Effect of hydrophobic modification on the catalytic performance of PdCl2/Cu-HMS with different silylation temperatures

Article abstract of DOI:10.1007/s10562-013-1115-2

A new class of organic-inorganic hybrid materials were prepared by combining Cu-HMS with a silylation agent, trimethylchlorosilane (TMCS) via a simple silylation process at different silylation temperatures. They were characterized by a series of techniques including FT-IR, powder XRD, Nitrogen adsorption-desorption, TG analysis and water adsorption capacity test. It was demonstrated that silylation of PdCl₂/Cu-HMS catalysts with TMCS enhanced their hydrophobicity, improved their activity and stability and importantly kept the excellent selectivity to diethyl carbonate (DEC) by oxidative carbonylation of ethanol in the gas-phase reaction. Moreover, the silylated samples obtained at 60 °C showed a better conversion of EtOH of 6.1 % and STY of DEC of 140.8 mg g^-1 h^-1. Springer Science+Business Media New York 2013.

Full text of DOI:10.1007/s10562-013-1115-2

Catal Lett (2014) 144:320–324
DOI 10.1007/s10562-013-1115-2
Effect of Hydrophobic Modification on the Catalytic Performance
of PdCl2/Cu-HMS with Different Silylation Temperatures
•
Pingbo Zhang Yan Zhou Mingming Fan
•
•
•
Pingping Jiang Xianglan Huang Jiang Lou
•
Received: 22 July 2013 / Accepted: 8 September 2013 / Published online: 12 October 2013
Ó Springer Science+Business Media New York 2013
Abstract A new class of organic–inorganic hybrid
materials were prepared by combining Cu-HMS with a
silylation agent, trimethylchlorosilane (TMCS) via a sim-
ple silylation process at different silylation temperatures.
They were characterized by a series of techniques includ-
ing FT-IR, powder XRD, Nitrogen adsorption–desorption,
TG analysis and water adsorption capacity test. It was
demonstrated that silylation of PdCl2/Cu-HMS catalysts
with TMCS enhanced their hydrophobicity, improved their
activity and stability and importantly kept the excellent
selectivity to diethyl carbonate (DEC) by oxidative car-
bonylation of ethanol in the gas-phase reaction. Moreover,
the silylated samples obtained at 60 °C showed a better
conversion of EtOH of 6.1 % and STY of DEC of
for tert-butyl ether (MTBE) as an attractive oxygen-contain-
ing fuel additive, and the gasoline/water distribution coeffi-
cients for DEC are more favorable than for dimethyl
carbonate and ethanol [1, 2]. In addition, as a chemical
intermediate, DEC is also drawing attention as a safe solvent
and an additive in lithium cell electrolyte [3, 4]. Currently,
oxidative carbonylation of ethanol in the gas-phase has been
deemed as one of the most promising routes for DEC syn-
thesis based on the ‘‘green chemistry’’ principles [5].
Various catalysts have been investigated for oxidative
carbonylation which were prepared by impregnating the
active carbon [2, 6, 7], oxides [8, 9] or other zeolites in
methanol solution of CuCl2 [5, 10–13]. In the previous
research [14, 15], PdCl2/Cu-HMS has been demonstrated an
excellent selectivity to DEC by oxidative carbonylation of
ethanol in the gas-phase reaction. Moreover, it was also
found that H2O in the feed decreased the conversion of
methanol and the selectivity of carbon monoxide to dime-
thyl carbonate [16, 17]. So removal of water from the cat-
alyst surface would reduce the hydrolysis of DEC and,
therefore, would benefit activity and stability of the reaction.
Catalytic activities can be improved by taking different
approaches to control of the surface hydrophobicity.
Recently, modification of MCM-41 by surface silylation
with various silylation agents has been extensively reported
and many researchers have studied their adsorption and
application [18–22]. The experimental results have shown
that surface silylation indeed improved the hydrophobicity
and thus benefited removal of water from the catalyst
surface in other systems. However, little has been written
about surface silylation of HMS and its application [23].
In this article, we described the synthesis of TMCS-
modified Cu-HMS and investigated the effect of different
temperatures. These materials were used to catalyze the
synthesis of DEC by oxidative carbonylation of ethanol
140.8 mg g-1 h-1
.
Keywords Organic–inorganic hybrid materials Á
Trimethylchlorosilane (TMCS) Á Hydrophobicity Á
Silylation temperature Á Diethyl carbonate (DEC)
1 Introduction
Diethyl carbonate (DEC) is recognized as an environmentally
benign chemical because of its negligible ecotoxicity and low
bioaccumulation and persistence. Because of its high oxygen
content (40.6 wt%), DEC has been proposed as a replacement
P. Zhang Á Y. Zhou Á M. Fan (&) Á P. Jiang Á X. Huang Á J. Lou
The Key Laboratory of Food Colloids and Biotechnology,
Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122,
People’s Republic of China
e-mail: fanmm2000@126.com
123

Effect of Hydrophobic Modification
321
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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322
P. Zhang et al.
Table 1 Catalytic performancea over PdCl2/Cu-HMS and PdCl2/Si-
Cu-HMS-x
Samples
Conversion STY of
of EtOH
(%)
SDEC/
EtOH (%)
DEC
(mg g-1 h-1
)
F
E
A (PdCl2/Cu-HMS)
4.6
97.2
100
100
100
100
100
100
D
C
B (PdCl2/Si-Cu-HMS-25) 4.9
C (PdCl2/Si-Cu-HMS-50) 5.4
D (PdCl2/Si-Cu-HMS-60) 6.1
E (PdCl2/Si-Cu-HMS-70) 5.9
F (PdCl2/Si-Cu-HMS-75) 5.6
a
115.8
125.6
140.8
136.5
130.2
B
A
Reaction condition: T = 423 K, P = 0.64 MPa, O2 = 10 sccm,
CO = 80 sccm, N2 = 50 sccm
2
4
6
8
10
2Theta (deg.)
Fig. 2 XRD patterns of PdCl2/Cu-HMS and PdCl2/Si-Cu-HMS-x
A PdCl2/Cu-HMS, B PdCl2/Si-Cu-HMS-25, C PdCl2/Si-Cu-HMS-50,
D PdCl2/Si-Cu-HMS-60, E PdCl2/Si-Cu-HMS-70, F PdCl2/Si-Cu-
HMS-75
F
E
D
enhanced. It was the surface hydrophobic groups that effi-
ciently reduced activity sites adsorbed by water, which was
beneficial to the improvement of catalytic performance of
catalysts.
C
B
A
-1
3.2.2 XRD Analysis
-1
847cm
2972cm
The XRD pattern (Fig. 2.) for parent PdCl2/Cu-HMS and
modified PdCl2/Si-Cu-HMS-x displayed the featured dif-
fraction peaks. It could be seen that the patterns of PdCl2/
Si-Cu-HMS-x were similar to that of the parent PdCl2/Cu-
HMS except the observed slight differences both in terms
of the width and intensities of the (100) peak, showing that
the pore structure integrity was kept after silylation. The
observed increase in the (100) peak width and decrease in
the peak intensity of PdCl2/Si-Cu-HMS-60 were probably
due to slight structure collapses during modification,
reflecting a worse crystallinity degree and order of pore
structure, which demonstrated that silylation was more
complete at 60 °C in consistent with Table 1.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1 The FT-IR spectra for PdCl2/Cu-HMS and PdCl2/Si-Cu-
HMS-x A PdCl2/Cu-HMS, B PdCl2/Si-Cu-HMS-25, C PdCl2/Si-Cu-
HMS-50, D PdCl2/Si-Cu-HMS-60, E PdCl2/Si-Cu-HMS-70, F PdCl2/
Si-Cu-HMS-75
conclusion, 60 °C was regarded as the best silylation
temperature.
3.2 Characterization
3.2.1 FT-IR
3.2.3 N2 Adsorption–Desorption and Water Adsorption
Capacity Test
The FT-IR spectra obtained for PdCl2/Cu-HMS and PdCl2/
Si-Cu-HMS-x was shown in Fig. 1. The bands at 1,087 and
804 cm-1 were for stretching vibration of Si–O–Si. In con-
trast with curve A, it can be seen that two new absorption
bands at 2,972 and 847 cm-1 respectively in other curves,
attributed to the attached CH3 groups, which indicated that –
Si(CH3)3 group had been successfully grafted on the Cu-
HMS surface. The intensities of the absorption band at 3,440
and 1,636 cm-1 which were ascribed to the free SiOH
groups and adsorbed water molecules, were obviously
decreased after silylation, which indicated that the SiOH
groups of Cu-HMS surface decreased and hydrophobicity
In order to further explore silalytion effect on pore
structure of Cu-HMS, N2 adsorption–desorption was
performed, shown in Fig. 3. All samples exhibited com-
plementary textural framework-confined mesoporosity, as
evidenced by the presence of type IV isotherms and H1
hysteresis loops, implying that silylation did not damage
the framework of Cu-HMS [27]. The rapid rise in the
adsorption branches for all the samples at a relative
pressure (P/P0) of 0.2–0.3 stemmed from capillary con-
densation in the mesopores. Moreover, the knees for the
123

Effect of Hydrophobic Modification
323
Table 2 The textural properties and water adsorption capacity of
PdCl2/Cu-HMS and PdCl2/Si-Cu-HMS-x
A
B
Samples
SBET
(m2 g-1
Pore Vpore
size
(nm)
Water
adsorption
capacity (%)
)
(cm3 g-1
)
C
D
A (PdCl2/Cu-HMS) 1,068.21 4.14
1.11
0.71
44.76
7.10
B (PdCl2/Si-Cu-
HMS-25)
1,053.10 2.70
E
F
C (PdCl2/Si-Cu-
HMS-50)
876.17
880.01
963.63
3.56
3.58
3.48
0.78
0.80
0.84
0.72
6.58
7.90
8.73
5.35
D (PdCl2/Si-Cu-
HMS-60)
0.0
0.2
0.4
0.6
0.8
1.0
E (PdCl2/Si-Cu-
HMS-70)
Relative Pressure (P/P )
0
F (PdCl2/Si-Cu-
HMS-75)
1,035.58 2.77
Fig. 3 N2 adsorption–desorption isotherms of PdCl2/Cu-HMS and
PdCl2/Si-Cu-HMS-x A PdCl2/Cu-HMS, B PdCl2/Si-Cu-HMS-25,
C PdCl2/Si-Cu-HMS-50, D PdCl2/Si-Cu-HMS-60, E PdCl2/Si-Cu-
HMS-70, F PdCl2/Si-Cu-HMS-75
physisorbed water in the voids of mesoporous structure.
The second weight loss between 250 and 350 °C was
attributed to the template. Finally, the third weight loss in
the temperature range of 400–550 °C was assigned to the
water loss from the condensation of adjacent silanol groups
to form siloxane bond [29, 30]. Moreover, all the weight
loss of silylated samples decreased, indicating that they had
a better hydrophobicity and a fewer existence of –OH on
the catalyst surface than PdCl2/Cu-HMS. So silylated cat-
alysts exhibited a better activity because decline of -OH
might benefit the removal of water, thus reducing activity
sites adsorbed by water and hydrolysis of DEC, which
improved catalytic performance of catalysts in a certain
degree.
modified samples were not as steep as that before modi-
fication, suggesting a slightly decline hexagonal order
after silylation.
Table 2 showed the textural properties and water
adsorption capacity on various samples, reflecting the
success of the modification. It could be seen that the sub-
sequent modifications performed on the Cu-HMS material
led to a concomitant decrease of the specific surface areas
(SBET) and mesoporous volumes (Vpore), which was ascri-
bed to decrease of efficient pore channels by large tri-
methylsilyl groups grafted. Pore diameter reached the
largest among silylated samples at 60 °C, which might be
another reason for the best catalytic performance besides
hydrophobicity (table 1), because large pore was more
favorable to the contact between reactants and active sites
in pore channel of catalyst. As expected, the TMCS-mod-
ified Cu-HMS samples contained a much stronger hydro-
phobicity compared to the parent sample because of
hydrophobic groups grafted on the surface of Cu-HMS.
The small amount of adsorbed water possibly occurred at
the residual SiOH sites through hydrogen bonding and/or
the strained siloxane bridges by rehydroxylation [19].
However, there was a little difference in water adsorption
capacity of the silylated samples under different silylation
temperatures, which was attributed to the difference of pore
volume [28]. Surprisedly, for PdCl2/Si-Cu-HMS-25 cata-
lyst, there did not exist such a relevance between water
adsorption capacity and pore volume. It was surmised that
there was more residual –OH on PdCl2/Si-Cu-HMS-25
catalyst due to low degree of silylation, which increased its
water adsorption capacity.
F
E
D
C
B
A
0
100
200
300
400
500
600
T(
)
Fig. 4 TG patterns of PdCl2/Cu-HMS and PdCl2/Si-Cu-HMS-x
A PdCl2/Cu-HMS, B PdCl2/Si-Cu-HMS-25, C PdCl2/Si-Cu-HMS-50,
Figure 4 showed the weight loss of PdCl2/Cu-HMS and
PdCl2/Si-Cu-HMS-x through thermogravimetric analysis.
The first weight loss below 150 °C was due to the loss of
D PdCl2/Si-Cu-HMS-60,
Cu-HMS-75
E
PdCl2/Si-Cu-HMS-70,
F
PdCl2/Si-
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P. Zhang et al.
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This work synthesized a family of organic–inorganic
hybrid materials by combining TMCS with Cu-HMS, and
investigated the influence of temperature to silylation. The
obtained PdCl2/Si-Cu-HMS-x exhibited a higher conver-
sion and activity in comparison with PdCl2/Cu-HMS,
coupled with an excellent selectivity for the synthesis of
DEC by oxidative carbonylation of ethanol. It was obvi-
ously demonstrated that silylation had modified the Cu-
HMS surface and improved hydrophobicity, which bene-
fited the removal of water and thus reduced activity sites
adsorbed by water and hydrolysis of DEC. It was note-
worthy that there existed a higher catalytic activity at
60 °C, which was due to that 60 °C was just above boiling
point of TMCS, where silylation was considered as vapor–
liquid–solid reaction, benefiting fully silylating. This sim-
ple but effective strategy for designing organic–inorganic
hybrid materials may draw an outline for applying in the
removal of water.
Acknowledgments The financial supports from the National Nat-
ural Science Foundation of China (no. 21106054) and Science
Research Foundation for New Scholars of Ministry of Education
(20100093120003) are gratefully acknowledged.
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