New method for catalyst preparation based on metal-organic framework MOF-5 for the partial hydrogenation of phenylacetylene

Article abstract of DOI:10.1007/s11172-014-0443-8

Small (less than 2 nm) Pd nanoparticles immobilized in the matrix of the microporous phenylenecarboxylate metal-organic framework MOF-5 were prepared for the first time by the fluid method. The catalytic properties of samples Pd@MOF-5 were studied in the

Full text of DOI:10.1007/s11172-014-0443-8

Russian Chemical Bulletin, International Edition, Vol. 63, No. 2, pp. 396—403, February, 2014
396
New method for catalyst preparation based on metalꢀorganic framework
MOFꢀ5 for the partial hydrogenation of phenylacetylene*
E. V. Belyaeva,a V. I. Isaeva,a E. E. SaidꢀGaliev,b O. P. Tkachenko,a S. V. Savilov,c
A. V. Egorov,c L. M. Kozlova,a V. Z. Sharf,a and L. M. Kustova
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: sharf@ioc.ac.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation
cChemical Department, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119991 Moscow, Russian Federation
Small (less than 2 nm) Pd nanoparticles immobilized in the matrix of the microporous
phenylenecarboxylate metalꢀorganic framework MOFꢀ5 were prepared for the first time by the
fluid method. The catalytic properties of samples Pd@MOFꢀ5 were studied in the selective
hydrogenation of phenylacetylene to styrene (methanol, 20 С, PH2 = 1 atm). The catalytic
experimental data and results of physicochemical studies indicate that palladium nanoparticles
are mostly localized in pores of the composite material 1%Pd@MOFꢀ5 obtained by the fluid
synthesis. The specific positions of active sites in the intracrystalline volume results in the
suppression of the undesirable conversion of styrene to ethylbenzene.
Key words: metalꢀorganic frameworks, fluid synthesis, partial hydrogenation, phenylacetylꢀ
ene, Pdꢀcontaining catalysts, nanoparticles.
Metal nanoparticles as components of active and seꢀ
Various methods were developed to prepare heterogeꢀ
neous catalysts of this type: impregnation to incipient wetꢀ
ness, coprecipitation, infiltration of chemical vapors
of organometallic compounds with labile ligands into
MOF pores (Metal Organic Chemical Vapor Deposition,
MOCVD), solidꢀstate mechanical mixing, and consecuꢀ
tive deposition—reduction to form eggꢀshell type bimetalꢀ
lic particles (for example, Au@Ag).7—9 Nanoparticles of
Pd, Pt, Fe, Au, Cu, Zn, Sn, Ru, and Ni can be immobiꢀ
lized in the MOF matrices using these methods.10—14
One of the promising methods for preparing heterogeꢀ
neous catalytic system is the fluid synthesis for which suꢀ
percritical carbon dioxide (SCF) is used as a solvent. The
studies on the synthesis of the Pt/C catalysts for fuel
cells15—24 and some heterogeneous catalysts25,26 in a meꢀ
dium of SCF are known. It the recent time, the fluid synꢀ
thesis was successfully applied to prepare the rutheniumꢀ
containing catalysts based on МОF.27,28 The catalysts preꢀ
pared in the SCF medium are superior to catalytic systems
obtained by traditional methods in activity and selectivity.
Fluid СО2 has a series of substantial advantages over
liquid solvents. Supercritical СО2 is characterized by
chemical inertness and low critical parameters. Since the
interface and, hence, surface tension are absent, SCF has
an absolute wettability and does not give capillary effects.
Due to these properties, functional additives dissolved in
lective catalysts for various processes are of substantial
interest.1 However, mobility of metal nanoparticles in the
processes of nanocomposite preparation results in their
aggregation, which prevents the full utilization of their
unique properties. In heterogeneous catalysis, the agglomꢀ
eration of nanoparticles before and after the catalytic reacꢀ
tion can be avoided by immobilizing nanoparticles on the
support.2,3 Crystalline materials with the ordered structure,
such as zeolites and mesoporous silicates, as well as metꢀ
alꢀorganic coordination polymers (metalꢀorganic frameꢀ
works, MOFs), make it possible to stabilize metal nanoꢀ
particles by their immobilization in the porous matrix.4
The family of MOFs represents hybrid zeoliteꢀlike maꢀ
terials formed by metal ions or clusters that are linked
by polydentate organic ligands.5 A high porosity and
high specific surface area, as well as a regular pore size of
these systems, create conditions for the uniform size disꢀ
tribution of metal nanoparticles (NPs) in a narrow range
and high dispersion of metalꢀcontaining catalysts immoꢀ
bilized in the MOF matrix. These properties are interestꢀ
ing for using composites NPs@MOF in heterogeneous
catalysis.6
* On occasion of the 80th anniversary of the N. D. Zelinsky
Institute of Organic Chemistry of the Russian Academy of Sciences.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 0396—0403, February, 2014.
1066ꢀ5285/14/6302ꢀ0396 © 2014 Springer Science+Business Media, Inc.

Phenylacetylene hydrogenation over PdꢀMOFꢀ5
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
397
and Pd(acac)2 (4.3 mg, 1.4•10–5 mol) were loaded into a reꢀ
actor. The reactor was sealed and purged with a flow of dry СО2
followed by heating the mixture to 150 С with stirring. Highly
pure СО2 was liquefied in a High Pressure Equipment press and
fed to the reactor thus creating a pressure of 10.8 MPa. The
reaction mixture was kept under these conditions with stirring
for 10 h. Then the reactor was cooled down and depressurized.
The second step, metal reduction, was carried out in a flow of
dry argon (10—20 cm3 min–1) (220 С, 4—6 h). Finally, the
reactor was cooled down and opened, and the palladiumꢀconꢀ
taining material was dried in a vacuum drying box (80 С, 2 h).
The palladium content of the synthesized samples was deterꢀ
mined on a VRA 30 Xꢀray fluorescence analyzer (Germany)
using the PdꢀK line of the Xꢀray fluorescence (XRF) specꢀ
trum. An Xꢀray tube with the Mo anode was used in a mode of
50 kV—20 A to excite XRF.
Methods of catalyst investigation. The specific surface area
of the supports was calculated from the data on nitrogen adsorpꢀ
tion using the BET equation.41 The Xꢀray diffraction powder
patterns were collected on a DRON 3M diffractometer with
CuꢀK radiation at a scanning rate of 1 deg min–1 ( = 1.54 Å).
The XRF spectra were recorded in the transmission mode at
77 K at the Center of Synchrotron Radiation DESY (Hamburg,
Germany, project IIꢀ2007/0022). The energy positions of the
PdKꢀedge absorption in the XANES Pd/MOFs spectra and Pd
foil coincide; however, a comparison of the EXAFS spectra
showed that the oscillation intensity in the spectra of the cataꢀ
lysts is much lower and depends on the support.
the fluid easily penetrate into pores of polymer, carbon,
and inorganic matrices.29
Selective hydrogenation of polyunsaturated comꢀ
pounds has a great applied significance.30 In the industrial
scale, the hydrogenation step is a component of hydroꢀ
fining processes, for instance, acetylene and diene impuriꢀ
ties are removed by hydrogenation from olefins.31—33 The
presence of phenylacetylene in the production of polyꢀ
styrene exerts a negative effect on the polymerization catꢀ
alyst and, for this reason, the admissible level of phenyꢀ
lacetylene in polymer is below 10 ppm.34
It should be mentioned that the partial hydrogenation
of phenylacetylene (PA) (Scheme 1) is a convenient modꢀ
el reaction for designing novel catalysts.35,36 Various cataꢀ
lytic systems were proposed for this reaction: Pd/SiO2,37
Pd nanoparticles supported on zeolites and mesoporous
silicates (MCMꢀ41 and AlꢀMCMꢀ41)34; amorphous metꢀ
al alloys (metallic glasses," for example, Pd81Si19)38; and
Rh nanoparticles encapsulated into polyvinylpyrrolidone
[RhꢀPVP].39
Scheme 1
The microstructure of the samples was studied by fieldꢀemisꢀ
sion scanning electron microscopy (FEꢀSEM) on a Hitachi
SU8000 electron microscopy. The images were detected in the
secondary electron detection mode at an accelerating voltage of
10 kV and a working distance of 8—10 mm. The morphology of
the samples was considered with allowance for a correction to
the surface effect of conducting layer sputtering.42
The samples were studied by highꢀresolution transmission
electron microscopy on a JEOL JEM 2100F/Cs microscope
(JEOL Co. Ltd., Japan). The TEM images were detected at an
accelerating voltage of 200 kV and an exposure time of 90 s in the
flightꢀfield transmission microscopy mode. When a sample was
prepared for analysis, its weighed sample was dispersed for
20 min (20 С) in water in a Sapfir ultrasonic bath (150 W, 22 kHz).
The obtained suspension (100 L) was deposited on a copper
grid preꢀcovered with polyvinylformal.
In this work, we studied the catalytic properties of
the Pd nanoparticles immobilized in the matrix of the
phenylenecarboxylate metalꢀorganic framework MOFꢀ5
(Zn4O(BDC)3, where BDC is benzeneꢀ1,4ꢀdicarboxylat).
The hydrogenation of phenylacetylene to styrene was chosen
as a model reaction. The obtained Pdꢀcontaining samples
of MOFꢀ5 were characterized by Xꢀray diffraction analyꢀ
sis, Xꢀray absorption spectroscopy, scanning electron
microscopy (SEM), highꢀresolution transmission electron
microscopy (HR TEM), and measurements of lowꢀtemꢀ
perature nitrogen adsorption.
Catalytic experiments. Hydrogenation was carried out in
a glass temperatureꢀcontrolled twoꢀnecked reactor with stirring
at 800 rpm.
Experimental
Samples of the microporous metalꢀorganic framework MOFꢀ5
were synthesized according to a previously published proceꢀ
dure.40
A weighed sample of the catalyst (100 mg, Pd content 1—5%)
was saturated with hydrogen for 30 min, and phenylacetylꢀ
ene was introduced (460 mg, 5 mmol). The reaction was carried
out in methanol (20 mL) under an atmospheric pressure of H2
(20 С).
Experimental verification of the catalyst stability. After comꢀ
pletion of the phenylacetylene hydrogenation cycle, the catalyst
was removed from the reactor, separated from the reaction soluꢀ
tion on a centrifuge, and washed with methanol (2×10 mL).
Then a fresh portion of methanol (~20 mL) was added, and the
system was maintained for 24 h. Then the solvent was removed
by decantation, and the catalyst was refluxed for 9 h in methanol
(25 mL). The catalyst was isolated on a centrifuge, washed with
methanol (2×10 mL), and evacuated (4 h, 150 С, 10–2 Torr).
Preparation of palladiumꢀcontaining systems. Impregnation
to incipient wetness. The Pdꢀcontaining samples were prepared
using a known procedure.7 A solution of palladium acetate
(Pd(OAc)2) or palladium acetylacetonate (Pd(acac)2)
(5.0—50.0 mg, 0.02—0.2 mmol) in absolute chloroform (0.30 mL)
was gradually added to the MOFꢀ5 samples (500 mg), and then
the solvent was distilled off under reduced pressure. Finally,
MOFꢀ5 samples containing the supported precursor Pd(ОАс)2
were evacuated (10–3 Torr, 4 h, 150 C).
Fluid synthesis. Preꢀextracted in SCF (70 °С, 15 MPa, 6.5 h) and
evacuated samples of the MOFꢀ5 support (0.33 g, 1.4•10–3 mol)

398
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Belyaeva et al.
The initial specific rate for hydrogenation of phenylacetꢀ
Table 1. Characteristics of the synthesized palladiumꢀcontaining
ylene (W01) and styrene (W02) (mole of substrate (mole
of Pd)–1 min–1) was calculated by the graphical method from
the curves describing converstion (<20%) as a function of time.
The selectivity at the first step of the hydrogenation process was
determined as a ratio of the concentrations of styrene and ethylꢀ
benzene in the reaction mixture
samples of the metalꢀorganic framework MOFꢀ5
Catalyst
Preparation
method
Precursor
Content of Pd
(wt.%)
I
Impregnation
Impregnation
Impregnation
SCF
Pd(OAc)2
Pd(OAc)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
1.0
5.0
1.0
1.0
3.5
II
III
IV
V
S = [styrene]/([styrene] + [ethylbenzene])
at the phenylacetylene conversion 50 and 95%.
SCF
The GLC analysis was carried out on a Model´ 3700 chroꢀ
matograph with a flameꢀionization detector on an FFAP capilꢀ
lary column (25 m×0.25 mm) at 65 С.
of palladium nanoparticles in the MOFꢀ5 matrix changes
the specific surface area of the samples. For example, after
Pd was introduced in sample I, the specific surface area
decreased by ~280 m2 g–1 compared to the value obꢀ
tained for the initial metalꢀorganic framework. This deꢀ
crease seems to indicate a partial localization of palladium
nanoparticles inside the cavities of MOFꢀ5.44,45 A more
noticeable decrease in the specific surface area (by
~1900 m2 g–1) was observed when the amount adsorbed
was measured on the Pd/MOFꢀ5 sample prepared from
Pd(acac)2 by the impregnation to incipient wetness methꢀ
od.7 This significant change in the texture properties can
be related to both palladium immobilization in the frameꢀ
work pores and the partial degradation of the structure.
Metalꢀorganic frameworks containing metal nanoparꢀ
ticles are conventionally divided into three classes. Class A
(metal/MOF) includes systems with nanoparticles localꢀ
ized on the external surface of microcrystallites of MOF
or near the surface. In these systems, the nanoparticle size
considerably exceeds the pore dimensions of the metalꢀ
organic framework.5 Class B (metal@MOFs) is associatꢀ
ed with systems containing nanoparticles distributed in
the pore volume of MOF microcrystals. Since the size of
these particles is larger than pore dimensions, a partial
Results and Discussion
Physicochemical study of the synthesized palladiumꢀconꢀ
taining catalysts. The synthesized samples of MOFꢀ5, used
as supports for palladium, have high specific surface area
(1800—2600 m2 g–1). The Xꢀray diffraction patterns of
the sample of the initial metalꢀorganic framework activatꢀ
ed in vacuo to remove solvent molecules from pores before
palladium deposition and the MOFꢀ5 sample treated in
SCF after evacuation are different. The ratio of reflection
intensities at small angles (6.9/9.7) is higher for the samꢀ
ple additionally treated under the supercritical conditions
(Fig. 1). A comparison of the Xꢀray diffraction pattern of
the "supercritical" material with the theoretical Xꢀray difꢀ
fraction pattern of MOFꢀ5 shows that the preliminary
treatment in SCF before palladium introduction enhances
the degree of crystallinity, i.e., improves the quality of the
sample of the metalꢀorganic framework. The efficiency of
this method for MOF activation for removal of guest molꢀ
ecules from pores was reported earlier for НССꢀ2.43
The characteristics of the synthesized palladiumꢀconꢀ
taining samples are given in Table 1. The immobilization
disintegration of the framework occurs. Class
C
I (arb. units)
(metal@MOF) represents the systems containing metal
nanoparticles immobilized in the microcrystallite bulk of
the metalꢀorganic framework. In this case, the average
diameter of immobilized nanoparticles is close to or smaller
than the pore size.
The Xꢀray diffraction and SEM studies confirm that
the metalꢀorganic framework of the synthesized palladiꢀ
umꢀcontaining samples remains unchanged (see Figs 1—3).
The cubic topology of the metalꢀorganic framework MOFꢀ5
(with minor changes in the lattice parameter) is retained
for samples I and III prepared with the palladium comꢀ
pounds using the impregnation to incipient wetness methꢀ
od (Fig. 2). The experimental Xꢀray diffraction patterns of
palladiumꢀcontaining samples I and III prepared by the
impregnation to incipient wetness method correspond to
the theoretical Xꢀray diffraction pattern of MOFꢀ5 (see
Fig. 2). The intensity ratio of the first reflection (6.9) and
second reflection (9.7) at small angles is ~1.5 in both the
1
2
3
5
10 15 20 25 30 35 40
2/deg
Fig. 1. Xꢀray diffraction patterns of the initial sample of the
metalꢀorganic framework МOFꢀ5 (1), samples MOFꢀ5 after
evacuation and additional treatment in SCF (2), and palladiumꢀ
containing materials V obtained by the fluid synthesis (3).

Phenylacetylene hydrogenation over PdꢀMOFꢀ5
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
399
I•104(arb. units)
palladium is fully reduced in the studied materials. It is
most likely that the precursor is completely decomposed
to form Pd0 nanoparticles when the palladiumꢀcontaining
materials are prepared.
10
8
The influence of the method of catalyst preparation on
the palladium particle size in the metalꢀorganic structure
of MOFꢀ5 was studied by a combination of XAS and HR
TEM methods. The processing of the EXAFS spectra by
the spherical approximation method indicates a very
small average diameter of the palladium particles in the
samples with the metalꢀorganic framework. The EXAFS
spectra of all samples contain a peak at the uncorrected
distance from the central Pd atom near 1.5 Å. The peak in
the spectrum of palladium oxide is at the same distance
and is attributed to the atomic pair Pd—O and/or Pd—C.
The second peak at the distance about 2.5 Å can be asꢀ
signed to the presence of the atomic pair Pd—Pd. Assumꢀ
ing the spherical shape of the particles, we can calculate
the formal average diameter of metallic palladium partiꢀ
cles. The calculation shows that nanoparticles with sizes
about 1 nm result from the reduction of the precursor on
the MOFs surface. The size of particles immobilized in
the metalꢀorganic framework matrix depends on the prepꢀ
aration method and palladium precursor. So, very small
(0.5 nm) palladium particles are formed in the MOFꢀ5
matrix of samples I. Larger particles (0.6 nm, Table 2) are
immobilized in the matrix of sample III prepared by imꢀ
pregnation from palladium acetylacetonate, When preꢀ
paring palladiumꢀcontaining sample V (3.5% Pd) in suꢀ
percritical СО2, the average size of the palladium nanoꢀ
particles attains values of 1.2—1.4 nm.
6
4
2
2
1
6
10
14
18
22
26
2/deg
Fig. 2. Xꢀray diffraction patterns of sample I (1) prepared using
palladium acetate and sample III (2) prepared from palladium
acetylacetonate by the impregnation to incipient wetness method.
2 nm
Fig. 3. SEM image of palladiumꢀcontaining sample I.
The HR TEM studies of palladiumꢀcontaining samꢀ
ples Pd@MOFs indicate the presence of small Pd nanoꢀ
particles (about 1 nm) in sample I (Fig. 4, а). Larger
palladium nanoparticles (to 2 nm, Fig. 4, b) are formed in
sample III. A comparison of these data with the XRF
results suggests that smaller palladium particles are formed
in the case of catalyst I (Pd acetate as a precursor). In
sample III (Pd acetylacetonate as a precursor), the bimoꢀ
dal nanoparticleꢀsize distribution is observed: along with
small particles (0.6 nm, see Table 2), the palladiumꢀconꢀ
taining system also contains larger particles (1.5—2 nm).
Probably, the content of smaller particles considerably
exceeds the amount of larger particles. This follows from
theoretical diffraction picture of MOFꢀ5 and experimenꢀ
tal Xꢀray diffraction of powders of samples I and III.
However, the Xꢀray diffraction pattern of sample V obꢀ
tained by the fluid synthesis exhibits the change in the
intensity ratio of the characteristic reflections at small
angles (6.9/9.7). It is known that for materials with large
pores accessible for solvent and adsorbate molecules the
Xꢀray reflection intensities at angles below 20 (2) deꢀ
pend substantially on the amount and ability to Xꢀray
scattering of the particles in pores.46 Thus, a decrease in
the intensity of the peak at 6.9 relatively to the reflection
9.7 in the Xꢀray diffraction pattern of sample V (synthesis
in SCF) compared to the theoretical Xꢀray diffraction patꢀ
tern of MOFꢀ5 can indicate the presence of guest moleꢀ
cules (palladium nanoparticles) in the pores of the metalꢀ
organic framework. On the contrary, in the case of comꢀ
posite samples I and III (impregnation from Pd(acac)2),
it can be suggested that palladium nanoparticles are locatꢀ
ed on the external surface of crystallites of the metalꢀ
organic framework.
Table 2. Parameters of composite samples Pd@MOFꢀ5 obtained
by the EXAFS method
2
Sample Pair
r/Å
CN*  •10–3 E
DPd/Å
/Å2
/eV
I
III
V
Pd—Pd 2.74 0.04 3.5 0.2 12
1
4
1
2
5
1
1 15.2 0.4
2 16.1 0.6
1 12.4 2.4
Pd—Pd 2.76 0.03 4.3 1.0
Pd—Pd 2.75 0.01 8.1 0.6
9
6
The characteristic reflections of PdO and Pd(OAc)2
are absent from the Xꢀray diffraction patterns of the
Pd@MOFꢀ5 samples. According to the EXAFS data,
CN is coordination number.

400
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
a
Belyaeva et al.
Table 3. Rates (W01, W02) and selectivity (S) at the initial
step of the partial hydrogenation of phenylacetylene on
palladiumꢀcontaining composites Pd/MOFꢀ5
a
b
Cataꢀ
lyst
W01
W02
S1
S2
mol (mol min)–1
I
11.6
19.9
45.6
16.0
13.7
47.9
17.7
96.3
10.2
10.5
98
97
92
95
94
97
95
86
90
92
II
III
IV
V
Note. Reaction conditions: 100 mg of the catalyst, 460 mg
of phenylacetylene, and 20 mL of methanol; 1 atm H2,
20 C; selectivity at phenylacetylebe conversions of 50%
(S1) and 90% (S2) conversion, respectively.
ladiumꢀcontaining materials prepared in SCF (samples
IV and V, see Table 3).
2 nm
As follows from Scheme 1, phenylacetylene hydrogeꢀ
nation proceeds in two steps: at first phenylacetylene is
hydrogenated to styrene, which corresponds to the conꢀ
sumption of the first mole of hydrogen, and then styrene is
converted to ethylbenzene when the second mole of hyꢀ
drogen is consumed. The ratio of hydrogenation rates of
acetylene and monoene is substantial for the isolation of
the target product upon the partial reduction of polyunꢀ
saturated compounds.47
b
It is known that the selectivity of partial hydrogenation
of compounds with the multiple С—С bond to monoene
in the presence of the palladiumꢀcontaining catalysts is
determined by two factors: (1) mechanism of hydroꢀ
gen addition to acetylene/diene compound (selectivity
by mechanism) and (2) adsorption displacement at
which adsorption of polyunsaturated compound prevents
reꢀadsorption of alkene (thermodynamic factor of selecꢀ
tivity.48 The selectivity at the first step of hydrogenation to
2 nm
Fig. 4. TEM images of palladiumꢀcontaining samples I (а) and
III (b) prepared by the impregnation to incipient wetness method.
Mole equiv.
I
II
2.0
1.5
1.0
0.5
the principal approaches on which two complementary
methods, HR TEM and XRF, are based.
III
Catalytic properties of the synthesized composite mateꢀ
rials Pd@MOFꢀ5. The synthesized composites
Pd NPs @MOF are highly active in the partial hydrogeꢀ
nation of phenylacetylene to styrene (Table 3, Fig. 5). The
highest rate for phenylacetyelene hydrogenation is achievꢀ
ed in the presence of catalyst III prepared by the impregꢀ
nation to incipient wetness method using palladium acetylꢀ
acetonate. The use of the palladiumꢀcontaining system
prepared by impregnation from palladium acetate (sample I)
noticeably decreases the hydrogenation rate of an acetylꢀ
ene compound. A sharp decrease in the phenylacetylene
hydrogenation rate is observed in the presence of the palꢀ
V
IV
20
40
60 t/min
Fig. 5. Kinetic curves of the partial hydrogenation of phenꢀ
ylacetylene on synthesized palladiumꢀcontaining samples
Pd@MOFꢀ5 I—V. Mole equiv. is the mole equivalent of abꢀ
sorbed H2.

Phenylacetylene hydrogenation over PdꢀMOFꢀ5
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
401
styrene is determined by the mechanism of hydrogen adꢀ
dition to phenylacetylene. Adsorption displacement is the
thermodynamic factor of selectivity and determines the
rate for styrene hydrogenation at the end of the first step
and at the second step. In the presence of the Pd@MOFꢀ5
samples prepared by impregnation to incipient wetness,
the hydrogenation rate at the second step substantially
increases, most likely, due to a decrease in the strength of
styrene adsorption on the metalꢀorganic framework after
the complete conversion of phenylacetylene (see Table 3).
As the palladium content increases, the rate of styrene
conversion to ethylbenzene decreases sharply in the presꢀ
ence of the Pd@MOFꢀ5 sample prepared by impregnation
from palladium acetate (sample II).
The optimum ratio of the hydrogenation rates of
phenylacetylene and styrene to ethylbenzene is achieved
in the presence of catalysts IV and V prepared in SCF. In
this case, the hydrogenation of styrene to ethylbenzene is
nearly completely suppressed (see Table 3).
The observed differences in activity of the obtained
catalysts in the partial hydrogenation of phenylacetylene
are probably related to the dispersion and localization of
palladium nanoparticles in the MOFꢀ5 matrix.
Several authors reported that the reaction rate decreases
with an increase in the dispersion of the metal.30 The efꢀ
fect of the metal particle size in the case of alkyne hydroꢀ
genation on the palladiumꢀcontaining catalysts is that the
selectivity and activity decrease with an increase in the
dispersion of the metal.49 We have recently showed that
the activity in the hydrogenation of butynediolꢀ1,4 on the
palladiumꢀcontaining MOFs increases with an increase in
the Pd nanoparticle size.50
A comparison of the results of catalytic experiments
and XRF and TEM data reveals a correlation between the
size of palladium particles localized on the support surface
and the activity of the obtained catalysts. Sample I
with the smallest average particle diameter (0.52 nm
according to the XRF data or less than 1 nm according
to the HR TEM data) exhibits the lowest activity in the
partial hydrogenation of phenylacetylene to styrene
(see Table 3).
The selectivity of the obtained catalysts toward styrene
depends substantially on several factors: the nature of the
precursor, method of precursor introduction, and palladiꢀ
um content in the MOFꢀ5 matrix. To the end of the first
step of hydrogenation palladiumꢀcontaining samples IV
and V prepared in SCF demonstrate selectivity higher than
90% (see Table 3). The highest selectivity (S1 ~98%) for
the partial hydrogenation of phenylacetylene is observed
in the presence of catalyst I prepared by the impregnation
to incipient wetness method from palladium acetate (see
Table 3, conversion of phenylacetylene 50%). To the end
of consumption of the first mole of hydrogen (conversion
of phenylacetylene 95%) the selectivity of this system reꢀ
mains high (S2 ~97%, see Table 3). As the palladium conꢀ
tent in this system increases to 5 wt.% (catalyst II), the
selectivity remains almost unchanged (S1 ~97%, converꢀ
sion of phenylacetylene 50%). In this case, an insignifiꢀ
cant decrease in the selectivity (S2 ~95%) is observed to
the end of the first hydrogenation step (see Table 3). The
replacement of palladium acetate by Pd(acac)2 in the case
when impregnation to incipient wetness was used results
in a noticeable decrease in the selectivity for styrene on
palladiumꢀcontaining material III (S2 ~86% at 95% conꢀ
version). Probably, the formation of larger palladium parꢀ
ticles in the Pd@MOFꢀ5 system, the possible bimodal
nanoparticle size distribution (see above) and the ensuing
heterogeneity of active sites result in a decrease in the
selectivity for monounsaturated compound. Catalysts IV
and V prepared in supercritical CO2 using the same preꢀ
cursor (palladium acetylacetonate) are characterized by
a higher selectivity of 90—92% (conversion of phenylꢀ
acetylene 95%) for styrene compared to sample III. Most
likely, this is related to the presence of active sites in pores
of these Pd@MOFꢀ5 samples prepared in SCF and to the
assumed monomodal size distribution of palladium nanoꢀ
particles (characteristic of the introduction of metal under
the fluid synthesis conditions). Thus, the selectivity of the
synthesized palladiumꢀcontaining materials Pd@MOFꢀ5
is probably determined by differences in dispersion and
position of palladium nanoparticles affected by the prepaꢀ
ration method of the catalytic system.
An increased activity of system III compared to samꢀ
ple I is due to the accessibility of active sites, i.e., their
immobilization on the external surface of the metalꢀorꢀ
ganic framework, and to the formation of larger palladium
particles (0.6 nm according to the XRF data or 1.5—2 nm
according to the HR TEM data). In the case of systems IV
and V prepared in a medium of SCF, the active sites are
predominantly immobilized, most likely, in pores of the
metalꢀorganic framework. Thus, taking into account the
pore size accessible for the adsorbate in the metalꢀorganic
framework MOFꢀ5 equal to 1.2—1.5 nm, samples I and
III should be ascribed to class А (according to R. A.
Fischer´s terminology5) and samples IV and V are attribꢀ
utable to class B (or С).
The obtained palladiumꢀcontaining materials
Pd@MOFꢀ5 are stable in the studied reaction. For examꢀ
ple, in the presence of sample III, the initial specific hyꢀ
drogenation rate of phenylacetylene to styrene (W01) reꢀ
mained unchanged within three cycles (Fig. 6). Upon the
multiple use of this catalyst, the rate of the undesirable
styrene conversion to ethylbenzene (W02) decreases noꢀ
ticeably.
Thus, the palladiumꢀcontaining systems involving
small Pd nanoparticles (less than 2 nm) immobilized in
the matrix of the microporous phenylenecarboxylate metꢀ
alꢀorganic framework MOFꢀ5 were synthesized for the
first time by the fluid synthesis method. The activity and
selectivity of the Pd@MOFꢀ5 catalysts in the partial hydroꢀ

402
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
Belyaeva et al.
W/mol ((mole of Pd) min)–1
120
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W01
W02
96.3
100
80
60
40
20
75.9
45.6
45.0
42.9
38.6
35.8
29.8
22.7
18.8
1
2
3
4
5
Cycles
Fig. 6. Stability of palladiumꢀcontaining sample MOFꢀ5 in the
partial hydrogenation of phenylacetylene; W01 and W02 are the
initial specific rates for the hydrogenation of phenylacetylene
and styrene, respectively.
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genation of phenylacetylene depend substantially on the
preparation method (fluid synthesis in supercritical CO2
and impregnation to incipient wetness) and on the nature
of the palladium precursor. A comparison of the catalytic
experimental data and results of physicochemical studies
suggested that small palladium nanoparticles (about
1—2 nm) are predominantly immobilized in pores of
the metalꢀorganic framework of composite material
1%Pd@MOFꢀ5 synthesized in supercritical CO2. This
position of active sites favors the suppression of the undeꢀ
sirable styrene conversion to ethylbenzene. The highest
selectivity for styrene (~95—97% at a phenylacetylene conꢀ
version of 95%) is observed in the presence of palaldiumꢀ
containing materials 1%Pd@MOFꢀ5 and 5%Pd@MOFꢀ5
prepared by the impregnation to incipient wetness method
from palladium acetate. The development of the composꢀ
ite materials, being palladium nanoparticles immobilized
in the matrix of the metalꢀorganic framework MOFꢀ5,
makes it possible to control the catalytic properties of the
Pd@MOFꢀ5 system in the partial hydrogenation of acetylꢀ
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This work was financially supported by the Russian
Russian Foundation for Basic Research (Project No. 12ꢀ
03ꢀ01097ꢀa).
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Received November 15, 2013;
in revised February 6, 2014