Clay Minerals (198I) 16, 173-179
A C T I V A T I O N OF A S E P I O L I T E W I T H D I L U T E
S O L U T I O N S OF H N O 3 A N D S U B S E Q U E N T H E A T
T R E A T M E N T S : II. D E T E R M I N A T I O N OF S U R F A C E
ACID CENTRES
J. L. B O N I L L A , J. DE D. L O P E Z - G O N Z , ~ L E Z , A. RAMIREZ-SA.ENZ,
F. R O D R I G U E Z - R E I N O S O AND C. V A L E N Z U E L A - C A L A H O R R O *
Department of lnorganic Chemistry, Faculty of Sciences, University of Granada, Spain
(Received9 September 1980)
ABSTRACT: A sepiolite from Toledo, Spain, was activated with dilute solutions of HNO3
(0.25, 0.50 and 0.75 M) followed by heat treatment at 110, 200, 300, 400 and 500~ for 10 h. The
retention of n-butylamine in cyclohexane solution was used to follow the variation of surface acid
centres of the sepiolite as a function of the acid and heat treatments, these reaching a maximum
for the product treated with 0.75 r~ HNO3 and heated at 300~ Acid treatments within the range
0-50-0-75 M and subsequent heat treatment at 200-300~ for 10 h yielded products with the
highest surface activity.
The catalytic activity of a solid is related to the total number of surface acid centres
present and their relative strengths. The number of such surface acid centres may be
modified by acid and heat treatment (Kayser & Bloch, 1952). This paper describes the
variation in surface acidity of a sepiolite from Toledo, Spain, following acid (0.25, 0-50
and 0.75 M HNO3) and subsequent heat (10 h at temperatures between 110 and 500~
treatments. The increase in surface area of this sepiolite following these treatments has
been described in a previous paper (L6pez-Gonzalez et al., 1981).
EXPERIMENTAL
Material used for this and the previous (L6pez-Gonzalez et al., 1981) series of experiments
was a natural sepiolite from Toledo, Spain. Its exchange capacity was 8 mEq/100 g and its
structural formula
-4+
3+
3+
2+
3+
[Sis.s0 A10.20] [Alo.05 Mg3.83 Fe0.03] O16 ~
This formula was derived from the chemical analysis of a carbonate-free sample which
had been ignited at 1000~
The natural sepiolite, crushed to a particle size between 2.38 and 4.76 mm, was boiled
with HNO3 (0.25, 0-50 and 0.75 M) for 1 h. Air-dried portions of the resulting products
were treated at different temperatures (room temperature, 110, 200, 300, 400 and 500~
for 10 h. The products are designated by an S followed by two subscripts; the first refers to
the concentration of the acid used and the second to the temperature at which the product
was subsequently heated.
* Present address: Faculty of Sciences, University of Extremadura, Badajoz, Spain.
0009-8558/81/0600-0173 $02.00
9 1981 The Mineralogical Society
174
J. Bonilla et al.
Determination of the total number o f surface acid centres was carried out following the
method described by L6pez-Gonz~tlez et al. (1978), which is based on analysis of the
retention isotherms at 20~ of n-butylamine in cyclohexane. The retention of n-butylamines by activated sepiolite is a first-order reversible process (L6pez-Gonz~lez et al.,
1977) and consequently the experimental data should fit the Langmuir equation
(Brunauer, 1945):
C
1
C
X - bXm + Xrn
where C = equilibrium concentration (mol/1),
X = moles of n-butylamine retained per g of sample,
b = constant related to the retention energy.
In fact, the plot of C / X versus C is a straight line with an intercept equal to 1/bXm and
slope equal to 1/Xm, leading immediately to the values of X m and b. The number (a) of
acid centres per g of sample is calculated from Xm, since the n-butylamine molecules
(Lewis bases) are retained on the surface acid centres (both Lewis and Br6nsted acids),
thereby neutralizing them.
Experimental data were obtained as follows. 150-500 mg of sample were immersed in
cyclohexane for seven days up to saturation. The samples were then transferred to a
stoppered conical flask containing 50 ml of a 9 x l0 3 ~ solution of n-butylamine in
cyclohexane and the flask kept in a thermostatted shaker bath at 20~ for seven days
(L6pez-Gonzfilez et al., 1977). Spectrophotometric analysis of n-butylamine remaining in
solution was carried out at 225 nm, the wavelength of maximum absorbance.
RESULTS AND DISCUSSION
The retention ofn-butylamine (in cyclohexane solution) at 20~ was measured for all the
activated products. Analysis of the experimental data, as noted above, yielded the
concentration (C) of the n-butylamine solution in equilibrium with the products as well as
the number of moles of n-butylamine (X) retained per g of sample.
Since the retention isotherms of n-butylamine on activated sepiolite fitted the
Langmuir equation, the C / X values corresponding to each C value could be calculated.
These C / X values were plotted as C / X = f ( C ) for each product (there were 18 such plots,
one of which is shown in Fig. 1). In each case the plot was a straight line from which the
slope and the intercept could be calculated. These data were then used to determine the
retention capacity (Xm) and the constant (b) of the Langmuir equation (Table 1). The
number of surface acid centres per g of activated sepiolite (a) calculated from the X m
values is also included in this table. This number of acid centres accessible to the
n-butylamine molecules is of the order of 102~ the maximum value corresponding to the
product S0.75-300.
In order to show the effect of the acid treatment on the number of surface acid centres of
the sepiolite, the values o f a (Table 1) have been plotted in Fig. 2 as a function o f the
concentration of HNO3 solution for each temperature used during the heat treatments.
These plots show that, in general, the number of acid centres increases in relation to the
concentration of acid used for activation. In all cases the increase in a is governed by the
temperature of the heat treatment used following the acid treatment.
Surface acid centres in activated sepiolite
175
6-
4.
L
2o
,
{
,
,
,
C,103(mot. L-~)
FIG. 1. Langrnuir plot for the retention of n-butylamine on sample 50-75 300.
TABLE 1. Some parameters of the activated sepiolite samples
Heat treatment (~
Room
temperature
Sample
110
200
300
400
500
So-25
Xm x 104
b x 10 -4
K
a x 10 -2~
S
8.44
0.44
3.7
5.1
127
9.55
6-1
58-5
5.8
144
9.94
3.1
31.3
6.0
150
10.67
1.6
16.9
6.4
161
10.97
0-16
1-8
6.6
165
7.12
0.19
1.3
4-3
107
S0~5o
X m x 104
b x 10 -4
K
a x 10 -20
S
9.68
0.20
2.0
5.8
146
11.81
0.43
5.1
7.1
178
14.33
0.29
4.2
8.6
216
12.66
0.57
7.2
7.6
191
13.25
0.28
3.7
8.0
200
9.75
0.25
2.4
5-9
147
S0.75
Xm • 104
b x 10 -4
K
a • 10 -20
S
10.32
0.31
3.2
6.2
155
14.03
0.50
7-0
8.5
211
13.90
0-45
6-2
8.4
209
15.14
0-78
11.9
9.1
228
14.05
0-43
6.1
8.5
211
10.84
0-27
2.9
6.5
163
Xm = retention capacity of the activated samples (mol/g).
b = Langmuir equation constant (l/tool).
K = apparent equilibrium constant (l/g).
a = number of acid centres per g.
S = surface area from n-butylamine retention (m2/g).
176
J. Bonilla et al.
Fig. 3 shows the data of Table 1 in the form a = f i T ) for each of the acid solutions used.
It can be seen that a increases with temperature, reaching m a x i m u m values in the
200-400~ range and decreasing thereafter. More intensive acid treatments (1-5 N HNO3)
followed by the same heat treatments used here (Jim6nez-L6pez et al., 1977) yielded, in
general, lower numbers of surface acid centres than those corresponding to the present
samples. On the other hand, Fig. 3 shows that, regardless of the concentration of the acid
solution used, there is a considerable decrease in a when the temperature of the heat
treatment increased from 400 to 500~
Values of the b constants in the Langmuir equation are related to the retention energy.
Table 1 shows that the variation in b does not follow the same trend as that of the number
of acid centres; this is not surprising since the adsorbent-adsorbate interactions are more a
function of the relative strength of the acid centres than of their number.
Table 1 shows that b varies from 0.16 x 104 to 6.1 x 104___2~o 1/mol, depending on the
temperature of the heat treatment and concentration of acid used. The maximum b value
(6.1 x 1041/mol) was found in product S0.25 ~10.As this was not the product with the largest
number of acid centres it is possible that the retention mechanism of n-butylamine varies
from product to product. Table 1 also shows that in the S0.2s series of products the
Room temperoture
9-
II0 * C
(D
200* C
|
300* C
aO0* C
0
500 * C
8-
7-
A
I
o
6
5-
4-
I
0.25 M
!
0.50 M
I
0.75 M
FIG. 2. T h e effect o f a c i d t r e a t m e n t s o n the n u m b e r o f a c i d centres.
Surface acid centres in activated sepiolite
177
maximum b value corresponds to l l0~ decreasing thereafter to much lower values
( ~ one-tenth) at 400 and 500~ In the other two series (S0.50 and S0.75) the maximum b
values are found at 300~ The product S0.75-300had the largest number of acid centres of
all samples studied.
Further information on the magnitude of the retention energy of the n-butylamine on
activated sepiolite and, consequently, on the strength of the acid centres, could be
obtained from the equilibrium constants of the retention process of the molecules at
different temperatures; this is so since this retention is a reversible process (L6pez-Gonz~ilez et al., 1977). However, it was not possible to calculate the thermodynamic equilibrium
constants since the activity coefficients of n-butylamine in solution and in the retained
phases were not known; consequently, only apparent equilibrium constants could be
calculated.
However, qualitative information about the free energy changes of the n-butylamine
retention on sepiolite was obtained by comparing the apparent equilibrium constants (K)
determined at the same temperature (20~ In adsorption process from solution in which
0
0.25 M
9
0.50 M
0.75 M
9-
B-
I
{m
7tn
0
ww
6-
'o
5-
4-
|
I00
I
200
I
300
I
400
5
T{*C)
FIG. 3. The effectof temperature on the number of acid centres.
178
J. Bonilla et al.
the isotherms (at least for low C values) follow the L a n g m u i r equation, the apparent
equilibrium constant is given (L6pez-Gonzb.lez et al., 1978) by
K=
Xm•
The K values were calculated from the X m and b values given in Table 1 and have been
included in this table; they range from 58.5 1/g to 1.3 l/g, depending on the acid and heat
treatments used. In the S0.50 and S0.75 series the m a x i m u m values o f K correspond to the
products treated at 300~ whereas in the S0.25 series the m a x i m u m values correspond to
the p r o d u c t S0.2s-j 10; the K value f o u n d for this sample ( K - 58.5 l/g) is the largest o f all the
values obtained and this suggests, again, that this p r o d u c t exhibits the strongest affinity
for the n-butylamine molecules. The fact that this p r o d u c t has a relatively low n u m b e r o f
acid centres (a = 5.8 x 1020 acid centres per g) indicates that the strength o f such centres is
not directly related to their abundance, as reported elsewhere (L6pez-Gonzhlez et al.,
1978).
Values found for the retention capacity X m (from which the n u m b e r o f acid centres was
calculated) can be used to determine the p r o p o r t i o n o f the surface area o f the sepiolite
which is accessible to the n-butylamine molecules, provided that the cross-sectional area
o f this molecule is k n o w n - - i n this case, the cross-sectional area was taken as 25 A 2
(L6pez-Gonzfilez et al., 1978). The resulting surface areas o f the activated samples o f
sepiolite are given in Table 1.
L6pez-Gonzfilez et al. (1981) f o u n d that the sepiolite products 50.50-200, 50.75-200 and
S0.75 300 gave the highest BET(N2) surface areas (449, 438 and 434 m2/g, respectively).
These results, taken in conjunction with those o f the present investigation, indicate that
treatment o f sepiolite with 0-50~
M HNO3 followed by heating at 200-300~ for 10 h
greatly enhances the surface area and numbers o f surface acid centres and yields material
o f the highest potential surface activity yet described (cf. Jimen6z-L6pez et al., 1978;
L6pez-Gonz~ilez et al., 1978).
REFERENCES
BRUNAUERS. (1945) The Adsorption of Gases and Vapors, pp. 60-82. Princeton University Press.
JIMENEZ-LOPEZ A., LOPEZ-GONZALEZ J. DE D., RAMIREZ-SAENZ A., RODRIGUEZ-RE1NOSO F., VALENZUELACALAHORROC. & ZURITA-HERRERAL. (1978) Evolution of surface area in a sepiolite as a function of acid
and heat treatments. Clay Miner. 13, 375-385.
KAYSERF. & BLOCHI.M. (1952) Some catalytic properties of montmorillonite. Comp. Rend. 234, 1885-1887.
LOPEZ-GONZALEZ J. DE O., RAMIREZ-SAENZ A., RODRIGUEZ-REINOSO F., VALENZUELA-CALAHORROC. &
ZURITA-HERRERAL. (1981) Activaci6n de una sepiolita con disoluciones diluidas de NO3H y posteriores
tratamientos t6rmicos: I. Estudio de la superficie especifica. Clay Miner. 16, 103-113.
LOPEZ-GONZALEZJ. DE D., VALENZUELA-CALAHORROC., JIMENEZ-LOPEZ A. ~r RAMIREZ-SAENZ A. (1977)
Retenci6n de n-butilamine sobre una sepiolita activada. I. cin&ica del proceso. An. Quire. 73, 1266-1270.
LOPEZ-GONZALEZJ. DE D., VALENZUELA-CALAHORROC., JIMENEZ-LOPEZA., RAMIREZ-SAENZA. t~r RODRIGUEZ-
REINOSOF. (1978) Retenci6n de n-butilamina sobre una sepiolita activada. II. Isotermas de retenci6n. An
Quim. 74, 220-224.
LOPEZ-GONZALEZ J. DE D., VALENZUELA-CALAHORROC., JIMENEZ-LoPEZ A. • RAMIREZ-SAENZ A. (1978)
Retenci6n de n-butilamina sobre una sepiolita activada. III. Aspectos termodinhmicos. An. Quim. 74,
225-228.
RI~SUMI~: Une s6piolite de Tol6de (Espagne) a ~t6 activ6e par des solutions dilutes de HNO3
(0.25 M, 0"50 Met 0.75 M) et un traitement thermique fi 1l0 ~ 200 ~ 300 ~ 400 ~ et 500 ~
pendant l0 heures. La r&ention de n-butylamine en solution cyclohexanique, a 6t6 utilis6e pour
Surface acid centres in activated sepiolite
suivre la variation du nombre de centres acides de surface de la s~piolite en fonction des
traitements acides et thermiques, ce nombre &ant maximal pour l'6chantillon S0.75 300. Des
traitements acides dans la g a m m e de 0.50 N-0.75 M, suivis de traitement thermique/t 200-300~
pendant 10 hres, conduisent/k la s~piolite d'activit~ de surface la plus forte.
K U R Z R E F E R A T : Ein Sepiolit aus Toledo, Spaien, wurde mit verdfinter HNO3-L6sung (0.25,
0-50 und 0.75 M) aktiviert und einer nachfolgenden 10-stfindigen Hitzebehandlung bei 110, 200,
300, 400 und 500"C unterzogen. Das Aneignungsverm6gen yon n-Butylamin in Cyclohexanl6sung wurde benutzt u m am Sepiolit den Effekt von S~iureund Hitzebehandlungen a u f die
OberflS.chens/iurezentren zu verfolgen, deren Anzahl demnach ein M a x i m u m fiir dasjenige
Produkt annimmt, welches mit 0.75 M HNO3 und 300~ behandelt wurde. S/iurebehandlungen
innerhalb des Bereiches von 0.5-0.75 m und nachfolgende Hitzebehandlung bei 200-300~ ffir 10
Std. ergab Produkte mit der gr6Bten Oberfl/ichenaktivitS.t.
R E S U M E N : U n a sepiolita procedente de Toledo, Espafia, se ha sometido a un proceso de
activaci6n con disoluciones diluidas de NO3H (0.25, 0.50 y 0.75 M) seguido de un tratamiento
t+rmico a 110, 200, 300, 400 y 500~ durante 10 horas. Para la determiniaci6n del nfimero de
centros ~cidos superficiales se ha realizado la retenci6n de n-butilamina en disoluci6n
ciclohex~mica a 20~ De esta forma se ha podido conocer la evoluci6n de la acidez superficial de
la sepiolita en funci6n de los tratamientos de activaci6n. Los tratamientos con disoluciones
0-50-0.75 M de NO3H y posterior calentamiento a 200-300~ conducen a l a s muestras de m a y o r
actividad superficial.
179