THE JOURNAL OF
P H Y S I C A L CHEMISTRY (Registered in U. S. Patent Office)
(Copyright, 1953, by the American Chemical Society)
Founded by Wilder D.Bancroft FEBRUARY 2, 1953
VOLUME;57
NUMBER 1
A MECHANISM OF CATION AND ANION EXCHANGE CAPACITY1 BY MELVINA. COOK,IVANB. CUTLER,GEORGE RICHARD HILLAND MILTON E. WADSWORTH College of Mines and Mineral Industries, University of Utah, Salt Lake City AND
ALEXG. OBLAD
Houdry Process Corporation, Linwood, Pennsylvania Received July W , 1061
Cation and anion exchange capacity in aluminous clay minerals and polyamine resins, respectively, are attributed to an (acid-base) “ion-pair” adsorption process in which one ion of the “ion-pair” is adsorbed on the solid or colloidal particle i n thc “compact double layer” while the other remains hydrated and occupies the “diffuse double layer.” In cation exchange clays the “compact double layer” ion is the hydroxyl ion, in anion exchange resins it is the hydrogen ion. This mechanism for ion exchange is here developed thermodynamically and correlations with available experimental data are discussed. Equilibrium constants and adsorption potentials obtained in these analyses are shown to have the magnitudes re uired by the theory. Previously obscure effects of the ionic strength of the aqueous solution are derived quantitatively an% the general feature of ion exchange explained.
Introduction
ing from the theory proposed, (2) the general correlation of the data, and (3) the explanation of the effects of ionic ‘strength which hitherto have remained obscure. No attempt will be made here to discuss cation exchange by the sulfonated resin type of ion exchange medium. This type of cation exchange is well understood. Discussion
In this paper the mechanism of cation and anion exchange in (1) aluminous clay mineral and synthetic zeolites and (2) polyamine resins, respectively, is considered. The factors affecting exchange capacity are then presented. The phenomena of ion exchange and especially the mechanisms involved in the systems mentioned have been objects of great activity by workers in many fields. Recent published material which initially was concerned with the mechanism of heterogeneous catalysis has shed very considerable light on the subject of the mechanism of ion exchange. In this article we shall employ the model proposed by Milliken, Mills and Oblad2 to discuss the chemistry and thermodynamics of base adsorption and BEC (base exchange capacity) of ,,lays and synthetic zeolites and anion exchange capacity (AEC) of resins. It will be seen that strong support for the mechanism proposed is obtained from (1) the equilibrium constantsand adsorption potentials
Milliken, ill^ and Oblad (MMO)1 associate catalytic activity of natural and synthetic alumina-silica and related acid catalysts with “promoted” structural changes on the surface of the catalyst. These changes are shown to correspond to a shift in coordination of the aluminum ions in alumina “micelles” from the 6-coordinate.d (octahedral) t,o the 4-coordinated (tetrahedral) forms. In the 6-coordinated state the calcined catalyst is a potential Lewis acid. The catalyst is prepared in such a way as to minimize the “activation energy” for the transformation. This is accomplished by forming the catalyst under conditions where the aluminum ions exist as extensively as possible in the 4-coordinat8ed or acid st.ate by means of promot,ional inI n the calcining fluences of (cristobalite) silica and ”4.’ process NH, and water are removed and the alumina conse( 1 ) Presented before the twenty-sixth National Colloid Symposium quently goes over to the 6-coordinated state. Because of which was held under the auspices of the Division of Colloid Chemistry the influence of cristobalite a t the linear interface between of the American Chemical Society in Los Angeles, California, June alumina and silica micelles and the fact that alumina in the 16-18, 1952. original or precalcined hydroaluminum silicate is originally (2) T. H. Milliken, G. A. Mills, and A. G. Oblad, “Heterogeneous largely 4-coordinat,ed, the system acquires appreciable strain Catalysis,” Faraday Society Discussion, 279, No. 8 (1950); Volume or surface warp when the alumina ‘contained therein goes 111, “Advances in Catalysis,” Academic Press, Ino., New York, N. Y., over to the 6-coordinated structure on calcination. The in press. influence of this strain is to lower the energy level of the
1
2
M. A. COOK,1. €3. CUTLER,C. 12. HILL,M. E. WADSWORTH AND A. G. OBLAD
transition state so that it becomes easier for the alumina to change over from the 6~coordinatedto the 4-coordinated ‘state under the influence of basic materials. Under the (promoted) condition in which even a hydrocarbon may act as a free base toward the catalyst, however, stronger bases such as alkali, quinoline, etc., “poison” the catalyst by rendering the 4-coordinated state energetically more stable than the 6-coordinated one. I n order that the catalyst remain spontaneously regenerative it is necessary for the 6-coordinated state to be the more stable one; for catalytic activity, it is necessary for the energy difference between the two states to be sufficiently small that the surface may oscillate rapidly back and forth between the two structures under the influence of temperature and the weak base adsorbate. The warped or strained surface is one in which the surface unsaturation is satisfied under the restrictions of limited mobility of a solid by sharing of the unsaturation bonds, between, for example, two sites on the unstrained structure. An adsorption site, represented by S, is thys broken into two joined sites (kinetically still a single site) in the transition to the higher energy level by adsorption of the weak base. The adsorption reaction may, therefore, be represented by the, equation
VOl.
57
may all be superimposed when plotted according to the equation 0 = g(&
+d;4e-)
(4)
Here tj is the fraction of S sites occupied by base, f is a function of the activity of the M + n alone and g is a function of the product (M+”) ( O H - ) . (rM+,yZH-). Equation ( 3 ) at constant pH and equation (4) resemble the Langmuir isotherm except that they do not become constant a t high base concentrations, instead experimentally 6 continues t o increase slowly as the base concentration increases. Equation (4) implies that base adsorption by clays is an “ion-pair” process and that BEC, therefore, depends not only on the exchangeable ion concentrations, but also on the hydroxyl ion concentration. We recognize three possible types of adsorption conforming t o this condition, namely :(a) hydrated ion-pair adsorption, (b) hydrated-dehydrated ion-pair adsorption, (e) dehydrated ionS (R:)+8’R - (8”) (1) pair (or free base-free acid) adsorption. where R: :s the (chemisorbed) radical or ion carrying the Type (a) involves only DDL (“diffuse-double free electron pair. I n the reaction of the alumina-silica layer”) adsorption and is perhaps best exemplified catalyst with hydrocarbon, R: is the hydride ion (H:)-. The benefit of the catalyst then results from the rearran e- by KfOH- adsorption on mercury resulting in the inents which take place in the carbonium ion under &e “ideal” electrocapillarity curve. It is clear that highly polar environment of the catalyst surface. Here S this type is not involved in base adsorption by represents the strained catalyst surface. Milliken, Mills and Obladz further showed that natural clays. Type (b) involves the adsorption of one and synthetic hydroaluminum silicates contain “acid” ion in the CDL (“compact-double layer”) in a desites and that the catalytic activity of the calcined form of hydrated condition and the other in the DDL in a these materials is a function of this “acidity,” implying that hydrated condition. The MMO model of clays the “acid” sites in the hydroaluminum silicates correspond to (but are not identical with) the seat of catalytic activity indicates that this type of adsorption is involved in of the calcined catalyst. Greensfelder, et uZ.,~ have given the extraction of bases from aqueous solutions by clays. Type (c) occurs prominently in cationic a discussion of the reactions occuwing on these “acid” catalysts. and anionic collectors and correspondingdepressants Milliken, Mills and Oblpdz also showed that base adsorp- in f l o t a t i o ~ i , ~and - ~ in acid and base adsorption by I ion by clays and synthetic hydroaluminum silicates is related t o the number of “acid” sit8esin a one to one corre- wool.8 Cook and Wadsworth discussed these three spondence with the active alumina content of the adsorptive types of acid and base adsorption and showed how material. They propose that base adsorption, like catalytic free energy and selectivity considerations may be activity by aluminum silicates, involves a change of co- used to distinguish between them. The equivalent of ordination of the aluminum from a 6-state to a 4-state. They point out that base adsorption by these materials is model (b) was proposed for anion exchangers by related not only to the cation involved but is also dependent BishopQand was implied by the equations for anion on the OH- concentration. For example, the amphoteric exchange capacity written by Kunin and Myers‘O nature of aluminum hydroxide, AI(OH)s, is related to the and by Jenny.li concentration of OH On the basis of the MMO model of clays one NaAlOe 2 H ~ 0 AI(0H)s NaOH concludes that a imre (dialyzed) clav has no BEG: The amphoteric nature of aluminum is not lost when alumina this property is acquirkd 6y the CDL adsorptioii is combined with silica. However, according to the MMO (chemisorption) of OH-, according to the equamodel, the pH level a t which the amphoteric or coordination tion (l),specifically
I
+
+
+
shift takes place is lowered from 10-12 to 3-4 or lower.
It is well known that BEC is related to pH. The conventional exchange equilibrium expressed by the equation
+
nSrnM~(+~)mMp(+”>
mSnMe(*)
+ nMl(tlnf
(2)
however, does not account for the BEG. Here and Mz(+n) are two cationic species of charge + m and +n, respectively, and the site S corresponds to the 8“ of equation (1) and carries a (-E) charge, where e is the electronic charge. It may be shown that the family of adsorption curves one obtains in the titration of a pure clay (usually referred to as H+ clay) represented at constant p H by the equation e
= P(AY +.)
(3)
13) B. L. Greensfelder, H. H. Voge and G. M. Good, Ind. Enp. Chem., 41, 2573 (1945).
s +oH-+SOH-
(5)
where S represents an inactive site and SOH- the active site. The symbol SOH- is intended to imply no particular structure for the hydroxyl ion activated site. Reaction ( 5 ) allows the accumulation of charge on the surface which in turn causes adsorption of the counter-ions ( M + n ) in the DDL: (4) M. A. Cook and J. C. Nixon, THISJOURNAL, 54, 445 (1950). (5) M. A. Cook and A. W. Last, Utah Engineering Expwiment Station BuUetin No. 47, 40; No. 14 (May, 1050). (6) G. A. Last and M. A. Cook, “Theory of Collector Depressant Equilibria,” Tnre JOURNAL, 66, 637, 643 (1952). (7) M. E. Wadsworth, R. G . Conrady and M. A. Cook, THIS 55. 1219 (1951). JOURNAL, (8) M. A. Cook and M. E. Wadsworth, Utah Enoineenng Experiment Station Bulletin No. 61, 41; No. 9 (May, 1951), part I. (9) J. A. Bishop, THIS JOURNAL, SO, 6 (1945); 54, 697 (1950). (10) R. IC. Kunin and R. J. Myers, ibid., 51, 1111 (1947) (11) H. Jenny, Colloid Sci., 1. 33 (1945)
L
A MECHANISM OF CATION AND ANIONEXCHANGE CAPACITY
Jan., 1953
MMO pointed out that the dialyzed clay is really not a H + clay. They show that H + must be adsorbed, because of ionic dimensions, as a hydronium ion H30+. The 4-coordinate structure of aluminum thus formed with H30+ as the positive ion is not a stable structure and shifts over to the 6coordinate hydragillite form of alumina, the molecular composition being unchanged in this shift. Anticipating the results of the present treatment oiie finds additional evidence for this shift from potential energy considerations. It will be shown that the OH- adsorption potential on silica-alumilia is in the range 12-14 kcal., or at least has this \ d u e as a minimum. The DDL adsorption of H + would amount to less than 3 kcal. Hence, the bum of the CDL adsorption of OH- and DDL adsorption of H + probably is less than the potential for formation of water from OH- and H+, unless the lower limit given above differs by more than 3-5' kcal. from the true value which seems unlikely. Hence, the H+ clay should be unstable. Thus, if the counter-ion to the 8 0 H - site were a H+, either mater would form regenerating the S site, or the H + would chemisorb to eliminate the surface charge depending on whether the potential for the latter reaction were less or greater than that for free water formation. The work of MMO seems to establish the latter. The hydroaluminum silioate is acidic and one must, therefore, take into account the Zeaction (bWH)
eSOH- + H +
(6)
The BEC is related directly to the concentration of SOH- sites which by virtue of their ( - E ) charge interact electrostatically with the hydrated counterion M + ? b with a negative potential energy approximately -ne2/DF. (D is the dielectric constant and Y the average or effective distance between the CDL and DDL). The formation of this DDL structure is described by the equation n(mH-)
+ AI+" Jr (SOH; ...JT+")
(7)
Equations (5), (6) and ( 7 ) , together with the surface site balance equation So
=
(S)
+ ( m H ) + (SOH~T...&I+")+KOH-
(8)
constitute the conditions determining the BEC. The actual exchange reaction (2) may be applied iiccura tely only when all of these conditions are simultaneously determined. Anion Exchange Capacity (AEC).-While detailed surface structure relations are unknown in this case the assumption is made that the same factors apply in determining AEC as in the BEC of zeolites. The equations in this case may be written as follows: For the CDL the important reaction in the polyamine resins is
__
R+H+=RH+
(9)
The surface charge may be removed by the reactioii (RHOH)
m++ OH-
(10)
implying the instability of t,lie DDL when the counter-ion is the OH-. This is the exact counterpart of the non-existence of the H + clay discussed by MMO. The third equation for AEC is the
3
DDL formation with exchangeable aiiioiis, ~i:tmoly nFH+
+ A-"
(m,?.. . A ( - ' & ) )
(11)
The surface balance equation is, as before Ro
=
(R)
+ ( W H ) + (RH?.
,
,A(-"))
+n H +
(12)
Finally the anion exchange reaction is
. m(RHf.. .A(-"))
+ nB-m
+
n(FH$. . .B(-"&)) ~ L A - " (13)
The g Function.-By introducing the appropriate equilibrium constants and solving equations ( 5 ), ( 6 ) ) ( 7 ) and (8) simultaneously one obtains
\\.here K I , K z and K3 are the equilibrium constants of eyustions ( 5 ) , (6) and (7), respectively. K , is the dissociation constant of water, e = (SOH,. . .XI+.)/ So,and y l , yp, y3 and y. are the activity coefficients of the s i t e s m H - and SHOH, (SOH,. . . M"+) and S, respectively. The equilibrium coilstants are here defined as k'l = AGa-/AsAoti& = AE+SGE-/ASH~H KI = AM+ I I A ~ ~ O H - / A Z H. ,- M ~.
+In
(15)
A plot of the left side of equation (14) (all quantities of which are measurable) against UOH- should give a straight line of intercept
under conditions where the activity coefficients are constant. The condition under which I and S are coiistants is that the total ionic strength of the aqueous sohtion must remain constant. This is not a priori ob\ious but will be shown to be true lat8erin this paper. The equations of this section all have their counterpart in the corresponding anion exchange systems. These are
where yIJ yLy3 and yr are the actirrity coefficients of the sites RHf, RHOH, (RH,+. . . A-") and R, respectively, t9 is (RH,'. . .A(-n)/Roand the K's are defined as Kl = A&?/ARAFI+ Kz = AoH-ARFI+/ARHoII K3 = AA-~A"RB+/A(G+,, . . . A-n
(19)
Base Adsorption on Clays.-The data of Marshall, et aE.,le~13 describing the titration of various dialyzed clay minerals by NaOH, KOH and ",OH (12) C. E. Marshall and W. E. Bergman, THISJOURNAL, 46, 52, 327 (18423. (13) C . E. Marshall and C. A. Krinbill, $bid., 46, 1072 (1942).
M. A. COOK, I. B. CUTLER,G. R. HILL,M. E. WADSWORTH AND A. G. OBLAD
4
were studied in detail by Cutler and C00k.l~ For these bases equation (14) becomes
TABLE I CONSTANTS O B Ea. 14a VOR BASEADSORPTION BY CLAYS
where Kt3= K3y3/y1, K’z = K2y2/y1 and K’1 = K1ys/yl. Values of (STH-. . . M f ) were obtained by means of membrane electrodes12 and the original concentration of base before clay additions; values of SO were determined semiempirically by Cutler and Cook.14
-
-6
0
10.0%Kaolinite titrated with KOH 0
f
-
- -7 8 b ? =
I1
II
. I1
I1
.
- I
9
-9
1 95”
-11
- 12 -9
hIinera1 Kaolinite Kaolinite Kaolinite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Beidellite Beidellite Beidellite Beidellite Beidellite Beidellite Beidellite Beidellite Illite Illite Illite Illite Illite Illite
-
-gl-
-8
-7
I
I
-6
-5
+
I
-4
-3
log (aOHKO). Fig. 1 .-Titration data of dialyzed Kaolinite, plotted according to the logarithmic forin of equation (14a).
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Suspension ooncn.,
%,
10.0 10.0 10.0 2.8 1.0 3.28 2.0 1.31 3.0 .l.O 0.5 10.0 7.0 11.42 4.45 1.14 8.0 5.0 1.0 10.0 5.0 10.0 5.0 10.0 5.0
S O (Ineq. per Base NaOH KOH NHiOH NaOH NaOH KO H KOH KOH NHiOH NHdOH NHiOH NaOH NaOH KOH KOH IiOH NHiOH “4OH NHiOH NaOH NaOH KOH KOH N Ha0 H NHaOH
100 g. K’n x clay) 103 3.0 0.6 3.0 0.6
3.0 100 100 100 100 100 150 150 150 95 95 95 95 95 95 95 95 35 35 35 35 35 35
0.45 5.0 0.6 5.0 2.5 1.6 8.0 1.6 8.0 10.0 5.5 9.0 3.5 1.0
3.2 1.9 0.25 1.4 0.55 2.5 1.0 1.0 1.0
K w / K ’ ; 41/K 1 (X 109 100 8.3 4 . 5 X 10’ .4.0 5.0 2.0 4.0 6.2 0.5 2.5 5.0 1.0 9.0 1.1 3.0 10.0 3.0 5.3 40.0 43.0 90.0 4.0 13.0 10.0 10.0
sible to evaluate the separate components in the first term of eq. (16) with the information so far developed. However, i t may be possible to do so by carrying out appropriate calorimetric measurements. Anion Exchange Capacity in Polyamine Resins. -For univalent anions in the DDL equation (18) becomes
+
Table I lists the value of SO,K’3 and (Kw/K’2 /PI)obtained from plots made according to equa-
tion 14a. I n all cases the data were found to conform to this theory. A sample plot MOLARITY OF BUFFER according to equation 14a is given in 0 ,453 Fig. 1 for the titration of kaolinite by o ,227 0 .04 KOH, NaOH and NH40H. 5 4 0 .02 The observed magnitude of K’3 is essen- X I tially correct for the DDL formation potential, assuming that y3/y1 1.0. If it is 3 assumed that KW/Kt2
