A. Fernandes et al. / Catalysis Communications 95 (2017) 16–20
17
variety of chemical reactions [19–21]. To attain this objective, several
2.3. Catalytic tests
strategies have been developed including the use of soft and hard co-
templates and post-synthesis treatments such as alkaline desilication
[22]. In the case of silicoaluminophosphates, although post-treatments
may be used to create porosity [23], other strategies using organosilane
surfactants in the conventional synthesis procedure [24–26] have been
attempted and proved successful for selected structures.
In this work, we have introduced an additional porosity in SAPO-40
using [3-(trimethoxysilyl)propyl]-octadecyldimethyl-ammonium as
secondary template. Its catalytic performance was compared with the
conventionally prepared SAPO-40 in the gas-phase dehydration of
glycerol.
Dehydration of glycerol was performed at 320 °C under atmospheric
pressure, in a fixed-bed quartz reactor (i.d. 1.5 cm) using 300 mg of
catalyst.
Before each test, the catalyst was maintained for 2 h at 500 °C under
a flux of dry nitrogen (30 mL·min−1). The reaction feed, an aqueous so-
lution containing 10 wt% of glycerol, was introduced into the reactor by
a syringe pump KD Scientific at a WHSV of 0.85 h−1 and diluted in a
flow of dry nitrogen (30 mL·min−1). The reaction products were col-
lected in an ice trap followed by two additional water traps. The reaction
products were analyzed on a Chrompack CP9001 gas chromatograph
equipped with a 25 m OPTIMA FFAP Macherey Nagel capillary column
and a FID detector. For quantitative measurements 1-propanol (for
low boiling point products) and 1,4-butanediol (for glycerol) were
used as internal standards.
2. Experimental
2.1. Synthesis
The conversion and selectivity were calculated as follows:
ngtotal−ngt
ngtotal
Pseudoboehmite (Pural SB, Condea, 74% Al2O3), fumed silica
(Cab-OSil M5, Fluka), ortho-phosphoric acid (85% Merck) and
tetrapropylammonium hydroxide (40% aq. sol., Alfa) were used to
prepare SAPO-40 according to the method described by N. Dumont
et al. [27], which comprises the preparation of two solutions. Briefly,
for the preparation of solution A, 10.53 g of pseudoboehmite was
mixed with 12.30 g of water and 17.63 g of ortho-phosphoric acid.
The mixture was left under magnetic stirring for 4 h. A second solu-
tion, solution B, was prepared by mixing 9.88 g of water, 20.0 g of
tetrapropylammonium hydroxide and 0.77 g of fumed silica. After
stirring for 2 h, 9.73 g of solution A were added to solution B and
the final mixture was aged for 2 h. The gel, with a molar composition
of 0.93Al2O3/0.93P2O5/0.65SiO2/2TPAOH/74H2O was submitted to a
hydrothermal treatment at 200 °C during 144 h in static conditions.
Mesoporous SAPO-40 (mSAPO-40) was prepared following
closely the method used for the preparation of SAPO-40, replacing
part of the silica source (Cab-OSil-M5) in the synthesis gel by [3-
(trimethoxysilyl)propyl]-octadecyldimethyl-ammonium chloride
(Aldrich, 78%) (TPOAC) and adjusting the other quantities (amount
of solution A and water). The required amount of TPOAC was
mixed with part of the water and let to hydrolyze overnight before
mixing with TPAOH and the silica. The final gel composition was
0.98Al2O3/0.98P2O5/0.5SiO2/0.15TPOAC/2TPAOH/120H2O. The final
mixture was submitted to a hydrothermal treatment at 200 °C dur-
ing 144 h.
%Conv ¼
%Seli ¼
ꢀ 100
ngi;t
ngtotal−ngt
ꢀ 100
where ngtotal is the total number of moles of glycerol injected into the re-
actor during the time on stream t, ngt the number of moles of glycerol in
the products recovered at time on stream t and ngi,t the number of moles
of glycerol converted to the product i during the time on stream t. In
order to obtain a significant amount of products, each analysis corre-
sponds to the products usually recovered for 3 h. The products obtained
for the first hour were rejected due to a poor mass balance. The time on
stream indicated in Table 2 and Figs. 4 and 5 correspond to the middle
point of the recovering time interval.
3. Results and discussion
Fig. 1 shows the XRD patterns obtained for as-prepared SAPO-40 and
mSAPO-40. The patterns agree with the presence of the AFR as the sole
crystalline phase (ICDD PDF2 # 01-087-1146). Although the pattern of
mSAPO-40 is perfectly defined, the shape of the baseline also suggests
the presence of some amorphous phase, which is confirmed by SEM
(Supplementary data, Fig. SD1). In what concerns the crystal morpholo-
gy, mSAPO-40 sample shows interconnected plate-like crystal aggre-
gates which contrasts with the isolated plate-like crystals typical of
SAPO-40 [18].
Both samples were calcined at 550 °C for 12 h under a flux of dry air.
This procedure assures the complete removal of the organic moieties.
The structural and textural parameters of the samples mSAPO-40
and SAPO-40 are presented in Table 1. When compared with SAPO-40,
2.2. Characterization
All the samples were checked for phase purity and crystallinity by
powder X-ray diffraction on a Panalytical X'Pert Pro diffractometer
using Cu Kα radiation filtered by Ni and an X'Celerator detector. 29Si
MAS NMR spectra were recorded on a Bruker Avance III 400 (9.4 T) mul-
tinuclear spectrometer. 27Al MAS NMR spectra were recorded on a
Bruker Avance III HD 700 (16.4 T) multinuclear spectrometer. SEM mi-
crographs were obtained on a JEOL JSM-7001F equipment equipped
with an Oxford light elements EDS detector. Pyridine (Py) adsorption
was followed by FTIR spectroscopy using a home-made quartz cell
allowing sample vacuum and temperature pretreatment and subse-
quent Py adsorption at 150 °C. Quantitative measurements were
done as described elsewhere [28]. Thermogravimetric data (TG) were
obtained with a TG92 Setaram apparatus, under air at a heating rate of
10 °C/min. Nitrogen sorption experiments were performed using an
ASAP 2010 series equipment from Micromeritics. Prior to measure-
ments, samples were outgassed first at 90 °C and then at 350 °C, during
1 and 4 h, respectively.
Fig. 1. XRD patterns of as-synthetized SAPO-40 (a) and mSAPO-40 (b).