The effect of sorption on the ionic conductivity of zeolites

£ inorg, nucl. Chem., 1976, Vol. 38, pp. 2091-2095. Pergamon Press. Printed in Great Britain

THE EFFECT OF SORPTION ON THE IONIC CONDUCTIVITY OF ZEOLITES D. VUCELIC and N. JURANI(~ Institute of Physical Chemistry and Institute of Chemistry, Faculty of Science University Belgrade, Yugoslavia (Re(ei~'ed 31 January 1975) Abstract--The effect of the sorbed molecules C2H2, C6H6, CH3COCH3, H20, CH3OH, CH3CH2OH, CI-ICL, butadiene 1,3, cyclohexane, etc. on the electrical conductivity of X zeolites was investigated in the temperature range 150-370°K. In all cases the sorption of the molecules leads to an increase in the ionic conductivity and to a change in the activation energy. The activation energy depends slightly on the degree of surface coverage. Depending on the kind of molecule, two processes may occur: a low-temperature process with an activation energy of about 30 kJ/mol and a high-temperature one with an activation energy of the order of 50 kJ/mol. It turned out that these results are in agreement with the hypothesis of a"free ionic conduction zone" which was proposed earlier for an anhydrous zeolite.

INTRODUCTION THE SORPTION of various molecules on a zeolite was found to affect the conductivily and state of cations at the surface. In hydrated samples the number of cations the site of which can be determined by X-ray crystal analysis decreases. The electric conductivity of hydrated samples increases, but there are large discrepancies in the absolute value of the conductivity [2, 3,11 ]. N.Stamires found that as the number of water molecules in a zeolite cage increases the activation energy decreases from 12 to 6 kcal/mol. He obtained similar results for the sorption of ammonia[2]. On the contrary, Beattie and Dyer[4] report a jump-like increase in activation energy with increasing hydration. Glazun et al. found that increasing hydration first leads to a decrease and then to an increase in the activation energy [5]. On the basis of dielectric measurements Jansen and Schoonheydt conclude that a partial hydration favours diffusion through site 111. For completely hydrated zeolites they found activation energies of 8-14 kcal/mol, depending on the kind of zeolite[6, 14]. For other sorbates only scarce experimental data are available. Stamires measured the electric conductivity of X-zeolite with sorbed acetonitrile, 2,2-dimethyl butane and triethylamine. For the first sorbate he found an increase in conductivity, whereas for the other two sorbates he observed a decrease in conductivity with an increasing degree of surface coverage [2]. A specially designed apparatus[7] enabled us to determine isosteric activation energies over a wide temperature range, whereby a fuller insight into electric conductivity of zeolites with sorbed molecules is gained. In a previous paper[8] the electrical conductivity of an anhydrous synthetic zeolite was considered. The electrical conductivity was attributed to the motion of counter. ions in a "free conduction zone". A further support to this assumption is given in the present paper, which reports the results obtained with some polar and non-polar adsorbates. EXPERIMENTAL In this experiment use was made of an apparatus with metallic spirals which serve at the same time for measurement of sorption and as electrodes for measurement of conductivity. The details of the apparatus have been described previously[7,8]. A zeolite sample was prepared by the standard procedure. Union Carbide

4A and 13X powders were pressed into appropriate tablets. The edges of the tablets were coated first with a layer of colloidal graphite and then with a layer of colloidal silver to which the ends of the metallic spirals were sealed on. The samples were degassed for 24 hr at 400°C and under a pressure of about 10 'mm Hg. After cooling the sample was brought into contact with adsorbate vapours by means of teflon valves in the absence of air. Sorbates of p.a. purity were further purified through columns with 3A. 4A or 5A zeolite pellets, depending on the kind and size of the molecules to be sorbed. Temperature was measured with a ChromeI-Alumel thermocouple immediately below the sample. For measurement of electrical conductivity use was made of a standard bridge at 2.5 kHz. The degree of surface coverage was taken to have the values of 153 and 180 mg of H20 per gram of the 4A and 13X zeotites respectively obtained by the BET method. For other sorbates it was given in percentages by weight. RESULTS The effect of sorption of various molecules on the electrical conductivity of zeolites was investigated. It was found that the conductivity of zeolites depends in all cases on the sorption of molecules, i.e. on the kind and number of the molecules sorbed. These results are presented in Fig. 1. The conductivity of zeolites first increases with the number of molecules on the surface, and then attains saturation. In the case of sorption of chloroform, butadiene, benzene, ether and cyclohexane, saturation is attained much more rapidly than in the case of acetone, ethyne and ethanol. For the molecules of water and methanol, which in the liquid state have the highest conductivity, no saturation could be achieved. In general, the higher the conductivity of molecules in the liquid state, the higher is the conductivity of the zeolite with the corresponding sorbed molecules. However, the difference in specific conductivity of a zeolite with different sorbates is less than two order of magnitude, whereas in the liquid state this difference is even larger than 10 orders of magnitude (water, cyclohexane). Results for a sorption of - 1 8 % are presented in Table 1. The second column of Table 1 shows the conductivity of a zeolite sample with 180 mg of sorbate per gram of sorbent. The difference o--o-: (the total conductivity minus the conductivity of the anhydrous zeolite) may be ascribed either to the contribution of the molecules of the sorbate itself, or to the increase in the number of

2091

2092

D. VUVELICand N. JURANIC Table I. The effect of sorbed molecules on the electrical conductivity of 13X-zeolite H20 Kind of

molecules CH3 OH

Change in the speciflc con-

Conductivity of NaX zeoHte at

ductiv[ty of NaX zeolite at 25°C -lcm-]

Conductivity at 25oc calculated

25oc calculated per molecule in per sorbed rote- the liquid s t a t e cule ~-Icm-1/r~lec" ~-lcm-I/rnolec.

H20

8x10-8

2,11x10-28

2x10-30

CH30H

7x10-8

3.35x%0.28

5x10-30

CH3CH20H

2xi0 "8

1.36xl0 -28

10-30

(CH3)2C0

]xlO -8

0.86xt0 -28

10 -30

CHCI3

0.23x10-8

0.40x10-28

10-30

C2H2

0.55x10-8

0.21x10-28

8utad[ene

O,lxl0 -8

0.16x10-28

Benzene

0,2xl0 -8

0.20x10-28

10-38 10-34

(C2H5)20

0.18x10-8

0,20x10-28

7x10-36

cyc,oho ....

o.,x,o-8

o.,,xlo-28

;~:]~

-8.0

_

,

CH,C[3

. - - - - - o - CH 2 = C H - CH= C H2 C6H6 C2 H5-O-C2 H5 ~

C6H12

Zeolite -9. 0 ~ - ~

~

~

12

16

2~0 Weight°/o

Fig. 1. Logarithm of the conductivity of NaX zeolite (at room temperature) with various sorbates as a function of the sorbed quantity. counterions--the carriers of current. The values of the relative specific conductivities in the liquid state vary from author to author within one order of magnitude[12, 13], hence in further observations they were taken with an accuracy to within the order of magnitude. Comparisons made on the basis of data of column 2 are not quite correct, because 1 c m 3 of the zeolite not only contains a considerably lower number of molecules than 1 cm 3 of the respective liquid, but it also contains different quantities of various sorbates depending on the size of their molecules. A correct measure of the effect on the conductivity is afforded by comparing a molecule on the

2oo,

surface with a molecule in the liquid, columns 3 and 4 of Table 1 refer to these cases. The effect of water sorption on electrical conductivity was investigated for LiX, NaX, KX and KA zeolites in great detail. In all cases the qualitative picture remains unchanged regardless of the kind of zeolite. A typical diagram is shown in Fig. 2. For 0 - 0 . 2 an inflection characteristic of the anhydrous zeolite may still be conjectured. However, begining from 0 - 0.3 the inflection already disappears. Subsequently sorbed portions of water affect only slightly the slope of the curves. For 0 - 1 and for all values 0 >1 beside this process in conduction one more process occurs with a very low activation energy which is lower, the higher the degree of surface coverage. Quantitative results are presented in Table 2. Column 3 of the table gives activation energies which appear for any degree of surface coverage. Their values lie between those of the low-temperature (values in parentheses) and high-temperature activation energies for the anhydrous zeolite. Column 4 gives the values of the activation energy which appear beginning from 0 - 1 and which evidently decrease with increasing 0, approaching the values for the corresponding solutions. Of the molecules from the lower part of Table 1, which ToK

3?0

2so,

_~-

~oo

0=1.4

8 =0

,

B=0.9 8=0.7

-1C

o/

;

g

Z

5

,ooo

5

T

Fig. 2. Logarithm of the conductivity of KX zeolite for different degrees of surface coverage as a function of reciprocal temperature.

2093

The effect of sorption on the ionic conductivityof zeolites Table 2, Dependence of the activation energy on the degree of surface coverage with H20 moleculesand on the kind of zeolite Kind of z e o l i t e

13X - Na

•

El+ - 3 kd/rnol

£2+ 3 kJ/t~o|

o

57(30)*

-

o. I

4~

o

57

29

28

Q,3 0.6 0.8 1.0

42 45 42 44

6

46

33

13

10

47

34

i3

32

]2

50

35

15

l.t

t~2 42 43

27 19

14

53

37

16

8

16

1.2 ] .3

0

45 72(30)*

0.1

43

0.25

47

0.65 O.95 I,] I, 3

53 5~ 48 ~8

0

62(21)*

0.3 0.5

35 36

0.7 0.9 1.0

38 40 45 43 38 48 48 50

.5,

13X-L[

13X-K

I .I

1.2 0.8 3A-K

Table 3. Activation energy for conduction when ethyne is sorbed on 13X-Nazeolite

1.0 1.1

5orbate

ethyne

benzene

19 19 12

30 22

1.2

54

23

58

21

• Low temperature a c t i v a t i o n energy of dry zeol ire,

equally but considerably less then those from the upper part affect the conductivity, only ethyne and benzene were investigated. Especially ethyne was investigated in great detail,'and the corresponding results are presented in Fig. 3. The shape of the curves is similar to that for a completely dehydrated zeolite, even though the differences are considerably less and gradually disappear as the sorption of ethyne increases; for a sorption of 14-16% already quite a straight line is obtained. The corresponding activation energies are given in Table 3. The high-temperature activation energies are lower and the low-temperature ones higher than those for the anhydrous zeolite, the value of E, increasing with increasing degree of surface coverage. .~-

250

!

E2+_4 kJ/tool

E1+-4 k J/tool

E=E2*E1 k J/tool

35 lz

35

47

4

Finally we have to recall the investigation of the behaviour of LV counterions in a LX zeolite by means of lithium nuclear magnetic resonance reported in a previous paper[8]. It was found that in the anhydrous LiX-zeolite beside slowly moving Li+ ions there is a certain number of highly mobile Li+ ions. The number of the mobile ions rapidly increases with sorption of water, 2-propanone and benzene. The NMR spectral line belonging to slowly moving Li ÷ ions disappears completely only for high degrees of surface coverage and after a large lapse of time.

37 33 22

1.3

~

DISCUSSION AND CONCLUSIONS From the results presented more or less definite conclusions may be drawn as to the effect of sorption on the electrical conductivity of zeolites. It is evident that the sorption of various molecules in all cases leads to an increase in the electrical conductivity (Fig. 1 and Table 1). It may be seen that Table 1 can be divided into two parts. In the lower part of the table there are sorbates which also in the liquid state have'low specific electrical conductivities: cyclohexane, diethylether, benzene, butadiene and ethyne, for which the effect (calculated per molecule; see column 3) on the electrical conductivity is almost completely independent of the kind of molecules sorbed. This shows that they are not carriers of current, but that they only affect in the same manner the increase of the electrical conductivity of counterions in the zeolite. In the upper part of the table there are substances the conductivity of which in the liquid state is higher by

290

300

To K

~

,

~b'

.//~ 1°°/°

-;"°//~

-701

I

/ eo/o

o o/o

-80}

! -90 ~

o

T

Fig. 3. Logarithm of the conductivity of NaX zeolite for different quantities of sorbed ethyne as a function of reciporcaltemperature.

2094

D. VU~ELI~and N. JURANIC

several orders of magnitude. The effect of sorption of these substances on the electrical conductivity of the zeolite (calculated per molecule; column 3) depends on the molecular species and follows exactly the order of their conductivities in the liquid state (column 4). This result shows that these molecules are involved in part in the conduction process. It is evident that they are not the only carriers of current, otherwise the difference between the electrical conductivities of zeolites with substances from the upper and from the lower part of Table 1 should be larger by several orders of magnitude. In the light of these conclusions the difference in behaviour of these molecules for different degrees of surface coverage (Fig. 1) is clear. Molecules which do not directly take part in conduction, after attaining a definite number on the surface of the zeolite no longer affect the mobility of counterions. For such molecules the existence of a saturation plateau is evident. Molecules which themselves take part in conduction, after ceasing to exert influence on the counterions lead to an increase in the conductivity, because as their number increases so does their own contribution to the electrical conductivity at the surface. Investigations with sorbed water (Table 2 and Fig. 2) show that water removes the difference between the low-temperature and high-temperature conductivities which was observed in anhydrous zeolite, as has been reported previously[8]. The activation energy does not vary considerably for different countereions at a given degree of surface coverage and amounts to about 40 kJ/mol, which is in agreement with the value found by Jansen a n d Schoonheydt[14]. Nevertheless, a slight minimum may be observed in the activation energies at 0 ~ 0.4, which is consistent with the result obtained by Glazun et al.[5]. For higher degrees of surface coverage and at a higher temperature another mechanism of conduction with a low activation energy is evident. The activation energy for this process decreases with the degree of surface coverage, and for 8 ~ 1.5 becomes

practically unobservable. At these temperatures and degrees of surface coverage a conduction process identical with that in a solution of given counterion of corresponding concentration is probably predominant. The effect of sorption of substances from the lower part of Table 1 is much weaker, as is shown in Fig. 3 and Table 3 for the sorption of ethyne. The basic characteristics of electrical conduction of the anhydrous zeolite are preserved, and only the respective activation energies change. When ethyne is sorbed the corresponding low-temperature activation energies increase considerably with the degree of surface coverage (15 kJ/mol). The high-temperature activation energies are as usual in the case of sorption lower than those for the anhydrous zeolite. Taking into account all these results and the NMR data on lithium relaxation, where it is evident that sorption leads to an increase in the number of mobile counterions, we may examine them from the point of view of the hypothesis of a "free cationic conduction zone" which has been proposed previously[8] (it is schematically presented in Fig. 4). The conductivity of a zeolite on which molecules are sorbed having in the liquid state a low specific electrical conductivity is still exclusively cationic. Sorption of such molecules reduces the interaction of counterions with the aluminosilicate framework, thereby increasing in the statistical distribution the number of molecules in the free cationic zone at the center of the zeolite cage. Owing to sorption the difference between the free and bound cationic levels is thus reduced. Further sorption in the region of the multilayer probably has little effect on this difference, but this is difficult to notice within the limits of error in measurement. A good indication that this statement is valid is the constant difference Er-E, (Table 3, column 5). The conductivity at lower temperatures is still determined by the activation energies for diffusion through the zeolite framework. The fact that the activation energy is higher and that it increases with the

( Cal/mol ) 17 16 15 I/, 13

c o

12

~3

"g

H

~ ~ :6

o u

DIFUSSION THROUGH ZEOLITE STRUCTURE

-o~c 0

10 g

o ,~

~,=,>, "

~

.o =-

~,~-

~,.

S 7

Free c a t i o n i c level in zeolite cage

6 Basic

NaX zeolite

"--wqffff "~-2 ~ - - s~FB ~d-

5 t,

c a t i o n i c level of

:li °

w

REGION OF EXCITATION

g

Basic with

cationic ethyne

Basic

cationic

level of NoX zeotite sorbed

'~

Level of d r y

Fig. 4. Schemeof the electricalconductivityof zeolitein the case of sorptionof H:O and ethyne.

NaXzeolite

The effect of sorption on the ionic conductivity of zeolites degree of surface coverage is evidently due to sorbed molecules which hamper diffusion by filling the otherwise open zeolite framework. The high-temperature process involves excitation from the bound level to a free level and further diffusion through the framework. Because the rise of the cationic bound level due to sorption of molecules is larger than the increase in the activation energy for diffusion the total activation energy in this region is lower than that in the anhydrous zeolite. The molecules of water, alcohol etc., which are characterized by high heats of sorption, (i.e. by a high energy of interactionn with the framework), reduce even more the interaction of counterions with the aluminosillicate framework as already suggested by Jansen and Scoonheydt[14]. The cationic bound level becomes so close (of the order of 3/2kT) to the free ionic level that the mechanism of conduction with a preceding excitation is unnoticeable. Because of the interaction of counterions with these molecules it may be expected that the activation energy (with respect to other molecules) for diffusion through the zeolite will be increased as has been assumed by Glazun et al.[5]. The increase of the activation energy as the degree of surface coverage of KX-zeolite increases (Table 2) is a good indication that the above conclusion is valid. Water and other substances of this class in a multilayer probably begin to conduct predominantly as in solutions of salts of corresponding concentration, as has already been assumed by Barrer[9.10] and Beattie[4]. In conclusion it may be

2095

assumed that a similar mechanism of electrical conduction might exist in some cationic ion exchangers. However, for such investigations it is necessary to improve the technique of measurement, since many fine details and reduced effects escape notice because of the relatively large error in measurement. REFERENCES 1. W. J. Mortier, H. J. Bosmens and J. B. Uytterhoeven, J. Phys. Chem. 76, 5 (1972). 2. D. N. Stamires, J, Chem. Phys. 86, 12 (1962). 3. B. Morris, J. Phys. Chem. Solids 30, 103 (1969). 4. I.R. Beattie and A. Dayer, Trans. Faraday Soe. 53, 6l (1957), 5. B. A. Glazun, I, V. Zilenkov and M. F. Rakitkjanskaja, Zh. Fiz. Khim. 43, 2397 (1969). 6. F. J. Jansen and R. A. Schooneydt, J. Chem. Soc. (Faraday I) 69 (1973). 7. N. Juranid. D, Karaulid and D. Vu~elid,J, Therm. Amd. 7, 119 (1975). 8. D. Vu~eliC N. Juranid, S. Macura and M. ~;u~id, J. lnorg. Nucl. Chem. 37, 1277 (1975). 9. R. M. Barter and E, A. Saxon-Napier, Trans. Faraday. Soc. 56, 709 (1960). 10. R. M. Barrer and L. V. C. Rees, Trans. Faraday Soc. 58, 709 (1960). 11. B. Morris, J. Phys. Chem. Solids 30, 73 (1%9). 12. Handbook for Chemistry, p. 937. Vol. 1, Chemistry, Moscow (1%6). 13. Landolt-B6rnstein, Zweiter Band, 7. Teil. p. 15. Springer, Berlin (1960). 14. F. J. Jansen and R. A. Schoonheydt, The Third Inter. Conf. on Molec. Sieve 96 (1973).