Properties of Modified Zirconia Used as Friedel-Crafts-Acylation Catalysts

JOURNAL OF CATALYSIS 177, 164–174 (1998)

ARTICLE NO. CA982098

Properties of Modiﬁed Zirconia Used as Friedel–Crafts–Acylation Catalysts

V. Quaschning, J. Deutsch, P. Druska, H.-J. Niclas, and E. Kemnitz¹

Humboldt-Universita¨t zu Berlin, Hessische Strasse 1-2, D-10115 Berlin, Germany; Institut fu¨r Angewandte Chemie Adlershof,

Rudower Chaussee 5, D-12484 Berlin, Germany

Received January 22, 1998; revised March 17, 1998; accepted March 17, 1998

ucts that must be destroyed by hydrolysis. Workup proce-

dures result in the loss of catalyst, acidic wastes, and corro-

sion problems. An improved procedure was demonstrated

by Effenberger et al. using only catalytic amounts of triﬂuo-

romethanesulfonic acid (4) and nonaﬂuorobutanesulfonic

acid (5) under homogeneous conditions.

Just the use of solid catalysts, instead of the above-

mentioned ones, offers some additional technical advan-

tages besides the already discussed environmental factors.

Solid acids are not corrosive and can be handled and sep-

arated easily from the liquid reaction mixtures. There are

some attempts to replace the above-mentioned Lewis or

mineral acids by solid acids like acidic zeolites in elec-

trophilic aromatic substitutions (6).

Recently, some general aspects of the use of solid acids

in environmentally clean technologies have been published

by Clark and Macquarrie (7). Therefore, only the use of sul-

fated zirconia as a heterogeneous catalyst may be reﬂected

here. Already in 1979 Arata et al. (8, 9) realized the en-

hanced acidity of solid zirconia treated with diluted sulfuric

acid and then calcined at elevated temperatures. For many

years, those sulfated zirconia were considered as solid super

acids. Today, it seems to be doubtless that sulfated zirconia

reveal high acidity, but they are not super acids (10–12).

Many publications appeared during the past reﬂecting the

wide interest in applicable solid acids and especially docu-

menting the pronounced role of sulfated zirconia (13–17).

We started to investigate the use of sulfated zirconia, em-

ploying several sulfating reagents for the formation of iso-

octanes which are of some interest as additives for gasoline

(18).

Both precipitated and aerogel synthesized zirconia were mod-

iﬁed employing gaseous and solid compounds, respectively, as

well as concentrated mineral acids. The sulfur and phosphorous-

containing samples were characterized by nitrogen adsorption, X-

ray diffraction, FTIR pyridine adsorption, ammonia TPD, and

1-butene isomerization. Modiﬁcation of zirconia samples results in

enhanced Brønsted acidity and catalytic activity. Especially am-

monium sulfate and sulﬁte generate strong acidic centers at the

surface in high concentrations. As was shown on the basis of XPS

measurements the ﬁnal calcination step in air converts the sulﬁte

into surface sulfate, resulting ﬁnally in the same, or at least very

similar, surface states of both differently treated samples. Some few

degrees enhanced catalytic activities were obtained using sulfated

zirconia aerogels. Applied to the benzoylation of anisole, a slurry

of these catalytically active zirconia samples yields similar high

conversion degrees as nonaﬂuorobutanesulfonic acid known as an

excellent catalyst for this reaction in homogeneous solution. There

is evidence to assume that the benzoylation reaction under the con-

ditions used occurs exclusively at Brønsted acid sites but not at

Lewis sites. A hypothetical path for the action of a sulfated zirconia

surface in the catalytic step is proposed.

ꢀc 1998 Academic Press

Key Words: modiﬁed zirconia; aerogel; acylation catalyst;

Brønsted acidity.

INTRODUCTION

Increasingly demanding environmental pressure and the

resulting drive towards clean technologies force us to de-

velop ecologically friendly alternatives. There are at least

two different ways, to synthesize alternative products or to

ﬁnd quite new clean synthesis routes to the same product.

A considerable part of synthesis processes in the chemi-

cal industries is based on acid catalyzed reactions. Com-

monly, these reactions are classiﬁed in Lewis and Brønsted

catalyzed ones. For both kinds there are numberless exam-

ples using homogeneous reaction conditions. These include

Friedel–Crafts reactions, where Lewis (1) or Brønsted acids

(2, 3) are used in stochiometric or even excess amounts

to achieve satisfactory reaction rates and yields. Homoge-

neous catalysts form strong complexes with reaction prod-

This paper deals with modiﬁed zirconia catalysts used for

acylation reactions which are of some technical importance.

EXPERIMENTAL

Sample Preparation

Standard procedure. I. Preparation of zirconia oxides.

Zirconia oxide was prepared by adding aqueous ammonia

to an aqueous solution of ZrOCl₂until a pH of 8 was

achieved. The solid was ﬁltered, washed several times, and

¹To whom correspondence should be addressed.

164

0021-9517/98 $25.00

c

Copyright ꢀ 1998 by Academic Press

All rights of reproduction in any form reserved.

FRIEDEL–CRAFTS–ACYLATION CATALYSTS

165

dried at 110^ꢁC for 15 h (ZR/1). A sample of this oxide was ﬂuoride contents were measured according to the Seel

calcined at 500^ꢁC in air (ZR/c1). Additionally, a ZrO₂sam- method (20).

ple was prepared by hydrolyzing Zr(i-OPr)₄, followed by

The surface analytical studies were performed by an

ESCALAB 220 iXL spectrometer (Fisons Instruments)

consisting of two vacuum chambers: the analyser and the

fast entry air lock/preparation chamber. The powdered

samples were ﬁxed on a carbon tape (carbon conduc-

tive tape, Pelco International) at the top of the sample

holder and transferred into the UHV. The X-ray source

was monochromatic focused AlKꢀ radiation (1486.6 eV)

with an input power of 150 W. The emerging charge of the

sample was equalised with the installed charge compensa-

tion. The ﬁnal peak position was determined using the C 1s

peak (shifted to 285.0 eV) corresponding absorbed carbon

species. The XPS-measurements were performed at a con-

stant pass energy of 25 eV. The ESCALAB was calibrated

routinely with the appropriated XPS lines of Au, Ag, and

Cu, as given in Ref. (21).

For determining the nature of the acid sites (LZ, Lewis

acid sites; BZ, Brønsted acid sites) all samples were

characterized by FTIR photoacoustic spectroscopy with

chemisorbed pyridine. In detail, about 70 mg of the sam-

ple were pretreated under nitrogen (35 ml/min) at 150^ꢁC;

30 ꢁl of pyridine were adsorbed after 15 min at this tem-

perature. Each sample was ﬂushed with nitrogen for an-

other 15 min to remove physisorbed pyridine. Spectra of the

sample (pyridine-loaded sample and the unloaded sample,

background) were taken at room temperature over a range

of 4000–400 cm^ꢅ1, using a MTEC-cell and FTIR system

2000, Perkin Elmer. All spectra were normalized, setting

the almost strongest peak in the zirconia system at 530–

630 cm^ꢅ1to 100% transmission and multiplying the whole

spectrum by this factor. To compare the amount of acid sites

of the different samples we determined the intensity of the

bands at about 1450 cm^ꢅ1(LZ) and 1490 cm^ꢅ1(LZ + BZ)

of the normalized spectra. If there are no BZ at the sur-

face the intensity of the 1490 cm^ꢅ1vibration is one-third of

that of the 1450 cm^ꢅ1band. Therefore the experimentally

determined height of the 1490-cm^ꢅ1band was reduced by

one-third of the 1450 cm^ꢅ1band height to get the BZ in-

tensity.

drying and ﬁnal calcination at 550^ꢁC (ZR/c2).

II. Synthesis of modiﬁed zirconia. The sulfation of zir-

conia was performed

ꢂ by kneading ZrO₂ꢃ ꢄ H₂O (sample ZR/1) thor-

oughly with solid (NH₄)₂SO₄(ZR/amSO₄) or solid

(NH₄)₂SO₃(ZR/amSO₃) or by kneading anhydrous ZrO₂

(sample ZR/c2) with (NH₄)₂SO₄(ZR/c-amSO₄), respec-

tively. The three samples were calcined at 500^ꢁC in air for

1 h.

ꢂ by passing an air ﬂow, saturated with SOCl₂

(ZR/SOCl₂) or SF₄(ZR/SF₄), respectively, for 2 h at 150^ꢁC.

A subsequent air ﬂow was used to remove physisorbed

gaseous products.

In some cases the catalysts were investigated in relation

to a commercially available sulfated zirconia catalyst of the

MELgroup(MELXZO682/01, MagnesiumElektronLtd.),

hereafter noted as ZR/com.

Sol–gel procedure. Zirconia aerogels were prepared

following the standard procedure described by Ward and

Ko (13). In a glovebox Zr(n-OPr)₄(70 wt% in propanol)

was diluted in a mixture of propanol and nitric acid. The

water–alcohol mixture, prepared in a second beaker, was

added to the alkoxide–alcohol–acid solution and stirred un-

til gelation occurred. After an aging period of about 30 min

the gel was put into an autoclave. The aerogels were pre-

pared by heating these mixtures up to 300^ꢁC and 160 bar

and maintaining the supercritical conditions for 1 h. The al-

cohol and further components were removed by controlled

expansion. The whole procedure is described in detail else-

where (19). After removal of the solvent in the ﬁrst heat

treatment step the aerogel powder was dried under vac-

uum at 300^ꢁC, followed by ﬁlling the system with nitrogen.

These two steps were repeated several times. The calcina-

tion step was performed under ﬂowing air for 2 h at about

500^ꢁC for pure and phosphated zirconia and at about 550^ꢁC

for sulfated zirconia, respectively.

The components Zr(n-OPr)₄/water/propanol/acid were

used in an overall molar ratio of 1 : 3.6 : 13.7 : 0.8. Modiﬁed

aerogels were prepared by replacing the appropriate molar

amount of nitric acid by sulfuric or phosphoric acid.

The temperature-programmed desorption (TPD) of am-

monia was used to characterize the acid strength distribu-

tion of the solids. The desorbed ammonia was absorbed in

0.1 N sulfuric acid and than titrated, allowing determination

of the overall amount of acid sites of the solids.

Inanickeltubeapproximately300mgofthegrainedcata-

lyst (0.4–0.5-mm diameter fraction) was pretreated under

nitrogen (70 ml/min) up to 500^ꢁC. Following the sample was

cooled to 120^ꢁC and exposed to a stream of nitrogen and

ammonia. The physisorbed ammonia was removed over 1 h

at 120^ꢁC. After cooling down to 80^ꢁC the TPD program

(10 K/min, up to 600^ꢁC) was started. The desorption of am-

monia was detected by continuous running IR spectroscopy

Sample Characterization

Crystallographic identiﬁcation of the samples was per-

formed using X-ray powder diffraction with CuKꢀ radia-

tion (XRD 7, Rich. Seifert & Co., Freiberg). The speciﬁc

surface areas were measured using nitrogen adsorption

at 77 K by the standard BET method (ASAP 2000

system, Micromeritics). The sulfur, phosphor, and chlo-

rine contents were determined by elemental analysis. The

166

QUASCHNING ET AL.

(FTIR system 2000, Perkin Elmer). The band at 930 cm^ꢅ1tion time of 7 h the analysis of the products was performed

was used to obtain the ammonia desorption proﬁle. Behind gaschromatographically, using a HP 5890 A with a ﬂame-

the IR-cell an absorption cell was situated, containing a ionization detector (column: Rtx 1, methylsilicon, Restec

known amount of sulfuric acid. The amount of desorbed corp., I.D. 0.25 mm, length, 30 m). Conversion is given as

ammonia was determined by a titration with sodium hy- the conversion into the products, p- and o-methoxybenzo-

droxide to get the quantitative amount of acid sites at the phenone.

catalysts’ surface.

For comparison, typical solid Lewis acids like ꢂ-AlF₃(22)

and calcined ꢃ -Al₂O₃were included into the measurements

of the acylation activity.

Catalytic Reactions

I. Double-bond-isomerization of 1-butene as probe

reaction. Butene isomerization was performed in a down-

ward-ﬂow, ﬁxed-bed glass-reactor. Approximately 250 mg

of the grained catalyst (0.4–0.5-mm diameter fraction)

was pretreated under nitrogen (10 ml/min) for 20 min at

100^ꢁC. At this temperature the sample was exposed to the

feed stream mixture of nitrogen (10 ml/min) and 1-butene

(0.6 ml/min). A residence time of 1.2 s was set up. The com-

position of the product stream at the exit of the reactor,

consisting of 1-butene and E/Z-2-butene, was determined

by a Shimadzu gas chromatograph GC-14A with a ﬂame-

ionization detector (column: Di-n-propylsulfon on 0.2–

0.4-mm Poralith, I.D. 2.4 mm, length, 4 m) after 3, 6,

9, . . . min, time-on-stream. Conversion is given as the con-

version into the product 2-butene.

RESULTS

Bulk Properties of the Zirconia Samples

Tables 1 and 2 summarize the notations and some charac-

teristics of the catalysts prepared by standard and aerogel

synthesis.

Standard preparation. It was veriﬁed that a drastic de-

crease in the speciﬁc surface area occurred after calcination

of pure Zr(OH)₄at about 773 K (340 → 40 m²/g) due to the

crystallization process. Exclusively the monoclinic crystal-

lized ZrO₂was detected. The sulfate and sulﬁte treatment

of amorphous Zr(OH)₄reduced the decrease in the speciﬁc

surface area (150 m²/g) and led to the tetragonal crystallized

phases, whereas the sulfate treatment of monoclinic ZrO₂

II. Benzoylation of anisole. The reaction between no longer showed this effect on crystallization and the sur-

anisole and benzoic anhydride or benzoyl chloride was car- face area. A modiﬁcation with gases at comparatively low

ried out in liquid phase in a round-bottom 50-ml 3-necked temperatures resulted in some amorphous material with a

ﬂask, provided with a magnetic stirrer, a thermometer, and large surface area in the case of SOCl₂and a small surface

a reﬂux condenser with a CaCl₂-tube. For each reaction area in the case of SF₄, respectively. The sulfur content was

a mixture of 12 ml of anisole, 0.00265 mol of the acyla- about 1% (gaseous modiﬁcation) to 4% (solid sulfate treat-

tion agent, 0.2 g of the catalyst, and 0.1 g of n-tetradecane ment). An enrichment of sulfur and ﬂuorine at the surface

added as an internal standard for the GC-detection was was detected by comparison between elemental analysis

used. The temperature was kept at 140^ꢁC. After a reac- (bulk) and XPS-measurements (surface).

TABLE 1

Preparation Conditions and Sample Characteristics of Modiﬁed Zirconia Samples

Modifying

agent

Temperature

(^ꢁC)

X-ray detected

phases

Speciﬁc surface

area (m²/g)

S-content bulk/

surface (wt%)

X-content bulk/

surface (wt%)

Sample

ZR/1

—

110

500

Amorphous

341

40.5

—

ZR/c1

Monoclinic, some

tetragonal ZrO₂

Monoclinic ZrO₂

Tetragonal ZrO₂

ZR/c2

ZR/amSO₄

—

(NH₄)₂SO₄

solid

(NH₄)₂SO₃

solid

(NH₄)₂SO₄

solid

SOCl₂

550

500

38.9

139

—

4.2

surface: 9.0

3.4

surface: 4.7

2.5

surface: n.d.

1.7

—

ZR/amSO₃

ZR/c-amSO₄

ZR/SOCl₂

ZR/SF₄

500

150

Tetragonal ZrO₂

Monoclinic ZrO₂

Amorphous

157

28.9

367

40.4

—

Cl: 7.5

surface: n.d.

F: 23

gaseous

SF₄

surface: 2.4

0.9

Amorphous

gaseous

surface: 1.2

surface: 31

Note. n.d., not determined.

FRIEDEL–CRAFTS–ACYLATION CATALYSTS

167

TABLE 2

Preparation Conditions and Sample Characteristics of the Aerogel Samples before and after the Calcination Step

Contents of

Modifying

agent

Temperature

(^ꢁC)

X-ray detected

phases^a

Speciﬁc surface

area (m²/g)

CHN

S

P

Sample

(wt%)

A

A/SO₄

A/PO₄

—

H₂SO₄

H₃PO₄

300

Tetragonal ZrO₂

232

346

306

7.5

7.6

6.7

—

3.8

—

0.6

A/c

—

500

Tetragonal, some

monoclinic ZrO₂

Tetragonal ZrO₂

134

<1.2

—

A/c-SO₄

A/c-PO₄

H₂SO₄

H₃PO₄

550

500

138

267

<1.2

1.6

—

1.6

^aAll aerogel samples are not crystalline but show broad and low-intensity reﬂexes.

Aerogel samples. All starting aerogel samples are ma-

Two characteristic proﬁles of the ammonia TPD spectra

terials with very large speciﬁc surface area. They are not are given in Fig. 2. The spectra are normalized allowing a

crystalline but show broad and low-intensity reﬂexes in direct comparison of the investigated samples by compar-

the diffraction pattern. The calcination at about 500^ꢁC— ing the areas under the curves which are proportional to

in order to reduce the content of organic compounds— the concentration of acid sites (given in millimoles of am-

also leads to a decrease in the speciﬁc surface area. But monia per gram of sample). A typical TPD proﬁle for an

even after calcination the pure zirconia aerogel has quite a unmodiﬁed zirconia, like for the sample ZR/c2, has only

high BET area of 140 m²/g, in comparison to the results of

the standard preparation. The speciﬁc surface area of the

calcined sulfated aerogel is also—despite the 50-K higher

temperature—in the same range as that obtained by a stan-

dard preparation of sulfated zirconia. The phosphated zir-

conia aerogel exhibits a high BET area of 267 m²/g after

the calcination at 500^ꢁC. Again, at this temperature the

pure zirconia aerogel already contained monoclinic crys-

tallized components, whereas the modiﬁed aerogels crys-

tallized only in the tetragonal form.

Acidic Properties

Two methods were used to characterize the acidic prop-

erties of the samples: FTIR photoacoustic spectroscopy

was carried out in order to distinguish between Lewis and

Brønsted acid sites. The temperature-programmed desorp-

tion (TPD) of ammonia was used to characterize the acid

strength distribution of the solids and, furthermore, to ob-

tain the quantitative amount of acid sites in the speciﬁed

temperature range.

The FTIR photoacoustic spectra of pyridine, chemis-

orbed at the solid surfaces, are shown in Fig. 1. The quantita-

tive results are listed in Table 3. The surface of unmodiﬁed

zirconia (ZR/1, ZR/c1, ZR/c2, A/c) contains only Lewis

acid sites. The sulfur treatment results in the formation

of Brønsted acid sites for all other samples. The biggest

quantity of Brønsted acidity can be achieved as a result

of modiﬁcation using SF₄(ZR/SF₄). Furthermore, there is

no signiﬁcant wavenumber shift for the pyridine adsorbate

complexes. Consequently, the strength of the acid sites of

all samples is comparable (cf. also TPD proﬁles).

FIG. 1. Normalized FTIR photoacoustic spectra of pyridine chemi-

sorbed at zirconia surfaces (70 mg catalyst weight, 30 ꢁl pyridine adsorp-

tion at 150^ꢁC, ﬂow system).

168

QUASCHNING ET AL.

TABLE 3

Quantitative Results of the FTIR Photoacoustic Spectroscopy of Pyridine Adsorbat Complexes and of the Ammonia-TPD

NH₃-TPD concentration/

density of acid sites

Preparation

FTIR-PAS

temperature

Speciﬁc surface

area (m²/g)

a

Sample

(^ꢁC)

LZ (1450 cm^ꢅ1

17.5 (high)

)

BZ (1490 cm^ꢅ1

)

(mmol/g)

(ꢁmol/m²)

ZR/1

ZR/c1

ZR/c2

ZR/amSO₄

ZR/amSO₃

ZR/c-amSO₄

ZR/SOCl₂

ZR/SF₄

110

500

550

500

150

341

40.5

38.9

139

157

28.9

367

40.4

0.0 (low)

0.28

0.16

0.30

0.33

0.09

n.d.

0.82

6.91

4.11

2.16

2.10

3.11

n.d.

8.5 (medium)

9.0 (medium)

0.0 (low)

14.0 (high)

12.0 (high)

5.5 (small)

24.5 (high)

0.0 (low)

13.0 (high)

0.0 (low)

64.0 (very high)

A/c

A/c-SO₄

A/c-PO₄

500

550

500

134

138

267

27.5 (high)

9.5 (medium)

20.5 (high)

0.0 (low)

13.0 (high)

4.0 (small)

0.38

0.32

0.52

2.84

2.32

1.95

Note. n.d., not determined, ammonia-TPD measurements were not possible.

^aClassiﬁcation of the intensities in the FTIR spectra: 0 ꢃ ꢃ ꢃ 2, low; 2 ꢃ ꢃ ꢃ 6, small; 6 ꢃ ꢃ ꢃ 12, medium; 12 ꢃ ꢃ ꢃ 30, high; >30, very high.

one maximum at a temperature of about 240^ꢁC. The proﬁle ZR/SOCl₂and ZR/SF₄. These two samples were prepared

of the sulfated zirconia sample, e.g. ZR/c-amSO₄, is more by a low temperature modiﬁcation with SOCl₂or SF₄. The

differentiated, exhibiting two maxima at temperatures of original gases as well as their hydrolysis and decomposition

about 220 and 340^ꢁC. The amount of desorbed ammonia products were released during the progress of the temper-

and, hence, the total number of acid sites is not higher but ature program of the ammonia desorption. A reaction be-

comparable for both the sulfated and the unmodiﬁed sam- tween the gases and ammonia was observed (probably the

ple (cf. Table 3). However, there is a distinct difference in formation of ammonium halides). Therefore, it was impos-

the distribution of the acid strength. In relation to the ZR/c2 sible to determine the acidity of these samples on basis of

sample the sulfated sample ZR/c-amSO₄clearly provides a TPD measurements. However, all other samples were in-

higher concentration of stronger acid sites (cf. Fig. 2). This vestigated by NH₃-TPD and provided ammonia desorption

fact is demonstrated by the larger area under the desorption proﬁles which were classiﬁed in one of these two modes. All

curve in the range between 450 and 550^ꢁC.

sulfated and the sulﬁted zirconia (ZR/amSO₄, ZR/amSO₃,

It was not possible to get the desorption proﬁles and the ZR/c-amSO₄, A/c-SO₄) were similar and showed a two-

quantitative amount of desorbed ammonia of the samples maxima-type of TPD-proﬁle with a higher concentration

of stronger acid sites. The other samples, unmodiﬁed ZR/1,

ZR/c1, ZR/c2, and A/c and the phosphated aerogel A/c-

PO₄, gave desorption proﬁles containing only one maxi-

mum and more weaker acid sites.

The results of the ammonia-TPD of all zirconia

samples—theconcentrationofacidsitesandtheirdensity—

are summarized in Table 3. Comparing the quantitative re-

sults of the ammonia-TPD, the concentration of acid sites is

approximately 0.3 mmol/g for each modiﬁed and unmodi-

ﬁed sample. Only the ZR/c2 and the resulting ZR/c-amSO₄

sample possess a lower concentration, due to a lower spe-

ciﬁc surface area. All aerogel samples reveal higher con-

centrations of acid sites and a larger surface area. For that

reason, the normalized concentration of acid sites of the

catalysts are given as speciﬁc concentrations. The samples

exhibit a mean concentration of acid sites between 2 and

3 ꢁmol/m². Lower values were found for the starting mate-

rial ZR/1, probably caused by its extremely high surface

area. Higher values of the acid site concentration were

found for the calcined zirconia ZR/c1 and ZR/c2 with a

FIG. 2. Normalized ammonia TPD proﬁles of unmodiﬁed and sul-

fated zirconia.

FRIEDEL–CRAFTS–ACYLATION CATALYSTS

169

aerogel A/c-SO₄, the ammonium sulﬁte (ZR/amSO₃),

and ammonium sulfate treated samples (ZR/amSO₄and

ZR/c-amSO₄). But even the SF₄modiﬁed zirconia was a

good catalyst for this reaction due to its very large amount

of Brønsted sites detected by FTIR spectroscopy.

TABLE 4

Catalytic Activity of the Zirconia Samples

Butene

isomerization

Reaction between anisole and

(conversion into Benzoyl chloride Benzoic anhydride

All catalyst samples were also tested in the benzoyla-

tion of anisole. The conversion degrees of the reaction be-

tween benzoyl chloride or benzoic anhydride and anisole

forming p- and o-methoxybenzophenone are shown in

Table 4. The commercial catalyst ZR/com and the solid

Lewis acids ꢂ-AlF₃and ꢃ -Al₂O₃were also included for

comparison. Again all pure zirconia samples (ZR/1, ZR/c1,

ZR/c2, and A/c) were catalytically inactive regarding the

acylation reaction. Only little activity was observed using

the phosphated zirconia aerogel A/c-PO₄(in case of the

acyl chloride) as well as the samples ZR/SOCl₂and ZR/SF₄.

Best results were obtained using the sulfated zirconia aero-

gel. The sulfated and the sulﬁted zirconia were also very

good catalysts, depending somehow on the kind of the acy-

lating agent. The ZR/com sample showed a conversion of

about 60% under the conditions used. Finally, some of our

samples have the same or a better catalytic activity regard-

ing the benzoylation of anisole than the commercially avail-

able catalyst.

Sample

ZR/1

ZR/c1

ZR/c2

ZR/amSO₄

ZR/amSO₃

ZR/c-amSO₄

ZR/SOCl₂

ZR/SF₄

2-butene in %)

(conversion into ketones in %)

0

66

90

82

18

56

0

76

62

49

1

80

33

20

1

12

17

A/c

A/c-SO₄

A/c-PO₄

0

93

0.7

0

86

7

0

95

0

ZR/com

n.d.

57

65

ꢂ-AlF₃

ꢃ -Al₂O₃

0

n.d.

0

2

0

n.d.

Note. Isomerization of 1-butene to E-/Z-2-butene (T = 100^ꢁC, resi-

dence time = 1.2 s) and benzoylation of anisole after 7 h at T = 140^ꢁC.

n.d., not determined.

AlF₃and Al₂O₃were hardly or not able to catalyze the

Friedel–Crafts acylation under the same conditions. Both

comparative samples exhibit only Lewis acidity.

The dependency of the conversion degree of the acyla-

tion reaction on the time and on the reaction temperature

was investigated for some representative zirconia samples.

Figure 3 illustrates the conversion degree in dependence on

the time of reaction employing the ZR/com sample and the

quite low speciﬁc surface area. Obviously, the creation of

high surface material does not result in a proportionally

enhanced number of acid sites at the surface.

Catalytic Properties

Two reactions were used to evaluate the catalytic activ-

ity of the modiﬁed and unmodiﬁed zirconia samples: the

isomerization of 1-butene to E/Z-2-butene and the ben-

zoylation of anisole to 4- and 2-methoxybenzophenone.

The double-bond isomerization of 1-butene depends on the

presence of Brønsted acid sites. At relatively low tempera-

tures (100^ꢁC) E/Z-2-butene is the only product. This test

reaction was consequently used in order to characterize

the Brønsted acidity. The reaction between benzoyl chlo-

ride or benzoic anhydride with anisole is of some applica-

tional interest. Friedel–Crafts reactions are commonly con-

sidered to be Lewis acid catalyzed reactions, but principally

they can occur at Brønsted sites (H₂SO₄, CF₃SO₃H), too

(3–6).

The results of the double-bond isomerization of 1-butene

are given in Table 4. Regarding this reaction, unmodiﬁed

zirconia is catalytically inactive. This fact does not de-

pend on the preparation conditions like the calcination

temperature or the procedure of synthesis (aerogel syn-

thesis providing high speciﬁc surface area samples). The

phosphate modiﬁed zirconia aerogel is also catalytically

inactive. In contrast, all of the sulfur containing samples

catalyze the isomerization of butene. Best results were

obtained using the sulfated zirconia samples: the sulfated

FIG. 3. Reaction between anisole and bezoic anhydride on ammo-

nium sulfate modiﬁed zirconia ZR/amSO₄in relation to Zr/com catalyst:

dependency of the conversion degree on the reaction time at two different

temperatures.

170

QUASCHNING ET AL.

As expected, pure zirconia crystallizes in the monoclinic

modiﬁcation, exhibiting a comparatively low BET surface

area (cf. Table 1). On the contrary, modifying Zr(OH)₄with

solid ammonium sulfate or sulﬁte results in the tetragonal

crystallized modiﬁcation with a larger surface area. Proba-

bly, the presence of sulfate or related compounds prevents

the zirconia from an undisturbed crystallization and from a

decrease of the speciﬁc surface area. The modiﬁcation with

gaseous reagents like SOCl₂or SF₄at 150^ꢁC leads to the

formation of amorphous materials.

Similar results were obtained with the aerogel phases. All

samples are less crystalline (tetragonal zirconia) and exhibit

a large speciﬁc surface area after the ﬁrst thermal treatment

at 300^ꢁC. Again, the pure zirconia aerogel contains some

monoclinic components after the calcination step, whereas

the modiﬁcation prevents it from a transformation, as was

already found by Ward and Ko (13), who investigated ex-

tensively the dependencies of aerogel properties on several

synthesis parameters. Even after calcination pure zirconia

aerogel exhibits a quite large BET surface area of about

140 m²/g as opposed to ZR/c1 and ZR/c2 which show only

about 40 m²/g. The speciﬁc surface areas of the calcined sul-

fated aerogels are, despite the 50 K higher temperature, in

the same range as those for the modiﬁed zirconia samples

ZR/amSO₄and ZR/amSO₃(140 m²/g). The phosphated

zirconia aerogel reveals a very high BET surface area of

267 m²/g after a calcination at 500^ꢁC.

It is remarkable that the samples ZR/amSO₄and ZR/

amSO₃exhibit very similar properties. Their powder

diffraction patterns are equal and also the sulfur contents

and BET surface areas, respectively, are the same. Identi-

cal FTIR spectra and similar results for the ammonia des-

orption were obtained. The catalytic activities are compa-

rable, too. These two samples were modiﬁed using solid

ammonium salts: the sulfate in the case of ZR/amSO₄and

the sulﬁte in the case of ZR/amSO₃. We suppose the rea-

son for these similarities is an oxidation of the sulﬁte (S^IV)

resulting in the formation of surface sulfate (S^VI). This is

very probable under the conditions used for the sample

preparation (calcination at high temperatures in presence

of air). Evaluation of the XPS measurements gave some

interesting effects (Fig. 5). Irrespective of their prepara-

tion with SOCl₂(ZR/SOCl₂), SO²₃^ꢅ(ZR/amSO₃), and SO²₄^ꢅ

(ZR/amSO₄), the sulfur signal shows a peak position of S^VI;

that means an oxidation of a lower valent sulfur species.

In contrast, a modiﬁcation with SF₄(ZR/SF₄) did not re-

veal any change of the binding energy of sulfur, show-

FIG. 4. Benzoylation of anisole on ammonium sulfate modiﬁed zir-

conia ZR/amSO₄in relation to Zr/com catalyst: dependency of the con-

version degree on the temperature after 7 h reaction time, using benzoic

anhydride (᭿ and ᮀ) and benzoyl chloride (᭡ and 4) and anisole as

reagents.

ammonium sulfate modiﬁed zirconia ZR/amSO₄. The reac-

tion between benzoic anhydride, commonly considered to

be less reactive in relation to benzoyl chloride, and anisole

was carried out at 100 and 140^ꢁC, respectively. ZR/amSO₄

results in a higher yield of ketones at both temperatures

than the ZR/com catalyst. High conversion degrees were

already achieved for the ZR/amSO₄sample after 2 (140^ꢁC)

or 3 h (100^ꢁC) reaction time. Using the ZR/com catalyst, at

least 4 h at 140^ꢁC were necessary to get similar yields of the

ketone.

The ZR/amSO₄catalyst is also able to catalyze the ben-

zoylation of anisole at lower temperatures. Figure 4 shows

the conversion results in dependence on the reaction tem-

perature after 7 h. It is remarkable that, particularly when

using the acid anhydride in the reaction, the conversion de-

gree is still very high at 60^ꢁC only. For example, the ZR/com

catalyst yields, even at 80^ꢁC, only about 18% using benzoyl

chloride and only 7% if the anhydride is used for the acy-

lation reaction. Therefore, the ketone yields are constantly

higher for the ZR/amSO₄catalyst than in the case of the

ZR/com catalyst.

DISCUSSION

Surface and Bulk Properties of the Zirconia Samples

Due to the variety of modiﬁcation reagents zirconia sam- ing clearly that no oxidation happened under these con-

ples with quite different properties were synthesized and ditions.

tested. Two preparation methods (standard and aerogel

The (NH₄)₂SO₃-modiﬁed sample ZR/amSO₃shows a sig-

synthesis) and the different modiﬁcation reagents inﬂu- niﬁcant shift of its XPS Zr 3d signal to higher binding en-

ence not only the bulk properties but also the acidity of ergies, in comparison with the signal of unmodiﬁed ZrO₂.

the samples and, therefore, their applicability as solid acid The same experimental results were achieved using SOCl₂

catalysts.

and SF₄, respectively, as modifying agents. In contrast, the

FRIEDEL–CRAFTS–ACYLATION CATALYSTS

171

FIG. 5. Selected XP spectra of pure and surface modiﬁed zirconia catalysts, given in arbitrary units: left, Zr 3d signal; right, S 2p signal. Dotted

lines give the peak positions of typical Zr(IV), S(IV), and S(VI) compounds, respectively.

(NH₄)₂SO₄-modiﬁed sample has the same peak position A Correlation between Brønsted Acid Sites

like unmodiﬁed zirconia. The peak shift to higher binding

energies is an indication for a lower electron density at the

surface zirconium atoms, tantamount to a higher acidity at

the surface.

and the Catalytic Activity

Figure 6 illustrates the dependencies of the observed con-

version degrees of the isomerization of 1-butene on the

kind and the amount of acid sites of the solids. The sample

ZR/SF₄is not plotted in this ﬁgure because of its very high

overall Brønsted acidity mainly inﬂuenced by adsorbed HF,

as discussed above. For comparison, the data for this sample

are: LZ, 0; BZ, 64; conversion of 1-butene, 56%.

The FTIR photoacoustic spectra of the pyridine ad-

sorbate complexes (cf. Fig. 1) of the modiﬁed samples

ZR/SOCl₂and ZR/SF₄show a very large amount of

Brønsted acid sites. Unfortunately, a distinct determination

of the acid strength by ammonia TPD was impossible due to

their preparation at 150^ꢁC only. During heating the release

of hydrogen halides (HX) was observed. It is not entirely

clear whether adsorbed HX is already present at the sur-

face, formed by the reaction of SF₄and SOCl₂with surface

hydroxyl groups during the treatment, or if it will be formed

–

as a result of hydrolysis of Zr X bonds during calcination.

Water for hydrolysis can be originated either by desorption

or from surface hydroxyl groups. Therefore, both adsorbed

HX and surface OH groups cause a high concentration of

Brønsted acid sites.

Whereas HCl is commonly more weakly adsorbed at sur-

face sites, HF should result in stronger bonded surface com-

plexes being consequently able to contribute to the surface

Brønsted acidity of these samples. However, it represents

only a weak Brønsted acid. Even the lower catalytic activ-

ity of these samples in the butene isomerization (ZR/SF₄,

about 56%) and in the acylation reaction (only about 15%)

is obvious on this basis because adsorbed HF should exhibit

a lower acidity than sulfate species on the surface, although

theoverallconcentrationofBrønstedacidsitesisverylarge.

FIG. 6. Conversion degrees for the butene isomerization in a Lewis

versus Brønsted acidity presentation. The conversion degrees are reﬂected

by the height of the columns, whereas the acidic properties deﬁne the

position of zirconia samples: (a) A/c; (b) ZR/1; (c) ZR/c2; (d) ZR/c1;

(e) A/c-PO₄; (f) ZR/SOCl₂; (g) ZR/c-amSO₄; (h) A/c-SO₄; (i) ZR/amSO₃;

(k) ZR/amSO₄.

172

QUASCHNING ET AL.

All samples which have only Lewis acid sites are unable exhibit two maxima. For the inactive samples of group A,

to catalyze the isomerization reaction (points on the left- there is only one maximum being broader and located with

hand side of Fig. 6). As expected, all samples with Brønsted the peak maximum at slightly lower temperatures. Addi-

acid sites are good catalysts for the butene isomerization tionally, the proﬁles are not differentiated evidently. More

(columns on the right-hand side), as was also reported by important, in relation to group A samples more ammonia is

Ward and Ko (14, 15). On this basis it is possible now to desorbed at higher temperatures (about 500^ꢁC) from those

summarize all zirconia samples in three groups:

catalysts which belong to group B. Therefore, the catalyti-

cally active samples possess a higher number of strong acid

sites in comparison with the unmodiﬁed nonactive samples.

Generally, it was impossible to carry out TPD measure-

ments on samples of group C.

Group A. Samples with little or no Brønsted acidity, cata-

lytically inactive:

all unmodiﬁed zirconia samples: ZR/1, ZR/c1, ZR/c2

the unmodiﬁed and the phosphated aerogels: A/c and

A/c-PO₄;

The Benzoylation of Anisole

Group B. Samples with a medium concentration of

Brønsted acidity, catalytically very active:

The reaction between anisole and benzoic anhydride or

the sulfated and sulﬁted zirconia samples: ZR/amSO₄, benzoyl chloride was investigated (Eq. [1]).

ZR/amSO₃, ZR/c-amSO₄

the sulfated aerogel: A/c-SO₄;

[1]

Group C. Samples with a very high concentration of

Brønsted acidity due to adsorbed HX, only medium catalytic

activity:

SF₄- and SOCl₂-modiﬁed zirconia samples: ZR/SF₄,

ZR/SOCl₂. These two samples are amorphous at 150^ꢁC; HX

molecules are adsorbed at the surface, generating the large

amount of Brønsted acidity in the FTIR spectra. The calci-

nation leads in the case of the ZR/SF₄sample, for instance,

to a ZrO_xF_yphase. Consequently, the samples ZR/SF₄and

ZR/SOCl₂can be considered to belong to a different cate-

gory of materials.

The conversion degrees obtained in the acylation reaction

are widely spread. In contrast to Patil et al. (23), all pure

zirconia samples are catalytically inactive with respect to

the acylation reaction. Using the phosphated zirconia aero-

gel, as well as the SOCl₂- and SF₄-modiﬁed zirconia sam-

ples, only low activity was observed. The ammonium sulﬁte

The kind of crystallization of zirconia is not so impor- (ZR/amSO₃) and sulfate (ZR/c-amSO₄) modiﬁed zirconia

tant for this classiﬁcation. Whereas most of the monoclinic samples possess a medium catalytic activity. The conver-

zirconia samples are to be found in group A, there is also sion raised to about 80%, employing the sulfated samples

one monoclinic sulfated zirconia (ZR/c-amSO₄) in group B. A/c-SO₄and ZR/amSO₄as catalysts: The sulfated zirco-

The tetragonal crystallized sample A/c-PO₄is not arranged nia aerogel was the best catalyst due to its high concentra-

in group B of the catalytically very active samples but in tion of strong Brønsted acid sites combined with a large

group A. That means that the priority impact of (sulfate) BET area. The modiﬁcation of Zr(OH)₄with solid am-

modiﬁcation is not the formation of a tetragonal crystal- monium sulfate resulted also in a very effective catalyst

lized zirconia bulk. The formation of strong acidic surface sample.

sites is the most important impact of the sulfation process.

Applied to the benzoylation of anisole, some of the in-

Although sample ZR/c-amSO₄possess a small amount vestigated samples have the same or better catalytic activity

of Brønsted sites—on the basis of photoacoustic spectro- than the ZR/com catalyst, showing only conversion degrees

scopy—it belongs to the catalytically very active samples. of about 60%. Even in comparison with ZR/com, also, con-

This can be explained by its comparatively small surface siderably higher conversion degrees were achieved using

area (about 29 m²) and the limited sensitivity of photoa- the ZR/amSO₄sample at lower temperatures and shorter

coustic spectroscopy being obviously not sufﬁcient enough reaction times (due to faster kinetics).

to detect this small absolute amount of acid sites. However,

the TPD-proﬁle clearly indicates the strong acidity of the tion. Two possible mechanisms of activation are presented

surface of this sample.

in Scheme 1. The benzoic compound may be activated on

The TPD measurements should also be interpreted in both Brønsted and Lewis acid sites.

ThetestreactionusedisclassiﬁedasFriedel–Craftsacyla-

categories of these groups with distinct differences in the

High catalytic activity in the acylation reaction was ob-

ammonia desorption proﬁles. Comparing the normalized served for the samples that show high conversion degrees

proﬁles of the samples from groups A and B there are two in the butene isomerization reaction, too. Therefore, it

main differences (cf. Fig. 2). The ammonia desorption pro- can be concluded that both reactions, the isomerization of

ﬁles of all catalytically active samples of the second group 1-butene as a test reaction for Brønsted acidity and the

FRIEDEL–CRAFTS–ACYLATION CATALYSTS

173

SCHEME 1. Proposed model for the catalytic action of a sulfated zirconia surface (activation at both Brønsted and Lewis acid sites). The surface

sulfate was drawn in simpliﬁed terms as chelating bidentate sulfate only.

acylation, take place on the same kind of acid sites of the

The gaschromatographically detected para to ortho ra-

zirconia samples used. This clearly indicates the dominating tio of the product isomers is about 94 : 6 for all catalytically

impact of Brønsted acid sites for the benzoylation of anisole active samples investigated here. The same result for the

using modiﬁed zirconia as a heterogeneous catalyst. If the benzoylation reaction was observed in the commonly per-

isomerization of 1-butene and the Friedel–Crafts acylation formed homogeneous catalysis (6).

proceed at the same kind of acid sites of similar strength,

both conversion lines should be proportional. Figure 7

illustrates the relation between the conversions of these

CONCLUSIONS

two reactions. The dashed line (not calculated) was drawn

as an optical assistance. A good correlation was found for

the conversion degrees of both catalyzed reactions, as a re-

sult of obviously similar activation at the catalyst surface.

Therefore, the mechanism of the acylation is supposed

as follows: The activation occurs on Lewis or Brønsted acid

sites when typical Lewis acids, e.g. AlCl₃, or Brønsted acids,

e.g. CF₃SO₃H, are employed as catalysts in the homoge-

neous phase. However, using solid acids (heterogeneous

catalysts) like modiﬁed solid zirconia the reaction proceeds

at Brønsted acid surface sites.

Comparing different modifying reagents it can be stated

that zirconia kneaded with solid ammonium sulfate or

ammonium sulﬁte generates very high Brønsted acidity

at the zirconia surface after a certain calcination proce-

dure. An increased acidity can be achieved using aerogel

techniques for the synthesis of sulfated zirconia. Here, a

comparatively large BET surface area, combined with a

high concentration of Brønsted acid sites has been ob-

tained, resulting in an even slightly higher active phase.

All these three sulfated samples yield comparatively high

conversion degrees in the employed benzoylation reac-

tion of anisole. Generally, the conversion degrees of the

benzoylation exhibits a good correlation for all catalysts

investigated with the used probe reaction, the double-

bond isomerization of 1-butene. Under the conditions used

the above-described benzoylation reaction occurs exclu-

sively on Brønsted acid sites. In contrast, solid Lewis acids

showed little or no conversion of the benzoylation agents

into the desired aromatic ketones under the applied condi-

tions.

ACKNOWLEDGMENTS

E. Kemnitz and V. Quaschning thank the Verein der Chemischen Indus-

trie (VCI), the Fond der Chemischen Industrie (FCI), and the Bundesmin-

isterium fu¨r Bildung, Wissenschaft, Forschung, und Technologie (BMBF)

for ﬁnancial support. Assistance in aerogel synthesis by Dr. Tomaz Skapin

from the Josef-Stefan-Institute, Ljubljana, is gratefully acknowledged.

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Article Doi

Properties of Modified Zirconia Used as Friedel-Crafts-Acylation Catalysts

DOI: 10.1006/jcat.1998.2098

Source and publish data:

Authors:

Article abstract of DOI:10.1006/jcat.1998.2098

Full text of DOI:10.1006/jcat.1998.2098

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