7
2
M.J. Schneider et al. / Journal of Catalysis 309 (2014) 71–78
the organic base 4-(dimethylamino)pyridine, and methyl trifluoro-
methanesulfonate were purchased from Sigma–Aldrich. The
precursor copper tris(pentafluoroethyl)trifluorophosphate and the
ionic liquids 1-butyl-3-methylimidazolium bis(trifluoromethylsul-
Fig. 1. Copper-catalyzed dimethyl carbonate synthesis.
fonyl)imide [BMIM][NTf
2
]
(melting point: ꢀ2 °C), 1-butyl-3-
promoted the catalyst activity yielding up to 5.6% conversion of
methanol at 110 °C, 7 bar and a copper loading of 3 wt.% Cu. This
positive influence of hydroxide solutions was attributed to the for-
methylimidazolium triflate [BMIM][OTf] (melting point: 16 °C),
1-butyl-3-methylimidazolium acetate [BMIM][OAc] (melting
point: ꢀ20 °C), 1-butyl-3-methylimidazolium chloride [BMIM][Cl]
mation of Cu(OH)Cl and paratacamite Cu
2
Cl(OH)
3
structures [17].
4
(melting point: 70 °C), tetrabutylammonium chloride [Bu N][Cl]
Additional promoting effects were found for alkali acetates [18]
and palladium chloride [19]. Itoh et al. [14] studied the influence
of the water content on the catalytic performance and found a
strong limitation through the amount of water present in the meth-
anol feed. A water content of only 5.5% significantly reduced the
methanol conversion, which was attributed to the hydrolysis of
DMC.
Ionic liquids on solid support were first applied for DMC synthe-
sis by Wang et al. [20] who immobilized copper(I) and copper(II)
halides on SBA-15 silica support and studied the influence of sur-
face modification by means of silanization with the ionic liquid
(melting point: 83 °C), N-propylpyridinium chloride [PrPyr][Cl]
(melting point: ꢁ 110 °C), tetraethylammonium chloride tetrahy-
drate [Et
4
N][Cl] 4 ꢂ H
2
O (melting point: 37.5 °C), trioctylmethy-
lammonium chloride [OMA][Cl] (melting point: ꢁꢀ20 °C), and
trioctylmethylammonium bromide [OMA][Br] (melting point:
<ꢀ20 °C) were obtained from Merck KGaA. The Polymer-Based
Spherical Activated Carbon (PBSAC) was supplied by Blücher
GmbH.
2.2. Synthesis of [DMMAP][OTf]
3
-trimethoxysilylpropyl-pyridinium chloride. CuBr
2
yielded the
4.86 mL (43 mmol) of methyltriflate was slowly added to a
cooled (ꢀ78 °C) solution of 5.0 g (40.9 mmol) 4-dimethylamino-
pyridine in dry dichloromethane. The resulting clear solution was
stirred for 30 min, and subsequently, the cooling was stopped
and the solution was stirred until ambient temperature was
reached. Afterward, the solvent was removed in vacuo to yield
11.7 g (40.9 mmol; 99%) of the desired product 4-N,N-dimetylami-
highest methanol conversion (17%) as well as the highest selectiv-
ity for DMC (97.5 %) on ionic liquid-modified SBA-15 reaching
ꢀ1
turnover frequencies of up to 7.09 h . At the same time, Stricker
et al. [21] reported on the use of task-specific catalytic ionic liquids
containing copper(I) in the complex cation or anion or in both, an-
ion and cation, for the liquid-phase dimethyl carbonate synthesis.
Up to 62% conversion of methanol at 89% selectivity was obtained
0
no-N -methylpyridinium triflate [DMMAP][OTf] as a white solid.
for copper(I)–bromide ionic liquid [Cu(C12-imidazole)
2 2
][CuBr ]
(
5 mol% Cu with respect to MeOH, 120 °C, 50 bar CO, 3 bar O
2
). In
2.3. SILP preparation
the current work, we want to take advantage of the negligible va-
por pressure of ionic liquids and extend their application in DMC
synthesis from the liquid phase to the continuous gas phase using
the Supported Ionic Liquid-Phase (SILP) technology (see Fig. 2). In
SILP systems, a well-defined catalyst complex is dissolved in an io-
nic liquid, which is dispersed as a thin film on the surface of a por-
ous solid support with a large surface area. The resulting SILP
catalyst powders are used as microscopically homogeneous, but
macroscopically heterogeneous catalysts in the gas-phase oxycarb-
onylation of methanol in continuous operation mode.
SILP catalysts were prepared by impregnation under argon
atmosphere using standard Schlenk techniques. 5 g of the PBSAC
solid supporting material (Vpore = 1.18 mL/g; BET surface
2
area = 2005 m /g) was stirred in 100 mL of a 1:1 mixture of water
and methanol and the respective amounts of copper precursor
(equal to 2–4 wt.% of Cu on PBSAC), the additive (KBr or organic
base in equimolar amounts relative to copper) and the ionic liquid
(equal to a pore-filling degree aIL = VIL/Vpore of 20–50%) were slowly
added. The solution was stirred for 30 min. Finally, the solvent was
slowly removed under reduced pressure at 50 °C.
2
. Materials and methods
2.4. Catalyst testing procedure
2
.1. Chemicals
The MeOH oxycarbonylation experiments were conducted in
continuous gas-phase operation mode at 110 °C and 10 bar using
a fixed-bed reactor set-up (see Fig. 3). The gaseous feedstocks
(CO 78.2 N mL/min) and synthetic air (36.5 N mL/min) were fed
by El-Flow mass flow controllers (MFC) from Bronkhorst. Liquid
methanol (0.05 mL/min) was pumped through a Knauer K-120
HPLC pump, evaporated and mixed with the feed streams of CO
and synthetic air. The gaseous feed stream was led to a reactor
where a fixed bed of 4 g SILP material catalyzed the reaction in a
residence time of 14 s. The product feed stream was continuously
analyzed by a Varian GC-3900 equipped with a FID-detector. All
gas flow meters and the GC were calibrated prior to use.
The applied gases carbon monoxide (purity 3.7), synthetic air
(
purity 4.0), and helium (purity 4.6) were obtained from Linde
AG. The copper precursors CuCl, CuCl , CuBr, CuBr , copper acetate,
2
2
2
.5. Isolation and characterization of the catalytically active species
4-O)(
l-Cl) Cl ]: [BMIM]Cl (2.24 g, 12.8 mmol)
6 4
[
BMIM]
4
[Cu
4
(l
and CuCl (1.27 g, 12.8 mmol) were dissolved in 20 mL MeCN and
placed in a glass liner of an stainless steel autoclave. Dry MeOH
(
4.11 g, 128 mmol) and water (230 mg, 12.8 mmol) were added;
the mixture was pressurized with 0.5 bar O and 4.5 bar N and
2
2
heated with stirring for 6 h at 110 °C. The autoclave was opened,
Fig. 2. Schematic view of the Supported Ionic Liquid-Phase (SILP) catalyst material
applied in this work.
and a dark green solution with some black finely suspended