First stoichiometric large-pore chromium(III) silicate catalyst

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Microporous and Mesoporous Materials 69 (2004) 209–215

First stoichiometric large-pore chromium(III) silicate catalyst

Paula Brand~ao a, Anabela Valente a, Artur Ferreira b, V�ıtor S. Amaral c, Jo~ao Rocha a,*

a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb ESTGA, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

c Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

Received 21 November 2003; received in revised form 16 February 2004; accepted 17 February 2004

Abstract

The first example of a large-pore framework chromium-silicate containing stoichiometric amounts of hexa-coordinated chro-

mium(III) has been reported. Samples were characterised by powder X-ray diffraction (XRD), scanning electron microscopy (SEM),

diffuse reflectance spectroscopy ultraviolet–visible (DR UV–VIS) spectroscopy, thermogravimetry (TG), adsorption of different

probe molecules and model catalytic tests; AV-15 is an active catalyst for the liquid phase epoxidation of cyclohexene to cyclohexene

oxide, the oxidation of cyclohexane to cyclohexanol and cyclohexanone, and the oxidative dehydrogenation of cyclohexanol to

cyclohexanone, under mild reaction conditions (using H2O2, at 50 �C). The catalyst is recyclable without significant loss of catalytic

activity, but leaching tests indicate that catalysis is mainly due to some leached chromium species. In gas phase, AV-15 exhibits

mainly oxydehydrogenation activity in the conversion of ethanol, 2-propanol and cyclohexanol, using air, yielding the corre-

sponding carbonyl product.

� 2004 Elsevier Inc. All rights reserved.

Keywords: Chromium silicate; AV-15; Large-pore; Hydrothermal synthesis; Oxidation reactions

1. Introduction

Heterogeneous catalytic oxidation, both in the va-

pour and liquid phase, is an important technological

area in the field of processes for the production of bulk

organic chemicals, the production of fine chemicals andfor pollution abatement [1]. A main advantage in he-

terogeneous catalysts is the easy and economical sepa-

ration of products from the catalyst. A large number of

studies have been devoted to the development of new

materials for catalytic oxidation, principally transition

metal substituted zeolites, zeotypes and mesoporous

materials. A primary difficulty relates to the stability of

these materials in the reaction environment. Some of themajor commodity chemicals, such as formaldehyde and

ethylene oxide, are produced by heterogeneous oxida-

tion in the gas phase at temperatures higher than 600 K

[1]. Hence, high thermal stability is a constraint to

materials developed for such applications. In liquid

phase, the stability of the catalyst to metal leaching

*Corresponding author. Tel.: +351-234-370730; fax: +351-234-

370084.

E-mail address: [email protected] (J. Rocha).

1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2004.02.011

during operation in the presence of oxidizing agents is

crucial, but often questionable [2].

It is well known that chromium supported on various

inorganic oxides, such as c-Al2O3, SiO2, ZrO2, and

molecular sieves, are interesting catalysts for a wide

range of redox and polymerisation reactions [3,4]. At-tempts have been made to prepare stable chromium-

based heterogeneous catalysts by incorporating small

amounts of metal in the framework of molecular sieves

[5–11]. For most of these materials framework insertion

requires chromium to be tetra-coordinated, but instead

they contain hexa-coordinated CrIII, most likely present

as extra-framework metal species [3,7].

As a result of a comprehensive effort for prepar-ing novel mixed tetrahedral–octahedral microporous

framework silicates [12], we wish to report the synthesis

and characterization of a large-pore chromium-silicate

containing stoichiometric amounts of hexa-coordinated

CrIII in the framework (AV-15, Aveiro microporous

solid number 15). The catalytic properties of AV-15 are

studied for the liquid phase oxidation of cyclohexene,

cyclohexane and cyclohexanol, under mild conditionsand gas phase conversion of ethanol, 2-propanol and

cyclohexanol using air.

mail to: [email protected]

210 P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215

2. Experimental

2.1. Synthesis

AV-15 was synthesised in teflon-lined autoclaves understatic hydrothermal conditions. Typically, an alkaline

solution was prepared by mixing 6.11 g sodium silicate

solution (Na2O 8 wt.%, SiO2 27 wt.%, Merck), 9.26 g

H2O and 0.90 g NaOH (Merck). A second solution was

prepared by mixing 6.22 g H2O with 1.20 g Cr2(SO4)3 Æ15H2O (Merck). These two solutions were combined

and stirred thoroughly. The gel, with a composition

10.6Na2O:15.1SiO2:Cr2O3:475.1H2O, was autoclavedfor seven days at 230 �C. The green crystalline pow-

der was filtered off, washed and dried at 60 �C over-

night. The synthesis of AV-15 is easily reproducible.

2.2. Characterisation

Powder XRD data were collected on a X’Pert MPD

Philips difractometer (CuKa X-radiation) with a curvedgraphite monochromator, a automatic divergence slit

(irradiated length 20.00 mm), a progressive receiving slit

(slit’s height 0.05 mm) and a flat plate sample holder, in

a Bragg–Brentano para-focusing optics configuration.

Intensity data were collected by the step counting

method (step 0.02�, time 38 s) in the range 2h 3–32�. The

in situ work was carried out using an Anton Parr high-

temperature chamber and a 10 �C/min heating rate.SEM images were recorded on a Hitachi S-4100 Field

Emission Gun tungsten filament working with a voltage

of 25,000 V. Chemical composition was determined by

energy dispersive analysis of X-rays (EDAX) on a SEM

instrument. DR UV–VIS was performed on a Jasco

V-560 PC spectrometer using BaSO4 as the reference

material. Magnetic susceptibility measurements were

performed on a SQUID (superconducting quantuminterference device) magnetometer, model MPMS5

from Quantum Design. The measurements were taken

under an applied magnetic field 100 Oe on heating

from 5 to 300 K. The sample was previously cooled with

the magnetic field applied from room temperature to

5 K. TGA and DSC curves were measured with TGA-

50 and DSC Shimadzu analysers. The samples were

heated under inert atmosphere and air with a rate of5 �C/min. Nitrogen adsorption measurements at 77 K

were performed using a Micromeritics ASAP 2010

V1.01 B automatic instrument. Pore size distribution

was determined using the density-functional theory

(DFT) Plus Software for data files generated from the

ASAP instrument. Adsorption of benzene and m-xylene

was measured at 298 K, using a gravimetric ad-

sorption apparatus equipped with a CI electronic MK2-M5 microbalance and an Edwards Barocel pressure

sensor. Before analysis, the solid was degassed at

523 K.

2.3. Catalytic tests

The catalytic performance of AV-15 was evaluated

for the liquid phase and gas phase reactions of organic

compounds. The liquid phase oxidations of cyclohexene,cyclohexane and cyclohexanol were carried out in a

micro-reaction vessel equipped with a magnetic stirrer,

using H2O2 (30% aq.) as oxidant, at 50 �C. In a typical

run, the reactor was loaded with 50 mg catalyst, 1.2

mmol substrate, 2.6 mmol H2O2, and 1 cm3 acetonitrile,

used as solvent. Blank reactions under the same condi-

tions without a catalyst were also performed. Samples

were withdrawn periodically and analyzed using a gaschromatograph (Varian 3800) equipped with a semi-

capillary CP WAX 52CB column (30 m · 0.53 mm) and

a flame ionization detector.

The gas phase conversion of ethanol, 2-propanol and

cyclohexanol was carried out in a fixed bed continuous

flow reactor (reaction tube of 7 mm inner diameter).

Prior to the reaction the catalyst (50 mg) was placed in

the reactor bed and heated in situ at 300 �C in flowingN2 (20 cm3/min) for 30 min. Afterwards 8 mmol/h

alcohol was introduced using a syringe pump and fed to

the reactor in a stream of air (20 cm3/min). For the

reactions carried out in the absence of oxygen only the

alcohol was introduced after dehydrating the catalyst.

After 30 min, the effluent gas was periodically injected

into the GC by a six-port VICI gas-sampling valve.

3. Results and discussion

3.1. Characterization

The powder XRD pattern of AV-15 is shown in Fig.

1 and the d-spacing and intensities of the main reflec-

tions are given in Table 1. The several peaks were in-dexed to different reflections with DICVOL [13] using

the first 20 lines. The reflections were poorly resolved

making it difficult to obtain a precise solution for the

unit cell. Our best result (M(20)¼ 5) was achieved by

indexing AV-15 on a triclinic cell with a ¼ 15:008 �A,

b ¼ 13:738 �A, c ¼ 11:896 �A, a ¼ 59:78�, b ¼ 110:84�,c ¼ 120:73� (volume 1806 �A3). In situ powder XRD

pattern remains unchanged until 500 �C in vacuum,whereas in air some loss of crystallinity is observed.

SEM images reveals that AV-15 crystals are narrow

plate-like particles with ca. 10–20 lm in length and ca. 2

lm width (Fig. 2). EDAX (energy dispersive absorption

of X-rays) yields Si/Cr and Na/Cr molar ratios of ca. 6

and 3, respectively. The DR UV–VIS spectrum shows

three bands centred at 290, 445 and 640 nm, typical of

trivalent chromium in octahedral coordination (Fig. 3).These results explain the green colour of the as-syn-

thesised sample, similar to most common minerals

containing octahedral CrIII, such as chromite [3]. Mag-

6 9 12 15 18 21 24 27 30 33 36 39 42 452θ /º

I rel

Fig. 1. Powder XRD pattern of chromium-silicate AV-15. The tick marks depict Bragg reflections.

Table 1

Powder XRD data of AV-15

d/�A I=I0

12.641 100

10.595 1

7.135 26

6.828 6

6.361 3

6.301 2

5.903 1

5.746 1

5.352 1

5.242 2

4.961 1

4.647 1

4.485 1

4.289 2

4.261 2

4.228 3

3.790 1

3.680 2

3.490 3

3.450 3

3.388 7

3.205 2

3.176 4

3.152 2

3.070 2

3.041 2

Fig. 2. SEM image of AV-15 crystals.

250 350 450 550 650 750

Wavelength (nm)

% R

(a.u

.)

640445

290

Fig. 3. Diffuse reflectance UV–VIS spectrum of AV-15.

P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215 211

netic susceptibility measurements (SQUID) show that

AV-15 is paramagnetic with a pseudo-Curie temperature

of 0.71 K and magnetic moment of 3.69 lB, indicating

CrIII is present. These results indicate that chromium

in AV-15 is present as Cr3þ in octahedral coordination

and a possible formula for dehydrated AV-15 is

Na3CrSi6O15. TGA analysis under inert atmosphere

shows a weight loss in the range 50–400 �C corre-sponding to the loss of ca. 4 mol H2O per formula unit

(Fig. 4). The DSC curve obtained under similar condi-

tions exhibits two endothermic peaks at ca. 80 and 130

�C ascribed to the loss of water located on the external

surface and in the pores of AV-15.

The N2 adsorption–desorption isotherm of AV-15 at

77 K is shown in Fig. 5. The significant adsorption at

very low relative pressures is supposedly due to micro-

pore filling. Since the isotherm increases further with

85

90

95

100

0 100 200 300 400 500Temperature (ºC)

Wei

ght (

%)

-1.5

-0.5

0.5

1.5

2.5

DSC

(mW

)

Fig. 4. TGA and DSC curves of AV-15 recorded in inert atmosphere.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.2 0.4 0.6 0.8 1.0

p/p0

mm

ol/g

0 2 4 6 8 10pore width (nm)

sorp

tion

(a.u

.)

Fig. 5. Nitrogen adsorption (þ)–desorption (�) isotherm at 77 K,

benzene adsorption (�) at 298 K and pore size distribution of AV-15.


increasing p=p0, the presence of some mesoporosity is

evident, probably corresponding to inter-particle voids.

A hysteresis loop with a very steep region of the

desorption branch which leads to a lower closure point

of the loop at p=p0 � 0:45 is characteristic of aggregates

of plate-like particles leading to slit shaped pores. These

results are in accord with the AV-15 morphology re-

vealed by SEM. The apparent micropore volume, esti-mated from either the t-plot or DR-plot methods, is 0.06

cm3/g. The Langmuir specific surface area is 183 m2 g�1,

of which less than 10% corresponds to external surface

area (16 m2 g�1 calculated from the t-plot). When so-

dium ions are partially substituted by the more bulky

potassium ions the specific surface area decreases ca.

50%. The maximum of the pore size distribution curve

corresponds to a pore width of 8 �A (Fig. 5). The AV-15

sorption capacity for benzene (kinetic diameter 6.8 �A) is

ca. 1 mmol g�1solid (taken at p=p0 � 0:4, Fig. 5), while for

larger adsorbate molecules such as m-xylene (7.4 �A) it is

much smaller (ca. 0.4 mmol g�1solid).

3.2. Catalytic tests

3.2.1. Liquid phase

Several chromium catalysts are known to effectively

oxidize organic compounds with hydrogen peroxide or

tert-butyl hydroperoxide [14–16]. The catalytic perfor-

mance of AV-15 was evaluated for the liquid phase

oxidation of cyclohexene, cyclohexane and cyclohexa-nol, using dilute H2O2 as oxidant, at 50 �C and the

catalytic results are shown in Table 2. Control experi-

ments without a catalyst gave negligible substrate con-

sumption at 24 h.

The oxidation of cyclohexene in the presence of AV-

15 proceeds to 40% conversion within 7 h. Cyclohexene

oxide is the main product, but undergoes acid-catalysed

epoxide ring opening and consecutive oxidation reac-tions, decreasing selectivity.

The catalytic oxidation of cyclohexane yields cyclo-

hexanone and cyclohexanol as the only products in a

molar ratio of 1.7. The turnover frequency of cyclo-

hexane conversion in the presence of AV-15 is 0.46

mmol g�1cat h

�1. The activity of AV-15 may be compared

to that of CrIII incorporated into the framework of

zeolite-b, in which case cyclohexane conversion usingH2O2 as oxidant, at 85 �C, was reported to be 7.8% after

6 h [17]: with AV-15 ca. 11% conversion is achieved. The

oxidation of cyclohexane in the presence of chromium

silicalite-1 (CrS-1, bearing a MFI structure) using a

stronger oxidizing agent than H2O2, i.e., tert-butyl

hydroperoxide (TBHP), at 100 �C, gives equimolar

amounts of cyclohexanone and cyclohexanol at 8.5%

cyclohexane conversion, achieved after 12 h [18].Cyclohexanol conversion in the presence of AV-15

produces cyclohexanone with 100% selectivity at 29%

conversion (Table 2). Similar results have been reported

by Parentis et al. [19] for the oxidation of cyclohexanol

by TBHP in the presence of silica supported CrIII, at 70

�C, though the reaction rate in the latter case is faster

(3.5 mmol g�1cat h

�1, TOF estimated at 7 h reaction) than

that observed for AV-15 (0.8 mmol g�1cat h

�1). The cata-lytic results with AV-15 show that cyclohexanol is more

easily converted than cyclohexane and in the latter

reaction cyclohexanol is subsequently converted to

cyclohexanone.

A recycling test was carried out for AV-15 in the

oxidation of cyclohexanol. Before reuse the solid was

separated from the reaction solution by centrifugation,

washed with acetonitrile and acetone and dried at 60 �C.The reaction rate and selectivity remained practically the

same (Table 2). No major morphological and structural

changes were observed after catalysis, as ascertained by

Table 2

Liquid phase oxidation of organic compounds in the presence of AV-15

Substrate TOFa (mmol g�1cat h

�1) Conversionb(%) Product Selectivityc(%)

Cyclohexene 1.35 51 Cyclohexene oxide 41

2-Cyclohexen-1-one 31

1,2-Cyclohexanediol 4

2-Hydroxy-1-cyclohexanone 14

Cyclohexane 0.46 24 Cyclohexanol 37

Cyclohexanone 63

Cyclohexanol

1st run 0.82 29 Cyclohexanone 100

2nd run 0.83 32 Cyclohexanone 100

a Turnover frequency calculated for 7 h reaction.b Substrate conversion after 24 h.c Products identified by GC–MS. For cyclohexene oxidation minor amounts of other products such as adipic aldehyde and 2-cyclohexen-1-ol are

formed.

Table 3

Gas phase reactions of alcohols in the presence of AV-15 at 300 �C

Alcohol Experimental

conditions

Conversiona

(%)

Selectivityb (%)

ENE ONE

Ethanol AV-15/air 27 9c 85

AV-15/N2 12 100 –

No catal./air 4 100 –

2-Propanol AV-15/air 45 20 80

AV-15/N2 50 86 14

No catal./air 16 69 31

Cyclohexanol AV-15/air 26 33 67

AV-15/N2 10 100 –

No catal./air 7 66 34

a Based on conversion at 260 min on-stream.b Selectivity to the corresponding olefin (ENE) and carbonyl prod-

uct (ONE).c Remaining products are ethyl acetate (5% selectivity) and acetic

acid (1% selectivity).


SEM and powder XRD. However, the Si/Cr molar ratio

of the recycled catalyst increased approximately 10%,

indicating some loss of chromium ions. To test for

leaching the reaction medium containing fresh AV-15

was filtered at the reaction temperature and the filtrate

was allowed to react further. The hot filtrate reacted at a

similar rate to that observed in the presence of AV-15.

The same is observed when a similar experiment iscarried out for the solid recovered from the second

reaction cycle, as well as for AV-15 washed with

ammonium acetate to remove possible non-framework

chromium species. These results indicate that, under the

applied experimental conditions, AV-15 is subject to

leaching and the soluble chromium species are capable

of catalysing the reaction. Chromium(III) can be oxi-

dized to CrVI in solution, which is an extremely activecatalyst and concentrations as low as 1–2 ppm CrVI can

catalyse the reaction efficiently [2]. When tert-butyl

hydroperoxide was used as oxidant no reaction oc-

curred, probably because no leaching occurs. The

chromium species present in solution may be removed

from the framework of AV-15 or from impurities of

unreacted gel.

3.2.2. Gas phase

The relatively high thermal stability of AV-15 makes

it an interesting candidate for gas phase catalytic oxi-

dation reactions. The oxidation of alcohols to carbonyl

compounds is a key reaction in organic synthesis [20].

Redox and nonredox CrIII sites in chromium catalysts

have been proposed as active for dehydrogenation

reactions [21, and references therein]. Herein, the cata-lytic performance of AV-15 is evaluated for the gas

phase conversion of alcohols, namely ethanol, 2-pro-

panol or cyclohexanol using air, at atmospheric pres-

sure, in a fixed bed continuous flow reactor, at 300 �C.

In these experiments the main product is the corre-

sponding carbonyl compound, which results from the

(direct and/or oxidative) dehydrogenation of the alco-

hol.

The conversion of ethanol produces mainly acetal-

dehyde with 85% selectivity at 27% conversion, after 260

min on-stream (Table 3). Some by-products such as

ethylene, ethyl acetate and acetic acid are formed. The

conversion of cyclohexanol over AV-15 gives cyclohex-

anone in 67% selectivity at 26% conversion, and the onlyby-product formed is cyclohexene. The oxydehydro-

genation activity (OD) of AV-15 for these reactions was

demonstrated by using nitrogen instead of air, under

identical experimental conditions, which resulted exclu-

sively in the dehydration of the alcohol to the corre-

sponding olefin. AV-15 exhibits OD activity for the

conversion of 2-propanol, giving acetone with 80%

selectivity at 45% conversion (TOS (time-on-stream)¼260 min). Without oxygen, the acetone yield is lower

(7% compared to 36% with air), whereas the propene

yield is higher (43% compared to 9% with air).

0

20

40

60

80

100

0 100 200 300

TOS (min)0 100 200 300

TOS (min)0 100 200 300

TOS (min)

Con

v., S

elec

t. (%

)

0

20

40

60

80

100

Con

v., S

elec

t. (%

)

0

20

40

60

80

100

Con

v., S

elec

t. (%

)

A B C

Fig. 6. Catalytic performance of AV-15 in the conversion (�) of ethanol (A), 2-propanol (B) or cyclohexanol (C), and selectivity to the corresponding

carbonyl (�) or olefin (þ) compound as a function of time-on-stream (TOS).


The observed OD activity may be due to the presenceof basic sites in AV-15. It has been found that basic sites

of molecular sieves play a very important role in the

dehydrogenation of alcohols [22–24]. On the other hand,

the dehydration activity, observed especially for the

conversion of 2-propanol to propene under N2 stream, a

typical acid-catalysed reaction [23], indicates that AV-15

possesses some surface acidity. Products resulting from

consecutive reactions (such as isomerization, dispro-portionation) of, for example, cyclohexene were not

detected, suggesting that the acid sites in AV-15 are

weak [25].

The conversion profiles shown in Fig. 6 indicate that

AV-15 slowly deactivates with TOS and simultaneously

the dehydrogenation activity tends to decrease whereas

the dehydration activity increases. The catalyst deacti-

vation may be partly due to the formation of coke. Infact, the originally green catalyst powder turned to a

brownish colour after the reaction, probably due to

accumulation of undesorbed dimeric and heavier olig-

omers. The DSC analysis carried out on the used sample

under air atmosphere, shows a broad exothermic peak

centred at 408 �C that does not appear for the as-syn-

thesized sample. Hence, this band may be assigned to

the decomposition of organic matter present in the solid,indicating that during gas-phase experiments the prod-

ucts desorption is hindered. Under the applied reaction

conditions AV-15 lost 36% crystallinity (estimated by

normalizing the sum of the areas of the powder XRD

peaks to the corresponding areas of the as-synthesised

AV-15 sample), which may also account for the ob-

served catalyst deactivation.

4. Conclusions

We report the synthesis and characterisation ofAV-15,

which contains stoichiometric hexa-coordinated chro-

mium(III). This material is, to the best of our knowl-

edge, the first example of a large-pore chromium silicate.

AV-15 is an active catalyst for the liquid phase epoxida-tion of cyclohexene to cyclohexene oxide, the oxidation

of cyclohexane to cyclohexanol and cyclohexanone, and

the oxidative dehydrogenation of cyclohexanol to cyclo-hexanone, undermild reaction conditions (usingH2O2, at

50 �C). The catalyst could be recycled without significant

loss of catalytic activity, but leaching tests confirmed that

catalysis is mainly due to some chromium species leached

from AV-15 during the oxidative transformations. The

catalytic performance of AV-15 was further evaluated for

the gas phase conversion of primary and secondary

alcohols using air (atmospheric pressure), at 300 �C. AV-15 exhibits mainly oxydehydrogenation activity in the

conversion of ethanol, 2-propanol and cyclohexanol,

yieldingmainly the corresponding carbonyl product. AV-

15 also exhibits some dehydration activity responsible for

the formation of the corresponding olefin as a by-prod-

uct. The olefins did not undergo any consecutive reac-

tions, suggesting that the acid sites in AV-15 are weak.

Coke formation and partial loss of crystallinity accountsfor some catalyst deactivation at 300 �C.

Acknowledgements

The authors thank FCT, PRAXIS XXI, POCTI and

FEDER (Portugal) for financial support and the

Materials Institute of Porto (IFIMUP) for performingthe SQUID magnetic measurements.

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First stoichiometric large-pore chromium(III) silicate catalyst

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