High-resolution single-turnover mapping reveals intraparticle diffusion limitation in Ti-MCM-41-Catalyzed epoxidation

Article abstract of DOI:10.1002/anie.200905039

(Figure Presented) Shedding light on catalysis: For the first time, transport phenomena and their influence on catalysis are directly observed for individual catalytic particles by single-turnover mapping using highresolution fluorescence microscopy. The Thiele modulus could be measured with just one experiment, proving the existence of diffusional limitations for the Ti-MCM-41 catalyzed epoxidation of a fluorescent reporter molecule (see picture; TBH P = tert-butylhydroperoxide).

Full text of DOI:10.1002/anie.200905039

Communications
DOI: 10.1002/anie.200905039
Heterogeneous Catalysis
High-Resolution Single-Turnover Mapping Reveals Intraparticle
Diffusion Limitation in Ti-MCM-41-Catalyzed Epoxidation**
Gert De Cremer, Maarten B. J. Roeffaers,* Evelyne Bartholomeeusen, Kaifeng Lin,
Peter Dedecker, Paolo P. Pescarmona, Pierre A. Jacobs, Dirk E. De Vos, Johan Hofkens, and
Bert F. Sels*
Micro- and mesoporous materials offer unique opportunities
for catalysis thanks to their large surface area. By introducing
active elements inside the pore walls of such materials, a wide
range of acid–base or redox catalysts has been developed.[1,2]
For example, incorporation of Ti sites in silicalite resulted in
the TS-1 catalyst, which is known for its high performance in
the selective oxidation and epoxidation of hydrocarbons.[3]
However, the small (0.55 nm) micropores of this catalyst
hinder the uptake of larger olefins as substrates for the
epoxidation.[4] To circumvent this limitation of TS-1, titano-
silicates with larger pores, such as Ti-Beta[5] and Ti-MWW[6]
zeolites, have been synthesized. Even mesoporous titanosili-
cates such as Ti-MCM-41 were developed with the aim of
faster diffusion of more bulky substrates towards the inner
active sites.[7,8] MCM-41 materials are characterized by a
hexagonal array of pores with a uniform diameter that can be
tuned between 1.5 and 10 nm.[9] Despite the relatively large
pore size, maximal utilization of the Ti sites in diffusion
unlimited conditions remains a major challenge. Typically, Ti-
MCM-41 is prepared in the form of particles with sizes of a
few micrometers. It was recently demonstrated that a
decrease in particle size to about 100 nm was accompanied
with a relevant increase in selectivity and reaction rate for the
epoxidation of cyclohexene and cholesterol.[10,11] It was
reasoned that intraparticle diffusion limitations in the mes-
opores of the large particles hindered an optimal use of the
active titanium sites, similarly as previously described for the
microporous TS-1 catalyst.[4]
The kinetics of a catalytic process are often governed by
the interplay between diffusion and reaction. Such insights
are classically gathered by macroscopic kinetic experiments,
for example, by comparing reaction rates using crystals with
different sizes, by varying space velocities of the feed, or by
measuring apparent activation energies.[12,13] Pulsed-field
gradient NMR spectroscopy has been used to determine
intraparticle diffusion coefficients during catalysis, but this
technique is restricted to extremely large particles (> 10 mm)
and only yields ensemble-averaged results.[14,15] Recent tech-
nological evolutions in optical microscopy now offer the
opportunity to confront these insights with in situ observa-
tions for single catalyst particles.
The high spatiotemporal resolution (submicrometer and
milliseconds) of (single-molecule) fluorescence microscopy
has proven to be extremely useful to study catalysis at the
level of individual particles or even at the level of individual
reaction events,[16–24] as well as to investigate diffusion
processes in mesoporous materials.[25–29] However, so far
these two phenomena, catalytic conversion and diffusion in
porous materials, were treated separately in single-molecule
studies; no direct information on the interplay between these
two processes has been obtained.
Moreover optical microscopy is subject to the laws of
diffraction, limiting the spatial resolution to a few hundred
nanometers, whereas the interesting processes related to
catalysis within porous particles typically occur on smaller
length scales. The present contribution circumvents the
resolution discrepancy by applying a single-turnover-based
strategy in fluorescence microscopy to provide diffraction-
unlimited resolution.[29] This approach allows mapping the
catalytic activity with nanometer-scale spatial resolution, that
is, in the order of 10 to 30 nm, which is competitive with the
most recent, but more complex nanoscopy tools such as
PALM, STORM, STED, and related techniques.[30–36] The
high spatial resolution provides the direct visualization, and
thus the immediate localization of active sites within individ-
ual particles, while recording the catalytic process under
realistic conditions. By exploiting the milliseconds time
resolution of the technique, the direct evaluation and
quantification of the kinetics is within reach with a very
limited number of experiments, as will be demonstrated
below for epoxidation over Ti-MCM-41. Typical parameters
such as the Thiele modulus and the related effectiveness
[*] G. De Cremer, E. Bartholomeeusen, Dr. K. Lin,
Prof. Dr. P. P. Pescarmona, Prof. Dr. P. A. Jacobs,
Prof. Dr. D. E. De Vos, Prof. Dr. B. F. Sels
Department of Microbial and Molecular Systems
Katholieke Universiteit Leuven
Kasteelpark Arenberg 23, 3001 Heverlee (Belgium)
Fax: (+32)16-321-998
E-mail: bert.sels@biw.kuleuven.be
Dr. M. B. J. Roeffaers, Dr. P. Dedecker, Prof. Dr. J. Hofkens
Department of Chemistry, Katholieke Universiteit Leuven
Celestijnenlaan 200F, 3001 Heverlee (Belgium)
E-mail: maarten.roeffaers@biw.kuleuven.be
[**] G.D.C., P.D., and M.B.J.R. thank the FWO (Fonds voor Weten-
schappelijk Onderzoek) for two doctoral fellowships and a post-
doctoral fellowship, respectively. This work was performed within
the framework of the IAP-VI program “Supramolecular Chemistry
and Catalysis” of the Belgian Federal government and of GOA-2/01.
We also gratefully acknowledge financial support from a long-term
structural funding “Methusalem” by the Flemish government and
from K.U. Leuven in the framework of the Centre of Excellence
CECAT.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905039.
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Chemie
factor[12,13] for catalysis in the condensed phase were calcu-
lated on the basis of the obtained experimental data.
Although the reaction mixture was not stirred during the
in situ measurements, the relatively low volumetric reaction
ꢀ1
Individual reactions were monitored by a versatile
method based on a fluorogenic reporter. A phenylbuta-
dienyl-substituted boron dipyrromethene difluoride probe
(PBD-bodipy; see the Supporting Information for synthesis),
the red fluorescence of which is not detected by the setup, was
used as substrate for the catalytic reaction (see Figure 1).
rate (0.1–1 mmolmꢀ3
s ) in the applied conditions
catalyst
assures a constant substrate concentration in the solution
directly surrounding the particle without limitations of
substrate supply from the bulk solution towards the particle
(see the Supporting Information for calculations).
As expected, no fluorescent spots were observed on the
particles in the absence of either of the two reactants (tert-
butylhydroperoxide or PBD-bodipy). This result confirms
that the observed activity does not result from photocatalytic
generation of reactive oxygen species such as hydroxyl
radicals from molecular oxygen, as observed in other Ti-
containing systems.[38,39] When Ti-MCM-41 particles were
contacted with both reactants, the formation of individual
emissive product molecules, followed by fast photobleaching
or diffusion out of the focal plane, was seen as short (in
general < 100 ms), but very intense flashes of light with a spot
size of about 250 nm, which is in agreement with the
diffraction limits of the technique. A snapshot of one such
epoxidation event is presented in Figure 2a. Mapping of the
Figure 1. Strategy for monitoring individual epoxidation events. The
catalyst is immobilized on a glass cover slide and mounted on a wide-
field fluorescence microscope. The red fluorescent bodipy probe is
epoxidized with tert-butylhydroperoxide (TBHP) on the Ti sites in the
mesopores of a Ti-MCM-41 particle. Upon catalytic conversion, yellow-
emitting bodipy products P1 and P2 are formed.
Upon epoxidation of the probeꢀs butadienyl bridge, the
emission undergoes a strong blue shift of 50 to 80 nm
depending on the regioselectivity of the reaction (for spectral
details, see Figure 1 and the Supporting Information).[37] By
spectral selection one can thus specifically monitor the
epoxidation in the fluorescence microscope. The high extinc-
tion coefficient (ca. 90000mꢀ1 cmꢀ1) and fluorescence quan-
tum yield (ca. 1) of the yellow-emitting epoxides allow
detection of their formation with single-molecule sensitivity.
In addition, the choice of the PBD-bodipy probe is justified
on the basis of its molecular size (ca. 1.8 nm and 0.5 nm along
the long and short axes, respectively), which is comparable to
that of large olefinic substrates such as cholesterol, which are
frequently used for epoxidation with Ti-MCM-41.
The formation of individual emissive product molecules
was monitored in a wide-field fluorescence microscope with
488 nm illumination. The micrometer-sized Ti-MCM-41 cata-
lysts (see the Supporting Information for synthesis) were
deposited on a glass cover slide by spin-coating from a
suspension in n-butanol. Subsequently, a freshly prepared
n-butanol solution containing the reactants (500 nm PBD-
bodipy and 35 mm tert-butylhydroperoxide (TBHP)) was
added. Images were recorded by moving the focal plane to
the middle of a Ti-MCM-41 particle to avoid emission from
catalytic events occurring at the bottom or top surfaces (see
the Supporting Information for experimental details).
Figure 2. a) Wide-field fluorescence microscopy snapshot image upon
488 nm illumination of Ti-MCM-41 particles (indicated by the red
circles) in the presence of the two reactants (tert-butylhydroperoxide
and PBD-bodipy). The contours of the particles are vaguely visible
because of scattering effects. The yellow arrow indicates a fluorescent
spot representing one turnover event. b) Left: Enlargement of the
emissive spot representing one epoxidized PBD-bodipy molecule.
Right: Fitting the intensity profile with a 2D Gaussian function
(colored contour lines; full width at half maximum=223 nm) yields
the central (x,y) position (red dot) of the single emitter in the focal
plane.
epoxidation on a single particle reveals distinct spots within a
short time frame. This stochastic behavior of the catalytic
events allows accurate determination of the central (x,y)
position in the focal plane, by simply fitting the intensity
profile with a 2D Gaussian function, such as illustrated in
Figure 2b.[29,40–41] It is important to note that, although in a
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wide field microscope emission from out-of-focus planes is in
spots originating from individual product molecules. Note
that this spread is determined not only by the accuracy of the
Gaussian fit, but also by possible diffusion of the product
molecule inside the Ti-MCM-41 channels and by vibration in
the experimental setup. The obtained resolution provides, to
our knowledge for the first time, unique spatial information
about the actual active regions for oxidation catalysis in
individual porous particles (Figure 3a,c).
principle also collected, only turnovers occurring in the focal
plane will yield well-defined sharp spots with diffraction-
limited size. These will be recognized and fitted by the
analysis software (see the Supporting Information).
By superposition of many single catalytic turnover events
and their respective fitted positions in one image, the spots
where the product molecules are actually formed can be
mapped with a resolution that is only limited by the signal-to-
noise ratio of the productꢀs emission. This mapping is
illustrated in Figure 3a, in which a transmission microscopy
An enlargement on the active region of one particle
reveals that product formation takes place only in the outer
300 nm (Figure 3c). A more accurate determination of the
reactivity decay from the particleꢀs outer surface towards the
particleꢀs center can be found in the Supporting Information.
Since this value is considerably larger than the accuracy in
determining the position of the individual turnovers, the
measured width is not biased by the resolution. XPS measure-
ments prove that Ti is homogeneously distributed throughout
the whole particle (see the Supporting Information). It can
therefore be excluded that Ti enrichment at the surface is the
cause of the peripheral active zone. Moreover, sorption
experiments in presence of excess of PBD-probe and in the
absence of TBHP reveal complete accessibility of the inner
part of the particle, thus excluding the hypothesis that pore
discontinuities are the origin of the observed catalytic zoning
effects (see the Supporting Information). Intraparticle diffu-
sion must be the limiting factor in case of Ti-MCM-41
catalyzed epoxidation of substrates such as PBD-bodipy. In
other words, the probability for the reactants to reach the
inner parts of the micrometer-sized particle before being
epoxidized is negligible and hence only the titanium sites in
the outer 300 nm of the particle are responsible for the
observed catalytic activity. In situ monitoring of the epoxida-
tion activity with fluorescence microcopy thus provides direct
evidence for the proposed size-dependent activity of Ti-
MCM-41 catalysts.[10]
The data obtained from the microscopic experiment
(Figure 3) also allow estimation of the effective intraparticle
diffusion coefficient Deff of the bodipy probe. The value is in
the order of 10ꢀ16 m2 sꢀ1 (see the Supporting Information for
calculations), which is a plausible value when compared with
diffusion coefficients obtained by single-molecule tracking in
other mesoporous materials.[27–28] For the first time, parame-
ters such as the Thiele modulus and the effectiveness factor,
which are commonly used in heterogeneous catalysis to
identify diffusion limitations at the bulk scale, were success-
fully determined at the single-particle level. The dimension-
less Thiele modulus (f) is defined by Equation (1).
Figure 3. a) Three examples of individual turnover measurements on
Ti-MCM-41 particles. The transmission images (grayscale) are overlaid
with a scatter plot (red dots) of the fitted positions of fluorescent
spots originating from individual product molecules. b) For two
representative examples, the fitted positions of an individual adsorbed
product molecule, emitting from the same location during several
frames (up to 2 s), are displayed in a scatter plot. The standard
deviations (s) in the x and y dimensions are indicated, together with
the obtained spatial accuracy (R). c) Enlargements of two regions of
the images displayed in panel (a), indicated by the white rectangles.
The positions of product formation are distributed over the outer 200
to 300 nm of the particles.
image is overlaid with the fitted positions of individual
reaction events acquired over 150 s (1500 frames) for a few
representative particles. As every spot corresponds to the
formation of one product molecule, the reaction rate or
catalytic activity per particle can easily be determined by
counting the spots. For example, the first particle in Figure 3a
produces 60 spots within 150 s seconds. The catalytic reaction
clearly occurs predominantly at the outer rim of the Ti-MCM-
41 particles.
The actual accuracy of the fitted position was estimated by
calculating the spread on the center of the emission from the
same product molecule that appears immobile for several
successive frames. The fitted center positions for two such
immobile molecules are depicted in Figure 3b. The spread in
the x and y dimensions is typically in the order of a few
nanometers, yielding an artificial resolution R of below 20 nm
(see the Supporting Information for calculations). The high
positioning accuracy in the microscopic measurements is the
result of the excellent signal-to-noise ratio of the fluorescent
L
c
pffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ ¼
ð1Þ
Deff=k
This modulus represents the characteristic size of the
particle (Lc = r/3) relative to the typical length scale over
pffiffiffiffiffiffiffiffiffiffiffiffiffi
which the concentration in the particle decays (
Deff=k, in
which Deff is the intraparticle diffusion coefficient and k is the
rate constant). The value of f is estimated at about 3,
corresponding to an effectiveness factor (h) of only 30% (see
the Supporting Information for calculations). This result
seems to agree visually with the volume fraction of the
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[15] R. Q. Snurr, A. Hagen, H. Ernst, H. B. Schwarz, S. Ernst, J.
catalyst which actually contributes to the observed activity in
the conditions applied, as observed in the fluorescence
microscopic image. The Biot number (Bi) which compares
the rate of film diffusion with intraparticle diffusion, was
found to be in the order of 105 (see the Supporting
Information for calculations), thus confirming that the con-
centration drop from the bulk solution towards the particleꢀs
surface, that is, film mass transfer limitation, is negligible.
In summary, we introduced a new versatile approach for
studying diffusion limitations in realistic catalytic conditions
for catalysis in the condensed phase using porous materials. A
probe molecule was selected that upon epoxidation gives rise
to bright fluorescent bursts, which represent the formation of
individual epoxide molecules. Thanks to the distinct positions
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Received: September 8, 2009
Published online: December 22, 2009
Keywords: diffusion · epoxidation · fluorescent probes ·
.
heterogeneous catalysis · single-molecule studies
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