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labelled tert-butyl chloride. FTIR spectroscopic measurements
on SF4-fluorinated γ-alumina using pyridine as a probe mole-
cule and mass spectrometric measurements of HCl and H2O
desorption from β-AlF3 have been used to supplement the
results from radiotracer experiments. Interpretations are offered
for the behaviour of β-AlF3 in terms of its structure6 and the
structural model previously proposed for the surfaces of β-
MF3, M = Cr and Al, fluorides.11,24 The data are used also to
propose a description for a fluorinated γ-alumina surface that is
the result of the fluorination by SF4.
Reactions under Friedel–Crafts conditions
These were performed in a Pyrex three-necked flask, equipped
with a septum cap for the introduction of reagents, an He gas
inlet and a gas outlet, fitted with a condenser. β-Aluminium()
fluoride or fluorinated γ-alumina (0.5 g in each case) was
loaded in the glove box and the apparatus flushed with He. A
mixture of dried toluene (224.0 mmol) and dried ButCl was
introduced via the septum and the mixture stirred magnetically
at a constant rate to minimise diffusion effects. Samples of the
liquid phase (0.5 cm3) were withdrawn at regular intervals and
analysed by GC (AMS Model 93, 15 m capillary column, FID),
response factors being determined by calibration with authentic
samples of the products. Confirmatory measurements were
made using GCMS (Hewlett Packard 5971 mass selective quad-
rupole detector at 70 eV interfaced to a Hewlett Packard 5890
series II GC, HP1 15 m column) and by 13C{1H} NMR
spectroscopy.
Experimental
Standard vacuum and glove box (H2O < 1 ppm) were used
throughout. Except where described below, instrumentation
and experimental methods have been described previously.12,25
Preparation of materials
β-Aluminium() fluoride was prepared by the temperature
programmed dehydration6 of AlF3ؒ3H2O (5.0 g, Aldrich,
purity 97%) under He (30 cm3 minϪ1), heating from room
temperature to 493 K at 5 K minϪ1, held at 493 K for 1 h,
heating to 723 K at 10 K minϪ1, held at 723 K for 2 h, finally
allowed to cool to room temperature under He flow. The β-AlF3
so formed was transferred in a sealed vessel to a N2 glove box
(H2O ca. 1 ppm) where subsequent manipulations were per-
formed. Its identity was confirmed by XRD. Samples prepared
in Berlin or Glasgow showed identical behaviour. Its BET area =
26 m2 gϪ1.12
Fluorinated γ-alumina was prepared under both flow and
static conditions. γ-Alumina (Degussa, BET area = 110 m2 gϪ1)
was calcined under N2 flow at 523 K, then fluorinated under
SF4/N2 flow for 2 h at 523 K (F, 47.1%; BET area = 67 m2 gϪ1).12
Fluorinations under static conditions were performed in a
Monel metal pressure vessel (Hoke, 90 cm3) attached to a
Monel vacuum line.23,25 Typically, γ-alumina (1.5 g), previously
caked, sieved to produce 500–1000 µm particles and calcined in
vacuo for 8 h at 523 K, was allowed to react with SF4 (9.0 mmol,
99%, Fluorochem) for 2 h, nominally at room temperature.
Volatile products, a mixture of OSF2 and SO2, whose com-
ponents were identified by FTIR spectroscopy, were removed
by distillation and the process repeated twice. The product, an
off-white solid, was transferred to and handled subsequently in,
a glove box. It could be stored for short periods in FEP; storage
in Pyrex led to etching, indicating that HF was lost slowly from
the solid. For this reason, smaller quantities (0.5 g) were pre-
pared for use in situ, with the appropriate adjustment of
the quantity of SF4. Fluorinations were carried out also using
SF4/OSF2 mixtures. Single point determinations of BET area
(Coulter SA 3100 instrument) gave values in the range 80–
90 m2 gϪ1. The imprecision was possibly a result of the corrosive
nature of the material. Fluorine content was not determined
directly but a value of ca. 22% was inferred from a previous
[18F] study of the fluorination carried out under very similar
conditions.23
Radiotracer experiments
The preparation of the isotope [18F], t1/2 110 min, βϩ(γ) emitter,
the preparation of H18F and the counting procedures used have
been described elsewhere.28 Interactions between H18F and
β-AlF3 or fluorinated γ-alumina (0.5 g for both solids) were
studied using an evacuable Ni tube reactor equipped with a
tube furnace, valves (Whitey), an FEP counting tube and a
Monel vessel to contain H18F or CCl2FCClF2. The apparatus
was calibrated and well seasoned with HF before use. The
reactor was loaded with solid in the glove box, supporting the
solid on a plug of fine Monel gauze, and transferred to a
Monel vacuum line. A measured quantity of H18F (normally
1–2 mmol), whose [18F] specific count rate (values in the range
45000–8000 count minϪ1 [mg atom F]Ϫ1) had been determined,
was introduced and the reactor heated at 573 K for 0.5–0.75 h.
Uncombined H18F was removed by distillation, quantified and
counted. The solid was tipped into the FEP tube and counted.
After [18F] had decayed completely, the solid was weighed.
Labelled β-AlF3 or F-γ-alumina (0.5 g), prepared as above, were
exposed to CCl2FCClF2 (0.4–0.8 g) for 0.75–1.0 h at 523–548 K
using a similar procedure.
The behaviour of β-AlF3 or fluorinated γ-alumina towards
[36Cl]-labelled HCl or ButCl was examined using the Geiger–
Müller direct monitoring method, developed in Glasgow for
[14C] adsorption measurements29 and used subsequently for a
variety of inorganic applications, including those with [36Cl]
and [35S].21 An evacuable Pyrex counting vessel with a gas hand-
ling facility was used for measurements at ambient temperature.
Two Geiger–Müller counters were positioned to enable [36Cl]
activity from the vapour phase and from the vapour plus sur-
face (due to self-absorption of the βϪ emitter [36Cl], activity
from the bulk was not detected) to be monitored concurrently.
The counting tubes were intercalibrated using H36Cl, counts
being recorded simultaneously on two scalers, enabling [36Cl]
counts from the surface of a solid placed below one of the
counters to be determined by subtraction. Powdered β-AlF3 or
fluorinated γ-alumina (0.5 g) samples were spread as thinly as
possible in order to approach the required criterion of an infin-
itely thin solid layer. Cell and solid were thoroughly degassed
before a measured pressure of labelled H36Cl or [36Cl]-ButCl
vapour was added via a calibrated gas-handling manifold.
Counting times were chosen to enable substantial counts (nor-
mally 104 to minimise counting errors) to be accumulated. Pres-
sures of volatile components were in the range 1300–6700 Pa.
At the conclusion of an adsorption isotherm determination or
of an addition sequence, volatile material was removed by distil-
lation and the count from [36Cl] material retained on the solid
determined.
Anhydrous [36Cl]-labelled hydrogen chloride was prepared
from conc. aqueous HCl (10 cm3), to which was added H36Cl
(1–2 cm3, specific activity ca. 925 kBq cmϪ3) and 98% H2SO4.
Trace H2O was removed by trap to trap distillation over P2O5,
the product being stored in an evacuated stainless steel vessel
over P2O5.26
2-Methylpropan-2-ol (1.66 g, 23.0 mmol) was shaken with
conc. aqueous HCl (5.66 cm3) containing aqueous H36Cl
(1.0 cm3, 925 kBq) over a 2 h period. The lower aqueous layer
was discarded and the organic layer washed with aqueous
NaHCO3 then H2O.27 The tert-butyl [36Cl]chloride so formed,
was dried over CaSO4 then over 3A sieves in vacuo; the yield
was ca. 70%. The [36Cl] specific count rate of the vapour was
195 count minϪ1 kPaϪ1. No impurities in an inactive sample
were detected using 1H, 13C NMR or FTIR spectroscopy.
The interaction between β-AlF3 or fluorinated γ-alumina and
ButCl vapour was also studied by FTIR using a 10 cm Pyrex cell
containing a depression to hold solid below the beam. It was
J. Chem. Soc., Dalton Trans., 2002, 40–47
41