Progress in Reaction Kinetics and Mechanism, 2015, 40(2), 119–127 doi:10.3184/146867815X14259937892267 Paper: 1400283
Density functional theoretical study on the mechanism of adsorption of 2-chlorophenol from water using γ-Fe2O3 nanoparticles A. Heshmati Jannat Maghama, A. Morsalib*, Z. Es’haghia, S. A. Beyramabadib and H. Cheginib aChemistry bDepartment
department, Payame Noor University, 19395-4697 Tehran, Iran
of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran *E-mail:
[email protected];
[email protected]
ABSTRACT Using density functional theory, the mechanism of adsorption of 2-chlorophenol from water in the presence of γ-Fe2O3 nanoparticles was investigated. Fe2O3 nanoparticles were modelled using Fe6(OH)18(H2O) 6 ring clusters. 2-chlorophenol can coordinate to the γ-Fe2O3 nanoparticles via its own OH or Cl groups. The process produces two intermediates which will be converted into final products through two pathways (Cl pathway and OH pathway). The activation energy and activation Gibbs free energy of the two pathways have been calculated and compared with each other. It was found that the OH pathway is under thermodynamic control and the C1 pathway is under kinetic control. All of the calculations were performed using a hybrid density functional method (B3LYP) in the solution phase (PCM model).
KEYWORDS: γ-Fe2O3 nanoparticles, 2-chlorophenol, adsorption mechanism, activation energy, density functional theory
1. INTRODUCTION The removal of aromatic pollutants from water is of great importance environmentally and industrially. 2-chlorophenol and phenol-like compounds are categorised as persistent pollutants. Many techniques have been presented for the removal of phenolic pollutants from water [1-5]. Among the most important of such techniques is the application of nanoparticles of magnetic metals such as Fe2O3 [613]. The huge surface, easy separation and low cost are the reasons to use γ-Fe2O3 nanoparticles as a strongly adsorbent material. Regarding the increasing use of nanotechnology in today’s life, understanding the fundamental mechanism of action of nanoparticles is of great importance [14, 15]. Jayarathne et al. presented a model for γ-Fe2O3 nanoparticles on the basis of the Fe6(OH)18(H2O) 6 ring cluster which gave good consistency with the experimental data including vibration 119
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frequencies and bond lengths [16]. In spite of the extensive use of magnetic nanoparticles, so far, the molecular mechanism of adsorption of pollutants in water by these nanoparticles has not been investigated. In this work, using density functional theory (DFT), the mechanism of adsorption of 2-chlorophenol from water in the presence of γ-Fe2O3 nanoparticles has been studied. The activation parameters for the different pathways are calculated and compared with each other.
2. COMPUTATIONAL DETAILS All of the present calculations have been performed with the B3LYP [17-19] hybrid density functional level using the GAUSSIAN 03 package [20]. The 6-31G(d,p) basis sets were employed except for Fe where the LANL2DZ basis set [21] was used with effective core potential (ECP) functions. The solvent has an important role in chemical reactions explicitly [22-25] or implicitly. The implicit effects of the solvent were considered by using the polarised continuum model (PCM) [26]. In the PCM method, the molecular cavity is made up of the union of interlocking atomic spheres. All degrees of freedom for all geometries were optimised in solution (water). The transition states obtained were confirmed to have only one imaginary frequency of the Hessian. The zero-point corrections were also considered in obtaining activation energies.
3. RESULTS AND DISSCUSSION The properties of γ-Fe2O3 nanoparticles were modelled using Fe6(OH)18(H2O) 6 ring clusters of six-edge sharing octahedra joining via 12 OH groups. The six water molecules and six surface OH groups were expected to form a network of H-bonded interactions (Figure 1) [16]. Six H2O molecules, due to their weaker bonding relative to OH, are proper choices to leave the cluster and to be replaced by 2-chlorophenol. The 2-chlorophenol molecule can coordinate to the Fe6(OH)18(H2O) 6 cluster via its own OH group (OH pathway ) or its Cl atom (Cl pathway) and thus the pollutant is removed from water. First, we investigated the Cl pathway. As shown in Scheme 1 this pathway corresponds to the substitution of H2O from Fe6(OH)18(H2O) 6 cluster by the Cl of 2-chlorophenol to give product C . In this scheme kCl is the rate constant and KCl and K´Cl are the relevant equilibrium constants. Figure 1 shows the optimised structure of the Fe6(OH)18(H2O) 6 cluster in the vicinity of 2-chlorophenol (species A). In order to find the transition state of the Cl pathway (Scheme 1), the product C in the vicinity of H2O should be optimised (species B). The optimised structure of species B is presented in Figure 2. Considering species A and species B, the transition state of the Cl pathway is obtained which we call TSAB. Figure 3 presents the optimised structures of TSAB. www.prkm.co.uk
DFT of adsorption of 2-chlorophenol in H2O with γ-Fe2O3 NPs
OH2
Cl Pathway
HO
OH
Cl H2O OH Fe
Fe
H2O OH Fe
OH2
+ C6H4ClOH Fe6(OH)18(H2O)6
KCl
OH
Fe
kCl species A
HO
H HO 2O
OH2
OH
Cl
Fe
Fe
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OH2
OH
species B
OH
Cl
Fe
Fe
K Cl /
OH + H2O
product C
Scheme 1
Using Figures 1–3, the Fe–Cl and Fe–O bond lengths decrease (increase) from 4.467 Å and 4.153 Å (2.563 Å and 2.046 Å) for species A and species B (species B and species A ) to 4.098 Å and 3.891Å for TSAB, respectively. The relative energies, including the electronic plus zero point energy (E), enthalpy (H) and Gibbs free energy (G) for the different structures in the Cl and OH pathways are presented in Table 1. The energy profile for these pathways are shown in Figure 4. The activation energy (Ea) and activation Gibbs free energy (DG‡) related to the step kCl (TSAB) are 35.42 kJ mol-1 and 36.79 kJ mol-1, respectively (Table 1 and Scheme 1). The total rate constant for Cl pathway is equal to kCl × KCl , so the total activation energy (Ea (Cl path)) and total activation Gibbs free energy (DG‡)Cl path)) are obtained from Eqns (1) and (2): Ea(Cl path) = Ea(kCl step)+DE(KCl step) = 35.42–18.86 = 16.66 kJ mol-1 (1) DG‡(Cl path) = DG‡(kCl step)+DG(KCl step) = 36.79+29.64 = 66.43 kJ mol-1 (2) The other pathway is the OH pathway (Scheme 2). This step commences with the attachment of the OH of 2-chlorophenol to H2O in the Fe6(OH)18(H2O) 6 cluster,
Figure 1 Optimised structure of species A.
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Figure 2 Optimised structure of species B.
Cl
OH Pathway OH2
OH
Fe
H2O OH Fe
+C6H4ClOH
Fe6(OH)18(H2O)6
OH2
KOH
OH2
Fe
HO HO OH Fe
H
2O
OH2
kOH species D
Cl
Cl
OH2
O
Fe
Fe
OH
species E
OH2
OH2
O
Fe
Fe
K/OH
OH + H2O
product F
Scheme 2
and then one hydrogen from H2O is transferred to the surface OH group (species D in Scheme 2). Similar to the Cl pathway, species D is initially formed with an equilibrium constant KOH , following which the species E and product F are formed in kOH and K´OH paths, where K´OH and kOH are the equilibrium constant and rate constant, respectively. The optimised structures of species D and species E are shown in Figures 5 and 6, respectively. Using species D and species E, a transition state is obtained which we call TSDE (Figure 7). Considering Figures 5–7, the Fe–O and O–H bond lengths increase (decrease) from 1.915 Å and 0.995 Å (3.733 Å and 1.599 Å ) for species www.prkm.co.uk
DFT of adsorption of 2-chlorophenol in H2O with γ-Fe2O3 NPs
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Figure 3 Optimised structure of TSAB.
Figure 4 Energy profile for the adsorption of 2-chlorophenol from water using γ-Fe2O3 nanoparticles.
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D and species E (species E and species D) to 3.035 Å and 1.021 Å for TSDE, respectively. (Ea) and (DG‡) related to kOH step are 85.98 kJ mol-1 and 93.37 kJ mol-1, respectively (Table 1 and Scheme 2). The total rate constant for the OH pathway is equal to kOH × KOH , so the total activation energy (Ea (OHpath)) and total activation Gibbs free energy (DG‡ (OHpath) ) are obtained from Eqns (3) and (4): Ea(OHpath) = Ea(kOHstep) + DE(KOHstep) = 85.98 – 36.53 = 49.45 kJ mol-1 (3) DG‡(OHpath) = DG‡ (kOHstep) + DG(KOHstep) = 93.37 + 7.34 = 100.71 kJ mol-1 (4) Ea and DG‡ for the OH pathway are higher than for the Cl pathway by 32.79 kJ mol-1 and 34.28 kJ mol-1, respectively. On the other hand, product F (OH pathway) is more stable than product C (Cl pathway) by 44.87 kJ mol-1, so product F and product C are thermodynamic and kinetic products, respectively. Table 1 Relative energies (kJ mol-1) for different structures in the proposed mechanisms. E, H and G are electronic plus zero point energy, enthalpy and Gibbs free energy, respectively Species
E
Fe6 (OH)18 (H2O) 6 + 2-chlorophenol Species A TSAB Species B Product C+H2O
0.00
0.00
0.00
–18.86 16.66 7.75 35.35
–17.38 17.24 7.63 37.36 OH pathway –35.48 50.85 –52.91 –0.03
29.64 66.43 59.69 49.58
Species D
TSDE
Species E Product F+H2O
–36.53 49.45 –55.58 –6.11
Figure 5 Optimised structure of species D.
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H Cl pathway
G
7.34 100.71 –4.90 4.71
DFT of adsorption of 2-chlorophenol in H2O with γ-Fe2O3 NPs
Figure 6 Optimised structure of species E.
Figure 7 Optimised structure of TSDE.
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In other words, kinetic and thermodynamic control act in opposition. Although in the Cl pathway, the pollutant coordinates to the Fe2O3 nanoparticle with a lower energy barrier, but since the reverse energy barrier is proportionately small, the pollutant is easily separated as well. Therefore, the passage of time acts against the kinetic control. The high energy barrier of the OH pathway is related to the proton transfer from OH of 2-chlorophenol to OH of the cluster. Since the reverse energy barrier is proportionately high as well, the probability of separation of the pollutant from the nanoparticle is very small. Different techniques such as using ultrasonic irradiation, helps to increase the contribution of the OH pathway and is in favour of thermodynamic control. Ultrasonic irradiation causes cavitations in a fluid medium where occur the formation, growth and implosive collapse of bubbles. This sequence of events leads to intense local heating around 5,000 °C and high pressure of over 1800 kPa [5].
4. CONCLUSIONS The mechanism of adsorption of 2-chlorophenol from water in the presence of γ-Fe2O3 nanoparticles has been studied in detail in a solvent (aqueous) environment, using the PCM model. γ-Fe2O3 nanoparticles were modelled using Fe6(OH)18(H2O) 6 ring clusters. There are two possibilities for the formation of a complex between Fe6(OH)18(H2O) 6 and 2-chlorophenol, in the first possibility, 2-chlorophenol is coordinated to Fe6(OH)18(H2O) 6 through its OH group (OH pathway) and in the second one, through its Cl atom (Cl pathway). The final product of the OH pathway (product F) is more stable, but the final product of the Cl pathway (product C) is formed faster and therefore product C is a kinetic product.
ACKNOWLEDGMENT We thank the Center of Theoretical Research of Kharazmi Institute for an allocation of computer time.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
Ahmaruzzaman, M. (2008) Adv. Colloid Interface Sci., 143, 48. Chaliha, S. and Bhattacharyya, K.G. (2008) Chem. Eng. J., 139, 575. Hameed, B. (2007) Colloids Surf., A, 307, 45. Caqueret, V., Bostyn, S., Cagnon, B. and Fauduet, H. (2008) Bioresour. Technol., 99, 5814. Entezari, M.H., Heshmati, A. and Sarafraz-Yazdi, A. (2005) Ultrason. Sonochem., 12, 137. Ji, J., Jia, Y., Wu, W., Bai, L., Ge, L. and Gu, Z. (2011) Colloids Surf., A, 390, 56. Panda, N., Sahoo, H. and Mohapatra, S. (2011) J. Hazard. Mater., 185, 359. Tang, W., Li, Q., Gao, S. and Shang, J.K. (2011) J. Hazard. Mater., 192, 131. Zhu, H., Jiang, R., Xiao, L. and Zeng, G. (2010) Bioresour. Technol., 101, 5063.
www.prkm.co.uk
DFT of adsorption of 2-chlorophenol in H2O with γ-Fe2O3 NPs [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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Zhu, H., Jiang, R., Fu, Y.-Q., Jiang, J.-H., Xiao, L. and Zeng, G.-M. (2011) Appl. Surf. Sci., 258, 1337. Zhu, H.-Y., Jiang, R., Xiao, L. and Li, W. (2010) J. Hazard. Mater., 179, 251. Chen, Y., Ai, Z. and Zhang, L. (2012) J. Hazard. Mater., 235, 92. Xu, P., Zeng, G.M., Huang, D.L., Feng, C.L., Hu, S., Zhao, M.H., Lai, C., Wei, Z., Huang, C. and Xie, G.X. (2012) Sci. Total Environ., 424, 1. Xue, Y., Wang, X. and Cui, Z. (2011) Prog. React. Kinet., 36, 329. Tasviri, M., Ghourchian, H., Gholami, M.R. and Rafiee-Pour, H.-A. (2012) Prog. React. Kinet., 37, 161. Jayarathne, L., Ng, W.J., Bandara, A., Vitanage, M., Dissanayake, C. and Weerasooriya, R. (2012) Colloids Surf., A, 403, 96. Becke, A.D. (1993) J. Chem. Phys., 98, 5648. Becke, A.D. (1988) Phys. Rev. A, 38, 3098. Lee, C., Yang, W. and Parr, R.G. (1988) Phys. Rev. B, 37, 785. Frisch, M., Trucks, G., Schlegel, H., Scuseria, G., Robb, M., Cheeseman, J., et al. (2003) Gaussian 03 Gaussian Inc., Pittsburgh, PA. Hay, P.J. and Wadt, W.R. (1985) J. Chem. Phys., 82, 299. Hooman Vahidi, S., Morsali, A. and Ali Beyramabadi, S. (2012) Comput. Theor. Chem., 994, 41. Morsali, A., Hoseinzade, F., Akbari, A., Beyramabadi, S.A. and Ghiasi, R. (2013) J. Solut. Chem., 42, 1902. Mohseni, S., Bakavoli, M. and Morsali, A. (2014) Prog. React. Kinet., 39, 89. Akbari, A., Hoseinzade, F., Morsali, A. and Ali Beyramabadi, S. (2013) Inorg. Chim. Acta, 394, 423. Cammi, R. and Tomasi, J. (1995) J. Comput. Chem., 16, 1449.
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