In
this study, the influence of physical and chemical surface properties of acidic
zeolites on ozone removal is evaluated. It is well known that ozone is
responsible of outdoor and indoor pollution [1]. Processes based on heterogeneous
catalytic decomposition using materials such as metal oxides, activated carbons
or zeolites have been developed in order to maintain air quality standards [2,
3, 4]. However, few investigations dealing with gaseous ozone abatement over zeolites [2,
5] have been reported to be effective at ambient
conditions. Two mechanisms have been described to explain ozone elimination over
zeolites [2,5].
Firstly, strong Lewis acid sites located at the surface of the zeolite (Z-Al)
have been claimed to be able to convert molecular ozone into surface atomic
oxygen species Z-Al···O (Eq. 1) [5]. After that, a series
of reactions have been proposed (Eq. 2 and Eq. 3), where Lewis acid sites can
be regenerated and be available for a new catalytic cycle [5]:

Secondly, ozone can be
adsorbed on weak Lewis acid sites present on zeolite surface. Unfortunately, the role of zeolite structure on gaseous ozone removal is still not
clear. Thus, in this study, the effect of physical and chemical properties of
three different types of zeolite frameworks on gaseous ozone removal were investigated,
using a fixed bed reactor.

Three kinds of hydrophobic microporous zeolites were compared: mordenite
(MOR), ZSM-5 and faujasite Y (FAU). Mordenite type zeolites consist of 12-membered straight
channels and apertures of 6.5 x 7 Ã… with the presence of side pockets (3.5 x
6.7 Ã…). ZSM-5 zeolites exhibit a three-dimensional pore network with
10-membered straight and sinusoidal ring channels and apertures of 5.3 x 5.6 Ã…
and 5.1 x 5.5 Ã…, respectively. FAU consist of β-cages link together which give 13 Ã… diameter supercages. Thus, the FAU structure is more open than mordenite one or ZSM-5 one. Zeolite samples were
characterized using pyridine sorption IR studies in order to determine the
concentration of Lewis acidic sites. Physical properties were determined using
nitrogen adsorption at -196 °C. Table 1 lists physical and chemical properties
of these zeolites.

Table 1 : Physical and chemical properties of zeolites used
in this study

Gaseous ozone removal experiments
were conducted in a fixed-bed flow reactor, at 20ºC and 101 kPa.
Zeolites samples were collected from the fixed bed during ozone exposure and
were further analyzed by Fourier
Transform Infrared (FTIR) spectroscopy
in order to follow the
evolution of intermediate species formed on the surface of the zeolites. Figure
1 shows the evolution of FTIR spectra of MOR during ozone exposure. A variation
of the intensity of the peak located at 1384 cm^-1 was observed. This
peak could correspond to atomic oxygen formed on strong Lewis acid sites. Its
intensity oscillated during a certain exposure time meaning that the Lewis acid
sites were regenerated. Then, a deactivation of the zeolites occurred
progressively as shown by the intensity of the peak at 1384 cm^-1
which remained high. These results seem to confirm the mechanism for ozone
decomposition described above. All the 3 zeolites exhibited a similar profile
as that observed for MOR with a slightly smaller intensity for FAU zeolite.

Figure 1: Evolution of IR signal of
MOR surface in contact with gaseous ozone (2 % weight zeolite / 98 % weight KBr) - Operating conditions for ozonation:
Mass of solid: 38.5 g ; O_3in: 24.7 g.m^-3;
relative humidity: 0 % ; inlet flow rate 0.18 m³.h^-1 ;
Inlet temperature: 19 °C

Figure 2 presents the variation of the outlet concentration of ozone as
a function of time during ozone elimination over a fixed bed reactor filled
with three different kinds of zeolites. The amounts of ozone removed were determined
from mass balances. A higher amount of ozone was decomposed over MOR (2.24 g O₃.g^-1)
than over FAU (0.72 g O₃.g^-1) or ZSM-5 (0.50 g O₃.g^-1).
The presence of Lewis acid sites can partly explain these results. Although,
Lewis acid sites are known to be related to  ozone
decomposition; it  could not be found a linear relationship between Lewis
acid site concentration and ozone removal. Neither microporous
volume could be linked to ozone removal. These
results indicate that another factor linked to zeolite structure could play a
fundamental role on ozone removal. Thus, steric effects of zeolite
framework could be a key parameter on ozone elimination. The ratio of the ozone
molecule size (5.8 Ȧ) to the pore diameter of the zeolites, is defined as λ. This value is slightly inferior to 1 (0.88) for MOR. Ozone molecule can enter
freely into the pores of this zeolite sample and the interactions of ozone with
the surface, where the Lewis acid sites are located, could be probably strong.
For FAU zeolite which has pore diameter much larger than the ozone molecule
(λ: 0.55), ozone can easily diffuse inside the pores but with a lower
contact between ozone molecule and the Lewis acid sites because of the larger
difference of sizes. In
the case of ZSM-5 (λ :1.05), ozone diffusion
could be hindered [6]. These results could indicate that the closeness
of the sizes between ozone and the pores could be related to the enhancement of
the observed ozone elimination at the surface of zeolite samples.

Figure 2: Influence
of structure properties on ozone removal: (■) MOR, (▲) FAU, (♦) ZSM-5.

Operating conditions for ozonation: 0.1 g
zeolite; O_3in:14 g.m^-3; 50 cm³.min^-1;
101 kPa; 20°C; O_3t/O_3in
represents the dimensionless concentration of ozone.

Finally,
the influence of the inlet ozone concentration on ozone removal was also
studied. For MOR and ZSM-5, the removal of ozone was slightly improved when the
inlet ozone concentration was increased whereas for FAU zeolite, the amount of
decomposed ozone were similar whatever the inlet ozone concentration was
applied. These results confirm that ozone removal over FAU was due to the weak
interaction of ozone with the surface, whereas it was overall due to strong
ozone interactions in the case of MOR and ZSM-5. In the case of ZSM-5, the
elimination of ozone was limited by the difficulty of ozone to enter the ZSM-5
pores.

This
work not only highlight the  role of the Lewis acid sites of zelite surface on ozone decomposition, but also the
importance of zeolite framework on ozone removal efficiency.

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