Dehydration of acetic acid and esterification product by acid-stable ZSM-5 membrane

Dehydration of acetic acid and esterification product by acid-stable ZSM-5 membrane

Microporous and Mesoporous Materials 181 (2013) 47–53 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepag...

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Microporous and Mesoporous Materials 181 (2013) 47–53

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Dehydration of acetic acid and esterification product by acid-stable ZSM-5 membrane q Mei-Hua Zhu, Izumi Kumakiri, Kazuhiro Tanaka, Hidetoshi Kita ⇑ Department of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Tokiwadai 2-16-1, Ube, Yamaguchi 755-8611, Japan

a r t i c l e

i n f o

Article history: Received 4 October 2012 Received in revised form 20 December 2012 Accepted 31 December 2012 Available online 9 January 2013 Keywords: ZSM-5 membrane Dehydration Pervaporation Acetic acid Esterification

a b s t r a c t ZSM-5 membranes were prepared on mullite supports by a static secondary hydrothermal synthesis without applying organic structure directing agents. The ZSM-5 membrane had Si/Al ratio of 13.1 and selectively permeated water from the binary acetic acid/water mixtures and the quaternary acetic acid/ethanol/water/ethyl acetate (HAc/EtOH/H2O/AcOOEt) mixtures by pervaporation. The deviation of the dehydration performance of the membranes prepared in different batch was small, suggesting the high reproducibility of the synthesis. Part of the Na+ in the ZSM-5 membrane was replaced to H+ during the dehydration of acetic acid solution. Cation type was changed back to Na+ by immersing the membranes in 0.1 M sodium hydroxide (NaOH) or sodium chloride (NaCl) solutions. The perm-selectivity of the membranes was maintained even after applying pure HAc liquid for 77 h and applying esterification product of ethanol and acetic acid for 98 days. No change in the membrane structure was observed before and after the dehydration of acidic solutions by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis characterizations. These results confirm the high acidic stability of the membrane. Ó 2013 Published by Elsevier Inc.

1. Introduction Acetic acid is one of the top 20 organic intermediates used in chemical industry. However, due to the small differences in volatility between water (H2O) and acetic acid (HAc), separation of the HAc/H2O mixture by traditional distillation is an energy-expense procedure [1]. Pervaporation (PV) is an energy-efficient separation process and has been recognized as a promising candidate for the dehydration of HAc aqueous solution in recent years [2]. Esterification reaction is another widely used industrial process where the process efficiency is limited by the equilibrium. It is reported that coupling the esterification process and the continuous removal of H2O by membrane separation process obtained the higher yields [3–5]. Nevertheless, the poor stability of commercialized membranes in acidic media, including the polymeric membranes and the commercialized NaA-type zeolite membranes [6], restricts the application of PV process. Therefore, a chemically inert and acid-stable membrane for dehydration of acidic mixtures is of great interests. It is well known that the acid stability of zeolite membranes is enhanced with increasing SiO2/Al2O3 ratio in the framework, trading-off hydrophilicity. Due to the medium SiO2/Al2O3 ratio, the high silica CHA [7], T q

This paper was presented in ZMPC 2012.

⇑ Corresponding author. Tel.: +81 0836 859661; fax: +81 0836 859601. E-mail address: [email protected] (H. Kita). 1387-1811/$ - see front matter Ó 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.micromeso.2012.12.044

[8], MOR [9,10] and ZSM-5 zeolite membrane [11,12] have been applied to dehydrate the carboxylic acidic mixtures or couple with the esterification process. Due to the high Si/Al ratio of the precursor gel, the best flux and separation factor (aH2 O=HAc ) of the as-synthesized ZSM-5 membranes were 0.63 kg m2 h1 and 23 for the dehydration of 54 wt.% HAc/H2O solution by PV at 343 K [11]. Using the high aluminum content (SiO2/Al2O3 = 15) synthesis gel, the acid-stable ZSM-5 membrane has been prepared in this laboratory [12]; however, the crystallization process of the membrane was carried out as long as 3 days under a continuous horizontal rotation at 37.5 rpm, which could cause the high cost to limit the industrial application of the membrane. Recently, hydrophilic ZSM-5 membranes were successfully prepared from the aluminum-rich precursor gel (Si/Al = 7.5) by static hydrothermal synthesis. The as-synthesized membranes displayed excellent dehydration performance for 10 wt.% H2O/ iso-propanol (IPA) mixture with separation factor over 5300 [13]. The synthesis conditions were optimized further in this study to apply the ZSM-5 membranes for acidic solutions. The optimized precursor gel had more silica content (Si/Al = 11.2). The stability and reproducibility of ZSM-5 membranes for the HAc/H2O mixture and quaternary HAc/EtOH/H2O/AcOOEt mixtures were investigated by PV operation. In addition, membranes were post-treated with 0.1 M NaOH or NaCl solution to explore the ion-exchange effects on the membrane separation and permeation properties.

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Nomenclature and units J

aw/o m A t Xw

total flux [kg m2 h1] separation factor [–] mass of permeate [kg] effective area of the membrane [m2] pervaporation time [h] mass fractions of water at the feed side [–]

Xo Yw Yo

mass fractions of organic component at the feed side [–] mass fractions of water at the permeation side [–] mass fractions of organic component at the permeation side [–]

water and organic at the feed and permeate sides, respectively. It is noted that the PV performance of the membranes for all mixtures achieved and kept a stable value after about 6 h consecutive operation.

2. Experimental 2.1. Preparation of ZSM-5 membrane and crystals Hydrophilic ZSM-5 zeolite membranes were prepared on mullite supports (o.d.: 12 mm; i.d.: 9 mm, porosity: 43%, pore size: 1.3 lm and length: 100 mm, Nikkato Corporation) by the static secondary hydrothermal synthesis. Prior to the synthesis, outer surface of mullite supports were rubbed with the water slurry of ZSM-5 crystals (SiO2/Al2O3 = 23.5, Tosoh) and dried at 80 °C. The precursor synthesis gels were prepared as our previous work [13]. NaOH (97 wt.%, Wako), sodium fluoride (NaF, 99 wt.%, Wako) and aluminum sulfate (Al2(SO4)314–18H2O, 51–57 wt.%, Wako) were dissolved in deioned water, and then the colloidal silica (40 wt.%, AS-40, Aldrich) was added into the above white gel. Finally, the aging of the precursor gel was carried out at room temperature for 2 h. The molar composition of the synthesis gel was 1 SiO2: 0.0447 Al2O3: 0.134 Na2O: 0.67 NaF: 33.3 H2O. The synthesis gel was transferred into stainless steel autoclave, in which two pieces of seeded mullite supports were immersed vertically. The hydrothermal syntheses were carried out at 180 °C for 48 h. After the hydrothermal synthesis, the membranes were rinsed thoroughly in boiling water and dried at 80 °C for several hours.

2.4. Stability of ZSM-5 membrane for the esterification product of ethanol and acetic acid The stability of the ZSM-5 membrane in the esterification mixture of ethanol and acetic acid was investigated as follows. First, the PV performance of the membrane was checked by dehydration of a 1 L esterification mixture with a composition shown in Table 4 at 75 °C for 6 h to confirm the steady-state permeation. Thereafter, the mixture feed solution was cooled down and the membrane was kept completely immersed into the mixture at ambient temperature for 98 days. The PV performance of the membrane was examined after certain time of immersion by heating the solution to 75 °C. In order to maintain the amounts and composition of feed mixture, the permeations were recycled to the feed mixture during the PV tests. 3. Results and discussion

2.2. Characterization of ZSM-5 membrane and crystal

3.1. XRD and SEM characterization of ZSM-5 membrane

The as-synthesized ZSM-5 membranes were characterized by X-ray diffraction (XRD, Rigaku, Smartlab) with Cu-Ka radiation. The spectra was scanned in the range of 2h = 5–45° at a scanning rate of 4°/min. Morphology of the membranes was observed with field emission scanning electron microscopy (FE-SEM, JEOL JSM 6335F). Energy dispersive X-ray analysis (EDAX, Shimadzu, EPMA-1720) was used to analyze the elemental composition of the ZSM-5 membrane surface.

Fig. 1a shows the XRD pattern of the as-synthesized ZSM-5 membrane. All the peaks are well corresponding to the MFI zeolites structure and the mullite support, which indicates that the membrane is consisting of pure ZSM-5 phase. The surface and the cross sectional SEM images of the membrane are displayed in Figs. 2a and b. The support surface was fully covered with fine and sand-rose-like zeolite crystals. The thickness of the zeolite

2.3. Pervaporation of ZSM-5 membrane for HAc/H2O solutions and esterification products of ethanol and acetic acid ZSM-5 membranes were applied to dehydrate the various compositional HAc/H2O solutions and HAc/EtOH/H2O/AcOOEt mixtures. The PV set-up was depicted in our previous studies [14,15]. The permeate vapor was collected by a cold trap condensed with liquid nitrogen. The compositions of the feed and permeate were analyzed by gas chromatograph (Shimadzu, GC-17A). The total flux (J, kg m2 h1) and separation factor (aw/o, –) of the membranes were determined as:

J ¼ m=ðA  tÞ

ð1Þ

aw=o ¼ ðY w =Y o Þ=ðX w =X o Þ

ð2Þ

where m, A, t, Xw, Xo, Yw and Yo denoted the mass of permeate condensed in the cooled trap for a known PV time (kg), effective area of membrane (m2), test time (h), the mass fractions of components

Fig. 1. XRD patterns of the ZSM-5 membrane before (a) and after (b) immersing in the rude quaternary esterification product of ethanol and acetic acid for 98 days.

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layer was about 5 lm. Meanwhile, the molar ratio of Si/Al and Na/ Al of the ZSM-5 membrane surface were 13.1 and 1.0 by the result of EDAX characterization (Table 1). 3.2. PV performance of ZSM-5 for binary HAc/H2O mixtures Table 2 summarizes the PV performance of the ZSM-5 membrane for the HAc/H2O mixtures at 75 °C with varying the HAc concentration in the feed side from 0 to 100 wt.%. The membrane showed water perm-selectivity with separation factors >100 for the whole HAc concentration range studied. The total fluxes and water content in the permeation side of the ZSM-5 membrane gradually decreased with elevating the HAc content in the feed side (Table 2). The membranes showed higher perm-selectivity compared to the ZSM-5 membranes reported by Li et al. [11]: flux and separation factor (aH2 O=HAc ) of the membrane were 0.63 kg m2 h1 and 23 for the dehydration of 54 wt.% HAc/H2O solution by PV at 70 °C. The membrane morphologies, especially

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the surface structures, were significantly different. While the membrane surface was rather smooth in the reported study, many voids can be seen as shown in Fig. 2. Such morphology may facilitate the permeation as observed in our previous paper [13]. The difference in selectivity may be explained by the difference in the Si/Al ratio of the membrane. Li et al. [11] prepared membranes with silica-rich precursor gel (Si/Al = 480) than this study. Accordingly, the ZSM-5 membranes could have higher Si/Al ratio than this study that can reduce the hydrophilic properties. Fig. 3a shows the H2O and HAc fluxes of the ZSM-5 membrane for 0–100 wt.% HAc/H2O mixtures at 75 °C. The H2O fluxes of the membrane gradually decreased as increasing the HAc content in the feed side. On the contrary, the corresponding HAc fluxes linearly increased with the increase of the HAc content over 0– 30 wt.% HAc mixtures whereas almost kept as a constant over 30–90 wt.% HAc/H2O mixtures. Fig. 3b shows the plot of the mole fraction of HAc in the permeation side as a function of that in the feed side. The mole ratio of HAc in the permeation side on the basis

Fig. 2. Surface and cross sectional SEM images of the ZSM-5 membrane: (a) and (b) as-synthesized ZSM-5 membrane; (c) and (d) ZSM-5 membrane after immersing in the esterification product of ethanol and acetic acid for 98 days; (e) and (f) ZSM-5 membrane was post-treated by the 0.1 M NaOH solution at 25 °C for 2 h.

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Table 1 Variation of the membrane composition after sequential treatments measured by EDAX. Membrane treatment conditions

As synthesized After PV with esterification product for 98 days After post-treatment by 0.1 M NaOH solution at 25 °C for 2 h After post-treatment by the 0.1 M NaCl solution at 75 °C for 2 h

Elemental composition of the membrane Si/Al

Na/Al

13.1 13.4

1.0 0.3

12.7

1.0

13.3

1.0

Table 2 PV performance of the ZSM-5 membrane for different composition HAc/H2O mixtures at 75 °C. Feed composition (HAc wt.%)

Jtotal (kg m2 h1)

H2O content in the permeate (wt.%)

aH2 O=HAc

0 15 30 50 75 90 100

5.05 3.4 2.6 2.21 0.85 0.25 0.026

100 99.86 99.63 99.51 98.72 94.82 –

– 125 115 200 230 165 –

(–)

(Fig. 4b). These results indicated the as-synthesized ZSM-5 membrane had a long-term stability and durability for the high HAc concentration HAc/H2O mixtures. 3.3. PV performance of ZSM-5 membrane for esterification product of ethanol and acetic acid Fig. 5 shows the PV performance of the ZSM-5 membrane for dehydration of the HAc/EtOH/H2O/AcOOEt mixture over a temperature range from 30 to 75 °C. As shown in Fig. 5, both the H2O and organic components fluxes of the ZSM-5 membrane gradually increased as elevating the operational temperature, but the increasing trend of the H2O flux of the membrane was much faster than that of the other three organic components (EtOH, HAc and AcOOEt). The vapor pressures of permeations at the feed side increase as increasing the operational temperature, and the vapor pressures of them at the permeate side are not affected [16]. Hence, both the H2O and HAc fluxes of the membrane gradually increased as elevating the operational temperature for the increasing interrelated driving forces in this study. Li et al. [12] had synthesized the long-term acid-stable ZSM-5 zeolite membrane from the aluminum-rich precursor gel by the rotational hydrothermal synthesis, and the as-synthesized membranes were applied to dehydrate the esterification production of methanol and acetic acid. The best total permeation flux and water content in the permeation side of the membrane were 1.25 and 95 wt.% for the 18 wt.% HAc/9.6 MeOH/58.2 AcOMe/14 H2O mixtures at 60 °C. In the present work, even though the operational temperature was increased to 75 °C, the H2O content in the permeation side of the ZSM-5 membrane achieved to 99.38 wt.% for the quaternary HAc/EtOH/H2O/AcOOEt mixture, and the corresponding H2O flux achieved to 1.49 kg m2 h1. The present work indicates that the ZSM-5 membranes have the excellent water permselectivity for the quaternary esterification production. 3.4. Long-term stability for esterification product of ethanol and acetic acid

Fig. 3. (a) H2O and HAc permeation of the ZSM-5 membrane as a function of HAc mole fraction in feed side and (b) the mole fraction of HAc in permeation side as a function of that in feed.

of PV results of the membrane was much lower than that on the basis of vapor–liquid equilibrium over the entire range of HAc concentration, which further confirmed that the ZSM-5 membrane had well dehydration performance for the 0–100 wt.% HAc/H2O mixtures. Long-term stability of the membranes for the high HAc concentration HAc/H2O mixtures was also examined in this study. Figs. 4a and b show the continuous PV performances of the membrane for 90 wt.% HAc/H2O mixture and pure HAc liquid at 75 °C as a function of continuous PV time, respectively. As shown in Fig. 4a, both the H2O flux and separation factor of the membrane achieved to a valley and almost kept constant values after 20 h. The HAc flux of the membrane was almost kept constant during the PV operation. Meanwhile, the performance of the membrane was stable for the pure HAc liquid as a function a continuous 77 h operation

Fig. 6 summarizes the long-term stability of a ZSM-5 membrane for the HAc/EtOH/H2O/AcOOEt mixture. The original HAc, EtOH, H2O and AcOOEt fluxes of the ZSM-5 membrane for the mixture at 75 °C were 0.006, 0.001, 1.23 and 0.002 kg m2 h1, respectively. It is noted that the permeations of the membrane were added to the feed mixture to maintain the amounts and composition of the feed mixture during the PV operation. As shown in Fig. 6, compared to the H2O fluxes, the HAc, EtOH and AcOOEt fluxes of the ZSM-5 membrane were still quite low after 98 days immersion, which suggested that the ZSM-5 membrane had a well durability and stability for the feed mixture. The corresponding H2O fluxes of the membrane were achieved to a stable value after the 21st day, and kept at 1.07 kg m2 h1 even though the immersion period was prolonged to 98 days. Clearly, the H2O flux only had a 12.9 wt.% reduction after 98 days, and the HAc, EtOH and AcOOEt fluxes were still lower than 0.006 kg m2 h1. Besides, the membrane was analyzed by the XRD, SEM and EDAX before and after the long-term immersed in the HAc/EtOH/H2O/ AcOOEt mixture. Similar XRD patterns (Fig. 1) and the Si/Al ratios (Table 1) of the membrane before and after the 98 days testing suggested that the as-synthesized membrane was long-term stable, which also proved that the membrane was stable and durable for dehydration of the acidic aqueous mixtures. As shown in Figs. 2c and d, the thicknesses of zeolite layers were the same, while some parts of the membrane surface were covered with gel-like materials. Water removal by a membrane enhanced the formation of ester that has low solubility in the mixture. If the esterification at the membrane surface was faster than the diffusion of ester to the bulk,

M.-H. Zhu et al. / Microporous and Mesoporous Materials 181 (2013) 47–53

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Fig. 4. PV performance of the ZSM-5 membrane for (a) 90 wt.% HAc/H2O mixture and (b) pure HAc liquid as a function of PV operational time.

Fig. 5. Fluxes of the ZSM-5 membrane for esterification product of ethanol and acetic acid as a function of temperature.

the concentration of ester could easily be over its solubility and form deposits on the membrane. The gel-like materials were removed by washing the membrane with NaOH solution. This corresponds well with the assumption that ester precipitated on the membrane, for this substance can be easily hydrolyzed in basic conditions and forms acetic acid and ethanol, which are highly soluble to the mixture. The coverage of membrane with gel-like materials is considered to be one of reasons for the reduction of the total flux of the membrane. In addition, the Na/Al ratio of the ZSM-5 membrane surface decreased to 0.3 after the 98 days immersion in the HAc/ EtOH/H2O/AcOOEt mixture (Table 1), which was supposed to be another cause for the12.9 wt.% reduction of H2O flux. 3.5. Post-treatment and PV performance of ZSM-5 membrane Table 3 shows the PV performance changes by post-treating the membrane with 50 wt.% HAc/H2O mixture, 0.1 M NaOH or NaCl solution at a certain condition. The as-synthesized membrane Z1 showed a well water perm-selectivity in 90 wt.% IPA/H2O separation, the initial total flux and H2O content in the permeate were 2.45 kg m2 h1and 99.49 wt.%, respectively. Besides, the separation performance of the membrane Z1 for the 90 wt.% IPA/H2O mixture was reinvestigated after immersing in 50 wt.% HAc/H2O mixture at 75 °C for 6 h, the membrane had almost the same separation factor as the as-synthesized membrane for the

Fig. 6. Long-term stability of ZSM-5 membrane for dehydration of esterification product of ethanol and acetic acid at 75 °C.

90 wt.% IPA/H2O mixture; however, about 20% reduction in the total flux was observed. It is noted that the membrane was cleaned with the deioned water at 75 °C for 2 h in between each PV test. Interestingly, the total flux of the membrane for the 90 wt.% IPA/ H2O mixture separation was almost recovered to the original value and maintained a constant separation factor after immersing by the 0.1 M NaOH solution at 25 °C for 2 h. Similar behavior was observed with Z2 membrane: immersing the membrane into the 50 wt.% HAc/H2O solution at 75 °C for 6 h, reduced the total flux with maintaining the separation properties, and total flux of the membrane for 90 wt.% EtOH/H2O separation was also recovered to the original value and kept a constant separation factor after cleaning by the 0.1 M NaCl solution at 75 °C for 2 h. As shown in Table 1, after a long-term immersed in the quaternary HAc/EtOH/H2O/AcOOEt mixture, the Na/Al ratio of the membrane promptly decreased by the results of EDAX characterization, from 1.00 to 0.33, namely, plenty of Na+ cations of the membrane surface were ion-exchanged with the H+ cations under the acidic case. It is also considered to be one of reasons for the total fluxes decrease of the membranes Z1 and Z2 for the 90 wt.% IPA/H2O or EtOH/H2O separation after immersion in 50 wt.% HAc/H2O mixture. After post-treating with the 0.1 M NaOH solution at room temperature for 2 h, the Si/Al ratio of the membrane surface had a few changes (from 13.4 to 12.7). In addition, the Na/Al values of the membrane surface were recovered to the original value after treated by 0.1 M NaCl and NaOH solutions at certain conditions (1.0,

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Table 3 Post-treatment conditions and PV performance of the ZSM-5 membranes at 75 °C. No.

Post-treatment conditions Solution

PV performance T (°C)

t (h)

Jtotal (kg m2 h1)

H2O content in permeater (wt.%)

aH2 O /organic (–)

99.49 99.32 99.53

1760 1310 1910

99.15 99.11 99.18

1050 1000 1090

As synthesized 50 wt.% HAc/H2O 0.1 M NaOH

– 75 25

– 6 2

90 wt.% IPA/H2O 2.45 2.00 2.40

As synthesized 50 wt.% HAc/H2O 0.1 M NaCl

– 75 75

– 6 2

90 wt.% EtOH/H2O 1.35 1.14 1.32

Z1

Z2

Table 4 PV performance of the ZSM-5 membranes for 50 wt.% HAc/H2O mixture and esterification product of ethanol and acetic acid at 75 °C. No.

Feed (wt.%)

J (kg m2 h1)

Permeate composition (wt.%)

Separation factor

aW/A (–) M1 M2 M3 M4 M5

50A/50W

2.05 1.86 2.21 2.36 1.77

0.65A/99.35W 0.55A/99.45W 0.40A/99.60W 0.75A/99.25W 0.48A/99.52W

M6 M7 M8 M9 M10

54A/5B/13W/28E

1.45 1.66 1.52 1.55 1.24

0.33A/0.03B/99.33W/0.31E 0.25A/0.05B/99.56W/0.14E 0.20A/0.04B/99.68W/0.09E 0.32A/0.06B/99.39W/0.22E 0.50 A/0.09 B/99.25W/0.16E

aW/B (–)

aW/E (–)

150 180 250 130 210

– – – – –

– – – – –

1250 1650 2070 1290 820

1270 770 960 640 420

690 1530 2380 970 1340

Note: A = HAc; B = EtOH; E = AcOOEt; W = H2O.

Table 1), and the fouling on the membrane was eliminated after cleaning with 0.1 M NaOH solution (Fig. 2e). Therefore, the Na+ amounts of the ZSM-5 membrane surface could be regained by ion-exchanging with 0.1 M NaOH or NaCl solution at a certain condition, resulting the fluxes of the membrane Z1 and Z2 were recovered to the original values for dehydration of 90 wt.% IPA/H2O and EtOH/H2O mixtures. Moreover, the Si/Al ratio of the membranes almost unchanged after post-treating with 0.1 M NaOH or NaCl solutions, which proved that the ZSM-5 membranes were not deteriorated during the post-treatment. Groen et al. [17] have reported that desilication of ZSM-5 zeolite in alkaline solution was hardly occurred for the aluminum-rich ZSM-5 zeolite (Si/Al = 17), which suggested that the almost unchanged Si/Al ratio of the ZSM5 membrane in 0.1 M NaOH solution could be attributed to the low Si/Al ratio (Si/Al = 13.1) of the membrane surface in this study.

Table 5 Comparison of PV performance of the zeolite membranes for carboxylic acid mixtures. Membrane

PV conditions

PV performance

Ref. [–]

Feed (wt.%)

T (°C)

Jtotal (kg m2 h1)

H2O content in permeates (wt.%)

ZSM-5

50HAc/50H2O

75

2.21

99.60

ZSM-5

54HAc/5EtOH/ 13H2O/ 28AcOOEt 50HAc/50H2O 54HAc/46H2O 18HAc/ 9.6MeOH/ 14H2O/ 58.2AcOOMe 50HAc/50H2O

75

1.66

99.56

80 70 60

0.614 0.63 1.19

99.67 95.14 95.00

[9] [11] [12]

75

1.46

99.45

[18]

MOR ZSM-5 ZSM-5

T

This work This work

3.6. Reproducibility of the ZSM-5 membrane preparation The reproducibility of the zeolite membrane quality is critical for its industrial application. Table 4 summarizes the PV performance of 10 pieces of tubular ZSM-5 membranes (M1–M10) prepared with the identical synthesis condition in this paper. For dehydration of the binary 50 wt.% HAc/H2O solution at 75 °C, the total fluxes and H2O contents in the permeate sides of the membranes (M1–M5) are 1.77–2.36 kg m2 h1 and 99.25–99.60 wt.%, respectively, and the standard deviations for flux and H2O content in permeat were 9.2% and 0.11%. Besides, the other ZSM-5 membranes (M6–M10) also exhibited well H2O perm-selectivity for the HAc/EtOH/H2O/AcOOEt mixture, the fluxes and the H2O content in the permeate side of the membranes are above 1.24– 1.66 kg m2 h1 and 99.25–99.68 wt.%, the corresponding standard deviations for the flux and H2O content in the permeat were 7.5% and 0.14%, respectively. As aforementioned, the ZSM-5 membranes prepared in this study had a well reproducible dehydration performance for the acidic aqueous mixtures.

Table 5 summarizes the comparison of the PV performance of zeolite membranes for the carboxylic acid containing mixtures. Compared with the previous reports, the present ZSM-5 membranes had a better H2O perm-selectivity for the binary HAc/H2O mixtures or quaternary esterification productions in the present work. Apparently, the as-synthesized ZSM-5 membranes have an excellent H2O perm-selectivity, long-term acid-stability and reproducibility in this study, which makes them to be a promising candidate in industrial application for dehydration of carboxylic acidic mixtures and/or esterification reactions.

4. Conclusions Hydrophilic ZSM-5 membranes were prepared on mullite supports. Membranes showed selectivity over 100 in the dehydration of binary HAc/H2O and quaternary esterification mixtures by PV. The partial flux of each component of the membrane was linearly

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increasing with temperature. As the partial flux of H2O increased faster than other three organic components (EtOH, HAc and AcOOEt), both the flux and selectivity became larger with temperature and achieved 1.49 kg m2 h1 and 99.38 wt.% at 75 °C. The membrane showed a long-term stable PV performance when 90 wt.% HAc/H2O mixture, pure HAc solution and HAc/EtOH/H2O/AcOOEt mixture were applied. No structure change was observed after immersing the ZSM-5 membrane in the HAc/EtOH/H2O/AcOOEt mixture for 98 days by the results of XRD, SEM and EDAX characterization. These results confirmed high acidic stability of the membrane. About 20% flux reduction was observed at the beginning of the dehydration tests using acetic acid solution. The flux was recovered after post-treating the membranes by the 0.1 M NaCl or NaOH solution. Ion-exchange between Na+ and H+ is the presumed cause of the flux change. The ZSM-5 membrane synthesis was reproducible. The acidic-stable ZSM-5 membranes having excellent H2O perm-selectivity would be a promising candidate in industrial application for dehydration of carboxylic acidic mixtures and esterification reactions. Acknowledgement This work was supported by NEDO project of Development of Fundamental Technologies for Green and Sustainable Chemical Processes.

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References [1] R.Y.M. Huang, A. Moreira, R. Notarfonzo, Y.F. Xu, J. Appl. Polym. Sci. 35 (1988) 1191–1200. [2] R.W. Helsel, Chem. Eng. Prog. 73 (1977) 55–59. [3] H. Kita, K. Tanaka, K. Okamoto, M. Yamamoto, Chem. Lett. 16 (1987) 2053– 2056. [4] Y. Zhu, R.G. Minet, T.T. Tsotsis, Chem. Eng. Sci. 51 (1996) 4103–4113. [5] Q.L. Liu, H.F. Chen, J. Membr. Sci. 196 (2002) 171–178. [6] Y. Hasegawa, T. Nagase, Y. Kiyozumi, T. Hanaoka, F. Mizukami, J. Membr. Sci. 349 (2010) 189–194. [7] Y. Hasegawa, C. Abe, F. Mizukami, Y. Kowata, T. Hanaoka, J. Membr. Sci. 415– 416 (2012) 368–374. [8] K. Tanaka, R. Yoshikawa, Y. Cui, H. Kita, K. Okamoto, Chem. Eng. Sci. 57 (2002) 1577–1584. [9] G. Li, E. Kikuchi, M. Matsukata, Sep. Purif. Technol. 32 (2003) 199–206. [10] Z. Chen, Y.H. Li, D.H. Yin, Y.M. Song, X.X. Ren, J.M. Lu, J.H. Yang, J.Q. Wang, J. Membr. Sci. 411–412 (2012) 182–192. [11] G. Li, E. Kikuchi, M. Matsukata, J. Membr. Sci. 218 (2003) 185–194. [12] X.S. Li, H. Kita, H. Zhu, Z.J. Zhang, K. Tanaka, J. Membr. Sci. 339 (2009) 224–232. [13] M.H. Zhu, Z.H. Lu, I. Kumakiri, K. Tanaka, X.S. Chen, H. Kita, J. Membr. Sci. 415– 416 (2012) 57–65. [14] X.L. Zhang, M.H. Zhu, R.F. Zhou, X.S. Chen, H. Kita, Sep. Purif. Technol. 81 (2011) 480–484. [15] R.F. Zhou, Z.L. Hu, N. Hu, L.Q. Duan, X.S. Chen, H. Kita, Micropor. Mesopor. Mater. 156 (2012) 166–170. [16] M.T. Sanz, J. Gmehling, Chem. Eng. J. 123 (2006) 1–8. [17] J.C. Groen, J.A. Moulijn, J. Pérez-Raml´rez, Ind. Eng. Chem. Res. 46 (2007) 4193– 4201. [18] K. Tanaka, R. Yoshikawa, Y. Cui, H. Kita, K. Okamoto, Catal. Today 67 (2001) 121–125.