Methacrylic acid routes

Dehydrogenation of Propionates. Oxidative dehydrogenation of propionates to acrylates employing vapor-phase reactions at high temperatures (400—700°C) and short contact times is possible. Although selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid have been developed in recent years (see Methacrylic ACID AND DERIVATIVES) and a route to methacrylic acid from propylene to isobutyric acid is under pilot-plant development in Europe, this route to acrylates is not presentiy of commercial interest because of the combination of low selectivity, high raw material costs, and purification difficulties. 

Selected physical properties of various methacrylate esters, amides, and derivatives are given in Tables 1—4. Tables 3 and 4 describe more commercially available methacrylic acid derivatives. A2eotrope data for MMA are shown in Table 5 (8). The solubiUty of MMA in water at 25°C is 1.5%. Water solubiUty of longer alkyl methacrylates ranges from slight to insoluble. Some functionalized esters such as 2-dimethylaniinoethyl methacrylate are miscible and/or hydrolyze. The solubiUty of 2-hydroxypropyl methacrylate in water at 25°C is 13%. Vapor—Hquid equiUbrium (VLE) data have been pubHshed on methanol, methyl methacrylate, and methacrylic acid pairs (9), as have solubiUty data for this ternary system (10). VLE data are also available for methyl methacrylate, methacrylic acid, methyl a-hydroxyisobutyrate, methanol, and water, which are the critical components obtained in the commercially important acetone cyanohydrin route to methyl methacrylate (11). 

Propylene-Based Routes. The strong acid-catalyzed carbonylation of propylene [115-07-1] to isobutyric acid (Koch reaction) followed by oxidative dehydration to methacrylic acid has been extensively studied since the 1960s. The principal side reaction in the Koch reaction is the formation of oligomers of propylene. Increasing yields of methacrylic acid in the oxydehydration step is the current focus of research. Isobutyric acid may also be obtained via the oxidation of isobutyraldehyde, which is available from the hydroformylation of propylene. The -butyraldehyde isomer that is formed in the hydroformylation must be separated. 

Only with propanal are very high conversions (99%) and selectivity (> 98 0) to MMA and MAA possible at this time. Although nearly 95% selective, the highest reported conversions with propionic acid or methyl propionate are only 30—40%. This results in large recycle streams and added production costs. The propanal route suffers from the added expense of the additional step required to oxidize methacrolein to methacrylic acid. 

Acetone Cyanohydrin. This cyanohydrin, also known as a-hydroxyisobutyronitnle and 2-methyUactonitrile [75-86-5], is very soluble in water, diethyl ether, and alcohol, but only slightly soluble in carbon disulfide or petroleum ether. Acetone cyanohydrin is the most important commercial cyanohydrin as it offers the principal commercial route to methacrylic acid and its derivatives, mainly methyl methacrylate [80-62-6] (see Methacrylic acid AND derivatives). The principal U.S. manufacturers are Rohm and Haas Co., Du Pont, CyRo Industries, and BP Chemicals. Production of acetone cyanohydrin in 1989 was 582,000 metric tons (30). 

Because of limitations on the ready availability of HCN, particularly in Japan, processes involving the oxidation of C4 intermediates have been developed and are now replacing the older route developed by Crawford. One important process is based on the two-stage oxidation of isobutylene or -butyl alcohol to methacrylic acid, which is then separated and esterified Figure 15.5a). 

Isobutane reactions, 13 698 Isobutane route, to methacrylic acid,... 

This chapter describes some aspects of the reactivity of POMs as catalysts for the selective oxidation of isobutane to methacrylic acid. If developed at the industrial level, this reaction could represent an alternative to the current production method via the ACH route. P/Mo Keggin-type POMs are active and selective catalysts for this reaction. 

Compared with propene, the oxidation of isobutene is more rapid but less selective, yet selectivities of over 75% appear feasible. Combustion is the main side reaction. One would expect that some considerable attention would be shown in the literature to isobutene oxidation as a route to the industrially important methacrylic acid, but this is not the case. Nor is it with the production of methacrylonitrile, analogous to the propene ammoxidation. Only in the patent literature is a high activity noticeable. 

Ethylene-Based (C-2> Routes. MMA and MAA can be produced from ethylene as a feedstock via propanol, propionic acid, or melhyl propionate as intermediates. Propanal may be prepared by hydrofonnylalion of ethylene over cobalt or rhodium catalysts. The propanal then reads in the liquid phase with formaldehyde in the presence of a secondary amine and. optionally, a carboxylic acid. The reaction presumably proceeds via a Mannich base intermediate which is cracked to yield methacrolcin. Alternatively, a gas-phase, crossed akin I reaelion with formaldehyde cataly zed by molecular sieves [Pg.988]

Isobutylene-Based butyl alcohol can be converted to methacrylic acid in a iwo-siage. gas-phase oxidation process via methacrolein as an intermediate. The alcohol and isobutylene may he used interchangeably in Ihe processes since fe/f-bnlyl alcohol readily dehydrates lo yield isobutylene under Ihe reaction conditions in the initial oxidation. Variations of this process have been commercialized. 

This reaction is another possible route for the production of methacrylic acid, since isobutyric acid can be obtained by an oxo process from propene and CO. Heteropoly compounds and iron phosphates are so far the most efficient catalysts for the reaction. The favorable role of the presence of an a-methyl group is remarkable for oxidative dehydrogenation, as the heteropoly compounds are not good catalysts for the dehydrogenation of propionic acid (338, 339). 

Direct esterification of methacrylic acid with alcohols using sulfuric acid or other catalysts can be used to prepare methyl methacrylate (MMA) and other esters. Commercial routes for the direct preparation of MMA and some lower alkyl esters also exist. In the 1990s, researchers at Shell developed a direct route to MMA from propyne (methylacetylene), carbon monoxide, and methanol using a Pd(II) catalyst. The limited availability of propyne may slow the expansion of this highly efficient route to high purity MMA. Transesterification of MMA is often the preferred route for the preparation of other esters. 

The competitiveness of the oxidation of isobutene compared to the conventional acetone cyanohydrin route (Equation 33) is not only related to its performance and better environmental standards but has to contend with the demands of other users for isobutene, particularly for MTBE and ETBE production. In fact the predominant methacrylic acid process is still the hydrolysis of acetone cyanohydrin however, the change of mood on the use of MTBE in gasoline blends in the USA, could signal a future shift of isobutene availability making it a more attractive feedstock for methacrylic acid production. 

Typically, carboxylate ionomers are prepared by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene or similar comonomers by free radical copolymerization (65). More recently, a number of copolymerizations involving sulfonated monomers have been described. For example, Weiss et al. (66-69) prepared ionomers by a free-radical, emulsion copolymerization of sodium sulfonated styrene with butadiene or styrene. Similarly, Allen et al. (70) copolymerized n-butyl acrylate with salts of sulfonated styrene. The ionomers prepared by this route, however, were reported to be "blocky" with regard to the incorporation of the sulfonated styrene monomer. Salamone et al. (71-76) prepared ionomers based on the copolymerization of a neutral monomer, such as styrene, methyl methacrylate, or n-butyl acrylate, with a cationic-anionic monomer pair, 3-methacrylamidopropyl-trimethylammonium 2-acrylamlde-2-methylpropane sulfonate. 

Figure 2.63 Acetone cyanohydrin (a) and alternative routes (b) in the synthesis of methacrylic acid.

Other technologies, already commercially applied or under development, are summarized in Figure 2.63b. Alternative routes of synthesis include (i) ethene hydroformylation to propionaldehyde, which then forms methacrolein by condensation with formaldehyde methacrolein is then oxidized to methacrylic acid (BASF process) (ii) isobuthyraldehyde conversion into isobutyric acid and then oxidative dehydrogenation to methacrylic add (Mitsubishi Kasei/Asahi process) and (iii) oxidation of terf-butyl alcohol to methacrolein followed by oxidation to methacrylic acid and esterification. 

Ionomers of practical interest have been prepared by two synthetic routes (a) copolymerization of a low level of functionalized monomer with an olefinically unsaturated monomer or (b) direct functionalization of a preformed polymer. Typically, carboxyl containing ionomers are obtained by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene and similar comonomers by free radical copoly-merization. Rees (22) has described the preparation of a number of such copolymers. The resulting copolymer is generally available as the free acid which can be neutralized to the degree desired with metal hydroxides, acetates and similar salts. Recently, Weiss et al.(23-26) have described the preparation of sulfonated ionomers by copolymerization of sodium styrene sulfonate with butadiene or styrene. 

In the first group, different routes to the C,4 alcohol, 2R,6R,10-trimethylundecanol and its halides have been significant. Thus, readily available methacrylic acid was hydrochlorinated to afford racemic 3-chloro-2-methylpropionic acid and the (R)-(+)-isomer, obtained by resolution with (+)-ephedrine, reduced to the alcohol, and the bromide obtained with PBrj then reacted with isopentylmagnesium bromide to afford a Cg halide. The Grignard... 

Another route to methacrylic acid is via oxidative dehydrogenation of isobutyric acid (equation 13). This reaction is catalyzed by molybdovanadophosphoric acid (H3+ PMoi2-nV 04o n = 0-3), whose redox potential and acidity are well-balanced for effecting this reaction. The acidity is necessary, although excess acidity accelerates the decomposition of isobutyric acid into CO and propene. 

Vinyl compounds are widely used in the industry in manufacture of various resins and polymers and the like. Methacrylic acid and methyl methacrylate are especially attractive as row materials of polymethyl methacrylate that is an important polymer so-called "organic glass." Until a new process consisting of two-step oxidation of isobutylene was commercially practiced in 1982, methyl methacrylate had been produced by the "Acetone Cyanohydrine Process," which uses acetone, hydrogen cyanide, methanol, and sulfuric acid as raw materials. Technical and economical drawbacks of this process have spurred a considerable industrial research effort to develop an alternate route to methacrylic acid and methyl methacrylate. Therefore, many attempts have been focused on the production of these compounds by aldol-type condensation using HCHO. 

Dr. Ai reviews the condensation of formaldehyde and methanol with other hydrocarbons to form the widely used vinyl compounds, such as methacrylic acid and methyl methacrylate. The development of active and stable solid catalysts for these reactions can eliminate the significant environmental problems faced with the current industrial routes to these chemicals. 

The current industrial production of methylmethacrylate by the acetone-cyanohydrin process suffers from a number of drawbacks, which make it environmentally unfriendly. In particular, it makes use of a very toxic reactant (HCN) and intermediate (acetone cyanohydrin), and coproduces large amounts of impure ammonium sulphate, contaminated with organic compounds. Among the several alternative synthetic routes which have been proposed, particularly interesting from both the practical and scientific points of view is the single-step oxidation of isobutane to methacrylic acid, intermediate in the synthesis of methylmethacrylate. Several industrial companies have studied this reaction (and the selective oxidation of propane to acrylic acid, as well), and it has been established that the most active and selective catalysts are those which are based on Keggin-type polyoxometalates (POM s), containing phosphorus and molybdenum as the main components [1-18]. 

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