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Effect of Alkalinity Towards the Formation of NaX Zeolite Membranes
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ScienceDirect Materials Today: Proceedings 19 (2019) 1287–1293
www.materialstoday.com/proceedings
ICCSE 2018
Effect of Alkalinity Towards the Formation of NaX Zeolite Membranes Liyana Salwa Mohd Nazira,b *, Yin Fong Yeong a, Thiam Leng Chew a a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak. b PETRONAS Research Sdn. Bhd, Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor.
Abstract In this study, the effect of solution alkalinity towards zeolite crystallization and growth were investigated for NaX zeolite crystals and NaX zeolite membrane grown via secondary growth. NaX zeolite crystals were synthesized by using different Na2O/Al2O3 ratios of 15, 16, 17, 18 and 19. XRD and FESEM results indicated that Na2O/Al2O3 ratio of 17 and above produced NaX zeolite crystal with high crystallinity ad single phase NaX zeolite. NaX zeolite membrane were also synthesized by varying Na2O/Al2O3 ratios of 17 and 19 in order to study the growth of NaX zeolite membrane via secondary growth. Na2O/Al2O3 ratio of 17 resulted in better intercrystalline growth of the membrane layer compared with the membrane synthesized using mixture solution with Na2O/Al2O3 ratio of 19. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: Zeolite membrane; Faujasite; NaX membrane; Alkalinity; zeolite crystallization; zeolite growth
1. Introduction Zeolites are crystalline, hydrated aluminosilicates having microporous and regular structures [1]. They are built up from of TO4 (T = Si or Al) tetrahedral which result in the various zeolite pore sizes and structures [2]. The zeolite micropores are of molecular size which give them adsorption, catalytic and ion exchange properties. Due to these characteristics, they are widely used in various environmental and industrial application such as ion exchanged [3– 5], catalysts [2], adsorbents [6] and membrane separation [7–9]. The application of zeolite membrane for gas
*
Corresponding author. Tel.: +605-3688000
E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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separation is favorable among other type of membrane because of their high chemical resistance and superior selectivity compared to polymeric membranes [10–12]. Zeolite membranes synthesized on porous supports has been studied for applications in separations of gas, vapour or liquid mixtures. Example of different types of zeolite membrane such as CHA, FAU, DDR and LTA has been reported in literature [3,10,13–20]. FAU is a large pore zeolite with pore diameter of 7.4 Å [6], where it can be used for separation involving larger molecules [14]. FAU type zeolite can be classified into NaX zeolite and NaY zeolite depending on their Si/Al ratios [15]. As for the synthesis of FAU type zeolite membrane, many studies has been reported for various application such as CO2/N2 separation [15], hydrocarbon separation [14], water vapour separation [21], and CO2/CH4 separation [22] Zeolite crystals and zeolite membranes are commonly prepared by hydrothermal synthesis in which reaction mixture is heated at temperature of 80 – 230 ˚C, depending on the zeolite to be crystallized for several hours or even days under autogenous pressure in an autoclave [7]. There are numerous variables that affects the growth and formation of zeolite membrane such as molar composition of synthesis gel, reactant source, Si/Al ratio, alkalinity, water content, inorganic cations, organic templates, solvents, temperature, aging stirring and seeding [8]. It is important understand how these parameters affects the formation of zeolite membrane in order to control the synthesis process and produce zeolite membrane with a continuous, defect free layer of zeolite crystals so that only transport through the zeolite pore takes place [9]. The existence of defects will significantly affect the separation performance of the zeolite membranes. Defects are define as intercrystalline spaces (sometimes called non zeolitic pores) that are larger than zeolite pores [10]. Both defect number and size must be minimized in order to take advantage of molecular sieving and preferential adsorption properties of zeolite pores and to reduce flux through defects [10]. Herein, NaX zeolite crystal and NaX zeolite membrane were synthesis using synthesis solution at varied
Na2O/Al2O3 ratios. The relationship between Na2O/Al2O3 ratios which denotes the solutions alkalinity and the growth and formation of NaX zeolite crystals and NaX zeolite membrane layer were discussed. 2. Experimental 2.1. NaX zeolite seed Preparation NaX zeolite synthesis was carried out via in-situ crystallization method following procedure as described in literature [17]. The materials used were as follows: (1) sodium silicate (100% (w/w) Na2SiO3; Fisher Chemical, UK); (2) sodium aluminate (100% (w/w) NaAlO2. Fisher Chemical, UK); (3) sodium hydroxide pellets (90-100% (w/w) NaOH; Avantor Performance Materials, USA) and (4) Deionized water. All materials were used as purchased. The aluminosilicate gel was prepare by mixing the sodium silicate, sodium aluminate, sodium hydroxide (NaOH) and deionized (DI) water. The mole ratio of the aluminosilicate gel composition were Al2O3 : 4.8SiO2 : xNa2O : 975H2O where x is varied from 15 – 19.The gel mixture was stirred for 4 hours at room temperature prior to the hydrothermal synthesis. Then, the synthesis gel was filled into an autoclave and hydrothermal synthesis was performed in an oven at temperatures of 90 °C and for 24 hours under autogenous pressure. The resultant crystals were recovered by gravitational filtration and washing with DI water until the pH is less than 10. Afterwards the filtered crystal was dried in oven at 90 ˚C 2.2. Zeolite Seed Layer Preparation and Synthesis of NaX zeolite membrane NaX zeolite membrane were grown on a porous α-alumina disc-type support via secondary growth method. Prior to hydrothermal synthesis, a thin layer of zeolite seed was coated onto the α-alumina disc-type support via dead end vacuum filtration method. The vacuum coating method were performed using zeolite solution concentration of 0.5 wt% as describe in literature [25]. The aluminosilicate gel used for the synthesis of NaX membrane were prepared by mixing sodium silicate, sodium aluminate, NaOH and DI water with composition of Al2O3 : 4.8SiO2 : xNa2O : 975H2O where x is varied at 17 and 19. The solution was stirrer for 4 hours at room temperature. The seeded α-
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alumina disc was placed in autoclave, which was filled with the synthesis solution. The autoclave was then placed into an oven for hydrothermal synthesis at 90˚C for 24 hours. Upon completion of hydrothermal synthesis the membrane was collected, wash with DI water and dried in air at ambient temperature. The NaX zeolite powder samples were characterized using powder X-ray diffraction (XRD; Shimadzu XRD7000) with Cu-Kα radiation, operated at 40.0 kV and 30.0 mA in order to identify the X-ray diffraction patterns of the crystal structure and determine the zeolite phase present in the samples. The zeolite membrane morphology was determined by using field emission scanning electron microscope (FESEM; Hitachi SU8020). Elemental analysis was also conducted using energy dispersive X-ray spectroscopy (EDS; Horiba EMAX) to determine the Si/Al ratio of the sample 3. Results and Discussion 3.1. Synthesis of NaX zeolite with varied alkalinity Crystallization of zeolites which occurs through nucleation and crystal growth is very much depends on alkalinity of the starting solution [26]. A higher alkalinity solution increases the solubility of the Si and Al sources, decreases the degree of polymerization of the silicate anions, and accelerates the polymerization of the polysilicate and aluminate anions. Thus, higher solution alkalinity will result in reduction of the induction and nucleation periods and speed up the crystallization of zeolites [23]. The effect of alkalinity on the properties of NaX zeolites crystal was verified by varying the Na2O/Al2O3 molar ratios of 15, 16, 17, 18 and 19 in the starting solution. Hydrothermal synthesis treatment was set at 90 °C for 24 hours. Figure 1 shows the diffraction patterns of samples prepared at various Na2O/Al2O3 ratios. It can be observed from Figure 1 that NaX zeolites can be obtained at Na2O/Al2O3 ratio of 16 and above. At low amount of Na2O/Al2O3 ratio of 15, mixture of crystal and amorphous phase was observed, indicating that lower Na2O/Al2O3 ratios may inhibit the formation of zeolite crystals. This is probably because low alkalinity reduced the dissolution of silicate and led to a poor conversion to zeolites [26]. As the Na2O/Al2O3 ratio was increased to 16, a mixture of NaX and NaP zeolites crystals started to form. Increasing the Na2O/Al2O3 ratio to 17 and above resulted in high crystalline single phase NaX zeolite crystal, indicated by the sharp and intense XRD peaks and low background radiation line. This result suggests that low Na2O/Al2O3 ratio could induce the formation of NaP zeolites and higher Na2O/Al2O3 ratio is favoured for the formation of NaX zeolite phase. Subsequently, the samples were characterized using FESEM in order to observe the morphology of the zeolite crystals formed. The FESEM images are shown in Figure 2. It can be observed from Figure 2 that increasing the Na2O/Al2O3 ratio resulted in the decreased of particle size and the particle size of the samples is indicated in Table 1. Referring to Figure 2, samples synthesized with Na2O/Al2O3 ratio of 17, 18 and 19 exhibit well define particles with octahedral morphology, resembling FAU zeolite phase [27]. Whereas, samples with Na2O/Al2O3 ratio of 16 (Fig. 2 b) resulted in the formation of NaP zeolite, which resemble the shape of spherelutic particles. This is in agreement with results reported by Chaves et. Al [28]. Chaves et al also described the changes in zeolite morphology with increased solution alkalinity. Increasing alkalinity resulted in more irregular particles. This can be observed for sample with Na2O/Al2O3 of 19 shown in Figure 2e. Meanwhile, the Si/Al ratio of the samples are shown in Table 1. All samples exhibited Si/Al ratio of less than 1.5, which confirmed the formation of NaX zeolite. A slight reduction on the Si/Al ratio is observed as the Na2O/Al2O3 ratio of the solution increased.
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Fig. 1. XRD patterns of zeolite samples synthesized at varied Na2O/Al2O3 ratios (▼: NaP zeolite, ●: NaX zeolite)
Fig. 2. FESEM images of zeolites particle synthesized at Na2O/Al2O3 ratio of (a) 15, (b) 16, (c) 17, (d) 18 and (e) 19
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Table 1. Properties of NaX zeolite synthesized at different Na2O/Al2O3 ratios Na2O/Al2O3
Si/Al
Particle diameter (µm)*
15
1.44
NA
16
1.44
5- 9
17
1.39
4-9
18
1.35
2-5
19
1.33
1-3
* Particle size was measured using FESEM
3.2. Formation of NaX zeolite membrane layer at different Na2O/Al2O3 ratios The effects of solution alkalinity towards membrane layer formation was studied and the zeolite membranes were synthesized by varying the Na2O/Al2O3 ratios of 17 and 19. The membranes synthesis were performed 2 times and indicated as 1st layer membrane growth and 2nd layer membrane growth, respectively. Figure 3 shows the images obtained from SEM imaging of the resultant membranes. At Na2O/Al2O3 ratio of 17 (Fig. 3b & c), it is observed that after 1st layer synthesis, a continuous layer of zeolite membrane with good zeolite intergrowth is observed. Upon further synthesis for 2nd layer, zeolite crystal intergrowth improved as indicated by increment in crystals size on the membrane layer. Increasing Na2O/Al2O3 ratio to 19 resulted in the reduction of membrane intergrowth between zeolite crystals as observed in Fig. 3e and f. Defects in term of intercrystalline gaps can be observed on the membrane layer. Further synthesis to 2nd layer improved the membrane layer intergrowth between the zeolite crystals. Based on the SEM results, it can be concluded that increasing alkalinity of the synthesis solution leads to lowered zeolite crystal growth rate. Na2O/Al2O3 ratio of 17 is more favorable to reduce intercrystalline gaps formed on the membrane layer.
Fig. 3. NaX zeolite membrane synthesized using synthesis solution with Na2O/Al2O3 ratio of 17; a) seed layer, b) 1st layer membrane, c) 2nd layer membrane and Na2O/Al2O3 of 19; d) seed layer e) 1st layer membrane f) 2nd layer membrane
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4. Conclusion In this work, optimum solution alkalinity with Na2O/Al2O3 ratios of 17 to 19 were found suitable for the formation of high crystalline NaX zeolite crystals and membranes. The NaX zeolite membrane synthesized using mixture solution with Na2O/Al2O3 ratio of 17 showed good crystals intergrowth and lower intercrystalline gaps. Acknowledgements The authors would like to acknowledge the financial support from PETROLIAM Research Fund (PRF), Universiti Teknologi PETRONAS and PETRONAS Research Sdn Bhd. References [1]
C.S. Cundy, P.A. Cox, The Hydrothermal Synthesis of Zeolites : History and Development from the Earliest Days to the Present Time,
[2]
C. Feng, K.C. Khulbe, T. Matsuura, R. Farnood, A.F. Ismail, Recent Progress in Zeolite / Zeotype Membranes, J. Membr. Sci. Res. 1
[3]
G. Guan, K. Kusakabe, S. Morooka, GAS PERMEATION PROPERTIES OF ION-EXCHANGED LTA-TYPE ZEOLITE
[4]
M. Asghari, T. Mohammadi, A. Samimi, M. Fouladi, Ion-exchanged zeolite X membranes : synthesis and characterisation, Membr.
[5]
A.M. Tarditi, E.A. Lombardo, Influence of exchanged cations ( Na + , Cs + , Sr 2 + and Ba 2 + ) on xylene permeation through ZSM-5
Chem. Rev. 103 (2003) 663–701. (2015) 49–72. MEMBRANES, Sep. Sci. Techology. 36 (2001) 2233–2245. doi:10.1081/SS-100105915. Technol. (2008) 2–4. / SS tubular membranes, 61 (2008) 136–147. doi:10.1016/j.seppur.2007.10.008. [6]
M. Tagliabue, D. Farrusseng, S. Valencia, S. Aguado, U. Ravon, C. Rizzo, A. Corma, C. Mirodatos, Natural gas treating by selective adsorption: Material science and chemical engineering interplay, Chem. Eng. J. 155 (2009) 553–566. doi:10.1016/j.cej.2009.09.010.
[7]
J. Caro, M. Noack, Zeolite membranes - Recent developments and progress, Microporous Mesoporous Mater. 115 (2008) 215–233.
[8]
J. Coronas, J. Santamaria, Separations using zeolite membranes, Sep. Purif. Methods. 28 (1999) 127–177.
doi:10.1016/j.micromeso.2008.03.008. doi:10.1080/03602549909351646. [9]
N. Kosinov, J. Gascon, F. Kapteijn, E.J.M. Hensen, Recent developments in zeolite membranes for gas separation, J. Memb. Sci. 499 (2016) 65–79. doi:10.1016/j.memsci.2015.10.049.
[10]
T. Wu, M.C. Diaz, Y. Zheng, R. Zhou, H.H. Funke, J.L. Falconer, R.D. Noble, Influence of propane on CO2/CH4 and N2/CH4
[11]
K.A. Lokhandwala, Nitrogen Removal from Natural Gas, 1999.
[12]
P. Gorgojo, Ó. de la Iglesia, J. Coronas, Preparation and Characterization of Zeolite Membranes, Membr. Sci. Technol. 13 (2008) 135–
[13]
K. Weh, M. Noack, I. Sieber, J. Caro, Permeation of single gases and gas mixtures through faujasite-type molecular sieve membranes,
separations in CHA zeolite membranes, J. Memb. Sci. 473 (2015) 201–209. doi:10.1016/j.memsci.2014.09.021.
175. doi:10.1016/S0927-5193(07)13005-9. Microporous Mesoporous Mater. 54 (2002) 27–36. [14]
V. Nikolakis, G. Xomeritakis, A. Abibi, M. Dickson, M. Tsapatsis, D.G. Vlachos, Growth of a faujasite-type zeolite membrane and its
[15]
X. Gu, J. Dong, T.M. Nenoff, Synthesis of defect-free FAU-type zeolite membranes and separation for dry and moist CO2/N2
[16]
K. Kusakabe, T. Kuroda, S. Morooka, Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeolite
[17]
Y. Hasegawa, K. Watanabe, K. Kusakabe, S. Morooka, The separation of CO 2 using Y-type zeolite membranes ion-exchanged with
[18]
Z. Zong, M.A. Carreon, Thin SAPO-34 membranes synthesized in stainless steel autoclaves for N2/CH4 separation, J. Memb. Sci. 524
application in the separation of saturated / unsaturated hydrocarbon mixtures, J. Memb. Sci. 184 (2001) 209–219. mixtures, Ind. Eng. Chem. Res. 44 (2005) 937–944. doi:10.1021/ie049263i. membranes formed on porous support tubes, J. Memb. Sci. 148 (1998) 13–23. alkali metal cations, Sep. Purif. Technol. 22–23 (2001) 319–325. (2017) 117–123. doi:10.1016/j.memsci.2016.11.011. [19]
J.V.A.N.D.E.N. Bergh, W. Zhu, F. Kapteijn, Separation of CO 2 and CH 4 by a DDR membrane, Res. Chem. Intermed. 34 (2008) 467–
[20]
J. van den Bergh, W. Zhu, J. Gascon, J.A. Moulijn, F. Kapteijn, Separation and permeation characteristics of a DD3R zeolite
474. membrane, J. Memb. Sci. 316 (2008) 35–45. doi:10.1016/j.memsci.2007.12.051.
L.S.M. Nazir et al. / Materials Today: Proceedings 19 (2019) 1287–1293 [21]
1293
T.F. Mastropietro, A. Brunetti, P.F. Zito, T. Poerio, H. Richter, M. Weyd, S. Wöhner, E. Drioli, G. Barbieri, Study of the separation properties of FAU membranes constituted by hierarchically assembled nanozeolites, Sep. Purif. Technol. 156 (2015) 321–327. doi:10.1016/j.seppur.2015.10.018.
[22]
Y. Hasegawa, T. Tanaka, K. Watanabe, B. Jeong, Separation of CO2 - CH4 and CO2 - N2 Systems Using Ion-exchanged FAU-type
[23]
J. Yu, Synthesis of zeolites, in: Introd. to Zeolite Sci. Pract. -3rd Revis. Ed., Elsevier B.V., 2007: pp. 39–103. doi:10.1016/S0167-
[24]
M. Yu, R.D. Noble, J.L. Falconer, Zeolite membranes: Microstructure characterization and permeation mechanisms, Acc. Chem. Res.
Zeolite Membranes with Different Si/Al Ratios, Korean J. Chem. Eng. 19 (2002) 309–313. 2991(07)80791-9. 44 (2011) 1196–1206. doi:10.1021/ar200083e. [25]
L.S.M. Nazir, Y.F. Yeong, T.L. Chew, A. Ahmadon, The effect of zeolite suspension concentration on the formation of seed layer via vacuum seeding prior to NaX zeolite membranes growth, IOP Conf. Ser. Mater. Sci. Eng. 380 (2018) 012019. doi:10.1088/1757899X/380/1/012019.
[26]
L. Liu, T. Du, G. Li, F. Yang, S. Che, Using one waste to tackle another: Preparation of a CO2capture material zeolite X from laterite residue and bauxite, J. Hazard. Mater. 278 (2014) 551–558. doi:10.1016/j.jhazmat.2014.06.041.
[27]
H. Robson, Verified Syntheses of zeolitic materials, 2016. http://www.iza-online.org/synthesis/VS_3rdEd.pdf.
[28]
T.F. Chaves, H.O. Pastore, D. Cardoso, A simple synthesis procedure to prepare nanosized faujasite crystals, Microporous Mesoporous Mater. 161 (2012) 67–75. doi:10.1016/j.micromeso.2012.05.022.
ScienceDirect Materials Today: Proceedings 19 (2019) 1287–1293
www.materialstoday.com/proceedings
ICCSE 2018
Effect of Alkalinity Towards the Formation of NaX Zeolite Membranes Liyana Salwa Mohd Nazira,b *, Yin Fong Yeong a, Thiam Leng Chew a a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak. b PETRONAS Research Sdn. Bhd, Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor.
Abstract In this study, the effect of solution alkalinity towards zeolite crystallization and growth were investigated for NaX zeolite crystals and NaX zeolite membrane grown via secondary growth. NaX zeolite crystals were synthesized by using different Na2O/Al2O3 ratios of 15, 16, 17, 18 and 19. XRD and FESEM results indicated that Na2O/Al2O3 ratio of 17 and above produced NaX zeolite crystal with high crystallinity ad single phase NaX zeolite. NaX zeolite membrane were also synthesized by varying Na2O/Al2O3 ratios of 17 and 19 in order to study the growth of NaX zeolite membrane via secondary growth. Na2O/Al2O3 ratio of 17 resulted in better intercrystalline growth of the membrane layer compared with the membrane synthesized using mixture solution with Na2O/Al2O3 ratio of 19. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: Zeolite membrane; Faujasite; NaX membrane; Alkalinity; zeolite crystallization; zeolite growth
1. Introduction Zeolites are crystalline, hydrated aluminosilicates having microporous and regular structures [1]. They are built up from of TO4 (T = Si or Al) tetrahedral which result in the various zeolite pore sizes and structures [2]. The zeolite micropores are of molecular size which give them adsorption, catalytic and ion exchange properties. Due to these characteristics, they are widely used in various environmental and industrial application such as ion exchanged [3– 5], catalysts [2], adsorbents [6] and membrane separation [7–9]. The application of zeolite membrane for gas
*
Corresponding author. Tel.: +605-3688000
E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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separation is favorable among other type of membrane because of their high chemical resistance and superior selectivity compared to polymeric membranes [10–12]. Zeolite membranes synthesized on porous supports has been studied for applications in separations of gas, vapour or liquid mixtures. Example of different types of zeolite membrane such as CHA, FAU, DDR and LTA has been reported in literature [3,10,13–20]. FAU is a large pore zeolite with pore diameter of 7.4 Å [6], where it can be used for separation involving larger molecules [14]. FAU type zeolite can be classified into NaX zeolite and NaY zeolite depending on their Si/Al ratios [15]. As for the synthesis of FAU type zeolite membrane, many studies has been reported for various application such as CO2/N2 separation [15], hydrocarbon separation [14], water vapour separation [21], and CO2/CH4 separation [22] Zeolite crystals and zeolite membranes are commonly prepared by hydrothermal synthesis in which reaction mixture is heated at temperature of 80 – 230 ˚C, depending on the zeolite to be crystallized for several hours or even days under autogenous pressure in an autoclave [7]. There are numerous variables that affects the growth and formation of zeolite membrane such as molar composition of synthesis gel, reactant source, Si/Al ratio, alkalinity, water content, inorganic cations, organic templates, solvents, temperature, aging stirring and seeding [8]. It is important understand how these parameters affects the formation of zeolite membrane in order to control the synthesis process and produce zeolite membrane with a continuous, defect free layer of zeolite crystals so that only transport through the zeolite pore takes place [9]. The existence of defects will significantly affect the separation performance of the zeolite membranes. Defects are define as intercrystalline spaces (sometimes called non zeolitic pores) that are larger than zeolite pores [10]. Both defect number and size must be minimized in order to take advantage of molecular sieving and preferential adsorption properties of zeolite pores and to reduce flux through defects [10]. Herein, NaX zeolite crystal and NaX zeolite membrane were synthesis using synthesis solution at varied
Na2O/Al2O3 ratios. The relationship between Na2O/Al2O3 ratios which denotes the solutions alkalinity and the growth and formation of NaX zeolite crystals and NaX zeolite membrane layer were discussed. 2. Experimental 2.1. NaX zeolite seed Preparation NaX zeolite synthesis was carried out via in-situ crystallization method following procedure as described in literature [17]. The materials used were as follows: (1) sodium silicate (100% (w/w) Na2SiO3; Fisher Chemical, UK); (2) sodium aluminate (100% (w/w) NaAlO2. Fisher Chemical, UK); (3) sodium hydroxide pellets (90-100% (w/w) NaOH; Avantor Performance Materials, USA) and (4) Deionized water. All materials were used as purchased. The aluminosilicate gel was prepare by mixing the sodium silicate, sodium aluminate, sodium hydroxide (NaOH) and deionized (DI) water. The mole ratio of the aluminosilicate gel composition were Al2O3 : 4.8SiO2 : xNa2O : 975H2O where x is varied from 15 – 19.The gel mixture was stirred for 4 hours at room temperature prior to the hydrothermal synthesis. Then, the synthesis gel was filled into an autoclave and hydrothermal synthesis was performed in an oven at temperatures of 90 °C and for 24 hours under autogenous pressure. The resultant crystals were recovered by gravitational filtration and washing with DI water until the pH is less than 10. Afterwards the filtered crystal was dried in oven at 90 ˚C 2.2. Zeolite Seed Layer Preparation and Synthesis of NaX zeolite membrane NaX zeolite membrane were grown on a porous α-alumina disc-type support via secondary growth method. Prior to hydrothermal synthesis, a thin layer of zeolite seed was coated onto the α-alumina disc-type support via dead end vacuum filtration method. The vacuum coating method were performed using zeolite solution concentration of 0.5 wt% as describe in literature [25]. The aluminosilicate gel used for the synthesis of NaX membrane were prepared by mixing sodium silicate, sodium aluminate, NaOH and DI water with composition of Al2O3 : 4.8SiO2 : xNa2O : 975H2O where x is varied at 17 and 19. The solution was stirrer for 4 hours at room temperature. The seeded α-
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alumina disc was placed in autoclave, which was filled with the synthesis solution. The autoclave was then placed into an oven for hydrothermal synthesis at 90˚C for 24 hours. Upon completion of hydrothermal synthesis the membrane was collected, wash with DI water and dried in air at ambient temperature. The NaX zeolite powder samples were characterized using powder X-ray diffraction (XRD; Shimadzu XRD7000) with Cu-Kα radiation, operated at 40.0 kV and 30.0 mA in order to identify the X-ray diffraction patterns of the crystal structure and determine the zeolite phase present in the samples. The zeolite membrane morphology was determined by using field emission scanning electron microscope (FESEM; Hitachi SU8020). Elemental analysis was also conducted using energy dispersive X-ray spectroscopy (EDS; Horiba EMAX) to determine the Si/Al ratio of the sample 3. Results and Discussion 3.1. Synthesis of NaX zeolite with varied alkalinity Crystallization of zeolites which occurs through nucleation and crystal growth is very much depends on alkalinity of the starting solution [26]. A higher alkalinity solution increases the solubility of the Si and Al sources, decreases the degree of polymerization of the silicate anions, and accelerates the polymerization of the polysilicate and aluminate anions. Thus, higher solution alkalinity will result in reduction of the induction and nucleation periods and speed up the crystallization of zeolites [23]. The effect of alkalinity on the properties of NaX zeolites crystal was verified by varying the Na2O/Al2O3 molar ratios of 15, 16, 17, 18 and 19 in the starting solution. Hydrothermal synthesis treatment was set at 90 °C for 24 hours. Figure 1 shows the diffraction patterns of samples prepared at various Na2O/Al2O3 ratios. It can be observed from Figure 1 that NaX zeolites can be obtained at Na2O/Al2O3 ratio of 16 and above. At low amount of Na2O/Al2O3 ratio of 15, mixture of crystal and amorphous phase was observed, indicating that lower Na2O/Al2O3 ratios may inhibit the formation of zeolite crystals. This is probably because low alkalinity reduced the dissolution of silicate and led to a poor conversion to zeolites [26]. As the Na2O/Al2O3 ratio was increased to 16, a mixture of NaX and NaP zeolites crystals started to form. Increasing the Na2O/Al2O3 ratio to 17 and above resulted in high crystalline single phase NaX zeolite crystal, indicated by the sharp and intense XRD peaks and low background radiation line. This result suggests that low Na2O/Al2O3 ratio could induce the formation of NaP zeolites and higher Na2O/Al2O3 ratio is favoured for the formation of NaX zeolite phase. Subsequently, the samples were characterized using FESEM in order to observe the morphology of the zeolite crystals formed. The FESEM images are shown in Figure 2. It can be observed from Figure 2 that increasing the Na2O/Al2O3 ratio resulted in the decreased of particle size and the particle size of the samples is indicated in Table 1. Referring to Figure 2, samples synthesized with Na2O/Al2O3 ratio of 17, 18 and 19 exhibit well define particles with octahedral morphology, resembling FAU zeolite phase [27]. Whereas, samples with Na2O/Al2O3 ratio of 16 (Fig. 2 b) resulted in the formation of NaP zeolite, which resemble the shape of spherelutic particles. This is in agreement with results reported by Chaves et. Al [28]. Chaves et al also described the changes in zeolite morphology with increased solution alkalinity. Increasing alkalinity resulted in more irregular particles. This can be observed for sample with Na2O/Al2O3 of 19 shown in Figure 2e. Meanwhile, the Si/Al ratio of the samples are shown in Table 1. All samples exhibited Si/Al ratio of less than 1.5, which confirmed the formation of NaX zeolite. A slight reduction on the Si/Al ratio is observed as the Na2O/Al2O3 ratio of the solution increased.
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Fig. 1. XRD patterns of zeolite samples synthesized at varied Na2O/Al2O3 ratios (▼: NaP zeolite, ●: NaX zeolite)
Fig. 2. FESEM images of zeolites particle synthesized at Na2O/Al2O3 ratio of (a) 15, (b) 16, (c) 17, (d) 18 and (e) 19
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Table 1. Properties of NaX zeolite synthesized at different Na2O/Al2O3 ratios Na2O/Al2O3
Si/Al
Particle diameter (µm)*
15
1.44
NA
16
1.44
5- 9
17
1.39
4-9
18
1.35
2-5
19
1.33
1-3
* Particle size was measured using FESEM
3.2. Formation of NaX zeolite membrane layer at different Na2O/Al2O3 ratios The effects of solution alkalinity towards membrane layer formation was studied and the zeolite membranes were synthesized by varying the Na2O/Al2O3 ratios of 17 and 19. The membranes synthesis were performed 2 times and indicated as 1st layer membrane growth and 2nd layer membrane growth, respectively. Figure 3 shows the images obtained from SEM imaging of the resultant membranes. At Na2O/Al2O3 ratio of 17 (Fig. 3b & c), it is observed that after 1st layer synthesis, a continuous layer of zeolite membrane with good zeolite intergrowth is observed. Upon further synthesis for 2nd layer, zeolite crystal intergrowth improved as indicated by increment in crystals size on the membrane layer. Increasing Na2O/Al2O3 ratio to 19 resulted in the reduction of membrane intergrowth between zeolite crystals as observed in Fig. 3e and f. Defects in term of intercrystalline gaps can be observed on the membrane layer. Further synthesis to 2nd layer improved the membrane layer intergrowth between the zeolite crystals. Based on the SEM results, it can be concluded that increasing alkalinity of the synthesis solution leads to lowered zeolite crystal growth rate. Na2O/Al2O3 ratio of 17 is more favorable to reduce intercrystalline gaps formed on the membrane layer.
Fig. 3. NaX zeolite membrane synthesized using synthesis solution with Na2O/Al2O3 ratio of 17; a) seed layer, b) 1st layer membrane, c) 2nd layer membrane and Na2O/Al2O3 of 19; d) seed layer e) 1st layer membrane f) 2nd layer membrane
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4. Conclusion In this work, optimum solution alkalinity with Na2O/Al2O3 ratios of 17 to 19 were found suitable for the formation of high crystalline NaX zeolite crystals and membranes. The NaX zeolite membrane synthesized using mixture solution with Na2O/Al2O3 ratio of 17 showed good crystals intergrowth and lower intercrystalline gaps. Acknowledgements The authors would like to acknowledge the financial support from PETROLIAM Research Fund (PRF), Universiti Teknologi PETRONAS and PETRONAS Research Sdn Bhd. References [1]
C.S. Cundy, P.A. Cox, The Hydrothermal Synthesis of Zeolites : History and Development from the Earliest Days to the Present Time,
[2]
C. Feng, K.C. Khulbe, T. Matsuura, R. Farnood, A.F. Ismail, Recent Progress in Zeolite / Zeotype Membranes, J. Membr. Sci. Res. 1
[3]
G. Guan, K. Kusakabe, S. Morooka, GAS PERMEATION PROPERTIES OF ION-EXCHANGED LTA-TYPE ZEOLITE
[4]
M. Asghari, T. Mohammadi, A. Samimi, M. Fouladi, Ion-exchanged zeolite X membranes : synthesis and characterisation, Membr.
[5]
A.M. Tarditi, E.A. Lombardo, Influence of exchanged cations ( Na + , Cs + , Sr 2 + and Ba 2 + ) on xylene permeation through ZSM-5
Chem. Rev. 103 (2003) 663–701. (2015) 49–72. MEMBRANES, Sep. Sci. Techology. 36 (2001) 2233–2245. doi:10.1081/SS-100105915. Technol. (2008) 2–4. / SS tubular membranes, 61 (2008) 136–147. doi:10.1016/j.seppur.2007.10.008. [6]
M. Tagliabue, D. Farrusseng, S. Valencia, S. Aguado, U. Ravon, C. Rizzo, A. Corma, C. Mirodatos, Natural gas treating by selective adsorption: Material science and chemical engineering interplay, Chem. Eng. J. 155 (2009) 553–566. doi:10.1016/j.cej.2009.09.010.
[7]
J. Caro, M. Noack, Zeolite membranes - Recent developments and progress, Microporous Mesoporous Mater. 115 (2008) 215–233.
[8]
J. Coronas, J. Santamaria, Separations using zeolite membranes, Sep. Purif. Methods. 28 (1999) 127–177.
doi:10.1016/j.micromeso.2008.03.008. doi:10.1080/03602549909351646. [9]
N. Kosinov, J. Gascon, F. Kapteijn, E.J.M. Hensen, Recent developments in zeolite membranes for gas separation, J. Memb. Sci. 499 (2016) 65–79. doi:10.1016/j.memsci.2015.10.049.
[10]
T. Wu, M.C. Diaz, Y. Zheng, R. Zhou, H.H. Funke, J.L. Falconer, R.D. Noble, Influence of propane on CO2/CH4 and N2/CH4
[11]
K.A. Lokhandwala, Nitrogen Removal from Natural Gas, 1999.
[12]
P. Gorgojo, Ó. de la Iglesia, J. Coronas, Preparation and Characterization of Zeolite Membranes, Membr. Sci. Technol. 13 (2008) 135–
[13]
K. Weh, M. Noack, I. Sieber, J. Caro, Permeation of single gases and gas mixtures through faujasite-type molecular sieve membranes,
separations in CHA zeolite membranes, J. Memb. Sci. 473 (2015) 201–209. doi:10.1016/j.memsci.2014.09.021.
175. doi:10.1016/S0927-5193(07)13005-9. Microporous Mesoporous Mater. 54 (2002) 27–36. [14]
V. Nikolakis, G. Xomeritakis, A. Abibi, M. Dickson, M. Tsapatsis, D.G. Vlachos, Growth of a faujasite-type zeolite membrane and its
[15]
X. Gu, J. Dong, T.M. Nenoff, Synthesis of defect-free FAU-type zeolite membranes and separation for dry and moist CO2/N2
[16]
K. Kusakabe, T. Kuroda, S. Morooka, Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeolite
[17]
Y. Hasegawa, K. Watanabe, K. Kusakabe, S. Morooka, The separation of CO 2 using Y-type zeolite membranes ion-exchanged with
[18]
Z. Zong, M.A. Carreon, Thin SAPO-34 membranes synthesized in stainless steel autoclaves for N2/CH4 separation, J. Memb. Sci. 524
application in the separation of saturated / unsaturated hydrocarbon mixtures, J. Memb. Sci. 184 (2001) 209–219. mixtures, Ind. Eng. Chem. Res. 44 (2005) 937–944. doi:10.1021/ie049263i. membranes formed on porous support tubes, J. Memb. Sci. 148 (1998) 13–23. alkali metal cations, Sep. Purif. Technol. 22–23 (2001) 319–325. (2017) 117–123. doi:10.1016/j.memsci.2016.11.011. [19]
J.V.A.N.D.E.N. Bergh, W. Zhu, F. Kapteijn, Separation of CO 2 and CH 4 by a DDR membrane, Res. Chem. Intermed. 34 (2008) 467–
[20]
J. van den Bergh, W. Zhu, J. Gascon, J.A. Moulijn, F. Kapteijn, Separation and permeation characteristics of a DD3R zeolite
474. membrane, J. Memb. Sci. 316 (2008) 35–45. doi:10.1016/j.memsci.2007.12.051.
L.S.M. Nazir et al. / Materials Today: Proceedings 19 (2019) 1287–1293 [21]
1293
T.F. Mastropietro, A. Brunetti, P.F. Zito, T. Poerio, H. Richter, M. Weyd, S. Wöhner, E. Drioli, G. Barbieri, Study of the separation properties of FAU membranes constituted by hierarchically assembled nanozeolites, Sep. Purif. Technol. 156 (2015) 321–327. doi:10.1016/j.seppur.2015.10.018.
[22]
Y. Hasegawa, T. Tanaka, K. Watanabe, B. Jeong, Separation of CO2 - CH4 and CO2 - N2 Systems Using Ion-exchanged FAU-type
[23]
J. Yu, Synthesis of zeolites, in: Introd. to Zeolite Sci. Pract. -3rd Revis. Ed., Elsevier B.V., 2007: pp. 39–103. doi:10.1016/S0167-
[24]
M. Yu, R.D. Noble, J.L. Falconer, Zeolite membranes: Microstructure characterization and permeation mechanisms, Acc. Chem. Res.
Zeolite Membranes with Different Si/Al Ratios, Korean J. Chem. Eng. 19 (2002) 309–313. 2991(07)80791-9. 44 (2011) 1196–1206. doi:10.1021/ar200083e. [25]
L.S.M. Nazir, Y.F. Yeong, T.L. Chew, A. Ahmadon, The effect of zeolite suspension concentration on the formation of seed layer via vacuum seeding prior to NaX zeolite membranes growth, IOP Conf. Ser. Mater. Sci. Eng. 380 (2018) 012019. doi:10.1088/1757899X/380/1/012019.
[26]
L. Liu, T. Du, G. Li, F. Yang, S. Che, Using one waste to tackle another: Preparation of a CO2capture material zeolite X from laterite residue and bauxite, J. Hazard. Mater. 278 (2014) 551–558. doi:10.1016/j.jhazmat.2014.06.041.
[27]
H. Robson, Verified Syntheses of zeolitic materials, 2016. http://www.iza-online.org/synthesis/VS_3rdEd.pdf.
[28]
T.F. Chaves, H.O. Pastore, D. Cardoso, A simple synthesis procedure to prepare nanosized faujasite crystals, Microporous Mesoporous Mater. 161 (2012) 67–75. doi:10.1016/j.micromeso.2012.05.022.