Active and regioselective rhodium catalyst supported on reduced graphene oxide for 1-hexene hydroformylation

Article abstract of DOI:10.1039/c5cy01355k

Alkene hydroformylation with syngas (CO + H₂) to produce aldehydes is one of the most important chemical reactions. However, designing heterogeneous catalysts to realize comparable performance with mature homogeneous catalysts is challenging. In this report, a reduced graphene oxide (RGO) supported rhodium nanoparticle (Rh/RGO) catalyst was successfully prepared via a one-pot liquid-phase reduction method and first applied in 1-hexene hydroformylation. 1-Hexene hydroformylation reaction under different reaction conditions with this Rh/RGO catalyst was investigated in detail. Low reaction temperature and short reaction time effectively enhanced the n/i (normal to iso) ratio of heptanal in the products. The catalytic performance of the Rh/RGO catalyst was also compared with those of Rh supported on other carbon materials, including activated carbon and carbon nanotubes (Rh/AC and Rh/CNTs). The results showed that the Rh/RGO catalyst exhibited the highest 1-hexene conversion and the largest n/i ratio of 4.0 among the tested catalysts. The special 2D nanosheet structure of the Rh/RGO catalyst, rather than the 3D porous and 1D nanotube structures of Rh/AC and Rh/CNTs, respectively, principally contributed to its excellent catalytic performance. These findings disclosed that reduced graphene oxide could be a promising catalyst support for designing heterogeneous hydroformylation catalysts.

Full text of DOI:10.1039/c5cy01355k

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Page 1 of 35
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Active and Regioselective Rhodium Catalyst Supported on Reduced
Graphene Oxide for 1-Hexene Hydroformylation
Minghui Tan,a,b Guohui Yang,b Tiejun Wang,d Tharapong Vitidsant,e Jie Li,b Qinghong
Wei,b Peipei Ai,b Mingbo Wu,a* Jingtang Zheng,a* and Noritatsu Tsubakib,c*
a State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, PR China.
b Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-
8555, Japan.
c JST, CREST, Gobancho 7, Chiyoda-ku, Tokyo 102-0076, Japan
d CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of
Sciences, 510640 Guangzhou, PR China
e Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
ABSTRACT
Alkene hydroformylation with syngas (CO+H2) to produce aldehyde is one of the
most important chemical reactions. However, designing heterogeneous catalysts to
realize comparable performance with the mature homogeneous catalysts is
challenging. In this report, a reduced graphene oxide (RGO) supported rhodium
nanoparticles (Rh/RGO) catalyst was successfully prepared via one-pot liquid phase
reduction method and first applied in 1-hexene hydroformylation. 1-Hexene
hydroformylation reaction under different reaction conditions with this Rh/RGO
*Corresponding Author: Phone: +(81)-76-445-6846. Fax: +(81)-76-445-6846. E-mail: tsubaki@eng.u-
toyama.ac.jp; wumb@upc.edu.cn; jtzheng03@163.com.
† Electronic Supplementary Information (ESI) available: Experimental Section (Preparation of Graphite Oxide,
Pre-treatment of AC and CNTs and CO-TPD ); FTIR, XPS and TEM for Rh/AC and Rh/CNTs
1

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catalyst was investigated in detail. Low reaction temperature and short reaction time
effectively enhanced the n/i (normal to iso) ratio of heptanal in products. The
catalytic performance of Rh/RGO catalyst was also compared with those of Rh
supported on other carbon materials, including activated carbon and carbon
nanotubes (Rh/AC and Rh/CNTs). The results showed that the Rh/RGO catalyst
exhibited the highest 1-hexene conversion and the largest n/i ratio of 4.0 among the
tested catalysts. The special 2D nano-sheet structure of Rh/RGO catalyst, rather than
3D porous Rh/AC and 1D nanotubed Rh/CNTs, principally contributed to its
excellent catalytic performance. These findings disclosed reduced graphene oxide as
a promising catalyst support for designing heterogeneous hydroformylation catalyst.
Keywords: Reduced graphene oxide (RGO), Rhodium, Hydroformylation, Syngas,
Nano sheet
2

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1. Introduction
Alkene hydroformylation, the addition of CO/H2 to the carbon-carbon double bond
of alkene to form aldehyde, is one of the main homogeneously metal-catalyzed
reactions, as shown by Scheme 1.1-3 Generally, the hydroformylation of 1-alkenes
results in the mixed products of linear n-aldehydes (n) and branched iso-aldehydes (i),
in which the linear n-aldehydes are required in large quantities for commodity
applications.4 Both cobalt (Co) and rhodium (Rh) complexes are usually used as
catalysts in industry for the homogeneously catalyzed hydroformylation. Especially
the ligand-modified Rh-based catalyst shows considerably excellent
hydroformylation activity and regioselectivity of linear products under mild reaction
conditions.5
Scheme 1 Hydroformylation of 1-alkene to aldehydes
Even though the traditional homogeneously catalyzed hydroformylation is
effective, developing heterogeneous catalysis systems is always in challenge to
overcome the drawbacks of homogeneous catalysis systems, such as high pressure,
metal loss, catalyst separation and recovery
.
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Until now, Rh supported on many solid supports, like SiO2,6-8 Al2O3,9, 10 zeolite,11
MOF,12, 13 or carbon materials,11, 14 as catalysts has been studied for heterogeneous
alkene hydroformylation. Due to the stable physical and chemical properties, such as
large specific surface area and easy recovery of noble metals from spent catalysts,
carbon materials are commonly used as supports for noble metals catalysts
preparation.15 Activated carbon (AC)14 and carbon nanotubes (CNTs)11 supported Rh
catalysts were investigated for alkene hydroformylation to obtain aldehyde. Although
these catalysts exhibited high conversion of alkene, aldehyde selectivity and n/i ratio
were depressed in the formed products. As a novel carbon material, graphene has
attracted much attention because of its unique 2 dimension (2D) structure,
mechanical, electronic and electrochemical properties.16, 17 Besides the above
properties, graphene oxide (GO), derived from graphene, has more advantages due to
the abundant oxygen-containing functional groups on its sheet surface,18 which can
improve the hydrophilicity and stability of GO, therefore ensuring its utilization in
liquid-phase reaction system. Meanwhile, on the other hand, these functional groups
on the surface of GO can act as nucleating sites to anchor metal ions.19, 20 Thus GO
has been regarded as an excellent support to develop a variety of heterogeneous
catalysts.21 Furthermore, the GO combined with metal ions on its surface can be
4

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simultaneously reduced to form reduced graphene oxide (RGO) supported metal
nanoparticles for various applications.22 However, to our knowledge, until now no
RGO is reported as catalyst support to prepare heterogeneous catalysts for alkene
hydroformylation.
In this work, we prepared Rh nanoparticles loaded RGO (Rh/RGO) catalyst via
one-pot liquid-phase reduction method. The as-prepared Rh/RGO catalyst was first
tested as heterogeneous catalyst for 1-hexene hydroformylation. The catalytic
performance of this Rh/RGO catalyst under varied reaction conditions, such as
reaction temperature or time, was investigated in detail. The hydroformylation
activity and regioselectivity of n-heptanal over Rh/RGO catalyst were also compared
with other carbon materials supported Rh catalysts (Rh/AC and Rh/CNTs).
2. Experimental
2.1 Catalysts preparation
The reduced graphene oxide supported Rh nanoparticles (Rh/RGO) catalyst was
prepared by a one-pot method. The preparation process of Rh/RGO catalyst is shown
in Scheme 2. First, 0.5 g graphite oxide obtained from graphite powder by a modified
Hummers’ method23 (see ESI†) was added into 120 mL glycol-water solution (20
mL deionized water, 100 mL ethylene glycol) that was beforehand mixed with 0.25 g
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of Rh(NO3)3 water solution containing 5.1 mg Rh3+. After ultrasonic treatment for 2 h,
the mixed solution was heated in 105 oC oil bath for 10 h with magnetic stirring and
water reflux. Then the mixture was filtered and washed by deionized water and
ethanol, respectively, for several times. At last the catalyst was dried in vacuum at 60
oC for 12 h. The obtained sample was the Rh/RGO catalyst with Rh loading amount
of 1.07 wt% detected by inductively coupled plasma atomic emission spectrometer
(ICP-AES, Optima 7300DV, PerkinElmer Inc., USA).
Oxidization
Graphite
Rh3+ ion
Graphite oxide
Rh particle
Reduction
Rh loaded on the reduced graphene oxide
(Rh/RGO)
Graphene oxide (GO)
(Mixed with Rh3+ in glycol-water solution)
Scheme 2 Illustration of the preparation process of Rh/RGO catalyst
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The AC and CNTs supported Rh catalysts (Rh/AC and Rh/CNTs) were also
prepared by the same method and used as the reference catalysts of Rh/RGO catalyst
in the followed sections. For further details, please see the ESI†. The metal loading
amount of Rh was about 1.02 wt% and 1.04 wt% (detected by ICP-AES) for Rh/AC
and Rh/CNTs, respectively.
2.2 Catalyst characterization
The crystal structure of samples was confirmed by X-ray diffraction (XRD) (RINT
2400 diffractmeter, Rigaku, Japan) with Cu Kα radiation source (λ = 1.54 Å,
scanning rate of 0.02 °/s in the range of 5−80°) at 40 kV and 20 mA. Raman
spectroscopy was recorded using an Ar+ ion laser at 514.5 nm (Renishaw inVia 2000
Raman microscope, Renishaw, U.K.) to characterize the graphitic structure of
samples. The catalyst morphology was characterized by transmission electron
microscopy (TEM) (JEM-2100UHR, JEOL, Japan) with an accelerating voltage of
200 kV. The oxygen-containing functional groups on catalysts were characterized by
Fourier transform infrared spectrometry (FTIR) (Nicolet NEXUS 670, Thermo
Scientific, USA). X-ray photoelectron spectroscopy (XPS) was recorded on
ESCALAB 250Xi spectrometer (Thermo Scientific, USA).
The metal surface area and metal dispersion were determined by a H2 chemical
adsorption experiment with Quantachrome Autosorb-1 vacuum apparatus
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(Quantachrome Instruments, USA). Samples were degassed at 200 oC in vacuum for
1 h and then reduced in H2 at 400 oC for 2 h. The chemisorption was carried out at
100 oC for 30 min to equilibrium. Rh dispersions, as well as the exposed Rh surface
areas and sizes, were determined by assuming a Rh/H adsorption stoichiometry equal
to 1 and a spherical geometry. The carbon monoxide temperature programmed
desorption (CO-TPD) at a heating rate of 5 oC·min-1 was performed on a BELCAT-
B-TT apparatus (Bel Japan Inc.) equipped with a mass spectrometry (BELMass).
2.3 Catalytic activity test of 1-hexene hydroformylation
The 1-hexene hydroformylation reaction was carried out in an autoclave with an
inner volume of 85 mL. 0.10 g catalyst and 3.73 g 1-hexene (40 mmol) were loaded
in the autoclave. The molar ratio of the utilized syngas was CO:H2 = 1:1. After
purging with 1.0 MPa syngas three times to remove the residual air, another 5.0 MPa
syngas containing 80 mmol CO and 80 mmol H2 was sealed in the reactor at the
room temperature. The hydroformylation reaction was conducted at various
temperatures in the range of 70-110 oC, for different time of 1-10 h, under continuous
stirring. After the reaction, the reactor was cooled down in an ice water mixture for
20 min and then depressurized to atmospheric pressure. The liquid products were
analyzed quantitatively with a gas chromatograph (Shimadzu GC-2014, Japan)
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equipped with a capillary column (InertCap 5, length: 30 m) and a flame ionization
detector (FID).
3. Results and discussion
3.1 Catalyst characterization
(002)
(002)
(001)
▽Rh(111)
(100)
▽
Rh/RGO
(100)
Graphite oxide
(004)
Graphite
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2 Theta (degree)
Figure 1 XRD patterns of graphite, graphite oxide and Rh/RGO
The XRD patterns of graphite, graphite oxide and Rh/RGO are compared in
Figure 1. The diffraction pattern of graphite shows two peaks at about 26.6° and
54.7°, which can be ascribed to the (002) and (004) reflections of graphite (JCPDS
No. 41-1487), respectively. For graphite oxide, the peak (001) at 10.7° is the
characteristic of graphite oxide through liquid oxidation from graphite. Because of
the introduction of abundant oxygen-containing functional groups, the interlayer
spacing of graphite oxide reaches 0.84 nm, much larger than 0.34 nm of the original
graphite. Due to the exfoliation of 3D graphite oxide to 2D GO sheets and then the
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reduction of GO by ethylene glycol to RGO, the peak around 10.7o of graphite oxide
will shift to large angle for RGO. Therefore the Rh/RGO catalyst has two peaks at
24o and 43o respectively, belonging to the crystalline structures (002 and 100) of
RGO (JCPDS No. 01-0646). The peak at 24o is much broader, which indicates that
the catalyst has some micro-structural defects and less crystallite carbon atoms,
possibly generated by the reduction process with ethylene glycol and the followed Rh
nanoparticles loading. There is no obvious Rh peak except the weak peak (111) of Rh
(denoted by

in Figure 1, JCPDS No. 05-0685) at 41o, because of the low loading
amount and fine dispersion of Rh nanoparticles on the RGO support.
For carbon materials, Raman spectroscopy is well recognized as a sensitive probe
to detect their defects and disorder.23 Figure 2 presents Raman spectra of graphite,
graphite oxide and Rh/RGO. Two strong bands observed in graphite are the G band
at 1580 cm-1 and the 2D band at 2700 cm-1.24 The G band is the first-order spectrum
due to the bond stretching of all pairs of sp2 atoms in both rings and chains.25 For
graphite oxide and Rh/RGO, two noticeable broad bands (D-band at 1340 cm-1 and
G-band at 1580 cm-1) are observed, and the D-band is ascribed to the defects or
imperfections of graphite oxide and RGO. The intensity ratio of D band to G band
(ID/IG) is related to the degree of disorder and defects.26 The ID/IG increases obviously
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from graphite oxide to Rh/RGO, indicating the increasing amount of defects on the
RGO support through a series of treatment processes. The increased defects, here, are
beneficial to form more catalytic active sites on the Rh/RGO catalyst for the followed
1-hexene hydroformylation.
D
G
Rh/RGO
Graphite oxide
Graphite
500
1000
1500
2000
2500
3000
Raman shift (cm-1)
Figure 2 Raman spectra of graphite, graphite oxide and Rh/RGO
C-O(carboxy)
C=C
Rh/RGO (spent)
C-H
Rh/RGO (fresh)
-O-H
C=O
C-O
Graphite oxide
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Figure 3 FTIR spectra of graphite oxide and Rh/RGO
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The reduction process can be confirmed by comparing the oxygen-containing
functional groups between graphite oxide and Rh/RGO by FTIR, as shown in Figure
3. The graphite oxide exhibits obvious peaks of oxygen-containing functional groups,
such as –OH (at 3000-3500 cm-1), C=O (carbonyl/carboxy at 1733 cm-1) and C-O
(carboxy at 1418 cm-1, epoxy at 1225 cm-1, and alkoxy at 1054 cm-1) groups,27, 28
which can act as nucleating sites to anchor Rh ions. After reduction, the peaks of
C=O (1733 cm-1) and some C-O (epoxy at 1225 cm-1, and alkoxy at 1054 cm-1) have
disappeared on the Rh/RGO catalyst, however, the peaks of –OH (3450 cm-1) and C-
O (carboxy at 1418 cm-1) groups still remain. It indicates that the oxygen-containing
functional groups on GO have not been removed fully and the GO is partly reduced
to RGO.
X-ray photoelectron spectra (XPS) analysis is also used to elucidate the surface
change and reduction degree of Rh/RGO from graphite oxide. As shown in Figure
4(a) and 4(b), after the reduction process, the oxygen content of Rh/RGO decreases
obviously from 29.16% of graphite oxide to 12.41%. The C1s can be fitted to
investigate the species and numbers of oxygen-containing functional groups. For
graphite oxide in Figure 4(a), the C1s peak consists of five components assigned to
C=C (284.4 eV), C-C (285.2 eV), C-O (286.7 eV), C=O (287.2 eV) and O=C-O
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(288.5 eV),29 and the two strong peaks (C-O and C=O) indicates abundant oxygen-
containing functional groups on graphite oxide. Compared with graphite oxide, the
C=O peak disappears and the C-O, O=C-O peaks of Rh/RGO decrease significantly
in Figure 4(b), indicating that most of oxygen-containing functional groups have
been removed after reduction except for some C-O and O=C-O. The result is
consistent with FTIR results.
(a)
(b)
O1s
C1s
C1s
C1s
C-O
C-C
C=C
C=O
O=C-O
C=C
C-C
C-O
O=C-O
O1s
C1s
293 292 291 290 289 288 287 286 285 284 283 282
293 292 291 290 289 288 287 286 285 284 283 282
C: 86.17%
O: 12.41%
C: 68.22%
O: 29.16%
1200
1000
800
600
400
200
0
1200
1000
800
600
400
200
0
Binding energy (eV)
Binding energy (eV)
(d)
C1s
308.6 eV
Rhn+
(c)
C1s
C=C
C-C
O=C-O
C-O
O1s
fresh
293 292 291 290 289 288 287 286 285 284 283 282
Rhn+
308.3 eV
C: 86.81%
O: 11.83%
spent
304
1200
1000
800
600
400
200
0
318
316
314
312
310
308
306
Binding energy (ev)
Binding energy (eV)
Figure 4 XPS spectra of graphite oxide and Rh/RGO: (a) survey spectra and C1s of
graphite oxide, (b) survey spectra and C1s of fresh Rh/RGO, (c) survey spectra and
C1s of spent Rh/RGO, (d) Rh3d region XPS spectra of fresh and spent Rh/RGO
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Due to the high electronegativity of GO, it is difficult to remove all oxygen-
containing functional groups and reduce Rh3+ to Rh0 completely under our mild
preparation condition.30 These C-O and O=C-O groups on Rh/RGO are stable and
still remain after reaction, as shown by Figure 3 and Figure 4(c). But for Rh/CNTs
and Rh/AC, these groups are changed after reaction and less stable than that on
Rh/RGO as shown in Figure S1†.The residual groups on Rh/RGO can be used for
anchoring Rh nanoparticles and are beneficial for the stability of the catalyst in
heterogeneous reaction. Usually, in the Rh-ligands homogenous catalyzed
hydroformylation, the electron withdrawing ability of substituents can influence the
selectivity of linear aldehyde.31 Therefore, as a kind of electron withdrawing groups,
the carbonyl group on the surface of catalyst may be in favour of n-heptanal
production. As shown in Figure 4(d), the binding energy of Rh 3d5/2 was 308.6 eV
that was assigned to a nonstoichiometric state Rhn+ between Rh3+ and Rh0.32, 33 This
result indicated that Rh3+ was partly reduced on RGO because of the high
electronegativity of GO. The similar result was also obtained from Rh/CNTs as
shown in Figure S2†. Without electronegativity of AC support, the Rh/AC catalyst
exhibits higher reduction state Rh0 (Figure S3
†) under the same preparation condition,
which also verifies the effect of electronegativity on Rh3+ reduction. Rhn+ as an
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electro-deficient species indicated that oxygen linkages exist between Rh
nanoparticles and RGO.22, 34 After reaction, the binding energy of Rh3d5/2 decreased
slightly to 308.3 eV and was still maintained at the Rhn+ state, demonstrating the high
stability of Rh/RGO catalyst.
Figure 5 TEM images of Rh/RGO catalyst obtained with different scale bar: (a) 200
nm, (b) 100 nm, (c) 5 nm
Figure 5 shows the TEM images of Rh/RGO catalyst at different magnifications. It
seems that the Rh particles have large size in 40-60 nm, as exhibited at low
magnification in Figure 5(a). However, in Figure 5(b) and 5(c), the magnified TEM
analysis on the catalyst discloses that the large Rh particle in fact has a cluster-like
structure comprising of dozens of smaller Rh nanoparticles with the size of 6 nm.
This cluster-like structure has proved its significant potential and prospects in
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catalysis reaction.35 The cluster-like structure of Rh particles supported on RGO
possesses not only large metallic surface area and high dispersion, but also abundant
active crystal defects and synergetic effects between small building units,36 which can
provide more active sites and higher activity for the targeted hydroformylation.
Moreover, because the residual groups can be used for anchoring Rh nanoparticles,
the cluster-like structure of Rh particles supported on RGO are stable and no obvious
aggregation of Rh particles is detected (Figure S4†)
.
3.2 Catalyst performance in 1-hexene hydroformylation reaction using Rh/RGO
catalyst
Scheme 3 1-Hexene hydroformylation and the co-existed side reactions
The major products of 1-hexene hydroformylation are heptanal, including n-
heptanal and i-heptanal, as given by Scheme 3. The side reactions of 1-hexene
hydroformylation mainly include 1-hexene isomerization through its carbon-carbon
double bond shift to generate internal hexene and the further hydroformylation of
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internal hexene on the same catalyst producing i-heptanal. Other side reaction, like
hexene hydrogenation, can produce some hexane.
100
90
80
70
100
90
80
70
60
60
1-Hexene
Internal hexene
Hexane
Heptanal
50
50
40
40
30
20
10
0
30
20
10
0
70
80
90
100
110
Temperature (oC)
Figure 6 Influence of reaction temperature on 1-hexene hydroformylation over
Rh/RGO catalyst. Reaction conditions: 0.10 g Rh/RGO catalyst, 3.73 g 1-hexene, 5
MPa syngas (CO:H2 = 1:1), 1h
Figure 6 shows the effect of reaction temperature on the catalytic properties of
Rh/RGO catalyst. The 1-hexene conversion increases with increasing reaction
temperature and reaches up to 100% when the reaction temperature is higher than 90
oC. The heptanal yield (n-heptanal & i-heptanal) also exhibits similar trend with the
maximum yield of 71.9% at 100 oC. Moreover, hydrogenation of hexene is promoted
slightly at high reaction temperature (

80 oC), producing more hexane.
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60
50
40
30
20
10
0
n-heptanal
i-heptanal
n/i ratio of heptanal
4
3
2
1
0
70
80
90
100
110
Temperature (oC)
Figure 7 Influence of the reaction temperature on the selectivity of n-heptanal, i-
heptanal and the n/i ratio of heptanal over Rh/RGO catalyst. Reaction conditions:
0.10 g Rh/RGO catalyst, 3.73 g 1-hexene, 5 MPa syngas (CO:H2 = 1:1), 1h
Figure 7 gives the selectivity and n/i ratio of heptanal at different reaction
temperature. Although the total heptanal yield increases with increasing reaction
temperature, the selectivity of n-heptanal decreases obviously, as shown by Figure 7.
It can be found that, due to the accelerated production of i-haptanal, the selectivity of
n-heptanal decreases from 46.6 to 29.6% and the n/i ratio of heptanal decreases from
4.0 to 0.8. Higher reaction temperature usually stimulates the shift of the carbon-
carbon double bond of 1-hexene and then improves the formation of internal hexene
(refer to Scheme 3).30 On the other hand, higher reaction temperature also
significantly accelerates the hydroformylation of internal hexene to form i-heptanal.
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As given by Figure 6, the yield of internal hexene increases initially and then
decreases rapidly with increasing reaction temperature. The hydroformylation of
internal hexene produces only i-heptanal, thus leading to the lower n/i ratio of
heptanal at higher reaction temperature.
Higher reaction temperature promotes the isomerization and hydrogenation of 1-
hexene to produce internal hexene and hexane, instead of the desired 1-hexene
hydroformylation reaction to generate n-heptanal. Moreover, the followed
hydroformylation of internal hexene will be accelerated at high reaction temperature,
which also produces more i-heptanal and decreases the n/i ratio directly. Therefore,
here, we can conclude that lower reaction temperature is beneficial to higher n/i ratio
of heptanal over the Rh/RGO catalyst. In order to avoid the undesired isomerization
and hydrogenation of 1-hexene, low reaction temperature is essential to 1-hexene
hydroformylation over the Rh/RGO catalyst. To further investigate the catalytic
performance of Rh/RGO catalyst, we performed more catalyst tests at the reaction
temperature of 70 oC with varied reaction time, and the reaction results are compared
in Figure 8.
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100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
1-Hexene
Internal hexene
Hexane
Heptanal
0
1
2
3
4
5
6
7
8
9
10
11
Reaction time (h)
Figure 8 The effect of the reaction time on 1-hexene hydroformylation over
Rh/RGO catalyst. Reaction conditions: 0.10 g Rh/RGO catalyst, 3.73 g 1-hexene, 5
MPa syngas (CO:H2 = 1:1), 70 oC
The 1-hexene conversion over Rh/RGO catalyst increases quickly with increasing
reaction time and reaches up to 100 % at 4 h. The heptanal (n-heptanal and i-heptanal)
yield increases sharply before 4h, similar to that of 1-hexene conversion, but rises
slowly after 4 h. At the initial reaction stage before 4 h, the formed heptanal mainly
comes from 1-hexene hydroformylation. But after 4 h reaction, the hydroformylation
of internal hexene derived from 1-hexene isomerization contributes to the rest of
heptanal yield. This hypothesis is well supported by the change of internal hexene
yield as in Figure 8. It is clear that the internal hexene yield increases before 3 h and
then decreases abruptly. The used low reaction temperature, here, can neither
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absolutely restrain 1-hexene isomerization to form internal hexene, nor depress the
hydroformylation of internal hexene to generate i-heptanal. Although the formation
of internal hexene is not inhibited completely,37 the isomerization rate of 1-hexene at
o
70 C is obviously lower than that at high reaction temperature, therefore more 1-
hexene is converted to produce n-heptanal though hydroformylation reaction.
The total heptanal yield increases with increasing reaction time as given in Figure
8. The yield profiles of n-heptanal and i-heptanal, respectively, are shown in Figure 9.
Both of two heptanal yield increase with increasing reaction time, and the n-heptanal
yield is always higher than the i-heptanal yield, that is, the n/i ratio of heptanal is
larger than 1.0 on this Rh/RGO catalyst under the used low reaction temperature.
At the initial stage of reaction, the n/i ratio of heptanal is as high as 4.0, which
means that n-heptanal formation is the major reaction route of 1-hexene
hydroformylation. However, as the reaction proceeds, the 1-hexene isomerization
forming internal hexene and the followed internal hexene hydroformylation to
generate i-heptanal dominate the reaction process, which therefore decease the n/i
ratio of heptanal in products. After the reaction for 4 h, the original 1-hexene is
depleted, the subsequent n-heptanal in theory may be produced only by the
hydroformylation of newly formed 1-hexene from the reverse isomerization of
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internal hexene. After the reaction for 10h, the n/i ratio of heptanal on the Rh/RGO
catalyst still remains at 1.1, similar to that of unmodified Rh complexes in
homogeneous catalysis system,38 but higher than those in the reactions performed at
high temperatures or some other Rh-based heterogeneous system.39, 40
60
4
50
40
30
20
10
0
3
2
1
0
n-heptanal
i-heptanal
n/i ratio of heptanal
0
1
2
3
4
5
6
7
8
9
10 11
Reaction time (h)
Figure 9 Effect of the reaction time on the yield of n-heptanal, i-heptanal and n/i ratio
of heptanal over Rh/RGO catalyst. Reaction conditions: 0.10 g Rh/RGO catalyst,
3.73 g 1-hexene, 5 MPa syngas (CO:H2 = 1:1), 70 oC
Considering the thermodynamic equilibrium between 1-hexene and internal
hexene, it is not easy to completely avoid 1-hexene isomerization and the subsequent
hydroformylation of internal hexene, irregardless of reaction temperature and
reaction time. Higher selectivity of n-heptanal can be obtained only at low
temperature for short reaction time. Therefore, the catalytic activity and
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regioselectivity of n-heptanal by 1-hexene hydroformylation over different catalysts
should be evaluated at low temperature for short time, ensuring that only 1-hexene
takes part in reaction.
3.3 Comparison of Rh/RGO with Rh/CNTs and Rh/AC catalysts
(b)
(a)
(002)
Rh(111)
(100)
Rh(111)
(100)
(002)
spent
spent
fresh
70
fresh
70
10
20
30
40
50
60
80
10
20
30
40
50
60
80
2 Theta (degree)
2 Theta (degree)
(002)
(c)
Rh(111)
(100)
spent
(004)
fresh
70
10
20
30
40
50
60
80
2 Theta (degree)
Figure 10 XRD patterns of fresh and spent catalysts: Rh/RGO (a), Rh/AC (b) and
Rh/CNTs (c)
In addition to the presented Rh/RGO catalyst, we also prepared other carbon
materials supported Rh, like Rh/AC and Rh/CNTs, as the reference catalysts. The
XRD patterns of these catalysts are compared in Figure 10. All the catalysts exhibit
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no obvious Rh peaks except the weak peak (111) of Rh at 41o due to the low loading
of Rh. After reaction, the XRD patterns of these spent catalysts are similar to their
fresh samples.
Table 1 Catalytic performances of 1-hexene hydroformylation over different
Rh/carbon catalysts
Catalyst Conversion (%)
Selectivity (%)
Internal hexene Hexane Heptanal
n/i ratio of
heptanal
Rh/RGO
Rh/AC
38.4
3.6
40.1
36.8
24.1
2.6
57.3
--
4.0
--
63.2
39.7
Rh/CNTs
10.7
36.2
2.8
Reaction conditions: 0.10 g catalyst, 3.73 g 1-hexene, 5 MPa syngas (CO:H2 = 1:1),
70 oC, 1h
Table 1 compares the detailed catalytic performances of 1-hexene
hydroformylation over these carbon materials supported Rh catalysts at 70 oC for 1 h.
All the pure carbon supports (RGO, AC and CNTs), without Rh, were also tested
under same reaction conditions and had no activity for 1-hexene hydroformylation or
other side reactions. The Rh/RGO catalyst realizes the highest 1-hexene conversion
of 38.4% among the tested catalysts. For 1 h reaction, the 1-hexene conversion
obtained by Rh/RGO catalyst is significantly higher than those of Rh/AC and
Rh/CNTs catalyst, indicating the excellent activity of Rh/RGO catalyst. Extending
the reaction time from 1 h to 2 h can increase the 1-hexene conversion of Rh/AC and
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Rh/CNTs, but their activities are still lower than that of Rh/RGO catalyst for 1 h, as
indicated in Figure 11(b).
(b)
(a)
1-Hexene Conversion
n/i of Heptanal
1-Hexene Conversion
n/i of Heptanal
40
30
20
10
0
4
3
2
1
0
40
30
20
10
0
4
3
2
1
0
Rh/CNTs
Rh/RGO
Rh/AC
Rh/CNTs
Rh/RGO
Rh/AC
Figure 11 1-Hexene conversion(black bar) and n/i ratio of heptanal(red bar) of
Rh/RGO, Rh/AC and Rh/CNTs catalysts: (a) under the same reaction time of 1 h; (b)
with similar 1-hexene conversion for different reaction time, Rh/RGO(1 h), Rh/AC(2
h) and Rh/CNTs(2 h). Reaction conditions: 0.10 g catalyst, 3.73 g 1-hexene, 5 MPa
syngas (CO:H2 = 1:1), 70 oC
For the Rh/RGO catalyst, the highest n-heptanal selectivity together with the
lowest i-heptanal selectivity is achieved. And the n/i ratio of heptanal obtained by
Rh/RGO catalyst reaches up to 4.0, the highest one among the tested catalysts. All
these findings indicate that this Rh/RGO catalyst has not only excellent activity, but
also good n-heptanal regioselectivity, by which to get the highest n/i ratio.
The Rh/RGO catalyst shows distinguished catalytic performance, better than the
reference catalysts Rh/AC and Rh/CNTs. This result can be explained by the unique
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structure, larger Rh exposed surface area and good nanoparticles dispersion of
Rh/RGO catalyst.
Table 2 The textural properties of different carbon materials supported Rh catalysts
Catalyst
Rh/RGO
Rh/AC
Exposed Rh surface area (m2 g-1)
Rh dispersion (%)
Size (nm)
6.1
1.60
0.68
1.25
18.12
7.76
14.2
Rh/CNTs
14.16
7.8
As well known, the exposed surface area and dispersion of Rh nanoparticles are
very important to the catalytic activity and n-heptanal selectivity in 1-hexene
hydroformylation. Therefore, H2 chemisorption was utilized to analyze the properties
of the tested catalysts, and Table 2 lists the analysis results. The Rh/RGO catalyst
owns the largest Rh exposed surface area of 1.60 m2 g-1 and the highest dispersion of
18.12% among these catalysts. Similar result is also detected from TEM images
shown in Figure S4†. The layer structure and surface functional groups of GO
support should be beneficial to loading Rh nanoparticles uniformly during Rh/RGO
catalyst preparation. The high exposed surface area, good dispersion and smaller size
of Rh nanoparticles favor CO adsorption and make CO insertion into C=C couple
bonds more feasible, therefore leading to the excellent catalytic performance of
Rh/RGO catalyst in 1-hexene hydroformylation. Furthermore, the abundant crystal
defects of Rh/RGO catalyst and synergetic effects between small building units of Rh
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nanoparticles in the cluster-liked loose structure (Figure 5) also provide more active
sites and ensure its higher activity.
Rh/CNTs
Rh/AC
Rh/RGO
30
45
60
75
90 105 120 135 150
Temperature (oC)
Figure 12 CO-TPD curves of different carbon materials supported Rh catalysts
CO adsorption and the subsequent CO insertion into the C=C couple bonds are the
key steps for alkenes hydroformylation. The activities of catalysts on CO insertion
can be investigated by CO-TPD.41-43 In this report, CO-TPD was employed to
analyze all catalysts, and the results are compared in Figure 12. The CO desorption
o
peaks locate at 61, 60 and 80 C for Rh/RGO, Rh/CNTs and Rh/AC catalysts,
respectively. The desorption temperatures of CO on Rh/RGO and Rh/CNTs catalysts
are clearly lower than that of Rh/AC, indicating that the CO activation is performed
easier on Rh/RGO and Rh/CNTs catalysts. The CO activation is closely related with
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the catalyst activity for 1-hexene hydroformylation.44 Therefore Rh/RGO catalyst and
Rh/CNTs catalyst exhibit higher activity than Rh/AC as shown in Table 1.
In addition, the catalysts structure should also be considered to elucidate their
different catalytic performance. The used AC and CNTs supports have complicated
physical structure, which in fact suppress the diffusion of reactants and products,
especially in a liquid-phase reaction system here. However, the RGO support differs
with AC and CNTs on structure. It has only 2D structure, that is, a flat nano-sheet.
Due to the unique sheet structure and abundant surface functional groups of GO, Rh
nanoparticles can be loaded on its surface uniformly, at the same time with smaller
nanoparticle size, whereby the prepared Rh/RGO catalyst owns larger Rh exposed
surface area and fine dispersion. On the other hand, the 2D structure of Rh/RGO
catalyst will facilitate the diffusion of reactants and products, considerably better than
other Rh/AC and Rh/CNTs catalysts. Unsuppressed diffusion of reactants and
products on this Rh/RGO catalyst will in part contribute to its good activity and
regioselectivity.45 The 1-hexene can reach the active sites of Rh/RGO catalyst more
easily than Rh/AC and Rh/CNTs catalysts, which is beneficial for improving the
reaction rate and producing more n-heptanal. Moreover, the products, especially
internal hexenes, can diffuse and move away from Rh/RGO catalyst immediately,
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reducing the chances of the side reactions (internal hexene hydroformylation) as
shown in Scheme 4. Conversely, for Rh/AC and Rh/CNTs catalysts, when these
internal hexenes diffuse out of the pores, they will contact other active sites to
produce more i-heptanal leading to the lower n/i ratio of heptanal.
Rh
Rh/RGO
Rh/AC or Rh/CNTs
Scheme 4 Different schematic of 1-hexene hydroformylation between Rh/RGO
catalyst and Rh/AC or Rh/CNTs catalyst
Another important factor to enhance the n/i heptanal ratio of Rh/RGO catalyst is
from the difference of diffusion rate of H2 vs CO on each Rh catalyst. On Rh/AC
and Rh/CNTs with a lot of nanopores filled with liquid medium, H2 diffuses more
quickly than CO inside these nanopores, leading to rather higher H2/CO ratio inside
pores even if H2/CO bulk ratio is 1. This enhanced H2/CO ratio can boost H2 partial
pressure around the Rh sites inside the nanopores of AC and CNTs, accelerating 1-
hexene isomerization to internal hexene, consequently increasing i-heptanal
formation rate and decreasing n/i ratio on these two catalysts.
On the other hand, 2D structure of RGO can lower the ratio of H2/CO around the
Rh sites, due to its flat nano-sheet morphology. Relatively lower H2 partial pressure
29