Applied Catalysis A: General 268 (2004) 25–31

In situ preparation of zirconium sulfate pillared clay:study of acidic properties

S. Ben Chaabenea, L. Bergaouia,∗, A. Ghorbela, J.F. Lambertb, P. Grangec

a Laboratoire de chimie des matériaux et catalyse, Faculté des sciences de Tunis, Université Tunis-El Manar,Campus universitaire, 1060 Le Belvédère, Tunis, Tunisia

b Laboratoire de réactivité de surface, UMR 7609 CNRS, Université Pierre et Marie Curie,4 place Jussieu, 75252 Paris Cedex 05, France

c Unité de catalyse et chimie des matériaux divisés, Université Catholique de Louvain, Croix du Sud2/17,B-1348 Louvain-la-Neuve, Belgium

Received 14 March 2003; received in revised form 14 March 2003; accepted 10 March 2004

Abstract

Sulfated zirconium clays were prepared by adding ammonium sulfate to the intercalation solution. The main controlled parameter in thisstudy is the SO4:Zr ratio in solution. The characterization of these catalysts, their acidic properties and their catalytic activities was examinedby N2-BET adsorption, chemical analysis, the adsorption–desorption of basic molecules, the conversion ofn-hexane and the transformationof isopropanol. Two different types of SO4–Zr polycation binding were advanced and correlated with the acidic properties of these solids.To enhance the acidity of the sulfated zirconium clay, the SO4:Zr molar ratio must be up than 0.125. The higher this ratio, the higher is theactivity of these solids. It appears that highly polymerized entities are more active than individual sulfated polycations.

The isopropanol dehydration to propene as well as the isomerization ofn-hexane seems to be related to the number and the strength ofBrönsted acidity.© 2004 Published by Elsevier B.V.

Keywords: Sulfated Zr-pillared clay; Acidity (TPD and IR);n-Hexane conversion; Isopropanol dehydration

1. Introduction

In order to protect the environment, several sets of regula-tions have been established. Owing to this legislation, greatinterest has been devoted to the substitution of unfriendlyand corrosive liquids (used in chemical and petrochemicalindustries for important reactions) by solid catalysts. Onthis basis, clays may constitute very promising substitutes.When inorganic species are introduced into the interlayersof the clay, the resulting nanocomposite can be used as acatalyst for specific reactions. For instance, pillared inter-layered clays (PILCs) including metal clusters such as (Zr,Al and Ti) generate micropores[1–5] larger than those ofzeolites[6,7]. Besides, clay constitutes a good and cheap

∗ Corresponding author. Present address: INSAT, Cimie et BiologieAppliques, Centre Urbain Nord, B.P. 676, Tunis 1080, Tunisia.Tel.: +216-71-703-829; fax:+216-71-704-329.

E-mail address: latifa.bergaoui@insat.rnu.tn (L. Bergaoui).

support to obtain a good dispersion of metal species. Thedispersion of sulfated metal oxide such as zirconium oxide,which is known to have a very strong acidity[8–10], andthe elucidation of their acidic properties is the aim of thiswork.

Synthesis of this type of materials has been the sub-ject of few studies[11–16]. Sulfated Zr-PILCs have beenprepared and impregnated with sulfates[11–14]. Neverthe-less, this method of sulfation leads to solids having lowsurface areas and low thermal stability of sulfates and de-veloping moderate acidity. Farfan-Torres and Grange mod-ified the acidity of the ZrOCl2-montmorillonite by adding(NH4)2SO4 during the intercalation reaction[11,13]. Theincorporation of zirconium sulfate hydroxyl complex inNa-montmorillonite using zirconium acetate as a precur-sor was also studied[15]. In this work, a new preparationmethod is investigated, including aninitial sulfation bycreation of sulfated species in the ZrOCl2 intercalationsolution.

0926-860X/$ – see front matter © 2004 Published by Elsevier B.V.doi:10.1016/j.apcata.2004.03.015

26 S. Ben Chaabene et al. / Applied Catalysis A: General 268 (2004) 25–31

2. Experimental

2.1. Preparation method

The preparation of sulfated zirconium pillared clay is anattempt to intercalate zirconium sulfate species between theclay layers. A sample of montmorillonite (KC2) was ob-tained from the CECA (France) and its<2�m fraction wasseparated by gravity sedimentation. It was then exchangedthree times with a 1 mol/l NaCl solution and washed thor-oughly. Chemical analysis yielded an exchange capacity of80 meq/100 g.

Ammonium sulfate was added to freshly prepared0.1 mol/l ZrOCl2 solutions, with SO4:Zr molar ratios vary-ing from 0 to 0.2. The solution was then refluxed during 4 h.Intercalated clays were then prepared by adding the inter-calation solution dropwise to 10 g/l clay suspensions. Theslurry was stirred for 4 h at boiling temperature, washed bysuccessive dialyses, and dried during one night at 120◦C.The samples were then calcined in flowing air. The temper-ature was raised at 60◦C/h to 500◦C, and the final temper-ature was maintained for 5 h. The term of “pillared” clayswill here be reserved to samples stabilized by calcination.The pillared montmorillonite will be called ZrP-r (whereris the SO4:Zr molar ratio in the intercalation solution).

2.2. Textural properties

Surface area measurements were performed by nitrogenphysisorption at 77 K using a static volumetric apparatus(Micromeritic ASAP 2000 adsorption analyzer); the BETequation was applied to the adsorption isotherm. All sam-ples were evacuated in vacuum at 110◦C prior to nitrogenphysisorption.

2.3. Chemical analysis

The total zirconium was analyzed by atomic adsorptionafter dissolution of the solids. Sulfur was analyzed by using aCoulomat 702 apparatus. Sulfur was thermally decomposedat high temperature, carried away by an oxygen flow anddissolved in solution. The amount of dissolved sulfur wasmeasured by a specific electrode.

The analysis of the surface for zirconium, silicon andsulfur was performed in an SSX 100/206 photoelectronspectrometer. The data treatment was performed with theCasaXPS program.

2.4. Temperature-programmed desorption of ammonia

The total acidity of the sulfated zirconium pillared clayswas determined by temperature-programmed desorption ofammonia (NH3-TPD).

Before adsorption of ammonia, the samples were treatedunder oxygen at 500◦C. The samples were then cooled downto 80◦C in He flow, then treated with NH3 pulses until sat-

uration. Weakly adsorbed NH3 was eliminated by treatmentunder He at the same temperature for 1 h.

The temperature-programmed desorption of ammonia wasperformed between 80 and 500◦C at 10◦C/min and followedby an on-line catharometer.

2.5. Pyridine thermodesorption

Surface acidity was determined using probe moleculesadsorption (pyridine). The calcined sample was evacuatedat 500◦C for 2 h, then cooled and contacted for 15 min withpyridine at room temperature. After this step, the sample wasoutgassed for 1 h, and then heated to the desired temperatureusing a linear temperature programmation. IR spectra wererecorded at each stage on a Bruker IFS 66V spectrometer.

2.6. n-Hexane conversion test

Then-hexane isomerization was performed in a microflowreactor under atmospheric pressure, using a mechanical mix-ture (1 w/w) of catalyst and a standard Pt/Al2O3 reformingcatalyst (0.35% Pt in weight). The latter was previously re-duced at 500◦C for 4 h. The catalytic test was performed asfollows: the mixture of Pt/Al2O3 + catalyst was pretreatedfirst under O2 flow at 500◦C for 1.5 h and then under H2flow for 30 min at 220◦C. H2 saturated withn-hexane waspassed over the catalyst. The analysis of the products wascarried out on-line using FID gas chromatography. At thereaction temperature (280◦C) Pt/Al2O3 alone is not active.

2.7. Isopropanol conversion test

The relative acidity of the catalysts was also evaluatedusing the 2-propanol transformation reaction. In a fixed bedreactor, 100 mg of catalyst were treated under O2 flow dur-ing 3 h at 400◦C and with a temperature ramp of 10◦C/min.The sample was then cooled under He down to 100◦C.Isopropanol-saturated helium was passed over the catalystand the reaction was followed at 100◦C using on-line chro-matography.

3. Results and discussion

3.1. Composition and textural properties of the catalysts

The amount of zirconium fixed by ZrP0 is 18.5 wt.% or25 wt.% of ZrO2 (Fig. 1). Former studies of Zr-pillared clayshave reported very different amounts of intercalated Zr, de-pending on the host clay and the pillaring method used. Inthe pioneer work of Yamanaka and Brindley, where the in-tercalation reaction was carried out at room temperature, theamount of zirconium fixed was about 13.9 wt.% of ZrO2[17]. The higher amount of fixed zirconium in our work isprobably do to the higher temperature used for the interca-lation reaction. An amount of fixed zirconium closer to our

S. Ben Chaabene et al. / Applied Catalysis A: General 268 (2004) 25–31 27

15

20

25

30

35

40

45

0 0.05 0.1 0.15 0.2

To

tal

zir

co

niu

m c

on

ten

t (w

t %

)

SO4:Zr

solution

Fig. 1. Fixed zirconium (wt.%) vs. SO4:Zr ratio in the intercalationsolution.

value has been reported where the suspension was stirred at40◦C during the intercalation[18,19].

In fact, it has been reported by Ohtsuka et al. that atroom temperature the intercalated species in a synthetic clayis the zirconium tetramer [Zr4(OH)8(H2O)16]8+, giving a7 Å interlayer spacing. More highly polymerized zirconiumspecies are generated after an elevated temperature agingstep[20]. Singhal et al. have suggested the existence in aZrOCl2 solution of an equilibrium between tetrameric andoctameric species[21]. In our case, we postulate that the in-tercalated species are octamers (Zr8) because an interlayerspacing higher than 7 Å is obtained. The speciation in solu-tion will be discussed elsewhere in more detail.

The textural and chemical properties of the catalysts arereported inTable 1. All the solids have a BET surface areabetween 200 and 273 m2/g and a microporous volume be-tween 0.03 and 0.05 cm3/g. For r values higher than 0.125a decrease in the surface is noticed whenr increases.

The zirconium and the sulfur contents (Figs. 1 and 2, re-spectively) remain constant when the sulfated zirconia pil-lared clays are prepared with SO4:Zr ratios ranging from0.05 to 0.125. For SO4:Zr ratios lower then 0.125, less thenone sulfate ion per Zr8 unit are present. This could be inter-preted by equilibrium, in the intercalation solution, betweensulfate-free Zr8 polycations and polycations with at least onesulfate attached. Since the sulfate-free polycations are morecharged than the sulfated polycations and cation-exchangeis expected to favor high-charge species, the amount of re-

Table 1Surface area and chemical analysis for different ZrPr solids

SO4:Zrmolar ratioin solution

Surfacearea(m2/g)

Si atomic% at thesurface

S/Zr atomicratio of thebulk

S/Zr atomicratio at thesurface

0 228 19.8 0 00.025 222 18.8 0.04 0.050.05 273 16.7 0.04 0.090.075 245 15.8 0.03 0.120.1 246 17.3 0.04 0.150.125 267 15.8 0.05 0.130.15 242 15.6 0.06 0.570.175 223 16.7 0.08 0.650.2 199 10.9 0.08 1.03

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.05 0.1 0.15 0.2

To

tal

sulf

ur

co

nte

nt

(wt

%)

SO4:Zr

solution

Fig. 2. Fixed sulfur (wt.%) vs. SO4:Zr ratio in the intercalation solution.

tained zirconium and sulfur can remain almost constant evenif the concentration of sulfated species increases. Forr >

0.125, the sulfur and zirconium amounts increase rapidlywhich suggest the presence of a more polymerized species.

Table 1also shows that the surface S/Zr ratios are higherthan the bulk S/Zr ratios. We have to precise here that thesurface region probed by XPS corresponds mainly to thesurface of tactoıds consisting in the stacking of several claylayers.

We have previously demonstrated for aluminum pil-lared clay, that when the intercalation solution is addedto a clay suspension, the solid flocculates quickly and theintercalation process takes place during the dialysis step[22]. The same phenomenon may be operating for sulfatedzirconium-intercalated montmorillonite. Since the unsul-fated polycations are more charged and less voluminousthan other, they have probably higher mobility during thedialysis process, and can migrate easier between clay layers.Therefore sulfated polycations should be fixed preferen-tially at the surface of tactoıds, while unsulfated polycationsare intercalated between clay layers.

For r = 0.2, the low Si content detected at the surface,the high total amounts of zirconium and sulfur and the lowsurface area suggest that a bulk Zr-containing polymericphase has been precipitated. This phase is highly sulfatedsince the S/Zr atomic ratio at the surface is equal to 1.03.

In summary, we can say that forr lower than 0.125the clay contains both unsulfated and sulfated polycationicspecies. This sulfated polycations will be called type Ispecies (Fig. 3). Whenr is higher than 0.125, the amountof SO4

2− in solution is sufficient to form bridges betweenthe zirconium polycations and develops a polymeric phase(type II in Fig. 3). For the highest amounts of SO4

2−, ahighly polymeric phase is observed. The presence of twomodes of interaction between sulfate ions and zirconiumpolycations was confirmed elsewhere[16].

3.2. TPD of NH3

The data displayed inTable 2show that the bulk con-centration of acidic sites is more important than for sulfatedzirconia [23]. Since the zirconium in pillared clay is more

28 S. Ben Chaabene et al. / Applied Catalysis A: General 268 (2004) 25–31

n Unsulfated polycation

Type I sulfated polycation

Type II sulfated polycations

Sulfate ion0.05<r<0.125 0.125<r<0.2

Fig. 3. Presentation of different zirconium species in interaction with the clay layers.

Table 2Concentration of ammonia desorbing between 80 and 500◦C for differentZrPr samples

SO4:Zr ratio in solution [NH3] (mmol/g)

0 0.7130.075 0.8030.125 0.9810.15 0.8660.175 0.8480.2 1.057

dispersed, this result is not surprising. The amounts of ad-sorbed NH3 obtained for our solids are very close to thosereported in a former study of zirconium-pillared clay sul-fated by impregnation[13].

The presence of sulfate in the intercalating solution ob-viously enhances the concentration of acid sites. It is noteasy to correlate this concentration with any single parame-ter because it depends on the amount of both zirconium andsulfur between the layers and at the surface of tactoıds, andalso on the surface area of the solids.

If we plot the surface concentration of acid sites (in�mol/m2 of solid) and the temperature of maximum des-orption versus the SO4:Zr ratio (Fig. 4), we can notice that,as long as there is no formation of a species type II (i.e.,for 0.05 < r < 0.125), the amount of adsorbed ammoniais relatively constant and the energy of interaction withthe surface does not vary either. We can hypothesize thatthe first type of bridging sulfate has not any evident effecton the number and the strength of the acidic sites. How-

1

2

3

4

5

6

80

100

120

140

160

180

0 0.05 0.1 0.15 0.2

Tem

pera

ture

of

max

imu

md

eso

rpti

on

(˚C

)

mo

l N

H3 /m

2

SO4

2-:Zr

4+

Solution

Fig. 4. The amount of retained ammonia and the temperature of maximumdesorption for different ZrPr.

14001450150015501600

fe

d

c

b

a

Weavenumber (cm-1

)

Tra

nsm

itta

nce

L

L+B

B

Fig. 5. IR spectra of ZrP0 sample after pyridine adsorption followed bydesorption at room temperature (a), 150◦C (b), 225◦C (c), 300◦C (d),375◦C (e) and 450◦C (f). B: Brönsted acidity; L: Lewis acidity.

ever, when the polymeric phase is present(r > 0.125), theamount and the strength of the acid sites abruptly increase.

3.3. Adsorption–desorption of pyridine

Pyridine has been used as a probe molecule for the deter-mination of the nature of acid sites present on the surfaceof catalysts.

Figs. 5–7present the evolution of the transmittance IRspectrum in the 1600–1400 cm−1 range upon pyridine ad-sorption followed by desorption at increasing temperature,respectively for the ZrP0, ZrP0.125 and ZrP0.2 samples.

14001450150015501600

L+B

B

Wavenumber (cm-1

)

Tra

nsm

itta

nce

fe

d

cb

a

L

Fig. 6. IR spectra of ZrP0.125 sample after pyridine adsorption followedby desorption at room temperature (a), 150◦C (b), 225◦C (c), 300◦C(d), 375◦C (e) and 450◦C (f). B: Brönsted acidity; L: Lewis acidity.

S. Ben Chaabene et al. / Applied Catalysis A: General 268 (2004) 25–31 29

14001450150015501600

L+B

Ba

b

c

def

Wavenumber (cm-1

)

L

Tra

nsm

itta

nce

Fig. 7. IR spectra of ZrP0.2 sample after pyridine adsorption followed bydesorption at room temperature (a), 150◦C (b), 225◦C (c), 300◦C (d),375◦C (e) and 450◦C (f). B: Brönsted acidity; L: Lewis acidity.

The spectrum recorded after pyridine desorption at 150◦Cshows the existence of both Lewis and Brönsted acidity. Thedecrease of the intensity of the 1448 cm−1 band after evacua-tion at 300◦C for ZrP0 and ZrP0.125 indicates that the Lewisacid sites are not very strong. For ZrP0.2 the 1448 cm−1 bandis still observed after desorption at 375◦C which indicatesa higher Lewis acidity for this solid. The Brönsted acidityof ZrP0.125 is lower than that of ZrP0. This fact suggeststhat SO4

2− substitutes groups which are responsible for theBrönsted acidity in the sulfate-free polycations. In sampleswhere we have postulated a polymeric phase (ZrP0.2), a newtype of Brönsted acidity appears, probably as a result of adifferent type of sulfate/polycation binding.

It is believed that dissolution of ZrOCl2·8H2O inwater give rise to polycations having a structure builtup from [Zr4(OH)8(H2O)16]8+ tetramers [24]. Indeed,Miehé-Brendlé and al. have shown by EXAFS that thenearly square frame of Zr4 zirconyl units is preserved afterintercalation between clay layers and calcination at moder-ate temperatures[25]. Using NMR to study the tetranuclearhydroxo zirconium complex in aqueous solution, Abergand Glaser have proposed that there are two inert and twolabile water molecules per Zr. In addition two exchangingprotons per Zr were observed and assigned to a terminalwater molecule in the tetramer[26]. They also suggest thatoctamer can form by stacking two tetramers on top of eachother and substituting some labile water molecules to givehydroxy bridges. But some labile waters are still present inthe octamer[26]. The release of the protons of labile wa-ter molecules makes the tetramer a strong acid. When thepolycation is intercalated, the corresponding Zr–OHterminalgroups are probably responsible for the Brönsted acidityobserved for ZrP0 solid.

Now, in our experimental conditions, when a low amountof SO4

2− is added to the intercalation solution, these labilegroups are probably substituted by sulfate groups. Thesesulfate groups cannot give rise to a Brönsted acidity becauseall sulfate oxygens are probably coordinated to Zr atoms(Fig. 8a). When the amount of sulfate becomes higher, a new

O

SO

O

O

Zr Zr

ZrZr

ZrZr

Zr Zr

(a) (b) (c)

SO

O

O H

O

Zr ZrO H

H O

Zr ZrO H

H O

SO

O

O

H

O

Zr ZrO H

H O

O

O

ZrO

Zr

H

H

Fig. 8. Schematic view of the first type of SO4–Zr polycations binding(a) and the second type of SO4–Zr binding (b, c).

source of Brönsted acidity appears. The type II of SO4–Zrpolycation binding provides most likely this new Brönstedacidity. In this second type of binding, two oxygen atomsmay be linked to zirconium, while the other two are free.The SO4 groups may be bridging between zirconium atomsof two different polycations which explain the formationof a high polymeric phase. The latter type of linkage, isprobably locally similar to surface groups of bulk SO4–ZrO2catalysts. An important number of studies have attemptedto determine the nature of acid sites in these catalysts[8].The Brönsted acidity can come from an OH groups linkedto zirconium (Fig. 8b). The donation ability of the hydroxylgroup is strengthened by the electron-inductive effect of theS=O double bonds in the sulfate group[27,28]. Alternatively,Brönsted acidity has been proposed to arise directly fromthe S–O–H group (Fig. 8c) in Clearfield’s model[29].

3.4. n-Hexane isomerization

The catalytic behavior of the sulfated zirconium pillaredclays in then-hexane isomerization reaction has been studiedin order to evaluate the acidity of the catalysts. The evolutionof n-hexane conversion as a function of sulfur amount isplotted in Fig. 9. A good correlation is observed betweenthe amount of fixed sulfur and the catalytic activity of theresulting solids. When SO42− groups are the first type(0 <

r < 0.125), the conversion is very low. On the other hand,the higher the amount of sulfate type II, the higher is theconversion.

The isomerization products selectivity is not correlatedwith the total amount of sulfate retained by the clay, norwith the S/Zr total ratio or the S/Zr surface ratio. In contrast,

0

5

10

15

20

0.6 0.8 1 1.2 1.4 1.6 1.8

n-h

ex

an

e c

on

vers

ion

(%

)

Total sulfur content (% wt)

0 < r < 0.125

r = 0.125

r = 0.15

r = 0.175

r = 0.2

Fig. 9.n-Hexane conversion as function of the total amount of fixed sulfur.

30 S. Ben Chaabene et al. / Applied Catalysis A: General 268 (2004) 25–31

10

20

30

40

50

60

70

0 1 2 3 4 5

Iso

meri

zati

on

Sele

cti

vit

y (

%)

Sulfur contentsurface

(atom. %)

Fig. 10. Isomerization selectivity inn-hexane transformation reactions onsulfated zirconium clays as function of the amount of fixed sulfur at thesurface.

it does appear to be positively correlated with the amountof sulfur at the surface, as illustrated inFig. 10. We canconclude that the isomerization reactions need the Brönstedacidity generated by the second type of sulfur.

Only the ZrP0.2 solids can be considered to exhibit astrong acid behavior because it has an appreciable isomer-ization activity at low temperature. In other words, it appearsthat only the polymeric phase (possessing the second typeof SO4

2−–Zr polycation bonding) is able to catalyze the iso-merization ofn-hexane efficiently. The activity of these pil-lared clay catalysts remains substantially lower than that ofbulk SO4–ZrO2 solids[30,31].

3.5. Isopropanol conversion test

The transformation of isopropanol is a widely used reac-tion to test the acid–base and redox properties of catalysts.The reaction can give diisopropyl ether, propene and ace-tone as product. Acetone is formed in the presence of ba-sic or redox sites via oxidative dehydrogenation. Ether for-mation must involve an inner-molecular coupling reaction.Many contradictory interpretations concerning the mecha-nism of isopropanol dehydration have been presented andisopropanol transformation cannot be a simple test of acid-ity [32].

In our experimental conditions only propene and ether areproduced at a temperature reaction of 100◦C. As expected,our solids do not have any basic or redox properties.

In Fig. 11 the propene activity is plotted in regardwith S/Zr surface ratio. A parallel evolution betweenthese two parameters is observed. The propene activ-ity seems to depend on the strength of acid sites at thesurface.

To confirm this observation, the activity of propene is alsoplotted for some catalysts versus the isomerization productsactivity inFig. 12. The good correlation between the propeneand the isomerization products activities shows that the tworeactions need the same active sites. Since it is assumed thatthe isomerization reactions need Brönsted sites, the samesites are involved in propene formation.

0

1

2

3

4

5

6

7

8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.05 0.1 0.15 0.2

Pro

pen

e a

cti

vit

y

(10

-8.m

ol.

g-1

.s-1

)

S/Z

r ato

m.

rati

o surf

ace

SO4:Zr

Solution

Fig. 11. Evolution of the propene activity and the total S/Zr ratio at thesurface for different SO4:Zr ratios in solution.

0

1

2

3

4

5

6

7

0 5 10 15 20

Pro

pen

e a

cti

vit

y

(10

-8.m

ol.

g-1

.s-1

)

Isomerization products activity

(10-5

.mol.g-1

.s-1

)

Fig. 12. Evolution of the propene activity as a function of the isomerizationproducts activity for different ZrPr catalysts.

4. Conclusion

The total number of acid sites as measured using a smallsized probe molecule (NH3) is very important in sulfatedZr-clay catalysts. Those sites are present in both sulfated andunsulfated Zr oxyhydroxide species. Unsulfated Zr speciesare mostly located between the layers while sulfated speciesare situated on the surface of tactoıds. Two types of SO4–Zrpolycations binding have been evidenced. The first one (typeI) does not generate any new Brönsted acidity because alloxygen atoms of SO42− are linked to zirconium ions. Thistype of binding is predominant forr < 0.125 where lessthen one sulfate ion per Zr8 unit are present. The secondtype of SO4–Zr polycation binding (type II) generates bothBrönsted and Lewis acidity; it probably corresponds to a sul-fate group bridging two zirconium atoms and still possess-ing free S–O groups. Type I species are not very active forn-hexane and isopropanol transformation. Then-hexane iso-merization occurs principally at the surface of tactoıds. Thedehydration of isopropanol seems to depend on the numberand strength of Brönsted acidic sites.

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