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

Подождите немного. Документ загружается.

680 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

17.3.6

Ar - Adsorption [126, 127]

The heat of adsorption of Ar was also measured for acidity evaluation. In the case

of Ar - TPD, an effect of the probe molecule diffusion in micropores is observed

with some samples, such as zeolites, at high temperature - programmed rates. The

adsorption method is not inﬂ uenced by diffusion of the adsorbed molecule because

the Ar isotherm is measured at static equilibrium. It is also advantageous that the

usual BET apparatus can be used to obtain the adsorption isotherm. In addition,

the adsorption behavior of Ar is of the Henry type at temperatures around room

temperature.

Langmuir ’ s adsorption equation can be converted into the following equation

by the introduction of an approximation at low coverage:

ln ln lnVP HRT b V

()

=−

()

++∆

(17.6)

where V is the volume adsorbed, P is the equilibrium pressure, ∆ H is the heat of

adsorption, R is the gas constant, T is the adsorption temperature, b

is a constant,

and V

is the volume corresponding to a monolayer. The heat of adsorption ( ∆ H )

is calculated from the gradient of a plot of ln ( V / P ) vs 1/ T . This method of calcula-

tion is known as the H - method.

Experimental results by means of the volumetric method using a conventional

BET system were observed to be Henry type in the temperature range 233 – 313 K

and the pressure range P = 5 – 30 kPa, achieving the condition of small coverage.

An example adsorption isotherm, for H - mordenite, is shown in Figure 17.3 .

In addition to the above technique, the heat of adsorption can also be determined

from Langmuir - type adsorption isotherms. Langmuir ’ s adsorption equation pro-

Figure 17.3 Adsorption isotherm of Ar on H - mordenite at various temperatures.

vides information regarding the saturated adsorption amount. It is expected that

the number of adsorbed atoms translates into the number of acid sites on the solid

surface. In this case, the heat of adsorption ( ∆ H ) is related to the adsorption con-

stant ( b ) as follows:

bb HRT=−

()

exp ∆

(17.7)

and the adsorption heat is calculated from the gradient of a plot of ln b vs 1/ T .

This method of calculation is known as the L - method.

On comparing the two methods of measurement, the advantage of the H -

method is its ease of operation and its shorter experimental time. However, the

adsorption isotherm must be measured with care, as the concentration of adsor-

bate is quite small in the low equilibrium pressure range.

The calculated heats of adsorption are summarized in Table 17.5 [125, 128] . The

heats of adsorption presented are comparable with those obtained by applying the

Henry and Langmuir equations. The relative order of SO

/ZrO

, mordenite, and

ZSM - 5 is consistent with that of the acid strength evaluated by the heat of ammonia

adsorption [93] . The value of SO

/SnO

is higher than that of SO

/ZrO

, indicating

higher acidity for the former, the strongest acidity among the solid metal oxide -

based superacids. For both materials, the heats of adsorption determined by the

H - method are lower than those by the L - method. The heats of adsorption of Ar

obtained from the L - method indicate the highest acid strength. On the other hand,

the H - method provides an average value that includes the weak acid sites, owing

to the wide pressure range used in the measurement. The saturated adsorption

amount is the number of acid sites with high acidity. In general, ammonia cannot

be used as a probe molecule for solid acids containing a component that actively

decomposes ammonia, for example platinum. The table indicates that the present

technique is applicable for metal - containing solid acids.

Table 17.5 Adsorption heats of Ar and acid amounts of various solid acids.

Sample

− ∆ H

(kJ mol

− 1

) − ∆ H

(kJ mol

− 1

) V

(mmol g

− 1

)

/SnO

23.5 29.6 0.10

/ZrO

22.4 26.3 0.009

/ZrO

21.6

/Fe

18.9

ZSM - 5 17.3 17.9 0.39

Mordenite 17.3 17.7 1.44

Beta 17.1 0.10

HY 14.8 15.6 0.35

SiO

- Al

14.4 14.9 0.35

3 wt% Pt - SO

/ZrO

22.8

7.5 wt% Pt - SO

/ZrO

24.6

a Calculated by H - method.

b Calculated by L - method.

17.3 Determination of Acid Strength 681

682 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

17.4

Nature of Acid Sites

The skeletal isomerization of butane to isobutane is a typical reaction catalyzed by

superacidity. Early in the history of this work, SO

/Fe

, SO

/TiO

, and SO

/ZrO

were termed superacids owing to their ability to isomerize butane at room tem-

perature or below [32, 37, 39] The formation of isobutane from butane, however,

does not necessarily require superacidic strength. A bimolecular reaction pathway

based on the intermediacy of butane is energetically lower than a monomolecular

mechanism [129 – 133] . The monomolecular and bimolecular mechanisms are

shown in Schemes 17.1 and 17.2 , respectively, using pentane as a model.

The processes are the monomolecular reaction through a protonated cyclopro-

pane produced by the abstraction of H

−

over Lewis acid sites and the bimolecular

mechanism where an oleﬁ n takes part in the reaction. The oleﬁ n is produced over

Br ø nsted acid sites. In the case of butane in the monomolecular mechanism, iso-

butane is formed through protonated methylcyclopropane with an activation

energy of 8.4 kcal mol

− 1

followed by the formation of the primary isobutyl cation

with high energy [134] .

Scheme 17.1 Isomerization of pentane via monomolecular intermediate.

Scheme 17.2 Disproportionation of pentane via bimolecular intermediate.

On the Br ø nsted acid sites, H

is added to the C

–

H bond of the substrate, fol-

lowed by elimination of H

and formation of the secondary butyl cation. The cata-

lyst surface would be in a proton - deﬁ cient condition owing to the elimination of

as H

. As a result, the secondary butyl cation releases H

to convert into an

alkene, which is an intermediate of the bimolecular reaction. The thermodynamic

stability of the tertiary cation is higher than that of the secondary cation. The

former cation is easily formed by methyl migration followed by a 1,2 - hydrogen

shift. The tertiary carbenium ion releases H

readily, converting into isopentene,

and this would be the intermediate of the bimolecular reaction. The reaction pro-

ceeds via the bimolecular mechanism of oligomerization – cracking involving the

formation of a C

intermediate. This is formed from a C

alkene and C

cation

followed by rearrangement and β - scission to yield isobutane as the ﬁ nal product,

as shown in Scheme 17.2 . Butane is also converted into isobutane in the same

manner.

In order to compare the catalytic action of SO

/ZrO

with that of H - mordenite

( H - Mor ), the reaction of pentane was performed in a closed recirculation system.

The changes of product yields against reaction time are shown in Figure 17.4 [135] .

The reaction rate with SO

/ZrO

at 0 ° C is almost constant during the reaction,

and the main product is isopentane, with small amounts of isobutane and hexanes

being detected.

In contrast, the reaction over H - Mor at 200 ° C has different characteristics than

that over SO

/ZrO

. H - Mor is inactive at 0 ° C, and at 200 ° C a short induction

period is observed for the catalytic activity and product selectivity. Although the

production of isopentane is predominant in the induction period, an increase in

the activity is observed along with the formation of isobutane, butane, and hexanes.

The main product is isobutane after the induction period, highlighting the effect

of Br ø nsted acid sites. The surface alkenes for the bimolecular mechanism

Figure 17.4 Isomerization and disproportionation of pentane

on SO

/ZrO

at 0 ° C and H - mordenite at 200 ° C; conversion

(䊊 ), butane ( 䊐 ), isobutane ( 䊏 ), isopentane ( 䊉 ), hexanes ( 䉭 ).

17.4 Nature of Acid Sites 683

684 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

are produced on the Br ø nsted acid sites, and alkenes accumulate on the surface

during the induction period. The activity increases with increase in the amount of

the accumulated alkenes. This leads to the conclusion that the monomolecular

reaction proceeds on SO

/ZrO

and the bimolecular mechanism proceeds on

H - mordenite.

In order to examine the nature of acid sites on SO

/ZrO

, the heats of adsorption

of N

and Ar were measured and compared with those of H - Mor and a heteropoly

acid (H

SiW

) supported on SiO

( SW/S ). Both heats plotted against the quan-

tity adsorbed are shown in Figure 17.5 [136] . The heat of adsorption of N

is larger

than that of Ar, with the difference being 2 – 3 kJ mol

− 1

on H - Mor and SW/S. Both

heats decrease gradually with increasing adsorption amounts, with an almost

constant relationship. On the other hand, SO

/ZrO

behaves differently. In par-

ticular, a very large heat of adsorption, more than 60 kJ mol

− 1

, is obtained for N

adsorption on SO

/ZrO

, which is much larger than that of H - Mor and the differ-

ence in the adsorption heats of the two gases is more than 30 kJ mol

− 1

The nitrogen molecule is strongly adsorbed on Lewis acid sites by interaction

between the 5 σ electron pair and the vacant molecular orbital of the Lewis site

[137] . The stabilization of the N

–

N bond in addition to the stabilization by adsorp-

tion on the acid sites is responsible for the large heat of N

adsorption. The acid

sites on SW/S are of Br ø nsted type [138] , and the acidity on H - Mor is also regarded

as Br ø nsted type, though a small number of Lewis sites are generated by treatment

at high temperatures [139] . This is consistent with the formation of isobutane from

pentane. Therefore it is concluded that the acidity of sulfated zirconia is of Lewis

type.

Figure 17.5 Heats of adsorption of Ar and N

on SO

/ZrO

, H - Mor, and SW/S.

17.5

Isomerization of Butane Catalyzed by Sulfated Zirconia [140]

The skeletal isomerization of butane catalyzed by SO

/ZrO

was carried out in a

closed recirculation system under mild conditions, at 0 ° C for a long reaction

period; the results are shown Figure 17.6 . A long induction period, ∼ 4 h, is

observed with the sole formation of isobutane. At longer times on stream, an

increase in the activity is observed along with the formation of propane and

pentanes. The ratio of butane to isobutane is close to the equilibrium value after

8 h. The apparent activation energy is 13.0 kcal mol

− 1

in the induction period and

8.8 kcal mol

− 1

after that period. The activation energy of the monomolecular

mechanism is higher than that of the bimolecular mechanism. The isomerization

of butane proceeds via a primary carbenium ion in the former mechanism and

via a secondary carbenium ion in the latter. The monomolecular reaction is pre-

dominant on superacidic Lewis sites in the induction period, and the reaction

changes to the bimolecular process on Br ø nsted sites by the formation of surface

alkene intermediates, giving rise to the additional C

and C

products. The

number of Br ø nsted sites over SO

/ZrO

is reduced, and the proportion of Lewis

sites is increased with increasing evacuation temperature. On the catalyst pre -

treated in vacuum at 250 ° C, some Br ø nsted acidity remains and causes the bimo-

lecular reaction. However, the activity increase disappears and only isobutane is

obtained on catalysts treated at 450 ° C or higher, which is indicative of the mono-

molecular reaction.

Figure 17.6 Reaction of butane over SO

/ZrO

at 0 ° C; the

catalyst was pre - treated at 250 ° C in vacuum for 3 h.

17.5 Isomerization of Butane Catalyzed by Sulfated Zirconia 685

686 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

17.6

Isomerization of Cycloalkanes [141]

Cyclohexane is known to be isomerized to methylcyclopentane when catalyzed by

strong acids. In fact, the SO

/ZrO

catalyst converts cyclohexane into methylcyclo-

pentane and methylcyclopentane into cyclohexane [119, 142, 143] . The reactions

proceed by the monomolecular mechanism via the intermediacy of secondary and

tertiary carbenium ions followed by protonated cyclopropanes.

The isomerization of open - chain alkanes with more than six carbon atoms gives

isobutane as the main product, together with disproportionated materials, even

though the reaction proceeds by the monomolecular pathway [144] . On the other

hand, for cyclic alkanes the monomolecular process with preservation of the cyclic

structure seems to be the most probable, judging from the results for cyclohexane.

The absence of isobutane in the products indicates that the reaction path does not

involve open - chain intermediate species. Therefore, it is of interest to try cycloal-

kanes larger than cyclohexane for clariﬁ cation of the reaction mechanism along

with the catalytic action of SO

/ZrO

The skeletal isomerization of cycloalkanes with more than six carbon atoms,

that is cycloheptane, cyclooctane, cyclodecane, and cyclododecane, was performed

over SO

/ZrO

in the liquid phase at 50 ° C.

The results for cycloheptane after 30 min reaction time are shown in Table 17.6 .

Methylcyclohexane is the major product, with a selectivity of 97%, in addition to

small amounts of four dimethylcyclopentanes and ethylcyclopentane. The reaction

of methylcyclohexane under the same conditions also produced the latter ﬁ ve

compounds, and the system reached its equilibrium state after 90 min of

reaction.

The reaction results of cyclooctane are shown in Table 17.7 . The major

product is ethylcyclohexane with a selectivity of 93%. Small amounts of ﬁ ve

dimethylcyclohexanes and methylcycloheptane were formed in addition. The

comparable reaction of ethylcyclohexane produced dimtheylcyclohexanes and

methylcycloheptane.

The reaction results of cycloheptane and cyclooctane indicate that the monomo-

lecular pathway is followed, preserving the cyclic structure, without the formation

Table 17.6 Product distribution for the reaction of cycloheptane

at 50 ° C after 30 min reaction time over SO

/ Z r O

Product Yield (%) Selectivity (%)

Methylcyclohexane 53 97

trans - 1,2 - Dimethylcyclopentane 0.7 1.3

trans - 1,3 - Dimethylcyclopentane 0.4 0.7

1,1 - Dimethylcyclopentane 0.3 0.4

cis - 1,3 - Dimethylcyclopentane 0.2 0.3

Ethylcyclopentane 0.2 0.3

of isobutane through protonated cyclopropane and cyclobutane intermediates, as

shown in Scheme 17.3 . The structure of the cyclic hydrocarbon has a large effect

on reaction. No isomerization was observed with cyclodecane, with decalins being

formed by dehydrogenation, and cyclododecane was converted into more than 30

product species by rearrangement, dehydrogenation, and cracking.

17.7

Structure of Sulfated Zirconia

In the very early stages of this work, we studied the catalytic surface using mainly

XPS and IR spectroscopy [32] , and proposed the surface structure to be SO

com-

bined with two zirconium species in a bridging bidentate state (refer to Scheme

17.4 ). An analogous model was proposed by Segawa and coworkers [53] , and Bolis

and coworkers [145] .

Table 17.7 Product distribution in the reaction of cyclooctane

at 50 ° C after 30 min reaction time over SO

/ Z r O

Product Yield (%) Selectivity (%)

Ethylcyclohexane 57 93

trans - 1,2 - Dimethylcyclohexane 2.1 3.3

Methylcycloheptane 1.1 1.7

cis - 1,3 - Dimethylcyclohexane 1.0 1.3

trans - 1,4 - Dimethylcyclohexane 0.2 0.3

1,1 - Dimethylcyclohexane 0.1 0.2

trans - 1,3 - Dimethylcyclohexane 0.1 0.2

Scheme 17.3 Intermediates for the isomerizations of

cycloheptane into methylcyclohexane and of cyclooctane into

ethylcyclohexane.

17.7 Structure of Sulfated Zirconia 687

688 17 Preparation of Superacidic Metal Oxides and Their Catalytic Action

Later, a number of studies attempted to determine the nature of acid sites in

the catalyst. Also applying XPS and IR spectroscopy, Tanabe and others proposed

a structure of chelating bidentate complexes, in which the sulfate species chelates

to a single Zr atom [102, 146] . This model, a chelating bidentate species, was also

proposed by Ward and Ko [61] .

Morrow and coworkers proposed a structure, on the basis of

O exchange using

O combined with IR analysis, in which three oxygens from the sulfate groups

are bonded to Zr species in a tridentate form [147] . They also pointed out the pos-

sibility of the formation of a polysulfate structure at high sulfate loading [148] .

This structure was supported by Morterra and coworkers using IR studies of

adsorbed pyridine [149] .

A monodentate structural model, which contains a bisulfate group, has been

proposed by several workers [103, 104] . The bisulfate OH group is hydrogen -

bonded to an oxygen on the surface of zirconia. A similar model has been proposed

for the surface of sulfated alumina on the basis of NMR studies [150] .

Another bisulfate structure was proposed by Riemer and coworkers using NMR

and Raman spectroscopies, in which two sulfate oxygens are bonded to Zr atoms

in a bridged bidentate state [106] . The same model was also proposed by Lunsford

[151] and Clearﬁ eld [152] .

Models in which SO

species are coordinated with zirconia have also been pro-

posed. One of them, suggested by Vedrine and coworkers [52] , is coordination of

the SO

sulfur with lone pairs of the oxygen in ZrO

in addition to one of the SO

oxygens coordinating with a Zr. Another proposal by White and coworkers [153]

is depicted in such a way that two of the SO

oxygens are coordinated with surface

zirconium atoms, leaving a single S

O moiety. A thionyl tetraoxide species with

four oxygens bonded to zirconia together with a single S

O has been proposed at

low sulfate content [154] .

Finally, sulfated zirconia was investigated by thermal analyses in addition to XPS

in order to provide additional information on the surface. On the basis of the

observations made, we have proposed possible surface structures of sulfated zirco-

nia [155a] . The active species of sulfated zirconia are decomposed over a quite broad

range of temperature, from 700 ° C to > 1000 ° C. XPS data indicates that the S species

comprise SO

2−

. An example of the models is shown in Scheme 17.4 , where two

oxygens are bonded to Zr in addition to coordination of an S

O group with Zr,

resulting in three grafting bonds in total. The addition of water causes the breakage

Scheme 17.4 A monosulfate structure.

of this coordination, generating Br ø nsted acid sites. Another example, consisting

of a cyclic trimer of SO

, is shown in Scheme 17.5 , where two terminal S

O anions

are bonded to Zr cations including three coordinations of S

O with Zr. These

coordination sites at Zr are also positions for water molecules giving Br ø nsted acid

sites, as shown in Scheme 17.4 in the case of monosulfate species.

The surface is composed of a wide range of coordinated oligomeric species,

predominantly species containing 3 or 4 S atoms, with two ionic bonds between

–

O − and Zr being formed, which is identical to the models for monomer and

trimer (Schemes 17.4 and 17.5 ). The active site is not on the metal species, but

rather on the S atoms.

The addition of water causes the breakage of the coordination bonds to yield

Br ø nsted acid sites strengthening Lewis acid sites, as shown in Scheme 17.4 , for

example. Many research groups report the simultaneous existence of Br ø nsted and

Lewis acid sites or the reversible transformation between Br ø nsted and Lewis

acidity upon hydration or dehydration [61, 106, 152] . Fraenkel suggests that in

order to be an effective superacid, sulfated zirconia should contain a critical

amount of moisture [155b] . Several workers propose that the strong acidity requires

the presence of both Lewis and Br ø nsted sites.

17.8

Promoting Effect

17.8.1

Effect of Addition of Metals to Sulfated Zirconia on the Catalytic Activity

A large number of metal - promoted superacids, which are highly active for butane

conversion, have been prepared by the addition of metal salts to SO

/ZrO

. Metal

promoters include Fe - Mn [156 – 162] : Pt, Pd [163] : Pd [164] : Ga, In, Tl [165] : Pt - Ni

[166] : Ni [167] : Mn [168] : Ce [169] : V, Cr, Mn, Fe, Co, Ni, Cu, Zn [170] : Pt, Pd, Ir

[171] : Al, Ga [172] : Cu [173] : Al [174, 175a – 175d] : Ni - Al [176] : Ga [177 – 181] : Pt, Nb

Scheme 17.5 A structural model of a trimer.

17.8 Promoting Effect 689