Mixed metal phosphorus oxide coated catalysts for the oxidative dehydrogenation of carboxylic acids

A process is provided for the preparation of coated iron phosphorus oxide containing catalysts useful in the oxydehydrogenation of saturated carboxylic acids, wherein the catalysts are prepared by partially wetting a carrier or support in an aqueous silica solution/suspension, contacting the partially wet carrier with a powder of the iron phosphorus oxide catalyst, drying and calcining.

BACKGROUND OF THE INVENTION 
This invention relates to the catalytic, oxidative dehydrogenation of 
saturated carboxylic acids to their corresponding unsaturated acids. More 
particularly, it is directed to the production of unsaturated carboxylic 
acids such as methacrylic acid from saturated carboxylic acids such as 
isobutyric acid utilizing coated iron phosphorus oxide catalysts. 
The production of unsaturated carboxylic acids from their corresponding 
saturated acids using iron phosphorus oxide catalysts, with or without 
various promoters, is disclosed in the art. 
U.S. Pat. No. 3,948,959 discloses the preparation of unsaturated acids by 
oxidation of the corresponding saturated acid using iron phosphorus oxide 
catalysts promoted with Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba. U.S. Pat. 
Nos. 3,634,494; 3,652,654; 3,855,279; 3,917,673 and and 4,029,695 disclose 
the preparation of unsaturated acids and esters from saturated acids and 
esters using iron phosphorus oxide catalysts, containing bismuth and/or 
lead promoters, optionally with other promoter elements, including Mn, U, 
Pr, Ca, Sr, and Cr. Prior art catalysts characteristically have exhibited 
short life and thermal instability. 
Coated catalysts useful for exothermic reactions and a method of coating 
such catalysts are disclosed in U.S. Pat. No 4,077,912. Coated catalysts 
comprising the oxides of molybdenum, phosphorus, arsenic, and copper, 
useful for the production of methacrylic acid from methacrolein, are 
disclosed in U.S. Pat. No. 4,110,369. 
These prior art coated catalysts are prepared by contacting an inert 
support with a liquid, preferably water, to produce a partially wet 
support and rolling the partially wet support in a powder of the catalytic 
material. 
Coated catalysts, prepared by the above technique with either water or 
ethanol as the liquid wetting agent, are disclosed in copending patent 
application U.S. Ser. No. 221,859 by S. E. Pedersen, J. L. Callahan and H. 
F. Hardman directed to Mixed Metal Phosphorus Oxide Catalysts for the 
Oxidative Dehydrogenation of Carboxylic Acids, and assigned to our common 
assignee. 
It is an object of the present invention to provide coated catalysts useful 
for the oxydehydrogenation of saturated carboxylic acids, which coated 
catalysts exhibit improved activity and attrition resistance. 
It is a further object of the present invention to provide a process for 
the production of unsaturated acids from their corresponding saturated 
acids. 
SUMMARY OF THE INVENTION 
We have found that iron phosphorus oxide catalysts, when coated according 
to the method set forth below, using an aqueous solution or colloidal 
suspension of silica containing liquid as a wetting or binding agent, 
results in the improvement of catalyst attrition resistance and physical 
integrity, as well as an increase of catalyst activity over such catalysts 
coated according to the prior art. 
In general, the present invention includes the preparation of catalysts 
containing the mixed oxides of iron and phosphorus coated upon a carrier 
by partially wetting the carrier with a liquid which comprises an aqueous 
solution or colloidal suspension of silica by contacting the carrier with 
the liquid such that at least some liquid is absorbed by the carrier, 
contacting the partially wet carrier with a powder of the iron phosphorus 
oxide to form a mixture, agitating the mixture to form the coated 
catalyst, and drying the coated catalyst. 
The present invention includes the preparation of unsaturated acids by 
contacting their corresponding saturated acids with molecular oxygen or an 
oxygen-containing gas in the vapor phase, at a reaction temperature of 
about 250.degree. C. to about 600.degree. C. in the presence of a catalyst 
containing iron phosphorus oxides coated on a carrier prepared by wetting 
the carrier surface with a liquid which comprises an aqueous solution or 
colloidal suspension of silica by contacting the carrier with the liquid 
such that at least some liquid is absorbed by the carrier, contacting the 
partially wet carrier with a powder of the iron phosphorus oxide to form a 
mixture, agitating the mixture to form the coated catalyst and drying the 
coated catalyst. 
DETAILED DESCRIPTION OF THE INVENTION 
Saturated carboxylic acids are oxidatively dehydrogenated according to the 
process of the present invention in the vapor phase, in the presence of 
promoted iron phosphorus oxide catalysts to form the corresponding 
unsaturated acid. The saturated acids preferably correspond to the formula 
##STR1## 
wherein R.sub.1, R.sub.2, and R.sub.3 are each independently selected from 
the group consisting of hydrogen and alkyl groups containing 1 to 4 carbon 
atoms. The acids may contain other functional groups such as aryl or 
nitrile, provided the functional groups do not interfere with the 
dehydrogenation reaction under the reaction conditions required. The 
dehydrogenation occurs essentially in the alpha, beta position. 
The process of the present invention is highly suitable for the oxidative 
dehydrogenation of isobutyric acid to methacrylic acid. 
Iron phosphorus oxide catalysts may be coated according to the procedure 
set forth above, using as the wetting liquid, an aqueous solution or 
colloidal suspension of silica (SiO.sub.2). Preferred are promoted iron 
phosphorus catalysts have the empirical formula 
EQU A.sub.a Fe.sub.b P.sub.c D.sub.d O.sub.x 
wherein 
A is selected from the group Al, B, Be, Cd, Co, Cr, Ga, Ge, In, Ni, Te, Th, 
Ti, Tl, U, V, Zn, Zr, rare earths and mixtures thereof, 
wherein 
D is selected from the group Ag, Cu, Mn and mixtures thereof, 
and wherein 
a=0-1.0 
b=0.75-1.5 
c=1.0-2.0 
d=0-2.0 
a+d is greater than zero and 
x is the number of oxygens needed to satisfy the valence requirements of 
the remaining elements. 
Preferably a equals 0.15-0.5 and d equals 0.2-1.5. Most preferred are 
catalysts of the empirical formula 
EQU A.sub.a Fe.sub.b P.sub.c D.sub.d O.sub.x 
wherein 
A is selected from the group Cd, Cr, Ge, Te, Th, Ti, U, V, Zr, rare earths 
and mixtures thereof; 
wherein 
D is selected from the group Ag, Cu, Mn and mixtures thereof, 
and wherein 
a=0-1.0 
b=0.75-1.5 
c=1.0-2.0 
d=0-2.0 
a+d is greater than zero and 
x is the number of oxygens needed to satisfy the valence requirements of 
the remaining elements. 
Preferably a equals 0.15-0.5 and d equals 0.2-1.5. Preferred rare earth 
metal promoters are La, Ce, Nd, Sm, Eu, Dy, Ho, Tm, Yb and Lu. 
The catalyst oxide powder may be prepared according to methods known in the 
art. 
One method of preparing the catalyst oxide powder includes introducing a 
compound of iron and a compound containing the promoter element, if any, 
into water and contacting with a phosphorus compound, or the iron and 
promoter containing compound are introduced into an aqueous solution of 
phosphoric acid. Preferably, the compounds used containing iron and the 
promoter elements are soluble in water, and may include salts such as 
nitrates, halides, sulfates, acetates, carbonates, formates and the like. 
The resulting solution or slurry is evaporated to dryness, and the 
resulting solid may be calcined at from about 300.degree. to 700.degree. 
C. Alternatively, the catalyst may be prepared in an organic liquid 
medium. Alternatively, the aqueous solution or slurry can be adjusted to a 
pH of about 5-6 before drying. 
The resulting iron phosphorus oxide catalyst may be ground to form the 
oxide powder which is used in the preparation of the coated catalyst. The 
catalyst may additionally be combined with inert diluents. 
When the catalyst is prepared according to the technique of the present 
invention, the carrier or support material for the catalyst forms the 
inner core of the catalyst. This is preferably an essentially inert 
support and may have substantially any particle size although a diameter 
of at least 20 microns is preferred. Especially preferred in the present 
invention for use in a commercial reactor are those supports which are 
spherical and which have a diameter of about 0.2 cm. to about 2 cm. 
In the preferred procedure of the invention, the support material employed 
is at least partially porous. By this is meant the support material must 
be susceptible to the penetration of liquid. Preferred support materials 
are capable of absorbing at least about 1% by weight of water based upon 
the weight of the support. Suitable examples of essentially inert support 
materials include: Alundum (Norton Company), silica, alumina, 
alumina-silica, silicon carbide, titania and zirconia. Especially 
preferred among these supports are Alundum, silica, alumina and 
alumina-silica. 
The catalysts may contain essentially any portions of support and 
catalytically active material. Preferred catalysts contain about 5 to 
about 50 percent by weight of catalytically active material based on the 
total weight of the support and active material. 
The total coated catalyst of the present invention is conveniently prepared 
by partially wetting the inert support with a liquid which comprises 
silica (SiO.sub.2) in water, such as in an aqueous soution or colloidal 
suspension of silica. The colloidal suspension may be acid or base 
stabilized. The liquid may contain about 5-60% silica by weight, and 
preferably contains about 10-30% silica by weight. 
The partially wet support should contain some liquid, but there should be 
no surface liquid visible. The partially wet support is contacted with a 
powder of the active ingredient composition, and the inert support is 
rolled or agitated in the active ingredients. In one embodiment of the 
invention, the contact between the powder and inert support is easily 
accomplished by placing the support in a closed container, rotating the 
container in an inclined plane and adding portions of the powder. 
Preferably, substantially all of one portion of the powder is coated on 
the support before another portion is added. 
The coated catalyst is dried and then calcined at from about 300.degree. C. 
to 700.degree. C. The final coated catalyst thus prepared generally 
contains about 1% to 10% SiO.sub.2 by weight. The coated iron phosphorus 
oxide containing catalyst prepared according to the process of the present 
invention exhibits excellent physical integrity, high attrition 
resistance, and enhanced activity and selectivity for the 
oxydehydrogenation of saturated carboxylic acids, particularly isobutyric 
acid, compared with coated catalysts prepared with water alone as the 
wetting agent. 
The saturated acids are contacted with the catalyst in the vapor phase, 
together with molecular oxygen. The molecular oxygen is most conveniently 
added as air, but synthetic streams containing oxygen are also suitable. 
In addition to the carboxylic acid feed and molecular oxygen, other gases 
may be added to the reactant feed. For example, steam is preferably added 
to the reactant feed to aid in the reaction, although the mechanism by 
which it does so is not certain. Inert diluents such as nitrogen, carbon 
monoxide, carbon dioxide and argon may also be added. 
The molar ratio of the reactants may vary widely and are not critical. The 
ratios of carboxylic acid:air:steam are in the range of 1:2.5-50:0-50 and 
are preferably 1:3-10:10-30. Diluents may be present in the range of 0-40 
moles per mole of carboxylic acid. 
The reaction temperature may vary widely and is dependent upon the 
particular carboxylic acid and catalyst employed. Normally, temperatures 
of about 250.degree. to 600.degree. C. are employed with temperatures of 
325.degree.-450.degree. C. being preferred. 
The contact time may vary from a fraction of a second to about 50 seconds. 
In fixed bed reactions the contact time is preferably about 0.5 seconds to 
about 10 seconds, and for fluid bed, preferably from about 2 seconds to 
about 20 seconds. The reaction may be conducted at atmospheric, 
superatmospheric or subatmospheric pressure, preferably from about 1 psia 
to about 100 psia, most preferably between about 10 to about 30 psia. 
In the production of methacrylic acid from isobutyric acid, the major 
by-product is acetone (generally about 5-15% yield) which may be removed 
from the product by conventional methods.

SPECIFIC EMBODIMENTS OF THE INVENTION 
Coated catalysts tested in the below examples were prepared according to 
the following procedure. 
Fe(NO.sub.3).sub.3.9H.sub.2 O, the appropriate promoter metal nitrate and 
H.sub.3 PO.sub.4 (85%) were added to water in the amounts necessary to 
provide the molar ratios set forth in the tables below, for each of the 
particular catalysts reported. The solution of the components was 
evaporated to a dry paste with heating and stirring. The paste was dried 
for about 16 hours at 110.degree. C., and the resulting solid was calcined 
for about 2 hours at 540.degree. C. The solid was crushed and ground to a 
fine powder to pass through about 50 mesh (0.3 mm). 
EXAMPLES 1-4 
20 grams of 1/8" (0.31 cm) Norton SA 5209 Alundum carrier was partially 
wetted with 4.5 g Nalco silica sol (10 weight % silica in water). The 
partially wetted carrier was contacted and agitated with 10.8 grams of 
catalyst oxide, which oxide had been powdered (to pass 50 mesh), having 
the empirical formula Ag.sub.0.8 Fe.sub.1.2 P.sub.1.84 O.sub.x and which 
catalyst oxide had been calcined for 2 hours at 540.degree. C. The 
resulting coated catalyst had a firm, hard coating of active material, 
about 35% by weight, and contained about 1.5% by weight SiO.sub.2. 
COMATIVE EXAMPLES 5-7 
The procedure of examples 1-4 was repeated, except that water alone was 
used as the wetting liquid. 
EXAMPLES 8-10 
Coated catalysts of the formula 45% Ag.sub.0.8 Fe.sub.1.2 P.sub.1.84 
O.sub.x /55% Alundum were prepared according to the procedure of examples 
1-4. 
EXAMPLES 11-12 
Coated catalysts of the formula 45% Ag.sub.0.8 Fe.sub.1.0 P.sub.1.84 
O.sub.x /55% Alundum were prepared according to the procedure of examples 
1-4. 
EXAMPLES 13-14 
Coated catalysts of the formula 35% Ag.sub.0.8 Fe.sub.1.0 P.sub.1.84 
O.sub.x /65% Alundum were prepared according to the procedure of examples 
1-4. 
EXAMPLES 15-17 
Coated catalysts of the formula 35% Mn.sub.0.5 Fe.sub.1.0 P.sub.1.84 
O.sub.x /65% Alundum were prepared according to the procedure of examples 
1-4. 
COMATIVE EXAMPLES 18-20 
Coated catalysts of the formula 35% Mn.sub.0.5 Fe.sub.1.0 P.sub.1.84 
O.sub.x /65% Alundum were prepared as in examples 15-17, except that water 
alone was used as the wetting liquid. 
The coated catalysts prepared in the above examples were tested for the 
oxydehydrogenation of isobutyric acid to methacrylic acid in a 20 cc fixed 
bed reactor. The reactor consisted of a length of stainless steel tubing 
having an outer diameter of about 1.3 cm, and containing a full length 
0.31 cm diameter axial thermowell. The reactor was heated with a split 
stainless steel block furnace. 
The isobutyric acid was fed to the reactor by passing air through a 
saturator filled with isobutyric acid and maintained at a temperature of 
108.degree. C. Water was fed by means of a tubing pump and vaporized in a 
compartment maintained at about 154.degree. C. before entering the 
reactor. Liquid products were analyzed on a Hewlett Packard 5710A F.I.D. 
gas chromatograph. Gaseous products were analyzed on a conventional split 
column system. 
The test reactions were run at atmospheric pressure. Reaction conditions 
such as temperature, feed ratios, contact time and catalyst working rate 
(WWH=weight of isobutyric acid/weight of catalyst/hour) are listed in the 
Table below. Results of the tests reported in the Table below are reported 
in terms as follows: 
##EQU1## 
As is demonstrated by the test results reported in the Table, coated iron 
phosphorus oxide catalysts according to the present invention exhibit high 
activity and selectivity in the oxydehydrogenation of saturated carboxylic 
acids, particularly isobutyric acid, to the corresponding unsaturated 
acid. The coated catalysts of the invention additionally exhibit improved 
attrition resistance and physical integrity compared to coated catalysts 
prepared according to the prior art. 
Thus it should be apparent to those skilled in the art that the subject 
invention accomplishes the objects set forth above. It is to be understood 
that the subject invention is not to be limited by the examples set forth 
herein. These have been provided merely to demonstrate operability, and 
the selection of iron and phosphorus-containing compounds, promoter 
element-containing compounds, support or carrier materials, preparation 
techniques, reaction feedstocks and reaction conditions can be determined 
from the total specification disclosure provided without departing from 
the spirit of the invention herein disclosed and described, the scope of 
the invention including modifications and variations that fall within the 
scope of the attached claims. 
TABLE 
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OXYDEHYDROGENATION OF ISOBUTYRIC ACID TO METHACRYLIC ACID 
OVER COATED IRON PHOSPHORUS OXIDE CATALYSTS 
Example 
% Active 
Feed Ratio 
Temperature 
Contact Time 
Methacrylic Acid Hours On 
No. Coating 
IBA/Air/H.sub.2 O 
.degree.C. 
(sec.) WWH % Yield 
% Selectivity 
% Conversion 
Stream 
__________________________________________________________________________ 
Ag.sub.0.8 Fe.sub.1.2 P.sub.1.84 O.sub.x 
1 35 1/4.9/25.3 
406 0.9 0.19 
73.2 74.6 98.2 18 
2 35 1/4.9/25.3 
406 0.9 0.19 
73.5 75.5 97.4 25 
3 35 1/4.9/32.1 
403 0.7 0.19 
75.1 76.7 97.9 48 
4 35 1/5.0.37.3 
403 0.6 0.19 
73.8 76.0 97.1 72 
C 5 35 1/4.9/25.3 
453 0.8 0.18 
51.5 52.7 97.8 2 
C 6 35 1/4.9/25.3 
452 0.9 0.18 
49.6 50.9 97.4 9 
C 7 35 1/4.9/25.3 
454 0.9 0.18 
41.2 45.8 90.0 27 
8 45 1/4.9/26.1 
410 0.8 0.18 
70.4 71.2 98.8 1 
9 45 1/4.9/26.1 
394 0.9 0.18 
70.5 73.7 95.6 25 
10 45 1/4.9/26.2 
415 0.8 0.18 
64.8 68.7 94.3 33 
Ag.sub.0.8 Fe.sub.1.0 P.sub.1.84 O.sub. x 
11 45 1/4.4/33.2 
436 0.7 0.17 
74.0 77.5 95.5 3 
12 45 1/3.2/33.2 
436 0.7 0.17 
68.2 79.4 86.0 27 
13 35 1/5.1/26.2 
442 0.8 0.18 
74.2 76.1 97.6 6 
14 35 1/3.5/33.2 
447 0.7 0.18 
74.1 77.2 96.0 16 
Mn.sub.0.5 Fe.sub.1.0 P.sub.1.84 O.sub.x 
15 35 1/5.1/25.3 
403 0.9 0.28 
70.4 72.0 97.8 7 
16 35 1/4.3/25.3 
404 0.9 0.28 
73.0 76.2 95.7 26 
17 35 1/4.3/25.3 
409 0.9 0.28 
70.8 73.6 96.2 29 
C 18 35 1/5.0/25.9 
409 0.9 0.21 
69.5 72.2 96.2 23 
C 19 35 1/4.7/25.3 
421 0.9 0.21 
66.8 67.7 98.7 48 
C 20 35 1/4.7/32.4 
414 0.7 0.21 
68.8 71.5 96.2 52 
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