J. Li et al. / Journal of Catalysis 339 (2016) 123–134
125
where AVi and AVf were the initial and final acid value of the mix-
ture, respectively.
After reaction, the solid catalyst was separated and washed
with methanol to remove absorbed oil and water prior to drying
in the oven at 80 °C for 2 h. The obtained catalyst was reused in
the next run to investigate its reusability.
Agilent Cary 660 FT-IR instrument. The C, N, H, S elemental analysis
was conducted on a Vario EL cube elemental analyzer. X-ray pho-
toelectron spectroscopy (XPS) spectra were obtained on a PHI 5000
Versa Probe X-ray photoelectron spectrometer with Al K
a radia-
tion (1486.6 eV). Thermogravimetric (TG) analysis was carried
out with an STA409 instrument in dry air at a heating rate of
10 °C minꢁ1. The acidity of the solid samples in aqueous solution
was measured by potentiometric titration with an automatic
potentiometric titrator using a pH composite electrode. The solid
was titrated with a solution of 0.05 mol Lꢁ1 n-butylamine in
acetonitrile. Liquid-state 1H and 13C nuclear magnetic resonance
(NMR) spectra were measured with a Bruker DPX 500 spectrome-
ter at ambient temperature in MDSO using TMS as internal refer-
ence. A solid state 31P NMR spectrum was originally recorded in
a Bruker Avance III 400 M spectrometer at a magnetic field
strength of 9.4 T corresponding to the Larmor frequency of
The turnover number (TON) was based on the esterification
product (mol) produced per molar acid site on the respective
catalysts [49,50]. TON = (mmol of oleic acid ꢀ conversion of oleic
acid)/(lmol of acid sites/1000). For PzS-Cl, PzS-PW, Amberlyst-
15, PzS-PW(EtOH), PzS-PW(0), PzS-PW(50), and PzS-PW(100), each
sulfonic acid group provides one acid site; thus the TON = (mmol of
oleic acid ꢀ conversion of oleic acid)/(
lmol of S/1000). For the two
S-free catalysts (H3PW and Cs2HPW12O40), each H+ equals one acid
site; thus TON = (mmol of oleic acid ꢀ conversion of oleic acid)/
(l
mol of H+/1000).
162 MHz with
a cross-polarization (CP)/magic-angle-spinning
(MAS) unit at room temperature. The spinning rate and contact
time were 10 kHz and 5.0 ms. A 1H NMR spectrum was obtained
at 400 MHz using the Hahn echo at room temperature. The spin-
ning rate was 10 kHz. A 13C NMR spectrum was performed at
400 MHz using high-powered decoupling at room temperature.
The spinning rate and contact time were 10 kHz and 2.5 ms. Scan
numbers were 20, 1.2, and 2 for 31P, 1H, and 13C, respectively.
Transmission electron microscopy (TEM) images were obtained
using a JEOL JEM-2010 (200 kV) TEM instrument. NH3 TPD curves
were recorded on a BELCAT-B temperature-programming unit
equipped with a thermal conductivity detector (TCD). The sample
was first pretreated in He at 300 °C for 2 h. After cooling down to
60 °C in He flow, it was saturated with ammonia (5% NH3 balanced
with He) at 60 °C for 30 min. Next, the sample was exposed to He
for 45 min to remove the physically adsorbed ammonia. Finally,
the TPD profile was recorded while the sample was heated at a rate
of 10 °C minꢁ1 up to 400 °C. Nitrogen (N2) adsorption isotherms
and Brunauer–Emmett–Teller (BET) surface areas were measured
using a BELSORPMINI analyzer. Before measurement, the samples
were degassed at 30 °C for 3 h under high vacuum. Electrospray
ionization mass spectrometry (ESI-MS) was recorded on a Fin-
nigam Mat APISSQ 710 mass spectrometer. Single-crystal diffrac-
tion analysis was measured by a Bruker Apex II CCD. Inductively
coupled plasma spectroscopy (ICP) was performed on a Jarrell-
Ash 1100 ICP-AES spectrometer.
3. Results and discussion
3.1. Formation of crystal-like IL-POM hybrids
Scheme 1 illustrates the synthesis procedure for PzS-PW. The
asymmetrical sulfoacid ionic liquid PzS-Cl is obtained from the
precipitation of pyrazine and chlorosulfonic acid in the solution
of anhydrous dichloromethane at 0 °C. Elemental analysis (wt.%)
of PzS-Cl finds C 21.2, N 11.8, H 3.4, and S 13.9, in good accordance
with Calcd: C 20.6, N 12.0, H 2.6, and S 13.8. The IL-POM hybrid
sample PzS-PW is obtained by slowly dropping aqueous H3PW into
the aqueous solution of PzS-Cl (PzS-Cl to H3PW molar ratio = 1.5).
The preparation, carried out at room temperature without using
any seeds, surfactants, or templates, is a slow precipitation process,
possibly because both the organic cation and inorganic anion have
large volumes, similar to an ionic self-assembly process [51]. The
cubic tetrakaidecahedron-like crystals for PzS-PW are achieved
after stirring for 24 h. Elemental analysis (wt.%) of PzS-PW finds
C 2.38, N 1.31, H 0.48, and S 1.49, which is very close to Calcd: C
2.31, N 1.35, H 0.29, and S 1.54. The calculated data are based on
the suggested chemical composition for PzS-PW in Scheme 1,
where the molar ratio of organic cation to inorganic anion is 1.5.
As shown in the positive-ion and negative-ion ESI-MS spectra of
PzS-PW (Fig. S1 in the Supplementary Material), the base peak at
m/z 81.0 is due to the twofold charged PzS2+ cation and the one
3ꢁ
at m/z 958.80 reveals the triply charged PW
O
, indicating that
12 40
PzS-PW is composed of organic IL cations (PzS2+) and inorganic
2.4. Catalysis tests
3ꢁ
heteropoly anions (PW
O
12 40
) through ionic linkage.
Catalytic activity of different IL-POM hybrids in the esterifica-
tion of oleic acid with methanol was evaluated. Taking the major
catalyst PzS-PW as an example, in a typical batch experiment,
25 mg of PzS-PW was added into a mixture of oleic acid (1 mmol,
0.28 g) and anhydrous methanol (10 mmol, 0.32 g) in a 25 mL
round-bottom flask equipped with a magnetic stirrer and a water
condenser system. At the end of the reaction, the catalyst was
separated by filtration.
Fig. 1A shows the XRD patterns of PzS-PW and its precursors
H3PW and PzS-Cl. PzS-Cl exhibits diffraction lines of a highly crys-
talline structure (curve a). H3PW displays a set of well-resolved
sharp diffraction peaks for a HPA crystal, an index of the typical
secondary cubic structure of Keggin anions (curve b). The crystal
phase of H3PW is taken from the JCPDS powder diffraction file
(Card 50-0304). The resultant PzS-PW sample exhibits X-ray
diffraction peaks different from those of raw H3PW and PzS-Cl,
revealing the formation of a crystal structure for PzS-PW (curve
c). Only the major peak at 9.60° is tentatively assigned to the
[110] phase, located at 10.44° for parent H3PW. This shift of the
[110] phase reflects an expanded unit cell due to the replacement
of protons with the bulky organic cation [52]. Single-crystal analy-
sis is performed; however, PzS-PW demonstrates no definite
single-crystal structure. The results of XRD and single-crystal
diffraction indicate that PzS-PW is polycrystalline in structure
rather than being a single crystal. The reason is that the arrange-
ment of ion pairs in PzS-PW presents a certain regularity but is
not as regular as the form of a single crystal. Based on these
characterizations, PzS-PW is composed of IL cations (PzS2+) and
FFA conversion can be evaluated using standard base-solution
titration. The acid value (AV) was calculated using the equation
AV ¼ VNaOH ꢀ CNaOH
;
ð1Þ
where VNaOH (mL) was the consumed volume of standard NaOH
(in ethanol) in every titration and CNaOH (mol Lꢁ1) was the concen-
tration of standard NaOH (in ethanol).
Oleic acid conversion (Xffa) in the corresponding esters was
estimated by calculating the product of an acid by titration with
NaOH (0.01 mol Lꢁ1), according to the following equation:
Xffa ¼ ðAVi ꢁ AVf Þ=AVi ꢀ 100;
ð2Þ