Orientation and Mobility of Hydrated Metal Ions in Layer Lattice Silicates

10 Orientation and Mobility of Hydrated Metal Ions in Layer Lattice Silicates

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T. J. P I N N A V A I A D e p a r t m e n t of C h e m i s t r y , M i c h i g a n State U n i v e r s i t y , E a s t L a n s i n g , M i c h . 48824

The swelling layer lattice silicates known as smectites possess mica-like structures in which two-dimensional silicate anions are separated by layers of hydrated cations. Unlike the micas, however, the interlayer cations can be readily replaced by simple ion-exchange methods with almost any desired cation, including transition metals, organometallic, and carbonium ions. In addition, the interlayer space occupied by the exchange cations can be swelled from zero to several hundred Å, depending on the nature of the interlayer cations, the charge density on the silicate sheets, and the partial pressure of water in equilibrium with the solid phase. Similar swelling over a more limited range can be achieved by replacing the water molecules with a variety of polar solvents such as alcohols and ketones. Our interest in these naturally occurring minerals has centered on their possible use as supports for homogeneous metal ion catalysts (1,2). To achieve this objective it was essential to determine the swelling conditions necessary to achieve the solution-like properties in the intracrystal environment. Earlier crystallographic studies have indicated that solvation of the exchange cations by two molecular layers of water resulted in a solid-like interlayer structure with the aquo complexes adopting well defined orientations in the intracrystal space (3). On the other hand, proton magnetic resonance studies (4-6) suggest that when the interlayers are swelled to several hundred Åunits, the adsorbed water possesses appreciable solution-like properties. Under these conditions, however, the minerals are gel-like and are not especially well suited as solid supports for a homogeneous catalyst. We have undertaken electron spin resonance investigations of the orientation and mobility of interlayer cations under different degrees of swelling to determine the minimum interlayer thickness necessary to achieve rapid tumbling of the exchange cations and diffusion of small substrate molecules into the intracrystal environment. Copper(II) or manganese(II) exchange forms of hectorite, montmorillonite and vermiculite have received 94

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

10.

PINNA

νΑ Ί Α

Hydrated

Metal Ions in Silicates

95

g r e a t e s t a t t e n t i o n . Considerable i n f o r m a t i o n on the nature o f diamagnetic exchange forms has a l s o been obtained from the e f f e c t s which these c a t i o n s have on the nature o f the esr s i g n a l s of paramagnetic i r o n i n orthorhombic environments i n the s i l i c a t e sheet s t r u c t u r e . The present d i s c u s s i o n reviews o n l y the nature o f the Cu2+ and Mn + exchange forms. The i d e a l i z e d u n i t c e l l formulas and c a t i o n exchange capac i t i e s o f the m i n e r a l s employed i n these i n v e s t i g a t i o n s are g i v e n i n Table I . Based on the experimental c a t i o n exchange c a p a c i t i e s and the t h e o r e t i c a l s u r f a c e areas 800 m^/g) the average d i s tance between d i v a l e n t exchange c a t i o n s i n the a i r - d r i e d m i n e r a l s v a r i e s from ^6 Â ( v e r m i c u l i t e ) t o 14 Â ( h e c t o r i t e ) . Hydrated C u * M i n e r a l s . When a c o p p e r ( I I ) exchange form o f h e c t o r i t e o r m o n t m o r i U o n i t e i s allowed to e q u i l i b r a t e i n a i r under ambient c o n d i t i o n s the 001 spacing i s 12.4 A, and the water to copper r a t i o i s approximately 8:1. Since the s i l i c a t e l a t t i c e c dimension i s 9.6 Â, the t h i c k n e s s o f the i n t e r l a y e r r e g i o n (2. a i ) i n d i c a t e s that the c o p p e r ( I I ) i o n s are hydrated by a monol a y e r o f water. The e s r s p e c t r a o f the C u ( I I ) i o n s under these c o n d i t i o n s are i l l u s t r a t e d i n F i g u r e 1. Randomly o r i e n t e d powder samples a t room temperature and a t 77°K c o n s i s t o f c l e a r l y def i n e d gj^ and g J J components as expected f o r Cu(II) w i t h t e t r a g o n a l symmetry. When the spectrum of an o r i e n t e d sample i s recorded w i t h the magnetic f i e l d d i r e c t i o n p a r a l l e l t o the s i l i cate l a y e r s only the g ι component i s observed. On the other hand, when the sample x s o r i e n t e d w i t h the magnetic f i e l d perpen d i c u l a r t o the s i l i c a t e sheets, o n l y the g I | component i s obser ved. These r e s u l t s i n d i c a t e t h a t the i n t e r c a l a t e d C u O ^ O ) ^ * i o n i s o r i e n t e d i n the i n t e r l a m e l l a r r e g i o n w i t h the f o u r - f o l d sym metry a x i s o f the i o n p o s i t i o n e d p e r p e n d i c u l a r t o the s i l i c a t e sheets. I f water i n outer spheres o f c o o r d i n a t i o n i s removed by h e a t i n g the m i n e r a l to 110° the esr s p e c t r a l f e a t u r e s o f the com p l e x remain unchanged. Thus the amount o f outer sphere water under c o n d i t i o n s where a monolayer i s b e i n g formed does not a l t e r the o r i e n t a t i o n of the i o n . Although the average copper-copper d i s t a n c e s among the montmorilonites and h e c t o r i t e d i f f e r , o r i e n ted samples o f each l a y e r s i l i c a t e show the same esr s p e c t r a l changes when the f i l m i s p o s i t i o n e d p a r a l l e l and p e r p e n d i c u l a r t o the a p p l i e d magnetic f i e l d . I n each case the magnitude o f giι i s g r e a t e r than g| (see Table I I ) . '' 2

2

2

Vermiculité nas a higher charge d e n s i t y than h e c t o r i t e and m o n t m o r i U o n i t e and i s more d i f f i c u l t to dehydrate. However, a monolayer o f i n t e r l a y e r water f o r the C u exchange form can be achieved by dehydration o f the m i n e r a l over P4O10 a t room tempe r a t u r e . As i n the case of h e c t o r i t e and m o n t m o r i U o n i t e an o r i e n t e d sample o f the m i n e r a l shows t h a t the t e t r a g o n a l Cu + i o n adopts a r e s t r i c t e d o r i e n t a t i o n on the s i l i c a t e sheet w i t h the symmetry a x i s p e r p e n d i c u l a r t o the l a y e r s . V e r m i c u l i t e i s w e l l s u i t e d f o r o b t a i n i n g two molecular l a y ers of water i n the i n t e r l a m e l l a r r e g i o n because the h i g h s u r f a c e 2 +

2


M A G N E T I C


RESONANCE

10.

Hydrated

PINNA VAIA

Metal Ions in

97

Silicates

charge permits a maximum of only two l a y e r s of water when the m i n e r a l i s f u l l y hydrated. The presence o f two l a y e r s of water i n the i n t e r l a m e l l a r regions o f an a i r d r i e d sample o f C u verm i c u l i t e i s v e r i f i e d by observing x-ray r e f l e c t i o n s o f s e v e r a l r a t i o n a l orders t h a t correspond t o an 0 0 1 spacing o f 14.2 Â . The esr spectrum o f an o r i e n t e d sample a t room temperature c o n s i s t s of g ι ι and g| components when the s i l i c a t e l a y e r s are p o s i t i o n e d both p a r a l l e l and p e r p e n d i c u l a r t o the a p p l i e d magnetic f i e l d . The absence o f an a p p r e c i a b l e change i n the r e l a t i v e i n t e n s i t i e s of g I J and g J i n d i c a t e s t h a t the symmetry a x i s o f the t e t r a g o n a l hexaaquo complex, CuOÔ)^ , i s i n c l i n e d w i t h r e s p e c t t o the s i l i c a t e sheets a t an angle near 4 5 ° . A l s o the Cu^ -water bonds along the symmetry a x i s are longer than those i n the X-Y plane o f the i o n as g|I i s g r e a t e r than g l (g| = 2 . 1 0 , ΔΗ I = 4 5 G ; g l l 2 . 4 0 A/c « 0.0115 cm" ). A n i s o t Y o p y H n the g f a c t o r o f Cu(H 0)^+ i s r a r e l y observed a t room temperature. I s o t r o p i c thermal motions are normally s u f f i c i e n t l y r a p i d above 50°K t o g i v e a s i n g l e esr l i n e ( 1 7 ) . Rapid i n t r a m o l e c u l a r exchange can occur between three e q u i v a l e n t J a h n - T e l l e r d i s t o r t e d s t a t e s which c o r respond t o a x i a l e l o n g a t i o n along the three p o s s i b l e s e t s of water-copper-water axes (18). I n absence o f r a p i d tumbling t h i s motion w i l l not lead t o averaging o f glι and gi when the i o n i s adsorbed on a planar s u r f a c e . I t may be concluded t h e r e f o r e t h a t the i n t e r l a y e r ions adopt r e s t r i c t e d o r i e n t a t i o n s on the s u r f a c e even when the i o n i s hydrated by two molecular l a y e r s of water. T h i s r e s u l t i s c o n s i s t e n t w i t h the r e s u l t s of e a r l i e r x-ray d i f f r a c t i o n studies of layer l a t t i c e s i l i c a t e s : under c o n d i t i o n s o f two l a y e r h y d r a t i o n the i n t e r l a y e r water i s highly structured. 2+ The g values f o r i n t e r l a y e r CuOÔ)^ are l a r g e r than those found f o r the planar aquo complex, and the c a l c u l a t e d average values o f g are i n good agreement w i t h the observed values o f g f o r Cu(II) i n aqueous s o l u t i o n (19) and f o r C u ( H 0 ) a t the exchange s i t e s o f r e s i n s (20-21). As the r e l a t i v e humidity i s i n c r e a s e d from approximately 40% t o 1 0 0 % , the C u exchange forms o f h e c t o r i t e and m o n t m o r i U o n i t e undergo a continuous t r a n s i t i o n from a mono l a y e r phase (dooi = 12.4 A) t o a m u l t i m o l e c u l a r i n t e r l a y e r water phase ( C Q O I A)'without forming a w e l l d e f i n e d i n t e r m e d i a t e h y d r a t i o n s t a t e . However, the magnesium exchange forms o f these minerals e x h i b i t an i n i t i a l h y d r a t i o n s t a t e which i s w e l l ordered with d = 1 5 . 0 Â . Complete replacement o f magnesium by copper causes the i n t e r l a y e r t o c o l l a p s e from three molecular l a y e r s of water t o a s i n g l e water l a y e r . However, when copper i s doped i n t o magnesium h e c t o r i t e a t the 5% l e v e l , the 0 0 1 spacing remains at 1 5 . 0 Â under a i r d r i e d c o n d i t i o n s . The esr spectrum of Cu doped i n t o the 1 5 . 0 Â phase o f magnesium h e c t o r i t e e x h i b i t s g|I(2.335) and g £ ( 2 . 0 6 5 ) v a l u e s , r e s p e c t i v e l y , f o r the magnetic f i e l d o r i e n t e d p e r p e n d i c u l a r and p a r a l l e l t o the s i l i c a t e sheets. I t may be concluded t h a t the 2 +

2 +

+

B

1

2

a v

2 +

2

6

2 +

38 2

0

0

0

1

2 +


M A G N E T I C

RESONANCE

Figure 2. Orientations of intercalated aquo copper(II) complexes formed by hydration with one, two, and three molecular layers of water. Open circles represent oxygen atoms of the silicate structure and ligand water molecules.


10.

Hydrated

piNNAVAiA

99

Metal Ions in Silicates 2+

symmetry axis of the tetragonal Cu(H20)£ ion i s perpendicular to the s i l i c a t e sheets. The ion i s not i n a solution-like environment insofar as dynamic Jahn-Teller distortions or rapid tumbling do not average the anisotropy or broaden the signal. Well resolved S O l i t t i n g of gl(B/C=-0.0022 cm" ) as well as g||(A/C= -0.0156 cm**) i s observed. Splitting of g i s not well resolved when the copper ions exclusively occupy an inner layer, presumably because of Cu(II)-Cu(II) dipolar broadening. The above spectral studies of Cu + ions hydrated by 1, 2, and 3 molecular layers of water indicate that the ions adopt restricted orientations i n the interlayer environment. The restricted orientations are illustrated i n Figure 2. The ions are best described as being i n a solid-like environment rather than a solution-like environment. Under these conditions one should not expect a catalytically active ion to be accessible for reaction with substrate species diffusing from solution into the interlayer region. The anisotropic esr signal of C u doped into magnesium hectorite i s lost upon solvation of the mineral in liquid water or ethanol. Jahn-Teller distortion or random tumbling of the ion in the expanded in ter layers WQQ-^ 2L 20-22 A) averages the anisotropy and relaxation due to modulation of the g tensor and spin rotation interactions result in a broad signal analogous to that observed for the ion in water or methanol solution. Hectorite f u l l y exchanged with C u and f u l l y swelled by water to a dgoi value of 22 A exhibits a single signal with a width near 120 gauss. These results suggest that when the interlayer region i s swelled to a thickness of 10-12 Â the interlayer ions begin to adopt solution-like properties. In order to more f u l l y estimate the tumbling motion of divalent ions i n the restricted water layers, an esr linewidth study of Mn exchange forms was undertaken . Hydrated Mn"*" Minerals. It has been previously reported that the linewidths of the hyperfine lines of hydrated Mn are broader on the exchange sites of montmoriUonite than i n bulk solution (22). The increase in line width was attributed to relaxation effects of the more restricted surface adsorbed ions. Also, M n montmoriUonite has been reported to exhibit broader hyperfine lines when larger molecules (e.g. pyridine) replace water in ligand positions(23,24). Again, the result was interpreted in terms of reduced mobility of the Mn ion. However, i n addition to mobility effects other factors such as dipolar relaxation can also contribute to the observed esr linewidths. The esr spectrum of Mn in solution normally consists of 6 hyperfine lines due to coupling of the S-5/2 electron spin with the 1=5/2 nuclear spin. Each hyperfine component consists of 3 superimposed Lorentzian lines due to the five Am =l transitions which are not resolved at X-band frequencies. The nondegeneracy of the Am =l transitions leads to inhomogeneous line broadening. The widths are the sum of two contributions: 1) solvent 1

1

2

2 +

2 +

2+

2

2+

2 +

2+

2+

s

g


M A G N E T I C

RESONANCE

American Mineralogist

Figure 3. ESR spectra at room temperature for (A) MnCl, in methanol (5.0 X 10~ M) and for powder samples of hectorite (B) fully hydrated, (C) air-dried, and (D) dehydrated at 200°C for 24 nr. Vertical lines indicate the resonance position of standard pitch (g — 2.0028) (9). 5


10.

piNNAVAiA

Hydrated

Metal Ions in

101

Silicates

c o l l i s i o n a l r e l a x a t i o n processes and 2) d i p o l a r i n t e r a c t i o n s between neighboring M n i o n s . The d i p o l a r e f f e c t i s concentrat i o n dependent and p r o p o r t i o n a l t o r~3 where r i s the average manganese-manganese d i s t a n c e . I n d i l u t e s o l u t i o n the l i n e s a r e narrow and determined e x c l u s i v e l y by c o l l i s i o n a l r e l a x a t i o n processes. I n c r e a s i n g the c o n c e n t r a t i o n causes the s i x h y p e r f i n e components t o broaden markedly u n t i l a t a c o n c e n t r a t i o n o f 2.3 M or g r e a t e r (r < 9.0 Â) , the h y p e r f i n e s t r u c t u r e i s l o s t and the spectrum appears as a s i n g l e broad l i n e . A t y p i c a l spectrum f o r Mn(II) i n d i l u t e s o l u t i o n i s shown i n F i g u r e 3 along w i t h s p e c t r a f o r M n h e c t o r i t e under v a r i o u s cond i t i o n s . I t i s seen that t h e f u l l y hydrated m i n e r a l e x h i b i t s a " s o l u t i o n - l i k e " spectrum, except that the h y p e r f i n e l i n e s a r e broader. Reducing the amount o f i n n e r - l a y e r water from s e v e r a l to two molecular l a y e r s by a l l o w i n g the m i n e r a l t o dry a t approximately 50% r e l a t i v e humidity causes the l i n e t o broaden markedly. Thermal dehydration a t 200° leads to s t i l l f u r t h e r l i n e broadeni n g and almost complete l o s s of h y p e r f i n e s t r u c t u r e . Similar changes i n l i n e w i d t h are found f o r M n - s a t u r a t e d montmoriUonite. Since the average i n t e r l a y e r exchange i o n d i s t a n c e f o r Mn^ m o n t m o r i U o n i t e and h e c t o r i t e i s i n the range 10 - 14 Â, the widths of the M n s i g n a l s should be determined by d i p o l a r r e l a x a t i o n e f f e c t s . This i s v e r i f i e d by the comparison i n F i g u r e 4 of the average width o f the mT*+5/2 l i n e s f o r MnCl2 i n methanol s o l u t i o n and f u l l y hydrated minerals of d i f f e r i n g Mn -Mn2+ d i s t a n ces . I n Mn2+ v e r m i c u l i t e , where the exchange i o n d i s t a n c e i s approximately 7 Â, o n l y a s i n g l e broad l i n e w i t h a w i d t h o f 710G i s observed. The broadening i s s i m i l a r t o that found f o r M n C l s a l t (830G) and c o n s i s t e n t w i t h d i p o l e - d i p o l e coupling between magnetic ions 3-8 Â apart. D i p o l a r i n t e r a c t i o n s between the Mn ^ exchange ions and i r o n ions i n the s i l i c a t e network may a l s o cont r i b u t e somewhat t o the broadening of the manganese resonances. The l i n e w i d t h of the mj=+5/2 l i n e s d i f f e r s by 15G when an o r i e n t e d sample i s p o s i t i o n e d p a r a l l e l and p e r p e n d i c u l a r to the a p p l i e d magnetic f i e l d . The d i f f e r e n c e i n l i n e widths i s a t t r i b uted t o an i s o t r o p i c d i p o l a r r e l a x a t i o n by s t r u c t u r a l i r o n along the c r y s t a l l o g r a p h i c c dimension. The s m a l l e s t d i f f e r e n c e i n l i n e widths i s observed f o r M n h e c t o r i t e which has the lowest Fe^ content (< 0.14%). I n t h i s case the i n c r e a s e i n l i n e widths w i t h decreasing h y d r a t i o n s t a t e can only be i n t e r p r e t e d i n terms of reduced m o b i l i t y o f the i n t e r l a y e r . However, i t i s d i f f i c u l t t o assess q u a n t i t a t i v e l y the i n t e r l a y e r m o b i l i t y because the l i n e w i d t h s are s t i l l determined by an i n t e r i o n i c d i p o l a r r e l a x a t i o n mechanism i n v o l v i n g neighboring M n exchange i o n s . I n order t o e l i m i n a t e d i p o l a r r e l a x a t i o n , was doped i n t o Mg2 h e c t o r i t e a t the 5% l e v e l . In absence o f d i p o l a r i n t e r a c t i o n s , s p i n r e l a x a t i o n o f M n ( H 0 ) 5 i n s o l u t i o n r e s u l t s from molecular c o l l i s i o n s between the s o l v a t e d i o n and s o l v e n t molecules which cause random d i s t o r t i o n 2 +

2 +

2+

2 +

z

2

2=

2 +

+

2 +

+

2+

2


M A G N E T I C

RESONANCE

American Mineralogist

Figure 4. Average m = ±5/2 line widths for Mn * vs. interionic distance. Open points are for MnCl in methanol, solid points are for nontronite (N), Upton (U) and Chambers (C) montmorillonites, and hectorite (H) under fully hydrated conditions (9). 2

r

t


10.

PINNA

νA i A

Hydrated

103

Metal Ions in Silicates

of the complex (25,26). By observing the r e l a t i v e e s r l i n e widths i n the two environments, one can estimate the c o r r e l a t i o n t i m e T f o r t h e i o n on the exchange s u r f a c e s r e l a t i v e t o the i o n i n b u l k s o l u t i o n . When IÙQT«1 which i s g e n e r a l l y t r u e f o r MnOÔ)^ "*" a t room temperature and a t X-band frequency (CÛQ=0.58X 10 radians/second), the w i d t h o f the -1/2 4

(OH)

(OH

( O H )

0.22 7.78 20

A1

](A1

S1

) O

l.O0 7.00 20

0.10

F

' >4

0 . 3 0 ^ 0 . 0 2 % . 6 6 i 2.28 5.72 20

M

( ]A1

A 1

(0H

2

0

0

1 1 6

1 0 4

9 2

7 3

Meq/100g

ϋ

^

±*

^

I*

Reference

Water of hydration i s omitted; M i s the interlayer exchange cation; elements i n brackets f i l l octahedral positions i n the s i l i c a t e l a t t i c e , elements i n parentheses f i l l tetragonal positions.

ΐ!θΟ/η

Vermiculite (Llano, Texas)

Fe

0

7.90

2.84 0.35 8 .85

[A1

Orientation and Mobility of Hydrated Metal Ions in Layer Lattice Silicates

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