In situ IR and X-ray high spatial-resolution microspectroscopy measurements of multistep organic transformation in flow microreactor catalyzed by Au nanoclusters

Article abstract of DOI:10.1021/ja412740p

Analysis of catalytic organic transformations in flow reactors and detection of short-lived intermediates are essential for optimization of these complex reactions. In this study, spectral mapping of a multistep catalytic reaction in a flow microreactor was performed with a spatial resolution of 15 μm, employing micrometer-sized synchrotron-based IR and X-ray beams. Two nanometer sized Au nanoclusters were supported on mesoporous SiO₂, packed in a flow microreactor, and activated toward the cascade reaction of pyran formation. High catalytic conversion and tunable products selectivity were achieved under continuous flow conditions. In situ synchrotron-sourced IR microspectroscopy detected the evolution of the reactant, vinyl ether, into the primary product, allenic aldehyde, which then catalytically transformed into acetal, the secondary product. By tuning the residence time of the reactants in a flow microreactor a detailed analysis of the reaction kinetics was performed. An in situ micrometer X-ray absorption spectroscopy scan along the flow reactor correlated locally enhanced catalytic conversion, as detected by IR microspectroscopy, to areas with high concentration of Au(III), the catalytically active species. These results demonstrate the fundamental understanding of the mechanism of catalytic reactions which can be achieved by the detailed mapping of organic transformations in flow reactors.

Full text of DOI:10.1021/ja412740p

Article
pubs.acs.org/JACS
In Situ IR and X‑ray High Spatial-Resolution Microspectroscopy
Measurements of Multistep Organic Transformation in Flow
Microreactor Catalyzed by Au Nanoclusters
Elad Gross,† Xing-Zhong Shu,† Selim Alayoglu,† Hans A. Bechtel,‡ Michael C. Martin,‡ F. Dean Toste,*,†
and Gabor A. Somorjai*,†
†Department of Chemistry, University of California, Berkeley, California 94720, United States, and Chemical Sciences Division,
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
‡Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: Analysis of catalytic organic transformations in flow reactors
and detection of short-lived intermediates are essential for optimization of
these complex reactions. In this study, spectral mapping of a multistep
catalytic reaction in a flow microreactor was performed with a spatial
resolution of 15 μm, employing micrometer-sized synchrotron-based IR and
X-ray beams. Two nanometer sized Au nanoclusters were supported on
mesoporous SiO2, packed in a flow microreactor, and activated toward the
cascade reaction of pyran formation. High catalytic conversion and tunable
products selectivity were achieved under continuous flow conditions. In situ synchrotron-sourced IR microspectroscopy detected
the evolution of the reactant, vinyl ether, into the primary product, allenic aldehyde, which then catalytically transformed into
acetal, the secondary product. By tuning the residence time of the reactants in a flow microreactor a detailed analysis of the
reaction kinetics was performed. An in situ micrometer X-ray absorption spectroscopy scan along the flow reactor correlated
locally enhanced catalytic conversion, as detected by IR microspectroscopy, to areas with high concentration of Au(III), the
catalytically active species. These results demonstrate the fundamental understanding of the mechanism of catalytic reactions
which can be achieved by the detailed mapping of organic transformations in flow reactors .
Catalytic reactions along flow reactors can be analyzed by a
variety of spectroscopic tools, such as fluorescence, IR, Raman,
NMR, and X-ray spectroscopy.22−27 However, these methods
usually do not have sufficient spatial or spectral resolution for
detailed kinetic analysis of the catalytic reaction.7 Synchrotron-
based spectroscopy can fill this gap with a diffraction-limited IR
beam diameter of <10 μm.28,29 Several initial studies have
demonstrated the use of synchrotron IR and X-ray micro-
spectroscopy to characterize simple gas phase catalytic reactions
while employing zeolites or metal clusters as catalysts.24,27,30−35
However, in situ kinetic mapping of complex organic
transformation within a flow reactor under reaction conditions
has yet to be performed.
In this work, we demonstrate that synchrotron-based
microspectroscopy mapping can reveal detailed mechanistic
information about catalytic organic transformation. Two
nanometer sized Au nanoclusters were loaded on mesoporous
SiO2 support, packed in a flow microreactor, and employed as a
catalyst for dihydropyran formation (Scheme 1). The
heterogeneous Au catalyst activated this cascade reaction with
high yield and selectivity which could not be gained by its
homogeneous counterpart. 1D mapping was performed with
1. INTRODUCTION
Carrying out catalytic transformation in flow reactors instead of
batch reactors enhances the sustainability of the catalytic
reaction, providing a highly recyclable, scalable, and efficient
setup.1−3 Moreover, the spatial confinement of reactants inside
a flow reactor increases the catalytic conversion and enables
high control over the residence time of the reactants which can
improve product selectivity.4−7 As a consequence, flow reactors
are being widely utilized in the pharmaceutical and chemical
industries for the synthesis of complex organic molecules.
The advantages of flow reactors over batch reactors facilitate
the substitution of heterogeneous catalysts for their homoge-
neous counterparts because they can be easily loaded and
recycled in flow reactors.8−11 The heterogenization of
homogeneous catalysts is performed by two main approaches.
In the first approach, transition metal complexes are grafted on
solid supports or directly on the walls of the flow reactor.12,13
Another, more challenging, approach is to replace the
homogeneous catalysts with supported metallic nanoclus-
ters.5,8,14−19 As part of this effort, we recently reported that
small (<2 nm) metallic nanoclusters encapsulated by
polypeptides and supported on mesoporous SiO2 show unique
reactivity and selectivity toward a wide range of π-bond
activation reactions that were previously solely catalyzed by
homogeneous catalysts.5,14−16,20,21
Received: December 18, 2013
Published: February 5, 2014
© 2014 American Chemical Society
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SBA-15 (Au@G4OH/SBA-15), a mesoporous SiO2 support with a
surface area of 760 m2 g−1 and a pore diameter of 7 1 nm. The
dendrimer-encapsulated Au nanoclusters were easily deposited inside
the pores of the mesoporous support, without any modifications in the
cluster size.5 Anchoring of dendrimer-encapsulated Au clusters within
the mesoporous support ensured the catalyst’s stability under liquid-
flow reaction conditions.15,16 Au@G4OH/SBA-15 catalyst was then
packed in a 20 mm long fixed-bed stainless-steel plug flow
microreactor with an inner diameter of 0.7 mm. The catalyst was
flushed with a solution of 0.02 M of PhICl2, an inorganic oxidizer, in
order to oxidize the Au nanoclusters into the catalytically active
Au(III) species.14,16,21 No leaching of metal ions to the solution phase
was detected during this step or during the catalytic reaction.
Scheme 1. Cascade Dihydropyran Synthesis
synchrotron-based 10 μm spot-size X-ray and IR beams in
order to follow the propagation of the cascade reaction along
the flow microreactor with high spatial resolution. We took
advantage of the slow evolution of the liquid phase, organic
reactants in order to acquire a direct kinetic analysis of products
and intermediate formation. The catalytically active species and
their distribution along the flow reactor were detected by in situ
X-ray absorption microspectroscopy under reaction conditions.
3. RESULTS AND DISCUSSION
3.1. IR Microspectroscopy: Catalytic Yield and
Products Selectivity. Samples consisting of 0.5 mmol of
vinyl ether 1, 0.5 mmol of butanol-d10, and 0.05 mmol of
PhICl2 were solvated in 10 mL of toluene-d8 and flowed
through the preoxidized Au catalyst at different flow rates at
room temperature. As detected by GC and NMR analysis of the
products solution, by tuning the residence time of the reactants
inside the flow reactor the chemoselectivity of the products can
be modified.5 For example, at a flow rate of 10 mL/h, the short
residence time resulted in low conversion (20%) with a
primary:secondary (2:3) products ratio of 5:1. Higher yield
(75%) was obtained by decreasing the flow rate by 50-fold to
0.2 mL/h while the selectivity of the products (2:3) was
transformed to 0:100. These results demonstrate the advantage
in performing catalytic transformations in flow microreactors.
In these systems the chemoselectivity can be widely tuned by
the flow rate, enabling isolation of either the primary or
secondary product. Tunable selectivity between different
products in cascade reactions is not easily achieved while
employing homogeneous or heterogeneous catalysts in a batch
reactor. Though isolation of the secondary product is possible
in batch mode reactions, the exclusive formation of primary
product is not a simple task.4−6
2. EXPERIMENTAL SETUP
Cascade dihydropyran synthesis was chosen as a test reaction for 1D
microspectroscopy IR mapping of organic transformation in a flow
microreactor. In this reaction, propargyl vinyl ether 1 is catalytically
rearranged by the Au catalyst to the primary product allenic aldehyde
2. Activation of the primary product by the Au catalyst is followed by
nucleophilic attack of butanol-d10 resulting in the formation of the
secondary product, acetal 3. It was previously shown that the cascade
dihydropyran reaction can be catalyzed by homogeneous Au
complexes with limited product selectivity.36
One of the main advantages in choosing cascade dihydropyran
formation as a test reaction for microspectroscopy mapping is that
each of the reactants and products show distinguishable IR signatures
(Figure 1). As a consequence, the evolution of reactants into products
To analyze the kinetic evolution of products in the flow
microreactor the catalyst was deposited in a designated IR cell
(the dimensions of the rectangular flow cell were x = 20 mm, y
= 3 mm, z = 0.2 mm, a detailed scheme of the IR cell can be
found in the Supporting Information, Figure S1). IR absorption
spectroscopy measurements were conducted at beamline 1.4.4
of the Advanced Light Source (ALS) at Lawrence Berkeley
National Laboratory (LBNL) employing a Thermo Nicolet
Nexus 870 and Continuμm XL IR microscope. Line scanning
of up to 15 mm was performed along the flow reactor with 15
μm steps, 4 cm−1 spectral resolution, and averaging of 256
scans. IR measurements were performed with continuous
reactants flow, ensuring that the reactant and product
concentrations at each point along the flow reactor were at
steady state. IR absorption measurements were conducted as
close to the inlet of the flow reactor as possible. Due to physical
limitations of the IR flow cell, the initial 40 μm of the flow
reactor could not be resolved by IR. As a consequence, the
actual zero point of the flow reaction is 40 μm before the
beginning of the IR scans.
Figure 1. FTIR spectra of the reactant, vinyl ether 1 (red), the primary
product, allenic aldehyde 2 (green), and the secondary product, acetal
3 (blue). High-energy regime of IR absorption spectra of butanol-d10
(black), the second reactant, is shown as well. The black rectangles
mark the areas in which the IR spectra of the different reactants and
products can be easily distinguished from each other.
can be precisely monitored by IR spectroscopy. Decrease of the vinyl
ether 1 absorption peaks at 1220 and 1650 cm−1 indicates the
consumption of the reactant, while the formation of the primary
product 2 is identified by the CO stretch of the aldehyde at 1720 cm−1.
The secondary product 3 is mainly detected by a decrease in the
amplitude of the OD stretch of the nucleophilic butanol-d10 at 2450
cm−1. The OD bond is consumed by the nucleophilic attack of butanol
during the formation of acetal 3.
Subsequent to the flow reaction the products were analyzed
by GC. All the conclusions that derive from IR measurements
were supported by the GC analysis. The synchrotron-based IR
microreactor and the lab-based stainless-steel microreactor
were designed to have similar inner volume, ensuring
comparable residence time of the reactants. GC analysis
Au clusters (2.0
0.3 nm) were synthesized within a fourth-
generation polyamidoamine (PAMAM) dendrimer and loaded on
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Figure 2. IR absorption microspectroscopy scans along the flow reactor with reactants flow rates of 10 (a), 1 (b), and 0.2 (c) mL/h. Markers were
added to direct the eye toward the changes in the IR spectra at different flow rates. A scheme of the micro-IR flow reactor is shown in part d.
verified that the yield and selectivity of the products in these
two microreactors were comparable (with variations of up to
15%).
1D IR absorption spectra mappings were performed along
the flow microreactor at three different flow rates (Figure 2a−
c). The IR absorption spectra between 2000 and 2150 cm−1
were removed due to overabsorption by the toluene-d8 solvent
in this region. IR scans were conducted along the flow reactor,
following the reactants into products transformation (Figure
2d).
While employing a high flow rate (10 mL/h) of the reactants
solution through the microreactor (Figure 2a) few changes
were observed in the lower energy regime (1150−2000 cm−1).
A new absorption peak was detected at 1720 cm−1, indicating
the formation of the primary product, allenic aldehyde 2.
Interestingly, a maximum in the absorption peak of the primary
product was detected at 0.1 mm, before setting into steady
absorption amplitude at 0.2 mm. This gradual decrease in
aldehyde absorption is correlated to partial catalytic trans-
formation of the primary product into the secondary product,
acetal 3. A minor decrease was obtained in the IR absorption
lines which are correlated to vinyl ether 1 (1200−1400 and
1600−1700 cm−1), indicating that a relatively small fraction of
the reactant was consumed within this short reactants residence
time. Almost no changes were observed in the higher energy
range (2150−2750 cm−1), demonstrating negligible consump-
tion of butanol-d10 and no IR detectable formation of secondary
products under these conditions. These IR measurements
indicate that high flow rate conditions resulted in low
conversion and mostly the formation of the primary product.
These conclusions are supported by the above-mentioned GC
and NMR results.
aldehyde 2. After 0.2 mm, a gradual decrease in the aldehyde
absorption peak along with a comparable decrease in the higher
energy (between 2300 and 2700 cm−1) absorption spectra
regime indicated the parallel consumption of butanol-d10 and
aldehyde 2 to the formation of the secondary product, acetal 3.
It can be concluded that under these flow conditions, longer
residence time of the reactants improved the catalytic
conversion along with the formation of both the primary 2
and secondary 3 products.
Higher yield was obtained when further increasing the
residence time of the reactants by 5-fold with a slower flow rate
of 0.2 mL/h (Figure 2c), as obtained by the complete loss of
the reactant’s absorption peaks at 1150−1250 cm−1 after 0.6
mm. Though primary product, 2, was initially formed with a
maximum local IR absorption peak at 0.05 mm, after 0.4 mm
the aldehyde IR signature was fully lost. This result indicates
the catalytic transformation from primary into secondary
product. The formation of the secondary product, acetal 3, is
demonstrated as well by the total consumption of butanol-d10
as observed by the gradual decrease of the O−D absorption
peak at 2300−2650 cm−1 and its disappearance after 0.8 mm. It
can be concluded that at this slow flow rate, continuous
consumption of vinyl ether 1 was followed by formation of the
primary product, allenic aldehyde 2, which then further reacts
with the catalyst and the nucleophile alcohol to form the
secondary product, acetal 3.
Combining GC analysis of the products with IR micro-
spectroscopy mapping indicated that longer residence times of
the reactants were followed by an increase in the products yield.
Moreover, product selectivity was tuned by changing the
residence time of the reactants for the formation of either
secondary or primary products.5
More significant changes in the 1D IR mapping spectra were
detected when the residence time of the reactants was increased
by an order of magnitude by slowing the flow rate to 1 mL/h
(Figure 2b). A decrease in the reactant’s, vinyl ether 1,
absorption peaks amplitude (around 1150−1250 cm−1)
indicated higher consumption of the reactant, which was
followed by a stronger absorption peak of the primary product,
3.2. Kinetic Analysis of the Cascade Reaction. A
detailed kinetic study of the catalytic reaction was performed by
integrating the IR absorption peaks of the reactants and
products as a function of time on stream and location along the
flow reactor. The IR absorption peaks were converted to
concentration values (mmol/L) according to GC analysis of the
solution before and after the catalytic reaction (Figure 3).
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mm) vinyl ether 1 was consumed in the formation of allenic
aldehyde 2. During the second part of the reaction, a linear
decrease (zero order kinetics) in the concentration of the two
reactants was measured. The third phase of the reaction was
not detected under these flow conditions. A shorter residence
time of the reactants resulted in lower catalytic yield and high
concentration of the reactants along the flow reactor, which
prevented the transformation from zero to first order kinetics.
3.3. X-ray Microspectroscopy: Characterization of Au
Catalyst. While the total length of the flow microreactor was 2
cm, the catalytic transformation was completed within the
initial 1−2 mm of the reactor. As demonstrated in Figure 4a,
Figure 3. Analysis of distribution of reactants and products as a
function of time on stream and location along the flow reactor at a
flow rate of 0.2 mL/h. The absorption values of the reactants and
products were integrated from the matching IR absorption peak areas
and converted to concentration based on GC analysis of the solution.
The dotted lines indicate the three different stages of the catalytic
reaction.
The kinetic evolution of the reaction at a flow rate of 0.2
mL/h was divided into three stages. During the initial phase of
the reaction (0−1.3 s, 0−0.14 mm) the concentration of
reactant 1 gradually decreased due to formation of primary
product 2. During this incubation time, no formation of
secondary product 3 or consumption of butanol-d10 were
detected. The early stages of allenic aldehyde 2 formation could
not be resolved by IR due to limitations in the detection of the
initial 40 μm of the IR microreactor cell.
Through the second part of the reaction (1.3−4.7 s, 0.14−
0.52 mm) the two reactants (vinyl ether 1 and butanol-d10) and
the primary product (allenic aldehyde 2) were consumed
concurrent with formation of the secondary product, acetal 3.
Zero order kinetics with similar reaction rate coefficient values
of −4.2 and −5.0 mM/s were measured for vinyl ether 1 and
butanol-d10, respectively. First order kinetics was fitted to the
primary product allenic aldehyde 2 with k = 1.9 s−1. At this
stage of the reaction the local concentration of the two
reactants (vinyl ether 1 and butanol-d10) saturate the catalyst,
inducing zero order kinetics. The similarities in the
consumption rate of the two reactants are consistent with the
fast transformation of primary product (allenic aldehyde 2) into
secondary product (acetal 3) indicating that the rate-
determining step is the conversion of vinyl ether 1 into allenic
aldehyde 2.
At the third phase of the reaction (4.7−9 s, 0.52−1 mm) the
concentration of the reactants is low enough to result in the
conversion from zero to first order kinetics for the two
reactants. The fitted reaction rate coefficient values were 3.1
and 2.2 s−1 for vinyl ether 1 and butanol-d10, respectively. At
this stage, a continuous low, steady state concentration of
allenic aldehyde 2 was detected, demonstrating its pivotal role
as an intermediate for the formation of the secondary product,
acetal 3. These results further verified that the rate-determining
step in this reaction is the transformation of vinyl ether 1 to
allenic aldehyde 2. This analysis excluded the competitive
reaction mechanism that was previously suggested,36 in which it
was proposed that interaction between reactant 1 and butanol
can directly lead to the formation of acetal 3.
Figure 4. (a) IR microspectroscopy scans along the flow reactor at a
flow rate of 5 mL/h. (b) The relative amount of catalytically active
Au(III) along the flow reactor as detected by μm-NEXAFS
spectroscopy under reaction conditions, with a reactant flow rate of
5 mL/h.
with a reactants flow rate of 5 mL/h, no additional
transformation of either the reactant or the primary product
was detected after 1.5 mm, although the Au nanoclusters were
evenly dispersed all along the reactor. To understand the
reason for this localized enhanced catalytic conversion in a
specific part of the flow reactor, the oxidation state of the
supported catalyst was in situ analyzed by μm-NEXAFS
spectroscopy.
X-ray absorption spectroscopy measurements were con-
ducted at beamline 10.3.2 of the ALS at LBNL. The data were
collected in fluorescence mode at the Au L3 edge (11.918 KeV)
with a spot size of 16 × 6 μm (details of the experimental flow
setup used in these measurements can be found in the
Supporting Information, Figure S3). A quartz tube with an
inner diameter of 0.96 mm was packed with Au@G4OH/SBA-
15 and employed as a flow cell for the NEXAFS experiments.
Similar residence times of the reactants in the IR and NEXAFS
flow cells (with inner diameters of 0.6 and 0.72 mm2,
respectively) indicate that comparable kinetic rates would be
accomplished during the catalytic reaction in these two setups.
The products were analyzed by GC following the catalytic
reaction within the X-ray microreactor. The measured products
yield and selectivity were similar to those obtained following
the catalytic reaction within the IR microreactor. The analysis
Similar kinetic analysis was performed at a higher reactants
flow rate of 1 mL/h (Supporting Information, Figure S2). In
this case, only the two initial phases of the catalytic reaction
were observed. In the first part of the reaction (0−0.5 s, 0−0.3
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of Au(III)/Au(0) ratio was performed by fitting the acquired
spectra, which were measured at different points along the flow
reactor, to the reference spectra of Au(III) and Au(0). The
measured spectra could not be fitted to the reference Au(I)
spectrum, indicating that Au(I) was not a long-lived active
species during this reaction (Supporting Information, Figure
S4).
NEXAFS measurements along the flow reactor revealed that
after 2 mm the concentration of the catalytically active Au(III)
species was gradually decreasing due to catalyst reduction to
Au(0) (Figure 4b, raw NEXAFS spectra can be found in the
Supporting Information, Figure S4). NEXAFS measurements
that were conducted following oxidation of Au nanoclusters by
PhICl2 and prior to reactants flow, did not detect any major
modifications in the concentration of Au(III) along the flow
reactor. The reduction of Au(III) to Au(0) can be correlated to
the nucleophilic properties of butanol-d10, which can act to
some extent as a reducing agent. Butanol-d10 can either directly
reduce the Au(III) back to its metallic state or attack the
oxidizer, PhICl2, decreasing its concentration along the flow
reactor. The consequence in both scenarios is similar, the
catalyst would be reduced and butanol would be oxidized into
butyraldehyde, as indeed was detected in small amounts (∼5%
of the product) by NMR spectroscopy.
The oxidation of butanol-d10 into butyraldehyde-d8 following
the interaction with the oxidized Au catalyst was in situ
detected by IR spectroscopy. Butanol-d10 (0.25 M) and PhICl2
(5 mM) were flowed at 2 mL/h through the catalyst-packed
microreactor. Initial accumulation of butyraldehyde was
detected by IR microspectroscopy after 2 mm (Supporting
Information, Figure S5). The lower inlet concentration of
butanol (0.05 M) employed in the cascade reaction and the
slow formation rate of butyraldehyde make it difficult to
directly detect the butyraldehyde formation by IR micro-
spectroscopy under conditions employed in the rearrangement
reaction.
Interestingly, similar deactivation along the stream of flow
reactor was previously reported while employing Rh catalyst for
the methane oxidation reaction. In this example, the Rh catalyst
was reduced and deactivated by the product molecules which
were produced during the reaction along the flow reactor.27
3.4. Optimization of the Catalytic Reaction. Based on
our spectroscopic analysis we aimed to increase the catalyst
stability by reducing its deactivation rate. By decreasing the
inlet concentration of the reactants by 5-fold from 0.05 to 0.01
M (at a flow rate of 2 mL/h) the range of the active catalyst
along the flow reactor was extended from 2 mm to more than 6
mm, respectively. As a consequence, the catalytic trans-
formation of primary to secondary product was detected up
to 6 mm along the flow reactor (Supporting Information,
Figure S6). GC analysis of the products indicated that by
decreasing the reactants concentration from 0.05 to 0.01 M the
primary:secondary (2:3) products ratio was changed from 3:4
to 1:4 and conversion was increased from 40% to 58%,
respectively. These results indicate that by decreasing the inlet
concentration of the alcohol, the reduction rate of the active
catalyst was decreased, resulting in longer interaction times
between the reactants and the catalyst and improved products
yield and selectivity.
the amount of catalyst by 10-fold and packed it within the
initial 2 mm of the flow reactor. The remaining microreactor
volume was packed with pure SiO2. Following a catalytic
reaction with a flow rate of 0.2 mL/h and a concentration of
reactants of 0.05 M, the measured yield of the products was
71% with a primary:secondary (2:3) products ratio of 0:100.
Indicating that the residence time of the reactants can be
decreased by an order of magnitude from 200 to 20 s without
any major changes in the yield and selectivity of products.
4. CONCLUSIONS
We have utilized micrometer-sized synchrotron-sourced IR and
X-ray beams for 1D mapping of the organic phase and metallic
catalyst during the multistep reaction of dihydropyran
formation in a flow microreactor. It was demonstrated that
Au@G4OH/SBA-15, a heterogeneous catalyst, activates the
cascade reaction with high yield and tunable selectivity, which
was gained by adjusting the residence time of reactants in the
flow microreactor. The kinetic evolution of the organic
transformation and the important role of the primary product
as an intermediate were analyzed via in situ IR micro-
spectroscopy with spatial resolution of 15 μm. Direct
correlation was obtained between the catalytically active areas
along the flow reactor, as detected by IR, to local high
concentrations of the catalytically active species, Au(III), as
analyzed by μm-NEXAFS spectroscopy. These measurements
revealed that the oxidation state of the catalyst and as a
consequence its catalytic properties were altered along the flow
reactor. As demonstrated in this work, by mapping the gradual
evolution of the multistep catalytic process, a better under-
standing of the mechanism of catalytic reactions and the kinetic
role of the intermediates were obtained. These in situ
spectroscopy measurements of organic transformation in a
flow reactor with high spatial resolution are potentially
applicable to numerous catalytic and stoichiometric organic
reactions, fulfilling a major challenge in flow chemistry.7 This
novel analysis of catalytic reactions can be directly utilized to
probe and improve the catalytic yield and selectivity of organic
transformations in flow reactors.
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of the experimental procedure and spectroscopic and
microscopic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
fdtoste@berkeley.edu; somorjai@berkeley.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geological and Biosciences of the US DOE under contract DE-
AC02-05CH11231. The Advanced Light Source is supported
by the Director, Office of Science, Office of Basic Energy
Sciences, of the U.S. Department of Energy under Contract No.
DE-AC02-05CH11231.
The NEXAFS and FTIR measurements shown above
indicated that with a reactants concentration of 0.05 M the
catalytically active Au clusters reside in the initial 10% of the
flow reactors volume. According to these results we decreased
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(35) Tada, M.; Ishiguro, N.; Uruga, T.; Tanida, H.; Terada, Y. Phys.
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