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Synthesis of highly selective zeolite topology molecular sieve for adsorption of benzene gas
Solid State Sciences 16 (2013) 39e44
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis of highly selective zeolite topology molecular sieve for adsorption of benzene gas Lin Wei, Yunlin Chen*, Baoping Zhang, Zhinan Zu School of Science, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2012 Received in revised form 21 October 2012 Accepted 24 October 2012 Available online 6 November 2012
Shangdong fly ash (SFA), Fangshan fly ash (FFA) and Heilongjiang fly ash (HFA) were selected as the raw materials to be used for synthesis of highly selective zeolite topology molecular sieve. Twice foaming method was studied in terms of synthetic zeolite. The experimental products were characterized by means of X-ray fluorescence (XRF), scanning electron microscope (SEM), X-ray diffraction (XRD), and automated surface area & pore size analyser. The results indicated that 10 M NaOH was chosen as modification experiment condition to process SFA. Crystallization temperature and time were 140 C and 8 h, respectively. Zeolite topology molecular sieve was prepared with Si/Al molar ratio of 7.9, and its adsorption ratio of benzene gas was up to 66.51%. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Fly ash Zeolite topology molecular sieve Twice foaming method Adsorption ratio of benzene gas
1. Introduction Mesoporous molecular sieve is a kind of multi-functional adsorbent material, with large surface area and high porosity [1]. Due to their adjustable micro-structures and surface chemical properties, mesoporous molecular sieve has been widely used in purification fields. FA may contain non-degradable or conservative mobile inorganic compounds, which are previously sequestered geochemically but through combustion are mobilized. The structures of inorganic compounds compose the zeolite precursor. Therefore, it’s necessary to look for low cost and economically versatile adsorbents from cheap available raw materials [2,3], namely selecting fly ash (FA) as the raw material for preparing zeolite mesoporous molecular sieve. The preparations have some common methods, such as high temperature firing method [4], crystallization directing agent method [5], co-precipitation method [6], ionic liquid method [7], refluxing and hydrothermal methods [8]. Hydrothermal activation method can be used to synthesize zeolites from FA. The subsequent precipitation principle of the zeolitic material is the dissolution of aluminium-silicate phases from coal fly ashes in an alkaline solution (mainly NaOH and KOH solutions) [9]. Zeolites are microporous crystalline hydrated aluminosilicates that can be considered as inorganic polymers built from an infinitely extending threedimensional network (similar to a honeycomb) of tetrahedral TO4 units, where T is Si or Al, which form interconnected tunnels and * Corresponding author. Tel.: þ86 010 51688483 18. E-mail address: [email protected] (Y. Chen). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.10.022
cages [10]. Those structures adsorb some VOCs through the surface energy and the bonding action. Some previous work done with twice foaming method was used in preparation for porous resin [11] and HDPE [12], etc. This study proposed a new application of the twice foaming method, which is used to prepare highly selective zeolite topology molecular sieve for adsorbing benzene gas. Twice foaming method was achieved by changing the conditions of modification [13] and crystallization. This study made innovations in zeolite preparation experiments, which develop the traditional preparation methods through adding coupling agents and the zeolite-polymer [14e17] to get the zeolite materials. 2. Materials and methods 2.1. Materials Three FA samples, from specific locations in the Shangdong, Fangshan, and Heilongjiang were chosen for the study, which were specified as SFA, FFA, and HFA. The raw materials were sieved to obtain a particle size range between 0.8 and 0.2 mm, and then dried in an air dry oven at 150 C for 24 h. These samples were cooled down to room temperature and sealed. 2.2. Synthesis methods All chemical reagents (including HCl, NaOH and SiO2) in the experiment were purchased from Beijing Chemical Works and used without further purification. Dry samples were placed in a three-
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Table 1 Chemical composition (wt. %) of the raw materials and SFA treated by 20% HCl (SFA_S). Oxide
SFA
FFA
HFA
SFA_S
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 Fe2O3 Total
0.62 1.47 29.64 44.80 1.61 2.29 2.17 7.26 1.81 7.59 99.26
0.72 0.34 17.02 59.86 1.71 0.15 6.23 3.65 1.40 8.28 99.36
0.00 0.18 28.08 42.95 2.05 0.66 2.23 8.37 4.42 9.91 98.85
0.87 0.55 26.71 59.49 1.21 0.68 1.69 0.94 1.53 2.58 96.26
The loss on ignition (LOI, wt.%) of SFA, FFA, and HFA are 2.33, 3.05, and 3.42, respectively.
neck flask, and 20% HCl was added by solution/FA ratio (L/S) of 10 ml/g. The mixtures were placed in a shaker water bath for 2 h at 80 C, filtered through filter paper, rinsed some times with distilled water and finally air-dried to room temperature. Modification experiments preformed at 80 C for 2 h under stirring with 6, 8, 10, 12 M aqueous NaOH solution (L/S of 10 ml/g). Then the mixture was rinsed, air-dried and returned to room temperature. Chemical compositions of samples were calculated. The Si/Al ratio was changed by adding analytical grade SiO2, NaOH and distilled water in the reaction medium. High and low Si/Al ratio materials were made, and the compositions were n(SiO2)/ n(Al2O3) ¼ 7.9, n(Na2O)/n(SiO2) ¼ 1.9, n(H2O)/n(Na2O) ¼ 40 and n(SiO2)/n(Al2O3) ¼ 3.0, n(Na2O)/n(SiO2) ¼ 1.9, n(H2O)/n(Na2O) ¼ 40, respectively. With crystallization temperature of 80e140 C and time of 6e8 h, the samples were crystallized in a closed system (Teflon reactors without stirring).
Fig. 2. The XRD pattern of SFA_S.
treated FA were identified by X-ray diffraction (XRD, Bruker D8 Advance). Scanning electron microscopy (SEM, Hitachi the S-4800) was used to ascertain the surface textures, particle size, pore size, particle uniformity and degree of reunion. N2 adsorption-desorption measurements were conducted on an automated surface area & pore size analyser (Quadrasorb SI). The pore diameter was calculated with the BJH method. Total pore volume was determined at P/P0 ¼ 0.99. The surface area was determined by the BET method. 2.4. Absorption ratio Q Synthetic zeolite molecular sieve as adsorbent was experimented for benzene gas adsorption test, using a fixed fluidized bed unit, which gas velocity always was set to b (ml/min). Sampling time were successively Dt ¼ 4 min (the first six groups), Dt ¼ 10 min (the last two groups). So the benzene gas input Q1 was calculated by the equation (1):
2.3. Characterization Chemical composition of the FA samples was determined by Xray fluorescence (XRF, Bruker S4 pioneer). The major oxide content and impurities were confirmed. The crystalline phases of FA and
The benzene gas input Q1 ¼ b t C0
(1)
where C0 is initial benzene gas concentration before through the adsorbent. Through the gas chromatograph, a CeS (concentratione
Fig. 1. XRD patterns of SFA (a), FFA (b) and HFA (c).
L. Wei et al. / Solid State Sciences 16 (2013) 39e44
41
Fig. 3. SEM pictures of SFA (a), SFA_S (b).
area) standard curve was made, and then a teC (timee concentration) curve was found out with the value of the concentration on it to establish the relationship between the t and C. The teC relationship was integrated to get a value d. The benzene gas output Q2 was calculated using the following equation (2):
The benzene gas output Q2 ¼ b d
(2)
mullite. Otherwise, it is not easy to making bonds in sodium silicate. Bonding instructions will be showed in 3.2. Thus, SFA was chosen as raw material for preparation of zeolite. 3.2. Activation of FA
Chemical and mineralogical compositions of FA are showed in Table 1. The main chemical composition of the FA is silica and alumina. Table 1 shows the sums of SiO2 and Al2O3 of SFA, FFA, and HFA are 74.44%, 76.88%, and 71.03% and the Si/Al ratios are 1.51, 3.52, and 1.53, respectively. Nevertheless, it has been shown that the Si/Al ratio is an important parameter for zeolite synthesis and, as FA has a Si/Al ratio of 1.6, it is appropriate for the synthesis of low Si-zeolite with high ion exchange capacity [18]. Besides, the FA has a higher Fe content (7.5e10%) than usual (4e10%) and a low Ca content but within the usual range (5e30%). Fe content is associated to presence of magnetite which can behave as inert material for zeolite synthesis while Ca-compounds could act as a zeolite synthesis inhibitor through the formation of calcium silicate [19]. Fig. 1 shows the main mineral phases in FA. As deduced from XRD analysis, the sample (Fig. 1a) has clear quartz and mullite phase. In Fig. 1b, there are no evident mullite phase and sharp crystal faces. And HFA has peak overlap between quartz and
After the treatment by 20% HCl, the iron content was significantly reduced from 7.59% to 2.58%. The iron distributes in the vitreous body shell and can be directly dissolved [20]. The Al is present in Al-rich vitreous (Al2O3SiO2) form. Stirring and leaching at the certain temperature can increase the leaching rate of silicon and aluminium from FA. So it can not only increase the silicon and aluminium content effectively, but also generates a large number of new micro holes. The acid treatment has two advantages. On one hand, it can remove metal oxide impurities effectively. On the other hand, it can increase the ratio of silica and alumina (from 1.51 to 2.23), mainly because the leaching rate of silicon is more than that of aluminium. Fig. 2 shows that free silica was significantly reducing, but it correspondence with is quartz and mullite phase increased obviously. Besides, crystal phase was stable, and lattice was clear. So activation increases main phase but reduces impurities. Fig. 3 shows that there were many heterogeneous pores caused by some carbon combustion. But at high temperatures, unreacted FA particles embedded into the semicircular molten particles, so the pores are heterogeneous, and even some channels are blocked. Acid activation will further open the pores and generate high surface activation energy, which lead to the silicon-oxygen bond recombining. It is thus clear that SFA_S has more advantages over SFA. Table 2 shows that the pore volume and pore size were increased, and the surface area increased by 6.2 times. The data of the Table 2 are consistent with SEM analysis, which proved the benefit of activation.
Table 2 Pore structural parameters of the samples.
Table 3 Chemical composition (wt. %) of SFA_N6, SFA_N8, SFA_N10 and SFA_N12.
The adsorption ratio was determined by the equation (3):
The adsorption ratio Q ¼
Q1 Q 2 100% Q1
(3)
3. Results and discussion 3.1. Composition of FA
Samples Pore diametera (nm) Pore volumeb (cm3 g1) Surface areac (m2 g1)
Oxide
SFA_N6
SFA_N8
SFA_N10
SFA_N12
SFA SFA_S SFA_N6 SFA_N8 SFA_N10 SFA_N12
SiO2 Al2O3 Fe2O3 Na2O MgO P2O5 SO3 K2O CaO TiO2 Total
40.82 42.11 4.53 2.20 1.03 1.42 0.78 0.78 1.85 3.31 98.84
39.81 40.31 4.26 5.08 1.16 1.38 0.85 0.73 2.08 3.15 98.81
36.56 35.46 10.17 6.23 0.90 1.25 0.99 0.52 1.38 2.28 95.74
40.98 37.19 3.30 10.20 0.80 1.40 0.71 0.53 1.47 2.52 99.10
2.20 3.06 5.38 6.19 6.73 6.78
0.03 0.07 0.18 0.14 0.13 0.13
8.97 55.51 62.09 53.36 39.28 34.06
SFA_N6, SFA_N8, SFA_N10, SFA_N12: the modified FA by NaOH (6, 8, 10, 12 M respectively). a Calculated by the BJH model from the desorption isotherm. b Total pore volume at P/P0 ¼ 0.99. c Calculated by the BET model.
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Fig. 4. (a) Optimized structure of SieOeAl; (b)The results of alkali modification and the icon of benzene adsorption.
3.3. Modification experiment The effect of NaOH on the alumino-silicate materials is tremendously intense, mainly because of the presence of aluminium [21]. In fact, it causes differentiation in the distribution of the electric charge between the AleO and SieO bonds that resulted in the polarization of the chemical bonds and the enhancement of the chemically active centre (of positive and negative charge) in the lattice. Thus, terminal groups such as hSieOH, hSieOe, h(SieO) 3AleOe are developed by NaOH or KOH, leading to the formation of more complex products (i.e. zeolites) [22]. Table 3 shows that in the 10 M modified sample the percentages of Al2O3 and SiO2 were reduced and iron oxide was reversely increased. Under certain conditions, FA reacted with alkali metals and alkaline-earth metal hydroxides. At the appropriate time, temperature and concentration, Al2O3 and SiO2 release from FA. The reaction can also lead to the relaxation of SieO or AleO tetrahedral structure. Because of the disordered structure, active core was exposed, the FA surface energy was increased, and this is helpful for sodium filling into the SieO or AleO [20]. When SFA_S was modified by 12 M NaOH, the passivation reaction inhibited the overflow of Al2O3 and SiO2. Although the Si or Al positions were
influenced by Na, the SieO or AleO bond distance were adjusted by Al [23]. The chemical bond of Na and SieO or AleO was adjusted constantly so as to form the specific channel (Fig. 4). The geometry depends on the coordination of the exchangeable cations with the oxygen atoms of the zeolite molecular framework [24]. Fig. 5 shows the XRD of SFA_N6, SFA_N8, SFA_N10 and SFA_N12. From the angle of crystal transformation, modification has influence on the aperture adjustment. In low alkali concentration, glass phase is consumed in a part of alkali, and moganite was formed. With the increase of NaOH concentration, some new peaks appeared. The peak located in the positions of SPA (19.8 ) and zeolite 4A (24.2 ). As the alkali concentration increased from 10 M to 12 M, the two positions gradually processed crystal transformation. So modification completed the first foaming to form zeolite topology. It can be seen that skin hole was disappearing in SEM image (Fig. 6) of SFA_N. With alkali concentration ranging from 6 M to 12 M, the mineral particles of SFA tended to adhere and formed agglomerates. SFA_N8 consists of irregular shaped particles and the surface is hollow and broken. The shapes of SFA_N10 and SFA_N12 samples have further been regular. Meanwhile, the isotherms (Fig. 7) show that the rates of adsorption [25] increased and the
Fig. 5. XRD patterns of SFA_N6 (a), SFA_N8 (b), SFA_N10 (c) and SFA_N10 (d).
L. Wei et al. / Solid State Sciences 16 (2013) 39e44
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Fig. 6. SEM pictures of SFA_N6 (a), SFA_N8 (b), SFA_N10 (c), and SFA_N12 (d).
amount of N2 adsorption reduced. So the pore diameter, pore volume and surface areas were changed (Table 2). The average pore diameter increased from 5.38 nm to 6.78 nm, the total pore volume decreased from 0.18 cm3 g1 to 0.13 cm3 g1, and the surface area of decreased from 62.09 m2 g1 to 34.06 m2 g1. Comparatively, SFA_N12 and SFA_N10 samples were chosen for crystallizing furthermore. 3.4. Crystallization and adsorption Under different conditions (Table 4) of crystallization, SFA_N10 and SFA_N12 were chosen as raw materials, the high Si/Al ratio and the low Si/Al ratio mixture were treated as reactant, different zeolite topology samples were prepared. The different absorption ratios have been obtained by means of parallel tests. By comparing
Fig. 7. N2 adsorptionedesorption isotherms of SFA_N6, SFA_N8, SFA_N10, and SFA_N12.
the absorption ratio of these materials, zeolite material with a high ratio of selective adsorption has been selected. Firstly, the influencing factor of the synthesis of zeolite is the molar of Si/Al. Secondly, sufficient ageing time is a necessary condition of the synthesis of high purity zeolite. Finally, the crystallization process was actually a process of reintegration of AleO and SieO tetrahedron in FA, and completed twice foaming. Table 4 Different crystallization conditions and adsorption ratios for products. Number
N/M
Si/Al
tc/h
Tc/ C
ta/h
Q/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 12 12 12 12 12 12 12 12 12 12 12 12
3.0 7.9 3.0 7.9 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0
6 6 6 6 6 6 6 6 6 8 8 8 8 8 8 6 6 6 6 6 6 8 8 8 8 8 8
100 100 80 80 80 100 100 120 120 100 100 120 120 140 140 100 100 120 120 140 140 100 100 120 120 140 140
24 24 24 24 0 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24
20.10 22.21 e e e 26.14 24.21 30.82 34.17 46.20 54.12 63.32 62.61 54.67 66.51 45.45 30.20 56.08 31.82 47.21 20.31 38.54 26.09 21.03 33.20 51.82 36.21
N: NaOH concentration; tc: crystallization time; ta: aging time.
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Fig. 8. The absorption ratio figures of high Si/Al samples (a) and low Si/Al samples (b).
Zeolite topology prepared may include Na-Pl, Na-X and Na-A, etc. When 1.0 SiO2/A12O3 2.0, a single Na-A zeolite was obtained, when SiO2/A12O3 2.5, Na-X zeolite was appearing. And with Si/Al ratio rising, Na-X zeolite also increased, meanwhile, Na-A zeolite began to decrease [26]. The more Na-X zeolite was, the more absorption value of the benzene gas was. In Table 4, according to different ratios of Si/Al, the samples were divided into two groups, and each absorption ratio was shown in Fig. 8(a, b) respectively. As seen from Fig. 8, the absorption ratio of zeolite prepared by high Si/Al ratio components was higher than that of low Si/Al ratio. With the increase of Si/Al ratio, zeolite X production has increased. The absorption ratios of number 12 and 15 are 63.32% and 66.51% respectively, which is the best of all. The result shows that changes in synthesis temperatures lead to different zeolite phases [27]. When the Si/Al is lower, zeolite materials which have good adsorption properties can be synthesized with a lower crystallization temperature. Of course, with higher Si/Al, the required temperature was higher, and this zeolite material has obvious advantages in the side of adsorption ratio. 4. Conclusions The study demonstrates the application of SFA for zeolite synthesis by hydrothermal treatment. Statistical analysis shows that an increase of alkalinity, Si/Al ratio, time and temperature in zeolite synthesis tends to influence the adsorption capacity of benzene gas. Optimal experimental conditions of twice foaming were that modified experimental condition was 10 M NaOH and crystallization condition was Si/Al ratio of 7.9 for 8 h at 140 C. In addition, all the synthetic materials were tested for their ability to absorb benzene gas and presented very encouraging results concerning their future, possible large-scale utilization. Acknowledgements All authors would like to thank Prof. J. P. Wen and Dr. Ch. Sh. Qiu for assistance with the measurements and discussion. References [1] N. Tancredi, N. Medero, F. Moller, J. Piriz, C. Plada, T. Cordero, Phenol adsorption on powdered and granular activated carbon prepared from Eucalyptus wood, J. Colloid Interface Sci. 279 (2004) 357e363. [2] S.B. Mishra, S.P. Langwenya, B.B. Mamba, M. Balakrishnan, Study on surface morphology and physicochemical properties of raw and activated South African coal and coal fly ash, Phys. Chem. Earth 35 (2010) 811e814. [3] A. Marocco, Gianfranco Dell’Agli, S. Esposito, Metal-ceramic composite materials from zeolite precursor, Solid State Sci. 14 (2012) 394e400. [4] K.M. Fu, H. Zhu, M.X. Lu, Influences of different disposal fly-ash methods on hydrothermal preparing zeolites, J. Synth. Cryst. 36 (4) (2007) 943e946.
[5] C.H. Du, Y.P. Ye, M.P. Wang, Synthesis of low-silica X-zeolite (LSX) via crystallization directing agent under ageing at room temperature, J. Univ. Sci. Technol. Beijing 30 (4) (2008) 414e419. [6] F. Qiu, D.Y. Wei, Y.L. Chen, Preparation of L zeolite from water glass and its influence factors research, Chem. Ind. (Henan) 4 (27) (2010) 43e46. [7] R. Cai, Y. Liu, Sh. Gu, Ambient pressure dry-gel conversion method for zeolite MFI synthesis using ionic liquid and microwave heating, JACS 132 (2010) 12776e12777. [8] K. Pimraksa, P. Chindaprasirt, N. Setthaya, Synthesis of zeolite phases from combustion by-products, Waste Manage. Res. 28 (12) (2010) 1122e1132. [9] N. Moreno, X. Querol, C. Ayora, C. Fernández Pereira, M. Janssen-Jurkovicova, Utilization of zeolites synthesized from coal FA for the purification of acid mine waters, Environ. Sci. Technol. 35 (2001) 3526e3534. [10] M. Nascimento, Paulo Sérgio Moreira Soares, Vicente Paulo de Souza, Adsorption of heavy metal cations using coal fly ash modified by hydrothermal method, Fuel 88 (2009) 1714e1719. [11] H.L. Yang, Z.Q. Peng, Y. zhou, Z.Q. Wang, F. Zhao, X.Y. Cao, Preparation and humidity-controlling characterization of composite CMC-g-PAM/PAAS porous resin by twice foaming method, Ciesc J. 61 (12) (2010) 3302e3308. [12] J.X. Li, G. Zhang, Z.S. Li, X.L. Wang, X.Q. Liu, Resistivity-temperature behavior of CB-filled HDPE foaming composites, Chem. Res. Chin. Univ. 24 (2) (2008) 215e219. a, F. Plana, A. Alastuey, A. López-Soler, A. Medinaceli, [13] X. Querol, J.C. Uman Synthesis of zeolites from fly ash in a pilot plant scale. examples of potential environmental applications, Fuel 80 (2001) 857e865. [14] H.D. Guan, Erick Bestlanda, C.Y. Zhu, Variation in performance of surfactant loading and resulting nitrate removal among four selected natural zeolites, J. Hazard. Mater. 183 (2010) 616e621. [15] Jeremy C. White, Prabir K. Dutta, Krenar Shqau, Henk Verweij, Synthesis of ultrathin zeolite Y membranes and their application for separation of carbon dioxide and nitrogen gases, Langmuir 26 (12) (2010) 10287e10293. [16] D. Karami, S. Rohani, A novel approach for the synthesis of zeolite Y, Ind. Eng. Chem. Res. 48 (2009) 4837e4843. [17] Tarek S. Jamil, Tarek A. Gad-Allah, Hanan S. Ibrahima, Adsorption and isothermal models of atrazine by zeolite prepared from Egyptian kaolin, Solid State Sci. 13 (2011) 198e203. [18] N. Moreno, Valorisation of fly ashes for zeolite synthesis from silica extraction and direct conversion. Environmental applications. PhD thesis, Polytech. Univ. Catalunya, Spain (in Spanish with English abstract), 2002. [19] R. Juan, S. Hernández, J.M. Andrés, C. Ruiz, Synthesis of granular zeolitic materials with high cation exchange capacity from agglomerated coal fly ash, Fuel 86 (2007) 1811e1821. [20] Y.N. Cao, F.F. Nan, The situation and development prospect of FA surface modification, Chem. Eng. Equip. 4 (2009) 100e105. [21] N. Koukouzas, C. Vasilatos, G. Itskos, I. Mitsis, A. Moutsatsou, Removal of heavy metals from wastewater using CFB-coal fly ash zeolitic materials, J. Hazard. Mater. 173 (2010) 581e588. [22] D. Wu, B. Zhang, L. Yan, H. Kong, X. Wang, Effect of some additives on synthesis of zeolite from coal fly ash, Int. J. Miner. Process. 80 (2006) 266e272. [23] L.B. McCusker, C. Baerlocher, S.T. Wilson, R.W. Broach, Synthesis and structural characterization of the aluminosilicate LZ-135, a zeolite related to ZSM10, J. Phys. Chem. C 113 (2009) 9838e9844. [24] M. Danilczuk, A. Lund, Adsorption of NO in Li-exchanged zeolite A. A density functional theory study, Chem. Phys. L 490 (2010) 205e209. [25] C. Cai, H. Wang, J.Y. Han, Synthesis and characterization of ionic liquidfunctionalized alumino-silicate MCM-41 hybrid mesoporous materials, Appl. Surf. Sci. 257 (2011) 9802e9808. [26] J.J. Tong, J.H. Fang, Y.M. Liu, Recent progrese on the synthesis of zeolite molecular sieve with fly ash, FA Compr. Util 5 (2003) 52e54. [27] X. Querol, A. Alastuey, N. Moreno, E.A. Ayuso, A.G. Sáchez, J. Cama, Immobilization of heavy metals in polluted soils by the addition of zeolitic material synthesized from coal fly ash, Chemosphere 62 (2006) 171e180.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis of highly selective zeolite topology molecular sieve for adsorption of benzene gas Lin Wei, Yunlin Chen*, Baoping Zhang, Zhinan Zu School of Science, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2012 Received in revised form 21 October 2012 Accepted 24 October 2012 Available online 6 November 2012
Shangdong fly ash (SFA), Fangshan fly ash (FFA) and Heilongjiang fly ash (HFA) were selected as the raw materials to be used for synthesis of highly selective zeolite topology molecular sieve. Twice foaming method was studied in terms of synthetic zeolite. The experimental products were characterized by means of X-ray fluorescence (XRF), scanning electron microscope (SEM), X-ray diffraction (XRD), and automated surface area & pore size analyser. The results indicated that 10 M NaOH was chosen as modification experiment condition to process SFA. Crystallization temperature and time were 140 C and 8 h, respectively. Zeolite topology molecular sieve was prepared with Si/Al molar ratio of 7.9, and its adsorption ratio of benzene gas was up to 66.51%. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Fly ash Zeolite topology molecular sieve Twice foaming method Adsorption ratio of benzene gas
1. Introduction Mesoporous molecular sieve is a kind of multi-functional adsorbent material, with large surface area and high porosity [1]. Due to their adjustable micro-structures and surface chemical properties, mesoporous molecular sieve has been widely used in purification fields. FA may contain non-degradable or conservative mobile inorganic compounds, which are previously sequestered geochemically but through combustion are mobilized. The structures of inorganic compounds compose the zeolite precursor. Therefore, it’s necessary to look for low cost and economically versatile adsorbents from cheap available raw materials [2,3], namely selecting fly ash (FA) as the raw material for preparing zeolite mesoporous molecular sieve. The preparations have some common methods, such as high temperature firing method [4], crystallization directing agent method [5], co-precipitation method [6], ionic liquid method [7], refluxing and hydrothermal methods [8]. Hydrothermal activation method can be used to synthesize zeolites from FA. The subsequent precipitation principle of the zeolitic material is the dissolution of aluminium-silicate phases from coal fly ashes in an alkaline solution (mainly NaOH and KOH solutions) [9]. Zeolites are microporous crystalline hydrated aluminosilicates that can be considered as inorganic polymers built from an infinitely extending threedimensional network (similar to a honeycomb) of tetrahedral TO4 units, where T is Si or Al, which form interconnected tunnels and * Corresponding author. Tel.: þ86 010 51688483 18. E-mail address: [email protected] (Y. Chen). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.10.022
cages [10]. Those structures adsorb some VOCs through the surface energy and the bonding action. Some previous work done with twice foaming method was used in preparation for porous resin [11] and HDPE [12], etc. This study proposed a new application of the twice foaming method, which is used to prepare highly selective zeolite topology molecular sieve for adsorbing benzene gas. Twice foaming method was achieved by changing the conditions of modification [13] and crystallization. This study made innovations in zeolite preparation experiments, which develop the traditional preparation methods through adding coupling agents and the zeolite-polymer [14e17] to get the zeolite materials. 2. Materials and methods 2.1. Materials Three FA samples, from specific locations in the Shangdong, Fangshan, and Heilongjiang were chosen for the study, which were specified as SFA, FFA, and HFA. The raw materials were sieved to obtain a particle size range between 0.8 and 0.2 mm, and then dried in an air dry oven at 150 C for 24 h. These samples were cooled down to room temperature and sealed. 2.2. Synthesis methods All chemical reagents (including HCl, NaOH and SiO2) in the experiment were purchased from Beijing Chemical Works and used without further purification. Dry samples were placed in a three-
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Table 1 Chemical composition (wt. %) of the raw materials and SFA treated by 20% HCl (SFA_S). Oxide
SFA
FFA
HFA
SFA_S
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 Fe2O3 Total
0.62 1.47 29.64 44.80 1.61 2.29 2.17 7.26 1.81 7.59 99.26
0.72 0.34 17.02 59.86 1.71 0.15 6.23 3.65 1.40 8.28 99.36
0.00 0.18 28.08 42.95 2.05 0.66 2.23 8.37 4.42 9.91 98.85
0.87 0.55 26.71 59.49 1.21 0.68 1.69 0.94 1.53 2.58 96.26
The loss on ignition (LOI, wt.%) of SFA, FFA, and HFA are 2.33, 3.05, and 3.42, respectively.
neck flask, and 20% HCl was added by solution/FA ratio (L/S) of 10 ml/g. The mixtures were placed in a shaker water bath for 2 h at 80 C, filtered through filter paper, rinsed some times with distilled water and finally air-dried to room temperature. Modification experiments preformed at 80 C for 2 h under stirring with 6, 8, 10, 12 M aqueous NaOH solution (L/S of 10 ml/g). Then the mixture was rinsed, air-dried and returned to room temperature. Chemical compositions of samples were calculated. The Si/Al ratio was changed by adding analytical grade SiO2, NaOH and distilled water in the reaction medium. High and low Si/Al ratio materials were made, and the compositions were n(SiO2)/ n(Al2O3) ¼ 7.9, n(Na2O)/n(SiO2) ¼ 1.9, n(H2O)/n(Na2O) ¼ 40 and n(SiO2)/n(Al2O3) ¼ 3.0, n(Na2O)/n(SiO2) ¼ 1.9, n(H2O)/n(Na2O) ¼ 40, respectively. With crystallization temperature of 80e140 C and time of 6e8 h, the samples were crystallized in a closed system (Teflon reactors without stirring).
Fig. 2. The XRD pattern of SFA_S.
treated FA were identified by X-ray diffraction (XRD, Bruker D8 Advance). Scanning electron microscopy (SEM, Hitachi the S-4800) was used to ascertain the surface textures, particle size, pore size, particle uniformity and degree of reunion. N2 adsorption-desorption measurements were conducted on an automated surface area & pore size analyser (Quadrasorb SI). The pore diameter was calculated with the BJH method. Total pore volume was determined at P/P0 ¼ 0.99. The surface area was determined by the BET method. 2.4. Absorption ratio Q Synthetic zeolite molecular sieve as adsorbent was experimented for benzene gas adsorption test, using a fixed fluidized bed unit, which gas velocity always was set to b (ml/min). Sampling time were successively Dt ¼ 4 min (the first six groups), Dt ¼ 10 min (the last two groups). So the benzene gas input Q1 was calculated by the equation (1):
2.3. Characterization Chemical composition of the FA samples was determined by Xray fluorescence (XRF, Bruker S4 pioneer). The major oxide content and impurities were confirmed. The crystalline phases of FA and
The benzene gas input Q1 ¼ b t C0
(1)
where C0 is initial benzene gas concentration before through the adsorbent. Through the gas chromatograph, a CeS (concentratione
Fig. 1. XRD patterns of SFA (a), FFA (b) and HFA (c).
L. Wei et al. / Solid State Sciences 16 (2013) 39e44
41
Fig. 3. SEM pictures of SFA (a), SFA_S (b).
area) standard curve was made, and then a teC (timee concentration) curve was found out with the value of the concentration on it to establish the relationship between the t and C. The teC relationship was integrated to get a value d. The benzene gas output Q2 was calculated using the following equation (2):
The benzene gas output Q2 ¼ b d
(2)
mullite. Otherwise, it is not easy to making bonds in sodium silicate. Bonding instructions will be showed in 3.2. Thus, SFA was chosen as raw material for preparation of zeolite. 3.2. Activation of FA
Chemical and mineralogical compositions of FA are showed in Table 1. The main chemical composition of the FA is silica and alumina. Table 1 shows the sums of SiO2 and Al2O3 of SFA, FFA, and HFA are 74.44%, 76.88%, and 71.03% and the Si/Al ratios are 1.51, 3.52, and 1.53, respectively. Nevertheless, it has been shown that the Si/Al ratio is an important parameter for zeolite synthesis and, as FA has a Si/Al ratio of 1.6, it is appropriate for the synthesis of low Si-zeolite with high ion exchange capacity [18]. Besides, the FA has a higher Fe content (7.5e10%) than usual (4e10%) and a low Ca content but within the usual range (5e30%). Fe content is associated to presence of magnetite which can behave as inert material for zeolite synthesis while Ca-compounds could act as a zeolite synthesis inhibitor through the formation of calcium silicate [19]. Fig. 1 shows the main mineral phases in FA. As deduced from XRD analysis, the sample (Fig. 1a) has clear quartz and mullite phase. In Fig. 1b, there are no evident mullite phase and sharp crystal faces. And HFA has peak overlap between quartz and
After the treatment by 20% HCl, the iron content was significantly reduced from 7.59% to 2.58%. The iron distributes in the vitreous body shell and can be directly dissolved [20]. The Al is present in Al-rich vitreous (Al2O3SiO2) form. Stirring and leaching at the certain temperature can increase the leaching rate of silicon and aluminium from FA. So it can not only increase the silicon and aluminium content effectively, but also generates a large number of new micro holes. The acid treatment has two advantages. On one hand, it can remove metal oxide impurities effectively. On the other hand, it can increase the ratio of silica and alumina (from 1.51 to 2.23), mainly because the leaching rate of silicon is more than that of aluminium. Fig. 2 shows that free silica was significantly reducing, but it correspondence with is quartz and mullite phase increased obviously. Besides, crystal phase was stable, and lattice was clear. So activation increases main phase but reduces impurities. Fig. 3 shows that there were many heterogeneous pores caused by some carbon combustion. But at high temperatures, unreacted FA particles embedded into the semicircular molten particles, so the pores are heterogeneous, and even some channels are blocked. Acid activation will further open the pores and generate high surface activation energy, which lead to the silicon-oxygen bond recombining. It is thus clear that SFA_S has more advantages over SFA. Table 2 shows that the pore volume and pore size were increased, and the surface area increased by 6.2 times. The data of the Table 2 are consistent with SEM analysis, which proved the benefit of activation.
Table 2 Pore structural parameters of the samples.
Table 3 Chemical composition (wt. %) of SFA_N6, SFA_N8, SFA_N10 and SFA_N12.
The adsorption ratio was determined by the equation (3):
The adsorption ratio Q ¼
Q1 Q 2 100% Q1
(3)
3. Results and discussion 3.1. Composition of FA
Samples Pore diametera (nm) Pore volumeb (cm3 g1) Surface areac (m2 g1)
Oxide
SFA_N6
SFA_N8
SFA_N10
SFA_N12
SFA SFA_S SFA_N6 SFA_N8 SFA_N10 SFA_N12
SiO2 Al2O3 Fe2O3 Na2O MgO P2O5 SO3 K2O CaO TiO2 Total
40.82 42.11 4.53 2.20 1.03 1.42 0.78 0.78 1.85 3.31 98.84
39.81 40.31 4.26 5.08 1.16 1.38 0.85 0.73 2.08 3.15 98.81
36.56 35.46 10.17 6.23 0.90 1.25 0.99 0.52 1.38 2.28 95.74
40.98 37.19 3.30 10.20 0.80 1.40 0.71 0.53 1.47 2.52 99.10
2.20 3.06 5.38 6.19 6.73 6.78
0.03 0.07 0.18 0.14 0.13 0.13
8.97 55.51 62.09 53.36 39.28 34.06
SFA_N6, SFA_N8, SFA_N10, SFA_N12: the modified FA by NaOH (6, 8, 10, 12 M respectively). a Calculated by the BJH model from the desorption isotherm. b Total pore volume at P/P0 ¼ 0.99. c Calculated by the BET model.
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Fig. 4. (a) Optimized structure of SieOeAl; (b)The results of alkali modification and the icon of benzene adsorption.
3.3. Modification experiment The effect of NaOH on the alumino-silicate materials is tremendously intense, mainly because of the presence of aluminium [21]. In fact, it causes differentiation in the distribution of the electric charge between the AleO and SieO bonds that resulted in the polarization of the chemical bonds and the enhancement of the chemically active centre (of positive and negative charge) in the lattice. Thus, terminal groups such as hSieOH, hSieOe, h(SieO) 3AleOe are developed by NaOH or KOH, leading to the formation of more complex products (i.e. zeolites) [22]. Table 3 shows that in the 10 M modified sample the percentages of Al2O3 and SiO2 were reduced and iron oxide was reversely increased. Under certain conditions, FA reacted with alkali metals and alkaline-earth metal hydroxides. At the appropriate time, temperature and concentration, Al2O3 and SiO2 release from FA. The reaction can also lead to the relaxation of SieO or AleO tetrahedral structure. Because of the disordered structure, active core was exposed, the FA surface energy was increased, and this is helpful for sodium filling into the SieO or AleO [20]. When SFA_S was modified by 12 M NaOH, the passivation reaction inhibited the overflow of Al2O3 and SiO2. Although the Si or Al positions were
influenced by Na, the SieO or AleO bond distance were adjusted by Al [23]. The chemical bond of Na and SieO or AleO was adjusted constantly so as to form the specific channel (Fig. 4). The geometry depends on the coordination of the exchangeable cations with the oxygen atoms of the zeolite molecular framework [24]. Fig. 5 shows the XRD of SFA_N6, SFA_N8, SFA_N10 and SFA_N12. From the angle of crystal transformation, modification has influence on the aperture adjustment. In low alkali concentration, glass phase is consumed in a part of alkali, and moganite was formed. With the increase of NaOH concentration, some new peaks appeared. The peak located in the positions of SPA (19.8 ) and zeolite 4A (24.2 ). As the alkali concentration increased from 10 M to 12 M, the two positions gradually processed crystal transformation. So modification completed the first foaming to form zeolite topology. It can be seen that skin hole was disappearing in SEM image (Fig. 6) of SFA_N. With alkali concentration ranging from 6 M to 12 M, the mineral particles of SFA tended to adhere and formed agglomerates. SFA_N8 consists of irregular shaped particles and the surface is hollow and broken. The shapes of SFA_N10 and SFA_N12 samples have further been regular. Meanwhile, the isotherms (Fig. 7) show that the rates of adsorption [25] increased and the
Fig. 5. XRD patterns of SFA_N6 (a), SFA_N8 (b), SFA_N10 (c) and SFA_N10 (d).
L. Wei et al. / Solid State Sciences 16 (2013) 39e44
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Fig. 6. SEM pictures of SFA_N6 (a), SFA_N8 (b), SFA_N10 (c), and SFA_N12 (d).
amount of N2 adsorption reduced. So the pore diameter, pore volume and surface areas were changed (Table 2). The average pore diameter increased from 5.38 nm to 6.78 nm, the total pore volume decreased from 0.18 cm3 g1 to 0.13 cm3 g1, and the surface area of decreased from 62.09 m2 g1 to 34.06 m2 g1. Comparatively, SFA_N12 and SFA_N10 samples were chosen for crystallizing furthermore. 3.4. Crystallization and adsorption Under different conditions (Table 4) of crystallization, SFA_N10 and SFA_N12 were chosen as raw materials, the high Si/Al ratio and the low Si/Al ratio mixture were treated as reactant, different zeolite topology samples were prepared. The different absorption ratios have been obtained by means of parallel tests. By comparing
Fig. 7. N2 adsorptionedesorption isotherms of SFA_N6, SFA_N8, SFA_N10, and SFA_N12.
the absorption ratio of these materials, zeolite material with a high ratio of selective adsorption has been selected. Firstly, the influencing factor of the synthesis of zeolite is the molar of Si/Al. Secondly, sufficient ageing time is a necessary condition of the synthesis of high purity zeolite. Finally, the crystallization process was actually a process of reintegration of AleO and SieO tetrahedron in FA, and completed twice foaming. Table 4 Different crystallization conditions and adsorption ratios for products. Number
N/M
Si/Al
tc/h
Tc/ C
ta/h
Q/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 12 12 12 12 12 12 12 12 12 12 12 12
3.0 7.9 3.0 7.9 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0 7.9 3.0
6 6 6 6 6 6 6 6 6 8 8 8 8 8 8 6 6 6 6 6 6 8 8 8 8 8 8
100 100 80 80 80 100 100 120 120 100 100 120 120 140 140 100 100 120 120 140 140 100 100 120 120 140 140
24 24 24 24 0 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24
20.10 22.21 e e e 26.14 24.21 30.82 34.17 46.20 54.12 63.32 62.61 54.67 66.51 45.45 30.20 56.08 31.82 47.21 20.31 38.54 26.09 21.03 33.20 51.82 36.21
N: NaOH concentration; tc: crystallization time; ta: aging time.
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L. Wei et al. / Solid State Sciences 16 (2013) 39e44
Fig. 8. The absorption ratio figures of high Si/Al samples (a) and low Si/Al samples (b).
Zeolite topology prepared may include Na-Pl, Na-X and Na-A, etc. When 1.0 SiO2/A12O3 2.0, a single Na-A zeolite was obtained, when SiO2/A12O3 2.5, Na-X zeolite was appearing. And with Si/Al ratio rising, Na-X zeolite also increased, meanwhile, Na-A zeolite began to decrease [26]. The more Na-X zeolite was, the more absorption value of the benzene gas was. In Table 4, according to different ratios of Si/Al, the samples were divided into two groups, and each absorption ratio was shown in Fig. 8(a, b) respectively. As seen from Fig. 8, the absorption ratio of zeolite prepared by high Si/Al ratio components was higher than that of low Si/Al ratio. With the increase of Si/Al ratio, zeolite X production has increased. The absorption ratios of number 12 and 15 are 63.32% and 66.51% respectively, which is the best of all. The result shows that changes in synthesis temperatures lead to different zeolite phases [27]. When the Si/Al is lower, zeolite materials which have good adsorption properties can be synthesized with a lower crystallization temperature. Of course, with higher Si/Al, the required temperature was higher, and this zeolite material has obvious advantages in the side of adsorption ratio. 4. Conclusions The study demonstrates the application of SFA for zeolite synthesis by hydrothermal treatment. Statistical analysis shows that an increase of alkalinity, Si/Al ratio, time and temperature in zeolite synthesis tends to influence the adsorption capacity of benzene gas. Optimal experimental conditions of twice foaming were that modified experimental condition was 10 M NaOH and crystallization condition was Si/Al ratio of 7.9 for 8 h at 140 C. In addition, all the synthetic materials were tested for their ability to absorb benzene gas and presented very encouraging results concerning their future, possible large-scale utilization. Acknowledgements All authors would like to thank Prof. J. P. Wen and Dr. Ch. Sh. Qiu for assistance with the measurements and discussion. References [1] N. Tancredi, N. Medero, F. Moller, J. Piriz, C. Plada, T. Cordero, Phenol adsorption on powdered and granular activated carbon prepared from Eucalyptus wood, J. Colloid Interface Sci. 279 (2004) 357e363. [2] S.B. Mishra, S.P. Langwenya, B.B. Mamba, M. Balakrishnan, Study on surface morphology and physicochemical properties of raw and activated South African coal and coal fly ash, Phys. Chem. Earth 35 (2010) 811e814. [3] A. Marocco, Gianfranco Dell’Agli, S. Esposito, Metal-ceramic composite materials from zeolite precursor, Solid State Sci. 14 (2012) 394e400. [4] K.M. Fu, H. Zhu, M.X. Lu, Influences of different disposal fly-ash methods on hydrothermal preparing zeolites, J. Synth. Cryst. 36 (4) (2007) 943e946.
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