Exceptionally stable Rh-based molecular catalyst heterogenized on a cationically charged covalent triazine framework support for efficient methanol carbonylation

Article abstract of DOI:10.1039/c8cy00294k

Direct carbonylation of methanol into methyl acetate and acetic acid using Rh-based heterogeneous catalysts is of great interest due to their effective levels of activity and stability. Here, a Rh-based molecular catalyst heterogenized on a charged 1,3-bis(pyridyl)imidazolium-based covalent triazine framework (Rh-bpim-CTF) was synthesized and characterized to have a single-site distribution of metal molecular species throughout the support by its ligation to abundant N atom sites. Methanol carbonylation was performed using the Rh-bpim-CTF catalyst in a plug-flow reaction in the gas phase, affording a turnover frequency of up to 3693 h^-1 and a productivity of 218.9 mol kg^-1 h^-1 for acetyl products with high stability.

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Exceptionally Stable Rh-based Molecular Catalyst Heterogenized
on a Cationically charged Covalent Triazine Framework Support
for Efficient Methanol Carbonylation
Received 00th January 20xx,
Accepted 00th January 20xx
Kwangho Parka†, Sangyup Lima†, Joon Hyun Baikb, Honggon Kimc, Kwang-Deog Jungc, and Sungho
Yoon*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Direct carbonylation of methanol into methyl acetate and acetic acid using Rh-based heterogeneous catalysis is of great
interest due to the effective levels of activity and stability. Here, a Rh-based molecular catalyst heterogenized on a charged
1, 3-bis(pyridyl) imidazolium-based covalent triazine framework (Rh-bpim-CTF) was synthesized and characterized to have
a single-site distribution of metal molecular species throughout the support by its ligation to abundant N atom sites.
Methanol carbonylation was performed using the Rh-bpim-CTF catalyst in plug-flow reaction in gas phase, affording a
turnover frequency up to 3693 h-1 and productivity of 218.9 mol kg-1 h-1 for acetyl products with high stability.
segregated from the reaction mixture and can be used in con-
tinuous liquid- and gas-phase reactions. In particular, the total
economic cost can be reduced if the products can be easily
Introduction
Acetic acid (AA) and methyl acetate (MA) are highly versatile
organic compounds that find wide applications ranging from
basic solvents for industrial processes to reagents for manufac-
turing more value-added materials such as polymers and fi-
bers.1 In particular, MA has garnered recent interest as an al-
ternative carbon feedstock for its conversion into ethanol via
hydrogenation and for cell growth in biotechnology.2-6 AA and
MA can be produced via a number of synthetic routes, typical-
ly dimethyl ether (DME) or methanol carbonylation. However,
the DME carbonylation is still under development because its
low efficiency hinders its industrialization.7
separated from the catalysts in gas-phase reaction process.11,
12
However, the development of heterogeneous catalysts with
high efficiency and long-term stability is considerably challeng-
ing nowadays because of the efficiency gap between homoge-
nous and heterogeneous systems.
The development of novel heterogeneous catalytic systems
via the heterogenization of molecular metal complexes on
various catalytic supports has achieved remarkable progres-
sion.13-24 The well-defined ligand sites on the supports in Rh-
catalyzed methanol carbonylation could provide abundant
binding sites for the metal complexes and enhanced the nu-
cleophilicity of the Rh(I) active site, thereby increasing the cat-
alytic activity in such reactions. In particular, the positively
charged ligand structures on the surface of the support could
dramatically increase the stability of the catalyst even at high
temperatures and pressures via the electrostatic interaction
with the catalytic active species, i.e., [Rh(I)CO2I2]−. In this con-
text, devising novel heterogeneous catalyst supports with the
aim of increasing the catalytic efficiency of manufacturing pro-
cesses is currently garnering substantial interest in the indus-
trial fields.
Methanol carbonylation using Rh- and Ir-based catalysts
stands out as the more relevant industrial process for the
worldwide production of AA and MA; it is successfully applied
by many chemical companies in homogeneous systems, i.e.,
the Monsanto and Cativa processes, as well as in heterogene-
ous systems, i.e., the Acetica and BP SaaBre processes.8-12
However, despite reaching effective levels of activity, the ho-
mogenous systems present a major drawback. Such systems
increase the complexity and cost of the manufacturing process
such as the recycling of catalyst from corrosive product mix-
ture.9 In contrast, in heterogeneous systems, the catalysts are
Recently, covalent triazine frameworks (CTFs) have gar-
nered increasing attention as catalytic supports because they
constitute a highly porous structure with robustness under
high pressure, thermal, and chemical conditions (i.e., acidic or
basic).25-36 In particular, CTFs possess abundant nitrogen-based
basic ligand sites that function similar to those in homogene-
ous ligands. Upon heterogenization with a metal complex,
CTFs afford strong coordination sites and provide the metal
center with σ-donor capability. Furthermore, recent reports
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illustrate that the incorporation of ionic building blocks on the CTFs resulted in high level of homogeneous charge distribution
Active state
Resting state
MeOH +CO
w/ flow of
CO, CH3I
Anionic
Acetic acid +
active species
Methyl acetate
In air
2
1
Figure. 1 Structural representations of Rh-bpim-CTF catalyst in resting state under ambient atmosphere (
state under the carbonylation conditions. ( ).
1) and the suggested structure of in situ generated active
2
on the surface.37, 38 The charged CTFs could tightly bind anionic Al Ka as the excitation source (hγ=1486.6 eV) with concentric
complexes in various reaction conditions via ion-pair interac- hemispherical analyzer. Rh content was determined by an in-
tion, thereby showing high potential as catalytic supports.39, 40
ductively coupled plasma optical emission spectroscopy (ICP-
The first-ever synthesis and characterization of a Rh-based OES) using iCAP 6000 (Thermo schientific, USA). The Diffuse
single-site catalyst heterogenized on a charged 1,3-bis(pyridyl) Reflectance Infrared Fourier Transform (DRIFT) spectra were
imidazolium-based CTF support (Rh-bpim-CTF) for its applica- obtained by using Nicolet 6700 (Thermo Electron Co.)
tion in the carbonylation of methanol in a continuous plug- equipped with an MCT detector and the Praying MantisTM
flow reaction system in the gas phase is presented herein. The high temperature reaction chambers (Harrick Scientific Prod-
catalytic species was implemented as a single-site catalyst dis- ucts Inc.). The sample was pre-heated before the measure-
tributed via the support by its ligation to abundant N atom ments at 200°C for 1 h to remove the moisture captured inside
sites (Figure 1, 1). As a result of the high electron donation of the support. Elemental analysis (EA) was performed with an
from the N-rich environment of the pore surface, the Rh-bpim- elemental analyser (Vario Micro cube, Germany). Transmission
CTF catalyst featured higher efficiency compared with other electron microscopy (TEM) analyses were performed on a Jeol
heterogeneous catalytic systems. Moreover, the long-term JEM-2100F HRTEM and operated at 200 kV. The samples were
stability of the catalyst during the continuous process was con- prepared by dispersing with Ethanol and placed on a holey
firmed: perceptible catalytic deactivation was not detected, carbon film grid by dropping. After dried under vacuum at 60°C,
which is most likely caused by the strong ion-pair interaction samples were analysed in both bright field and dark field imag-
between the positive-charged [imidazolium]+ moiety and the ing modes.
catalytic active species [Rh(CO)2I2]− (Figure 1,
2).
Synthetic procedures
Synthesis of bpim-CTF support. The bpim-CTF was prepared ac-
cording to the previously reported procedure.35 In a glove box,
1,3-bis(5-cyanopyridyl)-imidazolium bromide (bpim) (1.00 g,
2.83 mmol) and zinc chloride (1.89 g, 14.2 mmol) were charged
into an 50 ml of ampoule and flame sealed under vacuum,
which is heated to 500°C in a furnace with heating rate of 1°C
Experimental
Materials and methods
All chemicals were purchased from commercial suppliers and
used without further purification. 2-Bromo-5-cyanopyridine
(99%) was purchased from T.C.I. chemicals. 1-Methyl-1H-
imidazole (99%) and zinc chloride (98%) were purchased from min-1 and maintained for 48h. The crude product was ground
Sigma Aldrich. Methanol, methyl iodide were purchased from
DAEJUNG. Rhodium(III) chloride was purchased from Alfa Ae-
sar. Rh(I) carbonyl iodide dimer was synthesized as per a pre-
vious report.41 The mixture of CO and N2 (9:1 ratio) was ob-
tained from Sinyang gas industries.
finely and stirred with 400 mL of 1M aqueous HCl solution for
3h. Then the black solid was filtered and extensively washed
with 100 mL of water and acetone for five times. The resulting
solid was refluxed with 1M HCl (250 mL) for overnight, then
filtered and washed with 1M of HCl (3 x 100 mL), H2O (3 x 100
mL), THF (3 x 100 mL) and acetone (3 x 100 mL). Finally, the
obtained black powder was dried under vacuum at 200°C for 6
h.
Fourier-transform infrared (FT-IR) analysis was performed
on a Nicolet iS 10 (Thermo Fisher Scientific) using KBr pellet.
scanning electron microscopy (SEM) and energy-dispersive
spectroscopy (EDS) were performed on a JEOL LTD (JAPAN)
JEM-7610F operated at an accelerating voltage of 20.0 kV. N2
adsorption and desorption measurements were conducted at
77 K using a BELSORP mini II (JAPAN). The samples were de-
gassed for 1 day at 200°C before the measurements. X-ray
photoelectron Spectroscopy (XPS) analysis was performed on
ESCA 2000 (VG microtech) at a pressure of ~3 x 10-9 mbar using
Synthesis of Rh-bpim-CTF 5.00 g of bpim-CTF and 139 mg of
[Rh(CO)2I]2 (1 Wt%) was charged into 250 ml round bottom
flask. Then 150 ml of anhydrous methanol was added to the
mixture and stirred for overnight under CO atmosphere. After
the reaction finished, the resulting powder was filtered and
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washed with methanol (200 ml x 5). Finally collected black
powder was dried under vacuum for overnight.
Table. 1 Porosity parameters and the inductively coupled plasma-optical emission
spectroscopy (ICP-OES) analysis result for the support and the Rh-bpim-CTF catalyst.
Typical carbonylation procedure Methanol carbonylation was
carried out in homemade stainless steel bar-type reactor. In a
typical run, the Rh-bpim-CTF (200 mg) was fixed on the middle
of reactor by filling the remaining empty space with glass
wools and beads. After flushing with CO, the reactor was pres-
surized to the desired pressure (7.5-15 bar) with feed gas
(CO:N2=9:1) flow of 100 ml min-1 and heated at 150-270°C be-
fore the reaction. The pressure was maintained at the desired
pressure using back-pressure regulator and then gas flow rate
was controlled by the mass transfer controller (14.5-58 ml min-
1). Methanol and methyl iodide (molar ratio of 10:1) were
mixed preliminarily and supplied to the reactor using high-
pressure liquid pump.(0.186- 0.744 ml h-1) The liquid product
was collected and analyzed with gas chromatography (TCD
detector, series 6000, Younglin & Co.). The gas product was
analyzed with an on-line GC (FID detector, series 6000, Young
lin & Co.).
a
a
as,BET
m2 g-1
Vpore, tot
Vmean
ICPRh
[
]
[
cm3 g-1
]
[
nm
]
[Wt%
]
bpim-CTF
1764
1651
0.84
1.91
1.91
-
Rh-bpim-CTF
0.79
0.61
aDetermined by t-plot analysis.
a)
b)
c)
Results and discussion
The bpim-CTF support was synthesized as per a previous re-
port38. Briefly, bpim-CTF was synthesized by trimerization of
monomer in ionothermal reaction with ZnCl2 as Lewis acid as
well as solvent at 500°C for 48 h. The afforded black powder
was extensively washed and refluxed with diluted HCl and or-
ganic solvents to remove the ZnCl2, featuring moderate chemi-
cal stability in acidic condition. The porous and N-rich charac-
ters of the bpim-CTF support were verified via SEM, EDS, FT-IR
spectroscopy, and EA (Figure S1, S2, and Table S1).
Figure. 2 EDS mapping of the Rh atoms (top left), HR-TEM images (top right, a
and b), and HAADF-STEM image(top right, c) for the Rh-bpim-CTF catalyst. Rh 3d
XPS spectra (bottom left) and the DRIFT spectroscopy analysis result under a CO
flow (bottom right) for the Rh-bpim-CTF catalyst.
In particular, the FT-IR results exhibited the broad peaks in
the range at 1,450-1,650 cm-1 and 1,000-1,400 cm-1, which is
due to the partial graphitization experienced during the syn-
thetic procedure. However, the peaks still included the region-
al peaks for the triazine and imidazolium, i. e., 1,450–1,600
cm−1 (C=N or C=C) and 1,200–1,400 cm−1 (C-N or C-C), indicat-
ing their presence in the bpim-CTF support. Further, bpim-CTF
support synthesized at 500°C was used only despite the partial
thermal loss of functional groups, due to the high porosity.
Initially, the bpim-CTF support was synthesized at 400 ° C and
used for catalyst preparation (Table S2). However, the efficien-
cy for methanol carbonylation with the catalyst was low, which
may be attributed to limited mass transfer of gas phase feed
reagents to the catalytic active site due to the low porosity.
The catalyst was prepared by treating the dimer of Rh car-
bonyl iodide, [Rh(CO)2I]2, with the bpim-CTF support dispersed
in methanol under CO atmosphere; this reaction afforded a
black powder. SEM analysis of the thus-prepared catalyst re-
from 1763 m2 g−1 and 0.84 cm3 g−1 to 1651 m2 g−1 and 0.79 cm3
g−1, respectively, indicating a partial occupation of the pore by
the ligated Rh complex in the framework (Table 1). It is none-
theless worth mentioning that the catalyst still exhibits enough
porosity for the reagents to access the active metal site.
Furthermore, an EDS analysis was performed to confirm
the distribution state of Rh on the support. A homogenous
distribution of Rh throughout the framework was found, sug-
gesting the uniform ligation of Rh metal center to the abun-
dant N- binding sites (Figure 2, top right). In addition, iodine
was also homogenously distributed throughout the framework,
indicating the formation of a RhI complex (Figure S3).
High-resolution transmission electron microscopy (HR-TEM)
and high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) analysis were performed to
understand the dispersion state of Rh complex (Figure 2, top
right). However, the detection of the Rh single atoms on the
bpim-CTF support was unsuccessful. Meanwhile, it has been
reported that Rh atoms can be reduced to zero-valent metal
nanoparticles in the preparation steps.16 Consequently, the
HR-TEM image showed that no nanoparticle was found in the
Rh-bpim-CTF catalyst with different magnifications. HAADF-
STEM analysis revealed the same result.
vealed irregular block-shaped morphologies (>10 μm) (Figure 2,
top left). ICP-OES analysis was performed to determine the
precise loading amount of Rh in the catalyst, which was de-
termined to be 0.61 wt% of Rh (Table 1). Brunauer–Emmett–
Teller (BET) surface area analysis revealed the presence of Rh
complex in the CTF supports. Thus, the surface area and pore
volume of the catalyst was found to decrease after metalation
XPS analysis was performed to precisely elucidate the chemi-
cal state of the compositions in the catalyst, and the results
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Table 2. Catalytic performances of the Rh-bpim-CTF under various reaction conditions.a
were referenced relative to the C1s value (284.6 eV, C-C). The
deconvoluted C 1s spectrum was fitted using four peaks with
electron-binding energy (EBE) of 284.6 eV (C–C/C–H species),
285.4 eV (C–N species), 286.5 eV (C–O species), and 288.2 eV
(O–C=O species), (Figure S6, top left). Similarly, the deconvo-
luted N 1s spectrum consists of four peaks with binding energy
with 398.8 eV (pyridinic N species), 400.0 eV (pyrrolic N spe-
cies), 401.1 eV (quaternary N species), and 403.3 eV (oxidized
N-O species), (Figure S6, top right). 39 The peaks are attributed
to the triazine, pyridine, and imidazolium in bpim-CTF, which is
binding site with Rh species. Further, to elucidate the change
of electronic environment of the Rh around the bpim-CTF sup-
port, Rh-bpim-CTF and other Rh species, i.e., [Rh(CO)2I]2 and
RhCl3 were also confirmed (Figure 2, bottom left and Figure S6
bottom left and right). The Rh 3d5/2 EBE was found to be 308.4
eV, 308.7 eV, and 309.8 eV respectively. Thus, the results
proved that EBE of Rh on bpim-CTF support was similar to
[Rh(CO)2I]2, which can be inferred to Rh(I) oxidation state (Fig-
ure 2, bottom left).42, 43 Moreover, this EBE value decreased by
0.3 eV from that of [Rh(CO)2I]2, which is most likely caused by
the increased electron density in the Rh center owing to the
electron donation from the N-rich bpim-CTF support. XPS
measurements also showed the Rh/I atomic ratio to be 1:1.26
(Table S3), indicating the formation of a RhI complex incorpo-
rated on the bpim-CTF support.
b
W/Fm
T
Pc
Conv.d
TOFe
Produc.f
[molAcOR
Select.d
Entry
[
oC
]
[ bar
]
MeOH
[ %]
[molAcOR
MA
AA
g h mol−1
molRh-1 h-1
]
kgCat-1 h-1
]
[%]
[%]
1
2
3.8
3.8
3.8
3.8
3.8
3.8
3.8
3.8
1.9
2.5
7.6
150
180
210
240
270
240
240
240
240
240
240
10
10
10.3
24.8
47.6
62.1
72.5
40.6
77.0
83.2
35.7
43.0
94.6
457
27.1
88.0
12.0
5.7
1101
2113
2756
3218
1802
3417
3693
3169
2863
2100
65.2
94.3
89.1
75.3
76.5
89.1
67.7
62.5
92.7
85.8
44.0
3
10
125.2
163.4
190.7
106.8
202.6
218.9
187.8
169.7
124.4
10.9
24.7
23.5
10.9
32.3
37.5
7.3
4
10
5
10
6
7.5
12.5
15
7
8
9
10
10
11
10
14.2
56.0
10
aReaction conditions: 0.2 g of Rh-bpim-CTF was fixed on the stainless-bar reactor,
and pressurized up to the desired pressure and temperature. bA ratio of catalyst
weight and methanol feed gas flow, i.e., contact time. cTotal pressure at the
desired temperature dConversion and Selectivity of the products were analyzed
Meanwhile, an IR spectroscopy analysis was performed to
track the presence of CO in the Rh-bpim-CTF catalyst using KBr
pellet. The results revealed the absence of CO; the loss of CO
may have occurred in ambient atmosphere (Figure S7). How- using a gas chromatography. eTurn over frequency (TOF) = mole of acetyl prod-
ucts per mole of Rh per hour at steady-state conversion. fProductivity = mole of
ever, a diffuse reflectance infrared Fourier transform (DRIFT)
acetyl products per kilogram of the catalyst per hour at steady-state conversion.
spectroscopy analysis under a CO flow exposed the presence
of CO peaks in the region of 1980–2030 cm−1, which are the
ratio of catalyst weight and methanol feed gas flow, i.e., con-
characteristics of the symmetric and asymmetric stretching of
tact time, (W/Fm) of 3.8 g h mol−1 (Table 2, entry 1-5). The
a terminal CO ligand (Figure 2, bottom right). This result con-
catalytic methanol conversion was found to be low (yield <10%)
firms that the incorporated Rh complex readily provided the
at 150°C; however, increasing the reaction temperature led to
appropriate coordination site for free CO, thereby constituting
a gradual increase of the methanol conversion with 62% yield
the catalytic active species in the methanol carbonylation pro-
at 240°C and reached a maximum of 72.5% at 270°C, affording
cess.
a TOF of 2756 and 3218 h-1 respectively. MA and AA were the
In a typical methanol carbonylation experiment, the catalyst
major products of the reaction. The effect of the reaction tem-
was fixed at the center of the stainless-bar reactor (Figure S8);
perature on the selectivity of the methanol carbonylation was
the catalyst was then pressurized up to targeted pressure (7.5-
also considered. The selectivity for MA increased noticeably at
15 bar) with CO and heated to targeted temperature. After
180°C (94.3%), whereas that for AA decreased (5.7%). As the
reaching the targeted level, the pressure was controlled using
temperature increased, the formation of AA gradually rose up
a back-pressure regulator under a continuous flow of a mix-
to a maximum of 24.7%, and the MA formation reached a min-
ture of CO and N2 (9:1) with desired feed flow rate. Methanol
imum of 75.3% at 240°C. Further catalytic activity tests were
and methyl iodide were preliminarily mixed in liquid phase
performed at 240°C to avoid possible deactivation of the cata-
(10:1 molar ratio) and supplied to the reactor using a high-
lyst derived by the thermal degradation of bpim-CTF support.38
pressure liquid pump following the feed flow conditions (Table
Additional experiments were conducted to evaluate the de-
S4). The liquid phase reagents were pre-heated up to 240°C to
pendence of the catalytic activity on the pressure change (Ta-
their vapor phase and subsequently mixed with the CO before
ble 2, entry 4, 6-8). A linear increase of the methanol conver-
they entered the reactor. The gas-phase reaction media was
sion was observed as the pressure increased. Notably, such an
cooled down after passing through the reactor to be collected
effect was more evident at a high pressure, which may be at-
in the liquid form.
tributed to the increased accessibility and concentration of
To evaluate the effect of the temperature on the Rh-bpim-
reagents. In particular, the increased methyl iodide partial
CTF catalytic carbonylation, the reactions were performed in
pressure imparts rate-accelerating properties to the methanol
the temperature range of 150°C–270°C at 10 bar and with a
carbonylation, which is known to be the first order reaction for
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methyl iodide.12, 44, 45 The boiling points of methanol and me-
thyl iodide also rise at high pressures and turn the reaction
into its liquid phase. Owing to this, a 10-bar pressure was
adopted as the reaction condition to maintain the reaction in
the gas phase.
100
100
80
60
40
20
0
80
60
40
Further reactions were investigated to evaluate the influence
of the contact time of a substrate to active site on the reactivi-
ty and selectivity of the catalyst (Table 2, entry 4, 9-11). The
results showed that the methanol conversion increased rapidly
with W/Fm ratio = 7.6 g h mol−1 and reached a 94.6% conver-
sion, reaching a TOF of 2100 h-1 and productivity of 124.4 mol
kg-1 h-1. At the same time, it was observed that the selectivity
for AA reached its maximum at 56.0% and that for MA at
44.0%. This is likely caused by an increase in the hydrolysis
ratio of MA to AA with the increased reaction time.22, 24 The
methanol conversion was almost linearly reduced to a mini-
mum value of 35.7% at W/Fm = 1.9 g h mol−1 as the contact
time decreased. The selectivity for AA dramatically reduced up
to 7.3%; in contrast, the best MA yield of 92.7% was obtained
with a TOF of 3169h-1 and productivity of 187.8 mol kg-1 h-1.
This efficiency was higher than that of other catalysts that
were previously reported 13, 14, 16-24; this phenomenon can be
explained in terms of the increased electron donation effects
from the bpim-CTF to the Rh metal center.
MeOH Conversion
MeOAc selectivity
AcOH selectivity
20
0
0
10
20
30
40
Time / h
Figure 3. Stability test of the Rh-bpim-CTF for heterogeneous methanol car-
bonylation
Conclusions
The heterogenization of a Rh-based molecular catalyst on a
charged bpim-CTF support has been explored for the car-
bonylation of methanol. It was found that the Rh complex was
incorporated as a single-site catalyst throughout the support
via the ligation between the Rh centers and the N-derived
binding sites on the pore surface. The Rh-bpim-CTF catalyst
exhibited an excellent catalytic conversion of methanol of
93%, corresponding to the TOF of 2100 h-1 and productivity of
124.4 mol kg-1 h-1 with selectivities of 56% for AA and 44% for
MA at 240°C temperature, 10-bar pressure, and a W/Fm ratio
of 7.6 g h mol−1. Besides, in terms of methyl acetate synthesis,
the selectivity for MA increased to 92.7% at a W/Fm ratio of 1.9
g h mol−1, featuring TOF of 3169 h-1 and productivity of 187.8
mol kg-1 h-1 which constitutes an excellent value for the con-
tinuous methanol carbonylation in vapor phase. We attribute
this result to the electron donation effect of the abundant N
atoms on the pore surface of the CTF support. Furthermore,
the CTF supported heterogeneous catalytic system exhibited
an outperforming stability in a 40-h process, which is attribut-
ed to the ion-pair interaction between Rh centers and support,
thereby providing extra stability to the catalyst. Thus, the pro-
posed charged CTF is a promising candidate as a highly effi-
cient and robust catalytic support for the continuous manufac-
turing process of acetyl compounds in industrial applications.
Catalyst stability is one of the most significant factors to es-
tablish the potential usefulness of a catalyst industrial scale.
We therefore evaluated the stability of the Rh-bpim-CTF sys-
tem in the methanol carbonylation process by screening the
catalytic activity under the conditions of W/Fm = 7.6 g h mol−1,
at the temperature of 240°C, and at 10-bar pressure with con-
tinuous flow of reagents. To assess the effect of cationic
charge on the bpim-CTF support, the Rh-based catalyst het-
erogenized on a bipyridine based CTF support (Rh-bpy-CTF)
was also synthesized and used for screening the catalytic activ-
ity (Figure S10). The Rh-bpy-CTF showed less catalytic activity
with the slight deactivation over time. This may be attributed
to the weak binding of Rh active species during the flow reac-
tion, resulting in leaching of metal. In contrast, the present
catalyst showed consistent catalytic activity during the process
(Figure 3); the Rh-bpim-CTF system showed long-term stability
and high efficiency in a 40-h process. Thus, the conversion of
methanol was 93.5% ± 1 with selectivities of 49% ± 5 for AA
and of 40% ± 3 for MA. This behaviour can be ascribed to the
strong ion-pair interaction between the [imidazolium]+ moiety
on the bpim-CTF support and the active species [RhI2CO2]−,
providing extra stability to the catalyst.
Acknowledgements
The authors acknowledge the financial support of National
Research Foundation under “Next Generation Carbon Upcy-
cling Project” (Project No. 2017M1A2A2043134) of the Minis-
try of Science and ICT, Republic of Korea. Gas physisorption
analysis were conducted under the framework of Research
and Development Program of the Korea Institute of Energy
Research (KIER, B6-2434). Finally, we gratefully thank Miyeon
Jang for performing SEM measurements.
The thermal gravimetric analysis (TGA) results also exhibited
that the overall weight of the Rh-bpim-CTF was well-
maintained at the temperature of 240°C except for the slight
evaporation of water in the time to raise the temperature (<
7%), featuring high thermal stability of Rh-bpim-CTF (Figure
S11). Further, the catalyst was confirmed by TEM analysis after
the methanol carbonylation to check the transformation of Rh
atoms to nanoparticles under the catalytic reaction conditions
(Figure S12). In conclusion, none of the Rh nanoparticles was
observed by HR-TEM image in different magnification and by
HAADF-STEM.
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