An active and selective alkane isomerization catalyst: iron- and
platinum-promoted tungstated zirconia
Stefan Kuba,ab Bruce C. Gates,* Robert K. Grasselli* and Helmut Knözinger*
b
a
a
a
Department Chemie, Physikalische Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, Haus E,
D-81337 München, Germany. E-mail: helmut.knoezinger@cup.uni-muenchen.de;
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294, USA.
E-mail: bcgates@ucdavis.edu
b
Received (in Cambridge, UK) 12th December 2000, Accepted 16th January 2001
First published as an Advance Article on the web 31st January 2001
Addition of iron sulfate to Pt-promoted tungstated zirconia
increases the activity and selectivity of the catalyst for
10 ml min21. As the Pt precursor in the catalyst was observed
to be reduced in H to give zero-valent Pt even at room
temperature, we infer that it was also reduced in the presence of
the H -containing reactant mixture at 523 K during the initial
stages.
2
isomerization of n-pentane in the presence of H
98% have been observed.
2
; selectivities
>
2
Environmental concerns are motivating the use of motor fuels
with increased amounts of high-octane-number branched al-
kanes. These are made by alkylation and by isomerization of
straight-chain alkanes, the latter typically carried out with
bifunctional catalysts incorporating hydrogenation/dehydroge-
nation functions and acidic functions or by very strong acids,
such as aluminium chloride supported on alumina, which has
the disadvantages of being corrosive and expensive to dispose
of without environmental detriment. There is a need for alkane
isomerization catalysts with improved activities and selectiv-
ities. Recently investigated candidates include tungstated
zirconia (WZ), which has a high activity,1 especially when
The dependence of n-pentane conversion on time-on-stream
(TOS) in the flow reactor is shown for the three catalysts in
Fig. 1. Fe/Pt/17WZ(S) almost immediately attained a nearly
stable activity corresponding to a conversion of ca. 64%. In
contrast, induction periods of ca. 15 and 60 min were observed
for Pt/17WZ and Fe/Pt/17WZ(N), respectively, before a nearly
stable conversion of ca. 48% was attained. These activities are
in the reverse order of the BET surface areas of the catalysts and
show that the differences are associated with the catalyst
compositions and not just physical properties.
The selectivities of the catalysts for isopentane formation
measured during these experiments are shown in Fig. 2. The
selectivity of Pt/17WZ dropped from ca. 97% to a stable value
of ca. 95% after 1 h, whereas that of Fe/Pt/17WZ(N) reached a
stable selectivity of ca. 99.7% after 30 min and that of Fe/
Pt17WZ(S) approached 99% after 2.5 h. This small difference
in selectivies may be caused by the higher conversion on the
sulfate-containing catalyst. Thus, the data show that promotion
by Fe enhances the catalytic activity of WZ for n-pentane
conversion, with the iron sulfate providing a higher activity than
iron nitrate but a somewhat lower selectivity.
,2
promoted with Pt and when H
use of group 8 metals as promotors of WZ has been claimed in
is contained in the feed.3–5 The
2
6–8
recent patents.
Fe has also been incorporated in WZ
catalysts, but promotion was not observed in the absence of Pt
in the catalyst and H in the feed. We now report WZ catalysts
9
2
improved by promotion with both Pt and Fe and compare their
performance with that of WZ promoted by Pt only.
Catalysts were prepared by slurry impregnation of amor-
phous Zr(OH)
ammonium metatungstate, (NH
In the synthesis of Fe-promoted catalysts, the appropriate
amount of either FeSO or Fe(NO was added to the slurry.
4
(MEL Chemicals, XZO880/01) with aqueous
4
)
6
H
2
W
12
O
40·nH O (Aldrich).
2
The importance of H
was shown that Fe had no promoting effect on WZ when H
2
in the reactant feed is emphasized. It
9
2
4
3
)
3
was absent from the feed. Only 1% conversion of n-pentane and
30% selectivity for isopentane were measured under our
conditions for the Fe- and Pt-promoted catalyst in the absence of
The resultant suspensions were refluxed overnight at 393 K,
dried in an oven at 353 K, and then calcined at 923 K in static
air for 3 h. Separate batches of these calcined materials were
impregnated by the incipient wetness method with 0.6 M
aqueous Pt(NH
Each catalyst contained W in an amount corresponding to 17
wt% as WO , which is close to the theoretical monolayer
4 3 2
)(NO ) and calcined at 723 K in air.
3
2
capacity (19 wt%). The Pt content was 1 wt%. The catalysts
contained either no Fe (denoted as Pt/17WZ) or Fe in amounts
2 3
corresponding to 1.0 wt% as Fe O ; the latter are denoted as Fe/
Pt/17WZ(N) and Fe/Pt/17WZ(S), prepared from iron nitrate or
sulfate, respectively. The BET surface areas of Pt/17WZ, Fe/Pt/
2
21
1
7WZ(N) and Fe/Pt/17WZ(S) were 126, 110 and 80 m g
,
respectively.
Catalytic conversion of n-pentane was carried out in a once-
through packed-bed flow reactor under the following condi-
tions: temperature, 523 K; pressure, 101 kPa; n-pentane partial
2
pressure, 0.84 kPa; H partial pressure, 16.8 kPa; catalyst mass,
2
1
2
00 mg; feed flow rate (at NTP), 10 ml min of 1% n-pentane
2
1
2 2
in N mixed with 2 ml min of H . Under these conditions, the
predominant catalytic reaction product was isopentane, formed
with small amounts of methane, ethane, propane, butane,
isobutane and neopentane. Catalysis was also carried out in the
absence of H
2
under the same conditions, except that the feed n-
Fig. 1 Conversion of n-pentane catalyzed by Fe-promoted and Fe-free Pt/
WZ catalysts in a flow reactor (conditions stated in text).
pentane partial pressure was 0.84 kPa and the total flow rate was
DOI: 10.1039/b009900g
Chem. Commun., 2001, 321–322
321
This journal is © The Royal Society of Chemistry 2001
Kuba
