GB2094782A - Production of alkyl methacrylates - Google Patents

Abstract

A process for the production of alkyl (especially methyl) methacrylates from propylene is provided which involves fewer steps than hitherto. The propylene is catalytically hydroformylated to isobutyraldehyde and then converted by oxidative dehydrogenation to methacrolein; oxidative esterification of the latter in the presence of alkanol gives alkyl methacrylate. The preferred catalyst for oxidative dehydrogenation comprises optionally doped gamma -phase bismuth molybdate and for oxidative esterification comprises optionally doped molybdenum oxide. In an alternative route, alkyl isobutyrate is converted by oxidative dehydrogenation to alkyl methacrylate in the presence of optionally doped gamma -phase bismuth molybdate.

Description

SPECIFICATION A process for the production of alkyl methacrylates This invention relates to a process for the production of alkyl methacrylates, especially methyl methacrylate.

Hitherto methyl methacrylate has been produced commercially by reacting acetone cyanhydrin with sulphuric acid to form methacrylamide sulphate which is then reacted with methanol to produce methyl methacrylate. Acetone cyanhydrin is a sophisticated compound and so it is a relatively expensive starting material. A simple compound is isobutylene (ie. 2-methyl propene) and a proposed alternative process for producing methyl methacrylate comprises the catalytic oxidation of isobutylene. However, isobutylene is ceasing to be abundant and is becoming more expensive.

Alkyl methacrylates may also be produced by costly transesterification processes.

An object of this invention is to provide a simple process for the production of alkyl (for example lower alkyl such as alkyl groups containing from 1 to 4 carbons and especially methyl) methacrylates from a simple, abundant and inexpensive starting material without resorting to a transesterification process.

Accordingly this invention provides a process for the production of alkyl methacrylates (ie.

alkyl 2-methyl propenoate) characterised in that alkyl methacrylate is produced from propylene by catalyic hydroformylation of propylene in the presence of hydrogen and carbon monoxide to isobutyraldehyde (ie. 2-methyl propanal) followed by conversion of the isobutyraldehyde to alkyl methacrylate by performing a catalytic oxidative dehydrogenation in the presence of oxygen to create 1-methyl ethenyl moiety and a catalytic oxidative esterification of formyl moiety in the presence of alkanol and oxygen to create alkyl carboxylate moiety. The discovery that isobutyraldehyde was susceptible to catalytic oxidative dehydrogenation leading eventually to alkyl melthacrylates stimulated the secondary discovery that a simple catalytic route existed from propylene to alkyl methacrylates. Propylene is of course a cheap and readily available product of the cracking of petroleum naptha.

It seems probable that the reaction which creates the methyl ethenyl moiety abstracts oxygen from the oxidative dehydrogenation catalyst and the abstracted oxygen is presumably replaced by oxygen gas diffusing into the catalyst. For this reason it is preferred to use an oxidative dehydrogenation catalyst of the type which imparts oxygen to the oxidative dehydrogenation reaction and permits easy diffusion of oxygen gas into and through the catalyst.

Accordingly it is preferred to use a catalyst such as an optionally doped phase bismuth molybdate (for example Bi203. MoO3 some forms of which are known as koechlinite). The y- phase has an approximately lamellar structure and it is possible that the spaces between adjacent lamellae facilitate diffusion of oxygen gas into the catalyst.

Diffusion of oxygen gas into the catalyst may also be facilitated by doping the catalyst with metals thought to be capable of forming complex oxides with molybdenum trioxide. Such metals include vanadium, chromium manganese, iron, cobalt, nickel, copper, zinc, tungsten, cadmium, antimony, tin and magnesium. It is tentatively suggested that illustrative complex oxides might be: Co203. MoO3 Fe2 03. MoO3 and perhaps NiO2. MoO3 Possibly compex oxides create defects in the lamallae of the a-phase bismuth molybdate through which oxygen gas can diffuse more eaily. The precise concentration of these dopes will depend on the speed at which it is preferred to perform the oxidative dehydrogenation reaction which in turn will depend on the conditions of temperature and pressure chosen and the useful lifetime required of the catalyst.The catalyst may comprise just one of the above listed doping metals or it may comprise a mixture of any two or more. Preferably the total concentration of these doping metals in the catalyst should range from 0 to 1.3 atoms of doping metal(s) per atom of molybdenum. Better results are obtained if the catalyst comprise koechlinite together with some other structure involving molybdenum and hence it is preferred that there be an excess of molybdenum metal which in turn means that most preferably the range of concentration of doping metal(s) should range from 0 to 0.8 atoms per atom of molybdenum.

Possibly metal dopes which can form redox systems (for example the Fe+ +/Fe+ + + system) assist in associating the diffusing oxygen gas with the oxygen-depleted catalyst and hence of the doping metals listed above, those capable of forming redox systems are preferred, namely vanadium, chromium, manganese, iron, cobalt, nickel and copper. Some of the doping metals, especially iron, are throught to promote formation of y-phase bismuth molybdate at the expense of catalytically less active a and phase and hence are preferred. Antimony and/or tungsten appear to improve the selectivity of the catalyst.

Although y-phase bismuth molybdate catalysts perform well, they begin to lose activity and/or selectivity after a few hours operation. For reasons which are not understood, the lifetime of the catalyst can be prolonged by doping with tellurium and/or phosphorus. The concentrations of these two dopes is preferably from 0 to 0.5 atoms per atom of molybdenum.

However, vanadium also has a life-prolonging effect on the catalyst, so if this metal is used as one of the metal dopes mentioned earlier, then lesser concentrations of tellurium or phosphorus may be preferred. Accordingly vanadium is one of the more preferred of the metal dopes mentioned earlier.

The catalyst also appears to benefit from the presence of dopes which possibly neutralise Lewis acid-type sites, namely alkali, alkaline earth and rare earth metals including lithium, sodium, potassium, rubidium, calcium, strontium and cerium. Preferably these dopes should be present in concentrations of from 0 to 0.5 atoms per atom of molybdenum. Potassium is the preferred alkali metal.

Overall, it is preferred that the ratio of molybdenum atoms to all other atoms other than oxygen in the catalyst should range from 1 to 4:1 so as to ensure an adequate proportion of aphase bismuth molybdate. Typical catalysts might comprise components as follows: CcAaBijetPpMomOx where A is one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, W, Cd, Sb, Sn and Mg, C is one or more alkali, alkaline earth and/or rare earth metals.

a is O to 12 mis 10 to 12 b is 1 to 24 (but when a doped catalyst t is O to 2 is used, b is preferably p is O to 2 1 to 4) c is O to 2 and x is the number of oxygen atoms needed to provide notional charge neutrality to the catalyst and x will vary depending on the state of oxysten abstraction or replacement at any particular point in time or location within the catalyst as the oxidative dehydrogenation reaction proceeds. In practical terms, x is the number of oxygen atoms present after the catalyst composition has been for example calcined but during use it could fall by 10 or even 25% or possibly more depending upon reaction conditions.Specific catalyst compositions include: Fe2PO 5BiMo'2ox CrSbBiMo,,O, TeSb2PBiMo12O PTeBiMo100x Fe4VPO,6BiTeO 5MOt2OX Ko 1 Ni4Sn2CrVBiMo120x V2TeBiMo120x In a modification of the catalyst, some or all of the molybdenum content may be replaced by the catalytically analogous antimony and/or tungsten.

Catalytic oxidative dehydrogenation is preferably performed at temperatures of from 300 to 600"C (especially 400 to 500"C) and at pressures of from 1 to 1 5 N/m2. The partial pressure of the organic compounds is preferably 0.1 to 10 N/m2. Preferably the reaction mixture is fed to the catalyst at a rate of from 80 to 200 (especially 100 to 140) mls per minute per 2.5 g of catalyst.

It also seens that one possible mechanism for the oxidative esterification reaction likewise involves abstraction of oxygen from a catalyst and the abstracted oxygen is again presumably replaced by oxygen gas diffusing through the catalyst. Accordingly catalysts based on molyb denumoxides are also preferred for this reaction too. However, the more preferred catalysts have compositions slightly different from the catalysts more preferred for oxidative dehydrogenation. For example a bismuth content is still preferred but less so than for the oxidative dehydrogenation catalysts. Likewise the metal dopes may additionally include aluminium, titanium and/or the metalloids germanium and arsenic. In short the list of preferred dopes comprises.

a) V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Sn and/or Bi b)Al, Ti, Ge, As, Si, Te and/or P c) Sb and/or W which may in whole or in part replace the molybdenum content of the catalyst.

These dopes may in total be present in the catalyst in a concentration of from 0 to 1 atoms per atom of molybdenum. An illustrative preferred catalyst composition is as follows: DdVvSbsMOnOx where D is one or more of Cr, Mn, Fe, Co, Ni, Cu, Sn, Bi, Al, Ti, Ge, As, Te, P and/or W V is vanadium Sb is antimony d is 0 to 12 v is O to 6 (preferably 1 to 4) and nis 12-di(Oto4) x is as defined hereinbefore.

Specific examples include: SbBi3MO3Ox T%.35Cr2P0.7MO7Ox Ni4.5CO4MO12Ox AsNi4,5Co4FeBiMo12ox K02NiCo3V2P2BiMo12Q K06Si135W2FeCo4BiMo12O Mg1,5TeO,1 Ni6Co1 .Fe2PBiMo12O These various dopes possibly modify oxidative esterification catalyst in much the same way as they modify the oxidative dehydrogenation catalyst. Catalysts comprising at least some vanadium appear to be especially effective.

Oxidative esterification using such catalysts is preferably performed under the same range of conditions of temperature, pressure and flow rate as is the oxidative dehydrogenation except that in this reaction at least one alkanol (for example a butanol (including isobutanol) propanol, ethanol but preferably methanol) is present as an additional reactant. The partial pressure of the alkanol is preferably from 0.2 to 10 N/m2.

When using molybdenum oxide catalysts, the presence of the 1-methyl ethenyl moiety promotes the production of the alkyl carboxylate moiety and so the oxidative dehydrogenation reaction should be performed at least momentarily before the oxidative esterification reaction.

Nevertheless it is possible to design a composite catalyst suitable for catalysing both reactions. A preferred composite catalyst would have the following composition: zZYyEeBhSbiVjMOkOx where Z is one or more alkali, alkaline earth and/or rare earth metals, preferably potassium, Y is one or more of Cr, Mn, Fe, Co, Ni, Cu, Sn and/or W E is Te and/or P z is O to 1 y is Oto 12 e is O to 2 h is O to 4 i is O to 12 j is O to 4 k is 8 to 12-ii(Oto 6) and preferably 10 to 12.

x is as defined hereinbefore Specific compound catalysts include Bi2Sb6VMOi2Ox Fe2P0.5BiVMo12Ox Cr4Co4Sb4Bi2VMo1 20x NiTeBiVPMo12O K0,1VNi3Co2Fe3BiPO.5Mo12Px Optionally the catalysts may be carried on a support in order to create a structure of better mechnical strength. Typical supports include silica and a-alumina which has a low surface area.

If for any reason it is required to perform the oxidative esterification before the oxidative dehydrogenation, then this may be carried out using oxidative esterification catalysts known to be suitable for converting saturated aldehydes into saturated alkyl carboxylates. For example the isobutyraldehyde may be catalytically oxidatively dehydrogenated in the presence of oxygen and an alkanol using a catalyst comprising palladous chloride activated by a redox system, for example a system based on oxides, chlorides, carboxylates especially acetates, acetylacetonates or napthenates of chromium, manganese, iron, cobalt or copper. Such systems as more fully described in United States patent specification 3,257,448 and British patent specification 1,575,541 the contents of which are herein incorporated by reference.

The catalytic hydroformylation of propylene in the presence of hydrogen and carbon monoxide to, for example, isobutyraldehyde is easily performed using catalysts such as phosphine complexes of combined platinum tin halides or the carbonyl complexes of cobalt, ruthenium or rhodium optionally activated by the presence of for example, phosphine ligands. Fuller descriptions of the processes are given in pages 1 to 60 of the book "Advances in Organometallic Chemistry" Volume 1 7 edited by F.G.A. Stone and R. West published in 1979 by Academic Press of New York, in articles in ''Industrial and Engineering Chemistry Product Research and Development", the first being by E.R. Tucci on pages 286 to 290 of the edition of 3 September 1969 and the second being by H.F.Schulz et al on pages 1 76 to 183 of Volume 12 No. 3 of 1973 and in British patent specification 1,298,331, the contents of all four of these references being herein incorporated by reference. Hitherto the reaction conditions of such hydroformylation processes have been adjusted to optimise the production of normal butyraldehyde whereas for the present invention it is the isobutyraldehyde which is required.

Hence it may be desirable to re-adjust the conditions to increase the yield of isobutyraldehyde. It is preferred to separate the isobutyraldehyde from the other products.

Alkyl methacrylates produced by the present process have many potential uses for example in the manufacture of polymers and copolymers (especially poly methyl methacrylate or paints.

The invention is illustrated by the following examples.

Example 1 A y-phase bismuth molybdate catalyst was made as follows: 48.5 g of Bi(NO3)3.5H20 were dissolved in 1 50 mls of dilute (10 volume percent) nitric acid.

The solution was added to a dilute aqueous solution of ammonia to cause precipitaion of Bi(OH)3. The precipitate was washed free of alkali. The washed precipitate was slurried, boiled and stirred with 8.2 g of H2MoO4 in 1 litre of water for 10 hours. A pale yellow product was obtained, filtered and dried at 110 C. The dried product was calcined at 450"C in air for 2 hours and then crushed until it could be passed through a British Standard mesh No. 20.

Isobutyraldehyde recovered from a commercial hydroformylation process was mixed with oxygen and nitrogen in the ratio 3:5:12 respectively. The mixture was passed through the catalyst at a rate such that 2.5 x 10-2 g of catalyst was contacted with 1 ml of the mixture per minute. The reaction zone was maintained at 450"C and atmospheric pressure. Catalytic oxidative dehydrogenation of the isobutyraldehyde occurred creating 1-methyl ethenyl moieties and converting the isobutyraldehyde to 2-methyl-2-propenal and achieving a selectivity to the propenal of 55 + 15% and a conversion of 75 + 15%. The selectivity and conversion were reproduced on the same catalyst on at least five successive days when running at six hours per day.

Example 2 A second y-phase bismuth molybdate catalyst was made as follows: 48.5 g of Bi(NO3).5H20 were dissolved in 1 50 mis of dilute (10 volume percent) nitric acid.

The solution was added to a solution of 8.8 g of (NH4)6Mo7024.4H20 in water. The pH was adjusted to 5 with ammonia. The precipitate was filtered, dried at 110"C and calcined at 450"C in air for 2 hours and then crushed until it could be passed through a British standard mesh No.

20 The procedure of Example 1 was then repeated using the second catalyst. This time the conversion to 2-methyl-2-propenal was approximately 16% but the selectivity was 100%. This conversion rate and selectivity were maintained when the process was repeated on five subsequent days.

Clearly by adjusting the catalyst composition it is possible to achieve a pre-selected compromise between conversion rate and selectivity.

Example 3 A y-phase bismuth molybdate catalyst was made as in Example 1 and then used as follows to illustrate the conversion of methyl isobutyrate (ie. methyl 2-methyl propanoate) to methyl methacrylate.

Methyl isobutyrate made by a conventional catalytic oxidative esterification of isobutyraldehyde was mixed with oxygen and nitrogen in the ratio 3:5:12 respecively. The mixture was passed through the catalyst at a rate such that 2.5 x 10-2 g of catalyst was contacted with 1 ml of the mixture per minute. The reaction zone was maintained at 350"C and atmospheric pressure. Catalytic oxidative dehydrogenation of the methyl isobutyrate occurred creating 1methyl ethenyl moieties and converting the methyl isobutyrate to methyl methacrylate achieving a maximum selectivity to methyl methacrylate of 50 + 15% and a maximum conversion of 50+ 15%.

The invention also provides a process for the production of alkyl methacrylate characterised in that alkyl methacrylate is produced from isobutyrladehyde by catalytic oxidative dehydrogenation in the presence of oxygen to create a 1-methyl ethenyl moiety and by catalytic oxidative esterification in the presence of alkanol and oxygen to create a alkyl carboxylate moiety. The invention further provides a process for producing 2-methyl-2-propenal characterised in that the 2-methyl-2-propenal is produced from isobutyraldehyde by catalytic oxidative dehydrogenation of the isobutyraldehyde in the presence of oxygen and a catalyst comprising an optionally doped y-phase bismuth molybdate.The invention also provides a process for producing alkyl methacrylates characterised in that an alkyl 2-methyl propanoate is converted to the alkyl methacrylate by catalytic oxidative dehydrogenation of the alkyl 2-methyl propanoate in the presence of oxygen and a catalyst comprising an optionally doped y-phase bismuth molybdate.

Although the detailed reaction mechanisms involved in the performance of the invention are no boubt complicated, an overall scheme is summarised as follows:

propylene CH2 = CH - CH3 h ydroformylati on CH3 isobutyraldehyde < CH3- H CHO oxidative oxidative dehydrogenation esterificat ion alkanol 1-methyl CH3 ethenyl moiety I CH2=C -C02Alkyl alkyl carboxylate alkyl methacrylate moiety Intermediates may include: : CH3 I 2-methyl-2-propenal CH2 = C - CHO or CH3 I alkyl 2-methyl propanoate CH3 - CH - CO2 Alkyl

Claims (6)

1. A process for the production of alkyl methacrylates characterised in that alkyl methacrylate is produced from propylene by catalytic hydroformylation of propylene in the presence of hydrogen and carbon monoxide to isobutyraldehyde followed by conversion of the isobutyraldehyde to alkyl methacrylate by performing a catalytic oxidative dehydrogenation in the presence of oxygen to create 1-methyl ethenyl moiety and a catalytic oxidative esterification of formyl moiety in the presence of alkanol and oxygen to create alkyl carboxylate moiety.

2. A process as claimed in claim 1 wherein the catalytic oxidative dehydrogenation is performed using a catalyst comprising an optionally doped y-phase bismuth molybdate.

3. A process according to claim 2 wherein the y-phase bismuth molybdate catalyses the conversion of isobutyraldehyde to 2-methyl-2-propenal which is then catalytically oxidatively esterified to the alkyl methacrylate using a catalyst comprising an optionally doped molybdenum oxide.

4. A process for the production of alkyl methacrylates characterised in that alkyl methacry late is produced from isobutyraldehyde by catalytic oxidative dehydrogenation in the presence of oxygen to create 1-methyl ethenyl moiety and by catalytic oxidative esterification in the presence of alkanol and oxygen to create alkyl carboxylate moiety.

5. A process for producing alkyl methacrylates characterised in that an alkyl 2-methyl propanoate is converted to the alkyl methacrylate by catalytic oxidative dehydrogenation of the alkyl 2-methyl propanoate in the presence of oxygen and a catalyst comprising an optionally doped y-phase bismuth molybdate.

6. A process for producing 2-methyl-2-propenal characterised in that the 2-methyl-2propenal is produced from isobutyraldehyde by catalytic oxidative dehydrogenation of the isobutyraldehyde in the presence of oxygen and a catalyst comprising an optionally doped y- phase bismuth molybdate.