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CTAB ratio on crystal morphology, pore structure and adsorption performance of hierarchical (H) ZSM-11 zeolite
Microporous and Mesoporous Materials 271 (2018) 146–155
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Effect of Si-ATP/CTAB ratio on crystal morphology, pore structure and adsorption performance of hierarchical (H) ZSM-11 zeolite
T
Hong-Ji Lia,b,∗, Xiao-De Zhoua, Yu-Hui Dib, Jian-Min Zhangb, Yu Zhangb a b
School of Water Resources & Hydraulic Power, Xi'an University of Technology, Xi'an, 710048, China College of Town Planning & Municipal Engineering, Xi'an Polytechnic University, Xi'an, 710048, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Si-ATP/CTAB ratio Hierarchical (H)ZSM-11 Hierarchy factor Adsorption isotherm
In this manuscript, the micro-mesoporous crystals with the properties of (H) ZSM-11 zeolites were successfully obtained via one-pot synthesis in the hydrothermal system by using acidified attapulgite (Si-ATP) and cetyltrimethylammonium bromide (CTAB) as a precursor source (PS) and structure-directing agent (SDA), respectively. The effect of Si-ATP/CTAB ratio on controlling the crystal morphology and hierarchical pore structure of products was systematically investigated. The synthesized materials were characterized with XPS, XRD, FTIR, NMR, SEM, TEM and N2 adsorption-desorption analysis. Results demonstrate that the products possess not only a classical zeolite framework but also an adjustable mesoporous structure accompanied with a variety of Si-ATP/ CTAB ratios. Meanwhile Si-ATP/CTAB ratio has a well linear dependence with hierarchy factor (HF) in lower or higher ranges. The MFI crystal transforms gradually to the MEL-structure under the action of CTAB when SiATP/CTAB ratio increases to 80: 5. In addition, (H) ZSM-11 zeolite, this ratio of 80: 5, mesoporosity and adsorption capacity of MB molecules was significantly enhanced. Advanced adsorption performance is attributed to the abundance hydroxyl group and the generated mesoporosity of the zeolite surface under the participation of CTAB. The experimental data are well fitted to the Langmuir isotherm model, with a correlation coefficient of 0.9962. The excellent adsorption performance makes such hierarchically porous (H) ZSM-11 zeolites attractive for applications in the field of energy and environment.
1. Introduction
intersecting direct and sinusoidal channels, compared with that, ZSM11 zeolite with single straight channels (5.3 Å × 5.4 Å), possess the important values in MB wastewater treatment [9]. Three-dimensional topology structure of ZSM-11 zeolite consists of straight channels along a- and b-axis with lone pore openings along them [10]. The synthesis of hierarchical ZSM-11 zeolite is viewed as a convenient strategy to make full use of less diffusion resistance of consecutive channels, which overcome the drawback of blocking the molecules from entering the channel along the c-axis. Moreover, hierarchical porous materials may be favorable to keep the balance between mesoscopic and MEL-type zeolite ordering. Much effort has been devoted to developing a typical method that obtains hierarchical micro-mesoporous ZSM-11 zeolites [11,12], such as the hard-casting template techniques [13,14], chemical leaching approaches [15] [16], and the assembly of zeolite nanoparticles [17,18]. The acidity and crystalline framework of zeolites are well preserved in the template synthesis processes via the introduction of mesopores into microporous zeolite [19–21]. There are obvious disadvantages for the hard-casting template synthesis route, such as
The pollutions of dyeing wastewater from textile, tannery, paper and other industry are given more and more attention due to their high chemical oxygen demand (COD), low bioavailability, high perniciousness and strong persistent color [1–3]. Among of them, especially methylene blue (MB) wastewater has been a potential threaten to human and environment [4,5]. Significant characteristics of zeolites include high efficiency, rapid reaction rate and regeneration, capability to be used as a support for MB wastewater treatment as sorbent materials in the recent years. The hierarchic pore structure of zeolites with lots of channels and cavities increase the accessibility of zeolitic internal surface area and shorten their diffusion resistance [6]. However, there are few studies on the adsorption mechanism of hierarchical crystals. It is significant essential to reveal the influence of morphology and porosity of zeolites on the adsorption capability of MB. ZSM-11 zeolite is a typical member of the pentasil family with 10membered ring openings [7], which shows similar framework densities and pore sizes with ZSM-5 zeolite [8]. ZSM-5 zeolite has been
∗
Corresponding author. School of Water Resources & Hydraulic Power, Xi'an University of Technology, Xi'an, 710048, China. E-mail address: [email protected] (H.-J. Li).
https://doi.org/10.1016/j.micromeso.2018.05.039 Received 7 September 2017; Received in revised form 14 January 2018; Accepted 26 May 2018 Available online 01 June 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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2.2. Characterizations
intricate kinds of templates [22], relatively high-cost [23], and environmental pollution due to removing the template [24]. Moreover, ZSM-11 zeolite is synthesized with the traditional tetrabutyl-ammonium templated, to greater or lesser degrees, intergrowths containing ZSM-5 zeolite [25]. Selectively extraction of aluminum and silicon atoms from the framework by chemical leaching treatment through alkali or acid [26,27] leads to the formation of porous ZSM-11 zeolite [28–30]. However, porous zeolites are acquired via the chemical leaching treatment, which is usually at the expense of destroying microporosity in the traditional zeolite crystals [30–32]. The assemblage of zeolite nanocrystals may possess weak ordering from spontaneous and random assembling. It is difficult that keeping the balance of mesoscopic and the order in MEL-type zeolite in this situation. The similar frameworks, abundant pores and structural properties make phyllosilicate mine resources, including kaolin, diatomite and attapulgite, successful attempts taken to synthesize a hierarchical zeolite [33–35]. Compared with the above-mentioned methods, this method possesses the prominent characteristics of low-cost, environmentally friendly and high productivity. In the synthesis of ATPbased zeolite, the effects of crystal morphology and pore structure, such as Si/Al ratio, alkalinity and template have been researched by traditional hydrothermal synthesis. However, adding Si-ATP with abundant porosity as a precursor source, hierarchal zeolites are prepared by onepot synthesis, and less studied is the influence of Si-ATP and the structure-directing agent on zeolite morphology and hierarchic pore structure. To study the action of hierarchical zeolites on the adsorption capability, a highly ordered hierarchical (H) ZSM-11 zeolite with wellcrystallized was prepared under interaction of Si-ATP and CTAB. Crystalline type, surface morphology and pore characteristic were analyzed by XPS, FTIR, XRD, NMR, SEM, TEM and N2 adsorption-desorption. Further investigate the effect of Si-ATP/CTAB ratio on the morphology and pore structure, and reveal the relationship of this ratio and the hierarchy factor (HF) to show the mesoporous structure. It is worth noting that hierarchically porous (H) ZSM-11 zeolites are a relatively high external surface area and larger pore volume. In addition, we demonstrate that such a (H) ZSM-11 zeolite can be effectively applied during the MB adsorption process. This will be a qualitative discussion of details.
The study of the composition and properties was performed using Xray photoelectron spectroscopy (XPS). The XPS spectra were obtained by Axis Ultra (UK) using Monochromatic Al Kα (150 W, 15 kv, 1486.71 ev). Fourier-transform infrared (FT-IR) spectrum of samples was gathered on a Nicolet 5700 FT-IR spectrometer to characterize the surface functional groups, using KBr wafer technique. The infrared absorbance spectra were registered from 4000 to 400 cm−1. X-ray diffraction (XRD) experiments were obtained at room temperature on a Cu Κα source operated at 40 kV and 40 mA. A scan rate is 8°min−1 with the scan step of 0.02°, and the scan range is 10°–60°. A Bruker AV-400 nuclear magnetic resonance (NMR) spectrometer was used to collect 27Al and 29 Si MAS NMR spectra at 79.495 MHz at a spinning frequency of 27.7 kHz and 10 s intervals between successive accumulations. Scanning electron microscopy (SEM) images were collected on a Quanta-450- FEG electron microscope, to investigate the surface morphology with fabricated by FEI of the UK. Transmission electron microscopy (TEM) images were performed on a Tecnai G2 F20 transmission electron microscope operating at an accelerating voltage of 200 k V. Nitrogen adsorption-desorption isotherms were consulted on an ASAP-2020 HD88 Accelerated Surface Area and Porosimetry System. Surface area of the sample powders was analyzed from Brunauer, Emmett and Teller (BET) method at 77 K and the nitrogen adsorptiondesorption isotherm curves utilizing Autosorb-1 Quantachrome Instrument. Total pore volume of the sample powders was analyzed from single point adsorption total pore volume of pores. Both the micropore volume (Vmicr) and micropore surface area (Smicr) were calculated by application of the t-plot method. The mesopore volume (Vmeso) and external surface area (Smeso) were calculated by the aggregate data minus the corresponding micropore data. The hierarchy factor (HF) [36] was cited here to present the effects of textural properties, plotted as a function of the relative surface area of mesopores (Smeso/STOT) and the relative micropore volume (Vmicr/VTOT), which was described as Eq. (1): HF = (Vmicr / VTOT) × (Smeso / SBET)
(1)
2.3. MB adsorption
2. Experimental and instrumentation
Adsorption experiments were conducted by adding a fixed mass of zeolites into 100.0 ml MB (concentration of 50 mg L−1) solution in 250 ml conical flasks. This flasks were placed on a water bath shaker at 293 K with 100 rpm. Sub-samples were collected at gave time intervals via centrifuged to separate from the liquid phases. The supernatant solution was subsequently determined after diluted. The absorbance of the filtrate was measured by a 720S UV-vis spectrophotometer with a maximum absorbance wavelength for MB at 665 nm. The amount of MB adsorbed on zeolites for some time was calculated using Eq. (2):
2.1. Synthesis of hierarchical zeolite The precursor of hierarchical zeolites was prepared as following: 2.00 g natural attapulgite powders (ATP, Gansu Hao Di Co., Ltd., China) were dispersed in 1.00 mol/L hydrochloric acid (HCl) to a beaker contained and stirring for 30 min at room temperature. Hydrothermal acidified reaction was carried in a sealed reactor at 453 K for 12 h. Products were filtered by centrifuging repeatedly with 200 mL deionized water, and subsequently dried at 353 K under ambient condition. The acquired product was characterized as Si-ATP. Hierarchical zeolites were disposed according to the literature [27] with some modifications. Briefly, a mixture of TPAOH (25%, 7.7 mL) and NaAlO2 (AR, 0.011 g) was added to 5.20 mL deionized water and stirred perfectly. Then adding the different amount Si-ATP and CTAB and stirring the mixture at 60 °C for 4 h. The initial gel was transferred to a 25 mL Teflon-lined autoclave and hydrothermal acted at 453 K for 12 h. After cooled down, the template was removed by calcination at 823 K for 5 h. The effect of CTAB was better investigated on the properties of the final products. Seven samples with different Si-ATP/CTAB ratios (80: 1, 80: 2, 80: 3, 80: 4, 80: 5, 80: 6 and 80: 7, respectively) were added in the precursor solution, and synthesized depending on the aforementioned synthesis method. They were denoted as S n (n = 80: 1, 80: 2, 80: 3, …represents Si-ATP/ CTAB mass ratio).
qe (mg g−1) = (C0eCe) × (V/m)
(2)
Here, C0 and Ce are the initial and equilibrium MB concentration (mg L−1), respectively. V is the volume of MB solution (L), and m is the mass of zeolites (g). 3. Results and discussion The chemical states of different elements of ATP and Si-ATP were investigated by XPS characterization. The wide survey spectrum of ATP, which reveals the presence of Na, Fe, O, Ca, Si and Al elements, is displayed in Fig. 1 and Table 1. The full-range XPS spectra results seen in Fig. 1 qualitatively revealed the existence of Na 1s, Fe 2p, O Auger, O 1s, Ca 2p, C 1s, Si 2s, Si 2p and Al 2p with atomic percent accounts for 1.37%, 3.89%, 18.26%, 34.50%, 1.89%, 11.91%, 10.25%, 9.55% and 8.28%, respectively. High-resolution XPS spectra of acidified 147
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Fig. 1. XPS survey spectra of ATP and Si-ATP (a) high-resolution Si 2p and (b) Al 2p before and after acidified treatment.
two bonding environments, which are Si-O (101.85 e V) and Si O2 (104.3 e V), respectively [37]. Fig. 1b shows that the peak of Al 2p XPS spectra in Si-ATP appears only one bonding environments, which are Al-O (72.8 e V) with the disappearance of Al-OH (77.4 e V) from ATP in Fig. 1B [38]. Si-ATP with a well-preserved Si, Al and O element and pristine crystal structure can be used as the Si/Al source, which is the similar to those in zeolites. Therefore, the zeolite from Si-ATP is obviously synthesized. FT-IR spectra of ATP, Si-ATP and S80:5 (Si-ATP/CTAB ratio of 80: 5, (H) ZSM-11 zeolite) are illustrated in Fig. 2. The structural OH stretching vibrations appear in the range of 3420 cm−1 and 1632 cm−1, which confirms a great number of active hydroxyl groups exist on the surface of ATP, Si-ATP and S80:5 [33,39,40]). For Si-ATP, the characteristic band of carbonates, at 1464 cm−1, disappears from the
Table 1 Atomic percent accounts of ATP and Si-ATP from XPS survey spectra (Unit: %). Sample
Na1s
Fe2p
OAuger
O1s
Ca2p
C1s
Si2s
Si2p
Al2p
ATP Si-ATP
1.37 0.00
3.89 0.00
18.26 11.35
34.50 37.78
1.89 0.00
11.91 27.23
10.25 9.14
9.55 10.79
8.28 3.70
attapulgite (Si-ATP), which reveals only the existence of O Auger, O 1s, C 1s, Si 2s, Si 2p and Al 2p with atomic percent accounts for 11.35%, 37.78%, 27.23%, 9.14%, 10.79% and 3.70%, respectively, on the surface of the modified samples. Thus, it can be concluded that the partial metal cations, such as Na, Fe and Ca, of the attapulgite component are removed by acidification. Fig. 1A and (a)shows that the peak of Si 2p XPS spectra in ATP and Si-ATP is quite broad and appears to consist of 148
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Fig. 2. FTIR spectra of ATP, Si-ATP and hierarchical zeolites at the Si-ATP/ CTAB ratio of 80:5.
Fig. 4. Multinuclear solid state NMR spectra of zeolites obtained at Si-ATP/ CTAB ratio of 80:3, 80:4, 80:5 and 80:6 (a) 29Si SP MAS NMR and (b) 27Al MAS NMR.
amorphous silica (bending vibration of Si-OH), which appears after the dissolution of the octahedral sheet [41]. The adsorption bands at 800 cm−1 appears in the structure of Si-ATP under HCl treatment, which is attributed to the vibration of Si-O-Si in the formed free silica (SiO2) [42]. At 550 cm−1, 800 cm−1 and 3661 cm−1, the typical peak reveals the trend of transitionally Si-ATP to hierarchical zeolite. In particular, the vibration band at around 550 cm−1 indicates the presence of the double five rings of the characteristic structure of pentasil family zeolite (MFI and MEL) [43,44], rather than that of ATP or conventional chemical Si/Al sources (e.g., silicon dioxide (SiO2), tetraethoxysilane (TEOS), etc.). The Si-O tetrahedra bonds in Si-ATP are similar to those in zeolites, which facilitate the formation of zeolite frameworks but is absent in conventional chemical Si/Al sources [33]. The band at 3661 cm−1 is attributed to the hydroxyl stretching vibrations of the Al3+ cations in dioctahedral coordination (Al2OH). All the samples reveal characteristic adsorption bands at around 450, 550, 800, 1100 and 1225 cm−1. The adsorption bands at 1225, 1100, 800, 550 and 450 cm−1 ascribe to the external asymmetric stretching of Si-O-T linkage, the asymmetric stretching vibration of Si-O tetrahedra, Si-O-Si symmetric stretching, Si-O bending of the double fiv-rings and T-O bending of Si tetrahedral, respectively [45,46]. A wide absorption band of Si-O-Si and Si-O-Al for ATP near 1030 cm−1 is transferred to 1100 cm−1 in Si-ATP and hierarchical zeolite, revealing that the intensity of the Si-O absorption band becomes slightly stronger under hydrothermal condition. The inference is corroborated by the results of
Fig. 3. XRD patterns of ATP, Si-ATP and hierarchical zeolites with different SiATP/CTAB ratio.
sample subjected to the treatment of hydrochloric acid, which suggests that the carbonate impurities are eliminated. This band of 950 cm−1 is attributed to the silanol groups vibration related to the formed 149
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Fig. 5. SEM images of hierarchical zeolites with the different Si-ATP/CTAB ratio (a). The product without CTAB. (b–h). The products at the Si-ATP/CTAB ratio of 80:1 to 80:7.
the XRD patterns of these zeolites are in good agreement with the database MFI structure. There are five diffraction peaks at 2θ = 8.84, 17.68, 23.14, 23.77 and 24.29 of MFI zeolite. With Si-ATP/CTAB ratio increased to 80: 5, the XRD pattern shows the obvious difference in the intensity of individual peaks at 22.5–25° and 45.2°. Evidently, there are only two shape peaks at 2θ = 23.03° and 24.36°, meanwhile a single diffraction peak at 45.2° was observed in the S80:5, S80:6 and S80:7, without strong reflections at 2θ of 23.14°, 23.77° and 24.29° with arising from ZSM-5, which are consistent with that reported in the literature [47], indicating that high Si-ATP/CTAB ratio can result in the formation of (H) ZSM-11. CTAB with the ammonium heads absorb in the micropores of the small zeolite crystals, while the surfactant tails outside inhibit the further growth of these small zeolite crystals [48]. One may argue that CTAB is the structure-directing agent which also
the FTIR analyses, which proves the surface functional groups of Si-ATP are consisted with that of zeolites especially to some extent. Fig. 3 shows the XRD patterns of ATP, Si-ATP, products in the absence of CTAB and at different Si-ATP/CTAB ratio. Si-ATP as precursor source, the pristine crystal structure and fiber-shaped morphology of the raw material were well transformed through acid treatment. In Fig. 3, the characteristic diffraction peaks of ATP, such as 2θ = 12.46°, 20.15° and 32.10° have almost disappeared. Meanwhile, the distinct peaks both 15.56° and 21.92° of zeolites can be clearly observed, indicating the formation of a similar zeolitic framework. Free of CTAB, the crystal structure of a product tends to ATP rather than a zeolite structure having a typical diffraction peak at 2θ = 12.46° and 26.7°. Finally, all the intense and sharp peaks of zeolites appeared with adding the amount of CTAB. Si-ATP/CTAB ratios range from 80: 1 to 80: 4, and 150
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Fig. 6. TEM images of hierarchical zeolites at the Si-ATP/CTAB ratio of 80:5.
increased Si-ATP/CTAB ratio, all the samples show that they each contained three peaks with chemical shifts centered at approximately δ = −116, −113, and −104. These chemical shifts correspond to Si (3Si, 1Al), Si (4Si), and Q3 sites arising from Si (OH) on the zeolite surface, respectively [52]. Fig. 4b shows the 27Al MAS NMR spectra of these samples. To be specific, the resonance centered at around 53 ppm is commonly assigned to four-coordinated framework aluminum, implying that aluminum is in the framework of zeolite. Fig. 5 compares the particle sizes and morphologies of the synthesis products with different Si-ATP/CTAB ratios. In the absence of CTAB, the products are just as irregular crystals, apparently depending on Fig. 5a. After adding CTAB, the morphology of the sample (Fig. 5b ∼ h) shows a significant change compared with that free of CTAB. At a relatively low Si-ATP/CTAB ratio, SEM micrograph is shown that the sample is a classical twin hexahedron with smooth morphological surface, which is usually observed for ZSM-5 zeolite. The increase in the average particle size distribution (352.9 ± 0.5 × 407.1 ± 0.5 nm for S1 and 677.7 ± 0.5 × 805.2 ± 0.5 nm for S80:3) is this ratio from 80: 1 to 80: 3, indicating that CTAB may not only interact with the crystal nucleus but also promote the crystal growth. In comparison, the samples of S80:5, S80:6 and S80:7 exhibit microspheres similar to those of (H) ZSM-11 crystals, while the uniform particle size decreases gradually with the extra amount of CTAB increased. The excellent particle size is assigned to S80:5 with 900.0 ± 0.5 nm. The likely explanation is that excessive amounts of CTAB lead to altering the zeolitic morphology and restraining the crystal size. Therefore, the structure and morphology of Si-ATP are different from those of conventional silicon sources (i.e. silicon dioxide, tetraethyl orthosilicate and silica gel-sol), the interaction of Si-ATP and CTAB has a special effect on the crystal type and particle morphology of the products. To further confirm the morphology composition of a single nanocrystal, the TEM images of (H) ZSM-11 zeolite at Si-ATP/CTAB ratio of 80: 5 are shown in Fig. 6. TEM images (Fig. 6a and b) of (H) ZSM-11 reveal the relatively regular crystal edge, confirming that these crystals
Fig. 7. The nitrogen adsorption-desorption isotherms of hierarchical zeolites with the different Si-ATP/CTAB ratio.
has quaternary ammonium ions like TPA+ ions [49]. The results demonstrate that high Si-ATP/CTAB ratio first prevent the formation of ZSM-5, which makes the structure having the chance to promote the transformation of hierarchical (H) ZSM-11 zeolite. To investigate the coordination state of the local bonding environment and the Si and Al species contained in the samples, 29Si and 27Al MAS NMR spectra were carried out and the spectra of various samples are given in Fig. 4. Figure 4a shows the 29Si single pulse (SP) NMR spectra of the selected S80:3, S80:4, S80:5 and S80:6 zeolite samples. The changes of NMR spectra were qualitatively discussed with an increase of Si-ATP/CTAB ratio. In Fig. 4a, two well-resolved peaks (at −114 and −116 ppm) can be observed in the 29Si SP NMR spectrum of S80:3 and S80:4, which correspond to crystallographically nonequivalent Q4 tetrahedral sites [Si (Si-O4)] [50,51] and Si (O Al) groupings. With an 151
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Fig. 8. Pore size distribution of hierarchical zeolites with the different Si-ATP/CTAB ratio.
Fig. 9. Hierarchy factor of hierarchical zeolites with the different Si-ATP/CTAB ratio.
Fig. 10. Adsorption properties of hierarchical zeolites to MB.
do not contain any clearly visible amorphous materials. The particle size of (H) ZSM-11 zeolite is uniform (Fig. 6c), the TEM images indicate that the zeolites are between 800 and 1000 nm (mostly around 900 nm) which is in good agreement with average particle size obtained from SEM images (Fig. 5e). In addition, it is found that the surface of (H) ZSM-11 possessed many obvious cracks and the whiter tonality of zeolite crystals (Fig. 6d), which could be ascribed that significant mesoporosity has been successfully created. Nitrogen adsorption-desorption isotherms record for the texture properties of samples using initial mixture with the Si-ATP/CTAB ratio equal to 80: 1 to 80: 7 in Fig. 7. According to the IUPAC classification [53], a steep rise shape of the isotherm recorded for S1 is of type I, while the phenomenon of slightly adsorption hysteresis at a relative pressure
P/P0 greater than 0.4, indicating that the material is a main microporous phase with less meso porosity. With the increase of Si-ATP/ CTAB ratio, the isotherms of samples (i.e., S80:3, S80:4, S80:5 and S80:6) show an obvious hysteresis loop at 0.40 < P/P0 < 0.80 locations. They exhibit features of both types I and IV (a) profiles with two steep uptake steps, responding to micropore filling and mesopore capillary condensation, respectively. The Si-ATP/CTAB ratio of 80: 2 to 80: 4 result only in a slight development of mesoporosity, else samples S5 and S6 with higher Si-ATP/CTAB ratio is characterized by hysteresis loops typical of mesoporous materials. The result confirms the existing of hierarchical pore structure in our synthesized samples, farther more illustrates the increasing ratio of Si-ATP/CTAB benefited for the formation of mesopores incorporating the following the analysis of pore size distribution. However, excessive amount of CTAB will continue to
Table 2 Textural properties of Hierarchical zeolites with the different ratio of Si-ATP/CTAB. Sample
Si-ATP/ CTAB
SBET/ (m2·g−1)
Smicro/ (m2·g−1)
Smes/ (m2·g−1)
Vtot/ (cm3·g−1)
Vmicro/ (cm3·g−1)
Vmes/ (cm3·g−1)
Mean pore size/(Å)
S-1 S-2 S-3 S-4 S-5 S-6 S-7
80:1 80:2 80:3 80:4 80:5 80:6 80:7
324.73 321.85 301.06 313.20 432.02 372.38 300.28
132.34 118.35 177.49 112.75 165.65 189.08 212.81
192.39 203.50 123.57 200.45 266.37 183.30 87.47
0.20 0.21 0.21 0.16 0.40 0.34 0.19
0.07 0.07 0.10 0.06 0.08 0.11 0.12
0.13 0.14 0.11 0.10 0.32 0.23 0.07
43.36 37.30 43.36 26.11 46.72 54.79 43.71
152
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act on the crystal synthesis rather than the mesoporosity construction. This is the reason that the isotherm of S80:7 is classified as type I. It shows that the amount of CTAB is not able to be more than a certain value, otherwise has a negative effect with the mesopore volume being decreased. The characteristics of micropores and mesopores obtained by the analysis of the nitrogen sorption isotherms can be confirmed by the DFT pore size distribution (Fig. 8). The mesopores generate in the case of all the samples (except for S80:1 and S80:7) are within a wide range of pore sizes between about 20 and 80 Å. It is extremely important to notice that pore size distribution for the case of samples is slightly narrower (about 20–30 Å) at Si-ATP/CTAB ratio of 80: 1 and 80: 7. Insufficient or excessive CTAB amount results in a slimmer mesopore and zeolite structural contribution. In the case of hierarchical samples, their average mesopore sizes increase from 30 to 50 Å with change of SiATP/CTAB ratio of 80: 2 to 80: 5. More importantly, the microporous structure is well-protected among samples. This is because CTAB addition enhances the combination of mesopores, yet poses no negative effect on the micropores of Si-ATP. These results indicate that the mesoporous structure can be successfully prepared without the loss of micropores. In addition, the mesoporosity is related to the Si-ATP/ CTAB ratio and will be further discussed in the hierarchy factor. The hierarchy factor is cited here to research the impact of Si-ATP/ CTAB ratio on pore structural properties. Fig. 9 shows HF of samples at different Si-ATP/ CTAB ratio plotted against a function of the comparative surface area of mesopores (Smeso/STOT) and the relative micropore volume (Vmicr/VTOT). The porous nature of sample determines its position in the graph. One sample of the principal mesopore is situated in a top left corner, while the other sample with primary micropore is located in a bottom right corner. The relative surface area of mesopores has a well additive increase in the Si-ATP/CTAB ratio range of 80: 1 to 80: 3. The linear correlation coefficient is 0.9977. However, it has an observable additive reduction when Si-ATP/CTAB ratio is in the range of 80: 5 to 80: 7. The linear correlation coefficient is 0.9773. Otherwise, there is no order in the intermediate range of 80: 3 to 80: 4. It proves that the positive role of the structure-directing agent (CTAB) under the premise of using Si-ATP as the precursor source, which builds the mesoporous nature of the samples and simultaneously completes the crystalline transformation. An exception to this tendency is S80:5, the Si-ATP/CTAB ratio is 80: 5, located in the upper left corner. This proves its significant mesoporous character. S80:5 exhibits a high specific surface area (432.02 m2 g−1) and a large mesoporous volume (0.30 cm3 g−1) according to Table 2. It is worth to notice that the further increase Smeso/STOT is in relation to the decrease Vmicr/VTOT, the optimal proportion exists between of them. Basing on the analysis to the results presented in Fig. 9, the following parts of the studies are focused upon the hierarchy factor effect of distinct samples on adsorption performance. Discrete samples (Si-ATP/CTAB of 80: 1 to 80: 7) co-existed with MB are analyzed by adsorption on some contact time as showed in Fig. 10. It is evident that the adsorption capacity found to increase with the contact time lengthening for all samples. However, it is manifest that the sample (i.e., S80:5 or S80:6) with mesoporous mainly has a higher adsorption capacity and a shorter equilibrium time. This can be explained by the fact that the amount of CTAB would advance to form the mesopores in structure, which improves the adsorbent capacity for MB removal. At the same time, this rapid adsorption can be assigned to the sample with high BET surface area, which can be brought into rapid contact with the aqueous solution, such as S5 at 432.02 m2g-1 (according to Table 2). Excepting the comparable surface area and pore structure between S80:3 with the MFI-structure and S80:7 with the MELstructure, the adsorption capacity of (H) ZSM-11 is higher than that of ZSM-5 due to its shorter diffusion path lengths and more hydroxyl groups existing on the surface of the sample, such as Si-OH, and so on (according to FTIR and NMR results). The result shows that the adsorption capacity relay on not only BET surface area and pore structure,
Fig. 11. The effect of Si-ATP/CTAB ratio on MB adsorption capacity and hierarchy factor.
Fig. 12. Adsorption isotherms for the adsorption of MB molecules onto (H) ZSM-11 zeolites at the Si-ATP/CTAB ratio of 80: 5. (a). Langmuir isotherm model. (b). Freundlich isotherm model.
Table 3 Adsorption isotherm model parameters and error analysis for the adsorption of MB molecules onto (H) ZSM-11 zeolites. qm (mg g−1)
370.70
Langmuir model
Freundlich model 2
kL
RL
R
0.0027
0.79
0.9962
1/nF
KF
R2
0.96
1.02
0.9823
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MB-(H) ZSM-11 system from isotherm model studies (R2 = 0.9962). The RL value in Table 3 for MB adsorption on adsorbents is 0.79, which is within the favorable range of 0 < RL < 1 (Eq. (4)), indicating that the adsorption process for MB removal is favorable [58,59]. On the other hand, the Freundlich isotherm model provides a correlation coefficient (R2 = 0.9823), it is found that this adsorption progress can be usually used to describe a heterogeneous system. As heterogeneity factor, the values of 1/n is 0.96, smaller than 1.0 indicate that the adsorbate is easily adsorbed [60].
but also type of crystalline phase and active groups on the surface of the sample. In short, Si-ATP/CTAB ratio as the key factor, affects the pore structure and zeolite type, which plays a major role in MB adsorption. To investigate the effect of Si-ATP/CTAB ratio on the sorbent and the relationship of the hierarchy factor (HF), the adsorption capacity and hierarchy factor changes in Si-ATP/CTAB ratio are examined in Fig. 11. When Si-ATP/CTAB ratio is 80: 5, S5 has a higher adsorption capacity of MB and a lower hierarchy factor, its value is 366.18 mg g−1 and 0.12, respectively. In general, the mesoporosity dominants mainly in pore structure accompanied with a small hierarchy factor value according to the calculation results. However, outstanding MB adsorption capacity suggests that the mesopores help for the adsorption and separation of large-sized molecules from the liquid solution. Otherwise, the hierarchy factor of S80:4 is greater than that of S80:3, yet the adsorption effect is obviously better than that of S80:3. The result reveals that the crystalline transformation from ZSM-5 to (H) ZSM-11 according to XRD and SEM analysis also affects the adsorption capacity due to Si-ATP/CTAB ratio increasing in some range. The vertical channels of (H) ZSM-11 zeolite overcome the diffusion resistance of molecules entering into channels. We conclude that more mesopores and shorter diffusion paths benefit for the better adsorption performance of the zeolite. Adsorption properties and equilibrium parameters, commonly known as adsorption isotherms, describe how the adsorbate interacts with adsorbents, and comprehensive understanding of the nature of interaction. Isotherms help to provide information about the optimum use of adsorbents [54]. In order to evaluate the different Si-ATP/CTAB ratio in enhancing the adsorption performance of zeolite, it necessitates a similar equilibrium condition to provide a better comparison and understanding of the adsorption process. Fig. 12 shows a significant difference in adsorption capacity of MB dye from the samples. Two important adsorption isotherm models of the Langmuir and Freundlich were used to analyze an adsorption system to remove dye from solutions [55]. The linear form Langmuir isotherm model can be described as following Eq. (3):
1 1 1 1 = ⋅ + qe kL⋅qm ce qm
4. Conclusions In summary, hierarchical (H) ZSM-11 zeolites are successfully prepared by one-pot synthesis process with mesoporous Si-ATP in the presence of CTAB. Surface functional groups and XRD analysis of SiATP indicate the similarity to zeolite with Si-O tetrahedra and 2θ 23.14°, respectively. The change in the Si-ATP/CTAB ratio makes the types of crystalline phases and the mesopores structure significantly improved. The synthesized (H) ZSM-11 zeolite owns less diffusion resistance due to straight channels instead of directly intersecting and sinusoidal channels of ZSM-5 zeolite, and when this ratio is between 80: 3 and 80: 5. The Si-ATP/CTAB ratio has a well linear dependence with hierarchy factor (HF) in the both lower and higher ranges, which illustrate the amount of CTAB plays an important role of the mesoporous construction. It is obvious that hierarchical (H) ZSM-11 zeolites dominated with straight channels and mesoporosity, when the Si-ATP/CTAB ratio is 80: 5, possess a higher adsorption capacity of MB wastewater. This result is further confirmed that hierarchical zeolites pose a huge potential on adsorption performance for dyeing wastewater. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21573171), the Shaanxi Provincial Natural Science Foundation of China (No. 17JK0327), and the Research and Innovation Training Project for Graduate in General Universities of Shaanxi Province of China (No. 1731).
(3)
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Where qe is the amount of adsorbed dye at equilibrium (mg g ), qm is the maximum of adsorption capacity (mg g−1), Ce is the concentration of dye solution at adsorption equilibrium (mg L−1), and kL is the Langmuir constant (L mg−1) related to adsorption energy. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL hat is given by the following equation [56]:
RL =
1 1 + kL⋅C0
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(4) −1
Where C0 (mg L ) is the initial adsorbate concentration in aqueous solution, kL (L mg−1) is the Langmuir constant, and RL indicates the type of isotherm to be irreversible (RL < 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1). The Freundlich isotherm model can be described in its linearized form as:
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Effect of Si-ATP/CTAB ratio on crystal morphology, pore structure and adsorption performance of hierarchical (H) ZSM-11 zeolite
T
Hong-Ji Lia,b,∗, Xiao-De Zhoua, Yu-Hui Dib, Jian-Min Zhangb, Yu Zhangb a b
School of Water Resources & Hydraulic Power, Xi'an University of Technology, Xi'an, 710048, China College of Town Planning & Municipal Engineering, Xi'an Polytechnic University, Xi'an, 710048, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Si-ATP/CTAB ratio Hierarchical (H)ZSM-11 Hierarchy factor Adsorption isotherm
In this manuscript, the micro-mesoporous crystals with the properties of (H) ZSM-11 zeolites were successfully obtained via one-pot synthesis in the hydrothermal system by using acidified attapulgite (Si-ATP) and cetyltrimethylammonium bromide (CTAB) as a precursor source (PS) and structure-directing agent (SDA), respectively. The effect of Si-ATP/CTAB ratio on controlling the crystal morphology and hierarchical pore structure of products was systematically investigated. The synthesized materials were characterized with XPS, XRD, FTIR, NMR, SEM, TEM and N2 adsorption-desorption analysis. Results demonstrate that the products possess not only a classical zeolite framework but also an adjustable mesoporous structure accompanied with a variety of Si-ATP/ CTAB ratios. Meanwhile Si-ATP/CTAB ratio has a well linear dependence with hierarchy factor (HF) in lower or higher ranges. The MFI crystal transforms gradually to the MEL-structure under the action of CTAB when SiATP/CTAB ratio increases to 80: 5. In addition, (H) ZSM-11 zeolite, this ratio of 80: 5, mesoporosity and adsorption capacity of MB molecules was significantly enhanced. Advanced adsorption performance is attributed to the abundance hydroxyl group and the generated mesoporosity of the zeolite surface under the participation of CTAB. The experimental data are well fitted to the Langmuir isotherm model, with a correlation coefficient of 0.9962. The excellent adsorption performance makes such hierarchically porous (H) ZSM-11 zeolites attractive for applications in the field of energy and environment.
1. Introduction
intersecting direct and sinusoidal channels, compared with that, ZSM11 zeolite with single straight channels (5.3 Å × 5.4 Å), possess the important values in MB wastewater treatment [9]. Three-dimensional topology structure of ZSM-11 zeolite consists of straight channels along a- and b-axis with lone pore openings along them [10]. The synthesis of hierarchical ZSM-11 zeolite is viewed as a convenient strategy to make full use of less diffusion resistance of consecutive channels, which overcome the drawback of blocking the molecules from entering the channel along the c-axis. Moreover, hierarchical porous materials may be favorable to keep the balance between mesoscopic and MEL-type zeolite ordering. Much effort has been devoted to developing a typical method that obtains hierarchical micro-mesoporous ZSM-11 zeolites [11,12], such as the hard-casting template techniques [13,14], chemical leaching approaches [15] [16], and the assembly of zeolite nanoparticles [17,18]. The acidity and crystalline framework of zeolites are well preserved in the template synthesis processes via the introduction of mesopores into microporous zeolite [19–21]. There are obvious disadvantages for the hard-casting template synthesis route, such as
The pollutions of dyeing wastewater from textile, tannery, paper and other industry are given more and more attention due to their high chemical oxygen demand (COD), low bioavailability, high perniciousness and strong persistent color [1–3]. Among of them, especially methylene blue (MB) wastewater has been a potential threaten to human and environment [4,5]. Significant characteristics of zeolites include high efficiency, rapid reaction rate and regeneration, capability to be used as a support for MB wastewater treatment as sorbent materials in the recent years. The hierarchic pore structure of zeolites with lots of channels and cavities increase the accessibility of zeolitic internal surface area and shorten their diffusion resistance [6]. However, there are few studies on the adsorption mechanism of hierarchical crystals. It is significant essential to reveal the influence of morphology and porosity of zeolites on the adsorption capability of MB. ZSM-11 zeolite is a typical member of the pentasil family with 10membered ring openings [7], which shows similar framework densities and pore sizes with ZSM-5 zeolite [8]. ZSM-5 zeolite has been
∗
Corresponding author. School of Water Resources & Hydraulic Power, Xi'an University of Technology, Xi'an, 710048, China. E-mail address: [email protected] (H.-J. Li).
https://doi.org/10.1016/j.micromeso.2018.05.039 Received 7 September 2017; Received in revised form 14 January 2018; Accepted 26 May 2018 Available online 01 June 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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2.2. Characterizations
intricate kinds of templates [22], relatively high-cost [23], and environmental pollution due to removing the template [24]. Moreover, ZSM-11 zeolite is synthesized with the traditional tetrabutyl-ammonium templated, to greater or lesser degrees, intergrowths containing ZSM-5 zeolite [25]. Selectively extraction of aluminum and silicon atoms from the framework by chemical leaching treatment through alkali or acid [26,27] leads to the formation of porous ZSM-11 zeolite [28–30]. However, porous zeolites are acquired via the chemical leaching treatment, which is usually at the expense of destroying microporosity in the traditional zeolite crystals [30–32]. The assemblage of zeolite nanocrystals may possess weak ordering from spontaneous and random assembling. It is difficult that keeping the balance of mesoscopic and the order in MEL-type zeolite in this situation. The similar frameworks, abundant pores and structural properties make phyllosilicate mine resources, including kaolin, diatomite and attapulgite, successful attempts taken to synthesize a hierarchical zeolite [33–35]. Compared with the above-mentioned methods, this method possesses the prominent characteristics of low-cost, environmentally friendly and high productivity. In the synthesis of ATPbased zeolite, the effects of crystal morphology and pore structure, such as Si/Al ratio, alkalinity and template have been researched by traditional hydrothermal synthesis. However, adding Si-ATP with abundant porosity as a precursor source, hierarchal zeolites are prepared by onepot synthesis, and less studied is the influence of Si-ATP and the structure-directing agent on zeolite morphology and hierarchic pore structure. To study the action of hierarchical zeolites on the adsorption capability, a highly ordered hierarchical (H) ZSM-11 zeolite with wellcrystallized was prepared under interaction of Si-ATP and CTAB. Crystalline type, surface morphology and pore characteristic were analyzed by XPS, FTIR, XRD, NMR, SEM, TEM and N2 adsorption-desorption. Further investigate the effect of Si-ATP/CTAB ratio on the morphology and pore structure, and reveal the relationship of this ratio and the hierarchy factor (HF) to show the mesoporous structure. It is worth noting that hierarchically porous (H) ZSM-11 zeolites are a relatively high external surface area and larger pore volume. In addition, we demonstrate that such a (H) ZSM-11 zeolite can be effectively applied during the MB adsorption process. This will be a qualitative discussion of details.
The study of the composition and properties was performed using Xray photoelectron spectroscopy (XPS). The XPS spectra were obtained by Axis Ultra (UK) using Monochromatic Al Kα (150 W, 15 kv, 1486.71 ev). Fourier-transform infrared (FT-IR) spectrum of samples was gathered on a Nicolet 5700 FT-IR spectrometer to characterize the surface functional groups, using KBr wafer technique. The infrared absorbance spectra were registered from 4000 to 400 cm−1. X-ray diffraction (XRD) experiments were obtained at room temperature on a Cu Κα source operated at 40 kV and 40 mA. A scan rate is 8°min−1 with the scan step of 0.02°, and the scan range is 10°–60°. A Bruker AV-400 nuclear magnetic resonance (NMR) spectrometer was used to collect 27Al and 29 Si MAS NMR spectra at 79.495 MHz at a spinning frequency of 27.7 kHz and 10 s intervals between successive accumulations. Scanning electron microscopy (SEM) images were collected on a Quanta-450- FEG electron microscope, to investigate the surface morphology with fabricated by FEI of the UK. Transmission electron microscopy (TEM) images were performed on a Tecnai G2 F20 transmission electron microscope operating at an accelerating voltage of 200 k V. Nitrogen adsorption-desorption isotherms were consulted on an ASAP-2020 HD88 Accelerated Surface Area and Porosimetry System. Surface area of the sample powders was analyzed from Brunauer, Emmett and Teller (BET) method at 77 K and the nitrogen adsorptiondesorption isotherm curves utilizing Autosorb-1 Quantachrome Instrument. Total pore volume of the sample powders was analyzed from single point adsorption total pore volume of pores. Both the micropore volume (Vmicr) and micropore surface area (Smicr) were calculated by application of the t-plot method. The mesopore volume (Vmeso) and external surface area (Smeso) were calculated by the aggregate data minus the corresponding micropore data. The hierarchy factor (HF) [36] was cited here to present the effects of textural properties, plotted as a function of the relative surface area of mesopores (Smeso/STOT) and the relative micropore volume (Vmicr/VTOT), which was described as Eq. (1): HF = (Vmicr / VTOT) × (Smeso / SBET)
(1)
2.3. MB adsorption
2. Experimental and instrumentation
Adsorption experiments were conducted by adding a fixed mass of zeolites into 100.0 ml MB (concentration of 50 mg L−1) solution in 250 ml conical flasks. This flasks were placed on a water bath shaker at 293 K with 100 rpm. Sub-samples were collected at gave time intervals via centrifuged to separate from the liquid phases. The supernatant solution was subsequently determined after diluted. The absorbance of the filtrate was measured by a 720S UV-vis spectrophotometer with a maximum absorbance wavelength for MB at 665 nm. The amount of MB adsorbed on zeolites for some time was calculated using Eq. (2):
2.1. Synthesis of hierarchical zeolite The precursor of hierarchical zeolites was prepared as following: 2.00 g natural attapulgite powders (ATP, Gansu Hao Di Co., Ltd., China) were dispersed in 1.00 mol/L hydrochloric acid (HCl) to a beaker contained and stirring for 30 min at room temperature. Hydrothermal acidified reaction was carried in a sealed reactor at 453 K for 12 h. Products were filtered by centrifuging repeatedly with 200 mL deionized water, and subsequently dried at 353 K under ambient condition. The acquired product was characterized as Si-ATP. Hierarchical zeolites were disposed according to the literature [27] with some modifications. Briefly, a mixture of TPAOH (25%, 7.7 mL) and NaAlO2 (AR, 0.011 g) was added to 5.20 mL deionized water and stirred perfectly. Then adding the different amount Si-ATP and CTAB and stirring the mixture at 60 °C for 4 h. The initial gel was transferred to a 25 mL Teflon-lined autoclave and hydrothermal acted at 453 K for 12 h. After cooled down, the template was removed by calcination at 823 K for 5 h. The effect of CTAB was better investigated on the properties of the final products. Seven samples with different Si-ATP/CTAB ratios (80: 1, 80: 2, 80: 3, 80: 4, 80: 5, 80: 6 and 80: 7, respectively) were added in the precursor solution, and synthesized depending on the aforementioned synthesis method. They were denoted as S n (n = 80: 1, 80: 2, 80: 3, …represents Si-ATP/ CTAB mass ratio).
qe (mg g−1) = (C0eCe) × (V/m)
(2)
Here, C0 and Ce are the initial and equilibrium MB concentration (mg L−1), respectively. V is the volume of MB solution (L), and m is the mass of zeolites (g). 3. Results and discussion The chemical states of different elements of ATP and Si-ATP were investigated by XPS characterization. The wide survey spectrum of ATP, which reveals the presence of Na, Fe, O, Ca, Si and Al elements, is displayed in Fig. 1 and Table 1. The full-range XPS spectra results seen in Fig. 1 qualitatively revealed the existence of Na 1s, Fe 2p, O Auger, O 1s, Ca 2p, C 1s, Si 2s, Si 2p and Al 2p with atomic percent accounts for 1.37%, 3.89%, 18.26%, 34.50%, 1.89%, 11.91%, 10.25%, 9.55% and 8.28%, respectively. High-resolution XPS spectra of acidified 147
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Fig. 1. XPS survey spectra of ATP and Si-ATP (a) high-resolution Si 2p and (b) Al 2p before and after acidified treatment.
two bonding environments, which are Si-O (101.85 e V) and Si O2 (104.3 e V), respectively [37]. Fig. 1b shows that the peak of Al 2p XPS spectra in Si-ATP appears only one bonding environments, which are Al-O (72.8 e V) with the disappearance of Al-OH (77.4 e V) from ATP in Fig. 1B [38]. Si-ATP with a well-preserved Si, Al and O element and pristine crystal structure can be used as the Si/Al source, which is the similar to those in zeolites. Therefore, the zeolite from Si-ATP is obviously synthesized. FT-IR spectra of ATP, Si-ATP and S80:5 (Si-ATP/CTAB ratio of 80: 5, (H) ZSM-11 zeolite) are illustrated in Fig. 2. The structural OH stretching vibrations appear in the range of 3420 cm−1 and 1632 cm−1, which confirms a great number of active hydroxyl groups exist on the surface of ATP, Si-ATP and S80:5 [33,39,40]). For Si-ATP, the characteristic band of carbonates, at 1464 cm−1, disappears from the
Table 1 Atomic percent accounts of ATP and Si-ATP from XPS survey spectra (Unit: %). Sample
Na1s
Fe2p
OAuger
O1s
Ca2p
C1s
Si2s
Si2p
Al2p
ATP Si-ATP
1.37 0.00
3.89 0.00
18.26 11.35
34.50 37.78
1.89 0.00
11.91 27.23
10.25 9.14
9.55 10.79
8.28 3.70
attapulgite (Si-ATP), which reveals only the existence of O Auger, O 1s, C 1s, Si 2s, Si 2p and Al 2p with atomic percent accounts for 11.35%, 37.78%, 27.23%, 9.14%, 10.79% and 3.70%, respectively, on the surface of the modified samples. Thus, it can be concluded that the partial metal cations, such as Na, Fe and Ca, of the attapulgite component are removed by acidification. Fig. 1A and (a)shows that the peak of Si 2p XPS spectra in ATP and Si-ATP is quite broad and appears to consist of 148
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Fig. 2. FTIR spectra of ATP, Si-ATP and hierarchical zeolites at the Si-ATP/ CTAB ratio of 80:5.
Fig. 4. Multinuclear solid state NMR spectra of zeolites obtained at Si-ATP/ CTAB ratio of 80:3, 80:4, 80:5 and 80:6 (a) 29Si SP MAS NMR and (b) 27Al MAS NMR.
amorphous silica (bending vibration of Si-OH), which appears after the dissolution of the octahedral sheet [41]. The adsorption bands at 800 cm−1 appears in the structure of Si-ATP under HCl treatment, which is attributed to the vibration of Si-O-Si in the formed free silica (SiO2) [42]. At 550 cm−1, 800 cm−1 and 3661 cm−1, the typical peak reveals the trend of transitionally Si-ATP to hierarchical zeolite. In particular, the vibration band at around 550 cm−1 indicates the presence of the double five rings of the characteristic structure of pentasil family zeolite (MFI and MEL) [43,44], rather than that of ATP or conventional chemical Si/Al sources (e.g., silicon dioxide (SiO2), tetraethoxysilane (TEOS), etc.). The Si-O tetrahedra bonds in Si-ATP are similar to those in zeolites, which facilitate the formation of zeolite frameworks but is absent in conventional chemical Si/Al sources [33]. The band at 3661 cm−1 is attributed to the hydroxyl stretching vibrations of the Al3+ cations in dioctahedral coordination (Al2OH). All the samples reveal characteristic adsorption bands at around 450, 550, 800, 1100 and 1225 cm−1. The adsorption bands at 1225, 1100, 800, 550 and 450 cm−1 ascribe to the external asymmetric stretching of Si-O-T linkage, the asymmetric stretching vibration of Si-O tetrahedra, Si-O-Si symmetric stretching, Si-O bending of the double fiv-rings and T-O bending of Si tetrahedral, respectively [45,46]. A wide absorption band of Si-O-Si and Si-O-Al for ATP near 1030 cm−1 is transferred to 1100 cm−1 in Si-ATP and hierarchical zeolite, revealing that the intensity of the Si-O absorption band becomes slightly stronger under hydrothermal condition. The inference is corroborated by the results of
Fig. 3. XRD patterns of ATP, Si-ATP and hierarchical zeolites with different SiATP/CTAB ratio.
sample subjected to the treatment of hydrochloric acid, which suggests that the carbonate impurities are eliminated. This band of 950 cm−1 is attributed to the silanol groups vibration related to the formed 149
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Fig. 5. SEM images of hierarchical zeolites with the different Si-ATP/CTAB ratio (a). The product without CTAB. (b–h). The products at the Si-ATP/CTAB ratio of 80:1 to 80:7.
the XRD patterns of these zeolites are in good agreement with the database MFI structure. There are five diffraction peaks at 2θ = 8.84, 17.68, 23.14, 23.77 and 24.29 of MFI zeolite. With Si-ATP/CTAB ratio increased to 80: 5, the XRD pattern shows the obvious difference in the intensity of individual peaks at 22.5–25° and 45.2°. Evidently, there are only two shape peaks at 2θ = 23.03° and 24.36°, meanwhile a single diffraction peak at 45.2° was observed in the S80:5, S80:6 and S80:7, without strong reflections at 2θ of 23.14°, 23.77° and 24.29° with arising from ZSM-5, which are consistent with that reported in the literature [47], indicating that high Si-ATP/CTAB ratio can result in the formation of (H) ZSM-11. CTAB with the ammonium heads absorb in the micropores of the small zeolite crystals, while the surfactant tails outside inhibit the further growth of these small zeolite crystals [48]. One may argue that CTAB is the structure-directing agent which also
the FTIR analyses, which proves the surface functional groups of Si-ATP are consisted with that of zeolites especially to some extent. Fig. 3 shows the XRD patterns of ATP, Si-ATP, products in the absence of CTAB and at different Si-ATP/CTAB ratio. Si-ATP as precursor source, the pristine crystal structure and fiber-shaped morphology of the raw material were well transformed through acid treatment. In Fig. 3, the characteristic diffraction peaks of ATP, such as 2θ = 12.46°, 20.15° and 32.10° have almost disappeared. Meanwhile, the distinct peaks both 15.56° and 21.92° of zeolites can be clearly observed, indicating the formation of a similar zeolitic framework. Free of CTAB, the crystal structure of a product tends to ATP rather than a zeolite structure having a typical diffraction peak at 2θ = 12.46° and 26.7°. Finally, all the intense and sharp peaks of zeolites appeared with adding the amount of CTAB. Si-ATP/CTAB ratios range from 80: 1 to 80: 4, and 150
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Fig. 6. TEM images of hierarchical zeolites at the Si-ATP/CTAB ratio of 80:5.
increased Si-ATP/CTAB ratio, all the samples show that they each contained three peaks with chemical shifts centered at approximately δ = −116, −113, and −104. These chemical shifts correspond to Si (3Si, 1Al), Si (4Si), and Q3 sites arising from Si (OH) on the zeolite surface, respectively [52]. Fig. 4b shows the 27Al MAS NMR spectra of these samples. To be specific, the resonance centered at around 53 ppm is commonly assigned to four-coordinated framework aluminum, implying that aluminum is in the framework of zeolite. Fig. 5 compares the particle sizes and morphologies of the synthesis products with different Si-ATP/CTAB ratios. In the absence of CTAB, the products are just as irregular crystals, apparently depending on Fig. 5a. After adding CTAB, the morphology of the sample (Fig. 5b ∼ h) shows a significant change compared with that free of CTAB. At a relatively low Si-ATP/CTAB ratio, SEM micrograph is shown that the sample is a classical twin hexahedron with smooth morphological surface, which is usually observed for ZSM-5 zeolite. The increase in the average particle size distribution (352.9 ± 0.5 × 407.1 ± 0.5 nm for S1 and 677.7 ± 0.5 × 805.2 ± 0.5 nm for S80:3) is this ratio from 80: 1 to 80: 3, indicating that CTAB may not only interact with the crystal nucleus but also promote the crystal growth. In comparison, the samples of S80:5, S80:6 and S80:7 exhibit microspheres similar to those of (H) ZSM-11 crystals, while the uniform particle size decreases gradually with the extra amount of CTAB increased. The excellent particle size is assigned to S80:5 with 900.0 ± 0.5 nm. The likely explanation is that excessive amounts of CTAB lead to altering the zeolitic morphology and restraining the crystal size. Therefore, the structure and morphology of Si-ATP are different from those of conventional silicon sources (i.e. silicon dioxide, tetraethyl orthosilicate and silica gel-sol), the interaction of Si-ATP and CTAB has a special effect on the crystal type and particle morphology of the products. To further confirm the morphology composition of a single nanocrystal, the TEM images of (H) ZSM-11 zeolite at Si-ATP/CTAB ratio of 80: 5 are shown in Fig. 6. TEM images (Fig. 6a and b) of (H) ZSM-11 reveal the relatively regular crystal edge, confirming that these crystals
Fig. 7. The nitrogen adsorption-desorption isotherms of hierarchical zeolites with the different Si-ATP/CTAB ratio.
has quaternary ammonium ions like TPA+ ions [49]. The results demonstrate that high Si-ATP/CTAB ratio first prevent the formation of ZSM-5, which makes the structure having the chance to promote the transformation of hierarchical (H) ZSM-11 zeolite. To investigate the coordination state of the local bonding environment and the Si and Al species contained in the samples, 29Si and 27Al MAS NMR spectra were carried out and the spectra of various samples are given in Fig. 4. Figure 4a shows the 29Si single pulse (SP) NMR spectra of the selected S80:3, S80:4, S80:5 and S80:6 zeolite samples. The changes of NMR spectra were qualitatively discussed with an increase of Si-ATP/CTAB ratio. In Fig. 4a, two well-resolved peaks (at −114 and −116 ppm) can be observed in the 29Si SP NMR spectrum of S80:3 and S80:4, which correspond to crystallographically nonequivalent Q4 tetrahedral sites [Si (Si-O4)] [50,51] and Si (O Al) groupings. With an 151
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Fig. 8. Pore size distribution of hierarchical zeolites with the different Si-ATP/CTAB ratio.
Fig. 9. Hierarchy factor of hierarchical zeolites with the different Si-ATP/CTAB ratio.
Fig. 10. Adsorption properties of hierarchical zeolites to MB.
do not contain any clearly visible amorphous materials. The particle size of (H) ZSM-11 zeolite is uniform (Fig. 6c), the TEM images indicate that the zeolites are between 800 and 1000 nm (mostly around 900 nm) which is in good agreement with average particle size obtained from SEM images (Fig. 5e). In addition, it is found that the surface of (H) ZSM-11 possessed many obvious cracks and the whiter tonality of zeolite crystals (Fig. 6d), which could be ascribed that significant mesoporosity has been successfully created. Nitrogen adsorption-desorption isotherms record for the texture properties of samples using initial mixture with the Si-ATP/CTAB ratio equal to 80: 1 to 80: 7 in Fig. 7. According to the IUPAC classification [53], a steep rise shape of the isotherm recorded for S1 is of type I, while the phenomenon of slightly adsorption hysteresis at a relative pressure
P/P0 greater than 0.4, indicating that the material is a main microporous phase with less meso porosity. With the increase of Si-ATP/ CTAB ratio, the isotherms of samples (i.e., S80:3, S80:4, S80:5 and S80:6) show an obvious hysteresis loop at 0.40 < P/P0 < 0.80 locations. They exhibit features of both types I and IV (a) profiles with two steep uptake steps, responding to micropore filling and mesopore capillary condensation, respectively. The Si-ATP/CTAB ratio of 80: 2 to 80: 4 result only in a slight development of mesoporosity, else samples S5 and S6 with higher Si-ATP/CTAB ratio is characterized by hysteresis loops typical of mesoporous materials. The result confirms the existing of hierarchical pore structure in our synthesized samples, farther more illustrates the increasing ratio of Si-ATP/CTAB benefited for the formation of mesopores incorporating the following the analysis of pore size distribution. However, excessive amount of CTAB will continue to
Table 2 Textural properties of Hierarchical zeolites with the different ratio of Si-ATP/CTAB. Sample
Si-ATP/ CTAB
SBET/ (m2·g−1)
Smicro/ (m2·g−1)
Smes/ (m2·g−1)
Vtot/ (cm3·g−1)
Vmicro/ (cm3·g−1)
Vmes/ (cm3·g−1)
Mean pore size/(Å)
S-1 S-2 S-3 S-4 S-5 S-6 S-7
80:1 80:2 80:3 80:4 80:5 80:6 80:7
324.73 321.85 301.06 313.20 432.02 372.38 300.28
132.34 118.35 177.49 112.75 165.65 189.08 212.81
192.39 203.50 123.57 200.45 266.37 183.30 87.47
0.20 0.21 0.21 0.16 0.40 0.34 0.19
0.07 0.07 0.10 0.06 0.08 0.11 0.12
0.13 0.14 0.11 0.10 0.32 0.23 0.07
43.36 37.30 43.36 26.11 46.72 54.79 43.71
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act on the crystal synthesis rather than the mesoporosity construction. This is the reason that the isotherm of S80:7 is classified as type I. It shows that the amount of CTAB is not able to be more than a certain value, otherwise has a negative effect with the mesopore volume being decreased. The characteristics of micropores and mesopores obtained by the analysis of the nitrogen sorption isotherms can be confirmed by the DFT pore size distribution (Fig. 8). The mesopores generate in the case of all the samples (except for S80:1 and S80:7) are within a wide range of pore sizes between about 20 and 80 Å. It is extremely important to notice that pore size distribution for the case of samples is slightly narrower (about 20–30 Å) at Si-ATP/CTAB ratio of 80: 1 and 80: 7. Insufficient or excessive CTAB amount results in a slimmer mesopore and zeolite structural contribution. In the case of hierarchical samples, their average mesopore sizes increase from 30 to 50 Å with change of SiATP/CTAB ratio of 80: 2 to 80: 5. More importantly, the microporous structure is well-protected among samples. This is because CTAB addition enhances the combination of mesopores, yet poses no negative effect on the micropores of Si-ATP. These results indicate that the mesoporous structure can be successfully prepared without the loss of micropores. In addition, the mesoporosity is related to the Si-ATP/ CTAB ratio and will be further discussed in the hierarchy factor. The hierarchy factor is cited here to research the impact of Si-ATP/ CTAB ratio on pore structural properties. Fig. 9 shows HF of samples at different Si-ATP/ CTAB ratio plotted against a function of the comparative surface area of mesopores (Smeso/STOT) and the relative micropore volume (Vmicr/VTOT). The porous nature of sample determines its position in the graph. One sample of the principal mesopore is situated in a top left corner, while the other sample with primary micropore is located in a bottom right corner. The relative surface area of mesopores has a well additive increase in the Si-ATP/CTAB ratio range of 80: 1 to 80: 3. The linear correlation coefficient is 0.9977. However, it has an observable additive reduction when Si-ATP/CTAB ratio is in the range of 80: 5 to 80: 7. The linear correlation coefficient is 0.9773. Otherwise, there is no order in the intermediate range of 80: 3 to 80: 4. It proves that the positive role of the structure-directing agent (CTAB) under the premise of using Si-ATP as the precursor source, which builds the mesoporous nature of the samples and simultaneously completes the crystalline transformation. An exception to this tendency is S80:5, the Si-ATP/CTAB ratio is 80: 5, located in the upper left corner. This proves its significant mesoporous character. S80:5 exhibits a high specific surface area (432.02 m2 g−1) and a large mesoporous volume (0.30 cm3 g−1) according to Table 2. It is worth to notice that the further increase Smeso/STOT is in relation to the decrease Vmicr/VTOT, the optimal proportion exists between of them. Basing on the analysis to the results presented in Fig. 9, the following parts of the studies are focused upon the hierarchy factor effect of distinct samples on adsorption performance. Discrete samples (Si-ATP/CTAB of 80: 1 to 80: 7) co-existed with MB are analyzed by adsorption on some contact time as showed in Fig. 10. It is evident that the adsorption capacity found to increase with the contact time lengthening for all samples. However, it is manifest that the sample (i.e., S80:5 or S80:6) with mesoporous mainly has a higher adsorption capacity and a shorter equilibrium time. This can be explained by the fact that the amount of CTAB would advance to form the mesopores in structure, which improves the adsorbent capacity for MB removal. At the same time, this rapid adsorption can be assigned to the sample with high BET surface area, which can be brought into rapid contact with the aqueous solution, such as S5 at 432.02 m2g-1 (according to Table 2). Excepting the comparable surface area and pore structure between S80:3 with the MFI-structure and S80:7 with the MELstructure, the adsorption capacity of (H) ZSM-11 is higher than that of ZSM-5 due to its shorter diffusion path lengths and more hydroxyl groups existing on the surface of the sample, such as Si-OH, and so on (according to FTIR and NMR results). The result shows that the adsorption capacity relay on not only BET surface area and pore structure,
Fig. 11. The effect of Si-ATP/CTAB ratio on MB adsorption capacity and hierarchy factor.
Fig. 12. Adsorption isotherms for the adsorption of MB molecules onto (H) ZSM-11 zeolites at the Si-ATP/CTAB ratio of 80: 5. (a). Langmuir isotherm model. (b). Freundlich isotherm model.
Table 3 Adsorption isotherm model parameters and error analysis for the adsorption of MB molecules onto (H) ZSM-11 zeolites. qm (mg g−1)
370.70
Langmuir model
Freundlich model 2
kL
RL
R
0.0027
0.79
0.9962
1/nF
KF
R2
0.96
1.02
0.9823
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MB-(H) ZSM-11 system from isotherm model studies (R2 = 0.9962). The RL value in Table 3 for MB adsorption on adsorbents is 0.79, which is within the favorable range of 0 < RL < 1 (Eq. (4)), indicating that the adsorption process for MB removal is favorable [58,59]. On the other hand, the Freundlich isotherm model provides a correlation coefficient (R2 = 0.9823), it is found that this adsorption progress can be usually used to describe a heterogeneous system. As heterogeneity factor, the values of 1/n is 0.96, smaller than 1.0 indicate that the adsorbate is easily adsorbed [60].
but also type of crystalline phase and active groups on the surface of the sample. In short, Si-ATP/CTAB ratio as the key factor, affects the pore structure and zeolite type, which plays a major role in MB adsorption. To investigate the effect of Si-ATP/CTAB ratio on the sorbent and the relationship of the hierarchy factor (HF), the adsorption capacity and hierarchy factor changes in Si-ATP/CTAB ratio are examined in Fig. 11. When Si-ATP/CTAB ratio is 80: 5, S5 has a higher adsorption capacity of MB and a lower hierarchy factor, its value is 366.18 mg g−1 and 0.12, respectively. In general, the mesoporosity dominants mainly in pore structure accompanied with a small hierarchy factor value according to the calculation results. However, outstanding MB adsorption capacity suggests that the mesopores help for the adsorption and separation of large-sized molecules from the liquid solution. Otherwise, the hierarchy factor of S80:4 is greater than that of S80:3, yet the adsorption effect is obviously better than that of S80:3. The result reveals that the crystalline transformation from ZSM-5 to (H) ZSM-11 according to XRD and SEM analysis also affects the adsorption capacity due to Si-ATP/CTAB ratio increasing in some range. The vertical channels of (H) ZSM-11 zeolite overcome the diffusion resistance of molecules entering into channels. We conclude that more mesopores and shorter diffusion paths benefit for the better adsorption performance of the zeolite. Adsorption properties and equilibrium parameters, commonly known as adsorption isotherms, describe how the adsorbate interacts with adsorbents, and comprehensive understanding of the nature of interaction. Isotherms help to provide information about the optimum use of adsorbents [54]. In order to evaluate the different Si-ATP/CTAB ratio in enhancing the adsorption performance of zeolite, it necessitates a similar equilibrium condition to provide a better comparison and understanding of the adsorption process. Fig. 12 shows a significant difference in adsorption capacity of MB dye from the samples. Two important adsorption isotherm models of the Langmuir and Freundlich were used to analyze an adsorption system to remove dye from solutions [55]. The linear form Langmuir isotherm model can be described as following Eq. (3):
1 1 1 1 = ⋅ + qe kL⋅qm ce qm
4. Conclusions In summary, hierarchical (H) ZSM-11 zeolites are successfully prepared by one-pot synthesis process with mesoporous Si-ATP in the presence of CTAB. Surface functional groups and XRD analysis of SiATP indicate the similarity to zeolite with Si-O tetrahedra and 2θ 23.14°, respectively. The change in the Si-ATP/CTAB ratio makes the types of crystalline phases and the mesopores structure significantly improved. The synthesized (H) ZSM-11 zeolite owns less diffusion resistance due to straight channels instead of directly intersecting and sinusoidal channels of ZSM-5 zeolite, and when this ratio is between 80: 3 and 80: 5. The Si-ATP/CTAB ratio has a well linear dependence with hierarchy factor (HF) in the both lower and higher ranges, which illustrate the amount of CTAB plays an important role of the mesoporous construction. It is obvious that hierarchical (H) ZSM-11 zeolites dominated with straight channels and mesoporosity, when the Si-ATP/CTAB ratio is 80: 5, possess a higher adsorption capacity of MB wastewater. This result is further confirmed that hierarchical zeolites pose a huge potential on adsorption performance for dyeing wastewater. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21573171), the Shaanxi Provincial Natural Science Foundation of China (No. 17JK0327), and the Research and Innovation Training Project for Graduate in General Universities of Shaanxi Province of China (No. 1731).
(3)
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Where qe is the amount of adsorbed dye at equilibrium (mg g ), qm is the maximum of adsorption capacity (mg g−1), Ce is the concentration of dye solution at adsorption equilibrium (mg L−1), and kL is the Langmuir constant (L mg−1) related to adsorption energy. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL hat is given by the following equation [56]:
RL =
1 1 + kL⋅C0
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(4) −1
Where C0 (mg L ) is the initial adsorbate concentration in aqueous solution, kL (L mg−1) is the Langmuir constant, and RL indicates the type of isotherm to be irreversible (RL < 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1). The Freundlich isotherm model can be described in its linearized form as:
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(5)
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