Langmuir 2005, 21, 3877-3880
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Dibenzodioxin Adsorption on Inorganic Materials Yejun Guan, Yan Liu, Weicheng Wu, Keqiang Sun, Ying Li, Pinliang Ying, Zhaochi Feng, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China Received December 19, 2004 Dibenzodioxin adsorption/desorption on solid surfaces is an important issue associated with the formation, adsorption, and emission of dioxins. Dibenzodioxin adsorption/desorption behaviors on inorganic materials (amorphous/mesoporous silica, metal oxides, and zeolites) were investigated using in situ FT-IR spectroscopy and thermogravimetric (TG) analysis. Desorption temperatures of adsorbed dibenzodioxin are very different for different kinds of inorganic materials: ∼200 °C for amorphous/mesoporous silica, ∼230 °C for metal oxides, and ∼450 °C for NaY and mordenite zeolites. The adsorption of dibenzodioxin can be grouped into three categories according to the red shifts of the IR band at 1496 cm-1 of the aromatic ring for the adsorbed dibenzodioxin: a shift of 6 cm-1 for amorphous/mesoporous silica, a shift of 10 cm-1 for metal oxides, and a shift of 14 cm-1 for NaY and mordenite, suggesting that the IR shifts are proposed to associated with the strength of the interaction between adsorbed dibenzodioxin and the inorganic materials. It is proposed that the dibenzodioxin adsorption is mainly via the following three interactions: hydrogen bonding with the surface hydroxyl groups on amorphous/mesoporous silica, complexation with Lewis acid sites on metal oxides, and confinement effect of pores of mordenite and NaY with pore size close to the molecular size of dibenzodioxin.
Introduction Dioxins are a family of compounds consisting of two benzene rings joined by two bridge-oxygen atoms with chlorine atoms on the rings. They have been classified as a known carcinogen by the International Agency for Research on Cancer as firmer evidence has emerged from studies of those workers exposed to them.1 Many results suggest that trace amounts of dioxins are formed in the cooler sections of the plants through pyrocondensation from the incompletely decomposed organic fragments catalyzed by the fly ash between 300 and 400 °C and then desorb from the surface to the environment.2,3 Adsorption is commonly used in emission control of dioxin-like compounds. Thus, the adsorption/desorption behavior on many inorganic materials and adsorbents has attracted much attention.4-10 Different adsorption affinities for dioxins on these materials were reported from different studies. Lasagni et al.4 have reported that about 80% of adsorbed dibenzodioxin and dibenzofuran desorbed from the silica surface at 200 °C. Altwicker et al.6 reported that polychlorinated dibenzodioxins/furans (PCDDs/Fs) started to evaporate from the fly ash between 300 and 350 °C and about 94% PCDDs/Fs were in the gas phase. Addink et al.7 found that there was no significant * Corresponding author. Phone: (+86) 411-84379070. Fax: (+86) 411-84694447. E-mail:
[email protected]. Home page: http:// www.canli.dicp.ac.cn. (1) Kaiser, J. Science 2000, 288, 1313. (2) McKay, G. Chem. Eng. J. 2002, 86, 343. (3) Tuppurainen, K.; Asikainen, A.; Ruokojarvi, P.; Ruuskanen, J. Acc. Chem. Res. 2003, 36, 652. (4) Lasagni, M.; Collina, E.; Tettamanti, M.; Pitea, D. Environ. Sci. Technol. 1996, 30, 1896. (5) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 1373. (6) Altwiker, E. R.; Xun, Y.; Milligan, M. S. Organohalogen Compd. 1994, 20, 381. (7) Addink, R.; Govers, H. A. J.; Olie, K. Chemosphere 1995, 31, 3945. (8) Yang, R. T.; Long, R. Q.; Padian, J.; Takahashi, A.; Takahashi, T. Ind. Eng. Chem. Res. 1999, 38, 2726. (9) Long, R. Q.; Yang, R. T. J. Am. Chem. Soc. 2001, 123, 2058. (10) Jager, R.; Schneider, A. M.; Behrens, P.; Henkelmann, B.; Schramm, K.; Lenoir, D. Chem.sEur. J. 2004, 10, 247.
desorption of PCDDs/Fs when the fly ash was treated at 150 °C even after a long time (16 h); however, all isomers of PCDDs/Fs evaporated from the fly ash surface at 398 °C with an equal rate. Yang and co-workers8,9 found that carbon and carbon nanotubes have the best adsorption ability of dibenzodioxin among the adsorbents investigated using a TPD method. Jager et al.10 found that all the PCDD isomers investigated were adsorbed on the zeosils at 300 °C. These results suggest that the chemical composition of materials has quite an effect on the adsorption behavior of dioxins. In this article, we compare the adsorption affinity for dibenzodioxin on different inorganic materials using in situ FT-IR spectroscopy and thermogravimetric (TG) analysis. IR spectroscopy is a sensitive technique for detecting the surface species on solid oxides.11-14 Amorphous/mesoporous silica, metal oxides, and zeolites with different pore sizes are investigated. Considering the high toxicity of dioxins, nonchloro dibenzodioxin is used as the model molecule, which has a similar structure to that of PCDD/Fs. Desorption temperatures for adsorbed dibenzodioxin on the inorganic materials investigated are found to follow the trend of NaY, mordenite > Na (H)-ZSM-5 > metal oxides > silica, SBA-15. Surface hydroxyl groups, Lewis acid sites, and pore effects are found to be responsible for the adsorption of dibenzodioxin on inorganic materials. Experimental Section Materials. γ-Al2O3, TiO2, SiO2, H-ZSM-5 (Si/Al ) 15), NaZSM-5 (Si/Al ) 15), NaY (Si/Al ) 6), and mordenite (Si/Al ) 25) are commercial products. SBA-15 was prepared in this laboratory according to the literature.15 (11) Liu, Y.; Wu, W.; Guan, Y.; Ying, P.; Li, C. Langmuir 2002, 18, 6229. (12) Krishnamoorthy, S.; Amiridis, M. D. Catal. Today 1999, 51, 203. (13) Krishnamoorthy, S.; Rivas, J. A.; Amiridis, M. D. J. Catal. 2000, 193, 264. (14) Boyd, S. A.; Mortland, M. M. Nature 1985, 316, 532.
10.1021/la0468545 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005
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Figure 1. Changes in ν(O-H) region vibration bands of SiO2 after adsorption and desorption of dibenzodioxin at different temperatures. Synthesis of Dibenzodioxin. A 13.6 g portion of o-chlorophenol, 10 g of anhydrous potassium carbonate, and 4 g of copper powder (reduced from CuO by hydrogen at 400 °C for 4 h) were heated for 6 h in an oil bath at 170-190 °C. The tarry mixture was further refluxed with aqueous potassium hydroxide, allowed to cool, and finally extracted with ether.16 The product was confirmed by GC-MS. The mass spectrum peak attributed to the molecule ion of dibenzodioxin is observed at m/z ) 184. The main IR peaks of the product pressed in KBr disk are observed at 1590, 1496, 1301, 1287, and 923 cm-1, which is consistent with the literature report.17 In situ IR Studies. All materials were pressed into selfsupporting wafers (∼15 mg/cm2) and treated at 500 °C in vacuo in a quartz IR cell with CaF2 windows. Then, they were cooled to room temperature and the dibenzodioxin/CH2Cl2 solution was dropped down to the wafers. After CH2Cl2 was removed from the surface, all samples were heated to complete desorption temperatures at a rate of 10 °C/min. The IR spectra were recorded in situ on a Thermo Nicolet NEXUS 470 FT-IR spectrometer in the range 4000-1000 cm-1 with a resolution of 4 cm-1. All spectra are differences between the spectra of samples with adsorbed dibenzodioxin and those of samples without adsorbed dibenzodioxin. TG Analysis. Thermogravimetric (TG) analysis experiments were performed on a Perkin-Elmer Pyris TG-DTA from room temperature to 500-600 °C at a heating rate of 10 °C/min under flow nitrogen. The materials without pretreatment were impregnated by the dibenzodioxin/CH2Cl2 solution that consists of 6 mg of dibenzodioxin dissolved in 0.5 g of CH2Cl2.
Results Changes of IR Spectra in the OH Stretching Region. Figure 1 displays the IR spectra in the range 3800-3500 cm-1 due to the surface OH vibration of silica. After interaction, a negative IR band at 3735 cm-1 is detected for SiO2, including a positive band at 3746 cm-1. Both peaks weakened as the temperature increased. A similar phenomenon is also observed in the case of SBA15, which is not shown here. Negative bands are also observed for metal oxides: 3768, 3728 cm-1 for Al2O3, as shown in Figure 2, in addition to the TiO2. No obvious changes are observed for zeolites except H-ZSM-5, for which a negative band at 3596 cm-1 is detected and disappears at 250 °C, as shown in Figure 3. (15) Zhang, W. H.; Lu, J. Q.; Han, B.; Li, M. J.; Xiu, J. H.; Ying, P. L.; Li, C. Chem. Mater. 2002, 14, 3413. (16) Gilman, H.; Dietrich, J. J. J. Am. Chem. Soc. 1957, 79, 1439. (17) Gastilovich, E. A.; Klimenko, V. G.; Korolkova, N. V.; Nurmukhametov, R. N. Chem. Phys. 2002, 282, 265.
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Figure 2. Changes in ν(O-H) region vibration bands of H-ZSM-5 after adsorption and desorption of dibenzodioxin at different temperatures.
Figure 3. Changes in ν(O-H) region vibration bands of Al2O3 after adsorption and desorption of dibenzodioxin at different temperatures.
Change of IR Spectra in the Aromatic Ring Vibration Region. Figures 4-6 display the IR spectra due to the vibration of the aromatic ring of dibenzodioxin in the range 1550-1450 cm-1 for silica, Al2O3, and NaY, respectively. There is no obvious difference except the red shift of the IR band at 1496 cm-1: 6 cm-1 for SiO2, 10 cm-1 for Al2O3, and 14 cm-1 for NaY. The detailed red shifts on all materials investigated are shown in Table 1: 6-7 cm-1 for SiO2, SBA-15, and H-ZSM-5, about 10 cm-1 for Al2O3 and TiO2, and 14 cm-1 for NaY and mordenite. The intensity of this band decreases in intensity upon heating and disappears at different temperatures on different materials. Results of TG Experiments. Thermogravimetric analysis of dibenzodioxin adsorbed on these materials has also been carried out, and the DTG results are shown in Figure 7. Below 100 °C, a weight loss due to water desorption is observed for all samples. After that, the desorption of dibenzodioxin is detected. On alumina and titania, the maximum desorption temperature is around 230 °C, while it is only 170 °C on amorphous silica. For mesoporous material such as SBA-15, the desorption temperature is maximized around 200 °C. After dibenzodioxin is adsorbed on the zeolites, such as H-ZSM-5, Na-ZSM-5, NaY, and mordenite, the maximum desorption
Dibenzodioxin Adsorption on Inorganic Materials
Figure 4. Fundermental aromatic ring C-C stretching vibration of dibenzodioxin at 1496 cm-1 after adsorption and desorption from SiO2 at different temperatures compared with that of dibenzodioxin pressed by KBr disk.
Figure 5. Fundermental aromatic ring C-C stretching vibration of dibenzodioxin at 1496 cm-1 after adsorption and desorption from Al2O3 at different temperatures.
temperatures are 290, 310, 450, and 470 °C, respectively. Among these materials, NaY and mordenite show the highest desorption temperatures. Discussion For SiO2 and SBA-15, the 3735 cm-1 IR band is assigned to the geminal silanol groups and the 3746 cm-1 band is attributed to the isolated silanol groups. As described in Scheme 1a, after dibenzodioxin is adsorbed on silica and SBA-15, the geminal silanol groups are perturbed and isolated silanol groups appear. For H-ZSM-5, the negative IR band shift from 3596 to 3610 cm-1 is ascribed to the interaction between the bridge OH of Al-OH-Si and dibenzodioxin (see Scheme 1b). When adsorbed dibenzodioxin totally desorbs from the surface, the negative bands
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Figure 6. Fundermental aromatic ring C-C stretching vibration of dibenzodioxin at 1496 cm-1 after adsorption and desorption from NaY at different temperatures.
Figure 7. (a) DTG curves of adsorbed dibenzodioxin on amorphous oxides. (b) DTG curves of adsorbed dibenzodioxin on zeolites and mesoporous silica.
disappear (Figures 1-3), suggesting the restoring of surface hydroxyl groups. For metal oxides such as Al2O3 and TiO2, similar phenomena are also observed. These results suggest that surface hydroxyl groups are involved in the adsorption. Perturbation of surface hydroxyl bonded on silica results in the 6 cm-1 shift of the IR band due to the aromatic ring of adsorbed dibenzodioxin (Figure 4). Similar results are obtained for SBA-15 and H-ZSM-5. For Na-ZSM-5 support, the shift of the IR band at 1496 cm-1 is 8 cm-1 (Table 1). This may be due to the interaction between the aromatic ring and Na+.18 The shifts of the IR band due to the aromatic ring are 9-10 cm-1 for metal oxides such as Al2O3 and TiO2, which are larger than that corresponding on silica, suggesting another mode of interaction between the aromatic ring and the metal oxide surface besides the
Table 1. Desorption Temperatures of Adsorbed Dibenzodioxin on Different Materials and the Shift of IR Band at 1496 cm-1 due to Vibration of the Aromatic Ring of Dibenzodioxin Compared with That of Solid Dibenzodioxin Pressed in KBr support temperature/°C IR shift/cm-1
SiO2 170 6
SBA-15 220 7
H-ZSM-5 290 6
Na-ZSM-5 310 8
Al2O3 230 9
TiO2 230 12
NaY 450 14
mordenite 470 14
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Scheme 1. Proposed Models for the Interaction between Adsorbed Dibenzodioxin and Different Inorganic Materialsa
a (a) H-bonding interaction on silica; (b) H-bonding interaction in zeolites; (c) H-bonding interaction and complexation on Lewis acid sites on metal oxides.
interaction between the aromatic ring and surface hydroxyl groups. As shown in Scheme 1c, this kind of interaction probably occurs involving the Lewis acid sites on the surface of Al2O3 and TiO2. Mao et al.19 have studied the interaction of dibenzodioxin molecules with Laponite surfaces using diffuse reflectance and fluorescence spectra. They found that the π-electrons of the phenyl rings in dibenzodioxin reacted with the surface Lewis acid sites on the Laponite surface to form π-electron charge transfer complexes. As a result, the strength of complexation of the aromatic ring with Lewis acid sites will be stronger than that of the hydrogen bonding interaction. Although NaY and mordenite are silica-based materials, the adsorption behavior of dibenzodioxin on NaY and mordenite is remarkably different compared to those observed on SiO2. The adsorption of dibenzodioxin results in a shift of 14 cm-1 of the IR band due to the aromatic ring (Table 1). It is much larger than the corresponding shift for silica and amorphous metal oxides. These results indicate that strong interaction may exist between the adsorbed dibenzodioxin and zeolites with a threedimensional and well-defined microporous structure such as mordenite and NaY. The framework structure of ZSM-5 (18) Jannifer, C.; M.; Dennis, A. D. Chem. Rev. 1997, 97, 103. (19) Mao, Y.; Pankasem, S.; Thomas, J. K. Langmuir 1993, 9, 1504.
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contains two types of intersecting channels, both formed by 10-ring with an opening of about 0.55 nm. NaY and mordenite contain a 12-ring window with a pore size of 0.6-0.7 nm that is larger than that of ZSM-5. The adsorption of smaller molecules such as benzene and toluene on ZSM-5 and NaY has shown that the adsorption strength in zeolites depends on the pore size. Bohuatong et al. found that the adsorption energy of benzene in ZSM-5 is higher than that on NaY due to the smaller pore size of ZSM-5 and the stronger confinement effect of the ZSM-5 zeolite framework.20 Su et al. have proposed a hypothesis that the adsorption of benzene in 12-ring window zeolites is governed by a molecular recognition effect.21 Corma et al. proposed that electronic confinement of molecules in micropores may be contributed to explain the unusual properties of zeolites when the size of the guest molecules matches the size of the cavities taking account of the pore effect of zeolites.22 They have provided convincing spectroscopic evidence of the electronic confinement of a larger molecule such as anthracene in the pores of zeolites such as NaY and mordenite.23 Thus, the interaction strength of dibenzodioxin with ZSM-5, NaY, and mordenite may result from the pore size of these materials. The molecular size of dibenzodioxin matches the pore size of NaY and mordenite, whereas it is much larger than the size of ZSM-5 pores. Thus, dibenzodioxin can enter into the pores of NaY and mordenite and results in the highest desorption temperatures and biggest red shift of the aromatic ring due to the effect of pore confinement. For ZSM-5, dibenzodioxin cannot enter into the pores and shows a lower desorption temperature than that for NaY. Since the size of SBA-15 is too large compared with the dibenzodioxin molecule, it does not give any pore confinement and results in the same interaction strength with amorphous SiO2. Conclusions The desorption temperatures of adsorbed dibenzodioxin on inorganic materials increase as the following trend: silica < metal oxides < ZSM-5 < NaY and mordenite. The different interaction strengths may result from different interaction mechanisms. On amorphous/mesoporous silica, surface hydroxyl groups are mainly involved in the interaction via hydrogen bonding, whereas both the hydroxyl groups and Lewis acid sites are involved in for Al2O3 and TiO2. In zeolites with a pore size close to the molecular size of dibenzodioxin, the confinement effect of pore is the dominant interaction, which results in the strong adsorption of dibenzodioxin. LA0468545 (20) Bobuatong, K.; Limtrakul, J. Appl. Catal., A 2003, 253, 49. (21) Su, B.; Norberg, V. Langmuir 2000, 16, 6020. (22) Corma, A.; Garcia, H.; Sastre, G.; Viruela, P. M. J. Phys. Chem. B 1997, 101, 4575. (23) Marquez, F.; Garcia, H.; Palomares, E.; Fernandez, L.; Corma, A. J. Am. Chem. Soc. 2000, 122, 6520.
