Capillary microreactors wall-coated with mesoporous titania thin film
catalyst supports
Rebrov, E., Berenguer-Murcia, A., Skelton, H. E., Johnson, B. F. G., Wheatley, A. E. H., & Schouten, J. C.
(2009). Capillary microreactors wall-coated with mesoporous titania thin film catalyst supports. Lab on a Chip,
9(4), 503-506. DOI: 10.1039/b815716b
Published in:
Lab on a Chip
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Download date:17. Jun. 2017
Volume 9 | Number 4 | 2009
Miniaturisation for chemistry, biology & bioengineering
www.rsc.org/loc
Volume 9 | Number 4 | 21 February 2009 | Pages 485–624
Lab on a Chip
As featured in:
Volume 9 | Number 4 | 2009
Featuring research from the Department of Biosystems Research
and Development at Sandia National Laboratories, Livermore,
California.
Miniaturisation for chemistry, biology & bioengineering
www.rsc.org/loc
Volume 9 | Number 4 | 21 February 2009 | Pages 485–624
Lab on a Chip
Title: Isotropically etched radial micropore for cell concentration,
immobilization, and picodroplet generation
Pages 485–624
Using an adaptation of single-level isotropic wet etching, small shallow
micropores are radially embedded in larger deeper microchannels for
cell concentration, immobilization, and picodroplet generation.
ISSN 1473-0197
Schouten
Wall-coated microreactors
Levenberg
Piezoelectric droplet generator
Patel
Isotropically etched micropores
Reid
Aerosol arrays
1473-0197(2009)9:4;1-Y
See Kamlesh D. Patel et al., Lab Chip,
2009, 9, 507–515
Registered Charity Number 207890
Pages 485–624
www.rsc.org
ISSN 1473-0197
Schouten
Wall-coated microreactors
Levenberg
Piezoelectric droplet generator
Patel
Isotropically etched micropores
Reid
Aerosol arrays
1473-0197(2009)9:4;1-Y
COMMUNICATION
www.rsc.org/loc | Lab on a Chip
Capillary microreactors wall-coated with mesoporous titania thin film catalyst
supports†
Evgeny V. Rebrov,‡a Angel Berenguer-Murcia,‡b Helen E. Skelton,b Brian F. G. Johnson,b
Andrew. E. H. Wheatleyb and Jaap C. Schouten*ac
Received 9th September 2008, Accepted 31st October 2008
First published as an Advance Article on the web 17th November 2008
DOI: 10.1039/b815716b
A new method for catalyst deposition on the inner walls of capillary
microreactors is proposed which allows exact control of the coating
thickness, pore size of the support, metal particle size, and metal
loading. The wall-coated microreactors have been tested in a selective hydrogenation reaction. Activity and selectivity reach values
close to those obtained with a homogeneous Pd catalyst. The catalyst activity was stable for a period of 1000 h time-on-stream.
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 to diverse areas, such
as bioengineering, optics, and electronics. 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. For example, the synthesis of a large number of fine
chemicals, particularly in the field of flavor and fragrance chemistry
and pharmaceuticals, involves selective hydrogenation 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, multi-phase reactions can be performed in microchannels
or microcapillaries with a catalytic coating deposited on the channel
wall. However, the geometric surface area of the walls of the
microchannels is not usually sufficiently large for the deposition of
a catalyst with a homogeneous distribution. Consequently, introduction of thin catalytic coatings with large surface areas to the walls
of the reactor channels is a sought after goal. Thin films of inorganic
mesoporous materials have recently attracted considerable attention
in this respect because of their large surface area and narrow pore size
distribution, making them attractive candidates for catalyst
supports.1 Further, it has been shown that metal nanoparticles are
excellent hydrogenation catalysts when they are deposited on such
mesoporous supports.
a
Department of Chemical Engineering and Chemistry, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands. E-mail: [email protected]
b
Department of Chemistry, Cambridge University, Lensfield Road,
Cambridge, UK CB2 1EW
c
Institute for Complex Molecular Systems, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
† Electronic supplementary information (ESI) available: The detailed
protocol for the preparation of the thin film precursor solution and
high resolution transmission electron micrographs of confined
nanoparticles. See DOI: 10.1039/b815716b
‡ These authors contributed equally to the manuscript
This journal is ª The Royal Society of Chemistry 2009
Chip- and capillary-based microreactor research for fine chemicals
synthesis is a rapidly growing field worldwide following the realization of the benefits of microfluidic technology over conventional
chemical synthesis; these include improved temperature control,
enhanced selectivity, and both environmental and safety benefits
resulting from the use of small quantities of reagents and solvents.2
Early studies in this field included the preparation of macroporous
monolithic capillary microreactors3 that anchored different catalysts
in cross-linked polymer networks,4 or the use of surface grafting as
a means of specifically altering the properties of the porous surface.5
It should be noted, however, that when a solution is forced through
a micro packed bed capillary, it will tend to circumnavigate the
beads instead of passing through them, thereby severely affecting
the chemical efficiency of the system and often requiring larger
capillaries. An alternative to this approach utilizes wall-coated
capillary reactors. Recently, an efficient way to stabilize Pd catalyst
on microchannel walls was proposed,6 and the resulting microreactor
was tested in the reduction of a variety of substrates. Other studies
have demonstrated the potential of coating the inner surface of
a capillary with a thin mesoporous film.7,8 These systems have been
shown to be reliable in enzymatic and photocatalytic reactions.
Despite the large diversity of methodologies available,9 reproducible
preparation of the catalytic coating in microchannels remains
a crucial requirement in the fabrication of these multi-phase microfluidic devices. Herein we present the first use of a continuous
capillary microreactor with mesoporous titania thin films incorporated as catalyst supports for multi-phase catalytic reactions. The
potential of these novel systems is demonstrated by the results
obtained in a representative selective hydrogenation reaction.
Colloidal nanoparticles of approximately 2.5 nm mean diameter
were synthesised using a polyol reduction method modified from that
described in previous work,10 such that the synthesised nanoparticles
were finally dispersed in ethanol instead of methanol.
Nanoparticle-doped mesoporous titania thin films were generated
following a similar methodology published elsewhere.11 A precursor
solution with the following composition was prepared: 1 Ti(OBu)4
[titanium tetrabutoxide (TTB)] : 0.005 Pluronic F127 (BASF
Chemical Company) : 40 EtOH : 1.3 H2O : 0.13 HNO3. The solution
was made by adding the appropriate amounts of F127, water,
concentrated nitric acid, and TTB to ethanol, in that order. See ESI.†
Since the concentration of nanoparticles in ethanolic suspension
(vide supra) was fixed, a fraction of the ethanol that was used to
prepare the thin film precursor solution could be removed and
replaced by the colloidal suspension such as to obtain a theoretical
1 wt.% nanoparticle loading in the eventual mesoporous titania thin
film. The resulting mixture was stirred at room temperature for 2 h.
Lab Chip, 2009, 9, 503–506 | 503
After ageing, it was used to coat the interior surface of a fused silica
capillary with an internal diameter of 250 mm and a length of 9 m
(Varian Inc.). It should be noted that in principle, following this
approach allows the metal loading in the resulting thin films to be
exactly controlled by changing the concentration of nanoparticles in
the ethanol suspension.
Prior to dip-coating, the internal surface of the capillary was
activated using a flow of 1 M NaOH aqueous solution (0.03 mL
min1) for 15 min to enhance the adhesion qualities of the channel
wall during subsequent coating. The capillary was then flushed for
15 min (0.03 mL min1) with demineralised water and the titania sol,
respectively. Finally, the titania sol was withdrawn from the capillary
at a rate of 1 cm s1. The capillary was dried and calcined in an oven
at 300 C at a residual pressure of 15 mbar. Manipulation of the rate
of solution removal from the capillary allowed precise control of the
coating thickness on the nanometer scale. For example, a withdrawal
rate of 1 cm s1 may provide a coating thickness of 120 nm. A similar
coating was deposited on 1 1 cm2 flat silicon substrates in order to
perform characterization.
The porous structure of the prepared film was determined by
ellipsometric porosimetry (EP) using the adsorption–desorption of
ethanol at 14 C. The saturation pressure of ethanol at this temperature (41 mbar) provides a similar number of data points on
adsorption–desorption isotherms as that obtained in nitrogen
adsorption. In these experiments both the effective refractive index
and the film thickness were measured. The latter was determined by
monitoring the change in the effective refractive index in the range
300–1200 nm by spectroscopic ellipsometry. The hydrogenation of
phenylacetylene in the Pd/TiO2 coated capillary microchannel was
studied in the 30–50 C temperature range. The gas and liquid flows
were carefully controlled in order to work under the annular flow
regime in all instances. See ESI.†
Taken together, the small size of the prepared nanoparticles and
the fact that the nanoconfined colloids were calcined at 300 C in
vacuo would lead to the expectation that significant nanoparticle
aggregation should occur. However, our observations indicate that
confinement within the mesopores of the inorganic matrix stabilizes
the particles and prevents agglomeration. Fig. 1 shows TEM images
obtained from the thin films collected from the flat silicon substrates,
in which both the porous mesostructure and the nanoparticles are
visible.
The TEM images reveal that the nanoparticles retain their original
sizes throughout thermal treatment, indicating that the synthetic
Fig. 2 FEG-SEM images of the cross-section of a coated fused silica
capillary with an inner diameter of 250 mm with a Pd-doped mesoporous
titania film produced by the sol-gel method.
strategy described is a very promising one for the more general
confinement of colloidal nanoparticles in porous inorganic matrices,
see ESI.† It was also necessary to confirm that the nanoparticles did
not have a negative influence on the development of the mesoporous
network. In order to verify the formation of the desired cubic
mesostructure (apparent from Fig. 1B), an ethanol adsorption–
desorption isotherm was obtained using one of the doped thin films
deposited on a silicon wafer. According to the Bruggeman effective
medium approximation model, the coating porosity was 0.17,
resulting in a total coating mass of 1.32 mg with 1 wt.% Pd content.
Considering that the images presented in Fig. 1 were obtained from
thin films deposited on flat substrates, one might argue that the
coating of a capillary wall could easily yield a different result.
However, Fig. 2 shows the cross-sectional view of the capillary, and it
is clear from the images that the thickness of the layer (90 nm) is
similar to that determined by EP (108 nm). Using HR-SEM images,
20 measurements were made at different locations along the channel
to obtain the average thickness of the deposited layer in the capillary.
This was found to be 92 12 nm.
The parameters of the thin films achievable using the route
described here can be adjusted very precisely in order to fit the
adsorption mode of specific organic substrates and to avoid diffusion
limitations. Table 1 summarizes some of the parameters that can be
fine-tuned to meet specific demands.
As Fig. 3 shows, the isotherms obtained by ethanol adsorption–
desorption are typical type IV isotherms and are characteristic of
mesoporous solids. Using these data, the pore sizes were determined
to be 2 and 3 nm, respectively, for the two thin films studied. While
this result shows that although the mesostructure is not completely
Table 1 Tunable parameters of thin films and selected procedures
Parameter
Procedure
Metallic ratio in the
nanoparticlea
Nanoparticle size
Change initial precursor
concentrations12
Change amount of protecting
agent (PVP)13
Change surfactant/fractional
volume of surfactant in solution14
Use of co-surfactants
(e.g. N-butanol)13,14
Change delivery flow rate/Perform
multiple depositions15
Porous structure of
the coating
Pore sizeb
Coating thickness
Fig. 1 TEM images of the porous titania thin film. (A) Image showing
Pd nanoparticles (dark spots) embedded in the inorganic matrix. (B)
Image showing the porous network of the deposited thin film. Scale bars:
50 nm.
504 | Lab Chip, 2009, 9, 503–506
a
An example of the effect of Pd/Zn molar ratio on the selectivity in
hydrogenation of terminal acetylenic alcohols is presented in the ESI.†
b
An example of a thin film with a pore size of 5 nm is described and
characterized in the ESI.†
This journal is ª The Royal Society of Chemistry 2009
Fig. 4 X-Ray diffractogram of a calcined mesoporous TiO2 thin film,
the ethanol isotherm of which is shown in Fig. 3B.
Fig. 3 Ethanol adsorption–desorption isotherm obtained for two
different TiO2 thin films dip-coated (withdrawal rate 1 cm s1) on silicon
wafers with average pore sizes of (A) 2 nm and (B) 3 nm.
developed, the sharp ethanol uptake clearly indicates the presence
of pores with a narrow size distribution within the mesoporous
regime. Use of the evaporation induced self-assembly method to
produce thin mesoporous films by dip-coating can thus be regarded
as suitable for the hosting of nanoparticles with different sizes. The
H2 hysteresis loops present in the isotherms indicate that both films
possess a cubic mesostructure and, therefore, agree with our TEM
observations.
In order to further demonstrate the structural order of the
prepared thin films, X-ray diffraction was performed on the samples
after calcination. Fig. 4 shows the diffractogram obtained for the
sample corresponding to Fig. 3B. Analysis of the (100) peak results in
a lattice spacing of 3.9 nm, which is consistent with the data obtained
from the ethanol adsorption isotherm. Furthermore, the presence of
the (110) and (200) peaks at 2q angles of approximately 4.8 and
5 degrees are indicative of structured materials of good quality.
The capillary systems were tested as microreactors in selective
hydrogenations under different flow and temperature conditions.
Table 2 shows the results obtained for the semi-hydrogenation of
phenylacetylene. As the liquid flow rate of the feedstock solution
inside the capillary is increased, the conversion of phenylacetylene is
significantly reduced, whilst the selectivity with respect to styrene
formation increases. As a result of the increased ratio between the
gas and liquid flow rates the flowing liquid film becomes thinner,
which results in a higher liquid linear velocity and shorter liquid
residence time in the reactor. This improves the selectivity towards
This journal is ª The Royal Society of Chemistry 2009
the partially hydrogenated product because at the lower liquid
residence time the subsequent hydrogenation of phenylacetylene
does not occur.
It has proved possible to control both the conversion and selectivity of the catalytic process by careful restriction of the flow and
temperature conditions in the reactor assembly. By doing this, it is
possible to obtain phenylacetylene conversions close to 95% with
selectivities towards styrene in excess of 85%. The reaction rate in
terms of TOF was found to be up to 2 s1, a value comparable to that
reported for the same Pd nanoparticles in homogeneous phase.10 The
reaction rates in terms of TOF of 2 s1 are typical for kinetically
limited systems. The maximum value of the overall mass transfer
1
coefficient (kov) in the capillary microreactor is 0.01 m3L m3
R s ,
where subscripts L and R stand for liquid and reactor, respectively.
This value is rather low as compared with bulk mass transfer limited
1
systems (kov ¼ 0.1 m3L m3
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 reactors. The
catalyst loading in the capillary reactor corresponds to 2 kgPd m3
R.
Comparing this loading with that in batch systems, one can conclude
that this is in the range of catalyst concentrations within which most
kinetic studies have been carried out. Our results demonstrate the
potential of efficiently integrating thin film technology with colloidal
chemistry to produce different mesoporous matrices that can host
a variety of mono- and bimetallic nanoparticles. Moreover, the
capillary microreactors presented in this study maintained both their
activity and selectivity after more than one month of continuous use
under a variety of conditions. These results are comparable to those
obtained by Ueno et al., whereby a polymeric capillary loaded with
a Pd catalyst has shown stable performance for 100 h after crosslinking at 180 C. However, the presently reported systems are not
restricted to the inclusion of monometallic nanoparticles, but have,
instead, demonstrated the possibility of incorporating bimetallic
nanoparticles prepared (with compositional and size control) by the
polyol reduction method. In this respect, further improvements to
this reactor are currently under development by ourselves. These are
focused on the confinement of bimetallic catalysts such as Pt:Zn
and Pd:Zn nanoparticles for the selective hydrogenation of citral
(a,b-unsaturated carbonyl groups) and 2-methyl-3-butyn-2-ol (to
produce a precursor to vitamins A and E).
Lab Chip, 2009, 9, 503–506 | 505
Table 2 Summarized performance of a capillary microreactor with 1 wt.% Pd in selective hydrogenation as a function of flow rate and temperature
Temperature
30 C
Flow rate
Conversion
Selectivitya
Conversion
Selectivitya
Conversion
Selectivitya
99.2
94.2
54.9
19.8
64.0
85.2
94.5
94.6
99.5
97.5
99.8
35.2
5.3
24.5
53.8
93.8
99.9
99.9
99.8
50.3
1.6
44.8
62.8
92.8
3 mL
4 mL
5 mL
6 mL
a
min1
min1
min1
min1
40 C
50 C
Relative to styrene (semi-hydrogenation product).
In summary, we report the direct incorporation of air-stable
nanoparticles into the mesoporous inorganic matrix of a thin film
delivered to the internal surface of a fused silica capillary. The
nanoparticles retain their small particle size even after calcination
in a vacuum furnace, and are accessible for reaction thereafter. A
number of parameters can be easily fine-tuned in order to prepare
capillary microreactors on-demand to fit different specifications.
These novel systems have shown promise in a representative selective
hydrogenation test reaction.
The authors would like to acknowledge the European Commission
(NOE EXCELL NMP3-CT-2005-515703), the British Research
Council and NWO (Projects PPS-888 and PPS-894) for financial
support.
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This journal is ª The Royal Society of Chemistry 2009