G Model
CATTOD-10652; No. of Pages8
ARTICLE IN PRESS
T. Liu et al. / Catalysis Today xxx (2017) xxx–xxx
3
and 30Si/Ti–MCM-41 have a great marked inflection in the rel-
ative pressures (P/P0) of 0.2–0.4. However, with the increase of
titanium content to Si/Ti ratio of 10, the PSD curves inflection
of 10Si/Ti–MCM-41 is much weaker which indicates that the
pore structure of this sample is not as good order as others. The
pore sizes are calculated by the BJH method from the desorption
isotherm branch [29]. The mesoporous pore size of 120Si/Ti–MCM-
41, 60Si/Ti–MCM-41, 30Si/Ti–MCM-41, 10Si/Ti–MCM-41 are about
2.5 nm, which does not change significantly with the increase of
titanium content. N2 ad/desorption curve shows hysteresis loop in
attributed to the length of Ti-O bond is longer than that of Si-O
bonds, and the addition of extra amount of titanium can partly
damage pore wall of mesopore structure, which makes the pore
wall produce defect [30]. As shown in Table 1, the values of surface
areas and pore volume of 120Si/Ti–MCM-41, 60Si/Ti–MCM-41 and
30Si/Ti–MCM-41 are closed to each other, which are significantly
that the increase of titanium contents deteriorates the mesoporous
structure of MCM-41. It has been reported that when the titanium
contents are greater than 3.7 wt%, the hexagonal p6 mm structure
of MCM-41is lost [28].
3. Results and discussion
3.1. The influence of Ti content on the pore structure of
xSi/Ti–MCM-41
As shown in Table 1, the XRF analysis results of the synthesized
xSi/Ti–MCM-41samples indicate their Si/Ti molar ratios are closed
to those of corresponding hydrothermal mixtures, which indicates
that most of Ti added in the hydrothermal process has been incor-
porated into xSi/Ti–MCM-41. Because the industrial fluosilicate are
which contains the impurities such as Fe, Al, Na, etc., the produced
mesoporous molecular sieve contains some impurities. Besides the
impurities of fluorine (about 2.9 wt%), it also contains other trace
amount of Fe, Al, Na and K. The content of impurities of XRF char-
titanosilicate with different Si/Ti ratio.
The XRD patterns of xSi/Ti–MCM-41 with different Si/Ti molar
ratios are show in Fig. 1. The (100), (110), (200) and (210) diffrac-
tion peaks in the XRD pattern of samples show highly ordered
MCM-41 hexagonal p6 mm structure [24]. It can be seen that the
peaks intensity is gradually reduced with the decrease of Si/Ti molar
3.2. The influence of Si/Ti ratio on the coordination state of
incorporated Ti
UV-vis diffuse reflectance spectroscopy is an effective means
for characterization of titanium coordination number of the incor-
porated Ti species in the synthesis samples as shown in Fig. 4.
According to the literatures [14,31], absorption peaks at 220 nm
are corresponding to four coordination of titanium; Absorption
peak at 260 nm is assigned to six coordination corresponding
titanium; Absorption peak at 300 nm is the characteristic peak
of anatase or rutile titanium oxide. The sample 120Si/Ti–MCM-
41 has only one absorption peak at 220 nm, inferring that all Ti
species are in tetrahedral (Td) coordination. Sample 60Si/Ti–MCM-
41, 30Si/Ti–MCM-41 have a sharp absorption peak around 220 nm
and a shoulder peak at 260 nm, indicating that most of Ti(IV) cations
are in Td coordination but a small portion presents in octahe-
dral (Oh) coordination. Sample 10Si/Ti–MCM-41 has a pronounced
absorption bond at about 260 nm, which extends to the range above
300 nm, inferring that the Ti(IV) cations in this sample are mainly
Oh coordination with the formation of TiO2 clusters or crystal-
lites. These results demonstrate that better dispersion of Ti(IV) is
achieved by reduce the addition amount of titanium in the synthe-
sized samples.
√
the hexagonal unit cell parameter (a0 = 2d100
/
3) of the samples
were estimated from the (100) position of the XRD diffraction peak,
be very sensitive to the degree of organization order of the prod-
uct [25,26]. Because the [TO4] cell is bigger than [SO4] unit, the
incorporation of [TO4] into the mesoporous molecular sieve would
make the [TO4] surrounding lattice expansion and the hexagonal
unit cell parameter (a0) become bigger [23]. It can be observed
that the (100) reflection peak move slightly to lower angle with the
increase of the Ti content in the xSi/Ti–MCM-41, which means that
the d100 spacing, lattice parameter (a0) and the wall thickness of the
strates that the Si/Ti ratio can influence the packing of the silicon
and consequently the degree of long-order structure of MCM-41.
TEM micrographs of calcined 60Ti/Si–MCM-41 are given in
Fig. 2. The images of the sample presents a well-defined hexagonal
arrangement with a fairly uniform pore structure. The Fig. 2 shows
Overloaded titanium ruined the highly ordered MCM-41 hexago-
nal p6 mm crystal structure, but this damage is not throughout the
crystal structure. This behavior, the presence of disordered regions,
in the literature for MCM-41 synthesis [27].
3.3. The influence of hydrothermal conditions on the pore
The N2 ad/desorption isotherm was studied to evaluate
the mesoporous structures of samples Si/Ti–MCM-41. A typi-
cal reversible type IV ad/desorption isotherm for mesoporous
solids are observed as shown in Fig. 3. N2 ad/desorption curve
of these samples shows the same trend. At low relative pres-
sure (P/P0 < 0.2) the N2 sorption update amount is smoothness,
between the relative pressures (P/P0) of 0.2–0.4, which is corre-
sponding narrow and uniform pore size distribution band in the
pore size distribution (PSD) curve, as shown in the BJH PSD curves
of Fig. 3 [12,28,14]. This is caused by capillary condensation dur-
ing the adsorption step and is an indication of the mesoporosity
of the samples. The sample 120Si/Ti–MCM-41, 60Si/Ti–MCM-41
The CTAB/Si mole ratio can greatly influent the mesoporous
structure of 60Si/Ti–MCM-41. The N2 ad/desorption and pore size
distribution curves of the samples with different CTAB/Si mole ratio
are shown in Fig. 5. With the relative pressure P/P0 in the range
between 0.25–0.35, a dramatic increase of N2 absorption occurs
for the three samples, which is the character of the N2 sorption
data of samples 1, 2 & 3 in Table 2 shows that the pore structure
of these samples change with the increase of CTAB contents, while
with the CTAB/Si ratio 0.81 the specific surface area and pore vol-
umes reaches the largest value among these samples. The XRD
patterns in Fig. 6 indicate that the synthesized 60Si/Ti–MCM-41
samples with different CTAB/Si mole ratio are also preserve the
hexagonal p6 mm structures.
Please cite this article in press as: T. Liu, et al., Synthesis of titanium containing MCM-41 from industrial hexafluorosilicic acid as