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Al ratio of natural zeolite using acid treatment
Applied Surface Science 498 (2019) 143874
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Quantitative arrangement of Si/Al ratio of natural zeolite using acid treatment
T
Cheng Wang , Shaozheng Leng, Huidong Guo, Jiale Yu, Wenjie Li, Liyun Cao, Jianfeng Huang ⁎
⁎
School of Materials Science and Technology, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi'an 710021, China
ARTICLE INFO
ABSTRACT
Keywords: Quantitative arrangement SiO2/Al2O3 ratio Hydrophilicity/hydrophobicity Acid treatment
Acid treatment is an effective way to arrange SiO2/Al2O3 ratio of natural zeolite. However, limited researches have been done on quantitative regulation of SiO2/Al2O3 ratio of natural zeolite. In this paper, an orthogonal test was designed to optimize the acid treated conditions. Acid concentration, treated time and temperature were then employed to quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite, respectively. The results showed that the primary and secondary factors of acid treatment were acid treated time, acid concentration and acid treated temperature, respectively. The extent of dealumination of zeolite and its corresponding SiO2/Al2O3 ratio of natural zeolite could rise up to 98.4% and 372.7 under the optimized condition for 48 h, 8 M and 70 °C, respectively. Acid concentration, treated time and temperature could effective quantitatively arrange the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The as-prepared acid treated zeolites with different SiO2/Al2O3 ratio regulated by acid concentration had higher specific surface area, micropore volume, silanol group content and hydrophobicity than those for the natural zeolite.
1. Introduction Zeolites can be categorized in relation to the Si/Al ratio, i.e. low silica zeolites (Si/Al mole ratio = 1–2, SiO2/Al2O3 mass ratio = 1.18–2.35), medium silica zeolites (Si/Al mole ratio = 3–10, SiO2/Al2O3 mass ratio = 3.53–11.76) and high silica zeolites (Si/Al mole ratio = 10-∞, SiO2/Al2O3 mass ratio = 11.76-∞) [1]. The physical and chemical properties of zeolites vary by the proportion of SiO2/ Al2O3 ratio [2,3]. A low ratio causes surface of zeolite to be hydrophilic while a high ratio causes the surface to be hydrophobic [3,4]. Zeolites with low SiO2/Al2O3 ratio and high hydrophilicity exhibit relative stronger interaction with polar molecules, e.g. water [5] and those with high SiO2/Al2O3 ratio and high hydrophobicity exhibit relative stronger interaction with nonpolar molecules, e.g. VOCs [6,7]. The zeolites with different SiO2/Al2O3 ratio can be obtained via two ways, i.e. hydrothermal synthesis and post synthesis methods. The hydrothermal synthesis method is a direct and effectively way to prepare zeolites with different SiO2/Al2O3 ratios with an extremely wide range (1~∞). However, this method is inapplicable to arrange SiO2/ Al2O3 ratios for a specific zeolite e.g. natural zeolite. This problem can be solved by the post synthesis method, which is carried out by dealuminating or desilicating of the zeolite using acid or alkali treatment [4,6,8–10]. ⁎
Recently, we found that acid treatment using nitric acid could remove part of aluminum from framework of natural zeolite and thus improved SiO2/Al2O3 ratio of the zeolite. However, the dealumination efficiency of acid treatment in the above study was insufficient, e.g. only 5.1–23.3 wt% of aluminum was removed from natural zeolite (SiO2/Al2O3 mass ratio = 5.88) using 0.1–3 M nitric acid under 60 °C for 24 h and the corresponding SiO2/Al2O3 ratios of zeolites increased to 6.18–7.66, which still belonged to medium silica zeolites. Therefore, how to improve the dealumination efficiency and develop a way to quantitatively arrange the SiO2/Al2O3 ratio of zeolite are needed to be further investigated. The aim of this study is to further investigate the influence factors of acid treatment on SiO2/Al2O3 ratio of natural zeolite and find a way to quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite. For the above purpose, an orthogonal test with three operational factors, i.e. acid concentration, acid treated time and temperature, each in five levels were designed to optimize the acid treated condition. Acid concentration, treated time and temperature were then employed to quantitatively arrange the SiO2/Al2O3 mass ratio (SiO2/Al2O3 ratio) of natural zeolite. Structure and hydrophilicity/hydrophobicity of the samples were also determined.
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (J. Huang).
https://doi.org/10.1016/j.apsusc.2019.143874 Received 11 May 2019; Received in revised form 12 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 498 (2019) 143874
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2. Experimental
Table 1 Design factors and levels for the orthogonal L25 test.
2.1. Materials and methods
Parameters
Zeolite sample (located in Weichang region, Hebei province, China) was supplied by Beijing Guotou Shengshi Technology Co., Ltd. The mineralogical and chemical composition of natural zeolite can be found elsewhere [4]. Nitric acid (65%) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. 10.0 g pre-dried (60 °C, 24 h) zeolite powders and 100 mL nitric acid solution (1–8 M) were mixed in sealed glass conical flask and then placed in a shaking water bath (HZS-HA, Changzhou Guohua instrument Manufacturing Co., LTD, China) with shaking frequency of 150 rpm under different temperatures (40–80 °C) and times (2–48 h). Finally, the powders were separated from the mixture and washed with deionized water at least three times, and then dried at 60 °C for 24 h.
Symbol
Concentration (mol/L) Time (h) Temperature (°C)
A B C
Range and levels 1
2
3
4
5
1 2 40
2 6 50
4 12 60
6 24 70
8 48 80
orthogonal experiment, experimental design matrix and results, and analysis of the orthogonal test are summarized in Tables 1, 2 and 3, respectively. As shown in Table 2, the orthogonal experiment displays a wide range of aluminum removal rate and SiO2/Al2O3 ratio, i.e. 21.2–98.4% for aluminum removal rate and 7.4–372.7 for SiO2/Al2O3 ratio. The ranges are far more wide than our previous study with varied acid concentrations (0.1-3 M) under fixed treated time (12 h) and temperature (60 °C). This indicates that the dealumination efficiency of acid treatment is affected by the combined influences of acid concentration, treated time and temperature. It is clear that factors of the highest aluminum removal rate and SiO2/Al2O3 ratio are 98.4% and 372.7 (No. 25), respectively. Therefore, the optimized acid treated conditions are concluded as: acid concentration:acid treated time:acid treated temperature = 8 M:48 h:70 °C. A further orthogonal analysis was performed in order to determine the key influential factor and the results are shown in Table 3. Two experimental indexes i.e. aluminum removal rate and SiO2/Al2O3 ratio are used to evaluate the effect of acid treatment. The k and R, k′ and R′ are employed to analyze the orthogonal test. Where the k and k′ are average of data in one level of single factor (k for aluminum removal rate and k′ for SiO2/Al2O3 ratio), and the R and R′ are difference between the maximal value and minimal value of k and k′ (R for aluminum removal rate and R′ for SiO2/Al2O3 ratio). The detailed formulas are as follows:
2.2. Sample characterization An Optima 7300 V ICP-AES spectrometer (PerkinElmer, USA) was used to analyze contents of Si and Al in the acid extracted solution. Xray diffraction analysis was conducted using a Bruker AXS D8-Focus diffractometer (Bruker, Germany) with Cu Kα radiation and a graphite monochromator. The operating electric current and electric voltage was 40 mA and 40 kV, respectively. The step size and exposure time were 0.01° 2θ and 0.05 s per step, respectively. The relative crystallinity of the acid treated zeolites was determined by measuring the intensity of the diffraction signal of the (020) peak and comparing it to that of the zeolite. Specific surface area and pore size distribution analysis was conducted on a Gemini VII2390 automated physisorption analyzer (Micromeritics Instrument Corp., USA). The specific surface area was calculated from the nitrogen desorption data on the basis of the multipoint BET equation. The micropore volume (Vmicro) was estimated using the t-plot method. Micropore and mesopore size distributions were determined using the DFT and BJH model, respectively. TG analysis was performed on a TGA Q500 thermal analyzer (TA Instruments, USA) from room temperature to 800 °C at a heating rate of 10 K/min. Fourier transform infrared (FTIR) spectra of the samples were collected by a FTIR A VECTOR-22 FTIR spectrometer (Bruker Corporation, Karlsruhe, Germany) and the wave number region of 4000–400 cm−1, using a KBr pressed disk technique. UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained with a Cary 500 spectrometer (Varian, USA) and the spectra were recorded in the 200–600 nm range.
k = ( aluminum removal rate of single factor)/5
(1)
R = max k
(2)
min k
k = ( SiO2 /Al2 O3 ratio of single factor)/5
(3)
R = max k
(4)
min k
The high R and R′ values mean that the corresponding factor is the major factor on the impact of indicator [12]. As shown in Table 3, the values of R and R′ for each influential factor have small differences and this suggests that the three factors, i.e. acid concentration, treated time and temperature are more or less equal in influence on the aluminum removal rate and SiO2/Al2O3 ratio of zeolite. It shows that the primary and secondary factors evaluated by the index of aluminum removal rate are factor C (temperature) > factor B (time) > factor A (concentration), respectively. This suggests that the optimized acid treated conditions of aluminum removal rate are C5B5A5 for 80 °C, 48 h and 8 M, respectively. However, result evaluated by the index of SiO2/Al2O3 ratio significantly changes as comparison of the above one. The primary and secondary factors are factor B (time) > factor A (concentration) > factor C (temperature), respectively. Therefore, the optimize acid treated conditions of SiO2/ Al2O3 ratio are B5A5C4 for 48 h, 8 M and 70 °C, respectively. The above analysis indicates that high acid concentration, long acid treated time and high temperature are beneficial for aluminum removal. However, excessive temperature can increase the silicon removal rate and thus would decrease SiO2/Al2O3 ratio of zeolite.
2.3. Hydrophilicity/hydrophobicity determination Water vapor adsorption method was used to determine hydrophilicity/hydrophobicity of the samples. The detailed description of the procedure can be found elsewhere [4]. About 1.0 g of powders on weighing dish was put in a BPS-50CL temperature humidity chamber (Shanghai Yiheng Technical Co., Ltd., China) under atmospheric conditions at 30 °C, 70% humidity. Weight changes of powders were tested after 48 h of adsorption. Hydrophilicity/hydrophobicity of the samples was determined using the following formulas [11]: Wmm = Wmg/S, (1), where Wmm is the water vapor adsorption per unit area, mg/m2; Wmg is the water vapor adsorption per unit mass, mg/g; S is specific surface area of the sample, m2/g. 3. Results and discussions 3.1. Optimization of acid treated condition An L25(35) orthogonal test with three operational factors, i.e. acid concentration (A), acid treated time (B) and acid treated temperature (C), each in five levels were designed and analyzed by orthogonality experiment assistant II 3.1 (software). The factors and levels of the
3.2. Quantitatively arrangement of SiO2/Al2O3 ratio of natural zeolite In consideration of 2
the
positive
correlation of
extent of
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Table 2 Experimental design matrix and results for the orthogonal L25 test. No.
A Concentration (mol/L)
B Time (h)
C Temperature (°C)
Aluminum removal rate (%)
Silicon removal rate (%)
SiO2/Al2O3 mass ratio
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
A1 A1 A1 A1 A1 A2 A2 A2 A2 A2 A3 A3 A3 A3 A3 A4 A4 A4 A4 A4 A5 A5 A5 A5 A5
B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5
C1 C2 C3 C4 C5 C2 C3 C4 C5 C1 C3 C4 C5 C1 C2 C4 C5 C1 C2 C3 C5 C1 C2 C3 C4
21.2 34.3 46.0 67.1 79.1 38.9 51.1 55.5 76.6 48.3 47.5 55.8 73.4 53.7 66.0 56.4 71.9 54.8 57.2 85.6 73.7 49.0 65.3 81.3 98.4
0.30 0.35 0.36 0.46 0.59 0.27 0.24 0.27 0.41 0.16 0.11 0.11 0.17 0.07 0.09 0.06 0.06 0.03 0.03 0.05 0.02 0.01 0.02 0.02 0.03
7.4 8.9 10.9 17.8 27.9 9.6 12.0 13.2 25.1 11.4 11.2 13.3 22.1 12.7 17.3 13.5 20.9 13.0 13.7 40.8 22.3 11.5 16.9 31.5 372.7
1 1 1 1 1 2 2 2 2 2 4 4 4 4 4 6 6 6 6 6 8 8 8 8 8
2 6 12 24 48 2 6 12 24 48 2 6 12 24 48 2 6 12 24 48 2 6 12 24 48
40 50 60 70 80 50 60 70 80 40 60 70 80 40 50 70 80 40 50 60 80 40 50 60 70
extent of dealumination of zeolite of zeolite. The fitted equations all have high correlation coefficients (R2 ≥ 0.94) which indicate that acid concentration, treated time and treated temperature can quantitatively regulate the extent of dealumination of zeolite of natural zeolite. The nonlinear equations fitted for relationship of acid concentration (treated time and temperature) and SiO2/Al2O3 ratio can be expressed as y = a*bx (x = C, t, T), where y is the SiO2/Al2O3 ratio of zeolite. The differences between these fitted equations (C, t, T vs SiO2/Al2O3 ratio) and the above corresponding fitted equations (C, t, T vs dealumination rate) are mainly attributed to the desilication behavior of zeolites. The fitted equations for acid concentration and treated time both have relative high correlation coefficients (R2 ≥ 0.8501) which indicate that acid concentration, treated time and treated temperature can quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite. The fitted equations and their conditions for quantitatively arrangement the Aluminum removal rate and SiO2/Al2O3 ratio are concluded in Table 4.
Table 3 Analysis of orthogonal L25 test. Experimental indexes Aluminum removal rate (%)
SiO2/Al2O3 mass ratio
k1 k2 k3 k4 k5 R Primary and secondary factors Optimization scheme k′1 k′2 k′3 k′4 k′5 R′ Primary and secondary factors Optimization scheme
Concentration (mol/L)
Time (h)
Temperature (°C)
49.54 54.08 59.28 65.18 73.54 24.00 CBA
47.54 52.42 59.00 67.18 75.48 27.94
45.4 52.34 62.30 66.64 74.94 29.54
12.80 13.32 15.22 20.16 94.02 81.22
11.20 13.28 21.28 86.10 23.66 74.90
C5B5A5 14.58 14.26 15.32 20.38 90.98 76.72 BAC
3.3. Structure and hydrophilicity/hydrophobicity of the zeolite samples In purpose of investigating the structure, hydrophilicity/hydrophobicity and their changes of zeolites samples with different SiO2/ AlO3 ratio after acid treatment, the XRD analysis, UV–vis DRS analysis, FTIR analysis, TG analysis, N2 adsorption-desorption technique, and water vapor adsorption method were employed. The zeolite samples treated with different acid concentration (1–8 M) were adopted due to the relative good quantitative regulation effect of acid concentration on extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The zeolite and acid treated zeolites samples were denoted as NZ, NZ-1, NZ-2, NZ-4, NZ-6, NZ-8, respectively.
B5A5C4
dealumination of zeolite (SiO2/Al2O3 ratio) and acid concentration (treated time and temperature), relationship of the parameters was established in purpose of quantitative arrangement the SiO2/Al2O3 ratio of natural zeolite, and the results are shown in Fig. 1. For the acid concentration, treated time and temperature investigations, the corresponding conditions are fixed on 70 °C and 48 h, 6 M and 70 °C, 6 M and 48 h, respectively. From Fig. 1, it is observed that the acid concentration (treated time and temperature) follows nonlinear dependence with the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The nonlinear equations fitted for the relationship of acid concentration (treated time and temperature) and extent of dealumination of zeolite can be expressed as y = a-b*cx (x = C, t, T), where C is the acid concentration; t is the treated time; T is the treated temperature; y is the
3.3.1. Structure of the zeolite samples Fig. 2 shows the XRD patterns (a) and relative crystallinity (b), UV–vis DRS spectra (c), FTIR spectra (d) of zeolite and acid treated zeolites with different acid concentrations. XRD patterns of the samples in Fig. 2a show that some typical characteristic peaks of clinoptilolite, e.g. (400) and (020) peaks for acid treated zeolites decrease with the increase of acid concentration. The relative crystallinity of clinoptilolite evaluated by intensity of (020) peak greatly decreases with the increase 3
Applied Surface Science 498 (2019) 143874
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Fig. 1. Dealumination and of zeolite as a function of acid concentration (a), SiO2/Al2O3 ratio of zeolite as a function of acid concentration (b), dealumination of zeolite as a function of acid treated time (c), SiO2/Al2O3 ratio of zeolite as a function of acid treated time (d), dealumination of zeolite as a function of acid treated temperature (e), SiO2/Al2O3 ratio of zeolite as a function of acid treated temperature (f).
of acid concentration and then keeps steady at the concentration of ≥4 M (see Fig. 2b), which suggests the reduction of crystallinity of clinoptilolite. UV–vis DRS spectra of the samples in Fig. 2c show that
intensity of peaks centered at 258 nm and 366 nm (assigned to absorptions of tetrahedral framework AleO bond and octahedral extraframework AleO bond, respectively [13,14]) for the acid treated
Table 4 Fitted equations for quantitatively arrangement the Aluminum removal rate and SiO2/Al2O3 ratio. Experimental index
Fitted equation
Condition
x
Aluminum removal rate (%)
y = a-b*c (x = C, t, T)
SiO2/Al2O3 mass ratio
y = a*bx (x = C, t, T)
x = C, t = 48 h, T = 70 °C, C = 1–8 M; a = 103.36, b = 45.33, c = 0.73 x = t, C = 6 M, T = 70 °C, t = 2–48 h; a = 103.53, b = 53.15, c = 0.95 x = T, C = 6 h, t = 48 h, T = 30–80 °C; a = 114.94, b = 226.88, c = 0.97 x = C, t = 48 h, T = 70 °C, C = 1–8 M; a = 24.78, b = 1.41 x = t, C = 6 M, T = 70 °C, t = 2–48 h; a = 14.82, b = 1.06 x = T, C = 6 h, t = 48 h, T = 30–80 °C; a = 1.65, b = 1.07
4
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Fig. 2. XRD patterns (a) and relative crystallinity (b), UV–vis DRS spectra (c), FTIR spectra (d) of zeolite and acid treated zeolites with different acid concentrations.
zeolites decrease as compared with those for the zeolite sample, which confirms the removal of both tetrahedral framework and octahedral extra-framework aluminum from the zeolite. FTIR spectra of the samples in Fig. 2d show that intensity of the peak at 3625 cm−1 (assigned to Si-O(H)-Al bond) for acid treated zeolites obviously decreases while the peak at 1053 cm−1 (assigned to T-O bond, T=Si and Al) for zeolite shifts to 1047 cm−1 after acid treatment, which further confirms the removal of aluminum from the zeolite. In addition, the intensity of peak at 3425 cm−1 (assigned to Si-OH in nest defects) for acid treated zeolites somehow increases, which suggests the increase of Si-OH groups due to the formation of aluminum vancanies [4,13]. In order to further verify the increase of Si-OH groups after acid treatment, TG analysis was conducted and the results are shown in
Fig. 3. It is clear that the total weight losses of acid treated zeolites are lower than that of zeolite sample, and the low weight losses are mainly owing to the weight losses at 100–400 °C dehydration stage. Based on the type of the dehydration stage [9,15], the low weight losses are probably due to the decrease of aluminum in the (extra) framework of zeolite after acid treatment. In addition, the weight losses at > 400 °C dehydration stage for the acid treated zeolites are higher than that for the zeolite sample and the weight losses gently increase with the increase of acid concentration, and this result further confirms the increase of Si-OH groups in zeolite on the effect of dealumination [4,16]. Fig. 4 shows the N2 adsorption-desorption isotherms and pore size distribution of zeolite and acid treated zeolites. The slight increases of N2 adsorption under low P/P0 (~0.005) and obvious hysteresis loops
Fig. 3. TG curves of the zeolite and acid treated zeolite, and relative weight loss percentage of water molecules from samples at each dehydration stage. 5
Applied Surface Science 498 (2019) 143874
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Fig. 4. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of zeolite and acid treated zeolites.
(IV and H4 types) in the isotherms (see Fig. 4a) suggest that the zeolite samples contain slit-like mesopores with few of micropores. Comparison of isotherms for zeolite and acid treated zeolites show that notable increases of N2 adsorption under low P/P0 appear in the isotherms for acid treated zeolites while only slight changes of N2 adsorption under high P/P0 are found, which suggests that acid treatment significantly increase the micropores in the zeolite while it has negligible effect on the mesopores. The pore size distributions in Fig. 4b show that the amount of micropores on 0.7 nm deceases while those on 1.2 nm increase with the increase of acid concentration. In addition, the amount of mesopores in the acid treated zeolites remain unchanged compared with that in the zeolite sample. The specific surface area and pore volume of the zeolite samples in Table 4 show that the specific surface area SBET of the acid treated zeolites greatly increase as compared with that of the zeolite sample (27.67 m2/g). The increases of SBET are owing to the increases of Smicro with slight increases of Sexternal. The pore volume results have the same trend as the specific surface area results (Table 5).
Fig. 5. Water vapor adsorptions per unit area on zeolite and acid treated zeolites.
3.3.2. Hydrophilicity/hydrophobicity of the zeolite samples Fig. 5 shows the water vapor adsorptions per unit area on zeolite and acid treated zeolites with different SiO2/Al2O3 ratio. It is clear that water vapor adsorptions on the acid treated zeolites with relative high SiO2/Al2O3 ratio are significantly lower than that on the zeolite sample, which suggests the increase of hydrophobicity for those zeolites with high SiO2/Al2O3 ratio. However, it is found that hydrophobicity of the acid treated zeolites with different SiO2/Al2O3 ratio are much the same and higher SiO2/Al2O3 ratio does not correspond to higher hydrophobicity. The reason is attributed to the combined influences of SiO2/ Al2O3 ratio and hydrophilic silanol groups. On the one hand, acid treatment with increasing acid concentration causes the increase of SiO2/Al2O3 ratio (decrease of polar Si-O-Al group); On the other hand, acid treatment with increasing acid concentration also causes the
increase of polar silanol groups. 4. Conclusions An L25 orthogonal test with three operational factors, i.e. acid concentration, time and temperature, each in five levels were designed to optimize the acid treated condition. The factors for acid concentration (treated time and temperature) were then employed to quantitatively regulate the SiO2/Al2O3 ratio of natural zeolite. The following conclusions have been obtained: (1) The primary and secondary factors based on the index of aluminum removal rate and SiO2/Al2O3 ratio are temperature > time > concentration and time > concentration > temperature, respectively, and the optimized acid treated conditions for aluminum removal rate and SiO2/Al2O3 ratio are 80 °C:48 h:8 M and 70 °C:48 h:8 M, respectively. (2) Acid concentration, treated time and temperature could effective quantitatively regulate the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite, and the equations for regulation of SiO2/Al2O3 ratio based on acid concentration, treated time and temperature are y = a-b*cx (x = C, t, T) and y = a*bx (x = C, t, T), respectively. (3) The acid treated zeolites with different SiO2/Al2O3 ratio (20.0–372.7) had higher specific surface area (> 124.13 m2/g), micropore volume (> 0.046 cm3/g), silanol group content
Table 5 Specific surface area and pore volume of zeolite and acid treated zeolites. Samples
Smicroa (m2/g)
Sexternala (m2/ g)
SBETb (m2/ g)
Vmicroa (cm3/g)
Vmesoc (cm3/g)
NZ NZ-1 NZ-2 NZ-4 NZ-6 NZ-8
5.16 115.42 88.82 100.65 108.62 94.51
22.52 49.54 35.31 43.91 38.97 40.77
27.67 164.97 124.13 144.56 147.59 135.27
0.0024 0.0570 0.0460 0.0497 0.0537 0.0492
0.0882 0.1171 0.0913 0.0965 0.0838 0.0832
a b c
By t-plot method. By BET method. By BJH method. 6
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(> 1.7606%) and hydrophobicity (< 0.3077 mg/m2) than those for the natural zeolite with SiO2/Al2O3 ratio for 5.9, specific surface area for 27.67 m2/g, micropore volume for 0.0024 cm3/g, silanol group content for 1.6465% and hydrophobicity for 0.9359 mg/m2, respectively. However, crystallinity of the aid treated zeolites obviously decreased as compared with that of the raw zeolite.
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Future works to regulate the silanol group content in the zeolite and increase the crystallinity of the zeolite need to be investigated. Acknowledgments The project supported by National Natural Science Foundation of China (No. 51604170), China Postdoctoral Science Foundation (No. 2016M602936XB), Natural Science Foundation of Shaanxi Province (No. 2018JM5025), Foundation for Selected Overseas Chinese Scholar (No. 2018043), Shaanxi Province Postdoctoral Science Foundation and Wenzhou Science and Technology Foundation of China (No. S20180008), National Undergraduate Innovation and Entrepreneurship Training Program (No. 201810708042). References [1] S.M. Csicsery, Catalysis by shape selective zeolites-science and technology, Pure Appl. Chem. 58 (1986) 841–856. [2] Y. Li, L. Li, J.H. Yu, Applications of zeolites in sustainable chemistry, Chem 36 (2017) 928–949. [3] N. Jiang, R. Shang, S.G.J. Heijman, L.C. Rietveld, High-silica zeolites for adsorption of organic micro-pollutants in water treatment: a review, Water Res. 144 (2018) 145–161.
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Quantitative arrangement of Si/Al ratio of natural zeolite using acid treatment
T
Cheng Wang , Shaozheng Leng, Huidong Guo, Jiale Yu, Wenjie Li, Liyun Cao, Jianfeng Huang ⁎
⁎
School of Materials Science and Technology, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi'an 710021, China
ARTICLE INFO
ABSTRACT
Keywords: Quantitative arrangement SiO2/Al2O3 ratio Hydrophilicity/hydrophobicity Acid treatment
Acid treatment is an effective way to arrange SiO2/Al2O3 ratio of natural zeolite. However, limited researches have been done on quantitative regulation of SiO2/Al2O3 ratio of natural zeolite. In this paper, an orthogonal test was designed to optimize the acid treated conditions. Acid concentration, treated time and temperature were then employed to quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite, respectively. The results showed that the primary and secondary factors of acid treatment were acid treated time, acid concentration and acid treated temperature, respectively. The extent of dealumination of zeolite and its corresponding SiO2/Al2O3 ratio of natural zeolite could rise up to 98.4% and 372.7 under the optimized condition for 48 h, 8 M and 70 °C, respectively. Acid concentration, treated time and temperature could effective quantitatively arrange the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The as-prepared acid treated zeolites with different SiO2/Al2O3 ratio regulated by acid concentration had higher specific surface area, micropore volume, silanol group content and hydrophobicity than those for the natural zeolite.
1. Introduction Zeolites can be categorized in relation to the Si/Al ratio, i.e. low silica zeolites (Si/Al mole ratio = 1–2, SiO2/Al2O3 mass ratio = 1.18–2.35), medium silica zeolites (Si/Al mole ratio = 3–10, SiO2/Al2O3 mass ratio = 3.53–11.76) and high silica zeolites (Si/Al mole ratio = 10-∞, SiO2/Al2O3 mass ratio = 11.76-∞) [1]. The physical and chemical properties of zeolites vary by the proportion of SiO2/ Al2O3 ratio [2,3]. A low ratio causes surface of zeolite to be hydrophilic while a high ratio causes the surface to be hydrophobic [3,4]. Zeolites with low SiO2/Al2O3 ratio and high hydrophilicity exhibit relative stronger interaction with polar molecules, e.g. water [5] and those with high SiO2/Al2O3 ratio and high hydrophobicity exhibit relative stronger interaction with nonpolar molecules, e.g. VOCs [6,7]. The zeolites with different SiO2/Al2O3 ratio can be obtained via two ways, i.e. hydrothermal synthesis and post synthesis methods. The hydrothermal synthesis method is a direct and effectively way to prepare zeolites with different SiO2/Al2O3 ratios with an extremely wide range (1~∞). However, this method is inapplicable to arrange SiO2/ Al2O3 ratios for a specific zeolite e.g. natural zeolite. This problem can be solved by the post synthesis method, which is carried out by dealuminating or desilicating of the zeolite using acid or alkali treatment [4,6,8–10]. ⁎
Recently, we found that acid treatment using nitric acid could remove part of aluminum from framework of natural zeolite and thus improved SiO2/Al2O3 ratio of the zeolite. However, the dealumination efficiency of acid treatment in the above study was insufficient, e.g. only 5.1–23.3 wt% of aluminum was removed from natural zeolite (SiO2/Al2O3 mass ratio = 5.88) using 0.1–3 M nitric acid under 60 °C for 24 h and the corresponding SiO2/Al2O3 ratios of zeolites increased to 6.18–7.66, which still belonged to medium silica zeolites. Therefore, how to improve the dealumination efficiency and develop a way to quantitatively arrange the SiO2/Al2O3 ratio of zeolite are needed to be further investigated. The aim of this study is to further investigate the influence factors of acid treatment on SiO2/Al2O3 ratio of natural zeolite and find a way to quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite. For the above purpose, an orthogonal test with three operational factors, i.e. acid concentration, acid treated time and temperature, each in five levels were designed to optimize the acid treated condition. Acid concentration, treated time and temperature were then employed to quantitatively arrange the SiO2/Al2O3 mass ratio (SiO2/Al2O3 ratio) of natural zeolite. Structure and hydrophilicity/hydrophobicity of the samples were also determined.
Corresponding authors. E-mail addresses: [email protected] (C. Wang), [email protected] (J. Huang).
https://doi.org/10.1016/j.apsusc.2019.143874 Received 11 May 2019; Received in revised form 12 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
Table 1 Design factors and levels for the orthogonal L25 test.
2.1. Materials and methods
Parameters
Zeolite sample (located in Weichang region, Hebei province, China) was supplied by Beijing Guotou Shengshi Technology Co., Ltd. The mineralogical and chemical composition of natural zeolite can be found elsewhere [4]. Nitric acid (65%) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. 10.0 g pre-dried (60 °C, 24 h) zeolite powders and 100 mL nitric acid solution (1–8 M) were mixed in sealed glass conical flask and then placed in a shaking water bath (HZS-HA, Changzhou Guohua instrument Manufacturing Co., LTD, China) with shaking frequency of 150 rpm under different temperatures (40–80 °C) and times (2–48 h). Finally, the powders were separated from the mixture and washed with deionized water at least three times, and then dried at 60 °C for 24 h.
Symbol
Concentration (mol/L) Time (h) Temperature (°C)
A B C
Range and levels 1
2
3
4
5
1 2 40
2 6 50
4 12 60
6 24 70
8 48 80
orthogonal experiment, experimental design matrix and results, and analysis of the orthogonal test are summarized in Tables 1, 2 and 3, respectively. As shown in Table 2, the orthogonal experiment displays a wide range of aluminum removal rate and SiO2/Al2O3 ratio, i.e. 21.2–98.4% for aluminum removal rate and 7.4–372.7 for SiO2/Al2O3 ratio. The ranges are far more wide than our previous study with varied acid concentrations (0.1-3 M) under fixed treated time (12 h) and temperature (60 °C). This indicates that the dealumination efficiency of acid treatment is affected by the combined influences of acid concentration, treated time and temperature. It is clear that factors of the highest aluminum removal rate and SiO2/Al2O3 ratio are 98.4% and 372.7 (No. 25), respectively. Therefore, the optimized acid treated conditions are concluded as: acid concentration:acid treated time:acid treated temperature = 8 M:48 h:70 °C. A further orthogonal analysis was performed in order to determine the key influential factor and the results are shown in Table 3. Two experimental indexes i.e. aluminum removal rate and SiO2/Al2O3 ratio are used to evaluate the effect of acid treatment. The k and R, k′ and R′ are employed to analyze the orthogonal test. Where the k and k′ are average of data in one level of single factor (k for aluminum removal rate and k′ for SiO2/Al2O3 ratio), and the R and R′ are difference between the maximal value and minimal value of k and k′ (R for aluminum removal rate and R′ for SiO2/Al2O3 ratio). The detailed formulas are as follows:
2.2. Sample characterization An Optima 7300 V ICP-AES spectrometer (PerkinElmer, USA) was used to analyze contents of Si and Al in the acid extracted solution. Xray diffraction analysis was conducted using a Bruker AXS D8-Focus diffractometer (Bruker, Germany) with Cu Kα radiation and a graphite monochromator. The operating electric current and electric voltage was 40 mA and 40 kV, respectively. The step size and exposure time were 0.01° 2θ and 0.05 s per step, respectively. The relative crystallinity of the acid treated zeolites was determined by measuring the intensity of the diffraction signal of the (020) peak and comparing it to that of the zeolite. Specific surface area and pore size distribution analysis was conducted on a Gemini VII2390 automated physisorption analyzer (Micromeritics Instrument Corp., USA). The specific surface area was calculated from the nitrogen desorption data on the basis of the multipoint BET equation. The micropore volume (Vmicro) was estimated using the t-plot method. Micropore and mesopore size distributions were determined using the DFT and BJH model, respectively. TG analysis was performed on a TGA Q500 thermal analyzer (TA Instruments, USA) from room temperature to 800 °C at a heating rate of 10 K/min. Fourier transform infrared (FTIR) spectra of the samples were collected by a FTIR A VECTOR-22 FTIR spectrometer (Bruker Corporation, Karlsruhe, Germany) and the wave number region of 4000–400 cm−1, using a KBr pressed disk technique. UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained with a Cary 500 spectrometer (Varian, USA) and the spectra were recorded in the 200–600 nm range.
k = ( aluminum removal rate of single factor)/5
(1)
R = max k
(2)
min k
k = ( SiO2 /Al2 O3 ratio of single factor)/5
(3)
R = max k
(4)
min k
The high R and R′ values mean that the corresponding factor is the major factor on the impact of indicator [12]. As shown in Table 3, the values of R and R′ for each influential factor have small differences and this suggests that the three factors, i.e. acid concentration, treated time and temperature are more or less equal in influence on the aluminum removal rate and SiO2/Al2O3 ratio of zeolite. It shows that the primary and secondary factors evaluated by the index of aluminum removal rate are factor C (temperature) > factor B (time) > factor A (concentration), respectively. This suggests that the optimized acid treated conditions of aluminum removal rate are C5B5A5 for 80 °C, 48 h and 8 M, respectively. However, result evaluated by the index of SiO2/Al2O3 ratio significantly changes as comparison of the above one. The primary and secondary factors are factor B (time) > factor A (concentration) > factor C (temperature), respectively. Therefore, the optimize acid treated conditions of SiO2/ Al2O3 ratio are B5A5C4 for 48 h, 8 M and 70 °C, respectively. The above analysis indicates that high acid concentration, long acid treated time and high temperature are beneficial for aluminum removal. However, excessive temperature can increase the silicon removal rate and thus would decrease SiO2/Al2O3 ratio of zeolite.
2.3. Hydrophilicity/hydrophobicity determination Water vapor adsorption method was used to determine hydrophilicity/hydrophobicity of the samples. The detailed description of the procedure can be found elsewhere [4]. About 1.0 g of powders on weighing dish was put in a BPS-50CL temperature humidity chamber (Shanghai Yiheng Technical Co., Ltd., China) under atmospheric conditions at 30 °C, 70% humidity. Weight changes of powders were tested after 48 h of adsorption. Hydrophilicity/hydrophobicity of the samples was determined using the following formulas [11]: Wmm = Wmg/S, (1), where Wmm is the water vapor adsorption per unit area, mg/m2; Wmg is the water vapor adsorption per unit mass, mg/g; S is specific surface area of the sample, m2/g. 3. Results and discussions 3.1. Optimization of acid treated condition An L25(35) orthogonal test with three operational factors, i.e. acid concentration (A), acid treated time (B) and acid treated temperature (C), each in five levels were designed and analyzed by orthogonality experiment assistant II 3.1 (software). The factors and levels of the
3.2. Quantitatively arrangement of SiO2/Al2O3 ratio of natural zeolite In consideration of 2
the
positive
correlation of
extent of
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Table 2 Experimental design matrix and results for the orthogonal L25 test. No.
A Concentration (mol/L)
B Time (h)
C Temperature (°C)
Aluminum removal rate (%)
Silicon removal rate (%)
SiO2/Al2O3 mass ratio
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
A1 A1 A1 A1 A1 A2 A2 A2 A2 A2 A3 A3 A3 A3 A3 A4 A4 A4 A4 A4 A5 A5 A5 A5 A5
B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5 B1 B2 B3 B4 B5
C1 C2 C3 C4 C5 C2 C3 C4 C5 C1 C3 C4 C5 C1 C2 C4 C5 C1 C2 C3 C5 C1 C2 C3 C4
21.2 34.3 46.0 67.1 79.1 38.9 51.1 55.5 76.6 48.3 47.5 55.8 73.4 53.7 66.0 56.4 71.9 54.8 57.2 85.6 73.7 49.0 65.3 81.3 98.4
0.30 0.35 0.36 0.46 0.59 0.27 0.24 0.27 0.41 0.16 0.11 0.11 0.17 0.07 0.09 0.06 0.06 0.03 0.03 0.05 0.02 0.01 0.02 0.02 0.03
7.4 8.9 10.9 17.8 27.9 9.6 12.0 13.2 25.1 11.4 11.2 13.3 22.1 12.7 17.3 13.5 20.9 13.0 13.7 40.8 22.3 11.5 16.9 31.5 372.7
1 1 1 1 1 2 2 2 2 2 4 4 4 4 4 6 6 6 6 6 8 8 8 8 8
2 6 12 24 48 2 6 12 24 48 2 6 12 24 48 2 6 12 24 48 2 6 12 24 48
40 50 60 70 80 50 60 70 80 40 60 70 80 40 50 70 80 40 50 60 80 40 50 60 70
extent of dealumination of zeolite of zeolite. The fitted equations all have high correlation coefficients (R2 ≥ 0.94) which indicate that acid concentration, treated time and treated temperature can quantitatively regulate the extent of dealumination of zeolite of natural zeolite. The nonlinear equations fitted for relationship of acid concentration (treated time and temperature) and SiO2/Al2O3 ratio can be expressed as y = a*bx (x = C, t, T), where y is the SiO2/Al2O3 ratio of zeolite. The differences between these fitted equations (C, t, T vs SiO2/Al2O3 ratio) and the above corresponding fitted equations (C, t, T vs dealumination rate) are mainly attributed to the desilication behavior of zeolites. The fitted equations for acid concentration and treated time both have relative high correlation coefficients (R2 ≥ 0.8501) which indicate that acid concentration, treated time and treated temperature can quantitatively arrange the SiO2/Al2O3 ratio of natural zeolite. The fitted equations and their conditions for quantitatively arrangement the Aluminum removal rate and SiO2/Al2O3 ratio are concluded in Table 4.
Table 3 Analysis of orthogonal L25 test. Experimental indexes Aluminum removal rate (%)
SiO2/Al2O3 mass ratio
k1 k2 k3 k4 k5 R Primary and secondary factors Optimization scheme k′1 k′2 k′3 k′4 k′5 R′ Primary and secondary factors Optimization scheme
Concentration (mol/L)
Time (h)
Temperature (°C)
49.54 54.08 59.28 65.18 73.54 24.00 CBA
47.54 52.42 59.00 67.18 75.48 27.94
45.4 52.34 62.30 66.64 74.94 29.54
12.80 13.32 15.22 20.16 94.02 81.22
11.20 13.28 21.28 86.10 23.66 74.90
C5B5A5 14.58 14.26 15.32 20.38 90.98 76.72 BAC
3.3. Structure and hydrophilicity/hydrophobicity of the zeolite samples In purpose of investigating the structure, hydrophilicity/hydrophobicity and their changes of zeolites samples with different SiO2/ AlO3 ratio after acid treatment, the XRD analysis, UV–vis DRS analysis, FTIR analysis, TG analysis, N2 adsorption-desorption technique, and water vapor adsorption method were employed. The zeolite samples treated with different acid concentration (1–8 M) were adopted due to the relative good quantitative regulation effect of acid concentration on extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The zeolite and acid treated zeolites samples were denoted as NZ, NZ-1, NZ-2, NZ-4, NZ-6, NZ-8, respectively.
B5A5C4
dealumination of zeolite (SiO2/Al2O3 ratio) and acid concentration (treated time and temperature), relationship of the parameters was established in purpose of quantitative arrangement the SiO2/Al2O3 ratio of natural zeolite, and the results are shown in Fig. 1. For the acid concentration, treated time and temperature investigations, the corresponding conditions are fixed on 70 °C and 48 h, 6 M and 70 °C, 6 M and 48 h, respectively. From Fig. 1, it is observed that the acid concentration (treated time and temperature) follows nonlinear dependence with the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite. The nonlinear equations fitted for the relationship of acid concentration (treated time and temperature) and extent of dealumination of zeolite can be expressed as y = a-b*cx (x = C, t, T), where C is the acid concentration; t is the treated time; T is the treated temperature; y is the
3.3.1. Structure of the zeolite samples Fig. 2 shows the XRD patterns (a) and relative crystallinity (b), UV–vis DRS spectra (c), FTIR spectra (d) of zeolite and acid treated zeolites with different acid concentrations. XRD patterns of the samples in Fig. 2a show that some typical characteristic peaks of clinoptilolite, e.g. (400) and (020) peaks for acid treated zeolites decrease with the increase of acid concentration. The relative crystallinity of clinoptilolite evaluated by intensity of (020) peak greatly decreases with the increase 3
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Fig. 1. Dealumination and of zeolite as a function of acid concentration (a), SiO2/Al2O3 ratio of zeolite as a function of acid concentration (b), dealumination of zeolite as a function of acid treated time (c), SiO2/Al2O3 ratio of zeolite as a function of acid treated time (d), dealumination of zeolite as a function of acid treated temperature (e), SiO2/Al2O3 ratio of zeolite as a function of acid treated temperature (f).
of acid concentration and then keeps steady at the concentration of ≥4 M (see Fig. 2b), which suggests the reduction of crystallinity of clinoptilolite. UV–vis DRS spectra of the samples in Fig. 2c show that
intensity of peaks centered at 258 nm and 366 nm (assigned to absorptions of tetrahedral framework AleO bond and octahedral extraframework AleO bond, respectively [13,14]) for the acid treated
Table 4 Fitted equations for quantitatively arrangement the Aluminum removal rate and SiO2/Al2O3 ratio. Experimental index
Fitted equation
Condition
x
Aluminum removal rate (%)
y = a-b*c (x = C, t, T)
SiO2/Al2O3 mass ratio
y = a*bx (x = C, t, T)
x = C, t = 48 h, T = 70 °C, C = 1–8 M; a = 103.36, b = 45.33, c = 0.73 x = t, C = 6 M, T = 70 °C, t = 2–48 h; a = 103.53, b = 53.15, c = 0.95 x = T, C = 6 h, t = 48 h, T = 30–80 °C; a = 114.94, b = 226.88, c = 0.97 x = C, t = 48 h, T = 70 °C, C = 1–8 M; a = 24.78, b = 1.41 x = t, C = 6 M, T = 70 °C, t = 2–48 h; a = 14.82, b = 1.06 x = T, C = 6 h, t = 48 h, T = 30–80 °C; a = 1.65, b = 1.07
4
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Fig. 2. XRD patterns (a) and relative crystallinity (b), UV–vis DRS spectra (c), FTIR spectra (d) of zeolite and acid treated zeolites with different acid concentrations.
zeolites decrease as compared with those for the zeolite sample, which confirms the removal of both tetrahedral framework and octahedral extra-framework aluminum from the zeolite. FTIR spectra of the samples in Fig. 2d show that intensity of the peak at 3625 cm−1 (assigned to Si-O(H)-Al bond) for acid treated zeolites obviously decreases while the peak at 1053 cm−1 (assigned to T-O bond, T=Si and Al) for zeolite shifts to 1047 cm−1 after acid treatment, which further confirms the removal of aluminum from the zeolite. In addition, the intensity of peak at 3425 cm−1 (assigned to Si-OH in nest defects) for acid treated zeolites somehow increases, which suggests the increase of Si-OH groups due to the formation of aluminum vancanies [4,13]. In order to further verify the increase of Si-OH groups after acid treatment, TG analysis was conducted and the results are shown in
Fig. 3. It is clear that the total weight losses of acid treated zeolites are lower than that of zeolite sample, and the low weight losses are mainly owing to the weight losses at 100–400 °C dehydration stage. Based on the type of the dehydration stage [9,15], the low weight losses are probably due to the decrease of aluminum in the (extra) framework of zeolite after acid treatment. In addition, the weight losses at > 400 °C dehydration stage for the acid treated zeolites are higher than that for the zeolite sample and the weight losses gently increase with the increase of acid concentration, and this result further confirms the increase of Si-OH groups in zeolite on the effect of dealumination [4,16]. Fig. 4 shows the N2 adsorption-desorption isotherms and pore size distribution of zeolite and acid treated zeolites. The slight increases of N2 adsorption under low P/P0 (~0.005) and obvious hysteresis loops
Fig. 3. TG curves of the zeolite and acid treated zeolite, and relative weight loss percentage of water molecules from samples at each dehydration stage. 5
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Fig. 4. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of zeolite and acid treated zeolites.
(IV and H4 types) in the isotherms (see Fig. 4a) suggest that the zeolite samples contain slit-like mesopores with few of micropores. Comparison of isotherms for zeolite and acid treated zeolites show that notable increases of N2 adsorption under low P/P0 appear in the isotherms for acid treated zeolites while only slight changes of N2 adsorption under high P/P0 are found, which suggests that acid treatment significantly increase the micropores in the zeolite while it has negligible effect on the mesopores. The pore size distributions in Fig. 4b show that the amount of micropores on 0.7 nm deceases while those on 1.2 nm increase with the increase of acid concentration. In addition, the amount of mesopores in the acid treated zeolites remain unchanged compared with that in the zeolite sample. The specific surface area and pore volume of the zeolite samples in Table 4 show that the specific surface area SBET of the acid treated zeolites greatly increase as compared with that of the zeolite sample (27.67 m2/g). The increases of SBET are owing to the increases of Smicro with slight increases of Sexternal. The pore volume results have the same trend as the specific surface area results (Table 5).
Fig. 5. Water vapor adsorptions per unit area on zeolite and acid treated zeolites.
3.3.2. Hydrophilicity/hydrophobicity of the zeolite samples Fig. 5 shows the water vapor adsorptions per unit area on zeolite and acid treated zeolites with different SiO2/Al2O3 ratio. It is clear that water vapor adsorptions on the acid treated zeolites with relative high SiO2/Al2O3 ratio are significantly lower than that on the zeolite sample, which suggests the increase of hydrophobicity for those zeolites with high SiO2/Al2O3 ratio. However, it is found that hydrophobicity of the acid treated zeolites with different SiO2/Al2O3 ratio are much the same and higher SiO2/Al2O3 ratio does not correspond to higher hydrophobicity. The reason is attributed to the combined influences of SiO2/ Al2O3 ratio and hydrophilic silanol groups. On the one hand, acid treatment with increasing acid concentration causes the increase of SiO2/Al2O3 ratio (decrease of polar Si-O-Al group); On the other hand, acid treatment with increasing acid concentration also causes the
increase of polar silanol groups. 4. Conclusions An L25 orthogonal test with three operational factors, i.e. acid concentration, time and temperature, each in five levels were designed to optimize the acid treated condition. The factors for acid concentration (treated time and temperature) were then employed to quantitatively regulate the SiO2/Al2O3 ratio of natural zeolite. The following conclusions have been obtained: (1) The primary and secondary factors based on the index of aluminum removal rate and SiO2/Al2O3 ratio are temperature > time > concentration and time > concentration > temperature, respectively, and the optimized acid treated conditions for aluminum removal rate and SiO2/Al2O3 ratio are 80 °C:48 h:8 M and 70 °C:48 h:8 M, respectively. (2) Acid concentration, treated time and temperature could effective quantitatively regulate the extent of dealumination of zeolite and SiO2/Al2O3 ratio of natural zeolite, and the equations for regulation of SiO2/Al2O3 ratio based on acid concentration, treated time and temperature are y = a-b*cx (x = C, t, T) and y = a*bx (x = C, t, T), respectively. (3) The acid treated zeolites with different SiO2/Al2O3 ratio (20.0–372.7) had higher specific surface area (> 124.13 m2/g), micropore volume (> 0.046 cm3/g), silanol group content
Table 5 Specific surface area and pore volume of zeolite and acid treated zeolites. Samples
Smicroa (m2/g)
Sexternala (m2/ g)
SBETb (m2/ g)
Vmicroa (cm3/g)
Vmesoc (cm3/g)
NZ NZ-1 NZ-2 NZ-4 NZ-6 NZ-8
5.16 115.42 88.82 100.65 108.62 94.51
22.52 49.54 35.31 43.91 38.97 40.77
27.67 164.97 124.13 144.56 147.59 135.27
0.0024 0.0570 0.0460 0.0497 0.0537 0.0492
0.0882 0.1171 0.0913 0.0965 0.0838 0.0832
a b c
By t-plot method. By BET method. By BJH method. 6
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(> 1.7606%) and hydrophobicity (< 0.3077 mg/m2) than those for the natural zeolite with SiO2/Al2O3 ratio for 5.9, specific surface area for 27.67 m2/g, micropore volume for 0.0024 cm3/g, silanol group content for 1.6465% and hydrophobicity for 0.9359 mg/m2, respectively. However, crystallinity of the aid treated zeolites obviously decreased as compared with that of the raw zeolite.
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Future works to regulate the silanol group content in the zeolite and increase the crystallinity of the zeolite need to be investigated. Acknowledgments The project supported by National Natural Science Foundation of China (No. 51604170), China Postdoctoral Science Foundation (No. 2016M602936XB), Natural Science Foundation of Shaanxi Province (No. 2018JM5025), Foundation for Selected Overseas Chinese Scholar (No. 2018043), Shaanxi Province Postdoctoral Science Foundation and Wenzhou Science and Technology Foundation of China (No. S20180008), National Undergraduate Innovation and Entrepreneurship Training Program (No. 201810708042). References [1] S.M. Csicsery, Catalysis by shape selective zeolites-science and technology, Pure Appl. Chem. 58 (1986) 841–856. [2] Y. Li, L. Li, J.H. Yu, Applications of zeolites in sustainable chemistry, Chem 36 (2017) 928–949. [3] N. Jiang, R. Shang, S.G.J. Heijman, L.C. Rietveld, High-silica zeolites for adsorption of organic micro-pollutants in water treatment: a review, Water Res. 144 (2018) 145–161.
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