Catalytic oxidation of methacrolein

Methacrolein is catalytically oxidized into methacrylic acid in the vapor phase at a temperature of 200.degree. to 450.degree. C. The catalyst used is comprised of molybdenum, phosphorus, potassium and/or cesium, vanadium, silver and/or tellurium, and oxygen, and represented by the formula: EQU Mo.sub.12 P.sub.a A.sub.b V.sub.c X.sub.d O.sub.e wherein A is K and/or Cs and X is Ag and/or Te, and a=0.5-5, b=0.1-4, c=0.05-3, d=0.001-2 and e is a positive number required by the valence states of the other elements present.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to a process for catalytically oxidizing 
methacrolein in the vapor phase with molecular oxygen to produce 
methacrylic acid, wherein an improved catalyst is used giving an enhanced 
yield of methacrylic acid and exhibiting a long catalyst life. 
(2) Description of the Prior Art 
Many catalysts have been heretofore proposed which are used for oxidizing 
unsaturated aldehydes, such as acrolein and methacrolein at an elevated 
temperature in the vapor phase with molecular oxygen to produce 
corresponding unsaturated carboxylic acids, such as acrylic acid and 
methacrylic acid. However, proposals of the catalysts used for the 
oxidation of methacrolein to methacrylic acid are fewer in number than 
those of the catalysts used for the oxidation of acrolein to acrylic acid. 
Certain catalysts were proposed as being capable of being used both for the 
oxidation of acrolein and the oxidation of methacrolein. It is generally 
accepted, however, that such catalysts exhibit reduced catalytic activity 
for the oxidation of methacrolein to methacrylic acid as compared with 
catalytic activity for the oxidation of acrolein to acrylic acid. That is, 
the yield of methacrylic acid is far lower than the yield of acrylic acid, 
provided that the same catalyst is used in the respective oxidation 
reactions. It is presumed that one of the reasons for which the yield of 
methacrylic acid is lower than the yield of acrylic acid is that 
methacrolein has a branched carbon chain, i.e., a methyl group, which is 
susceptible to oxidation, and thus, it is difficult to selectively oxidize 
the aldehyde group without the oxidation of the methyl group. 
Heretofore, proposed catalysts used for the oxidation of methacrolein to 
methacrylic acid have some of the following defects: (1) the conversion of 
methacrolein, the selectivity to methacrylic acid and the yield of 
methacrylic acid are low; (2) the reaction temperature is undesirably high 
which influences the utility and the life of the catalyst and further 
causes side reactions; and, (3) the durability is poor. Typical examples 
of conventional catalysts for use in the oxidation of methacrolein are 
those which contain, as the essential elements, molybdenum, phosphorus, 
vanadium and an alkali metal. Illustrations of such catalysts are 
enumerated, for example, as in the following catalyst compositions: 
Mo-P-V-X-O (X=at least one metal of K, Rb, Cs and Tl; Japanese Laid-open 
patent application No. 82,013/1975), Mo-P-V-X-Y-O (X=at least one metal of 
K, Rb, Cs and Tl, Y=at least one metal of Sr, Zn, Cd, Nb, B, Pb, Bi and W; 
Japanese Laid-open patent application No. 123,619/1975 corresponding to 
U.S. Pat. No. 4,075,244), Mo-P-Cs-X-O (X=at least one metal of V, Nb and 
Tl; Japanese Laid-open patent application No. 135,020/1975), Mo-P-X-Y-O 
(X=at least one metal of V, Nb and Ta, Y=at least one metal of K and Tl; 
Japanese Laid-open patent application No. 65,713/1976); P-Mo-X-Y-O (X=at 
least one metal of K, Rb, Cs and Tl, Y=at least one metal of V, Fe, Mn, 
Ni, Ta, W, Sb, Co, Nb, Zn, Cd, U, Bi and Sn; Japanese Laid-open patent 
application No. 115,413/1976); Mo-P-X-Y-O (X is at least one metal of V, 
Fe, Pb and Ni, Y=at least one metal of K, Rb, Cs and Tl; Japanese 
Laid-open patent application No. 52,120/1976); and Mo-P-X-Y-O (X=at least 
one metal of K, Cs, Rb and Tl, Y=at least one metal of Ni, Sn, V, W, In, 
Zr and Ba; Japanese Laid-open patent application No. 57,117/1977). 
Furthermore, a catalyst having the composition of Mo-V-P-O, which may 
optionally contain at least one metal of Bi, As, B, Ce, Cr, Ag, Fe, W, Pb, 
Mn, Tl, Te, Ni, Nb, B, Sn and Cu, is disclosed in U.S. Pat. No. 3,875,220. 
According to the working examples of this patent, the yield of methacrylic 
acid is not satisfactory, i.e., in the range of from 44.7% to 57.6%. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for 
catalytically oxidizing methacrolein in the vapor phase to methacrylic 
acid wherein both the conversion of methacrolein and the selectivity to 
methacrylic acid are high and thus the yield of methacrylic acid is high 
even when the catalytic oxidation is carried out at a relatively low 
reaction temperature. 
Another object of the present invention is to provide a catalyst exhibiting 
an enhanced catalytic activity for the oxidation of methacrolein to 
methacrylic acid and possessing an improved durability. 
In accordance with the present invention, there is provided a process for 
the preparation of methacrylic acid by catalytically oxidizing 
methacrolein in the vapor phase with molecular oxygen at a temperature of 
from 200.degree. C. to 450.degree. C., characterized by using a catalyst 
consisting essentially of (A) molybdenum, (B) phosphorus, (C) potassium 
and/or cesium, (D) vanadium, (E) silver and/or tellurium and (F) oxygen 
and represented by the formula: 
EQU Mo.sub.12 P.sub.a A.sub.b V.sub.c X.sub.d O.sub.e 
wherein Mo is molybdenum, P is phosphorus, A is at least one metal selected 
from potassium and cesium, V is vanadium, X is at least one metal selected 
from silver and tellurium, and O is oxygen, and the subscripts a through d 
are positive numbers falling within the following ranges: 
a=0.5 to 5, preferably 0.9 to 3, b=0.1 to 4, preferably 0.5 to 3, c=0.05 to 
3, preferably 0.1 to 2, and d=0.001 to 2, preferably 0.01 to 1, and e is a 
positive number required by the valence states of the other elements 
present, the e being usually in the range of 39 to 60. 
The advantages of the present invention over processes employing 
conventional molybdenum, phosphorus, potassium (and/or cesium) and 
vanadium-containing catalysts are summarized as follows. First, the 
conversion of methacrolein and the yield of methacrylic acid are enhanced 
to a considerable extent, i.e., approximately 10% or more, as will be 
apparent from the comparison of the Examples with Comparative Examples 1, 
2 and 5, mentioned below. Secondly, a satisfactory yield of methacrylic 
acid is attainable even when the catalytic oxidation of methacrolein is 
carried out at a relatively low reaction temperature, e.g. from 
300.degree. to 340.degree. C. and further even when the catalytic 
oxidation is continued over a long period of time. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The atomic ratios of the respective elements, expressed by the subscripts a 
through d in the above-mentioned formula are crucial for the intended 
advantages. For example, if the amount of the X ingredient, i.e., silver 
and/or tellurium, exceeds the above-mentioned range, the selectivity to 
methacrylic acid is reduced. In contrast, if the amount of the X 
ingredient is smaller than the above-mentioned range, the conversion of 
methacrolein is reduced. 
It is thought that the respective elements (other than oxygen) present in 
the catalyst used are predominantly in the form of compounds of the type, 
in which two or more of the elements are bonded with oxygen, such as 
phosphomolybdic acid salts. 
The catalyst used in the process of the invention may be prepared in any 
convenient manner by using, as the starting raw material, oxides, salts 
and other compounds, containing the respective elements. The general 
procedure for the preparation of the catalyst is as follows. Oxides, salts 
and other compounds, containing the respective elements, are mixed with 
each other in an aqueous medium to prepare a uniform solution or 
dispersion. The aqueous solution or dispersion is dried and then, calcined 
in an air atmosphere at a temperature of from 300.degree. to 550.degree. 
C., preferably from 350.degree. to 450.degree. C. for a period of from 2 
to 24 hours, preferably from 3 to 15 hours. 
The procedure for the preparation of the catalyst will be described in more 
detail in the following example. Predetermined amounts of a potassium salt 
and/or a cesium salt, e.g., potassium nitrate and/or cesium nitrate are 
combined with an aqueous solution of a molybdic acid salt, e.g., 
phosphomolybdic acid, while being stirred. Then, a predetermined amount of 
a vanadium salt, e.g., ammonium metavanadate, and predetermined amounts of 
a silver salt or oxide and/or a tellurium salt or oxide, e.g. silver 
nitrate and/or tellurium dioxide, are successively added to the above 
solution, while being stirred. The obtained aqueous slurry is evaporated 
to dryness, and then, the dried product is calcined at a temperature of 
from 300.degree. to 550.degree. C. for a period of from 2 to 24 hours. It 
should be understood, however, that the procedure by which the catalyst is 
prepared and the raw materials from which the catalyst is prepared are not 
limited to those described in this example, and other procedures and raw 
materials may be employed. 
Illustrations of the starting raw materials for use in the preparation of 
the catalyst are enumerated, for example, ammonium molybdate, molybdic 
acid, phosphomolybdic acid, phosphoric acid, ammonium phosphate, potassium 
nitrate, cesium nitrate, potassium carbonate, cesium carbonate, potassium 
hydroxide, cesium hydroxide, potassium phosphate, metavanadic acid, 
ammonium metavanadate, vanadium pentoxide, silver nitrate, silver sulfate, 
silver chloride, silver carbonate, telluric acid, potassium tellurate, 
tellurium chloride and tellurium dioxide. 
The catalyst may be used alone or in combination with a carrier. The use of 
a carrier is advantageous in enhancement of the mechanical strength of the 
catalyst. As carriers, those which are known for use in supporting 
conventional oxidation catalysts, such as diatomaceous earth, silica, 
alumina, silicon carbide, silica-alumina and water-soluble silica sol may 
be used. 
In general, the size and shape of the catalyst particulates used are not 
particularly critical because they do not greatly effect the catalytic 
activity. Pellets, tablets and other optional shapes may be used depending 
upon the conditions under which the catalysts are used. 
Molecular oxygen used in the catalytic oxidation of the invention is not 
necessarily highly purified; however, air or other oxygen-containing gases 
may also conveniently be used. Particularly, air may be advantageously 
used. The amount of molecular oxygen used is usually in the range of from 
0.5 to 7 moles, more preferably from 1 to 5 moles, per mole of 
methacrolein. 
Methacrolein used in the catalytic oxidation is also not necessarily highly 
purified, and its mixtures may be used. For example, a gaseous product 
obtained by the oxidation of isobutylene may be used. A gaseous product 
obtained by the oxidation of a hydrocarbon mixture containing n-butene and 
isobutylene, such as a spent BB which is a residue obtained by separating 
1,3-butadiene from the C.sub.4 fraction produced by the thermal cracking 
of naphtha, may also be used. However, a mixture containing salient 
amounts of unsaturated aldehydes other than methacrolein should not be 
employed because such a mixture not only retards the catalytic oxidation 
reaction involved but also produces polymer and other side-reaction 
products. 
A gaseous feed comprising methacrolein and molecular oxygen may contain a 
diluent gas which does not influence the catalytic oxidation reaction 
involved. Such a diluent gas includes, for example, steam, nitrogen and 
carbon dioxide. Among others the incorporation of steam in the gaseous 
feed is preferable, because steam not only acts as a diluent but also 
exhibits effects for enhancing the selectivity to methacrolein and further 
makes the catalytic activity durable. The amount of steam incorporated in 
the gaseous feed is preferably in the range of from 1 to 30 moles, more 
preferably from 2 to 10 moles, per mole of methacrolein. 
It is convenient to carry out the catalytic oxidation reaction under 
atmospheric pressure although superatmospheric or subatmospheric pressure 
may be employed if desired. The catalytic oxidation reaction may be 
carried out at a temperature in the range of from 200.degree. C. to 
450.degree. C., preferably 250.degree. to 400.degree. C. The optimum 
reaction temperature is in the range of from 300.degree. to 340.degree. C. 
The contact time is usually in the range of from 0.1 to 10 seconds, 
preferably from 0.5 to 5 seconds. 
The catalytic oxidation reaction may be carried out in a fixed bed, a 
moving bed or a fluidized bed. It is generally preferable to employ a 
fixed bed. This is because the catalyst used in the process of the 
invention not only exhibits a high catalytic activity for the oxidation 
reaction involved even when the oxidation reaction is carried out at a 
relatively low temperature, but also can maintain its activity over a long 
period of time. 
The methacrylic acid produced may be recovered by any convenient manner, 
such as condensation or extraction with a solvent. 
The present invention will be further clarified by the following examples 
and comparative examples, wherein conversion of methacrolein, selectivity 
to methacrylic acid and yield of methacrylic acid were calculated in 
accordance with the following equations. 
##EQU1##

EXAMPLE 1 
300 g of phosphomolybdic acid [H.sub.3 PMo.sub.12 O.sub.40.29H.sub.2 O] 
were dissolved in one liter of water maintained at 80.degree. C. A 
solution of 25.7 g of potassium nitrate [KNO.sub.3 ] in 100 ml of water 
was added to the aqueous molybdic acid solution, while the mixture was 
being stirred. 7.45 g of ammonium metavanadate [NH.sub.4 VO.sub.3 ] and 
2.0 g of tellurium dioxide [TeO.sub.2 ] were successively added to the 
mixed solution. The slurry so obtained was heated, while being stirred, to 
be thereby concentrated and thereafter, evaporated almost to dryness with 
a drum dryer. The product was maintained at 200.degree. C. thereby being 
completely dried. The dried product was shaped into tablets 5 mm in 
diameter and 5 mm in height by using a tableting machine. The tablets were 
calcined at 400.degree. C. for 5 hours in air to prepare a catalyst. The 
atomic ratio of the respective elements (other than oxygen) present in the 
catalyst was Mo:P:K:V:Te=12:1:2:0.5:0.1. 
Ten ml of the catalyst were packed in a tubular glass reactor having an 
inner diameter of 8 mm. A gaseous mixture comprised of, by volume, 4% of 
methacrolein, 10% of molecular oxygen, 30% of steam and 56% of nitrogen 
was passed through the catalyst-packed reactor maintained at 330.degree. 
C. at a flow rate of 150 ml/min. The contact time was 4.0 seconds. The 
catalytic oxidation was continued over a period of 5 hours. Results of the 
catalytic oxidation are shown in Table I, below. 
EXAMPLES 2 THROUGH 14 
Following a procedure similar to that mentioned in Example 1, catalysts 
having the compositions shown in Table I, below, were prepared. Besides 
the starting compounds used in Example 1, cesium nitrate [CsNO.sub.3 ] and 
silver nitrate [AgNO.sub.3 ] were used as cesium and silver sources, 
respectively. 
Using these catalysts separately, the catalytic oxidation of methacrolein 
was carried out under conditions similar to those mentioned in Example 1, 
wherein the reaction temperature was set, as shown in Table I, below. 
Results of the catalytic oxidation are shown in Table I, below. 
TABLE I 
__________________________________________________________________________ 
Conversion 
Selectivity 
Catalyst composition 
Reaction 
of to Yield of 
Example 
(exclusive of oxygen; atomic ratio) 
temperature 
methacrolein 
methacrylic 
methacrylic 
No. Mo P K Cs 
V Ag 
Te (.degree.C.) 
(%) acid (%) 
acid (%) 
__________________________________________________________________________ 
1 12 1 2 -- 
0.5 
-- 
0.1 
330 97.3 75.8 73.8 
2 12 1 2 -- 
0.5 
0.1 
-- 330 95.4 77.3 73.7 
3 12 1 2 0.2 
1 -- 
0.1 
330 94.0 78.6 73.9 
4 12 1 2 0.1 
1 0.2 
0.2 
320 96.8 79.4 76.9 
5 12 1.5 
1.5 
0.5 
0.5 
0.1 
-- 330 93.9 80.1 75.2 
6 12 0.9 
1.8 
-- 
0.4 
-- 
0.4 
330 97.6 76.8 75.0 
7 12 1.1 
1.6 
1.1 
0.6 
-- 
0.1 
320 95.2 78.9 75.1 
8 12 1 2 -- 
0.5 
0.1 
0.1 
320 94.7 76.8 72.8 
9 12 1 2 -- 
0.5 
0.5 
-- 330 95.0 76.1 72.3 
10 12 1 2 0.3 
0.4 
-- 
0.01 
330 98.0 80.1 78.5 
11 12 1 -- 
2 0.5 
-- 
0.1 
330 98.2 74.0 73.6 
12 12 2 2 -- 
0.5 
0.1 
-- 330 94.8 80.3 76.1 
13 12 1 1 -- 
0.1 
0.1 
-- 330 98.6 72.8 71.9 
14 12 2 2 -- 
0.75 
-- 
0.01 
330 94.8 76.7 72.8 
__________________________________________________________________________ 
Contact time: 4.0 seconds 
COMATIVE EXAMPLES 1 THROUGH 5 
Following a procedure similar to that mentioned in Example 1, catalysts 
having the compositions shown in Table II, below, were prepared which 
compositions were outside the range claimed in the present application. 
Using these catalysts separately, the catalytic oxidation of methacrolein 
was carried out under conditions similar to those mentioned in Example 1, 
wherein the reaction temperature was set as shown in Table II, below. 
Results of the catalytic oxidation are shown in Table II, below. 
TABLE II 
__________________________________________________________________________ 
Catalyst composition Conversion 
Selectivity 
Comparative 
(exclusive of oxygen; 
Reaction 
of of Yield of 
Example 
atomic ratio) Temperature 
methacrolein 
methacrylic 
methacrylic 
No. Mo P K Cs 
V Ag 
Te 
(.degree.C.) 
(%) acid (%) 
acid (%) 
__________________________________________________________________________ 
1 12 1 2 -- 
0.5 
-- 
-- 
340 85.4 74.8 63.9 
2 12 1 2 0.2 
1 -- 
-- 
340 82.6 76.7 63.4 
3 12 1 2 -- 
-- 
0.1 
-- 
330 82.3 64.1 52.8 
4 12 1 2 -- 
-- 
-- 
0.1 
330 85.1 60.3 51.3 
5 12 2 -- 
2 1.5 
-- 
-- 
330 70.4 75.3 53.0 
__________________________________________________________________________ 
Contact time: 4.0 seconds 
COMATIVE EXAMPLES 6 AND 7 
Following a procedure similar to that mentioned in Example 1, two catalysts 
were prepared wherein niobium pentoxide [Nb.sub.2 O.sub.5 ] (in 
Comparative Example 6) and tantalum pentoxide [Ta.sub.2 O.sub.5 ] (in 
Comparative Example 7) were separately used instead of ammonium 
metavanadate. The two catalysts contained the respective ingredients at 
the atomic ratios shown in Table III, below. 
Using the two catalysts separately, the catalytic oxidation of methacrolein 
was carred out under conditions similar to those mentioned in Example 1. 
Results are shown in Table III, below. 
TABLE III 
__________________________________________________________________________ 
Compar- 
Catalyst composition 
Conversion 
Selectivity 
ative 
(exclusive of Reaction 
of to Yield of 
Example 
oxygen; atomic ratio) 
temperature 
methacrolein 
methacrylic 
methacrylic 
No. Mo P K Nb 
Ta 
Te 
(.degree.C.) 
(%) acid (%) 
acid (%) 
__________________________________________________________________________ 
6 12 1 2 1 -- 
0.1 
330 89.3 60.3 53.8 
7 12 1 2 -- 
1 0.1 
330 98.1 58.1 57.0 
__________________________________________________________________________ 
Contact time: 4.0 seconds 
EXAMPLE 15 
Using a catalyst similar to that prepared in Example 1 and having the 
composition of Mo.sub.12 P.sub.1 K.sub.2 V.sub.0.5 Te.sub.0.1, the 
catalytic oxidation of methacrolein was carried out continuously over a 
period of 1,000 hours. After the 1,000 hours' operation, no deterioration 
of the catalyst was observed, and the conversion of methacrolein, the 
selectivity to methacrylic acid and the yield of methacrylic acid were 
97.0%, 76.2% and 73.9%, respectively.