Selective hydrogenation of 2-methyl-3-butyne-2-ol in a wall-coated capillary microreactor with a Pd25Zn75/TiO2 catalyst

Article abstract of DOI:10.1021/op900085b

Continuous flow capillary microreactors with embedded monometallic (Pd) or bimetallic (Pd₂₅Zn₇₅) catalysts have been tested in the selective hydrogenation of alkyne reagents. The catalysts were prepared in two steps. At first, polymer- stabilized metal nanoparticles (either Pd or Pd ₂₅Zn₇₅) were prepared by the reduction by solvent method. Then, a solution of colloidal nanoparticles with a desired concentration was added into a titania sol, which was destabilized by solvent evaporation during dip-coating of the inner wall of a fused silica capillary with an internal diameter of 250 μm. The wall-coated microreactors were tested in the hydroge- nation of 2-methyl-3-butyne-2-ol (0.011-0.45 M solution in methanol) in the 328-337 K temperature range. The highest selectivity towards the alkene product of 90% was obtained at 99.9% conversion on the Pd₂₅Zn ₇₅/TiO₂ catalyst. The selectivity was further increased to 97% by addition of pyridine into the reactant solution. No deactivation of the wall-coated catalysts was observed during one month of continuous operation at 333 K.

Full text of DOI:10.1021/op900085b

Organic Process Research & Development 2009, 13, 991–998
Selective Hydrogenation of 2-Methyl-3-butyne-2-ol in a Wall-Coated Capillary
Microreactor with a Pd Zn /TiO Catalyst
25
75
2
,†
†
‡,⊥
§
†
Evgeny V. Rebrov,* Ekaterina A. Klinger, Angel Berenguer-Murcia, Esther M. Sulman, and Jaap C. Schouten
Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen,
The Netherlands, Department of Chemistry, Cambridge UniVersity, Lensfield Road, CB2 1EW Cambridge, U.K., and TVer
Technical UniVersity, A. Nikitin Street, 22, TVer 170026, Russia
Abstract:
Scheme 1. Possible transformations of
-methyl-3-butyne-2-ol
2
Continuous flow capillary microreactors with embedded mono-
metallic (Pd) or bimetallic (Pd25Zn75) catalysts have been tested
in the selective hydrogenation of alkyne reagents. The
catalysts were prepared in two steps. At first, polymer-
stabilized metal nanoparticles (either Pd or Pd25Zn75) were
prepared by the reduction by solvent method. Then, a
solution of colloidal nanoparticles with a desired concentra-
tion was added into a titania sol, which was destabilized by
solvent evaporation during dip-coating of the inner wall of
a fused silica capillary with an internal diameter of 250 µm.
The wall-coated microreactors were tested in the hydroge-
nation of 2-methyl-3-butyne-2-ol (0.011-0.45 M solution in
methanol) in the 328-337 K temperature range. The highest
selectivity towards the alkene product of 90% was obtained
as a critical step. In these gas-liquid-solid reactions, the overall
production rate is often limited by interphase mass transfer. To
overcome this deficiency, multiphase reactions can be performed
in microchannels or microcapillaries with a catalytic coating
deposited on the channel wall. To perform hydrogenations in a
microreactor and to avoid mass- and/or heat-transport limita-
tions, both titania and silica thin films with embedded nano-
2
at 99.9% conversion on the Pd25Zn75/TiO catalyst. The
selectivity was further increased to 97% by addition of
pyridine into the reactant solution. No deactivation of the
wall-coated catalysts was observed during one month of
continuous operation at 333 K.
1,6,7
structured catalysts have been developed in our laboratory.
Introduction
They consist of an open mesoporous structure of a support with
significant open porosity (usually 30-50%) in which the active
material is dispersed.
During the last two decades, significant developments in the
field of miniaturized systems, so-called microfluidics or lab-
on-a-chip technologies, have been achieved and applied widely
The selective hydrogenation of acetylene alcohols is an
important step in the synthesis of fine chemicals, viz. vitamins
A and E. Supported Pd catalysts are known to be very efficient
for the hydrogenation of long-chain acetylene alcohols to
ethylene alcohols. However, under several reaction conditions
a number of side products can be formed (Scheme 1). Thus,
under nitrogen atmosphere, oxidative dimerization dominates,
and 2-methyl-3-butyn-2-ol (MBY) is converted over a Pd
catalyst into 2,7-dimethyl-3,5-octadiyne-2,7-diol (1) which can
1,2
3
to diverse areas, such as catalysis, fine chemicals synthesis,
4
5
polymerization, and sensors. In fine chemicals synthesis, these
microreaction systems have found a wide range of applications
in different chemical reactions in what is currently referred to
as process intensification, which is directly aimed at improving
the performance of existing processes. The synthesis of a large
number of fine chemicals, particularly in the field of fragrance
chemistry and pharmaceuticals, involves selective hydrogenation
further be hydrogenated to 2,7-dimethyl-5-octen-3-yne-2,7-diol
*
Corresponding author. E-mail: e.rebrov@tue.nl. Phone: +31 40 2472850.
Eindhoven University of Technology.
Cambridge University.
Tver Technical University.
Present address: University of Alicante, Departamento de Qu ´ı mica Inor-
8
(2). Isomerization of MBY into prenal can be achieved in the
†
‡
presence of mixtures of titanium alkoxides, copper (I) chloride,
§
9
and carboxylic acids. A commonly used catalyst for the
⊥
g a´ nica, Ap. 99 - 03080, Alicante, Spain.
(
1) Rebrov, E. V.; Berenguer-Murcia, A.; Skelton, H. E.; Johnson, B. F. G.;
(6) Rebrov, E. V.; Berenguer-Murcia, A.; Johnson, B. F. G.; Schouten,
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Wheatley, A. E. H.; Schouten, J. C. Lab Chip 2009, 9, 503–506.
(
(
(
(
2) Kobayashi, J.; Mori, Y.; Okamoto, K.; Akiyama, R.; Ueno, M.;
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Chem. 2005, 44, 2413–2416.
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P. J.; Lafont, U.; de Croon, M. H. J. M.; Schouten, J. C. Chem. Eng.
J. 2008, 135S, S99–S103.
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1
1
0.1021/op900085b CCC: $40.75  2009 American Chemical Society
Vol. 13, No. 5, 2009 / Organic Process Research & Development
•
991
Published on Web 06/01/2009

semihydrogenation of alkynes is a commercial Lindlar catalyst,
calcium carbonate-supported palladium, modified by the addi-
tion of lead and, often, organic bases (e.g., quinoline) to improve
Table 1. Characteristics of Pd/TiO
2
and Pd25Zn75/TiO
2
catalysts
Pd/TiO
2
Pd25Zn75/TiO
2
10
selectivity.
titania thickness, nm
titania apparent density , g/cm
105
2.13
0.878
1.00
-
110
a
3
2.13
0.919
0.36
0.64
0.33
2.5
To explain the activity/selectivity behavior of Pd catalysts
in the hydrogenation of alkynes, the specificity of the interaction
of the catalytic centers with the carbon-carbon multiple bonds
of the reagents and the electronic and morphological proper-
coating weight in the reactor, mg
Pd loading, wt %
Zn loading, wt %
Pd/Zn molar ratio, -
-
b
mean metal particle average size , nm
2.5
11
c
ties of the metal particles were considered. A high selectivity
to the alkene is related to an increased Pd electron density that
leads to a decreased alkene adsorption. A higher selectivity to
the alkene was achieved by employing electron-donor com-
pounds, such as N-bases (quinoline, pyridine, and ammonia)
or by the addition of electron-donor compounds (Pb, Zn) to
the Pd nanoparticles.
Pd dispersion
0.40
0.40
a
Calculated from the film porosity. b Size of nanoparticles in the colloidal
solution. c Estimated from the mean particle size.
Table 2. Microreactor and process characteristics
length, m
10
internal diameter, mm
catalyst loading, kgTiO2/m3
liquid flow range, mL /min
gas flow range, mL/min (STP)
0.250
It is possible to change the performance of a metal by
1.8
12
13
0.002-0.014
0.60-1.00
0.6-5.0
1.1-1.8
0.3-1.0
0.21-0.33
1.0
tailoring the particle size and the stoichiometry, although
the latter has not been thoroughly investigated. By changing
the metal ratio in a bimetallic nanoparticle, different arrange-
ments of the surface atoms can be obtained. The type and
relative amount of these surface atoms will determine the
catalytic behavior of the nanoparticle, which should be opti-
mized for obtaining the highest activity and selectivity possible.
Understanding the relationship between the stoichiometry of
bimetallic nanoparticles and their activity/selectivity is of
significant importance in assisting the development of new
catalytic processes in microstructured reactors.
Re
Re
L
, -
, -
partial pressure , -
G
a
H
2
liquid hold-up, -
pressure at the outlet, bar
pressure drop over the reactor, bar
1.2-2.7
a
Nitrogen was used as a balance.
Herein we present the use of a continuous capillary mi-
croreactor with mesoporous titania thin films containing either
Pd or bimetallic Pd25Zn75 nanoparticles for MBY hydrogenation.
The nanoparticles with an average size of 2.5 nm were
embedded in the titania support during sol-gel synthesis
1,6
following previously reported methodology. The effect of the
-methyl-3-butyne-2-ol concentration was studied in the range
2
from 0.011 to 0.45 M, the hydrogen partial pressure in the range
from 0.30 to 0.80, and the reaction temperature between 328
and 337 K. The results are compared with the hydrogenation
2
over Pd25Zn75/TiO /Si plates in a stirred reactor operating under
Figure 1. Effect of initial concentration of 2-methyl-3-butyne-
-ol (MBY) on its hydrogenation rate and selectivity towards
MBE formation over the Pd25Zn75/TiO catalyst at 333 K. The
2
a hydrogen pressure of 5 bar.
2
initial concentration of MBY was varied between 0.011 and 0.15
M, while the liquid flow rate was varied between 9.0 and 2.3
µL/min to provide a conversion below 15%. The hydrogen flow
was fixed at 0.9 mL/min (STP).
Results and Discussion
Optimization of Reaction Parameters. Table 1 summarizes
the characteristics of the studied catalysts, while the reactor and
process parameters are listed in Table 2. The main reaction
products are 2-methyl-3-buten-2-ol (MBE) and 2-methyl-3-
butan-2-ol (MBA). At low hydrogen partial pressure, significant
amounts of the C10-dimer (1) are produced. The adduct (2) is
not observed during the present experiments. The supposed side
product, prenal (3), is not observed-probably due to the absence
of oxygen donors which are required to form a titanium
three different concentrations (0.01, 0.20, and 0.45 M). Figure
1
shows the reaction rate as a function of the liquid flow rate
in the microchannel. The initial concentration of MBY does
not influence the selectivity to MBE nor the content of the
byproduct formed. Within the whole range of MBY concentra-
tions studied, the selectivity to MBE attains 96.5% at an MBY
conversion below 20%, and the fraction of MBY transformed
to byproducts amounts to 3.5%. This dependence of the reaction
rate on the MBY concentration is in line with the zero reaction
order in MBY. An addition of Zn at a Pd/Zn molar ratio of 1/3
considerably improves the selectivity to the semihydrogenated
product (MBE) at high MBY conversion (Figure 2). The MBE
selectivity is controlled by the ratio of the kinetic constants of
the first and second hydrogenation steps as well as by the MBY
9
propargyl-oxo complex. The effect of the MBY concentration
on the reaction rate was examined on the Pd/TiO catalyst using
2
(
(
10) Lindlar, H.; Dubuis, R. Org. Synth. 1966, 46, 89–92.
11) Molnar, A.; Sarkany, A.; Varga, M. J. Mol. Catal. A: Chem. 2001,
73, 185–221.
12) Silvestre-Albero, J.; Rupprechter, G.; Freund, H. J. J. Catal. 2006,
1
(
(
2
40, 58–65.
13) Aramendia, M. A.; Borau, V.; Jimenez, C.; Marinas, J. M.; Sempere,
M. E.; Urbano, F. J. Appl. Catal. 1990, 63, 375–389.
9
92
•
Vol. 13, No. 5, 2009 / Organic Process Research & Development

(Figure 3a). The curves at temperatures below 328 K are not
shown in Figure 3 as a lower selectivity to MBE was observed
at these conditions. The apparent activation energy of the MBY
hydrogenation is 64 kJ/mol. This was calculated from an
Arrhenius plot (not shown) derived from the temperature
dependence of the MBY hydrogenation reaction rate shown in
Figure 3. The selectivity to MBE increases with temperature;
however, the influence of the temperature on the selectivity was
found to be far less decisive than that of the pressure (Figure
3b). The maximum MBE yield was observed at a temperature
of 337 K. However, fast catalyst deactivation was observed
close to the boiling point of methanol at 337 K; therefore, the
maximum temperature was fixed at 333 K during the kinetic
study. After reaching full MBY conversion, further increase in
hydrogen pressure resulted in a higher selectivity to MBA
Figure 2. Selectivity to MBE as a function of MBY conversion
over Pd/TiO (open symbols) and Pd25Zn75/TiO (closed sym-
bols) catalysts in the absence (solid lines and square symbols)
and in the presence of pyridine (0.1 M, dashed lines and circle
symbols) in the initial solution. Lines represent the kinetic
model. Symbols represent experimental data in the capillary
microreactor. Temperature: 333 K.
2
2
(Figure 3c) while the selectivity to the C10-dimers remained
unchanged and did not exceed 2% (Figure 3d).
It should be mentioned that at a liquid flow rate of, e.g., 5
µL/min, it takes 100 min to collect enough sample for the GC
analysis (4-6 sampling points per day with 100-110 points
per month for each catalyst, the flow being fed to the reactor
overnight as well). Therefore, the time-on-stream of each
catalyst was more than one month during which no deactivation
was observed at a temperature of 333 K with a few experimental
runs performed at lower temperatures.
An important characteristic of the catalytic material is the
amount of metal leaching during exposition to the fluid. This
undesirable effect can occur in three different ways, as pointed
out in ref 14: (i) very low metal leaching (under the limit of
detection of an adequately sensible analytical technique),
practically negligible; (ii) significant leaching of catalytically
active metal species; (iii) significant leaching of inactive metal
species. The last two cases can also include total or partial
readsorption of the metal species on the support surface
downstream of the reactor channel. It appeared from this study
that colloidal nanoparticles deposited on mesoporous titania
oxide are entirely leaching free (case (i)) due to their embedding
into the titania network. In an accompanying study (not shown
here as this is beyond the scope of this paper), we have
3+
demonstrated that the Ti sites were formed after calcination
of the mesoporous titania films containing metal nanoparticles
at 573 K. A strong metal support interaction (SMSI) in the
supported nanostuctured catalysts prevents metal leaching from
the mesoporous support.
Figure 3. (a) MBY conversion and (b-d) selectivity to MBY
(
2
b), MBA (c), and C10-dimer (d) over Pd25Zn75/TiO catalyst as
a function of hydrogen partial pressure at different tempera-
tures. Experimental data are given with symbols. The lines are
drawn as a guide for the eye. Solid lines and closed symbols
represent 337 K; dashed lines and half-open symbols: 333 K;
and dash/dotted lines and open symbols: 328 K. The total gas
flow rate (hydrogen and nitrogen) was kept constant at 1.0 mL/
min (STP). Liquid flow: 9.2 µL/min.
The rate of C10-dimer formation is, in general, an order of
magnitude lower than that of MBA, at least in the concentration
range studied; therefore, the concentration curves of the C10
-
dimer are not presented in several figures as its concentration
is below or just above the detection limit. The MBY conversion
decreases with increasing liquid flow rate (Figure 4). In the
range of gas and liquid velocities applied, an annular two-phase
flow was realized in the microchannel, such that the gas flowed
along the centre of the tube, while the liquid flowed along the
channel walls as a liquid film. The liquid hold-up and the liquid
residence time were calculated according to the Lockhart-
Martinelli-Chisholm correlation. The MBY conversion as a
and MBE adsorption constants. The origin of this high MBE
selectivity of the Pd25Zn75/TiO catalyst will be discussed in
2
the following section based on the reaction kinetics.
A systematic study of the effects of the reaction temperature
and the hydrogen partial pressure was performed over Pd25Zn75
/
TiO (Figure 3). A linear dependence of the reaction rate on
2
the hydrogen partial pressure up to a conversion of 95%
demonstrated that the reaction is first order in hydrogen, and
zero order in MBY, in the whole range of temperatures studied
(
14) Sheldon, R. A.; Wallau, M.; Arends, I. W. C. E.; Schuchardt, U. Acc.
Chem. Res. 1998, 31, 485–493.
Vol. 13, No. 5, 2009 / Organic Process Research & Development
•
993

Figure 4. MBY conversion and selectivity to major products
over Pd25Zn75/TiO catalyst as a function of liquid flow rate.
Hydrogen flow rate: 0.9 mL/min (STP). Temperature: 333 K.
2
Figure 6. Product distribution in the absence of pyridine (solid
lines and square symbols) and in the presence of pyridine
(
0
dashed lines and circle symbols, pyridine/MBY molar ratio:
.1) in the reactant solution over (a) Pd/TiO , (b) Pd25Zn75/TiO
2
2
catalysts as a function of the liquid residence time in the
microreactor. Lines represent the concentration profiles ob-
tained with the kinetic model. Experimental data for MBY are
given with closed symbols; for MBE with half-open symbols;
and for MBA with open symbols. Temperature: 333 K.
Hydrogen flow: 0.9 mL/min (STP).
Figure 5. MBY conversion and liquid hold-up over Pd25Zn75
/
TiO catalyst as a function of liquid residence time.
2
function of the liquid residence time gives a straight line passing
through the origin (Figure 5). This sustains the correctness of
the applied approach for the calculation of the liquid residence
time.
The effect of the liquid residence time, the presence of Zn,
and the addition of pyridine in the initial solution is investigated
by comparing the Pd/TiO and Pd25Zn75/TiO catalysts. A
2 2
nitrogen base (NB) is typically used in the concentrations
corresponding to a NB/Pd molar ratio in the range from 4 to
1
3
1
500 and an NB/reactant molar ratio of 0.1-1. Typical
Figure 7. MBE selectivity at the maximum yield of MBE as a
concentration curves of the alkyne, alkene and alkane are shown
in Figure 6. The reaction proceeds through two well-separated
stages clearly observable on the MBE curve. In the initial stage
lasting until ∼96% conversion on the Pd catalyst and almost
until 99.5% conversion on the Pd25Zn75 catalyst, the semihy-
drogenation of MBY to MBE (Ct C to C)C) is the main
reaction. Then, the second stage started. It can be seen that the
second stage proceeds much faster on the Pd catalyst as
compared with the Pd25Zn75 catalyst. An addition of pyridine
at a pyridine/MBY ratio of 0.1 decreases the MBY hydrogena-
tion rate, and it results in a wider maximum on the MBE
concentration curve. It should be pointed out that in the presence
of pyridine the initial MBY concentration was reduced by 4.5
times to achieve full MBY conversion (see Figure 6a, right
y-axis). On the Pd25Zn75 in the absence of pyridine, the first
and second hydrogenation steps have approximately the same
reaction rate as it can be seen from the slope of the correspond-
function of the pyridine/MBY molar ratio over Pd/TiO
2
and
Pd25Zn75/TiO
2
catalyst. Lines represent the kinetic model.
Symbols represent experimental data. Temperature: 333 K.
the Pd nanoparticles drastically decreases the number of multiple
palladium sites responsible for the dissociative adsorption of
MBY. This pathway is reported to lead to the formation of C10
-
dimers and further hydrogenation of C10-dimers adsorbed on
8,15
the Pd sites. As opposed to the Pd curves, the addition of
pyridine at a pyridine/MBY ratio of 0.1 does not change the
rate of MBY hydrogenation while it almost completely sup-
presses the second hydrogenation step (Figure 6b). In this case
equal initial concentrations of MBY were used. In Figure 7 the
concentration dependence of the selectivity is presented by
decreasing the amount of modifier in the reactant stream.
ing concentration curves in Figure 6b. The formation of C10
dimers is suppressed. It appears that the insertion of Zn into
-
(
15) Leviness, S.; Nair, V.; Weiss, A. H.; Schay, Z.; Guczi, L. J. Mol.
Catal. 1984, 25, 131–140.
9
94
•
Vol. 13, No. 5, 2009 / Organic Process Research & Development

Scheme 2. Kinetic Model
well as that the K
the following terms: K
C
term is much smaller than at least one of
, K , K , or K in the whole
D
D
Y Y
C
C
E E
C
A A
D D
C
range of experimental conditions studied, the reaction rate
equations can be written as eqs 1-3 (see Scheme 2):
k K C C
Y Y H2
1
r1 )
(1)
(2)
(3)
1
+ K C + K C + K C
Y
Y
E
E
P
P
1
Between experiments with different concentrations of pyridine,
an experiment was always performed without addition of
pyridine.
In analogy to Figure 2, a higher selectivity was always
accompanied by a substantial decreasing MBY hydrogenation
rate on the Pd catalyst, while the hydrogenation rate remained
practically unchanged on the Pd25Zn75 (not shown in Figure
k K C C
2
E
E
H2
r2 )
1
+ K C + K C + K C + K C
Y Y E E A A P P
1
2
Y
2
Y
k K C
3
r3 )
2
(1 + K C + K C + K C )
Y
Y
E
E
P
P
1
7
). Both catalysts have a similar nanoparticle size of ca. 2.5
nm, while the Pd content in the Pd25Zn75/TiO catalyst was 4
times lower than that in the Pd/TiO catalyst. However, the
initial reaction rate at 333 K in terms of TOF was 2.4 s on
2
n Y E A D
Where k is the rate constant for reaction n; K , K , K , K ,
and K are the adsorption constants, and C , C , C , C , and
P1 Y E A D
2
-1
C are the concentrations of alkyne, alkene, alkane, dimers and
P
-1
the Pd/TiO
2
versus 0.15 s on the Pd25Zn75/TiO
2
. In this
pyridine, respectively. The hydrogen concentration (C ) 16.5
H2
3
19
estimation it was assumed that Pd and Zn are equally distributed
throughout the bimetallic nanoparticles, or in other words there
is no rearrangement of metals during the calcination step. The
reaction rate is comparable to that reported for the unsupported
mol/m ) was calculated from existing solubility data. To model
the reaction satisfactory for the selectivity as well, it is necessary
to add a direct alkyne to alkane hydrogenation step (Pathway
4). Pathway 4 involves the reaction of MBY with hydrogen
adsorbed on the catalyst to form an intermediate species
responsible for the formation of the alkane and thus detracting
16
Pd nanoparticles. The reaction rates in terms of TOF of ∼2
are typical for kinetically limited systems. The maximum
value of the overall mass transfer coefficient (kov) in the capillary
-1
s
13
from the MBE selectivity.
3
-3
s-1. This value is rather low as
microreactor is 0.01 m
L
m
R
The effect of pyridine on the selectivity can not be described
with only one type of pyridine adsorption site. On the basis of
previously published results, an increase of the selectivity in
the presence of organic bases can be explained by a preferential
compared with bulk mass transfer-limited systems (kov is of the
3
-3 -1
order of 0.1 m
L
m
R
s ) and hardly varies with varying
hydrodynamics (there being no difference in kov between slug
and annular flow). This justifies the conclusion that interface
mass transfer is not the rate determining factor in these capillary
reactors and intrinsic kinetics is observed. The TOF value
2
0
adsorption of the additive over alkenol on the selective sites
and strong adsorption on the nonselective sites.
21,22
Assuming
that step 4 takes place on the nonselective sites, two different
adsorption constants of pyridine (on selective and nonselective
sites) are introduced in the model, while other constants are
assumed to be the same on both sites to limit the number of
fitted parameters (eq 4).
observed on Pd25Zn75/TiO
on the commercial Lindlar catalyst (5 wt % Pd/CaCO
more then 3 times lower than that observed on a spent Pd/ZnO
2
was comparable with that observed
3
) but
17
catalyst at 308 K.
Kinetic Model. From the study of the hydrogenation of
acetylene alcohols carried out by various authors the occurrence
of three possible pathways can be inferred. Pathway 1 takes
place through the associative adsorption of MBY and yields
the semihydrogenated product, MBE (Scheme 2). Pathway 2
takes place through the associative adsorption of MBE and
yields the fully hydrogenated product (MBA). Pathway 3
involves the dissociative adsorption of MBY on neighboring
Pd sites and results in the formation of C10-dimers. Many
k K C C
H2
4
Y
Y
r4 )
(4)
1
+ K C + K C + K C + K C
Y Y E E A A P P
2
A similar step occurred on the nonselective sites that were
included in the mechanism of acetylene hydrogenation.
23,24
The
authors concluded that the relative significance of the two paths
and, therefore, the selectivity can be controlled by the catalyst
and reaction conditions.
Fitting the model containing these equations to a number of
experiments in the presence of pyridine resulted in a value for
the equilibrium constants for adsorption of pyridine on the
catalysts are deactivated as a result of the absorption of the C10
-
dimers close to the Pd surface and the consequent blocking of
13
the active sites. As it is shown in the previous section, mass
transfer effects on the reaction rate are negligible compared to
the overall reaction rate. Therefore, kinetic equations can be
applied without including mass transfer in the modeling. Taking
selective and the nonselective sites, KP1,
KP2, of 100 and 960
(
(
19) Descamps, C.; Coquelet, C.; Bouallou, C.; Richon, D. Thermochim.
Acta 2005, 430, 1–7.
18
into consideration the weak hydrogen adsorption on Pd, as
20) Nijhuis, T. A.; van Koten, G.; Kapteijn, F.; Moulijn, J. A. Catal. Today
2
003, 79-80, 315–321.
(
16) Dom ´ı nguez-Dom ´ı nguez, S.; Berenguer-Murcia, A.; Pradhan, B. K.;
(21) Tschan, R.; Schubert, M. M.; Baiker, A.; Bonrath, W.; Rotgerink, L. H.
Catal. Lett. 2001, 75, 31–36.
´
Cazorla-Amo r´ ; os, D.; Linares-Solano, A. J. Catal. 2006, 243, 74–
8
1.
(22) Siegel, S.; Hawkins, J. A. J. Org. Chem. 1986, 51, 1638–1640.
(23) Bos, A. N. R.; Westerterp, K. R. Chem. Eng. Process. 1993, 32, 1–8.
(24) Al-Ammar, A. S.; Webb, G. J. Chem. Soc., Faraday Trans. 1979, 75,
1900–1911.
(
(
17) Semagina, N.; Grasemann, M.; Xanthopoulos, N.; Renken, A.; Kiwi-
Minsker, L. J. Catal. 2007, 251, 213–222.
18) Singh, U. K.; Vannice, M. A. J. Catal. 2000, 191, 165–180.
Vol. 13, No. 5, 2009 / Organic Process Research & Development
•
995

2 2
Table 3. Parameter values for the kinetic model in the hydrogenation of MBY on Pd/TiO and Pd25Zn75/TiO catalysts at 333 K
catalyst
Pd/TiO
2
confidence interval
Pd25Zn75/TiO
2
confidence interval
3
-1
-2
-3
k
k
k
k
K
K
K
K
K
1
2
3
4
, m /mol/s
1.8 × 10
(0.02
(0.5
1.1 × 10
(1.3 × 10
3
-2
-5
-3
, m /mol/s
4.0
9.3 × 10
1.0 × 10
(0.012
-1
-4
-2
, s
2.3 × 10
1.1 × 10
20
fixed
fixed
3
-3 a
-4
, m /mol/s
(0.002
1.2 × 10 (2.6 × 10 )
(2.0 × 10
3
2
Y
, m /mol
(2.8
2.1 × 10
(25
3
-2
-2
2
-2
E
, m /mol
3.5 × 10
1.3 × 10
1.0 × 10
9.6 × 10
1.7 × 10
0.045
(0.006
fixed
2.3 × 10
(0.002
fixed
(12
3
-2
A
, m /mol
1.3 × 10
3
2
P1, m /mol
(12
(80
1.0 × 10
3
2
2
P2, m /mol
9.6 × 10
(80
3
-1
-2
-4
Pd concentration, mol/m
6.1 × 10 (2.1 × 10 )
k
1
/k
2
, -
, -
0.12
9100
K
Y
/K
E
570
a
Values in the brackets are given for the batch reactor.
3
m /mol, respectively (Table 3). Apparently pyridine adsorbs on
Pd even more strongly than the alkyne, which is the explanation
of the reduction in the overall reaction rate. On the contrary,
the absorption constant of MBY on the Pd25Zn75 catalyst more
than twice exceeds that of pyridine, therefore its addition up to
the 0.1 pyridine/MBY ratio does not influence the hydrogenation
rate.
Due to the higher adsorption enthalpy of the alkynes, (K
) 570 and 9100 on Pd and Pd25Zn75, respectively), the
surface coverage of alkyne exceeds that of alkene until 99 and
9.9% conversion of alkyne on the Pd and Pd25Zn75 catalysts,
Y
/
K
E
9
respectively. This means that the alkyne either displaces the
alkene from the surface or blocks its readsorption. As a
consequence, an alkene is not hydrogenated in the presence of
an alkyne on the selective sites, while it undergoes ready
hydrogenation in the absence of the alkyne.
Figure 8. Product distribution in hydrogenation of MBY over
catalyst as a function of the reaction time in the
batch reactor. Lines represent the kinetic model. Symbols
represent experimental data. Temperature: 333 K. Pressure: 5
bar. Catalyst weight: 0.98 mg.
Pd25Zn75/TiO
2
The K
Y E
/K ratio of 570 observed in this study is close to
the same set of kinetic and adsorption constants as those in the
microreactor. The only significant difference was obtained in
the value of the rate constant of the direct hydrogenation on
the nonselective sites in accordance with a slightly lower
selectivity to MBE of 81% obtained in the batch reactor. A
lower selectivity can be explained by the different preparation
procedures of the thin films (dip-coating in the microreactor vs
spin-coating in the batch reactor) which might result in a
different concentration of the nonselective sites.
Table 4 summarizes the best results obtained on the Pd and
Pd25Zn75 catalysts. These results allow to conclude that wall-
coated capillary microreactors can be used as an efficient tool
in fine chemicals synthesis. The selectivity towards the semi-
hydrogenated product in the microreactor is even higher than
that observed in the batch reactor, while there is no need for a
subsequent catalyst separation step. It seems to be possible to
further improve the productivity in the hydrogenation of
acetylene alcohols by changing the stoichiometry of the Pd25Zn75
nanoparticles embedded in the mesoporous titania framework
by increasing the Pd/Zn ratio in the nanoparticles.
that obtained in the hydrogenation of 1-hexyne over a 3 wt %
25
Pd/C catalyst where a ratio of 133 was reported. In the same
study, a k /k ratio of 0.12 was found which is more than 2
times higher than that in the present work. An even higher
/k ratio of 2.3 is reported in a recent study on MBY
hydrogenation over a Pd/ZnO catalyst deposited on sintered
metal fibers, while the K /K ratio of 950 is almost 2 times
higher than that in the present study. These results suggest
that the kinetic constants for the Pd/TiO thin films are in the
range of earlier reported values for supported Pd catalysts.
As the MBY conversion increases from 5 to 95%, the (r
)/(r + 2r + r ) ratio, which determines the selectivity to
MBE, drops from 0.94 to 0.52 over the Pd/TiO catalyst, while
it remains virtually the same (0.90 and 0.89) over the Pd25Zn75
TiO catalyst (see Figure 2). In the presence of pyridine, this
ratio drops from 0.98 to 0.56 over the Pd/TiO catalyst, while
over the Pd25Zn75/TiO it drops only from 0.98 to 0.96. This is
mainly due to the fact that the adsorption constant K is an
order of magnitude larger on the Pd25Zn75/TiO compared to
that on the Pd/TiO catalyst.
1
2
25
k
1
2
Y
E
17
2
1
-
r
2
1
3
4
2
/
2
2
2
Y
2
2
An important parameter in industrial hydrogenation is the
catalyst cost per kilogram of product produced. The costs of
the in-house-made thin films with the embedded nanoparticles
are between 3 and 4 Euros per gram of support for the Zn/
An additional experimental run was performed in the batch
reactor to compare the results with those in the capillary
microreactor with the Pd25Zn75 catalyst (Figure 8). The experi-
mental curves for MBY, MBE, and MBA can be modeled with
2 2
TiO and pure Pd/TiO thin films, respectively. These costs do
not take into account manpower and electricity costs required
for their preparation. In general, the price of the in-house-made
ˇ
(
25) Dobrovoln a´ , Z.; Ka cˇ er, P.; Cerveny, L. J. Mol. Catal. A: Chem. 1998,
1
30, 279–284.
9
96
•
Vol. 13, No. 5, 2009 / Organic Process Research & Development

2 2
Table 4. Comparison of maximum MBE selectivity over Pd/TiO and Pd25Zn75/TiO catalysts
catalyst
reactor
batch
microreactor
microreactor
microreactor
additives
MBY conversion, %
MBE selectivity, %
Pd25Zn75/TiO
2
-
96.0
96.0
99.9
99.9
81.1
83.0
89.5
97.0
Pd/TiO
2
-
Pd25Zn75/TiO
Pd25Zn75/TiO
2
2
-
pyridine
a
Conversion from the range of 96-100% at which maximum selectivity was obtained.
2
TiO catalyst. Further fine-tuning of the nanoparticle
composition by increasing the Pd/Zn molar ratio can
improve the catalyst performance. The challenge here is
to find an optimum Pd/Zn ratio which would still provide
a high activity of the Pd catalyst while maintaining a high
selectivity observed on the Pd25Zn75 catalyst.
Experimental Section
Microreactors. The incorporation of the metal nano-
particles in the mesoporous titania thin films has been done
by addition of a fraction of the colloidal solution contain-
ing either Pd or Pd25Zn75 nanoparticles to the initial
solution containing the titania precursor, the surfactant,
and the solvent. This approach was successfully applied
in a previous study for the synthesis of catalytic thin films
containing Au nanoparticles. After ageing, the solution
was used to dip-coat the internal surface of a fused silica
capillary with an internal diameter of 250 µm and a length
Figure 9. Ethanol adsorption-desorption isotherms on the
mesoporous titania support (without nanoparticles) at 287 K
and the corresponding pore size distribution.
1
catalytic coatings remains competitive with the current price
of the commercial Lindlar catalyst produced at a similar scale
26
(
about 4-5 Euros per gram according to Sigma-Aldrich ). It
of 10 m. The characteristics of the Pd/TiO
TiO catalysts are listed in Table 1. In the kinetic tests, a
mixture of 2-methyl-3-butyn-2-ol (MBY) in methanol
0.011-0.45 M with or without addition of pyridine) was
2
and Pd25Zn75/
should be mentioned however, that the weight of the catalytic
coating in a 10 m capillary microreactor is about 0.9 mg. This
makes the catalyst costs less than 1% of the microreactor
hardware (capillary, mixing unit, and fittings). Of course the
contribution from the microreactor hardware will further
decrease during scale-up of a single capillary to a multiple
capillary microreactor assembly, consisting of several hundreds
or thousands of capillaries running in parallel.
2
(
mixed with a hydrogen/nitrogen mixture. The gas and
liquid were mixed in a T-mixer, the internal diameters of
the inlet and outlet tubes were 250 µm. The gas was fed
at an angle of 90° with respect to the liquid flow. The
liquid flow was varied between 2 and 12 µL/min and the
H
2
/N
2
flow between 0.250 and 1.0 mL/min (STP).
Conclusions
Batch Reactor. Silicon substrates with a cross section
Novel capillary microreactors that were wall-coated
with mesoporous titania thin films containing embedded
Pd or Pd25Zn75 (with a molar Pd/Zn ratio of 0.33)
nanoparticles have been tested in the hydrogenation of
2
of 9.8 × 9.8 mm and with a thickness of 0.5 mm were
used. The films were deposited on both sides of the
substrates by spin-coating at 1500 rpm at a relative
humidity (RH) of 80%. The as-deposited films were kept
at 80% RH for 24 h; then the temperature was increased
to 573 K under a residual pressure of 10 mbar at a heating
rate of 1 K/min followed by a dwelling interval of 4 h at
2
-methyl-3-butyne-2-ol in methanol by hydrogen. The
highest selectivity to the semihydrogenated product was
obtained at 333 K with pure hydrogen. Increasing the
nitrogen fraction in the gas phase resulted in a substantial
formation of 2,7-dimethyl-3,5-octadiyne-2,7-diol via oxi-
5
73 K. Ten Pd25Zn75/TiO
autoclave reactor with a total volume of 270 mL and were
in situ reduced at 523 K at a H pressure of 10 bar for
h. Then, the reactor was cooled to room temperature. A
.011 M solution of MBY in methanol (130 mL) was
2
/Si samples were placed in an
2
dated dimerization. The Pd/TiO catalyst demonstrated an
2
order of magnitude higher hydrogenation reaction rate
compared to the commercial Lindlar catalyst and gave an
alkene selectivity of 89% at 96% alkyne conversion at 3
bar hydrogen pressure in the presence of pyridine. The
selectivity was considerably improved towards 97% at
1
0
deoxygenated and transferred into the reactor. The hy-
drogenation reaction was performed at 333 K and 5 bar
hydrogen pressure under intensive stirring at 1500 rpm.
The analysis of the reaction mixture (2-methyl-3-butyn-
9
2
9.9% conversion over the Pd25Zn75/TiO catalyst, also
in the presence of pyridine. The subsequent MBE hydro-
genation step and the C10-dimer formation were sup-
pressed. However, in the latter case, the reaction rate
dropped by a factor of 16 compared to that with the Pd/
(27) Protasova, L. N.; Rebrov, E. V.; Ismagilov, Z. R.; Schouten, J. C.
Microporous Mesoporous Mater. 2009, doi:10.1016/j.microme-
so.2009.04.006.
(
28) Haverkamp, V.; Hessel, V.; L o¨ we, H.; Menges, G.; Warnier, M. J. F.;
Rebrov, E. V.; De Croon, M. H. J. M.; Schouten, J. C.; Liauw, M. A.
Chem. Eng. Technol. 2006, 29, 1015–1026.
(
26) http://www.sigmaaldrich.com.
Vol. 13, No. 5, 2009 / Organic Process Research & Development
•
997

2
-ol, 2-methyl-3-buten-2-ol, 2-methylbutan-2-ol, 2,7-di-
3 3
in HNO . The samples were treated with 2 mL of HNO
methyl-3,5-octadiyne-2,7-diol) was performed off-line
with a Varian CP-3800 gas chromatograph equipped with
a CP-Sil 5 CB capillary column (diameter: 1 mm, length:
diluted with ultrapure water and analyzed for their metal
content.
Acknowledgment
3
0 m) and an FID detector. Each compound was quantified
We thank Prof. D. Yu. Murzin from Abo Akademi Univer-
sity for his comments on the manuscript. The financial support
by the British Council-NWO Partnership Programme in Science,
Projects PPS 888 and 894, is gratefully acknowledged.
by authentic reference substances. To calculate standard
deviations, four samples were taken for analysis.
Characterization. The mesopore volume (porosity) and
the pore size distribution of the films deposited on the
flat substrates were determined by ellipsometric porosim-
etry (EP) from desorption isotherms of ethanol at 287 K
using the Bruggerman effective media approximation
Supporting Information Available
Calculation of the liquid hold-up and residence time
in the microreactor; mass balances in the batch reactor;
preparation of Pd and Pd25Zn75 bimetallic nanoparticles;
preparation of a mesoporous thin film with embedded
nanoparticles; Figure S1: TEM images of the prepared
Pd25Zn75 colloidal nanoparticles; Figure S2: particle size
distribution of Pd25Zn75 nanoparticles. This material is
availablefreeofchargeviatheInternetathttp://pubs.acs.org.
(
(
BEMA) and the modified Deriaguin-Broekhoff-de Boer
BDdB) theory. The mean pore size of mesoporous
2
7
titania prepared without addition of metal nanoparticles
in the synthesis solution was 3.0 nm (see Figure 9). The
bulk composition of the nanoparticles was measured by
emission spectroscopy (ICP-OES in a Perkin-Elmer
Optima 4300) after an aliquot of the colloids was dissolved
(
(
29) Chisholm, D. J. Heat Mass Transfer 1967, 10, 1767–1778.
30) Lu, P.; Teranishi, T.; Asakura, K.; Miyake, M.; Toshima, N. J. Phys.
Chem. B 1999, 103, 9673–9682.
Received for review April 7, 2009.
OP900085B
9
98
•
Vol. 13, No. 5, 2009 / Organic Process Research & Development