Synthesis of C2oxygenates from syngas over UiO-66 supported Rh-Mn catalysts: The effect of functional groups

Article abstract of DOI:10.1039/d0nj04994h

UiO-66 and its modified forms (UiO-66-NH2 and UiO-66-OH) were used as supports to prepare Rh-Mn catalysts by using a co-impregnation method, and their catalytic activities were investigated for the direct synthesis of ethanol-based C2+ oxygenates from CO hydrogenation. The catalysts were comprehensively characterized using N2 sorption, XRD, XPS, DRIFT, and TPSR analyses. The structural and textural properties of the catalysts clearly show that Rh and Mn are highly dispersed in the pores of MOFs, but the order of the original structure is partially sacrificed due to the loading of metals and the synergistic effect of Rh, Mn, and Zr would be influenced by various functional groups present in UiO-66. On combining both in situ FT-IR and TPSR analyses, it is confirmed that the higher number of active sites of Rh0 on RM/UiO-66 promotes the CO dissociation ability and hydrogenation rate, which result in its best catalytic activity with the highest yield of C2+ oxygenates, 197.3 g (kg h)-1. This journal is

Full text of DOI:10.1039/d0nj04994h

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Synthesis of C oxygenates from syngas over
2
UiO-66 supported Rh–Mn catalysts: the effect
Cite this: New J. Chem., 2021,
of functional groups
45, 696
Ying Han, Jun Yu, * Qiangsheng Guo, Xiuzhen Xiao, Xiaoming Guo,
Haifang Mao and Dongsen Mao
*
2
UiO-66 and its modified forms (UiO-66-NH and UiO-66-OH) were used as supports to prepare Rh–Mn
catalysts by using a co-impregnation method, and their catalytic activities were investigated for the direct
+
2
synthesis of ethanol-based C
oxygenates from CO hydrogenation. The catalysts were comprehensively
characterized using N
2
sorption, XRD, XPS, DRIFT, and TPSR analyses. The structural and textural properties
of the catalysts clearly show that Rh and Mn are highly dispersed in the pores of MOFs, but the order of the
original structure is partially sacrificed due to the loading of metals and the synergistic effect of Rh, Mn, and
Zr would be influenced by various functional groups present in UiO-66. On combining both in situ FT-IR
Received 11th October 2020,
Accepted 29th November 2020
0
and TPSR analyses, it is confirmed that the higher number of active sites of Rh on RM/UiO-66 promotes
DOI: 10.1039/d0nj04994h
the CO dissociation ability and hydrogenation rate, which result in its best catalytic activity with the highest
+
À1
rsc.li/njc
2
yield of C oxygenates, 197.3 g (kg h) .
extensively investigated for catalytic reactions such as
1
. Introduction
1
4
15
esterification,
cross-aldol condensation,
citronellal
1
6
17
Today, much attention is paid towards the exploration of cyclization, and hydrogenation. In our previous studies,
1
8
2
alternative feedstock for fossil fuel resources, as we have compared with the traditional carriers such as SiO ,
1
9,20
21
become more aware of the impact of the energetic dependence TiO
2
and ZrO
2
,
the promoted catalytic performance of
on oil. Ethanol, as a high-quality liquid fuel, is considered for the Rh–Mn catalyst supported on UiO-66 for the synthesis of
2
2
use as a fuel additive. Coal (syngas) to ethanol is the most C oxygenates from CO hydrogenation has been found.
promising new ethanol production route. Rh-based catalysts
2
1
,2
On the other hand, it is widely accepted that the promoter/
have become the hotspot and core of this catalytic process due Rh boundary plays a key role in generating the required
2
3,24
to their superior selectivity to ethanol-based C
(
2
oxygenates active sites for the selective synthesis of C
e.g., ethanol, acetaldehyde and acetic acid). The Rh catalysts Furthermore, previous studies have shown that UiO-66
promoted by Mn have been found to provide increasing activity modified by various functional groups or metal ions can play
and selectivity towards C oxygenates, which revealed that the a key role in generating the required active sites. For example,
auxiliary of Mn plays an important role in this behavior.
Therefore, continuous efforts focus on improving the efficiency catalysts to study the reaction of benzaldehyde with heptalde-
of Rh–Mn active sites by modulating the carrier. hyde aldol to form a-n-pentyl citronellol. As a result, it was
Metal–organic frameworks (MOFs) are a series of periodic found that UiO-66-NH showed higher catalytic activity.
2
oxygenates.
3
–6
2
7
–10
25
Vermoortele et al. used UiO-66 and UiO-66-NH
2
as acid
2
2
6
multi-dimensional crystal materials which are self-assembled Hu et al. reported that mixed matrix membranes containing
by metal ions and organic ligands. Owing to the intrinsic nature UiO-66(Hf)-(OH) nanoparticles have both increased H perme-
of the topologies, functionality, and ultrasmall metal sites of ability and H /CO for efficient H /CO separation.
MOFs, they can effectively interact with the loading metals and Therefore, in this work, we tried to use UiO-66 and its modified
restrict their size distribution. MOFs, in particular Zr-based forms (UiO-66-NH and UiO-66-OH) as supports to prepare Rh–Mn
2
2
2
2
2
2
2
1
1–13
MOFs with outstanding structural stability,
have been catalysts, and the effects on CO hydrogenation on the selective
synthesis of C oxygenates were investigated. The structure and
2
surface properties of the supports and the corresponding catalysts
Research Institute of Applied Catalysis, School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai 201418, China.
E-mail: yujun@sit.edu.cn, dsmao@sit.edu.cn; Fax: +86-21-6087-7231;
Tel: +86-21-6087-7223
were analyzed by means of various characterization studies, to gain
a better understanding of the relationships between the catalytic
performance and physicochemical properties of the catalysts.
6
96 | New J. Chem., 2021, 45, 696--704
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Table 1 Conversion and selectivity for various products from CO hydrogenation over catalysts
Selectivity (%)
+
a
+
b
+
À1
Catalyst
CO conv. (C%)
CO
2
CH
4
MeOH
ACH
EtOH
C
2
HC
C
2
Oxy
2
STY (C Oxy) g (kg h)
RM/UiO-66
RM/UiO-66-NH
RM/UiO-66-OH
14.7
8.3
6.3
2.4
5.2
2.8
29.5
49.7
34.9
1.7
2.5
2.9
8.1
4.5
9.0
29.2
20.7
29.4
20.7
14.1
15.1
45.7
28.5
44.3
197.3
66.1
77.6
2
À1
Reaction conditions: T = 300 1C, P = 3 MPa, catalyst: 0.3 g, and flow rate = 50 ml min (H
2
/CO = 2). Data taken 13 h after the steady state was
a
+
b
+
reached. Experimental error: Æ5%.
C
2
HC denotes hydrocarbons containing two and more carbon atoms.
C
2
Oxy denotes oxygenates
containing two and more carbon atoms.
Table 2 Comparison of the catalytic properties of Rh–Mn catalysts in the literature
ab
Catalyst
T (1C)
P (MPa)
SV
XCO (%)
SCH4 (%)
SEtOH (%)
Ref.
a
Rh–Mn/Al
2
O
2
3
(Rh 3.0 wt%, Mn 1.6 wt%)
(Rh 2.5 wt%, Mn 1.5 wt%)
(Rh 3.0 wt%, Mn 3.0 wt%)
-Ar (Rh 2.0 wt%, Mn 2.0 wt%)
250
260
300
280
300
2.0
5.4
3.0
3.0
3.0
6000b
5833
5.9
9.4
3.6
6.8
14.7
20.0
41.0
15.6
10.1
29.5
29.0
21.0
24.4
20.0
29.2
24
29
22
20
Rh–Mn/SiO
Rh–Mn/ZrO
Rh–Mn/TiO
Rh–Mn/UiO-66 (Rh 3.0 wt%, Mn 3.0 wt%)
a
2
10 000a
10 000a
10 000
2
This work
a
À1
b
À1
Space velocity in mL (gcat h)
.
Space velocity in h .
2
. Experimental
catalyst (0.3 g) diluted with inert a-alumina (0.6 g) to avoid
channeling and hot spots was loaded between quartz wool and
axially centered in the reactor tube; the temperature was
monitored using a thermocouple close to the catalyst bed. Prior
2
.1. Catalyst preparation
UiO-66 can be prepared by referring to the literature. Briefly,
50 mg of ZrCl (99.95 + %, J&K) and 246 mg of benzene-
dicarboxylic acid (H BDC, 99%, Damas-Beta) were dissolved
2
7
2
4
to the reaction, the catalyst was heated to 400 1C (heating rate
2
À1
B3 1C min ) and reduced with H
/N
2 2
(molar ratio of H
/N
2 2
= 1/9,
in 30 mL of N,N-dimethylformamide (DMF, 99%, General-
Reagent) premixed with 2 mL of concentrated HCl (37%,
General-Reagent) by using ultrasonic treatment for 15 min
until the solution turned pellucid. Then, the solution was
transferred to a glass bottle, and placed in an oven at 80 1C
under static conditions for 10 h. Afterwards, the obtained
precipitates were cooled down to room temperature and
separated by centrifugation. The solids were washed several
times with DMF and methanol by centrifugation, and dried
À1
total flow rate = 50 mL min ) for 2 h at atmospheric pressure.
The catalyst was then cooled down to 300 1C and the reaction
started as the gas flow was switched to a H /CO mixture (molar
2
À1
2
ratio of H /CO = 2, total flow rate = 50 mL min (STP)) at 3 MPa.
All post-reactor lines and valves were heated to 150 1C to prevent
product condensation. The products were analyzed for both
hydrocarbons and oxygenates on-line (FL GC 9720) using a
HP-PLOT/Q column (30 m, 0.32 mm ID) with a flame ionization
detector (FID) and a TDX-01 column with a thermal conductiv-
ity detector (TCD). The CO conversion was calculated based on
the fraction of CO that formed carbon-containing products and
under vacuum at room temperature. UiO-66-NH and UiO-66-
OH were prepared using the same method as UiO-66,
2
while H
-dihydroxyterephthalic acid, respectively. Noteworthy, this
synthetic reaction can be performed as a 10-fold scale up.
RhCl hydrate (Rh B 39 wt%, Fluka), Mn (NO 6H
99.99%, SCRC) and the supports synthesized above were used
2
BDC was changed to 2-aminoterephthalic acid and
2
3
3
)
2
2
O
(
in the preparation of catalysts. 2 g of support was weighed and
immersed in a mixed solution of RhCl and Mn(NO ) hydrate,
3
3 2
the weight percent of Rh and Mn were respectively 3 wt% and
wt%. After drying at room temperature, the catalyst was
shifted to an oven at 80 1C for 12 h, and then calcined at
00 1C for 4 h. The obtained samples were denoted as RM/UiO-66,
RM/UiO-66-NH , and RM/UiO-66-OH.
3
3
2
2
.2. Reaction
CO hydrogenation was performed in a fixed-bed microreactor
of length B350 mm and internal diameter B5 mm. The Fig. 1 The catalytic performance vs. time-on-stream for RM/UiO-66.
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Fig. 2
2
N sorption isotherms (A) and the pore size distributions (B) of different carriers.
4
0 kV and 40 mA using Ni b-filtered Cu Ka radiation. Two theta
Table 3 BET surface area and pore volume of different carriers
À1
angles (2y) ranged from 51 to 751 with a speed of 61 min
.
2
À1
3
À1
Surface area (m g
)
Pore volume (cm g
)
X-ray photoelectron spectra (XPS) experiments were per-
formed using a Kratos Axis Ultra DLD spectrometer equipped
with an Al Ka (1486.6 eV) X-ray source. The background
Sample
Total Micropore External Total Micropore Mesopore
UiO-66
UiO-66-NH
UiO-66-OH 282.8 228.4
1320.8 1115.8
2
1105.8 924.8
205.0
181.0
54.1
0.79 0.57
0.68 0.48
0.19 0.12
0.22
0.20
0.07
À9
pressure in the analysis chamber was kept below 7 Â 10
mbar during data acquisition. The powder of samples was
pressed into copper holders and then mounted on a support
rod placed in the pretreatment chamber. For the reduced
samples, they were pretreated in situ at 400 1C for 1 h under
the selectivity of a certain product was calculated based on
2
8
carbon efficiency, as reported previously.
2
200 mbar H flow in the pretreatment chamber. The binding
energies were calibrated relative to the C 1s peak from carbon
contamination of the samples at 284.9 eV to correct for contact
potential differences between the sample and the spectrometer.
CO adsorption was investigated using a Nicolet 6700 FTIR
spectrometer equipped with a diffuse reflectance infrared Four-
2
.3. Catalyst characterization
The specific surface area (SBET), pore volume (V ), and pore
p
diameter (D ) were measured by N sorption at À196 1C using
p
2
Micromeritics ASAP 2020 HD88 adsorption apparatus. All
samples were degassed at 150 1C for 10 h prior to sorption
testing.
ier transform (DRIFT) cell with CaF
samples pressed in the cell was pretreated using a mixed H
2
windows. The powder of
/N
2
2
À1
flow (molar ratio of H
at 400 1C for 2 h, followed by He flow (50 mL min ) flushing at
00 1C for 0.5 h. During cooling down to room temperature, a
2 2
/N = 1/9, total flow rate = 50 mL min )
The X-ray powder diffraction (XRD) patterns of samples were
recorded using a PANalytical X’Pert diffractometer operated at
À1
4
series of background spectra were captured at required tem-
À1
peratures. Followed by introducing 0.5% CO/He (50 mL min
)
Fig. 3 XPS spectra (region Zr 3d) for three supports.
Fig. 4 XRD patterns of different supports.
6
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Fig. 5
2 2
N sorption isotherms (A) and the pore size distributions (B) of RM/UiO-66 (red), RM/UiO-66-NH (magenta) and RM/UiO-66-OH (blue).
into the IR cell, the IR spectra at the specified temperatures
For comparison, the available results of Rh–Mn catalysts
were recorded. Moreover, after treating with the CO/N
at 300 1C for 60 min, an additional H2 flow (1 mL min
2
mixture supported on various carriers for CO hydrogenation are col-
À1
)
lated in Table 2. Compared to the UiO-66 supported Rh–Mn
was added to the flow, and the IR spectra were recorded as catalyst, the Rh–Mn catalysts supported on traditional oxides
À1
a function of time. The spectral resolution was 4 cm and the exhibit relatively low CO conversion and ethanol selectivity.
scan times were 64.
This shows that our catalyst is very promising.
+
The temperature-programmed surface reaction (TPSR)
Typical time dependent changes of CO conversion and C
2
experiments were carried out as follows: The sample (0.1 g) oxygenate selectivity over the representative RM/UiO-66
was reduced in H /He (molar ratio of H /He = 1/9) at 400 1C for catalyst are shown in Fig. 1. It can be seen that both the
h, followed by He flow (50 mL min ) flushing at this CO conversion and C oxygenate selectivity decrease dis-
2
2
À1
+
1
2
temperature for 0.5 h. After cooling down to room temperature, tinctly during the first 15 h on stream. However, the catalyst
À1
diluted CO (1% CO in Ar, 50 mL min ) was introduced for keeps good stability after 15 h, and the catalytic activity is
adsorption for 0.5 h. Afterwards, the H
again, and the temperature was increased at the rate of 10 1C min
2
/He mixture was swept hardly reduced until 150 h.
À1
.
The exhaust gas was detected using a quadrupole mass spectro-
meter (QMS, Balzers OmniStar 200) as the detector to monitor
the signal of CH (m/z = 15).
4
3
.2 Structural and textural properties of supports
Fig. 2 shows the N sorption curves and corresponding pore size
distribution of the supports. As seen from Fig. 2A, all N sorption
curves are I-type isotherms, which is typical for the MOFs. UiO-
6 has the maximum amount of N adsorption. In contrast, the
2
2
11
6
2
smallest N adsorption capacity is obtained on the UiO-66-OH
support. Correspondingly, the specific surface area and pore
volume of UiO-66-OH are far lower than those of the other two
2
3
. Results and discussions
3.1. Catalytic performance
Table 1 shows the effect of the Rh–Mn catalysts supported on carriers. It is suggested that because of the strong interaction
UiO-66, UiO-66-NH , and UiO-66-OH for the catalytic hydroge- between the polar hydroxyl group with metal ion and solvent, the
2
nation performance at 300 1C. As observed, the CO conversion movement of the organic substance and the formation of the 3D
of the catalysts follows the order: RM/UiO-66 4 RM/UiO-66- pores are restrained, resulting in a large decrease in the specific
3
0
NH 4 RM/UiO-66-OH. With respect to selectivity, the selectiv- surface area and pore volume. The pore size distribution of
2
+
ity of CH for RM/UiO-66-NH is the highest and that of C
UiO-66 is similar to that of UiO-66-NH , which is concentrated in
2
4
2
2
oxygenates is the lowest. Further calculations show that the the ranges of 0.52–1.0 nm and 1.5–2.0 nm. The pores in the range
+
space–time yield (STY) of the C
small in the following order: RM/UiO-66 4 RM/UiO-66-OH 4 tetrahedral cages. And the partial micropores in the range of
RM/UiO-66-NH 1.5–2.0 nm might be likely caused by the missing-linker defects in
2
oxygenates is from large to of 0.5–1.0 nm should consist of the regular octahedral and
2
.
Table 4 BET surface area and pore volume of different catalysts
2
À1
3
À1
Surface area (m g
)
Pore volume (cm g )
Sample
Total
Micropore
External
Total
Micropore
Mesopore
RM/UiO-66
RM/UiO-66-NH
RM/UiO-66-OH
129.4
47.2
45.6
83.5
21.3
18.5
45.9
25.9
27.1
0.15
0.10
0.06
0.04
0.01
0.01
0.11
0.09
0.05
2
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Fig. 6 (A) XPS spectra (region Rh 3d) of fresh catalysts; (B) XPS spectra (region Rh 3d) of the reduced catalysts; (C) XPS spectra (region Mn 2p) of catalysts
before and after reduction; and (D) XPS spectra (region Zr 3d) of catalysts before and after reduction.
11
the structure. Compared with UiO-66 and UiO-66-NH
2
, the lower
X-ray photoelectron spectroscopy was performed to provide
intensity of the micropores in the range of 0.5–1.0 nm is obtained information about the oxidation state of Zr present on the
on UiO-66-OH, which is consistent with the results of N
tion capacity and pore volume (Table 3).
2
adsorp- surface of the supports. The Zr 3d5/2 peaks of UiO-66 and its
modified forms are all centered at about 183 eV as seen in
Table 5 Surface status and content of Rh for the different catalysts
d+ a
0 a
d+
0
b
b
b
Catalyst
Rh
Rh
Rh /Rh
CO(dgc)
CO(l)
CO(b)
CO(dgc)/CO(l)
RM/UiO-66
RM/UiO-66-NH
RM/UiO-66-OH
1532.5
1749.7
2296.6
2812.1
2783.7
3047.9
0.54
0.63
0.75
7.6
8.6
7.8
1.9
1.7
1.2
9.1
2.3
2.3
4.1
5.1
6.7
2
a
+
0
b
Peak areas of Rh and Rh are derived from XPS peak data. Peak areas of CO (dgc), CO (l) and CO (b) are derived from IR spectra.
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The surface chemistry of the catalysts before and after
reduction at 400 1C was obtained by XPS characterization. As
shown in Fig. 6A, the Rh species in all the fresh catalysts
exist in the form of Rh O (310.0 eV). After reduction, the peak
2
3
shifts to the lower binding energy and can be deconvoluted
0
d+
into two components, Rh (306.8–307.4 eV) and Rh
3
3,34
0
(307.7–308.5 eV).
The proportion of Rh is relatively higher
d+
0
on the catalyst surface, and the ratios of Rh /Rh for the
catalysts are listed in Table 5. The binding energy of the
Mn 2p3/2 peaks are centered at about 642 eV on fresh catalysts.
Generally, the XPS spectra around the Mn 2p3/2 peak between
645 eV and 641 eV cannot be deconvoluted into various
Mn components of the signal, suggesting that Mn exists in
2
+
3+
4+ 20
the form of Mn or higher valence states (Mn and Mn ).
After reduction, the binding energy of Mn 2p3/2 shifts towards
lower values, indicating that the electronic density on Mn
particles increases caused by the interaction between Mn and
Fig. 7 XRD patterns of different catalysts.
Fig. 3, which is consistent with the coordination structure of Rh (or Zr). Fig. 6D shows the chemical states of Zr for the
3
1
4+
UiO-66 series materials.
catalysts. After loading Rh–Mn, the peaks of Zr in the three
Fig. 4 shows the XRD patterns of the supports. The three samples have hardly changed compared with those of the state
4
+
different carriers have characteristic diffraction peaks unique of Zr for carriers (Fig. 3). However, the peaks of Zr after
to the UiO-66 series materials at 2y = 7.31, 8.51, 25.71, 43.31, reduction shift to the lower binding energy to some extent. It is
1
1,32
etc.
Nevertheless, the peak intensity of UiO-66 is much further revealed that the interaction among Zr node, Rh, and
stronger than those of the others, which is the result of the Mn may occur during the reduction process, which, in turn, is
d+
change in the unique connection state of the original structure reflected by the presence of Rh and shift of Mn 2p3/2 and Zr
of UiO-66 due to the insertion of the relevant functional groups. 3d5/2 peaks after reduction.
Fig. 7 shows the XRD patterns of the different catalysts.
Compared with the supports, the diffraction peaks of the over-
3.3. Properties of catalysts
Fig. 5 shows the N
2
sorption curves of different catalysts and all framework structure almost disappear after loading of
their corresponding pore size distribution. After loading of metals and the diffraction peaks attributed to ZrO
Rh and Mn, the amount of N adsorption of catalysts decreases, 2y of about 30.51, 50.61 and 60.31 (PDF 17-0923). This indicates
while the law of N adsorption is satisfied with the rule: that the linkers of MOFs would be partially distorted or missing
RM/UiO-66 4 RM/UiO-66-NH E RM/UiO-66-OH. Compared because of the introduction of Rh and Mn particles, and partial
2
appear at
2
2
2
with pore size distribution of the supports, the micropores Zr nodes would be agglomerated, which destroyed the order of
of catalysts with the pore size bigger than 0.6 nm almost the original structure. In addition, no characteristic diffraction
disappear due to the filling of metals, which can be further peaks of Rh and Mn are found in all XRD patterns, indicating
supported by the data in Table 4.
that the Rh and Mn are highly dispersed in the pores of MOFs.
2
Fig. 8 The infrared spectra of chemisorbed CO in CO/N flow over different catalysts at 30 1C (A) and 300 1C (B).
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Fig. 9 The infrared spectra of absorbed CO over the catalysts at 300 1C followed by hydrogenation with hydrogen.
Fig. 8 shows the infrared spectra of the in situ reduced after CO reaction with H
catalysts after CO adsorption at 30 1C and 300 1C in a flowing shown in Fig. 9. It is found that the CO(l) band decreases
CO/N atmosphere. The change of the active component Rh in significantly due to the reaction between H and CO and
2
as a function of time at 300 1C are
2
2
different catalysts can be inferred based on the change of the remains basically unchanged after 30 min. It is further
CO adsorption peak in the infrared spectra. As observed in observed that the reaction rate of adsorbed CO on RM/UiO-66
Fig. 8A, the IR spectra of the adsorbed CO at 30 1C all exhibit is faster than that of the others, indicating that although the CO
À1
À1
two main peaks at B2100 cm and B2036 cm , which can be adsorption amount of RM/UiO-66 is smaller than that of the
attributed to the symmetric and asymmetric carbonyl stretch- others, the higher yH on RM/UiO-66 promotes the hydrogena-
+
35
ing of the gem-dicarbonyl Rh (CO)
2
[CO(gdc)]. Besides, two tion rate, resulting in a higher CO conversion.
À1
À1
weak bands at B2060 cm and B1860 cm correspond to the
4
Fig. 10 shows the patterns of CH formation for different
linear CO [CO(1)] and bridge CO bonded to Rh(111) [CO(b)], catalysts measured using a quadrupole mass spectrometer in
3
6,37
respectively.
catalysts is in the order of RM/UiO-66 4 RM/UiO-66-NH
RM/UiO-66-OH, which can be confirmed by the data found in tool for measuring the CO dissociation and hydrogenation
The total CO adsorption amounts of different the TPSR test. Since the hydrogenation of dissociated CO into
2
4
CH is rapid, the desorption of CH in TPSR can be used as a
4
4
2
4
Table 5. Under a reaction temperature of 300 1C, the CO(gdc) ability over the Rh-based catalyst. It can be seen that the
species disappears, and the adsorbed CO of catalysts exist as temperature of the CH peak is in the following order: RM/UiO-
4
the species of CO(l) and CO(b) (Fig. 8B). It is clear that the 66 o RM/UiO-66-OH o RM/UiO-66-NH
2
, suggesting that RM/
adsorption of CO is dominated by linear adsorption at the UiO-66 has better CO dissociation ability. However, the CO
reaction temperature. In addition, the CO adsorption amount dissociation of all the catalysts can be accomplished within the
of RM/UiO-66 is smaller than the other two catalysts. On the reaction temperature. On the other hand, the areas of the CH
4
other hand, the position of CO (l) under the reaction tempera- peak produced by the catalysts are summarized in Table 6,
ture decreases in the following order: RM/UiO-66-OH 4 RM/ following the order: RM/UiO-66 E RM/UiO-66-OH o RM/UiO-
UiO-66-NH2 4 RM/UiO-66, which can be interpreted by the 66-NH . This result reveals that there are more active sites for
2
3
9
decrease in surface coverage.
+
Because the CO(gdc) is considered to be formed on the Rh
0
38,39
sites and CO(l) is on the Rh sites,
it is obvious in Fig. 8A
+
0
that the ratio of CO(gdc)/CO(1) (Rh /Rh ) in RM/UiO-66 is much
smaller than that of the other two catalysts, which is consistent
with the ratio of chemical state distribution of Rh using XPS
+
0
(
Table 5). Between the active sites of Rh and Rh , H is more
2
0
40
easily adsorbed on Rh . Therefore, the coverage of H* species
yH) on RM/UiO-66 should be higher during the reaction
(
process. In the whole process of CO hydrogenation on Rh-
based catalysts, the rate of CO hydrogenation to formyl species
(
HCO*) is the slowest, that is to say, it controls the whole rate of
4
1
H
the entire reaction. Thus, it is inferred that high y on RM/
UiO-66 is likely to promote the formation of formyl species,
resulting in the higher CO conversion.
In order to further investigate the state and change of
adsorbed CO in the reaction condition, the IR spectra taken Fig. 10 CH4 desorption profiles from TPSR of three catalysts.
702 | New J. Chem., 2021, 45, 696--704
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Table 6 Relative quantities of CH
catalysts
4
formed in TPSR experiments on
torrefaction pretreatment with chemical looping gasifica-
tion, Appl. Energy, 2020, 260, 114315.
a
3 M. Haider, M. Gogate and R. Davis, Fe-promotion of sup-
ported Rh catalysts for direct conversion of syngas to
ethanol, J. Catal., 2009, 261, 9–16.
Catalyst
Area of CH
4
RM/UiO-66
RM/UiO-66-NH
RM/UiO-66-OH
202.5
247.8
203.4
2
4 J. Gong, H. Yue, Y. Zhao, S. Zhao, L. Zhao, J. Lv, S. Wang and
a
X. Ma, Synthesis of ethanol via syngas on Cu/SiO
2
catalysts
with balanced Cu -Cu sites, J. Am. Chem. Soc., 2012, 134,
3922–13925.
4
The CH peak area is obtained by integration.
0
+
1
CH
4 2
formation on RM/UiO-66-NH , which is consistent with the
5
6
J. Liu, Z. Guo, D. Childers, N. Schweitzer, C. L. Marshall,
R. F. Klie, J. T. Miller and R. J. Meyer, Correlating the degree
of metal-promoter interaction to ethanol selectivity over
MnRh/CNTs CO hydrogenation catalysts, J. Catal., 2014,
results of catalytic performance.
4
. Conclusion
313, 149–158.
W. Liu, S. Wang and S. Wang, Effect of impregnation
sequence of Ce promoter on the microstructure and perfor-
A series of Zr-based metal–organic framework (UiO-66, UiO-66-
NH , UiO-66-OH) supported Rh–Mn catalysts were prepared
using the incipient wetness impregnation method, and their
catalytic performance was investigated for the direct synthesis
of ethanol-based C
catalyst supported on UiO-66 is more active than those sup-
ported on UiO-66-NH and UiO-66-OH, and the maximum STY
of C2 oxygenates is obtained with the RM/UiO-66 catalyst.
The structural results show that Rh and Mn are highly
dispersed in the pores of MOFs, but the order of the original
structure is partially sacrificed by the loading of metals. The
XPS result indicates that interaction among Zr node, Rh, and
Mn occurs during the reduction process, and the degree can be
regulated by the different functional groups in UiO-66. On
combining both in situ FT-IR and TPSR analyses, it is further
confirmed that the higher number of active sites of Rh on RM/
UiO-66 promotes the CO dissociation ability and hydrogenation
rate, resulting in a higher CO conversion. Furthermore, the
2
mance of Ce-promoted Rh-Fe/SiO
2
for the ethanol synthesis,
Appl. Catal., A, 2016, 510, 227–232.
+
2
7 P. Lin, D. Liang, H. Luo, C. Xu, H. Zhou, S. Huang and
L. Lin, Synthesis of C2+ -oxygenated compounds directly
from syngas, Appl. Catal., A, 1995, 131, 207–214.
8 Y. Wang, H. Luo, D. Liang and X. Bao, Different Mechan-
isms for the Formation of Acetaldehyde and Ethanol on the
Rh-Based Catalysts, J. Catal., 2000, 196, 46–55.
H. Yin, Y. Ding, H. Luo, H. Zhu, D. He, J. Xiong and L. Lin,
Influence of iron promoter on catalytic properties of Rh-Mn-Li/
2
SiO for CO hydrogenation, Appl. Catal., A, 2003, 243, 155–164.
oxygenates from CO hydrogenation. The
2
+
9
1
1
1
0 W. Mao, J. Su, Z. Zhang, X. Xu, D. Fu, W. Dai, J. Xu, X. Zhou
and Y. Han, A mechanistic basis for the effects of Mn
loading on C2+ oxygenates synthesis directly from syngas
0
over Rh-MnO/SiO
01–311.
2
catalysts, Chem. Eng. Sci., 2015, 135,
3
1 J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti,
S. Bordiga and K. P. Lillerud, A New Zirconium Inorganic
Building Brick Forming Metal Organic Frameworks
with Exceptional Stability, J. Am. Chem. Soc., 2008, 130,
greater number of active sites for CH
NH can be responsible for its higher CH
4
formation on RM/UiO-66-
selectivity.
2
4
13850–13851.
Conflicts of interest
2 H. Wu, Y. Chua, V. Krungleviciute, M. Tyagi, P. Chen,
T. Yildirim and W. Zhou, Unusual and highly tunable
missing-linker defects in zirconium metal-organic frame-
work UiO-66 and their important effects on gas adsorption,
J. Am. Chem. Soc., 2013, 135, 10525–10532.
3 Y. Bai, Y. Dou, L. Xie, W. Rutledge, J. Li and H. Zhou,
Zr-based metal-organic frameworks: design, synthesis, structure,
and applications, Chem. Soc. Rev., 2016, 45, 2327–2367.
4 F. G. Cirujano, A. Corma and F. X. Llabr ´e s i Xamena,
Zirconium-containing metal organic frameworks as solid
acid catalysts for the esterification of free fatty acids:
Synthesis of biodiesel and other compounds of interest,
Catal. Today, 2015, 257, 213–220.
There are no conflicts to declare.
Acknowledgements
1
This project was financially supported by the National Natural
Science Foundation of China (21808142), Shanghai Institute of
Technology (ZQ2018-3, CHJJ-7).
1
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