Optimization of Plasma-Catalysis Two-Stage
Removal using Ag/ZSM-5 and ZSM-5 Catalysts
System
for
Toluene
Masami Sugasawaa*, Hiroshi Hirataa, Hyun-Ha Kima, Katsunori Kosugea, and Atsushi Ogataa
a
Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science
and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan
Abstract: VOC emission control techniques for small and medium facilities have
not yet been established. A plasma-catalysis two-stage system has the potential to
be one of best available technologies for such facilities. Thus, we investigated the
decomposition of toluene (C6H5CH3) by using ozone (O3)-assisted catalytic
oxidation on ZSM-5 and Ag/ZSM-5. On the ZSM-5 catalyst, the C6H5CH3
conversion was more than 90 % by increasing the reaction temperature (100 oC
 150 oC) and/or catalyst amount (0.5 g  1.0 g). However, ZSM-5 could not
oxidize C6H5CH3 to CO2 completely. In contrast, Ag/ZSM-5 showed a CO2
selectivity in COx higher than 95 % with an increase in inlet O3 concentration
regardless of other experimental conditions, although the conversion was lower
than that of ZSM-5. Ag species clearly promoted oxidizing HCOOH and CO to
CO2 by heat and/or O3. A layered catalyst in which ZSM-5 was followed by
Ag/ZSM-5 showed 91% of C6H5CH3 conversion and 94 % of the CO2 selectivity
in COx without HCOOH formation when supplied with 2700 ppm O3 at 100 °C.
The layered catalyst successfully used both advantages of ZSM-5 and Ag/ZSM-5.
Keywords: Toluene, Ozone, ZSM-5, Ag/ZSM-5, C6H5CH3 conversion, CO2
selectivity
1. Introduction
Volatile organic compounds (VOCs) are a major
source of secondary organic aerosol and
photochemical oxidants. In Japan, air pollution
control laws have regulated the emission of VOCs
by large emission facilities in the coating, drying,
industrial cleaning, printing, and storage sectors
since 2006. Conventional pollution control methods,
including incineration, catalytic combustion, and
adsorption techniques, have already been put to use
in those exhaust sources. However, such techniques
are expensive and require a large installation space,
making them unsuitable for many small and medium
facilities. Appropriate techniques for these smaller
facilities have not yet been established.
The catalyst method has the potential to be one
of best available technologies for such facilities if
the equipment can be made small and economical.
Recently, researchers have studied oxidation
reactions that use ozone (O3)-assisted catalysts—
such as Mn oxides (MnOx), Mn-based catalysts, a
Cu/Cr catalyst, and others—at low temperatures
(e.g., room temperature or 100 oC ). We found that
Ag supported on zeolite ZSM-5 (Ag/ZSM-5)
efficiently decomposes toluene (C6H5CH3) [1] and
suggested that Ag-based catalysts were also useful
for O3-assisted catalytic oxidation.
In this study, we investigated the catalytic
properties of ZSM-5 and Ag/ZSM-5 on the C6H5CH3
decomposition varying inlet O3 concentration,
catalyst amount and reaction temperature, in order to
clarify roles of Ag species. Furthermore, we
proposed layered catalysts in which ZSM-5 was
followed by Ag/ZSM-5 to use both advantages of
these catalysts for O3-assisted catalytic oxidation.
2. Experimental Conditions
2.1. Catalyst Preparation
A ZSM-5 catalyst was hydrophobic type,
SiO2/Al2O3 > 1000 and specific surface area = 313
m2/g (HiSiv-3000; Union Showa K. K., Japan). An
Ag/ZSM-5 catalyst was prepared by impregnating
ZSM-5 pellets in an aqueous solution of AgNO3,
drying at 40 oC in a rotary evaporator, and then
calcining in air at 500 oC for 10 h. A prepared
Ag/ZSM-5 catalyst (294 m2/g) contained 5.0 wt%
Ag.
(C) 1.0 g of catalyst at 100 oC, and
2.2. Experimental Apparatus
(D) 1.0 g of catalyst at 150 oC.
Fig. 1 shows the experimental flow diagram.
Ozone was supplied by using a surface discharge
type of plasma reactor. A spiral platinum wire coil
was set in contact with the inner wall of a quartz
tube (o.d. = 11.5-mm, i.d. = 9.5-mm, length = 150mm) as a barrier, and aluminum foil was wrapped on
the outer side of the tube as the ground electrode.
The desired O3 concentration was obtained by
passing pure O2 through this surface discharge
reactor. The catalysts were placed in a catalyst
reactor made of a quartz tube with a 12.5-mm outer
diameter, a 10.0-mm inner diameter, and a 210-mm
length The tubes were placed vertically, allowing the
descending gas to flow into the reactor.
In addition, to clarify the oxidizing powers of
ZSM-5 and Ag/ZSM-5 for undesired partial
oxidation products, O3-assisted catalytic oxidation
was carried out separately for 100 ppm HCOOH and
160 ppm CO on 0.5 g of catalyst at 100 and 150 oC.
(A) 0.5 g of catalyst at a reaction temperature of
100 oC,
(B) 0.5 g of catalyst at 150 oC,
For layered catalysts, ZSM-5 and Ag/ZSM-5
were packed such as 0.9 g ZSM-5 + 0.1 g Ag/ZSM5, 0.8 g ZSM-5 + 0.2 g Ag/ZSM-5, and 0.5 g ZSM-5
+ 0.5 g Ag/ZSM-5 in the direction of the gas flow at
100 °C.
Liquid C6H5CH3 in a glass container was
immersed in a water bath thermostatically
maintained at 5 oC to obtain the corresponding
C6H5CH3 vapor pressure in a N2 flow. To obtain the
desired C6H5CH3 concentration and flow rate, the
flows of C6H5CH3/N2 and another N2 source were
independently mixed by flow control units. Ozone
was generated by a surface discharge reactor with
0.20-2.00 W of discharge power.
Then, the C6H5CH3/N2 gas (0.4 L/min) and the
O3/O2 gas (0.1 L/min) were mixed at room
temperature. The C6H5CH3 was not decomposed by
the O3 in the gas phase at room temperature. The
inlet concentrations of C6H5CH3 and O3 were
adjusted to 200 ppm and 660–4100 ppm,
respectively. The total flow rate of the mixed gas
was set at 0.5 L/min. The mixed gas stream was then
fed into the catalyst reactor. Ozone generation, i.e.,
O3-assisted catalytic oxidation, was carried out after
reaching the adsorption-desorption equilibrium for
C6H5CH3 on the catalytic surfaces.
2.3. Experimental Conditions
Four experimental conditions were set up for
ZSM-5 and Ag/ZSM-5;
Figure 1. Experimental flow diagram.
2.4. Analysis
The concentrations of C6H5CH3 and reaction
products were measured continuously at downstream
of the catalyst reactor by an online Fourier-transform
infrared spectrophotometer (FT/IR-4200; Jasco Co.,
Japan) equipped with a gas cell with an optical
length of 2.5 m (Sirocco series 24102; Specac Inc.,
UK) and a Mercury Cadmium Telluride (MCT)
detector. The concentrations of C6H5CH3, CO2, CO,
and HCOOH were determined from the absorption
peaks centered at 729.2 cm-1, 2296.3 cm-1, 2190.0
cm-1, and 1103.0 cm-1, respectively. In addition, the
outlet O3 concentration was measured at downstream
of the spectrophotometer by an O3 monitor (EG-550;
Ebara Jitsugyo Co., Japan). The amount of O3
consumed in the catalyst reactor was evaluated as
the difference between the concentrations of inlet O3
and outlet O3. The results show the data at 30 min
after O3 was first supplied to the catalyst reactor,
unless otherwise noted.
3. Results and Discussion
3.1 Ozone-Assisted Catalytic Oxidation of C6H5CH3
on ZSM-5
The C6H5CH3 conversion on ZSM-5 was plotted
against inlet O3 concentration as shown in Fig. 2.
For all four experimental conditions, the conversion
was saturated at inlet O3 concentrations higher than
2700 ppm. For condition A, the conversion remained
at about 70 % even at 4100 ppm O3. For the other
three conditions, the conversions exceeded 90% with
inlet O3 concentrations higher than 2700 ppm.
Particularly for condition D, the conversion reached
100% at 3970 ppm of inlet O3 concentration. The
reaction was promoted effectively by increasing the
reaction temperature (100 oC  150 oC) and/or
catalyst amount (0.5 g  1.0 g).
The concentrations of oxidation products (COx
and HCOOH) obtained with the ZSM-5 catalyst was
clearly related to the inlet O3 concentration. For all
reaction conditions, the product concentrations
increased as the concentration of inlet O3 increased.
Product concentrations were highest in condition C,
regardless of inlet O3 concentration. Thus, both 1.0 g
of catalyst amount and 150 oC of reaction
temperature increased the concentrations of all
oxidation products. Furthermore, it was found that
the catalyst amount affected the activity more than
reaction temperature.
On the catalyst, HCOOH was observed as a
minor oxidation product. Increasing the catalyst
amount did not suppress HCOOH formation
substantially, even though it promoted C6H5CH3
decomposition as shown in Fig. 2. However,
HCOOH formation was suppressed to some extent
by increasing temperature from 100 to 150 oC. In
addition, HCOOH did not form at all at inlet O3
concentrations higher than 2700 ppm.
Meanwhile, CO2 selectivity in all products
(100×[CO2]/{[CO] + [CO2] + [HCOOH]}) increased
and reached at approximately 80 % at inlet O3
concentrations above 2700 ppm regardless of the
catalyst amount and reaction temperature. It was
suggested that the oxidation of C6H5CH3 to CO2
completely by ZSM-5 was difficult, because of its
limited oxidizing power as an O3-assisted catalyst.
3.2 Ozone-Assisted Catalytic Oxidation of C6H5CH3
on Ag/ZSM-5
The C6H5CH3 conversion on Ag/ZSM-5
saturated at inlet O3 concentrations higher than 2700
ppm and remained at approximately 70 % regardless
of experimental conditions. As different from the
decompositions on ZSM-5 (Fig. 2), the increasing
the catalyst amount or reaction temperature
minimally affected C6H5CH3 conversion on
Ag/ZSM-5.
Figure. 2. Relationship between C6H5CH3 conversion and Inlet
O3 concentration. Catalyst: ZSM-5. C6H5CH3 conversion
(%)=100×[1-[C6H5CH3]/ [C6H5CH3]0 ]. [C6H5CH3]0 indicates
initial concentration of C6H5CH3.
Fig. 3 shows the CO2 selectivities on Ag/ZSM-5
as a function of inlet O3 concentration. Because
HCOOH did not form at all on Ag/ZSM5, the values
were equal to 100×[CO2]/[COx]. Increasing either
the catalyst amount (0.5 g  1.0 g) or the reaction
temperature (100 oC  150 oC) raised CO2
selectivity. For condition D, in particular, the
selectivity showed an extremely high value of 99 %
at inlet O3 concentrations higher than 2300 ppm.
Furthermore, CO2 selectivity rose with the increase
in inlet O3 concentration on Ag/ZSM-5 regardless of
other experimental conditions. Ag species seemed to
promote CO2 formation.
Figure. 3. Relationship between CO2 selectivity in all products
and inlet O3 concentration. Catalyst: Ag/ZSM-5. CO2 selectivity
in all products (%)=100×[CO2]/[COx].
From the experimental results of O3-assisted
catalytic oxidation of 100 ppm HCOOH and 160
ppm CO, it was known that Ag clearly promoted the
oxidation of HCOOH and CO to CO2 by heat and/or
O3.
3.3 Ozone-Assisted Catalytic Oxidation of C6H5CH3
on Layered Catalysts of ZSM-5 and Ag/ZSM-5
On ZSM-5, CO2 selectivity in all products
remained at 80 % despite increases in C6H5CH3
conversion and concentration of oxidation products.
On Ag/ZSM-5, by contrast, CO2 selectivity
exceeded 95 % with no remarkable changes in
C6H5CH3 conversion and concentration of oxidation
products regardless of catalyst amount and reaction
temperature.
Therefore, we proposed layered catalysts
composed of a ZSM-5 bed followed by a Ag/ZSM-5
bed to use both advantages of of ZSM-5 and
Ag/ZSM-5. Table 1 shows the experimental results
on layered catalysts.
Table 1.
Ozone-assisted catalytic oxidation of 200 ppm
C6H5CH3 on layered ZSM-5 and Ag/ZSM-5 at 100 °C
Catalyst
Consumed O3
(ppm)
ZSM-5 (g) Ag/ZSM-5 (g)
C6H5CH3
conversion
(%)
CO2 selectivity
HCOOH
concentration in all products
(ppm)
(%)
0.5
-
1222
67
58
1.0
-
2067
96
12
73
80
-
0.5
2550
61
0
95
-
1.0
2677
68
0
97
0.9
0.1
2087
86
23
83
0.8
0.2
2153
86
11
86
0.5
0.5
2591
91
0
94
The C6H5CH3 conversion on 0.5 g ZSM-5 + 0.5
g Ag/ZSM-5 reached 91%, which corresponded to
that (96%) on 1.0 g ZSM-5. First, 67% of C6H5CH3
probably decomposed in the ZSM-5 layer. Then,
additional 24% of C6H5CH3 decomposed by the
remaining O3 in the Ag/ZSM-5 layer because 1369
ppm of O3 remained behind the ZSM-5 layer. In
addition, the HCOOH formation was suppressed
completely, i.e., 0 ppm. From the data on 0.5 g
ZSM-5 in Table 1, 58 ppm of HCOOH have formed
behind the ZSM-5 layer. A lot of CO remained, too.
However, the layered catalyst showed 94 % of CO2
selectivity without HCOOH formation. From these
results, Ag/ZSM-5 oxidized not only remained
C6H5CH3 but also CO and HCOOH to CO2 as
described in 3.2.
The other layered catalysts, i.e., “0.9 g ZSM-5 +
0.1 g Ag/ZSM-5” and “0.8 g ZSM-5 + 0.2 g
Ag/ZSM-5”, were insufficient to oxidize partial
oxidized products deeply.
4. Conclusions
A layered catalyst in which ZSM-5 was
followed by Ag/ZSM-5 successfully used both
advantages of ZSM-5 and Ag/ZSM-5 in
decomposition of 200 ppm C6H5CH3 by using O3assisted catalytic oxidation.
References
[1]M. Sugasawa and A. Ogata, Ozone: Science
& Engineering, vol.33, no.2, pp.158-163, 2011.