Selective oxidations with hydrogen peroxide and titanium silicalite catalyst

Article abstract of DOI:10.1016/S1381-1169(99)00115-6

Titanium silicalite (TS-1) samples with increasing amount of titanium incorporated into the silicalite framework (from 1 to 3.45 wt.% TiO₂) have been synthesized and characterized by UV-DRS spectroscopy. The samples have been tested on propylen

Full text of DOI:10.1016/S1381-1169(99)00115-6

Journal of Molecular Catalysis A: Chemical 146 Ž1999. 223–228
www.elsevier.comrlocatermolcata
Selective oxidations with hydrogen peroxide and titanium
silicalite catalyst
a,)
b
a
a
a
M.A. Mantegazza , G. Petrini , G. Span o` , R. Bagatin , F. Rivetti
a
EniChem, Via G. Fauser 4, 28100 NoÕara, Italy
Via C. Battisti 39A, 28066 Galliate (NO), Italy
b
Abstract
Titanium silicalite ŽTS-1. samples with increasing amount of titanium incorporated into the silicalite framework Žfrom 1
to 3.45 wt.% TiO . have been synthesized and characterized by UV-DRS spectroscopy. The samples have been tested on
2
propylene epoxidation, cyclohexanone ammoximation and ammonia oxidation reactions. It has been found that the yield
obtained with TS-1, expressed as turnover number, increases with the Ti content. The trend is common for all the reactions
investigated. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Ti-silicalite; Hydrogen peroxide; Ammoximation; Cyclohexanone; Epoxidation; Propylene oxide; Ammonia oxidation
1
. Introduction
turns into water, and the reactions, operated in
liquid phase under mild conditions, show very
high selectivities and yields reducing problems
and costs of by-product treatment.
Titanium silicalite TS-1 , a zeolitic material
Ž .
discovered by ENI researchers w1x, is one of the
first examples of a MFI structure in which the
framework heteroatom is different from Al.
TS-1 catalyzes a variety of useful oxidation
reactions. Such reactions include phenol hy-
droxylation w2x, olefin epoxidation w3x, cyclohex-
anone ammoximation w4x, alkane oxidation w5x,
oxidation of ammonia to hydroxylamine w6x,
secondary alcohols to ketones w2x, secondary
amines to dialkylhydroxylamines w7x.
The confinement of the metal species in the
well defined silicalite pore system endows TS-1
with shape selectivity properties analogous to
enzymes. For these features the application of
the terms ‘mineral enzyme’ or ‘zeozyme’ to
TS-1 is appropriate w8x.
The outstanding catalytic activity and selec-
tivity of TS-1 have made it a commercial cata-
lyst for the manufacture of catechol and hydro-
quinone from phenol. Enichem is now develop-
ing the propylene epoxidation and the ammoxi-
mation of cyclohexanone as low-salt by-produc-
tion alternatives to the conventional processes
TS-1 catalyzed processes are advantageous
from the environmental point of view as the
oxidant is aqueous hydrogen peroxide, which
for propylene oxide
oxime.
PO and cyclohexanone
Ž .
)
Corresponding author
1
381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S1381-1169Ž99.00115-6

2
24
M.A. Mantegazza et al.rJournal of Molecular Catalysis A: Chemical 146 (1999) 223–228
The ammoximation reaction proceeds accord-
ing to a mechanism involving two steps w9x.
. In the first step, ammonia is catalytically
oxidized by hydrogen peroxide to hydroxyl-
amine, the reaction intermediate:
In order to elucidate the structure of the Ti
centre in vacuum and in the reaction conditions
several techniques have been used, such as
UV–Vis, EPR, IR and Raman spectroscopies,
as well as XRD, XANES and EXAFS w9,12–15x.
1
TS-1
Tetrahedral titanium atoms located in the
framework sites are characterized by the 960
NH qH O
NH OHqH O
3
2
2
2
2
y
1
1
2
. In the second step, hydroxylamine reacts
cm
band in the IR spectra and the 48 000
y
with cyclohexanone to give cyclohexanone
oxime:
cm band in the UV–Vis spectra w9–15x.
The unit cell parameters of TS-1 were found
to increase linearly as a function of Ti content
TS-1
C H OqNH OH
C H NOHqH O
6
10
2
6
10
2
Ž
.
w
x
XRD 1,12 .
Cyclohexanone and other ketones small
Recently it has been found that the coordina-
enough to diffuse into the zeolite channels can
immediately react with hydroxylamine inside
the TS-1 pores. Bulky ketones, which are not
able to penetrate the zeolite channels, can react
only with the hydroxylamine migrated to the
external surface w9x.
For epoxidation reaction a mechanism in-
volving a solvent assisted transition state is
proposed and detailed in Ref. w10x. A protic
solvent, actually methanol, stabilizes hydroper-
oxo Ti species with a five member cyclic con-
figuration suitable for the approach of the olefin
to the oxygen farther from Ti atom.
The unique activity and selectivity of TS-1 is
believed to be due to isolated sites of tetrahedral
titanium atoms inserted in vicariant position of
silicon in the silicalite framework. The isolated
and tetracoordinated Ti centres are able to ex-
pand their coordination sphere up to six by
tion number of Ti, in a dehydrated TS-1 sample,
is larger than 4. EXAFS data analysis gives a
value of 4.44"0.25 w15x. This result indicates
the presence of Ti species different from the
perfect ‘closed’, four-fold coordinated Ti
ŽOSi.4
species. In full agreement with the EXAFS re-
sults, from photoluminescence spectroscopy two
bands have been observed in both emission and
excitation scans, so indicating the presence of
two slightly different families of framework Ti
species which differ for their local environment
w15x.
In this paper we present and discuss new
results on TS-1 reactivity as a function of the Ti
content. TS-1 samples with increasing amount
of titanium incorporated into the silicate frame-
work have been synthesized and characterized.
The samples have been tested in ammonia
oxidation, cyclohexanone ammoximation, and
propylene epoxidation.
interaction with extra ligands
monia . It has also been demonstrated that Ti
Ž
i.e., water, am-
.
The aim of this work is to provide new
information on the catalytic behaviour of the
active sites. In particular, very useful informa-
tion have been obtained from the comparison of
the behaviour of the catalyst used in the differ-
ent investigated reactions.
centres interact with hydrogen peroxide to form
hydroperoxo species w9x.
The framework composition of TS-1 can be
defined as: xTiO2 P
limit for x the Ti mole fraction is around
.
0
Ž .
1yx SiO , and the upper
2
Ž
.025 w12x. Attempts to produce TS-1 with sig-
nificantly higher Ti content fail, as the excess
titanium segregates as TiO . The catalytic acti-
2
2
. Experimental
vity of TS-1 is affected by the presence of extra
framework Ti, which can promote extensive
side reactions including decomposition of hy-
drogen peroxide.
TS-1 samples were synthesized and charac-
terized according to Ref. w1x.

M.A. Mantegazza et al.rJournal of Molecular Catalysis A: Chemical 146 (1999) 223–228
225
Table 1
Ammoximation runs were performed in a
List and composition of TS-1 samples
semibatch apparatus. Details of the method of
operation and analytical procedures have been
provided previously w4x.
TS-1 sample
Crystalline product
x ŽTi mole fraction.a
a
TiO Žwt.%.
2
A
B
C
D
E
F
G
H
0.0073
0.0105
0.0152
0.0192
0.0220
0.0226
0.0229
0.0262
0.97
1.39
2.01
2.54
2.90
2.99
3.02
3.45
Different amounts of catalyst were used in
order to have the same amount of Ti w0.228 g
Ž
TiO
.rlx in each experiment. Hydrogen perox-
2
ide
suspension wcyclohexanone 9.8 g: t-butanol
ml : H O 12 ml :NH 30 wt.%, 13 ml
88C.
On completion of the H O addition the
Ž
30 wt.%
.
was fed over 1 h to the reaction
25
x at
Ž
.
Ž
.
Ž
3
.
2
7
a
Determined by chemical analysis ŽXRF..
2
2
suspension was cooled, filtered and analyzed.
Ammonia oxidation experiments were carried
out as described in detail in Ref. w6x. Hydrogen
tured and broader towards lower wavenumbers
upon increasing the Ti content.
This suggests the presence of slightly differ-
ent framework Ti species, in agreement with
what found with photoluminescence spec-
troscopy w15x.
peroxide was added over 20 min, at 708C, to the
reaction suspension wt-butanol
ml : NH 30 wt.%, 13 ml
x.
The NH rH O mole ratio was 20, the Ti
Ž . Ž
25 ml : H O 12
2
.
Ž
.
3
3
2
2
concentration was 0.335 g
Propylene epoxidation was performed in a
Mettler RC1 calorimeter 1 l CSTR reactor
The procedure is described in Ref. w3x.
The reaction conditions were: Ts408C, P 4
atm. The reaction solvent was MeOH:H O 92:8
, the H O concentration in the reaction
Ž .
TiO rl.
2
3
.2. ActiÕity test
Ž
.
.
The TS-1 samples were tested in ammonia
oxidation, cyclohexanone ammoximation and
propylene epoxidation reactions.
As the Ti content of the samples was not
constant, different amounts of catalyst were used
in order to have the same amount of Ti in each
series of experiments.
Ž
2
wt.%
.
2
2
mixture was 1 MrKg, the Ti concentration was
0
.135 g TiO rKg.
Ž .
2
The yield is expressed as turnover number
Ž
TON , given as moles of product oxime, hy-
. Ž
3
. Results and discussion
droxylamine or propylene oxide wPOx.
per Ti
site per hour.
3
.1. TS-1 synthesis and characterization
All reported data
lectivity are expressed as a percentage of the
Ž
TON, conversion and se-
.
The composition of the catalysts used, in
maximum value observed, i.e. for the turnover
terms of Ti content are reported in Table 1.
The Ti mole fraction range from 0.0073
number:
TON%sample i
s
Ž
TONsample irTONmaximum
100
.
Ž
sample A
In Fig. 1 the DRS UV–Vis spectra of sam-
ples A xs0.0073 , C xs0.0152 and H
xs0.0262 are reported. They exhibit only the
to 0.0262 sample H .
. Ž .
Lines in the plots are an aid for visualization
and do not represent any mathematical interpre-
tation.
Ž
.
Ž
.
Ž
.
y
1
characteristic 48 000 cm band of tetracoordi-
nated Ti species, which confirms that no extra
framework Ti is present in the TS-1 samples.
Anyway it is possible to see that the band
shape changes: the band becomes more struc-
3.2.1. Ammonia oxidation to hydroxylamine
The results are reported in Fig. 2. The TON%
is plotted vs. the molar fraction of Ti in the
TS-1 sample.

2
26
M.A. Mantegazza et al.rJournal of Molecular Catalysis A: Chemical 146 (1999) 223–228
Fig. 1. DRS UV–Vis spectra of TS-1 samples A, C and H.
In the ammonia oxidation reaction almost all
the TS-1 but sample A
similar yield.
xs0.0073 show a
Ž .
Fig. 3. Cyclohexanone ammoximation. Ža. Activity data, Žb. cy-
clohexanone conversion and selectivity to oxime as a function of
the Ti mole fraction in the TS-1 samples.
The hydrogen peroxide conversion is almost
complete )98 mol% in all the experiments.
Ž
.
The lower yield of sample A is due to a
lower selectivity to hydroxylamine, based on
hydrogen peroxide.
oxime, based on cyclohexanone, as shown in
Fig. 3b.
Therefore, in the two related reactions, am-
monia oxidation and overall ammoximation re-
action, an apparently different trend is observed
as a function of Ti content.
3
.2.2. Cyclohexanone ammoximation
In the cyclohexanone ammoximation the yield
increases by increasing the Ti content up to x
about 0.020 Fig. 3a , with an almost linear
relationship between the two variables. A re-
markable scattering of values larger than the
is observed in the region of
Ž
.
3
.2.3. Propylene epoxidation
Ž
As depicted in Fig. 4, the effect of Ti content
in the propylene epoxidation shows a similar
trend as in the ammoximation reaction.
experimental error
.
high x.
The increase of the catalyst yield is due to an
increase of both conversion and selectivity to
Fig. 2. Ammonia oxidation. Activity data as a function of the Ti
mole fraction in the TS-1 samples.
Fig. 4. Propylene epoxidation. Activity data as a function of the Ti
mole fraction in the TS-1 samples.

M.A. Mantegazza et al.rJournal of Molecular Catalysis A: Chemical 146 (1999) 223–228
227
As in ammoximation, the increase of the
catalyst yield is due to an increase of both
propylene conversion and selectivity to the
epoxide. The scattering of values in the region
of high x is here even more evident.
monia oxidation doesn’t show a very evident
trend by increasing Ti concentration, which
makes b) less likely.
However it cannot be excluded that both
hypotheses play a role in the overall activity of
TS-1. Indeed, the observed scattering of activity
3
.3. Discussion
data at similar high Ti content, cannot be
Ž .
explained by taking into consideration only the
Attempting to explain the different catalytic
first hypothesis.
results on increasing Ti content, two evidences
from characterization should be taken into ac-
count:
4. Conclusion
1
. the unit cell parameters of TS-1 increase
linearly with Ti content XRD
. at least two different framework Ti species
Ž
.
w1,12x
A dependence of the activity of Ti site upon
framework Ti content in TS-1 for cyclohex-
anone ammoximation and propylene epoxida-
tion has been found. In ammonia oxidation all
Ž .
the samples except at very low Ti content
show a similar activity.
Two explanations for this behaviour are pos-
sible on the basis of the available characteriza-
tion data. The first deals with morphological
parameter, actually the pore diameter, which
increases with increasing Ti content. The sec-
ond, merely chemical, suggests a different acti-
vity for the slightly different Ti species ob-
served in TS-1.
The trend found for the catalytic activity
highlights the paramount importance of produc-
ing well structured TS-1 with the highest load-
ing of framework titanium.
2
can be detected in TS-1 w15x, one of which
seems to increase with Ti content
On the basis of the above evidences two
hypotheses can be proposed.
a) Ti content affects the catalytic actiÕity by
affecting the unit cell parameters.
In fact the first evidence implies an increase
in the micropore diameter, which can influence
the diffusion rate of reagents inside the chan-
nels.
It was already reported that channels dimen-
sion is affected by framework Ti and that
molecules with large kinetic diameter
xylene are sensitive probe for slight changes in
micropore diameter w16x.
Ž
m-
.
This could explain the differences found in
the three test reactions. Low or no effect has
been found in ammonia oxidation, the smallest
reagent. The effect is well evident in propylene
epoxidation and becomes remarkable in cyclo-
hexanone ammoximation, the reagent with the
largest molecular size. In fact it has been re-
ported that cyclohexanone reacts with the hy-
droxylamine intermediate in the TS-1 channels
w9x.
b) The different framework Ti species haÕe a
different catalytic actiÕity.
This hypothesis appears difficult to support
since no correlation has been drawn out as the
two species are only qualitatively recognized
but not quantitatively measured. Moreover am-
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