M.-Y. Huang et al. / Journal of Catalysis 320 (2014) 42–51
43
high reaction temperature, diversified by-products, and high
energy consumption, continue to hamper the use of aforemen-
tioned solid acid catalysts for esterification reactions. Moreover,
the high polarities of the reactants also tend to provoke leaching
of the solid acid catalysts [25–27].
withdrawn of the ester products at the upper layer after the
removal of excessive HOAc and water, while the PPS–TPA catalyst
retained at the bottom could be recycled, facilitating a continuous
reaction under controlled operation conditions. It is noteworthy
that for the catalyst system designed herein, acetic acid is
exploited not only as a cofeed but also a cosolvent during acetyla-
tion reaction. The effects of acidic strength, TPA pretreatment tem-
perature, HOAc/GL ratio, and reaction temperature on catalytic
performances of the PPS–TPA–HOAc catalysts during acetylation
reaction were also investigated.
Heteropolyacids (HPAs) are complex protic acids with versatile
structural and physicochemical properties [28–35]. HPAs have
very high solubility in polar solvents and ultra-strong Brønsted
acidity surpassing the classification of solid superacids [30,31].
The acidic property of HPAs has been explored by a variety of dif-
ferent techniques, such as Hammett indicator [36], thermal
desorption [36,37], microcalorimetry [37–43], H/D exchange [44],
FT-IR [37,45–47] and NMR [31,43,48–55] spectroscopy, and theo-
retical density functional theory (DFT) calculations [54–56]. It
has been found that HPA behaves like monobasic acids in HOAc
solution due to the low degree of dissociation [57]. HPAs have been
widely used as homogeneous and heterogeneous catalysts [30]
owing to their unique properties such as good thermal stability,
strong acidity, and excellent oxidizing capability. These together
with the stable and relatively nontoxic characteristics rendering
HPA widely appreciated as environmentally benign catalysts for
practical replacement of hazardous liquid acids conventionally
used in industrial processes. Furthermore, when immobilized on
a suitable support having a large surface area, the well-dispersed
HPAs not only facilitate high catalytic activity but also provide an
opportunity for easy separation of catalyst from reaction products
2
. Experimental
2.1. Preparation of PPS–TPA–HOAc catalysts
Tungstophosphoric acid (TPA, H
cals), pyridinium propyl sulfobetaine (PPS, C
3
PW12
O
40ÁnH
2
O; Fluka Chemi-
8 3
H11NO S; Wuhan
Fengfan Trading Co.), and research grade chemicals, such as glyc-
erol (>98%; J. T. Baker), acetic acid (P99.8%; Sigma–Aldrich), sulfu-
ric acid (95–97%; Merck), and trifluoromethanesulfonic acid
(
CF
TPA was pretreated by either a calcination treatment at 100–
00 °C in air for 2 h or drying at 100 °C for 2 h in a vacuum oven
3 3
SO H; P98%; Merck), were used in this study. Prior to use,
4
to remove crystalline water.
The TPA-immobilized IL catalysts were prepared by mixing
desirable amounts of calcined TPA with PPS, denoted as PPS–
TPA–x–y, where x represents the calcination temperature (in °C)
of the pristine TPA and y denotes the PPS/TPA molar ratio. In brief,
stoichiometric amounts of calcined TPA and PPS were placed in a
two-necked round-bottom flask together with excessive amount
of acetic acid (HOAc). The mixture was heated at 100 °C for 0.5 h
under continuous stirring, leading to a biphasic liquid layers
[
22,58,59]. The main feature that compromises the use of sup-
ported HPA catalysts is their high solubility in polar solvents
e.g., H O or HOAc), which in turn leads to leaching of HPA from
(
2
the solid support. Thus, to facilitate easy separation of products
and recovery and recycling of the HPA catalyst, an innovative
design of a homogeneous catalyst would be desirable. Tungsto-
phosphoric acid (TPA) and some zwitterion compound were used
to synthesis HPA salts and applied on esterification [60,61] and
acetylization [62] reactions. The HPA salt can readily be dissolved
in the presence of alcohol/acid feeds and subsequently precipitates
from the ester products. Thus, such catalyst system represents the
concurrent advantages of homogeneous and heterogeneous cataly-
sis. However, notable decrease in activity was commonly observed
for the HPA salts during esterification; thus, catalyst degradation
and recyclability remain as challenging and demanding issues.
We report herein the synthesis of a water-tolerable catalyst,
(Fig. 1a) after ‘‘holding’’ for ca. 10 min. The bottom layer being
the PPS–TPA IL catalyst dissolved in saturated HOAc, while the
upper layer is the residual HOAc.
2.2. Characterization methods
The structural properties of the pristine crystalline TPA material
were confirmed by X-ray diffraction (XRD), diffuse reflectance
infrared Fourier-transform spectroscopy (DRIFTS), thermogravi-
1
13
31
metric analysis (TGA), and solid-state H, C, and P nuclear mag-
netic resonance (NMR) spectroscopy. The XRD experiments were
performed on a Philip X’Pert PRO apparatus with Cu K radiation
3
namely tungstophosphoric acid (H PW12O40, TPA)-immobilized
pyridinium propyl sulfobetaine (PPS) ionic liquid (IL) immerged
in acetic acid. The acid properties of such homogeneous PPS–
TPA–HOAc catalysts were characterized by 31P NMR using trimeth-
ylphosphine oxide (TMPO) as the probe molecule [63–71]. It will
be shown later that during catalytic acetylation of glycerol, two
immiscible liquid phases, namely a hydrophilic catalyst phase
and a product/reactant phase, were readily formed. The intense
mass transfer between the two distinct liquid phases renders facile
(k = 0.15418 nm) operated at 40 mA and 45 kV.
The DRIFT spectra were recorded on a Bruker IFS-28 spectrom-
eter, whereas the TGA study was conducted on a Bruker TG-209
apparatus. All NMR experiments were conducted on a solid-state
Avance 300 spectrometer (Bruker-Biospin). The structure of pris-
1
31
tine TPA was characterized by solid-state H and P magic-
angle-spinning (MAS) NMR operating at a Larmor frequency of
O
OH
O
O
+
H
OH
OCC
3
HO
OH
+
+
+ H O
1
2
HO
OCCH
3
2
CH COH
HO
O
OH
3
GMA
O
O
OH
O
OCCH
OH
GMA
GDA
OCCH3
+
3
+ H O
2
H CCO
3
CH COH
H CCO
3
3
O
GDA
O
O
O
OCCH
OCCH
3
+
+ H
O
3
2
CH COH
H CCO
3
3
3
GTA O
Scheme 1. Acetylation of glycerol with acetic acid.