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M. Okamoto, Y. Taniguchi / Journal of Catalysis 261 (2009) 195–200
Fig. 1. Time evolution of acetaldehyde yield during oxidation of ethylene using
PdCl2–CuCl2–PEG/SiO2. PEG contents were (a) 0, (b) 10, (c) 20, and (d) 30% (w/w).
The reaction was carried out at 9.6 kPa of ethylene, 33.3 kPa of oxygen, and 48.0 kPa
Fig. 2. Effect of Pd content on acetaldehyde yield in oxidation of ethylene using
PdCl2–CuCl2–PEG/SiO2. Pd contents were (a) 3, (b) 6, (c) 7.5, and (d) 9% (w/w). The
reaction was carried out at 9.6 kPa of ethylene, 33.3 kPa of oxygen, and 48.0 kPa
◦
of water at 100 C using 0.86 g (excluding PEG weight) of the catalyst. The contact
◦
of water at 100 C using 0.86 g (excluding PEG weight) of the catalyst. The con-
time, W /F , was 8.8 g h mol−1. Pd and Cu contents were 6 and 3% (w/w), respec-
tact time, W /F , was 8.8 g h mol−1. Cu and PEG contents were 3 and 20% (w/w),
tively.
respectively.
Industries), CuCl2 (>99.0%, Kanto Chemical Co.), and PEG (aver-
age molecular weight 8200, Kanto Chemical Co.) were dissolved
in 3 mL of deionized water, following which 1 g of dried silica
gel was added and allowed to settle for 24 h. Finally, the cata-
lyst was dried using a rotary evaporator. When an additive such as
LiCl (>99.0%, Kanto Chemical Co.) was used, it also dissolved with
PdCl2, CuCl2 and PEG in water.
2.2. Reaction procedure
The catalyst (0.86 g excluding PEG weight) was placed in a re-
actor tube (quartz; i.d., 15 mm) of a fixed-bed flow reactor system.
◦
In the case of ethylene oxidation, the catalyst was heated to 100 C
at an incremental rate of 2.5 C min
◦
−1
in a helium stream, follow-
Fig. 3. Effect of Cu content on acetaldehyde yield in the oxidation of ethylene us-
ing PdCl2–CuCl2–PEG/SiO2. Pd and Cu contents were (a) 9 and 6, (b) 9 and 3, (c) 2
and 6, and (d) 2 and 3% (w/w), respectively. The reaction was carried out at 9.6 kPa
ing which ethylene (9 kPa), oxygen (36 kPa), and water (46 kPa)
were fed to the reactor at 100 C. The products were identified by
gas chromatography-mass spectrometry and analyzed by gas chro-
matography. Yields of oxidation products were calculated based on
ethylene fed to the reactor.
◦
◦
of ethylene, 33.3 kPa of oxygen, and 48.0 kPa of water at 100 C using 0.86 g (ex-
cluding PEG weight) of the catalyst. The contact time, W /F , was 8.8 g h mol−1. PEG
content was 20% (w/w).
deactivation was observed. SLPCs using PEG liquid showed stable
activity.
2.3. Catalyst characterization
The effects of Pd and Cu content were examined. Fig. 2 shows
the effects of Pd content on acetaldehyde yield. The yield increased
with increasing Pd content to 7.5% (w/w). However, use of 9%
(w/w) Pd decreased catalytic activity, i.e., catalyst deactivation was
observed when using >7.5% (w/w) of Pd. Cu content, on the other
hand, did not affect catalytic activity. As shown in Fig. 3, when
either 2 or 9% (w/w) of Pd was used, the time courses of acetalde-
hyde yield using different Cu contents were almost the same. This
indicates that even 3% (w/w) Cu is excessive for continuing the re-
dox cycle of Pd between (+2) and (0).
Nitrogen adsorption was measured with a BELSORP-mini (BEL
Japan). Prior to the measurement, the catalyst was dried at 120 C
◦
for 3 h under evacuation. Pore size distributions were calculated
using desorption isotherms. X-ray diffraction (XRD) patterns of the
catalysts were recorded using CuKα with a MiniFlex (Rigaku Co.).
XRD samples were not ground to avoid changes in the catalyst due
to heat generated by grinding. EXAFS analysis was performed at
the BL-7C facility of the Photon Factory at the High Energy Ac-
celerator Research Organization, Japan, for Cu K -edge analysis. Pd
K -edge absorption spectra were obtained at BL-10B and PF AR-10C.
PdCl2–CuCl2–PEG/SiO2 catalyst (Pd 6%, Cu 3%, PEG 20%), which
showed stable activity, was examined using powdered XRD. Fig. 4
shows the XRD patterns of the catalyst before and after the reac-
tion. Before the reaction, no peak was observed. This implies that
PdCl2 and CuCl2 were highly dispersed in PEG. After the reaction,
peaks assigned to Pd metal particles appeared, and their intensities
increased with time on stream. In a Wacker-type reaction, redox
of Pd is indispensable for continuing the reaction. Formation of Pd
metal particles, which cannot be oxidized to Pd(II) ions, causes de-
activation of the catalyst system. Thus, a slight deactivation of the
PdCl2–CuCl2–PEG/SiO2 catalyst (Pd 6%, Cu 3%, PEG 20%) system as
shown in Fig. 1c is caused by the formation of Pd particles.
The PdCl2–CuCl2–PEG/SiO2 catalyst system was also analyzed
by EXAFS. Fig. 5 shows Pd K -edge EXAFS Fourier transforms of the
catalysts before and after the reaction. In the as-synthesized cata-
3. Results and discussion
3.1. Ethylene oxidation
3.1.1. Ethylene oxidation over PdCl2–CuCl2–PEG/SiO2
Fig. 1 shows time courses of acetaldehyde yield obtained from
ethylene oxidation over catalysts with various PEG contents. In all
cases, the main product was acetaldehyde, and the selectivity of
carbon oxide and dioxide was low (<0.5%). The catalyst without
PEG showed high activity at the beginning of the reaction; how-
ever, the yield rapidly decreased with time on stream. Addition
of 10% (w/w) PEG improved catalytic stability. The yield was lower,
and decreased after 5 h. When >20% (w/w) of PEG was used, slight