Hydrotalcite anchored ruthenium catalyst for CO2 hydrogenation reaction

Article abstract of DOI:10.1515/chem-2018-0094

We developed a series of new organic-inorganic hybrid hydrotalcite functionalized Ru catalytic systems. All the developed materials have been studied by FTIR, N₂ physisorption, ICP-OES, XPS, NMR (¹H, ¹³C, ²⁹Si) and TEM analysis were performed to know the physiochemical behavior and structural morphology of functionalized hydrotalcite materials. XPS results strongly suggest that it involves the formation of N-Ru coordination bonds. We applied these well analyzed materials for CO₂ hydrogenation reaction as catalyst (with and without ionic liquid medium). We found that Ru metal containing functionalized hydrotalcite materials were highly active and stable (in terms of catalyst leaching and recycling). The heterogeneous catalyst can be easily recovered and reused 8 times without significant loss of catalytic activity and selectivity, which is a better green alternative for practical applications.

Full text of DOI:10.1515/chem-2018-0094

Open Chem., 2018; 16: 853–863
Research Article
Open Access
Vivek Srivastava*
Hydrotalcite Anchored Ruthenium Catalyst for CO2
Hydrogenation Reaction
https://doi.org/10.1515/chem-2018-0094
received February 26, 2018; accepted May 13, 2018.
different industries such as leather, rubber, and fragrance
[1,3].
Abstract: We developed a series of new organic-inorganic
Many reports have been published to utilize the
hybrid hydrotalcite functionalized Ru catalytic systems. application of homogenous as well as heterogeneous
All the developed materials have been studied by FTIR, catalysts with and without additives to hydrogenate CO2
N2 physisorption, ICP-OES, XPS, NMR (1H, 13C, 29Si) and gas to formic acid [4,8]. The main obstacle to achieve
TEM analysis were performed to know the physiochemical selective and easy hydrogenation of CO2 is the positive
behavior and structural morphology of functionalized standard free energy of the hydrogenation reaction [1,4-8].
hydrotalcite materials. XPS results strongly suggest A series of supported and unsupported transition metal
that it involves the formation of N-Ru coordination complexes or nanocatalysts were utilized with or without
bonds. We applied these well analyzed materials for CO2 a base to achieve a reasonable conversion of CO2 to formic
hydrogenation reaction as catalyst (with and without acid [9,10]. Nevertheless, the easy recovery of formic acid
ionic liquid medium). We found that Ru metal containing and catalyst recycling, as well as the requirement of the
functionalized hydrotalcite materials were highly active base as an additive, are still a challenge for the scientific
and stable (in terms of catalyst leaching and recycling). community. This problem is also considered as one of
The heterogeneous catalyst can be easily recovered and the main shortcomings for the industrialization of CO2
reused 8 times without significant loss of catalytic activity hydrogenation reaction [10,11].
and selectivity, which is a better green alternative for
practical applications.
A series of unsupported, homogeneous transition
metal catalysts such as Ru, Rh, Ir and Pt were effectively
used to catalyze the CO2 hydrogenation reaction [1,9].
Unfortunately, very few scientific reports are offered on
heterogeneous catalysts for CO2 hydrogenation reaction.
Organic-inorganic hybrid catalysts are considered as a
promising type of tethered heterogenous catalysts and are
designed to retain the selectivity of homogeneous catalysts
while being immobilized on heterogeneous support to
Keywords: Hydrotalcite; Ru metal; Carbon dioxide; Ionic
Liquid; Formic acid.
1 Introduction
The escalating level of carbon dioxide is one of the obtain easy separation [9]. In one of the recent reports,
important pressing environmental concerns of our age Baiker et al. reported that the synthesis of transition
and CO2 is considered as a major greenhouse gas [1,2,3]. metals incorporated silica material as catalyst followed by
Carbon capture and storage gave an alternative to reduce the co-condensation method to catalyze the preparation
CO2 concentration in the environment but surprisingly of N, N-diethylformamide [12]. In another report, Hicks
the carbon capture process alone increases the demand and Jones prepared a series of silica tethered Ir metal
for energy by 25-40% [3]. In most of the reports, CO2 is catalyst for the selective hydrogenation of CO2 gas [13].
utilized as a cheap C1 source in the production of value- Although, the mentioned catalytic systems were found
added chemicals like formic acid, methanol, urea, and to be effective for CO2 hydrogenation, these systems also
polycarbonates [1]. Formic acid is widely applicable in suffered in terms of long reaction, high catalysts loading
and catalysts leaching.
Hydrotalcite clay (HTc) is a naturally occurring
material having layered double hydroxide (LDH) structure
*Corresponding author: Vivek Srivastava, NIIT University, NH-8
[14,15]. The general formula of hydrotalcite material is
Jaipur/Delhi Highway, Neemrana, Rajasthan, Pin Code: 301705,
[M1-x2+Mx3+ (OH)2]x+ (An-)x/n.mH2O, where A are anions like
-
India, E-mail: vivek.shrivastava@niituniversity.in
Open Access. © 2018 Vivek Srivastava, published by De Gruyter.
NonCommercial-NoDerivatives 4.0 License.
This work is licensed under the Creative Commons Attribution-
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Vivek Srivastava
2-
2-
CO3 , OH-, Cl- or SO4 . The morphology of natural and adsorption analyzer (Micromeritics Instruments Inc).
synthetic HTc is like brucite clay but the basicity varies All the samples were degaussed at 250°C for 2 h before
with the concentration of cations [13-16]. Such variation taking the measurements. The surface area and pore size
makes the HTc more interesting in terms of the catalyst as distribution (PSD) were measured from the BET and BJH
well as catalyst support [16,18]. Therefore, HTc materials equations, respectively, by the instrument software. All
have been effectively applied in numerous areas such the hogenation reactions were carried out in a 100 mL
as adsorption materials for gases (CO2, H2S, and SO2), stainless steel autoclave (Amar Equipment, India).
precursors for functional materials, photochemistry and
electrochemistry [15,16]. In addition, the presence of
hydroxyl groups makes the HTc as an active and recyclable 2.1 Synthesis of Hydrotalcite material
solid base catalyst for a variety of organic reactions [18]. In
recent years, the applications of HTc have been increased The hydrotalcite with Mg and Al species was synthesized
not only as heterogeneous catalyst or catalyst support using the coprecipitation method [16]. An aqueous
but also as anion exchanger, CO2 absorbent and in water solution containing Mg(NO3)2. 6 H2O (80.0 g) and Al(NO3)3.
treatment[18,19].Asacatalystsupport,HTshavebeenused 9 H2O (37.74 g) in 225 ml of water (solution A) was prepared
to immobilize various transition metal nanoparticles and (while maintaining a Mg/Al molar ratio of 3). A separate
homogeneous catalysts [19-21]. Alkoxysilane linkage is one solution B, containing NaOH (54 g) and Na2CO3 (21.6 g) in
of the easiest ways to link homogeneous or heterogeneous 675 ml of water was prepared separately. Solution A and B
catalysts over a hydrotalcite surface [21-23]. Surprisingly, were simultaneously added dropwise into distilled water
anchoring the homogeneous or heterogeneous catalysts under vigorous mechanical stirring while maintaining the
over silica and alumina is well studied but there are still pH of the resulting solution in the range of 9.5-10 at 55°C
very few reports available with respect to alkoxysilane temperature. The slurry was ripened for 30 min under
interaction with hydrotalcite clay [21].
forceful stirring at 55°C and was allowed to stand in its
In this report, we are offering the synthesis of mother liquor for 3 h. The precipitate was washed several
hydrotalcite tethered Ru metal catalyst (HRUC) for the times until a pH of 7 was reached and was then dried at
hydrogenation of CO2 gas in the functionalized ionic liquid 110°C for 12 h. The calcined hydrotalcite (HT) was obtained
medium. The HRUC catalytic system was synthesized by by calcining the hydrotalcite (HTc) at 600°C for 4 h.
a multistep grafting process using an iminophosphine
ligand tethered to a hydrotalcite inorganic support.
2.2 Synthesis of iminophosphine ligands
-Moiety A and iminophosphine ligands with
2 Experimental
Ru metal- Ru-Moiety A
Reagent Plus® grade chemicals were purchased from A 100 ml round bottom flask was charged with
Sigma Aldrich and other chemical suppliers. FTIR 2-(diphenylphosphine) benzaldehyde (2.5 mmol) and
measurements were conducted with Bruker Tensor 27 in 1-propylamine (10 mL). All reactants were refluxed
DRIFT mode (KBr powder) with a scanning range 400- under nitrogen for 5 hours to obtain the Schiff- base.
4000 cm-1. Perkin Elmer Optima 3300 XL ICP-OES was After cooling, the reaction mass to room temperature,
used to study the elemental analysis. Nuclear Magnetic monophosphate ligand was isolated as a red-brown oil
Resonance (NMR) spectra were recorded on a standard after vacuum distillation and column chromatography
Bruker 300WB spectrometer with an Avance console at (yield 91%).
400 and 100 MHz for 1H NMR. The morphology of catalysts
1H NMR (400 MHz, CD2Cl2): δ= 9.02 (s, 1H), 8.04 (s, 1H),
was investigated by transmission electron microscopy 7.44 - 7.31 (m, 12 H), 6.97 (s, 1 H), 3.42 (t, 2 H), 1.57 - 1.53
13
(TEM) using a Philips CM12 instrument. Kratos-Axis 165 (q, 2 H), 0.82 – 0.77 (t, 3 H); C NMR (100 MHz, CD2Cl2):
with Mg Kα radiation 1254 eV was used to perform X-ray δ= 160.02, 134.25, 128.73, 63.34, 23.95, 11.79 ppm. 31P NMR:
photoelectron spectroscopy (XPS). DTA-TGA thermal (300 MHz, CD2Cl2, ppm) δ= - 13.02 ppm.
analyzer apparatus (Shimadzu DTG-60H) was used to
Ru-Moiety-A was obtained by reacting moiety -A (2 g)
study the thermal stability of an ionic liquid. BET surface with RuCl3.3H2O (2.1 g) in dry ethanol (25 mL) under an inert
area, pore size, and pore volume measurements of the atmosphere. The combined reaction mass was refluxed
catalysts were determined from a physical adsorption of for 12 hours. After cooling, the reaction mass was washed
N2 using liquid nitrogen by an ASAP2420 Micromeritics with dry ethanol (10 x 2 mL) under a nitrogen atmosphere.
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Dried material (Ru-Moiety-A) through lyophilizer was kept 2.6 Synthesis of Ru-Tethered Pre- catalyst
under nitrogen atmosphere.
(HRUC-A to D)
The ABIL-HT-A to D was added to RuCl3. 3H2O in anhydrous
ethanol (25 mL) under nitrogen by keeping the ratio of Ru
and phosphorus (1:1). The resulting reaction mixture was
2.3 Synthesis of Alkoxysilane -Moiety B
A 100 mL round bottom flask was charged with refluxed for 10 hours. After cooling the reaction mass, the
2-(Diphenylphosphino) benzaldehyde (0.75 g), resulting solid material was carefully washed several times
3-(Trimethoxysilyl)propylamine (0.375 g) and 40 mL dry with dry ethanol (10 x 2 mL) under a nitrogen atmosphere.
THF under a nitrogen atmosphere. The combined reaction The recovered black solid was dried using a lyophilizer and
mass was reacted at 100oC for 5 hours. After completion then carefully stored under inert atmosphere (yield 98%).
of the reaction, we obtained an alkoxysilane compound
with a bidentate iminophosphine ligand as a yellow oily
liquid (after the careful removal of solvent and volatile 2.7 CO2 hydrogenation reaction and
impurities followed by vacuum distillation and column
recycling experiment
chromatography) (yield 95%).
1H NMR (400 MHz, CD2Cl2): δ = 8.9 (s, 1H), 8.03 (s, 1H), After performing the catalysts pretreatment process at
7.45-7.28 (m, 12 H), 6.93 (s, 1 H), 3.52 (s, 9 H), 3.47 (m, 2 H), 45oC for 20 minutes under 10 MPa pressure of hydrogen
1.68 - 1.61 (m, 2 H), 0.58 – 0.51 (m, 2 H); 13C NMR (100 MHz, gas, all the gases were completely replaced with nitrogen
CD2Cl2): δ= 158.58, 134.15, 128.78, 65.15, 50.80, 24.61, 8.74 gas at room temperature. Then all the reactants (except
ppm.; 31P NMR : (300 MHz, CD2Cl2, ppm) δ= -13.05ppm.
H2 and CO2 gas) were added along with a known quantity
of dioxane (internal standard for HNMR analysis) to the
1
reaction vessel (as per Table 3 and 4) without opening the
autoclave. Then nitrogen gas was completely replaced by
CO2 gas (by 2-3 times flushing). Absorption of CO2 gas was
carried out at 80oC with 20 bar pressures for 1 hour. Later,
2.4 Synthesis of phosphine functionalized
calcined hydrotalcite (ABIL-HT-A to C)
A post-synthetic grafting method was applied to obtain after reducing the temperature of the autoclave to 40oC,
phosphine functionalized calcined hydrotalcite material hydrogen gas was added into the reactor. The combined
under an inert atmosphere. Phosphine ligand (2 mmol) reaction mass was permitted to cool at room temperature
was allowed to react with calcined hydrotalcite (1 g) in dry before opening the reaction vessel. A small quantity
1
toluene under (20 mL) nitrogen atmosphere. The reaction of crude reaction mass was used for HNMR analysis
mass was stirred for the next 24 hours at room temperature and titration to quantify the amount of formic acid in
(30-35oC). The resulting solid mass was washed several the reaction sample. No signs of ionic liquid, as well as
1
times to dry the toluene (10 x 2 mL). The recovered solid dioxane decomposition, were recorded during H NMR
was carefully dried in a lyophilizer. The pale-yellow solid analysis. The results obtained from 1H NMR analysis were
(ABIL-HT-A to C) was carefully placed under a vacuum at found to be in good agreement with the titration method.
room temperature (yield 98%).
Recovery of formic acid was carried out easily under
reduced pressure. At 50oC initially, all the water and other
o
volatile impurities were removed at 75-80 C, and under
2.5 Synthesis of amine functionalized
nitrogen flow the formic acid was isolated.
calcined hydrotalcite (ABIL-HT-D)
Calcined hydrotalcite (1 g) was mixed with 2.7.1 Catalyst recycling with ionic liquid
3-aminopropyltrimethoxysilane (1.2 mmol) in dry toluene
(20 mL). The combined reaction mass was heated and After careful isolation of formic acid, the ionic liquid
vigorously stirred at room temperature (30-35oC) for next immobilized catalytic system was washed several times
24 hours under a nitrogen atmosphere. The resulting with dry diethyl ether (5 x 2 mL) to remove all the organic
solid material was washed and filtered with dry toluene. impurities and then further dried in a lyophilizer. The ionic
(10 x 2 mL) The solid was then dried in a lyophilizer. The liquid immobilized catalytic system was again pretreated
light yellow solid (ABIL-HT-D) was sensibly stored under at 45oC for 20 minutes under 10 MPa pressure of hydrogen
vacuum in a nitrogen atmosphere (Yield 98%).
gas, before going to recycling for CO2 hydrogenation.
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3 Results and Discussion
P
N
Synthetic hydrotalcite was prepared via a well-reported
coprecipitationmethodusingMg(NO3)2 andAl(NO3)2,under
basic condition. The corresponding white precipitates
were dried at 110oC and further calcined at 600oC for
4 hours. The alkoxysilane containing the bidentate
iminophosphine ligand [o-Ph2PC6H4CH=N(CH2)3Si(OMe)3]
A was synthesized by reacting 2-(Diphenylphosphino)
benzaldehyde with 3-(Trimethoxysilyl)propylamine under
a nitrogen atmosphere in dry THF. All the analytical
data obtained from NMR analysis (1H/13C/31P) were found
in agreement with the reported data for the same [9,21].
Again, iminophosphine ligand A was grafted on the
surface of a calcined hydrotalcite material by refluxing
the reaction mass in dry toluene for 24 hours. Unreacted
or loosely coordinated iminophosphine ligand A was
isolated by Soxhlet extraction method. The resulting
solid material was dried under vacuum at 100oC to
obtain ABIL-HT-A (Figure 1). At last, ABIL-HT-A went to
the metalation step. The resulting HRUC-A material was
obtained in the black color solid and properly stored in
an argon atmosphere. In addition, we synthesized a series
of hydrotalcite anchored monodentate phosphine-based
materials such as HRUC-B, HRUC-C and HRUC-D using
ABIL-HT-B, ABIL-HT-C and ABIL-HT-D respectively (Figure
1). We also created an iminophosphine ligand without out
alkoxysilane moiety A to use them as a reference material
to understand the physiochemical properties of our
developed materials (Figure 2).
NH2
Si
P
P
Si
O
Si
Si
O
O
O
O
O
O
O
O
O
O
O
ABIL-HT-D
ABIL-HT-C
ABIL-HT-B
ABIL-HT-A
Figure 1: Structures of Hydrotalcite tethered phosphine ligands.
Figure 2: DRIFT analysis data for Moiety A and B.
We used various sophisticated techniques to
understand the physiochemical properties of HRUC
13
materials (Figure 2 and 3) like C & 29Si NMR, DRIFT,
FTIR, XPS, elemental analysis using AAS, BET surface
analysis and inductively coupled plasma optical emission
spectroscopy (ICP-OES).
Figure 3: 13 C analysis of HRUC-A (after removal of the solvent peak).
2.7.2 Catalyst recycling without ionic liquid
A solid state 13C & 29Si NMR analyss was applied
to understand the surface structure of functionalized
After the successful isolation of formic acid, the catalytic hydrotalcite materials (Figure 3 and 4). This analysis
system was washed several times with ether to remove helps to understand the physiochemical structure of
all the organic impurities from the catalysts and then the hydrotalcite with alkoxysilane moiety. The 13C NMR
catalyst was dried in a lyophilizer. The fresh catalyst was spectrum of an organic moiety of HRUC-A confirmed
added to the used catalyst, in case of any catalyst’s loss that the bidentate iminophosphine ligand is properly
during the workup process. After adjusting the quantity of intact with hydrotalcite support. This was also confirmed
catalyst again the autoclave was charged with a pretreated from the report published by Wang et al. [24]. No peak
catalyst and reactants as per the above-mentioned corresponding to the methoxy group of trimethoxysilane
13
method.
was found in C NMR analysis which also supports the
Ethical approval: The conducted research is not idea that all the three methoxy groups have been lost
related to either human or animal use.
during functionalization of hydrotalcite as HRUC-A. This
data confirms the tripodal (T3) type arrangement of silane
29
moiety over hydrotalcite. Si NMR analysis provided the
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Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
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Figure 4: 29Si analysis of HRUC-A.
Figure 6: DRIFT analysis data for HRUC-A to D with respect to
Hydrotalcite.
Before going to the metalation step, we performed
surface analysis using N2-physisorption analysis for
hydrotalcite (before and after calcination) and after the
functionalization calcined hydrotalcite clay. All the data
were tabulated in Table 1. A noticeable increase in the
Figure 5: DRIFT data for HRUC-A and ABIL-HT-A with respect to
Hydrotalcite.
insight nature of M-O-Si bonds on the hydrotalcite surface BET surface area, pore size, and pore (single point pore
[24,25]. The two distinct peaks confirmed in 29Si NMR volume measured at P/Po =0.97 on absorption) volume
confirmed the anchoring of silane groups to Mg3-OH and was recorded in the calcined hydrotalcite material with
AlMg2-OH environment respectively. Similar results were respect to normal hydrotalcite material. A visible drop in
observed in other HRUC-B to D materials [24-26].
terms of the BET surface area, pore size and pore volume
We performed diffuse reflectance infrared fourier were found in functionalized calcined hydrotalcite
transform spectroscopy (DRIFTS) for iminophosphine materials (ABIL-HT-A to D). The lowest value of BET
ligand (without alkoxysilane) and iminophosphine ligand surface area, pore size, and pore volume was noted for
with alkoxysilane moiety. Collectively, we used this data ABIL-HT-B. After the completion of the metalation process
to understand the structure of HRUC-A to D. We located for ABIL-HT-A to D materials, we also checked the BET
the position of a Schiff base (CH=NR) double bond near to surface area, pore size and pore volume along with the
1640 cm-1. The vibrations peaks of phenyl rings in HRUC-A elemental analysis. This data helped us to understand the
and B materials located in the following regions 3061, metallic composition of both ABIL-HT-A to D and HRUC-A
1580, 1430, 741 and 694 cm-1. Similar peaks for phenyl to D materials (Table 1). Considering the matrix effect, we
rings were observed in ABIL-HT-A and ABIL-HT-B. Such performed the elemental analysis using the ASS method
data confirms the presence of the Schiff base before and to confirm the quantity of Mg, Al, Si and Ru content in our
after the metalation of our developed materials. In FTIR developed materials. The silica content was only recorded
analysis, we also recorded different types of absorption in functionalized hydrotalcite clay, in the same pattern Ru
peaks for HRUC-A to D (Figure 5 and 6).
metal signal was only logged for HRUC-A to D materials. A
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slightly higher amount of Ru metal was found in HRUC-A
material with respect to other HRUC-B to D materials.
XPS analysis was performed to understand the
composition of different HRUC-A to D materials. The Ru
3d, C1, N 1S and P2p levels were calculated at a normal
angle with respect to the plane of the surface during XPS
spectra calculation. The binding energy was carefully
measured with a precision of 0.2 eV. Shirley background
subtraction, as well as Gaussian and Lorentzian principal
for peak shape, was considered while doing XPS
analysis of Ru 3d levels. The peaks near to 280.2 eV (Ru
3d5/2) and 284.3 eV (Ru 3d3/2) inveterate the attendance
of Ru (0) species. Unexpectedly, no signs of RuO2 were
recorded while performing the XPS analysis of HRUC-A
to D materials. This observation was also supported as no
peak was found to represent Ru 3p with binding energy
464 eV which confirmed the absence of Ru species with
+2 oxidation state. We also linked the XPS data of RuCl3/
calcined hydrotalcite material with XPS data of HRUC-A
to D materials and intimated the occurrence of Ru (0)
species.
Hydrotalcite
Calcined hydrotalcite
HRUC-A
HRUC-B
HRUC-C
HRUC-D
Some resemblance with respect to Ru 3d5/2 and Ru
3d3/2 was found between HURC-B and RuCl3/Calcined
hydrotalcite materials which established the low loading
of phosphine ligands on support. In contrast, much
higher phosphine loading was recorded in HRUC-C, which
resulted in a higher binding energy for Ru 3d5/2 and Ru 3d3/2.
The binding energies of HRUC-A for Ru 3d5/2 and Ru 3d3/2
HRUC-A after catalysis HRUC-A after catalysis HRUC-A after catalysis
(after 4st run) in amine
system
(after 7st run) in ionic
liquid system
(after 4th run) in ionic
liquid system
Figure 7: TEM images of HRUC catalysts
were found almost like Ru-PNPr1. These results indicate of perfume and fragrance [1]. Transition metal complex
that the location of Ru metal in HRUC-A is identical to catalysts were widely applied to utilize CO2 gas as a C1
Ru-PNPr1 species. TEM data also used to confirm the synthetic unit in formic acid synthesis. Although, this
morphology of the Ru metal loaded functionalized developed catalytic system offers the production of
hydrotalcite material (Figure 7). TEM image of all the formic acid, these catalytic protocols also suffer from
HRUC-A to D materials along with hydrotalcite and the selectivity and low reactivity of CO2 gas due to its
calcined hydrotalcite materials were recorded. The image high thermodynamic stability (+ΔGo298= 32.9 KJ/mol).
analysis of HRUC-A to D materials inveterate the presence Considering the above-mentioned facts, we tested our
of well dispersed Ru nanometal in the range of 8-10 nm (
developed catalytic systems (HRUC-A to D) for the selective
0.25) with a mean diameter of 4.5 nm. TEM image analysis hydrogenation of CO2 to formic acid under high-pressure
also disclosed the presence of highly crystalline nature of reaction condition in the presence of triethylamine and a
Ru nanometals.
small quantity of water at 80oC. We used cyclohexanone
as an internal standard in the reaction mass.
All results obtained while using HRUC-A to D catalysts
were tabulated in Table 3, entry 1-18, Scheme 1. The
quantity of formic acid was calculated using 1H NMR and
acid-base titration (Figure 8). Dioxane was added in the
3.1 Hydrogenation of CO2 to formic acid
without ionic liquid medium
Formic acid is one of the important chemicals in organic reaction mass before starting the reaction as an internal
chemistry. It has been reported as a starting chemical standard. After completion of the reaction, a small
for the synthesis of a large variety of beneficial chemical quantity of reaction mass was used for 1H NMR analysis,
derivatives such as aldehydes, ketones, amine and which confirms the formation of formic acid along with
carboxylic acid. It is also utilized in the manufacturing no formation of any side product during the reaction. The
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1300
1280
1260
1240
1220
1200
1180
1160
1140
First
Second
Third
Fourth
1196,01
TON value
1275,85
1220,71
1246,91
Figure 9: Results of Recycling tests of the HRUC-A catalytic system.
1
Figure 8: H NMR data after the hydrogenation of CO2.
used to perform the hydrogenation reaction. We found a
small quantity of formic acid with TOF value of 12 h-1. This
test confirmed the catalytic leaching during the reaction,
which was further supported by XPX and ICP-OES analysis
of Ru metal in the filtrate. The same observation was
observed while recycling the HRUC-A catalyst. We only
recorded the recycling test up to 4 cycles mainly because
Catalyst, Solvent, Amine
HCOOH (l)
CO2 (g) + H2 (l)
60-120oC, 2-7h
Scheme 1: Hydrogenation of CO2 in triethyl amine system.
1
quantity of formic acid calculated via H NMR was found of catalysts leaching, as well as Ru metal agglomeration
to be in good agreement with the data obtained from acid- (Figure 7). The size of Ru metal was increased from 8 to
base titration of reaction mass.
78 nm.
Asmallquantityofformicacidwasreportedinabsence
of triethylamine which confirmed that the basic nature of
hydrotalcite clay also influences the hydrogenation of CO2 3.2 Hydrogenation of CO2 to formic acid with
gas up to certain extent. RuCl3. 3H2O precursor, as well
as RuCl3, supported calcined hydrotalcite material gave
ionic liquid medium
formic acid with a low TON and TOF values. A low quantity Many attractive properties of ionic liquids like extremely
of formic acid was recorded with amine functionalized low volatility, high thermal stability, strong solvation
HRUC-Dcatalystswithrespecttophosphinefunctionalized nature for various substances and wide liquid-
HRUC-A to C catalytic system. This observation confirmed temperature range makes them promising alternative
the high activity of phosphine functionalized HRUC-A to solvents over conventional solvent systems. Additionally,
C catalytic systems for the hydrogenation of CO2 gas. The functionalized ionic liquid (by changing the anion or
bidentate phosphine functionalized HRUC-A catalyst gave cation or grafting functional group on to ions) extended
formic acid with a high TON and TOF value with respect the application ionic liquids in catalysis, extraction,
to other catalytic systems (HRUC-B to D). We performed absorption of gases such as CO2, H2, SO2 etc.
all the optimization steps with HRUC-A catalyst to get a
In our previous studies, we synthesized a series
high quantity of formic acid. Changing the temperature, of functionalized ionic liquid liquids such as 1-(N,N-
reaction time, quantity of reactant/catalyst quantity gave dimethylaminoethyl)
a noticeable change in formic acid production. Ru metal trifluoromethanesulfonate
2,3-dimethylimidazolium
([mammim][TfO]), and
loaded moiety-A gave the lowest value of formic acid due 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium
to the Ru-Ru metal dimerization in the reaction solution. bis (trifluoromethylsulfonyl) imide ([DAMI][NTf2])
A filtration test was performed to know the stability were synthesized as per the reported procedures [31-32]
of HRUC-A to D catalytic systems. After the completion while ionic liquids such as 1-(N,N-dimethylaminoethyl)
of the hydrogenation reaction under optimized 2,3-dimethylimidazolium bis (trifluoromethylsulfonyl)
reaction conditions (Table 3, entry 4), the solid part imide ([mammim][NTf2]), 1-(N,N-dimethylaminoethyl)-
of the reaction was isolated through 0.45 micrometer 2,3-dimethylimidazolium
polytetrafluoroethylene (PTFE) filter and the filtered was ([mammim]
nonafluorobutanesulfonate
1-(N,N-
[CF3CF2CF2CF2SO3]),
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also expected that the competence of the HRUC-A catalyst
was increased due to the presence of two -NH functional
groups on both the ends of the ionic liquid. The synergic
effect of ionic liquid with HRUC-A catalysts was also
studied and it was found that while using [bmim][NTf2]
ionic liquid (as nonfunctional ionic liquid) with HRUC-A
catalyst, formic acid was obtained with low TON/TOF
value (Table 4, entry 16).
After completion of CO2 hydrogenation, the reaction
mass was heated to isolate first water and then formic
acid with the help of N2 flow. The presence of a base (ionic
liquid and hydrotalcite material) and formic acid in the
reaction mass developed an equilibrium between free
formic acid, formic acid salt, ionic liquid and hydrotalcite
material. The amount of free formic acid was increased by
increasing the temperature as the acid-base neutralization
to salt is an exothermic process. Considering that fact,
separation of formic acid was carried out under nitrogen
Figure 10: TGA analysis data of [DAMI][CF3CF2CF2CF2SO3] ionic liquid.
dimethylaminoethyl)-
trifluoromethanesulfonate
2,3-dimethylimidazolium flow at optimized temperature.
([mammim]
[BF4]),
All the CO2 hydrogenation results were tabulated in
1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium Table 4 and Scheme 2. Surprisingly no by-product was
1
trifluoromethanesulfonate ([DAMI][TfO]), , 1,3-di(N,N- reported during hydrogenation reaction (confirmed by H
d i m e t h y l a m i n o e t h y l ) -2 - m e t h y l i m i d a z o l i u m NMR). We started our study by changing the molar ratio
nonafluorobutanesulfonate
([DAMI][CF3CF2CF2CF2SO3]) of formic acid/HRUC-A+ionic liquid at 80oC and 20 MPa
and1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium as a function of time. This is represented in Figure 3. The
tetrafluoroborate ([DAMI][BF4]) in order to obtain a almost linear increase was found between the formic acid/
high degree of chemo selectivity, easy catalyst recycling HRUC-A+ionic liquid ratio near to 1:81 at 5 hours. After
and informal product isolation [4-8]. Moreover, we also this time no drastic change in ratio was reported while
did a comprehensive study on the absorption of CO2 increasing the temperature and after 6 hours the ration
gas in above mentioned functionalized ionic liquid. In between formic acid/HRUC-A+ionic liquid was found to
our study, we found 1,3-di(N,N-dimethylaminoethyl)-2- be equal to 2.1 at 6-10 h. This observation showed that free
methylimidazolium nonafluorobutanesulfonate ([DAMI] formic acid combined with the basic HRUC-A+ionic liquid
[CF3CF2CF2CF2SO3]) ionic liquid as a promising reaction system and formed the formic acid salt. In starting, formic
medium [4-9,18,19,20]. The density of this ionic liquid was acid neutralization was fast with respect to the formic
measured by using a capillary pycnometer and found 1.235 acid formation. At a high ratio of formic acid/ionic liquid,
g/mL. We also calculated the glass transition temperature most of the ionic liquid got neutralized but HRUC-A+ionic
of this ionic liquid using differential scanning calorimetry liquid system and formic acid produced remained same in
and recorded it to be -41oC. This ionic liquid was also the system.
analyzed by means of thermogravimetric analysis which
indicates that the ionic liquid is found to be stable up to 1-7). It was clearly observed in our study that the TON-TOF
225oC (Figure 10).
value of formic acid was increased by adding water. This
The effect of water was also evaluated (Table 4, entry
In this manuscript, we took the ionic liquid as a increase can be explained in two different manners. First,
reaction medium to perform the CO2 hydrogenation using the addition of water will reduce the viscosity of the ionic
HRUC-A catalyst. All the experimental results of the CO2 liquid and second, presence water may increase the
hydrogenation reaction are represented in Table 4, entry chances of bicarbonate formation due to the reaction of
1-18, Scheme 2. The heterogeneous nature of HRUC-A the CO2 gas with water and the basic ionic liquid system. In
catalyst in ([DAMI][CF3CF2CF2CF2SO3]) ionic liquid was some of the reports, these bicarbonate species are treated
investigated as per the reported procedure of catalyst as the true substrate for hydrogenation reaction (Figure 11
poisoning experiments [6]. The above-mentioned protocol a). The bicarbonate species was identified in the 13C NMR
was found to be relatively simple and both ionic liquids as spectra and no other species were detected (Figure 11 b).
well as the catalyst were recycled straightforwardly. It is The errors in the peak areas of the spectra were estimated
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Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
ꢀ861
Catalyst, water, 2-10 h, 60-80o C
X
N
N
N
N
N
.
HCOOH
CO2 (g) + H2 (g)
HOOCH
.
X
N
N
N
N
Distillation
X
X=CF3CF2CF2CF2SO3
N
N
N
+ Catalyst
HCOOH (l)
Scheme 2: Ionic liquid mediated CO2 hydrogenation reaction.
to be at most 3% on the basis of the standard deviations of
peak areas from the different carbons of the same amine
species of ionic liquid. It should be noted that one peak
was detected for the carbon of the bicarbonate species in
13C NMR because of the fast exchange of protons.
The hydrogenation reaction was carried out at
different temperature and reaction pressure and time
(Table 4, entry 14). On increasing the temperature, the rate
of reaction increased and the TON value of formic acid
was increased up to 1581.62 at 100oC (Table 4, entry 7 & 8).
As pressure increased from 20 MPa to 30 MPa at 80oC, TON
and TOF values were also found to be higher and recorded
1886.94 and 317.4 respectively. The pressure effect in this
reaction can be explained according to Henry’s law, as
the solubility of two gases increases along with increasing
the pressure, resulting in the concentration of reactants
having greater importance on the rate of reaction [4-8].
We recovered the formic acid followed by the
distillation process under nitrogen flow. No sign of
ionic liquid decomposing and side product formation
was recorded during the reaction. This observation was
supported by 1HNMR analysis of crude reaction mass. We
used a simple titration method as well as 1HNMR analysis
to quantify the amount of formic acid. Data from both
analyses were found to be in good agreement.
Figure 11 a: FTIR data of format and carbonate species formed during
the reaction.
To
understand
the
stability
of
[DAMI]
[CF3CF2CF2CF2SO3]/HRUC-A catalytic system we performed
the filtration experiment. In this experiment, we mixed
our catalytic system with 10 mL of water and heated the
resulting mixture at 80oC for the next 6 hours under 40
bar hydrogen pressure in an autoclave. After cooling the
reaction mass and degassing the autoclave, the resulting
mixture was filtered, and the filtrate was used to for the
Figure 11 b: 13C NMR spectra of CO2-loaded [DAMI][CF3CF2CF2CF2SO3]
ionic liquid with peak interpretations for bicarbonate species.
Taking advantage of above experimental results,
CO2 hydrogenation experiment. Surprisingly, no sign of we exploited our catalytic system for a catalyst recycling
formic acid was found. This result was also supported by test. After completing the reaction, the reaction product
ICP-OES and XPS method that no Ru metal was detected was isolated, and the complete catalytic system was
in the solution.
recycled to the next run (after the pretreatment process).
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862ꢀ
Vivek Srivastava
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In summary, we synthesized a series of active hydrotalcite
supported Ru metal complexes (HRUC-A to D) to catalyze
hydrogenation of CO2 gas with and without ionic liquids.
HRUC-A catalyst was found to be highly active and gave
a good quantity of formic acid. The combination of
basic ionic liquid [DAMI][CF3CF2CF2CF2SO3] with HRUC-A
catalytic was found extremely efficient to get formic acid
with a high TON and TOF value. The addition of water and
increase in pressure, as well as temperature, resulted in a
high reaction rate. The molar ratio of formic acid to ionic
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formic acid is economical, energy efficient and has the
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Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
ꢀ863
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Supplemental Material: The online version of this article offers
supplementary material (https://doi.org/10.1515/chem-2018-0094).
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