Monodisperse CuB23 nanoparticles grown on graphene as highly efficient catalysts for unactivated alkyl halide Heck coupling and levulinic acid hydrogenation

Article abstract of DOI:10.1039/c4cy01331j

Monodisperse CuB₂₃ nanoparticles were successfully grown on graphene by the co-reduction of graphene oxide and Cu(NH₃)₄²⁺ using KBH₄ and with the assistance of solution plasma. Graphene is a powerful dispersion agent and a distinct support for CuB₂₃ nanoparticles. Unexpectedly, we found that the prepared CuB₂₃/graphene catalyst imparts exceedingly high activity to the Heck-type coupling of unactivated alkyl halides with alkenes and the liquid phase hydrogenation of levulinic acid to γ-valerolactone. Additionally, the as-prepared CuB₂₃/graphene also exhibited excellent recyclability during the hydrogenation of levulinic acid. The excellent catalytic performance might be caused by the combined effect of the good dispersion of CuB₂₃ nanoparticles and the interaction between graphene and CuB₂₃ nanoparticles. This not only facilitates electron transfer and mass transport but also improves thermal stability during the catalytic reaction process.

Full text of DOI:10.1039/c4cy01331j

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DOI: 10.1039/C4CY01331J
ARTICLE
Monodisperse CuB23 nanoparticles grown on
graphene as highly efficient catalysts for unactivated
alkyl halide Heck-couplings and levulinic acid
hydrogenation
Cite this: DOI: 10.1039/x0xx00000x
Received 00th October 2014,
Accepted 00th December 2014
a,b#
a,c#
d*
a,b
a,b
Shi Yan Fu,
Yuan Zhi Li,
Wei Chu, Chun Li and Dong Ge Tong *
DOI: 10.1039/x0xx00000x
Monodisperse CuB23 nanoparticles were successfully grown on graphene by the co-reduction
www.rsc.org/
2+
of graphene oxide and Cu(NH3)4 using KBH and with the assistance of solution plasma.
4
Graphene is a powerful dispersion agent and a distinct support for CuB23 nanoparticles.
Unexpectedly, we found that the prepared CuB /graphene catalyst imparts exceedingly high
2
3
activity to the Heck-type coupling of unactivated alkyl halides with alkenes and the liquid
phase hydrogenation of levulinic acid to γ-valerolactone. Additionally, the as-prepared
CuB /graphene also exhibited excellent recyclability during the hydrogenation of levulinic
2
3
acid. The excellent catalytic performance might be caused by the combined effect of the good
dispersion of CuB23 nanoparticles and the interaction between graphene and CuB23
nanoparticles. This not only facilitates electron transfer and mass transport but also improves
thermal stability during the catalytic reaction process.
7
1
. Introduction
performance catalysts. For example, Wang et al. reported that
the Rh Ni/graphene catalyst shows 100% H selectivity and
4
2
Over the past few decades, amorphous metal-boride (M–B) high activity during the decomposition of hydrous hydrazine at
7
a
alloy catalysts have been widely studied owing to their unique room temperature. Zhang et al. prepared Ag/graphene
properties such as their isotropic structure, high concentration composites that showed excellent catalytic performance in the
1
of coordinative unsaturated sites, and chemical stability.
reduction reaction between 2-nitroaniline and 1,2-
However, the amorphous M-B alloy catalysts produced by benzenediamine.7b However, the direct growth and the
regular chemical reduction methods agglomerate extensively anchoring of amorphous M-B alloy nanoparticles as
because of the exothermic nature of the reaction. Their catalytically-active components on graphene and the
2
catalytic activity is generally poor because of the low surface- exploitation of its catalytic performance has hardly been
3
to-volume ratio available after agglomeration. To solve this reported. Therefore, promoting the unique role of graphene for
problem, the development of support materials that ensure that use in the preparation of highly efficient amorphous M–B alloy
4
,5
the catalyst particles disperse without aggregation is required.
catalysts is of great interest.
Recently, supports such as carbon nanotubes, nickel foam and
Heck cross-couplings are one of the most important organic
mesoporous SiO , have been found to be efficient support reactions because of their role in the formation of
2
materials that decrease the extent of agglomeration and improve intermolecular C-C bonds between halides or sulfonate
4
,5
8
the dispersion of the M-B amorphous alloy catalysts.
electrophiles and alkenes under mild conditions. Recently,
However, they are far from meeting the demands of an ideal Heck-type reactions involving unactivated alkyl halides have
substrate for high-performance catalysts that facilitate electron been considered to be a major challenge in organic synthesis.
transfer and mass transport kinetics during catalytic reactions.
To date, a number of catalytic systems involving Pd and first-
Graphene, a single-atom-thick carbon material, holding many row transition metals (e.g., Ni, Ti, Co, Cu) have been developed
9
advantages such as outstanding charge carrier mobility, thermal to facilitate intermolecular alkyl-Heck processes. Among those
and chemical stability, high specific surface area, and superior catalysts, palladium complexes are the most efficient catalysts.
electrical conductivity, etc., could be an ideal substrate for However, palladium catalysts have catalyst/product separation
6
growing and anchoring metal nanoparticles. Graphene-based difficulties and palladium complex ligands are generally toxic
materials modified with metal nanoparticles have been a top- and unavailable. More efficient and recyclable catalysts for
hot topic recently in many fields, especially as high- Heck reactions need to be developed.
This journal is © The Royal Society of Chemistry 2014
J. Name., 2014, 00, 1‐12 | 1

Catalysis Science & Technology
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Levulinic acid (LA) is an important intermediate for the using the same route except that precursors, such as ferric
1
0
conversion of non-food biomass into chemicals and fuels. The citrate, cobalt acetate and nickel chloride, were usVeiedwiAnrtsictleeaOdnlionef
hydrogenation of LA to γ-valerolactone (GVL) is one of most copper nitrate, respectively. Graphene was prepared using the
DOI: 10.1039/C4CY01331J
1
0
important reactions in biorefinery systems. Although many same procedure but without the addition of copper nitrate.
1
1
12
catalysts, such as Ru, Ir and Au-based catalysts, Cu/CrOx,
Without KBH , GO cannot be reduced by SPP. Nano B with an
4
Cu/ZrO2 and Ni–MoOx/C , have been developed, they suffer average particle size of 5 nm was purchased from Changzhen
from catalyst/product separation difficulties or low efficiency. High Technology Limited Company (China). The high surface
For practical applications the development of novel, efficient area graphite (229.5 m g ) used in our work was kindly
and reusable catalysts for LA hydrogenation is urgently provided by the Institute of Coal Chemistry, Chinese Academy
1
3
14
2
-1
required.
of Sciences. According to the provider, the high surface area
Recently, a B-doped Cu catalyst has been found to have graphite was produced by mechanical treatment of synthetic
1
5
excellent catalytic activity. Based on this report, we attempted graphite.
to prepare a graphene supported amorphous Cu-B alloy as a
novel catalyst with high activity in Heck-type reactions and in 2.2. Characterization
LA hydrogenation.
X-ray diffraction (XRD) patterns were obtained using an X′Pert
In this work, a graphene-supported CuB23 catalyst was
successfully synthesized by a facile solution plasma process
X-ray powder diffractometer equipped with a CuKα radiation
source (λ=0.15406 nm). For compositional analyses, the dry
samples were dissolved in boiling aqua fortis using a
microwave digestion system. The Cu and B contents of the
samples were determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES; Irris, Advantage). C, N and
H elemental analyses were carried out using a Vario Micro
cube elemental analyzer. The reproducibility of the
compositional analyses was verified using at least three repeat
runs and was found to be within acceptable limits (±0.5%).
Thermogravimetric analysis (TGA) was carried out on a Seiko
Instruments EXSTAR TG/DTA 6300 using a heating rate of 10
1
6
(
SPP) route. Graphene plays an important role as a powerful
dispersion agent and a distinct support for the CuB23
nanoparticles (NPs). The catalytic properties of CuB /graphene
2
3
were evaluated in Heck-type carbon−carbon coupling reactions
and the hydrogenation of levulinic acid to γ-valerolactone.
These reactions provided evidence for the catalyst’s enhanced
performance compared with unsupported CuB23 and the
previously reported most efficient heterogeneous catalysts. A
correlation between the catalytic performance and the structural
properties was tentatively established.
−
1
−1
ºCmin in a stream of argon (100 mLmin ). The Brunauer–
Emmett–Teller (BET)-specific surface area of the samples was
2
. Experimental
determined using the N adsorption–desorption technique in
which the samples were degassed at 100 °C for 180 min before
2.1. Catalyst preparation
2
Graphene oxide (GO, C: 33.7 wt.%, O: 66.3 wt.%) was the measurements. Scanning transmission electron microscopy
prepared from graphite powder (Aldrich, powder, <20 mm, (STEM) images and selected-area electron diffraction patterns
1
7
synthetic) according to the method reported by Hummers. A 2 (SAED) of the samples were obtained using a FEI Tecnai G2
−1
gL GO aqueous suspension was obtained after 5 min of F20 S-Twin microscope. Samples for STEM analysis were
sonication. We described the self-designed reactor used for the prepared by depositing a single drop of diluted nanoparticle
1
8
preparation of CuB /graphene via SPP in our previous study.
dispersion in ethanol on an amorphous, carbon-coated nickel
2
3
The pulsed direct current voltage was 450 V (duty: 50%, grid. Near-surface EA was carried out using energy-dispersive
frequency: 12 kHz). The electrodes were tungsten wire X-ray spectroscopy (EDAX). The surface electronic states of
(
diameter: 2 mm) and the gap between the cathode and the the samples were investigated using X-ray photoelectron
anode was 1 mm. In a typical synthesis, 40 mL of 25% spectroscopy (XPS; Perkin–Elmer PHI 5000C ESCA, using
NH ·H O solution was mixed with 10 mL of copper nitrate AlKα radiation). The samples were placed in a homemade in
3
2
solution (2 mM, 3.7 mM, 5.5 mM, 7.6 mM and 11.3 mM) to situ XPS reactor cell and, after drying under an argon
2
+
form a Cu(NH3)4 complex. Then, 32 mL of a 1.0 M KBH4 atmosphere, each sample was transferred to the analysis
aqueous solution and a GO aqueous suspension was added at chamber and the XPS spectrum was recorded. All the binding
2
98 K under an argon atmosphere. No significant reaction energies were referenced to the C1s peak (binding energy of
occurred in the absence of plasma. In contrast, the product was 284.6 eV) of the surface adventitious carbon. The active surface
obtained 5 min after the application of plasma. The obtained area ( Cu) of the samples was measured by hydrogen
product was washed once with deionized water and then thrice chemisorption using a Quantachrome CHEMBET 3000 system
S
−20
2
with absolute ethanol. The sample was stored in ethanol until assuming H/Cu(s) = 1 and a surface area of 6.85×10 m per
1
9
use. Other CuB23 loaded catalysts were also prepared using the Cu atom. Raman spectra were obtained using a confocal
same procedure except that supports were used instead of GO. microscopy Raman spectrometer with 633 nm incident laser
For comparison, a conventional CuB23 sample was prepared light (inVia plus, Renishaw, United Kingdom). Time-of-flight
using the same method but without the addition of GO. Fe- secondary ion mass spectrometry (ToF-SIMS) experiments
B/graphene, Co-B/graphene and Ni-B/graphene were prepared were performed with a PHI TRIFT II(USA) equipped with a
2
| J. Name., 2014, 00, 1‐12
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pulsed liquid metal ion gun. Spectra were acquired in positive results were reproduced and the errors were within ±5%. For
ion mode over a mass range of 1–320 Da using a Ga+ primary the recycling study, the catalyst was separated fromVietwheArtriceleacOtniloinne
2
source with a 100×100 mm raster size, 15 kV applied voltage, mixture by centrifugation and washed with acetone followed by
DOI: 10.1039/C4CY01331J
6
00 pA aperture current, and 10 min acquisition time. At least drying under argon at 353 K for 2 h.
three different spots on each sample were analyzed, and the
most representative is presented.
3. Results and discussion
2
2
.3. Activity test
3.1 Structural and electronic characteristics
Fig. 1a shows a typical low magnification STEM image of
.3.1 Heck-type Carbon−Carbon Coupling Reactions.
The alkyl iodide (1.0 equiv), alkene (1.2 equiv),
CuB23/graphene (0.02 equiv), Na CO (2.0 equiv), and NMP
10 mL) together with a stirring bar were added to a sealed tube
2
3
(
in a glovebox. After removing the tube from the glovebox, it
was heated in an oil bath at 353 K for 12 h with stirring (800
rpm) followed by centrifuging. The solution was separated and
the precipitate was washed with ether (5 mL×3). The solutions
were combined and washed three times with water. The
combined organic layers were dried with MgSO4 and
concentrated in vacuo. The product was then obtained after
column chromatography on silica gel with hexane/ethyl acetate
(
20:1) as the eluent. The precipitate was further washed
sufficiently with ethanol and ether and then dried. The
Cu23B/graphene was recovered for further use.
Fig.1 (a) Low magnification STEM image; (b) High magnification
STEM image; (c) Particle size distribution histograms; and (d) SAED
pattern of the as-prepared 9.93 wt.% CuB23/graphene.
For the structural determination of the Heck reaction
1
products, H NMR spectra were obtained on a Bruker AV
1
13
spectrometer ( H NMR at 200 or 400 MHz and C NMR at
9
.93wt.%CuB23/graphene. Compared with the clear surface of
1
1
25 MHz) with solvent resonance as the internal standard ( H
NMR:CDCl at 7.28 ppm, C NMR:CDCl at 77.0 ppm). Mass
3 3
unloaded graphenes (Fig. S1a), the highly dispersed CuB23
successfully decorated the graphene sheets (Fig. 1b). The
unsupported CuB23 aggregated to form large particles (Fig.
S1b). Furthermore, we observed a crumpled silk wave-like
morphology, which is characteristic of single-layer graphene
sheets, as shown in Fig. S1a. A cross-section AFM analysis
indicates a height of 0.993 nm for the graphene sheet in our
CuB23/graphene (Fig. S2), which is similar to the reported
1
3
spectra were obtained using an Agilent 6850 series gas
chromatography system equipped with an Agilent 5973N mass
selective detector. Thin layer chromatography (TLC) was
performed on SiliaPlate 250 μm thick silica gel plates from
Silicycle. Visualization was accomplished with short wave UV
light (254 nm), aqueous basic potassium permanganate solution,
or ethanolic acidic p-anisaldehyde solution followed by heating.
20
apparent thickness of a single layer graphene sheet. Owing to
the irregular shape of some of the CuB23 NPs that formed in the
2
.3.2 Hydrogenation of Levulinic acid to γ-Valerolactone
9
.93wt.%CuB23/graphene, a determination of the exact particle
size is difficult. However, a rough estimation was obtained by
measuring 150 randomly chosen particles from magnified
STEM images. The loaded CuB23 NPs of the
The hydrogenation of levulinic acid was carried out in a 200
mL stainless autoclave equipped with a magnetic stirrer and an
electric heating system. After the addition of LA (20 mmol,
9
.93wt.%CuB23/graphene had an average particle size of 4.8
2
.322 g), n-dodecane (50 mL) and 0.4 mmol of the catalyst, the
nm (Fig. 1c and Table S1). Additionally, the corresponding
selected area energy dispersion patterns (Fig. 1d) indicated that
the CuB23 NPs are in an amorphous state. STEM images, size
distributions and the average particle size of CuB23/graphene
with different loading amounts are shown in Fig. S3, Fig. S4
and Table S1, respectively. Fig. S5 shows the degree of CuB23
NPs coverage on the surface of graphene, which was
determined from the STEM images for the different loading
amounts. From Fig. 1a, Fig. 1b, Fig. S3-5, and Table S1, it can
be concluded that varying the mass ratio of the reactant and
graphene leads to a gradual increase in the degree of coverage
and the particle sizes of the CuB23 NPs on graphene with an
reactor was sealed and flushed with H
exclude air. It was then filled with H
2
more than four times to
to 4.2 MPa followed by
2
heating up to 413 K. Once the pressure reached a steady state,
the hydrogenation reaction was initiated immediately by
stirring the reaction mixture vigorously (1100 rpm) to eliminate
any diffusion effects. During the reaction, samples were
withdrawn from the reaction mixture at specific intervals for
product analysis on a gas chromatograph equipped with a flame
ionization detector and an EC-WAX capillary column using n-
dodecane as an internal standard. The products were identified
by GC-MS equipped with the same column as the GC and by
comparison with commercially available pure products. All
2
+
increase in the concentration of Cu(NH
3
)
4
.
Serious
This journal is © The Royal Society of Chemistry 2014
J. Name., 2014, 00, 1‐12 | 3

Catalysis Science & Technology
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aggregation was observed for 20.58wt.%CuB23/graphene. This
possibly comes from the aggregation of neighboring particles at
View Article Online
DOI: 10.1039/C4CY01331J
2
+
higher concentrations of Cu(NH
3
)
4
where many more CuB23
NPs should be produced on the graphene. Very extensive
aggregation is not desired for the deposition of uniformly
dispersed CuB23 NPs on the surface of graphene. When using
Cu(NO ) in the absence of NH ligands only CuNPs grow on
3
2
3
the surface of graphene with a wide diameter distribution (Fig.
2
+
S6). Clearly, the use of Cu(NH
3
)
4
3
as a precursor or NH as a
ligand is important for the preparation of CuB23/graphene.
Interestingly, free CuB23 NPs were not detectable in the
centrifugal supernatants of either the reaction solutions or the
wash eluent suggesting that no or negligible CuB23 NPs formed
in the bulk solutions, and the deposited CuB23 NPs were stable
on or not washable from the graphene surface.
According to ICP-AES and elemental analyses, the relative
contents of Cu and B do not change after loading onto graphene,
which is similar behavior to that of unsupported CuB23
Fig.2 XRD patterns of (a) graphene oxide; (b) graphene; (c) 3.54 wt.%
CuB23/graphene; (d) 6.67 wt.% CuB23/graphene; (e) 9.93 wt.%
CuB23/graphene; (f) 13.62 wt.% CuB23/graphene; (g) 20.58 wt.%
CuB23/graphene and (h) CuB23
.
(
B/Cu=22.99) (Table S1). The formation of homogeneous alloy
CuB23 nanoparticles on the surface of 9.93wt.%CuB23/graphene
was confirmed by cross-sectional compositional line analyses
in high-angle annular dark-field-scanning TEM (STEM)
experiments (Fig. S7a), and energy-dispersive X-ray
spectroscopy (EDAX) that was performed at different points
(
Fig. S7b–d).
The XRD pattern (Fig. 2) also indicates the deoxygenation of
GO and the formation of amorphous CuB23 alloy NPs on the
surface of graphene because of the disappearance of the peak at
2
1-23
2
θ
=10.9° for graphene oxide
and the appearance of a peak
2
1-23
at 2θ = 26° for reduced graphene oxide
and a broad peak at
for amorphous CuB23 alloy nanoparticles.
Typical Raman spectra of graphite oxide and CuB23/graphene, (c) 9.93 wt.% CuB23/graphene, (d) 13.62 wt.%
2
4,25
2
θ = 45°
Fig.3 Cu2p3/2 XPS spectra of (a) 3.54 wt.% CuB23/graphene, (b) 6.67 wt.%
CuB23/graphene with different loading amounts are shown in CuB23/graphene , (e) 20.58 wt.% CuB23 /graphene and (f) CuB23
Fig. S8. They all contain two prominent peaks, which
.
correspond to the well-documented D and G bands. The D band
is attributed to a breathing k-point phonon with A1g symmetry
that corresponds to local defects and disorders while the G band
2
26
is assigned to the E2g phonon of C sp atoms. The G band
−
1
−1
moved from 1603 cm for graphite oxide to 1586 cm for
CuB23/graphene with different loading amounts. This is close to
the value of pristine graphite and confirms the reduction of
2
6
graphite oxide. The intensity ratio (I
/I ) of the D band to the
D G
2
21
G band correlates to the average size of the sp domains, that
is, smaller sp domains have a higher intensity ratio (I
2
/I ). The
D G
I /I ratios for graphite oxide and CuB /graphene with
D
G
23
different loading amounts are 1.09, 1.43, 1.45, 1.48, 1.50 and
.53, respectively. The higher I /I ratios for CuB23/graphene
1
D G
Fig.4 B1s spectra of (a) 3.54 wt.% CuB23/graphene, (b) 6.67 wt.%
with different loading amounts by comparison with graphite CuB23/graphene, (c) 9.93 wt.% CuB23/graphene, (d) 13.62 wt.%
CuB23/graphene, (e) 20.58 wt.% CuB23 /graphene and (f) CuB23
.
oxide comes from the smaller size of the re-established
graphene network (sp carbons) after chemical reduction
2
From Fig. S9a, three main types of C1s peaks are present at
284.7 eV, 286.8 eV and 288.3 eV, and these are attributed to
C–C, C–O and C=O, respectively, in the XPS spectrum of GO.
However, the spectra showed that all the C1s peaks of
CuB /graphene with different amounts of loaded composites
compared with the original graphite layer. This further indicates
that graphite oxide has been well deoxygenated and reduced.
2
3
A comparison of the C1s XPS spectra between GO and
CuB /graphene with different loading amounts is shown in Fig.
2
3
2
3
S9.
only have a main peak at 284.7 eV (Fig. S9b-f), confirming that
4
| J. Name., 2014, 00, 1‐12
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the oxygenated functional carbon groups on GO were removed shifts in the N1s and Cu2p3/2 binding energies for GO after
2
+
after co-reduction.
Cu(NH3)4 adsorption when compared with Cu(NO ) (Fig.
3 2
View Article Online
Fig. S10, Fig. 3 and Fig. 4 show the overall XPS spectra S13 and Fig. S14). Following the application of plasma within
containing the Cu2p3/2 and B1s peaks of the CuB23 particles 0.5 min, Cu(NH3)4 is reduced in situ by KBH4 to give
before and after loading, respectively. As shown in Fig. 3a, Cu CuB (NH ) (Fig. S12c), and this was confirmed by the
is present in the metallic state with a binding energy of 932.4 appearance of CuBNH peaks as shown in Fig. S12c-d. This
eV. In contrast, B (Fig. 4a) is present in both the elemental indicates that CuB (NH ) was temporarily stable on the
DOI: 10.1039/C4CY01331J
2
+
2
3
3 x
+
2
7
2
3
3 x
2
7
state (188.3 eV) and the oxidized state (192.5 eV). No surface of GO because of the role of the NH ligand during this
3
3
1,32
significant oxidation of metallic Cu occurred during the period.
The
sample be
can
designated
preparation process indicating that the presence of B effectively C8.165O10.165[Cu(NH ) ]
[CuB (NH ) ] . From 0.5 min
3 4 0.029
23
3 3.5 0.003
protects Cu from oxidation. As no oxidized B was detected by to 1 min, more CuB (NH ) was detected on the GO sample
2
3
3 x
XRD, the amount of oxidized B species was negligible. (Fig. 3d), and the composition of the sample was
Furthermore, the binding energy of the B species positively C8.165O7.783[Cu(NH ) ] [CuB (NH3)2.7]0.009. Apparently,
shifts to 1.3 eV suggesting that some electrons are transferred CuB (NH ) coalesces into clusters on the surface of GO
3 4 0.023
23
2
3
3 x
from B to Cu resulting in a Cu electron-enriched supported during this period and the newly created clusters can further
2
+
CuB23 catalyst, as found in most other metal–boron amorphous absorb Cu(NH3)4
from solution, which facilitates the
2
,3
alloys. These electron-enriched Cu sites are favorable for the continuous in situ reduction and the in situ growth of
3
2
oxidative addition of metallic Cu to the carbon−halogen bond CuB (NH ) NPs. After 2 min, more CuB23 was detected in
2
3
3 x
3
j,28
in Heck coupling reactions,
and are beneficial for the the samples while the NH content of CuB (NH ) decreased
3 23 3 x
2
9
formation of H−species. They also allow for the activation of from 2.7 to 1.6, and the final composition was
the adsorbed C=O group during the hydrogenation of levulinic C8.165O4.214[Cu(NH ) ] [CuB (NH ) ] . This indicated
3 4 0.011
23
3 2.7 0.021
3
0
acid to γ-valerolactone.
that the plasma accelerated the release of NH3 from
In CuB /graphene, the Cu peak shifts to a lower binding CuB (NH ) as the reaction proceeded. The exact mechanism
2
3
23
3 x
energy while the alloying B peak shifts to higher energy for the release of NH is not clear and theoretical calculations
3
compared with the B in the unsupported CuB23 (Fig. 3b–f and are required in further studies. After a 5 min reaction, 9.93 wt.%
Fig. 4b–f). Fig. S11 shows typical Cu2p3/2 spectra of 2.02 wt.% CuB /graphene, which can be expressed as C8.165(CuB23)0.032,
23
Cu/graphene, 2.02 wt.% Cu/C and 9.93 wt.% CuB /C. All of with a uniform distribution was obtained (Fig. S12f and Fig. 1).
2
3
three samples have an average particle size of about 5 nm, The composition of CuB /graphene did not change further with
2
3
which is similar to that of 9.93 wt.% CuB /graphene. SPP treatment up to 10 min (Fig. S12g). This further indicates
2
3
Compared with Fig. 3 and Fig. S11, the observed Cu binding that the anchored CuB23 nanoparticles on the surface of
energy shifts for the CuB /graphene, which implies that graphene are stable. Apparently, electrostatic repulsion among
2
3
2
+
graphene promotes electron transfer from B to Cu to form a Cu(NH3)4 ions is responsible for the uniform distribution of
7
b
more electron rich copper and thus increase the amount of CuB23 NPs on the surface of GO in our work. Additionally,
2
,3
2+
−
copper in the alloying state. Since an oxidized B species peak Cu(NH3)4 can inhibit the adsorption of ions such as NO
3
2
+
is not observed (Fig. 4b–f), we infer that only a small amount of onto GO sheets with the exception of NH and Cu (Fig. S13
3
CuB23 loading leads to oxidized B, and the amount formed is and Fig. S14), as shown by the comparison between the N1s
less than can be detected by the XPS equipment.
Based on the above experimental observation and analysis, Cu(NH3)4 adsorption from their solutions, respectively. This
and Cu2p
3 3 2
spectra of GO after NH , Cu(NO ) , and
3
+
/2
2
7
b
our present method can be used to selectively prepare uniform facilitates the reliable growth of dispersed CuB23 NPs on GO.
CuB /graphene composites. The smaller size and lack of However, the real mechanism of this unique CuB /graphene
2
3
23
agglomeration of CuB23 NPs in CuB /graphene with low requires further study, which is currently underway.
23
loading amounts (<20.58 wt.%) indicates that graphene can
successfully serve as a support and also as an efficient 3.2 Catalytic Performances
dispersing agent for the preparation of CuB23 NPs. This result is
3
.2.1 Heck-type carbon−carbon coupling reactions
attributed to the hydrophobic basal plane and the hydrophilic
phenyl epoxide and hydroxyl groups, which enable GO to
anchor the CuB23 NPs, and thus control their sizes and
distribution on graphene during the preparation. To further
understand the uniform CuB /graphene formation process we
The Heck-coupling of cyclohexyliodide has been carried out
under ligand-free conditions with NMP as a solvent and with
Na CO as the base at a molar ratio of cyclohexyliodide/Cu of
2
3
2
3
5
0. To limit the formation of a 2-fold coupling byproduct,
carried out time-dependent experiments where samples were
collected as a function of time and analyzed using ToF-SIMS
excess styrene with respect to cyclohexyliodide (molar
ratio=1.2) was used. No product was formed in the presence of
B, graphene, CuO or Cu O, indicating that metallic Cu is the
2
active site. To optimize the CuB23 content of the
CuB /graphene catalysts, catalysts with different CuB23
content (2.78 wt.%, 5.65 wt.%, 9.93 wt.%, 13.62 wt.% and
(
Fig. S12) and elemental analysis. From Fig. S12a and Fig.
2
+
S12b, the Cu(NH3)4 first anchored onto the surface of GO by
an electric interaction to form C8.165O12.031[Cu(NH ) ] in the
3 4 0.032
2
3
absence of plasma, and this was confirmed by the appearance
+
of a series of Cu(NH ) CO peaks. This was also confirmed by
3
4
2
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Fig.5 The yield of Heck cross-coupling between cyclohexyliodide and
F
ig.6 The time−ln(1−conversion) curve of Heck cross-coupling between
styrene over catalysts with different CuB23 contents. Reaction conditions:
cyclohexyliodide and styrene over 9.93 wt.% CuB23/graphene. Reaction
conditions: a catalyst containing 6.35 mg Cu, cyclohexyliodide (5.0 mmol),
1
25.64 mg catalysts, cyclohexyliodide (5.0 mmol), styrene (6.0 mmol),
Na CO (7.5 mmol), DMF (10 mL), T = 353 K, stirring rate =800 rpm, time
12 h.
2
3
styrene (6.0 mmol), Na
2 3
CO (7.5 mmol), DMF (10 mL), T = 353 K, stirring
=
rate =800 rpm.
2
0.58 wt.%) were evaluated in this reaction (Fig. 5). The yield
3
4
previously reported Pd complex catalysts (87% yield), 9.93
wt.% CuB23/graphene gave a much higher yield in the
described Heck coupling reactions. Considering that the
reaction temperature in our experiments was lower (353 K)
than that reported for Pd complexes (373 K), 9.93 wt.%
CuB /graphene was obviously more active.
was found to increase with CuB23 loading up to 9.93 wt.%.
However, the yield decreased with a further increase in CuB23
loading because of particle size growth at higher loading (Fig.
S1c and Fig. S4) resulting in a decrease in the active surface
area (SCu) (Table S1).
To ensure that the Heck coupling reaction that was carried
out on the surface of the CuB23 nanoparticles was truly
heterogeneous, a reaction between cyclohexyliodide and
2
3
The catalytic performance of 9.93 wt.% CuB23/graphene was
also compared with that of 9.93 wt.%CuB23/C (Table 1, entry
). The yield obtained in the Heck coupling reactions by 9.93
wt.% CuB23/C is about 2 times higher than that of 9.93 wt.%
CuB23/C, even though Vulcan XC-72C has a higher specific
surface area (250 m g ) (Table S1). Generally, a high surface
area favors the dispersion of active components; however,
CuB23 dispersion over Vulcan XC-72C is lower than that over
graphene, as measured by H chemisorption and the data are
2
listed in Table S1. The high dispersion of CuB23 on graphene
can be attributed to the unique structure of graphene, which
4
styrene was carried out for 1.5
h over 9.93 wt.%
CuB23/graphene upon which the solid was then filtered in a
glove box fulfilled with argon while the filtrate was allowed to
react for another 24 h. No conversion of cyclohexyliodide was
observed (Fig. S15). This confirmed that this reaction occurs on
the surface of CuB23 nanoparticles because catalysis by Cu
species that leached into solution can be ruled out.
2
-1
3
3
The reaction profiles (Fig. S16) indicate that the
7
9
.93wt.%CuB23/graphene was selective toward
(E)-(2-
allows for CuB23 to be dispersed. Meanwhile, the catalytic
performance of CuB23 particles deposited on graphite with
similar surface area instead of graphene is poor (Table 1, entry
5). It indicated that graphite is also not an ideal substrate for
growing and anchoring CuB23 nanoparticles as high-
performance catalysts in comparison with graphene. Therefore,
we conclude that graphene is more suitable for CuB23 loading
than Vulcan XC-72C and graphite.
cyclohexylvinyl)benzene (more than 97% throughout the
reaction). The obtained ln(1-conversion) increased almost
linearly with reaction time (Fig. 6) indicating that this reaction
was first-order with respect to cyclohexyliodide. Meanwhile,
there is no influence on the performances of CuB23/graphene
when using NaBH instead of KBH during SPP (Table 1,
4 4
entries 1 and 2). From the reaction time required to convert
cyclohexyliodide completely, the 9.93wt.% CuB23/graphene
3
3
It is worth noting that a slight loss in activity (4%) was
observed for CuB /graphene in the five recycling experiments.
(
Table 1, entry 1) showed up to 10 times more activity
23
compared with CuB23 (Table 1, entry 3). Clearly, the higher
reactivity may initially be attributed to the higher number of Cu
active sites (see SCu in Table S1). However, the SCu of 9.93wt.%
CuB23/graphene was only 4 times that of CuB23. Therefore, we
can argue that CuB23/graphene possesses enhanced intrinsic
reactivity compared with CuB23. As shown by the XPS results,
the Cu in CuB23/graphene was more electron-enriched than that
in CuB23, which would allow for a more favorable oxidative
addition of metallic Cu to carbon−halogen bonds and, therefore,
ICP analysis reveals that no Cu leaching occurred, which rules
out the loss of active phases after repeated use. Moreover,
STEM (Fig. S17a-c), SAED (Fig. S17d) and XRD (Fig. S18a)
results indicate that no changes occurred in the particle size
distribution, crystallization or structure of CuB /graphene after
2
3
repeated use. It appears that B oxidation on the active surface of
CuB23/graphene is responsible for the deactivation, as shown by
EDAX (Fig. S18b) and XPS (Fig. S19). In particular, the boron
oxide film results in a decrease in SCu as it blocks the Cu active
3
j,28
a more efficient catalyst was obtained.
By comparison with
3
3
sites and therefore reduces the electron density of Cu.
6
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a
Table 1 Heck-type coupling reaction between cyclohexyliodide and different styrenes.
catalysts
+
I
R
R
b,c
Entry
Catalyst
R
Time / h
12
Conversion / %
Yield / %
1
2
3
4
5
6
7
8
9
9.93 wt.% CuB23/graphene
9.93 wt.% CuB23/graphene
CuB23
H
100
100
99
97
d
H
12
97
H
120
24
96
9.93 wt.% CuB23/C
H
98
94
9.93 wt.% CuB23/graphite
9.93 wt.% CuB23/graphene
9.93 wt.% CuB23/graphene
9.93 wt.% CuB23/graphene
9.93 wt.% CuB23/graphene
9.93 wt.% CuB23/graphene
H
36
80
90
CH
CH
3
3
O
12
99
95
12
95
94
CF
CH
F
3
12
97
92
2
OH
12
99
93
1
a
0
12
98
92
Reaction conditions: a catalyst containing 6.35 mg Cu, cyclohexyliodide (5.0 mmol), alkenes (6.0 mmol), Na
2
CO
3
(7.5 mmol), DMF (10 mL),
b
c
1
T = 353 K, stirring rate =800 rpm; Yields of isolated product; Product ratios were determined by H NMR spectroscopy of crude reaction
d
mixtures; prepared by using NaBH
4
instead of KBH
4
Table 2 Heck-type coupling reaction between different alkylhalidide and styrenes (Table 1, entries 6-10). The electron-withdrawing or
a
styrene over 9.93 wt.% CuB23/graphene.
electron-donating groups did not obviously affect the coupling
yield between styrene and cyclohexyliodide (Table 1, entries 6-
10). This means that the reaction is relatively insensitive toward
the electronic characteristics of the substituents of styrene under
the present reaction conditions. Additionally, various alkyl
iodides such as cyclopentyl iodide, cycloheptyl iodide, and
iodooctane had high activities in addition to cyclohexyl iodide
during coupling with styrene (Table 2, entries 1-4).
CuB /graphene was also highly active during the Heck
R
I
CuB23/graphene
+
R
Entry
1
R-I
Conversion / %
Yield
b,c
/
%
99
94
I
2
3
2
3
98
99
92
95
I
coupling reaction between cyclohexyl bromide and styrene
Table 2, entry 5). However, it had low activity towards
cyclohexyl chloride in this reaction (Table 2, entry 6).
(
I
3
.2.2 Hydrogenation of levulinic acid to γ-valerolactone.
nHex
4
5
100
100
96
97
I
Liquid phase LA hydrogenation to γ-valerolactone was also
used to evaluate the performance of 9.93 wt.% CuB /graphene.
2
3
The hydrogenation of LA was confirmed to proceed on the
surface of CuB23 nanoparticles using the above-mentioned
filtration method (Fig. S20). The effect of various catalyst
parameters on the hydrogenation activity of LA to GVL under
Br
Cl
6
7
4
a
solvent-free conditions with 4.2 MPa H at 413 K for 4 h in the
2
Reaction conditions: a catalyst containing 6.35 mg Cu, alkyliodide (5.0
mmol), styrene (6.0 mmol), Na CO (7.5 mmol), DMF (10 mL), T = 353 K, presence of 2 mol% of the catalyst was investigated (Table 3).
b c
2
3
stirring rate =800 rpm, time =12 h; Yields of isolated product; Product
ratios were determined by HNMR spectroscopy of crude reaction mixtures.
First, we evaluated a series of 9.93 wt.% CuB -loaded catalysts
1
23
(
Table 3, entries 1–8). Generally, the CuB -loaded catalysts
23
gave a higher yield (55–99%) than unloaded CuB23 (Table 3,
entry 9, 28% yield). B and graphene (Table 3, entries 10 and 11)
CuB /graphene was also used an active catalyst for the
Heck-type coupling reaction of cyclohexyliodide with various
2
3
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a
Table 3 Hydrogenation of LA to GAL over different catalysts.
O
View Article Online
OH
catalysts
DOI: 10.1039/C4CY01331J
H
2
+
O
O
O
Entry
Catalyst
Conversion / %
Yield / %
1
2
3
4
5
6
7
8
9
9.93 wt.% CuB23/graphene
9.93 wt.%CuB23/C
100
79
68
59
75
77
69
68
33
0
99
76
64
55
70
72
63
60
28
0
9.93 wt.% CuB23/SiO
9.93 wt.% CuB23/Al
9.93 wt.% CuB23/TiO
9.93 wt.% CuB23/CeO
9.93 wt.% CuB23/MgO
2
2
O
3
2
2
9.93 wt.% CuB23/MoO
CuB23
x
Fig.7 Reaction profiles of LA (■) hydrogenation to GAL (●) over 9.93 wt.%
1
1
1
1
1
0
1
2
3
4
Nano B
Graphene
Fe-B/graphene
Co-B/graphene
Ni-B/graphene
CuB /graphene. Reaction conditions: Catalyst containing 2.54 mg Cu, 20
23
0
0
38
8
mmol LA, 50 mL of n-dodecane, PH
2
2
= 4.2 MPa H , T = 413 K, stirring rate
38
12
67
=
1100 rpm.
67
a
29
Reaction conditions: Catalyst containing 2.54 mg Cu, 20 mmol LA, 50mL favorable for the formation of H−species and the activation of
of n-dodecane, PH
rate = 1100 rpm.
2 2
= 4.2 MPa H , T = 373 K, Reaction time = 4 h, stirring
adsorbed C=O groups by electron back-donation from the d
orbital of Cu to the π*C=O antibonding orbital of the C=O
groups. This leads to the enhanced TOF. This is also
3
0
were completely inactive indicating that Cu catalyzes the
reaction. The effect of the support material on the activity of the
CuB23 nanoparticles (Table 3, entries 1–8) showed that
graphene is the best support. For the M-B (Cu, Fe, Co, Ni)
series loaded onto graphene (Table 3, entries 1,12–14), 9.93 wt.%
CuB23/graphene gave the highest yield. The solvent effect of
CuB23/graphene during the hydrogenation of LA was also
studied (Table S2). n-Dodecane gave the highest GVL yield.
From these results, we determined that 9.93 wt.%
CuB23/graphene with n-dodecane is the most effective catalytic
system for LA reduction.
Fig. 7 and Fig. S21 show the concentration change for both
the reactant and the product with reaction time during LA
hydrogenation over 9.93wt.%CuB23/graphene and CuB23
respectively. As shown in Fig. 7 and Fig. S21, LA conversion
increased almost linearly with reaction time over both the
CuB23 amorphous alloy catalysts implying that LA
hydrogenation was zero-order with respect to its
−1
supported by the fact that an abrupt decrease in TOF (0.36 s )
was observed for the 9.93 wt.% CuB23/graphene treated at 673
K for 2 h under Ar flow, and this was mainly because of the
crystallization of the CuB23 amorphous alloy (Fig. S22a). But,
the exact mechanism for the Cu sites towards the activation of
C=O bond in levulinic acid hydrogenation is not clear and
theoretical calculations are needed in further studies.
Fig. 8 shows the recycling tests undertaken using the CuB23
and 9.93 wt.% CuB23/graphene catalysts. A significant loss in
activity (70%) was observed for CuB23 after 3 cycles whereas
9
.93 wt.% CuB /graphene was used 7 times with only a slight
23
loss of activity (6%). ICP analysis reveals that no Cu leaching
occurred for CuB23 or 9.93 wt.% CuB23/graphene upon repeated
use, implying that both the catalysts were stable against the
chelating effect of the reactant and the product. Apparently, an
increase in particle size and crystallization (Fig. S23 and Fig.
S24) occurred for the un-supported CuB23 catalyst under high
,
3
3
concentration. A significant deviation from the straight line
was observed when the LA conversion was more than 80%, and
this is possibly due to the extremely low LA concentration. It is
also significant that the 9.93 wt.% CuB23/graphene showed
enhanced activity compared with CuB23, and this corresponds
to a shorter LA complete conversion time.
On the basis of the reaction rates at conversions between 0
and 10% for the hydrogenation of LA, the turnover frequency
(
TOF) of the catalysts was calculated and we found that the
intrinsic activity changed as follows: 9.93 wt.%
−1
−1
CuB /graphene (0.51 s ) > CuB (0.27 s ). Therefore, the
2
3
23
superior activity of 9.93 wt.% CuB23/graphene compared with
CuB23 may be attributed to both its higher metallic area (see the
S
Cu values) and its enhanced intrinsic activity (see the TOF
Fig.8 Recycling tests of 9.93 wt.% CuB23/graphene and CuB23 for LA
values). XPS analysis revealed that the Cu in CuB23/graphene
was more electron-enriched than that in CuB23. Accordingly,
the higher electron density on the Cu active sites might be
hydrogenation to GAL. Reaction conditions: Catalyst containing 2.54 mg Cu,
2
2 2
0 mmol LA, 50 mL of n-dodecane, PH = 4.2 MPa H , T = 413 K, Reaction
time = 4 h, stirring rate = 1100 rpm.
8
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ARTICLE
Table 4 Catalytic parameters of different heterogeneous catalysts for liquid hydrogenation of LA to GVL.
-
1
Catalysts
Mol%
Solvent
P(H
4.2
7
2
) / MPa
T / K
413
473
473
523
523
523
473
423
t / h
4
Yield / %
TON
7376.5
14
TOF/h
9
.93 wt.% CuB23/graphene
2
n-Dodecane
Water
99
91
90
99
3
1844.1
3.5
1
1
Cu/CrO
X
6.4
10
2
4
1
2
Cu/ZrO
2
Methanol
Solvent-free
Solvent-free
Solvent-free
Dioxane
3.4
5
5
9
1.8
1
3
Ni-MoO
X
/C
24
24
24
4
4951.2
151.2
100.8
3400
5872
206.3
6.3
1
3
Ni/C
2
5
1
3
Ni/HZSM-5
2
5
2
4.2
3
5
Ru-TiO
2
2.9
-
4
98
94
850.0
2936
3
6
Ru0.7Ni0.3−OMC
Solvent-free
4.5
2
1
4
35
MoOx/C and Ru/TiO2 toward LA hydrogenation (Table 1).
However, the TOF of our CuB2 33 /graphene is lower than that of
a heterogeneous catalysis of Ru Ni -OMC bimetal catalysts
0
.7
0.3
under 4.5MPa H
2
, which showed 94% yield of GVL with 0.022
mol% of the catalyst based on Ru, corresponding to a TOF of
-
1 36
2
936 h .
4
. Conclusions
In summary, we successfully prepared monodisperse CuB23
nanoparticles grown on graphene. The graphene proved to be a
distinct and powerful support for the CuB23 nanoparticles. A
structure–activity relationship study showed a combined good
dispersion effect for CuB23 and that the intimate interaction that
derived from graphene had a positive influence on its catalytic
activity in Heck-type carbon−carbon coupling reactions and in
the hydrogenation of LA to GAL by comparison with
unsupported CuB . Moreover, 9.93 wt.% CuB /graphene
exhibited superior performance by comparison with the most
efficient heterogeneous noble metal-free catalysts reported to
date for the liquid phase hydrogenation of LA. Our findings
demonstrate the advantages of graphene supports for the
preparation of efficient, uniform and stable amorphous alloy
catalysts on the basis of the relationship between their structure
and their catalytic performance.
Fig.9 DSC curves of (a) 9.93 wt.% CuB23/graphene and (b) CuB23 catalysts.
pressure and high temperature, and this was responsible for its
deactivation. The loss in activity for CuB23 because of
oxidation during the recycling process was only 10% in the
control experiment. 9.93 wt.% CuB23/graphene had far better
thermal stability than CuB23 because of its graphene support
and its uniform particle size, as confirmed by DSC (Fig. 9),
XRD (Fig. S22b and Fig. S22c), STEM (Fig. S25a-c) and
SAED results (Fig. S25d). Therefore, the slight loss of activity
for 9.93wt.% CuB23/graphene is mainly attributed to B
oxidation during the recycling process, as confirmed by XPS
2
3
23
(
Fig. S26). The catalytic performance of 9.93 wt.%
CuB /graphene was also compared with that of previously
2
3
1
2-14,35
Acknowledgements
reported LA hydrogenation catalysts (Table 4)
. Based on
the selected reaction conditions, the GAL yield, the turnover
number, the turnover frequency (TOF), and the catalytic
performance of our CuB23/graphene is superior to those of the
most effective single metal heterogeneous catalysts: Ni–
This work was financially supported by the National Natural Science
Foundation of China (21376033), the Sichuan Youth Science and
Technology Innovation Research Team Funding Scheme
(
2013TD0005), the Cultivating programme of Middleaged backbone
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teachers (HG0092), the Cultivating programme for Excellent
Innovation Team of Chengdu University of Technology (HY0084) and
Innovative Experimental Items for College Students of Sichuan
Province (SZH1106CX04).
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