Alkene hydrogenation over palladium supported on a carbon–silica material

Article abstract of DOI:10.1134/S0023158416050025

Palladium catalysts supported on a carbon–silica material were synthesized. Hydrogenation by molecular hydrogen was studied in the presence of straight-chain and cyclic olefins. As distinct from what is observed for olefins having a phenyl substituent, for aliphatic alkenes the reaction rate decreases with an increasing conversion due to the accumulation of hydrogenation products. The synthesized palladium catalysts show a higher hydrogenation activity than Pd/C.

Full text of DOI:10.1134/S0023158416050025

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 586–591. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © T.I. Akchurin, N.Z. Baibulatova, S.A. Grabovskii, P.P. Talipova, E.G. Galkin, V.A. Dokichev, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 5,
pp. 592–597.
Alkene Hydrogenation over Palladium Supported
on a Carbon–Silica Material
T. I. Akchurina, N. Z. Baibulatovaa, S. A. Grabovskiia, P. P. Tal ip ova a,
E. G. Galkina, and V. A. Dokichevb, *
aUfa Institute of Chemistry, Russian Academy of Sciences, Ufa, 450054 Bashkortostan, Russia
bUfa State Aviation Technical University, Ufa, 450008 Bashkortostan, Russia
*e-mail: dokichev@anrb.ru
Received July 8, 2015
Abstract—Palladium catalysts supported on a carbon–silica material were synthesized. Hydrogenation by
molecular hydrogen was studied in the presence of straight-chain and cyclic olefins. As distinct from what is
observed for olefins having a phenyl substituent, for aliphatic alkenes the reaction rate decreases with an
increasing conversion due to the accumulation of hydrogenation products. The synthesized palladium cata-
lysts show a higher hydrogenation activity than Pd/C.
Keywords: heterogeneous catalysis, palladium, carbon–silica support, hydrogenation reaction, molecular
hydrogen, olefins
DOI: 10.1134/S0023158416050025
INTRODUCTION
bornene (99%), and cyclopentene (96%) from
Aldrich, as well as 98% pure allylbenzene, vinylben-
zene, hexadecene-1, 1-heptene, 1-decene, 1-nonene,
1-octene, cyclohexene, and cyclooctene.
The supports were a carbon–silica adsorbent pro-
duced by Pilot Co. (Ufa) and activated carbon (BAU-
A brand, USSR State Standard GOST 6217–74) pro-
duced by Forus Co. Cherepovets.
In the catalytic hydrogenation of unsaturated com-
pounds with molecular hydrogen in the presence of
heterogeneous catalysts, choice of support plays a
major role because it has a significant effect on the
activity and selectivity of the catalyst. The most com-
monly used supports for Pd-catalysts are activated car-
bon, Al2O3, and zeolites [1]. In a number of studies
[2–4], microheterogeneous palladium catalysts con-
sisting of palladium phosphides and Pd(0) clusters
were used in hydrogenation. In recent years, there
have been data on the deposition of palladium on car-
bon materials, namely, fullerenes, carbon nanotubes
and nanofibers, and nanodiamonds [5–9] and on its
encapsulation in polypropylenimine dendrimers [10].
However, the literature contains no information on the
catalytic activity of heterogeneous palladium catalysts of water was refluxed for 1.5 h until a clear solution was
on carbon–silica supports that combine the properties obtained, which was diluted with 17 mL of water and
of silica and hydrophobic carbon materials [11–15].
The purpose of this work was to develop Pd-con-
taining catalysts on a carbon–silica support and to
study their usability in the hydrogenation of olefins
with straight-chain and cyclic structures by molecular
hydrogen.
Catalyst Synthesis
Pd/C–SiO2 catalysts with a metal content of 1, 2,
3, 4 and 5% were synthesized via a procedure devel-
oped for the synthesis of palladium on carbon [16].
For example, for the preparation of a 5% Pd sample, a
mixture of 1.0 g of PdCl2, 2.4 mL of HClconc and 6 mL
thoroughly stirred with 11.4 g of the carbon–silica sup-
port. The resulting material was evaporated in a water
bath and dried in an oven at 100°C (Table 1).
A 5%Pd/C reference catalyst was was synthesized
by the same method [16].
The weight of the sample obtained from 0.50 g of
PdCl2 and 6.60 g of activated carbon was 6.05 g
To synthesize a 5%Pd/C-SiO2 catalyst reduced
with diazomethane, 100 mL of a 0.5 M solution of dia-
zomethane in diethyl ether was added in small por-
EXPERIMENTAL
Materials
We used 99% pure PdCl2 and the following olefins: tions to 1 g of 5%Pd/C-SiO2. After gas evolution had
1,1-diphenylethylene (97%), indene (98%), nor- ceased, the catalyst was filtered and dried at room
586

ALKENE HYDROGENATION OVER PALLADIUM SUPPORTED
587
Table 1. Synthesis of the palladium catalysts Pd/C–SiO2
temperature in a vacuum (1 Torr). This yielded 0.88 g
of the final product.
Amounts of the initial
components, g
Weight
of the resulting
catalyst, g
Pd, wt %
Characterization Methods
PdCl2
support
The static adsorption capacity of the support was
determined by the desiccator method based on com-
plete saturation of the sample with adsorbate vapor
(water, benzene, n-heptane) under standard condi-
tions at a temperature of 20–25°C. The sample (0.2–
0.3 g) was dehydrated in a muffle furnace at 200 and
350°C for 2–3 h, placed in a desiccator with adsor-
bate, and incubated for 24–48 h at room temperature
until complete saturation with adsorbate vapor [17].
The static adsorption capacity of the sample for the
condensed adsorbate vapor was calculated from the
formula
1
2
3
4
5
0.25
0.50
0.60
0.80
1.00
14.85
14.70
11.64
11.50
11.40
15.30
15.40
12.34
12.60
12.42
Olefins Hydrogenation Experiments
The volumetric hydrogenation setup [21] consisted
of a thermostatically controlled jacketed glass reactor
and a gas burette. Ethanol (10 mL 95%), 5%Pd/C–
SiO2 (50 mg) containing 2.5 mg (0.024 mmol) if Pd,
and olefin (2.34 mmol) were placed in the jacketed
reactor, whose volume was 30 mL. The reaction was
performed under magnetic stirring at atmospheric
pressure and a temperature of 23 1°C. The volume
of hydrogen consumed during the reaction was mea-
C = m2 m1d420 ,
where m1 is the weight of the calcined sample (g), m2 is
the weight of the adsorbate adsorbed (g), and 20 is the
d4
adsorbate density (g/cm3). The discrepancy between
the results of replica measurements did not exceed
0.02 cm3/g.
The specific surface area (SBET) of the support was sured at a constant pressure, by comparing the levels of
the liquid in the burette and leveling vessel. Upon the
completion of hydrogenation (when the volume of
hydrogen ceased to change), 0.27 g (2.34 mmol) of
octane was added as a standard and the catalyst was
filtered out. The kinetics of olefin hydrogenation was
studied by a volumetric method, recording the change
in the amount of hydrogen. To determine reaction rate
constants, we plotted the absorbed amount of hydro-
gen as a function of the reaction time up to a conver-
sion of over 96%. The initial rate was calculated from
the slope of the tangent to the initial segments of
kinetic curves.
determined by the BET method by measuring the
equilibrium amount of nitrogen adsorbed at 77 K [18].
Adsorption isotherm in the relative equilibrium nitro-
gen vapor pressure (P/P0) range from 0.05 to 0.33 were
obtained using a Sorptomatic-1900 automated volu-
metric vacuum static analyzer (Fisons, Italy).
The IR spectra of the carbon–silica support were
recorded on a Vertex 70V Fourier spectrometer
(Bruker, Germany) in the 4000–400 cm-1 wavenum-
ber range. The sample, pressed into a KBr pellet, was
placed in a cuvette and was calcined at 200°C for
25 min to remove physically bound water.
The product composition and yield were deter-
mined in a GC-2014 gas chromatograph (Shimadzu,
Japan) with a 30 m × 0.25 mm × 0.25 μm capillary
column (HP-INNOWAX phase), raising the tempera-
ture from 50 to 250°C at a rate of 12°C/min. An MAT
95 XP gas chromatograph–mass spectrometer system
(Thermo Finnigan, Germany; voltage of 70 eV, ion-
ization chamber temperature of 250°C, temperature-
programmed mode, 50–250°C, heating rate of
10°C/min) was also used. Mixture components were
identified using the NIST 05 mass spectral library.
The elemental composition of the carbon–silica
support was determined using an EDX-800 HS
energy-dispersive X-ray (EDX) microprobe (Shi-
madzu, Japan).
The microstructure of the samples was studied
using field emission scanning electron microscopy
(FE-SEM) on an SU-8000 (Hitahi, Japan) electron
microscope. Imaging was performed in secondary-
electron detection mode at an accelerating voltage of
2 kV and a working distance of 4–5 mm. The measure-
ment results were optimized using the approach
described previously in Ref. [19]. The sample to be
examined was placed on the surface of an aluminum
stage 25 mm in diameter and wass secured with con-
ductive glue. The morphology of the samples tested
under native conditions to rule out the surface effects
arising from the deposition of the conductive layer
[20].
RESULTS AND DISCUSSION
Below, we report the results for the hydrogenation,
with molecular hydrogen, of the C=C bond in a series
of straight-chain and cyclic olefins over Pd/C–SiO2
catalysts with a metal content of 1 to 5% and over 5%
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

588
AKCHURIN et al.
Table 2. Static adsorption capacity of the carbon-silica sup-
rings of the carbons. The shift of the maximum of this
band to 1635 cm–1 may be due to the support being
similar in structure to the low metamorphosed coals or
to the presence of olefinic C=C groups on the surface.
However, the presence of this absorption band in the
spectrum of the carbon–silica support is most likely
fue to bending vibrations of water on the surface of
SiO2. The broad low-intensity absorption band at
port
Pore volume, cm3/g
Temperature,
°C
for benzene for heptane
for water
200
350
0.02
0.04
0.02
0.06
0.01
0.01
~1400 cm–1 can be assigned to bending vibrations of
COOH groups. The broad absorption band in the
1250−900 cm–1 range characterizes the stretching
vibrations of the C−OH bonds in the carbons. The
band at ~800 cm–1 can be assigned to out-of-plane
bending vibrations of C−H bonds in the substituted
benzene rings. In addition, in the 1250−400 cm–1
range there are absorption bands corresponding to
Si−O−Si stretching and bending vibrations: the
intense band is characteristic of the asymmetric
stretching vibrations of Si−O−Si (1090 cm–1); the
medium-intensity band at ~800 cm–1, to the symmet-
ric stretching vibrations of SiO4 tetrahedra; the band at
Pd/C–SiO2 pre-reduced with diazomethane. The
5%Pd/C sample was used as the reference catalyst.
The support was a hydrophilic low-porosity car-
bon–silica material with a particle size of 0.1–
0.63 mm, a bulk density of 1.25 g/cm3, a specific sur-
face area of 7.0 m2/g, and a water absorption capacity
of 10%. Static adsorption capacity data for the support
are presented in Table 2.
EDX microanalysis showed that the carbon–silica
support contains 71.78% C, 24.01% SiO2, and 3.91%
Al2O3.
~500 cm–1, to bending SiO4 vibrations.
In the FE-SEM images (Fig. 2), palladium parti-
cles can not be seen even at a very high magnification.
This may indicate that the particle size of Pd is less
than 3 nm and the distribution of palladium on the
support is relatively uniform, since the resolution limit
of the electron microscope is almost ~3 nm. The data
of X-ray microanalysis of the Pd/C-SiO2 catalyst indi-
cate the presence of palladium therein.
Absorption bands characteristic of both carbons [22]
and silica [23] were observed in the IR spectrum of the
support (Fig. 1). The absorption band at 3425 cm–1 is
due to the stretching vibrations of OH-groups bound
by hydrogen bonds. The absorption bands at 2920 and
2940 cm–1 are assignable to the stretching vibrations of
C−H bond in CH3 and CH2 groups. The weak peaks
at ~1700 cm–1 arise from the stretching vibrations of
the carbonyl group in the aromatic acids. The absorp-
The catalysts prepared were tested in the hydroge-
tion bands at ~1630 cm–1 are characteristic of the nation of 1-heptene, 1-octene, 1-nonene, 1-decene,
stretching vibrations of the C=C bonds in the aromatic 1-hexadecene, allylbenzene, vinylbenzene, 1,1-
Intensity, arb. units
34
32
30
28
26
24
22
20
18
16
14
12
10
3800 3400 3000 2600 2200 1800 1400 1000 600
ν, сm–1
Fig. 1. IR spectrum of the carbon–silica support.
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ALKENE HYDROGENATION OVER PALLADIUM SUPPORTED
(а) (b)
589
200 μm
100 nm
Fig. 2. FE-SEM images of the (a) support surface and (b) 5%Pd/C–SiO catalyst.
2
diphenylethylene,
cyclopentene,
cyclohexene, stant throughout the experiment. Thus, the rate con-
cyclooctene, norbornene, and indene with molecular stant decreases in the 1,1-diphenylethylene > allylben-
hydrogen. Their activity was characterized in terms of zene > vinylbenzene order (ralkene : Pd = 100 : 1,
the initial hydrogenation rate and 98% hydrogen con- 50 mg of 5%Pd/C–SiO2) and it is 2.2 0.1, 1.16
0.09 and 1.07 0.09 μmol L–1 s–1, respectively.
sumption time. The order of the reaction was deter-
mined by the integral method.
In the hydrogenation of straight-chain alkenes (1-
heptene, 1-octene, 1-nonene, 1-decene, and 1-hexa-
decene), the initial reaction rate was almost indepen-
dent of the length of the alkyl chain and was (1.1
The relative catalytic activity of the catalysts on
the carbon–silica support was determined using 1-
decene hydrogenation at a 1-decene : Pd ratio of 100:
1 (Fig. 3a). According to hydrogenation rate, the
catalysts are arranged in the following order:
5%Pd/C–SiO2 > 3%Pd/C–SiO2 > 4%Pd/C–SiO2 >
1%Pd/C–SiO2 > 2%Pd/C–SiO2 > 5%Pd/C–SiO2
reduced with diazomethane. The low activity of the
0.1)–(0.9
0.1) μmol L–1 s–1. However, with an
increasing substrate conversion, the hydrogenation
rate gradually decreased and, at 90% conversion, it
became 1.7−2.0 times lower than the initial rate; the
time to 95% conversion was 78−83 min. Apparently,
latter is probably due to the neutralization of its Brøn- the observed deceleration was due to the accumulation
sted acid sites (AlOH) by diazomethane.
of the hydrogenation product in the reaction mixture.
To verify this hypothesis, a series of experiments
was performed, in which 1-octene containing different
amounts of octane was hydrogenated. It was found
that the initial reaction rate does depend on the con-
centration of octane (Fig. 5), but when its content was
two times higher than the olefin concentration, the
hydrogen consumption rate became independent on
the substrate conversion and was described by a
zeroth-order equation (correlation coefficient of
>0.98). In the hydrogenation of alkenes containing a
phenyl substituent, the reaction products do not affect
the rate of the reaction. For vinylbenzene : ethylben-
zene = 1 : 2, k = 0.90 0.08 μmol L–1 s–1, and at the
same ratio of vinylbenzene to n-octane, the constant is
1.02 0.08 μmol L–1 s–1, which within the error coin-
A comparison of the 5%Pd/C–SiO2 and 5%Pd/C
catalysts showed that, when the carbon–silica support
is used, the reduction of 1,1-diphenylethylene takes
place 2.9 times more rapidly than in the case of
5%Pd/C (keff = 0.75 0.06 μmol L–1 s–1). The hydro-
genation of 1-hexadecene over 5%Pd/C–SiO2 (initial
rate of w0 = 1.06 0.09 μmol L–1 s–1) also occurs 1.4
times more rapidly than over 5%Pd/C. However, the
reaction rate over the 5%Pd/C–SiO2 catalyst is lower
and, at 90% conversion, it is 0.41 0.06 μmol L–1 s–1,
1.9 times lower than over 5%Pd/C. As a result, the
time to 98% conversion for both catalysts is the same
and is ~57 min (Fig. 3b).
Since the 5%Pd/C–SiO2 sample showed the high-
est catalytic activity, it is this catalyst that was used in cides with the rate constant of hydrogen consumption
all subsequent studies.
in the absence of ethylbenzene and octane, respec-
tively. The effect of the concentration of the hydroge-
nation product of the aliphatic olefins is probably
caused by the slightly higher static adsorption capacity
of the support for heptane versus benzene (Table 2).
The reactivity of monocycloalkenes in the hydro-
genation reactions over 5%Pd/C-SiO2 was the smaller
Among the straight-chain α-olefins, the alkenes
having a phenyl substituent are hydrogenated at the
highest rate (Fig. 4). The consumption of hydrogen
under the conditions examined is described by a
zeroth-order equation (correlation coefficient of
>0.98), and the rate of its consumption remains con-
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

590
AKCHURIN et al.
Volume of Н2 consumed, mL
(а)
Volume of Н2 consumed, mL
50
1
50
40
30
20
10
6
1
5
40
30
20
10
5
4
2
3
4
2
3
6
0
20
40
60
80
100
0
10
20
30
40
Time, min
Time, min
(b)
Volume of Н2 consumed, mL
60
Fig. 4. Time dependence of the volume of hydrogen con-
sumed for α-olefins hydrogenation oover 5%Pd/C–SiO :
2
(1) 1,1-diphenylethylene, (2) allylbenzene, (3) vinylben-
zene, (4) 1-hexadecene, (5) 1-decen, and (6) 1-octene.
1
50
40
30
20
10
2
w0, μmol L–1 s–1
4
3
1.0
0.8
0.6
0.4
0.2
0
20
40
60
80
Time, min
Fig. 3. (a) Time dependence of the volume of hydrogen
consumed for 1-decene hydrogenation over (1) 5%Pd/C–
0
0.2
0.4
0.6
0.8
1.0
SiO , (2) 3%Pd/C–SiO , (3) 4%Pd/C–SiO ,
2
2
2
(4) l%Pd/C–SiO , (5) 2%Pd/C–SiO , and (6)5%Pd/C–
[n-C8H18], mol/L
2
2
SiO reduced with diazomethane. (b) The same curves for
2
the hydrogenation of (1) 1,1-diphenylethylene and(2) 1-
hexadecene over 5%Pd/C–SiO and for the hydrogena-
Fig. 5. Dependence of the initial rate of the hydrogenation
2
tion of (3) 1-hexadecene and (4) 1,1-diphenylethylene over
of 1-octene over the 5%Pd/C–SiO catalyst on the octane
2
5%Pd/C.
concentration in the reaction mixture.
the greater the size of the ring. As in the case of phe-
nyl-substituted alkenes, the hydrogen uptake is
described by a zerothe-order equation (correlation
coefficient of >0.98), and the rate of hydrogen con-
sumption remains constant throughout the experiment.
The rate constant decreases in the cyclopentene >
cyclohexene > cyclooctene order and is 1.2 0.1 × 10–6,
The 5%Pd/C-SiO2 catalytic system is also suitable
of effectively hydrogenating bicyclic olefins, such as
norbornene and indene. Norbornene is hydrogenated
at a higher rate (k = 2.1 0.1 × 10–6 μmol L–1 s–1, and
96% conversion is reached in 34 min) than monocy-
cloalkenes. The hydrogenation rate of the double bond
in the cyclopentene ring of indene is two times lower
than in the case of norbornene: the rate constant is
7.6 0.1 × 10–7, and 4.8 0.1 × 10–7 μmol L–1 s–1, 7.5 0.1 × 10–7 μmol L–1 s–1 and is comparable with
the rate constant of cyclohexene hydrogenation.
Thus, in the present work new palladium catalysts
on the carbon–silica support were first synthesized.
respectively, while the 98% conversion of the initial
olefins is reached in 45, 59, and 80 min, respectively
(Fig. 6).
KINETICS AND CATALYSIS
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ALKENE HYDROGENATION OVER PALLADIUM SUPPORTED
591
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Time, min
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2
tene, (3) indene, (4) cyclohexene and (5) cyclooctene.
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ACKNOLEDGEMENTS
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The authors are grateful to the Department of
Structural Research of the Zelinsky Institute of
Organic Chemistry for the examination of the cata-
lysts by electron microscopy.
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Translated by A. Pashigreva
KINETICS AND CATALYSIS Vol. 57 No. 5 2016