Effect of Hydrophobic Modification
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aimed to improving the activity of catalysts while main-
taining the selectivity similar to unsilylated catalysts.
in terms of metallic palladium was 0.25 % by weight based
on the weight of carrier [26]. The prepared samples were
denoted as PdCl2/Si-Cu-HMS-x, where x described the si-
lylation temperatures (25, 50, 60, 70 and 75 °C,
respectively).
2 Experimental
2.1 Materials and Methods
2.3 Catalytic Performance
All chemicals were of analytical grade and used as received.
FT-IR spectra was carried out with an ABB Bomem
FTLA2000-104 spectrometer using KBr pellets in the
4,000–500 cm-1 region. The composition and phase of the
product were identified by powder X-ray diffraction (XRD)
on an D8 X-ray diffractometer (Bruker AXS, German) using
Catalytic activity was measured by a computer-controlled
continuous micro reactor system with a stainless steel
tubular reactor of 8 mm inner diameter. The reaction
conditions were as follows: 3 mL catalyst, 0.1 mL min-1
ethanol (as liquid), 10 sccm O2, 80 sccm CO, 50 sccm N2,
reaction temperature 423 K and reaction pressure
0.64 MPa. Analytical method for production in detail was
seen in the paper [14, 15].
˚
Cu Ka radiation (k = 1.5406 A) with a scanning rate of 2°/
min from 2h = 1° to 10°. The specific surface area of the
catalysts was measured according to the Brunauer–Emmet–
Teller (BET) method with nitrogen adsorption–desorption on
a ASAP 2020 instrument (Micromeritics, USA) and the degas
conditionwas200 °C for2 h. TG ofthe sampleswasrecorded
usingaMettlerTGA/SDTA851Eanalyzerinthe temperature
range 25–600 °C at a heating rate of 10 °C/min. Water
adsorption capacity test was seen in GB 6287-86.
3 Results and Discussion
3.1 Catalytic Performance of Different Catalysts
PdCl2/Cu-HMS and PdCl2/Si-Cu-HMS-x were used as cat-
alysts and compared in the synthesis of DEC by oxidative
carbonylation of ethanol (Table 1). Compared with PdCl2/
Cu-HMS, the silylated PdCl2/Si-Cu-HMS-x catalysts
maintained the advantage in the remarkable selectivity of
100 % to DEC based on ethanol, what’s more, the conver-
sion of EtOH and STY of DEC were all improved in some
degree. Based on these results, a conclusion could be drawn
that silylation had enhanced surface hydrophobicity of Cu-
HMS. On one hand, surface modification reduced the contact
between DEC and water, avoiding the hydrolysis of DEC. On
the other hand, the removal of water significantly increased
the reaction rates over the copper zeolite catalysts, both by
the effect of water on the equilibrium of ethoxide formation
also by water adsorption onto the active sites [26], resulting
in the increase of catalytic performance. In addition, as the
efficiency of the silylation increases with silylation temper-
ature until the boiling point of TMCS was obtained and at this
point decreased. When silylation temperature was 60 °C,
TMCS partly became vapor, in other words, silylation was
considered as vapor–liquid–solid reaction, while silylation
could be thought of as liquid–solid reaction below 60 °C.
Compared with liquid–solid reaction, vapor–liquid–solid
reaction had advantages of weak molecular force, small
influence of liquid viscosity and severe molecular motion,
which contributed to TMCS contacting hydroxy group on the
surface of Cu-HMS. However, when silylation temperature
was above 60 °C, catalytic performance began to decline,
because too severe molecular motion of TMCS and even
partly volatilizing resulted in the unsufficient contact
between TMCS and surface hydroxyl of Cu-HMS. In
2.2 Catalyst Preparation
2.2.1 Synthesis of Cu-doped Hexagonal Mesoporous Silica
(Cu-HMS)
The Cu-HMS was synthesized following the procedures
similar to those proposed by Tanev et al. [24, 25] via a
neutral templating pathway using dodecylamine (DDA) as
a surfactant. The method for synthesis of Cu-HMS in detail
was shown in paper [14, 15].
2.2.2 Surface Modification with Trimethylchlorosilane
(TMCS)
Cu-HMS (2 g) was impregnated into a trimethylchlorosilane
(TMCS) solution dissolved in benzene with a concentration
of 5 % (volume fraction) and total liquid volume was
100 mL under stirring for 8 h at a desired temperature (25,
50, 60, 70 and 75 °C) [19]. The mixture was then extensively
washed with acetone to rinse away any residual chemicals.
Finally, the suspension was dried at 80 °C for 2 h.
2.2.3 Loading Palladium Chloride
The catalysts were prepared by impregnating the silylated
Cu-HMS supports with methanol solution of palladium
chloride (PdCl2). The total contents of the metal compound
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