Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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670 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

The gel is obtained as ﬁ ne particles when it is washed with distilled water, and

a large part of the precipitate passes through a conventional ﬁ lter paper, resulting

in diminished yields. This difﬁ culty can be avoided by washing the gel with

aqueous ammonium acetate solution, which provides a quantitative yield.

17.2.1.3 Preparation of H

TiO

[30]

A volume of Ti[OCH(CH

)

]

(290 ml) is added to distilled water (2 l) with stirring,

and the white precipitates formed are dissolved by gradually adding conc. HNO

(250 ml) with stirring. Ammonia solution (28%, ∼ 300 ml), is added into the aqueous

solution with stirring until pH 8 is attained. The solution is then allowed to stand

for a day. Washing is then undertaken by decantation of a 5 l beaker of deionized

water twice. Finally the resultant material is dried at 100 ° C for 24 h.

Another method of preparing H

TiO

is by hydrolysis of TiCl

as follows. A

volume of TiCl

(80 ml) is gradually added to distilled water (2 l) in a 5 l beaker

cooled by ice water, with large amounts of HCl gas being formed. Ammonia solu-

tion (28%) is added at room temperature until a pH of 8 is attained. The resultant

precipitates are washed thoroughly by decantation using 60 l of water until no

chloride ions are detected in the ﬁ ltrate. The aqueous portion might become

cloudy during washing, but the white washings can be decanted.

17.2.1.4 Preparation of Fe(OH)

[30]

To a solution of Fe(NO

)

· 9H

O (500 g) dissolved in 2 l of water in a 5 l beaker,

ammonia solution (28%, ∼ 300 ml used, pH 8) is added with stirring to precipitate

Fe(OH)

. The aqueous portion is decanted from the precipitate after allowing the

solution to stand. The precipitates are washed by decantation until the liquid

portion becomes cloudy (7 – 8 times), and dried.

17.2.1.5 Preparation of Hf(OH)

[35]

HfCl

is gradually dissolved in distilled water with care, and the hydroxide is pre-

pared in the manner described above for Zr(OH)

17.2.1.6 Sulfation, Calcination, and Catalytic Action [30]

The above prepared materials are powdered below 100 mesh and treated with

sulfate ions by exposing 2 g of the hydroxides (gel) in 30 ml of aqueous sulfuric

acid for 1 h, ﬁ ltering, drying in a desiccator at room temperature, and ﬁ nally calcin-

ing [36] . The iron materials are again powdered because of solidiﬁ cation after

drying [36, 37] . The concentration of H

is 0.5 M for the hydroxides of Zr and

Ti [38, 39] , 3 M for Sn [40, 41] , 0.25 M for Fe [37] , and 1 M for Hf [35] . A recent

study shows that the optimum concentration for Zr is 0.25 N [42] .

After calcination of the sulfate - adsorbed materials in air, the substances are

catalytically active for the skeletal isomerization of butane to isobutane at room

temperature. The activities are dependent on the calcination temperature. The

maximum activity is observed with calcination at 575 – 650 ° C for the Zr catalyst

[32] , 500 – 550 ° C for Sn [34, 41] , 525 ° C for Ti [43] , 500 ° C for Fe [36, 37] , and 700 ° C

for Hf [35] .

17.2 Preparation 671

All the catalysts are calcined in Pyrex glass tubes in air for 3 h and sealed in

ampoules while being hot, to minimize exposure to humidity until use.

17.2.1.7 Preparation of Sulfated Silica [44]

Silica gel is obtained by hydrolyzing Si(OC

)

(100 ml) with water (100 ml) and a

few drops of HNO

. The mixture is stirred until gel formation. The precipitates are

obtained by evaporation of excess water and ethanol, formed by hydrolysis of

Si(OC

)

, followed by drying at 100 ° C, and powdering. The silica (3 g) is exposed

to SO

for 1 h followed by evacuating HCl evolved by the reaction of surface OH

group with SO

and excess SO

in vacuum, and calcining in air at 400 ° C.

17.2.1.8 Preparation of Sulfated Alumina

In the case of the sulfate - treated superacids of Zr, Sn, Ti, Fe, Hf, and Si, superacid

sites are not created by the treatment of sulfate ion on the crystallized oxides but

rather on the amorphous forms, followed by calcination to crystallization. The

superacid of Al

is prepared from the crystallized oxide, γ - Al

[45, 46] .

A highly active catalyst is obtained by hydrolysis of aluminum isopropoxide [47] .

Distilled water is added with stirring to a solution of Al[OCH(CH

)

]

(10 g) dis-

solved in isopropanol (300 ml) to precipitate Al(OH)

, followed by washing the

precipitates, drying, and calcining for crystallization at 500 ° C for 3 h. The crystal-

lized materials are then hydrated before sulfation. The sample (5 g) is suspended

in water (5 l) at 80 ° C for 3 h, ﬁ ltered, and dried at 100 ° C. SO

/Al

is obtained by

treatment with 2.5 M H

and calcination at 600 – 650 ° C for 3 h.

17.2.1.9 Property and Characterization

Properties of the sulfated materials thus prepared are summarized as follows

[43, 48] .

1. Superacidity is generally created by adsorbing sulfate ions onto amorphous

metal oxides followed by calcination in air to convert to the crystalline forms.

However in the case of Al

, a superacid is prepared from the crystallized

oxide.

2. Speciﬁ c surface areas of the catalysts are much larger than those of the oxides

without the sulfate treatment except for the Al

. A particularly large increase

in the area is observed on the highly active and acidic catalysts. The main reason

for the increase in surface area is the retardation of crystallization by sulfate

treatment. The areas of the Al

catalysts are smaller than those of the oxides

without the sulfate treatment.

3. By XRD analysis the degree of crystallization of the sulfated oxides is much

lower than that of the oxides without the sulfate treatment. Temperatures of

the crystallization or phase transformation for SO

/ZrO

and SO

/TiO

are circa

150 and 200 ° C higher than those for pure ZrO

and TiO

, respectively; the XRD

pattern of SO

/ZrO

(650 ° C) and SO

/TiO

(525 ° C), whose superacidities are

highest, are pure tetragonal and anatase forms, respectively.

672 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

4. The sulfate samples have IR spectra that are different from those of metal

sulfates; the materials show absorption bands at 980 – 990, 1040, 1130 – 1150,

and 1210 – 1230 cm

− 1

, which are assigned to the bidentate sulfate coordinated to

metal ions.

5. XPS spectra of SO

/ZrO

and SO

/Fe

show that the surface is not Zr(SO

)

and Fe

(SO

)

, but composed of ZrO

and Fe

with SO

2−

, respectively. On the

other hand, the spectra of SO

/TiO

and SO

/Al

are consistent with the

presence of surface Ti(SO

)

and Al

(SO

)

, respectively.

6. The IR spectra of pyridine adsorbed on SO

/ZrO

and SO

/TiO

show the facile

conversion of Lewis sites to Br ø nsted sites by water molecules. Lewis and

Br ø nsted sites are easily interchangeable by adsorption or desorption of water

molecules [49 – 55] .

7. Upon dehydration in N

at 375 ° C, a new IR peak centered at 1370 cm

− 1

appears;

the band is assigned to an S

O stretching vibration and is observed for the

catalytically active material [56, 57] .

8. The sulfated materials also show oxidizing action at elevated temperatures. In

particular the superacids of Fe

and SnO

demonstrate strongly oxidizing

potential at temperatures above 100 ° C [58, 59] .

9. Catalysts obtained by treatment with sulfuric acid are usually more active than

those obtained with ammonium sulfate treatment. However, a recent study

shows that the analogous activity is generated on SO

/ZrO

by the kneading

method with ammonium sulfate [60] .

17.2.1.10 One - Step Method for Preparation of SO

/ZrO

[61 – 65]

Ward and Ko investigated preparation of sulfate - zirconia aerogels in a one - step

synthesis by the sol – gel method followed by supercritical drying [61] . Sulfuric

acid is mixed with zirconium n - propoxide (16.2 ml of 70 wt% in propanol) in n -

propanol (30 ml) and reacted with water (1.3 ml) and nitric acid (1.9 ml of 70%

w/w) to form a zirconia - sulfate co - gel; supercritical drying with carbon dioxide

removes the alcohol solvent forming a high surface area aerogel. The properties

and catalytic activities are similar to those of the compounds we have prepared.

A single - step sol – gel method has also been used for the preparation of SO

/TiO

[66] .

17.2.1.11 Commercial Gels for Preparation of SO

/ZrO

and SO

/SnO

Commercial zirconia - gels are now supplied by chemical companies such as MEL,

Nakarai, and Daiichi Kigenso. For instance, XZO631 and 632 are the gels, and

XZO682 is a sulfate - adsorbed gel from MEL. According to our experience, com-

mercial gels have often led to catalysts superior to those prepared in the laboratory

[67, 68] . Similar observations have been made with SO

/SnO

. A commercial gel

prepared from meta - stannic acid gives satisfactory results for this type of catalysis

[69] . SO

/SnO

was prepared using meta - stannic acids (SnO

· H

O, commercial

17.2 Preparation 673

grade) supplied by Kojundo Kagaku, Ltd. and Yamanaka & Co. Ltd. The activity

for the acid - catalyzed conversion of methanol into dimethyl ether was higher than

for catalysts obtained by hydrolysis of SnCl

and much higher for the isomerization

of butane.

17.2.1.12 Effect of Drying and Calcination Temperatures on the Catalytic Activity

of SO

/ZrO

[68]

The catalytic activity of SO

/ZrO

varies with the type of zirconia gel and the drying

and calcination conditions. The calcination temperature showing the maximum

activity and acidity often varies with the type of prepared gel. For instance, the

maximum activity for the conversion of butane to isobutane is observed with cal-

cination at 575 and 650 ° C, respectively, for the materials prepared from ZrO(NO

)

and ZrOCl

as starting reagent [32] .

The activity also depends on the drying temperature of the gel before sulfation.

For the gels obtained by hydrolysis of ZrO(NO

)

followed by drying at 100 ° C and

300 ° C, the difference in the calcination temperature showing the maximum activ-

ity for the butane conversion was 50 ° C, and the difference in maximum activity

was a factor of two.

Several sulfated zirconias were prepared by changing the drying temperature of

gel in the range 100 – 400 ° C and the ﬁ nal calcination temperature in the range

475 – 700 ° C. It was found that the drying temperatures exhibiting the highest activ-

ity for the butane conversion are not always ﬁ xed, for instance 200 ° C for one and

300 ° C for another.

Figure 17.1 summarizes the effect of the calcination temperature on the catalytic

activity for butane isomerization over sulfated zirconias prepared from different

zirconia gels dried at the optimum temperatures. The ﬁ gure indicates that the

maximum activities are approximately the same for different catalysts even though

Figure 17.1 Activities for reaction of butane at 180 ° C over the

/ZrO

catalysts prepared from various Zr gels: MEL 631

dried at 300 ° C ( 䊐 ), MEL 632 dried at 200 ° C ( 䉱 ), Nakarai

dried at 300 ° C ( 䊉 ), and the gels prepared by hydrolysis of

ZrO(NO

)

( 䉭 ) and ZrOCl

( 䊊 ) followed by drying at 300 and

200 ° C, respectively.

674 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

the temperatures to give maximum activity are different. The calcination tempera-

tures required to give maximum activity for different catalysts fall within a tem-

perature range < 50 ° C. Residual species such as Cl

−

and NO

−

in the gel result in

differences in the optimum drying and calcination temperatures owing to differ-

ences in the state of dehydration of the zirconia gel or to reactivity of the zirconia

support with the sulfate species. However, selection of the optimum drying and

calcination temperature generate the same ultimate catalytic activity. The present

results point out that the optimum temperature for drying the Zr gel and the ﬁ nal

calcination should be determined according to the type of zirconia gel.

17.2.2

Tungstated, Molybdated, and Borated Metal Oxides

17.2.2.1 Preparation of WO

/ZrO

and MoO

/ZrO

[30]

A sample of Zr(OH)

(10 g), obtained from ZrOCl

, is heated at 300 ° C, the gels

are impregnated with aqueous ammonium metatungstate [(NH

)

] · n H

50 wt% WO

, 3.8 g] and water (15 ml) in a 100 ml beaker followed by evaporating

water at room temperature, drying, and calcining in air at 800 ° C for 3 h. The con-

centration is 15 wt% W based on the hydroxide, and 13 wt% W after calcination at

650 – 950 ° C. The analogous material is also formed by the kneading method with

tungstic acid (H

) which is insoluble in water; a wet mixture of Zr(OH)

(10 g)

and H

(2 g) with a little water is kneaded for 3 h [70, 71] .

After heating Zr(OH)

(10 g) at 300 ° C the sample is impregnated with molybdic

acid (H

MoO

, 2.5 g) dissolved in ammonium hydroxide (28%, 2 ml) and water

(15 ml) followed by evaporating water at room temperature, drying, and calcining

at 800 ° C in air for 3 h. The concentration is 5 wt% Mo metal based on the hydrox-

ide [72] .

17.2.2.2 Preparation of WO

/SnO

, WO

/TiO

, and WO

/Fe

[30]

After the hydroxides of Sn, Ti, and Fe, prepared by hydrolysis of SnCl

, TiCl

, and

Fe(NO

)

, respectively, are dried at 300 ° C, the gels are impregnated with aqueous

ammonium metatungstates [(NH

)

)] followed by evaporating water,

drying, calcining in air for 3 h at 1000, 700, and 700 ° C for the Sn, Ti, and Fe

materials, respectively. The concentration is 15 wt% W based on the hydroxides

(11 – 13 wt% W after calcination) [73, 74] .

/SnO

is also prepared from a commercial gel, used after drying meta -

stannic acid of Kojundo Kagaku, Ltd. at 100 ° C [69] . Calcination at 1000 ° C after

impregnation of the tungstate generates the highest activity for acid - catalyzed

reactions. High temperatures such as 1000 ° C for calcination do not discriminate

precursor stannia gels, though the calcination temperature giving the highest

activity for SO

/SnO

differs according to the gel.

17.2.2.3 Preparation of B

/ZrO

[75, 76]

Zirconium hydroxide is impregnated with aqueous boric acid followed by evapo-

rating water and calcining in air at 650 ° C (3 wt% B). The same catalyst is obtained

by suspending the hydroxide in 2 - propanol solution of trimethyl borate followed

by adding water to hydrolyze the borate.

17.2.2.4 Property and Characterization

Properties of the catalysts thus prepared are summarized as follows [43, 48] .

1. Superacid sites are not created by impregnation on the crystallized oxides, but

on the amorphous forms whose calcination then converts them to the crystalline

forms. Recent work shows that a WO

/ZrO

catalyst prepared by impregnation

of the crystalline zirconia (65% tetragonal, 35% monoclinic) exhibits comparable

behavior [77, 78] .

2. WO

/ZrO

(800 ° C) and MoO

/ZrO

(800 ° C), whose activities are highest, are

observed to be 100% tetragonal ZrO

by XRD, with the TiO

in WO

/TiO

(700 ° C)

being the anatase polymorph [74] .

3. Speciﬁ c surface areas of the WO

and MoO

catalysts are much larger than

those of the oxides without tungsten and molybdenum oxides, pure metal

oxides.

4. XPS spectra of the WO

and MoO

supported catalysts show their surface to be

or MoO

and ZrO

, SnO

, TiO

or Fe

[73] .

17.3

Determination of Acid Strength

Several methods have been used to determine the surface acidity of solid acids,

but each method has its limitations. Common methods are titration with the

Hammett indicators, temperature - programmed desorption ( TPD ) , adsorption

microcalorimetry, catalytic test reactions, and IR and NMR spectroscopies. These

techniques exhibit several advantages and disadvantages.

Titration with a variety of Hammett indicators is one of the most widely used

techniques to determine the distribution of acid strengths on the solid surface [79] .

However, many arguments have been raised in the past against the use of Hammett

indicators for evaluation of solid acidity [80 – 83] .

A commonly used technique is TPD of adsorbed bases such as ammonia and

pyridine [84 – 86] . However, there are critical questions in this method [87 – 89] . The

molecule is well known to interact with both the acidic OH group and the

basic oxygen [90] . Thus, NH

gives information on dual acid – base sites.

Instead of TPD, microcalorimetry of adsorption shows the heat evolved during

the adsorption of probe molecules, usually ammonia, on acid sites [91 – 94] . This

measurement can determine the distribution of adsorption enthalpies but cannot

differentiate between adsorption on Lewis and Br ø nsted acid sites.

Catalytic activity can be used to rank solid acidity, and activity for the skeletal

isomerization of butane is often used to indicate very strong acidity, in particular

superacidic strength [95] . A comparative study using the isomerizations of butane

17.3 Determination of Acid Strength 675

676 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

and pentane as test reactions gave good correlations between their rate constants

and the Hammett acid strengths [96] . These test reactions do not differentiate

between the acid strength and the number of acid sites. The mechanism of butane

or pentane isomerization, however, has been shown to be bimolecular, so rates of

alkane isomerization alone cannot be used to compare acidities [97 – 100] . One

paper indicates that the isomerization of α - pinene enables solid acids to determine

the Br ø nsted acid strength; superacidity promotes the formation of limonene over

camphene [101] .

A comparative study using IR spectroscopy can rank solid acidity by determining

the frequency shift of the adsorption band of pyridine [102] , the band shift of OH

groups due to the adsorption of benzene or CD

CN [103, 104] , and the shift of the

adsorbed CO on Lewis sites [105] . These IR techniques, however, are not widely

used for evaluation of solid acidity.

Solid - state NMR spectroscopy enables site - speciﬁ c characterization of solid acids

such as protonated zeolites [106, 107] . Attempts have been made to relate the acid

strength of solids to the

H chemical shift of surface OH groups, the shift brought

by the adsorbed bases such as CD

CN and CCl

CN, and the

P shift of the

adsorbed

P(CH

)

[108 – 113] . A disadvantage of NMR spectroscopy is the compli-

cated chemical shift due to hydrogen bonding.

17.3.1

Hammett Indicators

The acid strength of a solid is deﬁ ned as the ability of the surface to convert an

adsorbed neutral base into its conjugate acid. The strength is expressed by the

Hammett acidity function, H

, as explained in the Introduction. The color of suit-

able indicators adsorbed on a surface can give a measure of acid strength. If the

color is that of the acid form of the indicator, then the value of the H

function of

the surface is equal to or lower than the p

of the conjugate acid of the indica-

tor. Lower values of H

correspond to greater acid strength.

The acid strengths shown in Table 17.3 were examined by the visual color

change method using the Hammett indicators shown in Table 17.1 [43, 48] . The

indicator dissolved in solvent was added to the sample in powder form in a non -

polar solvent, sulfuryl chloride [38] or cyclohexane [40] . The strength of colored

materials such as SO

/Fe

and MoO

/ZrO

was estimated from their catalytic

activities in comparison with those of the catalysts determined by the Hammett -

indicator method.

Determination of the acid strength of solid catalysts using Hammett indicators,

however, has been criticized frequently because of the heterogeneity of the solid

surface [81, 104, 110, 114 – 116] . The principle of the Hammett acidity function is

based on the equilibrium equation in a homogeneous solution, and its application

to the heterogeneous condition is subject to severe criticism. In addition, the color

change of the adsorbed indicators on solids as determined by the naked eye is

subjective. The effects of interactions between the solvent and the solid surface

has also been raised [9] .

17.3.2

Test Reactions

Owing to the heterogeneity of solid superacids, accurate acidity measurements are

difﬁ cult to perform and interpret. The most simple and useful way to estimate the

acidity of a solid catalyst is to test its catalytic activity in acid - catalyzed reactions.

We usually compare activity with those of aluminosilicates such as silica - aluminas

and zeolites. These materials have strong enough acidities to cause the Friedel –

Crafts reactions, and their acidities are known to be in the range of superacidity

[117] . The Hammett - indicator method indicates that the acid strength of the SiO

used is in the range of − 12.70 < H

≤ − 11.35, whose acidity is H

= ∼ − 12 and

superacidic [37] . The acidity and catalytic activity of zeolites are generally higher

than the silica - aluminas, with mordenites being highest among them ( H

= ∼ − 14)

(ZSM - 5: H

= ∼ − 13) [67] .

In order to conﬁ rm the acidity results measured using the indicators shown in

Table 17.1 , we have investigated as many acid - catalyzed reactions as possible. The

reactions are summarized in Table 17.4 [43, 48, 118, 119] . Among them, the skel-

etal isomerization of light parafﬁ ns, in particular butane and pentane, has been

the most widely applied. The isomerization of butane at room temperature was a

well known test reaction for superacidity at the beginning of this work [43, 48,

118] . The activity for many of the reactions tested correspond to the acidities as

determined by use of the Hammett indicators.

17.3.3

Temperature - Programmed Desorption ( TPD )

TPD using base adsorbents such as ammonia and pyridine has been one of the

common techniques to evaluate the amount and strength of acid sites on solid

acids. However, the desorption temperature of ammonia and pyridine adsorbed

on solid superacids is so high (500 ° C and above) owing to the strong interaction

that the adsorbed compounds are decomposed or oxidized before desorption. The

surface structure can even be destroyed by reactions with probe molecules

[120 – 122] .

Table 17.4 Acid - catalyzed reactions.

Dehydration of alcohol → MeOH, EtOH, i - PrOH

Decomposition of alkylbenzene → Ph - Me, Ph - Et, Ph -

Friedel – Crafts acylation → acetylation, benzoylation, and so on

Isomerization of parafﬁ n → open - chain C

– C

, cyclic C

– C

Esteriﬁ cation → AcOH + MeOH, EtOH, and so on

OH + phthalic acid, and so on

Cationic polymerization → Me, Et,

Bu vinyl ether

Oligomerization → β - pinene, 1 - octene, 1 - decene

Others → aldol condensation, and so on

17.3 Determination of Acid Strength

677

678 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

TPD using pyridine was attempted for estimating the acidity of superacidic

materials, even though this method suffers a major drawback. An examination of

the termination temperature of pyridine desorption was made using TG - DTA

[123] . When the adsorbed compound is decomposed or oxidized to destruction

before desorption, its temperature must be below that of the real desorption. The

temperatures of pyridine desorption obtained were generally proportional to the

values of the highest acid strength determined by the Hammett method or

estimated by catalytic activity, except that of SO

/SnO

. All of the temperatures for

the superacids were higher than those of Al

, ZrO

- TiO

, SiO

- ZrO

, and SiO

, whose acidities are below superacidic, and whose oxidative action is quite

weak. In the case of SO

/SnO

whose temperature was unexpectedly low, oxidation

of the adsorbed pyridine occurred on the catalyst surface at a temperature below

that expected from the acid strength by the Hammett method.

17.3.4

Temperature - Programmed Reaction ( TPR a )

Another method similar to TPD, is temperature - programmed reaction ( TPRa )

using furan as a probe molecule [118] . Furan is resiniﬁ ed on the surface of solid

acids when heated at a temperature that depends on the acid strength. Solids with

weak acidity give rise to the reaction at elevated temperatures, and the reaction

temperature is dependent on the surface acidity. A relationship is observed between

decrease of the temperature and increase of the acid strength. The resiniﬁ cation

of furan adsorbed on the surface of catalyst is exothermic, and TPRa using DTA

is a technique for estimating the acid strength of superacids.

The initial temperatures of resiniﬁ cation were compared with the H

values of

superacids as determined by the Hammett method, and a linear relationship was

observed. The TPRa results gave the H

value of SO

/SnO

as − 18; the value

was not determined by the Hammett method because − 16.12 relates to 1,3,5 -

trichlorobenzene indicator which has the lowest p

value among the Hammett

indicators used to date (Table 17.1 ).

17.3.5

Ar - TPD [124, 125]

A new method has involved the use of Ar as a probe atom. We tried Ar as a probe

TPD and found that Ar - TPD could be applied to the evaluation of the acid strength

of solid superacids.

Noble gases such as Ar and Xe interact only with the acidic sites, giving infor-

mation on these sites. Hence these atoms are useful as probes of the intrinsic

nature of the acidic sites. For noble gases, polarization and dispersion are domi-

nant. Ar shows an acid – base - like interaction in a polarized state with acid sites at

low temperature owing to its induced dipole.

The acid strength was ﬁ rst evaluated as an activation energy of Ar desorption.

The activation energy was calculated by the following equation:

2ln lnTbERT

mdm

const−= +

(17.5)

Here T

is the peak temperature of desorption, b the rate of temperature

increase, and E

the activation energy. A linear plot is obtained between 2 ln

− ln b and 1/ T

, and E

is determined from the gradient.

The Ar - TPD experiments were performed in the temperature range 113 – 223 K

at the programmed rate of 2 – 5 K min

− 1

using a cooling system with N

gas bubbled

through liquid N

along with an electric heater regulated by a temperature control-

ler. The proﬁ les at the rate of 2 K min

− 1

are shown Figure 17.2 . The activation

energies of Ar desorption are calculated from the estimated values of T

to be 5.5,

6.0, 6.6, 6.7, 7.6, and 9.3 kJ mol

− 1

for SiO

- Al

, zeolites of H - Y, H - ZSM - 5, and

H - Mordenite, heteropoly acid (Cs

2.5

0.5

), and SO

/ZrO

, respectively. The

value for SO

/SnO

is determined to be 10.6 kJ mol

− 1

[33, 34] .

The heat of adsorption of ammonia correlates with the Hammett acidity func-

tion, H

. Similarly, the activation - energies of SiO

- Al

, three zeolites, and SO

ZrO

relate well with their H

values determined by the Hammett method and

represented by their highest acid strengths, with a linear plot being obtained. By

extrapolation of the linear plot to the activation energy 9.3 kJ mol

− 1

for SO

/ZrO

the H

value is estimated to be − 19 [68] , which agrees with that estimated from

the heat of adsorption of ammonia [93] . The value of 10.6 kJ mol

− 1

found for SO

SnO

is estimated to correspond to H

= − 21.

Figure 17.2 Ar - TPD proﬁ les of solid acids; temperature programmed rate: 2 K min

− 1

17.3 Determination of Acid Strength 679