Tuning the hydrogenation activity of Pd NPs on Al-MIL-53 by linker modification

Article abstract of DOI:10.1039/c3cy00910f

The hydrogenation activity of 3 wt.% Pd nanoparticles supported on various mono-group (H, OCH₃, NH₂, Cl, and NO₂) substituted Al-MIL-53 materials has been investigated. Substituents enhanced the dispersion of palladium nanoparticles on Al-MIL-53, leading to a narrow particle size distribution in the range of 2 to 4 nm. Pd nanoparticles on fresh catalysts were present as a mixture of Pd(ii) and Pd(0) with different ratios. These Pd species readily became metallic in a hydrogen flow even at room temperature. Their activities in hydrogenation of phenol and phenylacetylene are linked to the substituents on the aromatic ring of the framework. Catalysts with electron-donating groups (OCH₃ and NH₂) show much higher activity than those containing electron-withdrawing groups (Cl and NO₂). This behavior might be explained by the hydrogen dissociation abilities of metallic Pd nanoparticles affected by the organic linkers.

Full text of DOI:10.1039/c3cy00910f

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Tuning the hydrogenation activity of Pd NPs
on Al–MIL-53 by linker modification†
Cite this: DOI: 10.1039/c3cy00910f
a
a
b
a
a
Damin Zhang, Yejun Guan,* Emiel J. M. Hensen, Teng Xue and Yimeng Wang
The hydrogenation activity of 3 wt.% Pd nanoparticles supported on various mono-group (H, OCH
3
,
NH , Cl, and NO ) substituted Al–MIL-53 materials has been investigated. Substituents enhanced the
2
2
dispersion of palladium nanoparticles on Al–MIL-53, leading to a narrow particle size distribution in
the range of 2 to 4 nm. Pd nanoparticles on fresh catalysts were present as a mixture of Pd(II) and Pd(0)
with different ratios. These Pd species readily became metallic in a hydrogen flow even at room temper-
ature. Their activities in hydrogenation of phenol and phenylacetylene are linked to the substituents on
Received 11th November 2013,
Accepted 18th December 2013
the aromatic ring of the framework. Catalysts with electron-donating groups (OCH
3
2
and NH ) show
much higher activity than those containing electron-withdrawing groups (Cl and NO
2
). This behavior
DOI: 10.1039/c3cy00910f
might be explained by the hydrogen dissociation abilities of metallic Pd nanoparticles affected by the
organic linkers.
www.rsc.org/catalysis
linkers through metal–ligand coordination bonds. The possi-
Introduction
bility to use MOFs as supports for metal nanoparticles has
1
2,13
Palladium-based catalysis plays a vital role in fine chemical
already been studied in quite some detail.
Compared
1
,2
production. One of the predominant applications of palla-
dium catalysts is selective hydrogenation of unsaturated com-
pounds, e.g., phenol hydrogenation to cyclohexanone and
with conventional porous inorganic materials such as zeo-
lites, MOFs may display strong host–guest interactions
between the framework and metal nanoparticles via coordi-
nation and π–π forces. These kinds of interactions have been
proposed to enhance catalytic properties of supported cata-
3
–5
semi-hydrogenation of alkynes.
Cyclohexanone is of great
industrial interest for the production of caprolactam and
1
14
15
17
18
adipic acid. Recent studies have suggested that very high
lysts for hydrogenation of ethyl cinnamate and alkenes,
1
6
activity can be achieved in liquid phase phenol hydrogena-
tion at a relatively low temperature giving economic and
efficiency advantages.
cross-coupling reactions, aerobic oxidation of alcohols,
and especially hydrogenation of phenols and α-ketoesters.
1
0
4
,5
This also opens the possibility to
The introduction of various organic substituents in MOFs
has been argued to change their electronic properties, a main
effect being the modification of the electron density in the
phenyl moieties of the organic linker by use of electron-
produce bio-adipic acid from lignin-derived biomass through
6
hydrogenation of pyrolysis oil. Materials such as carbon
7
8
nitride, polyaniline-functionalized carbon-nanotubes, ionic
9
19
liquid-like copolymers, and metal organic frameworks
withdrawing or -donating groups. For instance, introducing
1
0,11
(MOFs)
have been reported to show improved hydrogena-
2
electron-withdrawing groups (e.g., NO ) in the organic linker
tion activity for Pd compared to the use of conventional
carrier materials. It is generally believed that an important
ability of these novel supports is to affect the electronic prop-
erties of Pd nanoparticles.
of UiO-66 strongly enhances the Lewis acidity of coordinated
2
0
Zr sites.
We surmise that such modifications in the
electronic structure of the MOF may also affect the properties
of Pd nanoparticles dispersed on their surface.
Metal organic frameworks (MOFs) comprise networks of
metal centers or inorganic clusters bridged by simple organic
To date, there has been no report on the effect of organic
linkers whose electronic properties are modified through
substituents on the catalytic properties of MOF-supported
noble metal nanoparticles. In this study, a series of mono-
substituted Al–MIL-53 have been synthesized using a one-
step hydrothermal method using substituted terephthalic
acid as precursor. Al-based MOFs are nontoxic and stable
against hydrolysis, which render it an outstanding host
material for metal nanoparticles. Upon dispersion of Pd over
their surface, the catalysts are characterized by Transmission
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
East China Normal University, North Zhongshan Road 3663, 200062 Shanghai,
China. E-mail: yjguan@chem.ecnu.edu.cn; Fax: +86 21 32530334
b
Schuit Institute of Catalysis, Department of Chemical Engineering and
Chemistry, Eindhoven University of Technology, 5612AZ Eindhoven,
The Netherlands
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00910f
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Electron Microscopy (TEM) and X-ray Photo-electron Spec-
troscopy (XPS). The effect of the organic linker modification
on the catalytic activity of supported Pd catalysts is explored
and discussed.
oven at 170 °C for 12 h. After cooling to room temperature,
the white microcrystalline product was obtained by filtration.
Activation of Al–MIL-53–X materials
As the thermal stability of product materials with different
substituents differs, the activation procedures have been tai-
lored. Pristine Al–MIL-53–H was purified by heating in air at
Experimental
Materials syntheses
3 2
300 °C for 3 days. Al–MIL-53 modified with OCH , NH , and Cl
AL–MIL-53–H. The synthesis was carried out under
groups were activated in two steps. 1.0 g of the as-synthesized
material was first heated in N,N-dimethylformamide (DMF;
hydrothermal
conditions
using
Al(NO ) ·9H O
and
3
3
2
1
,4-benzenedicarboxylic acid (abbreviated as BDC hereafter)
2
× 60 mL) at 155 °C for 24 h. The filtered solids were then
2
1
in deionized water. The molar composition of the starting
gel was 1 Al (5.20 g) : 0.5 H BDC (1.15 g) : 80 H O, which was
heated at 155 °C for 24 h. Al–MIL-53–NO2 (0.5 g) was first
heated in methanol (2 × 60 mL) at 80 °C for 48 h and filtered.
The white solid was heated at 150 °C for 12 h.
2
2
placed in a Teflon-lined autoclave for 3 days at 220 °C. The
solid product was obtained by filtration.
Al–MIL-53–NH2. Commercial 2-aminoterephthalic acid
Preparation of Pd catalysts
(
2
H BDC–NH , TCI) was used as the organic linker. Typically,
2 2
Supported Pd catalysts were prepared by a deposition–reduction
3 2 2 2
.9 g of AlCl ·6H O and 2.2 g of H BDC–NH in 30 mL of
H O were charged in a Teflon-lined autoclave. The autoclave
was kept at 150 °C for 5 h. After cooling to room tempera-
ture, a yellow precipitate was obtained by filtration.
method. Typically, 4.24 mL of H
2
4
PdCl aqueous solution
2
−
1
(
Pd: 11.8 mg mL ) was diluted to 40 mL with deionized water.
Then 0.95 g of the vacuum dried support was added to the
resulting solution and thoroughly stirred for 4 h. The final
pH value of the mixture was further adjusted to 9 by adding
NaOH solution (1 M). Then, NaBH4 solution was added to
Al–MIL-53–Cl. 2-Chloroterephthalic acid (H BDC–Cl) was
2
2
2
synthesized by oxidation of 2-chloroxylene with nitric acid.
In a typical synthesis, 6 ml (43 mmol) of 2-chloroxylene,
0 mL of deionized water and 16 mL of nitric acid (70%)
were charged into a Teflon-lined autoclave. The autoclave was
2
reduce the Pd(OH) under cooling in an ice bath with vigor-
6
1
1
ous stirring. The catalysts were filtered and dried at 90 °C
under vacuum overnight. The vacuum-dried catalysts were
cooled to room temperature. Before being taken out of the
oven, the vacuum valve was released slowly allowing for slow
exposure to air, which avoided undesired combustion.
−1
sealed and heated at 170 °C for 16 h (heating rate, 2.5 °C min ).
-Chloroterephthalic acid was recovered by filtration as a crys-
2
talline white powder, which was further thoroughly washed
with hot water and dried under vacuum at 70 °C overnight.
The synthesis of Al–MIL-53–Cl was done by a similar hydro-
thermal synthesis method. A mixture of AlCl ·6H O (1.0 g,
Characterization of Al–MIL-53–X and Pd/Al–MIL-53–X
3
2
4
.14 mmol), H
2
BDC–Cl (0.83 g, 4.14 mmol) and water
Powder X-ray diffraction (XRD) patterns were collected using
a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation
(λ = 1.5405 Å) operated at 35 kV and 25 mA. Scanning
electron microscopy (SEM) was performed using a Hitachi
S-4800 microscope. Transmission electron microscopy (TEM)
images were taken using a FEI Tecnai G2 F30 microscope
operating at 300 kV. The average Pd particle size was calcu-
(40 mL) was charged in a 100 mL Teflon-lined autoclave.
The autoclave was heated in an oven at 210 °C for 12 h. The
resulting precipitate was obtained by filtration.
Al–MIL-53–OCH . 2-Methoxyterephthalic acid (H BDC–OCH )
3
2
3
was used as the organic linker, which was prepared
2
3
according to the literature. In a typical synthesis, a mixture
of 2,5-dimethylanisole (30 g, 0.22 mol), potassium
permanganate (120 g, 0.76 mol) and distilled water (3000 ml)
was refluxed for 5 h. The mixture was cooled to room
temperature and poured into cold ethanol (2000 ml). This
mixture was then filtered, washed thoroughly with water,
reduced under vacuum, and acidified with concentrated
hydrochloric acid. The resulting white precipitate was collected
by filtration, washed with water and vacuum dried. For the
P
P
3
2
lated using dTEM = ( n d )/( n d ) by measuring at least
i
i
i i
100 particles. The Pd loading was determined using a Thermo
Elemental IRIS Intrepid II XSP inductively coupled plasma
emission spectrometer (ICP-AES). The desired amount of sam-
ple was dissolved in 10 mL of aqua regia. The solution was
heated to remove the residual acid and diluted in a 50 mL vol-
umetric flask. Pulse CO chemisorption was performed using a
Micromeritics AutoChem 2910 to determine the metal disper-
sion of the reduced catalysts. Prior to the measurement, an
synthesis of Al–MIL-53–OCH
3 3 2
, a mixture of AlCl ·6H O (1.0 g,
−1
4
.14 mmol), H BDC–OCH (0.75 g, 3.82 mmol), and water
amount of 100 mg catalyst was reduced in a flow of 80 mL min
2
3
(
40 mL) was charged in a 100 mL Teflon-lined autoclave, and
of 10 vol.% H2 in Ar at 150 °C for 3 h and then flushed
in He for 1 h. After cooling to 35 °C in He, the CO gas pulses
the autoclave was heated in an oven at 210 °C for 12 h.
The white product was obtained by filtration.
−
1
(5 vol.% in He) were introduced in a flow of 80 mL min . The
changes in the CO gas phase concentration were followed by
TCD. CO pulsing was repeated until the pulse area did not
change anymore. XPS spectra of fresh Pd catalysts were
recorded using a Kratos AXIS Ultra DLD spectrometer with a
Al–MIL-53–NO . In a typical synthesis, a mixture of
2
Al(NO
4
3 3 2 2 2
) ·9H O (1.49 g, 3.97 mmol), H BDC–NO (0.92 g,
.37 mmol) and water (50 mL) was placed in a 100 mL
Teflon-lined autoclave, and the mixture was heated in an
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monochromated Al Kα radiation source. The survey spectra
were recorded with a pass energy of 160 eV and the high-
resolution spectra with a pass energy of 40 eV to identify the
chemical state of each element. All the binding energies (BEs)
were referenced to the C 1s neutral carbon peak at 284.6 eV.
Phenol adsorption
The phenol adsorption behaviors on substituted Al–MIL–X
1
1
were performed as described elsewhere. Typically, 50 mg of
the dried sample was used for the adsorption in phenol solu-
tions with various initial phenol concentrations (0.05, 0.1,
0
.15, 0.2, and 0.25 M) at 20 °C. Before adsorption, MIL–X
was dried under vacuum at 140 °C for 2 h. The dried powder
50 mg) was added to the aqueous phenol solutions (50 mL)
Fig. 1 IR spectra of activated Al–MIL-53–X (X = H, OCH
and NO ) materials.
3 2
, NH , Cl,
(
2
and vigorously stirred for 6 h. After adsorption, the solution
was recovered by centrifugation and diluted, and the equilib-
rium phenol concentration was determined using the absor-
bance at 270 nm using a Shimadzu UV-2550 ultraviolet visible
spectrophotometer. The calibration curve was obtained from
the UV spectra of standard solutions (10–125 μM).
shown in Fig. 2 and clearly confirm that all materials had
the MIL-53 structure. The BET surface areas of Al–MIL-53–X
21
(
8
X = H, OCH
3
, NH
2
, Cl, and NO
2
) are 1728, 182, 113, 187, and
adsorption
2
−1
2 m g , respectively, calculated from their N
2
isotherm curves (see Fig. S1†). Introducing organic groups
resulted in a significant decrease of the surface area, partially
due to the blockage of micropores and/or pore collapse during
the pre-treatment.
Catalytic reaction tests
Phenol hydrogenation. A Teflon-lined (120 mL) batch reac-
tor was used to carry out the liquid phase hydrogenation. No
specific pretreatment was conducted prior to the reaction. An
amount of 100 mg of the catalyst was mixed with 10 mL of
aqueous phenol solution (0.25 M) into a Teflon-lined steel
batch reactor. The reactor was purged five times with H and
2
then pressurized with 5 bar H . The reaction was conducted
at 20, 35, 50, and 90 °C and kept for 24, 6, 5, and 2 h, respec-
Unlike Cr-based MOFs, Al-based MIL-53 has been consid-
ered to be an environmentally friendly and highly promising
24
candidate for industrial applications. It has been reported to
be a promising support of Pd catalysts for such organic reac-
tions as Heck and Suzuki–Miyaura cross-coupling reac-
2
25,26
tions.
The structure of MIL-53 is hydrophobic, therefore
tively. The products were analysed using a Shimadzu GC
limiting the wetting of its pores by the aqueous solution of
1
1
2
014 equipped with a DB-Wax capillary column (30 m length
palladium chloride. We have roughly estimated the hydro-
phobicity of Al–MIL-53–X by conducting TG analysis (Fig. 3).
Two weight losses are clearly observed for all materials. The
first weight loss at temperature below 100 °C is due to water
desorption, showing that Al–MIL-5–X materials are relatively
hydrophilic. From the weight loss percentage we can conclude
and 0.25 mm internal diameter). The main product was
cyclohexanone with cyclohexanol as the only by-product.
Phenylacetylene hydrogenation. The hydrogenation of
phenylacetylene was performed in a 50 mL three-neck flask
at 40 °C under atmospheric pressure. In a typical experiment,
9
.1 mmol phenylacetylene and 10 mg of catalyst was mixed
−
1
2
with 9 mL of methanol. 20 mL min of H was continuously
bubbled through the liquid phase with vigorous stirring.
Samples were withdrawn at the desired period, diluted with
methanol, and analysed using GC.
Results and discussion
Characterization of Al–MIL-53 structures
All Al–MIL-53 structures were activated and purified thoroughly
before use as supports. A previous study has shown that the
unreacted terephthalic acid gives an IR band in the region
−1
1
694–1711 cm assigned to its protonated form (–CO
2
H),
whereas DMF shows a strong absorption band in the range
−1
21
1
663–1673 cm due to the carbonyl stretching vibration. The
absence of these bands in the IR spectra (Fig. 1) suggests the
complete removal of solvents and residual terephthalic acid in
the activation steps. XRD patterns of the activated samples are
3 2 2
Fig. 2 XRD profiles of Al–MIL-53–X (X = H, OCH , NH , Cl, and NO )
catalysts.
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nanoparticles in Pd/Al–MIL-53–H had sizes in the range of
2
–5 nm with an average size of 4.3 ± 1.0 nm. By introducing
OCH , NH , Cl, NO groups, the average particle size
changed to 2.4 ± 0.45 nm, 3.8 ± 1.0 nm, 3.4 ± 0.77 nm, and
.4 ± 0.53 nm, respectively. Thus, the substituents clearly
improve the dispersion of Pd nanoparticles consistent with
3
2
2
2
2
5–27
previous reports.
was estimated by pulse CO chemisorption measurements.
The dispersion of Pd on Al–MIL-53–H, –OCH , –NH , –Cl, and
NO2 was 32%, 33%, 40%, 14% and 24%, respectively. The
average particle size calculated based on the dispersion was
.4, 3.3, 2.8, 7.5 and 4.6 nm, respectively. These values show
The total accessible metal surface area
3
2
–
3
distinct differences from the values obtained by HRTEM anal-
yses. For H and NH2 groups, similar Pd particle sizes were
observed, whereas a much larger particle size was found from
3 2
Fig. 3 TG curves of the activated Al–MIL-53–X (X = H, OCH , NH , Cl,
−1
2 2
and NO ) materials in N with a ramp rate of 10 °C min . (The inset is
the TG curves in the range of 20 to 130 °C).
CO chemisorption for Cl- and NO -modified Al–MIL-53. We
2
should like to note that the electron withdrawing groups may
inhibit CO chemisorption on Pd to some extent.
The fresh catalysts were characterized by XPS (Fig. 5),
which was used to establish the oxidation state of Pd under
ambient conditions. The doublet Pd 3d5/2 peaks at 335.3 and
that the organic substituents indeed change the hydrophobicity
of Al–MIL-53. Lower weight loss due to water desorption on
2 2
NH -, Cl-, and NO -modified MIL-53 points to their lower
21
hydrophilicity consistent with a previous report. In compari-
3
37.3 eV are clearly observed for all catalysts and are assigned
son, –OCH results in higher hydrophilicity, probably due to
3
2
5,28–30
to metallic Pd and oxidized Pd species, respectively.
result evidences the presence of both Pd and Pd in the nano-
particles. The Pd /Pd ratios obtained from the deconvolution
of the Pd 3d region spectra are summarized in Table 1. It can
be seen that Pd is the dominating species on MIL-53–H. Intro-
This
stronger H-bonding between water molecules and the methoxy
groups. Another observation obtained from TG analysis is
that these Al–MIL-53–X materials display different thermal
2
+
0
2
+
0
stabilities following the trend of –H (600 °C) > –NH
2
≈ –OCH
3
0
(
500 °C) > –Cl ≈ –NO
2
(400 °C). The lower thermal stability of
ducing substituent groups significantly increased the amount
Al–MIL-53–X with substituent groups is attributed to the disso-
ciation of the C–X bond of these functional groups.
2
+
2+
of Pd species of palladium catalysts. The proportion of Pd
2
increases following the trend of H < OCH < NH < Cl < NO .
3
2
It is well known that Pd nanoparticles can be easily reoxidized
upon exposure to air. Moreover, the smaller the particle size,
the more easily Pd is oxidized. Obviously, these results confirm
our speculation that the reason for the deviation between CO
chemisorption and TEM results may be incomplete reduction
of Pd or changes in its electronic nature when interacting with
the modified MOF surface. The trend is consistent with TEM,
evidencing that very small Pd particles were formed on the
modified Al–MIL-53–X.
Characterization of Pd/MIL-53 catalysts
No X-ray diffraction of palladium (not shown) was observed
for the Pd/MIL-53–X catalysts, probably due to the relatively
low loading, which was about 3 wt.% (Table 1). We measured
the BET surface areas of two supported Pd catalysts, namely
2
−1
Pd/Al–MIL-53–H and Pd/Al–MIL–NO , to be 1011 and 98 m g
2
calculated from N2 adsorption isotherms (Fig. S2†), respec-
tively. The BET area of Pd/Al–MIL-53–H is rather lower than that
of the pristine material, whereas a similar BET area is observed
for Pd/Al–MIL-53–NO2 and the corresponding support. This
result suggests that the Pd nanoparticles are not embedded in
the pores but are mainly located on the external surface.
The reducibility of these Pd nanoparticles was character-
2
ized by H -TPR and the results are shown in Fig. S3.† Oxidic
δ+
Pd was reduced readily at room temperature for all catalysts,
meaning that Pd nanoparticles were mainly in the metallic
The particle size distribution of the active Pd phase
was determined by high-resolution TEM (Fig. 4). The Pd
state in the hydrogenation process. Upon exposure to H
hydrides with various H/Pd ratios were formed depending on
2
, Pd
Table 1 Loading, particle sizes and oxidation states of supported Pd catalysts
0
2+
Pd
Pd
a
Pd/Al–MIL-53–X
Loading (wt.%)
dTEM (nm)
Dispersion (%)
BE (eV)
Ratio (%)
BE (eV)
Ratio (%)
H
OCH
NH
Cl
NO
2.8
3
3
2.9
3
4.3
2.4
3.8
3.4
2.4
32
33
40
14
24
335.4
335.9
335.7
335.5
335.6
70.0
63.4
62.8
54.5
45.0
337.2
337.4
337.5
337.5
337.6
30.0
36.6
37.2
45.5
55.0
3
2
2
a
0
Based on the total Pd loading and CO chemisorption amount. An assumption of CO/Pds = 1 was used.
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3 2 2
Fig. 4 TEM images of Pd nanoparticles supported on a) Al–MIL-53–H, b) Al–MIL-53–OCH , c) Al–MIL-53–NH , d) Al–MIL-53–Cl, and e) Al–MIL-53–NO .
3 2 2
Fig. 5 XPS spectra of Pd nanoparticles supported on a) Al–MIL-53–H, b) Al–MIL-53–OCH , c) Al–MIL-53–NH , d) Al–MIL-53–Cl, and e) Al–MIL-53–NO .
the electronic properties of the modified groups. Higher values
of H/Pd ratios on electron withdrawing groups (0.18 and 0.22
has been emphasized recently by an in situ XAFS study, which
was not perturbed by water and reaction intermediates.
32
for Cl and NO , respectively) than the values on electron donat-
2
2 3
ing groups (0.03 and 0.05 for NH and OCH , respectively) were
Catalytic activity tests
observed. The H/Pd ratio on the unmodified Pd/MIL-53 was
about 0.11. It is generally accepted that the H/Pd ratio is related
to the Pd particle size, with a higher value on bigger particles
Selective hydrogenation of phenol. Considering the
industrial importance of selective hydrogenation of phenol to
cyclohexanone, we tested the catalytic activity of Pd/MIL–Xs
in this reaction. The reactions were carried out in a Teflon-
(
ca. 0.67 maximum) and with smaller particles (<2.5 nm) show-
31
ing negligible Pd hydride formation. The catalytic role of these
Pd hydrides on Pd/C catalysts in liquid phase hydrogenation
lined steel autoclave under 0.5 MPa H with a phenol/Pd ratio
2
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of 83. We first investigated the effect of the reaction tempera-
ture on the hydrogenation activity of Pd–Al–MIL–X, i.e., 19,
Based on the TOFs at different temperatures, we calculated
the apparent activation energy (Eact) for the Pd catalysts in the
selective hydrogenation of phenol (Fig. 6), and the results are
3
5, and 50 °C. The results including phenol conversion and
−
1
cyclohexanone selectivity are collected in Table 2. All Pd cata-
lysts were active for phenol hydrogenation at 19 °C. The
phenol conversions after reaction for 24 h for various cata-
lysts were in the range of 8.4 to 36.3%, with selectivity to
cyclohexanone above 95% in all cases. We compared the
listed in Table 2. The Eact is in the range of 59–86 kJ mol .
These values are in good agreement with previous reports sug-
33,34
gesting that the reaction rate is not limited by mass transfer.
The activation energy and reactivity show a clear opposite
trend, with Pd/MIL-53 displaying the lowest activation energy
−
1
activity of the 5 wt.% Pd/Al
2
2
2
O
3
catalyst (from Alfa-Aesar);
7.0% of phenol conversion was observed after reaction for
4 h using 50 mg of the catalyst (phenol/Pd ratio of 100) and
of 59 kJ mol and having the highest activity. On the other
hand, the surface hydrophilicity and/or hydrophobicity may
also contribute to the E of the hydrogenation reaction.
a
3
5
at 25 °C under otherwise similar conditions, with a lower
cyclohexanone selectivity of 79.3%. This result further evi-
dences the advantage of MOFs as the support of Pd nano-
2
Wang et al. proposed a water promoted H activation mech-
anism with a very low Ea of 35.9 kJ mol inspired by the
H –D O exchange during hydrogenation process, which
−1
2
2
1
2,13
particles for hydrogenation under mild conditions.
explained well the superior catalytic activity of Pd catalysts in
aqueous phase than in organic solvents for hydrogenation. In
this regard, catalysts with higher hydrophilicity allowing for
The conversion follows the trend of H (36.3%) > OCH3
30.4%) > NH (25%) > Cl (11.5%) > NO (8.4%). The turnover
(
2
2
frequencies (TOFs) are calculated to be 1.5, 1.1, 0.94, 0.45, and
2 a
easier accessibility of H O will give lower E values, as is
−1
0
.3 h for H-, OCH -, NH -, Cl- and NO -modified Al–MIL-53
observed in this study. Much lower activation energies are
found on the unmodified catalyst, therefore enhancing the
hydrogenation activity.
At 90 °C these Pd catalysts showed very similar catalytic
activities (Table 3). It is likely that modified MOFs perform
better at higher temperatures. This might therefore represent
a case of enthalpy–entropy compensation.
3
2
2
with the corresponding conversions, respectively. Amongst all
Pd catalysts studied, the pristine MIL-53 supported Pd catalyst
showed the highest catalytic activity. When the reaction tem-
perature increased up to 35 °C, the reaction time was short-
ened four times to 6 h. The turnover frequencies are calculated
−1
to be 5.4, 4.9, 5.1, 2.2, and 2.0 h for H-, OCH -, NH -, Cl- and
3
2
NO
conditions, the reactivities of OCH
MIL-53s were the same as that of the pristine material,
whereas Cl- and NO -functionalized materials showed a signif-
2
-modified Pd catalysts, respectively. Under these reaction
Previous studies have shown that high phenol adsorption
capacity on the support is needed to ensure high catalytic activ-
ity and selectivity to cyclohexanone, as observed on polyaniline-
3
and NH substituted
2
8,11,35
2
functionalized carbon-nanotubes, MOFs and mpg-C
3 4
N .
icantly lower activity. With the temperature further increased
It has been assumed that phenol adsorbed in a nonplanar
8,35
to 50 °C, we found that the TOF increased to 15.2, 16.5, 16.3,
fashion gives rise to cyclohexanone.
In this study, the
−1
8
.9 and 9.5 h for H-, OCH
3 2 2
-, NH -, Cl- and NO - modified Pd
phenol adsorption capacities of Al–MIL-53–X have also been
catalysts, respectively. At 50 °C, the commercial 5 wt.% Pd/Al
2 3
O
catalyst gave a phenol conversion of 76.5% and cyclohexanone
selectivity of 98.4% after reaction for 4 h.
Table 2 Catalytic performance of Pd/Al–MIL-53–X catalysts for phenol
selective hydrogenationa
Sel./%
Pd/Al-MIL- T/°C
3-X (t/h)
a
E /kJ
mol
−1
−1
5
X
phenol/% CO C–OH TOF/h
H
19 (24) 36.3
97.2
98.2
96.6
94.1
95.8
94.2
95.0
97.9
97.0
97.4
98.6
96.4
95.2
96.2
96.2
2.8
1.6
3.4
5.9
4.2
5.8
5.0
2.1
3.0
2.6
1.4
3.6
4.8
3.8
3.8
1.5
5.4
15.2
1.1
4.9
16.5
0.94
5.1
16.3
0.45
2.2
8.9
0.3
2.0
9.5
59
3
5
5 (6)
0 (5)
33.9
79.0
OCH
3
19 (24) 30.4
68
72
76
86
3
5
5 (6)
0 (5)
38.5
91.8
NH
2
19 (24) 25.0
3
5
5 (6)
0 (5)
33.9
90.6
Cl
19 (24) 11.5
3
5
5 (6)
0 (5)
14.3
48.0
8.4
NO
2
19 (24)
3
5
5 (6)
0 (5)
13.2
53.0
Fig. 6 Arrhenius plots for the hydrogenation of phenol on 3 wt.%
Pd/Al–MIL-53–X (X = H, OCH
temperature range of 20–50 °C (100 mg catalyst, 10 ml of 0.25 M
aqueous phenol solution, and 0.5 MPa H ).
3 2 2
, NH , Cl, and NO ) catalysts in the
a
Reaction conditions: 100 mg catalyst, 10 ml 0.25 M aqueous phenol
solution, 0.5 MPa H
2
.
2
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Table 4 Semihydrogenation of phenylacetylene on Pd supported on
Al–MIL-53S with various organic linkers
compared. The pristine Al–MIL-53–H material showed the
highest phenol adsorption capacity (Fig. 7). Introducing
organic linkers remarkably decreased the phenol adsorption
capacities due to the significant loss of surface area. However,
the decrease of phenol uptake is not linear to the loss of sur-
face area, meaning that a strong interaction occurs between
phenol and the framework of MIL-53. Similar values on modi-
fied Al–MIL-53 materials were obtained regardless of the
linkers. Given the similar phenol adsorption behavior on Al–
MIL-53–X with mono-substituent, it is safe to conclude that
the activity of Pd/Al–MIL-53s is more likely related to the
electronic properties of the framework linkers rather than the
adsorption ability of phenol on different materials.
t = 0.5 h
t = 2 h
c
Pd/Al–MIL-
3–X
TOF
(s
a
b
−1
5
X
S
X
S
)
H
19.5
18.0
24.7
13.3
9.4
99
>99.9
80.0
>99.9
46.9
94.5
96.0
78.0
97.0
93.3
0.35
0.32
0.44
0.24
0.17
OCH
NH2
Cl
3
98.0
98.8
98.5
93.6
NO
2
34.5
Reaction conditions: 10 mg catalyst,
1
ml phenylacetylene
with a flow rate
dissolved in 10 ml of methanol, 43 °C, 1 bar H
2
−
1 a
b
of 20 mL min
. Conversion of phenylacetylene. Selectivity to
c
styrene. Calculated by Pd loading.
Phenylacetylene hydrogenation. The reactivity of these Pd
nanoparticles in semi-hydrogenation using phenylacetylene
as a substrate instead of phenol was conducted and the
results are shown in Table 4. This reaction has been widely
applied to characterize the catalytic activity of supported Pd
activation and transfer activity of electron withdrawing linkers,
Pd/Al–MIL-53–Cl (NO ) showed a rather lower hydrogenation
2
activity. These results further point to the effect of organic
linkers in hydrogen activation on Pd nanoparticles.
3
6
catalysts. Under the tested reaction conditions, H-, OCH3-,
and NH -functionalized Pd catalysts again showed much bet-
ter catalytic activity than Cl- and NO -modified Pd catalysts.
The utilization of a stable and environmentally benign
Al-based MOF as a support material for hydrogenation cata-
lysts would be interesting for future industrial applications.
The results presented in this study demonstrate that Pd
nanoparticles supported on Al–MIL-53 exhibit excellent cata-
lytic performance in selective hydrogenation reactions.
Organic substituent groups remarkably affect the catalytic
activities of supported Pd nanoparticles. Electron-donating
group (NH – and OCH )-modified catalysts show much
2
2
Amino functionalized Pd/MIL-53 showed the best catalytic
activity, giving 24.7% conversion of phenylacetylene and
98.8% selectivity to styrene after 0.5 h. The TOF is close to that
reported on cross-linked ionic liquid polymer supported Pd cat-
37
alysts. Extending the reaction time to 2 h led to nearly total
conversion (>99.9%) of phenylacetylene and selectivity of 78.0%
to styrene. The electron donating group-modified Pd/Al–MIL-53
catalysts showed higher catalytic activity than conventional inor-
2
3–
higher activity than their electron-withdrawing (Cl– and
NO –) counterparts. It should be noted that NH - and OCH -
2
2
3
36
ganic oxide-supported Pd catalysts. Due to the lower hydrogen
functionalized materials showed a much lower surface area
than the unmodified counterpart but a similar catalytic activ-
ity was achieved, thereby emphasizing the importance of the
electronic structure of supports in governing the catalytic
Table 3 Catalytic performance of Pd/Al–MIL-53–X catalysts for phenol
a
selective hydrogenation at 90 °C
3
8
activity of supported Pd nanoparticles.
−1
TOF (h
)
Sel. (%)
Pd/Al–MIL-53–X
Conv. (%)
CO
–OH Conclusions
H
OCH
NH
Cl
NO
97.5
99.4
98.8
97
46
44
44
45
44
94.8
81.9
95.1
96.2
87.7
5.2
18.1
4.9
3.8
12.3
In summary, the catalytic hydrogenation activity of 3 wt.%
Pd/Al–MIL-53–X catalysts with various organic linkers (X = H,
OCH , NH , Cl, and NO ) has been systematically studied. The
3
2
3
2
2
2
99.1
surface of Al–MIL-53–H is relatively hydrophilic and has a very
2
−1
a
high surface area (1728 m g ), which benefit the support of
Pd nanoparticles. Al–MIL-53–X with organic linkers containing
NH , Cl, and NO are more hydrophobic and show a much
Reaction conditions: 100 mg of catalyst, 10 ml of 0.25 M aqueous
phenol solution, 0.5 MPa H
2
, 90 °C, reaction for 2 h.
2
2
2
−1
lower surface area (varying from 82 to 187 m g ). Despite
the hydrophobic nature and lower surface area of modified
Al–MIL-53–X, Pd nanoparticles with very similar and narrow
distribution in the range of 2–5 nm were successfully supported
favored by the interaction between Pd and organic linkers.
Fresh Pd nanoparticles mainly existed as a mixture of Pd(II) and
Pd(0) when exposed to air, but easily became metallic upon
reduction at room temperature. Compared with electron donat-
ing groups (H, NH , and OCH ), electron withdrawing groups
2
3
2
(Cl and NO ) might inhibit the CO adsorption on the Pd sur-
MILs−
1
face, as well as hydrogen dissociation. Pd/Al–MIL-53–X showed
Fig. 7 Phenol adsorption capacities (mgphenol
g
) on Al–MIL-53
with various organic linkers under different equilibrium concentrations.
distinctly different catalytic hydrogenation activities at room
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Catalysis Science & Technology
temperature and 0.5 MPa H , with turnover frequencies of
18 H. Pan, X. Li, D. Zhang, Y. Guan and P. Wu, J. Mol. Catal. A:
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2
−
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.5, 1.1, 0.94, 0.45, and 0.3 h for H-, OCH -, NH -, Cl- and
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and the surface hydrophobicity brought by the linkers.
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Acknowledgements
We acknowledge the financial support from the National Nat-
ural Science Foundation of China (grant no. 21203065) and
the Fundamental Research Funds for the Central Universities.
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