Crosslinking of PVA Pervaporation Membrane by

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Abstract. Maleic acid (MA) crosslinked polyvinyl alcohol (PVA) membrane is prepared using a high temperature esterification reaction between PVA and MA in ...
TSINGHUA SCIENCE AND TECHNOLOGY nSSN 1007-0214 12/24 pp172 - 175 Volume 5, Number 2, June 2000

Crosslinking of PVA Pervaporation Membrane by Maleic Acid *

MENG Pingrui (ifu:+~), CHEN Cuixian (~*-¥{L1J) ,YU Lixin (*"iL~ff), LI Jiding (*~I*5E), JIANG Weijun (~~t$)])

Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Abstract

Maleic acid (MA) crosslinked polyvinyl alcohol

(PVA) membrane is prepared using a high

temperature esterification reaction between PVA and MA in the presence of sulfuric acid as a catalyst. The crosslinking reaction mechanism is investigated using FT -IR spectral analysis. The results indicate that maleic acid reacts with hydroxyl groups in PVA to form mono- and bis-ester in a two-step process.

Key words

pervaporation; polyvinyl alcohol (PVA); maleic acid (MA); crosslinking mechanism; Fourier transform-Infrared (FT -IR) spectrometry

conventional extraction dis tillation process [5J •

Introduction

Priority in pervaporation research has been

There has been much progress in the research

given to the development of polymer membranes

and development of membranes and their uses in

with high selectivity, acceptable flux, and good

pervaporation

stability and/or

processes

varIOUS organic

liquid

for

the

mixtures

separation and

of

for

the

dehydration

durability.

of

For example,

highly-concentrated

the

ethanol

dehydration of organic mixtures. Currently, much

solutions above 95 % (mass fraction)

a ttention is still being paid to pervaporation ,

membrane which preferentially allows the passage

because, as an energy saving separation method, it

of water.

might

used in ethanol-water separation was the GFT

partly

aqueous

replace

alcohol

traditional

solutions

distillation

and

of

azeotropic

membrane,

needs a

The pioneer pervaporation membrane which

was

developed

by

GFT

rnixt uresl v':'. In addition, it has the advantage of

(Germany) at the beginning of the 1980s. It is a

simplifying the process design and avoiding the

composite membrane

pollution of products caused by the substances

(PVA) active layer coated on a polyacrylonitrile

used in distillation processes to break up azeotropic

(PAN) ultrafiltration membrane.

mixtures.

polyvinyl alcohol

Huang and Y eom [6J reported the effect of the

Sander investigated a pilot plant combining pervaporation

with a

and

extraction

dis tillation ,

and

concentration of a cross-linking agent (arnic acid) on the PVA membrane performance. Nobrega et

reported that the operating cost of the hybrid

al.

process was 1/3 to

a

thermal treatment on the PVA membrane performance. Spitzen et al. [8,9J reported on the

Received: 1998-09-14; revised: 1999-06-11 by a Ninth-Five-Year National Project (No. 96-A13-01-06) and the National Natural Science

water permselectivity of PVA/PAN membranes

1/4 less than that of

* Supported

Foundation of China (No. 29231620-02)

* * To

whyjl::9frHndence should be addressed

[7]

investigated

the

effect

of

chemical

and

used in pervaporation processes. All the results of these investigations show that after crosslinking the water permselectivity and durability of a PVA membrane can be remarkably improved.

173

MENG Pingrui (~3f~) et al . Crosslinking oj PYA Pervaporation Membrane by Maleic Acid

This paper describes the preparation of a

(provided with the instrument).

C=O 1

group's

maleic acid (MA) crosslinked PVA membrane

IR spectra (vc=o 1750 - 1700 cm-

through an esterification reaction between PVA

the Calactic Peaksolve curve fitting program (also

and MA in the presence of sulfuric acid as catalyst,

provided with the ins trumen t ),

investigates the reaction mechanism using Fourier

2

transform-infrared correla tes

the

(FT- IR) spectrometry,

crosslinking

structure

with

and the

membrane's pervaporation properties.

1

Results and Discussion

2. 1

Hot water durability of PVA-MA membranes

PVA

homogeneous

Table 1

PVA aqueous solution is first prepared by dissolving PVA in hot water in a flask. Maleic acid and sulfuric acid are added to an 8 % PVA aqueous solution which is stirred to obtain a homogeneous

the

air

bubbles

are

60 p.m. The membrane is then heated in an oven at a specified temperature for a certain period of time for the esterification reaction to take place to form desired

crosslinking

structure.

Cooling

produces the final PVA membranes crosslinked by maleic acid. These PVA membranes are tested for water durability and analyzed using FT-IR analysis to determine the reaction mechanism. Spreading the PVA solution containing maleic acid and sulfuric acid on a PAN microporous support membrane instead of on a glass plate produces a composite membrane with a thin PVA active layer. The PVA thickness is several microns (3 - 4 p.m in general).

This type of composite

membrane was tested in a pervaporation unit to

o (not

crosslinked)

almost unchanged with a slight decrease of transparency

10

almost unchanged with a slight decrease of transparency

The results in Table 1 show that the water durability of crosslinked PVA membranes is much improved. This is mainly due to the crosslinking of PVA. The decrease of hydrophilic hydroxyl groups might also have some effect. 2. 2

Pervaporation experiment

Composite membranes with a PVA active layer crosslinked by maleic acid were tested in a pervaporation unit for the dehydration of ethanol. The concentration of the feed ethanol was 95 % (mass fraction). The operating temperature was 70 e . The experiment data are listed in Table 2. For comparison, the separation factor and the flux of the uncross linked membranes are Q' = 300 - 400 and J (flux) = 300 - 400 g/ (m 2 • h). o

Table 2

theoretical

F1L-][R spectral analysis

crosslinking

FT-IR spectral analysis can be used to obtain the structural information of polymcr s-l'".

PYA

and

were

PVA-MA

homogeneous

membranes

tested using an FT- IR spectroscope (Model Fis 165, Rio-Rad Co. , USA) with an analysis range of 500 - 4000 cm- 1 , a resolving power of 4 cm- 1 and a scanAl~jmf 16. Second derivative spectra were

obtained

completely dissolved serious wrinkles appear

determine the separation factor and flux data. 1. 2

hot water durability at 100°C

theoretical crosslinking

After the water evaporates,

membranes are obtained with a thickness of about

the

In

degree of PYA by MA( %)

removed from the solution, the solution is spread on a glass plate.

shown

Water durability of various PVA-MA

a desired theoretical degree of crosslinking in the After

are

and PV A homogeneous membranes

solution. The amount of maleic acid corresponds to membrane.

membranes

Table 1.

Preparation of PVA-MA membranes

PVA

were fit with

The water durability of various PVA-MA and

Experimental

1. 1

)

usmg

the

OMNIC

program

degree (%)

1 1 1 5 5 5 10 10 10

Pervaporation data of MA crosslinked PV Alp AN composite membranes crosslinking

cros linking

temperature reaction time

caC) 100 120 140 110 120 140 110 120 140

(h)

2 1

o. 5 1.5 1 o. 5 1.5 1 o. 5

flux

separation

J

factor

(g/ (m 2 • h))

135 88 321 61 98 197 55 59 158

a

688 344 116 478 191 306 205 236 49

Tsinghua Science and Technology, June 2000, 5 (2): 172 - 175

174

2. 3

F1L-][R analysis results

The FT-IR spectra of PVA-MA and PYA are shown in Fig. 1. For PVA-MA membranes, the v c=o stretching vibration absorption can be observed at 1750 - 1700 cm- 1, which indicates the existence of C=O groups in the membranes, but the crosslinking structure can not be fully identified from the single peak.

unreacted maleic acid. The absorption at 1 1712.29 cm- is caused by C=O groups in the bis-ester product. The absorption at 1703. 08 cm- 1 is caused by C=O groups in the mono-ester product[1l-13] .

Fig. 2

Absorption of three different types of C=O groups (cross linking conditions: 100°C, 2 h , theoretical crosslinking degree of 10%)

1750

1700

1650

1600

wave number tcm'")

Fig. 1

infrared spectra of PV A membranes crosslinked by MA at different crosslinking degrees (a, PVA; b , 1 % crosslinked; c , 5 %; d , 10%)

PV A and maleic acid can react to form either mono-esters or bis-esters as shown below[ll]: -CH-CHz-CH-CH z-

I

OH D

-~

I

OH

+

CH-COOH

I

CH-COOH

-CH-CHz-CH-CH zI I

o

0

I C=O

I C=O

I

CH

I

CH I COOH

I

CH

I

CH I C=O I

OH 0 I I -CH-CHz-CH-CH zThe C=O groups that cause the absorption at 1750 - 1700 ern -1 can come from either the mono-ester, the bis-ester and the unreacted maleic acid. However, the single peak can be clearly split into three peaks using second derivative spectra analysis (see Fig. 2 ). The a bsorption at 1722. 34 cW--ni~)[~sed by C=O groups in the carboxyl groups of the mono-ester product and the

Table 3 gives some results of the percentage of different types of C=O groups in maleic acid crosslinked PV A membranes at different crosslinking conditions. The percentage distribution shows that: (1) crosslinking reaction takes place to form the desired crosslinking s tructure which leads to good durability in hot water; (2) crosslinking reaction takes place in a twostep sequence with mono-ester as an intermediate product; (3) crosslinking reaction is a slow reaction. Unlike the ordinary reversible esterification reaction in the liquid phase, the esterification here can be regarded as irreversible due to the prompt removal of the water produced. Theoretically, increasing the crosslinking degree will reduce the flux of the PV A membranes because of the increased resistance imposed on the water and ethanol as they pass through the membrane while the separation factor will increase because the ethanol is a larger molecule. But if not all the maleic acid reacts to bis-ester structure, the unreacted maleic acid and the mono-ester structure in the membrane might lead to other tendencies in the variation of the flux and separation factor which complicates the analysis of the pervaporation data in Table 2. Interestification reaction which consumes hydroxyl groups in the same PV A chain might also take place, leading to a more complicated situation In explaining the pervaporation data.

MENG Pingrui (~3f~) et al . Crosslinking oj PYA Pervaporation Membrane by Maleic Acid

influence of reaction conditions on crosslinking

Table 3 crosslinking conditions temperature (OC)

time (h)

theoretical crosslinking degree (%) 1

5. 94

92. 21

1. 85

5

20. 21

75. 78

4.01

2

100

10

16.68

66. 75

16.57

120

10

30. 38

58.05

11. 57

140

10

26. 23

54. 23

19.54

5

[7J

Conclusions

[5J

[6J

[8J

1990,33: 127. (in Japanese) Sander U, Soukup P. Design and operation of a

membranes [J].

J

Membrane

[10J

Membrane Sci, 1990, 51: 259 - 272. Sander U. Experiences in design of a dehydration plant for ethanol-water mixtures [A]. Bakish R, ed. Proceedings of the 1st International Conference on Pervaporation Processes In the Chemical Industry[C]. Atlanta, Bakish Materials, Corp, GA, 1986. 163. Huang R Y M, Yeom C K. Pervaporation separation of aqueous mixtures using cross-linked polyvinyl alcohol (PVA) , IT • Permeation of ethanol-water mixtures[J]' J Membrane Sci, 1990, 51: 273 - 292.

Corp, NJ, 1987. 209. Spitzen J W F, Koops G H, Mulder M H V, et al. The influence of membrane thickness on pervaporation processes [A]. Bakish R, ed. Proceedings of the 3rd International Conference on Pervaporation Processes In the Chemical Industry[C]. Englewood, Bakish Materials Corp, NJ, 1988. 252. Li Quan , Wong Shifu, Wu Jinguang. FT-IR study on the hydration of sulfonate In water / AOT In-heptane microemulsion system[J]. Chinese Journal of Applied Chemistry, 1998, 15 (1) : 1 - 4. (in Chinese)

Sci,

1988, 36: 445 - 462. Boddeker K W. Terminology in pervaporation[JJ. J

Bakish Materials Corp, NJ, 1988. 326. Spitzen J W F, Elsinghorst E, Mulder M H V, et al. Solution-diffusion aspects In the separation of ethanol/water mixtures with PYA membranes[A]. Bakish R, ed. Proceedings of the 2nd International Conference on Pervaporation Processes In the Chemical Industry[C]. Englewood, Bakish Materials

[9J

pervaporation plant for ethanol dehydration[J]' J Membrane Sci, 1988, 36: 463 - 475. Rautenbach R, Herion C, Franke M, et al. Investigation of mass transport In asymmetric

Nobrega R, Harbert A C, Garcia M E F, et al. Separation of ethanol water mixtures by pervaporation through polyvinyl alcohol (PVA) membranes [A]. Bakish R, ed. Proceedings of the 3rd International Conference on Pervaporation Processes in the Chemical Industry[C]. Englewood,

Ohya H, Matsumoto K. Membranes for separation of aqueous alcohol solutions [J]. Sekiyu Gakkaishi,

pervaporation [4J

1703. 08 cm- 1

100

References

[3J

1712.29cm- 1

100

Maleic acid crosslinked homogeneous PVA membranes and PV AlP AN composite membranes were produced. The FT- IR analysis of the homogeneous membranes indicates that the crosslinking reaction mechanism is a two-step esterification sequence. The pervaporation data from the composite membranes were then analyzed using FT- IR analysis but the data could not be clearly interpreted because of the many factors which affect the process.

[2J

1722. 34 cm- 1

2 1

[lJ

(v c=o peak area percentage (%)

2

o.

3

175

[11 J

[12J

[13J

Zhang Yueting, Guan Guihe, Sun Tong. Super-absorbent modified PVA [J]. Synthetic Fiber Industry, 1987, 6: 1 - 6. (in Chinese) Chen Xianhai, Zhang Yifeng, Shen Zhiquan. Synthesis and structural characterization of MAn-PO compolymer prepared by rare earth coordination catalyst[J]. Acta Polymerica Sinica, 1994, 1· 70 - 75. (in Chinese) Shi Yanqiao, Chen Guanwen. The separation of water and alcohol by pervaporation with PVA/PAN[J]. Acta Polymerica Sinica, 1996, 2: 211 - 212. (in Chinese)