Methacrolein oxidation

It was proposed that the increase in activity during the equilibration period was due to the generation of new active sites,consisting of the Mo species located in the cationic position in the secondary framework of the POM. A similar hypothesis was formulated by other authors for the methacrolein oxidation to methacrylic acid." " More generally, it is currently believed that for exothermic reactions, and specifically for oxidations, the true working state of the POM, does not correspond to its crystalline form." The presence of steam and the large amount of heat released provoke an incipient surface decomposition, which leads to the expulsion of the Mo species from the anion as a metastable defective... 

It has been demonstrated that V5+ in Hj+JtPMi2-,V,04o (M = Mo, W) is eliminated from the polyanion framework upon thermal treatment or during catalytic oxidation, and the VOz+ salt of H3PM12O40 is formed (284). It has been reported (103) that H3PM012O40 is re-formed from thermally decomposed H3PM012O40 under the conditions of methacrolein oxidation. 

Solid heteropoly compounds are suitable oxidation catalysts for various reactions such as dehydrogenation of O- and N-containing compounds (aldehydes, carboxylic acids, ketones, nitriles, and alcohols) as well as oxidation of aldehydes. Heteropoly catalysts are inferior to Mo-Bi oxide-based catalysts for the allylic oxidation of olefins, but they are much better than these for oxidation of methacrolein (5). Mo-V mixed-oxide catalysts used commercially for the oxidation of acrolein are not good catalysts for methacrolein oxidation. The presence of an a-methyl group in methacrolein makes the oxidation difficult (12). The oxidation of lower paraffins such as propane, butanes, and pentanes has been attempted (324). Typical oxidation reactions are listed in Table XXXI and described in more detail in the following sections. 

Preparation of heteropolyacid/carbon catalyst and its application to methacrolein oxidation... 

Metal-oxide clusters such as polyoxometallates offer attractive prospects as catalysts, since these oxide clusters are robust, their structures can be systematically varied, sites of stable coordinative unsaturation can be created, and catalytically active metal centers can be incorporated at the cluster surfaces or within the cluster framework. Structures approximated as Cs2H2(PVMoi] 04q) are selective methacrolein oxidation catalysts. Further research is recommended it should include synthesis and development of methods for stable molecular dispersion on supports and systematic investigation of structure, reactivity, and catalytic activity. 

The oxidative dehydration of isobutyric acid [79-31-2] to methacrylic acid is most often carried out over iron—phosphoms or molybdenum—phosphoms based catalysts similar to those used in the oxidation of methacrolein to methacrylic acid. Conversions in excess of 95% and selectivity to methacrylic acid of 75—85% have been attained, resulting in single-pass yields of nearly 80%. The use of cesium-, copper-, and vanadium-doped catalysts are reported to be beneficial (96), as is the use of cesium in conjunction with quinoline (97). Generally the iron—phosphoms catalysts require temperatures in the vicinity of 400°C, in contrast to the molybdenum-based catalysts that exhibit comparable reactivity at 300°C (98). 

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. 

Isobutjiene [115-11-7] or tert-huty alcohol can be converted to methacrylic acid in a two-stage, gas-phase oxidation process via methacrolein as an intermediate. The alcohol and isobutjiene may be used interchangeably in the processes since tert-huty alcohol [75-65-0] readily dehydrates to yield isobutjiene under the reaction conditions in the initial oxidation. Variations of this process have been commercialized by Mitsubishi Rayon and by a joint venture of Sumitomo and Nippon Shokubai. Nippon Kayaku, Mitsui Toatsu, and others have also been active in isobutjiene oxidation research. 

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). 

The oxidation of methacrolein to methacrylic acid is most often performed over a phosphomolybdic acid-based catalyst, usually with copper, vanadium, and a heavy alkaU metal added. Arsenic and antimony are other common dopants. Conversions of methacrolein range from 85—95%, with selectivities to methacrylic acid of 85—95%. Although numerous catalyst improvements have been reported since the 1980s (120—123), the highest claimed yield of methacryhc acid (86%) is still that described in a 1981 patent to Air Products (124). 

Several variations of the above process are practiced. In the Sumitomo-Nippon Shokubai process, the effluent from the first-stage reactor containing methacrolein and methacrylic acid is fed directiy to the second-stage oxidation without isolation or purification (125,126). In this process, overall yields are maximized by optimizing selectivity to methacrolein plus methacrylic acid in the first stage. Conversion of isobutjiene or tert-huty alcohol must be high because no recycling of material is possible. In another variation, Asahi Chemical has reported the oxidative esterification of methacrolein directiy to MMA in 80% yield without isolation of the intermediate MAA (127,128). 

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. 

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). 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Monodentate dipolarophiles such as acrolein, methacrolein, and a-bromoacrolein could be successfully utilized in the l ,J -DBFOX/Ph-transition metal complex-catalyzed asymmetric nitrone cycloadditions [76]. The reactions of N-benzylideneani-line N-oxide with acrolein in the presence of the nickel(II) aqua complex R,R-DBF0X/Ph-Ni(C104)2 3H20 (10mol%) and MS 4 A produced a mixture of two regioisomers (5-formyl/4-formyl regioisomers ca 3 1). However, enantio-... 

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 ... 

In the second step, methacrolein is oxidized to methacrylic acid at a relatively lower temperature range of 250-350°C. A molybdenum-supported compound with specific promoters catalyzes the oxidation. 

To synthesize graft copolymers of PAN using oxidation-reduction systems, random copolymers of AN with methacrolein (MAC) can also be taken as the initial product. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

Figure 14.2 shows the simplified flow sheet of the process, as reported in patents issued to Sumitomo. CO2 is maintained in the recycle loop to act as a ballast component the desired concentration of CO2 is obtained by combustion of CO, while excess CO2 is separated. Methacrolein is separated and recycled to the oxidation reactor. An overall recycle yield of 52% to methacrylic acid is reported, with a recycle conversion of 96% and a per-pass isobutane conversion of 10%. The heat of reaction produced, mainly deriving from the combustion reaction, is recovered as steam. 

Figure 14.3 Different strategies for integration of isobutane (oxi)dehydrogenation to isobutene and isobutene oxidation to methacrolein and to methacrylic acid.

TABLE 14.2 Summary of Results Reported in the Scientific and Patent Literature for the Oxidation of Isobutane to Methacrolein and Methacrylic Acid Catalyzed by Keggin-Type POMs... 

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacrolein, which is transformed to acetic acid, methacrylic acid, and carbon oxides. ° Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. Isobutene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion. ... 

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. 

BASF led the development of a route based on ethylene and synthesis gas. Its four step process begins with the production of propionaldehyde from ethylene, CO, and H2 using a proprietary catalyst mixture that they aren t telling anything about. Reaction with formaldehyde gives methacrolein. The last two steps are the same as above—oxidation with air yields the MAA subsequent reaction with methanol yields MMA. 

The same rhodium precursor, (S Rh,/ c)-[(Tl -C5Me5)Rh (l )-Prophos (H20)] (SbFg)2, promotes the reaction between the nitrones A-benzylideneaniline A-oxide or 3,4-dihydroisoquinoline A-oxide with other enals different from methacrolein (Scheme 10). The cycloadducts were prepared with excellent regioselec-tivity, perfect endo selectivity, and enantiomeric excesses up to 94% [35]. 

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