Ion-exchange properties of weakly hydrated, crystalline tin dioxide: Ion-exchange of Cu2+, Zn2+, Co2+, Ni2+, Mn2+

L inorg, nucl. Chem.. 1978, Vol. 40, pp. 539-544. Pergamon Press.

ION-EXCHANGE

Printed in Great Britain

PROPERTIES

CRYSTALLINE OF

TIN

OF

DIOXIDE:

WEAKLY

HYDRATED,

ION-EXCHANGE

C u 2 + , Z n 2 + , C o 2 + , Nl'2+,

Mn2+

N. JAFFREZIC-RENAULT Groupe de Recherche de Radiochimie Analytique, C.N.R.S.-Laboratoire "Pierre Sue", C.E.N./Saclay B.P. No. 2, 91190-Gif sur Yvette, France

(Received 7 February 1977) Abstract--We have studied the effect of thermal treatment of tin dioxide (water content, specific area, Brtnstedt acidity). According to published data and our results, the Brtnstedt acidity of this hydrous oxide may be attributed to the surface hydroxyl groups. We have followed the retention of divalent transition elements by a tracer technique. In accordance with an ion-exchange model a thermodynamic study has been made and we have established that divalent ions exchange with part of the acidic protons in a reversible equilibrium, with stoichiometry 1M2+: 1H+. The thermodynamic constants of this exchange follow the order of stabilities of the M2+-Obonds. A study of the solid before and after the retention of the divalent ions has been conducted by ESCA and TGA. Our observations are in agreement with an M2+-O- bond and the divalent elements are retained in solvated form. It seems that the ion-exchange model fits the retention of divalent ions on tin dioxide.

INTRODUCTION Tin dioxide belongs to the family of the quadrivalent element oxides AO2 (SIP2, MnO2, Ti02, SnO2, ZrO2, Th02). These hydrous oxides show acidic and ionexchange properties[l] due to Br6nsted acidic groups. They behave either as cation-exchangers or anionexchangers, depending upon the basicity of the central atom and the strength of the M-O bond relative to that of the O-H bond in the hydroxyl group, if the hydroxyl groups are the only Br6nsted acidic groups present. The equilibria may be written as follo~vs:

with the pH of the medium. Thus, the distinction between an ion-exchange mechanism and a hydrolysis model is difficult to establish. In this work, we studied the thermal treatment of tin dioxide prepared by Carlo-Erba Society (Italy). In accordance with the ion-exchange model a thermodynamic study was made; the stoichiometry of the ion-exchange reaction and the formula of the complex after ion-exchange was established. A study of the solid before and after the retention of the divalent ions was conducted by ESCA and TGA; our results are in agreement with the exchange model.

EXPERIMENTAL Determination of the Brtnsted acidity The principle of the titration is the reaction:

The acidity scale of these different oxides is established by comparison of the pH of the isoelectric points (compare Table 1). We notice that the acidic strength decreases in the order SIP2, MnO2, SnO2, TIP2, ZrO2, ThO2 and so do the capacities for cation exchange in basic-media. The values of the pH of the isolectric points depend on the structures of the oxides and especially on their crystallinity. The divalent transition elements are easily retained in acidic media although these oxides are anion exchangers in such media; these elements are hydrolysable and have a great affinity for the oxygen atoms of the hydroxyl groups. The ion-exchange reaction may be represented:

and we determine the quantity of acetic acid released. The method used has been perfected by DeMourgues[13]: two grams of solid are equilibrated with 50 ml of 1 N ammonium acetate solution, by shaking for 30 rain. After separation of the solid from the solution by centrifugation, the pH of the solution is measured. A correlation between the pH and the quantity of the acetic acid released may be determined by means of a previously established calibration curve. The calibration solutions are also shaken and centrifuged so that they have the same degree of carbonation as the sample solutions. Since the uncertainty of the pH measurements is about 0.02, it involves a relative error of about 10% in the values of the acidity.

M2+ + ( ~ A _ _ O H ) . ~<._.~___]-/ ~

Determination of the stoichiometry of the ion-exchange reaction and the distribution coefficients (KD). The equilibrium is

\

](2-m)+

+

+.

+

CH3COOH solution+ NH4s~o2

obtained by shaking 50 ml of 0.1 N NaNO3 solutions (constant ionic strength) at a concentration of 10-4M in the labelled elements, with two grams of tin dioxide; the pH is fixed by additions of nitric acid or soda. The shaking is conducted for two hours at 20°C-+1°C. The radioactive tracers 64Cu*, 69mZn*, 6°Co*, 65Ni*,56Mn*are obtained by irradiating a part of the salt used (nitrate) in the EL3 Reactor of the C.E.N. Saclay-France (flux: 5 × 1012n cm-2 s-%. After separation of solid and liquid by centrifugation, the radioactivity A s is measured in an aliquot of the solution by a scintillator NaI (TI) connected to a multichan-

The stoichiometric coefficient m has been found to be equal to 2 for TiO216], ThO2[6], ZrO217] and MnO218, 9]. For SiO2[10] and MnO2[ll, 12], it has been found to be equal to 1; according to VYDRA[10], the ion-exchange reaction on SiP2 occurs with the MOH ÷ groups. However these stoichiometric coefficients are calculated from the variation of the exchanged quantity 539 JINC Vol. 40, No ~--L

~

H sno2+ CH3COONH 4~,u,ion

540

N. JAFFREZIC-RENAULT

Table 1. Ph of the isolectric points of different tetravalent oxides Natures of the oxides

Structures

pHi

Refs.

SiP2 MnO2 Hydrous precipitate MnO2 /3 MnO2 S^ /Hydrous precipitate

1.8 2.25 4.6 4.5

[2] [3] [4] [5]

nu2~Heated T._ / H y d r o u s

at 100°C precipitate

5.5 6.0

[5] [5]

lt)2~Heated ZrO2 ThO2

at 100°C

4.7 6.05 7

[5] [6] [1]

nel analyser. It is compared to A0 the radioactivity of the initial solution. The measurements of the pH is made at equilibrium. Determination o[ the capacities/or retention. They are calculated according to the Glaeser's method[M]. The experimental conditions are the same as those previously described. Physical measurements. Thermogravimetric analysis: the apparatus is a CI Microbalance, Thirty grams of the sample are heated in the air; the rate of heating is 5°C per min. The error on the water content is about 5%. Specific areas: they are determined from the nitrogen adsorption isotherm, according to the BET method. The apparatus is called "Isotherme-graphe" [ 15]. E.S.C.A.: the apparatus used is an IEE VARIAN spectrometer. The source is the K~ emission of the magnesium (energy: 1253,6eV). --Estimation of the quantities of retained ions[16]: Only the ions located near and in the surface layer are detected. The relative intensity coefficients are determined for the nitrates of the transition metals; they are defined by the ratio between the intensity of the 2p peak of each element and that of the ls peak of nitrogen. Taking Sn 3d peak as reference and using the intensity coefficients,we calculate the capacities of Cu, Zn and Ni relative to that of Mn (= 1 arbitrary unit). ---Characterization of the chemical environment: it is necessary to compare bond energy of the 2p electrons in hydroxides, oxides, sulfates, nitrates of the metals. The bond energy of the 2p electrons decreases following the order: nitrate, sulfate, hydroxide, oxide depending on the electronegativity of the environment. RESULTS AND DISCUSSION

The effect of thermal treatment o/the tin dioxide Several samples have been selected for us by CarloErba Society (Milan-Italy). The initial product is a stannic acid precipitate prepared according to an EURATOM

licence. Each part of the precipitate is heated at a different temperature (preparation temperature) for 6 hr at ambient pressure. The thermogravimetric analysis of these products shows two losses of weight: the free water is removed between 25 and 200°C [17]. The hydration water is removed between 200 and 850°C; the nature of this water has been studied by IR spectroscopy[18, 19]. We have established a linear relation between the concentration of the hydration water (E) and the specific area (S) of the samples: E = (1.49 x 10-' S) - (1.7 x 10-4) E (mole per g of SnO2), S (m 2 per g). This seems to indicate that the hydration water surrounds the crystallities of SnO2, held in the oxide in several modes described by Thornton [18]: incorporation in the pore structure, hydrogen bonding to surface hydroxyl groups, bonding to Lewis acidic sites as unsatured Sn(IV) sites, coming from condensation of surface hydroxyl groups. We have determined the quantity of Brfnsted acidic groups by the method previously described. The results are between 4.4 x 10-4 eq per g of SnO2 for the product heated at 100°C and 5.2 x 10-5 eq per g for the product heated at 600°C. The Brtnsted acidity has not been demonstrated by IR spectroscopy[19, 20]. There are no Lewis acidic groups at the surface of the tin dioxide heated in air, as has been shown by Thornton[18] and Primet[21]. Thus, the Brtnsted acidity must be attributed only to the surface hydroxyl groups. We have established a linear relation between the Brtnsted acidity (A) and the specific area (S) of our samples: A = (1.4 x 10-6 S) + (4 x 10-') A: equivalents H ÷ per gram of SnO2, S: m2/g. Such a relation may characterize the acidity due to the hydroxyl groups, these condense when the specific area decreases, on heating. This acidity gives the cation exchange properties to these hydrated oxides.

Variation of the capacity for retention of the divalent ions. The ratios between the retention capacities and the quantity of acid protons are presented in Table 2. On

Table 2. Ratios between the retention capacities for divalent ions and the quantity of the acidic protons of the samples Sample preparation temperatures Brtnsted acidity (A) (Nb eq. H÷ per g SnO2) (Cu____)(Nb ion. g Cu2÷ per g SnO2) (A) (Nb eq. H÷ per g SnO2)

100°C

200°C

250°C

400°C

500°C

600°C

TDO

4.4 X l0 -4 3.3 x 10-5 2.8 x 10-4 8.9 × 10-5 5.2 x 10-5 5.2 × 10-s 5.0 × 10-5 0.86

0.57

0.58

0.48

0.55

0.34

0.24

0.24

0.25

(Co___)(Nb ion. g CO2+ per g SnO2) (A) (Nb eq. H÷ per g SnO2)

--

--

--

0.25

(Ni) (Nb ion. g Ni2+ per g SnO2) (A) (Nb eq. H + per g SnO2)

--

--

--

0.25

(Mn___)(Nb ion. g Mn2+ per g SnO2) (A) (Nb eq. H+ per g SnO2)

--

--

--

0.25

(Zn)(Nb ion. g Zn2+per g SnO2) (A) (Nb eq. H+ per g SnO2)

0.57 0.17

0.50

541

Ion-exchangeprope~ies of weakly hydrated, crystallinetin dioxide average, the ratio is equal to 0.57 in the case of Cu2+ ions; these are exchanged with half of the acidic protons. The other divalent ions Znz+, Co2+, Ni2+ and Mn2+ are exchanged with a quarter of the acidic protons. For analytical applications, we notice that the retention capacity decreases when the preparation temperature increases. Variation of selectivity. The selectivity in relation to two ions Mi and M2, is defined by the ratio:

Ko(M,) Ko(Mg" According to Fig. 1, it appears that the selectivity of tin dioxide, for the separation Fe3+/Cu2+, depends on its thermal treatment. The best preparation temperature would be around 350°C.

Thermodynamic study o[ the ion-exchange reaction The study has been conducted with the commercial product TDO. The ion-exchange reaction may be represented as follows: q~ ~-oz~

M 2+ solution + a H

a H s+olution+ M SnO~. 2+

Assuming that the equilibrium is reversible and calculating the activities of the ions with some approximations [22], the following relation is established: M z+]

log r~2+1 JSnO2' = •tog (K~ x [r~2+ x [ n +]s,o2X(H +)s,o,_) "- i t lvx

J solution

+apH The symbols [] and 0 respectively represent the concentrations and the activities of the ions. K~ is the corrected thermodynamic exchange constant,/M2+ is the activity coefficient of the ion M2+. If we represent the isotherms of the reactions ® and @ in the form: [M2+ls.o z

og [M2+]~o,ut,o.= f(pH). We obtain a single line, with slope equal to a and the intercept equal to the constant expression: K0×fM2+X[H ' *]s-oz × ( H+) s*-' .ov

103-

As we did for Cu 2+[22], we have also established for Zn2+ that the experimental points obtained for reactions ® and ® fall on a line. For these two ions the ionexchange reaction is reversible. We have assumed that it is reversible for Co2+, Ni2+ and MnZ+; for these ions, the isotherm for reaction ® is a straight line. The stoichiometric coefficient c~ and the corrected thermodynamic exchange constant K~ are given in Table 3. For these five divalent ions, the stoichiometry of the exchange reaction is 1 M2÷ : 1 H +. The values of K~ follow the order of the hydrolysis constants of these divalent ions[23]: Cu 2+/> Zn 2+ > Co 2+ > Ni 2+/> Mn 2+.

This is in agreement with a bond -O--M2+ being formed after the exchange. It may be noted that the value of K~ for the ion-exchange reaction of Cu 2+ on alumina 0 is equal to 1.5 x 10 3[24]. This very weak affinity of alumina for Cu2+ may be explained by the weak acidity of this hydrous oxide: the pH of the isoelectric point of this oxide is 8.7 while it is around for 5 for tin dioxide. According to our results, the equilibrium for retention of M2+ may be written: ~ S n - - O H + M2+, ,, ~ [~Sn--O---M]+ + H +. For such a process the variation of the free energy (AG) with the metal cation will reflect the chemical interactions between exchanged ions and the functional groups. KH and KM being the dissociation constants of the complex forms ~ S n - - O H and ~Sn--O--M+, the thermodynamic exchange constant K~ will follow the relation: log K~ = pKM---pKH pKM has been determined by an acid-base titration with soda[25]; it is equal to 5.1 at 20°C. The value of pKM may be calculated; our results are presented in Table 4. Table 3. Values of the stoichiometric coefficients a and of the

exchange thermodynamicconstants K~ Ion

a

K6

r regression coefficient

Cu2+ Zn2+ Co2+ Ni2÷

0.99 1.2 1.2 0.9

22 21 6.4 2.6

0.999 0.990 0.999 0.9%

Mn2+

1.2

3.0

0.997

Table 4. Values of pK M, the dissociation constants of the complex forms ~_>

Sn--O--M

and

of PKMo., the dissociation constants of the forms (MOIl)+

io

Ion

I00

200

300 400 500 600 Preparation temperature, *C

Fig. I. Variation of the selectivity with relation to Fe 3+ and Cu 2÷ versus sample preparation temperature.

Cu2+ Zn z+ Co2+ Ni2+ Mn2+

pK M

pKMo.

6.4 6.4 5.9 55 5.5

11.1 10.7 8.1 7.1 5.1

542

N. JAFFREZIC-RENAULT

K ~ o . is the dissociation constant of the form (MOH)÷. According to the values of the constants PK~, the complex forms ~Sn--O--M

Table 5. Comparison of the capacities determined by ESCAand tracer techniques

are Jess stable than the

ESCA

hydroxyl forms (MOH)÷, except for the ion Mn2÷. This enables us to exclude hydrolysis phenomenon in our experimental conditions.

Mn: 1 (u.a.) Cu: 3.5-+0.9 Zn: 1.6-+0.4 Ni: 1.2+0.3

Study of the solid alter the retention o[ the ions The results obtained by ESCA have confirmed that the retained cations were located at the surface of the samples, as were the hydroxyl groups responsible of the exchange. The apparent superficial capacities obtained by ESCA are compared to the total capacities obtained by using radioactive tracers. The results, presented in Table 5, are reported relation to the capacity for manganese equal to 1 (arbitrary units). The results of both methods are in good agreement. Thus these cations are located at the surface of the samples. The chemical environment of these cations after retention on tin dioxide may be deduced from a comparison of the bond energy of the 2p electrons in the different compounds, presented in Fig. 2. The energies for the retained cations are near those of

1623

Mn: 1 (u.a.) Cu: 2.3-+0.7 Zn: 1.1-+0.4 Ni: 1.1-+0.4

the oxide and the hydroxide for zinc, near that of the hydroxide for nickel, and near the monovalent oxide for copper. In the case of copper, the Cu 2p peak of the retained ion is accompanied by satellites characteristic of divalent copper; although it seems to be in a weakly electronegative chemical environment. Supposing that the relaxation energies are the same for all the compounds, these observations would be in agreement with the bond M2÷-O being formed during the retention of the divalent ions. It may be noticed that the bond energies of the 2p electrons for the ions retained on tin dioxide are different from the bond energies of the hydroxides which allows us to exclude the phenomenon of simple hydrolysis, which was proposed by Bilinski[26].

I 1622I

Zn 2p

Tracers

/ I

1621

Ni 2p 855

8'54

t~

®~

Mn 2p 6'42

(/l Cu 2p

Cu LMM

'TT

i

I

i 641

.I-

II

9'34

I I 3'38

BOND ENERGY (eV)

I I

+

933

I

3'36

+ 04=

I

'

9;2

I I

C ls at 284eV

Fig. 2. 2p electron bond energies for different compounds.

I

IA

3;4

543

Ion-exchangeproperties of weakly hydrated, crsytalline tin dioxide Table 6. Water content of the samples before and after saturation by divalent ions Free water Sample (preparation)

Hydration water

Evacuation temperature

Quantity

25-100°C

3.0 X 10 -4 mole/g

200-835°C

3.7 x 10-4 mole/g

25-105°C

3.3 × 10-4 mole/g

160-805°C

5.3 X 10 -4 mole/g

25-100°C

2.7 ×

10 -4 mole/g

185-960°C

4.8 X 10 -4 mole/g

25-12&C

4.9 X 10 -4 mole/g

180--905°C

6.8 X 10-4 mole/g

Sample of tin dioxide (a) equilibrated with water (b) dried in the air at 25°C for two days Sample of tin dioxide (a) equilibrated with a Cu2+ 10-2 M solution (b) washed with water (c) dried in the air at 25°C for two days The quantity of Cu~+ retained is 2.7 × 10-5 ion. g/g Sample of tin dioxide (a) equilibrated with a Zn2÷ 10-2 M solution (b) washed with water (c) dried in the air at 25°C for two days The quantity of Zn2÷ retained is 1.25× 10-5 ion. gig Sample of tin dioxide (a) equilibrated with a Co2+ 10-5 M solution (b) washed with water (c) dried in the air at 25°C for two days The quantity of Co2+ retained is 1.25x 10-5 ion. g/g

Evacuation temperature

Quantity

We note the irregular position of the Auger peak CuLMM in the high bond energies (compare Fig. 2). The presence of the solvation water surrounding the divalent ions after their retention is shown by the appearance of a shoulder on the O ls peak of oxygen, in the high bond energies. This shoulder remains, even after the sample remains in the apparatus for a long time; it shows the stability of the water held to the surface of the samples under these experimental conditions (50°C, 4 x 10 - 6 torr). Samples of tin dioxide saturated with copper, were heated in air for one hour at 900°C; the thermogravimetric analysis shows that all the water is lost above 820°C and that the removal of the hydration water is irreversible. After heating there is no alteration of the Cu 2p and Cu LMM spectra. These results show that copper is retained in a very stable form and that the bond Cu2+-solid is not disturbed by the presence of the hydration water. By means of thermogravimetric analysis, we have determined the hydration water in the samples of tin dioxide before and after the retention of the divalent ions Cu 2+, Zn2+ and Co 2÷. Our results are listed in Table 6. In all the cases, the quantity of hydration water is higher after the retention of the ions M2÷ than before. This confirms that these ions are retained surrounded with solvation water.

tion sphere of water, we propose the following ionexchange reaction:

Proposed a retention mechanism According to published data and our results, the Brrnsted acidity of the tin dioxide may be attributed to the surface hydroxyl groups. We have established that the divalent ions are exchanged with a part of these protons according to a reversible equilibrium, with a stoichiometry IM2+:IH +. The exchange thermodynamic constants follow the order of the stabilities of the bonds M-O-. Under our experimental conditions, pH = 2 to 4, the hydroxyl forms of the divalent ions are in negligible concentrations with respect to the form M2÷. Thus we may consider that the exchanged form is M2+. The results obtained by ESCA and TGA show that the divalent ions are retained in solvated form. If we consider that the ion M2+ is surrounded with one coordina-

ler to be catalytically very active in SnO2-CuO gels [2728]. Since the Cu 2÷ ion is more acidic than the ions Zn2÷, Co 2÷, Ni2÷ and Mn2÷, it could exchange with protons which would not be exchangeable with the other ions because of their weak acidity; the acidity of the protons depends on the hydrogen bonds and their solvation[29]. According to the dissociation constants of the complex forms after exchange and according to the chemical shift determined by ESCA, the model of hydrolysis does not seem to fit to the retention of the divalent ions on tin dioxide under our experimental conditions; such hydrolysis was proposed by Bilinski[26] for other oxides.

H20

H

\o/s

\o / ~)/

HzO

H20

H20 q+ /H20 I

+

H30*

6 \ Such groups - ~ S n - - O - - C u - - have been found by Ful-

Acknowledgements--The author wishes to thank Mrs. Escard and Mr. Brion from the "Centre de Recherche de l'Ircha" (Vertle-Petit, France) who have conducted the ESCA study.

544

N. JAFFREZIC-RENAULT

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