Isobutene oxidations

Isobutene oxidation Isobutene or t-butyl alcohol, air Two catalytic oxidations Several in Japan... 

Isobutene oxid with I-Butyl alcohol, air Second oxidative step integrated with Asahi Kasei... 

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

Methylazo)isobutene, oxidation of, 359 2-(Methylazo)propene, 306 4 -Methylazoxybenzene, 353 Methyl-MVO-azoxybenzene, 352... 

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. 

Influence of the composition of FeoBibK0.2CrCo6Mo n0y on its performance is isobutene oxidation catalyst in a flow reactor at 400 C... 

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.)...

Fig. 6. The variation of initial percentage yields with surface at 270 and 300 °C. isobutene , propionaldehyde , acetone cp, propene 6, isobutyraldehyde , methacrolein >, acetaldehyde , isobutene oxide. (From ref. 44.)...

Fig. 25. The variation with initial pressure of the initial percentage yield of products from the oxidation of isobutane at 310 °C. Isobutane oxygen = 1 2 volume of reaction vessel = 500 cm. isobutene , acetaldehyde O, propionaldehyde propene t>, fcrf-butyl hydroperoxide , isobutene oxide o, acetone.

Using the same approach and interpretation, values of — jq-ii.io o.44 jjj3 molecule s and Eub = 161.2 6.4 kJ mol were obtained [45] from studies of isobutene oxidation, as predicted by the similar thermochemistry and inert nature of methylallyl radicals due to electron delocalization. The agreement is good, and moreover the Arrhenius parameters are entirety consistent with Aif= 10 " cm molecule s and Elf = 163 kJ mot , which were obtained from studies of HCHO oxidation under conditions where the chain length was reduced virtually to zero. In the initial stages of reaction, the mechanism in KCl-coated vessels, where HO2 and H2O2 are efficiently destroyed at the vessel surface, is very simple. 

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]. 

Figure 1.17 shows that the degree of autocatalysis is considerably less marked than with propene and isobutene oxidations. 

Major yields of propene (ca. 10%) are found in the initial products from isobutene oxidation between 673 and 773 K, but effectively no propene is observed initially from the oxidations of butene-1, 2-methylbutene-2 and 2,3-dimethylbutene-2. Structurally, propene formation is possible via C3H7 radicals in all cases through the hydroxy adduct. 

The major products from methyloxirane (propene oxide) are C2H5CHO, CH3COCH3 and CH2=CHCH20H [90], Similarly, isobutyraldehyde is the dominant product from isobutene oxide. Oxetanes are known to undergo homolysis through ring splitting, so that isobutene and HCHO are major products from 3,3-dimethyloxetane. 

Sb 7Sb, etc. Some metals (mainly Ag for ethylene epoxidation), noble metals (as Pt, Pd), zeolites (titanosilicalite TS-1 from ENI for phenol oxidation) and heteropolyoxometallates (e.g. H PMOjjVO u for isobutene oxidation to methacrolein) may also be used. 

In other examples, extensively studied by Delmon et al., SbaOa was used with M0O3 for isobutene oxidation to methacrolein [29, 30], and for the dehydration of N-ethyl formamide [46,47]. Antimony is one of the elements frequently found in selective oxidation catalysts, as in the pionneering work on uranium antimony oxides for ammoxidation of propene [48], and more recently in ammoxidation of propane on V-Sb-Al system [49]. 

The data obtained on isobutene oxidation are represented in the Table 1, Figures 1 and 2. To compare the catalytic activities of the samples synthesized by various methods and containing different amounts of metal, we calculated the atomic catalytic activity (ACA), i.e. the rate of methacrolein formation (mol/s) per one g of metal. 

A specific feature of isobutene oxidation on photoimmobilized catalysts obtained consists in the fact that this process takes place almost at room temperature. 

For example, on Cu-Ph reaction starts at 290 K, with increasing temperature the amount of methacrolein formed rises, attains its maximum value at 355-365 K and then at 430 K it decreases. The maximum degree of isobutene conversion is 25-30 %. On Cu-Iml and Cu-Sup isobutene oxidation starts at 420-450 K (Figure 1). Partial oxidation products (methacrolein, acetaldehyde, acetone - 0.05 vol. %) are formed with maximal rate at 520-550 K. Above 570 K isobutene is oxidized to CO2 and H2O The similar results are obtained over Ti-containing catalysts. 

Another peculiarity of isobutene oxidation on Cu, Mo, W, V, Ti catalysts is the high selectivity towards methacrolein formation (100 % selectivity is observed at 300-390 K). In the region of maximal yield of methacrolein there is no even trace amounts of CO2. In some cases (Mo-Ph, W-Ph) acetone and the cracking products are observed at temperatures above 390 K. 

Multicomponent reactions (MCRs) were applied to the synthesis of substituted isoxazolines. For example, 64 was obtained by addition of nitro-alkene 60 and acrylate 61 to a solution of isonitrile 59 generated in situ by reaction of trimethylsilyl cyanide and isobutene oxide in the presence of Pd(CN)2 <05OL3179>. This cascade MCR is believed to occur through [1+4] cycloaddition of 59 with 60, subsequent fragmentation of 62 and 1,3-DC of nitrile oxide 63 with 61. Under microwave irradiation, reaction times could be reduced from several hours to 15 min, with comparable yields. 

In isobutene oxidation the reaction conditions were catalyst weight 300 mg GHSV 3000 h1 normal composition ISOB/O2/N2 - 1/2/7 temp. = 420 and 45Q5C O2/ISOB ratio 2.0 (normal composition), 1 and 0.25 time of reaction 5 hrs. 

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