A molecular precursor route to active and selective vanadia-silica- zirconia heterogeneous catalysts for the oxidative dehydrogenation of propane [10]

Full text of DOI:10.1021/ja981798d

J. Am. Chem. Soc. 1998, 120, 9959-9960
9959
A Molecular Precursor Route to Active and Selective
Precursor 1 was obtained in 92% yield by reaction of OVCl
3
t
17
Vanadia-Silica-Zirconia Heterogeneous Catalysts
with HOSi(O Bu)
of various ratios of 1 and 2 in octane solution at 175 °C produced
gels that were dried in air and then calcined under O for 3 h at
00 °C, to quantitatively produce highly porous, amorphous V/Si/
3
in the presence of pyridine. Cothermolysis
for the Oxidative Dehydrogenation of Propane
2
Ron Rulkens and T. Don Tilley*
5
Zr/O xerogels containing 2-34% vanadia (eq 2). The porosities
Department of Chemistry
UniVersity of California, Berkeley
Berkeley, California 94720-1460
Chemical Sciences DiVision
octane/175 °C/48 h
t
nOV[OSi(O Bu) ] + mZr(OCMe Et) -C4H8, -C5H10, -H2O8
3
3
2
4
1
2
Lawrence Berkeley Laboratory
1
Cyclotron Road, Berkeley, California 94720
V Si Zr O (OH) (2)
n
3n
m
x
y
ReceiVed May 26, 1998
and surface areas of these xerogels vary with elemental composi-
tion (Table 1). Whereas the average pore size and total pore
volume increased with increasing vanadia content, the highest
surface areas were obtained for a sample composed of ca. 10%
A major challenge in chemistry is the design and development
of new catalysts for selective transformations of saturated
hydrocarbons.1
-3
For example, the high demand for C2-C4
alkenes has motivated interest in their production from inexpen-
sive C1-C4 alkanes. The direct, dehydrogenation of light alkanes
proceeds only at high temperatures, where cracking and coking
present serious problems. On the other hand, oxidative dehy-
drogenation (ODH) is thermodynamically favored at lower
temperatures and does not suffer from the deposition of carbon,
which decreases catalyst performance.1 Thus, the ODH of
propane (eq 1) is attractive as an alternative source of propene,
particularly given its high demand for the production of polypro-
pene, acrylonitrile, and propene oxide.4 However, this selective
oxidation is particularly challenging, given the high reactivity of
18
vanadia, 20% silica, and 70% zirconia. In general, this method
provides materials with properties approaching those observed
19
for aerogels. Infrared spectra of the V/Si/Zr/O xerogels, in the
region containing asymmetric Si-O-Si and Zr-O-Si stretches
-1
(
700-1250 cm ), resemble those reported for zirconia-silica
20
aerogels shown to be highly dispersed. For materials with a
high vanadia content (g18%), a band at ca. 950 cm is observed
for the V-O-Si stretch
can be assigned to silica. The position of the OfV charge-
-1
21,22
23
-1
and bands at 450, 550, and 800 cm
transfer band edge, measured by DRUV (Diffuse Reflectance
UV-vis) spectroscopy, was correlated with the vanadium coor-
1
,5
propene toward further oxidation.
Among the best catalysts
24,25
dination environment.
For xerogels with 2-8% vanadia, an
1,3,6
7
7
reported for the ODH of propane
are CoMoO
4
, MgMoO
4
,
absorption edge of ca. 4.1 eV is consistent with single-site,
Mg/V/O,8 V/Nb/O, NiMoO
,9
10
, and vanadia silicalite.
11
6,12
4
4
-coordinate vanadium. Xerogels with 10-18% vanadia exhibit
a gradual shift in the absorption edge from ca. 4.0 to ca. 3.6 eV,
indicating an increasing degree of linear polymerization for the
1
CH CH CH + / O
8 H CdCHCH + H O (1)
2 3 2
3
2
3
2
2
catalyst
VO
4
tetrahedra.25 For the sample containing 23% vanadia, band-
In search of synthetic methods that allow atomic-level control
over the local environment of reaction centers in mixed-metal
catalysts, we are investigating molecular precursor routes to
heterogeneous catalysts for the ODH of propane. Along these
edge features at ca. 3.6 and 2.9 eV suggest the additional presence
of 5-coordinate vanadium, and only for the vanadia-silica xerogel
(34% vanadia) was a prominent feature observed for crystalline
V O , at ca. 2.3 eV. The samples with <23% vanadia exhibit
2 5
29
lines, we have previously reported the conversion of metal-OSi-
MAS Si NMR shifts (δ -99 to -88) that reflect primarily
2 3
t
(
O Bu)
3
derivatives to high surface area, highly dispersed metal
contributions from Q and Q sites, and therefore a high degree
13-15
26
oxide-silica materials.
The approach described here focuses
of silicon dispersion.
on vanadia as the active component, since vanadia silicalite has
been reported as one of the most selective catalysts for propane
The results of the propane ODH catalysis are summarized in
Figure 1 and Table 1. The vanadia-silica xerogel exhibits low
ODH.6 More specifically, we describe the solution-phase trans-
27
2 5
activity and rapidly declining selectivity, as found for V O ,
t
formation of a two-component precursor system, OV[OSi(O Bu)
]
3 3
but the introduction of zirconia dramatically improves the catalyst
performance. In contrast to what has been observed for vanadia-
silica catalysts,27 the selectivity to propene increases for the V/Si/
(
2 4
1) and Zr(OCMe Et) (2), to vanadia-zirconia-silica catalysts
that exhibit impressive activities and selectivities for the ODH
of propane. The presence of zirconia-silica in these catalysts
2 5
Zr/O catalysts with increasing V O content. The best perfor-
mance was observed for xerogels with 18% and 23% vanadia,
which are characterized by high selectivities comparable to the
16
was expected to improve the vanadia dispersion while providing
high surface areas.1
3,14
(
1) Kung, H. H. AdV. Catal. 1994, 40, 1.
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quantitatively.
(
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(
S0002-7863(98)01798-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/11/1998

9960 J. Am. Chem. Soc., Vol. 120, No. 38, 1998
Communications to the Editor
Table 1. Characterization Data and Catalytic Activities for V/Si/Zr/O Xerogels and Two Reference Catalysts
activity (mmol g h-1
d
-1
)
V
2
O
wt (%)
5
/SiO
2
/bZrO
2
BET surface
area (m /g)
total pore
vol (cm /g)
av pore
n:ma
c
2
3
c
size (Å)c
673 K
773 K
0
0
0
0
0
0
1
.032
.064
.14
.19
.32
.51
.03
2/4/93
5/10/85
8/15/77
10/20/70
14/28/58
18/36/46
23/46/31
34/66/0
10/0/90
18/36/46
430 (302)
553 (428)
647 (436)
638 (447)
529 (466)
414 (404)
239 (268)
320 (311)
260 (122)
(121)
0.50
0.57
0.68
1.1
23.1
20.6
21.0
28
56
82
3.1
9.1
14
15
28
17
45
70
88
100
78
46
19
58
0.9
1.4
1.61
0.97
2.25
15
81
140
7.6
2.2
18
∞
2
8
1
1
0% V
8% V
2
O
O
5
on ZrO
on ZrO
2
e
2
5
2
/SiO
2
(0.29)
(51)
0.3
a
Molar ratio of precursor 1 and 2 in the thermolysis mixture. b For V
m
Si3nZr O
(8.5n+2m) as the formed composition after calcination. c The number
n
d
in parantheses is the surface area after calcination at 500 °C. Fixed bed microreactor, 0.020-0.100 g of catalyst + 0.500 g of quartz sand,
/C
-
1 e
O
2
3 8
H /He ) 8/25/200 mL min
.
Prepared by NH
4
VO
3
-oxalic acid wet impregnation of a Si/Zr/O xerogel (see text).
catalyst (18% vanadia). On a per-gram basis, the activities of
these catalysts far exceed those previously reported for propane
1
1
ODH catalysts (by factors of g18), however, â-NiMoO
4
is
approximately three times as active on the basis of catalyst surface
areas. Significantly, the activity and selectivity exhibited by the
2 5
catalyst with 18% V O were constant at 500 °C over a 2 h period,
and during this time the catalyst surface area was reduced by
only 15%.
To evaluate the utility of this molecular-precursor route to
catalysts, we compared data obtained from materials with a similar
composition, but prepared by more conventional methods (im-
pregnation). A high surface area, active V
prepared by NH VO -oxalic acid wet impregnation of ZrO
found to exhibit optimal performance at a loading of 10% V
and exhibited a selectivity at 400 °C that is comparable to the
V/Si/Zr/O catalyst containing 5% V . In addition, at low
conversion, the V /ZrO catalyst exhibited a decreasing selec-
tivity with increasing temperature. A second catalyst for com-
parison, with 18% vanadia, was prepared by NH VO -oxalic acid
wet impregnation of a zirconia-silica xerogel, which was obtained
2
O
5
/ZrO
2
catalyst
4
3
2
was
2
8
O ,
2 5
2
O
5
2
O
5
2
Figure 1. Plot of selectivity to propene as a function of propane
conversion, with V/Si/Zr/O xerogel catalysts (at 673 K).
4
3
t
14
via cothermolysis of Zr[OSi(O Bu)
3
]
4
2 4
and Zr(OCMe Et) . This
catalyst exhibited a very low activity (Table 1), and a selectivity
of ca. 80% at low conversions. Thus the molecular precursor,
cothermolysis procedure of eq 2 produces a catalyst with superior
and unique properties.
In summary, a novel method for the preparation of high surface
area, ternary metal oxide catalysts, has been developed. The V/Si/
Zr/O catalysts with 18 and 23% V
most selective and active catalysts reported for propane ODH,
2 5
O compare favorably to the
1
,3,6-12
and exhibit catalytic properties which are superior to those of
previously reported vanadium-based catalysts. This method is
inherently versatile, allows control over elemental composition,
and should be applicable to many catalyst formulations. Most
significantly, the novel features and impressive performance for
these materials suggest that molecular-level control over structure
can provide new generations of superior catalysts.
-
1
Figure 2. Correlation of vanadia content with activity (0, mmol h
g
0
-
1
at 773 K) and propene selectivity ([, % at 673 K, extrapolated to
% conversion) for the V/Si/Zr/O catalysts.
Acknowledgment. This work was supported by the Director, Office
of Energy Research, Office of Basic Energy Sciences, Chemical Sciences
Division, of the U.S. Department of Energy under Contract No. DE-
AC03-76SF00098. We thank Professor Alexis T. Bell, Professor Enrique
Iglesia, Professor Gabor Somorjai, and Dr. Andrei Khodakov for useful
discussions.
best yet reported for propane ODH.1,3,6-12 Interestingly, the
selectivities observed for these catalysts at 0% conversion are
invariant over the temperature range of 400-500 °C. For the
catalyst with 18% V
2 5
O , 8.0% conversion of propane (with 20%
conversion of O ) at a propene selectivity of 81.5% was achieved
2
at 550 °C. The zirconia-containing xerogels exhibit selectivities
for propene that decrease rather slowly with conversion, indicating
that the presence of zirconia suppresses the competing, deleterious
oxidation of propene in these structures. Activities for the V/Si/
Zr/O catalysts are presented in Table 1 and Figure 2. The highest
activity was observed for the catalyst containing 14% vanadia.
Figure 2 correlates selectivities and activities for the catalysts,
and shows that high activities are achieved for the most selective
Supporting Information Available: Synthetic procedures and char-
acterization data and IR, solid state 29Si NMR, and DRUV spectra for
the V/Si/Zr/O catalysts (6 pages, print/PDF). See any current masthead
page for ordering information and Web access instructions.
JA981798D
(
28) Khodakov, A.; Yang, J.; Su, S.; Iglesia, E.; Bell, A. T. J. Catal. A
1998, 177, 343.