Supplementary material to
Spatially resolved observation of crystal face dependent
catalysis by single turnover counting
by Roeffaers, Sels, Uji-i, De Schryver, Jacobs, De Vos, Hofkens
While the data given in the main body of the paper concern basic catalysis on layered
double hydroxides (LDHs), this supplementary section reports on spatially resolved in situ
observation of two different acid-catalyzed reactions on zeolite crystals. Zeolites have
previously been studied by fluorescence microscopy, but rather in the context of
supramolecular organisation1 or of diffusion2,3. However, direct in situ visualization of the
catalytically active domains in an acid zeolite crystal has not yet been achieved. The
following results show that fluorescence microscopy with suitable probes is capable to
distinguish between various spatial domains with different catalytic activity within a
zeolite crystal. As zeolite catalysts, commercial mordenite crystals were selected.
Experimental. Materials. The mordenite catalyst (H-MOR, ZM-980) was acquired from
Zéocat. It has a Si/Al ratio of 100; the crystals typically have dimensions of 10-20 μm.
Before use in the microscopic experiments, the mordenite crystals were thermally activated
under air. First, the sample was heated at 5 °C min-1 to 120 °C and kept at this temperature
for 3h, in order to remove physisorbed water. This treatment prevents hydrothermal
destruction of large crystals. Next, the samples were further heated to 520 °C (5 °C min -1)
and kept at this temperature for 24 h. The samples were cooled down and stored under dry
nitrogen in a desiccator for maximally one week.
Microscopic experiments. Measurement of the fluorescence intensity was combined with
observation of the transmission image using an IX70 Olympus microscope and the
Fluoview FV500 operating system (Olympus). For this an Ar+ laser (Spectra Physics)
giving continuous excitation at 488 nm was directed on the sample using an oil immersion
objective lens (Olympus, 100x, 1.4 NA). The fluorescence signal is separated through a
488/543 nm dichroic mirror and a 505 nm long pass filter (Chroma Technology) before it
reaches the photomultiplier tube. Analysis was carried out with the microscope’s operating
system.
Furfuryl alcohol oligomerisation. The mordenite crystals are dispersed in n-butanol
before deposition on cleaned coverglasses through spin coating (1000 rpm). In the
microscopic experiments, these catalyst-loaded coverglasses are submerged in 980 µl of
the reaction solvent n-butanol, to which a 20 µl aliquot of furfuryl alcohol is added.
Acid catalyzed dehydration. This reaction was conducted in a similar way, by exposing
the mordenite to a 8,75 10-5 M solution of 1,3-diphenyl-1,3-propanediol in
dichloromethane.
Suppl. Material to: ‘Spatially resolved observation of crystal face dependent catalysis …’
2
Results and discussion
First, the oligomerization of furfuryl alcohol on acid mordenite was studied. Furfuryl
alcohol oligomerization starts by alkylation of one furfuryl alcohol molecule by another
one in an electrophilic aromatic substitution (EAS):
2
O
CH2OH
fluorescent oligomers
O
O
CH2OH
Such EAS reactions are widely used, e.g. in industrial Friedel-Crafts reactions. After some
subsequent acid-catalyzed steps, fluorescent compounds are formed4. Figures 1 and 2
show the catalytic formation of fluorescent oligomers inside zeolite micropores, starting
from furfuryl alcohol monomers. Because of the high spatial resolution, one can clearly
observe that the reaction starts at two opposing crystal faces. Based on the transmission
image and additional electron microscopy evidence, these faces are readily identified as the
(001) faces. As the reaction is followed as a function of time, the fluorescence propagates
along the [001] direction, which corresponds to the 12-membered ring channels in
mordenite. This proves that the furfuryl alcohol diffuses inside the channels, and that
product molecules are also formed in the interior of the zeolite crystals (note that with
scanning probe methods, only reactions at the outer surface can be monitored).
Figure 3 shows results for a different acid-catalysed reaction, viz. the dehydration of 1,3diphenyl-1,3-propanediol (DP3). Reaction of this compound on acid sites results in
formation of a carbocation which is weakly fluorescent5. Similar zeolite crystals were used
as in Figures 1 and 2. Again, two crystal faces with a high activity are immediately
identified.
With other currently used methods, it is – to the best of our knowledge – impossible to
obtain such spatially detailed in situ information on acid zeolite catalysis.
References
1. Brühwiler, D. & Calzaferri, G. Molecular sieves as host materials for supramolecular
organization. Microporous Mesoporous Mater. 72, 1–23 (2004).
2. Hashimoto, S. & Kiuchi, J. Visual and spectroscopic demonstration of intercrystalline
migration and resultant photochemical reactions of aromatic molecules adsorbed in
zeolites. J. Phys Chem. B. 107, 9763-9773 (2003)
3. Hellriegel, C., Kirstein, J. & Bräuchle, C. Tracking of single molecules as a powerful
method to characterize diffusivity of organic species in mesoporous materials. New J.
Phys. 7, 1-14 (2005).
4. Choura, M., Belgacem, N. M. & Gandini, A.Acid-catalyzed polycondensation of
furfuryl alcohol: mechanisms of chromophore formation and cross-linking.
Macromolecules 29, 3839-3850 (1996).
5. Garcia, H., Garcia, S., Perez-Prieto, J. & Scaiano, J. C. Intrazeolite photochemistry. 14.
photochemistry of ,–diphenyl allyl cations within zeolites. J. Phys. Chem. 100,
18158-18164 (1996).
Suppl. Material to: ‘Spatially resolved observation of crystal face dependent catalysis …’
3
Figure 1 | Visualization of the furfuryl alcohol self-reaction in a mordenite crystal.
While the reagent is non-fluorescent, the product oligomers display strong fluorescence
upon formation. The reaction is monitored as a function of time (false color images, time
indicated in seconds). The reaction starts at two crystallographically identical (001) faces
(indicated by the red arrows). From these crystal planes the reaction and thus the
fluorescence spreads into the crystal interior via the micropores. The other crystal planes
do not show any fluorescence, which indicates that there is hardly any catalytic activity.
The inset is a transmission image of the same catalytic crystal.
Suppl. Material to: ‘Spatially resolved observation of crystal face dependent catalysis …’
4
Figure 2 | Three-dimensional visualization of acid-catalyzed furfuryl alcohol
oligomerization in a mordenite crystal under different viewing angles (a, b). For each
orientation a schematic representation of the crystal is given along with the corresponding
fluorescence and transmission images. Fluorescence images clearly show that reaction
starts at the crystal faces perpendicular to the accessible 12-MR pores.
Figure 3 | Reactive zones inside a mordenite crystal during dehydration of 1,3diphenyl-1,3-propanediol. For conditions, see experimental section; (left) false color
fluorescence image, (right) transmission image.