X. Zhou et al. / Journal of Molecular Catalysis A: Chemical 417 (2016) 71–75
73
Table 2
Acid strength and acidity of Ag1H2PW and (NH4)0.5Ag0.5H2PW.
2
Catalyst
SBET (m /g)
H0
Acidity (mmol/g)
H3PW
Ag1H2PW
5.1
3.3
10.8
−12.70 < H0 < −13.75
−11.35 < H0 < −12.70
−11.35 < H0 < −12.70
1.62
1.87
2.01
(
NH4)0.5Ag0.5H2PW
Table 3
Effect of catalyst on the synthesis of n-butyl levulinate.
Yield (%)a
TOF (h )b
−1
Entry
Catalyst
1
2
3
4
5
–
34.2
96.8
92.1
99.0
95.5
75.2
72.6
89.4
75.6
–
H3PW
Ag1H2PW
(NH4)0.5Ag0.5H2PW
(NH4)1H2PW
(NH4)0.5Ag0.5H2PW
(NH4)0.5Ag0.5H2PW
32.3
30.7
33.0
31.8
75.2
72.6
29.8
25.2
c
6
7
c
d
d
8
9
Ag1H2PW
(NH4)1H2PW
d
Reaction conditions: LA: 0.1 mol, n-butanol: 0.2 mol, catalyst: 1.5% (Based on the
mass of LA), 120 C, 2 h.
◦
Fig. 3. Effect of dopant amount on the catalytic activity of (NH4)xAgyH3-x-yPW. Reac-
tion conditions: LA: 0.1 mol, n-butanol: 0.2 mol, catalyst: 1.5% (Based on the mass
of LA), 120 C, 2 h.
a
The selectivity of n-butyl levulinate was nearly 100% in all tests.
Gram of LA converted per gram of catalyst in 1 h.
Catalyst: 0.5% (Based on the mass of LA).
The 4th run.
◦
b
c
d
also lower than (NH4)0.5Ag0.5H PW. In addition, the selectivity of
2
n-butyl levulinate was nearly 100% in all tests.
erty. Therefore, Ag H PW and (NH )0.5Ag0.5H PW catalysts were
The reusability of solid acid catalyst was one of the key fac-
tors for evaluating its catalytic performance. Herein, the reusability
of (NH4)0.5Ag0.5H2PW, Ag1H2PW and (NH4)1H2PW was investi-
gated for the esterification of LA with butanol. The amount of
(NH4)0.5Ag0.5H2PW was decreased to 0.5% for the evaluation of its
reusability, because the conversion of LA was too high (nearly100%)
in the presence of 1.5% (NH4)0.5Ag0.5H2PW. As shown in Table 3,
(NH4)0.5Ag0.5H2PW performed good reusability. And the yield of n-
butyl levulinate was decreased slightly from 75.2% in the first run
to 72.6% in 4th run. The catalytic activity of Ag1H2PW had also no
remarkable decrease after 4 runs, which was consistent with results
reported by literatures [21–23]. However, the yield of n-butyl lev-
ulinate decreased quickly from 95.5% to 75.6% after 4 recycles of
(NH4)1H2PW. According to the literatures [18], the hydrophilicity
of (NH4)1H2PW was better than that of Ag1H2PW, resulting higher
loss of (NH4)1H2PW during the reaction in polar mixture. And the
results of leaching test in filtration also proved that. The loss rate
of (NH4)1H2PW was up to 18% after 4 runs, while the loss rate
of (NH4)0.5Ag0.5H PW and Ag H PW was just about 2%. It should
1
2
4
2
tested by the method of n-butylamine titration using Hammett
indicators. As shown in Table 2, the surface acid strength (Ho) of
Ag H PW and (NH )0.5Ag0.5H PW was both in the range of −11.35
1
2
4
2
to −12.70, which was slightly lower than that of parent H PW.
3
However, the surface acid density of Ag H PW (1.87 mmol/g) was
1
2
higher than that of parent H PW (1.62 mmol/g), which was consis-
3
tent with previous report [21]. Among them, (NH4)0.5Ag0.5H PW
2
showed the highest surface acid density (2.01 mmol/g). Although
the surface Brønsted acid sites of H PW salts was lower than that
3
of parent H PW, H PW salts possess higher surface Lewis acid sites
3
3
than H PW, as reported by previous literatures [21–23]. In addi-
3
tion, the surface area was one of the influence factors on the surface
acid density. (NH4)0.5Ag0.5H PW showed the highest surface area
2
among these catalysts, which might be one of reasons for its highest
surface acid density.
3.2. Catalytic property test
2
1
2
Effect of catalyst on the conversion of renewable LA to n-butyl
be one of reasons for poor reusability of (NH4)1H2PW. Based on
the results of activity and reusability test, (NH4)0.5Ag0.5H2PW per-
formed better catalytic property than traditional single NH4+ or Ag+
doped H3PW for the synthesis of n-butyl levulinate.
◦
levulinate was investigated at 120 C within 2 h. The reaction equi-
librium was broken by removing water to improving the yield of
n-butyl levulinate in all tests. As shown in Table 3, the yield of n-
butyl levulinate was only 34.2% in the absence of catalyst. Then,
the yield of n-butyl levulinate could reach 96.8% and 92.1% using
H PW and Ag H PW as catalyst, respectively. Although the activity
3.3. Effect of dopant amount on the activity of catalyst
3
1
2
The exchange number of H+ in heteropoly acid had an impor-
tant effect on the catalytic property of heteropoly acid salts. When
the total doping amount of Ag+ and NH4+ was setting in the
range of 1–2.5, effects of doping amount of Ag and NH4+ on the
catalytic activity of (NH4)xAgyH3-x-yPW were investigated in the
present work. As shown in Fig. 3, the activity of (NH4)xAgyH3-x-yPW
decreasedwith increasingthe total doping amountof Ag and NH4 .
Among these catalysts, (NH4)0.5Ag0.5H2PW with remaining two H+
in the molecular structure performed the highest activity, which
was consistent with the results of Ag1H2PW [21].
of H PW was higher than that of Ag H PW, H PW was completely
3
1
2
3
dissolved in the reaction system resulting in a complicated process
for catalyst recycling. In contrast to H PW, Ag H PW performed
3
1
2
+
in heterogeneous state during the reaction, which was much more
favorable in terms of sustainable chemistry.
As shown in Table 3, the yield of n-butyl levulinate was up
to 99.0% with 33.0 turn over frequencies (TOF) value of LA using
+
+
(
NH4)0.5Ag0.5H PW as heterogeneous catalyst. And the results indi-
2
cated that the activity of (NH4)0.5Ag0.5H PW is higher than that of
2
Ag H PW. Although (NH )0.5Ag0.5H PW and Ag H PW possessed
1
2
4
2
1
2
similar Keggin structure and acid strength (Fig. 1, 2), the sur-
face acid density of (NH4)0.5Ag0.5H PW was higher than that of
3.4. Effect of catalyst dosage
2
Ag H PW (Table 2). It might be the reason for higher activity of
1
2
(
NH4)0.5Ag0.5H PW. The activity of (NH ) H PW for the synthe-
Effects of catalyst dosage on the esterification of LA with n-
2
4
1
2
sis of n-butyl levulinate was also investigated. And its activity was
butanol were investigated using (NH4)0.5Ag0.5H PW as catalyst.
2