Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon Sequestration Reaction

Article abstract of DOI:10.1007/s10562-016-1772-z

Abstract: A series of silica-tethered Ru complexes were easily prepared and further analyzed by sophisticated analytical techniques such as FTIR, N₂ physisorption, ICP-OES and XPS analysis. After proper characterization of catalyst structure, we exploited them to synthesize formic acid followed by CO₂ hydrogenation reaction. To determine the exact amount of reaction product (formic acid) in the reaction mixture we performed the ¹H NMR analysis, using dioxane as an internal slandered. No degradation of dioxane as well as the side product formation was recorded in this study. Reported catalytic systems were found active and stable in terms of formic acid formation and catalyst recycling experiment up to nine consecutive runs. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1772-z

Catal Lett (2016) 146:1478–1486
DOI 10.1007/s10562-016-1772-z
Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon
Sequestration Reaction
Praveenkumar Ramprakash Upadhyay1 Vivek Srivastava1
•
Received: 26 April 2016 / Accepted: 17 May 2016 / Published online: 2 June 2016
Ó Springer Science+Business Media New York 2016
Abstract A series of silica-tethered Ru complexes were
easily prepared and further analyzed by sophisticated
analytical techniques such as FTIR, N2 physisorption, ICP-
OES and XPS analysis. After proper characterization of
catalyst structure, we exploited them to synthesize formic
acid followed by CO2 hydrogenation reaction. To deter-
mine the exact amount of reaction product (formic acid) in
Keywords Silica Á Ru complex Á Formic acid Á Carbon
dioxide Á Carbon sequestration
1 Introduction
Carbon dioxide (CO2) gas has become a great global
interest from, both practical and theoretical point of view
as CO2 is the major backer to the greenhouse effect [1–3].
In recent years, CO2 exploitation and its conversion into
the value added chemicals have fascinated great consid-
eration of leading research groups [4–6]. Although, CO2
is extensively acknowledged as a reactant for the syn-
thesis of carbonates and other vital chemicals such as
methanol, aspirin, formic acid etc., but CO2 reduction is
still hurting because of its extreme thermodynamic sta-
bility. In the view of exceptional stability of CO2, it’s a
major challenge to develop new competent and highly
reductive protocol to make them ready for chemical
reaction under mild reaction condition [7–9]. Catalytic
hydrogenation of CO2 gas has become as a useful method
to fix CO2 either in chemicals or chemical intermediates
[10–12].
1
the reaction mixture we performed the H NMR analysis,
using dioxane as an internal slandered. No degradation of
dioxane as well as the side product formation was recorded
in this study. Reported catalytic systems were found active
and stable in terms of formic acid formation and catalyst
recycling experiment up to nine consecutive runs.
Graphical Abstract
P
RuCl3
RuCl3
NH2
N
RuCl3
P
RuCl3
P
Si (OMe)
O
Si (OMe)
Si (OMe)
O
O
Si (OMe)
Formic acid is considered as a significant chemical
feedstock for the number of chemical industries such as
perfume, dyeing and sanitary industry (as disinfectant and
preservatives) [10, 11, 13–16]. Apart from above men-
tioned applications of formic acid, it also applied in the
production format ester, which further utilized for the
synthesis of aldehydes, ketones, amides and carboxylic
acid [13–16]. The commercial process of formic acid
synthesis is based on the hydrolysis of methyl format
[17, 18] or by the reaction of CO with water [19]. All these
conventional approaches agonize with producing perilous
waste and consume heavy volumes of electricity because of
O
O
O
O
O
SRUC-1A
SRUC-2 A
SRUC-3A
SRUC-4A
& Vivek Srivastava
vivek.shrivastava@niituniversity.in
1
Basic Sciences, NIIT University, HN-8 Delhi-Jaipur
Highway, Neemrana, Rajasthan, India
123

Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon Sequestration Reaction
1479
that it’s a necessity to develop more environmentally clean
process for the production of formic acid [14–16].
analysis was conducted in a Perkin Elmer Optima 3300 XL
ICP-OES to determine the metal Ru and P content. BET
surface area, pore size, and pore volume measurements of
the catalysts were determined from a physical adsorption of
N2 using liquid nitrogen by an ASAP2420 Micromeritics
adsorption analyzer (Micromeritics Instruments Inc.). The
metal contents in the heterogenous catalysts were deter-
mined by using EDX, Quantax 200 Energy Dispersive
X-ray Spectrometer. Scanning electron microscopy (SEM)
images of support and catalyst were measured on SEM,
JEOL JSM-840 A.
A range of unsupported, homogenous transition metal
catalysts such as Ru, Rh, Ir and Pt (as hail or hybrids) to
efficiently catalyze the CO2 hydrogenation reaction
[10, 19–21]. Surprisingly, very few reports are available on
tethered heterogeneous catalysts for the hydrogenation of
CO2 gas. Organic–inorganic hybrid catalysts are the
promising class of tethered heterogeneous catalysts, are
designed to retain the selectivity of homogeneous catalysts
while being immobilized them on heterogeneous support to
obtain easy separation [19]. However, very few reports
have represented the applications of tethered heteroge-
neous catalysts for the reduction of CO2 gas. Baiker and
co-authors reported co-condensation method to incorporate
a transition metal complex based on Ru, Ir, Pt, or Pd within
a silica support [21]. As per report, Ru-phosphine hybrid
catalysts gave best catalytic activity to synthesize N,N-di-
ethylformamide from CO2, H2, and diethylamine. Ami-
nosilicate-tethered Ru complex was reported by Yu and co-
authors for hydrogenation of CO2 to formic acid with the
TON value of 1482 h-1 [22]. To achieve this TON value
they used PPh3 with supercritical CO2 at 80 °C. In another
report, Hicks and Jones reported the synthesis of a series of
silica tethered iridium catalyst for the CO2 hydrogenation
reaction. The achieve the formic acid as a reaction product
with good TON value (27 9 102 h-1) at 60 °C after 20 h
of reaction [23, 24]. Although, mentioned catalytic systems
were found effective for CO2 hydrogenation, but these
systems also suffered in terms of long reaction time, high
catalyst loading and catalyst recycling.
2.1 Synthesis of SBA-15 [23]
Poly(ethylene glycol)-blockpoly(propylene glycol)-block-
poly (ethylene glycol) (P-123) (15 g) was allowed to react
with distilled water (650 g) and concentrated HCl (80 g)
for 3 h. Afterward, tetraethyl orthosilcate (TEOS) (35 g)
was added to the above miceller solution and the total
reaction mass was stirred for 30 h at 40 °C in oven. To
achieve proper swelling pores and aging, the same reaction
mass was heated at 80 °C for another 24 h. Several
washings were used (by distilled water) to obtain resulting
solid. Further, solid material was kept under vacuum dry-
ing at 50 °C for 12 h. The process of calcination was
carried out at 550 °C for 7 h with a temperature ramp of
1.2 °C/min. The pretreatment was carried out before using
the SBA-15, and it was vacuum dried at 200 °C for 3 h
(82 % yield).The SBA-15 was kept in an N2 atmosphere
for storage.
In this report, we are offering the synthesis of silica-
tethered ruthenium catalyst (SRUC) for the hydrogenation of
CO2 to formic acid. The SRUC catalytic system was syn-
thesized by a multistep grafting process using iminophos-
phine ligand tethered to mesoporous SBA-15 inorganic
support. After activating the SRUC catalyst with hydrogen
gas, it was applied as hydrogenating catalyst for CO2 gas.
2.2 Synthesis of Iminophosphine Ligand Without
Alkoxylsilane Moiety A [25]
A Schiff–base reaction carried out under nitrogen atmo-
sphere by adding 2-(diphenylphosphino)benzaldehyde
(2.1 mmol) and 1-Propylamine (10 mL) in reaction appro-
priate vessel. The resulting reaction mixture was refluxed for
5 h. After cooling the reaction mass up to room temperature
the red, brown oil was obtained by removing the unutilized
1-propylamine under high pressure vacuum (89 % yield).
1H NMR (400 MHz, CD2Cl2): d = 9.01 (s, 1H), 8.03 (s,
1H), 7.45–7.29 (m, 12 H), 6.98 (s, 1 H), 3.43 (t, 2 H),
1.57–1.52 (q, 2 H), 0.82–0.78 (t, 3 H); 13C NMR
(100 MHz, CD2Cl2): d = 160.01, 134.24, 128.74, 63.33,
23.95, 11.78 ppm; 31P NMR: (300 MHz, CD2Cl2, ppm)
d = -13.01 ppm.
2 Experimental
Reagent PlusÒ grade chemicals were purchased from
Sigma Aldrich and other suppliers. All the Nuclear Mag-
netic Resonance (NMR) (1H/13C/31P) spectra were recor-
ded on a standard Bruker 300WB spectrometer with an
Avance console at 400 and 100 MHz. The FTIR mea-
surements were conducted with a Bruker Tensor 27 in
DRIFTS mode (KBr powder) with a scan range from 400 to
4000 cm-1. All the hydrogenation reactions were carried
out in a 100 mL stainless steel autoclave (Amar Equip-
ment, India). Energy-dispersive FTIR data for all the
samples were studied with Bruker Tensor-27. Elemental
2.3 Synthesis of Alkoxylsilane-Containing Bidentate
Iminophosphine Ligand Moiety B
The synthesis of the iminophosphine ligand carrying
alkoxylsilane moiety A was performed through the reactive
123

1480
P. R. Upadhyay, V. Srivastava
distillation
of
2-(diphenylphosphino)benzaldehyde
3 Synthesis of Ru Complex with Iminophosphine
Ligand (Ru-Moiety A)
(0.500 g) with 3-(aminopropyl)trimethoxysilane (0.250 g)
in dry toluene (25 mL). After 5 h, we obtained a yellow
oily liquid by careful removal of toluene under vacuum
(85 % yield).
1H NMR (400 MHz, CD2Cl2): d = 8.8 (s, 1H), 8.02 (s,
1H), 7.45–7.27 (m, 12 H), 6.91 (s, 1 H), 3.53 (s, 9 H), 3.46
(m, 2 H), 1.68–1.60 (m, 2 H), 0.58–0.52 (m, 2 H); 13C
NMR (100 MHz, CD2Cl2): d = 158.57, 134.14, 128.79,
65.13, 50.82, 24.62, 8.73 ppm; 31P NMR: (300 MHz,
CD2Cl2, ppm) d = -13.07 ppm.
The reaction vessel was charged with moiety A (2.0 g),
RuCl3Á3H2O (2.10 g) and dry ethanol (25 mL). The reac-
tion mass was allowed to reflux under Argon atmosphere
10 h. The resulting reaction mixture was filtered and then
with dry ethanol. All the process was completed under
argon atmosphere. The material (98.5 % yield) was dried
and stored under argon atmosphere.
3.1 CO2 Hydrogenation Reaction and Catalyst
Recycling [28, 29]
2.4 Synthesis of Phosphine-Functionalized SBA-15
(IPPL-SBA-15 1A-3A) [24, 26, 27]
The high pressure autoclave (100 mL) was charged with
catalyst, triethylamine (2 mL) and water (as per require-
ment). In a typical reaction, a 150 lM catalyst concen-
tration was loaded in autoclave. A known amount of
dioxane was added with reactants (as an internal stan-
dard), to quantify the reaction kinetics with 1H NMR
spectroscopy. It is important to notify that no decompo-
sition of dioxane was found under all reaction conditions.
After careful addition of all the required reactants, the air
of reaction vessel was replaced with nitrogen gas (five
times nitrogen purging was carried out). Then the catalyst
activation step was done at 20 bar hydrogen pressure for
20 min at 35 °C. Nitrogen gas was used to replace all the
gasses after completion of catalyst activation step. Then
only, the reaction vessel was charged with CO2 and H2
gas (desired quantity) by replacing the nitrogen gas and
the total reaction mass was allowed to stir for next 2 h at
100 °C. After cooling the reaction mass using ice cold
water (2–5 °C), gas pressure was released. A small
Post-synthetic grafting technique was used to obtain
phosphine functionalized SBA-15 under a nitrogen
atmosphere. Phosphine ligand (2.1 mmol) was added to
the dry SBA-15 powder (1 g) in dry toluene (25 mL)
under N2. The reaction mass was stirred for next 20 h at
room temperature (25–30 °C). The resulting solid was
washed with anhydrous toluene (10 9 2 mL). The
recovered solid was carefully dried under vacuum at room
temperature and stored in nitrogen atmosphere (87 %
yield).
2.5 Synthesis of Amine-Functionalized SBA-15
(IPPL-SBA-15 4A) [25]
3-Aminopropyltrimethoxysilane (1.15 g) was added to the
dried SBA-15 powder (1 g) in toluene (50 mL) under N2
atmosphere. The combined reaction mass was allowed to
stir for 24 h at room temperature (20–30 °C) under N2
atmosphere. The subsequent solid was washed and filtered
with anhydrous toluene (10 9 2 mL) and dried under
vacuum at 80 °C. The final product was stored in a nitro-
gen gas (85 % yield).
1
amount of crude reaction mass was used for H NMR
analysis. Water was evaporated from the reaction mass at
110 °C, and then formic acid was isolated from the
reaction mass with the help of nitrogen gas flow at
125–130 °C, passing through the water trap, in order to
capture formic acid [28, 29]. Acid base titration was used
to calculate the amount formic acid in water trap. The
1
results obtained from H NMR analysis as well as from
2.6 Synthesis of Ru-Tethered Pre-catalysts (SURC
1A-4A)
titration method were found in good agreement to each
other.
In recycling experiment, catalytic system was washed
with dry diethyl ether (5 9 2 mL) and kept under vacuum
12 h at 40 °C (all the product isolation steps as well as
catalyst cleaning steps were carried out in the autoclave to
avoid the catalyst loss). Then, the catalytic system went for
recycling experiment where, recovered catalyst was
allowed to stir under nitrogen atmosphere (2 bar) for 1 h at
room temperature. Later, nitrogen was replaced by hydro-
gen gas and reaction mass was stirred under hydrogen
The reaction was carried out by refluxing in anhydrous
ethanol with RuCl3Á3H2O under nitrogen atmosphere for
12 h. The Ru/P molar ratio was maintained to 1/1. The
resulting reaction mixture was degassed and kept under
nitrogen atmosphere. The cleaning of material was carried
out carefully with anhydrous ethanol in the presence of N2
gas. The recovered solid was then dried under vacuum at
room temperature for 24 h, lastly warehoused in the N2
atmosphere (91 % yield).
123

Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon Sequestration Reaction
1481
atmosphere (4 bar) for 20 min at 35 °C. Then only, all the
steps were completed as per above mentioned CO2
hydrogenation protocol.
was obtained in black color and wisely stored in nitrogen
atmosphere. Some more monodentate phosphine pre-cata-
lysts such as SRUC-2A, SRUC-3A and SRUC-4A (pri-
mary amine pre-catalyst) (Fig. 2) were created along with
IPPL-SBA-15 2A, IPPL-SBA-15 3A and IPPL-SBA-15
4A materials respectively to go for proper comparative
study. Iminophosphine ligand without alkoxylsilane moiety
B was also synthesized to go for relative study of hybrid
materials.
4 Result and Discussion
4.1 Synthesis and Characterization of Catalysts
In this report, we are offering the synthesis of SRUC for the
hydrogenation of CO2 gas to formic acid. The SRUC cat-
alytic system was synthesized by a multistep grafting
process using iminophosphine ligand tethered to meso-
porous SBA-15 inorganic support. After activating the
SRUC catalyst with hydrogen gas, it was applied as
hydrogenating catalyst for CO2 gas. It is worth noted that,
SBA-15 was synthesized as per reported protocol and the
important IV-type isotherm was recorded while performing
N2 physisorption analysis. The acceptable BET surface
area (950 m2/g) and Barrett–Joyner–Halenda pore size
(6.1 nm) was recorded for synthesized SBA-15. The
alkoxysilane-containing bidentate imonophosphine ligand
[o-Ph2PC6H4CH=N(CH2)3Si(OMe)3] A was obtained as
per previously reported procedure by reacting o-
Sophisticated and important analytical techniques were
applied to understand the physiochemical properties of the
SRUC (SRUC-1A, SRUC-2A, SRUC-3A and SRUC-4A)
such as FTIR, XPS, SEM–EDS, N2 physisorption and
inductively coupled plasma optical emission spectroscopy
(ICP-OES).
In DRIFT spectra, while comparing the recorded data of
iminophosphine ligand without alkoxylsilane moiety A
(Ru(Cl3)Ph2PC6H4CH=N(CH2)2CH3) and moiety
B
iminophosphine Ligand (Ph2PC6H4CH=N(CH2)2CH3) with
SRUC 1A-4A, the Schiff base (CH=NR) double-bond
adsorption appears at 1640 cm-1, while phenyl ring
vibration caught near 3062, 1585, 1431, 742, and
695 cm-1. Same absorption peaks were again recorded for
tethered materials IPPL-SBA-15 1A and SRUC-1A. These
data reflected the presence of Schiff base before and after
the metallation. The monodentate phosphine pre-catalysts
SRUC-2A, 3A and 4A gave different absorption data in
there FTIR analysis (Figs. 3, 4, 5).
(diphenylphosphino)benzaldehyde
with
3-(amino-
propyl)trimethoxysilane under nitrogen atmosphere in dry
toluene solvent system (Schiff–base reaction) [24–27]. All
the NMR (1H/13C/31P) data were found in good agreement
with previously published reports [24–27]. In the next step,
imonophosphine ligand A was sensibly grafted on well
characterized SBA-15 to get IPPL-SBA-15 1A (Fig. 1).
Then finally, IPPL-SBA-15 1A went to metallation steps,
where IPPL-SBA-15 1A was allowed to reflux with
hydrated RuCl3 in dry ethanol. The resulting SRUC-1A
Before going to the metallation step, N2-physiosorption
analysis was carried out for SBA-15 and bifunctional
ligands modified SBA-15. All the recovered data were
summarized in Table 1, where with respect to SBA-15 the
pore size was reduced after modifying it to bifunctional
ligands.
P
NH2
N
P
P
Si (OMe)
O
Si (OMe)
O
Si (OMe)
O
Si (OMe)
O
O
O
O
O
IPPL-SBA-15 4 A
IPPL-SBA-15 3 A
IPPL-SBA-15 1A
IPPL-SBA-15 2 A
Fig. 1 Structure of SBA-15 tethered phosphine ligands
123

1482
P. R. Upadhyay, V. Srivastava
Fig. 2 SBA-15 tethered
ruthenium catalysts
P
N
RuCl3
RuCl3
NH2
RuCl3
P
RuCl3
P
Si (OMe)
O
Si (OMe)
Si (OMe)
O
O
Si (OMe)
O
O
O
O
O
SRUC-1A
SRUC-2 A
SRUC-3A
SRUC-4A
To understand the composition of synthesized materials,
we performed XPS analysis. In XPS spectra Ru 3d, C 1s, N
1s and P 2p levels were measured at a normal angle with
respect to the plane of the surface. High-resolution spectra
were corrected for charging effects by assigning a value of
284.6 eV to the C 1s peak (adventitious carbon). Binding
energies were determined with an accuracy of 0.2 eV.
Synthetic components on Ru 3d–C 1s were analyzed with a
Shirley background subtraction and a peak shape with a
combination of Gaussian and Lorentzian (30 % Lor-
entzian). The peaks at 280.2 (Ru 3d5/2) and 284.3 eV (Ru
3d3/2) were confirmed the presence of Ru (0) [20]. Sur-
prisingly, in XPS study no peaks were recorded near to
281.1 and 287.1 eV, which confirmed the absence of Ru2O
species. Also, no Ru 3p core level peak with a binding
energy of 464 eV was noted for the neat SRUC 1–4A
samples. This observation also confirmed the absence of
ruthenium in ?2 oxidation state.
Fig. 3 FTIR spectra of unsupported moiety 1 and 2
As shown in Table 2, XPS analysis of supported RuCl3/
SBA-15 exhibited a low binding energy of Ru 3d3/2 and
3d5/2 confirmed the presence of Ru (0) species relative to
the binding energies of SRUC-1A and SRUC-3A. Sur-
prisingly, there is some resemblance in the Ru 3d3/2 and
3d5/2 binding energies between SRUC-2A and RuCl3/
SBA-15, which could be the result relatively low loading of
the phosphine ligand on the support (Fig. 6). This would
allow RuCl3 to interact with residual silanol groups on the
support (Fig. 6). In opposite, SRUC-3A gave much higher
loading of the anchored phosphine, which resulted in a
higher binding energy for Ru 3d3/2 and 3d5/2. Furthermore,
SRUC-1A had slightly higher binding energy for 3d3/2 and
3d5/2.than SRUC-3A. Interestingly, the shift in the 3d3/2
and 3d5/2 peak of the unsupported analogue was 284.7 eV
(Ru-PNPr1), which was similar to SRUC-1A. These results
Fig. 4 DRIFT data for SURC-1A and IPPL-SBA-15 1A with respect
to SBA-15
123

Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon Sequestration Reaction
1483
indicate that the environment of Ru in the tethered biden-
tate material is perhaps comparable to that in the unsup-
ported Ru-PNPr1 analogue. SEM–EDX spectrum of
SRUC-1A with respect to IPPL-SBA-15 1A further con-
firmed the presence of Ru in the SRUC-1A (Fig. 6).
ICP-OES and TGA analysis were carried out to calcu-
late the loading of phosphine on SBA-15 as well as ligand
loading on SBA-15 respectively (Fig. 7). Maximum
phosphine piling was recorded in IPPL-SBA-15 2A and
the lowest loading was noted with IPPL-SBA-15 1A. This
observation was also found in good agreement with XPS
analysis. Higher ligand loading on SBA-15 was observed
in IPPL-SBA-15 1A.
4.2 Application of Catalysts for CO2 Hydrogenation
Reaction
The catalytic efficiency of SRUCs were screened for the
hydrogenation of CO2 to formic acid (Table 3). The
hydrogenation reaction was carried out in 100 mL auto-
clave with triethyl amine (NEt3) and water under the
pressure of CO2 and hydrogen gas (40 bar, CO2:H2 = 1:1)
at 75 °C. The formation of formic acid (or formate) was
1
calculated through H NMR spectroscopy where DMSO
was used as solvent and dioxane as an internal standard
(Fig. 8) [24, 27–29].
Prior to the hydrogenation reaction, the catalyst was
activated by hydrogen (10 bar) for 30 min at 30 °C. The
Fig. 5 DRIFT data for SURC-1A-4Awith respect to SBA-15
Table 1 Physical properties of
SBA-15 modified materials
Materials
SBET (m2/g)
Pore volume (cm3/g)
Pore size (nm)
IPPL-SBA-15 1A
IPPL-SBA-15 2A
IPPL-SBA-15 3A
IPPL-SBA-15 4A
SBA-15
688
271
419
365
950
0.74
0.36
0.51
0.46
0.85
5.9
4.0
4.9
5.0
6.1
Table 2 XPS analysis of
developed materials
Samples
Ru 3d3/2 (eV)
Ru 3d5/2 (eV)
Cl2 2p (eV)
P 2p (eV)
N 1s (eV)
RuCl3
284.6
284.4
284.7
–
280.2
280.1
280.8
–
199.2
199.1
198.6
–
–
RuCl3/SBA-15
Ru-PNPr1
–
–
131.9
131.8
–
399.8
IPPL-SBA-15 1A
IPPL-SBA-15 2A
IPPL-SBA-15 3A
IPPL-SBA-15 4A
SRUC-1A
398.1
–
–
–
–
–
–
–
131.1
131.1
131.8-
131.7
131.6
–
–
–
–
–
284.7
284.7
284.6
284.6
284.6
280.8
284.8
284.5
284.5
284.4
197.1
198.6
199.3
199.3
198.5
400.1-
SRUC-2A
–
SRUC-3A
–
SRUC-4A
400.2-
–
SRUC-2A
131.6
(after catalyst recycling)
Measurement error of 0.2 eV; XPS data of Si 2p at 103.3 eV. XPS data referenced to C 1s at 284.5 eV
123

1484
P. R. Upadhyay, V. Srivastava
Fig. 6 SEM-EDS analysis
value (Table 3, entry 5–13). The tethered bidentate catalyst
SRUC-1A showed high catalytic activity and we isolated
formic acid with high TON value [28, 29]. After screening
all the developed catalysts, we can arrange them with
respect to their catalytic performance in the following order
SRUC-1A [ SRUC-3A [ SRUC-2A [ unsupported Ru-
moiety A. The lowest catalytic performance was recorded
with SRUC-1A mainly because of low loading of phos-
phine group on the support. While low activity of unsup-
ported Ru-moiety A precatalyst was recorded mainly
because of metal–metal dimerization in solution [24].
It is worth noted here that, tethered catalysts can be
easily recovered followed by filtration. In catalyst recycling
experiment of SRUC-1A (Fig. 9), we successfully
obtained the formic acid up to nine consecutive runs. The
slender drop in catalyst activity was recorded due catalyst
loss during work-up, mainly because of metal leaching
(observed in AAS analysis) (Fig. 10). The XPS analysis
was also performed for SRUC-1A catalyst after the catal-
ysis and surprisingly no changes in the binding energy of
Ru 3d3/2 and 3d5/2 was found almost similar to the fresh
SRUC-1A catalyst. Such observation indicates that, our
catalytic system is stable in aqueous solution and for high
pressure reaction condition.
3
2.5
2
Ligand loading on SBA-
15 (mmol P/g material)
P loading on SBA-15
(mmol P/g material)
1.5
1
0.5
0
IPPL-SBA-15 1A
IPPL-SBA-15 2A
IPPL-SBA-15 3A
Material
Fig. 7 Phosphine ligand (P) loding on IIPL-SBA-15 material
results obtained while utilizing the silica-tethered Ru cat-
alysts for CO2 hydrogenation were concluded in Table 3.
In the absence of a catalyst, no sign of hydrogenation
reaction was observed (Table 2, Entry 1). The unsupported
RuCl3 precursor gave only a small amount of formic acid
(Table 2, Entry 2). Even formic acid with low TON value
was also attained while applying RuCl3/SBA-15 and
amine-functionalized SBA-15 (SRUC-4A) (Table 3, entry
3) and 4). In our study it is clear that, catalytic materials
with phosphine ligands gave formic acid with good TON
To understand the stability of the bindentate catalyst
over monodentate catalyst, we performed the filtration
experiment. In this experiment, at 75 °C, aqueous solution
of (1 M, 5 mL) catalysts were mixed with hydrogen gas at
5 MPa for 1 h. The resulting mixture was filtered and the
filtrate was used for the hydrogenation reaction. No
123

Heterogeneous Silica Tethered Ruthenium Catalysts for Carbon Sequestration Reaction
1485
Table 3 Catalyst optimization
for CO2 hydrogenation reaction
°
b
)
Entrya Catalyst
T C t (h) TON 9 102 (molFA/molRu
TON 9 102 (molFA/molRu 9 h-1
)
1
None
75
75
75
75
75
75
1
1
1
1
2
2
–
–
RuCl3Á3H2O
RuCl3/SBA-15
SRUC-4A
SRUC-2A
SRUC-3A
SRUC-3A
SRUC-1A
SRUC-1A
SRUC-1A
SRUC-1A
SRUC-1A
Moiety-A
RuCl2 (PPh3)3
11.1
18.7
41.6
67.8
87.1
96.4
122.7
143.1
167.2
131.2
148.9
28.6
16.1
11.1
18.7
41.6
33.9
43.55
6.4
2
3
4
5
6
75 15
7
75
75
1
2
122.7
71.5
11.1
131.2
74.4
14.3
8.1
8
9
75 15
10
11
12
13
a
100
1
2
2
2
120
75
75
Reaction condition: 100 mL (Amar autoclave) charged with double distilled water (15 mL), distilled
water (15 mL), NEt3 (2 mL), concentration of Ru metal in catalyst = 0.2 9 10-2 g/g catalyst, total
pressure 20 Bar (H2/CO2 = 1:1)
b
FA formic acid
20
Ru loading in μmol (Before catalysis)
Ru loading in % (After catalysis)
15
10
5
0
SURC-1A
SURC-2A
SURC-3A
Fig. 8 1H NMR analysis for formic acid from crude reaction mass
Pre-catalyst
Fig. 10 Leaching study of Ru metal in SRUC catalysts
12750
formation of formic acid confirmed negligible catalyst
leaching of active catalyst. This result was also supported
by ICP-OES analysis result of filtrate where Ru metal was
not detected in the solution. Monodentate catalysts gave
prominent metal leaching in the solution with respect to
SRUC-1A. It was also confirmed by ICP-OES analysis.
The trend of metal leaching from highest to lowest was
TON value
12740
12730
12720
12710
12700
12690
found
in
following
order
SRUC-3A [ SRUC-
2A [ SRUC-1A (Fig. 10). Even, while performing the
hydrogenation reaction using the filtrate of monodentate
catalysts, we also obtained formic acid. Here we obtained
the formic acid with higher TON value with SRUC-3A
filtrate (TON = 0.8 9 102).
Catalytic run
Fig. 9 Catalytic recycling of SRUCA-1A catalyst
123

1486
P. R. Upadhyay, V. Srivastava
´
´
7. Kondratenko EV, Mul G, Baltrusaitis J, Larrazabal GO, Perez-
´
5 Conclusion
Ramırez J (2013) Energy Environ Sci 6(11):3112
´
8. Gomes CDN, Jacquet O, Villiers C, Thuery P, Ephritikhine M,
In this manuscript, we reported a new protocol to synthe-
size mesoporous silica-tethered Ru complexes (SRUC
1–4A). Among these, materials, SRUC 1A was found as an
effective heterogeneous catalyst for the selective CO2
hydrogenation reaction to attain formic acid under normal
reaction condition. In terms of catalyst recycling, this
catalytic system was found highly active for 6 catalytic
runs without any noteworthy loss of TON value of formic
acid. In parallel, we also performed the filtration experi-
ment and the obtained results were found in good agree-
ment with recycling test results. Furthermore, this tethered
bidentate catalyst gave high stability over the monodentate
catalysts.
Cantat T (2012) Angew Chem Int Ed 51(1):187
9. Lu Q, Rosen J, Zhou Y, Hutchings GS, Kimmel YC, Chen JG,
Jiao F (2014) Nat Commun 5:1
10. Saeidia S, Amina NAS, Rahimpour (2014) J CO2 Util 5:66
11. Jadhava SG, Vaidyaa PD, Bhanageb BM, Joshi JB (2014) Chem
Eng Res Des 92(11):2557
12. Hong SH (2013) Org Chem Curr Res 2:2
13. Gibson HW (1969) Chem Rev 69(5):673
14. Grasemanna M, Laurenczy
5(8):8171
G (2012) Energy Environ Sci
15. Johnson TC, Morrisa DJ, Wills M (2010) Chem Soc Rev 39(1):81
16. Laurenczy G, Dyson PJ (2014) J Braz Chem Soc 25(12):2157
17. Leitner W (1995) Angew Chem Int Ed Engl 34(20):2207
¨
˚
18. Jogunola O, Salmi T, Warna J, Mikkola J-P, Tirronen E (2011)
Ind Eng Chem Res 50(1):267
19. Valkenberg MH, Holdrich WF (2002) Catal Rev 44:321
20. Wang W, Wang S, Maa X, Gong J (2011) Chem Soc Rev
40(7):3703–3727
21. Schmid L, Rohr M, Baiker A (1999) Chem Commun 22:2303
22. Zhang Y, Jinhua JF, Yu FY, Zheng X (2004) Catal Lett 93(3/
4):231
Acknowledgments This work is financially supported by DST Fast
Track (SB/FT/CS-124/2012), India.
23. Hicks JC, Jones CW (2006) Langmuir 22(6):2676
24. Xu Z, McNamara ND, Neumann GT, Schneider WF, Hicks JC
(2013) ChemCatChem 5(7):1769
25. Ghilardi CA, Midollini S, Moneti S, Orlandini A, Scapacci G
(1992) J Chem Soc Dalton Trans 23:3371
26. Bek D, Balcar H, Zilkova N, Zukal A, Horacek M, Cejka J (2011)
ACS Catal 1(7):709–718
27. Behringer KD, Blumel J (1996) Inorg Chem 35(7):1814
28. Srivastava V (2014) Catal Lett 144(10):1745
29. Srivastava V (2014) Catal Lett 144(12):2221
References
1. Sedjo R, Sohngen B (2012) Annu Rev Res Econ 4(1):127
2. Lal R (2015) Philos Trans R Soc B 363(1492):815
´ ´
3. Quere CL, Raupach MR, Canadell JG, Marland G (2009) Nat
Geosci 2(12):831
4. Richards KR, Stokes C (2004) Clim Change 63(1/2):1
5. Kim SH, Kim KH, Hong SH (2014) Angew Chem Int Ed
53(3):771
´
6. Cuellar-Franca RM, Azapagic A (2015) J CO2 Util 9:82
123