Oxidative carbonylation of aniline with a mesoporous silica gel immobilised Se-functionalised ionic liquid catalyst

Article abstract of DOI:10.3184/030823410X12759267364502

A series of BMImBF₄-templated mesoporous silica gel immobilised Se-functionalised ionic liquids have been prepared. The catalysts were characterised by BET, FTIR and ICP-AES, and the results showed that the content of Se increased from 2.7 to 4.76 wt% with pore volume increasing from 0.83 to 1.26 cm³ g^-1. Preliminary tests of this catalyst system on the carbonylation of aniline to afford methyl phenyl carbamate (MPC) indicated that good catalytic activity (higher than 74% aniline conversion with 99% MPC selectivity) could be achieved with higher than 150 mol h^-1 mol ^-1 turnover frequency (TOF). TOF and aniline conversion slightly increased with increasing the content of Se from 2.7 to 3.0 wt%, then decreased when the content of Se increased to more than 3.0 wt%, this suggested that 2.7 to 3.0 wt% Se content should be appropriate for MPC synthesis. The appropriate Se content could be obtaineded by adjusting the range of pore volume of mesoporous silica gel from 0.83 to 0.85 cm³ g^-1. In addition, the recycling test indicated that there was almost no deactivation of the catalyst when it was reused four times.

Full text of DOI:10.3184/030823410X12759267364502

344 RESEARCH PAPER
JUNE, 344–347
JOURNAL OF CHEMICAL RESEARCH 2010
Oxidative carbonylation of aniline with a mesoporous silica gel
immobilised Se-functionalised ionic liquid catalyst
Yubo Maa,b, Feng Shia and Youquan Denga*
aCentre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000,
P. R. China
bGraduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China
A series of BMImBF4-templated mesoporous silica gel immobilised Se-functionalised ionic liquids have been pre-
pared. The catalysts were characterised by BET, FTIR and ICP-AES, and the results showed that the content of
Se increased from 2.7 to 4.76 wt% with pore volume increasing from 0.83 to 1.26 cm3 g−1. Preliminary tests of this
catalyst system on the carbonylation of aniline to afford methyl phenyl carbamate (MPC) indicated that good catalytic
activity (higher than 74% aniline conversion with 99% MPC selectivity) could be achieved with higher than 150 mol
h−1 mol−1 turnover frequency (TOF). TOF and aniline conversion slightly increased with increasing the content of
Se from 2.7 to 3.0 wt%, then decreased when the content of Se increased to more than 3.0 wt%, this suggested that
2.7 to 3.0 wt% Se content should be appropriate for MPC synthesis. The appropriate Se content could be obtaineded
by adjusting the range of pore volume of mesoporous silica gel from 0.83 to 0.85 cm3 g−1. In addition, the recycling
test indicated that there was almost no deactivation of the catalyst when it was reused four times.
Keywords: silica gel, ionic liquids, selenium, non-phosgene, methyl phenyl carbamate
Isocyanates are currently manufactured by the phosgenation of
the corresponding amine with toxic phosgene. This usually
causes serious environmental pollution and equipment cor-
rosion. As the key intermediates of phosgene-free synthesis
of isocyanates, carbamates have been extensively studied in
the past two decades. An important approach is the reductive
carbonylation of nitro compounds using transition metal com-
plexes as the catalyst.1–4 The methoxycarbonylation of amines
in the presence of dimethyl carbonate (DMC) has also been
reported.5,6 The deactivation of the noble metal catalysts is one
of the main problems in reductive carbonylation, while the
price of DMC inhibits its large-scale use in methoxycarbonyl-
ation. Besides the reductive carbonylation and methoxycar-
bonylation methods, the oxidative carbonylation of amines has
also been investigated extensively.7–13 However, the recovery
of the noble-metal catalysts was difficult. It has been reported
Fig. 1 The relationship between the pore structure and the
that selenium compounds and ionic liquids containing anionic
amounts of BMImBF4: (a) BET surface area, (b) pore volume,
selenium species could be used for the oxidative carbonylation
and (c) pore diameter.
of amines.14–18 This should be a good choice for the develop-
ment of an economic and recoverable noble-metal free catalyst
system for the oxidative carbonylation of aniline.
(BMImBF4) as the template, were examined, Fig. 1. It can
be seen that the BET surface areas decreased from ~1041 to
~250 m2 g−1, pore volumes and average pore sizes increased
from 0.61 to 1.26 cm3 g−1 and 2.8 nm to 13 nm respectively
when the amount of ionic liquid was increased from 0 to
58 wt%. These data suggested that all SiO2 used in this work
was mesoporous silica gel.
The content of selenium for 14.7 wt% Se–IL–SiO2-6.5 was
2.70 wt%, for 25.1 wt% Se–IL–SiO2-15 was 4.65 wt%, for
23.9 wt% Se–IL–SiO2-25 was 4.42 wt%, for 25.7 wt% Se–IL–
SiO2-45 was 4.76 wt%, and for 16.2 wt% Se–IL–SiO2-58 was
3.0 wt% (the numbers 6.5, 15, 25, 45 and 58 indicate the use of
6.5 wt%, 15 wt%, 25 wt%, 45 wt% and 58 wt% BMImBF4 as
the template).
Mesoporous silica materials with different structures have
attracted widespread attention due to their potential applica-
tions in catalysis, biosensors, chromatographic separation,
microelectronic and optoelectronic technologies.19–21 Recently,
ionic liquids were reported to be effective templates for pre-
paring mesostructured materials.22 This may offer new oppor-
tunities for catalyst preparation using mesoporous silica gel
(prepared with ionic liquids as the template) as a support or
catalyst.
Herein, a series of Se–IL–SiO2 catalysts were prepared.
According to ref 14 and our research experience, aniline and
methanol easily form a carbamate, so these were selected as
the reactants to test the Se–IL–SiO2 catalysts (Scheme 1).
From the FT-IR spectra of Se–IL and Se–IL–SiO2 (Fig. 2),
two characteristic peaks (1469 and 1571 cm−1) were observed:
the first ascribed to the in-plane C–H deformation vibration,
and the C–C and C–N stretching vibrations of the imidazole
Results and discusssion
Mesoporous silica gel, prepared with various amounts of the
ionic liquid 1-butyl-3-methylimidozolium tetrafluoroborate
Scheme 1 MPC synthesis via the oxidative carbonylation of aniline.
* Correspondent. E-mail: ydeng@licp.cas.cn

JOURNAL OF CHEMICAL RESEARCH 2010 345
(MPC) selectivity, and 232, 168, 181, 157, 238 mol h−1 mol−1
TOF were obtained with 14.7 wt% Se–IL–SiO2-6.5, 25.1 wt%
Se–IL–SiO2-15, 23.9 wt% Se–IL–SiO2-25, 25.7 wt% Se–
IL–SiO2-45, and 16.2 wt%Se–IL–SiO2-58 as the catalysts,
respectively (entries 1–5). These results suggested that the
Se–IL–SiO2 catalyst system had a high catalytic activity. TOFs
of all catalysts in this work exceeded 150 mol h−1 mol−1. Note
that TOFs for the selenium-based catalyst system reported
by other groups are usually lower than 100 mol h−1 mol−1.26–29
It should also be mentioned that TOFs for Se-functionalised
ILs catalyst systems reported by other groups are usually lower
than 180 mol h−1 mol−1 when aniline conversion was higher
than 70%.14
Since the TOF was higher than those of the selenium-
based catalyst systems and the Se-functionalised IL catalysts
systems reported by other groups, and the 16.2 wt% Se–
IL–SiO2-58 catalyst showed the highest TOF, MPC synthesis
was investigated in detail with the latter as the representative
catalyst.
Fig. 2 FT-IR spectra of (a)Se–IL; (b) Se–IL–SiO2; and (c)SiO2.
The results of 16.2 wt% Se–IL–SiO2-58 catalysed aniline
carbonylation are listed in Table 1. The conversion of aniline
decreased from 84% to 75%, while TOF increased from 238 to
425 mol h−1 mol−1 (entries 5 and 6) when the aniline/methanol
molar ratio was doubled. These results suggest that the molar
ratio of aniline/methanol has an important impact on MPC
synthesis.
The reuse of the catalyst was also investigated. To our
delight, 82% aniline conversion with 99% MPC selectivity
and 235 mol h−1 mol−1 TOF were obtained when the catalyst
was reused for the 4th time (entry 7). Almost the same aniline
conversion, MPC selectivity and TOF were achieved, which
suggests that the catalyst was stable enough for reuse.
As shown in Fig. 4, the aniline conversion increased from 74
to 94% when the content of Se increased from 2.7 to 4.42 wt%,
then decreased from 94 to 88% with further increasing of the
content of Se from 4.42 to 4.76 wt%, while the TOF increased
slightly from 232 to 238 mol h−1 mol−1 with the increasing of
the content of Se from 2.7 to 3.0 wt%. The TOF then decreased
from 238 to 157 mol h−1 mol−1 when the content of Se increased
from 3.0 to 4.76 wt%. These results suggest that 2.7–3.0 wt%
Se should be appropriate for MPC synthesis. As mentioned
above, the content of Se increased with the increasing of the
pore volume of mesoporous silica gel. Therefore, the appropri-
ate content of Se for higher catalytic activity on MPC synthesis
could be obtained by adjusting the pore volume of mesoporous
silica gel.
Fig. 3 The relationship between the Se content and pore
volume of mesoporous silica gel.
ring;25 the second to the CH3 bending vibration. This suggests
that Se–IL were anchored on the SiO2.
As shown in Fig. 3, it can be seen that the content of Se in
the catalysts increased from 2.7 wt% to 4.76 wt% as the pore
volume of the mesoporous silica gel increased from 0.83 to
1.26 cm3 g−1. It seems that there is a close relationship between
the Se content and the structure of mesoporous silica gel.
As shown in Table 1, 74%, 94%, 92%, 88%, 84% aniline
conversions with more than 99% methyl phenyl carbamate
Table 1 The results of oxidative carbonylation of aniline to
methyl phenyl carbamate
Entry Catalyst
Conversion Selectivity TOF
/%
/%
/mol h−1
mol−1c
1
2
3
4
5
6
7
14.7 wt% Se–IL–SiO2-6.5
74
92
94
88
84
75
82
>99
>99
>99
>99
>99
>99
>99
232
168
181
157
238
425
235
25.1 wt% Se–IL–SiO2-15
23.9 wt% Se–IL–SiO2-25
25.7 wt% Se–IL–SiO2-45
16.2 wt% Se–IL–SiO2-58
16.2 wt% Se–IL–SiO2-58a
16.2 wt% Se–IL–SiO2-58b
Aniline (0.5 mL), catalyst (50 mg), methanol (5 mL), P = 5 MPa
(CO/O2 = 9/1 V/V), t = 1 h, T = 393 K; a1 mL aniline; bthe catalyst
was reused for the 4th time, cTOF = mol substrate converted h−1
mol−1 Se.
Fig.4 TherelationshipbetweenTOFandtheanilineconversion
to the Se content.

346 JOURNAL OF CHEMICAL RESEARCH 2010
toluene (50 mL) and ethanol (150 mL) sequentially, and dried at 80 °C
for 12 h. The pretreated SiO2-6.5 (1 g), toluene (20 mL) and Se–IL
(3 g) were introduced into a conical flask and refluxed at 110 °C for
24 h. After being filtered off, washed with toluene (50 mL) and etha-
nol (150 mL) sequentially and dried at 80 °C for 12 h, the catalyst was
denoted as 14.7 wt% Se–IL–SiO2-6.5 (14.7 wt% was the content of
Se–IL) (Scheme 3). The same procedure was used for the preparation
of other Se–IL–SiO2 catalysts, i.e. 25.1 wt% Se–IL–SiO2-15, 23.9
wt% Se–IL–SiO2-25, 25.7 wt% Se–IL–SiO2-45 and 16.2 wt% Se–
IL–SiO2-58.
In summary, a series of Se–IL–SiO2 was prepared and
members used as highly efficient catalysts for the oxidative
carbonylation of aniline. Higher than 74% aniline conversion
with 99% MPC selectivity and up to 425 mol h−1 mol−1 TOF
could be obtained. There was no obvious deactivation of the
catalyst when the catalyst was reused four times. More impor-
tantly, the catalytic capability could be effectively regulated by
adjusting the pore volume of mesoporous silica gel.
The BET surface area, porous volume, and average pore diameter
of SiO2 in this work were measured by physisorption of N2 at 76 K
using a Micromeritics ASAP 2010 instrument. Before the measure-
ment, the samples were degassed at 200 °C for 12 h, to remove mois-
ture and adsorbed gases from the catalyst surface. The isotherms were
elaborated according to the BET method for surface area calculation,
with the Horwarth– Kavazoe and BJH methods used for micropore
and mesopore evaluation, respectively.
The Se content of the catalysts was determined on an inductively
coupled plasma-atomic emission spectrometer (ICP-AES) (Thermo-
Elemental Company in the USA) after the samples were dissolved in
aqueous nitric acid (5 M).
Experimental
CAUTION: Because of the toxic and irritant properties of
Se compounds, their handling and the catalyst preparation
and use should be carried out in a fume cupboard with
appropriate precautions.
Selenium dioxide (98.0%), K2CO3 (99.0%), hydrochloric acid
(36–38 wt%), Cl(CH2)3Si(OEt)3, methylimidazole, aniline and metha-
nol were all analytical reagent grade and used as received. Carbon
monoxide (99.99%), and oxygen (99.99%) were used as purchased.
BMImBF4, RMImSeO2(OCH3) (Se–IL) (BMIm = 1-butyl-3-methy-
limidazolium, RMIm = 1-Si(OEt)3(CH2)3-3-methylimidazolium) and
KSeO2(OCH3) were synthesised respectively according to previously
published papers with slight modifications,14,23 replacing Cl(CH2)3CH3
with Cl(CH2)3Si(OEt)3. The structural formula of Se–IL in this work
is shown in Scheme 2.
Fourier transform IR (FT-IR) spectra were recorded on a Nicolet
FTIR 5700 spectrophotometer using the KBr pellet technique. The
spectra were acquired by accumulating 64 scans at a resolution of
2 cm−1 in the range of 400–4000 cm−1.
All reactions were conducted in a 90 mL autoclave in a glass tube.
In each reaction, 50 mg Se–IL–SiO2 catalyst, 0.5 mL (or 1 mL) ani-
line, 5 mL methanol, and 5.0 MPa of mixture gases (CO purity
99.99%, 4.5 MPa and O2 99.99%, 0.5 MPa) were successively intro-
duced without any additional organic solvent. All reactions proceeded
at 393 K for 1 h. Qualitative analysis was conducted with an HP
6890/5793 GC-MS. Quantitative analysis was conducted over an Agi-
lent 6820 GC, the conversion and selectivities were determined by an
external standard, and were calculated according the chromatographic
peak areas of the resulting products given by the GC chemstation.
This work was financially supported by National Natural
Science Foundation of China (No. 2053308).
Scheme 2 The structure formula of Se–IL.
Received 6 March 2010; accepted 16 May 2010
Paper 101041 doi: 10.3184/030823410X12759267364502
Published online: 2 July 2010
BMImBF4-templated mesoporous silica gel was prepared as fol-
lows:24 a mixture of tetraethoxyorthosilicate (TEOS, 10 mL, 45 mmol),
EtOH (7 mL) and BMImBF4 (0.2–4.0 g, 1.2–24 mmol) were heated to
60 °C. Then, hydrochloric acid (5 M, 5 mL) was added and the mix-
ture coagulated. After ageing at 60 °C for 12 h, the solid material was
dried under reduced pressure at 150 °C for 3 h, and 3–7 g solid sam-
ples were obtained. In order to obtain pure mesoporous silica gel or
mesoporous silica gel skeleton, the samples were washed with ethanol
under vigorous refluxing for 10 h when all BMImBF4 should be
washed out (tested by elemental analysis), and BMImBF4-templated
mesoporous silica gel with different structures was achieved. The
samples were denoted as SiO2-6.5 (6.5 was 6.5 wt% BMImBF4 as the
template), SiO2-15, SiO2-25, SiO2-45 and SiO2-58.
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