6
28
M. Zimowska et al. / Materials Research Bulletin 83 (2016) 623–631
However, the procedure of converting the directly aluminated
materials into their hydrogen forms is associated with a certain
degree of dealumination, as evidenced by 27Al MAS NMR spectrum
of the H-Al-FSM-16(20)/D sample (Fig. 4a). A similar effect has
been reported for directly aluminated MCM-41 silica [26]. Analysis
of Si MAS NMR resonance of aluminated samples confirms that
incorporated aluminium is indeed substituting the Si centres in the
mesoporous silica [50]. The spectrum of purely siliceous sample
3
745
FSM-16
Al-FSM-16(20)/P
Al-FSM-16(20)/D
H-Al-FSM-16(20)/D
2
9
shows
a
single line with chemical shift of
d
= À112 ppm,
4
characteristic of Q (0Al) units, in which the central Si atom is
interconnected via four oxygen bridges with other Si atoms. A
certain asymmetry of this absorption indicates a small contribu-
tion from less condensed Si species. Insertion of Al is associated
with the appearance of a second maximum at less negative
d
values (Fig. 4b). Its position, at
d
= À101 and À102 ppm, in Al-FSM-
16(20)/D and Al-FSM-16(20)/P, respectively, points to the contri-
3
900
3800
3700
3600
3500
]
3400
3300
4
bution from tetrahedrally coordinated Si Q (1Al) units, in which
Wavenumber [cm-1
one of four bridging oxygen connects the Si center with an Al site. It
should be noted that the 29Si MAS-NMR spectra have much poorer
Fig. 6. FTIR spectra of the O ÀÀ H stretching mode region in aluminated FSM-16
materials.
signal to noise ratio than the 27Al ones. This is caused by the
difference in natural abundances of both isotopes, which are
respectively 4% and 100%, and equally large difference in T
1
substitution and formation of the aluminosilicate framework.
Noteworthy, the band at 960 cm due to Si ÀÀ O stretches within
relaxation times. Nevertheless, the quality of 29Si MAS-NMR
À1
spectra is sufficient for unequivocal interpretation.
terminal silanols becomes less pronounced, although this bonding
is not directly affected by Al substitution. Transformation of Al-
FSM-16(20)/D into its hydrogen form does not change the
spectrum of framework vibrations in any meaningful way. In
contrast to direct alumination, the post-synthesis incorporation of
Al has practically no effect on the FTIR detectable skeletal
vibrations of parent FSM-16. The only exception is the suppression
3.3.3. Infrared spectroscopy
Fig. 6 shows the framework vibration region of FTIR spectra
recorded for FSM-16, Al-FSM-16(20)/D, Al-FSM-16(20)/P, and H-
Al-FSM-16(20)/D. The most prominent band in the vicinity of
À1
À1
À1
1
1
SiO
080 cm , the band at 1243 cm
and a weak peak around
À1
198 cm are due to asymmetric Si ÀÀ O stretching modes within
of the 960 cm band associated with free silanols. In order to get
À1
4
tetrahedra. The less intense feature at 798 cm is due to
more insight into this effect, FTIR spectra in the O ÀÀ H stretching
À1
À1
symmetric stretching Si ÀÀ O modes, and the band at 456 cm is
mode range were analyzed (Fig. 6)
À1
associated with bending skeletal modes [51]. Around 960 cm , a
The sharp band at 3745 cm is due to free silanol groups [53],
À1
band assigned to Si-O stretching within terminal Si ÀÀ OH groups, is
while the broad band in the 3700–3300 cm range to hydrogen
observed [52]. In addition, in the spectrum of FSM-16 and post-
bonded hydroxyls, associated mainly with defects caused by
interrupted Si ÀÀ O ÀÀ Si linkages [54]. It is evident that both the
direct and the post-synthesis alumination result in a decrease of
synthesis aluminated Al-FSM-16(20)/P, a weak band is visible at
À1
6
20 cm . This band is absent in the directly aluminated materials
À1
(
Fig. 5). Analysis of other characteristic frequencies confirms that
the 3745 cm band intensity, which explains the diminution of the
À1
alumination affects the silica network in different way,
a
960 cm band in the skeletal modes range. In addition, both types
depending on the manner of Al insertion. Direct alumination
of alumination cause an increase of the broad band intensity,
which signifies enhanced contribution from H-bonded OH-groups,
and reflects an increased concentration of surface structural
defects in the aluminated silica framework. Transformation of Al-
FSM-16(20)/D into its hydrogen form brings about further increase
of the broad band intensity. Obviously, the partial loss of Al from
the mesoporous silica structure, occurring in the process of H-Al-
FSM-16(20)/D preparation and evidenced by Al MAS NMR, leads
to a higher content of structural defects, including the hydroxylat-
ed ones.
shows a clear impact on the framework vibrations. All stretching
À1
bands become broader, and, as a result, the 1243 cm
1
and
À1
198 cm peaks resolved in FSM-16, coalesce into a shoulder in Al-
FSM-16(20)/D. A downward shift of the dominant maximum from
À1
À1
1
087 cm to 1077 cm is observed, indicative of the Al for Si
2
7
1087
1077
H-Al-FSM-16(20)/D
Al-FSM-16(20)/D
Al-FSM-16(20)/P
FSM-16
3
.4. Acidic properties
1
198
Kanemite
456
As indicated in the introduction, formation of Si–O–Al linkages
1
234
in the silica-based solids is a means of enhancing the materials
acidity. In order to follow changes in the acidic properties of FSM-
16 induced by its alumination, adsorption/desorption of base probe
molecules, i.e. ammonia and pyridine, has been carried out. The
acidity study was completed with tests of the solids activity in the
catalytic decomposition of ethanol.
9
61
798
620
1
049
1198
1
012
6
86
9
05
3.4.1. Ammonia TPD-MS study
5
13
7
79
572
600
Substitution of Al for Si in the silica framework generates cation
exchange positions, which, when occupied by protons, are the
source of Brønsted acidity. Surface coordinatively unsaturated Al
sites may act as Lewis acid sites. To get insight into the acidic
1
400
1300
1200
1100
1000
900
800
700
500
400
-1
Wavenumber [cm ]
Fig. 5. FTIR spectra of the skeletal vibrations in aluminated FSM-16 materials.