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was prepared by dissolving of the triblock copolymer Pluronic
P123 (BASF, 4 g) in deionized water (107.5 g) before adding citric
acid monohydrate (Riedel-De Haen, 3.684 g) and trisodium citrate
(UCB, 2.540 g). The resulting solution was stirred for 24 h. A
sodium silicate solution (10% NaOH, 27% SiO2, Merck, 10.4 g) was
diluted in deionized water (30 g) and added to the surfactant solu-
tion. The solution was stirred for 5 min and aged at RT for 24 h.
The material was filtered, washed, and dried at 333 K overnight. Fi-
nally, it was calcined in air first at 573 K for 8 h, and then at 773 K
(1 Kminꢀ1) for 8 h.
radiation, l=0.154056 nm) in transmission geometry with a posi-
tion sensitive image plate detector (IP PSD). Measurements at RT in
Debye–Scherrer mode were carried out in capillaries with 0.7 mm
internal diameter.
The specific surface areas and porosities of the catalysts were de-
termined by nitrogen adsorption at 77 K on a Micromeritics Tristar
3000 instrument. All samples were degassed under nitrogen flow
at 673 K for 6 h before the measurements. The BJH model was ap-
plied on the adsorption branch of the nitrogen isotherm to obtain
the pore size distributions.
The method of Grꢁn et al. was used to synthesize spherical MCM-
41 particles.[18b] The starting surfactant solution was prepared by
dissolving n-hexadecyltrimethylammonium bromide (C16TMABr,
Fluka, 2.5 g) in deionized water (50 g) before adding aqueous am-
monia (32 wt%, Merck, 13.2 g) and absolute ethanol (VWR, 60.0 g).
The solution was stirred for 15 min, before tetraethyl orthosilicate
(TEOS, Aldrich, 4.7 g) was added. The resulting gel was stirred for
2 h, filtered and washed with deionized water (100 mL) and metha-
nol (Aldrich, 100 mL). The washed sample was dried overnight at
363 K and calcined at 823 K for 5 h (1 Kminꢀ1).
29Si MAS NMR spectra were recorded on a Bruker AMX300 spec-
trometer (7.0 T). At this field, the resonance frequency of 29Si is
59.6 MHz. 4,000 scans were accumulated with a recycle delay of
60 s, the pulse length being 5.0 ms. The spinning frequency of the
rotor was 5 kHz. The 29Si CP MAS NMR spectrum was recorded on
a Bruker Avance400 spectrometer (9.4 T). At this field, the reso-
nance frequency of 29Si is 79.5 MHz. 17,000 scans were accumulat-
ed with a recycle delay of 10 s. The CP contact time was 4.0 ms.
The spinning frequency of the rotor was 10 kHz. For both measure-
ments the samples were packed in 4 mm Zirconia rotors. Tetrame-
thylsilane was used as chemical shift reference.
Mg(OH)2 was obtained from the rehydration of commercial MgO
according to the method of Di Cosimo et al.,[38] involving dilution
of commercial MgO (Aldrich, 25 g) in deionized water (250 mL).
The mixture was then stirred and heated to 353 K for 4 h, and final-
ly dried at 358 K.
Basicity of the catalysts was determined by temperature-pro-
grammed desorption of CO2 (CO2-TPD). A flow apparatus was
equipped with a Pfeiffer Omnistar quadrupole mass spectrometer
for the detection of the desorbed gases. Prior to adsorption, the
sample (150 mg) was pretreated in a helium flow for 1 h at 673 K
(5 Kminꢀ1). Saturation of the samples was performed in a CO2 flow
at RT for 30 min. Weakly bound CO2 was removed by flushing with
helium for 1 h. The TPD experiments were carried out in a helium
flow of 20 mLminꢀ1 in the temperature range of 298–693 K with
The MgO-silica catalysts were prepared by dry milling of Mg(OH)2
with each of the silicas followed by calcination at 673 K for 3 h
(2.5 Kminꢀ1). Commercial silica gel (SiO2), COK-12, and MCM-41
were used as silica sources. All the mixed systems had a molar Mg/
Si ratio of 2. Pure MgO was obtained by calcining Mg(OH)2 in the
same manner as the mixed catalysts.
a heating rate of 10 Kminꢀ1
.
Silver-containing samples with different Ag loading were prepared
by incipient wetness impregnation of the calcined MgO-silica sup-
port with the appropriate amount of an aqueous silver nitrate solu-
tion ranging from 30 mm to 260 mm.[4] After impregnation the
samples were dried at 333 K and calcined at 673 K for 3 h
(2.5 Kminꢀ1). The samples were denoted as xAg/MgO-silica, where
x represents the silver loading in wt%. Blanco catalysts devoid of
silver (0Ag/MgO-silica) were also prepared using the same prepara-
tion method, using deionized water as wetting solution. A Mg-free
Ag-on-silica sample (1.0Ag/SiO2) was obtained by the same im-
pregnation method.
IR experiments were performed on a Nicolet 6700 spectrometer
equipped with a DTGS detector (128 scans; resolution of 2 cmꢀ1).
Self-supporting wafers were pretreated in vacuum at 673 K for 1 h
(5 Kminꢀ1) before measurements. Acidity of the catalysts was ana-
lyzed using pyridine as probe. After pretreatment at 673 K, the
samples were saturated with about 25 mbar of pyridine vapor at
323 K for 20 min. The evacuated samples containing the sorbed
pyridine were subjected to a temperature-programmed desorption
at 323, 423, 523 K for 20 min, respectively, with a heating rate of
5 Kminꢀ1 and the IR spectra were recorded in situ at these temper-
atures. CO2 was also used as probe to analyze the basicity. After
pretreatment at 673 K, the sample was saturated in a CO2 flow at
298 K for 30 min. The saturated sample was subjected to a temper-
ature-programmed desorption at 298, 373, 473, 573 and 673 K for
20 min, respectively, with a heating rate of 5 Kminꢀ1 and cooling
down to 298 K in between to record the IR spectra. For analysis,
the spectrum of the sample without CO2 is subtracted from the
spectra with adsorbed CO2.
Catalyst characterization
SEM images were recorded on an SEM Philips XL 30 FEG. The sam-
ples were fixed on a carbon tape and coated with a layer of gold.
ESEM/EDX mapping was performed on a ESEM Philips XL 30 FEG
equipped with an EXD detector (EDAX). The surface of the pow-
dered catalysts, embedded in a resin, was grinded, polished and
coated with carbon before the measurement.
The nature of the silver and magnesium oxide species was exam-
ined by UV/Vis DRS. Spectra were recorded on an Agilent Carry
5000 spectrophotometer using Halon white standard as reference.
Prior to the measurements, the samples were pretreated under a ni-
trogen flow at 673 K for 1 h (5 Kminꢀ1).
SAXS measurements were recorded at RT with a SAXSSess mc2 in-
strument (Anton Paar, Graz, Austria) equipped with a line-collimat-
ed CuKa X-ray source (l=0.154056 nm) and a 2D imaging plate
detector. The SAXS patterns were normalized to the incident beam
intensity. Background subtraction (from an empty capillary) and
correction for instrumental broadening were performed by using
the SAXSquant software.
Catalytic reactions
The catalytic conversion of ethanol to butadiene was carried out in
the reactor system described previously.[4] Briefly, a downstream
continuous-flow quadri-reactor containing four parallel alloy 600
reactor tubes with 3 mm internal diameter was used. Typically,
Powder XRD patterns were recorded on a STOE STADI P Combi dif-
fractometer equipped with a Ge (111) monochromator (CuKa X-ray
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