Meihua Zhu*, Xingguo An, Tian Gui, Ting Wu, Yuqin Li, Xiangshu Chen*

State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China

Keywords:

ABSTRACT

1. Introduction

Faujasite (FAU) zeolite membrane, including NaY and NaX types, is a low silica zeolite with 12 membered ring pores(～0.74 nm), which are broadly applied in separation, catalysis,and sensor [1–10]. FAU zeolite membrane showed high water perm-selectivity and permeance for dehydration of alcohol with large water contents mixture,and the membrane revealed a superior stability[1].The FAU-type zeolite membrane was used for the catalytic dehydrogenation of cyclohexane in a membrane reactor packed with a Pt/Al2O3catalyst, the conversion of cyclohexane in the FAU-type zeolite membrane reactor reached 72.1% at 200 °C[2]. A sandwich FAU-LTA zeolite dual-layer membrane was used as a catalytic membrane reactor for the synthesis of dimethyl ether(DME), a high methanol conversion (90.9% at 310 °C) and essentially 100% DME selectivity were achieved for the continuous removal of water [3, 4]. The hollow-fiber-supported Au/FAU catalytic membrane showed high catalytic activity and stability for removal of CO from hydrogen [5,6]. Besides, the FAU type zeolite membrane showed a high enough NH3permeance and NH3/N2separation factor[7,8].An ultra-thin FAU zeolite membranes were evaluated for desalination by pervaporation (PV), the observed water fluxes were as high as 28 kg?m-2?h-1at 90 °C [9].

Although the general hydrothermal synthesis factors have greatly influences on the synthesis and performance of the FAU zeolites and membrane,adequate understanding of the optimum synthesis condition is lacking, which is critical for preparation and application of NaY zeolite membranes [10,11]. Pre-treatment or post-treatment is necessary for the high quality FAU zeolite membranes, and a number of studies on the pre-treatment or posttreatment of zeolite membranes have appeared [12–16]. 3-Aminopropyltriethoxysilane (APTES) acts as a molecular linker for anchoring the FAU precursors onto the support surface,which can promotethe nucleation and growth of a thin,well intergrown zeolite FAU membrane.The obtained membranes displayed a high desalination performance for seawater by pervaporation[12].Mundstock et al.[13]reported that the NaX membrane layers grown on modified supports(with APTES or polydopamine)and showed an remarkably high H2/CO2separation factors.

Ion-exchange is a particularly important route for modifying the separation and catalytic properties of FAU zeolites/zeolite membranes [14–19]. It is reported that the adsorption capacity of CO2and N2O,ideal adsorbed solution theory CO2/N2O selectivity of the FAU zeolites are significantly increased with the alkali metal cation (from Li+to K+) [14]. The NaY-type membranes exchanged with K+and Li+ions gave higher and lower CO2/N2selectivity than the fresh NaY-type membrane [15]. Hasegawa et al. [16] reported that the NaY-type zeolite membranes were ion exchanged with K+, Rb+or Cs+ions, and the CO2/N2selectivity of the ionexchanged zeolite membranes was in the order: Rb+=K+>Cs+>Na+.Because there are a strong interaction between olefin and Ag+cations, the ion-exchanged Ag-X membrane exhibited superior and stable performance for separation of propylene/propane mixture (50:50), a maximum propylene selectivity and permeance were 55.4 and 4.13×10-8mol?m-2?s-1?Pa-1at 80 °C [17,18]. The LiX/Celgard membrane had a specific capacity and a capacity retention of 70% after 400 cycles, which is an enlightenment for the development of ion-selective membranes used in rechargeable battery [19].

In order to improve the PV performance of the NaY zeolite membrane for ethanol (EtOH) aqueous in this work, and Na+cations in NaY membranes are ion-exchanged by a series of mono- and di-valent cations for adjusting the channel structure and water affinity. The mono- and di-valent cations are inorganic nitrate solutions, including Co(NO3)2?6H2O, Mg(NO3)2?6H2O,Zn(NO3)2?6H2O, Ca(NO3)2?4H2O, Cu(NO3)2?6H2O, KNO3, and AgNO3in this study.

2. Experimental

2.1. Materials

Materials for preparation and ion-exchange of NaY zeolite membranes are home-made α-Al2O3supports (0.1 m length,Foshan, OD. 1.2×10-3m, ID. 8.0×10-3m), water glass (Na2SiO3,Aldrich, 26.5% (mass) SiO2,), sodium aluminate (Al/NaOH = 0.8,Wako), sodium hydroxide (NaOH, 96% (mass), Tianjin Fuchen),ammonia fluoride (96% (mass), Shanghai Guoyao), cobalt nitrate(Co(NO3)2?6H2O, 99% (mass), Tianjin Fuchen), magnesium nitrate(Mg(NO3)2?6H2O, 99% (mass), Tianjin Fuchen), zinc nitrate (Zn(NO3)2?6H2O, 99% (mass), Tianjin Fuchen), calcium nitrate (Ca(NO3)2?4H2O, 99% (mass), Tianjin Fuchen), potassium nitrate(KNO3, 99.0% (mass), Aladdin), silver nitrate (AgNO3, 99% (mass),Nanjing Chem) and deionized water.

2.2. Preparation and ion-exchange of NaY zeolite membrane

Preparation procedure of NaY zeolite membranes is similar to our previous studies[20,21].NaY zeolite membranes are prepared on the home-made α-Al2O3supports,and molar composition of the precursor synthesis gel is 25 SiO2: 1 Al2O3: 22 Na2O: 990 H2O: 7.5 NH4F. Amount of NaAlO2, NaOH and NH4F were completely dissolved with deionized water and formed a solution.Then the water glass is slowly added to the solution with stirring,and the mixture is stirred vigorously for 4 h at room temperature.Besides,the precursor synthesis gel is aged at 30°C for 14 h.Tubular α-Al2O3supports are coated with the NaY seed crystals crystals (Si/Al = 2.51,Aladdin, ～2000 nm) and dried at 60 °C air oven. Thereafter, the seeded supports and precursor synthesis gel are settled into an autoclave, and the hydrothermal synthesis is carried out at 100 °C for 6.5 h. The sample is washed with deionized water after hydrothermal synthesis.

Table 2 PV performance of Zn-NaY zeolite membranes for separation 10% (mass) H2O/EtOH mixture at 75 °C

Table 3 PV performance of Zn-NaY zeolite membranes for separation of different 10% (mass)H2O/organic mixtures

Na+cations of NaY zeolite membranes are ion-exchanged with K+, Ag+, Mg2+, Ca2+, Zn2+, and Co2+in this study. NaY zeolite membranes are immersed in the inorganic nitrate solutions (AgNO3,KNO3, Mg(NO3)2, Ca(NO3)2, Zn(NO3)2and Co(NO3)2) concentration of the inorganic nitrate solutions 0.1 mol?L-1, the ion exchange temperature and time are 50 °C and 2 h. In order to remove the excess metal ions on the surface of ion-exchanged NaY zeolite membranes, which are washed with the deionized water for three times.

2.3. Characterization of NaY zeolite membranes

NaY zeolite crystals and membranes are characterized by X-ray diffraction (XRD, Rigaku, Ultima IV) and the scanning electron microscopy (FE-SEM, Hitachi, SU 8020). N2adsorption–desorption isotherms and pore width distribution of the NaY zeolite crystals are tested by adsorption apparatus (Micromeritics, ASAP-2460) at-196°C.X-ray photoelectron spectroscopy(XPS,Thermo Scientific K-Alpha+) is applied to characterize Si/Al ratios of NaY zeolite crystals.

PV performance of NaY zeolite membrane are evaluated by separation of 10%(mass)H2O/EtOH mixture by PV.The PV installation is similar with our previous studies[19–20].The membrane is contacted with the liquid feed, the compositions of the feed and the permeate are analyzed by a gas chromatograph equipped with TCD detector (GC-14C, Shimadau). PV performance of membrane is evaluated by total flux (J, kg?m-2?h-1) and separation factor(αH2O/EtOH). Total flux and separation factor were defined as

where m was the mass of permeation, A was the effective surface area of membrane, t was the operating time; YH2Oand YEtOHwere the concentration of H2O and EtOH in permeation, XH2Oand XEtOHwere the concentration of H2O and EtOH in feed liquid.

Fig. 2. PV performance of Zn-NaY zeolite membranes for separation 10% (mass) H2O/EtOH mixture at 75 °C: (a) Q ; (b) αH2O/EtOH; (c) flux variation; (d) separation factor variation.

3. Results and Discussion

3.1. PV performance of fresh and ion-exchanged NaY zeolite membranes

Na+cations in MOR membranes were ion-exchanged by a series of mono- and di-valent cations, the channel structure and water affinity of membranes could be adjusted,and the H-MOR membrane showed the largest separation performance for dehydration of 90%(mass) acetic acid/water under 75 °C [22]. In order to improved the separation performance of NaY zeolite membranes, different mono-and di-valent nitrate salt are applied to improve the performance of NaY zeolite membranes by ion-exchange in this work.

Table 4 Textural properties of fresh and ion-exchange NaY zeolites by N2 adsorption–desorption isotherm

Table 1 summarizes the ion-exchange conditions and PV performance of NaY zeolite membranes for separation of 10% (mass)H2O/EtOH mixture at 75 °C, Fig. 1 presents trend chart of flux and separation factor variation. Mono-valent nitrate salt (KNO3/AgNO3) could not improve dehydration performance of the NaY membranes(M-1/M-2,Fig.1(a)and(b)).Especially,the membrane M-2 was ion-exchanged with AgNO3solution, which had no separation performance for dehydration of 10% (mass) H2O/EtOH mixture. While the separation factor of the membranes (M-3, M-4,M-5, M-6) are increased with di-valent nitrate salt, the increase order of Zn(NO3)2?Mg(NO3)2≈Ca(NO3)2> Co(NO3)2,while the flux variation of the membranes are -40%--50% (Fig. 1(c) and(d)). When the ion-exchange solution is Zn(NO3)2, the separation factor and separation factor variation of the NaY membrane (M-5) are up to 100% and 230%.

Seven pieces of NaY zeolite membrane are ion-exchanged with Zn(NO3)2solution in this work,Table 2 and Table 3 present PV performance of the Zn-NaY zeolite membranes for 10% (mass) H2O/EtOH mixture at 75 °C and different 10% (mass) H2O/organic mixtures.Besides,Fig.2 presents flux and separation factor variation of the membranes for 10% (mass) H2O/EtOH mixture. The separation factor of the membranes (M-5, M-7-M-12) are greatly improved by the ion exchange with Zn(NO3)2solution,flux variation and separation factor variation of the membranes are in the range of-40%--55% and 160%–230% (Fig. 2(c) and (d)). Besides, the Zn-NaY zeolite membranes have good PV performance for separation of 10% (mass) H2O/isopropanol, H2O/acetone and H2O/butanol mixtures. Hence, Zn(NO3)2is the alternative nitrate salt for improving dehydration performance of NaY zeolite membrane by ion-exchange, and the Zn-NaY zeolite membranes have a good reproducibility in this work.

3.2. Characterization of ion-exchanged NaY zeolites and zeolite membranes

Because the di-valent nitrate salt could enhance the dehydration performance of the NaY zeolite membrane in this work, the textural properties of ion-exchanged NaY zeolites and zeolite membranes were investigated by XRD, SEM, N2adsorption–desorption, XPS, and pore size distribution characterization technologies. Figs. 3 and 4 presents XRD patterns, surface and cross sectional SEM images of the fresh and ion-exchange NaY zeolite membranes. Both the fresh and ion-exchanged NaY zeolite layer consists of randomly oriented and octahedron morphology NaY

Table 5 Ion-exchange degree of NaY zeolites by XPS characterization

Table 4 shows the textural properties of fresh and ionexchanged NaY zeolites by N2adsorption–desorption isotherm,K-NaY Ag-NaY, Ca-NaY Mg-NaY and Zn-NaY are the number of NaY zeolites with KNO3, AgNO3, Ca(NO3)2, Mg(NO3)2, Zn(NO3)2and Co(NO3)2solution. It is noted that the preparation and ionexchange procedure of the NaY zeolites are identical with the NaY zeolite membranes. BET surface and total pore volume of the Zn-NaY zeolites is larger than the fresh NaY zeolites, while the BET surface and total pore volume of NaY zeolites become low after ion-exchange with other mono- and di-valent nitrate solution. Especially, BET surface area and total pore volume are greatly decreased by ion-exchange with AgNO3, Co(NO3)2and Mg(NO3)2solution, BET surface area and total pore volume of the Mg-NaY zeolites are only 7.4×104m2?kg-1and 0.4×10-4m3?kg-1.The orders of BET surface area and pore capacity of the fresh and ion-exchanged NaY zeolites is Zn-NaY > NaY > Ca-NaY > K-NaY >Ag-NaY > Co-NaY > Mg-NaY.

Besides, the XPS characterization element composition of NaY zeolites by characterized are presented and summarized in Fig. 5 and Table 5. Both mono-valent and di-valent metal ions successfully existed in the FAU zeolite structure by ion-exchange.Because Ag+and K+cations could enter into the sodalite and D6R cages of the NaY zeolites, which could totally exchanged Na+cations of NaY zeolites[23].As shown in Table 5,Ag+and K+cation exchange degree of NaY zeolites are achieved to 96.54% and 82.77%. It is reported that the ion-exchange order (mono-valent and di-valent cations) of NaA zeolites are as followings: Ag+>Tl+>K+>NH4+>Rb+>Li+>Cs+, Zn2+>Sr2+>Ba2+>Ca2+>Co2+>Ni2+>Cd2+>Hg2+>Mg2+[23,24].The order of ion-exchange degree of NaY zeolites is similar with those of NaA zeolites, which is Ag+>K+>Ca2+>Zn2+?Co2+>Mg2+in this work.

Fig.5. XPS patterns of the fresh and ion-exchanged NaY zeolites.A:(a)NaY zeolites,(b)K-NaY zeolites,(c)Ag-NaY zeolites;B:(d)Ca-NaY zeolites,(e)Mg-NaY zeolites,(f)Zn-NaY zeolites, (g) Co-NaY zeolites.

Yu et al.[24]reported that the diffraction peaks intensity of the LiY zeolites was gradually decreased with Ag+exchange, and the diffraction peaks was shifted to the small angle side,indicating that the introduction of Ag+cations could decrease the crystallinity and expansion of lattice parameters.As shown in Table 5,Fig.3(c)and Fig. 5 A(c), Ag+cation exchange degree of NaY zeolites is achieved to 96.54%in this work.There are few Ag2O nano particle are formed on the Ag-NaY zeolite membrane surface, which could greatly decrease the typical characteristic diffraction peak and separation performance of NaY zeolite membranes. Mg-related and Corelated species may reside on the outer surface of the zeolite or enter the pore channels of the zeolite,it is speculated that the reason for the sharp decline of BET surface area after Mg2+and Co2+ionexchange [25]. Therefore, the flux of NaY zeolite membranes are greatly decreased with Mg(NO3)2and Co(NO3)2solutions, and the separation factor has a slightly increase after ion-exchange.

Fig.S1 and Fig.S2(Supplementary Material)shows the pore size distribution and N2adsorption–desorption isotherm of the fresh and ion-exchanged NaY zeolites.The pore size of the Ag-NaY and K-NaY zeolites are similar with the fresh NaY zeolites,the Mg-NaY,Co-NaY and Ca-NaY zeolites have a larger pore size than the fresh NaY zeolite[26,27].Interestingly,the pore size of Zn-NaY becomes lower than the NaY zeolites, which proves that the dehydration performance(both the flux and separation factor) of NaY zeolite membrane is greatly improved by ion-exchange with Zn(NO3)2solution. The N2adsorption–desorption isotherm of the Mg-NaY and Co-NaY zeolites have hysteresis loop,which suggested that the Mg-NaY and Co-NaY zeolites have mesoporous[27].

4. Conclusions

NaY zeolite membrane is ion-exchanged with mon-divalent and di-valent nitrate salt, ion-exchange degree order of NaY zeolites is Ag+>K+>Ca2+>Zn2+?Co2+>Mg2+in this work.BET surface,total pore capacity, pore size distribution and water contact angle of NaY zeolites by mono- and di-valent cations exchange are all disordered. Dehydration performance of NaY zeolite membranes is improved by di-valent nitrate salt. The separation factor variation of the Zn-NaY membrane (M-5) is 230% for separation of 10%(mass) H2O/EtOH mixture, and the ion-exchanged Zn-NaY membranes showed good reproducibility.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21868012 and 21868013), Jiangxi Provincial Department of Science and Technology (20171BCB24005 and 20181ACH80003).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.12.006.