A new ternary mixed oxide catalyst for ammoxidation of propane:
Sn/V/Sb
S. Albonetti
a
,G.Blanchard
b
,P.Burattin
c
, S. Masetti
a
,A.Tagliani
a
and F. Trifir
o
a
a
Department of Industrial Chemistry andMaterials,Universityof Bologna, V. le Risorgimento 4, 40136 Bologna, Italy
b
Rhone Poulenc Chimie, Rue De La Haie Coq 52, 93308 Aubervilliers, France
c
Rhone Poulenc Chimie, Av. Des Freres 85, 69192 Saint Fons Cedex, France
Received 28 July 1997; accepted 18 November 1997
The catalytic properties of a Sn/V/Sb mixed oxide for propane ammoxidation to acrylonitrile are studied in this paper. In parti-
cular the antimony and vanadium amounts were changed in order to optimize the relative atomic ratio. The ternary sample with the
best catalytic performance was compared with the analogous binary samples (Sn/Sb and Sb/V). The results obtained indicate that
vanadium is responsible for the paraffin activation, nevertheless much vanadium produces a lot of carbon oxides. Antimony is
responsible for the insertion of nitrogen in the molecule (it can be considered as an acrylonitrile selectivity modulator); nevertheless,
too much antimony deactivates the catalyst. Tin does not act only as a dispersive matrix for the active sites, but leads to the forma-
tion of a polyfunctional catalyst, increasing the rate of acrylonitrile formation from the intermediate propene. From a structural
point of view, this catalytic system can be described as a homogeneous system, containing microfields of rutile type oxide (SnO
2
),
promoted with antimony and vanadium in substitutional solid solution, dispersed in an excess of amorphous antimony oxide.
Keywords: acrylonitrile, ternary mixed oxide, tin, vanadium, antimony
1. Introduction
Acrylonitrile (ACN) is at present being produced by
the ammoxidation of propene on catalysts made of pro-
moted Fe^Bi^Mo^O (SOHIO) or promoted Fe^Sb^O
(Nitto). Nevertheless, in recent years some companies
have decided to invest in research on the ammoxidation
of propane [1].
One of the more interesting catalytic systems for the
direct ammoxidation of the paraffin is Sb/V/O [2^7].
The synthesis of these catalysts is usually made by solid
state reaction between V
2
O
5
and Sb
2
O
4
at 700
Corby
reaction between NH
4
VO
3
and Sb
2
O
3
.M
ossbauer anal-
ysis shows that antimony is mainly in its pentavalent
state, so a large amount of vanadium is reduced, with the
formation of V
III
Sb
V
O
4
and the possible presence of V
IV
in substitutional or interstitial solid solution coordina-
tions. Centi et al. [3] and Nilsson et al. [4] have shown
that an excess of antimony brings about a decrease in
activity, but also a large increase in the selectivities and
in the yields in acrylonitrile and propene. The best cat-
alyst for the synthesis of acrylonitrile from propane has
a large excess of antimony (Sb/V 5.0). This excess
seems to quicken the transformation of the intermediate
propene to acrylonitrile. In contrast, an excess of vana-
dium brings about an increase in activity, but also a low
selectivity in acrylonitrile due to the increased produc-
tion of propene and carbon oxides.
The Sn/Sb/O system [7] has been widely studied in
recent years as a catalyst active in allylic oxidation and
ammoxidation. The best preparation method for these
compounds involves coprecipitation from a solution of
Sn(IV) and Sb(V) chlorides [8,9]. Calcination at tem-
peratures higher than 700^900
C (depending on the bulk
antimony/tin ratio) provokes segregation on the surface
of -Sb
2
O
4
particles [10]. An antimony surface enrich-
ment, especially for a concentration of antimony lower
than 20^30% [11] and the formation of Sb(III) [9] are evi-
denced during the thermal treatment. The catalytic
activity depends considerably on the Sn/Sb ratio and on
the calcination temperature [10,12,13]. In the literature
there is also disagreement about the nature of the active
sites for allylic oxidation on this system: isolated Sb
3
species surrounded entirely by Sn
4
ions in a specific
environment [9,10]; an oriented film of Sb
2
O
4
supported
on a solid solution of Sb
5
in Sn(IV) oxide [8,13]; a solid
solution of Sb
5
ions in a rutile type matrix [14]; and two
``gem'' Sb
5
O groups [15].
In order to create a new ternary mixed oxide catalyst
for propane ammoxidation we decided to mix these two
systems: Sb/V, in order to activate the paraffin, and Sn/
Sb, which has been already utilized to ammoxidate the
relative olefin. The samples characterized in this paper
have been synthesized by an original method of prepara-
tion [16,17] consisting in a pH-controlled coprecipita-
tion from an ethanolic solution of the starting materials.
In particular we have studied the effects of the antimony
and vanadium amounts in the Sn/V/Sb ternary system;
moreover, we have compared the ternary catalyst with
the best catalytic performance with the analogous binary
systems in order to better understand the role of the three
ions in the ternary sample.
Catalysis Letters 50 (1998) 17^23
17
Ä J.C. Baltzer AG, Science Publishers