Oxidation of isobutene

Isobutene can be oxidized to methacrolein or methaciylonitrile using the same catalysts as for propylene oxidatiom The reactions ate  

This process avoids the by-product ammonium sulfate formed from the acetone cyanohydrin route. 

TABLE 4.15. Acrylic Acid Production from Propylene in Two Steps. 

Catalyst (BijOj), (FejO,), (MoOj) silica support (V2O5), (W,Fe,Ni,Mn,Cu) (M0O3), silica support 

Note Steam added to propylene/air mixture with interbed cooling. 

Below 800 K, H abstraction from isobutene results almost uniquely in methylallyl (MA) radicals which undergo homolysis to a small degree to give allene and CH3 radicals, but otherwise are as unreactive as allyl radicals. With their resulting high radical concentration, significant yields (up to 30%) of 2,5-dimethylhexa-l,5-diene (DMHDE) are observed in the initial products of isobutene oxidation. 

Because /cubAip 5, then [HO2] is increased in isobutene oxidation, and significant yields of methacrolein are found through the sequence, 

Isobutene oxide is formed through HO2 addition, and acetone through OH addition in an analogous way to CH3CHO from propene. Measurement of the initial rates of formation of DMHDE and isobutene oxide coupled to relevant rate constants from the literature gave accurate values for [MA] and [HO2]. 

As indicated earlier, the basic mechanism is so simple that reliable values have been obtained for from the fundamental equation, 

The basic oxidation mechanism for propene and isobutene appears almost unique and apart from OH and HO2 addition involves the formation, through abstraction, of highly inert radicals which are almost completely consumed through radical-radical processes. The tremendous acceleration (over a factor of 100) shown between the initial and maximum rates for both alkenes (Eig. 1.17) can be explained through secondary initiation involving three reactions for each alkene. 

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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. 

Research on the selective oxidation of isobutene is mainly focussed on bismuth molybdate-based catalysts. Under conditions which are optimal... 

Mann and Ko [202] likewise examined the selective oxidation of isobutene on bismuth molybdate. With an integral flow reactor, the highest selectivity was obtained at over 30% conversions for an oxygen/olefin ratio of 2/1 and a W/F = 2.5 g h mol-1 (390°C). The data were correlated with a rather complicated Langmuir—Hinshelwood expression inconsistent with a redox mechanism. This was based on a rate-controlling step between adsorbed isobutene and adsorbed oxygen, and included an inhibiting effect of methacrolein by competitive adsorption with isobutene, viz. 

Table l. Evolution of Fe2(Mo04)3, pure or in a mixture with oSt>204 (50/50) in the oxidation of isobutene (ISOB) to methacrolein. The measurements are made after 5 hrs on stream. The experimental conditions are as indicated in the legend of fig. 1 except for the more severe O2/ISOB rates of 1 (instead of 2) and a Gas Hourly Space Velocity of 3000 h1, corresponding to about twice the value in fig. 1, account taken of the different dilutions by inert materials (3). Y yield C conversion S selectivity. 

Identical mechanical mixtures of a-Sb204 and M0O3 of various compositions (19) are active and relatively selective in the oxidation of isobutene to methacrolein (fig. 8 and 9) a-Sb204 is not active in this reaction. The lack of selectivity of pure M0O3 is due to reduction. The suboxides of M0O3 (M0O3.X) are indeed much less selective. 

Fig. 8. Yield in methacrolein in the oxidation of isobutene with identical weights of mechanical mixtures of a-Sh CM + M0O3 of various compositions. Partial pressures in the gas...

Fig. 9. Selectivity in methacrolein in the oxidation of isobutene. Same conditions as for fig. 8 (U9).

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. 

However, the nature of the active sites in antimony-molybdenum mixed oxide catalysts for the selective oxidation of isobutene to metha-crolein is not clear yet, notwithstanding significant research efforts in this area (Gaigneaux et al., 1998). There is evidence that the catalytically active site has to be part of the outermost layer of the catalyst (Gaigneaux et al., 1998), as expected. Recent catalytic experiments demonstrated that the TOF of the reaction on MoOx is much higher than that on SbOY, a result... 

Methacrylic acid is produced by a number of different processes, one of which is based on the oxidation of isobutene (or of t-butyl alcohol) via the intermediate formation of methacrolein (Equation 32). The general features and the catalyst for the first-stage process are not dissimilar to those for acrolein production, whereas the oxidation of methacrolein to MMA differs in that it is catalyzed by... 

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. 

Methyl methacrylate (MMA) is an important commodity since it is polymerized to give poly methylmethacrylate (PMMA), a strong, durable and transparent polymer sold under the trade-names Perspex and Plexiglas. Since the conventional routes to MMA involve either the reaction of acetone with HCN to give the cyanohydrin (which has environmental problems), or the oxidation of isobutene, alternative carbonylation routes to MMA are being developed. One of these is the Lucite Alpha process which is claimed to decrease production costs by ca. 40%. This first synthesizes methyl propionate by a methoxycarbonylation of ethylene (Equation 23), using a palladium catalyst with very high (99.8%) selectivity. In the second step, MMA is formed in 95% selectivity by the reaction of methyl propionate with formaldehyde (Equation 24). 

As the fuel consumption increased, the alkene concentration eventually reached a kinetically controlled stationary value [43]. Since the major product at all stages in the oxidation of isobutene is acetone [42] (Fig, 4), it was possible to follow the consumption of isobutene during the oxidation of isobutane by observing the formation of acetone. It can be seen from Fig. 5 that a marked increase in the yield of acetone in the later... 

Fig. 4. The variation with time of product formation during the oxidation of isobutene. Initial temperature = 293 °C initial pressure of isobutene = 100 torr initial pressure of oxygen = 100 torr. O, isobutene , oxygen , acetone , isobutene oxide , isobutyraldehyde , carbon dioxide , carbon monoxide , water. (From ref. 42.)...

Our catalysts were simple mixtures of a-Sb204 and Fc2(Mo04)3 powders prepared separately. The catalysts were tested in the oxidation of isobutene to meihacrolein (ISOB) and ethanol to acetaldehyde (ETH). In order to show more dramatically the influence of a-Sb204 the oxygen ratio in the feed (O2/ISOB) was kept at a value lower than that normally used in the first reaction, With respect to the second reaction, different values of O2/ETH were taken. A special long-run experiment (30 h) was made for the reaction of ethanol. Catalysts were characterized by XRD. XPS, electron microscopy, and Fc Mdssbauer spectroscopy before and after reaction. 

Mitsubishi Rayon in Japan has commercialized a three-step process on the basis of a two-step catalytic oxidation of isobutene, preferentially through f-butanol as primary intermediate. This process suffers not only from a relatively moderate overall MMA yield ( 80%), but also from increasing isobutene cost due to its alternative use for MTBE (methyl fert.-butyl ether) production as a gasoline additive. 

The so-called "multicomponent catalysts" used in selective oxidation are oxides, and they represent the vast majority of catalysts used in this field. All multicomponent industrial catalysts contain several phases. We discovered about ten years ago that simple mechanical mixtures of two oxides had much better performances than those of the two constituents [63-69,71-72,76,78]. This is illustrated by fig. 2 in the case of the oxidation of isobutene to methacrolein over mixtures of micron-size M0O3 and a-Sb204 particles. All experiments were made with the same total quantity of catalysts. The arrows show the increase of yield compared to the simple addition of the individual contributions of the catalyst components. 

Figure 2. Synergy between a-Sb204 and M0O3 particles in the selective oxidation of isobutene to methacrolein. The figure concerns yields (namely conversion x selectivity) in experiments where conversion was always below 25%. The catalysts were prepared by mixing the powders of a-Sb204 and M0O3, prepared separately, as a stirred suspension in n-pentane, and evaporating n-pentane. The same overall weight of mixture was used for all compositions and the experimental conditions were identical [63,78].

On the basis mainly of results obtained in the oxidation of isobutene to methacrolein, the oxidative dehydrogenation of butene to butadiene and the oxygen-aided dehydration of formamide to nitriles, it was possible to show that oxides present in catalysts are located on a scale reflecting donor-acceptor properties (fig. 5). Some oxides are essentially acceptors (e.g., M0O3, some tellurates) they can potentidly cany active and selective sites, provided they receive spillover oxygen. Others are essentidly donors a-Sb204, in this respect, is typical it produces spillover oxygen but carries no sites active for oxidation. Other oxides have mixed properties. The acceptors are relatively covalent, the donors are more ionic [63,77]. 

Figure 3. Synergy (selectivity) between a-Sb204 and Sn02 particles in the selective oxidation of isobutene to methacrolein. The preparation of the sample mixtures and the experimental conditions are described in the legend of fig. 2. More details are found in the original articles [63-65,83].

Vapour-phase catalytic oxidation of isobutene was carried out at atmospheric pressure in a completely automated laboratory setup, including a fixed bed reactor (700 mm length, 10 mm inner diameter) with corundum as wall material. In order to ensure isothermicity, the heated section (200 mm in length) was divided into five independently heated zones and the catalyst bed was diluted with inert pellets (a-Al203). Inert pellets were placed above and below the catalyst bed to ensure a well-mixed feed stream, and to preheat the gas to the reaction temperature. Bi203 catalyst (Merck) was pressed into thin wafers and broken into small particles. Granules with a diameter of 0.8 -1.2 mm were used. 

Oxidation of isobutene to methacrolein on metal complex catalysts prepared by various... 

The oxidation of isobutene on W- and V- containing catalysts proceeds in the same way as on Mo-containing samples. 

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