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Morphology control of ordered supermicroporous silicas
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Morphology control of ordered supermicroporous silicas Xiuxiu Zou, Peng Wang∗, Shangxing Chen, Zongde Wang, Guorong Fan, Shengliang Liao, Hongyan Si Collaborative Innovation Center of Jiangxi Typical Trees Cultivation and Utilization, College of Forestry, Jiangxi Agricultural University, Nanchang, 330045, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supermicroporous Silica Morphology Rosin-based quaternary ammonium bromide
A feasible method has been developed for the morphology control of ordered supermicroporous silicas by adopting a kind of rosin-based surfactant as the templating agent. By adjusting the molar ratio of sodium silicate (SS) and ethyl acetate (EA), the morphologies of the materials can be changed from hollow sphere to hollow tube, and solid rod, while the degree of ordering of the samples showed a trend of firstly increasing and then decreasing. When the molar ratio of SS/EA was 0.38, an ordered supermicroporous silica with a hollow tubular morphology was obtained. The material had a surface area of 645.5 m2/g, a pore volume of 0.3 cm3/g, and a pore size distribution centered at 1.8 nm. When the molar ratio of SS/EA was increased to 0.46, the resulting sample exhibited a solid-rod morphology with a highly ordered hexagonal supermicroporous structure. This material had a large surface area of 1478.1 m2/g, a high pore volume of 0.7 cm3/g, and a uniform pore size distribution centered at 2.0 nm.
1. Introduction Microporous zeolites have been widely used in the field of catalysis, adsorption, and separation, due to their good shape-selectivity and high thermal/hydrothermal stability [1]. Unfortunately, the sluggish diffusion problem resulted from their 1.0 nm limitation in pore size degrades the performance of the materials [2]. By contrast, ordered mesoporous molecular sieves with enlarged pore size allow the fast diffusion of bulky molecules that cannot pass through the pores of microporous zeolites. However, conventional mesoporous materials usually have a pore size of larger than 2.0 nm, which is too large to influence shapeselectivity in most chemical reactions [3]. Therefore, it is of great importance to synthesize ordered porous materials with pore sizes that are between the micropore and mesopore ranges, since such materials are expected to have better mass transfer performance than microporous zeolites and greater shape-selectivity potential than mesoporous molecular sieves [4,5]. Such materials, with pore size between the range of 1.0–2.0 nm, can be classified as ordered supermicroporous materials, a term introduced by Dubinin in 1974 [6]. However, due to the lack of a suitable templating agent, the synthesis of ordered supermicroporous materials remains a challenge [7]. In order to overcome this challenge, scientists have proposed several strategies. Post-synthesis routes, such as shrinking the pore-opening size of MCM-41 through a chemical vapor deposition system and decreasing the pore size of MCM-48 through high-temperature calcination with the filling of the pore
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channels by carbon, were reported by Lu et al. [8] and Kan et al. [9], respectively. Surfactant/co-surfactant systems were also adopted by using decyltrimethyl ammonium bromide/butanol [10], decyltrimethyl ammonium bromide/sodium octyl sulfate [11], and dodecyltriethylammonium bromide/polycarboxylic acids [12] as the composite templating agents. The results indicated that the introduction of the cosurfactants could effectively enhance the degree of ordering of the pore structure. Additionally, some amphipathic compounds with special molecular structures have been synthesized and used as templating agents, such as ω-hydroxybolaform surfactants [13], gemini surfactants [14], semifluorinated surfactants [15], rigid-benzene-core surfactants [16], and ionic liquids [17]. The materials synthesized by the above strategies had highly ordered hexagonal, cubic, and lamellar supermicroporous structures with surface areas larger than 1000 m2/g and narrow pore size distributions in the range of 1.5–2.0 nm. However, there are few reports on the morphology control of supermicroporous materials. Such control is of great importance for tailoring the morphology and pore structure of porous materials for desired applications [18,19]. As far as we know, only Yano et al. [20,21] have demonstrated the synthesis of supermicroporous silica spheres. The materials showed a mono-dispersed spherical morphology with particle sizes in the range of 0.52–1.25 μm. Although three peaks can be observed in the smallangle X-ray diffraction (XRD) characterization, no long-range ordered pore structure was visible from transmission electron microscopy (TEM) images.
Corresponding author. E-mail address: [email protected] (P. Wang).
https://doi.org/10.1016/j.ceramint.2019.05.220 Received 9 April 2019; Received in revised form 16 May 2019; Accepted 21 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiuxiu Zou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.220
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Our group has found that rosin-based surfactants can be good candidates for the synthesis of ordered supermicroporous materials, due to their strong self-assembling ability and small molecular diameter [22]. However, the materials we have previously synthesized did not exhibit any special shapes. In this study, ordered supermicroporous silicas with various morphologies were synthesized by using a kind of rosin-based surfactant, dehydroabietyltrimethyl ammonium bromine (DTAB), as the templating agent. Additionally, the effects of the molar ratio of sodium silicate (SS) and ethyl acetate (EA) on the morphology and pore structure of the silicas were investigated. 2. Experimental 2.1. Chemicals SS (AR) and EA (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. DTAB was self-synthesized according to the methodology of our previous study [22]. 2.2. Methods For the synthesis of ordered supermicroporous silicas with various morphologies, a certain amount of SS and DTAB were firstly dissolved in deionized water at 308 K. Under vigorous stirring, EA was quickly added to this solution. After further stirring the solution for about 30 s, the mixture was left to stand in a water bath at 308 K for 24 h. Then, the suspension liquid was aged at 373 K in a Teflon-lined autoclave for 24 h. The resulting white precipitate was filtered and washed several times with water and ethanol. The molar compositions of the mixtures were nSS:nEA:nDTAB:nH2O = 0.08–0.77:1:0.12:534. After drying in air at 343 K for 12 h, the solids were calcined at 823 K for 4 h to removal the surfactant.
Fig. 1. Small-angle XRD patterns of as-synthesized silicas synthesized with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
2.3. Characterization XRD patterns of the samples were recorded using a D8 Focus X-ray diffractometer (Bucker AXS Inc., Germany) with Cu Ka radiation (λ = 0.154 nm). The operating target voltage and the current were set to 40 kV and 40 mA, respectively. The sample was powdered and scanned at 2θ angles ranging from 0.5 to 10°. Scanning electron microscopy (SEM) images were obtained with a FEI Quanta 250 microscope (Thermo Fisher Scientific Inc., USA) operating at 15 kV. The samples were coated with gold before the measurements. TEM images were made of samples supported on carbon-coated copper grids using an FEI Talos F200X microscope (Thermo Fisher Scientific Inc., USA) operating at 200 kV. Nitrogen physisorption isotherms were measured using a Micromeritics ASAP2020 instrument (Micromeritics Instrument Corp., USA) to determine the surface areas and pore distributions of the samples. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-synthesized samples fabricated by adding different amounts of SS. When the molar ratio of SS to EA (denoted as x) is 0.08, no diffraction peaks are observed in sample a (Fig. 1a). As x is increased to 0.15 and 0.23, both of the samples (Fig. 1b and c) show only a broad peak at about 2θ = 2.98° (d100 = 2.96 nm), suggesting the formation of a disordered ‘worm-hole’ pore structure. With the further increase of x, the regularity of the pore structure is significantly increased. At x = 0.31 (Fig. 1d), the sample exhibits a sharp diffraction at 2θ = 2.96° (d100 = 2.98 nm). However, no other diffraction peaks can be observed. At x = 0.38 and 0.46 (Fig. 1e and f), three well-resolved diffraction peaks are observed at 2θ = 2.88°, 5.07°, and 5.83° (d100 = 3.07 nm, d110 = 1.74 nm, d200 = 1.51 nm) and 2θ = 2.94°, 5.11°, and 5.89° (d100 = 3.00 nm, d110 = 1.73 nm, d200 = 1.50 nm), respectively, which can be ascribed to the 100, 110,
Fig. 2. Small-angle XRD patterns of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77. 2
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Fig. 3. SEM images of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
samples. Rosin-based surfactants possess the characteristics of small molecular size and strong self-assembly ability. By using such surfactants, ordered supermicroporous materials can be easily formed without the requirement of special conditions. Therefore, we selected a kind of rosin-based surfactant, DTAB, as the templating agent to control the morphology of the ordered supermicroporous materials. As the introduced EA is firstly hydrolyzed in a solution of SS and DTAB, the resulting acetic acid will gradually neutralize SS to initiate the self-assembly of the silicate source and templating agent. This process
and 200 crystal plane reflections of the 2D-hexagonal phase, indicating that both of the samples have a long-range ordered pore structure. When the addition of SS is higher, at x = 0.54 and 0.77 (Fig. 1g and h), the degree of ordering of the pore structure decreases, with only broad and dispersive diffraction peaks being observed at around 2θ = 3.16° (d100 = 2.79 nm) and 2θ = 3.66° (d100 = 2.41 nm) for the samples g and h, respectively. The d100 values of the above samples are lower than those of the classical MCM-41 materials listed in literature [23,24], suggesting the formation of supermicroporous structures in the 3
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Fig. 4. TEM images of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
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polyanions to contact the micelles containing silicate polyanions, so that the anions and polyanions polymerize with each other to form disordered silicas. Additionally, the amount of silicate source also has a great influence on the pH of the reaction system. If the pH is too high or too low, it will affect the hydrothermal crystallization process in the materials, which is harmful to the formation of stable pore structures. The XRD characterizations of the calcined materials have fully proved this point of view. As can be seen from Fig. 2, after the calcination, only the diffraction peak intensities of samples e and f are significantly enhanced due to the removal of the surfactant. For the other six samples, the intensities of the diffraction peaks are not enhanced, or are even weakened. It can be concluded that choosing an appropriate pH value is conducive to the crystallization of the sample, thus strengthening the stability of the pore walls. The calcined samples e and f show three distinctive diffraction peaks (Fig. 2e and f) at 2θ = 3.03°, 5.34°, and 6.05° (d100 = 2.91 nm, d110 = 1.65 nm, d200 = 1.46 nm)and 2θ = 3.04°, 5.38°, and 6.14° (d100 = 2.90 nm, d110 = 1.64 nm, d200 = 1.44 nm), respectively, suggesting that both of the samples retained the ordered supermicroporous structures after calcination. Fig. 3 presents SEM images of the calcined samples fabricated by adding different amount of SS. When the amounts of DTAB and EA are fixed, supermicroporous silica particles with various morphologies can be readily synthesized by varying the amount of SS that is added. As can be seen from Fig. 3a, sample a is composed of well-dispersed hollow silica spheres with diameters of 150–200 nm. In samples b, c, and d, with the increase of SS content, hollow tubular structures are observed (Fig. 3b, c, and d). The structure of sample e (x = 0.38) is entirely composed of hollow tubular structures (Fig. 3e). In sample f (x = 0.46), a structural transformation from hollow tubes to solid rods occurs (Fig. 3f). The width of the solid rods is about 500 nm, and their sizes increase to 800–900 nm with the further increase in the amount of silicate source in samples g and h (Fig. 3g and h). The less SS is added, the more EA is retained in the system. The excessive amount of EA is inserted into the hydrophobic core of micelles, reducing their modulus of elasticity and increase their curvature [27]. When EA accumulates to a certain amount, it will exceed the swelling limit of micelles, leading to a transformation from a structure composed of hollow tubes to one composed of hollow spheres [28]. It is noteworthy that the morphology of the samples changes from a hollow tubular structure to a solid rod structure as x increases from 0.38 to 0.46. Regev [29] reported that, as the polymerization process takes place, the ordered arrays of micelles are firstly ‘wrapped’ in a silica film, which, during the course of the reaction, penetrates the micelles. If there are sufficient amounts of silicate anions and polyanions, every single micelle is wrapped by the silica film to form the solid rods. However, if there are insufficient amounts of silicate anions and polyanions, only hollow tubular structures can be obtained. The observed phenomenon that the size of the structural components increases with the use of increasing amounts of silicate source results from the high speed of polymerization of the excess silicate anions and polyanions. Fig. 4 shows TEM images of the calcined samples fabricated by adding different amounts of SS. From these images it can be seen that increasing the amounts of SS leads to morphological transformations from hollow spheres to hollow tubes, and to solid rods, which is consistent with the results obtained by SEM. Sample a shows a structure comprised of hollow spheres, with a cracked shell and solid core at the center, however without a long-range ordered pore structure (Fig. 4a). In samples b, c, and d, with the increase of SS content (x = 0.15, 0.23, and 0.31, respectively), hollow spheres begin to merge with each other and transform into a hollow tubular structure. However, no long-range ordered pore structure is observed for these three samples (Fig. 4b, c, and d). Sample e is composed of hollow tubes with highly ordered pore channels which are parallel to the elongation direction of the tubes (Fig. 4e). With the continued increase of SS content (x = 0.46), sample f exhibits a solid-rod morphology with large domains of well-ordered 2D hexagonal pore structure (Fig. 4f). With the further increase of SS
Fig. 5. Nitrogen adsorption–desorption isotherms of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) (sample e in previous results) x = 0.38; (b) (sample f in previous results) x = 0.46.
Fig. 6. DFT pore size distributions of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) (sample e in previous results) x = 0.38; (b) (sample f in previous results) x = 0.46.
includes three parallel processes: the silicate anions contact with the DTAB micelles and aggregate on their surfaces; the micelles containing silicate polyanions self-assemble to form ordered arrays by charge density matching; and the free silicate anions and polyanions further polymerize on the surfaces of the micelles containing silicate polyanions to form pore walls [25,26]. When SS and EA are at a suitable proportion, the silicate source will aggregate on the surfaces of DTAB micelles at an appropriate rate, which will promote the self-assembly of micelles containing silicate polyanions to form ordered arrays. Eventually, through the further polymerization of the free silicate anions and polyanions at a suitable concentration, ordered silica pore walls will be obtained. When the amount of silicate source is small, the concentrations of free silicate anions and polyanions will be too low to form stable silica pore walls among the micelles. Meanwhile, the excessive unhydrolyzed EA molecules remaining in the reaction system will be solubilized in the DTAB micelles, which will prevent the micelles from forming ordered arrays. When the amount of silicate source is too high, the concentrations of silicate anions and polyanions are significantly increased, which leads to the acceleration of the polymerization rate. Hence, it is difficult for the excessive quantities of silicate anions and
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References
content in samples g and h (x = 0.54 and 0.77), the resulting solid rods become thicker, and no well-ordered pore structure can be observed (Fig. 4g and h). Such an evolution of the degree of ordering of samples is consistent with the results of the XRD characterization. In order to further investigate the pore structure of the samples, nitrogen physisorption measurements were made. Fig. 5 shows the N2 adsorption–desorption isotherms for the two samples with well-ordered pore architectures (samples e and f in previous results). The isotherm of sample e (Fig. 5a) is found to be a transitional type between the Type I (b) and IV (b) isotherms, according to the IUPAC (2015) classification [30]. Although the isotherm of sample e is similar to the Type I (b) isotherm, the sample shows a linear range at P/P0 = 0.04–0.14, which is due to capillary condensation rather than micropore filling, suggesting the existence of supermicropores in sample e. Additionally, a significant hysteresis loop is detected in the range of P/P0 = 0.46–1.0 for sample e, indicating a broad distribution of large pores in the sample. For sample f, a Type IV (b) isotherm can be observed (Fig. 5b), the adsorption and desorption branches of the isotherm almost coincide with each other, and sharp changes in adsorption are observed at P/ P0 = 0.12–0.18, demonstrating that a uniform pore size is obtained in the ordered framework. In contrast to the conventional mesoporous material MCM-41 [23,24], the capillary condensation occurred at a lower value of P/P0 in sample f, indicating that the pore size of sample f is much smaller than that of MCM-41. The density functional theory (DFT) pore size distributions of samples e and f are depicted in Fig. 6. Both of the samples show narrow pore size distributions, centered at 1.7 nm (Fig. 6a) and 2.0 nm (Fig. 6b), which belong to the pore size range of supermicroporous materials. It should be noted that, in sample e, there are a small number of pores with sizes in the range of 20.0–100.0 nm, which are the centered caves of the hollow tubes. The Brunauer-Emmett-Teller (BET) specific surface area of sample e is 645.5 m2/g, and the pore volume is 0.3 cm3/g. Sample f has a larger surface area of 1478.1 m2/g and a higher pore volume of 0.7 cm3/g. There maybe two reasons for the differences in the BET surface area and pore volume between the two samples. Firstly, the hollow tubes of sample e have caves with large diameters (20.0–100.0 nm), which lead to a lower BET surface area and pore volume. On the other hand, in sample e, the orientation of the pore channels is parallel to the elongation direction of the hollow tubes (Fig. 4e), while in sample f the orientation of the pore channels is perpendicular to the elongation direction of the solid rods (Fig. 4f). The length of the pore channels is much larger in sample e than in sample f, which leads to a lower BET surface area and pore volume in sample e.
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4. Conclusions In the present work, a series of supermicroporous silicas with different morphologies was successfully synthesized by using a kind of rosin-based quaternary ammonium salt as the templating agent. The relationship between the amount of sodium silicate used and the microstructure of samples, mainly the morphology and the degree of ordering of pore channels, was investigated in detail. With increasing amounts of sodium silicate, the morphologies of the prepared samples changed from hollow spheres to hollow tubes, and solid rods. At the same time, the degree of ordering of the samples firstly increased and then decreased. This synthesis approach allows an effective way to control the morphology and pore structure of ordered supermicroporous materials, which can facilitate their further practical applications in adsorption, separation, and catalysis. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant Nos. 31860191 and 31500484) and the Innovation Fund Designated for Graduate Students of Jiangxi Province (grant No. YC2017-S184). 6
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Morphology control of ordered supermicroporous silicas Xiuxiu Zou, Peng Wang∗, Shangxing Chen, Zongde Wang, Guorong Fan, Shengliang Liao, Hongyan Si Collaborative Innovation Center of Jiangxi Typical Trees Cultivation and Utilization, College of Forestry, Jiangxi Agricultural University, Nanchang, 330045, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supermicroporous Silica Morphology Rosin-based quaternary ammonium bromide
A feasible method has been developed for the morphology control of ordered supermicroporous silicas by adopting a kind of rosin-based surfactant as the templating agent. By adjusting the molar ratio of sodium silicate (SS) and ethyl acetate (EA), the morphologies of the materials can be changed from hollow sphere to hollow tube, and solid rod, while the degree of ordering of the samples showed a trend of firstly increasing and then decreasing. When the molar ratio of SS/EA was 0.38, an ordered supermicroporous silica with a hollow tubular morphology was obtained. The material had a surface area of 645.5 m2/g, a pore volume of 0.3 cm3/g, and a pore size distribution centered at 1.8 nm. When the molar ratio of SS/EA was increased to 0.46, the resulting sample exhibited a solid-rod morphology with a highly ordered hexagonal supermicroporous structure. This material had a large surface area of 1478.1 m2/g, a high pore volume of 0.7 cm3/g, and a uniform pore size distribution centered at 2.0 nm.
1. Introduction Microporous zeolites have been widely used in the field of catalysis, adsorption, and separation, due to their good shape-selectivity and high thermal/hydrothermal stability [1]. Unfortunately, the sluggish diffusion problem resulted from their 1.0 nm limitation in pore size degrades the performance of the materials [2]. By contrast, ordered mesoporous molecular sieves with enlarged pore size allow the fast diffusion of bulky molecules that cannot pass through the pores of microporous zeolites. However, conventional mesoporous materials usually have a pore size of larger than 2.0 nm, which is too large to influence shapeselectivity in most chemical reactions [3]. Therefore, it is of great importance to synthesize ordered porous materials with pore sizes that are between the micropore and mesopore ranges, since such materials are expected to have better mass transfer performance than microporous zeolites and greater shape-selectivity potential than mesoporous molecular sieves [4,5]. Such materials, with pore size between the range of 1.0–2.0 nm, can be classified as ordered supermicroporous materials, a term introduced by Dubinin in 1974 [6]. However, due to the lack of a suitable templating agent, the synthesis of ordered supermicroporous materials remains a challenge [7]. In order to overcome this challenge, scientists have proposed several strategies. Post-synthesis routes, such as shrinking the pore-opening size of MCM-41 through a chemical vapor deposition system and decreasing the pore size of MCM-48 through high-temperature calcination with the filling of the pore
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channels by carbon, were reported by Lu et al. [8] and Kan et al. [9], respectively. Surfactant/co-surfactant systems were also adopted by using decyltrimethyl ammonium bromide/butanol [10], decyltrimethyl ammonium bromide/sodium octyl sulfate [11], and dodecyltriethylammonium bromide/polycarboxylic acids [12] as the composite templating agents. The results indicated that the introduction of the cosurfactants could effectively enhance the degree of ordering of the pore structure. Additionally, some amphipathic compounds with special molecular structures have been synthesized and used as templating agents, such as ω-hydroxybolaform surfactants [13], gemini surfactants [14], semifluorinated surfactants [15], rigid-benzene-core surfactants [16], and ionic liquids [17]. The materials synthesized by the above strategies had highly ordered hexagonal, cubic, and lamellar supermicroporous structures with surface areas larger than 1000 m2/g and narrow pore size distributions in the range of 1.5–2.0 nm. However, there are few reports on the morphology control of supermicroporous materials. Such control is of great importance for tailoring the morphology and pore structure of porous materials for desired applications [18,19]. As far as we know, only Yano et al. [20,21] have demonstrated the synthesis of supermicroporous silica spheres. The materials showed a mono-dispersed spherical morphology with particle sizes in the range of 0.52–1.25 μm. Although three peaks can be observed in the smallangle X-ray diffraction (XRD) characterization, no long-range ordered pore structure was visible from transmission electron microscopy (TEM) images.
Corresponding author. E-mail address: [email protected] (P. Wang).
https://doi.org/10.1016/j.ceramint.2019.05.220 Received 9 April 2019; Received in revised form 16 May 2019; Accepted 21 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiuxiu Zou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.220
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Our group has found that rosin-based surfactants can be good candidates for the synthesis of ordered supermicroporous materials, due to their strong self-assembling ability and small molecular diameter [22]. However, the materials we have previously synthesized did not exhibit any special shapes. In this study, ordered supermicroporous silicas with various morphologies were synthesized by using a kind of rosin-based surfactant, dehydroabietyltrimethyl ammonium bromine (DTAB), as the templating agent. Additionally, the effects of the molar ratio of sodium silicate (SS) and ethyl acetate (EA) on the morphology and pore structure of the silicas were investigated. 2. Experimental 2.1. Chemicals SS (AR) and EA (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. DTAB was self-synthesized according to the methodology of our previous study [22]. 2.2. Methods For the synthesis of ordered supermicroporous silicas with various morphologies, a certain amount of SS and DTAB were firstly dissolved in deionized water at 308 K. Under vigorous stirring, EA was quickly added to this solution. After further stirring the solution for about 30 s, the mixture was left to stand in a water bath at 308 K for 24 h. Then, the suspension liquid was aged at 373 K in a Teflon-lined autoclave for 24 h. The resulting white precipitate was filtered and washed several times with water and ethanol. The molar compositions of the mixtures were nSS:nEA:nDTAB:nH2O = 0.08–0.77:1:0.12:534. After drying in air at 343 K for 12 h, the solids were calcined at 823 K for 4 h to removal the surfactant.
Fig. 1. Small-angle XRD patterns of as-synthesized silicas synthesized with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
2.3. Characterization XRD patterns of the samples were recorded using a D8 Focus X-ray diffractometer (Bucker AXS Inc., Germany) with Cu Ka radiation (λ = 0.154 nm). The operating target voltage and the current were set to 40 kV and 40 mA, respectively. The sample was powdered and scanned at 2θ angles ranging from 0.5 to 10°. Scanning electron microscopy (SEM) images were obtained with a FEI Quanta 250 microscope (Thermo Fisher Scientific Inc., USA) operating at 15 kV. The samples were coated with gold before the measurements. TEM images were made of samples supported on carbon-coated copper grids using an FEI Talos F200X microscope (Thermo Fisher Scientific Inc., USA) operating at 200 kV. Nitrogen physisorption isotherms were measured using a Micromeritics ASAP2020 instrument (Micromeritics Instrument Corp., USA) to determine the surface areas and pore distributions of the samples. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-synthesized samples fabricated by adding different amounts of SS. When the molar ratio of SS to EA (denoted as x) is 0.08, no diffraction peaks are observed in sample a (Fig. 1a). As x is increased to 0.15 and 0.23, both of the samples (Fig. 1b and c) show only a broad peak at about 2θ = 2.98° (d100 = 2.96 nm), suggesting the formation of a disordered ‘worm-hole’ pore structure. With the further increase of x, the regularity of the pore structure is significantly increased. At x = 0.31 (Fig. 1d), the sample exhibits a sharp diffraction at 2θ = 2.96° (d100 = 2.98 nm). However, no other diffraction peaks can be observed. At x = 0.38 and 0.46 (Fig. 1e and f), three well-resolved diffraction peaks are observed at 2θ = 2.88°, 5.07°, and 5.83° (d100 = 3.07 nm, d110 = 1.74 nm, d200 = 1.51 nm) and 2θ = 2.94°, 5.11°, and 5.89° (d100 = 3.00 nm, d110 = 1.73 nm, d200 = 1.50 nm), respectively, which can be ascribed to the 100, 110,
Fig. 2. Small-angle XRD patterns of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77. 2
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Fig. 3. SEM images of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
samples. Rosin-based surfactants possess the characteristics of small molecular size and strong self-assembly ability. By using such surfactants, ordered supermicroporous materials can be easily formed without the requirement of special conditions. Therefore, we selected a kind of rosin-based surfactant, DTAB, as the templating agent to control the morphology of the ordered supermicroporous materials. As the introduced EA is firstly hydrolyzed in a solution of SS and DTAB, the resulting acetic acid will gradually neutralize SS to initiate the self-assembly of the silicate source and templating agent. This process
and 200 crystal plane reflections of the 2D-hexagonal phase, indicating that both of the samples have a long-range ordered pore structure. When the addition of SS is higher, at x = 0.54 and 0.77 (Fig. 1g and h), the degree of ordering of the pore structure decreases, with only broad and dispersive diffraction peaks being observed at around 2θ = 3.16° (d100 = 2.79 nm) and 2θ = 3.66° (d100 = 2.41 nm) for the samples g and h, respectively. The d100 values of the above samples are lower than those of the classical MCM-41 materials listed in literature [23,24], suggesting the formation of supermicroporous structures in the 3
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Fig. 4. TEM images of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) x = 0.08; (b) x = 0.15; (c) x = 0.23; (d) x = 0.31; (e) x = 0.38; (f) x = 0.46; (g) x = 0.54; (h) x = 0.77.
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polyanions to contact the micelles containing silicate polyanions, so that the anions and polyanions polymerize with each other to form disordered silicas. Additionally, the amount of silicate source also has a great influence on the pH of the reaction system. If the pH is too high or too low, it will affect the hydrothermal crystallization process in the materials, which is harmful to the formation of stable pore structures. The XRD characterizations of the calcined materials have fully proved this point of view. As can be seen from Fig. 2, after the calcination, only the diffraction peak intensities of samples e and f are significantly enhanced due to the removal of the surfactant. For the other six samples, the intensities of the diffraction peaks are not enhanced, or are even weakened. It can be concluded that choosing an appropriate pH value is conducive to the crystallization of the sample, thus strengthening the stability of the pore walls. The calcined samples e and f show three distinctive diffraction peaks (Fig. 2e and f) at 2θ = 3.03°, 5.34°, and 6.05° (d100 = 2.91 nm, d110 = 1.65 nm, d200 = 1.46 nm)and 2θ = 3.04°, 5.38°, and 6.14° (d100 = 2.90 nm, d110 = 1.64 nm, d200 = 1.44 nm), respectively, suggesting that both of the samples retained the ordered supermicroporous structures after calcination. Fig. 3 presents SEM images of the calcined samples fabricated by adding different amount of SS. When the amounts of DTAB and EA are fixed, supermicroporous silica particles with various morphologies can be readily synthesized by varying the amount of SS that is added. As can be seen from Fig. 3a, sample a is composed of well-dispersed hollow silica spheres with diameters of 150–200 nm. In samples b, c, and d, with the increase of SS content, hollow tubular structures are observed (Fig. 3b, c, and d). The structure of sample e (x = 0.38) is entirely composed of hollow tubular structures (Fig. 3e). In sample f (x = 0.46), a structural transformation from hollow tubes to solid rods occurs (Fig. 3f). The width of the solid rods is about 500 nm, and their sizes increase to 800–900 nm with the further increase in the amount of silicate source in samples g and h (Fig. 3g and h). The less SS is added, the more EA is retained in the system. The excessive amount of EA is inserted into the hydrophobic core of micelles, reducing their modulus of elasticity and increase their curvature [27]. When EA accumulates to a certain amount, it will exceed the swelling limit of micelles, leading to a transformation from a structure composed of hollow tubes to one composed of hollow spheres [28]. It is noteworthy that the morphology of the samples changes from a hollow tubular structure to a solid rod structure as x increases from 0.38 to 0.46. Regev [29] reported that, as the polymerization process takes place, the ordered arrays of micelles are firstly ‘wrapped’ in a silica film, which, during the course of the reaction, penetrates the micelles. If there are sufficient amounts of silicate anions and polyanions, every single micelle is wrapped by the silica film to form the solid rods. However, if there are insufficient amounts of silicate anions and polyanions, only hollow tubular structures can be obtained. The observed phenomenon that the size of the structural components increases with the use of increasing amounts of silicate source results from the high speed of polymerization of the excess silicate anions and polyanions. Fig. 4 shows TEM images of the calcined samples fabricated by adding different amounts of SS. From these images it can be seen that increasing the amounts of SS leads to morphological transformations from hollow spheres to hollow tubes, and to solid rods, which is consistent with the results obtained by SEM. Sample a shows a structure comprised of hollow spheres, with a cracked shell and solid core at the center, however without a long-range ordered pore structure (Fig. 4a). In samples b, c, and d, with the increase of SS content (x = 0.15, 0.23, and 0.31, respectively), hollow spheres begin to merge with each other and transform into a hollow tubular structure. However, no long-range ordered pore structure is observed for these three samples (Fig. 4b, c, and d). Sample e is composed of hollow tubes with highly ordered pore channels which are parallel to the elongation direction of the tubes (Fig. 4e). With the continued increase of SS content (x = 0.46), sample f exhibits a solid-rod morphology with large domains of well-ordered 2D hexagonal pore structure (Fig. 4f). With the further increase of SS
Fig. 5. Nitrogen adsorption–desorption isotherms of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) (sample e in previous results) x = 0.38; (b) (sample f in previous results) x = 0.46.
Fig. 6. DFT pore size distributions of calcined silicas fabricated with different amounts of SS, nSS:nEA:nDTAB:nH2O = x:1:0.12:534. (a) (sample e in previous results) x = 0.38; (b) (sample f in previous results) x = 0.46.
includes three parallel processes: the silicate anions contact with the DTAB micelles and aggregate on their surfaces; the micelles containing silicate polyanions self-assemble to form ordered arrays by charge density matching; and the free silicate anions and polyanions further polymerize on the surfaces of the micelles containing silicate polyanions to form pore walls [25,26]. When SS and EA are at a suitable proportion, the silicate source will aggregate on the surfaces of DTAB micelles at an appropriate rate, which will promote the self-assembly of micelles containing silicate polyanions to form ordered arrays. Eventually, through the further polymerization of the free silicate anions and polyanions at a suitable concentration, ordered silica pore walls will be obtained. When the amount of silicate source is small, the concentrations of free silicate anions and polyanions will be too low to form stable silica pore walls among the micelles. Meanwhile, the excessive unhydrolyzed EA molecules remaining in the reaction system will be solubilized in the DTAB micelles, which will prevent the micelles from forming ordered arrays. When the amount of silicate source is too high, the concentrations of silicate anions and polyanions are significantly increased, which leads to the acceleration of the polymerization rate. Hence, it is difficult for the excessive quantities of silicate anions and
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References
content in samples g and h (x = 0.54 and 0.77), the resulting solid rods become thicker, and no well-ordered pore structure can be observed (Fig. 4g and h). Such an evolution of the degree of ordering of samples is consistent with the results of the XRD characterization. In order to further investigate the pore structure of the samples, nitrogen physisorption measurements were made. Fig. 5 shows the N2 adsorption–desorption isotherms for the two samples with well-ordered pore architectures (samples e and f in previous results). The isotherm of sample e (Fig. 5a) is found to be a transitional type between the Type I (b) and IV (b) isotherms, according to the IUPAC (2015) classification [30]. Although the isotherm of sample e is similar to the Type I (b) isotherm, the sample shows a linear range at P/P0 = 0.04–0.14, which is due to capillary condensation rather than micropore filling, suggesting the existence of supermicropores in sample e. Additionally, a significant hysteresis loop is detected in the range of P/P0 = 0.46–1.0 for sample e, indicating a broad distribution of large pores in the sample. For sample f, a Type IV (b) isotherm can be observed (Fig. 5b), the adsorption and desorption branches of the isotherm almost coincide with each other, and sharp changes in adsorption are observed at P/ P0 = 0.12–0.18, demonstrating that a uniform pore size is obtained in the ordered framework. In contrast to the conventional mesoporous material MCM-41 [23,24], the capillary condensation occurred at a lower value of P/P0 in sample f, indicating that the pore size of sample f is much smaller than that of MCM-41. The density functional theory (DFT) pore size distributions of samples e and f are depicted in Fig. 6. Both of the samples show narrow pore size distributions, centered at 1.7 nm (Fig. 6a) and 2.0 nm (Fig. 6b), which belong to the pore size range of supermicroporous materials. It should be noted that, in sample e, there are a small number of pores with sizes in the range of 20.0–100.0 nm, which are the centered caves of the hollow tubes. The Brunauer-Emmett-Teller (BET) specific surface area of sample e is 645.5 m2/g, and the pore volume is 0.3 cm3/g. Sample f has a larger surface area of 1478.1 m2/g and a higher pore volume of 0.7 cm3/g. There maybe two reasons for the differences in the BET surface area and pore volume between the two samples. Firstly, the hollow tubes of sample e have caves with large diameters (20.0–100.0 nm), which lead to a lower BET surface area and pore volume. On the other hand, in sample e, the orientation of the pore channels is parallel to the elongation direction of the hollow tubes (Fig. 4e), while in sample f the orientation of the pore channels is perpendicular to the elongation direction of the solid rods (Fig. 4f). The length of the pore channels is much larger in sample e than in sample f, which leads to a lower BET surface area and pore volume in sample e.
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4. Conclusions In the present work, a series of supermicroporous silicas with different morphologies was successfully synthesized by using a kind of rosin-based quaternary ammonium salt as the templating agent. The relationship between the amount of sodium silicate used and the microstructure of samples, mainly the morphology and the degree of ordering of pore channels, was investigated in detail. With increasing amounts of sodium silicate, the morphologies of the prepared samples changed from hollow spheres to hollow tubes, and solid rods. At the same time, the degree of ordering of the samples firstly increased and then decreased. This synthesis approach allows an effective way to control the morphology and pore structure of ordered supermicroporous materials, which can facilitate their further practical applications in adsorption, separation, and catalysis. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant Nos. 31860191 and 31500484) and the Innovation Fund Designated for Graduate Students of Jiangxi Province (grant No. YC2017-S184). 6