HYDROXYLATION OF PHENOL OVER Ti-MCM-41 AND TS-1

Article abstract of DOI:10.1135/cccc19950451

In the oxidation of phenol by hydrogen peroxide over titanium containing MCM-41 materials, practically only para isomers are formed.The exclusive para selectivity is proposed to be due to the different strength of adsorption for the para and ortho isomers influencing the overall reaction rate or the faster polymerization of the ortho product.The former mechanist is more probable.

Full text of DOI:10.1135/cccc19950451

Hydroxylation of Phenol
451
HYDROXYLATION OF PHENOL OVER Ti-MCM-41 AND TS-1*
a
a
b
b
Kornelia KULAWIK , Gunter SCHULZ-EKLOFF , Jiri RATHOUSKY , Arnost ZUKAL
and Jiri HAD
c
a Institut fur Angewandte und Physikalische Chemie,
Universitat Bremen, 28359 Bremen, Germany
b J. Heyrovsky Institute of Physical Chemistry,
Academy of Sciences of the Czech Republic, 182 23 Prague 8, The Czech Republic
c Central Laboratories,
Prague Institute of Chemical Technology, 166 28 Prague 6, The Czech Republic
Received October 20, 1994
Accepted December 5, 1994
In the oxidation of phenol by hydrogen peroxide over titanium containing MCM-41 materials, prac-
tically only para isomers are formed. The exclusive para selectivity is proposed to be due to the
different strength of adsorption for the para and ortho isomers influencing the overall reaction rate
or the faster polymerization of the ortho product. The former mechanism is more probable.
In the hydroxylation of phenol, hydroquinone (1,4-dihydroxybenzene) is the desired
product used for the production of photographic developers, polymerization inhibitors
and dyes1. Therefore, an increased interest is given to the catalysts exhibiting improved
p/o product ratio.
Recently, titanium containing MCM-41 mesoporous molecular sieves (Ti-MCM-41)
have been synthesized2–5 and used in the oxidation of several organic molecules (α-ter-
pineol, norbornene2,3, 2,6-di-tert-butylphenol and benzene4).
In the present work the catalytic properties of Ti-MCM-41 in the phenol oxidation
are studied and compared with those of titanium silicalite (TS-1). The latter may be
regarded as a standard catalyst in liquid phase heterogeneous oxidations6–8
.
EXPERIMENTAL
Two samples of Ti-MCM-41 differing in the content and state of titanium were prepared. The standard
zeolite material TS-1 was synthesized according to the procedure given in a patent9.
* Presented at the XXVIth Czech and Slovak Symposium on Catalysis, Prague, November 7 – 8, 1994.
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452
Kulawik, Schulz-Ekloff, Rathousky, Zukal, Had:
Preparation of Ti-MCM-41
The reaction mixture was prepared as follows: 19.26 g of Ludox AS-40 (Du Pont, 40 wt.% colloidal
silica in water) were put into a 400 ml polypropylene beaker and stirred by a magnetic stirrer at ca
1 200 rpm. Afterwards, 18.52 g of tetraethylammonium hydroxide solution (TEAOH, Merck, 20 wt.%
solution in water) and subsequently 16 g of hexadecyltrimethylammonium chloride solution
(HDTMACl, 25% water solution, Fluka, 1/3 of the total amount) were added. To the gel formed, the
other 2/3 of the surfactant and either 0.84 or 1.68 g of tetrabutyl orthotitanate (TBOT, Merck) diluted
with 0.9 g of 2-propanol (Merck) were added simultaneously. All vessels, except that with TBOT,
were washed with 10 g of deionized water to achieve a quantitative transfer of the reagents. As the
vessel with TBOT could not be washed with water because of the hydrolysis, extra 0.05 g of TBOT
were always added to make up for the losses. All components were cooled down to 10 °C in an ice
bath. The gel formation also occurred at this temperature.
The gel was agitated for 1 min and then allowed to react with stirring (150 rpm) in a 250 ml
polypropylene autoclave at 110 °C for 24 h. The resulting solid product was recovered by filtration,
extracted with ethanol for 4 h in a Soxhlet apparatus and finally calcined under air flow at 600 °C
for 20 h. The reaction mixture of both samples contained the components in the following oxide
molar ratio: 32.94 SiO2 : 0.317 or 0.634 (TBOT)2 : 3.3 (TEA)2O : 4.72 (HDTMA)2O : 1 021 H2O.
The differing amounts of TBOT given correspond to samples Ti-MCM-41-a and Ti-MCM-41-b.
Preparation of TS-1
The synthesis was carried out according to a patent9, i.e. tetraethyl orthosilicate (Merck) and tetra-
ethyl orthotitanate (Merck) were used as silicon and titanium sources. The template tetrapropylam-
monium hydroxide was prepared from the corresponding bromide (Aldrich) by anion exchange
(Amberlite IRA-400). The crystallization was carried out without stirring in a Teflon-coated auto-
clave (166 °C, 10 days). The obtained crystalline product was washed, dried and calcined under
oxygen flow at 400 °C for 6 h.
Characterization
The titanium content was determined by atomic absorption spectroscopy (AAS Philips PU 9200).
Prior to the analysis, samples were digested by a mixture of HCl, HF and H2SO4 in an autoclave
(180 °C, 2 h).
Powder X-ray diffraction data were obtained on a Seifert 3000 P diffractometer in the Bragg–
Brentano geometry arrangement, using CoKα radiation with a graphite monochromator and a scintil-
lation detector.
The adsorption isotherms of nitrogen at −196 °C were measured with an Accusorb 2100E instru-
ment (Micromeritics). Each sample was degassed at 330 °C for at least 20 h until a pressure of 10−4 Pa
was attained.
The dispersion of titanium species in titanosilicates was determined by the diffuse reflectance
UV/VIS spectroscopy (DRS), which was performed on a Varian Cary-4 double beam spectrometer
with BaSO4 as the standard.
Catalytic Hydroxylation
The hydroxylation of phenol by hydrogen peroxide to give dihydroxybenzenes was performed in a
batch reactor under reflux at 65 °C for 6 h. A suspension of 9.4 g of phenol, 0.5 g of a catalyst, and
28 ml of methanol was uniformly stirred by a nitrogen flow. A total of 4.4 ml of H2O2 (34 wt.%,
Jansen) were added by a Hamilton syringe over 10 min. Prior to the sample withdrawal (0.15 ml) for
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Hydroxylation of Phenol
453
analysis, the nitrogen flow was interrupted, to allow the catalyst to deposit. The products were ana-
lyzed by high-performance liquid chromatography (Merck), using a LiChrosorb RP 18 column and a
UV detector L-4000 at 280 nm. The conversion of hydrogen peroxide was determined by a standard
iodometric titration.
RESULTS AND DISCUSSION
The characteristic feature of powder X-ray diffractograms of both Ti-MCM-41 samples
containing 2.2 and 4.3 mole % of TiO2 (Table I) is that they exhibit reflections only at
small angles 2 θ (Fig. 1). From the comparison with published data it follows that they
can be indexed10 on a hexagonal lattice typical of MCM-41. The lattice constant a0 and
the structure parameters derived from adsorption data are given in Table I. The meso-
pore volume Vme of Ti-MCM-41 was obtained as the adsorption at the relative pressure
of 0.75 corresponding to the horizontal plateau of the adsorption isotherm, converted to
the liquid volume. The size (diameter) of mesopores was calculated using a tubular
TABLE I
Chemical composition and structure parameters
Samplea
x
SBET, m2 g−1 V, cm3 g−1
Dme, nm
a0, nm
t, nm
Ti-MCM-41-a
Ti-MCM-41-b
TS-1
0.022
0.043
0.021
770
827
–
0.621
0.712
0.205
3.22
3.44
–
4.46
4.38
–
1.24
0.94
–
a
x = TiO2/(TiO2 + SiO2) molar fraction of TiO2, SBET surface area, V pore volume, Dme mesopore
size (see text), t = a0 − Dme pore wall thickness.
I
a.u
FIG. 1
X-Ray diffractogram of Ti-MCM-41-a
0
5
10
15
20
2 θ, °
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454
Kulawik, Schulz-Ekloff, Rathousky, Zukal, Had:
model as 4V/S, where S stands for the BET surface area minus the external surface area.
The pore wall thickness of Ti-MCM-41 was estimated as a difference between the
lattice constant a0 and the diameter. With the TS-1 sample, the volume of micropores
was obtained similarly as with Ti-MCM-41.
From the above mentioned data and a detailed analysis of nitrogen isotherms5, it
clearly follows that both titanium-containing MCM-41 sieves are characterized by a
porous structure identical with that of aluminosilicate MCM-41 materials. They contain
a regular, hexagonal array of uniform channels with the pore size of 3.2 – 3.4 nm, the
pore wall thickness being about 1 nm.
In Fig. 2, the diffuse reflectance UV/VIS spectra of all samples are presented and
compared with that of anatase. All individual absorption bands in the diffuse reflectance
spectra of titanium containing molecular sieves have not yet been assigned un-
equivocally. There is a general agreement that titanium(IV) atoms in a lower coordina-
TABLE II
Oxidation of phenol
Conversion, mole %a
Sample
Y, mole %b
p/o Selectivity
phenol
H2O2
Ti-MCM-41-a
Ti-MCM-41-b
TS-1
1.0
1.4
47
54
83
30
14
81
only p
only p
2.7
31.8
a
b
Conversion of phenol and hydrogen peroxide determined after 6 h. Y yield of dihydroxybenzenes
per mole of the reacted phenol.
3
1
F(R)
a.u.
2
4
FIG. 2
Diffuse reflectance spectra of: bulk anatase 1,
Ti-MCM-41-a 2, Ti-MCM-41-b 3, TS-1 4. F(R)
is the Kubelka–Munk function
200
300
400
λ, nm
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Hydroxylation of Phenol
455
tion state, probably tetrahedrally coordinated, should be responsible for the peak at ca
205 nm, and the band at ca 330 nm corresponds to anatase11–15. The bands between
these two limits could be assigned to different molecular TiOx-species15.
From Fig. 2 it follows that the diffuse reflectance spectra of Ti-MCM-41 containing
2.2% of TiO2 and TS-1 are similar, both containing titanium mostly in the tetrahedral
coordination. The spectrum of Ti-MCM-41 with 4.3% of TiO2 is also blue shifted in
comparison with that of anatase. It contains a broad band between 230 and 280 nm
which can be assigned to the above mentioned TiOx-species.
In Table II the results of catalytic experiments are summarized. While the conversion
of phenol and hydrogen peroxide over both samples of Ti-MCM-41 is almost the same,
it is much lower than that achieved with TS-1. In the case of Ti-MCM-41, the main
products were hydroquinone and p-quinone (which was not determined quantitatively),
no ortho products being found. The catalytic activity and selectivity of the TS-1 sample
are comparable to the values obtained in other laboratories16–19
.
The practically exclusive formation of para isomers in the oxidation of phenol over
Ti-MCM-41 is surprising. This, and also the p/o ratio over TS-1 higher than 2, require
the assumption of additional effects favouring formation of para isomer molecules.
On the basis of a study of phenol oxidation20 over TS-1, the product and transition-
state shape-selectivity as well as the different diffusivities of the para and ortho mole-
cule can be excluded as a reason of the increase in the p/o ratio. It has been suggested20
that the increased p/o ratio over TS-1 might be explained by the different adsorption
ability of the para and ortho products on the catalyst or by different rates of polymeri-
zation of both products. The stronger adsorption of the ortho molecule can be expected
from the ortho hydroxyl groups enabling a twofold attachment to an active centre
(structure A).
Therefore, one can suggest that the kinetics is influenced by the rate law comprising
a term which considers the different strength of adsorption of both products. Alterna-
tively, the faster polymerization of the ortho product can also be the cause of the ob-
served selectivity.
As follows from DRS, both Ti-MCM-41 samples contain the titanium component in
different coordination, but always in a highly dispersed state. This is in agreement with
their similar catalytic properties. The explanation suggested for the p/o selectivity of
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

456
Kulawik, Schulz-Ekloff, Rathousky, Zukal, Had:
the phenol oxidation over TS-1 is also confirmed by the results obtained over Ti-MCM-41,
because a very high para selectivity has been found on the purely mesoporous catalyst.
The authors (K. K., G. S.-E., J. R., A. Z.) are grateful to the Volkswagen Foundation for financial
support (Grant No. I/69 159).
REFERENCES
1. Weissermel K., Arpe H.-J.: Industrial Organic Chemistry, p. 358. VCH Publ., Weinheim 1993.
2. Corma A., Navarro M. T., Perez Pariente J.: J. Chem. Soc., Chem. Commun. 1994, 147.
3. Corma A., Navarro M. T., Perez Pariente J., Sanchez F.: Stud. Surf. Sci. Catal. 84, 69 (1994).
4. Tanev P. T., Chibwe M., Pinnavaia T. J.: Nature 368, 321 (1994).
5. Franke O., Rathousky J., Schulz-Ekloff G., Starek J., Zukal A.: Stud. Surf. Sci. Catal. 84, 77
(1994).
6. Perego G., Bellussi G., Corno C., Taramasso M., Esposito A.: Stud. Surf. Sci. Catal. 28, 129
(1986).
7. Notari B.: Stud. Surf. Sci. Catal. 37, 413 (1987).
8. Bellussi G., Carati A., Clerici M. G., Maddinelli G., Millini R.: J. Catal. 133, 220 (1993).
9. Taramasso M., Perego G., Notari B.: U.S. 4 410 501 (1983); Chem. Abstr. 95, 137 (1981).
10. Beck J. S., Vartuli J. C.,. Roth W. J., Leonowicz M. E., Kresge C. T., Schmitt K. D., Chu C.
T.-W., Olson D. H., Scheppard E. W., McCullen S. B., Higgins J. B., Schlenker J. L.: J. Am.
Chem. Soc. 114, 10834 (1992).
11. Boccuti M. R., Rao K. M., Zecchina A., Leofanti G., Petrini G.: Stud. Surf. Sci. Catal. 48, 133
(1989).
12. Zecchina A., Spoto G., Bordiga S., Ferrero A., Petrini G., Leofanti G., Padovan M.: Stud. Surf.
Sci. Catal. 69, 251 (1991).
13. Geobaldo F., Bordiga S., Zecchina A., Giamello E., Leofanti G., Pertini G.: Catal. Lett. 16, 109
(1992).
14. Blasco T., Camblor M. A., Corma A., Perez-Pariente J.: J. Am. Chem. Soc. 115, 11806 (1993).
15. Klaas J., Kulawik K., Schulz-Ekloff G., Jaeger N. I.: Stud. Surf. Sci. Catal. 84, 2261 (1994).
16. Tuel A., Moussa-Khouzami S., Ben Taarit Y., Naccache C.: J. Mol. Catal. 68, 45 (1991).
17. Thangaraj A., Kumar R., Ratnasamy P.: J. Catal. 131, 294 (1991).
18. van der Pol A. J. H. P., Verduyn A. J., van Hooff J. H. C.: Appl. Catal., A 92, 113 (1992).
19. Kraushaar B., van Hooff J. H. C.: Catal. Lett. 2, 43 (1989).
20. Vetter S., Schulz-Ekloff G., Kulawik K., Jaeger N. I.: Chem. Eng. Technol., in press.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)