Methyl methacrylate from methallyl alcohol


phosphorus pentoxide for removal of the water of esteri- fication at reflux .... furnace was lagged with sheet asbestos to diminish the tempera- ture ...
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Methyl methacrylate from methallyl alcohol JAMES M. CHURCH AND LAWRENCE LYNN' COLUMBIA UNIVERSITY. NEW YORK 27. N. Y.

T h e feasibility of ohta-ng methyl methacrylate from methallyl alcohol i n satisfactory yielda for a possible commercial process has been investigated. T h e first step of the synthesis consists of a catalytic air oxidation of methallyl alcohol which in readily converted to methacrolein i n good yields. It was found that the best results were obtained at a temperature of about 350' C. using a 50% excess of the theoretical requirement of alcohol. Under these conditions a 71y0 conversion of the alcohol was obtained; this represents a 95y0 theoretical yield of the aldehyde. T h e second step involves a catalytic oxidation of t h e methacrolein to methacrylic acid with oxygen under pressure. T h e maximum conversion of aldehyde was 62y0,representing a 99% theoretical yield based on aldehyde

consumed. T h e best results were obtained at room tempersture, with the oxygen pressure a t 210 pounds per quare inch and in a reaction time of5 hours. T h e final step was an esterification of the methacrylic acid with excess methanol, using phosphorus pentoxide for removal of the water of esterification at reflux temperatures. By this method practically quantitative yields of methyl methacrylate were obtained. T h e favorable yields obtained i n this synthesis, based on isobutylene as the starting material, are most promising for the development of a sound technical process for the manufacture of methyl methacrylate. Based o n this work further study of the process possibilities under conditions more closely approximating actual plant operation is contemplated.

M

with :m estimated produet,ion oi 15,oM),000 pounds of the monomer in 194R (82).

.

ETHYL methacrylate, in its polymeric forms, is one of the most mdely known of all of the present-day synthetic organic plastics. It resulted from the researches of Rohm of Germany performed a t the turn of the century but did not achieve commercial ~ u c c e suntil tho devolopment of a satisfactory procegs by Bauer of Gcrmnny in 1927. Currently it is used almost entirely as the polymer or in numerous copolymers under such trade names as Aeryloid, Vernonite, Lucite, Cryritallitc, and Plexiglas (720,88, 84, 31 ). As a result of its transparency and other excellent properties, polymeric methyl methacrylate has found widespread use for a variety of applications. Its usefulnoss has grown rapidly during the past 20 years, particulady during World War I1 when tremendous quantities were utilized far the transparent sections of airplanes and boats, such as cockpit cnclosnrns, windows, and bomber nases. The demand for polymeric methyl methacrylate in the postwar period has continued a t a n ever increasing rate I

Prssent add-,

C0l-e

Methods of Synthesis Although numerous syntheses have been devised, all thc methyl methacrylate produced in the United States today is made by the acetone cyanohydrin process. This method was originally devclopod in Germany (87) and first appeared in the United States about 1030. I n this process (8, 19, 720, 38) the starting material is acetone; this is prepared either synthetically from propylene or by fermentation along with butanol from cerbohydmteu. Thc acetone is first converted to the cyan* hydrin by B rrartion with hydrocyanic acid, either directly or in situ from sodium cyanide and an acid. The acetone Cyanohydrin is then hydrolyzed to n-hydroryisobutyric acid which is dehydrated t o methacrylic acid and ostorified with methanol in the presence of an acid catalyst to form the methyl methacrylate (31).

Corporation. Clarkwmd. Tez.

768

May 1950

INDUSTRIAL AND E N G INEERING CHEMISTRY

Xlost of the proposed processes are complex, involving a multiplicity of operations and reactions, utilizing expensive starting materials, or resulting in poor yields of the product, all of which prohibit any commercial usage. Recently, a process was suggested (27) which combines acetone, acetylene, and potassium ucetylides in the presence of excess potassium hydroxide to form dimethylhexynediol on subsequent hydrolysis. An oxidation of the diol in the presence of a metal oxide using air highly enriched N ith ozone yields a-hydroxyisobutyric acid on hydrolysis. The final step is a combined dehydration and esterification of the acid witfi methanol in the presence of phosphorus pentoxide for the formation of the methyl methacrylate. Other processes likewise involse a-hydroxyisobutyric acid as the intermediate. In one of these (17, 18) the acid is mixed with acetic anhydride and dehydrated by passing the heated mixture over pumice containing phosphoric acid a t 400" to 600" C. to form methacrylic acid, which in turn is esterified to methyl methacrylate. Another synthesis (20) reacts phosgene with methanol to first form methyl chloroformate. This is further reacted with propylene in the presence of a Friedel-Crafts catalyst to form methyla-cthloromethyl propionate, which when treated with caustic soda forms methyl methacrylate. Similarly the isomeric achloroisobutyric acid may be prepared (20) by oxidation of tertbutyl chloride obtained from isobutylene. The acid is likewise esterified and the chloroester dehydrohalogenated by heating in the presence of anhydrous ferric chloride to form the methyl methacrylate.

oxygen using temperatures of 500' to 550" C. This range of temperature is considerably higher than that employed by Zimakov and Pokrovskii for allyl alcohol. They discouragingly mention that although the conversion was fairly high at the lower temperature, polymerization to the extent of over 6% of the charge occurred and that isobutyraldehyde occurred as a byproduct with the methacrolein. Under certain conditions this by-product yield was as high as 20 to 25%. Bludworth (4), in an alcohol Oxidation patent, states that if methallyl alcohol is mixed with ethanol and both are subjected to oxidation, acetaldehyde and isobutyraldehyde result, but he makes no mention of any methacrolein.

m a lcohol

\

\

/

U

A

L

Oxidation of Methallyl Alcohol Previous Studies. Methallyl alcohol or isobutenol wm first synthesized in 1884 by Sheshukov (30). It remained essentially a laboratory curiosity, attracting very little attention for over 50 years. Today it is being produced commercially in limited quantities by a high temperature chlorination of isobutylene, followed by an alkaline hydrolysis of the resulting methallyl chloride (11). Since the starting materials are inexpensive and readily available and the yields quite good, methallyl alcohol is potentially a low cost chemical and should be available for the synthesis of methyl methacrylate a t a reasonable price if a sufficient market should develop. Since methallyl alcohol possesses the carbon configuration of methacrylic acid, with a carbon-to-carbon double bond similarly located, its structure readily suggests the possibility of oxidation to the acid. A successful oxidation of the alcohol followed by an esterification of the acid with methanol would afford a comparatively direct process for the manufacture of methyl methacrylate. Although several patents (3, 4,18, 32) have been issued covering the oxidation of methallyl alcohol to methacrolein, as well as the oxidation of methacrolein to methacrylic acid, little is disclosed in the literature relative to these oxidations. Some general information is found concerning the oxidation of methallyl alcohol with chemical oxidizing agents such as potassium persulfate or silver oxide, but there is little information concerning the use of air or oxygen for the oxidation (12). Some mention is made of the corresponding allyl compounds, and their similarity to the methallyl derivatives in their physical properties and chemical reactions was t o some extent useful in this investigation (6,6, 36). Perhaps the most important of the researches with the methallyl homologs was that of Zimakov and Pokrovskii (89) who studied the air oxidation of allyl alcohol to acrolein. They found that a mixture of air and allyl alcohol vapor passed through a large bed of finely divided silver a t temperatures of 200' to 240 C. would give a good yield of acrolein at a maximum conversion of 76% of the alcohol. The similarity of the problem of oxidation of allyl alcohol with that of methallyl alcohol would indicate that the similar conditions might be employed for the latter. In the paper by Hearne (12) the use of a silver or copper catalyst is suggested in the oxidation of methallyl alcohol with

769

Figure 1. Laboratory A paratus for Va or Phase Oxidation of d t h a l l y l Alcohof A = Valve

B = Valve and vent line C = Capillary flowmeter

D = Manometer I E = Buret for alcohol feed F = Thermometer G = Electric heater L H = Rheostats M

= Reactor-heater

.k === Condensers Condensate trap Absorptionflasks

-

Alcoholvaporiaer

No information is available on the oxidation of methacrolein to methacrylic acid except in a few patents (3, 19,32) covering variations of this and similar reactions. These patents differ from those dealing with the oxidation of methallyl alcohol to methacrolein in that closer agreement exists between them. They all concur in the use of oxygen a t room temperatures in a solvent medium. Bauer and Weisert ( 3 )claim that the oxidation of methacrolein with oxygen by means of a silver oxide catalyst is not feasible because of low yields, although Herstein (IS) claimed yields up to 90% of acrylic acid from an alkaline solution of acrolein using this catalyst. Staudinger and Tuerck (32) claim a 64% conversion of acrolein to acrylic acid using a temperature of less than 45 O C. and oxygen a t 1 atmosphere, with a mixture of acetone and acetic anhydride as the solvent and vanadic acid as the catalyst. The esterification of acrylic and methacrylic acids with various alcohols is described by Rehberg, Dixon, and Fisher (28, 29). However they present no data for the actual formation of the methyl ester of methacrylic acid. Armitage ( 1 ) and Thoinas and Oxley (33) also suggest various modifications of the equilibrium esterification, increasing the velocity of the reaction by the use of various catalysts. In other instances dehydrating agents are employed to increase the velocity and favor the equilibrium for a more complete reaction. Perhaps the most notable of the latter in which phosphorus pentoxide is employed is described by Porter (27).

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

770

This Investigation. The design of the apparatus used for the oxidation of methallyl alcohol, as shown in Figures 1 and 2, was mo'deled after that used by Faith and Schaible (9) in their studies of crotonaldehyde oxidation. The reactor tube was a 2-foot length of 0.75-inch red brass pipe (85% copper) and red brass pipe fittings. The forward opening was restricted to 0.25 inch for entrance of the air-alcohol mixture and the other end contained an inner brass tube used as a thermocouple well. The side T-tube consisted of a 0.25-inch brass tubing, attached to the brass pipe tee located 3 inches from the end containing the thermometer well, and served as the exit for the reaction products.

1 -2.b

1 . -

+li>

Figure 2.

-

All-Metal Reactor fcr Vapor Phase Oxidation of Methallyl Alcohol

A = 2-inch copper tubing B I / + X 1 1 4 inch brass coupling C = ?It X inch brass bushing D = 1 1 % x 3 1 4 inch brass coupling E = Z/s X 3 1 4 inch brass coupling

= X 3 1 4 X a l a inch brass tee G = sls-inrh copper pipe

F

H = a/,-inch copper pipe I = Copper gauze catalyst

J = Closed-end thermometer well

The air used for this oxidation was supplied by an ordinary air compressor commonly employed in the laboratory. A by-pass tube and valve were placed on the air stream in order to obtain finer control of the rate of flow of air as shown in Figure 1. A mercury manometer was used to determine the static pressure of the air downstream of the by-pass unit. Since the pressure drop in the equipment was very small, the air pressure was never far removed from atmospheric, and the manometer was not really necessary for most of the runs. The manometer was useful, however, in the calibration of the flowmeter, which was placed immediately downstream of the manometer. The flowmeter used consisted of the usual capillary-tube U-tube manometer containing mercury. The flowmeter was calibrated for air by displacement of water, correction being made for the water vapor pressure. As the temperature of the air would not change by more than a few degrees centigrade over the course of time and the air pressure was substantially atmospheric, Whitwell corrections ( 3 7 )were not employed. Following the flowmeter, the air was passed through a 250-ml. three-necked flask which operated as a carburetor or vaporizer. The flask contained methallyl alcohol which could be fed into it either continuously or in small and frequent additions from a 50ml. buret attached to the flask through a rubber stopper. Provision was made to keep the quantity of alcohol, and hence its level in the flask, constant at 100 ml. The rate of feed of alcohol was thus determined by readings of the buret while maintaining a constant level in the flask. The temperature of the alcohol was maintained a t a selected temperature within the range 50" to 70' C. for the desired alcohol-air ratio, by means of an electric heater placed under the flask. The ratio of oxygen to alcohol in the existing vapor was determined by a calculation from the air flo3 and the amount of alcohol charged. From the vaporizer the alcohol-air mixture was passed to the ieaction tube which was heated by means of an electrical combustion furnace. The furnace heat was controlled by means of a rheostat built into the furnace and a fine adjustment rheostat in series with the furnace. The temperature a t the center of the reaction tube was measured with a calibrated thermocouple. That part of the reaction tube which projected beyond the furnace was lagged with sheet asbestos to diminish the temperature differential between the center and the ends of the reactor. The catalyst consisted of a porous copper plug made by rolling up a 1.5 X 10 inch strip of 15-mesh copper gauze. This spiral was placed in the center of the reactor. The exit gases from the reactor were partially condensed in two 12-jnch vertical water-cooled Liebig condensers connected in series. The condensate was collected in a flask immersed in a bath of toluene and dry ice. The noncondensed gases were then passed through three absorber flasks placed in series, packed with glass beads and containing cyclohexanol as the absorber liquid in

Vol. 42, No. 5

the flask. The choice of this organic solvent resulted from a consideration of its high boiling point, low vapor pressure at room temperature, nonreactivity, nonodor, and nontoxicity, as well as its excellent solvent properties for both methallyl alcohol and methacrolein. The noncondensable gases leaving t,he last absorber were measured and vented to a near-by hood.

A group of several preliminary runs not only proved the feasibility of the equipment but also indicated fairly high conversions and yields of product by this method of oxidation. The best results m r e obtained when using an oxygen to alcohol ratio below 1.0 and a temperature within the range of 300" t o 400" C. Accordingly, the next group of runs was performed at. a temperature of 340" to 350" C. to find the maximum conversion and yield for this temperature and to establish the optimum mole ratio of oxygen to alcohol at 0.6 to 0.7. Following this a group of runs was performed keeping the mole ratio fixed within this 'range and varying the temperature over a wide range of 200" to 400" C., confirming the previous temperature of 340' to 350' C. to be optimum. The recovery of methacrolein vas made by a quantitative distillation of the condensate and absorber products. The comparatively low boiling products absorbed in the cgclohexanol included methacrolein, methallyl alcohol, water of reaction, and a small amount of water contained in the original technical methallyl alcohol charged. 911 these materials have boiling points below 115' C. and can be distilled with ease, leaving the solvent behind (boiling point, 161" C.). The condensate collected directly from t.he reactor formed two dist'inct layers, a lower aqueous layer and an upper organic layer rich in methacrolein. The low boiling materials stripped from the cyclohexanol were added to the organic layer of the original condensate for recovery. The two layers were separated and each subjected to a slow analytical distillation through a short packed column. Csing this method of analysis it was possible to obtain sufficient data for an accurate determination of the composition of the products from the reaction. A check of the method was made by the distillation of known compositions of t,wo different,samples.

110

100

5

90

c-

t

2 80 70 60

0

20

'40

60

80 100 Dietillate

120

140

160

180 200

Figure 3. Distillation Curve of Oxidation Products-Methallyl Alcohol to Methacrolein

A lower temperature fraction consisting of the azeotrope of methacrolein and water was f i s t collected a t 63" t o 65' C.; a higher boiling azeotrope of methallyl alcohol and watrr was collected at 92" C . as the second fraction; and finally a fraction containing methallyl alcohol was collected at 114" to 116" C. An intermediate fraction, from about 66" to 91" C., amounting only to about 1% of the total, was equally divided between the two first fractions. il distillation curve of a typical sample is shown in Figure 3. The combined first fractions from the distillations of both the aqueous and organic layers of each run were analyzed for methacrolein by the use of 2,4-dinitrophenylhydraxine on an aliquot sample according to the gravimetric method of Iddlps and co-

INDUSTRIAL AND ENGINEERING CHEMISTRY

May 1950

Table I. Vapor Phase Oxidation of Methallyl Alcohol to Methacrolein (Using air with copper gauze catalyst) Expt. 1 2 3 4 5

;

8 9 10 11 12 13 14 15 16

17 18 19 20 21

T:m6:,

440 430 468 308 284 394 264

Mole Ratio, Oz/Alcohol

Conversion,

%

Preliminary Runs 1.0 0 1.0 Trace 1 .o Trace 1.113 61.5 0.880 69.3 1,130 57.4 1.005 60.6

Variation of Mole Ratio, Oz/Alcohol 341 0.674 70.9 345 69.9 0.462 0,855 67.6 348 344 69.5 0.800 64.5 345 0.996 51.3 344 0.522 65.5 344 0.352 346 60.4 0.295 66.7 346 0.403 341 214 278 236 391

Variation of Temperature ' 0.674 70.9 0,663 1.2 0.632 67.4 0.640 40.9 0.642 65.3

Yield,

%

0 Trace Trace 68.6 77.4 74.4 70 3 95.3 91.6 86.3 91.0 76.5 97.0 93.2 95.2 91.8

'

95.3 99.7 98.1 97.8 90.6

w o kers ~ (14, 15). The 2,4-dinitrophenylhydrazine readily combines with methacrolein to form the methacrolein-2,4-dinitrophenylhydrazone which appears as a flocculent orange precipitate. The melting point of hydrazone samples ranged from 180" to 182' C., corresponding to the reported 182' C. of Fuson and Shriner (IO). Likewise the higher boiling azeotrope fraction was tested according t o the method of Zappi (58) for the determination of methallyl alcohol. This method involves a titration with standard bromine solution in the cold and absence of light, with a back titration of the excess bromine with standard thiosulfate solution. The chemical analysis agreed with the distillation analysis within I % in both cases. The results of the investigation of the oxidation of methallyl alcohol are summarized in Table I.

Oxidation of Methacrolein to Methacrylic Acid

1

771

gases. Thus the vapors, after passing through the reflux condenser, were sent through two nitrogen flask absorbers containing the Rame solvent as used in the reaction. The unabsorbed gases were then passed through a 15-inch glass helical coil which w w immersed in a dry ice and toluene bath for final condensation of any organic vapors. The following catalysts were employed in the liquid phase oxidation experiments: vanadium pentoxide, cobalt acetate, cobalt acetate (60.7%) and copper oxide (39.3%), cobalt acetate (22.4%) and copper acetate (77.6%), copper acetate (80.3%) and copper oxide (19.7%). The catalysts were employed in concentrations ranging from 1.9 to 4.8% based on the weight of methacrolein; the temperature range was 22" to 33" C. and the aldehyde concentrations in the solvent were from 35.5 to 43%. The time of oxidation, approximately 3.5 hours, was the same for all runs. The results of these experiments are presented in Table I1 showing the amounts of acid formed as determined by titration of the total acidity with standard alkali. In the second attempt a vapor phase oxidation of the aldehyde was tried because of the poor results of the liquid phase air oxidation. The same vapor phase apparatus used for the alcohol oxidation was employed with minor modifications. After several u n s u c c e s s f u l runs attempts were discontinued since the results definitely indicated that this method of oxidation was not suitable for methacrolein. The third method of oxidation involved the use of oxygen rather than air under pressure in an autoclave-type e x p e r i m e n t . A standard Parr hydrogenation bomb mounted in a shaker-type mechanism and equipped L2 . 4 with an electrical heater was Figure 4. Liquid Phase Reemployed. Agitation was acactor Oxidation of Methacrocomplished by means of a lein motor which imparted a Glass internal cooling coil not shown rocking motion to the bomb. A 100-mI. glass flask was used as a liner for the bomb; this required that the shaker autoclave be tilted to a more vertical position in order to prevent spillage. The temperature was measured by a thermometer placed in the thermometer well which projected into the flask. and the oxygen pressure was measured with a Bourdon-type pressure gage. The first of the experiments was performed using 2.7% copper acetate, based on the weight of the methacrolein, as the catalyst,

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Three distinct procedures were attempted for the oxidation of the aldehyde to the acid with a different type of equipment employed for each. The first attempt involved a liquid phase oxidation at substantially atmospheric pressures using air as the oxidizing agent. The air, taken from an air compressor was sent through a purification system similar to that used for the oxidation of the alcohol. The air was measured by passage through a capillary flowmeter before entering the reactor shown in Figure 4. The reactor was a 2 X 10 inch glass test tube equipped with a reflux condenser, thermometer well, gas inlet tube with a porous distributor, internal U-tube for a cooling coil, and a propellor-type agitator. The methacrolein and a selected solvent were first charged to the reactor along with the catalyst. The air used in the oxidation, on leaving - the distributor at the bottom of the reactor, passed up through the mass of liquid and added considerTable 11. Liquid Phase Oxidation of Methacrolein to Methacrylic Acid ably to the agitation provided by the (Air a t atmospheric pressures) stirrer. On completion of the run hydroAverage, Catalyst Aldehyde quinone was added to prevent polymeriTemp., Concn.a, Concn., Yield, Expt. C. Catalyst Compn. % Solvent % 5% zation of any remaining methacrolein as 1 32.4 Vanadium pentoxide 4.8 Acetic acid 35.5 0well as the acid formed. In the one 2 23.9 Cobalt acetate 1.9 Acetic acid and 43.1 0 anhydride experiment where the inhibitor was 3 24 Cobalt acetate, 60.7; COP3.0 Toluene 57.7 0 omitted the formation of a white solid per oxide, 39.3% 4 22 1 Cobalt acetate, 22.4%; 2.3 Toluene 43 Trace mass of polymer resulted. copper acetate 77.6% 5 32.8 Copper acetate, ' 80.3%; 2.3 Toluene 44 2 0 Since the air removed considerable copper oxide, 19.7% methacrolein, an efficient recovery By weight based on methacrolein. system was employed to wash the exit

INDUSTRIAL A N D ENGINEERING CHEMISTRY

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Table 111. Liquid Phase Oxidation of Methacrolein to Methacrylic Acid

Expt. 1

2 3

Time, HI.Min.

(Oxygen under Pressure) PresCatasure, Concn. lyst Lb./Sq. AldeTemp., Cone.=, Inch hyde, C. yG Abs. 70

4-35 4-30 0-45

33.5 32.8 28.0

hliacellaneous 2.7 60.7 1.6 180.9 2.4 188.7

5-10 5-10 5-10 5-15 040 5-15 5-10

27.5 3 7.1

37.5

22 .. 22 2.2 2.4

40.3 33.1

2.2 2.2

7.-.0 1-0 5-10 3-0

Variation of Time of Reaction 38.8 2.2 160.7 63.6 32.0 2.2 160.7 63.; 39.9 2 160.7 63..1 32.0 2 .. 22 l5Q.Q 63 5

48.4

63.6 61.4

Conversion,

Yield,

t0.3 LRc8

lOoc.0

70

70 b

Yariation of Pressure 4

5 6

7 8 9 10 11 12 13

14

39.9 25.1

2.4

65.8 113.8 160.7 211.4 274.2 193.3 183.9

63.; 63.: 63.0 63.6 63.6 63.5 63.5

42 28 .. 83 04.0 61i7

100.0 98.5 90.3

60.1 57.8

b

%.O b

61.1 14.9

96.5 99.6

54.0 45,Q

99.3 99.1

a Based on weight of methacrolein~ catalyst for rnnn 1 and 2 was CnAca; for run 3 a mixed catalyst of CuA& SiAcz, and AgO (69:26:5 by wt.); for remainder of runs a mixture of CuAcz and ?ii.icz (73:27 by wt.). Yields not determined; assuined to be practically quantitative hased on high conversion and corresponding yields of other experirnente. Explosion occurred; pressure and teinperatuve extremely high iustantaneously; conversion and yield presunied nil.

,

since it had proved to be the best of the liquid phase air oxidation catalysts previously investigated. A concent,ration of about 50y0of methacrolcin was employed using benzene BY the solvrnt. The temperature was initially 28' C. nnd slight,lyduring the course of the reaction. The average temperature over a, period of some 4 honrs and 35 minutes of the iwwtiori was 33.5' C. Khile t h e pressure stayed fairly constant a1 a n average of 70 pounds. Under these conditions the conwrsioii was only 10.3y0,as measured by titration of the acid formed. In the next, run with the aldehyde concentration raised to 63.57, and the pressure increased to 180 pounds, the reaction was allowed to continue for 4.5 hours. With the incw?ased concentration and pressure a much higher conversion, 25.87,, \?-a? obtained. In the following run 2.49, of a mixed rat,alyst composcd of 68.8% copper acet,ate, 26.5% nickel acetate, and 4.7% silver oxide TTas employed. Under almost identical conditions as in the previous run, an explosion occurred which fortunately did little damage to the equipment. On examination of the bomb it was found to contain shiny black flocks o f carbon adniixed n-ith the broken glass of the flask. S o trace of the odor of met,hacrolein was not,iceable. Because of the apparent promoter action of the silver oxide it was decided to use mixed acet,ates of copper and nickel alone in the following runs. The catalyst chosen was a mixture of 73% copper acetate and 277, nickel acetate with the catalyst concentration fixed a t 2.2 to 2.4%, based on the weight of methacrolein. The concentration of methacrolein remained the same as before, 63.5% by weight, but toluene \vas substituted in place of b e n z e n ~ as the solvent. Likewise the temperature increased, becauw of the exothermic reaction, from room temperature to a maxiniuiii of 50" C. over a period of about 5 hours. The pressure was varied over the course of several expcriiiieiiti from 65 to 210 pounds with iiirreasing conversion at, each higher pressure and an essentially constant yield of acid resulting. However a t a higher pressui'e, 2 i 5 pounds, another explosion occurred, evidently due to excessive oxidation. Thereafter the oxygen pressure n-as not allowed to exceed 210 pounds. Chemical analysis of the product,s of oxidation of niet,hacrolein, using the two methods cited for unreacted methacrolein, checked within o.770 of each ot,her. The first method involved the quantitative precipitation of the 2,4-dinitrophenylhydrazone

Vol. 42, No. 5

derivat,ive of inethacrolein Kith a melting point check for ident,ification and the other method was a titration with bromine according to the met,hod of Zappi (38). The acid in turn was determined b y titration with standard alkali and confirmed by the formation of the amide according to t,he procedure of Fusoii and Shriner ( I O ) which gave a melting point of 106O C., corrcsponding to that of the amide. An alternate procedure was employed which involved the 6ep:iration of t,he solvent and residual methacrolein from the acid by fractional distillat,ion. Since the boiling points for t8healdehgdc, 7 3 " C., and for thc solvent,, benzene, 80" C., or toluene, 111' C., are compai~ativelyloner than the 163" C. boiling point, of the acid, this separation n-as easily accomplished. The first fract,ion \vas, analyzed for aldeliyde using the hydrazone method nientioned above, and the acid in the residue fraction was determined by the bromine unsat,uration test, obtained from 11'Alelio ( 7 ) . The results of these pressure oridation experiments are s h o w in Table 111.

Esterification of Methacrylic Acid Two esterification experiments were performed using the methacrylic acid obtaincd from t,he oxidation of mct.liacrolt:in. The esterification Jyas peyfornie,j using a flask equipped :t reflux column and employing 2S07, excess of methanol ovcr tlmt, required theoretically. Phosphorus pentoxide was added in sufficient quantit,y t,o combine with thc witel' of reaction to ensure a complet,e esterification. During the course of t,h(> reaction the phosphorus pcntoside is converted to orthophosphoric acid and is recovered as a residue on removal of t h e ester product by distillat,ion. The niixt,ure of niet,hanol, niethscu-ylic acid. and phosphorus pentoside \vas refluxed gently for about 4 hours, after which the excess inethanol n;as slowly ICmoved by dist,illatioii a t a teniperat,ure of 64' to 66" C. Thc iiiain product, methyl niet,hacryla,te, \vas recovered at OT t o 101 C. as an almost water white, sweet, sniclling liquid, vliic-h could readily be polymerized, by warming with ;t irace of benzoyl peroxide, to a hard transparent polymer. I'ractically quantitative yields, 98 and 99%, respehvely, of l h e ester were obtained in the t,noruns, as shown in Table I T T . O

O

Table IV. Esterification of Methacrylic Acid to Methyl Methacrylate Mole Ratio, Expt. 1 2

Mole Ratio, Alcohol/Acid 3.50 3.50

PzO~

Siethacrj-lie Acid 0.333 0,333

Time Ilour$ 4 4

Yield,

% 98.8

97.9

Discussion of Results Methacrolein may be formed readily by the oxidation of niethallyl alcohol in fairly high conversions and excellent yields as s h o w in Table I and Figure 5 . The aldehyde was easily iccovered as a water azeotrope by dist,illation of the reaction products and collected over the range of 62.5" to 65.5" C., as indicated by the distillation curve of Figure 3. Although Bludworth (4)and IIearne (I?) reported considci,;ible isobutyraldehyde, no appreciable qumtity of the isomeric aldehyde was obtained in any of thn expei,iinorits, judging from the fact that the first drop in ilio analytical distillation came ovcr above 62" C. The azeot,rope of isohutyrnldchyde and \vatcar boils a t 57" C. Moreover had any isoi)uty-raldehyde foi.rned it would have been converted t o the acid in the next rcaction and the presence of isobutyric acid, which possesses a11 unmistakablo rancid odor, was not detect,ed among the products obtained. In addition to the formation of isobutyraldehyde, Hearne (18) also reported some difficulty with the polymerization of the

INDUSTRIAL AND ENGINEERING CHEMISTRY

May 1950

methacrolein. At no time during any of the experiments yielding methacrolein did polymerization occur. The addition of hydroquinone to the condensate trap and the cyclohexanol of the absorber flasks was probably instrumental in preventing polymerization in these sections of the apparatus. Perhaps polymerization Ras retarded in the reactor because the reactor was constructed of brass containing 85% copper, which is a known polymerization inhibitor, whereas Hearne used a glass reactor. 100 80

2U

60

d E

1 40

c

6 20 0 200

220

240

260

280

300

Temperature,

' C.

320

340

360

380

Figure 5. Effect of Temperature in Oxidation of Methally Alcohol Vapor phase; air-copper Catalyst; upper curve, yield; lower curve, conversion

w

The conversion and yield of methacrolein are plotted against temperature in Figure 5 . Using a mole ratio of oxygen to alcohol of between 0.63 and 0.67 the maximum conversion obtained was 70.9% a t 341" C. The optimum conversion temperature under the fixed conditions used appears to be within the range 300" to 350" C. Not much variation in the conversion is shown within this range. Below 280" C. the conversion falls off rapidly as. the temperature is decreased, with practically no conversion at 212" C. Above 380" C. the conversion likewise declines as the temperature is raised. The yield on the other hand is most favorable a t the lower temperatures with the yield decreasing from 99.7% at 214" C. to 95.3% a t 341" C. and decreasing further to 90% a t 390" C. At the maximum conversion of 70.9%, however, the yield is still quite high, amounting to over 95% of theory of aldehyde consumed. The low conversion of the alcohol to the aldehyde at the lower temperatures indicates that under these conditions there is little activation of the alcohol group. On the other hand, extremely high temperatures promoted the decomposition of both the alcohol and aldehyde, resulting in the formation of a number of lower oxidation and pyrolysis products which account for the low yield. In one of the preliminary runs, shown in Table I, when a temperature of 468" C. was used, there was considerable evidence of decomposition occurring. This is in agreement with the results reported by Peytral (26) who found that allyl alcohol, when passed through a platinum tube a t very high temperatures, formed not only acrolein but also propylene, ethylene, methane, hydrogen, carbon monoxide, and carbon dioxide. Thompson and Frewing (34) likewise found that, acrolein underwent pyrolysis a t high temperatures, 490" C., decomposing to form ethane, ethylene, carbon monoxide, carbon dioxide, carbon, and hydrogen. The same would be expected of methacrolein because of its similar structure. Nevitt et al. (66) concluded from their studies that the presence of a side chain increases the ease of decomposition of an aldehyde. The graph of Figure 6 shows the variation of the conversion and yield of methallyl alcohol to methacrolein, with different mole ratios of oxygen to alcohol. When employing a low mole ratio of oxygen to alcohol, of 0.3 to 0.4 a t a temperature of 340' to 350" C., the conversion was low, but the yield of aldehyde

773

was exceptionally high. This may be the result of insufficient oxygen for any excessive oxidation or degradation at the temperature employed. However, with mole ratio above 0.7 the yield falls off very sharply because of increased tendency toward overoxidation. The conversion rises rapidly a t f i s t as the mole ratio is raised from 0.4 to about 0.6, where it becomes a maximum for a 70% conversion, then decreases a t higher ratios. Only one of the three different procedures attempted in the oxidation of methacrolein to methacrylic acid gave satisfactory results. In the fist method, involving a liquid phase air oxidation, only one of the experiments gave any acid product-that using a mixed acetate catalyst where a trace of the acid was detected. A vapor phase oxidation was attempted as a second method but no yield of acid was obtained. Instead a violent decomposition occurred resulting in complete oxidation of the methacrolein. From the results obtained in the liquid phase air oxidation, shown in Table 11, it is apparent that under the conditions employed, with substantially atmospheric pressures and temperatures below the boiling point of methacrolein, insufficient activation of the aldehyde group is obtained. On the other hand, in vapor phase oxidations a t the higher temperatures, overactivation with excessive oxidation occurred. Thus, in the latter case it was obvious that the conditions were too extreme, and it would appear unlikely that any favorable results could be expected from a vapor phase type of oxidation of methacrolein.

100

se

$

90

j;

2

80

e a

0

'E

E s

70

60 0.2

0.3

0.5 0.6 0.7 Mole Ratio, Oxygen-Alcohol

0.4

0.8 .

0.9

1.0

Figure 6. Effect of Mole Ratio of Oxygen-Alcohol i n Oxidation of Methallyl Alcohol Vapor phase; 350° C.; air-copper catalyst

Recourse was then taken to a third method where oxygen was employed under pressure in a liquid phase type reaction which proved most successful, giving high yields of the acid. In the fist of the experiments with this method a rather low conversion was obtained with the temperature a t 33" C., a low pressure of 70 pounds, and a reaction period of 4.5 hours using copper acetate LLS the catalyst. When the pressure was increased to 180 pounds a higher conversion of 25.8% was obtained, resulting in practically a quantitative yield of acid on the basis of consumed aldehyde. In order to increase the conversion further, an experiment was performed using a mixed catalyst of copper and nickel acetates containing 4.7% of silver oxide. This was similar to the catalyst of Dinelli (8) which contained a trace of silver oxide for promoting the oxidation of furfuraldehyde to pyroinucic acid. It was hoped that with this type of catalyst, increase in the activity of the system for a higher conversion might be achieved. Such was the case, but the result was a mild explosion with not a trace of the odor of methacrolein remaining on opening the autoclave. The pressure and temperature were increased instantaneously just previous to the explosion which destroyed the pressure gage when the pressure exceeded the 600-pound capacity of the instrument. The presence of a shiny black mass

I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY

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of carbon among the broken glass in the autoclave was evidence

of the extreme temperature which must have resulted. It was quite evident that this type of catalyst rvas much too active for a well controlled oxidation of methacrolein.

Vol. 42, No. 5

The esterification step ~r-asinvestigated mainly to drmoiistratc that the acid could be converted to the desired ester product without difficulty. Many references in the literature present data concerning esterification equilibrium, indicating that various methods for a complete esterification have been thoroughly investigated. The esterification results obtained in the two experiments of this work gave yields of 97.9 and 98.8%, confirming the previous results reported ( 2 7 ) . In all probability the yields are quantitative, but slight losses of material occurred during the transfer.

Proposed Commercial Process

40

SO

120 160 Oxygen Pressure

200

240

280

Figure 7. Effect of Pressure in Oxidation of Methacrolein Liquid phase; oxygen; copper-nickel catalyst

The use of mixed acetates of copper and nickel as the catalyst was found to give satisfactory result5 if the pressure was not excessive. The results, showing the percentage conversion and yield as a function of the pressure are presented in Figure 7 and show a gradual increase in conversion with increasing pressure up to 210 pounds. At excessive pressure, however, the extreme conditions were again encountered, resulting in another violent decomposition a t 275 pounds oxygen pressure. i2s the graph shows, increasing pressures up to 210 pounds resulted in a maximum conversion of 62%, a t this pressure, then a rapid decline in conversion and overoxidation a t pressures above 210 pounds. The dotted lines of the curves of Figure 7 indicate a region within which this overoxidation occurs. At the maximum conversion of 62%, a theoretical yield of over 99% of the acid was obtained. The structure of methacrolein consists of a conjugated system of the C=C-C=O type. Such a structure represents a very reactive type of compound with possible attack of oxygen at both conjugated double bonds. At the lower intensities, with the oxygen concentration represented by pressures below 210 pounds, the attack is evidently restricted entirely to the carbonyl portion of the molecule as denoted by the almost theoretical yields of acid obtained. With higher oxygen concentrations, a t the excessive pressures above 210 pounds, sufficient intensity of oxidation must occur to permit attack of the oxygen a t the ethylenic double bond as well as the carbonyl group, resulting in complete oxidation and decomposition of the methacrolein. The data represented in Figure 8 show the relationship between percentage conversion and yield rTith the time of reaction, a t constant temperatures and pressures of 35 C. and 160 pounds, respectively. 8 s might be expected the conversion rises very rapidly a t f i s t and gradually approaches a maximum a t about 7 hours. The yield of methacrylic acid is highest a t the lower times of reaction and remains practically quantitative up t o 5 hours. However, increasing the time of the reaction to longer periods results in a decrease in the yield although the percentage conversion increases considerably, as shown in Figure 8; 7 hours of reaction gave a 96.5% theoretical yield of the acid and a 61% conversion. This may possibly be attributed to some decomposition of the methacrylic acid with prolonged reaction.

The results of this research have definitely demonstrated that methallyl alcohol may be converted to met'hacrolein and the aldehyde in turn to methacrylic acid by oxidation processes vith fairly good conversions and exceptionally high yields. The first of these reactions was carried out in a semicontinuous manner for a laboratory investigation and could be made entirely continuous byu se of convent'ional industrial-type equipment. However, the second oxidat,ion, which was performed with oxygen under pressure, was restricted to a batch operation because of the length of time of reaction involved under the opt'imum conditions found. Wit,h further investigation this step likewise could be made a cont'inuous operation. The flow sheets of Figures 9 and 10 illustrat,ethe plant, operations which might be employed for the manufacture of methyl methacrylate according to the met,hods developed in this research. The operat,ions shown are, for tmhemost part, similar to those eniployed in the laboratory experiments and therefore are not necessarily those which might result, from a more extensive proceri development. I n Figure 9, after preliminary drying and preheating, the air is mixed with the alcohol contained in a steam-jacket,ed carburetor by passage through a distributor near the bottom. The airalcohol vapor mixture from the carburet'or then enters a heated tubular converter containing rolls of copper gauze employed as the catalyst. The exit vapors consist,ing of methacrolein, niethallyl alcohol, and water are chilled in a condenser !There the major

n

i

2

3

4

5

6

7

S

Reaction Time, HOLWS

Figure 8. Effect of Reaction Timein Oxidation of Methacrolein Liquid phase; oxygen, 160 Ib./sq. inch; copper-nickel catalyst

part of the alcohol and aldehyde are condensed and the lest of thr vapors are absorbed in a packed tower countercurlently with cyclohexanol as the solvent. The cyclohevanol extract from the absorber tover is sent to a contmuous distillation column where methacrolein, alcohol, and iTater are recovered and the residue solvent is recycled to storage for re-use. The distillate is combined with the liquid from the first condensate and constitutes thc crude methacrolein product which is stored in a refrigerated tank prior to refinement. Recovery is accomplished by a distillation in which the methacrolein is obtained as a water azeotrope a t 65' C. containing 6.7y0 of water. After removal of the water b r dehv-

May 1950

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

dration it may be desirable to stabilize the aldehyde by the use of a polymerization inhibitor such as hydroquinone. The residue from the aldehyde distillation, consisting of methallyl alcohol and water, is sent to another column in which the alcohol is recovered first as a water azeotrope a t 94" C. and finally as the anhydrous alcohol a t 115" C.; this remains as the

+

H1O TO

775

leaving the high boiling phosphoric acid behind as a residue which may be recovered. The ester is then neutralized and purified by a redistillation. An inhibitor such as hydroquinone (0.05% by weight) is added to the crude methyl methacrylate distillate as well as to the purified monomer product. For the formation of the polymer, the un-

TO SALE AND

WASTE

FURTHER PROCESSINQ

Figure 9 . Production of Methacrolein

bottom fraction for recycling to the carburetor as shown in Figure 9. The alcohol-water azeotrope requires only a partial dehydration before recycling to the carburetor. The further oxidation of methacrolein to methacrylic acid and subsequent esterification are shown in Figure 10. Before the methacrolein enters the reactor it should be freed by distillation of any inhibitor which may have been added. If this were not done the inhibitor might interfere with the absorption of oxygen in the oxidation of the aldehyde. The distilled aldehyde is first dissolved in benzene, used as an inert diluent for the oxidation reaction which is performed in an autoclave under an oxygen pressure of 200 pounds. A close temperature control a t 30' C. is maintained by an efficient cooling of the autoclave for removal of the heat evolved in this reaction. The safety gage or rupture disk included in the autoclave design is standard practice to prevent any damage to the equipment should an explosive decomposition occur. In order to prevent localized overheating and to ensure proper catalyst distribution as well as to help disperse the oxygen, good agitation of the autoclave contents must be provided. After 5 hours of reaction the autoclave charge is cooled before filtering to recover most of the catalyst, and it is then sent to a batch distillation unit where the methacrolein and benzene are recovered leaving the high boiling acid (boiling point, 163 O C.) as bottoms. The crude methacrylic acid is charged directly to the esterification unit which is provided with a reflux-type condenser. A large excess of methanol is added along with sufficient phosphorus pentoxide to combine with the water of reaction. A lively reflux is maintained for several hours a t the boiling point of methanol, after which the resultant mixture is sent to a distillation column for recovery of the excess methanol for re-use. The ester boiling a t 100' C. is separated from the phosphoric acid in another column

stabilized purified product is heated in the presence of a trace of peroxide catalyst to form the attractive crystalline clear transparent plastic.

Summary and Conclusions ' Methaliyl alcohol is readily converted to methacrolein, in the presence of copper at 300" to 350" C. by a vapor phase air oxidation. The optimum mole ratio of oxygen t o alcohol was found t o be 0.6 to 0.7 for a maximum conversion of 71% a t this temperature resulting in a theoretical yield of 95.3% based on alcohol consumed. At temperatures below 280" C. the conversions are insufficient for practical considerations. Above 390' C., however, a very rapid decomposition occurs resulting in a substantial decrease in yield. When the mole ratio is below the optimum of 0.6 to 0.7, a t 340" t o 350" C., the yield remains high but the conversion is decreased. With mole ratios exceeding the optimum, both yield and conversion rapidly decrease because of further osidation of the aldehyde. Likewise a liquid phase pressure oxidation of methacrolein with oxygen gives high yields of methacrylic acid when the oxidation is performed a t substantially room temperatures under pressures of 200 pounds of oxygen using mixed acetates of copper and nickel as the catalyst. The conversion increases rapidly with higher gressures up to a maximum critical pressure of about 250 pounds where very rapid decomposition takes place. The maximum conversion obtained was 61.7% at 210 pounds pressure or a theoretical yield of methacrylic acid of 96% or better. The conversion of the aldehyde to the acid increases as a function of the time of reaction, with maximum yield a t about 5 hours, after which the yield decreases markedly. The esterification of methacrylic acid to methyl methacrylate is

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INDUSTRIAL AND ENGINEERING CHEMISTRY SCREW

INHIBITOR

CONVEYOR

I

Vol. 42, No. 5

CONDENSER-

TEAM REFINING

STILL

E R U O E

STRIPPING STILL

Figure 10. Production of Methyl ivethacrylate

readily achieved employing excess methanol and phosphorus pentoxide as the dehydrating catalyst at reflux temperatures and gives practically quantitative yields of the ester. A commercial process, based on the results of this research, is proposed in view of the excellent yields obtained in the various reactions comprising the synthesis of methyl methacrylate from methallyl alcohol. This method of synthesis is believed t o be both economically and technically feasible as a new process for the production of methyl methacrylate and superior in many respects to the present method.

Bibliography (1) Armitage, F., Paint Technol., 7, 205 (1943). (2) Barnes, C. E., U. 8. Patent 2,241,175 (1941). (3) Bauer and Weisert, Brit. Patent 373,326 (1932); Fiench Patent 713,261 (1931); U. S. Patent 1,911,219 (1933). (4) Bludworth, J. E., U.S. Patent 2,263,607 (1941). ( 5 ) Caldwell, W. T., “Organic Chemistry,” pp, 231-2, 244, 246-9, Boston, Houghton-RIifflin Company, 1943. (6) Constable, F. H., Proc. R o y . SOC.(London), 113A, 254-8 (1926). (7) D’Alelio, “Experimental Plastics and Synthetic Resins,” p. 177, New York, John Wiley & Sons, Inc., 194ti. (8) Dinelli, D., Ann. chim. applicuta, 29, 448-61 (1939). (9) Faith, W. L., and Schaible, A. M,, J . Am. Chem. Soc., 60, 52 4 (1938). (10) Fuson and Shriner, “Systematic Identification of Organic Compounds,” 3rd ed., pp. 157-9, New York, John Wiley & Sons, Inc., 1948. (11) Groggins, P. H., “Unit Processes in Organic Synthesis,” 3rd ed.. pp. 256-7, New York, McGraw-Hill Book Company, 1947. (12) Hearne, Tamele, and Converse, IND.ENG.CHEM.,33, 805-9 (1941). (13) Herstein, K. M., U. S.Patent 2,288,566 (1942). (14) Iddles, H. A., and Jackson, C. E., IXD.ENG.CHEX.,ANAL.ED., 6, 454-6 (1934). (15) Iddles, H. A., Low, A. W., et al., Ibid., 11, 102-3 (1939). (16) Kautter, German Patent 634,501 (1936). (17) Kirk and Jones, U. S. Patent 2,303,842 (1943). (18) Kirk, P. M., and McCellan, P. P., Ibid., 2,143,941 (1941). (19) Marks, B. M., Ibid., 2,336,493 (1943). (20) Mason, J. P., and Manning, J. F., “Technology of Plastlcs and Resins,” pp. 240-2, New York, D. Van Nostrand Company, 194.5.

(21) Mode;nPZastics, 26, 70-1 (1949).

(22) Modern Plastics Encyclopedia, pp. 145-8, New Pork Plastics Catalog Co., 1948.

(23) Muskat, I. E., Brit. Patent 540,940 (1941); U. S. Patent8 2,332,461 (1941) and 2,320,533 (1943). (24) Seher, H. T., IND. E m . CHEY.,28, 269 (1936). (26) Xevith et al., J . Chem. Soc., 1939, pp. 1703-10. (26) Peytral, E., Bull. SOC.Chim. (France),8, 507-19 (1941). (27) Porter, R. W., Chem. Eng., 54, 102-5 (April 1947). (28) Rehberg, C. E., Dixon, M. B., and Fisher, C . H., Ibid.,68, 5446 (1946). (29) Rehberg, C. E., and Fisher, C. H A m . Chem. Soc., 66, 1203 (1944). (30) Sheshukov, J . Russ. Phys. Chenb., 16, 478 (1884). (31) Simmons, H. R., Weith, 9.J., and Higelow, M. H., “Handbook of Plastics” 2nd ed., p. 726, New York, D. Iran Nostrand Co., 1949. (32) Staudinger and Tuerck, Brit. Patent 560,166 (1944). (33) Thomas and Oxley, U. S. Patent 2,226,646 (1941). (34) Thompson, H. W., and Frewing, J. J., J . C h m . Soc., 1935, 111). 1443-51. (35) Trillat, A., Bull. Soc. Chim., (France), 29, 41 (1903). (36) Varne, H. E., U. S. Patent, 2,352,582 (1944). (37) Whitwell, J. C., IND. E m . CHmi., 30, 1157 (1938). (38) Zappi, E. V., Anales Assoc. qudrn. aigentina, 18, 43--.5 1930). (39) Zimakov and Pokrovskii, Compt. Rend. A c n d . Sci. 41 (1945). RECEIVED October 5, 1949. Contribution from t h e Department of Chen~ical Engineering of Columbia Unirersity, New York, N. Y. Submitted by Lawrence Lynn in partial fulfillment of the requirements for the degree of master of science.