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SITY

Illillll'illlfili‘llllrtlrlllflmll‘lll

3 1293 00786 4931

 

 

LIBRARY
Michigan State
University l
K _r

 

 

This is to certify that the
dissertation entitled

Pillaring of Layered Double Hydroxide

with Polyoxometalates

presented by

Taehyun Kwon

has been accepted towards fulfillment
of the requirements for

Ph . D degree in Chemistry

/
WM 7 Manama;

M ajo ofessor

 

Date September 22, 1988

 

MSU is an Affirmative Action/Equal Opportunity Institution 0-12771

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before due due.

    
  
 

DATE DUE DATE DUE DATE DUE

CE:
I IZZY—fl
L_]l::ll__
I! EC?
.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

usu Is An Aflirmdive Action/Equal Opportunity Institution
cumulus-e:

 

PILLARING OF LAYERED DOUBLE HYDROXIDE WITH
POLYOXOMETALATES

By
TAEHYUN KWON

A DISSERTATION

Submitted to
Michigan State University
in partiaL fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry
1988

6,

ABSTRACT
PILLARING OF LAYERED DOUBLE HYDROXIDE WITH
POLYOXOMETALATES
Taehygg Kwon

Pure Zn/Al layered double hydroxides (LDHS) of the type Zn2Al(OH)6-A, where
A = C1-, N03“, have been prepared by an induced hydrolysis method. Several
polyoxometalates (POMS) have been intercalated in the Zn/Al LDH gallery by controlled
ion-exchange reactions to form new classes of two-dimensional microporous solids. The
following POMS have been successfully intercalated to yield pure phases as the sole
reaction products : H2W120406-, a-SiV3W9O407', PM02W9O397‘,
BV(IV)W1104o7-. BCo(II)W110397-. BCu(II)W1 10397-. SiFe(III)(SO3)W110397-.
SiW110393-. BW110399-. PW9O349-. PV140429-. H2W1204210-.
NaP5W30011014‘. The following POMS undergo partial exchange and yield
incompletely exchanged reaction products : PCu(II)W110395-, BV(V)W110406',
BCo(III)W110396-, PV3W9O4o6-, Bl-SiW110398', [32-SiW110398', B-
SiV3W9O407-.

The intercalation reaction exhibits novel ion exchange selectivities. A complete
exchange reaction takes place only under acidic reaction conditions. The a isomer of
Keggin-type POM (eg. a-SiV3W9O40 ') is more favorably intercalated than other
stereo isomers. Such favorable ion exchange behavior for a-Keggin POM can be
explained in terms of the commensurate relationship of oxygen layers in the POM and
the LDH hydroxyl lattice. The rhombohedral stacking pattern of the hydroxyl layer,
which is more thermodynamically favorable than the hexagonal stacking polytype, is

retained when the a-isomer is intercalated. The minimum charge carried by the Keggin-

type POM for complete exchange is observed to be 6-.

The POMs intercalated in LDH galleries are hydrolytically more stable than POM
in the homogeneous solution. In the solid state, the POM-pillared LDH (LDH-POM) is
generally stable up to about 200°C. An amorphous phase is formed around 300°C, and
new crystalline inorganic phases are formed around 500°C.

Fl‘IR studies indicate that the POMS in LDH galleries experience two major types
of interactions : anion-anion repulsions and hydrogen bonding interaction with the LDH
layer. These two competing interactions determine the direction of the v(M=O) band
shift compared to normal potassium or ammonium salts. It has been observed that
hydrogen bonding dominates, resulting in a red shift for the v(M=O) band.

LDH-POMS have been found to swell in water, the extent of swelling ranging
from one to two mono layers. Non-pillared LDH intercalate are not swell in water.

LDH-POMS exhibit high surface areas, compared to a non-pillared LDH
derivatives. This result indicates that extra space in the gallery has been introduced by
pillars. The surface area of the LDH-POM depends on the charge of POM in the gallery.
The highest surface area has been found for the Keggin-type POM with an 8- charge. The
nonrigidity of the host layer could lead to low surface areas in the case of LDH-POMS in
which the POMS carry higher charges than 8-.

LDH-POMS are photochemically reacitve. They oxidize iso-propanol to acetone
under irradiation by a Hg(Xe) lamp. The reactivity of the LDH-POM is different from
that for POM in homogeneous solution. The higher the charge of POM in the gallery, the
more space in the gallery, and the higher the photoreactivity of LDH-POM. The
diffusivity of both the reactant and the products in the LDH gallery appears to determine

the efficiency of the photoreaction.

TO MY FAMILY

ii

ACKNOWLEDGEMENTS

I would like to thank Dr. T]. Pinnavaia for patience, encouragement, and
guidance offered tluought the years of graduate school. I am also grateful to Dr. H.A.
Eick for his editorial assistance in improving my writing.

I would like to express my gratitude to all past and present group members with
whom I have come in contact with.

Finally, I would like to thank my parent and my wife for their financial support as

well as encouragement.

iii

Chapter

LIST OF TABLES

CHAPTER I.
A.
B.

CHAPTERII.

F1905”?

TABLE OF CONTENTS

 

INTRODUCTION

 

Pillaring of Lamellar Solids

 

 

Layered Double Hydroxide (LDH)

 

1. Structure

2. Synthesis of LDH

 

3. Ion-exchange Property of LDHS

 

4. Comparison of LDH and Smectite Swelling Properties -

 

5. Thermal Stability of LDHS

 

6. Application of LDHS

 

Polyoxometalate
1. Structure and Property of Polyoxometalate mm»

2. Charge Distribution of POMS

 

 

3. Photochemistry of POMS

 

Adsorption onto Porous Materials

 

Experimental

Materials

 

 

Preparation of LDH-Cl and LDH-N03

 

Preparation of Guest Anions POMS
Synthesis of LDH-POMS

 

Physical Measurements

 

1. X-ray Powder Diffraction

 

 

2. Gas Adsorption Measurement

3. Thermal Stability Study of LDH-POMS -..----------

 

4. Nuclear Magnetic Resonance Studies

iv

Page

xii

F.
G.
H.
CHAPTER III. RESULT AND DISCUSSION
A.

F9371?“

H
O

Photocatalytic Reaction

 

 

Elemental Analysis

Transmission Electron Microscopy (TEM) ............

 

Synthesis of LDH-Cl, N03

 

Synthesis of LDH-POMS

 

 

Characterization of LDH-POMS

 

1. Electron Microscope Studies
2. X-ray Powder Diffraction (XRD) Studies -----------
3. Elemental Analysis

 

4. Nuclear Magnetic Resonance (NMR) Studies ------

 

5. Infared Spectroscopy Study

 

a) Vibrations of Host Layer
b) Vibrations of Guest POM Anion ------------
c) Vibrations of LDH-POMS

 

Selectivities of Ion-Exchange Reaction and

 

Orientation of the Intercalated POMS
1. Charge Selectivity

 

2. Streo Selectivities

 

3. Orientation of POMS in the Gallery
Thermal Stabilities of LDH-POMS
Adsorption Study of LDH-POMS

 

 

 

Theoretical Calculation of Surface Area Pore Volume
Photoreactivity of LDH-POMS

 

Swelling Properties of LDH-POMS

 

PTOPOSCd Mechanism of Ion-Exchange Reactio ------

Appendix A: XRD pattern of LDH-N03 and LDH-POMS. ............

V

54
54
55
56
56
58
61
61
66
66
66
82
82
84
90

111
111
116
121
123
208
219
224
228
228

223

Appendix B: T-plots and DR or D-A plots of LDH-POMS. ---------- 242

References 251

 

vi

Table

10
11
12
13

14
15
16

LIST OF TABLES

Some Natural Minerals of the Pyroaurite—Sjogrenite ---------
pH values for Synthesis by the
Induced Hydrolysis Method

 

Classification of 2:1 Minerals according
to the Swelling in Water and the

Exchangeable Cation (after Suquet)39

 

Swelling of [anCr(OH)6)][C12H25804‘-nH20]

in n-Alcohol

 

Thcnnal Stability of [M(II)1-xM(III)(OH)2][An-] ............
Heteropoly Tungstate and Molybdates with

a-Keggin Structure

 

Polarographic Half-Wave Potentials of

 

Lacunary Tungstosilicate Anions

 

Lacunary 1:11 and 2:17 Polyanion Ligands
Heteroatoms and Terminal Ligands observed

in (XM11)ZL and (X2M17)ZL

 

Heteroatoms of Z(XM11)2 and Z(X2M17)2 Complexes ----

 

Elemental Analysis Data for LDH-POM

 

Vibrational Frequency of X04
Infrared Spectral Data of Free-POMS
and LDH-POMS

 

LDH-POM Intercalates for N2 Adsorption Studies -----------
N2 Gas Adsorption Data and Calculated Values -------------

 

Photochemical Reactivities of LDH-POMS

vii

Page

13

13
14

19

23
26

28
29
69
86

91

217
223
226

Figure

LIST OF FIGURES

Page

Layer sequence in pyroaurite and sjogrenite.

 

The brucite-like layers are Shaded. 4

\O

Basal Spacings(A) of Na-saturated bcidllite at different p/po--
Change of approximate mean spacing between layers
of montrnorillonite with reciprocal of square root

of concentration for N a-saturated clay (after Norrish).

 

x in N aCl solutions, 0 in N a2804 solution. 11
Two types of M06 octahedra. a) Type I and b) Type ---------- 16
The five Baker-Figgis isomers of the Keggin structure (a).

In [3-, 7-, 8, and e—structures one, two, three and

four M3013 groups (shown unshaded) have been

rotated by 1t/3. 17

 

a and [5 Structure of heteropolyanions of the 12-series -------- 20
Polyhedral models and relationships between the

different known tungstosilicates. This scheme is

based partly on structural determination by

single-crystal X-ray diffraction studies and partly

on proposed structures. Hydrolysis of the [3-8insz ‘

anion only gives B2-SiW110398‘ and, then,

the y—SiW100368' anion. 21

 

Polyhedral representation of a) a-SiV3W9O407'
and b) B—SiV3W90406'. 3o

 

viii

Figure

10
11
12

13

14
15

 

 

 

Page
XRD pattern of LDH-Cl prepared at different pH.
* represents a Zn5(OH)8C12 phase. 57
It spectrum of LDH-N03. 59
XRD pattern of LDH-NaP5W3oO110, prepared at pH = 7 .4.- 60
XRD pattern of a) LDH-SiFe(SO3)W11039 and
b) LDH-PW9034. * represents an extra phase. 62
Transmission electron microscope picture of LDH-POMS.--- 64
(top, left), LDH-N03; (top, right), LDH-BCuW11039;
(below, left), LDH-PM02W9039; (below, right),
LDH-PW9034.
Polyhedral representation of H2W1204210-. 65

 

(top, left), An ORTEP view of [NaP5W3001 10114»
looking approximately along the virtual C5 symmetry
axis. Oxygen and tungsten atoms are represented as
large and small open circules, respecrively, and

the sodium ion by the central closed circle. The

phophorus atoms are shown as closed circles.

(top, right), an idealized view along C5 axis

showing the W06 octahedra in the upper half of

the anion. (below), The PW6022 unit perpendicular to
the anion’s virtual Cs axis. The phosphorus atom is

represented by the closed circle. 68

 

ix

Figure

16

17

18
19

20

21

22

23

24

25

Page

Solution 51V NMR spectra for a) LDH-SiV3W904o at
pH = 2.0 1.10104 solution and b) a-K6HSiV3W9O40 at
pH = 1.7. 71

 

Solid state 295i MAS NMR for a) LDH-SiV3W904
and b) a-K6HSiV3W9O40. 72
Solid state 51v MAS NMR for LDH-SiV3W904o. ---------- 73

 

Solid State 31P MAS NMR for a) LDH-PM02W9039
and b) K7PM02W9O39. 75

 

Solid state 31p MAS NMR for a) LDH-PW9034

 

and b) NagHPW9034. 77
Solid state 31p MAS NMR for a) LDH-PV14042

 

and b) (NH4)9PV14O4. 78
Solid state 113 MAS NMR for a) LDH-BW11039
and b) KgBW11O39. 80

 

Solid state 23Na MAS NMR for a) LDH-NaP5W3oO110

 

 

and b) (NH4)14NaP5W300110. 81
Solid state 31? MAS NMR for a) LDH-NaP5W3oO110
and b)(NH4)14NaP5W300110. 83

Idealized Structure of or-XM12 (Keggin structure) (a)

and a-XMll (defect Keggin structure) (b)

(central X04tetrahedron is not Shown).

(0a = oxygen shared by 3 M06 octahedra and

X04 tetlahedron. Ob and 0c = oxygen linked

to two different M atokms. 0d = terminal unshared oxygen.- 85

Figure

26
27
28
29
30
31
32
33
34
35

36
37
38
39

41
42

43

 

The polyhedral model of the PV140429' anion.
Ir spectra of l) (NH4)6W12040 and 2) LDH-H2W12042.--
Ir spectra of l) K6BVW11040 and 2) LDH-BVW1104o.----
Ir spectra of 1) K6HSiV3W9040 and 2) LDH-SiV3W9040.
Ir spectra of 1) K381W11039 and 2) LDH-SiW11039. -----
Ir spectra of 1) K7PM02W9039 and 2) LDH-PM02W9039.
Ir spectra of l) K9BW11039 and 2) LDH-BW11039.-------
Ir Spectra of 1) K7BCuW11039 and 2) LDH-BCuW11039.
Ir spectra of 1) K7BCoW11O39 2) LDH-BCoW11039. -----
Ir spectra of 1) K7SiFe(SO3)W11039 and

2) LDH-SiFe(SO3)W1103.

 

Ir spectra of 1) B-NagHPW9034 and 2) LDH-PW9034.---

Ir Spectra of 1) (NH4)3C02W12042 and 2) LDH-Con12042.
Ir spectra of 1) (NH4)9PV 14042 and 2) LDH-PV14O42.---

Ir Specua of 1) (NH4)10H2W12042 2) LDH-H2W12042.--

Ir Spectra of 1) (NH4)14NaP5W300110 and

2) LDH-NaP5W3oO11.
XRD pattern of LDH-PCuW11039.

 

 

XRD pattern of a) LDH-BCo(III)W11039.

b) LDH-BV(V)W11040 and C) LDH-PV3W9040.-----------
XRD pattern of a) LDH-B1-SiW11039, b) IDH-BZ'SiW11039.
c) LDH-B3-SiW11039, d) LDH-a-SiW11039,

e) LDH-B—SiV3W9O4.

 

xi

Page

89
93
95
96
97
99
100
101
103

104
113

107
108
109

110
112

114

118

Figure

45

47

48
49
50

51
52
53

54

XRD pattern of LDH-H2W12040 at different temperatures.-
a) 25°C, b) 120°C, c) 180°C: dotted line for the

rehydrated sample, solid line for dehydrated sample,

d) 225°C, e) 320°C, and 1) 470°C.

Ir spectra of LDH-H2W1204o at different temperatures.----

 

DSC and TGA of IDH-H2W1204o.
XRD pattern of LDH-SiV3W9040 at different temperatures.
a) 25°C, b) 165°C, c) 220°C: dotted line for the

rehydrated sample, solid line for the dehydrated sample,

d) 250°C, e) 360°C, and f) 500°C.

Ir Spectra of LDH-SiV3W9040 at different temperatures-«-

 

DSC and TGA of LDH-SiV3W9O4o.

127

130
131
133

136
138

XRD pattern of LDH-BV(IV)W11039 at different temperatures.- 140

a) 25°C, b) 150°C, c) 225°C: dotted line for the

rehydrated sample, solid line for the dehydrated

sample, d) 255°C, e) 360°C, and 1) 470°C.

Ir Spectra of LDH-BV(IV)W11040 at different temperatures.

 

DSC and TGA of LDH-BV(IV)W11040.

XRD pattern of LDH-BCo(II)W11039 at different temperatures.

a) 25°C. b) 185°C: dotted line for the rehydrated sample,
solid line for the dehydrated sample, c) 200°C: dotted

line for the rehydrated sample, solid line for the

dehydrated sample, d) 250°C, e) 360°C, and f) 500°C.

Ir spectra of LDH-BCo(II)W11039 at different temperatures.-

xii

143
144
146

149

Figure

55
56

57
58
59

61
62

63

65

DSC and TGA of LDH-BCo(H)W11039.

 

XRD pattern of LDH-BCu(II)W11039 at different temperatures.

a) 25°C, b) 125°C, c) 160°C, d) 190°C: dotted line for

the rehydrated sample , solid line for the dehydrated

sample, e) 230°C, and 0 500°C.

Ir spectra of LDH-BCu(II)W11039 at different temperatures.
DSC and TGA of LDH-BCu(II)W11039.

 

XRD pattern of LDH-PM02W9039 at different temperatures.
a) 25°C, b) 150°C, 0) 205°C: dotted line for the

rehydrated sample, solid line for the dehydrated

sample, d) 340°C, and e) 500°C.

Ir spectra of LDH-PM02W9039 at different temperatures.---
DSC and TGA of LDH-PM02W9039.
XRD pattern of LDH-SiFe(III)(SO3)W11039 at

 

different temperatures.

a) 25°C, b) 150°C, c) 185°C, d) 200°C: dotted line

 

for the rehydrated sample, solid line for the
dehydrated sample, C) 303°C, and 1) 500°C.
Ir spectra of LDH-SiFe(SO3)W11039 at different temperatures.
DSC and TGA of LDH-SiFe(SO3)W11039.

 

XRD pattern of LDH-SiW11039 at different temperatures.-
a) 25°C, b) 160°C. c) 180°C: dotted line for the

rehydrated sample, solid line for the dehydrated

sample, (1) 230°C, e) 300°C, and 1) 500°C.

xiii

Page

151
153

156
157
159

162
164

166

169
170
172

Figure

66
67
68

69
7O
71

72
73
74

75
76

Ir Spectra of LDH-SiW11039 at different temperatures. ------
DSC and TGA of LDH-SiW1 1039.

 

XRD pattern of LDH-BW11039 at different temperatureS.---
a) 25°C, b) 150°C, c) 180°C: dotted line for

the rehydrated sample, solid line for the dehydrated

sample, (1) 210°C, e) 260°C, and f) 500°0C.

Ir spectra of LDH-BW11039 at different temperatures. ------
DSC and TGA of LDH-BW11039.

 

XRD pattern of LDH-Co2W12042 at different temperatures.
a) 25°C, b) 120°C, c) 180°C: dotted line for the

rehydrated sample, solid line for the dehydrated

sample, d) 225°C, e) 320°oc, and t) 500°0C.

Ir spectra of LDH-Co2W12042 at different temperatureS.--—
DSC and TGA of LDH-C02W12042.

 

XRD pattern of LDH-PW9034 at different temperatures-«-
a) 25°C, b) 110°C: dotted line for the rehydrated

sample, solid line for the dehydrated sample,

c) 175°C, d) 215°C, e) 320°C, and f) 500°0C.

Ir spectra of LDH-PW9034 at different temperatures.--------
DSC and TGA of LDH-PW9O34.

 

xiv

Page

175
176
178

181
183
185

188
189
191

194
196

Figure

77
78

79
80

81
82
83

84
85
86
87
88

Ir Spectra 0f LDH-PV14042 at different temperatures. ------
XRD pattern of LDH-PV14042 at differrent temperatureS.---
a) 25°C, b) 125°C, c) 185°C, (1) 320°C, and e) 500°C.

DSC and TGA of LDH-PV14042.

 

XRD pattern of LDH-NaP5W300110 at different

 

temperatures.
a) 25°C, b) 120°C: dotted line for the rehydrated
sample, solid line for the dehydrated sample,
c) 180°C: dotted line for the rehydrated sample,
solid line for the dehydrated sample, d) 250°C,
and e) 500°C.
Ir spectra of LDH-NaP5W300110 at different temperatures.-
DSC and TGA of LDH-NaP5W300110.

 

XRD pattern of LDH-H2W12042 at different temperatures.-
a) 25°C, b) 120°C, c) 180°C, d) 230°C, e) 320°C

and f) 500°C.

1r spectra of LDH-H2W12042 at different temperatures.----
DSC and TGA of LDH-H2W12042.

 

 

N2 adsorption isotherm of LDH-POMS.
T-curve of LDH-C1.

 

XRD pattern for wet samples of l) LDH-PV14042
and 2) LDH-NaP5W300110, wet sample.

 

XV

Page

194
199

202

204

207
209
21 1

214
215
216
218

229

CHAPTER I
INTRODUCTION

A. Pillaring of Lamellar Solids

The concept of pillaring lamellar solids was first introduced by Barrer and
Mcleod in 19551 by showing that permanent porosity could be induced in smectite clay
(e.g., montrnorillonite) by replacing the interlayer N 3+ ions with alkylammonium ions.
The alkylammonium ions functioned as molecular props or pillars between the silicates
and provided the intracrystalline free spaces between the pillars. The pillaring agents
range from thermally unstable organic or organometallic cations, e.g., bicyclic amine
cationszs3 and tris metal chelates4:5 to thermally stable polyoxo cations,6-11 e.g.,
A11304(0H)24(HzO)127+, and related chromium- and zirconium-polyoxocations. In
fact, the term ‘pillaring clay’ is being used frequently in reference to clays interlayered
with thermally stable polyoxy aggregates. Since the charge density of smectite clays is
relatively low, the lateral distance between pillars is sufficiently large and the layers are
sufficiently rigid to allow for the existence of free void volume between the pillars. Such
pore volume then becomes available for adsorption and possible catalytic reaction of
substrates sufficiently small to reach the intracrystalline pores. The adsorption study
provides the insight of such porous materials.

A few pillarable host lattices other than smectite clay have been reported. a.-
Zirconium phosphate, in which the layers are cross-linked by bis-alkyl phosphonic
group,12- 14 are another example of a pillared lamellar solid. But, in general, pillared

2'-

lamellar solids are still quite rare. The major reason for such rarity is that there has not
been found suitable host lattices which meets the basic requirements for a pillarable
lattice, that is, 1) a reasonably low layer charge density, and 2) a reasonably rigid lattice
layer.

In this work, the first pillared oxide structure other than smectite clays will be
presented. The host lattice is a layered double hydroxide (LDH), which has about three to
four times higher surface charge density (about 25-35 A2/e) than smectite clay.
Therefore, the guest anion Should have high charge density to compensate the high
surface charge density of the host layer. Polyoxometalates, which have a wide range of
charge density and a variety of Sizes, would be a suitable class of guest anions which

meet this requirement.

B. Layered Double Hydroxide (LDH)

1. Structure

There are many natural minerals and synthetic compounds having compositions
of the type:
[M(II)l-xM(III)x(0H)2]+X[A“‘x/n-YH201 where
M(II) = Mg, Fe, Ni, Zn, Cu
M(III) = A1, Fe, Cr
An- = 0032-, 8042-, N033 x = halides, Fe(CN)53-, 0042—, ClO4-, etc
0.2 < x < 0.33

They are referred to as pyroaurite-sjogrenite mineral groups,15 hydrotalcite-like
minerals,16 mixed metal hydroxides,17 and layered double hydroxides. 13 The latter
term, abbreviated LDH, will be used in the present work.

They can be derived by the substitution of trivalent ion, M(III) into brucite-like
layers, M(II)(OH)2. This introduction imparts a net positive charge on the sheet which is

balanced by hydrated anions intercalated between the brucite-like layers.

3

The most common natural LDHS are hydrotalcite and manasseite polymorphs
with composition [Mg6A12(OH)16][CO3.4H20]. They are actually stacking polymorphs
that have the same basic layer structure. Mg and A1 are randomly distributed among the
octahedral sites. In hydrotalcite, the layers are stacked with rhombohedral symmetry
(BC-«CAmABu-BC for the brucite-like mainlayers), and three brucite-like layers are
present per unit cell (c = 3c’ = 23.4 A). In manasseite, they are stacked with hexagonal
symmetry (BC---CB«-BC---) and two brucite layers are present per unit cell (c = 20’ =
15.6 A).19s20 In nature the polymorphs are commonly intergrown. Manasseite generally
forms the core and hydrotalcite the outer part of the grain. Thus, hydrotalcite appears to
form later than the coexisting manasseite, and presumably, at lower temperature. To date,
synthetic hydrotalcite has been prepared, but manasseite has never been synthesized.

The crystal structures of the Mg(II)/Fc(III) pyroaurite and sjogrenite polymorphs
are the most extensively studied natural LDH’S.20-23 They possess the same morphology
as the Mg(II)/Al(III), hydrotalcite and manasseite polymorphs. In both compounds,
brucite-like layers carrying a net positive charge alternate with layers in which the
oxygen atoms of carbonate groups and water molecules are Statistically distributed on a
single set of sites. Adjacent brucite-like layers are stacked so that the hexagonal groups
on the lower surface of one layer are directly above those on the upper surface of the
layer below, as in gibbsite, and not as in brucite. The 2H (hexagonal) and 3R
(rhombohedral) subgroups represent the two simplest sequences satisfying this condition.
The sequence of the OH layers in pyrourite and sjogrenite are the same as with those of
hydrotalcite and manasseite, respectively. The distance between two metal layers in
pyroaurite equals c/3 = 7.80 A. Because the brucite layers are only about 4.8 A thick, a
Space of 3.0 A is left between them. This space is occupied by a disordered interlayer,
[1/8003-1/2H2O]0-25'. The distances parallel to the z-axis between layers are the same
in both polymorphs: Mg + Fe layer to OH layer, 1.02 A; between 0H layers in the
brucite layer, 2.04 A; OH layer to interlayer, 2.88 A. Fig. 1 shows the layer sequences in

O
7'8A

 

 

 

 

 

 

 

 

 

0-125 cog' + ()5 H20

OH
075 Mp2° + 025 1303'
OH-

0125 cog' + 05 H20

OH
0-75 Mgz°+ 0:25 Fe”
014'

Fig. 1. Layer sequences in pyroaurite and sjogrenite.

The brucite-like layers are shaded.

5

pyroaurite and sjogrenite. Some natural minerals with the relevant crystal symmetries are

given in Table 1.

Table 1. Some Natural Minerals of the Pyroaurite-Sjogrenite

 

 

 

Group22
Name Approximate Composition
3R-Typ 2H-Type Hydroxide Interlayer '—
Sheet Sheet

Pymam‘itc sjogrenite [Mg6F62(OH)16]°[(C03)(HzO)4l
Hydrotalcite Manasseite [Mg6A12(OH)16]-[(CO3)(H20)4]
Stichtite Barbertonite [Mg6Cr2(0H)16]-[(CO3)(H20)4]
Reevestite ------ [Ni6Fe2(OH)16]-[(C03)(H20)4]
TakOVitc ------ [Ni6A12(OH)16]:[(C03)0120)4l

 

2. Synthesis of LDHS

Feitchnecht24 has Shown that a large number of synthetic LDHS can be prepared
by titrating the solution containing divalent and trivalent cations with NaOH. Other
workers have been involved in the modification and refinement of Feitchnecht’s
coprecipitation method.

Mortland and GastuschZS prepared the mixed Mg/Al hydroxides from the
corresponding chlorides aged in a dialyzed medium, achieving in this way a much better
crystallization and a high purity. They found that two mixed Mg/Al hydroxides having
Mg/Al ratios of about 5:1 and 2:1 were formed from solution with Mg/Mg-I-Al molar
ratios of 0.8 and 0.7, respectively. They have layer dimensions a = 3.048 A, layer
thickness 7.6 A for the Al-rich compound and a = 3.072 A, layer thickness 7.92 A for the
Al-poor compound.

6

Coprecipitation, followed by hydrothermal treatment, has been used by Miyata.26
The hydrothermal treatment of freshly coprecipitatcd products results in highly
crystalline LDHS in a relatively short period of time.

The hydrothermal and high temperature ‘dry’ technique was introduced by Roy et
al.27 Solid MgO and A1203, which were the source of cations, were heated in the
presence of H20 and C02 or N205, which were the source of C032- and N03- anions,
respectively. The products were LDH-C032- and LDH-NO3- respectively, where LDH
stands for the positively charged Mg/Al brucite layer. This dry technique is useful when
carbonate-free LDH is required.

Recently, R.M. T aylor23 reported a new method, called "induced hydrolysis",
which involves the hydrolysis of a cation in solution by a fully hydrolyzed and
precipitated hydroxide of a second metach cation. The term induced hydrolysis was
coined because the fully hydrolyzed cation, that is, the hydroxide, caused complete
hydrolysis of the second cation at a pH below that at which this reaction would normally
occur. This technique utilized one cation typically M(III) as a hydroxide, formed from a
solution of its chloride or nitrate salt. The advantage of this method is that crystalline
compounds with reasonably sharp x-ray powder diffraction (XRD) patterns are formed
within a few hours at room temperature. The pH values for precipitation of some typical
LDH systems are listed in Table 2.

3. Ion-exchange Property of LDHS

Since the exchangeability of the interlayer C032' of takovite
[Ni3Al(0H)8]2-C03-4H20 was first found by Bish,29 a variety of anion exchanged
LDHS have been prepared. Miyata30 has reported quantitative Studies of ion exchange
properties of LDHS. The ion selectivities of monovalent anions are in the order of OH- >
F- > C1- > Br- > N03- > I-. Ion selectivity of bivalent anions, which are higher than those
of mono- valent anions, order as 0032- > 8042-. Such anion-exchanged forms of LDHS

provide better XRD patterns than freshly precipitated LDHS.

Table 2.

pH Values for Synthesis of LDHS by the Induced
Hydrolysis Method

 

 

LDH pH
N i-Al-CO32- 6.9
Co-Al-Cl- 7.4
Mn-Al-Cl- 7.5
Mg-Al CI- 8.4
Co-Fe-Cl- 7,4
Co-Si(IV) 7.4
Fe-V(IV) 4.0

 

8

4. Comparison of LDH and Smectite Swelling Pr0perties

The unidimensional swelling of smectite clays is their most interesting and
important property. A number of polar molecules can be intercalated in multilayer form
and thereby expand the galleries to substantial heights. The molecules may enter in
stages, that is, one mono layer at a time.31

The adsorption of water on sodium beidellite is shown in Fig. 2.32 One mono
layer is formed in the interlayer gallery at H20 partial pressures between approximately
0.1 to 0.6, and one more mono layer is built up to yield two mono-layers at partial
pressures between 0.6 and 0.9. With sodium montrnorillonite and hectorite the water
layers can be built up infinitely and the clay layers are essentially exfoliated. Bradley et
al-33 showed that water molecules could be sorbed in monomolecular sheets in
montrnorillonite, and that one, two, three, or four water layers were possible.

The occurrence of "Steps" in the basal spacing of swelling minerals as a function
of water content is now a generally accepted phenomenon. The first water to enter the
interlayer positions is the result of hydration of the ions34 after which the water forms
distinct layers which increase in number. The water molecules of, at least, the first layers,
are probably arranged in a hexagonal network35 whose order is determined by hydrogen
bonding to oxygens of the clay. This type of swelling is commonly referred to as Type
1,36 Short range swelling,37 or crystalline swelling.38 In this range, the adsorbed water
increases to about 0.5g H20/g clay while the interlayer Spacing increases from 9.5 A for
the dry material to about 20 A, corresponding to four layers of water. The mechanism of
this Type 1, Short range, or crystalline swelling has been explained in terms of the
hydration energy of both interlayer cations and silicate layers.37 When the interlayer
cations in montrnorillonite become hydrated, and the layer energy involved is able to
overcome the attractive forces between layers, Type 1 swelling takes place. Since in the
prototype minerals interlayer cations are absent, there is no cation hydration energy

available to separate the layers and no swelling occurs in these types of minerals.

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10

There is another type of swelling. Vermiculites and smectites when saturated with
certain ions are capable of swelling to give much larger spacings between the layers than
those normally observed with interlamellar complexes (Type 1). This type of swelling is
called osmotic or type 2 swelling.36 Its essential features are not only the large spacings
observed but also the different nature of the forces between the layers. It is due to the
diffuse double-layer repulsion by which the particles or the layers may be pushed further
apart. In smectite the net negative layer charge is compensated by cations which are
located on the layer surface in the dried clay. In the presence of water, these
compensating cations have a tendency to diffuse away from the layer surface since their
concentration will be smaller in the bulk solution. On the other hand, they are attracted
electrostatically to the charged layers. The result of opposing trends is creation of an
atmospheric distribution of compensating cations in a diffuse electrical double layer on
the exterior layer surface of a clay particle. The compensating cations between the layers
of the stack are confined to the narrow space between opposite layer surfaces. These
forces are osmotic and result from a balance of electrostatic forces, van der Waals forces
and the osmotic pressure of the interlamellar ions. The distance between the layers is not
even approximately constant but shows a distribution covering a considerable range of
distances. The transition from normal or Type 1 interlamellar swelling to Type 2 swelling
is sharp. The swelling in the Type 2 is dependent on the concentration of electrolyte in
the solution. The silicate-layer separation normally increases linearly as function of the
reciprocal of the square root of the salt concentration}2 Normal electrical double layers
are presumably developed between the layers. As shown in Fig. 3, N orrish32
demonstrated that in N a-montrnorillonite the individual layers dissociate completely as
the water content is increased. The spacing first increases in steps corresponding to
various organized structures of water molecules until 20 A is reached and then jumps to
40 A, after which it increases linearly.In the zone of large spacing ( i.e. > 40 A), single

spacing is replaced by a statistical distribution. Osmotic swelling in the swellable

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12

minerals depends largely on the surface charge density and charge localization as well as
the nature of the exchangeable cation. Table 3 shows the swelling property of expandable
2:1 minerals.39
There have been two reported cases of LDHS that swell. One of these is
[Ca2A1(OH)6][OH-2H2O]40 with a basal spacing of 7.4 A. When it takes up water to
produce [Ca2Al(OH)6][OH.6H20], the basal spacing increases up to 10.7 A indicative
that one mono layer of water has been built up in the gallery. The other case is
[Zn2Cr(OH)6][org.]4l in which org. represents organic anions, for example, alkyl
sulfates. Table 4 shows the swelling of [Zn2Cr(OH)6][org.] with polar organic solvents.
The behavior is analogous to that of alkylammonium layer silicate.42 The basal Spacing
after swelling of dodecyl sulfate with dodecyl alcohol corresponds to the value calculated
for alkyl double layer (41.3 A).

5. Thermal Stability of LDHS

The thermal stability of LDHS are very important in their application for
heterogeneous catalysts at elevated temperature. A thermal study of the hydrotalcite was
done by G.W. Brindley and S. Kikkwa.43 They reported that the XRD pattern showed
little or no change up to 240°C, but at 260-2800C there was an abrupt decrease of basal
spacing to 6.2-6.4 A which was followed by a continued decrease until complete
decomposition occurred at around 370°C. Following the abrupt basal spacing decrease
to 6.4 A at 260°C, and further decrease to 6.2 A at 280°C, the mineral could be
rehydrated to give the initial basal spacing. This initial collapse was due to
decomposition of the CO3 groups, CO32- ---—> C02 + 02'; the oxygen anions remained
between the hydroxide layers. The hydroxide layers were more closely packed than the
bulky CO3 groups. At this state, the layers were sufficiently separated to permit water
molecule penetration and reexpansion to the original spacing. The continued decrease in

basal Spacing to about 5.5 A at higher temperature was attributed to hydroxide layer

13

 

 

 

Table 3. Classification of 2:1 Minerals according to the

Swelling in Water and the Exchangeable Cation

(after Suquet)39
Hydratign state Exchangeable Cation

Li Na K CL Mg Bg

Infinite MBSV MB M
3 water layers MB(S) MBS MBS
2 water layers V BSV M (B)SV (S)V (S)V
1 water layer BSV
0 water layer V

 

M = Montmorillonite, B = Beidellite, S = Saponite,

V = Vermiculite

 

 

Table 4. Swelling of [Zn2Cr(OH)6][C12H25804'-nH20] in

n-Alcohol
swelling agent d(A) swelling agent d(A)
C3H7OH 28.3 C10H21OH 38.2
C4H90H 29.2 C12H250H 41.1
C6H13OH 30.8 C14H290H 42.4

C3H17OH 33.7 C16H33OH 44.9

 

l4

decomposition and accommodation of 02' anions more or less within these layers. The
minimum spacing of 5.5 A approached that of brucite.

Generally speaking, the thermal Stability of LDHS depends on both interlayer
anions and the compositions of brucite-like layers. Table 5. shows the dehydroxylation
temperature.

Table 5. Thermal stability of [M(ll)1-xM(III)(OH)2][An-]

 

 

 

M(II) M(III) x An- Dehydroxylation ref.
temperathC)
Mg Al 0.25 8042- 455 44
Mg Al 0.25 Cr042- 460 44
Mg Al 0.25 CO32‘ 440 44
Ni Al 0.26 Cl- 300 45
Zn Al 0.25 Cl- 280 45
Mg Al 0.25 N03- 455 45
Mg Al 0.33 Cl- 430 45
Mg Al 0.25 CI’ 415 45
Mg Al 0.24 CI' 400 45
Mg Fe 0.25 CO32' 250 46

 

6. Application of LDHS

Synthetic LDHS, particularly carbonate derivatives, are utilized as acid
adsorbents,47 antipeptins,48 Ziegler-Nana type catalyst supports,49 in combination with
a polyolefins as nontoxic and non smoke flame retardants,50 catalyst precursors of syn
gas-to-methanol production,51 and catalyst precursors for the aldol condensation.52 The
pristine LDHS, e.g., [Zn2Cr(OH)6]XonH20 have been tested for the triphase catalyst.53
Thin films of LDHS deposited on electrodes have also been tested for the modification of
electrode.54

15

B. Polyoxometalate

1. Structure and Properties of Polyoxometalates

Polyoxomelates are represented by the general f0rmula55 [MmOylp'
(isopolyanions) and [XmeOyN‘ (x < m) (heteropolyanions), where M is usually
molybdenum or tungsten, and less frequently vanadium, niobium 0r tantalum, or mixture
of these elements in their highest oxidation states.

Such polyoxometalates form a structurally distinct class of complexes based
predominantly, although not exclusively, upon quasi-octahedral-coordinated metal atoms
(M06). There are two types of polyoxometalates formed by two different M06 units.
Type I has only one terminal oxygen as shown in Fig. 4 a). On the other hand, Type II
has two terminal oxygens as shown Fig. 4 b). Many WW I) and M0(VI) polyoxoanions
are seen to be on larger, neutral MnO3n cages that encapsulate anionic subunits and are
linked to them only by weak ( > 2.2 A) b0nds.56 For example, among the Type I class of
polyoxoanions, W60192' contains a W6013 cage encapsulating 02’. Thus, W60192-
can be expressed by (W6018)(02-). PM0120403- by (M012036)(PO43-). W100324*
by (W 10030)(02')2. P2M0180626' by (M018054)(P043')2. and so on. In the limit an
infinitely large cage is formed by the linking of Type I M06. Precisely such a compound
is found in the M003-2H2O structure.57 There is another type of polyoxoanions formed
by the linking of Type II octahedra. For examples, consider M04012(O2)24- in which
the M04012 ring is encapsulating (022-)2, expressed by (M04012)(022')2,
82M050244r by (M05015)(SO32-). P4W804012' by (W 8024)(PO43-)4. and so on. In
the limit of a large ring of Type H polyanion one obtains an infinite chain structure of the
composition M03. Precisely such chains are found in the M003 st11.lcture.58

The most extensively studied polyoxometalate compounds are those with Keggin
structures, XM1204on-. There are five possible isomers as shown in Fig. 5. Since

Keggin reported the structure of 12-tungst0ph0sph0ric acid in 1933,59 it has customarily

16

O
u “i
l— W\:{ g '''''''''' Mo:5
8:)? A;
a) b)

Fig. 4. Two types of M06 octahedra a) Type I and b) Type II.

 

Fig. 5. The five Baker-Figgis isomers of the Keggin Structure (a). In B—, y—, 8- and 8-

two, three and four M3013 groups (Shown unshaded) have been rotated

SUUCIUI'CS one,

by 1t/3.

18

been referred to as the a-form. This structure is based on a cenual X04 tetrahedron
surrounded by twelve M06 octahedra arranged in four groups of three edge shared
octahedra, M3013 (M3 triplet). These groups are linked by shared comers to each other
and to the central X04 tetrahedron. The majority of heteropoly tungstates adopt either the
Keggin structure or structures derived from fragments of that structure. Table 6 shows
numerous heteropoly molybdates and tungstates with the tit-Keggin Structure.

An isomer of the Keggin structure is known for several tungstates (X = B, Si, Ge,
H2) and molybdates; these appear to contain the B-Keggin Structure. Fig. 6 shows the 0t-
and B-Keggin Structures. The B-Keggin structure has one of the three edge-shared
M3013 triplets of the a-structure rotated by 600 around the C3 axis (designated by R),
thereby reducing the overall symmetry of the anion from Td to C3v. The new comer-
shared W-O-W linkage between the rotated group (R) and the rest of the anion involves a
shorter W--W separation (3.65 vs 3.72 A) and a more acute W-O-W angle(-145° vs
155°) than the a-Structure. Both of these features may account for the lower stability of
B vs at i.e., increased coulombic repulsion6O and less favorable pn-dn interactions,61
respectively. The B-structure is one of several proposed by Baker and Figgis;55 the
others involve the 60° rotation of two, three, and all four M3013 as shown in Fig. 5. All
oxidized B—forms spontaneously isomerize to 01-forms at rates which vary from seconds
for B—PM012 at room temperature62 to hours for B—SiW12 at 150°C.63 The
polarographic reduction waves for B-isomers parallel those for tit-isomers, but they
exhibit slightly more positive potentials. It follows that the reduced B-structure is more
stable than the a-structurc. The optical absorption Spectra of reduced B—isomers are
similar and are unlike those of the corresponding reduced a-isomers.

When solutions of XW 12040 are treated with base (pH > ca 5) a complex series
of hydrolysis reactions ensues, leading to a variety of lacunary anions. Fig. 7 Shows the

reaction pathway as well as the species involved in each step.

19

 

 

Table 6. Heteropoly Tungstates and Molybdates with tit-Keggin
Structure 4

Md X

[XW12040]n- H. H2. B. A1. 030103. Si. Ge(IV). P(V).

AS(V). V(V)b. Cr(III). Mn(IV)C. Fe(III).
C0(III), Cu(II), Cu(I), Zn, Se(IV)c, Te(IV),
Sb(III)C, Bi(III)C

[XM012040]"' Si. Ge(IV). P(V). AS(V). V(V)b. WWW.
Zr(IV)C, ln(nl)c, sz, Mod

 

a Known as lacunary (GeW11) anion only.
b In mixed addenda (V + W, V + Mo) anions only.
c Confirmation desirable.

d Existence questionable.

20

 

   

a (7;)

Fig. 6. a and B structures of heteropolyanions of the 12-series

21

 

 

2- 2- A
51013 +1NO4 ]

 

 

 

 

 

 

 

 

 

 

Fig. 7. Polyhedral models and relationships between the different known tungstosilicatas.
This scheme is base partry on structural determination by single-crystal X-ray diffraction
studies and partly on proposed structures. Hydrolysis of the B—SiW120404‘ anion only
gives B2-SiW110393' and , then, the y-SinoO368‘ anion.

22

Since each of the above polyanions is electrochemically reducible, the
interconversions and isomerizations can be followed by polarography. The half-wave
potentials of each tungstosilicate species are given in Table 7.

All of the lacunary anions, except a-XW 11 and, perhaps, Siwlo appear to be
metastable in solution. But conditions of pH, temperature, etc., can be arranged so that
decomposition and isomerization reactions are slow, and intermediate salts can be
crystallized. The B-XW12 structure can give rise to three isomeric KW 11 anions shown
in Fig. 7. For X = Si and Ge all three isomers have been observed and isolated.
Historically, the isomers were named to corresponding to the spontaneous direction of
isomerization B1 --—> B3. The structure were assigned on the basis of electrostatic
arguments.60 The structures Show a diminution of the number of short W---W
separations between rotated and unrotated W3 groups in the sequence B1 —> B2 —9 B3 -)
0.. The properties of the B-Sinl isomers parallel those of a-Sinl except that
irreversible isomerization occurs in solution. Estimates of the half-lives of the SiW11
isomers at pH = 5 and 25°C are 01, 11hr; 02, 7.5hr; 03, 4.5hr.64

Alkaline degradation of a-XW11 and B-XW11 at pH > 8 leads to a-XW9 and
B-XW9 anions, respectively, as shown in Fig. 7. There are no stable B-isomers of or-
[XW12O4013- and a-[XW11039]7‘ (X = P, As). In aqueous solutions PW12 has a
limited stability range. At pH = 1.5 - 2, PW12 is rapidly (and reversibly) converted to the
lacunary PW11 anion. The properties of further degradated species XW9 (X = P, As)
seem to parallel those of SiW9 and GeW9 in many, but not all aspects. The following

scheme Shows the presently known interconversions.

23

 

 

 

Table 7. Polarographic Half-Wave Potentialsa of Lacunary
Tun gstosilicate Anions55

Anion E 1Q}; Ma of electrons)
a-SiW9 -0.78 (4)

B-SiW9 -0.80 (2) -0.90 (2)
a—Sinl -0.65 (2) -091 (2)
B1-SiW11 -0.63 (2) -0.83 (2)
B2-SiW11 -O.63 (2) -0.77 (2)
B3-SiW11 -0.69 (2) -0.89 (2)

 

b

Dropping mercury electrode; 1.0 M sodium acetate/acetic

acid pH 4.7.

vs. SCE

liXW9

X043-,WO42-

\

24

 

 

l

l

rapid

ot-xw9 .-.=== [x2w19068114-e==2 a-XWll

OH“

 

l

[X2W2107116-

 

a-XW12

Besides the instability of B-[XW11039J7’ and the non-existence (to date) of B-XW12,

the above scheme differs from that of the tungstosilicates by the inclusion of X2W19 (X

= P) and X2W21 (X = P, As) complexes.66

The only authenticated heteropolyanions containing boron are the tungstoborates.

The interconversion of isomers is shown below.67

25

H+, W042'

_\
_—’

 

8033-, w042- F==$ h-BWll ........=: h’-BW12 e===e h-BW12

I a-BW12

 

Unlike other or-XWIZ species, or-BW12 decomposes above pH = 6.5 directly to borate
and tungstate; no a-BWll or BW9 species is known. The 13er species is readily
isolated as a potassium salt, K8HBW11033-17H2O,67 from mixture of borate and
tungstate at pH = 6.5, and has been shown to belong to the h-series by its reversible
interconversion with h-BW12. The isomerization h --—> (1, occurs in solution at pH < 5,
but it becomes very slow in acidic solution. At pH = 5.5, where h-BW12 has been
converted to h’-BW12, no isomerization occurs even on boiling.

Complexes are known in which the ‘ligand’ is a polyanion structure that is
deficient in single M06 octahedron. Metal ion binding occurs at the vacant site. Mono
vacant Keggin derivatives form by far the largest and versatile class of lacunary
polyanions, and their complexes have been intensively Studied since the first examples
were recognized in 1966.68 A list of polyanion ligands that have been observed in such
complexes is given in Table 8.

Two global Stochiometries are observed for the complexes, metalz‘ligand’ = 1:1
and 1:2. The 1:1 complexes are formed predominantly with ‘octahedral’ metal ions, with

the polyanion ligand functioning in a dentate manner. In the 1:2 complexes the metal ion

26

Table 8. Lacunary 1:11 and 2:17 Polyanion Ligandsa.55

 

 

 

90mm X Isomer
[XW12039]n- P, AS 01
Si. Ge (1.131. 132. B3
B hb
Al, Ga, Fe(III), Co(III) 010
Co(II), Zn, H2 ad
Sb(III), Bi(lII) -e
[XL/[011039]n- P. As, Si, Ge 01
[x2w17061110- P a1. a2
AS a
a Species of which complexes have been isolated.
b The free ligand has the currently unknown ‘h’ Structure.

Complexes appear to contain the or form.

c a-structure not proved in every case, but considered very
probable.
d Free ligand not known.

e True structure not known.

27

Table 9. Heteroatoms and Terminal Ligands observed in
(XMl 1)ZL and (Z2M17)ZL Complexesa. 55

 

 

 

M.L Heteroatom. Z Refs.

02- w6+/5+, Mo6+/S+, v5+/4+
Ti4+, Sn4+, Nb5+, Rh3+, 69, 70, 71, 72
Ge4+, Re7+/6+, Sb5+ 73, 74

H20b v3+, Cr3+, Mn2+/3+, Ni2+ 68, 74, 75, 76
Fe2+/3+, C02+/3+, Cu2+, 77, 78, 79, 80
Zn2+, A13+, Ga3+, In3+, 81, 82,
Rh3+
Ru3+/4+ 83

N3' 086+ 84

NO Ru2+l3+ 85

N113, NCS-, N02-,

N33 CN', pyridine, C02+/3+, Fe3+, Ni2+, 75, 86

pyrazine, thiourea Zn2+d

Fe(CN)54-, 5032- Fe2+ 74

a This table excludes complexes where ZL represents an

organometallic moiety.
b In several cases the sz are such that stable

hydroxometal derivatives are isoable.
c Unsubstituted, 2- and 4-methyl, 3-cyano.

d Not all metal-ligand combinations.

28

Table 10. Heteroatoms of Z(XM11)2 and Z(X2M17)2

 

 

 

Complexesas55
mm Refs.
cc4+l3+, pr3+, Nd3+, 3313+, 5113+, 1103+ 90
Th4+, U5+/4+ 91, 92, 93, 94
P144“, Tb4+/3+, Pu4+/3+, Np4+, Am4+/3+, 95, 96, 97
Cm4+/3+, Cf4+/3+
1n3+b 98
La3+ 99
Sr2_+,_B_a2_t 100
a All tungstates.
b Note that two types of complex, with six- and eight

coordinate In3+, appear to be formed.

29

can be viewed as 8—coordinate. The Sixth coordination site on the metal in the 1:1
complexes may be occupied by a variety of ligands L, as shown in Table 9.

Although the lacunary anions Spontaneously isomerize (B1 --—> "-9 0t; a1 -—9
(12), the substituted species appear to be quite stable in this respect. The substituted
complexes are generally Stable at pH ca. 4-8 although most of the molybdate complexes
are much less stable than the tungstate complexes. In acidic solution, the complexes lose
the 211+ cations and the lacunary ligands are converted to XM12 0r X2M18. In neutral or
basic solution, the water molecules coordinated to 211+ may be deprotonated before the
heteropolyanion is hydrolyzed. Replacement of coordinated water on Zn+ by a number of
ligands has been studied by Weakley36 and Baker et al.-75:33. Since the molybdenum
and tungsten atoms in the Keggin and XM11 Structure are reducible (to form heteropoly
blues), certain complexes have intense low energy charge transfer bands that correspond
to (Zn+,M5+)---(Z(n-1),M5+) transitions. Such bands are observed when Z is V4+, V34",
Mn2+, Fe2+, M05+, Re6+, Re5+. In some cases, reduction potentials of Z and M are so
evenly balanced that valence isomers can exist, e.g., B1-[SiW1 1Vm(1120)039]5‘ (at pH
< 6) and B1-[SiW10WVV1VO4OJ7’ (at pH > 10).39

A second type of complex shown in Table 10 is formed by the lacunary ligands
XM11 and X2M17 with lanthanide and actinide cations. These complexes have the
stoichiometry metal(ligand)2 and the first examples were reported by Peacock and
Weakley.100 The high charge carried by the bis-ligand complexes enables the
stabilization of ‘unusual’ oxidation states for the heteroatom, e.g., tetravalent Pr, Tb, Am,
Cm, and Cf and pentavalent U.

2. Charge Distribution of Polyoxometalates

It is commonly asserted that heteropoly anions, particularly those with the almost
Spherical Keggin structure, are unsolvated in aqueous solution and do not participate in
hydrogen bonding in the crystalline state. Various lines of experimental evidence,

including viscosity102 and diffusion studies,102 and crystal structure determination,104

30

A\
;;\\\\\\\\‘vA\\\v
“(WNQA

    
      

   

 

T
“\\\\v ‘ “V—
\\ "
A/

   

._,t\\\
I],

 
   
 
 
 

 

 

 

Fig. 8. Polyhedral representation of a) a-SiV3W9O407' and b) B—SiV3W9O4O6'.

31

tend to support that assertion. l2-molybdosilicate and 12-molybdophophate anions are
less susceptible to hydrogen bonding than is the perchlorate ion.104 Since there is no
apparent Steric hindrance to heteroconjugation, these results confirm the very low surface
charge density of such polyanions.

It has been known that the negative charge on the surface of the polyoxoanion is
concentrated on specific oxygen atoms. Klemperer et al.106 determined the protonated
oxygen site of HV2W40193- by 170 NMR. They observed an upfield Shift of only the
0V2 (bridging oxygen) resonance upon protonation of V2W40194-. This change in
chemical shift identified the 0V2 oxygen in the V2W40194- cluster as the protonation
site. All resonances except the 0V2 resonance in V2W40194- shifted downfield upon
protonation of the cluster. This downfield shift reflects a strengthening of metal-oxygen
bonds and concomitant reduction of negative charge on the oxygen atoms in
HV2W40193'. More recently, Klemperer et al.107 reported the most basic site of
oxygen atoms of cis-Nb2W4O194- by its alkylation reaction pattern. They concluded that
the bridging oxygen (Nb-O-Nb) was the most basic site, i.e., site with most
nucleophilicity. Also they found that charge delocalization substantially occurred in the
anion when the product distribution was considered. The enhanced reactivity of
Nb2W40194‘ compared to tha of W60192- is additional supporting evidence for the
idea of a charge delocalization. The 0W2 oxygen in W60192- were not alkylated by
dimethyl sulfate in acetonitrile even after 2 days at ambient temperature. In contrast, the
0W2 oxygens in Nb2W40194' were alkylated under the same condition in less than 5
minutes.

It has been reported“)8 that the protonation site of SiV3W9O407- was assigned
to the 0V2 oxygen (bridging oxygen) by 51V NMR, as shown in Fig. 8.

3. Photochemistry of POMS

The photocatalytic process of polyoxoanions was first examined in detail by

Papaconstantinou et al.,109 who subsequently demonstrated that reduced polyoxoanions

32

could be reoxydized by air, thereby completing a catalytic cycle. They suggested the
following mechanism for the photooxidation of isopropanol by the polyoxoanion

PW1204031110

 

 

 

hv
PW123' + MeZCHOH ------------- —9 PW124' + Me2C0H + H2“
hv
PW124" + M62C0H =- PW125‘ + Me2CO + H+
PW125- + PW123- EA 2PW124'
2PW124- + 1/202 + 2H+ ------- -) zpw123- + H20
PW125- + 1/202 + 2H+ --------- —> PW123' + H20
Me2CI-IOH + 1/202 => Me2CO + H20

 

In the absence of 02, PW124’ can react with H+ according to

 

PW124- + H+ => PW123‘ + 1/2H2

Therefore, the total reaction in the absence of 02 will be dehydrogenation :

hv
R1R2CHOH => RleCO + H2

 

33

Recently, Hill and Bouchard111 reported that polyoxometalates (POMS) could be

classified according to their photocatalytic reactivities:

Type I : Nb, Ta oxometalates, for example, M60198- can not be photoreduced by any

organic material even with u.v. light ( < 290 nm)

Type II : These are readily photoreduced by a wide variety of organic substrates, but the
reduced species are relatively and thermodynamically incapable of evolving hydrogen.
Nearly all heteropoly and isopolymolybdates as well as polyvanadates are in this
category. Thermal reoxidation of the reduced form of this type POM with O2 is usually

kinetically slow and sometimes thermodynamically unfavorable.
Type III : Nearly all heteropoly- and isopoly-tungstates undergo both facile
photoreduction at 25°C by a variety of organic substrates, and the reduced forms

undergo facile hydrogen evolution or air (02) oxidation.

He suggested the following mechanism for the photooxidation of organic

 

 

substrate by POMS.
Pn- + 8H2 =5 (P—H)“' + SH-
PIT- + 8H2 :wfi PH'.SH2
hv
Pn'°SH2 ‘___. __.s__ p(n+l)-. SH2+°

 

P(n+1)‘SI-I2+. ----------- —) P(n+1)' + 31-124"-

34

 

p(n+1)-.SH2+. S. p(n+2)- + S + 2H+
SH2+~ -----------——> HS- + W“

Pn' + HS. ----------- -> p(n+l)- + S + H+
P(n+2)~ + Pn- -------- -+ 2p(n+1)-

They emphasized that the two most likely mechanisms for the key substrate oxidation
steps were radical hydrogen abstraction and electron transfer :

Pn- + R2CHOH -----_, P-H(“‘1)' + RZCOH

Pn- + R2CHOH --------- —> P<n+1)- + [RZCHOH]+°

Substrate activation takes place in part by irradiation of the intermolecular charge
transfer band of a POM-substrate complex, but also to some extent by a process most
accurately characterized as hydrogen atom abstraction from the substrate by an
intermolecular oxygen to metal charge transfer exited state of the POM.

The complex charge-transfer electron absorption manifold of some soluble metal
oxide species, including heteropoly acids of Mo and W, is known to shift substantially to
lower energy on going from water to organic solvents. These medium-induced
bathochromic spectral shifts render some POM species highly photosenstive. Hill et
al.112 reported the photoreactivity of l,1,3,3,-tetramethyl urea (TMU)-0t-H3PM012040
the charge transfer complex. They observed that although the xmax was little perturbed
on going from water (Xmax = 310 nm) to TMU (Xmax = 308 nm), the low energy
absorption tail was quite shifted to the visible in TMU. The electronic interactions

between the polyoxo moiety and the TMU moiety were responsible for the shifts in the

35

absorption spectrum. The shift were disrupted when the complex was dissolved in
acetonitrile. Irradiation into this low energy absorption tail resulted in the oxidation of
TMU and the production of the reduced POM ‘heteropoly blue’.

Most recently, Fox et a1. 1 13 reported a new mechanism for photooxidation of
alcohols by POMs, which supports Hill’s idea of POM-organic substrate complex
formation. She claimed that the preassociation complex formation between alcohol and
POM should precede photoactivation. She put forward several evidences for such a
preassociation complex, including N MR and fluorescence life time measurements. This
concept can explain the photoreactivity of POM-alcohol systems. The tightness of the
preassociation complex is responsible for the reactivity. It depends on both alcohols and
POMs. Less bulky alcohols favor the formation of such complexes and the following
orders of reactivities are observed :
primary alcohol > secondary alcohol >> tertiary alcohol

When V5+ is substituted for W6+ as in P2V5W15062, the compound will bear
three extra negative charges so that the electron density shifts toward the vanadium oxide
terminal oxygen (-O4V!O’). The oppositely charged terminal oxygen will interfere
electrostatically with the necessary complexation at the terminal VO site. Since negative
charge is delocalized in the polyoxoanions by the bond alternation mechanism,56 the
tungsten sites should also exhibit diminished reactivity. The reactivity also depends on
the heteroatom in the same type of POM. The order of decreasing reactivity is H > Fe >>
Co, Si. The diminished reactivity has an electronic basis that allows for more efficient
back electron transfer to the adsorbed oxidized species.

Papaconstantinou reported114 that the rate of electron donation from alkyl radical
to POM depended on the charge of the POMs and their reduction potentials. Increasing
the negative charge on the complex and increasing the negative reduction potentials of

the POM leads to a decrease in the electron transfer rate.

36

Fox finally proposed Scheme 1 for the photooxidation of 2-butanol by
PW1204o3-(1).113 She suggested that path a, the two electron, two proton route, was
more likely on the basis of several evidences. For example, the order of reactivity of

substrate paralleled that of substrate ionization potential.

C. Adsorption onto Porous Materials

Since the pillared LDHS to be discussed in this work contain micropores, it is
appropriate to review briefly the method used to measure micropore volume and size.

The pore-types of porous materials are classified into three categories according

to their sizes :115

Macropores : widths > 500 A
Mesopores : 20 A < widths < 500 A
Micropores : widths < 20 A

Their adsorption mechanisms are different. The mesopore surface is the scene of
monomolecular and polymolecular adsorption of vapors, i.e., layer-by-layer coverage
ending in the volume filling of this pore type by the capillary condensation mechanism.
The specific surface areas of mesopore structures vary within the range 10 to 400 m2/g.
Adsorption on a microporous adsorbent involves not only the successive formation of
adsorption layers on the surface of the micropores, but also the filling of their adsorption
spaces. The concept of a surface area of microporous adsorbents loses its physical
significance.

A characteristic feature of adsorption on microporous adsorbents is the substantial
increase in the adsorption energy and, consequently, the adsorption potentials in
micropores, as compared to the corresponding values for mesopores or nonporous

adsorbents of a similar chemical nature. An increase in adsorption energy in micropores

37

.8538. .5 .o as. :8. a .25 3.8.3 25.0 s chlé 8 22 358a: .. .I 2.

 

 

 

 

 

 

 

 

 

n\
{fwd . @9wa
We . from? A x. roam“... from: . W0 i§~
z an.“ 3: ..._
from: 2W... frown“... .qu v 5%: . Wu isms W. L
.é/ a i.e.... 7 are; than deV
a“. .3 ,2... i... .. . L... We \............,

 

 

tigid

4

reward

 

oi
.flNU 4”" .Wzv. . for“:

. L .3 .o:2:m.n .3 52350 wax—58225 2: 3.. Exes—002 v0.89... ._ 8.39m

38

leads to a considerable increase in the value of vapor adsorption in the region of low
equilibrium pressures.

There have been two equations extensively used to describe the adsorption of
vapors onto microporous materials to determine the micropore volume. One of them is an
empirical equation proposed by Dubinin and Radushkevich.115 Their equation (D-R

equation) is expressed as

a = ao cxPl-k(A/B)2],

where a/ao is a fraction of adsorption space filled at relative pressure p/po. A =
RTln(p/po) and k and B are constants. The micropore volume (a0) is determined by
plotting log a against log2(po/p) (so called D-R plot). Marsh and Rand117 demonstrated
that this D-R plot extrapolation must be carried out with care, since, although the plot
might show linear regions, it is very rarely linear over a wide pressure range. These
deviations can be linearized by using a more generalized equation proposed by Dubinin

and Astakhov.118 It is expressed by an equation (D-A equation),

a = ao cxvl-(A/B)"]

where a/ao is the degree of pore filling, A is a differential molar work of adsorption,
which is the negative value of Gibb’s free energy of adsorption and is expressed by
RTln(po/p), B is a constant parameter which is a characteristic free energy of adsorption,

and n is a constant. This D-A equation can also be expressed in another form,

log[d(log a)/dA] = log(-nD) — (n-1)log A

where D = 1/(2.3OBBn)

39

By plotting log A against log[d(log a)/dA], n can be determined and then by plotting log
a against A a0 can be determined.

Brunauer et al. have proposed a method (so called MP method, micropore
analysis method) for calculating,119 on the basis of an experimental isotherm for vapor
adsorption on a microporous adsorbent, the specific surface area, the pore volume and the
volume distribution of the micropores according to their sizes. It is based on the so-called
t-curve, which is a plot of the statistical thickness of the adsorbed film against p/po for
nonporous adsorbents. The statistical thickness t, which is the ordinate of the t-curve, is
obtained by dividing the volume of nitrogen adsorbed as liquid at a given p/po by the
BET surface area. The isotherm of microporous material is converted into a V1(volume of
liquid nitrogen adsorbed)-thickness plot (t—plot) in which the t values are read from a t-
curve for standard nonporous material. By analyzing the t-curve, the surface area, pore
volume and pore size distribution can be determined. If the t-plot is a linear, then the

system consists of a nonporous material. If the t-plot is a non-linear, then the material is

porous.

CHAPTER II
EXPERIMENTAL

A. Materials

Zinc nitrate hexahydrate, zinc chloride, aluminum nitrate nonahydrate,
ammonium chloride, and sodium hydroxide were obtained from Fisher Scientific
Company. Aluminum chloride hexahydrate, nitric acid, acetic acid, sodium molybdate
dihydrate, ammonium acetate, and methanol were purchased from Mallinckrodt. Cobalt
acetate tetrahydrate, tungstic acid (HzWO4), vanadyl sulfate trihydrate, and gold label
iso-propyl alcohol were purchased from Aldrich Chemical Company. Ammonium
hydroxide, potassium chloride, sodium metasilicate nonahydrate (NazSiO3), and
potassium bisulfite from IT Baker Chemical Company were used. Boric acid and
hydrochloric acid were obtained from Columbus Chemical Company. Sodium meta-
vanadate was purchased from Alfa-Products-Ventron. Sodium tungstate dihydrate was a

gift from GTE Sylvania Company.

B. Preparation of LDH-Cl and LDH-N03

Pristine layered double hydroxides of the type Zn2Al(OH)6-Cl or -NO3 (LDH-
NO3, or -Cl) were prepared by a modification of the induced hydrolysis method of
Taylor.28 Deionized water, pre-boiled for 2 hours under N2, was used for the preparation
of all solution. All manipulations were carried out under a N2 atmosphere to avoid

contact with C02.

1+0

41

To 200 ml of 0.1M Al3+ solution containing the desired anion in a 2-liter three
neck round bottom flask was added 1M NaOH solution from a dropping funnel with
vigorous stirring. The delivery of the NaOH solution was completed at pH = 7. The white
slurry was stirred for one hour. Then, 200 ml of 0.3M Zn2+ solution was added dropwise
from a dropping funnel. As the Zn2+ solution was added to the slurry, the pH of the
reaction mixture dropped. The pH of the reaction mixture was adjusted to 6.2 by adding
N aOH during the delivery of the Zn2+ solution. After the complete delivery of the Zn2+
solution, the slurry was boiled for one week under a N2 atmosphere. The product was
washed several times with water by centrifugation. After the final washing, the stock
slurry was prepared by redispersing the product well into 800 ml of water. Even this
latter operation was carried out under nitrogen.

An oriented film sample of the Zn2Al LDH was prepared on a glass slide for x-
ray examination. About 50 mg of solid product was dissolved into 100 ml of 20% (VA!)

HNO3 for elemental analysis by atomic absorption.

C. Preparation of Guest Anions POMs

All the guest anions were prepared according to literature methods. The details of
syntheses are summarized below.
B-NasHPW9O34-24H20120 : Sodium tungstate Na2W04-2H20 (120 g, 0.36 mole)
was dissolved in water (150 ml). Orthophosphoric acid, H3PO4 (3 ml, 14.7M), and
concentrated acetic acid (22 ml, 17.4M) were successively added to the solution. The

desired white salt precipitated.

a-K7PW9M02039mH20120 : B—NagI-IPW9034'24HZO (11 g, 3.9 mmole) was
dissolved in an aqueous mixture of sodium molybdate, Na2MoO4.2H20 (20 ml, 1M) and
HCl (16 ml, 1M). Then HCl (about 12 ml, 1M) was added dropwise until the pH was
between 6 and 6.5. The desired product was precipitated by addition of solid KCl.

42

K7BCo(II)W11039-nH20121 : NaZWO4-2HZO (36.3 g, 0.11 mole) was dissolved in
150 ml water and the pH adjusted to 6.3 with acetic acid. H3BO3 (2 - 3 g, 0.03 - 0.05
mole) was added and the mixture was heated to 80 - 90°C. A solution containing 0.01
mole Co(II) was added dropwise, followed by the addition of KC] (15 - 20 g).
Recrystallization from hot 2% KCl resulted in the formation of the desired product.

K7BCu(II)W110390~nH2032 : This compound was prepared in exactly the same way
as K7BCoW11040H2, except that Cu(II) replaced Co(II). Fibrous, clear green crystals
separated from the hot solution, followed on cooling by the separation of pale green

cubic crystals.

q-KSSiWuO39-12H20122 : Na2WO4-2HZO (182 g, 0.55 mole) and 14.7 g of
metasilicate, NaSiO3~9H20 were dissolved in 300 ml of cold water, HCl (195 ml, 4M)
was added slowly, then the solution was boiled for an hour. The potassium salt was

precipitated by addition of 75 g of solid KCl.

0t-l,2,3,-K6HSiV3W9O40123 : NaVO3 (5.5 g, 45 mole) and 40 g of a-

NaloSiW9034o12H2O (15 mole)73 were mixed as dry powders and added to 400 ml
of water at room temperature. The solution was stirred vigorously and 6M HCl was
added dropwise to bring the pH to 1.5. A clear wine-colored solution developed. Solid
KCl (50 g) was stirred into the solution to leave a clear solution. Methanol (1.5 L) was
then added and the solution was allowed to age 60 hours, after which an orange
precipitate was collected. The solid was dissolved in water at 65°C and filtered. The

filtrate was cooled to 00C to form 2.3 g of an orange-brown powdered product. Purity
was checked by 51V NMR.

43

K7SiFe(III)(SO3)W1 1039-91120124 : To a warm (60°C) solution of

K58iFe(III)(H2O)W11O39-14H20124 (10.0 g, 3.1 mmole) in 100 ml of water was added
KHSO3 (15.0 g, 0.125 mole). The solution was stirred at 60°C for 10 minutes and then

cooled rapidly to 2°C. The sought salt separated upon adding 10 ml of methanol to the

cold solution. It was not recrystallized.

(NH4)8C02W12042o20H20126 : Na2WO4-2H2O (180 g, 0.55 mole) was dissolved in
400 ml of water. The pH of the solution was adjusted to a value between 6.5 and 7.5 by
addition of glacial acetic acid. A separate solution was prepared by dissolving cobaltous
acetate tetrahydrate (24.9 g, 0.1 mole) in 125 ml of warm water to which a few drops of
glacial acetic acid had been added. The sodium tungstate solution was brought to boil and
the cobaltous acetate solution was added to it slowly with stirring. After all of the
cobaltous acetate had been added, the solution was boiled for 10 minutes and then
filtered hot to remove traces of insoluble matter. The solution was heated to boiling again
and 135 g of ammonium acetate dissolved in a small amount of boiling water was added
to it. Upon cooling, an 85% yield of dark emerald green cubic crystals separated. The
product was recrystallized from hot acetic acid (0.5 ml of glacial acetic acid per 100 ml
of water). Five recrystallizations gave a product which dissolved without leaving a solid

residue.

(NH4)6H2W12040-14H20127 : A slurry of tungstic acid, H2WO4 (90.8 g, 0.36 mole)
in 240 ml of deionized water was stirred and heated to 80 - 90°C. While maintaining

temperature, 10.5 ml of NH40H (28%) was added during a period of 1 hour. Digestion
was continued for 2 hours, and the mother liquor was separated from the ammonium

metatungstate solution by filtration.

44

Nalonzwlzonozonzom : A solution of Na2WO4-2HZO (50 g, 0.15 mole) and
Al(NO3)3-9H2O (5.16 g, 14 mole) in 20 ml water was adjusted to pH = 7.0 with 6M
HNO3. A clear solution was formed after 10 - 15 minutes at the boiling temperature. The
solution was filterd and evaporated on a steam bath to obtain crystals. The product was

twice recrystallized and dried in air.

(NH4)14NaP5W300110.3lH20129 : Na2WO4.2H20 (250 g, 0.76 mole) was dissolved
in 300 ml of boiling water. 85% H3PO4 (400 g) was slowly added to it. The solution was
transferred to a Pyrex glass-lined high pressure reactor, and heated to 120°C for 5 hours.

After cooling, the ammonium salt was precipitated by the addition, with stirring, of 250 g
of solid NH4C1. The precipitate was separated by filtration, then redissolved into 400-500
ml of warm water (60°C). The salt was reprecipitated by addition of 200 g of solid

NH4Cl, then redissolved into 700 ml of water. In general, in this step, a white insoluble
deposit which formed was filtered out. The clear filtrate was then slowly cooled. The

white crystals which formed first were the desired product.

_ K7BV(V)W11040~7H20130 : The pH of a solution containing Na2WO4-2H2O (36.6 g,
0.11 mole) was adjusted to 6.3 with acetic acid and boric acid (2.47 g, 0.04 mole) was
added. The solution was heated to 80-90°C and vanadyl(lV)sulfate trihydrate (2.17 g,
0.01 mole) was added, whereupon the color changed to dark red-brown. Solid KCl was
added to the hot solution and the product was crystallized upon cooling. The salt was
recrystallized from a 0.5M potassium acetate-acetic acid buffer (pH = 5).

(K,H)9[PV14O42]-nH20131 : NaVO3 (4.5 g, 37 mmole) was dissolved in 250 ml of hot
water and 6.2 ml of 1.5M H3PO4 was added. The pH of the deep red solution was
adjusted to 2—3 by 3M HNO3. The solution was warmed to 50°C, and an aqueous

45

solution at 50°C containing 10 g KCl was added slowly. After stirring, the solution was

left standing overnight at room temperature to form fine black crystals.

B-K6HSiW9V3O4o-3H20132 : Sodium metavanadate, NaVO3 (6.4 g, 52 mole) was
dissolved in 900 ml of hor water and cooled to room temperature. To this colorless,
homogeneous solution was added 8.4 ml (101 mole) of 12M HCl. This resulted in a
pale yellow, homogeneous solution (pH about 1.5). This was followed by the addition of
solid A-B—NagHSinO34-23HZO (48 g, 16.9 mmole) to the vigorously stirred solution.

This solution rapidly developed a deep, cherry-red color as the SiW9O3410' dissolved.
The resulting homogeneous solution was reacdified with 2.8 ml (33.6 mmole) of 12M
HCl, bringing the total H+ added to 134.6 mmole (8.0 equiv.). Next, solid KCl (60 g, 0.8
mole) was added and the solution was stirred until homogeneous. Methanol was added
until the total volume was approximately 2 L. The resulting precipitate was separated by
filtration and recrystallized overnight from a hot, saturated water/methanol solution at pH
= 1.5. Filtration followed by air drying at 60°C yielded 37 g of the product. The solution

51v NMR contained a single peak at -570 ppm (1 g/4 ml of D20, pH = 2.3).

a—l,2,3,-K6PV3W9O40-4H20123 : NaVO3 (6.1 g, 50 mole) was added to 200 ml of
1.0M sodium acetate/acetic acid buffered at pH = 4.8. N a3HPW9034 (40 g, 15 mole)
was added and the solution was stirred at 25°C for 48 hours. The potassium salt was
precipitated by the addition of 30 g of solid KCl. After the solution had been stirred for
30 min., methanol (500 ml) was added to produce a precipitate which was filtered. The
product was examined by NMR. NMR (31P) -13.41 ppm. NMR(51V) -566.1 ppm at pH
= 1.8, 30°C.

KsBW1103gan2067 : A mixture of Na2WO4.2H2O (36.3 g, 0.11 mole) and H3303
(3 g, 0.05 mole) was acidified by acetic acid to pH = 6.5. The potassium salt was

46

precipitated by addition of solid KCl. The product was filtered, washed with 100 ml of

50:50 water/methanol solution, and air dried.

Bl-NagsiWnO39-nH20122 : A suspension of NagHSiW9034°23HzO (5.8 g, 2.04
mole) was added to 40 ml of 0.1M N a2WO4 solution. Slow addition of 5 ml of 1M
HCl reduced the solution pH to about 6.0. This was then followed by addition of 50 ml
ethanol. The oily portion was decanted and the desired product was precipitated as a

white powder by addition of ethanol.

Bz-K881W11039-12H20122 : Na2WO4°2HZO (182 g, 0.55 mole) was dissolved in 300
ml of water. The solution was cooled to 50C and 165 ml of 4M HCl was added Slowly.
Then a solution containing 14.7 g of Na2$iO3-9H2O dissolved in 100 ml of water was
added. The pH of solution was adjusted to 5.5 by adding 4M HCl. This value was
maintained for about 1 hour (about 30 ml acid was needed). The potassium salt,
contaminated with B-paratungstate, was precipitated by adding solid KCl. The precipitate
was redissolved into water in an amount equal to 10 times the weight of salt. The
insoluble salt was discarded and solid KCl was added to precipitate the desired product.

The precipitate was filtered and washed with dilute KCl solution.

B3-K38iW11039-12H20122 : This salt was obtained by the isomerization of B2-

SiW11039-12H20. B2-K38iW11039-12H20 (30 g, 9.4 mmole) was dissolved into 300
ml of water. The pH of the solution was adjusted to 5.0 by adding HCl. After allowing
the solution to age 4.5 hour at room temperature, the product, which was contaminated
with (at-SiW11, was placed into water. The insoluble a-isomer was discarded. The
desired product was precipitated by adding solid KCl to the clear solution. The solution
was filtered and washed with dilute KCl solution.

47

D. Synthesis of LDH-POM’S

The general method employed for the preparation of
Zn1-xAl(OH)2 (=LDH)-POM was an ion exchange reaction. The pristine LDH-N03 was
used as a starting material. The extent of the ion-exchange reaction was checked by
infrared spectroscopy, which provided the characteristic bands of NO3 at 1380 cm-1 in
case of incomplete reaction. All of the manipulations were carried out under a nitrogen

atmosphere to prevent carbonate contamination.

LDH-BW11039 : A 50 ml (5.03 mequiv.) portion of boiling LDH-N03 suspension was
added dropwise to 100 ml (8.56 mequiv.) of K9BW11039 solution at pH = 7.2. The pH
of the reaction mixture was maintained at 6.2 by addition of dilute HN O3. After an hour
of stirring at room temperature, the product was treated as usual way, in which the
product was washed with deionized water by several cenuifugations. The product was

examined by ir spectroscopy and XRD.

LDH-PM02W9039 : This was prepared in the same way as LDH-BW11039 except that
10.6 milliequivalents of PMo2W9039 were used.

LDH-BV(IV)W11040 : A 50 ml (5.03 mequiv.) portion of LDH-N03 suspension was
diluted to 110 ml and the pH was brought to 6 by addition of dilute HNO3. The boiling
LDH-N03 suspension was added dropwise to 50 ml (7.5 mequiv.) of K7BVW11040
solution at pH = 5.0. The pH of the reaction mixture after 90 minutes of stirring at room
temperature was adjusted to 6.0 by addition of dilute HNO3. The product was treated as
above.

LDH-NaP5W300110 : A boiling 100 ml (2.6 mequiv.) portion of LDH-N03 suspension
at pH = 6.0 was added dropwise to 200 ml (4.08 mequiv.) of (NH4)14NaP5W300110

48

solution. The pH of the reaction mixture after two hours of stirring was adjusted to 6.2 by

adding dilute HNO3 during the reaction. The product was treated as above.

LDH-PV14042 : A 100 ml (2.6 mequiv.) portion of boiling LDH-N03 suspension at pH
= 6.0 was added dropwise to 100 ml (2.6 mequiv.) of (NH4)9PV14042 501116011 at PH =

4.0. After an hour of stirring at room temperature, the product was treated as above.

LDH-PW9034 : A 100 ml solution (7 mequiv.) of B-NagHPWgO34 at pH = 7.6 was
added dropwise to 60 ml (4.1 mequiv.) of LDH-N03 suspension at room temperature.
After complete delivery of PW9034 solution, the pH of reaction mixture was 7.1. Dilute
HNO3 was then added to the reaction mixture to bring its pH to 6. The final pH of the

reaction mixture was 5.9. The product was treated as in the other cases.

LDH-SiFe(III)(SO3)W11040 : A 100 ml (3.55 mequiv.) of Fe(III)(SO3)W11039
solution at pH = 7.5 was added to 50 ml (2.51 mequiv.) of LDH-N03 suspension at room
temperature. After 20 minutes of stirring, the pH of the reaction mixture was brought to
6.1 by adding dilute HNO3. The final pH of the reaction mixture after an additional 35

minutes of Stirring was 6.2. The product was treated in the usual way.

LDH-a-SiV3W9O40 : A boiling 150 ml (5 mequiv.) portion of LDH-N03 suspension at
pH = 6.0 was added dropwise to 100 ml (7.1 mequiv.) of K5HSiV3W9040 solution at
pH = 3.0. After an hour stirring, the product was washed twice with deionized water by
centrifugation. The product was redispersed into 50 ml of water and then was added
dropwise to 20 ml of solution containing 0.53 milliequivalents of K6HSiV3W904o at pH
= 3.0. After an hour stirring , the pH of reaction mixture was 5.7. The product was treated

as usual.

49

LDH-H2W12040: A boiling 80 ml portion (8.04 mequiv.) of LDH-N03 suspensiom at
pH = 6.2 was added dropwise to 160 ml (10 mequiv.) of (NH4)6H2W12040 solution at
pH = 4.6. After 90 minutes of stirring, the product was washed twice with water by
centrifugation. The product was redispersed into 50 ml of water and was added dropwise
to 70 ml (4.5 mequiv.) of (NH4)6H2W12040 solution at pH = 4.2. After an hour of

stirring the product was treated as usual.

LDH-BCO(II)W11040 : A boiling 50 ml portion (5 mequiv.) of LDH-N03 suspension
at pH = 6.0 was added dropwise to 100 ml (7.5 mequiv.) of K7BCoW11040 solution at
pH = 6.0. After 90 minutes stirring, the pH of the reaction mixture was 6.7. The product
was washed twice with water by centrifugation. The product was redispersed into 50 ml
of water and was added to 100 ml (3.16 mequiv.) of BCoW11040 solution at pH = 6.2.

After an hour Stirring, the product was treated as usual.

LDH-SiW11039 : A boiling 100 ml (5 mequiv.) portion of LDH-N03 suspension at pH
= 6.2 was added dropwise to 100 ml (7.6 mequiv.) of KgsiW11039 solution at pH = 6.3.
The pH of the reaction mixture was monitored by adding dilute HNO3 during the
reaction so that after 90 minutes of stirring, the pH was 6. The product was washed twice
with water by centrifugation. The product was redispersed into 50 ml of boiling water
and was added dropwise to 80 ml (7 mequiv.) of SiW1lO39 solution at pH = 6.3. The
reaction condition was monitored by the addition of dilute HNO3 so that after an hour of

stirring, the pH of the reaction mixture was 6.0. The final product was treated as usual.

LDH-BCu(II)W11039 : An 80 ml (8 mequiv) portion of boiling LDH-N03 suspension
at pH = 6.1 was added dropwise to 160 ml (10 mequiv.) of K3Na4BCuW11040H2
solution at pH = 5.5. After 80 minutes of stirring, the pH of the reaction mixture was 5.5.

The product was washed twice with water by centrifugation. It was redispersed into 50

50

ml of boiling water and was added dropwise to an 80 ml (4.3 mequiv.) solution of
BCuW11039 at pH = 5.5. After another 80 minutes of stirring, the pH was 5.8. The

product was treated as usual.

LDH-H2W12042 : A 50 ml (5 mequiv.) portion of boiling LDH-N03 suspension at pH
= 6.3 was added dropwise to 150 ml (7.6 mequiv.) of Na10H2W12042 solution at pH =
6.2. After complete delivery of the LDH-N03 slurry to the POM solution, the pH of the
reaction mixture was brought to 6.0 by addition of dilute HNO3. After an hour stirring,
the product was washed twice with water by centrifugation. The product was redispersed
into 50 ml of water and was again added to 100 ml (3.9 mequiv.) of H2W12042 solution
at pH = 6.2. After 80 minutes of stirring, the pH of the reaction mixture was adjusted to
6.1 by addition of dilute HNO3 during Stirring. The product was washed twice with water
by centrifugation. The product was redispersed into 100 ml of water and was added
dropwise to 150 ml (1.1 mequiv.) of H2W12042 solution at pH = 6.2. The pH of the
reaction mixture was adjusted to 5.9 by addition of dilute HNO3 with Stirring. Finally,

the Slurry was treated as usual.

Reaction of LDH-N03 with B-SiV3W9O4o : A 5 ml of (0.5 mequiv.) portion of
boiling LDH-N03 slurry at pH = 6.1 was added dropwise to 20 ml (0.75 mequiv.) of B-
K6HSiV3W904o solution at pH = 2.3. After 80 minutes of Stirring, the product was
washed throughly with water by centrifugation. The exchange reaction was incomplete,
as judged by XRD. Thus, the product was redispersed into 20 ml of water and was added
again dropwise to 20 ml of the solution containing 0.75 milliequivalents of B-
SiV3W904o at pH = 3.6. After an hour Stirring, the product was treated as usual.

Reaction of LDH-N03 with a-PV3W9O40 ' : A 10 ml (0.5 mequiv.) portion boiling
LDH-N03 suspension at pH = 6.1 was added dropwise to 20 ml (1.73 mequiv.) of or-

51

K6PV3W9O4O solution at pH = 3.6. After 75 minutes of stirring, the pH of the reaction
mixture was 5.3. The excess of PV3W9040 was washed throughly with water by
centrifugation. The reaction was incomplete, as verified by XRD. The incompletely
exchanged product was redispersed into 20 ml of boiling water and was added dropwise
to 20 ml of solution containing 1.3 milliequivalents of PV3W9040 at pH = 3.4. After an

hour of stirring at room temperature, the product was routinely treated as usual.

Reaction of LDH-N03 with BV(V)W110406' : A 15 ml (0.5 mequiv.) portion of
boiling LDH-N03 suspension at pH = 6 was added dropwise to 20 ml (0.75 mequiv) of
K6BV(V)W11040 solution at pH = 5. After an hour, the pH of the mixture was 5.6. The
product was treated as above. The reaction was incomplete, as verified by XRD and ir.
The incompletely exchanged product was redispersed into boiling 20 ml of water and
was added dropwise to 20 ml of solution containing 1.2 milliequivalents of
BV(V)W11040 at pH = 5.3. After an hour of stirring, the excess BVW11040 was
throughly washed out with water by centrifugation. The product was examined by XRD,
ir and 51V NMR. A sample for N MR experiment was prepared by dissolving the product
in LiClO4 solution at pH = 2.0

Reaction of LDH-N03 with BCo(IIDW110396- : A boiling 20 ml suspension
containing 0.5 mequiv. of LDH-N03 was added to a 20 ml solution containing 0.75
milliequivalents of K6BCo(III)W11039 at pH = 5.5 and at ambient temperature. After 90
minutes of stirring, the product was washed in a routine way. The excess of
BCo(III)W11039 was contained in the supernatant. The pH of the reaction mixture at the

end of the reaction was 6.1. The product was examined by XRD and ir Spectroscopy.

Reaction of LDH-N03 with Bl-NagsiW11039 : A boiling 20 ml (0.39 mequiv.)
portion of LDH-N03 suspension at pH = 6.2 was added to 20 ml (0.61 mequiv.) of

52

SiW11039 solution at pH = 6.4. After an hour of stirring, the reaction product was

treated as described previously.

Reaction of LDH-N03 with Bz-KSSiW11039 : A boiling 20 ml (0.5 mequiv.) portion
of LDH-N03 suspension was added dropwise to SiW11039 solution containing 0.75
milliequivalents at pH = 7.6. After 30 minutes of Stirring, the pH of the reaction mixture
was lowered to 6.1 by addition of dilute HNO3. After an additional 15 minutes of
stirring, the product was washed with water by centrifugation. The product was
redispersed into 20 ml of water and added to a 20 ml solution containing 0.63
milliequivalents of SiW11039 at pH = 6.4. After 35 rrrinutes of stirring, the pH of the
reaction mixture was adjusted to 6 by adding dilute HNO3. After 50 minutes of Stirring,

the final product was collected and washed in the usual way.

Reaction of LDH-N03 with B3-K8SiW11039 : This reaction was carried out exactly
in the same way as described for the 131-SiW1 1039 intercalate. During reaction, the pH
was adjusted to 6.1 by adding dilute HNO3.

E. Physical Measurements

l. X-Ray Powder Diffraction

X-ray diffractograms were recorded using a Philips model APD 3720
diffractometer. Ni—filtered Cu K; radiation was used as the X-ray source. Typically, thin
film sample was prepared by dropping a water slurry onto either a glass Slide or a quartz
sample holder, followed by air drying.

For the swelling experiments, samples were prepared on ceramic plates and kept

wet to maintain the spacing characteristic of the most product.

53

2. Gas Adsorption Measurement

N2 gas adsorption at 77K was measured using either a Quantasorb Jr. Surface
Area Analer or a Cahn electrical balance. Samples were outgassed under vacuum at the
desired temperature, usually in the range 120°C to 135°C. He and N2 gas were passed
through a trap maintained at dry ice temperature. Measurement performed in the Cahn
balance provided data points at low p/po, which are useful especially for the micropore
analysis of the material. The BET N2 surface areas were determined by the multiple
point method in a p/po region where the data conformed to a linear BET plot.

3. Thermal Stability Study of LDH-POMS

Oriented film samples supported on either a glass Slide or a quartz plate were
heated at a given temperature for 5 hours in a Fisher Model 497 Isotemp Programmable
Ashing Furnace in the open air. The heated samples were examined by X-ray diffiaction
and FTIR spectroscopy. The rehydration properties were studied by soaking the heated
samples in water for one day, followed by air drying. Then, these rehydrated samples
were reexamined by X-ray powder diffraction. Fourier transform infrared spectra of all
samples were taken using an IBM model IR 40S. All sample were prepared by the
Standard KBr technique.

Differential scanning calorimeu'y and thermal gravimetric analysis were carried
out on a Dupont 9900 thermal analyzer under N2 purge. Temperature was ramped at a
rate of 5°C/min from room temperature to 500°C.

4. Nuclear Magnetic Resonance Studies

A Bruker WH-180 Spectrometer was used to obtain 29Si, 51V, 31P, 11B, and
23Na at 35.77 MHz, 47.32 MHz, 72.88 MHz, 57.77 MHz, and 47.62 MHz, respectively.
The WH-180 spectrometer was equipped with Nicolet NTCFT-180 computer software
and a Doty solid State probe operated at the magic angle of 55°44’. In the case of solid
samples, the sample spinning frequency was between 3.5 KHz and 5 KHz. TMS, VOCl3,
85% H3PO4, saturated H3BO3, and NaCl were used as references for Si, V, P, B, and

54

Na, respectively. The Na chemical Shift of NaCl was taken to be 7.7 ppm relative to the
infinitely dilute aqueous NaCl solution. The chemical shifts of other references were
taken to be 0.0 ppm. The solution sample derived from LDH-SiV3W904o were prepared
by dissolving the intercalate in acidic (pH = 1.5 - 2) LiClO4 aqueous solution.

F. Photocatalytic Reaction

The pre-dispersed LDH-POM in H2O was transferred to a 45 ml of centrifuge
tube and washed several times with isopropanol by centrifugation. The sample was
redispersed in iSOpropanol and transferred to one of the chambers of an H-type
photoreactor. The LDH-POM slurry was outgassed by pumping and refilled with Ar or
02. Photolysis was carried out with an Oriel le Hg(Xe) ozone-free lamp equipped
with a 295 nm long pass filter and a water filter. The radiation was focused on the
photoreactor by use of a secondary focussing lens. After 24 hours irradiation, the liquid
reactant and products were separated from the solid LDH-POM by dipping the empty
chamber of the H-type reactor into liquid nitrogen. After completely transferring the
liquid, it was analyzed using a Varian aerograph model 920 gas chromatograph equipped
with a 3 m carbopack B/3% Sp-1500 column purchased from Supelco. Helium was used
as a carrier gas with a flow rate of 0.25 ml/sec. The column-, detector- and injection-
temperatures were 90°C, 180°C and 90°C, respectively. A thermocouple detector was
used at a filament current of 200 mA. The solid LDH-POM used in the reaction was

weighed after being dried under vacuum.

G. Elemental analysis
Zinc and aluminum analyses of LDH-N03 or -Cl were done at the inorganic
chemistry laboratory of the Department of Toxicology at Michigan State University. The

analysis of the other samples was done by Galbraith Laboratories

55

H. Transmission Electron Microscopy (TEM)
Specimens for TEM were obtained by dropping a dilute slurry of the sample in
water on copper grids. TEM was carried out a Jeol 100cx at the Center for Electron

Optics at Michigan State University.

CHAPTER III
RESULT AND DISCUSSION

A. Synthesis of LDH-Cl, N03

When a solution containing Zn2+ and Al3+ and C1- or NO3- counter anions with
Zn/Al = 3 is precipitated at pH = 10 by the addition of OH: a LDH phase Zn2Al(OH)6A
and a secondary phase are formed upon hydrothermal treatment of the gel. The second
phase is either Zn5(OH)3C12 or Zn5(OH)3(NO3)2, as verified by their XRD patterns
presented in Fig. 9. The ratio Zn/Al in the mixed solid product was 3. When the mixed
product was redispersed in water and H+ was added to bring the pH to about 6, the
second phase disappeared. That is, the second phase dissolved at this acidic condition.
The excess Zn2+ resulting from the dissolution of second phase could be reprecipitated
by adding OH-.

When a solution containing Zn2+, Al3+ and Cl- with Zn/Al = 3 was precipitated
at pH = 6, no secondary phase was formed. The Zn/Al ratio in the solid precipitate was 2.

When a solution containing Zn/Al = 5 was precipitated at pH = 10 the second
phase was observed again. Even a solution containing Zn/Al = 2 upon precipitation at pH
= 10 exhibited the second phase. Therefore, the second phase is not due to excess Zn, but

due to the coprecipitation of Zn5(OH)3A2 at a too high pH.

56

57

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am

 

 

 

 

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No A mm 3 omonn 6.568 do ca; 51394 Ill

58

B Synthesis of LDH-POMS

Ion-exchange was the general method employed for the preparation of LDH-
POMS. The pristine LDH-N03 was used as a Starting material. This derivative is a high
crystalline material, providing a highly crystalline exchange product. Because of the low
ion exchange selectivity toward NO3', the exchange reaction takes place easily. It is easy
to check the extent of reaction, because the unexchanged NO3- can be detected easily by
ir spectroscopy due to its characteristic N-O Stretching absorption band at 1380 cm'l, as
shown in Fig. 10.

When an exchange reaction is carried out, two major parameters should be
considered in order to get high purity, crystalline products :
l) the pH of reaction mixture
2) the reaction temperature
Because LDH-N03 is amphoteric, it is stable only within a limited pH range, i.e., 5.8 <
pH < 10, otherwise it will dissolve. AS discussed in the previous chapter, POMs are also
generally hydrolytically unstable. However, they are quite stable within a certain pH
range. The hydrolytic instabilities of both LDHS and POMs impose a restricted pH range
on exchange reaction conditions. In addition to the hydrolytic instability problem, the
product crystallinity can be affected by pH. In the case of LDH-NaP5W300110, in which
N aP5W3oO1 1o, unlike most POMS, is hydrolytically stable even under basic conditions
at elevated temperature, the crystallinity of the product obtained at neutral pH is very
much ' lower than that obtained under acidic condition, as is shown in Fig. 11.
Furthermore, under basic conditions, the exchange reaction of POMS does not produce a
completely exchanged crystalline product. Therefore, it is necessary to keep the reaction
pH acidic.

The exchange reaction also depends on temperature. Exchange carried out at low
temperature is always incomplete. When exchange is executed at elevated temperature,

reaction occurs rapidly and completely. Therefore, it is desirable to carry out the

59

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61

exchange reaction at high temperature, provided that the POM is Stable at this
temperature. In general, an aqueous suspension of LDH-N03 in boiling water is added
dropwise to a POM solution at ambient temperature. When a drop of LDH-N03 slurry is
added to the POM solution, the local temperature at the contacting spot will be
momentarily high enough to cause a fast exchange reaction, yet not cause POM
decomposition. Once the POM is intercalated into the LDH gallery, it is hydrolytically
Stabilized. When a completely exchanged LDH-POM is heated in water to the boiling
point. there is no indication of POM degradation in the gallery as demonstrated by ir and
XRD. An exception is LDH-PV14042, which is unstable under these conditions. In the
case of LDH-SiFe(SO3)W11039, if the hot LDH-N03 suspension is added to the POM
solution, a two phase containing mixed product is formed. The XRD is shown in Fig. 12.
The impurity phase LDH-803 intercalate (dool = 7.98 A) was formed from the
decomposition of Si(SO3)W11039 to SO32-and SiW110393-at such temperatures. An
impurity phase is also obtained in case of LDH-PW9034. Therefore, in those cases
where POMs are not hydrolytically stable, the POM solution should be added to the

LDH-N03 Slurry at room temperature to prevent impurity phase formation.

C. Characterization of LDH-POMS

1. Electron Microscope Studies

The ion exchange reaction employed for the preparation LDH-POMS was a
topotatic reaction, This was proved by an electron microscopy study of the products,
LDH-POMS, and the starting material. The electron micrographs are provided in Fig. 13.
The crystal shapes of the products are hexagonal, just as for the starting. material, also the
crystal Sizes of the products are of the order of one micron, just as for the starting
material. On the basis of these electron micrographs, it is obvious that the exchange

reaction has taken place in a topotatic way.

 

62

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41

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Fig. 12. XRD pattern of a) LDH-SiFe(SO3)W1 1039 and b) LDH-PW9034 prepared

using a hot LDH-N03. * represents an impurity phase.

63

Fig. 13. Transmission electron microscope pictures for LDH-POMS. (top, left),
LDH-N03; (top, right), LDH-BCuW11039; (below, left), LDH-PM02W9039;
(below, right), LDH-PW9034.

 

65

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66

2. X-ray Powder Diffraction (XRD) Study

This technique is one of the most powerful methods for characterizing the LDH-
POM intercalates. It provides information on the completeness of the exchange reaction,
as well as on the phase homogeneity of the product, LDH-POM.

The XRD patterns of the LDH-POMS described in this work, which are provided
in Appendix A, are divided into two categories. One is that in which the POM is of the
Keggin-ion type. In this case the gallery height 14.8 A - 4.8 A = 10 A corresponds to the
size of a POM consisting of four oxygen layers. In the second class of LDH-POM the
POM is a non-Keggin-type, for example, LDH-H2W12042 and LDH-NaPsW300110.
Although the basal spacing of the former is the same as that for an LDH-POM in which
the POM is a Keggin-type, the Structure of the H2W12042 anion is not a Keggin-type, as
shown in Fig. 14. When this anion is oriented in the gallery so that a mirror plane is
parallel to the c direction, the resulting gallery height Should give a value corresponding
to four oxygen layers. In the case of LDH-NaP5W300110, the gallery height is 16.5 A,
which corresponds to a value of seven oxygen layers. AS given in Fig. 15, when this
anion is oriented with the C5 axis parallel to the c direction, the resulting gallery height
Should correspond to seven oxygen layers.

3. Elemental Analysis

Elemental analysis data (weight %) for some representative LDH-POMS are
provided in Table 11.

4. Nuclear Magnetic Resonance (NMR) Studies

It has been demonstrated that NMR is a powerful technique for the elucidation of
the Structures of POMS. In this work, 298i, 31P, 11B, 51V, and 23Na were accessible
nuclei with our N MR facility. Even though this technique provides fine structural
information, it was not applicable to any LDH-POMS containing paramagnetic atoms
such as Co(II), Fe(III), and Cu(II). The following summarizes the NMR properties of

each system accessible with our facility.

67

Fig. 15. a) An ORTEP view of [NaP§W300110]14‘ looking approximately along the
virtual C5 symmetry axis. Oxygen and tungsten atoms are represented as large and small
Open cireles, respectively, and the sodium ion by the central closed circle. The
phosphorus atoms are Shown as closed circles that are almost entirely obscured by the
inner pentagon of five tungsten atoms. b) An idealized view along C5 Showing the W06
octahedra in the upper half of the anion. c) The PW6022 unit view perpendicular to the
anion’s virtual C5 axis. Oxygen and tungsten atoms are represented by the large and

small open circles, respectively, and the phosphorus atom by the closed circle.

 

69

Table 11. Elemental Analysis Data for LDH-POMS

 

 

 

wt % wt %
POM observed calculated n*
Zn Al W Zn Al W
H2W1204o 14.8 3.1 41.6 15.6 3.9 40.0 22
SiW11039 18.9 3.9 36.9 17.4 3.7 38.5 19
PW9034 21.0 4.4 29.5 18.4 4.8 31.7 28
BW11039 21.0 3.8 38.1 21.5 4.4 37.0 26
H2W12042 19.2 3.5 38.7 21.2 4.4 35.8 38
NaP5W300110 12.1 2.1 48.7 14.9 3.1 44.9 67

 

n* represents a hydration number, as estimated from the TGA curves for each sample.

70

LDH-SiV3W9O40 : The NMR spectra of this system could be obtained both in the state
and in solution. For a solution Spectrum, a sample was prepared by dissolving solid LDH-
SiV3W9040 at pH = 2.0. At this pH the host layer was destroyed, but the anion
SiV3W904o was stable. The 51V NMR spectra of LDH-SiV3W904o and that of the
potassium salt in solution at pH = 1.7 are given in Fig. 16. The observed peak positions
of the LDH-SiV3W9040 and cr-K6HSiV3W9O4O are -567 ppm and -572 ppm,
respectively. The reported value for the potassium salt at 30°C is -573 ppm at pH =

1.5.122 The NMR chemical shift for the 51V has been reported to be sensitive to sample
solution pH, so that 8(51V) Shifts to upfield (Shielding) upon protonation.133
Protonation increases the energy of a ligand-to-metal charge transfer, and hence
decreases the absolute resonance frequency by reducing the deshielding contribution of
the temperature-independent paramagnetism. Therefore, the small change in the chemical
Shift for the dissolved sample at pH = 2.0 means that the structure of the anion in the
LDH-SiV3W904o is retained. The spectrum for the LDH-SiV3W904o, has a single
peak reasonably close to the peak position observed for the potassium salt, Strongly
suggestive that in the gallery the guest anion, SiV3W904o has the same structure as the
potassium salt of the anion.

In the solid state state, the 29Si NMR spectra of both LDH-SiV3W904o and the
potassium salt of the anion, as provided in Fig. 17, demonstrate that the anion structure
under the two different surroundings is identical. There is only one peak at -84 ppm in
both cases. The reported value is -84 ppm at pH = 1.5 and at 30°C.122

The solid State 51V MAS NMR of LDH-SiV3W9040 does not provide clear
structural information due to the complexity of the spectrum arising from spinning Side
bands, as shown in Fig. 18. On the basis of the above data it can be concluded that the
structure of the guest anion in the LDH gallery is identical to that of the potassium salt.
LDH-PM02W9039 : It has been reported that the peak position of the 31P NMR of this
POM correlates with the Structure of the POM.64 In the case of orthophosphate, PO43',

71

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74

the o-bond between a P atom and an oxygen atom is formed with Sp3 hybrid orbitals on
the phosphorus atom. In addition to the o-bond, 1t bonding forms through participation
of the p orbitals of the oxygen atoms. This rt-bond formation substantially reduces the
positive charge on the P atom. Increasing the degree of 1t-interaction leads to deshielding
of the P nucleus, and its resonance absorption is observed at lower field. Consequently,
any interaction of the oxygen atoms with the external surroundings will influence the
electron density around the P atom. This idea can be applied to the Keggin anion,
PM12040, in which each oxygen atom of P04 is bounded to three metal atoms of the
coordination sphere. The interaction of those atoms with phosphorus is influenced by the
ability of the metal to remove electron density. Thus, oxygen atoms will Shield the
phosphorus atom to different degrees. In all cases except that of M = V, the 31P NMR
line is shifted to higher field, reflecting a reduction of the n-interaction in the POM. The
increase of 8 values according to the sequence PW12 < PM012 < PV12 is easily
understood because the electron-accepter properties increase in the series V < Mo < W. It
is also known that the greater the shielding, the higher the v(P-O) frequency.64 With
defect Keggin-type compounds, for example, PM11039, occurrence of a hole (defect)
leads to a weakening of the PO bond and thus induces an increase in the 8 value. When
the hole is filled by a cation that forms Strong ionic bonding with the defect POM, for
example, Zn2+, the oxygen atoms of the hole Site are not substantially affected. Thus, the
chemical Shifts are similar to those of the parent "defect Structure". On the other hand, if
the hole is filled by a metal covalently bonded to the POM, the defect character is greatly
attenuated, and the chemical shift approaches that of saturated Keggin-type compounds.
The solid state 31P MAS NMR spectra for LDH-PMo2W9039 and
K7PMo2W9039 are provided in Fig. 19. Only one peak is observed at -10.8 ppm for
LDH-PM02W9039 and at - 10.0 ppm for the potassium salt of anion. The reported value
for the potassium salt is -9.4 ppm.64 The chemical Shift for the LDH-PMo2W9039 is

slightly lower than that of the potassium salt. The P tit-electron density in the P04

 

75

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76

tetrahedron has been reduced upon intercalation of the anion in the LDH gallery.
Hydrogen bonding between the POM anion and the hydroxyl layer may be a source of
such electronic perturbation around the P nucleus.

On the basis of these NMR data, it is certain that the structures of the
PMo2W9039 anions in the LDH intercalate and in the potassium salt are identical.
LDH-PW9034 : The solid state 31P MAS NMR Spectra for the LDH-PW9034 and the
potassium salt are Shown in Fig. 20. In the case of LDH-PW9034, there are two peaks at
-13.3 ppm and at 0.4 ppm. The latter peak is due to an impurity and cannot be assigned to
a POM containing a P atom. It has been observed that the size of impurity peak increases
with aging. POMS containing phosphorus have not been observed to evidence such 31P
chemical Shifts. The major peak at —l3.3 ppm for the LDH-PW9034 is observed at lower
8 region (higher field) than that of the corresponding potassium salt (97 ppm). The
hydrogen bonding between the anion and the hydroxyl layer in the LDH gallery may be
responsible for the lower 5 value, as in the LDH—PMo2W9O34 case. However, an
alternative explanation is that the PW9034 anion is transformed to a phosphorus
containing POM which has the same size as the parent PW9034. In this case the major
peak at -13.3 ppm and the impurity peak at 0.4 ppm could arise from the POM containing
a phosphorus atom and the phosphate anion, respectively. The hydrolysis of the PW9034
anion in the gallery proceeds to some extent by aging. The latter argument is more
plausible than the former. On the basis of the LDH-PW9034 ir data, which will be
discussed in the next section, the LDH product formed from the PW9034 anion
resembles that formed from PW9+xO34+y (0 < x < 3).

LDH-PV14042 : The solid state 31P MAS NMR spectra for both LDH-PV14042 and
(NH4)9PV14042 are given in Fig. 21. Both spectra exhibit only one peak at -1.25 ppm
for the LDH-PV14042 and at -1.38 ppm for the ammonium salt. The Similarity of these

spectra suggests that the anion under the two different circumstances is identical.

77

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593.6

 

 

 

 

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78

-1.3 - .
(a) PPm (b) l 1 4PPm

 

 

 

 

L)

. J . .21 2
PPM ° -20 PPM 0 --20

Fig. 21. Solid state 31P MAS NMR for a) LDH-PV14O42 and b) (NH4)9PV14042.

79

LDH-BW11039 : Boron atom has a spin 3/2. Thus, it has an electric quadruple moment
which will lead to line broadening. The 11B solid state MAS NMR for both LDH-
BW11039 and K9BW11039 are provided in Fig. 22. The spectrum for the potassium
salt of the BW11039 anion shows a typical powder pattern containing a peak doublet. In
this case, the quadruple interaction Still contributes to the line broadening due to the
asymmetric environment around the central B atom (Cs symmeu'y). Therefore, even
though the Spectrum was obtained by the MAS technique, its Shape still shows
appreciable line broadening. On the other hand, in the case of LDH-BW11039, the
spectrum contains a much more narrow single line at ~17.3 ppm. The reported chemical
Shift value for the BW120405' anion in solution is -17.6 ppm relative to a saturated
solution of boric acid.134 Such a narrow single line indicates a highly symmetric
environment around the B central atom in the BW11039 anion. Consequently, line
broadening due to the quadruple interaction will vanish. The highly symmetric
environment around the central B atom upon intercalation was indicated in the ir
spectrum. Split bands assigned as v(B-O) due the low symmetry environment around the
B atom in the potassium salt merged into one single band at 985 cm"1 for the intercalated
anion in LDH. The number of ir absorption bands for the potassium salt (Cs symmetry)
has been found to be greater than that for the intercalated anion, as given in Fig. 32.

The increased symmetry of the anion may be due to the filling of the vacancy in
the defect Keggin structure with a group which may come from the hydroxyl layer (e.g.,
OH)
LDH-NaP5W300110 : The solid state 23Na MAS NMR spectra of both LDH-
NaP5W300110 and the ammonium salt are given in Fig. 23. These Spectra also Show a
typical powder pattern. The similarity of the two spectra suggests an identical anion
structure under the two different circumstances. The Slight line broadening that occurs
upon intercalation may be due to a less symmetrical surrounding around the Na nucleus

due to shift of the Na atom out of the central position in the POM. The shift in Na+

80

 

 

 

 

 

- 17.3ppm
M —22.2ppm
- 19.2ppm
(a) (b)
g l l I l L l 4 l l L I L
10 25 10 25

Fig. 22. Solid state 118 MAS NMR for a) LDH-BW11039 and b) K9BW11039.

81

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82

position may arise because of interaction of the POM with the hydroxyl layer in the
gallery. For instance, H-bonding may result in an increase in the electric field gradient
around Na nucleus. Thus, the quadruple interaction will be increased and the line will be
broadened.

The solid state 31P MAS NMR spectra of both LDH-NaP5W300110 and the
ammonium salt of the POM are provided in Fig. 24. Since both Spectra Show a Single
peak at -9.8 ppm for LDH-NaP5W300110 and at -9.7 ppm for the ammonium salt, it is
obvious that the POMS in the different environments are identical.

5. Infrared Spectroscopic Studies

The ir Spectra of LDH-POMS should consist two main components. One should
be the contribution of the host layer, which is a layered octahedral metal hydroxide
lattice. The other should be the contribution of the guest POM anion.

a) Vibrations of the host layer

Information on the octahedral layers can be obtained from lattice vibrations. In
principle, these vibrations can be assigned to the translational motions of the oxygen
layers. A vibrational study of layers having the composition M(II)2M(III)(OH)6 was
done by Sema et 31.135 They assumed that the layer had ideal D3d symmetry. Under this
assumption, seven it active modes, namely, 3A2u + 4B“, are possible. Only five
vibrations (2A2u + 3Eu) can be observed above 250 cmrl. Within the frequency range
accessible with our IBM-40 FI‘IR instrument, that is, above 400 cm-1, only three bands
are observable. The most distinctive band, a M(II)-O-M(III) stretch, occurring in the
range 460-410 cm-1 is dependent on the metals occupying the octahedral interstices. For
example, a Strong band is observed at 435 cm-1 for Ni2Al(OH)6.l/2CO3, 455 cm-1 for
Mg2Al(OH)6-1/2CO3 and 430 cm-1 for Fe2Al(OH)6-1/2CO3. For our Zn2Al(OH)6-NO3
system, the band is observed at 427 cm-1. The other two bands for Zn-O-Al stretching
occur at 678 and 558 cm-1 as shown in Fig. 10. The other type of layer contribution is a

6(OH) vibration at about 620 cm'l.

83

-9.8pprn ‘9-7PPm
(a) (b)

 

 

 

 

 

gawk.

- J O -;
PPM O 20 PPM 20

Fig. 24. Solid state 31p MAS NMR for a) LDH-NaP5W3OOl 10
and b) (NH4)14NaP5W300110.

84

b) Vibrations of guest POM anion

Ir spectroscopy is extensively used in polyoxometalate chemistry for structural
elucidation. Especially, the vibrational spectra of Keggin structured POMS (XM12040)
have been extensively studied by Rocchiccioli-Deltcheff et al..135 As discussed in the
introduction chapter, the (rt-Keggin structure consists of one X04 tetrahedron surrounded
by four M3013 (M3 triplet) sets formed by three edge-sharing octahedra. These
structural features are illustrated in Fig. 25. Theoretically, there are 22 it active
modes.135f) These modes belong to vibrations of the T2 type. In view of the complexity
of this anion, many vibrations may not be observed on account of either random
degeneracy, low intensity of the bands, or occurrence beyond the measurable range limit.

Three major groups contribute to the POM spectrum, namely, the 12 terminal
M=Od bonds, the 24 bridging M-O-M bonds and one central XOa4 tetrahedron. In the
bridging M-O—M groups, there are two types of bonding in the bridging M-O-M groups:
12 almost linear M-Oc-M "intra" bridging bonds135b) contained within four M3 triplets :
12 bent M-Ob-M "inter" bridging bonds connecting the four M3 triplets to each other.
The band due to the M=Od group occurs in the 1100-900 cm-1 range. For example, the
band v(W=O) is observed at 935 cm‘1 for H2W120406', 960 cm'1 for BW120405',
982 cm-1 for SiW12O404- and 985 cm-1 for PW12O403’.136 d) The valence vibrations
of the second M-O-M group (symmetry C2v) are observed in the range of 900 - 200 cm“
1. Three bands can be observed, namely, the symmetric and asymmetric valence
vibrations vasy and vsy and one deformational vibration 8 when vasy > vsy.137 In
general, two bands at 920 - 850 cm-1 and 800 - 750 cm-1 are assigned to the
antisymmetric valence vibrations of M-Ob-M and M-Oc-M, respectively. The latter band
is usually Strong. The vibrations of the tetrahedral X04 group in the anion appear in the
region of 1100 - 800 cm-1 (A1 and T2) and 600 - 400 cm-1 (E and T2). Only three with

T2 symmetry are ir active modes. They include two vibrational modes v3 and V4 which

85

O. 0.

 

 

 

 

‘- O... . ob

 

 

 

 

 

 

“XM12

 

 

 

 

 

 

 

 

 

 

0,

(X141.
b

Fig. 25. Idealized structures of (rt-XM12 (Keggin structure) (a) and or-XMll (defect
Keggin Structure) (b) (the central XO4 tetrahedron is not shown). (Oa = oxygen Shared by
3 M06 octahedra and the x04 tetrahedron, ob and 06 = oxygen linked to two different
M atoms, Od = terminal unshared oxygen).

86

represent a stretching and a deformation morion, respectively. In general, v3 > v4. Table
12 provides the vibrational frequencies of the most frequently occurring XO4.

Table 12. Vibrational Frequency of XO4.135 d)

 

X B Si Ge P As

 

M W W Mo W Mo W Mo W Mo

 

v(cm'l) 920 930 910 815 802 1080 1070 915 900
x x x 460 460 598 598 472 472

 

x: Complicated bands in the region of 500—530 cm-l.

Except for X = P, the X-Oa band assigned as vasy(X-Oa) often has a discussed shape. It
is rather broad because of band mixing with v(M-Ob-M). In the case of X=P, vasy(P-
03) is well resolved from other bands, thereby providing an accurate assignment.

In the case of a lacunary Keggin structure, XM11039, the symmetry will
decrease from Td to Cs. The decrease in symmetry will result in band broadening and the
splitting of degeneracies, thereby increasing the number of bands. However, such
splittings do not take place as much as expected.135 CLC) A general decrease of
Stretching frequencies is observed on account of a weakening of the anion cohesion. In
particular, the strong band around 800 cm-1 attributed to vasy(M-Ob-M) is split into two
bands at 797 and 725 cm-1 for SiW11039 and into two bands at 810 and 725 cm-1 for
PW11039. For the PM11039 anion, vs(P-Oa), which is well separated from the other
bands of the spectra for PM12040 as explained above, is split into two components. The
two bands occur at 1060 and 1040 cm-1 for PM011O39 and at 1085 and 1040 cm-1 for
PW11039. This splitting is also due to the symmetry decrease of the P04 tetrahedron.

When a lacunary POM XM11039 forms complexes of the type XM11M’O39, in
which M’ are the 3d series ions, A1111, SnIV or BIII , the ir Spectra of these complexes
Show that the ligands retain a structure related to the Keggin structure.135 6)

Complexation always induces an increase in stretching frequencies, but the shifted

87

frequencies remain lower than the frequencies observed for XM12040. This increase
shows a strengthening of the anion cohesion and an opening of bridge angles relative to
those of the free ligand, because the du-p,t interaction of the M-O-M bond is enhanced

by increasing the bridge angle M-O—M. In the case of PM11039, the v3 P04 splitting
value, Av, is always lower than that of the free ligand. Therefore the filling of the vacant

site by an M’ atom restores to some extent the symmetry of the central tetrahedron,
owing to the interaction between the M’ and 0a atoms.1356)

When a XM11039 degradates further to XM9034, the symmetry changes from
Cs to C3v. The general decrease of stretching frequencies is also observed when
XMle4o changes to XM11039. For example, v(W=Od)s are observed at 985 cm’1 for
PW12040,135 d) 950 cm-1 for PW11039135 C) and 931 cm-1 for PW9034.137
Furthermore, v(P—Oa) decreases in the same way ; 1080 cm'1 for PW12040,135 d) 1085
and 1040 em-1 for PW11039,135 c) and 1056 and 1002 em-1 for a-PW9034.138

When some of the tungstens in the XW12040 Keggin Structure are substituted by
M0 or V, for example, SiW12-xMoxO40 (x = 1 - 3), their ir spectra are intermediate
between that of SiW12040 and that of SiM01204o, without the appearance of new
bands due to the decrease of the molecular symmetry.135 d) The band around 800 cm-1
attributed to v(M-Ob-M) still remains discussed without Splitting as is observed for
SiW1204o and SiM012040.

The detailed description of the ir spectroscopic features of each LDH-POM will
be presented below (cf. Table 13). However, a few general conclusions will be provided
first. Generally Speaking, when POMs are intercalated into the gallery of the LDH, their
ir bands shift either to higher or lower frequencies. Rocchiccioli-Deltcheff has disscussed
the influence of counterions on the vibrational spectra of POMS in the solid State.135 b)
He defined two opposing effects, labelled effect A and effect B. The former leads to an
increase in the Stretching frequencies, for example, v(M=Od), with respect to the

reference spectra (effect AS) and a decrease in the vibrational frequency for the bending

88

motion of the bridging M—O-M (effect Ab), whereas the latter leads to an decrease in the
the stretching frequencies. For example, the anion-anion interaction belongs to effect A
and hydrogen bonding belongs to effect B. Therefore, the band shifts will be determined
by these two competing effects. When effect A is predominant, a blue shift will result in
v(M=Od). When they contribute equally, there will be no band shifts.

Generally speaking, in this work, red Shift of vagy(M=Od) for the POMS in the
gallery were observed most commonly due to the predomination of hydrogen bonding
between M=O in POMS and the hydroxyl group in the host layer. The size of the POMS
in the gallery can be calculated by subtracting the thickness of the hydroxyl layer (4.8 A)
from the basal spacings of the LDH-POMS determined from the XRD data. The size of
the intercalated Keggin type POM is about 9.6 A to 9.8 A, which is smaller than that
obtained for POMs (10.4 A) in crystallographic Studies.135b) This observation strongly
suggests that the interaction between the guest POM and the host hydroxyl layer is
strong. Such a Strong interaction causes d-spacing contraction, with resulting stabilization
of POMS in the gallery. Most POMS except PV14042 are Stable in the gallery even in
boiling water. In this latter case, a blue shift of the v(V=Od) band was observed when
the compound was in the gallery. This blue Shift arises from the predomination of the
anion-anion interactions in the gallery. These interactions cause an increase in the
potential energy of POM network, thereby increasing the vibrational frequency. In fact,
PV14042 is not a keggin structure but is a derivative, having two more V=Od units
located on opposite Sides of the C3 axis of the original Keggin Structure as Shown in Fig.
26. The distance between these two V=Od is about 13.4 A.139 The resulting shape
resembles a football. The long axis is about 13.4 A and the Short axis 10.4 A. The
PV14042 ion is oriented in the gallery so that the long axis is parallel to the hydroxyl
layer. Such orientation will result in a decrease of the distance between POMS, thereby

enhancing the anion-anion interaction.

89

 

"Q
i‘

 

 

 

 

 

 

Fig. 26. The polyhedral model of the PV140429‘ anion.

90

In order to explain the anon-anion interaction in the gallery, we define a new
quantity, the average distance between the POMS in the gallery. This quantity is obtained
on the basis of the hexagonal array of the POM in the gallery. Its detail will be discussed
in the other section. In the case of PV14042, which has a football like shape, the average
distance is 13.2 A, which is smaller than the longest axis (13.4 A) and larger than the
shortest axis (10.4 A). Thus, two neighboring POMs can be oriented so that they can
contact each other. Rocchiccioli-Deltcheff et a1135 b) reported that the interaction would
be effective if the distance between POMS is less than 6 A. Therefore, the perturbation
will Spread over the POM framework. This anion-anion interaction results in unusual
behavior, such as the instability of POMS in the gallery at high temperature in water, its
thermal instability in the solid state, and a blue shift of the v(V=Od) band on
intercalation.

c) Vibrational Study of LDH-POMS

The details of each system are discussed below. All band positions with their
assignments except those for NaP5W3oO110 and LDH-NaP5W300110 are listed in
Table. 13.

LDH-H2W12O4o6- : When H2W12040 is intercalated into the LDH, it experiences two
opposing effects, that is, effect A and effect B as discussed above. In this case, effect B is
predominant, inducing a red Shift (about 11 cm-l) for the band assigned as v(W=Od),
even though the average distance between the POMS in the gallery is Short enough (10.7
A) to promote a effect A. The ir Spectra of POMs under the two different surroundings
are shown in Fig. 27. There is also a red shift of v(W-Oc-W) from 760 cm‘1 for the
NH4+ salt to 757 cm:1 for H2W1204o in the intercalated State. The red Shift of this band
is due to the effect Ab and effect B, as was pointed out by Rocchiccioli-Deltcheff.135 b)
The unlisted bands are 8(OH) at 620 cm‘1 and v(Zn-O-Al) at 427 cm‘l.

LDH-BVW110407- : In this complex there is almost no observable Shift for v(W=Od)
even though the average distance between the POMS in the gallery is 11.6 A. Therefore,

91

Table 13
Infrared Spectral Data of Free-POMS and LDH-POMS

 

 

Compounda Assignmentfi
vas(X-Oa)9 vas(M=Od) v(M-O-M)

(NH4)6H2W12042 - " 936 ‘ 901, 871,
760

LDH-H2W1204o - 925 895, 7 57 11

K7BVW11040 990, 908 947 885, 811. -
693. 673

LDH-BVW11040 998, 905 949 807, 695 -2
668

K5HSiV3W904o 1007 915, 965 782, 634 -

LDH-SiV3W9O4o 1003 913, 959 780 e

a-KSSiW11039 995 960 891, 799 -
728

LDH-SiW11039 1001 955 791, 762 5
7 15

K7PM02W9O39 1082, 1046 945 897, 855 -

1033 811, 734

LDH-PMo2W9039 1081, 1050 946 803, 745 -1

KgBW11039 995, 917 954 884, 870 -
836, 811
788, 751
7 19

LDH-BW11039 985 939 896, 786 5
783, 716

K7BCuW11O39 987, 907 940 875, 810 -
711

LDH-BCuW11039 986 937 897 , 807 3
7 10

K7BCoW11O39 984, 912 940 885, 820 -
775

LDH-BCoW11039 984 937 897, 815 3
782

K7SiFe(SO3)W11039 996 953 885, 797 -
727

LDH-SiFe(SO3)W11039 1000 952 903, 784 1
7 16

B-NagHPW9034 1055, 1014 936 882. 819 -
748

LDH-PW9034 1083, 1057 947 809, 685 -11

92

Table 13 continued

(NH4)3C02W12042 - 937' 885, 790 -
75 ,

LDH-C02W12042 - 917 872, 787 20
735,

(NH4)9PV14042 1060 938 867, 810 -
810, 770

LDH-PV14042 1057 943 805, 757 -5

(NH4)10H2W12042 - 950, 934 871, 861 -
765, 699

LDH-H2W12042 932 863, 798, e
702

 

DJ

Hydrated waters were omitted for simplicity and the data

for (NH4)14NaP5W300110 and LDH-NaP5W300110 are not

listed because of the ambiguity of band assignments.

Their band positions are in the text.

b Band assignments were done only for three characteristic
vibrations modes of POMs.
0 Frequency unit is cm'l.

d Avis obtained by substracting v(M=Od) of free POM from
that of LDH-POM.

e Details are in the text.

93

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F P _ _ L _

 

 

 

 

 

BONVLLINSNVUL

94

the two opposing effects are almost balanced. A band for v(W—Oc-W) shifts from 811
cm"1 for the K+ salt to 807 cm-1 for the intercalated anion due to effect A and effect B.
The unlisted remaining bands are 8(OH) at 623 cm‘l, an unassigned band at 559 cm‘1
and v(Zn-O-Al) at 427 cm‘l. The ir spectra under the two different environment are very
similar, as presented in Fig. 28.

LDH-SiV3W9O4o7- : There are two strong bands assigned as v(M=Od) at 965 and 915
cm-1. They are shifted from 959 and 913 cm-1 for the K+ salt, respectively. The former
bands shift more than the latter. The most basic sites of SiV3W904o7-, that is, the
oxygen atoms bound at V atoms, prefer to hydrogen bond with the hydroxyl group of the
layer. Therefore, V=Od will participate in the hydrogen bonding more strongly than
W=Od, with the result that the shift is greater than those of other W=Od. As a result, a
band at 959 cm-1 for LDH-SiV3W9040 and a band at 965 cm-1 for the K+ salt of
SiV3W9040 can be assigned as v(V=Od). As in the other cases, v(M-Oc-M) shifts from
782 cm-1 in the K+ salt to 780 cm“1 for the anion in the gallery due to effect Ab and
effect B. The unlisted remaining bands are 8(OH) at 622 cm'l, an unassigned band at
549 cm-1 and v(Zn-O-Al) at 428 cm‘l. The similar ir spectra for SiV3W9O40 under the
two different environments, as shown in Fig. 29, indicate identical anion structures.
LDH-SiW110393- : In this case the B effect is predominant, so the v(W=Od) band in
the K+ salt at 960 cm-1 shifts to 955 cm-1 for the anion in the LDH. The average
distance between the POM in the gallery is 12.4 A. As in the other cases, there are red
shifts in the v(W-Oc-M) band from 799 and 728 cm-1 for the 10‘ salt to 791 and 715
cm-1 for the LDH intercalated anion , as shown in Fig. 30. The band splitting of v(W-
Oc-W) is conserved, suggestive that the POM in the gallery still has a lacunary Keggin
structure. The unlisted remaining bands are 6(OH) at 621 cm‘l, an unassigned band at
545 cm-1 and v(Zn-O-Al) at 429 cm’l. The similarities in the ir spectra of the anion

under the two different environments confirms their identical structures.

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LDH-PM02W90397' : In this complex two opposing effects are well balanced, so no
band shift is observed. However, similar to other cases, there is a red shift in v(M-O-M)
from 811 cm'1 for the K" salt to 803 cm-1 for the intercalated anion due to an effect Ab
and an effect B. The three well-resolved bands attributed to v(M-Oc-M) are merged into
two bands for the intercalated ion, as given in Fig. 31. The unlisted remaining bands are
8(OH) at 621 cm'l, v(Zn-O-Al) at 559 and 427 cm’l, and an unassigned band at 519
cmrl.

LDH-BW110399' : In this complex, the effect B is predominant so v(W=Od) shifts
from 954 cm'1 in the K+ salt to 939 cm-1 in the intercalated anion. The average distance
between the POMs in the gallery is 13.2 A. In the 10' salt there are several bands
corresponding to v(W-Oc-W), as listed in Table 13. The other band positions at 625 cm‘
1, 513 and 479 cm-1 are unassigned, see Fig. 32. Due to effect Ab and effect B, the two
bands at 788 and 719 cm-1 in the K+ salt are shifted to 783 and 716 cm-1 in the
intercalated anion. The remaining bands are 5(OH) at 620 cm-1 and v(Zn-O-Al) at 562
and 427 cm-1.

LDH-BCu(H20)W110397- : In this complex the effect B is predominant, so v(W=Od)
shifts from 940 cm-1 in the K+ salt to 937 cm-1 in the intercalated anion, as provided in
Fig. 33. As in other examples, v(W-Oc-W) bands shift to lower frequencies due to effect
Ab and B. The remaining bands are 8(OH) at 622 cm'1,v(Zn-O-Al) at 562 and 426 cm"
1, and an unassigned band at 495 cm-1. The similarities of the ir spectra of POMs under
two different environments suggests the structure of the POM is conserved in the layer.
LDH-BCo(I-IzO)W110397' : This complex exhibits a dominant B effect, so a red shift
(3 cm-1) is observed; see Table 12. As in the other cases, v(W-Oc-W) at 820 cm'1 for
the K+ salt shifts to a lower frequency, 815 cm-1, in the LDH intercalate due to effect Ab
and B. The unlisted remaining bands are 6(OH) at 623 cm-1 and v(Zn-O-Al) at 562 and
427 cm-1. A band at 492 cm—1 can be assigned as 603-0,) shifted from 499 cm-1 for the

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K+ salt. The similarities of the ir spectra of the POM under the two different
circumstances are apparent in Fig. 34.

LDH-SiFe(SO3)W110397- : In this system, there is almost no band shift in v(W=Od),
reflective that the two opposing effects are well balanced. However, as in the other cases,
there are red shifts of v(W-Oc-W) from 797 and 727 cm'1 in the K+ salt to 784 and 716
cm-1 in the intercalated anion. The other bands are 8(OH) at 621 cm'1 and v(Zn-O-Al)
at 543 and 427 cm'l. The similarities of the ir spectra of the POM under the two different
surroundings can be easily noticed in Fig. 35.

LDH-PW90349' : Remarkable blue shifts are observed in this system. The v(P-Oa)
bands shift from 1055 and 1014 cm'1 in the Na+ salt to 1083 and 1057 cm-1 in the
intercalated anion. Such blue shifts indicate strengthening of the P-Oa bond of the POM
in the gallery. The v(W=Od) also exhibits a blue shift from 936 cm‘1 in the Na+ salt to
947 cm-1 in the intercalated anion. Such blue shifts may not be due to effect A because
the average distance between the POMs in the gallery is 13.2 A, as in the LDH-
BW11039 where an effect B was found to dominate. In general, it has been observed
that v(W=Od) depends on the degree of the degradation of XM12040, that is,
v(XM12040) > v(XM11039) > v(XMgO34).135°)»c) From consideration of the band
positions, it is plausible to suggest that the anion in the gallery is stabilized by forming a
XM11M’O39-like surrounding. The band broadening in the 1000 - 650 cm'1 region,
which corresponds to the v(M-O—M) range, may be attributed to such an unusual
surrounding of PW9034 in the gallery. As shown in Fig. 36 the differences in the ir
spectra of the anion under the two different circumstances are easily recognizable. The
unlisted remaining bands are 8(OH) at 620 cm‘1 and v(Zn-O-Al) at 565 and 428 cm‘l.
LDH-C02W120423' : In this case, the B effect is remarkably predominant so that
v(w=od) shifts from 937 cm-1 in the NH4+ salt to 917 cm-1 in the intercalated anion.
There are also red shifts in v(W-Oc-W) bands as in the other cases. Remaining bands

include unassigned bands at 520 and 488 cm-1, 5(OH) at 625 cm’1 and v(Zn-O—Al) at

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428 cm'l. The ir spectra of the anion under the two different circumstances are presented
in Fig. 37.

LDH-PV140429- : This system was discussed previously. The unlisted remaining bands
are 5(OH) at 612 cm‘1 and v(Zn-O—Al) at 565 and 427 cm’l. The ir spectra of the anion
under the two different circumstances are given in Fig. 38.

LDH-H2W1204210' : This is another system in which the effect B dominates. The
NH4+ salt of the anion evidences two v(W=Od) bands due to the two different W=Ods.
One of them is engaged in hydrogen bonding with crystalline water. This bonding lowers
v(W=Od) to 934 cm'l, as discussed by Kiss.”O Thus, the band at 950 cm'1 corresponds
to the non-hydrogen bonded W=Od. This band shifts to 932 cm-1 in the LDH-
H2W12042, a value which is almost the same as that of the hydrogen bonded W=Od.
Therefore, the ir spectrum of the intercalated anion in the LDH gallery shows only one
W=Od band, as in the case of an anion in a highly hydrated state. As a result, it is
obvious that the hydrogen bonding for W=Od in the POM is driven by its intercalation
into the gallery. For the NH4+ salt, the remaining bands are 8(W-OH) at 610, 555 and
532 cm-1 and 8(W-O—W) at 498 and 435 cm'l. The bands for LDH-H2W12042 are
5(OI-I) at 618 cm’l, v(Zn-O-Al) at 560 and 427 cm’l, and 5(W-O-W) at 493 cm‘l. The
ir spectra of the anion under the two different environments are provided in Fig. 39.
LDH-NaP5W30011014- : Even though the band assignments for this system cannot be
made clearly due the complexity of the anion, the band shapes and positions of the NH4+
salt of the anion and its LDH intercalate are very similar, as is shown in Fig. 40.
Therefore, the two opposing effects must be well balanced. The structure of the POM in
the gallery should be identical with that in the NH4+ salt. Bands positions are as follows :
(NH4)14NaP5W300110 : 1164, 1079, 1017, 985, 935, 911, 778, 737, 656, 573, 541,
473, 464 cm°1

LDH-NaPsW300110 : 1165, 1080, 1018, 984, 936, 915, 787, 737, 618, 574, 545, 466.
427 cm-1

 

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In the latter case the bands due to the LDH layer are 5(OH) at 618 cm‘1 and v(Zn-O-Al)
at 427 cm-1.

D. Selectivities of ion exchange reaction and orientation

of the intercalated POMs

1. Charge Selectivity

As discussed in the introduction chapter, the higher the charge of the anions, the
more the ion is preferred by the LDH. For example, the charge density of PW120403- is
lower than that of ClO4‘. Such low charge densities are unfavorable for intercalation in
LDH. As will be discussed in the gas adsorption study section, the minimum charge for
the intercalation of POM with a Keggin structure is 4.8 - 5.0. This calculation was based
on a comparison of the surface charge density of the LDH and POM. However, in this
calculation any repulsive interactions between POMs in the gallery were not taken into
account. When repulsions are taken into account, the POMs repel each other in the same
gallery.

When POMs having a 5- charge were allowed to react with LDH-N03, only
partial ion exchange was observed, even when a large excess of POMs was used. Fig. 41
shows the existence of two phases, one for the LDH-POM and the other for unexchanged
LDH-N03. Such partial ion exchanges occurred even with POMs having 6- charge, but
H2W1204o6‘ was an exception even though very small amount of unexchanged NO3'
was detected by ir spectroscopy. Fig. 42 illustrates the XRD of LDH-POMs with POM =
BV(V)W1 104063 BCo(III)W110396- and PV3W9O4o6-. The same types of XRD
patterns are shown with that of Fig. 41, except that the peaks due to LDH-N03 phase are
much weaker. The decreased intensities of LDH-N03 peaks indicate a smaller amount of
this phase present here than in LDH-POM5-. Therefore, the POM6- is more favorably
intercalated into the LDH lattice than POM5-. The more favorable ion exchange reaction

of H2W120406- relative to other POMs having the same charge can not be explained by

112

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Fig. 42. XRD pattern of a) LDH-BCo(III)W11039, b) LDH-BV(V)W11040
and c) LDH-PV3W9040.

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surface charge density consideration alone. There must be other factors governing the ion
exchange reaction, for example, the hydration energy of the POM. If the energy gained
by the intercalation of H2W120405- is greater than those of other POMs having same
charge, the former must be more favorably intercalated than the latter.

2. Stereo Selectivities

Except for B3-SiW11039, when B—12- and B-ll-POM with Keggin structure
POMs reacted with LDH-N O3, incomplete exchange was observed. The XRDs of the
partially exchanged LDH-B-SiV3W9O40, LDH-Bl- and Bz-SinlO39 and also fully
exchanged LDH-B3-SiW11039 are shown in Fig. 43. In contrast to the above B-POMs ,
a-POMs such as a-SiV3WgO40 and 01-8in 1039 are completely intercalated between
the brucite-like lattice layers.

As discussed in the introduction, there are two observed stacking polymorphs for
naturally occurring LDHs. One has a three—layer rhombohedral oxygen stacking
symmetry, represented by (BC)(CA)(AB)(BC)-«, the other has a two-layer hexagonal
oxygen stacking symmetry, represented by (BC)(CB)(BC)(CB)---. For the synthetic
LDHS, only the three-layer rhombohedral stacking pattern, which is a low temperature
product, has been observed. Thus, it is reasonable to expect this polymorphorism for
LDH-POMs. The oxygen stacking symmetry of the 3-1ayer polymorph is so much more
favorable than that the two-layer polymorph that the latter has not been observed in the
laboratory.

It is necessary to consider possible commensurate stacking symmetry
relationships between the surface oxygens of POMs and the oxygen sheets of the brucite
layers, i.e., the stereo relationship between the top and bottom oxygen sheets of POMs
and the brucite hydroxyl. The resulting symmetry with LDH-oc-POMs is always
rhombohedral, when the POM in the gallery is oriented so that C2 axis is orthogonal to
the LDH layer. This orientation provides the maximum contact between the POM and

LDH layer. The stacking pattern yields (BC)((CA))(AB)((BC))(CA)~, where the oxygen

117

F2- 43. XRD pattern of 3‘) LDH-B1-SiW11039. bl LDH‘BZ‘Siw11039'

A
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c: LDH-B3-Si\\’11039. d) LDH-a-Si\\'11039 and LDH-8-5iv3w9040

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121

layers contained within the double parentheses correspond to the t0p and bottom oxygen
sheets of the POM in contact with the oxygen sheets of the brucite layers. Each oxygen
sheet consists of eight oxygen atoms. With B—POM, either of the oxygen sheets from the
POM consists of only six oxygen atoms. This six oxygen atom sheet is not commensurate
with the hydroxyl group of the brucite-like layer, resulting in higher d—spacings, e.g.,
d(LDH-a-SiV3W9O40), 14.4 A; d(LDH-B~SiV3W9O40), 14.8 A. The reduced contact
area between the POM and the LDH layer also makes the B-isomers’ ion exchange
tendencies less favorable. When one of the six oxygen atoms, which initially define the
top or bottom faces of the POM, is removed from the B-lZ-Keggin structure, it results in
the same sequence with LDH-B3-SiW11039. The mismatching of the oxygen layers is
vanished, so a favorable stacking is resulted as in the LDH-a-POM. The ion exchange
behavior of B3-SiW11039 is similar to that of a-isomer. Therefore, complete exchange
occurs with the b3 isomer. Its d-spacing also changes to a lower value (14.4 A) as in the
a-isomer intercalates. However, the other two B-l l-isomer intercalates result in the
same sequence with the B-SiV3W9O40 intercalate. Thus, only partial exchange occurs
with these isomers.

3. Orientation of Keggin-Type POMs in the Gallery

As seen in Fig. 8, the structure of the a- and B- isomer of SiV3W9O40 contain
three vanadium atoms located adjacent to each other. Rotated M3 triplet unit is located
on the opposite side. The ion exchange behavior difference between a- and B-isomers
described in the previous section indicates that the orientation of the Keggin-type POM
in the gallery must be such that the a- and B—isomers can be differentiated. There are
two possible orientations, i.e., either C3 or C2 axis is orthogonal to the LDH layer. As
discussed previously, the most possible orientation is one in which C2 axis is
perpendicular to the LDH-layer. Therefore, in LDH-SiV3W904o C3 axis is tilt along 0

direction. Such an orientation will be extended to other systems such as a- and B-

122

isomers of ll-Keggin structures. These have been already discussed in the previous
section.

In para-tungstate, H2W12042, there are two M3 triplets on opposite sides
connected by two other types of units, as shown in Fig. 14. On the basis of the XRD
study (d001 = 14.1 A) these two M3 triplet must directly contact the brucite layer. The
oxygen layer stacking arrangement of the whole system approximates rhombohedral
symmetry even though two M3 triplets are not exactly superimposed, but are shifted by
about half an oxygen perpendicular to the direction connecting two M3 triplets.

In the case of PV14042, whose structure is shown in Fig. 26, there are two more
V=O units than in Keggin-type POMs. These V=O units are located along the C2 axis of
the PV12040 ion. On the basis of both commensuration of the oxygen sheets of
PV14042 in the brucite layer and d-spacing consideration ((1 = 14.4 A) the orientation of
this ion must be different from those of the other Keggin-type POMs. Therefore, the C2
axis connecting the two extra V=O units is parallel to the brucite layer lattice.

The compound NaP5W300110, whose structure is shown in Fig. 15, is oriented
in the gallery so that the C5 axis of molecule is perpendicular to the brucite layer, as
judged from the observed d-spacing (d001 = 21.3 A). This orientation provides the
maximum contact between the brucite layer and NaPsW300110 and the minimum d-
spacing.

With PW9034, whose structure is shown in Fig. 7, there is only one way to
explain the observed d-spacing (d001 = 14.5 A), a value almost the same as those of the
other Keggin-type POMs. The face, which is formed by removing three W02 units, and
the M3 triplet face located on the opposite side of this face are not contacting the brucite
layer.

In other cases in which one W=O was replaced by Mn+(O, H20, or 803), where
M is Cu(II), Co(II), V(IV), or Fe(III), their gallery orientations are uncertain. But with
BCo(II)(I-I20)W11039, there was a color change from pinkish to greenish during the

123

heating of the sample under vacuum. This phenomenon is reversible. There is no d-
spacing change during the dehydration process. This color change indicates the
coordination number of Co(II) has changed from six to live. This type of color change in
related molecules has been observed during removal of coordinated water in
homogeneous solution by azeotropic boiling or by blowing hot nitrogen tluough their
solid samples. 141 From the above observations made on LDH-BCo(II)W11039, the side
of the ion containing such coordinated water in the POM frame must not be contacting

the brucite layer but must be positioned in the middle of the gallery.

E. Thermal stabilities of LDH-POMs

Generally speaking, LDH-POMs are stable up to 200°C even though their
thermal Stabilities are dependent on the POMs in the gallery. Both LDH-H2W12042 and
LDH-PV14042 are exceptional cases, because neither is thermally stable even at low
temperature. They undergo degradation of POMs in the gallery, resulting in unassignable
XRD patterns.

When samples are heated at certain temperatures, water molecules occupying the
free space between POMs in the gallery are removed from LDH-POMs. As the water
molecules are removed, their XRD patterns are also changed. Both the relative intensities
of peaks and their peak positions are affected by dehydration. In general, the relative
intensity of the 001 reflection increases, while the other 001 reflections decrease in
intensity as the heating temperature increases. Specially, the relative intensity of 002
reflection decreases much more than those of the others. Peak broadenings are also
observed as temperature increases. The higher the charge on the POM in the gallery, the
greater the changes in the XRD patterns upon heating. When samples are dehydrated at
temperatures where they are stable and then rehydrated by soaking in water, XRD peak
positions and relative intensities are reestablished, even though absolute intensities are

not regained. A slight decrease in d-spacing of the dehydrated sample is generally

 

124

observed. Such changes result only from the change water content in the gallery. The
dehydration and rehydration process are reversible even though the rate of rehydration is
dependent on the POMs in the system. Differences in scattering contributions of water
molecules is the source of changes in XRD intensities. When water molecules are
removed, there is no water contribution to peak intensity. When the system is rehydrated,
the water contribution is restored. In order for the water molecules to contribute to the
XRD pattern of system, they should be positioned regularly in the gallery. In the
rehydrated system the water molecules reoccupy gallery positions equivalent to those for
an unheated sample. Therefore, the same type of XRD pattern as that observed for an
unheated sample is obtained.

The following noteworthy observations for LDH-POMs are summarized : 1) the
higher the charge of the POM in the gallery, the more the XRD peaks are broadened, 2)
the XRD pattern upon dehydration and rehydration processes is reversible, 3) upon
dehydration the d-spacin g decreases, 4) the calculated surface areas and pore volumes for
systems containing POMs with more than 9- charge are much larger than the observed
values.

Layer nonrigidity will result in layer sagging when LDH-POMs are dehydrated.
The higher the charge of POMs in the gallery, the greater distance between the POM
pillars and the more the layer will sag. When systems with sagged layers are rehydrated,
the layer will reswell much like the swelling of a sponge. The decrease in the absolute
intensities of the XRD peaks comes from the decrease in the regularity of the lattice
array, which arises partially from layer sagging. Severe layer sagging may result in bond
breaking in the layer network. Such partial bond breakings will also give rise to a
decrease in the crystal size, which, in turn, will result in both peak broadening and a
decrease in peak intensity. When such fractured crystals are rehydrated, bond reformation

does not occur. Therefore, complete recovery of absolute peak intensities is not observed.

 

125

When LDH-POMs are heated at about 500°C, new inorganic phases appears.
The only assignable major phase is a ZnWO4 type phase which appears for all complexes
except LDH-PV14042, in which the only a Zn2V207-type phase can be identified.

The thermal stability of LDH-POMs is discussed below.
LDH-H2W12040 : This complex is stable to 180°C, and samples heated below this
temperature can be rehydrated. XRDs at different temperatures are shown in Fig. 44. By
225°C, the decomposition of H2W12040 has occurred. The ir spectra in Fig. 45 and the
XRD study shown in Fig. 45 verify the decomposition temperature. Only one
unidentified peak at 26 = 6.9 remains in the sample heated to 225°C. The ir pattern of
this sample becomes simpler below 1000 cm-1 and structure due to POM frequencies is
no longer observed. The peak at 426 cm-1, which is attributed to an Al-O-Zn stretching
motion, disappears after heating this temperature. An amorphous phase is observed up to
450°C. At 470°C, crystalline inorganic phases appear.

TGA and DSC data indicative of phase changes upon heating are shown in Fig.
46. The dehydration processes occur up to 216°C, followed by layer dehydroxylation up
to 354°C. An exothermic peak starting at 446°C corresponds to the formation of new
inorganic crystalline phases.
LDH-SiV3W9040 : This complex is stable up to 220°C. As shown in Fig. 47
rehydration of the heated sample is possible. The POM SiV3W9040 decomposes at
250°C, as verified by it spectroscopy and XRD data. The ir spectrum, presented in Fig.
48, shows a reduction in the number of bands below 1000 cm-1 and disappearance of the
428 cm-1 band due to the degradation and disappearance of both the POM and the
hydroxyl layer upon heating. Therefore, the Zn-OH-Al lattice network is absence. There
are only two detectable XRD peaks at 29 = 6.560 and 19.03° in the 220°C specimen,
but these peaks cannot be related each other. Therefore, more than one unidentifiable

phases must exist. An amorphous phase is observed below 450°C. The shape of ir

126

Fig. 44. XRD pattern of LDH-H2W12040 at different temperatures. a) 25°C, b) 120°C,
c) 180°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, d)
225°C, e) 320°C and f) 470°C

127

18.25 Dogs
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0.81‘
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3) XRD of LDH—HZWRO“ at 25°C

 

m

 

 

 

1.001
0.01‘
0.641
0.491
0.361
0.25‘
0.16‘
0.091
0.04
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30.0 45.0

b) XRD or LDH-H.150, at 12%

33.0

128

rehydrated

 

 

  

 

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30 ' 35 4o 45 50
C) XRD of LDH-Htwuo. at IBO'C and rehydrated.

”3 L
1.001
0.31‘
0.044
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0.361
0.25‘
0.16‘
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0.3

0.231
0.161
0.091
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0.01‘

 

 

1 v I T 1 V V

23.0 30.0 38.0 40.0

d) XRD of LDH—3,1! 0 at zzs-c

1040

129

19’ ‘
1.001
0.81‘
0.641
9-‘9‘ 7.28 Doss
0.36‘
0.25"
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0.251
0.161
0.091
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30.0 35.0 40.0 45.0 50.0

 

e) XRD or LDH-HJuo“ at 320‘C

 

t0" 1 -
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0.981
0.72‘
0.50‘
0.32‘
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0.724
0.501
0.321
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40.0 45.0 50.0 55.0 60.0 65.0 70.0

1') XRD of LDH-mluoa at 47o-c

 

130

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132

Fig. 47. XRD pattern of LDH-SiV3W9O40 at different temperatures. a) 25°C, b)
165°C, c) 220°C: dotted line for the dehydrated sample, solid line for the rehydrated
sample, d) 250°C, e) 360°C and 1') 500°C.

 

133

 

 

 

 

 

 

 

 

 

 

 

 

 

x104 L
1.001 10.44 0.9:
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. 25.0
30.0 35.0 40.0 45.0 50.0
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30.0 35.0 40.0 45.0 50.0

b) XRD of LDH—swam“, at res-c

 

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rehydrated
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5.0 10.0 15.0 20.0 25.0

 

 

d) XRD of LDH-swam, at 250’C

135

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.621
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.901
0.721 7-1? 005!

 

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0.321
0.181
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0.321
0.101
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30.0 35.0 40.0 45.0 50.0

e) XRD of LDH-SiVJp“ at 360’C

 

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1.001
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0.641

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0.251

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5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
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50.0 55.0 60.0 65.0 70.0 75.0

0 XRD of LDH—SiVJp“ at 500'C

 

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137

spectra of the sample heated to 250°C is consistent with a single band centered at 870
cm-1. At 500°C, new crystalline inorganic phases appear.

The DSC and TGA data are shown in Fig. 49. The dehydration process occurs up
to 200°C, followed by dehydroxylation of the host lattice beyond this point up to
365°C. An exothermic process starts at 460°C and new crystalline inorganic phases are
observed by XRD.

LDH-BV(IV)W11040 : This complex is stable up to 225°C and the rehydration of
heated sample is possible, as illustrated in Fig. 50. Beyond 225°C, the POM in the
gallery BV(IV)W11040 is decomposed and according to the ir spectrum and XRD
pattern amorphous phase results. As in the other cases, in the amorphous phase there is
one band centered at 862 cm-1, as shown in Fig. 51. The ir spectra at higher temperatures
are also shown in Fig. 51. There is only one unidentified peak at 20 = 7.24° in the
specimen heated to 360°C, as shown in Fig. 50. At 470°C, new crystalline inorganic
phases appear.

The DSC and TGA data showing phase changes are given in Fig. 52. The
dehydration of lattice water occurs up to 227°C, followed by dehydroxylation of the
lattice between 285°C and 355°C. An exothermic process related to the formation of
new crystalline inorganic phases starts at 442°C.

LDH-BCo(II)W11039 : This complex is stable up to 200°C and below this point, the
rehydration of the heated sample is possible, as shown in Fig. 53. The decomposition of
BCo(II)W11039 in the gallery is observed beyond this point and verified by the ir
spectra shown in Fig. 54. As in the other LDH-POMs cases, the ir spectrum at 250°C
shows only one *absorption band centered at 842 cm-1. Thermal decomposition of these
LDH-POMs typically results in an amorphous phase with an absorption band positioned
at 855 cm'l. In the specimen heated to 250°C there is only one unidentified XRD peak
at 20 = 703°. The XRDs of LDH-BCo(II)W1 1039 at different temperatures are shown

 

138

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139

Fig. 50. XRD pattern of LDH-BVW11040 at different temperatures. a) 25°C, b)
150°C, c) 225°C: dotted line for the rehydrated sample, solid line for the dehydrated
sample, d) 255°C, e) 360°C and 1‘) 470°C.

140

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b) XRD or LDH—BV(IV)Wuo“ at 150-c

 

141

rehydrated

 

 

 

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30 35 40 45 50

 

c) XRD of LDH—BV(IV)w 0 at zzs-c

1140

 

 

 

 

(1) XRD of LDH-BV(IV)W O at 255°C

1140

142

 

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1’) XRD of LDH-BV(IV)Wnoa at 470°C

 

143

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145

Fig. 53. XRD pattern of LDH-BCoW11039 at different temperatures. a) 25°C, b)
185°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, c)
200°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, (1)
250°C, e) 360°C and 1) 500°C.

 

 

 

146

17105 Cnts
8.44 0095

  
 
   
 

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5.0 10.0 15.0 20.0 25.0

37.44 0095
3623 Cnts

 

30.0 35.0 40.0 45.0 50.0
b) XRD or LDH-BCo(II)W“Oa at 25'C

rehydrated f";

 

 

 

 

 

30 ° 35 4o 45 50

a) XRD of LDH-BCo(II)W c at 105°C

11 80

 

L47

rehydrated

 

 

 

    

 

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c) XRD of LDH-BCo(II)W 0 at ZOO'C

t1 8.

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1.00%
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0.361
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30.0 35.0 40.0 45.0 50.0

(0 XRD of LDH-BCo(II)W 0 at 250%

It 8.

148

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2.“3

1.62‘
1.20‘
0.93
0.727
0.50‘
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40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0

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x103 1
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4.051
3.201
2.453
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0.451
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40 .0 45 .0 50.0 55.0 60.0 65 .0 70.0 75.0

1) XRD of LDH-BCo(II)Wuo, at soc-c

0830382 820:6 a ammo" ~>>oum+~a mo «58% .: .% .wE

 

 

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150

in Fig. 53. At 360°C a complete amorphous phase appear. At 500°C new crystalline
inorganic phases are observed.

The DSC and TGA results showing phase changes are presented in "Fig. 55. The
dehydration process occurs up to 225°C, followed by the dehydroxylation of the host
lattice up to 365°C. A small exothermic peak starting at 450°C indicates the onset of
new inorganic phase formation.

LDH-BCu(II)W11039 : This complex is stable up to 190°C. Rehydration of the heated
sample is possible, as shown in Fig. 56. The BCu(II)W11039 in the gallery decomposes
at 230®C, as verified by changes in the ir spectra shown in Fig. S7 and in the XRD
pattern at 230°C shown Fig. 56. As in the other cases, there is only one ir absorption
band centered at 835 cm-1 below 1000 cm-1. The XRD pattern of the 230°C specimen
shows two unidentified peaks at 20 = 6.630 and 19.080. Heating beyond this point
results in complete formation of an amorphous phase. At 500°C, new crystalline
inorganic phases appear, as shown in Fig. 56.

The DSC and TGA of LDH-BCu(II)W11039 are provided in Fig. 58. The
dehydration process occurs up to 230°C, followed by dehydroxylation of the host layer
up to 360°C. There is a very small exothermic peak starting at 450°C, which
corresponds to new inorganic phase formation.

LDH-PM02W9039 : This complex is stable at 205°C and also shows the reversible
rehydration process, as indicated by their XRD pattern in Fig. 59. Its ir spectrum, as
given in Fig. 60, shows that the intensity of an absorption band around 427 cm-1
decreases substantially upon heating, reflecting to some extent layer network
degradation. At 340°C an amorphous phase is formed, as shown in the XRD pattern in
Fig. 59. As in the other cases, the ir spectrum at the amorphous region shows no fine
absorption structure with only one band centered at 860 cm'l. At 500°C the crystalline

inorganic phases appear, as shown in the XRD pattern of the 500°C material.

151

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152

Fig. 56. XRD pattern of LDH-BCuW11039 at different temperatures. a) 25°C, b)
125°C, c) 160°C, d) 190°C: dotted line for the rehydrated sample, solid line for the
dehydrated sample, e) 230°C and f) 500°C.

153

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a) XRD of LDH-BCu(II)W“O. at 25'C
,0. L

.003 18.50 0090
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.64‘
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0.25‘ 1704 C038
0.164 33038 0.93

0.09‘
0.041
0.01

 

 

 

b) XRD of LDH-BCu(II)W“O. at 1251c

 

154

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1.003
0.813

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0.361
0.251
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5.0 10.0 15.0 20.0 25.0

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30.0 35.0 40.0 45.0 50.0

 

 

c) XRD of LDH-BCu(II)W“O. at 100°C

rehydrated ,1‘.‘

 

 

 

5 10 15 20 25

 

 

(1) XRD of LDH-BCu(II)W O at 190'C

I! II

.68 3
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65.0 70.0

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60.0 75.0

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50.0 55.0

 

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4.053
3.203
2.453
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1.253
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0.453
0.20‘
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45.0 60.0

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Fig. 59. XRD pattern of LDH-PM02W9039 at different temperatures. a) 25°C, b)
150°C, c) 205°C: dotted line for the rehydrated sample, solid line for the dehydrated
sample, (1) 340°C, and e) 500°C.

 

159

 

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2.001 18.31 0095
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30.0 , 35.0 40.0 45.0 H I 50.0
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3103 L

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3.0 35.0 40.0 45 .0 50.0

b) XRD of LDH-Puomp, at two

 

160

rehydrated

 

 

 

 

O
ego--0-..
'0--.-¢o --. -0. "’

1 1 1 I l

30 35 40 45 50
c) XRD Of LDH-PMO'W.O. at 205°C
3 L_______ "

bl

 

113

$900—-
15¢
‘0‘

0.301
0.251
0.16‘
0.091
0.041
0.011

 

 

5.0 10.0 15.0 20.0 25.0 30.0 35.0
1.” ‘
0.01 ‘

0.64 '
0.49 ‘
0.33

0.251
0.161
0.091
0.041
0.011

 

 

“35.0 40.0 45.0 50.0 55.0 60.0 65.0

(1) XRD of LDH-PMO.W.O' at 340'C

161

 

.1"

OOOOOQQ—F‘N
M
0

 

 

 

5.0 10.0 15.0

 

" U T V r I V V I

60.0 65.0

I V V

45.0 50 .0 55 .0

0) XRD of LDH—P110319. at. 500‘C

162

08880932 Bogota. 3 amOoBaEZQfin—A .«o 0.58% b .8 .wE

 

 

 

 

mmmmxazu><z
8.. 80 8o 82 85
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BONVLLIHSNVHJ.

163

The DSC and TGA of LDH-PM02W9O39 are shown in Fig. 61. The dehydration
process occurs up to 205°C, followed by dehydroxylation of the layer up to 340°C.
There is a small exothermic peak starting at about 450°C which corresponds to the
proce~1s of new crystalline inorganic phase formation.

LDH-SiFe(SO3)W11039 : This complex is stable up to 200°C and rehydration is
possible below this point shown in Fig. 62. The XRD pattern of the specimen heated to
268°C shows only one unidentified small peak at 29 = 7.17°. Its ir spectrum shows
only one absorption band centered at 863 cm-1 without a band around 427 cm-1,
indicative of degradation of both the POM in the gallery and the hydroxyl layer, as
shown in Fig. 63. An amorphous phase is observed until new crystalline inorganic phases
appear at 500°C.

The DSC and TGA of LDH-SiFe(SO3)W11039 are provided in Fig. 64. The
dehydration process occurs up to 207°C, followed by dehydroxylation of hydroxyl layer
up to 338°C. At 450°C new crystalline inorganic phases appear.

LDH-SiW11039 : This system is stable at 180°C. Rehydration of heated samples is

possible, as indicated by their XRD pattern shown in Fig. 65. The decomposition of both
the hydroxyl layer and the POM in the gallery occurs beyond 180°C, as shown in the ir

spectra provided in Fig. 66. At 300°C LDH-SiW11039 becomes amorphous; new

crystalline inorganic phases appear1at'500°C, as shown in Fig. 65

The DSC and TGA of LDH-SiW11039 are provided in Fig. 67. The dehydration
process occurs up to 227°C, followed by the dehydroxylation of layer up to 356°C. An
exothermic peak starting at 450°C corresponds to the onset of new crystalline inorganic
phases formation.

LDH-BW11039 : This complex is stable at 210°C. Rehydration of heated samples is
possible below this point, as shown by XRD patterns in Fig. 68. At 260°C, the LDH-
BW11039 turns into an amorphous phase; new crystalline inorganic phases appear at

500°C. As in the other cases, the ir spectrum Fig. 69 of an amorphous material shows

164

680332023 2e <2. e5 08 .8 .02

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165

Fig. 62. XRD pattern of SiFe(SO3)W11039 at different temperatures. a) 25°C, b)
150°C, c) 185°C, d) 200°C: dotted line for the rehydrated sample, solid line for the
dehydrated sample, e)303°C and 1) 500°C.

166

410‘ L
5.8‘ 18.31 0035
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1.253 11674 Cnts

0.003
0-45‘ 6.03 0.9.
“'3'" 431 Cats

0.05" M

 
  

 

 

45.0 50.0

   

30.0 " 35.0 .0 '
a) XRD of LDH—Sire(m)(so,)wuo, at 200
x10‘ L

1.001 _
0.011 10.44 0.9.
0.641 5001 out:
0.491 12.25 0.9.
0.361
0.251
0.161
0.091
0.041
0.011

   
 

24 .67 0030
1450 cm;

 

 

-¢ , . .
5.0 10.0 15.0 20.0 25.0

1.001
0.011
0.641
0.491
0.361
0.253 37.44 0035
0.161 30.97 Dogs 1129 Cat:
’1'," 301 Cnts

0.041
0.011

 

30.0 35.0 40.0 45.0 50.0

b) x121) or LDH-Sire(m)(so,)wuo, at. iso-c

167

 

.19.
5. 1

18.36 D 5
4.051 '9

3.203 2852 Ont:

3,45 1 12.14 0095

1.83 5 97 1273 cm, 24.61 Dogs
:'::j C . 750 Cnts
0.453

0.203
0.053

 

 

 

5.0 10.0 15.0 20.0 25.0
5.00”
4.05‘
3.20‘
2.45‘
1.00‘ 37.31 0090

"'25" 30-92 0050 737 cm:

0.003
0.434 336 wt.

0.20‘ _

0.05‘

30.0 35.0 40.0 45.0 50.0 ‘

 

 

c) XRD of LDH—Sire(lll)(so,)w 0 at res-c

11 I.

rehydrated

 

 

 

 

 

 

(1) XRD of LDH—SiFeOIIXSOJWuou at. ZOO'C

168

 

 

5.0 10.0 15.0 . 20.0 25.0

"‘° 9’?’:‘!°
fiflifl 83911610

0.323

 

 

I' I r I Y 7 I

30.0 ' 35.0 40.0 45.0 50.0

e) XRD o1 LDH-Sire(m)(so,)w“o. at 303°C
.31

2.001
1.621 30.56 0450
1.283
0.901
0.721

.503

.321
0.101
0.08‘
0.023

 

 

 

2.”‘
1.623
1.20‘

. 50.31 0095
0.98 36.4%” . 31.68 0.9, 61.90 0095

we W
0.503
0.32

0.103 _ 53.71 0.95
0.001
0.021

 

 

35.0 40.0 45.0 50.0 55.0 60.0 65.0

f) XRD of LDH—SiFe(III)(SO,)W“O. at 500‘C

169

022802 222% 0 502 5009.553 .6 88% a .8 .02

 

 

 

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BONVllIHSNVHl

170

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8.9 Padang—Eon.

 

 

 

 

 

 

 

 

 

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171

Fig. 65. XRD pattern of LDH-SiW11039 at different temperatures. a) 25°C, b) 160°C,
c) 180°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, (1)
230°C, e) 300°C and f) 500°C.

172

 

3°°°' 18.25 0093
12093 Cnts

 
  

24.36 Dog:
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0.501 37.” Dogs
9.32l 3°°67 ”'9' 2173 Cat:
9-19‘ 939 Cats

”1&1 0.324

 

 

r r

A": 30.0 ‘ 350° “.0 ‘500 “00

3) XRD of LDH-SiWno. at 25°C

 

um;

11

‘-°°‘ 19.31 0.9.
0.81‘

0.64‘
0.49*
0.36‘
9.25‘
0.16‘
0.09‘
0.04*
0.011

5947 Cnts

 

 

 

 

5.0 10.0 15.0 29.0 25.0

I.” ‘
0.81‘
0.64 ‘
0.49 ‘
0.36‘
0.32154
0.16‘
0.09 ‘
0.04 ‘
0.01‘

 

 

30.0 . 35.0 40.0 45.0 30.0

b) XRD of LDH—Siwuo. at 1600

l

 

 

173

rehydrated

 

 

4O

 

 

45

c) XRD OF LDH-SiW O at IBO’C

 

 

. 6.6? 0093
599 Cats

18.72 0093
151 Ont:

 

 

r
3..

10.0

(1) XRD of LDH-Si' O at 230‘C

It I.

174

-' ‘ 7.09 0093

0.324

 

 

5.0 10.0 13.0 20.0 23.0 30.0 33.0
2.00
1.62
1.20
0.90
0.72
0.501
0.321
0.101
0.001
0.021

 

 

V T V V V V V V V r V

40.0 45 .0 50.0 53.0 60.0 63.0 70.0

0) XRD of LDH-Siwuo. at 30051:
1.9 L

2.001
1.62‘
1.201
0.904
0.72‘
0

0

 

 

 

 

V Y T V I I r I I r ‘

50.0 53.0 60.0 65.0 70.0 75.0

1

40 .0 45 .0

V

f) XRD of LDH—sump. at 500~c

.mBBSomEB Subway 3 mmO— 5,2933 .«o .383 h .8 .wE

mmwmxazm><z
oov ocm com oco« coma

 

175

_ _ _ _ b h _ p P _ _ _ _
V.
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Decca u
Cocoa ”
Doom." u
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/

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176

can

.32 55:5 .6 <0... 93 can .3 .5

AOL 93:00:80.—

 

 

 

 

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CC - . “NOIP - P p - *.°Iu
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0.8.. 98»
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1 mud—um gala—hog i T Gan—nu 38.125 r
83..—
030933
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177

Fig. 68. XRD pattern of LDH-BW11039 at different temperatures. a) 25°C, b) 150°C,
c) 180°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, (1)
210°C, e) 260°C and 0 500°C.

 

5°C, b) 1501

31011 5331,91:

 

 

x103 L
5.”‘
4.05‘
3.20‘
2.45‘
1.80‘
1.25‘
0.%‘
0.451
0.30‘
0.05‘

 

178

18.50 0090
81727 Cats

37.44 0090

31.33 0.9, 3195 Cats

62? Cats

 

 

W V V Y

30.0 35.0 40.0 45.0 . 30.0

a) XRD of LDH-Blnou at 25‘C

18.55 009:

12.35 Dogs
6.25 0093

 

5.001
4.051
3.."

2.451
1.001
1.251

0.“ 4¥“ _‘

0.451
0.201
0.051

 

5.0 10.0 1510 ' 20.0

31.10 0093 37.50 003:

 

35.0

b) XRD of LDH—Bluo. at. 1501c

 

179

rehydrated f...

 

 

 
    

 

30 35 4o 45 50
c) XRD of LDH—amp, at 1ao-c

A
—

O 0"

37828560111081"...

18.67 009:
649 Cats

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A A A A A A A

12.44 Doss

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A L

 

 

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5.0 ‘OCO ‘500 ”.0 6..

A A A A A A A A A

A
O
‘
. 1
. d
. d
d
.

9999999775"
83888383338

 

 

(1) XRD of LDH-BWuo, at 21010

 

180

'16:

9.811
0.641
0.491

0.251
0.16‘
0.091
0.041
0.01‘

 

 

5.0 10.0 15.0 20.0 25.0 30.0
1.00
0.01
0.64‘
0.49‘
0.36‘
0.25‘
0.16‘
0.09‘
0.04‘
0.011

 

 

T I I

30.0 35.0 40.0 45.0 50.0 35.0

0) XRD of LDH-3W O at 260°C

 

«10
2.00‘
1.62‘
1.28‘
0.90‘
0.72‘
0.50‘
0.32‘
0.18‘
0.001
0.021

 

 

 

510 10.0 15.0 20.0 25.0 30.0 35.0
2.001

1-63‘ 3.5 009: 61.900093
1.281 .41009341.280090 48.760093: 3

9.931 38.18 009: “'24
0.721
2'31 0 51.79 0.9:
0.18‘
0.00‘
0.021

 
    
 

 

 

35.0 40.0 45.0 50.0 55.0 60.0 65.0

1) XRD of LDH—amp. at soc-c

181

cow

2228382 80.5me 8 amofi H 39mg .3 «beam .: do .wE

mmmmxazm><3
0cm com

ocw«

 

 

 

 

 

 

 

 

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./
0.3m ” v
063 u m
0.93 n m
9mm u H

 

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mvn.o

BONVLLIHSNVHL

182

only one absorption band centered at 855 cm'1 without the band around 427 cm‘l, which
is attributed to the Zn-OH-Al lattice vibration. Thus, species in an amorphous region do
not contain an Al-O-Zn network.

The DSC and TGA of LDH-BW11039 are provided in Fig. 70. The dehydration
process occurs up to 208°C, followed by dehydroxylation of the hydroxyl layer up to
342°C. At 450°C new crystalline inorganic phases appear.

LDH-C02W12042 : This system is stable to 180°C. Rehydration of sample heated to
180°C is possible as shown by their XRD pattern in Fig. 71. At 225°C, The ir spectrum
provided in Fig. 72 shows that LDH-C02W12042 decomposes at 225°C. There is only
one absorption band centered at 841 cm-1 as in the other cases. Its XRD pattern shows
only one unidentified peak at 20 = 6.90. At 320°C It becomes completely amorphous,
as shown in its XRD pattern. The ir spectrum of this amorphous material below 1000
cm-1 shows only one absorption band centered at 856 cm'l. At 500°C new crystalline
inorganic phases appear.

The DSC and TGA of LDH-C02W12042 are provided in Fig. 73. Dehydration
occurs up to 223°C, followed by dehydroxylation of the hydroxyl layer up to 356°C.
At 450°C new crystalline inorganic phases appear.

LDH-PW9034 : When this complex is heated even at 110°C, its XRD pattern shows
remarkable peak broadening , as shown in Fig. 74. However, there is no substantial
difference in its ir spectrum from that of an unheated samples shown in Fig. 75. Thus, it
is obvious that heating causes no structural change in PW9034 in the gallery by heating.
The XRD peak broadening mainly originates from the layer sagging as discussed in a
previous section in this chapter. Such a broadened XRD pattern is observed for the
215°C heated specimen; peak broadening is so severe that it is difficult to locate peaks.
Rehydration is possible even though recovery of XRD pattern is very slow. Heating at
320°C causes it completely to an amorphous phase. As in the other cases, the ir

spectrum below 1000 cm-1 shows a typical single band centered at 852 cm-1 below 1000

183

.22 gain: 8 <09 Ea 0mm .2 .5

AOL 0539—2—38.

 

 

 

 

 

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184

Fig. 71. XRD pattern of LDH-C02W12042 at different temperatures. a) 25°C,
b) 120°C, c) 180°C: dotted line for the rehydrated sample, solid line for the dehydrated
sample, (1) 225°C, e) 320°C and 1) 500°C.

.1371
1.00‘
0.91‘
0.64‘
0.49‘
0.36
0.251

18.33 0095
4516 Cnts

12.13 Dog:
1704 Ont:

0.16‘
0.09‘
0.04‘
0.01

 

24.58 Doss
1004 Cnts

 

5.0 10.0 15.0
1.001
0.011
0.641
0.491
0.301
0.251
0.161
0.091
0.041
0.011

37.25 0095
38.83 0090 785 Cnts

288 Cnts

 

 

25.0

 

I if

30.0 35.0 40.0 45.0

3) XRD of LDH—CoJuO. at we

‘
'1...) A

5.001
4.051
3.201
2.451
1.001
1.251
0.001
0.451
0.201
0.051

I

 

5.001
4.051
3.201
2.451
1.88‘
1.251
0.001
0.451
0.201
0.051

 

18.39 009:
3768 Ont:

 

 

5.0 10.0 15.0

37.33 0090
586 Ont:
38.97 0095
191 Cnts

 

 

45.0

b) XRD of LDH—Coyuoa at 120°C

186

rehydrated

 

T5 2' “0 25

cm
pa
0

 

 

‘ 30 35 46 is

C) XRD of LDH-COW O at 180°C

01:.
1L

xlB'
1.00‘

0.81‘

0.64+ 6.90 0.93

0.49‘

9.3.1 W
0.25‘ .

0.16‘
0.09‘
0.04‘
0.01*

 

 

5.0 10.0 15.0 20.0 25.0
1.00‘
0.81‘

0.64
0.49‘
0.36‘

0.251
0.161
0.091
0.041
0.011

 

 

25.0 30.0 35.0 40.0 45.0

d) XRD of LDH-CoJuO. at 225’C

203 l
.001
.81‘
.64‘
.49‘
.36‘
.25‘
0.16‘
0.09‘
0.04‘
0.01‘

00000“'

 

187

7.46 009:

 

1.08‘
0.81‘
0.64
0.49

0.25‘
0.16‘
0.09‘
0.04‘
0.01‘

 

5.0 15.0

9.36%

 

v v v 5 v V V "

25.0 35.0

e) XRD 01 LDH-CoJuO. at. :3sz

 

 

 

 

2.00‘

0.90‘
0.721
0.50‘
0.32‘
0.18‘
8.881
0.021

 

10.0

41.38 0093

 

40.0

f

65.0

I I

45.0 55.0 60.0

W

I 10

0 XRD of LDH—Co 0, at 50%

188

oov

 

 

3580983 82850 “a NvONH 390-33 mo «88% A .Nh .wE

 

 

mammzszm><z
coo coo“ cam“
1‘ / _ _ _ . .. 88...
a n.
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r.cco o
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H.806
/ H
:.oo~.c
0.8m ” v n 82.
obmm u m n1
0.2: u m n
” ruooo.o
9mm a T 83 o

 

BONVLLINSNVUL

189

802390-35 .3 <2. 22 can .2 .5

8b 0.3.1.3333.

 

 

 

 

 

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. .... 1 2T.
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T.
on L
0nd
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2:03.35
can

 

(3/1) sou 3003

190

Fig. 74. XRD pattern of LDH-PW9034 at different temperatures. a) 25°C. b) 110°C:
dotted line for the rehydrated sample, solid line for the dehydrated sample, c) 175°C, (1)
215°C, e)320°C and f) 500°C.

104 -~
1 001
0.811
8.64‘
0.491
0.361
0.251
0.161
0.09‘
0.041
0.01

 

191

 

18.33 009!

 

 

1.001
0.011
0.641
0.491
0.361

0.04‘
0.01‘

 

5.0 10.0 ' 15.0 ‘8 20.0 . 25.0 30.0

37.15 0093

9.351 33.82
0.16‘
0.09‘ “W

 

30.0

35.0 40.0 45.0 50.0 55.0

0) XRD of LDH-PW,O“ at. 25°C
rehydrated

'/
0
I 0
g 0
, 0
, 0
'

 

255 1?? 1'5 2‘5 * 25

 

ovmm'

‘ #1.__ AAA—___
30 35 4O 45

b) XRD of LDH-mm“ at 11o-c

192

18.60 Dogs

12.52
1 6.80 0.9. M

 

 

5.0 10.0 15.0 20.0 25.0

1.28‘ 37.48 0095

 

 

V f 5 fi 5

25.0 30.0 35.0 40.0 45,n

c) XRD of LDH-mm“ at 175-c

g.
A__‘

19.” 0033

A A A A

C I
A

°°°°PPPrrP
52200022222

 

2.001
1.62‘
1.28‘
0.”'
0.72‘

0.50'rhh~'hqu*d‘nnh'In.~".~Iflu‘y‘gflflabuhuFfll'fifl‘flflwu~ilflif-flufla’
0.321
0.18 ‘

0.001
0.021

 

 

V fir r

30.0 35.0 40.0 45.0 50.0 53.0

d) XRD of LDH—mp“ at. 215-c

193

 

-'-'9' 30.63 00915
.01 1
.64‘
.491
.361
.251
.161
.091
.041
.011

 
 

18.82 009: 23.91 009:

  
   

cocoons-9.3.1...-

 

 

' ' V V V7 Y

5.0 10.0 15.0 20.0

V V v Y

25.0 30.0 35.0
1.001
0.011
0.64
0.491
9'3“ 51.79 0096
0.251
0.16‘
0.091
0.041
0.011

.48 003641.43 0.93 43.59 0.3. 53.78 0036

   

 

 

i 1 V I f f r f v 1 t Y

35.0 40.0 45.0 50.0 55.0 60.0 65.0

f) XRD of LDH—PWp“ at. soc-c
21
l0

1.00
0.011
0.641
0.491
0.361
0.251
0.161
0

0

0

 

I” ‘
.04 ‘
.01 1

 

1.00

0.01

8.64

0.491
0.361
0.251
0.161
0.091
0.041
0.01 1

 

 

30.0 t 35.0 40.0 45.0 50.0 . 53.0

e) XRD of LDH—211,0“ at. 320°C

194

2838382 Santa 8 30325-33 .«o «88% h .3. .me

 

 

 

 

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com

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BONVllINSNVUl

195

cm‘l. At 500°C new crystalline inorganic phases appear. As the heating temperature
increases, the frequency attributed to Al-O-Zn vibrational motion increases : 428 cm-1 at
25°C, 429 cm‘1 at 110°C, 430 cm-1 at 150°C, 432 cm-1 at 175°C and 434 cm-1 at
215°C.

The DSC and TGA of LDH-PW9034 are given in Fig. 76. The dehydration
process occurs up to 217°C, followed by dehydroxylation of the brucite-like layer up to
350°C. At 450°C new crystalline inorganic phases appear.

LDH-PV14042 : This complex is very thermally unstable even at 125°C. The
PV14042 in the gallery is decomposed at this point, verified by its ir spectrum provided
in Fig. 77. However, the hydroxyl layer still remains intact; an absorption band at 429
cm-1 manifests the preservation of the hydroxyl network. Heating at 120°C changes the
XRD pattern to one that is uninterpretable, as shown in Fig. 78. Such decomposed phases
remain after heating to 185°C. At 320°C the LDH-PV14042 turns completely to an
amorphous phase, as shown in its XRD pattern in Fig. 78. But unlike the other systems
there are two absorption bands centered at 934 cm.1 and 775 cm-1. At 500°C new
crystalline inorganic phases appear.

The DSC and TGA of LDH-PV14042 are provided in Fig. 79. The dehydration
of the lattice water and the decomposition of PV14042 in the gallery occurs up to
180°C, followed by dehydroxylation of the hydroxyl layer up to 374°C. An exothermic
peak starting at 450°C indicates the onset of new inorganic phases formation.
LDH-NaP5W300110 : When this system is heated at 120°C, a remarkable change in
XRD pattern is observed, as shown in Fig. 80. The first order peak increases, while the
second order peak disappears. When the sample heated at 120°C is rehydrated, its XRD
pattern is restored to that of an unheated sample. That is, the absent second order peak
reappears and the increased intensity of first order peak decreases, as shown in its XRD
pattern. However, there is no Change in the ir spectrum upon heating at 120°C provided

(see Fig. 81), indicative that no structural change has taken place. At 180°C there only

196

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198

Fig. 78. XRD pattern of LDH-PV14042 at different temperatures. a) 25°C, b) 125°C,
c) 185°C, d) 320°C and e) 500°C.

 

199

 

 

 

 

 

 

 

 

 

 

 

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3) XRD of LDH-13V O at 25°C

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201

 

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203

Fig. 80. XRD pattern of LDH-NaP5W300110 at different temperatures. a) 25°C, b)
120°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, c)

180°C: dotted line for the rehydrated sample, solid line for the dehydrated sample, d)
250°C and 500°C.

 

 

 

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two peaks at 20 = 5.20 and 10.40 remain. These may be assigned as the first and
second order peak , respectively. At 250°C, this specimen turns into an almost
amorphous phase even though a trace of the LDH-NaP5W3oO110 phase remain apparent
in its XRD pattern. Its ir spectrum shows evidence of structural change in the
NaP5W300110 in the gallery. New crystalline inorganic phases appear at 500°C.

The DSC and TGA of LDH-NaP5W300110 are given in Fig. 82. The
dehydration process occurs up to 176°C, followed by dehydroxylaton of the hydroxyl
layer up to 322°C. An exothermic peak starting at 466°C corresponds to the onset of
new crystalline inorganic phase formation.

LDH-H2W12042 : This compound is moderately thermally unstable even at low
temperature. The XRD pattern of this complex heated at 120°C shows a decomposed
phase, as evidenced in Fig. 83. The decomposition of this complex is also easily verified
by ir spectroscopy, as illustrated in Fig. 84. A decomposed phase is observed even at
120°C, and the complex turns to an almost amorphous material at 230°C. At 320°C
the material is completely amorphous. Its ir spectrum shows one typical simplified
absorption band centered at 856 cm"1 as do other complexes. At 500°C new crystalline
inorganic phases appears.

The DSC and TGA of LDH-H2W12042 are provided in Fig. 85. The dehydration
process occurs up to 230°C, folloWed by dehydroxylation of the brucite-like layer to
355°C. At 500°C new crystalline inorganic phase appears.

F. Adsorption Study of LDH-POMs

The LDH-POMs can be classified according to the charges of POMs in the
gallery. The N2 adsorption isotherms of the representative LDH-POMs are shown in
Table 14.

Fig. 86 shows the N2 adsorption isotherms, providing the isotherm type and the
BET (N2) surface area. The shape for the first four LDH-POMs, type 1, indicates that

209

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Fig. 83. XRD pattern of LDH-H2W12042 at different temperatures. a)25°C, b) 120°C,
c) 180°C, (1) 230°C, e) 320°C and 500°C.

 

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217

Table 14. LDH-POM Intercalates for N2 Adsorption Studies.

 

 

POM Isotherm Type Surface Area
BET N2(m2/g)

H2W1204o6- I . 63
SiV3W9O4o7- I 155
SiW110398- I 211
BW1 10399- I 96
PW90349- II 26
H2W12042“). II 15

NaP5W3001 1014‘ II 3

 

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P/Po

Fig. 87. T-curvc of LDH-Cl.

219

these systems are microporous materials in which the pores are nearly filled at low p/po.
These have relatively high surface areas compared to the other three LDH-POMs whose
adsorption isotherm shapes are type II. The latter type II isotherms are typical for
nonporous materials. One of lowest adsorptions capacities was observed for LDH-Cl,
which is a reference nonporous material used for the t-curve analysis, which is provided
in Fig. 87.

The pore volumes of the porous LDH-POMs were obtained from either a DA or
D-R plot and a t-plot. These are shown in Appendix B. All t-plots have the same shape.
Such sharp transitions in the t-plot curves indicate that each system has a unique pore
size. It can be said that POMs are uniformly distributed in the gallery, thereby providing

an unique pore size.

G. Theoretical Calculation of Surface Area and Pore

Volume

Certain POMs can be intercalated in LDH galleries, thereby providing free space
in the gallery. The available extra space or area can be estimated in the following way.
Here only LDH-POM in which POMs have the Keggin structure are considered.

The general chemical formula of LDH-POMs is
[Zn2A1(OH)6]nPOMn-, where n is1th‘e negative charge carried by POM. The following
quantities are defined :
81 = area of anA1(OH)6 surface in the absence of'any gallery ion
Sc = maximum cross area of POM
The free area created by the intercalation of one molecule of POM in the gallery is equal
to nS1 - Sc. The molecular weight of [Zn2A1(OH)6]nPOMn' is equal to 260n + MpoM,
where MPOM is the molecular weight of the POM. Assuming that l) the host layer is
perfectly rigid, 2) the repulsive interaction between POMs in the gallery is free and 3) the
POM is spherical with 2r = d(gallery height) = 9.6 A. Then,

220

S(m2/g) = 6.0 x 103 x {n81 - (ud2/4) }/(260n + MPOM) --(1)

The calculated values for selective systems including observed ones are tabulated in
Table 15. Here, the equation (1) should be greater than zero, Therefore,

n81 - 1td2/4 > o

n > 4.4

Since the area occupied by the POM in the gallery should be less than that provided by
the LDH layer (n81), the above result implies that the minimum charge of a POM with
Keggin structure to be intercalated is 4.4. Because of the integrality of molecular charge,
the minimum must be 5. However, in this calculation the free area includes all the area
available regardless of the size and shape of the free area. For more precise calculation,
the assignment of the POMs in the gallery should be taken into account. The most
efficient array of the POMs in the given area is a hexagonal one. When the POMs are
closely packed hexagonally, the line connecting the nearest three POMs will make an
equilateral triangle with an interstice in the center. The area of this equilateral triangle is
equal to 0.433 d2, corresponding to one half of the area given to one POM. Thus, the area
given to one POM is 79.8 A2 for d = 9.6 A, This area consists of the area occupied by
one POM as well asthe area of the interstice. When the area n81 given by the LDH layer
is equal to that given to one POM (2 x 0.433 d2), the POM molecules in the gallery are
most efficiently arranged, so maximum POM number is accommodated by the host layer.

Thus,

n81 = 2 x 0.433 d2

For (1 = 9.6 A and 81 = 16.5 A2, 11 is equal to 4.8.
In reality, the POMs with 5- charge have not been completely exchanged into the
gallery, but only partial exchange has been observed. The observed minimum charge for

the complete exchange reaction is 6-. This discrepancy arises from the oversimplification

221

of the above model. If the repulsive interaction between POMs is considered, the van der
Waals radius would increased, and the calculated minimum charge would be greater than
5.

From the above derivation of the minimum charge carried by POM, the average
distance between the POMs in the gallery can be also obtained. For the simplicity, let’s
take the minimum charge is 5-, which corresponds to a value obtained using (1 = 9.8 A
and the area given to a POM 83.2 A2. As n increases from 5- to 6-, the given area to the
POM also increases proportionally. Thus, the resulted area is 99.8 A2. Now, the quantity
(1 corresponds to the distance between the POM. Therefore,

2 x 0.433 d2 = 99.8
d is equal to 10.7 A. In the same way, the following distances can be obtained for the
othernvalues : n =7, d= 11.6 A; n = 8, d =12.4A;n =9,13.2A;n=10,13.9A.

The free volume can also be estimated in a following manner. The free space
generated by the intercalation of one molecule of POM is V, expressed as below, where

the symbols and definitions have the same meaning, except that dis again a d-spacing.
V = (d x n81) - volume occupied by a POM ----- (2)

If the shape of POM in the gallery is assumed to be spherical and if the pore volume
(Vsp)is expressed as a ml/g unit, then

v = vSp = 0.6{16.5nd -( rtd3/6)}/(26On + MPOM) ----- (3)

If the shape of POM in the gallery is cylindrical with height = d(gallery height) and cross

area = So, then

v = vcy = 0.6[16.5nd - (M3/4) }/(260n + MpoM) ----- (4)

222

All calculated pore volumes are tabulated along with observed values in Table 15.

As the charge, 11, on the POM increases, the calculated pore volume and surface
area also increase, because the lateral distance between POMs in the gallery increases.
However, the denominators in both equation (3) and (4) also increase with increasing
charge. The contribution of the weight term does not change the trend of calculated
values : the higher the charges of POMs in the gallery, the larger the surface areas and
pore volumes. The observed values of surface area and pore volume increase as the
charge on the POM increases only in the range of n = 6 - 8. When n reaches 9, all the
values abruptly decrease. Therefore, the maximum value of surface area and pore volume
is observed at n = 8. The largest difference in the absolute value between the observed
and calculated ones occurs at n = 8. The structure of the POM at n = 8 is no longer a
Keggin structure, because one out of 12 W required in the original Keggin structure is
missing. The reduction of the lateral diameter of the pillaring POM will contribute extra
space in the gallery and result in larger observed than calculated values of both surface
area and pore volume.

The big drop in observed surface area and pore volume at n > 8 probably results
because one of the assumptions made in the calculation does not hold. The assumption of
perfect layer rigidity is not realized in these systems. Imperfect layer rigidity results in
layer sagging after the zeolitic water in free space is removed prior to the gas adsorption.
Such layer sagging will result in a reduction of the free space, thereby blocking the
adsorbate from reaching the gallery space. The dependence of layer sagging on the POM
charge densities also is observed in the case of POMs having the same total charge but
different size. For the n = 9 case, the 9- charges are delocalized on 39 oxygens in
BW11039 and on 34 oxygens in PW9034. Therefore, the charge density of the former is
lower than that of the latter. The higher charge density and small size of the latter will

cause greater layer sagging than the lower charge density POM. The dependence of layer

223

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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sagging on charge density is also observed in the XRD data for both systems heated at
temperatures employed for the adsorption studies. These XRD patterns are in Fig. 68 (c)
and Fig. 74 (b). As the charges of POMs increased further, observed values decreased.
The details of layer sagging will be discussed in the other section.

The pore volumes calculated on the basis of a cylindrical model are closer to the
observed values than those based on a spherical model. Since nitrogen used as an
adsorbate in the adsorption study has a finite size, there are dead spaces that nitrogen
molecules can not reach. Thus, a cylindrical model, in which the volume occupied by
POMs will include some of such dead spaces, will be more realistic than a spherical

model, in which the dead spaces are not taken into account.

G. Photoreactivity of LDH-POMs

Following the scheme of Hill for classifying POM photoreactivity in
homogeneous solution, we can classify LDH-POMs by the visual changes which occur
during the photoreaction.
Class I : This category of LDH-POMs neither develops any color change even under Ar
nor yields any significant photoproduct. The yields are so low even under 02 that in
some cases product production is below reasonable limits of detection. The LDH-POMs
containing transition metals are in this category. For example, LDH-SiFe(SO3)W11039,
LDH-BCuW11O39, LDH-BCoW11039 and LDH-Con12042 belong to this class.
Class II : This category of LDH-POMs changes its color to mostly blue under Ar. The
original colors are not completely recovered when the reduced LDH-POMs are exposed
to air. Even after several days there is still slow recovery of the original color, but
complete recovery is not achieved. The LDH-POMs containing V(V) and Mo(IV) belong
to this category. Thus, when photoreactions are carried out even under Oz, color changes

are developed. In the catalytic cycle, the rates of reoxidation of the reduced LDH-POMs

225

are very slow compared to those of Class III. LDH-PV14042, LDH-SiV3W9O40 and
LDH-PM02W9039 belong to this category.

Class III : This category of LDH-POMs changes its color from mostly white to blue
under an argon atmosphere. When these blue-colored forms are exposed to the air, their
colors change back to the original white color. When photoreactions are carried out under
O_2_, no color change develops. Therefore, the catalytic cycle, which involves reduction of
LDH-POM by organic substrates, formation of a blue colored LDH-POM and
reoxidation of the LDH-POM by Oz, is completed without appearance of a color change.
For example, LDH-NaP5W300110, LDH-H2W12042, LDH-SiW11039, LDH-
PW9034, LDH-H2W1204o and LDH-BW11O39 are in this category.

The photoreactivities of LDH-POMs for the oxidation of isopropyl alcohol are
summarized in Table 16.

The general trend of photoreactivity in Class III complexes is that the higher the
charge on the POMs in the gallery, the greater the reactivity. As discussed in the previous
section, the higher the charge on the POM in the gallery, the more free space available
between POMs. Therefore, it can be said that the more spaceous the gallery, the more
reactive is the system. In the case of LDH-NaP5W300110, the increase in gallery height
provides more space compared to LDH-POMs of the Keggin-type. Therefore, for the
Class III systems, the greater the free space within the gallery, the higher the
photoreactivity. This observation is contradictory to Fox’s result in which the higher the
charges or the larger the sizes of POMs, the lower the reactivities. The photoreactivities
of LDH-POMs have been observed to be very much different from those of
homogeneous POM solutions. It has been claimed that a pre-association complex
formation between the organic substrate and POM should precede photoactivation and
that the photoreactivities depend on the Stabilities of these complexes. The rate of
formation of the pre-association complex in the LDH-POM will be dependent upon the

mobility of the organic substrate within the free space in the gallery. It is certain that the

226

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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227

more spaceous the gallery is, the faster the organic substrate will travel through it.
Therefore, higher reactivity will be observed in the high charged system containing a
large POM rather than in a low charged system with small sized POM. The increased
space also facilitates diffusion of products from the reaction site out of the gallery. The
ability of product to leave easily minimizes back electron transfer from the reduced POM
to product or other transient species. Therefore, both easy access of reactant and easy
leaving of product in the gallery make the system well suited for high photoreactivities.
Fox has also claimed that the photoreactivities of POM depend on the heteroatom of the
POMs, as well as on their reduction potentials. The same observation has been made for
the LDH-POMs. For example, the following order of photoreactivity was observed :
PW9034 > H2W1204o > SiV3W9O4o > C02W12042.. Therefore, the photoreactivities
of LDH-POMs depend on the electrochemical properties of POMs, as in the
homogeneous solution. When the photoreactivities of LDH-POMs under Ar are
compared with those under Oz, the highest ratio of (turnover # under Oyturnover #
under Ar) is observed in LDH-H2W12040. A trace of product was produced under Ar,
while under 02 a large quantity of product was obtained as shown in Table. 16. The
behavior of this system shows that reoxidation of reduced POM in the gallery by 02 is
highly efficient even though the total yield is lower than that in other systems which have
more free space in the gallery due to the high charge of POM, e.g., LDH-H2W12042.
Therefore, this system illustrates that the photoreactivities of LDH-POMs associate
strongly with the electrochemical properties of POMs.

The LDH-POMs in which boron is a heteroatom are very photochemically
unreactive compared to those in which P is the heteroatom.

Class I system shows almost no photoreactivity, perhaps because insufficient
number of photons is available to carry out photoreaction on the W-O centers due to the

high absorption of photons by the transition metals.

228

H. Swelling Properties of LDH-POMs

It is known that LDH-A intercalation compounds, in which A is a general
inorganic anion, do not swell in any solvent. However, LDH-Or derivatives, in which Or
is an organic anion, for example, alkyl sulfate, can be swelled by polar organic solvents,
as was mentioned in the introduction.

The LDH-POM derivatives show unique, but limited swelling by water, in
contrast to the conventional LDH-A. Wet samples of LDH-PV14042 and LDH-
NaP5W300110 were examined by XRD as shown in Fig. 88. There were two phases
present in each sample. One phase gives a high d spacing corresponding to the swelled
phase, the other exhibits a low d-spacing, corresponding to the non-swelled phase. The
differences in d-spacings of the two hydrated phases in LDH-PV14042 and LDH-
NaP5W300110 are 1.5 A and 2.3 A, respectively. These values correspond to one and
two monolayers of water, respectively. This unusual swelling property of LDH-POMs
can provide insights into the ion exchange mechanism for LDH-A with POMs and will

be discussed later.

1. Proposed Mechanism of Ion Exchange Reaction.

Three important observations can be used to support a mechanism for the ion
exchange reaction of LDH-N03 with POMs. First of all, the ion exchange reaction takes
places only in neutral or acidic solution. Secondly, the LDH layer is not strictly rigid.
Finally, LDH-POMs can be swelled over a limited range, for example, 1 - 2 monolayers
of water, whereas LDH-N03 is not swellable.

’ LDH layers have broken edge sites in which the aluminums are coordinatively
unsaturated. Such edges may have chemical property similar to those of aluminum
hydroxide whose colloidal particles carry positive charge at acidic and neutral condition
and negative charge at basic condition.37 Therefore, negatively charged POMs will

electrostatically interact with positively charged edges of LDH layer at acidic and neutral

229

Fig. 88. XRD pattern for wet samples of 1) LDH-PV14042
and 2) LDH-NaP5W300110.

 

 

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232

conditions. It is obvious that the interaction will be stronger with highly charged POMs
than with lower charged POMs. POMs will consequently bind to the edge of the layer.
Exchange of N03 with edge-bound POMs will take place. Once POMs replace NO3 at
the edge of the layer, two phases coexist. One is a LDH-POM phase and the other is a
LDH-N03 phase. The LDH-POM is the only swellable phase and thereby only it can
provide sufficient space for easy access of incoming POMs into the gallery to replace
N03. The flexibility of the LDH layer helps the whole system accommodate two
different phases in the same gallery. As the reaction proceeds, the LDH-POM phase
extends toward the center of the particles and the exchange reaction becomes more
favorable because the preformed LDH-POM phase will help the exchange reaction to
proceed due to the swellability of this phase. Consequently, the LDH-POM phase plays

the role of a catalyst by facilitating the exchange reaction.

233

Appendix A: XRD pattern of LDH-N03 and LDH-POMs at 25°C.

-a) LDH-N03, b) LDH-H2W12040, c) LDH-BV(IV)W11040, d) LDH-BCoW11039, e)
LDH-BCuW11039, f) LDH-SiFe(SO3)W11039, g) LDH-SiV3W9O4o, h) LDH-
PM02W9039, i) LDH-SiW11039, j) LDH-BW11039, k) LDH-PW9034, l) LDH-
Con12042, m) LDH-PV14042, n) LDH-NaP5W300110 and o) LDH-H2W12042.

234

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235

1|
.JO‘ 18.25 0093
“63‘ 12809 cm:
;.ae*
‘3-93‘ 12.14 begs

2:22, 4547 cm: 3450 Does

0.32‘
0.18‘

0.02‘

 

 

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0.50l
0.18l 713 Cnts

0.08‘
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c.90‘
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1.88‘ 10646 Cnts
0 98 12.19 0095

 

 

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1.28‘
0.98i
0.?2‘
0.50. 37.44 0093
0.32‘ 30.97 009: 2345 Cut:
0.181
0.08‘
0.02

 

 

c) XRD of LDH—BV(IV)W“O” at 25°C

 

8 62‘
C

 

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2.06‘
1.62‘
1.88‘
0.981
8.721
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236

17105 Cnts
8.44 0093

 
 
  
  
 

24.67 Dogs
2998 Cats

6.19 009:
149 Cnts

 
 

5.0 10.0 15.0 20.0 25.0
37.44 0093
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31.03 0093
762 Ont:

 

30.0 35.0 45.0 50.0

(1) XRD or LDH-BCo(II)W“O‘. at. 25'C

18.44 0093
10184 Cnts

 

2.00‘
1.62‘
1.28‘
0.981
0.72‘
0.501
0.32‘

 

 

 

 

 

 

e) XRD or LDH—BCu(II)Wuo, at 25'C

237

5.00' 18.31 Doss
4.851 35415 Cats

3-80‘ 12.14 0.9.
2"5 16959 Cnts 34-56 0093
11674 Cats

   

0.451 6.03 0.93
°°3°l 431 Cnts

 

 

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01‘5‘ 2230 Cnts

 

 

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30.0 35.0

40.0 45.0 50.0
1) XRD of LDH—Sirearrxsogwno, at 25'C

x104 P
1.80‘ 18.44 009:

9-91‘ 12.19 0.9:

006‘T

0.491 24.61 Dogs
8.36‘

8.23‘

8.16‘

3.991 6.88 0093

0.04‘ "-212 - :

 

 

0.01

 

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5.9 19.9 15.9 29.9 25.9
1.991
9.911
9.641
9.491 .
8.36‘
9.251

9.161
9.991
9.941 “:—~;:‘“*‘ "‘1“"

0.01‘

 

 

 

30.0 35.0 40.0 43.0 50.0

9) XRD of LDH-Sivawpw at 25°C

238

 

1-63‘ 12999 Cnts

 

 

 

 

30.0 35.0 40.0 45.0 50.0

h) XRD or LDH-PMon. at 25°C

 

2.08‘ 18.25 0095
1162l 12993 Cnts

 

 

 

 

 

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9.321 3°16? 0'9” 2173 Cuts
0°19‘ 939 Ont:
9.991
9.921
39.9 35.9 49.9 45.9 59.9

i) XRD or LDH—Siwno. at. 25'C

23S)

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1.991
1.251
9.991
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9.291
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5.00‘
4.05‘
3.20‘
2.451
1.80‘

1
:2:1 37.44 0095

9.451 31.93 0.9. 3195 ents

0.20‘ 62? Cats
0.05

30.0 35.0 40.0 43.0 50.0

j) XRD of LDH-ewuo, at 25-c

 

 

 

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1.36‘ 37.15 0093

3.25 ‘ 33082
0.16‘
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30.0 35.0 40.0 45.0 50.0 55.0

k) XRD of LDH-P119“ at 25-c

 

 

 

240

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1.901

0.91‘
10.33 0093
0.64‘

4516 Cnts

0.491
0.36‘
0.25‘
0.16‘
0.09‘
0.041
0.01‘

 

 

 

5.9 19.9 15.9 29.9 25.0
1.001
0.81‘
9.641
9.491
0.361
9.251
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0.011

 

 

 

30.0 35.0 40.0 45.0 50.0

1) XRD of LDH—Co W O at 25‘C .

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1.28

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0.18
0.08
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2.00‘
1.62‘

1.28
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0.72
0.50
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1*

    
 
   

 

 

 

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30.0 35.0 40.0 45.0 50.0

0) XRD of LDH—H3120“ at 25°C

242

Appendix B: T-plots and D-R or D-A plots of LDH-POMs.

a) t-plot of LDH-H2W1204o. b) D-A plot of LDH-H2W1204o.
c) t-plot of LDH-SiV3W904o. (1) DA plot of LDH-SiV3W904o.
e) t-plot of LDH-SiW11039. 1) DR plot of LDH-SiW11039.
g) t-plot of LDH-BW11039. h) D-A plot of LDH-BW11039.

243

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