Zeolite-coated optical microfibers for intrazeolite photocatalysis studied by in situ solid-state NMR

Full text of DOI:10.1021/ja992683s

404
J. Am. Chem. Soc. 2000, 122, 404-405
coat various substrates such as gold electrodes or quartz crystals
with zeolite-silica thin films. However, it remained a challenge
to prepare a zeolite-coated substrate system that could decrease
light scattering and would be robust enough to tolerate sample
spinning at 3000 Hz during the SSNMR experiments. We report
a new approach to the study of intrazeolite photochemistry which
uses the combination of a novel zeolite-coated optical microfiber
catalyst and in situ SSNMR. We demonstrate that the use of
zeolite composite catalysts results in increased photoefficiency
for selective oxidation reactions of chlorinated hydrocarbons such
as TCE and methylene chloride.
Zeolite-Coated Optical Microfibers for Intrazeolite
Photocatalysis Studied by in Situ Solid-State NMR
Ajit R. Pradhan, Megan A. Macnaughtan, and Daniel Raftery*
H. C. Brown Laboratory, Department of Chemistry
Purdue UniVersity, West Lafayette, Indiana 47907-1393
ReceiVed July 29, 1999
The use of zeolites in photochemistry has attracted significant
interest as they host a variety of organic transformations within
their cavities and channels that often lead to product distributions
considerably different from those in solution.1-4 For example,
Frei et al. used large-pore alkali or alkaline-earth zeolites to
achieve the partial oxidation of small alkanes, olefins, and
alkylbenzenes with unprecedented selectivity using visible light.5,6
These reactions are conducted at ambient temperature and in the
absence of solvent or photosensitizers. Important applications of
hydrocarbon partial oxidation include the production of valuable
products and the transformation of unwanted compounds into
benign species.7,8 Despite the high potential of the zeolites as a
novel, solid reaction medium for photochemical transformations,
there still remains a gap between our understanding of the
mechanistic aspects of the zeolite-adsorbate photophysics and the
remarkable specificity and selectivity observed.8 Moreover, to
scale-up the micromolar-quantity experiments, it is essential to
improve upon the scattering of visible light by the zeolite matrix,
the ability of reactants and products to diffuse through the catalyst
bed, and the development of a continuous-flow reaction system.6
Recently, we demonstrated9-11 that in situ solid-state nuclear
magnetic resonance (SSNMR) is useful to study the near-UV
photooxidation reactions of organic molecules such as trichloro-
ethylene (TCE) and ethanol and that it provides valuable
information on the complex reaction chemistry at the surface of
TiO2 photocatalysts. SSNMR methods are advantageous as they
allow an atom specific and quantitative examination of the
reactions on the catalyst surface.12 To overcome the inherent
problem presented by light scattering and limitations due to the
presence of dark regions within the interior of the material, we
also reported the use of quartz optical microfibers as a TiO2
photocatalyst support.13 Although numerous literature is available
on the deposition of zeolite crystallites on various substrates from
a hydrothermal reaction gel, there are very few examples of three-
dimensional objects that have been coated with films of pre-
synthesized zeolites.14,15 Among these is the approach reported
by Bein et al.16,17 in which silane-coupling agents are used to
Zeolite crystals were coated onto optical microfibers from a
tetraethyl orthosilicate (TEOS) solution via the sol-gel process.
An alcohol solution of TEOS was subjected to acid-catalyzed
hydrolysis of the alkoxide groups into hydroxyl groups followed
by a condensation reaction of these groups to form -Si-O-
Si- linkages. In a typical procedure, the stock solution of the
silica sol was prepared by adding 5 mL of TEOS and 5 mL of
ethanol. A small amount of acid (about 0.5 mL of 0.04 M HCl)
was added to the silica sol as a silicate oligomerization catalyst.
Additional experiments were also carried out using 0.04 M HNO3
(about 0.5 mL) and 5 M NH4OH (about 1.0 mL) as a base catalyst.
The dip-coated zeolite-silica composite was prepared by adding
0.4 g of the cleaned and cut microfibers18 (1.5 cm length) to a
suspension of 0.5 g of BaY19 zeolite in 1.0 mL of silica sol, which
was then diluted with 15 mL of ethanol, and stirred for 1 h at
298 K. The coated fibers were then drained and dried in a vacuum
oven at 343 K for 4 h to remove the solvent. The resulting silicate
films converted to a gel by calcining at 753 K for 8 h in the
presence of flowing air and locked into the particular configuration
of zeolite crystals. The covalent bond between the native oxide
surface of the optical fibers and the organosilicon compounds
makes these films particularly robust.20 The amount and nature
of the composite zeolite-silica coating was controlled by varying
the silica sol content, ethanol concentration, pH, and the zeolite
content in the reactant gel. Figure 1 shows scanning electron
micrograph (SEM) images of the quartz optical microfibers be-
fore (Figure 1a) and after (Figure 1b and c) coating with BaY
zeolite.
In situ SSNMR experiments were performed on a 300 MHz
Varian Unity Plus NMR spectrometer with a home-built double
resonance magic angle spinning (MAS) probe. In a typical
experiment, 60 mg of BaY zeolite-coated optical fibers were
packed into a 5 mm glass NMR tube (Norell), which was then
attached to a glass manifold. The catalyst sample was evacuated
to 2 × 10-5 Torr at 403 K and then cooled to 298 K. Typically,
30 µmol of 13C-labeled TCE or CH2Cl2 (Cambridge Isotope
Laboratories) and 90 µmol O2 were introduced onto the zeolite
sample using a liquid nitrogen trap. The NMR tube was then
sealed above the catalyst sample. Light produced by a 300 W Xe
arc lamp (ILC Technology) was filtered by a dichroic mirror (Oriel
Corporation, 420-630 nm) and was delivered evenly over the
spinning sample via a liquid light guide.10
* Corresponding author.
(1) Turro, N. J. Pure Appl. Chem. 1986, 58, 1219.
(2) Ramamurthy, V.; Lakshminarasimhan, P.; Grey, C. P.; Johnston, L. J.
Chem. Commun. 1998, 2412.
(3) Yoon, K. B. Chem. ReV. 1993, 93, 321.
(4) Xiang, Y.; Larsen, S. C.; Grassian, V. H. J. Am. Chem. Soc. 1999,
121, 5063.
(5) Blatter, F.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297.
(6) Frei, H.; Blatter, F.; Sun, H. CHEMTECH 1996, 24.
(7) Sheldon, R. A., van Santen, R. A., Eds. In Catalytic Oxidation, Principle
and Applications; World Scientific Publishing: Singapore, 1995.
(8) Centi, G.; Misono, M. Catal. Today 1998, 41, 287.
(9) Hwang, S.-J.; Petucci, C.; Raftery, D. J. Am. Chem. Soc. 1997, 119,
7877.
13C MAS NMR results for the in situ photocatalytic oxidation
of TCE and CH2Cl2 are presented in Figure 2. The narrow line
widths of the peaks indicate that TCE (Figure 2a) and the
photooxidized products (Figure 2b) are quite mobile. The spec-
tra indicate 78% degradation of TCE after 100 min and the
(10) Hwang, S.-J.; Petucci, C.; Raftery, D. J. Am. Chem. Soc. 1998, 120,
4388.
(18) The polyimide cladding present on the quartz microfibers (Quartzel
Fibers; 9 µm diameter) was removed first by calcination at 753 K for 8 h in
the presence of flowing oxygen and by treatment with piranha solution (7:3
concentrated H2SO4/30% H2O2) at 363 K for 1 h.
(11) Hwang, S.-J.; Raftery, D. Catal. Today 1999, 1579.
(12) See, for example, Haw, J. F.; Xu, T. AdV. Catal. 1998, 42, 115 and
references therein.
(13) Rice, C. V.; Raftery, D. Chem Commun. 1999, 895.
(14) Bein, T. Chem. Mater. 1996, 8, 1636 and references therein.
(15) Jansen, J. C. et al. Microporous Mesoporous Mater. 1998, 21, 213.
(16) Bein, T.; Brown, K. J. Am. Chem. Soc. 1989, 111, 7640.
(17) Kurth, D. G.; Bein, T. J. Phy. Chem. 1992, 96, 6707.
(19) Zeolite BaY was prepared from NaY zeolite (Zeolyst International,
CBV-100, SiO2/Al2O3 ) 5.1) by ion-exchange using a 0.5 N BaCl2 solution
(Alfa Aesar).
(20) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J. Non-
Cryst. Solids 1982, 48, 47.
10.1021/ja992683s CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/31/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 405
However, the significant concentration of oxalyl chloride suggests
that the reaction mechanism may involve the formation of a
hydroperoxide intermediate as proposed by Frei et al.6 When the
photocatalytic experiment was repeated using 40 mg BaY zeolite
powder alone (i.e., without optical fibers) as the catalyst, a similar
product distribution was observed; however, in situ irradiation
with visible light resulted in only 34% degradation of TCE even
after 533 min. The difference in the activity demonstrates the
superior photocatalytic performance of the coated optical fibers
versus the powder catalyst; moreover, the performance is even
more significant, considering that the BaY zeolite constitutes less
than 50% of the composite catalyst. After the reaction, the zeolite
composite catalyst was regenerated,21 and the catalytic activity
was tested again. It was found that the catalyst could degrade
69% of TCE during the experiment. The restoration of catalytic
activity (compared to 78% over the fresh catalyst) indicates that
there is no significant loss of zeolite particles from the optical
fibers while spinning during the SSNMR experiments or during
the regeneration process. The robust nature of this catalyst will
be important for future development of a continuous flow reactor
system.
The intrazeolite photooxidation of CH2Cl2 was also examined.
The results in Figure 2c and d indicate 51% degradation of CH2-
Cl2 (δ 54.8) after 390 min. CO2 (δ 124.4) is observed as the major
product of the photocatalysis. Control experiments using BaY
zeolite powder as the catalyst (i.e., without optical fibers) showed
that the initial rate of CH2Cl2 degradation was roughly 20 times
slower than that observed using the zeolite-coated optical fibers.
This finding is significant as it suggests that compounds previously
dismissed as unreactive with powder zeolites may react with
coated optical fiber composite catalysts. Higher performance of
the fiber catalyst can be attributed to a decrease in the light
scattering due to the presence of a monolayer of zeolite particles
supported on the transparent optical fibers. Moreover, loose
packing of the optical fibers facilitates easy transport of reactants
to the active sites and desorption of products, which significantly
increases the activity of the catalyst system. In contrast to the
zeolite-coated optical microfiber catalysts, the photooxidation of
CH2Cl2 using TiO2 is less inefficient and requires high energy
UV irradiation. Separate experiments were performed to measure
the catalytic activity using acid (HNO3) and base (NH4OH) silicate
oligomerization catalysts during the preparation of the zeolite
fibers. These experiments showed that the catalytic activity of
these optical fibers is comparable (within a factor of 2) to the
HCl-prepared fiber catalysts. Further detailed experiments are
planned to study the reaction kinetics and the possible effects of
the acid or base used in the reaction gel on the photocatalytic
activity of the composite catalysts.
Figure 1. SEM images of (a) bare quartz optical microfibers, (b) BaY
zeolite-coated optical fibers, and (c) a magnified image of (b). The BET
surface area of this composite catalyst was determined to be 140 m2/g.
In summary, we have demonstrated that pre-synthesized and
ion-exchanged zeolites such as BaY can be coated on optical
microfibers via a sol-gel process from TEOS. The use of zeolite-
coated optical fibers reduces light scattering by the zeolite matrix,
facilitates free transport of reactant and product molecules, and
creates the possibility of transferring light photons to the active
sites via optical fibers. It also significantly increases photocatalytic
efficiency compared to bulk powders. Moreover, the use of
zeolite-coated optical microfibers provides an efficient methodol-
ogy to study zeolite photochemistry with SSNMR, which will be
useful for a wide range of fundamental investigations.
Figure 2. Proton-decoupled 13C MAS NMR spectra. (a) TCE and O2
adsorbed on BaY-coated optical fibers before irradiation and (b) after
irradiation with visible light for 100 min; (c) CH2Cl2 and O2 adsorbed
on BaY-coated optical fibers and (d) after irradiation with visible light
for 390 min.
formation of dichloroacetyl chloride (Cl2CHCOCl, DCAC, dou-
blets at δ 69.3 and 164.5) and oxalyl chloride (ClCOCOCl, OC,
δ 158.3) as major products. Additionally, minor products 1,1,2-
trichloroethan-2-ol (Cl2CHCHClOH, TCEOH, doublets at δ 72.7
and 84.7), phosgene (CCl2O, adsorbed δ 142.4 and gas-phase δ
139.2), trichloroactaldehyde (CCl3CHO, TCAA, doublet at δ 93.0
and 176.0), and carbon monoxide (CO, δ 182.6) were also formed.
The reaction intermediate TCEOH was observed to be very
unstable and was converted into TCAA during storage. The
products observed over BaY-coated optical fibers in the presence
of visible light is similar to that observed over semiconductor
photocatalysts, such as TiO2, in the presence of UV irradiation.9,10
Acknowledgment. This work was supported by the NSF (CHE 97-
33188). M.M. thanks the DOD for a fellowship. D.R. is an Alfred P.
Sloan Research Fellow (1999-2001).
JA992683S
(21) After the reaction, the sealed NMR tube was broken open, and the
zeolite composite catalyst was regenerated in the presence of O2 at 753 K for
8 h. The fibers were repacked in the NMR tube, and the catalytic activity
was tested following the procedure presented earlier.