7
86
Chemistry Letters Vol.36, No.6 (2007)
þ
Boron-modified Chlorine-free K –FeO /SBA-15 as Highly Effective Catalyst
x
for Propylene Epoxidation by Nitrous Oxide
ꢀ
Sufen Yang, Qinghong Zhang, and Ye Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, P. R. China
(Received March 19, 2007; CL-070283; E-mail: wangye@xmu.edu.cn)
þ
þ
Modification of chlorine-free alkali metal ion (K , Rb , or
Cs )-containing FeOx/SBA-15 catalysts with boron enhanced
Table 1. Effect of boron on catalytic results of alkali metal
a
þ
ion-modified FeOx/SBA-15 for C3H6 epoxidation by N2O
their catalytic performances in propylene epoxidation by nitrous
þ
Selectivity/%
C3H6
oxide. A B–K –FeOx/SBA-15 catalyst (K/Fe = 2.5, B/Fe =
0.5) showed the best propylene oxide (PO) formation activity,
providing PO selectivities of 79 and 55% at propylene conver-
b
Catalyst
conv.
/
c
PO
Others
COx
%
ꢁ
sions of 4.8 and 13% at 350 and 400 C, respectively.
FeOx/SBA-15
1.3
2.3
4.1
3.5
4.2
4.4
3.8
4.8
1.6
4.9
8.9
13
0
0
50
59
62
60
18
21
14
12
11
12
14
15
13
13
18
14
ꢁ
50
41
22
24
13
13
22
9.4
8.9
11
18
30
33
13
37
10
B–FeOx/SBA-15
þ
Li –FeOx/SBA-15
16
16
69
66
64
79
80
77
68
55
54
74
45
76
þ
The epoxidation of propylene using a ‘‘green’’ oxidant has
1
B–Li –FeO /SBA-15
x
attracted much attention in recent years. Although oxygen or
air is the most ideal oxidant, the epoxidation of C3H6 by O2 is
still not successful, and PO selectivity can hardly exceed 60%
þ
Na –FeOx/SBA-15
þ
B–Na –FeOx/SBA-15
þ
K –FeOx/SBA-15
2
even at a low C3H6 conversion. A few efficient catalysts have
þ
þ
þ
þ
B–K –FeOx/SBA-15
been reported for the epoxidation of C3H6 by H2O2 in liquid
phase,3 but the high cost of H2O2 and the difficulty in its
handling are problems. Many studies have contributed to the
in situ production of H2O2 from O2–H2 gas mixture for C3H6
d
e
B–K –FeOx/SBA-15
B–K –FeOx/SBA-15
B–K –FeOx/SBA-15f
þ
g
B–K –FeOx/SBA-15
4
epoxidation. Among the catalysts reported, the Au–Ti-based
vapor-phase catalysts developed by Haruta and co-workers have
þ
Rb –FeOx/SBA-15
2.9
4.2
2.8
4.4
þ
B–Rb –FeOx/SBA-15
4
b–4d
attracted much attention.
Recently, we studied an iron-based C3H6 epoxidation
þ
Cs –FeOx/SBA-15
þ
B–Cs –FeOx/SBA-15
5
catalytic system with N2O as the oxidant. We reported that a
KCl-modified 1 wt % FeOx/SBA-15 (K/Fe = 5) catalyst could
provide PO selectivities of 72 and 50% at C3H6 conversions
a
Reaction conditions: W(catalyst) = 0.2 g, T ¼ 350 C, P(C3H6) =
ꢂ1
2
min. Fe content = 1 wt %; alkali metal ion/Fe (molar ratio) = 2.5,
.5 kPa, P(N2O) = 25.3 kPa, F(total) = 60 mL min , TOS = 30
b
5
a,5b
of 4.5 and 10%, respectively.
potassium salt such as KAc as the modifier led to remarkably
However, the use of other
c
B/alkali metal ion (molar ratio) = 0.5. Others include acrolein,
d
allyl alcohol, acetone, and acetaldehyde. TOS = 230 min.
e
5
b
lower PO formation activities. Because chlorine may go to
the gas phase to act as a radical promoter and may be lost during
the reaction, it is useful to develop a highly efficient chlorine-
free catalyst. Herein, we report our recent finding that the pres-
ence of boron can significantly enhance the catalytic properties
Regenerating the catalyst after 230 min of reaction, followed by
f
ꢁ
g
ꢁ
reaction for 30 min. T ¼ 375 C. T ¼ 400 C.
PO was observed over the FeOx/SBA-15 or the B–FeOx/SBA-
15. The modification with a no-halogen alkali metal ion could
also induce the PO formation and enhance C3H6 conversion.
þ
þ
þ
of the no-chlorine K (Rb or Cs )–FeOx/SBA-15 catalysts.
SBA-15 and FeOx/SBA-15 were prepared using the proce-
þ
þ
Among the alkali metal ions, Na and K showed similar
þ
5
b
þ
þ
dure reported previously. The modified FeOx/SBA-15 cata-
lysts were prepared by impregnation of the FeOx/SBA-15 pow-
der with an aqueous solution of alkali metal acetate or a mixed
aqueous solution containing alkali metal acetate and H3BO3 in
a fixed ratio, followed by drying and calcination at 823 K for
enhancing effects and were better than Li , Rb , and Cs as
modifiers. As compared with the KCl–FeOx/SBA-15 catalyst
5
a
þ
reported previously, the present chlorine-free K –FeOx/
SBA-15 catalyst exhibited a lower C3H6 conversion and a lower
PO selectivity. However, the presence of boron remarkably
increased the PO selectivities as well as C3H6 conversions for
6
h. The contents of iron and potassium in the catalysts were de-
þ þ þ
termined by ICP. The catalysts were characterized by XRD and
N2 sorption. Catalytic reactions were carried out using a fixed-
bed flow reactor (quartz tube) operated at atmospheric pressure.
The products were analyzed by two on-line gas chromatographs.
Table 1 shows the catalytic performances of the 1 wt %
FeOx/SBA-15 catalysts modified by various alkali metal ions
with and without boron for the epoxidation of C3H6 by N2O.
It should be noted that no reaction occurred without iron irre-
spective of the presence or absence of boron or alkali metal ions
on SBA-15. In the absence of an alkali metal ion, no formation of
the K -, Rb -, and Cs -modified FeOx/SBA-15 catalysts al-
þ
þ
though the catalytic performances of the Li – and Na –FeOx/
SBA-15 catalysts were not obviously altered by boron. PO
þ
þ
þ
selectivities over the B- and K (Rb or Cs )-doubly modified
5
,6
catalysts were higher than 70%. Similar to other catalysts, the
activity of the present catalyst also decreased with time on
stream (TOS), but the activity can be recovered by regenerating
the catalyst with a gas flow containing O2 at 823 K (Table 1).
We have carried out further studies on the effect of boron on
þ
catalytic performances of the K –FeOx/SBA-15 catalysts with
Copyright Ó 2007 The Chemical Society of Japan