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Solvent-free synthesis of zeolite LTA monolith with hierarchically porous structure from metakaolin
Materials Letters 248 (2019) 28–31
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Solvent-free synthesis of zeolite LTA monolith with hierarchically porous structure from metakaolin Yi Liu, Xiaohui Yang, Chunjie Yan ⇑, Hongquan Wang, Sen Zhou Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 12 December 2018 Received in revised form 16 March 2019 Accepted 30 March 2019 Available online 1 April 2019 Keywords: Solvent-free Self-supporting Hierarchical structure Zeolite LTA Cast Porous materials
a b s t r a c t We report a simple and effective process for preparing self-supporting zeolite LTA monoliths with hierarchical structure via the solvent-free crystallization route followed by a calcination process, using polymethyl methacrylate microbeads as sacrificial template. SEM images demonstrate the formation of interconnected macropores. Cube-shaped zeolite particles are intergrown at their interfaces, establishing a stable porous system. XRD results confirm the purity and crystallinity of zeolite LTA. BET and MIP results indicate three levels of pores (micro-, meso- and macrospores) present in the zeolite monolith. In addition, the BET surface area, bulk density, true density of the hierarchical zeolite are 57.67 m2/g, 0.684 g/cm3 and 2.771 g/cm3, respectively, and the total porosity is up to 75.3 vol%. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are crystalline aluminosilicates with uniform channels and cavities (sizes usually <1.3 nm) [1,2]. In the past decades, owing to its remarkable properties, such as uniform pore size, high surface area, strong acidity and high thermal/hydrothermal stability etc., zeolites have been successfully used as highly selective adsorbents, ion exchangers and catalysts in many important applications [1,3,4]. By introducing photoactive guest to the internal voids of zeolite pores, zeolites have opened the possibilities for the design of novel photonic materials [5–7]. Generally, zeolites are synthesized in the form of fine powders. In most industry applications, they are agglomerated into granules using various inert binding agents [8]. However, several problems arise due to the binding. The mass transport is slow and the active sites located inside the agglomerated products are hardly accessible [9,10]. Besides, the heavy use of binders dilutes the active zeolite and blocks the zeolite pores. Those drawbacks can severely limit the performance of zeolites. To overcome the problem of diffusion limitations, ‘‘hierarchical zeolites” were developed by zeolite scientists. Hierarchical zeolites possess at least one additional level of pores, i.e. mesopores or macropores. Up to now, the synthesis methods can be classified into two categories: ‘‘bottom-up” and ‘‘top-down” routes [1,9,11,12]. In particular, ‘‘bottom-up” routes comprise hard⇑ Corresponding author. E-mail address: [email protected] (C. Yan). https://doi.org/10.1016/j.matlet.2019.03.135 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
templating methods, soft-templating methods, assembly of nanosized zeolites and zeolitization of preformed materials. While ‘‘top-down” routes include the removal of framework atoms methods (dealumination, desilication and irradiation), and recrystallization methods. Among them, an interesting process to synthesis self-supporting hierarchical zeolite monoliths is using nonreactive or reactive solid foams as supports. Hierarchical zeolite monoliths are prepared by in situ or ex situ coating zeolite crystals on the support surface in hydrothermal solutions [11]. Nevertheless, this method poses several challenges, such as low zeolitesupport mass ratio, easy to lose zeolite due to the difference in thermal expansion coefficients etc. Thus, new effective method of preparing self-supporting hierarchical zeolite monoliths should be developed to solve the practical problems. The focus of this study therefore aims to explore a feasible approach for the synthesis of self-supporting hierarchical zeolites without using any foamed supports.
2. Materials and methods Metakaolin (MK) was collected from Super Technology Co. Ltd., Hunan. Commercially available polymethyl methacrylate microbeads (PMMA, 2000 mesh) obtained from Jinyunlai plastics business department, Dongguan, were used as sacrificial template. Sodium hydroxide (analytical purity) was purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used for making NaOH solution.
29
Y. Liu et al. / Materials Letters 248 (2019) 28–31
Fig. 1. Schematic illustrations of hierarchical zeolite LTA monoliths preparation process.
Table 1 Chemical composition of metakaolin by XRF technique (wt%) LOI = loss on ignition. Sample
Al2O3
SiO2
Fe2O3
TiO2
K2O
Na2O
CaO
MgO
P2O5
MnO
LOI
MK
45.4
51.4
0.4
1.3
0.1
0.2
0.1
<0.1
0.4
<0.1
0.4
Fig. 2. SEM images of (a) MK; (b) PMMA; (c) PMMA/zeolite composites; (d-e) hierarchical zeolite LTA, (f) the corresponding EDS results and (g) elemental mapping analysis.
30
Y. Liu et al. / Materials Letters 248 (2019) 28–31
Hierarchical zeolite monoliths were synthesized according to the following process (as illustrated in Fig. 1). Firstly, 2.50 g MK and 2.50 g PMMA were thoroughly mixed with 3.8 ml of 9.7 mol/ L NaOH solution for 8–10 min to make a homogeneous paste. The molar ratio of Na2O/Al2O3 and SiO2/Al2O3 of the paste are 1.2 and 1.9, respectively. The paste shows favorable workability in this condition. In flowing, the paste was cast into a silicone rubber mold, vibrated (to remove entrained air bubbles) and sealed with preservative film. The PMMA/zeolite composites were obtained after curing in an oven at 60 °C for 24 h. There is no limitation in size and shape for the preparation of PMMA/zeolite composites. Zeolite LTA monoliths with hierarchical structure were obtained by further heating the PMMA/zeolite composites to 550 °C for 2 h, at a heating rate of 10 °C/min. The chemical composition of MK was determined using X-ray fluorescence (AXIOSmAX, PANalytical Netherlands). The crystalline phase of the zeolite was recorded on a D8-Focus type X-ray powder diffractometer (BrokerAXS Germany). Microstructure observation of the samples were performed using a SU8010 type scanning electron microscopy (Hitachi Japan). Nitrogen gas adsorption–desorption experiments were performed using an automatic ASAP2020 surface area and porosimetry system (Micromeritics, America), and the specific surface area was calculated by the Brunauer–Em met–Teller (BET) method. The pore structure of hierarchical zeolite was also tested by an AutoPore IV 9510 mercury intrusion porosimeter (Micromeritics, America). 3. Results and discussion The main elemental components of MK are silica and alumina (Table 1). The molar ratio of SiO2/Al2O3 is 1.9, which is close to the theoretical value of 2.0 in molecular of zeolite A
(Na12(AlO2)12(SiO2)1227H2O). Therefore, MK is the ideal Si and Al sources for the synthesis of zeolite A. SEM micrographs of MK, PMMA, PMMA/zeolite composites, hierarchical zeolite LTA and the EDS of hierarchical zeolite are demonstrated in Fig. 2. It is observed that MK particles show agglomerated or pseudohexagonal plate like morphology (Fig. 2a). The diameters of the spherical PMMA microbeads are varying from 1 to 6 lm (Fig. 2b). Fig. 2c shows PMMA/zeolite composites are dense. There is no agglomeration of PMMA microbeads, indicating PMMA microbeads are well embedded within the zeolite matrix. After calcination, PMMA microbeads are removed, leaving behind interconnected voids (Fig. 2d). The morphology of those voids resembles the morphology of PMMA microbeads. Zeolite particles are intergrown at their interfaces, establishing a stable porous system. No significant cracks can be seen in the hierarchical zeolite specimens (Fig. 2d inserted image), which indicate the sacrificial template method is a feasible method to prepare hierarchical zeolites. Fig. 2e presents typical zeolite LTA crystals. They are cubeshaped with particle size around 3 lm. Moreover, some particles with irregular shapes and different sizes can also be observed. These particles might be zeolite precursor-geopolymer, which failed to transform into well crystalline zeolites. These geopolymer particles can act as binder in the monoliths, giving the samples certain strength. Fig. 2f is the EDS spectra of zeolite LTA. It shows a Na: Al:Si:O molar ratio of 1:1.08:1.10:3.45, which is close to the molar ratio of the theoretical zeolite LTA formula (Na12Al12Si12O48(H2O)27, as identified in Fig. 3a). Elemental mapping analysis reveals that the O, Na, Al and Si elements were dispersed uniformly in zeolites. Fig. 3a presents the XRD patterns of MK and hierarchical zeolite. There is a broad hump between 2h = 18 and 30°, indicating MK is
Fig. 3. (a) XRD patterns of metakaolin (MK) and hierarchical zeolite (HZ), (b) BJH pore size distribution, (c) N2 adsorption-desorption isotherms and (d) MIP results of hierarchical zeolite.
Y. Liu et al. / Materials Letters 248 (2019) 28–31
amorphous. After the curing and calcination process, the amorphous phase is converted into crystalline phase. The characteristic diffraction peaks of the zeolite match well with zeolite LTA (JCPDS card number 73-2340). The BJH pore size distribution (PSD) and N2 adsorptiondesorption isotherms of hierarchical zeolite are plotted in Fig. 3b-c. The PSD curve (Fig. 3b) shows that the hierarchical zeolite possesses a broad distribution of pore sizes, and three types of pores (micro-, meso- and macrospores) present in the zeolite products. The N2 adsorption-desorption isotherms of hierarchical zeolite are shown in Fig. 3c. It is a type II isotherm with a H3 hysteresis loop according to International Union of Pure and Applied Chemistry (IUPAC) classification [13]. Type II isotherms represent adsorbents are non-porous or macroporous materials. At P/Po = 0.96–1.00, the isotherm rises nearly horizontal and parallel to the gas adsorption axis, indicating the presence of very wide range of macropores. The phenomenon can also be confirmed by the PSD curve. The Type H3 loop reveals that the zeolite product having nano-sized slit-like pores, which might be caused by the intercrystalline aggregation of zeolite crystals. The BET surface area and total pore volume of the hierarchical zeolite are 57.67 m2/g and 0.0569 cm3/g, respectively. The BET surface area is quite lower than zeolite 4A synthesized from hydrothermal strategies [14], however, it is in accordance with zeolites fabricated from solvent-free method [15]. The bulk density, true density and porosity of hierarchical zeolite were tested by MIP. The measured bulk density, true density of hierarchical zeolite are 0.684 g/cm3 and 2.771 g/cm3, respectively. The total porosity is up to 75.3 vol%. The average pore dimeter of the hierarchical zeolite tested by MIP is 4.73 lm, which is compatible to the diameters of the sacrificial templates. The pore size distribution and differential curves of hierarchical zeolite are shown in Fig. 3d. The sample has bi-modal pore size distribution. Most of the pore diameters centered at around 3 lm and a small amount of pore diameters are distributed around 280 lm. As it is evident, the broad peak centered around 3 lm is due to the PMMA burn out. The pore diameters around 280 lm in the hierarchical zeolite are probably caused by air bubbles introduced during the mixing and casting process. From the above observations, we can reasonably conclude that it is able to manufacture hierarchical zeolite with desired macropores by choosing appropriate PMMA mocrobeads diameters. 4. Conclusions In summary, hierarchical zeolite LTA monoliths were successfully synthesized using PMMA microbeads as the transitional
31
template through solvent-free crystallization method followed by a calcination process. The zeolite/PMMA composites were heated to 550 °C for 2 h to eliminate PMMA microbeads, leaving behind zeolite monoliths with interconnected macropores. This method extends the diversity of synthetic procedures. The material for zeolite synthesizes and the templates for macropores are commercially available, abundant and inexpensive. Moreover, the synthetic route is very simple, even suitable for large-scale production. 5. Declarations of interest None. Acknowledgements This work was supported by National Key R&D Program of China under the project No. 2017YFB0310805, Engineering Research Center of non-metallic minerals of Zhejiang Province, Key Laboratory of Clay Minerals, Ministry of Land and Resources, and Engineering Research Center of Nano-Geo Materials of Ministry of Education (NGM2019KF016). References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15]
D.P. Serrano, J.M. Escola, P. Pizarro, Chem. Soc. Rev. 42 (9) (2013) 4004–4035. Y. Liu, C. Yan, Z. Zhang, H. Wang, S. Zhou, W. Zhou, Fuel 185 (2016) 181–189. E.T. Vogt, B.M. Weckhuysen, Chem. Soc. Rev. 44 (20) (2015) 7342–7370. L. Peng, X. Xu, X. Yao, H. Liu, X. Gu, J. Membrane Sci. 549 (2018) 446–455. K. Kanimozhi, S. Khaleel Basha, V. Sugantha Kumari, K. Kaviyarasu, M. Maaza, Appl. Surf. Sci. 449 (2018) 574–583. K. Kombaiah, J.J. Vijaya, L.J. Kennedy, M. Bououdina, R.J. Ramalingam, H.A. Al Lohedan, Ceram. Int. 43 (10) (2017) 7682–7689. S.K. Jesudoss, J.J. Vijaya, L.J. Kennedy, P.I. Rajan, H.A. Al Lohedan, R.J. Ramalingam, K. Kaviyarasu, M. Bououdina, J. Photochem. Photobiol. B 165 (2016) 121–132. Y. Liu, C. Yan, Z. Zhang, L. Li, H. Wang, S. Pu, Mater. Lett. 196 (2017) 328–331. A. Feliczak Guzik, Microporous Mesoporous Mater. 259 (2018) 33–45. S.M. Al Jubouri, S.M. Holmes, Chem. Eng. J. 308 (2017) 476–491. S. Lopez Orozco, A. Inayat, A. Schwab, T. Selvam, W. Schwieger, Adv. Mater. 23 (22–23) (2011) 2602–2615. L. Chen, X. Li, J.C. Rooke, Y. Zhang, X. Yang, Y. Tang, F. Xiao, B. Su, J. Mater. Chem. 22 (34) (2012) 17381. K.S. Sing, Pure Appl. Chem. 57 (4) (1985) 603–619. X. Ni, Z. Zheng, X. Wang, S. Zhang, M. Zhao, J. Alloys Compd. 592 (2014) 135– 139. Y. Liu, C. Yan, J. Zhao, Z. Zhang, H. Wang, S. Zhou, L. Wu, J. Clean. Prod. 202 (2018) 11–22.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Solvent-free synthesis of zeolite LTA monolith with hierarchically porous structure from metakaolin Yi Liu, Xiaohui Yang, Chunjie Yan ⇑, Hongquan Wang, Sen Zhou Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 12 December 2018 Received in revised form 16 March 2019 Accepted 30 March 2019 Available online 1 April 2019 Keywords: Solvent-free Self-supporting Hierarchical structure Zeolite LTA Cast Porous materials
a b s t r a c t We report a simple and effective process for preparing self-supporting zeolite LTA monoliths with hierarchical structure via the solvent-free crystallization route followed by a calcination process, using polymethyl methacrylate microbeads as sacrificial template. SEM images demonstrate the formation of interconnected macropores. Cube-shaped zeolite particles are intergrown at their interfaces, establishing a stable porous system. XRD results confirm the purity and crystallinity of zeolite LTA. BET and MIP results indicate three levels of pores (micro-, meso- and macrospores) present in the zeolite monolith. In addition, the BET surface area, bulk density, true density of the hierarchical zeolite are 57.67 m2/g, 0.684 g/cm3 and 2.771 g/cm3, respectively, and the total porosity is up to 75.3 vol%. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are crystalline aluminosilicates with uniform channels and cavities (sizes usually <1.3 nm) [1,2]. In the past decades, owing to its remarkable properties, such as uniform pore size, high surface area, strong acidity and high thermal/hydrothermal stability etc., zeolites have been successfully used as highly selective adsorbents, ion exchangers and catalysts in many important applications [1,3,4]. By introducing photoactive guest to the internal voids of zeolite pores, zeolites have opened the possibilities for the design of novel photonic materials [5–7]. Generally, zeolites are synthesized in the form of fine powders. In most industry applications, they are agglomerated into granules using various inert binding agents [8]. However, several problems arise due to the binding. The mass transport is slow and the active sites located inside the agglomerated products are hardly accessible [9,10]. Besides, the heavy use of binders dilutes the active zeolite and blocks the zeolite pores. Those drawbacks can severely limit the performance of zeolites. To overcome the problem of diffusion limitations, ‘‘hierarchical zeolites” were developed by zeolite scientists. Hierarchical zeolites possess at least one additional level of pores, i.e. mesopores or macropores. Up to now, the synthesis methods can be classified into two categories: ‘‘bottom-up” and ‘‘top-down” routes [1,9,11,12]. In particular, ‘‘bottom-up” routes comprise hard⇑ Corresponding author. E-mail address: [email protected] (C. Yan). https://doi.org/10.1016/j.matlet.2019.03.135 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
templating methods, soft-templating methods, assembly of nanosized zeolites and zeolitization of preformed materials. While ‘‘top-down” routes include the removal of framework atoms methods (dealumination, desilication and irradiation), and recrystallization methods. Among them, an interesting process to synthesis self-supporting hierarchical zeolite monoliths is using nonreactive or reactive solid foams as supports. Hierarchical zeolite monoliths are prepared by in situ or ex situ coating zeolite crystals on the support surface in hydrothermal solutions [11]. Nevertheless, this method poses several challenges, such as low zeolitesupport mass ratio, easy to lose zeolite due to the difference in thermal expansion coefficients etc. Thus, new effective method of preparing self-supporting hierarchical zeolite monoliths should be developed to solve the practical problems. The focus of this study therefore aims to explore a feasible approach for the synthesis of self-supporting hierarchical zeolites without using any foamed supports.
2. Materials and methods Metakaolin (MK) was collected from Super Technology Co. Ltd., Hunan. Commercially available polymethyl methacrylate microbeads (PMMA, 2000 mesh) obtained from Jinyunlai plastics business department, Dongguan, were used as sacrificial template. Sodium hydroxide (analytical purity) was purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used for making NaOH solution.
29
Y. Liu et al. / Materials Letters 248 (2019) 28–31
Fig. 1. Schematic illustrations of hierarchical zeolite LTA monoliths preparation process.
Table 1 Chemical composition of metakaolin by XRF technique (wt%) LOI = loss on ignition. Sample
Al2O3
SiO2
Fe2O3
TiO2
K2O
Na2O
CaO
MgO
P2O5
MnO
LOI
MK
45.4
51.4
0.4
1.3
0.1
0.2
0.1
<0.1
0.4
<0.1
0.4
Fig. 2. SEM images of (a) MK; (b) PMMA; (c) PMMA/zeolite composites; (d-e) hierarchical zeolite LTA, (f) the corresponding EDS results and (g) elemental mapping analysis.
30
Y. Liu et al. / Materials Letters 248 (2019) 28–31
Hierarchical zeolite monoliths were synthesized according to the following process (as illustrated in Fig. 1). Firstly, 2.50 g MK and 2.50 g PMMA were thoroughly mixed with 3.8 ml of 9.7 mol/ L NaOH solution for 8–10 min to make a homogeneous paste. The molar ratio of Na2O/Al2O3 and SiO2/Al2O3 of the paste are 1.2 and 1.9, respectively. The paste shows favorable workability in this condition. In flowing, the paste was cast into a silicone rubber mold, vibrated (to remove entrained air bubbles) and sealed with preservative film. The PMMA/zeolite composites were obtained after curing in an oven at 60 °C for 24 h. There is no limitation in size and shape for the preparation of PMMA/zeolite composites. Zeolite LTA monoliths with hierarchical structure were obtained by further heating the PMMA/zeolite composites to 550 °C for 2 h, at a heating rate of 10 °C/min. The chemical composition of MK was determined using X-ray fluorescence (AXIOSmAX, PANalytical Netherlands). The crystalline phase of the zeolite was recorded on a D8-Focus type X-ray powder diffractometer (BrokerAXS Germany). Microstructure observation of the samples were performed using a SU8010 type scanning electron microscopy (Hitachi Japan). Nitrogen gas adsorption–desorption experiments were performed using an automatic ASAP2020 surface area and porosimetry system (Micromeritics, America), and the specific surface area was calculated by the Brunauer–Em met–Teller (BET) method. The pore structure of hierarchical zeolite was also tested by an AutoPore IV 9510 mercury intrusion porosimeter (Micromeritics, America). 3. Results and discussion The main elemental components of MK are silica and alumina (Table 1). The molar ratio of SiO2/Al2O3 is 1.9, which is close to the theoretical value of 2.0 in molecular of zeolite A
(Na12(AlO2)12(SiO2)1227H2O). Therefore, MK is the ideal Si and Al sources for the synthesis of zeolite A. SEM micrographs of MK, PMMA, PMMA/zeolite composites, hierarchical zeolite LTA and the EDS of hierarchical zeolite are demonstrated in Fig. 2. It is observed that MK particles show agglomerated or pseudohexagonal plate like morphology (Fig. 2a). The diameters of the spherical PMMA microbeads are varying from 1 to 6 lm (Fig. 2b). Fig. 2c shows PMMA/zeolite composites are dense. There is no agglomeration of PMMA microbeads, indicating PMMA microbeads are well embedded within the zeolite matrix. After calcination, PMMA microbeads are removed, leaving behind interconnected voids (Fig. 2d). The morphology of those voids resembles the morphology of PMMA microbeads. Zeolite particles are intergrown at their interfaces, establishing a stable porous system. No significant cracks can be seen in the hierarchical zeolite specimens (Fig. 2d inserted image), which indicate the sacrificial template method is a feasible method to prepare hierarchical zeolites. Fig. 2e presents typical zeolite LTA crystals. They are cubeshaped with particle size around 3 lm. Moreover, some particles with irregular shapes and different sizes can also be observed. These particles might be zeolite precursor-geopolymer, which failed to transform into well crystalline zeolites. These geopolymer particles can act as binder in the monoliths, giving the samples certain strength. Fig. 2f is the EDS spectra of zeolite LTA. It shows a Na: Al:Si:O molar ratio of 1:1.08:1.10:3.45, which is close to the molar ratio of the theoretical zeolite LTA formula (Na12Al12Si12O48(H2O)27, as identified in Fig. 3a). Elemental mapping analysis reveals that the O, Na, Al and Si elements were dispersed uniformly in zeolites. Fig. 3a presents the XRD patterns of MK and hierarchical zeolite. There is a broad hump between 2h = 18 and 30°, indicating MK is
Fig. 3. (a) XRD patterns of metakaolin (MK) and hierarchical zeolite (HZ), (b) BJH pore size distribution, (c) N2 adsorption-desorption isotherms and (d) MIP results of hierarchical zeolite.
Y. Liu et al. / Materials Letters 248 (2019) 28–31
amorphous. After the curing and calcination process, the amorphous phase is converted into crystalline phase. The characteristic diffraction peaks of the zeolite match well with zeolite LTA (JCPDS card number 73-2340). The BJH pore size distribution (PSD) and N2 adsorptiondesorption isotherms of hierarchical zeolite are plotted in Fig. 3b-c. The PSD curve (Fig. 3b) shows that the hierarchical zeolite possesses a broad distribution of pore sizes, and three types of pores (micro-, meso- and macrospores) present in the zeolite products. The N2 adsorption-desorption isotherms of hierarchical zeolite are shown in Fig. 3c. It is a type II isotherm with a H3 hysteresis loop according to International Union of Pure and Applied Chemistry (IUPAC) classification [13]. Type II isotherms represent adsorbents are non-porous or macroporous materials. At P/Po = 0.96–1.00, the isotherm rises nearly horizontal and parallel to the gas adsorption axis, indicating the presence of very wide range of macropores. The phenomenon can also be confirmed by the PSD curve. The Type H3 loop reveals that the zeolite product having nano-sized slit-like pores, which might be caused by the intercrystalline aggregation of zeolite crystals. The BET surface area and total pore volume of the hierarchical zeolite are 57.67 m2/g and 0.0569 cm3/g, respectively. The BET surface area is quite lower than zeolite 4A synthesized from hydrothermal strategies [14], however, it is in accordance with zeolites fabricated from solvent-free method [15]. The bulk density, true density and porosity of hierarchical zeolite were tested by MIP. The measured bulk density, true density of hierarchical zeolite are 0.684 g/cm3 and 2.771 g/cm3, respectively. The total porosity is up to 75.3 vol%. The average pore dimeter of the hierarchical zeolite tested by MIP is 4.73 lm, which is compatible to the diameters of the sacrificial templates. The pore size distribution and differential curves of hierarchical zeolite are shown in Fig. 3d. The sample has bi-modal pore size distribution. Most of the pore diameters centered at around 3 lm and a small amount of pore diameters are distributed around 280 lm. As it is evident, the broad peak centered around 3 lm is due to the PMMA burn out. The pore diameters around 280 lm in the hierarchical zeolite are probably caused by air bubbles introduced during the mixing and casting process. From the above observations, we can reasonably conclude that it is able to manufacture hierarchical zeolite with desired macropores by choosing appropriate PMMA mocrobeads diameters. 4. Conclusions In summary, hierarchical zeolite LTA monoliths were successfully synthesized using PMMA microbeads as the transitional
31
template through solvent-free crystallization method followed by a calcination process. The zeolite/PMMA composites were heated to 550 °C for 2 h to eliminate PMMA microbeads, leaving behind zeolite monoliths with interconnected macropores. This method extends the diversity of synthetic procedures. The material for zeolite synthesizes and the templates for macropores are commercially available, abundant and inexpensive. Moreover, the synthetic route is very simple, even suitable for large-scale production. 5. Declarations of interest None. Acknowledgements This work was supported by National Key R&D Program of China under the project No. 2017YFB0310805, Engineering Research Center of non-metallic minerals of Zhejiang Province, Key Laboratory of Clay Minerals, Ministry of Land and Resources, and Engineering Research Center of Nano-Geo Materials of Ministry of Education (NGM2019KF016). References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15]
D.P. Serrano, J.M. Escola, P. Pizarro, Chem. Soc. Rev. 42 (9) (2013) 4004–4035. Y. Liu, C. Yan, Z. Zhang, H. Wang, S. Zhou, W. Zhou, Fuel 185 (2016) 181–189. E.T. Vogt, B.M. Weckhuysen, Chem. Soc. Rev. 44 (20) (2015) 7342–7370. L. Peng, X. Xu, X. Yao, H. Liu, X. Gu, J. Membrane Sci. 549 (2018) 446–455. K. Kanimozhi, S. Khaleel Basha, V. Sugantha Kumari, K. Kaviyarasu, M. Maaza, Appl. Surf. Sci. 449 (2018) 574–583. K. Kombaiah, J.J. Vijaya, L.J. Kennedy, M. Bououdina, R.J. Ramalingam, H.A. Al Lohedan, Ceram. Int. 43 (10) (2017) 7682–7689. S.K. Jesudoss, J.J. Vijaya, L.J. Kennedy, P.I. Rajan, H.A. Al Lohedan, R.J. Ramalingam, K. Kaviyarasu, M. Bououdina, J. Photochem. Photobiol. B 165 (2016) 121–132. Y. Liu, C. Yan, Z. Zhang, L. Li, H. Wang, S. Pu, Mater. Lett. 196 (2017) 328–331. A. Feliczak Guzik, Microporous Mesoporous Mater. 259 (2018) 33–45. S.M. Al Jubouri, S.M. Holmes, Chem. Eng. J. 308 (2017) 476–491. S. Lopez Orozco, A. Inayat, A. Schwab, T. Selvam, W. Schwieger, Adv. Mater. 23 (22–23) (2011) 2602–2615. L. Chen, X. Li, J.C. Rooke, Y. Zhang, X. Yang, Y. Tang, F. Xiao, B. Su, J. Mater. Chem. 22 (34) (2012) 17381. K.S. Sing, Pure Appl. Chem. 57 (4) (1985) 603–619. X. Ni, Z. Zheng, X. Wang, S. Zhang, M. Zhao, J. Alloys Compd. 592 (2014) 135– 139. Y. Liu, C. Yan, J. Zhao, Z. Zhang, H. Wang, S. Zhou, L. Wu, J. Clean. Prod. 202 (2018) 11–22.