2
B. Liu et al. / C. R. Chimie xxx (2016) 1e9
Owing to its superior properties, aluminum phosphate
2.2. Characterization of catalysts
is regarded as a high-quality support material and has been
applied in a variety of catalysis systems [25e28]. Amor-
phous aluminum phosphates commonly have high surface
areas and large pore volumes. The skeleton of amorphous
aluminum phosphate consists of tetrahedral units of AlO4
and PO4, which have a similar structure to that of silica [29].
This type of structure results in the high thermal stability of
AlPO4 [30]. All these above are believed to be vital prop-
erties for a catalyst support. In addition, the surface struc-
tural defects of AlPO4 promote the formation of surface OH
groups, which are similar to the SieOH groups of zeolites.
Thus an appropriate loading of heteroelements can
remarkably enhance the catalytic performance of the AlPO4
supported catalysts, due to their satisfactory dispersion and
specific interactions with the support [31]. However, pub-
lications about AlPO4 supported solid acid catalyst are rare
in the literature [32]. Till now, to the best of our knowledge,
investigations of sulfated aluminum phosphate solid acid
used in esterification reactions have not been available.
Moreover, acting as a support of solid acid, aluminum
phosphate has a relatively lower cost compared with some
frequently-used metal oxides such as ZrO2 and TiO2.
Accordingly, in the present work, amorphous aluminum
phosphate was prepared and used as support for the
immobilization of sulfate groups. Physicochemical proper-
ties of the resulting solid acid catalysts (SO42ꢁ/AlPO4-T)
were investigated with the help of various characterization
techniques. Catalytic activities of these catalysts were
evaluated by the esterification of propanoic acid with n-
butanol. The effects of esterification reaction parameters
such as reaction time, reaction temperature, catalyst
amount and molar ratio of alcohol to acid on the conversion
of propanoic acid were studied in detail. Besides, recycling
tests as well as reactivation of the catalyst were also
performed.
The crystalline structure and phase composition of
these prepared catalysts were analyzed by powder X-ray
diffraction (XRD) technique on a Bruker AXS D8-Focus X-
ray diffractometer equipped with a nickel filtered Cu K
a
(0.15418 nm) radiation source and a scintillation counter
detector. The XRD patterns were recorded by scanning the
samples in a 2q
range of 3e60ꢀ. The morphologies and sizes
were observed using a Hitachi S-4800 field emission
scanning electron microscope (FE-SEM). Nitrogen adsorp-
tionedesorption measurements were performed at liquid
nitrogen temperature (77 K), using a Micromeritics ASAP
2020 gas sorption analyzer. Specific surface areas were
calculated from the linear part of adsorption data via the
BrunauereEmmetteTeller (BET) equation. The total pore
volumes were estimated according to the adsorbance of
nitrogen at a relative pressure (P/P0) around 0.99. Fourier
transform infrared (FT-IR) spectra of catalysts and the
matrix were recorded on a Nicolet 6700 IR spectrometer in
the spectral range of 2000e500 cmꢁ1 with a resolution of
4 cmꢁ1 using the conventional KBr pellet technique.
Chemical states of the elements present on the catalyst
surface were analyzed by X-ray photoelectron spectroscopy
(XPS) on a Thermo ESCALAB 250 Xi instrument with
monochromatized Al K
a
line (h
y
¼ 1486.6 eV) as the exci-
tation source. Analyses were done at room temperature
and the samples were maintained in a high vacuum (less
than 10ꢁ8 Pa) to avoid noise in the spectra. The C 1s peak of
hydrocarbon at 284.8 eV was used as an internal standard
to which the binding energy values were referenced. The
acidic properties of the catalysts were evaluated by tem-
perature programmed desorption of ammonia (NH3-TPD)
using
a Micromeritics AutoChem II 2920 instrument
equipped with a TCD detector. Each sample was pretreated
under a helium flow (50 mL/min) at 300 ꢀC for 1 h and then
cooled down to 100 ꢀC. Subsequently, the sample was
exposed to a flowing 10% NH3/He gas mixture (50 mL/min)
for 30 min. The excess physisorbed NH3 was flushed out for
1 h with a pure helium gas flow. Measurements were
conducted by heating the sample up to 650 ꢀC with a
ramping rate of 10 ꢀC/min under a helium flow. To identify
the Brønsted (B) and Lewis (L) acid sites on these catalysts,
FT-IR spectra of pyridine adsorbed on the catalysts were
recorded on a Nicolet 6700 spectrometer equipped with an
in situ quartz cell. Before measurement, the sample was
pretreated at 350 ꢀC for 2 h in a vacuum of 10ꢁ2 Pa. After the
quartz cell was cooled to room temperature, the sample
was saturated with pyridine vapor. Excess physisorbed
pyridine was removed by degassing for 30 min at room
temperature under vacuum. The FT-IR spectrum of the
adsorbed pyridine was then recorded. The quantities of
acid sites present on the catalysts were estimated from the
total amount of sulfate groups determined by an ion-
exchange/titration method [33,34]. In a typical experi-
ment, 0.30 g of catalyst and 3.0 g of NaCl, as an ion-
exchange agent, were added into 60 mL of deionized
water and stirred for 24 h. Afterwards, the solid was
filtered. Using phenolphthalein as an indicator, the filtrate
was titrated with 0.05 M NaOH solution. The amounts of
acid sites were thus determined.
2. Experimental
2.1. Preparation of catalyst
All chemicals in this study were of analytical grade and
were used as received. Aluminum phosphate support was
prepared via an incipient precipitation method under basic
conditions. Typically, 15.01 g of Al(NO3)3$9H2O and 4.61 g
of H3PO4(85%) (corresponding to theoretical Al/P molar
ratio of 1:1) were dissolved in 100 mL deionized water. To
this solution, a diluted NH3 solution (10 wt %) was added
dropwise under vigorous stirring to attain a final pH of 9
and the resulting suspension was further stirred for 1 h. The
precipitate was filtered, washed with deionized water to
remove soluble impurities, and dried at 110 ꢀC overnight.
The thus obtained white solids were ground to fine pow-
ders in a carnelian mortar and calcined at 400 ꢀC for 3 h. For
targeted catalysts preparation, 0.8 g of the prepared AlPO4
was added into a solution of 0.4 g (NH4)2SO4 dissolved in
15 mL deionized water in a 50 mL beaker. After standing fꢀor
5 h at room temperature, this mixture was dried at 110 C
for 16 h and finally calcined at 400e550 ꢀC for 3 h. The as-
prepared catalysts were denoted as SO42ꢁ/AlPO4-T, where T
denote the calcination temperature (ꢀC).
Please cite this article in press as: B. Liu, et al., Aluminum phosphate-based solid acid catalysts: Facile synthesis, characterization
10.1016/j.crci.2016.07.006