GB2104512A - Process for producing methacrylic acid - Google Patents

Abstract

In the vapor phase oxidation of methacrolein to methacrylic acid with molecular oxygen over a catalyst containing Mo, P, Cu, Sb, and Cs and/or Ca, the amount of methacrylic acid produced per unit weight of catalyst is improved by carrying out the oxidation in the presence of a greater molar ration of CO2/CO than is produced by the oxidation. When unreacted gases are recycled, the ratio of carbon dioxide to carbon monoxide can be increased by selective oxidation of the CO to CO2.

Description

SPECIFICATION Process for producing methacrylic acid It is well known that unsaturated acids, such as acrylic acid and methacrylic acid, can be produced by the vapor-phase oxidation of the corresponding unsaturated aldehydes by means of molecular oxygen in the presence of a suitable oxidation catalyst. A variety of catalyst compositions have been proposed for thins purpose. Many such compositions comprise the oxides of molybdenum and phosphorous in association with the oxides of various other elements, both metallic and non-metallic. Of particular interest with respect to the present invention are the catalysts disclosed in the commonly assigned patents and applications.

British patent specification 2040717 and U.S.

Patents 4,252,682,4,252,681, 4,252,683,4,240,930, 4,261,858,4,261,859, 4,261,860, and 4,271,040.

It has been found that catalysts for oxidation of methacrolein to methacrylic acid have the characteristic property of remaining stable for a period of time and then, rapidly declining in activity. The formulations of such catalysts have been adjusted to extend their useful life and to attain improved production of methacrylic acid from a given amount of catalyst. Now it has been found that the composition ofthe reaction feed gas, and in particular the carbon dioxide content, affects the useful life and productivity of the catalysts.

The art shows little with respect to the effect of the composition of the reaction gases. In U.S. 3,171,859 recirculation of by-product gases is shown to provide a high concentration of carbon oxides in the reaction feed gas when oxygen is supplied as pure oxygen. The reaction disclosed is the oxidation of propylene to acrolein rather than the oxidation of methacrolein to methacrylic acid. No effect of the gas composition on the life of the catalyst is disclosed.

The oxidation of olefins, including isobutylene, is discussed in U.S. 3,883,588. It is stated that diluents are desirable to facilitate control of the exothermic reaction. Carbon dioxide produced by the reaction was suggested, as well as nitrogen from the air. No effect of the use of such diluents on the catalyst performance was suggested.

The oxidation of acrolein to acrylic acid is discussed in U.S. 3,985,800, where it is disclosed that use of a feed gas containing a large fraction of carbon oxides (Example 2) gave results similar to those when nitrogen was the diluent (Example 1).

Although the catalyst was reported to have good life, no effect of the use of carbon oxides was noted and gas composition was among the operating data said to be outside the invention being disclosed.

The adverse effect of including carbon oxides in the reaction feed gas is taught in connection with the oxidation of propylene to acrylic acid in U.S.

4,147,885.

As will be seen in the discussion which follows, including carbon dioxide as a large proportion of the reaction feed gas has now been found to have a beneficial effect on the catalyst life and productivity when employed in the vapor phase oxidation of methacrolein to methacrylic acid over suitable catalysts.

It has been discovered that the productivity of molybdenum-based catalysts in the vapor phase oxidation of methacrolein can be substantially improved by increasing the molar ratio of carbon dioxide to carbon monoxide above that produced by the oxidation reaction. This may be done by adding carbon dioxide from an external source, selectively removing carbon monoxide, or oxidizing carbon monoxide to carbon dioxide.

Broadly, the invention applies to catalysts characterized as molybdenum-based heteropoly acids, such as phosphomolybdates. Preferably, the oxidation reaction is carried out over a suitable catalyst comprising oxides of molybdenum, copper, phosphorus, antimony, and cesium and/or calcium and preferably including one or more elements of Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, w, Mn, Re, and rare earth metals including La. The reaction feed gas will contain 5-20 percent by volume methacrolein, 3-15 percent by volume oxygen, and up to 50 percent by volume steam. The ratio of carbon dioxide to carbon monoxide in the feed gas will be greater than is produced by the oxidation of methac robin.

In a preferred embodiment, fresh oxygen is supplied to the reaction as substantially pure molecular oxygen and the unreacted methacrolein and oxygen are recycled until an equilibrium is established. The carbon dioxide content of the recycled gases is allowed to increase to its equilibrium value while the carbon monoxide is selectively oxidized over a suitable catalyst, in the presence of methacrolein so that the reactor feed gas contains a ratio of carbon dioxide to carbon monoxide greater than is produced by the oxidation of methacrolein, typically in a CO2/CO molar ratio above about 1.

The preferred catalyst composition used in the process of the invention also may be defined by the following general formula: Mo,2CuaPbSbcAdBeOx wherein A is cesium and/or calcium and B is one or more members of the group consisting of Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re, and rare earth metals including La, and where a-e and x indicate the atomic ratio of each component relative to Mo12 and, when a is 0.05-3, b is 0.1-5, c is 0.01-1, d is 0.1-3, e is 0-3, and x has a value which is determined by the valence and proportions of the other elements in the catalyst. Preferably, b will be 0.5-3, more preferably 1-2, and most preferably about 1.2-1.8., while c preferably is 0.01-1.

The invention may be viewed broadly as a process for oxidizing methacrolein to methacrylic acid in which the ratio of carbon dioxide to carbon monoxide is greater than that produced by the oxidation reaction. Practically, this means that the CO2/ ratio is changed by either adding carbon dioxide, removing carbon monoxide, or converting carbon monoxide to carbon dioxide.

The method selected vvill be affected by the form in which oxygen is supplied. There are two principal methods of providing oxygen needed for the oxida tion of methacrolein, namely, as substantially pure oxygen or as air. A combination of the two methods is less likely, but the invention could be adapted to suit such a process.

Since conversion of methacrolein generally is less than one hundred percent, it is probable that any commercial plant will provide for separation of uncoverted methacrolein and its recycle to the oxi dation reactor. It will be recognized by those skilled in the art that such a choice is essentially an economic one in which the full usage of the valuable methacrolein is balanced against the costs of recycling. Thus, although recirculation of a gas containing methacrolein is more likely to be chosen, it is possible that a once-through reactor could be selected for the oxidation. In such a case, the feed would be expected to contain little, if any, carbon monoxide and, thus, carbon dioxide, either alone or in combination with other gases, would be added to the feed gas to extend the useful life and productivity of the catalyst.Oxygen could be provided as molecular oxygen in a once-through reactor, but more likely in the form of air. Any amount of carbon dioxide could be added from some external source with beneficial effect For example, where pure oxygen is used, up to about 60 volume percent of the feed gas could be carbon dioxide, with the remainder being about 20 volume percent steam, 7 volume percent methacrolein and 13 volume percent oxygen.

Where oxygen is fed as air, again carbon dioxide may be added in any amount, but it also dilutes the feed gas since a substantial amount of nitrogen will be present, associated with the needed oxygen. Typically, the nitrogen would be allowed to displace a portion of the carbon dioxide so that a feed gas might contain, for example, 5 volume percent methacrolein, 9.30 volume percent oxygen, 14.3 volume percent steam, 42.9 volume percent nitrogen and 28.5 volume percent carbon dioxide. Since such a composition is more dilute in methacrolein, the mode of operating the oxidation reactor would likely be modified, as compared with the composition in which no nitrogen is present.

It is assumed in the foregoing discussion that essentially no carbon monoxide is present, since no recycling of effluent gases is used in a once-through reactor.

In the more typical situation, unreacted methacrolein will be recovered from the product methacrylic acid and recyled, along with other gases to the reactor. Where oxygen is supplied as air it is necessary to purge an amount of nitrogen equivalent to the amount fed and the concentration of nitrogen will equilibrate to a relatively high level, say about 60 volume percent of the recycle stream. Carbon oxides are produced by the combustion of some methac rolein or other organic materials present in the oxi dation reactor and will build up in the recycle stream until the amount purged with the nitrogen equals the amount of carbon oxides being produced. The total amount of carbon oxides in such a case will be rela tively small, say 4-6 volume percent of the recycle gas.In order to obtain the benefits of additional car bon dioxide it may be preferred to add the carbon dioxide from an external source since the absolute amount of carbon dioxide would be small, even if the carbon monoxide were to be oxidized to carbon dioxide, as will be discussed later. Carbon dioxide would be added to provide a feed stream having a composition similar to that described above where the reactor isto be operated once-through and the oxygen is fed as air.

An alternative and preferred mode of operation would be to use substantially pure oxygen instead of air and to recycle unconverted methacrolein, since little purge of gases is necessary. In such a situation, the carbon oxides build up and equilibrate at rather high concentrations, say about 60 volume percent of the recycle gas. While our data showthatsuch an high concentration of carbon oxides provides an improved catalyst life and productivity relative to the use of nitrogen in the feed gas, it is the purpose of our invention to carry out the oxidation of methacrolein with higher than the natural occurring ratio of carbon dioxide to carbon monoxide. In practice, this may be done by increasing the ratio of carbon dioxide to carbon monoxide in the recycle gas or feed stream.While the ratio will vary depending upon the particular operating conditions in the reactor, the natural ratio normally is found to be about 0.7:1 to 1.5:1. It may be adjusted according to our invention by at least three basic methods. First, carbon dioxide may be introduced from an external source of supply to increase the CO,!CO ratio. Second, carbon monoxide may be selectively removed, as by scrubbing with an adsorbent for carbon monoxide. Third, carbon monoxide may be selectively oxidized to carbon dioxide, which is doubly beneficial.Such an oxidation may be carried out by contacting the recycle gas at a temperature the range of 200-350"C with a suitable catalyst, such as a platinum-containing molecular sieve, which can carry out the oxidation without significantly oxidizing the methacrolein or other hydrocarbons present. Ideally, all of the carbon monoxide would be converted to carbon dioxide, but lesser amounts might be oxidized. In a recirulating system, an amount of carbon monoxide at least equal to the amount made in each pass through the reactor must be oxidized to avoid a buildup of carbon monoxide.

The process of the invention is believed to be broadly applicable to molybdenum-based heteropoly acid catalysts of the sort known in the art and particularly to the phosomolybdates. The effect of carbon dioxide has been found useful in processes for oxidizing methacrolien to methacrylic acid over the catalysts shown in the previous noted commonly-assigned patents.

Those catalysts comprise oxides or oxygencontaining compounds of molybdenum, copper, phosphorus, antimony, and cesium and/or calcium and optionally may include members of a group of elements in group "B" below. The catalysts may be represented by the general formula: Mo,2CuaPbSbcAdBeOx wherein A is cesium and/or cal cium and B is Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re and rare earth metals including La, and where a-e and x indicate the atomic ratio of each component relative to Mo12 and, when a is 0.05-3, b is 0.1-5, c is 0.01-3, d is 0.1-3, e is 0.3, preferably 0.01-3, x is a value determined by the valence and proportions of the other elements in the catalyst.

Preferably, b will be 0.5-3, more preferably 1-2, and most preferably about 1.2-1.8, while c will be 0.01-1.

Particularly preferred catalysts include those in which component B is tungsten or rhenium. Other elements may be included in minor amounts in the catalyst formulation in orderto promote catalyst activity or selectivity, provided that the advantages of the basic formula are retained. The catalyst composition may be regarded either as a mixture of oxides of the named elements or as oxygencontaining compounds of the elements or both. As prepared and/or under reaction conditions, the catalyst may contain either or both forms and both are intended to be included within the phrase "mixtures of oxides".

The catalyst composition is preferably used in supported form e.g. in the form of pellets or other like compressed shapes of various sizes although conventional supports could be employed instead.

The composition may be formed in conventional manner using techniques well known to persons skilled in the art. For example, compounds of molybdenum, copper, phosphorus antimony, cesium, and rhenium are dissolved in a small amount of water or other solvent, and the solutions are then combined and evaporated to dryness, e.g. in a rotary dryer. The several components can be introduced into solution in the form of various salts or other compounds of convenient types and no specific form for the catal ystprecursors is necessary. The use of ammonium salts, halides, e.g. chlorides, nitrates or acid forms of the elements, e.g. phosphoric acid, are however, particularly suitable. Preferably, however aqueous solutions are employed and water-soluble forms of the elements are used.In some cases the solutions may have acids and/or bases added to them to facilitate dissolution of the catalyst precursors. For example, acids such as hydrochloride or nitric acids, or bases such as ammonium hydroxide, can be used as desired. The resulting powder from the evaporation is then thoroughly dried and preferably screened to eliminate large particles which make it difficult to produce uniform compressed shapes, such as pellets. Typically, the powder is passed through a 20-mesh screen. The powder is then mixed with an organic binder which can be of any conventional type, such as polyvinyl alcohol, and the mixture is thoroughly dried and screened, typically to provide a 20-60 mesh size.The dried mixture is then preferably combined with a lubricant, again of any conventional type, such as strearic acid or graphite, and compressed into the desired shape, e.g. pelletized, the compressed shapes typically having heights and diameters of 1/16 inch to 3/8 inch.

Finally, the thus produced catalyst composition is activated at high temperature for a prolonged period in accordance with conventional practice in this art.

For example, the pellets are placed in an oven or kiln, or in a tube through which air is passed, at an elevated temperature (e.g. 300-500"C, preferably 325-450"C) for at least ten hours. In a particu lary preferred activation step, the temperature is raised at the rate of 20"C per hourto a maximum of 420"C, preferably 320-400 C, and this temperature is main tainedfor8 hours.

The oxidation conditions employed are similar to those generally associated with the oxidation of methacrolein, but differ in the intentional use of large proportions of carbon dioxide in the feed gas. It is preferred that the molar ratio of oxygen to methacrolein should be kept at a high value near the flammable range. Steam is employed to improve the selectivity to the desired product.

Methacrolein can be present in concentrations of more than 5 up to about 20 volume percent of the total feed with a preferred range of more than 5 up to about 15 volume percent. In general at least 6 volume percent of the aldehyde is used in the feed. The corresponding ranges for oxygen are 3 to 15 volume percent, preferably 5 to 12 volume percent and for steam up to 50 volume percent, preferably 5 to 35 volume percent. The molar ratio of carbon dioxide to carbon monoxide will be greater than the 0.7/1 1.5/1 typically found in the oxidation of methacrolein, when a recycle process is used according to the invention. Where a once-through process is employed carbon dioxide will be added to the feed gas in any desired amount. The balance will be various gases, such as insert gases or traces of by-products of the oxidation reaction.

The temperature of the reaction should, for best results, be within the range of from about 270 to 450"C, preferably 280-400"C, and most preferably 290 to 325"C. Because the reaction is exothermic, means for conducting the heat away from the reactor are normally employed to avoid a temperature increase which favors the destruction of methacrolein by complete oxidation to carbon oxides and water. The reactor temperature may be controlled by conventional methods such as by surrounding tubes containing catalyst with a molten salt bath. Alternatively other types of conventional reactions may be used.

The reaction may be conducted at atmospheric, super-atmospheric or subatmospheric pressure. Preferably, however, pressures are employed ranging from atmospheric up to about 8 kg/cm2 absolute, preferably up to about 6.3 kg/cm2 absolute, and most preferably up to about 4.5 kg/cm2 absolute.

The unsaturated acid product may be recovered by a number of methods well known to those skilled in the art. For example, the acid may be condensed, or scrubbed with water or other suitable solvents, followed by separation of the unsaturated acid product from the scrubbing liquid. The gases remaining afterthe acid-removal step may be recycled to the reaction to complete the oxidation of methacrolein, although the benefits of our invention may be obtained in a once-through reaction as well.

The features of the invention will be more readily apparent from the following specific examples. It will be understood, however, that these examples are for the purpose of illustration only and are for to be interpreted as limiting the invention.

EXAMPLE 1 Catalyst Preparation In 750 cc of water are dissolved 636 grams of (NH4) 6Mo7O24 AH2O. Then 21.7 grams of Cu(NO3)2 - 3H20 are dissolved in 100 cc of water, 58.4 grams of CsNO3 are dissolved in 150 cc of water, 20.5 grams of Sic13 are dissolved in a mixture of 30 cc of water and 10 cc of concentrated HC1, 5 grams of Re2O7 are dissolved in 100 cc of water, and 34.5 grams of H3PO4 are dissolved in a mixture of 100 cc of water and 50 cc of 58% NH4OH solution.These solutions are mixed with 400 cc of 58% NH4OH and fed to a rotary dryer of 4000 cc capacity and the mixture is evaporated to dryness at a temperature reaching a maximum of 140-200"C. The resulting powder is removed from the dryer and dried in an oven at 200"C for 4 hours.

The dried powder is screened through a 20-mesh screen, a 4% aqueous solution of polyvinyl alcohol is added in sufficient quantity to make a damp mixture and this mixture is dried at 75-80"C until the moisture content falls to 2-4 wt.%. The dried mixture is then screened to 20-60 mesh size particles, and about 2-6% of stearic acid powder is thoroughly mixed with it. The resulting mixture is then pelletized to form pellets of 1/8 inch height and diameter in which the catalyst components molybdenum, copper, phosphorus, antimony, cesium and rhenium are present (by calcuiation) in the atomic ratios of 12, 0.3, 1, 0.3, 1, and 0.07 respectively.The pellets are then activated in an oven by heating them to 100"C in one hour and then raising the temperature gradually at a rate of about 20 C per hour to 370"C and maintaining them at this temperature for 8 hours. The catalyst is tested according to the procedure of Example 2.

Example2 Catalyst Testing A 57 gms quantity of the catalyst composition of Example 1 is placed in a reactor defined by a 1/2" 90" stainless steel pipe, the reactor pipe being filled with 100 ce of the inert filler above the catalyst bed in conventional manner to insure uniform temperature contact with the catalyst. Feed gas mixtures containing methacrolein, oxygen and steam are fed to the reactor at a pressure of 1.74 kg/crrf (absolute) and at a space velocity of about 3800 her' . The term "space velocity" is used in its conventional sense to mean liters of gas (at standard temperature and pressure) per liter of catalyst per hour.The feed composition is approximately, by volume, 6-7% methacrolein, 11-12% oxygen and 20% steam, the balance being various combinations of nitrogen, carbon dioxide, and carbon monoxide determination being made on a wet basis. The reaction is run continuously and the exit gas is analyzed at intervals of several hours.

Analyses are carried out by means of gas chromatography and by infrared spectography using conventional techniques. The average amount of methacrylic acid produced is determined periodi cally and the reactor temperature is adjusted as necessary to obtain the desired yield, that is, the product of the conversion and the selectivity, which for purposes of the comparisons to be made is about 0.042 gm of methacrylic acid per hour per gram of catalyst. The operation of the catalyst is continued until it is no longer possible to produce the desired amount of methacrylic acid, thereby determining the useful life of the catalyst under these severe conditions. It should be noted that this test is not intended to represent conditions for a typical commercial application of such catalysts where an economic balance would govern the severity of operating conditions which are chosen.

Example 3 The results of a series of tests with the catalyst of Example 1 tested according to Example 2 is shown in the following table.

Table I Partial Feed Gas Composition Productivity, Test (Exluding 02, MCHO, H20) gms MAAlgm No. % CO % CO2 % N2 Catalyst 1 - - 60 71.6 2 - 60 - 240 3 6 54 - 197.9 4 30 30 - 92.6 It can be clearly seen that carbon dioxide has a beneficial effect on the catalyst productivity compared to operating with only nitrogen present. Test4 represents a ratio of CO2/CO close to that which typically results from recycling unreacted methacrolein and its accompanying gases when pure oxygen is used (i.e. no nitrogen is introduced). This condition is seen to be more favorable to catalyst life than carrying out the reaction once-through with air, similar to Test 1.To obtain the advantage of our discovery, the feed gas should contain more CO2 than naturally results from the oxidation reaction and preferably the amount of carbon monoxide will be as low as possible.

Examples In a preferred mode of carrying out the invention the oxidation reaction is carried out with recycle of the unreacted methacrolein and with selective oxidation of the carbon monoxide present to carbon dioxide, as shown in the accompanying figure.

The oxidation reaction is carried out in reactor 10, which may be of the tubular type, where the catalyst is formed into pellets and charged to the inside of vertical tubes which are surrounded on the outside by a heat transfer fluid such as the molten salts and special liquids typically used by those skilled in the art. Alternatively, othertypes of reactors could be used, provided that the heat released by the reaction is adequately removed. For purposes of this example oxygen is assumed to be substantially pure and no substantial purge of inerts is shown. It will be understood that a minor purge of inert gases would be likely in a commercial design, but this has not been shown in the simplified figure.

Fresh methacrolein is fed to the reactor 10 via line 12 and oxygen make up via line 14. These gases join the recycle stream 16 and mix before entering the reactor 10. The recycle stream 16 is substantially carbon dioxide, carbon monoxide, unreacted methacrolein, unreacted oxygen and steam, plus minor amounts of inert gases and light reaction by-products. The composition ofthe recycle stream 16 is about 67 volume percent carbon dioxide, 13 volume percent carbon monoxide, 4 volume percent methacrolein, 8 volume percent oxygen, and 7 volume percent steam, + 1% impurities. The amount of steam may be adjusted by control of the quench column 20 and if additional steam is needed it is added via line 18.

The combined feed gas to the oxidation reactor 10 has a composition as follows: 7 volume percent methacrolein, 12 volume percent oxygen, 20 volume percent steam, 50 volume percent carbon dioxide and 10 volume percent carbon monoxide + 1% impurities. The temperature entering the reactor is about 280"C. It will be understood that the heat evolved is removed by circulating a heat transfer fluid (not shown) through the shell side of reactor 10 as is well known to those skilled in the art. The operating pressure is about 1.8 kg/cnf absolute.

The effluent gases pass through heat exchanger 19 for preliminary cooling and then enter quench tower 20 where they are cooled and condensed by countercurrent contact with a recirculating stream, which is substantially aqueous methacrylic acid. The heat of condensation is removed by circulating the liquid via purge 22 through heat exchanger 24 and returning the liquid to quench tower 20. A portion of the liquid is removed as stream 26 and sent to other facilities (not shown) for a recovery of methacrylic acid. The uncondensed gases at a temperature of about 40"C are sent via compressor 29 and line 28 to carbon monoxide oxidation reactor 32. Depending upon the catalyst used, heat may be added in heat exchanger 30 (optional) to raise the gas temperature to the desired level.In this example, a fixed bed of 0.02 wt % platinum disposed in the pores of a molecularseive (described below) is used in reactor 32, which is capable at a temperature of about 300"C of converting about 10% of the carbon monoxide in the uncondensed gases leaving quench tower 20 to carbon dioxide. In a continuous equilibrated reaction system, the gases in line 28 will contain about 13 volume percent carbon monoxide and after the oxidation the gases in line 16 will contain about 11 volume percent carbon monoxide.

The amount of carbon monoxide converted to carbon dioxide will be controlled by the type and amount of catalyst used and the operating conditions. In order to avoid loss of methacrolein by oxidation the catalyst used must be capable of selectively oxidizing carbon monoxide to carbon dioxide without substantial oxidation of methacrolein. One such catalyst employs a precious metal oxidation catalyst such as platinum disposed within the pores of a molecular sieve having pores ofabout4-5Ang- stroms diameter. The pores of such sieves are too small to readily admit methacrolein to enter and be oxidized, but the carbon monoxide molecules can be oxidized to carbon dioxide. Operation at a sufficiently lowtemperature permits the desired selective oxidation of carbon monoxide.It will be understood that the reaction conditions in reactor 32 would be established to convert an amount of carbon monoxide which produces an optimum effect on the performance of reactor 20 in accordance with the principles of our invention. this may be only so much carbon monoxide as is produced in each pass through the reactor or it may be more if it is desired to adjust the ratio of carbon dioxide to carbon monoxide.

Another method of increasing the amount of CO2 in the methacrolein reactor feed gas isto selectively remove CO from the recycle gas. Since the molar ratio of CO/CO2 is naturally about 0.7/1 to 1.5/1 (typically about 1/1) underthe reactor conditions described, when pure oxygen is feed the recycle concentrations would rise to about 30% of each carbon oxide. If CO can be selectively removed, then the CO2 concentration can be allowed to build up to a substantial level. Processes for removal of CO are known in connection with other processes and these may be adapted to use with the present invention.

Examples of processes which may be considered are the cryogenic methods used to recover carbon monoxide from synthesis gas and the extraction of carbon monoxide with a cuprous aluminiumchloride-toluene solvent or other related copper based compounds used for scrubbing flue gases.

Claims (13)

1. A process for gas-phase oxidation of methacrolein to methacrylic acid with molecular oxygen in the presence of a catalyst comprising molybdenum, phosphorus, copper, antimony, cesium and/or calcium and optionally one or more elements selected from the group consisting of Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re and rare earth metals including La, wherein said oxidation is carried out in the presence of a greater molar ratio of CO2/CO than is produced by the oxidation of methacrolein.

2. The process of claim 1 wherein carbon dioxide is introduced from a source external to the oxidation reaction.

3. The process of claim 1 wherein unreacted gases from the oxidation are recycled and carbon monoxide is removed from said recycle gases.

4. The process of claim 3 wherein said carbon monoxide in said recycle gas is selectively oxidized to carbon dioxide in the presence of methacrolein.

5. The presence of claim 4 wherein said selective oxidation is carried out in the presence of a catalyst.

6. The process of claim 5 wherein said catalyst is a precious metal disposed on a molecular sieve.

7. The process of claim 6 wherein said catalyst is platinum disposed on a molecular sieve having pores of about 4-5 Angstrom diameter.

8. The process of any one of claims 1 to 7 wherein said catalyst comprises molybdenum, phosphorus, copper, antimony, cesium and rhenium.

9. The process of any one of claims 1 to 8 wherein said molar ratio of CO2/CO is greater than about 1.

10. The process of any one of claims 1 to 9 wherein the catalyst is defined by the formula Mo,2CuaPbSbcAdBeOx where A is cesium and/or cal cium and B is one or more members of the about consisting of Ni, Zn, Ru, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re, and rare earth metals including La and where a-c and x indicate the atomic ratio of each component relative to Mot2 a when a = 0.05-3, b = 0.1-5, c = 0.01-1, d = 0.1-3, e = 0-3, and x has a value which is determined by the valence and proportions of the other elements.

11. A process as claimed in claim 1, substantially as hereinbefore described.

12. A process as claimed in claim 1, substantially as illustrated in any one of the Examples.

13. Methacrylic acid when prepared by the process claimed in any one ofthe preceding claims.