Applied CutnZ~sis,14(1985)15-21 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

15

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.

16

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

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-

18

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.

19

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,

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

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.