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an automated porosimeter (Micrometrics ASAP 2000). The surface
OH groups were characterized by FTIR spectroscopy in the region
of 3000–4000 cmÀ1. TEM images were obtained by using a JEOL
1200 EX II electron microscope operated at 100 kV. Samples were
placed on a microgrid carbon polymer supported on a Cu grid by
deposition of a few droplets of a suspension of the ground sample
in ethanol on the grid followed by drying under ambient
conditions.
hyde, which regenerates the active sites (Scheme 2). Although
this mechanism is consistent with the data reported, more
studies are needed to confirm all the reaction steps. Additional
insight can be obtained by the investigation of the reaction
over Ag catalysts with different Ag dispersions and different
supports.
Although the mechanism proposed is somewhat speculative,
it provides useful information for the further design of efficient
catalysts for ethanol dehydrogenation. In particular, it implies
that such catalyst should exhibit the following features: 1) a
high density of SiÀOH groups in silica, 2) a high Ag dispersion
on the silica surface, and 3) a close proximity of the SiÀOH and
Ag sites.
Catalyst evaluation
The Ag/SiO2 catalyst was tested for ethanol dehydrogenation
under atmospheric pressure. Catalytic experiments were performed
in flow-type fixed-bed reactor. In a typical experiment, the catalyst
(2 g, fraction 0.5–1 mm) was loaded into the quartz tubular reactor,
which was purged with N2 at 873 K for 0.5 h followed by reduction
under a H2 flow at 573 K for 0.5 h. Ethanol was fed in to the reac-
tor by using a Razel syringe pump. He (20 mLminÀ1) was used as
a carrier gas, and methane was used as an external standard. The
reaction temperature was 573 K, and the weight hourly space ve-
locity (WHSV) was varied within 2.0–6.0 hÀ1. The reaction products
were analyzed online by using GC (Crystal 2000M) equipped with
a 50 m capillary column with SE-30.
Work is in progress for the development of such catalysts.
Conclusions
A silica-supported Ag catalyst has been shown to be an effi-
cient heterogeneous catalyst for the oxidant-free dehydrogen-
ation of ethanol into acetaldehyde. The catalyst shows
a higher activity (TOF=3.4Æ0.8 sÀ1) than more expensive sup-
ported Au catalysts.
A mechanism for the oxidant-free dehydrogenation of etha-
nol into acetaldehyde over the silica-supported Ag catalyst is
proposed based on the results of in situ FTIR spectroscopic
studies and kinetic experiments. The first step involves the acti-
vation of ethanol on silica to yield a hydrogen-bonded surface
complex. The next step is concerted CÀH bond cleavage on
a Ag site and proton abstraction on a silica site. This step is
the rate-determining step of the reaction as evidenced by the
observation of the primary isotopic effects for the conversion
of CH3CD2OH and CH3CH2OD. The final step of the reaction in-
cludes desorption of H2 and acetaldehyde, which regenerates
the active sites.
In situ spectroscopic studies
In situ IR spectra were recorded by using a Nicolet Protꢃgꢃ 460 in-
strument fitted with a stainless-steel IR cell with CaF2 windows
connected to a conventional flow-reaction system. The sample was
pressed into a self-supporting wafer (20 mg) and placed in the IR
cell. Spectra were recorded with an accumulation of 64 scans at
4 cmÀ1 optical resolution. Before each experiment, the catalyst disk
was heated in a He flow (40 mLminÀ1) at 573 K for 2 h and reduced
in a H2 flow (20 mLminÀ1) at 573 K for 0.5 h followed by cooling to
373 K and purging with He for 20 min. Ethanol was dosed to
a flow of He by using a microsyringe pump, and the adsorption
time was 30 s at 373 K. After adsorption, the weakly bonded etha-
nol was purged with a He flow (20 mLminÀ1) for 50 s. Then the He
flow was stopped, and the spectra were recorded every 20 s. A ref-
erence spectrum of the catalyst wafer under He recorded at 373 K
was subtracted from each spectrum.
The mechanism proposed suggests that an efficient Ag cata-
lyst for ethanol dehydrogenation should have a high density
of SiÀOH groups on the silica support, a high Ag dispersion,
and it should be characterized by the close proximity of the
SiÀOH and Ag sites. The results demonstrate that less expen-
sive catalysts than Pt- or Au-based catalysts can be designed
by the optimization of the acid–base properties of the support,
Ag particle size, and by control of the distance between the
metal and the active sites of the support.
Acknowledgements
V.L.S. gratefully acknowledges Haldor Tøpsoe A/S for a PhD fel-
lowship. The authors thank the Federal Principal Program of the
Russian Ministry of Education and Science “Research and educa-
tional specialists of innovative Russia” for financial support.
Experimental Section
Catalyst preparation
Keywords: dehydrogenation
· IR spectroscopy · silver ·
The Ag/SiO2 catalyst was prepared by the incipient wetness im-
pregnation of SiO2 (Karpov Chemical Plant) with an aqueous solu-
tion of AgNO3 to attain a Ag content of 10 wt.%. After impregna-
tion, the catalyst was dried at 393 K and calcined at 773 K for 3 h
in a flow of air.
reaction mechanisms · supported catalysts
[1] R. A. Sheldon, H. van Bekkum, Fine Chemicals Through Heterogeneous
Catalysts, New York, 2001.
196–200; f) K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K.
Catalyst characterization
The chemical composition of the sample was determined by AAS.
N2 sorption–desorption isotherms were measured at 77 K by using
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