CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–Reactivity Relationship Study

Article abstract of DOI:10.1007/s10562-021-03609-5

We gave an effective protocol to support Ru NPs on amine-functionalized SBA-15 mesoporous silica to catalyze the CO₂ hydrogenation reaction. The amine groups present in the catalytic system performed an essential role in stabilizing the Ru NPs, delivering the robust metal-support interaction and improved catalytic activities to material in formic acid synthesis. We also demonstrated a comprehensive study of different amine groups on the catalytic performance of ultrafine uniformly dispersed Ru NPs over mesoporous SBA-15 support. The effect of various compositional and steric properties of amine groups on the size/distribution of the Ru NPs were closely studied and correlated with their catalytic performance in the CO₂ hydrogenation reaction. The in situ DRIFTS analysis of CO₂ hydrogenation into formic acid in presence of developed CATALYST-1 showed active surface species bonded to support sites and to Ru NPs. This interaction proposed the formation of important intermediates such as hydrides, formates and bicarbonates, which are significant for the formation of formic acid. We successfully recycled the catalysts up to 5 runs with good catalytic activity. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-021-03609-5

Catalysis Letters
https://doi.org/10.1007/s10562-021-03609-5
CO2 Hydrogenation over Ru‑NPs Supported Amine‑Functionalized
SBA‑15 Catalyst: Structure–Reactivity Relationship Study
Vivek Srivastava1
Received: 19 January 2021 / Accepted: 18 March 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
We gave an efective protocol to support Ru NPs on amine-functionalized SBA-15 mesoporous silica to catalyze the CO2
hydrogenation reaction. The amine groups present in the catalytic system performed an essential role in stabilizing the Ru
NPs, delivering the robust metal-support interaction and improved catalytic activities to material in formic acid synthesis.
We also demonstrated a comprehensive study of diferent amine groups on the catalytic performance of ultrafne uniformly
dispersed Ru NPs over mesoporous SBA-15 support. The efect of various compositional and steric properties of amine
groups on the size/distribution of the Ru NPs were closely studied and correlated with their catalytic performance in the
CO2 hydrogenation reaction. The in situ DRIFTS analysis of CO2 hydrogenation into formic acid in presence of developed
CATALYST-1 showed active surface species bonded to support sites and to Ru NPs. This interaction proposed the formation
of important intermediates such as hydrides, formates and bicarbonates, which are signifcant for the formation of formic
acid. We successfully recycled the catalysts up to 5 runs with good catalytic activity.
Graphic Abstract
Keywords SBA-15 · Formic acid · Carbon sequestration · Hexaammineruthenium (III) complexes · Amine-functionalized
silica
1 Introduction
Carbon dioxide gas is one of the important products of the
combustion process and is not counted as a benign molecule
* Vivek Srivastava
of the atmosphere. The quantity of CO2 in the atmosphere
vivek.shrivastava@niituniversity.in
has touched extraordinary levels and remains to rise owing
1
Mathematics & Basic Sciences, NIIT University, NH-8
Jaipur/Delhi Highway, Neemrana, Rajasthan 301705, India
to a rising rate of fossil fuel burning, triggering apprehension
Vol.:(0123456789)
1 3

V. Srivastava
about climate change, and increasing sea levels [1]. Consid-
ering the above, a conceivable solution to this problem is
the utilization of carbon dioxide. Scholars in this area are
using diferent approaches to convert CO2 gas into useful
chemicals. However, this creates the challenge of employing
a large volume of seized CO2, which earlier has not had any
industrially practical uses at such a large scale [2]. Realiz-
ing that, historically, fossil reserves were created via natural
carbon-hydrogenation during photosynthesis, synthetic CO2
hydrogenation is likely the best way to regenerate combusted
hydrocarbons [3, 4].
catalyst support were also found as an important factor to
promote the reaction kinetics of CO2 hydrogenation reac-
tion due to the presence of surface functional groups with
supports, such as the presence of Schif base on the Au/SiO2-
Schif catalyst [28] and nitrogen functionalities on the Pd/
C3N4 catalyst [29]. However, the described heterogeneous
catalysts still sufer from some demerits such as restricted
chemical designability, low reactivity comparing with homo-
geneous ones, comparatively low product concentration, and
requirement of toxic polar organic solvents [30]. Therefore,
it is necessary to create a cost-efective heterogeneous cata-
lytic system for CO2 hydrogenation to format or formic acid.
In line with this, wide eforts have been engrossed in the
reaction protocol to improve the reactivity by increasing the
density of supported metal, reducing the size of metal par-
ticles, and regulating the morphologies as well as surface
characteristics of support [23, 31–33]. As a result, competent
heterogeneous catalysts were typically designed by immo-
bilizing metal nanoparticles (NPs) on a variety of supports
such as ionic liquid, polymers, MMT clay, metal oxides,
TiO2, graphene, and silica, etc. [17, 27, 29–36]. However,
the stacked metal NPs in catalysts are suitable to aggregate
thus leading to a decrease in their catalytic capacity. This is
principally attributing to their large surface-to-volume ratio
and higher surface free energy in contrast with their bulk
equivalents [23, 33]. To defeat the challenges mentioned
above and enhance the catalytic activity as well as stability,
metal NPs-based catalysts were prepared by embedding the
NPs on the inner surface of the interlayer of the support.
In this report, we have developed a series of amine-func-
tionalized nonporous silica materials to anchor Ru NPs.
We also studied the relationship among diferent types of
amine functionality, Ru NPs, and the catalytic property of
the developed material for CO2 hydrogenation reaction.
We also demonstrated and designed amine-functionalized
SBA-15 supported Ru NPs where we used all three types
of amines such as primary, secondary, and tertiary. After
performing the comprehensive analysis of all the developed
amine-functionalized SBA-15 supported Ru NPs, their cata-
lytic performance was tested for CO2 hydrogenation reac-
tion and correlated the efect of amines, size of Ru NPS,
physiochemical properties of support in terms of formic acid
quantity.
While CO2 hydrogenation is difcult due to the high ther-
mal stability of CO2 molecules, substantial development has
been made for transforming CO2 to single carbon (C1) prod-
ucts such as formic acid, methane, and methanol using direct
hydrogen reduction [5–7].
Formic acid has gained much consideration due to its
several applications as raw material/intermediate, in food
technology, agriculture, leather, and rubber industries [8].
Furthermore, formic acid is considered as one of the encour-
aging hydrogen storage materials, which has a volumetric
hydrogen density of 53 g/L of H2 and is suitable to store
and transport in the liquid state under ambient conditions
[9, 10]. As per the literature reviews, diferent techniques for
the synthesis of formic acid from CO2 have been established,
such as photocatalytic reduction, electrochemical processes,
enzymatic conversion, and hydrogenation [11–16]. The
above procedures comprise of two steps: (1) carbonylation
of methanol to methyl format (HCOOCH3) and (2) hydroly-
sis of HCOOCH3 to HCOOH [11, 12, 16]. Unfortunately,
the above method also sufers due to the use of toxic CO for
methanol carbonylation. Also, the formic acid can be made
from CO2 using the following reactions such as hydrogena-
tion, carbonation, and hydration [5, 7, 10–12].
Various homogeneous metal complexes have been widely
explored as a catalyst for CO2 hydrogenation to formic
acid, but they normally sufer from the difculty to isolate
and reuse of the catalyst [17–21]. Although some hybrid
heterogenized homogeneous catalysts have been reported
to address the demerits of the homogeneous such as post-
immobilizing homogenous metal complexes on inorganic
or organic supports such as silica and polymers [9, 18, 22].
Unfortunately, the above approach repeatedly ends with low
activity and selectivity due to changes in the chemical and
electronic structure of the metal catalytic centers. Moreover,
the requirement of costly ligands also hinders its large-scale
application in CO2 hydrogenation.
2 Experimental
Heterogeneous catalysts showed noticeable improve-
ments in easy product separation and catalyst recycling
[23]. Various Au, Pt, Ru, Ir, Cu, etc. metals were supported
on diferent inorganic supports such as MMT clay, Silica,
TiO2, etc. to catalyze CO2 hydrogenation to format or formic
acid [24–27]. It was also noticed that in some of the reports
All the chemicals were purchased from Sigma Aldrich and
other chemical suppliers. Elemental analysis was carried
out with Perkin Elmer Optima 3300 XL ICP-OES. Stand-
ard Bruker 300WB Nuclear Magnetic Resonance (NMR)
spectrometer with an Avance console at 400 and 100 MHz
was used to record 1H NMR. The morphology of catalysts
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
was investigated by transmission electron microscopy
(TEM) using a Philips CM12 instrument. Kratos-Axis 165
with Mg Kα radiation 1254 eV was used to perform X-ray
photoelectron spectroscopy (XPS). DTA-TGA thermal ana-
lyzer apparatus (Shimadzu DTG-60H) was used to study the
thermal stability of all the developed materials. BET surface
area, pore size, and pore volume measurements of the cata-
lysts were determined from physical adsorption of N2 using
liquid nitrogen by an ASAP2420 Micromeritics adsorption
analyzer (Micromeritics Instruments Inc). The surface area
and pore size distribution were calculated using BET and
BJH equations by the instrument software. All the hydro-
genation reactions were performed in a 100-mL stainless
steel autoclave (Amar Equipment, India). UV–Vis difuse
refectance spectroscopy was carried out with a Cary 5000
UV–Vis spectrometer (Agilent).
2.3 Synthesis of Hexaammineruthenium (III)
Chloride and Derivatives [38]
The reaction vessel was charged with RuCl3.3H2O (0.7 g),
water (5 mL), and hydrazine hydrate (5 mL, 85% solution).
The combined reaction was refuxed for 2 h, then after cool-
ing the reaction mass was fltered by gravity. Easy reaction
workup was performed using ammonium chloride solu-
tion. The canary-yellow color precipitate was collected
by simple fltration and the same was washed with a small
quantity of ice-cold aqueous ammonia solution (3×2 mL),
ethanol (3 × 2 mL), and in last with ether (3 × 2 mL). The
resulting materials were dried at 40 °C under vacuum for
2 h (0.40 g, 60% yield). Please note the material radially
decomposes on exposure to air. [Ru (NH3)6 Cl3] Anal. Calcd.
For H18Cl3N6Ru: N, 27.13; H, 5.81; Cl−, 34.41. Found N,
27.04; H, 5.80; Cl−, 34.1.
Two hexaammineruthenium (III) complexes were syn-
thesis, by mixing RuCl3.3H2O (0.65 g) with three diferent
types of amines such as methylamine (1.8 gm) or dimethyl-
amine (1.91 g) in presence of hydrazine hydrate (5 mL, 85%
solution). The above reaction mass was refuxed for 2 h, then
after cooling the same, it was washed with a small quantity
of ice-cold aqueous ammonia solution (3×2 mL), ethanol
(3 × 2 mL), and last with ether (3 × 2 mL). The resulting
materials [Ru (NH3)6 Cl3] was dried at 40 °C under vacuum
for 2 h (0.40 g, 60% yield). Please note the material radially
decomposes on exposure to air.
2.1 Synthesis of SBA‑15 Silica
The SBA-15 was synthesized using Pluronic ® P123 as a
templating agent in acidic solution as per the reported lit-
erature [37]. In a simple experimental procedure, the reac-
tion vessel was charged with Pluronic ® P123, 10 M HCl,
tetraethyl orthosilicate (TEOS), and deionized water with
the molar mass ratio of 1:7:2:18. The combined reaction
was vigorously stirred at 50 °C for 24 h. Then the resulting
solution was aged at 70 °C for the next 24 h. After cooling
the reaction mass, a solid mass was recovered by washing
with deionized water (5×2 mL). The unutilized Pluronic ®
P123 templet was isolated from the resulting white solid by
dispersing the same in the mixture of ethanol and diethyl
ether (1:1 ratio). The slurry was stirred for 5 h at 50 °C. Then
the resulting white solid material was washed with diethyl
(5×2 ml) ether and ethanol (7×2 mL). Later, the perfectly
washed materials were dried at 40 °C in a vacuum oven and
the fnal material was named ET-SBA-15.
[Ru (NH2 CH3)6 Cl3] Anal. Calcd. For C6H30Cl2N6Ru:
C, 18.30; H, 7.68; Cl, 27.01; N, 21.34; Ru, 25.67 Found C,
18.20; H, 7.47; Cl, 27.21; N, 21.30; Ru, 25.78.
[Ru (NH (CH3)2)6 Cl3] Anal. Calcd. For C12H42Cl2N6Ru:
C, 30.16; H, 8.86; Cl, 22.25; N, 17.58; Ru, 21.15 Found C,
30.01; H, 8.76; Cl, 22.30; N, 17.41; Ru, 21.10.
2.4 Synthesis of Ru Complex Ion Anchored
Amine‑Functionalized SBA‑15 Mesoporous
Silica Materials
2.2 Synthesis of Amine‑Functionalized SBA‑15
Silica Using Three Diferent Types of Amine
The reaction fask was charged with [Ru (NH2)6Cl3] (0.5 g),
ethanol (2 mL), deionized water (5 mL) and amine-func-
tionalized SBA-15 (1 g) such as SBA-15 -PAN (primary
amine, -NH2) SBA-15-SAN (secondary amine, -NHCH3)
and SBA-15-TAN (tertiary amine, -N(CH3)2). The mixture
was stirred for 5 h at 80 °C. Then solid material was fltered
through centrifugation (at 2000 rpm for 5 min) and washed
with water (5×2 mL). The perfectly washed material was
dried under vacuum at 40 °C. We obtained the desired prod-
uct SBA-15-PAN@Ru (III), SBA-15-SAN@Ru (III), and
SBA-15-TAN@Ru (III) in good quantity.
The reaction vessel was charged with ET-SBA-15 (1 g),
dry toluene (70 mL) and (3-Aminopropyl) trimethoxysi-
lane (APTMS) or N-Methylaminopropyltrimethoxysilane
(MAPTMS) or 3-(N,N-dimethylamino propyl) trimethox-
ysilane (DMAPTMS) (1.4 mmol). The combined reaction
mass was vigorously stirred for 5 h at 75 °C. After colling
the reaction mass, the resulting solid mass was washed with
toluene (5 × 2 mL) and then ethanol (5 × 2 mL). The per-
fectly washed solid materials were dried in a vacuum oven at
40 °C. The resulting white powder was named SBA-15 -PAN
(primary amine, -NH2) SBA-15-SAN (secondary amine,
-NHCH3), and SBA-15- TAN (tertiary amine, -N(CH3)2).
The controlled sample ET-SBA-15 @Ru (III) was also
prepared as per the above protocol by mixing ET-SBA-15
with [Ru (NH2)6Cl3].
1 3

V. Srivastava
reactants, the reaction mass was stirred for the appropriate
time and temperature as per Table 3. After completion of the
reaction, the reaction mass was allowed to cool. Later the
catalyst was isolated by centrifugation method. Then using
nitrogen gas at ambient pressure, formic acid was distilled
at 130 °C.
2.5 Synthesis of Ru (0) Anchored
Amine‑Functionalized SBA‑15 Mesoporous
Silica Materials
The SBA-15-PAN@Ru (III), SBA-15-SAN@Ru (III), and
SBA-15-TAN@Ru (III) and ET-SBA-15- @Ru (III) materi-
als were reduced by using 15% of H2/N2 at 250 °C for 2 h to
convert Ru (III) ions into the Ru (0) NPs. The representa-
tive materials isolated were named as SBA-15-PAN@Ru
(0), SBA-15-SAN@Ru (0), and SBA-15-TAN@Ru (0) and
ET-SBA-15@Ru (0) (control material).
The recovered catalyst was washed with ether to remove
the organic impurity from the surface of the catalyst. Then,
the catalyst was pretreated at 45 °C for 10 min under 10 MPa
pressure of hydrogen gas. After the pretreatment of the cata-
lyst, all the reaction steps were repeated as per the above
procedure to obtain the recycling result.
2.6 CO2 Hydrogenation Reaction and Formic Acid
and Catalyst Recycling
3 Result and Discussion
All the catalytic reactions were carried out in a 50 mL stain-
less steel autoclave equipped with a magnetic stirrer. In a
simple experimental procedure, the reactor was charged
with catalyst, water, and other reactants as per Table 3, entry
1–19. Then after closing the reactor, the air in the reactor
was replaced with CO2 and the reactor was kept at 60 °C
for 30 min. Then the reactor was pressurized with H2 gas
to get the desired pressure. After charging all the required
3.1 Preparation and Physicochemical Analysis
of Amine‑Functionalized SBA‑15 Mesoporous
Silica
We developed a series of SBA-15 mesoporous silica mate-
rials using three diferent types of amines (primary amine
(PAN), secondary (SAN), and tertiary (TAN)) as per the
Scheme 1 Synthetic procedure for diferent SBA-15 materials with and without Ru metal
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
Table 1 Physiochemical analysis of diferent SBA-15 materials
reported procedure with slight modifcation (scheme 1) [39].
The surface Si–OH groups of surfactant-extracted SBA-15
mesoporous silica (ET-SBA-15) were anchored with three
types of 3-aminoaloxysilanes such as (3-Aminopropyl) tri-
methoxysilane (APTMS), N-Methylaminopropyltrimeth-
oxysilane (MAPTMS), and 3-(N,N-dimethylaminopropyl)
trimethoxysilane (DMAPTMS).
Material
Na (mmol/
BETb surface Pore sizec
Pore
gcat
)
area (m2/g)
(nm)
volumec
(cm3/g)
ET-SBA-15 N/A
348
278
6.2
4.8
0.49
0.21
SBA-15-
2.7
PAN
The ET-SBA-15 was developed by eliminating the Plu-
ronic ® P123 templets from the as -formulated mesoporous
silica using a solvent extraction method [37, 40, 41]. Even
if it does not get free of the templates, this technique “as
countered to calcination” was preferred for eliminating the
templates here, because it leaves behind the signifcantly
desired (in our case), higher density of Si–OH groups. A
better density of Si–OH functional groups is favorable in
our case because it allows the grafting of a higher density
of organoamine functional group onto the SBA-15 mate-
rial. Here, the above functional groups can be utilized as
co-catalysts in CO2 reduction reaction and formic acid syn-
thesis. Additionally, the large density organoamine func-
tional groups are helpful to upload the higher density of Ru
(III) ions over SBA-15 materials. Moreover, a substantial
amount of the outstanding Pluronic ® P123 templates comes
out from ET-SBA-15 material during the following steps:
subsequent grafting step at 80 °C for 5 h, the solvent extrac-
tion process, and reduction step at 250 °C for 2 h (when Ru
NPS formed) (Scheme 1). TGA analysis of Pluronic ® P123
showed the thermal degradation of the material at 250 °C.
This data confrms that we can obtain the Pluronic ® P123
impurity-free material at this temperature. Hence, the cata-
lytic activity of the material does not compromise due to
the presence of residual templates as an impurity. While
performing the grafting process of aminoorganoaloxysi-
lane, the Si–OH functional groups in ET-SBA-15 material
work as nucleophiles. As a result, they create organoamine
functional groups that are covalently linked to the surface
of ET-SBA-15 followed by Si–O-Si bonds. In this process,
the formation of the methanol molecule was noticed as a
by-product (Scheme 1). As per the above protocol, three
diferent types of amine groups (PAN, SAN, and TAN) were
attached to the surface of ET-SBA-15 by using their cor-
responding alkoxysilane precursors and gave three types of
amine-functionalized SBA-15 such as SBA-15 -PAN, SBA-
15-SAN, and SBA-15-TAN respectively as per scheme 1.
We performed a comprehensive analytical study such
as elemental analysis (EA), thermogravimetric analysis
(TGA), and N2 porosimetry to characterize the physico-
chemical properties of all the above-synthesized mate-
rials. As per Table 1, the details of elemental analysis
for all types of amine-functionalized SBA-15 (SBA-15-
PAN, SBA-15-SAN, and SBA-15-TAN) were tabulated.
It is worth notify that a nearly similar amount of nitro-
gen (2.0–2.6 mmol gcat−1) was recorded in all types of
SBA-15-
SAN
SBA-15-TAN 2.9
2.6
231
298
4.1
3.8
0.17
0.11
aCalculated based on EA
bCalculated based on N2 adsorption/desorption data
cCalculated using BJH adsorption isotherm equation
Fig. 1 TGA analysis data of neat ET-SBA-15 and other amine-func-
tionalized SBA-15 materials
amine-functionalized SBA-15 materials. TGA analysis
data of neat ET-SBA-15 and other amine-functional-
ized SBA-15 materials were displayed in Fig. 1. The 1st
weight loss of 0.75% in ET-SBA-15 was recorded from
room temperature to 120 °C due to the evaporation of
water molecules from the ET-SBA-15 structure. A 2nd
weight loss (near to 17%) was noticed from the tem-
perature range 120–700 °C due to the loss of outstand-
ing polymers and water followed by the condensation of
Si–OH groups. While performing the TGA analysis for
SBA-15-PAN, SBA-15-SAN, and SBA-15-TAN materials
to distinct weight loss from room temperature to 120 °C
due to evaporation of residual water from the surface of
the material and then from 120–700 °C the degradation
of template polymers and water (by the condensation of
Si–OH groups) as well as the decomposition of organoam-
ine group (ca. 12%) respectively. The above TGA analysis
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V. Srivastava
results confrmed the almost similar quantity of organoam-
ine moieties in all SBA-15-PAN, SBA-15-SAN, and SBA-
15-TAN materials.
3.2 Preparation and Physicochemical
Analysis of a Series of Ru Ion Anchored
Amine‑Functionalized SBA‑15 Mesoporous
Silica Materials
In, N2 adsorption/desorption measurements of amine-
functionalized SBA-15 materials lower surface area was
reported with respect to ET-SBA-15 material due to the
pores in the SBA-15-PAN, SBA-15-SAN, and SBA-15-
TAN materials were partially occupied surface organoam-
ine groups as a result of organosilane grafting (Fig. 2a and
b). Additionally, the N2 gas adsorption/desorption data of
all the materials shown a type IV isotherm with hyster-
esis loops, signifying the existence of mesoporous mate-
rials as well as also confrmed the memory efect of the
mesoporous structure in the materials during organoam-
ine functionalization. Also, due to the grafting of bulky
organoamines over SBA-15 increasingly reduced the pore
size of the materials. Notably, a drop in average BJH
pore diameters for all the materials (ET-SBA-15, SBA-
15-PAN, SBA-15-SAN, and SBA-15-TAN) was found in
the range of 6.9–3.8 nm respectively (Table 1 and Fig. 2a
and b). Despite the presence of heavier grafting groups,
the slightly higher surface area was recorded in SBA-15-
PAN, and SBA-15-SAN. This discrimination was recorded
due to the functionalization of SBA-15-TAN with more
nonfunctionalized groups. BET surface area analysis con-
frmed the efective cross-sectional area of N2 on silica
surface ranging between 16 Å and 21 Å (may vary with
the surface modifcation of silica). Although several types
of corrections were considered to obtain the efective
cross-sectional area of N2 interaction, the BET surface
area analysis cannot be accurate, and it may vary with the
nature of nanoporous silica material modifed with non-
polar functional moieties. Considering the above, the BET
surface area value was higher for SBA-15-TAN than SBA-
15-PAN and SBA-15-SAN.
The Ru ions were immobilized into amine-functionalized
SBA-15 materials (SBA-15-PAN, SBA-15-SAN, and SBA-
15-TAN) was achieved by reacting the solution of SBA-15
-PAN or SBA-15-SAN or SBA-15-TAN with a solution of
hexaammineruthenium (III) chloride complex ([Ru (NH3)6]
Cl3). To monitor the interaction of hexaammineruthenium
(III) complex with the grafted amine functional groups of
ET-SBA-15 material were characterized by UV–Vis difuse
refectance spectroscopy. We obtained the absorption maxi-
mum (λmax) for all SBA-15-PAN, SBA-15-SAN, and SBA-
15-TAN was recorded in the range between 280–300 nm
(attributable to interbank transition in the metal-amine
complex) (Fig. 3a and b). Notably, the absorption data for
all the above-mentioned materials gave a red-shift com-
pared with the control sample of amine-free SBA-15@Ru
(λmax =282 nm). A clear increase in absorption maxima was
recorded while increasing the number of methyl substituents
in the amine group: SBA-15-PAN (λmax = 286 nm), SBA-
15-SAN (λmax =294 nm) and SBA-15-TAN (λmax =301 nm).
Comparably, red-shift was recorded in the three control
samples synthesized by mixing unmodifed ET-SBA-15
with three types of hexaammineruthenium (III) chloride
complexes, [Ru (NH3)6]Cl3, [Ru (NH2CH3)6]Cl3, and [Ru
(NH(CH3)2)6]Cl3. As per the UV–Vis spectra of the above
complexes, the redshift is often due to the following three
possible reasons; a. N-Methylation of amine groups of metal
amine complex creates an anodic shift in the redox potential;
b. N-Methylation of amine groups generally increases the
electron density due to the increase of N atom makes the
amine functionality to be good σ-donor.; c. N-Methylation
Fig. 2 a N2 Adsorption/desorption profles, b N2 Adsorption/desorption profles of SBA-15 materials
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
Fig. 3 a UV–Vis data for amine functionalized SBA-15 with Ru (II) metal, b UV–Vis data for amine functionalized SBA-15 with ruthenium
complex species
also lowers the degree of solvation of metal- amine complex
(Fig. 3a and b). As per the above reasons, in presence of
polar protic solvents, tertiary amines are widely examined
as poorer σ-donor and poorer ligand in compression with
primary and secondary amines. Thus, because of it, the
formation of Metal-Nitrogen-H- -O hydrogen bonds to the
solvent molecules make the primary and secondary amines
stronger σ-donors. Additionally, the steric hindrance due
to the increase of methyl groups elongated the M–N bonds
and distorted the coordination sphere around Ru ions in the
complexes. As per the above discussion the red-shift in the
charge transfer band upon more N-Methylation of the ligands
to be due to the gradually weakening σ electron-donating
nature of the nitrogen atoms of the amine ligands.
metal varied from 0.16–0.41 mmol gcat−1. Although the same
molar quantity of amine-functionalized SBA-15 materials
(SBA-15-PAN, SBA-15-SAN, and SBA-15-TAN) were used
to react with the identical moles of Ru precursors, the fnal
product SBA-15-PAN@Ru (0) (CATALYST-1), SBA-15-
SAN@Ru (0) (CATALYST-2) and SBA-15-TAN@Ru (0)
(CATALYST-3) showed the variation in the quantity of
Ru metal due to the diferences in the hydrophobicity of
organoamines which were grafted on the materials. More
precisely, as per Table 2, the Ru metal loading in catalyst 2
and catalyst 3 was lesser than catalyst 1 due to the weaker
ability of hexaammineruthenium (III) ions to anchor onto
SBA-15-SAN and SBA-15-TAN than SBA-15-PAN.
The N2 adsorption/desorption data of CATALYST-1,
CATALYST-2, and CATALYST-3 gave type IV isotherm
with hysteresis loops. This data confirmed the original
intact structure of SBA-15-PAN, SBA-15-SAN, and SBA-
15-TAN materials even at comparatively high temperatures
under H2 pressure. Additionally, the BET surface areas of
CATALYST-1, CATALYST-2, and CATALYST-3 were
found lower than their corresponding amine-functionalized
SBA-15 materials (SBA-15-PAN, SBA-15-SAN, and SBA-
15-TAN) (Table 2). Moreover, these materials also showed
lower average BJH pore diameters and pore volumes while
increasing the bulky organoamine groups in amine-function-
alized SBA-15 materials.
3.3 Preparation and Physicochemical
Analysis of a Series of Ru (0) Anchored
Amine‑Functionalized SBA‑15 Mesoporous
Silica Materials/Catalysts
After the grafting of the organoamine groups and immobi-
lization of Ru ions on the surface of ET-SBA-15, the cor-
responding material was further reduced under molecular H2
gas at 250 °C. As a result of the above process, we obtained
grayish colored powder of Ru metal immobilized-amine
functionalized SBA-15 with diferent amine groups SBA-
15-PAN@Ru (0) (CATALYST-1), SBA-15-SAN@Ru (0)
(CATALYST-2), and SBA-15-TAN@Ru (0) (CATALYST-3)
(Fig. 4a and b). All the synthesized materials were analyzed
by EA, ICP-OES, N2 porosimetry and the corresponding
results were tabulated in Table 2. The EA and ICP-OES
experimental results indicated the quantity of nitrogen was
identical (1.3–1.5 mmol gcat−1) while the amount of Ru
All the catalysts and amine-free SBA-15@Ru(0) were
characterized by TGA. All the catalysts gave 0.62% of ini-
tial weight loss while elevating the temperature from room
temperature to 120 °C due to residual water in the structure
of the catalysts (Fig. 5). A second weight loss of 12% was
recorded while increasing the temperature from 120 and
700 °C mainly because of the organoamine decomposition
1 3

V. Srivastava
Fig. 4 a N2 Adsorption/desorption profles, b N2 Adsorption/desorption profles of diferent SBA-15 materials with Ru(0) NPs
and water (followed by condensation of Si–OH groups in
the materials). While in the control sample of SBA-15@
but in CATALYST-3 much bigger Ru NPs were found
(17.9 nm ( 0.25)). Biggest Ru NPs was recoded in SBA-
Table 2 Physiochemical
analysis of diferent Ru
anchored SBA-15 materials
Material
Na
BETb surface
Pore sizec (nm)
Pore volumec Ru (0)d
(mmol/gcat
)
area (m2/g)
(cm3/g)
(mmol/
gcat)
ET-SBA-15
N/A
2.6
2.7
3.1
163
185
197
243
9.3
7.8
4.5
4.8
0.22
0.27
0.24
0.35
–
CATALYST -1
CATALYST -2
CATALYST -3
0.43
0.15
0.16
aCalculated based on EA
bCalculated based on N2 adsorption/desorption data by BET method
cCalculated using BJH adsorption isotherm equation by BET method
dCalculated by ICP-OES analysis
Ru(0) 1.3% weight loss was recorded between the temper-
ature range between room temperature to 120 °C (due to
the removal of water) and 7.6% weight loss between the
temperature range from 120 to 700° (due to the removal of
residual Pluronic polymer templates and water due to the
condensation of Si–OH groups).
15@Ru(0) (32.7 nm ( 0.25)). We also performed SBA-15-
PAN@Ru (III) material using NaBH4 at room temperature
−
in the molar ratio of 1:1.7 (Ru+3:BH4 ) as per the reported
literature [42]. Unfortunately, we obtained the Ru NPs in
CATALYST 1A is quite a bigger size ((27.8 nm ( 0.25))
due to incomplete reduction.
To inspect the efect of diferent amine groups on the
size and the dispersion of Ru NPS over the support in all the
catalysts were analyzed by transmission electron microscopy
(TEM) (Fig. 6). Narrow size and uniformly distributed Ru
NPS were recorded over the SBA-15@Ru(0), CATALYST-1,
CATALYST-2, and CATALYST-3 materials. However, the
Ru NP size varies with respect to amine-functionalized
SBA-15 materials. The smallest Ru NPs were noticed in
CATALYST-1 (4.5 nm ( 0.25)) and an almost identical Ru
NP size was recorded in CATALYST-2 (9.6 nm ( 0.25)),
The above-mentioned variations in the size of the Ru NPs
in the diferent catalysts can be explained based on the stabi-
lizing ability of diferent types of amines in amine-function-
alized SBA-15 material. Just like many other organic ligands
and surfactants, the amine group in amine-functionalized
SBA-15 was used as capping agents to synthesize ultrafne
Ru nanoparticles. The Ru complex or Ru NPs in catalyst
1–3 are expected to have diferent degrees of interaction
with the different amine ligand functionalized SBA-15
materials and therefore end up with diferent sizes. More
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
specifcally, tertiary amine ligands are weaker than primary
and second to Ru complexions due to their poor solvation,
hence they formed less thermodynamically stable Ru ion
complex. Ru complexions or Ru NPs showed less stabil-
ity within the pores of SBA-15 functionalized with more
N-methylated amine ligands concerning less N-methylated
or unmethylated counterparts. In other words, less interac-
tion was recorded between the tertiary amine group with Ru
NPs than a primary amine with Ru NPS. As a result of the
above discussion, catalyst 3 gave bigger NPs.
The potential electronic interaction between the amine
group and Ru NPs was elucidated using X-ray photoelec-
tron spectroscopy (XPS) and all peaks related to Ru were
carefully analyzed and showed in Fig. 7. Due to the pres-
ence of organoamine functional groups in the catalysts 1,
2, and 3 the peaks for the Ru 3d region showed two intense
narrow peaks at binding energies of 280.4 and 284.3 eV for
Ru 3d5/2 and 3d3/2, respectively (Fig. 7). These peaks can be
assigned to metallic Ru as per the literature [43–45], which
indicates that the composition of the NPs is mainly Ru (0)
NPs. No noticeable change among the peak of Ru 3d in the
Fig. 5 TGA data for Ru doped amine functionalized SBA-15
Fig. 6 TEM data of all the developed materials
1 3

V. Srivastava
increases with temperature rise and passes through a
maximum at 125 °C. The increase in H2 uptake with tem-
perature rise is mainly due to multiple bond adsorption
or reversible H2 adsorption on ruthenium sites science it
is well established that strong and irreversible H2 uptake
corresponds to 1:1 stoichiometry of Hirr/Ru (0) does not
change with temperature. The capacity of ruthenium
atoms to form multiple bonds with hydrogen atoms on the
catalyst surface is enhanced by raising the temperature
of adsorption. The hydrogen chemisorption isotherm of
CATALYST-1 is presented in Fig. 9. In which it is shown
that the total hydrogen uptake at 740 torr increases up
to 125 °C above which decreases drastically. Thus, the
results of the present studies suggest that the reversible H2
chemisorption plays an important role in the hydrogena-
tion reaction on group VIII metals [46, 47, 47, 48].
3.3.1 Catalytic Activities of the Materials CO2
Hydrogenation Reaction and Efect of Their
Physicochemical Properties
Fig. 7 XPs data of Ru doped amine functionalized SBA-15 materials
CATALYST-1, 2, and 3 were recorded while varying the
nature of methyl substituents.
The CO2 hydrogenation reaction was carried out by placing
the series of SBA-15-PAN@Ru (0) (CATALYST-1), SBA-
15-SAN@Ru (0) (CATALYST-2), and SBA-15-TAN@Ru
(0) (CATALYST-3) as per the given reaction conditions in
We also performed, the total hydrogenation chemisorp-
tion and the reversible hydrogen chemisorption at 25° C to
identify the physical properties of all the developed CAT-
ALYST-1, 2 and 3. It was recorded that the total and the
reversible H2 adsorption isotherms became parallel above a
pressure of 500 torr indicating the presence of equilibrium in
adsorption above this pressure. The diference between the
two isotherms at 740 torr is taken as a measure of irrevers-
ible H2 uptake [46–48]. The reversible and irreversible H2
uptake and their ratios (Hr/Hirr) for the catalyst of diferent
metals lodgings are presented in Table 3.
Table 3, confrmed a direct correlation between Ru
metal loading and hydrogen uptake. The irreversible
hydrogen uptake which corresponds to a stoichiometry
of Hirr/Ru (0) = 1:1 increases with Ru loading and found
maximum in the case of CATALYST-1.
The hydrogen chemisorption is also studied with CAT-
ALYST-1 at a diferent temperature from 25 °C to 300 °C
(Fig. 8). The hydrogen uptake taken from the isotherm
at 740 torr pressure is plotted against the temperature
of adsorption in Fig. 9. It is seen that hydrogen uptake
Fig. 8 H2 chemisorption isotherm at diferent temperature
Table 3 Hydrogen
Material
BETa surface area Ru (0)b
Reversible H2
uptake (Hr)
μ mol/g
Irreversible H2
uptake (Hirr)
μ mol/g
Hr/Hirr
chemisorption study
(m2/g)
(mmol/gcat)
CATALYST -1
CATALYST -2
CATALYST -3
185
197
243
0.43
0.15
0.16
85.0
62.7
63.5
25.2
10.2
7.8
3.37
6.14
8.14
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
Fig. 9 1HNMR analysis of
Reaction mass and formic acid
calculation
Table 4. Among all the three catalysts, catalyst 1 gave higher
catalyst activity and synthesized the formic acid in good
quantity. After completion, the reaction and catalyst were
isolated from the reaction mass followed by the centrifuga-
tion method, then a small quantity of reaction mass was used
for titration and 1H NMR analysis to quantify the amount of
formic acid in the reaction mass. It is worth notify that the
result obtained by the titration method and 1HNMR (Fig. 9),
were found in good agreement with each other. The activ-
ity of all three catalysts was measured in terms of formic
acid quantity, TON, and TOF value, and all the results were
summarized in Table 4. The catalytic performance of all
three catalysts can be explained based on the physicochemi-
cal properties of the catalysts. In some of the recent reports,
amine-functionalized mesoporous silica-supported metals
Table 4 Application of CATALYST 1, 2 and 3 for CO2 hydrogenation reaction
S.No
Catalytic system
P (H2) (P total
)
ToC
t (h)
Formic acid (g)
TON (molFA/molRu
)
TON
(CATALYST 0.01 g+5 ml Water)
(MPa)
(molFA/
molRu x
h−1
)
1
2
3
4
5
6
7
8
CATALYST-1
CATALYST-2
CATALYST-3
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
10 (20)
30 (60)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
20 (40)
80
80
80
80
80
80
80
80
80
80
80
80
100
50
80
80
80
80
80
80
80
80
80
8
8
8
8
8
8
8
8
8
8
8
8
8
8
4
10
8
8
8
8
8
8
8
0.94
0.88
0.67
0.91
0.36
0.51
0.17
0.91
0.90
0.12
0.30
0.89
0.82
0.23
0.28
0.85
0.82
0.41
0.12
0.32
0.22
0.19
0.87
10,356.5
885.2
5204.1
5090.2
2013.7
2909.7
950.9
5035.3
5030.4
1342.5
1678.1
4978.3
4586.8
1390.4
1566.2
4754.6
4586.8
1062.5
17.8
1294.6
110.7
650.5
636.3
251.7
363.7
118.9
629.4
628.8
167.8
209.8
622.3
573.4
173.8
391.6
475.5
573.4
132.8
2.2
CATALYST-1+0.2 mL Et3N+5 mL Water
CATALYST-1+0.2 g NaOH+5 mL Water
CATALYST 1+5 mL DMSO
CATALYST-1+2 mL water
CATALYST-1+10 mL water
CATALYST-1 (0.1 g)
CATALYST-1 (0.005 g)
CATALYST-1
CATALYST-1
CATALYST-1
CATALYST-1
CATALYST-1
CATALYST-1
CATALYST-1+PPh3 (0.2 g)
SBA-15@Ru
RuCl3.3H2O
[Ru (NH3)6 Cl3]
[Ru (NH2 CH3)6 Cl3]
[Ru (NH (CH3)2)6 Cl3]
CATALYST-1a
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1810.2
1231.8
987.1
226.3
154.0
123.4
591.3
4730.4
1 3

V. Srivastava
were found highly active for CO2 hydrogenation reaction
[49, 50].
reactants in comparison with the optimized condition, and
a clear drop in formic acid quantity was recorded. Drop-
in formic acid quantity at low pressure can be explained
by the weak interaction between catalyst and reactants.
An increase in reactant pressure also lowers the quantity
of formic acid due to the decomposition of formic acid
at high pressure. This observation was also supported by
previously reported literature [51]. A very low quantity of
formic acid was noticed while performing the reaction at
50 °C, this confrms the low activation of reactants at the
above temperature. Similarly, a small drop in formic acid
quantity was recorded at a high temperature which supports
the decomposition of formic acid at 100 °C under pressure
or reverses water gas shift reaction. The optimized reac-
tion condition of CO2 hydrogenation was also checked with
PPh3 (triphenylphosphine) as an additive, but no signifcant
change in the quantity of formic acid was recorded. We also
utilized [Ru (NH3)6 Cl3], [Ru (NH2 CH3)6 Cl3] and [Ru (NH
(CH3)2)6Cl3] materials to catalyze the CO2 reduction reac-
tion but we obtained formic acid in low quantity (Table 4,
entry 20–22). This confrms the importance of support in the
catalytic system which also worked as a co-catalyst.
The high catalytic performance of catalysts 1, 2, and 3
was ascribed to the electron density transfer from the pri-
mary amine groups to the Ru NPs, and thus the electron-
richness of the Ru centers in it. The same electron transfer
progression from secondary and tertiary amine groups to Ru
NPs and comparable peak positions for Ru 3d in XPS analy-
sis were also recorded for the Ru NPs in catalyst 2 and 3.
Hence the catalytic activity of CATALYST-2 and 3, can be
justifed as per the above argument. Though, moreover, we
can correlate the catalytic performance of CATALYST-1, 2,
and 3 in the CO2 hydrogenation reaction. A drop in HCOOH
quantity was recorded while increasing the Ru particle size
from 4.5–32.7 nm.
The CO2 hydrogenation reaction is reported as a thermo-
dynamically unfavorable transformation [17, 23]. To shift
the reaction towards the formic acid formation, high pressure
(> 2 MPa) and additives as amine, ammonia, or an alkali
were explored to promote the formation of adduct or complex
which support the easy reduction of CO2 to formic acid. We
also used the organic and inorganic base to achieve the same,
despite considering the presence of amine functionality in
our catalysts. No positive change in the quantity of formic
acid was recorded, this confrms the additional benefts of
using amine-functionalized support in our catalytic system.
We also examined the efect of the solvent system in our
catalytic system. Apart from water, we also tested the polar
aprotic DMSO solvent system in place of water. The low for-
mic acid quantity was noticed while replacing the water with
DMSO. In our study, we found that water plays an impor-
tant role to accelerate the rate of CO2 reduction as it reacts
with water as well as amine during the reaction and form
the bicarbonate species, which may work as a true reaction
substrate for the hydrogenation [27, 51, 52]. Although, the
presence of water not only increases the quantity of formic
acid but also increases the energy consumption to remove
water from the reaction mass. Hence, it is important to use
water in an optimized quantity. During the water optimiza-
tion, 5 mL of water was found sufcient to get the maximum
amount of formic acid.
In the catalytic CO2 hydrogenation reaction, we pre-
sented the in‐situ Difuse Refectance Infrared with Fourier
Transform Spectroscopy (DRIFT) analysis to explore the
creation of surface species in the catalytic processes and to
fnd the reaction mechanism. In this analysis, DRIFT spec-
tra were scrutinized at increasing reaction temperatures, in
the existence of the reactant mixture/products. Figure 10a,
revealed the spectrum of ET-SBA-15 after the CO2 adsorp-
tion (Table 5). The band at 1715 cm−1 could be allocated to
the νS modes of bridge carbonate [28, 53–56]. While, the
bands at 1573 and 1308 cm−1 are ascribed, respectively, to
the νas as (CO3) and νS (CO3) modes of bidentate carbonate
[57, 58]. The band at 1555 cm−1 resembles νCO modes of
bicarbonate species and the band at 1662 cm−1 to the νCO
mode of carboxylate species [28, 54, 57, 58]. The 1628 cm−1
band is appointed to the ẟOH mode of physisorbed water on
ET-SBA-15. This shows the various interface confgurations
of CO2 molecules with ET-SBA-15.
Figure 10b, exposes the interaction spectra between
ET-SBA-15 and HCOOH. The bands were specifed based
on IR surveys registered for SBA-15. In the spectrum ET-
SBA-15, the bands of 2945 and 1735 cm−1 could be named
to the νCH and νCO modes of molecularity adsorbed HCOOH
[59–61]. In the arena of Fig. 11, the noticed bands at 3715
and 3648 cm−1are allocated to νOH modes of isolated OH
groups on ET-SBA-15. The strength of these bands declines
with HCOOH adsorption, coupled with an expansion of
the wide-ranging 3000–3400 cm−1 band of νOH modes of
hydrogen bonding. This indicates that the insular OH groups
on ET-SBA-15 contributed to the molecular adsorption of
HCOOH via hydrogen bonding. New bands emerged at
It is well documented that the quantity of catalyst plays
an important role to change the rate of any chemical reaction
[17, 23]. The same observation was recorded while lowering
or increasing the catalyst quantity the noticeable change in
the amount of formic acid was recorded. A drop in formic
acid quantity was found while increasing the catalyst quan-
tity signifes the formation of Ru agglomerates during the
reaction. While using the optimized catalyst quantity, the Ru
NPs get dispersed efectively in reaction mass and provides
better contact between reactants and catalyst active sites.
It is well documented that CO2 hydrogenation is a pres-
sure-dependent reaction, but while changing the pressure of
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
Fig. 10 a DRIFT analysis after
CO2 adsorption on CATA-
LYST-1, b DRIFT analysis after
CO2 adsorption on ET-SBA-15
(1) and CATALYST-1 (2) Inset
spectra of νOH a. CATALYST-1
before HCOOH adsorption, b.
CATALYST-1 after HCOOH
adsorption and ET_SBA-15
after HCOOH adsorption
2875, 1595, and 1375 cm−1 credited to the νCH, νasOCO, and
sOCO modes of adsorbed HCOO− respectively [55, 57–62].
summaries in Table 4 and the related species composition is
ν
displayed in Fig. 12.
These fndings suggest that HCOOH adsorbs in both molec-
ular and dissociative forms on the ET-SBA-15 surface. In
the CATALYST-1 spectrum, the aforementioned HCOOH
adsorption bands on ET-SBA-15 are marginally dislocated.
Bands of molecularly adsorbed HCOOH have redshift to
2935 cm−1 showed, assigned to the νCO mode of HCOOH
adsorbed on the surface. These trapped band shifts could be
a result of changes in the degree of surface hydration or sam-
ple pre-treatment. In this analysis, the CATALYST-1 were
pre-reduced in H2 at 45 °C while ET-SBA-15 was analyzed
as prepared. Despite the dislocation, the band assignments
are consistent with the development of surface species on the
support site and not on the Ru site. The band assignments are
The plausible mechanism of CO2 hydrogenation to for-
mic acid can be proposed in the following steps a. catalyst
activation, b. H2 dissociation, c. CO2 insertion, d. hydride
attack, e. formic acid desorption, and f. catalyst regeneration
step (Scheme 2). Nanocrystalline Ru anchored in the cage-
like structure of ET-SBA-15 can easily absorb the H2 mol-
ecule on their active surface sites. These H2 molecules were
further dissociated and formed the highly reactive complex
molecule with CO2 gas. As per the reaction at the catalysts
surface, followed by hydride attack on CO2 molecule formic
acid adduct formation takes place. The formic acid adduct
molecule further gets desorbed from the catalyst surface via
the catalyst regeneration step.
Table 5 Capture frequencies and band assignments after adsorption
of CO2 and HCOOH to the reduced CATALYS-1
Probe molecule
Captured frequen- Assignment
cies (cm−1
)
CO2
1715
1662
1573
1555
1308
2945–2935
2875
νCO bridged carbonate
νCO carboxylate
νasCO bidentate carbonate
νCO bicarbonate
νsCO bidentate carbonate
a
HCOOH
νCH HCOOHabs
νCH HCOOabs
1735–1720
1665
1595
νCO HCOOHabs
νCH HCOOHsabs
νasoco HCOOHabs
b
1375–1381
νssoco HCOOHabs
aAdsorbed
bStrongly adsorbed
Fig. 11 H2 chemisorption isotherm as a function of temperature
1 3

V. Srivastava
Fig. 12 Types of species identi-
fed in DRIFT study
Scheme 2 Plausible mechanism of CATALYST-1 promoted CO2 hydrogenation into the formic acid
recycling study. This study showed that the growth in Ru
particle size as agglomeration was recorded after the 5th
run in CATALYST-1 (25.8 nm ( 0.25)) and CATALYST-2
(39.9 nm ( 0.25)) and CATALYST-3 (45.9 nm ( 0.25))
gave the same result after their 3rd recycling run (Fig. 13).
Also, a sign of Ru metal leaching was noticed maximum
in the case of catalyst 2 and 3 in compression with catalyst
1. The above observations were found in good agreement
with the application of amine group interaction with Ru
metal. While achieving the satisfactory interaction of the
amine group with Ru metal in CATALYST-1 lowered the
process of agglomeration and metal leaching with respect
to CATALYST- 2 and 3.
3.4 Catalyst Recycling Study
Catalyst recyclability studies were conducted for all the
catalysts to determine their stability and reusability under
high-pressure reaction conditions. In the case of catalyst 1,
we obtained efective recyclability up to 5 times in terms
of formic acid quantity as well as TON and TOF values.
While the other two catalysts were not found active after
the 3rd run and we obtained a very low quantity of formic
acid after recycling the CATALYST-2 and CATALYST-3
in the 4th recycling run (Fig. 10). We performed the TEM
and ICP-OES analysis to understand better the physi-
ochemical properties of all three catalysts during their
1 3

CO2 Hydrogenation over Ru-NPs Supported Amine-Functionalized SBA-15 Catalyst: Structure–…
1
Fig. 13 Catalysts recycling
experiment result
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1st run
2nd run
3rd run
4th run
5th run
6th run
Formic acid Quanꢀty @ CATALYST- 1
Formic acid Quanꢀty @ CATALYST- 3
Formic acid Quanꢀty @ CATALYST-2
4 Conclusion
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