CA1194014A

CA1194014A - Process for preparing methacrylic acid

Info

Publication number: CA1194014A
Application number: CA000415415A
Authority: CA
Inventors: Wilhelm Gruber
Original assignee: Roehm GmbH Chemische Fabrik
Current assignee: Roehm GmbH Darmstadt
Priority date: 1981-11-13
Filing date: 1982-11-12
Publication date: 1985-09-24
Also published as: EP0079491B1; DE3145091A1; DE3264684D1; JPS5896040A; JPH0531533B2; EP0079491A1; ES517285A0; ES8307703A1

Abstract

ABSTRACT OF THE DISCLOSURE

A catalyst for the oxidation of isobutyric acid to methacrylic acid in the vapor phase comprises a molybdenum-phosphorus oxide catalyst, optionally containing vanadium, supported on a porous support having a porosity of 10 to 80 % and an interior surface of less than 1 m2/g. The catalyst may optionally contain at least one metal from the first through fourth principal group or second through eighth secondary group of the periodic table. The oxidative dehydrogenation is carried out by contacting a gaseous mixture of isobutyric acid and oxygen with the catalyst at a temperature above 300°C.

Description

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Field of the Invention:
This invention relates to the preparation of methacrylic acid and more particularly to processes for preparing methacrylic acid by oxidative dehydro-~enation of isobutyric acid.

_escription of -the Prior Art:
German Patent 26 33 593 discloses a process for preparation of methacrylic acid by oxidative dehydro-genation of isobutyric acid wherein a catalyst is used which is supported on a carrier comprised of at least 60 % SiO2 having a water absorptivity of at least 60 %. ~o special importance is ascribed to the interior surface of the carrier. The carrier used has a surface area of 2.8 to 370 m2/g.
German Patent 27 22 375 discloses a process of this type but generally no data are given regarding the interior surface of the support. It is apparent that the significance of the interior surface of the catalyst support for the course of the gas-phase oxidative de-hydrogenation of isobutyric acid has hitherto been un-recognized.

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European Patent 15,569 discloses a coated catalyst for gas-phase oxidation processes wherein the exterior surface of a support carries a shell of active catalyst 10 to 1500 ~m thick. The support for the catalyst, which is used at tamperatures of 350C or above, is a non-porous material or a porous material having a pore diameter oE at least 20 ~m . In each case the pore volume does not exceed 10 ~ and the interior surEace does not exceed 5 m2/g. In many cases, the interior surface is less than 0.1 m2/g. The active material is contacted with the support in the form of an aqueous suspension in such a way that it forms a solid outer shell on the support and may be anchored in the pores. The use of this type of catalyst for dehydrogenation has not been disclosed.
Hence a need has continued to exist for an improved catalyst for the vapor-phase oxidative dehydrogenation of isobutyric acid to form methacrylic acid.

SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide an improved catalyst for the oxidative dehydrogenation of isobutyric acid.
A further object is to provide a catalyst for the oxidative dehydrogenation of isobutyric acid which improves the yield of methacrylic acid.
A further object is to provide a catalyst for oxidative dehydrogenation of isobutyric acid which improves the selectivity for methacrylic acid.
Further objects of the invention will become apparent Erom the description of the invention which follows.
These objects of the invention are attained by the use of a catalyst for oxidative dehydrogenation of isobutyric acid to methacrylic acid which comprises a support having porosity of 10 to 80 % and an interior surface of less -than 1 m2/g impregnated with a catalyst comprising oxides of phosphorus and molybdenu~.
The catalyst is prepared by impregnating the porous support with a homogeneous aqueous solution of the active components of the catalyst, drying the i~pregnated support and calcining the dried support.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED
EMBODIMENTS
The catalysts of the invention use a support which has a large pore volume, but which has a relatively small interior surface. This surface is chiefly located in large pores in the interior of the support. Unlike the catalyst oE European Patent 15,569, which is prepared by application of the active materials in the form of an aqueous suspension, and wherein the pores of the suppor~
serve only Eor mechanical anchoring of a thick catalyst coating, in this invention the active material is introduced into the pores in the form of a homogeneous solution. The pores are essentially completely filled with the ac-tive catalytic material. The volume of the ac-tive material introduced, after drying and calcining, should accordingly no-t be larger than the volume of the pores.
The activity of catalysts in gas-phase oxidation processes depends on a great many factors. Besides the variations in the arrangement of the aciive rnaterial discussed above, these factors also include the composition and proportions of the active material relative to the support, the arrangement of the catalyst iD the reactor, the composition of the gaseous reaction mixture~ the reaction conditions and o her factors as well. Because of these multiple factors, there is always a rather broad range of results which is attained with a particular type of catalyst. This applies equally to the values of yield, selectivity and space-time yield. For a catalyst to be an improvement, it is not a necessary condition that -the entire range of results attainable by its use surpass the best values reached by the use of known catalysts. Rather, an improvement is present if the whole range is shifted to a somewhat higher level which overlaps the comparison level but attains higher peak values.
In the process of German Patent 26 33 593 a maximum value of the selectivity of 70 ~ with a conversion of 99 % is attained using a catalyst on a support having a surface of 20 m2/g. ~lis result is not surpassed `when such catalysts applied to carriers having less surface area are used. Also, with the catalysts disclosed in European Patent 13,578 on supports having unknown surface areas, selectivity greater than 70 % for methacrylic acid was not attained.
An increased selectivity was at-tained with the catalysts of German Patent 27 2~ 375, which are prepared from homogeneous solutions of the active ingredients and, for example, diatomaceous earth as a support. The peak values reached a sele~tivity for methacrylic acid of over 70 %, in the best instance 75.6 ~. Diatomaceous earth has a surface area between 2.8 and 20 m2/g depending on the source and the preliminary treatment.
The invention has now made possible a further increase in the selectivity for methacrylic acid to a peak value of over 80 ~ at a conversion of 97 %. These results illustrate the improvement attained by the catalyst of this invention.
The_ ataly3t a) Active ingredients The catalysts of the invention contain as active ingredients phosphomolybdic heteropolyacids, optionally containing vanadium. These compounds have a red to orange color, but change their color when calcined, so that it may be assumed that the heteropolyacid iB no longer present in the prepared catalyst. Methods for preparing ~hese heteropolyac:ids are disclosed, e.g., in German Patent 27 22 375. ~ley are obtained in the form of strongly acid solutions by lengthy boiling of oxides or oxyacids of molybdenum and vanadium in aqueous phosphoric acid. Salts of these heteropolyacids are obtained, e.g., by incorporating other metal compounds such as salts, oxides or hydroxides together with the molybdenum, vanadium and phosphorus compounds mentioned above.
Metal salts which are additional active ingredients can also be added -to the acid solutions of the heteropolyacids. These salts are preferably derived from those acids which upon drying or calcining, or later, under the conditons of the oxidative dehydration, vaporize without leaving a residue. Nitrates and hydroxides are preferred salts. ~xamples of suitable metals are those of the first to the fourth principal groups (A-groups) and second to eighth secondary groups (B-groups) of the periodic table of the elements~
Especially beneficial are the metals K, My, Ca, Ba, Al, Ce, La, Pb, Cu, cr, Fe, Co, Ni. It is possible to treat the support with the heteropolyacid and the metal salt separately, but this procedure i8 not generally practical. However, the separate treatment may be necessary if the addition of the metal salt to the solution of heteropolyacid produces a precipitation or flocculation, since only the impregna~ion of the support with a homogeneous solution is according to the invention.
The atomic ra-tio Mb:V:P in the heteropolyacids in question can lie in the range of ~8 to 12):(0 to 3):1.
The preferred heteropolyacids to be used correspond to t~e formula H5Molov2po4o or H2Mlov2Po38~5- Meta additives are preferably so controlled that one to two hydrogen atoms in these formulas are replaced by one equivalent of metal atoms each. An example of such a composition is CoO 5H4MolOV2PO40 b) Support Suitable support materials having a porosity of 10 to 80 % and an inner surface less than 1 m2/g are known and are commercially available. LiXewise, the methods for determining these values are, in general, known. m e porosity, given in %, is defined as Weight of the water- ~Jeight of the saturated support - dry support . 100 %
Porosity =
Bouyancy of the support in water The interior surface is determined by the BET method.
The preferred values for the pore volume lie between 30 and 70 volume %. The interior surface is preferably less than 1 m2/g, preferably less than 0.5 m2/g. The support o~ten has the form of spheres, cylinders, saddles or granules having a particle size of about 3 to 10 mm. The chemical nature of the support is not critical; however, the support m~st be sufficiently resistant to the loads imposed in the preparation and use o~ the catalyst.
Supports which have proved to be satisactory are made from, e.g., silicon dioxide, aluminum oxide, zirconium dioxide or mixtures of these compounas. Supports having a large proportion of silicon carbide , optionally mixed with silicon dioxide, are especially advantageous.
c~ Process of preparation The amount of active ingredient contained in the catalyst should be as great as possible, howver not so great that the volume of the active ingredient is greater than the pore volume. ~ne most concentrated solution possible of the active component is used. m e impregnation of the support is advantageously carried out under conditions in which the air in the pores can escape. Often mere immersion in the aqueous solution is not sufficient: the impregnation vessel should be at least briefly evacuated one or more times in order to draw the air out of the pores. The solution can also be allowed to trickle over the support mate}ial for an extended period of time.
After a suitable impregnation, the portion of the solution which was not taken up is allowed to drain and f~

then the support is dried. This can be carried out in air, but preferably is perEormed in a drying apparatus, if possible with continuous agitation and air circulation at an elevated temperature, e.g., 50 to lO0~C. The temperature can then be increased to 120C, under vacuum, if necessary. Thereafter the dried material is calcined at 250 to 350C for 2 to 10 hours. After this treatment the catalyst is ready for use. The proportion of ac-tive ingredients can be determined by the weight increase after drying or calcination ~compared to a blank sample). It should not be less than lO ~, calculated as the total weight of prepared catalyst, and preferably amounts to 25 to 50 % by weight.
Oxidative dehydrogenation process According to the invention the catalyst is used in the continuous oxidative dehydrogenation of isobutyric acid to methacrylic acid in the vapor phase at temperatures above 300C, preferably at 320 to 360C.
The reaction gas contains 1 to 2 moles of oxygen, generally in the form of air, per mole of isobutyric acid and 0 to 3 moles of water vapor. If the reaction is carried out, at least in the first stage, in a recirculation reactor wherein a por-tion of the reaction gas leaving the reaction volume is recycled to the feed ga6 stream, the addition of water vapor or other inert gases in addition to the nitrogen fraction derived from the air used for oxidation can be largely or completely 4(~

omitted. A conventional tube reactor can advantageously be attached to the recirculating rea~tor in order to continue the process. If the entire reaction is carried out in one or more tube reactors, the addition of inert gases, such as water vapor, carbon dioxide or nitrogen, is generally advantageous.
In con-tinuous operation, the catalyst can be fed with about 1 x 10 2 to 20 x 10 2 moles of isobutyric acid per kg of catalyst per hour. In a recirculating reactor conversions of 50 to 75 % of the isobutyric acid feed can be obtained, ~hile in a tub~ reactor almost 100 ~ can be attained.
Having generally described the invention, a more complete understanding can be obtained by reference to ~ertain specific examples, which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Examples A) General procedures for catalyst preparation A solution of the heteropolyacid or a salt oE the heteropolyacid having a concentration of 10-80 %, by weight, of catalytically active material, is continuously pumped through a reactor charged with a layer of the prepared catalyst support. The impregna-tion can be carried out from room temperature up to about 100C.
Then the impregnated material so obtained is dried at a ~emperature of about 120DC, in vacuum if necessary.
Before the oxidative dehydrogenation is carried out, the catalyst can be calcined at a tem~erature ranging from ~50 to 350C.

B) Oxidative dehy-lrogenation oE isobutyric acid I. Operation in a recirculating reactor The catalyst to be -tested was introduced into a recirculating reactor having a diameter of 7 cm. The composition and the form of the catalyst, data regarding the interior surface of the support, as well as the experimental conditions and the results obtained are summarized in Table 1 below (Examples 1 to 4). The reaction temperature in Examples 1 to 4 was 340 i 10C.
In each case isobutyric acid and oxygen (as air) were introduced in a mole ratio of 1 : 1.5.

II. Operation in a tube reactor A gaseous mixture of isobutyric acid, water, oxygen and nitrogen in a mole ratio of 1 : 2 : 1.5 : 20 wa~
passed over a catalyst charge of 58 ml in a steel t~be reactor having an inner diameter of 2.5 cm, with a residence time of 0.2 to 0.5 sec and at a reaction temperature of 320 to 350DC. The results obtained (Table

2, Examples 5 to 7) show, in this case also, the clear superiority of catalysts which are prepared with carriers having lesser interior surface, as is established by g~ ~4 comparison with the catalyst of the com~arative Example All catalysts contained the heteropolyacid MolOV2P040 as the catalytically active component.

III. Operation in a small reactor Catalysts having various materials as the support and H5MolOV2P0~0 as -the active material were tested in a reac-tor which contained 2.5 g of catalyst having a particle size of about 2 mm under the same conditions as in II. at a reaction temperature o~ 330C by passing therethrough a gas-phase mixture of isobutyric acid, water, and oxygen (as air) in a molar ratio of 1 : 2 :
1.7 with a residence time of 0.3 sec. The results of Examples 9 to 11 can be seen in Table 3, wherein Examples 10 and 11 are comparative experiments with supports having greater interior surface.
~ aving now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set ~orth herein.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A supported catalyst for the gas-phase oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising a) a porous support having a porosity of 10 to 80 %
and an interior surface less than 0.5 m2/g, and b) supported thereon, a phosphorus-molybdenum oxide catalyst.

2. The supported catalyst of Claim 1 wherein said catalyst additionally comprises vanadium oxide.

3. The supported catalyst of Claim 1 wherein said support contains a major proportion of silicon carbide.

4. The supported catalyst of Claim 1 wherein said catalyst additionally contains a metal selected from the group consisting of metals of the first through fourth principal groups and metals of the second through eighth secondary groups of the periodic table of the elements.

5. The supported catalyst of Claim 4 wherein said metal is selected from the group consisting of K, Mg, Ca, Ba, Al, Ce, La, Pb, Cu, Cr, Fe, Co, and Ni.

6. A supported catalyst for the oxidative dehydrogenation of isobutyric acid to methacrylic acid, comprising:
a) a porous support having a porosity of 10 to 80 % and an interior surface of less than 0.5 m2/g, and b) supported thereon, a phosphorus-molybdenum hetero-polyacid or salt thereof.

7. The supported catalyst of Claim 6 wherein said heteropolyacid additionally comprises vanadium.

8. The supported catalyst of Claim 7 wherein said heteropolyacid is selected from the group of heteropolyacids having the formulas H5Mo10V2PO40 and H2Mo10V2PO38.5.

9. The supported catalyst of Claim 8 wherein a salt of said heteropolyacids is used wherein one to two hydrogen atoms of the acid are replaced by a metal atom.

10. The supported catalyst of Claim 9 wherein said metal atom is selected from the group consisting of metals of principal groups 1 through 4 and secondary groups 2 through 8 of the periodic table of the elements.

11. The supported catalyst of Claim 10 wherein said metal is selected from the group consisting of K, Mg, Ca, Ba, Al, Ce, La, Pb, Cu, Cr, Fe, Co, and Ni.

12. A process for oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising contacting a gaseous mixture comprising isobutyric acid and oxygen with the supported catalyst of Claim 1, at a temperature above 300°C.

13. A process for oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising contacting a gaseous mixture comprising isobutyric acid and oxygen with the supported catalyst of Claim 2, at a temperature above 300°C.

14. The process of Claim 12 wherein said gaseous mixture additionally comprises an inert gas.

15. The process of Claim 14 wherein said inert gas is selected from the group consisting of nitrogen and water vapor.

16. The process of Claim 12 wherein said gaseous mixture contains atmospheric air.

17. The process of Claim 12 wherein said catalyst is contained in a reaction volume and a continuous stream of said gaseous mixture is fed to said reaction volume and a continuous stream of product gas is withdrawn from said reaction volume.

18. The process of Claim 17 wherein a portion of said product gas stream is recycled to said feed gas stream.

19. A process for oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising contacting a gaseous mixture comprising isobutyric acid and oxygen with the supported catalyst of Claim 6 at a temperature above 300°C.

20. A process for oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising contacting a gaseous mixture comprising isobutyric acid and oxygen with the supported catalyst of Claim 7 at a temperature above 300°C.

21. The process of Claim 20 wherein said gaseous mixture additionally comprises an inert gas.

22. The process of Claim 21 wherein said inert gas is selected from the group consisting of nitrogen and water vapor.

23. The process of Claim 19 wherein gaseous mixture contains atmospheric air.

24. The process of Claim 19 wherein said catalyst is contained in a reaction volume and a continuous stream of said gaseous mixture is fed to said reaction volume and a continuous stream of product gas is withdrawn from said reaction volume.

25. The process of Claim 24 wherein a portion of said product gas is recycled to said feed gas stream.

26. A process for preparing supported catalyst for the oxidative dehydrogenation of isobutyric acid to methacrylic acid comprising:
a) impregnating a porous support having a porosity of 10 to 80% and an interior surface of less than 0.5 m2/g with a homogeneous aqueous solution of a molybdenum-phosphorus heteropolyacid or salt thereof;
b) drying said impregnated support, and c) calcining said dried, impregnated support.

27. The process of Claim 26 wherein said heteropolyacid additionally comprises vanadium.