Isomerism of butenes

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz ... 

In this connection, a recent article by Phillipson and Wells 44) dealing with the isomerization of butenes is of interest. The Group VIII metals catalyze both the hydrogenation and isomerization of butenes, cobalt... 

Commercial processes for the isomerization of butene and/or higher olefins were aruiounced by, among others, UOP (UOP Butesom and Pentesom proc-esses[50j), MW Kellogg (ISOFIN process[40]), Texaco (ISOTEX process [51]) and Lyondell [52]. The process scheme is quite simple and the Butesom process scheme is shown in Figure 14.7. 

The epoxidation of propylene is discussed in Chapter 10, Section 2. Some isobutane can be made by isomerizing -butane. The isomerization of -butenes to isobutylene is also being commercialized. 

As in the case of double bond isomerization of butenes, the double bond isomerization of VBH is considered to be initiated by abstraction of an allylic proton from a tertiary carbon atom to give an allylic anion that may be stabilized by metal ions, yielding the E- and Z-EBH isomers. 

Fig. 2. Possible elementary steps for isomerization of butenes. This diagram corresponds to listing (39).

The other secondary reaction that we observed is isomerization of butene formed in reaction +V (and, perhaps, in the supposed reaction IV). Table II shows the isomeric distribution of butenes that are formed from w-butyl alcohol upon dehydration at 423 K in the HZSM-5 zeolites with different crystallite sizes. It is seen that, for Sample 1 (with the smallest crystallite size), the isomeric distribution is very far from equilibrium (Table II). However, with the increase of the crystallite size or by inserting rather bulky Na+ ions into the... 

The isomerization of butene-1 and butene-2 has been studied over conventional ion exchange resins 110 in> as well as over the sulfonated and phosponated polyphenyls 110>. A careful kinetic study showed that the reaction over these materials as well as over silica-alumina catalysts goes by way of a common intermediate, which can be understood best as the secondary butyl carbonium ion. 

Rosynek et al. (32) reported that neither ammonia nor pyridine had a significant poisoning effect on the double-bond shift or cis-trans isomerization of butenes. This result appears to be quite questionable, since the retarding effects on the double-bond shift of ammonia (312, 376), triethylamine (365), and pyridine (377, 378) have clearly been demonstrated. 

The isomerization of butenes over MgO is a catalytic reaction which has been studied by a number of workers and is thought to occur via an anionic mechanism which involves basic sites on the surface. Baird and Lunsford (167) were able to show a correlation between the concentration of electron donors on the surface and the 1-butene isomerization rate, and they suggested that the formation of a carbanion occurred on O2- ions located on corner... 

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

A comparable study of the Ni-catalyzed reactions of isobutene, 1-butene, and cis-butene-2 in the presence of H2 and D2 has revealed a very complex series of chemical reactions that take place, including induced isomerization of 1-butene to 2-butene, deuterium exchange, induced cis-trans isomerization of butene-2, and finally, addition to the double bond. Below 200 mm Hg pressure, the rates of exchange, addition, and isomerization are about equal for 1-butene and are about % order in olefin and order in H2. With increasing excess of H2 this approaches zero order in olefin and % order in H2, while for large excess of 1-butene, all reactions become inhibited (30 to 150°C). Although the authors have attempted to discuss mechanisms in connection with the data, the lack of information on the isotherms makes that of dubious value. 

The acid strength of oxoacid salts in the solid state increases with the electronegativity of the metal ion, and hence catalytic activity increases with electronegativity. Figure 14 shows an example of selectivity, in which the selectivity of double bond isomerization of butene over metal sulfates changes with the electronegativity. ... 

Sigal studied the benzene d-6 sensitized isomerization of butene-2 at pressures below 0.1 mm., where the interval between collisions is long relative to the fluorescence lifetime. The butene-2 isomer concentration was monitored by long-path infrared spectrometry. The data are consistent with a kinetic scheme based on collisions being rate determining for intersystem crossover at low pressures. 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

Additional experiments have enabled researchers to distinguish between the monomolecular and bimolecular mechanisms in the skeletal isomerization of butenes. The experiments were based on two different approaches, one based on the use of C-labeled linear butenes and the identification of the label in the isobutylene product and the other based on the identification of the products resulting from the reactions of various possible Cg-olefin intermediates when the bimolecular mechanism prevails. 

Meriaudeau and Naccache conclude the volume with a concise description of skeletal isomerization of butenes catalyzed by medium-pore zeolites and molecular sieves. This isomerization is a relatively new industrial process. and it is remarkable how fast a good fundamental understanding of it has developed in a few years the chapter is an account of catalysis by well-defined acidic groups in pores that exert a subtle control over catalyst performance, including selectivity. It is a story that was deeply appreciated by Werner Haag. 

Step (7) was suggested by Groh et while Cundall and Davies preferred step (8). Investigating the acetone-sensitized cis-trans isomerization of butene-2, Cundall and Davies determined values for ks+k [A]) T (where t is the yield of the triplet acetone) at various acetone pressures. They found that the change in acetone pressure (in the range 25-100 torr) does not affect this ratio (at 3130 A... 

Primary step I The relative efficiencies of biacetyl and butene-2 in the quenching of reaction I were found to be not far from unity in the case of the ketones studied consequently, primary process I occurred from the high vibrational levels of the triplet state, possessing sufficient energy to induce efficiently the isomerization of butene-2. The efficiency of energy transfer to cw-butene-2 compared with that to biacetyl, measured in terms of reaction I, was found to be in the following order 2-pentanone > 2-butanone > acetone. It is possible that this order reflects the facility with which the vibrational energy of the donors may be made available to... 

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