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In order to achieve this goal, new materials such as non-halide
copper containing catalysts [11] and lanthanide (oxy)chlorides
[12], particularly lanthanum oxychlorides, have been reported in
the patent literature. However, none of these systems is realized
on an industrial scale, most likely due to the low per pass VCM
yields and/or stability hurdles faced by these catalysts. Recently,
we have uncovered the high stability and remarkable yield of chlo-
rinated compounds (25% VCM, 25% EDC) over CeO2. This outstand-
ing performance was attributed to the integration of both redox
sites, which catalyze the ethylene oxychlorination to EDC, and acid
sites, responsible for the dehydrochlorination of EDC to VCM, on
the same catalyst surface.
In spite of these encouraging results, CeO2 offers significant
margins for improvement. In particular, a considerable amount of
over-chlorinated compounds (1,2-dichloroethene, commonly
known as 1,2-dichloroethylene and hereafter denoted as 1,2-
DCE) and combustion products (COx) were also formed, decreasing
the overall selectivity of EDC and VCM. Moreover, only moderate
dehydrochlorination activity was observed. Thus, it can be antici-
pated that materials exhibiting milder oxidative properties and
higher density of strong cites than ceria could lead to enhanced
performance. Surprisingly, despite their extensive use as dopants
represents the molar fraction of Eu in the range of 0.3–0.9) were
synthesized by precipitation (p, single oxides) and coprecipitation
(cp, mixed oxides) following a protocol reported elsewhere [13].
Briefly, the metal nitrates (Eu2(NO3)3ꢁ6H2O (ABCR, 99.9%) for
Eu2O3-p, Ce(NO3)3ꢁ6H2O (ABCR, 99.9%) for CeO2-p, or mixtures
for EuxCe1ꢀxO2ꢀ0.5x-cp) were dissolved in deionized water under
stirring and H2O2 (Acros Organics, 35%) was added to the solution
to obtain a molar H2O2:M ratio of 3 (M = Eu, Ce, or Eu + Ce). The
coprecipitation was achieved by the dropwise addition of aqueous
NH4OH (Sigma–Aldrich, 30%) until a pH of 10.5 was reached. The
slurry was stirred for 4 h and washed with deionized water. Upon
filtration, the precipitate was dried at 393 K for 12 h and calcined
at 773 K for CeO2-p, 773–1173 K for Eu2O3-p, and 773 K for
EuxCe1ꢀxO2ꢀ0.5x-cp in flowing air using a heating rate of 5 K minꢀ1
and an isothermal step of 5 h.
2.2. Characterization
The metal content was determined by X-ray fluorescence (XRF)
spectroscopy using an Orbis PC Micro-EDXRF analyzer with a Rh
source (15 kV, 500
diffraction (XRD) was measured using a PANalytical X’Pert PRO-
MPD diffractometer and Cu K radiation (k = 0.15418 nm). The
lA) and a silicon drift detector. Powder X-ray
of the CuCl2/c-Al2O3 catalysts, lanthanide compounds were never
systematically investigated as the main active phase for the con-
version of ethylene to VCM. Besides, they were investigated in sev-
eral other catalyst formulations mainly as dopants and supports
[14–18], with the exception of cerium oxide and lanthanum oxide
(or (oxy)chloride) which are also studied as the primary catalytic
phase in oxidative processes, including CO oxidation [19,20],
isobutane oxidation [21], HCl oxidation [22,23], methane oxidative
coupling [24], selective reduction of nitrogen oxides [19,25], and
methane oxychlorination [26,27].
Herein, the comparison of the performance of a broad set of the
most abundant rare-earth compounds leads to the discovery of the
exceptional performance of europium oxychloride, exhibiting 96%
VCM selectivity at 20% conversion for over 100 h on stream. Struc-
tural, redox, and acidic properties are investigated to rationalize
the superior performance of europium oxychloride with respect
to other lanthanides and parametric studies give insight to the dis-
tribution of products. In order to attain superior VCM yields, this
novel active phase is combined with the high activity of CeO2 by
synthesis of mixed oxides and through dual-bed reactor concepts.
This study comprises the first practically-relevant application of
europium in heterogeneous catalysis and the materials presented
here have great potential to be explored in challenging catalyzed
reactions, particularly toward the functionalization of
hydrocarbons.
a
data were recorded in the 10–70° 2h range with an angular step
size of 0.017° and a continuing time of 0.26 s per step. N2 sorption
at 77 K was measured in a Quantachrome Quadrasorb-SI analyzer.
Prior to the measurements, the samples were outgassed to 50 mbar
at 573 K for 3 h. The Brunauer-Emmett-Teller (BET) method [28]
was applied to calculate the total surface area, SBET, in m2 gꢀ1
.
High-resolution transmission electron microscopy (HRTEM) and
elemental mapping using energy-dispersive X-ray spectroscopy
(EDX) were conducted on a FEI Talos microscope operated at
200 kV. All samples were dispersed as dry powders onto lacey car-
bon coated nickel or molybdenum grids. X-ray photoelectron spec-
troscopy (XPS) measurements were performed on a Physical
Electronics Quantum 2000 X-ray photoelectron spectrometer using
monochromatic Al Ka radiation generated from an electron beam
operated at 15 kV, and equipped with a hemispherical capacitor
electron-energy analyzer. The powdered sample was firmly
pressed onto the foil. The area analyzed was 150 lm in diameter
and the electron take-off angle was 45°. The pass energy used for
the detailed spectra of the C 1s, O 1s, Cl 2p, Eu 3d, Eu 4d, and
Ce 3d core levels was 46.95 eV to yield a total analyzer energy res-
olution of 0.95 eV. The spectrometer energy scale was calibrated
for the Au 4f electrons to be at 84.0 0.1 eV. Partial compensation
of surface charging during spectra acquisition was obtained by the
simultaneous operation of electron and argon ion neutralizers. Ele-
mental concentrations are given in atomic percent using the mea-
sured photoelectron peak areas after Shirley background
subtraction and the built-in sensitivity factors for calculation.
Temperature-programmed desorption of ammonia (NH3-TPD)
and temperature-programmed reduction with hydrogen (H2-TPR)
were performed using a Micromeritics Autochem II 2920 unit
equipped with a thermal conductivity detector coupled to a MKS
Cirrus 2 mass spectrometer. The powder sample (0.1 g) was loaded
into a U-shaped quartz micro-reactor, pretreated in He (20 cm3
STP minꢀ1) at 573 K for 3 h, and cooled to 373 K in He. For
NH3-TPD experiments, ammonia was chemisorbed at 473 K in
three consecutiꢀv1e cycles of saturation with 5 vol.% NH3/He
2. Experimental methods
2.1. Catalyst preparation
Commercial La2O3 (Alfa Aesar, 99.99%), Pr2O3 (Alfa Aesar,
99.9%), Nd2O3 (Sigma–Aldrich, 99.9%), Sm2O3 (Sigma–Aldrich,
99.9%), Eu2O3 (Sigma–Aldrich, 99.5%), Gd2O3 (Alfa Aesar, 99.99%),
Tb2O3 (Strem Chemicals, 99.9%), Dy2O3 (ABCR, 99.99%), Ho2O3
(Fluka Chemie, 99.9%), and Er2O3 (Fluka Chemie, 99.9%) were cal-
cined at 773 K, and CeO2 (Sigma–Aldrich, 99.9%) at 773 K and
1173 K in static air using a heating rate of 5 K minꢀ1 and an
isothermal step of 5 h prior to their use in catalytic studies. Analy-
sis by X-ray diffraction revealed that the commercial praseody-
mium oxide actually consisted of Pr4O7 and thus it is denoted as
such hereafter. Europium oxide (Eu2O3-p-Tcal, where Tcal denotes
the calcination temperature in K), cerium oxide (CeO2-p-Tcal), and
mixed europium-cerium oxides (EuxCe1ꢀxO2ꢀ0.5x-cp-Tcal, where x
(20 cm3 STP min
) for 30 min followed by purging with He
(20 cm3 STP minꢀ1) at the same temperature for 30 min. Desorp-
tion of NH3 was monitored in the range of 473–1273 K using a
heating rate of 20 K minꢀ1 and a He flow of 20 cm3 STP minꢀ1
.
For H2-TPR experiments, the sample was pretreated in He
(20 cm3 STP minꢀ1
)
at 423 K for 1 h, and cooled to room