Parallel analysis of the reaction products from combinatorial catalyst libraries

Article abstract of DOI:10.1002/1521-3773(20010817)40:16<3028::AID-ANIE3028>3.0.CO;2-X

Rapid-scan FT-IR spectral imaging allows high-throughput screening of the activities of components of a catalyst library in a truly parallel fashion. The gasphase reaction products from 16 supported catalysts can be investigated simultaneously with the re

Full text of DOI:10.1002/1521-3773(20010817)40:16<3028::AID-ANIE3028>3.0.CO;2-X

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1
pling.[3] Reactants, products, and carrier gas are withdrawn
from each microreactor channel using a capillary sampling
probe. By repeating this approach for each microreactor, the
entire library can be screened. Another experimental method
is based on photoionization of reaction products using tunable
[
15] Compound 6: 31P{ H} NMR (121.5 MHz, [D
8
]xylene, 508C, H
3
PO
4
):
2
2
1
d ˆ 36.6 (d, J(P,P) ˆ 25.4 Hz), 0.6 (d, J(P,P) ˆ 25.4 Hz). H NMR
3
(300 MHz, [D
8
]xylene, 508C, TMS): d ˆ 0.71(d, J(P,H) ˆ 21 Hz, 3H;
), 0.73(d, 3J(P,H) ˆ 23 Hz, 3H; PCCH
), 4.12 (d, J(P,H) ˆ
2
PCCH
3
3
4
1
5.4 Hz, 1H; NH), 4.50 (br, 2H; NH), 7.18 (d, J(H,H) ˆ 2.3 Hz,
4
1
H; Aryl-H), 7.29 (d, J(H,H) ˆ 2.0 Hz, 1H; Aryl-H), 7.32 (d,
4
4
J(H,H) ˆ 2.0 Hz, 1H; Aryl-H), 7.44 (d, J(H,H) ˆ 2.3 Hz, 1H; Aryl-
[4]
UV lasers. The resulting photoions are detected by a
1
31
H). Signal assignments were aided by 2D H{ P}-HMQC spectra; the
remaining resonance signals could not be unequivocally assigned.
16] See: J. Böske, E. Niecke, B. Krebs, M. Läge, G. Henkel, Chem. Ber.
microelectrode in close proximity to the sample. One
disadvantage of this technique is that a suitable laser
frequency for each species of interest must be known and
accessible. In general, all of these techniques have the
shortcoming that the screening time is proportional to the
library size.
[
1992, 125, 2631.
In contrast, truly parallel screening techniques gather
information simultaneously from all the elements in a library.
This category so far only includes heat-sensing techniques.
Infrared thermography and thermistor arrays detect heat
evolved from active library members and have been used to
detect activity for exothermic reactions in combinatorial
Parallel Analysis of the Reaction Products from
Combinatorial Catalyst Libraries**
Chris M. Snively, Gudbjorg Oskarsdottir, and
Jochen Lauterbach*
[5]
libraries. These techniques, however, cannot chemically
resolve product composition, which is often the most impor-
tant issue when studying catalytic reactions, and therefore
cannot determine the selectivity of a catalyst. Also, the
assumption is made that the exothermicity is derived solely
from the desired reaction, and not from any unforeseen side
reactions, limiting these techniques to the study of well-
known, simple reactions.
The combinatorial approach has great potential in many
disciplines to optimize systems that have large parameter
spaces. Recently, this concept has been introduced to the field
of materials science.[ The ultimate goal of the combinatorial
approach is to efficiently optimize and discover new formu-
lations, be they pharmaceutical products, catalysts, or other
materials. Practically, this is accomplished by a systematic and
efficient exploration of the parameter space that controls the
properties of the final product. The two key components to a
successful combinatorial approach are the controlled syn-
thesis of a collection of materials with systematic variations in
properties and the subsequent high-throughput analysis of
libraries of these materials. Speed, through parallel synthesis
and characterization, consequently becomes critical for the
success of the combinatorial discovery process. Herein, we
report the first analytical technique for truly parallel high-
throughput screening of the reaction products from libraries
of heterogeneous supported catalysts.
1]
FT-IR imaging has the ability to gather chemically sensitive
information from all library elements simultaneously. This
approach has been demonstrated in recent work, where our
group has pioneered infrared spectral imaging for the rapid
[6]
analysis of reactions on solid bead materials. IR spectros-
copy is a well-established tool for the analysis of the
composition of gas mixtures. The lower detection limit
depends on several factors, such as the absorptivity of the
absorption bands of the specific gas, the path length of the cell,
the concentration of the species of interest, the spectral noise
level, and the strength and structure of the bands of
interfering gases. The true power of FTIR imaging for high-
throughput analysis lies in its capability for parallel examina-
tion of product streams from multiple reactors. For this
purpose, we have developed a novel gas-phase array attached
to a multiple sample reactor, which currently allows us to
perform parallel screening of the product stream of 16
supported catalyst samples.
A number of experimental approaches have been reported
for screening catalyst libraries. These are based on conven-
tional serial techniques, which have been automated to
decrease the screening time. Scanning mass spectrometry is
based on rapidly analyzing the gases from one sample in a
combinatorial library at a time, in a sequential manner. One
approach uses a single probe composed of coaxial gas delivery
To demonstrate the applicability of gas-phase IR imaging to
parallel reaction product analysis, we present results, in which
the parallel FTIR analysis was used to determine conversion
during temperature-programmed complete oxidation of pro-
pene. This reaction is important for the automotive three-way
catalyst and, in general, hydrocarbon oxidation is mainly
[
2]
and gas analysis tubes. Libraries are analyzed by sequen-
tially placing the tube over each element of the library,
feeding reactant gases, and analyzing product gases. This
approach is applicable to the initial screening of libraries
deposited onto flat, solid substrates due to its gas delivery
design. A second approach uses array microreactors with
supported catalysts, coupled with capillary microprobe sam-
[7]
catalyzed by platinum group metals.
The samples examined in this study were commercial
catalyst monoliths as well as custom-synthesized supported
catalyst powders. Additionally, some channels of the reactor
were filled with blank support material. After approximately
[
*] Dr. C. M. Snively, G. Oskarsdottir, Prof. Dr. J. Lauterbach
School of Chemical Engineering, Purdue University
West Lafayette, IN 47907-1283 (USA)
Fax : (‡1)765-494-0805
0.2 g of each catalyst sample was loaded into the 16-catalyst
parallel reactor; all samples were pretreated simultaneously
by alternate oxidation and reduction cycles. After the reactant
E-mail: jochen@ecn.purdue.edu
[
**] This work was supported by the National Science Foundation (grant
CTS-0071020).
�
1
gases were introduced, a temperature ramp of 10 Kmin was
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established, and absorbance images of the exit stream of all 16
samples were taken every 30 s over a temperature range from
650 K 12 catalysts produce CO . All of the inactive array
2
elements are those containing the silica and alumina support
materials, which are expected to show no reactivity. Notably,
3
00 K to 670 K. Figure 1 shows a representative FTIR
�
1
spectrum of the exhaust stream at 300 K. The spectrum is
dominated by the absorption bands of propene. This spectrum
was taken from one pixel in the detector array using one scan,
that is, no coaddition or averaging of spectra was performed.
This demonstrates the excellent spectral quality that can be
achieved with this instrumental setup.
one entire data set (i.e., 4096 IR spectra with 8 cm spectral
resolution) takes less than 20 s to collect and process.
Integration of the IR bands belonging to the desired
reaction products from each array element in the image allows
quantitative analysis of the product stream. Data for the
amount of CO produced as a function of reactor temperature
2
are shown in Figure 3 for seven selected catalysts from the
Figure 1. Representative gas-phase spectrum of propylene recorded with
the FTIR imaging spectrometer equipped with the gas-phase sampling
array.
Figure 3. Results for the production of CO
propene for four g-Al catalysts with 1 wt% Pd (
60 K), 5 wt% Pt (^, 450 K), 5 wt% Ru (^, 420 ± 435 K), a SiO
with 2.67 wt% Pd (~), pure SiO
support (*), and two commercial catalyst
2
during the oxidation of
, 520 K), 1 wt% Pt (*,
catalyst
&
2
O
3
5
2
2
samples (3, 420 ± 435 K). The temperature or temperature range given in
Figure 2 shows absorbance images for the gas-phase CO2
vibration band for catalyst temperatures of 440 K and 650 K.
Plotting absorbance intensities for a specific reaction product
for the entire array reveals the position of the active catalysts.
High intensity in the image indicates an active catalyst
parentheses indicates the light-off temperature of the catalyst. (Â) propene
signal for the 5 wt% Pt on g-Al
2
O
3
catalyst. Yˆ conversion [%].
array. The pure support shows no measurable reactivity, as
expected. The dashed line shows the decrease of propene in
converting propene to CO . Each pixel in the image contains
2
�
1
a full IR spectrum from 2600 to 1300 cm . At 440 K, seven
catalysts are active for total oxidation of propene, while at
the reactor effluent for the 5 wt% Pt on g-Al O catalyst. This
2 3
line follows very closely the CO production and the material
2
balance closes within an error of less than 5%. The
reproducibility of the method is demonstrated by comparing
two identical catalysts (commercial samples; Figure 3).
With the temporal resolution of our analytical technique, it
becomes possible to monitor the effect of changing control
parameters of the reaction (i.e., temperature, gas composi-
tion) on a time scale of seconds. Additionally, this allows the
high-throughput evaluation of the light-off characteristics of
catalysts. For example, it took only around 30 min to assess
the light-off characteristics and reactivity of an array of 16
supported catalyst samples over a temperature range of
350 K.
It has been demonstrated that rapid-scan FTIR spectral
imaging is the first technique that is capable of monitoring
gas-phase reaction products from multiple members of a
supported catalyst library in a truly parallel fashion. This
experimental methodology is fully scalable, and with the
coming availability of larger FPA detectors, this approach will
concentration (2360 cm 1) measured by
�
Figure 2. Spectral images of CO
2
FTIR imaging at 440 K (A) and at 650 K (B). Light-off between these two
temperatures can be seen for several elements of the array. C) Schematic
representation of the location of the individual cells in images A and B.
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be capable of screening 100 s to 1000 s of supported catalyst
samples on a time scale of seconds. In addition, rapid-scan
FTIR imaging is a flexible technique, which can be used for
analysis of many combinatorial systems, that is, the technique
is independent of the specifics of the reactor setup.
for 6 h, and reduced at 473 K in 20% H
pretreated simultaneously in situ by flowing oxygen in nitrogen at 625 K for
0 min. Reaction conditions were established with flow ratios of prope-
ne:O :N of 0.1:0.57:1 with a total flow of 210 sccm. The reactor was heated
to the desired temperature, allowed to equilibrate, and infrared spectral
imaging data of the gas-phase array were collected.
2
/He for 12 h. All catalysts were
9
2
2
Received: February 28, 2001 [Z16698]
Experimental Section
The optical setup consists of a Nicolet Magna 860 FTIR spectrometer,
2
barium fluoride (BaF ) optics, a long pass optical filter, a KBr diffuser, and
[1] a) W. F. Maier, G. Kirsten, M. Orschel, P. A. Weiss, Chim. Oggi 2000,
a 64 Â 64 pixel mercury cadmium telluride (MCT) focal plane array (FPA)
detector. A more detailed description of the rapid-scan imaging setup can
be found in reference [8]. The reactor has 16 individual channels that each
hold one powdered catalyst sample. The individual gas outlets are
connected to the gas sampling array constructed from 16 stainless steel
tubes, each with a separate gas inlet and outlet. The ends of the cells are
equipped with IR transparent windows sealed by gaskets. The cells are
arranged in an array fashion, so that all 16 can be studied simultaneously
using our FTIR imaging system. The entire path that the reactant and
product gases travel through is composed of stainless steel, teflon, buna
rubber, and calcium fluoride, all of which are not catalytically active at the
temperatures used in the experiments. Currently, each cell has a path length
of 15 cm. However, due to the fact that a parallel IR beam travels through
the array, it is possible to extend the cells. This might become necessary, for
example, to study gas concentrations below the current detection limit,
which was determined to be around 1000 ppm for the case of propene. This
may also become necessary when studying mixtures of gases with similar IR
signatures, where weak vibrational bands have to be followed to distinguish
various components of the gas mixture. Also, due to the inherent nature of
IR vibrational spectra, this technique is not suited for the detection of
1
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1
5
5
% Pt on g-Al
% Pt on g-Al
% Ru on g-Al
2
O
3
, and 1% Pd on g-Al
, (from Alfa ásar), 5% Pd on g-Al
(from Alfa ásar), pure silica support powder, and a
2
O
3
(all synthesized in house), and
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2
O
3
2 3
O
(from Alfa ásar),
2
O
3
commercial automotive monolith catalyst. The synthesized catalysts were
prepared by using the incipient wetness technique, calcined in air at 573 K
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