Process for preparing vanadyl pyrophosphate catalyst

Details Process for preparing vanadyl pyrophosphate catalyst Process for preparing vanadyl pyrophosphate catalyst

2003-03-20

2004-08-10

Bell, Mark L. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Phosphorus or compound containing same

C502S209000, C423S305000, C423S306000

Reexamination Certificate

active

06774081

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for preparing vanadyl pyrophosphate catalyst. More particularly, the present invention relates to a process for the preparation of vanadyl pyrophosphate catalyst with improved structural characteristics for the selective oxidation of butane to maleic anhydride.
BACKGROUND OF THE INVENTION
Layered vanadyl hydrogen phosphate hemihydrate, VOHPO
4
.0.5H
2
O has tremendous technological importance as the precursor to the vanadyl pyrophosphate, (VO)
2
P
2
O
7
which is used as the catalyst for the commercially established selective oxidation of butane to maleic anhydride [Ref. G. Centi, F. Trifiro, J. R. Ebner, V. M. Franchetti,
Chem.Rev.,
1988, vol. 58, p 55].
Maleic anhydride is a valuable intermediate for the production of some commercially important fine chemicals and polymers. The largest use of maleic anhydride occurs in the production of unsaturated polyesters. In the fine chemicals industry, maleic anhydride is used as a raw material for the production of succinic anhydride. &ggr;-butyrolactone, 1,4-butanediol, tetrahydrofuran, fumaric acid, malic acid and D-L tartaric acid.
Vanadyl pyrophosphate (VO)
2
P
2
O
7
catalyzed process of maleic anhydride synthesis by selective oxidation of butane replaced the earlier process of gas phase oxidation of benzene on a supported V—Mo—O catalyst due to economic and environmental reasons. Under typical industrial conditions 65-70% selectivity towards maleic anhydride is achieved for butane conversion of 70-85%. Carbondioxide and trace amounts of acetic acid are the only other byproducts.
Vanadyl pyrophosphate (VO)
2
P
2
O
7
is regarded to be the active phase of the VPO catalyst since it is the only bulk phase that exists in equilibrated VPO catalysts. The catalytic activity of (VO)
2
P
2
O
7
is known to be sensitive to its morphological characteristics and in particular the preferential exposure of the (100) crystallographic plane of the (VO)
2
P
2
O
7
crystallites is highly desirable since this plane has been established to be the most active and selective for the oxidation of butane to maleic anhydride [Ref. “Vanadyl Pyrophosphate Catalysts”. ed. G. Centi,
Catal.Today,
1993, vol. 16]. The morphology of the VO
2
P
2
O
7
phase is indirectly controlled through that of the VOHPO
4
.0.5H
2
O phase, since it undergoes a topotactic transformation at ~450° C. to the (VO)
2
P
2
O
7
phase with retention of the microstructure and morphological characteristics of the precursor VOHPO
4
.0.5H
2
O phase [Ref. J. W. Johnson, D. C. Johnston, A. J. Jacobson, J. F. Brody,
J. Am. Chem. Soc.,
1984, vol. 106, p. 8123]. During this transformation the (001) plane of VOHPO
4
.0.5H
2
O is transformed to the active (100) plane of (VO)
2
P
2
O
7
with retention of the morphology/surface defect of the (001) plane of the hemihydrate phase.
In view of the well established role of the (100) plane of the (VO)
2
P
2
O
7
phase in catalyzing the selective oxidation of butane to maleic anhydride, the synthesis of phases with preferential exposure of this plane would be of great significance in increasing the activity of (VO)
2
P
2
O
7
catalyst. The intensity ratio of the interplaner (100) and in-plane (021) x-ray reflection of (VO)
2
P
2
O
7
i.e. (I
100
:I
021
) is used to determine the preferential exposure of the surface (100) planes proposed to contain the active and selective catalytic sites for selective oxidation of butane to maleic anhydride. The conventional methods of synthesis of (VO)
2
P
2
O
7
catalysts typically exhibits low intensity ratios I
100
:I
021
ca. 0.4-2 indicating that the surface (100) planes are not preferentially exposed by these methods [Ref. V. V. Guliants, J. B. Benziger, S. Sundaresan, I. E. Wachs, J. M. Jehng, J. E. Roberts,
Catal. Today,
1996, vol. 28, p. 275].
There are very few reports in literature on the preferential exposure of catalytically important (100) crystallographic plane of (VO)
2
P
2
O
7
phase.
Reference is made to a procedure of one pot synthesis of VOHPO
4
.0.5H
2
O with high growth of the (001) plane and its subsequent transformation to a (VO)
2
P
2
O
7
phase with preferentially exposed (100) plane [N. Mizuno, H. Hatayama, M. Misono,
Chem. Mater.,
1997, vol. 9, no. 12, p. 2697]. The drawback of this procedure is the requirement of a costly surfactant and hydrothermal condition during the crystallization of the precursor VOHPO
4
.0.5H
2
O phase.
Reference is made to colloidal templated synthesis of (VO)
2
P
2
O
7
phase [M. A. Carreon, V. V. Guliants,
Chem.Commun.,
2001, p 1438] with I
100
:I
021
=2.48 suggesting preferential exposure of surface (100) planes. However, this method comprises a complex procedure using mono dispersed polystyrene spheres as templating agents for formation of VO)
2
P
2
O
7
with selectively exposed catalytically important (100) plane.
OBJECTS OF THE INVENTION
The main object of the invention is to provide a process for the preparation of vanadyl pyrophosphate catalysts with improved structural characteristics for the selective oxidation of butane to maleic anhydride.
Another object of the invention is to provide a simple method for the selective exposure of catalytically active (100) plane of the (VO)
2
P
2
O
7
catalyst used for selective oxidation of butane to maleic anhydride.
Yet another object of the invention is to provide a process for the preparation of vanadyl pyrophosphate catalyst which exhibits high selective exposure of the catalytically active (100) plane in the (VO)
2
P
2
O
7
catalyst in comparison to catalyst prepared by conventional methods.
It is another object of the invention to provide a process for the preparation of vanadyl pyrophosphate which does not require hydrothermal reaction conditions for the preparation of (VO)
2
P
2
O
7
catalyst with selective exposure of the catalytically important (100) plane.
It is further object of the invention to provide a process for the preparation of vanadyl pyrophosphate which does not require costly surfactant and mono dispersed colloidal templating agents during the preparation of (VO)
2
P
2
O
7
catalyst with selectively exposed (100) plane.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a process for the preparation of vanadyl pyrophosphate catalyst with improved structural characteristics and useful for the selective oxidation of butane to maleic anhydride, which comprises:
i) reducing V
2
O
5
solid by an aqueous solution of NH
2
OH.HCl in the presence of H
3
PO
4
to obtain a blue solution,
ii) evaporating the solution to obtain a pasty mass,
iii) aging the pasty mass to obtain a blue solid,
iv) washing the solid with boiling water to remove the water soluble phases,
v) drying the solid to form VOHPO
4
.0.5H
2
O phase,
vi) grinding the dry VOHPO
4
.0.5H
2
O phase to a fine powder.
vii) dispersing the VOHPO
4
.0.5H
2
O powder into a mixture of dimethyl formamide (DMMF) and water (H
2
O) and stirring the resulting slurry,
viii) recovering the dispersed VOHPO
4
.0.5H
2
O powder from the slurry and washing with hot water,
ix) drying the powder to obtain a VOHPO
4
.0.5H
2
O solid with selectively enhanced (001) plane,
x) calcining the VOHPO
4
.0.5H
2
O solid with selectively enhanced (001) plane to obtain the desired (VO)
2
P
2
O
7
phase with selectively exposed (100) plane.
In one embodiment of the invention. V
2
O
5
solid is reduced in step (i) at a temperature in the range of 70-100° C. while maintaining a P:V ratio in the reaction mixture in the range of 1:0.8 to 1:1.4.
In another embodiment of the invention, the pasty mass obtained in step (ii) is aged in step (iii) by heating to a temperature in the range of 100-150° C. in air atmosphere for a period in the range of 12-36 hours.
In yet another embodiment of the invention, the washed solid obtained in step (iv) is dried in step (v) at a temperature in the range of 100-150° C. for 6-16 hours in air atmosphere to form VOHPO
4
.0.5H
2
O phase.
In yet another embodiment of the invention, the dry VOHPO
4
.0

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