Chapter 6
Gelation Mechanism of Chromium(III)
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
Paul Shu Mobil Research and Development Corporation, Central Research Laboratory, Princeton, NJ 08540
The chromium(III) i o n is a common c r o s s l i n k e r of many polymer g e l s used i n r e s e r v o i r p e r m e a b i l i t y p r o f i l e c o n t r o l . I t has been generally recognized that olated chromium(III) species are involved i n the c r o s s l i n k i n g , but the d e t a i l e d mechanism is not fully understood. Also, Cr(III) s a l t s and redox generated Cr(III) species do not always y i e l d the o l a t e s . Furthermore, C r ( I I I ) olates can vary greatly i n t h e i r crosslinking r e a c t i v i t y and therefore r e s u l t i n gels of d i f f e r e n t p r o p e r t i e s . After studying the gelation of xanthan gum with various Cr(III) species ranging from simple s a l t s to Cr olates of varying degrees of h y d r o l y s i s , we propose that the formation of Cr olates is the rate-determining step i n a gelation reaction and the simple binuclear Cr olate is the most reactive species for crosslinking.
Chromium(III) i s a commonly-used c r o s s l i n k e r f o r preparing p r o f i l e c o n t r o l g e l s with polymers having c a r b o x y l a t e and amide f u n c t i o n a l i t i e s ( l a , b ) . C r ( I I I ) i s a p p l i e d i n many forms. F o r example, i t can be used i n the form of simple chromic s a l t s of chloride and sulfate, or as complexed Cr(III) used i n leather tanning (2), or as i n s i t u generated C r ( I I I ) from the redox r e a c t i o n of dichromate and b i s u l f i t e or t h i o u r e a . The g e l a t i o n r a t e and g e l quality depend on which form of Cr(III) i s used. We have found t h a t the Cr o l a t e s produced by h y d r o l y s i s of Cr(III) ions are the r e a c t i v e c r o s s l i n k i n g species. The d i f f e r e n t gelation rates are due to the d i f f e r e n t degrees of o l a t i o n . Furthermore, by c o n t r o l l i n g the degree of h y d r o l y s i s , Cr(III) derived from various sources mentioned above can exhibit the same gelation rate. Hydrolysis of Cr(III) Due to the high charge-to-radius r a t i o , a hexaaqua C r ( I I I ) c a t i o n loses protons to form olates (3a,b) i n t h i s hydrolysis process. One, two and three protons can be l o s t from Cr-coordinated HgO t o y i e l d the mono-, d i - and t r i - h y d r o x i d e s o f h y d r o u s Cr s p e c i e s , 0097-6156/89/0396-0137$06.00/0 o 1989 American Chemical Society In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
138
OIL-FIELD CHEMISTRY
respectively. These hydroxides then dimerize or polymerize to form Cr o l a t e s (Equation 1) through OH or " o l " bridges. I s o l a t i o n and i d e n t i f i c a t i o n of dimer, trimer, and tetramer were reported by Stunzi and Marty (4a) and higher oligomers by Marty and S p i c c i a (4b).
Cr(H 0)^
+
[Cr(H 0) 0H]
2
2
2+
5
J * [Cr(H 0) (0H)]* +H +H 2
+H Olation
Olation
I D
C
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
[( 2°)4
( 2°)4]
0 H
Dimer
Cr (OH)3(HgO) 3
4
H
( 2 )4 0
C r
^
n H
0 H
1
+
0
>( 2 )2jC
Linear Polymer
0 H
W
^ g ^ n
3-Dimensional Polymer
The hydrolysis reaction i s very slow at ambient temperatures and i s a c c e l e r a t e d by b o i l i n g chromium s a l t s o l u t i o n s (5) . The h y d r o l y s i s r e a c t i o n i s c h a r a c t e r i z e d by the transformation of the deep blue c o l o r e d Cr(HoO)g t o green c o l o r e d h y d r o l y z e d o l a t e s . Another i n d i c a t i o n i s tnat an aged or b o i l e d Cr(III) s a l t s o l u t i o n has a higher n e u t r a l i z a t i o n equivalent than a f r e s h one due t o the h y d r o l y t i c a l l y produced protons. One way t o e s t a b l i s h h y d r o l y t i c e q u i l i b r i a quickly i s to add appropriate equivalents of bases such as NaOH to Cr(III) s a l t solutions. Olated Cr(III) reagents were prepared according t o Equation 2 by r e a c t i n g CrCNO^), with a c a l c u l a t e d equivalent of NaOH. Chromic n i t r a t e was used Decause the f r e s h l y prepared s o l u t i o n a f f o r d s the hexaaqua Cr(III) cations. +
Cr(H 0)g 2
+
+
n
nNaOH + Cr(0H)^" ; n = 0-3
(2)
The pH and UV-VIS s p e c t r a l data are l i s t e d i n Table I . For n=3, the product C r ( 0 H ) i s a p r e c i p i t a t e . T h e r e f o r e , t h e UV-VIS spectrum of Cr(0H)« was not obtained. 3
Table I. UV-VIS of Olated C r ( I I I ) * (Cr(N0 ?3 + n NaOH 3
n=0 1/3 2/3 1 2 3 *0.088M
XI 406 410 414 418 420 NA
Al 1.367 1.551 1.811 2.080 2.75 NA
Olates)
X2 574 576 578 580 584 NA
A2 1.159 1.256 1.372 1.498 1.837 NA
pH 2.48 2.66 2.78 2.87 3.25 4.89
A gradual s h i f t of absorption maxima to ^.onger wavelength and an increased absorbance were observed when Cr reacted with more and more NaOH. The s h i f t of peak p o s i t i o n and the change i n absorbance were also found by Ardon and S t e i n (6) . The s p e c t r a of o l a t e d Cr prepared by us agree with the l i t e r a t u r e .
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
6. S H U
139
Gelation Mechanism of Chromium(III)
As increasing amounts of NaOH are added to the Cr(N0g)o solution, the hydrolyzed Cr forms dimeric, polymeric and three-dimensional species. G e l l e d , amorphous and c o l l o i d a l C r ( 0 H ) i s e v e n t u a l l y formed. £. M a t i j e v i c reported the p r e p a r a t i o n of a monodispersed C r ( 0 H ) s o l by f o r c e d h y d r o l y s i s of C r ( I I I ) s a l t a t 90°C ( 7 ) . Because of the d i f f e r e n c e s i n s t r u c t u r a l f e a t u r e s , each o l a t e d species (n=l,2,3) should react d i f f e r e n t l y with polymers and form gels of d i f f e r e n t properties. 3
3
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
Gelation Mechanism and Rate of Cr(III) Olates The gel time of a 2000 ppm Flocon 4800 (a P f i z e r xanthan polymer) i n 2% NaCl s o l u t i o n was measured with various Cr(III) c r o s s l i n k e r s a t room temperature (Table I I ) . In t h i s s e r i e s of experiments C r ( I I I ) concentration was 90 ppm. The most r e a c t i v e C r ( I I I ) species were olates derived from C r ( N 0 ) with one and two equivalents of NaOH. Gels formed within 5 minutes and the r e a c t i o n r a t e appeared t o be d i f f u s i o n - c o n t r o l l e d . C r ( N 0 ) without NaOH r e q u i r e d 48 hours t o gel the polymer solution. This r e f l e c t s the time needed to hydrolyze C r ( N 0 ) i n Equation 3. 3
3
3
3
C r
3
3
(N0 )3 ^ > 3
Table I I .
[Cr (OH),(HjP) ]
+
Cr(H 0)3 ^
2
2
4
4+
^g^>
Gel
(3)
V a r i a t i o n of Gelation Time by Different Cr(III) Sources
Polymer = 2000 ppm P f i z e r Flocon 4800 i n 2% NaCl Cr = 90 ppm Cr Source Simple Cr olate
Gel Time
0 H
2°
4+
2VI/° ^ I / 2°
H
| "-H 0 H0 ^
H
H
C
H 0^ | ^OH^ | \ H0 P
9
*
2°
2+
2°^i -
EJO^
— — >
9
V H
3+ 2
9
J
2
H0 9
i
2
Crosslinked (B) H
H 0. 9
2P +
* H0 9
H
2° 2° | OH ^ | QT Cr | ^OH |^ H0 H0 2
4 H0
+
9
Z
H0 9
2
P = Polymer Gelation Reaction of Redox Generated Cr(III) 2Cr 0^" + 3S 0g" + H 0 2
2
2
•> 2 " C r 0 " + 2H + 6S0^" +
2
3
(5)
G e l a t i o n time of a 2000 ppm F l o c o n 2% NaCl s o l u t i o n w i t h 90 ppm Cr(III) according to Equation 5 was 2 weeks (Table I I ) , which i s i n the range of the Cr c o l l o i d gelation discussed e a r l i e r . Based on the e a r l i e r discussion, the gelation reaction of redox generated C r ( I I I ) can also be accounted for with the o l a t i o n mechanism. However i t i s
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
6. S H U
141
Gelation Mechanism of Chromium(HI)
very important to have a better understanding of Cr(VI) •+ C r ( I I I ) r e d u c t i o n , s i n c e the r e a c t i o n r a t e and the r e a c t i o n product are highly dependent on the amount of acid present i n the redox reaction. The pH Dependence of Cr(VI) Reduction and I t s Products Chromate i s a strong o x i d i z i n g agent i n a c i d i c media and produces Cr(III) ions i n the hydrate form, Equation 6. In n e u t r a l and b a s i c media, chromate i s a r a t h e r weak o x i d i z e r , as evidenced by i t s negative oxidation potential (Equation 7), and the product i s chromic hydroxide (8). Cr 0^~ + 14 H Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
CrQ^~
+
3+
+ 6e" -> 2 C r ( a q ) + 7 HgO E°=1.33V
(6)
+ 4 H 0 + 3e~ + Cr(0H) (s) + 50H" E°=-0.13V
(7)
2
2
3
None of the Cr(III) products from Equations 6 or 7 are e f f e c t i v e c r o s s l i n k e r s since a chromic aqua i o n must be hydrolyzed f i r s t to form o l a t e d Cr t o become r e a c t i v e . C o l l o i d a l and s o l i d chromium hydroxides react very slowly with ligands. In many gelation studies, t h i s c r i t i c a l condition was not c o n t r o l l e d . Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). By a d j u s t i n g t h e r e a c t i o n pH, one can a c h i e v e a t h e r m a l dynamically favorable redox r e a c t i o n and produce r e a c t i v e Cr o l a t e s in the dimeric or l i n e a r polymer forms for crosslinking. Gelation Reactions by Acidity-Controlled Redox Reactions The approach i s based on proper c o n t r o l of the a c i d i t y of the redox r e a c t i o n mentioned e a r l i e r . A general equation of the dichromated i s u l f i t e reaction as a function of a c i d i t y i s expressed i n Equation 8 where Cr(0H) and H are the hypothetical products. 3
2 Cr 0^~ +3 S 0j?~+ n H 2
2
+
+ 7 H 0 ^ 4 Cr(0H) 2
3
+ (n+2) H
+
+ 6 S0^~
+
(8)
The reaction of C r ( 0 H ) and H i s the reverse of the h y d r o l y s i s of Cr(H 0)g (Equation 1). Therefore by adjusting the a c i d i t y of the redox reaction (Equation 8), Cr olates of a l l oligomerizations can be prepared. At HCl s t o i c h i o m e t r i c s of n=0, 2, 6 and 10 (Equation 8 ) , the r e d u c t i o n product showed wide v a r i a t i o n s i n g e l a t i o n r e a c t i v i t y (Table I I I ) . At n=2 and 6, the reaction products were very e f f e c t i v e in g e l l i n g a 2000 ppm Flocon 4800 xanthan polymer because dimeric and linear polymeric Cr olates are formed. On the other hand, at n=0 and 10, g e l a t i o n was very slow, because h i g h l y h y d r o l y z e d m a t e r i a l similar to Cr(0H) i s the product when n=0, and aqua i o n of C r ( I I I ) with blue color i s the product when n=10. Furthermore, i n the n=10 case, where no g e l a t i o n occurred a f t e r 24 hrs, a g e l formed i n one hour a f t e r enough NaOH was added to y i e l d the suggested d i m e r i c Cr olate. A l l the above r e s u l t s have shown the importance of a c i d i t y i n determining the r e a c t i v i t y of Cr(III) produced i n a redo* reaction. 3
2
3
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
142
OIL-FIELD CHEMISTRY Table I I I .
Gelation by Acidity-Adjusted Redox Reactions
Polymer = Flocon 2000 ppm 2Cr 0y 2
+
2
nH + 7H 0 -> 4Cr(0H)
+
+
2
ppm
3
+ (n+2)H + 6S0
2
Gelation Time
Cr (III) Product
Degree of Cr Polymerization
0
Cr(0H)
3
3-d Polymer
No gel i n 2 weeks
2
Cr(0H)
2
2-d Polymer
5 Minutes
6
Cr(0H)
Dimer
30 Minutes
Monomer
2-3 Days
nH
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
+ 3S 0J?
i n 2% NaCl ; Cr(III) = 90
+
2+
Cr(H 0)3
10
+
2
Similar species are formed from both the acid-ad jus ted redox and the C r ( I I I ) s a l t - NaOH r e a c t i o n s . A comparison i s given i n Table IV. The pH of each corresponding p a i r at the same Cr concentration i s very close, futher supporting t h i s theory. The UV absorption at -400 nm of the redox products s h i f t e d to shorter wavelength when the s t a r t i n g redox mixture was made more a c i d i c , suggesting t h a t l e s s hydrolyzed Cr was formed at higher a c i d i t y . This trend was observed i n the p r e p a r a t i o n of Cr o l a t e s by the nNaOH + C r ( N 0 ) r e a c t i o n (Table I ) . Q
Q
6
Table IV.
Similar Cr Olates Derived From Redox and From Cr(N0 ) + xNaOH 3
3
Cr conct 90
Redox Reaction nH
+
ppm
Cr(III) Product
CrfNOpU + xNaOH
pH
0
5.5
Cr(0H)
2
3.6
Cr(0H) +
6
2.8
Cr(0H)
10
6
3
2+
Cr(H 0)3
1.5
+
2
pH
xNaOH
4.9
3
3.3
2
2.9
1
2.5
0
Further Evidence on A c i d i t y Influence of the Cr Redox Reaction We n o t i c e d t h a t the g e l a t i o n of polymers by the redox method i s promoted i f 2-3 times the c a l c u l a t e d molar r a t i o of t h i o s u l f i t e i s used (see h a l f Equation 9 below). The g e l a t i o n rate was very slow when x=l or x=4 (Table V). 2
2
2 Cr 0 " + x(3S 0 ) " 2
2
5
+ HgO
+ Polymer -> Gel
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
(9)
6. SHU
Gelation Mechanism ofChromium(III)
Table V.
Gelation Rate as a Function of SgO,.
143 Concentration
Polymer = 1500 ppm Flocon i n 2% NaCl Cr=90 ppm 2 C r 0 " + x (3 S 0 ) ~ + Polymer 2
2
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
2
2
5
x
Gel Time, hr.
Color of Reaction Mixture
1 2 3 4
No g e l i n 2 weeks 4 2 24
Yellow Green Green Blue
Here, d i s u l f i t e i s f u n c t i o n i n g as a l a t e n t a c i d , r e l e a s i n g protons and b i s u l f i t e upon h y d r o l y s i s (Equation 10). At the proper proton c o n c e n t r a t i o n s , (x=2, 3), r a p i d Cr(VI) r e d u c t i o n and f a s t g e l a t i o n take p l a c e . Therefore at x=2 to 3, the redox r e a c t i o n should be the same as i f a c i d were added at n=2 to 6 (Equation 9) . The g e l a t i o n r e a c t i v i t y of the two are comparable under t h e s e conditions. S
2°5~
+
H
2°
*
2 H +
+
2 S 0
3~
10
( )
A t x = l , the redox r e a c t i o n was v e r y slow and t h e UV-VIS absorption showed no change with time. The orange - y e l l o w i s h c o l o r persisted f o r weeks, indicating that there was l i t t l e or no reduction ( i . e . , poor conversion) of Cr(VI). At x=4 or more, development gf blue c o l o r o c c u r r e d i n s t a n t l y , which i s e v i d e n c e of C r ( H 0 ) g production v i a the a c i d i c redox mechanism. At x=2 to 3, the green color of olated Cr(III) developed i n minutes, followed by gelation of the polymer. 2
Conclusions *
The r e a c t i v e C r ( I I I ) s p e c i e s i n polymer c r o s s l i n k i n g are the olates derived from the hydrolysis of hydrated Cr(III) cations.
*
The rate-gleterming step i s the deprotonation i n the h y d r o l y s i s of the Cr hydrate.
*
In the presence of NaOH or other basic materials, dimerization to form olates becomes the rate-determining step.
*
Various C r ( I I I ) o l a t e s can be generated from the r e d u c t i o n of Cr(VI) by c o n t r o l l i n g the amount of the acid.
*
The gelation mechanism of redox-Cr (III) follows the same pathway as Cr(III) s a l t gelation.
Acknowledgments The author wishes to thank the management of Mobil Research and Development Corporation f o r permission to present t h i s work. The d i l i g e n t work of Marie J . Wszolek i s greatly appreciated.
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
144
OIL-FIELD CHEMISTRY
Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch006
Literature Cited 1. (a) Abdo, M. K.; Chung, H. S.; Phelps, C.H.; K l a r i c , T.M. SPE/DOE Paper 12642, 1984. (b) Hessert, J . E.; Fleming, P. D. 1979 T e r t i a r y O i l Recovery Conf., Wichita, KS, Apr. 25-26, 1979; p 58-63. 2. Udy, M. J . Chemistry of Chromium and I t s Compounds; V o l . 1, ACS Monograph Series No. 132, 1956; p 302. 3. (a) Bailar, J . C., Jr., Ed. The Chemistry of C o o r d i n a t i o n Compounds; Reihold Publishing Co.: New York, 1986; Chapter 13, p 448-471. (b) C o l t o n , R. C o o r d i n a t i o n Chemistry Reviews 1985, 62, p 85-130. 4. (a) Stunzi H.; Marty, W. Inorg. Chem. 1983, 22, p 2145. (b) Spiccia, L.; Marty, W. Inorg. Chem. 1986, 25, p 266. 5. H a l l , H. T.; Eyring, H. J . Am. Chem. Soc. 1950, 72, p 782. 6. Ardon, M.; Stein, G. J . Chem. Soc. 1956 78, p 2095. 7. Popey, C. G.; M a t i j e v i c , E.; Patel, R. C. J . Colloid and Interface Science 1981, 80, No. 1, p 74. 8. C o t t o n , F. A.; W i l k i n s o n , G. F.R.S., Advanced I n o r g a n i c Chemistry, Interscience Publishers, 1972; 3rd E d i t i o n , p 841. 9. Prud'homme, R. K.; Uhl, J . T.; Poinsatte, J . P.; Halverson, F. Soc. Pet. Eng. J . 1983, p 804. 10. Southard, M. Z.; Green, D. W.,; W i l l h i t e , G. P. Paper 12638 SPE/DOE 4th Symposium on Enhanced Oil Recovery, Apr. 16-18, 1984, Tulsa, OK. R E C E I V E D January 27,
1989
In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
