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The rate of photocatalytic oxidation of aromatic volatile organic compounds in the gas-phase
Atmospheric Environment 42 (2008) 7844–7850
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
The rate of photocatalytic oxidation of aromatic volatile organic compounds in the gas-phase Aikaterini K. Boulamanti, Christos A. Korologos, Constantine J. Philippopoulos* Chemical Process Engineering Laboratory, National Technical University of Athens, 9 Heroon Polytechniou Str., Zografou Campus, 157 80 Athens, Greece Department of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Str., Zografou Campus, 157 80 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2008 Received in revised form 9 July 2008 Accepted 10 July 2008
In the present study, the gas-solid heterogeneous photocatalytic oxidation (PCO) of six aromatic species of volatile organic compounds (VOCs), benzene, toluene, ethylbenzene, m-, o- and p-xylene over illuminated titania was carried out at ambient temperature in a continuous stirring-tank reactor. Initial VOC concentrations were in the low parts per million (ppm) range. Maximum conversions were over 90% for all compounds except from benzene, ethylbenzene and o-xylene, while the residence time varied from 50 to 210 s. Intermediates were detected only in the case of the xylenes, but catalyst deactivation occurred for all six compounds. The PCO kinetics were well fit by a Langmuir–Hinshelwood (L–H) model for monomolecular surface reaction and it was proved that the reaction rate is related to both constants. The rate constants ranged from 0.147 ppm s1 gcat1 for benzene 1 for to 1.067 ppm s1 g1 cat for m-xylene, while the adsorption constants from 0.424 ppm 1 ethylbenzene to 0.69 ppm for toluene. The molecular structure of the compounds was found to play an important role in the reaction. Finally the efficiency of the procedure in the case of a mixture of these aromatic substances was tested. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Aromatic VOCs Titanium dioxide Langmuir–Hinshelwood kinetics Molecular structure
1. Introduction Volatile organic compounds (VOCs) are an important class of pollutants usually found in the atmosphere of all urban and industrial areas. The Geneva Convention (1979) on atmospheric pollution has defined VOCs as all organic compounds of anthropogenic nature, other than methane, that are capable of producing photochemical oxidants by reactions with nitrogen oxides in the presence of sunlight. Several of these compounds can induce odor pollution (Doucet et al., 2006) and are considered to be toxic, carcinogenic, mutagenic, or teratogenic (Alberici and Jardim, 1997), therefore directly harmful to human beings. Among various methods which are effective in the abatement of environment problems, heterogenous photocatalytic oxidation (PCO) has appeared to be promising,
* Corresponding author. Tel.: þ30 210 772 3224; fax: þ30 210 772 3155. E-mail address: [email protected] (C.J. Philippopoulos). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.07.016
due to the mild experimental conditions under which the runs are usually carried out. During this process the illuminated semiconductor absorbs light and generates free radicals. Titanium dioxide in various forms is almost exclusively used as a photocatalyst (Alberici and Jardim, 1997; Doucet et al., 2006), probably due to the fact that it is relatively inexpensive, shows efficient destruction of toxic contaminants and operates at ambient temperature and pressure. It has a large bandgap energy (typically <380 nm), and as a consequence it is able to absorb ultraviolet light. The final reaction products are CO2 and H2O or HCl in the case of chlorinated organic compounds. Several studies have been conducted on the PCO of compounds including aromatics in the gas-phase, mainly benzene and toluene. Doucet et al. (2006) chose benzene and methanol as model pollutants to check the effectiveness of a standardized reactor they have built up, while Wang et al. (2003) examined the effect of process parameters on the PCO of benzene in an annular photoreactor.
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
The photodegradation of toluene was studied in a plug flow reactor by Alberici and Jardim (1997) and in a batch reactor by Kim and Hong (2002). d’Hennezel and Ollis (1997) demonstrated that trichloroethylene enhances the rate of conversion of a broad range of common air pollutants. The purpose of this study was to evaluate the technical feasibility of the appliance of TiO2 photocatalysis for the six simpler aromatic VOCs: benzene, toluene, ethylbenzene, m-, o- and p-xylene. The differences in the behavior of all possible C8H10 isomers, as well as benzene and toluene were investigated. We focused in the low part per million (ppm) range (less than 10–20 ppm) whereas most previous studies have dealt primarily with concentrations in higher parts per million ranges. The correlation between reaction rate and molecular structure was investigated and a kinetics analysis was conducted. In addition experiments were conducted with a mixture of five of these compounds, so as to correlate the behavior of the catalyst in the presence of each compound separately and in mixture. 2. Experimental The VOC mixture in gas-phase was prepared in a sealed 1000 mL high pressure Parr vessel (model 4531 Parr Instrument Inc., USA) made of SS316 stainless steel. The desired volume of pollutant in liquid form was injected through an injection port and the vessel was filled with N2. The stream from the Parr vessel, controlled by a Brooks Smart Series digital mass flow controller (Brooks Instrument, Emerson) was diluted with N2 and O2 to deliver the desired VOC and O2 ratio to the reactor. Both the used gases (N2 and O2) and the chemicals were of chromatographic quality (Air Liquide and Merck respectively). The basic experimental set up used in this study is shown in Fig. 1. The experiments were performed in an annular photocatalytic reactor, with inner diameter 23.0 mm, outer diameter of the lamp 14.75 mm and effective volume 20 mL. A low-pressure Hg lamp of 12 W, which emits mainly (85–90%) ultraviolet light at 254.7 nm and 7–10% at 184.9 nm, was positioned coaxially in the reactor. The catalyst is immobilized in the internal surface of the cylindrical tube. The method used for immobilization is the one based on titania powder (Degussa P25) and proposed by Doll and Frimmel (2004). The TiO2 surface concentration obtained was 3.5 103 g cm2 as estimated by weighing the reactor before and after coating. A recycle loop that consists of the photoreactor and the circulation pump (Model SP 200 EC–LC, Schwarzer Precision GmbH u. Co.) allowed us to resemble the behavior of a continuous stirring-tank reactor (cstr). The recycle ratio was maintained over 6 to ensure successful mixing. Illumination was initialized after steady-state conditions were established. Because of the direct contact of the treated stream with the lamp, the temperature of the reactor was 50 2 C. The residence time was determined by the influent flow. The outlet stream was analyzed by gas chromatography (Shimadzu GC-17A, column HP1-MS, FID detector) for measuring the concentrations of reactants and gas chromatography–mass spectroscopy techniques (Hewlett Packard GC 6890–MS 5973, column HP1–MS) for detecting any possible stable intermediate species and
7845
reaction byproducts and determining the CO2 produced during the reaction. To ensure that no photoactivity decay occurred due to strongly adsorbed, oxidizable intermediates, the catalyst was regenerated after each experiment by continuous illumination and flow of pure air through the reactor overnight and stabilized by feeding the reactor with a continuous flow of some VOC concentration under illumination until steady state is achieved. 3. Results 3.1. Photocatalytic oxidation – intermediates observed It was established through preliminary experiments that the photodegradation of all compounds occurred only in the presence of titania catalyst, oxygen and near UV irradiation. The conditions for all experiments were 23% v/v O2 concentration and no presence of humidity. Table 1 summarizes some observations and the data obtained in the degradation of the six aromatic compounds, after the conditioning of the catalyst, which is described in the experimental section, while Fig. 2 shows the dependence of the conversion on the residence time. Conversions were above 90% for all VOCs tested, except from benzene, ethylbenzene and o-xylene. In particular, maximum conversion was 82% after 209 s for benzene, 83% after 60 s for ethylbenzene, 85% after 49 s for o-xylene, 92% after 60 s for m-xylene, 97% after 92 s for toluene and 98% after 75 s for p-xylene. No gas-phase intermediates were detected for benzene, toluene and ethylbenzene. Martra et al. (1999) and Xie et al., 2004 reported the production of benzaldehyde as the main and benzene, benzyl alcohol and traces of benzoic acid as the secondary oxidation products of toluene, but their experiments were conducted under much higher toluene initial concentrations as well as in the presence of water. On the contrary, in the case of xylenes, ethylbenzene was produced, in low concentrations for o- and p-xylene and in higher concentrations for m-xylene. d’Hennezel and Ollis (1997) detected no intermediates in the PCO of m-xylene. It is therefore obvious that the byproduct formation can be a function of the experimental conditions, of the configuration of the reactor used and of different analytical methods. Substantial deactivation was observed for all compounds tested, a result which is in good agreement with previous studies (Alberici and Jardim, 1997; Doucet et al., 2006; Wang et al., 2003). Since all the experiments were operated in the absence of water vapor, the decrease in the catalyst activity can be attributed either to the coating of the surface active sites by some strongly adsorbed less-reactive, poisonous intermediates, or to the consumption of the surface hydroxyl groups. 3.2. Kinetics and comparison Fig. 3 shows the effect of the VOC concentration in the apparent oxidation rates, which is, in increasing order: benzene, o-xylene, toluene, ethylbenzene, p- and m-xylene.
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VOC
PI 2
P
FIC
P
PI 4
I
I
FT 2
Photoreactor N2
PI 2
GC-FID
PI 4
GC-MS
Exhaust
O2
Fig. 1. Diagram of the experimental set up.
This order is in accordance with the results of d’Hennezel and Ollis (1997) who observed the following order: benzene, toluene, ethylbenzene and m-xylene. It can be noticed that the compound the most easily oxidized was m-xylene, while the compound least oxidized was benzene. Although usually it becomes more difficult to completely oxidize hydrocarbons with higher molecular weight, due to the number of oxygen atoms required for total oxidation (7.5 for benzene, 9 for toluene and 10.5 for the rest), this is not the case for aromatic compounds. Apparently the stereochemical structure of the molecules plays a really significant role in the heterogeneous catalytic reactions. The difference in the oxidation rates of the three xylenes can be attributed to the fact that o- and p-xylene are more stable than m-xylene. In these intermediates one of the contributing carbocationic structures is tertiary. This structure is more stable than the others because the
electrons on the methyl group can directly stabilize the electron deficient carbocationic carbon (Fig. 4) (McMurry, 2000). As a result m-xylene is more easily decomposed. The low oxidation rate of benzene can also be explained according to its stereochemical structure, since the electronical resonance of the aromatic ring and the lack of any substituent on it makes the structure stable and less susceptible to the attack of reactive species, while the adsorption on the catalyst can imply up to six active sites since the six carbon atoms on its molecule are equivalent. Two regimes are observed: first order kinetics at low concentrations and saturation (zero order) at high concentrations. Such behavior is characteristic of the simple Langmuir–Hinshelwood (L–H) model. The equation for the simplest monomolecular reaction can be presented as follows:
r ¼ k0 , Table 1 Observations and data for each of the six compounds tested Compound
Cin (ppmv)
Max conversion achieved
Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene
21.2 8.57 19.00 7.53 10.20 23.58
0.82 0.97 0.83 0.85 0.92 0.98
KLH ,C 1 þ KLH ,C
(1)
where KLH is an adsorption constant reflecting the proportion of solute molecules which adhere to the catalyst surface, C the VOC concentration in the gas-phase and k0 a kinetics (or rate) constant, related to the limiting rate of reaction at maximum coverage for the experimental conditions. The reaction coefficients of the reaction kinetics rate form can be obtained by regressing the experimental data (Yang et al., 2007).
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
b
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Conversion
Conversion
a
20
40
60
80 100 120 140 160 180 200
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2
0
10
20
Residence time (s)
d
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 5
10 15 20 25 30 35 40 45 50 55 60 65
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
10
15
f Conversion
Conversion
0.7 0.6 0.5 0.4 0.3 20
30
40
50
60
70
80
90 100
25
30
35
40
45
50
1.0
0.8
10
50
Residence time (s)
0.9
0.2
40
20
Residence time (s)
e
30
Residence time (s)
Conversion
Conversion
c
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60
0.8 0.6 0.4 0.2 0.0
0
10
20
30
40
50
60
70
80
Residence time (s)
Residence time (s)
Fig. 2. Conversion versus residence time for (a) benzene (Cin ¼ 23 ppm), (b) toluene (Cin ¼ 11 ppm), (c) ethylbenzene (Cin ¼ 23 ppm), (d) m-xylene (Cin ¼ 12 ppm), (e) o-xylene (Cin ¼ 10 ppm) and (f) p-xylene (Cin ¼ 25 ppm).
Oxidation rate (ppm s-1 gcat)
The inverse rate r 1 versus the inverse VOC concentration C 1 should be linear:
r 1 ¼
1.0
Benzene Toluene Ethylbenzene p Xylene o Xylene m Xylene total mixture
0.8 0.6 0.4 0.2 0.0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Concentration (ppm) Fig. 3. Photocatalytic oxidation rates as a function of the concentration for the six VOCs separately and the total oxidation rate of the mixture of the five VOCs.
1 1 ,C 1 þ 0 k0 ,KLH k
(2)
From Fig. 5 it can be seen that the experimental data are in good agreement with this assumption. It is therefore verified the suitability of the Langmuir–Hinshelwood model. A non-linear, least-squares optimization procedure was used to fit the experimental data. Values of the constants KLH and k0 are given in Table 2, while the best fit curves obtained were compared to the experimental results (Fig. 6). It is interesting to note that the photocatalytic oxidation rate relates to both k0 and KLH . As seen in Table 2 maximum adsorption onto the surface of the catalyst occurs with toluene, while ethylbenzene is the least adsorbed. From the adsorption constants it is shown that the species are adsorbed on the catalyst in the following order: ethylbenzene < benzene < o-xylene < p-xylene < m-xylene < toluene. However the reaction rates described by k0
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values show a different behavior. Thus, although toluene is more strongly adsorbed, the destruction rate is lower than the one observed for ethylbenzene, which indicates that a higher adsorption constant does not always result in a higher reaction rate. It can also be observed that the adsorption constants for toluene, m- and p-xylene are approximately the same. Taking into consideration the molecular structure of these substances, this may mean that they are adsorbed on similar sites and under similar ways, for example if the methyl is adsorbed on the catalyst. The remaining aromatic compounds have lower constants,
Fig. 4. Comparison of the para-, ortho- and meta-intermediates.
12
b Benzene Linear fit
11
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
a
10 9 8 7 6 0,00
R2 = 0,9697 0,05
0,10
0,15
0,20
9
Toluene Linear Fit
8 7 6 5 R2 = 0,9998
4
0,25
0,0
0,5
Concentration-1 (ppm-1)
d
Ethylbenzene Linear fit
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
c 3,2 3,0
2,8
2,6 0,10
0,15
0,20
4
0,25
5,5 5,0 R2 = 0,9553 0,3
0,4
0,5
0,6
Concentration-1 (ppm-1)
3,5
2
R2 = 0,975 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
6,0
0,2
3,0
3
f 2,8
o Xylene Linear fit
0,1
2,5
Concentration-1 (ppm-1)
6,5
4,5 0,0
2,0
m Xylene Linear fit
Concentration-1 (ppm-1)
e 7,0
1,5
1
R2 = 0,9603 0,05
1,0
Concentration-1 (ppm-1)
0,7
0,8
p Xylene Linear fit
2,6
2,4
2,2
2,0
R2 = 0,9294 0,05
0,10
0,15
0,20
0,25
0,30
Concentration-1 (ppm-1)
Fig. 5. Inverse rate versus inverse concentration for (a) benzene, (b) toluene, (c) ethylbenzene, (d) m-xylene, (e) o-xylene and (f) p-xylene.
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
were 15% for benzene, 48% for toluene, 40% for ethylbenzene 55% for m-xylene and 48% for o-xylene (Fig. 3). The oxidation rates ranged from 0.01 to 0.06 ppm s1 g1 cat for all compounds, which showed the same behavior in the reaction. Taking into consideration this observation and because of the low concentration level of each aromatic in the mixture and the similarities in their molecular structures, they are assumed to be equivalent. This assumption allows us to consider the total oxidation rate for every total VOC concentration present in the reactor, which is shown in Fig. 3.
Table 2 Langmuir–Hinshelwood parameters obtained in the photocatalytic degradation of six VOCs
Benzene Toluene Ethylbenzene m-xylene o-xylene p-xylene
Rate constant ½ppmðs$gcat Þ1
Adsorption constant ½ppm1
0.147 0.286 0.47 1.067 0.277 0.531
0.448 0.69 0.424 0.68 0.493 0.654
which may also be explained according to their molecular structure. In the case of ethylbenzene, ethyl is a bigger substituent than methyl, while in the case of o-xylene the two methyls are too close to each other. These structures may induce bigger stereochemical hindrance during adsorption on the catalyst or need a different type of site.
4. Conclusions Photocatalytic oxidation has been proved to be an efficient process for the treatment of aromatic compounds with concentrations lower than 20 ppmv, while their molecular structure played a significant role in the reaction rates. The possibility of homogeneous photodestruction (direct photolysis) was negligible as concluded by control experiments in a catalyst-free reactor. Benzene was the least easily oxidized compound, with the lowest conversion achieved in the longest retention time (82% after 209 s of illumination), while m-xylene was the most easily oxidized, a behavior that can be attributed to the substitution on the aromatic ring and to the stability of the molecules. As far as it concerned intermediates, in the case of benzene, toluene and ethylbenzene none was detected, while ethylbenzene was produced during the PCO of all three xylenes. All six aromatic compounds caused substantial deactivation of the catalyst. The Langmuir–Hinshelwood kinetic model was successfully applied to correlate the obtained data and the kinetics and adsorption constants were determined by
3.3. Photocatalytic oxidation of mixture After studying the photocatalytic degradation of each compound as single component, the reaction of a mixed feed of them was examined. The mixture in the feed contained benzene, toluene, ethylbenzene, o- and m-xylene with initial concentrations 7.2 1.5 ppm. p-Xylene was not included because it could not be distinguished from m-xylene in the analysis. Residence times varied from 9.5 to 24.5 s, while the maximum conversion achieved at 24.5 s was 20% for benzene, 38.6% for toluene, 52.6% for ethylbenzene, 62.3% for m-xylene and 66% for o-xylene. The conversions achieved at residence time about 25 s in the photocatalytic degradation of each compound individually
Benzene exp Benzene theor o Xylene exp o Xylene theor Toluene exp Toluene theor
0.25 0.20 0.15
Oxidation rate (ppm s-1 gcat-1)
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0.10 0.05 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Concentration (ppm) m Xylene exp m Xylene theor
1.20 1.05
Ethylbenzene exp Ethylbenzene theor p Xylene exp p Xylene theor
0.90 0.75 0.60 0.45 0.30 0.15 0
2
4
6
8
10
12
14
16
18
20
22
Fig. 6. Comparison of experimental (symbols) and theoretical (lines) values. (a) For -/, benzene, benzene, +/j o-xylene and m/n p-xylene.
B/C
24
26
28
Concentration (ppm) toluene and L/M m-xylene and (b) for :/8 Ethyl-
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A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
a mathematical procedure. It was observed that the reaction rate related to both constants and a higher adsorption constant did not always result in a higher reaction rate. mXylene had the highest rate constant (1.067 ppm s1 g1 cat) and benzene the lowest (0.147 ppm s1 g1 cat), while toluene had the highest adsorption constant (0.69 ppm1) and ethylbenzene the lowest (0.424 ppm1). Finally the reaction was performed in the case of a mixture of the target aromatics, showing that their behavior is not showing big differences in this case. The results presented here suggest that the gas-phase photocatalytic oxidation can be considered in pollution control strategies concerning aromatic compounds for single components and in the case of mixtures synergistic and inhibition effects must be taken into consideration. Acknowledgements The Project is co-funded by the European Social Fund (75%) and National Resources (25%) (PENED 2003). References Alberici, R.M., Jardim, W.E., 1997. Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide. Applied Catalysis B-Environmental 14, 55–68.
d’Hennezel, O., Ollis, D.F., 1997. Trichloroethylene-promoted photocatalytic oxidation of air contaminants. Journal of Catalysis 167, 118–126. Doll, T.E., Frimmel, F.H., 2004. Development of easy and reproducible immobilization techniques using TiO2 for photocatalytic degradation of aquatic pollutants. Acta Hydrochimica et Hydrobiologica 32, 201– 213. Doucet, N., Bocquillon, F., Zahraa, O., Bouchy, M., 2006. Kinetics of photocatalytic VOCs abatement in a standardized reactor. Chemosphere 65, 1188–1196. Geneva Convention, 1979. Protocol to the 1979 Convention on long-range transboundary air pollution concerning the control of emissions of Volatile Organic Compounds or their transboundary fluxes. Geneva, 13–18. November 1979. Kim, S.B., Hong, S.C., 2002. Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst. Applied Catalysis B: Environmental 35, 305–315. Martra, G., Coluccia, S., Marchese, L., Augugliaro, V., Loddo, V., Palmisano, L., Schiavello, M., 1999. The role of H2O in the photocatalytic oxidation of toluene in vapour phase on TiO2 catalyst – A FTIR study. Catalysis Today 53, 695–702. McMurry, J., 2000. Organic Chemistry, Vol. 1. University of Crete Press, Herakleion, pp. 721-722. Wang, W., Chiang, L.W., Ku, Y., 2003. Decomposition of benzene in air streams by UV/TiO2 process. Journal of Hazardous Materials B101, 133–146. Xie, C., Xu, Z., Yang, Q., Li, Na, Zhao, D., Wang, D., Du, Y., 2004. Comparative studies of heterogeneous oxidation and toluene on pure titania, titania–silica mixed oxides and sulfated titania. Journal of Molecular Catalysis A: Chemical 217, 193–201. Yang, R., Zhang, Y.P., Xu, Q.J., 2007. A mass transfer based method for measuring the reaction coefficients of a photocatalyst. Atmospheric Environment 41, 1221–1229.
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
The rate of photocatalytic oxidation of aromatic volatile organic compounds in the gas-phase Aikaterini K. Boulamanti, Christos A. Korologos, Constantine J. Philippopoulos* Chemical Process Engineering Laboratory, National Technical University of Athens, 9 Heroon Polytechniou Str., Zografou Campus, 157 80 Athens, Greece Department of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Str., Zografou Campus, 157 80 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2008 Received in revised form 9 July 2008 Accepted 10 July 2008
In the present study, the gas-solid heterogeneous photocatalytic oxidation (PCO) of six aromatic species of volatile organic compounds (VOCs), benzene, toluene, ethylbenzene, m-, o- and p-xylene over illuminated titania was carried out at ambient temperature in a continuous stirring-tank reactor. Initial VOC concentrations were in the low parts per million (ppm) range. Maximum conversions were over 90% for all compounds except from benzene, ethylbenzene and o-xylene, while the residence time varied from 50 to 210 s. Intermediates were detected only in the case of the xylenes, but catalyst deactivation occurred for all six compounds. The PCO kinetics were well fit by a Langmuir–Hinshelwood (L–H) model for monomolecular surface reaction and it was proved that the reaction rate is related to both constants. The rate constants ranged from 0.147 ppm s1 gcat1 for benzene 1 for to 1.067 ppm s1 g1 cat for m-xylene, while the adsorption constants from 0.424 ppm 1 ethylbenzene to 0.69 ppm for toluene. The molecular structure of the compounds was found to play an important role in the reaction. Finally the efficiency of the procedure in the case of a mixture of these aromatic substances was tested. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Aromatic VOCs Titanium dioxide Langmuir–Hinshelwood kinetics Molecular structure
1. Introduction Volatile organic compounds (VOCs) are an important class of pollutants usually found in the atmosphere of all urban and industrial areas. The Geneva Convention (1979) on atmospheric pollution has defined VOCs as all organic compounds of anthropogenic nature, other than methane, that are capable of producing photochemical oxidants by reactions with nitrogen oxides in the presence of sunlight. Several of these compounds can induce odor pollution (Doucet et al., 2006) and are considered to be toxic, carcinogenic, mutagenic, or teratogenic (Alberici and Jardim, 1997), therefore directly harmful to human beings. Among various methods which are effective in the abatement of environment problems, heterogenous photocatalytic oxidation (PCO) has appeared to be promising,
* Corresponding author. Tel.: þ30 210 772 3224; fax: þ30 210 772 3155. E-mail address: [email protected] (C.J. Philippopoulos). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.07.016
due to the mild experimental conditions under which the runs are usually carried out. During this process the illuminated semiconductor absorbs light and generates free radicals. Titanium dioxide in various forms is almost exclusively used as a photocatalyst (Alberici and Jardim, 1997; Doucet et al., 2006), probably due to the fact that it is relatively inexpensive, shows efficient destruction of toxic contaminants and operates at ambient temperature and pressure. It has a large bandgap energy (typically <380 nm), and as a consequence it is able to absorb ultraviolet light. The final reaction products are CO2 and H2O or HCl in the case of chlorinated organic compounds. Several studies have been conducted on the PCO of compounds including aromatics in the gas-phase, mainly benzene and toluene. Doucet et al. (2006) chose benzene and methanol as model pollutants to check the effectiveness of a standardized reactor they have built up, while Wang et al. (2003) examined the effect of process parameters on the PCO of benzene in an annular photoreactor.
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
The photodegradation of toluene was studied in a plug flow reactor by Alberici and Jardim (1997) and in a batch reactor by Kim and Hong (2002). d’Hennezel and Ollis (1997) demonstrated that trichloroethylene enhances the rate of conversion of a broad range of common air pollutants. The purpose of this study was to evaluate the technical feasibility of the appliance of TiO2 photocatalysis for the six simpler aromatic VOCs: benzene, toluene, ethylbenzene, m-, o- and p-xylene. The differences in the behavior of all possible C8H10 isomers, as well as benzene and toluene were investigated. We focused in the low part per million (ppm) range (less than 10–20 ppm) whereas most previous studies have dealt primarily with concentrations in higher parts per million ranges. The correlation between reaction rate and molecular structure was investigated and a kinetics analysis was conducted. In addition experiments were conducted with a mixture of five of these compounds, so as to correlate the behavior of the catalyst in the presence of each compound separately and in mixture. 2. Experimental The VOC mixture in gas-phase was prepared in a sealed 1000 mL high pressure Parr vessel (model 4531 Parr Instrument Inc., USA) made of SS316 stainless steel. The desired volume of pollutant in liquid form was injected through an injection port and the vessel was filled with N2. The stream from the Parr vessel, controlled by a Brooks Smart Series digital mass flow controller (Brooks Instrument, Emerson) was diluted with N2 and O2 to deliver the desired VOC and O2 ratio to the reactor. Both the used gases (N2 and O2) and the chemicals were of chromatographic quality (Air Liquide and Merck respectively). The basic experimental set up used in this study is shown in Fig. 1. The experiments were performed in an annular photocatalytic reactor, with inner diameter 23.0 mm, outer diameter of the lamp 14.75 mm and effective volume 20 mL. A low-pressure Hg lamp of 12 W, which emits mainly (85–90%) ultraviolet light at 254.7 nm and 7–10% at 184.9 nm, was positioned coaxially in the reactor. The catalyst is immobilized in the internal surface of the cylindrical tube. The method used for immobilization is the one based on titania powder (Degussa P25) and proposed by Doll and Frimmel (2004). The TiO2 surface concentration obtained was 3.5 103 g cm2 as estimated by weighing the reactor before and after coating. A recycle loop that consists of the photoreactor and the circulation pump (Model SP 200 EC–LC, Schwarzer Precision GmbH u. Co.) allowed us to resemble the behavior of a continuous stirring-tank reactor (cstr). The recycle ratio was maintained over 6 to ensure successful mixing. Illumination was initialized after steady-state conditions were established. Because of the direct contact of the treated stream with the lamp, the temperature of the reactor was 50 2 C. The residence time was determined by the influent flow. The outlet stream was analyzed by gas chromatography (Shimadzu GC-17A, column HP1-MS, FID detector) for measuring the concentrations of reactants and gas chromatography–mass spectroscopy techniques (Hewlett Packard GC 6890–MS 5973, column HP1–MS) for detecting any possible stable intermediate species and
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reaction byproducts and determining the CO2 produced during the reaction. To ensure that no photoactivity decay occurred due to strongly adsorbed, oxidizable intermediates, the catalyst was regenerated after each experiment by continuous illumination and flow of pure air through the reactor overnight and stabilized by feeding the reactor with a continuous flow of some VOC concentration under illumination until steady state is achieved. 3. Results 3.1. Photocatalytic oxidation – intermediates observed It was established through preliminary experiments that the photodegradation of all compounds occurred only in the presence of titania catalyst, oxygen and near UV irradiation. The conditions for all experiments were 23% v/v O2 concentration and no presence of humidity. Table 1 summarizes some observations and the data obtained in the degradation of the six aromatic compounds, after the conditioning of the catalyst, which is described in the experimental section, while Fig. 2 shows the dependence of the conversion on the residence time. Conversions were above 90% for all VOCs tested, except from benzene, ethylbenzene and o-xylene. In particular, maximum conversion was 82% after 209 s for benzene, 83% after 60 s for ethylbenzene, 85% after 49 s for o-xylene, 92% after 60 s for m-xylene, 97% after 92 s for toluene and 98% after 75 s for p-xylene. No gas-phase intermediates were detected for benzene, toluene and ethylbenzene. Martra et al. (1999) and Xie et al., 2004 reported the production of benzaldehyde as the main and benzene, benzyl alcohol and traces of benzoic acid as the secondary oxidation products of toluene, but their experiments were conducted under much higher toluene initial concentrations as well as in the presence of water. On the contrary, in the case of xylenes, ethylbenzene was produced, in low concentrations for o- and p-xylene and in higher concentrations for m-xylene. d’Hennezel and Ollis (1997) detected no intermediates in the PCO of m-xylene. It is therefore obvious that the byproduct formation can be a function of the experimental conditions, of the configuration of the reactor used and of different analytical methods. Substantial deactivation was observed for all compounds tested, a result which is in good agreement with previous studies (Alberici and Jardim, 1997; Doucet et al., 2006; Wang et al., 2003). Since all the experiments were operated in the absence of water vapor, the decrease in the catalyst activity can be attributed either to the coating of the surface active sites by some strongly adsorbed less-reactive, poisonous intermediates, or to the consumption of the surface hydroxyl groups. 3.2. Kinetics and comparison Fig. 3 shows the effect of the VOC concentration in the apparent oxidation rates, which is, in increasing order: benzene, o-xylene, toluene, ethylbenzene, p- and m-xylene.
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VOC
PI 2
P
FIC
P
PI 4
I
I
FT 2
Photoreactor N2
PI 2
GC-FID
PI 4
GC-MS
Exhaust
O2
Fig. 1. Diagram of the experimental set up.
This order is in accordance with the results of d’Hennezel and Ollis (1997) who observed the following order: benzene, toluene, ethylbenzene and m-xylene. It can be noticed that the compound the most easily oxidized was m-xylene, while the compound least oxidized was benzene. Although usually it becomes more difficult to completely oxidize hydrocarbons with higher molecular weight, due to the number of oxygen atoms required for total oxidation (7.5 for benzene, 9 for toluene and 10.5 for the rest), this is not the case for aromatic compounds. Apparently the stereochemical structure of the molecules plays a really significant role in the heterogeneous catalytic reactions. The difference in the oxidation rates of the three xylenes can be attributed to the fact that o- and p-xylene are more stable than m-xylene. In these intermediates one of the contributing carbocationic structures is tertiary. This structure is more stable than the others because the
electrons on the methyl group can directly stabilize the electron deficient carbocationic carbon (Fig. 4) (McMurry, 2000). As a result m-xylene is more easily decomposed. The low oxidation rate of benzene can also be explained according to its stereochemical structure, since the electronical resonance of the aromatic ring and the lack of any substituent on it makes the structure stable and less susceptible to the attack of reactive species, while the adsorption on the catalyst can imply up to six active sites since the six carbon atoms on its molecule are equivalent. Two regimes are observed: first order kinetics at low concentrations and saturation (zero order) at high concentrations. Such behavior is characteristic of the simple Langmuir–Hinshelwood (L–H) model. The equation for the simplest monomolecular reaction can be presented as follows:
r ¼ k0 , Table 1 Observations and data for each of the six compounds tested Compound
Cin (ppmv)
Max conversion achieved
Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene
21.2 8.57 19.00 7.53 10.20 23.58
0.82 0.97 0.83 0.85 0.92 0.98
KLH ,C 1 þ KLH ,C
(1)
where KLH is an adsorption constant reflecting the proportion of solute molecules which adhere to the catalyst surface, C the VOC concentration in the gas-phase and k0 a kinetics (or rate) constant, related to the limiting rate of reaction at maximum coverage for the experimental conditions. The reaction coefficients of the reaction kinetics rate form can be obtained by regressing the experimental data (Yang et al., 2007).
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
b
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Conversion
Conversion
a
20
40
60
80 100 120 140 160 180 200
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2
0
10
20
Residence time (s)
d
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 5
10 15 20 25 30 35 40 45 50 55 60 65
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
10
15
f Conversion
Conversion
0.7 0.6 0.5 0.4 0.3 20
30
40
50
60
70
80
90 100
25
30
35
40
45
50
1.0
0.8
10
50
Residence time (s)
0.9
0.2
40
20
Residence time (s)
e
30
Residence time (s)
Conversion
Conversion
c
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60
0.8 0.6 0.4 0.2 0.0
0
10
20
30
40
50
60
70
80
Residence time (s)
Residence time (s)
Fig. 2. Conversion versus residence time for (a) benzene (Cin ¼ 23 ppm), (b) toluene (Cin ¼ 11 ppm), (c) ethylbenzene (Cin ¼ 23 ppm), (d) m-xylene (Cin ¼ 12 ppm), (e) o-xylene (Cin ¼ 10 ppm) and (f) p-xylene (Cin ¼ 25 ppm).
Oxidation rate (ppm s-1 gcat)
The inverse rate r 1 versus the inverse VOC concentration C 1 should be linear:
r 1 ¼
1.0
Benzene Toluene Ethylbenzene p Xylene o Xylene m Xylene total mixture
0.8 0.6 0.4 0.2 0.0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Concentration (ppm) Fig. 3. Photocatalytic oxidation rates as a function of the concentration for the six VOCs separately and the total oxidation rate of the mixture of the five VOCs.
1 1 ,C 1 þ 0 k0 ,KLH k
(2)
From Fig. 5 it can be seen that the experimental data are in good agreement with this assumption. It is therefore verified the suitability of the Langmuir–Hinshelwood model. A non-linear, least-squares optimization procedure was used to fit the experimental data. Values of the constants KLH and k0 are given in Table 2, while the best fit curves obtained were compared to the experimental results (Fig. 6). It is interesting to note that the photocatalytic oxidation rate relates to both k0 and KLH . As seen in Table 2 maximum adsorption onto the surface of the catalyst occurs with toluene, while ethylbenzene is the least adsorbed. From the adsorption constants it is shown that the species are adsorbed on the catalyst in the following order: ethylbenzene < benzene < o-xylene < p-xylene < m-xylene < toluene. However the reaction rates described by k0
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values show a different behavior. Thus, although toluene is more strongly adsorbed, the destruction rate is lower than the one observed for ethylbenzene, which indicates that a higher adsorption constant does not always result in a higher reaction rate. It can also be observed that the adsorption constants for toluene, m- and p-xylene are approximately the same. Taking into consideration the molecular structure of these substances, this may mean that they are adsorbed on similar sites and under similar ways, for example if the methyl is adsorbed on the catalyst. The remaining aromatic compounds have lower constants,
Fig. 4. Comparison of the para-, ortho- and meta-intermediates.
12
b Benzene Linear fit
11
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
a
10 9 8 7 6 0,00
R2 = 0,9697 0,05
0,10
0,15
0,20
9
Toluene Linear Fit
8 7 6 5 R2 = 0,9998
4
0,25
0,0
0,5
Concentration-1 (ppm-1)
d
Ethylbenzene Linear fit
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
c 3,2 3,0
2,8
2,6 0,10
0,15
0,20
4
0,25
5,5 5,0 R2 = 0,9553 0,3
0,4
0,5
0,6
Concentration-1 (ppm-1)
3,5
2
R2 = 0,975 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Rate-1 (s gcat ppm-1)
Rate-1 (s gcat ppm-1)
6,0
0,2
3,0
3
f 2,8
o Xylene Linear fit
0,1
2,5
Concentration-1 (ppm-1)
6,5
4,5 0,0
2,0
m Xylene Linear fit
Concentration-1 (ppm-1)
e 7,0
1,5
1
R2 = 0,9603 0,05
1,0
Concentration-1 (ppm-1)
0,7
0,8
p Xylene Linear fit
2,6
2,4
2,2
2,0
R2 = 0,9294 0,05
0,10
0,15
0,20
0,25
0,30
Concentration-1 (ppm-1)
Fig. 5. Inverse rate versus inverse concentration for (a) benzene, (b) toluene, (c) ethylbenzene, (d) m-xylene, (e) o-xylene and (f) p-xylene.
A.K. Boulamanti et al. / Atmospheric Environment 42 (2008) 7844–7850
were 15% for benzene, 48% for toluene, 40% for ethylbenzene 55% for m-xylene and 48% for o-xylene (Fig. 3). The oxidation rates ranged from 0.01 to 0.06 ppm s1 g1 cat for all compounds, which showed the same behavior in the reaction. Taking into consideration this observation and because of the low concentration level of each aromatic in the mixture and the similarities in their molecular structures, they are assumed to be equivalent. This assumption allows us to consider the total oxidation rate for every total VOC concentration present in the reactor, which is shown in Fig. 3.
Table 2 Langmuir–Hinshelwood parameters obtained in the photocatalytic degradation of six VOCs
Benzene Toluene Ethylbenzene m-xylene o-xylene p-xylene
Rate constant ½ppmðs$gcat Þ1
Adsorption constant ½ppm1
0.147 0.286 0.47 1.067 0.277 0.531
0.448 0.69 0.424 0.68 0.493 0.654
which may also be explained according to their molecular structure. In the case of ethylbenzene, ethyl is a bigger substituent than methyl, while in the case of o-xylene the two methyls are too close to each other. These structures may induce bigger stereochemical hindrance during adsorption on the catalyst or need a different type of site.
4. Conclusions Photocatalytic oxidation has been proved to be an efficient process for the treatment of aromatic compounds with concentrations lower than 20 ppmv, while their molecular structure played a significant role in the reaction rates. The possibility of homogeneous photodestruction (direct photolysis) was negligible as concluded by control experiments in a catalyst-free reactor. Benzene was the least easily oxidized compound, with the lowest conversion achieved in the longest retention time (82% after 209 s of illumination), while m-xylene was the most easily oxidized, a behavior that can be attributed to the substitution on the aromatic ring and to the stability of the molecules. As far as it concerned intermediates, in the case of benzene, toluene and ethylbenzene none was detected, while ethylbenzene was produced during the PCO of all three xylenes. All six aromatic compounds caused substantial deactivation of the catalyst. The Langmuir–Hinshelwood kinetic model was successfully applied to correlate the obtained data and the kinetics and adsorption constants were determined by
3.3. Photocatalytic oxidation of mixture After studying the photocatalytic degradation of each compound as single component, the reaction of a mixed feed of them was examined. The mixture in the feed contained benzene, toluene, ethylbenzene, o- and m-xylene with initial concentrations 7.2 1.5 ppm. p-Xylene was not included because it could not be distinguished from m-xylene in the analysis. Residence times varied from 9.5 to 24.5 s, while the maximum conversion achieved at 24.5 s was 20% for benzene, 38.6% for toluene, 52.6% for ethylbenzene, 62.3% for m-xylene and 66% for o-xylene. The conversions achieved at residence time about 25 s in the photocatalytic degradation of each compound individually
Benzene exp Benzene theor o Xylene exp o Xylene theor Toluene exp Toluene theor
0.25 0.20 0.15
Oxidation rate (ppm s-1 gcat-1)
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0.10 0.05 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Concentration (ppm) m Xylene exp m Xylene theor
1.20 1.05
Ethylbenzene exp Ethylbenzene theor p Xylene exp p Xylene theor
0.90 0.75 0.60 0.45 0.30 0.15 0
2
4
6
8
10
12
14
16
18
20
22
Fig. 6. Comparison of experimental (symbols) and theoretical (lines) values. (a) For -/, benzene, benzene, +/j o-xylene and m/n p-xylene.
B/C
24
26
28
Concentration (ppm) toluene and L/M m-xylene and (b) for :/8 Ethyl-
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a mathematical procedure. It was observed that the reaction rate related to both constants and a higher adsorption constant did not always result in a higher reaction rate. mXylene had the highest rate constant (1.067 ppm s1 g1 cat) and benzene the lowest (0.147 ppm s1 g1 cat), while toluene had the highest adsorption constant (0.69 ppm1) and ethylbenzene the lowest (0.424 ppm1). Finally the reaction was performed in the case of a mixture of the target aromatics, showing that their behavior is not showing big differences in this case. The results presented here suggest that the gas-phase photocatalytic oxidation can be considered in pollution control strategies concerning aromatic compounds for single components and in the case of mixtures synergistic and inhibition effects must be taken into consideration. Acknowledgements The Project is co-funded by the European Social Fund (75%) and National Resources (25%) (PENED 2003). References Alberici, R.M., Jardim, W.E., 1997. Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide. Applied Catalysis B-Environmental 14, 55–68.
d’Hennezel, O., Ollis, D.F., 1997. Trichloroethylene-promoted photocatalytic oxidation of air contaminants. Journal of Catalysis 167, 118–126. Doll, T.E., Frimmel, F.H., 2004. Development of easy and reproducible immobilization techniques using TiO2 for photocatalytic degradation of aquatic pollutants. Acta Hydrochimica et Hydrobiologica 32, 201– 213. Doucet, N., Bocquillon, F., Zahraa, O., Bouchy, M., 2006. Kinetics of photocatalytic VOCs abatement in a standardized reactor. Chemosphere 65, 1188–1196. Geneva Convention, 1979. Protocol to the 1979 Convention on long-range transboundary air pollution concerning the control of emissions of Volatile Organic Compounds or their transboundary fluxes. Geneva, 13–18. November 1979. Kim, S.B., Hong, S.C., 2002. Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst. Applied Catalysis B: Environmental 35, 305–315. Martra, G., Coluccia, S., Marchese, L., Augugliaro, V., Loddo, V., Palmisano, L., Schiavello, M., 1999. The role of H2O in the photocatalytic oxidation of toluene in vapour phase on TiO2 catalyst – A FTIR study. Catalysis Today 53, 695–702. McMurry, J., 2000. Organic Chemistry, Vol. 1. University of Crete Press, Herakleion, pp. 721-722. Wang, W., Chiang, L.W., Ku, Y., 2003. Decomposition of benzene in air streams by UV/TiO2 process. Journal of Hazardous Materials B101, 133–146. Xie, C., Xu, Z., Yang, Q., Li, Na, Zhao, D., Wang, D., Du, Y., 2004. Comparative studies of heterogeneous oxidation and toluene on pure titania, titania–silica mixed oxides and sulfated titania. Journal of Molecular Catalysis A: Chemical 217, 193–201. Yang, R., Zhang, Y.P., Xu, Q.J., 2007. A mass transfer based method for measuring the reaction coefficients of a photocatalyst. Atmospheric Environment 41, 1221–1229.