A highly active solid superacid catalyst for n-butane isomerization: persulfate
modified Al O –ZrO
2
3
2
Y. D. Xia, W. M. Hua, Y. Tang and Z. Gao*
Department of Chemistry, Fudan University, Shanghai 200433, P.R. China
Received (in Cambridge, UK) 15th June 1999, Accepted 23rd August 1999
A new solid superacid catalyst of persulfate modified Al
2
O
3
–
2
an H +butane molar ratio of 10+1. Before testing, each catalyst
ZrO
2
has been prepared for the first time; it displays
was pretreated in situ in dry air at 723 K for 3 h. The products
were analyzed with an on-line gas chromatograph equipped
with a flame ionization detector.
extraordinarily high catalytic activity and stability for the
isomerization of n-butane.
The major reaction product of n-butane isomerization at 523
K is isobutane and by-products are propane and isopentane. The
In recent years, sulfated zirconia superacids have attracted
increasing attention because they can give high activity at low
temperature for n-butane isomerization, which has recently
become a very important reaction in the petrochemical industry
owing to growing environmental constraints and requirements
for high-octane-number gasoline.1–3 However, a rapid deactiva-
tion of this catalyst has often been observed at high temperature.
22
22
selectivity to isobutane for the SO
4
/ZrO
2
and S O
2 8
2 3
/Al O –
ZrO catalysts is > 95%. Table 1 shows the variation of the
2
conversion of n-butane at 523 K with time on stream for both
catalysts. During the initial 1 h of reaction both catalysts are
rapidly deactivated. After being on stream for 2 h the
conversions of both catalysts then drop more slowly. The
22
22
In order to improve the lifetime of SO
4
/ZrO
2
catalysts for n-
S
2
O
8
2 3 2
/Al O –ZrO catalyst reaches a steady state for n-butane
butane isomerization, the presence of hydrogen and/or addition
of a small amount of Pt or Ni have been suggested.4 Although
Fe- and Mn-promoted sulfated zirconia are ca. three orders of
isomerization after 2 h on stream (although it deactivates more
,5
22
rapidly than SO
indicating that it is more stable than SO
of n-butane isomerization. Both the initial and steady state
4
2
/ZrO during the initial 1 h of reaction),
22
4
2
/ZrO for the reaction
22
magnitude more active than SO
4
/ZrO
merization at low temperature, they are deactivated quickly at
50 °C in the presence of hydrogen or at 60 °C in the presence
2
for n-butane iso-
6
22
activities of the S
2
O
4
8
/Al
22/ZrO
–ZrO is 2.1 times more active after being on
stream for 6 h.
The stability of the S –ZrO catalyst has been
22/Al
investigated by running the reaction at 523 K continuously for
2
O
3
–ZrO
2
catalyst are much higher
22
2
than those of SO
2
. As compared with SO
4
2
/ZrO ,
7,8
22
8
of nitrogen. We found that sulfated oxides of Cr–Zr, Fe–Cr–
Zr and Fe–V–Zr were 2–3 times more active than sulfated Fe–
Mn–Zr for n-butane isomerization, but these transition metal-
doped sulfated zirconias also deactivated rapidly in the presence
S
2
O
/Al
O
2 3
2
2
O
8
2
O
3
2
of hydrogen at high temperature.8 In our previous results,
,9
10,11
200 h. As illustrated in Fig. 1, the initial conversion on S
O
22
/
2
8
the addition of Al to sulfated zirconia significantly enhanced the
activity and stability of the catalyst for n-butane isomerization
at 250 °C in the presence of H . Persulfate modified zirconia is
2
more active than sulfated zirconia for the isomerization of n-
butane. The main problem in this research area is the durability
of the catalyst during isomerization of n-butane. Here, we report
a new solid superacid catalyst that is more active and stable for
n-butane isomerization than any sulfated zirconia-based cata-
lysts yet reported.
Al –ZrO is 51.8%, dropping to 37.8% after 2 h, and then
2
O
3
2
remaining constant at ca. 37.4% up to 200 h without further
observable deactivation. In other words, n-butane isomerization
22
proceeds steadily on S
O
2 8
/Al
2
O
3
–ZrO
2
at a level of 72% of
its equilibrium conversion. In view of its high isomerization
22
activity and stablity, S
2
O
8
/Al
2
O
3
–ZrO
2
can be regarded as an
excellent candidate for a commercial-scale n-butane isomeriza-
tion catalyst.
After running on stream for 6 h, the amount of coke deposited
22
The new catalyst was prepared as follows: aqueous ammonia
on the S
2
O
8
2 3 2
/Al O –ZrO catalyst is 1.0 wt%, which is
was added dropwise to a mixed solution of ZrOCl
2 2
·8H O and
Al(NO ·9H O until pH = 9–10. After washing the mixed
3
)
3
2
hydroxide and drying at 383 K overnight it was immersed in 0.5
M ammonium persulfate solution for 30 min. The persulfated
Al(OH)
overnight and calcined at 923 K in static air for 3 h. The new
3 4
–Zr(OH) was then filtered off, dried at 383 K
2
21
catalyst, a white solid, had a surface area of 80.8 m g and
contained 3.0 mol% Al and 3.5 wt% sulfate. Sulfated
zirconia was made for comparison in the same manner by
immersing dried Zr(OH) in 0.5 M sulfuric acid, followed by
calcination at 923 K in static air for 3 h. The sulfated zirconia,
2 3
O
4
2
21
also a white solid, had a surface area of 113.0 m g and
contained 4.0 wt% sulfate. n-Butane isomerization on the
catalysts was carried out at 523 K in a fixed-bed continuous
Fig. 1 Long-term test of S O822/Al O –ZrO catalyst for n-butane
isomerization at 523 K.
2
2
3
2
2
1
flow reactor under ambient pressure with WHSV = 0.3 h and
Table 1 Activities of solid superacid catalysts for n-butane isomerization at 523 K
Conversion (%)
Catalyst
2 min
10 min
60 min
120 min 180 min 240 min 300 min 360 min
2
8
2
SO
4
/ZrO
/Al O –ZrO
2 3 2
2
27.7
51.8
25.2
43.7
23.5
38.2
21.6
37.8
20.4
37.7
19.3
37.7
18.1
37.4
17.5
37.3
2
2
2
S O
Chem. Commun., 1999, 1899–1900
1899
Xia
