FULL PAPER
2
ꢀ
Lewis and Brønsted base sites due to the O and OH
groups. Additionally, Lewis acid sites in the form of incom-
3 4
(Bruker AV 400 spectrometer; samples in CDCl solution with Me Si as
internal standard). 2,3,5-Trimethylhydroquinone (97 wt.%) and isophytol
(95 wt.%) were purchased from Acros Organics. The other reagents (an-
alytical grade) were obtained from Merck.
2
+
pletely coordinated Mg sites on the surface are also pres-
ent. Such materials have been successfully applied as bifunc-
Characterisation
[6]
tional catalysts in Michael addition reactions.
XRD: X-ray powder diffraction (XRD) patterns were recorded using an
XRD-7 Seiffert-FPM diffractometer equipped with a CuKa radiation
source.
In this paper, we report the structure and properties of
nanoscopic, partly hydroxylated magnesium fluorides pre-
pared by a novel sol–gel synthetic route. In a single step,
19
F MAS NMR: Solid-state MAS NMR experiments were performed on
1
9
aqueous HF solution was added to Mg
A
H
U
G
R
N
U
G
a Bruker AVANCE 400 spectrometer by using a 2.5 mm rotor for
(n ACHTNUTGRENUNGN
Larmor
F
3
2
1
9
the concentration of the aqueous HF solution but keeping
the molar ratio of HF to Mg constant at 2:1, different acidic
materials were obtained. The bulk structure of these materi-
als was investigated by XRD, TEM, and solid-state NMR.
The thermal behaviour of the compounds was investigated
by DTA/TG coupled with MS measurements. The surface
1
9
brated with standard reference substances ( F: C
dard diso =ꢀ166.61 ppm against CFCl ). Background F signals could be
suppressed by application of a phase-cycled depth pulse sequence accord-
6 6
F as secondary stan-
19
3
[
12]
ing to Cory and Ritchey.
Elemental analysis: The C, H, N contents were determined using a Leco
CHNS-932 analyser. The fluoride content was determined by means of a
fluoride-sensitive electrode. Before the analysis, the samples were digest-
properties were examined by means of N and Ar adsorp-
2
tion–desorption isotherms, XPS, and infrared investigations
with probe molecules. Furthermore, catalytic test reactions
have been carried out to investigate the different acid sites.
To our astonishment, the resulting magnesium hydroxyl
groups on magnesium fluoride are purely acidic, and to the
best of our knowledge this is the first time that this has been
reported on. Magnesium hydroxide groups on magnesium
fluoride and general magnesium hydroxide have been dis-
ed by fusion with Na
ples were determined by the ICP OES method (IRIS Intrepid HR
DUO) after a microwave-assisted (ETHOS plus) digestion with HNO
2 3 2 3
CO /K CO . The magnesium contents of the sam-
3
.
Determination of the surface area and porosity: The surface properties
of the samples were determined by acquiring adsorption–desorption iso-
therms of N
2
at 77 K and of Ar at 87 K on a Micromeritics ASAP 2020
instrument. Samples were first degassed in vacuo at 708C. Surface areas
[13]
were calculated according to the BET method.
[7,8]
Thermal analysis: Thermal analysis experiments were performed on a
Netzsch STA 409C apparatus. A DTA-TG sample-holder system (Pt/
PtRh10 thermocouple) was used, and measurements were performed in
cussed in the literature as basic species
plied as basic catalysis.
and have been ap-
In contrast, our new materials
[9–11]
have proved to be very active in reactions requiring acidic
sites. Thus, we have successfully applied these compounds as
heterogeneous catalysts for the synthesis of (all-rac)-[a]-to-
copherol.
2
an atmosphere of N . The thermo analyser was equipped with a Balzers
QMG429 quadrupole mass spectrometer for the multiple ion detection
modes.
2
Transmission electron microscopy (TEM): MgF -71 nanoparticles for
TEM measurements were prepared by dispersing the powder in metha-
nol and applying ultrasound. A conventional carbon film of thickness
about 20 nm supported by a copper grid was then dipped into the suspen-
sion. The grid was slowly moved sideways to pick up as many particles as
possible. After complete evaporation of the methanol from the specimen,
the particles were studied on a JEOL TEM 2200FS operated at an accel-
eration voltage of 200 kV. In high-resolution mode TEM (HRTEM), the
point-to-point resolution was 0.19 nm and the information limit for the
visualisation of crystal lattice planes was 0.14 nm.
Experimental Section
Sample preparation
x
Synthesis of MgF2ꢀx(OH) samples: Five magnesium fluoride catalysts
were prepared from metallic Mg using the sol–gel method, as follows:
metallic Mg (Aldrich, 99.98% powder) (1.56 g, 64 mmol) was treated
with dry methanol (50 mL) at room temperature overnight. After heating
7
FTIR spectroscopy: Samples were pressed (10 Pa) into self-supported
2
ꢀ2
discs (2 cm area, 10–20 mgcm ). All samples were then degassed in situ
before measurement at 373 K. The infrared cell was made of quartz and
under reflux conditions for 3 h,
130 mmol) dissolved in different amounts of water (HF solution concen-
trations: 40, 57, 71, 87, and 100 wt% HF) was added to the formed Mg-
(OCH solution. The mixtures reacted to form highly viscous transpar-
a stoichiometric amount of HF
(
2
was equipped with CaF windows. A movable quartz sample holder al-
lowed the pellet to be moved into the infrared beam for spectral acquisi-
tion or into a furnace at the top of the cell for thermal treatment. Trans-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3 2
)
ent gels. After ageing for 12 h, each gel was dried under vacuum at room
temperature. The solid product thus obtained was then further dried
under vacuum at 708C for 5 h. The prepared catalysts are referred to
ꢀ
1
mission IR spectra were recorded in the range 650–4000 cm on a Nico-
let Nexus spectrometer equipped with an extended KBr beam-splitting
device and a mercury cadmium telluride (MCT) cryodetector. The cell
hereafter as MgF
cating the different concentrations of HF solutions used. For comparison,
crystalline commercial MgF (Aldrich, 99.8%) was also tested (MgF -C).
2 2 2 2 2
-40, MgF -57, MgF -71, MgF -87, and MgF -100, indi-
was connected to
a
vacuum line for evacuation, calcination steps
ꢀ
3
ꢀ4
(
Presidual =10 –10 Pa), and for the introduction of CO gas into the infra-
2
2
red cell. Spectra were recorded at room temperature. In the CO adsorp-
tion experiments, the temperature of the pellet was decreased to about
Synthesis of (all-rac)-[a]-tocopherol: In a typical procedure, 2,3,5-trime-
thylhydroquinone (TMHQ, 152 mg, 1 mmol) was dissolved in a 1:1 mix-
ture of heptane/propylene carbonate (6 mL) in a glass vial with a stan-
dard capacity of 8 mL, equipped with a magnetic stirrer. Isophytol (IP,
1
2
00 K by cooling the sample holder with liquid N after quenching the
sample from the thermal treatment temperature. CO (>99.997% pure,
supplied by Air Liquide, France) was provided from a gas balloon, dried
with liquid N , and incrementally dosed to the sample.
2
2
96 mg, 1 mmol) and catalyst (50 mg) were then added. After sealing the
vial, it was immersed in an oil bath at 1008C, and the reaction mixture
was stirred at 1500 rpm for 60–1200 min. After separating the catalyst
from the two-phase solvent mixture, the heptane phase containing the to-
copherol was isolated and the solvent was removed in vacuo to give the
crude product. The crude products were analysed by HPLC (column: EC
The addition of accurately known increments of CO probe molecules to
the cell (a typical increment corresponded to 100 mmol of CO per gram
3
of material) was possible by means of a calibrated volume (2.15 cm ) con-
nected to a pressure gauge for the control of the probe pressure (1–
ꢀ
1
4
1
25/4.6 Nucleosil 120-5 C18; eluent: acetonitrile; flow rate: 0.8 mLmin
;
10 Pa range). The CO pressure inside the IR cell was controlled by an-
1
13
3
wavelength: 280 nm; volume of sample: 15 mL) and by H and C NMR
other pressure gauge (1–10 Pa range).
Chem. Eur. J. 2008, 14, 11488 – 11499
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11489