Pt nanoparticles entrapped in mesoporous metal-organic frameworks MIL-101 as an efficient catalyst for liquid-phase hydrogenation of benzaldehydes and nitrobenzenes

Article abstract of DOI:10.1016/j.molcata.2015.01.018

Metal organic-framework MIL-101 and inorganic mesoporous composites Al₂O₃@SBA-15 supported Pt catalysts, Pt/MIL-101 and Pt/Al₂O₃@SBA-15 catalysts, were prepared and characterized by means of X-ray diffraction (XRD), N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO chemisorption and thermo-gravimetric (TG) analysis. Pt nanoparticles were highly dispersed on both supports. In liquid-phase hydrogenation of nitrobenzene, benzaldehyde and their derivatives, the Pt/MIL-101 catalyst was superior to the Pt/Al₂O₃@SBA-15 catalyst in water. For liquid-phase hydrogenation of nitrobenzene with the Pt/MIL-101 catalyst, owing to high solubility of nitrobenzene in ethanol, the reaction in ethanol went much faster than that in water, furnishing a turnover frequency (TOF) in ethanol up to 18,053 h^-1, almost triple of that obtained in water under similar conditions. The highest TOF of 25,438 h^-1 was obtained in ethanol for hydrogenation of 3-chloro-nitrobenzene with the Pt/MIL-101 catalyst. As for hydrogenation of benzaldehyde series, 2-fluoro-benzaldehyde and 3-fluoro-benzaldehyde gave the highest TOFs of 5146 h^-1 and 3165 h^-1 in water with the Pt/MIL-101 and Pt/Al₂O₃@SBA-15 catalysts, respectively. We deduce that surface property of MIL-101 with high hydrophobicity is helpful to enrich reactants around the Pt/MIL-101 catalyst in water, where nitrobenzene or benzaldehyde and its derivatives have a limited solubility, so that high catalytic performance was achieved with the Pt/MIL-101 catalyst in water. Of particular note is that the Pt/MIL-101 catalyst can be reused at least four times without loss in activity or selectivity.

Full text of DOI:10.1016/j.molcata.2015.01.018

Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Pt nanoparticles entrapped in mesoporous metal–organic frameworks
MIL-101 as an efficient catalyst for liquid-phase hydrogenation of
benzaldehydes and nitrobenzenes
Huiyan Pan, Xiaohong Li∗, Yin Yu, Junrui Li, Jun Hu, Yejun Guan, Peng Wu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 North Zhongshan Rd.,
Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal organic-framework MIL-101 and inorganic mesoporous composites Al2O3@SBA-15 supported Pt
catalysts, Pt/MIL-101 and Pt/Al2O3@SBA-15 catalysts, were prepared and characterized by means of
X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), CO chemisorption and thermo-gravimetric (TG) analysis. Pt nanoparticles
were highly dispersed on both supports. In liquid-phase hydrogenation of nitrobenzene, benzaldehyde
and their derivatives, the Pt/MIL-101 catalyst was superior to the Pt/Al2O3@SBA-15 catalyst in water.
For liquid-phase hydrogenation of nitrobenzene with the Pt/MIL-101 catalyst, owing to high solubility of
nitrobenzene in ethanol, the reaction in ethanol went much faster than that in water, furnishing a turnover
frequency (TOF) in ethanol up to 18,053 h−1, almost triple of that obtained in water under similar condi-
tions. The highest TOF of 25,438 h−1 was obtained in ethanol for hydrogenation of 3-chloro-nitrobenzene
with the Pt/MIL-101 catalyst. As for hydrogenation of benzaldehyde series, 2-fluoro-benzaldehyde and
3-fluoro-benzaldehyde gave the highest TOFs of 5146 h−1 and 3165 h−1 in water with the Pt/MIL-101
and Pt/Al2O3@SBA-15 catalysts, respectively. We deduce that surface property of MIL-101 with high
hydrophobicity is helpful to enrich reactants around the Pt/MIL-101 catalyst in water, where nitroben-
zene or benzaldehyde and its derivatives have a limited solubility, so that high catalytic performance was
achieved with the Pt/MIL-101 catalyst in water. Of particular note is that the Pt/MIL-101 catalyst can be
reused at least four times without loss in activity or selectivity.
Received 4 December 2014
Received in revised form 13 January 2015
Accepted 18 January 2015
Keywords:
Pt/MIL-101
Pt/Al2O3@SBA-15
Hydrogenation
Benzaldehyde
Nitrobenzene
Liquid-phase
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
catalysts are often used and supports are of varieties [6–8,12,13].
For instance, carbon materials supported Pt nanoparticles were
Hydrogenations of nitroarenes and aromatic aldehydes to cor-
responding aryl amines and aromatic alcohols are important
processes in synthesis of dyes, pesticides, fine chemicals and phar-
maceuticals. These transformations can be achieved either through
hydrogen-transfer reduction [1–3] or by direct hydrogenation in
heterogeneous system using atmospheric or compressed H2 as
hydrogen resource [4–8]. These heterogeneous hydrogenation pro-
cesses are conducted usually in gas or liquid phase. Liquid phase
hydrogenation systems have drawn much attention recently. Iron
used to be a commonly applied catalyst in hydrogenation of
nitroarenes [9–11], but was restricted due to relatively low catalytic
ability and environmental pollution. With regard to heterogeneous
liquid-phase catalytic hydrogenation of these compounds, Pt-based
active in hydrogenation of both benzaldehyde and nitrobenzene
[7,12]. Especially for selective hydrogenation of nitrobenzene on
Pt/MWCNTs (multi-walled carbon nanotubes), up to 66,900 h−1
TOF was obtained [12]. Other supports, including mesoporous com-
posites TiO2@SBA-15 [6] and metal-organic frameworks (MOFs)
[8], were employed to immobilize Pt nanoparticles as efficient cat-
alysts for the tested hydrogenation reaction.
In order to compare support effect on catalytic performance
in liquid-phase hydrogenation of benzaldehydes and nitroarenes
with supported Pt catalysts, herein, we chose two different meso-
porous supports: the one is the MOFs and the other is an inorganic
mesoporous composites. Recently, a great deal of attention has
been attracted to design and application of MOFs because MOFs
often possess large specific surface area and pore volume, poros-
ity, well-defined structure, chemical tunability and flexibility and
they can be used in different fields such as gas storage and separa-
tion, heterogeneous catalysis, molecular recognition and sensing,
∗
Corresponding author. Tel.: +86 21 62238590; fax: +86 21 62238590.
E-mail address: xhli@chem.ecnu.edu.cn (X. Li).
http://dx.doi.org/10.1016/j.molcata.2015.01.018
1381-1169/© 2015 Elsevier B.V. All rights reserved.

2
H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
autoclave and heated at 493 K for 8 h, and then further purified by
CHO
NO2
CH2OH
NH2
Pt Catalyst
298 K, Solvent, H2
twice solvothermal treatment in DMF (N,N-dimethylformamide) at
393 K for 8 h, and sequentially twice in ethanol at 353 K for 8 h. The
resulting green powder was finally dried overnight at 373 K in oven
for further use.
R
R
The 5 wt.% Pt/MIL-101 catalyst was prepared via a facile impreg-
nation method [24]: MIL-101 was impregnated with an ethanolic
solution containing H2PtCl6 and stirred for 4–6 h. Then, the mix-
ture was evaporated to remove excess solvent, followed by drying
at 373 K overnight. The Pt precursor was reduced in an aqueous
solution of sodium formate at 368 K for 2 h and then washed by
plenty of water to remove chlorine anion. Finally, the sample was
dried at 373 K overnight for further use. The 35AS was prepared
using an ultrasonic impregnation method according to Ref. [38].
Procedure for preparing the 5 wt.% Pt/35AS catalyst was similar to
that of the Pt/MIL-101 catalyst.
Pt Catalyst
R
R
Solvent, H2, 298 K
Scheme 1. Liquid-phase hydrogenation of benzaldehyde, nitrobenzene and their
derivatives with Pt/MIL-101 or Pt/35AS catalyst.
and optics [14–18]. Due to high thermal stability, hydrothermal sta-
bility, acid–base resistance and mesoporous pores, MIL-101 MOFs,
which were constructed by the link of terephthalic acid and the ver-
tex of chromium oxo trime, have been extensively investigated in
heterogeneous catalysis as catalyst alone [19–23] or as catalyst sup-
port [8,24–36]. For instance, Au nanoparticles, Pd nanoparticles, Pt
nanoparticles and some bimetallic metals supported on MIL-101
exhibited good performances in hydrogenation [8,24–27], oxida-
tion [28–31], decomposition [32–34] and other reactions [35,36].
In our previous work, Pt/MIL-101 catalyst was proved to be effi-
cient and recyclable in asymmetric hydrogenation of ␣-ketoesters
after chirally modified with cinchona alkaloids and also showed
comparable performance to commercial Pt/C and Pt/Al2O3 catalysts
[24].
The other concern is a kind of inorganic mesoporous com-
posites, Al2O3@SBA-15, which was obtained by coating alumina
inside mesoporous channels of SBA-15 through different meth-
ods. Al2O3@SBA-15 composites can not only take advantage of
mesoporous materials SBA-15 with large specific surface area and
uniform mesopore channel, but also exhibit properties not found
in Al2O3 alone. To be most important, Al2O3@SBA-15 supported Pt
catalysts have been proved to be superior to pristine Al2O3 sup-
ported one in enantioselective hydrogenation of ␣-ketoesters after
chirally modified with cinchona alkaloids as well [37,38].
Encouraged by these achievements, we were motivated to
extend application of the Pt/MIL-101 and Pt/35AS (35AS denotes
the Al2O3@SBA-15 composites where Al2O3 loading is 35 wt.%)
catalysts in liquid-phase hydrogenation of nitrobenzene and ben-
zaldehyde and their derivatives (Scheme 1). Influences of support
and solvent on catalytic performance were investigated. Mainly due
to high hydrophobicity of MIL-101, the Pt/MIL-101 catalyst was
more active than the Pt/35AS catalyst in water for the liquid-phase
hydrogenation of nitrobenzene, benzaldehyde and their deriva-
tives.
2.2.2. Catalyst characterization
XRD patterns of samples were collected on
a Bruker
D8 Advance instrument using Cu-K␣ radiation. The nitrogen
adsorption–desorption isotherms were measured at 77 K on a
Quantachrome Autosorb-3B system, after the samples were evacu-
ated for 10 h at 473 K. The Brunauer–Emmett–Teller (BET) specific
surface area was calculated using adsorption data in the relative
pressure range from 0.05 to 0.35. The pore size distribution curves
were calculated from the analysis of the adsorption branch of the
isotherm using the Barrett–Joiner–Halenda (BJH) algorithm. The
TEM images were taken on an FEI Tecnai G2-TF30 microscope at
an acceleration voltage of 300 kV. The SEM images were taken on
a Hitachi S4800 electromicroscope with an acceleration voltage of
20 kV. The composition of samples was characterized by energy
dispersive X-ray spectroscopy (EDX) using Horiba EMAX spec-
troscopy. The degree of dispersion and the mean particle size (cubic
model) were estimated from the measured CO uptake, assuming a
cross-sectional area for a surface platinum atom of 8.0 × 10−20 m2
and a stoichiometric factor of one, using the nominal platinum con-
centrations. The TG analysis of the samples was conducted from
ambient temperature to a required temperature under nitrogen
atmosphere with Mettler Toledo TGA/SDTA851e apparatus. The Pt
loading on samples was detected with a Thermo Elemental IRIS
Intrepid II XSP inductively coupled plasma–atomic emission spec-
troscopy (ICP–AES).
2.3. Catalytic tests
For liquid-phase hydrogenation with the Pt/MIL-101 catalyst,
0.04 g or 0.05 g catalyst was pretreated under a hydrogen flow
(40 mL/min) at 503 K for 2 h before use; while for the Pt/35AS
catalyst involved hydrogenation, 0.05 g catalyst was used after pre-
treated under the similar conditions at 673 K for 2 h. The catalyst
was then transferred to a 100 mL autoclave and mixed with sol-
vent and substrate. The reaction began when hydrogen (4.0 MPa)
was introduced with stirring at 298 K. The reaction was stopped
after a proper time and products were analyzed by GC–FID (GC-
2014, Shimadzu) equipped with a capillary column (DM-WAX,
30 m × 0.25 mm × 0.25 ␮m).
2. Experimental
2.1. Materials
Chloroplatinic acid hydrate (H2PtCl6·6H2O) (Pt ≥ 37%), tereph-
thalic acid and chromium nitrate (Cr(NO3)3·9H2O) were purchased
from Shanghai Experiment Reagent Co., Nitrobenzene and its
derivatives were purchased from Aladdin and used as received.
Benzaldehyde was of analytical grade and used as received and its
derivatives were purchased from Alfa Aesar and used as received.
3. Results and discussion
2.2. Catalyst preparation and characterization
3.1. Characterization of Pt catalysts
2.2.1. Catalyst preparation
The MIL-101 support and the Pt/MIL-101 catalyst were char-
acterized by XRD, N2 sorption, SEM and TEM. The XRD pattern of
Pt/MIL-101 catalyst demonstrates that the structure of the MIL-
101 was well kept after Pt nanoparticles depositing (Fig. 1a). N2
adsorption–desorption isotherms of MIL-101 and Pt/MIL-101 are
MIL-101 was synthesized according to Ref. [19] with traditional
hydrothermal method. Briefly, terephthalic acid (1.66 g, 10 mmol),
Cr(NO3)3·9H2O (4.00 g, 10 mmol), 5 M aqueous HF (2 mL, 10 mmol)
and deionized water (48 mL), were placed in a 200 mL Teflon-linear

H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
3
a
Pt(111)
MIL-101
Pt/MIL-101
10 20 30 40 50 60 70 80
2 theta (degree)
30
40
50
60
70
80
2 theta (degree)
Fig. 1. XRD patterns of (a) MIL-101 and Pt/MIL-101; (b) 35AS and Pt/35AS.
almost same (Fig. 2a). There are no obvious changes in BET specific
surface areas (above 2000 m2/g) and pore size distribution after Pt
nanoparticles deposited on MIL-101. SEM images in Fig. 3a and b
show that morphology of MIL-101 and Pt/MIL-101 crystals is nearly
same and both of them are homogeneously dispersed with irregular
shapes. The distribution of Pt nanoparticles was characterized by
TEM firstly. As displayed in Fig. 4a, the Pt nanoparticles are highly
dispersed on the MIL-101. The dispersion of Pt nanoparticles was
also measured by CO chemisorption and average Pt particle size is
2.9 nm with a dispersion of 37.9%.
Similarly, the 35AS composites and the Pt/35AS catalyst were
also characterized using these techniques. The Pt/35AS catalyst
reserves structure of 35AS composites (Fig. 1b) and BET specific
surface area is decreased from 307 to 296 m2/g after Pt nanoparti-
cles deposition (Fig. 2b). Moreover, morphology of catalyst is almost
as same as that of 35AS composites (Fig. 3c and d). TEM image in
Fig. 4b illustrates that Pt nanoparticles are highly dispersed on sup-
port and calculated results according to CO chemisorption give an
average Pt particle size of 2.4 nm with a dispersion of 45.8%. The
detailed physicochemical parameters for these two catalysts are
also listed in Table 1 for clarity.
3.2. Hydrogenation of nitrobenzene and its derivatives
In order to investigate catalytic performance of Pt catalysts
supported on different materials in liquid-phase hydrogenation of
nitrobenzene and its derivatives, kinetic profile for hydrogenation
of nitrobenzene with the Pt/MIL-101 catalyst in water was tested
firstly. As shown in Fig. 5, conversion of nitrobenzene increased
with reaction time almost linearly. About 32.5% conversion was
obtained within 15 min and 62.9% of nitrobenzene was converted
to aniline after 30 min. The reaction nearly completed after 45 min
with 96.1% conversion, affording a TOF of 6673 h−1. The reaction
rate did not change with proceeding of nitrobenzene reduction, so
that we deduce that the hydrogenation of nitrobenzene with the
Pt/MIL-101 catalyst was an apparent zero-order reaction. Besides
Fig. 2. N2 sorption isotherms and pore size distributions (inset) of (a) MIL-101 and Pt/MIL-101 catalyst; (b) 35AS and Pt/35AS catalyst.

4
H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
Fig. 3. SEM images of (a) MIL-101, (b) Pt/MIL-101, (c) 35AS and (d) Pt/35AS.
Fig. 4. TEM images of (a) Pt/MIL-101 catalyst and (b) Pt/35AS catalyst and their particle size distributions.

H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
5
Table 1
The relevant physicochemical parameters for supports and catalysts
a
a
Entry
Sample
SBET
DP
Pt size
b(nm)
Pt particle
disp. (%)
Pt loading (wt.%)
EDX
(m2 g−1
)
(nm)
ICP
1
2
3
4
MIL-101
Pt/MIL-101
35AS
2295
2217
307
1.6, 2.1
1.6, 2.1
6.8
–
2.9
–
–
37.9
–
–
4.6
–
–
4.9
–
Pt/35AS
296
5.8
2.4
45.8
3.5
3.7
a
Determined by N2 adsorption.
Average Pt particle size was determined by CO chemisorption.
b
Table 2
100
Hydrogenation of nitrobenzenes with the Pt/MIL-101 catalyst in ethanol
NH2
NO2
Pt/MIL-101
80
60
40
20
0
R
R
Ethanol, H2, 298 K
.
b
Entry
R-group
Conv. (%)a
Sel. (%)
TOF (h−1
)
1
2
3
4
5
6
7
8
4-Cl
3-Cl
2-Cl
2-CH3
3-CH3
4-CH3
2-CH3O
4-CH3O
99.8
99.9
5.5
21.5
97.6
62.7
0.3
77.3
96.7
94.8
>99.0
>99.0
>99.0
>99.0
>99.0
25,412
25,438
1,400
5,474
25,036
15,965
77
Water
5.8
1,477
Ethanol
Reaction conditions: 21 mmol nitroarenes, 4 MPa H2, 298 K, 0.05 g Pt/MIL-101 cata-
lyst (0.06 mol% Pt), 20 mL ethanol, 10 min.
0
10
20
30
40
a
Conversion was based on GC–FID.
Time (min)
b
The TOF was calculated as converted substrate per mol of exposed Pt atoms per
hour according to Pt loading detected by ICP–AES.
Fig. 5. Kinetic profiles of hydrogenation of nitrobenzene in different solvents with
the Pt/MIL-101 catalyst. Reaction conditions: 21 mmol nitrobenzene, 4 MPa H2,
298 K, 0.04 g Pt/MIL-101 for the reaction in 20 mL water or 0.05 g Pt/MIL-101 for
the reaction in 20 mL ethanol.
almost fully hydrogenated within 10 min, although selectivities
to their target products were slightly lower, affording 96.7% and
77.3%, respectively (Table 2, entries 1 and 2). On the contrary,
their ortho-substituted isomer only gave a 5.5% conversion with
selectivity of 94.8% to 2-chloro-aniline (Table 2, entry 3). For
an electron-donating substituent, such as methyl group substi-
tuted nitrobenzenes, the hydrogenation of the ortho-substituted
nitrobenzene obtained 21.5% conversion (Table 2, entry 4); lower
than those obtained with its meta- and para- isomers. As a result,
3-methyl-nitrobenzene and 4-methyl-nitrobenzene showed 97.6%
and 62.7% conversions, respectively (Table 2, entries 5 and 6). For
methoxy substituted nitrobenzene derivatives, both 2-methoxyl-
nitrobenzene and 4-methoxyl-nitrobenzene were inactive under
the tested conditions, only giving 0.3–5.8% conversions (Table 2,
entries 7 and 8). These results showed that in ethanol, whatever
the electron property of the substituent, the meta- and para- sub-
stituted isomers were more active than their ortho- substituted
isomer with the Pt/MIL-101 catalyst. This might be explained in
terms of steric hindrance of substituents. The Cl , CH3 or CH3O
at the ortho-position of NO2 group with a comparatively larger
size would bring stronger steric repulsion, so that attack of hydro-
gen to nitro group became more difficult under this circumstance.
As for selectivity to the hydrogenated product, it is almost over
99% except for those obtained with Cl substituted nitrobenzenes.
By-products in these cases were phenyl amines derived from a side
reaction of hydrodechlorination. Among all of the nitrobenzene
derivatives, the 3-chloro-nitrobenzene was most active, furnishing
a TOF of about 25,500 h−1. Whereas the 2-methoxy-nitrobenzene
was most inactive, only furnishing a TOF of 77 h−1 under the same
conditions. It seemed that there was no obvious trend in electronic
effect of substituted groups at different positions of phenyl ring in
the catalytic performance of the Pt/MIL-101 catalysts, although the
reason is still unclear at this moment.
in water, we also conducted the reaction in ethanol for compar-
ison. To our delight, the hydrogenation of nitrobenzene with the
Pt/MIL-101 catalyst went much faster in ethanol. It afforded 32.3%
conversion within 5 min, 70.9% conversion in 10 min and nearly full
conversion within 15 min. The highest TOF reached 18,053 h−1 with
the Pt/MIL-101 catalyst in ethanol. The rate in ethanol was almost
triple of that in water. The reason that nitrobenzene hydrogenation
went faster in ethanol than in water could be explained in terms of
higher solubility of nitrobenzene in ethanol, giving rise to a higher
diffusion rate of nitrobenzene in ethanol.
For comparison, we also carried out the liquid-phasehydrogena-
tion of nitrobenzene with the Pt/35AS catalyst. Under the same
conditions, the hydrogenation of nitrobenzene with the Pt/35AS
catalyst in water went quite slowly, only obtaining a conversion of
75% within 1 h, affording a TOF of 3626 h−1, which was about half
of that obtained with the Pt/MIL-101 catalyst.
Based on the results discussed above, we found that using
the Pt/MIL-101 catalyst and using ethanol as solvent are a better
choice for hydrogenation of nitrobenzene. Therefore, we adopted
the Pt/MIL-101 catalyst to extend hydrogenation of nitrobenzene
derivatives in ethanol.
To understand an effect of substituent group at aromatic
ring, the hydrogenation of nitrobenzenes with different sub-
stituents at phenyl ring was carried out with the Pt/MIL-101
catalyst in ethanol. Due to high reactivity of the Pt/MIL-101 cat-
alyst in ethanol, the hydrogenation of nitrobenzene derivatives
in ethanol was stopped after 10 min. Table 2 lists the detailed
results obtained for the hydrogenation of nitrobenzene derivatives
with different substituents at the phenyl ring with the Pt/MIL-
101 catalyst. For an electron-withdrawing substituent such as Cl
group, the meta- and para-substituted chloronitrobenzene were

6
H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
Table 3
Table 4
Hydrogenation of benzaldehyde with the Pt/MIL-101 catalyst in different solvents.
Hydrogenation of benzaldehyde derivatives with different catalysts
CHO
CH2OH
Entry
Solvent
Conv. (%)a
Pt Catalyst
298 K, water, H2
R
R
1
2
3
4
Water
Ethanol
Cyclohexane
Toluene
59.4
61.1
44.1
14.4
.
Entry
R-group
Catalyst
Conv. (%)a
Sel. (%)
Reaction conditions: 50 mg 5 wt.% Pt/MIL-101 catalyst (0.06 mol% Pt), 21 mmol ben-
zaldehyde, 20 mL solvent, 1200 rpm, 4.0 MPa H2, 1 h.
1
2
3
4
5
6
7
8
2-F
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
Pt/MIL-101
Pt/35AS
97.0
64.4
96.4
98.2
88.2
72.5
74.5
55.7
87.5
49.0
85.3
49.1
94.1
95.4
80.1
91.2
49.9
66.4
100
100
100
99.0
100
99.0
77.0
54.3
100
80.4
97.5
81.5
97.5
99.0
84.9
99.0
91.2
98.4
a
Conversion was based on GC–FID.
3-F
3.3. Hydrogenation of benzaldehyde and its derivatives
4-F
2-Cl
Besides nitrobenzene and its derivatives, we also want to extend
to liquid-phase hydrogenation of benzaldehyde and its derivatives
with the Pt/MIL-101 and the Pt/35AS catalysts. We firstly screened
solvent with the Pt/MIL-101 catalyst for hydrogenation of ben-
zaldehyde in different solvents. As shown in Table 3, when the
reaction was carried out in toluene, the hydrogenation of benzalde-
hyde only gave 14.4% conversion within 1 h (Table 3, entry 4). If
cyclohexane was used as solvent, 44.1% conversion of benzalde-
hyde was achieved under the similar conditions (Table 3, entry 3).
When the reaction was performed with the Pt/MIL-101 catalyst
in water or in ethanol, comparable conversions (59.4–61.1%) were
obtained (Table 3, entries 1 and 2). Therefore, water was applied as
solvent for the follow-up studies.
9
3-Cl
10
11
12
13
14
15
16
17
18
4-Cl
2-CH3O
3-CH3O
4-CH3O
Pt/MIL-101
Pt/35AS
Reaction conditions: 21 mmol substrate, 4 MPa H2, 20 mL water, 298 K, 0.04 g Pt/MIL-
101 catalyst (0.048 mol% Pt), 1 h; 0.05 g Pt/35AS catalyst (0.044 mol% Pt), 1.5 h.
a
Conversion was based on GC–FID.
Just like for the liquid-phase hydrogenation of nitrobenzene,
the kinetic profile for the hydrogenation of benzaldehyde with the
Pt/MIL-101 catalyst in water was investigated firstly. As shown
in Fig. 6, at the beginning, only 14.6% conversion was obtained
after 15 min. With reaction proceeding, the reaction was slightly
accelerated. Conversions of benzaldehyde increased to 56.6% after
45 min and to 83.3% after 60 min. The reaction went slowly after
1 h. For the hydrogenation of benzaldehyde with the Pt/35AS cat-
alyst, the kinetic profile demonstrated a nearly linear relationship
between conversion and reaction time. About 31.0% conversion was
obtained in the first 30 min and the reaction proceeded steadily,
obtaining 59.0%, 78.0% and 92.0% conversions after 60 min, 90 min
and 120 min, respectively.
kinetic profile of benzaldehyde hydrogenation on the Pt/MIL-101
catalyst, the hydrogenation of derivatives of benzaldehyde was
conducted in 1 h. As listed in Table 4, for a strong electroneg-
ative substituent such as F-, conversions for the ortho-, meta-
and para-fluorine substituted benzaldehydes were 97.6%, 96.4%
and 88.2% with selectivities of 100% to the corresponding ben-
zyl alcohol, respectively, (Table 4, entries 1, 3 and 5) with the
Pt/MIL-101 catalyst. Among the F-substituted benzaldehydes, the
para-substituted isomer was most inactive. While for a weak elec-
tronegative substituent Cl-, the ortho-Cl- substituted benzaldehyde
was comparatively inactive, obtaining a conversion of 74.5%, lower
than its meta- and para-substituted isomers, which afforded 87.5%
and 85.3% conversions, respectively (Table 4, entries 7, 9 and
11). For an electron-donating group, such as methoxy, the ortho-
and meta-substituted benzaldehydes obtained 94.1% and 80.1%
conversions, respectively, much higher than that of their para-
substituted isomer, only achieving 49.9% conversion under the
same conditions (Table 4, entries 13, 15 and 17). Among all the
substituted benzaldehydes, 2-fluoro-benzaldehyde afforded the
highest activity, affording a 97.6% conversion within 1 h; while 4-
methoxy-benzaldehyde showed the lowest activity, nearly a half
of that for 2-fluoro-benzaldehyde.
Conversions obtained with both Pt catalysts are comparable
for the hydrogenation of benzaldehyde in water, especially at the
beginning. In order to equally compare catalytic performances of
the two Pt catalysts, both Pt catalysts were applied to the hydro-
genation of benzaldehyde derivatives in water. According to the
100
80
Similar to those behaviors in the hydrogenation of nitroarenes,
the Pt/35AS catalyst afforded lower conversions than the Pt/MIL-
101 catalyst for the hydrogenation of benzaldehyde derivatives
in most cases; even the reaction time was prolonged to 1.5 h
for the Pt/35AS catalyst. For instance, for a strong electron-
withdrawing substituent F-, the results were quite different from
those obtained on the Pt/MIL-101 catalyst. The meta-substituted
fluorobenzaldehyde was the most active isomer, obtaining a
98.2% conversion within 1.5 h, higher than the ortho- and para-
substituted isomers, which afforded 64.4% and 72.5% conversions,
respectively (Table 4, entries 2, 4 and 6). For another electron-
withdrawing substituent Cl group substituted benzaldehydes,
all the three isomers gave comparable conversions (49.1–55.7%)
(Table 4, entries 8, 10 and 12). For an electron-donating group
methoxy-substituted benzaldehydes, the results were similar to
those afforded by the Pt/MIL-101 catalyst. Both the ortho- and meta-
substituted isomers achieved higher conversions (91.2–95.4%) than
60
40
20
Pt/MIL-101
Pt/35AS
0
0
20
40
60
80
100
120
Time (min)
Fig. 6. Kinetic profile of benzaldehyde hydrogenation in water with the Pt/MIL-
101 catalyst and the Pt/35AS catalyst. Reaction conditions: 21 mmol benzaldehyde,
4 MPa H2, 20 mL water, 298 K, 0.04 g Pt/MIL-101 catalyst (0.048 mol% Pt) or 0.05 g
Pt/35AS catalyst (0.044 mol% Pt).

H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
7
6000
4000
2000
0
70
60
50
40
30
20
10
0
Pt/35AS
Pt/MIL-101
1
2
3
4
5
Number of Run
R-group
Fig. 8. Reusability of the Pt/MIL-101 catalyst toward the hydrogenation of ben-
zaldehyde in ethanol. Reaction conditions: 0.04 g Pt/MIL-101 catalyst (0.048 mol%
Pt), 21 mmol benzaldehyde, 20 mL ethanol, 4 MPa H2 and 298 K, 1 h.
Fig. 7. Hydrogenation of benzaldehyde and its derivatives with the Pt/MIL-101 and
Pt/35AS catalysts. The reaction conditions are identical to those in Table 4.
their para-substituted isomer, which only showed a conversion
of 66.4% (Table 4, entries 14, 16 and 18). Just like nitroben-
zene series, the chlorine substituted benzaldehydes also afforded
much lower selectivities to the corresponding benzyl alcohol
and the by-product was benzyl alcohol, also originated from the
hydrodechlorination side reaction. In addition, it seemed that there
is no distinct trend in the steric effect for hydrogenation of ben-
zaldehyde and its derivatives with two Pt catalysts as well.
Nevertheless, we noticed that even with the Pt/MIL-101 cata-
lyst, hydrogenation of nitrobenzenes and benzaldehydes showed
different trends in the case of methoxy-substituted isomers. This
can be interpreted as follows: the ortho-methoxy-substituted
nitrobenzene showed lower activity mainly due to larger steric hin-
drance of ortho-methoxy for NO2 group. For methoxy-substituted
benzaldehyde derivatives, both Pt/MIL-101 and Pt/35AS showed
inferior activity for benzaldehyde derivative with methoxy group at
the para- group. Although methoxy group at ortho- or para-position
has similar conjugated effect, the methoxy group at ortho-position
of aldehyde group would produce stronger inductive effect than
the one at para-position, so that ortho-methoxy substituted ben-
zaldehyde was more active than the para-substituted isomer.
In order to compare intrinsic activity of Pt catalysts supported on
different materials in hydrogenation of benzaldehyde derivatives,
the TOF values calculated as converted benzaldehydes per mol of
exposed Pt atoms per hour according to the Pt loadings detected by
ICP–AES were compared. As shown in Fig. 7, the Pt/35AS catalyst
afforded the TOF values in the range of 1580–3165 h−1, varying
with different substituent at phenyl ring. While for the Pt/MIL-
101 catalyst, the TOF values varied from 2647 h−1 to 5146 h−1 with
different substituents. Obviously, the Pt/MIL-101 catalyst afforded
much higher activity than the Pt/35AS catalyst. The highest TOF of
5146 h−1 was afforded by 2-fluoro-benzaldehyde with the Pt/MIL-
101 catalyst. It is worth noting that these results are comparable
with, in some cases even higher than, those obtained in our previous
studies with Pt catalysts supported on other materials, such as peri-
odic mesoporous resols (FDU-14), ordered mesoporous carbons
(CMK-3 and CMK-8) and mesoporous composites (TiO2@SBA-15)
[5–7].
liquid-phase hydrogenation of benzaldehydes and nitrobenzenes.
Nevertheless, reusability of the Pt/MIL-101 catalyst for liquid-phase
hydrogenation of nitrobenzenes or benzaldehydes is also important
to consider. Therefore, we performed recycling experiments of the
Pt/IL-101 catalyst for the liquid-phase hydrogenation of benzalde-
hyde. Considering that benzaldehyde and its hydrogenated product
had a limited solubility in water compared with in ethanol, the
recycling experiments were conducted in ethanol for convenient
separation of benzaldehyde and its hydrogenated product from
the Pt/MIL-101 catalyst after each cycle. As shown in Fig. 8, the
Pt/MIL-101 catalyst could be used for at least five cycles without
loss in activity. About 99% selectivity to benzyl alcohol was also
kept constant during the recycling. The used Pt/MIL-101 catalyst
was further characterized by ICP–AES to detect the Pt loading after
five reaction cycles. As a result, the Pt loading was still 4.9 wt.% for
the used Pt/MIL-101 catalyst. Compared with the fresh one, the Pt
loading on the MIL-101 support was unchanged, indicating that the
Pt nanoparticles were stable enough on the MIL-101 support.
3.5. Discussion
The Pt/MIL-101 catalyst was much more active than the Pt/35AS
catalyst in liquid-phase hydrogenation of nitrobenzene series and
100
90
Pt/35AS
80
70
Pt/MIL-101
60
50
3.4. Reusability of the Pt/MIL-101 catalyst
300
400
500
600
700
800
900
1000 1100
Temperature (K)
Based on the discussions mentioned above, the Pt/MIL-101 cat-
alyst was proved to be more active than the Pt/35AS catalyst for the
Fig. 9. TG curves of Pt/MIL-101 and Pt/35AS catalysts.

8
H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
Fig. 10. The soakage properties of the Pt/MIL-101 and Pt/35AS catalysts.
benzaldehyde series and could be reused for several cycles. Based
on characterization results, both Pt catalysts had an average Pt
particle size of 2.4–2.9 nm with high dispersions. Thus, the two
supported Pt catalysts can provide sufficient exposed Pt atoms to
participate in hydrogenation reaction, especially for the Pt/35AS
catalyst. Nevertheless, the Pt/MIL-101 catalyst gave higher activi-
ties than the Pt/35AS catalyst. We can deduce that the Pt particle
size cannot be the determining parameter for two Pt catalysts
with different performances; however, the support effect cannot
be neglected. On the one hand, the MIL-101 MOFs had much larger
specific surface area compared with the 35AS composites. On the
other hand, MIL-101 and 35AS composites had different composi-
tions: the former was constructed by the link of terephthalic acid
and the vertex of chromium oxo trime; while the latter was com-
posed of alumina and silica essentially. As well known, the different
compositions would result in different surface properties in turn.
In order to understand surface hydrophilicity & hydrophobicity of
these materials, TG analysis was conducted under nitrogen atmo-
sphere. As shown in Fig. 9, TG curves showed that weight loss of
MIL-101 was mainly from physisorbed water because most of the
weight loss took place below 373 K. The weight loss of MIL-101 at
above 600 K was originated from decomposition of MIL-101. For
the 35AS, weight loss was derived from desorption of chemisorbed
water, as the weight loss was continuing till much higher tem-
perature. This indicates that MIL-101 was more hydrophobic than
35AS.
differences were obvious. The Pt/MIL-101 catalyst was dispersed in
oil phase, while the Pt/35AS catalyst was located at the interface of
water–oil due to gravitational settling. Although additional ethanol
was added to these two tubes and shaken violently, the dispersion
of these two catalysts did not change at all. Thus, either nitroben-
zenes or benzaldehydes could be easily aggregated around the
Pt/MIL-101 nanoparticles when reaction was performed in water,
so that substrate concentration in water would be enriched around
the Pt/MIL-101 catalyst, giving rise to a fast reaction compared with
that with the Pt/35AS catalyst [5].
4. Conclusion
Both the Pt/MIL-101 and Pt/35AS catalysts were active in the
liquid-phase hydrogenation of nitroarenes and benzaldehyde and
their derivatives. Nevertheless, the Pt/MIL-101 was much more
active than the Pt/35AS catalyst under the same conditions mainly
due to the more hydrophobic properties of MIL-101. For the hydro-
genation of nitroarenes on Pt/MIL-101 catalyst in ethanol, up to
25,438 h−1 TOF was achieved. With respect to benzaldehyde and its
derivatives, the TOF reached 5146 h−1 for 2-fluoro-benzaldehdye
with the Pt/MIL-101 catalyst in water. Besides, the Pt/MIL-101 cat-
alyst can be reused for at least four times without loss in activity or
selectivity for the hydrogenation of benzaldehyde in ethanol.
Acknowledgements
Detailed soakage properties of these two catalysts can also be
revealed from photographs displayed in Fig. 10. Because nitroben-
zene is not soluble in water, nitrobenzene and water are kept in
two phases, thus nitrobenzene is at the bottom and water is in the
up phase. After Pt/MIL-101 and Pt/35AS catalysts were added sepa-
rately and shaken violently, the Pt/MIL-101 catalyst was dispersed
around the mixture; while the Pt/35AS catalyst was only dispersed
in the aqueous phase. After the tubes were stood for a while, the
This work was supported by the National Natural Science Foun-
dation of China (NSFC grant nos. 21273076 and 21373089), the
Open Research Fund of Top Key Discipline of Chemistry in Zhejiang
Provincial Colleges and Key Laboratory of the Ministry of Education
for Catalysis Materials (Zhejiang Normal University) (ZJHX2013),
Programs Foundation of Ministry of Education (2012007613000),
and Shanghai Leading Academic Discipline Project(B409).

H. Pan et al. / Journal of Molecular Catalysis A: Chemical 399 (2015) 1–9
9
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