Ultrasound- and microwave-assisted preparation of lead-free palladium catalysts: Effects on the kinetics of diphenylacetylene semi-hydrogenation

Article abstract of DOI:10.1002/cctc.201402999

The effect of environmentally benign enabling technologies such as ultrasound and microwaves on the preparation of the lead-free Pd catalyst has been studied. A one-pot method of the catalyst preparation using ultrasound-assisted dispersion of palladium acetate in the presence of the surfactant/capping agent and boehmite support produced the catalyst containing Pd nanoparticles and reduced the number of pores larger than 4 nm in the boehmite support. This catalyst demonstrated higher activity and selectivity. The comparison of kinetic parameters for diphenylacetylene hydrogenation showed that the catalyst obtained by using the one-pot method was seven times as active as a commercial Lindlar catalyst and selectivity towards Z-stilbene was high. Our work also illustrated that highly selective Pd/boehmite catalysts can be prepared through ultrasound-assisted dispersion and microwave-assisted reduction in water under hydrogen pressure without any surfactant.

Full text of DOI:10.1002/cctc.201402999

DOI: 10.1002/cctc.201402999
Full Papers
Ultrasound- and Microwave-Assisted Preparation of Lead-
Free Palladium Catalysts: Effects on the Kinetics of
Diphenylacetylene Semi-Hydrogenation
[
a]
[b]
[a]
[a]
[b]
Zhilin Wu, Nikolay Cherkasov, Giancarlo Cravotto,* Emily Borretto, Alex O. Ibhadon,*
[c]
[c]
Jonathan Medlock, and Werner Bonrath*
The effect of environmentally benign enabling technologies
such as ultrasound and microwaves on the preparation of the
lead-free Pd catalyst has been studied. A one-pot method of
the catalyst preparation using ultrasound-assisted dispersion of
palladium acetate in the presence of the surfactant/capping
agent and boehmite support produced the catalyst containing
Pd nanoparticles and reduced the number of pores larger than
rameters for diphenylacetylene hydrogenation showed that
the catalyst obtained by using the one-pot method was seven
times as active as a commercial Lindlar catalyst and selectivity
towards Z-stilbene was high. Our work also illustrated that
highly selective Pd/boehmite catalysts can be prepared
through ultrasound-assisted dispersion and microwave-assisted
reduction in water under hydrogen pressure without any sur-
factant.
4
nm in the boehmite support. This catalyst demonstrated
higher activity and selectivity. The comparison of kinetic pa-
Introduction
Hydrogenation reactions are performed in many areas of the
chemical industry with hydrogen gas activated by nickel-, plati-
high yield (i.e., with high substrate conversion and selectivity)
while minimizing the reaction duration, the number of reac-
tions, and separation steps such as protection of certain
groups and separation of products or catalysts from the prod-
[
1–3]
num-, or palladium-based catalysts.
Pd catalysts require the
mildest of conditions and are the most widely used. In many
cases, it is desired to obtain alkenes from the reduction of al-
kynes (i.e., to halt the reaction at the carbon–carbon double
bond stage); this class of reactions is termed as selective hy-
drogenation or semi-hydrogenation and is widely used in the
fine chemical industry because it is one of the vital steps in
the synthesis of vitamins (vitamins A, E, and K), terpenes
[5]
uct. In addition, alkynes as impurities in the feedstock are
strong poisons for Ziegler–Natta polymerization catalysts and
should be converted into alkenes by using semi-hydrogenation
[2,6,7]
reactions.
Therefore, semi-hydrogenation reactions are im-
portant for polyethylene and polypropylene production.
Conventional Pd catalysts for alkyne hydrogenation do not
provide high selectivity in these processes and are usually
modified with adsorbates such as quinoline, silver, and lead,
which increase selectivity while decreasing activity. The Lindlar
catalyst (Pd poisoned with lead and quinoline supported on
[
2–4]
(linalool), and other products.
The main challenge in this
process is to obtain the products of semi-hydrogenation in
[
a] Dr. Z. Wu, Prof. G. Cravotto, Dr. E. Borretto
Dipartimento di Scienza e Tecnologꢀa del Farmaco
and NIS—Centre for Nanostructured Interfaces and Surfaces
University of Turin
CaCO ) was an industrial standard in fine chemistry for more
3
[8,9]
than 50 years.
However, there is a drive to develop new
lead-free semi-hydrogenation catalysts for environmental rea-
[10]
Via P. Giuria 9
sons because lead is toxic. Addressing the problem of toxici-
ty and trying to increase performance, extensive research has
been performed to search for other modifier metals such as Ga
1
0125 Turin (Italy)
E-mail: giancarlo.cravotto@unito.it
[
b] Dr. N. Cherkasov, Dr. A. O. Ibhadon
Catalysis and Reactor Engineering Research Group
Department of Chemistry and School of Biological
Biomedical and Environmental Sciences
University of Hull
[11–14]
and Zn.
However, bimetallic catalysts require complex syn-
thesis routes; Pd–Ga intermetallic compounds were recently
shown to be oxygen sensitive even in carefully controlled air-
[15]
free laboratory conditions. In contrast, monometallic Pd cat-
Cottingham Road, Hull HU6 7RX (UK)
E-mail: a.o.ibhadon@hull.ac.uk
alysts provide the highest catalytic activity and were demon-
strated to provide high selectivity toward semi-hydrogena-
[
c] Dr. J. Medlock, Dr. W. Bonrath
Research and Development
DSM Nutritional Products Ltd
P.O. Box 2676
[16]
tion. The hydrogenation of alkynes on solid Pd surfaces is
a structure-sensitive reaction, an important feature for the un-
[17,18]
derstanding of such catalytic processes.
The unselective
4
002 Basel (Switzerland)
hydrogenation proceeds on hydrogen-saturated b-hydride,
E-mail: werner.bonrath@dsm.com
whereas selective hydrogenation is possible only after decou-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402999.
[19]
pling bulk properties from the surface events and only if sur-
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Full Papers
face hydrogen from the gas phase is available to generate the
[
20]
alkene. For this reason, effects of various supports such as
[
21]
[22]
silica and alumina on the catalytic hydrogenation behavior
have been extensively studied.
Non-conventional enabling technologies such as microwaves
(MW) and ultrasound (US) foster process intensification, com-
bine safer protocols, and result in cost reduction and energy
[
23]
saving. In comparison to conventional conductive heating,
MW irradiation causes volumetric heating via the direct cou-
pling of the electromagnetic field with polar solvents and cata-
lysts in the reaction mixture. As a result, fast selective heating
[24]
can be attained by irradiating polar materials in an MW field.
MW irradiation has proved to be a suitable energy source in
catalyst preparation because it enables one to generate nano-
scale colloids and clusters of greater size and shape uniformi-
[
25]
[26,27]
ty —an important advantage for catalysis.
US enables the rapid dispersion of solids and facilitates the
2
Figure 1. SEM images of a) Pdsolv/boehmite and b) Pdsolv/CeO catalyst
particles.
[
28]
formation of porous materials and nanostructures. Moreover,
sonication can inhibit particle aggregation because of intense
implosion, which produces extreme chemical environments,
of the catalyst particles are shown in Figure 1. Boehmite con-
sists of non-uniform (mainly spheroidal) particles of diameter
ranging from 1 to 100 mm (Figure 1a). In contrast, ceria parti-
cles are uniform with the dimensions of approximately 50 mm
(Figure 1b).
[
29–31]
caused by acoustic cavitation.
If a cavitation bubble col-
lapses violently near a solid surface, the high-speed jets of the
[
32]
liquid are driven into the surface of a particle. Thus, US is
usually used to prepare catalytic materials and modify existing
[
33–35]
metal catalysts.
The excellent results of catalyst preparation by using US- or
The nitrogen physisorption data of the catalysts obtained
are presented in Figure 2. Both catalysts demonstrated type IV
adsorption isotherms according to the IUPAC classification,
[
36]
MW-assisted protocols in previous studies prompted us to
explore new promising US- and MW-assisted synthetic routes
for the preparation of Pd-supported catalysts to be used in
[44]
which are characteristic of many mesoporous sorbents. How-
[
5,37]
alkyne semi-hydrogenation.
The semi-hydrogenation of di-
phenylacetylene (DPA) is a conventional model reaction used
to evaluate the behavior, mechanisms, and kinetics of the cata-
[
38–40]
lytic hydrogenation of internal alkynes.
Herein, we investi-
gated how non-conventional preparation methods (US and
MW) of lead-free Pd catalysts may affect the kinetics of DPA
semi-hydrogenation.
Results and Discussion
Effect of the catalyst support with US-assisted dispersion
and reduction
The role of catalyst supports is a multifaceted issue that in-
volves phenomena of chemical and physical catalyst–support
[
41]
interactions. On the basic level, however, a catalyst support
acts as a matrix preventing sintering of the catalytically active
Pd particles; thus, high surface area and thermostability are im-
portant characteristics of the catalyst support. Hence, alumina
(along with carbon) is one of the widely used supports for hy-
drogenation catalysts. In contrast, strong metal–support inter-
actions between Pd and the catalyst support such as
cerium(IV) oxide were shown to enhance activity and selectivi-
[
42,43]
ty of hydrogenation reactions.
The boehmite-supported (Pdsolv/boehmite) and ceria-sup-
ported (Pdsolv/CeO ) catalysts were obtained by reducing palla-
2
dium(II) acetate with ethylene glycol at 1008C in the presence
Figure 2. a) Nitrogen physisorption isotherms of Pdsolv/CeO
2
and
of boehmite and ceria supports, respectively. The SEM images
Pdsolv/boehmite catalysts; b) BJH desorption pore distribution.
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whereas boehmite contained
a significant amount of meso-
pores that can be accessible to
Pd nanoparticles.
Table 1. Pd content, textural properties, and catalytic performance of the catalysts studied.
[
a]
[b]
[c]
[d]
[e]
[f]
Catalyst
W
Pd
S
BET
2
V
pore
d
pore
d
av.pore
A
init
S
DPE
[%]
ꢁ1
3
ꢁ1
ꢁ1 ꢁ1
Pd
[%]
[m g
]
[cm g
]
[nm]
[nm]
[molg
h
]
Pdsolv/boehmite
0.72
1.33
0.69
0.78
0.88
4.22
234
3.6
115
115
231
3.3
0.43
0.02
0.29
0.29
0.43
0.01
3.9
3.2
3.5
3.5
3.9
2.4
6.8
12.9
7.7
6.7
6.0
5.4
3.0
16.8
18.6
6.6
81.0
81.7
87.0
88.0
87.5
93.2
TEM analysis of the Pdsolv/
Pdsolv/CeO
2
boehmite catalyst revealed that
Pd nanoparticles of diameter
PdLV/boehmite
PdLV-1/boehmite
PdUS-MW/boehmite
Lindlar
(
20ꢀ5) nm formed agglomer-
ates on the surface of the
boehmite catalyst support
13.0
2.4
[a] Pd content in % mass; [b] BJH desorption cumulative volume of pores between 1.7 and 300 nm in diame-
ꢁ1
ter; [c] The most frequent BJH desorption pore diameter; [d] BJH desorption average pore diameter (4 VA );
e] Initial activity of DPA consumption; [f] Z-DPE selectivity at 90% DPA conversion.
(Figure 3). The Pdsolv/CeO cata-
2
[
lyst was also studied, but be-
cause Ce is heavier than Pd, the
contrast was not high enough
ever, the amount of adsorbed nitrogen on these catalysts was
different (Figure 2a) because specific surface area of the
to identify Pd nanoparticles. However, because of a higher Pd
content and a much lower surface area of the ceria support,
Pdsolv/CeO catalyst was almost 2 orders of magnitude lower
the diameter of the obtained nanoparticles in Pdsolv/CeO was
2
2
than that of the Pdsolv/boehmite catalyst (Table 1). Similarly, Bar-
rett–Joyner–Halenda (BJH) desorption pore distribution (Fig-
ure 2b) showed that Pdsolv/CeO2 was practically non-porous
whereas Pdsolv/boehmite was mesoporous. Hence, ceria parti-
cles had only external surface area available for the catalyst,
expected to be much higher than that in the Pdsolv catalyst.
The concentration profiles of DPA hydrogenation on Pdsolv
/
CeO and Pdsolv/boehmite catalysts are shown in Figure 4. The
2
profiles are typical of Langmuir–Hinshelwood hydrogenation
reactions with quasi-zero-order kinetics during the initial
stages. The performance of the catalysts was similar: full con-
version of the same amount of DPA required approximately
300 min with the highest Z-1,2-diphenylethene (Z-DPE) selec-
tivity of approximately 75%. However, considering that the Pd
content in the Pdsolv/CeO2 catalyst is double (Table 1), the
Pdsolv/boehmite catalyst was more active per mole of Pd.
Effect of the reducing agent and surfactant with US-assisted
dispersion
Figure 3. TEM image of the Pdsolv/boehmite catalyst. Energy-dispersive X-ray
spectrometry analysis confirmed that dark areas are Pd nanoparticles.
With the aim to further increase the activity and improve the
selectivity toward Z-DPE, a Pd /boehmite catalyst was ob-
LV
tained with Luviquatꢁ that acts as both a reducing and a cap-
[45–47]
ping agent.
A representative TEM image of the Pd /boehmite catalyst
LV
with Pd nanoparticles of diameter (3.0ꢀ1.1) nm is shown in
Figure 5. In comparison to the surfactant-free Pd /boehmite
solv
catalyst, the diameter of Pd nanoparticles significantly de-
creased and the particles were uniformly distributed on the
catalyst surface.
Interestingly, the total BET surface area had a twofold de-
crease in comparison to the surfactant-free Pdsolv/boehmite cat-
alyst (Table 1). The pore distribution had also changed
(
Figure 6): the proportion of the pores smaller than 2 nm (mi-
Figure 4. Concentration profile of the hydrogenation of DPA on
a) Pdsolv/boehmite and b) Pdsolv/CeO
, E-DPE; ^, diphenylethane.
2
catalysts. *, DPA; !, Z-DPE;
~
LV
Figure 5. TEM image of the Pd /boehmite catalyst.
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erties of the catalyst obtained were identical to those of the
Pd /boehmite catalyst (Table 1 and Figure 6). However, the
LV
higher Pd content in PdLV-1/boehmite indicates that sonication
facilitated Pd deposition on boehmite (Table 1).
According to TEM data (Figure 8), the average diameter of
Pd nanoparticles obtained was (2.4ꢀ0.7) nm, that is, smaller
than that of the Pd /boehmite catalyst despite the higher Pd
LV
loading, likely owing to the stabilization of the nanoparticles
with Luviquatꢁ and the effect of sonication in the one-pot
method.
Figure 6. BJH desorption pore distribution of PdLV/boehmite,
PdLV-1/boehmite, PdUS-MW/boehmite catalysts.
cropores) and the pores larger than 4 nm decreased. These ef-
fects can be attributed to the strong adsorption of the surfac-
tant molecules in the catalyst pores or to restructuring of the
[48,49]
boehmite structure directed by the surfactant molecules.
As expected, the Pd /boehmite catalyst demonstrated much
LV
Figure 8. TEM image of the PdLV-1/boehmite catalyst.
higher activity than the Pdsolv/boehmite catalyst, although the
Pd content in Pd /boehmite was much lower than that in
LV
Pdsolv/boehmite (Table 1); full conversion of DPA occurred
within 80 min (Figure 7).
The concentration profiles of DPA hydrogenation shown in
Figure 9 correlate with the decreased Pd particle size: activity
and selectivity of the catalyst were higher (Table 1). These data
are in excellent agreement with the smaller diameter of Pd
nanoparticles observed by using TEM owing to a higher selec-
[52]
tivity of edge sites toward the internal Z-alkene formation.
Figure 7. Concentration profile of the hydrogenation of DPA on the
PdLV/boehmite catalyst. *, DPA; !, Z-DPE; ~, E-DPE; ^, diphenylethane.
Such a drastic increase in activity corresponds with the TEM
data, because much smaller Pd nanoparticles found in the
Figure 9. Concentration profile of the hydrogenation of DPA on the
PdLVꢁ1/boehmite catalyst. *, DPA; !, Z-DPE; ~, E-DPE; ^, diphenylethane.
Pd /boehmite catalyst expose more active surface Pd sites.
LV
Moreover, Z-DPE selectivity of the Pd /boehmite catalyst at full
LV
Effect of US-assisted dispersion and MW-assisted reduction
on Pd/boehmite preparation
DPA conversion increased from 81 to 87% for the Pdsolv
/
boehmite catalyst. This effect can also be explained, consider-
ing smaller Pd particles. As the diameter of the nanoparticle
decreases, a higher fraction of Pd atoms occupies step
As the diameter of Pd nanoparticles decreased on ultrasonic
synthesis of the PdLV-1/boehmite catalyst, a similar but surfac-
tant-free synthesis was tried. The PdUS-MW/boehmite catalyst
was obtained through dispersion of palladium(II) acetate in
water by sonication and then reduction in the hydrogen at-
mosphere by MW heating. The catalyst demonstrated surface
and porosity properties identical to those of the
Pdsolv/boehmite catalyst obtained with the reduction by sol-
vent of palladium(II) acetate (Table 1). However, the Pd content
of the PdUS-MW/boehmite catalyst was higher than that of the
Pdsolv/boehmite catalyst, likely owing to enhanced Pd reduc-
tion by US and MW treatments. The pore diameter distribution
of PdUS-MW/boehmite and Pdsolv/boehmite catalysts were also
[
50,51]
sites
while these step sites provide higher alkene selectivi-
ty owing to the suppression of undesirable full hydrogenation
[
52]
reactions.
Effect of US in a one-pot method
To simplify the preparation process and improve the hydroge-
nation activity and selectivity towards Z-DPE, a PdLV-1/boehmite
catalyst was prepared by using a one-pot method, which dis-
perses palladium(II) acetate and boehmite using sonication si-
multaneously in the presence of Luviquatꢁ. The textural prop-
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identical (Figure 6); that is, MW treatment did not affect the
properties of the boehmite support.
Kinetic modeling: Comparison with the Lindlar catalyst
Effect of mass transfer on reaction rates
The PdUS-MW/boehmite catalyst contains Pd nanoparticles
that form clusters of approximately 100 nm in diameter
The Weisz–Prater criterion is widely used to ensure that a cata-
lytic reaction is not diffusion limited. If the Weisz–Prater
number is below 6 for zero-order reactions for every major re-
action and product component, it proves that the reaction is
(Figure 10). The formation of the agglomerates indicates that
US-assisted dispersion and MW-assisted reduction cannot pre-
vent agglomeration of the forming nanoparticles without
a capping agent.
[53,54]
not diffusion limited.
The Weisz–Prater number is calculat-
ed by using Equation (1):
2
p
<
R
N
¼
ð1Þ
WꢁP
CD
eff
ꢁ
3
ꢁ1
in which < is the effective reaction rate (molm s ), R is
p
the radius of the catalyst particle (m), C is the concentration of
ꢁ
3
a reactant or a product (molm ), and D is the effective diffu-
eff
2
ꢁ1
sion coefficient of a reactant or a product (m s ).
To ensure that mass transfer limitations did not apply for all
catalytic systems studied, the upper boundary estimation of
the Weisz–Prater number [Eq. (1)] was performed by using the
Figure 10. TEM image of the PdUS-MW/boehmite catalyst.
ꢁ
3
ꢁ1
The concentration profiles of DPA hydrogenation on the
PdUS-MW/boehmite catalyst are shown in Figure 11. US-assisted
dispersion and MW-assisted reduction with the PdUS-MW/boehm-
ite catalyst were shown to result in enhanced activity and in-
largest possible values for < and R : <=0.02 molm
s
ob-
P
ꢁ
4
served for the Pd /boehmite catalyst and R =10 m, the
LV-1
p
largest catalyst particle size observed by using SEM (Fig-
ure 1b).
The bulk diffusion coefficients for hydrogen and DPA were
estimated by using the methods described in detail by Van-
[54]
nice. The resulting coefficients and the Weisz–Prater num-
bers for these molecules presented were 0.004 for hydrogen
and 0.036 for DPA, which were several orders of magnitude
lower than the boundary value of 6; hence, kinetic modeling
was performed considering only intrinsic kinetics rather than
mass transfer phenomena.
Kinetic modeling
Figure 11. Concentration profile of the hydrogenation of DPA on the
PdUS-MW/boehmite catalyst. *, DPA; !, Z-DPE; ~, E-DPE; ^, diphenylethane.
Hydrogenation kinetics of semi-hydrogenation reactions are
modeled by considering the sequential reduction reactions of
alkynes to alkenes, followed by alkene to alkane hydrogena-
tion. In addition, a number of authors have considered and
modeled a direct route of full hydrogenation of alkynes to al-
kanes, which is independent of the hydrogenation sequence
creased selectivity toward Z-DPE without a capping agent in
comparison to the Pdsolv/boehmite catalyst (Figure 4a). Full
DPA conversion for the same amount of DPA reduced from
300 to 170 min for the PdUS-MW/boehmite catalyst in compari-
[13,55–57]
son to the Pdsolv/boehmite catalyst, whereas in the DPA conver-
sion range of 85–95%, Z-DPE selectivity was approximately 5%
higher for the PdUS-MW/boehmite catalyst in comparison to the
Pdsolv/boehmite catalyst. However, Z-DPE selectivity was close
and may occur even at low alkene concentrations.
We
believe that this is mechanistically unlikely; however, to ensure
that a possible hydrogenation pathway is not ignored and to
enable direct comparison with previous works, we have also
included a term for the direct hydrogenation of the alkyne to
the alkane in our kinetic model. In the case of DPA, alkenes
can be formed in their either Z or E geometries; hence, the full
reaction scheme consists of 5 possible stages, as shown in
Scheme 1.
for PdUS-MW/boehmite, PdLV-1/boehmite, Pd /boehmite catalysts;
LV
thus, it may be concluded that individual Pd nanoparticles
formed during MW/US-assisted synthesis were similar to those
obtained with the capping agent. In the case of PdLV-1/boehm-
ite and Pd /boehmite catalysts, small Pd nuclei formed were
LV
stabilized by the capping agent, which gave rise to small Pd
nanoparticles. In the case of the PdUS-MW/boehmite catalyst,
quick and uniform heating with MW led to simultaneous nu-
cleation of Pd nanoparticles. However, owing to the lack of
a capping agent, the formed nanoparticles quickly agglomerat-
ed (Figure 10), still retaining the large number of surface step
sites that can explain high DPE selectivity.
Each of the stages is, in turn, described by the Langmuir–
Hinshelwood mechanism that consists of quasi-equilibrium ad-
sorption of hydrogen and organic species, and the rate-limiting
step of the addition of the hydrogen species followed by de-
[58,59]
sorption.
A full derivation of the model is presented in the
Supporting Information. Briefly, the model consists of a system
of 5 ordinary differential equations that includes 7 parameters:
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Scheme 1. DPA hydrogenation reactions.
Figure 12. Concentration profile of the hydrogenation of DPA on the Lindlar
catalyst. *, DPA; !, Z-DPE; ~, E-DPE; ^, diphenylethane.
5
apparent rate constants (Scheme 1) and 2 relative adsorption
constants, Q =KDPE/KDPA and Q =KDiphenylethane/KDPA. The system
1
2
of differential equations was integrated by using a program
written in MATLAB with a fourth-order Runge–Kutta method,
and the model parameters were optimized by using the Leven-
berg–Marquardt algorithm to fit the experimental data. The
objective function for minimization was calculated as a statistic
weighted squared residual by using Equation (2):
Table 2. Comparison of the kinetic parameters of the Lindlar and
PdLVꢁ1/boehmite catalysts.
[
a]
[
b]
[c]
Parameter
PdLV-1/boehmite
Lindlar
Ratio
ꢁ1 ꢁ1
[
s g ]
cat
k
k
k
k
k22
Q
Q
11
21
3
174ꢀ5
131ꢀ3
1.33ꢀ0.07
2.64ꢀ0.62
2.42ꢀ1.47
2.37ꢀ0.42
14.6ꢀ13.75
1.07ꢀ0.13
1.42ꢀ0.39
11 ꢀ1
4.3ꢀ0.5
2.9ꢀ1.1
1.7ꢀ0.2
1.8ꢀ1.7
0.37ꢀ0.02
0.26ꢀ0.05
5.4ꢀ1.7
4.0ꢀ0.3
4.3ꢀ1.3
0.39ꢀ0.02
0.35ꢀ0.03
X
ꢀ
ꢁ
1
12
2
S ¼
w C
ꢁ Cmodeled;i
;
w ¼
ð2Þ
i
exp erimental;i
i
sexperimental;i
[
d]
1
[
d]
2
in which Cexperimental,i and Cmodeled,i are experimental and calcu-
[a] 90% confidence intervals were obtained by using the Monte Carlo
[
60]
lated concentrations of components, respectively, and w are
error analysis; [b] Apparent rate constant corresponding to Scheme 1;
c] The ratio of the corresponding kinetic parameters of PdLV-1/boehmite
and Lindlar catalysts; [d] Q and Q are the relative adsorption constants
i
[
statistical weights calculated by using experimental uncertain-
1
2
ties (s
) of 2% of the experimental concentration or
experimental,i
of alkenes to alkynes and of alkanes to alkynes, respectively. See the Sup-
porting Information for a full model description.
ꢁ3
0.3 molm , whichever is larger.
Confidence intervals of the model parameters were deter-
[
60]
mined by using the Monte Carlo method: 500 sets of the ini-
tial data normally distributed with the standard deviations
However, other rate constant ratios were approximately 2.5,
which means that all undesired reactions such as full hydroge-
nation and Z/E isomerization were faster (even in relative
(
sexperimental,i) were generated, and kinetic parameters were ob-
tained by fitting the experimental data. Resulting uncertainties
were calculated as 90% confidence intervals.
t
e
r
m
s
)
w
i
t
h
t
h
e
P
d
/
b
o
e
h
m
i
t
e
c
a
t
a
l
y
s
t
.
T
h
e
r
a
t
e
c
o
n
s
t
a
n
t
k
*
L
V
-
1
2
2
of E-DPE hydrogenation demonstrated wide confidence inter-
vals because of high errors of E-DPE determination due to its
low concentration in the reaction mixture. The relative adsorp-
Comparison of PdLV-1/boehmite with the Lindlar catalyst
tion constant Q that shows poisoning of the catalytic surface
1
The Lindlar catalyst has been an industrial standard for semi-
hydrogenation reactions for more than 50 years; therefore, the
PdLV-1/boehmite catalyst was compared with it. To obtain quan-
titative information on the reaction stages, DPA was hydrogen-
ated with the Lindlar catalyst and kinetic modeling was per-
formed by using the relative constants of the Langmuir–Hin-
shelwood model. The results of this model were in excellent
agreement with the experimental data of the Lindlar and
PdLV-1/boehmite catalysts: lines in Figures 9 and 12 represent
modeled concentration profiles, whereas dots represent experi-
mental values.
with the alkene was similar for both systems, whereas constant
Q is higher for the PdLV-1/boehmite catalyst; that is, alkane
2
molecules adsorb strongly on the Pd surface rather than on
Pb-doped Pd. As a result, the maximum alkene selectivity of
the Lindlar catalyst is higher than that of the PdLV-1/boehmite
catalyst (91 vs 85% at 95% DPA conversion). Hence, the data
demonstrate that lead in the Lindlar catalyst does not signifi-
cantly change the ratio of alkene to alkyne adsorption con-
stants while possibly changing the absolute values of the con-
stants. The most important role of lead is usually explained by
the significant change in the adsorption energies of intermedi-
[61]
The obtained model parameters are given in Table 2. The ap-
parent rate constant k* of the hydrogenation of DPA into Z-
ate reaction species. However, the similar relative adsorption
constants obtained in the present study indicate that this ther-
modynamic explanation does not apply to the studied DPA hy-
drogenation reaction. In contrast, lower apparent reaction
rates observed for the Lindlar catalyst imply that the active site
poisoning plays a more important role in enhancing the semi-
11
DPE indicates that the PdLV-1/boehmite catalyst provided
.3 times higher DPA hydrogenation rate than the Lindlar cata-
lyst per unit of the catalyst mass. Considering more than five
times higher Pd content in the Lindlar catalyst, the PdLV-1
1
/
[52]
boehmite catalyst demonstrated a significant increase in activi-
ty.
hydrogenation selectivity.
ChemCatChem 2015, 7, 952 – 959
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A Luviquatꢁ-free synthesis with the MW-assisted reduction with
Conclusions
hydrogen produced the PdUS-MW catalyst. Pd(OAc) (30 mg) was sus-
2
pended in water (10 mL) and sonicated in a cup-horn apparatus
The effect of sonication, the capping agent (Luviquatꢁ), and
the reduction by microwave heating on the Pd catalyst prepa-
ration has been studied. The combination of the capping
agent and sonication enabled us to obtain monodisperse
(cavitation tube, 19.9 kHz, 100 W, Danacamerini sa.) at 308C to
obtain a dispersion, which was then heated using MW under hy-
drogen (1 MPa, Sapio, grade 4.5) with a SynthWAVE reactor (MLS
GmbH, Milestone Srl) at 408C and 300 W MW power. After cooling
the mixture, the obtained black dispersion was added dropwise to
a suspension of boehmite (1.5 g) in water (10 mL). The mixture was
stirred overnight at RT, and then, the solid catalyst was filtered,
washed with water, and dried under vacuum.
(2.4ꢀ0.7) nm Pd nanoparticles supported on boehmite during
the one-pot synthesis. The catalyst was approximately 7 times
as active as a commercial Lindlar catalyst per unit of Pd mass
taken and only 6% (at 95% conversion) less selective toward
semi-hydrogenation. Kinetic modeling performed indicates
that the improved selectivity of the Lindlar catalyst is attribut-
ed to the active site poisoning rather than to thermodynamic
effects.
A commercial Lindlar catalyst was purchased from Alfa Aesar and
used as a reference catalyst without the addition of quinoline.
Characterization
The microwave-assisted synthesis of the PdUS-MW/boehmite
catalyst demonstrated that although large agglomerates of Pd
nanoparticles were formed, semi-hydrogenation selectivity was
significantly higher than that of the conventionally heated
Pdsolv/boehmite catalyst. This difference suggests that the cata-
lyst contains many step Pd sites, like the catalysts containing
smaller nanoparticles.
BET measurements
Surface areas and pore distributions were measured by using nitro-
gen physisorption with a Micromeritics TriStar 3000 surface area
and porosity analyzer using standard multipoint BET analysis and
BJH pore distribution methods. All specimens were dried at 1408C
for 4 h under nitrogen flow before the measurements.
SEM analysis
Experimental Section
The catalysts were applied on a conductive carbon adhesive tape
and coated with a thin layer of carbon to prevent charging. The
SEM analysis of the samples obtained was performed with the
Zeiss EVO 60 instrument equipped with an Oxford Instruments
INCA 350 energy-dispersive X-ray spectrometer system under the
Catalyst synthesis
Pdsolv/boehmite and Pdsolv/CeO2 catalysts were obtained through
US-assisted dispersion and reduction by solvent on boehmite and
ceria supports, respectively. Palladium(II) acetate (20 mg, Pd(OAc)2,
Alfa Aesar, 99%) was suspended in ethylene glycol (20 mL, Alfa
Aesar, 99%) by sonication, which produced an orange salt suspen-
sion in the transparent solvent. The suspension was sonicated with
a titanium immersion horn (21.1 kHz, 100–150 W) at approximately
ꢁ2
pressure of 10 Pa and electron acceleration voltage of 20 kV.
TEM analysis
1
008C, which became a homogeneous black dispersion of Pd
The catalyst samples were dispersed in ethanol under sonication
and applied on polymer-coated copper grids. The TEM analysis of
the grids was performed with a JEOL JEM-2010 high-resolution
transmission electron microscope equipped with an Oxford Instru-
ments energy-dispersive X-ray microanalysis system.
nanoparticles. The dispersion obtained was added dropwise to
a stirring suspension of support (1 g) in ethylene glycol (10 mL).
The mixture was stirred overnight at room temperature (RT), and
then the solid was filtered, washed with methanol, and dried
under vacuum.
The Pd /boehmite catalyst was obtained by the reduction of
LV
Inductively coupled plasma analysis
Pd(OAc) with Luviquatꢁ (Sigma–Aldrich, 30% in water). Pd(OAc)
2
2
(
(
30 mg) was added to an aqueous solution of Luviquatꢁ
0.0365 mm), followed by sonication in a cup-horn apparatus (cavi-
The Pd content was determined with the PerkinElmer Optima 5300
DV inductively coupled plasma optical emission spectrometer. The
tation tube, 19.9 kHz, 100 W) at 308C to obtain a pale orange sus-
pension. The suspension was heated in a silicon oil bath at 808C,
samples were dissolved in the solution of HF/HCl/HNO in 1:1:3
3
ratio and heated at 2008C for 10 min with a CEM MARSXpress Plus
MW digestion system. After cooling the samples, the saturated
aqueous solution of boric acid was added to complex excess HF;
then, the vessels were heated again at 1808C for 10 min. The solu-
tions were diluted with water and analyzed.
and Pd(OAc) was reduced with Luviquatꢁ. The obtained homoge-
2
neous black dispersion was added dropwise to a stirring suspen-
sion of boehmite (1.5 g) in water (10 mL). The mixture was stirred
overnight at RT, and then the solid was filtered, washed with
water, and dried under vacuum.
A one-pot synthesis of the PdLV-I/boehmite catalyst was performed
by combining the dispersion with Luviquatꢁ and deposition on to
Catalyst testing
ꢁ1
the support under sonication. A mixture of Pd(OAc) (30 mg) and
The catalyst (6.0 mg) was added to a DPA (12.0 mL, 56 mmolL ,
Alfa Aesar, 98%) solution in hexane (Sigma–Aldrich, 97%). Before
hydrogenation, the reactor (glass vacuum desiccator containing 4
pieces of 48 mL polypropylene cylindrical centrifuge tubes) was
evacuated and filled with nitrogen gas (Sapio, grade 6.0) to
remove air followed by evacuation and filling with hydrogen gas
(Sapio, grade 4.5). The reaction was performed at RT [(23ꢀ1)8C]
2
boehmite (1.5 g) was dispersed in the aqueous solution of Luvi-
quatꢁ (20 mL, 0.0365 mm) in a cup-horn apparatus (cavitation
tube, 19.9 kHz, 100 W) at 308C. Then, the reduction and impregna-
tion was performed in an oil bath at 808C for 2 h. The mixture was
stirred overnight at RT, and then the solid was filtered, washed
with water, and dried under vacuum.
ChemCatChem 2015, 7, 952 – 959
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958
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
with stirring rates of 600 rpm and atmospheric hydrogen pressure
hydrogen balloon). Aliquots (100 mL) of the solution were periodi-
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim