Isobutene oxide, from oxidation

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. 

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]. 

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. 

Oxidative coupling of isobutene suffers from severe deep oxidation. As in many other partial oxidation reactions selectivity remains low, despite intensive optimization of catalysts and reaction conditions. Among various new reactor concepts, the separation of catalyst reduction and reoxidation is very promising (two step process). Reaction engineering investigations of the two step process have been done. The influence of reaction conditions and reversibility of reduction/reoxidation cycles have been investigated. Based on the reaction engineering results a first approach to a kinetic model of both reaction steps has been developed. 

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. 

Apart from dimerization and copolymerization, linear butenes have a few other important apphcations, including hydratization to isobutene, oxidation to maleic anhydride, and dehydrogenation to butadiene (Figure 5.3.7). 

The ylide (35), prepared from base treatment of the methylenetriphenyl-phosphorane-isobutene oxide adduct, has been shown to react with steroidal aldehydes to give the (E)-homoallylic alcohol stereoselectively [equation (6)], especially at low temperatures. 

Citral is prepared starting from isobutene and formaldehyde to yield the important C intermediate 3-methylbut-3-enol (29). Pd-cataly2ed isomeri2ation affords 3-methylbut-2-enol (30). The second C unit of citral is derived from oxidation of (30) to yield 3-methylbut-2-enal (31). Coupling of these two fragments produces the dienol ether (32) and this is followed by an elegant double Cope rearrangement (21) (Fig. 6). 

Extraction of sec-butanol from isobutene Hydrothermal oxidation of organic wastes in water... 

Like propane, n-hutane is mainly obtained from natural gas liquids. It is also a hy-product from different refinery operations. Currently, the major use of n-hutane is to control the vapor pressure of product gasoline. Due to new regulations restricting the vapor pressure of gasolines, this use is expected to he substantially reduced. Surplus n-butane could be isomerized to isobutane, which is currently in high demand for producing isobutene. Isobutene is a precursor for methyl and ethyl tertiary butyl ethers, which are important octane number boosters. Another alternative outlet for surplus n-butane is its oxidation to maleic anhydride. Almost all new maleic anhydride processes are based on butane oxidation. 

The three isomers constituting n-hutenes are 1-hutene, cis-2-hutene, and trans-2-hutene. This gas mixture is usually obtained from the olefinic C4 fraction of catalytic cracking and steam cracking processes after separation of isobutene (Chapter 2). The mixture of isomers may be used directly for reactions that are common for the three isomers and produce the same intermediates and hence the same products. Alternatively, the mixture may be separated into two streams, one constituted of 1-butene and the other of cis-and trans-2-butene mixture. Each stream produces specific chemicals. Approximately 70% of 1-butene is used as a comonomer with ethylene to produce linear low-density polyethylene (LLDPE). Another use of 1-butene is for the synthesis of butylene oxide. The rest is used with the 2-butenes to produce other chemicals. n-Butene could also be isomerized to isobutene. ... 

Fig. 3. FT-IR spectra of the adsorbed species arising from the interaction of (a) rerr-butanol and (b) isobutane over a combustion catalyst (MgCr204) at 423 K, and from rerr-butanol (373 K, c), isobutene (300 K, d) and isobutane (380 K, e) on a selective oxidation catalyst.

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Very many acidic solids and liquids, immiscible with hydrocarbons, will catalyse the oligomerisation of isobutene at ambient temperatures. Among the more common are syncatalysts prepared from boron fluoride and a protonic substance BH (B = OH, CHsO, C2H50, t-C4H90, CH3C02, etc.) mineral acids natural and synthetic alumino-silicates, (e.g., Fuller s earth, bentonite, attapulgite) and metal oxides containing small quantities of water. 

If the initial intermediate or the original fuel is a large monoolefin, the radicals will abstract H from those carbon atoms that are singly bonded because the CH bond strengths of doubly bonded carbons are large (see Appendix D). Thus, the evidence [12, 32] is building that, during oxidation, all nonaromatic hydrocarbons primarily form ethene and propene (and some butene and isobutene) and that the oxidative attack that eventually leads to CO is almost solely from these small intermediates. Thus the study of ethene oxidation is crucially important for all alkyl hydrocarbons. 

In the oxidation of f-butanol, acetone and isobutene appear [46] as intermediate species. Acetone can arise from two possible sequences. In one,... 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

In the second scheme, the alkane is transformed to the olefin by oxidehydro-genation, and the outlet stream is sent to the second oxidation reactor without any intermediate separation." Isobutane and isobutene are recycled, together with oxygen, nitrogen, and carbon oxides. Finally, the third scheme differs from the first one in that hydrogen is separated from propane/propylene after the dehydrogenation step, and oxygen is preferably used instead of air in the oxidation reactor." ... 

CO is derived from a variety of feedstocks such as petroleum gas, fuel oil, coal, and biomass. The industrial scale production of PO starts from propylene, which is mainly obtained from crude oil. However, due to the high importance of this compound, many pathways from renewable sources have additionally been developed [54]. PP is converted to PO by either hydrochlorination or oxidation [55]. The use of chlorine leads to large amounts of salts as by-products, therefore oxidation methods are more important, such as the co-oxidation of PP using ethylbenzene or isobutene in the presence of air and a catalyst. However, this process is economically dependent on the market share of these by-products, thus new procedures without significant amounts of other side-products have been developed, such as the HPPO (hydrogen peroxide to propylene oxide) process in which propylene is oxidized with hydrogen peroxide to give PO and water [56, 57] (Fig. 14). 

For many reactions the type of intermediate that is involved may be deduced from a study of a family of reactants. For example, by noting that in allylic oxidation the order of reactivity is isobutene > trans-2-butene > cis-2-butene > 1-butene one may deduce that an allyl radical or cation is an intermediate. For other oxidations, if the reaction rate order is primary > secondary > tertiary, then an anionic intermediate is implicated. However, care must be taken that the formation of the intermediate is involved in the ratedetermining step and that there are no adsorption equilibrium effects. To rule out the latter, the reaction should be carried out at conditions of low coverage. 

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). 

Infrared spectra of propene and isobutene on different catalysts were measured by Gorokhovatskii [143]. Copper oxide, which converts olefins to butadiene and aldehydes, shows adsorption complexes different from structures on a V2Os—P2Os catalyst which produces maleic acid anhydride. Differences also exist between selective oxidation catalysts and total oxidation catalysts. The latter show carbonate and formate bands, in contrast to selective oxides for which 7r-allylic species are indicated. A difficulty in this type of work is that only a few data are available under catalytic conditions most of them refer to a pre-catalysis situation. Therefore it is not certain that complexes observed are relevant for the catalytic action. 

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