Synthesis and properties of Ti-MOR molecular sieve on the

oxidation of cyclohexene: Influence of the Ti source

Romilda Fernandez, Dilson Cardoso *

Department of Chemical Engineering, Universidade Federal de S. Carlos, P.O. Box 676, 13565-905 S. Carlos, Brazil

Available online 12 September 2005

Abstract

The incorporation of Ti in a molecular sieve having aMFI structure has generated a newmaterial, TS-1. This newmaterial is very active for

the oxidation of organic compounds in the liquid phase, using H2O2 as oxidizing agent. However, TS-1 has medium diameter micropores,

which adversely affects the diffusion of molecules with kinetic diameters larger than 5.5 A. This limitation has provided a challenge for

researchers to develop titanium-silicates with larger pore diameter. The purpose of this study is to compare physical–chemical and catalytic

properties of Ti-MOR synthesized through Ti(OC2H5)4 (TEOT) and H2TiF6 as Ti sources. The results showed the formation of a MOR

structure using the H2TiF6 as the Ti source. The incorporation of Ti to Ti-MOR structure synthesized using H2TiF6 is lower than for TEOT,

however with a larger formation of pentacoordinated Ti in the framework. The Ti species incorporated to the catalyst synthesized using

H2TiF6 are more active for the oxidation of cyclohexene than TEOT. However a sample synthesized with TEOT provides a higher selectivity

for the formation of cyclohexene oxide.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Ti-MOR; TEOT; H2TiF6; Oxidation of cyclohexene

www.elsevier.com/locate/cattod

Catalysis Today 107–108 (2005) 844–848

1. Introduction

The incorporation of Ti to the framework of a molecular

sieve with MFI structure, which was first accomplished by

researchers from Enichem in 1983, opened a new research

area for redox catalysis [1]. The titanium-silicate obtained

was called TS-1. Results have demonstrated that tetrahedrally

co-ordinated Ti atoms are responsible for the behaviour seen

during oxidation of organic substrates, when hydrogen

peroxide is used as an oxidazing agent [2]. Since then,

titanium-silicates have received the interest of researchers due

to the fact that they can be used in the oxidation of organic

compounds with the use of oxidazing agents having a low

environmental impact. For this reason, these catalysts can be

efficiently employed in the so-called ‘‘clean processes’’.

TS-1 is the most widely studied titanium-silicate.

However, due to the small size of its pores (5.5 A diameter),

the use of TS-1 is limited to reactions that make use of

* Corresponding author. Fax: +55 16 3351 8266.

E-mail address: dilson@power.ufscar.br (D. Cardoso).

0920-5861/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cattod.2005.07.020

organic compounds with a larger kinetic diameter. This

limitation has provided an incentive for researchers to

develop titanium-silicates with larger pore diameter (e.g. Ti-

Beta [3], TS-2 [4] and Ti-MCM-41 [5]).

Another molecular sieve that has large pore diameter

(dp � 7 A) and permits the incorporation of Ti to its

framework is mordenite. The incorporation of Ti to MOR

structurehas been carried out byhydrothermal synthesis using

tetraethyl orthotitanate (TEOT) as a Ti source. TEOT is an

expensive compound and is difficult to handle because it

hydrolyses easily [6,7]. However, Jahn et al. have successfully

performed the synthesis of Ti-Beta, using a cheaper andmore

stable Ti source, hexafluorotitanic acid (H2TiF6) [8].

The present studies are designed to synthesize Ti-MOR

with H2TiF6 as Ti source and compare them with those

synthesized with TEOT. This will be done to verify the

influence of the two different Ti sources on the properties of

the materials formed. In addition, the catalysts will be tested

for the oxidation of cyclohexene using hydrogen peroxide as

the oxidazing agent so that their catalytic properties can also

be compared.

R. Fernandez, D. Cardoso / Catalysis Today 107–108 (2005) 844–848 845

Fig. 1. Diffractograms from samples containing 0 and 2% Ti.

Table 1

Crystallinity level and chemical composition of catalysts

Sample Crystallinity

(%)

Chemical composition

100 � PTi 100 � PAl 100 � PSi Al/Ti Na/Al

M00 100 0.0 23.6 76.4 1 1.0

Ac02 54 0.6 13.3 86.1 22.2 1.0

T02 78 1.2 29.4 69.4 24.5 1.2

PTi = molar fraction of Ti in solid = Ti/(Ti + Si + Al); similar for PAl and

PSi.

2. Experimental

2.1. Synthesis

The synthesis of Ti-MOR was based on the method of

Chandwadkar et al. [6], while changing the Ti source and the

pH control. The reaction mixture contains colloidal silica

(30% SiO2—Nalco 1030) as the Si source, which was slowly

added pseudoboehmite (Catapal B) as Al source. Then, the

Na source (NaOH—diluted in half the volume of water

needed for the total synthesis) was added. At this point 1,4-

diazabicyclo[2,2,2] octane (DABCO, Aldrich, diluted in the

remaining water), used as template, was added to the

mixture. Finally, the Ti source (H2TiF6 60% or TEOT,

Aldrich) was added. The mixture was stirred for 5 h at room

temperature, and then pH was adjusted to pH � 13.7 by

dropping addition of concentrated NaOH (10 M). The

composition of the reaction mixture with Si/Al ratio = 3.75

and 2% Ti, is given by:

0:150 TiO2: 7:5 SiO2: Al2O3: DABCO :

1:35Na2O : 110H2O:

Reaction mixtures were placed in PTFE cylinders, which

were previously conditioned in stainless steel autoclaves, and

crystallized at 160 8C for periods of time ranging from 120 to

168 h. Formed solids were washed until a pH of �9 was

reached, then filtered and dried at 110 8C. To remove the

template, the sampleswere calcinated at 540 8C in an inert and

then oxidant atmosphere, as previously described [9]. In order

to reduce Na content and leave the catalyst in the protonic

form, the samples were subjected to ion exchange with a

solution of NH4Cl (1 M) at 90 8C. The ammonium cations

were eliminated by calcination in an oxidant atmosphere.

2.2. Characterization

After calcination, the materials were characterized by the

following techniques: (a) X-ray diffraction (Siemens DCC

50) using Cu Ka radiation, 40 kV, 40 mA and 2u in the rangeof 2–408 at a scan-rate of 28/min; (b) plasma emission induced

spectroscopy (Varian-Spectra AA640) for determination of

the ofAl,Na andTi content in the solid; (c) thermogravimetric

measurements (SDTSimultaneousDSC-TGA) in the range of

25–900 8C, at 10 8C/m increment and 100 mL/min air follow,

using 20 mgof the original sample; and (d) diffuse reflectance

UV–vis spectroscopy, in the region of 200–400 nm. TheUV–

vis spectroscopy was carried out in a Varian Cary 5

spectrometer using a constant mass of 50 mg.

2.3. Catalytic tests

The catalysts, in a protonic form, had their performance

assessed in the oxidation of cyclohexene, using hydrogen

peroxide as the oxidazing agent. Catalytic activity tests were

performed using 20 mmoles of cyclohexene, 5 mmoles of

hydrogen peroxide, 100 mg of catalyst and 30 mL of

acetonitrile (solvent). Oxidation was carried out at 60 8C for

up to 4 h. Products were analysed by gas chromatography

(Varian model 3400) with a flame ionisation detector and a

capillary column.

3. Results and discussion

3.1. Catalysts characterization

Fig. 1 shows XRD patterns of solids containing 2% Ti

synthesized from TEOT (sample T02), or H2TiF6 (sample

Ac02) as theTi source andwith 0%Ti (sampleM00). Samples

M00 and T02 were hydrothermally treated for 120 h, and

sample Ac02 for 168 h of crystallization. Samples’ XRD

shows peaks that are characteristic of mordenite, thus

confirming the formation of the MOR structure.

Incorporation of Ti to MOR structure reduces peak

intensity. This suggests that Ti absorbs X-ray more intensely

or that incorporating Ti reduces the development of MOR

structure and therefore decreases the level of crystallinity

(Table 1). From these results crystallinity decrease is

stronger when H2TiF6 is used as the Ti source.

Table 1 also shows results from the chemical analysis

carried out on the samples having 0 and 2% Ti (M00, T02

R. Fernandez, D. Cardoso / Catalysis Today 107–108 (2005) 844–848846

Table 2

Mass loss in samples containing 2% Ti in an oxidant atmosphere

Sample Mass loss (%)

I (25–150 8C) II (150–350 8C) III (350–500 8C) IV (500–700 8C) Total (700–900 8C)

T02 6.8 6.2 2.0 0.6 15.5

Ac02 6.6 5.5 2.9 0.9 15.9

and Ac02). As the initial and final pH readings for both

reaction mixtures were similar throughout the synthesis,

while Ti content is much lower when H2TiF6 is used, it may

be possible that there is a greater difficulty to organize the

zeolite framework due to the stability of Ti and Al

complexes containing fluoride. In fact, the sample obtained

from F-containing mixture has also less Al when compared

to those achieved through TEOT.

From Table 1 it can be seen that the Ti source does not

have a significant influence on the Al/Ti ratio in a solid.

However, the Al/Ti ratio in the reaction mixture is 13.3,

which is a much lower value than that found in the solid. As

Al is known to be responsible for the nucleation of the

zeolite phase [3], this suggests that the incorporation of Al to

the structure is preferential to Ti. Except for sample

synthesized using TEOT, the Na/Al ratio is approximately

1.0, which suggests that all Al is present in the framework

and is neutralized by the Na.

Table 2 shows mass loss in samples synthesized with 2%

Ti, carried out by thermogravimetric analysis under an

oxidant atmosphere. All samples show mass losses in four

different temperature ranges [10].

From the results showed in Table 2, total mass loss is

approximately 15% for both samples. Mass loss in

temperature ranges I and II corresponds to water loss and

desorption of physically occluded template in the zeolite

channels. The mass loss in temperature ranges III and IV

may be related, respectively, to (a) the decomposition of the

protonated amine that is compensating for the negative

charge generated by the pentacoordinated Ti in the

framework [11] and (b) removal of the coke formed during

Fig. 2. UV–vis Spectra of samples containing 2% Ti synthesized with

H2TiF6 (sample Ac02) and TEOT (sample T02).

polymerisation of the olefins formed in previous temperature

range. Both losses are higher in sample synthesized using

H2TiF6. It is important to observe that despite the fact that

this sample shows to have lower Ti content (Table 1), the

thermal analysis shows a larger formation of pentacoordi-

nated Ti when compared to the T02 sample.

Fig. 2 shows UV–vis spectra of samples containing 2% Ti

(T02 and Ac02). This spectra display two absorption bands

that can be related to framework Ti. The absorption band at

approximately 210 nm (A) can be related to tetracoordinated

Ti isolated in the framework [12], while band (B) for the T02

sample (�260 nm) can be related to partially polymerised,

hexacoordinated species that make up Ti–O–Ti bonds

[11,12]. From Fig. 2, it can be seen that band (B) of the

sample synthesized with H2TiF6 (Ac02) is broader than for

T02. This could be due to the contribution from

pentacoordinated Ti species [11,13], as suggested by the

thermal analysis results, shown in Table 2. In the spectra

presented in Fig. 2, the band related to extra framework Ti

(320 nm) is not seen and confirms that extra framework Ti

was not formed in both samples.

3.2. Oxidation of cyclohexene

Fig. 3 shows the conversion of cyclohexene as a function

of the reaction time for protonic samples containing 0 and

2% Ti. It can be seen that the sample that does not contain Ti

has an almost negligible conversion, reaching 0.5% as its

highest value. It can also be seen that after 4 h of reaction,

samples containing Ti show a conversion close to 3%, which

Fig. 3. Conversion (�0.1%) of cyclohexene for samples containing 0 and

2% Ti.

R. Fernandez, D. Cardoso / Catalysis Today 107–108 (2005) 844–848 847

Fig. 4. Specific activity (�20) for samples containing 2% Ti.

Table 3

Selectivity of oxidation products after 4 h of reaction

Sample Selectivity Si (%)

a b c

M00H – 100 –

T02H 16.6 83.4 –

Ac02H 11.9 64.0 24.1

Si = 100 � mol of i formed/mol of cyclohexene consumed (a) cyclohexeneoxide, (b) 2-cyclohexene-1-ol, (c) cyclohexanol.

is a typical value for titanium-silicates under similar

conditions [7].

Fig. 3 shows the conversion of cyclohexene using

catalysts containing 2% Ti increases along the reaction time,

indicating that the system has not reached a balance.

Additionally, despite Ti contents being very different, the Ti-

containing catalysts shows to have a very similar behaviour,

regardless of the Ti source used.

Fig. 4 shows the specific activity, which is defined as the

ratio between the number of moles of cyclohexene converted

and the number of moles of Ti present in the catalyst, per h of

reaction time (Xc/Ti � h). As can be seen, the catalyst

synthesized with H2TiF6 displays a much higher specific

activity (up to 2.5 times) than that synthesized with TEOT.

These results confirm that despite the lower incorporation of

Ti to the Ac02 sample (see Table 1), Ti species are more

active than those in the sample synthesized with TEOT. As

the UV–vis and thermogravimetric results show that the

Ac02 sample contains larger quantity of pentacoordinated

Ti, this is the reason for increased catalytic activity.

In what concerns selectivity, these titanium-silicates can

form the following products during the oxidation of cyclo-

hexene, as shown in the following scheme: (a) cyclohexene

oxide, (b) 2-cyclohexene-1-ol and (c) cyclohexanol.

Table 3 shows the selectivity of the products formed

during oxidation of cyclohexene, after 4 h of reaction and

using samples containing 0 and 2% Ti. Because the presence

of some amount of Al is needed to get the MOR structure,

behaviour of the Ti-MOR catalyst is influenced by elements

such as the acidity and hydrophilic nature of the surface.

This can be confirmed by following the catalyst synthesized

without Ti (M00H), which, although not very active, results

exclusively from the formation of 2-cyclohexene-1-ol.

According to Balkus et al. [14], the acidic zeolite in the

presence of H2O2 is responsible for the development of this

type of product. Thus, for all the catalysts, 2-cyclohexene-1-

ol is formed as the main product, followed by cyclohexene

oxide (Table 3).

Selectivity of the catalysts prepared with 2% Ti indicates

that despitethe fact that the catalyst synthesized with H2TiF6showed to have more activity than that prepared with TEOT,

the latter is more selective with regards to the formation of

epoxide. Actually, sample Ac02H presents the formation of

cyclohexanol, which is simply the result from the hydrating

the olefin catalysed by acidic sites. These results suggest that

Ti species, which are present in the Ac02H sample, help

increase the acidity of the catalysts [15]. A higher acidity

could result from the fluoride in the Ti source.

4. Conclusions

Results from thiswork show that it is possible to synthesize

Ti-MOR through a cheaper and more stable Ti source

(H2TiF6). XRD patterns show that incorporating Ti to the

framework adversely affects the formation ofMOR structure.

The incorporation of Ti to the solid is larger in samples

synthesized with TEOT than with H2TiF6. Conversely, the

thermogravimetric analysis indicates a larger quantity of

pentacoordinated Ti species in the framework for the Ac02

sample. TheUV–vis spectra confirm the incorporation ofTi to

the MOR structure. Catalytic tests show that cyclohexene

conversion is similar for both the catalysts, regardless of theTi

source used in the preparation. The catalyst synthesized with

H2TiF6 showed to have higher specific activity in the

oxidation of cyclohexene than theTEOT-synthesized catalyst.

This shows that Ti species incorporated from a reaction

mixture containing fluoride are more active in this reaction.

The catalyst synthesized usingTEOThasgreater selectivity in

the formation of cyclohexene oxide.

R. Fernandez, D. Cardoso / Catalysis Today 107–108 (2005) 844–848848

Acknowledgements

The authors would like to thank CNPq and the Pronex

program for their financial support. We would also like to

thank Nalco for providing the colloidal silica used in this

study.

References

[1] M. Taramasso, G. Perego, B. Notari, US Patent 4,410,501 (1983).

[2] R.A. Sheldon, Heterogeneous catalytic oxidation and fine chemicals,

Stud. Surf. Catal. 59 (1991) 33.

[3] M.A. Camblor, A. Corma, J. Perez-Pariente, Zeolites 13 (1993) 82.

[4] G. Bellussi, A. Carati, M.G. Clerici, A. Esposito, R. Millini, F.

Buonomo, Belgian Patent 1,001,038 (1989).

[5] A. Corma, M.T. Navarro, J. Perez-Pariente, J. Chem. Soc., Chem.

Commun. 147 (1994) 147.

[6] A.J. Chandwadkar, A.A. Belhekar, T.K. Das, K. Chaudhari, S.G.

Hegde, Stud. Surf. Sci. Catal. 113 (1998) 195.

[7] R. Ryoo, J.H. Kwak, S.J. Cho, Catal. Lett. 37 (1996) 217.

[8] S.L. Jahn, D. Cardoso, 12th International Zeolite Conference, vol. III,

Material Research Society, Baltimore, USA, 1998, p. 1885.

[9] T. Yashima, P. Wu, T. Komatsu, J. Catal. 168 (1997) 400.

[10] J. Perez-Pariente, J.A. Martens, P.A. Jacobs, Appl. Catal. 31 (1987)

35.

[11] T. Blasco, M.A. Camblor, A. Corma, J. Perez-Pariente, J. Am. Chem.

Soc. 115 (1993) 11806.

[12] G. Petrini, A. Cesana, G. De Alberti, F. Genoni, G. Leofanti, M.

Padovan, G. Paparatto, P. Rofia, Stud. Surf. Sci. Catal. 68 (1991)

761.

[13] S.L. Jahn, P.A.P. Nascente, D. Cardoso, Zeolites 19 (1997)

416.

[14] K.J. Balkus, A.K. Khanmamedova, J. Shi, Stud. Surf. Sci. Catal. 110

(1997) 999.

[15] T. Blasco, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A.

Martinez, J. Perdigon-Melon, S. Valencia, J. Phys. Chem. B 102

(1998) 75.