Solvent-free selective hydrogenation of 1,5-cyclooctadiene catalyzed by palladium incorporated TUD-1

Article abstract of DOI:10.1016/j.catcom.2017.07.026

Palladium (Pd) was incorporated into TUD-1 mesoporous siliceous material by using one-pot synthesis procedure. The catalytic activity of the prepared samples was evaluated in the selective hydrogenation of 1,5-cyclooctadiene (COD) at 80 °C in a solvent-free condition. Pd-TUD-1 showed > 95% conversion of COD with a selectivity > 90% towards cyclooctene (COE).

Full text of DOI:10.1016/j.catcom.2017.07.026

CatalysisCommunications101(2017)62–65
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Catalysis Communications
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Short communication
Solvent-free selective hydrogenation of 1,5-cyclooctadiene catalyzed by
palladium incorporated TUD-1
Mhamed Benaissa, Abdullah M. Alhanash, Murad Eissa, Mohamed S. Hamdy⁎
Chemistry Department, Science College, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Palladium (Pd) was incorporated into TUD-1 mesoporous siliceous material by using one-pot synthesis proce-
dure. The catalytic activity of the prepared samples was evaluated in the selective hydrogenation of 1,5-cy-
clooctadiene (COD) at 80 °C in a solvent-free condition. Pd-TUD-1 showed > 95% conversion of COD with a
selectivity > 90% towards cyclooctene (COE).
Heterogeneous catalysis
Green chemistry
Solvent-free
Hydrogenation
Palladium
TUD-1
1. Introduction
TUD-1 is a siliceous mesoporous material which patented in 2001
by Shan and coworkers [9]. The importance of TUD-1 came from its
Hydrogenation reactions attract significant attention in industrial
catalytic processes which produce chemicals with high purity and at
low costs [1,2]. The selective hydrogenation of consecutive hydro-
genation reaction is very complex, particularly if one reaction inter-
mediate is the desired product. This requires the conversion to be kept
very low [2–4] in order to maintain a high selectivity of the desirable
intermediate (product). This requirement reduces the economic value
of the overall process. The selective hydrogenation of cyclooctadiene
(COD) to cyclooctene (COE) is a good example of this kind of reactions
from an academic and industrial point of view [5]. Cyclooctene is
produced by partial hydrogenation of 1,5-cyclooctadiene as an inter-
mediate in the consecutive reaction which is normally over‑hy-
drogenated to form the undesired product; cyclooctane (COA). The
challenge here is to design a catalyst which is able to catalyze the hy-
drogenation of COD to selectively produce COE as a major product and
COA as a minor product, i.e. minimize the consecutive hydrogenation
reaction.
easy synthetic procedure where no surfactant is needed (cost effective).
Moreover, TUD-1 is a unique siliceous material and different from the
conventional mesoporous materials such as MCM-41 and SBA-15 due to
its open and 3D pore system. TUD-1 was used as a support for different
metal ions or oxide particles, such as Au [10] Mo [11] Cr [12], etc., and
the resultant materials exhibited high activity in different catalytic re-
actions such as selective oxidation reactions [10,11] and photocatalytic
elimination of short chain hydrocarbons [12].
In the current study, TUD-1 mesoporous material was functionalized
by Pd in one-step synthesis procedure and the prepared material was
characterized by physical and chemical techniques. The catalytic ac-
tivity of the prepared materials was investigated in the selective hy-
drogenation of COD under different reaction conditions. Optimization
of reaction conditions, such as temperature, H2 pressure, and time is
reported. Moreover, recycling of used catalyst is also discussed.
2. Experimental
Supported palladium nanoparticles were reported earlier as active
species for COD hydrogenation, such as Pd nanoparticles supported on
carbon nitride functionalized silica [5], nitrogen-doped carbon nano-
tubes [6], salen/FAU [7] and γAl2O3[8]. These catalysts exhibited high
activity in the conversion of COD with a good selectivity towards COE,
however, the synthesis of these catalysts was carried out in two-step
synthesis procedure. Hence, the ease one-step procedure to prepare Pd
nanoparticles incorporated in a high surface area support is -indeed- an
advantageous.
2.1. Pd-TUD-1 synthesis
Pd-TUD-1 was prepared by using one-pot synthesis procedure based
on hydrothermal sol-gel technique. A sample with a Si/Pd ratio of 50,
was
prepared
by
using
the
molar
oxide
ratio
of
1SiO2:0.02PdO:0.5TEAOH:1TEA:15H2O. In a typical synthesis, 12.9 g
of triethanolamine (TEA, 97%, ACROS) diluted by 7 ml of deionized
water was added to palladium (II) nitrate dihydrate (Pd(NO3)2·2H2O,
⁎
Corresponding author.
E-mail address: m.s.hamdy@gmail.com (M.S. Hamdy).
http://dx.doi.org/10.1016/j.catcom.2017.07.026
Received 15 April 2017; Received in revised form 14 July 2017; Accepted 27 July 2017
Availableonline28July2017
1566-7367/©2017ElsevierB.V.Allrightsreserved.

M. Benaissa et al.
CatalysisCommunications101(2017)62–65
98% Sigma-Aldrich) solution (0.46 g of Pd salt dissolved in 5 ml of
deionized water) under stirring. The obtained solution was added
dropwise to 17.8 g of tetraethyl orthosilicate (TEOS, +98%, ACROS)
under vigorous stirring. After 30 min, 17.6 g of tetraethyl ammonium
hydroxide (TEAOH, 35%, Aldrich) was added dropwise and the overall
mixture was stirred for at least two hours at room temperature. The
resulting homogeneous solution/gel was aged at room temperature for
24 h, and then dried at 98 °C for another 24 h. The obtained brownish
solid was grounded and hydrothermally treated in a 50 ml Teflon-lined
stainless steel autoclave at 178 °C under autogenous pressure for 4 h.
The obtained solid was grounded again, and then calcined in static air
at 600 °C for 10 h by applying a heating ramp rate of 1 degree/min.
2.2. Characterization
XRD was performed by using Schimadzu 6000 DX instrument dif-
fractometer equipped with a graphite monochromator using CuKα ra-
diation (λ = 0.1541 nm). Nitrogen adsorption/desorption isotherms
were recorded on a QuantaChrome NOVA 2000e instrument. ICP
technique was used to quantify the amount of Pd present in the pre-
pared material by using Thermo Scientific, ICAP 7000 series, part No:
1340910, Qtegra Soft wear, Germany. Raman spectrum was collected at
ambient conditions by using a Renishaw Raman Imaging Microscope,
system 2000. The green (λ = 514 nm) polarized radiation of an argon
ion laser beam of 20 mW was used for excitation. DR UV–Vis spectra
were collected at ambient conditions on a CaryWin 300 spectrometer in
the wavelength range of 200–800 nm by using BaSO4 as a reference
material. Scanning electron microscopy (SEM) Jeol Model 6360
LVSEM, USA, was used to observe the pore structure of the synthesized
sorbent materials.
2.3. Catalytic performance study
The solvent-free hydrogenation experiments were carried out in a
stirred batch Parr (300 ml capacity) reactor. In a typical run, 0.25 g of
catalyst was placed inside the reactor with particle diameter < 40 μm
and covered with 25 ml of COD. Then, the autoclave was closed and
degassed twice with nitrogen and heated to the working temperature.
After stabilization of the temperature to a desired value, the reactor was
pressurized to the necessary pressure with hydrogen gas. The reaction
was stirred by a high agitation speed (1000 rpm) to avoid external mass
transfer limitations. After the reaction time was elapsed, the reactor was
allowed to cool-down to the room temperature and then a liquid sample
was withdrawn and filtrated. The concentration of the product and the
reactants were analyzed by using a SHIMADZU GC-17 instrument
equipped with RTX-5 capillary column (30 m × 0.25 mm × 0.25 μm)
and a flame ionization detector (FID).
Fig. 1. a) The XRD patterns of Pd-TUD-1 and the neat TUD-1 materials, b) N2 physi-
sorption isotherms of Pd-TUD-1 and TUD-1.
3. Results and discussion
3.1. Characterization results
The Si/Pd ratio in the synthesized gel and in the calcined product
(determined by ICP elemental analysis) are very close, i.e. in the syn-
thesized gel Si/Pd ratio = 50 and in the obtained final solid powder Si/
Pd ratio = 48.3. This indicates that all the palladium amount which
added during the synthesis were found in the final solid product. Fig. 1a
shows the XRD patterns of Pd-TUD-1 compared with that of neat TUD-1
sample. The amorphous TUD-1 pattern shows one broad peak at 25° 2θ
which indicates the amorphous phase of SiO2[13]. This broad peak was
also observed in Pd-TUD-1 patterns, in addition to other peaks at 33.7,
41.8, 54.5, 60.7 and 71.2 2 theta. These peaks can be corresponding to
the crystal planes of (101), (110), (112), (200) and (202), which ac-
cording to JCPDS card no. 43–1024 is an indication to the presence of
palladium oxide species [14]. Fig. 1b shows the N2 sorption isotherm of
the Pd-TUD-1 sample compare with that of neat siliceous TUD-1
sample. The two isotherms are of type IV and showed a hysteresis loop
of H1 type indicating the meso-structured character of the two samples
[15]. Moreover, the similarity of the two isotherms is an indication that
the mesoporous property of TUD-1 did not change as a result of Pd
incorporation. The measured BET surface area of Pd-TUD-1 was
868 m2/g which is higher than neat TUD-1 which was 655 m2/g. The
Conversion of cyclooctadiene, and selectivity towards cyclooctene
were calculated according to the following equations:
[COD]0 − [COD]t
Conversion =
× 100
[COD]0
where [COD]0 is the initial cyclooctadiene concentration, [COD]t is the
concentration of cyclooctadiene at time (t).
[COE]
Selectivity =
× 100
[COD]0 − [COD]t
where [COE] is the cyclooctene concentration.
On the other hand, the turnover frequency (TOF) was calculated
according to the equation
[COD]0 − [COD]t
TOF =
[Pd] × 3600
where numerator represents the number of converted moles of COD
divided by the total mole number of Pd active phase and the reaction
time in seconds.
63

M. Benaissa et al.
CatalysisCommunications101(2017)62–65
texture properties of Pd-TUD-1 are compared with that of TUD-1 in
Table S1 (Supplementary information).
led to an improvement in the catalytic performance of Pd-TUD-1 where
97% of COD was converted with a 96% selectivity towards COE, and
the TOF was improved to 1.167 s−1. However, increasing the tem-
perature to 100 °C decreases the selectivity of COE to 80.3% (Fig. 2a).
This can be explained by activating the over-hydrogenation reaction of
COE to COA. Hence, the optimum reaction temperature was set to be
80 °C.
In Fig. S1 (Supplementary information), Raman spectrum of Pd-
TUD-1 is compared with that of neat TUD-1 sample. TUD-1 did not
show any Raman bands, while Pd-TUD-1 showed a single intensive
band around 650 cm−1. This band can be attributed to the presence of
PdO as reported earlier by Baylet and coworkers [16].
The morphological structure of TUD-1 particles (Fig. S2a) is com-
pared with that of Pd-TUD-1 (Fig. S2b) by using scanning electron
microscopy. TUD-1 exhibited the irregular-shape bulky particles with a
smooth surface and edge length of 100–200 μm. Pd-TUD-1 particles
exhibited almost the same morphological structure as TUD-1 but the
particles became smaller (> 50 μm) with rough surface. This result
shows the effect of Pd on the integrity of TUD-1 particles. Moreover,
elemental mapping is introduced in Fig. S3 which clearly shows the
high distribution of PdO particles throughout the silica matrix.
The optimum H2 pressure was evaluated by conducting the reaction
at 2, 5, and 10 (Fig. 2b). Under 2 atm of hydrogen pressure, 84% of
COD was converted with 99.4% selectivity towards COE and a total
TOF of 1.009 s−1. Increase the pressure to 10 atm improved the con-
version to 100%, however, selectivity decreased to 91%. Again, this can
be attributed to promote the over-hydrogenation reaction to form COA.
Hence, based on the obtained results, the optimum pressure was es-
tablished to be 5 atm.
On the other hand, it is obviously clear that by increasing the re-
action time, the selectivity is reduced (Fig. 2c). And hence, the reaction
time was set to 60 min only.
3.2. Catalytic activity
Recycling of the catalyst was carried out by reuse the catalyst
without any treatment in four consecutive runs. As seen in Fig. 2d, the
conversion % of COD was almost constant without any significant
change during the four runs, however, selectivity was decreased from
96% to 82%, and then it remained stable in the next two runs. Ele-
mental analysis of the resultant solution showed that only negligible
amount of Pd in the solution, i.e. no leaching for Pd from the meso-
porous silica was observed.
The catalytic performance of Pd-TUD-1 was evaluated in the hy-
drogenation of cyclooctadiene (COD) to the desired product cyclooc-
tene (COE) and the undesired product cycloctane (COA). Few pre-
liminary experiments were carried out in the beginning of the catalytic
activity measurements. First, COD did not show any conversion either
in the absence of catalyst (Pd-TUD-1), or in the presence of hydrogen
gas. In another experiment, no conversion was obtained in the presence
of Pd-TUD-1 and in the absence of hydrogen gas. Hence, hydrogenation
of COD was performed successfully only in the presence of Pd-TUD-1
under hydrogen pressure and at elevated temperatures. In a typical
experiment, under 5 atm of H2 gas and at 60 °C, 76% of COD was
converted to COE with a selectivity of 98% and the calculated TOF was
0.906 s−1. More importantly, the intermediate 1,3-COD was not de-
tected as an indication for the very fast conversion of 1,3-COD (if
formed) to the desired product COE. This result clearly shows that Pd-
TUD-1 is an efficient catalyst for the selective hydrogenation of COD.
Reaction conditions, mainly time, temperature, H2 pressure, were
optimized. The obtained results are listed in Table S2 (Supplementary
information) and in Fig. 2. Increasing temperature from 60 °C to 80 °C
For internal mass transfer limitations, Weisz modulus was used
2
r
R
0
[17,18], Mw
=
where r represents the radius of the spherical
C
H
D
e
particle of Pd-TUD-1, and because we sieved the catalyst through
40 μm mesh, then we can say that r = 20 μm. R0 is the calculated initial
reaction rate, and it equals 22.2 mol/m3 s. Moreover, CH is the con-
centration of hydrogen in the liquid phase, CH = 14.1 mol/m3, and fi-
nally, De is the effective diffusivity of hydrogen which is equal to the
molecular
De = 30 × 10−9 m2/s. Therefore, the calculated Weisz modulus is
w = 0.02. It has been reported [18] that if Mw is higher than 0.3 this
diffusivity
for
powder
catalyst
particle,
M
means that the catalytic system suffers from mass transfer limitation.
However, it obviously clear that Pd-TUD-1 did not suffer from internal
Fig. 2. The optimization reactions of the selective hydro-
genation of COD over Pd-TUD-1 in terms of a) reaction
temperature, b) H2 gas pressure, c) reaction time, and fi-
nally d) recycling test.
64

M. Benaissa et al.
CatalysisCommunications101(2017)62–65
Table 1
Comparison between the catalytic performance of Pd-TUD1 catalyst and other reported catalyst.
Catalyst
Solvent
T (C)
Patm
COD con. (%)
COE sel. (%)
TOF (S−1
)
Ref.
Pd-TUD-1
Pd-CN/SBA-15
Rh/PEG
Free
n-Heptane
Toluene, n-heptane
Toluene, n-heptane
Benzene
80
80
70
90
110
60
50
5
20
1
10
20
15
10
97.3
99
96
89
83
98
67
90
100
1.16
0.65
1.37
0.37
0.78
–
This study
[5]
[19]
[20]
[21]
[7]
100
100
100
100
100
Pd/PEG
Ir4 clusters
Pd salen/FAU
Pd/γAl2O3
n-Heptane
n-Heptane
0.31
[8]
mass transfer limitation.
Acknowledgement
Finally, in Table 1, a comparison between the catalytic data of Pd-
TUD-1 and the other reported homogeneous and heterogeneous cata-
lysts is introduced. The calculated TOF of Pd-TUD-1 is higher than most
of the reported catalysts with one more advantage which is no solvent is
needed to perform the reaction. Obviously, Pd-TUD-1 is a promising
catalyst in the selective hydrogenation of COD to COE, which open a
window to perform the reaction under one green-chemistry condition,
which is solvent-free condition.
To investigate the nature of the active sites after the reaction. XRD
and Raman spectroscopic studies were performed for the catalyst before
and after the first run. The results are introduced in Fig. S4. In XRD, the
Pd peaks at 40.1°, 46.6° and 68° are identified as the Pd(111), Pd(200)
and Pd(220) reflections, which can be attributed to the face-centered
cubic structure of Pd (JCPDS card no. 46-1043) [6]. Moreover, the
disappearance of the band around 650 cm−1 agrees the formation of
metallic Pd as reported by Baylet and coworkers [16]. The obtained
data confirmed the reduction of PdO to Pd nanoparticles during the first
run, i.e. in-situ pretreatment for the catalyst. More importantly, the
reduction of PdO into Pd nanoparticles during the run can explain the
decrease in selectivity of COE after the first run. In the fresh sample's
run, few minutes are consumed in the reduction of the catalyst to form
the Pd active sites, then the reaction starts. However, in the reused
catalyst, the catalyst will convert directly COD to COE and further time
will be available for over-reduction reaction to convert COE to COA.
The authors extend their appreciation to the Deanship of Scientific
Research at King Khalid University for funding this work through
General Research Project under grant number (G.R.P- 210-38).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2017.07.026.
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4. Conclusions
Here, a new catalytic material is introduced. Palladium oxide in-
corporated into amorphous siliceous TUD-1 material and denoted as
Pd-TUD-1. Pd-TUD-1 was synthesized in one-step synthesis procedure
and characterized by using several physical and chemical techniques.
Pd-TUD-1 shows a promising catalytic activity for selective hydro-
genation of COD to COE under solvent-free condition. 97% conversion
of COD towards cyclooctene (COE) with a selectivity up to 90% was
achieved. The optimum reaction conditions were achieved in terms of
reaction temperature, H2 gas pressure and reaction time. Pd-TUD-1
showed high stability and no leaching was observed.
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