GB2116063A

GB2116063A - Catalyst and process for producing methacrylic acid

Info

Publication number: GB2116063A
Application number: GB08307118A
Authority: GB
Inventors: Sargis Khoobiar
Original assignee: Halcon SD Group Inc
Current assignee: Halcon SD Group Inc
Priority date: 1978-12-26
Filing date: 1983-03-15
Publication date: 1983-09-21
Also published as: GB2116063B; IT7951168A0; DE2952455A1; FR2445310A1; GB2040717A; IT1164100B; BR7908476A; MX154396A; GB8307118D0; GB2121309B; NL7909142A; GB2121309A; DE2952455C2; GB2040717B; GB8307119D0; FR2445310B1

Abstract

A catalyst composition useful for the oxidation of unsaturated aldehydes, particularly the oxidation of methacrolein to produce methacrylic acid, consists of the combination of oxides of molybdenum, copper, phosphorus, antimony, and cesium and optionally may include one or more of the elements Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, Nb, W, Mn and rare earth metals including La.

Description

1 GB 2 116 063 A 1

SPECIFICATION

Catalyst and process for producing methacrylic acid This invention relates to a process and catalyst for the vapor-phase oxidation with molecular oxygen of 5 methacrolein to 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 alclehydes by means of molecular oxygen in the presence of a suitable oxidation catalyst. A variety of catalyst compositions have been proposed for this purpose. Many such compositions comprise the oxides of molybdenum and phosphorus in association with 10 the oxides of various other elements, both metallic and non-metallic.

The prior art catalysts generally are found to require elements not included in the catalysts of the present invention or some of the essential elements of the present catalysts are lacking in prior art catalysts.

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, without warning, of beginning a rapid decline in 15 activity. Consequently, an increase in the useful activity of such catalysts has been sought.

Despite the many disclosures of the prior art, an improved catalyst of this type is not developed merely by randomly selecting a group of the many elements which have been disclosed. Small changes in composition may be very important in achieving improved catalyst performance and particularly in optimizing the catalyst composition to suit, not only a specific reaction, but the desired operating conditions also. The point 20 is well illustrated by the improved catalyst formulations to be described hereinafter.

It has been discovered that when using the catalysts to be described to produce methacrylic acid by vapor phase oxidation of methacrolein, it is possible to achieve both high activity and high selectivity for extended periods of time. Broadly, the catalyst composition consists of oxides of molybdenum, copper, phosphorus, antimony, and cesium and optionally one or more elements of Ni, Zn, Ru, Rh, Pd, Pt,As, K, Rb, Sr, Ba, Cr, Nb, W, Mn and rare earth metals including La.

The catalyst composition used in the process of the invention also may be expressed by the following general formula:

M012CUaPbSbeMeOx wherein X is Cs and Z is at least one of Ni, Zn, Ru, Rh, Pcl, Pt, As, K, Rb, Ca, Sr, Ba, Cr, Nb, W, Mn and rare earth metals including La, and where a-e and x indicate the atomic ratio of each component and, when a is 0.05-3, b is 0.1 -5, c is 0.01 -3, cl 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 and more preferably 1-2, while 35 c preferably will be 0.01-1.

Particularly preferred are catalysts where Z is absent and b is 1.2-1.8.

When such a catalyst as has been described is in contact wifth a vaporphase mixture of methacrolein, molecular oxygen, steam, and nitrogen at typical temperatures in the range of 250-400'C and pressures in the range of 0-5 atmospheres, excellent activity and selectivity to the production of methacrylic acid is obtained for extended periods of time.

Catalyst Composition and Preparation -The catalyst of the invention consists of oxides or oxygen-containing compounds of molybdenum, copper, phosphorus, antimony and cesium and optionally may include members of a group of elements designated 45 "Z" below. The catalyst may be represented by the general formula:

M012CUa Pb SbeMeOx wherein Xiscesium anclZ is atleastoneof Ni,Zn, Ru, Rh, Pd, Pt,As, K, Rb, Sr, Ba, Cr, Nb,W, Mn and rare 50 earth metals including La, and where a-e and x indicate the atomic ratio of each component relative to M012 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, x is a value determined by the valence and proportions of the other elements in the catalyst. Preferably, b will be 0.5-3 and more preferably 1-2, while c preferably will be 0.01 -1. Preferred catalysts include those in which component Z is tungsten (with arsenic optional). The catalyst composition maybe regarded either as a mixture of oxides of the named elements or 55 as oxygen-containing 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 unsupported 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 and cesium 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 catalyst precursors is necessary. The use 65 2 GB 2 116 063 A 2 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 hydrochloric or nitric acid, or bases such as ammonium hydroxide, can be used as desired. The resulting powderfrom the evaporation is then thoroughly dried and preferably screened to eliminate large particles which make it difficultto produce uniform compressed shapes, such as pellets. Typically, the powder is passed through a 20-mesh screen (Tyler mesh). 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 again screened, typically to provide a 20-60 mesh size. The dried mixture is then preferably combined with a lubricant, again of any conventional type, 10 such as stearic acid or graphite, and compressed into the desired shape, e.g. pelletized, the compressed shapes typically having heights and diameters of 1/16 inch (1.6 mm) to 3/8 inch (9.5 mm). 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. 300-5000C, preferably 325-4500C) for at least ten hours. In a particularly preferred activation step, the temperature is raised at the rate of 20'C per hour to a maximum of 420'C, preferably 320-400'C, and this temperature is maintained for 8 hours.

It will be understood that the foregoing description regarding preparation of the catalyst in a form suitable for use in a vapor-phase oxidation reaction is merely illustrative of many possible preparative methods, although it is a particularly suitable method and is preferred.

Methods of Operation The catalysts described are generally useful forthe production of unsaturated acids by oxidation with molecular oxygen of unsaturated aldehydes, although the reaction of methacrolein to form methacrylic acid is of particular interest. Other possible starting materials are the monoethylenically unsaturated aliphatic 25 monoaldehydes of from 3 to 6 carbon atoms, such as acrolein, crotonalclehyde, 2-methyl-2-butenal, and the like, or mixtures thereof.

The reaction in which the catalyst compositions of this invention are of particular utility and in which they provide high conversions and selectivities involves contacting the catalyst with methacrolein and oxygen in the vapor phase, preferably also in the presence of steam and diluents. When the catalyst of this invention is 30 used in the vapor-phase oxidation of methacrolein to form methacrylic acid, the oxidation conditions employed are those generally associated with this reaction, although it is preferred thatthe molar ratio of oxygen to methacrolein should be kept at a high value near the flammable range. Once reaction is begun, it is self-sustaining because of its exothermic nature. A variety of reactor types may be employed such as fluid or fixed bed types, but reactors having the catalyst disposed inside a multiplicity of heat exchanger tubes are 35 particularly useful and convenient.

The gaseous feed to the reactor contains appropriate concentration of methacrolein, oxygen and steam and usually an inert gas is also present, such as nitrogen and the like. The oxygen is usually added as such or as air, which may be enriched with oxygen. As mentioned, conventional oxidation conditions can be employed but it is a feature of the catalyst of this invention that 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 balance being the inert gas or gases.

The temperature of the reaction should, for best results, be within the range of from about 270 to 450'C, preferably 280-4000C, and the optimum temperature range is 290 to 3250C. 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 the 50 catalyst-containing tubes with a molten salt bath.

The reaction may be conducted at atmospheric, superatmospheric or subatmospheric pressure. Preferably, however, pressures are employed ranging from atmospheric up to about 8 kg/cm 2 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 maybe recovered by a number of methods well known to those skilled in the 55 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 after the acid-removal step may be recycled to the reaction preferably after removal Of C02 by conventional means, e.g., absorption in aqueous carbonate solution.

The features of the invention will be more readily apparent from the following specific examples of typical 60 catalyst preparation and its use in the oxidation of methacrolein. It will be understood, however, that these examples are for the purpose of illustration only and are notto be interpreted as limiting the invention.

i 1 3 GB 2 116 063 A 3 EXAMPLE 1

Catalyst Preparation In 750 cc of water are dissolved 636 grams of (NH4)6MO7024.4H20. Then 21. 7 grams of Cu(N03)2.3H20 are dissolved in 100 cc of water, 58.4 grams of CsN03 are dissolved in 150 cc of water, 20.5 grams of SbC13 are dissolved in a mixture of 30 cc of water, and 10 cc of concentrated HCI and 34.5 grams of H3PO4 are dissolved 5 in a mixture of 100 cc of water and 50 cc of 58% NH40H solution. These solutions are mixed with 400 cc of 58% NH40H 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 10 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 3/16 inch (4.76 mm) height and diameter in which the catalyst components molybdenum, copper, phosphorus, antimony, cesium are present (by calculation) in the atomic ratios of 12, 0.3,1,0.3 and 1, respectively. The pellets are then activated in an oven by heating them to 1000C in one hour 15 and then raising the temperature gradually at a rate of about 200C 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.

EXAMPLE 2

Catalyst Testing A 150 cc quantity of the catalyst composition of Example 1 is placed in a reactor defined by a 1/211 x 90" (12.7 mm x 2286 cm) stainless steel pipe, the reactor pipe being filled with 50 cc of inert filler (silicon carbide) below the catalyst bed and 100 cc of the inert filler above the catalyst bed in conventional manner to insure uniform temperature contact with the catalyst. Nitrogen-diluted mixtures containing methacrolein, oxygen and steam are fed to the reactor at a pressure of 1.74 kg/cM2 (absolute) and at a space velocity of about 1200 25 hr-1. 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 nitrogen, 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 spectrography using conventional techniques. The average amount of methacrylic acid produced is determined periodically 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 0.15 gm of methacrylic acid per hour per gram of catalyst.

EXAMPLE 3

A catalyst is prepared according to the method of Example 1 except that the phosphorus content is increased to provide a catalyst having the following nominal composition (by calculation):

M012CU0.3PlAb03CS1 OX The catalyst is tested according to the method of Example 2.

EXAMPLE 4

A catalyst is prepared according to the method of Example 1 but having a higher phosphorus content than 45 the catalyst of Example 3, providing a catalyst having the following nominal composition (by calculation):

M 0 1 2CUO.3P2S bo.3CSI OX The catalyst is tested according to the method of Example 2.

The results of the tests carried out on the catalysts of Examples 1, 3, and 4 are summarized in the following Table 1.

1 4 GB 2 116 063 A 4 TABLE 1

Selectivity to Activity Phosphorus Temp. Methacrylic Coeff K Cat Content (a) Hours OC Acid (b) (C) 5 1 1 35 293 77 23 323 74.5 6 3 1.5 150 298 77.6 26 10 296 79.7 26 4 2 26 320 70 10 122 324 65 7 (a) relative to M012 (b) (c) percent of methacrolein reacted which is converted to methacrylic acid a value calculated from experimental data to provide a measure of catalyst activity and derived from the equation:

20 K = F. Q. S. e EIRT where:

F = methacrolein concentration in feed S = space velocity of feed gas Q = conversion of methacrolein E = activation energy, 25,000 k cal/mol. 25 R = gas constant T = absolute temperature As explained in Example 2 the catalysts are operated to provide a constant predetermined yield of methacrylic acid. In order to provide a proper comparison, the performance of a catalyst is reported at the 30 period of time where the catalyst activity has stabilized after an initial break-in period. This may mean, as in Table 1, that the time selected for comparisons is not the same if the catalyst performance differs significantly. A stable period at 150-200 hours could be established readily for the catalyst of Example 3, while the catalysts of Examples 1 and 4 had relatively poorer performance and their activities declined earlier, as indicated by the data. It is clear that the catalyst of Example 3 is superior to those of Examples 1 35 and 4, since it operated consistently at a lower temperature and with higher activity and selectivity after the other catalysts had lost significant activity. Thus, it is concluded that catalysts containing molybdenum, copper, antimony, and cesium are sensitive to the phosphorus content. An optimum level should be found between a phosphorus level of 1 and 2 (relative to M012)- It is believed that the optimum level is located 4() between P1.2 and P1.8, particularly between P1.3 and P1.7- EXAMPLE 5

A catalyst is prepared according to the general method of Example 1 except that the phosphorus level is increased and palladium is included to produce a catalyst having the following nominal composition:

M012CUO.3P2SbO.3CS1 PdO.030x The catalyst is tested under the conditions of Example 2 and the significance of the phosphorus to molybdenum ratio with respect to catalyst performance is shown.

EXAMPLE 6

A catalyst is prepared according to the general method of Example 1 except thatthe phosphorus level is increased and rubidium is added to produce a catalyst having the following nominal composition:

M012CUO.3P1.75SbO.3CSO.5Rbo.50x The catalyst is tested underthe conditions of Example 2 and the significance of the phosphorus to molybdenum ratio with respect to catalyst performance is shown.

k 4k 9 1 0 GB 2 116 063 A 5 EXAMPLE7

Other suitable catalysts in accordance with the invention are prepared according to the general method of Example 1 and have the following nominal compositions:

01 2CU0.21P2S bo.3CSO.3S rO.7N bo.30x 0 1 2CU0.21P1.7S bo.3CSO.3BaO.7M n030X M012CUO.3P1.2SbO.3CS, LaWN i030x M012CUO.3P1 Sbo3CS0.21Ceo.7Zn030x M012CUO.3P1 S160.3CS, Rug., Ox M012CUO.3Po.5Sbo.3Cs, RhO.030x M 0 1 2CL10. 3P1 S bo.3CSO.3PtO.030x M012CaO.3P1.5SbO.3CS1WO.5 The catalysts are tested under the conditions of Example 2 and the significance of the phosphorus to molybdenum ratio with respect to catalyst performance is shown.

Catalysts consisting essentially of the oxides of Mo, Cu, P, Sb, Re and Cs and/or Ca, and optionally one or more of Ni, Zn, Ru, Rh, Pcl, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn and the rare earth metals including La are the subject of our copending application No. 7944392.

Rhenium-free catalyst compositions consisting essentially of the oxides of Mo, Cu, P, Sb and either Ca or both Ca and Cs, and optionallyone ormore of Ni,Zn, Ru, Rh, Pd, Pt,As, K, Rb, Sr, Ba,Cr,V, Nb,W, Mn and 20 the rare earth metals including La are the subject of our copending application No.

Claims

1. A catalyst composition suitable for the vapor-phase oxidation of methacrolein to produce methacrylic 25 acid, said composition consisting of the oxides of Mo, Cu, P, Sb and Cs and optionally one or more of the elements selected from the group consisting of Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, Nb, W, Mn and the rare earth metals including La.

2. The catalyst of claim 1 expressed by the formula:

M012C1laPloSbcMeOx where: a =0.05-3; b = 0.1-5; c=0.01-3; d = 0.1-3; e=0-3; andx=avaluedeterminedbythevaience and proportions of the other elements of the formula, where X is Cs and Z is one or more elements selected from the group consisting of Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, Nb, W, Mn and rare earth metals 35 including La.

3. Thecatalystof claim 2where b = 1-2.

4. The catalystof claim 2 or3where c= 0.01-1.

5. The catalyst of claim 2, 3 or4where e is zero, and b is 1.2-1.8.

6. Anyone of the catalysts specifically disclosed in the Examples as an embodiment of the catalyst 40 claimed in claim 1.

7. A catalyst as claimed in claim 1, substantially as hereinbefore described with particular reference to the Examples.

8. A process for the preparation of methacrylic acid which comprises oxidizing methacroiein in the vapor-phase with molecular oxygen in the presence of a catalyst as claimed in any one of claims 1 to 7. 45

9. A process as claimed in claim 8, substantially as hereinbefore described.

10. Methacrylic acid when prepared by the process claimed in claim 8 or claim 9.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company limited, Croydon, Surrey, 1983.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.