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Pervaporation separation of water—ethanol through modified chitosan membranes. IV. Phosphorylated chitosan membranes
Journal of Membrane Sczence, 64 (1991) 145-152 Elsevrer Scrence Pubhshers B V , Amsterdam
145
Pervaporation separation of water-ethanol through modified chitosan membranes. IV. Phosphorylated chitosan membranes Young Moo Lee* and Eun Mi Shin Department of Industrtal Chemutry, College of Enganeermng,Hanyang UnwersLty, Seoul 133-791 (S Korea)
(Becewed December 13,1990, accepted m revised form June 5,1991)
Abstract The present study investigates the pervaporatron performance of novel phosphorylated chltosan membranes to separate water from aqueous ethanol solution Phosphorylated chltosan membranes were prepared from the reaction of orthophosphonc acid and urea on the surface of chrtosan membrane in N,Ndlmethylformamrde The phosphorus contents m the membrane varied from l-80 mg/m’ dependmg upon the reaction penod Chemical modrficatron, m this case phosphorylatron, contributed to an rmproved pervaporatron performance of chrtosan membranes Among the phosphorylated chrtosan membranes, PCS-30 containmg 56 mg/m2 of phosphorus showed the best pervaporatlon performance the flux was ca 0 2 kg/m2-hr and the selectrvlty toward water was ca 600 measured with 90 wt % ethanol at 70°C In comparrson with the previously reported sulfonated and carboxymethylated chltosan membranes, permeate flux through the present phosphorylated membrane showed a four-fold increase wrthout the loss of selectrvlty towards water Keywords
pervaporatron, separation of water-ethanol mixtures, chrtosan membranes, phosphorylated
Introduction We have previously reported on the pervaporation performance of modified chitosan membranes to separate water from aqueous ethanol mixtures. The membranes we evaluated were a chitosan-acetic acid complex membrane [ 11, carboxymethyl (CMCS ) , carboxyethyl (CECS), cyanoethyl (CNCS) and amidoxlme ( AMCS ) chitosan membranes [ 21, and sulfonyl chitosan membranes (SCS) [3] A basic assumption behind conducting these studies was that good selectivities for water *To whom all correspondence should be addressed
could be obtained using ionic or hydrophilic groups or by the mclusion of these groups m the base membrane structure. Among the chemically modified chitosan membranes, membranes containing carboxyl or sulfonyl groups show a high separation efficiency Several other researchers have reported on the pervaporation performance of chitosan membranes [471 Katsuura and Inagaki [8] reported the formation of phosphorylated cellulose by reacting the cellulose with orthophosphoric acid and urea. They applied this study to improve the flameproofing properties of cotton fabrics [ 9 I Ueda et al. [lo] presented the evidence and
0376-7368/91/$03 50 0 1991 Elsevler Science Publishers B V All nghts reserved
146
proposed the mechamsm for the formation of ammonium polyphosphate when orthophosphoric acid was reacted with urea. The contents of nitrogen depended upon the pH and temperature in the reaction mixture. The most probable mode of reaction seems to involve phosphorylation of the cellulose at the primary hydroxyls. Sakaguchi et al. [ 111 reported the improved adsorption of uranium by chitin and chitosan phosphate based on the assumption that the phosphorylation should occur mainly on the primary hydroxyl group in chitosan. More recently, Saito et al. studied the phosphorylated polyethylene hollow fibers synthesized by radiation grafting and crosslinking reaction [ 121. These fibers exhibited improved permeation flux and chemical stability after crosslinkmg However, the separation of water-ethanol through phosphorylated chitosan (PCS) membrane has not yet been reported. In general, selective separation of solute requires strong adsorptions through membranes caused by a strong interaction such as hydrogen bonding. It is expected that the phosphoryl group might have a strong interaction with water and thus a membrane containing a phosphoryl group might allow selective permeation of water. The present study investigates the pervaporation performance of phosphorylated chitosan membranes. Experimental Materzals Chitm was obtained from crab shell by a modification of Hackman’s method [ 131.The chitin was subsequently deacetylated with NaOH solution to obtain chitosan. A detailed manufacturing method for chitin and chitosan is described elsewhere [ 11. Acetic acid, sodn.rm hydroxide, methanol, ethanol, urea, and dimethylformamide (DMF) were obtained from Duksan Pharmaceutical Co. Phosphoric acid
YOUNG MOO LEE AND EUN MI SHIN
(100% ) was from Aldrich Chemical Company Inc.
For structural determination, an infrared spectrophotometer (FTIR Nicolet Model 5DX), a calorimeter (Bausch & Lomb Spectronic 21) and a differential scanning calorimeter (DSC, DuPont Model 912) were used. The phosphorus contents of chitosan membranes were measured usmg the molybdenum blue method [ 141. Preparatwn of phosphoryluted chrtosan membrane Orthophosphoric acid (2 g of 100%) and 100 g of urea were dissolved in 200 ml of DMF and added to the chitosan membrane in a reactor in which one side of the membrane surface was allowed to react. In this case, urea was added to the reaction media to act as a reaction promoter [ 8-101. The reaction was conducted at 70°C for 10 min-3 hr. The membranes were designated as PCS-lo, PCS-20, PCS-30, PCS40, PCS-60 and PCS-180, respectively, according to the reaction time in minutes. After the reaction, each membrane was washed with water and dried at room temperature. Pervaporatron experiment A detailed procedure for performing the pervaporation experiment can be found elsewhere [l&16]. Results and discussion Effect of phosphorylatzon Infrared spectra of chitosan and PCS membranes are shown in Fig. 1, where the stretching vibration of P-O appears at 1251 cm-l and increases markedly on phosphorylation. We have not observed any significant changes in X-ray scattermg patterns of chitosan phosphates depending upon the reaction time.
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
147
san membrane. Figure 3 shows the DSC thermograms of the PCS membranes. Table 1 shows the effect of phosphorylation on the phosphorus contents in chitosan and the chain relaxation energy as calculated from Figs. 2 and 3. The phosphorus content in the membrane increased from 40 mg/cm2 to 80 mg/cm2 as the membrane was reacted for periods from 10 min to 3 hr. The phosphorylation occurs rapidly in the first 30 min and obviously seems to slow down after that. In contrast, the cham
1350
1170
e F 180
Wavenum ber in cd
Fig 1 Infrared spectra for (a) chltosan and (b) phosphorylated chltosan membrane
40
20
10
50
150
250
TemperatureK
350
1
Fig 3 DSC thermogram for PCS membranes Numbers on the figure mdlcate the duration of reaction (mm) TABLE I 50
I
I
I
150
250
350
TemperatureP’C 1
Fig 2 DSC thermogram for (a) chltosan film and (b) quenched chltosan film
Figure 2 shows a DSC thermogram of the chitosan membrane, where a large endothermic peak appeared at 140” C. When this chltosan sample was heated to 250’ C and then rapidly quenched with liquid nitogen, and the DSC analysis was rerun up to 400 oC with a heating rate of 40”C/min, the endothermic peak disappeared. The endothermic peak is believed to be due to a chain relaxation in the base chito-
The relatlonshlp between phosphorus content and volume relaxation energy of polymer membrane Sample
Chltosan PCS-10 PCS-20 PCS-30 PCS-40 PCS-60 PCS-180
Reaction tune (mm)
Phosphorus content hdm2)
AH,”
0 10 20 30 40 60 100
00 39 42 55 62 67 79
3516 299 0 2014 906 193 0 262 6 264 9
00 18 53 93 59 05 33
(J/g)
“AH, = volume relaxation energy of polymer membrane samples measured from the DSC thermogram appearmg at around 120-150°C
YOUNG MOO LEE AND EUN MI SHIN
148
relaxation energy of chitosan was reduced for the membrane subjected to up to 30 min of phosphorylation and then increased for the samples treated further. Phosphate formation from the reaction of orthophosphoric acid and cellulose in the presence of urea is well described in the literature [ 8-101. The possible mechanism for formation of the phosphate may involve mainly the pnmary hydroxyl groups in cellulose because they are more reactive than the secondary hydroxyls Smce chrtosan has a similar chemical structure to cellulose and possesses primary hydroxyls in the C-6 positron of the glucosamine umts, the possible reaction mechamsm of chitosan phosphate in the presence of urea is proposed and illustrated in Fig. 4. That is, at the beginning of the reaction, the phosphate groups in the membrane may be formed at the C-6 position of the glucosamine units [see the product (4) in Fig. 41. As the
13
H$4--NH2
0 #
Hi&~
Hi
--_)
HN=C=O
HN=C=O
+
(1)
+
NH.j
i? HO-P-OH
reaction is prolonged to 3 hr, the phosphorus content increases logarithmically, meaning that in the first 30 min the phophorylation occurs very actively on the primary hydroxyls in glucosamine units and then slows down, due either to the consumptron of reactive hydroxyls or to the formation of pyrophosphates, triphosphates and finally polyphosphates [product (5) in Fig. 41. The polyphosphate may be formed, not only from mter- and mtramolecular condensation of phosphates m chltosan polymer chains, but also from condensation of chitosan phosphate and mcommg orthophosphoric acid from the reaction media. The former can be regarded as a crosslinking reaction. When the reaction time is prolonged, the latter reaction mode becomes more probable. This kind of reactron has been well documented for cellulose [&lo]. Chitosan reacting with phosphoric acid m the presence of urea has been also applied for the adsorption of uranmm [ 111. Table 2 shows the pervaporation performance of PCS membranes tested with 90 wt.% ethanol feed at 70’ C Compared with the chitosan membrane, the permeatron rate generally increased for PCS membranes except for PCS-160 Meanwhile, the separatron factor was enhanced for PCS-30 membranes having up to
)
TABLE 2 Effect of phosphorus content on pervaporatlon separation ablllty of phosphorylated chltosan membrane” Membrane
Total flux,
Water flux,
Ethanol flux,
8, In
Qw m
Q., m
(kg/m2-hr)
Separatlon factor, cx
0 1270 0 2143 0 1268 0 1822 0 1626 0 1996 0 0731
0 1140 0 1981 0 1251 0 1796 0 1960 0 1867 00631
0 013000 0 016202 0 001696 0 002600 0 025346 0 012903 0 010035
82 40 106 00 410 29 54108 77 31 89 94 53 76
(kg/m*-hr)
Fig 4 Proposed mechanism for the preparation of phosphorylated chltosan membrane (4 and 5) from the reaction of chltosan (3) wth orthophosphorlc acid (2) and urea (1)
Chltosan PCS-10 PCS-20 PCS-30 PCS-40 PCS-60 PCS-180
(kg/m*-hr)
“Membrane thickness z 20 m, feed ethanol concentration = 90 wt operatmg temperature = 70 ’ C %
,
149
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
56 mg/m2 of phosphorus content and then shifted to a decrease. An increase in flux after phosphorylation is believed to be due to the formation of phosphate and polyphosphate, which contributed to an enhanced capability of hydrogen bonding between chitosan phosphate and water. The fact that the selectivity was the highest for PCS-30 can be explained by comparison with the relaxation energy listed in Table 1. There is an inverse relationship between the cham relaxation energy and the pervaporation selectivity of PCS membranes. This result suggests a formation of phosphates and ammonium polyphosphates during the phosphorylation reaction. As the chitosan phosphates are formed m the C-6 position in glucosamine units, the chain relaxation is markedly reduced because of the bulky side chain and crosslinking If the reaction proceeds further, ammomum polyphosphates are formed from the condensation of chitosan phosphate and orthophosphoric acids from the reaction mixture, resultmg in a less packed structure in the chitosan polymer chains. Thus the membrane becomes more mobile and relaxed and the relaxation energy goes up. The ethanol could pass through the free volumes formed due to the motion of mobile chains, resultmg in a drop in separation factor. Effect of feed ethanol concentratton Figure 5 shows the effect of the feed ethanol concentration on the pervaporation separation of PCS-30 membrane tested at 70°C. It is a general tendency that the selectivity increases and the flux decreases as the feed ethanol concentration increases. Note that the permeate ethanol concentration was only 2-3 wt.% over the whole feed ethanol concentration range. The permeabilities of ethanol and water through a PCS-30 membrane are shown in Figs, 6 and 7, respectively, as a function of the feed ethanol and water concentrations. Figure 6
Ehanoi cone In the feed In wt%
Fig 5 Effect of ethanol concentration m the feed on total flux ( 0 ), water flux ( 0 ), ethanol flux ( n ) , and separaken factor (A ) through PCS-30 membrane Operatmg temperature 70 oC, membrane thickness 20 pm Ooz5Y
Feed ethanolcone (ml /dm3
)
Fig 6 Effect of ethanol concentration m the feed on ethanol flux through PCS-30 membrane Operating temperature 70 ’ C, membrane thickness 20 pm
Feed water cone (mol/6m3)
Fig 7 Effect of water concentration m the feed on water flux through PCS-30 membrane Operatmg temperature 7O”C, membrane thickness 7Opm
150
YOUNG MOO LEE AND EUN MI SHIN
TABLE 3 Pervaporatlon separation of modlfied chltosan membranes’ Sample
Total flux, Q, m (g/m’-hr)
Water flux, Q,, m (g/m’-hr)
Ethanol flux, Q., m (g/m’-hr)
Separation factor, a
PSI
CS complexb CMCS SCS” CNCS CECSd PCS”
148 8 36 7 519 89 2 30 3 182 2
142 4 36 4 517 79 9 29 5 179 6
6 33 0 28 0 29 9 26 0 79 2 60
242 1294 1560 72 301 541
360 47 5 810 65 91 98 6
Qff
“Feed ethanol concentration = 90 wt %, operating temperature = 25°C bChltosan-acetm acid complex membrane treated with alkah for 3 hr ‘Crosslmked with 0 025 wt % glutaraldehyde-70 wt % methanol aqueous solution for 15 mm dReacted with 1N NaOH-methanol solution for 5 hr ‘Phosphorus content = 56 mg/m’, operatmg temperature = 70” C
shows a shape similar to that of the ethanol permeability plot of a CMCS membrane [2]. This means that both the CMCS membrane and the present PCS-30 membrane have their maximum ethanol flux at 5-7 mol/dm3 or lo-20 wt.% feed ethanol concentration. This behavior is attributed mainly to the coupling or plasticizing effect, as observed previously for a poly (1-methyl-4vinylpyridinium iodide-coacrylonitrile) membrane [ 171 and for a poly (acrylomtrile-co-acrylic acid) membrane [ 181. The water flux of the PCS-30 membrane, however, tends to show an exponential profile with water concentration. It also showed the same tendency as for the CMCS membrane. Comparwon between modtfuzd chltosan membranes Pervaporatlon performances of the modified chitosan membranes studied m the present and m previous work are listed in Table 3. The pervaporation separation index (PSI ) , that is, the flux multiplied by the selectivity, was suggested by Huang and Rhim [ 191, and represented a good indication of the pervaporation performance. Among these membranes, PCS and SCS membranes show high PSI values. The flux of
0 Ethanol cone mfeed (wt%)
Fig 8 Pervaporatlon separation curve of modtied chltosan membranes, chltosan-acetic acid complex membrane ( 0 ) , carboxymethylchltosan membrane ( A ) , cyanoethylchltosan membrane ( n ) , and phosphorylated chltosan membrane ( 0 )
the CMCS membrane was lower than that of the PCS and SCS membranes, m spite of its high selectivity. This material should have a high PSI value, if we could reduce the membrane thickness. In the case of CECS and chitosan-acetic acid complex membranes, they show similar values to the CMCS membrane: that is, a high separation factor and a low flux. In Fig. 8, the permeate ethanol concentration measured for various feed ethanol concentrations through the chemically modified chl-
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
tosan membranes are compared. All the modified chitosan membranes show a low permeate ethanol concentration over the whole range of ethanol concentratron. In particular, the present PCS membrane shows the lowest permeate ethanol concentration
3
Conclusions
5
Phosphorylated chitosan (PCS) membranes were manufactured from the reaction of phosphoric acid on the surface of a chltosan membrane Phosphorus contents in the membrane ranged from l-80 mg/m2 depending upon the reaction period. A chemical modification (phosphorylation) conferred an improved pervaporatlon performance on chitosan membranes m general. Among the PCS membranes, PCS-30 contammg 56 mg/m2 of phosphorus showed the best pervaporatlon performance. The flux was 0.2 kg/m2-hr and the selectivity was ca 600. In comparison with the SCS and CMCS membranes, the permeate flux through PCS membranes is increased about four-fold without the significant loss of selectivity for water. Acknowledgement
4
6
7
8
9
10
11
This work was supported by the 1990 Korean Ministry of Education Research Fund for Advanced Materials.
12
References
13
Y M Lee and E M Shin, Pervaporatlon separation of water-ethanol through modlfied chltosan membrane I Chltosan-acetic acid and-metal complex membranes, Polhmo, 15 (2) (1991) 1 Y M Lee, E M Shm and S T Noh, Pervaporatlon separation of water-ethanol through modified chltosan membrane II Carboxymethyl, carboxyethyl, cyanoethyl, and amldoxune chltosan membranes, Angew Makromol Chem , m press
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15
16
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Y M Lee, E M Shm and C N Chung, Pervaporatlon separation of water-ethanol through modified chltosan membrane III Sulfonated chltosan membranes, Polhmo, 15(4) (1991) 497-500 A Mochlzukl, Y Sate, H Ogawara and S Yamashlta, Pervaporatlon separation of water/ethanol mixtures through polysacchande membranes II Permselectmlty of chltosan membrane, J Appl Polym Scl , 37 (1989) 3375 T Uragaml and T Monkawa, Studies on synthesis and permeablhkes of special polymer membranes Comparison of permeation and separation characterlstlcs for aqueous alcohol solution by pervaporatlon and new evapomeatlon method through chltosan membranes, Makromol Chem , Rapid Commun , 9 (1988) 361-365 M Maya, R Iwamoto, S Mama, S Yamashlta, A Mochlzukl and Y Tanaka, Chltosan membrane for separation of water-ethanol by pervaporatlon, Kobunshl Ronbunshu, 42 (2) (1985) 139-142 H Ishlda, M Watanabe and S Kamagaml, Effect of treatments of chltosan membrane on the separation of ethanol-water mixture by pervaporatlon, KobunshlRonbunshu,43(7) (1986)449-452 K Katsuura and N Inagakl, Formation of phosphorylated cellulose with urea and phosphonc acid, J Chem Sot Jpn , Ind Eng Chem Sect ,69(4) (1966) 681 K Katsuura and H Mlzuno, Flameproofmg of cotton fabrics with urea and phosphonc acid m organic solvent, Sen-I Gakkalshl, 22 (11) (1966) 510 S Ueda, K Oyama and K Koma, Nitrogen contammg phosphoric acid salt from condensation reactlon I Formation of ammonmm polyphosphate, J Chem Sot Jpn , Ind Eng Chem Sect, 66(5) (1963) 586 T Sakaguchl, T Horlkoshl and A Nakajima, Adsorption of uranium by chltm phosphate and chltosan phosphate, Agnc Blol Chem ,45 (10) (1981) 21912195 K Salto, T Kaga, H Yamwshl and S Furusakl, Phosphorylated hollow fibers synthesis by radlatlon grafting and cross-hnkmg, J Membrane Scl , 43 (1989) 131 R H Hackman, Enzymatic degradation of chltm esters, Aust J Blol Scl ,I (1954) 168-178 H H Furman, Standard Methods of Chemical Analysis, Vol 1, 6th edn , Van Nostrand, New York, NY, 1975, pp 818-819 Y M Lee, D Bourgeois and G Belfort, Sorption, dlffusion, and pervaporatlon of orgamcs m polymer membranes, J Membrane Scl ,44(2) (1989) 161-181 Y M Lee and K Won, Pervaporatlon separation of water-ethanol through modlfled polyacrylontltnle membranes, Polym J ,22 (7) (1990) 578-586
YOUNG MOO LEE AND EUN MI SHIN
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M Yoshlkawa, T Ohsawa, M Tamgakl, W Eguchi and N Ogata, Separation of aqueous ethanol mixtures through poly (cyclohexyl methacrylate-co-styrene) membranes, Sen-I Gakkalshi, 44( 11) (1988) 551 Y M Lee and W J Wang, Pervaporatlon of water-ehtan01 mixture through poly(acrylomtnle-co-acrylic
19
acid) membrane, Makromol Chem ,191 (1990) 31313138 R Y M Huang and J W Rhlm, Separation characterLstlcs of pervaporatlon membrane separation proceases, J Membrane Scl ,46 (1989) 335
145
Pervaporation separation of water-ethanol through modified chitosan membranes. IV. Phosphorylated chitosan membranes Young Moo Lee* and Eun Mi Shin Department of Industrtal Chemutry, College of Enganeermng,Hanyang UnwersLty, Seoul 133-791 (S Korea)
(Becewed December 13,1990, accepted m revised form June 5,1991)
Abstract The present study investigates the pervaporatron performance of novel phosphorylated chltosan membranes to separate water from aqueous ethanol solution Phosphorylated chltosan membranes were prepared from the reaction of orthophosphonc acid and urea on the surface of chrtosan membrane in N,Ndlmethylformamrde The phosphorus contents m the membrane varied from l-80 mg/m’ dependmg upon the reaction penod Chemical modrficatron, m this case phosphorylatron, contributed to an rmproved pervaporatron performance of chrtosan membranes Among the phosphorylated chrtosan membranes, PCS-30 containmg 56 mg/m2 of phosphorus showed the best pervaporatlon performance the flux was ca 0 2 kg/m2-hr and the selectrvlty toward water was ca 600 measured with 90 wt % ethanol at 70°C In comparrson with the previously reported sulfonated and carboxymethylated chltosan membranes, permeate flux through the present phosphorylated membrane showed a four-fold increase wrthout the loss of selectrvlty towards water Keywords
pervaporatron, separation of water-ethanol mixtures, chrtosan membranes, phosphorylated
Introduction We have previously reported on the pervaporation performance of modified chitosan membranes to separate water from aqueous ethanol mixtures. The membranes we evaluated were a chitosan-acetic acid complex membrane [ 11, carboxymethyl (CMCS ) , carboxyethyl (CECS), cyanoethyl (CNCS) and amidoxlme ( AMCS ) chitosan membranes [ 21, and sulfonyl chitosan membranes (SCS) [3] A basic assumption behind conducting these studies was that good selectivities for water *To whom all correspondence should be addressed
could be obtained using ionic or hydrophilic groups or by the mclusion of these groups m the base membrane structure. Among the chemically modified chitosan membranes, membranes containing carboxyl or sulfonyl groups show a high separation efficiency Several other researchers have reported on the pervaporation performance of chitosan membranes [471 Katsuura and Inagaki [8] reported the formation of phosphorylated cellulose by reacting the cellulose with orthophosphoric acid and urea. They applied this study to improve the flameproofing properties of cotton fabrics [ 9 I Ueda et al. [lo] presented the evidence and
0376-7368/91/$03 50 0 1991 Elsevler Science Publishers B V All nghts reserved
146
proposed the mechamsm for the formation of ammonium polyphosphate when orthophosphoric acid was reacted with urea. The contents of nitrogen depended upon the pH and temperature in the reaction mixture. The most probable mode of reaction seems to involve phosphorylation of the cellulose at the primary hydroxyls. Sakaguchi et al. [ 111 reported the improved adsorption of uranium by chitin and chitosan phosphate based on the assumption that the phosphorylation should occur mainly on the primary hydroxyl group in chitosan. More recently, Saito et al. studied the phosphorylated polyethylene hollow fibers synthesized by radiation grafting and crosslinking reaction [ 121. These fibers exhibited improved permeation flux and chemical stability after crosslinkmg However, the separation of water-ethanol through phosphorylated chitosan (PCS) membrane has not yet been reported. In general, selective separation of solute requires strong adsorptions through membranes caused by a strong interaction such as hydrogen bonding. It is expected that the phosphoryl group might have a strong interaction with water and thus a membrane containing a phosphoryl group might allow selective permeation of water. The present study investigates the pervaporation performance of phosphorylated chitosan membranes. Experimental Materzals Chitm was obtained from crab shell by a modification of Hackman’s method [ 131.The chitin was subsequently deacetylated with NaOH solution to obtain chitosan. A detailed manufacturing method for chitin and chitosan is described elsewhere [ 11. Acetic acid, sodn.rm hydroxide, methanol, ethanol, urea, and dimethylformamide (DMF) were obtained from Duksan Pharmaceutical Co. Phosphoric acid
YOUNG MOO LEE AND EUN MI SHIN
(100% ) was from Aldrich Chemical Company Inc.
For structural determination, an infrared spectrophotometer (FTIR Nicolet Model 5DX), a calorimeter (Bausch & Lomb Spectronic 21) and a differential scanning calorimeter (DSC, DuPont Model 912) were used. The phosphorus contents of chitosan membranes were measured usmg the molybdenum blue method [ 141. Preparatwn of phosphoryluted chrtosan membrane Orthophosphoric acid (2 g of 100%) and 100 g of urea were dissolved in 200 ml of DMF and added to the chitosan membrane in a reactor in which one side of the membrane surface was allowed to react. In this case, urea was added to the reaction media to act as a reaction promoter [ 8-101. The reaction was conducted at 70°C for 10 min-3 hr. The membranes were designated as PCS-lo, PCS-20, PCS-30, PCS40, PCS-60 and PCS-180, respectively, according to the reaction time in minutes. After the reaction, each membrane was washed with water and dried at room temperature. Pervaporatron experiment A detailed procedure for performing the pervaporation experiment can be found elsewhere [l&16]. Results and discussion Effect of phosphorylatzon Infrared spectra of chitosan and PCS membranes are shown in Fig. 1, where the stretching vibration of P-O appears at 1251 cm-l and increases markedly on phosphorylation. We have not observed any significant changes in X-ray scattermg patterns of chitosan phosphates depending upon the reaction time.
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
147
san membrane. Figure 3 shows the DSC thermograms of the PCS membranes. Table 1 shows the effect of phosphorylation on the phosphorus contents in chitosan and the chain relaxation energy as calculated from Figs. 2 and 3. The phosphorus content in the membrane increased from 40 mg/cm2 to 80 mg/cm2 as the membrane was reacted for periods from 10 min to 3 hr. The phosphorylation occurs rapidly in the first 30 min and obviously seems to slow down after that. In contrast, the cham
1350
1170
e F 180
Wavenum ber in cd
Fig 1 Infrared spectra for (a) chltosan and (b) phosphorylated chltosan membrane
40
20
10
50
150
250
TemperatureK
350
1
Fig 3 DSC thermogram for PCS membranes Numbers on the figure mdlcate the duration of reaction (mm) TABLE I 50
I
I
I
150
250
350
TemperatureP’C 1
Fig 2 DSC thermogram for (a) chltosan film and (b) quenched chltosan film
Figure 2 shows a DSC thermogram of the chitosan membrane, where a large endothermic peak appeared at 140” C. When this chltosan sample was heated to 250’ C and then rapidly quenched with liquid nitogen, and the DSC analysis was rerun up to 400 oC with a heating rate of 40”C/min, the endothermic peak disappeared. The endothermic peak is believed to be due to a chain relaxation in the base chito-
The relatlonshlp between phosphorus content and volume relaxation energy of polymer membrane Sample
Chltosan PCS-10 PCS-20 PCS-30 PCS-40 PCS-60 PCS-180
Reaction tune (mm)
Phosphorus content hdm2)
AH,”
0 10 20 30 40 60 100
00 39 42 55 62 67 79
3516 299 0 2014 906 193 0 262 6 264 9
00 18 53 93 59 05 33
(J/g)
“AH, = volume relaxation energy of polymer membrane samples measured from the DSC thermogram appearmg at around 120-150°C
YOUNG MOO LEE AND EUN MI SHIN
148
relaxation energy of chitosan was reduced for the membrane subjected to up to 30 min of phosphorylation and then increased for the samples treated further. Phosphate formation from the reaction of orthophosphoric acid and cellulose in the presence of urea is well described in the literature [ 8-101. The possible mechanism for formation of the phosphate may involve mainly the pnmary hydroxyl groups in cellulose because they are more reactive than the secondary hydroxyls Smce chrtosan has a similar chemical structure to cellulose and possesses primary hydroxyls in the C-6 positron of the glucosamine umts, the possible reaction mechamsm of chitosan phosphate in the presence of urea is proposed and illustrated in Fig. 4. That is, at the beginning of the reaction, the phosphate groups in the membrane may be formed at the C-6 position of the glucosamine units [see the product (4) in Fig. 41. As the
13
H$4--NH2
0 #
Hi&~
Hi
--_)
HN=C=O
HN=C=O
+
(1)
+
NH.j
i? HO-P-OH
reaction is prolonged to 3 hr, the phosphorus content increases logarithmically, meaning that in the first 30 min the phophorylation occurs very actively on the primary hydroxyls in glucosamine units and then slows down, due either to the consumptron of reactive hydroxyls or to the formation of pyrophosphates, triphosphates and finally polyphosphates [product (5) in Fig. 41. The polyphosphate may be formed, not only from mter- and mtramolecular condensation of phosphates m chltosan polymer chains, but also from condensation of chitosan phosphate and mcommg orthophosphoric acid from the reaction media. The former can be regarded as a crosslinking reaction. When the reaction time is prolonged, the latter reaction mode becomes more probable. This kind of reactron has been well documented for cellulose [&lo]. Chitosan reacting with phosphoric acid m the presence of urea has been also applied for the adsorption of uranmm [ 111. Table 2 shows the pervaporation performance of PCS membranes tested with 90 wt.% ethanol feed at 70’ C Compared with the chitosan membrane, the permeatron rate generally increased for PCS membranes except for PCS-160 Meanwhile, the separatron factor was enhanced for PCS-30 membranes having up to
)
TABLE 2 Effect of phosphorus content on pervaporatlon separation ablllty of phosphorylated chltosan membrane” Membrane
Total flux,
Water flux,
Ethanol flux,
8, In
Qw m
Q., m
(kg/m2-hr)
Separatlon factor, cx
0 1270 0 2143 0 1268 0 1822 0 1626 0 1996 0 0731
0 1140 0 1981 0 1251 0 1796 0 1960 0 1867 00631
0 013000 0 016202 0 001696 0 002600 0 025346 0 012903 0 010035
82 40 106 00 410 29 54108 77 31 89 94 53 76
(kg/m*-hr)
Fig 4 Proposed mechanism for the preparation of phosphorylated chltosan membrane (4 and 5) from the reaction of chltosan (3) wth orthophosphorlc acid (2) and urea (1)
Chltosan PCS-10 PCS-20 PCS-30 PCS-40 PCS-60 PCS-180
(kg/m*-hr)
“Membrane thickness z 20 m, feed ethanol concentration = 90 wt operatmg temperature = 70 ’ C %
,
149
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
56 mg/m2 of phosphorus content and then shifted to a decrease. An increase in flux after phosphorylation is believed to be due to the formation of phosphate and polyphosphate, which contributed to an enhanced capability of hydrogen bonding between chitosan phosphate and water. The fact that the selectivity was the highest for PCS-30 can be explained by comparison with the relaxation energy listed in Table 1. There is an inverse relationship between the cham relaxation energy and the pervaporation selectivity of PCS membranes. This result suggests a formation of phosphates and ammonium polyphosphates during the phosphorylation reaction. As the chitosan phosphates are formed m the C-6 position in glucosamine units, the chain relaxation is markedly reduced because of the bulky side chain and crosslinking If the reaction proceeds further, ammomum polyphosphates are formed from the condensation of chitosan phosphate and orthophosphoric acids from the reaction mixture, resultmg in a less packed structure in the chitosan polymer chains. Thus the membrane becomes more mobile and relaxed and the relaxation energy goes up. The ethanol could pass through the free volumes formed due to the motion of mobile chains, resultmg in a drop in separation factor. Effect of feed ethanol concentratton Figure 5 shows the effect of the feed ethanol concentration on the pervaporation separation of PCS-30 membrane tested at 70°C. It is a general tendency that the selectivity increases and the flux decreases as the feed ethanol concentration increases. Note that the permeate ethanol concentration was only 2-3 wt.% over the whole feed ethanol concentration range. The permeabilities of ethanol and water through a PCS-30 membrane are shown in Figs, 6 and 7, respectively, as a function of the feed ethanol and water concentrations. Figure 6
Ehanoi cone In the feed In wt%
Fig 5 Effect of ethanol concentration m the feed on total flux ( 0 ), water flux ( 0 ), ethanol flux ( n ) , and separaken factor (A ) through PCS-30 membrane Operatmg temperature 70 oC, membrane thickness 20 pm Ooz5Y
Feed ethanolcone (ml /dm3
)
Fig 6 Effect of ethanol concentration m the feed on ethanol flux through PCS-30 membrane Operating temperature 70 ’ C, membrane thickness 20 pm
Feed water cone (mol/6m3)
Fig 7 Effect of water concentration m the feed on water flux through PCS-30 membrane Operatmg temperature 7O”C, membrane thickness 7Opm
150
YOUNG MOO LEE AND EUN MI SHIN
TABLE 3 Pervaporatlon separation of modlfied chltosan membranes’ Sample
Total flux, Q, m (g/m’-hr)
Water flux, Q,, m (g/m’-hr)
Ethanol flux, Q., m (g/m’-hr)
Separation factor, a
PSI
CS complexb CMCS SCS” CNCS CECSd PCS”
148 8 36 7 519 89 2 30 3 182 2
142 4 36 4 517 79 9 29 5 179 6
6 33 0 28 0 29 9 26 0 79 2 60
242 1294 1560 72 301 541
360 47 5 810 65 91 98 6
Qff
“Feed ethanol concentration = 90 wt %, operating temperature = 25°C bChltosan-acetm acid complex membrane treated with alkah for 3 hr ‘Crosslmked with 0 025 wt % glutaraldehyde-70 wt % methanol aqueous solution for 15 mm dReacted with 1N NaOH-methanol solution for 5 hr ‘Phosphorus content = 56 mg/m’, operatmg temperature = 70” C
shows a shape similar to that of the ethanol permeability plot of a CMCS membrane [2]. This means that both the CMCS membrane and the present PCS-30 membrane have their maximum ethanol flux at 5-7 mol/dm3 or lo-20 wt.% feed ethanol concentration. This behavior is attributed mainly to the coupling or plasticizing effect, as observed previously for a poly (1-methyl-4vinylpyridinium iodide-coacrylonitrile) membrane [ 171 and for a poly (acrylomtrile-co-acrylic acid) membrane [ 181. The water flux of the PCS-30 membrane, however, tends to show an exponential profile with water concentration. It also showed the same tendency as for the CMCS membrane. Comparwon between modtfuzd chltosan membranes Pervaporatlon performances of the modified chitosan membranes studied m the present and m previous work are listed in Table 3. The pervaporation separation index (PSI ) , that is, the flux multiplied by the selectivity, was suggested by Huang and Rhim [ 191, and represented a good indication of the pervaporation performance. Among these membranes, PCS and SCS membranes show high PSI values. The flux of
0 Ethanol cone mfeed (wt%)
Fig 8 Pervaporatlon separation curve of modtied chltosan membranes, chltosan-acetic acid complex membrane ( 0 ) , carboxymethylchltosan membrane ( A ) , cyanoethylchltosan membrane ( n ) , and phosphorylated chltosan membrane ( 0 )
the CMCS membrane was lower than that of the PCS and SCS membranes, m spite of its high selectivity. This material should have a high PSI value, if we could reduce the membrane thickness. In the case of CECS and chitosan-acetic acid complex membranes, they show similar values to the CMCS membrane: that is, a high separation factor and a low flux. In Fig. 8, the permeate ethanol concentration measured for various feed ethanol concentrations through the chemically modified chl-
PERVAPORATION SEPARATION OF WATER-ETHANOL THROUGH MODIFIED CHITOSAN MEMBRANES
tosan membranes are compared. All the modified chitosan membranes show a low permeate ethanol concentration over the whole range of ethanol concentratron. In particular, the present PCS membrane shows the lowest permeate ethanol concentration
3
Conclusions
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Phosphorylated chitosan (PCS) membranes were manufactured from the reaction of phosphoric acid on the surface of a chltosan membrane Phosphorus contents in the membrane ranged from l-80 mg/m2 depending upon the reaction period. A chemical modification (phosphorylation) conferred an improved pervaporatlon performance on chitosan membranes m general. Among the PCS membranes, PCS-30 contammg 56 mg/m2 of phosphorus showed the best pervaporatlon performance. The flux was 0.2 kg/m2-hr and the selectivity was ca 600. In comparison with the SCS and CMCS membranes, the permeate flux through PCS membranes is increased about four-fold without the significant loss of selectivity for water. Acknowledgement
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This work was supported by the 1990 Korean Ministry of Education Research Fund for Advanced Materials.
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