Isobutylene to methacrolein

Methacrolein and Methacrylic Acid. A two-stage technology, essentially the same as the propylene oxidation process for the manufacture of acrolein and acrylic acid, was developed to oxidize isobutylene to methacrolein and methacrylic acid 949-951 Two different molybdenum-based multicomponent catalysts are used. In a typical procedure949 isobutylene is reacted with excess steam and air (5 30 65) at about 350°C to produce a mixture of methacrolein and methacrylic acid with 80-85% selectivity at a conversion of 98%. In the second stage this reaction mixture is oxidized at slightly above 300°C to yield methacrylic acid (80% selectivity at >90% conversion). 

Methacrylic acid has been used for the synthesis of polyfmethyl methacrylate). It has been synthesized industrially via a reaction of acetone with hydrogen cyanide (12, 17, 330, 331). However, the process produces ammonium bisulfate and uses the toxic hydrogen cyanide. Recently, an alternative, a two-step oxidation of isobutylene, has been developed. The first step is the oxidation of isobutylene to methacrolein, and the second is the oxidation of methacrolein to methacrylic acid ... 

Multitubular reactors are mainly used in gas-phase partial oxidation processes, such as the air oxidation of light olefins, paraffins, and aromatics. Examples of chemistries where these reactors are used include the partial oxidation of methanol to formaldehyde, ethylene to ethylene oxide, ethylene and acetic acid to vinyl acetate, propylene to acrolein and acrylic acid, butane to maleic anhydride, isobutylene to methacrolein and methacrylic acid, and o-xylene to phthalic anhydride. An overview of the multitubular reactor process for the partial oxidation of n-butane to maleic anhydride is given here. 

P2 Comments Two-stage catalytic oxidation (isobutylene to methacrolein and then to methacrylic acid final esterification with methanol) complex catalyst system. 

These few examples show an advantage of anaerobic oxidations for selected reactions, to minimize CO2 formation. A few other opportunities for further study should include the oxidation of o-xylene to phthalic anhydride, oxidative dehydrogenation of ethylbenzene to styrene, oxidation of isobutylene to methacrolein and methacrylic acid, and oxidative dehydrogenation of paraffins to olefins. 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. 

Much like the oxidation of propylene, which produces acrolein and acrylic acid, the direct oxidation of isobutylene produces methacrolein and methacrylic acid. The catalyzed oxidation reaction occurs in two steps due to the different oxidation characteristics of isobutylene (an olefin) and methacrolein (an unsaturated aldehyde). In the first step, isobutylene is oxidized to methacrolein over a molybdenum oxide-based catalyst in a temperature range of 350-400°C. Pressures are a little above atmospheric ... 

Reducing the number of steps in a chemical transformation can have a profound impact on its economics. Thus, there has been considerable interest in developing 1-step processes for propylene-to-acrylic acid or isobutylene-to-methacrylic acid, which currently involve the intermediate production of acrolein and methacrolein, respectively (9). Sometimes simplification of a process just involves the elimination of a purification step, so that the products of one reactor go straight to another without isolation of an intermediate. 

Catalytic oxidation and ammoxidation of lower olefins to produce a,/3-unsaturated aldehyde or nitrile are widely industrialized as the fundamental unit process of petrochemistry. Propylene is oxidized to acrolein, most of which is further oxidized to acrylic acid. Recently, the reaction was extended to isobutylene to form methacrylic acid via methacrolein. Ammoxidation of propylene to produce acrylonitrile has also grown into a worldwide industry. 

Isobutylidenediurea is used as a slow-release fertilizer. If there is a surplus of isobutyraldehyde, recent work has shown that it can be decarbonylated to propylene over a palladium on silica catalyst.220 There is also the possibility of dehydrogenation to methacrolein for conversion to methaciylic acid, then to methyl methacrylate. Dehydration of isobutyl alcohol would produce isobutylene for conversion to methyl-ferf-butyl ether, although this would probably be uneconomical. 

The handling of toxic materials and disposal of ammonium bisulfate have led to the development of alternative methods to produce this acid and the methyl ester. There are two technologies for production from isobutylene now available ammoxidation to methyl methacrylate (the Sohio process), which is then solvolyzed, similar to acetone cyanohydrin, to methyl methacrylate and direct oxidation of isobutylene in two stages via methacrolein [78-85-3] to methacryhc acid, which is then esterified (125). Since direct oxidation avoids the need for HCN and NH, and thus toxic wastes, all new plants have elected to use this technology. Two plants, Oxirane and Rohm and Haas (126), came on-stream in the early 1980s. The Oxirane plant uses the coproduct tert-huty alcohol direcdy rather than dehydrating it first to isobutylene (see Methacrylic acid). 

In Asia, Asahl and Mitsubishi have commercialized a process using isobutylene or tertiary butyl alcohol to malce methacrolein. Then they further oxidize it to methacryiic acid, MAA, which is then esterified with methanol to MMA. The same process might eventually start with iso butane oxidation to bypass the olefin step. 

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. 

Because of the problems with disposal of the bisulfate waste and the handling of HCN, much research has been devoted to alternative processes. The new processes range from using new feedstocks such as isobutylene / t-butyl alcohol, ethylene, isobutane or methylacetylene to techniques for recycling the HCN and / or ammonium bisulfate279 28°. In 1998 Asahi replaced 60,000 tonnes per year of MMA capacity based on direct oxidation of isobutylene with a new process that also starts with isobutylene. However the new direct oxidative esterification (DOE) process makes MMA by the simultaneous oxidation and esterification of methacrolein, which eliminates the intermediate production of methacrylic acid298. 

The technical routes of the commercialized processes of PMMA could be categorized by (i) the direct oxidation process which consists of catalytic oxidation of isobutylene or tert-butanol to methacrylic acid (MAA) in two steps (ii) the methacrylonitrile (MAN) route by ammoxidation of tert-butanol (iii) the BASF s method which employs ethylene, carbon monoxide, and formaldehyde as raw materials (iv) the new ACH process by Mitsubishi Gas Chemical Co. Inc., which does not generate acid waste and (v) the direct oxidative esterification of methacrolein by Asahi Chemical Co. Ltd.[l] For most of the newly developed processes, efforts have been made to minimize the impact of the production on the environment. 

When HY in Fig. 15 is HOH, acrylic acid is formed in high yields during mild liquid-phase oxidation over Pd/C [30,32-35]. Presumably, allyl alcohol is the initial product, which is rapidly oxidized further on the surface of the catalyst (Fig. 18). In a similar manner, isobutylene is oxidized to methacrylic acid and methacrolein, while 2-butene is oxidized to crotonic acid and crotonaldehyde. 

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