X.F. Shi et al. / Chinese Chemical Letters 23 (2012) 53–56
55
Table 1
Catalytic results for the epoxidation of styrene with O2 over CoAPO-5 and M/CoAPO-5.
Entry
Catalyst
M/Co (mol/mol)
Calcined temperature (8C)
Solvent
Conversion (%)
Product selectivity (%)
BA
PA
SO
1
2
Blank
–
–
–
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
DMF
21
38
74
81
82
85
69
77
80
50
70
72
75
80
82
84
68
88
84
53
27
23
12
12
17
12
14
26
34
24
9
4.0
12
37
55
56
66
69
60
67
65
66
49
58
59
52
71
71
51
26
CoAPO-5
0
10
18
21
22
19
23
21
21
8
3
Li/CoAPO-5
Na/CoAPO-5
K/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-5
Cs/CoAPO-52nd
Cs/CoAPO-53rd
Co/APO-5a
1.0
1.0
1.0
1.0
0.5
0.8
1.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
–
350
350
350
350
350
350
350
350
350
350
100
550
350
350
–
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Cyclohexanone
Trioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
17
18
32
25
17
17
25
55
23
12
12
24
19
Cs/Co/APO-5
1.0
350
Reaction conditions: 0.1 g catalyst, 10 mL solvent, 5 mmol styrene, 10 mL/min O2, 100 8C, 4 h.
a
Prepared by impregnation of Co salt on AlPO-5 and calcining at 550 8C for 5 h. Co content: 2.7 wt.%.
SO of 38% and 37% respectively, which are much higher than those of blank experiment (entry 1). Impregnation of alkali
metal salts on the CoAPO-5 led to a significant increase in both the catalytic activity and the selectivity, which, moreover,
increased with increasing alkalinity of alkali metal salts (entries 3–6). Cs/CoAPO-5 showed a styrene conversion and a
selectivity to SO of 85% and 69% respectively, which are comparable to those of the alkali metal or alkali earth metal ion-
exchangedCo–NaX[7]. AlthoughthepromotionaleffectofalkalimetalsaltsonthecatalyticperformanceofCoAPO-5is
not very clear, we speculate that it might result from the interaction between alkali metal cations and Co species, which
weaken the distortion extent of tetrahedral Co2+ caused by removal of template during calcination. In addition, the
promotion effect is closely related to the methods of introduction of Co species into APO-5. For Co-impregnated APO-5
(Co/APO-5), the impregnated CsCl could also increase styrene conversion and decrease BA selectivity as it did for
CoAPO-5, but it greatly promoted PA formation at the expense of SO (entries 17, 18).
The catalytic performance of Cs/CoAPO-5 was highly dependent on the solvent and the calcination temperature
employed after impregnation of alkali metal salts. 1,4-Dioxane as solvent was much more active and selective than
DMF, trioxane and cyclohexanone (entries 3, 10–12). This is different from that observed over Co–NaX and its alkali
metal or alkaline earth metal ion-exchanged analogues, which showed high activity and SO selectivity in DMF
solvent. The difference may be related to their different Co environment. Co2+ is located in the ion-exchange position
for the Co–NaX, while it is sited in the framework for the CoAPO-5. Despite that the selectivity to SO and PA of Cs/
CoAPO-5 decreased with increasing calcination temperature, its activity exhibited a volcanic trend (entries 3, 13, 14).
Considering the activity and selectivity, the sample should be calcined at 350 8C.
The Cs/Co molar ratio (entries 3, 7–9) also strongly influenced the catalytic performance of Cs/CoAPO-5. The
activity and selectivity both increased with increasing Cs/Co molar ratio up to 1.0. A further increase in the Cs/Co
molar ratio had no positive effect.
Table 1 also shows that Cs/CoAPO-5 (Cs/Co = 1.0, calcined at 350 8C) had high stability (entries 15, 16). Its
catalytic activity was kept at least within three repeated runs with regeneration (the catalyst was recovered from the
reaction mixture by centrifuging and washing with acetone). Neither Co nor Cs was detected in the reaction liquid.
This shows that Cs/CoAPO-5 is recyclable. This could be attributed to the incorporation of Co2+ in the framework of
CoAPO-5.
In conclusion, alkali metal salt impregnated CoAPO-5 is a highly active, selective and stable heterogeneous catalyst
for the liquid phase epoxidation of styrene with O2. The activity and selectivity to SO and PA increased with increasing