Propene isomerisation

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

The present paper is an attempt to unravel a rather confused aspect of cationoid polymerisations. This concerns the phenomenon comprised in the term monomer complexation of the growing cation . The idea seems to have occurred for the first time in the work of Fontana and Kidder on the polymerisation of propene by AlBr3 and HBr in w-butane [3]. The kinetics indicated a reaction of zero order with respect to monomer, M to explain this, it was assumed that the growing end of the chain, written as a carbenium ion, Pn+, is complexed with M and that the rate-determining growth step is an isomerisation of this complex ... 

Another application of an isomerisation reaction can be found in the production of the third monomer that is used in the production of EPDM rubber, an elastomeric polymerisation product of Ethene, Propene and a Diene using vanadium chloride catalysts. The starting diene is made from vinylnorbomene via an isomerisation reaction using a titanium catalyst. The titanium catalyst is made from tetravalent salts and main group hydride reagents, according to patent literature. 

The tppts process has been commercialised by Ruhrchemie (now Celanese), after the initial work conducted by workers at Rhone-Poulenc, for the production of butanal from propene. Since 1995 Hoechst (now Celanese) also operates a hydroformylation plant for 1-butene. The partly isomerised, unconverted butenes are not recycled but sent to a reactor containing a cobalt catalyst. The two-phase process is not suited for higher alkenes because of the... 

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

When the rate of polymerisation is dependent on the propene concentration, but site isomerisation is not, the concentration of propene effects the properties of the polymer. A faster polymerisation will lead to a higher stereospecificity in the polymer. 

The diphenylallyl carbanion is conveniently formed by the abstraction of an allylic proton from the corresponding diphenyl propene. Under certain conditions it was found that the initial product formed from trans 1,3 diphenyl-2-methyl propene was the trans,trans anion which isomerised completely into the cis,trans conformation within a few minutes [3]. Clearly, the abstraction reaction proceeds under kinetic and-not thermodynamic control. 

More information is available about orientation, when a second alkyl group is introduced into the aromatic ring, and about relative rates. As might be expected, propene reacts more easily than ethylene [342,346] and isobutene more easily than propene [342]. Normal butenes are sometimes isomerised in the process practically the same product composition, consisting mainly of 2,2,4-trimethylpentane, is obtained in the alkylation of isobutane whether the olefin component is isobutene or 2-butene [339]. In the alkylation of aromatic hydrocarbons, this side reaction is negligible. 

Pig. 8. Elimination-isomerisation-addition mechanism for the formation of 1,3 enchained propene units... 

Cyclopropane isomerises to propene at 1000°C. The rate is 0.92 s when the concentration of cyclopropane is 0.1 mol dm , and 0.46 s when the concentration of cyclopropane is 0.05 mol dm. Deduce the relationship between rate and cyclopropane concentration, giving the value of any constants. 

Scheme 2. Isomerisation of 2-ethyl propenal to trans-2-methyl-2-butenal.

In contrast to the cyclobutane systems, fluorinated cyclopropanes show a lesser thermal stability than the parent hydrocarbon. Trotman-Dickenson et al. (refs. ° )have studied thethermally induced unimolecular isomerisations of partly fluorinated cyclopropanes to mixtures of the corresponding fluoro-propenes. At 450 °C the rate of decomposition of monofluorocyclopropane is about three times that of cyclopropane products formed are 1-fluoropropene (79 %) cis-trans mixture), 2-fluoropropene (9 %) and 3-fluoropropene (11 %). The substitution of further fluorine atoms in the cyclopropane ring further increases the rate of isomerisation ° (see Table 2). Unlike the partly fluorinated cyclopropanes, perfluorocyclopropane does not isomerise to propene compounds but decomposes at 250-300 °C by a first-order reaction to perfluoroethylene . The rate coefficient is expressible as, k = 1.78 x 10 exp (-38,600//JjT) sec , and the decomposition is consistent with the mechanism... 

Propene oxide would appear to be a far more likely product of the isomerisation (50). However, an alternative mode for the dihydroxy radical decomposition (47) also gives rise to propene oxide. That is. 

Silica-supported rhodium hydrides are highly efficient isomerisation catalysts. They are prepared by the reaction of silica suspended in toluene with tris(allyl)rhodium. The intermediate complex reacts with hydrogen to eliminate propene and propane to give a material believed to have neighbouring rhodium sites connected by hydrogen bridges.164... 

Furthermore, silica-based systems received attention as effective catalysts for several photochemical reactions, e.g olefin photo-isomerisation [12], olefin photo-oxidation [13-15], photomethatesis [16] and methane coupling [17]. Recently, it was also reported that silica mesoporous materials are more active photocatalysts than amorphous silica [18]. Among them, Mg/Si02 systems [13] were found to act as catalysts for the photo-oxidation of propene to propene oxide in the presence of molecular oxygen, which is an attractive path for the production of this industrially important chemical. 

The cyclopropyl-hydrido complex 415 exhibits appreciable thermal instability, such that it rearranges quantitatively in ambient temperature benzene solutions to the rhodacyclobutane complex Tp Rh(cr,cr -CH2CH2CH2)(CNCH2 Bu) (154, Scheme 34, Section II-C.l). Thermal isomerisation of 154 (65 °C, 2.5 h) affords a 7i-propene complex (153) that is able to activate benzene. Alternatively, in the presence of excess CNCH2 Bu, 154 thermally inserts two equivalents of the isonitrile to ultimately afford the rhodacyclohexane complex 427 (Scheme 34). 

Other processes include the alkylation of phenol using alkenes, and the manufacture of acrylate and methacrylate esters from alcohols and the corresponding acids. Olefin hydration reactions require more extreme conditions but Deutsche Texaco have developed a resin-catalysed propene hydration process to form isopropyl alcohol [125]. The reaction is run at 130 C near the upper limit for sulphonic acid resins, but a species with sufficient lifetime is available. There is even some evidence that butene hydration is now carried out similarly. Finally, B.P. Chemicals have recently disclosed [126] a new olefin isomerisation process yielding 2,3-dimethylbut-l-ene. Here the conditions required to favour the isomerisation versus rapid oligomerisation had to be identified to establish a viable process. 

Upon heating, compound 71 (R = H) undergoes quantitative CR2 bridge collapse providing ethene whereas facile carbene isomerisation to propene occurs in the case of 71 (R = Me) upon thermolysis. 

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