SO42−-AAO difunctional catalytic-pervaporation membranes: Preparation and characterization

Journal Pre-proofs PVA/SO4 2-- AAO difunctional catalytic-pervaporation membranes: preparation and characterization Hanshuo Sun, De Sun, Xiumin Shi, Bingbing Li, Dongmin Yue, Rui Xiao, Ping Ren, Jinhui Zhang PII: DOI: Reference:

S1383-5866(19)35423-1 https://doi.org/10.1016/j.seppur.2020.116739 SEPPUR 116739

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

26 November 2019 17 February 2020 17 February 2020

Please cite this article as: H. Sun, D. Sun, X. Shi, B. Li, D. Yue, R. Xiao, P. Ren, J. Zhang, PVA/SO4 2-- AAO difunctional catalytic-pervaporation membranes: preparation and characterization, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116739

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PVA/SO42-- AAO difunctional catalytic-pervaporation membranes: preparation and characterization Hanshuo Sun, De Sun*, Xiumin Shi*, Bingbing Li* , Dongmin Yue, Rui Xiao, Ping Ren, Jinhui Zhang Department of Chemical Engineering, Changchun University of Technology, 2055 Yanan Street, Changchun 130012, P. R.China *Corresponding authors: De Sun; Xiumin Shi; Bingbing Li Email: [email protected]; Tel:86-431-85717211 Email: [email protected]; Tel:86-13578780198 Email: [email protected]; Tel:86-15526639820

Abstract In this paper, polyvinyl alcohol/SO42-- anodic aluminum oxide (PVA /SO42-AAO) membranes, the dual-functional flat composite membranes (DCMs) applied for the simultaneous catalytic reaction and pervaporation, were fabricated via dip-coating method. Firstly, the PVA /AAO pervaporation composite membranes (SCMs) were prepared by using PVA as the active separation layer and AAO as the support layer. Secondly, the SO42- was loaded in the pores of AAO by dipping the membranes in dilute sulfuric acid solution. Thirdly the membranes were calcinated at 400 ℃ to produce the solid acid catalyst layer (SO42--AAO). Finally, PVA solution was coated on one side of SO42--AAO, and then the PVA/SO42--AAO difunctional membranes 1

were prepared. The effects of PVA concentration on the morphology and hydrophilicity of SCM were characterized by SEM and water contact angle. The effects of the concentration of the sulfuric acid solution on morphology, acid sites, elements distribution and crystal form of DCMs were characterized by SEM, MIP, NH3-TPD, XPS and XRD. The catalytic activity of SO42--AAO was investigated through the esterification between ethanol and acetic acid. When the concentration of sulfuric acid was 2 mol/L, the SO42--AAO had the best catalytic behavior. For the separation performance, compared with SCMs, the DCMs had higher flux but lower separation factor. Using DCM4 to do the pervaporation-esterification coupling experiments, acetic acid conversion reached above 96% after the 16-hours’ reaction, in addition, more than 90% acetic acid conversion was achieved after the operation of 80 hours. Keywords: pervaporation; difunctional membrane; catalytic membrane; esterification reaction

1. Introduction The coupling of pervaporation with chemical reaction is widely studied [1-6], it can improve the reaction efficiency and increase the conversion of the reactants. The application of chemical integration can be classified into three kinds[7]: (1) coupling with parallel reactions to enhance the selectivity by means of controlling the flux of one reactant, (2) coupling with successive reactions to enhance the selectivity by the removal of intermediate product, (3) coupling with reversible reactions to intensify the

2

conversion by the removal of products or byproducts. For example, ethyl acetate is prepared by the esterification between acetic acid and ethanol, which is a typical reversible reaction limited by thermodynamic equilibrium and the production of the by-product of water. There have been several literatures on the coupling of esterification and pervaporation to remove by-product water in-situ in recent years [8-10]. Pervaporation membranes used for the integration of esterification reaction and membrane separation can be divided into two types[11]: one is the single-functional membrane which has only the separation function without catalyst activity; the other is the difunctional membrane, which has both catalytic activity and separation performance. For difunctional membrane, the coupling of reaction and pervaporation can be achieved by placing the membrane in a membrane reactor[3]. Nowadays, more and more literatures focus on membrane reactor have been published and kinds of membranes have been developed using different membrane materials [4-6]. Lots of natural and synthetic hydrophilic polymeric materials have been used to prepare membranes for the dehydration of organics and water mixtures, such as polyvinyl alcohol (PVA)[12, 13], cellulose[14, 15] and chitosan[16, 17]. As a kind of hydrophilic material, PVA has some excellent performances such as non-toxic, cheap and easy available[18], and it has been commonly used in the fabrication of pervaporation membranes to separate water from solutions with a trace amount of water. However, there exists a large number of hydroxyl group in PVA which makes PVA easily swelled in aqueous solution, consequently water selectivity would be 3

reduced because the dissolution and diffusion of organics are promoted [19, 20], so to overcome the swelling of PVA membrane is very important for pervaporation. Several studies have also demonstrated that the support layer has significant effects on the pervaporation [21, 22] for the reason that the support layer of present polymer membranes have the disadvantages of swelling and resisting which affect transfer efficiency. Tanaka et al. [23] prepared the hydrophobic composite membranes with

cross-linked

poly(dimethyl

siloxane)–poly(methyl

hydrogen

siloxane)

(PDMS/PMHS), for which the PDMS/PMHS polymer penetrated into and plugged the pores of the γ-alumina support so as to inhibit the swelling of membrane. Compared with polymeric support layer, the ceramic support layer has superior mechanical property and exhibits high surface porosity, strong structural stability, no swelling and compaction, negligible resistance of mass transfer and so on [24, 25]. Currently, most of the catalytic-separation difunctional membranes used in the coupling of esterification-pervaporation are blending membranes [11]. Shi et al. [26] prepared a new kind of organic–inorganic hybrid membrane by using sulfonated poly (ether sulfone) (SPES) as substrate and phosphotungstic acid (PWA) as heterogeneous acid catalyst for biodiesel production. Bo et al.[14] fabricated a bilayer catalytic composite membrane by coating sulfonated-polyvinyl alcohol (SPVA) casting solution onto the polyvinyl alcohol (PVA)-sodium alginate (SA) membrane. For the blending catalytic membranes, the catalyst tends to be covered by polymer, as a result, the liquid and the catalyst cannot be fully contacted, and the catalytic performance of the membrane would be reduced [27]. The method of combining catalyst layer with 4

separation layer is a challenge for the preparation of catalytic membrane, some researchers have prepared catalytic membranes by methods such as non-solvent induced phase inversion (NIPS) [1, 27-29], electrospinning [30] and dip-coating [31]. AAO membrane is a special ceramic membrane which has high porosity, lower channel sinuosity and regular pore structure [32], it has been utilized in the filtration areas such as gas separation[33] and water purification[34]. In this study, the PVA/AAO composite membranes with pervaporation function (SCMs) were prepared, and the PVA/ SO42--AAO difunctional composite membranes (DCMs) with pervaporation and catalytic functions were fabricated. For the DCMs, PVA was anchored in AAO holes to inhibit swelling, and the SO42--AAO as the substrate played the function of catalyst. In this study, we focused on the preparation as well as the characterizations of the membranes. In addition, the catalytic activity of SO42--AAO, and the pervaporation performances of SCMs and DCMs were investigated. The coupling of pervaporation and esterification was studied.

2. Experimental 2.1 Material PVA (polymerization degree 1750±50) was supplied by GuangFu Institute of Fine Chemicals (Tianjin, China), glutaraldehyde (50 %) was procured from GuangFu Institute of Fine Chemicals (Tianjin, China), hydrochloric acid (36 %-38 %) was obtained from Baicheng Reagent Plant (Jilin, China), and ethyl acetate, sulfuric acid, 5

ethanol, acetic acid and acetone were purchased from Beijing Chemical Works (Beijing, China). AAO (electrolyzed by Oxalic acid), provided by Guangzhou Lesson Nano Technology Company Limited (Guangzhou, China), was used as support layer and its structural parameters are listed in Table 1. All chemicals were of analytical grade and were used without further purification. Homemade deionized water was used throughout the work. Table 1 AAO structural parameters Pore

Porosity/

Thickness/

Membrane

Bursting

diameter/(nm)

(%)

(μm)

diameter/

strength/

(mm)

(psi)

25

65-110

200

60

60

2.2 Membrane preparation 2.2.1 Preparation of SCMs To prepare PVA solution, a certain amount of PVA was dissolved in deionized water under continuous magnetic stirring of 10 hours at 90 ℃, then the solution was cooled to room temperature and defoamed for 12 h. The surface-linking progress was as follows: one side of AAO was dipped in PVA solution for 90 s, then the membrane was taken out and dried at ambient temperature for overnight, after that the PVA layer of the membrane was immersed into the crosslinking solution for 30 min at room temperature for interfacial crosslinking. The crosslinking solution was acetone aqueous 6

solution which contained 4 wt.% glutaraldehyde (GA) as crosslinking agent and 1 wt.% Hydrochloric acid (HCl) as the catalyst. After the reaction, the membrane was taken out, washed with deionized water and dried in a dust-free room, the PVA/AAO membrane was prepared. Corresponding to the concentrations of the PVA in the solutions of 1 wt.%, 3 wt.% and 7 wt.%, the membranes were designed as SCM1, SCM3 and SCM7, respectively. The diagram of the preparation and the pictures of SCMs were shown in Fig.1 and Fig.2.

Fig.1 Schematic diagram of preparation of SCMs

Fig.2 Pictures of various membranes: a: AAO b: SCM1 c: SCM3 d: SCM7

2.2.2 Preparation of DCMs In order to prepare DCMs, firstly the AAO samples were immersed in different concentration dilute sulfuric acid solutions (0.5mol/L, 1.0mol/L, 1.5mol/L, 2.0mol/L, and 2.5mol/L). Taken out from the solutions, the samples were dried at ambient temperature and then were calcinated at 400 ℃ in a muffle furnace for 3 hours, then the SO42--AAO layers, which were used as the catalyst as well as the support layer were 7

produced. Finally, PVA solution was coated on the surfaces of the SO42--AAO layers according to the method in the above description. Corresponding to the molar concentration of the sulfuric acid which were 0.5mol/L, 1.0mol/L, 1.5mol/L, 2.0mol/L, and 2.5mol/L, the SO42--AAO catalyst layers were named as 0.5 SO42--AAO, 1 SO42--AAO, 1.5 SO42--AAO, 2 SO42--AAO and 2.5 SO42—AAO, and the dual functional membranes were designated as DCM1, DCM2, DCM3, DCM4 and DCM5, accordingly. The diagram for the preparation of the DCMs was illustrated in Fig.3. The pictures of DCMs have been shown in Fig.4. There are some black solids on the membrane surfaces, because there exists organic matter carbonizing on the SO42--AAO surfaces after the calcination.

Fig.3 Schematic diagram of preparation of DCMs

Fig.4 Picture of various DCMs (a-e: DCM1 to DCM5)

8

2.3 Characterizations of SCMs and DCMs The morphology and thickness of the membranes were examined by scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan). The samples were immersed in liquid nitrogen for a few minutes then were fractured and then gold-coated for SEM testing. The water contact angles on AAO surface and the surfaces of the PVA side of the SCMs membranes were measured by a contact angle meter (DSA30, KRuss GmbH, Germany) at room temperature using the static sessile drop method. Deionized water droplets were dropped onto the membrane surfaces at five different locations of the same sample, then the results were averaged. The acidity of the prepared catalyst layers was determined by the NH3 temperature-programmed desorption (TPD) (Auto Chem п2920, Micromeritics, US). The samples were swept by N2 at 200 ℃ for 20 minutes, dropped to ambient temperature, NH3 then was introduced for 30 minutes. After the sorption process, the samples were heated to 750 K at a rate of 10 K/min. The X-ray diffraction (XRD) spectrograms for the catalyst layers of various DCMs were performed on D/MAX 2000/PC (Japan) under 10mA and 40kV with Cu Kα radiation. The valence states of elements (O1s, S2p and Al2p) and the composition of the catalyst layers of DCMs were determined by XPS (VG Microtech 3000 Multilab), all XPS spectra were corrected to the C 1s peak at 284.6 eV. Average pore size was obtained by an automatic intrusion porosimeter (Autopore IV 9500, Micromeritics, USA).

9

2.4 Pervaporation experiments

Fig.5 (a) Scheme of the PV separation and coupling experiment, (1) feed tank (2) peristaltic pump (3) rotameter (4) membrane module (5) vacuum control valves (6) cold trap (7) surge flash (8) vacuum indicator (9) vacuum pump; (b) Schematic diagram of membrane module; (c) Partial enlarged drawing of module: (1) silicone gasket (2) membrane (3) PP porous support layer

The pervaporation experiment was carried out by a homemade experiment device, and the scheme diagrams of the device, module and partial enlarged drawing of the module were shown in Fig.5a, 5b, 5c, respectively. Membrane samples were placed in the membrane unit with effective membrane area of 3.14 cm2, the capacity of the feed tank was 20 L, and the downstream pressure of the membrane cell was kept at 2 kPa (absolute pressure) by a vacuum pump. The membrane was placed in the silicon gasket 10

in the membrane module. When the system reached a steady state, the permeate began to be collected by a cold trap that was immersed in liquid nitrogen. The feed concentration and permeate composition were measured by a gas chromatography (GC-2014c, Shimadzu, Japan) with a thermal conductivity detector (TCD). In addition, the permeate was diluted with deionized water to one phase before injection if there were two phases of the permeate. The permeation flux (Ji), separation factor (αi) and PV separation index (PSI) were calculated by the following equations: Ji =

αi =

Wi At

yi  1- yi  xi  1- xi 

PSI = J  α

(1) (2) (3)

Where Wi is the weight of the component i in permeate substance (g); A is the effective membrane area (m2); t is the time of pervaporation (h); xi and yi represent the mass fraction of the component i in feed and permeate, respectively.

2.5 Esterification reaction As given in Fig.6, the esterification reaction was carried out in a batch reactor. The feed solution with a constant molar ratio of ethanol to acetic acid of 4:1 was added to a 250 mL three-necked flask which had a temperature sensor and a reflux condenser. The reactive gas was condensed and flowed back to the liquid phase of the reaction system. For the reaction, a certain amount of SO42--AAO (0.2 g) was used as the catalyst, the reaction was carried out for 16 h, the temperature was kept at 70 ℃ controlled by water 11

bath, and the composition of the reaction product in the flask was tested by GC. The acetic acid conversion was used to assess the catalytic performance of DCMs: Acetic acid percent conversion= m0 - m × 100% m0

(4)

Where m0 is the initial mass of acetic acid (g), m is the mass of acetic acid at any time (g).

Fig.6 Set-up diagram of catalytic reaction: (1) Water bath (2) Temperature sensor (3) Reflux condenser (4) Reactor (5) Temperature controller instrument

2.6 Coupling experiment The coupling experiment was carried out using the same device as for the pervaporation experiment. In addition, the membrane was placed in the same way as for the pervaporation process. In order to make the catalyst fully contacted with the feed liquid, the catalyst layer contacted with the feed liquid. The reactions were operated for 16 hours under the conditions that membrane area was 29.43 cm2 (6

12

pieces) and the mole ratio of ethanol to acetic acid was 4:1. The partial fluxes and the acetic acid conversion were calculated as described in section 2.4 and 2.5, respectively.

3. Result and discussion 3.1 Characterizations of SCMs 3.1.1 Membrane morphology

Fig.7 Surface and cross-section SEM images and scheme picture of AAO, SCM1, SCM3, SCM7.

SEM images are used to analyze the morphology of the membranes, and Fig.7 13

indicated the surface and the cross-section images of AAO and PVA/AAO composite membranes. It could be observed that the surfaces of the membranes were very different in morphology. AAO had a regular array of the pores and a rough surface. When PVA solution was coated on the surface of AAO, its surface morphology changed. When PVA concentration was 1wt.%, the surface morphology changed slightly, there was only a little PVA on the surface. When PVA concentration was 3 wt.%, the surface structure changed obviously, it had a regular array of humps. When the concentration of PVA solution increased to 7 wt.%, the pores were difficult to be observed and the surface became smooth and dense. As the scheme diagrams showed, AAO possessed homogenous through-holes. When PVA solution was deposited on AAO surface, a part of the PVA solution entered the pores of the AAO layer, then an organic-inorganic transition region was formed between the PVA layer and the AAO layer, the transition region was regarded as the interface layer, and the PVA layer on AAO surface was called as surface layer. The thickness of the active layer was the total thickness of the interface layer and the surface layer. When PVA concentration was 1 wt.%, the viscosity of the PVA casting solution was very low ( Table 2 ), so much PVA entered the AAO pores and formed the interface layer, and just a very thin film was coated on the AAO surface which had no separation function. When PVA concentration increased, less and less PVA casting solution penetrated into the pores of support layer, so compared with SCM1, for SCM3, only a small amount of PVA solution permeated into the AAO holes and most of the PVA solution was left on the surface of membrane, so the interface layer became thinner and the surface layer started 14

to form. For SCM7, the surface of AAO support was completely covered by PVA, and it had the thinnest interface layer and the densest surface layer. From the cross-section image of AAO, it could be seen that AAO had the throughout holes with low tortuosity. When 1 wt.% PVA was coated on AAO, the pore channels of AAO were filled with a small amount of PVA solution, the active layer was 1 μm (Table 2). When the concentration of PVA was 3 wt.%, some PVA solution stayed on the surface and fewer solution permeated into the pore channel, so its separation layer was thicker than SCM1. When the concentration increased to 7 wt.%, PVA solution stayed on the support surface and a small amount of PVA penetrated the pore channels, it had a boundary between the support layer and the active layer. With the increase of PVA concentration, the thickness of separation layer increased but the thickness of interface layer decreased. Table 2 The thickness of separation layer and viscosity of coating solution of PVA/AAO membranes Membrane

Thickness of active layer(μm)

Viscosity of coating solution(mPa•s)

SCM1

1

5

SCM3

3

21

SCM7

34

340

15

3.1.2 Static contact angle Water contact angle was measured to estimate the hydrophilicity of the membranes. The contact angles of water droplets on AAO and the active separation layers of SCMs were measured and the results were shown in Fig.8. The contact angle of AAO membrane was 127 °. Obviously, with the increase of PVA concentration, the contact angle decreased and was only 59 ° for SCM7. Because of the increase of PVA concentration, more and more PVA covered the surface of AAO, so its performance of hydrophilicity increased.

Fig.8 Water contact angle for AAO and SCMs

16

3.2 Pervaporation performance 3.2.1 Effect of feed temperature

Fig.9 Effect of feed temperature on water flux, EAc flux, separation factor(α), PSI for PVA/AAO composite membranes (2 wt.% water-EAc)

17

Fig.10 Effect of feed temperature on water flux, ethanol flux, separation factor(α), PSI for PVA/AAO composite membranes (2 wt.% water-ethanol)

18

Fig.11 Effect of feed temperature on water flux, acetic acid flux, separation factor(α), PSI for PVA/AAO composite membranes (2 wt.% water-acetic acid)

Temperature is a crucial influential factor for pervaporation process, using different PVA/AAO composite membranes, the effects of feed temperature on pervaporation performance were carried out. For this experiment, three kinds solutions of ethyl acetate/water, ethanol/water, and acetic acid/water were used as the feeds respectively, feed water content was 2 wt.% and permeation side vacuum degree was 0.098 MPa, the result were shown in Fig.9, Fig.10 and Fig.11, respectively. For all membranes, water flux and organics fluxes showed almost the same trend, and SCM1 had the highest fluxes because its active layer thickness was the thinnest (Table 2). When feed temperature increased, water fluxes increased because mass transfer driving force through the membrane increased [35]. But for the fluxes of organics, different 19

membrane had different variation tendency, it was determined by the structure of membranes. For SCM1, contrary to the universal relationship between temperature and flux, organics fluxes decreased with the increasing temperature due to the obvious swelling inhibition of the PVA filled in the rigid AAO pore channels. The swelling inhibition hindered the penetration of the organics molecules, which was called confinement effect or swelling inhibition effect as Wei and Liu et al. proposed [36-39]. For SCM3, the swelling inhibition of the interface layer offset the swelling effect of the surface layer, so the organics fluxes kept invariable as temperature increased. For SCM7, there were typical pervaporation performance curves due to the swelling effect, organics fluxes increased with the increase of temperature. As for separation factor and PSI, for SCM1, due to the obvious swelling inhibition of its PVA interface layer, the swelling effect of the active layer was the weakest, so with the increasing temperature, water flux increased and organics fluxes decreased, as a result, separation factor and PSI of SCM1 increased. For SCM3, separation factor increased to its maximum at 50 ℃ and then decreased. The reason was that, below 50 ℃, the swelling effect was more obvious than the swelling inhibition effect, whereas, over 50 ℃, the swelling inhibition effect was more obvious. Similar results were concluded by Zhu et al. [35, 40-43]. For SCM7, when temperature increased, separation factor decreased, which was the typical pervaporation performance curve caused by swelling effect.

20

3.2.2 Effect of feed concentration

Fig.12 Effect of feed concentration on water flux, EAc flux, separation factor (α), PSI of AAO supported PVA composite membranes (at 50 ℃)

21

Fig.13 Effect of feed concentration on water flux, ethanol flux, separation factor (α), PSI of AAO supported PVA composite membranes (at 50 ℃)

22

Fig.14 Effect of feed concentration on water flux, acetic acid flux, separation factor (α), PSI of AAO supported PVA composite membranes (at 50 ℃)

Feed concentrate is another changeable parameter in pervaporation, and the effects of feed concentration on pervaporation performance for ethyl acetate/water, ethanol/water, and acetic acid/water were displayed in Fig.12, Fig.13, and Fig.14 at feed temperature 50 ℃ and permeation side vacuum degree 0.098 MPa. For all membranes, when water concentration increased, water flux and organics fluxes still showed almost the same trend. Water flux increased, because when water content increased, the driving force of water molecules penetration increased [18], but for organics fluxes, different membrane had different variation tendencies. For SCM1, the organics fluxes decreased caused by the swelling inhibition of the membrane. For SCM3, although the AAO support layer inhibited the swelling of the PVA interface 23

layer[44], but the coupling effect caused by the water adsorption in PVA surface layer made organics fluxes increased slightly. For SCM7, because AAO support layer had almost no inhibition on the swelling of PVA interface layer, water flux and the fluxes of organics all increased [41]. For all SCMs, with the increasing water concentration in feed, all separation factors and PSIs dropped. When feed content of water was 0.5 wt.%, the separation factor and PSI values were very high, which demonstrated that the SCMs were suitable for separating organics/water mixture with low water content. In addition, comparing the pervaporation performances of these membranes, SCM3 had the best pervaporation performance, it had the highest separation factor and PSI.

3.2.3 Pervaporation performance comparison with literatures Many research groups have been focused on the separation of ethyl acetate/water mixtures by pervaporation method. Table 3 listed a comparison of pervaporation performances of PVA/AAO composite membrane with the membranes in some literatures on the dehydration of ethyl acetate solution. From the performance parameters of the membranes, it could be seen that water flux and the separation factor of PVA/AAO composite membrane could reach 3137 g/(m2·h) and 7778 at 50 ℃ and 2 wt.% water content in feed, which were relatively high compared with reported composite membranes for ethyl acetate solution dehydration. This could attribute to the larger pore diameter and relatively straight channels which would limit membrane

24

swelling and decrease transport resistance. In addition, moderate viscosity of PVA limited the excessive penetration of PVA solution into AAO holes and made a defect-free and thin polymer separation layer. Table 3 Pervaporation performances for dehydration of ethyl acetate/water mixture through various membranes Membrane

Feed composition

Temperatu

Water flux

Separation

PSI

Referen

re

（g/(m2·h)）

factor

(105 g/m2

ce

a.u.

h)

（℃） PVA crosslinked

EAc/W=

by Tac

97.5/2.5

PU

EAc/W=

50

54

4000

2.2

[38]

40

148

42

0.06

[39]

40

205

496

0.9

[35]

40

348

2218

7.7

[40]

60

1019

633

6.4

[41]

40

315

163000

513.4

[42]

60

820

2478

20.3

[45]

92/8 PFSA-TEOS/PAN

EAc/W= 98/2

PVA-TEOS-PFSA/P

EAc/W=

AN

98/2

PVA/ceramic

EAc/W= 94.9/5.1

NaA zeolite

EAc/W=

membrane

98/2

PBI/PEI

EAc/W=

25

98/2 MOF@GO/PAN

EAc/W=

60

3632

6076

220.64

[46]

50

3137

7778

245.5

This

98/2 PVA/AAO

EAc/W= 98/2

work

3.3 Characterization of SO42--AAO and DCMs 3.3.1 XRD analysis of SO42--AAO

Fig.15 XRD spectra of the catalyst layers of DCMs

To investigate the effect of sulfuric acid concentration on crystal form of the support layers, different SO42--AAO layers of DCMs were analyzed by XRD measurements and the results were shown in Fig.15. When the concentration of sulfuric acid varied from 0 to 2 mol/L, there was no crystal characteristic peak such as γ-Al2O3, γ-AlOOH or other forms of Al2O3. All SO42--AAOs had an obvious diffraction peak at 26

the angle of 2θ about 25 degrees. However, for 2.5 SO42--AAO, it seemed that there was a small peak at 26 ° besides the diffraction peak at 25 °. When the sulfuric acid concentration increased from 0.5 to 2 mol/L, AAO was amorphous and the crystal form of catalyst layer would not change. When the concentration of sulfuric acid was 2.5 mol/L, the peak was assigned to Al2(SO4)3, this might because part of the sulfuric acid reacted with AAO and produced a small amount of Al2(SO4)3[47].

3.3.2 NH3-TPD analysis of SO42--AAO

Fig.16 NH3-TPD profiles of catalyst layers Table 4 Total quantities of NH3 desorbed on catalyst layers and every peak temperature of acid site for catalyst layers Sample

Total NH3 desorbed

Peak temperature of acid sites (K)

(mmol/g) 0.5 SO42--AAO

0.0099

375.8

27

1 SO42--AAO

0.1522

375.8

1.5 SO42--AAO

0.3058

403.3

2 SO42--AAO

0.4621

543

2.5 SO42--AAO

0.0478

432.08

The acidic sites of the catalyst are the active centers of the esterification reactions, ammonia temperature-programmed desorption (NH3-TPD) experiments were conducted to measure the performance of acidic sites. NH3 desorption peaks of the membranes were measured in the tested temperatures ranging from 310 K to 750 K, the results based on the Gaussian fitting were shown in Fig.16 and Table 4. With the increase of sulfuric acid concentration, the amount of acid sites firstly increased from 0.0099 to 0.4621 mmol/g and then decreased to 0.0478 mmol/g. The low temperature NH3 desorption peaks of 0.5 SO42--AAO, 1 SO42--AAO, 1.5 SO42--AAO, and 2.5 SO42--AAO were in a range of about 360-448 K, which were attributed to the weak acidic positions. The peak of 2 SO42--AAO at about 543 K was assigned to the medium temperature desorption signal corresponding to the desorbed NH3 on medium acid sites [48, 49]. These results demonstrated that 2 SO42--AAO owned the most amount of the acid sites and had the strongest acidity.

28

3.3.3 XPS analysis of SO42--AAO

Fig.17 (a) XPS spectrum of the 2 SO42--AAO layer (b) the core-level XPS spectra of Al2p, O1s, S2pof 2 SO42--AAO

Table 5 Electron binding energies (eV) and the ratio of S to Al of various catalyst layers Samples

Binding Energy (eV)

S/Al

Al2p (+3)

O1s (-2)

S2p (+6)

AAO

74.3

531.2

0.5 SO42--AAO

74.5

531.9

168.6

0.0867

1 SO42--AAO

74.4

531.9

169.6

0.1056

1.5 SO42--AAO

74.5

531.9

169.5

0.2497

2 SO42--AAO

75.1

532.2

169.8

0.415

2.5 SO42--AAO

74.7

532.1

169.0

0.5896

--

In order to analysis the valence states of the elements, XPS survey scan spectra of 2 SO42-AAO was taken as an example. As shown in Fig.17, the sample contained the elements of Al, O, S and C. Among these elements, the C element might be resulted 29

from the oxalic acid deposition in the process of anodic oxidation or the carbon contamination on the surface of the membrane, so we could see the black solid on the SO42-AAO membrane surface. In addition, the Al, O and S elements on the surfaces of the membranes were scanned in a narrow range and their photoelectron spectra were presented in Fig.17 (a),(b). According to these pictures, the elements Al and O in the sample only owned the characteristic peaks of Al(+3) and O(-2) corresponding to the binding energy of 75.1 eV and 532.2 eV, respectively. Element S also had a characteristic peak of S (+6) at 169.8 eV which was the highest valence state of S element, besides this, there was no other valence state. The result was coordinated with some references [50-52], which reveals that, in the solid acid catalyst, the S(+6), the oxidation state of sulfur, showed high activity in the acid catalyzed reaction. Table 5 also presented the binding energy of different elements and the ratio of elements S to Al in various SO42--AAO. Compared with AAO, the binding energy of Al element increased slightly in SO42--AAO layer. In general, a slight shift to higher binding energy could be seen in the case of sulfated catalyst, which might because that Al was connected with one or more atoms or groups of atoms with strong electrical absorption property[53]. The sulfate acid group had electron-withdrawing nature, so as an electron-acceptor group, the oxygen sulfur bond (S-O) connected with the Al in Al2O3 through the oxygen bridge. Moreover, the formation of acid centers was produced by the strong shifts of Al-O bond electron clouds caused by the coordination adsorption of the sulfate groups on the AAO surface. The schematic diagram of the ball-stick model of SO42--AAO was shown in Fig.18. The sulfuric acid concentration 30

had an important influence to the ratio of S to Al, with the increased concentration of sulfuric acid from 0.5 to 2.5mol/L, the S/Al increased from 0.0867 to 0.5896, which indicated that more and more S atoms were contained in the surface of catalyst layers.

Fig.18 Ball- stick model of SO42--AAO

3.3.4 Analysis of acid concentration For the further comparison of the catalytic activity of different catalyst layers and the study of the stability of SO42--AAO, the initial acid concentration (CA0) and the acid concentration after 16-hours’ esterification reaction (CA1) were studied by acid-base titration method using 0.05 mol/L NaOH aqueous solution, the results were indicated in Table 6. With the increased concentration of sulfuric acid from 0 mol/L to 2.0 mol/L, the initial acid concentration increased from 3.5 mmol/L to 1390 mmol/L, but when the sulfuric acid concentration increased to 2.5 mol/L, the acid concentration decreased to 1025 mmol/L because of the existence of Al2(SO4)3. From the results of the analysis for initial acid concentration, it was illustrated that 2.0 SO42--AAO had the best catalytic performance and this was in accordance with the XPS results. In order to study the stability of SO42--AAO, the loss of acid concentration after 16-hour’s esterification reaction was measured. According to the decrease of acid concentration, it was indicated the SO42--AAO had excellent stability 31

because the loss percentages of acid concentration were less than 0.8% [54]. Table 6 Acid concentration of AAO and SO42--AAO layer Item

Acid concentration (mmol/L)

Loss percentage of acid concentration (%)

CA0 (mmol/L)

CA1(mmol/L)

AAO

3.5

3.5

0

0.5 SO42--AAO

10.4

10.3

0.2

1 SO42--AAO

84.8

84.2

0.7

1.5 SO42--AAO

441.7

439.2

0.6

2 SO42--AAO

1390.0

1380.3

0.7

2.5 SO42--AAO

1025.0

1018.7

0.6

3.3.5 Membrane morphology of DCMs Fig.19 (a)-(e) showed the SEM micrographs of the cross-sections of DCMs. It could be seen that the surfaces of the SO42--AAO supports were completely covered with PVA, in addition, for the same concentration of PVA, the thickness of the PVA layers became thinner as the concentration of sulfuric acid increased. MIP (Mercury intrusion porosimeter) analysis was used to illustrate the change of pore channel after the membranes were dipped in sulfuric acid, and the values of the pore diameter of SO42--AAO layers are shown in Table 7. When the concentration of the dipping sulfuric acid increased, the pore diameter of the supports increased, as we can see, the average pore diameter of 2.5 SO42--AAO was much larger than that of AAO. SEM 32

micrograph of the lower surface of DCM5 was shown in Fig.19 (f), it could be clearly observed that sulfuric acid could corrode AAO, consequently, the pore channel of AAO would be damaged. From Fig.19 (e), there was a lot of PVA in the pore channels of the support layer.

Table 7 Average pore diameter of SO42--AAO layers Sample

Average pore diameter(nm)

0.5 SO42--AAO

216

1 SO42--AAO

233

1.5 SO42--AAO

268

2 SO42--AAO

342

2.5 SO42--AAO

830

Fig.19 SEM images of cross-section of DCMs (a-e), lower surface of DCM5 (f)

33

3.3.6 Catalytic performance of SO42--AAO Catalytic activities of AAO and different SO42--AAO were investigated by the esterification reaction between acetic acid and ethanol when the molar ratio of ethanol/acetic acid was 4:1, operation temperature was 70 ℃, and the mass of the catalyst was 0.2 g, the results were demonstrated in Fig.20. When the concentration of sulfuric acid increased from 0 to 2 mol/L, final conversion increased, and the time reached equilibrium shortened, as a result, with the highest acidity and the most acidity center in the catalyst layer, 2 SO42--AAO had the highest catalytic activity. Due to the formation of the Al2(SO4)3 in 2.5 SO42--AAO, the catalytic activity dropped slightly, and the conversion decreased. The results were consistent with the analysis of NH3-TPD and XPS in 3.3.2 and 3.3.3.

Fig.20 Catalytic activities of AAO and different SO42--AAO with reaction time (operation conditions: reaction temperature 70 ℃, molar ratio of ethanol to acetic acid 4:1 and the mass of catalyst 0.2 g) 34

3.3.7 Separation performance of DCMs Fig.21 showed the pervaporation results of the 2 wt.% water-EAc mixture at 50 ℃ by the SCM3 and DCMs. From DCM1 to DCM5, with the increasing concentration of sulfuric acid, the fluxes of water and EAc increased, but the separation factor decreased, it could be seen that the water flux of DCM5 was as high as 5605 g/(m2·h) but the separation factor was only 1676. It was indicated that the black solids on the membrane surface did not affect permeation flux. Compared with the SCMs, the thickness of PVA separation layer was thinner and the resistance of the molecules through membranes was smaller. Due to the intermolecular interaction, with water flux increasing, more EAc molecules went through the membrane, which resulted in the decrease of separation factor. Under the same operation condition, the water flux and separation factor of SCM3 were 3137g/(m2 · h) and 7779, respectively. Compared DCMs with SCM3, DCMs had increased flux, decreased separation factor, and decreased PSI.

Fig.21 Pervaporation results of 2 wt.% water-EAc mixture by SCM3 and DCMs at 50 ℃

35

3.4 Pervaporation-esterification coupling Temperature is a key factor in the pervaporation-esterification coupling process [55]. The coupling experiment of the esterification between acetic acid and ethanol using DCM4 were run for 16 hours under 50-70 ℃. Membrane area was 29.43 cm2 (6 pieces) and the mole ratio of ethanol to acetic acid was 4:1. Coupling experiment results were shown in Fig.22, compared with the ordinary catalysis experiment, the equilibrium time was longer, and the acetic acid conversion was obviously enhanced. This is because coupling experiment could catalyze the reaction and remove the byproduct water simultaneously, so the equilibrium could be broken. Besides, with the increase of temperature, the equilibrium time was increased, and the final acetic acid conversion increased from 82% to 96%. From Fig.23, it could be seen that the partial fluxes increased with the increase of operation temperature. In addition, under the same operate temperature, the order of the partial fluxes was Jwater>Jethanol>Jacetic

acid>JEAc≈0.

Table 8 listed some literatures about the coupling

pervaporation-esterification experiments in recent 10 years, the results showed that DCM had excellent coupling performance. To study the reusability, DCM4 was reused for five experiments under the same reaction condition of 70 ℃ for 16 hours, the results were shown in Fig.24, the final acetic acid conversion was essentially unchanged and was remained above 90 %.

36

Fig.22 Effect of reaction temperature on acetic acid conversion Condition: DCM4 29.43cm2 the mole ratio of ethanol to acetic acid 4:1 Table 8 Comparison of coupling performance with other literature Separation layer

Catalyst type

Temperature

Final conversion

(℃)

(%)

Reference

PVA

Maleic acid

85

91.4

[28]

PVA

Perfluorosulfon

60

91.2

[30]

ic acid resin Chitosan

Zr(SO4)2·4H2O

70

94.0

[56]

Sodium alginate/PVA

5-sulfoisophtha

75

96.0

[8]

lic acid α-Al2O3

Zeolite

85

89.0

[57]

NaAlg polymer

Zeolite

70

89.5

[58]

PI

MIL-101(Cr)

70

70.5

[59]

37

PVA

SO42--AAO

70

96.4

This work

Fig.23 Effect of temperature on fluxes of components in the permeat side Condition: DCM4 29.43cm2 the mole ratio of ethanol to acetic acid 4:1

Fig.24 Stability of DCM4 through five runs Condition: DCM4 29.43 cm2, the mole ratio of ethanol to acetic acid 4:1, reaction temperature 70 ℃, 16 h

38

4 Conclusion In this paper, PVA/AAO pervaporation membranes were prepared by coating PVA solution on AAO, compared with other separation membranes in the literature, the membranes prepared in this experiment have higher flux and bigger separation factor. For the pervaporation of water-EAc mixture, the SCM coated with 3wt.% PVA possessed water flux 2726.99 g/(m2 h), separation factor 37945.68 and PSI 1040.3×105 g/(m2 h) . PVA/SO42--AAO pervaporation catalytic membranes were successfully prepared by dip-coating method and the catalytic activities of these catalyst layers were evaluated by the esterification between ethanol and acetic acid. The preparation of the difunctional membrane is an effective way to combine catalyst and pervaporation function together. From the results of XRD, it could be seen that all SO42--AAOs were amorphous and 2.5 SO42--AAO had a little bit of Al2(SO4)3. With the increasing of sulfuric acid concentration, both the amount of the acid sites and the acid concentration first increased and then decreased; for membrane morphology, the pore diameter of SO42--AAO became larger and larger. From the results of XPS, the S element had one characteristic peak of S (+6) which was the highest valence state of the sulfur in solid acid catalyst. For the catalytic activity, 2.0 SO42--AAO reached the highest equilibrium conversion of ethanol, which illustrated that 2.0 SO42--AAO had the best catalytic activity. For DCMs separation activity, water flux and EAc flux increased, but separation factor and PSI decreased. For the pervaporation-esterification coupling 39

experiments, under the operation conditions of the DCM4 area 29.43 cm2, the mole ratio of ethanol to acetic acid 4:1 and the reaction temperature 70 ℃, the final acetic acid conversion could reach above 96%; DCM4 had an excellent stability that the final acetic acid conversion could still reach above 90% after 5 runs.

Acknowledge Authors acknowledge the financial support from National Key R&D Plan under grant 2018YFB0504600, 2018YFB0504603. The Foundation of Jilin Educational Committee (No: JJKH20191318KJ).

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48

Highlights: 1. PVA/AAO membranes were prepared and used in pervaporation. 2. SO42--AAO is catalyst and the support layer of the composite membrane. 3. PVA/SO42--AAO membranes were applied to the catalyze esterification and pervaporation. 4. Acid conversion was enhanced by the coupling experiment. 5. Catalytic membrane had high stability and excellent reusability.

49

Author Statement Manuscript: PVA/SO42-- AAO difunctional catalytic-pervaporation membranes: preparation and characterization Hanshuo Sun: Conceptualization, Methodology, Investigation, Formal analysis. Writing - Original Draft. De Sun: Writing - Review & Editing, Supervision. Xiumin Shi: Writing: Review & Editing, Supervision. Bingbing Li: Writing: Review & Editing, Supervision. Dongmin Yue: Formal analysis, Visualization, Rui Xiao: Validation, Formal analysis, Investigation. Ping Ren: Resources, Validation, Data Curation. Jinhui Zhang: Resources, Investigation.

We have no conflicts of interest to declare.

50