Enhancing catalytic performance of activated carbon supported Rh catalyst on heterogeneous hydroformylation of 1-hexene via introducing surface oxygen-containing groups

Article abstract of DOI:10.1016/j.apcata.2016.08.024

Activated carbon supported rhodium (Rh/AC) catalysts with different amounts of oxygen-containing functional groups were prepared by nitric acid (HNO₃) treatment at varied temperatures. Thermal analyses of Rh/AC catalysts with or without this acidic treatment were characterized by thermogravimetric analysis (TGA) and temperature programmed desorption (TPD). The change of surface oxygen-containing functional groups was characterized by Fourier transform infrared spectrometry (FTIR) and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the amount of oxygen-containing functional groups increased with the treatment temperature. The influence of these oxygen-containing functional groups on the products selectivities in heterogeneous hydroformylation reaction was investigated in detail. These abundant functional groups were benefited to improve the selectivity of n-heptanal, resulting in higher n/i (normal to iso) ratio of heptanal. The Rh/AC catalyst being treated at 80?°C had the highest n/i ratio of 2.3, due to the maximum amount of oxygen-containing functional groups, which was almost double to that of raw Rh/AC catalyst. Moreover, abundant functional groups on catalyst suppressed hydrogenation of hexene, decreasing the selectivity of hexane from 4.9% of raw Rh/AC to 0.2%. These findings disclosed that these oxygen-containing functional groups on catalysts played an extremely important role in improving the catalytic performance of heterogeneous hydroformylation reaction, providing a new viewpoint for the studies on heterogeneous hydroformylation.

Full text of DOI:10.1016/j.apcata.2016.08.024

Applied Catalysis A: General 527 (2016) 53–59
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Enhancing catalytic performance of activated carbon supported Rh
catalyst on heterogeneous hydroformylation of 1-hexene via
introducing surface oxygen-containing groups
Minghui Tana,b, Ding Wangc, Peipei Aib, Guoguo Liub, Mingbo Wua,∗, Jingtang Zhenga,∗
,
Guohui Yangb,d, Yoshiharu Yoneyamab, Noritatsu Tsubakib,∗
a State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
b Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
c College of Materials Science and Engineering, University of Shanghai for Science & Technology, Shanghai 200093, China
d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2016
Received in revised form 19 August 2016
Accepted 26 August 2016
Available online 28 August 2016
Activated carbon supported rhodium (Rh/AC) catalysts with different amounts of oxygen-containing
functional groups were prepared by nitric acid (HNO3) treatment at varied temperatures. Thermal
analyses of Rh/AC catalysts with or without this acidic treatment were characterized by thermo-
gravimetric analysis (TGA) and temperature programmed desorption (TPD). The change of surface
oxygen-containing functional groups was characterized by Fourier transform infrared spectrometry
(FTIR) and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the
amount of oxygen-containing functional groups increased with the treatment temperature. The influence
of these oxygen-containing functional groups on the products selectivities in heterogeneous hydroformy-
lation reaction was investigated in detail. These abundant functional groups were benefited to improve
the selectivity of n-heptanal, resulting in higher n/i (normal to iso) ratio of heptanal. The Rh/AC catalyst
being treated at 80 ◦C had the highest n/i ratio of 2.3, due to the maximum amount of oxygen-containing
functional groups, which was almost double to that of raw Rh/AC catalyst. Moreover, abundant func-
tional groups on catalyst suppressed hydrogenation of hexene, decreasing the selectivity of hexane from
4.9% of raw Rh/AC to 0.2%. These findings disclosed that these oxygen-containing functional groups on
catalysts played an extremely important role in improving the catalytic performance of heterogeneous
hydroformylation reaction, providing a new viewpoint for the studies on heterogeneous hydroformyla-
Keywords:
Hydroformylation
Syngas
Aldehyde
Surface modification
Activated carbon
tion.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
which have high catalytic activity and selectivity [7–10]. But they
also have some drawbacks, such as very high pressure, noble metal
Hydroformylation of alkene is one of the most important
homogenous metal-catalyzed reactions [1–4]. Aldehyde is formed
by addition of syngas across the double carbon bond (C C) of
1-alkene. Hydroformylation of 1-alkenes produces the mixed alde-
hydes, linear n-aldehydes (n) and branched i-aldehydes (i). But only
n-aldehydes are optimized products in commodity applications
[5,6].
loss, catalyst/product separation difficulty and so on [11]. There-
fore, the development of different heterogeneous catalyst systems
to replace homogeneous catalysts is promising. Heterogenizations
of homogeneous catalysts in aqueous phase, ionic liquids or fluo-
rous phase have been employed for hydroformylation in biphasic
systems [12–15]. Immobilization of homogeneous species onto the
solid Si has also been investigated for heterogeneous hydroformy-
lation [11]. These catalysts can be easily separated and recycled
from the reaction mixtures, but the catalytic active species in these
systems are usually less catalytically active than their homoge-
neous analogues [16]. Moreover, the synthesis procedures of these
catalysts are complicated. Thus, extensive efforts have been made
to exploit solid supported rhodium catalysts with high active by
simple preparation method. So far, many solid catalysts, such as
The traditional hydroformylation of 1-alkene usually uses
homogeneous catalysts such as the rhodium-ligand complexes,
∗
Corresponding authors.
E-mail addresses: wumb@upc.edu.cn (M. Wu), jtzheng03@163.com (J. Zheng),
tsubaki@eng.u-toyama.ac.jp (N. Tsubaki).
http://dx.doi.org/10.1016/j.apcata.2016.08.024
0926-860X/© 2016 Elsevier B.V. All rights reserved.

54
M. Tan et al. / Applied Catalysis A: General 527 (2016) 53–59
2. Experimental
2.1. Catalyst preparation
The Rh/AC catalyst was prepared by an incipient wetness
impregnation method. The AC (40–60 meshes, Kanto Chemical Co.
Inc., Japan) was impregnated by a certain amount of Rh(NO3)3
aqueous solution to yield a 5 wt% Rh loading. After impregnation,
the catalyst precursor was vacuumed at room temperature for 1 h
and dried at 120 ◦C for 12 h. Then the sample was calcined at 400 ◦C
for 2 h under the nitrogen flow. Finally, the catalyst was reduced
by hydrogen at 400 ◦C for 10 h. Surface oxidation treatments on
the obtained Rh/AC catalyst were conducted by being heated in
HNO3 solution (35%) at different temperature for 4 h with magnetic
stirring. After treatment, the catalyst was centrifuged and rinsed
with distilled water, and then dried at 60 ◦C under vacuum for
12 h. According to the temperature of oxidation treatment, these
catalysts were named as Rh/AC-40 (40 ◦C), Rh/AC-60 (60 ◦C) and
Rh/AC-80 (80 ◦C).
Scheme 1. Hydroformylation and isomerization of 1-hexene.
SiO2, Al2O3, zeolite, MOF or carbon materials supported Rh cata-
lysts, have been investigated for the 1-alkene hydroformylation,
which exhibit high reaction activity and aldehyde yields [17–20].
But the selectivity of n-aldehyde is always unsatisfactory. The low
selectivity of n-aldehyde is mainly derived from the competitive
production of i-aldehyde from hydroformylation of terminal alkene
(b, Scheme 1), hydrogenation of alkene (c, Scheme 1), isomeriza-
tion (the migration of C C) of terminal alkene (d, Scheme 1), and
hydroformylation of internal alkene (e, Scheme 1).
2.2. Catalyst characterization
In homogeneous catalysis, the catalyst selectivity can be
adjusted by some electron withdrawing aryl substituents, or func-
tional groups (such as carboxylic acid, ester or ketone groups)
[21–25]. For instance, Wakamatsu et al. found that the shift of car-
bon double bond was strongly influenced by the functional groups
in functionalized internal alkenes [22]. Casey et al. investigated the
different effects of aryl substituents linked at equatorial and apical
diphosphines ligand on the selectivity of n-aldehyde [25]. In het-
erogeneous catalyst, the surface properties of catalysts are also very
important, such as the surface functional groups, heteroatom dop-
ing and hydrophobicity-hydrophilicity balance, which can affect
the catalytic activity and product selectivity [26,27]. However, to
our knowledge, the influence of the surface properties on the cat-
alytic performance of heterogeneous hydroformylation is rarely
reported. For the previous heterogeneous hydroformylation, most
researches were focused on the selection of catalyst supports or
active metals, while the effect of catalyst surface property was often
ignored.
In order to clarify this effect, we selected activated carbon (AC)
as the catalyst support, which is easy to tune the surface prop-
erties by introducing some surface oxygen-containing functional
groups. Generally, HNO3 treatment is a facile and available way
for implanting of surface oxygen-containing functional groups on
the surface of AC. These functional groups on AC can act as nucle-
ation sites to anchor metal ions, which is beneficial the dispersion
of metal ions. Therefore, HNO3 treatment is often used to pretreat
AC before metal immobilization. Some works used AC pretreated
by HNO3 as support to investigate the effect of these groups on the
preparation of catalyst [27–29]. These fixed surface groups could
affect the particle size and dispersion of the loaded active metal on
AC, and alter catalytic performance consequently. However, these
groups introduced by pretreatment, which may play an important
role in adsorption and catalysis in reaction, were often removed
during the catalyst preparation process accompanied by heating
or reduction. Therefore, to investigate the direct influence of these
groups on the catalytic reaction, it is necessary to introduce the
groups on the final prepared catalyst. Herein, considering the sim-
ple modification of AC and strong resistance of Rh to HNO3, AC
supported Rh (Rh/AC) catalyst was treated by HNO3 to introduce
surface oxygen-containing groups on its surface. Then, these solid
catalysts were applied in hydroformylation of 1-hexene to investi-
gate the influences of these groups on catalytic performance. These
treated catalysts with abundant surface oxygen-containing func-
tional groups exhibited enhanced selectivity of n-heptanal.
N2 adsorption-desorption was carried out on Micromeritics
ASAP 2020 sorption analyzer (USA) to determine the BET spe-
cific surface area and the pore volume of samples. The crystalline
structure of samples was confirmed by X-ray diffraction (XRD)
(RINT 2400 diffract meter, Rigaku, Japan). Thermogravimetric anal-
ysis (TGA) was carried out on Shimadzu DTG-60 instrument. After
drying at 100 ◦C for 1 h, the catalysts were heated in the N2 atmo-
sphere from 100 ◦C to 800 ◦C at a rate of 5 ◦C/min. Fourier transform
infrared spectrometry (FTIR) (Thermo Nicolet NEXUS 670, USA)
and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific
ESCALAB 250Xi, USA) were employed to characterize the surface
oxygen-containing functional groups on the catalysts.
Temperature programmed desorption (TPD) and CO tempera-
ture programmed desorption (CO-TPD) were carried out on the
catalyst analyzer BELCAT-B-TT (BEL Co. Ltd., Japan) equipped with a
thermal conductivity detector (TCD) and a mass spectrometry (BEL-
mass). For CO-TPD, sample was first pretreated in He flow at 150 ◦C
for 1 h, cooled to 40 ◦C, then to be saturated with CO for 1 h. Sam-
ples were subsequently purged with helium gas at 40 ◦C for some
time to remove physical adsorption of CO until no CO signal in the
effluent could be detected by TCD. CO-TPD profiles were obtained
by heating the sample at a heating rate of 10 ◦C/min in He flow. On
the other hand, TPD curves were recorded with the same manner as
CO-TPD without pre-absorption of CO. The desorbed products were
CO and CO2 as detected by mass spectrometry. In order to elimi-
nate the decomposition of surface groups on catalysts, the curves
of desorbed CO and CO2 in CO-TPD subtracted those data from TPD.
2.3. Catalytic activity test
The hydroformylation reaction was conducted at 70 ◦C in an
autoclave under continuous stirring. 0.10 g of catalyst and 3.73 g of
1-hexene were loaded into the autoclave. After flux with 1.0 MPa
syngas thrice to purge the residual air, 5.0 MPa syngas was sealed
in the reactor at the room temperature. The molar ratio of the uti-
lized syngas was CO:H2 = 1:1. After the reaction, the liquid products
were analyzed quantitatively by a gas chromatograph (Shimadzu
GC2014) equipped with a capillary column (InertCap 5, length:
30 m) and a flame ionization detector (FID).
3. Results and discussion
The N2 adsorption/desorption isotherms obtained at 77 K for AC,
Rh/AC and treated Rh/AC catalysts were shown in Fig. S1 (Supple-

M. Tan et al. / Applied Catalysis A: General 527 (2016) 53–59
55
Table 1
The pore parameters and surface chemical properties of AC and catalysts.
Materials
Specific surface area (m2/g)
Pore volume (cm3/g)
Total amount (mmol/g)a
C1s(%)b
Total
Micropore
Mesopore
C
C(C C)
C
–
13.0
13.1
13.7
11.9
O
C
–
6.3
7.7
8.9
8.4
O
O
C O
AC
Rh/AC
Rh/AC-40
Rh/AC-60
Rh/AC-80
1064
1145
1017
951
0.62
0.67
0.63
0.58
0.57
0.54
0.56
0.48
0.46
0.45
0.08
0.11
0.15
0.12
0.12
–
–
–
7.6
8.9
10.3
12.5
2.15
3.82
4.24
5.57
73.1
70.3
70.1
68.7
944
a
The total amount of surface oxygen-containing functional groups obtained by TPD.
The relative contents of functional groups obtained from XPS C1s spectra.
b
Fig. 2. TPD profiles of catalysts.
Fig. 1. TGA curves of catalysts in N2.
mentary material). All samples exhibited typical type-I isotherm
according to IUPAC classification, which was the characteristic of
microporous material. The BET specific surface area and pore data
calculated from the isotherms were listed in Table 1. The specific
surface area and pore volume of Rh/AC catalyst were slightly larger
than those of raw AC, indicated that the low loading amount and
fine dispersion of Rh did not block the pore of raw AC and the
reduction process further developed the pore structure of raw AC.
After being treated by nitric acid, the surface areas and pore vol-
umes of treated Rh/AC catalysts decreased little with the increasing
treatment temperature. This slight decrease could be attributed
to the increase of catalyst mass due to the introduction of oxy-
gen by treatment, which led to the relative decrease of the surface
area and pore volume of per unit mass catalyst. In addition, the
decrease was very limited and negligible, indicating that the nitric
acid treatment did not destroy the structure of Rh/AC. The similar
result could also be obtained by XRD. The XRD patterns of these
catalysts were compared in Fig. S2 (Supplementary material). After
nitric acid treatment, the crystalline structures of treated Rh/AC
catalysts were similar to that of the raw Rh/AC catalyst.
Fig. 3. FTIR spectra of catalysts.
Nitric acid treatment can introduce a significant number of
oxygen-containing functional groups onto the carbon surface,
which include carboxylic, lactone, phenolic hydroxyl and so on
[30,31]. Due to the pyrolysis of these introduced groups, the
thermal weight losses of materials are different which can be inves-
tigated by TGA in N2. As compared in Fig. 1, it was obvious that the
weight losses of treated-catalysts gradually became larger with the
increasing treatment temperature, indicating that the amount of
surface oxygen-containing functional groups on Rh/AC increased
with the increasing treatment temperature.
ference of samples before and after the surface treatment. The TPD
profiles of catalysts were displayed in Fig. 2. The raw Rh/AC cat-
alyst exhibited only one peak at the lower temperature of 230 ◦C,
which might be attributed to CO2 decomposed from carboxylic acid
group on the AC [34]. Different from the raw Rh/AC, the treated cat-
alysts exhibited multiple peaks, due to the increasing groups after
treated by nitric acid. Except the broad peak at temperature lower
than 300 ◦C, the broad peak around 640 ◦C belonged to the lactone
groups or carbonyl groups, and the peak at 350 ◦C of Rh/AC-80 could
be attributed to anhydride [31,33].
As an effective technique to characterize surface properties of
carbon materials [31–35], TPD was also used to investigate the dif-
The surface properties of Rh/AC before and after the surface
treatment were also characterized by FTIR, as shown in Fig. 3.

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M. Tan et al. / Applied Catalysis A: General 527 (2016) 53–59
Fig. 4. Surface oxygen content obtained from XPS (a) and typical XPS spectra of catalysts: Rh3d (b), C1s (c), and O1s (d).
For the raw Rh/AC catalyst, there was only one peak of OH at
3430 cm−1, and there existed no obvious peaks for other oxygen-
containing functional groups, indicating the low content of oxygen
on the surface of Rh/AC. After being treated by nitric acid, some
new peaks appeared on the modified Rh/AC catalysts, such as
Rh metal, the treatment by HNO3 could produce abundant oxygen-
containing functional groups effectively. C1s and O1s peaks were
fitted to analyze the categories and amounts of functional groups
on catalysts as presented in Fig. 4(c) and (d), respectively. As in
Fig. 4(c), the binding energies of 284.3 and 285.2 eV were assigned
for the C C and C C bonds, respectively. And the binding ener-
gies of 286.4, 287.5 and 288.8 eV were attributed to the C O, C O,
C
O (epoxy) at 1220 cm−1, C O (quinone or ionoradical struc-
ture) at 1583 cm−1 and C O (carbonyl or carboxyl) at 1716 cm−1
(Rh/AC-60 and Rh/AC-80) [36]. This indicated that more oxygen-
containing groups were introduced on Rh/AC surface after nitric
acid treatment. To improve understanding the changes of sur-
face oxygen-containing functional groups on catalysts before and
after the treatment, XPS was employed to investigate the sur-
face property. As shown in Fig. 4(a), the surface oxygen content
increased along with the increasing treated temperature. The sur-
face oxygen content of Rh/AC-80 was as high as 23.81 at%, much
higher than 9.12 at% of untreated Rh/AC, demonstrating the intro-
duction of large number of oxygen-containing groups successfully
after treatment, being coincided well with the results obtained by
TGA. Moreover, for raw Rh/AC catalyst, Rh exhibited two states as
Rh0 (307.1 eV) and Rh+ (308.3 eV) in Fig. 4(b). Both Rh0 and Rh+
are active for the hydroformylation reaction. Moreover, a certain
amount of Rh1+ is beneficial to the CO insertion and the suppres-
sion of hydrogenation, which can improve the reaction rate and
activity for 1-alkene hydroformylation [37,38]. Even after being
treated by nitric acid, the state of Rh had almost no difference
between raw Rh/AC and Rh/AC-80, being attributed to the resis-
tance of Rh to HNO3. Based on avoiding the oxidation and loss of
and O
C OH oxygen-containing functional groups [34,35,39,40].
Table 1 listed the total amount and relative contents of these groups
obtained from TPD and XPS C1s spectra, respectively. It is clear
that the amount of functional groups increased obviously with the
increasing of treated temperature. Especially, the relative contents
of C O and O
C OH for Rh/AC-80 become much larger than those
of Rh/AC. The similar results can be observed from the O1s XPS
spectra as in Fig. 4(d). The peaks of C O in esters, carbonyls and
carboxyl (531.0–532.5 eV) increased obviously.
The catalytic performances of 1-hexenehydroformylation over
various catalysts were compared in Figs. 5 and 6. Fig. 5 showed
the effect of reaction time on the conversion of 1-hexene over
various catalysts. For all catalysts, the conversion of 1-hexene
increased with increasing reaction time. However, compared to
raw Rh/AC catalyst, the conversions of these modified catalysts
decreased slightly with the increased treatment temperature under
the same reaction time. There are two main probable reasons for
this decrease: 1) the increase of oxygen content led to the relative
decrease of Rh content in 0.1 g catalyst (5.05%, 4.84%, 4.59% and
4.22% for Rh/AC, Rh/AC-40, Rh/AC-60 and Rh/AC-80 respectively);

M. Tan et al. / Applied Catalysis A: General 527 (2016) 53–59
57
Scheme 2. Schematic diagram of Wilkinson’s hydroformylation mechanism.
Fig. 5. The effect of reaction time on the conversion of 1-hexene over varied cata-
lysts. Reaction conditions: 0.1 g catalyst, 3.73 g 1-hexene, 70 ◦C, 5 MPa, H2:CO = 1:1.
Scheme 3. Schematic diagram of the influence of groups on hydroformylation.
2) the increase of support’s hydrophilicity reduced the diffusion
rate of 1-hexene in the pores of AC. Although the conversions of the
modified catalysts were slightly lower than that of the raw Rh/AC
catalyst, they exhibited higher selectivity of n-heptanal and lower
selectivity of hexane as shown in Fig. 6.
With the increased treatment temperature, the n/i ratio of hep-
tanal was enhanced significantly. After treatment by nitric acid
at 80 ◦C for 4 h, Rh/AC-80 showed the highest selectivity of lin-
ear product and the n/i value of heptanal reached 2.3, higher than
1.2 of Rh/AC. The increased n/i ratio should be attributed to the
added functional groups, especially the carbonyl or carboxylgroup.
According to the Wilkinson’s dissociative hydroformylation mech-
anism, the key step to control aldehyde regioselectivity was the
hydride addition step as shown in Scheme 2 [41,42]. In order to
improve the selectivity of n-aldehyde, hydrogenation to the C at
␤ position (␤-C) of 1-alkeneshould be expected, as in Scheme 2.
However, according to the Markovnikov’s rule, the H was easier
to be attached to the carbon with more H atoms, which likely
resulted in the addition of H to the C at ␣position (␣-C) of 1-
alkene during the hydride addition step [43,44]. C O groups could
absorb and conjugate with C C of 1-hexene connected with Rh,
indirectly affecting the action of Rh on the rate and the regiose-
lectivity of the reaction. After being treated by nitric acid, these
introduced oxygen-containing functional groups around the Rh
active sites might conjugate with ␣-C of 1-alkene connected with
Rh (Scheme 3). Acting as electron-withdrawing groups, these car-
boxyl or carbonyl groups increased the negative charge of ␣-C
under the action of inductive effect. The intermediate formed from
hydrogenation of ␣-C in this state was unstable. Therefore, during
the hydride addition step, Rh might add the H to ␤-C of 1-alkene to
form a more stable intermediate, which altered the catalytic action
of Rh and achieved an anti-Markovnikov’s structure, in favor of
n-heptanal production and boosted up the n/i ratio of heptanal.
Fig. 6. (a) The conversion of 1-hexene and n/i ratio of heptanal of varied catalysts;
(b) the products distribution of varied catalysts. Reaction conditions: 0.1 g catalyst,
3.73 g 1-hexene, 70 ◦C, 1 h, 5 MPa, H2:CO = 1:1.

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M. Tan et al. / Applied Catalysis A: General 527 (2016) 53–59
Fig. 8. Reusability test of Rh/AC-80 catalyst for hydroformylation of 1-
hexene.Reaction conditions: 0.1 g catalyst, 3.73 g 1-hexene, 70 ◦C, 1 h, 5 MPa,
H2:CO = 1:1.
Rh/AC and Rh/AC-80 catalysts, respectively. And the CO desorp-
tion peak of Rh/AC-80 was rather broader than that of Rh/AC. The
higher desorption temperature and broader peak of CO desorption
for Rh/AC-80 elucidate the stronger adsorption of CO on Rh/AC-
80. For Rh/AC catalyst, the CO2 desorption peak located at 252 ◦C.
Different from Rh/AC catalyst, there was no obvious CO2 desorp-
tion peak for Rh/AC-80 catalyst. It suggests that these introduced
oxygen-containing functional groups are disadvantageous for CO
desorbed as CO2, which is alternatively in favor of CO insert reac-
tion and improving the hydroformylation of 1-hexene.Therefore,
the selectivity of hexane decreased from 4.9% of Rh/AC to 0.2%.
The recycling of Rh/AC-80 catalyst in hydroformylation of 1-
hexene was tested though four cycles under the same reaction
conditions, as shown in Fig. 8. It was found that the 1-hexene con-
version and products selectivities of the repeated catalyst were
maintained after 4 cycles. To investigate the leaching of Rh, the
Rh loadings of the recycled catalysts after each cycle were deter-
mined by ICP. No obvious decrease of Rh loading of Rh/AC-80 after
four cycles was observed, demonstrating its reusability. It should be
added that the liquid filtered and separated from the reaction mix-
ture after a reaction was added into 5 mL 1-hexene to react under
the same reaction conditions but no activity was detected, proving
that Rh leaching did not happen.
Fig. 7. CO-TPD-mass profiles of Rh/AC (a) and Rh/AC-80 (b) catalysts.
It is necessary to point out that the treated catalysts effec-
tively suppressed production of hexane, which was a common
byproduct inhydroformylation of 1-hexene. The selectivity of hex-
ane over treated catalysts was lower than that of raw Rh/AC. It
could be also attributed to the existence of the surface oxygen-
containing functional groups, which affected the chemisorbed CO
on Rh catalyst. It was reported that the thermal desorption pat-
tern of the chemisorbed CO on supported metal catalysts was
correlated closely with their catalytic activities for hydrogenation
and hydroformylation of alkenes [45,46]. The chemisorbed CO on
metal usual desorbed in two ways: desorption as CO at low tem-
perature or desorption as CO2 at high temperature. Desorption of
CO at low temperature indicated the strong carbon-oxygen bond
and weak metal-carbon bond, therefore the cleaving of metal-
carbon bond CO and keeping CO structure would be easy during the
reaction. In contrast, desorption of CO2 at high temperature indi-
cated the strong metal-carbon bond and weak carbon-oxygen bond.
The strong metal-carbon bond made CO difficult to desorb while
carbon-oxygen bond easy to dissociate. Therefore, desorbed CO2 at
high temperature need higher energy for cleaving CO bond, which
was disadvantageous for CO insert reaction, such as hydroformyla-
tion, consequently leading to produce more alkene hydrogenation
product. Here CO-TPD-mass was utilized to characterize the CO
desorption behavior of Rh/AC and Rh/AC-80 catalysts.
In order to verify the influence of catalyst’s oxygen-containing
functional groups on hydroformylation of 1-alkenes, other sub-
strates were used to hydroformylaton reaction. For 1-heptene,
1-octene and 1-decene, the n/i ratios of aldehydes obtained by
Rh/AC-80 catalyst were higher than those of Rh/AC catalyst as
shown in Fig. 9.
4. Conclusions
Rh/AC catalyst was treated by HNO3 at varied temperatures to
implant oxygen-containing functional groups. These catalysts were
investigated to clarify the influence of catalyst’s oxygen-containing
functional groups on hydroformylation of 1-hexene for the first
time. These oxygen-containing functional groups could alter the
surface property of catalysts, thus affect the adsorption and reac-
tion between the active sites and reactants, which was beneficial to
improve the selectivity of n-heptanal in hydroformylation. There-
fore, the treated catalysts exhibited enhanced n/i ratio of heptanal.
Especially Rh/AC catalyst treated by HNO3 at 80 ◦C realized the
highest n/i ratio as 2.3, demonstrating that more functional groups
The results for CO2 and CO desorption during the TPD were com-
pared in Fig. 7. The CO desorption peak located at 77 and 102 ◦C for

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