M. Trejda et al. / Catalysis Today 254 (2015) 104–110
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(i.e. processes that are also relevant for industry) were chosen.
Moreover, these processes were not much investigated using cat-
alytic system applied in this study, especially in relation to MCF
materials having high pore diameter. In this paper we obtained
a series of catalysts differing in the number of acid sites, using
the same methodology of preparation. For this purpose niobium
and manganese were chosen as modifiers of mesoporous materi-
als. Both metals play an important role in the oxidation of SH groups
to sulphonic ones by H2O2 as evidenced in [20]. On the other hand
manganese easily changes its oxidation state contrary to niobium
species and it is more hardly introduced to mesoporous silica struc-
ture if used in the form of cationic species and under the conditions
applied for SBA-15 and MCF syntheses. One can expect that such
differences will be reflected in the anchoring of MPTMS and oxida-
tion of SH groups to SO3H and moreover, will allow checking if the
effectiveness of oxidation of thiol groups by H2O2 depends or not
on the incorporation of transition metal into the silica structure.
X-ray fluorescence (XRF) using MiniPal-Philips instrument was
applied to determine the real molar ratio of Si/Nb or Ta in materials
prepared. The calculations were performed using calibration curve
based on reference mixtures of silica (Degussa) and metal oxide
Nb2O5 (Alfa Aesar). The calibration curves consisted of 10 points
related to different Si/Nb molar ratio in the range from 3 to 300.
The metal concentration in a sample was established by the amount
of emitted X-ray radiation related to the values in the calibration
curves.
Elemental analyses of materials obtained were performed using
Elementar Analyser Vario EL III.
Titration of acidic sites was performed using 100 mg of anhy-
drous material (dried at 423 K for 12 h). Catalyst was immersed in
a 2 M NaCl solution (60 cm3) and stirred at RT for 18 h. After this
time the solution was titrated with 0.005 M NaOH solution.
2.4. Esterification processes
2. Experimental
Esterification of acetic acid with ethanol or 2-propanol was per-
formed in a liquid phase in batch reactor without usage of any
solvents. The reactor was equipped with a condenser and a process
was conducted at autogenously pressure. The reaction was carried
out for 4 h at 363 K and 373 K for ethanol and 2-propanol, respec-
tively. Before reaction catalyst was activated in oven at 423 K for
12 h. 12 g of acetic acid was used for all reaction keeping molar
ratio of acetic acid to alcohol 2:1. The volume of solution after the
reaction was checked each time to indicate that there is no leak
in the system. For selected catalysts the reuse test was performed.
Prior this process the catalyst after the first run of reaction was sep-
arated from reactant mixture by centrifugation and dried in oven
overnight at 423 K. Then the catalysts were applied for the next use.
The composition of products were analysed by a gas chromatograph
(Thermo Scientific—Focus) equipped with 60 m DB-1 capillary col-
umn, worked at the temperature range of 313–523 K (temperature
ramp 10 K min−1), and MS detector. The quantitative analysis were
performed by acetic acid titration with sodium hydroxide solution.
2.1. Preparation of SBA-15 type catalysts
SBA-15 catalysts functionalized with MPTMS, (3-mercapto-
propyl)trimethoxysilane and modified with Nb or Mn were
prepared via hydrothermal synthesis. The synthesis was per-
formed in polypropylene bottle (PP). The synthesis procedure was
as follows. To the PP bottle the Pluronic P123 (Poly(ethylene
glycol)-block–poly(propylene glycol)-block–poly(ethylene glycol)
(Aldrich—8 g), HCl (Chempur 35%—17.52 g) and water (282.5 g)
were added. When the surfactant was dissolved
a TEOS
(Aldrich—17.054 g) was dropwise inserted. After 45 min MPTMS
(Aldrich—1.69 g) and hydrogen peroxide (Merck 35%—7.17 g) were
added. For metal containing samples, ammonium niobate(V)
oxalate (Aldrich) or manganese(II) nitrate tetrahydrate (Aldrich)
were also added to the gel (10 min after TEOS addition) keeping
Si/Nb or Si/Mn molar ratio 64. Final mixture was stirred at 313 K for
20 h and then heated at 373 K under static conditions for 24 h. The
product was washed with water and dried at RT. The template was
removed by constant extraction with ethanol in Soxhlet apparatus.
3. Results and discussion
3.1. Texture/structure parameters
2.2. Preparation of MCF type catalysts
Two different types of mesoporous materials, i.e. SBA-15 and
MCF, were functionalized with MPTMS using one-pot synthesis
procedure in the presence of hydrogen peroxide. H2O2 was applied
to oxidize thiol species. Moreover, the materials were also modi-
fied with Mn or Nb mainly to enhance the efficiency of sulphonic
species formation [22]. The choice of different structured materi-
als (SBA-15 with hexagonally ordered mesopores and mesoporous
cellular foams (MCF) containing big cells and windows) was dic-
tated by the expected differences in the location of the active sites
and in their availability to reagents. SBA-15 based samples show
typical XRD patterns characteristic of this solid material (Fig. 1).
An intensive peak assigned to (1 0 0) plane is observed at 2 theta
below 1◦. The structure of SBA-15 samples is also ordered in long
distance, which is testified by the presence of two additional peaks
at 2 theta between 1 and 2o. These peaks are assigned to (1 1 0)
and (2 0 0) planes, respectively. XRD patterns of MCF materials do
not show peaks in this region (data not shown here). This is typical
of samples with relatively high mesoporous diameter. It is known,
MCF catalysts functionalized with MPTMS, (3-mercapto-
propyl)trimethoxysilane and modified with Nb or Mn were
prepared via hydrothermal synthesis similar like for SBA-15. 1,3,5-
trimethylbnzene (Aldrich—12 g) and NH4F (Aldrich—0.0934 g)
were added under vigorous stirring after dissolving of Pluronic
P123. TEOS was added one hour after 1,3,5-trimethylbenzene and
NH4F addition. Next steps were the same as for SBA-15.
2.3. Characterization techniques
The materials prepared were characterized using different ana-
lytical techniques: N2 adsorption/desorption, XRD, XRF, elemental
analysis and amperometric titration.
XRD patterns were recorded at room temperature on a Bruker
AXS D8 Advance apparatus using Cu K␣ radiation (˛ = 0.154 nm),
with a step of 0.02◦ and 0.05◦ in the small-angle and wide-angle,
respectively.
N2 adsorption/desorption isotherms were obtained on a Micro-
vacuum at 423 K to remove water or solvent, like ethanol from
pores. The surface area was calculated using the BET method.
Pore volume, cells and windows diameter of MCF materials
were estimated according to Broekhoff-de Boer method with
Frenkel–Halsey–Hills approximation [21].
The structure of MCF materials can be confirmed by their
sorption properties. Fig. 2 presents the N2 adsorption/desorption
isotherms and the parameters calculated from it are shown
in Table 1. The shape of isotherms presented in Fig.
2 is