obviously, the pro-
duction of styrene oxide increased gradually within 5 h and
then slowly increased with time. As expected, the conversion
of styrene catalysed by Au @NH -GO was also strongly
0
.9%
2
aꢁected by the reaction temperature, which increased with
temperature varying from 30 to 60 °C. However, further
increasing the temperature to 70 °C, a low styrene conver-
sion of 70% with an 87% selectivity was attained (Fig. 5c).
Based on the above investigation, the optimized condition
for this protocol was the use of Au @NH -GO (25 mg)
Fig. 6 The results of the epoxidation of various alkenes over Au0
@
.9%
NH2-GO. Reaction conditions: alkene (5 mmol), catalyst (25 mg),
CH CN (5 mL), pH 9.0–9.5, H O /styrene=3/1(molar ratio), 60 °C,
3
2
2
5
h
3.5 Recycling Experiments
0
.9%
2
at 60 °C in CH CN for 5 h by using threefold of H O as
3
2
2
the oxidant.
Owing to the heterogeneous nature of Au @NH -GO,
0
.9%
2
the reusability of it was also evaluated by using styrene as
the substrate under the optimal reaction conditions. After
each run, the catalyst was recovered by simple centrifuga-
tion, washing with alcohol, and then drying at 50 °C under
vacuum. As shown in Fig. 7a, Au @NH -GO could be
3.4 Substrate Scope
Inspired by the excellent catalytic performance of Au0.9%
@
NH -GO, the scope of substrates was then examined by
2
0.9%
2
using various alkenes, such as styrene (1a), α-methylstyrene
reused for ꢃve times with a slight decrease in the conversion,
while the selectivity remained at >90%. After the 5th reuse,
a signiꢃcantly decreased conversion of styrene was obtained
(64%), probably due to the aggregation of AuNPs. This is
supported by the analysis of TEM for the diameter of AuNPs
in the fresh and 6th recovered Au @NH -GO (2–5 nm
(
1b), indene (1c), 1,2-dihydronaphthalene (1d), cyclohexene
1e) and β-pinen (1f). As depicted in Fig. 6, the correspond-
(
ing epoxides (2a–2d) were obtained in higher conversions
(
90–97%) with > 89% selectivities under the optimized
conditions. The results demonstrated that the ꢀexible inter-
layer of diamine-decorated graphene oxide with immobi-
lized AuNPs, avoiding the diꢁusion constraint eꢁectively.
However, the epoxidation of 1e and 1f with H O gave the
0
.9%
2
vs. 10–60 nm), as shown in Fig. 7b. Furthermore, ICP-OES
analysis of the 6th recovered catalyst showed the amount
of 0.88 wt% of Au (0.9 wt% of Au for the fresh catalyst),
indicating almost no leaching of Au.
2
2
decreased conversion, probably owing to the lower possi-
bility of electrophilic cycloaddition resulting from the low
electron density of non-conjugated alkenes [6].
Fig. 5 Eꢁects of molar ratio of
H O to styrene (a), reaction
1
00
100
80
60
40
20
0
100
80
60
40
20
0
2
2
time (b) and reaction tempera-
ture (c) on catalytic perfor-
mance of Au @NH -GO
8
0
0.9%
2
catalyst. Reaction conditions: a
Au @NH -GO (25 mg), sty-
60
40
(
A)
(B)
(C)
0
.9%
2
rene (5 mmol), CH CN (5 mL),
3
6
0 °C, 5 h. b Au @NH -GO
0.9% 2
(
25 mg), styrene (5 mmol),
2
0
0
H O (15 mmol), CH CN
Conv.
Select.
2
2
3
.9 %
Conv.
Select.
1 2 3 4 5 6
Conv.
Select.
(
5 mL), 60 °C. c Au
@NH -
0
2
GO (25 mg), styrene (5 mmol),
1
2
3
4
5
30 40 50 60 70
H O (15 mmol), CH CN
2
2
3
o
(
5 mL), 5 h
n (H O ) / n (styrene)
Time / h
Temperature / C
2 2
1
3

Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes…
Fig. 7 a Reusability of Au0.9%@
NH -GO; b TEM images of
2
fresh Au @NH -GO and
0.9%
2
reused Au @NH -GO after
0.9%
2
six cycles
4
Conclusions
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conꢀicts of
We developed the facile functionalization of graphene oxide
with diamines by covalently intercalated ethylenediamine
into the ꢀat planes and edge of the GO sheet. The diamines
could act as buꢁer layer between GO sheets, thereby avoid-
interest.
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Aꢀliations
Weiguo Zheng1 · Haining Hu · Yaju Chen2 · Rong Tan · Donghong Yin
1
3
3
2
*
*
Weiguo Zheng
College of Chemistry, Guangdong University
of Petrochemical Technology, Maoming 525000,
People’s Republic of China
zhengweiguo4433@126.com
Yaju Chen
3
chenyaju970@126.com
National & Local Joint Engineering Laboratory
for New Petro-chemical Materials and Fine Utilization
of Resources, Key Laboratory of Chemical Biology
and Traditional Chinese Medicine Research (Ministry
of Education), Hunan Normal University, Changsha 410081,
People’s Republic of China
1
College of Chemistry and Materials Engineering, Hunan
University of Arts and Science, Changde 415000,
People’s Republic of China
1
3