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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-
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
2.2. Catalyst characterization
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-
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-