Methacrolein from oxidation

Figure 24.5. Selectivity to the main partial oxidation products achieved during the partial oxidation of C2-C4 alkanes over MoVTeNbO (a) and VPO (b) catalysts. AA=Acrylic acid MA = maleic anhydride MTA = methacrolein. From Ref. 170.

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. 

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). 

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. 

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. 

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... 

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]. 

Morita et al. [222] compared bismuth molybdate (1/1) with U—Sb oxides (1 2) at 400°C in a continuous flow system. The methacrolein selectivity for U—Sb is significantly higher than in the case of Bi—Mo (see Table 20). These values increase slightly with increasing conversion of isobutene. Isobutene itself retards the oxidation. In contrast to the pro-pene oxidation, addition of steam accelerates the reaction up to a factor 4 with U—Sb and to a smaller degree with Bi—Mo. With the first catalyst, the activation energy decreases from 27 to 18 kcal mol-1 (0.23 atm steam). U—Sb seems to be less stable than Bi—Mo, but steam has a beneficial effect here too (Table 20). 

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. 

Fig. 62. Oxidation of methacrolein catalyzed by H4PM01 V040, untreated (O) and treated ( ) with pyridine. Catalyst, 10 cm3 reaction temperature, 553 K SV, 1000 h. Reactant composition methacrolein 2%, oxygen 6%, water 20%. (From Ref. 335.)...

Heteropoly catalysts have significant activities for the oxidation of isobutane into methacrolein and methacrylic acid. The yield increased up to 6% by vanadium substitution or salt formation, as follows. With Cs2.5Ni0.08H0.34+JrPV,Mo12 - O40, the highest conversion and selectivity were observed at x 1 (355). Increases in the reaction temperature to 613 K led to increased yields, up to 9.0%. A similar increase in the yield resulted from the substitution of As for P as a heteroatom or from the addition of various transition metals (106, 356). 

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. 

A different phenomenon has also been detected in tellurium-containing mixtures used in the oxidation of isobutene to methacrolein. The addition of a-St>204 inhibits sintering (35). Te02 appears as a weak acceptor (36). A hypothesis, still to be confirmed, is that the inhibition of sintering has to do with a spillover of oxygen from a-Sb204 to Te02. 

The acidity and oxidizing ability work cooperatively for oxidation of methacrolein, while they function competitively for oxidative dehydrogenation of isobutyric acid. These two reactions also differ in that the former is a surface-type and the latter a bulk-type. From this standpoint, different considerations in effective catalyst design are necessary. 

Applications of POMs to catalysis have been periodically reviewed [33 0]. Several industrial processes were developed and commercialized, mainly in Japan. Examples include liquid-phase hydration ofpropene to isopropanol in 1972, vapor-phase oxidation of methacrolein to methacrylic acid in 1982, liquid-phase hydration of isobutene for its separation from butane-butene fractions in 1984, biphasic polymerization of THE to polymeric diol in 1985 and hydration of -butene to 2-butanol in 1989. In 1997 direct oxidation of ethylene to acetic acid was industrialized by Showa Denko and in 2001 production of ethyl acetate by BP Amoco. 

---