Mendeleev Commun., 2013, 23, 110–112
Table 2 The surface area and H values of catalysts samples.
2
M NH4F/Al2O3
50 °C
0
2
+
+ C12H24
1
Surface area/
Catalyst
H0
2
–1
m g (±15%)
Scheme 3
Al O
—a
150
145
180
120
15
2
3
0
.5 m NH F/Al O
3
~ –13.75
4
2
conversion was 87% in 4 h, 90% of the products were C + C
8
12
2
m NH F/Al O
–13.75 < H < –12.7
4
2
3
3
0
fraction.
5
m NH F/Al O
> –12.7
—a
4
2
Xray diffraction studies showed that initial alumina had cubic
structure (PDF #10425) (Figure 1). Upon washing in diluted
NH F solutions (0.5 m and 2 m) and subsequent thermal treatment,
the crystalline structure of powders did not change. Treatment with
m NH F solution followed by calcination at 600°C resulted in
a partial formation of anhydrous aluminium fluoride. Thus, the
TiO2
‡
2
m NH F/TiO2
~ –12.7
—a
20
4
4
ZrO2
30
2
m NH F/ZrO2
~ –11.99
20
4
5
4
a
Not determined.
sample obtained from concentrated solution of NH F is biphasic.
4
Particle size of aluminium fluoride estimated using Scherrer equa
tion was about 20 nm.
According to EDX data, fluorine to aluminium molar ratio at
the surface of the fluorinated alumina catalysts increased from
1639
L
1445
1
487
1540
B
L
1
:4 to 2:1 with the increase in NH F concentration. These data are
4
B+L
in a good agreement with the tendency revealed by XRD study.
Fluorine to titanium molar ratio at the surface of the fluorinated
titania catalyst was 1:8, fluorine to zirconium molar ratio at the
surface of the fluorinated zirconia catalyst was 1:7, revealing that
fluorination of alumina surface proceeds much more effectively
compared to zirconia and titania.
1
300
1400
1500
1600
1700
n/cm–1
Figure 2 IR spectroscopy data of pyridine adsorbed on a surface of the
2 m NH F/Al O catalyst.
4
2
3
§
The Hammet acidity function H is used as a measure of acid
surface (Figure 2). The IR spectra of fluorinated alumina samples
contained bands which are characteristic of pyridinium ions
adsorbed on both Lewis (L) and Brønsted (B) acid centres (cm ):
1445 (L), 1490 (B+L), 1540 (B), 1639 (L). We suppose that
Lewis and Brønsted acid sites are formed according to a mecha
nism offered for sulfated oxides (Scheme 4).
0
strength in nonaqueous solvents. Values less than H for 100%
0
1
–1
H SO (–11.93) are considered superacidic. The determination
2
4
1
5
of the Hammet acidity function H revealed that the acidity of
0
NH F/Al O samples reached the value of ca. –13.75; it means
4
2
3
1
2
that fluorinated Al O can be considered as a solid superacid.
2
3
Fluorinated TiO (H is ca. –12.7) and ZrO (H is ca. –11.99)
2
0
2
0
F
OH
Al
OH
Al
appeared to be substantially weaker acids than fluorinated alumina.
The surface area and H0 values of catalysts samples are
presented in Table 2.
Al
– H2O
O
O
Brønsted
centre
To determine the types of acidic centres of the 2 m NH F/Al O
catalyst, we used IR spectroscopy data of pyridine adsorbed on a
4
2 3
Lewis
centre
H
H
F
O
F
O
H2O
– H2O
Al
Al
Al
Al
Al
Al
*
O
O
*
O
O
Scheme 4
*
*
*
*
The IR spectra of fluorinated titania and zirconia did not show
adsorbed pyridinium ions at all. This may be due to a very low
concentration of surface acidic centres.
*
*
*
*
*
4
A catalytic activity of fluorinated oxides is a function of their
acidity. Fluorinated alumina has a higher acid centres concentra
tion in comparison with those for fluorinated titania and zirconia,
and alumina centres are more acidic. We suppose that at least two
factors determine the absence of catalytic activity of fluorinated
titania and zirconia: (i) a low F:Ti/Zr ratio due to a thermohydro
lytic cleavage of Ti–F or Zr–F bonds during their calcination at high
temperature and (ii) significantly lower specific surface area which
cannot provide enough accessibility of substrate to acidic centres.
3
2
1
1
0
20
30
40
50
60
70
80
2
q/°
Figure 1 Xray diffraction data for samples of fluorinated alumina: (1)
initial alumina powder, (2)–(4) alumina powder after washing with 0.5, 2.0
and 5.0 m NH F solution, respectively, followed by thermal treatment at
In conclusion, soaking of alumina in aqueous NH F solution
4
with subsequent calcination can be used as a simple and effective
route to produce superacidic fluorinated alumina which appeared
to be an effective acid catalyst for olefin oligomerization.
4
6
00°C. Asterisks point diffraction peaks of AlF (PDF #43435), dashes –
3
Al O (PDF #10425).
2
3
‡
§
Powder XRay diffraction (XRD) analysis was carried out on a Rigaku
D/Max 2500 diffractometer with a rotating copper anode (CuKa irradiation,
° £ 2q £ 80°, 0.02° step). Diffraction maxima were indexed using the
IR spectroscopy was performed on a Spectrum One (Perkin Elmer, USA)
spectrometer in a 4000–350 cm–1 region (KBr pellets, 0.25–0.5% mass
sample content). Samples for IR analysis were prepared as follows.A catalyst
sample (~0.04 g) was calcined in a dry air flow in a tube reactor at 600°C.
After that the temperature was decreased to 150°C, a bubbler with 1 ml
of pyridine was embedded into the air line and a resulting gaspyridine
mixture was passed over the catalyst sample until all pyridine was evaporated.
5
PDF2 database. Energy dispersive XRay analysis (EDX) was performed
using a field emission scanning electron microscope Carl Zeiss NVision 40
equipped with Oxford Instruments XMAX analyzer operating at 20 kV
accelerating voltage.
–
111 –