8
2
Y. Qi et al. / Molecular Catalysis 445 (2018) 80–86
Table 1
Elemental analysis for catalysts .
Table 2
a
Effect of acidity on Prins condensation over various catalysts.
+
Catalysts
C(%)
N(%)
H(%)
Catalysts
H0
[H ] (mmol)
Conversion (%)
Selectivity (%)
[
[
[
[
[
NMC]3PW
NMC]3PMo
NMP]4SiW
NMC]4SiW
DP]2SiW
7.89(7.73)
11.70(11.42)
7.95(7.33)
9.97(9.92)
4.51(4.59)
1.36(1.29)
1.83(1.90)
1.83(1.71)
1.64(1.65)
1.69(1.78)
1.32(1.29)
2.11(1.90)
1.39(1.22)
1.75(1.65)
1.16(0.89)
H3PW
[NMC]3PW
H3PMo
[NMC]3PMo
H4SiW
3.11
3.19
3.27
3.31
3.09
3.08
3.08
3.06
1.20
0.22
1.20
0.20
1.60
1.10
0.53
1.60
88
96
66
83
75
80
80
86
99
80
100
98
100
100
99
[
[
[
NMP]4SiW
NMC]4SiW
DP]2SiW
a
Experimental results (theoretical concentrations).
100
Condition: 90 mmol of formaldehyde, 20 mmol of ␣-methylstyrene, 0.4 mmol of
◦
catalysts, at 95 C for 3 h.
Acidity strength of catalysts (Hammett acidity) were measured
based on previous reports (Eq. (2)) [38,39]. H related to strength of
0
+
acidity, [I] and [IH ] assigned to the unprotonated and protonated
3
. Results and discussion
indicator. With consideration of absorption proportional to species
concentration, [I]/[IH ] was calculated based on the absorbance
difference corresponding for addition of catalysts in the same con-
dition.
+
3.1. Characterization of catalysts
All catalysts were solid with different colors. TGA was employed
to character their thermal properties. As we all known that the loss
+
◦
H = pK(I)aq + log([I]s/[IH ]s)
(2)
under 100 C should attribute to water, thus starting weight loss
0
◦
temperature (Ts) of catalysts above 100 C was mainly investigated
(
Fig. S1). The results implied all catalysts can exist stably during
4
-Aminoazobenzene (AB, 128 mg/L, pKa = 2.8) was employed as
+
reaction process.
basic indicator, for all catalysts, the concentration of [H ] was fixed
−
4
As Fig. 1(a) presents, four characteristic bands in FT-IR spec-
at 1.67 × 10 mol/L in anhydrous ethanol. And then 0.5 mL of indi-
trum for Keggin structure of neat H SiW appeared at 1064, 961,
868 and 775 cm , assigned to vas (Si-O), vas (W = O), vas (W-Ob-
W) and vas (W-Oe-W) [33,37]. For [NMC] SiW, these feature peaks
appeared with shift of 961 cm to 957 cm , 868 cm to 877 cm
and 775 cm to 798 cm , which confirmed N-cation bonding with
heteropolyanion with ionic linkage. Moreover, H NMR data of
4
cator and 2.5 mL of catalysts solution were added in cuvette to get
−
1
+
absorbance. Acid concentration (H ) was measured according to
their solubility under reaction condition.
4
−1
−
1
−1
−1
−
1
−1
1
2
.4. Procedure of prins reaction
[
NMC] SiW indicated the NMC structure for the organic moiety.
4
XRD patterns of [NMC] SiW and pure H SiW were illustrated in
4
4
◦
In typical procedure, 1.35 g of catalyst was added into a
0 mL round-bottomed flask, followed by addition of 2.36 g of
◦
◦
◦
◦
◦
Fig. 1(b). The peaks located within 6 ∼ 10 , 15 ∼ 22 , 24 ∼ 30 and
5
◦
◦
3
3
∼ 36 attributed to characteristic diffraction peaks of Keggin-
␣
-methylstyrene and 7.30 g of formalin. Equipped with a reflux
type structures. The peaks (e.g., at 2ꢀ of 8.91, 17.65, 19.56, 20.84,
3.41, 33.11, 36.08 ) provided a strong proof for [NMC] SiW to for-
◦
condenser, the reaction was performed at 95 C for 3 h under stir-
ring. After reaction, the catalyst was separated and recycled by
decantation, and then washed with ethyl acetate and dried for next
run. Reaction product was extracted by the n-heptane solution of
ethyl acetate (v(n-heptane)/v(ethyl acetate) = 5/1), and analyzed by
gas chromatography (Kechuang, GC 9800, Agilent VF–5 ms) with
FID detector using n-dodecane as internal standard.
◦
2
4
mat Keggin-structure. These all features suggested lactam molecule
and HPA kept their own original structures. Moreover, the distinc-
tion between [NMC] SiW and H SiW in XRD demonstrated that the
4
4
protons of NMC changed the replacement of H SiW in crystal lat-
4
tice, which may explain the variation of Ts in TGA. Moreover, the
chemical composition of [NMC] SiW was well in accord with theo-
4
retical value based on element analysis (as summarized in Table 1).
For the other catalysts, characteristic bands for Keggin struc-
ture occurred in both FT-IR and XRD for all catalysts. Combined
with the data of 1H NMR, these features suggested that they not
only formed Keggin-type structure but also kept original structure.
The elemental analysis results of all catalysts were summarized in
Table 1. (More detail information was presented in Fig. S2, Tables
S1 and S2)
2
.5. Computational details
In this study, Gaussian03 suit of programs were employed for all
calculations, and DFT with B3LYP was conducted [40–42]. During
optimization procedure, some points were fixed in their position.
The nature of these points, which featured with the minima for
all real frequencies and just one imaginary frequency for all tran-
sition states, was characterized by frequency calculations at the
level of B3LYP/6-31G (d, p), the thermal corrections related to
Gibbs free energy at 298 K was also obtained in this way. The
The acidity of catalysts was characterized as the H0 value
+
(
Table S3) and [H ] concentration (Table S4) summarized in
Table 2. Firstly, the result of H0 indicated that set of H SiW
4
catalysts showed strongest relative acidity (H = 3.09 for H SiW,
0
4
6
–31G (d, p) basis set was used to optimize geometries in pres-
3
.08 for [NMP] SiW, 3.08 for [NMC] SiW, and 3.06 for [DP] SiW)
4 4 2
ence of some fixed points, and the results were examined by
larger 6–311 + G (2d, 2p) basis set to give more accurate energies.
All transition states remained local minima, which were carefully
checked by intrinsic reaction coordinate (IRC) calculations. These
hybrid organic-HPA catalysts can fully dissociate into polyanion
and organic cation under reaction condition [34,35,37], which can
owe to the hydrogen bond between the polyanion and organic
cation [43–51]. With consideration of Prins reaction activated by
protonic acid which derived from N-based cation in this condition,
effect of heteropolyanions was ignored during calculations.
among these catalysts, while set of H PMo catalysts presented
3
weakest relative acidity (H = 3.31 for [NMC] PMo). It was clear
0
3
that H0 of H SiW was much close to these catalysts with
4
4−
SiW
heteropoly-anions, indicating their acid strength in same
level. Besides, H0 of catalysts with different heteropoly-anions
showed order as [NMC] SiW < [NMC] PW < [NMC] PMo, which
4
3
3
demonstrated that H0 was more dependent on the heteropoly-
+
anions. On the other hand, the [H ] concentration of catalysts
4−
with SiW
exhibited as [DP] SiW > [NMP] SiW > [MNC] SiW,
2 4 4
+
and with respect to the catalysts with [NMC] , it showed as