Clays and Clay Minerals, Vol. 41, No. 6, 731-737, 1993.
SYNTHESIS A N D CO2 ADSORPTION FEATURES OF A
HYDROTALCITE-LIKE C O M P O U N D OF THE M g 2 + - A 1 3 + - F e ( C N ) 6 4S Y S T E M WITH HIGH LAYER-CHARGE DENSITY
GANG MAO, MASAMICHITSUJI, AND YUTAKA TAMAURA
Department of Chemistry, Research Center for Carbon Recycling & Utilization
Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan
Abstract--Hydrotalcite-like compounds (HT) with 240/o-48% A13+-substitution have been synthesized in
the Mge+-AP+-Fe(CN)64 system. Conditioning of the synthesized and air-dried compound with
K4Fe(CN)64 solution at 80~ was essential to obtain the 80O/o-90% pure ionic Fe(CN)64- form on an
equivalent basis. A linear decrease in ao with an increase in the mole ratio of R = A13+/(Mg2++A13+ )
was extended to R = 0.48. The formation of highly A13+-substituted HTs has been corroborated by the
decrease in the hexagonal lattice constant ao down to 3.016 ~. The ao value was independent of the
interlayer anions. The CO2 adsorption profiles were dependent upon both the A13+-substitution and the
interlayer distance. The isosteric heat of CO2 adsorption was 43.3 kJ tool t in the range of adsorption
of 20 to 40 cm 3 g-' at 298 K and 0.1 MPa.
Key Words--AP+-substitution, CO2 adsorption, Hydrotalcite-like compound, Ion exchange.
INTRODUCTION
CO2 emits from power, ceramic, and steel plants as
well as automobiles, contributing to the "green-house,"
or global warming, effect. Reduction of emission rates
may be possible, however, through chemical processes
that involve separation, recovery, and chemical transformation o f recovered CO2 combined with reduction
o f consumption, i m p r o v e m e n t o f energy utilization efficiency, and use o f solar energy. Recently, magnetiteintercalated mica has been found to show CO2 decomposition reactivity (Tsuji et al., 1993b). There exist a
large number o f layered compounds such as phosphates and tobermorite that are well known as cation
exchangers or H 2 0 adsorbers (Clearfield, 1988; Komarneni and Tsuji, 1989; Tsuji and Komarneni, 1989,
1991). Other layered compounds belonging to solid
bases, such as hydrotalcite-like compounds(HT) and
Al(OH)3-derived compounds, exhibit an anion-exchange property (Miyata, 1983; Sissoko et al., 1985).
Studies on the adsorption of acidic gases, e.g., CO2,
SO., and NOx, were carried out earlier on zeolite and
hydrotalcite (Sazarashi et al., 1992; Miyata and Hirose,
1978). The H T general formula is represented by:
[M 2+ , RM3+R(OH)2]R+ [(R/n)A"- ' m H20] r~where M 2+ and M 3+ are divalent and trivalent cations,
respectively, A" is an anion, and R is M3+-substitution defined by the mole ratio: R = M3+/(M2+ + M 3+ )
(Allmann, 1968; Bish and Brindley, 1977; Brindley and
Kikkawa, 1979; Gastuche et aL, 1967; Miyata, 1975,
1980; Taylor, 1973). A large layer-charge is desirable
if H T is to be a significant adsorbent for the above
gases.
Copyright 9 1993, The Clay Minerals Society
731
There have been reported Al-rich hydrotalcite-like
compounds in the CO32- form with the m a x i m u m substitution of R = 0.44 (Pausch et aL, 1986; Thevenot
et al., 1989; Tsuji et al., 1993a; Yamaoka et aL, 1989).
Moreover, contradictions and confusions have been
recently pointed out on the thermodynamic treatment
o f the anion exchange on this type o f material (Tsuji
et al., 1992). The standard state for the solid is not
clearly defined in the ion-exchange treatment, and the
ion-exchange isotherm is not consistent with the Kielland plot. Adsorption data for CO2 on the H T are
randomly scattered among H Ts with the same layer
charge synthesized by different authors (Miyata and
Hirose, 1978; Kikkawa and Koizumi, 1982). Therefore, the objectives o f the present study were 1) to
synthesize H Ts in the Fe(CN)64 form with as wide a
range o f A13+-substitution as possible and 2) to study
their C O 2 adsorption behavior.
EXPERIMENTAL METHODS
Synthesis o f hydrotalcite-like compound
Appropriate proportions o f 0.5 M Mg(NO3)z, 0.5 M
Al(NO3)3, and 0.25 M K4Fe(CN)6 solutions were mixed
in different glass vessels to give various mole ratios (r)
o f A13+/(Mg 2+ + A13+ ) and the mole ratio of Fe(CN)64to A13+ 1.0. After each mixed solution was degassed
with N2, 2 M N a O H was added slowly until pH 10.0
was reached while stirring. The precipitated amorphous hydrolysis produce was aged in the mother solution at room temperature overnight, washed with
water, filtered under suction, and air-dried at room
temperature for several days to allow slow polymer-
732
Clays and Clay Minerals
Mao, Tsuji, and Tamaura
"
6.... ~
I--
~////~Aq
/->
4
C~[7 5
--
- To Vacuum line
---~
Injection of CO2
Sampling to Gas
Chromatography
li
i
'k/
. r"q
3
sample
5. valve
i
,t
9
/
/
1
. v.,q
1. electlic tumace 2, Ihermocouplc
i
/
,
//~-'\
\
/
',,
i
!1
'1
i
C./3
/
i
/
\ / ..
4 pressme gauge
6. quarlz cell
/
Figure 1. Homemade apparatus for CO2 adsorption.
/
'i
i z a t i o n . D r y i n g at h i g h e r t e m p e r a t u r e (550-800C)
adopted by other authors prevented the growth of hydolysis products in crystals with a high degree o f A13+substitution.
(c~;t
X~-,
V~ (l'I,O) !
"
'f•\,\ C
"\
/
v~ (NO3-)
\/'
vvv:~ (K,O)
4000
1
v~ ( O-COp
3000
2000
./%'
i
i%
2 (HOC02)\<"
v s ( O-COp
1000
400
wavenumber ( cm-~ )
Conditioning ofair-dried products and
chemical analyses
The air-dried products were mixed ionic forms, as
will be discussed later, and were conditioned as follows
to obtain a well-defined ionic form. They were ground
and sieved to obtain a fraction o f 100-200 mesh size
(37-74 #am in diameter). A portion o f the sieved material (0.50 g) was immersed in 15 cm 3 of 1 M K4Fe(CN)6
solution at 80~ to exchange Fe(CN)64 for NO3 and
CO3 z- in the interlayer. The solution was replaced every 90 minutes for 4 cycles. The process is based on
the ion exchange. However, the ion exchange selectivity for these ions has not been investigated, and the
experimental conditions cannot be decided to obtain
each ionic form. The present procedures were taken
from our conventional m e t h o d (Tsuji and Komarneni,
1989). Then the samples were washed with water to
r e m o v e the excess Fe(CN)64- ions and air-dried.
Mg 2+, A13+, and Fe z+ were determined by inductively coupled atomic emission spectrometry (ICP-AES)
with a Seiko Instrument Spectrometer Model SPS 7000
after dissolving the compounds in hydrochloric acid.
Thermal analyses (TG and DTA) were undertaken with
a Rigaku Thermoflex-type thermal analyzer model 8001
at a heating rate o f 10~ rain -~ by using ~x-Al203 as
the reference material. The content of water in the
interlayer o f the samples was determined from the T G
weight loss curve. Powder X-ray diffraction was carried
out with a Rigaku X-ray diffractometer R I N T 1100 at
a scanning speed o f 2~
min-~ with Ni-filtered CuKc~
radiation. The infrared absorption spectra were recorded by a J AS CO F T / I R - 8 9 0 0 equipment with
pressed sample disks.
Figure 2. The infrared absorption spectra of sample K (R =
0.366): a) unconditioned, b) Fe(CN)64 -conditioned, and c)
N O s -conditioned.
COe adsorption
CO2 adsorption experiments were carried out with
a h o m e m a d e apparatus (Figure 1). Prior to the adsorption experiment, the samples were heated at 150~
for 10 min while evacuating to expel the interlayer
water molecules. The layer structure was not collapsed
upon heating at 150~ but degraded at > 200~ After
cooling to a required temperature, a given amount o f
CO : gas was injected into the quartz cell (20 cm long
x 3 cm diameter) and the equilibrium CO2 content in
the cell was determined with gas chromatography (Shimadzu GC-8A).
RESULTS AND DISCUSSION
Synthesis of l i T in the Fe(CN)64- form
A typical IR spectrum o f air-dried or unconditioned
H T is shown for sample K in Figure 2a. Broad absorption bands with peak tops at 3448 + 3 cm ~ and
1635 + 5 cm -l can be assigned to the stretching vibrations, v~ and v3, and the deformation, v2, o f the
interlayer water, respectively. They are the same as
those for liquid water irrespective o f the R value. Although these absorption bands should also stem from
the O H groups o f the main layer, the contribution is
hidden by the large interlayer H 2 0 band. This is because the interlayer distance is expanded by a large
Fe{CN)6 4 ion, and the O H groups are weakly hydrogen-bonded. The effect may be observed in H Ts with
Vol.
41, No. 6, 1993
ii d003
q
CO2 adsorption of hydrotalcite-like compounds
:Mg(OH)2
AD-Mg2++A1>
0.50
0.48
0.43
037
0.30
J
@
~s,
' '~
~
!
9
,
~ ,,
~
. . . .
%. ,i
....
~i
0.25
,'
-
R=
[
5
20
1
I
A
40
60
70
0.05
20 (deg)
Figure 3.
Sampie
I'*
R**
a0
(~)
dc~}3
(~)
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
0.05
0.10
0.15
0.18
0.22
0.25
0,25
0.25
0.29
0.33
0.33
0.33
0.40
0.42
0.45
0.124
0.171
0.241
0.251
0.270
0,273
0.276
0.299
0.346
0.366
0.325
0.334
0.431
0.481
3.089
3.084
3.048
3.052
3.057
3.059
3.052
3.040
3.046
3.042
3.053
3.032
3.021
3.016
8.162
8.059
10.64
10.63
10.96
10.91
10.96
10.61
10.96
10.89
10.91
10.77
t0.71
10.88
Observed phase
HT*** + brucite
HT + brucite
HT
HT
HT
HT
HT
HT
HT
HT
HT
HT
HT
HT
HT
0.t7
0.12
I
Table 1. Synthesis conditions and X R D data of the hydrotalcites in the Fe(CN)~ form.
* Mole ratio of A13+/(Mg2+ + A P + ) in the initial solution.
** Mole ratio fo AP+/(Mg 2+ + AP + ) in the product.
*** HT = hydrotalcite.
~,'
,"
733
X-ray diffraction patterns of products.
s m a l l e r i n t e r l a y e r d i s t a n c e a n d t h e larger A13+-substi t u t i o n . T h e b a n d at 2 0 4 0 + 4 c m - ~ is t h e s t r e t c h i n g
v i b r a t i o n o f u(CN) ( I d e m u r a et al., 1989), a n d b a n d s
at 1385 c m ~ a n d 798 c m ~ c a n b e a s s i g n e d to the
s t r e t c h i n g v i b r a t i o n s , v3 a n d v2, o f N O 3 - ion, respectively ( N a k a m o t o , 1986). T h e f o r m e r is n o t s h i f t e d to
a l o w e r w a v e n u m b e r . H e n c e , t h e h y d r o g e n b o n d is
w e a k b e t w e e n t h e i n t e r l a y e r Fe(CN)64 a n d t h e O H
g r o u p o f t h e m a i n layer. T h e b a n d at 777 c m 1 c a n be
a s s i g n e d to t h e v i b r a t i o n , vs, o f u n i d e n t a t e c a r b o n a t e O - C O 2 ( E v a n s a n d W h a t e l e y , 1967). A n u n i d e n t i f i e d
a b s o r p t i o n b a n d was also o b s e r v e d at 1085 c m -~ o n
t h e a i r - d r i e d H T . T h u s , a i r - d r i e d m a t e r i a l s are m i x e d
i o n i c f o r m s c o n t a i n i n g Fe(CN)64-, NO3 a n d CO32i n t h e interlayer.
A i r - d r i e d s a m p l e s were c o n d i t i o n e d w i t h t M
K 4 F e ( C N ) 6 or N a N O 3 s o l u t i o n s to o b t a i n e a c h ionic
f o r m (Figures 2 b a n d 2c). Fe(CN)64- c o u l d be exc h a n g e d for NO3 , b u t a s m a l l a m o u n t o f CO32- still
r e m a i n e d in t h e interlayer. N O 3 - - c o n d i t i o n e d H T
s h o w e d a n a d d i t i o n a l a b s o r p t i o n b a n d at 3 0 0 0 c m -~.
It is a s c r i b e d to t h e h y d r o g e n b o n d s b e t w e e n t h e M OH group and the interlayer anion, O-H...N,
and
b e t w e e n the M - O H g r o u p s o f t h e m a i n layer. H o w e v e r ,
t h e h y d r o g e n b o n d is n o t so strong, a n d t h e d e p e n d e n c e
o f t h e w a v e n u m b e r o n t h e R v a l u e was n o t o b s e r v e d .
T h e a b s o r p t i o n b a n d , v~, a s s i g n e d to u n i d e n t a t e carb o n a t e - O - C O 2 at 1364 c m -~ was also o b s e r v e d o n
b o t h t h e t r e a t e d s a m p l e s as well as t h e b a n d s for
Fe(CN)64- a n d NO3 . A n a b s o r p t i o n b a n d at 955 c m -1
was also o b s e r v e d , w h i c h c a n b e a s s i g n e d to t h e stretching v i b r a t i o n , v2 o f b i - c a r b o n a t e H O - C O 2 ( E v a n s a n d
W h a t e l e y , 1967). T h e u n i d e n t a t e c a r b o n a t e a n d bic a r b o n a t e c o u l d n o t c o m p l e t e l y b e r e m o v e d in the a b o v e
procedure.
M ~ + - s u b s t i t u t i o n in t h e b r u c i t e layer c a n b e evid e n c e d b y t h e d e c r e a s e in ao w i t h a n i n c r e a s e in the R
v a l u e b e c a u s e t h e crystal i o n i c r a d i u s o f a t r i v a l e n t
m e t a l i o n is n o r m a l l y s m a l l e r t h a n t h a t o f a d i v a l e n t
m e t a l ion, e.g., rA0+ = 0.535 fik < r~g2+ = 0 . 7 2 0
( S h a n n o n , 1976). T h e e x t e n t o f f o r m a t i o n o f t h e H T
in t h e M g 2 + - A P + - C O f - s y s t e m h a s b e e n b e l i e v e d to
r a n g e f r o m R = 0.20 to 0.33 (Brindley a n d K i k k a w a ,
1979; M i y a t a , 1980). T h e R v a l u e o f l i T is k n o w n to
b e l i m i t e d to 0.26 to 0.32 in t h e M g 2 + - A P + - F e ( C N h 4s y s t e m ( K i k k a w a a n d K o i z u m i , 1982). T a b l e 1 repr e s e n t s t h e lattice p a r a m e t e r ao a n d t h e layer thickness,
doo3, o f t h e Fe(CN)64- f o r m as well as t h e m o l e r a t i o
o f A P + / ( M g 2+ +A13+ ) i n t h e s t a r t i n g s o l u t i o n for p r e p a r a t i o n , r, a n d t h a t o f t h e c o n d i t i o n e d p r o d u c t , R. T h e
R v a l u e s are basically t h e s a m e as t h e r values. T h e
w h o l e m a s s o f p r e c i p i t a t e was crystallized to f o r m a
single H T p h a s e i n t h e h e x a g o n a l s y s t e m f r o m R =
0.171 to 0.481 (Figure 3). A p r o d u c t w i t h a s m a l l R
v a l u e o f 0.05 s h o w e d two H T p h a s e s a n d Mg(OH)2,
w h i l e a p r o d u c t w i t h a large R v a l u e o f 0.50 w a s c o m p o s e d o f t w o H T p h a s e s a n d MgAIa(OH)8. T h e A P +s u b s t i t u t i o n is e v i d e n c e d b y t h e l i n e a r d e c r e a s e in ao
w i t h a n i n c r e a s e in t h e R v a l u e (Figure 4). T h e s m a l l e s t
ao v a l u e o f 3.016 A was o b s e r v e d o n t h e s a m p l e O
w i t h R = 0.481. T h e s e results c o r r o b o r a t e t h e form a t i o n o f t h e H T in t h e wide r a n g e o f A13+-substitution
734
Mao, Tsuji, and Tamaura
310
]2
3.08
li
Clays and Clay Minerals
9
0
o<
%Oo
o
9
9
,o 3.06
o<
3.04
7:
< O9 O0 9
O
9
o
9
3.02
1
0. I
3 O0
0
1
I
I
I
0.2
0.3
0.4
0.5
7
0.6
0
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
R
Figure 4.
tion (R).
06
R
Plot of the lattice constant (ao) vs. AP +-substitu-
Figure 5. Plot of the doo3 spacing of hydrotalcites in the
Fe(CN)64- form vs. AP +-substitution (R).
(0.24-0.48 as t h e R value) in the Mg2+-AP+-Fe(CN)64
system.
Figure 5 s h o w s the c h a n g e in t h e doo 3 spacing w i t h
t h e R value. T h e s a m p l e s w i t h R > 0.24 e x h i b i t e d a n
e x p a n d e d layer t h i c k n e s s o f a b o u t 1 0 . 6 - 1 0 . 9 A. It is
e q u a l to t h e s u m o f t h e t h i c k n e s s o f t h e b r u c i t e layer
(4.77 ~ ) a n d t h e d i m e n s i o n o f Fe(CN)64- (5.94 A) (Wilson, 1951). H T s with R < 0.17 s h o w e d a s m a l l e r layer
t h i c k n e s s o f a b o u t 8.1 A. I n a n o t h e r words, H T s w i t h
h i g h c h a r g e d e n s i t y c a n easily i n c o r p o r a t e Fe(CN)64w i t h i n t h e interlayer, b u t t h e c o m p o u n d s w i t h l o w e r
R v a l u e s c a n n o t a c c o m m o d a t e t h e large t e t r a v a l e n t i o n
F e ( C N ) 6 4 a s a n i n t e r l a y e r a n i o n . A sufficient charge
i n t h e b r u c i t e layer is n e c e s s a r y to take u p the large
a n d highly c h a r g e d F e ( C N ) 6 4 i o n w i t h i n t h e interlayer.
W h e n t h e R v a l u e is lower, t h e l o c a t i o n s o f charge in
t h e b r u c i t e layer are d i s t a n t f r o m e a c h o t h e r a n d n o t
close e n o u g h to f o r m t h e ionic b o n d w i t h t h e F e ( C N ) 6 4 - .
I n such case, t h e m a i n layer o f H T s will b e able to
f o r m t h e i o n i c b o n d w i t h s m a l l a n i o n s s u c h as CO~ 2o r N O 3 - . H T b e t w e e n R = 0.17 a n d R = 0.24 c o u l d
n o t b e o b t a i n e d , t h o u g h v a r i o u s r v a l u e s were used for
H T p r e p a r a t i o n . T h e r v a l u e s o f 0.15 a n d 0.18 g a v e
t h e R v a l u e s o f 0.171 a n d 0.241 i n t h e p r o d u c t , respectively. T h e r e a s o n is n o t clear. T h u s , t h e e x t e n t o f
A13+-substitution in t h e b r u c i t e layer h a s b e e n d e m o n s t r a t e d to r a n g e f r o m R = 0.24 to 0.48 in t h e M g 2+A P + - F e ( C N ) 6 4 - system. It is m u c h w i d e r t h a n t h a t
reported previously.
T h e c h e m i c a l c o m p o s i t i o n s o f c o n d i t i o n e d H T s are
s h o w n in T a b l e 2. T h e c o n t e n t s o f CO32- were calcul a t e d b y b a l a n c i n g t h e charges b e t w e e n t h e b r u c i t e layer
Table 2. Chemical composition of synthesized hydrotalcites in the Fe(CN) 4 form.
Sample
C
D
E
F
G
H
I
J
K
L
M
N
O
Chemical composition
[Mgo.829A1..... (OH)2I [(Fe(CN)~ - )..... o(COJ- )...... 91.1 5H20]
[Mgo.759Alo.z4, (O H)2][ (Fe(CN)4- )o.0590(CO2- )o.o025 . 0.9 20 H201
[Mgo.749A1..... (OH)2] [(Fe(CN) 4- )0.o600(CO2- )...... 90.8 3 5 H20]
[Mgo.730Alo.27o(OH)2][(Fe(CN)4 - )0.05~(CO] -) ...... 91.40H201
[Mgo.727Alo.273(OH)j[(Fe(CN)~-)o.065dCO]-)o.0063- 1.26HzO]
[Mgo 724A1..... (OH)2][(Fe(CN)64-)o.06,9(CO32-)o..... .0.977H20]
[Mgo.7o~Alo.299(OH)j[(Fe(CN)64-)o.o67s(CO32-)o.o,4591.1 9HzO]
[Mgo.6~4A1..... (OH)2][(Fe(CN) 4- ) . . . . . . ( C O 2 - ) . . . . . . -0.803H20]
[Mgo.634Alo.366(OH)2][(Fe(CN)~-)o.o7s6(CO]-)..... 8" 1.11H20]
[Mgo.67~AI..... (OH)j[(Fe(CN)~-) ..... ,(fOE-) ...... -1.01H20]
[Mgo.666Alo.334(OH)2][(Fe(CN)~ - )...... (COt-) ...... 91.46H20]
[Mgo.569AI..... (OH)j[{Fe(CN)64 - )o.~9,7(CO]-) ...... - 1.15H20]
[Mgo.stgAl0.4s~(OH)2][(Fe(CN) 4-)0 o80dCO]-)o 08ot'0.893H20]
Vol. 41, No. 6, 1993
CO2 adsorption of hydrotalcite-like compounds
40
735
%
o...,
j J
s
6'
30
4O
j J+
f
so
(5
o 20
a
r...p
/
9
g~
9
0
<
<
c
~ lo
20
9
l0
9
9
<
@
+
~
5
~
I
10
+
I
15
+
I
20
+
I
25
<
r
30
Pco ~ ( cmHg )
@
@2
./
O----~_
o--q p
t
0.4
03
O _
05
R
Figure 6. Effect of the ionic form on isotherms for CO2
adsorption. Sample K (R = 0.366): a) unconditioned (do03 =
10.67 A), b) Fe(CN)64 -conditioned (d0o3 = 10.89 ~,), and c)
NO3 -conditioned (d0o3 = 8.01 ~).
Figure 7. Variation of amounts of adsorbed CO2 on HTs in
the Fe(CN)6 4- form with different A13+-substitutiOn. Adsorption temperature = 25~
and the interlayer based on the pure phase formula.
T he calculated e q u i v a l e n t fraction o f F e ( C N ) 6 4 - /
(CO3
2 - +Fe(CN)64-) was 0.8-0.9. These samples will
be referred to as the "Fe(CN)64 form". Incompleteness
of conditioning with K4Fe(CN)6 solution will be ascribed to the very high ion-exchange selectivity for
CO32 . The H 2 0 content was determined from T G
weight loss subtracted by the contribution of C N and
CO2. The chemical composition shows that approximately one mole o f water is contained per chemical
formula of hydrotalcite in the Fe(CN)64 form. The
amount o f H 2 0 is approximately two times of that of
the CO32 form reported (Tsuji et al., 1993a). The
expanded interlayer can accommodate more water than
the CO32 form with the close packing.
the amount o f C O 2 adsorption increased by 65%. It is
clear that the NO3 ions in the interlayer clogged the
adsorption spaces for COz in the interlayer o f hydrotalcite in the mixed ionic form. The expanded space
with the large anion Fe(CN)64 can accommodate more
CO2 gas. The interlayer space may be designed for the
adsorption field of COL and other gas molecules if it
could be tuned by suitable guest anions. Thus, the ionic
form o f H T is an important factor for the CO2 adsorption characteristics as well as the interlayer spacing. In the present research, in situ infrared adsorption
spectra of COL-adsorbed material were not recorded.
The technique is critical for probing the state o f CO2
adsorption on these materials, but the equipment was
not available.
CO2 adsorption isotherms were determined on other
samples with R = 0.24-0.48 and the amount of CO:
adsorption at 25 c m H g was plotted as a function of
the R value (Figure 7). The sample with R = 0.37
exhibited the m a x i m u m amount o f COL adsorption
and others showed a smaller amount of adsorption.
They possess nearly the s a m e doo 3 (10.6 A-10.9 ,~). If
the amount of CO2 adsorption was only determined
by the interlayer thickness, almost the same amount
o f CO2 adsorption should be observed for these sampies. Therefore, besides doo3, the effect o f the second
factor, the layer charge, will have to be taken into account for interpretation o f the m a x i m u m adsorption.
It also affects the amount o f CO2 adsorption because
the void space in the interlayer is determined by the
density and sizes of the incorporated ions. In this study,
the largest amount o f adsorbed CO2 was observed in
the sample with R = 0.366. Thus, the o p t i m u m space
C02 adsorption behavior
The COL adsorption isotherms were determined on
the mixed ionic form, Fe(CN)64 - and NO3 -conditioned product o f sample K to study the effect of the
ionic form on the adsorption behavior (Figure 6). The
smallest amount of CO2 adsorption (about 10 cm3/g
at 20 cmHg) was observed on the NO3--conditioned
form with a layer spacing ofdoo3 = 8.01 A. The amount
o f adsorption is given on a STP basis or at 298 K and
0.1 MPa. The air-dried product or mixed ionic form
with doo3 = 10.67 ,g, exhibited the saturated amount of
adsorbed CO2 o f about 21 cm3/g at 20 cmHg. The
F e ( C N ) 6 4- form with doo3 = 10.89 A showed a larger
adsorption for CO2, about 33 cm3/g at 20 cmHg. The
adsorption did not reach saturation in the present experiments. In comparison with the mixed ionic form,
the "doo3" o f the Fe(CN)64- is larger by only 2%, but
736
Mao, Tsuji, and Tamaura
and the charge density in the interlayer as a CO2 adsorption field could be found on the hydrotalcite with
the AP+-substitution of 0.37.
The isosteric heat o f C O 2 adsorption, -2xHaa, o f sample K was determined by the following equation (Ruthyen, 1984) from the adsorption isotherms at 0, 25 and
40~
P2
log PI
--
(•
l)
2.303RkT2 - ~
where ZXH,a is the enthalpy change on CO2 adsorption,
R is the gas constant, and P~ and P2 are the equilibrium
pressures at the corresponding temperatures (T~ and
T2) for the same amount o f adsorption. The isosteric
heat o f CO2 adsorption was determined to be 43.3 kJ
m o l - 1at the adsorption amount of 20--40 cm3/g, which
indicates the level o f chemical adsorption. This is almost the same as the isosteric heat of CO2 adsorption
(40 kJ tool l) at an infinitesimal coverage on oxygendeficient magnetite at 150-300~ (Nishizawa et aL,
1992). Therefore, CO 2 adsorption on H T is based on
the same chemical process as in the latter material.
Moreover, H T does not show any adsorption for N2
gas and is comparable to an activated carbon in the
separation performance o f CO2 and N2 (Ito, 1993).
CONCLUSION
The extent o f formation ofhydrotalcite in the Mg 2+A13+-Fe(CN)64- system has been determined to range
from 0.24 to 0.48. It was evidenced by a linear decrease
in the lattice constant ao with an increase in R value.
A lower temperature (room temperature) has been
demonstrated to be effective for the formation o f HTs
with high A13+-substitution. Slow polymerization at
lower temperature allows an amorphous hydrolysis
product to grow in crystals with the high degree of Al 3+substitution. Several anions coming from the starting
materials or atmosphere usually coexist in the interlayer. When the R value is lower than 0.17, the HTs
containing highly charged Fe(CN)64- ions were not
formed because the charge density o f the brucite layer
was not high enough to accommodate the large ion.
Anions in the interlayer could be mostly replaced by
other anions by ion-exchange treatment for obtaining
a required anion form. CO2 can be accommodated more
easily in an expanded interlayer space. The amount o f
CO2 adsorption is affected not only by the interlayer
spacing (doo3) but also by the layer charge and the ionic
form. The o p t i m u m space and the charge density for
CO2 adsorption by hydrotalcite in the MgZ+-A13+Fe(CN)6 a system were found at the Al3+-substitution
o f 0.37. The isosteric heat of CO2 adsorption on H T s
in the Mg2+-A13+-Fe(CN)64 system indicated a level
of chemical adsorption. The above H T compound with
the expanded interlayer will be taken advantage o f for
obtaining, for instance, the C104- form, which cannot
Clays and Clay Minerals
be synthesized directly from aqueous solutions of metal
salts.
ACKNOWLEDGMENT
The present work was partially supported by a grant
from the Salt Science Research Foundation.
REFERENCES
Allmann, R. (1968) The crystal structure ofpyroaurite: Acta
CpTstallogr. B24, 972-977.
Bish, L. and Brindley, G. W. (1977) A reinvestigation of
takovite, a nickel aluminum hydroxy-carbonate of the pyroaurite group: Amer. Mineral 62, 458-464.
Brindley, G. W. and Kikkawa, S. (1979) A crystal-chemical
study of Mg, A1, and Ni, AI hydroxy-perchlorates and hydroxy-carbonates: Amer. Mineral. 64, 836-843.
Clearfield, A. (1988) Role of ion exchange in solid-state
chemistry: Chem. Rev. 88, 125-148.
Evans, J. V. and Whateley, T. L. (1967) Infrared study of
adsorption of carbon dioxide and water on magnesium oxide: Trans. Faraday Soc. 63, 2769-2777.
Gastuche, M. C., Brown, G., and Mortand, M. M. (1967)
Mixed magnesium-aluminum hydroxides: Clay Miner. 7,
177-192.
Idemura, S., Suzuki, E., and Ono, Y. (1989) Electronic state
of iron complexes in the interlayer of hydrotalcite-like materials: Clays & Clay Minerals, 37, 553-557.
Ito, W. (1993) Recovery of carbon dioxide by adsorbents:
Handbook ofAdsorption Technology, H. Shimizhu, ed., NTS
Ltd., Tokyo, 761-765 (in Japanese).
Kikkawa, S. and Koizumi, M. (1982) Ferrocyanide anion
bearing Mg, AI hydroxide: Mater. Res. Bull. 17, 191-198.
Komarneni, S. and Tsuji, M. (1989) Selective cation exchange in substituted tobermorites: J. Am. Ceram. Soc. 72,
1668-1674.
Miyata, S. (1975) The synthesis of hydrotalcite-like compounds and their structure and physico-chemical properties-I: The systems Mg2+-AP+-NO3 , Mg2+-Ap+-C1 , Mg2+AI~+-CIO4-, Ni2+-A13+-C1 and Zn2+-AI3+-CI : Clays &
Clay Minerals 23, 369-375.
Miyata, S. (1980) Physico-chemical properties of synthesis
of new hydrotalcites in relation to composition: Clays &
Clay Minerals 28, 50-56.
Miyata, S. (1983) Anion-exchange properties of hydrotalcite-like compounds: Clays & Clay Minerals 31, 305-311.
Miyata, S. and Hirose, T. (1978) Adsorption of N2, O~, CO2
and Hz on hydrotalcite-like system: Mg2+-AI3+-Fe(CN)64 :
Clays & Clay Minerals 26, 441-447.
Nakamoto, K. (1986) Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., John Wiley
& Sons, New York, 106-124.
Nishizawa, K., Kodama, T., Tabata, M., Yoshida, T., Tsuji,
M., and Tamaura, Y. (1992) Adsorption of CO2 on oxygen-deficient magnetite: Adsorption enthalpy and adsorption isotherm: J. Chem. Soc. Faraday Trans. 88, 27712773.
Pausch, I., Lohse, H. H., Schurmann, K., and Allmann, R.
(1986) Synthesis of disordered and Al-rich hydrotalcitelike compounds: Clays & Clay Minerals 34, 507-510.
Ruthven, D.M. (1984) Thermodynamics of adsorption: in
Principles of Adsorption & Adsorption Process, John Wiley
& Sons, New York, 62-64.
Sazarashi, M., Takeshita, K., Kumagai, M., Tamura, T., and
Takashima, Y. (1992) Recovery of nitrogen oxides as nitric acid by mineral zeolite: J. At. EnergySoc. Jpn. 34, 529534 (in Japanese).
Shannon, R. D. (1976) Revised effective ionic radii and
Vol. 41, No. 6, 1993
CO2 adsorption of hydrotalcite-like compounds
systematic studies of inter-atomic distances in halides and
chalcogenides: Acta Crystallogr. A32, 751-946.
Sissoko, I., Iyagba, E. T., Sahai, R., and Biloen, P. (1985)
Anion intercalation and exchange in Al(OH)a-derived compounds: J. Solid State Chem. 60, 283-288.
Taylor, H. F. W. (1973) Crystal structure of some double
hydroxide minerals: Mineral. Mag. 39, 377-389.
Thevenot, F., Szymanski, R., and Chaumette, P. (1989)
Preparation and characterization of Al-rich Zn-A1 hydrotalciteqike compounds: Clays & Clay Minerals 37, 396402.
Tsuji, M. and Komarneni, S. 0989) Alkali metal ion exchange selectivity orAl-substituted tobermorite: J. Mater.
Res. 4, 698-703.
Tsuji, M., Komarneni, S., and Malla, P. (1991) Substituted
tobermorites: 27A1 and 29Si MASNMR, cation exchange,
and water sorption studies. J. Am. Ceram. Soc. 74, 274279.
Tsuji, M., Mao, G., and Tamaura, Y. (1992) On the ther-
737
modynamic treatment for anion exchange in hydrotalcitelike compounds: Clays & Clay Minerals 40, 742-743.
Tsuji, M., Mao, G., and Tamaura, Y. (1993a) Hydrotalcites
with an extended AP+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors: J. Mater. Res. 8, 1137-1142.
Tsuji, M., Tabata, M., and Tamaura, Y. (1993b) CO2 decomposition by oxide ceramics intercalated in layer compound: J. Am. Ceram. Soc. (in press).
Wilson, A. J. C. (1951) Structure Reports 11, p. 42.
Wykoff, R. W. G. (1963) Crystal Structure, Vot. 1, John
Wiley and Sons, New York, p. 268.
Yamaoka, T., Abe, M., and Tsuji, M. (1989) Synthesis of
Cu-AI hydrotalcite-like compound and its ion exchange
property: Mater. Res. Bull. 24, 1183-1199.
(Received 8 February 1993," accepted 20 August 1993; Ms.
2322)