comparison of the methods for the determination of surface acidity of solid catalysts

DETERMINATION OF THE ACID STRENGTH O F THE CENTERS According to Walling [4], the acid strength of a solid can be de- fined as its ability to convert a neutral base, adsorbed on its s u r

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Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts

Lucio Forni aa

Istituto di Chimica Fisica Universita di Milano,Milano, Italy

Version of record first published: 13 Dec 2006

To cite this article: Lucio Forni (1974): Comparison of the Methods for the

Determination of Surface Acidity of Solid Catalysts, Catalysis Reviews: Scienceand Engineering, 8:1, 65-115

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Comparison of the Methods for the Determination

of Surface Acidity of Solid Catalysts

LUCIO FORNI

Istituto di Chimica Fisica

Universitii d i Milano

Milano Italy

I INTRODUCTION 66

I1 DETERMINATION OF THE ACID STRENGTH OF THE CENTERS 67

A Method of Adsorption of Colored Indicators 67

B Spectrophotometric Method 69

C Adsorption of Gaseous Basic Substances 71

D Calorimetric Methods 73

E OtherMethods 78

I11 DETERMINATION OF THE SURFACE DENSITY OF ACID CENTERS 80

A 80 B Titration after Ionic Exchange 81

C Titration with Bases in Nonaqueous Solvents 82

D Calorimetric Titration 87

E Adsorption and Desorption of Gaseous Bases 88

91 94 96 Direct Titration of Aqueous Suspensions

F Method of the Poisoning of Specific Surface Reactions

G Hydrogen-Deuterium Exchange Reactions

H Indicator Reactions Method

I Spectroscopic Methods 98

J Reaction with Hydrides 100

K OtherMethods 102

65

Copyright 0 1973 by Marcel Dekker Inc All Rights Reserved Neither this work nor any part

may he reproduced or transmitted in any form or by any means electronic or mechanical includ- ing phococopying microfilming and recording or by any information storage and retrieval sys-

IV DETERMINATION OF THE NATURE OF ACID SITES:

BRBNSTED TYPE AND LEWIS TYPE 103

A Determination of Brhsted Sites Alone 1 0 3

B Determination of Lewis Sites Alone 104

V CONCLUSIONS 108 REFERENCES 111

I INTRODUCTION The concept of surface acidity was originally introduced with the aim of justifying the presence of some substances formed in catalytic chemical reactions, not as a consequence of suppositions about the nature of surface-active sites of solid catalysts The formation of such substances in some reactions (e.g., cracking, isomerization,

or polymerization) can be better explained by admitting the forma- tion of reaction intermediates having the structure of a carbonium ion, which can be formed by interaction between the reacting sub-

stance (hydrocarbon) and an acid center A s an example, in the

cracking of alkylaromatics catalyzed by decationated zeolites, the following reaction mechanism is generally accepted:

where the first stage can be interpreted as an electrophilic substi- tution of the proton onto the alkyl group

A complete description of the surface acid properties of a solid must involve the determination of the acid strength of the sites, their density (number of acid centers p e r unit surface a r e a of the solid), and their nature (Bransted o r Lewis type) Such a description is not easy to make, since the strength and the density of the sites a r e generally strictly connected to each other and, besides, the distribu- tion of the acid strength is usually heterogeneous Furthermore, most of experimental methods can distinguish the centers only on the grounds of their strength They cannot distinguish between Bransted and Lewis centers, but simply give a measure of total acidity of both types

lished in the literature [I-31 Nevertheless, none of them gives a

complete list of the available methods The scope of the present Some excellent reviews on the argument have recently been pub-

paper is then a collection and a critical comparison of all the methods actually employed fo r the determination of surface acidity of solids Each of them, in fact, taken by itself, allows some useful informa- tions to be collected, but can give rise to some criticisms The com- bination of the information obtainable f r o m two o r more of them can often be the only way to give a complete picture of the surface acid properties of the solid under examination

11 DETERMINATION OF THE ACID STRENGTH O F THE CENTERS According to Walling [4], the acid strength of a solid can be de- fined as its ability to convert a neutral base, adsorbed on its s u r - face, into the corresponding conjugated acid If the reaction takes place through the transfer of a proton from the solid surface to the adsorbed molecule (Brdnsted acidity) o r of an electron pair from the adsorbed molecule to the solid surface (Lewis acidity), the acid strength can be expressed, respectively, by means of the Hammett function H, in the following way [5-71:

o r

where K, is the equilibrium constant of the dissociation of the acid, and [B], [BH'], and [AB] are the concentrations of neutral base, its conjugated acid, and the addition product formed during the adsorp- tion of the base on the Lewis center, respectively

A Method of Adsorotion of Colored Indicators

An immediate application of Walling's analysis, originally adopted

by Walling, Weil-Malherbe, and Weiss [a, Ikebe et al [9], and many others, is the observation of the color shown by suitable indicators adsorbed on t h e solid surface, If t h e adsorbed indicator assumes the color of its acid form, the value of H, of the surface is lower or equal to the pK, of the indicator The lower the value of H, (and the lower the pKJ, the higher is the acid strength of the solid Benzene, isooctane, decalin, o r cyclohexane may be employed as solvents In Table 1 the most important indicators a r e reported In the last column of the table the wt% of H,SO, in sulfuric acid solution, which has the acid strength corresponding to the respective pK,, is given

for some of the indicators In Table 2 the acid strength, obtained by

such a method by various authors [lo-131, is given

TABLE 1 Basic Indicators Used for the Measurement of Acid

NiSO, xH,O heat-treated (350°C)

NiSO, xH,O heat-treated (460°C)

<-8.2

<-8.2 +1.5 - -3.0

t 1 5 - -3.0 -5.6 - -8.2

<-8.2 +6.8 - -3.0 +6.8 - +1.5 +6.8 - +4.0

+6.8 - +3.3 +6.8 - C3.3

In the case of black o r dark-colored solids, when the observation

of the indicators color is impossible o r very difficult, a small amount

of a white solid of known acidity may be added to the sample and the

acidity value obtained corrected for such an addition

reproducibility to be obtained, although some difficulty may a r i s e either in the determination of the exact end point of titration o r due

to moisture contamination Also, the acidity values obtained a r e not absolute because they a r e not related to energetic factors, but simply

to the pK, values of the indicators employed Other disadvantages of the method are t h e impossibility of making acidity determinations in the real working conditions of the catalyst and sometimes the long period required for the equilibrium between adsorbed and f r e e base

to be reached

The adsorption method is generally quite accurate and allows good

B Spectrophotometric Method Since the visual judgment of t h e color shown by the indicator in

t h e preceding method can be uncertain at times, some absorption spectra of dyeing materials, adsorbed on various solids, have been

determined [13,14] For example, Leftin and Hobson [13] recorded

the absorption spectra of phenylazonaphthylamine (pK, = +4.0) on a

12% alumina silica-alumina catalyst for both the basic and acid form

of the indicator Such spectra were recorded in pure isooctane and

in an ethanolic solution, acidified with HC1, respectively The re- sults are reported in Fig 1 One can observe that the spectrum of the adsorbate reveals that it is adsorbed exclusively in its acid form

In a similar way Dzisko and co-workers [15] determined the s u r -

face acid strength of mixtures of oxides They employed the indica- tors reported in Table 3 and established the following qualitative

scale of acid strength: SiO, A1,0, > ZrO, SiO, - GqO, SiO, >

B e 0 SiO, - MgO SiO, > Y,O, SiO, > L+O, SiO, > SnO SiO, -

PbO SiO,

Finally, Kobayashi [16-191 recorded the absorption spectra of dimethyl yellow, dimethyl red, and bromophenol blue adsorbed over partially n-butylamine covered silica-alumina in a nonpolar solvent

He determined not only t h e acid strength, but also the total number

of acid centers present on the catalyst surface Apart from some discrepancies due to the change in the activity of the adsorbed base when surface coverage became higher and higher, he confirmed that the values of H,, obtained by this method, are independent on the nature of the indicator employed

The spectrophotometric method gives good qualitative informa- tions on the form in which t he dyeing substance is adsorbed onto the solid surface, but does not eliminate the main disadvantages con-

of Acid Strength

Phenylazonaphthylamine pDimethylaminoazobenzene Aminoazobenzene

Benzeneazodiphenylamine pNitroaniline

o-Nitroaniline p-Nitrodiphenylamine

2,4-Dichloro-6-nitroaniline

p-Nitroazobenzene 2,4-Dinitroaniline Benzalacetophenone p-Benzoyldiphenyl Anthraquinone 2,4,6-Trinitroaniline

3-Chloro-2,4,6-trinitroaniline

p-Nitrotoluene Nitrobenzene 2,4-Dinitrotoluene

+4.0 +3.3 t2.8

t 1 5 +1.1

-0.2

-2.4 -3.2 -3.3 -4.4 -5.6

- 6 2 -8.1 -9.3 -9.7 -10.5 -11.4 -12.8

nected with the adsorption method, e.g., nonabsolute acidity value determinations and nonreal working conditions of the catalyst

C Adsorption of Gaseous Basic Substances The strength with which a base adsorbs on the surface acid cen-

t e r s of a solid is directly proportional to the acid strength of the centers If, after the adsorption, the solid is heated at growing tem- peratures and the quantity of desorbed base is recorded, a measure

of the acid strength of the centers can be obtained Before the exper- iment the solid must be pretreated in o r d e r to obtain reproducible results Such a treatment usually consists in the elimination of the volatile impurities by evacuation and/or heating and flushing in an inert gas flow The less volatile impurities can often be eliminated

by converting them in more volatile compounds by reaction with oxygen o r hydrogen The adsorption equilibrium of the base can often be reached at relatively high temperatures and low pressures

A real chemical reaction of the base with the surface may also occur Peri [20] and Wilmot [21], for example, showed that an exchange re- action between ammonia and OH surface groups, with the formation

of water and NH, surface groups, may take place together with the adsorption of ammonia Such a side reaction, on the other hand, usually wastes but a small fraction of the adsorbed ammonia, so that it is difficult to determine the amount of ammonia consumed in

t h i s way by simply analyzing the gaseous phase, particularly if the

volume of the gaseous phase is large A method for reducing the

e r r o r due to such a side reaction is to make a s e r i e s of cyclic ad- sorptions and desorptions on the sample by varying the temperature

o r the p re s su r e of the system In this way it is likely that only the reversibly adsorbed ammonia takes part in such cycles

The measures of acid strength can be performed by determining the amount of desorbed ammonia obtained by heating in vacuo [22,23],

o r in a closed system [24], o r by flash desorption in an inert gas flow [25-271 Webb [23] worked with HF-A1,0, samples at various

H F percentages After outgassing and dehydrating at 500°C and’ lo* T o r r for 16 h r , the solid was exposed for 30 min at 175°C to

10 T o r r s p re s su re of gaseous ammonia The desorption was made

by evacuating the sample up to 500°C and collecting the desorbed ammonia in a liquid-nitrogen cooled trap From the difference in weights of adsorbed and desorbed base, the amount of base remained

on the solid surface could be determined The results a r e reported

in Fig 2 One can observe that the higher the HF percentage in the solid, the higher the amount of adsorbed ammonia This means that the acid strength of the solid is directly proportional to the HF frac- tion

is given in a paper by Shirasaki et al [28] They worked on silica- alumina covered with pyridine, n-butylamine, o r acetone By plotting both the changes of solid temperature [with respect to the reference sample (DTA)] and of solid weight (TGA) vs temperature (see Fig 31,

they simultaneously determined the amount of adsorbed base (x) and adsorption heat (S) By plotting S v s x one can get (dS/dx), which is directly proportional to the acid strength of the centers By plotting

x vs (dS/dx), the number of centers of given acid strength can be ob- tained

By means of the DTA technique Bremer and Steinberg [29] also

observed an inverse dependence of the amount of adsorbed base (pyridine) on the pretreating temperature of the solid (MgO SiO,)

In fact, preheating at high temperature gave a pyridine desorption peak at a lower temperature, with progressively higher pyridine de- sorption peak temperatures as the preheating temperature was lowered

employed, particularly with ammonia The main advantage of this method is that the acidity measurements can be made in the real working conditions of the catalyst Another advantage, particularly The gaseous bases adsorption-desorption method has been widely

FIG 3 Schematic DTA and TGA curves [28]

when TGA and DTA techniques are employed, is the possibility of obtaining a measure of the gaseous base desorption activation energy, which allows an absolute determination of the acid strength of the surface sites to be obtained However, the results often cannot be related to the catalytic activity and, when ammonia is employed, its adsorption on the solid is so strong that a careful evaluation of the acid strength distribution becomes very difficult o r impossible In addition, the method cannot distinguish between physical and chemi- cal adsorption of the base Such a difficulty has been avoided by in- troducing some standard conditions (e.g., heating up to a given tem- perature and evacuating down to a given low pressure for a given time) with the aim of establishing a given limiting point between the two types of adsorption, but such a procedure is obviously empirical

D Calorimetric Methods Another method for measuring the acid strength of a catalyst s u r - face is based on the determination of t h e heat of adsorption of basic

substances Richardson and Benson [30] measured the heat of ad- sorption of trimethylamine over cracking catalysts calorimetrically

The values of AH&, obtained ranged from -33 to -38 kcal/mole

Zettlemoyer and Chessick [31] determined the energy distribution

of the acid centers for kaolin and attapulgite catalysts by means of

a relationship between the differential heat of adsorption and the amount of adsorbed base (n-butylamine) A procedure suggested by

Harkins [32] consists of the rapid immersion of the solid in the liquid base In such a way, after suitable corrections, an integral

heat of immersion is determined In order to obtain differential heats, orie must preequilibrate the sample with various amounts of base before the immersion From the slopes of the curves obtained

by plotting the integral heats of immersion so determined vs the fraction of the surface precovered before the immersion, one can obtain differential heats as a function of coverage By plotting such differential heats v s coverage (see Fig 4), the distribution of acid strength of the centers can be obtained The behavior of the curve for kaolin has been interpreted by assuming an interaction among the adsorbed molecules of base Such an exothermic interaction gives a maximum heat evolution for coverages ranging from 0.1 to 0.6, and the heat evolved adds to that evolved by the adsorption re- action

alumina 8391, but the acid strength was determined by means of a

chemical [38] or gas chromatographic analysis [39] of the gases evolved by heating the sample after the adsorption The work by

Hsieh [34] on silica-alumina is another example of interaction among the molecules of adsorbed base (ammonia) with a consequent defor- mation of the plot of differential heat of adsorption vs coverage (see Fig 5) The interpretation by Hsieh of the behavior of such a curve

is a s follows: For low coverages (8 < 0.1) t he ammonia adsorbed neutralizes all types of surface acid groups, first on stronger cen-

t e r s and then on weaker ones At 9 = 0.1 all the acid centers have been neutralized with the formation of either NH,+ ions or highly polarized NH, molecules On further adsorption, ammonia molecules interact with NH,' ions and polarized NH, molecules, to which they are bound by Coulombian forces; the new ammonia also may interact

by van der Waals forces with t h e catalyst surface The Coulombic interaction, due to its stronger force (inverse square law, with r e - spect to inverse sixth power law of van der Waals interaction), pro- vides for a much higher adsorption heat, thus explaining the large interaction heats shown for 8 > 0.1 Hsieh also discusses the effect

of acid strength of t h e centers on the catalytic activity of the solid

7t f

FIG 5 Differential heat of adsorption for NH, on silica-alumina vs surface coverage

The methods based on the determination of the immersion heat of

t341

the solid have the advantage of allowing an absolute measure of a sufficiently accurate acid strength to be obtained Their main dis- advantage is connected to the interactions among the adsorbed mole-

By means of the flash desorption method of preadsorbed bases in

an inert gas flow, plots of the type reported in Fig 8 can be obtained

[26] The apparatus employed by Amenomiya et al [26] is shown in Fig 9 Such a method has also been employed, with some modifica- tions, to the study of hydrogenation of ethylene over alumina, with

cules of the base, particularly at high coverages, and to the difficulty

of calculating the heat corresponding to such interactions

The method of desorption of a preadsorbed base by progressive heating of the solid in a closed system, followed for example by Ballou [24], leads to plots of the type reported in Figs 6 and 7

From such graphs it is possible to obtain both the acid strength distribution and the density of the acid centers

The activation energy of desorption reactions can be calculated

by performing some experiments at various heating rates ( p ) and recording the temperatures (TJ corresponding to the maxima of the peaks of Fig 8 graphs The formula employed for such calcula- tions is

where Ed is the desorption energy, R the gas constant, V, the maxi- mum volume of gas adsorbed on the solid, and k, a constant, inde- pendent on temperature When the energies of the centers a r e hetero- geneous, the value of T, must be for samples which have been pre- treated so that they have the same degree of surface coverage at the

start of flash desorption

A similar method was followed by Kubokawa [22], who calculated the desorption activation energies from desorption rates instead of doing experiments by flash desorption

E Other Methods The acid strength of a solid may be determined on the basis of its catalytic activity toward some suitably chosen reactions For example, Pines and Haag [41] estimated t h e acid strength of some alumina catalysts by measuring the rates of cyclohexane and di-

methyl-1-butene isomerization and 1 -n-butyl alcohol dehydration Another method, followed by Aonuma et al [42], s t a r t s with the de- termination of the equilibrium constant for the adsorption of am- monia on the solid surface Such a constant is obtained by experi- ments on the progressive and reversible poisoning of the catalyst with the base in t h e cumene-cracking reaction Chapman and Hair

[43] determined t h e acid strength from measures of the shifting of the characteristic absorption bands of the base when it adsorbs on

the solid The experiment was first performed with benzaldehyde

s o hexachloroacetone [44] was used and the observations were made following the shifting of the carbonyl group band during the desorp- tion of the ketone at growing temperatures Other experiments were

also made with various alcohols in CC1, [45,46] A comparison was

made of frequency shifts of the OH group band at various concentra- tions of the alcohol in the solvent, in the presence of the solid, with the known acidity constants of the alcohols From such a compari- son the authors determined the following pK, values for the solids

examined: magnesia, 15.5; boria, 8.8; silica, 7.1; silica-alumina,

7.1; phosphorus, -0.4

111 DETERMINATION OF THE SURFACE DENSITY

OF ACID CENTERS The number of acid centers present on a solid surface is usually expressed as surface density, e.g., a s the number of centers, o r millimoles, per unit weight o r unit surface area

A Direct Titration of Aqueous Suspensions When an acid solid is suspended in water, it often lowers the pH

of the aqueous phase A direct titration of the aqueous suspension with a standard base, either in the presence of an indicator or poten-

tiometrically, can then give a measure of the surface acidity [23,47,

does not measure the acidity of the solid A typical case is reported

by Oblad et al [49]: By titrating an aqueous suspension of silica-

alumina catalysts with NaOH, they observed that the first end point

is reached very rapidly But, after some hours, the pH of the sus- pension decreased, and a quantity of titre had to be added to reach

t h e new end point They interpreted such a phenomenon by postulat- ing the formation of the following equilibrium:

By neutralizing t h e acid alumino-silicate with the titrating base, the equilibrium slowly shifts to the left and a new quantity of acidity is formed Another typical case is given by the water itself, which re- acts with t he solid, e.g., by transforming the Lewis centers into Brbnsted centers Then titration of the catalyst in the aqueous phase gives a quantity of Brdnsted centers greater than that obtained in an-

hydrous solvents The e r r o r may be very large, as reported in Fig 10,

in the case of two different types of silica-alumina [50]

From the experimental point of view, titration methods in aqueous solution can be considered as the simplest ones for the determination

of surface acidity On the other hand, the presence of water, which cannot be considered an inert medium, introduces such heavy limita- tions that titration methods may be employed only in particular cases

FIG 10 Brdnsted acidity of synthetic (A) and commercial (B) silica-alumina

catalysts; (X) moist; (0) dry [50 1

B Titration after Ionic Exchange The previously described direct titration method is, in fact, based

on an ionic exchange between the acid solid and the aqueous phase But in the case of cracking catalysts based on molecular sieves, some

titration methods involving a direct neutralization of Ht cations have been developed In fact, in such catalysts the active sites are local- ized in some regions characterized by a strong local electrostatic

field [51], generated by the simultaneous presence of basic and acid centers The cationic exchange on molecular sieves takes place very quickly at room temperature The pH of exchanging solutions must not be too low to avoid damage to the crystal structure of the sieves Usually t h e pH must not be lower than 4 , although in some cases it is possible to operate down to pH = 2 According to Gren- hall [52], the Ht ion of the solid can be exchanged with N a t by means

of a 5% NaCl aqueous solution The exchanged solutions obtained

at various exchanging times a r e titrated and the results extrapolated

to t = 0 in order to eliminate the influence of the reaction between the solid and water, according to equilibrium reaction (4) Plank [53] performed the ion exchange with a 0.1-8 ammonium acetate solution

The ion exchange technique, followed by titration, was also employed

by Mahl [54], Trambouze et al [55], Holm et al [56], and Danforth

[57] Holm et al in particular performed an accurate study which demonstrated the independence of surface acidity on catalyst particle size They also outlined the influence of sample quantity on the re- sults of acidity determination Their results clearly indicate that the reaction reaches an equilibrium in which the specific quantity

of acid transferred from the solid surface to the solution increases with an increase in the amount of employed sample Since below 0.1 g

of sample the increase in acidity became negligible, the authors per- formed all their determinations on samples weighting less than 0.1 g They were also able to determine the relative acid strength of such centers from the change in the degree of exchange with the quantity

of sample

Titrimetric methods following ion exchange a r e interesting in that they do not need direct contact between the titrating base and the solid, but they do not avoid the presence of water This is an important limitation because, as with the direct titration method,

all the centers whose acid strength is lower than that of water itself cannot be titrated

C Titration with Bases in Nonaqueous Solvents This method, originally introduced by Tamele [58] and Benesi

lows: A small quantity of predehydrated solid is covered with an inert, anhydrous solvent in which a predetermined amount of a base

is dissolved After equilibration an indicator, which adsorbs on the solid surface, is added and assumes the color of its acidic o r basic form The quantity of base needed to impart to the solid the color

0

4

H

of the basic form of the indicator represents the measure of the

quantity of acid centers present on the catalyst surface Obviously

only the centers whose strength is higher than the pK, of the indi-

cator a r e titrated However, by employing indicators of various pK,

it is possible to titrate the centers of various acid strengths Ex-

perimental details can b e found in an example reported by Tanabe

and Katayama [60] Some subsequent modifications of the method

concerned only the employment of different solvents and/or bases

For example, Johnson [61] employed CC1, and isooctane as solvents

and observed that, for some solids such as silica-alumina, the solu-

tion equilibration times for drop by drop titrations may be very long

(2-3 days) Such a difficulty was overcome in the Benesi method [59]

by adding the indicator to the suspension aftek equilibrium had been

reached, and the end point was attained by successive approxima-

tions In addition, the Benesi method strongly reduces the danger of

moisture contamination because the amine is added all at once

Matsuzaki e t al [62] studied the effects of some parameters on

the results of titration One can observe (see Fig, 11A) that t he

amount of indicator added affects the results only if it is lower than

a given minimum (e.g., f o r dimethyl yellow 0.2-0.3 ml of 1% ben-

zene solution), while the titration time has an influence only if it is

FIG 11 (A) Effect of added indicator volume on measured acid amount Sample:

0.5 g of >lo0 mesh silica-alumina in 10 ml benzene Indicator: 1% benzene solution of

dimethyl yellow (B) Effect of titration time on measured acid amount Sample as in A

Indicator: 0.3 ml of 1% benzene solution of benzeneazodiphenylamine [ 621

shorter than 50 h r (Fig 11B) The latter difficulty (too long titration times) can be overcome if the solid is ground down to fine powder

( 2 100 mesh), as may be seen in Fig 12A The harmful effects of exposing the solid to a moist atmosphere a r e shown in Fig 12B The

stronger the acidity of the centers (H, +1.5), the stronger is the

latter effect, This is probably due to the transformation of a part

of the stronger centers into weaker ones In fact, the pK, of water

only the H, -1.7 centers

FIG 12 (A) Effect of powder size on acid amount Sample: 0.5 g silica-alumina in

10 ml benzene Indicator: 0.3 ml of 1% benzene solution of dimethyl yellow Titration time 2 hr (B) Effect of moisture on acid amount Sample: 0.5 g of >lo0 mesh silica- alumina in 10 ml benzene Indicators: 0.3 ml each of several pKa values (1% benzene solution) Titration time; 2 hr Predried catalyst left in 90% humidity at 20°C for (a) 0 min,(b)5 min,(c) 1 0 m i n [ 6 2 ]

Perhaps the best technique was developed by Bertolacini [63]

The main advantage of such a titration method consists of an ultra- sonic generated stirring by which the equilibrium conditions can be reached in a very short time (some tens of minutes, instead of days)

(see Table 4)

As repeatedly mentioned, the most common source of e r r o r in such determinations is contamination with water For example, on silica-alumina, even after dehydration in air at 600°C, at least 0.5

wt% of water still remains on the solid If all that water was asso- ciated with surface centers, and if the surface a r e a of the solid was

TABLE 4 Titration Time Dependence of Titrations by

2

4

8

24 0.5

Av 0.34 0.20 0.21 0.20 0.21 0.21

Av 0.21 0.70 0.68 0.68 0.69 0.70

Av 0.69

about 300 m2/g, this could account for 5 X 1013 sites/cm2, i.e., a num-

ber very close to the total number of s i t e s present on the solid s u r - face

If the solid surface is colored, the method can still be employed

by adding to the mixture a given quantity of a white acid solid The end point of titration is determined by observing the change in color

of the white solid surface A correction factor must obviously be introduced for the quantity of base consumed in the neutralization of such a white solid [61] A practical example of such a procedure is given by Tanabe and Watanabe [64] in which the surface acidity of titanium trichloride in the presence of a small amount of silica- alumina is determined In this case the sharpest change in,color

was observed for mixtures of about 0.02-0.05 g of TiCl, and 0.2 g of

Si0,-Al,03 Another example is given by Voltz et al [65] in which the surface acidity of a dark green sample of Crz03, previcusly dried for 4 h r at 500°C, was determined in the presence of a given amount

of alumina

strength and the number of surface sites The results are reported

in Fig 13 The three substances on which he worked were Si0,-MgO,

SiO,-Al,O,, and Filtrol Another example of correlations of this type,

Benesi [59] also tried to find a correlation between the acid

0.0 o*2* + 4 +2 0 - 2 - 4 - 6 -8

He

FIG 13 Butylamine titres vs acid strength for catalysts calcined at 500°C (1) Silica- magnesia, (2) silica-alumina (MS-A-l), (3) Filtrol SR [ 591

reported by Goldstein [l], refers to a sample of silica-alumina

seen that, in the steamed sample, the sites of strongest acidity still remain poisoned by water

FIG 14 Acid strength distribution for silica-alumina on unit area basis [ 11

Another possible source of e r r o r in such determinations is re- lated to the indicator employed A spectrophotometric study per-

formed by Drushel and Sommers [66] showed that some indicators

a r e not able to measure protonic-type acidity, some show two end points and some others a r e improper for the usually assigned pK, value Some aromatic alcohols have also been employed as indica- tors It has been reported that such aromatic alcohols, called HR indicators, are specifically employable for protonic-type acidity

In Table 5 a short list of such substances, reported by Hirschler [67],

is given They dissociate according to

ROH + H+ R+ + H,O

so that t h e definition of H, is

H R = PK, + log ([ROHI/ER+I)

In Fig 15 the plot of H, and H R vs the H,SO, wt% in aqueous solution

is reported The double end point observed with some indicators could then be due to such a different behavior of these substances with respect to the type of acidity present on the solid surface

TABLE 5

HR Indicators [67]

Indicator 4,4’,4”-Trimethoxytriphenylmethanol 4,4’,4“-Trimethyltriphenylmethanol Triphenylmethanol

3,3‘,3”-Trichlorotriphenylmethanol Diphenylmethanol

4,4’,4“-Trinitrophenylmethanol

2,4,6-Trimethylbenzy1 alcohol

PKR

+0.82 -4.02 -6.63 -11.03 -13.3 -16.27 -17.38

D, Calorimetric Titration The calorimetric titration method was first introduced by Tram- bouze and co-workers [68-701 and subsequently developed by Top- chieva e t al “711, and Tanabe and Yamaguchi “721 The bases em- ployed can be n-butylamine, ethyl acetate, or dioxane As previously reported, the method i s also useful for the determination of acid strength Experimental details are given by Tanabe and Yamaguchi [72] The

+ S O 1 I I ’ I ’ I ’ ’ I

H,SO, ( w t % )

FIG 15 H,, and HR values as a function of H,SO, wt% in aqueous solution [67]

results a r e usually plotted as temperature v s time (see Fig 16) The temperature increase A T, corresponding to each addition of titre, is given by the distance between the two straight lines parallel

to the YM” line (see Fig 16) recorded before and after each addition

to the base If the total AT,, given by the sum of the AT’S correspond- ing to the n additions of base, is plotted vs the volume of added titre,

a curve is obtained (Fig 17) which flattens in correspondence of the base volume needed for the titration of total acid amount of the solid sample The progressive flattening of the Fig 17 curve with the in- crease in the volume of added base may be due either to the hetero- geneity of the centers, as suggested by Topchieva et al [71], o r to the decrease in the diffusion rate of the base as the diameter of the catalyst pores decreases

E Adsorption and Desorption of Gaseous Bases

A s in the previously described determination of surface acid strength, various experimental techniques can be adopted, such as the determination of the weight of adsorbed base collected after de-