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Platinum catalysts supported on hydrothermally stable mesoporous aluminosilicate for the catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs)
Catalysis Communications 11 (2010) 1068–1071
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Platinum catalysts supported on hydrothermally stable mesoporous aluminosilicate for the catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs) Joo-Il Park a, Jihn-Koo Lee b, Jin Miyawaki a, Wei-Wei Pang a, Seong-Ho Yoon a,⁎, Isao Mochida a a b
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan 121 Bio-venture Center, Korea Research Institute of Bioscience and Biotechnology, 52 Eoun-dong, Yuseong-gu, Daejeon 305-806, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 9 April 2010 Accepted 23 April 2010 Available online 10 May 2010 Keywords: Polycyclic aromatic hydrocarbons (PAHs) Naphthalene Catalytic oxidation Hydrothermally stable mesoporous aluminosilicate MCM-41 Platinum
a b s t r a c t Catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs) was studied over platinum catalysts supported on the hydrothermally stable mesoporous aluminosilicate (SM-41). Naphthalene was chosen as a model reactant of PAHs, due to the simplest and the least toxic PAHs. Zeolite seeds crystallization method was used for synthesis of SM-41. Pt/SM-41 catalyst showed higher activity than Pt/MCM-41 for catalytic oxidation of naphthalene in the presence of 10 vol.% water vapor. Hydrothermal stability and hydrophobicity of Pt/SM-41 must be beneficial for the catalytic oxidation of naphthalene in the presence of water vapor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Even if meso-structured molecular sieves of MCM-41 type [1,2] have been suggested as good support due to their high surface area, pore volume and precisely controllable pore diameter, practical applications of mesoporous molecular sieves are severely limited by the weak hydrothermal stability. The instability of these structures has been attributed in part to the thickness and incomplete crosslinking of the pore walls [3]. Therefore, it is necessary for mesoporous molecular sieves to enhance the hydrothermal stability to be applied as a good catalyst support. There have been many attempts to enhance the hydrothermal stability of mesoporous materials, for examples, thickening the mesoporous wall [4], surface modification by sililation [5], salt treatment [6], grafting method [7]. Recent breakthrough has been made to improve the hydrothermal stability of mesoporous structures through the use of protozeolitic aluminosilicate nanoclusters or “zeolite seeds” as framework precursors [8,9]. Polycyclic aromatic hydrocarbons (PAHs) emitted from diesel engines are ubiquitous carcinogenic substances to which humans are exposed from the environment [10]. They are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs stem from the atmospheric deposition and diesel emission. Consequently, the elimination of
⁎ Corresponding author. Tel.: + 81 92 583 7959; fax: +81 92 583 7897. E-mail address: [email protected] (S.-H. Yoon). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.04.017
PAHs in the sources is one of the priority and emerging challenges. Naphthalene is the most volatile member of this class of pollutants [11] and also a constituent of diesel and jet fuel [12–14]. Li et al. [13] and Siegl et al. [14] have pointed out that PAHs in diesel emissions mainly consisted of two to five aromatic rings, especially, naphthalene, phenanthrene and pyrene. Most of the naphthalene is released into the atmosphere (90%), with a small portion into water (5%) and soil (3%) [12]. Naphthalene consists of a basic aromatic structure and is the simplest and the least toxic PAHs. Therefore, naphthalene can be chosen as a model compound for the development of abatement technique of PAHs emitted into the atmosphere. Common methods applicable for eliminating naphthalene from the waste stream include biodegradation [15], absorption [16], adsorption [17], ozonization [18,19] and catalytic oxidation [20,21]. Among them, the catalytic oxidation has been demonstrated as one of the cost effective and efficient technologies to destroy the troublesome VOCs. There are very few reports on the catalytic oxidation of naphthalene, mostly on the γ-alumina supported metal catalysts [20,21]. Among the metal catalysts, Pt-based and/or modified catalysts represented the best activities for the oxidation of naphthalene [22,23]. Moreover, the various supports have been investigated over for Pt catalyst for naphthalene oxidation [23]. In this work, hydrothermally stable mesoporous molecular sieves (SM-41) synthesized by the method of zeolite seed crystallization were used as catalyst support. The activity of Pt/SM-41 was investigated for the catalytic oxidation of naphthalene. Comparison was made with Pt/MCM-41 catalyst.
J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
2. Experimental 2.1. Preparation of catalysts and characterization MCM-41 was synthesized by the conventional hydrothermal pathway similar to the procedure described by Ryoo et al. [24]. The hydrothermally stable mesoporous molecular sieves (SM-41, Si/Al molar ratio; 75) were synthesized by the zeolite seed crystallization method using a two-template synthesis gel system (TEAOH and CTAB) [8]. A typical hexagonal aluminosilicate meso-structure was derived from zeolite seed that normally leads to the crystallization of ZSM-5 zeolite. The synthesis route of SM-41 was basically identical to that of MCM-41 except the use of zeolite nano-clustered seeds gel as silica source rather than sodium silicate solution. The stability of samples was examined by comparing XRD, N2 adsorption/desorption isotherm and FT-IR. Supported Pt catalysts were prepared by incipient wetness impregnation method. Pt(NH3)4Cl2·H2O (98%, Aldrich) was used as a precursor. 1 wt.% of Pt was loaded on the MCM-41 and SM-41, respectively, and they were dried at 100 °C and calcined (air) at 550 °C for 5 h. Pt dispersion was estimated by CO chemisorption. 2.2. Catalytic activities The catalytic activity for the oxidation of naphthalene was measured in fixed bed apparatus at a temperature range of 150–500 °C. The reaction feed mixture containing 400 ppmv naphthalene and 10 vol.% of water vapor with balance air was introduced through the evaporator at 100 °C. The space time (W/F) was 0.016 g-cat·h/L. The effluent gases were analysed by an on- or off-line GC (6890, Hewlett-Packard, USA) through HP-5 column with FID and MS for naphthalene and other hydrocarbons, and Carboxene 1006 column for CO and CO2 with methanizer. The conversion of naphthalene and CO2 yield [(CO2)e / (CO2)t × 100, %] were measured as a function of temperature. The (CO2)e denotes the CO2 concentration which was measured experimentally and (CO2)t the one predicted for complete oxidation of naphthalene (C10H8 + 12O2 → 10CO2 + 4H2O). Carbon species over the surface of catalysts were analyzed by dichloromethane method similar to the procedure described by Zhang et al. [25].
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structure assembled from nano-clustered zeolite seeds retains its mesoporous character upon hydrothermal treatment at 100 °C with autogeneous pressure. However, the meso-structure synthesized from conventional sodium silicate precursor (MCM-41) was totally collapsed from its original structure by hydrothermal treatment. This observation was consistent with the results of N2 sorption isotherm in Fig. 2. After hydrothermal treatment, the mesoporous structure of MCM-41 disappeared almost completely. This structural degradation was accompanied by reduction in surface area and pore volume of 74.7% and 38.5%, respectively, while those of SM-41 were maintained in large portion (i.e., 11.1% and 9.1%, respectively) as shown in Table 1. Enhanced hydrothermal stability of SM-41 might be attributed to the crystallization of amorphous wall of conventional MCM-41 due to zeolitic phase, which could be evidenced by the presence of characteristic FT-IR peak of zeolite's 5-ring as shown in Fig. 3. The vibrational band of a skeleton bending mode observed at ca. 500–550 cm− 1 for ZSM-5, which has been assigned to highly distorted double six-ring or five member ring [26,27] appeared in the spectrum of SM-41. The presence of similar band for SM-41 seems to suggest the presence of pentasil zeolite subunits, contributing to the improvement of hydrothermal stability. The catalytic performance of Pt/MCM-41 and Pt/SM-41 for the oxidation of naphthalene in dry condition is shown in Fig. 4. The blank tests were conducted by passing naphthalene (400 ppm) through an empty reactor that was heated from 150 to 450 °C at a rate of 10 °C/min. No naphthalene decomposition activity can be observed, indicating that
3. Results and discussion The Pt/MCM-41 and Pt/SM-41 catalysts were treated in boiling water for 24 h to test their hydrothermal stability. Fig. 1 shows the XRD patterns of MCM-41 and SM-41 before and after the hydrothermal treatment. The XRD results clearly indicate that the SM-41 meso-
Fig. 1. XRD patterns of Pt/MCM-41 and Pt/SM-41 before and after hydrothermal treatment (HT) at 100 °C.
Fig. 2. Nitrogen sorption isotherms of: (a) Pt/MCM-41; (b) Pt/SM-41 before and after hydrothermal treatment (HT).
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J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
Table 1 Texture properties of Pt/MCM-14 and Pt/SM-41 before and after hydrothermal stability test (HT) at 100 °C. Catalysts
Surface area (m2/g)
Pore volume (cm3/g)
Avg. pore diameter (Å)
Dispersion (%)
Pt/MCM-41 Pt/MCM-41(HT) Pt/SM-41 Pt/SM-41(HT)
1162 294 1361 1321
1.09 0.67 1.37 1.34
29.2 81.9 31.1 31.9
17.1 20.3
any oxidation activity below 450 °C can be fully attributed to the presence of the catalysts. In addition, no activity was observed for pure MCM-41 and SM-41 below 450 °C. It can be seen that Pt/SM-41 showed a higher catalytic activity than Pt/MCM-41. The high activity of Pt/SM-41 catalyst appears to be related to its higher Pt dispersion than that of Pt/MCM-41, as shown in Table 1, as well as due to the enhancement of surface acidity which was evidenced by NH3-TPD (even if not shown in this paper). Higher hydrothermal stability as well as higher catalytic activity should make the Pt/SM-41 catalyst a good candidate applicable to a catalytic oxidation process of PAHs. Fig. 4 also shows that some decrease of CO2 yield in wet condition was observed over the entire temperature range for both catalysts. The decrease of CO2 yield over the catalysts in the presence of water vapor might be attributed to the suppression of the adsorption of reactants on the catalyst surface by water molecules. It should be noted that the decrease in activity of Pt/MCM-41 and Pt/SM-41 in the presence of water vapor was similar at lower temperature region (below 180 °C), but the decrease in CO2 yield of Pt/SM-41 was found to be smaller than that of Pt/MCM-41 at higher temperature region (above 180 °C). It could be suggested that this little decrease in catalytic activity of Pt/SM-41 was ascribed to the different contribution of hydrophobicity with the temperature. The adsorption capacity for water at 35 °C and 25 Torr was 0.05 g-H2O/g-SM-41 and 0.4 g-H2O/g-MCM-41, respectively. This water rejection property of SM-41 may lead to the enhanced hydrothermal stability. Although Pt catalysts supported on SM-41 showed better activity under wet conditions than that of MCM-41, both catalysts represented the deficit in the carbon balance at lower temperature. It implies the incomplete oxidation to CO2 which can be led to the generation of CO and secondary pollutants as by-products. The differences between naphthalene conversion and CO2 yield was mainly due to CO (over the 95%) as a effluent gas, and the reaction
Fig. 3. FT-IR spectra of ZSM-5, SM-41 and MCM-41.
Fig. 4. (a) The conversion of naphthalene and (b) CO2 yield over Pt/SM-41(●, ○) and Pt/MCM-41(■, □) for catalytic oxidation of naphthalene. (400ppmv naphthalene and 10 vol.% water vapor (if necessary) balanced with air; dry condition: black, wet condition: open).
intermediates such as naphthalene derivates (1–methynaphthalene and 2,2′-binaphthalene) and polymerized polycyclic or polymerizedoxygenated polycyclic compounds (5,12-naphthacendion) less than 1 ppm were found in the reaction products, not only in the gas phase but also on the catalyst surface. Since the destruction of reactants may generate by-products as secondary pollutants, the disappearance of reactants may not represent the extent of oxidation. In fact, our GC-MS analysis confirmed that a larger amount of intermediate hydrocarbons was yielded at 200 °C, but the complicated compositions of by-products indicated that various reactions were involved, and provided limited quantitative information. Therefore, the catalyst activity based on final products such as CO2 was treated as a better measure of effectiveness of the catalysts in this study. Fig. 5 represents the changes in CO2 yield of Pt/MCM-41 and Pt/ SM-41 at 290 °C with a stepwise addition and removal of water vapor. There were gradual changes of activity with time-on-stream irrespective of water addition for both catalysts. The activity dropped instantaneously by the addition of 10 vol.% of water vapor. After switching back to water-free reaction conditions, the activity immediately returned to the initial value in the case of SM-41. It should be noted for Pt/MCM-41 that the changes of activity after 140 min and the response to water addition were more pronounced. The activity of Pt/MCM-41 could not be restored to its initial value and decreased gradually. The changes of physical properties of Pt/ MCM-41 after stepwise water treatment were larger than that of Pt/
J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
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Table 3 Carbon species over the surface of catalysts during time-on-stream. (With library mass spectrum match over than 90%). Compounds
Pt/SM-41 (ppm) b
c
– – 1.5 – 2.1 1.8
– – 1.7 – 2.1 2.2
– – 1.7 1.1 2.4 2.3
1st run
Benzoic acid Propanoic acid Phthalic acid ester 1-methyl naphthalene 2,2′-binaphthalene Benzo-pyrene
Pt/MCM-41 (ppm)
a
2nd run
3rd run 1st run 2nd run 3rd run – 0.4 3.2 – 2.3 1.2
– – 3.7 0.3 2.5 2.5
1.3 – 4.1 0.1 3.2 3.1
All carbon species at each step were extracted by dichloromethane (DCM). a 1st run: after dry run. b 2nd run: after dry–wet run. c 3rd run: after dry–wet–dry run.
Fig. 5. Changes of CO2 yield in the presence of water vapor over Pt/MCM-41 and Pt/SM-41 at 290 °C.
SM-41 as shown in Table 2. The Pt/MCM-41 catalyst should experience the structural damage as well as the decrease in Pt dispersion to a larger extent than Pt/SM-41 at that temperature. The reason why differences (surface area and pore volume) of “after dry– wet” and “after dry–wet–dry” runs are similar with one between “after dry” and “after wet run” in Pt/MCM-41 catalyst might be due to the restoring time to dry condition again, in which some more structural damage might be progressed, because of the remained water vapors. Carbon species over the surface of catalysts at each step during time-on-stream were analyzed using mass spectroscopy (Table 3). The intermediates identified in this study might be classified into three categories of naphthalene-decomposed products (benzoic acid, propanoic acid, and phthalic acid ester), naphthalene derivates (comprising alkyl substituents; 1-methyl naphthalene and 2,2′binaphthalene) and polymerized products (benzo-pyrene). Compounds contained heavier molecular weight (over than naphthalene) after 3rd run of Pt/MCM-41 were larger than that of Pt/SM-41, which might be attributed to the decreased activity resulted from pore blocking and collapse under wet conditions, in contrast to Pt/SM-41 catalyst. 4. Conclusion The improvement of hydrothermal stability of MCM-41 was achieved by adopting the zeolite seeds crystallization method. The increase in hydrothermal stability might be attributed to the Table 2 Physical properties of Pt/MCM-41 and Pt/SM-41 after stepwise water treatment. Catalysts
Surface area Pore volume Avg. pore diameter Dispersion (cm3/g) (Å) (%) (m2/g)
Pt/MCM-41 (fresh) Pt/MCM-41 (after dry run) Pt/MCM-41 (after dry–wet run) Pt/MCM-41 (after dry–wet–dry run) Pt/SM-41 (fresh) Pt/SM-41 (after dry run) Pt/SM-41 (after dry–wet run) Pt/SM-41 (after dry–wet–dry run)
1162 1158
1.09 1.10
29.3 29.3
17.1 17.0
1030
0.95
30.5
12.1
939
0.88
28.9
9.8
1361 1364
1.37 1.36
31.1 30.9
20.3 20.2
1346
1.39
31.8
18.9
1334
1.35
30.8
18.4
improved crystallization of amorphous walls of conventional MCM-41 with zeolitic phase. The Pt/SM-41 catalyst showed better catalytic activity than Pt/MCM-41 catalyst for catalytic oxidation of naphthalene, which was likely due to the higher Pt dispersion of Pt/SM-41. Even if the conversion of naphthalene and CO2 yield decreased in the presence of water vapor in both Pt/SM-41 and Pt/MCM-41 catalysts, the decrease was much smaller for the Pt/SM-41 catalyst above 200 °C. The Pt/SM-41 catalyst makes a promising catalyst for the removal of PAHs due to its hydrothermal stability together with hydrophobicity. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.04.017. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] A. Corma, M.S. Grande, V. GonzalezAlfaro, A.V. Orchilles, J. Catal. 159 (1996) 375. [4] D. Zhao, Q. Huo, J. Feng, B.F. Chemelka, G.D. Stuky, J. Am. Chem. Soc. 120 (1998) 6024. [5] X.S. Zhao, G.Q. Lu, X. Hu, Micropor. Mesopor. Mater. 41 (2000) 37. [6] J.M. Kim, S. Jun, R. Ryoo, J. Phy. Chem. B 103 (1999) 6200. [7] R. Mokaya, Angew. Chem. Int. Ed. 38 (1999) 2930. [8] Z.T. Zhang, Y. Han, L. Zhu, R.W. Wang, Y. Yu, S.L. Qiu, D.Y. Zhao, F.S. Xiao, Angew. Chem. Int. Ed. 40 (2001) 1258. [9] Y. Liu, T.J. Pinnavaia, J. Mater. Chem. 12 (2002) 3179. [10] International Program on Chemical Safety (IPCS), Environmental Health Criteria 2002, Selected Non-heterocyclic Polycyclic Aromatic Hydrocarbons, WHO, Geneva, 1998. [11] R. Preuss, J. Angerer, H. Drexler, Int. Arch. Occup. Environ. Health 76 (2003) 556. [12] S. Coons, M. Byrne, M. Goyer, An exposure and risk assessment for benzo(a)pyrene and other polycyclic aromatic hydrocarbons, Naphthalene, Final Draft Report, USEPA, II, Office of Water Regulations and Standards, Washington, DC, 1982. [13] H. Li, C.D. Banner, G.G. Mason, R.N. Westerholm, J.J. Rafter, Atmos. Environ. 30 (1996) 3537. [14] W.O. Seigl, R.H. Hammerle, H.M. Herrmann, B.W. Wenclawiak, B. Luers-Jongen, Atmos. Environ. 33 (1999) 797. [15] G. Chen, K.A. Strevett, B.A. Vanegas, Biodegradation 12 (2001) 433. [16] H. Lhuang, W.M. Lee, J. Environ. Eng. ASCE 128 (2002) 60. [17] S.Y. Lee, S.J. Kim, Appl. Clay Sci. 22 (2002) 55. [18] B. Legube, S. Guyon, H. Sugimitsu, M. Dore, Water Res. 20 (1986) 197. [19] Y. Hchen, C.Y. Chang, S.F. Huang, C.Y. Chiu, D. Ji, N.C. Shang, Y.H. Yu, P.C. Chiang, Y. Ku, J.N. Chen, Water Res. 36 (2002) 4144. [20] X.W. Zhang, S.C. Shen, L.E. Yu, S. Kawi, K. Hidajat, K.Y.S. Ng, Appl. Catal. A: Gen. 250 (2003) 341. [21] J.L. Shie, C.Y. Chang, J.H. Chen, Appl. Catal. B: Environ. 58 (2005) 289. [22] E.N. Ndifo, T. Garcia, S.H. Taylor, Catal. Lett. 110 (2006) 125. [23] E.N. Ndifo, A.F. Carley, S.H. Taylor, Catal. Today 137 (2008) 362. [24] R. Roo, S. Jun, J. Phy. Chem. B 10 (1997) 313. [25] X.W. Zhang, S.C. Shen, K. Hidajat, S. Kawi, L.E. Yu, K.Y. Simon Ng, Catal. Lett. 96 (2004) 87. [26] Y. Han, S. Wu, Y.Y. Sun, D.S. Li, F.S. Xiao, J. Liu, X.Z. Zhang, Chem. Mater. 14 (2002) 1144. [27] Y. Di, Y. Yu, Y.Y. Sun, X.Y. Yang, S. Lin, M.Y. Zhang, S.G. Li, F.S. Xiao, Micropor. Mesopor. Mater. 62 (2003) 221.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Platinum catalysts supported on hydrothermally stable mesoporous aluminosilicate for the catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs) Joo-Il Park a, Jihn-Koo Lee b, Jin Miyawaki a, Wei-Wei Pang a, Seong-Ho Yoon a,⁎, Isao Mochida a a b
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan 121 Bio-venture Center, Korea Research Institute of Bioscience and Biotechnology, 52 Eoun-dong, Yuseong-gu, Daejeon 305-806, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 9 April 2010 Accepted 23 April 2010 Available online 10 May 2010 Keywords: Polycyclic aromatic hydrocarbons (PAHs) Naphthalene Catalytic oxidation Hydrothermally stable mesoporous aluminosilicate MCM-41 Platinum
a b s t r a c t Catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs) was studied over platinum catalysts supported on the hydrothermally stable mesoporous aluminosilicate (SM-41). Naphthalene was chosen as a model reactant of PAHs, due to the simplest and the least toxic PAHs. Zeolite seeds crystallization method was used for synthesis of SM-41. Pt/SM-41 catalyst showed higher activity than Pt/MCM-41 for catalytic oxidation of naphthalene in the presence of 10 vol.% water vapor. Hydrothermal stability and hydrophobicity of Pt/SM-41 must be beneficial for the catalytic oxidation of naphthalene in the presence of water vapor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Even if meso-structured molecular sieves of MCM-41 type [1,2] have been suggested as good support due to their high surface area, pore volume and precisely controllable pore diameter, practical applications of mesoporous molecular sieves are severely limited by the weak hydrothermal stability. The instability of these structures has been attributed in part to the thickness and incomplete crosslinking of the pore walls [3]. Therefore, it is necessary for mesoporous molecular sieves to enhance the hydrothermal stability to be applied as a good catalyst support. There have been many attempts to enhance the hydrothermal stability of mesoporous materials, for examples, thickening the mesoporous wall [4], surface modification by sililation [5], salt treatment [6], grafting method [7]. Recent breakthrough has been made to improve the hydrothermal stability of mesoporous structures through the use of protozeolitic aluminosilicate nanoclusters or “zeolite seeds” as framework precursors [8,9]. Polycyclic aromatic hydrocarbons (PAHs) emitted from diesel engines are ubiquitous carcinogenic substances to which humans are exposed from the environment [10]. They are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs stem from the atmospheric deposition and diesel emission. Consequently, the elimination of
⁎ Corresponding author. Tel.: + 81 92 583 7959; fax: +81 92 583 7897. E-mail address: [email protected] (S.-H. Yoon). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.04.017
PAHs in the sources is one of the priority and emerging challenges. Naphthalene is the most volatile member of this class of pollutants [11] and also a constituent of diesel and jet fuel [12–14]. Li et al. [13] and Siegl et al. [14] have pointed out that PAHs in diesel emissions mainly consisted of two to five aromatic rings, especially, naphthalene, phenanthrene and pyrene. Most of the naphthalene is released into the atmosphere (90%), with a small portion into water (5%) and soil (3%) [12]. Naphthalene consists of a basic aromatic structure and is the simplest and the least toxic PAHs. Therefore, naphthalene can be chosen as a model compound for the development of abatement technique of PAHs emitted into the atmosphere. Common methods applicable for eliminating naphthalene from the waste stream include biodegradation [15], absorption [16], adsorption [17], ozonization [18,19] and catalytic oxidation [20,21]. Among them, the catalytic oxidation has been demonstrated as one of the cost effective and efficient technologies to destroy the troublesome VOCs. There are very few reports on the catalytic oxidation of naphthalene, mostly on the γ-alumina supported metal catalysts [20,21]. Among the metal catalysts, Pt-based and/or modified catalysts represented the best activities for the oxidation of naphthalene [22,23]. Moreover, the various supports have been investigated over for Pt catalyst for naphthalene oxidation [23]. In this work, hydrothermally stable mesoporous molecular sieves (SM-41) synthesized by the method of zeolite seed crystallization were used as catalyst support. The activity of Pt/SM-41 was investigated for the catalytic oxidation of naphthalene. Comparison was made with Pt/MCM-41 catalyst.
J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
2. Experimental 2.1. Preparation of catalysts and characterization MCM-41 was synthesized by the conventional hydrothermal pathway similar to the procedure described by Ryoo et al. [24]. The hydrothermally stable mesoporous molecular sieves (SM-41, Si/Al molar ratio; 75) were synthesized by the zeolite seed crystallization method using a two-template synthesis gel system (TEAOH and CTAB) [8]. A typical hexagonal aluminosilicate meso-structure was derived from zeolite seed that normally leads to the crystallization of ZSM-5 zeolite. The synthesis route of SM-41 was basically identical to that of MCM-41 except the use of zeolite nano-clustered seeds gel as silica source rather than sodium silicate solution. The stability of samples was examined by comparing XRD, N2 adsorption/desorption isotherm and FT-IR. Supported Pt catalysts were prepared by incipient wetness impregnation method. Pt(NH3)4Cl2·H2O (98%, Aldrich) was used as a precursor. 1 wt.% of Pt was loaded on the MCM-41 and SM-41, respectively, and they were dried at 100 °C and calcined (air) at 550 °C for 5 h. Pt dispersion was estimated by CO chemisorption. 2.2. Catalytic activities The catalytic activity for the oxidation of naphthalene was measured in fixed bed apparatus at a temperature range of 150–500 °C. The reaction feed mixture containing 400 ppmv naphthalene and 10 vol.% of water vapor with balance air was introduced through the evaporator at 100 °C. The space time (W/F) was 0.016 g-cat·h/L. The effluent gases were analysed by an on- or off-line GC (6890, Hewlett-Packard, USA) through HP-5 column with FID and MS for naphthalene and other hydrocarbons, and Carboxene 1006 column for CO and CO2 with methanizer. The conversion of naphthalene and CO2 yield [(CO2)e / (CO2)t × 100, %] were measured as a function of temperature. The (CO2)e denotes the CO2 concentration which was measured experimentally and (CO2)t the one predicted for complete oxidation of naphthalene (C10H8 + 12O2 → 10CO2 + 4H2O). Carbon species over the surface of catalysts were analyzed by dichloromethane method similar to the procedure described by Zhang et al. [25].
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structure assembled from nano-clustered zeolite seeds retains its mesoporous character upon hydrothermal treatment at 100 °C with autogeneous pressure. However, the meso-structure synthesized from conventional sodium silicate precursor (MCM-41) was totally collapsed from its original structure by hydrothermal treatment. This observation was consistent with the results of N2 sorption isotherm in Fig. 2. After hydrothermal treatment, the mesoporous structure of MCM-41 disappeared almost completely. This structural degradation was accompanied by reduction in surface area and pore volume of 74.7% and 38.5%, respectively, while those of SM-41 were maintained in large portion (i.e., 11.1% and 9.1%, respectively) as shown in Table 1. Enhanced hydrothermal stability of SM-41 might be attributed to the crystallization of amorphous wall of conventional MCM-41 due to zeolitic phase, which could be evidenced by the presence of characteristic FT-IR peak of zeolite's 5-ring as shown in Fig. 3. The vibrational band of a skeleton bending mode observed at ca. 500–550 cm− 1 for ZSM-5, which has been assigned to highly distorted double six-ring or five member ring [26,27] appeared in the spectrum of SM-41. The presence of similar band for SM-41 seems to suggest the presence of pentasil zeolite subunits, contributing to the improvement of hydrothermal stability. The catalytic performance of Pt/MCM-41 and Pt/SM-41 for the oxidation of naphthalene in dry condition is shown in Fig. 4. The blank tests were conducted by passing naphthalene (400 ppm) through an empty reactor that was heated from 150 to 450 °C at a rate of 10 °C/min. No naphthalene decomposition activity can be observed, indicating that
3. Results and discussion The Pt/MCM-41 and Pt/SM-41 catalysts were treated in boiling water for 24 h to test their hydrothermal stability. Fig. 1 shows the XRD patterns of MCM-41 and SM-41 before and after the hydrothermal treatment. The XRD results clearly indicate that the SM-41 meso-
Fig. 1. XRD patterns of Pt/MCM-41 and Pt/SM-41 before and after hydrothermal treatment (HT) at 100 °C.
Fig. 2. Nitrogen sorption isotherms of: (a) Pt/MCM-41; (b) Pt/SM-41 before and after hydrothermal treatment (HT).
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J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
Table 1 Texture properties of Pt/MCM-14 and Pt/SM-41 before and after hydrothermal stability test (HT) at 100 °C. Catalysts
Surface area (m2/g)
Pore volume (cm3/g)
Avg. pore diameter (Å)
Dispersion (%)
Pt/MCM-41 Pt/MCM-41(HT) Pt/SM-41 Pt/SM-41(HT)
1162 294 1361 1321
1.09 0.67 1.37 1.34
29.2 81.9 31.1 31.9
17.1 20.3
any oxidation activity below 450 °C can be fully attributed to the presence of the catalysts. In addition, no activity was observed for pure MCM-41 and SM-41 below 450 °C. It can be seen that Pt/SM-41 showed a higher catalytic activity than Pt/MCM-41. The high activity of Pt/SM-41 catalyst appears to be related to its higher Pt dispersion than that of Pt/MCM-41, as shown in Table 1, as well as due to the enhancement of surface acidity which was evidenced by NH3-TPD (even if not shown in this paper). Higher hydrothermal stability as well as higher catalytic activity should make the Pt/SM-41 catalyst a good candidate applicable to a catalytic oxidation process of PAHs. Fig. 4 also shows that some decrease of CO2 yield in wet condition was observed over the entire temperature range for both catalysts. The decrease of CO2 yield over the catalysts in the presence of water vapor might be attributed to the suppression of the adsorption of reactants on the catalyst surface by water molecules. It should be noted that the decrease in activity of Pt/MCM-41 and Pt/SM-41 in the presence of water vapor was similar at lower temperature region (below 180 °C), but the decrease in CO2 yield of Pt/SM-41 was found to be smaller than that of Pt/MCM-41 at higher temperature region (above 180 °C). It could be suggested that this little decrease in catalytic activity of Pt/SM-41 was ascribed to the different contribution of hydrophobicity with the temperature. The adsorption capacity for water at 35 °C and 25 Torr was 0.05 g-H2O/g-SM-41 and 0.4 g-H2O/g-MCM-41, respectively. This water rejection property of SM-41 may lead to the enhanced hydrothermal stability. Although Pt catalysts supported on SM-41 showed better activity under wet conditions than that of MCM-41, both catalysts represented the deficit in the carbon balance at lower temperature. It implies the incomplete oxidation to CO2 which can be led to the generation of CO and secondary pollutants as by-products. The differences between naphthalene conversion and CO2 yield was mainly due to CO (over the 95%) as a effluent gas, and the reaction
Fig. 3. FT-IR spectra of ZSM-5, SM-41 and MCM-41.
Fig. 4. (a) The conversion of naphthalene and (b) CO2 yield over Pt/SM-41(●, ○) and Pt/MCM-41(■, □) for catalytic oxidation of naphthalene. (400ppmv naphthalene and 10 vol.% water vapor (if necessary) balanced with air; dry condition: black, wet condition: open).
intermediates such as naphthalene derivates (1–methynaphthalene and 2,2′-binaphthalene) and polymerized polycyclic or polymerizedoxygenated polycyclic compounds (5,12-naphthacendion) less than 1 ppm were found in the reaction products, not only in the gas phase but also on the catalyst surface. Since the destruction of reactants may generate by-products as secondary pollutants, the disappearance of reactants may not represent the extent of oxidation. In fact, our GC-MS analysis confirmed that a larger amount of intermediate hydrocarbons was yielded at 200 °C, but the complicated compositions of by-products indicated that various reactions were involved, and provided limited quantitative information. Therefore, the catalyst activity based on final products such as CO2 was treated as a better measure of effectiveness of the catalysts in this study. Fig. 5 represents the changes in CO2 yield of Pt/MCM-41 and Pt/ SM-41 at 290 °C with a stepwise addition and removal of water vapor. There were gradual changes of activity with time-on-stream irrespective of water addition for both catalysts. The activity dropped instantaneously by the addition of 10 vol.% of water vapor. After switching back to water-free reaction conditions, the activity immediately returned to the initial value in the case of SM-41. It should be noted for Pt/MCM-41 that the changes of activity after 140 min and the response to water addition were more pronounced. The activity of Pt/MCM-41 could not be restored to its initial value and decreased gradually. The changes of physical properties of Pt/ MCM-41 after stepwise water treatment were larger than that of Pt/
J.-I. Park et al. / Catalysis Communications 11 (2010) 1068–1071
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Table 3 Carbon species over the surface of catalysts during time-on-stream. (With library mass spectrum match over than 90%). Compounds
Pt/SM-41 (ppm) b
c
– – 1.5 – 2.1 1.8
– – 1.7 – 2.1 2.2
– – 1.7 1.1 2.4 2.3
1st run
Benzoic acid Propanoic acid Phthalic acid ester 1-methyl naphthalene 2,2′-binaphthalene Benzo-pyrene
Pt/MCM-41 (ppm)
a
2nd run
3rd run 1st run 2nd run 3rd run – 0.4 3.2 – 2.3 1.2
– – 3.7 0.3 2.5 2.5
1.3 – 4.1 0.1 3.2 3.1
All carbon species at each step were extracted by dichloromethane (DCM). a 1st run: after dry run. b 2nd run: after dry–wet run. c 3rd run: after dry–wet–dry run.
Fig. 5. Changes of CO2 yield in the presence of water vapor over Pt/MCM-41 and Pt/SM-41 at 290 °C.
SM-41 as shown in Table 2. The Pt/MCM-41 catalyst should experience the structural damage as well as the decrease in Pt dispersion to a larger extent than Pt/SM-41 at that temperature. The reason why differences (surface area and pore volume) of “after dry– wet” and “after dry–wet–dry” runs are similar with one between “after dry” and “after wet run” in Pt/MCM-41 catalyst might be due to the restoring time to dry condition again, in which some more structural damage might be progressed, because of the remained water vapors. Carbon species over the surface of catalysts at each step during time-on-stream were analyzed using mass spectroscopy (Table 3). The intermediates identified in this study might be classified into three categories of naphthalene-decomposed products (benzoic acid, propanoic acid, and phthalic acid ester), naphthalene derivates (comprising alkyl substituents; 1-methyl naphthalene and 2,2′binaphthalene) and polymerized products (benzo-pyrene). Compounds contained heavier molecular weight (over than naphthalene) after 3rd run of Pt/MCM-41 were larger than that of Pt/SM-41, which might be attributed to the decreased activity resulted from pore blocking and collapse under wet conditions, in contrast to Pt/SM-41 catalyst. 4. Conclusion The improvement of hydrothermal stability of MCM-41 was achieved by adopting the zeolite seeds crystallization method. The increase in hydrothermal stability might be attributed to the Table 2 Physical properties of Pt/MCM-41 and Pt/SM-41 after stepwise water treatment. Catalysts
Surface area Pore volume Avg. pore diameter Dispersion (cm3/g) (Å) (%) (m2/g)
Pt/MCM-41 (fresh) Pt/MCM-41 (after dry run) Pt/MCM-41 (after dry–wet run) Pt/MCM-41 (after dry–wet–dry run) Pt/SM-41 (fresh) Pt/SM-41 (after dry run) Pt/SM-41 (after dry–wet run) Pt/SM-41 (after dry–wet–dry run)
1162 1158
1.09 1.10
29.3 29.3
17.1 17.0
1030
0.95
30.5
12.1
939
0.88
28.9
9.8
1361 1364
1.37 1.36
31.1 30.9
20.3 20.2
1346
1.39
31.8
18.9
1334
1.35
30.8
18.4
improved crystallization of amorphous walls of conventional MCM-41 with zeolitic phase. The Pt/SM-41 catalyst showed better catalytic activity than Pt/MCM-41 catalyst for catalytic oxidation of naphthalene, which was likely due to the higher Pt dispersion of Pt/SM-41. Even if the conversion of naphthalene and CO2 yield decreased in the presence of water vapor in both Pt/SM-41 and Pt/MCM-41 catalysts, the decrease was much smaller for the Pt/SM-41 catalyst above 200 °C. The Pt/SM-41 catalyst makes a promising catalyst for the removal of PAHs due to its hydrothermal stability together with hydrophobicity. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.04.017. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] A. Corma, M.S. Grande, V. GonzalezAlfaro, A.V. Orchilles, J. Catal. 159 (1996) 375. [4] D. Zhao, Q. Huo, J. Feng, B.F. Chemelka, G.D. Stuky, J. Am. Chem. Soc. 120 (1998) 6024. [5] X.S. Zhao, G.Q. Lu, X. Hu, Micropor. Mesopor. Mater. 41 (2000) 37. [6] J.M. Kim, S. Jun, R. Ryoo, J. Phy. Chem. B 103 (1999) 6200. [7] R. Mokaya, Angew. Chem. Int. Ed. 38 (1999) 2930. [8] Z.T. Zhang, Y. Han, L. Zhu, R.W. Wang, Y. Yu, S.L. Qiu, D.Y. Zhao, F.S. Xiao, Angew. Chem. Int. Ed. 40 (2001) 1258. [9] Y. Liu, T.J. Pinnavaia, J. Mater. Chem. 12 (2002) 3179. [10] International Program on Chemical Safety (IPCS), Environmental Health Criteria 2002, Selected Non-heterocyclic Polycyclic Aromatic Hydrocarbons, WHO, Geneva, 1998. [11] R. Preuss, J. Angerer, H. Drexler, Int. Arch. Occup. Environ. Health 76 (2003) 556. [12] S. Coons, M. Byrne, M. Goyer, An exposure and risk assessment for benzo(a)pyrene and other polycyclic aromatic hydrocarbons, Naphthalene, Final Draft Report, USEPA, II, Office of Water Regulations and Standards, Washington, DC, 1982. [13] H. Li, C.D. Banner, G.G. Mason, R.N. Westerholm, J.J. Rafter, Atmos. Environ. 30 (1996) 3537. [14] W.O. Seigl, R.H. Hammerle, H.M. Herrmann, B.W. Wenclawiak, B. Luers-Jongen, Atmos. Environ. 33 (1999) 797. [15] G. Chen, K.A. Strevett, B.A. Vanegas, Biodegradation 12 (2001) 433. [16] H. Lhuang, W.M. Lee, J. Environ. Eng. ASCE 128 (2002) 60. [17] S.Y. Lee, S.J. Kim, Appl. Clay Sci. 22 (2002) 55. [18] B. Legube, S. Guyon, H. Sugimitsu, M. Dore, Water Res. 20 (1986) 197. [19] Y. Hchen, C.Y. Chang, S.F. Huang, C.Y. Chiu, D. Ji, N.C. Shang, Y.H. Yu, P.C. Chiang, Y. Ku, J.N. Chen, Water Res. 36 (2002) 4144. [20] X.W. Zhang, S.C. Shen, L.E. Yu, S. Kawi, K. Hidajat, K.Y.S. Ng, Appl. Catal. A: Gen. 250 (2003) 341. [21] J.L. Shie, C.Y. Chang, J.H. Chen, Appl. Catal. B: Environ. 58 (2005) 289. [22] E.N. Ndifo, T. Garcia, S.H. Taylor, Catal. Lett. 110 (2006) 125. [23] E.N. Ndifo, A.F. Carley, S.H. Taylor, Catal. Today 137 (2008) 362. [24] R. Roo, S. Jun, J. Phy. Chem. B 10 (1997) 313. [25] X.W. Zhang, S.C. Shen, K. Hidajat, S. Kawi, L.E. Yu, K.Y. Simon Ng, Catal. Lett. 96 (2004) 87. [26] Y. Han, S. Wu, Y.Y. Sun, D.S. Li, F.S. Xiao, J. Liu, X.Z. Zhang, Chem. Mater. 14 (2002) 1144. [27] Y. Di, Y. Yu, Y.Y. Sun, X.Y. Yang, S. Lin, M.Y. Zhang, S.G. Li, F.S. Xiao, Micropor. Mesopor. Mater. 62 (2003) 221.