Selective hydrogenation of 2-methyl-3-butyn-2-ol over Pd-nanoparticles stabilized in hypercrosslinked polystyrene: Solvent effect

Article abstract of DOI:10.1016/j.cattod.2014.01.045

Selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-butene-2-ol (MBE) over Pd supported on hypercrosslinked polystyrene was studied in polar (ethanol, isopropanol, water) and non-polar (cyclohexane, toluene, octane, hexane, heptane and m-xylene) solvents. The catalytic activity and selectivity were found to be strongly affected by solvent properties such as dipole moment and dielectric constant, but cannot be explained by solvent polarity only. Hydrogen solubility and solvent-catalyst interaction also influential factors. The catalyst activity decreases in the series: alcohols > cyclohexane > water/ethanol mixture > octane ≥ hexane ≥ xylene > toluene > heptane. The highest values of MBE selectivity of 99.6% and 98.7% at 95% MBY conversion were obtained in toluene and in ethanol, respectively.

Full text of DOI:10.1016/j.cattod.2014.01.045

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Selective hydrogenation of 2-methyl-3-butyn-2-ol over
Pd-nanoparticles stabilized in hypercrosslinked polystyrene: Solvent
effect
Linda Nikoshvilia,∗, Elena Shimanskayaa, Alexey Bykova,
Igor Yuranovb, Lioubov Kiwi-Minskerb, Esther Sulmana
a Department of Biotechnology and Chemistry, Tver Technical University, Tver 170026, Russian Federation
b Group of Catalytic Reaction Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-butene-2-ol (MBE) over Pd
supported on hypercrosslinked polystyrene was studied in polar (ethanol, isopropanol, water) and
non-polar (cyclohexane, toluene, octane, hexane, heptane and m-xylene) solvents. The catalytic activ-
ity and selectivity were found to be strongly affected by solvent properties such as dipole moment
and dielectric constant, but cannot be explained by solvent polarity only. Hydrogen solubility and
solvent–catalyst interaction also influential factors. The catalyst activity decreases in the series: alco-
hols > cyclohexane > water/ethanol mixture > octane ≥ hexane ≥ xylene > toluene > heptane. The highest
values of MBE selectivity of 99.6% and 98.7% at 95% MBY conversion were obtained in toluene and in
ethanol, respectively.
Received 27 September 2013
Received in revised form 13 January 2014
Accepted 24 January 2014
Available online xxx
Keywords:
Heterogeneous catalytic hydrogenation
Solvent effect
Palladium
© 2014 Elsevier B.V. All rights reserved.
Hypercrosslinked polystyrene
1. Introduction
and cyclohexane as solvents. It was shown that in the case of a
positive interaction, the strength of the reagent adsorption on cat-
Selective carbon–carbon triple bond alkynol hydrogenation is
one of the important reactions in manufacturing of fine chemicals.
It is usually carried out in a liquid phase using catalysts containing
noble (Pd, Pt, Rh) or transition (Ni) metals [1]. The catalyst activity
and selectivity are known to be strongly influenced by a solvent. The
choice of a suitable solvent requires understanding of relationships
between the solvent nature and the interactions with participation
of the solvent that take place in three phase catalytic hydrogenation
system [1].
The problem of choosing a suitable solvent for catalytic hydro-
genation of various organic compounds has been considered by
many researchers. As a result, several factors determining the influ-
ence of the solvent on hydrogenation process were identified [2,3].
The first factor is a substrate–solvent–catalyst interaction. For
example, Rajadhyaksha and Karwa [3] studied the reaction of
selective hydrogenation of 2-nitrotoluene in the presence of the
2 wt% Pd/C catalyst using different alcohols, benzene, n-hexane,
alytically active centers decreases, while in the case of a negative
interaction the increase of adsorption is observed causing the cor-
responding decrease of selectivity. Augustine et al. [4] investigated
hydrogenation of alkenes over Pt- and Pd-containing catalysts in
alcohols. MeOH and EtOH were found to compete with hydrogen
and to adsorb on the Pd surface, resulting in occupying the catalytic
active sites, while i-PrOH enters into the reaction as a hydrogen
donor. Toukoniitty et al. [5] found that hydrogenation rate depends
on the ability of solvent to interact with substrate and to compete
with other reagents while present on catalytic surface.
The second factor is a solvent polarity. The attempts were
undertaken to explain the differences in catalyst activities con-
sidering correlations between the reaction rate and traditional
solvent parameters such as a dipole moment (ꢀ) and a dielectric
constant (ε) [6]. Drelinkiewicza et al. [7] studied hydrogenation
of acetophenone (AP) on polymer supported Pd catalysts using
different alcohols and cyclohexane as solvents. The hydrogenation
rate was found to be higher in alcohol solutions in comparison to
cyclohexane. It was proposed that the catalyst activity is rather
influenced by a solvent polarity than H2 solubility. Enantioselective
hydrogenation of AP on chiral Ru-based catalysts using different
solvents was investigated by Cheng et al. [8]. Hydrogenation
∗
Corresponding author. Tel.: +7 4822449317; fax: +7 4822449317.
E-mail addresses: nlinda@science.tver.ru (L. Nikoshvili),
lioubov.kiwi-minsker@epfl.ch (L. Kiwi-Minsker).
http://dx.doi.org/10.1016/j.cattod.2014.01.045
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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in hypercrosslinked polystyrene: Solvent effect, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.045

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was revealed to proceed fast in C1–C4 alcohols in a contrast to
aprotic polar and non-polar solvents. Mukherjee and Vannice [9]
studied hydrogenation of citral on Pt/SiO2 using eight solvents
belonging to different chemical groups in order to find a rela-
tionship between the catalyst activity and dielectric constants
and dipole moments of the chosen solvents. Hajek et al. [10]
used 12 different solvents in hydrogenation of cinnamaldehyde
on Ru/Y zeolite and obtained a satisfactory correlation between
the solvent polarity and hydrogenation rate, but only in the
case of polar solvents. For hydrogenation of benzyl alcohol over
Ru/Al2O3 and Pt/Al2O3 catalysts, the hydrogenation rate was
found to be in a good agreement with ı (defined as a difference
between the donor and acceptor numbers) [11]. It was deter-
mined that both non-polar solvents and polar solvents having
negative ı, such as MeOH, EtOH, and acetic acid, had no influence
on hydrogenation of benzyl alcohol over Ru/Al2O3. However,
polar solvents with positive ı, such as acetone, tetrahydrofuran,
1,4-dioxane, and diethyl ether, inhibited the reaction. Besides,
alcohols were found to strongly inhibit the activity of the Pt/Al2O3
catalyst [11]. The selectivity and reaction rate of Pd catalyzed
hydrogenation of ketoisophorone were shown to be strongly
dependent on the solvent polarity [12]. Investigated solvents
formed a certain series according to their influence on catalytic
active phase, i.e., metal nanoparticles are inclined to aggregation
and leaching [17–19]. Besides, they often require additional mod-
ifiers [17] that results in pollution of target products. The use of
nanostructured polymers of different types as supports for catalytic
species allows formation of microheterogeneous catalysts, which
combine the advantages of both homo- and heterogeneous systems
and are suitable for various catalytic applications.
In this paper, we propose hypercrosslinked polystyrene (HPS) as
a prospective polymeric support for hydrogenation catalyst. Due
to its extremely high surface area and unique porous structure
[20,21], HPS allows control over incorporation of various metal-
containing compounds and also control nucleation and growth of
metal nanoparticles within the polymer matrix. This allows one to
prevent metal leaching. However, in spite of numerous studies of
HPS as a catalyst support in different oxidation and hydrogenation
reactions [21–26], none of them explored the solvent effect. In this
paper we are assessing the influence of both polar and non-polar
solvents and correlating the solvent characteristics (ꢀ, ε) and cat-
alytic performance (activity/selectivity) of Pd supported on HPS in
catalytic hydrogenation of a triple bond in MBY to a double one in
2-methyl-3-butene-2-ol (MBE).
2. Materials and methods
activity
(toluene = acetonitrile ≤ cyclohexane = butanol < i-
PrOH < EtOH < MeOH)
and selectivity
2.1. Materials
(butanol < cyclohexane < toluene = i-PrOH < EtOH < MeOH). Masson
et al. [13] studied liquid-phase selective hydrogenation of AP
on Ni-Raney catalyst using C1–C3 alcohols and cyclohexane as
solvents. Hydrogenation rate was found to increase linearly with
the increase of the dielectric constant of alcohols. At the same
time, the reaction rate was higher for cyclohexane (non-polar
solvent) than for PrOH (polar solvent), but lower than for i-PrOH,
which did not allow a complete correlation between the catalytic
activity and solvent polarity. Bertero et al. [2] also investigated AP
hydrogenation over Ni-containing catalyst (Ni/SiO2) using a wide
range of solvents and found that for protic solvents, the activity
pattern was: i-PrOH > PrOH > EtOH ꢀ MeOH. Besides, the highest
selectivity was observed in protic solvents in comparison with
aprotic polar and non-polar ones. Hydrogen-bond donor ability (˛)
as well as solvent–catalyst interaction was suggested to influence
the dependence of catalytic activity and selectivity on solvent
nature.
Before the use, HPS Macronet MN270 (Purolite Int., United King-
dom) and HPS Optipore, OP, (Sigma-Aldrich, Switzerland) were
washed with water and acetone and dried under vacuum [21].
Ethanol (EtOH, >99.5%), m-xylene (>98%), hexane (>99%), hep-
tane ((>99%), MBY (>99%), MBE (>97%), 2-methyl-2-butanol (MBA,
>96%) were obtained from Fluka (Switzerland). Metal precur-
sor (Pd(CH3COO)2, >99%), cyclohexane (>99%), acetone (>99.9%)
were obtained from Sigma-Aldrich (Switzerland). n-Octane (>99%),
dodecane (>99%), 1-butanol (>99.5%) were obtained from Acros
Organics (Belgium). 2-Propanol (i-PrOH, >99%) was obtained from
Merck KGaA (Germany). Toluene (>99%) was obtained from AnalaR
NORMAPUR (Ireland). All chemicals were used as received. Distilled
water was purified with an Elsi-Aqua water purification system.
2.2. Catalysts synthesis
The last factor determining the solvent influence on hydrogena-
tion reactions is the solubility of hydrogen. The solvent influence via
thermodynamic interactions was proposed by many researchers.
Favorable thermodynamic interactions between the solvent and
reactants were found to allow reducing the adsorption of reactants
on the catalyst [14]. Rautanen et al. [15] revealed that different
hydrogenation rates can be achieved depending on the hydro-
gen solubility: catalytic activity was the same for isooctane and
n-heptane, while it was lower for cyclohexane, especially at high
temperatures [15].
The influence of the solvent nature on hydrogenation of 2-
methyl-3-butyn-2-ol (MBY) was studied by Zakumbaeva et al. [16].
It was shown that solvents affect not only the mechanism, but also
the enthalpy of hydrogen adsorption on the catalyst surface, which
causes a difference in the catalyst activity and selectivity.
Thus, numerous studies showed the influence of solvent on
the catalyst performance in selective hydrogenation reactions.
However, these studies were mainly carried out on conventional
heterogeneous catalysts where noble metals were deposited on
carbon or metal oxide supports. As a rule, such catalysts have high
metal loading (from 2 up to 10 wt.%) and show rather good corre-
lation between the hydrogenation rate and solvent parameters.
It is noteworthy that heterogeneous catalysts based on inorganic
or carbon supports do not usually provide stability of catalytically
0.2 wt.% Pd/MN270 catalyst was synthesized according to the
procedure described elsewhere [21] upon variation of the HPS pow-
dering degree: the granules with the size of <47 ␮m and <63 ␮m
as well as unpowdered HPS (0.2–1 mm) were investigated [21]. In
a typical experiment, 3 g of pretreated, dried and crushed poly-
mer granules were impregnated with 8.5 mL of a Pd(CH3COO)2 THF
solution of a certain concentration. The Pd-containing polymer was
dried at 70 ◦C, treated with Na2CO3, washed with distilled water till
neutral pH, and dried again. Then the resulting catalyst was reduced
by H2 at 300 ◦C for 2 h.
The Pd/OP catalyst with 0.5 wt.% of Pd was synthesized using the
same impregnation procedure. The resulting Pd-containing poly-
mer was dried at room temperature and reduced by H2 at 300 ◦C
for 2 h.
The catalyst compositions were confirmed by atomic absorption
spectroscopy (AAS) analysis.
2.3. MBY hydrogenation
Hydrogenation experiments were carried out in stainless
steel semi-batch reactor (150 mL autoclave, Buchi AG, Uster,
Switzerland) equipped with a heating jacket, a hydrogen supply
system, a 8-blade disk turbine impeller and a pressure controlled H2
Please cite this article in press as: L. Nikoshvili, et al., Selective hydrogenation of 2-methyl-3-butyn-2-ol over Pd-nanoparticles stabilized
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3
+H2
Cat
supply system. At the beginning of each experiment, an MBY solu-
tion (CMBY = 75 g/L, Vtot = 80 mL, CST = 63 g/L (dodecane or 1-butanol
(in the case of water/EtOH mixture) were used as a standard for
chromatography)) was charged into the reactor and flushed three
times with N2 under stirring. The temperature was set and allowed
to stabilize (ca. 30 min). H2 was then introduced and the sys-
tem was pressurized (time t = 0 for reaction). In a series of blank
tests, reactions carried out in the absence of a catalyst did not
show any measurable conversion. The consumption of hydrogen
was monitored using pressflow gas controller (BPC-6002) (Buchi,
Switzerland).
Experiments were carried out at 3 bar of hydrogen partial pres-
sure at various reaction temperatures (313 K or 333 K) and stirring
rates (from 500 up to 2000 rpm). Both polar (EtOH, i-PrOH, H2O)
and non-polar (alkanes: hexane, cyclohexane, heptane, octane and
aromatic hydrocanbons: toluene, xylene) solvents were used. It is
noteworthy that the total pressure in the reactor was varied accord-
ing to the achieved solvent vapor pressure to maintain the equal
hydrogen partial pressure in each experiment.
+H2
Cat
+H2
Cat
OH
OH
OH
2-methyl-3-butyn-2-ol
(MBY)
2-methyl-3-buten-2-ol
(MBE, target product)
2-methylbutan-2-ol
(MBA, side product)
Scheme 1. Reaction pathway of MBY hydrogenation.
1,0
0,8
0,6
0,4
0,2
0,0
MBE
MBY
MBA
A non-invasive liquid sampling system via syringe allowed a
controlled removal of aliquots (≤0.3 cm3) from the reactor. The
samples were analyzed by gas chromatography (GC) using a Perkin-
Elmer Auto System XL equipped with a 30-m Stabilwax (Crossbond
Carbowax-PEG, Restek) 0.32-mm capillary column with a 0.25-
␮m coating. The carrier gas (He) pressure was 0.1 MPa. Injector
and flame ionization detector temperatures were 473 K and 523 K,
respectively. The oven temperature was maintained at 323 K for
4 min, and then increased to 473 K at a ramp rate of 30 K/min.
The concentrations of the reaction mixture components Yi (where
i represents MBY, MBE, and MBA) was calculated from the peak
areas assuming similar GC response factors. Selectivity to MBE was
0
10
20
30
40
Time, min
Fig. 1. Kinetic curves of MBY hydrogenation in ethanol over 0.2%Pd/MN270 catalyst
(T = 333 K, PH = 3 bar, stirring–1500 rpm).
2
defined as SMBE = YMBE × X−1 × 100%, where X is the MBY conversion.
Activity was calculated as TOFX = CMBY × CPd × X × 0.01 × ꢁ−1
,
−1
3. Results and discussions
where CMBY and CPd are the molar amounts of MBY and Pd and
ꢁ is the reaction time for achieving the certain MBY conversion X.
Scheme 1 represents the reaction pathway of MBY hydrogena-
tion.
Kinetic curves of MBY hydrogenation obtained in a typical
experiment using EtOH as a solvent are shown in Fig. 1. Clearly, the
time dependence of the MBE accumulation goes through a max-
imum at the MBY conversion of about 85%. A further substrate
conversion results in over-hydrogenation of MBE to MBA.
2.4. Catalyst characterization
Pd nanoparticle size was evaluated by transmission electron
microscopy (TEM) using a JEOL JEM1010 instrument at electron
accelerating voltage of 80 kV. Samples were prepared by embed-
ding the catalyst in epoxy resin with following microtomming at
ambient temperature. Images of the resulting thin sections (ca.
50 nm thick) were collected with the Gatan digital camera and ana-
lyzed with the Adobe Photoshop software package and the ImageJ
software.
BET area was determined using the Sorptomatic 1990 (Carlo
Erba). Prior to analysis, the sample was out gassed at 523 K for 2 h
under vacuum (#14×19#× 10−2 Torr). BET area was obtained by
nitrogen adsorption at 77 K according to the method of Dollimore
and Heal.
3.1. Effect of the support and characterization of the catalysts
A comparative study of the catalysts based on HPS purchased
from different suppliers was carried out using EtOH and toluene as
solvents (Table 1). From the data presented in Table 1 it is seen that
the catalyst based on HPS purchased from Purolite Ltd. (MN270)
provides higher activity and selectivity in the MBY selective hydro-
genation in comparison with that based on OP. Thus all the further
experiments were carried out using 0.2%Pd/MN270 catalyst as the
most promising one.
Pd loading was determined by AAS. The sample was dissolved
in aqua regia at continuous stirring and heating and obtained solu-
tion was analyzed using Shimadzu AA-6650 spectrometer with an
air–acetylene flame.
Chosen HPS supports and the HPS-based catalysts synthesized
were characterized by the low-temperature nitrogen physisorption
and TEM. The BET surface areas (SSA) of both MN270 and OP were
very high: 1373 m2 g−1 and 1065 m2 g−1, respectively. Besides, the
Table 1
MBY hydrogenation (333 K, PH = 3 bar, 1500 rpm) over the HPS-based catalysts.
2
Solvent
Catalyst
Dm, nm
TOF50, mol(MBY)/(mol(Pd)*s)
MBE selectivity, %
Maximum MBE yield, %
at 50% MBY conversion
at 95% MBY conversion
EtOH
0.2%Pd/MN270
0.5%Pd/OP
0.2%Pd/MN270
0.5%Pd/OP
3.7
4.2
3.7
4.2
85.5
45.3
35.0
36.3
94.8
91.0
99.9
99.5
93.2
83.0
99.6
93.5
88.8
76.9
96.4
86.8
Toluenee
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40
30
20
10
0
nitrogen physisorption measurements suggest the presence of both
micro- (<2 nm) and meso-porosity (2–50 nm). The predominant
pores measure 4–6 nm allowing control over the Pd nanoparticle
formation (see also Refs. [20,21])). For the catalyst 0.2%Pd/MN270,
after the first catalytic use, the porosity dramatically changed
with an increase of the fraction of mesopores and the appear-
MN270
0.2%Pd/MN270
ance of macropores (Fig. 2). The BET SSA dropped to 1070 m2 g−1
.
According to the t-plot the decrease of SSA was mainly caused
by the decrease of SSA of micropores (from 1121 m2 g−1 up to
836 m2 g−1), while external SSA changed slightly (from 288 m2 g−1
up to 260 m2 g−1).
The TEM images (Fig. 3) showed that the Pd nanoparticles with
a mean diameter of about 4 nm and fairly narrow size distribution
were formed in both MN270 and OP.
3.2. Influence of stirring rate on MBY hydrogenation
Under 6 6-8
8 - 10 10 - 12 12 - 16 16 - 20 20 - 80 Over 80
D, nm
For MN270-based palladium catalyst, the influence of a stir-
ring rate, which was varied from 500 rpm up to 2000 rpm, on
MBY hydrogenation rate was investigated to exclude external dif-
fusion limitations (Fig. 4). The catalyst (Pd) concentration was
chosen as 7 × 10−6 mol(Pd)/L at the substrate (MBY) concentration
of 0.9 mol/L in all the experiments.
Fig. 2. Pore size distribution for MN270 and Pd-containing sample.
Fig. 3. TEM images and particle size distribution for 0.2%Pd/MN270 (a, b) and 0.5%Pd/OP (c and d) (the samples were taken for TEM analysis after one catalytic use).
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5
120
100
80
100
EtOH
Toluene
80
60
40
60
50% of MBY conversion
80% of MBY conversion
40
95% of MBY conversion
20
0
20
400
600
800
1000
1200
1400
1600
1800
2000
2200
1
2
3
rpm
Powdering degree
Fig. 4. Dependence of TOF of MBY hydrogenation in EtOH using 0.2%Pd/MN270 on
stirring rate (333 K, 3 bar of H2 pressure).
Fig. 6. Dependence of reaction rate (TOF50) on the degree of HPS powdering for
0.2%Pd/MN270: 1–unpowdered MN270 (0.2–1 mm), 2–size of HPS granules less
than 63 ␮m, 3–size of HPS granules less than 47 ␮m.
As it can be seen from Fig. 4, there are no mass transfer lim-
itations above 1000 rpm independently of the degree of MBY
conversion. Thus all further experiments were carried out at
1500 rpm.
the decrease of TOF, which corresponds to the achievement of the
kinetic regime for the chosen catalyst, i.e., all the Pd species partic-
ipate in the reaction (note that external diffusion was excluded at
this stage, see Section 3.2). Thus all the further experiments were
carried out at the MBY concentration of 0.9 mol(MBY)/L.
3.3. Effect of catalyst and MBY loading
3.4. Influence of powdering degree of the catalyst
Before investigation of solvent influence the optimal substrate-
to-catalyst ratio should be found. For the catalyst 0.2%Pd/MN270,
the MBY loading was varied from 0.3 up to 0.9 mol(MBY)/L at
following reaction conditions: 333 K, 3 bar of hydrogen partial pres-
sure, 1500 rpm, catalyst concentration 7 × 10−6 mol(Pd)/L, EtOH
was used as a solvent. The dependence of the catalytic activity on
the MBY initial concentration is presented in Fig. 5.
An increase of the MBY loading from 0.3 up to 0.6 mol(MBY)/L
showed a slight increase of the reaction rate (Fig. 5). Such effect
can be observed for rapid reactions when an insufficient amount of
substrate may result in incomplete participation of catalytic sites.
In this case, the increase of the substrate concentration results
in an apparent increase of TOF, which is most likely due to the
participation of larger number of catalytic sites in the process. A
further increase of the MBY loading up to 0.9 mol(MBY)/L results in
To ensure that for the chosen catalyst (0.2%Pd/MN270) internal
diffusion limitations are excluded, we investigated the influence
of the HPS particle size, which was varied from unpowdered ini-
tial HPS (granules with the diameter of 0.2–1 mm) to < 63 ␮m
and < 47 ␮m. Although HPS is able to swell in any solvent [27], the
swelling may be very different. We tested two solvents, EtOH and
toluene, which completely differ in their properties (see Fig. 6) to
assess the influence of the HPS powdering degree.
The decrease of the HPS particle size from initial granules to
the particles with the size less than 63 ␮m provides increase of
TOF almost by a factor of 1.5 both for EtOH and toluene. Selectivity
also increases from 87.3% up to 94.8% (at 50% of MBY conversion).
Further powdering of HPS up to < 47 ␮m has almost no effect on
catalytic activity and selectivity in accordance with the data pub-
lished by Doluda et al. [28] for lactose hydrogenation over Ru/HPS
catalysts in water. Thus, all further experiments were carried out
using the catalyst based on the HPS granules with the size < 63 ␮m.
106
104
102
100
98
96.3%
3.5. Solvent effect
86.9%
Fig. 7 shows the solvent effect on MBY conversion. For the reader
convenience, we used different designations to curves correspond-
ing to solvents of different groups: dashed lines for polar solvents;
dash-dot lines for aromatic non-polar solvents (xylene and toluene)
and solid lines for other non-polar solvents.
96
94
One can see (Fig. 7) that the rate of the MBY conversion changes
with time, e.g., at the high conversion, the conversion rate decreases
in many cases. Thus, for a fair comparison, we mainly considered
the values of hydrogenation rate and selectivity at the MBY conver-
sion of 50% and also at 95%.
92
90
94.8%
88
86
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
3.5.1. Influence of solvent polarity
Initial concentration of MBY, mol/L
The data of Fig. 7 show that solvent nature strongly influences
the MBY conversion. To find correlations between solvent nature
and catalytic activity and selectivity, we considered solvents to
Fig. 5. Dependence of TOF50 on MBY loading for 0.2%Pd/MN270 (corresponding
values of selectivity are also indicated).
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100
80
cyclohexane
333 K
313 K
70
80
60
40
20
0
60
hexane
50
octane
octane
EtOH
i-PrOH
40
30
20
10
water+EtOH
hexane
heptane
octane
toluene
xylene
cyclohexane
heptane
hexane
cyclohexane
heptane
1,90
0
10
20
30
40
50
60
1,88
1,92
1,94
1,96
1,98
2,00
2,02
2,04
ε
Time, min
Fig. 9. Dependence of reaction rate (TOF50) on dielectric constant of non-polar sol-
Fig. 7. Solvent effect on MBY conversion over 0.2%Pd/MN270 (333 K, 3 bar of H2
vents.
pressure).
(with the exception of hexane, for which the highest selectivity
(100% at 50% of MBY conversion) was observed).
be divided into the three following groups: polar (water, EtOH, i-
PrOH), non-polar (hexane, heptane, octane and cyclohexane) and
aromatic non-polar (m-xylene and toluene) solvents.
The results of the MBY hydrogenation in the polar solvents over
the 0.2%Pd/MN270 catalyst are presented in Table 2. It is notewor-
thy that pure water could not be used as a solvent because of the
limited solubility of the hydrogenated products and the formation
of an emulsion under the reaction conditions.
The temperature increase leads to the increase of the hydro-
genation rate (TOF) while the selectivity decreases (Table 2). The
decrease of selectivity is likely due to the increase of the over-
hydrogenation reaction rate with temperature.
Besides, a clear correlation between the hydrogenation rate
(TOF50) and the solvent dielectric constant (ε) is observed (Fig. 8).
The increase of the solvent polarity was found to result in the appro-
priate decrease of the reaction rate independently of the reaction
temperature. At the same time the selectivity does not seem to be
influenced by the solvent polarity. The maximum selectivity was
observed for EtOH.
The solvent influence on the catalyst performance was also
tested for non-polar aromatic hydrocarbons, such as m-xylene and
toluene with nonzero dipole moments. The results of the MBY
hydrogenation in these solvents are presented in Table 4 and Fig. 10.
Similarly, the influence of aromatic non-polar solvents on catalytic
activity becomes noticeable only at a higher temperature. It can be
seen that at 333 K the higher dipole moment and dielectric con-
stant result in the lower activity but higher selectivity. The highest
selectivity (close to 100%) was reached in toluene and the highest
TOF50 (47.0 mol(MBY)/(mol(Pd)*s)) was observed in m-xylene.
However, for all the non-polar solvents, no correla-
tion can be found (see Fig. 11): pairs heptane/toluene
and octane/xylene revealed similar TOF50 (33.1/35.0 and
49.3/47.0 mol(MBY)/(mol(Pd)*s),
respectively)
for
sol-
vents with dielectric constants. The highest activity
(68.6 mol(MBY)/(mol(Pd)*s)) and the lowest selectivity (85.8%
at 50% of MBY conversion) were observed for cyclohexane.
At the same time, the increase of the dielectric constant for
polar solvents, in contrast to non-polar ones (such as hexane,
heptane, octane and cyclohexane), results in the corresponding
decrease of TOF50. For the water/EtOH mixture, the observed value
In the case of non-polar solvents (Table 3), the increase of the
dielectric constant was found to result in a corresponding increase
of TOF at temperature of 333 K (Fig. 9) while selectivity diminished
100
50
i-PrOH
333 K
90
333 K
313 K
45
40
35
30
25
20
xylene
313 K
EtOH
80
70
60
toluene
i-PrOH
50
EtOH + H2O
EtOH
40
xylene
toluene
2,380
30
EtOH + H2O
20
20
40
60
80
2,355
2,360
2,365
2,370
2,375
2,385
ε
ε
Fig. 8. Dependence of reaction rate (TOF50) on dielectric constant of polar solvents.
Fig. 10. Dependence of TOF50 on dielectric constant of aromatic non-polar solvents.
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Table 2
MBY hydrogenation in polar solvents over 0.2%Pd/MN270 (PH = 3 bar, 1500 rpm, catalyst concentration 7 × 10−6 mol(Pd)/L, initial concentration of MBY 0.9 mol/L).
2
Solvent
T, K
ε
ꢀ
50% MBY conversion
95% MBY conversion
Maximum MBY
yield, %
TOF50, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
TOF95, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
313
333
313
333
313
19.9
24.6
76.7
1.66
1.69
–
49.6
94.0
48.1
85.5
27.9
97.3
95.8
99.5
94.8
95.5
68.2
109.1
64.1
107.7
34.1
92.3
86.0
98.7
93.2
92.4
89.1
82.4
96.4
88.8
89.1
i-PrOH
EtOH
H2O (70 mL) + EtOH (10 mL)
Table 3
MBY hydrogenation in non-polar solvents over 0.2%Pd/MN270 (PH = 3 bar, 1500 rpm, catalyst concentration 7 × 10−6 mol(Pd)/L, initial concentration of MBY 0.9 mol/L).
2
Solvent
T, K
ε
50% MBY conversion
95% MBY conversion
Maximum MBE
yield, %
TOF50, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
TOF95, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
Heptane
Hexane
313
333
313
333
313
333
313
333
1.900
1.904
1.948
2.020
19.3
33.1
21.7
48.8
20.2
49.3
19.8
68.6
96.9
96.3
96.3
100
92.8
93.6
93.9
85.8
27.2
45.6
25.5
64.2
22.9
58.8
23.6
78.4
95.9
92.8
92.7
93.9
90.4
92.8
85.4
81.7
91.6
90.4
92.2
90.1
89.7
87.8
84.5
78.5
Octane
Cyclohexane
Table 4
MBY hydrogenation in aromatic non-polar solvents over 0.2%Pd/MN270 (PH = 3 bar, 1500 rpm, catalyst concentration 7 × 10−6 mol(Pd)/L, initial concentration of MBY
2
0.9 mol/L).
Solvent
T, K
ε
ꢀ
50% MBY conversion
95% MBY conversion
Maximum MBE
yield, %
TOF50, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
TOF95, mol(MBY)/
(mol(Pd)*s)
MBE
selectivity, %
313
333
313
333
2.359
2.380
0.33
0.37
24.2
47.0
23.0
35.0
92.4
90.6
99.7
100
33.5
64.2
32.2
40.0
89.3
89.6
99.5
99.6
86.3
82.4
96.0
96.4
m-Xylene
Toluenee
80
70
60
50
40
Thus, the difference in TOF cannot be explained simply by the
difference in solvent polarity (in contrast to the data reported by
Bertero et al. [2] and Drelinkiewicza et al. [7] for AP hydrogena-
tion, when H-bond donor ability and alcohol solvation hindered
the substrate adsorption on noble metal).
cyclohexane
85.8%
We suggest that the absence of correlation between the
observed activity and solvent polarity can be due to the unusual
properties of MBY. The presence of a heteroatom in the alcohol with
unpaired electrons in vicinity of triple carbon–carbon bond causes
the effect of mesomerism or conjugation. Two atomic orbitals give
molecular orbitals (MO) with different energy. The MO with a low
binding energy will be the bonding one (␲-bond of C–C), and the MO
with a high binding energy is the antibonding one (C–OH). A neg-
ative charge is localized on oxygen, while carbon and hydrogen of
the OH-group are partially positively charged [29]. For example, the
energy of the C–O-bond increases in the following order: propar-
gyl alcohol (74.3 kcal/mol) < allylic alcohol (78.9 kcal/mol) < PrOH
(91.8 kcal/mol) [30,31], i.e. the mesomeric effect results in the
decrease of the C–O binding energy.
octane
93.6%
hexane
xylene
90.6%
100%
toluene
heptane
96.3%
100%
30
1,8
2,0
2,2
2,4
ε
Fig. 11. Dependence of TOF50 on dielectric constant for non-polar solvents (corre-
sponding values of selectivity are also indicated).
In the case of MBY, the C–O binding energy will be also
decreased. Besides, due to alkyl groups in MBY, the induction effect
(acting in opposite direction of mesomerism) will take place, i.e.,
the alkyl groups additionally push the electron density in the direc-
tion of the triple bond [31].
of TOF50 (52.0 mol(MBY)/(mol(Pd)*s) is comparable with the value
found in octane (49.3 mol(MBY)/(mol(Pd)*s), though the dielectric
constant of octane is about 40 times lower than that of the mix-
ture.
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3.5.3. Influence of solvent–catalyst interactions
It is known that in many cases the interaction of a solvent
with a catalyst is responsible for changes of the reaction rates
[2,4,5,35–37].
However, in our case, while dealing with highly porous poly-
meric network, the measurement of the enthalpy of the solvent
desorption from palladium is complicated for several reasons such
as capillary effect hindering the solvent evacuation from inner HPS
pores and the possible solvent interaction with benzene rings of
the polymeric matrix. As the effect of support cannot be excluded,
e.g., like it was done by Bertero et al. [2] for SiO2, we suggested
that due to the features of the catalytic support used and low Pd
content, there is no possibility to provide quantitative estimation
of solvent–catalyst interactions, and for further discussion we only
used literature data.
It was reported that in the case of the AP hydrogenation over
Ni-containing catalyst, there is a difference in adsorption enthalpy
for toluene and benzene. The authors found that weaker adsorption
of toluene on Ni in comparison with benzene can explain the lower
activity [2 and Refs. therein]. Thus we can also propose that the
same kind of the dependence takes place in the case of Pd-HPS. We
suggested that m-xylene adsorbs weaker on the Pd surface in com-
parison with toluene due to the existence of an additional methyl
group, and hence the TOF observed in m-xylene was higher and
the selectivity was lower than those in toluene (see Table 4 and
Fig. 10). For cyclohexane, which likely has the lowest adsorption
strength on palladium among the non-polar solvents, the highest
activity was observed (Fig. 11). Moreover, the difference in catalytic
activity for solvents belonging to different groups (polar and non-
polar), e.g. EtOH and toluene, can be also due to the differences in
their adsorption strength.
As a result, in the case of MBY, the polarity of OH-group
will be decreased (␦- for O will be higher and hence the elec-
tron donation from H will be weaker) due to the close proximity
of the triple bond as well as two CH3-groups. For example, a
dipole moment of MBY (1.59) is lower than that of propargyl
alcohol (1.78) [29]. Thus, we suggest solvent–substrate inter-
actions do not influence the differences in catalytic activity
observed.
It is noteworthy that when the reaction was carried out under
solvent-free conditions, relatively high values of selectivity were
observed (93.8% and 99.8% at 95% and 50% conversions, respec-
tively) at rather low TOF50 (33.2 s−1 at 333 K, respectively).
3.5.2. Influence of hydrogen solubility
According to the investigations carried out by several research
groups [2,7,15], hydrogen solubility cannot explain the difference
in catalytic activity. The same conclusion can be made also for the
MBY selective hydrogenation.
For example, a mole fraction of soluble H2 in heptane is higher
in comparison with that in octane at the same conditions (0.087
vs. 0.091, at 295 K and 13.88 MPa), while in toluene it is higher
in comparison with that in m-xylene (0.041 vs. 0.045, at 295 K
and 13.88 MPa) [32]. However, we observed that catalytic activities
in heptane and toluene at 333 K and 3 bar are lower in compar-
ison with those in octane and m-xylene, respectively. Hydrogen
solubility in cyclohexane almost 1.5 times lower than that in hep-
tane and octane [32], but TOF50 of the MBY hydrogenation is by
a factor of 1.3–1.4 higher at 333 K and comparable at 313 K (see
Table 3). Solubility of hydrogen in hexane is lower than that in
octane (0.074 vs. 0.079 at 298 K and 5 MPa [33]), but TOFs at 50%
of the MBY conversion are comparable. Although the above men-
tioned data on the H2 solubility are presented for relatively low
temperatures (295–298 K) and high partial pressures, it is note-
worthy that at the certain partial pressure the difference between
hydrogen solubilities decreases with the increase of temperature.
For example, the difference in mole fraction solubilities of H2 in
octane and hexane at 5 MPa changes in the following order: 0.0049
(298 K) > 0.0025 (323 K) > 0.0016 (373 K) [33]. Thus it can be con-
cluded that the evident increase of the differences in TOFs of the
MBY hydrogenation with the temperature for non-polar solvents
(see Figs. 8 and 9) is not due to hydrogen solubility and can be
explained only by the solvent–catalyst interaction, as all the exper-
iments were carried out at the constant H2 partial pressure of
3 bar.
In the case of polar solvents, higher TOF for i-PrOH in comparison
with EtOH can be explained by the ability of i-PrOH to be a hydrogen
donor rather than EtOH in H-transfer reactions, as was reported by
different research groups [2,38–40].
3.6. Activation energy of the reaction
For the best chosen solvents (EtOH and toluene), the values
of apparent activation energy of the MBY hydrogenation over
the 0.2%Pd/MN270 catalyst were found to be 26 5 kJ/mol and
40 5 kJ/mol, respectively (see Fig. 12).
The thermodynamic interaction can influence the reaction rate
in two ways. Higher activity in the liquid phase can result in an
increase of the adsorbed phase concentration at the certain concen-
tration in the liquid phase. This can result in higher rates by a mere
mass-transfer effect in the adsorbed phase. While the mass-transfer
effect could result in the rate increase without any significant
change in the activation energy, the latter would be reflected in
the values of the apparent activation energy. As the mass-transfer
effects were excluded (Sections 3.2, 3.3 and 3.4), the difference in
the valuesof apparentactivationenergyis due to the solventnature.
Besides, according to the data of Ghavre et al. [34], at a low
hydrogen partial pressure of 0.101 MPa and 293 K, the solubility
of H2 in EtOH (2.98 × 10−3 mol/L) is lower than that in toluene
(3.50 × 10−3 mol/L) and in cyclohexane (3.63 × 10−3 mol/L), that
contradicts to the TOFs values observed. While comparing the cat-
alytic activity observed in EtOH and i-PrOH (Table 2) one can see
that TOF in i-PrOH is higher than that in EtOH, but hydrogen solubil-
ity in i-PrOH is lower in comparison to the value in EtOH according
to the data of Bertero et al. [2].
The effect of hydrogen solubility can only explain the lowest TOF
in the water/EtOH mixture, as in water the H2 solubility by several
orders of magnitude lower (0.81 × 10−3 mol/L [34]) than in other
polar solvents.
3.7. Catalyst reuse
Stability of the 0.2%Pd/MN270 catalyst was investigated in
repeated reaction runs with the same catalyst in EtOH. After each
run the catalyst was washed with EtOH and dried in air. The results
obtained are presented in Table 5.
According to the AAS analysis, after all runs the Pd loading in
the catalyst did not change. One can conclude that there is no
metal leaching during the hydrogenation. The HPS-based catalyst
demonstrates a combination of stability and selectivity toward the
desired product.
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-5,5
-6,0
-6,5
-7,0
-7,5
-8,0
-8,5
-5,8
-6,0
toluene
ethanol
Ea = 40 kJ/mol
-6,2
Ea = 26 kJ/mol
-6,4
-6,6
-6,8
-7,0
-7,2
-7,4
2,7
2,8
2,9
3,0
3,1
3,2
3,3
2,8
2,9
3,0
3,1
3,2
3,3
1/T . 103, K
1/T . 103, K
Fig. 12. Arrhenius plots for MBY hydrogenation over the 0.2%Pd/MN270 in toluene (a) and EtOH (b).
Table 5
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Please cite this article in press as: L. Nikoshvili, et al., Selective hydrogenation of 2-methyl-3-butyn-2-ol over Pd-nanoparticles stabilized
in hypercrosslinked polystyrene: Solvent effect, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.045

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Please cite this article in press as: L. Nikoshvili, et al., Selective hydrogenation of 2-methyl-3-butyn-2-ol over Pd-nanoparticles stabilized
in hypercrosslinked polystyrene: Solvent effect, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.045