Potentiometric Method for Determining the Number and Relative Strength of Acid
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Transcript of Potentiometric Method for Determining the Number and Relative Strength of Acid
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Applied CutnZ~sis,14(1985)15-21 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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POTENTIOMETRIC METHOD FOR DETERMINING THE NUMBER AND RELATIVE STRENGTH OF ACID
SITES IN COLORED CATALYSTS
Ruby CID and Gina PECCHI
Facultad de Ciencias, Universidad de Concepci&, Casilla 3-C, Concepcibn, Chile.
(Received 26 September 1983, accepted '21 September 1984)
ABSTRACT
A potentiometric method was used to determine the number of acid centers and their relative strengths in colored catalysts. The validity of the results was established by using measured white solids by the Benesi method and the proposed method.
INTRODUCTION
The Benesi method is often used to measure the number and strength of acid
sites in a solid [I]. Unfortunately, this method is difficult to apply to coloured
solids, especially if they are weak acids. For these solids, which include simple
and mixed oxides with catalytic properties [2,3], a potentiometric method was
developed which has been briefly reported by Goldstein [4]. A new criterion to
interpret the data obtained is also suggested.
The physical system consists of a solid dispersed in a nonaqueous solvent,
which is titrated with a solution of an amine in the same nonaqueous solvent.
The electrode potential variation is a function of the environment in which the
electrode is immersed.
MATERIALS
The reagents used were pure n-butylamine and pure acetonitrile. The surface
acidity was determined for the following solids: AlE03, Si02-A1203, MoO3, FeE03,
Te02, Fe2(Mo04)3, Fe2(Mo04)3 + 5% Te02, Te2M007, Te2Mo07 + 5% Fe203, Fe2(Te0313,
and Fe2(Te03)3 + 5% MoO3, prepared according to a technique already described C51.
A type-Y zeolite has also been used, exchanged with 71% of NH; [6]. The solids
used had a grain size between 106 and 150 urn, calcined at 500C for 4 h. A Beckman
digital pH meter with a combination glass and Ag/AgCl electrode was used.
METHOD
A small quantity of 0.1 N n-butylamine in acetonitrile was added to a known
mass of solid, and agitated for 3 h. Later, the suspension was titrated with the
same base at 0.05 ml min -1 . The electrode potential variation was registered on
0166-9834/85/$03.30 0 1985 Elsevier Science Publishers B.V.
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E(mv)
0
320
240
160
0.
-80.
-160 _
meq. g.cat.
gx 103
I-
FIGURE 1 Potentiometric titration, NH4(71)NaY.
H(71)-NaY
H(71)-NaY
I I I I 320 160 0
ElmVl
FIGURE 2 The acid strength distribution, Benesi method and potentiometric method,
for NH4(71)NaY.
a Beckman digital pH meter. The mass of the solid and the quantity of base used
to reach equilibrium depends on the acidity of the solid used. The reproducibility
of the electrode potential curve was 3 mV.
We tested benzene, isooctane and acetonitrile as a solvent. We chose a polar
solvent such as acetonitrile to eliminate the problem of irreversible adsorption
of butylamine from an inert solvent [7].
DISCUSSION AND RESULTS
In this potentiometric method, the measured electrode potential may arise from
several sources. For example, if the electrodes used were of glass and calomel [S],
the measured potential could be due to:
(a) the standard electrode potential of the reference electrode
(b) the liquid junction potential between the aqueous KC1 solution inside the
calomel electrode and the dispersion in acetonitrile outside the electrode
(c) the difference in potential across the glass membrane separating the dispersion
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17
t-4 t SiOZ-A1203
1.6 I
SiOZ -A$03
LO
X
21) u X X J !60 80 0 -80 Gv,
FIGURE 3 The acid strength distribution of Si02-A1203, Benesi method and potentio-
metric method.
in acetonitrile and the aqueous buffer solution inside the electrode, and
(d) the potential of the sensing electrode within the glass electrode, the Ag-AgCl
interface
Since (a) and (d) remain unchanged during an experiment, effect (b), if present,
does not appear to change appreciably during a titration. Therefore, it appears
that the electrode potential is determined primarily by the acid environment
imposed on the outside of the glass electrode membrane by immersion in the acid
particles.
To prove the validity of this potentiometric method, the surface acidity of
three white solids was determined: A1203, Si02-Al203 and- NH4(71)NaY by both
methods, the Benesi method and the potentiometric method in order to see if equal
values of total acidity could be obtained. This NH4(71)NaY which has already been
measured through the Benesi method, gave potentiometrically (Figure 1) the same
value of total acidity, 2.9 meq g -1
cat [9]. In order to obtain an acid strength
distribution as obtained by the Benesi method, the potentiometric method was
compared with Chessick and Zettlemoyer's method [IO]. They noted that the potentio-
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meq/g.caf x 102
Te 02
<
30 MOOg
20
II) (I C 0.0 100 60 20 -20 -60 -100 -110
EImVl
I
L
I
FIGURE 4a) The titration curves for the simple oxides of the Mo-Fe-Te-0 system.
FIGURE 4b) The distribution of acid strength for the simple oxides of the Mo-Fe-
Te-0 system.
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metric curve was similar to the differential heat curve of adsorption of n-
butylamine on attapulgite.
Following a procedure used by Drain and Morrison [Ii] for heat of adsorption
of argon on rutile, Chessick and Zettlemoyer calculated a site energy distribution
function, g(E). The heat of adsorption is calculated from the slope of the adsorbed
n-butylamine vs. heat of adsorption (nHd) curve as:
The potentiometric titration curve can be handled in the same way, although it
does not mean that there is an equivalence of thermodynamic values.
The acid strength distribution was calculated for the zeolite NH4(71)NaV, using
the Benesi method with Hammett indicators. This is shown in Figures 2a and 2b
where potentiometric titration was used. It can be observed that there is a con-
cordance between them and, based on the Hirschler and Schneider acid sites classi-
fication 1121, it can be concluded that there are sites of moderate acidity in the
solid.
The acid distribution of Si02-A1203 by the potentiometric and Benesi methods
is shown in Figure 3. In our opinion the concordance obtained supports the validity
of the potentiometric method used in this investigation, although the strong sites
of the Si02-A1203 sample seem to be undetected clearly by the potentiometric
method. These sites can be detected by the Benesi method, though there is not
concordance, in the bibliography, if the silica-alumina has strong or intermediate
acid strength sites [7].
Then the potentiometric method was applied to the determination of the acidity
of colored solids. The titration curve obtained for the simple oxides of the Mo-Fe-
Te system is shown in Figure 4a. It can be observed that not only the value of
acidity found changes, but also the millivolt range. The latter can be related
to the acid strength of the solid. Thus, the Moo3 has only a few acid sites, but
strong ones. The result obtained using the potentiometric method agrees with
the one obtained by P. Ratnasamy et al. [13]. They determined by titration with
n-butylamine and Hammett indicators that pure MoO3, obtained similarly to the
one used in this work, showed strong acidity. The Te02 is the oxide which shows
the lowest density of acid sites and these are very weak. In the bibliography
available, Te02, as the promoter of mixed oxides, is given the role of destroying
surface acid sites. The distribution of acid strength of each of the oxides is
shown in Figure 4b. In the case of Mo03, the inflection near 20 mV may be an
experimental mistake, although the same type of curve was obtained three times.
In Figure 4 the great difference in acid strength between the three oxides can
be noted from the millivolt range. Te02 shows only one type of acid site and
very weak, but Fe203 shows clearly two types of acid sites, very weak and weak,
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20
-20.
-20
: i 2
w -60
Q t
0
0 -60
OL 08 12 16
mrq/g cat I to2
FIGURE 5 The titration curves for oxides and promoted oxides of the Mo-Fe-Te-0
system.
and Moo3 seems to show only one type of site, not very strong.
As a criterion for interpreting the results obtained, it is suggested that the
initial electrode potential indicates the maximum acid strength of the surface
sites, and the range N(millieq base/g solid) where the plateau is reached indicates
the total number of acid sites. Knowing the specific area of each solid, acid
site density can be calculated.
On the other hand, by this method the acidity of a given catalyst can be
compared with the same catalyst promoted. This is shown in Figure 5, where it is
clearly seen that an addition of Te02 (5%) does not decrease the total number of
acid sites of Fe2(Mo04)3, as suggested in the bibliography [141, but the strength
of thesites varies. In this same figure, acidities of a-Te2M00, and a-Te2Mo07 +
5X FezOX are shown. It can be observed that the addition of Fe203 generated strong
acid sites which are not present either in a-TepMo07 or in Fe203.
In the case of Fe2(Te03)3 and Fe2(Te03)3 t 5% Mo03, it can be observed that the
addition of Moo3 does not create strong acid sites, although the pure Moo3 shows
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then. The data shown in Figure 5 agree with the activity of these promoted mixed
oxides in the total oxidation of propylene [15].
It is apparent that the potentiometric method enables determination of the total
number of acid sites and their distribution, The color of the catalytic solids is
not a limitation.
ACKNOWLEDGEMENTS
This work was supported by Direcci& de Investigaci;n, Universidad de Concepci6n,
Chile,
REFERENCES
1 2 3
4
5
IO 11 12 13
H.A. Benesi, J. Am. Chem. Sot., 78 (1956) 5490. J. Bart, G. Petrini and N. Giordano, Z. Anorg. Allg. Chem., 412 (1975) 258. Y. Arnaud, J. Guidot, J.Y. Robin, M. Romand and Y.E. German, J. Chim. Phys., 73 (1976) 615. M.S. Goldstein, in Exp. Methods in Catalytic Research, R.B. Anderson 1, (1968) 370 Academic Press, 1968. R. Arriagada, J. Godoy, R. Cid and R. Garcia, Proceedings 7 Simposio Ibero- americano de C$t6lisis, La Plata, Argentina, Ed. Grafos 1980, 506. R. Cid. M.E. Konig and R. Arriagada, Appl. Catal., 60 (1979) 417. M. Deeba and W.K. Hall, J. Catal., 60 (1979) 417. R.O. Clark, E.V. Ballow and R.T. Banth, Anal. Chim. Acta., 23 (1960) 189. R. Cid, G. Pecchi and M.E. K&iig, Proceedings XIV Congreso Latinoamericano de Quimica, Costa Rica, 1981, p.180. J.J. Clessick and A.C. Zettlemoyer, J. Phys. Chem., 62 (1958) 1717. L.E. Drain and J.A. Morrison, Trans. Faraday Sot., 48 (1959) 316. A.E. Hirschler and A. Schneider, J. Chem. Eng. Data, 6 (1961) 313. P, Ratnasamy, D.K. Sharma and L.D. Sharma, J. Phys. Chem., 78 No.20 (1974) 2069. P. Forzatti, F. TrifirG and P.L. Villa, J. Catal., 52 (1978) 389. R. Cid, R. Arriagada, G. Pecchi and J. Villasezor, Proceedings 8 Simposio Iberoamericano de Catblisis, Huelva, Espana, Julio 1982, p.439.