CuCl catalyst heterogenized on diamide immobilized SBA-15 for efficient oxidative carbonylation of methanol to dimethylcarbonate

Article abstract of DOI:10.1039/b301375h

CuCl has been successfully immobilized on a novel diamide modified SBA-15, and proven to be an efficient heterogenized catalyst for the oxidative carbonylation of methanol to dimethylcarbonate.

Full text of DOI:10.1039/b301375h

CuCl catalyst heterogenized on diamide immobilized SBA-15 for efficient
oxidative carbonylation of methanol to dimethylcarbonate†
Yong Cao,* Jun-Cheng Hu, Ping Yang, Wei-Lin Dai and Kang-Nian Fan*
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry,
Fudan University, Shanghai 200433, P. R. China. E-mail: yongcao@fudan.edu.cn; Fax: 86-21 65641740;
Tel: 86-21 65643792-5
Received (in Cambridge, UK) 4th February 2003, Accepted 25th February 2003
First published as an Advance Article on the web 10th March 2003
CuCl has been successfully immobilized on a novel diamide
modified SBA-15, and proven to be an efficient hetero-
genized catalyst for the oxidative carbonylation of methanol
to dimethylcarbonate.
EO20PO70EO20+2 M HCl+TEOS+H O = 2+60+4.25+15 (mass
2
ratio) was prepared, stirred for several hours at 40 °C, and then
heated at 95 °C for 3 days.13 The solid products were filtered off
and calcined at 500 °C for 5 h. The aminosilylation procedure
was performed with N-(2-aminoethyl-3-aminopropyl)trime-
thoxysilane (aapts) according to the procedure given in ref. 14
with some minor modification. SBA-15 dehydrated at 200 °C
was contacted with a refluxing of toluene solution containing 10
wt% aapts for 5 h. This mixture was stirred vigorously at room
temperature for 1 h and refluxed for 6 h. The solid was isolated
by filtration to obtain the amine-grafted SBA-15. The obtained
diamine-grafted SBA-15 (SBA-15–aapts) was then treated with
50 ml of a refluxing toluene solution containing 5 wt% ethyl
formate for 6 h to obtain diamide-functionalized solids. Finally
the amide modified SBA-15 (SBA-15–diamide) was dispersed
in 150 ml of toluene, and 2 g CuCl were added and refluxed for
12 h, the heterogenized CuCl catalyst (SBA-15–diamide–CuCl)
was obtained by filtration, washed and dried. Elemental analysis
of the SBA-15–amide–CuCl using ICP gave a 3.38 wt% loading
of copper. Into a 150 ml autoclave fitted with a Teflon® liner
and containing 10 ml of methanol, the powdered catalysts were
Dimethylcarbonate (DMC) is an environmentally benign chem-
ical compound and unique intermediate with versatile chemical
reactivity which has attracted increasing interest from both
practical and fundamental points of view in recent years.1
Simple copper salts are widely used as catalysts in the liquid
phase oxidation carbonylation of methanol to dimethylcarbo-
nate. Recently, new efficient homogeneous complex catalyst
systems for DMC synthesis have been developed by us and
others.5 However, a homogeneous catalytic system often has
disadvantages, such as difficult separation of the products and
corrosive effect of the catalyst on the reactor materials. In
developing large-scale processes, it is extremely important to
heterogenize homogeneous catalyst systems, because heteroge-
neous catalytic systems have attractive advantages over homo-
geneous counterparts in liquid-phase reactions, including easy
removal of catalysts from reaction mixtures and recycling/reuse
of catalysts.7 Recently, Sato and co-workers have developed
–4
,6
,8
I
3 2
charged (Cu( )/CH OH molar ratio = 1/240). Then, O (PO2 =
heterogenized homogeneous catalyst systems by using poly-
0.24 MPa) and CO (PCO = 2.76 MPa) were introduced. After
carrying out the reaction at around 120 °C for 5 h with a stirring
rate of 1000 rpm, the liquid products were analyzed by GC
(Shanghai Analysis GC-122) equipped with an FID using a
mers as supports,9
–11
but the catalytic results in terms of
methanol conversion and DMC selectivity are far from
satisfactory. SBA-15 is a newly discovered mesoporous silica
molecular sieve with tunable uniform hexagonal channels
ranging from 5 to 30 nm and thick framework walls (3–6 nm),
which is therefore thermally and hydro-thermally more robust
than a polymer support.12 Here we report a novel heterogeneous
catalyst system based on CuCl immobilized on a diamide-
modified mesoporous SBA-15 silica (Scheme 1), which is air-
stable and reusable, and exhibits excellent catalytic perform-
ance for the oxidative carbonylation of methanol to DMC.
The mesoporous silica material SBA-15 was prepared
according to the literature procedure12 using Pluronic P123
triblock polymer (EO20PO70EO20, Mav = 5800, Aldrich) as
template under acidic conditions. Briefly, a solution of
SGE capillary column (0.5 µm 3 30 m). Gaseous products (O
2
,
CO and CO) were analyzed with TCD using a TDX-01 packed
2
column (3.0 mm 3 6 m). Products were also identified by GC-
MS.
A number of analytical studies have been carried out to gain
information on the copper loading and distribution, the material
structure and the surface species. The powder X-ray diffraction
patterns (XRD) of the all SBA-15 materials show a very intense
peak and two weak peaks characteristic of 2-d hexagonal
(P6mm) structure with excellent textural uniformity. There were
no significant changes upon diamide and CuCl immobilization
except for the expected changes in XRD peak intensity. Optical
and scanning electron microscopic studies on the material
reveal a homogeneous mass with elemental analysis (EDAX)
carried out at randomly selected spots showing a consistent
Cu+Si ratio of ca. 3+11. This suggests an evenly distributed
copper complex rather than a simple mixture of a copper
complex with the SBA-15 silica. It was confirmed that the
nitrogen adsorption isotherms are type IV in nature and
exhibited a H1 hysteresis loop, which is typical of mesoporous
solids. The adsorption isotherms show a sharp inflection at
relative pressures of 0.65–0.85, which is a characteristic of
capillary condensation within uniform mesopores. Pore sizes
2
21
are 6.5 nm and 6.4 nm and BET surface areas are 548 m g to
2
21
3
64 m g for SBA-15–diamide and SBA-15–diamide–CuCl,
respectively. All these results indicate that the immobilization
of CuCl does not affect the pore structures of the SBA-15
material or cause the decrease of surface area that is consistent
with copper loading. TG–DTA studies on SBA-15–diamide and
SBA-15–diamide–CuCl reveal that the organic moieties begin
to decompose at temperatures above 523 K, which means the
catalyst are stable at reaction temperature. The XPS–Auger
Scheme 1
†
Electronic supplementary information (ESI) available: XPS and Auger
spectra of the SBA–diamide–CuCl sample. See http://www.rsc.org/
suppdata/cc/b3/b301375h/
9
08
CHEM. COMMUN., 2003, 908–909
This journal is © The Royal Society of Chemistry 2003

Table 1 Catalytic performance of the amide-immoblized CuCl catalysts in the DMC synthesisa
DMCd
sel.(CH
DMC (mass%) (%)
c
e
f
Cu loading
(wt%)
CH
(%)
3
OH Conv.
3
OH)
DMC yield
(O ) (%)
DMC sel.(O
(%)
2
)
Entry
Catalyst
Pore size/nm
2
1
2
3
4
5
CuCl
—
—
9.7
22.1
19.6
16.5
14.5
7.1
12.8
28.0
25.5
21.7
19.3
8.7
97.0
99.2
98.4
97.9
97.5
95.5
97.5
97.2
41.6
96.5
85.8
72.6
65.1
30.0
48.3
44.3
97.5
99.8
99.7
99.4
99.2
92.6
98.3
97.6
SBA-diamide-CuCl
Removed from #2
Removed from #3
Removed from #4
SBA-15–CuCl
6.4
6.4
6.4
6.4
7.0
1.9
11.0
3.38
3.22
3.19
3.15
3.35
3.05
2.80
b
6
7
8
a
MCM–diamide–CuCl
11.2
10.3
15.0
13.8
SiO
2
–diamide–CuCl
Reaction conditions: Cu(
Methanol conv. % = reacted methanol/introduced methanol. DMC sel.(CH
I
)/methanol molar ratio = 1/240, 0.24 mol of methanol, 120 °C, 5 h. b Prepared by the conventional wet-impregnation method.
c
d
e
3
OH) % = 2 produced DMC/reacted methanol. Yield of DMC (O
2
) % =
f
produced DMC/2 introduced O
2
.
Selectivity of DMC (O
2
) % = produced DMC/(produced DMC + produced CO ).
2
characterization confirms the presence of Cu( ) species over the
I
surface of SBA-15–diamide–CuCl (ESI†).
The remarkable activity of the SBA-15–diamide–CuCl
catalyst can be seen from Table 1. Entry 2 shows that SBA-
1
5–diamide–CuCl is very active with a 22.1% conversion of
methanol and a 96.5% yield of DMC based on O , which are
more than twice those of the commercial CuCl catalyst
Aldrich, 99.9%, average size ~ 0.5–1 µm). Heterogeneous
2
(
catalysts often suffer extensive leaching of active metal species
during reactions and eventually lose their catalytic activity. To
our surprise, SBA-15–diamide–CuCl maintained its high cata-
lytic activity even after four recyclings and reuse of the catalyst
(entries 3–5 in Table 1) although a Cu leaching of about ~ 7%
occurred. SBA-15–diamide–CuCl maintained much higher
catalytic activity than CuCl even after four runs. CuCl was also
loaded onto SBA-15 using the conventional impregnation
method. The catalytic activity of the impregnated catalyst was
found to be inferior to that of CuCl. MCM-41 (surface area 940
2
21
2
21
m g ) and amorphous silica gel (surface area 340 m g
,
mean pore size 11 nm) were also applied as supports. Clearly
the catalytic performance of SBA-15–diamide–CuCl (mean
pore size 6.4 nm) is superior to that of MCM-41–diamide–CuCl
Fig. 1 DRIFT spectra of (a) SBA-15–aapts, (b) SBA-15–diamide, (c) SBA-
15–diamide–CuCl.
(
mean pore size 1.9 nm) and amorphous silica–diamide–CuCl.
This work was supported by the National Natural Science
Foundation of China (Grant No. 20203003) and the Committee
of Shanghai Science and Technology (Grant No. 0112NM076,
The higher catalytic activity of SBA-15–diamide–CuCl might
be due to the easy access and discharge of reactants in larger
regular mesopores. It is also interesting to note that SiO –
2
diamide–CuCl and MCM-41–diamide–CuCl exhibit compara-
ble activities although the former has a much larger pore size;
this could possibly be due to their similar low CuCl loadings.
The dramatic increase in the reaction efficiency of the CuCl
catalyst upon immobilization in the present case can be
understood by a strong ligand interaction between the copper
0
2QA14006).
Notes and references
1
2
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5
species and the amide moieties in the SBA-15 material. The
DRIFT spectrum of SBA-15–diamide material shows a CNO
3 M. A. Pacheco and C. L. Marshall, Energy Fuels, 1997, 11, 2.
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1
stretching absorption band at ca. 1680 cm and N–H bending
2
1
band at ca. 1600 cm (Fig. 1). Compared to that of SBA-
5–diamide, the amide carbonyl band was shifted to lower
2
002, 185, 1.
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1.
1
6
7
8
2
1
wavenumber at 1640 cm for SBA-15–diamide–CuCl, in-
dicating the existence of strong ligand interactions between the
amide carbonyl group and the copper species. Thus, the
promotional effect of CuCl catalyst upon immobilization in the
5
H. U. Blaser, A. Baiker and R. Prins, Heterogenerous Catalysis and
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Solid Supports and Catalysts in Organic Synthesis, ed. K. Smith, Ellis
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present reaction system is to facilitate the intrinsic Cu(II)/Cu( )
I
redox processes by a strong electronic modification of the
9 Y. Sato, M. Kagotani, T. Yamamoto and Y. Souma, Appl. Catal. A,
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material,15 leading to dramatically enhanced methanol oxida-
1
1 Y. Sato and Y. Souma, Catal. Surveys Jpn., 2000, 4, 65.
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tive carbonylation activity in DMC synthesis.
1
In conclusion, we have successfully demonstrated for the first
time that CuCl immobilized on diamide modified SBA-15 is an
excellent heterogeneous candidate for oxidative carbonylation
of methanol to DMC. The catalyst systems can be separated
easily and are reusable, showing attractive potential for
practical applications.
1
1
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