Ruthenium nanoparticles supported over mesoporous TiO2 as an efficient bifunctional nanocatalyst for esterification of biomass-derived levulinic acid and transfer-hydrogenation reactions

Article abstract of DOI:10.1039/c6ra10233f

We have supported ruthenium nanoparticles (NPs) over a mesoporous TiO₂ material (Ru@MTiO₂) having nanoscale porosity and a high BET surface area. Wide-angle powder X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible absorption spectra (UV-vis) and N₂ sorption studies are used to characterize the Ru@MTiO₂ material. This nanostructured ruthenium supported over a TiO₂ material exhibited excellent bifunctional catalytic activity in the esterification of levulinic acid and selective transfer hydrogenation reactions of nitroarenes under mild reaction conditions. This Ru@MTiO₂ catalyst can be easily recovered from the reaction mixtures and reused five times without a significant loss of its reactivity and selectivity.

Full text of DOI:10.1039/c6ra10233f

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Ruthenium nanoparticles supported over mesoporous TiO as an
2
efficient bifunctinal nanocatalyst for esterification of biomassꢀderived
levulinic acid and transferꢀhydrogenation reactions
a
a,b
c
c,
Usha Mandi, Noor Salam, Sudipta K. Kundu, Asim Bhaumik * and Sk.
a,
Manirul Islam *
a
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
b
Department of Chemistry, The University of Burdwan, Burdwan, West Bengal, India
c
Department of Materials Science, Indian Association for the Cultivation of Science,
Kolkata - 700032, India
Abstract
We have supported ruthenium nanoparticles (NPs) over mesoporous TiO
2
material
(
Ru@MTiO ) having nanoscale porosity and high BET surface area. The wideꢀangle powder
2
Xꢀray diffraction (XRD), transmission electron microscopy (TEM), energyꢀdispersive Xꢀray
spectroscopy (EDX), Fourier transform infrared spectroscopy (FTꢀIR), ultravioletꢀvisible
absorption spectra (UVꢀvis) and N sorption studies are used to characterize the Ru@MTiO
2
2
material. This nanostructured ruthenium supported over TiO
2
material exhibited excellent
bifunctional catalytic activity in the esterification of levulinic acid and selective transfer
hydrogenation reactions of nitroarenes under mild reaction conditions. This Ru@MTiO
2
catalyst can be easily recovered from the reaction mixtures and reused five times without a
significant loss of its reactivity and selectivity.
2
Keywords: Mesoporous TiO , ruthenium nanoparticles, biofuel, transferꢀhydrogenation,
nitroarenes.
*
Authors to whom correspondences should be addressed.
1

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S. M. Islam, Fax: +91ꢀ33ꢀ2582ꢀ8282, Eꢀmail: manir65@rediffmail.com
A. Bhaumik, Fax: +91ꢀ33ꢀ2473ꢀ2805, Eꢀmail: msab@iacs.res.in
Introduction
Depleting fossil resources and increasing concerns over greenhouse gas emissions
have been attracted a wideꢀscale interest in producing fuels and valuable chemicals from
1
renewable biomass feedstocks. The chemical industry view biomass as a promising
feedstock in future and thus biomass conversion into the value added chemicals is a matter of
growing interest today. Among these routes, the synthesis of levulinate esters from levulinic
acid (LA) has attracted widespread interests because LA is one of the top biomass derived
platform molecules that can be made via C6 sugar carbohydrates derived from renewable
2
lingoꢀcellulose. Levulinates such as ethyl, methyl, and nꢀbutyl levulinate are obtained by
3
esterification of levulinic acid (LA) or ethanolysis of furfuryl alcohol. These levulinate
esters have several potential applications in flavour and fragrance industry. Moreover, ethyl
levulinate (EL) can be used up to 5 wt% as the diesel miscible biofuel (DMB). Addition of
EL to the diesel leads to considerable improvement in the fuel properties such as a clean
4
burning fuel with high lubricity, stability in flashpoint and improvement in viscosity.
On the other hand selective reduction or transfer hydrogenation of nitroarenes to
5
anilines is a very important reaction both in industry and academia. In the past few years,
several protocols for the reduction of nitroarenes employing various catalysts using a variety
6,7
of metals including Cu, Ni, Fe, Pd, Ag, Ru and Rh have been investigated. But catalytic
hydrogenation has two major drawbacks: the danger and inconvenience of handling highly
flammable hydrogen gas and the low selectivity in the reduction of functionalized
nitroarenes. Apart from hydrogen, several other stoichiometric reducing agents have been
introduced that allow an efficient reduction of nitroarenes to anilines when used in
2 4 2
combination with metal catalysts, Therefore, hydrazine hydrate (N H ·H O) is a very suitable
2

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8
2
reagent for the reduction of nitro groups, as it generates only N gas as a byproduct.
Compared to its unstable anhydrous form, hydrazine hydrate is relatively safe and easy to
handle.
Nanometerꢀsized transition metal particles have emerged as one of the most useful
and promising material because of their unique surface properties together with porosity and
9
high surface area, which are largely different from their corresponding bulk materials. Their
large surface areaꢀto volume ratio, which increases when decreasing particle size, could
10
greatly increase their specific catalytic activity. Supported ruthenium catalysts have been
widely used in heterogeneous catalysis, owing to their high activity and/or selectivity in many
industrially important catalytic processes, including ammonia synthesis and decomposition,
11
hydrogenation, etc. The effectiveness of the Ruꢀbased catalysts in these heterogeneous
reactions is strongly dependent on the metalꢀsupport interaction, which imposes significant
influences on the morphology, surface and electronic structure, and the resulting catalytic
12
performance. Thus, these nanoparticle based catalysts are often attached with different solid
supports such as carbon nanotube, graphene, inorganic materials as SiO , Al O , ZrO , TiO ,
2
2
3
2
2
hydrotalcite, Montmorillonite clay and other materials for easier separation and presence of
1
3
wellꢀdistributed Ru active sites. Recently, Weckhuysen et al. has employed Ru/HꢀZSMꢀ5 as
a heterogeneous catalyst to directly convert LA to valeric acid (VA) and its esters but a yield
13f
of 45.8% of VA was achieved in dioxane as solvent. Therefore, the development of a novel
catalytic system for esterification of levulinc acid with ethanol aiming for biofuel derived
ethyl levulinate production in one pot with high efficiency is of great significance. Moreover
2
TiO supported Ru catalyst is considered as one of the most promising catalyst support
materials due to high surface area, its low cost, high photocatalytic activity, environmentally
benign characteristics, high reflective index, noncorrosion, good stability.
3

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2
In that context, TiO ꢀsupported catalysts have been attracted considerable attention in
many reactions of both environmental and industrial importance, because of their excellent
14
catalytic activities compared with other supported catalysts. Although Ru NPs are known
15,16
for their unique catalytic activity in wide range of chemical reactions
and nanoporous
17
TiO as a robust catalyst support, there are only few very recent reports on Ru NPs
2
18
supported over TiO
photocatalytic oxidation of bezylalcohol. To the best of our knowledge, there are only a few
reports of high surface area mesoporous TiO materials composed of tiny titania
2
materials for partial hydrogenation of benzene to cyclohexene and
1
9
2
nanoparticles with crystalline pore walls. In recent years, with the increasing environmental
restrictions and the green development of industrial processes TiO ꢀsupported nanocatalysts
2
2
0
offer a wide scope. Many research studies are focusing on fabricating various morphologies
21
of TiO
2 2 2
, such as nanotubes, porous nanofibers, metal doped TiO , etc. Furthermore, TiO
was found to be a good metal oxide catalyst support due to the strong metal support
21
interaction, chemical stability, and acidꢀbase property. Herein, we wish to demonstrate the
synthesis of a new and highly stable ruthenium nanoparticles embedded on mesoporous TiO2
material and its excellent catalytic activity towards the esterification of levulinc acid with
ethanol aiming for biofuel derived ethyl levulinate production and also in efficient and
selective reduction of nitroarene mixed in the presence of styrene by using hydrazine as
reducing agent at room temperature.
Experimental
Chemicals
3 2
Ruthenium trichloride hydrate (RuCl ·nH O) and Titanium (IV) isopropoxide
[
Ti(OC
3 7 4
H ) ] were purchased from SigmaꢀAldrich. Hydrochloric acid (HCl) and sodium
borohydrate (NaBH
4
) were obtained from EꢀMerck, India. Sodium dodecyl sulfate (SDS) was
4

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obtained from Spectrochem, and used as anionic structureꢀdirecting agent. All other
chemicals were also purchased from EꢀMerck, India.
Characterization techniques
Powder Xꢀray diffraction (XRD) patterns of the samples were investigated with a
Bruker D8 Advance Xꢀray diffractometer using Ni–filtered Cu K (λ=0.15406 nm) radiation.
α
Transmission electron microscopy (TEM) images of the nanocomosite were analyzed to
determine the particle morphology, using a JEOL JEM 2010 transmission electron
microscope operating at 200 kV. A PerkinꢀElmer FTꢀIR 783 spectrophotometer was used to
record the FTꢀIR spectra of the samples. UVꢀVis spectra were carried out using a Shimadzu
UVꢀ2401PC doubled beam spectrophotometer having an integrating sphere attachment for
2
solid samples. N adsorption/desorption isotherms were obtained by using a Quantachrome
autosorbꢀ1C at 77 K. Prior to gas adsorption, the sample was degassed for 4 h at 393 K under
high vacuum conditions. Thermogravimetry (TG) analysis of the samples was taken in a
TGA instruments thermal analyzer Mettler Toledo TGA/DTA 851e under N flow. A Varian
2
3400 gas chromatograph equipped with a 30 m CPꢀSIL 8CB capillary column and a flame
ionization detector was used to analyze the reaction products and a Trace DSQ II GCꢀMS
equipped with a 60 m TRꢀ50MS capillary column was used to identify the products.
2
Preparation of mesoporous TiO nanoparticles
17
Synthesis of mesoporous TiO was carried out according to the reported procedure.
2
In this process, 20 mL distilled water was acidified with 35 wt% of hydrochloric acid (1.2 g).
After that, 0.01 M titanium (IV) isopropoxide (2.84 g) was added slowly into the aqueous
acidic solution under vigorous stirring. The stirring was continued until a clear solution was
observed. Then, in a round bottom flask this clear solution was taken and allowed to stir for
20 h at 313 K. After 20 h of stirring, milky highly dispersed nanoparticles were obtained. 20
5

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mL distilled water was taken in another beaker and 0.12 M SDS (0.345 g) was added. After
that, the prepared titania nanoparticles were slowly added to the micellar solution of SDS.
The whole solution was then allowed to stir for 2 h at room temperature. Finally, this solution
was transferred in a round bottom flask and allowed to stir for 3 h at 353 K. After cooling the
solution at room temperature centrifugation was done to separate out the selfꢀassembled
nanoparticles. Then this was dried and calcined at 773 K for 5 h in presence of air to get the
templateꢀfree self assembled mesoporous titania nanoparticles.
Synthesis of Colloidal Ruthenium Nanoparticles (Ru NPs)
Ruthenium NPs have been synthesized through the reduction of aqueous solution of
ruthenium trichloride by NaBH
was dissolved in 50 mL distilled water under vigorous stirring. In another round bottom flask,
mM SDS (1.15 g) was dissolved into 50 mL of water with constant stirring followed by the
addition of 6 mM NaBH (0.23 g). Then the solution of prepared RuCl was slowly added
4 . 3
as a reducing agent The 0.2 mM RuCl (0.042 g) solution
4
4
3
into the mixture solution of NaBH and SDS under continuous stirring for 1h. Then the
4
precipitates were collected by centrifugation and washed with double distilled water for
several times, yielding colloidal ruthenium nanoparticles.
2
Synthesis of Ru@MTiO nanocatalyst
The prepared ruthenium NPs were dispersed into the methanol by sonication for 10
minutes. Then 1.0 g mesoporous TiO was added into the solution. The mixture was then
2
stirred for 24 h at room temperature to obtain the homogeneous suspension. Finally, the
mixture was washed with water and acetone, dried in oven and calcined at 300 °C for 6 h to
obtain the Ru@MTiO
2
nanocatalyst (Scheme 1).
6

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HCl, SDS
RuCl . nH O, NaBH
3 2
O
4
O
Ti
O O
Calcination
Reduction
Ru@MTiO2
TiO nanoparticles
2
Ru nanoparticles
Scheme 1. Schematic diagram for the formation of Ru@MTiO
2
nanocatalyst.
General experimental procedure for esterification of levulinic acid with alcohol using
Ru@MTiO nanocatalyst
2
Esterification reaction was carried out in a 50 mL round bottom flask equipped with a
reflux condenser. In a typical synthesis, levulinic acid (1.474 g, 12.7 mmol), methanol (4.1
mL, 101.6 mmol) and catalyst (40 mg) were heated to 338 K. The progress of the reaction
was monitored by GC. After 8 h the solid catalysts were separated out from reaction mixture
by centrifugation. Then, the obtained liquid was dried over anhydrous sodium sulphate and
finally, the product was analyzed by GCꢀMS.
General experimental procedure for transfer-hydrogenation of nitroarenes over
2
Ru@MTiO nanocatalyst
In a typical synthesis, nitrobenzene (0.5 mmol, 0.05 mL), solvent (2.0 mL) and
catalyst (30 mg) were taken in a 50 mL round bottom flask. Then, hydrazine hydrate (5
mmol, 0.16 mL) was added into the reaction mixture. After completion of the reaction the
catalyst was removed from the mixture by filtration. Then, the filtrate was washed with ethyl
acetate and dried over anhydrous sodium sulphate. The residue obtained was purified by flash
column chromatography on silica gel to afford the desired product, and then analyzed by GCꢀ
MS.
Results and Discussion
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Characterization of catalyst
Powder XRD studies
The wideꢀangle powder Xꢀray diffraction patterns for mesoporous TiO2 and
Ru@MTiO materials are shown in Figure 1. The wide angle XRD patterns of the TiO
nanoparticles showed highly crystalline planes corresponding to the anatase TiO . The peaks
at 2θ values of 25.35 , 37.77 , 48.20 54.65 , 62.92 , 69.52 and 75.40 agree well with the
anatase (101), (004), (200), (211), (204), (116) and (215) crystal planes of TiO (JCPDS,
CAS number 21ꢀ1272). Therefore, this powder XRD result indicates the successful
preparation of the stable crystalline TiO nanoparticles. The comparison of the XRD patterns
for TiO and Ru@MTiO samples shows that the characteristic diffraction peaks of titania
2
2
2
o
o
o
o
o
o
o
2
22
2
2
2
anatase remains unchanged. This observation clearly suggests that the loss in the crystallinity
or change in lattice framework of anatase titania after reduction of ruthenium(III) ions do not
occurs. Furthermore, we observe two peaks for (100) and (101) crystal planes corresponding
2
3
to hexagonal ruthenium NPs on titania anatase in Figure 1 (JCPDS CAS number 89ꢀ4903).
Figure 1. Wideꢀangle powder XRD patterns of mesoporous TiO2 and Ru@MTiO2
3
nanomaterials. The asterisk sings indicate the presence of unreacted RuCl .
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o
o
o
In the Figure 1, the peaks at 2θ values of 18.20 , 27.40 and 31.29 (indicated by asterisk
signs) have come due to (011), (104) and (015) crystal planes of unreacted ruthenium
trichloride, respectively (JCPDS CAS number 01ꢀ085ꢀ0528).
TEM analysis
The particle size and morphology of the samples are investigated by TEM analysis.
The TEM images (Figure 2 A, B) showed the uniformly distribution of TiO
2
particles of size
in between 6 to 9 nm. The presence of ruthenium NPs are confirmed from EDX analysis
(Figure 2C).
Figure 2. TEM images (A,B) and EDX pattern (C) of Ru@MTiO nanocatalyst.
2
ꢀ1
2
The loading Ru over TiO is around 0.1637 mmolg from ICPꢀAES measurement. ICPꢀAES
measurement has also been carried out for the reused catalyst after five consecutive catalytic
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cycles. It is found that, after the five catalytic cycles the Ru metal loading on TiO
2
is around
ꢀ1
0
.1550 mmolg .
N
2
adsorption-desorption analysis
The nitrogen adsorptionꢀdesorption isotherms of calcined TiO and Ru@MTiO2
2
materials describe the porosity of these materials, which is shown in Figure 3. The BET
BrunauerꢀEmmettꢀTeller) surface areas of calcined TiO and Ru@MTiO materials are 113
(
2
2
2
ꢀ1
2 ꢀ1
m g and 92 m g , respectively. Both isotherms can be classified as type IV isotherm
corresponding to the mesoporous nature and these isotherms display H2 type hysteresis
22
2 2
loops. The decrease in surface area of Ru@MTiO than that of calcined TiO indicates the
2 2 2
Figure 3. N adsorptionꢀdesorption isotherms of TiO and Ru@MTiO materials (A) and the
pore size distributions of TiO and Ru@MTiO materials (B).
2
2
1
0

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successful grafting of ruthenium nanoparticles at the surface of the mesopores TiO
2
material.
The pore size distribution is measured by nonꢀlocal density functional theory (NLDFT)
ꢀ1
2 2
method. The total pore volumes of calcined TiO and Ru@MTiO materials are 0.1796 cc g
ꢀ1
and 0.1196 cc g , respectively.
FTIR spectroscopy
2
The FT IR spectra of mesoporous titania and Ru@MTiO is shown in Figure 4.
ꢀ1
Mesoporus titania showed a broad peak at 3300 to 3400 cm which could be assigned due to
the OꢀH stretching vibrations of the residual water present in the mesoporous titania. The
–1
mesoporous titania also shows a broad band in the 450ꢀ800 cm region confirming the
2
4
anatase phase of TiO . The IR spectra of Ru@MTiO shows small shift towards lower wave
2
2
number of these peaks, this may be due the interaction of ruthenium nanoparticle with
anatase TiO . Further, we have added UVꢀvisible absorption spectroscopic data (ESI: Figure
S1) to know the effect of the loading of Ru NPs on TiO matrix and TGA analysis (ESI:
Figure S2) to understand the thermal stability of our Ru@MTiO catalyst.
2
2
2
2 2
Figure 4. FTIR spectra of mesoporous TiO and Ru@MTiO nanomaterials.
Catalytic activity
1
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Recently, significant interest has been paid on designing the nanostructured materials
of definite size and shape because of their various potential applications as catalysts, sensors,
and optoelectronics and so on. Mesoporous materials have great demand now a day due to its
25
high surface area, uniform pore size distribution and high catalytic activity. So, we have
explored the catalytic activity of the mesoporous Ru@MTiO in the esterification of biomass
2
derived levulinic acid and selective transfer hydrogenation reactions of nitroarenes under
mild conditions.
2
Esterification of levulinic acid using alcohol catalyze by Ru@MTiO nanomaterials
The esterification of levulinic acid (LA) is an important chemical transformation to
produce the biobased chemicals using environmentally friendly methods. Biofuel derived
levulinate ester is widely used as a fuel in oil and gas industry, additive in some foods and
fragrance in cosmetics. To explore the catalytic activity of Ru@MTiO
esterification of levulinic acid with methanol was chosen initially as a model reaction
Scheme 2).
2
nanocatalyst,
(
O
O
Ru@MTiO2
OH
MeOH
OMe
o
6
5 C, 8 h
O
O
Scheme 2. Esterification of levulinic acid with methanol.
Optimization of reaction parameters
Effect of Catalyst Loading
The effect of catalyst loading on LA esterification has been studied to optimize the
reaction conditions. The catalyst loading was varied from 10 to 60 mg at a temperature of 338
K, reaction time of 8 h and methanol to LA molar ratio 8:1. With increasing of catalyst
loading overall conversion was also increased from 45 to 95% (Figure 5). In this case,
catalyst loading of 40 mg is observed to be optimum. Figure 7 illustrated the effect of catalyst
1
2

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loading on LA conversion. Above 40 mg catalyst loading, the almost steady LA conversion is
observed, suggested this optimum loading of Ruꢀcatalyst necessary for the esterification
reaction.
Figure 5. Effect of catalysis loading at reaction conditions: solvent to LA ratio: 8:1,
temperature: 338 K, reaction time: 8 h.
Effect of Reaction Temperature
The effect of temperature on LA esterification was studied. The reaction temperature
was varied from 298 K to 378 K. The maximum conversion (95%) was obtained at 338 K,
after 8 h using 40 mg catalyst and methanol to LA molar ratio 8:1. Figure 6 displayed the
effect of reaction temperature on LA conversion.
Effect of molar ratio of solvent to levulinic acid
The molar ratio of solvent to levulinic acid on LA esterification was investigated. The
2
methanol to LA molar ratio was varied between 4:1 and 10:1 over Ru@MTiO catalyst at 338
K for 8 h using 40 mg catalyst. Figure 9 illustrated the effect of molar ratio of solvent on LA
conversion. LA conversion increased with increase in ethanol to LA molar ratio from 4:1 to
8:1. Figure 7 showed that the maximum conversion of 95% was observed when methanol to
1
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Figure 6. Effect of temperature at reaction conditions: solvent to LA ratio: 8:1, catalyst: 40
mg, reaction time: 8h.
LA molar ratio was 8:1. However, further increase in molar ratio to 12:1, LA conversion was
decreased from 95% to 78%. This is because of mass transfer limitation, as more methanols
may dilute the reactant and lowering LA conversion. In present case, molar ratio of methanol
to LA 8:1 is observed to be optimum.
Figure 7. Effect of solvent to LA molar ratio at reaction conditions: catalyst: 40 mg,
temperature: 338 K, reaction time: 8 h.
Esterification of levulinic acid over Ru@MTiO
2
1
4

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2
To explore the activity of Ru@MTiO catalyst for the esterification of LA in the
presence of different alcohols, methanol, ethanol, isoꢀpropanol and 2ꢀbutanol have also been
investigated. As seen from Table 1, conversions of levulinic acid to corresponding alkyl
levulinate in the presence of different alcohols were studied under optimized reaction
conditions. Esterification of levulinic acid with methanol and ethanol afforded the methyl
levulinate and ethyl levulinate in high yields of 94% and 95% respectively (Table 1, entry 1
and 2). However, alcohols with bulkier groups slowed down the LA esterification. Thus, isoꢀ
propanol and 2ꢀbutanol gave the esterification product in 85% and 80% respectively (Table 1,
entry 3 and 4).
a
Table 1. Levulinic acid conversion using different solvents over Ru@MTiO nanomaterial
2
b
Entry
1
Reactants
Solvents
Conversion (%)
95
O
Methanol
OH
O
O
O
O
2
3
4
O
O
O
Ethanol
94
85
80
OH
OH
OH
Isoꢀpropanol
2ꢀbutanol
a
Reaction conditions: solvent to LA ratio: 8:1, temperature: 338 K, catalyst: 40 mg, reaction
b
1
time: 8 h. The products were confirmed by GCꢀMS, the yields was determined by GC. H
NMR spectra of methyl and ethyl levulinates are provided in the supporting information.
Esterification of levulinic acid over different TiO and Ru@TiO materials
2
2
a
Table 2. Levulinic acid conversion to methyl levulinate in presence of different catalysts
1
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b
Entry
Catalyst
Conversion (%)
1
2
3
4
5
Bulk TiO
Bulk Ru@TiO
Mesoporous TiO
2
30
2
38
55
2
Mesoporous Ru@MTiO
2
95
Without catalyst
No reaction
a
Reaction conditions: Methanol to LA ratio: 8:1, temperature: 338 K, catalyst: 40 mg,
b
reaction time: 8 h. The products were confirmed by GCꢀMS, the yields was determined by
GC.
We have performed the esterification of levulinic acid with methanol in the presence
of titanium dioxide (TiO
Table 2. When we have carried out the esterification of levulinic acid by using bulk (TiO
and Ru@TiO ) and mesoporous TiO catalysts, then the conversion of the desired product are
2 2
) and Ru@TiO as the catalysts and the results are summarized in
2
2
2
very low compared to the mesoporous Ru@MTiO
2
material (Table 2, entry 4). High catalytic
activity of mesoporous Ru@MTiO material for the production of methyl levulinate in high
2
yields as seen from the Table 2 could be attributed to its high surface area, porosity and selfꢀ
assembled nanostructure.
Transfer hydrogenation of nitrobenzene over Ru@MTiO
2
nanomaterial
Catalytic activity of Ru@MTiO nanocatalyst was tested in the selective transfer
2
hydrogenation of various nitroarenes. For this purpose, we have choosen transfer
hydrogenation of nitrobenzene in the presence of styrene as model reaction (Scheme 3)
where, nitrobenzenes only selectively reduces to the corresponding anilines.
NO2
Ru@MTiO2
NH
2
MeOH, RT, Air
N H (5 mmol)
2
4
>99 % Yield <1 % Yield
Scheme 3. Selective transfer hydrogenation of nitrobenzene mixed with styrene.
1
6

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Selective transfer hydrogenation of nitrobenzene in the presence of styrene has been
carried out to understand the optimized reaction conditions over Ru@MTiO nanocatalyst.
2
The reaction is governed by number of factors such as solvent, amount of catalyst, reaction
time. Firstly, transfer hydrogenation of nitrobenzene with different solvents such as MeOH,
EtOH, H O, CH CN and PEGꢀ400 were investigated. All the results are summarized in Table
2
3
2
. In the presence of MeOH at room temperature, after 2h >99 % aniline (Table 2, entry 1)
was formed. Using ethanol as solvent 98% product (Table 3, entry 2) was obtained after 3h of
reaction. When water, CH CN and PEGꢀ400 were used as solvent the yield of anilines were
5%, 90% and 88% respectively (Table 3, entries 3ꢀ5) but it took longer time to give the
3
9
desired reduced product. Therefore, the best result was obtained when MeOH was used as
solvent at room temperature under atmospheric pressure. The effect of catalyst amount for the
transfer hydrogenation of nitrobenzene was also investigated. It was observed that using 30
mg of catalyst gave the higher product (Table 3, entry 1). When we decrease the catalyst
amount from 20 mg to 10 mg lower conversion of aniline was obtained (Table 3, entries 6, 7).
Using mesoporous TiO as catalyst only 35% yield was observed after 12h. Without using the
2
catalyst 10% anilines was observed (Table 3, entry 9). So, the best result observed for the
transfer hydrogenation of nitrobenzene when MeOH was used as solvent for 2h reaction
2
using 30 mg Ru@MTiO catalyst at room temperature.
Table 3. Optimization of reaction conditions for the selective transfer hydrogenation of
a
nitrobenzene using Ru@MTiO nanomaterial
2
b
Entry
Catalyst (mg)
Ru@MTiO (30)
Solvent
MeOH
EtOH
Reaction Time (h)
Yield (%)
1
2
3
2
2
3
7
>99
98
Ru@MTiO
2
2
(30)
(30)
Ru@MTiO
H
2
O
95
1
7

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4
5
6
7
8
Ru@MTiO (30)
CH CN
10
12
2
90
88
75
58
35
10
2
3
Ru@MTiO
Ru@MTiO
Ru@MTiO
2
2
2
(30)
(20)
(10)
PEGꢀ400
MeOH
MeOH
MeOH
MeOH
2
MTiO
_
2
(50)
12
12
9
a
Reaction conditions: Nitrobenzene (0.5 mmol), styrene (0.5 mmol), 30 mg catalyst, solvent
b
(
2 mL) and N
2
H
4
(5 mmol), at room temperature. The products were confirmed by GCꢀMS
and the yields were determined by GC.
After optimization of the reaction conditions, selective transfer hydrogenation of other
2
nitroarenes were carried out by using Ru@MTiO nanocatalyst and MeOH at room
temperature under air. It was seen from the Table 4 that, the transfer hydrogenation of
nitroarenes took place very smoothly and chemoselectively to produce the desired anilines in
high yields. Nitrobenzene gave the aniline in high yield of >99% (Table 4, entry 1).
Substituents like –CH , –OCH , –CH OH, –OH were successfully reduced to obtain the
3
3
2
product in good yield (Table 4, entries 2ꢀ5). The nitroaniline reduction produced phenyl
diamine in 90% yield (Table 4, entry 6). The reduction of halogenꢀsubstituted nitrobenzenes
afforded the corresponding amines in high yields 91ꢀ95% (Table 2, entries 7 and 8).
Nitrobenzaldehyde and nitrobenzoic acid gave the respective amines in 90ꢀ94% yield without
reducing the –CHO and –COOH groups (Table 4, entries 9 and 10). This result suggested that
2
Ru@MTiO is highly selective in this transfer hydrogenation reaction.
a
Table 4. Transfer hydrogenation of nitrobenzene catalysed by Ru@MTiO
2
nanomaterial
b
Entry
Nitroarene
Product
Yield (%)
1
NO
2
NH
2
>99
1
8

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2
NO
2
NH2
CH3
95
CH3
NO
3
4
2
NH
2
92
92
OMe
NO
OMe
NH
2
2
CH OH
CH OH
2
2
5
6
NO
2
NH
2
88
90
OH
NO
OH
NH
2
2
NH2
NH2
7
8
9
NO
2
2
2
NH
2
95
91
94
Cl
Cl
NO
NH
2
F
F
NO
NH
2
CHO
CHO
1
9

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DOI: 10.1039/C6RA10233F
10
NO
2
NH
2
90
COOH
COOH
a
2 4
Reaction conditions: Nitroarene (0.5 mmol), N H (5 mmol), 30 mg catalyst and MeOH (2
b
mL) at room temperature for 2h. The products were confirmed by GCꢀMS, the yields were
determined by GC.
Recyclability of Ru@MTiO nanomaterial
2
The recyclingꢀefficiency of ruthenium nanocatalyst has been investigated to
understand its potential for reuse as this is one of the most advantageous features for a
heterogeneous catalyst. For this purpose, esterification of levulinic acid with methanol has
2
been carried out over Ru@MTiO catalyst under optimized reaction conditions. After the
completion of the reaction, solid catalyst was separated by centrifugation, washed with water
o
and acetone. Then dried in 100 C and used in the next catalytic run. As seen from Figure 8
that the catalyst can be efficiently recycled and reused for five times without any appreciable
loss in its product yields.
2
Figure 8. Recycling efficiency of Ru@MTiO in the esterification of levulinic acid.
Heterogeneity Test
2
0

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Hot Filtration Test
An experiment for heterogeneity test was carried out in the esterification of
levulinic acid with methanol with our Ru@MTiO nanocatalyst. The catalyst was filtered
2
o
from the reaction mixture after 3 h reaction at 65 C and the filtrate was allowed to react
further. We observed that no change in the conversion of levulinic acid after 6 h reaction
2
time. This result suggests that our Ru@MTiO catalyst is heterogeneous in nature. It is
also observed that ruthenium was not leached out or decomposed from the mesoporous
support during the catalytic reaction as no ruthenium was observed in the filtrate of the
reaction mixture in the atomic absorption spectrometric analysis. Thus, we can conclude
from this hot filtration test that ruthenium nanoparticles are strongly bound at the
mesoporous TiO
2 2
support and Ru@MTiO is a robust catalyst for carrying out liquid
phase catalytic reactions.
Comparison of catalytic activity of Ru@MTiO for the esterification of
2
levulinic acid with other reported systems
We have studied the esterification of levulinic acid with alcohols over a variety of
catalysts. In Table 5, a comparative results for the esterification of levulinic acid is shown for
different catalysts to understand the catalytic activity of our present catalytic system with
26ꢀ29
those reported in the literature.
It is clear from the Table 5, that our Ruꢀcatalyst showed
26ꢀ29
higher yields in the esterification reaction compared to the other reported systems.
Table 5. Catalytic conversion of levulinic acid with other reported catalysts
Catalysts
Reaction Conditions LA conversion (%)
Reference
2
0 wt%
H SiW O ꢀSiO
2
Levulinic acid: 205
mg, ethanol: 2 mL,
cyclohexane: 5 mL,
temp: 75 °C, time: 6
75
26
4
12 40
2
1

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h, catalyst: 104 mg
1
5% (w/w)
LA to ethanol molar
ratio: 1:8, catalyst to
94
40
27
DTPA/DHꢀZSMꢀ597
LA
ratio:
0.25,
temperature: 351 K,
reaction time: 4 h
LA to ethanol molar
ratio=1:6, temp: 78
H/BEA zeolite
28
°C, time : 5 h,
Catalyst loading : 20
wt.% of LA
4
0ꢀWDꢀS
LA: 2 mmol, ethanol:
76
29
1
28 mmol, temp: 78
°C, time : 10 h,
catalyst: 0.01 mmol
LA to ethanol molar
ratio : 1:8, temp :
Ru@MTiO
2
95
This work
Nanomaterial
3
38 K, time : 8 h,
Catalyst loading :
0 mg
4
Comparison of catalytic activity of Ru@MTiO2 for the transfer
hydrogenation of nitrobenzene with other reported system
Table 6 showed the comparison data for the transfer hydrogenation of nitrobenzene
30ꢀ33
with our catalyst and other reported catalytic system.
The catalytic activity of our catalyst
showed higher conversion than other reported catalysts in the transfer hydrogenation of
nitrobenzene.
Table 6. Catalytic conversion of transfer hydrogenation with other reported catalysts
Catalysts
Cu(Oac)
Reaction Conditions
Nitrobenzene conversion (%) Reference
97 30
2
Nitrobenzene (1 mmol),
Ethylene glycol (10 mL),
N H (5 mmol), Catalyst
2
4
o
(
50 mg), 125 C, 3.5
mins, CMUI
2
2

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Zinc
Nitrobenzene (5 mmol),
96
97
31
32
phthalocyanine PEG 400, N H
(4
(1
2
4
equiv.),
catalyst
mol%), 100 C, 8 h
Nitrobenzene
o
o
Ru ꢀ
(0.2
(0.5
Montmorillonite mmol),
NaOH
clay
mmol), Isopropanol (20
mL), catalyst (50 mg), 85
C, 12 h
o
Iron(0)
Nitrobenzene (1 mmol),
95
33
nanoparticles
water,
Catalyst
(3
mmol), RT, 2ꢀ3 h
Nitrobenzene
Ru@MTiO
2
(0.5
99
This work
nanocatalyst
mmol), MeOH (2 mL),
(5 mmol), Catalyst
30 mg), RT, 2 h
2 4
N H
(
Conclusions
In summary, we can conclude that a new Ruꢀnanocatalyst has been prepared by
grafting of Ru NPs on the surface of mesoporous TiO . This Ruꢀnanocatalyst has been
2
successfully used as a highly efficient recyclable heterogeneous catalytic system for the
esterification of levulinic acid and transfer hydrogenation of nitroarenes under mild reaction
2
conditions. Furthermore, the Ru@MTiO nanomaterial could be easily separated from the
reaction mixture by simple filtration and reused several times with minimal loss of its activity
and selectivity. The catalytic process is green, economically advantageous and environment
friendly. Thus, green and cost effective bifunctional catalytic pathways reported over
2
mesoporous Ru@MTiO nanomaterial suggested a sustainable future and large scale
application for the synthesis of a wide range of value added fine chemicals.
Acknowledgements
S.M.I. acknowledges the Department of Science and Technology (DSTꢀSERB), New
Delhi, India, University Grant Commission (UGC), New Delhi, India, Department of Science
2
3

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and Technology, West Bengal (DSTꢀW.B) for funding. UM is thankful to the UGC, New
Delhi and SKK is also thankful to the CSIR, New Delhi, India for their senior research
fellowship. NS acknowledges the Science and Engineering Research Board (SERB), DST,
Govt. Of India, for his National PostꢀDoctoral Fellowship Scheme (File No.:
PDF/2015/000460). AB wishes to thank DST SERB for funding. AB wishes to thanks DST
for funding through DSTꢀUKIERI research grant. We gratefully acknowledge the DST and
UGC, New Delhi, Govt. of India, for award of grant under FIST, PURSE and SAP program
to the Department of Chemistry, University of Kalyani.
2
4

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Graphical Abstract
Ruthenium nanoparticles supported over mesoporous TiO as an
2
efficient bifunctinal nanocatalyst for esterification of biomass-derived
levulinic acid and transfer-hydrogenation reactions
a
a,b
c
c,
Usha Mandi, Noor Salam, Sudipta K. Kundu, Asim Bhaumik * and Sk.
a,
Manirul Islam *
a
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
b
Department of Chemistry, The University of Burdwan, Burdwan, West Bengal, India
c
Department of Materials Science, Indian Association for the Cultivation of Science,
Kolkata - 700032, India
2
Ru NPs has been supported over mesoporous TiO to obtain a bifunctional nanocatalyst,
which displayed excellent catalytic activity and recyclability in the esterification of levulinic
acid and selective transfer hydrogenation of nitroarenes under mild reaction conditions.