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Selective oxidation of pyridine to pyridine-N-oxide with hydrogen peroxide over Ti-MWW catalyst
Wei Xie,
Yuting Zheng, Song Zhao, Junxia Yang, Yueming Liu∗, Peng
Wu∗
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, North Zhongshan Rd.
3663, Shanghai 200062, China
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a r t i c l e i n f o a b s t r a c t
Article history:
Available online 15 March
2010
Keywords: Oxidation Pyridine Pyridine-N-oxide Ti-MWW
Hydrogen peroxide
The oxidation of pyridine to pyridine-N-oxide (PNO)
with hydrogen peroxide has been investigated on various titanosilicate
catalysts. Superior to other titanosilicates like TS-1, Ti-Beta and Ti-MOR,
Ti-MWW showed a higher catalytic activity and product selectivity. The reaction
parameters such as solvent, tem- perature, hydrogen peroxide/substrate ratio,
the amount of catalyst and Ti content were optimized to maximize the PNO yield.
Ti-MWW was capable of giving pyridine conversion and PNO selectivity both over
99% under optimum reaction conditions. Ti-MWW was a highly active, selective,
and reusable cat- alyst for the synthesis of PNO. The mechanism for the
oxidation of pyridine over titanosilicates has also been considered. Ti-MWW was further structurally modified to posses
open reaction spaces
by interlayer expansion,
which reduced the diffusion limitations in the oxidation of pyridine
derivatives with bulkier molecular dimensions.
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© 2010 Elsevier B.V. All rights reserved.
1.
Introduction
Pyridine-N-oxide (PNO) is an important fine
chemical with a constantly increasing world market owing to its usefulness as syn-
thetic intermediates and biological importance. Heterocyclic PNO is also
used as protecting groups, auxiliary agents,
oxidants, ligands in metal
complexes and catalysts [1]. In addition, PNO is used as a functional chemical building block
for synthesizing agrochemicals and pharmaceuticals.
The conventional processes for manufacturing PNO are of mul-
tisteps involving sulfuric
acid-catalyzed oxidation of pyridine with H2O2 as an oxidant, acetic acid as the solvent, and the
separa- tion of PNO by neutralizing sulfuric acid with sodium hydroxide [2]. These processes are encountering serious
disadvantages, such as using
excessive solvent like acetic acid and corrosive sulfuric acid, producing onslaughts of valueless sodium sulfate by-product together with toxic waste
solvent and bringing about environ- mental problems. Therefore, it
is urgent to develop costless and environmentally benign methods of
zero-emission for PNO syn- thesis.
The
discovery of TS-1 titanosilicate has opened up new pos- sibilities of
developing zeolite catalyst-based green processes, as the material is capable
of catalyzing a variety of organic com- pounds
with H2O2 and with
water as almost
the sole by-product [3].
 |
Recently, TS-1 is reported
to be useful for the oxidation of primary,
secondary amines and pyridine derivatives [4]. However, TS-1 is still facing some drawbacks. Its manufacture cost is generally
high due to the use of organic silicon
source and tetrapropylammonium hydroxide as
structure-directing agent. Moreover, the MFI
topol- ogy of TS-1 is composed of medium-sized pores of 10-membered ring (MR), which limits the
application of TS-1 to the oxidations involving the substrates and oxidants
both with relatively small molecular sizes.
Many
other titanosilicates with various crystalline topologies have been developed
after TS-1. Up to date, only Ti-MWW with the
MWW structure seems
to be intrinsically more active
than TS- 1 [5]. The advantage of Ti-MWW is
firmly related to its unique pore system consisting of two independent 10-MR
channels, 12- MR supercages and external side cups as well. Since the MWW
zeolite has structural diversity, it is possible
to transform Ti-MWW into the titanosilicates with open
porosity, such as Del-Ti-MWW [6] and IEZ-Ti-MWW [7] through phase delamination and inter- layer expanding, respectively. Ti-MWW has been shown to be an
efficient for the epoxidation of functional alkenes
and ammoxima- tion of cyclohexanone. These
successes encourage us to investigate its application in the
synthesis of valuable oxygenated chemicals containing other functional groups.
In this
study, we have carried out the oxidation of pyridine and its derivatives on
Ti-MWW, TS-1, Ti-Beta and Ti-MOR with H2O2
or tert-butyl hydroperoxide (TBHP) with the
purpose to synthe- size N-oxides
actively and selectively. The stability and reusability of Ti-MWW have been
investigated in detail. In particular, IEZ-Ti- MWW with an open reaction space
has been prepared to improve
0920-5861/$ – see front
matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.045
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No.
|
Oxidant
|
Pyridine
conv. (%)
|
PNO
sel. (%)
|
XH2 O
|
2 (%)
|
UH2 O
|
2 (%)
|
||
|
1
|
Ti-MWW
(43)
|
H2 O2
|
97.7
|
99.4
|
96.0
|
77.8
|
252
|
||
|
2
|
TS-1 (40)
|
H2 O2
|
99.2
|
95.5
|
97.1
|
75.1
|
238
|
||
|
3
|
Ti-Beta (70)
|
H2 O2
|
51.5
|
95.7
|
70.2
|
54.0
|
216
|
||
|
4
|
Ti-MOR (92)
|
H2 O2
|
24.5
|
90.2
|
60.3
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28.2
|
135
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||
|
5
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Silicalite-1
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H2 O2
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0
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0
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–
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–
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0
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||
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6
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none
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H2 O2
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0
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0
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–
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–
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0
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||
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7
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Ti-MWW (43)
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TBHP
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13.0
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99.0
|
–
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–
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33
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||
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8
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TS-1
(40)
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TBHP
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1.4
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98.0
|
–
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–
|
3
|
||
|
Ti-Beta
(70)
|
TBHP
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2.4
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97.6
|
–
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–
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10
|
|||
a Reaction conditions: cat., 0.15 g;
pyridine, 30 mmol; H2 O2 (30%), 39 mmol; temp., 348 K; time, 2
h; without solvent.
b The number in parentheses indicates the Si/Ti molar ratio.
c TOF: turnover frequency.
the catalytic activity in
the oxidation of bulky amine substrates. This study may provide ideas for
developing cleaner processes for the production of PNO and other oxides of
pyridine derivatives.
2.
Experimental
2.1. Catalyst
preparation and characterization
Ti-containing
MWW lamellar precursor was hydrothermally synthesized following the procedures reported previously [5]. The precursor was then refluxed with 2 M HNO3 to result in the samples with Si/Ti ratios of 43–110 and Si/B ratios
of 36–56. The acid-treated
sample was washed with deionized
water and subsequently dried at 373 K (Ti-MWW-dry) or further calcined at 823 K (Ti-MWW-cal.).
Interlayer-expanded zeolite, IEZ-Ti-MWW, was postsynthesized by alkoxysilylating Ti-MWW-dry with diethoxydimethylsilane [7]. TS-1 with a Si/Ti molar ratio of 40 was
hydrothermally synthe- sized with Enichem
method [8]. Al-free Ti-Beta was
hydrothermally synthesized by a seeding method [9], while Ti-MOR was postsyn-
thesized by vapor-phase treatment of delaminated mordenite with TiCl4 [10].
The samples were characterized by inductively
coupled plasma
(ICP) on a Thermo IRIS Intrepid
II XSP atomic emission spec- trometer, by X-ray diffraction (XRD) on a Bruker
D8 Advance diffractometer
(Cu Ka), by
N2 adsorption
(Autosorb Quancachrome 02108-KR-1) and by UV–vis
(Shimadzu UV-2400PC) and IR (Nicolet NEXUS 670) spectroscopies.
2.2.
Catalytic reactions
The
liquid-phase oxidation of pyridine was carried out under vigorous stirring
in a 50 mL glass
flask connected to a cooling
con- denser. In a typical run, a desirable
amount of solvent, 30 mmol of pyridine, 39 mmol of H2O2 (30 wt% aqueous solution) or TBHP (70 wt%
aqueous solution) and 0.15 g catalyst were mixed in the flask and heated at
different temperature under agitation for 2 h. The reaction mixture was converted into a homogeneous phase by mixing with
enough ethanol. After removal of the catalyst pow- der, the products formed were then determined using authentic chemicals on a GC–MS (Agilent GC–MS (6890-5973N)). And their
amounts were quantified by GC (Shimadzu, FID
detector and 30-m OV-1 column). The amount
of unconverted H2O2
was quantified by iodometry.
3.
Results and discussion
3.1.
Summary of catalyst characterization
The XRD
patterns confirmed that all titanosilicates had objec- tive zeolite
structures with a relatively high crystallinity (Fig. S1).
No impurity phase was detected.
The samples showed the charac- teristic adsorption bands around
220 nm in UV–vis spectra
(Fig. S2) and at 960 cm−1 in
IR spectra, respectively (not shown), which indicated that they contained the tetrahedral Ti species dispersed isolatedly in the zeolite
framework [3]. The Langmuir specific
sur- face areas determined from N2 adsorption were in the range of 590–635 m2 g−1 for
the Ti-MWW samples with the Si/Ti molar ratios of 43–110, 530 m2 g−1 for
TS-1, 553 m2 g−1 for
Ti-MOR and 653 m2 g−1 for Ti-Beta. Above characterizations verified
that these titanosilicates were good in quality as liquid-phase oxidation cat- alysts.
3.2. A comparison of oxidation among various titanosilicates
Table 1 compares the results of pyridine oxidation over
dif- ferent titanosilicates using hydrogen peroxide or TBHP as an oxidant.
Ti-MWW showed a pyridine conversion of 97.7%
and PNO selectivity of 99.4% with
H2O2 as an oxidant. The absolute amount of
by-products other than PNO was extremely small (No. 1). Under the reaction
conditions same as those adopted for Ti- MWW, TS-1 showed a comparably high
pyridine conversion but a lower PNO
selectivity than Ti-MWW (No. 2). The by-products were mainly 2-hydroxypyridine,
4-hydroxypyridine, oxaziranes, and azoxy compounds, which were consistent with
those reported in previous studies [11]. The specific activity with respect to Ti species,
i.e. turnover frequency (TOF), indicated that
TS-1 was less active. The superiority of Ti-MWW to
TS-1 has already been observed in the epoxidation of various kinds of alkenes [12], which is attributed to the unique pore system of
MWW zeolite favoring the adsorption and access of substrate molecules to the Ti
active sites. The efficiency of H2O2 utilization was 75–78% for Ti-MWW and TS-1.
Nonproductive decomposition of H2O2 was mainly due to the basic media with the presence
of pyridine.
Ti-MOR and Ti-Beta turned
out to be much less active and selec-
tive than Ti-MWW and TS-1 (Nos. 3 and 4). Ti-Beta exhibited highly hydrophilic features due
to stacking defaults, which made it to be an
unsuitable catalyst for
pyridine oxidation. Since
Ti-MOR had larger crystal size and only one-dimensional channels,
both of which hin- der the diffusion and access of substrate molecules to the Ti sites,
it was also an unsuitable catalyst for pyridine oxidation. In spite of the
difference in optimum reaction conditions, the titanosili- cates exhibited in
pyridine oxidation a catalytic activity order of Ti-MWW > TS-1 ∗ Ti-Beta
> Ti-MOR. In particular, no product was obtained either in the absence of any catalyst
or in the presence of Ti-free Silicalite-1 (Nos. 5 and 6).
The results clearly indicate that the isolated tetrahedral Ti4+ ions in titanosilicates are the active
sites for pyridine oxidation.
In comparison with H2O2, the use of TBHP with a bulky molec- ular size made the reaction
greatly retard (Nos. 7–9). In general, the titanosilicate-catalyzed reactions
are considered to involve the 5-MR intermediates which are formed
through the coordination
of a solvent molecule such as alcohol
or H2O to a Ti peroxo species
[13]. Species I and II shown
in Scheme 1 are thus presumed
to be the intermediates for the oxidations with TBHP and H2O2, respectively, since both oxidants
contained water. The species I is much larger
than the species II owing to a larger molecular dimension of tert- butyl groups. An actual reaction
occurs only when the molecules can
reach the above intermediates. The medium pores of TS-1 imposed a serious
steric restriction for the intermediate of
TBHP, which led to a very low activity. Similarly, the oxidation of pyridine with
TBHP was restricted seriously on Ti-Beta. Although the cat- alytic performance of Ti-MWW was also affected
by the molecular sizes of the oxidant, it showed a higher pyridine
conversion than TS-1 and Ti-Beta in the case of TBHP. This is because in addition to two-dimensional 10-MR sinusoidal channels
running throughout the structure
parallel to the ab-plane, the MWW structure
contains also an independent channel system which is comprised of large supercages (0.7 nm × 0.7
nm × 1.8
nm), as well as the pocket or cup
moieties (0.7 nm × 0.7 nm) of the supercages on the crystal
exte- rior. These open reaction spaces make Ti-MWW more active than
TS-1 and Ti-Beta for the pyridine oxidation with TBHP.
3.3. Effects of reaction parameters on the oxidation
of pyridine over Ti-MWW
3.3.1.
Effect of H2O2/pyridine ratio
Fig. 1 shows the effect of H2O2/pyridine ratio on the pyridine oxidation over Ti-MWW.
The pyridine oxidation stoichiometrically needs equivalent moles of H2O2
and pyridine, but the pyridine con- version was only 75% at H2O2/pyridine ratio of 1.0. With increasing
H2O2/pyridine ratio, the pyridine conversion increased simultane- ously. Due to unproductive
decomposition of H2O2, the reaction needed more H2O2 than pyridine to proceed to a high level. A high
PNO selectivity (>97%) was retained at different H2O2/pyridine ratios. The pyridine conversion reached nearly 100% when the
 |
Fig. 1. Effect of H2 O2 /pyridine ratio on the pyridine
oxidation over Ti-MWW (43). Oxidation conditions:
see Table 1.
Fig. 2. Effect of reaction
temperature on the pyridine oxidation
over Ti-MWW (43). Oxidation conditions: see Table 1.
H2O2/pyridine ratio was above 1.3, and showed
no change at higher
ratios.
3.3.2. Effect of reaction temperature
The
reaction temperature had a great effect on the oxidation of pyridine over
Ti-MWW. As shown in Fig. 2, the conversion of pyridine increased with increasing temperature and
reached the maximum at 353 K, whereas
the selectivity to PNO remained
over 98.0% at the temperatures investigated. Further increasing the tem-
perature to 358 K, no further changes
were observed for either the pyridine conversion or the
PNO selectivity.
3.3.3. Effect of
solvent
Titanosilicate-catalyzed reactions are well known
to depend greatly on the nature of solvent. The oxidation of pyridine over
Ti-MWW has been carried out in the solvents such as methanol, ethanol,
isopropanol, t-BuOH, water, acetonitrile, acetone,
ether, or free of any additional solvent (Fig. 3). The PNO selectivity was over 99.4% in all cases except for
acetonitrile and acetone which showed a selectivity of 96–98.0%. With methanol
and water as effective solvents for the oxidation of pyridine, the PNO yield
decreased in the order of methanol ≈ H2O >
non-solvent > ethanol
> acetonitrile > isopropanol > t-BuOH ∗ acetone
> ether. The effect of solvent is one of the most complicated
issues in the catalytic sys-
 |
Fig. 3. The results of pyridine oxidation on Ti-MWW free of any
additional sol- vent (a), and in the presence of methanol (b), ethanol (c),
isopropanol (d), t-BuOH (e), water (f), acetonitrile (g), acetone (h), and
ether (i). Oxidation conditions: the amount of solvent, 0.19 mol; others see Table 1.
Fig. 4. Dependence of the pyridine oxidation
on the Ti content of Ti-MWW. Reaction conditions: see Table 1.
tem of titanosilicate/H2O2. Although MeCN has been reported to be a suitable
solvent for the epoxidation of alkenes on Ti-MWW, it only led to a moderate activity (96.4%
pyridine conversion). In contrast, when the system was free of solvent except for
the water originally coming from the aqueous H2O2 solution, the conversion increased to 97.7% at 348
K. In comparison to other solvent, iso- propanol or t-BuOH with a large molecular dimension made the
oxidation retard greatly. The pyridine conversion decreased signif- icantly
with non-polar solvents. The conversion turned to be 50.6% and 15.1% in acetone
and ether, respectively. Above results suggest the reaction rate is closely
related to the polarity and molecular size of the solvents, and also the
solubility of the substrates, products, etc.
A series
of Ti-MWW catalysts with Si/Ti ratios of 43–110 were prepared and used in the
oxidation of pyridine (Fig. 4). Reasonably, the yield of PNO increased with an increasing amount of the
Ti active sites in Ti-MWW. It should be noted that the oxidation did not
proceed in the presence of Ti-free MWW
zeolite.
3.4. The stability and regeneration of Ti-MWW
In
addition to the catalytic activity and PNO selectivity, the reusability of Ti-MWW is also important issue as it determines the real
applicability. The stability and reusability of Ti-MWW in the pyridine
oxidation have thus been checked. The experiments were initiated with an
enlarged reaction scale using 0.3 g of Ti-MWW, and the used catalyst was
subjected to ICP analyses and repeated reactions under the same conditions. After
calcination in air at 823 K
for 6 h, the color of the used catalyst
changed from light brown to
Fig. 5. The reuse of Ti-MWW in pyridine oxidation. The used
catalyst was regen- erated by water washing and further calcination in air at
823 K for 6 h. Reaction conditions: see Table 1.
snow white, indicating a
total removal of heavy organic species. This
kind of regeneration actually prevented the catalyst from deac-
tivating. As shown in Fig. 5, no obvious Ti leaching occurred
after the repeated reaction while the content of boron was gradually
decreased as the boron species
moved out of the framework easily due to a small ionic radius.
To
further verify the heterogeneous nature of catalysis and assess the absence of
soluble active species, we have carried out the hot filtration test following the suggestion in the literature [14]. The experiments involved performing two parallel reactions in the presence of the solid catalyst (Fig. S4). When the conversion of 62% was reached
at 20 min, the solid catalyst was removed by filtration
for one reaction. The reaction was terminated since the pyridine conversion was
almost unchanged with prolonging the reaction time. On contrast, the pyridine conversion for the reaction
without hot filtration increase with reaction time and reached about 98%
at 120 min. This result indicates that the liquid-phase oxidation of pyridine actually
takes place in a heterogeneous manner, and also rules out the leaching
of Ti species during the reaction.
The used
catalyst showed similar
XRD pattern and
UV–vis. spec- trum to the
fresh one (Figs. S5 and S6), indicating
no obvious changes occurred in the structure and Ti coordination state. The
catalyst remained to be highly active and selective to the pyridine oxidation after reused for
four times. A slight decrease in conversion is probably due to a partial dissolution of framework silicon
in basic reaction media.
Very similar results have already been obtained with Ti-MWW in the epoxidation of various alkenes
[12].
3.5. Oxidation of pyridine derivatives
The
oxidation of pyridine derivatives has been carried out on different titanosilicate catalysts (Table 2). The larger the substrate
Table
2
|
No.
|
Catalyst
|
3-Picoline oxidation
|
|
4-Picoline oxidation
|
|
||
|
1
|
TS-1
(40)
|
29.9
|
97.0
|
72
|
30.4
|
96.5
|
73
|
|
2
|
Ti-MWW-cal.
(43)
|
34.0
|
98.9
|
87
|
38.8
|
97.6
|
100
|
|
3
|
Ti-MWW-dry
(43)
|
57.0
|
99.5
|
147
|
45.3
|
97.2
|
117
|
|
IEZ-Ti-MWW
(40)
|
71.9
|
99.0
|
177
|
71.3
|
98.9
|
176
|
|
a Reaction conditions: cat., 0.15 g;
3-picoline or 4-picoline, 30 mmol; H2 O2 (30%), 39 mmol; temp., 348
K; time, 2 h; without solvent.
c TOF: turnover frequency.
d 4-PNO: 4-picoline-N-oxide.
Fig. 6. Effect of catalyst
amount on the 4-picoline oxidation. Oxidation conditions:
solvent was methanol; others see Table 2.
is the more open reaction
space is needed. The conversion of 3- picoline and
4-picoline was only
29.9% and 30.4%
over medium pore TS-1 (No. 1).
Ti-MWW in the
calcined form was
more active than
TS- 1 by showing
the conversion of 34.0% and 38.8% for 3-picoline and 4-picoline, respectively (No.
2). This is mainly because
it has 12-MR supercages as well as external half cups. Nevertheless, the catalytic
activity of Ti-MWW for these methyl-substituted derivatives was obviously lower than for pyridine
as shown in Table 1.
We reported that the sample without calcination,
that is Ti- MWW-dry, may still contain layered
structure and then have open pore system [15]. It is thus superior to the calcined Ti-MWW in the oxidation of cyclic alkenes. The same
phenomenon was also observed in the oxidation of pyridine derivatives. The
conversion of 3-picoline and 4-picoline
over Ti-MWW-dry increased to 57.0% and 45.3%,
respectively (Table 2, No. 3). However, a
subsequent calcination would cause the
dehydroxylation between the layer sheets to narrow the interlayer pore
entrance. Ti-MWW-dry suf- fers a disadvantage that
its structure and catalytic activity is not totally
maintained when subjected to regeneration by calcination. It is desirable to
develop the titanosilicate which is thermally and hydrothermally stable and
contains the basic MWW structure and larger pores as well.
Very
recently, a methodology has been established to make the interlayer
structure expanded for the MWW, FER, CDO, and
MCM- 47 topologies by the silylation of
corresponding precursors [7]. The interlayer expanded zeolites thus obtained are composed
of the fundamental building units identical to directly calcined precur- sors
but exhibit a larger porosity. Interlayer expanded MWW-type titanosilicate,
i.e. IEZ-Ti-MWW, was then prepared and applied to the oxidation of picolines. IEZ-Ti-MWW showed a much
higher conversion than other
catalysts (Table 2, No. 4). The conversion of 3-
picoline and 4-picoline reached high 71.9% and 71.3%,
respectively. With the interlayer spacing expanded by
silylation by 0.25 nm [7], IEZ-Ti-MWW possessed
larger pore windows which were accessi- ble to the
substrates with large molecular dimensions and were also benefit for product desorption. As a result,
the 3-PNO or 4-PNO yield decreased in the order of
IEZ-Ti-MWW > Ti-MWW-dry > Ti-MWW- cal. > TS-1.
Fig. 6 shows the effect
of catalyst amount on the 4-picoline oxidation over
different titanosilicates. The reaction was carried out by using the same
amount of substrate, while varying the amount of catalysts in the range of
0–0.3 g. The 4-PNO selectiv- ity was always over 97%. The 4-picoline conversion increased with
increasing amount of
catalyst. It is noteworthy that the conversion of 4-picoline reached 97.1% when
the amount of the IEZ-Ti-MWW catalyst was 0.3 g (corresponding to ca. 11 wt% of
4-picoline). How- ever, the conversion of 4-picoline was only 75% and 88.3%
over TS-1 and Ti-MWW-cal.,
respectively, when the same amount of catalyst was used. The interlayer pore
expansion of the MWW structure is thus an effective way to enhance the activity
for the oxidation of pyridine derivatives.
4. Conclusion
Ti-MWW is capable of catalyzing the liquid-phase oxidation
of pyridine to pyridine-N-oxide at a conversion and selectivity >99% in the presence of water or methanol, and proves to be a reusable
catalyst for pyridine-N-oxide synthesis. The catalytic behavior
of Ti- MWW depends
greatly on the amount of Ti active
sites. Ti-MWW is more effective than TS-1, Ti-Beta
and Ti-MOR. The catalytic per- formance of Ti-MWW for bulky substrates pyridine derivatives is greatly improved by interlayer
silylation of Ti-MWW precursor. Thus, MWW structure-based titanosilicates are
promising cata- lysts for the clean synthesis of PNO and other oxides of
pyridine derivatives.
Acknowledgments
We
gratefully acknowledge NSFC of China (20925310, 20873043), Science and
Technology Commission of
Shang- hai Municipality
(08JC1408700, 09XD1401500), 973 Program (2006CB202508), 863
Program (2007AA03Z34, 2008AA030801),
and Shanghai Leading Academic Discipline Project (B409).
Appendix A. Supplementary data
Supplementary data
associated with this
article can be found, in the online version, at doi:10.1016/j.cattod.2010.02.045.
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