Polymeric pseudocrown ethers. II. Complexation with acids in aqueous and organic solvents

Hydrometallurgy, 4 (1979) 83--92 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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POLYMERIC PSEUDOCROWN ETHERS II. C O M P L E X A T I O N W I T H A C I D S I N A Q U E O U S A N D O R G A N I C S O L V E N T S

ABRAHAM WARSHAWSKY and BERTA BORER Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot (Israel) (Received February 24rd, 1978; in revised form May 23rd, 1978)

ABSTRACT

Warshawsky, A. and Borer, B., 1979. Polymeric pseudocrown ethers. II. Complexation with acids in aqueous and organic solvefts. Hydrometallurgy, 4: 83--92. A macroreticular polystyrene incorporating polyoxyethylene (14) units (Pseudocrown14) was shown to complex strongly with both protic and Lewis acids. The distribution coefficients depend on the dielectric constant of the solvent. In non polar organic solvents such as benzene or chloroform, the capacity of the polymer for acid reaches the theoretical value of 13.60 mmole hydrogen ions per gram polymer, Column tests have shown that the polymers can be used to reduce acidity in solutions to 10-4M or less, and that the polymers can be fully regenered using polar solvents such as methanol or water. The comparison of HBr complexation with pseudocrown-14 and linear or macrocyclic ethers, shows that pseudocrown-14 behaves similarly to a macrocyclic crown ether.

INTRODUCTION T h e g r o w i n g i m p o r t a n c e o f m a c r o c y c l i c ligands o f t h e c r o w n e t h e r t y p e , resulted in a v a r i e t y o f s y n t h e t i c strategies, as discussed b y L e h n ( 1 9 7 3 ) in a c o m p r e h e n s i v e review o n this subject. T h e s y n t h e t i c steps usually involve s e q u e n t i a l b i m o l e c u l a r or even tri- o r t e t r a m o l e c u l a r t y p e r e a c t i o n s , carried o u t at v e r y high d i l u t i o n s and w i t h u n f a v o u r a b l e yields. Polymers incorporating pendant crown groups prepared from 4'-vinyl derivatives o f b e n z o - 1 5 - c r o w n - 5 and b e n z o - 1 8 - c r o w n - 6 , a n d t h e i r i n t e r a c t i o n w i t h ions, w e r e i n v e s t i g a t e d e x t e n s i v e l y b y S m i d and c o w o r k e r s ( K o p o l o w et al., 1 9 7 3 ; Smid, 1 9 7 6 ; W o n g a n d Staid, 1977). R e c e n t l y we h a v e s h o w n t h a t p o l y m e r i c " p s e u d o c r o w n " ethers, s y n t h e s i z e d b y r e a c t i o n b e t w e e n a flexible c h l o r o m e t h y l a t e d p o l y m e r a n d a linear p o l y o x y a l k y l e n e , h a v e e x c e l l e n t p r o p e r t i e s as m e t a l c o m p l e x i n g agens ( W a r s h a w s k y et al., 1978). T h e y are p r e p a r e d b y t h e f o l l o w i n g s c h e m e :

84

o/

CH2CI

+

HO--CH2--CH2[-O--CH2--CH2]n--O--CH2--CH2--ON

Ct42CI

/ 0 -- C H2--C FI2 C N2

\0 cH2

~CH 2

CIN2

"~-O_ C N2-- CH2~/nO

The polymeric pseudocrown ethers (PPCE for short) were shown to extract HFeC14, as an ion pair, from mixed phosphoric--hydrochloric acids (Warshawsky et al., 1975 and 1978}. The mechanism of the reaction involves the protonated form of the PPCE. In this study, the interaction of polymeric pseudocrown ethers with Lewis acids, including protic acids, in both organic and aqueous media is presented. EXPERIMENTAL

Acids used were 32% HC1 (B.D.H.A.R. grade), 98% H2SO4 A.R., 45% HBr in acetic acid, trifluoroacetic acid A.R. grade and boron-trifluoride etherate A.R. grade. For organic solvents, A.R. grade solvents were used after distillation.

Acidity titration Aqueous solutions were titrated with 0.4 M NaOH with phenolphthalein as indicator. Organic solutions were titrated with 0.1 M NaOCH3 in CH3OH, with thymol blue as indicator.

Equilibrium absorption experiments 2 g dry polymer samples in 50 ml of solution were shaken for 24 hours at 25 ° C. Duplicate 5 ml aliquots of the supernatant were withdrawn and analyzed. The results are given in Figs. 1 and 2 and in Tables 1--3.

Pseudocrown-14 The polymeric pseudocrown ether, incorporating polyethyleneglycol 600 units (13.6 mmol --O--CH2--CH2-- units per gram polymer) was prepared from Amberlite XE-305, a macroreticular styrene--divinyl benzene copolymer, as described by Warshawsky et al. (1976).

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Absorption on columns 20 g of dry pseudocrown-14 were placed in a 2.0 × 26.5 cm column. The solutions were passed in a downflow rate of between 0.4--3.0 ml/min. The column was treated with the neat solvent before and after the absorption and then the acid was eluted with water or methanol. For results see Table 4 and Fig. 3.

Polymer volume expansion To 25 ml of dry pseudocrown-14 was added 50 ml of chloroform in a stoppered graduated cylinder. After 24 hours the polymer volume had expanded to 29.0 ml. To the chloroform swollen polymer was added 50 ml 0.88 M trifluoroacetic acid in chloroform. After 20 hours, the polymer volume had increased to 43 ml, corresponding to 72% volume increase from dry polymer, and 56% volume increase from chloroform swollen polymer.

RESULTS

AND

DISCUSSION

1. Protonation of ethers The complexing properties of ethers, including basicity, are discussed in a comprehensive review by Searles Jr., and Tamres (1967). Several ether--acid complexes were actually isolated, such as (C4Hg)20"HNO3, (C2Hs)20"H2SO,, HC104"dioxane.H20 and (CH3)20"HBr. As the ethers are very weak bases, they were usually studied in very concentrated acids. Generally, under such conditions, aliphatic ethers have pKa values ranging between --5.4 to --2.0 (Arnett, 1963).

2. Complexation of protic acids by polymeric pseudocrown ethers (PPCE) The complexation of protons by PPCE can be easily described as a multiprotonation process as follows: /o-

cH~

c , ~ - c H ~ - o.,.

/o-

cH~ ~HA I

C H'~r-o_ CIA2_ C H ._']n_o/C FI2

-

(~IH~

CH~--CH2 -- o...

CH~

-

I

mHA

CH2 "~2[O_ C IA2_ C kl2_']n__O/

In this case m = 14 and n = 12. The equilibrium constant for the complexation reaction is defined by

86

[mHA-PPCE] m gp =

(1)

[PPCE] [HA]m

In very dilute acid solutions, the term [PPCE] can be taken as constant, and eq. 1 may be rewritten as: [mHA.PPCE] gp =

[HA] p -D -

HA

(2) [HA] s

where D is the equilibrium distribution coefficient of the acid between the polymer phase and solvent phase, and [HA] p and [HA] s are the equilibrium acid concentration in the polymer and solution, respectively. A priori, it can be assumed that the proton complexation reaction is strongly dependent on the nature of the solvent. We have therefore selected for this study solvents with a wide range of dielectric constants. Distribution coefficients were determined by batch equilibration of polymer samples for 24 hours, followed by titrimetric analysis of the liquid phase. The results are TABLE 1

The d i s t r i b u t i o n of CF3CO~H in various solvents Benzene

Chloroform

Dioxane

[nA]0

[HA]s

[nA]p

D

[HA] 0

[HA] s

[HA]p

0.016 0.185 0.394 0.98 1.17 1.57

0.001 0.022 0.130 0.550 0.680 1.030

0.40 4.075 6.65 11.00 12.25 13.62

400 185 51.1 20.0 20.4 13.2

0.022 0.22 0.88 0.72 1.21 3.87

0.013 0.116 0.4 0.351 0.704 3.4

0.22 2.6 12.0 9.2 12.75 11.65

D 16.9 22.4 30.0 26.2 18.1 3.4

[HA]0

[HA] s

[SA]p

D

0.34 0.68 0.80

0.3 0.60 0.70

2.1 2.1 2.1

3.3 3.5 3.0

[ H A ] 0 = original acid c o n c e n t r a t i o n , tool/l; [ H A ] s = e q u i l i b r i u m s o l u t i o n c o n c e n t r a t i o n , tool/l; [ H A ] p = e q u i l i b r i u m p o l y m e r c o n c e n t r a t i o n , m o l / k g ; D = d i s t r i b u t i o n c o e f f i c i e n t between polymer and solvent phases.

TABLE 2

The d i s t r i b u t i o n of HBr a in various solvents Benzene

Chloroform

Dioxane

['-HA]0

[HA]s [HAip D

[HA]o-iHA]s [HA]p-D--

0.092

0.001 0.022 0.017 0.089 0.184 0.239

0.09 0.15 0.32 0.44 0.68 0.78 1.64

0.123 0.167 0.353 0.435 0.67

2.35 2.52 3.75 6.61 8.74 10.78

235.0 114.5 220.5 73.4 47.5 45.1

0.001 0.044 0.075 0.136 0.33 0.41 1.25

1.3 2.75 6.13 7.59 8.5 9.2 9.6

2300 62.5 81.8 55.8 25.7 22.4 7.7

0.12 0.17 0.80

0.03 0.05 0.67

2.23 3.0 3.0

74.3 60 4.5

a S t o c k s o l u t i o n o f 45% W/V in acetic acid was used. [ H A ] 0 = original acid c o n c e n t r a t i o n , tool/l; [ H A ] s = e q u i l i b r i u m s o l u t i o n c o n c e n t r a t i o n , tool/l; [ H A ] p = e q u i l i b r i u m p o l y m e r c o n c e n t r a t i o n , m o l / k g ; D = d i s t r i b u t i o n c o e f f i c i e n t between polymer and solvent phase.

87

1 i

"•15,

HBr-C6H6 HOr-CHCI3 c---.o HBr -Dioxon a~am TFA.C6H6 m(P-~TFA-CHCJ 3 TFA-Oioxon BF3- CHCI3 - C6H6

z

0.5

1.0

1,5

2.0

in Solutionat Equlhbrium(mmog ) Fig. 1. Acid absorption from solvents. [HA]

presented in Table 1 for CF3CO2H and Table 2 for HBr. The distribution coefficients vary in the various solvents in the following order: Dbenzene > Dchloroform > Ddioxane > Dwater Since the D values are inversely related to the apparent constant 1/Kp, this means that the HA-PPCE complexes dissociate better in the polar solvents. The polymer capacity, as determined from the extraction isotherms presented in Figs. 1 and 2, reaches the theoretical value of 13.6 mmol/g for HBr and CF3CO2H in benzene and chloroform. It is worthwhile noting the shape of the isotherms which is very favourable for extraction from very dilute solutions. In dioxane, the polymer capacity falls to 3 mmol/g and the range of effective extraction is shifted to higher acid concentrations. In water, complexation starts practically at HC1 concentration exceeding 1 M.

~2n

E

E

5

I

I

I

I

I

I

2

3

4

5

I

I

I

-

4--

32-

-r

I --

HCl

in Solution at

6

I

I

7

8

Equilibrium(mmol/ml)

Fig. 2. HCI absorption from water.

88

TABLE 3

The distribution of BF 3'etherate in various solvents Benzene

Chloroform

Dioxane

iliA] 0

[HA] s

[HA]p

D

[HA]0

[HA]s

[HA]p

D

[HA]0

[HAJs

[HA]p

D

0.145 0.29 0.53 0.96 0.12 0.89

0.052 0.171 0.406 0.82 0.97 1.75

2.32 2.86 3.11 3.46 3.77 3.44

44.6 16.8 7.8 4.2 3.9 2.0

0.04 0.166 0.27 0.44 0.52 0.74 1.25 1.92

0.016 0.104 0.197 0.374 0.47 0.66 1.18 1.85

0.54 1.65 1.80 1.64 1.30 1.~2 1.56 1.95

33.7 15.9 9.1 4.4 2.8 2.4 1.3 1.05

0.161 0.343 0.572 1.14

0.14 0.312 0.536 1.10

0.52 0.77 0.91 0.94

3.7 2.5 1.7 0.8

[ HA ] 0 = original acid concentration, mol/l; [ HA ] s = equilibrium solution concentration, tool/l; [HA ] p = equilibrium polymer concentration, mol/kg; D = distribution coefficient, between polymer and solvent

phases.

3. Complexation o f Lewis acids Group III elements, such as boron, aluminum and others, form addition compounds with ethers. For example, boron trifluoride etherates are isolated as stable liquids. In the present study, BF3 was chosen to represent the Lewis acids. Since it is well known that the exchange of BF3 between ethers occurs at high rates, comparable to proton acid--base reactions (Rutenberg and Palko, 1965), BF3-etherate could be conveniently used as a source of BF3. The equilibrium distribution coefficients, as described in Table 3, show that, as in the case of the complexation of protic acids, the complexation of BF3 is inversely dependent on the polarity of the solvent. The relative order of equilibrium distribution coefficients is: Dbenzene

~ Dchloroform ~ Ddioxane

The polymer capacity for BF3 is 3.5 mmol/g, indicating participation of four oxygen atoms in the BF3"PPCE complex. 4. Column tests From the equilibrium distribution work presented in Tables 1--3, it is evident that pseudocrown-14 is capable of efficiently removing acids from organic solvents. It is also clear that the acids can be regenerated by polar solvents. The column tests were necessary to obtain: (1) More accurate information on polymer loading capacity, particularly in aqueous media, where the low polymer capacity indicated the need for experiments with larger polymer quantities. (2) Data on polymer regeneration.

89

(3) Data on the efficiency of acid removal. In all the column experiments, three fractions containing all the column effluents were collected: (a) The barren solution, after breakthrough. (b) The washing solution. (c) The elutriating solution. The results of the column tests are presented in Table 4. The breakthrough capacities obtained from the column tests are generally in good agreement with those obtained in the batch distribution experiments. They are 13.50 m m o l e CF3CO2H and 4.1 mmoles BF3 per gram polymer from chloroform, and 2.41 mmole HC1 and 15.3 mmoles H2SO4 per gram polymer from water. The regeneration of the polymers is possible with methanol but the volumes required are excessive, and the best elution is achieved with water. The polymers in Test Nos. 3 and 5 were regenerated with water and reused in

TABLE 4

The absorption of various acids on columns; column dimensions: 2 × 26.5 cm Expt. No.

1 2 3 4 5 6

Prewashing

Polymer (g)

H20 H20 CHCl 3 CHC13 b CHCI 3 CHCI~ c

17.5 20 17.5 17.5 20 20

Absorption a HA

(tool/l)

(ml)

HCI/H20 H~SO4/H20 CF3CO2H/CHCl 3 CF3CO2H/CHC13 BF3/CHC13 BF3/CHCI 3

6.08 6 0.56 0.41 0.158 0.156

76.5 300 700 800 1100 800

aFlow was stopped when initial level of concentration was achieved. bSecond cycle on regenerated column from expt. 3. CSecond cycle on regenerated column from expt. 5.

TABLE 4 (continued)

Exp. No.

1 2 3 4 5 6

Elution

Breakthrough capacity (mmol HA/g polymer)

Eluant

(ml)

% eluted

H~O H20 CH3OH CH3OH CH~OH CH3OH

100 300 200 200 200 300

100 96.7 75.2 60.7 73.4 93.9

2.42 15.33 12.54 13.50 4.1 3.81

Rate (ml/min)

HA absorbed (mmol)

1.0 0.5 0.4 0.4 3.6 0.8

42.44 306 219.5 237.6 81.9 55.6

90

Test Nos. 4 and 6 respectively, w i t h o u t loss of any capacity, indicating the effectiveness o f the water regeneration step. To establish the acid removal efficiency, the acid concent rat i on in the column effluents o f Test Nos. 3 and 5 was monitored, and is plotted against volume in Fig. 3. A p p r o x imately 8 bed volumes (bV) of 0.159 M BF3 and 17 bV o f 0.56 M CF3CO2H were passed before the acid c o n c e n t r a t i o n rose above 10 -3 M. To summarise, the column tests have shown that the polymeric pseudocrown ethers have high capacity for acids, they are totally regenerable, and effective in reducing acid levels to unde r 10 -3 M concentration. I

I

I

I

I

o.8~0.7 E

0.6

o E E

05

o---o

BF 3 in CHCI 3

IBF3]o=OI59M

~

CF3CO2Hin

[CF3CO2H]o=O156M(~=I75 CelpoClty:125mrnol/cj

CHCI 3

(~=20gCapactty:41mmol/g_

g ~

0.4

-6

.E

03

o.2

0

I00

200 Volume

300 400 p f l s s e d (ml)

500

Fig.3. Absorption of CF3CO2H and BF 3 on columns.

5. Comparison between pseudocrown-14, linear and macrocyclic polyethers It is of considerable interest to compare data on polymeric pseudocrown ethers with similar data on linear and macrocyclic polyethers. Table 5 shows a comparison o f p r o t o n c om pl exat i on constants between the various ether classes and HBr. The work of Shchori and Jagur-Grodzinski (1972) was carried out in very dilute solutions ( 10- 6- - 10 -3 M) using very sensitive c o n d u c t o m e t r i c and s p e c t r o p h o t o m e t r i c methods, whilst in the present work, HBr was complexed from an initial c o n c e n t r a t i o n of 0.09 M HBr in c h l o r o f o r m (Table 2) and equilibrium measured at 10 -3 M HBr. However, fair conclusions may still be derived from this comparison. Pseudocrown-14 (/9 = 2.3 × 103) is four orders of magnitude stronger as a base than diethylene glycol-dimethylether (Kp = 0.17 M -1 ) or triethyleneglycoldimethylether (Kp = 0.20 M -1 ) and one

91

TABLE 5 Complexation constants for linear ethers, polymeric pseudocrown ethers and crown ethers in chloroform Ether

Kp a (M-')

Diethyleneglycol-dimethylether

0.17 b

Triethyleneglycol-dimethylether Dibenzo- 18-crown-6 Pseudocrown-14

0.20 b 210 b 2300 c

Dicyclohexyl-18-crown-6

106 b

KK ÷

(M- I )

~ 10' d l0 s d N.E.

106.01 d

aKp ffi proton complexation constant in CHCI 3 at 25 ° C. KK+ = potassium complexation constant in methanol at 25°C. bAfter Shchori and Jagur-Grodzinski, 10-6--10 - 3M HBr/CHC13 (din 3 mole -I). CTable 2, this work, 10-3M HBr/CHCI 3 D value equals Kp. dAfter Frensdroff (1971 ). N.E. = Not established.

order of magnitude stronger than dibenzo-18-crown-6 (Kp = 210 M - ' ). On the other hand, dicyclohexyl-18-crown-6, is three orders of magnitude stronger than pseudocrown-14 (Kp = 106 M -1 ). This comparison suggests, that pseudocrown-14 behaves like a true crown ether, and secondly, that conformational mobility is very important for strong complexation to occur. Pseudocrown-14, with a rigid aromatic chain structure on one side, and a free aliphatic oligooxyethylene chain on the other side, behaves as an intermediate between the rigid dibenzo-18-crown-6 and the flexible dicyclohexyl-18-crown-6 (DCC). This trend is in good agreement with results from the complexation of metal ions by the same ligands, such as the complexation of potassium ions by DBC and DCC as reported by Frensdorff (1971) and reproduced in Table 5.

REFERENCES Arnett, E.M., 1963. Quantitative comparisons of weak organic bases. In: S.J. Cohen, A. Streitwieser Jr. and R.W. Taft (editors), Progress in Physical Organic Chemistry, Interscience, New York, pp. 356--365. Frensdorff, H.K., 1971. Stability constants of cyclic polyethers. J. Am. Chem. Soc., 93: 600--606. Izatt, R.M., Eatough, D.J. and Christensen, J.J., 1973. Cation-macrocyclic compound interaction. In: J.D. Dunitz (editor). Structure and Bonding. Springer-Verlag,New York, pp. 161--189. Kopolow, S., Machacek, Z., Takaki, V. and Staid, J., 1973. Interactions of ions and ion pairs with crown ethers and their polymers. J. Macromol. Sci. Chem., A-7: 1015--1033. Lehn, J.M., 1973. Design of organic complexing agents. In: J.D. Dunitz (editor), Structure and Bonding, Springer-Verlag,New York, pp. 1--69. Rutenberg, A.C. and Palko, A.A., 1965. Nuclear magnetic resonance studies of borontrifluoride addition compounds. J. Phys. Chem., 69: 527--531.

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Searles, S. (Jr.) and Tamses, M., 1967. Basicity and complexing ability of ethers. In: S. Patai (editor), The chemistry of the ether linkage. Interscience, New York, pp. 243--257, and references cited therein. Shchori, E.and Jagur-Grodzinski, J., 1972. Protonation of macrocyclic polyethers. Complexes with hydrogen bromide and hydrogen tribromide in chloroform. J. Am, Chem. Soc., 94: 7957--7962. Staid, J., 1976. Solute binding to polymers containing macroheterocyclic rings. Pure App. Chem., 48: 343--353. Warshawsky, A., Kalir, R. and Patchornik, A., 1975. Purification of phosphoric acid by new oxonium type polymers. XXV IUPAC Congress. 6--11 July, Jerusalem, p. 262. Warshawsky, A., Patchornik, A. and Kalir, R., 1976. Polyether compounds. Israeli Appl. 50122, 25th July. Warshawsky, A., Bercovitz, H., Kalir, R. and Patchornik, A., 1978. Polymeric pseudocrown ethers. I. Synthesis and complexation with transition metal anions. J. Am. Chem. Soc., submitted. Wong, L.H. and Staid, J., 1977. Binding of organic solutes to polycrown ethers. J. Am. Chem. Soc., 99: 5637--5642.