Novel periodic mesoporous silica chlorides (PMSCL) with 2D P 6 mm hexagonal structures: Efficient catalysts for the beckmann rearrangement

Article abstract of DOI:10.1055/s-0030-1258484

A family of ordered mesoporous silica chloride (SBA-Cl) with 2D P ₆mm hexagonal structures and high surface areas was prepared through treatment of SBA-15 with thionyl chloride. The materials were shown to have excellent catalytic activity in Beckmann rearrangements of ketoximes. The catalyst (SBA-Cl-2) was recovered and reused for at least three reaction cycles without significant change in its nanostructure as evidenced by surface analysis. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0030-1258484

CLUSTER
2019
Novel Periodic Mesoporous Silica Chlorides (PMSCl) with 2D P6mm
Hexagonal Structures: Efficient Catalysts for the Beckmann Rearrangement
Efficient
Catalys
a
B
b
R
ea
a
rrangemen
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t
Karimi,* Hesam Behzadnia
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-6731, Iran
Fax +98(241)4153232; E-mail: karimi@iasbs.ac.ir
Received 12 May 2010
chlorine contents. While several types of solid silica chlo-
Abstract: A family of ordered mesoporous silica chloride (SBA-Cl)
rides have already been prepared from the corresponding
amorphous silica, to the best of our knowledge, there is no
report about the same transformation with the use of or-
with 2D P6mm hexagonal structures and high surface areas was pre-
pared through treatment of SBA-15 with thionyl chloride. The ma-
terials were shown to have excellent catalytic activity in Beckmann
rearrangements of ketoximes. The catalyst (SBA-Cl-2) was recov- dered mesoporous silicas. If such a transformation could
ered and reused for at least three reaction cycles without significant
change in its nanostructure as evidenced by surface analysis.
be attained without significant change in structural fea-
tures of the parent ordered silica, it would be possible to
Key words: Beckmann rearrangement, silica chloride, ketoximes,
amides, heterogeneous catalysis
prepare a variety of sophisticated ordered mesoporous
chlorosilicates with attractive properties including high
surface area, high surface Si–Cl loading, and large or-
dered pores with narrow size distributions. Here, we have
Since the first reports on surfactant-templated mesopo- chosen to employ mesoporous silica SBA-152 as the pre-
rous materials appeared in the past decade,1 a wide variety cursor for preparation of chlorinated silica, not only be-
of these materials, which possess uniform channels with cause it has larger and more accessible channels but also
large pore size (2–40 nm), high surface area (up to 2000 because it displays higher thermal and hydrothermal sta-
m2 g–1), and tuneable structure have been synthesized.2 bility than M41S family.1 SBA-15 was obtained from plu-
Furthermore, organically functionalized ordered mesopo- ronic P123 and (EtO)4Si under acidic conditions
rous silicas3 with tailored composition has also been wide- according to the reported procedure.2 The surfactant-free
ly investigated for applications in adsorbent synthesis,4 SBA-15 was obtained by subsequent removal of pluronic
gas separation techniques,5 photocontrollable molecular P123 through extensive extraction with ethanol for 48
storage,6 catalysis,7 molecular recognition,8 nanoreac- hours. The resulting SBA-15 was then separately allowed
tors,9 and biological uses.10 The surface properties of me- to react with excess of thionyl chloride under reflux con-
soporous silicates can be easily modified with different ditions for 24 and 48 hours, to afford the corresponding
chemical groups, by which their functionality and textural surface chlorinated SBA-Cl-1, SBA-Cl-2, respectively.
properties could be changed. In earlier studies, these The excess of thionyl chloride was distilled off and the re-
chemical groups were incorporated into ordered mesopo- sulting product was calcined at ca. 550 °C and stored in a
rous silicas through direct co-condensation or post-graft- sealed vessel, under dry argon atmosphere. The materials
ing of trialkoxysilane R1Si(OR2)3 which resulted in were characterized by simultaneous thermal analysis, sur-
terminally bonded organic groups inside the channels.7,11 face analysis, and transmission electron microscopy.
Another less frequently used method was direct chlorina-
Table 1 Characterization of Materials
tion of surface Si–OH groups to generate Si–Cl functions
as precursors for grafting of the organic moiety onto silica
a
c
Entry Compd
SBET
870
710
682
363
Pore Sizeb VP
Cl contentd
surfaces.7,11 In this approach organic groups were bound
robustly, and owing to higher stability of Si–C bond be-
tween the organic moiety and the surface, they were typi-
cally less prone to leaching than those attached via
conventional grafting. Furthermore, many potential appli-
cations that exploited high surface activities of these chlo-
rinated silicas were demonstrated in catalysis,12 water
softening,13 chromatography,14 and functional-group in-
terconversions.15 However, most of these amorphous sili-
ca chlorides often had low surface areas (up to 100 m2 g–1),
large distributions in pore diameters and void volumes.
They also suffer from the disadvantage of low surface
1
2
3
4
SBA-15
6.3
5.1
5.3
7.9
0.87
0.80
0.79
0.72
–
SBA-Cl-1
SBA-Cl-2
1.7
3.4
2.8
recovered
SBA-Cl-2
5
silica-Cl
408
5.98
0.61
2.2
a BET surface area (m2 g–1).
b BJH average pore diameter (nm).
c Total pore volume (cm3 g–1).
d Surface Si–Cl content (mmol g–1) determined by back titration using
(0.01 M NaOH) after hydrolysis and elemental analysis.
SYNLETT 2010, No. 13, pp 2019–2023
x
x
.x
x
.2
0
1
0
Advanced online publication: 09.07.2010
DOI: 10.1055/s-0030-1258484; Art ID: Y01310ST
© Georg Thieme Verlag Stuttgart · New York

2020
B. Karimi, H. Behzadnia
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Figure 1 Nitrogen adsorption–desorption isotherms for mesoporous A) SBA-15, B) SBA-Cl-1, C) SBA-Cl-2, and D) recovered SBA-Cl-2.
Inset: BJH pore size distributions obtained from the corresponding isotherms.
Nitrogen adsorption–desorption of two samples provided As it clearly could be seen from BJH average pore-size
very clear type IV profiles with sharp hysteresis loop, calculations, the pore diameters decreased by up to 1 nm,
which is characteristic of the highly ordered mesoporous when the parent SBA-15 was chlorinated under the de-
materials (Figure 1). Interestingly, the capillary conden- scribed chlorination protocol.
sation step (observed at relative pressure between 0.6–
The BJH calculations also indicated a narrow pore-size
0.8) was clearly remained even after calcination of the
distributions for both SBA-Cl-1 and SBA-Cl-2 in the me-
samples at ca. 550 °C, which provided clear evidence for
soporous region (Figure 1, B and C). These distributions
the preservation of ordered mesopore structures in both
interestingly showed that the nanoarchitecture of the sam-
ples (ordered mesoporous channels) largely survived even
SBA-Cl-1 and SBA-Cl-2.
It is also noteworthy that even after surface chlorination after prolonged reflux in excess thionyl chloride during
followed by calcination at ca. 550 °C, the resulting surface the preparation stage and also after calcinatation at 550
chlorinated materials exhibited a specific surface area of °C.
ca. 710 m2 g–1 and 682 m2 g–1 and total pore volumes of
The chlorine contents of the materials were also deter-
0.8 and 0.79 cm3 g–1, respectively (Table 1, entry 2 and 3).
mined with the use of back-titration of the resulting HCl
BJH calculation showed an average pore diameter of 6.3
upon the treatment of the materials with deionized water
nm for the starting SBA-15 (Table 1, entry 1), 5.1 nm for
for 12 hours. These values were further corroborated with
SBA-Cl-1, and 5.3 nm for SBA-Cl-2 (Table 1, entry 1–3),
elemental analysis to be 1.7 and 3.4 mmol g–1 for SBA-Cl-
which were in good agreement with pore diameters esti-
1 and SBA-Cl-2, respectively. All samples were preheat-
mated from TEM images (Figure 2). Moreover, the TEM
ed at 550 °C under dry argon atmosphere, before perform-
images of all chlorinated samples showed ordered do-
ing hydrolysis and titration experiments. These results
mains that appeared to have a two-dimensional hexagonal
clearly excluded any interference that might be resulted
from physically adsorbed HCl in the calculation of total
chlorine content. Consequently the observed data for
(P6mm) symmetry similar to the parent SBA-15 materials.
The accurate images also provided further evidence that
the ordered phase dominated any disordered (amorphous)
chlorine content could be truly related to Si–Cl moieties
silicate phases in all of the samples that were investigated.
throughout the structure. It is also worth mentioning that
these materials, regardless to their Cl content, were highly
stable upon storage for months under an inert atmosphere.
Encouraged by high surface area, high chlorine content of
the surface, and highly stable and ordered structures of
SBA-Cl, we speculated whether these materials might
also be suitable catalysts (or reagents) in some organic
tranformations. Along the line of this hypothesis, we were
also interested to study the catalytic performance of SBA-
Cl in the Beckmann rearrangement. The Beckmann rear-
rangment is a common organic transformation to convert
ketoxime into amides.16 One of the most important indus-
trial version of this reaction is the rearrangement of cyclo-
hexanone oxime with oleum to e-caprolactam sulfate that
is a precursor for production of Nylon-6.17 However, this
method is accompanied by the co-production of million
tons of undesired ammonium sulfate and also suffers from
serious corrosion problems during liquid-phase industrial
processes. As a result, several efforts have been directed
Figure 2 TEM images of (A, B) SBA-Cl-1; (C, D) SBA-Cl-2
toward the development of milder methods for Beckmann
Synlett 2010, No. 13, 2019–2023 © Thieme Stuttgart · New York

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Efficient Catalysts for the Beckmann Rearrangement
2021
rearrangement by using catalysts that replace the more tra-
ditional and stoichiometric ones.18 However, most of
these protocols need expensive reagents, or suffer from
the tedious workup procedures. If this rearrangement
could be performed by the use of solid catalysts, because
of the ease with which the catalyst could be separated
from product and recycled, the reaction would be consid-
erably cheaper and environmentally more friendly. Along
this line, several catalysts have been investigated and in-
clude silica-supported molybdenum(VI) oxide,19 silica
sulfate,20 solid sulfonic acid resin,21 Al-MCM-41 molecu-
lar sieves,22 zeolites,23,24a–h MCM-22,24i silicoalumino
phosphates24j and titano silicate.24k While these methods
have provided considerable improvement in the Beck-
mann rearrangement, they often require high temperature
(over 130 °C), high catalyst loading, high concentrations
of strongly acidic additives and/or special apparatus for
vapor-phase Beckmann rearrangements. It is also noted
that the chemical yields and selectivities of the corre-
sponding products are not always satisfactory. Therefore,
it seems that there is still much room to develop new
methods for heterogeneous and catalytic Beckmann rear-
rangements under mild and more appropriate reaction
conditions. As summarized in Table 2, we found that
SBA-Cl-2 is an efficient catalyst for the Beckmann rear-
rangement of a large number of ketoximes. Different sol-
vents were investigated for this transformation and it
turned out that refluxing toluene could led to higher yields
of products. Most of the products were easily separated in
almost pure form with a simple filtration of catalyst and
evaporation of the solvent under reduced pressure.
Table 2 Beckmann Rearrangement of Various Oximes Using
SBA-Cl-2
R2
O
SBA-Cl-2 (20 mol%)
toluene, reflux
R1
HO
R1
N
R2
N
H
Entry R1
R2
Time (h) Yield (%)a,b
1
2
Ph
Ph
2.5
2.5
5
100
98c
7d
Ph
Ph
3
Ph
Ph
4
Ph
Me
3
100
92e
83f
95
5
Ph
Me
3
6
Ph
Me
3
7
4-MeOC6H4
3-MeOC6H4
4-MeOC6H4
2-MeOC6H4
4-MeC6H4
Me
10
8
Me
12
5
68
9
4-MeOC6H4
82
10
11
12
13
14
Me
Me
2
100
95
3
3,4-(MeO)2C6H3 Me
12
8
89
Ph
Et
92
4-PhC6H4
Me
8
60
HO
N
15
35
88
The catalyst (SBA-Cl-2) was recovered in the Beckmann
rearrangement of acetophenone oxime by simple separa-
tion and was used directly for the next round of reaction
(Table 2, entries 5 and 6). Interestingly, the nanostructure
of the catalyst remained unchanged after the third reaction
recycle, although, a significant change in the surface area
(from 682 to 383 m2 g–1) and a slight decrease in the total
pore volume from 0.79 to 0.72 cm3 g–1 and chlorine con-
tent (from 3.4 to 2.8 mmol g–1) occurred (Table 1,
Figure 1, D). These results clearly showed that the cata-
lyst was highly stable and no significant restructuring of
the catalyst took place on multiple uses. Interestingly, we
observed that the stored sample showed essentially the
same reactivity as that of the fresh catalyst. This observa-
tion definitively proved that ordered mesoporous silica
chlorides were indeed stable at room temperature under an
inert asmosphere. It was very important to note that upon
the use of amorphous silica chloride (SiO2Cl)12 under the
same reaction conditions, the corresponding products
were obtained in very low yields (Table 2, entry 3). This
result clearly showed that the SBA-Cl was a much more
active catalyst in comparison with the amorphous silica
chloride in the model reaction. Particularly, SBA-Cl-2
also showed excellent activity in the selective Beckmann
rearrangement of cyclohexanone oxime to e-caprolactam,
without the formation of any byproducts (Table 2, entry
15).
a All the products are known.25
b Isolated yield.
c Reaction was carried out using a three months stored sample of
SBA-Cl-2 under an inert atmosphere.
d Reaction using amorphous silica chloride.
e Second run.
f Third run.
The precise mechanism for this reaction is unclear at this
stage. However, it seems reasonable that SBA-Cl-2 might
act as a Lewis acid (through its surface Si–Cl bonds) since
it has calcined at 550 °C in order to minimize the contri-
bution of physically adsobed HCl. Therefore, a plausible
idea is that SBA-Cl-2 reacts with oxime to produce the
corresponding O-silylated oxime 1 and HCl. The resulted
HCl is then captured by the rearranged intermediate 2 to
afford the Beckmann products and concomitant release of
SBA-Cl-2 that in turn re-enter to the catalytic cycle
(Scheme 1). Nevertheless, it is difficult at this stage to ex-
actly attribute the actual catalytic activity to solely surface
Si–Cl bonds. Based on the moisture-sensitive nature of
SBA-Cl-2, it would not be also surprise if this material
serves as a source for in situ formation of a trace HCl. Fur-
ther studies on this particular subject are currently under
way to clarify this issue, and the results will be appearing
in due course.
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2022
B. Karimi, H. Behzadnia
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O
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General Procedure for the Beckmann Rearrangement Using
SBA-Cl-2
In a 10 mL glass flask equipped with a magnetic stirrer and con-
denser, we placed oxime (1 mmol) and SBA-Cl (20 mol%) in tolu-
ene (3 mL). The mixture was refluxed in appropriate time. The
reaction was monitored by TLC, and after completion of the reac-
tion, most of products were easily extracted with a simple filtration
of catalyst and evaporation of the solvent under reduced pressure.
In other cases the crude products were required to purification on
silica gel using a mixture of n-hexane and EtOAc to give the isolat-
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tion was performed using large scale of acetophenone oxime (10
mmol, 1.35 g) the corresponding amide was obtained in 92% yield
(1.24 g) under similar reaction conditions.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
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The authors acknowledge IASBS and INSF Research Councils for
support of this work. We also acknowledge Dr. Saeed Emadi for
correcting the manuscript.
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