Promotional effects of Cr and Fe on Rh/SiO2 catalyst for the preparation of ethanol from CO hydrogenation

Article abstract of DOI:10.1039/c5ra28075c

A series of Cr and Fe promoted Rh-based silica supported catalysts were prepared by an incipient co-impregnation method. The performance of the catalysts was investigated by the hydrogenation of carbon monoxide to ethanol. The catalysts were characterized by N₂ adsorption-desorption, X-ray diffraction, transmission electron microscopy, H₂ temperature-programmed reduction, temperature programmed surface reaction, and Fourier transform infrared spectroscopy. The results revealed that addition of 0.2-0.4 wt% Cr to Rh/SiO₂ increased its activity significantly. However, further addition of 0.1-0.2 wt% Fe lowered CO conversion slightly, while the selectivity towards ethanol was enhanced significantly. Therefore, high catalytic activity and selectivity toward ethanol were achieved by simultaneous introduction of 0.4 wt% Cr and 0.2 wt% Fe to Rh/SiO₂. This may be due to improved dispersion of Rh particles, moderate ability to dissociate CO molecules, and high ratio of Rh⁺ sites to Rh⁰ sites.

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Promotional effects of Cr and Fe on Rh/SiO2 catalyst for the
preparation of ethanol from CO hydrogenation
Received 00th January 20xx,
Accepted 00th January 20xx
Fei Xuea, c, Weimiao Chena, Xiangen Songa, Xianbo Chenga, Yunjie Dinga, b
*
A series of Cr and Fe promoted Rhꢀbased silica supported catalysts were prepared by an incipient coꢀimpregnation method.
The performance of the catalysts was investigated by the hydrogenation of carbon monoxide to ethanol. The catalysts were
characterized by N2 adsorptionꢀdesorption, Xꢀray diffraction, transmission electron microscopy, H2 temperatureꢀ
programmed reduction, temperature programmed surface reaction, and fourier transform infrared spectroscopy. The results
revealed that addition of 0.2–0.4 wt% Cr to Rh/SiO2 increased its activity significantly. However, further addition of 0.1–0.2
wt% Fe lowered CO conversion slightly, while the selectivity towards ethanol was enhanced significantly. Therefore, high
catalytic activity and selectivity toward ethanol were achieved by simultaneous introduction of 0.4 wt% Cr and 0.2 wt% Fe
to Rh/SiO2. This may be due to improved dispersion of Rh particles, moderate ability to dissociate CO molecules, and high
ratio of Rh+ sites to Rh0 sites.
DOI: 10.1039/x0xx00000x
www.rsc.org/
and many promoters such as Mn25, V26, La6, and Zr27 have been
used with Rhꢀbased catalysts. Cr2O3 has been used as a
1. Introduction
catalyst28,29 for alkane dehydrogenation or
a structural
promoter30–32 for methanol synthesis from syngas. On the other
hand, some studies1,33 have suggested that the ethanol yield was
improved as Rhꢀbased catalysts were supported over Cr2O3.
However, detailed studies on the role of Cr2O3 in the Rhꢀbased
catalysts for CO hydrogenation have not yet been addressed.
Moreover, the promotion effect of Fe on Rhꢀbased catalysts has
been extensively studied.34–36 Many studies37,38 have
demonstrated that the addition of Fe to Rhꢀbased catalysts can
boost the hydrogenation of acetaldehyde to form ethanol and
decrease methane selectivity. Therefore, we speculated that the
selectivity of the catalyst towards ethanol formation might be
increased by combining the merits of both Cr and Fe.
Ethanol, as a clean fuel, offers the same chemical energy as that
of gasoline with less emission of greenhouse gases and other
environmental pollutants attracting increasing attention.
Currently, ethanol is produced by fermentation of sugars and
hydration of petroleumꢀbased ethylene.1 However, the poor
energy efficiency of the fermentation process and depletion of
crude oil resources globally have limited the use of ethanol in
automobiles. Therefore, development of alternate technologies
to synthesize ethanol from syngas (CO and H2) derived from
coal, natural gas, or biomass has attracted global attention in
recent years. So far, four types of catalysts have been primarily
employed for this conversion: (a) Rhꢀbased catalysts2–8, (b)
modified methanol synthesis catalysts9,10, (c) modified Fischer–
Tropsch catalysts based on Co11–13, Fe14, and Ru15, and (d)
modified Moꢀbased catalysts16. Among these catalysts, Rhꢀ
based catalysts have been demonstrated to be the most selective
catalysts for the synthesis of C2+ oxygenates due to the unique
CO adsorption behavior on Rh particles. In other words, Rh
species are able to simultaneously adsorb CO in both molecular
and dissociated states.17–24
In this study, we investigated the promoting effects of Cr and
Fe on silica supported Rhꢀbased catalysts for the synthesis of
ethanol from CO hydrogenation.
2. Experimental
2.1 Catalyst preparation
First, SiO2 (20–40 mesh) was calcined at 800 °C for 6 h prior to
its use. Then, the catalysts were prepared by impregnating SiO2
with an aqueous solution of RhCl3·3H2O, Cr(NO3)3·9H2O and
Fe(NO3)3·9H2O. Following impregnation, the catalysts were
first dried at room temperature, then dried at 100 °C for 12 h,
and finally calcined at 300 °C for 3 h. The amount of Rh in all
catalysts was 1.5 wt%. The asꢀprepared catalysts are labeled as
Extensive studies have shown that promoters are essential for
Rhꢀbased catalysts to synthesize C2+ oxygenates from syngas
RCF(
x,
y)/SiO2, where
x,
y
indicates the contents of Cr and Fe
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respectively (
0.4 wt%, based on SiO2).
x
= 0.0, 0.2, 0.4 or 0.6 wt%;
y
= 0.0, 0.1, 0.2, or was passed through a condenser and subsequently analyzed onꢀ
line by an Agilent 3000A Micro GC with thermal
a
conductivity detector. After the reaction reached stability, the
liquid products dissolved in deionized water in the condenser
2.2 Catalyst characterization
were collected for
5 h and analyzed offꢀline by an
The specific surface area, pore diameter, and pore volume of
the samples were obtained by N2 adsorptionꢀdesorption at
−196 °C using a physical adsorption instrument (Quantachrome,
USA). Prior to analysis, each sample was preꢀtreated under
vacuum at 120 °C for 3 h.
Agilent 7890 GC with a flame ionization detector using nꢀ
pentanol as an internal standard. The CO conversion and
product selectivity were calculated according to the following
equations:
n × M
∑
i
i
Xꢀray diffraction (XRD) patterns were recorded on a
PANalytical X’ PertꢀPro diffractometer operated at 40 kV and
CO conversion [%] =
×100 ,
MCO
40 mA using Niꢀfiltered Cu Kα (λ = 0.15406 nm) radiation.
ni × Mi
Selectivity [%] =
×100
Transmission electron microscopy (TEM) images were
recorded on a JEOL JEMꢀ2100 electron microscope operated at
200 kV.
H2 temperatureꢀprogrammed reduction (TPR) was carried out
on Altamira Instruments AMIꢀ300 U. First, approximately 100
mg of the catalyst sample was pretreated at 120 °C for 1 h
under Ar (99.999% purity) and then cooled to 50 °C.
Subsequently, 10% H2/Ar was introduced with a flow rate of 30
ml/min and the reduction was carried out at a temperature ramp
rate of 10 °C/min.
n × M
∑
i
i
Where
n
i was the number of carbon atoms in product
i, Mi is the
percentage of product
i
detected, and MCO is the percentage of
CO in the syngas feed. The experimental error is about ±5%.
3. Results and discussion
3.1 Catalytic performance
Table 1 lists the results of the CO hydrogenation reaction over
RhꢀCrꢀFe/SiO2 catalysts with different loading amounts of Cr
and Fe. The CO conversion over Rh/SiO2 was only 4.6%, and
the main product was CH4. As Cr was added to the Rh/SiO2, the
CO conversion and ethanol selectivity increased significantly,
reaching 14.5% and 11.7%, respectively when Cr loading
increased to 0.4 wt%. Further increase in the Cr loading amount
lowered the catalytic performance. Therefore, the Cr loading
was fixed at the optimal 0.4 wt% in the Rh/SiO2 catalysts when
we intended to add the second promoter Fe. It was found that
CO conversion decreased slowly. On the other hand, the
selectivity towards ethanol reached a maximum of 26.0% as 0.2
wt% Fe was added to Crꢀpromoted Rh/SiO2. Further increase in
Fe loading caused an abrupt decrease in the CO conversion to
6.1% and the selectivity towards ethanol to 16.1%. In addition,
the referenced Feꢀpromoted Rh/SiO2 with Fe loading of 0.2 wt%
was also prepared and investigated in CO hydrogenation. It was
found that the CO conversion changed little compared with
Rh/SiO2; and that the selectivity towards ethanol was only
14.8%.
Temperature programmed surface reaction (TPSR) analysis
was also performed on Altamira Instruments AMIꢀ300 U.
Approximately 100 mg of the catalyst sample was used for each
test. Each sample was reduced in situ by 10% H2/Ar at 350 °C
for 2 h, purged with Ar (99.999% purity) at the same
temperature for 0.5 h, and then cooled to 50 °C. Then, 10%
CO/He was introduced for adsorption for 0.5 h. Subsequently,
the sample was flushed with 10% H2/Ar, while the temperature
was increased to 800 °C at the rate of 10 °C /min. The signal of
CH4 (m/z = 16) was recorded by a quadruple mass spectrometer
simultaneously.
Fourier transform infrared (FTIR) spectra were collected
with a Nicolet iS50 spectrometer. Before each experiment, the
samples were pressed into selfꢀsupported wafers (ca. 20
mg/cm2) and reduced in situ in the IR cell under H2 gas flow
(99.999% purity) at 350°C for 2 h. Subsequently, the sample
was swept with N2 (99.999% purity) at 350 °C for 0.5 h, and
then cooled to 50 °C. After cooling to 50 °C in N2, a
background spectrum was recorded with spectral resolution of 4
cm–1 and scan times of 32. Then the sample was purged with
CO (99.999% purity) for 0.5 h, followed by flushing with N2.
3.2 N2 adsorption results
2.3 Activity tests
The N2 adsorption results of the support and catalysts are
shown in Table 2. The specific surface area, pore size, and total
pore volume for the support and catalysts were in the range of
133.0–138.0 m2/g, 8.60–8.75 nm, and 0.78–0.85 cm3/g,
respectively. The composition of catalysts had little effect on
the physical properties of the catalysts, which might be due to
the low metal loading. Therefore, the physical properties of the
catalysts are likely not the major factor influencing the catalytic
performance.
CO hydrogenation was performed in a fixedꢀbed microꢀreactor
(length of 300 mm and internal diameter of 9 mm) at 280 °C,
5.0 MPa (H2/CO = 2), and gas hourly space velocity (GHSV) =
5000 h−1. The catalyst (0.95 g) and quartz sand (3.0 g) were
mixed together to avoid channeling and hot spots, and axially
centered in the reactor tube with the temperature monitored by
a thermocouple. The catalysts were first reduced in situ in a
pure H2 atmosphere at 350 °C for 2 h, and then the catalyst bed
was cooled down to the reaction temperature, followed by the
introduction of syngas. The effluent from the fixedꢀbed reactor
2 | J. Name., 2012, 00, 1-3
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Table 1 Results of CO hydrogenation over Rhꢀbased catalysts with loading amounts of promoters a
Selectivity of products (C %)
Cr
(wt%)
Fe
(wt%)
CO conversion
(%)
CH4
71.3
69.1
64.6
72.1
43.4
66.8
58.2
62.0
C2+ HC b
14.7
12.0
11.1
9.1
CO2
0.2
0.0
0.0
0.0
0.8
0.0
0.0
0.5
MeOH c
HAc d
EtOH e
3.0
HOAc f
0.0
C2+Oxy g
2.0
0.0
0.2
0.4
0.6
0.0
0.4
0.4
0.4
0.0
0.0
0.0
0.0
0.2
0.1
0.2
0.4
4.6
10.5
14.5
9.1
0.0
8.8
0.5
8.6
7.9
0.1
1.8
0.8
8.3
11.7
10.3
14.8
19.7
26.0
16.1
1.2
2.3
0.6
5.5
0.6
1.8
5.4
0.8
39.6
3.0
0.0
0.0
0.6
14.0
10.2
6.1
5.6
2.6
0.7
1.6
2.5
10.5
19.2
0.9
0.0
1.9
1.2
0.0
0.0
1.0
a Reaction conditions: 0.95 g catalyst, 3.0 g quartz sand, 280 °C, 5 MPa, H2/CO = 2, GHSV = 5000 h−1
Acetaldehyde. e Ethanol. f Acetic acid. g Oxygenates with more than two carbon atoms.
.
b Hydrocarbons with more than two carbon atoms. c Methanol. d
Table 2 Physical properties of the support and catalysts
Fig. 2 TEM images of reduced RCF(
x
,
y
)/SiO2 catalysts: (a)
x
x
=
=
0.0,
0.4,
y
y
= 0.0; (b)
= 0.2.
x
= 0.0,
y
= 0.2; (c)
x
= 0.4, = 0.0; and (d)
y
Specific surface
area (m2/g)
Pore size
(nm)
Pore volume
(cm3/g)
Catalysts
SiO2
137.5
134.4
133.6
134.9
135.9
8.69
8.73
8.69
8.71
8.66
0.84
0.80
0.80
0.78
0.80
Table 3 Average Rh particle size (nm) in the catalysts estimated
from XRD and TEM
RCF(0.0, 0.0)/SiO2
RCF(0.0, 0.2)/SiO2
RCF(0.4, 0.0)/SiO2
RCF(0.4, 0.2)/SiO2
Catalysts
XRD a
4.4
4.4
–
TEM b
4.3
RCF(0.0, 0.0)/SiO2
RCF(0.0, 0.2)/SiO2
RCF(0.4, 0.0)/SiO2
RCF(0.4, 0.2)/SiO2
4.2
3.3 XRD and TEM analysis
2.3
–
2.5
a Calculated by using the Scherrer equation. b Estimated from TEM micrographs
by counting more than 150 particles.
Fig. 1 shows the XRD patterns of the reduced catalysts. Two
diffraction peaks at 41° and 47° were observed over nonꢀ
promoted Rh/SiO2, which are attributed to Rh crystal39. Also, a
broad diffraction peak attributed to amorphous SiO2 (2θ = 22°)
was observed. The addition of Fe to Rh/SiO2 did not cause any
significant changes compared with nonꢀpromoted Rh/SiO2.
However, as the Cr was added to the Rh/SiO2, the intensity of
the diffraction peaks attributed to Rh crystal decreased
significantly, indicating that the addition of Cr increased the
dispersion of Rh. Fig. 2 shows the TEM images of reduced Rhꢀ
CrꢀFe/SiO2 catalysts with different compositions. The Rh
particles were well dispersed over the SiO2 support in all cases.
The sizes of Rh particles calculated from XRD and TEM are
listed in Table 2. It can be seen clearly that the size of Rh
particles decreased from approximately 4.3 nm to 2.4 nm by
addition of Cr to Rh/SiO2. Some promoters such as Sm26, Zr27
and La37 could also increase the dispersion of Rh particle on
silica. Nonetheless, little change in Rh particle size was
observed after addition of Fe to Rh/SiO2. This might be
attributed to the fact that the mode of contact between Rh and
Cr is different to that between Rh and Fe, which is discussed in
more details in section 3.5.
Fig. 1 XRD patterns of reduced RCF(
x
,
y
)/SiO2 catalysts: (1)
x
=
= 0.0,
0.4,
y
= 0.0; (2)
x
= 0.0,
y
= 0.2; (3)
x
= 0.4, = 0; and (4)
y
x
y
= 0.2.
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3.4 TPR
temperature compared with nonꢀpromoted Rh/SiO2. Therefore,
the ability of Rh to dissociate CO molecules was enhanced by
the addition of Cr, while it was decreased by the addition of Fe.
For Crꢀ and Feꢀ promoted Rh/SiO2, the low temperature
desorption peak was located between RCF(0.4, 0.0)/SiO2 and
RCF(0.0, 0.2)/SiO2, indicating a moderate ability to dissociate
CO molecules.
Fig. 3 TPR profiles of RCF(
x
,
y
)/SiO2 catalysts: (1)
x
x
= 0.0,
= 0.4,
y
y
=
=
0.0; (2)
0.2.
x
= 0.0,
y
= 0.2; (3)
x
= 0.4, = 0.0; and (4)
y
The TPR profiles of RCF(x, y)/SiO2 catalysts are shown in Fig.
3. A single peak at 110 °C was observed in Rh/SiO2 which is
due to the reduction of Rh3+. This peak shifted to 118 °C after
the addition of Fe to Rh/SiO2, indicating the close contact
between Rh and Fe, which is in agreement with previous
reports in literature37. In addition, a weak peak between 300 °C
and 350 °C caused by the reduction of Fe3+ was also observed
over RCF(0.0, 0.2)/SiO2. In the case of RCF(0.4, 0.0)/SiO2, a
broad peak with a shoulder at around 153 °C was observed. The
peak at 128 °C was assigned to the reduction of Rh3+, while the
shoulder peak can be attributed to the reduction of Cr species.
The results indicated that the addition of Cr or Fe to Rh/SiO2
inhibited the reduction of Rh3+. Similar phenomenon was also
observed over V26 or La37 promoted Rh/SiO2 catalysts. On the
other hand, simultaneous introduction of Cr and Fe shifted the
broad peak to higher temperature with higher peak area,
indicating that the reduction of Rh species over RCF(0.4,
0.2)/SiO2 catalyst was further suppressed and more promoter
species got reduced. These results showed that Rh is in intimate
contact with Cr and Fe, which might contribute to the improved
CO conversion and the selectivity towards ethanol.
Fig. 4 CH4 desorption from TPSR of RCF(
x
,
y
)/SiO2 catalysts:
(1)
(4)
x
x
= 0.0,
= 0.4,
y
y
= 0.0; (2)
= 0.2.
x
= 0.0,
y
= 0.2; (3)
x
= 0.4, = 0.0; and
y
FTIR spectroscopy of adsorbed CO provides an efficient tool
to study the interaction between CO and catalysts, which could
help determine the surface site that CO resides on.41 Fig. 5
shows the FTIR spectra of CO adsorbed on RCF(x, y)/SiO2
catalysts. For nonꢀpromoted Rh/SiO2, two broad peaks at ca.
2042 cm–1 and 1897 cm–1 were clearly observed, which were
attributed to linear CO [CO(l)] and bridged CO [CO(b)] formed
on Rh0 sites, respectively.41,42 Moreover, two weak peaks at ca.
2092 cm–1 and 2026 cm–1 also appeared in the COꢀFTIR of
Rh/SiO2, which was associated with germinal CO [CO(gdc)]
formed on the Rh+ sites.41,42 After the addition of Cr to Rh/SiO2,
CO adsorption was enhanced and the position of CO(l) shifted
to higher wavenumber slightly. These results indicated that
introduction of Cr improved the dispersion of Rh particles43,
which is in line with the results of XRD and TEM. However,
after the addition of Fe to Rh/SiO2, the formation of CO(b) was
suppressed. It is well known that CO(b) forms on large Rh
particles.41 Nevertheless, the introduction of Fe to Rh/SiO2 had
negligible effect on the size of Rh particles (Table 3). Thus,
FeOx might tend to cover Rh particle, which does not change
the size of Rh particles but decreases the CO(b). On the other
hand, introduction of Cr increased the dispersion of Rh particle
and the CO adsorption. Therefore, CrOx likely has a tendency
to form in the interface between Rh particles and the support.
The proposed contact mode between Rh and the promoters over
3.5 TPSR and FTIR study
As the hydrogenation of carbon to methane is very fast on the
Rhꢀbased catalyst, CH4 formation from chemisorbed CO in H2
gas flow can provide useful information about the ability of Rh
to dissociate CO molecules.40 Fig. 4 displays the TPSR profiles
of the samples. A large CH4 desorption peak at 100~300 °C and
a very small one at 450~550 °C were observed for all the
samples studied, which is in lined with the results reported by
Liu et al.2. The low temperature desorption peak might be
attributed to fast hydrogenation of carbon species derived from
CO dissociation while the high temperature desorption peak
might be due to the hydrogenation of less active carbon species
formed at lower temperatures.2,40 As shown in Fig. 4, Cr and Fe
had different effects on the temperature of low temperature
desorption peak. In other words, Cr decreased the temperature
of the low temperature peak while Fe shifted it to high
RCF(x, y)/SiO2 catalysts was shown in Scheme 1.
The ratio of CO(gdc) versus CO(l) is dependent on the
composition of catalysts and these values are listed in Table 2.
The ratio was increased to 2.09 and 1.23 for RCF(0.4, 0.0)/SiO2
and RCF(0.0, 0.2)/SiO2, respectively, compared with nonꢀ
promoted Rh/SiO2. After simultaneous introduction of Cr and
Fe to Rh/SiO2, the ratio of CO(gdc) to CO(l) increased
significantly. Therefore, the ratio of Rh+ sites to Rh0 sites was
4 | J. Name., 2012, 00, 1-3
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higher over RCF(0.4, 0.2)/SiO2 compared with RCF(0.4, Scheme 1 The proposed contact mode between Rh and
0.0)/SiO2 and RCF(0.0, 0.2)/SiO2.44 A growing consensus promoters over RCF(
)/SiO2 catalysts: (1) = 0.0, = 0.0; (2)
= 0.4, = 0.0; and (4) = 0.4, = 0.2.
x,
y
x
y
regarding CO hydrogenation over Rhꢀbased catalysts is that CO
dissociates at Rh0 sites, and that Rh+ sites are responsible for
x
= 0.0,
y
= 0.2; (3)
x
y
x
y
CO insertion to form intermediates of C2 oxygenates.41,44,45 Table 4 Ratios of peak areas for the adsorbed CO species
This likely explains why the efficiency of CO insertion over
Catalysts
CO(gdc)/CO(l) a
RCF(0.4, 0.2)/SiO2 was enhanced significantly. It is widely
accepted that the main steps for the synthesis of C2 oxygenates
from CO hydrogenation over Rhꢀbased catalysts are: CO
dissociation and hydrogenation to form CHx species, and then
CO insertion to form CHxCO species, followed by
hydrogenation, although the exact mechanism is under
debate.46,47 Thus, a good balance between the levels of CO
dissociation and insertion is important for efficient synthesis of
C2 oxygenates from CO hydrogenation. Furthermore, addition
of Fe to Rh/SiO2 has been reported to improve the efficiency of
the hydrogenation process.48,49 Thus, high activity and
selectivity towards ethanol were obtained over RCF(0.4,
0.2)/SiO2 with moderate levels of CO dissociation and a high
degree of CO insertion.
RCF(0.0, 0.0)/SiO2
RCF(0.0, 0.2)/SiO2
RCF(0.4, 0.0)/SiO2
RCF(0.4, 0.2)/SiO2
0.089
1.23
2.09
3.28
a
Peak area ratio of CO(gdc) to CO(l) in Fig. 4. Experimental
error: ±5%.
4. Conclusions
A series of Crꢀpromoted and (or) Feꢀpromoted Rh/SiO2
catalysts were prepared and investigated in the synthesis of
ethanol from CO hydrogenation. TPR showed that Rh was in
close contact with Cr and Fe during the catalytic reaction.
Furthermore, Cr species showed a tendency to form in the
interface between Rh particles and the silica support, while Fe
was inclined towards covering the Rh particles, as indicated by
the results of XRD, TEM, and FTIR. Addition of Cr to Rh/SiO2
increased the dispersion of Rh particle and the ability of Rh
particles to dissociate CO molecules, while addition of Fe to
Rh/SiO2 increased the degree of CO insertion. A high CO
conversion of 10.2% and ethanol selectivity of 26.0% could be
obtained over RCF(0.4, 0.2)/SiO2 with moderate levels of CO
dissociation and high efficiency of CO insertion.
Fig. 5 Infrared spectra of chemisorbed CO on RCF(
x
,
y
)/SiO2
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The mode of contact between Rh and Cr is different to that between Rh and Fe.