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
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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
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100
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