On the e.s.r. line widths of hydrated Cu2+ in zeolites

On the e.s.r, line widths of hydrated Cu 2+ in zeolites F. Ucun, F. K6ksal, and R. Tapramaz Physics Department, Faculty of Arts and Sciences, Ondokuz Mayis University, Samsun, Turkey In this study, the electron spin resonance (e.s.r.) of Cu(H20)~+ ions were used to investigate the mobility and the freezing properties of water adsorbed on the synthetic zeolites of types 3A, 4A, 5A, and 13X with pore diameters of 3, 4, 5, and 10/~, respectively, and the natural zeolites, heulandite and clinoptilolite. The temperature dependencies of the e.s.r, line widths of the zeolites with pore diameters of 3, 4, and 5/~ indicated that the first layers of water molecules from the surfaces are restricted in motion even at room temperature, and beyond these, the water molecules are mobile. The water molecules in the 13X zeolite, heulandite, and clinoptilolite behave like liquid water. Keywords: Cu ion, e.s.r.; line width; zeolites

INTRODUCTION It is known that the dynamics of water molecules adsorbed on porous systems can be studied by using metal ion complexes as paramagnetic probes. ~-3 Such studies give information about the localization and structure of surface-adsorbed complexes and about the behavior of the adsorbed liquids. In this study, we report electron spin resonance (e.s.r.) line width results of the Cu(H20) 2+ spin probe dissolved in water and adsorbed on several zeolites. Narrow-pore, wide-pore, and natural zeolites were used.

EXPERIMENTAL The synthetic zeolites of the types 3A, 4A, 5A, and 13X were purchased from the British Drug House (BDH) and the natural zeolites heulandite and clinoptilolite were obtained from the Institute of Geology, the Hacettepe University, Ankara. The unit cell content of type 4A zeolite is4-6 Nax2 [(AIO2)12 (8i02)12] 27 H 2 0 , and types 3A and 5A are the potassium- and calcium-exchanged forms of type 4A. The unit cell contents of type 13X, heulandite, and clinoptilolite, are Na86 [(A102)86 (8i02)106] 264 H20, Ca4 [(A102)8 (SIO2)28] 24 H20, and Na6 [(A102)6 (SIO2)30] 24 H20, respectively. To avoid the contributions from C u 2 + - C u 2+ dipolar interactions to the e.s.r, line width, aqueous solutions of CuCI2 of 5.10 -3 M were prepared in Address reprint requests to Dr. K6ksal at the Physics Dept., Faculty of Arts and Sciences, Ondokuz Mayis University, Samsun, Turkey. Received 21 January 1991; accepted 16 August 1991 © 1992 Butterworth-Heinemann 420

ZEOLITES, 1992, Vol 12, April/May

distilled water. These solutions were adsorbed in the zeolites of pore diameters 3, 4, 5, and 10/~ and the natural zeolites heulandite and clinoptilolite. In each adsorption, 5 g of zeolite was put into 20 cm 3 of the appropriate solution and stirred and stored at room temperature for 24 h before filtering. The surface areas of the zeolites were found to be 16, 10, 4, 37, 67, and 20 m2/g, respectively, for types 3A, 4A, 5A, 13X, heulandite, and clinoptilolite. These specific surface areas of the zeolite aggregates were determined by the BET (Brauner, Emmett, Teller) volumetric adsorption technique. The water molecules in the pores of the zeolite were removed by keeping them in a furnace at 400 K for 24 h and were kept under vacuum for 12 h at room temperature. The surface areas were measured at 77 K by measuring the volumes of the nitrogen gas at equilibrium pressures. The BET equation is given as 7's"

P

1 - -

V(Po - P)

C-1 -4-

V,,C

-

-

V,,C

P .

-

-

(1)

Po

where P is the partial pressure of adsorbed nitrogen gas; P0, the saturated vapor pressure at working temperature; V, the amount of the adsorbed nitrogen gas; V,,, the monolayer coverage; and C, a constant related to the heat of adsorption. The monolayer coverage V,, can be calculated from the linear plot of P/V[(Po - P)] against P/Po in the 0.05-0.35 range. The measurements in this interval are evaluated using a computer program, and Vm values of each zeolite were obtained. The specific surface area is equal to the number of molecules at 77.4 K given as 16.2 ,~.2 Therefore, the specific surface area is:

E.s.r. line widths of hydrated Cu 2÷ in zeolites: F. Ucun et al.

250G

4

Type-3A

drawn and the distance between the intersections of these two lines with the base line is taken to be half of the peak-to-peak line width, as shown in Figure 1. E.S.R L I N E W I D T H I N S O L U T I O N

Type-4A

T h e e.s.r, line width of any line of quantum n u m b e r ml is given byg: AH(ml) = ix' + a," + [3ml + ym2

Type-SA

(3)

where ct' represents essentially the spin-rotation contribution and the last three terms represent the anisotropic dipolar and g tensor interactions• An approximate expression for 0~' is I°'l l:

13X(10~) 0L" ~ - -

~ g 2 .UN = - -

3

Heutandite

(4)

9~c

In this expression, 5 g 2 = Y'i(gi -- ge) 2 and ge is the free-electron spectroscopic splitting factor; K, the molecular m o m e n t of inertia; ~U, the spin-rotational correlation time; and ~c, the reorientational correlation time. T h e other parameters in Equation (3) are2: 2h

CLinoptiloLite

1 • { - - ( A g ~ o H / h ) 2 [4~c + 3~J(1 + co2zY)] 45 +.

Figure 1 The e.s.r, spectra of Cu(H20)~ + ions in all the zeolites at room temperature. The drawing on the lower left side indicates the form of half of the line width

V,,,N 10 -20

(AA/h) 2 [3~c + 7"~fl(1 + ~02"~2)]}

(5)

24

J

S(m2/g) -

1

16•2

(2)

1 • {--~-(AgAAH[3o/h 2) [4~ + 3t J(1 + ¢o~2)]} (6)

y = _~o(AA/h)2

[5.Uc __

1:c](1 + t02~)]

(7)

22414 where N is Avogadro's number. T h e measurements were carried out only for adsorption isotherms, and desorptions were not studied• T h e e.s.r, spectra were recorded by an X-band Varian E-109 C model spectrometer equipped with a Varian t e m p e r a t u r e control unit. T h e g values were determined by comparison with a D P P H sample of g = 2•0036. T h e peak-to-peak line width measurements were made from the right half of the perpendicular c o m p o n e n t of the first derivative of the C u ( H 2 0 ) 2+ spectra, assuming that only this part was undistorted by the line of the free complex• U n d e r this assumption, a tangent to the descending side at the point of intersection with the base line o f the right half of the first derivative spectrum and a perpendicular line to the base line that passes through the peak point is

where Ag = gll - g± and AA = All - A± are anistropic g and hyperfine interactions, respectively; H, the magnetic field; C0o, the electron spin resonance frequency; and ~o, the Bohr magneton. T h e ml independent part of Equation (3) is: H(0) = ct' + a/'

(8)

and this indicates the separability of 0t' and az', since at high temperatures, the spin-rotational mechanism (ix'), and at low temperatures, anisotropic g and hyperfine interactions (a;') are effective in broadening of the e.s.r, lines• RESULTS AND DISCUSSION T h e room temperature e.s.r, spectra of Cu(H20) 2+

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421

E.s.r. line widths of hydrated Cu 2+ in zeolites: F. Ucun et al. Table I The e.s.r, parameters for the zeolites of pore diameters 3, 4, 5, and 10 A and the natural zeolites heulandite and clinoptilolite at 133 K Zeolite

OoO Oo

Pore diameter (A)

All (G)

gll

g±

3

130

2.40

4 5 10 -

115 130 120 130 138

2.39 2.39 2.41 2.40 2.38

2.10 2.10 2.09 2.11 2.10 2.09

Type 3A Type 4A Type 5A 13X Heulandite Clinoptilolite

D E3

AHpp(G

coo

o

o8 ~° ID

oo°

[]

0

0

0 [] []

0 0

ions in CuCl2 + water-adsorbed zeolites are given in Figuro 1. As seen from these, the spectra o f pore diameters 3, 4, and 5 ~ are axially symmetric and the measured values of All, g]l, and g± at 133 K are given in Table 1. T h e r e f o r e , we can state that the large part of the water adsorbed in zeolites of pore diameters 3, 4, and 5 /~ is immobile even at room temperature. However, the spectra o f the zeolite 10 ~ pore diameter and the natural zeolites heulandite and clinoptilolite are almost isotropic with a g value of 2.15. If this g value is compared with the measured value at 133 K in Table 1, it can be concluded that the water molecules in these last three zeolites are mobile. T h e measured e.s.r, line widths of Cu(H20) 2+ ions are shown in Figure 2 as a function of t e m p e r a t u r e for the zeolites of pore diameters of 3, 4, 5, and 10 ~. As seen from these plots, the line widths of all the samples increase as the t e m p e r a t u r e increases. This is in a g r e e m e n t with Equations (4) and (8) since AH oc 1/~. T h e line width increases continuously for the zeolite of pore diameter 10 ~, and this behavior is in a g r e e m e n t with the reported behavior of Cu(H20) 2+ in bulk water. 2 T h e r e f o r e , we state that for this sample the line width b r o a d e n i n g is due to the spin-rotational interaction 0t', and the adsorbed water behaves as liquid water in the bulk phase. However, the line widths of the zeolites of pore diameters 3, 4, and 5 ~ pass t h r o u g h shallow minima a r o u n d room 400

~H~G~ o o O

BDe OI

DQ []

D

D

DDDD~

2~

e •

@@

tOO

. . . .

~0

o

g

8

oo

A

A

A

AAA

go

e

0 0

0 O0

o

~AAAAAAAA

3~o

go

,= T(K)

Figure 2 The e.s.r, line width plots of Cu(HzO)~ + ions in (O) type 3A, (rq) 4A, (A) 5A and (0) 13X zeolites

422

ZEOLITES, 799?, Vol 12, April~May

f~ m ~

olo 0

i

r~ En

i 200

i

I 300

i

400

T(K)

Figure 3 The e.s.r, line width plots of Cu(H20)~ + ion in natural zeolites heulandite (©) and clinoptilolite (FI)

temperature. This means that part of the adsorbed water in narrow-pore zeolites is mobile and part of it immobile at these temperatures. F u r t h e r m o r e , from the plot that displays a m i n i m u m at 283 K in Figure 2, we obtain the correlation time of Cu(H20)26 + as ~c = 17.10- l i s for the zeolite of type 3A, and for the same temperature, we find ~c ~ 2 -8.10-1] s for Cu(H20)26 + in the bulk water solution from Martini and Burlamacchi's work. 2 If we compare these two results, we can state that the water in the narrow-pore zeolites is more viscous than the bulk water solution since ~c oc ~I/T. This means that the mobility of water decreases as adsorbed by narrow-pore zeolites. T h e e.s.r, parameters measured for the zeolites in this study are given in Table I. These parameters indicate that the low-field anisotropic n u c l e u s electron dipolar and the g tensor parameters are larger than the high-field parameters (All ~> A±, gll > gl); therefore, their contributions to electronic relaxation and line width are higher. T h e activation energies, E~, for the reorientation motion of the zeolites investigated in this study [by plotting In AH against 1/T, since ~ = "c~.vexp (EJRT)]. were f o u n d to be 1.87, 0.96, 0.77 kcal/mol, respectively, for the zeolites o f pore diameters 3, 4, and 5 ~. This indicates that the activation energies decrease as the pore diameter increases. T h e line w i d t h - t e m p e r a t u r e plots for the natural zeolites heulandite and clinoptilolite are shown in Figure 3. As seen from this figure, the plots do not pass t h r o u g h minima and increase continuously as the t e m p e r a t u r e is i n c r e a s e d . T h e r e f o r e , the adsorbed water in these zeolites behaves as in the zeolite of pore diameter 10 /~. Hence, the pore diameters o f these natural zeolites should be at least over 5 ~. In conclusion, the above experimental results indicate the presence o f at least two types o f water in the narrow-pore zeolites. These are mobile and

E.s.r. line widths of hydrated Cu2+ in zeolites: F. Ucun et al.

immobile water. The water in 13X, heulandite, and clinoptilolite zeolites behave as liquid water in the bulk phase.

REFERENCES 1 Burlamacchi, L., Martini, G. and Ottaviani, M.F.J. Chem. Soc., Faraday Trans. II 1976, 72, 324 2 Martini, G. and Burlamacchi, L. Chem. Phys. Lett. 1976, 41, 129

3 Burlamacchi, L. and Martini, G. Magnet. Resonance Colloid Interface ScL 1980, 621 4 Mumton, F.A. Mineral. Soc. Am. Course Notes 1977, 4, 221 5 Scott, J. Zeolites Technol. Appl. 1980, 33 6 Dyer, A. An Introduction to Zeolite Molecular Sieves, Wiley, New York, 1988, p. 95 7 Brauner, S., Emmett, P.H. and Teller, E.S. Am. Chem. Soc. 1938, 60, 309 8 Dollimore, D. Thermochim. Acta 1980, 38, 1 9 Wilson, R. and Kivelson, D. J. Chem. Phys. 1966, 44, 154 10 Nyberg, G. Mol. Phys. 1967, 12, 69 11 Poupko, R. and Luz, Z. J. Chem. Phys. 1972, 57, 3311

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