Method for making heteropolyacid catalysts and catalysts made by the method

Catalysts having an improved long-term activity behavior in the oxydehydrogenation of, for example, isobutyric acid to methacrylic acid, are prepared from heteropolyacids of molybdenum or from their metal salt derivatives by the concurrent use of a water soluble organic compound which is of low volatility or is practically non-volatile, such as a polymer, followed by calcination of the preparation at 200.degree. C. to 400.degree. C. in the presence of oxygen.

The present invention relates to methods for making an oxidation catalyst 
comprising a heteropolyacid of molybdenum, to catalysts so made, and to 
the use of such catalysts. 
Such heteropolyacids and their metal salts are known as selectively acting 
catalysts in oxydehydrogenation reactions, for example in the oxidative 
dehydrogenation of isobutyric acid to methacrylic acid. 
Phosphomolybic acid (12-molybdophosphoric acid, H.sub.3 PMo.sub.12 
O.sub.24, and particularly its vanadium derivatives, that is 
molybdovanadophosphoric acids of the formula H.sub.3+x PMo.sub.12-x 
V.sub.x O.sub.40, wherein x=1,2, or 3, are known as heteropolyacid 
catalysts for carrying out selective oxidations, including the 
oxydehydrogenation of isobutyric acid or its esters to methacrylic acid or 
its esters. The preparation of such heteropolyacids is described in Inorg. 
Chem. 7 (1968), 437-441, for example, or in U.S. Pat. No. 4,146,574. For 
use as catalysts in vapor phase oxidations, the heteropolyacids, also in 
the form of salts or metal derivatives, are advantageously formed into 
manageable shapes together with an inert inorganic support material, such 
as silica or alumina, which possesses internal surfaces and porosities 
adapted for specific uses. 
Catalysts of this type and their uses, such as in particular the oxidative 
dehydrogenation of isobutyric acid to methacrylic acid, are described in 
German patent publication 27 22 375 and European Pat. publication No. 0 
113 084. However, catalysts containing H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 
or CU.sub.0.2 PMo.sub.10 VO.sub.35.2, for example, which have been 
prepared by prior art methods exhibit a serious drawback in use: namely 
they become deactivated relatively quickly, which means that their service 
life is too short for practical purposes. Deactivated catalysts can be 
regenerated by the method disclosed in published German Pat. application 
No. P 36 26 255, for example. 
The present invention seeks to improve the activity time behavior of 
oxidation catalysts comprising heteropolyacids of molybdenum using 
measures adopted during their preparation. Oxidation catalysts prepared 
according to the invention are advantageously used for the 
oxydehydrogenation of isobutyric acid and/or its lower esters to 
methacrylic acid and/or its lower esters in the vapor phase in the 
temperature range from 250.degree. C. to 400.degree. C. 
It has been found that if a water soluble organic compound which has low 
volatility or is practically non-volatile is added during the preparation 
of catalysts from heteropolyacids or their metal salts, optionally 
together with inert inorganic supports or extenders, followed by drying 
and calcination of the catalysts, the activity of the novel catalysts so 
prepared will be preserved for a substantially longer period of time. 
Prior to calcining, the catalyst mixture may be shaped by tableting, 
extrusion, or pelletizing, for example. 
Thus, the invention relates to a method for making an oxidation catalyst 
consisting essentially of a heteropolyacid of molybdenum having Mo, P, and 
V as its principal elements, or metal salts of such acids, by mixing a 
catalytically active heteropolyacid of this kind, optionally with the 
addition of an inert inorganic support material, with a water soluble 
organic compound which is practically non-volatile or of low volatility, 
the mixture then being dried and calcined, possibly after being formed 
into shapes, at a temperature from 200.degree. C. to 400.degree. C. in the 
presence of oxygen. Oligomers and polymers have been found to be 
particularly suitable water soluble organic compounds.

EXAMPLE 1 
H.sub.3.6 CU.sub.0.2 PMo.sub.11 VO.sub.40 
A mixture of 198.31 g of MoO.sub.3, 11.39 g of V.sub.2 O.sub.5, 14.26 g of 
H.sub.3 PO.sub.4, 2.0 g of CuO, and 2034 g of distilled water is 
maintained at boiling temperature for 16 hours with stirring. The solution 
is then cooled to about 20.degree. C. and, after filtration, is mixed with 
6.73 g of a 40 weight percent solution of polyacrylic acid having a 
molecular weight of 20,000. After 45 minutes, the solution is mixed with 
80.07 g of diatomaceous earth and 16 g of silica gel ("Aerosil 200")in a 
weight ratio of diatomaceous earth to silica gel of 5:1. The mixture is 
then concentrated by evaporation to form a paste. The latter is then dried 
and preheated in a circulating air over for 1 hour at 110.degree. C. and 
for 3 hours at 300.degree. C. in the presence of atmospheric oxygen. The 
catalyst material so obtained is then cooled and comminuted to a particle 
size from 2 to 5 millimeters. 
EXAMPLES 2 TO 4 
By the reaction of MoO.sub.3, V.sub.2 O.sub.3, H.sub.3 PO.sub.4, and CuO, 
optionally together with Cs.sub.2 CO.sub.3, in the molar ratios given in 
the formulas of following Table 1, solutions of these heteropolyacid 
compounds are prepared as in Example 1. A water soluble nonvolatile 
organic compound is added in an amount of 3 parts by weight per 100 parts 
by weight of active component. Then, as in Example 1, a 70:30 mixture by 
weight is formed between the active component and diatomaceous 
earth/silica gel. This mixture is dried to form a paste and is calcined as 
described in Example 1. 
TABLE 1 
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Ex- Organic compound 
ample Composition of active component 
added 
______________________________________ 
2 H.sub.3.6 Cu.sub.0.2 PMo.sub.11 VO.sub.40 
Polyvinyl alcohol 
(MW 72,000) 
3 H.sub.4.6 Cu.sub.0.2 PMo.sub.10 V.sub.2 O.sub.40 
Polyacrylic acid 
(MW 15,000) 
4 H.sub.3.2 Cs.sub.0.1 PMo.sub.11.5 V.sub.0.5 O.sub.40 
Polyacrylic acid 
(MW 32,000) 
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EXAMPLE 5 
A heteropolyacid, H.sub.5 PMo.sub.10 V.sub.2 O.sub.40, containing water of 
crystallization and prepared according to U.S. Pat. No. 4,146,574 is 
pelletized in powder form in a solution of 5 parts by weight of 
polyvinylpyrrolidone ("Kollidon 25") in a mixture of 55 parts by weight of 
water and 40 parts by weight of isopropanol. (See Ullmans Encyclopadie der 
technischen Chemie, 4th edition, vol. 2, pp. 321-327). In the process, 5 g 
of polyvinylpyrrolidone are added per 100 g of H.sub.5 PMo.sub.10 V.sub.2 
O.sub.40. The pellets, which have an average diameter of 3.5 millimeters, 
are dried for 21 hours at 110.degree. C. and then for 2 hours at 
150.degree. C., and are then calcined for 4 hours at 310.degree. C. in the 
presence of atmospheric oxygen. 
EXAMPLE 6 
A CS.sub.2 Mo.sub.12 V.sub.1.5 P.sub.2 O.sub.45.8 catalyst free of 
diatomaceous earth/silica gel is prepared according to Example 1 of U.S. 
4,370,490 omitting the calcination step there indicated. 5 g of 
polyvinylpyrrolidone ("Kollidon 25") are added in the form of a 20 percent 
by weight aqueous solution to 100 g of the catalyst powder and the whole 
is mixed thoroughly, dried, and calcined for 5 hours at 320.degree. C. in 
the presence of atmospheric oxygen. 
CATALYST TESTS IN THE OXYDEHYDROGENATION OF ISOBUTYRIC ACID 
In each case, a vaporous mixture of isobutyric acid and oxygen (as air) in 
a molar ratio of 1:1.5, sometimes together with nitrogen in an amount of 2 
moles per mole of isobutyric acid, is reacted in a circulating reactor 
over a catalyst and under the conditions given in following Table 2. The 
reaction gas is continuously analyzed by gas chromatography and the 
conversion of isobutyric acid and the selectivity of the catalyst for 
methacrylic acid are calculated from the values obtained. These measures 
of catalytic activity are plotted in accompanying FIGS. 1-7. 
TABLE 2 
__________________________________________________________________________ 
IBA feed 
(g of IBA per 
Catalyst Tem- 1,000 g of 
Result 
(catalytically active 
perature 
active component 
plotted 
Test component) .degree.C. 
and hour) 
in FIG. 
__________________________________________________________________________ 
1 H.sub.3.6 Cu.sub.0.2 PMo.sub.11 VO.sub.40 
340 1,250 plus N.sub.2 
1 and 2 
2 As above 340 1,250 plus N.sub.2 
1 and 2 
(Comp.) 
As in Ex. 1 but with- 
out polyacrylic acid 
3 As above 340 1,250 plus N.sub.2 
3 
As in Example 1 
4 As above 340 1,250 plus N.sub.2 
3 
As in Example 2 
5 H.sub.4.6 Cu.sub.0.2 PMo.sub.10 V.sub.2 O.sub.40 
340 834 4 and 5 
As in Example 3 
6 As above 340 834 4 and 5 
(Comp.) 
As in Example 3 but 
without polyacrylic 
acid 
7 H.sub.3.2 Cu.sub.0.1 Cs.sub.0.1 PMo.sub.11.5 - 
320 2,500 6 and 7 
V.sub.11.5 O.sub.40 as in Ex. 4 
8 As above 320 2,500 6 and 7 
(Comp.) 
As in Example 4 but 
without polyacrylic 
acid 
__________________________________________________________________________ 
TESTS 9 AND 10 (COMATIVE) 
In keeping with the procedure followed in Tests 1 to 8, the catalyst of 
Example 5 is tested at a reaction temperature of 340.degree. C. and with 
an hourly feed of isobutyric acid of 600 g per 1000 g of active component 
in the presence of 1.5 moles of O.sub.2 (as air) plus 2 moles of N.sub.2 
per mole of isobutyric acid. The result of this test is compared with that 
of an identical test run with a catalyst containing the same catalytically 
active component, but produced without the addition of 
polyvinylpyrrolidone. In the first 100 hours of reaction, both catalysts 
give isobutyric acid conversions from 82 to 85 percent with selectivities 
for methacrylic acid of 72 percent. The catalyst prepared according to the 
invention maintains these activity and selectivity levels for many 
hundreds of hours, whereas the isobutyric acid conversion of the catalyst 
made by prior art methods drops steadily and considerably after about 100 
hours of reaction and its selectivity for methacrylic acid also declines. 
TESTS 11 AND 12 (COMATIVE) 
A mixture of isobutyric acid/O.sub.2 (as air)/H.sub.2 O in a molar ratio of 
1:1.7:2 is passed over three different 2 milliliter samples of a Cs.sub.2 
Mo.sub.12 V.sub.1.5 P.sub.2 O.sub.45.8 catalyst. The catalyst used in Test 
11 was prepared according to the invention: that of Test 12 was made 
without addition of polyvinylpyrrolidone. Analyses made during the first 
50 hours of the tests give the results with respect to activity for 
isobutyric acid and selectivity for methacrylic acid reported in following 
Table 3. 
TABLE 3 
______________________________________ 
Selectivity for 
Temperature IBA conversion 
methacrylic acid 
Test .degree.C. % % 
______________________________________ 
11 350 99.4 63.5 
340 98.0 62.8 
330 95.3 63.0 
12 350 91.0 66.6 
340 90.1 66.1 
330 77.8 65.3 
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